Growth of 1-D ZnO Nanowires/ Nanorods

Vapor-Liquid-Solid (VLS) Method. Among all vapor based methods, the VLS

method, which is a growth mechanism based on chemical vapor transport, seems to be the most successful for fabricating nanowires with single crystalline structures and in relatively large quantities. This process was first developed by Wagner et al. to produce Si single crystalline micro-whiskers in 1960s [1], and recently re-examined successfully by Lieber [2] and Yang [3]. The key factor is needed to deposit metal clusters such as Fe, Co, Ni, Cu, Sn and Au as the catalysts. A typical VLS process starts with the dissolution of gaseous reactants into nano-sized liquid droplets of catalyst metal while the liquid droplets are supersaturated with the guest material, followed by nucleation and growth of single crystalline nanorods and then nanowires.

The 1D growth is mainly induced and dictated by the liquid droplets, whose size remains essentially unchanged during the entire process of nanowire growth. In the sense, each of liquid droplets serves as a soft template to strictly limit the lateral growth of an individual nanowire. As a major requirement, there should exist a good solvent capable of forming liquid alloy with the target material, ideally they should be able to form eutectic compounds. All of the major steps involved in a VLS process for a Ge nanowire case are schematically illustrated in Fig. 2-1 [3]. In the beginning Ge and Au form liquid alloys when the temperature is raised above the eutectic point.

Once the liquid droplet is supersaturated with Ge, growth of nanowire takes place at the solid-liquid interface. The vapor pressure of Ge in the chemical-vapor-deposition

Figure 2-1. Schematic illustration of vapor-solid growth mechanism including three stages (I) alloying, (II) nucleation and (III) axial growth. Three stages are projected onto the coventional Au-Ge phase diagram; (b) shows the compositional and phase evolution during the nanowire growth process. [3]

system has to be kept sufficiently low so that the second ordinary nucleation will be completely suppressed. Figure 2-2 shows a sequence of real-time TEM images during the growth of a Ge nanowire.

Both physical methods (thermal evaporation and laser ablation) and chemical methods (chemical vapor transport and deposition) have been employed to generate the vapor species required for the growth of nanowires, and no significant difference was found in the quality of nanowires produced by these methods.

Figure 2-2. In situ TEM images recorded during the process of nanowire growth.

(a) Au nanoclusters in solid state at 500 °C; (b) alloying initiated at 800 °C, at this stage Au exists mostly in solid state; (c) liquid Au/Ge alloy; (d) the nucleation of Ge nanocrystal on the alloy surface; (e) Ge nanocrystal elongates with further Ge condensation and eventually forms a wire (f).[3]

The VLS processes are usually carried out in a horizontal tube furnace, as shown in Fig. 2-3. In this schematic, the carrier gas, Ar, is introduced from the left end of the alumina tube and is pumped out from the right end. The source material is loaded on an alumina boat and positioned at the center of the highest temperature zone in the alumina tube. The substrate temperature usually drops with the distance from the position of the source material(s). The local temperature where the substrate is situated (usually 500-700 °C) determines the type of product that will be obtained. To reduce the decomposition temperature, ZnO powder is usually mixed with graphite powder to form the source mixture. At temperatures 800-1100 °C, graphite reacts with ZnO to form Zn, CO, and CO2 vapors, which then react on the substrate to form ZnO nanostructures.

Self-organized [0001]-oriented ZnO nanowires have been successfully

Figure 2-3. A schematic diagram of the horizontal tube furnace for growth of ZnO nanostructures by the solid-vapor phase process.

[4]. As mentioned above, selective nanowire growth could be readily achieved by patterning the Au thin film before growth. Typical scanning electron microscope (SEM) images of nanowire arrays grownon a-plane sapphire (112 0) substrates with patterned Au thin film are presented in Fig. 2-4. By adjusting the growth time, nanowires could be grownup to 10 mmin length. The diameters of these wires range from 20 to 150nm, although more than 95% of themhave diameters between 70 and 100nm.

Klingshirn’s group also identified well-defined lasing modes under optical excitation of ZnO nanorods grown in a similar fashion using catalyst-assisted VLS technique by employing self-organized polystyrene spheres on GaN substrates as a mask during Au evaporation [5]. Ordered arrays of [0001]-oriented ZnO nanorods with 200nm diameter and 4.7 mm length were obtained with 500nm rod-to-rod spacing as shown in Fig. 2-5. As alluded earlier, well-aligned ZnO nanorods provide a perfect gain medium as well as act as waveguides and Fabry-Perot resonators with well-defined cavity ends.

Figure 2-4. Scanning electron microscope images of ZnO nanowire arrays grown on sapphire substrates (a–e). A top view of the well-faceted hexagonal nanowire tips is shown in (e). (f) High-resolution TEM image of an individual ZnO nanowire showing its [0001] growth direction.

Figure 2-5. (a) 45˚ side-view SEM images of ordered ZnO nanorod arrays and (b) hexagonally ordered ZnO nanorod arrays grown by the VLS method on patterned Au-covered GaN/Al2O3 substrates. The inset is the top view of the nanorod arrays.

Vapor-Solid (VS) Method. The vapor-solid (VS) method is another chemical

vapor transport mechanism compared with VLS, depending on the presence of a metal catalyst. VS growth also holds for the growth of 1D ZnO nanomaterials. In this process, evaporation, chemical reduction or gaseous reaction first generates the vapor.

The vapor is subsequently transported and condensed onto a substrate. The VS method has been used to prepare whiskers of oxide, therefore, possible to synthesize the 1D nanostructures if one can control the nucleation and the subsequent growth process. According to the difference on nanostructure formation mechanisms, the extensively used vapor transport process can be categorized into the catalyst free VS process and catalyst assisted VLS process. Synthesis utilizing VS process is usually capable of producing a rich variety of nanostructures, including nanowires, nanorods, nanobelts, nanohelix and other complex structures.[6-7]

Using a solid state thermal sublimation process and controlling the growth kinetics (VS growth mechanism), local growth temperature, and the chemical composition of the source materials, a wide range of nanostructures of ZnO have been synthesized by Wang’s group, as shown in Fig. 2-6 [7]. The two important characteristics of the wurtzite structure are the noncentral symmetry and polar surfaces. The structure of ZnO, for example, can be described as a number of alternating planes composed of tetrahedrally coordinated O^2- and Zn²⁺ ions, stacked alternately along the c-axis. The oppositely charged ions produce positively charged (0001)-Zn and negatively charged (0001)-O polar surfaces, resulting in a normal dipole moment and spontaneous polarization along the c-axis, as well as a divergence in surface energy. As a result, the formation of a self-coiled, coaxial, multilooped nanoring structure is spontaneous, which means that the self-coiling along the rim proceeds as the nanobelt grows under the driving force of stacking the polar surfaces.

Figure 2-6. A collection of nanostructures of ZnO synthesized under controlled conditions by thermal evaporation of solid powders. [7]

A variety of novel hierarchical nanostructures with 6-, 4-, and 2-fold symmetries have been successfully grown by a vapor transport and condensation technique from Ren’s group, as shown in Fig. 2-7 [8] . It was found that the major core nanowires are single-crystal In2O3 with 6, 4, and 2 facets, and the secondary nanorods are single-crystal hexagonal ZnO and grow either perpendicular on or slanted to all the facets of the core In2O3 nanowires. Because no catalyst is used in the system, the In2O3 nanowire growth should be based on the vapor-solid mechanism. On the other hand, it is more difficult to define the ZnO nanorod growth mechanism. Probably ZnO nanorods also grow based on the vapor-solid mechanism because the In2O3 core is covered by a ZnO layer that can be the base for further ZnO nanorod growth.

Compared to the aligned ZnO grown by the vapor-liquidsolid mechanism [4] with source temperature around 900 °C, the metal and/or metal oxide vapor pressure here

Figure 2-7. SEM images of the 6-fold ZnO nanostructures. (a) SEM image showing the abundance of the 6S-fold symmetry. Scale bar, 10 µm. (b) SEM image showing the 6M-fold symmetry. (c) High magnification SEM image of the 6S-fold symmetry.

(d) High magnification SEM image of the 6M-fold symmetry. (e) Head on look at a 6S-fold symmetry to show the hexagonal nature of the major core nanowire. [8]

is much higher. This high vapor pressure is necessary for the growth of the hierarchical structures. The growth conditions such as temperature, pressure, and source component ratios are correlated to affect the supersaturation rate and the structure formed.

Metal-Organic Chemical Vapor Deposition (MOCVD) Method. For device

fabrication, heteroepitaxial growth with control over impurities and thickness down to nanometer scale is required. VLS method is limited and cannot completely meet these requirements. Growth of complex structures for device applications can only be accomplished by advanced epitaxial methods such as metal-organic chemical vapor deposition (MOCVD) or metal-organic vapor phase epitaxy (MOVPE) and molecular

beam epitaxy (MBE). Particularly, MOCVD has been proven to be a powerful technique for large-scale production with accurate control over doping and thickness.

For MOCVD growth of ZnO nanorods, usually diethylzinc and oxygen are employed as the reactants and argon as the carrier gas [9]. N2O as the oxygen source and nitrogen as the carrier gas have also been used [10]. Typical growth temperatures range from 400 to 1050 °C. The growth occurs without a catalyst, and flat terraces and steps are observed at the nanorod tips resulting from the layer-by-layer growth mode, instead of the metallic nanoparticles characteristic to catalyst-assisted VLS processes.

Figure 2-8 shows ZnO nanorods grown by Park et al. [9] using low-pressure MOVPE on Al2O3 (0001) substrates at 400 °C without any metal catalysts. Very thin ZnO buffer layers were deposited at a low temperature before the nanorod growth.

The mean diameter of nanorods obtained by MOVPE was as small as 25 nm, smaller than the typical diameters of 50-100nm for those prepared by other deposition methods [4]. Furthermore, ZnO nanorods were well aligned vertically, showing uniformity in their diameters, lengths, and densities as revealed from electron microscopy. These ZnO nanorods were epitaxially grown with homogeneous in-plane alignment as well as c-axis orientation. The room-temperature PL spectra of the nanorods showed strong and narrow excitonic emission with a dominant peak at 3.29 eV and an extremely weak deep level emission at 2.5 eV, indicating the high optical quality of the nanorods. Free exciton emission lines were still clearly visible at 10 K, and no quantum confinement effect was evident for the nanorods with diameters exceeding 20nm [11].

For ZnO nanorods grown at relatively higher temperatures (700-1050 °C), vertical alignment of the c-axis-oriented nanorods was observed only on a-plane sapphire substrates, whereas the use of c-plane sapphire, Si (111), SrTiO3 (100), and

Figure 2-8. (a) plan-view and (b) tilted images of ZnO nanorods with a mean diameter of 25nmand (c) tilted and (d) cross-sectional images of ZnO nanorods with a mean diameter of 70 nm. In (c), hexagon-shaped pyramids with flat terraces and steps are seen at the ends of the nanorods. [9]

SrTiO3 (111) substrates resulted in rather random alignment [10]. To test the possibility of a catalyst-assisted process, nanorods were also grown on a-plane sapphire substrates partially coated with a thin (1-3 nm) gold layer. Close to 100%

vertical orientation of ZnO nanorods with a diameter of 50±5nm and a length of several micrometers was observed in areas without gold metallization, while growth with no preferential direction occurred in the areas coated with gold, demonstrating that MOVPE growth of ZnO nanorods is different from the VLS process.

Li et al. [12] prepared ZnO nanoneedles on silicon through chemical vapor deposition. The diameters of the needle tips were in the range of 20-50 nm.

High-resolution transmission electron microscopy revealed that the nanoneedles were single crystals along the [0001] direction. They exhibited multiple tip surface perturbations, and were just 1-3nm in dimension. Field emission measurements showed fairly low turn-on and threshold fields of 2.5 and 4.0 V mm^-1, respectively.

The nanosize perturbations on the nanoneedle tips have been assumed to be the cause for such excellent field emission performance. High emission current density, high stability, and low turn-on field make ZnO nanoneedle arrays one of the promising candidates for high brightness field emission electron sources and flat panel displays.

Lee [13] and Wu [14] also successfully demonstrated the ZnO nanorods directly on GaAs and fused silica substratesvia MOCVD, repectively. Hence, MOCVD is one of the candidates to grown high-density and well-ordered 1-D ZnO nanostructures for practical applications toward manufacturability.

Aqueous Solution-Based Method. For cost-effective growth of ZnO

nanostructures, many groups have reported the growth of highly oriented ZnO nanowires and other nanostructures by using aqueous solution methodology. On the other hand, the control of morphology and the positioning of the nanostructures using these techniques are challenging. Solvothermal is extensively employed as a solution route to produce semiconductor nanowires and nanorods. In this process, a solvent is mixed with metal precursors and crystal growth regulating or templating agents, such as amines. This solution mixture is placed in an autoclave maintained at relatively high temperatures and pressures to carry out the crystal growth and the assembly process. The methodology is quite versatile and has enabled the synthesis of

Figure 2-9. FESEM image of 3-dimensional arrays of ZnO nanorods by using aqueous solution chemical method. [16]

crystalline nanowires of semiconductors and other materials. Andrés-Vergés et al. [15]

reported the solvothermal growth method for the first time in 1990. More than 10 years later, Vayssieres et al. [16] used this method to grow nanorods on conducting glass and Si substrates. For this type of growth, a ZnO seed layer is needed to initialize the uniform growth of oriented nanowires. Often, a solution of ZnO(NO3)2

(or other Zinc salt) mixed with hexamethyltetramine (HMT) is used:

(CH2)6N4 + 6H2O ↔ 6HCHO + 4NH3

NH3 + H2O ↔ NH4⁺ + OH -2OH^- + Zn²⁺→ ZnO(s) + H2O

Hydroxide ions are formed by the decomposition of HMT and they react with the Zn²⁺

to form ZnO. Figure 2-9 shows some typical nanowire arrays grown using this method. For technical applications, it is important to note that this method operates at low temperature and a homogenous coverage of nanowires can be achieved over large areas.

Figure 2-10. (upper) ZnO nanowire array on a four-inch (ca. 10 cm) silicon wafer.

(down) ZnO nanowire array on a two-inch (ca. 5 cm) PDMS substrate. [17]

For demonstrate the ease of commercial scale-up, a growth of ZnO arrays on four-inch (ca. 10 cm) silicon wafers and two-inch plastic (polydimethylsiloxane, PDMS) substrates was presented by Yang’s group, as shown in Fig. 2-10 [17]. In addition, unlike previous reports [15] where crystal growth occurs within enclosed flasks, the current process is carried out in open vessels, which points to the possibility of large-scale reel-to-reel production of such nanowire arrays.This low-temperature hydrothermal method is substrate independent and produces high-quality nanowire arrays on ITO glass, sapphire, titanium foil, and polymer surfaces.

Table 2-1 summarizes some of the studies on solution growth and the resulting structures. [17]

TABLE 2-1.

Figure 2-11. Micrograph of an anodic alumina membrane (AAM) ( Zheng et al. [18]).

Template-Assisted Method. Template-directed synthesis represents a

convenient and versatile method for generating 1D nanostructures. In this technique, the template serves as a scaffold against which other materials with similar morphologies are synthesized. That is, the in situ generated material is shaped into a nanostructure with a morphology complementary to that of the template. The templates could be nanoscale channels within mesoporous materials, porous alumina and polycarbonate membranes. The nanoscale channels are filled using, the solution, the sol-gel or the electrochemical method. The nanowires so produced are released from the templates by removal of the host matrix. Unlike the polymer membranes fabricated by track etching, anodic alumina membranes (AAMs) containing a hexagonally packed 2D array of cylindrical pores with a uniform size are prepared using anodization of aluminium foils in an acidic medium (Fig. 2-11).

For example, with the help of an electrodeposition method [19-20], AAM with highly ordered nanopores was used as a template, zinc nanowires were fabricated into the nanopores via electrodeposition, forming zinc nanowires array, then the nanowire array was oxidized at 300°C for 2 hours and ZnO nanowire array was obtained. In a sol-gel synthesis method [21], AAM was also used as the template and immersed into a suspension containing zinc acetate for 1 minute, then heated in air at 120 °C for 6 hours. ZnO nanofibers were eventually obtained after removing the AAM template.

These methods are complementary to the vapor transport synthesis of ZnO nanostructure, and also employ less rigorous synthesis conditions and provide great potential for device applications.

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