Low-temperature Scanning Tunneling Microscopy Research Articles

Unveiling the Correlation between Defects and High Mobility in MoS2 Monolayers.

Defects in semiconductors play a crucial role in modifying their electronic structure and transport properties. In transition metal dichalcogenides, atomic chalcogen vacancies are a primary source of intrinsic defects. While the impact of these vacancies on electrical transport has been widely studied, their exact role remains not fully understood. In this work, we correlate optical spectroscopy, low-temperature electrical transport measurements, scanning tunneling microscopy (STM), and first-principles density functional theory (DFT) calculations to explore the effect of chalcogen vacancies in MoS2 monolayers grown by chemical vapor deposition. We specifically highlight the role of disulfur vacancies in modulating electrical properties, showing that these defects increase the density of shallow donor states near the conduction band, which facilitates electron hopping conduction, as evidenced by low-temperature transport and STM measurements. These findings are further supported by DFT calculations, which reveal that the electronic states associated with these defects are relatively delocalized, promoting hopping conduction and inducing n-type doping. This mechanism accounts for the observed high field-effect mobility (>100 cm2 V-1s-1) in the samples. These findings highlight the potential for defect engineering as a universal approach to customizing the properties of 2D materials for various applications.

Stripe charge order and its interaction with Majorana bound states in 2M-WS2 topological superconductors.

To achieve logic operations via Majorana braiding, positional control of the Majorana bound states (MBSs) must be established. Here we report the observation of a striped surface charge order coexisting with superconductivity and its interaction with the MBS in the topological superconductor 2M-WS2, using low-temperature scanning tunneling microscopy. By applying an out-of-plane magnetic field, we observe that MBSs are absent in vortices in the region with stripe order. This is in contrast to adjacent underlaying layers without charge order, where vortex-bound MBSs are observed. Via theoretical simulations, we show that the surface stripe order does not destroy the bulk topology, but it can effectively modify the spatial distribution of MBSs, i.e. it pushes them downward, away from the 2M-WS2 surface. Our findings demonstrate that the interplay of charge order and topological superconductivity can potentially be used to tune the positions of MBSs, and to explore new states of matter.

Importance of Ion Size on the Dominance of Water-Ion Versus Water-Water Interactions in Au-Supported Solvatomers.

The solvation of ions at interfaces is important to areas as diverse as atmospheric sciences, energy materials, and biology. Despite the significance, fundamental understanding, particularly at the molecular level, remains incomplete. Here, we probe the initial solvation of two singly charged but differently sized ions (Li and Cs) on a Au(111) by combining low-temperature scanning tunneling microscopy with density functional theory. Real-space molecular scale information reveals that water-ion interactions dominate the Li-water system, whereas water-water interactions dominate in the Cs case, and in both cases, the Au(111) surface confines the formed solvatomers to two dimensions. The difference in prevalent interactions leads to disparate symmetry and binding patterns of the solvation shells observed, as well as significantly different ion-surface interactions. The relationship between water number, geometry, and electronic structure of the solvatomers obtained here is an essential step toward understanding heterogeneous interfaces on the nanoscale.

Characterization of an Unexpected μ3 Adsorption of Molecular Oxygen on Ag(100) with Low-Temperature STM.

Precise description of the interaction between molecular oxygen and metal surfaces is one of the most challenging topics in quantum chemistry. In this work, we use low-temperature scanning tunneling microscopy (STM) to identify and characterize an adsorption state of molecular oxygen that coordinates to three Ag atoms (μ3) on Ag(100). Surprisingly, μ3-O2 cannot be identified as a stable configuration with generalized gradient approximation (GGA)-level density functional theory (DFT) calculations. Through inelastic electron tunneling spectroscopy (IETS), we identify three vibrational modes of individual μ3-O2 and assign them to out-of-plane hindered rotation (HR) at 38.0 meV, in-plane HR at 32.4 meV, and in-plane hindered translation (HT) at 22.0 meV. We determine the barrier for rotational isomerization of μ3-O2 to be 69.3 meV from tunneling electrons-induced rotations. The inability of theory to predict the experiment stems most likely from self-interaction errors inherent to GGA-DFT, which leads to an inaccurate description of localized charges. We speculate that the μ3-O2 configuration represents a formal molecular oxygen anion and assign the ±11 meV excitation in the IETS to a transition between spin-orbit states of the surface-bound anion.

Imaging Dihydrogen Bond-Driven Assembly of Borazine on Au(111).

Dihydrogen bonding (DHB) is a peculiar type of attractive interaction occurring between a partially positively charged hydrogen atom and a partially negatively charged hydrogen atom. Borazine represents a prototypical molecule exhibiting dihydrogen bonding in both gas phase, as well as in its crystalline form. For borazine assemblies on solid surfaces, a direct observation and characterization of dihydrogen bonding has remained elusive, possibly due to an intricate interplay of substrate-molecule and intermolecular interactions. Here we present evidence of dihydrogen bonding occurring in borazine assemblies on a Au(111) surface. By means of low-temperature scanning tunneling microscopy, we unveiled distinct configurations, exhibiting single and double dihydrogen bonding. Density functional theory calculations elucidate the interplay between substrate adsorption and intermolecular interactions to stabilize the formation of borazine dimers on Au(111). The dimers constitute the building blocks for the formation of larger assemblies.

Endoscope-assisted manipulator for precooling and deposition in low-temperature scanning tunneling microscope systems

We present the design and implementation of an advanced manipulator system for use in low-temperature scanning tunneling microscope (STM) setups, specifically in an ultrahigh vacuum (UHV) chamber equipped with a bottom-loading cryostat and a superconducting magnet. This system integrates UHV-compatible endoscope assemblies, providing visual assistance for the precise alignment and transfer of the STM scanner shuttle to its receptacle. Additionally, the manipulator features an 85 K precooling stage, enabling molecule deposition on metallic substrates and STM imaging to verify surface conditions before transferring the shuttle to its receptacle at 4.2 K. This new approach enhances the operational efficiency of STM systems, particularly for low-temperature measurements in high magnetic fields, and addresses the challenges of sample contamination and thermal shock during the transfer process.

Exploring structural and electronic properties of topological insulator/graphene nano-heterostructures

There is great interest in the study of topological insulator-based heterostructures due to expected emerging phenomena. However, a challenge of topological insulator (TI) research is the contribution of bulk conduction to the TI surface states. Both strain engineering and thickness control routes, which have been proposed to compensate for bulk doping, can be accessed through the use of nano-heterostructures consisting of topological insulator nanostructures grown on 2D materials. In this work, we report the synthesis of TI/graphene nano-heterostructures based on Bi2Te3 and Sb2Te3 nanoplatelets (NPs) grown on single-layer graphene. Various techniques were used to characterize this system in terms of morphology, thickness, composition, and crystal quality. We found that most of the obtained NPs are mainly <30 nm thick with thickness-dependent crystal quality, observed by Raman measurements. Thinner NPs (1 or 2 quintuple layers ) tend to replicate the topography of the underlying single-layer graphene, according to roughness analysis. Finally, we show preliminary studies of their band structure obtained by Low Temperature Scanning Tunneling Microscopy, Scanning Tunneling Spectroscopy, and by Density Functional Theory. We observe a highly negative ED value which can be attributed to the presence of defects.

Mechanisms for the Spin-State Switching of Strapped Ni-Porphyrin Complexes Deposited on Metal Surfaces: Insights from Quantum Chemical Calculations.

The incorporation of molecular switches on nanodevices requires both the intactness of the molecule once deposited on a substrate and the persistence of the reversible switching feature. Recently, the reversible spin-switching of strapped Ni(II)-porphyrin complexes deposited on Ag(111) surface is demonstrated with low-temperature scanning tunneling microscopy (STM). The spin transition is accompanied by the coordination change of the metal center, a phenomenon denominated in coordination-induced spin-state switching (CISSS). In this contribution, the spin switching of the deposited strapped Ni-porphyrin molecules using different quantum chemistry approaches is explored. This calculations inform about the geometry and electronic structure of the adsorbed molecules and the origin of the voltage-dependent switching promoted by the STM tip. Two different mechanisms are inspected to elucidate the key role of the tip, mainly the electron injection between the tip and the molecule and the differential stabilization of the two spin states by the applied electric field between the tip and the silver surface. This study puts in evidence the relevance of the pyridine ligand contained in the strap in the transport properties as in the CISSS process itself.

Atomically Precise Control of Topological State Hybridization in Conjugated Polymers.

Realization of topological quantum states in carbon nanostructures has recently emerged as a promising platform for hosting highly coherent and controllable quantum dot spin qubits. However, their adjustable manipulation remains elusive. Here, we report the atomically accurate control of the hybridization level of topologically protected quantum edge states emerging from topological interfaces in bottom-up-fabricated π-conjugated polymers. Our investigation employed a combination of low-temperature scanning tunneling microscopy and spectroscopy, along with high-resolution atomic force microscopy, to effectively modify the hybridization level of neighboring edge states by the selective dehydrogenation reaction of molecular units in a pentacene-based polymer and demonstrate their reversible character. Density functional theory, tight binding, and complete active space calculations for the Hubbard model were employed to support our findings, revealing that the extent of orbital overlap between the topological edge states can be finely tuned based on the geometry and electronic bandgap of the interconnecting region. These results demonstrate the utility of topological edge states as components for designing complex quantum arrangements for advanced electronic devices.

Realization of a 2D Lieb Lattice in a Metal-Inorganic Framework with Partial Flat Bands and Topological Edge States.

Flat bands and Dirac cones in materials are the source of the exotic electronic and topological properties. The Lieb lattice is expected to host these electronic structures, arising from quantum destructive interference. Nevertheless, the experimental realization of a 2D Lieb lattice remained challenging to date due to its intrinsic structural instability. After computationally designing a Platinum-Phosphorus (Pt-P) Lieb lattice, it has successfully overcome its structural instability and synthesized on a gold substrate via molecular beam epitaxy. Low-temperature scanning tunneling microscopy and spectroscopy verify the Lieb lattice's morphology and electronic flat bands. Furthermore, topological Dirac edge states stemming from pronounced spin-orbit coupling induced by heavy Pt atoms are predicted. These findings convincingly open perspectives for creating metal-inorganic framework-based atomic lattices, offering prospects for strongly correlated phases interplayed with topology.

The development of a low-temperature terahertz scanning tunneling microscope based on a cryogen-free scheme.

We have developed a cryogen-free, low-temperature terahertz scanning tunneling microscope (THz-STM). This system utilizes a continuous-flow cryogen-free cooler to achieve low temperatures of ∼25K. Meanwhile, an ultra-small ultra-high vacuum chamber results in the reduction of the distance from sample to viewport to only 4cm. NA = 0.6 can be achieved while placing the entire optical component, including a large parabolic mirror, outside the vacuum chamber. Thus, the convenience of optical coupling is much improved without compromising the performance of STM. Based on this, we introduced THz pulses into the tunnel junction and constructed the THz-STM, achieving atomic-level spatial resolution in THz-driven current imaging and sub-picosecond (sub-ps) time resolution in autocorrelation signals during pump-probe measurements. Experimental data from various representative samples are presented to showcase the performance of the instrument, establishing it as an ideal platform for studying non-equilibrium dynamic processes at nanoscale.

From BN-Doped Molecular Carbon Scaffolds to BNC Sheets

In carbon nanoscience, the on-surface synthesis strategy emerged as a promising route to achieve structural control at the atomic scale. This approach might be applied to fabricate well-defined two-dimensional BN-substituted carbon nanomaterials with tunable electronic properties [1]. In this talk, I will review our activities on the self-assembly of BN-doped molecular precursors, including borazine [2] and BN-doped carbon scaffold [3], on noble metal supports. Temperature-induced dehalogenation and dehydrogenation reactions are applied to introduce intramolecular ring-closing and intermolecular coupling, e.g., yielding random BNC sheets, probed by low-temperature scanning tunneling microscopy and X-ray photoelectron spectroscopy. The potential transfer and characterization of such two-dimensional BNC materials in devices will be discussed.[1] Sánchez-Sánchez et al., ACS Nano 9, 9228 (2015)[2] Weiss et al., Adv. Mat. Interfaces, accepted, doi: 10.1002/admi.202300774[3] Schwarz et al., Chem. Eur. J. 24, 9565 (2018)

Atomic-precision control of plasmon-induced single-molecule switching in a metal–semiconductor nanojunction

Atomic-scale control of photochemistry facilitates extreme miniaturisation of optoelectronic devices. Localised surface plasmons, which provide strong confinement and enhancement of electromagnetic fields at the nanoscale, secure a route to achieve sub-nanoscale reaction control. Such local plasmon-induced photochemistry has been realised only in metallic structures so far. Here we demonstrate controlled plasmon-induced single-molecule switching of peryleneanhydride on a silicon surface. Using a plasmon-resonant tip in low-temperature scanning tunnelling microscopy, we can selectively induce the dissociation of the O–Si bonds between the molecule and surface, resulting in reversible switching between two configurations within the nanojunction. The switching rate can be controlled by changing the tip height with 0.1-Å precision. Furthermore, the plasmon-induced reactivity can be modified by chemical substitution within the molecule, suggesting the importance of atomic-level design for plasmon-driven optoelectronic devices. Thus, metal–single-molecule–semiconductor junctions may serve as a prominent controllable platform beyond conventional nano-optoelectronics.

Cryogen-free modular scanning tunneling microscope operating at 4-K in high magnetic field on a compact ultra-high vacuum platform.

One of the daunting challenges in modern low temperature scanning tunneling microscopy (STM) is the difficulty of combining atomic resolution with cryogen-free cooling. Further functionality needs, such as ultra-high vacuum (UHV), high magnetic field (HF), and compatibility with μm-sized samples, pose additional challenges to an already ambitious build. We present the design, construction, and performance of a cryogen-free, UHV, low temperature, and high magnetic field system for modular STM operation. An internal vibration isolator reduces vibrations in this system, allowing for atomic resolution STM imaging while maintaining a low base temperature of ∼4K and magnetic fields up to 9T. Samples and tips can be conditioned in situ utilizing a heating stage, an ion sputtering gun, an e-beam evaporator, a tip treater, and sample exfoliation. In situ sample and tip exchange and alignment are performed in a connected UHV room temperature stage with optical access. Multisite operation without breaking vacuum is enabled by a unique quick-connect STM head design. A novel low-profile vertical transfer mechanism permits transferring the STM between room temperature and the low temperature cryostat.

Unveiling the Interlayer Interaction in a 1H/1T TaS2 van der Waals Heterostructure.

This study delves into the intriguing properties of the 1H/1T-TaS2 van der Waals heterostructure, focusing on the transparency of the 1H layer to the charge density wave of the underlying 1T layer. Despite the sizable interlayer separation and metallic nature of the 1H layer, positive bias voltages result in a pronounced superposition of the 1T charge density wave structure on the 1H layer. The conventional explanation relying on tunneling effects proves insufficient. Through a comprehensive investigation combining low-temperature scanning tunneling microscopy, scanning tunneling spectroscopy, non-contact atomic force microscopy, and first-principles calculations, we propose an alternative interpretation. The transparency effect arises from a weak yet substantial electronic coupling between the 1H and 1T layers, challenging prior understanding of the system. Our results highlight the critical role played by interlayer electronic interactions in van der Waals heterostructures to determine the final ground states of the systems.

Spatially Dependent in-Gap States Induced by Andreev Tunneling through a Single Electronic State.

By using low-temperature scanning tunneling microscopy and spectroscopy (STM/STS), we observe in-gap states induced by Andreev tunneling through a single impurity state in a low carrier density superconductor (NaAlSi). The energy-symmetric in-gap states appear when the impurity state is located within the superconducting gap. In-gap states can cross the Fermi level, and they show X-shaped spatial variation. We interpret the in-gap states as a consequence of the Andreev tunneling through the impurity state, which involves the formation or breakup of a Cooper pair. Due to the low carrier density in NaAlSi, the in-gap state is tunable by controlling the STM tip-sample distance. Under strong external magnetic fields, the impurity state shows Zeeman splitting when it is located near the Fermi level. Our findings not only demonstrate the Andreev tunneling involving single electronic state but also provide new insights for understanding the spatially dependent in-gap states in low carrier density superconductors.

Hidden Charge Order and Multiple Electronic Instabilities in EuTe4.

The rare-earth telluride compound EuTe4 exhibits a charge density wave (CDW) and an unconventional thermal hysteresis transition. Herein, we report a comprehensive study of the CDW states in EuTe4 by using low-temperature scanning tunneling microscopy. Two types of charge orders are observed at 4 K, including a newly discovered spindle-shaped pattern and a typical stripe-like pattern. As an exotic charge order, the spindle-shaped CDW is off-axis and barely visible at 77 K, indicating that it is a hidden order developed at low temperature. Based on our first-principles calculations, we reveal the origins of the observed electronic instabilities. The spindle-shaped charge order stems from a subsequent transition based on the stripe-like CDW phase. Our work demonstrates that the competition and cooperation between multiple charge orders can generate exotic quantum phases.

Orientation transitions and chiral assemblies of para-terphenyl molecules on Cd(0001).

The interplay between orientation transition and chiral self-assemblies of para-terphenyl (P3P) molecules on the Cd(0001) surface has been investigated using low temperature scanning tunneling microscopy and density functional theory calculations. Three distinct molecular orientations have been discerned from the self-assembled thin films of P3P. At the low coverage, flat-lying molecules appear in the homochiral domains with the incommensurate registry to the substrate. With the coverage increasing, the incoming molecules are incorporated into the first layer with edge-on orientation and form the self-assembled zigzag chains. The alternative arrangement of zigzag chains with opposite chirality leads to the formation of a c(4 × 2) superstructure, in which the tilted molecules exhibit orientational frustration and fuzzy noises. The analysis of the tunneling spectra reveals that the electronic structure of P3P layers is contingent upon the hybridization between the electronic states of P3P molecules and the Cd(0001) surface. These results provide important insights into the interplay between orientational transition and chiral assembly of P3P molecules on metal substrates.

Atomic-scale characterization of defects in oxygen plasma-treated graphene by scanning tunneling microscopy

Defects in graphene are important nanoscale pathways for metal atoms to enter the interface between epitaxial graphene and SiC in order to form stable ultrathin metal layers with new exotic physical properties. However, the atomic-scale details of defects that mainly govern the intercalation process remain modest. In this work, we present the first atomic investigation of point defects generated by oxygen plasma treatment on epitaxial graphene grown on SiC using low-temperature scanning tunneling microscopy, corroborated by density functional theory calculations. We found a broad spectrum of point defects that varies in size, shape, and symmetry and is dominated by triangular species. Tunneling spectroscopy identified defect-induced states in the vicinity of the Fermi level that significantly perturb the graphene electronic properties at the defect site. Based on the well-defined defect symmetry, we simulated the local density of states of the triangular defects and their corresponding scanning tunneling microscopy images which further helped us to identify the exact atomic configurations of monovacancy defects. The combination of atomic-scale scanning tunneling microscopy experiments and reliable density functional theory simulations provides ultimate microscopic details and opens a new way to identify the atomic configurations of defects in oxygen plasma-treated graphene. Our work might shed light on precise control of defect engineering in graphene for metal intercalation by controlling the defect types based on a deep understanding of each configuration.

Fully Reprogrammable 2D Array of Multistate Molecular Switching Units.

Integration of molecular switching units into complex electronic circuits is considered to be the next step toward the realization of novel logic and memory devices. This paper reports on an ordered 2D network of neighboring ternary switching units represented by triazatruxene (TAT) molecules organized in a honeycomb lattice on a Ag(111) surface. Using low-temperature scanning tunneling microscopy, the bonding configurations of individual TAT molecules can be controlled, realizing up to 12 distinct states per molecule. The switching between those states shows a strong bias dependence ranging from tens of millivolts to volts. The low-bias switching behavior is explored in active units consisting of two and more interacting TAT molecules that are purposefully defined (programmed) by high-bias switching within the honeycomb lattice. Within such a unit the low-bias switching can be triggered and accessed by single-point measurements on a single TAT molecule, demonstrating up to 9 and 19 distinguishable states in a dyad and a tetrad of coupled molecules, respectively. High experimental control over the desired state, owing to bias-dependent hierarchical switching and pronounced switching directionality, as well as full reversibility, make this system particularly appealing, paving the way to design complex molecule-based memory systems.