Scanning Tunneling Microscopy Spectroscopy Research Articles

Using k-means to sort spectra: Electronic order mapping from scanning tunneling spectroscopy measurements

Hyperspectral imaging techniques have a unique ability to probe the inhomogeneity of material properties whether driven by compositional variation or other forms of phase segregation. In the doped cuprates, iridates, and related materials, scanning tunneling microscopy/spectroscopy (STM/STS) measurements have found the emergence of pseudogap “puddles” from the macroscopically Mott insulating phase with increased doping. However, categorizing this hyperspectral data by electronic order is not trivial and has often been done with ad hoc methods. In this paper, we demonstrate the utility of k-means, a simple and easy-to-use unsupervised clustering method, as a tool for classifying heterogeneous scanning tunneling spectroscopy data by electronic order for Rh-doped Sr2IrO4, a cuprate-like material. Applied to STM data acquired within the Mott phase, k-means was able to identify areas of Mott order and of pseudogap order. The unsupervised nature of k-means limits avenues for bias and provides clustered spectral shapes without a priori knowledge of the physics. Additionally, we demonstrate the use of k-means as a preprocessing tool to constrain phenomenological function fitting. Clustering the data allows us to reduce the fitting parameter space, limiting over-fitting. We suggest k-means as a fast, simple model for processing hyperspectral data on materials of mixed electronic order.

Investigation of perovskite materials for solar cells using scanning tunneling microscopy.

The issue of energy scarcity has become more prominent due to the recent scientific and technological advancements. Consequently, there is an urgent need for research on sustainable and renewable resources. Solar energy, in particular, has emerged as a highly promising option because of its pollution-free and environment-friendly characteristics. Among the various solar energy technologies, perovskite solar cells have attracted much attention due to their lower cost and higher photoelectric conversion efficiency (PCE). However, the inherent instability of perovskite materials hinders the commercialization of such devices. The utilization of scanning tunneling microscopy/spectroscopy (STM/STS) can provide valuable insights into the fundamental properties of different perovskite materials at the atomic scale, which is crucial for addressing this challenge. In this review, we present the recent research progress of STM/STS analysis applied to various perovskites for solar cells, including halide perovskites, two-dimensional Ruddlesden-Popper perovskites, and oxide perovskites. This comprehensive overview aims to inspire new ideas and strategies for optimizing solar cells.

Epitaxial Growth of Monolayer WS2 Single Crystals on Au(111) Toward Direct Surface-Enhanced Raman Spectroscopy Detection.

The epitaxial growth of wafer-scale two-dimensional (2D) semiconducting transition metal dichalcogenides (STMDCs) single crystals is the key premise for their applications in next-generation electronics. Despite significant advancements, some fundamental factors affecting the epitaxy growth have not been fully uncovered, e.g., interface coupling strength, adlayer-substrate lattice matching, substrate step-edge-guiding effects, etc. Herein, we develop a model system to tackle these issues concurrently, and realize the epitaxial growth of wafer-scale monolayer tungsten disulfide (WS2) single crystals on the Au(111) substrate. This epitaxial system is featured with good adlayer-substrate lattice matching, obvious step-edge-guiding effect for the unidirectionally aligned nucleation/growth, and relatively weaker interfacial interaction than that of monolayer MoS2/Au(111), as evidenced by the evolution of a uniform Moiré pattern and an intrinsic band gap, according to on-site scanning tunneling microscopy/spectroscopy (STM/STS) characterizations and density functional theory calculations. Intriguingly, the unidirectionally aligned monolayer WS2 domains along the Au(111) steps can behave as ultrasensitive templates for surface-enhanced Raman scattering detection of organic molecules, due to the obvious charge transfer occurred at substrate step edges. This work should hereby deepen our understanding of the epitaxy mechanism of 2D STMDCs on single-crystal substrates, and propel their wafer-scale production and applications in various cutting-edge fields.

Electronic States of Single Perovskite Quantum Dots in Weak and Strong Interaction Regimes: Implications in Electrically Pumped Quantum Emitters.

We investigate the effect of Coulomb interactions on the electronic states of a single perovskite quantum dot (PQD), CsPbBr3, through scanning tunneling microscopy/spectroscopy (STM/S). Under a weak interaction regime, where the time-averaged occupation of electrons in a PQD remains zero, the peaks observed in the differential tunneling conductance (dI/dV) spectrum correspond to the single-particle density of states (DOS) without any electron-electron correlation. However, with a shorter tunnel distance between the STM tip and PQD, additional electrons are trapped in the QD, leading to a strong interaction regime with well-defined electronic fine structures due to the lifting of spin degeneracy in the conduction bands. Interestingly, we observe that the strong Coulomb interaction can modify the spin-orbit coupling (SOC) strength in the PQDs. We have concluded that the energy levels under a strong electron-electron interaction regime are of utmost importance since they will be applicable to electrically pumped PQD-based single photon quantum emitters.

Quantum States Induced by Strong Interface Coupling in a 2D VSe2/Bi2Se3 Heterostructure.

We have successfully fabricated single-layer (SL) 1T-VSe2/Bi2Se3 heterostructures using molecular beam epitaxy (MBE), which exhibits uniform moiré patterns on the heterostructure surface. Scanning tunneling microscopy/spectroscopy (STM/STS) reveals a notable quantum state near the Fermi energy, robust across the entire moiré lattice. This quantum state peak shifts slightly across different domain ranges, suggesting an elastic strain dependence in SL VSe2, confirmed by geometric phase analysis (GPA) simulations. Density functional theory (DFT) calculations indicate that the enhanced quantum state results from charge redistribution between the substrate and the epifilm with the orbitals of Se atoms in the deformed VSe2 playing a dominant role.

On-Surface Synthesis of 2D Radical Frameworks

The design of atom thick two-dimensional (2D) frameworks by on-surface synthesis has recently contributed to the development of advanced materials with high control over their structural, electronic, optical and magnetic properties. Countless 2D frameworks, with a large variety of applications, can be synthesized made of purely organic molecules (covalent organic and supramolecular frameworks) or combining both metals and organic molecules (metalorganic frameworks). Here, we focus on the design and synthesis of radical 2D frameworks, composed of open-shell nanographenes equipped with azaindole units, on an Au(111) surface. Interestingly, the deposition of transition metal atoms on such frameworks allow the tuning of their magnetic properties, expanding their possible applications. The structural, electronic and magnetic characterization of the frameworks was performed by scanning tunneling microscopy/spectroscopy (STM/STS) and non-contact atomic force microscopy, and supported by density functional theory (DFT) calculations. Additionally, the magnetic characterization was complemented by nickelocene-functionalized tips investigations, which act as a magnetic sensor that unveils the magnetism of the frameworks at the single molecule level.

Substantial Energy Band Modulation of Semiconducting Hexagonal GaTe Quantum Wells by Layer Thickness and Mirror Twin Boundaries.

Exploring emerging two-dimensional (2D) van der Waals (vdW) semiconducting materials and precisely tuning their electronic properties at the atomic level have long been recognized as crucial issues for developing their high-end electronic and optoelectronic applications. As a III-VI semiconductor, ultrathin layered hexagonal GaTe (h-GaTe) remains unexplored in terms of its intrinsic electronic properties and band engineering strategies. Herein, we report the successful synthesis of ultrathin h-GaTe layers on a selected graphene/SiC(0001) substrate, via molecular beam epitaxy (MBE). The widely tunable quasiparticle band gaps (∼2.60-1.55 eV), as well as the vdW quantum well states (QWSs) that can be strictly counted by the layer numbers, are well characterized by onsite scanning tunneling microscopy/spectroscopy (STM/STS), and their origins are clearly addressed by density functional theory (DFT) calculations. More intriguingly, distinctive 8|8E and 4|4P (Ga) mirror twin boundaries (MTBs) are identified in the ultrathin h-GaTe flakes, which can induce decreased band gaps and prominent p-doping effects. This work should deepen our understanding on the electronic tunability of 2D III-VI semiconductors by thickness control and line defect engineering, which may hold promise for fabricating atomic-scale vertical and lateral homojunctions toward ultrascaled electronics and optoelectronics.

Towards revealing intrinsic vortex-core states in Fe-based superconductors through statistical discovery

In type-II superconductors, electronic states within magnetic vortices hold crucial information about the paring mechanism and can reveal non-trivial topology. While scanning tunneling microscopy/spectroscopy (STM/S) is a powerful tool for imaging superconducting vortices, it is challenging to isolate the intrinsic electronic properties from extrinsic effects like subsurface defects and disorders. Here we combine STM/STS with basic machine learning to develop a method for screening out the vortices pinned by embedded disorder in iron-based superconductors. Through a principal component analysis of large STS data within vortices, we find that the vortex-core states in Ba(Fe0.96Ni0.04)2As2 start to split into two categories at certain magnetic field strengths, reflecting vortices with and without pinning by subsurface defects or disorders. Our machine-learning analysis provides an unbiased approach to reveal intrinsic vortex-core states in novel superconductors and shed light on ongoing puzzles in the possible emergence of a Majorana zero mode.

Visualization of Multiple-Resonance-Induced Frontier Molecular Orbitals in a Single Multiple-Resonance Thermally Activated Delayed Fluorescence Molecule.

The spatial distribution and electronic properties of the frontier molecular orbitals (FMOs) in a thermally activated delayed fluorescence (TADF) molecule contribute significantly to the TADF properties, and thus, a detailed understanding and sophisticated control of the FMOs are fundamental to the design of TADF molecules. However, for multiple-resonance (MR)-TADF molecules that achieve spatial separation of FMOs by the MR effect, the distinctive distribution of these molecular orbitals poses significant challenges for conventional computational analysis and ensemble averaging methods to elucidate the FMOs' separation and the precise mechanism of luminescence. Therefore, the visualization and analysis of electronic states with the specific energy level of a single MR-TADF molecule will provide a deeper understanding of the TADF mechanism. Here, scanning tunneling microscopy/spectroscopy (STM/STS) was used to investigate the electronic states of the DABNA-1 molecule at the atomic scale. FMOs' visualization and local density of states analysis of the DABNA-1 molecule clearly show that MR-TADF molecules also have well-separated FMOs according to the internal heteroatom arrangement, providing insights that complement existing theoretical prediction methods.

Intrinsic Grain Boundary Structure and Enhanced Defect States in Air-Sensitive Polycrystalline 1T'-WTe2 Monolayer.

Monolayer WTe2 has attracted significant attention for its unconventional superconductivity and topological edge states. However, its air sensitivity poses challenges for studying intrinsic defect structures. This study addresses this issue using a custom-built inert gas interconnected system, and investigate the intrinsic grain boundary (GB) structures of monolayer polycrystalline 1T' WTe2 grown by nucleation-controlled chemical vapor deposition (CVD) method. These findings reveal that GBs in this system are predominantly governed by W-Te rhombi with saturated coordination, resulting in three specific GB prototypes without dislocation cores. The GBs exhibit anisotropic orientations influenced by kinks formed from these fundamental units, which in turn affect the distribution of grains in various shapes within polycrystalline flakes. Scanning tunneling microscopy/spectroscopy (STM/S) analysis further reveals metallic states along the intrinsic 120° twin grain boundary (TGB), consistent with computed band structures. This systematic exploration of GBs in air-sensitive 1T' WTe2 monolayers provides valuable insights into emerging GB-related phenomena.

Ghost states and surface structures of the charge density wave kagome metal ScV[formula omitted]Sn[formula omitted]

We investigate the high-temperature phase of the kagome metal ScV6Sn6 using scanning tunneling microscopy/spectroscopy (STM/S) and density functional theory calculations. STM topographic images of the cleaved sample reveal two distinct surface terminations: flat islands with Sn termination and trenches terminated by kagome layers with Sn as the outermost atomic layer. STS measurements on the Sn-terminated and kagome-terminated surfaces show significant differences, in particular the presence of large density of states near the Fermi level in the former case. Our first-principles calculations reveal that the charge density on the kagome-terminated surface gives rise to “ghost states” which show intensity away from surface atoms, arising due to hybridization of orbitals above the surface. These states can obscure the intrinsic properties of the surface, potentially leading to misattribution of the surface termination. This underscores the need for careful interpretation in STM studies, especially when discerning surface states of localized states. Understanding the surface structure of this versatile quantum material provides essential information for interpreting surface-sensitive experiments, tailoring material properties, engineering interfaces, and controlling stability and reactivity. This knowledge paves the way for further exploration and potential applications of kagome lattice materials in various fields, including quantum computing, topological physics, and advanced electronic devices.

Atomic-scale magnetic doping of monolayer stanene by revealing Kondo effect from self-assembled Fe spin entities

Atomic-scale spin entity in a two-dimensional topological insulator lays the foundation to manufacture magnetic topological materials with single atomic thickness. Here, we have successfully fabricated Fe monomer, dimer and trimer doped in the monolayer stanene/Cu(111) through a low-temperature growth and systematically investigated Kondo effect by combining scanning tunneling microscopy/spectroscopy (STM/STS) with density functional theory (DFT) and numerical renormalization group (NRG) method. Given high spatial and energy resolution, tunneling conductance (dI/dU) spectra have resolved zero-bias Kondo resonance and resultant magnetic-field-dependent Zeeman splitting, yielding an effective spin Seff = 3/2 with an easy-plane magnetic anisotropy on the self-assembled Fe atomic dopants. Reduced Kondo temperature along with attenuated Kondo intensity from Fe monomer to trimer have been further identified as a manifestation of Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between Sn-separated Fe atoms. Such magnetic Fe atom assembly in turn constitutes important cornerstones for tailoring topological band structures and developing magnetic phase transition in the single-atom-layer stanene.

Strain-induced superstructure evolution and Te vacancy accumulation in AgTe monolayer grown on Ag(111) substrate

Two-dimensional (2D) materials have attracted increasing attention for frontier science and industrial applications in the past few decades. As a 2D materials with honeycomb lattice, AgTe monolayer grown on Ag (111) substrate belongs to the transition metal monochalcogenides (TMMs) family possessing various superstructures and exotic electronic structure. In this report, the AgTe monolayer superstructure is mainly studied using scanning tunneling microscope/spectroscopy (STM/S) measurements. With the increase of Te deposition, the planar structure of the AgTe monolayer becomes the one-dimensional (1D) moiré pattern structure, which transformation is related to the change of strain combing with theoretical analysis. When there is an isotropic strain on the surface, the AgTe monolayer is a flat honeycomb structure. In contrast, the lattice structural strain is released under a uniaxial strain as the AgTe monolayer is transformed into a 1D moiré pattern structure. In addition, Te vacancies in the AgTe monolayer are also affected by the synergistic effect of strain and Ag (111) substrate, and the concentration of Te vacancy changes significantly with the strain transformation. Moreover, both experimental statistics and theoretical calculations verify that Te vacancy tend to appear in the bright ridge (BR) region of the AgTe monolayer with a 1D moiré pattern structure, which is also the result of strain modulation. Our findings open up a new thought for strain transformation control of 2D materials driven by elemental concentration.

Spontaneous Line Defect‐Induced Co4Te7 Superlattices on SrTiO3(001) Featuring Flat Bands

Abstract1D structures/patterns (e.g., line defect arrays, 1D Moiré patterns) embedded in 2D materials provide fascinating platforms for exploring versatile intriguing phenomena, for example, 1D Luttinger liquids and charge density waves (CDWs). Despite persistent efforts, incorporating periodic 1D patterns into 2D materials remains an ongoing pursuit. Herein, the direct preparation of monolayer 1D‐defect‐induced Co4Te7 superlattices (with periodic Te defect lines in the upper Te layer in 1T‐CoTe2) is reported, on lattice‐matched SrTiO3(001) (STO(001)) substrates via molecular beam epitaxy (MBE). Utilizing on‐site scanning tunneling microscopy/spectroscopy (STM/STS) combined with density functional theory (DFT) calculations, the detailed atomic structure of monolayer Co4Te7 is identified, and its formation mechanisms from the synergistic effects of the Co/Te precursor ratio and adlayer‐substrate interfacial coupling are uncovered. The potential flat‐band feature of the monolayer Co4Te7 is also unveiled. This work should hereby offer valuable insights into the engineering of periodic 1D‐defect patterns in 2D materials, as well as the atomic‐scale structure and electronic property characterizations, thus paving ways for their intriguing property investigations.

Twist-Angle Tuning of Electronic Structure in Two-Dimensional Dirac Nodal Line Semimetal Au2Ge on Au(111).

Topological semimetals have emerged as quantum materials including Dirac, Weyl, and nodal line semimetals, and so on. Dirac nodal line (DNL) semimetals possess topologically nontrivial bands crossing along a line or a loop and are considered precursor states for other types of semimetals. Here, we combine scanning tunneling microscopy/spectroscopy (STM/S) measurements and density functional theory (DFT) calculations to investigate a twist angle tuning of electronic structure in two-dimensional DNL semimetal Au2Ge. Theoretical calculations show that two bands of Au2Ge touch each other in Γ-M and Γ-K paths, forming a DNL. A significant transition of electronic structure occurs by tuning the twist angle from 30° to 24° between monolayer Au2Ge and Au(111), as confirmed by STS measurements and DFT calculations. The disappearing of DNL state is a direct consequence of symmetry breaking.

Deceptive orbital confinement at edges and pores of carbon-based 1D and 2D nanoarchitectures.

The electronic structure defines the properties of graphene-based nanomaterials. Scanning tunneling microscopy/spectroscopy (STM/STS) experiments on graphene nanoribbons (GNRs), nanographenes, and nanoporous graphene (NPG) often determine an apparent electronic orbital confinement into the edges and nanopores, leading to dubious interpretations such as image potential states or super-atom molecular orbitals. We show that these measurements are subject to a wave function decay into the vacuum that masks the undisturbed electronic orbital shape. We use Au(111)-supported semiconducting gulf-type GNRs and NPGs as model systems fostering frontier orbitals that appear confined along the edges and nanopores in STS measurements. DFT calculations confirm that these states originate from valence and conduction bands. The deceptive electronic orbital confinement observed is caused by a loss of Fourier components, corresponding to states of high momentum. This effect can be generalized to other 1D and 2D carbon-based nanoarchitectures and is important for their use in catalysis and sensing applications.

Growth of Dichalcogenide Layers on TiO2(110)—MoSe2 or TiSe2

Transition‐metal dichalcogenide (TMDC) islands and thin films are grown on TiO2(110) via physical vapor deposition of Mo and Se and are analyzed with electron diffraction, Auger spectroscopy, and scanning tunneling microscopy (STM). Surprisingly, large, crystalline TiSe2 patches develop on the TiO2 surface instead of the expected MoSe2 islands. They are identified by a hexagonal lattice with 3.4 Å periodicity, a unique (2 × 2) superstructure related to charge‐density waves, and an empty conductance doublet in STM spectroscopy. Density‐function‐theory calculations assign the imaging contrast and the conductance doublet to tunneling into the 3p orbitals of the topmost Se layer and Ti 3d‐related electronic states, respectively. Control experiments without Mo deposition confirm that the observed TMDC growth indeed results from a surface reaction between gas‐phase Se and Ti3+ atoms, segregating out of the reduced TiO2 crystal. From the unique shape and orientation of the TiSe2 islands in the low‐coverage regime, a distinct nucleation scheme is proposed, in which the Se(3 × 1) superstructure on TiO2 provides pinning centers for the TMDC. This article aims at increasing the awareness that Mo and Se co‐deposition does not automatically result in MoSe2 formation, but parasitic processes may trigger the growth of other TMDCs.

Observation of Two-Dimensional Type-II Superconductivity in Bulk 3R-TaSe2 by Scanning Tunneling Spectroscopy.

Here we report a low-temperature and vector-magnetic-field scanning tunneling microscopy/spectroscopy (STM/S) study on 3R-TaSe2. The sample surface was obtained by exfoliating a bulk 3R-TaSe2 single crystal in an ultrahigh-vacuum (UHV) chamber and then transferred in situ to STM. It was observed that the topmost layer shows a 3 × 3 charge density wave pattern at T = 4.2 K with metallic character in STS. The electronic characterization study by variable-temperature and magnetic field STS revealed that 3R-TaSe2 behaves as a type-II superconductor. More intriguingly, such superconductivity (SC) can survive under strong in-plane magnetic fields even up to 2.5 T and out-of-plane magnetic fields up to 0.7 T, exhibiting an anisotropic superconducting property. Temperature-dependent STS showed that 3R-TaSe2 undergoes a transition above 0.58 K. Our results may be important for understanding the intriguing SC properties of the 3R-phase van der Waals materials.

Searching for Majorana quasiparticles at vortex cores in iron-based superconductors

Abstract The unambiguous detection of the Majorana zero mode (MZM), which is essential for future topological quantum computing, has been a challenge in recent condensed matter experiments. The MZM is expected to emerge at the vortex core of topological superconductors as a zero-energy vortex bound state (ZVBS), amenable to detection using scanning tunneling microscopy/spectroscopy (STM/STS). However, the typical energy resolution of STM/STS has made it challenging to distinguish the MZM from the low-lying trivial vortex bound states. Here, we review the recent high-energy-resolution STM/STS experiments on the vortex cores of Fe(Se,Te), where the MZM is expected to emerge, and the energy of the lowest trivial bound states is reasonably high. Tunneling spectra taken at the vortex cores exhibit a ZVBS well below any possible trivial state, suggesting its MZM origin. However, it should be noted that ZVBS is a necessary but not sufficient condition for the MZM; a qualitative feature unique to the MZM needs to be explored. We discuss the current status and issues in the pursuit of such Majorananess, namely the level sequence of the vortex bound states and the conductance plateau of the ZVBS. We also argue for future experiments to confirm the Majorananess, such as the detection of the doubling of the shot-noise intensity and spin polarization of the MZM.

Electronic Decoupling and Single‐Molecule Charging of C60 on h‐BN/Rh(111)

AbstractA detailed understanding of the interaction between molecules and 2D materials is crucial to implement molecular films into next‐generation 2D material‐organic hybrid devices effectively. In this regard, energy level alignment and charge transfer processes are particularly relevant. This work investigates the interplay between a hexagonal boron nitride (h‐BN) monolayer on an Rh(111) single crystal and self‐assembled C60 thin films. The influence of the corrugated topography and electrostatic surface potential originating from the h‐BN/Rh(111) Moiré superstructure on the electronic level alignment and charging characteristics of C60 is being studied. A combination of scanning tunneling microscopy/spectroscopy (STM/STS) and a theoretical tight‐binding approach is used to gain insight into the C60 bandstructure formation and electronic decoupling of specific C60. This decoupling results from adsorption site‐dependent variations of the molecular energy level alignment, which controls the strength of intermolecular hybridization. The decoupling of specific C60 enables the direct observation of single‐electron charging processes via STS and Kelvin probe force microscopy. The charging of the C60 is enabled by combining two gating mechanisms: the electrostatic surface potential of the monolayer h‐BN/Rh(111) Moiré and the electric field of the STM tip.