Atomic Reconstruction Research Articles

Compositional and electrical properties of line and planar defects in Cu(In,Ga)Se2 thin films for solar cells – a review

The present review gives an overview of the various reports on properties of line and planar defects in Cu(In,Ga)(S,Se)2 thin films for high‐efficiency solar cells. We report results from various analysis techniques applied to characterize these defects at different length scales, which allow for drawing a consistent picture on structural and electronic defect properties. A key finding is atomic reconstruction detected at line and planar defects, which may be one mechanism to reduce excess charge densities and to relax deep‐defect states from midgap to shallow energy levels. On the other hand, nonradiative Shockley–Read–Hall recombination is still enhanced with respect to defect‐free grain interiors, which is correlated with substantial reduction of luminescence intensities. Comparison of the microscopic electrical properties of planar defects in Cu(In,Ga)(S,Se)2 thin films with two‐dimensional device simulations suggest that these defects are one origin of the reduced open‐circuit voltage of the photovoltaic devices. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Atomic Image Reconstruction from Atomic Resolution Holography Using <i>L</i><sub>1</sub>-Regularized Linear Regression

Inverse x-ray fluorescence holography provides new knowledge about the local atomic structure around dopants in crystals. The three-dimensional atomic image is reconstructed from the inverse x-ray fluorescence hologram using a calculation. The performance of the algorithm used for this calculation is important. The Barton method, which is based on the Fourier transform, is typically used. However, images reconstructed by this method sometimes contain artifacts. I developed an algorithm (SPEA-L1) using an iterative method with L1 regularization and report its performance. [DOI: 10.1380/ejssnt.2016.158]

Spin-polarized gapped Dirac spectrum of unsupported silicene

We study effects of the spin–orbit interaction and the atomic reconstruction of silicene on its electronic spectrum. As an example we consider unsupported silicene pulled off from Pb(111) substrate. Using first principles density functional theory we show that the inversion symmetry broken arrangement of atoms and the spin–orbit interaction generate a spin-polarized electronic spectrum with an energy gap in the Dirac cone. These findings are particularly interesting in view of the quantum anomalous and quantum valley Hall effects and should be observable in weakly interacting silicene-substrate systems.

Microstructuring of silicon single crystals by fiber laser in high-speed scanning mode

Subject of Study. The surface structure of the silicon wafers (substrate) with a thermally grown silicon dioxide on the surface (of SiO2/Si) is studied after irradiation by pulse fiber laser of ILI-1-20 type. The main requirements for exposure modes of the system are: the preservation of the integrity of the film of silicon dioxide in the process of microstructuring and the absence of interference of surrounding irradiated areas of the substrate. Method. Studies were carried out on silicon wafers KEF-4,5 oriented in the crystallographic plane (111) with the source (natural) silicon dioxide (SiO2) with thickness of about 4 nm, and SiO2 with 40 nm and 150 nm thickness, grown by thermal oxidation in moist oxygen. Also, wafers KHB-10 oriented in the plane (100) with 500 nm thickness of thermal oxide were investigated. Irradiation of SiO2/Si system was produced by laser complex based on ytterbium fiber pulse laser ILI-1-20. Nominal output power of the laser was 20 W, and the laser wavelength was λ = 1062 nm. Irradiation was carried out by a focused beam spot with a diameter of 25 microns and a pulse repetition rate of 99 kHz. The samples with 150 nm and 40 nm thickness of SiO2 were irradiated at a power density equal to 1,2·102 W/cm2, and the samples of SiO2 with 500 nm thickness were irradiated at a power density equal to 2,0·102 W/cm2. Scanning was performed using a two-axis Coordinate Scanning Device based on VM2500+ drives with control via a PC with the software package "SinMarkTM." Only one scan line was used at the maximum speed of the beam equal to 8750 mm/s. Morphology control of the irradiated samples was conducted by an optical microscope ZeissA1M with high-resolution CCD array. A scanning probe microscope Nanoedicator of the NT-MDT company was used for structural measurements. Main Results. It has been shown that at a single exposure of high-frequency pulsed laser radiation on SiO2/Si system, with maintaining the integrity of the SiO2 film, similar symmetric microstructures are formed on the silicon surface. We suggest that the reason for their appearance is the reconstruction of the surface of the silicon arising in the process of recrystallization of the silicon melt surface in the irradiated region of the substrate. The morphology of these microstructures is due to several factors: crystallographic orientation of the plate, original (before the thermal oxidation) reconstruction of the plate surface, elastic stresses of SiO2/Si, laser treatment (thermal, corpuscular, wave). Exposure modes for the observed structures have been determined. Practical Relevance. For the first time in the microstructuring of SiO2/Si by fiber laser in the high-speed scanning mode, images of pieces have been obtained, which can indirectly imply that the reconstruction of atoms on the silicon substrate surface occurs before the thermal oxidation. Clearly marked localization of laser energy at selected irradiation modes enables to assert the possibility of detection and control of structural defects for the elements of semiconductor electronic devices.

B21-O-02In situTEM observation of Cu-doped graphene

We directly observed atomic-scale transformations of Cu-doped graphene by aberration-corrected TEM, operated at 80 kV. Many metals, including Cr, Ti, Pd, Ni, and Al, were experimentally reported to etch graphene [ 1 ]. Cu atoms, however, promoted graphene reconstruction rather than etching in our experiments. They preferentially replaced C atoms in defective graphene areas and modified their structures via repeated C–C rotations, formation and mending of nanopores, resulting in the rotation of grains mediated by contaminants and lattice defects. Furthermore, Cu atoms promoted to connect the neighbouring edges of two graphene layers and created stable nanopores in bilayer graphene. Edge-closed nanopores were predicted to exhibit wide range electronic properties (from semiconductor to metallic), depending on the size and distance from neighbouring nanopores [ 2 ]. We also observed bilayer graphene with other impurity atoms, such as Si and Pt; they, however, etched only one layer and did not create through-pores. Our results suggested that individual Cu atoms can catalyse the reconstruction of carbon atoms and create novel carbon nanostructures.

Atomic characterization of Au clusters in vapor-liquid-solid grown silicon nanowires

By correlating atom probe tomography with other conventional microscope techniques (scanning electron microscope, scanning transmission electron microscope, and scanning tunneling microscopy), the distribution and composition of Au clusters in individual vapor-liquid-solid grown Si nanowires is investigated. Taking advantage of the characteristics of atom probe tomography, we have developed a sample preparation method by inclining the sample at certain angle to characterize the nanowire sidewall without using focused ion beam. With three-dimensional atomic scale reconstruction, we provide direct evidence of Au clusters tending to remain on the nanowire sidewall rather than being incorporated into the Si nanowires. Based on the composition measurement of Au clusters (28% ± 1%), we have demonstrated the supersaturation of Si atoms in Au clusters, which supports the hypothesis that Au clusters are formed simultaneously during nanowire growth rather than during the cooling process.

Young’s Modulus Variation of Carbon Nanotubes Due to Defects Associated with Atomic Reconstruction of Random Vacancies

This paper studies the effect of three different types of defects on the Young’s modulus of zigzag and armchair single-walled carbon nanotubes. The defects are randomly introduced into the structure with a maximum distribution density of 2%. The defects include monovacancies, before and after reconstruction of dangling bonds, resulting finally to the Stone-Wales defect. As the length of model effects computation time significantly, finite element models are created in pristine and very short tube lengths to investigate the effect of model length on the results obtained. The investigation shows that the introduction of defects generally weakens the elastic strength of tubes but to rather different degrees. However, the structure is capable of restoring its elastic strength by rearranging its atoms in form of the Stone-Wales defect. The use of shorter nanotube models on the other hand can lead to accurate results if appropriate cylindrical boundary conditions are defined.

Identifying Atomic Reconstruction at Complex Oxide Interfaces Using Quantitative STEM

We present a novel electron microscopy characterization of atomic reconstruction at epitaxial complex oxide interfaces. One of the most important aspects of oxide heterostructures is the discontinuity in polarity at the interface, which can lead to many exciting new properties that cannot be observed in bulk materials, such as metal-insulator transitions and superconductivity (e.g. [1]). The conflict of polarity may also result in atomic and electronic reconstructions at the interface, which may directly affect the transport properties of the interface. However, identifying the exact atomic reconstruction at the interface is a challenging task because the reconstruction is usually confined to an interfacial region thinner than 1 unit cell. Cross-sectional TEM or STEM images can reveal some information about the interfacial reconstruction, but it is difficult to obtain the reconstruction pattern along the depth direction (parallel to the beam) from the projected image. Therefore the identification of interfacial reconstruction requires a technique that can acquire atomic scale 3-dimensional (3D) information.

Cross-sectional geometry dependence of spontaneous phase transformation of copper nanowires

Molecular dynamics simulations have been performed to investigate the spontaneous phase transformation of copper nanowires. It is found that the spontaneous phase transformation exhibits distinct dependence on the cross-sectional geometry of the nanowires and can lead to the reconstruction of atoms into different atomistic configurations, e.g., pure hexagonal-close-packed crystals, fivefold deformation twins, and core/shell structures. For single-crystal copper nanowires, the critical cross-sectional size, above which no spontaneous phase transformation can occur, is determined. The physical mechanisms underlying the complicated transformation behavior are analyzed from the viewpoints of energy and stresses.

Effect of CC bond breakage on diamond tool wear in nanometric cutting of silicon

It is well known that diamond is one of the most ideal cutting tool for materials, but the rapid tool wear can make surface integrity of the machined surface decline sharply during the nanometric cutting process for a single crystal silicon. Thus, a research on the wear mechanism of the diamond tool is of tremendous importance for selecting measures to reduce tool wear so as to extend service life of the tool. In this paper, the molecular dynamics simulation is applied to investigating the wear of the diamond tool during nanometric cutting for the single crystal silicon. Tersoff potential is used to describe the CC and SiSi interactions, and also the Morse potential for the CSi interaction. The rake and flank faces are diamond (111) and (112) planes respectively. A new method, by the name of 6-ring, is proposed to describe the bond change of carbon atoms. This new method can extract, all the worn carbon atoms in diamond tool, whose accuracy is higher than the conventional coordination number method. Moreover, the graphitized carbon atoms in the diamond tool also can be extracted by the combination of these two methods. Results show that during the cutting process, the CC bond's breaking in the surface layer of the diamond tool leads to the transformation of hybrid structure of the carbon atoms at both ends of the broken bond, from sp3 to sp2. Following to the bond breaking, the bond angle between the surface carbon atoms increases to 119.3 whose hybrid structure has changed, and the length between nearest neighboring atoms quickly decreases to 0.144 nm, indicating that the space structure formed by these carbon atoms has changed from 3D net structure of diamond to plane structure of graphite. Hence, the carbon atoms in the tool surface whose space structure has changed due to bond breaking should be defined as worn carbon atoms, but not only the carbon atoms whose hybrid structure has changed. The structure defects at both edges of the diamond tool are much more serious, which make the energy of CC bonds at the edges weakened with the enhancement of defects. The bonds with lower energy are broken under the effect of high temperature and shear stress, which also produces the tool wear. The graphitization occurs at the tool of the cutting tool because the structure defects there are the most serious. The reconstruction of the carbon atoms with lower coordination number causes its neighboring region to produce serious distortion, which leads to easy breaking of CC bonds in this region.

Atomic resolution tomography reconstruction of tilt series based on a GPU accelerated hybrid input-output algorithm using polar Fourier transform.

Advances in diffraction and transmission electron microscopy (TEM) have greatly improved the prospect of three-dimensional (3D) structure reconstruction from two-dimensional (2D) images or diffraction patterns recorded in a tilt series at atomic resolution. Here, we report a new graphics processing unit (GPU) accelerated iterative transformation algorithm (ITA) based on polar fast Fourier transform for reconstructing 3D structure from 2D diffraction patterns. The algorithm also applies to image tilt series by calculating diffraction patterns from the recorded images using the projection-slice theorem. A gold icosahedral nanoparticle of 309 atoms is used as the model to test the feasibility, performance and robustness of the developed algorithm using simulations. Atomic resolution in 3D is achieved for the 309 atoms Au nanoparticle using 75 diffraction patterns covering 150° rotation. The capability demonstrated here provides an opportunity to uncover the 3D structure of small objects of nanometers in size by electron diffraction.

Vacancy-Induced Intramolecular Junctions and Quantum Transport in Metallic Carbon Nanotubes

First-principles calculations are carried out to study atomic reconstruction and the electronic structures of metallic armchair and zigzag single-walled carbon nanotubes (SWCNT) in the presence of hexa-vacancy defects. The introduction of hexa-vacancy defects can effectively give rise to the formation of intramolecular junctions in both armchair- and zigzag-type SWCNTs. Two kinds of vacancy distribution, six atoms removed along the zigzag chain and a pristine hexagonal lattice, lead to asymmetric and symmetric intramolecular junctions with respect to the axis of the SWCNT after reconstruction, respectively. It is found that the (7,7) symmetric intramolecular junction exhibits more favorable electronic transport compared to the corresponding asymmetric intramolecular junction, whereas the (12,0) symmetric exhibits smaller current in contrast to the asymmetric configuration. This unexpected behavior is attributed to a competition between a widening of the transmission windows that opens additional transmission channels and transport quenching due to orbital misalignment induced by the applied bias.

Atomic reconstruction of niobium (111) surfaces

The {111} surface of niobium single crystals has been investigated by ultra-high vacuum scanning tunneling microscopy. An atomic reconstruction has been experimentally observed and has been characterized by a lattice constant two times larger than that expected for a body-centered cubic structure. Moreover, an upper incipient structure is observed on these (2×2) surfaces. The upper unit cell is 23 times larger than the bulk one and is also rotated by 30° with respect to the bulk structure. A model is proposed to explain the competition between a covering of the free surface by a non-dense atomic structure and an island formation of niobium atoms in the dense configuration. The experimental results are finally discussed with the help of ab initio calculations used to determine the surface energies of the associated atomic configurations.

Alleviating polarity-conflict at the heterointerfaces of KTaO3/GdScO3 polar complex-oxides

We have synthesized and investigated the heterointerfaces of KTaO3 (KTO) and GdScO3 (GSO), which are both polar complex-oxides along the pseudo-cubic [001] direction. Since their layers have the same, conflicting net charges at interfaces, i.e., KO(−1)/ScO2(−1) or TaO2(+1)/GdO(+1), forming the heterointerface of KTO/GSO should be forbidden due to strong Coulomb repulsion, the so-called polarity conflict. However, we have discovered that atomic reconstruction occurs at the heterointerfaces between KTO thin-films and GSO substrates, which effectively alleviates the polarity conflict without destroying the hetero-epitaxy. Our result demonstrates one of the important ways to create artificial heterostructures from polar complex-oxides.

Optical in Situ Study of InP(100) Surface Chemistry: Dissociative Adsorption of Water and Oxygen

Semiconductors designated for solar water-splitting need to be simultaneously stable and efficient in the charge transfer over the interface to the aqueous electrolyte. Although InP(100) has been employed as photocathode for several decades, no experimental data on its initial interaction with water is available. We study reaction mechanisms of well-defined surfaces with water and oxygen employing photoelectron and in situ reflection anisotropy spectroscopy. Our findings show that reaction path and stability differ significantly with atomic surface reconstruction. While the mixed-dimer In-rich surface exhibits dissociative water adsorption featuring In–O–P rather than unfavorable In–O–In bond topologies, the H-terminated, P-rich surface reconstruction is irreversibly removed. Oxygen exposure attacks the In-rich surface more efficiently and additionally modifies, unlike water exposure, bulk-related optical transitions. Hydroxyl is not observed, which suggests a dehydrogenation of adsorbed species already at ambient temperature. Our findings may benefit the design of InP(100) surfaces for photoelectrochemical water splitting.

Surface Structure and Morphology of M[CoM′]O4(M = Mg, Zn, Fe, Co and M′ = Ni, Al, Mn, Co) Spinel Nanocrystals—DFT+U and TEM Screening Investigations

Plane wave periodic GGA-PBE+U density functional theory calculations were used to study the structure, surface energy, and equilibrium shape of faceted nanocrystals for a series of cubic (Fd3m) 2–3 AB2O4 spinels with the following formula: Co[Co2]O4, Mg[Co2]O4, Zn[Co2]O4, Co[NiCo]O4, Co[MnCo]O4, Fe[FeCo]O4, and Co[Al2]O4. Their bulk geometries (lattice constants and oxygen u parameters) as well as electronic and magnetic properties were computed and compared with experimental data. All planes, (100), (110), and (111), exposed by the spinel nanocrystallites of equilibrium morphology were taken into account, and their atomic structure, reconstruction, and stabilization were elucidated and systematized in terms of the structural oxygen u parameter. The strongest relaxation of the A cations was observed for the (100) plane, whereas that for the B cations was on the (111) plane. By using the calculated surface energy values, the shapes of the spinel nanocrystallites were predicted by means of the Wulff constru...

Investigation of the red-shift of the emission of microwave solvothermally-grown europium doped calcium sulphide crystals

A distinct red-shift to 660 nm-featured longer wavelength of europium doped calcium sulphide nanocrystals was observed in a microwave (MW)-assisted solvothermal synthesis procedure. This red-shift was probably due to a combined effect from Mg2+ codoping and strain accumulation at grain boundaries. The latter originated from the formation of small grains with an average size of around 200 nm in micrometer sized crystals upon Mg2+ codoping. In particular, MW electromagnetic field suppressed grain growth and enabled a reconstruction of atoms at the inner grain boundary at which the field strength was intensified around rare earth ions in host lattices. This MW synthesis route provided an option to prepare luminescent crystals with the desired blue-excitable longer wavelength emission.

Physical properties of quasi-one-dimensional MgO andFe3O4-based nanostructures

We have studied the properties of several representative one-dimensional structures---MgO nanowires, ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ hollow nanotubes, ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ nanowires, and $\mathrm{Mg}\mathrm{O}/{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ core/shell nanotubes---by means of first-principles-based calculations. Each of these nanostructures reveals different electronic properties with novel electronic states due to the large surface/interface of the nanocylinders. Electronic states of the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ nanowire are localized around small clusters of atoms, and its bands appear with almost no energy dispersion. Localization is not a direct consequence of structural disorder; instead, it seems to be induced by an enhanced charge transfer due to the undercoordination on the surface. The combined effect of the $\mathrm{Mg}\mathrm{O}/{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ nanostructure shows that the MgO is well coated and even bulk-like states can be observed. However, the magnetite suffers important atomic reconstructions losing symmetries and increasing its atomic-like behavior. Effects of axial deformations on the properties and the relation of the results to potential technological applications are discussed.

Spontaneous polarization driven Mg concentration profile reconstruction in MgZnO/ZnO heterostructures

Atomic reconstruction at the interface of MgZnO and ZnO in molecular beam epitaxy grown heterostructures is investigated. Using secondary ion mass spectroscopy, we experimentally find that Mg atomic reconstruction depends on the polarity of the interface; it is not observed in n-type interfaces (MgZnO on Zn-polar ZnO) owing to electron accumulation, while in p-type interfaces (ZnO on Zn-polar MgZnO), Mg drastically redistributes into the ZnO layer. Combined with self-consistent calculation of band profiles and carrier distributions, we reveal that the observed Mg reconstruction is not due to thermal diffusion but consequences in order to avoid hole accumulation. This tendency implies inherent significant asymmetry of energy scales of atomic and electronic reconstructions between n-type and p-type interfaces.

Features of atomic images reconstructed from photoelectron, Auger electron, and internal detector electron holography using SPEA-MEM

Three-dimensional atomic images can be reconstructed from photoelectron, Auger electron, and internal detector electron holograms using a scattering pattern extraction algorithm using the maximum entropy method (SPEA-MEM) that utilizes an integral transform. An integral kernel function for the integral transform is the key to clear atomic image reconstruction. We composed the kernel function using a scattering pattern function and estimated its ability. Image distortion caused by multiple scattering was also evaluated. Four types of Auger electron wave functions were investigated, and the effect of these wave function types was estimated. In addition, we addressed refraction at the surface, the effects of data limitation, and energy and angular resolutions.