Selective area growth of GaN nanowires using metalorganic chemical vapor deposition on nano-patterned Si(111) formed by the etching of nano-sized Au droplets

We report on the selective area growth of GaN nanowires (NWs) on nano-patterned Si(111) substrates by metalorganic chemical vapor deposition. The nano-patterns were fabricated by the oxidation of Si followed by the etching process of Au nano-droplets. The size of formed nano-pattern on Si(111) substrate was corresponding to the size of Au nano-droplet, and the diameter of GaN NWs grown was similar to the diameter of fabricated nano-pattern. The interesting phenomenon of using the nano-patterned Si(111) substrates is the formation of very clear substrate surface even after the growth of GaN NWs. However, in the case of GaN NWs grown using Au nano-droplets, there was several nanoparticles including GaN bulk grains on the Si(111) substrates. The smooth surface morphology of nano-patterned Si(111) substrates was attributed to the presence of SiO₂ layer which prevents the formation of unnecessary GaN particles during the GaN NW growth. Therefore, we believe that nano-patterning method of Si(111) which was obtained by the oxidation of Si(111) substrate and subsequent Au etching process can be utilized to grow high-quality GaN NWs and its related nano-device applications.     

Growth and characterization of GaAs nanowires on carbon nanotube composite films: toward flexible nanodevices

Poly(ethylene imine) functionalized carbon nanotube thin films, prepared using the vacuum filtration method, were decorated with Au nanoparticles by in situ reduction of HAuCl4 under mild conditions. These Au nanoparticles were subsequently employed for the growth of GaAs nanowires (NWs) by the vapor-liquid-solid process in a gas source molecular beam epitaxy system. The process resulted in the dense growth of GaAs NWs across the entire surface of the single-walled nanotube (SWNT) films. The NWs, which were orientated in a variety of angles with respect to the SWNT films, ranged in diameter between 20 to 200 nm, with heights up to 2.5 microm. Transmission electron microscopy analysis of the NW-SWNT interface indicated that NW growth was initiated upon the surface of the nanotube composite films. Photoluminescence characterization of a single NW specimen showed high optical quality. Rectifying asymmetric current-voltage behavior was observed from contacted NW ensembles and attributed to the core-shell pn-junction within the NWs. Potential applications of such novel hybrid architectures include flexible solar cells, displays, and sensors.     

Growth of SiO(x) nanowires by laser ablation

Amorphous SiO(x) nanowires (NWs) were synthesized using laser ablation of silicon-containing targets. The influence of various parameters such as target composition, substrate type, substrate temperature and carrier gas on the growth process was studied. The NWs were characterized using high resolution scanning and transmission electron microscopes (HRSEM and HRTEM) with their attachments: electron dispersive spectroscopy (EDS) and energy electron loss spectroscopy (EELS). A metal catalyst was found essential for the NW growth. A growth temperature higher than 1000?°C was necessary for the NW formation using an Ar-based carrier gas at 500?Torr. The use of Ar-5%H(2) instead of pure Ar resulted in a higher yield and longer NWs. Application of a diffusion barrier on top of the Si substrate guaranteed the availability of metal catalyst droplets on the surface, essential for the NW growth. Ni was found to be a better catalyst than Au in terms of the NW yield and length. Two alternative sequences for the evolution of the amorphous SiO(x) NWs were considered: (a)?the formation of Si NWs first and their complete oxidation afterwards, which seems to be doubtful, (b)?the direct formation of SiO(x) NWs, which is more likely to occur. The direct formation mechanism was proposed to advance in three stages: preferential adsorption of SiO(x) clusters on the catalyst surface first, a successive surface diffusion to the catalyst droplet lower hemisphere, and finally the formation and growth of the NW between the catalyst and the substrate.     

Gold‐Catalyzed Vapor–Liquid–Solid Germanium‐Nanowire Nucleation on Porous Silicon

Nanoporous Si(111) substrates are used to study the effects of Au catalyst coarsening on the nucleation of vapor–liquid–solid‐synthesized epitaxial Ge nanowires (NWs) at temperatures less than 400 °C. Porous Si substrates, with greater effective interparticle separations for Au surface diffusion than nonporous Si, inhibit catalyst coarsening and agglomeration prior to NW nucleation. This greatly reduces the variation in wire diameter and length and increases the yield compared to nucleation on identically prepared nonporous Si substrates.     

Synthesis of single-crystalline Zn metal nanowires utilizing cold-wall physical vapor deposition

Zinc metal nanowires (NWs) of two different morphologies have been synthesized in a cold-wall physical vapor deposition (CWPVD) chamber at high vacuum conditions and growth temperatures of 150 degrees C. Substrates initially seeded by gold or platinum crystals show NWs of wool-like and/or unidirectional morphologies. Transmission electron microscopy (TEM) studies revealed that the rodlike NWs consist of single-crystalline Zn covered with a thin native oxide. NWs of wool-like morphology are suppressed using platinum as the seed metal. NW growth proceeds via vapor-solid (VS) kinetics without any catalyst particles on the wire tips. The highest observed growth rates exceed the Zn deposition rate by factors up to 860, indicating the dominant role of surface diffusion of Zn adatoms, also along the NWs. The surface diffusion length of Zn adatoms on the NW side facet is determined to be 39 mum. Direct impingement of precursor atoms on the NW tip is not significant for the growth process.     

Lateral, Ge, nanowire growth on SiO2

Vapor-liquid-solid (VLS) nanowires (NWs) typically grow in [111] directions. Previously, the authors have demonstrated guided Si NW growth, engineering the VLS NWs to grow in a [110] direction against a SiO(2) surface. In this work, the authors demonstrate guided high-quality Ge nanowire growth against a SiO(2) surface in the substrate plane to bridge between two Si mesas. The authors explore the interfaces between a Ge NW and the two Si device-layer mesas and report high-quality, epitaxial interfaces between the Ge NW and both Si mesas.     

Shape-controlled Au particles for InAs nanowire growth

We present a study of InAs nanowire (NW) growth with shape-controlled Au seed particles. In comparison to more conventional spherical particles, the highly faceted, shaped Au particles are found to enhance the initial growth kinetics of InAs NWs at identical growth conditions. Analysis of the NWs after growth by transmission electron microscopy and energy-dispersive spectroscopy suggests that while In diffuses into the bulk of the shaped Au particles, in accordance with the vapor-liquid-solid (VLS) growth mechanism, the surface faceting is preserved. A key difference is that the shaped Au particles are characterized by a thicker In shell on their surfaces than the spherical Au particles, indicating that increased adsorption of In leads to the observed growth rate enhancement. On the basis of these results, we propose that our picture of VLS growth in regards to liquefaction and droplet formation is incomplete and that the initial particle morphology can be used to tailor NW growth.     

Growth of Silver Nanowires from Controlled Silver Chloride Seeds and Their Application for Fluorescence Enhancement Based on Localized Surface Plasmon Resonance

A “Polyol” method has granted low‐cost and facile process‐controllability for silver‐nanowire (Ag‐NW) synthesis. Although homogenous and heterogeneous nucleation and growth during Ag‐NW synthesis are possible using polyol methods, heterogeneous nucleation and growth of Ag NW guarantees highly selective growth of nanostructures using silver chloride (AgCl) seeds, which provides a stable source of chloride ions (Cl?) and thermodynamic reversibility. In this paper, a microdroplet has been adopted to synthesize uniform AgCl seeds with different diameter that are used for seed‐mediated Ag‐NW synthesis. The concentration of two precursors (AgNO₃ and NaCl) in the droplets is modulated to produce different sizes of AgCl seeds, which determines the diameter and length of Ag NWs. The process of the seed‐mediated growth of Ag NWs has been monitored by observing the peak shift in the time‐resolved UV–vis extinction spectrum. Furthermore, the distinct plasmonic property of Ag NWs for transverse and longitudinal localized‐surface‐plasmon‐resonance (LSPR)‐mediated fluorescence enhancement is utilized. The high aspect ratio and sharp tips work as simple antennas that induce the enhanced fluorescence emission intensity of a fluorophore, which can be applied in the fields of biological tissue imaging and therapy.     

Physical consequences of the equivalence of conditions for the steady-state growth of nanowires and the nucleation on triple phase line

A new theoretical model describing the steady-state growth and crystalline structure of semiconductor nanowires (NWs) is proposed and its physical consequences are considered. It is demonstrated that the Nebol’sin-Shchetinin condition (nonwetting of the NW side surface by the liquid drop) necessary for the steady-state growth of NWs according to the vapor-liquid-solid (VLS) mechanism is equivalent to the Glas condition of nucleation on the triple phase line for the monocentric NW growth. An energy criterion for the steady-state growth of NWs is formulated in the general case of faceted NW side surface. Effective surface energies are found that determine the activation barrier for nucleation at the NW top. Based on the proposed model, the issue of determining the III–V semiconductor NW crystal structure (cubic zinc blende type versus hexagonal wurtzite type) is considered. In particular, it is shown that a decrease in the surface energy of a catalyst must lead to the predominant formation of a cubic phase, which is confirmed by experimental data on the growth of GaAs nanowires according to the VLS mechanism with Au and Ga catalysts.     

Two‐Dimensional Self‐Organization of an Ordered Au Silicide Nanowire Network on a Si(110)‐16 × 2 Surface

A well‐ordered two‐dimensional (2D) network consisting of two crossed Au silicide nanowire (NW) arrays is self‐organized on a Si(110)‐16 × 2 surface by the direct‐current heating of ≈1.5 monolayers of Au on the surface at 1100 K. Such a highly regular crossbar nanomesh exhibits both a perfect long‐range spatial order and a high integration density over a mesoscopic area, and these two self‐ordering crossed arrays of parallel‐aligned NWs have distinctly different sizes and conductivities. NWs are fabricated with widths and pitches as small as ≈2 and ≈5 nm, respectively. The difference in the conductivities of two crossed‐NW arrays opens up the possibility for their utilization in nanodevices of crossbar architecture. Scanning tunneling microscopy/spectroscopy studies show that the 2D self‐organization of this perfect Au silicide nanomesh can be achieved through two different directional electromigrations of Au silicide NWs along different orientations of two nonorthogonal 16 × 2 domains, which are driven by the electrical field of direct‐current heating. Prospects for this Au silicide nanomesh are also discussed.     

Peculiarities of the MBE growth of nanowhiskers on GaAs(100) substrates

We have studied the formation of nanowhiskers (NWs) by molecular beam epitaxy (MBE) on GaAs(100) substrates. The MBE growth of NWs exhibits two stages (initial and developed) and leads to the formation of NWs with surface morphology of two types (nucleation and intergrowth). The stage of developed growth is characterized by the predominant formation of intergrown NWs oriented in the 〈111〉B direction, having (110) habit (including the NW tip surface) and hexagonal cross sections with a transverse size within 50–300 nm. It was found that the transverse size of a hexagonal NW may significantly differ from that of an Au-GaAs melt droplet. The ratio of longitudinal and transverse dimensions of intergrown NWs can be on the order of 150 and above. When the transverse size of NWs exceeds a certain value (about 200 nm), the crystal length exhibits a slight decrease. The existence of two types of morphology is indicative of inhomogeneous character of the NW growth on a GaAs(100) surface, which depends on the catalyst droplet size, effective thickness of the deposited GaAs layer, and the growth temperature.     

Mass Transport Determined Silica Nanowires Growth on Spherical Photonic Crystals with Nanostructure‐Enabled Functionalities

A robust and facile method has been developed to obtain directional growth of silica nanowires (SiO₂NWs) by regulating mass transport of silicon monoxide (SiO) vapor. SiO₂NWs are grown by vapor–liquid–solid (VLS) process on a surface of gold‐covered spherical photonic crystals (SPCs) annealed at high temperature in an inert gas atmosphere in the vicinity of a SiO source. The SPCs are prepared from droplet confined colloidal self‐assembly. SiO₂NW morphology is governed by diffusion‐reaction process of SiO vapor, whereby directional growth of SiO₂NWs toward the low SiO concentration is obtained at locations with a high SiO concentration gradient, while random growth is observed at locations with a low SiO concentration gradient. Growth of NWs parallel to the supporting substrate surface is of great importance for various applications, and this is the first demonstration of surface‐parallel growth by controlling mass transport. This controllable NW morphology enables production of SPCs covered with a large number of NWs, showing multilevel micro‐nano feature and high specific surface area for potential applications in superwetting surfaces, oil/water separation, microreactors, and scaffolds. In addition, the controllable photonic stop band properties of this hybrid structure of SPCs enable the potential applications in photocatalysis, sensing, and light harvesting.     

Heterojunction solar cells with integrated Si and ZnO nanowires and a chalcopyrite thin film

ZnO nanowires (NWs) have been successfully synthesized using a hydrothermal technique on both glass and silicon substrates initially coated with a sputtered ZnO thin film layer. Varying ZnO seed layer thicknesses were deposited to determine the effect of seed layer thickness on the quality of ZnO NW growth. The effect of growth time on the formation of ZnO NWs was also studied. Experimental results show that these two parameters have an important effect on formation, homogeneity and vertical orientation of ZnO NWs. Silicon nanowires were synthesized by a Ag-assisted electroless etching technique on an n-type Si (100) wafer. SEM observations have revealed the formation of vertically-aligned Si NWs with etching depth of ∼700 nm distributed over the surface of the Si. An electron-beam evaporated chalcopyrite thin film consisting of p-type AgGa_0.5In_0.5Se₂ with ∼800 nm thickness was deposited on the n-type ZnO and Si NWs for the construction of nanowire based heterojunction solar cells. For the Si NW based solar cell, from a partially illuminated area of the solar cell, the open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency were 0.34 V, 25.38 mA cm⁻², 63% and 5.50%, respectively. On the other hand, these respective parameters were 0.26 V, 3.18 mA cm⁻², 35% and 0.37% for the ZnO NW solar cell.     

Tunable catalytic alloying eliminates stacking faults in compound semiconductor nanowires

Planar defects in compound (III-V and II-VI) semiconductor nanowires (NWs), such as twin and stacking faults, are universally formed during the catalytic NW growth, and they detrimentally act as static disorders against coherent electron transport and light emissions. Here we report a simple synthetic route for planar-defect free II-VI NWs by tunable alloying, i.e. Cd(1-x)Zn(x)Te NWs (0 ≤ x ≤ 1). It is discovered that the eutectic alloying of Cd and Zn in Au catalysts immediately alleviates interfacial instability during the catalytic growth by the surface energy minimization and forms homogeneous zinc blende crystals as opposed to unwanted zinc blende/wurtzite mixtures. As a direct consequence of the tunable alloying, we demonstrated that intrinsic energy band gap modulation in Cd(1-x)Zn(x)Te NWs can exploit the tunable spectral and temporal responses in light detection and emission in the full visible range.     

The growth of porous ZnO nanowires by thermal oxidation of ZnS nanowires

The growth of porous ZnO nanowires (NWs) via phase transformation of ZnS NWs at 500-850 degrees C in air was studied. The ZnS NWs were first synthesized by thermal evaporation of ZnS powder at 1100 degrees C in Ar. On subsequent annealing at 500 degrees C in air, discrete ZnO epilayers formed on the surface of ZnS NWs. At 600 degrees C, polycrystalline ZnO and the crack along the (0001) interface between the ZnO epilayer and ZnS NW were observed. At 700-750 degrees C ZnS NWs transformed to ZnO NWs, meanwhile nanopores and interfacial cracks were observed in the ZnO NWs. Two factors, the evaporation of SO2 and SO3 and the stress induced by the incompatible structure at the interface of ZnO epilayer and ZnS NW, can be responsible for the formation of porous ZnO NWs from ZnS NW templates on annealing at 700-750 degrees C in air. Rapid growth of ZnO at 850 degrees C could heal the pores and cracks and thus resulted in the well-crystallized ZnO NWs.     

Plasma-plasmonics synergy in the Ga-catalyzed growth of Si-nanowires

This paper reports on the growth of Si nanowires (NWs) by SiH₄/H₂ plasmas using the non-noble Ga-nanoparticles (NPs) catalysts. A comparative investigation of conventional Si-NWs vapour–liquid–solid (VLS) growth catalyzed by Au NPs is also reported. We investigate the use of a hydrogen plasma and of a SiH₄/H₂ plasma for removing Ga oxide shell and for enhancing the Si dissolution into the catalyst, respectively. By exploiting the Ga NPs surface plasmon resonance (SPR) sensitivity to their surface chemistry, the SPR characteristic of Ga NPs has been monitored by real time spectroscopic ellipsometry in order to control the hydrogen plasma/Ga NPs interaction and the involved processes (oxide removal and NPs dissolution by volatile gallium hydride). Using in situ laser reflectance interferometry the metal catalyzed Si NWs growth process has been investigated to find the effect of the plasma activation on the growth kinetics. The role of atomic hydrogen in the NWs growth mechanism and, in particular, in the SiH₄ dissolution into the catalysts, is discussed. We show that while Au catalysts because of the re-aggregation of NPs yields NWs that do not correspond to the original size of the Au NPs catalyst, the NWs grown by the Ga catalyst retains the diameter dictated by the size of the Ga NPs. Therefore, the advantage of Ga NPs as catalysts for controlling NWs diameter is demonstrated.     

Self-assembled GaN nanowires on diamond

We demonstrate the nucleation of self-assembled, epitaxial GaN nanowires (NWs) on (111) single-crystalline diamond without using a catalyst or buffer layer. The NWs show an excellent crystalline quality of the wurtzite crystal structure with m-plane faceting, a low defect density, and axial growth along the c-axis with N-face polarity, as shown by aberration corrected annular bright-field scanning transmission electron microscopy. X-ray diffraction confirms single domain growth with an in-plane epitaxial relationship of (10 ?10)(GaN) [parallel] (01 ?1)(Diamond) as well as some biaxial tensile strain induced by thermal expansion mismatch. In photoluminescence, a strong and sharp excitonic emission reveals excellent optical properties superior to state-of-the-art GaN NWs on silicon substrates. In combination with the high-quality diamond/NW interface, confirmed by high-resolution transmission electron microscopy measurements, these results underline the potential of p-type diamond/n-type nitride heterojunctions for efficient UV optoelectronic devices.     

Unravelling a solution-based formation of single-crystalline kinked wurtzite nanowires: The case of MnSe

The search for a novel strategy to sculpt semiconductor nanowires (NWs) at the atomistic scale is crucial for the development of new paradigms in optics,electronics,and spintronics.Thus far,the fabrication of single-crystalline kinked semiconductor NWs has been achieved mainly through the vapor-liquid-solid growth technique.In this study,we developed a new strategy for sculpting single-crystalline kinked wurtzite (WZ) MnSe NWs by triggering the nonpolar axial-oriented growth,thereby switching—at the atomistic scale—the NW growth orientation along the nonpolar axes in a facile solution-based procedure.This presents substantial challenges owing to the dominant polar c axis growth in the solution-based synthesis of one-dimensional WZ nanocrystals.More significantly,the ability to continuously switch the nonpolar axial-growth orientation allowed us to craft the kinking landscape of types 150°,120°,90°,and 60°.A probabilistic analysis of kinked MnSe NWs reveals the correlations of the synergy and interplay between these two sets of nonpolar axial growth-orientation switching,which determine the actual kinked motifs.Furthermore,discriminating the side-facet structures of the kinked NWs significantly strengthened the spatially selected interaction of Au nanoparticles.We envisage that such a facile solution-based strategy can be useful for synthesizing other single-crystalline kinked WZ-type transition-metal dichalcogenide NWs to develop novel functional materials with finely tuned properties.     

Germanium nanowires with 3-nm-diameter prepared by low temperature vapour-liquid-solid chemical vapour deposition

We report the growth of germanium nanowires (Ge NWs) with single-step temperature method via vapour-liquid-solid (VLS) mechanism in the low pressure chemical vapour deposition (CVD) reactor at 300 degrees C, 280 degrees C, and 260 degrees C. The catalyst used in our experiment was Au nanoparticles with equivalent thicknesses of 0.1 nm (average diameter approximately 3 nm), 0.3 nm (average diameter approximately 4 nm), 1 nm (average diameter approximately 6 nm), and 3 nm (average diameter approximately 14 nm). The Gibbs-Thomson effect was used to explain our experimental results. The Ge NWs grown at 300 degrees C tend to have tapered structure while the Ge NWs grown at 280 degrees C and 260 degrees C tend to have straight structure. Tapering was caused by the uncatalysed deposition of Ge atoms via CVD mechanism on the sidewalls of nanowire and significantly minimised at lower temperature. We observed that the growth at lower temperature yielded Ge NWs with smaller diameter and also observed that the diameter and length of Ge NWs increases with the size of Au nanoparticles for all growth temperatures. For the same size of Au nanoparticles, Ge NWs tend to be longer with a decrease in temperature. The Ge NWs grown at 260 degrees C from 0.1-nm-thick Au had diameter as small as approximately 3 nm, offering an opportunity to fabricate high-performance p-type ballistic Ge NW transistor, to realise nanowire solar cell with higher efficiency, and also to observe the quantum confinement effect.     

Kinetics of nanowhisker growth via the vapor-liquid-solid mechanism

Kinetics of the main physical processes controlling the growth of nanowhiskers (NWs) via a sequence of vapor-liquid-solid phase transitions is considered. The roles of thermodynamics and kinetics of cluster nucleation in the initial stage of NW formation are studied. Approximate expressions for the NW length are obtained in the one-dimensional approximation. The influence of transfer processes in the gas phase on the growth of NW is evaluated.