Vacancy Islands Research Articles

A new look at metal–carbon hybrids: Molecular-scale study of covalently grafted gold

The reliable integration of new nanocarbons in future molecular and hybrid electronics requires mastery of chemical binding between dissimilar materials down to the individual bond. Here, we present a detailed study of covalent grafting of aryl radicals on single crystal Au(111), and investigate their electrochemical stability and oxidative desorption. Copper underpotential deposition and Gaussian deconvolution of its current–potential trace are used to map the thermodynamic and kinetic landscape of the modified Au surface. Scanning tunnelling microscopy in a high-boiling organic solvent suggests that grafting is accompanied by vacancy island formation on the Au surface, while iodide adsorption heals these vacancies and allows high-contrast imaging of individual covalent grafts.

Impact of annealing treatment on adlayer structures of terphenylmethanethiol self-assembled monolayers on Au(111)

The scanning tunneling microscopy (STM) technique was employed to investigate the adlayer structure and morphology of Self-assembled monolayers (SAMs) fabricated from terphenylmethanethiol (TP1) molecules on Au(111) by preparing from an ethanolic solution at room temperature and subsequently annealed at various temperatures, reaching up to 468 K. Upon heat treatment, the TP1 adlayer exhibited a sequence of distinct structural phases characterized by varying molecular arrangements, namely α, β, γ, δ, ε, σ, and θ, emerging successively for both the as-prepared and annealed samples. The progressive transformation from α→β→ γ→δ→ε→σ→α→θ phases, driven by escalating annealing temperatures, entailed a gradual decrease in molecular packing density until the transition from γ→δ, followed by a subsequent increase leading to the formation of the most densely packed molecular network with molecules standing almost upright, referred to as the α phase, which is the same phase observed for non-heat-treated films. Interestingly, the desorption process for the annealed SAMs occurred from the densest packing TP1 molecular structure of the α phase, contrary to those of biphenyl-based organothiol and n-alkanethiol SAMs where desorption takes place from lying-down striped phases. Almost all TP1 molecules were desorbed at an annealing temperature of 468 K, leading to a significant alteration in the surface structure. During the annealing process, our results showed that the gold surface underwent remarkable changes, including the disappearance of the vacancy islands and the emergence of surface cracks in the form of trenches, along with the formation of rounded monoatomic gold islands with a densely packed α phase.

Nanoisland Formation on the Low Index Planes of Pt By Repeated Electro-Oxidation and-Reduction

Pt-based electro-catalysts are the most common cathode material in proton exchange membrane fuel cells given their high oxygen reduction reaction activity and stability. However, practical applications are limited by catalyst performance loss and degeneration. A main cause for this is nanoscale Pt surface restructuring, which occurs upon formation and reduction of a Pt surface oxide. During oxidation, a fraction of the Pt surface atoms are extracted in a process known as “place-exchange” from the electrode and are bound at specific sites in an oxide about 1 – 4 Å above the surface [1]. Upon subsequent oxide reduction, these atoms as well as the vacancies from which they originate can agglomerate, leading to the nucleation of adatom and vacancy islands. If this process is repeated, nanoislands with a well-defined lateral size of about 20 – 60 Å are grown [2-6]. Detailed studies of this growth process have been performed on the Pt(111) electrode [2-6], revealing two distinct growth regions which are characterized by different vertical scaling exponents [3].For the development of catalysts with superior long-term stability, a more detailed knowledge of the restructuring of the other low index planes of Pt is required. In our recent study of the Pt oxides on Pt(111) and Pt(100) we already showed a strong dependence of the structural stability of the surface on differences in the atomic-scale oxide structure formed on those two surface orientations [1]. However, the influence of surface structure on the nanoisland growth after repeated oxidation and reduction is largely unclear.We present a comparative operando grazing incidence small angle X-ray scattering (GISAXS) study of the nanoisland formation on Pt(111), Pt(100) and Pt(110) in acidic electrolyte. The GISAXS measurements of the nanoisland growth were performed at a high photon energy of >70 keV, which lowered background scattering from the electrolyte, enabling us to resolve the islands after the very first ox./red. cycle. Differences in island shape and growth behaviour on the low index planes will be discussed.We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft via project no. 418603497, MA 1618/23-1 and the BMBF via 05K19FK3.References Fuchs et al., Nat. Catal. 3, 754–761 (2020)Sugawara and K. Itaya, J. Chem. Soc., Faraday Trans. 185, 1351-1356 (1989)Jacobse et al., Nat. Mater. 17 (3), 277-282 (2018)J. Rost, et al., Nat. Commun. 10, 5233 (2019)Ruge et al., J. Am. Chem. Soc. 139, 4532 (2017)Jacobse, et al. ACS Cent. Sci. 5 (12), 1920–1928 (2019)

Understanding the Two-Dimensional Mixing Behavior of 1-Naphthalenethiol and Octanethiol.

A two-dimensional (2D) mixture in the form of a self-assembled monolayer composed of two distinct organothiol compounds was created by sequentially depositing 1-naphthalenethiol (1NT) and octanethiol (OT) on a gold surface. By varying the sequence of deposition, two mixed surface systems were created. The surface structure of the resulting mixed monolayer was characterized with Scanning Tunneling Microscopy (STM) and showed surface disorder across all investigated domains. Elemental analysis was carried out with X-ray Photoelectron Spectroscopy (XPS) and indicated that the 1NT monolayer was prone to significant oxidation. Reductive desorption (RD) was used to characterize the binding strength and electrochemical environments of the molecular components in the mixture, and confirmed disordered molecular layers. Due to the presence of oxidized species in the 1NT monolayer, 1NT was displaced by OT resulting in a novel surface structure composed of either OT or 1NT. Monolayers of OT that were exposed to a solution of 1NT resulted in disordered surface structures with a significant amount of gold vacancy islands. To date, there is no experimental phase diagram explaining the chemical behavior of two-dimensional mixtures. This study addresses the need for an experimental understanding of the phase behavior of mixed organothiol self-assembled monolayers (SAMs).

Room Temperature Migration of Ag Atoms to Cover Pd Islands on Ag(111)

The structures of Pd islands at three different Pd coverages (0.028, 0.064, and 0.150 monolayer (ML)) on Ag(111) were studied at room temperature with scanning tunneling microscopy (STM). While previous studies have shown that the structure and composition of Pd islands on Ag(111) change at elevated temperatures, we found that Ag atoms migrate to cover the Pd islands even at room temperature. These Ag atoms occupy sites in the middle of the islands, and second layer growth begins at these sites. The migration of Ag atoms leads to the formation of vacancy islands in the Ag(111) terraces. Upon annealing to 340 K, the majority of the Pd islands are encapsulated by Ag atoms to form an Ag/Pd/Ag(111) structure. However, upon further annealing the composition of some islands at a Pd coverage of 0.150 ML changed to Ag/Ag/Pd/Ag(111).

Drift of two-dimensional vacancy islands on the Si(100) surface under electromigration conditions

The paper presents an experimental study of morphological changes of the Si(100) surface under electromigration conditions that was carried out using the in situ method involving ultrahigh vacuum reflection electron microscopy. The study aims to determine the temperature dependence of the effective electric charge of an adsorbed atom on the Si(100) surface. A system of concentric two-dimensional vacancy islands was formed on the surface of Si(100) samples by low-energy argon ion sputtering and subsequent high-temperature annealing. Quasi-equilibrium conditions were created on the sample surface by compensating the sublimating flow from the surface from an external silicon source. The video images of the drift of vacancy islands were recorded under the conditions of electromigration with the compensation of sublimation. Based on the processing and analysis of video images, the authors described the dependence of the velocity of motion of vacancy islands on the Si(100) surface for various temperatures and the direction of the electric current along and across the dimer rows with the (2 × 1) superstructure inside the island. It is shown that the drift rate of vacancy islands does not depend on their size under quasi-equilibrium conditions. A simplified one-dimensional theoretical model has been constructed. It includes one atomic step moving by a detachment of atoms from the step and their drift under the force of electromigration in the absence of desorption and deposition of atoms on the surface. Based on the proposed model, the effective electric charge is estimated, and the temperature dependence of the effective charge in the temperature range of 1010 to 1120 °C is obtained. The absolute value of the effective charge decreases linearly with increasing temperature. The sign of the effective charge is negative, and its average value is Z = –0.5 ± 0.3 elementary charges. The obtained results can be used for creating structures with a countable number of atomic steps and act as secondary measures of height with reference to the silicon crystal lattice.

Time evolution of GaAs(111) surface morphology and desorption rate during Langmuir evaporation: Monte Carlo simulation

The high-temperature GaAs annealing kinetics is studied by Monte Carlo simulation. The evaporation rate dependences on the annealing time are obtained for (111)A and (111)B substrate surfaces in a 800–1200 K temperature range. The surface morphology evolution and evaporation kinetics are shown to be dependent on the GaAs substrate orientation. The congruent evaporation of (111)A surface corresponds to the layer-by-layer evaporation mode, while, on the (111)B surface, multilayer vacancy islands are formed. At temperatures exceeding the congruent evaporation temperature, the initial evaporation stage before the gallium droplet formation agrees with the congruent evaporation mode. The appearance of Ga droplets on the surface, corresponding to the start of incongruent evaporation regime, results in a sharp fall of the GaAs evaporation rate. It is found that the Ga droplet on vicinal surfaces locally inverts the step movement direction.

Sub-100 nm high spatial frequency periodic structures driven by femtosecond laser induced desorption in GaAs

This paper presents a previously unreported mechanism for the formation of High Spatial Frequency Laser Induced Periodic Surface Structures (HSFL) in GaAs upon irradiation by femtosecond laser pulses (repetition rate = 1 kHz, τ = 150 fs, λ = 390 nm) that is driven by point defect diffusion, desorption of surface atoms, and roughening of the surface. The HSFL have trenches that are 100 nm deep, an average spatial period of 65 nm, and are completely below the original surface. Sub-100 nm periodicity with high depth to period aspect ratio has not been previously observed in GaAs. In the proposed mechanism, laser irradiation generates point defects that diffuse to the surface. Interstitials that reach the surface can be easily desorbed and the remaining vacancies coalesce into vacancy islands. This results in a rough surface, which can excite surface plasmon polaritons. Despite our observations of periodicity corresponding to SPPs, calculations done using the excited dielectric function indicate that SPPs should not be supported. This points to the need to incorporate the cumulative effects of laser irradiation in existing models. This paper also presents evidence that desorption can also occur during HSFL formation in GaAs when irradiated with 780 nm in vacuum. The HSFL have the similar spatial period as GaAs irradiated in air with 780 nm, but completely below the original surface.

Shedding Light on the Interfacial Structure of Low-Coverage Alkanethiol Lattices

A comprehensive description of the self-assembly process of alkanethiols on Au(111) is presented, focused on the initial formation of the lying down phases. Low-coverage monolayers are prepared by the disintegration of Au144(RS)60 nanoclusters on the reconstructed (22 × ?3)-Au(111) surface. The method provides a limited number of thiols together with a large excess of gold adatoms. Scanning tunneling microscopy and density functional theory calculations were employed to study the transition between low to high thiolate coverage phases. The process involves different lattices and surface transformations, including thiyl radicals on the herringbone reconstruction, radical-induced herringbone lifting, and the formation of energetically similar metastable phases formed by RS-Au-RS moieties. Results also show that the transition is slow, and different surface structures can coexist on the same sample. Along the process, the first source of Au adatoms to form the RS-Au-SR moieties is the lifting of the herringbone reconstruction because of the lower energetic cost to extract the extra Au atom. However, for hexanethiol (and shorter alkanethiols) at low coverage, additional Au adatoms must be taken from terraces leading to vacancy islands. This process can be entirely suppressed by growing the lying down phases in the presence of an excess of Au adatoms. Taken together, our results shed light on the elusive initial steps of thiol adsorption on clean reconstructed Au, showing that the RS-Au-SR staple motif is also present at the interface of low-coverage self-assembled monolayers.

Surface-Alloying during Pb Underpotential Deposition on Au

Surface-alloys can form spontaneously during the deposition of one metal onto another, especially in systems with a large lattice mismatch that do not have bulk counterparts (so-called immiscible systems). The spontaneous surface alloying has been widely studied in Ultra High Vacuum (UHV) [1], but these materials have not been exploited much in practical applications. Surface-alloying in an electrochemical environment is an ideal route to the design of new functional materials. Electrochemically controlled surface-alloying would allow room-temperature conditions and better scalability making it more viable and versatile for practical applications.The focus of our study was the surface-alloying during underpotential deposition (UPD) of Pb on Au(111) [2,3]. As shown in Fig 1.a, the cyclic voltammetry (CV) of Pb UPD on Au (111) has asymmetric anodic and cathodic peaks (labelled in the order of their associated appearance). The most positive anodic peak A2 is often associated with the dealloying of Pb incorporated in the Au surface. The electrochemical measurements and the in-situ Scanning Tunnelling Microscopy (STM) characterisation during Pb UPD on Au(111) have shown that the Pb ML stripping is followed by a significant Au surface roughening [2]. During Pb ML formation and dissolution, the top-surface Au atoms get displaced, creating highly mobile Au ad-islands and vacancy islands [2]. The in-situ surface stress measurements have shown a tensile stress relaxation caused by the alloying/dealloying of Pb-Au during Pb ML formation and dissolution [2,4-5].In this study, we used two different electrochemical protocols to examine the surface alloying process: 1) repeated potential cycling within the UPD range and 2) polarisation at a fixed potential in the UPD region. By following the changes in electrochemical behaviour, we established the range of potentials and conditions at which the surface-alloys form. In addition to that, we investigated the structural and compositional changes by surface sensitive techniques such as X-ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron Spectroscopy (UPS), Energy-Filtered Photoemission Electron Microscopy (EF-PEEM) and Work Function (WF) mapping.To study the 'dynamic' alloying, the potential was cycled repeatedly between 0.0 V and 0.5 V (vs Pb/Pb2+), as marked in Fig 1.a, over the potential range negative from the A2 anodic peak. The potential was cycled up to 400 times. After each 100th cycle, the potential was extended to the whole Pb UPD region to observe the changes in the cathodic and anodic peaks. For illustration, Fig 1.b and Fig 1.c show the changes of peaks A2 and A3 with the number of cycles. While both peaks show changes of the shape and total charge, we observe a significant shift of the peak A2 suggesting dramatic changes associated with the morphology of the surface and the amount of alloyed Pb in the top surface layer. The Pb-Au surface alloy composition, based on the charge under the A2 peak, increase from 17% up to 25% of Pb (after 400 cycles) as shown in Fig 1.d. Similar changes of the CV peaks have been observed after the polarisation at the fixed potential of 0.2 V (vs Pb/Pb2+) in the Pb UPD region for different times (15 min - 60 min).Furthermore, the Pb UPD on Au(111) has been studied in the presence of chloride. These results show gradual changes in positions and shapes of all characteristic peaks of the Pb UPD leading to complete disappearance of the A2 dealloying peak, which have not been reported before.

Langmuir evaporation of GaAs(1 1 1)A and GaAs(1 1 1)B: Monte Carlo simulation

The Langmuir evaporation (LE) of GaAs vicinal surfaces with various crystallographic orientations is analyzed at the atomistic level by Monte Carlo simulation. The evolution of the surface morphology and LE characteristics (gallium and arsenic evaporation rates, congruent evaporation temperature Tc, effective activation energy of Ga desorption) are studied under the increasing annealing temperature for GaAs(111)A and GaAs(111)B singular, and vicinal surfaces with various atomic step density. At low temperatures T < Tc, the step-terraced morphology of a vicinal surface is preserved in the LE process. At high temperatures T > Tc, the characteristics and crystallographic orientation specificity of surface morphology are determined by the formation of liquid Ga droplets. It is shown by simulation that, as compared to the (111)B surface, on the (111)A surface, the congruent evaporation temperature is substantially higher and is more sensitive to the atomic step density, as well as to the concentration of intentionally introduced defects on the terraces of the initial step-terraced surface. These observations are explained by two morphological peculiarities of LE on (111)A and B surfaces: (i) on the (111)A surface, Ga droplets penetrate deep into the substrate; (ii) on the (111)B surface, vacancy islands are more readily formed on terraces.

Structural and morphological instabilities of the Si(1 1 1)-7 × 7 surface during silicon growth and etching by oxygen and selenium

Using in situ reflection electron microscopy and ex situ atomic force microscopy, we have studied the morphological stability of large-scale (~10–100 μm) Si(111)-7 × 7 terraces during silicon growth and etching by oxygen and selenium. On the large-scale terraces, silicon growth at substrate temperatures T = 600–770 °C and Si deposition rates R = 0.002–0.2 BL/s proceeds in multilayer mode. Based on RMS surface roughness scaling W ∝ Θβ, we have discerned three modes of morphological instability caused by (I) effective adatom diffusion along step edges at low T and R (β ≈ 0.33), (II) effective diffusion and fast step motion along disordered “1 × 1” regions in 7 × 7 domain boundaries at intermediate T and R (β ≈ 0.2), and (III) accumulation of Si adatoms in high-atom-density “1 × 1” regions on the uppermost terraces at high T and R (β ≈ 0.5). The etching of the singular Si(111)-7 × 7 surface by oxygen leads to the slow development of multilayer morphology, while selenium-induced etching preserves flat surface morphology with periodic 2D vacancy island nucleation, growth, and coalesce, which is attributed to Se adatom diffusion. On the step-bunched surface, the Si or Se adatom diffusion to the step bunches leads to the self-organization of pyramidlike or valley-like morphology during Si growth or Se-induced etching, respectively.

Etching of step-bunched Si(1 1 1) surface by Se molecular beam observed by in situ REM

Using in situ ultrahigh vacuum reflection electron microscopy, we have first visualized the evolution of step-bunched Si(1 1 1) surface during Se beam etching depending on substrate temperature T and terrace size required for survival of etching-induced vacancies. Below T ~ 630 °C, the etching proceeds in layer-by-layer mode via periodic nucleation of small 2D vacancy islands without noticeable motion of steps. However, in 630–830 °C interval, the etching of the 7 × 7 reconstructed Si(1 1 1) step-bunched surface is realized by both step flow and continuous nucleation and growth of 2D vacancy islands on wide terraces, which leads to the excavation of initially flat surface between step bunches. At T > 830 °C, the etching proceeds by step flow with preservation of initial surface morphology. We have found that the Si(1 1 1) surface etching by Se beam is characterized by 2.65 eV activation energy that limits etching kinetics in low-temperature range when the whole Si(1 1 1) surface is converted to disordered impurity-induced Si(1 1 1)“1 × 1”-Se phase.

Electrochemical Design of Nanoporous Gold with Tunable Porosity Using Lead Underpotential Deposition

Nanoporous gold (NPG) is a metamaterial with unique physicochemical, mechanical, and plasmonic properties of interest for various applications in catalysis and electrocatalysis, energy conversion and storage, (bio)sensing, actuation, and surface-enhanced Raman scattering.1-3 A monolithic structure with the nanoscales size of ligaments and rich surface electrochemistry3 make the NPG materials promising for many energy applications such highly active electrocatalysts for energy conversion, and three-dimensional substrates for energy storage in supercapacitors and lithium ion batteries. The length scale of NPG porosity (ligaments & open pores) and its effect on the properties is an essential aspect for such a broad range of applications. The size of porosity can range from a few nanometres to several microns and can be controlled by parent alloy composition, the method of dealloying (chemical, electrochemical) as well as the temperature of dealloying and annealing. Using the electrochemical surface processes to change and fine-tune size, uniformity and hierarchy of the ligament network has been attracting attention as a possible tool for manipulation and design of new and improved NPG functionalities 4, 5 Here we present an all-electrochemical approach to creating NPG thin films with different size of porosity from AgxAu1-x alloys. AgxAu1-x alloys of various compositions were electrodeposited at the constant potential from a thiosulfate-based solution by controlling the potential and concentration of Ag+ and Au+ ions in solution6. Following the structural (SEM, AFM) and composition (XPS) characterisation of the electrodeposited alloys, the selective dissolution of Ag under electrochemical control was used to create NPG with different size of porosity (5 - 15 nm). A process of Pb monolayer underpotential deposition (UPD) has been shown to be an excellent analytical tool to measure the surface area of as created NPG structures7. In this work, we demonstrate that this UPD process featuring surface stress driven surface allying can also be used as an electrochemical tool to shape and change NPG structure. The in-situ STM and electrochemical studies on Au(111) have shown that Pb ML formation is accompanied by significant Au surface restructuring. The top-surface atoms get displaced by alloying at low Pb ML coverage, followed by dealloying at high Pb-coverage creating a highly mobile Au ad-island and vacancy islands that with repeated cycling can lead to a significant surface morphology changes8. We observed that repeated potential cycling of Pb UPD on NPG resulted in ‘coarsening’ of the nanoporous structure with the effect similar to a so called ‘electrochemical annealing’. Combining the electrochemical and structural characterisation were followed the evolution of porosity and showed that changes of the surface area and ligament/pore size could be controlled by the range of the potentials, scan rate and a number of cycles applied.

Electromigration Effect on Vacancy Islands Nucleation on Si(100) Surface during Sublimation

By means of in situ ultrahigh vacuum reflection electron microscopy, the nucleation of vacancy islands on wide terraces of the Si(100) surface is investigated. The temperature dependence of the displacement of a vacancy island nucleation center is determined in the process of heating a sample with a dc electric current. On the basis of a theoretical model, the effective electric charge of addimers is estimated in the direction across dimer rows of the surface. The effective charge has a positive sign and does not exceed 15 units of the elementary charge in the temperature range of 1020–1120°C.

New aspects of the surface chemistry of sulfur on Au(111): Surface structures formed by gold-sulfur complexes

Sulfur adsorption on gold surfaces has been extensively studied because of the key role of sulfur species in heterogeneous catalysis, and, more recently, due to the interest in the synthesis of anisotropic gold nanoparticles with potential applications in medicine that involves sulfide reduction. Here we report new surface structures for sulfur on Au(111) by combining in situ scanning tunneling microscopy in aqueous sodium sulfide solutions and density functional theory calculations. Our results show two related lattices, (3√3 × 3√3) R30° (θ = 0.22) and (√7 × √7) R19.1° (θ = 0.57), that involve AuS3 complexes as building blocks. Gold‑sulfur complexes are formed by the lifting of gold atoms from the substrate surface as revealed by density functional theory calculations. These species, intermediate between adsorbed S in the well-known (√3 × √3)-R30○ lattice and adsorbed polysulfides in organized rectangular structures, explain the surface coverage of gold vacancy islands, a fingerprint of S adsorption on Au(111).

Importance of point defect reactions for the atomic-scale roughness of III–V nanowire sidewalls

The surface morphology of III–V semiconductor nanowires (NWs) protected by an arsenic cap and subsequently evaporated in ultrahigh vacuum is investigated with scanning tunneling microscopy and scanning transmission electron microscopy. We show that the changes of the surface morphology as a function of the NW composition and the nature of the seed particles are intimately related to the formation and reaction of surface point defects. Langmuir evaporation close to the congruent evaporation temperature causes the formation of vacancies which nucleate and form vacancy islands on {110} sidewalls of self-catalyzed InAs NWs. However, for annealing temperatures much smaller than the congruent temperature, a new phenomenon occurs: group III vacancies form and are filled by excess As atoms, leading to surface AsGa antisites. The resulting Ga adatoms nucleate with excess As atoms at the NW edges, producing monoatomic-step islands on the {110} sidewalls of GaAs NWs. Finally, when gold atoms diffuse from the seed particle onto the {110} sidewalls during evaporation of the protective As cap, Langmuir evaporation does not take place, leaving the sidewalls of InAsSb NWs atomically flat.

Effective determination of surface potential landscapes from metal-organic nanoporous network overlayers

Determining the scattering potential landscape for two-dimensional superlattices provides key insight into fundamental quantum electron phenomena. Theoretical and semiempirical methods have been extensively used to simulate confinement effects of the two-dimensional electron gas (2DEG) on superlattices with a single scatterer in the form of vicinal surfaces and dislocation networks or isolated structures such as quantum corrals and vacancy islands. However, the complexity of the problem increases when the building blocks (or scatterers) are heterogeneous, as in metal-organic nanoporous networks (MONNs), since additional potentials may come into play. Therefore, the parametrization of the surface potential landscape is often inaccurate, leading to incorrect scattering potentials. Here, we address this issue with a combination of scanning tunneling microscopy/spectroscopy, angle resolved photoemission spectroscopy and Kelvin probe force microscopy measurements together with electron plane-wave expansion simulations on a MONN grown on Cu(111). This experimental-theory approach, enables us to capture the 2DEG response to the intricate scattering potential landscape, and reveals systematic modeling procedures. Starting from a realistic geometry of the system, we determine the repulsive scattering potentials for both molecules and coordinated metal adatoms, the latter contradicting the established simulation framework. Moreover, we reveal local asymmetries and subtle renormalization effects of the 2DEG that relate to the interaction of the MONN and the underlying substrate.

Evolution of Micropits on Large Terraces of the Si(111) Surface during High-Temperature Annealing

The transformation of micropits on large terraces of the Si(111) surface containing no vicinal atomic steps has been investigated by in situ ultrahigh-vacuum reflection electron microscopy upon thermal annealing of the substrate in the range of 1200–1400°C. A procedure for the formation of micropits on large terraces of the Si(111) surface with the application of focused-ion-beam (Ga+) technology has been proposed. It has been found that the micropit decay kinetics varies upon reaching the critical radius Rcrit, which is caused by the activation of nucleation of two-dimensional vacancy islands on the micropit bottom. A theoretical model describing variations in the lateral sizes of the micropit both before and after reaching Rcrit has been proposed. Based on analysis of the found temperature dependence of the nucleation frequencies of two-dimensional vacancy pits on the micropit bottom, the effective energy of nucleation of a vacancy island has been determined to be 4.1 ± 0.1 eV.

Эволюция микролунок на широких террасах поверхности Si(111) в процессе высокотемпературного отжига

We have investigated in situ the morphological transformation of the Si(111) surfaces with micro-pits at large terraces during high-temperature annealing at T = 1200–1400°C. Experimental observation of the micro-pits kinetic decay have been performed by means of ultrahigh vacuum reflection electron microscopy. Focused ion beam system have been used for micro-pits creation at Si(111) terraces of large size. We have found that kinetic of micro-pit decay processes is affected by two dimensional vacancy islands nucleation at the micro-pit bottom when the micro-pit reaches the critical lateral size. The simple theoretical model has been proposed for describing the changes of the lateral size of micro-pit. The temperature dependence of two-dimensional vacancy islands nucleation frequency at micro-pit bottom is found to be described by the activation energy of 4.1 ± 0.1 eV.