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Study of nanometer-scale structures and electrostatic properties of InAs quantum dots decorating GaAs/AlAs core/shell nanowires
摘要： The configurations of core/shell nanowires (NWs) and quantum dots (QDs) decorating NWs have found great applications in forming optoelectronic devices thanks to their superior performances, and the combination of the two configurations would expect to bring more benefits. However, the nanometer-scale electrostatic properties of the QD/buffer layer/NW heterostructures are still un-revealed. In this study, the InAs QDs decorating GaAs/AlAs core/shell NWs are systemically studied both experimentally and theoretically. The layered atomic structures, chemical information, and anisotropic strain conditions are characterized by comprehensive transmission electron microscopy (TEM) techniques. Quantitative electron holography analyses show large number of electrons accumulating in the InAs QD especially at the dot apex, and charges of reversed signs and similar densities are observed to distribute at the sequential interfaces, leaving great amount of holes in the NW core. Theoretical calculations including simulated heterostructural band structures, interfacial charge transfer, and chemical bonding analysis are in good accordance with the experimental results, and prove the important role of AlAs buffer layer in adjusting the heterostructural band structure as well as forming stable InAs QDs on the NW surfaces. These results could be significant for achieving related optoelectronic devices with better stability and higher efficiency.
关键词： quantum dot,transmission electron microscopy,atomic structure,core/shell nanowire,electrostatic property
更新于2025-09-23 15:19:57
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Characterization of the chips generated by the nanomachining of germanium for X-ray crystal optics
摘要： Micro-Raman spectroscopy, scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HR-TEM) were used to study the effect of cutting speed and cutting depth on the mode of the single-point diamond fly cutting of Ge(110) surface via crystallinity of the chips. Reducing the cutting depth from 15 to 2 μm and concurrently cutting speed from 10 to 2 mm/min at 2000 rpm, the content of amorphous phase in the chips increased at the expense of the crystalline one from 28 to 46%. Simultaneously, the chip morphology visible by SEM suggested transition from a brittle to a mixed brittle-ductile mode of nanomachining. The damage transition line indicates 1/3 portion of the ductile component at 2-μm cutting depth that produced twisted lamellae of a width of 18–20 μm without any signs of a fracture. As the feed rate here was 1 μm/rev, the tool made 18–20 revolutions while passing the same point of the nanomachined surface that was enough to gradually remove the surface region damaged by the brittle cutting component along with the entire amorphous region beneath, both being delaminated by the chips. This explains the dislocation-free single-crystal lattice beneath the Ge(110) surface machined under these conditions. A close relationship between the brittle mode of nanomachining and crystallinity of the chips observed by micro-Raman spectroscopy and SEM was confirmed by HR-TEM showing dense occurrence of nanocrystals in the chips coming from the nanomachinings with 5-μm and 15-μm cutting depths. These results demonstrate potential of the single-point diamond machining for the preparation of high-quality X-ray surfaces with undistorted single-crystal lattice beneath for next-generation X-ray crystal optics.
关键词： Micro-Raman spectroscopy,X-ray crystal optics,Scanning electron microscopy,Germanium,Transmission electron microscopy,Single-point diamond machining
更新于2025-09-19 17:15:36
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Synthesis of Nanostructured PLD AlN Films: XRD and Surface-Enhanced Raman Scattering Studies
摘要： Thin films of AlN on Si were fabricated by pulsed laser deposition in vacuum and in nitrogen ambient, and at laser repetition rate of 3 Hz or 10 Hz. The films were nanostructured according to the X-ray diffraction analysis and TEM imaging. Films deposited in vacuum were polycrystalline with hexagonal AlN phase and with columnar structure, while films deposited in nitrogen were predominantly amorphous with nanocrystallites inclusions. The Al-N phonon modes in the surface-enhanced Raman spectra were largely shifted due to stress in the films. Phonon mode of Al-O related to film surface oxidation is observed only for deposition at low pressures.
关键词： microstructure,nanostructured thin film,Transmission electron microscopy,pulsed laser deposition,Aluminium nitride,Raman spectroscopy,X-ray diffractometry
更新于2025-09-19 17:15:36
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Facile fabrication of well-polarized Bi <sub/>2</sub> WO <sub/>6</sub> nanosheets with enhanced visible-light photocatalytic activity
摘要： Designing a templating strategy for directing mesopore growth along different crystallographic directions is essential for fabricating two- or three-dimensional single-crystalline mesoporous zeolites. However, so far, mesopores formed in MFI zeolites by soft templates have mostly been generated by disrupting growth along the b axis; generating mesopores by disrupting growth along the a axis is rare. Herein, a single-crystalline mesoporous MFI zeolite (SCMMZ) with sheet-like mesopores layered along the a and b axes was synthesized using a triply branched surfactant with diquaternary ammonium groups connected to 1,3,5-triphenylbenzene by a six- and eight-carbon alkyl chain (TPB-6 and 8). The sheet-like mesopores were embedded in the MFI framework and were retained even after calcination. Molecular mechanics calculations provided evidence of low binding energy configurations of the surfactant that directed the growth of straight and zigzag channels along the b and a axes, respectively. The formation of nanosheets was attributed to the geometric matching of the arrangement of the aromatic groups to the zeolite framework.
关键词： mesoporous zeolite,transmission electron microscopy,surfactants,self-assembly,MFI
更新于2025-09-19 17:15:36
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The maximum a posteriori probability rule for atom column detection from HAADF STEM images
摘要： Recently, the maximum a posteriori (MAP) probability rule has been proposed as an objective and quantitative method to detect atom columns and even single atoms from high-resolution high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) images. The method combines statistical parameter estimation and model-order selection using a Bayesian framework and has been shown to be especially useful for the analysis of the structure of beam-sensitive nanomaterials. In order to avoid beam damage, images of such materials are usually acquired using a limited incoming electron dose resulting in a low contrast-to-noise ratio (CNR) which makes visual inspection unreliable. This creates a need for an objective and quantitative approach. The present paper describes the methodology of the MAP probability rule, gives its step-by-step derivation and discusses its algorithmic implementation for atom column detection. In addition, simulation results are presented showing that the performance of the MAP probability rule to detect the correct number of atomic columns from HAADF STEM images is superior to that of other model-order selection criteria, including the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC). Moreover, the MAP probability rule is used as a tool to evaluate the relation between STEM image quality measures and atom detectability resulting in the introduction of the so-called integrated CNR (ICNR) as a new image quality measure that better correlates with atom detectability than conventional measures such as signal-to-noise ratio (SNR) and CNR.
关键词： Atom detection,Scanning transmission electron microscopy (STEM),Atom detectability,Model selection
更新于2025-09-19 17:15:36
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Photoluminescence emission from nanostructured porous preparations of CdSa??ZnTiO <sub/>3</sub> assembled nanoparticles
摘要： This study reports the structural and optical properties of CdS/ZnTiO3 nanocomposites prepared using a chemical bath and different titanate concentrations. Commercial ZnTiO3 nanoparticles were introduced into a chemical bath that had been used to produce CdS semiconductor nanoparticles (NPs). Here, the growing CdS crystallites precipitated onto the suspended zinc titanate NPs. X-ray diffraction patterns revealed that samples of CdS/ZnTiO3 nanopowders were made of cubic ZnTiO3 and hexagonal CdS wurtzite. The morphology of the particles was studied using transmission electron microscopy and scanning electron microscopy images. These images demonstrated the different characteristics of the CdS/ZnTiO3 nanocomposites and their dependence on titanate concentration when placed into the CdS-growing solution. Photoluminescence spectra showed three main emission bands for the electron transitions in the CdS/ZnTiO3 composite. This composite produced three photoluminescence bands, the intensities of which depended on composite shape, which in turn depended on the relative concentrations of CdS and ZnTiO3.
关键词： CdS/ZnTiO3 nanocomposites,transmission electron microscopy,X-ray diffraction,chemical bath,photoluminescence,scanning electron microscopy
更新于2025-09-19 17:13:59
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Plasmonic Pt Superstructures with Boosted Near‐Infrared Absorption and Photothermal Conversion Efficiency in the Second Biowindow for Cancer Therapy
摘要： Defects are commonly found in two-dimensional (2D) transition-metal dichalcogenide (TMD) materials. Such defects usually dictate the optical and electrical properties of TMDs. It is thus important to develop techniques to characterize the defects directly with good spatial resolution, specificity, and throughput. Herein, we demonstrate that Kelvin probe force microscopy (KPFM) is a versatile technique for this task. It is able to unveil defect heterogeneity of 2D materials with a spatial resolution of 10 nm and energy sensitivity better than 10 meV. KPFM mappings of monolayer WS2 exhibit interesting work function variances that are associated with defects distribution. This finding is verified by aberration-corrected scanning transmission electron microscopy and density functional theory calculations. In particular, a strong correlation among the work function, electrical and optical responses to the defects is revealed. Our findings demonstrate the potential of KPFM as an effective tool for exploring the intrinsic defects in TMDs.
关键词： transition-metal dichalcogenides,density functional theory,defects,Kelvin probe force microscopy,scanning transmission electron microscopy,work function,two-dimensional materials
更新于2025-09-19 17:13:59
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Laser-Based Phase Contrast for Transmission Electron Microscopy
摘要： Laser control of free electrons has been used to advance the exploration of matter on the atomic scale. For example, temporal modulation of electron waves with light has enabled the study of transient processes with attosecond resolution. By contrast, laser-based spatial shaping of the electron wave function has not yet been realized, even though it could be harnessed to probe radiation-sensitive systems, such as biological macromolecules, at the standard quantum limit and beyond. We demonstrate increased image contrast by laser control of the spatial phase profile of the electron wave function in transmission electron microscopy (TEM). We first realize an electron interferometer, using continuous-wave laser-induced retardation to coherently split the electron beam, and capture TEM images of the light wave. We then demonstrate Zernike phase contrast by using the laser beam to shift the phase of the electron wave scattered by a specimen relative to the unscattered wave. Electrons interact with light via the repulsive ponderomotive potential arising from stimulated Compton scattering. Due to the short electron-light interaction time in a micron-scale laser focus, retardation of the relativistic electrons used in TEM requires an intensity of tens of GW/cm2. Such intensities have so far only been attained with pulsed lasers, but a cw laser is needed in order to work state-of-the-art, continuously operating TEM. The requisite laser intensity is generated by 4000-fold resonant power enhancement in a near-concentric Fabry-Perot optical cavity with a mode waist of w0 = 13 μm. A laser system consisting of a fiber amplifier seeded by a low-power master laser supplies an input laser beam at a wavelength of λ = 1064 nm. The experiments are carried out with 80 keV electrons, in a custom-modified TEM (FEI Titan) equipped with additional electron optics that magnify the diffraction pattern to an effective focal length of f = 20 mm. The cavity is suspended in the TEM column, with its axis orthogonal to the electron beam propagation direction and with the mode waist positioned close to the center of the magnified electron diffraction plane, as shown in Fig. 1. Zernike phase contrast is evident in a typical close-to-focus image (Fig. 1), showing the structure of the carbon film. A high-intensity CW laser field generates Zernike phase contrast in a TEM and significantly increases the image contrast at low spatial frequencies. Such a phase plate will enable dose-efficient data collection in single-particle analysis of biological macromolecules, electron tomography of vitrified cells, and imaging of sensitive materials science specimens. The controllable phase shift in this device can also be used for holographic reconstruction of the post-specimen wave function. Work in the immediate future will include working with 300-keV electrons, which requires constructing a new cavity with lower-loss optical coatings to reach higher laser intensity. We will also study and optimize the imaging properties of the phase-contrast TEM, and apply it to structural biology.
关键词： Zernike phase contrast,Laser control,Fabry-Perot optical cavity,Transmission Electron Microscopy,electron wave function
更新于2025-09-19 17:13:59
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Elucidation of Donor:Acceptor Phase Separation in Nonfullerene Organic Solar Cells and Its Implications on Device Performance and Charge Carrier Mobility
摘要： In bulk-heterojunction solar cells, the device performance strongly depends on the donor and acceptor properties, the phase separation in the absorber layer, and the formation of a bicontinuous network. While this phase separation is well explored for polymer:fullerene solar cells, only little is known for polymer:nonfullerene acceptor solar cells. The main hurdle in this regard is often the chemical similarity of the conjugated polymer donor and the organic nonfullerene acceptor (NFA), which makes the analysis of the phase separation via atomic force microscopic (AFM) phase images or conventional transmission electron microscopy difficult. In this work, we use the donor polymer PTB7-Th and the small molecule acceptor O-IDTBR as the model system and visualized the phase separation in PTB7-Th:O-IDTBR bulk-heterojunctions with different donor:acceptor ratios via scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) images and electron energy loss spectroscopy (EELS) based elemental mapping, which resulted in a good contrast between the donor and the acceptor despite very low differences in the chemical composition. AFM as well as grazing-incidence wide-angle X-ray scattering (GIWAXS) investigations support the electron microscopic data. Furthermore, we elucidate the implications of the phase separation on the device performance as well as charge carrier mobilities in the bulk-heterojunction layers, and a high performance of the solar cells was found over a relatively broad range of polymer domain sizes. This can be related to the larger domain sizes of the acceptor phase with higher amounts of O-IDTBR in the blend, while the polymer donor phase still forms continuous pathways to the electrode, which keeps the hole mobility at a relatively constant level.
关键词： nanomorphology,organic photovoltaics,charge carrier mobility,bulk-heterojunction,scanning transmission electron microscopy
更新于2025-09-19 17:13:59
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Evidence of Low Temperature Joints in Silver Nanowire Based Transparent Conducting Layers for Solar cells
摘要： The primary stage of joint formation of silver nanowires (AgNWs) at 60 °C is investigated using rotary scanning transmission electron microscopy (STEM with tomographic reconstruction images), and super large-scale molecular dynamic (MD) simulation (2×106 atoms). This study proves to establish that silver nanowires do not require the conventional high temperature post treatment process at 200 °C to form fused contacts at the intersections. In fact, a low temperature annealing at 60 °C facilitates formation of highly conductive networks. The connection between the nanowires is made through a stage called thinning, shown in this report for the first time, which occurs before broadening of the nanowires and is caused due to simultaneous effects of loads from the top nanowires and the heating, as confirmed by STEM and MD result. The outcomes of our investigation significantly promote the application of AgNWs as a transparent conductive layer for solar cells with requirement of low temperature processing such as Kasterite, Perovskite and Organic solar cells.
关键词： Low temperature process,Scanning transmission electron microscopy,Molecular dynamic simulation,Junction resistivity,Transparent conductive layer,Silver nanowire
更新于2025-09-19 17:13:59