Scanning electron microscopy observation of surface acoustic wavepropagation in the LiNbO3 crystals with regular domainstructures

This paper reports a scanning electron microscopy investigation of surface acoustic wave propagation in the LiNbO₃ crystals with regular domain structures. The regular domain structures in the LiNbO ₃ crystals were formed by the method of the thermo-electric treatment after growth. We investigated two modes of the interaction between the surface acoustic waves and regular domain structures in the LiNbO₃ crystals: excitation of the surface acoustic waves by the curved regular domain structure, and propagation of the surface acoustic waves along and across the regular domain structures. It is shown that the regular domain structure in the first case can be used as an interdigital transducer for excitation of the surface acoustic waves through the longitudinal piezoelectric effect. In the second mode of the interaction we observed that the regular domain structure can be used as an acoustic wave-guide in the process of the propagation of the traveling surface acoustic waves along the regular domain structure. Also, we demonstrate the reflection of the surface acoustic waves from the domain walls in the process of surface acoustic wave propagation across the regular domain structure     

Domain Structure and Phase Symmetry of Multiferroic BiFeO3-PbTiO3 Single Crystals Studied by Piezoresponse Force Microscopy

Piezoresponse force microscopy (PFM) was used to investigate the domain structure of multiferroic BiFeO₃- PbTiO₃ single crystals with morphotropic phase boundary (MPB) compositions. The PFM data demonstrate the piezoelectricity and the PFM domain responses under the designed experimental setup reveal the coexistence of a rhombohedral, a tetragonal, and an orthorhombic phases in the MPB crystals, which is consistent with the results of structural refinements based on X-ray diffraction data.     

Morphology,structure, and mechanical properties of the surface of PbTe crystals after etching with H<Subscript>2</Subscript>O<Subscript>2</Subscript>–HBr–ethylene glycol solutions

The morphology and microstructure of PbTe surfaces polished with H₂O₂–HBr–ethylene glycol solutions have been studied by atomic force and electron microscopy techniques. The surface of PbTe single crystals after chemical–mechanical and dynamic chemical polishing with bromine-releasing etchants has been examined by optical microscopy and profilometry. The structural perfection and composition of the PbTe surface after chemical treatment have been determined by X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. The mechanical properties of the crystals have been studied using microindentation. Using X-ray microanalysis, we have monitored the concentrations of the host elements (Pb and Te) and possible contamination with chemical compounds present in the etchants and the solutions used to rinse the samples.     

Growth structure investigation of MgF2 and NdF3 films grown by molecular beam deposition on CaF2(111) substrates

The growth structure of MgF₂ and NdF₃ films grown on polished CaF₂(111) substrates deposited by molecular beam deposition has been investigated using transmission electron microscopy (TEM) of microfractographical and surface replications as well as cross-sectional TEM, atomic force microscopy, packing density, and absorption measurements. It has been shown that by taking advantage of ultrahigh vacuum environments and a special stratification property of MgF₂ and NdF₃ films, the preparation of nanocrystalline films of high packing density and low optical absorption is possible at a substrate temperature of 425 K.     

Using atomic-step-structured 6H-SiC(0001) surfaces for the calibration of nanotranslations in scanning probe microscopy

The silicon surfaces of 6H-SiC(0001) single crystals subjected to stepwise high-temperature annealing in vacuum have been studied by atomic force microscopy and scanning tunneling microscopy techniques. A special annealing procedure is proposed that yields a structured surface featuring regular atomicsmooth steps with heights of 0.75 and 1.5 nm. We propose using these structured crystal surfaces for calibrating vertical translations in scanning probe microscopes.     

Investigation of twin-wall structure at the nanometre scale using atomic force microscopy

The structure of twin walls and their interaction with defects has important implications for the behaviour of a variety of materials including ferroelectric, ferroelastic, co-elastic and superconducting crystals. Here, we present a method for investigating the structure of twin walls with nanometre-scale resolution. In this method, the surface topography measured using atomic force microscopy is compared with candidate displacement fields, and this allows for the determination of the twin-wall thickness and other structural features. Moreover, analysis of both complete area images and individual line-scan profiles provides essential information about local mechanisms of twin-wall broadening, which cannot be obtained by existing experimental methods. The method is demonstrated in the ferroelectric material PbTiO(3), and it is shown that the accumulation of point defects is responsible for significant broadening of the twin walls. Such defects are of interest because they contribute to the twin-wall kinetics and hysteresis.     

Periodic micro- and nanostructures in LiNbO<Subscript>3</Subscript>〈Gd〉 single crystals grown under unsteady-state conditions

Gd-doped lithium niobate single crystals having a regular domain structure with a period from 100 to 1 μm have been grown under highly unsteady thermal conditions. Examination by atomic force microscopy shows that such lithium niobate single crystals contain periodic nanostructures with a period from 10 to 100 nm in the region of regular domain structures. Raman scattering data suggest that the cation sublattice of the lithium niobate crystals contains a superstructural sublattice of defect clusters with a step of 1–2 nm. We conclude that lanthanide-doped lithium niobate single crystals contain periodic fractal structures in a range of length scales, from ≃ 1 nm to 100 μm.     

Structural defects as a result of Zn diffusion into CdTe single crystals

The presence of defects in CdZnTe crystals is detrimental for optoelectronic devices fabrication and therefore should be minimized. In this paper we present the characterization of structural defects on the surface and the cross-section of CdTe single crystals that were subjected to high temperature (up to 950 °C) diffusion of Zn. The defects were characterized by various X-ray techniques, optical microscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM). Quantitative data are obtained, a practical solution for reducing the defects is suggested and some implementations are discussed. Further effort is currently being made to investigate the lattice sites which are involved with the diffused Zn atoms near the surface and in the bulk.     

Domain structure and electrical properties of Gd-and Tm-doped lithium niobate single crystals

We have grown lithium niobate single crystals doped with rare-earth elements (Gd and Tm) and have examined etch patterns on their surfaces by atomic force microscopy. The thermal stability of their regular domain structure has been assessed, and their anomalous electrical properties have been studied in the temperature range 300–380 K.     

X-ray surface reflection and transmission topography of magnetic domain walls in Czochralski-grown nickel single crystals

Ferromagnetic domain walls are observed in large Czochralski-grown nickel single crystals by X-ray double crystal diffraction topography in the surface reflection geometry as well as in the transmission geometry. The images of magnetic domain walls in surface reflection topographs possess almost as good contrast as those in transmission topographs, and even reveal fine detailed structure. Based on preliminary arguments, the images observed in the surface reflection topographs are attributed to 180° walls intersecting the crystal surface obliquely, while the transmission topographs easily image 71° and 109° walls in the interior of the crystals.     

Direct imaging of the surfaces of poly(β) hydroxybutyrate and hydroxybutyrate oligomers by atomic force microscopy

Attempts have been made to image the fold surface of a single crystal of polyhydroxybutyrate (PHB) using the relatively new technique of atomic force microscopy (AFM). To overcome the obscuring of the fold surface by loose loops of polymer and chain ends, two different approaches were used. We first studied the single crystals of an oligomer of 32 HB units, which is known to fold once very tightly within a crystal, using AFM. Secondly, studies were made of single crystals of PHB which have been chemically degraded with methylamine to etch away the amorphous layer of loosely folded material, in an attempt to expose the fold surface. The crystals of the 32-mer had a similar morphology to those of the polymer PHB. However, at high magnification, lines of ridges were observed which ran parallel to the crystallographic b axis with a spacing of 0.7 nm, similar to the dimensions of the unit cell (0.58, 1.32, and 0.60 nm). It was not possible to differentiate between chain ends and folds. The partially etched PHB crystals maintained enough integrity to permit imaging by AFM, although surface detail could not be resolved on a molecular scale.     

Effect of amino acid doping on the growth and ferroelectric properties of triglycine sulphate single crystals

Effect of amino acids (l-leucine and isoleucine) doping on the growth aspects and ferroelectric properties of triglycine sulphate crystals has been studied. Pure and doped crystals were grown from aqueous solution by low temperature solution growth technique. The cell parameter values were found to significantly vary for doped crystals. Fourier transform infrared analysis confirmed the presence of functional groups in the grown crystal. Morphology study reveals that amino acid doping induces faster growth rate along b-direction leading to a wide b-plane and hence suitable for pyroelectric detector applications. Ferroelectric domain structure has been studied by atomic force microscopy and hysteresis measurements reveal an increase of coercive field due to the formation of single domain pattern.     

GaN single crystals grown on HVPE seeds in alkaline supercritical ammonia

Ammonothermal growth (synthesis in supercritical (sc) ammonia fluid) has the promise of producing large low defect gallium nitride crystals through the application of techniques similar to those used in hydrothermal growth. Retrograde solubility of GaN greater than 5% by weight using group I amides as mineralizers is demonstrated in high nickel content autoclaves at pressures of one to three kilobars and temperatures between 300 to 600°C. The above conditions were optimized to grow single-crystal GaN at rates up to 40 μm per day on one cm² seeds. Gallium nitride Hydride Vapor Phase Epitaxy (HVPE) seeds are placed in the higher temperature zone below the nutrient basket employing the same configurations used in reverse gradient hydrothermal growth of berlinite (AlPO₄). GaN single crystals grown by the ammonothermal technique were characterized by X-ray diffraction, photoluminescence, scanning electron microscopy (SEM), atomic force microscopy (AFM), and chemical etching. The nitrogen-terminated face tends to exhibita flatter surface morphology than the gallium-terminated face, which is made up of a series of hexagonal columns. Major impurities in the crystal include potassium from the mineralizer, metals from the autoclave, and oxygen. The nitrogen-terminated face incorporated a lower level of metallic impurities in comparison with the gallium-terminated face. Finally, several process phenomena such as ammonia decomposition, parasitic nucleation of GaN on the autoclave walls, impurity incorporation, and defect generation in single-crystal GaN layers grown on HVPE seeds are identified and their possible mechanisms are discussed.     

Atom inlays performed at room temperature using atomic force microscopy

The ability to manipulate single atoms and molecules laterally for creating artificial structures on surfaces is driving us closer to the ultimate limit of two-dimensional nanoengineering. However, experiments involving this level of manipulation have been performed only at cryogenic temperatures. Scanning tunnelling microscopy has proved, so far, to be a unique tool with all the necessary capabilities for laterally pushing, pulling or sliding single atoms and molecules, and arranging them on a surface at will. Here we demonstrate, for the first time, that it is possible to perform well-controlled lateral manipulations of single atoms using near-contact atomic force microscopy even at room temperature. We report the creation of 'atom inlays', that is, artificial atomic patterns formed from a few embedded atoms in the plane of a surface. At room temperature, such atomic structures remain stable on the surface for relatively long periods of time.     

Nanoscale current imaging of the conducting channels in proton exchange membrane fuel cells

The electrochemically active area of a proton exchange membrane fuel cell (PEMFC) is investigated using conductive probe atomic force microscopy (CP-AFM). A platinum-coated AFM tip is used as a nanoscale cathode in an operating PEMFC. We present results that show highly inhomogeneous distributions of conductive surface domains at several length scales. At length scales on the order of the aqueous domains of the membrane, approximately 50 nm, we observe single channel electrochemistry. I-V curves for single conducting channels are obtained, which yield insight into the nature of conductive regions across the PEM. In addition, we demonstrate a new characterization technique, phase current correlation microscopy, which gives a direct measure of the electrochemical activity for each aqueous domain. This shows that a large number ( approximately 60%) of the aqueous domains present at the surface of an operating Nafion membrane are inactive. We attribute this to a combination of limited aqueous domain connectivity and catalyst accessibility.     

A microstructural study on the surface and interface of CdTe/CdS solar cells

The surface and interface properties of CdTe/CdS solar cells, including interfacial mixing, surface and interface geometrical morphology, CdTe grain size and preferential crystal orientation of CdTe layers were studied using Auger electron spectroscopy (AES) depth profiling, atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, optical reflectance (OR) and X-ray diffraction (XRD) techniques. The correlation between the surface and interface properties and CdTe/CdS solar cell performance was also investigated. AES depth profiling was used to analyse the interdiffusion between the CdTe and CdS layers. Atomic force microscopy (AFM) suggests that the interfacial geometrical morphology has a significant influence on the photovoltaic property of CdTe/CdS solar cells. Rough interfaces tend to increase the photovoltaic conversion efficiency of solar cells because of multiple reflections. X-ray diffraction shows that polycrystalline CdTe/CdS solar cells with higher efficiencies appear to be orientated with more (1 1 1) planes of CdTe parallel to the macrosurface, but CdTe single crystals with differently indexed surface planes show almost the same reflection behaviour. Further theoretical and experimental analyses are therefore needed to clarify this observation. This revised version was published online in July 2006 with corrections to the Cover Date.     

An investigation on chain mobility in solid state polymer systems

In this paper we have investigated chain mobility in polyethylene below its melting temperature. The investigation techniques like atomic force microscopy (AFM), transmission electron microscopy (TEM), time-resolved small/wide angle X-ray scattering (SAXS/WAXS), time-resolved longitudinal acoustic mode (LAM) Raman spectroscopy have been used to follow the chain mobility within individual single crystals and regularly stacked crystals, as a function of temperature and time. Our observations on single crystals are that crystal thickness increases immediately on heating just above the crystallization temperature. In the regularly stacked polyethylene single crystals wherever the overlapping of at least two lamellae arises, thickening occurs during annealing via a mutual chain rearrangement between the adjacent crystals, which leads (ultimately) to a quantum increase, i.e., doubling, of the lamellar thickness. A model has been proposed to explain this quantum increase in the lamellar thickness. The fundamental findings have been extended for some applications.     

Atomic-scale study of electric dipoles near charged and uncharged domain walls in ferroelectric films

Ferroelectrics are materials exhibiting spontaneous electric polarization due to dipoles formed by displacements of charged ions inside the crystal unit cell. Their exceptional properties are exploited in a variety of microelectronic applications. As ferroelectricity is strongly influenced by surfaces, interfaces and domain boundaries, there is great interest in exploring how the local atomic structure affects the electric properties. Here, using the negative spherical-aberration imaging technique in an aberration-corrected transmission electron microscope, we investigate the cation-oxygen dipoles near 180 degrees domain walls in epitaxial PbZr(0.2)Ti(0.8)O(3) thin films on the atomic scale. The width and dipole distortion across a transversal wall and a longitudinal wall are measured, and on this basis the local polarization is calculated. For the first time, a large difference in atomic details between charged and uncharged domain walls is reported.     

Microstructural analysis of WO3 thin films on alumina substrates

WO₃ thin films have been deposited by thermal evaporation on an alumina oxide single crystal and annealed either in oxygen or in air. The morphology and the crystallographical structure for the as-deposited and the annealed films have been investigated by reflection high energy electron diffraction, atomic force microscopy and transmission electron microscopy. During annealing the WO₃ thin films recrystallise and undergo important morphological and structural changes: the annealed films exhibit large grains which have the monoclinic structure in epitaxial orientations. These grains are made of twinned microdomains, which are elongated in the [100] direction resulting of a preferential growth along the direction that corresponds to the smallest lattice parameter of WO₃.     

PTCDA molecules on an InSb(001) surface studied with atomic force microscopy

PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) molecular structures assembled on an InSb(001) c(8 × 2) reconstructed surface have been studied using frequency modulated atomic force microscopy. The high-resolution imaging of the structures is possible through repulsive interactions, using the constant height scanning mode. During initial stages of growth the [110] diffusion channel dominates as indicated by formation of long PTCDA molecular chains parallel to the [110] crystallographic direction on the InSb surface. For a single monolayer coverage a wetting layer of PTCDA is formed. Finally it is shown that the PTCDA/InSb is a promising system for building molecular nanostructures by manipulation of single molecules with the AFM tip.