Scanning Tunneling Research Articles

(η8-Cyclooctatetraene)(η5-fluorenyl)titanium: a processable molecular spin qubit with optimized control of the molecule-substrate interface.

Depositing single paramagnetic molecules on surfaces for sensing and quantum computing applications requires subtle topological control. To overcome issues that are often encountered with sandwich metal complexes, we exploit here the low symmetry architecture and suitable vaporability of mixed-sandwich [FluTi(cot)], Flu = fluorenyl, cot = cyclooctatetraene, to drive submonolayer coverage and select an adsorption configuration that preserves the spin of molecules deposited on Au(111). Electron paramagnetic resonance spectroscopy and ab initio quantum computation evidence a d z 2 ground state that protects the spin from phonon-induced relaxation. Additionally, computed and measured spin coherence times exceed 10 μs despite the molecules being rich in hydrogen. A thorough submonolayer investigation by scanning tunneling microscopy, X-ray photoelectron and absorption spectrocopies and X-ray magnetic circular dichroism measurements supported by DFT calculations reveals that the most stable configuration, with the fluorenyl in contact with the metal surface, prevents titanium(iii) oxidation and spin delocalization to the surface. This is a necessary condition for single molecular spin qubit addressing on surfaces.

On-Surface Synthesis of 2D Polymers at the Liquid-Solid Interface

Unraveling the structure and dynamics of the formation of covalent and non-covalent organic-based 2D crystalline materials is key to controlling the quality of these materials, such as the defect density and their size. Understanding these characteristics is important for controlling their properties. This contribution highlights our efforts in using scanning probe microscopy, particularly scanning tunneling microscopy (STM), to visualize the structure and dynamics of substrate-supported metal-organic frameworks (sMOFs) and substrate-supported covalent organic frameworks (sCOFs) at the liquid-solid interface.We will focus on the growth of covalent organic frameworks, primarily based on boroxine chemistry, and reveal both qualitative and quantitative details of the nucleation-elongation processes in real-time and under ambient conditions. Sequential data analysis enables the observation of the amorphous-to-crystalline transition, the time-dependent evolution of nuclei, the existence of 'non-classical' crystallization pathways, and, importantly, the experimental determination of essential crystallization parameters with excellent accuracy, including critical nucleus size, nucleation rate, and growth rate. Other aspects that will be addressed include sCOFs and chirality, as well as the formation of multilayered sCOFs. Additionally, we demonstrate that in specific cases, the electric field between the STM tip and the substrate can induce, on demand, the polymerization or depolymerization process.

Single Atom Platforms on Surfaces: A Step Towards Future Catalysts

Single (metal) atoms (SAs) have attracted great attention in virtue of their intriguing physical and chemical properties, representing a fertile playground for fundamental studies of electronic, magnetic and chemical phenomena. Placing SAs on clean, crystalline and atomically flat substrates allows the study of their properties and functions via the whole plethora of surface science tools, offering direct access to their features with ultimate resolution. Moreover, the isolated metal atoms exhibit peculiar properties and tend to behave similarly to reactive centres in solution or gas phase, bridging homogeneous and heterogeneous catalysis. In these regards, atomically dispersed metal catalysts (ADCs) have recently received great attention [1, 2]. However, they suffer some intrinsic bottlenecks such as a poor tunability and limited maximum density.Here we show the successful achievement of single atom platforms (SAPs), where SAs are coordinated to atomically precise organic templates, confined to a two dimensional surface. The templates are obtained via the on-surface synthesis (OSS) of carefully designed molecular precursors, and consist of carbon-based, covalent polymers equipped with side moieties that act as coordination sites. These groups capture metal atoms that are deposited on the substrate in a post-synthetic step, and together form active site with rich potential applications. Conceptually, the obtained SAP offers rich tunability prospects by varying not only the substrate and the SA, but also the organic ligand. Its shape and composition (e.g. heteroatoms, functional groups, strain, etc.) will influence the properties of the SA and enable their fine tuning. Moreover, the versatility of the OSS strategy offers a wide plethora of possible architectures, allowing also the precise modification of the SA density.We have characterized each stage of the SAP preparation via high-resolution scanning tunnelling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) with functionalized tips [3]. The trapping and conversion ability of these well-defined active sites towards CO and CO2 was then studied. Our investigation allows the direct visualization of reactants and binding motifs with unprecedented resolution, and opens new avenues for the study of chemical reactions localized at the active sites of our SAP.[1] Wang, A.; Li, J.; Zhang, T. Heterogeneous Single-Atom Catalysis. Nature Reviews Chemistry 2018, 2 (6), 65–81.[2] Hannagan, R. T.; Giannakakis, G.; Flytzani-Stephanopoulos, M.; Sykes, E. C. H. Single-Atom Alloy Catalysis. Chem. Rev. 2020, 120 (21), 12044–12088.[3] Gross, L.; Mohn, F.; Moll, N.; Liljeroth, P.; Meyer, G. The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy. Science 2009, 325 (5944), 1110–1114. Figure 1. Single atom platform concept. Sketch of surface-adsorbed single atoms (orange dots) and of a single atom platform. Here, the organic part is represented by 1D polymers adsorbed on a surface, and the single atoms decorate the coordination sites (U-shaped motifs). Figure 1

(Invited) Dissolution of Organic Overlayers Modified Platinum Single Crystal Interfaces in Acidic Medium

Electrocatalysts design has been an important researching discipline in the filed of electrochemical energy conversion and storage, among which, surface (organic) modification has becoming an efficient, selective, and flexible approach in electrocatalysis study and applications. In the focus of direct modulation on electrocatalysis interfaces, the functionalization of metallic catalysts with organic ligands (i. e. functional organic molecule, ionomers, ionic liquids) has demonstrated its capacity on improving catalytic activity and selectivity in electrochemical reactions such as ORR, CO2RR, as well as oxidation processes. However the electrocatalysts stability behavior and surface degradation dynamics of such modified systems are still not sufficiently studied.To fully decipher the potentials of surface/interface modification in electrocatalysis, we focus our studies on the dissolution behavior of functional organics (i. e. melamine, immidazolium ionic liquids) modified model Pt (hkl) single crystal facets, in acidic medium. In regard of the excellent system sensitivity of our inductively coupled plasma mass spectrometer (ICP-MS, detection limit down to 0.1-1 ng/L), we employed a customized scanning flow cell (SFC) coupled on-line monitoring system to probe the potential-resolved surface metal dissolution dynamics. With designed electrochemical system configuration, we acquired results demonstrating the delicate stability behavior changes over transient and accumulative dissolution study on (modified) Pt facets. In combination of other state-of-art operando spectroscopy methods (electrochemical infrared reflection absorption spectroscopy, electrochemical scanning tunneling microscopy, Raman spectroscopy, etc.), we further comprehended the surface oxidation process, surface blockage/modification mechanisms on electrocatalysis interface. Our research conferred a model stability study approach on surface modification of electrodes and electrocatalysts, providing important insights into the dynamic interfacial interaction between functional modifier and electrocatalysts.

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)

Topology Selectivity of a Conformationally Flexible Precursor by Selenium Doping

Conformational arrangements within nanostructures play a crucial role in shaping the overall configuration and determining the properties, for example in covalent/metal organic framework (COF/MOF) synthesis. In on-surface synthesis, conformational diversity often leads to uncontrollable or disordered structures. Therefore, exploration on controlling and directing the conformational arrangements is significant in achieving desired nanoarchitectures. In this study, a conformationally flexible precursor 2,4,6-tris(3-bromophenyl)-1,3,5-triazine (mTBPT) was employed, and a random phase consisting of C3h and Cs conformers was first obtained after deposition of mTBPT on Cu(111) from room temperature (RT) to ~365 K as expected. By low coverage (0.01 ML) selenium (Se) doping, we achieved the selectivity of the C3h conformer greatly and improved the nanopore structural homogeneity. The ordered 2D metal organic nanostructure from mTBPT could be fulfilled by Se doping from RT to ~365 K, and the deposition sequence of mTBPT and Se did not make any difference. Through the combination of high-resolution scanning tunneling microscopy and non-contact atomic force microscopy, we explored the regulation of the conformationally flexible mTBPT on Cu(111) and achieved the high topology selectivity by low coverage Se doping. Density functional theory calculations revealed the energy regulation of organometallics before and after Se doping at atomic level. These results could enrich the on-surface synthesis toolbox of conformationally flexible precursors, for the design of complex nanoarchitectures, and for future development of engineered nanomaterials and nanodevices. Figure 1

Combining Nitrogen Doping and Vacancies for Tunable Resonant States in Graphite.

We investigate the combination of nitrogen doping and vacancies in highly ordered pyrolytic graphite (HOPG), to engineer defect sites with adjustable electronic properties. We combine scanning tunneling microscopy and spectroscopy and density functional theory calculations to reveal the synergistic effects of nitrogen and vacancies in HOPG. Our findings reveal a remarkable shift of the vacancy-induced resonance peak from an unoccupied state in pristine HOPG to an occupied state in nitrogen-doped HOPG. This shift directly correlates with the shift of the charge neutrality point resulting from the n-doping induced by substitutional nitrogen. These results open new avenues for defect engineering in graphite or graphene and achieving novel functionalities for chemical activity or electronic properties.

Mechanistic Aspects of Selenate Reduction on CU(UPD) on AU Electrodes

Recent findings in our laboratories have shown that copper1and cadmium2 underpotential deposited on polycrystalline Au electrodes, can mediate the reduction of in 0.1 M HClO4 aqueous solutions, to yield irreversibly adsorbed elemental Se allowing its detection down to the nM range. Current efforts are focused on gaining a better understanding of the the factors that govern this unique electrocatalytic phenomenon by employing a combination of electrochemical, microgravimetric, and optical techniques, using Cu as the UPD species. Correlations have been sought between the rates of this process and the coverage of Cu(UPD), the concentration of and, to a more limited extent, the applied potential in 0.1 M HClO4 solutions. Experimental conditions were selected to unveil the functional dependence of the kinetics on each of the aforementioned factors, while keeping the others constant, in solutions both quiescent and in the presence of convective flow. A number of interesting observations were made based on the data collected. In particular, shown in Panel C, Fig. 1, are plots of the charge associated with the oxidation of adsorbed Cu and Se, QCu and QSe, respectively (see Panel C), obtained from the area under the peaks shaded in light blue and yellow in Panels A and B, Fig, 1, respectively, vs (see below). These data were recorded during linear potential scans following polarization of the electrode at Ehold = 0.325 V for various times, thold (see Panels A-C, Fig. 1), where the linear character of the data in solutions is consistent with Cu(UPD) proceeding under strict diffusion control. As indicated by the QSe data, the electrode displayed electrocatalytic activity for reduction only for QCu > ca. 80 μC cm2-. Additional insight was obtained from measurements of the electrode weight as a function of using a quartz crystal microbalance (see Panel D, Fig.1), which yielded evidence for adsorption only for QCu > ca. 80 μC cm2-, pointing to the formation of a surface-bound adduct, as a necessary step for reduction to ensue. On this basis, we propose that for QCu > ca. 80 μC cm2 the mechanism for reduction involves, as a first step, the reversible formation of the adduct, Eq. (1) followed by its irreversible reduction, generating adsorbed elemental Se, Cu|Se(ads), Eq. (2).Atomically resolved images of Cu(UPD) on Au(111) obtained with a scanning tunneling microscope for the closely related sulfate ion, strongly suggest that the active site involved adsorption on the empty Au sites of the so-called √3´x√3 superstructure (see Insert in Panel A, Fig. 1). A similar conclusion was drawn based on measurements performed with rotating Cu ring-polycrystalline Au disk electrodes, RRDE, at constant Cu(UPD) coverage and applied potential for various solution compositions. Quantitative analyses of all the data, which included numerical simulations, yielded kf/kr, and kET values in the range (2.4 – 3.23) ´ 106 cm3 mol-1 and (2.5 – 9) ´ 10-3 s-1, respectively. Acknowledgments Support for this work was provided by NSF CHEM 1808592

Identifying Target Molecule and Trace Amount of the Byproduct by Two-Dimensional Self-Assembly with Different Solution Concentrations.

Scanning tunneling microscopy (STM) is a powerful way to realize the recognition of self-assembled nanostructures on the atomic scale. In this article, dihexadecyl 6,9-bis((4-(hexadecyloxy)phenyl)ethynyl) phenanthro[9,10-c]thiophene-1,3-dicarboxylate (D-PT) and dihexadecyl 6-bromo-9-((4-(hexadecyloxy) phenyl)ethynyl)phenanthrol[9,10-c]thiophene-1,3-dicarboxylate (S-BrPT) with different substituents were chosen as the target system. D-PT with four side chains as the target molecule and S-BrPT with three side chains and a bromine substituent as the byproduct were mixed in a molar concentration ratio of 20:1. The effect of solution concentration on the molecular self-assembly of the mixture was investigated by STM at the hexadecane/HOPG interface. At high concentrations, only D-PT molecules formed a dimer pattern resulting from the intermolecular van der Waals force and self-adaption. Further diluting the solution, D-PT formed the coexisting dimer and linear structures, in which the linear pattern was formed via solvent coadsorption. At low concentrations, S-BrPT molecules forming N-shaped dimers appeared and filled the linear structure fabricated by D-PT molecules. With further decrease in the concentration, S-BrPT molecules formed N-shaped dimers covering almost half of the surface area, resulting from the C-Br···π and Br···H-C bonds. At very low concentrations, S-BrPT molecules formed N-shaped dimers to arrange the matrix architecture due to the coadsorption of more hexadecane molecules. Density functional theory (DFT) calculations demonstrated that the stronger intermolecular C-Br···π and Br···H-C bonds were significant factors in determining the formation of N-shaped dimers and the stability of this nanostructure. This work enriches the diversity of self-assembled motifs and provides a strategy to characterize different symmetric molecules with trace amounts in a mixed system by STM.

Robust Edge States of Quasi-1D Material Ta2NiSe7 and Applications in Saturable Absorbers.

The helical edge states (ESs) protected by underlying Z2 topology in two-dimensional topological insulators (TIs) arouse upsurges in saturable absorptions thanks to the strong photon-electron coupling in ESs. However, limited TIs demonstrate clear signatures of topological ESs at liquid nitrogen temperatures, hindering the applications of such exotic quantum states. Here, we demonstrate the existence of one-dimensional (1D) ESs at the step edge of the quasi-1D material Ta2NiSe7 at 78 K by scanning tunneling microscopy. Such ESs are rather robust against the irregularity of the edges, suggesting a possible topological origin. The exfoliated Ta2NiSe7 flakes were used as saturable absorbers (SAs) in an Er-doped fiber laser, hosting a mode-locked pulse with a modulation depth of up to 52.6% and a short pulse duration of 225 fs, far outstripping existing TI-based SAs. This work demonstrates the existence of robust 1D ESs and the superior SA performance of Ta2NiSe7.

19]Starphene: Combined In‐Solution and On‐Surface Synthesis Towards the Largest Starphene

Starphenes are structurally appealing three‐fold symmetric polycyclic aromatic compounds with potential interesting applications in molecular electronics and nanotechnology. This family of star‐shaped polyarenes can be regarded as three acenes that are connected through a single benzene ring. In fact, just like acenes, unsubstituted large starphenes are poorly soluble and highly reactive molecules under ambient conditions making their synthesis difficult to achieve. Herein, we report two different synthetic strategies to obtain a starphene formed by 19 cata‐fused benzene rings distributed within three hexacene branches. This molecule, which is the largest starphene that has been obtained to date, was prepared by combining solution‐phase and on‐surface synthesis. [19]Starphene was characterized by high‐resolution scanning tunneling microscopy (STM) and spectroscopy (STS) showing a remarkable small HOMO‐LUMO transport gap (0.9 eV).

Size-Increased All-Phenylene Molecular Spoked Wheels - A Combined Theoretical and Experimental Approach.

A lateral expansion of molecular spoked wheels (MSWs) based on an all-phenylene backbone is described. The MSWs contain a central hub, six spokes, and a rim that is formed by a sixfold Yamamoto coupling of the respective non-cyclized dodecabromo precursor yielding MSWs with up to 30 phenylene rings in the perimeter. Attempts to prepare compounds of such size without flexible side groups at the spokes were unsuccessful, most probably due to an aggregation and accompanying oligomerization of the precursors during the cyclization. To overcome these problems, fluorene units are inserted into the spokes. These contain additional alkyl chains and lead to a curvature of the wheels. Quantum chemical calculations on the mechanism of the Yamamoto coupling lead to geometry and strain-related criteria for the successful rim closure to the respective MSW. Subsequently, MSWs are prepared with four and even six phenylene units at each edge of the hexagonal wheels. The resulting MSWs are characterized by spectroscopic methods, and additionally some of them are visualized via scanning tunneling microscopy (STM).

19]Starphene: Combined In-Solution and On-Surface Synthesis Towards the Largest Starphene.

Starphenes are structurally appealing three-fold symmetric polycyclic aromatic compounds with potential interesting applications in molecular electronics and nanotechnology. This family of star-shaped polyarenes can be regarded as three acenes that are connected through a single benzene ring. In fact, just like acenes, unsubstituted large starphenes are poorly soluble and highly reactive molecules under ambient conditions making their synthesis difficult to achieve. Herein, we report two different synthetic strategies to obtain a starphene formed by 19 cata-fused benzene rings distributed within three hexacene branches. This molecule, which is the largest starphene that has been obtained to date, was prepared by combining solution-phase and on-surface synthesis. [19]Starphene was characterized by high-resolution scanning tunneling microscopy (STM) and spectroscopy (STS) showing a remarkable small HOMO-LUMO transport gap (0.9 eV).

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.

The nontrivial effects of annealing on superconducting properties of Nb single crystals

The effect of annealing on the superconducting properties of niobium single crystals was studied using optical, magnetic, and scanning tunneling microscopy (STM) methods. Pieces of the same crystal boule were studied before and after the annealing at 800 ∘C , 1400 ∘C , and near the melting point of niobium (2477 ∘C ). The initial samples had a high hydrogen content and low-temperature imaging revealed large hydrides (hundreds of micrometers) appearing below 190 K. The formation of these large precipitates is already completely suppressed by annealing at 800 ∘C . However, the overall superconducting properties of the annealed samples did not improve and, in fact, worsened. In particular, the superconducting transition temperature decreased, the upper critical field increased, and the pinning strength increased. In the STM study, the sample was annealed initially at 400 ∘C , measured, annealed at 1700 ∘C , and measured again. The STM revealed a ‘dirty’ superconducting gap with a significant spatial variation in tunneling conductance after annealing at 400 ∘C . The clean gap was recovered after annealing at 1700 ∘C . This is likely due to oxygen redistribution near the surface, which is always covered by oxide layers in as-grown crystals. Our results indicate that vacuum annealing at least up to 1400 ∘C , while removing a large percentage of hydrogen, introduces additional nanosized defects, likely hydride precipitates, that act as efficient pair-breaking and pinning centers. The dimensionless scattering rate is estimated to have increased from Γ=0.2 to about Γ=0.4 after annealing at 1400 ∘C . These results on single crystals differ drastically from those obtained in polycrystalline bulk niobium (i.e. cut from superconducting radio-frequency cavities), where annealing is known to have a significant positive effect that is attributed to the improvement of the crystalline structure masking the more subtle influence of the hydrides.

Challenges to extracting spatial information about double P dopants in Si from STM images

The design and implementation of dopant-based silicon nanoscale devices rely heavily on knowing precisely the locations of phosphorous dopants in their host crystal. One potential solution combines scanning tunneling microscopy (STM) imaging with atomistic tight-binding simulations to reverse-engineer dopant coordinates. This work shows that such an approach may not be straightforwardly extended to double-dopant systems. We find that the ground (quasi-molecular) state of a pair of coupled phosphorous dopants often cannot be fully explained by the linear combination of single-dopant ground states. Although the contributions from excited single-dopant states are relatively small, they can lead to ambiguity in determining individual dopant positions from a multi-dopant STM image. To overcome that, we exploit knowledge about dopant-pair wave functions and propose a simple yet effective scheme for finding double-dopant positions based on STM images.

Determining the density and spatial descriptors of atomic scale defects of 2H–WSe2 with ensemble deep learning

We have demonstrated atomic-scale defect characterization in scanning tunneling microscopy images of single crystal tungsten diselenide using an ensemble of U-Net-like convolutional neural networks. Coordinates, counts, densities, and spatial extents were determined from almost 16 000 defect detections, leading to the rapid identification of defect types and their densities. Our results show that analysis aided by machine learning can be used to rapidly determine the quality of transition metal dichalcogenides and provide much needed quantitative input, which may improve the synthesis process.

Influencing the surface quality of free-standing wurtzite gallium nitride in ultra-high vacuum: Stoichiometry control by ammonia and bromine adsorption

Gallium nitride (GaN), a wide bandgap semiconductor with absorption and emission in the ultraviolet/visible range, is proposed as an alternative to metallic surfaces for assembling organic molecular structures aiming at optoelectronic applications. However, the formation of a persistent surface oxide layer in air considerably limits the use of GaN for well-defined interfaces. In this work, we have investigated, characterized and processed n-type free-standing c-plane hexagonal wurtzite GaN crystals grown by hydride vapor phase epitaxy and ammonothermal growth methods. Surface cleaning and full removal of the oxide layer on GaN surfaces could be reproducibly achieved via sputtering and annealing cycles, as evidenced by X-ray photoelectron spectroscopy and low-energy electron diffraction. Scanning tunneling microscopy, however, indicated substantial roughening of the GaN surface and the formation of unwanted Ga-rich islands and clusters. Although ammonia (NH3) and bromine (Br) treatments compensated the N/Ga atoms ratio reduced by sputtering, the surface morphology remained rough, exhibiting randomly shaped and distributed hillocks. In addition, we studied the effect of electron bombardment on the surface quality of GaN during NH3 annealing, on-surface debromination and polymerization of 1,3,5-tris(4-bromophenyl) benzene on GaN, and the removal of Ga atoms by Br atoms during the desorption.

Evidence for Two-Dimensional Weyl Fermions in Air-Stable Monolayer PtTe1.75.

The Weyl semimetals represent a distinct category of topological materials wherein the low-energy excitations appear as the long-sought Weyl Fermions. Exotic transport and optical properties are expected because of the chiral anomaly and linear energy-momentum dispersion. While three-dimensional Weyl semimetals have been successfully realized, the quest for their two-dimensional (2D) counterparts is ongoing. Here, we report the realization of 2D Weyl Fermions in monolayer PtTe1.75, which has strong spin-orbit coupling and lacks inversion symmetry, by combined angle-resolved photoemission spectroscopy, scanning tunneling microscopy, second harmonic generation, X-ray photoelectron spectroscopy measurements, and first-principles calculations. The giant Rashba splitting and band inversion lead to the emergence of three pairs of critical Weyl cones. Moreover, monolayer PtTe1.75 exhibits excellent chemical stability in ambient conditions, which is critical for future device applications. The discovery of 2D Weyl Fermions in monolayer PtTe1.75 opens up new possibilities for designing and fabricating novel spintronic devices.

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.