Surface evolution of thermoelectric material KCu_(4)Se_(3) explored by scanning tunneling microscopy

Novel two-dimensional thermoelectric materials have attracted significant attention in the field of thermoelectric due to their low lattice thermal conductivity.A comprehensive understanding of their microscopic structures is crucial for driving further the optimization of materials properties and developing novel functional materials.Here,by using in situ scanning tunneling microscopy,we report the atomic layer evolution and surface reconstruction on the cleaved thermoelectric material KCu_(4)Se_(3) for the first time.We clearly revealed each atomic layer,including the naturally cleaved K atomic layer,the intermediate Se^(2-)atomic layer,and the Se^(-)atomic layer that emerges in the thermodynamic-stable state.Departing from the maj ority of studies that predominantly concentrate on macroscopic measurements of the charge transport,our results reveal the coexistence of potassium disorder and complex reconstructed patterns of selenium,which potentially influences charge carrier and lattice dynamics.These results provide direct insight into the surface microstructures and evolution of KCu_(4)Se_(3),and shed useful light on designing functional materials with superior performance.

Assessing the dynamics of O_(2) desorption from cobalt phthalocyanine by in situ electrochemical scanning tunneling microscopy

We report here the in situ electrochemical scanning tunneling microscopy(ECSTM) study of cobalt phthalocyanine(CoPc)-catalyzed O_(2) evolution reaction(OER) and the dynamics of CoPc-O_(2) dissociation.The self-assembled CoPc monolayer is fabricated on Au(111) substrate and resolved by ECSTM in 0.1 M KOH electrolyte.The OH^(-)adsorption on CoPc prior to OER is observed in ECSTM images.During OER,the generated O_(2) adsorbed on Co Pc is observed in the CoPc monolayer.Potential step experiment is employed to monitor the desorption of OER-generated O_(2) from CoPc,which results in the decreasing surface coverage of CoPc-O_(2) with time.The rate constant of O_(2) desorption is evaluated through data fitting.The insights into the dynamics of Co-O_(2) dissociation at the molecular level via in situ imaging help understand the role of Co-O_(2) in oxygen reduction reaction(ORR) and OER.

The scanning tunneling microscopy and spectroscopy of GaSb_(1-x)Bi_(x) films of a few-nanometer thickness grown by molecular beam epitaxy

The ultrahigh vacuum scanning tunneling microscope(STM)was used to characterize the GaSb_(1-x)Bi_(x) films of a few nanometers thickness grown by the molecular beam epitaxy(MBE)on the GaSb buffer layer of 100 nm with the GaSb(100)substrates.The thickness of the GaSb_(1-x)Bi_(x) layers of the samples are 5 and 10 nm,respectively.For comparison,the GaSb buffer was also characterized and its STM image displays terraces whose surfaces are basically atomically flat and their roughness is generally less than 1 monolayer(ML).The surface of 5 nm GaSb_(1-x)Bi_(x) film reserves the same terraced morphology as the buffer layer.In contrast,the morphology of the 10 nm GaSb_(1-x)Bi_(x) film changes to the mound-like island structures with a height of a few MLs.The result implies the growth mode transition from the two-dimensional mode as displayed by the 5 nm film to the Stranski-Krastinov mode as displayed by the 10 nm film.The statistical analysis with the scanning tunneling spectroscopy(STS)measurements indicates that both the incorporation and the inhomogeneity of Bi atoms increase with the thickness of the GaSb_(1-x)Bi_(x) layer.

SCANNING TUNNELING MICROSCOPY OF （CrFe）7C3

The microstructure of(CrFe)_7C_3 has been studied with scanning tunneling microscopy.It shows that a carbide consists of colonies which are full of stacking faults. The stacking faults in one colony are parallel while those in different colonies lie at angle with each other.

Graphene/Rh(111) Structure Studied Using In-Situ Scanning Tunneling Microscopy

Scanning tunnel microscopy （STM） is performed to verify if an Rh ＇nails＇ structure is formed accompanying the graphene growing during chemical vapor deposition. A structure of a graphene island in an Rh vacancy island is used as the start. While the graphene island is removed by oxygenation, the variations of the Rh vacancy island are imaged with an in-situ high-temperature STM. By fitting with our model and calculations, we conclude that the best fit is obtained for 0% Rh, i.e., for the complete absence of nails below graphene on Rh（111）. That is, when graphene is formed on Rh（111）, the substrate remains fiat and does not develop a SUPPorting nail structure.

Scanning tunneling microscopy study of molecular growth structures of Gd@C_(82) on Cu(111)

The coverage and temperature-dependent nucleation behaviors of the Gd@C82 metallofullerenes on Cu（111） have been studied by low-temperature scanning tunneling microscopy （LT-STM） in detail. Upon molecular deposition at low temperature, Gd@C82 molecules preferentially decorate the steps and nucleate into single layer islands with increasing coverage. Further annealing treatment leads some of the Gd@C82 molecules to assemble into bright and dim patches, which are correlated to the adsorption induced substrate reconstruction. Upon sufficient thermal activation, Gd@C82 molecules sink into the Cu（111） surface one-copper-layer-deep, forming hexagonal close-packed molecular islands with intra-molecular details observed as striped patterns. By considering the commensurability between the Gd@C82 nearest-neighbor distance and the lattice of the underlying Cu（111）, we clearly identified two kinds of in-plane molecular arrangements as （19（1/2）×19（1/2））R23.4°and （19（1/2）×19（1/2））R36.6°with respect to Cu（111）. Within the assembled Gd@C82 molecular, island molecules with dim–bright contrast are spatially distributed, which may be modulated by the preexisted species on Cu（111）.

Electronic phase diagram of NaFe_(1-x)Co_xAs investigated by scanning tunneling microscopy

Our recent scanning tunneling microscopy （STM） studies of the NaFelxCoxAs phase diagram over a wide range of dopings and temperatures are reviewed. Similar to the high-Tc cuprates, the iron-based superconductors lie in close proximity to a magnetically ordered phase. Therefore, it is widely believed that magnetic interactions or fluctuations play an important role in triggering their Cooper pairings. Among the key issues regarding the electronic phase diagram are the properties of the parent spin density wave （SDW） phase and the superconducting （SC） phase, as well as the interplay between them. The NaFe l-xCoxAs is an ideal system for resolving these issues due to its rich electronic phases and the charge-neutral cleaved surface. In our recent work, we directly observed the SDW gap in the parent state, and it exhibits unconventional features that are incompatible with the simple Fermi surface nesting picture. The optimally doped sample has a single SC gap, but in the underdoped regime we directly viewed the microscopic coexistence of the SDW and SC orders, which compete with each other. In the overdoped regime we observed a novel pseudogap-like feature that coexists with supercon- ductivity in the ground state, persists well into the normal state, and shows great spatial variations. The rich electronic structures across the phase diagram of NaFel_xCoxAs revealed here shed important new light for defining microscopic models of the iron-based superconductors. In particular, we argue that both the itinerant electrons and local moments should be considered on an equal footing in a realistic model.

FABRICATION OF THE TIP FOR ELECTROCHEMICAL SCANNING TUNNELING MICROSCOPY

A nover technique for the fabrication of the tip for e tectrochemical scanning tunneting microscopy(ECSTM)is presented. The curvature radius of the fabricated tip is smatter than 1 μM. Faradaic leakage current is tess than 0.1nA in the sotution of 1 mol/L NaCl. The atomic image of highty oriented pyrotytic graphite (HOPG)has been taken using the prepared tip.

Direct Observation of Transition Metal Dichalcogenides in Liquid with Scanning Tunneling Microscopy

We present atomic-resolution images of TiSe2,MoTe2 and TaS2 single crystals in liquid condition using our home-built scanning tunneling microscopy(STM).By facilely cleaving of single crystals in liquid,we were able to keep the fresh surface not oxidized within a few hours.Using the high-stable home-built STM,we have obtained atomic resolution images of TiSe2 accompanied with the single atom defects as well as the triangle defects in solution for the first time.Besides,the superstructure of MoTe2 and hexagonal chargedensity wave domain structure in nearly commensurate phase of TaS2 were also obtained at room temperature(295 K).Our results provide a more efficient method in investigating the lively surface of transition metal dichalcogenides.Besides,the high stable liquid-phase STM will support the further investigations in liquid-phase catalysis or electrochemistry.

SURFACE PRESSURE,SURFACE POTENTIAL AND SCANNING TUNNELING MICROSCOPY STUDIES FOR N-DOCOSYLPYRIDINIUM-TCNQ MONOLAYER

Simultaneous measurements of surface pressure and surface potential and scanning tunneling microscopy study for N-docosylpyridinium- TCNQ monolayer were carried out.These methods allow us to get more informations on properties of the monolayer.The molecules at the final stage of compression are really in compact stack although a voluminous hydrophilic head exists in the molecule.

Probing the Majorana bound states in a hybrid nanowire double-quantum-dot system by scanning tunneling microscopy

We propose an interferometer composing of a scanning tunneling microscope(STM),double quantum dots(DQDs),and a semiconductor nanowire carrying Majorana bound states(MBSs)at its ends induced by the proximity effect of an s-wave superconductor,to probe the existence of the MBSs in the dots.Our results show that when the energy levels of DQDs are aligned to the energy of MBSs,the zero-energy spectral functions of DQDs are always equal to 1/2,which indicates the formation of the MBSs in the DQDs and is also responsible for the zero-bias conductance peak.Our findings suggest that the spectral functions of the DQDs may be an excellent and convenient quantity for detecting the formation and stability of the spatially separated MBSs in quantum dots.

Molecular Beam Epitaxy Growth and Scanning Tunneling Microscopy Study of Pyrite CuSe2 Films on SrTiO3

We perform molecular beam epitaxy growth and scanning tunneling microscopy study of copper diselenide （CuSe2 ） films on SrTiO3 （001）. Using a Se-rich condition, the single-phase pyrite CuSe2 grows in the Stranski-Krastanov （layer-plus-island） mode with a preferential orientation of （111）. Our careful inspection of both the as-grown and post-annealed CuSe2 films at various temperatures invariably shows a Cu-terminated surface, which, depending on the annealing temperature, reconstructs into two distinct structures 2 ×√3 and √x ×√3-R30°. The Cu termi- nation is supported by the depressed density of states near the Fermi level, measured by in-situ low temperature scanning tunneling spectroscopy. Our study helps understand the preparation and surface chemistry of transition metal pyrite dichalcogenides thin films.

Some Unusual Features of Highly Oriented Pyrolytic Graphite Observed by Electrochemical Scanning Tunneling Microscopy

Highly oriented pyrolytic graphite (HOPG) is the substrate often used in scanningtunneling Ancroscopy (STM). It is well known that STM images of the basal plane of HOPG showsome unusual structUral patterns. In this letter, we present in situ STM images of some unusualfeatures on HOPG in solutions, including normal or abnormal chain-like featUres and hexagonal oroblique superPeriodic structures. These features emerge both next to and apart from the step ofHOPG.

Electrochemical Scanning Tunneling Microscopy──An Ex－situ Study of the Growth of Polypyrrole Films on Electrodes

his paper covers the electropolymerized polypyrrole doped with p-toluenesul-fonate studied using scanning tunneling microscope. The growth processes of polypyrrole films on the electrode surface were observed. Polypyrrole chain struc-ture with many separated micro-holes of 0. 15 nm diameter at the initial state ofelectropolymerization can be seen from obtained STM images. This structure is ex-tremely similar to the pioneer-proposed ideal structure of polypyrrole chains. Thereexisted some helical structure and simple strands on the electrode surface, which isdue to different existing states of polypyrrole chains on electrode surface. STM ex-periments showed that polypyrrole chains were firstly formed on the electrode sur-face, then, they growed gradually by 3-dimensional mode during slow oxidation toform many separated micro-islands. Finally, a large corrugation area of polymerfilms occurred on the electrodes.

Topography Structure and Scanning Tunneling Spectrum of Nickel( Ⅱ )-tetraphenylporphyrin Molecules on Au(111)

Scanning tunneling microscopy （STM） and scanning tunneling spectroscopy （STS） were performed on monolayer film of NiTPP supported on Au（111） under ultrahigh vacuum （UHV） conditions. The constant current STM images show remarkable bias dependence. High resolution STM data clearly show the individual NiTPP molecules and allow easy differentiation between NiTPP and CoTPP reported before. Scanning tunneling spectra, as a function of molecule-tip separation, were acquired over a range of tip motion of 0.42 nm. Spectra do not show the variation in band splitting with tip distance. It appears for molecules such as NiTPP that the average potential at the molecule is essentially the same at the same metal substrate. For molecules of the height of NiTPP, the scanning tunneling spectra should give reliable occupied and unoccupied orbital energies over a wide range of tip-molecule distances.

Image charge effect on the light emission of rutile TiO2(110) induced by a scanning tunneling microscope

The plasmon-enhanced light emission of rutile TiO2（110） surface has been investigated by a low-temperature scanning tunneling microscope （STM）. We found that the photon emission arises from the inelastic electron tunneling between the STM tip and the conduction band or defect states of TiO2（110）. In contrast to the Au（111） surface, the maximum photon energy as a function of the bias voltage clearly deviates from the linear scaling behavior, suggesting the non-negligible effect of the STM tip on the band structure of TiO2. By performing differential conductance （dl/dV） measurements, it was revealed that such a deviation is not related to the tip-induced band bending, but is attributed to the image charge effect of the metal tip, which significantly shifts the band edges of the TiO2（110） towards the Femi level （EF） during the tunneling process. This work not only sheds new lights onto the understanding of plasmon-enhanced light emission of semiconductor surfaces, but also opens up a new avenue for engineering the plasmon-mediated interfacial charge transfer in molecular and semiconducting materials.

Bimodal growth of Fe islands on graphene

Magnetic metals deposited on graphene hold the key to applications in spintronics. Here, we present the results of Fe islands grown on graphene/Si C(0001) by molecular beam epitaxy, which are investigated by scanning tunneling microscopy. The two types of islands distinguished by flat or round tops are revealed, indicating bimodal growth of Fe. The atomic structures on the top surfaces of flat islands are also clearly resolved. Our results may improve the understanding of the mechanisms of metals deposited on graphene and pave the way for future spintronic applications of Fe/graphene systems.

Superconducting state in Ba_((1-x)) Sr_(x)Ni_(2)As_(2) near the quantum critical point

In the phase diagram of the nickel-based superconductor Ba_(1-x)Sr_(x)Ni_(2)As_(2),T_(C) has been found to be enhanced sixfold near the quantum critical point(QCP) x=0.71 compared with the parent compound.However,the mechanism is still under debate.Here,we report a detailed investigation of the superconducting properties near the QCP(x≈0.7) by utilizing scanning tunneling microscopy and spectroscopy.The temperature-dependent superconducting gap and magnetic vortex state were obtained and analyzed in the framework of the Bardeen-Cooper-Schrieffer model.The ideal isotropic s-wave superconducting gap excludes the long-speculated nematic fluctuations while preferring strong electron-phonon coupling as the mechanism for T_(C) enhancement near the QCP.The lower than expected gap ratio of Δ/(k_(B) T_(C)) is rooted in the fact that Ba_(1-x)Sr_(x)Ni_(2)As_(2) falls into the dirty limit with a serious pair breaking effect similar to the parent compound.

Growth and characterization of Bi(110)/CrTe_(2) heterostructures:Exploring interplay between magnetism and topology

The interplay between topology and magnetism is vital for realizing exotic quantum phenomena,significant examples including quantum anomalous Hall effect,axion insulators,and high-order topological states.These states host great potential for future applications in high-speed and low-consumption electronic devices.Despite being extensively investigated,practical platforms are still scarce.In this work,with molecular beam epitaxy(MBE),we provide the first experimental report on high-quality Bi(110)/CrTe_(2) magnetic heterostructure.By employing in-situ high-resolution scanning tunneling microscopy,we are able to examine the interaction between magnetism and topology.There is a potential edge state at an energy level above the Fermi level,but no edge states observed near the Fermi level The absence of high-order topological corner states near EF highlights the importance of lattice matching and interface engineering in designing high-order topological states.Our study provides key insights into the interplay between two-dimensional magnetic and topological materials and offers an important dimension for engineering magnetic topological states.

Field induced Chern insulating states in twisted monolayer–bilayer graphene

Unraveling the mechanism underlying topological phases, notably the Chern insulators(Ch Is) in strong correlated systems at the microscopy scale, has captivated significant research interest. Nonetheless, Ch Is harboring topological information have not always manifested themselves, owing to the constraints imposed by displacement fields in certain experimental configurations. In this study, we employ density-tuned scanning tunneling microscopy(DT-STM) to investigate the Ch Is in twisted monolayer–bilayer graphene(t MBG). At zero magnetic field, we observe correlated metallic states.While under a magnetic field, a metal–insulator transition happens and an integer Ch I is formed emanating from the filling index s = 3 with a Chern number C = 1. Our results underscore the pivotal role of magnetic fields as a powerful probe for elucidating topological phases in twisted Van der Waals heterostructures.