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All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Atomic-Scale Manipulation and In Situ Characterization with Scanning Tunneling Microscopy

Scanning tunneling microscope (STM) has presented a revolutionary methodology to nanoscience and nanotechnology. It enables imaging of the topography of surfaces, mapping the distribution of electronic density of states, and manipulating individual atoms and molecules, all at atomic resolutions. In particular, atom manipulation capability has evolved from fabricating individual nanostructures toward the scalable production of the atomic-sized devices bottom-up. The combination of precision synthesis and in situ characterization has enabled direct visualization of many quantum phenomena and fast proof-of-principle testing of quantum device functions with immediate feedback to guide improved synthesis. Several representative examples are reviewed to demonstrate the recent development of atomic-scale manipulation, focusing on progress that addresses quantum properties by design in several technologically relevant materials systems. Integration of several atomically precisely controlled probes in a multiprobe STM system vastly extends the capability of in situ characterization to a new dimension where the charge and spin transport behaviors can be examined from mesoscopic to atomic length scale. The automation of atomic-scale manipulation and the integration with well-established lithographic processes further push this bottom-up approach to a new level that combines reproducible fabrication, extraordinary programmability, and the ability to produce large-scale arrays of quantum structures.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

Simplified feedback control system for scanning tunneling microscopy

A Scanning Tunneling Microscope (STM) is one of the most important scanning probe tools available to study and manipulate matter at the nanoscale. In a STM, a tip is scanned on top of a surface with a separation of a few Å. Often, the tunneling current between the tip and the sample is maintained constant by modifying the distance between the tip apex and the surface through a feedback mechanism acting on a piezoelectric transducer. This produces very detailed images of the electronic properties of the surface. The feedback mechanism is nearly always made using a digital processing circuit separate from the user computer. Here, we discuss another approach using a computer and data acquisition through the universal serial bus port. We find that it allows successful ultralow noise studies of surfaces at cryogenic temperatures. We show results on different compounds including a type II Weyl semimetal (WTe2), a quasi-two-dimensional dichalcogenide superconductor (2H–NbSe2), a magnetic Weyl semimetal (Co3Sn2S2), and an iron pnictide superconductor (FeSe).

Martín-Vega, Francisco (ORCID:0000000333177833)↗
DOE OSTI2023

Viewing Optical Processes at the Nanoscale: Combining Scanning Tunneling Microscopy and Optical Spectroscopy

The combination of optical spectroscopy and scanning tunneling microscopy techniques has the potential to yield the unambiguous single-molecule level insight required to fully understand the complex structure–property relationship of emerging material systems. In this Perspective, we highlight the recent progress of single-molecule absorption detected by scanning tunneling microscopy (SMA-STM) to investigate light–matter interactions of supported (nano)particles, quantum dots, and molecular and thin films at the nanoscale. We further show how the SMA-STM technique has been recently extended to expand the classes of materials that can be investigated as well as recent design modifications that enable both time-resolved imaging and tomography. Finally, we hypothesize how these modifications can be further leveraged to advance our current knowledge of light–matter interactions and how the resulting nanoscale insight can be implemented for future material applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Intrinsic Defect-Induced Local Semiconducting-to-Metallic Regions Within Monolayer 1T-TiS2 Displayed by First-Principles Calculations and Scanning Tunneling Microscopy

Using density functional theory (DFT) and scanning tunneling microscopy (STM), the intrinsic point defects, formation energy, and electronic structure of 1T-TiS2 were investigated. Defect systems include single-atom vacancies, interstitial and adatom additions, and direct atomic substitution. Using a collective approach for analyzing realistic systems for point defect investigation, we provide a more straightforward comparison to the experimental measurements, reproducing more realistic environmental conditions related to thin film growth. STM images are compared to computationally simulated electron density images to identify specific geometries that result from favorable point defects. DFT suggests that titanium interstitials are the most energetically favorable intrinsic defect, and sulfur vacancies are more likely to form than titanium vacancies within this realistic analysis, which is in agreement with STM data. A pristine, stoichiometric monolayer system is calculated to have a direct band gap of 0.422 eV, which varies based on local point defects. Local semiconducting-to-metallic electronic transitions are predicted to occur based on the presence of Ti interstitials.

Keeney, P. J.↗
DOE OSTI2023

In situ plasmonic tip preparation and validation techniques for scanning tunneling microscopy

Among the many parts constituting a scanning tunneling microscope, the metallic tip is the component that directly interacts with the specimen and plays a critical role in visualizing the physical quantity of interest. While tip materials such as W and Pt–Ir are commonly used for topographic imaging and their preparation is well-documented, the preparation of plasmonic materials such as Ag for tip-enhanced Raman spectroscopy is relatively less standardized. Furthermore, we present several in situ Ag tip preparation and validation techniques for the microscopist to use depending on their intended application, including atomic resolution imaging, scanning tunneling spectroscopy (STM), and tip-enhanced Raman spectro-microscopy in ultrahigh vacuum. Besides optical applications, these methods are not limited to Ag but also applicable to other STM tip materials.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Topography inversion in scanning tunneling microscopy of single-atom-thick materials from penetrating substrate states

Scanning tunneling microscopy (STM) is one of the indispensable tools to characterize surface structures, but the distinction between atomic geometry and electronic effects based on the measured tunneling current is not always straightforward. In particular, for single-atomic-thick materials (graphene or boron nitride) on metallic substrates, counterintuitive phenomena such as a larger tunneling current for insulators than for metal and a topography opposite to the atomic geometry are reported. Using first-principles density functional theory calculations combined with analytical modeling, we reveal the critical role of penetrating states of metallic substrates that surpass 2D material states, hindering the measurement of intrinsic 2D materials states and leading to topography inversion. Our finding should be instrumental in the interpretation of STM topographies of atomic-thick materials and in the development of 2D material for (opto)electronic and various quantum applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Evidence for chiral superconductivity on a silicon surface

Tin adatoms on a Si(111) substrate with a one-third monolayer coverage form a two-dimensional triangular lattice with one unpaired electron per site. These electrons order into an antiferromagnetic Mott-insulating state, but doping the Sn layer with holes creates a two-dimensional conductor that becomes superconducting at low temperatures. Although the pairing symmetry of the superconducting state is currently unknown, the combination of repulsive interactions and frustration inherent in the triangular adatom lattice opens up the possibility of a chiral order parameter. Here we study the superconducting state of Sn/Si(111) using scanning tunneling microscopy, scanning tunneling spectroscopy and quasiparticle interference imaging. Here we find evidence for a doping-dependent superconducting critical temperature with a fully gapped order parameter, the presence of time-reversal symmetry breaking and a strong enhancement in zero-bias conductance near the edges of the superconducting domains. Although each individual piece of evidence could have a more mundane interpretation, our combined results suggest the possibility that Sn/Si(111) is an unconventional chiral d-wave superconductor.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Probing topological quantum matter with scanning tunnelling microscopy

The search for topological phases of matter is evolving towards strongly interacting systems, including magnets and superconductors, where exotic effects emerge from the quantum-level interplay between geometry, correlation and topology. Over the past decade or so, scanning tunnelling microscopy has become a powerful tool to probe and discover emergent topological matter, because of its unprecedented spatial resolution, high-precision electronic detection and magnetic tunability. Scanning tunnelling microscopy can be used to probe various topological phenomena, as well as complement results from other techniques. We discuss some of these proof-of-principle methodologies applied to probe topology, with particular attention to studies performed under a tunable vector magnetic field, which is a relatively new direction of recent focus. Finally, we then project the future possibilities for atomic-resolution tunnelling methods in providing new insights into topological matter.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Scanning tunneling microscopy of ferrocenecarboxylic acid assemblies on Ag(111): a comparison to Au(111)

Scanning tunneling microscopy was used to investigate the arrangement of ferrocenecarboxylic acid (FcCOOH) monolayers on the Ag(111) surface. Four distinct structures were observed, none of which had previously been observed on other surfaces. Structural analysis indicates that these assemblies are primarily composed of dimers. VASP calculations support the molecular assignments of the monolayer structures, while ESI-MS experiments confirm that dimers are the predominant species in solution. Of particular note, the cyclic pentamers and aperiodic packing observed for FcCOOH on Au(111) were not observed, despite (1) the similarity of the Ag(111) and Au(111) surfaces in reactivity, flatness, and lattice constant, and (2) prior explanation of the Au(111) monolayer in terms of molecule–molecule interactions alone. It is clear that while the surface does not have a template structure, it has a significant influence on which structures are formed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

XTIP – the world's first beamline dedicated to the synchrotron X-ray scanning tunneling microscopy technique

In recent years, there have been numerous efforts worldwide to develop the synchrotron X-ray scanning tunneling microscopy (SX-STM) technique. Here, the inauguration of XTIP, the world's first beamline fully dedicated to SX-STM, is reported. The XTIP beamline is located at Sector 4 of the Advanced Photon Source at Argonne National Laboratory. It features an insertion device that can provide left- or right-circular as well as horizontal- and vertical-linear polarization. XTIP delivers monochromatic soft X-rays of between 400 and 1900 eV focused into an environmental enclosure that houses the endstation instrument. This article discusses the beamline system design and its performance.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Detecting Symmetry Breaking in Magic Angle Graphene Using Scanning Tunneling Microscopy

A growing body of experimental work suggests that magic angle twisted bilayer graphene exhibits a "cascade" of spontaneous symmetry-breaking transitions, sparking interest in the potential relationship between symmetry breaking and superconductivity. However, it has proven difficult to find experimental probes which can unambiguously identify the nature of the symmetry breaking. Here, we show how atomically resolved scanning tunneling microscopy can be used as a fingerprint of symmetry-breaking order. Further, by analyzing the pattern of sublattice polarization and "Kekulé" distortions in small magnetic fields, order parameters for each of the most competitive symmetry-breaking states can be identified. In particular, we show that the "Kramers intervalley coherent state," which theoretical work predicts to be the ground state at even integer fillings, shows a Kekulé distortion which emerges only in a magnetic field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Deriving Material Properties from Feedback Error Signals in Scanning Tunneling Microscopy

Imperfections in measurements, e.g., deviations and broadening, are not devoid of information; rather, they can reveal valuable physical properties and processes. Scanning tunneling microscopy leverages a negative feedback loop to regulate the tunneling current. Practically, current fluctuates around its set point, and such deviations are considered insignificant and ignored. Here, in this study, we investigate the information embedded in these deviations. In the constant-current mode with an active feedback loop, we observe an unexpected persistent DC current offset from its set point when the tunneling junction is periodically perturbed. We demonstrate both experimentally and theoretically that such error signals encode local tunneling barrier heights and the square of local differential conductance as a consequence of the interplay between rectification and active feedback compensation. We provide evidence on the generalizability of this phenomenology to other negative feedback systems. This new approach has the potential to broadly impact physical sciences by allowing rapid measurements without lock-in amplifiers.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Determination of rutile transition metal oxide (110) surface terminations by scanning tunneling microscopy contrast reversal

The surfaces of rutile transition-metal oxides (TMO 2 ) are widely investigated for catalysis, photoelectrochemical solar cells, memristors, and supercapacitors, but their structures have remained controversial. Here we employ density functional theory to predict that a universal behavior of metallic TMO 2 surfaces, i.e., the stoichiometric TMO 2 surfaces, exhibit a contrast reversal in simulated scanning tunneling microscopy (STM) images at different scanning biases. The predictions are verified by experimental STM imaging of RuO 2 (110) surfaces and this feature is shown to enable accurate determinations of the TMO 2 (110) surface structures under various conditions. This research provides different insights into the electronic properties of TMO 2 (110) surfaces and offers an effective method to directly map the surface structure and point defects using bias-dependent STM.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Size Effect of Local Current-Voltage Characteristics of MX 2 Nanoflakes: Local Density of States Reconstruction from Scanning Tunneling Microscopy Experiments

Local current-voltage characteristics for low-dimensional transition-metal dichalcogenides (LDTMD), as well as the reconstruction of their local density of states (LDOS) from scanning tunneling microscopy (STM) experiments, are of fundamental interest and can be useful for advanced applications. Most of the existing models either have limited applicability for complex-shaped LDTMDs (e.g., those based on the Simmons approach) or require solving of an ill-defined integral equation to deconvolute the unknown LDOS (e.g., those based on the Tersoff approach). Using a serial expansion of the Tersoff formulas, we propose a flexible method to reconstruct the LDOS from local current-voltage characteristics measured in STM experiments. We establish a set of key physical parameters, which characterize the tunneling current of a STM-probe–sample contact and the sample LDOS expanded in Gaussian functions. Using a direct variational method coupled with probabilistic analysis, we determine these parameters from the STM experiments for MoS2 nanoflakes with different numbers of layers. The main result is the reconstruction of the LDOS in a relatively wide energy range around the Fermi level, which allows us to gain insight into the local band structure of LDTMDs. The reconstructed LDOS reveal pronounced size effects for the single-layer, two-layer, and three-layer MoS 2 nanoflakes, which we relate to the low dimensionality and strong bending or corrugation of the nanoflakes. We hope that the proposed elaboration of the Tersoff approach, allowing LDOS reconstruction, will be of critical interest for the quantitative description of STM experiments and also of use to better understand the microscopic physical aspects of the surface, strain, and bending contributions to the LDTMDs’ electronic properties.

42 ENGINEERING↗
DOE OSTI2019

X-ray Assisted Scanning Tunneling Microscopy and Its Applications for Materials Science: The First Results on Cu Doped ZrTe 3

Synchrotron X-ray Scanning Tunneling Microscopy (SX-STM) is a novel imaging technique capable of providing real space chemically specific mapping with a potential of reaching atomic resolution. Determination of chemical composition along with ultra-high resolution imaging by SX-STM can be realized through excitation of core electrons by incident X-rays when their energy is tuned to an absorption edge of a particular atom during raster scanning, as is done in the conventional STM experiments. In this work, we provide a brief summary and the current status of SX-STM and discuss its applications for material science. In particular, we discuss instrumentation challenges associated with the SX-STM technique and present early experiments on Cu doped ZrTe 3 single crystals.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Fingerprints of composite fermion Lambda levels in scanning tunneling microscopy

A composite fermion (CF) is a topological quasiparticle that emerges from a nonperturbative attachment of vortices to electrons in strongly correlated two-dimensional materials. Similar to noninteracting fermions that form Landau levels in a magnetic field, CFs can fill analogous “Lambda” levels, giving rise to the fractional quantum Hall (FQH) effect of electrons. Here, we show that Lambda levels can be directly visualized through the characteristic peak structure in the signal obtained via spectroscopy with scanning tunneling microscopy (STM) on a FQH state. Complementary to transport, which probes the low-energy properties of CFs, we show that high-energy features in STM spectra can be interpreted in terms of Lambda levels. We numerically demonstrate that STM spectra can be accurately modeled using Jain's CF theory. Our results show that STM provides a powerful tool for revealing the anatomy of FQH states and identifying physics beyond the noninteracting CF paradigm.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗