K w. Kolasinski/ Current Opinion in Solid State and Materials Science 10(2006)182-191 185 Ga2O3 nanowires appear to have a nearly spherical particle In, no detectable Se and is slightly larger than the nano- at their wire. When a 30 nm In film is used for catalysis, no In par Mohammad [60] has grown a variety of Ga and In con- ticle is found at the end of grown nanowires. At the le growth taining nanowires including GaN, InAs, InN, In GaAs, temperature either Au or In would be liquids In GaN, In gaasn using self-catalyzed growth involving Chen et al. [36]have grown CsN nanotubes from pyri either liquid Ga or liquid In droplets. Depending on the dine over a Fe-Co catalyst deposited on y-Al2O3. A mix- conditions either multipronged root growth or single- ture of N2 and pyridine passes over the catalyst while pronged float growth (defined in the next section) are heated to 550-950oC. In this case, the catalyst particles observed. A combination of carrier gas(N2 or H2) and have an unusual conical shape. Not only do the appear NH3, if required, flows over the metallic reagents placed to poke into the growing nanotube, they also are attached in Bn boats. One of the reactant metals may also be placed to the substrate rather than floating to the top of the nano- on the substrate. The distance between boats and the sub- tube even though only one tube grows from one particle. A strate, the flow rates and the temperatures of the boats and tapered particle that pokes into the core of a Cnt has also the substrate are all important variables that affect the been noted by Hofmann et al. [22] during plasma enhanced omposition and characteristics of the nanowires. Autocat- growth from C2H, or CH alytic growth shares much in common with and may actu ally be the mechanism behind what is called vapor-solid (VS)growth, in which no catalyst is intentionally added 3. Mechanisms to the system. Nonetheless, if one of the components is a low melting point metal such as Ga, In or Zn, a liquid cat Let us first start out with some generalities. The singular alyst particle may result during growth even if now none is attribute that usually leads to the conclusion that VLS intentionally added growth has occurred is that a metal particle of roughly Yun et al. [35] have heated a mixture of B+40 wt% the same diameter as the nanowire is found at the end of 9203 (which is a liquid at the growth temperature)in vac- the wire. This, of course, does not determine the phase uum over a 5-20 nm film of Au on Si to 600-950C for of the particle during growth and this controversial aspect 30 min B nanowires coated with an oxide layer are formed. will be dealt with further below Interestingly, a root growth mechanism, in which several Nanowires produced by catalytic growth are often nanowires emanate from a single catalytic particle, occurs found to have a uniform diameter. The wires are not at low temperature and float growth, in which one catalyst always round but might also exhibit other crystallograph particle sits atop each nanowire, occurs at high tempera- cally defined shapes, such as hexagonal ZnO or rectangular ture. The catalyst particle is a Au-B eutectic and dissolu- In,O3. Under some conditions, particularly for long tion of not only B into the Au particles but also the growth times, tapering of diameter to smaller (or less com- interaction of the eutectic with liquid B2O3 may be impor- monly larger) values is found. Often growth requires a bit tant in describing the growth dynamics of bundles of nano- of an induction period before uniform nanowires begin to bes and the switching between root growth and float grow. These considerations are represented schematically growth Laser ablation has been used by Jia et al. [55]to produce The initial period before uniform growth commences is ZnSe nanowires. Contrary to previous experiments involv- associated with any of a number of processes. In some g nanowire growth during laser ablation [61-63], the cases, the catalytic particles must be formed by vapor phase wafer target was not placed in a furnace. The evidence and/or surface diffusion transport or else their surfaces for self-catalytic VLS growth, as suggested by the authors, have to be cleansed of impurities(oxides or terminating thi- is not conclusive ols). The particles may be deposited directly, for instance, ZnTe nanowires with an average diameter of 30 nm from the evaporation of a colloidal solution with a well- and lengths >l um can be grown under the influence of a defined size. Alternatively, a thin film of metal can be evap- Au catalyst. Janik et al. [57] used a MBE source and a 3- orated directly onto a substrate and if the metal does not 20 A film of Au coated onto a GaAs substrate. The nano- wet the substrate, it will ball up into islands either immedi- wires, which are inclined about 55 to the(100)substrate ately as the result of Volmer-Weber growth [64] or else normal, have a zincblende crystal structure and their subsequently when the system is annealed, the onset of ost- growth axis is ( 111. The growth temperature is at slightly wald ripening [65] will lead to a distribution of island sizes above350°C The particles might also result from an evaporation and In2Se] nanowires have been grown either with an Au growth process that occurs during the initial stage, such talyst or with In acting as a self-catalyst by Sun et al. as when carbothermal reduction is used to generate a vol [49]. In2Se3 powder was held at 900-950C and placed atile metal that proceeds to condense elsewhere in the reac- upstream in flowing Ar from a Au or In coated silicon tor. Indeed, catalytic particles form readily under a variety wafer held at 650-700C. The nanowires are 40-80 nm in of conditions from any number of high vapor pressure, low diameter and up to 100 um in length. The spherical Au par- melting point metals. Instead of being the exception, ticle found at the tip of the nanowire contains less than 5% rather seems to be the case that the onus is on an investigaGa2O3 nanowires appear to have a nearly spherical particle at their tip. Mohammad [60] has grown a variety of Ga and In con￾taining nanowires including GaN, InAs, InN, InGaAs, InGaN, InGaAsN using self-catalyzed growth involving either liquid Ga or liquid In droplets. Depending on the conditions either multipronged root growth or single￾pronged float growth (defined in the next section) are observed. A combination of carrier gas (N2 or H2) and NH3, if required, flows over the metallic reagents placed in BN boats. One of the reactant metals may also be placed on the substrate. The distance between boats and the sub￾strate, the flow rates and the temperatures of the boats and the substrate are all important variables that affect the composition and characteristics of the nanowires. Autocat￾alytic growth shares much in common with and may actu￾ally be the mechanism behind what is called vapor–solid (VS) growth, in which no catalyst is intentionally added to the system. Nonetheless, if one of the components is a low melting point metal such as Ga, In or Zn, a liquid cat￾alyst particle may result during growth even if now none is intentionally added. Yun et al. [*35] have heated a mixture of B + 40 wt% B2O3 (which is a liquid at the growth temperature) in vac￾uum over a 5–20 nm film of Au on Si to 600–950 C for 30 min. B nanowires coated with an oxide layer are formed. Interestingly, a root growth mechanism, in which several nanowires emanate from a single catalytic particle, occurs at low temperature and float growth, in which one catalyst particle sits atop each nanowire, occurs at high tempera￾ture. The catalyst particle is a Au–B eutectic and dissolu￾tion of not only B into the Au particles but also the interaction of the eutectic with liquid B2O3 may be impor￾tant in describing the growth dynamics of bundles of nano￾tubes and the switching between root growth and float growth. Laser ablation has been used by Jia et al. [55] to produce ZnSe nanowires. Contrary to previous experiments involv￾ing nanowire growth during laser ablation [61–63], the wafer target was not placed in a furnace. The evidence for self-catalytic VLS growth, as suggested by the authors, is not conclusive. ZnTe nanowires with an average diameter of 30 nm and lengths >1 lm can be grown under the influence of a Au catalyst. Janik et al. [57] used a MBE source and a 3– 20 A˚ film of Au coated onto a GaAs substrate. The nano￾wires, which are inclined about 55 to the (1 0 0) substrate normal, have a zincblende crystal structure and their growth axis is h111i. The growth temperature is at slightly above 350 C. In2Se3 nanowires have been grown either with an Au catalyst or with In acting as a self-catalyst by Sun et al. [49]. In2Se3 powder was held at 900–950 C and placed upstream in flowing Ar from a Au or In coated silicon wafer held at 650–700 C. The nanowires are 40–80 nm in diameter and up to 100 lm in length. The spherical Au par￾ticle found at the tip of the nanowire contains less than 5% In, no detectable Se and is slightly larger than the nano￾wire. When a 30 nm In film is used for catalysis, no In par￾ticle is found at the end of grown nanowires. At the growth temperature either Au or In would be liquids. Chen et al. [36] have grown C5N nanotubes from pyri￾dine over a Fe–Co catalyst deposited on c-Al2O3. A mix￾ture of N2 and pyridine passes over the catalyst while heated to 550–950C. In this case, the catalyst particles have an unusual conical shape. Not only do the appear to poke into the growing nanotube, they also are attached to the substrate rather than floating to the top of the nano￾tube even though only one tube grows from one particle. A tapered particle that pokes into the core of a CNT has also been noted by Hofmann et al. [22] during plasma enhanced growth from C2H2 or CH4. 3. Mechanisms Let us first start out with some generalities. The singular attribute that usually leads to the conclusion that VLS growth has occurred is that a metal particle of roughly the same diameter as the nanowire is found at the end of the wire. This, of course, does not determine the phase of the particle during growth and this controversial aspect will be dealt with further below. Nanowires produced by catalytic growth are often found to have a uniform diameter. The wires are not always round but might also exhibit other crystallographi￾cally defined shapes, such as hexagonal ZnO or rectangular In2O3. Under some conditions, particularly for long growth times, tapering of diameter to smaller (or less com￾monly larger) values is found. Often growth requires a bit of an induction period before uniform nanowires begin to grow. These considerations are represented schematically in Fig. 1. The initial period before uniform growth commences is associated with any of a number of processes. In some cases, the catalytic particles must be formed by vapor phase and/or surface diffusion transport or else their surfaces have to be cleansed of impurities (oxides or terminating thi￾ols). The particles may be deposited directly, for instance, from the evaporation of a colloidal solution with a well￾defined size. Alternatively, a thin film of metal can be evap￾orated directly onto a substrate and if the metal does not wet the substrate, it will ball up into islands either immedi￾ately as the result of Volmer–Weber growth [64] or else subsequently when the system is annealed, the onset of Ost￾wald ripening [65] will lead to a distribution of island sizes. The particles might also result from an evaporation and growth process that occurs during the initial stage, such as when carbothermal reduction is used to generate a vol￾atile metal that proceeds to condense elsewhere in the reac￾tor. Indeed, catalytic particles form readily under a variety of conditions from any number of high vapor pressure, low melting point metals. Instead of being the exception, it rather seems to be the case that the onus is on an investiga￾K.W. Kolasinski / Current Opinion in Solid State and Materials Science 10 (2006) 182–191 185