Scanning Tunneling Microscopy Study Research Articles

Adsorption of molecular oxygen on the Ag(111) surface: A combined temperature-programmed desorption and scanning tunneling microscopy study.

The adsorption of O2 on Ag(111) between 300 and 500 K has been studied with temperature-programmed desorption (TPD) and scanning tunneling microscopy (STM). At the first stage of adsorption, the disordered local oxide phase (commonly looking in STM as an array of black spots) is formed on the surface irrespective of the substrate temperature. The maximum concentration of black spots was found to be ≈0.11 ML, which corresponds to an oxygen coverage of ≈0.66 ML. Taking into account that the nucleation of the Ag(111)-p(4 × 4)-O phase starts after the saturation of the disordered phase, one can conclude that its coverage is at least not less than 0.66 ML. The analysis of STM and TPD data shows that the thermodesorption peak (m/e = 32) at 570 K is related exclusively to the decomposition of the p(4 × 4) phase, while the local oxide phase does not contribute to desorption.

Atomic visualization of copper oxide structure in the infinite-layer cuprate SrCuO2

We report the atomic-scale structure of epitaxial films of the parent infinite-layer compound $\mathrm{SrCu}{\mathrm{O}}_{2}$ prepared on ${\mathrm{SrTiO}}_{3}$ by molecular beam epitaxy. In situ scanning tunneling microscopy study reveals a stoichiometric copper oxide (${\mathrm{CuO}}_{2}$) -terminated surface featured by $2\ifmmode\times\else\texttimes\fi{}2$ reconstruction, caused primarily by structural distortions of four adjacent ${\mathrm{CuO}}_{2}$ plaquettes. Furthermore, the subsurface Sr atoms have been rarely discernible, showing intra-unit-cell rotational symmetry breaking. These observations can be reasonably modeled by a periodic up-down buckling of oxygen ions on the ${\mathrm{CuO}}_{2}$ plane. Further postannealing leads to the removal of surface oxygens and an incommensurate stripe phase. Our study provides indispensable structural information to help understand the exotic properties of cuprate superconductors.

X-ray structural analysis of an epitaxially grown Ag film/Si(111)-B substrate interface

We investigated the structure of the Si(111)-B surface and epitaxially grown Ag(111) film/Si(111)-B substrate interface using surface X-ray diffraction. An analysis of their crystal truncation rod and fractional order rod scatterings revealed a good agreement between the structure of the Si(111)-B surface and previous theoretical studies when taking into account structural inhomogeneity caused by subsurface defects. At the Ag film/Si(111)-B substrate interface, the Si(111)-B surface reconstruction was maintained intact. The experimentally determined positions of Ag atoms were displaced from the ideal Ag(111) bulk sites, and a 3 × 3 periodicity was introduced to the bottom layer of the Ag film at the interface. The 3 × 3 periodic corrugation was transferred, albeit damped, to the upper layer. This transfer was considered to be the origin of the 3 × 3 periodic pattern at the Ag film surface on the Si(111)-B substrate, a pattern observed in our previous scanning tunneling microscope study.

Quasiparticle Interference Studies of Quantum Materials.

Exotic electronic states are realized in novel quantum materials. This field is revolutionized by the topological classification of materials. Such compounds necessarily host unique states on their boundaries. Scanning tunneling microscopy studies of these surface states have provided a wealth of spectroscopic characterization, with the successful cooperation of ab initio calculations. The method of quasiparticle interference imaging proves to be particularly useful for probing the dispersion relation of the surface bands. Herein, how a variety of additional fundamental electronic properties can be probed via this method is reviewed. It is demonstrated how quasiparticle interference measurements entail mesoscopic size quantization and the electronic phase coherence in semiconducting nanowires; helical spin protection and energy-momentum fluctuations in a topological insulator; and the structure of the Bloch wave function and the relative insusceptibility of topological electronic states to surface potential in a topological Weyl semimetal.

Tunneling Spectra of a Quasifreestanding Graphene Monolayer

With considering the great success of scanning tunnelling microscopy (STM) studies of graphene in the past few years, it is quite surprising to notice that there is still a fundamental contradiction about the reported tunnelling spectra of quasi-free-standing graphene monolayer. Many groups observed V-shape spectra with linearly vanishing density-of-state (DOS) at the Dirac point, whereas, the others reported spectra with a gap of 60 meV pinned to the Fermi level in the quasi-free-standing graphene monolayer. Here we systematically studied the two contradicted tunnelling spectra of the quasi-free-standing graphene monolayer on several different substrates and provided a consistent interpretation about the result. The gap in the spectra arises from the out-of-plane phonons in graphene, which mix the Dirac electrons at the Brillouin zone corners with the nearly free-electron states at the zone center. Our experiment indicated that interactions with substrates could effectively suppress effects of the out-of-plane phonons in graphene and enable us to detect only the DOS of the Dirac electrons in the spectra. We also show that it is possible to switch on and off the out-of-plane phonons of graphene at the nanoscale, i.e., the tunnelling spectra show switching between the two distinct features, through voltage pulses applied to the STM tip.

Anisotropic superconductivity and elongated vortices with unusual bound states in quasi-one-dimensional nickel-bismuth compounds

We report low temperature scanning tunneling microscopy and spectroscopy studies of Ni-Bi films grown by molecular beam epitaxy. Highly anisotropic and twofold symmetric superconducting gaps are revealed in two distinct composites, Bi-rich NiBi3 and near-equimolar NixBi, both sharing quasi-one-dimensional crystal structure. We further reveal axially elongated vortices in both phases, but Caroli-de Gennes-Matricon states solely within the vortex cores of NiBi3. Intriguingly, although the localized bound state splits energetically off at a finite distance ~10 nm away from a vortex center along the minor axis of elliptic vortex, no splitting is found along the major axis. We attribute the elongated vortices and unusual vortex behaviors to the combined effects of twofold superconducting gap and Fermi velocity. The findings provide a comprehensive understanding of the electron pairing and vortex matter in quasi-one-dimensional superconductors

(Invited) In Situ scanning Tunneling Microscopy Studies of hBN Layer Growth Kinetics and the Influence of Substrate on Electronic Structure of the Layers

Two-dimensional (2D) layered materials, owing to a wide range of properties (e.g., graphene is metallic, hBN is insulating, and MoS2 is semiconducting), have attracted considerable attention over the past decade. hBN is isostructural to graphene and is considered an ideal dielectric for the fabrication of vertical 2D layered heterostructures with potential applications in optoelectronics and nanoelectronics. hBN layers are commonly grown on metal substrates. In contrast to free-standing hBN layers, substrate-supported hBN can be metallic, semiconducting, or insulating, depending on the chemistry and electronic structure of the hBN/substrate interface. Moreover, due to relatively weak van der Waals interactions between the 2D layer and the substrate, orientational registry of the layers with the substrate is not expected and is often difficult to control. While the growth and electronic structure of graphene layers have been fairly well studied, relatively little is known concerning the mechanisms underlying the growth of the hBN layers and the influence of hBN domain orientation (with respect to the substrate) on its electronic properties. Using in situ ultra-high vacuum scanning tunneling microscopy (UHV STM), we investigated the chemical vapor deposition (CVD) kinetics of hBN on Pd(111). STM images are acquired during CVD of borazine as a function of substrate temperature, borazine flux, and deposition time. We observe the nucleation and growth of chemisorbed borazine islands on the Pd surfaces. Furthermore, we investigated the surface structure of hBN domains on Pd(111) using STM, scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. STM images acquired from the hBN/Pd(111) sample reveal moiré patterns with at least four different periodicities λ corresponding to four rotational domains of hBN. From the STM images, we measured the surface corrugations in each of the moiré patterns and found that the corrugation amplitude Δz depends on the tunneling bias and increases with increasing λ. We suggest that Δz is a measure of hBN-Pd(111) interaction strength and attribute the higher corrugation amplitudes to stronger interactions between the hBN domains and the Pd(111). We expect that similar approach could be used to investigate growth kinetics and orientation-dependent interactions in other substrate-supported 2D layers with large lattice mismatch with respect to the substrate.

Green's function approach to the Kondo effect in nanosized quantum corrals

We present a theoretical study of the Kondo effect for a magnetic atom placed inside nanocorrals using Green's function calculations. Based on the standard mapping of the Anderson impurity model to a one-dimensional chain model, we formulate a weak-coupling theory to study the Anderson impurities in a hosting bath with a surface state. With further taking into account the multiple scattering effect of the surrounding atoms, our calculations show that the Kondo resonance width of the atom placed at the center of the nanocorral can be significantly tuned by the corral size, in good agreement with recent experiments [Q. L. Li et al., Phys. Rev. B 97, 035417 (2018)]. The method can also be applied to the atom placed at an arbitrary position inside the corral where our calculation shows that the Kondo resonance width also oscillates as the function of its separation from the corral center. The prediction is further confirmed by the low-temperature scanning tunneling microscopy studies where a one-to-one correspondence is found. The good agreement with the experiments validates the generality of the method to the system where multiadatoms are involved.

Lateral Heterostructures Formed by Thermally Converting n-Type SnSe2 to p-Type SnSe.

Different two-dimensional (2D) materials, when combined together to form heterostructures, can exhibit exciting properties that do not exist in individual components. Therefore, intensive research efforts have been devoted to their fabrication and characterization. Previously, vertical and in-plane 2D heterostructures have been formed by mechanical stacking and chemical vapor deposition. Here, we report a new material system that can form in-plane p-n junctions by thermal conversion of n-type SnSe2 to p-type SnSe. Through scanning tunneling microscopy and density functional theory studies, we find that these two distinctively different lattices can form atomically sharp interfaces and have a type II to nearly type III band alignment. We also demonstrate that this method can be used to create micron-sized in-plane p-n junctions at predefined locations. These findings pave the way for further exploration of the intriguing properties of the SnSe2-SnSe heterostructure.

Electronic states of domain structure in 1T-TaS2-xSex observed by STM/STS

We report on a systematic scanning tunneling microscopy and spectroscopy (STM/STS) study on 1T–TaS2-xSex (x = 0, 0.3, 1.0) at 4.2 K. While the compounds with x = 0 and 0.3, which undergoes the Mott transition, showed the commensurate charge density wave (CDW) with the period of (a0 is in-plane lattice constant), the compound with x=1, which shows superconductivity at 3.5 K, exhibits anomalous domain structure: The domain structure consists of regions with regular array of David-stars divided by bright contrasted walls at positive bias voltage. We found the domain wall showed the different electronic state from that of the domain.

Influence of the State of the Tungsten Tip on STM Topographic Images of SnSe Surfaces

Tin selenide (SnSe) has recently attracted significant attention because of its excellent thermoelectric properties with a figure of merit (ZT) of 2.6. Previous scanning tunneling microscopy (STM) studies of SnSe surfaces showed that only Sn atoms are resolved in topographic images due to the dominant contribution of the Sn 5pz states in tunneling. However, when the state of the tungsten (W) tip changes from a typical four-lobe d state such as d xy or $${d_{{x^2} - {y^2}}}$$ to a two-lobe $${d_{{z^2}}}$$ state, the atomic features observed on the SnSe surface in STM topography can be dramatically altered. In this report, we present the results of a systematic study on the influence of the W tip’s states on the STM images of SnSe surfaces. Sn atoms are observed with much stronger corrugation amplitude and smaller apparent radius when the tip is in a $${d_{{z^2}}}$$ state. In addition, the atomic features of the Se atoms become visible because of the sharply focused shape of the W $${d_{{z^2}}}$$ state. We expect our results to provide important information for establishing a better understanding of the microscopic nature of SnSe surfaces.

Superposition of and 3 × 3 supermodulations in TaS2 probed by scanning tunneling microscopy

We report on a scanning tunnelling microscopy study of TaS2 at 4.2 K. A surface prepared by cleavage showed a superimposed pattern of two types of charge density waves with 3a0 × 3a0 and periodicity, which had never been observed previously. We attribute the superposition to regular stacking of 4Hb polytypes or irregular stacking of 2H and 4Hb layers.

Stability of M3S3 complexes on fcc M(111) surfaces: M = Au, Ag, Cu, and Ni

Density Functional Theory is utilized to assess the stability of metal (M)–sulfur (S) complexes adsorbed on fcc M(111) surfaces, specifically considering S-decorated planar M trimers, M3S3. Scanning Tunneling Microscopy studies have identified structures proposed to be Ni3S3 on Ni(111), and Au3S3 on Au(111). Also, Cu3S3 on Cu(111) has been suggested to facilitate enhanced Cu surface mass transport. Our analysis considers M3S3 complexes for M = Au, Ag, Cu, and Ni, assessing key measures of stability on surfaces, and also comparing behavior with trends in gas-phase stability. These surface and gas-phase analyses are systematically related within the framework of Hess’s law, which allows elucidation of various contributions to the overall energetics. In all cases, the adsorbed complex is stable relative to its separated constituents adsorbed on the terrace. However, only for Ag does one find a negative energy of formation from excess S on terraces and M extracted from kink sites along step edges, implying spontaneous complex formation for this pathway. We interpret various experimental observations in the context of our results for energetics.

Adsorption of Terarylenes on Ag(111) and NaCl(001)/Ag(111): A Scanning Tunneling Microscopy and Density Functional Theory Study

Photoswitching materials are building blocks of next generation optoelectronic devices which may require molecule deposition on a solid substrate. However, molecule properties change upon adsorption due to surface-molecule interactions and symmetry considerations. Scanning tunneling microscopy (STM) and density functional theory (DFT) offer techniques to address interactions of functional molecules down to the single-molecular level on solid substrates. In this paper, we present a combined STM and DFT study of a tert-butyl functionalized terarylene molecule on Ag(111) and NaCl(001)/Ag(111) at ∼5 K. tert-Butyl groups aided in identifying three conformations of the compound upon adsorption on the surface. DFT calculations showed that two of these conformations refer to different adsorption geometries of the trans conformation in the gas phase. The other was assigned to the nonreactive cis conformation. For the first time, this conformation was isolated and imaged at the single-molecular level. Calculations ...

Scanning tunneling microscopy and spectroscopy studies of the heavy-electron superconductor TlNi2Se2

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Studies of scanning tunneling spectroscopy on iron-based superconductors

Since the discovery of iron-based superconductors in 2008, it has been a hot topic to research the pairing mechanism of superconductivity. Scanning tunneling microscopy (STM) can be used to detect the electronic information in nano-scale, hence, it is an important tool to do research on superconductivity. In recent 10 years, many valuable works have been carried out by STM in iron-based superconductors. In this paper, we try to make a brief introduction of the STM works in iron-based superconductors. Since the iron-based superconductors have multiple bands and superconducting gaps, the Fermi surface topology can change significantly among different materials. There are some evidences to prove a nodeless s-wave pairing in the optimally-doped iron-based superconductors with both electron and hole pockets by STM experiments. Furthermore, it has been demonstrated that FeSe-based materials with only electron pockets also have a sign-change order parameter, which provides a robust evidence for the unified picture of the electron pairing in iron-based superconductors. Besides, STM experiments provide fruitful information about the novel electronic properties including the electronic nematicity, shallow band effect, and possible topological superconductivity. Finally, we also give perspectives about the STM studies in iron based superconductors.

Scanning Tunneling Microscopy Study of the Spin Hall Effect in Platinum and Highly Resistive Tungsten Films

The results of a nanoscale experimental study of the spin Hall effect in platinum and highly resistive tungsten films are reported. These results are obtained by using scanning tunneling microscopy based measurements with tungsten and iron-coated tungsten tips. These measurements reveal an appreciable asymmetry of tunneling currents observed by changing the polarity of tunneling voltages or the direction of electric currents through the films. This asymmetry is a manifestation of accumulations of spin-polarized electrons on conducting film boundaries occurring as a result of the spin Hall effect.

How a tertiary diamine molecule chelates the silicon dimers of the Si(001) surface: a real-time scanning tunneling microscopy study.

The patterning of silicon surfaces by organic molecules emerges as an original way to fabricate innovative nanoelectronic devices. In this regard, we have studied how a diamine, N,N,N',N'-tetramethylethylenediamine (TMEDA, (CH3)2N-[CH2]2-N(CH3)2), chelates the silicon dimers of the Si(001)-2 × 1 surface. Starting from very low coverage to surface saturation (at 300 K), we used real-time scanning tunneling microscopy (STM) in a scanning-while-dosing approach. The images show that the molecules can adopt two bonding configurations: the cross-trench (CT) configuration by bridging two adjacent dimer rows, and the end-bridge (EB) configuration by chelating two adjacent dimers in the same row. However, while CT dominates over EB at low coverage, the percentage of EB adducts steadily increases, until it becomes largely dominant at high molecular coverage. Above a critical coverage θmol of ∼0.13 monolayer (ML), a sudden change in the molecular imprints is seen, likely due to a change in the tunneling conditions. The EB stapling of two adjacent dimers in a row via a dual-dative bond (as shown by XPS) is achieved efficiently by the TMEDA molecule with a very high chemical selectivity. The EB is a unique configuration in amine adsorption chemistry, as it leads to the formation of a pair of first-neighbor, doubly-occupied dangling bonds. Further reactivity of the EB site with other molecules remains to be explored, and possible reaction schemes are envisaged.

On the effect of local barrier height in scanning tunneling microscopy: Measurement methods and control implications.

A common cause of tip-sample crashes in a Scanning Tunneling Microscope (STM) operating in constant current mode is the poor performance of its feedback control system. We show that there is a direct link between the Local Barrier Height (LBH) and robustness of the feedback control loop. A method known as the "gap modulation method" was proposed in the early STM studies for estimating the LBH. We show that the obtained measurements are affected by controller parameters and propose an alternative method which we prove to produce LBH measurements independent of the controller dynamics. We use the obtained LBH estimation to continuously update the gains of a STM proportional-integral (PI) controller and show that while tuning the PI gains, the closed-loop system tolerates larger variations of LBH without experiencing instability. We report experimental results, conducted on two STM scanners, to establish the efficiency of the proposed PI tuning approach. Improved feedback stability is believed to help in avoiding the tip/sample crash in STMs.

Carbon Precursor Structures and Graphene on Palladium Nanoparticles

Due to their stable catalytic properties graphene-encapsulated metal nanoparticles (NPs) have come into the focus of heterogeneous catalysis. Research on graphene’s atomic structure is a prerequisite for understanding and controlling catalytic processes; however, it is still at its very beginning considering metal NPs. This atomic-scale scanning tunnelling microscopy study documents preliminary growth stages and the structural properties of graphene on highly oriented pyrolytic graphite (HOPG) supported palladium NPs. It is shown that an annealing of the NPs at ∼650 °C in a few tens of Langmuir of ethylene (ethene) leads first to a carbon precursor structure covering the NP’s (111) facets. At higher ethylene exposures, the precursor structure turns into single-layer graphene encapsulating the NPs. Graphene-encapsulated palladium NPs with a size larger than ∼30 nm exhibit mostly one single and an almost defect-free graphene sheet on a facet. In most of the cases, such a perfect growth does not take place o...