2x1 Surface Research Articles

Dissociation of N2 on a Si(111)-7x7 Surface at Room Temperature.

We demonstrate that the strong N2 bond can be efficiently dissociated at low pressure and ambient temperature on a Si(111)-7x7 surface. The reaction was experimentally investigated by scanning tunnelling microscopy and X-ray photoemission spectroscopy. Experimental and density functional theory results suggest that relatively low thermal energy collision of N2 with the surface can facilitate electron transfer from the Si(111)-7x7 surface to the π*-antibonding orbitals of N2 that significantly weaken the N2 bond. This activated N2 triple bond dissociation on the surface leads to the formation of a Si3 N interface.

Effect of surface morphology and interfaces on longitudinal phonon thermal conductivity in Ge(001) and Si/Ge(001) thin-film structures

In this work the method of non-equilibrium molecular dynamics was used to study the longitudinal phonon thermal transport at 300 K in nanoscale homogenous Ge(001) and layered Si/Ge(001) thin films with p(2x1) surface reconstruction along different directions. The appearance of thermal transport anysotropy in the films under consideration, which is due to both the surface morphology and sharp Si/Ge interfaces, has been established. For the direction when the dimers and Si-Ge bonds at the interface lie in a plane parallel to the direction of the heat flow, the lowest thermal conductivity is observed (~5-18 W/(m·K) in the range from ~1 to 27 nm). It is shown that for films with thicknesses of more than 13 nm for all directions, layered films have a lower thermal conductivity compared to homogenous ones. In this case, the role of the surface morphology and interfaces is reduced to different degrees of phonon localization and compensation for more heat-conducting Si layers, respectively. Keywords: thermal conductivity, molecular dynamics, Si/Ge thin-film, surface, interfaces.

Diamond (111) surface reconstruction and epitaxial graphene interface

The evolution of the diamond (111) surface as it undergoes reconstruction and subsequent graphene formation is investigated with angle-resolved photoemission spectroscopy, low energy electron diffraction, and complementary density functional theory calculations. The process is examined starting at the C(111)-(2x1) surface reconstruction that occurs following detachment of the surface adatoms at 920 {\deg}C, and continues through to the liberation of the reconstructed surface atoms into a free-standing monolayer of epitaxial graphene at temperatures above 1000 {\deg}C. Our results show that the C(111)-(2x1) surface is metallic as it has electronic states that intersect the Fermi-level. This is in strong agreement with a symmetrically {\pi}-bonded chain model and should contribute to resolving the controversies that exist in the literature surrounding the electronic nature of this surface. The graphene formed at higher temperatures exists above a newly formed C(111)-(2\times1) surface and appears to have little substrate interaction as the Dirac-point is observed at the Fermi-level. Finally, we demonstrate that it is possible to hydrogen terminate the underlying diamond surface by means of plasma processing without removing the graphene layer, forming a graphene-semiconductor interface. This could have particular relevance for doping the graphene formed on the diamond (111)surface via tuneable substrate interactions as a result of changing the terminating species at the diamond-graphene interface by plasma processing.

Indirect mechanism of Au adatom diffusion on the Si(100) surface

Calculations of the diffusion of a Au adatom on the dimer reconstructed Si(100)-2x1 surface reveal an interesting mechanism that differs significantly from a direct path between optimal binding sites, which are located in between dimer rows. Instead, the active diffusion mechanism involves promotion of the adatom to higher energy sites on top of a dimer row and then fast migration along the row, visiting ca. a hundred sites at room temperature, before falling back down into an optimal binding site. This top-of-row mechanism becomes more important the lower the temperature is. The calculations are carried out by finding minimum energy paths on the energy surface obtained from density functional theory within the PBEsol functional approximation followed by kinetic Monte Carlo simulations of the diffusion over a range of temperature from 200 K to 900 K. While the activation energy for the direct diffusion mechanism is calculated to be 0.84 eV, the effective activation energy for the indirect mechanism is on average 0.56 eV.

The growth processes and crystal structure of Ca silicides films grown by MBE at 500 °C on a Si(001) substrate

This article investigates the growth of calcium (Ca) silicides on the Si(001)2x1 surface during molecular beam epitaxy (MBE) from two sources of Ca and Si with a variable flow ratio from three to 10 at a fixed substrate temperature of 500 °C. According to atomic force microscopy (AFM), the grown films consist of grains with a mutually perpendicular orientation and have pronounced grain boundaries with an inhomogeneous relief. The best crystalline qualities of the films, both CaSi and CaSi2, were confirmed for the minimum ratio (about 3.0) of Ca to Si deposition flows by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) data in both cross-sections and in planar sections. At the same time, according to XRD and HRTEM data, for all formed films over the CaSi(010) layer on Si(001), the growth of hR3-CaSi2 was observed with epitaxial relationships: hR3-CaSi2(001)//CaSi(010)//Si(001). An analysis of these studies showed that calcium monosilicide (CaSi) is the first silicide phase in the Ca–Si(001) system at 500 °C due to (1) the maximum modulus of its heat of formation, (2) a decrease in the Ca accommodation coefficient to silicon less than unity and the formation of a mixture close to CaSi, (3) a better matching between the parameters of the CaSi(010) plane lattice and the Si(001) plane lattice, and (4) lower CaSi (010)/Si (001) interface energy compared with the CaSi2(001)/Si(001) system. It is assumed that the formation of CaSi2 occurs in the subsurface region of the growing film at 500 °C due to increased diffusion of Ca atoms into the Si substrate and increased intergranular diffusion of Si atoms, which leads to the formation of a mixture with the CaSi2 composition. Both factors create conditions for the crystallization of CaSi2 on the surface of the CaSi layer at Т = 500 оС. The epitaxial ordering of CaSi2 on the CaSi surface and the decrease in the energy of hR3-CaSi2(001)/CaSi(010) interface is associated with small distortions of their lattices during superstructural matching, which is proved by modeling their matching.

Chlorine insertion and manipulation on the Si(100)- 2×1 -Cl surface in the regime of local supersaturation

We insert and manipulate a single chlorine atom in chlorine monolayer on a Si(100)-2x1 surface using a scanning tunneling microscope. Two objects were created - a Cl atom in a groove between two dimer rows, and bridge-bonded Cl on a silicon dimer. Changing the voltage polarity leads to conversion of the objects into each other. Anisotropic movement of the objects at 77 K is mediated by two different diffusion mechanisms: hopping and crowdion-like motion. Insertion of a Cl atom in a groove between two dimer rows leads to the formation of a dangling bond on a third-layer Si atom. At positive sample voltage bias, the first object is positively charged, while the second object can be neutral or negatively charged depending on silicon sample doping.

A Theoretical Study of the Energetic Stability and Electronic Structure of Terminated and P-Doped Diamond (100)-2x1 Surfaces

The combined effect of P-doping and surface-termination on the energetics, and especially the electronic structure, of a diamond (100) surface, has in the present study been investigated by using a Density Functional Theory method. The diamond surface was terminated with any of the following species: H, F, OH, Obridge and NH2 . These adsorbates have earlier experimentally been proven crucial for e.g. applications based on surface electrochemistry. The observed results were analysed with the purpose to obtain a deeper knowledge about the atomic-level cause to the observed effects by the P doping and surface termination. The P dopant was found to have a very minor influence on the averaged adsorption energy for the various terminating species (i.e. with less than 0.17 eV). Moreover, the adsorbates were found to reduce the stability of the P-dopant in the diamond lattice. When analysing the results of the calculated partial density of states, the P dopant was found to contribute with band gap states below the conduction band, out of which one is a donor band. In addition, the surfaces with their terminating species will contribute with empty band gap states just below the CBM of the diamond surfaces. Hence, the combination of P doping and surface termination will induce both donor and acceptor states in the diamond surface band gap, which will improve the usefulness of these specific diamond surfaces in various electronic devices. The work function of a diamond surface is one specific properties that will be affected by substitutional doping and surface termination. Within the present study, the terminating species were found to render a strong impact on the surface work function (as compared to a non-terminated diamond (100)-2x1 surface), with calculated work function values that were even further decreased by P-doping.

PM-19 Deep learning analysis of Si(111)-7x7 surface in atomic force microscopy

PM-19 Deep learning analysis of Si(111)-7x7 surface in atomic force microscopy

Effect by Diamond Surface Modification on Biomolecular Adhesion.

Diamond, as material, show very attractive properties. They include superior electronic properties (when doped), chemical inertness, controllable surface termination, and biocompatibility. It is thus clear that surface termination is very important for those applications where the implant material is based on diamond. The present theoretical work has focused on the effect of diamond surface termination, in combination with type of surface plane, on the adhesion of important biomolecules for vascularization and bone regeneration. These biomolecules include Arginine-Glycine-Aspartic acid (RGD), Chitosan, Heparin, Bone Morphogenetic Protein 2 (BMP2), Angiopoietin 1 (AGP1), Fibronectin and Vascular Endothelial Growth Factor (VEGF). The various surface planes are diamond diamond (100)-2x1 and (111). The theoretical results show that the non-covalent binding of these biomolecules is in proportion with their molecular weights. Moreover, three groups of biomolecules were observed for both types of surface planes. The most strongly binding biomolecule was the BMP2 molecule. The smaller polypeptides (RGD, Chitosan and Heparin) formed a less strongly binding group. Finally, the biomolecules VEGF, Fibronectin and Angiopoietin showed bond strengths numerically in between the other two groups (thereby forming a third group). Moreover, the (111) surface was generally observed to display a stronger bonding of the biomolecules, as compared with the (100)-2x1 surface.

Fingerprints of single nuclear spin energy levels using STM – ENDOR

We performed STM-ENDOR experiments where the intensity of one of the hyperfine components detected in ESR-STM is recorded while an rf power is irradiated into the tunneling junction and its frequency is swept. When the latter frequency is near a nuclear transition a dip in ESR-STM signal is observed. This experiment was performed in three different systems: near surface SiC vacancies where the electron spin is coupled to a next nearest neighbor 29Si nucleus; Cu deposited on Si(111)7x7 surface, where the unpaired electron of the Cu atom is coupled to the Cu nucleus (63Cu, 65Cu) and on Tempo molecules adsorbed on Au(111), where the unpaired electron is coupled to a Nitrogen nucleus (14N). While some of the hyperfine values are unresolved in the ESR-STM data due to linewidth we find that they are accurately determined in the STM-ENDOR data including those from remote nuclei, which are not detected in the ESR-STM spectrum. Furthermore, STM-ENDOR can measure single nuclear Zeeman frequencies, distinguish between isotopes through their different nuclear magnetic moments and detect quadrupole spectra. We also develop and solve a Bloch type equation for the coupled electron-nuclear system that facilitates interpretation of the data. The improved spectral resolution of STM - ENDOR opens many possibilities for nanometric scale chemical analysis.

Atomic force microscope manipulation of Ag atom on the Si(111) surface

We present first-principles total-energy electronic-structure calculations that provide the microscopic mechanism of the Ag atom diffusion between the half unit cells (HUCs) on the Si(111)-(7x7) surface with and without the tip of the atomic force microscope (AFM). We find that, without the presence of the AFM tip, the diffusions between the two HUCs are almost symmetric with the energy barrier of about 1 eV in the both directions. The diffusion is a two-step process with an intermediate metastable configuration in which the Ag atom is at the boundary of the HUCs. With the presence of the tip, we find that the reaction pathways are essentially the same, but the energy barrier in one direction is substantially reduced to be 0.2 - 0.4 eV by the tip, while that of the diffusion in the reverse direction remains larger than 1 eV. The Si tip reduces the energy barrier more than the Pt tip due to the flexibility of the tip apex structure. In addition to the reduction of the barrier, the tip traps the diffusing adatom preventing the diffusion in the reverse direction. Also we find that the shape of the tip apex structure is important for the trapping ability of the adatom. When the tip apex structure is blunt, the adatom interacts with the tip atom other than the tip apex atom. The bond formation between the AFM tip atom and the surface adatom is essential for the atom manipulation using the AFM tip. Our results show that the atom manipulation is possible with both the metallic and semiconducting AFM tips.

Molecular and atomic manipulation mediated by electronic excitation of the underlying Si(111)-7x7 surface

We report the local atomic manipulation properties of chemisorbed toluene molecules on the Si(111)-7x7 surface and of the silicon adatoms of the surface. Charge injected directly into the molecule, or into its underlying bonding silicon adatom, can induce the molecule to change bonding site. The voltage dependence of the rates of these processes match closely with scanning tunnelling spectroscopy of the toluene and adatom species. The branching ratio between toluene molecules which are moved to a neighbouring site, or those that travel further is invariant to voltage, suggesting a common final manipulation step for both injection into the molecule and into the bonding adatom site. At low temperatures the rate of silicon adatom manipulation matches that of toluene manipulation, further suggesting that all these manipulation processes are driven by electronic excitation of the underlying silicon surface. Our results therefore suggest that a common non-adiabatic process mediates atomic and molecular manipulation induced by the STM on the Si(111)-7x7 surface and may also mediate similar manipulation induced by the laser irradiation of the Si(111)-7x7 surface.

Multistep atomic reaction enhanced by an atomic force microscope probe on Si(111) and Ge(111) surfaces

We present first-principles total-energy electronic-structure calculations that provide the microscopic mechanism of the adatom interchange reaction on the Sn- and Pb-covered Ge(111)-(2x8) and the Sb-covered Si(111)-(7x7) surfaces with and without the tip of the atomic force microscope (AFM). We find that, without the presence of the AFM tip on the Ge surface, the adatom interchange occurs through the migration of the adatom, the spontaneous formation of the dimer structures of the two adatoms, the dimer-dimer structural transitions that induce the exchange of the positions of the two adatoms, and then the backward migration of the adatom. We also find that the dimer structure is unfeasible at room temperature on the Si surface and the adatom interchange are hereby unlikely. With the presence of the tip, we find that the reaction pathways are essentially the same for the Ge surface but that the energy barriers of the migration and the exchange processes are substantially reduced by the AFM tip. We further find that the AFM tip induces the spontaneous formation of the dimer structure even on the Si surface, hereby opening a channel of the interchange of the adatoms. Our calculations show that the bond formation between the AFM tip atom and the surface adatom is essential for the atom manipulation using the AFM tip.

Effect of carbon doping on magnetic properties of Mn/Si interface

High-resolution photoelectron spectroscopy with synchrotron radiation and magnetic linear dichroism in Mn 3p core-level photoemission has been used to study the effect of carbondoping on the initial stages of Mn/Si interface formation and its ferromagnetic ordering. It is shown that the manganese deposition on Si(100)2x1 surface with and without carbon doping results in the formation of the interfacial MnSi silicide and the solid solution of silicon in manganese. The Mn film covered with segregated Si begins to grow on the samples surface at coverage of 9 Å of Mn. It is shown that in-plane ferromagnetic ordering of the interfaces arises after of 2-4 Å Mn in both systems, however, the remanent magnetization of Mn-C/Si is higher than that of Mn/Si.

Formation of Potassium Nanostructures on Reconstructed High Index Si (5512)2x1 Surface for Night Vision Devices (NVD)

In the present study potassium nanostructure was grown on the high index reconstructed Si (5512) 2x1 surface due to their potential application in the night vision devices (NVD). The formation of potassium atoms (K) on high index Si semiconductor under controlled kinetic and thermodynamic growth conditions having desired shape and size with low work function were studied. The studies have been performed on a reconstructed high index silicon surface at room temperature as well as on high substrate temperatures such as (375, 550 and 680 oC). The structural properties of the high index trenched template have been exploited for the development of desired nanostructures of different size and shape. It is constructed to a planner surface having a single domain with 1-D symmetry and relatively largest unit cell (5.35 nm). In the present study, growth of potassium nanostructures on the high index reconstructed Si (5 5 12) surface occur, which can be potentially utilized in night vision devices. The experiments were performed in-situ in a Ultra high vacuum (UHV) system in-housed with various surface sensitive techniques such as Auger Electron Spectroscopy (AES), Electron Energy Loss Spectroscopy (EELS), and Low Energy Electron Diffraction (LEED).

Atomistic Frictional Properties of the C(100)2x1-H Surface

Density functional theory- (DFT-) based ab initio calculations were used to investigate the surface-to-surface interaction and frictional behavior of two hydrogenated C(100) dimer surfaces. A monolayer of hydrogen atoms was applied to the fully relaxed C(100)2x1 surface having rows of C=C dimers with a bond length of 1.39 Å. The obtained C(100)2x1-H surfaces (C–H bond length 1.15 Å) were placed in a large vacuum space and translated toward each other. A cohesive state at a surface separation of 4.32 Å that is stabilized by approximately 0.42 eV was observed. An increase in the charge separation in the surface dimer was calculated at this separation having a 0.04 e transfer from the hydrogen atom to the carbon atom. The Mayer bond orders were calculated for the C–C and C–H bonds and were found to be 0.962 and 0.947, respectively.σC–H bonds did not change substantially from the fully separated state. A significant decrease in the electron density difference between the hydrogen atoms on opposite surfaces was seen and assigned to the effects of Pauli repulsion. The surfaces were translated relative to each other in the (100) plane, and the friction force was obtained as a function of slab spacing, which yielded a 0.157 coefficient of friction.

Ge(100)-2ｘ1表面への室温酸素初期吸着確率の並進エネルギー依存性

Ge(100)-2ｘ1表面への室温酸素初期吸着確率の並進エネルギー依存性

放射光XPSを用いたGe(100)-2×1表面の室温酸化物の時分割観察

放射光XPSを用いたGe(100)-2×1表面の室温酸化物の時分割観察

Ge(100)-2×1表面の超音速酸素分子線による室温酸化促進

Ge(100)-2×1表面の超音速酸素分子線による室温酸化促進

Noninvasive embedding of single Co atoms in Ge(111)2 × 1 surfaces

We report on a combined scanning tunneling microscopy (STM) and density functional theory (DFT) based investigation of Co atoms on Ge(111)2x1 surfaces. When deposited on cold surfaces, individual Co atoms have a limited diffusivity on the atomically flat areas and apparently reside on top of the upper pi-bonded chain rows exclusively. Voltage-dependent STM imaging reveals a highly anisotropic electronic perturbation of the Ge surface surrounding these Co atoms and pronounced one-dimensional confinement along the pi-bonded chains. DFT calculations reveal that the individual Co atoms are in fact embedded in the Ge surface, where they occupy a quasi-stationary position within the big 7-member Ge ring in between the 3rd and 4th atomic Ge layer. The energy needed for the Co atoms to overcome the potential barrier for penetration in the Ge surface is provided by the kinetic energy resulting from the deposition process. DFT calculations further demonstrate that the embedded Co atoms form four covalent Co-Ge bonds, resulting in a Co4+ valence state and a 3d5 electronic configuration. Calculated STM images are in perfect agreement with the experimental atomic resolution STM images for the broad range of applied tunneling voltages.