Scanning Tunneling Microscope Tip Research Articles

Moiré-modulated band gap and van Hove singularities in twisted bilayer germanene

Twisting bilayers of two-dimensional topological insulators has the potential to create unique quantum states of matter. Here, we successfully synthesized a twisted bilayer of germanene on Ge2Pt(101) with a 21.8° twist angle, corresponding to a commensurate (√7×√7) structure. Using scanning tunneling microscopy and spectroscopy, we unraveled the structural and electronic properties of this configuration, revealing a moiré-modulated band gap and a well-defined edge state. This band gap opens at AB/BA stacked sites and closes at AA stacked sites, a phenomenon attributed to the electric field induced by the scanning tunneling microscopy tip. Our study further revealed two van Hove singularities at −0.8 eV and +1.04 eV, resulting in a Fermi velocity of (8 ± 1) × 105 m s−1. Our tight-binding results uncover a unique quantum state, where the topological properties could be regulated through an electric field, potentially triggering two topological phase transitions.

Scanning Tunneling Microscopy for Molecules: Effects of Electron Propagation into Vacuum.

Using scanning tunneling microscopy (STM), we experimentally and theoretically investigate isolated platinum phthalocyanine (PtPc) molecules adsorbed on an atomically thin NaCl(100) film vapor deposited on Au(111). We obtain good agreement between theory and constant-height STM topography. We theoretically examine why strong distortions of STM images occur as a function of distance between the molecule and the STM tip. The images of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) exhibit for increasing distance, significant radial expansion due to electron propagation in the vacuum. Additionally, the imaged angular dependence is substantially distorted. The LUMO image has substantial intensity along the molecular diagonals where PtPc has no atoms. In the electronic transport gap, the image differs drastically from HOMO and LUMO even at energies very close to these orbitals. As the tunneling becomes increasingly off-resonant, the eight angular lobes of the HOMO or of the degenerate LUMOs diminish and reveal four lobes with maxima along the molecular axes, where both, HOMO and LUMO have little or no weight. These images are strongly influenced by low-lying PtPc orbitals that have simple angular structures.

Tuneable Current Rectification Through a Designer Graphene Nanoribbon.

Unimolecular current rectifiers are fundamental building blocks in organic electronics. Rectifying behavior has been identified in numerous organic systems due to electron-hole asymmetries of orbital levels interfaced by a metal electrode. As a consequence, the rectifying ratio (RR) determining the diode efficiency remains fixed for a chosen molecule-metal interface. Here, a mechanically tunable molecular diode exhibiting an exceptionally large rectification ratio (>105) and reversible direction is presented. The molecular system comprises a seven-armchair graphene nanoribbon (GNR) doped with a single unit of substitutional diboron within its structure, synthesized with atomic precision on a gold substrate by on-surface synthesis. The diboron unit creates half-populated in-gap bound states and splits the GNR frontier bands into two segments, localizing the bound state in a double barrier configuration. By suspending these GNRs freely between the tip of a low-temperature scanning tunneling microscope and the substrate, unipolar hole transport is demonstrated through the boron in-gap state's resonance. Strong current rectification is observed, associated with the varying widths of the two barriers, which can be tuned by altering the distance between tip and substrate. This study introduces an innovative approach for the precise manipulation of molecular electronic functionalities, opening new avenues for advanced applications in organicelectronics.

Correlation-Induced Symmetry-Broken States in Large-Angle Twisted Bilayer Graphene on MoS2.

Strongly correlated states commonly emerge in twisted bilayer graphene (TBG) with "magic-angle" (1.1°), where the electron-electron (e-e) interaction U becomes prominent relative to the small bandwidth W of the nearly flat band. However, the stringent requirement of this magic angle makes the sample preparation and the further application facing great challenges. Here, using scanning tunneling microscopy (STM) and spectroscopy (STS), we demonstrate that the correlation-induced symmetry-broken states can also be achieved in a 3.45° TBG, via engineering this nonmagic-angle TBG into regimes of U/W > 1. We enhance the e-e interaction through controlling the microscopic dielectric environment by using a MoS2 substrate. Simultaneously, the width of the low-energy van Hove singularity (VHS) peak is reduced by enhancing the interlayer coupling via STM tip modulation. When partially filled, the VHS peak exhibits a giant splitting into two states flanked by the Fermi level and shows a symmetry-broken LDOS distribution with a stripy charge order, which confirms the existence of strong correlation effect in our 3.45° TBG. Our result demonstrates the feasibility of the study and application of the correlation physics in TBGs with a wider range of twist angle.

Submolecular-scale control of phototautomerization.

Optically activated reactions initiate biological processes such as photosynthesis or vision, but can also control polymerization, catalysis or energy conversion. Methods relying on the manipulation of light at macroscopic and mesoscopic scales are used to control on-surface photochemistry, but do not offer atomic-scale control. Here we take advantage of the confinement of the electromagnetic field at the apex of a scanning tunnelling microscope tip to drive the phototautomerization of a free-base phthalocyanine with submolecular precision. We can control the reaction rate and the relative tautomer population through a change in the laser excitation wavelength or through the tip position. Atomically resolved tip-enhanced photoluminescence spectroscopy and hyperspectral mapping unravel an excited-state mediated process, which is quantitatively supported by a comprehensive theoretical model combining ab initio calculations with a parametric open-quantum-system approach. Our experimental strategy may allow insights in other photochemical reactions and proof useful to control complex on-surface reactions.

Atomic-scale manipulation of polar domain boundaries in monolayer ferroelectric In2Se3

Domain boundaries have been intensively investigated in bulk ferroelectric materials and two-dimensional materials. Many methods such as electrical, mechanical and optical approaches have been utilized to probe and manipulate domain boundaries. So far most research focuses on the initial and final states of domain boundaries before and after manipulation, while the microscopic understanding of the evolution of domain boundaries remains elusive. In this paper, we report controllable manipulation of the domain boundaries in two-dimensional ferroelectric In2Se3 with atomic precision using scanning tunneling microscopy. We show that the movements of the domain boundaries can be driven by the electric field from a scanning tunneling microscope tip and proceed by the collective shifting of atoms at the domain boundaries. Our density functional theory calculations reveal the energy path and evolution of the domain boundary movement. The results provide deep insight into domain boundaries in two-dimensional ferroelectric materials and will inspire inventive applications of these materials.

Atomic-scale manipulation of buried graphene–silicon carbide interface by local electric field

Precision of scanning tunneling microscopy (STM) enables control of matter at scales of single atoms. However, transition from atomic-scale manipulation strategies to practical devices encounters fundamental problems in protection of the designer structures formed atop the surface. In this context, STM manipulation of subsurface structures on technologically relevant materials is encouraging. Here, we propose a material platform and protocols for precise manipulation of a buried graphene interface. We show that an electric field from the STM tip reversibly controls breaking and restoring of covalent bonds between the graphene buffer layer and the SiC substrate. The process involves charge redistribution at the atomically sharp interface plane under the epitaxial graphene layer(s). This buried manipulation platform is laterally defined by unit cells from the corresponding (6×6)SiC moiré lattice of the epitaxial graphene. Local and reversible electric-field-induced patterning of graphene heterostructures from the bottom interface creates an alternative architecture concept for their applications.

Microscopy aided detection of the self-intercalation mechanism and in situ electronic properties in chromium selenide.

Two-dimensional (2D) chromium-based self-intercalated materials Cr1+nX2 (0 ≤ n ≤ 1, X = S, Se, Te) have attracted much attention because of their tunable magnetism with good environmental stability. Intriguingly, the magnetic and electrical properties of the materials can be effectively tuned by altering the coverage and spatial arrangement of the intercalated Cr (ic-Cr) within the van der Waals gap, contributing to different stoichiometries. Several different Cr1+nX2 systems have been widely investigated recently; however, those with the same stoichiometric ratio (such as Cr1.25Te2) were reported to exhibit disparate magnetic properties, which still lacks explanation. Therefore, a systematic in situ study of the mechanisms with microscopy techniques is in high demand to look into the origin of these discrepancies. Herein, 2D self-intercalated Cr1+nSe2 nanoflakes were synthesized as a platform to conduct the characterization. Combining scanning transmission electron microscopy (STEM) and scanning tunneling microscopy (STM), we studied in depth the microscopic structure and local electronic properties of the Cr1+nSe2 nanoflakes. The self-intercalation mechanism of ic-Cr and local stoichiometric-ratio variation in a Cr1+nSe2 ultrathin nanoflake is clearly detected at the nanometer scale. Scanning tunneling spectroscopy (STS) measurements indicate that Cr1.5Se2/Cr2Se2 and Cr1.25Se2 exhibit conductive and semiconductive behaviors, respectively. The STM tip manipulation method is further applied to manipulate the microstructure of Cr1+nSe2, which successfully produces clean zigzag-type boundaries. Our systematic microscopy study paves the way for the in-depth study of the magnetic mechanism of 2D self-intercalated magnets at the nano/micro scale and the development of new magnetic and spintronic devices.

Vacuum cleaving of superconducting niobium tips to optimize noise filtering and with adjustable gap size for scanning tunneling microscopy

Superconducting (SC) tips for scanning tunneling microscopy (STM) can enhance a wide range of surface science studies because they offer exquisite energy resolution, allow the study of Josephson tunneling, or provide spatial contrast based on the local interaction of the SC tip with the sample. The appeal of a SC tip is also practical. An SC gap can be used to characterize and optimize the noise of a low-temperature apparatus. Unlike typical samples, SC tips can be made with less ordered materials, such as from SC polycrystalline wires or by coating a normal metal tip with a superconductor. Those recipes either require additional laboratory infrastructure or are carried out in ambient conditions, leaving an oxidized tip behind. Here, we revisit the vacuum cleaving of an Nb wire to prepare fully gapped tips in an accessible one-step procedure. To show their utility, we measure the SC gap of Nb on Au(111) to determine the base temperature of our microscope and to optimize its RF filtering. The deliberate coating of the Nb tip with Au fully suppresses the SC gap and we show how sputtering with Ar+ ions can be used to gradually recover the gap, promising tunability for tailored SC gaps sizes.•Oxide free superconducting STM tips•RF filter optimization

On the magnetic bistability of small iron clusters used in scanning tunneling microscopy tip preparation

The combination of electron spin resonance with scanning tunneling microscopy has resulted in a unique surface probe with sub-nm spatial and neV energy resolution. The preparation of a stable magnetic microtip is of central importance, yet, at the same time remains one of the hardest tasks. In this work, we rationalize why creating such microtips by picking up a few iron atoms often results in magnetically stable probes with two distinct magnetic states. By using density functional theory, we show that randomly formed clusters of five iron atoms can exhibit this behavior with magnetic anisotropy barriers of up to 73 meV. We explore the dependence of the magnetic behavior of such clusters on the geometrical arrangement and find a strong correlation between magnetic and geometric anisotropy—the less regular the cluster the higher its magnetic anisotropy barrier. Finally, our work rationalizes the experimental strategy of obtaining stable magnetic microtips.

Atomic-scale tailoring of chemisorbed atomic oxygen on epitaxial graphene for graphene-based electronic devices

Graphene, with its unique band structure, mechanical stability, and high charge mobility, holds great promise for next-generation electronics. Nevertheless, its zero bandgap challenges the control of current flow through electrical gating, consequently limiting its practical applications. Recent research indicates that atomic oxygen can oxidize epitaxial graphene in a vacuum without causing unwanted damage. In this study, we have investigated the effects of chemisorbed atomic oxygen on the electronic properties of epitaxial graphene using scanning tunneling microscopy (STM). Our findings reveal that oxygen atoms effectively modify the electronic states of graphene, resulting in a bandgap at its Dirac point. Furthermore, we demonstrate that it is possible to selectively induce desorption or hopping of oxygen atoms with atomic precision by applying appropriate bias sweeps with an STM tip. These results suggest the potential for atomic-scale tailoring of graphene oxide, enabling the development of graphene-based atomic-scale electronic devices.

Junctions and Superconducting Symmetry in Twisted Bilayer Graphene.

Junctions provide a wealth of information on the symmetry of the order parameter of superconductors. We analyze junctions between a scanning tunneling microscope (STM) tip and superconducting twisted bilayer graphene (TBG) and TBG Josephson junctions (JJs). We compare superconducting phases that are even or odd under valley exchange (s- or f-wave). The critical current in mixed (s and f) JJs strongly depends on the angle between the junction and the lattice. In STM-TBG junctions, due to Andreev reflection, the f-wave leads to a prominent peak in subgap conductance, as seen in experiments.

Molecular Alignment and Chiral Bias under Dynamic Nanoconfinement Conditions: The Impact of Molecular Symmetry

When molecules assemble in self-assembled molecular networks on highly oriented pyrolytic graphite, typically six orientational domains are formed, which can be grouped into two sets of three, each set characteristic of a packing chirality. Under dynamic nanoconfinement conditions, where the scanning tunneling microscopy (STM)-based lithography process gradually increases the area available for molecule adsorption and self-assembly, the orientational domain equivalence is often disrupted. Additionally, this phenomenon can sometimes be accompanied by the selective formation of one of the two enantiomorphic packings. Until now, the preferential orientation of domains, sometimes accompanied by the favored formation of enantiomorphic packing patterns, has been investigated for anisotropic molecules with a single alkyl chain. We reasoned that a system where different alkyl chains may adsorb along various directions on the substrate, while also adopting different orientations concerning the line-wise movement of the STM tip, can potentially hinder molecular alignment in the nanocorrals and eventually limit the extent of enantiomorph-selective adsorption. To address this hypothesis, in this study, we investigate the self-assembly of an achiral molecule with four alkyl chains, in pairs directed along two different directions, under dynamic nanoconfinement conditions. Despite of this increased molecular structural complexity, dynamic nanoconfinement conditions lead to a clear bias in the formation of rotational domains, as well as partial chiral selection, although in a complex fashion. The factors that contribute to this bias are discussed. This highlights the significance of utilizing dynamic nanoconfinement conditions to control domain orientation and to disturb chirality equivalence, even for intricate systems.

Search for large topological gaps in atomic spin chains on proximitized superconducting heavy-metal layers

One-dimensional systems comprising s-wave superconductivity with meticulously tuned magnetism realize topological superconductors hosting Majorana modes whose stability is determined by the gap size. However, for atomic spin chains on superconductors, the effect of the substrate’s spin-orbit coupling on the topological gap is largely unexplored. Here, we introduce an atomic layer of the heavy metal gold on a niobium surface combining strong spin-orbit coupling and a large superconducting gap with a high crystallographic quality, enabling the assembly of defect-free iron chains using a scanning tunneling microscope tip. Scanning tunneling spectroscopy experiments and density functional theory calculations reveal ungapped Yu–Shiba–Rusinov bands in the ferromagnetic chain despite the heavy substrate. By artificially imposing a spin spiral state, the calculations indicate minigap opening and zero-energy edge state formation. The methodology enables a material screening of heavy-metal layers on elemental superconductors for ideal systems hosting Majorana edge modes protected by large topological gaps.

Strain Effects in Twisted Spiral Antimonene.

Van der Waals (vdW) layered materials exhibit fruitful novel physical properties. The energy band of such materials depends strongly on their structures, and a tremendous variation in their physical properties can be deduced from a tiny change in inter-layer spacing, twist angle, or in-plane strain. In this work, a kind of vdW layered material of spiral antimonene is constructed, and the strain effects in the material are studied. The spiral antimonene is grown on a germanium (Ge) substrate and is induced by a helical dislocation penetrating through few atomic-layers of antimonene (β-phase). The as-grown spiral is intrinsically strained, and the lattice distortion is found to be pinned around the dislocation. Both spontaneous inter-layer twist and in-plane anisotropic strain are observed in scanning tunneling microscope (STM) measurements. The strain in the spiral antimonene can be significantly modified by STM tip interaction, leading to a variation in the surface electronic density of states (DOS) and a large modification in the work function of up to a few hundreds of millielectron-volts (meV). Those strain effects are expected to have potential applications in building up novel piezoelectric devices.

Ferroelectricity in Niobium Oxide Dihalides NbOX2 (X = Cl, I): A Macroscopic- to Microscopic-Scale Study.

2D materials with ferroelectric and piezoelectric properties are of interest for energy harvesting, memory storage and electromechanical systems. Here, we present a systematic study of the ferroelectric properties in NbOX2 (X = Cl, I) across different spatial scales. The in-plane ferroelectricity in NbOX2 was investigated using transport and piezoresponse force microscopy (PFM) measurements, where it was observed that NbOCl2 has a stronger ferroelectric order than NbOI2. A high local field, exerted by both PFM and scanning tunneling microscopy (STM) tips, was found to induce 1D collinear ferroelectric strips in NbOCl2. STM imaging reveals the unreconstructed atomic structures of NbOX2 surfaces, and scanning tunneling spectroscopy was used to probe the electronic states induced at defect (vacancy) sites.

Reversible Tuning of Surface Properties of Graphene-like Material via Covalently Functionalized Hydrophobic Layer

Nanoscale tuning of the surface properties of graphene-like materials is essential to optimize their application in electronic devices and protective technologies. The covalent modification method has recently been established as the most effective approach for tailoring the interface structure and properties, which are key aspects for fine-tuning the processability and performance of graphene-like materials. In this work, we demonstrate systematic exploration of the reversible covalent functionalization of a highly oriented pyrolytic graphite (HOPG) surface, a model system of multi-layered graphene, at the molecular scale. This is achieved using 3,5-trifluoromethyl benzenediazonum (3,5-TFD) and experimental investigations via cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning tunneling microscopy (STM), and Raman spectroscopy. The degree of functionalization could be tuned by varying the concentration of 3,5-TFD dissolved in the grafting electrolyte. The covalently functionalized layer of 3,5-TFD was either locally degrafted by the STM tip or globally detracted upon thermal treatment, leaving the defect-free graphitic surfaces behind. Our findings open a new pathway for reversibly and robustly functionalizing graphene and other 2D materials for multiple uses in high-end applications.

Activation, Transportation, and Reaction of Alkyl Radicals on a Si(111)-B Surface by a Scanning Tunneling Microscope Tip

Activation, Transportation, and Reaction of Alkyl Radicals on a Si(111)-B Surface by a Scanning Tunneling Microscope Tip

Investigation of ultrafast excited-state dynamics at the nanoscale with terahertz field-induced electron tunneling and photon emission

The combination of terahertz (THz) pulses with scanning tunneling microscopy (STM) enables us to investigate ultrafast dynamics at material surfaces with high spatial (nanoscale) and temporal (femtosecond) resolution. In this Perspective, we review the basic principles of THz-STM, outline the related literature, and discuss its future outlook capable of advancing nanoscience, especially by introducing further combination of THz-STM with scanning tunneling luminescence spectroscopy, which can extract more fruitful information about nanoscale materials via obtaining luminescence spectra compared to just observing the net tunneling current induced by THz pulses in THz-STM. By shining THz pulses onto an STM tip, we can observe plasmon luminescence locally confined between the tip and the surface, whose spectrum changes with tip conditions. The combined approach is a stepping stone toward investigating the ultrafast excited-state dynamics of nanoscale materials with high spatiotemporal resolution.

A Novel Atomically Resolved Scanning Tunneling Microscope Capable of Working in Cryogen-Free Superconducting Magnet.

We present a novel homebuilt scanning tunneling microscope (STM) with atomic resolution integrated into a cryogen-free superconducting magnet system with a variable temperature insert. The STM head is designed as a nested structure of double piezoelectric tubes (PTs), which are connected coaxially through a sapphire frame whose top has a sample stage. A single shaft made of tantalum, with the STM tip on top, is held firmly by a spring strip inside the internal PT. The external PT drives the shaft to the tip-sample junction based on the SpiderDrive principle, and the internal PT completes the subsequent scanning and imaging work. The STM head is simple, compact, and easy to assemble. The excellent performance of the device was demonstrated by obtaining atomic-resolution images of graphite and low drift rates of 30.2 pm/min and 41.4 pm/min in the X-Y plane and Z direction, respectively, at 300K. In addition, we cooled the sample to 1.6 K and took atomic-resolution images of graphite and NbSe2. Finally, we performed a magnetic field sweep test from 0 T to 9 T at 70 K, obtaining distinct graphite images with atomic resolution under varying magnetic fields. These experiments show our newly developed STM's high stability, vibration resistance, and immunity to high magnetic fields.