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- ItemElectron Tomography of Pencil-Shaped GaN/(In,Ga)N Core-Shell Nanowires(New York, NY [u.a.] : Springer, 2019) Nicolai, Lars; Gačević, Žarko; Calleja, Enrique; Trampert, AchimThe three-dimensional structure of GaN/(In,Ga)N core-shell nanowires with multi-faceted pencil-shaped apex is analyzed by electron tomography using high-angle annular dark-field mode in a scanning transmission electron microscope. Selective area growth on GaN-on-sapphire templates using a patterned mask is performed by molecular beam epitaxy to obtain ordered arrays of uniform nanowires. Our results of the tomographic reconstruction allow the detailed determination of the complex morphology of the inner (In,Ga)N multi-faceted shell structure and its deviation from the perfect hexagonal symmetry. The tomogram unambiguously identifies a dot-in-a-wire configuration at the nanowire apex including the exact shape and size, as well as the spatial distribution of its chemical composition. © 2019, The Author(s).
- ItemSynthesis of surfactant-free Cu–Pt dendritic heterostructures with highly electrocatalytic performance for methanol oxidation reaction(London [u.a.] : Taylor & Francis, 2016) Kang, Shendong; Gao, Guanhui; Xie, Xiaobin; Shibayama, Tamaki; Lei, Yanhua; Wang, Yan; Cai, LintaoA facile and free surfactant strategy is explored to synthesize Cu–Pt bimetallic nano-heterostructures with dendritic exterior. For comparison, the Cu–Pt coral-like nanoparticles are fabricated by using CTAC as a surfactant. The well-designed Cu–Pt dendritic spherical heterostructures exhibit superior enhanced electrocatalytic activity and stability toward methanol oxidation reaction in alkaline media, compared to the Cu–Pt coral-like nanoparticles and the commercial Pt/C, respectively. The advanced technique for fabricating Cu–Pt dendritic spherical heterostructures could pave a way to pursue low-cost Pt-based catalysts, maintaining highly promoted electrocatalytic performance and durability.
- ItemStrategies for Analyzing Noncommon-Atom Heterovalent Interfaces: The Case of CdTe-on-InSb(Weinheim : Wiley-VCH, 2019) Luna, Esperanza; Trampert, Achim; Lu, Jing; Aoki, Toshihiro; Zhang, Yong-Hang; McCartney, Martha R.; Smith, David J.Semiconductor heterostructures are intrinsic to a wide range of modern-day electronic devices, such as computers, light-emitting devices, and photodetectors. Knowledge of chemical interfacial profiles in these structures is critical to the task of optimizing the device performance. This work presents an analysis of the composition profile and strain across the noncommon-atom heterovalent CdTe/InSb interface, carried out using a combination of electron microscopy imaging techniques. Because of the close atomic numbers of the constituent elements, techniques such as high-angle annular-dark-field and large-angle bright-field scanning transmission electron microscopy, as well as electron energy-loss spectroscopy, give results from the interface region that are inherently difficult to interpret. By contrast, use of the 002 dark-field imaging technique emphasizes the interface location by comparing differences in structure factors between the two materials. Comparisons of experimental and simulated CdTe-on-InSb profiles reveal that the interface is structurally abrupt to within about 1.5 nm (10–90% criterion), while geometric phase analysis based on aberration-corrected electron microscopy images reveals a minimal level of interfacial strain. The present investigation opens new routes to the systematic investigation of heterovalent interfaces, formed by the combination of other valence-mismatched material systems. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemRole of hole confinement in the recombination properties of InGaN quantum structures([London] : Macmillan Publishers Limited, part of Springer Nature, 2019) Anikeeva, M.; Albrecht, M.; Mahler, F.; Tomm, J. W.; Lymperakis, L.; Chèze, C.; Calarco, R.; Neugebauer, J.; Schulz, T.We study the isolated contribution of hole localization for well-known charge carrier recombination properties observed in conventional, polar InGaN quantum wells (QWs). This involves the interplay of charge carrier localization and non-radiative transitions, a non-exponential decay of the emission and a specific temperature dependence of the emission, denoted as “s-shape”. We investigate two dimensional In0.25Ga0.75N QWs of single monolayer (ML) thickness, stacked in a superlattice with GaN barriers of 6, 12, 25 and 50 MLs. Our results are based on scanning and high-resolution transmission electron microscopy (STEM and HR-TEM), continuous-wave (CW) and time-resolved photoluminescence (TRPL) measurements as well as density functional theory (DFT) calculations. We show that the recombination processes in our structures are not affected by polarization fields and electron localization. Nevertheless, we observe all the aforementioned recombination properties typically found in standard polar InGaN quantum wells. Via decreasing the GaN barrier width to 6 MLs and below, the localization of holes in our QWs is strongly reduced. This enhances the influence of non-radiative recombination, resulting in a decreased lifetime of the emission, a weaker spectral dependence of the decay time and a reduced s-shape of the emission peak. These findings suggest that single exponential decay observed in non-polar QWs might be related to an increasing influence of non-radiative transitions.
- ItemAuthor Correction: Modulation of van der Waals and classical epitaxy induced by strain at the Si step edges in GeSbTe alloys([London] : Macmillan Publishers Limited, part of Springer Nature, 2018-3-19) Zallo, Eugenio; Cecchi, Stefano; Boschker, Jos E.; Mio, Antonio M.; Arciprete, Fabrizio; Privitera, Stefania; Calarco, Raffaella[no abstract available]