High-temperature Scanning Tunneling Microscopy Research Articles

Thermal oxidation process on Si(113)-(3 × 2) investigated using high-temperature scanning tunneling microscopy.

Thermal oxidation of Si(113) in a monolayer regime was investigated using high-temperature scanning tunneling microscopy (STM). Dynamic processes during thermal oxidation were examined in three oxidation modes – oxidation, etching, and transition modes – in the third of which both oxidation and etching occur. A precise temperature–pressure growth mode diagram was obtained via careful measurements for Si(113), and the results were compared with those for Si(111) in the present work and Si(001) in the literature. Initial oxidation processes were identified based on high-resolution STM images.

The Pressure Gap for Thiols: Methanethiol Self-Assembly on Au(111) from Vacuum to 1 bar

Functionalizing noble metal surfaces with (bio)organic molecules is a vibrant field of research, with key applications in medicine, catalysis, and molecular electronics. Control over the molecular self-assembly is essential to creating functional devices. Here, we exploit our high-pressure, high-temperature scanning tunneling microscope (STM) to relate the effects of controllable parameters (temperature and pressure) to atomic-scale assembly mechanisms. Using methanethiol self-assembly on Au(111) as a model system, we monitor the formation and assembly of the ubiquitous (CH3S)2Au “staple” motif into row structures at pressures of up to 1 bar. We observe a pressure-induced transition from the usual 1/3 monolayer (ML) saturation coverage in vacuum to 3/8 ML at 1 bar, thus providing the first evidence for a pressure gap effect for thiol adsorption. Complementing our experiments, we employed dispersion-corrected density functional theory computations to model the formed surface adlayers, corresponding STM images, and underlying equilibrium thermodynamics.

Imaging the confined surface oxidation of Ni3Al(111) by in situ high temperature scanning tunneling microscopy.

The initial oxidation of Ni3Al(111) was imaged by in situ scanning tunneling microscopy (STM) at 700-750 K. At 740 K ± 10 K a moiré structure is formed as the major surface phase: high resolution STM data atomically resolve a top hexagonal lattice with a lattice constant of 2.93 ± 0.01 Å aligned or slightly rotated with respect to the substrate. Auger electron spectra acquired from the surface phase identify Al atoms in an oxidic environment together with Ni atoms unaffected by the oxidation of the Ni3Al(111) surface. A special mass balance analysis applied to STM images recorded during formation of the moiré structure allowed to extract the metal content of the surface phase. The moiré phase can be attributed to a single O/Al double layer of α-Al2O3 ontop of the Ni3Al(111) crystal. The surface double layer is laterally expanded by ∼7% with respect to α-Al2O3 and, relating to the next nearest neighbor distance of the substrate of 2.52 Å, it contains 0.73 ML oxygen and 0.49 ML aluminium atoms. The building principle of the surface phase is almost identical to the one of the reported Oi/Ali interface layer of the so called surface oxide, except for its rotational alignment with respect to the substrate as shown in a careful moiré analysis. It could be shown that this thinnest possible surface aluminum oxide layer is formed due to kinetic restrictions: the oxide grows within the first layer of the Ni3Al(111) surface ejecting 0.5 ML surface metal atoms, which are then converted into the surface oxide laterally separated at the ascending step edge of the same terrace. While the formation of the surface oxide is kinetically hindered most likely by the availability of Al adatoms, all rearrangement processes required for the surface oxide formation on each terrace are not rate limiting as identified by in situ STM. Instead, the local oxide growth rather follows the kinetics driven by the adsorption probability of the impinging oxygen molecules and provides the possibility to entirely cover whole Ni3Al(111) surface.

Microscopic mechanism of the homoepitaxy onSi(111)7×7

Very initial stages of the homoepitaxy on the $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ surface are studied at 310, 356, and $366{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ by in situ continuous observation using high-temperature scanning tunneling microscopy (HTSTM) at the atomic scale during Si deposition under a slow rate of 0.02 bilayer $(\mathrm{BL})\phantom{\rule{0.16em}{0ex}}{min}^{\ensuremath{-}1}$. The substrate surface is reconstructed to the well-established dimer-adatom-stacking fault (DAS) structure with the $7\ifmmode\times\else\texttimes\fi{}7$ unit cell consisting of two triangular half unit cells (HUCs): a stacking faulted (F)-HUC and a normally stacked one. It is expected that the complex, large unit cell compels the initial homoepitaxy to proceed in a quite different manner from that on unreconstructed surfaces. Formation and growth of various adsorbed clusters are pursued by the continuous observation of the same narrow areas during the deposition, avoiding the tip-shadow effect. The most anomalous finding is a quasiliquid cluster (QLC) spreading to plural HUCs (spread QLC) on the $7\ifmmode\times\else\texttimes\fi{}7$ DAS substrate. This appears as a result of the difficulty of the F-HUC to be transformed into the normally stacked BL, being essential for the homoepitaxy. After the transformation, the spread QLC undergoes the following structural changes with an increase of the Si deposition: crystallization to a small epitaxial BL \ensuremath{\rightarrow} surface reconstruction to the DAS structure. Validity and reasons of the transitional formation of the spread QLC are discussed. The spread QLC mediated homoepitaxy mechanism is concluded and is a new mode in the crystal growth. Real in situ HTSTM observations of the same areas on the nanoscale during Si deposition are indispensable to explore for the dynamic atomistic mechanism of the homoepitaxial growth on the $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$.

Tailoring metal–organic hybrid interfaces: heteromolecular structures with varying stoichiometry on Ag(111)

The physical properties of interfaces between organic semiconductors and metal surfaces crucially influence the performance of organic electronic devices. In order to enable the tailoring of such metal–organic hybrid interfaces we study the adsorption of heteromolecular thin films containing the prototypical molecules copper-II-phthalocyanine (CuPc) and 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) on the Ag(111) surface. Here, we demonstrate how the lateral order can be tuned by changing the relative coverage of both adsorbates on the surface. The layer growth has been studied in real time with low energy electron microscopy, and—for different stoichiometries—the geometric properties of three heteromolecular submonolayer phases have been investigated using high resolution low energy electron diffraction and low temperature scanning tunneling microscopy. Furthermore, we have used a theoretical approach based on van der Waals and electrostatic potentials in order to reveal the influence of the intermolecular and the molecule–substrate interactions on the lateral order of heteromolecular films.

In situ high-pressure high-temperature scanning tunneling microscopy of a Co(0 0 0 1) Fischer–Tropsch model catalyst

The Fischer–Tropsch synthesis of hydrocarbons from carbon monoxide and hydrogen over cobalt catalysts is expected to become the key method for the future production of liquid fuels. Despite decades of research on the reaction mechanism, the state of the surface of the operating catalyst is still uncertain. Using in situ high-temperature high-pressure scanning tunneling microscopy, we have investigated the Fischer–Tropsch reaction over a Co(0001) single crystal model catalyst in the methanation limit. Atomically resolved images show that the surface does not transform into an oxide or carbide, but remains metallic under reaction conditions. The data are consistent with a mobile layer of reversibly adsorbed particles on the surface. The surface morphology under reaction conditions is unchanged from the surface in ultra-high vacuum. Widespread assumptions about a surface restructuring that seemed to explain the activity of cobalt-based Fischer–Tropsch catalysts are not confirmed.

The ReactorSTM: atomically resolved scanning tunneling microscopy under high-pressure, high-temperature catalytic reaction conditions.

To enable atomic-scale observations of model catalysts under conditions approaching those used by the chemical industry, we have developed a second generation, high-pressure, high-temperature scanning tunneling microscope (STM): the ReactorSTM. It consists of a compact STM scanner, of which the tip extends into a 0.5 ml reactor flow-cell, that is housed in a ultra-high vacuum (UHV) system. The STM can be operated from UHV to 6 bars and from room temperature up to 600 K. A gas mixing and analysis system optimized for fast response times allows us to directly correlate the surface structure observed by STM with reactivity measurements from a mass spectrometer. The in situ STM experiments can be combined with ex situ UHV sample preparation and analysis techniques, including ion bombardment, thin film deposition, low-energy electron diffraction and x-ray photoelectron spectroscopy. The performance of the instrument is demonstrated by atomically resolved images of Au(111) and atom-row resolution on Pt(110), both under high-pressure and high-temperature conditions.

High-Temperature Scanning Tunneling Microscopy Study of the Ordering Transition of an Amorphous Carbon Layer into Graphene on Ruthenium(0001)

The ordering transition of an amorphous carbon layer into graphene was investigated by high-temperature scanning tunneling microscopy. A disordered C layer was prepared on a Ru(0001) surface by chemical vapor deposition of ethylene molecules at ~660 K. The carbon layer grows in the form of dendritic islands that have almost the same density as graphene. Upon annealing of the fully covered surface, residual hydrogen desorbs and a coherent but still disordered carbon layer forms, with almost the same carbon coverage as in graphene. The ordering of this layer into graphene at 920 to 950 K was monitored as a function of time. A unique mechanism was observed that involves small topographic holes in the carbon layer. The holes are mobile, and on the trajectories of the holes the disordered carbon layer is transformed into graphene. The transport of C atoms across the holes or along the hole edges provides a low-energy pathway for the ordering transition. This mechanism is prohibited in a dense graphene layer, which offers an explanation for the difficulty of removing defects from graphene synthesized by chemical methods.

Visualization of Surfaces of Pt and Ni Model Catalysts in Reactive Environments Using Ambient Pressure High Temperature Scanning Tunneling Microscopy and Understanding the Restructurings of Surfaces of Model Metal Catalysts under Reaction Conditions at Near Ambient Pressure

Catalysis science has emerged as one of the crucial fields in energy science responsible for a sustainable energy world. Fundamental study of surface structures of catalysts at atomic scale under reaction conditions or during catalysis is critical in understanding catalytic mechanisms because a single catalysis event is performed on a specific site comprising one or several atoms with appropriate geometric and electronic structures. Ambient pressure high temperature scanning tunneling microscopy (APHT-STM) can identify structural details of catalyst surfaces at nano or atomic level when catalysts are under reaction conditions or during catalysis. By using APHT-STM, structures of the step edge of Pt(111) and the surface of Ni(557) were studied under reaction conditions. For Pt(111) model catalyst in a CO environment at a pressure of 0.1 Torr or higher, Pt atoms at step edges exhibit a dynamic restructuring. They form kink sites at 0.1 Torr and create nanoclusters near the step edges at 1–10 Torr within 1–2 min. A pressure-dependent restructuring of step edges of Pt(111) was revealed. In in situ studies of the vicinal surface, Ni(557) shows a pressure-dependent restructuring in CO, resulting from reorganization of all surface atoms. Nickel nanoclusters are formed on the whole surface, consistent with the increased coverage of CO chemisorbed on the Ni surface at a relatively higher pressure. Restructuring of atoms at a step edge of terraces of a flat surface and all atoms of a vicinal surface suggest the dynamic nature of model metal catalyst surfaces. Essentially, the surface structure of a metal catalyst in a reactive environment is determined by its reaction or catalysis condition.

Role of ethylene on surface oxidation of TiO2(110)

Using in situ high-temperature (700-1000 K) scanning tunneling microscopy (STM), we studied the influence of ethylene on the surface dynamics of oxygen-deficient, rutile-structured TiO2(110). STM images were acquired during annealing the sample as a function of time, oxygen and ethylene pressures, and temperature. With increasing oxygen pressure and/or decreasing temperature, TiO2(110) surface mass increased, consistent with previous results. Interestingly, annealing the sample in ethylene with traces of oxygen also results in the growth of TiO2 at higher rates than those observed during annealing in pure oxygen. Our results indicate that ethylene promotes oxidation of TiO2(110).

Robust Surface Nano-Architecture by Alkali–Carboxylate Ionic Bonding

Ionic bonding in supramolecular surface networks is a promising strategy to self-assemble nanostructures from organic building blocks with atomic precision. However, sufficient thermal stability of such systems has not been achieved at metal surfaces, likely due to partial screening of the ionic interactions. We demonstrate excellent stability of a self-assembled ionic network on a metal surface at elevated temperatures. The structure is characterized directly by atomic resolution scanning tunneling microscopy (STM) experiments conducted at 165 °C showing intact domains. This robust nanometer-scale structure is achieved by the on-surface reaction of a simple and inexpensive compound, sodium chloride, with a model system for carboxylate interactions, terephthalic acid (TPA). Rather than distinct layers of TPA and NaCl, angle resolved X-ray photoelectron spectroscopy experiments indicate a replacement reaction on the Cu(100) surface to form Na-carboxylate ionic bonds. Chemical shifts in core level electron states confirm a direct interaction and a +1 charge state of the Na. High-temperature STM imaging shows virtually no fluctuation of Na-TPA island boundaries, revealing a level of thermal stability that has not been previously achieved in noncovalent organic-based nanostructures at surfaces. Comparable strength of intermolecular ionic bonds and intramolecular covalent bonds has been achieved in this surface system. The formation of these highly ordered structures and their excellent thermal stability is dependent on the interplay of adsorbate-substrate and ionic interactions and opens new possibilities for ionic self-assemblies at surfaces with specific chemical function. Robust ionic surface structures have potential uses in technologies requiring high thermal stability and precise ordering through self-assembly.

In situ high-temperature scanning tunneling microscopy study of bilayer graphene growth on 6H-SiC(0001)

Using in situ high-temperature (1395K), ultra-high vacuum, scanning tunneling microscopy (STM), we investigated the growth of bilayer graphene on 6H-SiC(0001). From the STM images, we measured areal coverages of SiC and graphene as a function of annealing time and found that graphene grows at the expense of SiC. Graphene domains were observed to grow, at comparable rates, at (I) graphene-free SiC step edges, (II) graphene–SiC interfaces, and (III) the existing graphene domain edges. Based upon our results, we suggest that the rate-limiting step controlling bilayer graphene growth is the desorption of Si from the substrate.

Probing the Buried Pb/Si(111) Interface with SPA LEED and STM on Si(111)-Pbα√3×√3

High resolution spot profile analysis low energy electron diffraction (SPA-LEED) and variable temperature scanning tunneling microscopy (STM) have been used to observe the growth of Pb on the Pb/Si(111)-α√3×√3 phase, which is driven by quantum size effects (QSE). A change in the rotation of the Pb grown islands with respect to the Si substrate has been observed with increasing coverage θ. At lower coverage, separated two-step islands are grown and are aligned with the [110] axis of the substrate. With increasing coverage above 1.5 ML, of the islands coalesce and form a bilayer, with additional islands grown on top. The preferred Pb island orientation changes to 5.6° with respect to the [110] direction. These changes at the metal/semiconductor buried interface are obtained both with SPA LEED and STM as changes to the period of the Moire pattern. The method of analysis of the corrugation period and rotation angle of the Moire pattern measured with diffraction and STM can be applied to obtain the structure of buried metal/substrate interfaces in other epitaxial systems.

Single Terrace Growth of Graphene on a Metal Surface

The epitaxial growth of graphene by chemical vapor deposition of ethylene on a Ru(0001) surface was monitored by high-temperature scanning tunneling microscopy. The in situ data show that at low pressures and high temperatures the metal surface facets into large terraces, leading to much better ordered graphene layers than resulting from the known growth mode. Density functional theory calculations show that the single terrace growth mode can be understood from the energetics of the graphene-metal interaction.

Probing the thermal decomposition behaviors of ultrathinHfO2 films by an in situ high temperature scanning tunneling microscope

The thermal decomposition of ultrathinHfO2 films(∼0.6–1.2 nm) on Si by ultrahigh vacuum annealing (25–800 °C) is investigated in situ in real time by scanning tunneling microscopy. Twodistinct thickness-dependent decomposition behaviors are observed. When theHfO2 thicknessis ∼ 0.6 nm, no discernible morphological changes are found below ∼ 700 °C. Then an abruptreaction occurs at 750 °C with crystalline hafnium silicide nanostructures formed instantaneously.However, when the thickness is about 1.2 nm, the decomposition proceedsgradually with the creation and growth of two-dimensional voids at800 °C. The observed thickness-dependent behavior is closely related to the SiO desorption, whichis believed to be the rate-limiting step of the decomposition process.

Defect Structure of Ultrathin Ceria Films on Pt(111): Atomic Views from Scanning Tunnelling Microscopy

Atomically resolved scanning tunnelling microscopy (STM) images have been obtained on ultrathin films of CeO2(111) supported on Pt(111). The ultrathin films were grown in two ways, by reactive deposition in an oxygen atmosphere and by postoxidation of Ce/Pt surface alloys. STM results are compared with previously reported high-temperature STM and noncontact atomic force microscopy (NC-AFM) images of the native CeO2(111) surface. The similarity between these images is striking and allows a number of defects and adsorbates in our ultrathin film to be assigned. Moreover, the similarity in structure between the native oxide and the ceria ultrathin film indicates that it is an excellent topographic mimic of the native oxide.

Investigation on the deoxidation process of SrO/Si(100) surface

Using high temperature scanning tunneling microscopy (STM) and XPS, we investigated the dynamic process of SrO/Si(100) changing to Sr/Si(100) reconstructed surface, which plays a critical role in the growth of crystalline oxide on silicon substrate. During this process we find some interesting phenomenan: there appears crystalline SrO on Si(100) substrate at low annealing temperature of 500 ℃; at higher annealing temperature of 550—590 ℃, the oxygen in the SrO/Si(100) interface will react with silicon and form volatile SiO, leading the surface with a large quantity of line vacancies. In the later case, there appears abnormal metallic property of this surface, which results from dangling bonds of silicon atoms in the surface.

As-rich reconstruction stability observed by high temperature scanning tunnelling microscopy

Atomic resolution scanning tunnelling microscopy (STM) has been used to study in-situ the As-terminated reconstructions formed on GaAs(0 0 1) surfaces in the presence of an As 4 flux. The reconstructions c(4×4), (2×4) and (3×1) are long established for GaAs(0 0 1) between 400 and 600 °C for varying Ga and As flux, however the stoichiometry of incommensurate transient reconstructions is still uncertain. By performing high temperature STM on an initial (2×4) surface between 250 and 450 °C in the absence of an As flux, small domains with varying reconstruction are observed in a similar manner to the InAs/GaAs(0 0 1) wetting layer. The local storage of excess Ga in Ga-rich domains could provide insight into sub ML homo- and hetero-epitaxial growth.

High temperature scanning tunneling microscopy of purely ion conducting yttria stabilized zirconia (YSZ)

Scanning tunneling microscopy (STM) on wide band gap insulators is generally not possible. Here we demonstrate that ionic insulators with high ion mobility, such as yttria stabilized zirconia, are an exception to this rule. The ion conductivity ensures that the sample maintains a defined potential relative to the STM tip and thus a stable tunnel junction. Consequently, ion conductors provide an opportunity for characterizing wide band gap insulators, and thin films and nanostructures supported on them by STM.

Fluorine diffusion assisted by diffusing silicon on the Si(111)-(7×7) surface

The diffusion process of fluorine (F) atoms on the Si(111)-(7x7) surface is investigated using high-temperature scanning tunneling microscopy. The kinetic parameters of F hopping agree well with those of the diffusing silicon (Si) atoms, which implies that of all reaction processes, the Si diffusion serves as the rate-determining one. Deposition of Si on the surface is found to enhance F hopping, which supports the above-mentioned observation. Theory reveals that the replacement of F adsorption sites by diffusing Si atoms is the key process in the diffusion mechanism.