Full utilization of the high storage capacity of conversion electrode materials as tin oxide (SnO2) in lithium‐ion batteries is hindered by the high volumetric expansion due to the high lithium storability which can lead to major cell damage and consequent safety issues. To overcome this issue, two promising approaches, nanostructures and buffer layers, are combined and evaluated. SnO2 nanowires (NWs) are coated with an aluminum oxide (Al2O3) buffer layer to investigate the combination SnO2–Al2O3. Strong differences in the crystallinity after cycling between the SnO2/Al2O3 core/shell NW‐based heterostructure and uncoated SnO2 NWs based on detailed structural analysis are shown via transmission electron microscopy (TEM) and determination of the elemental distribution of tin, oxygen, lithium, and aluminum via energy‐dispersive X‐Ray spectroscopy and energy‐filtered TEM in the as‐prepared and postmortem nanostructures. The core/shell NWs exhibit two different states after charge/discharge cycling, amorphous or crystalline, depending on the NW diameter; for the uncoated SnO2 NWs, only an amorphous postmortem structure is found. Additionally, differences in the elemental distribution for the amorphous and crystalline postmortem SnO2/Al2O3 core/shell NWs, especially for tin, are measured. Consequently, the structures and effects of the Al2O3 coating on the lithiation behavior of SnO2 NW‐based heterostructures are discussed.
Highly textured ZnO thin films were successfully grown on Si(111) by atomic layer deposition using an epitaxial AlN buffer layer at deposition temperatures between 100 and 300 °C. X-ray diffraction analysis proves an epitaxial relationship of ZnO[0001]//AlN[0001] and ZnO[112¯0]//AlN[112¯0]. Omega scans of the (0002) and (101¯0) reflections of ZnO demonstrate an improving crystalline quality for increasing deposition temperatures. An additional thermal postannealing step at 800 °C is found to be beneficial to further improve the crystal structure.
Metal-assisted chemical etching (MACE) of Si with unstable metals like Cu can be described with reference to the oxidation effect of aqueous Cu2+ metal cations, in combination with the dissolution of Si species by HF. For 1-step MACE, salt precursors introduce possibly oxidizing additional anionic species, such as NO3− ions from Cu(NO3)2, that cannot be ignored when explaining MACE. This study for the first time unites Cu(NO3)2-MACE with isotropic hydrofluoric/nitric/acetic acid (HNA) Si etching, by purposely avoiding extra oxidants like H2O2, and contrasting the etching characteristics with etchants containing CuSO4, Cu(OAc)2, or CuCl2 precursors. A new model involving Cu2+-promoted autocatalysis of reactive nitrogen species (RNSs) is discussed, which explains the high etch rates, the NO3− anion concentration dependence, and the induction period specific to Cu(NO3)2 etchants. These results highlight the importance of Cu salt precursor selection when rationally designing new Cu-containing wet chemical etchants for key applications like MEMS and black silicon.
We demonstrate the epitaxial coating of GaN NWs with an epitaxial ZnO shell by atomic layer deposition at 300 °C. Scanning transmission electron microscopy proves a sharp and defect-free coherent interface. The strain in the core–shell structure due to the lattice mismatch and different thermal expansion coefficients of GaN and ZnO was analyzed using 4D-STEM strain mapping and Raman spectroscopy and compared to theoretical calculations. The results highlight the outstanding advantages of epitaxial shell growth using atomic layer deposition, e.g., conformal coating and precise thickness control.
This report deals with the electrical properties of a heterojunction between self‐assembled InN nanowires (NWs) grown by plasma‐assisted molecular beam epitaxy and Si (111) with different doping types and concentrations. Multiple NWs are contacted in a vertical sandwich structure and current–voltage (IU) measurements show a rectifying behavior which strongly depends on the doping level of the Si substrate. Comparable results are obtained for single NWs contacted with a conductive atomic force microscope tip. The transport processes can be described by thermionic emission theory of a Schottky diode. However, field emission and recombination can not be neglected, especially for high doping concentrations of the Si substrate. The temperature dependence of the barrier height and the ideality factor have been determined by measuring IU‐curves from 200 K up to 300 K. Furthermore, electrical changes based on aging and on different environmental conditions, like contact with water, are discussed. The underlying band diagram of the InN/Si heterojunction has been simulated with the software package Nextnano++ and is in agreement with the experimental data. The simulations consider both polarities of the InN NWs and both doping types of the Si substrates.
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