2024
Topologically Localized Vibronic Excitations in Second-layer Graphene Nanoribbons
Zhengya Wang, Ruoting Yin, Zixi Tang, Hongjian Du, Yifan Liang, Xiaoqing Wang, Qing-Song Deng, Yuan-Zhi Tan, Yao Zhang, Chuanxu Ma, Shijing Tan & Bing Wang
Physical Review Lwtters Accepted (2024)
It is of fundamental importance to characterize the intrinsic properties, like the topological end states, in the on-surface synthesized graphene nanoribbons (GNRs), but the strong electronic interaction with the metal substrate usually smears out their characteristic features. Here, we report our approach to investigate the vibronic excitations of the topological end states in self-decoupled second-layer GNRs, which are grown using an on-surface squeezing-induced spillover strategy. The vibronic progressions show highly spatially localized distributions at the second-layer GNR ends, which can be ascribed to the decoupling-extended lifetime of charging through resonant electron tunneling at the topological end states. In combination with theoretical calculations, we assign the vibronic progressions to specific vibrational modes that mediate the vibronic excitations. The spatial distribution of each resolved excitation shows evident characteristics beyond the conventional Franck−Condon picture. Our work by direct growth of second-layer GNRs provides an effective way to explore the interplay between the intrinsic electronic, vibrational, and topological properties.
Artificial Kagome Lattices of Shockley Surface States Patterned by Halogen Hydrogen-bonded Organic Frameworks
Ruoting Yin†, Xiang Zhu†, Qiang Fu, Tianyi Hu, Lingyun Wan, Yingying Wu, Yifan Liang, Zhengya Wang, Zhen-Lin Qiu, Yuan-Zhi Tan, Chuanxu Ma, Shijing Tan, Wei Hu, Bin Li, Z. F. Wang, Jinlong Yang & Bing Wang
Nature Communications 15, 2969 (2024)
Artificial electronic kagome lattices may emerge from electronic potential landscapes using customized structures with exotic supersymmetries, benefiting from the confinement of Shockley surface-state electrons on coinage metals, which offers a flexible approach to realizing intriguing quantum phases of matter that are highly desired but scarce in available kagome materials. Here, we devise a general strategy to construct varieties of electronic kagome lattices by utilizing the on-surface synthesis of halogen hydrogen-bonded organic frameworks (XHOFs). As a proof of concept, we demonstrate three XHOFs on Ag(111) and Au(111) surfaces, which correspondingly deliver regular, breathing, and chiral breathing diatomic-kagome lattices with patterned potential landscapes, showing evident topological edge states at the interfaces. The combination of scanning tunnelling microscopy and noncontact atomic force microscopy, complemented by density functional theory and tight-binding calculations, directly substantiates our method as a reliable and effective way to achieve electronic kagome lattices for engineering quantum states.
Unveiling Diverse Coordination-defined Electronic Structures of Reconstructed Anatase TiO2(001)-(1 × 4) Surface
Xiaochuan Ma†, Yongliang Shi†, Zhengwang Cheng†, Xiaofeng Liu, Jianyi Liu, Ziyang Guo, Xuefeng Cui, Xia Sun, Jin Zhao, Shijing Tan & Bing Wang
Nature Communications 15, 2326 (2024)
Transition metal oxides (TMOs) exhibit fascinating physicochemical properties, which originate from the diverse coordination structures between the transition metal and oxygen atoms. Accurate determination of such structure-property relationships of TMOs requires to correlate structural and electronic properties by capturing the global parameters with high resolution in energy, real, and momentum spaces, but it is still challenging. Herein, we report the determination of characteristic electronic structures from diverse coordination environments on the prototypical anatase-TiO2(001) with (1 × 4) reconstruction, using high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/atomic force microscopy, in combination with density functional theory calculation. We unveil that the shifted positions of O 2s and 2p levels and the gap-state Ti 3p levels can sensitively characterize the O and Ti coordination environments in the (1 × 4) reconstructed surface, which show distinguishable features from those in bulk. Our findings provide a paradigm to interrogate the intricate reconstruction-relevant properties in many other TMO surfaces.
2023
Graphene nanoribbons (GNRs) are strips of graphene, with widths of a few nanometers, that are promising candidates for future applications in nanodevices and quantum information processing due to their highly tunable structure-dependent electronic, spintronic, topological, and optical properties. Implantation of periodic structural heterogeneities, such as heteroatoms, nanopores, and non-hexagonal rings, has become a powerful manner for tailoring the designer properties of GNRs. The bottom-up synthesis approach, by combining on-surface chemical reactions based on rationally designed molecular precursors and in situ tip-based microscopic and spectroscopic techniques, promotes the construction of atomically precise GNRs with periodic structural modulations. However, there are still obstacles and challenges lying on the way toward the understanding of the intrinsic structure–property relations, such as the strong screening and Fermi level pinning effect of the normally used transition metal substrates and the lack of collective tip-based techniques that can cover multi-internal degrees of freedom of the GNRs. In this Perspective, we briefly review the recent progress in the on-surface synthesis of GNRs with diverse structural heterogeneities and highlight the structure–property relations as characterized by the noncontact atomic force microscopy and scanning tunneling microscopy/spectroscopy. We furthermore motivate to deliver the need for developing strategies to achieve quasi-freestanding GNRs and for exploiting multifunctional tip-based techniques to collectively probe the intrinsic properties.
Revealing Intramolecular Isotope Effects with Chemical-bond Precision
Xiang Zhu†, Jiayu Xu†, Yao Zhang, Bin Li, Yishu Tian, Yingying Wu, Zhiwei Liu, Chuanxu Ma, Shijing Tan & Bing Wang
Journal of the American Chemical Society 145, 13839–13845 (2023)
Isotope substitution of a molecule not only changes its vibrational frequencies but also changes its vibrational distributions in real-space. Quantitatively measuring the isotope effects inside a polyatomic molecule requires both energy and spatial resolutions at the single-bond level, which has been a long-lasting challenge in macroscopic techniques. By achieving ångström resolution in tip-enhanced Raman spectroscopy (TERS), we record the corresponding local vibrational modes of pentacene and its fully deuterated form, enabling us to identify and measure the isotope effect of each vibrational mode. The measured frequency ratio νH/νD varies from 1.02 to 1.33 in different vibrational modes, indicating different isotopic contributions of H/D atoms, which can be distinguished from TERS maps in real-space and well described by the potential energy distribution simulations. Our study demonstrates that TERS can serve as a non-destructive and highly sensitive methodology for isotope detection and recognition with chemical-bond precision.
Remote-triggered Domino-like Cyclodehydrogenation in Second-layer Topological Graphene Nanoribbons
Chuanxu Ma†, Jufeng Wang†, Huanhuan Ma†, Ruoting Yin†, Xin-Jing Zhao, Hongjian Du, Xinyong Meng, Yifan Ke, Wei Hu, Bin Li, Shijing Tan, Yuan-Zhi Tan, Jinlong Yang & Bing Wang
Journal of the American Chemical Society 145, 10126–10135 (2023)
Cyclodehydrogenation reactions in the on-surface synthesis of graphene nanoribbons (GNRs) usually involve a series of Csp2–Csp2 and/or Csp2–Csp3 couplings and just happen on uncovered metal or metal oxide surfaces. It is still a big challenge to extend the growth of second-layer GNRs in the absence of necessary catalytic sites. Here, we demonstrate the direct growth of topologically nontrivial GNRs via multistep Csp2–Csp2 and Csp2–Csp3 couplings in the second layer by annealing designed bowtie-shaped precursor molecules over one monolayer on the Au(111) surface. After annealing at 700 K, most of the polymerized chains that appear in the second layer covalently link to the first-layer GNRs that have partially undergone graphitization. Following annealing at 780 K, the second-layer GNRs are formed and linked to the first-layer GNRs. Benefiting from the minimized local steric hindrance of the precursors, we suggest that the second-layer GNRs undergo domino-like cyclodehydrogenation reactions that are remotely triggered at the link. We confirm the quasi-freestanding behaviors in the second-layer GNRs by measuring the quasiparticle energy gap of topological bands and the tunable Kondo resonance from topological end spins using scanning tunneling microscopy/spectroscopy combined with first-principles calculations. Our findings pave the avenue to diverse multilayer graphene nanostructures with designer quantum spins and topological states for quantum information science.
Revealing the Polaron State at the MoS2/TiO2 Interface
Miaomiao Xiang†, Xiaochuan Ma†, Chang Gao†, Ziyang Guo, Chenxi Huang, Yue Xing, Shijing Tan, Jin Zhao, Bing Wang & Xiang Shao
The Journal of Physical Chemistry Letters 14, 3360–3367 (2023)
Interfacial polarons determine the distribution of free charges at the interface and thus play important roles in manipulating the physicochemical properties of hybridized polaronic materials. In this work, we investigated the electronic structures at the atomically flat interface of the single-layer MoS2 (SL-MoS2) on the rutile TiO2 surface using high-resolution angle-resolved photoemission spectroscopy. Our experiments directly visualized both the valence band maximum and the conduction band minimum (CBM) of SL-MoS2 at the K point, which clearly defines a direct bandgap of ~2.0 eV. Detailed analyses corroborated by density functional theory calculations demonstrated that the CBM of MoS2 is formed by the trapped electrons at the MoS2/TiO2 interface that couple with the longitudinal optical phonons in the TiO2 substrate through an interfacial Fröhlich polaron state. Such an interfacial coupling effect may register a new route for tuning the free charges in the hybridized systems of two-dimensional materials and functional metal oxides.
Self-limited Embedding Alternating 585-Ringed Divacancies and Metal Atoms into Graphene Nanoribbons
Zhengya Wang†, Ruoting Yin†, Jie Meng†, Jianing Wang, Yifan Liang, Chuanxu Ma, Shijing Tan, Qunxiang Li, Jinlong Yang & Bing Wang
Journal of the American Chemical Society 145, 8445–8454 (2023)
Because of their theoretically predicted intriguing properties, it is interesting to embed periodic 585-ringed divacancies into graphene nanoribbons (GNRs), but it remains a great challenge. Here, we develop an on-surface cascade reaction from periodic hydrogenated divacancies to alternating 585-ringed divacancies and Ag atoms via intramolecular cyclodehydrogenation in a seven-carbon-wide armchair GNR on the Ag(111) surface. Combining scanning tunneling microscopy/spectroscopy and noncontact atomic force microscopy combined with first-principles calculations, we in-situ-monitor the evolution of the distinct structural and electronic properties in the reaction intermediates. The observation of embedded Ag atoms and further nudged elastic band calculations provide unambiguous evidence for Ag adatom-mediated C–H activation in the intramolecular cyclodehydrogenation pathway, where the strain-induced self-limiting effect contributes to the formation of the GNR superlattice with alternating 585-ringed divacancies and Ag atoms, which shows a band gap of about 1.4 eV. Our findings open an avenue to introducing periodic impurities of single metal atoms and nonhexagonal rings in on-surface synthesis, which may provide a novel route for multifunctional graphene nanostructures.
In-situ Measurements of Reconstructed Anatase TiO2(001) Surface by Variable-temperature STM
Jianyi Liu, Xiaochuan Ma, Xintong Li, Zhengwang Cheng, Xuefeng Cui & Bing Wang
Chinese Journal of Chemical Physics 36, 125-131 (2023)
The catalytic performance of metal oxide surface mainly depends on its atomic surface structure, which usually changes under various treatment conditions and during catalytic reactions. Therefore, it is quite important to acquire the atomic geometries of the surfaces under different treatments for further understanding the catalytic mechanisms in the surfaces with complicated reconstructions. Here, we report the investigation on the evolution of surface geometries of the Ar+-ion-sputtered anatase TiO2(001) films followed by heating treatments at various temperatures, characterized using variable-temperature scanning tunneling microscopy. Our experimental results reveal the different surface morphologies at different heating temperatures. During the heating treatment, the migrations of O atoms from the bulk to the surface of TiO2(001) play an important role in the reoxidation of the Ti2+ and Ti3+ states for the formation of (1×4) reconstruction. The atomic-resolution images of the ridges show asymmetric features, which well support the fully oxidized structural model of the reconstructed TiO2(001)-(1×4) surface.
2022
Two-dimensional Dirac-line Semimetals Resistant to Strong Spin–orbit Coupling
Deping Guo†, Pengjie Guo†, Shijing Tan, Min Feng, Limin Cao, Zheng-Xin Liu, Kai Liu, Zhong-Yi Lu & Wei Ji
Science Bulletin 67, 1954-1957 (2022)
There is no abstract for this article.
Structural and Superconducting Properties of Low-temperature Ultrathin PbBi3 Films
Juefeng Wang, Mingyang Tian, Hongjian Du, Chuanxu Ma & Bing Wang
Acta Physica Sinica 71, 127401 (2022)
Bismuth (Bi), as a stable heaviest element in the periodic table of elements, has strong spin-orbit coupling, which has attracted a lot of attention as the parent material of various known topological insulators. Previous calculations predicted that Bi(111) with a thickness less than eight bilayers and the ultrathin black-phosphorus-like Bi(110) films are single-element two-dimensional (2D) topological insulators. However, it is generally believed that these crystalline bismuth phases are not superconducting or their transition temperature should be lower than 0.5 mK. Lead (Pb) is a good superconducting elementary material, and there is a relatively small difference in radius between the Bi atom and Pb atom. According to the Hume-Rothery rule, it is expected that Pb/Bi alloys in an arbitrary ratio should be superconducting. One may thus expect to form crystalline Bi based superconductors by Pb substitution, which might host intriguing topological superconductivity. While our previous work has demonstrated a low-temperature stable Pb1–xBix (x~0.1) alloy phase in which Pb in the Pb(111) structure is partially replaced by Bi, the Bi crystalline structure-based phases of the superconducting alloys still lack in-depth research. Here, we report a new low-temperature phase of Pb-Bi alloy thin film, namely PbBi3, on the Si(111)-(7×7) substrate, by co-depositing Pb and Bi at a low temperature of about 100 K followed by an annealing treatment of 200 K for 2 h. Using low-temperature scanning tunneling microscopy and spectroscopy (STM/STS), we characterize in situ the surface structure and superconducting properties of the Pb-Bi alloy film with a nominal thickness of about 4.8 nm. Two spatially separated phases with quasi-tetragonal structure are observed in the surface of the Pb-Bi alloy film, which can be identified as the pure Bi(110) phase and the PbBi3 phase, respectively, based on their distinct atomic structures, step heights and STS spectra. The PbBi3 film has a base structure similar to Bi(110), where about 25% of the Bi atoms are replaced by Pb, and the surface shows a √2×√2R45° reconstructed structure. The superconducting behavior of the PbBi3 phase is characterized using variable-temperature STS spectra. We obtain that the superconducting transition temperature of PbBi3 is about 6.13 K, and the 2Δ(0)/kBTC ratio is about 4.62 using the fitting parameter of Δ(0)=1.22 meV at 0 K. By measuring the magnetic field dependent superconducting coherence length, the critical field is estimated at larger than 0.92 T. We further investigate the superconducting proximity effect in the normal metal-superconductor (N-S) heterojunction consisting of the non-superconducting Bi(110) domain and the superconducting PbBi3 domain. The N-S heterojunctions with both in-plane configuration and step-like configuration are measured, which suggest that the atomic connection and the area of the quasi-2D Josephson junctions and the external magnetic field can affect the lateral superconducting penetration length. We also observe the zero-bias conductance peaks (ZBCPs) in the superconducting gap of the PbBi3 surface in some cases at zero magnetic field. By measuring dI/dV spectra at various temperatures and by adopting a superconducting Nb tip, we identify that the ZBCP originates from the superconductor-insulator-superconductor (S-I-S) junction formed between a superconducting tip and the sample. Nevertheless, the Bi(110)-based PbBi3 phase may provide a possible platform to explore the intriguing topological superconducting behaviors at the vortexes under magnetic fields, or in the vicinity of the potentially topological superconducting Bi(110) islands by considering the proximity effect.
Step-assisted On-surface Synthesis of Graphene Nanoribbons Embedded with Periodic Divacancies
Ruoting Yin†, Jianing Wang†, Zhen-Lin Qiu†, Jie Meng, Huimin Xu, Zhengya Wang, Yifan Liang, Xin-Jing Zhao, Chuanxu Ma, Yuan-Zhi Tan, Qunxiang Li & Bing Wang
Journal of the American Chemical Society 144, 14798–14808 (2022)
The bottom-up approach through on-surface synthesis of porous graphene nanoribbons (GNRs) presents a controllable manner for implanting periodic nanostructures to tune the electronic properties of GNRs in addition to bandgap engineering by width and edge configurations. However, owing to the existing steric hindrance in small pores like divacancies, it is still difficult to embed periodic divacancies with a nonplanar configuration into GNRs. Here, we demonstrate the on-surface synthesis of atomically precise eight-carbon-wide armchair GNRs embedded with periodic divacancies (DV8-aGNRs) by utilizing the monatomic step edges on the Au(111) surface. From a single molecular precursor correspondingly following a trans- and cis-coupling, the DV8-aGNR and another porous nanographene are respectively formed at step edges and on terraces at 720 and 570 K. Combining scanning tunneling microscopy/spectroscopy, atomic force microscopy, and first-principles calculations, we determine the out-of-plane conformation, wide bandgap (∼3.36 eV), and wiggly shaped frontier orbitals of the DV8-aGNR. Nudged elastic band calculations further quantitatively reveal that the additional steric hindrance effect in the cyclodehydrogenative reactions has a higher barrier of 1.3 eV than that in the planar porous nanographene, which also unveils the important role played by the monatomic Au step and adatoms in reducing the energy barriers and enhancing the thermodynamic preference of the oxidative cyclodehydrogenation. Our results provide the first case of GNRs containing periodic pores as small as divacancies with a nonplanar configuration and demonstrate the strategy by utilizing the chemical heterogeneity of a substrate to promote the formation of novel carbon nanomaterials.
Hydrogen-bond Network Promotes Water Splitting on the TiO2 Surface
Xiaochuan Ma†, Yongliang Shi†, Jianyi Liu, Xintong Li, Xuefeng Cui, Shijing Tan, Jin Zhao & Bing Wang
Journal of the American Chemical Society 144, 13565–13573 (2022)
Breaking the strong covalent O–H bond of an isolated H2O molecule is difficult, but it can be largely facilitated when the H2O molecule is connected with others through hydrogen-bonding. How a hydrogen-bond network forms and performs becomes crucial for water splitting in natural photosynthesis and artificial photocatalysis and is awaiting a microscopic and spectroscopic understanding at the molecular level. At the prototypical photocatalytic H2O/anatase-TiO2(001)-(1×4) interface, we report the hydrogen-bond network can promote the coupled proton and hole transfer for water splitting. The formation of a hydrogen-bond network is controlled by precisely tuning the coverage of water to above one monolayer. Under ultraviolet (UV) light irradiation, the hydrogen-bond network opens a cascaded channel for the transfer of a photoexcited hole, concomitant with the release of the proton to form surface hydroxyl groups. The yielded hydroxyl groups provide excess electrons to the TiO2 surface, causing the reduction of Ti4+ to Ti3+ and leading to the emergence of gap states, as monitored by in situ UV/X-ray photoelectron spectroscopy. The density functional theory calculation reveals that the water splitting becomes an exothermic process through hole oxidation with the assistance of the hydrogen-bond network. In addition to the widely concerned exotic activity from photocatalysts, our study demonstrates the internal hydrogen-bond network, which is ubiquitous at practical aqueous/catalyst interfaces, is also indispensable for water splitting.
Ultrafast Charge Transfer Coupled to Quantum Proton Motion at Molecule/Metal Oxide Interface
Weibin Chu, Shijing Tan, Qijing Zheng, Wei Fang, Yexin Feng, Oleg V. Prezhdo, Bing Wang, Xin-Zheng Li & Jin Zhao
Science Advances 8, eabo2675 (2022)
Understanding how the nuclear quantum effects (NQEs) in the hydrogen bond (H-bond) network influence the photoexcited charge transfer at semiconductor/molecule interface is a challenging problem. By combining two kinds of emerging molecular dynamics methods at the ab initio level, the path integral–based molecular dynamics and time-dependent nonadiabatic molecular dynamics, and choosing CH3OH/TiO2 as a prototypical system to study, we find that the quantum proton motion in the H-bond network is strongly coupled with the ultrafast photoexcited charge dynamics at the interface. The hole trapping ability of the adsorbed methanol molecule is notably enhanced by the NQEs, and thus, it behaves as a hole scavenger on titanium dioxide. The critical role of the H-bond network is confirmed by in situ scanning tunneling microscope measurements with ultraviolet light illumination. It is concluded the quantum proton motion in the H-bond network plays a critical role in influencing the energy conversion efficiency based on photoexcitation.
Electronic and Topological Properties of Bi(110) Ultrathin Films Grown on a Cu(111) Substrate
Jufeng Wang, Xia Sun, Hongjian Du, Chuanxu Ma & Bing Wang
Physical Review B 105, 115407 (2022)
The electronic and topological properties of Bi(110) ultrathin films epitaxially grown on a Cu(111) substrate are investigated using scanning tunneling microscopy/spectroscopy, combined with density functional theory calculations. Bilayer-by-bilayer growth of the ultrathin Bi(110) films is observed, which manifests a structure of black-phosphorus-like Bi bilayers (BLs), with thickness up to 4 BLs. The surface atomic buckling heights in the 1-BL and 2-BL films are clearly identified to depend on the stacking modes with respect to the well-ordered Bi atoms in the adlayer covered on the Cu(111) surface. Our results demonstrate that while the electronic and topological properties of 1-BL films greatly depend on the stacking modes between the Bi(110) bilayer and the adlayer, the 2-BL films show well-decoupled electronic properties from the Cu substrate and nontrivial topologies robust against surface atomic buckling height benefitting from the interbilayer coupling. Our calculations further show that besides the parameter of buckling heights, the topological nontrivial-to-trivial transition can also be induced by changing the interbilayer distance and the vertical intralayer bond length.
2021
Determining Structural and Chemical Heterogeneities of Surface Species at the Single-bond Limit
Jiayu Xu†, Xiang Zhu†, Shijing Tan , Yao Zhang, Bin Li, Yunzhe Tian, Huan Shan, Xuefeng Cui, Aidi Zhao, Zhenchao Dong, Jinlong Yang, Yi Luo, Bing Wang & J. G. Hou
Science 371, 818-822 (2021)
The structure determination of surface species has long been a challenge because of their rich chemical heterogeneities. Modern tip-based microscopic techniques can resolve heterogeneities from their distinct electronic, geometric, and vibrational properties at the single-molecule level but with limited interpretation from each. Here, we combined scanning tunneling microscopy (STM), noncontact atomic force microscopy (AFM), and tip-enhanced Raman scattering (TERS) to characterize an assumed inactive system, pentacene on the Ag(110) surface. This enabled us to unambiguously correlate the structural and chemical heterogeneities of three pentacene-derivative species through specific carbon-hydrogen bond breaking. The joint STM-AFM-TERS strategy provides a comprehensive solution for determining chemical structures that are widely present in surface catalysis, on-surface synthesis, and two-dimensional materials.
On-surface Cyclodehydrogenation Reaction Pathway Determined by Selective Molecular Deuterations
Chuanxu Ma, Zhongcan Xiao, Peter V. Bonnesen, Liangbo Liang, Alexander A. Puretzky, Jingsong Huang, Marek Kolmer, Bobby G. Sumpter, Wenchang Lu, Kunlun Hong, Jerzy Bernholc & An-Ping Li
Chemical Science 12, 15637-15644 (2021)
Understanding the reaction mechanisms of dehydrogenative Caryl–Caryl coupling is the key to directed formation of π-extended polycyclic aromatic hydrocarbons. Here we utilize isotopic labeling to identify the exact pathway of cyclodehydrogenation reaction in the on-surface synthesis of model atomically precise graphene nanoribbons (GNRs). Using selectively deuterated molecular precursors, we grow seven-atom-wide armchair GNRs on a Au(111) surface that display a specific hydrogen/deuterium (H/D) pattern with characteristic Raman modes. A distinct hydrogen shift across the fjord of Caryl–Caryl coupling is revealed by monitoring the ratios of gas-phase by-products of H2, HD, and D2 with in situ mass spectrometry. The identified reaction pathway consists of a conrotatory electrocyclization and a distinct [1,9]-sigmatropic D shift followed by H/D eliminations, which is further substantiated by nudged elastic band simulations. Our results not only clarify the cyclodehydrogenation process in GNR synthesis but also present a rational strategy for designing on-surface reactions towards nanographene structures with precise hydrogen/deuterium isotope labeling patterns.
Graphene Nanoribbons for Quantum Electronics
Haomin Wang†, Hui Shan Wang, Chuanxu Ma†, Lingxiu Chen, Chengxin Jiang, Chen Chen, Xiaoming Xie, An-Ping Li & Xinran Wang
Nature Reviews Physics 3, 791–802 (2021)
Graphene nanoribbons (GNRs) are a family of one-dimensional (1D) materials with a graphitic lattice structure. GNRs possess high mobility and current-carrying capability, sizeable bandgap and versatile electronic properties, which make them promising candidates for quantum electronic applications. In the past 5 years, progress has been made towards atomically precise bottom-up synthesis of GNRs and heterojunctions that provide an ideal platform for functional molecular devices, as well as successful production of semiconducting GNR arrays on insulating substrates potentially useful for large-scale digital circuits. With further development, GNRs can be envisioned as a competitive candidate material in future quantum information sciences. In this Perspective, we discuss recent progress in GNR research and identify key challenges and new directions likely to develop in the near future.
Time- and Momentum-Resolved Image-potential States of 2H-MoS2 Surface
Jianyi Liu, Xiang Jiang, Xintong Li, Xiaochuan Ma, Xia Sun, Qijing Zheng, Xuefeng Cui, Shijing Tan, Jin Zhao & Bing Wang
Physical Chemistry Chemical Physics 23, 26336-26342 (2021)
Rydberg-like image potential states (IPSs) form special series surface states on metal and semiconducting surfaces. Here, using time-resolved and momentum-resolved multi-photon photoemission (mPPE), we measured the energy positions, band dispersion, and carrier lifetimes of IPSs at the 2H-MoS2 surface. The energy minima of the IPSs (n = 1 and 2) were located at 0.77 and 0.21 eV below the vacuum level. In addition, the effective masses of these two IPSs are close to the rest mass of the free electron, clearly showing nearly-free-electron character. These properties suggest a good screening effect in the MoS2parallel to the surface. The multi-photon resonances between the valence band and IPS (n = 1) are observed, showing a k‖-momentum-dependent behavior. Our time-resolved mPPE measurements show that the lifetime of photoexcited electrons in the IPS (n = 1) is about 33 fs.
Lead-bismuth (Pb-Bi) alloys, as a superconducting material, have been widely studied at their superconducting transition temperatures and the critical magnetic fields for different composition ratios. Most of experimental studies focused on the stable ε phase formed at high temperatures, but less on the Pb-Bi alloys grown at low temperatures. So far, the structural and superconducting properties of the low-temperature Pb-Bi phases are far from good understanding. Here, we report our investigation of structural and superconducting properties of a low-temperature phase of Pb-Bi alloy. The Pb-Bi alloy films with a nominal thickness of about 6 nm are prepared by co-depositing Bi and Pb on Bi(111)/Si(111)-(7×7) substrates at a low temperature of 100 K followed by annealing at a treatment of 200 K for 2 h. The structural and superconducting properties of the Pb-Bi alloy films are characterized in situ by using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS). It is observed that the spatially separated phases of nearly pure Bi(111) domains and Pb1–xBix alloy domains are formed in the films, where these phases can be identified by their distinct differences in the atomic structure and the distributions of step heights in the atomically resolved STM images, as well as by their distinguished STS spectra. The Pb1–xBix. alloy phase presents the structure of Pb(111), in which about x ≈ 0.1 Bi is substituted for Pb. The STS spectra show that the Pb1–xBix alloy phase is superconducting, with a transition temperature TC = 7.77 K derived from the variable-temperature measurements. This transition temperature is higher than that in pure Pb film (6.0–6.5 K), which can be well explained by the Mattias rules, with considering the fact that the average number of valance electrons increases after Bi atoms with five valance-electrons have been substituted for Pb atoms with four valance-electrons. The analysis shows that the ratio 2Δ(0)/kBTC is about 4.94 with the superconducting gap Δ(0)=1.66 meV at 0 K, indicating that the Pb1–xBix alloy is a strongly-coupled superconductor. The non-superconducting Bi(111) and the superconducting Pb1–xBix alloy domains form an in-plane superconductor-normal metal-superconductor (S-N-S) Josephson junction. The proximity effect in the Bi(111) domains is measured at different N-S junctions, which suggests that the lateral superconducting penetration length in Bi(111) might be affected by the area of the quasi-two-dimensional interface. The superconducting gap in the Bi(111) region with a narrow width of 23 nm in an S-N-S Josephson junction is found to be greatly enhanced due to the existence of multiple Andreev reflections. Since Bi can host potential topological properties, the lateral Bi(111)-Pb1–xBix heterostructures, because of the existing proximity effect, could have potential applications in exploring the novel topological and superconducting phenomena.
Patterning of Transition Metal Dichalcogenides Catalyzed by Surface Plasmons with Atomic Precision
Xiaoli Zhou†, He Hao†, Ying-Jie Zhang, Qijing Zheng, Shijing Tan, JinZhao, Hai-Bo Chen, Jie-Jie Chen, Ying Gu, Han-Qing Yu & Xian-Wei Liu
Chem 7, 1626-1638 (2021)
Plasmon-induced charge transfer has drawn considerable interests and spurred rapid developments of photovoltaics and photocatalysis. Various plasmonic metal/transition metal dichalcogenide (TMD) heterostructures have been developed to promote charge separation. For plasmon-induced catalysis, the transformation of TMDs is rare, and the erosion of TMDs has not yet been reported. Here, we report the etching of two-dimensional TMDs by planar surface plasmon polaritons (SPPs). We found that SPPs can etch various TMDs into desired layers and lateral size through controlling the light power and incident direction. In aqueous media, the strong plasmonic coupling at Au/MoS2 interface generates holes in the valence band of MoS2 that can weaken its interlayer interaction. As a synergistic effect, the plasmonic hot electrons produce oxidizing H2O2 to dissolve MoS2 from the top layers. Our results provide a new perspective for understanding the plasmonic coupling at metal-TMD interfaces and a reliable route toward fabricating well-defined TMDs.
Formation of Plasmonic Polarons in Highly Electron-doped Anatase TiO2
Xiaochuan Ma†, Zhengwang Cheng†, Mingyang Tian, Xiaofeng Liu, Xuefeng Cui, Yaobo Huang, Shijing Tan, Jinlong Yang & Bing Wang
Nano Letters 21, 430–436 (2021)
The existence of various quasiparticles of polarons because of electron–boson couplings plays important roles in determining electron transport in titanium dioxide (TiO2), which affects a wealth of physical properties from catalysis to interfacial superconductivity. In addition to the well-defined Fröhlich polarons whose electrons are dressed by the phonon clouds, it has been theoretically predicted that electrons can also couple to their own plasmonic oscillations, namely, the plasmonic polarons. Here we experimentally demonstrate the formation of plasmonic polarons in highly doped anatase TiO2 using angle-resolved photoemission spectroscopy. Our results show that the energy separation of plasmon-loss satellites follows a dependence on √n, where n is the electron density, manifesting the characteristic of plasmonic polarons. The spectral functions enable to quantitatively evaluate the strengths of electron–plasmon and electron–phonon couplings, respectively, providing an effective approach for characterizing the interplays among different bosonic modes in the complicate many-body interactions.
2020
Interfacial Polarons in van der Waals Heterojunction of Monolayer SnSe2 on SrTiO3(001)
Yahui Mao†, Xiaochuan Ma†, Daoxiong Wu, Chen Lin, Huan Shan, Xiaojun Wu, Jin Zhao, Aidi Zhao & Bing Wang
Nano Letters 20, 8067–8073 (2020)
Interfacial polarons have been demonstrated to play important roles in heterostructures containing polar substrates. However, most of polarons found so far are diffusive large polarons; the discovery and investigation of small polarons at interfaces are scarce. Herein, we report the emergence of interfacial polarons in monolayer SnSe2 epitaxially grown on Nb-doped SrTiO3 (STO) surface using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). ARPES spectra taken on this heterointerface reveal a nearly flat in-gap band correlated with a significant charge modulation in real space as observed with STM. An interfacial polaronic model is proposed to ascribe this in-gap band to the formation of self-trapped small polarons induced by charge accumulation and electron–phonon coupling at the van der Waals interface of SnSe2 and STO. Such a mechanism to form interfacial polaron is expected to generally exist in similar van der Waals heterojunctions consisting of layered 2D materials and polar substrates.
We report our investigation on the electron-phonon coupling in graphite using scanning tunneling microscopy/spectroscopy (STM/STS), in combination with angle-resolved photoemission spectroscopy (ARPES). The exhibition of a gaplike feature near the Fermi level both in the STS and ARPES spectra confirms its intrinsic origination because of the electron-phonon coupling in graphite, where the phonon modes at ±65 and ±160 meV are highly resolvable in the clean samples. Under our controlled experiment using W, Pt/Ir, and Ag tips, we find that the presence of hydrogen at the tip apex or in the tip-substrate gap is responsible for smearing out the gaps in the STS spectra by providing additional tunneling channels. Due to its small adsorption energy of hydrogen for Ag among these tip materials, our results suggest that the use of Ag tips can be more suitable for resolving the intrinsic phonon-induced property in carbon-based materials.
Electric field control of charge carrier density provides a key in situ technology to continuously tune the ground states and map out the phase diagram of correlated electron systems in one device. This technique is highly expected to be combined with the modern state-of-the art spectroscopic probes, such as angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy (STM/S), to efficiently address these states and the underlying physics. However, it is extremely difficult and not successful so far, mainly because the fabrication process of such devices makes them prohibitive for surface probes. Here, by using a solid Li-ion conductor (SIC) as gate dielectric, we have successfully developed gate-tunable STM/S and visualized the superconductor–insulator transition (SIT) in a thin flake of single crystal (Li, Fe)OHFeSe at the nanoscale. The gate-controlled Li-ion injection first enhances the superconductivity and then drives the flake into an inhomogeneous insulating state, where superconductivity is totally suppressed. This process can be reversed by applying an opposite gate voltage. Importantly, the atomically resolved images allow us to identify the critical role that the injected Li ions play in the tuning process. Our results not only provide clear evidence of the microscopic mechanism of the tunable superconductivity and SIT in the SIC-based (Li, Fe)OHFeSe devices, but also establish SIC-gating STM as a powerful tool for investigating the complicated phase diagram of correlated electron system spectroscopically in a single sample with the field-effect approach.
Engineering Edge States of Graphene Nanoribbons for Narrow-band Photoluminescence
Chuanxu Ma, Zhongcan Xiao, Alexander A. Puretzky, Hao Wang, Ali Mohsin, Jingsong Huang, Liangbo Liang, Yingdong Luo, Benjamin J. Lawrie, Gong Gu, Wenchang Lu, Kunlun Hong, Jerzy Bernholc & An-Ping Li
ACS Nano 14, 5090–5098 (2020)
Solid-state narrow-band light emitters are on-demand for quantum optoelectronics. Current approaches based on defect engineering in low-dimensional materials usually introduce a broad range of emission centers. Here, we report narrow-band light emission from covalent heterostructures fused to the edges of graphene nanoribbons (GNRs) by controllable on-surface reactions from molecular precursors. Two types of heterojunction (HJ) states are realized by sequentially synthesizing GNRs and graphene nanodots (GNDs) and then coupling them together. HJs between armchair GNDs and armchair edges of the GNR are coherent and give rise to narrow-band photoluminescence. In contrast, HJs between the armchair GNDs and the zigzag ends of GNRs are defective and give rise to nonradiative states near the Fermi level. At low temperatures, sharp photoluminescence emissions with peak energy range from 2.03 to 2.08 eV and line widths of 2–5 meV are observed. The radiative HJ states are uniform, and the optical transition energy is controlled by the band gaps of GNRs and GNDs. As these HJs can be synthesized in a large quantity with atomic precision, this finding highlights a route to programmable and deterministic creation of quantum light emitters.
Molecular Molds for Regularizing Kondo States at Atom/Metal Interfaces
Xiangyang Li†, Liang Zhu†, Bin Li, Jingcheng Li, Pengfei Gao, Longqing Yang, Aidi Zhao, Yi Luo, Jianguo Hou, Xiao Zheng, Bing Wang & Jinlong Yang
Nature Communications 11, 2566 (2020)
Adsorption of magnetic transition metal atoms on a metal surface leads to the formation of Kondo states at the atom/metal interfaces. However, the significant influence of surrounding environment presents challenges for potential applications. In this work, we realize a novel strategy to regularize the Kondo states by moving a CoPc molecular mold on an Au(111) surface to capture the dispersed Co adatoms. The symmetric and ordered structures of the atom-mold complexes, as well as the strong dπ–π bonding between the Co adatoms and conjugated isoindole units, result in highly robust and uniform Kondo states at the Co/Au(111) interfaces. Even more remarkably, the CoPc further enables a fine tuning of Kondo states through the molecular-mold-mediated superexchange interactions between Co adatoms separated by more than 12 Å. Being highly precise, efficient and reproducible, the proposed molecular mold strategy may open a new horizon for the construction and control of nano-sized quantum devices.
Creation of the Dirac Nodal Line by Extrinsic Symmetry Engineering
Mingyang Tian†, Jufeng Wang†, Xiaofeng Liu†, Weiwei Chen†, Zhao Liu, Hongjian Du, Xiaochuan Ma, Xuefeng Cui, Aidi Zhao, Qinwei Shi, Zhengfei Wang, Yi Luo, Jinlong Yang, Bing Wang & J. G. Hou
Nano Letters 20, 2157–2162 (2020)
The formation of the Dirac nodal line (DNL) requires intrinsic symmetry that can protect the degeneracy of continuous Dirac points in momentum space. Here, as an alternative approach, we propose an extrinsic symmetry protected DNL. On the basis of symmetry analysis and numerical calculations, we establish a general principle to design the nonsymmorphic symmetry protected 4-fold degenerate DNL against spin–orbit coupling in the nanopatterned 2D electron gas. Furthermore, on the basis of experimental measurements, we demonstrate the approximate realization of our proposal in the Bi/Cu(111) system, in which a highly dispersive DNL is observed at the boundary of the Brillouin zone. We envision that the extrinsic symmetry engineering will greatly enhance the ability for artificially constructing the exotic topological bands in the future.
Interfacial Hydrogen-bonding Dynamics in Surface-facilitated Dehydrogenation of Water on TiO2(110)
Shijing Tan†, Hao Feng†, Qijing Zheng, Xuefeng Cui, Jin Zhao, Yi Luo, Jinlong Yang, Bing Wang & J. G. Hou
Journal of the American Chemical Society 142, 826–834 (2020)
Molecular-level understanding of the dehydrogenation of interfacial water molecules on metal oxides and their interactive nature relies on the ability to track the motion of light and small hydrogen atoms, which is known to be difficult. Here, we report precise measurements of the surface-facilitated water dehydrogenation process at terminal Ti sites of TiO2(110) using scanning tunneling microscopy. Our measured hydrogen-bond dynamics of H2O and D2O reveal that the vibrational and electronic excitations dominate the sequential transfer of two H (D) atoms from a H2O (D2O) molecule to adjacent surface oxygen sites, manifesting the active participation of the oxide surface in the dehydrogenation processes. Our results show that, at the stoichiometric Ti5c sites, individual H2O molecules are energetically less stable than the dissociative form, where a barrier is expected to be as small as approximately 70–120 meV on the basis of our experimental and theoretical results. Moreover, our results reveal that interfacial hydrogen bonds can effectively assist H atom transfer and exchange across the surface. The revealed quantitative hydrogen-bond dynamics provide a new atomistic mechanism for water interactions on metal oxides in general.
2019
Step Edge-mediated Assembly of Periodic Arrays of Long Graphene Nanoribbons on Au(111)
Chuanxu Ma, Zhongcan Xiao, Wenchang Lu, Jingsong Huang, Kunlun Hong, Jerzy Bernholc & An-Ping Li
Chemical Communications 55, 11848-11851 (2019)
The influence of substrate steps on the bottom-up synthesis of atomically precise graphene nanoribbons (GNRs) on an Au(111) surface is investigated. Straight surface steps are found to promote the assembly of long and compact arrays of polymers with enhanced interchain π–π stacking interactions, which create a steric hindrance effect on cyclodehydrogenation to suppress the H passivation of polymer ends. The modified two-stage growth process results in periodic arrays of GNRs with doubled average length near step edges.
Direct Writing of Heterostructures in Single Atomically Precise Graphene Nanoribbons
Chuanxu Ma†, Zhongcan Xiao†, Jingsong Huang, Liangbo Liang, Wenchang Lu, Kunlun Hong, Bobby G. Sumpter, J. Bernholc & An-Ping Li
Physical Review Materials 3, 016001 (2019)
Precision control of interfacial structures and electronic properties is the key to the realization of functional heterostructures. Here, utilizing the scanning tunneling microscope (STM) both as a manipulation and characterization tool, we demonstrate the fabrication of a heterostructure in a single atomically precise graphene nanoribbon (GNR) and report its electronic properties. The heterostructure is made of a seven-carbon-wide armchair GNR and a lower band gap intermediate ribbon synthesized bottom-up from a molecular precursor on an Au substrate. The short GNR segments are directly written in the ribbon with a STM tip to form atomic precision intraribbon heterostructures. Based on STM studies combined with density functional theory calculations, we show that the heterostructure has a type-I band alignment, with manifestations of quantum confinement and orbital hybridization. Our finding demonstrates a feasible strategy to create a double-barrier quantum dot structure with atomic precision for functionalities, such as negative differential resistance devices in GNR-based nanoelectronics.
Electron-phonon Coupling in d-electron Solids: A Temperature-dependent Study of Rutile TiO2 by First-principles Theory and Two-photon Photoemission
Honghui Shang, Adam Argondizzo, Shijing Tan, Jin Zhao, Patrick Rinke, Christian Carbogno, Matthias Scheffler & Hrvoje Petek
Physical Review Research 1, 033153 (2019)
Rutile TiO2 is a paradigmatic transition-metal oxide with applications in optics, electronics, photocatalysis, etc., that are subject to pervasive electron-phonon interaction. To understand how energies of its electronic bands, and in general semiconductors or metals where the frontier orbitals have a strong d-band character, depend on temperature, we perform a comprehensive theoretical and experimental study of the effects of electron-phonon (e−p) interactions. In a two-photon photoemission (2PP) spectroscopy study we observe an unusual temperature dependence of electronic band energies within the conduction band of reduced rutile TiO2, which is contrary to the well-understood sp-band semiconductors and points to a so far unexplained dichotomy in how the e−p interactions affect differently the materials where the frontier orbitals are derived from the sp- and d orbitals. To develop a broadly applicable model, we employ state-of-the-art first-principles calculations that explain how phonons promote interactions between the Ti−3d orbitals of the conduction band within the octahedral crystal field. The characteristic difference in e−p interactions experienced by the Ti−3d orbitals of rutile TiO2 crystal lattice are contrasted with the more familiar behavior of the Si−2s orbitals of stishovite SiO2 polymorph, in which the frontier 2s orbital experiences a similar crystal field with the opposite effect. The findings of this analysis of how e−p interactions affect the d- and sp-orbital derived bands can be generally applied to related materials in a crystal field. The calculated temperature dependence of d-orbital derived band energies agrees well with and explains the temperature-dependent inter-d-band transitions recorded in 2PP spectroscopy of TiO2. The general understanding of how e−p interactions affect d-orbital derived bands is likely to impact the understanding of temperature-dependent properties of highly correlated materials.
Photoresponses of Supported Au Single Atoms on TiO2(110) through the Metal-Induced Gap States
Shihui Dong, Bin Li, Xuefeng Cui, Shijing Tan & Bing Wang
The Journal of Physical Chemistry Letters 10, 4683–4691 (2019)
When a metal single-atom (SA) catalyst is supported on a semiconducting photocatalyst, the charge transfer of the photoexcited carriers to metal SAs can provide a synergetic activity for the co-catalysts. Here, we report the interfacial electronic coupling of the Au SAs on the TiO2(110) surface using scanning tunneling microscopy/spectroscopy, in combination with first-principles calculations. Distinct energy and spatial distributions of the metal-induced gap states (MIGSs) are experimentally revealed for the Au SAs adsorbed at the terminal Ti sites and the oxygen vacancies. The localized MIGS below the Fermi level provides a dedicated channel for the transfer of a photoexcited hole from the TiO2 substrate to the adsorbed Au SAs. The hole can weaken the Ti–Au bonding and activate the diffusion of Au SAs. Our results shed light on combining the advantages of photocatalysis and metal SA catalysis using a co-catalyst, which is promising to promote chemical reactions at low temperatures.
Spin Polarization Enhancement of an Fe3O4(100) Surface by Coadsorption of Atomic Hydrogen and Molecular Nitric Oxide
Xia Sun, Muhammad Jibran, Andrew Pratt, Bing Wang & Yasushi Yamauchi
The Journal of Physical Chemistry C 123, 12813–12817 (2019)
The geometric structure, electronic states, and surface spin polarization of a (H, NO)-coadsorbed Fe3O4(100) surface have been studied using density functional theory calculations. H atoms saturate the surface dangling bonds through bonding with the O atom (O1) without a tetrahedral Fe neighbor (Fe(A)), inducing a deeper level shift of the spin-up surface state bands and a widening of the spin-up band gap between the Fermi level (EF) and the valence band maximum. NO molecules are adsorbed on surface octahedral Fe atoms (Fe(B)). The adsorbate/substrate and molecule–molecule interactions cause considerable filling and broadening of the spin-down 2π* states of the adsorbed NO molecule. A −100% spin polarization is obtained over the energy range of −0.8 eV to EF for the (H, NO)-coadsorbed Fe3O4(100) surface, indicating that this system has greater potential for application in spintronic devices than either the solely H-adsorbed or NO-adsorbed surfaces. Furthermore, the adsorbed NO molecule can provide a considerable density of −100% spin-polarized states. Both of these findings are significant for the application and design of spintronic devices.
Atomically dispersed iron hydroxide anchored on Pt for preferential oxidation of CO in H2
Lina Cao†, Wei Liu†, Qiquan Luo†, Ruoting Yin, Bing Wang, Jonas Weissenrieder, Markus Soldemo, Huan Yan, Yue Lin, Zhihu Sun, Chao Ma, Wenhua Zhang, Si Chen, Hengwei Wang, Qiaoqiao Guan, Tao Yao, Shiqiang Wei, Jinlong Yang & Junling Lu
Nature 565, 631–635 (2019)
Proton-exchange-membrane fuel cells (PEMFCs) are attractive next-generation power sources for use in vehicles and other applications, with development efforts focusing on improving the catalyst system of the fuel cell. One problem is catalyst poisoning by impurity gases such as carbon monoxide (CO), which typically comprises about one per cent of hydrogen fuel. A possible solution is on-board hydrogen purification, which involves preferential oxidation of CO in hydrogen (PROX). However, this approach is challenging because the catalyst needs to be active and selective towards CO oxidation over a broad range of low temperatures so that CO is efficiently removed (to below 50 parts per million) during continuous PEMFC operation (at about 353 kelvin) and, in the case of automotive fuel cells, during frequent cold-start periods. Here we show that atomically dispersed iron hydroxide, selectively deposited on silica-supported platinum (Pt) nanoparticles, enables complete and 100 per cent selective CO removal through the PROX reaction over the broad temperature range of 198 to 380 kelvin. We find that the mass-specific activity of this system is about 30 times higher than that of more conventional catalysts consisting of Pt on iron oxide supports. In situ X-ray absorption fine-structure measurements reveal that most of the iron hydroxide exists as Fe1(OH)x clusters anchored on the Pt nanoparticles, with density functional theory calculations indicating that Fe1(OH)x–Pt single interfacial sites can readily react with CO and facilitate oxygen activation. These findings suggest that in addition to strategies that target oxide-supported precious-metal nanoparticles or isolated metal atoms, the deposition of isolated transition-metal complexes offers new ways of designing highly active metal catalysts.
K Atom Promotion of O2 Chemisorption on Au(111) Surface
Jindong Ren, Yanan Wang, Jin Zhao, Shijing Tan & Hrvoje Petek
Journal of the American Chemical Society 141, 4438–4444 (2019)
Alkali atoms are known to promote or poison surface catalytic chemistry. To explore alkali promotion of catalysis and to characterize discharge species in alkali-oxygen batteries, we examine coadsorption of K and O2 on Au(111) surface at the atomic scale by scanning tunneling microscopy (STM) and density functional theory (DFT). On a clean Au(111) surface, O2 molecules may weakly physisorb, but when Au(111) is decorated with K+ ions, they chemisorb into structures that depend on the adsorbate concentrations and substrate templating. At low K coverages, an ordered quantum lattice of K2O2 complexes forms through intramolecular attractive and intermolecule repulsive interactions. For higher K and O2 coverages, the K2O2 complexes condense first into triangular islands, which further coalesce into rhombohedral islands, and ultimately into incommensurate films. No structures display internal contrast possibly because of high structural mutability. DFT calculations explain the alkali-promoted coadsorption in terms of three center, cation−π interactions where pairs of K+ coordinate the π-orbitals on each side of O2 molecules, and in addition O2 forms a covalent bond to Au(111) surface. The K promoted adsorption of O2 is catalyzed by charge transfer from K atoms to Au(111) substrate and ultimately to O2 molecules, forming O2–δ in a redox state between the peroxo and superoxo. Tunneling dI/dV spectra of K2O2 complexes exhibit inordinately intense inelastic progression involving excitation of the O–O stretching vibration, but absence of a Kondo effect suggests that the magnetic moment of O2 is quenched.
Efficient Plasmon-hot Electron Conversion in Ag–CsPbBr3 Hybrid Nanocrystals
Xinyu Huang†, Hongbo Li†, Chunfeng Zhang, Shijing Tan, Zhangzhang Chen, Lan Chen, Zhenda Lu, Xiaoyong Wang & Min Xiao
Nature Communications 10, 1163 (2019)
Hybrid metal/semiconductor nano-heterostructures with strong exciton-plasmon coupling have been proposed for applications in hot carrier optoelectronic devices. However, the performance of devices based on this concept has been limited by the poor efficiency of plasmon-hot electron conversion at the metal/semiconductor interface. Here, we report that the efficiency of interfacial hot excitation transfer can be substantially improved in hybrid metal semiconductor nano-heterostructures consisting of perovskite semiconductors. In Ag–CsPbBr3 nanocrystals, both the plasmon-induced hot electron and the resonant energy transfer processes can occur on a time scale of less than 100 fs with quantum efficiencies of 50 ± 18% and 15 ± 5%, respectively. The markedly high efficiency of hot electron transfer observed here can be ascribed to the increased metal/semiconductor coupling compared with those in conventional systems. These findings suggest that hybrid architectures of metal and perovskite semiconductors may be excellent candidates to achieve highly efficient plasmon-induced hot carrier devices.
2018
Design of Atomically Precise Nanoscale Negative Differential Resistance Devices
Zhongcan Xiao†, Chuanxu Ma†, Jingsong Huang, Liangbo Liang, Wenchang Lu, Kunlun Hong, Bobby G. Sumpter, An-Ping Li & Jerzy Bernholc
Advanced Theory and Simulations 2, 1800172 (2018)
Downscaling device dimensions to the nanometer range raises significant challenges to traditional device design, due to potential current leakage across nanoscale dimensions and the need to maintain reproducibility while dealing with atomic-scale components. Here, negative differential resistance (NDR) devices based on atomically precise graphene nanoribbons are investigated. The computational evaluation of the traditional double-barrier resonant-tunneling diode NDR structure uncovers important issues at the atomic scale, concerning the need to minimize the tunneling current between the leads while achieving high peak current. A new device structure consisting of multiple short segments that enables high current by the alignment of electronic levels across the segments while enlarging the tunneling distance between the leads is proposed. The proposed structure can be built with atomic precision using a scanning tunneling microscope (STM) tip during an intermediate stage in the synthesis of an armchair nanoribbon. An experimental evaluation of the band alignment at the interfaces and an STM image of the fabricated active part of the device are also presented. This combined theoretical–experimental approach opens a new avenue for the design of nanoscale devices with atomic precision.
Oxidization Stability of Atomically Precise Graphene Nanoribbons
Chuanxu Ma, Zhongcan Xiao, Alex A. Puretzky, Arthur P. Baddorf, Wenchang Lu, Kunlun Hong, J. Bernholc & An-Ping Li
Physical Review Materials 2, 014006 (2018)
The stability of graphene nanoribbons (GNRs) against oxidation is critical for their practical applications. Here we study both the thermal stability and the oxidation process of the ambient-exposed armchair GNRs with a width of seven carbon atoms (7-aGNR), grown on an Au(111) surface. The atomic scale evolution of the armchair edges and the zigzag ends of the aGNRs after annealing at different temperatures are revealed by using scanning tunneling microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and first-principles calculations. We observe evidence that the zigzag ends start to be oxidized and decomposed at 180 °C, while the armchair edges are intact at 430 °C but become oxidized at 520 °C. Two different oxygen species are identified at the armchair edges, namely the hydroxyl pair and the epoxy bonding motif with one oxygen bonded to two edge carbons. These oxidization species modify the electronic properties of the pristine 7-aGNRs, with a band-gap reduction from 2.6 to 2.3 eV and 1.9 eV for the hydroxyl pair- and epoxy-terminated edges, respectively. These findings demonstrate the oxidation stability of both the zigzag and armchair edges of GNRs, and they provide an opportunity to harness the high density of edge atoms in applications such as GNR-based high-temperature oxygen sensors.
Enhancement of the Spin Polarization of an Fe3O4(100) Surface by Nitric Oxide Adsorption
Z. Y. Li, M. Jibran, Xia Sun, A. Pratt, Bing Wang, Y. Yamauchib & Zejun Ding
Physical Chemistry Chemical Physics 20, 15871-15875 (2018)
The geometric, electronic and magnetic properties of a nitric oxide (NO) adsorbed Fe3O4(100) surface have been investigated using density functional theory (DFT) calculations. NO molecules preferentially bond with surface Fe(B) atoms via their N atoms. The generalized gradient approximation (GGA) is not recommended to be used in such a strongly correlated system since it provides not only an overestimation of the adsorption energy and an underestimation of the Fe(B)–N bond length, but also magnetic quenching of the adsorbate and the bonded Fe(B) atoms. In contrast, a tilted geometry and magnetization of the adsorbate and the bonded Fe(B) atom are obtained after including the strong on-site Coulomb interactions through a Hubbard term (GGA+U). The spin-down 2π* states of the NO molecule are filled and broadened due to the adsorbate–substrate interaction and the molecule–molecule interaction. The surface spin polarization close to the Fermi level is expected to be greatly enhanced by the NO adsorption which has significance for interface design in spintronic devices.
Epitaxial Growth of Highly Strained Antimonene on Ag(111)
Yahui Mao†, Lifu Zhang†, Huili Wang, Huan Shan, Xiaofang Zhai, Zhenpeng Hu, Aidi Zhao & Bing Wang
Frontiers of Physics 13, 138106 (2018)
The synthesis of antimonene, which is a promising group-V 2D material for both fundamental studies and technological applications, remains highly challenging. Thus far, it has been synthesized only by exfoliation or growth on a few substrates. In this study, we show that thin layers of antimonene can be grown on Ag(111) by molecular beam epitaxy. High-resolution scanning tunneling microscopy combined with theoretical calculations revealed that the submonolayer Sb deposited on a Ag(111) surface forms a layer of AgSb2 surface alloy upon annealing. Further deposition of Sb on the AgSb2 surface alloy causes an epitaxial layer of Sb to form, which is identified as antimonene with a buckled honeycomb structure. More interestingly, the lattice constant of the epitaxial antimonene (5 Å) is much larger than that of freestanding antimonene, indicating a high tensile strain of more than 20%. This kind of large strain is expected to make the antimonene a highly promising candidate for roomtemperature quantum spin Hall material.
Some metal–organic (MO) lattices with a strong spin–orbit coupling (SOC) have been predicted to behave as two-dimensional topological insulators (2D-TIs). The polarization of metallic edge states with the opposite electron spin in 2D-TIs, in otherwise insulating 2D MO sheets, is interesting for spintronic technology. The 2D-TI character of MO lattices, however, has not been confirmed by experiment. The main challenge has been that MO lattices usually self-organize on metal substrates, which can introduce interactions that modify and can even suppress the topological character. We calculate the topological properties of 2D metal–organic honeycomb lattice composed of noble metal atom vertices and bidentate 1,4-phenylene diisocyanide (PDI) linkers, which form metal–organic honeycomb–Kagome lattices (MOHKLs) band structure, free and on metal substrates. The selection of vertices and linkers can independently tune the SOC and transport properties. Calculations predict that unsupported 2D MOHKLs indeed possess SOC-induced gaps between the Dirac bands at the K-points. Furthermore, nanoribbons of such MOHKLs are calculated to possess metallic spin-polarized edge states. Supporting MOHKLs on a metal substrate, however, introduces an electric potential, giving rise to Rashba SOC, which can collapse the band gaps. Molecule-resolved measurements by scanning tunneling microscopy and spectroscopy test the electronic properties of Ag–PDI MOHKL self-assembled on Ag(111) surface, but find no evidence of the 2D-TI electronic band structure. Releasing MOHKLs from the electronic and chemical influences of substrates to preserve their TI properties remains a challenge. The Rashba SOC provides an additional tool for designing 2D-TI band structures.
Visualizing Elementary Reactions of Methanol by Electrons and Holes on TiO2(110) Surface
Shijing Tan†, Hao Feng†, Yongfei Ji†, Qijing Zheng, Yongliang Shi, Jin Zhao, Aidi Zhao, Jinlong Yang, Yi Luo, Bing Wang & J. G. Hou
The Journal of Physical Chemistry C 122, 28805–28814 (2018)
Direct visualization and comparison of the elementary reactions induced by electrons and holes are of importance for finding a way to conduct chemical reactions and reaction sequences in a controllable manner. As a semiconductor, TiO2 provides a playground to perform the measurements, and moreover, the information can be useful for design of high-performance TiO2-based catalysts and photocatalysts. Here, we present our investigation on the elementary reactions of CH3OH on TiO2 surface through visualization of specific elementary steps by highly controllable electron and hole injection using scanning tunneling microscopy. The distinct sequential routes and their kinetics, namely, breaking C–O and O–H bonds by electrons and breaking O–H and C–H bonds by holes, respectively, have been experimentally identified and well elucidated by density functional theory calculations. Our nonlocal h-injection experimental and theoretical results suggest that the delocalized holes in the TiO2 substrate should be responsible for the temperature-dependent h-route reactions. The locally triggered e-route reaction is associated with the fact that the location of the unoccupied hybridization states is much higher than that of the conduction band onset. Our findings resolve the long-standing debate about the intermediate species and reaction mechanism in photocatalytic oxidation of CH3OH. Our proposed protocol offers a powerful means to study elementary reactions induced by electrons and holes on a semiconductor surface in general.
Epitaxial Growth of Ultraflat Stanene with Topological Band Inversion
Jialiang Deng†, Bingyu Xia†, Xiaochuan Ma†, Haoqi Chen, Huan Shan, Xiaofang Zhai, Bin Li, Aidi Zhao, Yong Xu, Wenhui Duan, Shou-Cheng Zhang, Bing Wang & J. G. Hou
Nature Materials 17, 1081–1086 (2018)
Two-dimensional (2D) topological materials, including quantum spin/anomalous Hall insulators, have attracted intense research efforts owing to their promise for applications ranging from low-power electronics and high-performance thermoelectrics to fault-tolerant quantum computation. One key challenge is to fabricate topological materials with a large energy gap for room-temperature use. Stanene—the tin counterpart of graphene—is a promising material candidate distinguished by its tunable topological states and sizeable bandgap. Recent experiments have successfully fabricated stanene, but none of them have yet observed topological states. Here we demonstrate the growth of high-quality stanene on Cu(111) by low-temperature molecular beam epitaxy. Importantly, we discovered an unusually ultraflat stanene showing an in-plane s–p band inversion together with a spin–orbit-coupling-induced topological gap (~0.3 eV) at the Γ point, which represents a foremost group-IV ultraflat graphene-like material displaying topological features in experiment. The finding of ultraflat stanene opens opportunities for exploring two-dimensional topological physics and device applications.
Landau Quantization of a Narrow Doubly-folded Wrinkle in Monolayer Graphene
Chuanxu Ma†, Xia Sun†, Hongjian Du, Jufeng Wang, Mingyang Tian, Aidi Zhao, Yasushi Yamauchi & Bing Wang
Nano Letters 18, 6710–6718 (2018)
Folding can be an effective way to tailor the electronic properties of graphene and has attracted wide study interest in finding its novel properties. Here we present the experimental characterizations of the structural and electronic properties of a narrow graphene wrinkle on a SiO2/Si substrate using scanning tunneling microscopy/spectroscopy. Pronounced and nearly equally separated conductance peaks are observed in the dI/dV spectra of the wrinkle. We attribute these peaks to pseudo-Landau levels (PLLs) that are caused by a gradient-strain-induced pseudomagnetic field up to about 42 T in the narrow wrinkle. The introduction of the gradient strain and thus the pseudomagnetic field can be ascribed to the lattice deformation. A doubly-folded structure of the wrinkle is suggested. Our density functional theory calculations show that the band structure of the doubly folded graphene wrinkle has a parabolic dispersion, which can well explain the equally separated PLLs. The effective mass of carriers is obtained to be about 0.02 me (me: the rest mass of electron), and interestingly, it is revealed that there exists valley polarization in the wrinkle. Such properties of the strained doubly folded wrinkle may provide a platform to explore some exciting phenomena in graphene, like zero-field quantum valley Hall effect.
Hidden Order and Haldane-Like Phase in Molecular Chains Assembled from Conformation-Switchable Molecules
Jialiang Deng, Aidi Zhao, Ruiqi Zhang, Huan Shan, Bin Li, Jinlong Yang & Bing Wang
ACS Nano 12, 6515–6522 (2018)
Topological properties of matters have attracted tremendous interest in the past years due to the scientific and technological importance. It is of great interest to discover the analogs of topological phases in molecular architectures, if the key constituents of the phases are properly mimicked. Using scanning tunneling microscopy, we demonstrate that quasi-1D molecular chains assembled from conformation-switchable dibenzo[g,p]chrysene molecules show hidden antiparallel order analogous to the hidden antiferromagnetic order in the Haldane phase, a known topological phase of quantum spin-1 chains. This is realized by mimicking the spin degree of freedom with the intramolecular helicene chiral switches and by emulating the interspin antiferromagnetic coupling with intermolecular homochiral coupling. The discovery of the molecular analog of topological matters may inspire the search of molecular architectures with nontrivial topological properties.
Tuning the Doping Types in Graphene Sheets by N Monoelement
Chuanxu Ma†, Qing Liao†, Haifeng Sun, Shulai Lei, Yi Zheng, Ruoting Yin, Aidi Zhao, Qunxiang Li & Bing Wang
Nano Letters 18, 386–394 (2018)
The doping types of graphene sheets are generally tuned by different dopants with either three or five valence electrons. As a five-valence-electrons element, however, nitrogen dopants in graphene sheets have several substitutional geometries. So far, their distinct effects on electronic properties predicted by theoretical calculations have not been well identified. Here, we demonstrate that the doping types of graphene can be tuned by N monoelement under proper growth conditions using chemical vapor deposition (CVD), characterized by combining scanning tunneling microscopy/spectroscopy, X-ray/ultraviolet photoelectron spectroscopy, Hall effect measurement, Raman spectroscopy, and density functional theory calculations. We find that a relatively low partial pressure of CH4 (mixing with NH3) can lead to the growth of dominant pyridinic N substitutions in graphene, in contrast with the growth of dominant graphitic N substitutions under a higher partial pressure of CH4. Our results unambiguously confirm that the pyridinic N leads to the p-type doping, and the graphitic N leads to the n-type doping. Interestingly, we also find that the pyridinic N and the graphitic N are preferentially separated in different domains. Our findings shed light on continuously tuning the doping level of graphene monolayers by using N monoelement, which can be very convenient for growth of functional structures in graphene sheets.
Direct View of Cr Atoms Doped in Anatase TiO2(001) Thin Film
Haoqi Tang†, Yue Lin†, Zhengwang Cheng†, Xuefeng Cui & Bing Wang
Chinese Journal of Chemical Physics 31, 71–76 (2018)
Imaging the doping elements is critical for understanding the photocatalytic activity of doped TiO2 thin film. But it is still a challenge to characterize the interactions between the dopants and the TiO2 lattice at the atomic level. Here, we use high angle annular dark-field/annular bright-field scanning transmission electron microscope (HAADF/ABF-STEM) combined with electron energy loss spectroscopy (EELS) to directly image the individual Cr atoms doped in anatase TiO2(001) thin film from [100] direction. The Cr dopants, which are clearly imaged through the atomic-resolution EELS mappings while can not be seen by HADDF/ABF-STEM, occupy both the substitutional sites of Ti atoms and the interstitial sites of TiO2 matrix. Most of them preferentially locate at the substitutional sites of Ti atoms. These results provide the direct evidence for the doping structure of Cr-doped A-TiO2 thin film at the atomic level and also prove the EELS mapping is an excellent technique for characterizing the doped materials.
Coherent Electron Transfer at the Ag/Graphite Heterojunction Interface
Shijing Tan, Yanan Dai, Shengmin Zhang, Liming Liu, Jin Zhao & Hrvoje Petek
Physical Review Letters 120, 126801 (2018)
Charge transfer in transduction of light to electrical or chemical energy at heterojunctions of metals with semiconductors or semimetals is believed to occur by photogenerated hot electrons in metal undergoing incoherent internal photoemission through the heterojunction interface. Charge transfer, however, can also occur coherently by dipole coupling of electronic bands at the heterojunction interface. Microscopic physical insights into how transfer occurs can be elucidated by following the coherent polarization of the donor and acceptor states on the time scale of electronic dephasing. By time-resolved multiphoton photoemission spectroscopy (MPP), we investigate the coherent electron transfer from an interface state that forms upon chemisorption of Ag nanoclusters onto graphite to a 𝜎 symmetry interlayer band of graphite. Multidimensional MPP spectroscopy reveals a resonant two-photon transition, which dephases within 10 fs completing the coherent transfer.
2017
Seamless Staircase Electrical Contact to Semiconducting Graphene Nanoribbons
Chuanxu Ma, Liangbo Liang, Zhongcan Xiao, Alex A. Puretzky, Kunlun Hong, Wenchang Lu, V.Meunier, J. Bernholc & An-Ping Li
Nano Letters 17, 6241–6247 (2017)
Electrical contact to low-dimensional (low-D) materials is a key to their electronic applications. Traditional metal contacts to low-D semiconductors typically create gap states that can pin the Fermi level (EF). However, low-D metals possessing a limited density of states at EF can enable gate-tunable work functions and contact barriers. Moreover, a seamless contact with native bonds at the interface, without localized interfacial states, can serve as an optimal electrode. To realize such a seamless contact, one needs to develop atomically precise heterojunctions from the atom up. Here, we demonstrate an all-carbon staircase contact to ultranarrow armchair graphene nanoribbons (aGNRs). The coherent heterostructures of width-variable aGNRs, consisting of 7, 14, 21, and up to 56 carbon atoms across the width, are synthesized by a surface-assisted self-assembly process with a single molecular precursor. The aGNRs exhibit characteristic vibrational modes in Raman spectroscopy. A combined scanning tunneling microscopy and density functional theory study reveals the native covalent-bond nature and quasi-metallic contact characteristics of the interfaces. Our electronic measurements of such seamless GNR staircase constitute a promising first step toward making low resistance contacts.
Controllable Conversion of Quasi-Freestanding Polymer Chains to Graphene Nanoribbons
Chuanxu Ma†, Zhongcan Xiao†, Honghai Zhang, Liangbo Liang, Jingsong Huang, Wenchang Lu, Bobby G. Sumpter, Kunlun Hong, J. Bernholc & An-Ping Li
Nature Communications 8, 14815 (2017)
In the bottom-up synthesis of graphene nanoribbons (GNRs) from self-assembled linear polymer intermediates, surface-assisted cyclodehydrogenations usually take place on catalytic metal surfaces. Here we demonstrate the formation of GNRs from quasi-freestanding polymers assisted by hole injections from a scanning tunnelling microscope (STM) tip. While catalytic cyclodehydrogenations typically occur in a domino-like conversion process during the thermal annealing, the hole-injection-assisted reactions happen at selective molecular sites controlled by the STM tip. The charge injections lower the cyclodehydrogenation barrier in the catalyst-free formation of graphitic lattices, and the orbital symmetry conservation rules favour hole rather than electron injections for the GNR formation. The created polymer–GNR intraribbon heterostructures have a type-I energy level alignment and strongly localized interfacial states. This finding points to a new route towards controllable synthesis of freestanding graphitic layers, facilitating the design of on-surface reactions for GNR-based structures.
Electronic Properties of Cr-N Codoped Rutile TiO2(110) Thin Films
Zhengwang Cheng†, Lili Zhang†, Shihui Dong, Xiaochuan Ma, Huanxin Ju, Junfa Zhu, Xuefeng Cui, Jin Zhao & Bing Wang
Surface Science 666, 84-89 (2017)
We report our investigation on the electronic properties of Cr-N codoped rutile TiO2(110) single crystal thin films, homoepitaxially grown by pulsed-laser-deposition method, and characterized using scanning tunneling microscopy and spectroscopy (STM/STS), X-ray/ultraviolet photoemission spectroscopy (XPS/UPS), in combination with first-principles calculations. Our results show that the bandgap reduction of the TiO2(110) surface is mainly contributed by the delocalized states whose position is at 2.0 eV below the Fermi level, introduced by the substitutional codoped Cr-2N pair, which is evidenced by the accordance of the results between the STS spectra and the calculated DOS. The codoped Cr-N pair contributes the gap state at about 0.8 eV below the Fermi level, in consistent with the theoretical calculations. While, the monodoped Cr contributes the states either close to the valence band maximum or the conduction band minimum, which should not contribute to the bandgap reduction too much. Our experimental results joint with theoretical calculations provide an atomic view of the bandgap reduction of the rutile TiO2(110) surface, which indicates that the excess substitutional N atoms should be important to efficiently narrow the bandgap by introducing the Cr-2N pairs.
Identifying the Site-dependent Photoactivity of Anatase TiO2(001)-(1×4) Surface
Zhengwang Cheng†, Haoqi Tang†, Xuefeng Cui, Shihui Dong, Xiaochuan Ma, Bin Luo, Shijing Tan & Bing Wang
The Journal of Physical Chemistry C 121, 19930–19937 (2017)
We here report our investigation on the photocatalytic reactions of methanol on the anatase TiO2(001)-(1×4) surface using scanning tunneling microscopy and temperature-programmed desorption. Our results show that in the reduced surface the Ti-pair defect sites are photoinactive, but in the reoxidized surface the partially oxidized defect sites are photoactive to dissociate methanol and methoxy to formaldehyde. The perfect (1×4) lattice sites are photoactive, even though they are inactive in thermally driven reaction. The photocatalytic reaction of methanol (CD3OH) at the perfect lattice sites is evidenced by the conversion of CD3OH to various other deuterated methanol (CD3OD and CD2HOH) and a small amount of formaldehyde (CD2O) after ultraviolet light irradiation, in contrast with no such conversion reaction in the nonirradiated sample. The production of CD3OD and CD2HOH can be attributed to the reverse reaction between the produced CD2O and the H or D adatoms during or after ultraviolet light irradiation, which also leads to the small amount of formaldehyde in final products. The identification of the site-dependent photocatalytic reactions of methanol, in addition to our recently characterized site-dependent thermally driven reactions, may provide an insightful understanding about the activity and photoactivity of the anatase TiO2(001)-(1×4) surface.
Understanding the Intrinsic Chemical Activity of Anatase TiO2(001)-(1×4) Surface
Haoqi Tang†, Zhengwang Cheng†, Shihui Dong, Xuefeng Cui, Hao Feng, Xiaochuan Ma, Bin Luo, Aidi Zhao, Jin Zhao & Bing Wang
The Journal of Physical Chemistry C 121, 1272–1282 (2017)
We report our investigation on the intrinsic chemical activity of the anatase TiO2(001)-(1×4) reconstructed surface, using epitaxially grown anatase TiO2(001) thin films and using methanol molecules as a probe, characterized by combining scanning tunneling microscopy and temperature-programmed desorption. Our results provide direct evidence that the perfect (1×4) lattice sites of the surface are intrinsically quite inert for the reaction of methanol. We obtain that the activation energy for desorption of molecular methanol is about 0.55–0.64 eV, which is in good agreement with our first-principle calculations based on the structural model with 5-fold coordinated Ti atoms at the ridges of (1×4) reconstruction. We find that two types of defect sites, that is, reduced Ti pairs and partially oxidized Ti pairs, are responsible for the chemical activity of the surface, evidenced by the desorption of water due to the dehydrogenation of methanol at the defect sites. The methoxy left at the reduced Ti-pair sites further produced CH3 radical, and the methoxy near the partially oxidized Ti-pair sites produced formaldehyde and methanol through disproportionation reaction. The determination of these intrinsic properties can be important to understanding the conflicting results from this surface in the literature and thus to reveal the actual reaction mechanisms.
Plasmonic Coupling at a Metal/Semiconductor Interface
Shijing Tan, Adam Argondizzo, Jindong Ren, Liming Liu, Jin Zhao & Hrvoje Petek
Nature Photonics 11, 806–812 (2017)
Integrating plasmonic nanoparticles with semiconductor substrates introduces strong optical resonances that extend and enhance the spectrum of photocatalytic and photovoltaic activity. The effect of plasmonic resonances has been variously attributed to the field nanoconfinement, plasmon–exciton coupling, hot electron transfer, and so on, based on action spectra of enhanced photoactivity. It remains unclear, however, whether energized carriers in the substrate are generated by the transfer of plasmonically generated hot electrons from the metal, as broadly believed, or directly by dephasing of the plasmonic field at the interface. Here, we demonstrate the importance of the direct plasmonic coupling across the chemical interface for hot electron generation at a prototypical Ag nanocluster/TiO2 heterojunction by direct probing of the coherence and hot electron dynamics with two-photon photoemission spectroscopy. Energy, time and material distributions of excitations in the Ag nanocluster/TiO2 heterojunction indicate that dielectric coupling with the substrate renormalizes the plasmon resonance of the Ag nanoparticle, and its dephasing directly generates hot electrons in TiO2 on a <10 fs timescale.
Ultrafast Plasmon-enhanced Hot Electron Generation at Ag Nanocluster/Graphite Heterojunctions
Shijing Tan, Liming Liu, Yanan Dai, Jindong Ren, Jin Zhao & Hrvoje Petek
Journal of the American Chemical Society 139, 6160–6168 (2017)
Hot electron processes at metallic heterojunctions are central to optical-to-chemical or electrical energy transduction. Ultrafast nonlinear photoexcitation of graphite (Gr) has been shown to create hot thermalized electrons at temperatures corresponding to the solar photosphere in less than 25 fs. Plasmonic resonances in metallic nanoparticles are also known to efficiently generate hot electrons. Here we deposit Ag nanoclusters (NC) on Gr to study the ultrafast hot electron generation and dynamics in their plasmonic heterojunctions by means of time-resolved two-photon photoemission (2PP) spectroscopy. By tuning the wavelength of p-polarized femtosecond excitation pulses, we find an enhancement of 2PP yields by 2 orders of magnitude, which we attribute to excitation of a surface-normal Mie plasmon mode of Ag/Gr heterojunctions at 3.6 eV. The 2PP spectra include contributions from (i) coherent two-photon absorption of an occupied interface state (IFS) 0.2 eV below the Fermi level, which electronic structure calculations assign to chemisorption-induced charge transfer, and (ii) hot electrons in the π*-band of Gr, which are excited through the coherent screening response of the substrate. Ultrafast pump–probe measurements show that the IFS photoemission occurs via virtual intermediate states, whereas the characteristic lifetimes attribute the hot electrons to population of the π*-band of Gr via the plasmon dephasing. Our study directly probes the mechanisms for enhanced hot electron generation and decay in a model plasmonic heterojunction.
Ultrafast Multiphoton Thermionic Photoemission from Graphite
Shijing Tan, Adam Argondizzo, Cong Wang, Xuefeng Cui & Hrvoje Petek
Physical Review X 7, 011004 (2017)
Electronic heating of cold crystal lattices in nonlinear multiphoton excitation can transiently alter their physical and chemical properties. In metals where free electron densities are high and the relative fraction of photoexcited hot electrons is low, the effects are small, but in semimetals, where the free electron densities are low and the photoexcited densities can overwhelm them, the intense femtosecond laser excitation can induce profound changes. In semimetal graphite and its derivatives, strong optical absorption, weak screening of the Coulomb potential, and high cohesive energy enable extreme hot electron generation and thermalization to be realized under femtosecond laser excitation. We investigate the nonlinear interactions within a hot electron gas in graphite through multiphoton-induced thermionic emission. Unlike the conventional photoelectric effect, within about 25 fs, the memory of the excitation process, where resonant dipole transitions absorb up to eight quanta of light, is erased to produce statistical Boltzmann electron distributions with temperatures exceeding 5000 K; this ultrafast electronic heating causes thermionic emission to occur from the interlayer band of graphite. The nearly instantaneous thermalization of the photoexcited carriers through Coulomb scattering to extreme electronic temperatures characterized by separate electron and hole chemical potentials can enhance hot electron surface femtochemistry, photovoltaic energy conversion, and incandescence, and drive graphite-to-diamond electronic phase transition.
Molecule-Confined Engineering toward Superconductivity and Ferromagnetism in Two-dimensional Superlattice
Zejun Li†, Yingcheng Zhao†, Kejun Mu†, Huan Shan, Yuqiao Guo, Jiajing Wu, Yueqi Su, Qiran Wu, Zhe Sun, Aidi Zhao, Xuefeng Cui, Changzheng Wu & Yi Xie
Journal of the American Chemical Society 139, 16398–16404 (2017)
Superconductivity is mutually exclusive with ferromagnetism, because the ferromagnetic exchange field is often destructive to superconducting pairing correlation. Well-designed chemical and physical methods have been devoted to realize their coexistence only by structural integrity of inherent superconducting and ferromagnetic ingredients. However, such coexistence in freestanding structure with nonsuperconducting and nonferromagnetic components still remains a great challenge up to now. Here, we demonstrate a molecule-confined engineering in two-dimensional organic–inorganic superlattice using a chemical building-block approach, successfully realizing first freestanding coexistence of superconductivity and ferromagnetism originated from electronic interactions of nonsuperconducting and nonferromagnetic building blocks. We unravel totally different electronic behavior of molecules depending on spatial confinement: flatly lying Co(Cp)2 molecules in strongly confined SnSe2 interlayers weaken the coordination field, leading to spin transition to form ferromagnetism; meanwhile, electron transfer from cyclopentadienyls to the Se–Sn–Se lattice induces superconducting state. This entirely new class of coexisting superconductivity and ferromagnetism generates a unique correlated state of Kondo effect between the molecular ferromagnetic layers and inorganic superconducting layers. We anticipate that confined molecular chemistry provides a newly powerful tool to trigger exotic chemical and physical properties in two-dimensional matrixes.
Detecting the Photoactivity of Anatase TiO2(001)-(1×4) Surface by Formaldehyde
Bin Luo, Haoqi Tang, Zhengwang Cheng, Yuanyuan Ji, Xuefeng Cui, Yongliang Shi & and Bing Wang
The Journal of Physical Chemistry C 121, 17289–17296 (2017)
We report our investigation using temperature-programmed desorption to detect the activity and the photoactivity of the reduced anatase TiO2(001)-(1×4) surface by using CH2O as a probe. We obtain the adsorption energy of 0.55 eV for CH2O on the surface and find that the defect sites in the reduced surface are the only active sites for thermally driven reaction to produce C2H4. We also identify the pathways for photodecomposition of CH2O on the anatase TiO2(001)-(1×4) surface, where the C−H bond breaking to form the intermediates of HCOO− should be a key step in the reactions. After the ultraviolet light irradiation, the dissociation of HCOO– produces CO and H2O at elevated temperatures. Accompanied by desorption of H2O, we observe higher production of C2H4 than that in the non-irradiated sample. We interpret our experimental results by attributing the temporary change of the (1×4) ridge structure to water desorption, that is, the initially oxygen-rich five-fold coordinated Ti atoms change to four-fold coordinated Ti atoms. The four-fold coordinated Ti sites thus act as the highly active sites for the coupling reaction of CH2O to produce C2H4. Our findings here provide insightful understanding for the thermal and photocatalytic reactions of CH2O on the anatase TiO2(001)-(1×4) surface.
2016
Interplay Between Intercalated Oxygen Superstructures and Monolayer h-BN on Cu(100)
Chuanxu Ma, Jewook Park, Lei Liu, Yong-Sung Kim, Mina Yoon, Arthur P. Baddorf, Gong Gu & An-Ping Li
Physical Review B 94, 064106 (2016)
The confinement effect of intercalated atoms in van der Waals heterostructures can lead to interesting interactions between the confined atoms or molecules and the overlaying two-dimensional (2D) materials. Here we report the formation of ordered Cu(100) p(2×2) oxygen superstructures by oxygen intercalation under the monolayer hexagonal boron nitride (h-BN) on Cu after annealing. By using scanning tunneling microscopy and X-ray photoelectron spectroscopy, we identify the superstructure and reveal its roles in passivating the exposed Cu surfaces, decoupling h-BN and Cu, and disintegrating h-BN monolayers. The oxygen superstructure appears as a 2D pattern on the exposed Cu surface or quasi-1D stripes of paired oxygen intercalated in the interface of h-BN and Cu predominantly oriented along the moiré modulations. The oxygen superstructure is shown to etch the overlaying h-BN monolayer in a thermal annealing process. After extended annealing, the h-BN monolayer disintegrates into nanoislands with zigzag edges. We discuss the implications of these findings on the stability and oxidation resistance of h-BN and relate them to challenges in process integration and 2D heterostructures.
Surface Landau Levels and Spin States in Bismuth(111) Ultrathin Films
Hongjian Du†, Xia Sun†, Xiaogang Liu, Xiaojun Wu, Jufeng Wang, Mingyang Tian, Aidi Zhao, Yi Luo, Jinlong Yang, Bing Wang & J. G. Hou
Nature Communications 7, 10814 (2016)
The development of next-generation electronics is much dependent on the discovery of materials with exceptional surface-state spin and valley properties. Because of that, bismuth has attracted a renewed interest in recent years. However, despite extensive studies, the intrinsic electronic transport properties of Bi surfaces are largely undetermined due to the strong interference from the bulk. Here we report the unambiguous determination of the surface-state Landau levels in Bi(111) ultrathin films using scanning tunnelling microscopy under magnetic fields perpendicular to the surface. The Landau levels of the electron-like and the hole-like carriers are accurately characterized and well described by the band structure of the Bi(111) surface from density functional theory calculations. Some specific surface spin states with a large g-factor are identified. Our findings shed light on the exploiting surface-state properties of Bi for their applications in spintronics and valleytronics.
Dynamic Processes of Formaldehyde at Terminal Ti Sites on the Rutile TiO2(110) Surface
Hao Feng, Liming Liu, Shihui Dong, Xuefeng Cui, Jin Zhao & Bing Wang
The Journal of Physical Chemistry C 120, 24287–24293 (2016)
We investigate the dynamic processes of formaldehyde (HCHO) molecules on 5-fold-coordinated titanium (Ti5c) sites of rutile TiO2(110) surface using scanning tunneling microscopy (STM) together with density functional theory simulations. Our results show that the adsorbed HCHO molecules at Ti5c sites are present as two types of protrusions, either centered at Ti5c rows or centered at bridging oxygen (Ob) rows in the STM images, corresponding to the monodentate adsorption configuration through a O–Ti5c bond and to the bidentate adsorption configuration through both O–Ti5c and C–Ob bonds, respectively, which can be well supported by the simulated images. It is also observed that the monodentate adsorption tends to spontaneously switch to bidentate adsorption. Our results confirm the existence of the energetically more favored bidentate adsorption for HCHO at Ti5c sites. We obtain that the energy barriers are approximately 0.28 and 0.75 eV for the adsorbed HCHO molecules switching from monodentate adsorption to bidentate adsorption and reversely switching from bidentate adsorption to monodentate adsorption, respectively, from measurements of their dynamic processes. Our findings can well elucidate the missing signature of the energetically more favored bidentate configuration in some previous experiments and provide insightful understanding of formaldehyde on TiO2(110) surface.
We report our investigation of the reversible reaction of methanol and the migration of hydrogen adatom (Had) on TiO2(110)-(1×1) surface at various temperatures and methanol coverages using scanning tunneling microscopy joint with density functional theory (DFT) calculations. At a relatively low coverage measured at room temperature, the methanol species adsorbed at the oxygen vacancy (OV) sites are immobile and appear only as a dissociative form, and the observed Had migration events are very few. However, when the OV sites are fully filled by methanol in the methanol-overdosed sample, the methanol species at the OV sites keep immobile but frequently switch between molecular and dissociative forms, accompanied by dramatically enhanced Had migration. Meanwhile, an established equilibrium shows a concentration ratio of 1:3 between the molecular and dissociative methanol. At 235 K, we directly visualized and confirmed that the reversible reactions of methanol and the enhanced Had migration are mediated by the diffusive methanol adsorbed at the 5-fold coordinated Ti sites. Our DFT calculations well elucidate the experimental results using the modeled configurations by considering the exchange processes of H atoms, reaching a clear atomistic picture for the dynamic equilibrium of the reversible reactions and the Had migration.
Adsorption and Self-Assembly of the 2,3,5,6-Tetra(2'-pyridyl)pyrazine Nonplanar Molecule on a Au(111) Surface
Xiaohui Li, Bin Li, Yongfei Ji, Jing Zhang, Aidi Zhao & Bing Wang
The Journal of Physical Chemistry C 120, 6039–6049 (2016)
We report our investigation of adsorption and self-assembly of a nonplanar molecule 2,3,5,6-tetra(2'-pyridyl)pyrazine (TPPZ) on a Au(111) surface using ultrahigh vacuum low-temperature scanning tunneling microscopy joint with density functional theory (DFT) calculations. We find that the nonplanar TPPZ molecules exhibit various adsorption configurations depending on the coverage of molecules. The molecules mainly adsorb at step edges with a flat-lying configuration at low coverages and gather into chiral trimers almost equidistantly separated from each other in the fcc domains accompanied by diffusive molecules in the hcp domains of the herringbone reconstructed Au(111) surface at a coverage of about 0.2 monolayer (ML) and then form two dominant types of ordered domains, i.e., stripe-like (S-phase) and honeycomb-like (H-phase) superstructures, which may reflect the chiral separation characteristics at a coverage of about 1 ML. In the trimers and ordered domains, the adsorption configurations of molecules become declining or almost erect, i.e., an "edge-on" configuration, quite different from the flat-lying configuration at low coverages. After annealing to 380 K the S-phase transfers to the H-phase, and the H-phase may persist after annealing up to 410 K, which can be attributed to the existence of C–H···N hydrogen bonds between the TPPZ molecules with the same chirality. Our observations can be energetically interpreted by considering the interplay of molecule–substrate interaction and intermolecular interaction including van der Waals interaction and hydrogen bonds on the basis of the DFT calculations, where the hydrogen bonds should be a key factor for the formation of the stable ordered H-phase with chiral separation.
Temperature- and Coverage-Dependent Kinetics of Photocatalytic Reaction of Methanol on TiO2(110)-(1×1) Surface
Hao Feng, Shijing Tan, Haoqi Tang, Qijing Zheng, Yongliang Shi, Xuefeng Cui, Xiang Shao, Aidi Zhao, Jin Zhao & Bing Wang
The Journal of Physical Chemistry C 120, 5503–5514 (2016)
We systematically investigated the photocatalytic reaction of methanol on the TiO2(110)-(1×1) surface under irradiation with ultraviolet (UV) light performed at various conditions, using scanning tunneling microscopy (STM) jointed with temperature-programmed desorption (TPD) techniques. Our STM and TPD results show that the photocatalytic reaction is indeed initiated from the molecular methanol at the 5-fold coordinated Ti sites, as commonly ascribed to the methanol oxidation by the photogenerated holes, reflecting the highly photoactive nature of methanol. The formaldehyde yield from the TPD results is much smaller by a factor of 2/3 than the amount of dissociated methanol from the STM results at 80 K. This observation can be assigned to the reverse reaction during the TPD measurement, and may explain the lower yield of formaldehyde using molecular methanol than using methoxy. From the fractal-like reaction kinetics of methanol, we can associate the coverage-dependence of the spectral dimensions with the change for the diffusion of holes across the surface from a one-dimensional to a two-dimensional behavior because of the increased scattering species at higher coverages. Our results here provide a clear picture for the photocatalytic reaction of molecular methanol and may rationalize the different observations performed at various conditions.
Engineering Hybrid Co-picene Structures with Variable Spin Coupling
Chunsheng Zhou, Huan Shan, Bin Li, Aidi Zhao & Bing Wang
Applied Physics Letters 108, 171601 (2016)
We report on the in situ engineering of hybrid Co-picene magnetic structures with variable spin coupling using a low-temperature scanning tunneling microscope. Single picene molecules adsorbed on Au(111) are manipulated to accommodate individual Co atoms one by one, forming stable artificial hybrid structures with magnetism introduced by the Co atoms. By monitoring the evolution of the Kondo effect at each site of Co atom, we found that the picene molecule plays an important role in tuning the spin coupling between individual Co atoms, which is confirmed by theoretical calculations based on the density-functional theory. Our findings indicate that the hybrid metal-molecule structures with variable spin coupling on surfaces can be artificially constructed in a controlled manner.
Understanding the physics of surface plasmons and related phenomena requires knowledge of the spatial, temporal, and spectral distributions of the total electromagnetic field excited within nanostructures and their interfaces, which reflects the electromagnetic mode excitation, confinement, propagation, and damping. We present a microscopic and spectroscopic study of the plasmonic response in single-crystalline Ag wires grown in situ on Si(001) substrates. Excitation of the plasmonic modes with broadly tunable (UV–IR) femtosecond laser pulses excites ultrafast multiphoton photoemission, whose spatial distribution is imaged with an aberration-corrected photoemission electron microscope, thereby providing a time-integrated map of the locally enhanced electromagnetic fields. We show by tuning the wavelength, polarization, and k-vector of the incident laser light that for a few micrometers long wires we can selectively excite either the propagating surface plasmon polariton modes or high-order multipolar resonances of the Ag/vacuum and Ag/Si interfaces. Moreover, upon tuning the excitation wavelength from the UV to the near-IR spectral regions, we find that the resonant plasmonic modes shift from the top of the wires to selvedge at the Ag/Si interface. Our results, supported by numerical simulations, provide a better understanding of the optical response of metal/semiconductor structures and guidance toward the design of polaritonic and nanophotonic devices with enhanced properties, as well as suggest mechanisms for plasmonically enhanced photocatalysis.
2015
Quantitative o perando Visualization of the Energy Band Depth Profile in Solar Cells
Qi Chen, Lin Mao, Yaowen Li, Tao Kong, Na Wu, Changqi Ma, Sai Bai, Yizheng Jin, Dan Wu, Wei Lu, Bing Wang & Liwei Chen
Nature Communications 6, 7745 (2015)
The energy band alignment in solar cell devices is critically important because it largely governs elementary photovoltaic processes, such as the generation, separation, transport, recombination and collection of charge carriers. Despite the expenditure of considerable effort, the measurement of energy band depth profiles across multiple layers has been extremely challenging, especially for operando devices. Here we present direct visualization of the surface potential depth profile over the cross-sections of operando organic photovoltaic devices using scanning Kelvin probe microscopy. The convolution effect due to finite tip size and cantilever beam crosstalk has previously prohibited quantitative interpretation of scanning Kelvin probe microscopy-measured surface potential depth profiles. We develop a bias voltage-compensation method to address this critical problem and obtain quantitatively accurate measurements of the open-circuit voltage, built-in potential and electrode potential difference.
Structural and Electronic Properties of an Ordered Grain Boundary Formed by Separated (1,0) Dislocations in Graphene
Chuanxu Ma, Haifeng Sun, Hongjian Du, Jufeng Wang, Aidi Zhao, Qunxiang Li, Bing Wang & J.G. Hou
Nanoscale 7, 3055-3059 (2015)
We present an investigation of the structural and electronic properties of an ordered grain boundary (GB) formed by separated pentagon–heptagon pairs in single-layer graphene/SiO2 using scanning tunneling microscopy/spectroscopy (STM/STS), coupled with density functional theory (DFT) calculations. It is observed that the pentagon–heptagon pairs, i.e., (1,0) dislocations, form a periodic quasi-one-dimensional chain. The (1,0) dislocations are separated by 8 transverse rows of carbon rings, with a period of ~2.1 nm. The protruded feature of each dislocation shown in the STM images reflects its out-of-plane buckling structure, which is supported by the DFT simulations. The STS spectra recorded along the small-angle GB show obvious differential-conductance peaks, the positions of which qualitatively accord with the van Hove singularities from the DFT calculations.
We investigate the modification of electronic properties of single cobalt phthalocyanine (CoPc) molecule by an extra Co atom co-adsorbed on Au(111) surface using scanning tunneling microscopy (STM), joint with density functional theory (DFT) calculations. By manipulating CoPc molecules using the STM tip to contact individually adsorbed Co atom, two types of relatively stable complexes can be formed, denoted as CoPc-Co(I) and CoPc-Co(II). In CoPc-Co(I), the Co atom is at an intramolecular site close to aza-N atom of CoPc, which induces significant modifications of the electronic states of CoPc, such as energy shifts and splitting of nonlocal molecular orbitals. However, in CoPc-Co(II) where the Co atom is underneath a benzene lobe of CoPc, it only slightly modifies the electronic states of CoPc, and mainly local characteristics of specific molecular orbitals are affected, even though CoPc-Co(II) is more stable than CoPc-Co(I). Our DFT calculations give consistent results with the experiments, and related analyses based on the molecular orbital theory reveal mechanism behind the experimental observations.
2014
Evidence of van Hove Singularities in Ordered Grain Boundaries of Graphene
Chuanxu Ma†, Haifeng Sun†, Yeliang Zhao, Bin Li, Qunxiang Li, Aidi Zhao, Xiaoping Wang, Yi Luo, Jinlong Yang, Bing Wang & J. G. Hou
Physical Review Letters 112, 226802 (2014)
It has long been under debate whether the electron transport performance of graphene could be enhanced by the possible occurrence of van Hove singularities in grain boundaries. Here, we provide direct experimental evidence to confirm the existence of van Hove singularity states close to the Fermi energy in certain ordered grain boundaries using scanning tunneling microscopy. The intrinsic atomic and electronic structures of two ordered grain boundaries, one with alternative pentagon and heptagon rings and the other with alternative pentagon pair and octagon rings, are determined. It is firmly verified that the carrier concentration and, thus, the conductance around ordered grain boundaries can be significantly enhanced by the van Hove singularity states. This finding strongly suggests that a graphene nanoribbon with a properly embedded ordered grain boundary can be a promising structure to improve the performance of graphene-based electronic devices.
The initial step of O2 evolution reaction on a TiO2 surface is a long-standing puzzle. A recent scanning tunneling microscopy experiment showed that the H2O molecule adsorbed on rutile TiO2(110) surface could decompose under ultraviolet illumination. The underlying reaction mechanism is now examined by our GGA+U study, in which the oxidation of the H2O molecule by both free and trapped holes has been carefully investigated. It is found that the transfer of the hole trapped at the bridge oxygen to the molecule is hindered by the mismatch between the energy and spatial symmetry of the trapped hole orbital and the highest occupied molecule orbital of H2O. The entire oxidation reaction has a high energy barrier and is barely exothermic. In contrast, the oxidation of the molecule by the free hole is energetically more favorable. The free hole is transferred to the H2O molecule via the in-plane oxygen atom when the molecule stays in the transient dissociation state. This mechanism may also be applicable to the photooxidation of other R–OH type molecules adsorbed on the rutile TiO2(110) surface.
A new carbon-based two-dimensional crystalline nanostructure was discovered. The nanostructure was facilely constructed by chemical vapor deposition of benzene on Cu(111) in an ultrahigh vacuum chamber. A low temperature scanning tunneling microscopy and spectroscopy study of the nanostructure indicated that it has an orthorhombic superstructure and a semiconductor character with an energy gap of 0.8 eV. An X-ray photoelectron spectroscopy study showed that C–C(sp2) bonding is predominantly preserved, suggesting a framework consisting of π-conjugated building blocks. The periodic nanostructure was found to be a surprisingly excellent template for isolating and stabilizing magnetic atoms: Co atoms deposited on it can be well dispersed and form locally ordered atomic chains with their atomic magnetism preserved. Therefore the nanostructure may be suitable for organic spintronic applications. The most likely structural model for the nanostructure is proposed with the aid of density functional theory calculations and simulations, suggesting that the 2D nanostructure may consist of polyphenylene chains interconnected by Cu adatoms.
2013
Role of Point Defects on the Reactivity of Reconstructed Anatase Titanium Dioxide(001) Surface
Yang Wang†, Huijuan Sun†, Shijing Tan, Hao Feng, Zhengwang Cheng, Jin Zhao, Aidi Zhao, Bing Wang, Yi Luo, Jinlong Yang & J. G. Hou
Nature Communications 4, 2214 (2013)
The chemical reactivity of different surfaces of titanium dioxide (TiO2) has been the subject of extensive studies in recent decades. The anatase TiO2(001) and its (1×4) reconstructed surfaces were theoretically considered to be the most reactive and have been heavily pursued by synthetic chemists. However, the lack of direct experimental verification or determination of the active sites on these surfaces has caused controversy and debate. Here we report a systematic study on an anatase TiO2(001)-(1×4) surface by means of microscopic and spectroscopic techniques in combination with first-principles calculations. Two types of intrinsic point defects are identified, among which only the Ti3+ defect site on the reduced surface demonstrates considerable chemical activity. The perfect surface itself can be fully oxidized, but shows no obvious activity. Our findings suggest that the reactivity of the anatase TiO2(001) surface should depend on its reduction status, similar to that of rutile TiO2 surfaces.
The adsorption of O2/H2O molecules on the ZnO nanowire (NW) surface results in the long lifetime of photo-generated carriers and thus benefits ZnO NW-based ultraviolet photodetectors by suppressing the dark current and improving the photocurrent gain, but the slow adsorption process also leads to slow detector response time. Here we show that a thermally evaporated copper phthalocyanine film is effective in passivating surface trap states of ZnO NWs. As a result, the organic/inorganic hybrid photodetector devices exhibit simultaneously improved photosensitivity and response time. This work suggests that it could be an effective way in interfacial passivation using organic/inorganic hybrid structures
Characterization of Cr–N Codoped Anatase TiO2(001) Thin Films Epitaxially Grown on SrTiO3(001) Substrate
Yang Wang, Zhengwang Cheng, Shijing Tan, Xiang Shao, Bing Wang & J.G.Hou
Surface Science 616, 93-99 (2013)
We investigate the growth of Cr–N codoped anatase TiO2(001) thin films, prepared with a pulsed-laser-deposition (PLD) method using a mixed Cr2O3 and TiN ceramic target (6at.% Cr), and characterized using scanning tunneling microscopy (STM), X-ray and ultraviolet photoemission spectroscopy (XPS/UPS), and ultraviolet–visible (UV–Vis) absorption spectroscopy. We find that the doping concentration of N in the films can be finely tuned by the O2 pressure and the growth temperature. By optimizing the growth conditions, we obtain the anatase TiO2(001) films with relatively smooth (1×4) reconstructed surface at equally codoped contents of 6 at.% Cr and 6 at.% N. The roughness of the surface is about 0.9 nm in root mean square, and the typical size of the (1×4) terraces is about 20 nm. The XPS results indicate that Cr and N should be both substitutionally doped in the film. From the UPS spectrum for the codoped film, the valence band maximum is significantly lifted by about 1.3 eV, indicating a narrowing band gap of 1.9 eV. The optical absorption spectrum shows that the codoped film noticeably absorbs the light at less than 710 nm. Derived from the optical absorption spectrum, an estimated band gap value of 1.78 eV is obtained, which is consistent with the UPS result.
STM Tip-assisted Single Molecule Chemistry
Aidi Zhao, Shijing Tan, Bin Li, Bing Wang, Jinlong Yang & J. G. Hou
Physical Chemistry Chemical Physics 15, 12428-12441 (2013)
Scanning tunnelling microscopy (STM) has been a unique and powerful tool in the study of molecular systems among various microscopic and spectroscopic techniques. This benefits from the local probing ability for the atomically resolved structural and electronic characterization by the STM tip. Moreover, by using the STM tip one can modify a given structure and thus control the physical and chemical properties of molecules at a single-molecule level. The rapid developments in the past 30 years have extended the functions of STM far beyond characterization. It has shown the flexibility to combine STM with other techniques by making use of the advantages of the STM tip, demonstrating important applications in the growing nanotechnology. Here we review some recent progresses in our laboratory on single molecule chemistry by taking advantage of tip-assisted local approaches, such as the identification of specific orbitals or states of molecules on surfaces, tip-induced single-molecule manipulation, atomically resolved chemical reactions in photochemistry and tip-induced electroluminescence. We expect more joint techniques to emerge in the near future by using the unique advantages of STM tip, providing more powerful tools for the growing requirements of new materials design and the mechanism of chemical reactions at the molecular scale.
