Scanning Tunneling Spectroscopy Data Research Articles

Thermally-induced nanoscale phase change in chalcogenide glass Cr2Ge2Te6 revealed by scanning tunneling microscopy

Cr2Ge2Te6 (CrGT) is viewed as an important phase change material (PCM) for next-generation nonvolatile memory devices because of its superior properties, e.g., high thermal stability and low operation energy, compared to conventional PCMs. However, the phase change mechanism of CrGT remains unsolved, especially at the nanoscale. Here, we investigated thermally induced nanoscale phase changes of CrGT thin films using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). By performing statistical analysis of the measured STM topographic and STS data, we evaluated the inhomogeneity and distribution of the phase change characteristics of CrGT thin films. We also related the nanoscale phase change properties of CrGT to macroscopic phase changes by comparing the STM and STS results with experimental data from Raman spectroscopy.

Sub-5 nm Contacts and Induced p-n Junction Formation in Individual Atomically Precise Graphene Nanoribbons.

This paper demonstrates the fabrication of nanometer-scale metal contacts on individual graphene nanoribbons (GNRs) and the use of these contacts to control the electronic character of the GNRs. We demonstrate the use of a low-voltage direct-write STM-based process to pattern sub-5 nm metallic hafnium diboride (HfB2) contacts directly on top of single GNRs in an ultrahigh-vacuum scanning tunneling microscope (UHV-STM), with all the fabrication performed on a technologically relevant semiconductor silicon substrate. Scanning tunneling spectroscopy (STS) data not only verify the expected metallic and semiconducting character of the contacts and GNR, respectively, but also show induced band bending and p-n junction formation in the GNR due to the metal-GNR work function difference. Contact engineering with different work function metals obviates the need to create GNRs with different characteristics by complex chemical doping. This is a demonstration of the successful fabrication of precise metal contacts and local p-n junction formation on single GNRs.

Determining the temperature of a millikelvin scanning tunnelling microscope junction

Cooling the junction of a scanning tunneling microscope to millikelvin temperatures is fundamental for high-resolution scanning tunneling spectroscopy. However, accurately determining the junction temperature has proven elusive, due to the microscopic dimension of the junction and its continuous energy exchange with the surrounding environment. Here, we employ a millikelvin scanning tunnelling microscope cooled by an adiabatic demagnetization refrigerator. Using normal-metal and superconducting tips, we perform scanning tunnelling spectroscopy on an atomically clean surface of Al(100) in a superconducting state. By varying the refrigerator temperatures between 30 mK and 1.2 K, we show that the temperature of the junction is decoupled from the temperature of the surrounding environment. To corroborate our findings, we simulate the scanning tunnelling spectroscopy data with P(E) theory and determine that the junction has a temperature of 77 mK, despite its environment being at 1.5 K.

Charge density wave in single-layer Pb/Ge(111) driven by Pb-substrate exchange interaction

Single layer Pb on top of (111) surfaces of group IV semiconductors hosts charge density wave and superconductivity depending on the coverage and on the substrate. These systems are normally considered to be experimental realizations of single band Hubbard models and their properties are mostly investigated using lattice models with frozen structural degrees of freedom, although the reliability of this approximation is unclear. Here, we consider the case of Pb/Ge(111) at $1/3$ coverage, for which surface x-ray diffraction and angle-resolved photoemission spectroscopy data are available. By performing first-principles calculations, we demonstrate that the nonlocal exchange between Pb and the substrate drives the system into a $3\ifmmode\times\else\texttimes\fi{}3$ charge density wave. The electronic structure of this charge ordered phase is mainly determined by two effects: The magnitude of the Pb distortion and the large spin-orbit coupling. Finally, we show that the effect applies also to the $3\ifmmode\times\else\texttimes\fi{}3$ phase of Pb/Si(111) where the Pb-substrate exchange interaction increases the bandwidth by more than a factor $1.5$ with respect to density functional theory $+\mathrm{U}$, in better agreement with scanning tunneling spectroscopy data. The delicate interplay between substrate, structural, and electronic degrees of freedom invalidates the widespread interpretation available in literature considering these compounds as physical realizations of single band Hubbard models.

Revealing the Chemical Bonding in Adatom Arrays via Machine Learning of Hyperspectral Scanning Tunneling Spectroscopy Data.

The adatom arrays on surfaces offer an ideal playground to explore the mechanisms of chemical bonding via changes in the local electronic tunneling spectra. While this information is readily available in hyperspectral scanning tunneling spectroscopy data, its analysis has been considerably impeded by a lack of suitable analytical tools. Here we develop a machine learning based workflow combining supervised feature identification in the spatial domain and unsupervised clustering in the energy domain to reveal the details of structure-dependent changes of the electronic structure in adatom arrays on the Co3Sn2S2 cleaved surface. This approach, in combination with first-principles calculations, provides insight for using artificial neural networks to detect adatoms and classifies each based on their local neighborhood comprised of other adatoms. These structurally classified adatoms are further spectrally deconvolved. The unexpected inhomogeneity of electronic structures among adatoms in similar configurations is unveiled using this method, suggesting there is not a single atomic species of adatoms, but rather multiple types of adatoms on the Co3Sn2S2 surface. This is further supported by a slight contrast difference in the images (or slight size variation) of the topography of the adatoms.

Ultrafast method for scanning tunneling spectroscopy

A scanning tunneling microscope (STM) combines unique capabilities in imaging and spectroscopy with atomic precision, and it can obtain energy-resolved spectroscopic data with atomic resolution. In this paper, we utilize a recently proposed modification to the STM feedback control loop to acquire high quality d2I/dV2 images. We have developed a constant differential conductance imaging method by closing the STM feedback loop with a high precision dI/dV measurement. In this mode, the tip’s vertical position is adjusted so as to keep the differential conductance constant during raster scanning of the surface. Based on this imaging mode, we propose a new technique to acquire fast and reliable scanning tunneling spectroscopy (STS) data simultaneously with the imaging.

Measuring and Tuning the Potential Landscape of Electrostatically Defined Quantum Dots in Graphene.

We use Kelvin probe force microscopy (KPFM) to probe the carrier-dependent potential of an electrostatically defined quantum dot (QD) in a graphene/hexagonal boron nitride (hBN) heterostructure. We show that gate-dependent measurements enable a calibration scheme that corrects for uncertainty inherent in typical KPFM measurements and accurately reconstructs the potential well profile. Our measurements reveal how the well changes with carrier concentration, which we associate with the nonlinear dependence of graphene's work function on carrier density. These changes shift the energy levels of quasi-bound states in the QD which we can measure via scanning tunneling spectroscopy (STS). We show that the experimentally extracted energy levels closely compare with wave functions calculated from the reconstructed KPFM data. This methodology, where KPFM and STS data are simultaneously acquired from 2D materials, allows the quasiparticle response to an electrostatic potential to be determined in a self-consistent way.

Topological orders competing for the Dirac surface state in FeSeTe surfaces

FeSeTe has recently emerged as a leading candidate material for the two-dimensional topological superconductivity (TSC). Two reasons for the excitement are the high $T_c$ of the system and the fact that the Majorana zero modes (MZMs) inside the vortex cores live on the exposed surface rather than at the interface of a heterostructure as in the proximitized topological insulators. However, the recent scanning tunneling spectroscopy data have shown that, contrary to the theoretical expectation, the MZM does not exist inside every vortex core. Hence there are ``full'' vortices with MZMs and ``empty'' vortices without MZMs. Moreover the fraction of ``empty'' vortices increase with an increase in the magnetic field. We propose the possibility of two distinct gapped states competing for the topological surface states in FeSeTe: the TSC and half quantum anomalous Hall (hQAH). The latter is promoted by magnetic field through the alignment of magnetic impurities such as Fe interstitials. When hQAH takes over the topological surface state, the surface will become transparent to scanning tunneling microscopy and the nature of the vortex in such region will appear identical to what is expected of the vortices in the bulk, i.e., empty. Unmistakable signature of the proposed mechanism for empty vortices will be the existance of chiral Majorana modes(CMM) at the domain wall between a hQAH region and a TSC region. Such CMM should be observable by observing local density of states along a line connecting an empty vortex to a nearby full vortex.

Acquisition and analysis of scanning tunneling spectroscopy data—WSe2 monolayer

Acquisition and analysis are described for scanning tunneling spectroscopy data acquired from a monolayer of WSe2 grown on epitaxial graphene on SiC. Curve fitting of the data is performed, in order to deduce band edge energies. In addition to describing the details of the theoretical curves used for the fitting, the acquisition and analysis methods are also discussed within the larger context of the historical development of scanning tunneling spectroscopy techniques.

C60 self-orientation on hexagonal boron nitride induced by intermolecular coupling

A deep grasp of the properties of the interface between organic molecules and hexagonal boron nitride (h-BN) is essential for the full implementation of these two building blocks in the next generation of electronic devices. Here, using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), we report on the geometric and electronic features of C60 evaporated on a single layer of h-BN grown on a Rh(110) surface under ultra-high vacuum. Two different molecular assemblies of C60 on the h-BN/Rh(110) surface were observed. The first STM study at room temperature (RT) and at low temperatures (40 K) looked at the molecular orientation of C60 on a two-dimensional layered material. Intramolecular-resolution images demonstrate the existence of a phase transition of C60 over the h-BN/Rh(110) surface similar to that found on bulk solid C60. At RT molecules exhibit random orientations, while at 40 K such rotational disorder vanishes and they adopt a common orientation over the h-BN/Rh(110) surface. The decrease in thermal energy allows recognition between C60 molecules, and they become equally oriented in the configuration at which the van der Waals intermolecular interactions are optimized. Bias-dependent submolecular features obtained by means of high-resolution STM images are interpreted as the highest occupied and lowest unoccupied molecular orbitals. STS data showed that fullerenes are electronically decoupled from the substrate, with a negligible charge transfer effect if any. Finally, the very early stages of multilayer growth were also investigated.

Hut-shaped lead nanowires with one-dimensional electronic properties

The ultrathin Pb layer on the Si(110) surface might be converted into hut-shaped nanowires extending in a unique $[\overline{1}10]$ direction. Despite the fact that about 2.6 ML of lead is used to form an ordered array of nanowires, this system shows not only anisotropy of the electronic structure but also a small band gap in the [001] direction. Among several quasi-one-dimensional bands, two of them show pure one-dimensional character. Electron dispersion maps measured with photoelectron spectroscopy are supported by scanning tunneling spectroscopy data which confirm the absence of electronic states at the Fermi level in areas between lead nanowires. Based on first-principles density functional theory calculations four different structural models of lead nanowires are proposed reflecting a different number of Pb atoms in the unit cell.

Hexagonal Nanopits with the Zigzag Edge State on Graphite Surfaces Synthesized by Hydrogen-Plasma Etching

We studied, by scanning tunneling microscopy, the morphology of nanopits of monolayer depth created at graphite surfaces by hydrogen plasma etching under various conditions such as H$_2$ pressure, temperature, etching time, and RF power of the plasma generation. In addition to the known pressure-induced transition of the nanopit morphology, we found a sharp temperature-induced transition from many small rather round nanopits of ~150 nm size to few large hexagonal ones of 300-600 nm within a narrow temperature range. The remote and direct plasma modes switching mechanism, which was proposed to explain the pressure-induced transition, is not directly applicable to this newly found transition. Scanning tunneling spectroscopy (STS) measurements of edges of the hexagonal nanopits fabricated at graphite surfaces by this method show clear signatures of the peculiar electronic state localized at the zigzag edge (edge state), i.e., a prominent peak near the Fermi energy accompanied by suppressions on either side in the local density of states. These observations indicate that the hexagonal nanopits consist of a high density of zigzag edges. The STS data also revealed a domain structure of the edge state in which the electronic state varies over a length scale of ~3 nm along the edge. The present study will pave the way for microscopic understanding of the anisotropic etching mechanism and of spin polarization in zigzag nanoribbons which are promising key elements for future graphene nanoelectronics.

Analyzing scanning tunneling spectroscopy for Fe-based superconductors and extracting sample density of states

We extract the density of states (DOS) from the scanning tunneling spectroscopy data for Ba1−xKxFe2As2 superconductor. The obtained sample DOS is composed of two ordinary s-wave types from the band at point and a linear-like DOS within the s-wave gap from the band at M point in the Brillouin zone, and is consistent with the corresponding data from angle-resolved photoemission spectroscopy. We clarify that the major peak of the tunneling conductance is not related to the DOS but is rather the effect of nonequilibrium coherent tunneling including all coherent spins in the tip and sample.

Lateral scattering potential of the PTCDA/Ag(111) interface state

A modified scheme of scanning tunneling spectroscopy data analysis has been used to reconstruct the band structure of a strongly dispersive two-dimensional unoccupied electronic state located at the commensurate interface between a monolayer of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) and Ag(111) surface up to the fourth band in the reduced Brillouin zone of the molecular monolayer. The two-dimensional scattering potential, evaluated with the help of the measured band structure data, is highly corrugated. This shows that the accepted picture, describing the interface state of PTCDA/Ag(111) as an upshifted Shockley state of Ag(111), is incomplete. Our findings indicate that the hybridization of unoccupied molecular orbitals of PTCDA on one of the two sublattices with the metal states of Ag(111) substantially influences the interface state properties.

Effect of intrinsic electronic defect states on the morphology and optoelectronic properties of Sn-rich SnS particles

A small variation in the elemental composition of a chemical compound can cause the formation of additional electronic defect states in the material, thereby altering the overall microstructure and thus induced properties. In this work, we observed chemical constitution-induced modification in the morphology and optoelectronic properties of SnS. To this end, SnS particles were prepared using the solution chemical route and were characterized using a wide range of experimental techniques, such as x-ray diffractometry, field emission scanning electron microscopy, high resolution transmission electron microscopy, energy dispersive spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS), UV-Vis spectrophotometry, and scanning tunneling spectroscopy (STS). All these SnS particles are found to be Sn-rich and p-type. However, distinctly different morphologies (i.e., flower-like and aggregated ones) are observed. These are then correlated with the electronic defect states, which are induced because of the presence of Sn vacancies, Sn antisites, and/or Sn interstitials. A combination of EDS, XPS, and STS data confirmed the presence of a higher concentration of Sn vacancies along with lower quantities of Sn interstitials and/or antisites in the SnS particles with flower-like morphologies giving rise to higher hole concentration, which subsequently leads to reduced transport, optical band gaps, and barrier heights.

Zero-bias anomaly in nanoscale hole-doped Mott insulators on a triangular silicon surface

Adsorption of 1/3 monolayer of Sn on a heavily doped $p$-type Si(111) substrate results in the formation of a hole-doped Mott insulator, with electronic properties that are remarkably similar to those of the high-${T}_{c}$ copper oxide compounds. In this work, we show that the maximum hole-density of this system increases with decreasing domain size as the area of the Mott insulating domains approaches the nanoscale regime. Concomitantly, scanning tunneling spectroscopy (STS) data at 4.4 K reveal an increasingly prominent zero-bias anomaly (ZBA). We consider two different scenarios as potential mechanisms for this ZBA: chiral ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}+\phantom{\rule{4pt}{0ex}}\mathrm{i}{d}_{xy}$ wave superconductivity and a dynamical Coulomb blockade (DCB) effect. The latter arises due to the formation of a resistive depletion layer between the nanodomains and the substrate. Both models fit the tunneling spectra with weaker ZBAs, while the DCB model clearly fits better to spectra recorded at higher temperatures or from the smallest domains with the strongest ZBA. Consistently, STS spectra from the lightly doped substrates display oscillatory behavior that can be attributed to conventional Coulomb staircase behavior, which becomes stronger for smaller sized domains. We conclude that the ZBA is predominantly due to a DCB effect, while a superconducting instability is absent or a minor contributing factor.

Temperature-Triggered Sulfur Vacancy Evolution in Monolayer MoS2 /Graphene Heterostructures.

The existence of defects in 2D semiconductors has been predicted to generate unique physical properties and markedly influence their electronic and optoelectronic properties. In this work, it is found that the monolayer MoS2 prepared by chemical vapor deposition is nearly defect-free after annealing under ultrahigh vacuum conditions at ≈400 K, as evidenced by scanning tunneling microscopy observations. However, after thermal annealing process at ≈900 K, the existence of dominant single sulfur vacancies and relatively rare vacancy chains (2S, 3S, and 4S) is convinced in monolayer MoS2 as-grown on Au foils. Of particular significance is the revelation that the versatile vacancies can modulate the band structure of the monolayer MoS2 , leading to a decrease of the bandgap and an obvious n-doping effect. These results are confirmed by scanning tunneling spectroscopy data as well as first-principles theoretical simulations of the related morphologies and the electronic properties of the various defect types. Briefly, this work should pave a novel route for defect engineering and hence the electronic property modulation of three-atom-thin 2D layered semiconductors.

Self-limiting CVD of a passivating SiOx control layer on InGaAs(001)-(2x4) with the prevention of III-V oxidation

A thin passivating SiOx control layer has been deposited via self-limiting CVD on the InGaAs(001)-(2x4) surface by first depositing 2 monolayers of silicon with –Clx termination using Si2Cl6,and then subsequently oxidizing the silicon seed layer by employing anhydrous HOOH(g) at a substrate temperature of 350°C. After HOOH(g)) dosing, XPS spectra show a higher binding energy shoulder peak on Si2p indicative of SiOx bonding, while an unshifted Si 2p component remains, and In 3d, Ga 2p, and As 2p peaks show no higher binding energy components consistent with the prevention of III-V oxidation. Scanning tunneling spectroscopy (STS) measurements show after SiOx deposition on the InGaAs(001)-(2x4) surface, the bandgap broadens towards that of SiO2, with the electronic structure free of states in the bandgap leaving the surface ready for subsequent gate oxide ALD. Density functional theory calculations support the experimental STS data following TMA dosing, which shows TMA nucleates directly on the SiOx/InGaAs(001) surface and leaves an electrically passive interface with the bandgap free of defect states and the surface ready for high-K gate oxide nucleation.

Energy Level Alignment at the Fullerene/Titanium Oxide Ultrathin Film Interface

The performance of molecule based electronic devices can be improved by use of transition metal oxides (TMO) as charge injection buffer layers between electrodes and organic semiconductors. It is known that the appropriate energy level alignment at the molecule/TMO interface determines the efficiency of the respective TMO. Herein, scanning tunneling microscopy is employed to characterize the interface formed by fullerenes (C60) deposited on a titanium oxide (TiO) ultrathin film, which is created by oxidation of a Pt3Ti(111) surface. Individual C60 are identified with orbital resolution, and the interfacial and intermolecular interactions are characterized in detail. Furthermore, the energy level alignment at the C60/TiO ultrathin film interface is deduced based on scanning tunneling spectroscopy data. The results demonstrate that the C60/TiO interface corresponds to a type-I heterojunction and, thus, is useful to decouple C60 molecules from the metallic alloy surface.

Band Alignment in MoS2/WS2 Transition Metal Dichalcogenide Heterostructures Probed by Scanning Tunneling Microscopy and Spectroscopy.

Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), we examine the electronic structure of transition metal dichalcogenide heterostructures (TMDCHs) composed of monolayers of MoS2 and WS2. STS data are obtained for heterostructures of varying stacking configuration as well as the individual monolayers. Analysis of the tunneling spectra includes the influence of finite sample temperature, yield information about the quasi-particle bandgaps, and the band alignment of MoS2 and WS2. We report the band gaps of MoS2 (2.16 ± 0.04 eV) and WS2 (2.38 ± 0.06 eV) in the materials as measured on the heterostructure regions and the general type II band alignment for the heterostructure, which shows an interfacial band gap of 1.45 ± 0.06 eV.