Vapor Liquid Solid Model Research Articles

On a new method of heterojunction formation in III–V nanowires

The process of the formation of axial heterostructures in nanowires is considered on the basis of the vapor–liquid–solid model of growth. A new method of heterostructure formation in (Al, Ga)As nanowires by varying the arsenic flux is proposed.

Site-controlled VLS growth of planar nanowires: yield and mechanism.

The recently emerged selective lateral epitaxy of semiconductor planar nanowires (NWs) via the vapor-liquid-solid (VLS) mechanism has redefined the long-standing symbolic image of VLS NW growth. The in-plane geometry and self-aligned nature make these planar NWs completely compatible with large scale manufacturing of NW-based integrated nanoelectronics. Here, we report on the realization of perfectly site-controlled growth of GaAs planar NW arrays with unity yield using lithographically defined gold (Au) seed dots. The growth rate of the planar NWs is found to decrease with the NW width at fixed spacing, which is consistent with the conventional VLS model based on the Gibbs-Thomson effect. It is found that in general, the planar and out-of-plane NW growth modes are both present. The yield of planar NWs decreases as their lateral dimension shrinks, and 100% yield of planar NWs can be achieved at moderate V/III ratios. Based on a study of the shape of seed particles, it is proposed that the adhesion between the liquid-phase seed particle and the substrate surface is important in determining the choice of growth mode. These studies represent advances in the fundamental understanding of the VLS planar NW growth mechanism and in the precise control of the planar NW site, density, width, and length for practical applications. In addition, high quality planar InAs NWs on GaAs (100) substrates is realized, verifying that the planar VLS growth mode can be extended to heteroepitaxy.

Rapid sintering of silicon nitride foams decorated with one-dimensional nanostructures by intense thermal radiation

Silicon nitride foams were prepared by direct foaming and subsequent rapid sintering at 1600 °C. The intense thermal radiation generated under the pressureless spark plasma sintering condition facilitated necking of Si3N4 grains. The prepared foams possessed a porosity of ∼80 vol% and a compressive strength of ∼10 MPa, which required only ∼30 min for the entire sintering processes. Rapid growth of one-dimensional SiC nanowires from the cell walls was also observed. Thermodynamic calculations indicated that the vapor–liquid–solid model is applicable to the formation of SiC nanowires under vacuum.

Nucleation and growth of boron nanowires on diamond particles

Abstract Crystalline boron nanowires (BNWs) are grown on the entire surface of diamond powders by a solid–solid reaction approach. The as-grown BNWs have diameters in the range of 20–200 nm, lengths of several micrometers. High-resolution transmission electron microscopy (HRTEM) observation indicated the boron nanowires are composed of two structures, the interior of the nanowire is well crystalline in hexagonal structure and the surface of the nanowire is covered by a 5–10 nm thick tetragonal structure layer. Systematic analysis reveals that the conventional VLS (vapor–liquid–solid) growth model is not applicable to explain this kind of BNWs’ growth mechanism. Hereby, an extended VLS model is proposed which can perfectly interpret the BNW's growth process on diamond surface and match experiment results very well.

Crystalline boron nanowires grown on the diamond surface

Crystalline boron nanowires (BNWs) have been grown on the entire surface of diamond powders by a solid–solid reaction approach. A series of analysis technique were employed to investigate the as-grown BNWs. The as-grown BNWs are in length of several micron-meters with the diameter ranging from 20–200nm. And the BNWs crystalline structure is identified to be tetragonal structure. Systematic analyses reveal that the growth models such as classic VLS (vapor–liquid–solid) model are not applicable to explain this kind of BNW's growth mechanism. Hereby, an extended VLS model is proposed which can perfectly interpret the BNW′s growth process on diamond surface and match experiment results very well.

Synthesis of multiwalled carbon nanotubes from bamboo charcoal and the roles of minerals on their growth

Abstract Multiwalled carbon nanotubes (MWCNTs) were synthesized from bamboo charcoals by chemical vapor deposition in the presence of ethanol vapor. Fresh bamboo culms were first heat treated at 1000–1500 °C to form charcoals. The elemental composition and structure of mineral phases in the bamboo charcoal treated at different temperatures were analyzed. The results showed that Mg 2 SiO 4 and particularly calcium silicate were responsible for the nucleation and growth of MWCNTs at 1200–1400 °C. Transmission electron microscope and energy dispersive X-ray spectrometer observations indicated that the tips of nanotubes synthesized at 1200–1400 °C consist mainly of calcium silicate. Such silicate tips acted as effective catalysts for nanotubes. The growth of MWCNTs followed the vapor–liquid–solid model including an initial decomposition of ethanol vapor into carbon, dissolution of carbon inside molten silicate and final nucleation of nanotubes.

Synthesis and Photoluminescence of Silica Nanowires Grown on Si Substrate

A new simple method was developed to synthesize silica nanowires by physical thermal evaporation of a mixture of silica particles and iron trichloride. The silica nanowires have diameters from 30 to 50 nm, and lengths of ~10 μm, with iron particles at their tips. The growth mechanism of the amorphous silica nanowires follows vapor–liquid–solid model. A broad blue–green photoluminescence band with defined maximum peak energy of ~2.45 eV is observed at room temperature revealing that the silica nanowires could have potential applications in optoelectronic devices.

Unconventional vapor–liquid–solid growth of SiO2 nanooctopuses

In this study, we demonstrate the unconventional vapor–liquid–solid (VLS) growth of amorphous SiO2 nanooctopuses. The SiO2 nanooctopuses were successfully synthesized by a simple water-assisted reaction on Si wafers. The obtained aggregates have been systematically characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy. A novel VLS model has been proposed for the growth of SiO2 nanooctopuses based on the experimental observations and characterizations. The amorphous SiO2 nanooctopuses emit stable violet-blue light, implying their potential applications in photoelectronic devices.

The mechanism of ZnO nanorod growth by vapor phase transportation

We studied a kinetic model of the ZnO nanorod (NR) growth that occurs via vapor phase transportation (VPT). The mechanism of ZnO NR growth was discussed in terms of the length-to-diameter ( L– D) relationship of NRs, and it was confirmed by varying both the type of catalyst and source material used. Both theoretical calculations and experimental data indicated that elongation of NRs is primarily determined by diffusion of the absorbed atoms (adatoms) within a nominal migration length on the surface, which is not considered in the simple vapor–liquid–solid model.

Molecular dynamics study of SWNT growth on catalyst particles without temperature gradients

A temperature gradient (TG) is often included in the Vapor–Liquid–Solid model to describe catalyzed nucleation and growth of carbon nanotubes (CNTs). Although the TG may be important to describe the growth of large CNTs, it has been suggested that a carbon concentration gradient (CG) is more important for the growth of single-walled carbon nanotubes (SWNTs) on small catalyst particles. The molecular dynamics study performed here shows that the CG is more important than the TG for the growth of SWNTs on small metal particles and that SWNTs growth can, in fact, even occur in the presence of an opposing TG (i.e., the SWNTs grow from the hot region of the catalyst particle). This shows that a TG is not required for SWNT growth on small catalyst particles.

Structure- and size-controlled ultrafine ZnS nanowires

We report the first synthesis of ultrafine zinc sulfide (ZnS) nanowires with a sphalerite structure. Large-area, high- and uniform-density ZnS nanowires were grown on Au-coated silicon substrates by hydrogen-assisted thermal evaporation. The product was analyzed with scanning electron microscopy, X-ray diffraction, transmission electron microscopy and photoluminescence (PL). The ZnS nanowires had a sphalerite structure, a growth direction along [1 1 1], diameters of 10–20 nm, and lengths of several micrometers. The PL spectrum of the ZnS nanowires was measured. The growth of the ZnS nanowires was explained with the vapor–liquid–solid model, and their structure could be controlled by the deposition temperature.

Gallium nitride nanowires doped with silicon

High-quality GaN nanowires doped with silicon have been synthesized by hot-filament chemical vapor deposition on Au-coated Si (100) wafers. The GaN was systematically characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, Raman spectroscopy, and photoluminescence (PL). The GaN nanowires had a uniform concentration of 3% Si, a uniform diameter around 10 nm, and a hexagonal wurtzite structure grown along the [001] direction. The intense PL peak of GaN nanowires at 344 nm showed a distinct blueshift from the bulk bandgap emission, revealing a clear quantum confinement effect. The growth of GaN nanowires is discussed in terms of the oxide-assisted metal-catalyst vapor–liquid–solid model.

Microscopic mechanisms for the catalyst assisted growth of single-wall carbon nanotubes

Whatever the synthesis technique used, the growth of ropes of single-wall carbon nanotubes requires the assistance of a metallic catalyst. In this paper, the role played by the catalyst is studied both experimentally and theoretically. Experimentally, the similarities between the samples synthesized from different techniques suggest a common growth mechanism proceeding via the precipitation of excess carbon on metallic nanoparticles. In this paper, the correlation between ropes and catalytic particles is investigated in detail in the case of the Ni–Y catalyst used in the arc discharge technique by combining high resolution transmission electron microscopy, X-ray and electron energy loss spectroscopy. It is shown that the ropes are always found attached to metallic particles about ten times larger than the tube diameter. A further remarkable proof of this relationship is provided by the chemical analyses of the metallic particles. These are found to be free of carbon and to always display the same Ni:Y composition range, whatever the initial Ni:Y composition of the catalyst mixture used in the synthesis, whereas the composition of other particles is highly dispersed. These experimental results support a mechanism of formation based on a vapor–liquid–solid model, in which the tubes of a given bundle nucleate in a cooperative manner and grow at the surface of a same metallic particle. This phenomenological scheme is supported by quantum molecular dynamics simulations which show that carbon atoms are incorporated at the root of a growing tube by a diffusion-segregation process occurring at the surface of the catalytic particle.

Nucleation and growth of silicon nitride nanoneedles using microwave plasma heating

We report on needlelike silicon nitride nanowires grown on silicon using ammonia microwave plasma heating. Scanning electron microscope reveals that the nanoneedles are either straight, slightly bent, or kinked with sharp tips. Transmission electron microscope shows that the sizes of the nanowire tips are less than 5 nm and the structures are well crystallized. X-ray diffraction on the surface of the as-deposited structures indicates that both α-silicon nitride and β-silicon nitride exist. Catalyst particle splitting and merging explain observed structure derivation and bending. A vapor–liquid–solid model is employed to explain nucleation and growth mechanism.

Growth of amorphous silicon nanowires via a solid–liquid–solid mechanism

Amorphous silicon nanowires (a-SiNW) with an average diameter of ca. 20 nm were synthesized at about 950°C under an Ar/H 2 atmosphere on a large area of a (111) Si substrate without supplying any gaseous or liquid Si sources. The Si substrate, deposited with a layer of Ni (ca. 40 nm thick), served itself as a silicon source for the growth of the a-SiNWs. In contrast to the well-known vapor–liquid–solid (VLS) for conventional whisker growth, it was found that growth of the a-SiNWs was controlled by a solid–liquid–solid (SLS) mechanism, which is analogous to the VLS model.

Controlled Growth of Amorphous Silicon Nanowires Via a Solid-Liquid-Solid (SLS) Mechanism

ABSTRACTAmorphous silicon nanowires (a-SiNW's) with average diameter around 20 nm were synthesized at about 950° C under an Ar/H2 atmosphere on large area of a (11) Si substrate without supplying any gaseous or liquid Si sources. The Si substrate, deposited with a layer of Ni of about 40 nm thick, served itself as a silicon source for the growth of the a-SiNWs. Different from the well-known vapor-liquid-solid (VLS) for conventional whisker growth, it was found that growth of the a-SiNWs was controlled by a solid-liquid-solid mechanism, which is analogous to the VLS model.