Scanning Tunneling Microscopy Techniques Research Articles

Probe the Dynamic Adsorption and Phase Transition of Underpotential Deposition Processes at Electrode-Electrolyte Interfaces.

Electrochemical scanning tunneling microscopy (EC-STM) and electrochemical quartz crystal microbalance (E-QCM) techniques in combination with DFT calculations have been applied to reveal the static phase and the phase transition of copper underpotential deposition (UPD) on a gold electrode surface. EC-STM demonstrated, for the first time, the direct visualization of the disintegration of (√3 × √3)R30° copper UPD adlayer with coadsorbed SO42- while changing sample potential (ES) toward the redox Pa2/Pc2 peaks, which are associated with the phase transition between the Cu UPD (√3 × √3)R30° phase II and disordered randomly adsorbed phase III. DFT calculations show that SO42- binds via three oxygens to the bridge sites of the copper with sulfate being located directly above the copper vacancy in the (√3 × √3)R30° adlayer, whereas the remaining oxygen of the sulfate points away from the surface. E-QCM measurement of the change of the electric charge due to Cu UPD Faradaic processes, the change of the interfacial mass due to the adsorption and desorption of Cu(II) and SO42-, and the formation and stripping of UPD copper on the gold surface provide complementary information that validates the EC-STM and DFT results. This work demonstrated the advantage of using complementary in situ experimental techniques (E-QCM and EC-STM) combined with simulations to obtain an accurate and complete picture of the dynamic interfacial adsorption and UPD processes at the electrode/electrolyte interface.

Kinkless Electronic Junction along 1D Electronic Channel Embedded in a Van Der Waals Layer.

Here, the formation of type-I and type-II electronic junctions with or without any structural discontinuity along a well-defined 1 nm-wide 1D electronic channel within a van der Waals layer is reported. Scanning tunneling microscopy and spectroscopy techniques are employed to investigate the atomic and electronic structure along peculiar domain walls formed on the charge-density-wave phase of 1T-TaS2 . Distinct kinds of abrupt electronic junctions with discontinuities of the band gap along the domain walls are found, some of which even do not have any structural kinks and defects. Density-functional calculations reveal a novel mechanism of the electronic junction formation; they are formed by a kinked domain wall in the layer underneath through substantial electronic interlayer coupling. This work demonstrates that the interlayer electronic coupling can be an effective control knob over nanometer-scale electronic property of 2D atomic monolayers.

Self-Assembly of N-Rich Triimidazoles on Ag(111): Mixing the Pleasures and Pains of Epitaxy and Strain.

In the present report, homochiral hydrogen-bonded assemblies of heavily N-doped (C9H6N6) heterocyclic triimidazole (TT) molecules on an Ag(111) substrate were investigated using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) techniques. The planar and prochiral TT molecules, which exhibit a threefold rotation symmetry and lack mirror symmetry when assembled on the substrate, carry multiple hydrogen-bonding donor and acceptor functionalities, inevitably leading to the formation of hexameric two-dimensionally extended assemblies that can be either homo- (RR/SS) or heterochiral (RS). Experimental STM data showing well-ordered homochiral domains and experimental LEED data are consistent with simulations assuming the R19.1° overlayer on the Ag(111) lattice. Importantly, we report the unexpected coincidence of spontaneous resolution with the condensation of neighboring islands in adjacent "Janus pairs". The islands are connected by a characteristic fault zone, an observation that we discuss in the context of the fairly symmetric molecule and its propensity to compromise and benefit from interisland bonding at the expense of lattice mismatches and strain in the defect zone. We relate this to the close to triangular shape and the substantial but weak bonding scheme beyond van der Waals (vdW) of the TT molecules, which is due to the three N-containing five-membered imidazole rings. Density functional theory (DFT) calculations show clear energetic differences between homochiral and heterochiral pairwise interactions, clearly supporting the experimental results.

Scanning Tunneling Microscopy Study of Lipoic Acid, Mannose, and cRGD@AuNPs Conjugates.

The functionalization of AuNPs with different biological elements was achieved to investigate their possibility in biomedical applications such as drug delivery, vaccine development, sensing, and imaging. Biofunctionalized AuNPs are pursued for applications such as drug delivery, vaccine development, sensing, and imaging. In this study, AuNPs with diameters of 20 nm were functionalized with lipoic acid, mannose, or the cRGD peptide. By using UV-vis spectroscopy, Fourier transform infrared spectroscopy, dynamic light scattering, transmission electron microscopy, and scanning tunneling microscopy techniques, we showed that AuNPs can be functionalized by these biomolecules in a reliable way to obtain conjugates to explore potential biomedical applications. In particular, we demonstrate that the STM technique can be employed to analyze biofunctionalized AuNPs, and the obtained information can be valuable in the design of biomedical applications.

Scanning tunneling microscopy under chemical reaction at solid–liquid and solid–gas interfaces

The task of bridging the pressure gap between ideal ultrahigh vacuum conditions and more realistic reaction conditions involving gas and liquid phases is crucial in surface and interfacial chemistry. Scanning tunneling microscopy (STM) has played a key role in addressing this challenge by enabling atomic-scale probing of the interface. STM enabled us to study surface structure, electronic structure, atomic manipulation, dynamics of molecules and atoms, and chemical properties of the surface at the atomic scale. Over the past four decades, the field of STM has undergone explosive growth. This review article focuses on recent advances in operando STM, specifically in the study of solid–liquid and solid–gas interfaces. It highlights the latest works in ambient-pressure STM, which has enabled the observation of atomic features under various gas and reaction conditions. This information sheds light on the surface mobility of adsorbates and atomic structures of reaction intermediates. The review also addresses research on electrochemical STM, which investigates the evolution of surface morphology under electrochemical processes and provides insights into atomic-scale reaction mechanisms. Finally, the article outlines future challenges and perspectives for operando STM techniques.

Ultrafast Dynamics Revealed with Time-Resolved Scanning Tunneling Microscopy: A Review.

A scanning tunneling microscope (STM) capable of performing pump-probe spectroscopy integrates unmatched atomic-scale resolution with high temporal resolution. In recent years, the union of electronic, terahertz, or visible/near-infrared pulses with STM has contributed to our understanding of the atomic-scale processes that happen between milliseconds and attoseconds. This time-resolved STM (TR-STM) technique is evolving into an unparalleled approach for exploring the ultrafast nuclear, electronic, or spin dynamics of molecules, low-dimensional structures, and material surfaces. Here, we review the recent advancements in TR-STM; survey its application in measuring the dynamics of three distinct systems, nucleus, electron, and spin; and report the studies on these transient processes in a series of materials. Besides the discussion on state-of-the-art techniques, we also highlight several emerging research topics about the ultrafast processes in nanoscale objects where we anticipate that the TR-STM can help broaden our knowledge.

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. Lastly, 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.

Fabrication and Characterization of Pre-Defined Few-Layer Graphene

Graphene is one of the most well-known two-dimensional (2D) materials that has attracted significant interest due to its unique electrical and optical properties. Being a van der Waals substrate, the fabrication of few-layered graphene by stacking a pre-defined number of graphene monolayers is essential in the field. The thickness can influence the interface interaction and therefore tune the surface electronic properties. In the study, we demonstrate a bottom-up synthesis of pre-defined few-layer graphene on SiC substrate using the thermal decomposition method and carefully characterize its thickness by the non-damageable synchrotron-radiation-based X-ray photo-electron spectroscopy (SR-XPS). By varying the photon energy, we acquire different probe depths, resulting in the different intensity ratios of graphene to SiC substrate, which is then used to estimate the thickness of the few-layer graphene. Our calculation demonstrates that the thermal decomposition method in the study can repeatedly fabricate graphene samples with expected thickness. We further compare the obtained few-layer graphene to the single-layer graphene and HOPG using the scanning tunneling microscopy (STM) technique. Our work provides accurate methods for fabricating and characterizing pre-defined few-layer graphene, providing essential knowledge in future graphene-based thin film electronics.

Spin excitations of individual magnetic dopants in an ionic thin film

Individual magnetic transition metal dopants in a solid host usually exhibit relatively small spin excitation energies of a few meV. Using scanning tunneling microscopy and inelastic electron tunneling spectroscopy (IETS) techniques, we have observed a high spin excitation energy around 36 meV for an individual Co substitutional dopant in ultrathin NaCl films. In contrast, the Cr dopant in the NaCl film shows much lower spin excitation energy around 2.5 meV. Electronic multiplet calculations combined with first-principles calculations confirm the spin excitation induced IETS, and quantitatively reveal the out-of-plane magnetic anisotropies for both Co and Cr. They also allow reproducing the experimentally observed redshift in the spin excitations of Co dimers and ascribe it to a charge and geometry redistribution.

The effect of hydrogen bond or halogen bond on assemblies of TTF derivatives

We designed and synthesized a Tetrathiafulvalene (TTF) derivative molecule TTF-1, whose iodinated phenyl and pyridine groups are directly linked to the TTF core. TTF-1 was used to study the self-assembly performance at HOPG interface in different solvents (heptanoic acid and 1-phenyloctane). The diverse self-assembly nanostructures of TTF-1 in two different solvents were characterized by using scanning tunneling microscopy (STM) technique. The experimental results show that the structure of supramolecular self-assembly in liquid-solid interface varies significantly with the different solvents due to different intermolecular interactions: hydrogen bond or halogen bond. Density functional theory (DFT) calculations were implemented to explore the possible assembly models of the two-dimensional (2D) nanostructure.

Observing Electrocatalytic Processes via In Situ Electrochemical Scanning Tunneling Microscopy: Latest Advances.

Electrocatalysis is the foundation of many techniques currently used to address environmental and energy problems. Therefore, understanding the electrocatalytic processes is essential to guide the rational design of electrocatalysts. Scanning tunneling microscopy (STM), developed in the 1980s, remains one of the few techniques that allow surface imaging at the atomic level, making it incredibly useful in electrocatalytic research. In this review, we introduced the basic concept and latest applications of the STM technique for in situ studies of electrocatalytic processes, particularly its capability in active site identification, species adsorption/desorption analysis, surface reconstruction imaging, and electrocatalyst dissolution detection, as well as its advantages and limitations.

Metal Porphyrins as Single Site Catalyst Models Explored by Electrochemical Scanning Tunneling Microscopy: A New Perspective in Electrocatalysis

AbstractM–N–C materials featuring M–Nx single site catalysts, where for example M = Co, Mg, or Fe coordinating x atoms of nitrogen, have been intensely investigated as promising candidates to replace Pt group metals in a variety of electrocatalytic reactions, for example in the O2 reduction reactions (ORR). However, despite the recent emerging of atomic resolution techniques, there is still a lack of understanding of the precise configuration of the M–Nx single site. For example, metal porphyrins are known to act as catalysts for O2 and they are indeed good model systems for mimicking M–N4 sites. In particular, it would be of interest to gain more information on the precise dynamic of the catalytic process at atomic scale, especially in an electrochemical environment, i.e., at electrode/electrolyte interphase. An approach that is becoming more and more consolidated is to combine the information coming from electrochemistry and scanning tunneling microscopy techniques in electrochemical scanning tunneling microscopy (EC‐STM) and the result is that a precise dynamic of the catalytic site at atomic scale can be investigated. This review aims to emphasize the main results and advances of EC‐STM in investigating small molecule electrocatalysis and specifically O2 reduction at metal porphyrins M–N4 model system.

Effects of functional groups and side chains on assembly of “X”-shaped new aggregation-induced emission molecules

The self-assembly properties of aggregation-induced emission molecules play important roles in electroluminescence devices and fluorescence sensors because noncovalent interactions in self-assembly structures would accelerate the excitation energy consumption. However, there are only few studies to explore their self-assembly properties on the interface and there is still a great need for further understanding self-assembled mechanisms from the viewpoint of molecular design. Here, we presented three X-shaped aggregation-induced emission molecules X1, X2 and X3, which decorated with different functional groups and alkyl side chains. The self-assembly structures were revealed by scanning tunneling microscopy technique in combination with density functional theory. Results showed that X-shaped molecules self-assembled into different structures, depending on their molecular structure, especially the functional groups. Furthermore, self-assembly structures could be regulated by adjusting solution concentration. In more detail, parallel with gradually increasing solution concentration, the molecules approached closer and molecule–molecule interactions were enhanced, finally resulting in new nanostructures. The self-assembly properties of three X-shaped aggregation-induced emission molecules on the liquid/solid interface would give a guidance for further exploring the aggregation state in three-dimensional space. Meanwhile, the two-dimensional nanostructures might show special properties, which could be used in fabricating next generation functional films.

Atomic-scale visualization of chiral charge density wave superlattices and their reversible switching

Chirality is essential for various phenomena in life and matter. However, chirality and its switching in electronic superlattices, such as charge density wave (CDW) superlattices, remain elusive. In this study, we characterize the chirality switching with atom-resolution imaging in a single-layer NbSe2 CDW superlattice by the technique of scanning tunneling microscopy. The atomic arrangement of the CDW superlattice is found continuous and intact although its chirality is switched. Several intermediate states are tracked by time-resolved imaging, revealing the fast and dynamic chirality transition. Importantly, the switching is reversibly realized with an external electric field. Our findings unveil the delicate switching process of chiral CDW superlattice in a two-dimensional (2D) crystal down to the atomic scale.

Large-Gap Quantum Spin Hall State and Temperature-Induced Lifshitz Transition in Bi4Br4.

Searching for quantum spin Hall insulators with large fully opened energy gap to overcome the thermal disturbance at room temperature has attracted tremendous attention because of the robustness of one-dimensional (1D) spin-momentum locked topological edge states in the practical applications of electronic devices and spintronics. Here, we report the investigation of topological nature of monolayer Bi4Br4 by the techniques of angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy. The possible topological nontriviality of 1D edge state integrals within the large energy gap (∼0.2 eV) is revealed by the first-principle calculations. The ARPES measurements at different temperatures show a temperature-induced Lifshitz transition, corresponding to the resistivity anomaly evoked by the chemical potential shift. The connection between the emergency of superconductivity and the Lifshitz transition is discussed.

Engineering Domain Wall Electronic States in Strongly Correlated van der Waals Material of 1T-TaS2.

Although a few physical methods were demonstrated for domain wall engineering in various electronic or ferroic materials with broken discrete symmetries, the direct control over the electronic properties of individual domain walls has been extremely limited. Here, we introduce a chemical method to tune the electronic property of domain walls in 1T tantalum disulfide. By using scanning tunneling microscopy and spectroscopy techniques, we find that indium adatoms on 1T-TaS2 have distinct behaviors on the domains with different bulk terminations. Moreover, the adatoms form their own chains along the edges of neighboring domains. The density functional theory calculations reveal a 1D Mott insulating state on a modified domain wall, resulting from the degenerated spin-polarized bands with electron doping from adsorbates and charge transfer from neighboring domains. This work suggests that chemical decoration by adsorbates can be widely used to tune local electronic states of domain walls and various 2D materials.

Ultrafast Photon-Induced Tunneling Microscopy.

Unification of the techniques of ultrafast science and scanning tunneling microscopy (STM) has the potential of tracking electronic motion in molecules simultaneously in real space and real time. Laser pulses can couple to an STM junction either in the weak-field or in the strong-field interaction regime. The strong-field regime entails significant modification (dressing) of the tunneling barrier of the STM junction, whereas the weak-field or the photon-driven regime entails perturbative interaction. Here, we describe how photons carried in an ultrashort pulse interact with an STM junction, defining the basic fundamental framework of ultrafast photon-induced tunneling microscopy. Selective dipole coupling of electronic states by photons is shown to be controllable by adjusting the DC bias at the STM junction. An ultrafast tunneling microscopy involving photons is established. Consolidation of the technique calls for innovative approaches to detect photon-induced tunneling currents at the STM junction. We introduce and characterize here three techniques involving dispersion, polarization, and frequency modulation of the laser pulses to lock-in detect the laser-induced tunneling current. We show that photon-induced tunneling currents can simultaneously achieve angstrom scale spatial resolution and sub-femtosecond temporal resolution. Ultrafast photon-induced tunneling microscopy will be able to directly probe electron dynamics in complex molecular systems, without the need of reconstruction techniques.

Engineering atomic-scale magnetic fields by dysprosium single atom magnets

Atomic scale engineering of magnetic fields is a key ingredient for miniaturizing quantum devices and precision control of quantum systems. This requires a unique combination of magnetic stability and spin-manipulation capabilities. Surface-supported single atom magnets offer such possibilities, where long temporal and thermal stability of the magnetic states can be achieved by maximizing the magnet/ic anisotropy energy (MAE) and by minimizing quantum tunnelling of the magnetization. Here, we show that dysprosium (Dy) atoms on magnesium oxide (MgO) have a giant MAE of 250 meV, currently the highest among all surface spins. Using a variety of scanning tunnelling microscopy (STM) techniques including single atom electron spin resonance (ESR), we confirm no spontaneous spin-switching in Dy over days at ≈ 1 K under low and even vanishing magnetic field. We utilize these robust Dy single atom magnets to engineer magnetic nanostructures, demonstrating unique control of magnetic fields with atomic scale tunability.

Dual modulation STM: Simultaneous high-resolution mapping of the differential conductivity and local tunnel barrier height demonstrated on Au(111)

We present a scanning tunneling microscopy (STM) technique to simultaneously measure the topography, the local tunnel barrier height (dI/dZ), and the differential conductivity (dI/dV). We modulate the voltage and tip piezo with small sinusoidal signals that exceed the cut-off frequency of the STM electronics and feed the tunneling current into two lock-in amplifiers (LIAs). We derive and follow a set of criteria for the modulation frequencies to avoid any interference between the LIA measurements. To validate the technique, we measure Friedel oscillations and the subtle tunnel barrier difference between the hcp and fcc stacked regions of the Au(111) herringbone reconstruction. Finally, we show that our method is also applicable to open feedback loop measurements by performing grid I(V) spectroscopy.

Fast spin flips for entangled atoms

Spin Physics The integration of pump-probe spectroscopy with scanning tunneling microscopy (STM) tools has allowed studies of spin relaxation in atoms on the nanosecond time scale. However, following the free evolution of a pair of entangled spins requires faster initial excitation than can be delivered with microwave pulses. Veldman et al. sequentially combined electron spin resonance–STM and direct-current pump-probe techniques to instantaneously flip spins while preserving spin coherence. They used this method to follow free, coherent flip-flop evolution of two coupled spin-1/2 atoms (hydrogenated titanium atom dimers) on a magnesium oxide surface. Science , abg8223, this issue p. [964][1] [1]: /lookup/doi/10.1126/science.abg8223