High-resolution Transmission Electron Microscopy Lattice Research Articles

Te-rich Bi2Te3 thin films by electron−beam deposition: Structural, electrical, optical and thermoelectric properties

Tellurium-rich Bi2Te3 thin films are deposited by electron−beam evaporation technique at 300 K. These as-deposited thin films are further annealed at 100 °C, 200 °C and 300 °C for 1 h at a pressure of 3 × 10−4 Pa. X-ray diffraction (XRD) patterns of as-deposited films distinctly show Te phase along with Bi2Te3. Peak intensity ratio suggests the polycrystalline nature of as-deposited Bi2Te3 films. On vacuum annealing Te-rich Bi2Te3 films exhibit improved crystallinity with a c-axis preferred orientation. In addition, structural features related to Te and Bi2Te3 composition change with Te fraction diminishing on annealing at 300 °C. From Raman spectral studies, the presence of distinct Te-rich regions, predominantly within the interlayers of Bi2Te3, are discerned. Te becomes structurally integrated within the quintuples of Bi2Te3 lattice as intergrown layers. Disordered planar structures, mostly concentrated on crystallite surfaces result from Te accumulations as evidenced in high-resolution transmission electron microscopy lattice images and energy dispersive X-ray spectroscopy mapping. These are consistent with the observations from XRD and Raman studies further confirming Te-rich Bi2Te3 characteristics. Electrical properties of Te-rich Bi2Te3 thin films exhibit n-type semiconductor behaviour. Seebeck coefficient for as-deposited film is ~ 32 μV/K, which increases to ~ 97 μV/K on 200 °C annealing. Resistivity increases from 1.39 × 10−4 Ωcm to 18.76 × 10−4 Ωcm and power factor changes from 7.4×10−4W/K2m to 27.17×10−4W/K2m going through a maximum at 200 °C upon systematic annealing. Te-rich Bi2Te3 thin films annealed at 200 °C exhibit high power factor (∼29×10−4W/K2m) for a wide range of temperature gradients ( ΔT from 30 °C to 165 °C).

Improved quantification of curvature in high-resolution transmission electron microscopy lattice fringe micrographs of soots

Soot affects climate change and human health. Its structure is influenced by fuel and combustion conditions. Image analysis of high-resolution transmission electron microscopy lattice fringes indicates curvature is common and impacts reactivity. Here an improved curvature quantification determined the distributions of segment frequency, segment lengths, angles between segments, frequency of inflection points, relative inflection point locations, and overall angle changes for each fringe for three soots and a carbon black. Curvature was present in 28–49% of the fringes. Low curvature (<30°) account for the largest portion (61–76%). The most common inflection angles were 10–20°. Geometry optimized (ab initio) atomistic models containing centrally located pentagon rings or Stone-Thrower-Wales defects had angle changes 19–77° depending on viewing angle and molecule. The majority of curved fringes (71–78%) had a singular inflection point. Fringes were primarily within the range 6–15.0 Å length 4–100° cumulative angle. Tortuosity data demonstrated this commonly used parameter was limited for the evaluation of undulating curvature. Linking these data to structural defects will enhance linking reactivity to structure, capturing and improving structural features in atomistic representations. Changes in segment angles <45° however are not likely to be associated with centrally placed isolated pentagon rings.

Self-formed grain boundary healing layer for highly efficient CH3NH3PbI3 perovskite solar cells

Perovskite solar cells have attracted significant research efforts due to their remarkable performance, with certified power conversion efficiency now reaching 22%. Solution-processed perovskite thin films are polycrystalline, and grain boundaries are thought to be responsible for causing recombination and trapping of charge carriers. Here we report an effective and reproducible way of treating grain boundaries in CH3NH3PbI3 films deposited by means of a Lewis acid–base adduct approach. We show by high-resolution transmission electron microscopy lattice images that adding 6 mol% excess CH3NH3I to the precursor solution resulted in a CH3NH3I layer forming at the grain boundaries. This layer is responsible for suppressing non-radiative recombination and improving hole and electron extraction at the grain boundaries by forming highly ionic-conducting pathways. We report an average power conversion efficiency of 20.1% over 50 cells (best cell at 20.4%) together with significantly reduced current–voltage hysteresis achieved by this grain boundary healing process. The grain boundaries in thin-film perovskite solar cells are responsible for non-radiative carrier recombination, which is deleterious for the optoelectronic performance. Son et al. show how to passivate the grain boundaries by using excess CH3NH3I in the precursor solution, achieving efficiencies of 20.4%.

Quantifying Curvature in High-Resolution Transmission Electron Microscopy Lattice Fringe Micrographs of Coals

The observation of high-resolution transmission electron microscopy (HRTEM) lattice fringe images for coals has aided the rationalization of structure and order. Within the hundreds of lattice fringes (edge-on view of the aromatic structures) in a typical micrograph, apparent curvature is common. Traditional image analysis approaches do not appropriately quantify curvature. Tortuosity is functional for the quantification of single-inflection-point smooth lines but poor for complex or undulating lines. Here we present an image analysis method that can identify the points of inflection, angles, and segment lengths that constitute curved lattice fringes. Four coals from the Argonne Premium suite (Pocahontas No. 3, Upper Freeport, Illinois No. 6, and Beulah-Zap) and example anthracites were examined, and curvature was present in 17–24% of the fringes. These curved fringes were further classified as having low ( 90°) on the basis of cumulative angle changes. For the A...

Effect of Doping with Substituent Bi Atoms on the Electrical Transport Properties of a Bi0.4Sb1.6Te3 Film Fabricated by Molecular Beam Epitaxy

A highly crystalline Bi0.4Sb1.6Te3 film was fabricated on a Si substrate by molecular beam epitaxy (MBE) at a substrate temperature of 280°C. On the basis of study of x-ray diffraction patterns and high-resolution transmission electron microscopy lattice fringes it was inferred that Bi atoms were successfully incorporated into Sb lattice sites, forming substituent Bi impurities. Reduction of the carrier concentration was ascribed to the increased resistance to formation of antisite defects when Sb was substituted by Bi. The reduced mobility was a result of enhanced grain boundary scattering and attraction by substituent Bi atoms. Analysis of temperature-dependent electrical transport properties revealed that introduction of Bi atoms resulted in deeper energy level impurities in the Bi0.4Sb1.6Te3 film and higher activation energy (43.2 meV) than the normal value at room temperature, leading to semiconductor characteristics of the film in the temperature range −50°C to 150°C.

Growth of crystalline Al2O3 via thermal atomic layer deposition: Nanomaterial phase stabilization

We report the growth of crystalline Al2O3 thin films deposited by thermal Atomic Layer Deposition (ALD) at 200 °C, which up to now has always resulted in the amorphous phase. The 5 nm thick films were deposited on Ga2O3, ZnO, and Si nanowire substrates 100 nm or less in diameter. The crystalline nature of the Al2O3 thin film coating was confirmed using Transmission Electron Microscopy (TEM), including high-resolution TEM lattice imaging, selected area diffraction, and energy filtered TEM. Al2O3 coatings on nanowires with diameters of 10 nm or less formed a fully crystalline phase, while those with diameters in the 20–25 nm range resulted in a partially crystalline coating, and those with diameters in excess of 50 nm were fully amorphous. We suggest that the amorphous Al2O3 phase becomes metastable with respect to a crystalline alumina polymorph, due to the nanometer size scale of the film/substrate combination. Since ALD Al2O3 films are widely used as protective barriers, dielectric layers, as well as potential coatings in energy materials, these findings may have important implications.

Extracellular synthesis of mycogenic silver nanoparticles by Cylindrocladium floridanum and its homogeneous catalytic degradation of 4-nitrophenol

Green synthesis of extracellular mycogenic silver nanoparticles using the fungus, Cylindrocladium floridanum is reported. The synthesized mycogenic silver nanoparticles were characterized using UV–Vis absorption spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM) techniques. The nanoparticles exhibit fcc structure with Bragg’s reflections of (111), (200), (220) and (311) was evidenced by XRD pattern, high-resolution TEM lattice fringes and circular rings in selected-area electron diffraction (SAED) pattern. The morphology of nanoparticles was roughly spherical in shape with an average size of ca. 25nm. From FTIR spectrum, it was found that the biomolecules with amide I and II band were involved in the stabilization of nanoparticles. These mycogenic silver nanoparticles exhibited the homogeneous catalytic potential in the reduction of pollutant, 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) using sodium borohydride, which followed a pseudo-first-order kinetic model. Thus, the synthesis of metal nanoparticles using sustainable microbial approach opens up possibilities in the usage of mycogenic metal nanoparticles as catalysts in various chemical reactions.

Determination of critical thickness for defect formation of CdSe/ZnSe heterostructures by transmission electron microscopy and photoluminescence spectroscopy

We report on the investigation of CdSe/ZnSe heterostructures by transmission electron microscopy (TEM) and photoluminescence spectroscopy (PL). CdSe layers with nominal thicknesses tCdSe between 0.5 and 8 monolayers (ML) were embedded in a ZnSe matrix and grown on a GaAs(001) substrate by molecular-beam epitaxy at 280 °C. The Cd-distribution was obtained from high-resolution TEM lattice fringe images using composition evaluation by lattice fringe analysis technique. The measured minimum, average and maximum Cd-concentrations and the overall CdSe contents in the layers increase with the nominal CdSe layer thickness and reach a constant value at tCdSe = 4 ML. The measured CdSe content in the regions with the maximal Cd-concentration continues to increase for tCdSe ≥ 5 ML. The increasing of measured Cd-concentrations/CdSe contents is correlated with a red shift of PL spectra. In the CdSe layers with tCdSe ≥ 5 ML, formation of defects is observed. The increasing density of defects and decreasing intensity of PL spectra with tCdSe suggest that the critical thickness for defect formation during CdSe growth on the ZnSe(001) is between 4 ML and 5 ML. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of polycrystalline CuInSe 2 thin films deposited by sputtering and evaporation as a function of composition

In this study, a series of polycrystalline single-layer CulnSe 2 thin films with Cu/In atomic ratios between 1.7 and 0.5 were grown by using a physical vapor deposition system Cu, In sputtering, and Se evaporation. The variation in the microstructure (surface morphology, grain size, and defects) of the CulnSe 2 thin films on gradually changing the stoichiometry from Cu-rich to Cu-poor was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In general, Cu-rich thin films showed large grains with relatively low defect densities, while Cu-poor thin films exhibited highly planar defects, twins and stacking faults, with small grains. Polycrystalline bi-layer CulnSe 2 thin films were also grown by depositing a Cu-poor layer on the top of a Cu-rich layer. The bi-layer Cu-poor thin films exhibited large grains (>3 μm). High-resolution TEM lattice image shows a thin coherent layer (∼20 nm) at the both sides of the grain boundary for a slightly Cu-poor film. TEM energy dispersive X-ray spectroscopy (EDX) analyses indicate that the grain boundary region is more Cu-poor than the intra-grain for the slightly Cu-poor film. The coherent thin layer and more Cu-poor at the grain boundary of the slightly Cu-poor film could be related to a type inverse. The surface morphologies and the grain boundaries of the polycrystalline CuInSe 2 could provide an understanding of the growth behavior of CuInSe 2 and the performance of CuInSe 2 devices.

00/00931 Solar cells based on the heterojunction a-C/p-Si

In this study, a series of polycrystalline single-layer CulnSe2 thin films with Cu/In atomic ratios between 1.7 and 0.5 were grown by using a physical vapor deposition system Cu, In sputtering, and Se evaporation. The variation in the microstructure (surface morphology, grain size, and defects) of the CulnSe2 thin films on gradually changing the stoichiometry from Cu-rich to Cu-poor was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In general, Cu-rich thin films showed large grains with relatively low defect densities, while Cu-poor thin films exhibited highly planar defects, twins and stacking faults, with small grains. Polycrystalline bi-layer CulnSe2 thin films were also grown by depositing a Cu-poor layer on the top of a Cu-rich layer. The bi-layer Cu-poor thin films exhibited large grains (>3 μm). High-resolution TEM lattice image shows a thin coherent layer (∼20 nm) at the both sides of the grain boundary for a slightly Cu-poor film. TEM energy dispersive X-ray spectroscopy (EDX) analyses indicate that the grain boundary region is more Cu-poor than the intra-grain for the slightly Cu-poor film. The coherent thin layer and more Cu-poor at the grain boundary of the slightly Cu-poor film could be related to a type inverse. The surface morphologies and the grain boundaries of the polycrystalline CuInSe2 could provide an understanding of the growth behavior of CuInSe2 and the performance of CuInSe2 devices.

Composition Fluctuations in InGaN Analyzed by Transmission Electron Microscopy

A series of GaN/InxGa1—xN/GaN quantum well structures was investigated by transmission electron microscopy (TEM) and photoluminescence spectroscopy (PL). The structures were grown by metal organic chemical vapor deposition on Al2O3(0001) substrates varying only the substrate temperature between 800 and 860 °C for the InGaN growth under otherwise unaltered growth conditions. A strong shift of the PL emission wavelength from 390 to 480 nm is observed. Composition analyses were carried out by measuring local lattice parameters, which are directly related to the local In concentration, from high-resolution TEM lattice fringe images. An increase of the average In concentration from 9% to 18% is found as the growth temperature is lowered. A laterally inhomogeneous In concentration is observed in all samples with inclusions of high In content embedded in a quantum well with lower In concentration indicating that phase separation occurs even at a low average In concentration of 9%.

Composition Fluctuations in InGaN Analyzed by Transmission Electron Microscopy

A series of GaN/InxGa1—xN/GaN quantum well structures was investigated by transmission electron microscopy (TEM) and photoluminescence spectroscopy (PL). The structures were grown by metal organic chemical vapor deposition on Al2O3(0001) substrates varying only the substrate temperature between 800 and 860 °C for the InGaN growth under otherwise unaltered growth conditions. A strong shift of the PL emission wavelength from 390 to 480 nm is observed. Composition analyses were carried out by measuring local lattice parameters, which are directly related to the local In concentration, from high-resolution TEM lattice fringe images. An increase of the average In concentration from 9% to 18% is found as the growth temperature is lowered. A laterally inhomogeneous In concentration is observed in all samples with inclusions of high In content embedded in a quantum well with lower In concentration indicating that phase separation occurs even at a low average In concentration of 9%.

Quantitative determination of imaging parameters and composition from high-resolution transmission electron microscopy lattice images

A quantitative method for the direct determination of defocus Δf, local sample thickness t and local composition x from high-resolution transmission electron microscopy (HRTEM) lattice images of wedge-shaped samples is described. The method is applicable to a wide class of crystalline materials comprising elemental semiconductors, elemental metals and substitutional random alloys composed of these elements. The proposed method relies on the functional dependence of linear and non-linear image Fourier coefficients on the parameters defocus Δf, sample thickness t and composition x. This relationship is analytically derived by application of the Bloch wave formalism and the non-linear imaging theory to the HRTEM imaging process. Influences of inelastic electron scattering and partially coherent illumination conditions are taken into account explicitly.

Quantitative determination of defocus, thickness and composition from high-resolution transmission electron microscopy lattice images

A quantitative method for the direct determination of defocusΔf, local thickness t and local compositionx from high-resolution transmission electron microscopy lattice images of wedge-shaped crystal samples is proposed. The method relies on the analytically derived relation between the first-order linear and nonlinear image Fourier coefficientsJ1 andJ2 onδf,t andx. By plotting J1 VSJ2 for varying t, ellipses with defocus- and composition-specific geometry are obtained. By reconstructing the appropriate ellipse for image regions of homogeneous composition, Δf and t can be determined independently. At interfaces, local compositionsx can be determined within the full range 0 ≤ x ≤ 1 by utilizing systematic variations of the ellipse's geometry with varyingx.

Quantitative determination of defocus, thickness and composition from high-resolution transmission electron microscopy lattice images

Quantitative determination of defocus, thickness and composition from high-resolution transmission electron microscopy lattice images