ZnO nanorods

Nanostructured ZnO materials have attracted much attention due to their exceptional performance in electronics, optics and photonics. From the 1960s, synthesis of ZnO thin films has been an active field because of their applications as sensors, transducers and catalysts. In the last few decades, especially since the nanotechnology initiative led by the US, study of one-dimensional (1D) materials has become a leading edge in nanoscience and nanotechnology. With decreasing volume size, novel electrical, mechanical, chemical and optical properties are introduced, which are believed to be the result of surface and quantum confinement effects. Nanowire-like structures are the ideal system for studying the transport process in one-dimensionally (1D) confined objects, which are of benefit not only for understanding the fundamental phenomena in low dimensional systems, but also for developing new generation nano-devices. ZnO is a key technological material. The lack of a centre of symmetry in wurtzite, combined with large electromechanical coupling, results in strong piezoelectric and pyroelectric properties and the consequent use of ZnO in mechanical actuators and piezoelectric sensors. In addition, ZnO is a wide band-gap (3.37 eV) compound semiconductor that is suitable for short wavelength optoelectronic applications. The high exciton binding energy (60meV) in ZnO crystal can ensure efficient excitonic emission at room temperature and room temperature ultraviolet (UV) luminescence has been reported in disordered nanoparticles and thin films. ZnO is transparent to visible light and can be made

highly conductive by doping. ZnO is a versatile functional material that has a diverse group of growth morphologies, such as nanocombs, nanorings, nanosprings, nanobelts, nanowires and nanocages. The basic materials parameters of ZnO are shown in Figure 2.1.

2.1.2 Crystal and surface structure of ZnO

Wurtzite zinc oxide has a hexagonal structure (space group C6mc) with lattice parameters a

= 0.3296 and c = 0.52065 nm. The structure of ZnO can be simply described as a number of alternating planes composed of tetrahedrally coordinated O²⁻ and Zn²⁺ ions, stacked alternately along the c-axis (Figure 2.2[16]). The tetrahedral coordination in ZnO results in non central symmetric structure and consequently piezoelectricity and pyroelectricity.

Another important characteristic of ZnO is polar surfaces. The most common polar surface is the basal plane. The oppositely charged ions produce positively charged Zn-(0001) and negatively charged O-(0001) surfaces, resulting in a normal dipole moment and spontaneous polarization along the c-axis. To maintain a stable structure, the polar surfaces generally have facets or exhibit massive surface reconstructions, but ZnO ± (0001) are exceptions: they are atomically flat, stable and without reconstruction.[17,18] The other two most commonly observed facets for ZnO are {2110} and {0110}, which are non-polar surfaces and have lower energy than the {0001} facets.

2.1.3 Growth ZnO nanorods

The different surface structures of ZnO could induce anisotropic growth. Under thermodynamic equilibrium conditions, the facet with higher surface energy is usually small in area, while the lower energy facets are larger. Specifically, in the ZnO growth, the highest growth rate is along the c-axis and the large facets are usually {0110} and {2110}. Therefore, the ZnO nanorods can be synthesized by many process, including chemical bath deposition(CBD)[19,20], aqueous chemical solution[21], and vapour-liquid-solid (VLS)[22].

The growth mechanism from VLS method was proposed in the 1960s-1970s for large whisker growth. This method is promotion of anisotropic crystal growth using metal nanoparticles as catalysts. Yang et al. [22] reported the use of the vapor-phase transport process to grow ZnO nanowires via the VLS mechanism. The Zn vapor was generated using carbothermal or hydrogen reduction of ZnO. Size control of the nanowire diameters was achieved by varying the thickness of the thin film Au catalyst.

Another important growth method is aqueous chemical method. Vayssieres et al. [21] 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 nanorods. Often, a solution of Zn(NO3)2 and hexamethyltetramine (HMT) is used:

(CH2)6N4 + 6H2O ↔ 6HCHO + 4NH3 (1) NH3 + H2O ↔ NH4+ + OH^- (2) 2OH^- + Zn²⁺ → ZnO(s) + H2O (3)

Hydroxide ions are formed by the decomposition of HMT and they react with the Zn²⁺ to form ZnO.

Idealized growth habit of a ZnO crystal was first to describe by Laudise [23,24] by the hydrothermal method. It has been observed that the maximal crystal growth velocity is fixed in the <0001> direction and the following relationship between the velocities of crystal growth to different directions is found to be: V<0001> > V<0110> > V<0001>. Because the fastest growth plane (0002) will be disappeared, the shape of ZnO nanostructure would become wires or rods.

2.1.4 ZnO defect chemistry

There are a number of intrinsic defects with different ionization energies in ZnO structure.

The Kroger Vink notation uses: i = interstitial site, Zn = zinc, O = oxygen, and V = vacancy.

The terms indicate the atomic sites, and superscripted terms indicate charges, where a dot

indicates positive charge, a prime indicates negative charge, and a cross indicates zero charge, with the charges in proportion to the number of symbols. Figure 2.3 shows that there are a number of defect states within the bandgap of ZnO. The donor defects are:Zn_i^••,Zn_i^•,Zn_i^x,V_o^••,V_o^•,V_o and the acceptor defects are:V ′′_Zn,V ′_Zn. The defect ionization energies vary from ~0.05-2.8 eV. Zn interstitials and oxygen vacancies are known to be the predominant ionic defect types.[25]

Figure 2.4 [26] shows the corrected formation energies for the relevant native point defects in ZnO as a function of Fermi-level position. The kinks in the curves for a given defect indicate transitions between different charge states. The differences are related to the correction of absolute formation energies, where we now take into account the occupancy of the defect-induced states in the case of defects with partially occupied states in the band gap.

Oxygen vacancies have the lowest formation energy (see Fig. 2.4). Oxygen vacancies have frequently been invoked as the source of unintentional n-type conductivity in ZnO. Prof.

Vanheusden found that the green emission intensity is strongly influenced by free-carrier depletion at the particle surface, particularly for small particles and/or low doping. Their data suggested that the singly ionized oxygen vacancy is responsible for the green emission in ZnO;

this emission results from the recombination of a photogenerated hole with the singly ionized charge state of this defect. [26]

Because of the different ionization energies, the relative concentrations of the various defects depend strongly on temperature. However, the partial pressure of oxygen and zinc, pO2 and pZn, respectively, are also very important. Hence, under very reducing conditions and at high temperatures, oxygen vacancies may predominate, depending on the relative pO2/pZn ratio. During annealing, the variety of these defects with the oxygen pressure (P_O2) could be expressed as follows:

Equations (4) and (6) indicate that concentrations of the oxygen vacancy and the interstitial zinc decreased with the increase of oxygen pressure

O2

P : While Eqs. (5), (7) and (8) indicated that concentrations of the zinc vacancy, the interstitial zinc and antisite oxygen increased with the increase of the oxygen pressureP_O2. [27]

2.1.5 Applications of the ZnO nanorods

So far, there are many applications of the ZnO nanorods, such as sensor, light-emitting diodes, lasing, cantilevers and solar cells. Especially, solar cells and lasing are the two famous applications. Prof. Yang reported nanowire dye-sensitized solar cells in 2005. They introduced a version of the dye-sensitized cell in which the traditional nanoparticle film wass replaced by a dense array of oriented, crystalline ZnO nanowires. The nanowire anode wass synthesized by mild aqueous chemistry and features a surface area up to one-fifth as large as a nanoparticle cell. The direct electrical pathways provided by the nanowires ensured the rapid collection of carriers generated throughout the device, and a full Sun efficiency of 1.5% was demonstrated. The device structure is presented in Figure 2.5. [28]

Laser application of the ZnO nanorods is also quite attracted much attention up to now.

Prof. Choy show the room-temperature ZnO ultraviolet laser in 2003. They presented that a high-quality ZnO nanrods was grown on a Si wafer by a wet-chemical process at 95^oC, where the Si wafer was dip-coated with 4 nm sized ZnO nanoparticles s buffer and seed layer prior

(4) (5) (6) (7) (8)

th the crystal growth. The product ZnO nanrods showed threshold power density was ~70kW cm^-2. The result can be seen in the Figure. 2.6. [29] Moreover, Prof. Wang developed a direct-current nanogenerator driven by ultrasonic waves in 2007. The nanogenerator was fabricated with vertically aligned zinc oxide nanowire arrays that were placed beneath a zigzag metal electrode with a small gap. The wave drives the electrode up and down to bend and/or vibrate the nanowires. A piezoelectric semiconducting coupling process converts mechanical energy into electricity. The zigzag electrode acts as an array of parallel integrated metal tips that simultaneously and continuously create, collect, and output electricity from all of the nanowires. The approach presents an adaptable, mobile, and cost-effective technology for harvesting energy from the environment, and it offers a potential solution for powering nanodevices and nanosystems. The nanogenerator structure was shown in the Figure 2.7. [30]