Atomic-scale Investigations Research Articles

Atomic scale investigations on CdxZn1−xSe quantum dots: Correlation between the composition and emission properties

Atom probe tomography and photoluminescence spectroscopy have been used to study CdxZn1−xSe quantum dots embedded in a ZnSe layer grown on a (001) GaAs substrate. Atom probe tomography analyses show significant cadmium incorporation in the center of the dots surrounded by poor cadmium region. These measurements illustrate that the maximum cadmium concentration in the quantum dots is significantly higher than the concentration estimated by transmission electron microscopy. The composition and size of quantum dots obtained by atom probe tomography have been used to calculate the transition energies including excitonic and strain effects.

Revealing the Strain-Hardening Mechanisms of Advanced High-Mn Steels by Multi-Scale Microstructure Characterization

We have investigated the strain-hardening mechanisms across the relevant scales in a Fe-22Mn-0.6C (wt.%) twinning induced plasticity steel by multi-scale microstructure characterization. The approach makes use of electron microscopy techniques such as electron channeling contrast imaging (ECCI) to characterize microstructure features at the micro/nanoscale, and atomic-scale investigations of partitioning behavior across interfaces and solid solution/clustering effects by atom probe tomography (APT). The contribution of most relevant microstructure features to strain hardening is analyzed.

Probing Electronic Properties of Graphene on the Atomic Scale by Scanning Tunneling Microscopy and Spectroscopy

AbstractAtomic scale investigations of the electronic properties of graphene are playing a crucial role in understanding and tuning the exotic properties of this material for its potential device applications. Scanning tunneling microscopy (STM) and spectroscopy (STS) are unique techniques for atomic scale investigations and have been extensively used in graphene research. In this article, we review recent progresses in STM and STS studies of the electronic properties of suspended graphene as well as graphene supported by different substrates including graphite, metals, silicon carbide, silicon dioxide and boron nitride.

Atomic scale investigations of the gate controlled tunneling effect in graphyne nanoribbon

Configuration and transport properties of zigzag graphyne nanoribbon (n = 2) are investigated by means of the first-principles calculations and non-equilibrium Green's function in this work. We demonstrated the controllability of the graphyne's conductivity by gate bias, and the tunneling behavior induced by gate and drain voltages was investigated systemically. The characteristics of Id-Vd, Id-Vg, as well as the evolutions of current with electron temperature elevation were explored. The device exhibits a tunneling ratio around 103, and the state art of tunneling operations of the tunneling field effect transistor in this split-new material was achieved.

Atomic scale study of the life cycle of a dislocation in graphene from birth to annihilation

Dislocations, one of the key entities in materials science, govern the properties of any crystalline material. Thus, understanding their life cycle, from creation to annihilation via motion and interaction with other dislocations, point defects and surfaces, is of fundamental importance. Unfortunately, atomic-scale investigations of dislocation evolution in a bulk object are well beyond the spatial and temporal resolution limits of current characterization techniques. Here we overcome the experimental limits by investigating the two-dimensional graphene in an aberration-corrected transmission electron microscope, exploiting the impinging energetic electrons both to image and stimulate atomic-scale morphological changes in the material. The resulting transformations are followed in situ, atom-by-atom, showing the full life cycle of a dislocation from birth to annihilation. Our experiments, combined with atomistic simulations, reveal the evolution of dislocations in two-dimensional systems to be governed by markedly long-ranging out-of-plane buckling.

Atomic scale investigations of ultra-thin GaInN/GaN quantum wells with high indium content

Using scanning transmission electron microscopy (STEM), we have studied ultra-thin (<2 nm) GaInN quantum wells (QWs) on c-plane GaN with high indium content (>25 %) suitable for blue-green light emitting devices. We are able to analyze the QW on an atomic scale with high resolution STEM and derive the indium content quantitatively. In our analysis, we find that indium is not only incorporated into the QW but also into the barriers under certain growth conditions. We observe indium tails or even plateau-like structures in the barriers, caused by excess indium being supplied during quantum well growth.

Atomistic modeling of an impurity element and a metal–impurity system: pure P and Fe–P system

An interatomic potential for pure phosphorus, an element that has van der Waals, covalent and metallic bonding character, simultaneously, has been developed for the purpose of application to metal–phosphorus systems. As a simplification, the van der Waals interaction, which is less important in metal–phosphorus systems, was omitted in the parameterization process and potential formulation. On the basis of the second-nearest-neighbor modified embedded-atom method (2NN MEAM) interatomic potential formalism applicable to both covalent and metallic materials, a potential that can describe various fundamental physical properties of a wide range of allotropic or transformed crystalline structures of pure phosphorus could be developed. The potential was then extended to the Fe–P binary system describing various physical properties of intermetallic compounds, bcc and liquid alloys, and also the segregation tendency of phosphorus on grain boundaries of bcc iron, in good agreement with experimental information. The suitability of the present potential and the parameterization process for atomic scale investigations about the effects of various non-metallic impurity elements on metal properties is demonstrated.

Inversion boundary induced grain growth in ZnO ceramics: from atomic-scale investigations to microstructural engineering

In semiconducting materials special boundaries play the key role in crystal growth. They introduce an abrupt structural and chemical anisotropy, which is readily reflected in an unusual microstructure evolution, whereas their local structure affects the physical properties of semiconducting materials. These effects, however, can be exploited to tailor the electronic and optical properties of the materials, as demonstrated in this review. The presented topic fits in the field of preparatory stage of phase transformations, manifested through evolution of chemically induced structural faults. In the noncentrosymmetric structure of ZnO, inversion boundaries (IBs) are the most common type of planar faults that can be triggered by the addition of specific spinel-forming dopants (Sb2O3, SnO2, TiO2). In addition to conventional HRTEM techniques several new methods were developed to resolve crystallography and atomic-scale chemistry of IBs. The absolute orientation of the polar c-axes on both sides of the IB was determined by a novel quantitative microdiffraction method, providing a reliable identification of crystal polarity in noncentrosymmetric crystals. To determine sub-monolayer quantities of dopants on the IB, we developed a special technique of analytical electron microscopy using concentric electron probe (CEP) in EDS or EELS mode, providing more accurate and precise results than any other available technique. Knowing the local crystal chemistry of IBs we were able to design experiments to identify their formation mechanism. IBs nucleate in the early stage of grain growth as a dopant-rich topotaxial 2D reaction product on Zn-terminated surfaces of ZnO grains. Soon after their nucleation, ZnO is epitaxially grown on the inherent 2D phase in an inverted orientation, which effectively starts to dictate anisotropic growth of the infected crystallite. In very short time the grains with IBs dominate the entire microstructure in ZnO ceramics via IB-induced exaggerated grain growth mechanism. This phenomenon was used to design physical properties of ZnO-based varistor ceramics, whereas the bottom-up approach demonstrated here provides the basic tool for microstructural engineering of functional materials in virtually any system that is prone to formation of special boundaries.

Surface diffusion coefficient determination by uniaxial tensile strain in Pb/Cu(111) surface systems

Atomic-scale investigations on the surface diffusions of Pb adatoms on uniaxially strained Cu(111) were performed using molecular dynamics simulations and ab initio calculations methods. Tensile strains in [ 1 1 ¯ 0 ] reduced the surface diffusion coefficient, while the [112] strains increased the diffusion coefficient until the Cu(111) substrate was elongated 4% uniaxially. The surface diffusion energy barriers for the three different paths: [ 1 1 ¯ 0 ] , [112] and [ 2 1 ¯ 1 ] , were calculated using ab initio calculations. The energy barrier only for the [ 1 1 ¯ 0 ] direction was increased when tensile strain in the [ 1 1 ¯ 0 ] direction was applied, while the [112] strain decreased the energy barriers for all three diffusion paths. The [ 1 1 ¯ 0 ] strain altered the easy surface diffusion direction from the x-direction to the y-direction, while the [ 2 1 ¯ 1 ] strain created isotropic diffusion coefficients in lateral directions.

Point defects in multicomponent ordered alloys: Methodological issues and working equations

The aim of this work is to give the independent-point-defect thermodynamics of ordered compounds a sufficiently general flavour, adapted to and working for multicomponent alloys. Generalizing previous approaches, we first show that an appropriate description for a crystal with point defects allows treatment of the practically important pressure and defect volume parameters in the grand canonical framework, the equivalence of which is explicited with the closer to experiments isothermal–isobaric conditions. Since industrial applications often involve multialloyed compounds, we then derive an operational tool for atomic-scale investigations of long-range order alloys with complex crystallographies and multiple additions.

Applications of Atomic Force Microscopy to Surface Investigations in Liquids

Atomic force microscopy (AFM) using frequency modulation (FM) detection has been widely used for the atomic scale investigations of various materials. However, high-resolution imaging in liquids by FM-AFM is severely hindered by the extreme reduction of the Q-factor due to the hydrodynamic interaction between the cantilever and the liquid. Recently the use of the small amplitude mode and the large noise reduction in the cantilever deflection sensor brought a great progress in FM-AFM imaging in liquids. In this article the details of the problems in FM-AFM imaging in liquids as well as the general directions of the improvements are described. The present status of the high-resolution FM-AFM imaging is also presented. Furthermore, future prospects and possible applications of FM-AFM in liquids are discussed.

Aberration-corrected HRTEM of the Incommensurate Misfit Layer Compound (PbS)1.14NbS2

AbstractThe development of tunable spherical aberration (Cs) imaging correctors for medium-voltage transmission electron microscopes (TEM) offers new opportunities for atomic-scale in-vestigations of materials. A very interesting class of microstructures regarding a variety of dif-ferent physical properties are the transition metal dichalcogenide misfit layer compounds exhibit-ing a high density of incommensurate interfaces due to their stacked nature. In the present study, the benefits coming along with the set-up of negative CSimaging (NCSI) conditions (in TEM) are demonstrated by means of different examples regarding local inhomogeneities in (PbS)1.14NbS2crystals that can not be dissected in such detail by averaging x-ray techniques.

Atomic-scale theoretical investigations of compound semiconductor surfaces

Atomic-scale theoretical investigations of clean and covered low-index surfaces of compound semiconductors are presented. Particular emphasis is laid on the role of the electron counting rule (ECR) in governing plausible surface reconstructions. Trends are presented for the characeristic tilt of the topmost atomic layer and the highest localised phonon mode on nonpolar III–V(1 1 0) surfaces, including III-nitride compounds. Reconstructions and electronic properties of polar surfaces are explained in terms of dimer formation on (0 0 1), and trimer and/or chain formation on (1 1 1) faces. It is pointed out that some surface reconstructions stabilise as a result of a balance between the ECR and minimization of adsorbate-induced local distortion. This is demonstrated for the long-range ordered reconstruction on the Sb:GaAs(1 1 1)B surface.

Interface strain profiling in ultrathin SiO2 gate oxides with medium energy ion scattering spectroscopy

Medium energy ion scattering spectroscopy (MEIS) could identify ∼1 nm interface layer with compressive strain, which depends sensitively on the interface treatment conditions such as oxynitridation, ozone oxidation, tilt of Si(0 0 1) substrates. The interface strain relaxation always shows improvements in gate oxide reliability. Atomic scale investigations of strain profiles with MEIS are reviewed for SiO 2 /Si(0 0 1) interfaces.

Atomic-Scale Structural Investigations on the Nucleation of Cubic Boron Nitride from Amorphous Boron Nitride under High Pressures and Temperatures

By controlling the microstructure of the starting materials, i.e., by ball-milling a commercial hexagonal boron nitride (h-BN) to an amorphous boron nitride (a-BN), the subsequent high-pressure and high-temperature (HP−HT) induced phase transformation has been significantly facilitated. Namely, cubic boron nitride (c-BN) forms at 900 °C and achieves accomplishment at 1350 °C under 7.7 GPa, which are significantly less-extreme conditions than that of crystalline h-BN under similar HP−HT treatments. High-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) clarified the nucleation mechanism at an atomic scale. It demonstrated that the c-BN phase nucleates directly from the sp3-hybridized amorphous matrix, which is originally induced by ball-milling and is therefore responsible for the reduced HP−HT conditions. This c-BN nucleation mechanism is completely different from the so-called diffusionless “puckering” mechanism involved in the HP−HT experiments starting fro...

Imaging the atomic arrangements on the high-temperature reconstructed α-Al2O3(0001) surface

Alumina is a technologically important oxide crystal because of its use as a catalyst and as a substrate for microelectronic applications. A precise knowledge of its surface atomic structure is a prerequisite for understanding and controlling the physical processes involved in many of its applications. Here we use a dynamic scanning force microscopy technique to image directly the atomic structure of the high-temperature phase of the alpha-Al2O3(0001) surface. Evidence for a surface reconstruction appears as a grid of protrusions that represent a rhombic unit cell, and we confirm that the arrangement of atoms is in the form of surface domains with hexagonal atomic order at the centre and disorder at the periphery. We show that, on exposing the surface to water and hydrogen, this surface structure is important in the formation of hydroxide clusters. These clusters appear as a regular pattern of rings that can be explained by self-organization processes involving cluster-surface and cluster-cluster interactions. Alumina has long been regarded as the definitive test for atomic-resolution force microscopy of insulators so the whole class of insulating oxides should now open for direct atomic-scale surface investigations.

Atomic scale investigations of the Co/Pt(111) interface structure and magnetic properties

The interface structure of an ultrathin Co overlayer on a Pt(111) crystal was investigated with atomic-layer resolution medium-energy ion scattering spectroscopy and surface magneto-optical Kerr effect (SMOKE). For a 7 ML Co, interdiffusion begins at 673 K to form a heavily distorted Co–Pt surface alloy layer with little change in SMOKE intensity. However, annealing at 773 K formed a 30 atomic-layer-thick Co–Pt substitutional alloy with 3.7% maximum tensile strain, at which the SMOKE intensity increased more than 200%. The enhancement of the Kerr intensity is discussed with the interface alloy formation.