Scanning Tunneling Microscopy Investigation Research Articles

Comprehensive analysis of charge carriers dynamics through the honeycomb structure of graphite thin films and polymer graphite with applications in cold field emission and scanning tunneling microscopy

Polymer graphite electron sources have performed satisfactorily as field emission emitters and scanning tunneling microscopy probes in the past few years. However, the emission process was characterized by limited total emission currents. This paper introduces the elemental, vibrational, electronic structure, and optical analysis of polymer graphite and glass-graphite composite field emission cathodes to study these limitations. Moreover, the field emission characteristics are studied including the changes in the potential energy barrier of the used materials and structures. Among the studied structures, the cathodes prepared from graphite thin films deposited on a micropointed glass substrate (film-GMF) showed superior performance as random field emission arrays. This includes obtaining much higher emission current values ≈ 20 times) and lower threshold voltages ≈ 1/2) compared to the results obtained from polymer graphite samples. The enhancement factor in such emitters is believed to be the three-dimensional honeycomb structure of graphite. Moreover, the study includes applying graphite coatings to tungsten nano-field emission cathodes and scanning tunneling microscopy probes, which improves the performance of such cathodes/probes in both microscopic techniques.

Boosting spatial and energy resolution in STM with a double-functionalized probe.

The scattering of superconducting pairs by magnetic impurities on a superconducting surface leads to pairs of sharp in-gap resonances known as Yu-Shiba-Rusinov (YSR) bound states. Similar to the interference of itinerant electrons scattered by defects in normal metals, these resonances reveal a periodic texture around the magnetic impurity. The wavelength of these resonances is, however, often too short to be resolved even by methods capable of atomic resolution, i.e., scanning tunneling microscopy (STM). We combine a CO molecule with a superconducting cluster pre-attached to an STM tip to maximize both spatial and energy resolution, thus demonstrating the superior properties of such double-functionalized probes by imaging the spatial distribution of YSR states. Our approach reveals rich interference patterns of the hybridized YSR states of two Fe atoms on Nb(110), previously inaccessible with conventional STM probes. This advancement extends the capabilities of STM techniques, providing insights into superconducting phenomena at the atomic scale.

Advancing scanning probe microscopy simulations: A decade of development in probe-particle models

The Probe-Particle Model combine theories designed for the simulation of scanning probe microscopy experiments, employing non-reactive, flexible tip apices to achieve sub-molecular resolution. In the article we present the latest version of the Probe-Particle Model implemented in the open-source ppafm package, highlighting substantial advancements in accuracy, computational performance, and user-friendliness. To demonstrate this we provide a comprehensive review of approaches for simulating non-contact Atomic Force Microscopy. They vary in complexity from simple Lennard-Jones potential to the latest full density-based model. We compared those approaches with ab initio calculated references, showcasing their respective merits. All parts of the ppafm package have undergone acceleration by 1-2 orders of magnitude using OpenMP and OpenCL technologies. The updated package includes an interactive graphical user interface and seamless integration into the Python ecosystem via pip, facilitating advanced scripting and interoperability with other software. This adaptability positions ppafm as an ideal tool for high-throughput applications, including the training of machine learning models for the automatic recovery of atomic structures from nc-AFM measurements. We envision significant potential for this application in future single-molecule analysis, synthesis, and advancements in surface science in general. Additionally, we discuss simulations of other sub-molecular scanning-probe imaging techniques, such as bond-resolved scanning tunneling microscopy and kelvin probe force microscopy, all built on the robust foundation of the Probe-Particle Model. Altogether this demonstrates the broad impact of the model across diverse domains of on-surface science and molecular chemistry.

Modeling the phase boundaries in Cu(100)-(2√2 × √2)R45°-O missing row reconstruction (MRR) structure

Although the structure of Cu(100)-(22×2)R45°-O missing row reconstruction (MRR) has been well-established for decades, the detailed structure of its various boundaries remains an untilled area due to the difficulties in obtaining atomically resolved images. Herein, atomic arrangement of the phase boundaries existing in MRR structure was modeled on the basis of scanning tunneling microscopy (STM) investigations. By determining the periodicity and unit structure of MRR in STM images and extending them to boundary region, several types of phase boundaries were identified, resulted respectively from: (1) the mismatch between c(2 × 2)-O patches, (2) the regulation by step edges, and (3) the mismatch between Cu missing rows (MRs). With the modeled structure, it was revealed that the types of the c(2 × 2)-O mismatch induced phase boundaries (OMIPBs) are mainly dominated by the oxygen exposure and in-diffusion barrier. The step edge regulated phase boundaries (SERPBs) are always terminated with Cu-O chain and may represent an intermediate growth stage to larger MRR structure. Comparatively, Cu MRs mismatch is often reconciled by the differently oriented domains between them. As a result, the Cu MRs mismatch induced phase boundaries (CMRMIPBs) are only occasionally observed as Cu-O chains between mismatched Cu MRs that encounter shoulder-to-shoulder. For all studied boundaries, the surrounding MRR domains exhibit obvious orientation preference through inclined packing along the SP direction with the degree closely related with the width of the boundaries.

Transport Property Evolution in 2H‐MoTe2−x Mediated by Te‐Deficiency‐Induced Mirror Twin Boundary Networks

The 2H‐MoTe2 is a well‐known layered 2D semiconductor that is considered as a promising material for next‐generation microelectronic and optoelectronic devices. Te‐deficiency‐induced defective structures, like Te vacancy and mirror twin boundary (MTB), would be generated at elevated temperatures. However, the temperature‐dependent evolution of such defects and their influence on the macroscopic electrical transport property of 2H‐MoTe2 is unclear. Herein, the semiconductor–metal transition phenomenon in 2H‐MoTe2−x mediated by the evolving disordered MTB network with increasing Te deficiency is reported on. The samples are grown by molecular beam epitaxy, while the Te deficiency is tuned by post‐growth flash annealing in ultra‐high vacuum. Low‐temperature scanning tunneling microscope investigation discloses the medium‐range disorder evolution of the MTB network incorporated in the 2H‐MoTe2, which eventually transforms to an ordered metallic Mo5Te8 metastable phase. The scanning tunneling spectroscopy shows rich in‐gap states localized at the MTBs, which provide a conducting channel in the semiconductor. The ultra‐high vacuum in situ transport measurement shows a gradual decrease of resistance of the sample upon flash annealing from 50 to 480 °C, confirming the influence of Te deficiency on the transport property, which would play an essential role in the device performance and durability.

Scanning Tunneling Microscopy Investigation of Synaptic Behavior in AgInS2 Quantum Dots: Effect of Ion Transport in Neuromorphic Applications

Scanning Tunneling Microscopy Investigation of Synaptic Behavior in AgInS₂ Quantum Dots: Effect of Ion Transport in Neuromorphic Applications

Topologically engineering of π-conjugated macrocycles: Tunable emission and photochemical reaction toward multi-cyclic polymers

The topology of conjugated macrocycles had significant impacts on their photo-physical and photo-chemical properties. Herein, a series of π-conjugated macrocycles with diverse topology were synthesized via intramolecular McMurry coupling. Their chemical structure and macrocyclic topology were unambiguously confirmed via NMR, MALDI-TOF mass spectra, crystal analysis and scanning tunneling microscopy (STM). Depending on the structural topology and structural rigidity, these cyclic compounds display obviously distinctive emission behavior and photochemical reactions in the solution and in the solid state. Monocyclic phenylene vinylene macrocycle (denoted as MST) exhibiting aggregation-induced emission behavior, was more vulnerable to photo-cyclization in solution and triplet sensitizer promoted photo-dimerization due to lower strain and more flourishing intramolecular motions. After UV light irradiation, relatively more flexible MST could yield the anti-dimer via triplet excimer on the HOPG surface confirmed by STM investigation. By contrast, highly constrained bicyclic analogue (named as DMTPE) with central tetraphenylethene core, displayed high emission quantum yields of 68% both in solution and in the solid state, and was relatively inert to photochemical reactions and yield syn-dimer on the surface via singlet excimer involved [2 + 2] photo-dimerization. Based on the solution-mediated photo-polymerization of MST moiety, multicyclic porous carbon-rich ribbon connected with four-membered ring was successfully constructed and validated via STM imaging.

Scanning tunneling microscopy investigation of self-assembly behavior of C60 on an intrinsically patterned Ag5Se2 surface

Extensive research has been conducted on the use of surface nanotemplates, whether artificially or naturally patterned, to regulate the self-assembly processes on surfaces. It has become clear that in order to gain a better understanding and control over the formation of well-ordered two-dimensional nanostructure arrays, detailed investigations into the nucleation and growth of adsorbates (molecules, atoms, or clusters) on the surface nanotemplates are essential. The present study focuses on the self-assembly structure, nucleation, and growth process of C60 molecules on the patterned Ag5Se2/Ag(111) surface, utilizing low-temperature scanning tunneling microscopy. The findings reveal that C60 molecules exhibit a preference for appearing at the triangular domain boundaries on the Ag5Se2/Ag(111) surface. As coverage increases, the molecules continue to grow across regions, ultimately forming a densely packed island self-assembled structure. Owing to the weak interaction between C60 molecules and Ag5Se2/Ag(111), the molecules adopt various orientation configurations. This research contributes valuable insights into the fabrication of carbon nanodevices on patterned substrates.

Flexible Phenanthracene Nanotubes for Explosive Detection.

Phenanthracene nanotubes with arylene-ethynylene-butadiynylene rims and phenanthracene walls are synthesized in a modular bottom-up approach. One of the rims carries hexadecyloxy side chains, mediating the affinity to highly oriented pyrolytic graphite. Molecular dynamics simulations show that the nanotubes are much more flexible than their structural formulas suggest: In 12, the phenanthracene units act as hinges that flip the two macrocycles relative to each other to one of two possible sites, as quantum mechanical models suggest and scanning tunneling microscopy investigations prove. Unexpectedly, both theory and experiment show for 13 that the three phenanthracene hinges are deflected from the upright position, accompanied by a deformation of both macrocycles from their idealized sturdy macroporous geometry. This flexibility together with their affinity to carbon-rich substrates allows for an efficient host-guest chemistry at the solid/gas interface opening the potential for applications in single-walled carbon nanotube-based sensing, and the applicability to build new sensors for the detection of 2,4,6-trinitrotoluene via nitroaromatic markers is shown.

On-surface polymerization reactions of dibrominated hexaphenylbenzene influenced by densely packed self-assembly.

Controlled bottom-up fabrication of molecular nanostructures through on-surface reactions of tailor-made precursors is of scientific and technological interest. Recently, on-surface polymerization reactions influenced by precursor self-assembly have been reported. Thus, a fundamental understanding of the reaction process is a prerequisite for controlled formation. Herein, we report on the influence of molecular self-assembly of dibrominated hexaphenylbenzene (Br2-HPB) on the on-surface polymerization reactions on a Au(111) substrate. By using low-temperature scanning tunnelling microscopy (STM), we find that the polymerization of Br2-HPB proceeds while maintaining the long-range ordered self-assembly, despite a decrease in HPB-HPB distance due to debromination and successive covalent bonding of Br2-HPB. From the STM investigations of the polymerization process, we conclude that the polymerization of Br2-HPB is accompanied by molecular rotations to maintain the periodic array of the self-assembled structure, contrary to the conventional understanding of the polymerization of the self-assembled precursor layer.

Unraveling the Dynamic Processes of Methanol Electrooxidation at Isolated Rhodium Sites by In Situ Electrochemical Scanning Tunneling Microscopy.

Materials with isolated single-atom Rh-N4 sites are emerging as promising and compelling catalysts for methanol electrooxidation. Herein, we carried out an in situ electrochemical scanning tunneling microscopy (ECSTM) investigation of the dynamic processes of methanol absorption and catalytic conversion in the rhodium octaethylporphyrin (RhOEP)-catalyzed methanol oxidation reaction at the molecular scale. The high-contrast RhOEP-CH3OH complex formed by methanol adsorption was visualized distinctly in the STM images. The Rh-C adsorption configuration of methanol on isolated rhodium sites was identified on the basis of a series of control experiments and theoretical simulation. The adsorption energy of methanol on RhOEP was obtained from quantitative analysis. In situ ECSTM experiments present an explicit description of the transformation of the intermediate species in the catalytic process. By qualitatively evaluating the rate constants of different stages in the reaction at the microscopic level, we considered the CO transformation/desorption as the critical step for determining the reaction dynamics. Methanol adsorption was found to be correlated with RhOEP oxidation in the initial stage of the reaction, and the dynamic information was revealed unambiguously by in situ potential step experiments. This work provides microscopic results for the catalytic mechanism of Rh-N4 sites for methanol electrooxidation, which is instructive for the rational design of the high-performance catalyst.

Three-dimensional printed microcantilever with mechanical metamaterial for fiber-optic microforce sensing

Mechanical metamaterials can adjust mechanical properties of structures flexibly through a mechanical structural design based on the premise that the materials remain unchanged. Here, a cantilever probe microstructure is designed using mechanical metamaterials for an optical fiber microforce sensor tip that can be prepared by femtosecond laser-induced two-photon polymerization. The elastic constant k of the fabricated fiber-optic microforce sensor has been adjusted by two orders of magnitude from 0.165 to 46 N/m, and the geometric configuration of the cantilever beam can be tailored to match the mechanical properties of biological specimens. This fiber microforce sensor shows an ultra-high force sensitivity of 154 nm/µN and a force resolution of up to 130 pN. The optical fiber microforce sensor that shows the lowest force resolution in a direct-contact mode has high potential for biosensing applications, and the results reveal a potential design strategy for special scanning tunneling microscope probes with unique physical properties.

Accurate Simulations of Scanning Tunneling Microscope: Both Tip and Substrate States Matter.

Scanning tunneling microscope (STM) provides an atomic-scale characterization tool. To this end, high-resolution measurements and accurate simulations must closely cooperate. Emerging experimental techniques, e.g., substrate spacers and tip modifications, suppress metallic couplings and improve the resolution. On the other hand, development of STM simulation methods was inactive in the past decade. Conventional simulations focus on the electronic structure of the substrate, often overlooking detailed descriptions of the tip states. Meanwhile, the overwhelming usage of periodic boundary conditions ensures effective simulations of only neutral systems. In this Perspective, we highlight the recent progress that takes the effects of both tip and substrate into account under either Tersoff-Hamann or Bardeen's approximation, which provides an accurate analysis of measured high-resolution STM results, uncovers underlying concepts, and rationally designs experimental protocols for important chemical systems. We hope this Perspective will stimulate broad interest in advanced STM simulations, highlighting the way forward for STM investigations that involve complex geometrical and electronic structures.

Adsorption and reversible conformational change of a thiophene based molecule on Au(111)

We present a low temperature scanning tunneling microscope investigation of a prochiral thiophene-based molecule that self-assembles forming islands with different domains on the Au(111) surface. In the domains, two different conformations of the single molecule are observed, depending on a slight rotation of two adjacent bromothiophene groups. Using voltage pulses from the tip, single molecules can be switched between the two conformations. The electronic states have been measured with scanning tunneling spectroscopy, showing that the electronic resonances are mainly localized at the same positions in both conformations. Density-functional theory calculations support the experimental results. Furthermore, we observe that on Ag(111), only one configuration is present and therefore the switching effect is suppressed.

Impact of External Electronic Perturbations on Single-Walled Carbon Nanotube Electronic Structure: Scanning Tunneling Spectroscopy and Density Functional Theory

Understanding the local impact of environmental electronic perturbations on the local density of states (LDOS) of single-walled carbon nanotubes (CNTs) is critical for developing CNT-based devices. We present scanning tunneling microscopy and spectroscopy (STM/STS) investigations of CNTs adsorbed on both a metal, Au(111), and a dielectric, monolayer RbI on Au(111), serving as models for stronger and weaker electrostatic interactions, respectively. In both cases, STS revealed modulations in the CNT LDOS corresponding to features in the underlying material. We corroborate our STM/STS results with density functional theory calculations of the electronic structure of semiconducting CNTs in the presence and absence of an external dipole. DFT-calculated CNT LDOS qualitatively matched STM/STS results, providing key insight into the local impact external charges have on CNT electronic structure.

The quantum twisting microscope.

The invention of scanning probe microscopy revolutionized the way electronic phenomena are visualized1. Whereas present-day probes can access a variety of electronic properties at a single location in space2, a scanning microscope that can directly probe the quantum mechanical existence of an electron at several locations would provide direct access to key quantum properties of electronic systems, so far unreachable. Here, we demonstrate a conceptually new type of scanning probe microscope-the quantum twisting microscope (QTM)-capable of performing local interference experiments at its tip. The QTM is based on a unique van der Waals tip, allowing the creation of pristine two-dimensional junctions, which provide a multitude of coherently interfering paths for an electron to tunnel into a sample. With the addition of a continuously scanned twist angle between the tip and sample, this microscope probes electrons along a line in momentum space similar to how a scanning tunnelling microscope probes electrons along a line in real space. Through a series of experiments, we demonstrate room-temperature quantum coherence at the tip, study the twist angle evolution of twisted bilayer graphene, directly image the energy bands of monolayer and twisted bilayer graphene and, finally, apply large local pressures while visualizing the gradual flattening of the low-energy band of twisted bilayer graphene. The QTM opens the way for new classes of experiments on quantum materials.

Direct measurement of band offsets on selective area grown In0.53Ga0.47As/InP heterojunction with multiple probe scanning tunneling microscopy

The knowledge of the band alignment in semiconductor heterostructures is crucial, as it governs carrier confinement with many impacts on the performances of devices. By controlling the direction of the current flow in in-plane In0.53Ga0.47As/InP heterostructure nanowires, either horizontally along the nanowires or vertically into the InP substrate with low temperature multiple-probe tunneling spectroscopy, a direct measurement of the band offsets at the buried In0.53Ga0.47As/InP heterointerface is performed. Despite the unavoidable processing steps involved in selective area epitaxy, conduction and valence band offsets of 0.21 ± 0.01 and 0.40 ± 0.01 eV are, respectively, found, indicating the formation of an interface with a quality comparable to two-dimensional In0.53Ga0.47As/InP heterostructures.

Higher-indexed Moiré patterns and surface states of MoTe2/graphene heterostructure grown by molecular beam epitaxy

Stabilization of the 2H phase of MoTe2 during molecular beam epitaxy (MBE) growth on graphene terminated 6H-SiC(0001) is highly desirable in order to take advantage of its promising properties in electronic applications. By properly adjusting the conditions, direct growth of the highly crystalline 2H phase of MoTe2 has been achieved. In such van der Waals heterostructure, the atomically-clean interface between graphene and MoTe2 permits the electronic coupling between the adjacent layers and the emergence of a high variety of Moiré patterns. In this paper, we investigate a single layer of 2H-MoTe2 grown on graphene by MBE and we present scanning tunneling microscopy (STM) investigations combined with density functional theory (DFT) calculations and simulations of STM images. Our results show that the STM images of the MoTe2/graphene heterostructure surprisingly amplify the otherwise weak Moiré potential modulations leading to the appearance of unique higher-indexed Moiré patterns. These patterns are unusually rich with many Fourier-overtones and show a remarkable variety of different applied bias voltages, revealing the complex electronic features of the heterostructure.

Tuning the morphological and electrical properties of graphite surface by self-assembled viologen nanostructures

In this contribution, a combination of cyclic voltammetry (CV), atomic force microscopy (AFM), electrochemical scanning tunneling microscopy (EC-STM) and Kelvin probe force microscopy (KPFM) is used to characterize the electrochemical, structural and electronic properties of the dibenzyl viologen (DBV) molecules based adlayers electrochemically deposited on highly oriented pyrolytic graphite (HOPG) surface. The adlayer formations on HOPG are dependent on its redox states in-situ modulated by the electrode potential and the used concentration. Relying on the intrinsic solubility of individual reduced molecules one enables permanent modulation the modification degree of HOPG surface. In addition, at the potentials where HOPG surface is merely modified by DBV0 molecules, the electrical properties of HOPG surface can also be modulated by various DBV2+ concentrations as the variable.

Synthesis and Characterization of peri‐Heptacene on a Metallic Surface

AbstractThe synthesis of long n‐peri‐acenes (n‐PAs) is challenging as a result of their inherent open‐shell radical character, which arises from the presence of parallel zigzag edges beyond a certain n value. They are considered as π‐electron model systems to study magnetism in graphene nanostructures; being potential candidates in the fabrication of optoelectronic and spintronic devices. Here, we report the on‐surface formation of the largest pristine member of the n‐PA family, i.e. peri‐heptacene (n=7, 7‐PA), obtained on an Au(111) substrate under ultra‐high vacuum conditions. Our high‐resolution scanning tunneling microscopy investigations, complemented by theoretical simulations, provide insight into the chemical structure of this previously elusive compound. In addition, scanning tunneling spectroscopy reveals the antiferromagnetic open‐shell singlet ground state of 7‐PA, exhibiting singlet–triplet spin‐flip inelastic excitations with an effective exchange coupling (Jeff) of 49 meV.