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Electron-beam (e-beam) irradiation damage is often regarded as a severe limitation to atomic-scale study of two-dimensional (2D) materials using electron microscopy techniques. However, energy transferred from the e-beam can also provide a way to modify 2D materials via defect engineering when the interaction of the beam with the sample is precisely controlled. In this article, we discuss the atomic geometry, formation mechanism, and properties of several types of structural defects, ranging from zero-dimensional point defects to extended domains, induced by an e-beam in a few representative 2D materials, including graphene, hexagonal boron nitride, transition-metal dichalcogenides, and phosphorene. We show that atomic as well as line defects and even novel nanostructures can be created and manipulated in 2D materials by an e-beam in a controllable manner. Phase transitions can also be induced. The e-beam in a (scanning) transmission electron microscope not only resolves the intrinsic atomic structure of materials with defects, but also provides new opportunities to modify the structure with subnanometer precision.
We present comparative studies of optical properties of GaN nanowires (NWs) obtained by two different self-formation techniques: Plasma-Assisted Molecular Beam Epitaxy (PAMBE) growth; and plasma etching of GaN layers deposited by Metal-Organic Vapor Phase Epitaxy (MOVPE). The effects of the coalescence process on grown NW and plasma-induced defects in etched NWs have been studied by photoluminescence (PL) and Raman scattering. In MBE grown NWs, the coalescence-associated defects are extended toward the NW top for intermediate Ga flux. Using High Resolution Electron Microscopy of reactive plasma etching (RIE) NWs, it was found that NWs obtained with an optimal combination of inductive (ICP) and capacitive (RF) plasma are free of extended structural defects. The PL efficiency is strongly increased in plasma etched NWs. However, plasma-induced point defects have to be taken into account for explaining the changes of the PL spectra. Less plasma-induced degradation is observed for high ICP/RF power ratios.
We present a comparative study of the microstructure of Ca3Co4O9 single crystals and c-axis oriented Ca3Co4O9 thin films grown on glass substrates. Though both crystals and films have similar values of Seekbeck coefficient and electric resistivity at room temperature, their microstructures are rather different. Extensive high resolution transmission electron microscopy (TEM) studies reveal that the films grown on glass substrates have abundant stacking faults, which is in contrast to the perfect crystalline structure found in the single crystal sample. The c-axis lattice constants derived from the x-ray diffraction (XRD) and TEM measurements for the single crystal sample and the thin film are virtually the same, suggesting that the thin film on the glass substrate was not strained.
Atmospheric pressure chemical vapor deposition (APCVD) is being studied as an alternative for large-area manufacturing of CdTe thin films. High efficiency research cells have been constructed, but the fundamental materials properties and limitations have not been fully explored. APCVD material is examined with several techniques and compared with close-space sublimation (CSS). Transmission and scanning electron microscopy studies show a similar morphology to CSS CdTe. However high resolution TEM scans show the formation of a disordered layer between the CdTe and CdS, and the removal of defects within some grain structures upon annealing. Cathodoluminescence shows electronic defect states localized to grain boundaries. A large concentration of trap states was also observed with deep-level transient spectroscopy that may correspond to hole traps found in lower amounts in other materials. The presence of traps was also indicated in impedance spectroscopy measurements. The latter studies indicate a high grain boundary resistance contributes to transport.
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