Minimizing the Gibbs-Thomson effect in the low-temperature plasma synthesis of thin Si nanowires

An advanced combination of numerical models, including plasma sheath, ion- and radical-induced species creation and plasma heating effects on the surface and within a Au catalyst nanoparticle, is used to describe the catalyzed growth of Si nanowires in the sheath of a low-temperature and low-pressure plasma. These models have been used to explain the higher nanowire growth rates, low-energy barriers, much thinner Si nanowire nucleation and the less effective Gibbs-Thomson effect in reactive plasma processes, compared with those of neutral gas thermal processes. The effects of variation in the plasma sheath parameters and substrate potential on Si nanowire nucleation and growth have also been investigated. It is shown that increasing the plasma-related effects leads to decreases in the nucleation energy barrier and the critical nanoparticle radius, with the Gibbs-Thomson effect diminished, even at low temperatures. The results obtained are consistent with available experimental results and open a path toward the energy- and matter-efficient nucleation and growth of a broad range of one-dimensional quantum structures.     

Room-temperature quantum confinement effects in transport properties of ultrathin Si nanowire field-effect transistors

Quantum confinement of carriers has a substantial impact on nanoscale device operations. We present electrical transport analysis for lithographically fabricated sub-5 nm thick Si nanowire field-effect transistors and show that confinement-induced quantum oscillations prevail at 300 K. Our results discern the basis of recent observations of performance enhancement in ultrathin Si nanowire field-effect transistors and provide direct experimental evidence for theoretical predictions of enhanced carrier mobility in strongly confined nanowire devices.     

Diameter-dependent or independent: toward a mechanistic understanding of the vapor-liquid-solid si nanowire growth rate

Si nanowires have received continued increased attention because they keep the promise of monolithic integration of high-performance semiconductors with new functionality into existing silicon technology. Most Si nanowires are grown by vapor-liquid-solid mechanism, and despite many years of study, this growth mechanism remains under lively debate. For instance, contradictory results have been reported on the effect of diameter size on nanowire growth rate. Here, we developed a universal kinetic model of Si nanowire growth based on surface diffusion which takes into account adatom diffusion from the sidewall and substrate surface into the liquid droplet as well as the Gibbs-Thomson effect. Our analysis shows that the diameter independence for Si nanowires is affected by the interplay between the Gibbs-Thomson effect and the surface diffusion, whereas the diameter dependence is mainly influenced by the Gibbs-Thomson effect. The results based on the proposed model are in good agreement with experimental data.     

Diameter-dependent composition of vapor-liquid-solid grown Si(1-x)Ge(x) nanowires

Diameter-dependent compositions of Si(1-x)Ge(x) nanowires grown by a vapor-liquid-solid mechanism using SiH(4) and GeH(4) precursors are studied by transmission electron microscopy and X-ray energy dispersive spectroscopy. For the growth conditions studied, the Ge concentration in Si(1-x)Ge(x) nanowires shows a strong dependence on nanowire diameter, with the Ge concentration decreasing with decreasing nanowire diameter below approximately 50 nm. The size-dependent nature of Ge concentration in Si(1-x)Ge(x) NWs is strongly suggestive of Gibbs-Thomson effects and highlights another important phenomenon in nanowire growth.     

Template‐Free Growth of Well‐Ordered Silver Nano Forest/Ceramic Metamaterial Films with Tunable Optical Responses

Currently, the limitations of conventional methods for fabricating metamaterials composed of well‐aligned nanoscale inclusions either lack the necessary freedom to tune the structural geometry or are difficult for large‐area synthesis. In this Communication, the authors propose a fabrication route to create well‐ordered silver nano forest/ceramic composite single‐layer or multi‐layer vertically stacked structures, as a distinctive approach to make large‐area nanoscale metamaterials. To take advantage of direct growth, the authors fabricate single‐layer nanocomposite films with a well‐defined sub‐5 nm interwire gap and an average nanowire diameter of ≈3 nm. Further, artificially constructed multilayer metamaterial films are easily fabricated by vertical integration of different single‐layer metamaterial films. Based upon the thermodynamics as well as thin film growth dynamics theory, the growth mechanism is presented to elucidate the formation of such structure. Intriguing steady and transient optical properties in these assemblies are demonstrated, owing to their nanoscale structural anisotropy. The studies suggest that the self‐organized nanocomposites provide an extensible material platform to manipulate optical response in the region of sub‐5 nm scale.     

Localized temperature and chemical reaction control in nanoscale space by nanowire array

We introduce a novel method for chemical reaction control with nanoscale spatial resolution based on localized heating by using a well-aligned nanowire array. Numerical and experimental analysis shows that each individual nanowire could be selectively and rapidly Joule heated for local and ultrafast temperature modulation in nanoscale space (e.g., maximum temperature gradient 2.2 K/nm at the nanowire edge; heating/cooling time < 2 μs). By taking advantage of this capability, several nanoscale chemical reactions such as polymer decomposition/cross-linking and direct and localized hydrothermal synthesis of metal oxide nanowires were demonstrated.     

Semiconductor nanowire heterostructures

Recent progress on the synthesis and characterization of semiconductor nanowire heterostructures is reviewed. We describe a general method for heterostructure synthesis based on chemical vapour deposition and the vapour-liquid-solid growth of crystalline semiconducting nanowires. We then examine examples of nanowire heterostructures for which physical properties have been measured, considering the effects of synthetic conditions on the heterointerfaces as well as the electrical and optical characterization measurements that reveal heterointerface formation and quality. Finally, we identify areas of technical and conceptual progress that can contribute to the development of functional nanowire heterostructures.     

Modeling of InAs-InSb nanowires grown by Au-assisted chemical beam epitaxy

Interesting phenomena during the Au-assisted chemical beam epitaxy of InAs-InSb nanowire heterostructures have been observed and interpreted within the framework of a theoretical model. An unusual, non-monotonous diameter dependence of the InSb nanowire growth rate is demonstrated experimentally within a range of deposition conditions. Such a behavior is explained by competition between the Gibbs-Thomson effect and different diffusion-induced material fluxes. Theoretical fits to the experimental data obtained at different flux pressures of In and Sb precursors allow us to deduce some important kinetic coefficients. Furthermore, we discuss why the InAs nanowire stem forms in the wurtzite phase while the upper InSb part has a pure zinc blende crystal structure. It is hypothesized that the 30° angular rotation of nanowire when passing from InAs to the InSb part is driven by the lowest surface energy of (1100) wurtzite and (110) zinc blende facets.     

Controlling silicon nanowire growth direction via surface chemistry

We report on the first in situ chemical investigation of vapor-liquid-solid semiconductor nanowire growth and reveal the important, and previously unrecognized, role of transient surface chemistry near the triple-phase line. Real-time infrared spectroscopy measurements coupled with postgrowth electron microscopy demonstrate that covalently bonded hydrogen atoms are responsible for the (left angle bracket 111 right angle bracket) to (left angle bracket 112 right angle bracket) growth orientation transition commonly observed during Si nanowire growth. Our findings provide insight into the root cause of this well-known nanowire growth phenomenon and open a new route to rationally engineer the crystal structure of these nanoscale semi-conductors.     

一维硅锗纳米复合材料的制备及在场效应晶体管中的应用

一维硅锗纳米复合材料，主要包括硅锗纳米线异质结与纳米管，具有优异的电学、光学等性能，易与现代以硅为基础的微电子工业相兼容，所以在纳米器件等领域得到了广泛重视。总结了一维硅锗纳米复合材料的研究现状和相关的制备方法，重点评述了在纳米场效应晶体管中的应用，并对其研究前景做了展望。     

Size effect on Ge nanowires growth kinetics by the vapor-liquid-solid mechanism

Germanium nanowires were grown on germanium (111) substrate by ultra high vacuum chemical vapor deposition, via the vapor-liquid-solid growth mechanism, using digermane as gaseous precursor and gold as catalyst. The results show that the nanowire length depends on the diameter catalyst, the smaller the nanowire radius, the slower the nanowires grow.In order to fit the experimental data, we have used a simple model based on the Gibbs-Thomson effect and adapted to our growth conditions. This model was found to perfectly account for the catalyst size dependence of the experimental growth kinetics of germanium nanowires from digermane. From that, a critical radius of 6 nm was derived.     

Effect of point defects and impurities on the electronic transport of Au tipped ultranarrow PbS nanorods

Electronic transport through single nanowire/nanorod directly probes the fundamental limits of semiconductor device miniaturization. Point defects or impurity centers form easily during the growth of nanorods/nanowires which may strongly affect the electronic transport efficiencies. Existing models of electronic transport are often unable to determine the role of defects and impurities at the nanoscale because there are significant differences between nanostructures and bulk materials arising from unique geometries and confinement. The effect of defect and impurities on the conductance of a model ultranarrow PbS rod was modeled using density functional theory. It was observed that the introduction of defects and Au impurities modified the orbital energies of PbS nanorods and reduced the conductance compared to the defect-free rod. The conductance for the nanorods with defects and impurities were limited by the number of available conduction channels required for efficient electronic conduction.     

Highly-ordered supportless three-dimensional nanowire networks with tunable complexity and interwire connectivity for device integration

The fabrication of three-dimensional assemblies consisting of large quantities of nanowires is of great technological importance for various applications including (electro-)catalysis, sensitive sensing, and improvement of electronic devices. Because the spatial distribution of the nanostructured material can strongly influence the properties, architectural design is required in order to use assembled nanowires to their full potential. In addition, special effort has to be dedicated to the development of efficient methods that allow precise control over structural parameters of the nanoscale building blocks as a means of tuning their characteristics. This paper reports the direct synthesis of highly ordered large-area nanowire networks by a method based on hard templates using electrodeposition within nanochannels of ion track-etched polymer membranes. Control over the complexity of the networks and the dimensions of the integrated nanostructures are achieved by a modified template fabrication. The networks possess high surface area and excellent transport properties, turning them into a promising electrocatalyst material as demonstrated by cyclic voltammetry studies on platinum nanowire networks catalyzing methanol oxidation. Our method opens up a new general route for interconnecting nanowires to stable macroscopic network structures of very high integration level that allow easy handling of nanowires while maintaining their connectivity.     

The influences of technological conditions and Au cluster islands on morphology of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> nanowires grown by VLS method on GaAs substrate

The synthesis of semiconductor nanowires is more and more interested to the applications for building blocks of the innovative nano-sized devices and circuits, but the research and fabrication of these nanowires are also holding a number of difficulties and challenges. Among many different kinds of semiconductor nanowires, Ga₂O₃ is increasingly grown for many promising applications in nano-device production, namely nanowire LED and Laser. So far there are many synthesizing methods of semiconductor nanowires, among them the vapor–liquid–solid (VLS) method is simple, cheap and popular. However, when we use the VLS method for nanowire growth, various technological problems exist. This paper aims at investigating some influences of the growth technological conditions and Au metal catalyst on the morphology of Ga₂O₃ nanowire grown by VLS on GaAs substrate. The main considering factors include the different growing temperatures and times, the effects of Au diffusion, Au droplets formation, Au cluster islands formation, and gas volume of the growing tube/ampoule at the 10⁻¹ torr low air pressure. The obtained experimental results regarding the structural properties of nanowires under these effects investigated by scanning electron microscopy, field emission scanning electron microscopy, high angle annular dark field and bright field, scanning transmission electron microscopy, energy-dispersive X-ray techniques, and focus ion beam are presented and discussed.     

A Theoretical Review on Interfacial Thermal Transport at the Nanoscale

With the development of energy science and electronic technology, interfacial thermal transport has become a key issue for nanoelectronics, nanocomposites, energy transmission, and conservation, etc. The application of thermal interfacial materials and other physical methods can reliably improve the contact between joined surfaces and enhance interfacial thermal transport at the macroscale. With the growing importance of thermal management in micro/nanoscale devices, controlling and tuning the interfacial thermal resistance (ITR) at the nanoscale is an urgent task. This Review examines nanoscale interfacial thermal transport mainly from a theoretical perspective. Traditional theoretical models, multiscale models, and atomistic methodologies for predicting ITR are introduced. Based on the analysis and summary of the factors that influence ITR, new methods to control and reduce ITR at the nanoscale are described in detail. Furthermore, the challenges facing interfacial thermal management and the further progress required in this field are discussed.     

Transport modulation in Ge/Si core/shell nanowires through controlled synthesis of doped Si shells

Appropriately controlling the properties of the Si shell in Ge/Si core/shell nanowires permits not only passivation of the Ge surface states, but also introduces new interface phenomena, thereby enabling novel nanoelectronics concepts. Here, we report a rational synthesis of Ge/Si core/shell nanowires with doped Si shells. We demonstrate that the morphology and thickness of Si shells can be controlled for different dopant types by tuning the growth parameters during synthesis. We also present distinctly different electrical characteristics that arise from nanowire field-effect transistors fabricated using the synthesized Ge/Si core/shell nanowires with different shell morphologies. Furthermore, a clear transition in the modification of device characteristics is observed for crystalline shell nanowires following removal of the shell using a unique trimming process of successive native oxide formation/etching. Our results demonstrate that the preferred transport path through the nanowire structure can be modulated by appropriately tuning the growth conditions.     

Self-limited plasmonic welding of silver nanowire junctions

Nanoscience provides many strategies to construct high-performance materials and devices, including solar cells, thermoelectrics, sensors, transistors, and transparent electrodes. Bottom-up fabrication facilitates large-scale chemical synthesis without the need for patterning and etching processes that waste material and create surface defects. However, assembly and contacting procedures still require further development. Here, we demonstrate a light-induced plasmonic nanowelding technique to assemble metallic nanowires into large interconnected networks. The small gaps that form naturally at nanowire junctions enable effective light concentration and heating at the point where the wires need to be joined together. The extreme sensitivity of the heating efficiency on the junction geometry causes the welding process to self-limit when a physical connection between the wires is made. The localized nature of the heating prevents damage to low-thermal-budget substrates such as plastics and polymer solar cells. This work opens new avenues to control light, heat and mass transport at the?nanoscale.     

Effect of Thiolated Ligands in Au Nanowire Synthesis

Thiolated ligands are seldom used as morphology‐directing reagent in the synthesis of Au nanostructures due to their low selectivity toward the different facets. Recently, we developed a thiolated ligands‐induced synthesis of nanowires where the selective Au deposition only occurs at the ligand‐deficient Au–substrate interface. Herein, the structural effect of thiolated ligands in this active surface growth is systematically investigated. It is revealed that their ability of rendering surface is closely related to the molecular structure. Ligands with aromatic backbones are capable of inducing nanowire formation, whereas those with aliphatic backbones cannot, likely because the former can pack better at short time scale of the rapid growth. The substituents of the ligands are critical for the colloidal stability of the final structure. It is further demonstrated that aromatic and aliphatic ligands could be mixed to turn on the continual lateral growth, leading to nanowires with tapered ends. The ligand generality in this growth mode also allows the creation of superhydrophobic surface, with the nanowire forest providing the nanoscale surface roughness and the hydrophobic ligand offering the surface property. These applications of the thiolated ligands in the nanosynthesis open a new approach for controlled synthesis of Au‐based nanostructures with various morphologies and properties.