quasi - one dimensional metal oxide semiconductors

Quasi-one dimensional metal oxide semiconductors: Preparation, characterization and application as chemical sensors E. Comini, C. Baratto, G. Faglia, M. Ferroni, A. Vomiero, G. Sberveglieri * SENSOR Lab, CNR-INFM, Dipartimento di Chimica e Fisica per l’Ingegneria e per i Materiali, Brescia University, via Valotti 9, 25133 Brescia, Italy article info Article history: Accepted 16 June 2008 abstract The continuous evolution of nanotechnology in these years led to the production of quasi-one dimensional (Q1D) structures in a variety of morphologies such as nanowires, core–shell nanowires, nanotubes, nanobelts, hierarchical structures, nanorods, nanorings. In particular, metal oxides (MOX) are attracting an increasing interest for both fundamental and applied science. MOX Q1D are crystalline structures with well-defined chemical composition, surface terminations, free from dislocation and other extended defects. In addition, nanowires may exhibit physical properties which are significantly different from their coarse-grained poly- crystalline counterpart because of their nanosized dimensions. Surface effects dominate due to the increase of their specific sur- face, which leads to the enhancement of the surface related prop- erties, such as catalytic activity or surface adsorption: key properties for superior chemical sensors production. High degree of crystallinity and atomic sharp terminations make nanowires very promising for the development of a new genera- tion of gas sensors reducing instabilities, typical in polycrystalline systems, associated with grain coalescence and drift in electrical properties. These sensitive nanocrystals may be used as resistors, and in FET based or optical based gas sensors. This article presents an up-to-date review of Q1D metal oxide materials research for gas sensors application, due to the great research effort in the field it could not cover all the interesting works 0079-6425/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.pmatsci.2008.06.003 * Corresponding author. Tel.: +39 030 3715771; fax: +39 030 2091271. E-mail address: sbervegl@sensor.ing.unibs.it (G. Sberveglieri). URL: http://sensor.ing.unibs.it (G. Sberveglieri). Progress in Materials Science 54 (2009) 1–67 Contents lists available at ScienceDirect Progress in Materials Science journal homepage: www.elsevier.com/locate/pmatsci reported, theonesthat,accordingto the authors,are goingto contrib- ute to this field’s further development were selected and described. Ó 2008 Elsevier Ltd. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . 2 2. Deposition techniques and growth mechanisms . . . 3 2.1. Vapor phase growth . . . . . . . . . . . 4 2.1.1. Vapor–liquid–solid mechanism . . . . . . . . . 5 2.1.2. Vapor–solid mechanism. . . . . . . . . . . . . . . 9 2.2. Solution phase growth . . . . . . . . . 10 2.2.1. Template-assisted synthesis. . . . . . . . . . . . 11 2.2.2. Template-free methods . . . . . . . . . . . . . . . 12 3. Vertical and horizontal alignment techniques . . . . . 13 3.1. Electric field alignment . . . . . . . . . 18 3.2. Nanomanipulation. . . . . . . . . . . . . 21 4. Doping of quasi 1D metal oxide nanostructures . . . 22 5. Preparation of quasi 1D metal oxide heterostructures . . . . . . . . . . . . . . . . . 24 6. Applications of metal oxide nanostructures . . . . . . . 30 6.1. Metal oxide gas sensors . . . . . . . . 30 6.1.1. Surface adsorption 30 6.1.2. Detection through surface reactions . . . . . 31 6.1.3. DC resistance transduction . . . . . . . . . . . . 31 6.1.4. Conductometric gas sensors. . . . . . . . . . . . 31 6.1.5. Single nanowire transistor (SNT) based gas sensors . . . . . . . . . . . 45 6.1.6. PL based gas sensors . . . . . . . . . . . . . . . . . 50 6.2. Other application fields. . . . . . . . . 56 6.2.1. Lasers. . . . . . . . . . . 56 6.2.2. Solar cells . . . . . . . 57 6.2.3. Field emitters . . . . 57 6.2.4. Li-ion batteries . . . 59 6.2.5. Single nanowire transistors for biosensing 59 7. Conclusions. . . . . . . . . . . . . . 59 Acknowledgements . . . . . . . 60 References . . . . . . . . . . . . . . 60 1. Introduction The increasing concerns with pollution on health and safety stress the need of monitoring all as- pects of the environment in real time, and in turn led to a tremendous effort in terms of research and funding for the development of sensors devoted to several applications [1–9]. As far as chemical sensing is concerned, it has been known, from more than five decades, that the electrical conductivity of metal oxides semiconductors varies with the composition of the surrounding gas atmosphere. The sensing properties of semiconductor metal oxides in form of thin or thick films other than SnO 2 , like TiO 2 ,WO 3 , ZnO, Fe 2 O 3 and In 2 O 3 , have been studied as well as the benefits from the addition of noble metals – Pd, Pt, Au, Ag – in improving selectivity and stability. In 1991 Yamazoe showed that reduction of crystallite size went along with a significant increase in sensor performance [10]. In a nanosized grain metal oxide almost all the carriers are trapped in surface states and only a few thermal activated carriers are available for conduction. In this configuration the transition from activated to strongly not activated carrier density, produced by target gases species, has a huge effect on sensor conductance. Thus, the technological challenge moved to the fabrication of materials with small crystallize size which maintained their stability over long-term operation at high temperature. A huge variety of devices have been developed mainly by an empirical approach 2 E. Comini et al. / Progress in Materials Science 54 (2009) 1–67 and a lot of basic theoretical research and spectroscopy studies have been carried out to improve the well known ‘‘3S” of a gas sensor, namely sensitivity, selectivity and stability. Nanotechnology is nowadays producing sensing materials such as quasi-1D metal oxides (MOX), carbon nanotubes, and nano porous materials. In particular, metal oxides are an attractive and heter- ogeneous class of active materials covering the entire range from metals to semiconductors and insu- lators and almost all aspects of material science and physics in areas including superconductivity and magnetism. After the first publications demonstrating the ability of metal oxide nanowires in detecting a vari- ety of chemical species [179,188], the interest in this research area was growing exponentially in the past years as testified by literature. Significant progress has been made both in terms of our fundamental understanding of the inter- play between bulk and surface properties and processes in MOX nanowires sensors together with their development as real world sensing platforms. Q1D metal oxide nanostructures have several advantages with respect to traditional thin- and thick film sensors such as very large surface-to-volume ratio, dimensions comparable to the extension of surface charge region, superior stability owing to the high crystallinity [11], relatively simple prepa- ration methods that allow large-scale production [14], possible functionalization of their surface with a target-specific receptor species [190], modulation of their operating temperature to select the proper gas semiconductor reactions, catalyst deposition over the surface for promotion or inhibition of spe- cific reactions and finally the possibility of field-effect transistors (FET) configuration that allows the use of gate potential controlling the sensitivity and selectivity [188]. Preparation and performances of these emerging nanosized structures have been reviewed by a number of authors [12–15], but this research field is growing so fast that there is still the need of a review focused on sensing applications. This review article is focused on the description of metal oxide single crystalline Q1D nanostruc- tures used for gas-sensing application, specifically on the promising approaches that are going to con- tribute to the further development of this field. The overview will start from presenting the fabrication techniques and the growth mechanisms, focusing on their development and improvements, and pointing out the steps critical for application in real environments. Then the application as chemical sensors will be addressed. Furthermore an outlook on other possible new applications of metal oxide single crystalline nanowires will be presented. 2. Deposition techniques and growth mechanisms Nanocrystalline materials can be classified into different categories depending on the number of dimensions that are nanostructured (with dimensions lower than 100 nm); we will follow one of the possible classification: i.e. zero dimensional for clusters, mono dimensional for nanowires and two dimensional for films. There are two different approaches to the production of 1D structures: top-down and bottom up technologies. The first one is based on standard micro fabrication methods with deposition, etching and ion beam milling on planar substrates in order to reduce the lateral dimensions of the films to the nanometer size. Electron beam, focused ion beam, X-ray lithography, nano-imprinting and scanning probe microscopy techniques can be used for the selective removal processes. The advantages are the use of the well developed technology of semiconductor industry and the ability to work on planar sur- faces, while disadvantages are their extremely elevated costs and preparation times. In the top-down approach highly ordered nanowires can be obtained [16–19], but at the moment this technology does not fulfil the industrial requirements for the production of low cost and large numbers of devices. Furthermore the 1D nanostructures produced with these techniques are in gen- eral not single-crystalline. The second approach, bottom-up, consists of the assembly of molecular building blocks or chemical synthesis by vapor phase transport, electrochemical deposition, solution-based techniques or tem- plate growth. Its advantages are the high purity of the nanocrystalline materials produced, their small E. Comini et al. / Progress in Materials Science 54 (2009) 1–67 3 diameters, the low cost of the experimental set ups together with the possibility to easily vary the intentional doping and the possible formation of junctions. The main disadvantage regards their inte- gration on planar substrates for the exploitation of their useful properties, for example transfer and contacting on transducers can be troublesome. The bottom-up approach allows low cost fabrication although it could be very difficult to get them well arranged and patterned [20]. Furthermore more control and insight into the growth process must be achieved for their fruitful integration in functional devices. The most promising approach to produce functional nanowires will be the combination of the two preparation technologies. This review article will be focused on the bottom-up techniques for the preparation of 1D single- crystal nanostructures. Numerous one-dimensional oxide nanostructures with useful properties, compositions, and mor- phologies have recently been fabricated using bottom-up synthetic routes. Some of these structures could not have been created easily or economically using top-down technologies. A nomenclature for these peculiar structures has not been well established. In the literature a lot of different names have been used, like whiskers, fibers, fibrils, nanotubules, nanocable, etc. The defini- tion of these 1D nanostructures is not well established. A few classes of these new nanostructures with potential as sensing devices are summarized schematically in Fig. 1. The geometrical shapes can be tubes, cages, cylindrical wires, rods, nails, cables, belts, sheets and even more complex morphologies. When developing 1D nanocrystals the most important requirements are dimensions and morphol- ogy control, uniformity and crystalline properties. In order to obtain one-dimensional structures a preferential growth direction with a faster growth rate must exists. Achieving 1D growth in systems with a isotropic atomic bonding requires a break in the symmetry during the growth and not just stop- ping the growth process at an early stage (0 and 2D). In the past years the number of synthesis techniques has grown exponentially. We can divide these growth mechanisms in different categories, first of all catalyst-free and catalyst assisted procedures and then we can distinguish between vapor and solution phase growth. As far as metal oxides are con- cerned the most used procedure is the vapor phase one. But solution phase growth techniques provide a more flexible synthesis process with even lower production costs. There are different growth mechanism depending on the presence of a catalyst, i.e. vapor–liquid– solid (VLS), solution–liquid–solid (SLS) or vapor–solid (VS) process. 2.1. Vapor phase growth The vapor phase approach was used in the early 60’ for the preparation of micrometer-size whis- kers. These whiskers were prepared either by simple physical sublimation of the source material or Fig. 1. Schematic drawing of some of the possible morphologies: (a) nanowire, (b) core–shell nanowire, (c) nanotube, (d) nanobelt, (e) hierarchical structure, (f) nanorod and (g) nanoring. 4 E. Comini et al. / Progress in Materials Science 54 (2009) 1–67 through reduction of a volatile metal halide. In the last years this method was used to prepare differ- ent materials in form of nanowires. The growth was performed in tubular furnace studied to obtain the proper temperature gradient. The source material once evaporated is transported by a gas carrier towards the growth site where it nucleates. The nucleation can start from particles or catalyst, follow- ing the VS, VLS mechanisms. 2.1.1. Vapor–liquid–solid mechanism The controlled catalytic growth of whiskers, and more recently nanowires, was discovered by Wag- ner and Ellis in 1964 [21], they found that Si whiskers could be grown by heating a Si substrate cov- ered with Au particles in a mixture of SiCl 4 and H 2 and their diameters was determined by the size of Au particles. Wagner and Ellis named the VLS mechanism for the three phases involved: the vapor- phase precursor, the liquid catalyst droplet, and the solid crystalline product (Fig. 5). VLS in the last decades was one of the most important methods for preparing 1D structures, it is promising as a scalable, economical and controllable growth of different materials (oxide, semicon- ductors, ). Understanding the growth dynamics is important to have a greater control in the nano- wires shape, diameter and for a selective growth. In general the presence of a metal particle, of size comparable to the nanowire, at its apex leads to the conclusion that the growth mechanism followed the vapor–liquid–solid (VLS) process, but this does not determine the phase of the catalyst during growth. In most of the catalytic growths, nanowires have uniform diameters. The section can be rounded or polygonal with atomically sharp lateral terminations. The growth process takes some dead time, a starting period before the real growth begins, this was experimented also for vapor phase processes [22]. The catalytic particles can be formed by vapor phase and/or surface diffusion transport or be depos- ited from the evaporation of a colloidal solution or by deposition of a thin film onto the substrate. If the metal does not wet the substrate, it will form clusters as the result of Volmer–Weber growth [23] or when the substrate is kept at the high temperatures required for the growth process, the onset of Ostwald ripening [24] will lead to a distribution of cluster sizes. In some cases the catalyst clusters that initiate the NWs growth can also be formed at the initial deposition step; for example when car- bothermal reduction is used to generate a volatile metal that is transported from a carrier gas and then condense on the substrate. Sometimes the catalyst may undergo other processes before becoming active for the growth of nanowires after its formation or deposition. A mixture of the growth compound and the metal might be more active for the NW formation than the pure metal catalyst, and may be required to form an alloy, a true eutectic or some solid/liquid solution. In this case, saturation of the catalytic particle with the growth material or the formation of the proper composition may explain the dead time period be- fore growth. The incorporation of a significant amount of growth material into the catalytic particle is expected to change the volume and, in turn, the diameter of the catalyst from its initial value with a change in the NW section. Consequently Ostwald ripening and incorporation of growth material con- tribute in changing the size of the catalytic particles. A constant section NWs growth may correspond to a condensation on the catalyst surface and dif- fusion and segregation at the interface between catalyst and nanowire. When the condensation and incorporation is occurring only on the catalyst and not onto the NW sides, a constant catalyst section results in a constant nanowire section. The dimensions of the catalyst clusters can determine the NW section either by direct matching of the size or by mechanism involving the catalyst curvature in which strain and lattice matching are important. The NW section will decrease and eventually the growth process will end if the catalyst is consumed or evaporates during the growth, or when the material is no longer supplied, or if the temperature is reduced below a critical value necessary for the growth process. Temperature is a key factor in determining processes such as dissociative adsorption, surface diffusion, bulk diffusion through the catalyst, solubility and thermodynamic stability of certain phases. The catalyst cluster can offer a higher sticking coefficient, but the difference in sticking coefficients alone cannot account for the NWs growth process. Further considerations must be performed to ex- plain the preferential incorporation at the interface between nanowire and catalyst. For example E. Comini et al. / Progress in Materials Science 54 (2009) 1–67 5 the catalytic particle can lower the energy barrier for the incorporation of new material at the growth interface compared to the one needed for nucleation of an island on a sidewall or on the substrate. Adsorption occurs from the fluid (gaseous, liquid or supercritical) phase, it can be molecular or dis- sociative and may occur on nanowire, catalyst, or substrate. The catalyst can activate the growth with a sticking coefficient higher on its surface and vanishing elsewhere. After the adsorption there is the adatoms diffusion onto or into the catalyst, across the substrate, or on the NWs lateral sides. In order to have the unidirectional growth, the last two processes must be rapid and avoid secondary nucleation. The nanowires can grow from the top or the bottom of the catalyst cluster and as reported in Fig. 2, a catalyst cluster can give rise to single or multiple nanowires growth. The catalyst can be found at the bottom or top of the nanowire. In single NW growth there is a one-to-one correspondence between catalyst and nanowires. In single wire growth control over the nanowire diameter should be obtained controlling the catalyst radius. While in multiple nanowires growth the section must be related to other factors such as the curvature of the growth interface and lattice matching between the catalytic particle and the nanowire. Regardless of the phase of the catalyst, the major requirement is the mobility of the growth mate- rial that can allow reaching the growth interface with a low probability of nucleation in sites other than the nanowire–catalyst interface. The growth activation energy can be related to activated adsorption or with surface or bulk diffu- sion. The essential role of the catalyst appears to be lowering the activation energy of nucleation at the interface. There is a substantial barrier associated with the formation of the critical nucleation cluster at a random position on the substrate or nanowire according to classical nucleation theory. If the cat- alyst can lower the nucleation barrier at the particle/nanowire interface, then growth may only occur Fig. 2. The processes that occur during catalytic growth. (a) In root growth, the particle stays at the bottom of the nanowire. (b) In float growth, the particle remains at the top of the nanowire. (c) In multiple prong growth, more than one nanowire grows from one particle and the nanowires must necessarily have a smaller radius than the particle. (d) In single-prong growth, one nanowire corresponds to one particle. One of the surest signs of this mode is that the particle and nanowire have very similar radii. Reprinted from Ref. [22]. License number 1905961408030. 6 E. Comini et al. / Progress in Materials Science 54 (2009) 1–67 on the catalyst. The most important role of the catalyst particle is to ensure that the material is pref- erentially incorporated at the growth interface. Understanding the dynamics of VLS nanowires growth is essential in order to relate the properties of the wire to their processing conditions. A theory for VLS growth has been presented in reference [25] incorporating the surface energy of the solid–liquid, liquid–vapor, and solid–vapor interfaces. The catalyst concentration profile in the droplet, the degree of supersaturation, and the modification to the shape of the solid–liquid interface were predicted as functions of the material properties and process parameters. The calculated growth rate found has the same dependence on diameter as the flux of growth material at the liquid–vapor interface; thus, for radius independent flux, growth rate results also radius independent. It is often found that, the growth rate should decrease with decreasing diameter [26,27]. The effects of size on the growth kinetics of nanowires by the vapor–liquid–solid mechanism were addressed from the theoretical point of view in [27]. The dependences of the growth rate and the acti- vation energy of crystallization on size were given quantitatively. The obtained theoretical results showed that the smaller the nanowire radius, the slower the growth rate, and the activation energy of crystallization increases with decreasing radius of the nanowire. These theoretical predictions are in agreement with the experimental cases. However, this conclusion depends on the growth condi- tions [28] since the extent of supersaturation within the catalyst depends on the temperature and gas-phase composition. Transitions from smaller diameters having lower growth rates to smaller diameter having higher growth rates can occur as temperature and gas-phase composition are changed. Although it is commonly believed that in the VLS process, the size of the catalyst particles deter- mines the NWs width, this is not true for all deposition conditions. Experimental studies on ZnO NWs growth on Al 0.5 Ga 0.5 N substrate confirm that this rule only applies when the catalyst particles are reasonably small (<40 nm) [29]. A linear relationship between the density of the nanowires and the thickness of the catalyst layer was found, therefore catalyst thickness control could be a very simple and effective way to achieve density control of aligned nanowires over a large surface area. To reveal why the density varies, but the width remains constant, the wetting behavior of a gold layer on the substrate was investigated when heated to the growth temperature. The results classified the growth processes into three categories: separated dots initiated growth, continuous layer initiated growth, and scattered particle initiated growth. Because of the wetting sit- uation between the melted catalyst droplet and the substrate, more energy favorable sites were cre- ated for nanowire growth with thinner catalyst layers. Moreover, when the catalyst layer was sufficiently thick, a continuous ZnO network would be deposited simultaneously at the bottom of the nanowires (Fig. 3). Another important process controlling the cluster dimensions, that is in general forgotten, is the thermodynamic limit for the minimum radius of the metal liquid clusters at high temperature r min ¼ 2r LV V L =RT ln s where r LV is the liquid vapor surface free energy, V L is the molar volume of liquid, and s is the vapor phase supersaturation. Furthermore as well as the equilibrium vapor pressure of a solid surface also the solubility depends on the surface curvature. As the size are reduced the solubility increases, as a result higher supersat- uration in the vapor phase has to be created. Higher supersaturation may promote lateral growth on the NWs side or homogeneous nucleation in the gas phase. A procedure for controlling the radial and axial dimensions of SnO 2 NWs has been presented in [30,31] by combining VLS approach with molecule-based chemical vapor deposition. The synthesis was based on the decomposition of discrete molecular species, which allows growing nanowires at low temperatures with a precise control over their diameter and length. The precursor chemistry was chosen to facilitate the stripping of organic ligands and to achieve complete decomposition that is critical for maintaining the gas phase supersaturation necessary for 1D growth. Axial and radial dimensions of the NWs were varied by adjusting the precursor feedstock, deposition temperature, and catalyst size. E. Comini et al. / Progress in Materials Science 54 (2009) 1–67 7 Despite the success of all these growth procedures, there have been just few comparative studies on catalysts and substrates influence. Such studies are valuable because both the catalyst and substrate play important roles in NWs structure and properties. In reference [32], a case study of ZnO nanowire growth was performed; four different catalysts and substrates of different materials, structure, and crystal ori- entation were investigated. It was found that the growth depends on the choice of surface catalysts, e.g. for the Fe catalysts, the growth of ZnO nanowires may occur via a vapor–solid process, while, for the case of Au, Ag, and Ni catalysts, the vapor–liquid–solid process usually dominates the wire growth. Further- more differences in growth were also closely related to the differences in materials properties of these wires, including the degree of nanowire alignment on substrates and the atomic composition ratio of Zn/ O, as well as the relative intensity of the oxygen vacancy-related emission in PL spectra. The use of different catalysts provides the versatility of growth for one-dimensional ZnO nano- structures with different ranges of parameters such as diameters, areal densities, and aspect ratios. Fig. 3. (a) Variation of density (left-hand vertical axis) and width (right-hand vertical axis) of the aligned ZnO nanowires with the thickness of gold catalyst layer. Inset: Top-view SEM image of the aligned ZnO nanowires used for density calculation, the scale bar represents 200 nm. (b, c) TEM images of ZnO nanowires catalyzed by 1 and 8 nm gold layers, respectively. Inset: Selected area electron diffraction pattern recorded from a nanowire indicated by the circle in image c. catalyzed by the corresponding gold layers. Reprinted with permission from [29]. Ó American Chemical Society 2006 8 E. Comini et al. / Progress in Materials Science 54 (2009) 1–67 This works suggested that, compared to noble-metal catalysts, growth using transition metal catalysts occurs at a relatively faster rate and therefore typically yields thicker wires with higher aspect ratio. However, the NWs have more oxygen vacancies affecting other properties, such as electrical transport and surface chemistry. Few in situ studies were performed especially regarding the Si NWs growth. Si nanowires growth by the vapor–liquid–solid mechanism was monitored using real time in situ ultra high vacuum trans- mission electron microscopy [33]. A growth rate independent of wire diameter was found. Showing that the irreversible, kinetically limited, dissociative adsorption of disilane directly on the catalyst surface was the unique rate-limit- ing step (Fig. 4). The growth rates were independent of wire diameter, and increased linearly with pressure. From the growth rate measurements, the reactive sticking probabilities of Si 2 H 6 at the droplet surface and at the wire sidewall was determined. A novel dependence of growth rate on wire taper, which was attributed to the deposition of excess Si from the shrinking droplets, was observed. Many open questions still remain regarding the different experimental evidences on VLS growth, top or bottom catalyst cluster, single or multiple nanowires growth, the relation between the catalyst and nanowire size, the possibility of an auto-catalyzed growth. But, regardless of the catalyst phase (either liquid or solid) the growth dynamics does not change and catalytic growth still appears to be the most powerful method for producing 1D nanostructures. 2.1.2. Vapor–solid mechanism The VS growth takes place when the nanowire crystallization originates from the direct condensa- tion from the vapor phase without the use of a catalyser. At the beginnings the growth was attributed to the presence of lattice defects, but when defects-free nanowires were observed this explanation Fig. 4. (a) Representative bright field TEM images of a Si wire acquired at four successive times during deposition. White arrows highlight a reference point on the wire sidewall. (b) Length L (open squares) and diameter d (solid circles) of the same wire as a function of t. The straight line is a least-squares fit to the first 1200 s. Reprinted Fig. 2 with permission from [33]. http:// link.aps.org/abstract/PRL/v96/e096105. Ó American Physical Society 2006 E. Comini et al. / Progress in Materials Science 54 (2009) 1–67 9 cannot be any longer accepted. Another peculiar effect registered was a nanowire growth rate higher than the calculated condensation rate from the vapor phase. A possible interpretation is that all the faces of the nanowire adsorb the molecules that afterwards diffuse on the principal growth surface of the wire. VS process occurs in many catalyst-free growth processes. Quite a few experimental and theoret- ical works have proposed that the minimization of surface free energy primarily governs the VS pro- cess. Under high temperature condition, source materials are vaporized and then directly condensed on the substrate placed in the low temperature region. Once the condensation process happens, the initially condensed molecules form seed crystals serving as the nucleation sites. As a result, they facil- itate directional growth to minimize the surface energy. This self-catalytic growth associated with many thermodynamic parameters is a rather compli- cated process that needs quantitative modelling [34]. It was reported for indium oxide, In 2 O 3 wires were synthesized through thermal evaporation of pure In 2 O 3 powders and the effect of substrate seed- ing was studied for controlling density distribution and lateral dimensions of the wires. The wires ex- hibit uniform section, atomically sharp lateral facets, and pyramidal termination, typical of a VS growth mechanism assisted by oxidized nanocrystalline seeds. Other growth conditions have been reported, for example ZnO NWs were synthesized by a VP pro- cess using a thin film (10 nm) of tin as catalyst. Carbothermal reduction was used to reduce the source temperature needed for the vapor phase production. The tip of the NWs resulted without the catalyst and was attributed to VS process [35]. Many report also the NWs production by simple oxidation of the metal composing the metal oxide [36], for example [37] report the growth of CuO NWs from copper foils oxidized in wet air at temper- atures between 300 and 800 °C. Within the temperature range of 400–700 °C, the nanowires formed have two different morphologies, curved and straight, with diameters between 50 and 400 nm and lengths between 1 and 15 l m. The growth behavior was explained in terms of kinetics involving short-circuit diffusion, the strength of the nanowires, and the thickness ratio of the oxide scale and the metal. The formation kinetic of CuO nanowires was governed by the short-circuit diffusion of atoms or ions during the reaction. The deformation of thin oxide scale under thermal stresses may also contribute to the formation of curved nanowires. The vapor–solid VS growth was attributed also for a two-step high-temperature, catalyst-free, physical evaporation of tungsten oxide NWs [38]. The procedure consisted in heating tungsten pow- der, during the heating an oxide layer might be formed on the metal surface; the oxide then can evap- orate and redeposit on the substrate surface, forming one dimensional nanostructures. Alternatively, the tungsten metal was vaporized first; a subsequent oxidation during the deposition on the substrate may also form the nanostructure. 2.2. Solution phase growth Growth of nanowires, nanorods and nanoneedles in solution phase has been successfully achieved. These growth methods usually require ambient temperature so that complexity and cost of fabrication are considerably reduced. To develop strategies that can guide and confine the growth direction to form Q1D nanostructures, researchers have used a number of approaches that may be grouped into template-assisted and template-free methods. The solution-based catalyzed-growth mechanism is similar to the previously described VLS mechanism, in this case a nanometer-scale metallic droplet catalyze the precursors decomposition and crystalline nanowire growth. The variants of VLS growth in solutions SLS and supercritical fluid–liquid–solid (SFLS) growths provide nanowire solubility, control over surface ligation, and smaller diameters. But the VLS growth in general produce nanowires of the best crystalline quality. There exist strong indications that the catalyst droplets in the SLS, as well as in VLS, mechanisms play a catalytic role in precursor decomposition, in addition to catalyze the NWs growth (Fig. 6). The early VLS literature claimed such a role on the basis of various experimental observations [21,39,40], including that VLS crystal growth typically occurs at temperatures several hundreds of degrees lower than epitaxial film growth from the same precursors. 10 E. Comini et al. / Progress in Materials Science 54 (2009) 1–67 [...]... controlling the morphologies and sizes of the secondary-grown 1D ZnO nanostructures, suggesting that the nano-heterostructures of these nanostructures grown on SnO2 nanobelts may have potential applications in nano-optoelectronic devices 6 Applications of metal oxide nanostructures 6.1 Metal oxide gas sensors Metal oxides semiconductors are normally high gap metal oxides in which the semiconducting behavior arises... activated surface diffusion (see Fig 25) One further process for obtaining heterostructures starting from a single material is the sequential oxidation of metal nanowires Arrays of metal metal oxide core–shell nanowires and single-crystalline metal oxide nanotubes have been obtained [155] The process is based on the kinetic control of the conversion of single-crystalline Bi nanowires to Bi–Bi2O3 core–shell... characterization of metal oxide nanowires has been performed via two-contacts or four-contacts methods, where the conductive paths are provided by either patterned structures [112], piezo-actuated probes, or metallic deposits obtained by focused ion beam [131] These approaches allow the measurement of physical quantities such as free-electron concentration, electrical mobility, and conductivity in single tin oxide. .. superlattices in a single-crystalline nanowire have been obtained (Fig 23) Dendritic nanowires growth mediated by a self-assembled catalyst was also exploited for production of InAs structures [152] Typically, heterostructures of metal oxides are more difficult to be synthesized with respect to III–V semiconductors, and in fact the block-by-block mechanism was never reported for oxides E Comini et al... Similar technique was applied for creation of indium tin oxide (ITO) nanowires coated with TiO2 [161] Catalyst-free transport-and-condensation method was applied for nanowires growth, using a E Comini et al / Progress in Materials Science 54 (2009) 1–67 27 Ó American Chemical Society 2006 Fig 25 Zinc oxide nanowalls and nanowires (A) SEM image of quasi- 3D ZnO nanostructures grown on a sapphire using $4... SnO2–TiO2 system [164] A non-equilibrium synthesis technique has been developed to produce novel transition metal oxide single-crystal nanowires, including YBa2Cu3O6.66, La0.67Ca0.33MnO3, PbZr0.58Ti0.42O3, and Fe3O4 [156] Vertically aligned single-crystalline MgO nanowires, grown via VLS method, were applied as templates for epitaxial deposition of the desired transition metal oxides using pulsed laser... of several nanostructures [133–135] 4 Doping of quasi 1D metal oxide nanostructures Doping of nanowires is pursued to the controlled modification of the characteristics of the nanowires, in terms of morphological features as well as electrical or optical properties Differently from heterogeneous systems formed by metallic catalytic particles and metal- oxide nanowires, the introduction of dopants assumes... nanostructrures of metal oxide classified by sensing oxide Type of metal oxide Gases tested Multiple or single nanowire Wire cross section diametera (nm) Working temperature (°C)b References SnO2 SnO2 SnO2 SnO2 SnO2 SnO2 SnO2 SnO2–Pt/Ni SnO2:Sb ZnO ZnO ZnO Ethanol, CO, NO2 Ozone Ethanol CO, O2 CO, relative humidity NH3 e CO NO2 H2, CO Ethanol Ethanol,H2 Ethanol Ethanol, methane, 2butanone, triethylamine,... SAED pattern showing the cubic single-crystal arrangement for the indium oxide nanowire (2) CBED pattern and high-resolution image from the tin oxide nanowire, demonstrating its single-crystalline tetragonal (cassiterite) arrangement (3,4) highly magnified TEM view and corresponding SAED pattern of the heterojunction, where superimposition of both indium and tin oxides has been recorded Reprinted from... and three-dimensionally branched ITO nanowire arrays with a controlled crystallographic growth direction has been exploited [97] Yttrium-stabilized zirconia (YSZ) has been used as the substrate because YSZ, with a fluorite crystal Fig 7 FE-SEM micrographs of vertical In2O3 nanowire arrays on single crystalline optical a-sapphire substrates (a) A 45° perspective view of an array of nanowires on a-sapphire . Quasi-one dimensional metal oxide semiconductors: Preparation, characterization and application. 1D metal oxide nanostructures . . . 22 5. Preparation of quasi 1D metal oxide heterostructures . . . . . . . . . . . . . . . . . 24 6. Applications of metal