Ultrahigh Vacuum Scanning Tunneling Research Articles

Point Defects and Grain Boundaries in Rotationally Commensurate MoS2 on Epitaxial Graphene

With reduced degrees of freedom, structural defects are expected to play a greater role in two-dimensional materials in comparison to their bulk counterparts. In particular, mechanical strength, electronic properties, and chemical reactivity are strongly affected by crystal imperfections in the atomically thin limit. Here, ultra-high vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) are employed to interrogate point and line defects in monolayer MoS2 grown on epitaxial graphene (EG) at the atomic scale. Five types of point defects are observed with the majority species showing apparent structures that are consistent with vacancy and interstitial models. The total defect density is observed to be lower than MoS2 grown on other substrates, and is likely attributed to the van der Waals epitaxy of MoS2 on EG. Grain boundaries (GBs) with 30{\deg} and 60{\deg} tilt angles resulting from the rotational commensurability of MoS2 on EG are more easily resolved by STM than atomic force microscopy at similar scales due to the enhanced contrast from their distinct electronic states. For example, band gap reduction to ~0.8 eV and ~0.5 eV is observed with STS for 30{\deg} and 60{\deg} GBs, respectively. In addition, atomic resolution STM images of these GBs are found to agree well with proposed structure models. This work offers quantitative insight into the structure and properties of common defects in MoS2, and suggests pathways for tailoring the performance of MoS2/graphene heterostructures via defect engineering.

Effect of low-temperature-grown GaSb layer on formation of high-density and small GaSb islands on Si(100) substrate

The effect of a low-temperature-grown gallium antimonide (LT-GaSb) layer on the formation of high-density GaSb islands on a Si(100) substrate was studied using ultrahigh-vacuum scanning tunneling microscopy (UHV-STM) and atomic force microscopy. By using an LT-GaSb layer in the initial growth stage (new growth method), high-density and small GaSb islands were formed at 500 °C regardless of the type of the reconstructed surface. The density and size of the GaSb islands with the LT-GaSb layer were estimated to be ∼1.0 × 1011 cm−2 and ∼20 nm, respectively. The STM results showed that the LT-GaSb layer was composed of an Sb-rich amorphous GaSb layer. With a rapid increase in temperature under the Sb flux, the LT-GaSb layer immediately changed into GaSb nuclei with a high density (2.1 × 1011 cm−2) and small size (11 nm). It is suggested that these high-density and small GaSb islands grow from the GaSb nuclei, which act as crystal seeds.

Optimisation of anatase TiO2 thin film growth on LaAlO3(0 0 1) using pulsed laser deposition

Optimisation of epitaxial anatase TiO2 thin films grown on LaAlO3(0 0 1) substrates was performed using ultra-high vacuum based pulsed laser deposition (PLD) and studied by in-situ reflection high-energy electron diffraction (RHEED). In addition, ex-situ X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM) were performed to characterise the bulk properties of these thin films. The deposited TiO2 thin film is demonstrated to have anatase phase and bonded directly to the LaAlO3(0 0 1) substrate. In a separate ultra-high vacuum system low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) measurements were performed and a well-ordered two-domain (1×4) and (4×1) reconstruction of anatase surface was observed. Analysis of the STM measurements indicates the coexistence of atomic steps of both 2.5Å and 5.0Å, confirming the existence of two TiO2 domains. The atomic resolution STEM images reveal that the TiO2/LaAlO3 interface to be terminated with LaO layer and that the anatase-TiO2 reconstruction was found to be stable during the film growth.

Chemically selective formation of Si–O–Al on SiGe(110) and (001) for ALD nucleation using H2O2(g)

Passivation and functionalization via atomic hydrogen, hydrogen peroxide (H2O2(g)), and trimethylaluminum (TMA) on clean silicon–germanium (Si0.5Ge0.5(110) and Si0.47Ge0.53(001)) surfaces were studied and compared at the atomic level using ultra-high vacuum (UHV) scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and X-ray photoelectron spectroscopy (XPS) to understand the topological, electronic, and chemical structures of the surfaces. STM and XPS indicate that a sputter-cleaned SiGe(110) surface is terminated with adatoms, while a SiGe(001) surface is terminated with germanium dimers. STS demonstrates that the Fermi level on a clean SiGe(110) surface is pinned near mid-gap due to surface dangling bonds, while the Fermi level on a clean SiGe(001) surface is consistent with unpinning. A saturation dose of H2O2(g) at 25°C chemisorbs to SiGe surfaces, leaving the Fermi level at the surface consistent with unpinning, and the surface is functionalized mainly with Si–OH, Ge–OH, and Si–O–Ge bonds on both SiGe(110) and (001). After a subsequent TMA dose at 25°C, XPS and STM verify that a thermally stable and well-ordered monolayer of Al2O3 is formed on SiGe(110) and (001) surfaces, resulting in the formation of Al–O–Si bonds. The H2O2(g) functionalization provides three times more oxygen sites on the surface and three times as great a TMA nucleation density than does H2O(g) at both 25°C and 120°C. STS demonstrates that H2O2(g)- and TMA-dosed SiGe surfaces show a Fermi level consistent with unpinning and a local density of states (DOS) without any states between the conduction and valence band edge, indicating an ideal template for further atomic layer deposition (ALD) nucleation of high-k materials on SiGe(110) and (001) surfaces.

Current-Driven Hydrogen Desorption from Graphene: Experiment and Theory.

Electron-stimulated desorption of hydrogen from the graphene/SiC(0001) surface at room temperature was investigated with ultrahigh vacuum scanning tunneling microscopy and ab initio calculations in order to elucidate the desorption mechanisms and pathways. Two different desorption processes were observed. In the high electron energy regime (4-8 eV), the desorption yield is independent of both voltage and current, which is attributed to the direct electronic excitation of the C-H bond. In the low electron energy regime (2-4 eV), however, the desorption yield exhibits a threshold dependence on voltage, which is explained by the vibrational excitation of the C-H bond via transient ionization induced by inelastic tunneling electrons. The observed current independence of the desorption yield suggests that the vibrational excitation is a single-electron process. We also observed that the curvature of graphene dramatically affects hydrogen desorption. Desorption from concave regions was measured to be much more probable than desorption from convex regions in the low electron energy regime (∼2 eV), as would be expected from the identified desorption mechanism.

A Suggestion of the Graphene/Ge(111) Structure Based on Ultra-High Vacuum Scanning Tunneling Microscopy Investigation

We report on the 5.5√3 × 5.5√3 − R30 ∘ overlayer superstructure observed by the scanning tunneling microscopy on the Ge(111) surface. It shows pronounced effects of the local density of states leading to the strong dependence of STM images on the bias voltage and some dynamic changes of images at 300 K. This overlayer is tentatively interpreted as graphene formed in small submonolayer amounts due to the pyrolysis of hydrocarbon constituents of the residual atmosphere of the vacuum chamber during the annealing of a Ge(111) sample at 900 K. We suggest a model of the graphene/Ge(111)-5.5√3 × 5.5√3 − R30 ∘ heteroepitaxial interface, featuring the reconstructed Ge(111) substrate with no long-range order under the graphene layer, the latter being corrugated due to spatial variations of the interatomic geometry of the Ge(111) and graphene(0001) atomic lattices with extremely large mismatch.

Electronic and Mechanical Properties of Graphene-Germanium Interfaces Grown by Chemical Vapor Deposition.

Epitaxially oriented wafer-scale graphene grown directly on semiconducting Ge substrates is of high interest for both fundamental science and electronic device applications. To date, however, this material system remains relatively unexplored structurally and electronically, particularly at the atomic scale. To further understand the nature of the interface between graphene and Ge, we utilize ultrahigh vacuum scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) along with Raman and X-ray photoelectron spectroscopy to probe interfacial atomic structure and chemistry. STS reveals significant differences in electronic interactions between graphene and Ge(110)/Ge(111), which is consistent with a model of stronger interaction on Ge(110) leading to epitaxial growth. Raman spectra indicate that the graphene is considerably strained after growth, with more point-to-point variation on Ge(111). Furthermore, this native strain influences the atomic structure of the interface by inducing metastable and previously unobserved Ge surface reconstructions following annealing. These nonequilibrium reconstructions cover >90% of the surface and, in turn, modify both the electronic and mechanical properties of the graphene overlayer. Finally, graphene on Ge(001) represents the extreme strain case, where graphene drives the reorganization of the Ge surface into [107] facets. From this work, it is clear that the interaction between graphene and the underlying Ge is not only dependent on the substrate crystallographic orientation, but is also tunable and strongly related to the atomic reconfiguration of the graphene-Ge interface.

Formation of Pt and Rh Nanoclusters on a Graphene Moiré Pattern on Cu(111)

The formation of Pt and Rh nanoclusters on a graphene moire pattern on Cu(111) was studied with ultrahigh vacuum scanning tunneling microscopy (UHV-STM). Isolated graphene islands with different periodicities were successfully grown on the Cu surface. As a result of weak coupling to the Cu substrate, different graphene rotational domains were observed including one with a periodicity of 4 nm that has not been previously reported. Furthermore, our results have shown that Pt and Rh form dispersed nanoclusters on graphene/Cu(111) with Pt forming more regular structures trapped mostly at the hollow sites. This variation in growth behavior is mainly attributed to differences in Pt–carbon and Rh–carbon interaction energies. Thermal stability experiments were also performed, and Pt clusters were found to be structurally stable up to 700 K.

Solution-Cast Monolayers of Cobalt Crown Ether Phthalocyanine on Highly Ordered Pyrolytic Graphite

15-Crown-5-ether-substituted cobalt(II) phthalocyanine (CoCrPc) is an atomically thin and flat-laying, electrically insulating molecule that can solvate ions; these properties are desirable for nanoelectronic devices. A simple, solution-phase deposition method is demonstrated to produce a monolayer of CoCrPc on highly ordered pyrolytic graphite (HOPG). A uniform and continuous CoCrPc layer is obtained on freshly cleaved HOPG by solution drop casting, followed by thermal annealing under ambient pressure in Ar in the temperature range of 150–210 °C. While the quality of the monolayer is independent of annealing time, the composition of the annealing atmosphere is critical; exposure to ambient air degrades the quality of the monolayer over the time scale of minutes. Using ultrahigh vacuum scanning tunneling microscopy, a highly ordered and flat CoCrPc layer with hexagonal symmetry and average spacing of 4.09 ± 0.2 nm is observed. The band gap of the CoCrPc, measured by scanning tunneling spectroscopy, is 1.3...

Cyclic Hydrogen Bonding in Indole Carboxylic Acid Clusters

Monolayers of indole-2-carboxylic acid and indole-3-carboxylic acid on gold are studied using ultrahigh-vacuum scanning tunneling microscopy. Both molecules form symmetric, cyclic, hydrogen-bonded pentamers, a structure that is stabilized by the presence of a weak hydrogen-bond donor (NH or CH) adjacent to the carboxylic acid on the five-membered ring. In addition to pentamers, indole-2-carboxylic acid forms hexamers and catemer chains, while indole-3-carboxylic acid monolayers are generally disordered. Density functional theory calculations show that pentamers and hexamers have stability comparable to dimers or short catemers. The coexistence of all of these structures likely arises from the nonequilibrium conditions present in solution during pulse deposition of the monolayer.

Preserving the 7 × 7 surface reconstruction of clean Si(111) by graphene adsorption

We employ room-temperature ultrahigh vacuum scanning tunneling microscopy and ab-initio calculations to study graphene flakes that were adsorbed onto the Si(111)–7 × 7 surface. The characteristic 7 × 7 reconstruction of this semiconductor substrate can be resolved through graphene at all scanning biases, thus indicating that the atomistic configuration of the semiconducting substrate is not altered upon graphene adsorption. Large-scale ab-initio calculations confirm these experimental observations and point to a lack of chemical bonding among interfacial graphene and silicon atoms. Our work provides insight into atomic-scale chemistry between graphene and highly reactive surfaces, directing future passivation and chemical interaction work in graphene-based heterostructures.

Investigation of local tunneling current noise spectra on the silicon crystal surfaces by means of STM/STS

We report on a careful analysis of the local tunneling conductivity by means of ultra-high vacuum scanning tunneling microscopy/spectroscopy (STM/STS) technique in the vicinity of low-dimensional structures on the Si(111)–(7 × 7) and Si(110)–(16 × 2) surfaces. The power-law exponent α of low-frequency tunneling current noise spectra is investigated for different values of the tunneling contact parameters: relaxation rates, the localized state coupling, and the tunneling barrier width and height.

Tunneling electron induced molecular electroluminescence from individual porphyrin J-aggregates

We investigate molecular electroluminescence from individual tubular porphyrin J-aggregates on Au(111) by tunneling electron excitations in an ultrahigh-vacuum scanning tunneling microscope (STM). High-resolution STM images suggest a spiral tubular structure for the porphyrin J-aggregate with highly ordered “brickwork”-like arrangements. Such aggregated nanotube is found to behave like a self-decoupled molecular architecture and shows red-shifted electroluminescence characteristics of J-aggregates originated from the delocalized excitons. The positions of the emission peaks are found to shift slightly depending on the excitation sites, which, together with the changes in the observed spectral profiles with vibronic progressions, suggest a limited exciton coherence number within several molecules. The J-aggregate electroluminescence is also found unipolar, occurring only at negative sample voltages, which is presumably related to the junction asymmetry in the context of molecular excitations via the carrier injection mechanism.

Combined wet and dry cleaning of SiGe(001)

Combined wet and dry cleaning via hydrofluoric acid (HF) and atomic hydrogen on Si0.6Ge0.4(001) surface was studied at the atomic level using ultrahigh vacuum scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and x-ray photoelectron spectroscopy to understand the chemical transformations of the surface. Aqueous HF removes native oxide, but residual carbon and oxygen are still observed on Si0.6Ge0.4(001) due to hydrocarbon contamination from post HF exposure to ambient. The oxygen contamination can be eliminated by shielding the sample from ambient via covering the sample in the HF cleaning solution until the sample is introduced to the vacuum chamber or by transferring the sample in an inert environment; however, both processes still leave carbon contaminant. Dry in-situ atomic hydrogen cleaning above 330 °C removes the carbon contamination on the surface consistent with a thermally activated atomic hydrogen reaction with surface hydrocarbon. A postdeposition anneal at 550 °C induces formation of an atomically flat and ordered SiGe surface observed by STM. STS verifies that the wet and dry cleaned surface has an unpinned Fermi level with no states between the conduction and valence band edge comparable to sputter cleaned SiGe surfaces.

Effects of Ga-induced reconstructed surfaces and atomic steps on the morphology of GaSb islands on Si(1 0 0)

The effects of the Ga-induced reconstructed surface and atomic step type on the shape and morphology of GaSb islands on a Si(100) surface were studied using ultrahigh-vacuum scanning tunneling microscopy and atomic force microscopy. Though both anisotropic elongated islands and isotropic islands were formed on clean Si and Ga/Si(100)-2×3 substrates, isotropic islands were dominantly formed on Ga/Si(100)-2×2 substrates at 300°C. The density and size of GaSb islands on 2×2-Ga at 300°C were estimated to be 4.9×1010cm−2 and 29.1nm, respectively. The difference in the GaSb island shapes could be caused by the terrace and step of the substrate surface being changed by step rearrangement due to the deposition of Ga atoms at high temperatures. Above 350°C, the density and size of islands decreased to 2.7×109cm−2 and increased to 62.0nm, respectively. In the initial growth stage, scanning tunneling microscopy results revealed that a Sb/Si(100)-2×1 reconstructed surface was formed above 350°C. The large islands were assumed to aggregate from the surface diffusion of each atom because the Sb-terminated Si surface is inactive against Ga and Sb atoms. The type of the reconstructed surface is also suggested to affect the density and size of the islands.

Molecular-Resolution Interrogation of a Porphyrin Monolayer by Ultrahigh Vacuum Tip-Enhanced Raman and Fluorescence Spectroscopy.

Tip-enhanced Raman scattering (TERS) and optically excited tip-enhanced fluorescence (TEF) of a self-assembled porphyrin monolayer on Ag(111) are studied using an ultrahigh vacuum scanning tunneling microscope (UHV-STM). Through selectively exciting different Q-bands of meso-tetrakis- (3,5-ditertiarybutylphenyl)-porphyrin (H2TBPP), chemical information regarding different vibronic excited states is revealed by a combination of theory and experiment; namely, TERS and time-dependent density functional theory (TDDFT) simulations. The observed TEF spectra suggest a weak coupling of H2TBPP to the substrate due to the bulky t-butyl groups and a possible alternative excited state decay path. This work demonstrates the potential of combining TERS and TEF for studying surface-mounted porphyins on substrates, thus providing insight into porphyrin-sensitized solar cells and catalysis.

The transition from 3C SiC (1 1 1) to graphene captured by Ultra High Vacuum Scanning Tunneling Microscopy

In this paper we clarify the transformation mechanism of 3C-SiC into graphene upon thermal decomposition, by a combination of high resolution Scanning Tunneling Microscopy (STM) images and first principle calculations. We studied the transition from 3C-SiC to graphene by high temperature annealing of C-terminated 3C SiC(111)/Si(111) samples in Ultra High Vacuum. By using STM we were able to observe very clear atomic resolution images of the transition from SiC (√3×√3)R30° to a new intermediate stage SiC 32×√3R30° (very close to the graphene (2×2) reconstruction) after annealing at 1250°C. We also obtained images of the transformation of the intermediate structure into a (1×1) monolayer graphene, caused by further sublimation of atoms in the subsurface layer. We have interpreted the results by using Density Functional Theory – Local Density Approximation calculations, which give full account of the SiC (√3×√3)R30° reconstruction, but fail to describe the SiC 32×√3R30° structure due to its incommensurability with the 3C-SiC(111) lattice.

Domain fracture and recovery process of metal phthalocyanine monolayers via NO2 and H2O

CuPc ultrathin films (5 monolayers) are employed to detect NO2 in chemFETs [organic thin film transistors (OTFTs)]; while the NO2 causes OTFT degradation, H2O restores OTFT performance. To develop an atomic understanding of this H2O induced performance recovery, NO2/CuPc/Au(111) was exposed to H2O, then observed using ultrahigh vacuum scanning tunneling microscopy. After dosing NO2 (10 ppm for 5 min) onto CuPc monolayers under ambient conditions, domain fracture is induced in CuPc monolayers, and CuPc aggregates are formed near new grain boundaries, consistent with dissociative O adsorption between CuPc molecules and Au(111). Conversely, after exposing H2O onto a fractured CuPc monolayer for 30 min, fractured domains merge, then large area domains are generated. As the duration of H2O exposure increases to 4 h, second layer growth of CuPc molecules is observed on the CuPc monolayers consistent with H2O breakdown of CuPc aggregates which have formed at the domain boundaries. The results are consistent with H2O driving the removal of atomic O between CuPc molecules and Au(111) consistent with previous sensing results.

Influence of annealing condition and multicycle AlGaAs/GaAs structures on the Al0.26Ga0.74As surface morphology

The influence of annealing temperature, As4 beam equivalent pressure and multi-runs growth on AlGaAs/GaAs structures was investigated. The real space ultrahigh vacuum scanning tunneling microscopy images showed that AlGaAs/GaAs surface morphology greatly depends on annealing conditions and initial state of surface. The reasons of the surface phenomenon are proposed, and a physical model was proposed to explain why the multi-runs growth structures can increase AlGaAs surface roughness. The reasonable preparation conditions for AlGaAs/GaAs structures were proposed.

N and p type character of single molecule diodes

Looking for single molecule electronic devices, we have investigated the charge transport properties of individual tetra-phenylporphyrin molecules on different substrates by ultrahigh-vacuum scanning tunneling microscopy and spectroscopy and by first-principles calculations. The tetra-phenylporphyrins with a Co atom (Co-TPP) or 2 hydrogens (H2-TPP) in the central macrocycle when deposited on Cu3Au(100) substrates showed a diode-like behavior with p and n type character, respectively. After removing the central hydrogens of H2-TPP molecule with the STM tip an ohmic behavior was measured. The rectifying effect was understood from the theoretical point of view by assuming for Co-TPP HOMO conduction and for H2-TPP LUMO conduction, both selectively elected by the hybridization of states between molecule and substrate surface.