Search Results (16)

The broad and fascinating properties of nanowires and their synthesis have attracted great attention as building blocks for functional devices at the nanoscale. Silicon and germanium are highly interesting materials due to their compatibility with standard CMOS technology. Their combination provides optimal templates for quantum applications, for which nanowires need to be of high quality, with carefully designed dimensions, crystal phase, and orientation. In this work, we present a detailed study on the growth kinetics of silicon (length 0.1–1 $\mathsf{\mu }$m, diameter 10–60 nm) and germanium (length 0.06–1 $\mathsf{\mu }$m, diameter 10–500 nm) nanowires grown by chemical vapor deposition applying the vapour–liquid–solid growth method catalysed by gold. The effects of temperature, partial pressure of the precursor gas, and different carrier gases are analysed via scanning electron microscopy. Argon as carrier gas enhances the growth rate at higher temperatures (120 nm/min for Ar and 48 nm/min H${}_2$), while hydrogen enhances it at lower temperatures (35 nm/min for H${}_2$ and 22 nm/min for Ar) due to lower heat capacity. Both materials exhibit two growth regimes as a function of the temperature. The tapering rate is about ten times lower for silicon nanowires than for germanium ones. Finally, we identify the optimal conditions for nucleation in the nanowire growth process. Full article

Germanium–cobalt–indium nanostructures were synthesized via cathodic electrodeposition from aqueous complex solutions of Ge (IV) and Co (II) with drop-like indium crystallization centers. This approach features simplicity, avoids heating and allows using cheaper GeO₂ instead of pure Ge as starting material. Further, in this case, target nanostructures grow directly upon the substrate. Various analytical methods (scanning electron microscopy, transmission electron microscope and X-ray diffraction) were used for characterization of the nanostructures under study. The samples obtained consist of an array of globular particles of 200 to 800 nm, with nanowires in between. The globules, in turn, contain primary particles of 5 to 10 nm consisting of cobalt, germanium and oxygen. Nanowires consist of germanium and indium. The electrochemical properties of the above-mentioned nanostructures were assessed with cyclic voltammetry and galvanostatic cycling. The germanium–cobalt–indium nanostructures are characterized by a high specific capacity upon lithium insertion, which is approximately 1350 mAh/g at C/8, and a high Coulomb cycling efficiency in the first cycle (approximately 0.76). Germanium–cobalt–indium nanostructures show the ability to operate at high rates up to 16 C at a wide temperature range from +20 to −35 °C. Full article

Germanium-based multi-metallic-oxide materials have advantages of low activation energy, tunable output voltage, and high theoretical capacity. However, they also exhibit unsatisfactory electronic conductivity, sluggish cation kinetics, and severe volume change, resulting in inferior long-cycle stability and rate performance in lithium-ion batteries (LIBs). To solve these problems, we synthesize metal-organic frameworks derived from rice-like Zn₂GeO₄ nanowire bundles as the anode of LIBs via a microwave-assisted hydrothermal method, minimizing the particle size and enlarging the cation’s transmission channels, as well as, enhancing the electronic conductivity of the materials. The obtained Zn₂GeO₄ anode exhibits superior electrochemical performance. A high initial charge capacity of 730 mAhg⁻¹ is obtained and maintained at 661 mAhg⁻¹ after 500 cycles at 100 mA g⁻¹ with a small capacity degradation ratio of ~0.02% for each cycle. Moreover, Zn₂GeO₄ exhibits a good rate performance, delivering a high capacity of 503 mA h g⁻¹ at 5000 mA g⁻¹. The good electrochemical performance of the rice-like Zn₂GeO₄ electrode can be attributed to its unique wire-bundle structure, the buffering effect of the bimetallic reaction at different potentials, good electrical conductivity, and fast kinetic rate. Full article

Si_1−xGe_x nanowires (NWs) were prepared by gold-supported chemical vapor deposition (CVD) using a single-source precursor with preformed Si–Ge bonds. Besides the tamed reactivity of the precursor, the approach reduces the process parameters associated with the control of decomposition characteristics and the dosing of individual precursors. The group IV alloy NWs are single crystalline with a constant diameter along their axis. During the wire growth by low pressure CVD, an Au-containing surface layer on the NWs forms by surface diffusion from the substrate, which can be removed by a combination of oxidation and etching. The electrical properties of the Si_1−xGe_x/Au core-shell NWs are compared to the Si_1−xGe_x NWs after Au removal. Core–shell NWs show signatures of metal-like behavior, while the purely semiconducting NWs reveal typical signatures of intrinsic Si_1−xGe_x. The synthesized materials should be of high interest for applications in nano- and quantum-electronics. Full article

Layers of germanium (Ge) microrods with a core–shell structure on titanium foils were grown by a metal-assisted electrochemical reduction of germanium oxide in aqueous electrolytes. The structural properties and composition of the germanium microrods were studied by means of scanning and transmission electron microscopy. Electrochemical studies of germanium nanowires were carried out by impedance spectroscopy and cyclic voltammetry. The results showed that the addition of vinylene carbonate (VC) in the electrolyte significantly reduced the irreversible capacity during the first charge/discharge cycles and increased the long-term cycling stability of the Ge microrods. The obtained results will benefit the further design of Ge microrods-based anodes that are formed by simple electrochemical deposition. Full article

Layers of germanium (Ge) nanowires (NWs) on titanium foils were grown by metal-assisted electrochemical reduction of germanium oxide in aqueous electrolytes based on germanium oxide without and with addition of sodium silicate. Structural properties and composition of Ge NWs were studied by means of the scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Raman spectroscopy. When sodium silicate was added to the electrolyte, Ge NWs consisted of 1–2 at.% of silicon (Si) and exhibited smaller mean diameter and improved crystallinity. Additionally, samples of Ge NW films were prepared by ultrasonic removal of Ge NWs from titanium foils followed with redeposition on corundum substrates with platinum electrodes. The electrical conductivity of Ge NW films was studied at different temperatures from 25 to 300 °C and an effect of the silicon impurity on the thermally activated electrical conductivity was revealed. Furthermore, the electrical conductivity of Ge NW films on corundum substrates exhibited a strong sensor response on the presence of saturated vapors of different liquids (water, acetone, ethanol, and isopropanol) in air and the response was dependent on the presence of Si impurities in the nanowires. The results obtained indicate the possibility of controlling the structure and electrical properties of Ge NWs by introducing silicate additives during their formation, which is of interest for applications in printed electronics and molecular sensorics. Full article

In this article, we review the main features of nonlocal and nonlinear heat transport in nanosystems and analyze some celebrated differential equations which describe this phenomenon. Then, we present a new heat-transport equation arising within the so-called thermomass theory of heat conduction. We illustrate how such a theory can be applied to the analysis of the efficiency of a thermoelectric energy generator constituted by a Silicon–Germanium alloy, as the application and new results for a nanowire of length $L=100$ nm, are presented as well. The thermal conductivity of the nanowire as a function of composition and temperature is determined in light of the experimental data. Additionally, the best-fit curve is obtained. The dependency of the thermoelectric efficiency of the system on both the composition and the difference of temperature applied to its ends is investigated. For the temperatures $T=300$ K, $T=400$ K, and $T=500$ K, we calculate the values of the composition corresponding to the optimal efficiency, as well as the optimal values of the thermal conductivity. Finally, these new results are compared with recent ones obtained for a system of length $L=3$ mm, in order to point out the benefits due to the miniaturization in thermoelectric energy conversion. Full article

This study demonstrated the deposition of size-controlled gold (Au) nanoclusters via direct-current magnetron sputtering and inert gas condensation techniques. The impact of different source parameters, namely, sputtering discharge power, inert gas flow rate, and aggregation length on Au nanoclusters’ size and yield was investigated. Au nanoclusters’ size and size uniformity were confirmed via transmission electron microscopy. In general, Au nanoclusters’ average diameter increased by increasing all source parameters, producing monodispersed nanoclusters of an average size range of 1.7 ± 0.1 nm to 9.1 ± 0.1 nm. Among all source parameters, inert gas flow rate exhibited a strong impact on nanoclusters’ average size, while sputtering discharge power showed great influence on Au nanoclusters’ yield. Results suggest that Au nanoclusters nucleate via a three-body collision mechanism and grow through a two-body collision mechanism, wherein the nanocluster embryos grow in size due to atomic condensation. Ultimately, the usefulness of the produced Au nanoclusters as catalysts for a vapor–liquid–solid technique was put to test to synthesize the phase change material germanium telluride nanowires. Full article

The fabrication of individual nanowire-based devices and their comprehensive electrical characterization remains a major challenge. Here, we present a symmetric Hall bar configuration for highly p-type germanium nanowires (GeNWs), fabricated by a top-down approach using electron beam lithography and inductively coupled plasma reactive ion etching. The configuration allows two equivalent measurement sets to check the homogeneity of GeNWs in terms of resistivity and the Hall coefficient. The highest Hall mobility and carrier concentration of GeNWs at 5 K were in the order of 100 ${\mathrm{cm}}^2/\left(\mathrm{Vs}\right)$ and $4\times {10}^{19}{\mathrm{cm}}^{-3}$, respectively. With a decreasing nanowire width, the resistivity increases and the carrier concentration decreases, which is attributed to carrier scattering in the region near the surface. By comparing the measured data with simulations, one can conclude the existence of a depletion region, which decreases the effective cross-section of GeNWs. Moreover, the resistivity of thin GeNWs is strongly influenced by the cross-sectional shape. Full article

Ge nanowires are playing a big role in the development of new functional microelectronic modules, such as gate-all-around field-effect transistor devices, on-chip lasers and photodetectors. The widely used three-phase bottom-up growth method utilising a foreign catalyst metal or metalloid is by far the most popular for Ge nanowire growth. However, to fully utilise the potential of Ge nanowires, it is important to explore and understand alternative and functional growth paradigms such as self-seeded nanowire growth, where nanowire growth is usually directed by the in situ-formed catalysts of the growth material, i.e., Ge in this case. Additionally, it is important to understand how the self-seeded nanowires can benefit the device application of nanomaterials as the additional metal seeding can influence electron and phonon transport, and the electronic band structure in the nanomaterials. Here, we review recent advances in the growth and application of self-seeded Ge and Ge-based binary alloy (GeSn) nanowires. Different fabrication methods for growing self-seeded Ge nanowires are delineated and correlated with metal seeded growth. This review also highlights the requirement and advantage of self-seeded growth approach for Ge nanomaterials in the potential applications in energy storage and nanoelectronic devices. Full article

Controllable growth of wafer-scale in-plane nanowires (NWs) is a prerequisite for achieving addressable and scalable NW-based quantum devices. Here, by introducing molecular beam epitaxy on patterned Si structures, we demonstrate the wafer-scale epitaxial growth of site-controlled in-plane Si, SiGe, and Ge/Si core/shell NW arrays on Si (001) substrate. The epitaxially grown Si, SiGe, and Ge/Si core/shell NW are highly homogeneous with well-defined facets. Suspended Si NWs with four {111} facets and a side width of about 25 nm are observed. Characterizations including high resolution transmission electron microscopy (HRTEM) confirm the high quality of these epitaxial NWs. Full article

The thermoelectric performance of nanostructured low dimensional silicon and silicon-germanium has been functionally compared device-wise. The arrays of nanowires of both materials, grown by a VLS-CVD (Vapor-Liquid-Solid Chemical Vapor Deposition) method, have been monolithically integrated in a silicon micromachined structure in order to exploit the improved thermoelectric properties of nanostructured silicon-based materials. The device architecture helps to translate a vertically occurring temperature gradient into a lateral temperature difference across the nanowires. Such thermocouple is completed with a thin film metal leg in a unileg configuration. The device is operative on its own and can be largely replicated (and interconnected) using standard IC (Integrated Circuits) and MEMS (Micro-ElectroMechanical Systems) technologies. Despite SiGe nanowires devices show a lower Seebeck coefficient and a higher electrical resistance, they exhibit a much better performance leading to larger open circuit voltages and a larger overall power supply. This is possible due to the lower thermal conductance of the nanostructured SiGe ensemble that enables a much larger internal temperature difference for the same external thermal gradient. Indeed, power densities in the μW/cm² could be obtained for such devices when resting on hot surfaces in the 50–200 °C range under natural convection even without the presence of a heat exchanger. Full article

In this paper, we model the electrical properties of germanium nanowires with a particular focus on physical mechanisms of electrical molecular sensing. We use the Tibercad software to solve the drift-diffusion equations in 3D and we validate the model against experimental data, considering a p-doped nanowire with surface traps. We simulate three different types of interactions: (1) Passivation of surface traps; (2) Additional surface charges; (3) Charge transfer from molecules to nanowires. By analyzing simulated I–V characteristics, we observe that: (i) the largest change in current occurs with negative charges on the surfaces; (ii) charge transfer provides relevant current changes only for very high values of additional doping; (iii) for certain values of additional n-doping ambipolar currents could be obtained. The results of these simulations highlight the complexity of the molecular sensing mechanism in nanowires, that depends not only on the NW parameters but also on the properties of the molecules. We expect that these findings will be valuable to extend the knowledge of molecular sensing by germanium nanowires, a fundamental step to develop novel sensors based on these nanostructures. Full article

In this paper, a germanium-based gate-metal-core vertical nanowire tunnel field effect transistor (VNWTFET) has been designed and optimized using the technology computer-aided design (TCAD) simulation. In the proposed structure, by locating the gate-metal as a core of the nanowire, a more extensive band-to-band tunneling (BTBT) area can be achieved compared with the conventional core–shell VNWTFETs. The channel thickness (T_ch), the gate-metal height (H_g), and the channel height (H_ch) were considered as the design parameters for the optimization of device performances. The designed gate-metal-core VNWTFET exhibits outstanding performance, with an on-state current (I_on) of 80.9 μA/μm, off-state current (I_off) of 1.09 × 10⁻¹² A/μm, threshold voltage (V_t) of 0.21 V, and subthreshold swing (SS) of 42.8 mV/dec. Therefore, the proposed device was demonstrated to be a promising logic device for low-power applications. Full article

Silicon and germanium pillar structures (i.e., micro- and nanowires) were fabricated by a top-down approach including nanoimprint lithography and cryogenic dry etching. Various etching parameters were tested to ensure a reliable fabrication process. The impression of nanomechanical properties of such 3-D structures were extracted experimentally by nanoindentation showing promising and comparative results to utilize such nanostructures as small force artefacts. Full article