Growth of NWs

6.3.1 The vapor–liquid–solid growth method

Semiconductor NWs are generally synthesized by employing metal nanoclusters as catalysts via a vapor–liquid–solid (VLS) process (Fig. 96). In this process, the metal nanoclusters are heated above the eutectic temperature for the metal–semiconductor system of choice in the presence of a vapor-phase source of the semiconductor, resulting in a liquid droplet of the metal/semiconductor alloy. The continued feeding of the semiconductor reactant into the liquid droplet supersaturates the eutectic, leading to nucleation of the solid semiconductor. The solid–liquid interface forms the growth interface, which acts as a sink causing the continued semiconductor incorporation into the lattice and, thereby, the growth of the nanowire with the alloy droplet riding on the top. The gaseous semiconductor reactants can be generated through decomposition of precursors in a chemical vapor deposition (CVD) process or through momentum and energy transfer methods such as pulsed laser ablation or molecular beam epitaxy (MBE) from solid targets. So far, CVD has been the most popular technique. In CVD–VLS growth, the metal nanocluster serves as a catalyst at which site the gaseous precursor decompose, providing the gaseous semiconductor reactants. In the case of SiNW growth (Fig. 96), silane (SiH4) and Au nanoparticles are normally used as the precursor and catalysts, respectively. Besides group IV materials, compound III–V and II–VI NWs have also been produced with the VLS method, in which pseudo binary phase diagrams for the catalyst and compound semiconductor of interest are employed. In the compound semiconductor case, metal-organic chemical vapor deposition (MOCVD) or pulsed laser ablation are typically used to provide the reactants. There are two competing interfaces during nanowire growth, the liquid/solid interface between the eutectic and the nanowire and the gas/solid interface between the reactants and the exposed surface of the growing nanowire. Precipitation through the first interface results in the VLS growth and axial elongation of the nanowire, while dissociative adsorption on the second interface results in vapor–solid growth and thickening in the radial direction. Either mechanism can be dominating in an actual growth process, depending on the detailed growth condition such as the pressure, flow rate, temperature, reactant species and background gases that

are by-products of growth reactions. For example, in the abovementioned SiNW growth process, low temperature growth can reduce the rate of direct thermal dissociation of silane; hence, axial nanowire growth is favored.

Hydrogen has also been found to mitigate radial growth through suppression of either the adsorption of the reactants by terminating the Si surface or of the dissociation of silane. The use of H2 as the carrier gas also passivates the NWsurface in a manner similar to that observed in thin-film growth and reduces roughening along the NW. Uniform NWs with negligible diameter variation can thus be achieved through careful control of the growth conditions, including the employment of local heaters to reduce uncontrolled decomposition of silane. On the other hand, tapered NWs are products from simultaneous growth in both the axial and radial directions and are generally not desirable for most electrical and optical applications. In the CVD–VLS growth process the diameter of the nanowire is determined by that of the starting nanocluster, and uniform, atomic-scale NWs can be obtained in a well controlled growth process as nanoclusters with diameters down to a few nanometers are now commercially available. Wu et al reported growth of uniform SiNWs with diameters down to 3 nm using SiH4 as the precursor and H2 as the carrier gas. Wu performed detailed high-resolution transmission electron micrography (HRTEM) studies on these SiNWs and observed that the NWs are single-crystalline with little or no visible amorphous oxide even at this scale. The resulting SiNWs show narrow size distributions of 13.2±1.7 nm, 5.9±1.1 nm, and 4.6 ± 1.2 nm, respectively when gold nanoclusters of diameters of 10 (9.7 ± 1.5) nm, 5 (4.9 ± 0.7) nm, and 2 (3.3 ± 1) nm are used.

The increase in the NW diameters compared with those of the starting nanoclusterswas attributed to the supersaturation of silicon in gold nanoclusters during the formation of the liquid droplet prior to nucleation, an effect observed previously with in situ observations of the growth of germanium NWs.

Fig. 96 Schematic of VLS growth of Si nanowires (SiNWs). (a) A liquid alloy droplet AuSi is first formed above the eutectic temperature (363 ◦C) of Au and Si. The continued feeding of Si in the vapor phase into the liquid alloy causes oversaturation of the liquid alloy, resulting in nucleation and directional nanowire growth. (b) Binary phase diagram for Au and Si illustrating the thermodynamics of VLS growth.

【Wagner R S 1970 Whisker Technology (New York: Wiley)】

6.3.2 Nanowire heterostructures

Compared with nanostructures fabricated from other approaches such as vapor–solid growth or solution based liquid–solid growth, the VLS process offers one key advantage—heterostructures can be achieved at the individual device level in a controlled fashion. Both axial heterostructures (Fig. 97(c) and (e)), in which sections of different materials with the same diameter are grown along the wire axis , and radial heterostructures (Fig. 97(d) and (f )), in which core/shell and core/multi-shell form along the radial direction , have been realized on VLS–CVD grown NWs. To understand the rational formation of nanowire heterostructures within the context of the VLS method, consider the possible effects of a change in reactant vapor once nanowire growth has been established (Fig. 97). If vapor decomposition/adsorption continues exclusively at the surface of the catalyst nanocluster site, crystalline growth of the new semiconductor will continue along the axial direction (Fig. 97(c)). On the other hand, if the decomposition of the new vapor/reactant on the surface of the semiconductor nanowire cannot be neglected, a shell of material will grow on the original nanowire surface (Fig. 97(d)). Repeated changing of reactants in a regime favoring axial growth will lead to the formation of a nanowire superlattice, as shown in Fig. 97(e), while changing reactants in a radial-growth regime will result in core-multi-shell radial structures, as shown in Fig. 97(f). It is important to mention that there are few constraints on the

composition of the shell growth; any material suitable for planar film deposition can be deposited on the surface of a nanowire, and crystalline radial heteroepitaxy can be achieved as the nanowire surface is crystalline.

Fig. 97 Nanowire heterostructure synthesis. (a) Preferential reactant incorporation at the catalyst (growth end) leads to 1D axial growth. (b) A change in the reactant leads to either (c) axial heterostructure growth or (d) radial heterostructure growth depending on whether the reactant is preferentially incorporated (c) at the catalyst or (d) uniformly on the wire surface. Alternating reactants will produce (e) axial superlattices or (f) core-multi-shell structures.

【Kong Y C, Yu D P, Zhang B, Fang W and Feng S 2001 Appl. Phys. Lett. 78 407】

6.3.2.1 Radial nanowire heterostructures

Radial core / shell heterostructures can be achieved if dissociation of the reactants is promoted at the grown nanowire surface (Fig. 97(d)), analogous to the layered growth of planar heterostructures. Compared with NWs in the simple homogeneous form, core/shell heterostructure NWs offer better electrical and optical properties as they can now be tailored through band structure engineering.

6.3.2.2 Axial nanowire heterostructures

Unlike radial heterostructures in which the shell growth does not involve reaction with the nanocluster catalyst, axial nanowire heterostructures can be obtained by alternative introduction of vapor phase reactants that react with the same nanocluster catalyst, as illustrated in Fig. 97(c) and (e). A critical requirement of the axial nanowire heterostructure growth is then that a single

nanocluster catalyst can be found which is suitable for growth of the different components under similar conditions.

6.3.3 Growth of metal oxide NWs

Besides the VLS approach, a simple thermal evaporation/ vapor transport deposition approach has also been shown to be effective in growing 1D structures, in particular metal oxide (e.g. ZnO, In2O3 and SnO2) NWs. Such NWs have been studied in applications ranging from optoelectronics devices, field-effect transistors, ultra-sensitive nanoscale gas sensors and field emitters.

In particular, ZnO NW shave attracted a lot of interest due to the large exciton binding energy (60meV), high electromechanical coupling constant and resistivity to harsh environment. The first attempt to produce ZnO NWs was based on an electrochemical method using anodic alumina membranes (AAM).

Later on, high-density, ordered ZnO nanowire arrays were obtained using AAM in a vapor-deposition process. However, the AAM approach is typically limited to generating polycrystalline NWs. To produce singlecrystalline 1D ZnO nanostructures, a number of methods have been explored such as thermal evaporation / vapor transport deposition, hydrothermal process, MOCVD, pulsed laser deposition and MBE. Among all methods used to grow metal oxide NWs, thermal evaporation / vapor transport deposition has gained the most popularity. The thermal evaporation technique is based on a simple process in which source materials in the condensed or powder form are first vapourized at an elevated temperature; the resulting materials in vapor phase then condense under the right conditions (temperature, pressure, atmosphere, substrate, etc) to form the desired product. The processes are usually carried out in a setup as shown in Fig. 98, which includes a horizontal tube furnace, a quartz or alumina tube, gas supplies and control system. Single-crystalline ZnO NWs grown with the thermal evaporation method (referred also as the physical vapor deposition method) were first reported in 2001, with an average diameter of about 60 nm. In 2002, Yao et al reported mass production of ZnO NWs, nanoribbons and needle-like nanorods by thermal evaporation of ZnO powders mixed with graphite. Yao found that temperature was the critical experimental parameter for the formation of different morphologies of ZnO nanostructures. Banerjee et al succeeded in producing grams of ZnO NWs via thermal evaporation of ZnO powder in a tube furnace at high temperatures.

The graphite flakes were found to be the key.

Fig. 98 Schematic diagram of the horizontal tube furnace for growth of ZnO nanostructures.

Reprinted with permission from .

【Qian C, Kim F, Ma L, Tsui F, Yang P and Liu J 2004 J. Am. Chem. Soc. 126 1195】