Nanostructured Zinc Oxide and related materials
- Thomas Frauenheim (University of Bremen, Germany)
- Jan M. Knaup (University of Bremen, Germany)
- Andreia Luisa da Rosa (University of Bremen, BCCMS, Germany)
ZnO is a semiconductor compound with a wide direct band gap of 3.4 eV with a large exciton binding energy, whose main interest is for optoelectronics applications. ZnO can exist as thin film as well as in several nanostructured forms, such as nanowires, nanoparticles and nanobelts, which has extended the range of its potential applications to biosensing , ultraviolet (UV) lasers , light-emitting diodes , high performance nanosensors , solar cells , photocatalysis , and more recently piezoelectric nanogenerators . It is also a potential candidate for commercial purposes, since its inexpensive, abundant and has much simpler crystal-growth technologies. For all the applications mentioned above, ZnO needs to be put into contact with its functional environment. Therefore, the underlying physics and chemistry of the interaction of the material with their environment needs to be understood and controlled. Although the physical and chemical properties of ZnO have been investigated for several decades, many questions regarding how surface treatment, doping, and defects in ZnO can influence, improve or degrade the material/device performance and functionality remain a challenge and need to be overcome. For that, a scientific exchange between the theorists and experimentalists is primordial. It is equally important to be mindful of the insights obtained on other metal oxide materials under similar conditions, to be able to distinguish between effects which are specific to ZnO and those that are generic to transition metal oxides.
Optoelectronics: The use of ZnO in optoelectronics has been hindered by the lack of control over its electrical conductivity. The nature of the residual n-type conductivity in undoped ZnO films, whether being due to impurities of some native defects (vacancies, interstitials) and impurities (such as hydrogen) is still under debate. Interstitial H donors have been observed with IR spectroscopy, while substitutional H donors have been predicted from first-principles calculations but not observed directly. The well-known broad green band in ZnO luminescence spectra around 500-530 nm , observed in most samples regardless of growth conditions, is related oxygen vacancies, but it has also been regarded as residual copper impurities. Beside the huge effort of the experimental community and theoretical investigations using first-principles calculations [12,13,14] a deeper understanding of the role of native point defects and impurities on the unintentional n-type conductivity in ZnO, still remains an open question. In order to obtain p-type doping, several impurities and complexes have been investigated. According to first-principles calculations, N substitutes for an O and is a deep acceptor. Theory suggests that the binding energy might be lowered by codoping with isovalent Mg or Be. Usually these acceptors are negatively ionized due to compensating donors such as group-III impurities. N acceptors may also be compensated via the formation of defects such as O vacancies, complexes with Zn interstitials, or N2 molecules. The key challenge is to introduce acceptors without being overwhelmed by compensating donors. In addition to optoelectronic and electronic devices, ZnO may become a key material for spintronic applications when doped with transition metals. However, the ferromagnetism behavior observed in ZnO is complicated by the presence of secondary phases, grain boundaries, and native defects.
Solar cells, photovoltaics: Overall water splitting to form hydrogen and oxygen over a heterogeneous photocatalyst using solar energy is a promising process for clean and recyclable hydrogen production at large scale. In recent years, numerous attempts have been made for the development of photocatalysts that work under visible-light irradiation to efficiently utilize solar energy. Sun is the primary source of our energy but the biggest challenge is to convert such a huge energy into electricity. Photovoltaic solar cells give us such opportunity which can convert sunlight directly into electricity. The currently used photovoltaic cells are silicon based but the high cost and high energy consumption limits their large scale utilizations. Presently, the thin film solar cells are more promising for future solar cell applications, as it uses high absorption materials as an absorption layer which is cheap and environmentally friendly. So far, several kinds of thin film solar cell have been produced in the market and some of the thin films such as CIGS, CdTe, need a transparent conducting oxide (TCO) as a window layer, which is very important as the high quality TCO works as an antireflection layer thus allowing light reaching the absorption layer without much loss. Indium tin oxide (ITO) is being widely used for TCO material, specially flat panel displays. The most important TCO semiconductors are impurity-doped ZnO, In2O3, SnO2 and CdO, as well as the ternary compounds Zn2SnO4, ZnSnO3, Zn2In2O5, Zn3In2O6, In2SnO4, CdSnO3, and multi-component oxides consisting of combinations of ZnO, In2O3 and SnO2 . The expanding use of TCO materials, especially for the production of transparent electrodes for optoelectronic device applications, is endangered by the scarcity and high price of In. Besides, its performance decays due to the indium diffusion. On the other hand, ZnO based TCO are cheap and easily available. This situation drives the search for alternative TCO materials to replace ITO. Currently, ZnO:Al and ZnO:Ga (AZO and GZO) semiconductors could become good alternatives to ITO for thin-film transparent electrode applications. However, development of large area deposition techniques are still needed to enable the production of AZO and GZO films on large area substrates with a high deposition rate.
Hybrid interfaces - catalysis, photocatalysis: A new emerging field is the combination of organic molecules and inorganic semiconductors, the so called hybrid materials. Attachment of organic dyes, biomolecules and polymers to semiconductor oxide surfaces and nanostructures will enable the production of hybrid systems with novel properties, such as biosensors with high selectivity and sensitivity, solar cells and light-emitting diodes. Nanostructured ZnO offer several advantages due to its high surface-to-volume ratio. Since the interface between the organic material and the semiconductor surface is rather complex, the chemistry of the interface is difficult to determine. At present, it is an undeniable fact that the efficiency of DSSCs based on ZnO is lower than that of DSSCs based on TiO2 . Nevertheless, considerable interest is focused on ZnO-based solar cells, due to significantly higher electron mobility and greater flexibility in comparison with TiO2. Better electron transport can in principle result in more efficient electron collection. There are for example many open issues regarding which molecules better work for surface functionalization of ZnO, what is their conformation and how their conformation affects the device performance. An important issue is the fact that experimental results indicate that the functionalization of semiconductor surfaces may be quite susceptible to experimental conditions, which led to a limited understanding of the interface chemistry. Due to the complexity of the systems, it is not always straight forward to obtain all aspects of semiconductor-organic interface by experimental characterization. In this context, it is primordial to bring theorists and experimentalists to discuss and identify possible strategies for successful functionalization of ZnO surfaces and nanostructures.
Finally, photocatalytic oxidation of organic compounds using semiconductor materials is an alternative to conventional methods for the removal of organic pollutants from water . The uses of ZnO as a photocatalytic degradation material for environmental pollutants has also been extensively studied, because of its nontoxic nature, low cost, and high photochemical reactivity. However, for higher photocatalytic efficiency and many practical applications, it is desirable that ZnO photocatalyst should absorb not only UV but also visible light. In order to absorb visible light, the band gap of ZnO has to be tuned, which can be achieved by doping with transition metal ions or other light elements such as nitrogen. The ZnO with different oxygen defects exhibited excellent activity toward the degradation of several pollutants . Again the role of surface defects is still an issue for further improvement of ZnO catalysts. It is therefore essential to promote collaborative work which aims to elucidate important aspects of the organic-inorganic interfaces, such as binding modes, role of defects, band gap engineering, optical properties, allowing opimization of materials for real applications.
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