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End date: 2022 × Start date: 2022 × WGL Institute: PDI ×

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    Protection Mechanism against Photocorrosion of GaN Photoanodes Provided by NiO Thin Layers
    (Weinheim : Wiley-VCH, 2020) Kamimura, Jumpei; Budde, Melanie; Bogdanoff, Peter; Tschammer, Carsten; Abdi, Fatwa F.; van de Krol, Roel; Bierwagen, Oliver; Riechert, Henning; Geelhaar, Lutz
    The photoelectrochemical properties of n-type Ga-polar GaN photoelectrodes covered with NiO layers of different thicknesses in the range 0–20 nm are investigated for aqueous solution. To obtain layers of well-defined thickness and high crystal quality, NiO is grown by plasma-assisted molecular-beam epitaxy. Stability tests reveal that the NiO layers suppress photocorrosion. With increasing NiO thickness, the onset of the photocurrent is shifted to more positive voltages and the photocurrent is reduced, especially for low bias potentials, indicating that hole transfer to the electrolyte interface is hindered by thicker NiO layers. Furthermore, cathodic transient spikes are observed under intermittent illumination, which hints at surface recombination processes. These results are inconsistent with the common explanation of the protection mechanism that the band alignment of GaN/NiO enables efficient hole-injection, thus preventing hole accumulation at the GaN surface that would lead to anodic photocorrosion. Interestingly, the morphology of the etch pits as well as further experiments involving the photodeposition of Ag indicate that photocorrosion of GaN photoanodes is related to reductive processes at threading dislocations. Therefore, it is concluded that the NiO layers block the transfer of photogenerated electrons from GaN to the electrolyte interface, which prevents the cathodic photocorrosion. © 2020 The Authors. Solar RRL published by Wiley-VCH GmbH
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    Electron 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, Achim
    The 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).
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    Effective group dispersion of terahertz quantum-cascade lasers
    (Bristol : IOP Publ., 2020) Röben, Benjamin; Lü, Xiang; Biermann, Klaus; Schrottke, Lutz; Grahn, Holger T.
    Terahertz (THz) quantum-cascade lasers (QCLs) are based on complex semiconductor heterostructures, in which the optical gain is generated by intersubband transitions. Using the spacing of the laser modes in the emission spectra, we have determined the effective group refractive index for more than one hundred THz QCLs of the hybrid design with Fabry-Pérot resonators based on single-plasmon waveguides. The experimentally obtained values of for emission frequencies between 2.5 and 5.6 THz generally follow the trend of derived from electromagnetic simulations. However, for a certain number of QCLs, the experimental values of exhibit a rather large deviation from the general trend and the simulation results. From a thorough analysis, we conclude that differences in the optical gain/loss spectra are responsible for this deviation, which lead to a modification of the dispersion in the active region and consequently to altered values of. The analysis also provides evidence that these differences in the gain/loss spectra originate from both, the details of the design and the gain broadening due to interface roughness. © 2020 The Author(s). Published by IOP Publishing Ltd.
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    Synthesis 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, Lintao
    A 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.
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    Strong confinement-induced engineering of the g factor and lifetime of conduction electron spins in Ge quantum wells
    ([London] : Nature Publishing Group UK, 2016) Giorgioni, Anna; Paleari, Stefano; Cecchi, Stefano; Vitiello, Elisa; Grilli, Emanuele; Isella, Giovanni; Jantsch, Wolfgang; Fanciulli, Marco; Pezzoli, Fabio
    Control of electron spin coherence via external fields is fundamental in spintronics. Its implementation demands a host material that accommodates the desirable but contrasting requirements of spin robustness against relaxation mechanisms and sizeable coupling between spin and orbital motion of the carriers. Here, we focus on Ge, which is a prominent candidate for shuttling spin quantum bits into the mainstream Si electronics. So far, however, the intrinsic spin-dependent phenomena of free electrons in conventional Ge/Si heterojunctions have proved to be elusive because of epitaxy constraints and an unfavourable band alignment. We overcome these fundamental limitations by investigating a two-dimensional electron gas in quantum wells of pure Ge grown on Si. These epitaxial systems demonstrate exceptionally long spin lifetimes. In particular, by fine-tuning quantum confinement we demonstrate that the electron Landé g factor can be engineered in our CMOS-compatible architecture over a range previously inaccessible for Si spintronics.
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    Engineering grain boundaries at the 2D limit for the hydrogen evolution reaction
    ([London] : Nature Publishing Group UK, 2020) He, Yongmin; Tang, Pengyi; Hu, Zhili; He, Qiyuan; Zhu, Chao; Wang, Luqing; Zeng, Qingsheng; Golani, Prafful; Gao, Guanhui; Fu, Wei; Huang, Zhiqi; Gao, Caitian; Xia, Juan; Wang, Xingli; Wang, Xuewen; Zhu, Chao; Ramasse, Quentin M.; Zhang, Ao; An, Boxing; Zhang, Yongzhe; Martí-Sánchez, Sara; Morante, Joan Ramon; Wang, Liang; Tay, Beng Kang; Yakobson, Boris I.; Trampert, Achim; Zhang, Hua; Wu, Minghong; Wang, Qi Jie; Arbiol, Jordi; Liu, Zheng
    Atom-thin transition metal dichalcogenides (TMDs) have emerged as fascinating materials and key structures for electrocatalysis. So far, their edges, dopant heteroatoms and defects have been intensively explored as active sites for the hydrogen evolution reaction (HER) to split water. However, grain boundaries (GBs), a key type of defects in TMDs, have been overlooked due to their low density and large structural variations. Here, we demonstrate the synthesis of wafer-size atom-thin TMD films with an ultra-high-density of GBs, up to ~1012 cm−2. We propose a climb and drive 0D/2D interaction to explain the underlying growth mechanism. The electrocatalytic activity of the nanograin film is comprehensively examined by micro-electrochemical measurements, showing an excellent hydrogen-evolution performance (onset potential: −25 mV and Tafel slope: 54 mV dec−1), thus indicating an intrinsically high activation of the TMD GBs.
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    Exchange-Striction Driven Ultrafast Nonthermal Lattice Dynamics in NiO
    (College Park, Md. : APS, 2021) Windsor, Y.W.; Zahn, D.; Kamrla, R.; Feldl, J.; Seiler, H.; Chiang, C.-T.; Ramsteiner, M.; Widdra, W.; Ernstorfer, R.; Rettig, L.
    We use femtosecond electron diffraction to study ultrafast lattice dynamics in the highly correlated antiferromagnetic (AFM) semiconductor NiO. Using the scattering vector (Q) dependence of Bragg diffraction, we introduce Q-resolved effective temperatures describing the transient lattice. We identify a nonthermal lattice state with preferential displacement of O compared to Ni ions, which occurs within ∼0.3  ps and persists for 25 ps. We associate this with transient changes to the AFM exchange striction-induced lattice distortion, supported by the observation of a transient Q asymmetry of Friedel pairs. Our observation highlights the role of spin-lattice coupling in routes towards ultrafast control of spin order.
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    Role 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.
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    Indocyanine Green-Loaded Polydopamine-Reduced Graphene Oxide Nanocomposites with Amplifying Photoacoustic and Photothermal Effects for Cancer Theranostics
    (Wyoming, NSW : Ivyspring, 2016) Hu, Dehong; Zhang, Jingnan; Gao, Guanhui; Sheng, Zonghai; Cui, Haodong; Cai, Lintao
    Photoacoustic (PA) imaging and photothermal therapy (PTT) as light-induced theranostic platforms have been attracted much attention in recent years. However, the development of highly efficient and integrated phototheranostic nanoagents for amplifying PA imaging and PTT treatments poses great challenges. Here, we report a novel phototheranostic nanoagent using indocyanine green-loaded polydopamine-reduced graphene oxide nanocomposites (ICG-PDA-rGO) with amplifying PA and PTT effects for cancer theranostics. The results demonstrate that the PDA layer coating on the surface of rGO could effectively absorb a large number of ICG molecules, quench ICG's fluorescence, and enhance the PDA-rGO's optical absorption at 780 nm. The obtained ICG-PDA-rGO exhibits stronger PTT effect and higher PA contrast than that of pure GO and PDA-rGO. After PA imaging-guided PTT treatments, the tumors in 4T1 breast subcutaneous and orthotopic mice models are suppressed completely and no treatment-induced toxicity being observed. It illustrates that the ICG-PDA-rGO nanocomposites constitute a new class of theranostic nanomedicine for amplifying PA imaging and PTT treatments.
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    Phonon anharmonicities and ultrafast dynamics in epitaxial Sb2Te3
    ([London] : Macmillan Publishers Limited, part of Springer Nature, 2020) Bragaglia, V.; Ramsteiner, M.; Schick, D.; Boschker, J.E.; Mitzner, R.; Calarco, R.; Holldack, K.
    In this study we report on the investigation of epitaxially grown Sb2Te3 by employing Fourier-Transform transmission Spectroscopy (FTS) with laser-induced Coherent Synchrotron Radiation (CSR) in the Terahertz (THz) spectral range. Static spectra in the range between 20 and 120 cm−1 highlight a peculiar softening of an in-plane IR-active phonon mode upon temperature decrease, as opposed to all Raman active modes which instead show a hardening upon temperature decrease in the same energy range. The phonon mode softening is found to be accompanied by an increase of free carrier concentration. A strong coupling of the two systems (free carriers and phonons) is observed and further evidenced by exciting the same phonon mode at 62 cm−1 within an ultrafast pump-probe scheme employing a femtosecond laser as pump and a CSR single cycle THz pulse as probe. Separation of the free carrier contribution and the phonon resonance in the investigated THz range reveals that, both damping of the phonon mode and relaxation of hot carriers in the time domain happen on the same time scale of 5 ps. This relaxation is about a factor of 10 slower than expected from the Lorentz time-bandwidth limit. The results are discussed in the framework of phonon scattering at thermal and laser induced transient free carriers.