K.w. Kolasinski/ Current Opinion in Solid State and Materials Science 10(2006)182-191 Carbothermal reduction, vapor phase transport of in contrast to, for instance, Au catalyzed growth of Si and growth material and reaction with trace amounts of oxygen SiGe nanowires. Also in this system, both the growth and have been utilized by Kuo and Huang in the growth of catalyst phases are transported via the vapor phase Taper Cdo nanowires that are 40-80 nm in diameter and 30- ing to larger diameters is observed in this system, most 50 um in length. Cdo and graphite powders are mixed in likely because the Sn nanoparticles are growing during a 4: 1 ratio. The silicon substrate is coated with a I nm growth. Au film and then placed 25 cm downstream from the reac Xu et al. [53] formed Zno nanowires with a hexagonal tant mixture. The reactants are held at 500C and the cross section. Interestingly no catalytic particle is found substrate at 400C. The Au particle at the end of the at the tip. Rather the catalytic action is provided by a nanowire has a diameter slightly smaller than the nano- ZnBil thin film(a combination of tetragonal ZnI, and wire. Most of the nanowires have a smooth surface but hexagonal Bil3) Mixtures of either Bil3 and Zn powder I an area of the reactor where there was probably a greater or Bi and Zn powders were heated in flowing Ar to 250- amount of oxygen available a jagged necklace structure 300C. The presence of I changes the growth direction of formed by a lateral growth of the rhombohedral Cdo the nanowires. The O can be supplied by an impurity in nanocrystals over the smooth nanowires. the Ar but no growth is observed in the absence of Bi when Pulsed laser deposition has been used by Morber et al. only Zn or Znl2 powders are used [39]to synthesize FeOy and Mg doped a-Fe2O3 nanorods, Carbothermal reduction can be used to produce ZnO nanowires and nanobelts. The ablation target was a pressed nanowires on a silicon substrate. Moreover, what Yang powder of magnetite(Fe3 O4), which was placed next to a et al. have shown is that growth can be switch away from quartz boat containing polycrystalline alumina wafer sub- Zno nanowires to Mn doped ZnSiO4 if MnCl2 4H2O is strates coated with a 2 nm Au film. Au particles are found added to the reaction mixture. The furnace system was at the ends of the nanowires, therefore it appears that the flushed with high-purity Ar gas to eliminate O2 and heated u film spontaneously breaks up to form the catalytic to 1 100C under a constant flow of Then. th nanoparticles quartz boat was placed in the centre of the furnace and An arc discharge has been used by Li et al. [34] to form held at 1100C under the same Ar flow. After reaction ubic 7-Al2O3 nanorods. The source material is a pressed for 50-60 min at 800-900C, the Si wafers, which are situ- powder mixture of Fe and Al, surprisingly in a 60: 40 ratio. ated downstream of the reaction mixture, were coated with Fe catalyst particles are found at the ends of the nanorods. a layer of nanowires. The majority of this material is com- The discharge is run in a mixture of 0.018 MPa Ar (99.9% posed of a willemite phase(a-Zn2SiO4 with rhombohedral purity) and 0.008 MPa H2(99.99% purity ). The growth structure) forming well-aligned nanorods with lengths of 2- conditions are highly non-equilibrium with the o being 4 um and diameters of 70-150 nm. The Zn catalyst is con- pplied at the trace level as an impurity in the process sumed so there is a severe taper at the end of the wires and gases. The diameters of the 7-Al2O3 nanorods are relatively no metal particle is found uniform, ranging from 20 to 30 nm Simultaneous growth of ZnO and LiF nanowires has Carbothermal reduction of a mixture of an Al com- been reported by Jiang et al. [50]. Zn acts as the cata olex with Fe powder has been used by Jung and Joo [33] ly hen LiF t Zno powders are heated in Ar to to create AIn whiskers. The mixture of the Al complex, 750-850C. The downstream deposition region has a Fe and graphite was calcined at 1200-1500C for 5 h in temperature in the range of 400-500C. Cubic-structured flowing N2. The whiskers often show modulations in their single-crystalline LiF nanowires grew along the (001) diameters along the lengths of the whisker and the shapes and(110) crystallographic directions with diameters of obtained depend on the growth temperature. This system 100-500 nm and lengths of tens of microns. The authors exhibits rather complex chemistry on account of the carbo- propose that there must be a barrier to the incorporation thermal reduction to produce Al, dissociative adsorption of of Lif into the lattice and the Zn particle acts to lower this N2, presumably on the Fe particles, and the formation of barrier and cause growth to be preferential at its ba Fe/Al/N alloy particles of the appropriate size. Very similar conditions can be used to grow Dendritic ZnO nanowires can be grown from Sn catalyst Ga2O3 and SnO, nanowires. Indeed, In,O3 and particles as shown by Gao et al. [59]. They heated ZnO and nanowires can be grown simultaneously without cross con- SnO, powders(1: I ratio) for I h under a pressure of 300- tamination or doping as demonstrated by Johnson et al 400 Torr of Ar carrier gas. The nanostructures grew on [45]. Fifty nanometer Au particles were deposited from the top of the inner alumina wall of the tube furnace in a solution onto the si substrate placed downstream from region located downstream M15 cm away from the source high-purity(6 N)metal reactant species (In, Ga, or Sn) material, which was located in the middle of the furnace placed separately in a quartz boat. The furnace was heated at 1300C, and the local growth temperature was in the to 800-1000C under flowing N2. No oxygen was supplied range of 700-800C. The Sn particle is significantly larger tly to the system other than as an impurity in the n than the Nw diameter. Note that in this system, as in sev- or that which desorbs from the surfaces inside the furnace. eral others, there is a high proportion of the catalyst start- Rectangular In]O3 rods are capped with a rectangular Au ing material compared to the growth material. This stands particle that is slightly smaller than the rod, whereasCarbothermal reduction, vapor phase transport of growth material and reaction with trace amounts of oxygen have been utilized by Kuo and Huang in the growth of CdO nanowires that are 40–80 nm in diameter and 30– 50 lm in length. CdO and graphite powders are mixed in a 4:1 ratio. The silicon substrate is coated with a 1 nm Au film and then placed 25 cm downstream from the reac￾tant mixture. The reactants are held at 500 C and the substrate at 400 C. The Au particle at the end of the nanowire has a diameter slightly smaller than the nano￾wire. Most of the nanowires have a smooth surface but in an area of the reactor where there was probably a greater amount of oxygen available, a jagged necklace structure formed by a lateral growth of the rhombohedral CdO nanocrystals over the smooth nanowires. Pulsed laser deposition has been used by Morber et al. [39] to synthesize FexOy and Mg doped e-Fe2O3 nanorods, nanowires and nanobelts. The ablation target was a pressed powder of magnetite (Fe3O4), which was placed next to a quartz boat containing polycrystalline alumina wafer sub￾strates coated with a 2 nm Au film. Au particles are found at the ends of the nanowires, therefore it appears that the Au film spontaneously breaks up to form the catalytic nanoparticles. An arc discharge has been used by Li et al. [34] to form cubic c-Al2O3 nanorods. The source material is a pressed powder mixture of Fe and Al, surprisingly in a 60:40 ratio. Fe catalyst particles are found at the ends of the nanorods. The discharge is run in a mixture of 0.018 MPa Ar (99.9% purity) and 0.008 MPa H2 (99.99% purity). The growth conditions are highly non-equilibrium with the O being supplied at the trace level as an impurity in the process gases. The diameters of the c-Al2O3 nanorods are relatively uniform, ranging from 20 to 30 nm. Carbothermal reduction of a mixture of an Al3+ com￾plex with Fe powder has been used by Jung and Joo [33] to create AlN whiskers. The mixture of the Al complex, Fe and graphite was calcined at 1200–1500 C for 5 h in flowing N2. The whiskers often show modulations in their diameters along the lengths of the whisker and the shapes obtained depend on the growth temperature. This system exhibits rather complex chemistry on account of the carbo￾thermal reduction to produce Al, dissociative adsorption of N2, presumably on the Fe particles, and the formation of Fe/Al/N alloy particles of the appropriate size. Dendritic ZnO nanowires can be grown from Sn catalyst particles as shown by Gao et al. [59]. They heated ZnO and SnO2 powders (1:1 ratio) for 1 h under a pressure of 300– 400 Torr of Ar carrier gas. The nanostructures grew on the top of the inner alumina wall of the tube furnace in a region located downstream 15 cm away from the source material, which was located in the middle of the furnace at 1300 C, and the local growth temperature was in the range of 700–800 C. The Sn particle is significantly larger than the NW diameter. Note that in this system, as in sev￾eral others, there is a high proportion of the catalyst start￾ing material compared to the growth material. This stands in contrast to, for instance, Au catalyzed growth of Si and SiGe nanowires. Also in this system, both the growth and catalyst phases are transported via the vapor phase. Taper￾ing to larger diameters is observed in this system, most likely because the Sn nanoparticles are growing during growth. Xu et al. [53] formed ZnO nanowires with a hexagonal cross section. Interestingly no catalytic particle is found at the tip. Rather the catalytic action is provided by a ZnBiIx thin film (a combination of tetragonal ZnI2 and hexagonal BiI3). Mixtures of either BiI3 and Zn powder or Bi and Zn powders were heated in flowing Ar to 250– 300 C. The presence of I changes the growth direction of the nanowires. The O can be supplied by an impurity in the Ar but no growth is observed in the absence of Bi when only Zn or ZnI2 powders are used. Carbothermal reduction can be used to produce ZnO nanowires on a silicon substrate. Moreover, what Yang et al. have shown is that growth can be switch away from ZnO nanowires to Mn doped ZnSiO4 if MnCl2 Æ 4H2O is added to the reaction mixture. The furnace system was flushed with high-purity Ar gas to eliminate O2 and heated to 1100 C under a constant flow of Ar gas. Then, the quartz boat was placed in the centre of the furnace and held at 1100 C under the same Ar flow. After reaction for 50–60 min at 800–900 C, the Si wafers, which are situ￾ated downstream of the reaction mixture, were coated with a layer of nanowires. The majority of this material is com￾posed of a willemite phase (a-Zn2SiO4 with rhombohedral structure) forming well-aligned nanorods with lengths of 2– 4 lm and diameters of 70–150 nm. The Zn catalyst is con￾sumed so there is a severe taper at the end of the wires and no metal particle is found. Simultaneous growth of ZnO and LiF nanowires has been reported by Jiang et al. [50]. Zn acts as the cata￾lyst when LiF + ZnO powders are heated in Ar to 750–850 C. The downstream deposition region has a temperature in the range of 400–500 C. Cubic-structured single-crystalline LiF nanowires grew along the h001i and h110i crystallographic directions with diameters of 100–500 nm and lengths of tens of microns. The authors propose that there must be a barrier to the incorporation of LiF into the lattice and the Zn particle acts to lower this barrier and cause growth to be preferential at its base. Very similar conditions can be used to grow In2O3, Ga2O3 and SnO2 nanowires. Indeed, In2O3 and SnO2 nanowires can be grown simultaneously without cross con￾tamination or doping as demonstrated by Johnson et al. [45]. Fifty nanometer Au particles were deposited from solution onto the Si substrate placed downstream from high-purity (6 N) metal reactant species (In, Ga, or Sn) placed separately in a quartz boat. The furnace was heated to 800–1000 C under flowing N2. No oxygen was supplied directly to the system other than as an impurity in the N2 or that which desorbs from the surfaces inside the furnace. Rectangular In2O3 rods are capped with a rectangular Au particle that is slightly smaller than the rod, whereas 184 K.W. Kolasinski / Current Opinion in Solid State and Materials Science 10 (2006) 182–191