Scanning Tunneling Microscopy Study Research Articles

Lattice Dynamics Driven by Tunneling Current in 1T′ Structure of Bilayer VSe2

Interesting lattice dynamics were recently reported in 1T′ structure of bilayer VSe2 attributed to the vibration of Se atoms, but it was limited to topographic observation using scanning tunneling microscopy (STM). We conduct further systematic STM study to understand the origin of lattice dynamics in bilayer VSe2. Time spectroscopy of tunneling current is measured with the feedback loop disabled, while holding the position of the STM tip over a single Se atom. The time spectroscopy indeed shows random telegraph signals that tunneling current fluctuates between high and low currents. Such fluctuations of current reflect the vibrational motion of the Se atom. Statistical analysis described by the Markov process provides average residence time (τ) for high and low current states, and transition rate (R = 1/τ). Interestingly, the transition rate as a function of tunneling current (I) follows a power law of R = IN (N = constant), and the N value is close to 1. This linearity of transition rate indicates that the lattice dynamics are mostly induced by tunneling current. Our result confirms the role of tunneling current that drives the observed lattice dynamics in the bilayer VSe2.

Single-water-dipole-layer-driven Reversible Charge Order Transition in 1T-TaS2.

Water-solid interactions are crucial for many fundamental phenomena and technological processes. Here, we report a scanning tunneling microscopy study about the charge density wave (CDW) transition in 1T-TaS2 driven by a single water dipole layer. At low temperature, pristine 1T-TaS2 is a prototypical CDW compound with × charge order. After growing a highly ordered water adlayer, a new charge order with 3 × 3 periodicity emerges on water-covered 1T-TaS2. After water desorption, the entire 1T-TaS2 surface appears as localized × CDW domains that are separated by residual-water-cluster-pinned CDW domain walls. First-principles calculations show that the electric dipole moments in the water adlayer attract electrons to the top layer of 1T-TaS2, which shifts the phonon softening mode and induces the × to 3 × 3 charge order transition. Our results pave the way for creating new collective quantum states of matter with a molecular dipole layer.

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.

Observation of Topological Electronic Structure in Quasi-1D Superconductor TaSe3

Topological superconductors (TSCs), with the capability to host Majorana bound states that can lead to non-Abelian statistics and application in quantum computation, have been one of the most intensively studied topics in condensed matter physics recently. Up to date, only a few compounds have been proposed as candidates of intrinsic TSCs, such as doped topological insulator CuxBi2Se3 and iron-based superconductor FeTe0.55Se0.45. Here, by carrying out synchrotron and laser based angle-resolved photoemission spectroscopy (ARPES), we systematically investigated the electronic structure of a quasi-1D superconductor TaSe3, and identified the nontrivial topological surface states. In addition, our scanning tunneling microscopy (STM) study revealed a clean cleaved surface with a persistent superconducting gap, proving it suitable for further investigation of potential Majorana modes. These results prove TaSe3 as a stoichiometric TSC candidate that is stable and exfoliable, therefore a great platform for the study of rich novel phenomena and application potentials.

Magnetic anisotropy of one-dimensional Co nanostructures

We report a combined scanning tunneling microscopy and x-ray magnetic circular dichroism study to investigate the structural properties and magnetic behavior of a Co ultrathin film composed of dimer nanolines. These Co nanolines, \ensuremath{\sim}6 nm in length, are grown on a Si nanotemplate composed of nanoribbons self-organized on Ag(110). The first two Co layers present a weak magnetic response while upper Co layers exhibit an enhanced magnetization. Orbital and spin moments are experimentally determined. We show that an in-plane magnetization is favored and the magnetic anisotropy energy associated with the directions parallel and perpendicular to the nanolines are measured. The Co ultrathin film is shown to behave as a superparamagnetic system composed of one-dimensional segments containing each \ensuremath{\sim}170 ferromagnetically coupled Co atoms, with a blocking temperature estimated to be between 20 and 40 K. Our set of experiments allows for a comprehensive description of the magnetic behavior of the Co nanoline ultrathin film grown on a functionalized metallic substrate.

Scanning Tunneling Microscopy Studies of Potassium-Doped Picene Films on Au(111) Surface

Potassium-doped picene films grown on an Au(111) surface have been investigated using low-temperature scanning tunneling microscopy. The evolution of both the morphologies and the electronic proper...

Spin-Polarized Scanning Tunneling Microscopy Study of Non-Collinear Magnetic Coupling in Layerwise Antiferromagnetic Mn(001) Ultra-Thin Films on Fe(001) due to Interface Roughening

Spin-Polarized Scanning Tunneling Microscopy Study of Non-Collinear Magnetic Coupling in Layerwise Antiferromagnetic Mn(001) Ultra-Thin Films on Fe(001) due to Interface Roughening

Surface Phase Nucleation of Lead Monoatomic Layers on Si(111) Induced by Manganese Phthalocyanine Molecules

Hybrid interfaces where organic molecules are adsorbed on metallic substrates are very interesting to understand the fundamental interactions that might modify the chemical–physical properties of molecules or substrate. Here, we explore the adsorption of manganese phthalocyanines (MnPcs) on different structural phases of a Pb monatomic layer, namely, the √7 × √3-Pb and the striped incommensurate phase (SIC-Pb) phase, grown on Si(111). Surprisingly, the deposition of a minute amount of MnPc molecules (∼0.18 molecules/100 nm2) nucleates a macroscopic structural transition of the √7 × √3-Pb phase into the SIC-Pb phase. Our combined scanning tunneling microscopy, low-energy electron diffraction, and density functional theory study revealed that the mechanism behind this surface transformation is related to a strong and local molecule–substrate interaction. The structural phase transition is finally driven by the strained nature of the Pb phases and the energetic stability of the MnPc/SIC-Pb/Si(111) system with respect to the MnPc/√7 × √3-Pb/Si(111) one. The molecule–substrate interaction found in the present study is stronger than the one observed on Pb(111) bulk or thin films, highlighting the implication of the Pb/Si(111) interface in the interaction process. Hence, our results reveal that playing with the substrate dimensionality to tune the molecule–substrate coupling has strong impact on the electronic/magnetic properties of organic hybrid systems.

Equilibrium shapes of facetted 3D metal nanoclusters intercalated near the surface of layered materials

Experimental studies indicate that 3D crystalline metal nanoclusters (NCs) intercalated under the surface of graphite have flat-topped equilibrated shapes. We characterize the shapes of these facetted NCs sandwiched between a blanketing graphene layer and the underlying graphite substrate. Specifically, we focus on the cases of fcc Cu and hcp Fe NCs. The analysis involves numerical minimization of the system energy for a specified NC volume and NC height, the latter corresponding to the separation between parallel top and bottom facets. Our numerical analysis quantifies how the distance of the side facet planes from center of the nanocluster varies linearly with a natural characteristic linear dimension of the nanocluster. Calculated shapes of fcc Cu and hcp Fe NCs are consistent with the hexagonal footprints observed in scanning tunneling microscopy studies.

Growth Processes and Reductive Desorption Behaviors of 4-Fluorobenzenethiol Self-Assembled Monolayers on Au(111).

Growth processes and electrochemical behaviors of 4-fluorobenzenethiol (4-FBT) self-assembled monolayers (SAMs) on Au(111) prepared by vapor deposition at 323 K were examined using scanning tunneling microscopy (STM) and cyclic voltammetry (CV). STM imaging revealed that 4-FBT SAMs at the initial growth stage (deposition for 1 min) were mainly composed of bright molecular aggregates and liquid-like disordered phase. After longer deposition for 3 min, 4-FBT SAMs had three distinct surface features: a few molecular aggregates, small ordered domains, and disordered phase. These small ordered domains with sizes ranging from 5 to 10 nm had a (4× √3)R30° packing structure. As deposition time increased to 24 h, long-range ordered domains larger than 40 nm were formed on Au(111) surfaces. From this STM study, we demonstrate that phase transitions of 4-FBT SAMs on Au(111) occur from molecular aggregates to large ordered domains via formation of small ordered domains as deposition time increases. CV measurements showed reductive desorption peaks for 4-FBT SAMs in the range of -638~-648 mV regardless of SAM morphology, suggesting that S-Au binding strength of 4-FBT SAMs on Au electrodes is a dominant factor for electrochemical stability.

Atomic adsorption of Sn on mechanically cleaved WS2 surface at room temperature

Room temperature (RT) adsorption of tin (Sn) under ultra-high vacuum (UHV) conditions on mechanically cleaved tungsten disulfide (WS2) surfaces is reported here. In-situ low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) studies indicate commensurate adsorption of Sn on the hexagonal surface of WS2. Bare WS2 displays an atomic periodicity of lattice constant 0.34 nm. Growth of Sn reveals monomer like a single atom or unpaired (U) and dimer like- or paired (P) adsorptions on the top (`T’) sulfur (S) sites of the trigonal prismatic sublattice within the honeycomb lattice of WS2. The lateral distance between two nearest adsorbed monomers is around 0.34 nm which is exactly similar to the atomic periodicity of the underneath WS2 substrate crystal. For the case of dimer like- adsorption, the separation between two adspecies is 0.293 nm. The bond length of the adsorbed Sn with the surface S atoms is around 40 (±10) pm. We do not find any evidence of Sn adsorption on the bridge (`B’) or hollow (`H’) site of the WS2 lattice. Analysis of the LEED spots and the FFT images of the STM micrographs after Sn adsorption discloses a stretched hexagonal surface which is most likely due to the overall consequence of a perfectly commensurate monomer and nearly commensurate dimer like- adsorptions of the Sn atoms on the WS2 surface.

Melting of charge density wave and Mott gap collapse on 1T−TaS2 induced by interfacial water

Microscopically revealing the interactions between interfacial water and the quantum states of matter is an important task from both the materials science and the physics points of view. Here we report a low-temperature scanning tunneling microscopy (STM) and spectroscopy study of water adsorption on the charge density wave compound $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, which has a Mott-insulating ground state. Interfacial water forms monolayer islands with $6\ifmmode\times\else\texttimes\fi{}6$ superstructures on the surface of $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, and the charge order under water islands can be directly imaged in STM topographies taken with negative bias voltages. Compared with the original $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}$ charge order in $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, the charge order under water islands becomes significantly disordered and denser. A $\mathsf{V}$-shaped gaplike feature emerges in water-covered $1T\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, which may be due to the enhanced dielectric constant of interfacial water, which reduces short-range Coulomb repulsion and induces Mott gap collapse. Our observations open the way to microscopically understanding the interactions between interfacial water and the correlated quantum states of matter.

Sublimation of Li@C60

Experiments that probe the fundamental properties of endohedral fullerenes often require the preparation of molecular beams or thin films of the neutral molecules. It is challenging to cleanly sublime this class of molecules without producing some thermal degradation. We report combined gas phase and scanning tunnelling microscopy studies that probe the thermal decay of commercial [Li+C60]PF6- in a quartz ampoule and provide treatment conditions that will allow the sublimation of intact, neutral Li@C60 accompanied by a well-characterised component of neutral C60. The decay of the material at appropriate temperatures can be modelled with the assumption of a second order decay process in the oven yielding Arrhenius parameters that can predict the ratio of Li@C60 to C60 in the sublimed material.Graphical abstract

Impurity-pinned incommensurate charge density wave and local phonon excitations in 2H−NbS2

Here we report a scanning tunneling microscopy (STM) and spectroscopy (STS) study in the superconducting state of 2H-NbS2. We directly visualize the existence of incommensurate charge density wave (CDW) that is pinned by atomic impurities. In strong tunneling conditions, the incommensurate CDW is de-pinned from impurities by the electric field from STM tip. We perform STM-based inelastic tunneling spectroscopy (IETS) to detect phonon excitations in 2H-NbS2 and measure the influence of atomic impurities on local phonon excitations. In comparison with the calculated vibrational density of states in 2H-NbS2, we find two branches of phonon excitations which correspond to the vibrations of Nb ions and S ions, and the strength of the local phonon excitations is insensitive to the atomic impurities. Our results demonstrate the coexistence of incommensurate CDW and superconductivity in 2H-NbS2, and open the way of detecting atomic-scale phonon excitations in transition metal dichalcogenides with STM-based IETS.

Appearance of unidirectional superstructure in silver intercalated 2H-NbSe2

We report scanning tunneling microscopy study to clarify the modulation structure in silver intercalated stage 2 2H-NbSe2 at low temperature. In addition to the surface structure expected from previous electron diffraction measurements, we observed unidirectional superstructure with period of 23a0. Appearance of this new superstructure indicates that the instability of original charge density wave in pristine 2H-NbSe2 against strain induced by intercalants.

Scanning tunneling microscopy study on phase behavior of self-assembled monolayers formed by coadsorption of octanethiol and octyl thiocyanate on Au(111)

To understand headgroup effects on phase behaviors of self-assembled monolayers (SAMs) prepared by coadsorption of octanethiol [OT, CH3(CH2)7–SH] and octyl thiocyanate [OTC, CH3(CH2)7–SCN] on Au(111) as a function of the OT:OTC molar concentration ratio, we investigated the surface structures of the resulting SAMs formed in 1 mM ethanol solutions at 50 °C for 24 h using scanning tunneling microscopy (STM). The STM study showed that the single-component OT SAMs were composed of an ordered c(4 × 2) phase, whereas single-component OTC SAMs had a binary phase consisting of a disordered phase and a striped phase with a distance of 1.3 ± 0.1 nm between rows. The fully covered c(4 × 2) phase on Au(111) formed by OT molecules was observed even a molar concentration ratio of OT is equal to OTC (50:50) or is much lower than OTC (10:90). The partially covered striped phase formed by OTC molecules was observed when the molar concentration ratios of OTC were much higher than OT (OT:OTC = 5:95 or OT:OTC = 1:99). Molecular-scale STM results clearly reveal the phase transitions of SAMs on Au(111) via coadsorption of OT and OTC depending on OT:OTC molar ratio. These results will help advance understanding of SAM formation and phase behavior on Au(111) by the coadsorption of alkanethiol and alkyl thiocyanates.

Orbital-collaborative charge density waves in monolayer VTe2

Charge density waves in transition metal dichalcogenides have been intensively studied for their close correlation with Mott insulator, charge-transfer insulator, and superconductor. VTe2 monolayer recently comes into sight because of its prominent electron correlations and the mysterious origin of CDW orders. As a metal of more than one type of charge density waves, it involves complicated electron-electron and electron-phonon interactions. Through a scanning tunneling microscopy study, we observed triple-Q 4-by-4 and single-Q 4-by-1 modulations with significant charge and orbital separation. The triple-Q 4-by-4 order arises strongly from the p-d hybridized states, resulting in a charge distribution in agreement with the V-atom clustering model. Associated with a lower Fermi level, the local single-Q 4-by-1 electronic pattern is generated with the p-d hybridized states remaining 4-by-4 ordered. In the spectroscopic study, orbital- and atomic- selective charge-density-wave gaps with the size up to ~400 meV were resolved on the atomic scale.

A hybrid magnet based scanning tunneling microscope.

In this paper, a scanning tunneling microscope (STM) is presented that operates in a 27.5 T magnetic field within a hybrid magnet. The coarse approach of the STM is realized by using an inertial piezoelectric motor, and the scanning is realized by using a miniature scanner, which stands alone on a sapphire base. A combined vibration isolation system consisting of a brick-rubber-brick stack and two springs is used to isolate the vibration generated from the magnet. An enclosed copper shield is used to prevent sound from entering the tip-sample junction. The sound and vibration isolation measures highly improve the stability of the STM imaging. All the materials selected to construct the STM head are nonmagnetic. The drift rates of the STM in the X-Y plane and Z direction are as low as 26.2 pm/min and 34.6 pm/min, respectively, under ambient conditions. The high performance of the homebuilt STM was demonstrated by graphite hexagonal lattice images obtained in magnet fields ranging from 0 T to 27.5 T even without the protection of a vacuum and low temperatures. As far as known, this is the first STM that operates in a hybrid magnet. It is also the first STM that can obtain graphite hexagonal lattice images in magnetic fields up to 27.5 T. Our results greatly contribute to the further STM studies under ambient conditions and ultrahigh magnetic fields.

Transition Metal Doping of MoS2: A Correlated Experimental and Theoretical Study

Transition metal dichalcogenides (TMDs) are two-dimensional (2D) layered materials covalently bonded within the layers, but with only weak van-der-Waals (vdW) interactions between individual monolayers [1]. Substantial progress has been made in better understanding of these materials, from the nature of their defects [2] to the achievement of large area epitaxially grown films [3-5]. These materials have attracted great attention for applications such as next-generation electronics - including sub-60 mV sub-threshold slope transistors, flexible electronics, and optoelectronics, novel applications in spintronic devices [6] and the use of heterostructures of TMDs for tunnel field effect transistors (TFETs) [7]. However, issues including control of channel and source/drain doping have impeded their implementation into device. Similar to three-dimensional semiconductors, doping of the TMDs is required to modulate carrier concentration, to achieve Ohmic contacts, and to generate n-type and p-type materials which are required for complementary metal-oxide-semiconductor (CMOS) technology and TFET applications. One of the most studied TMDs is MoS2. The transition metals Nb and Re are two candidate dopants for MoS2 with theoretical results showing their suitability as p- and n-type dopants, respectively [8]. Experimental results have confirmed that Nb substitutes at the Mo-site and acts as a p-type dopant in MoS2 [9-11]. While Re has been confirmed as an n-type dopant, with the additional benefit of reducing sulfur vacancies and defect-related gap states [12-13].This study reports on the band structure and electrical characteristics of doped and unintentionally doped chemical vapor transport (CVT) grown MoS2 bulk crystals. We present a direct determination of the valence band structure of the MoS2 and the impact of transition metal doping (Nb and Re) using high-resolution angle-resolved photoemission spectroscopy (ARPES). Structural defects in the form of vacancies are widely known to strongly alter the MoS2 electronic structure. Therefore, we have performed highly-efficient density functional theory (DFT) based simulations to provide insight into the impact of vacancies in addition to the incorporation of transition metal dopants on the MoS2 band structure. Unfolded band structures obtained through our simulations [14], in comparison with the experimentally obtained occupied band structure of doped and un-doped MoS2 have shown excellent agreement and revealed that there has been significant distortion to the band structure due to the presence of vacancies, as well as the introduction of degenerate Nb-doping. Scanning tunneling microscopy (STM) studies revealed high quality crystals with point defects established by our first-principle calculations to be mainly Mo vacancies. We also report our Hall effect analysis to obtain the electrical metrics for the crystals. Secondary ion mass spectrometry (SIMS) shows the impurities present in the crystals, which is then used to explain the difference in transport behaviour and dopant types between similar crystals from different material sources.Figure 1. (a) Unfolded band structure of un-doped MoS2 with Mo vacancy obtained by DFT calculations shown using contour plot of total weight intensity. Black curve: primitive-cell of pristine MoS2 band structure. Mo vacancy induced localized states are located close to the valence band edge - shown by white rectangles. (b) ARPES spectra for a non-intentionally-doped MoS2 crystal acquired along the high symmetry Γ-K direction overlapped with DFT results showing excellent agreement considering the effect of a Mo vacancy. (c) DFT-obtained STM images of a pristine MoS2 (left), and Mo vacancy in MoS2 (right). (d) Charge density difference between pristine MoS2 and MoS2 with Mo vacancy. Red and blue indicate charge accumulation and depletion, respectively, at Mo vacancy sites.Figure 2. In situ STM, XPS and LEED measurements of MoS2. (a) Large-scale STM image with bright and dark defects (300×300 nm). (b) STM image (100×100 nm) with line profiles over the defects 1, 2, and 3. (c) STM image illustrates atomic resolution with interatomic distance 0.32 nm. (d) & (e) corresponds to the binding energies of Mo 3d and S 2s, and S 2p core levels, respectively. (f) LEED showing highly ordered structure.

Multimorphism and gap opening of charge-density-wave phases in monolayer VTe2

Vanadium dichalcogenides have attracted increasing interests for the charge density wave phenomena and possible ferromagnetism. Here, we report on the multiphase behavior and gap opening in monolayer VTe2 grown by molecular beam epitaxy. Scanning tunneling microscopy (STM) and spectroscopy study revealed the (4×4) metallic and gapped (2√3×2√3) charge-density wave (CDW) phases with an energy gap of ≈ 40 meV. Through the in-plane condensation of vanadium atoms, the typical star-of-David clusters and truncated triangle-shaped clusters are formed in the (4×4) and (2√3×2√3) phases respectively, resulting in different surface morphologies and electronic structures as confirmed by density functional theory (DFT) calculations with on-site Coulomb repulsion. The CDW-driven reorganization of the atomic structure weakens the ferromagnetic superexchange coupling and strengthens the antiferromagnetic exchange coupling on the contrary, suppressing the long-range magnetic order in monolayer VTe2. The electron correlation is found to be important to explain the gap opening in the (2√3×2√3) phase.