Synthesis and applications of one-dimensional semiconductors
Introduction
Semiconductors are widely used in electronic, catalytic, photonic and energy related applications. In recent years, ongoing miniaturisation of electronic circuits led to an emerging interest in nanoscaled materials. In addition, inorganic structures confined in several dimensions within the nanometre range, exhibit peculiar and unique properties superior to their bulk counterparts. These unique properties can be attributed to the limited motion of electrons in the confined dimensions of the nanomaterial (quantum effects) [1].
The interest in utilising the unique properties of nanostructures for practical applications increases with the deeper understanding and tailoring of these materials. To date, a huge variety of materials have been synthesised and incorporated in devices demonstrating their potential to overtop the performance of currently used technology. The material classes of inorganic 1D structures include metallic elements [2], [3], metal nitrides [4], [5], oxides [6], [7], carbides [8], [9], and sulphides [10], [11]. However the transition from fundamental science to industrial application requires an even deeper understanding and control of morphology and composition at the nanoscale. Size reduction of well known materials into the nanometre regime or the realisation of novel nanostructures can improve device performance and lead to novel discoveries. For instance, in microelectronics the increased number of transistors per area of a silicon chip, has led to faster operation and lower power consumption [12]. Size-dependent physical properties observed in 1D nanomaterials have included photon absorption and emission, such as nanoscale avalanche photodiodes [13], metal-to-insulator transition in a material [14] and quantised or ballistic transport characteristics [15]. However literature reports describe divergent behaviour of some intrinsic material properties, such as the elastical modulus [16], [17], [18], for nanosized materials ranging from diminishing to increased values with shrinking radial dimensions, which implies that reliable instrumentation has to be established to gain precise insight in the effects present in the nanometre regime. Besides the opportunity to investigate and evaluate novel physical properties of 1D materials, the controlled fabrication of high quality nanowires and their growth mechanisms has attracted tremendous attention. Though the number of reports describing the growth of novel 1D structures has increased rapidly over the last 10 years, the fundamental understanding of their formation is still limited. In addition, the integration of high aspect-ratio nanostructures into devices requires ongoing efforts in both engineering and materials science to control the processes on the atomic scale [19].
In this article, state-of-the-art strategies for engineering one-dimensional functional semiconductors, which can be used for photonic, sensor and energy applications, are reviewed. Viable growth strategies and mechanisms are outlined in this review and the synthesis of Si, Ge, GaN, GaAs, CdS, ZnO, and SnO2 is discussed. For controlled use of individual or bundles of defined numbers of nanostructures alignment techniques are a mandatory requirement. A description of the most effective in situ and post-alignment methods is therefore included in this review. In addition, we emphasise the applicability of nanowire-based devices for gas and biochemical detection, nanophotonics, energy harvesting, such as nanogenerators and solar cells, as well as components in Li ion batteries.
Section snippets
Synthetic approaches to 1D nanostructures
During the last decade several approaches for synthesising 1D nanostructures have been described in detail, however generic methodologies are limited. Herein we delineate rational strategies used to facilitate the formation of inorganic high aspect-ratio nanostructures. Fig. 1 illustrates different types of 1D nanostructures and the terms/abbreviations used to describe them.
Synthesis of Si, Ge, GaN, GaAs, CdS, ZnO and SnO2 1D nanostructures
Inorganic semiconductors spanning a large variety of material classes are known to act as active components in functional devices. We have summarised in this section reliable strategies (Table 1) for the synthesis of the most popular semiconductors for nanowire-based sensors, energy related applications and nanophotonics.
In situ alignment of 1D nanostructures
Most in situ alignment techniques are based on either templates, etching or epitaxial metal-promoted growth techniques. The template based technique is already described in Section 2.5 in detail. However hybrid approaches, such as the homo and hetero-epitaxial growth of Si nanowires, with unconventional growth directions, within AAO membranes on Si substrates has not been discussed and is achieved by Shimizu et al. [142], [350]. Etching techniques are a viable approach to produce horizontally
Nanowire sensors
The fundamental background of sensing is based on changes in the proximity of the active material, which leads to changes in the electrical or optical properties. In most cases the interaction between adsorbed (physi- or chemisorbed) species is responsible for these effects. The effective change of the local charge density can be detected by variations in the conductivity of the devices used to detect the species of interest. In the molecule–surface interaction the term ionosorption is used
Summary
This article reviews viable approaches for the synthesis of one-dimensional semiconductors and highlights the formation of high aspect-ratio semiconductors. The efforts to understand the fundamental underlying mechanisms for the controlled formation of high quality 1D nanostructures have been tremendous. However, addressing fundamental scientific questions will also lead to both expected and unexpected advances in this field of research.
The improved theoretical support already helped to
Acknowledgement
SB and JDH acknowledge financial support from Science Foundation Ireland (Grants 07/RFP/MASF710 and 08/CE/I14320). FHR thanks the European Community for funding (7th Framework Programme, Grant #247768).
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