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[en] We report increased radiation in the visible and terahertz (THz) regimes in silicon(Si)-based nanostructures. The nanostructures, Si nanowires (SiNWs) and porous Si (PSi), were synthesized via electroless and electrochemical surface modification, respectively. In particular, picosecond (ps) radiative lifetimes in the order of 250 ps were obtained from time-resolved photoluminescence (PL) measurements. The fast radiative lifetimes are associated with increased surface defect density in PSi. Reflectance measurements confirmed that optical absorption of the nanostructured Si samples increased relative to bulk Si. Both nanostructured Si exhibit THz emission, albeit weaker in PSi due to higher density of defects. An inverse relationship between PL and THz emission strength was therefore observed. Lastly, the wider bandwidth of the THz emission in SiNWs is attributed to the directionality of the transient photocurrent compared to the more disordered carrier transport in PSi.
[en] One-dimensional semiconducting nanowires (NWs) have great potential for applications in the fields of optoelectronic and sensor devices, mainly due to the remarkable electronic and optical properties that they exhibit. The key for the future of semiconducting NW-based devices is the detail understanding of their carrier relaxation and transport processes. The main objective of this research work is the detail understanding of ultrafast carrier dynamics in GaN, In2O3 and SnO2 NWs, due to their attractive potential in various optoelectronic applications. This has been accomplished through the use of the powerful techniques of ultrafast transient differential absorption and THz spectroscopy. Atmospheric pressure chemical vapour deposition has been utilized for the synthesis of GaN, In2O3 and SnO2 NWs grown via the vapour–liquid-solid method. Optimization of these NWs has been accomplished through scanning electron microscopy, x-ray diffraction and steady state transmission spectroscopy. Transient differential absorption spectroscopy has been used as the key ultrafast characterization technique for the investigation of carrier dynamics on a sub-picosecond time scale in these NWs. Measurements as a function of various parameters including photon energy, incident pump fluence and probing wavelength provided invaluable information on carrier relaxation. It appears that defect-related states play a crucial role in the relaxation of the photogenerated carriers. Intensity dependent measurements revealed the role of Auger recombination in these nanostructures. Following the experimental findings, schematics of the band diagrams, which incorporate carrier dynamics are presented. Furthermore, THz transmission spectroscopies have been used to investigate the transport properties of SnO2 NWs. A transient THz transmission spectroscopy based on the pump-probe technique has been developed for measuring the complex response function of the nanostructures in equilibrium state or following excitation with ultrashort pulses. Analysis of the conductivity measurements in these NWs have been achieved using the Drude-Smith model. These measurements reveal localization of carriers in the NWs and strong suppression of long transport. (author)
[en] A comprehensive investigation of quantum confinement in nanowires has been carried out. Though applied to silicon nanowires (SiNWs), it is general and applicable to all nanowires. Fundamentals and applications of quantum confinement in nanowires and possible laws obeyed by these nanowires, have been investigated. These laws may serve as backbones of nanowire science and technology. The relationship between energy band gap and nanowire diameter has been studied. This relationship appears to be universal. A thorough review indicates that the first principles results for quantum confinement vary widely. The possible cause of this variation has been examined. Surface passivation and surface reconstruction of nanowires have been elucidated. It has been found that quantum confinement owes its origin to surface strain resulting from surface passivation and surface reconstruction and hence thin nanowires may actually be crystalline-core/amorphous-shell (c-Si/a-Si) nanowires. Experimental data available in the literature corroborate with the suggestion. The study also reveals an intrinsic relationship between quantum confinement and the surface amorphicity of nanowires. It demonstrates that surface amorphicity may be an important tool to investigate the electronic, optoelectronic and sensorial properties of quantum-confined nanowires. (topical review)
[en] The interaction of a current-driven domain wall with an anisotropy boundary in nanowires with perpendicular magnetic anisotropy is investigated. A local reduction of the anisotropy constant is used to create an artificial boundary where the domain wall gets pinned. Micromagnetic simulations and analytical calculations, based on a one-dimensional model, are employed to describe the interaction of the domain wall and the anisotropy boundary and to determine the depinning current densities. Two different pinning regimes—an intrinsic and an extrinsic—can be identified in dependence with the characteristic of the boundary. A very good agreement between simulated and analytically obtained data is achieved. (paper)
[en] Highlights: • Cold welding processes of copper nanowires were explored by molecular simulations. • Twinned nanowires can be successfully welded through small loadings. • Welding processes were compared between single-crystalline and twinned nanowires. In this article, molecular simulations were adopted to explore the cold welding processes of copper nanowires with both single-crystalline and fivefold twinned structures. It was verified that the twinned nanowires exhibited enhanced strength but lowered elastic limit and ductility. Both nanowires could be successfully welded through rather small loadings, although their stress–strain responses toward compression were different. Meanwhile, more stress was accumulated in the twinned nanowire due to repulsive force of the twin boundaries against the nucleation and motions of dislocations. Moreover, by characterizing the structure evolutions in the welding process, it can be ascertained that perfect atomic order was finally built at the weld region in both nanowires. This comparison study will be of great importance to future mechanical processing of metallic nanowires.
[en] A simple, inexpensive and wafer-scale method to obtain Si microstructures is proposed. The method consists in a sequence of steps that include a selective metal-assisted chemical etching process to create regions of Si nanowires that are sacrificed in a post-etching process, leaving microstructures standing. As a proof of concept, Si micropillars with length of 7 µm and diameter of 1.4 µm were fabricated. The advantage of the proposed method is its simplicity, allowing the production of microstructures in a basic chemical laboratory.
[en] The electronic band structures of Si and Ge low-dimensional nanostructure such as nanofilms and nanowires have been calculated using first principles based on density functional theory (DFT) with the generalized gradient approximation (GGA). The calculation results show that a direct band gap can be obtained from Si orientation  or in Ge orientation  confined low dimensional nanostructure. However, an indirect band gap is still kept in the Si orientation  or in the Ge orientation  confined low dimensional nanostructure. The calculation results are interesting and the transition mechanism from indirect to direct band gap in low dimensional nanostructures is given in the physical structures model.
[en] The effects of the shape of a nanowire terminus on the excited surface plasmon polariton (SPP) modes are investigated. The conical terminus and terminus cut at a certain angle are studied. For the first time, the quantitative mode decompositions are carried out to derive the full information about excited SPP modes. It is demonstrated that tuning the shape of the terminus provides an effective method to control the composition of excited SPP modes on metal nanowires. It is especially found that some important patterns, such as the pure TM0 mode and the superposition of TM0 and HE+1 or HE−1 modes, can be generated by some specific shapes of the terminus, whereas there is no way to produce these patterns using flat-end nanowires. (condensed matter: electronic structure, electrical, magnetic, and optical properties)
[en] The magnetization behavior of nanowires arranged in an array is strongly influenced by the strength and shape of the demagnetizing field experienced by the individual nanowires. In order to understand the initial stage of the reversal process in nanowire arrays a hybrid method is proposed. First, an expression for the numerical calculation of the demagnetizing field in nanowire arrays is presented and compared to already available approaches. The derived expression calculates the demagnetizing field as a function of position along the length of an individual nanowire embedded in the array. In a second step, a micromagnetic simulation of the reversal process of a an isolated nanowire (1 μ m) is superimposed with the earlier obtained field distribution. This enables including the inhomogeneity of the demagnetizing field of a large array (≈108 nanowires) into the micromagnetic simulation without the need of particularly large computing power. The simulation results show that in contrast to the switching in a single nanowire, where the reversal starts at the nanowire ends, the reversal under the influence of the array field strongly depends on the magnetization state of the whole array. Reversal can start either at the nanowire’s central region or at its ends. (paper)
[en] The fluctuation of the stretched resistance of composites based on inkjet-printed silver nanowires (AgNW) and polydimethylsiloxane (PDMS) is controlled by different ratios of a curing agent in PDMS, with an optimized normalized resistance change as low as 4.67% in a 50% tensile testing cycle. The results confirm that the properties of PDMS including the Young’s modulus and creep deformation play key roles in controlling the resistance changes of stretched AgNW–PDMS composites based on inkjet-printed AgNW, which opens up applications of inkjet-printable elastic AgNW conductors in stretchable electronics. (letter)