1.2.1 General of Nanostructures
During the last decade, the growth of low-dimensional semiconductor structures has made it possible to reduce the dimension from three (bulk material) to the quasi-zero dimensional semiconductor structures. Generally, the crystal structure of a solid restricts the movement of carriers. In a semiconductor material, the outer electrons of the atoms are delocalized over the entire crystal, with the periodicity of the crystal structure limiting their movement. For certain electron energy, the carrier is allowed to move in one direction, but its motion in a different direction is restricted as a result of destructive interaction from the atomic lattice. This dependence of the electron energy on the momentum of the carrier results in a structure of energy bands where the carrier can exist. The electron energy of low-dimensional semiconductor structures becomes quantized and depends on the structural size. The band gap and density of states (DOS) associated with a quantum-structure differs from that associated with bulk material, determined from the magnitude of the three-dimension wave vector. Figure 1-4 illustrated the density of states in systems of differing dimensionality: bulk, quantum well, quantum wire, and quantum dot.
Figure 1-4. Density of states in different dimensional systems.
The degree of quantum confinement is determined by the interaction length over which the bond between an electron and a hole extends in an exciton, compared with the size of the materials. Take quantum dot for example, in weak quantum confinement, the interaction length is shorter than the dimensions of the quantum dot, resulting in very closely spaced quantum-confined energy states. In this case, the quantum dot seems more bulk-like because the electron and hole can separate beyond the exciton interaction length, thus breaking up the exciton and making the transition energy independent of the quantum dot size. In the strong quantum-confinement regime, the quantum dot size is smaller than the electron hole interaction length, resulting in widely spaced energy states. The quantum dot sizes can be so small that the energy spacing between the allowed states in the conduction and valence bands is large enough to cause a so-called phonon bottleneck.
In most semiconductors, excited electrons can relax down to the bottom of the conduction band by losing energy through carrier-carrier and carrier-phonon interactions. However, a phonon bottleneck occurs when the spacing between energy states is much larger than these interelectronic energies (as is the case in strong quantum confinement). In this case, the probability for a carrier in a higher energetic state to lose its energy to a lower energetic state through these interactions becomes very low. As a result, electrons cannot relax to the bottom of the conduction band, and so quantum dots with strong quantum confinement should not be expected to emit light from the normal transition across the band gap. However, this phonon bottleneck has been observed experimentally first in recent years. [16]
According to these above mentions, the quantum confinement effect come from nanostructures become predominant and give rise to many interesting electronic and optical properties.
1.2.2 One-Dimensional (1-D) ZnO Nanostructures
One-dimensional (1-D) nanostructures have attracted intensive attention because of their unique physical, optical and electrical properties resulting from their low dimensionality. The electron-hole interaction will have orders of magnitude enhancement in a nanostructure, due to the dramatically increased electron density of states near the van Hove singularity. ZnO has an effective electron mass of ~0.24 me, and a large exciton binding energy of 60 meV. Tremendous progress has been made to understand the quantum-size behavior and to investigate the size- and morphology-dependent properties. Compared with bulk ZnO, the reported 1-D ZnO nanowires or nanorods have the same lattice constant. However, the significant characteristics of 1-D ZnO nanostructures are their high surface-to-volume ratio and anisotropic carrier transport. With this respect great efforts and contribution from the fruitful group such as Yang’s [17] and Wang’s group [18] for ZnO nanostructures, have been made persistently to enrich the diversiform morphological world of nanostructures and show their possibilities for versatile utilizations, such as room temperature laser [19], waveguide [20] and piezoelectric nanogenerators [21, 22].
1-D ZnO nanowires and nanorods are also being intensively investigated because they possess a combination of attractively optical, electronic, mechanical, magnetic properties and so on. Extensively potential applications as light-emitting diodes [23, 24], UV photodetectors [25-27], field-effect transistors [28-31], field electron emitters [32-35] gas sensors [36-40], and solar cells [41-47] are continuously be evaluated.
Moreover, the observations of quantum confinement [48] and the discrete energy levels [49] are demonstrated in ZnO/ZnMgO nanorod heterostructures. Therefore, the ZnO nanowires and nanorods have become one of the most promising elemental building blocks in nanotechnology applications. Other 1-D ZnO nanostructures such
as nanoneedle, nanopencil, nanobelt, nanotube, nanospiral, nanohelix, nanonail and dendritic nanowire have also been discovered [50, 51], but lack of applications at present merely interesting growth mechanisms have been emphatically demonstrated.
To date, most of the work on 1-D nanostructures ZnO has focused on the synthesis methods. For application of nano-photonics and electronics, it is needed to create ZnO nanowires that are selective area growth, highly aligned and orientation-ordered on substrates. The fundamental optical properties, including the origin of luminescence, carrier-carrier interaction, stimulated emission and lasing are also needed to understand. Furthermore, band-gap engineering, which is the process of controlling or altering the band gap, and fabrication of heterostructure or quantum-structures of ZnO-based nanowires and nanorods are important issues.
1.2.3 Zero-Dimensional (0-D) ZnO Nanostructures
Zero-dimensional (0-D) ZnO nanoparticles (NPs) or quantum dots (QDs) with several nanometers in diameter and large surface to volume ratio are becoming more and more attractive due to their quantum size effect and therefore unique properties different from the bulk ZnO materials. The physical properties in ZnO nanostructures changed dramatically as the diameter closes to the exciton Bohr radius (~2.43 nm) of bulk ZnO [52].
Accordingly, a great deal of attention has been paid to nanocrystalline materials for the study of the size-dependent quantum confinement effects. However, the ZnO nanocrystals have not been investigated as much as other semiconductor nanocrystals such as CdSe despite the possibility of UV/blue applications. The greater part of investigations for 0-D ZnO nanostructures focus on the fundamentally physical phenomenons, such as band edge emission [53-59], phonon quantum confinement
[60-64], random Laser [65-67] and nonlinear susceptibility [68, 69].
Recently, several applications such as photocatalytic activity [70, 71] and cell labeling [72] have been reported owing to successfully faceted control and surface functionality of ZnO NPs and QDs, respectively. Organization of ZnO NPs and QDs building blocks into premeditated one- (1-D), two- (2-D), and three-dimensional (3-D) architectural systems should be considered as one of the key challenges in today’s science and engineering.