Fabrication of nanowires of multicomponent oxides: Review of recent advances

Materials Science and Engineering C 25 (2005) 738 – 751 www.elsevier.com/locate/msec

Fabrication of nanowires of multicomponent oxides: Review of recent advances K. Shantha Shankar *, A.K. Raychaudhuri Department of Physics, Indian Institute of Science, Bangalore-56012, India Available online 26 October 2005

Abstract We review the state of the art in nanowire synthesis with special emphasis on multicomponent oxide nanowires and profile the latest advances. We emphasize the advantages of both template-aided and template-free chemical solution methods for the synthesis of functional oxide nanowires. We analyse some of the key issues facing the practical realization of nanowire-based products from the synthesis point of view and present potential solutions. The objective of our paper is to provide key facts that can bridge the gap between the Science and Technology of nanowires fabrication. D 2005 Published by Elsevier B.V. Keywords: Nanowires; Multicomponent oxides; Fabrication

1. Introduction Nanowires, nanorods, nanowhiskers, it does not matter what you call them, they are the hottest property in nanotechnology (Nature 419 (2002) 553). One-dimensional nanostructures that include wires, rods, belts and tubes have attracted rapidly gro wing interest due to their fascinating properties and unique applications. Nanowires, the focus of our review, are emerging as important building blocks serving as interconnects and active components in nanoscale electronic, magnetic and photonic devices. It is expected that the nanowire based quasi one-dimensional materials will be the focus of the next decade of nanomaterials research [1]. The recent achievements in the fabrication of nanowires and the demonstration of nanocircuits built using semiconductor nanowires are scientific breakthroughs fast maturing into technology marvels. Success in fine-tuning the properties of nanowires through rational design and intelligent synthesis methods has motivated researchers to envision radical innovations and enabled technologists to implement novel * Corresponding author. Tel.: +91 80 23608653; fax: +91 80 23602602. E-mail address: [email protected] (K.S. Shankar). 0928-4931/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.msec.2005.06.054

applications. There are numerous challenges associated with the synthesis of 1D nanostructures with well-controlled size, phase purity, crystallinity and chemical composition. The key to fabricating precisely designed nanostructures is to understand and thereby control the nucleation and growth processes at the nanoscale. We review the state of the art and profile the latest breakthroughs in the synthesis of nanowires of multicomponent materials. The focus of our review is on solution-based chemical processing methods and templatedirected synthesis of nanowires. This review is organized as follows: We begin with a discussion on the unique applications of 1D nanostructures and then proceed to elucidate the basic principles of fabrication. We present two case studies of chemical solution processing of single and multicomponent oxide nanowires of technologically important materials like zinc oxide and rare-earth manganite. The review concludes with a brief overview of happenings at the technology frontier of nanowires.

2. Unique applications of 1D nanostructures One-dimensional nanostructures are attractive candidates for nanoscience studies as well as nanotechnology applica-

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tions. The unique feature of nanowires, compared to other low dimensional systems, is that they have two quantum confined directions while still leaving one unconfined direction for electrical conduction. This allows nanowires to be used in applications where electrical conduction rather than tunneling transport is required. Because of the unique density of electronic states nanowires in the limit of small diameters are expected to exhibit significantly different optical, electrical and magnetic properties from their bulk 3D crystalline counterparts. The attractive properties of onedimensional systems arise from their unique chemistry and physics [2]. Recent demonstration of ballistic electron transport in many metallic nanowires [3], size controlled semimetal to semiconductor transition in bismuth nanowires [4], electrically controllable UV lasing from a single zinc oxide nanowire [5], size dependent excitation or emission for photoluminescence in semiconducting nanowires like those of InP [6], improved sensitivity and overall performance of FETs based on semiconducting nanowires [7], etc., have given new impetus to the research efforts on nanowires. Recently magnetic oxide nanowires are entering a new domain of highly sophisticated biomedical applications including targeted drug delivery, ultra-sensitive disease detection, gene therapy, genetic screening, rapid toxicity cleaning [8]. Multicomponent oxides are technologically important materials with proven applications in electronic, magnetic and photonic devices. However the progress in the growth of nanowires of muticomponent oxides is not proportionately significant. Keeping in mind, the multi-faceted functional properties of these materials that include electronic and ionic conductivity, superconductivity, ferroelectricity, piezoelectricity, optical non-linearity and magnetoresistance properties and the innumerable applications these nanowires could bring forth, it is worth reviewing the recent progress made towards this direction. We will discuss the bottlenecks and challenges involved in the growth of complex multicomponent oxides.

3. Introduction to synthesis of nanowires The synthesis of 1D nanostructures in general and nanowires in particular is all about constraining the growth of material in two directions to a few nanometers and allowing the growth in the third direction. The key to achieving 1D growth in materials, where atomic bonding is relatively isotropic, is to break the symmetry during the growth rather than simply arresting growth at an early stage. While it appears plausible for single component materials (elemental materials) the complexity of the task scales up proportionately in multicomponent materials as we need to achieve the desired stoichiometry within the nanodimensions. Attempts to break the growth symmetry either physically or chemically have been successful and resulted in the bulk synthesis of nanowires. The key idea behind all these attempts to direct chemical reaction and material growth in

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1D is the use of linear templates, including the edges of surface steps [9], nanofibers [10], and porous membranes [11] or sufactants. Different nanomanipulation techniques to obtain nanostructures and importantly nanowires are discussed in a topical review by Rao et al. [12]. An alternative strategy is to employ a Fcatalyst_ to confine the growth. These methods are named based on the phases involved in the reaction—vapor –liquid – solid (VLS) [13], solution – liquid – soild (SLS) [14], vapor – solid (VS) growth [15,16]. Xia et al. [17] have recently reviewed the synthesis, characterization and applications of one-dimensional nanostructures, their assembly and have addressed the key issues in utilizing nanomaterials in nanodevices. The exhaustive review on inorganic nanowires by Rao et al. [18] is a repository of valuable synthesis methods. Inspite of the persistent efforts by research groups across the globe, there is still a wide gap between the science and technology of nanowire synthesis of technologically important materials. To adopt successful nanowire synthesis methodologies to a manufacturing environment, we need to ensure that these methods are easy to scaleup and are cost-effective. The reactant and the byproducts of the nanowires synthesis should be environmentally benign. We should model the influence of each of the process parameters and develop methods of precisely controlling the process parameters during large scale synthesis. We should design a robust process of synthesizing nanowires that is built around the noise parameters. The noise parameters referred to here are process parameters that we do not have control over or it is too expensive to control. We have made great strides in the recent past in developing a rigorous understanding of the material chemistry at the nanoscale and modeling the physics of the system. Nanowire synthesis is all set to enter its second phase of growth that involves optimization of the methodologies for easy adoption in a manufacturing environment. Over the last few years there has been a tremendous progress in the growth of 1D nanostructures of metals, semiconductors and simple oxides. We are focusing on oxide nanowires as they are promising for nanoscale building blocks because of their interesting properties, diverse functionalities, surface cleanliness and chemical/ thermal stability. The earliest development in this field was the fabrication of oxide nanowires of MgO, Al2O3, ZnO, SnO2 by carbon-thermal reduction process [19]. Wang et al. later demonstrated that nanowires and nanobelts could be prepared by simple thermal evaporating commercial metal oxide powders at high temperatures [20]. Table 1 lists some of the important materials grown in the form of nanowires by different methods. The potential of these nanowires for application in gas sensors [21], chemical and biological sensors [22], micro lasers and displays [5] has been realized. Nanowire superlattices and pn junction within a single nanowire [23] have been demonstrated. Assembling nanowires into device architectures to yield nano FETs [7], light emitting diodes [24], bipolar junction transistors [25] and logic circuits [25] are quite promising.

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Table 1 Oxide nanowires applications and synthesis (this is not an exhaustive, but a representative list) Material

Method

Properties and applications

Reference

MgO

VS

[105]

Cu2O

Vapor phase Surfactant-assisted VLS CVD Laser ablation Vapor phase Molten phase Vapor phase Etching of AAO template Vapor phase Electrodeposition – oxidation Laser ablation Polyol Vapor phase Flux growth Electrodepositon – oxidation Microemulsion Hydrothermal Sonochemical Microemulsion

High melting point (2400 -C) and high heat capacity—functional composite as a reinforcement agents and pinning centers Direct bandgap semiconductor (2 eV)—conversion of optical, electrical and chemical energy Optical wave guiding

SiO2

Ga2O3 Al2O3 In2O3

SnO2

MnO2 Sb2O3 TiO2

V2O5 WOx ZnO

Sol – gel Anodic oxidative hydrolysis Cathodically induced sol – gel Electrophoretic Hydrothermal Polyol Sol – gel Surfactant-assisted Surfactant-assisted Electrodeposition – oxidation Microemulsion – hydrothermal Low temperature precipitation

BaTiO3 PZT

Sol – gel template based hydrothermal Sol – gel template-based

LiNiO2 LiMn2O4 LiCoO2 LiNi0.5Co0.5O2 La1
x Cax MnO3

Sol – gel template-based

La1
x Srx MnO3 La1
xBax MnO3

Hydrothermal Hydrothermal

Sol – gel template based

Wide band gap semiconductor (4.9 eV)—blue light emission and gas sensing, catalytic converter High temperature—insulation Applications Transparent conducting oxide—solar cells, LEDs, gas detector UV light detector Flash memory Gas sensors, solar cells

Electrodes for Li ion batteries High-efficiency flame-retardant—synergist in plastics, paints, adhesives, and textile back coatings Pollution control, molecular detection, tips for nano probe

Electrochromic Fsmart_ windows Electrochromic Fsmart_ windows Wide band gap semiconductor (3.3 eV) and large exciton binding energy (60 meV)— dye-sensitized solar cell (DSC) electrodes, antireflection (AR) coatings, photocatalysts, photonic crystals, surface acoustic wave (SAW) fiters, ultraviolet (UV) semiconductor diode lasers (SDLs), UV photodetectors, photodiodes, optoelectronic devices, and gas sensors Ferroelectric, piezoelectric—FERAM, temperature sensors, DRAM capacitors Ferroelectric, piezoelectric— FERAM, pressure sensors, actuators, micromotors, acousto-optic modulators, accelerometers, displacement sensors for AFMs, IR bolometers, photoacoustic gas sensors, sonar transducers, etc. High energy electrochemical storage—nano form – longer life – >1400 cycles

Magnetoresistance—magnetic field sensors, MRAM

From Table 1 we find that there are relatively few multicomponent materials whose nanowires have been made. One obvious reason for this is the inherent difficulty in controlling the reaction and achieving stoichiometry at the nanoscale.

4. Growth of oxide nanowires Fig. 1 shows the schematic of different techniques adopted for the growth of 1D nanostructures.

[106] [107] [108] [109] [110] [111] [112] [113] [114] [115] [50] [116] [76] [19,20] [117] [51] [83] [94] [88] [118] [58] [53,54] [73] [70] [82] [76] [58] [95] [99] [52] [85] [100 – 102]

[86,98] [71]

[60] [61] [62] [63] [65,66] [98]

4.1. Vapor phase growth Vapor phase synthesis is the most extensively explored approach to synthesize 1D nanostructures such as whiskers, nanorods, and nanowires. The key to 1D growth in a controlled way is to keep the supersaturation at a relatively low level. Vapor phase growth has been exploited to synthesize nanowires of many technologically important oxide nanowires (Table 1) [26,27]. In a typical process, vapor species is first generated by evaporation, reduction and other kinds of gaseous reaction. These species are

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Fig. 1. Schematic illustration of different methods used for nanowire synthesis; (A) VLS method, (B) Sol – gel synthesis, (C) Electrodeposition and (D) Surfactant assisted.

subsequently transported and condensed onto a substrate kept at a lower temperature. Carbothermal reduction [28], physical vapor deposition (PVD) [29], chemical vapor deposition (CVD) [30], and metallorganic chemical vapor deposition (MOCVD) [31] have also been used for nanowire synthesis. Vapor –liquid – solid (VLS) method generally utilizes a proper catalyst, which defines the diameter of the nanowire and directs preferentially the addition of reactants (Fig. 1(A)). The critical steps in the catalytic growth of nanowires has been clearly outlined by Wang [1] (page 7). Lieber’s group has demonstrated the potential of this technique to grow semiconductor nanowires of many semiconductors. A major breakthrough in this field was the fabrication of nanowire superlattices like GaAs/GaP [20]. Pn junction within individual Si nanowires have been fabricated by goldnanocluster-catalyzed CVD and dopant modulation. We have included only a few highlights of vapor phase synthesis. A detailed discussion on this topic is not within the scope of this paper. The demonstration of growth of single crystalline nanowires of numerous semiconducting materials and doped NW superlattices by VLS method is an important milestone in realizing functional nanodevices. However, this method is likely to be limited to simple oxides. Laser assisted VLS method requires expensive experimental setup unlike chemical based methods. VLS growth of nanowires is restricted to systems that can form eutectic with catalysts at growth temperature. Many of the oxides

possess high melting point and as a result form eutectic liquid at very high temperatures, necessitating high temperature for nanowire growth. For many of the complex oxides, there is very limited information about the formation of eutectic liquids. In case of complex multicomponent oxides, precise control over stoichiometric composition is possible only by chemical solution methods. Further, when combined with template-aided synthesis, it offers the possibility of fabricating aligned unidirectional and uniformly sized oxide nanorods over large area, which is attractive for device fabrication and study of collective phenomenon. 4.2. Chemical solution growth 4.2.1. Template-assisted synthesis The template-assisted synthesis of nanowires is a conceptually simple and intuitive way to fabricate nanostructures [32 – 34]. A typical template contains very small cylindrical pores. Nanowires can be fabricated by filling the pores with the desired material and crystallizing them. 4.2.1.1. Physical and chemical templates. In templateassisted synthesis of nanostructures, the chemical stability and mechanical properties of the template, as well as the diameter, uniformity and density of the pores are important characteristics to consider. Template-based methods make use of either Fhard_ templates or Fsoft_ templates. The hard

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templates include inorganic mesoporous materials such as anodic aluminium oxides (AAO) and zeolites, mesoporous polymer membranes, block copolymers, carbon nanotubes, etc. Soft templates generally refer to surfactant assemblies such as monolayers, liquid crystals, vesicles, micells, etc. (Fig. 2(C)) and the synthesis based on these soft templates are referred to alternately as template-free or chemical template methods. 4.2.1.2. Anodized aluminum oxide (AAO) membranes. Anodized alumina templates are the most extensively used porous membranes used for nanowire synthesis. They are produced by anodizing pure Al in various acids [35,36]. Under carefully chosen anodization conditions, the resulting oxide possesses a regular hexagonal array of parallel and nearly cylindrical channels. The top and cross sectional view of a typical AAO membrane is shown in Fig. 2(A) (a and b). The intricacies of pore formation have been extensively studied over the past four decades and there are very good reviews on this including the most

recent review on nanometric superlattices by Chik and Xu [38]. Depending on the anodization conditions, the pore diameter can be systematically varied from 10 up to 200 nm with a pore density in the range of 109 –1011 pores/cm2. With intensive research effort over the years (including two step anodization process), anodization of alumina is almost perfected to yield templates most suitable for nanowire fabrication. Recently, fabrication of AAO membranes with Y-branched nanopores are also reported [39]. One can contemplate on three terminal nanoscale transistor, by applying different voltages to the different arms, which would be an invaluable component of nanocircuits. A major step towards integration of the nanowires in devices was achieved with the tailored growth of AAO membranes on glass and Si substrates [40]. 4.2.1.3. Other membranes. Another class of porous templates commonly used for nanowire synthesis are those fabricated by chemically etching particle tracks originating from ion bombardment [41], such as track-etched polycarbonate membranes. Mesoporous molecular sieves [42], termed MCM-41, possess hexagonally-packed pores with very small channel diameters which can be varied between 2 and 10 nm [43]. Diblock copolymers, which consist of two different polymer chains with different properties, have also been utilized as templates for nanowire growth (Fig. 2(B)) [44,45]. More recently, the DNA molecule has also been used as a template for growing nanometer-sized metal nanowires (Fig. 2(D)) [46]. Many other biological templates can be used for the fabrication of nanowires [47]. Commercial availability of AAO and polycarbonate membranes (Anopore and Nucleopore, respectively, www.whatman. com) has greatly accelerated the progress of template-aided synthesis of nanowires.

5. Filling of membranes The deposition of the material inside the pores of the template can be achieved by pressure injection, electrodeposition or capillary-rise. 5.1. Pressure injection

Fig. 2. Schematic illustration of the different templates used in nanowire synthesis; (A) AAO membrane, (B) Copolymer template and (C) Micelle soft templates.

The pressure injection technique is often employed for fabricating highly crystalline nanowires from a lowmelting point material or when using porous templates with robust mechanical strength Metal nanowires (Bi, In, Sn, and Al) and semiconductor nanowires (Se,Te, GaSb, and Bi2Te3) have been fabricated in anodic aluminum oxide templates using this method [48]. Nanowires produced by the pressure injection technique usually possess high crystallinity and a preferred crystal orientation along the wire axis. Not suitable for metal oxides because of their high melting point.

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5.2. Electrochemical deposition Electrodeposition offers a simple and viable alternative to the cost-intensive methods such as laser ablation. In particular, the growth occurs closer to equilibrium than those high temperature vacuum deposition techniques. Both AC and DC electrodeposition are used for filling the pores. In the electrochemical methods, a thin conducting metal film is first coated on one side of the porous membrane to serve as the cathode for electroplating. Schematic picture of electrodeposition is given in Fig. 1(B). This method has been used to synthesize a wide variety of nanowires, e.g., metals and semiconductors [49]. In case of oxide nanowires, electrodeposition of the metal into the pores is followed by oxidation. Examples are In2O3 [50], SnO2 [51] and ZnO [52] nanowires. Advantages of utilizing this method are that it is costeffective and simple. However, composition modulation is difficult and this method is not suitable for multicomponent oxide nanowires as different cations would have different ionic sizes and diffusivity. There are a very few examples wherein single crystalline nanowires are obtained: TiO2 nanowires by anodic oxidative hydrolysis of acidic aqueous TiCl3 solutions followed by annealing [53,54]. Zu et al. have reported the growth of CdTe single crystalline nanowires by electrodeposition [55]. 5.3. Sol – gel deposition Sol – gel processing has evolved into a general and powerful approach to prepare highly stoichiometric nanocrystalline materials and has proved to be a very good method to prepare nanocrystalline materials of multicomponent oxides [56]. Sol – gel synthesis combined with template-aided synthesis and electrodeposition (electrophoretic deposition) has proved to be an excellent method for the preparation of ordered array of multicomponent nanowires. The basis of sol –gel processing is the hydrolysis of a solution of precursor molecules to obtain first a suspension of colloidal particles (the sol) and then condensation of sol particles to yield a gel. Precursors can be either organic metal alkoxides in organic solvents or inorganic salts in aqueous media. Inorganic route involves the formation of condensed species from aqueous solutions of inorganic salts by adjusting the pH, by increasing the temperature or by changing the oxidation state. Most of the time precipitation rather than gel formation occurs. This kind of precipitation is extensively used in the synthesis of nanowires of semiconductors such as ZnO on seeded substrates (Section 8). However, stable sols can also be prepared by this method by utilizing polymerizing agents such as ethylene glycol. An alternative method is to use metal alkoxides which dissolve in organic solvents. These sol – gel processes

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involve hydrolysis, i.e., substitution of alkoxy ligands by hydroxylated species XOH as follows: MðORÞz þ y XOH ! ½MðORÞz
y ðOXÞy  ! MOx where X stands for hydrogen (hydrolysis), a metal atom (condensation) or even an organic or inorganic ligand (complexation). The biggest advantage of sol – gel processing is the ability to process multicomponent complex oxides. A proper control over hydrolysis and condensation is very essential. The constituent materials should be homogeneousely mixed at the molecular level. Moreover, each precursor may have different reactivities, hydrolysis and condensation rates. Consequently, each precursor may form nanoclusters of its own metal oxide, yielding composite of multiple oxide phases, instead of a single phase complex oxide. There are several ways of avoiding homocondensation and achieve a homogeneous mixture of multicomponents at the molecular level. Polymer precursor sol – gel processing, wherein a polymerizing agent like ethylene glycol is utilized to form a polyethylene-cation complex consisting of uniformly arranged cations through a polymer network is very effective in obtaining homogenous distribution [57]. Polymeric precursor route gives sols ideal for template synthesis. In this preparation route, it is possible to control the viscosity of the sol easily and it is possible to prepare sols that are stable over many months. 5.3.1. Direct sol filling The growth method typically involves the hydrolysis of a solution of a precursor molecule to obtain a suspension of colloidal particles. Due to capillary action, the pores are filled with the sol particles that slowly condense to form a gel. The gel on thermal treatment yields the desired material (Fig. 1(C)). Nanowire array of many oxides are prepared through sol – gel template-aided processing. Examples include TiO2, V2O5, WO3, ZnO [58], Ga2O3, In2O3 [59]. In-template nanowires of LiNiO2 [60], LiMn2O4 [61], LiCoO2 [62], and LiNi0.5Co0.5O2 [63] have been prepared by sol – gel synthesis. Highly ordered zirconia nanowire arrays have been demonstrated by the AAO template method using sol –gel synthesis [64]. Sol –gel processing has proved to be very efficient to prepare nanowires of complex oxides of the kind lanthanum calcium manganese oxide [65,66] and lanthanum strontium manganese oxide [67] within AAO templates. A typical processing of manganite nanowire through sol –gel processing is shown as a flow chart in Fig. 3. The scanning electron micrograph and transmission electron micrograph taken on the LCMO nanowire array along with the magnetic susceptibility data is shown in Fig. 4. These nanowires exhibited enhanced Tc (Tc enhancement of 80 K) due to the size induced lattice contraction (Fig. 4). The reduction in unit cell volume was close to 2.6% in the nanowires. Tc ehancement arises mainly from the hardening of the Jahn – Teller (JT) phonon

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Fig. 3. Schematic illustration and the flow chart of the procedure used in nanowire synthesis of manganites by sol – gel template-aided sythesis.

mode X ph as the size is reduced. Increase in bandwidth due to decrease in Mn – O bond length and decrease in Mn –O – Mn bond angle also contributes to Tc enhancement. Manganite nanowires with enhanced Tc are attractive for sensor applications. A major limitation of the template-aided synthesis is that the nanowires tend to be polycrystalline due to the heterogeneous nucleation on the pore walls; there are very few reports on the synthesis of single crystalline nanowires through this method [68]. We have explored ways of overcoming this limitation. One strategy is to electrostatically confine the sol particles within the center of the pores to enhance homogeneous nucleation and thereby limit the heterogeneous nucleation on the pore walls. We have demonstrated the growth of oriented nanowires of manganites [67]. A specific example of growth of oriented lanthanum strontium manganese oxide nanowire is given below. This was achieved by choosing a suitable sol and template combination. Sol consisting of polyethylene glycol-cation complex was found to be suitable for anodized alumina template. As the walls of AAO templates are positively charged due to oxygen deficiency, choosing sol particles that are also positively charged helped to confine the sol particles to the center of the pores. In our method, the polymer-cation complex and more importantly the choice of the polymer plays a key role in obtaining oriented growth of nanowires. Polyethylene glycol (PEG) is extensively used to prepare nanowires in polyol method (details given in Section 6.1). It was found in the polyol method preparation

of metal oxide nanowires, ethylene glycol forms a chain-like complex with cations attaching only at specific sites and they readily aggregates into 1D nanostructures. In the present case, during pore filling by capillary action, it is reasonable to expect that the linear chains of PEG-cation complex align along the axis of the pores due to their high aspect ratio (shown as a schematic in Fig. 5). The linear alignment of the PEG-cation chain along the pore axis and the attachment of cations to only at specific sites along the chain limit the number of nuclei formed along a given cross section. The fewer number of nuclei is favorable for single crystal/oriented nanowire formation. A likely scenario can be that grains or stable nuclei with preferred orientation form near the center of the pore. As more grains form, they attach and grow along the preferred growth axis. During the nucleation process, the crystallites not oriented along this axis can rotate or reorient if sufficient volume is available. In the present case, the polymer matrix surrounding the crystallites provides enough space for such reorientations (Fig. 5(C)). Thus this technique could be considered as chemical (polymer) physical template (AAO membrane) method of preparation of nanowires. Achieving oriented growth is a significant milestone in the growth of single crystalline nanowires of complex multicomponent materials by template sol –gel synthesis. During direct filling of the sol into pores, capillary action is the only driving force to fill the pores. Moreover, the solid content in typical sols is low and hence on heating it may yield porous nanostructures or hollow tubes. Electrophoretic

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Fig. 4. (a) Transmission electron micrograph of LCMO nanowires along with the selected area electron diffraction, (b) Scanning electron micrograph of LCMO nanowire array within AAO template, and (c) Temperature variation of magnetic susceptibility of LCMO nanowires and single crystals.

sol – gel process was developed in order to improve the packing density of sol particles within the pores. 5.3.2. Electrophoretic sol –gel processing In electrophoretic sol – gel processing, the charge on the sol particles is utilized and an electric field is applied to

induce electrophoretic motion of the sol particles into the pore channels. This can substantially increase the solid content within the pores and hence yield better nanowires. Nanowires of many technologically important oxides like BaTiO3, TiO2, SiO2, Lead Zirconium Titanate (PZT), and Sr2Nb2O7 [69 – 72] are prepared by this method.

Fig. 5. Schematic illustration of filling of AAO membranes with PEG-cation complex and subsequent nanowire growth.

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The biggest advantage of preparing ordered array of nanowires in templates is that for sensor and nanoelectrode applications, nanowires could be retained within the membranes as an array and for other applications membranes can be dissolved to obtain individual nanowires. However, removal of the template may cause damage to the nanowires and also introduce impurities such as sodium ions (from NaOH which is generally used for dissolving AAO membranes) into the system. Moreover, since the size of the sol particles ranges from 10 to 100 nm, when the pore size is very small filling them becomes difficult even by electrophoretic process. In order to overcome this problem, Maio et al. have adopted electrochemically induced sol – gel processing to prepare TiO 2 nanowires, wherein alumina membrane was immersed in titania alkoxide solution and sol formation was induced electrochemically inside the pores [73]. Where there is problem of pore filling, template-free solution methods like hydrothermal, sonochemical, microemulsion or soft template methods like surfactant assemblies and micelles come handy.

6. Template-free solution based methods 6.1. Polyol method Polyol process involves boiling metal precursors or salts in ethylene glycol. Ethylene glycol is extensively used in the preparation of nanoparticles by polyol process as it is a reducing agent and has high boiling point (195 -C) [74]. Recently Xia and Sun demonstrated that by reducing silver nitrate with ethylene glycol in the presence of poly(venyl pyrrolidone) with the introduction of Pt nanoparticles as seed particles [75]. Jiang et al. have utilized poyol method for the large scale synthesis of metal oxide TiO2, SnO2, In2O3, and PbO nanowires with diameters around 50 nm and lengths up to 30 Am [76]. In most of the cases, alkoxides were transformed into a chain-like, glycolate complex that subsequently crystallized on heating into uniform nanowires. The key to the success of this synthesis was the use of ethylene glycol to form chain-like complexes with appropriate metal cations, which could readily aggregate into 1D nanostructures within an isotropic medium. Polyol seems to be an attractive route for the synthesis of a wide variety of oxide nanowires. 6.2. Surfactant assemblies Surfactants are conveniently used to promote the anisotropic 1D growth of nanocrystals. Solution phase synthetic routes have been optimized to produce monodispersed quantum dots, i.e., zero-dimensional isotropic nanocrystals [77]. Surfactants are necessary in this case to stabilize the interfaces of the nanoparticles and retard

oxidation and aggregation processes. Detailed studies on the effect of growth conditions have revealed that they can be manipulated to induce a directional growth of the nanocrystals, usually generating nanorods (aspect ratio of 10), and in favorable cases, nanowires of high aspect ratios. The use of surfactants to obtain nanowires is demonstrated in case of many semiconductors like CdSe [78], PbSe and CdS [79]. Solution based surfactant assisted method is used to prepare oxide nanorods by Yan et al. [80]. 6.2.1. Micells Microemulsion system consists of an oil phase, a surfactant phase and an aqueous phase. It is basically a thermodynamically stable isotropic dispersion of an aqueous phase in the continuous oil phase. These reverse micells act like microreactors for confining the growth of nanomaterials. Li et al. have adopted microemulsion method using the microemulsions of NaCl, cyclohexane as the oil phase, a mixture of poly(oxyehylene), nonyl phenol ether (NPS) and poly(oxyethylene)-9-nonyl phenol ether (NP9) as nonionic surfactants to prepare single crystalline nanowires of TiO2 [82]. Microemulsion method is also used to prepare nanowires of SnO2 [83]. Even nanowires of complex polyoxometalate of the kind Ag4SiW12O40 are prepared using microemulsion technique consisting of ethanol and AOT (sodium bis-(2-ethyexylsulfosuccinate) [84]. Zhang et al. have used microemulsion mediated hydrothermal process to prepare nanowires of ZnO [85]. Single crystalline BaTiO3 and SrTiO3 nanowires of 5– 70 nm in diameter and lengths exceeding 10 Am is prepared by solution based template free method [86]. The method is based on the solution-phase decomposition of bimetallic precursor in the presence of coordinating ligands. In a typical reaction to prepare BaTiO3 nanowires, an excess of H2O2 was added at 100 -C to heptadecane solution containing a 10:1 molar ratio of BaTi[OCH(CH3)2]6 to oleic acid. The reaction mixture was then heated to 260 -C for 6 h, resulting in a white precipitate, which composed of nanowire aggregates. These nanowires were found to be ferroelectric exhibiting hysterisis loop with coercive field of 7 kV/cm
1. It is proposed that the anisotropic growth takes place most likely due to precursor decomposition and crystallization in a structured inverse micelle medium formed by precursors and oleic acid under these reaction conditions. How surfactant molecules influence 1D growth is very interesting. Though there are efforts to understand the mechanism of 1D growth, in many studies it is confined to specific cases. Moreover, there are too many parameters to control such as the nature and amount of surfactants, concentration of the reactants, temperature and pH of the solution as all these have influence on the 1D growth. And, not all the surfactants work in the same way. For example, in case of ZnO nanorod formation [85], CTAB only accelerates the hydrothermal oxidation

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and guides the growth direction and does not serve as a microreactor. The behavior of surfactant molecules depends on the charge and stereochemistry properties of reactants. There is a lack of concrete understanding of the nanowire formation, which is essential to extend it to many more complex oxide systems. This technique deserves further study in order to extend this process to many other systems. 6.3. Sonochemical synthesis Sonochemistry, the use of power ultrasound to stimulate chemical process in liquid, is currently the focus. The chemical effects of ultrasound arise from acoustic cavitation (the formation, growth, and implosive collapse of bubbles in a liquid). During cavitational collapse, intense heating of the bubbles occurs. These hot spots have temperatures of roughly 5000 K, pressures of about 1000 atmospheres, and cooling rates above 1010 K/s. These extreme conditions attained during bubble collapse have been exploited to prepare nanoparticles of metals, alloys, metal carbides, metal oxides, and metal sulfides [87]. Recently sonochemical synthesis is used to prepare high aspect ratio nanoparticles and nanorods. Examples include nanowires of MnO2 [88], Fe2O3 [89], and V2O5 [90]. 6.4. Microwave irradiation Microwave irradiation is also used in the synthesis of high aspect ratio nanoparticles and nanorods. For example Liao et al. have reported the growth of Bi2S3 nanorods by microwave irradiation of formaldehyde solution of bismuth nitrate and thiorea through the formation of bismuth thiorea complexes [91]. Nanostructures of CuS including nanotubules were prepared by microwave synthesis without the help of any surfactant [92]. Microwave irradiation is a powerful technique, which still remains unexplored for large scale synthesis of nanowires. 6.5. Hydrothermal and solvothermal reactions Hydrothermal precipitation entails heating an aqueous solution containing soluble metal species or aqueous slurry in an autoclave. Temperatures normally greater than 100 -C and pressures exceeding atmospheric pressure are chosen to promote the formation and precipitation of the desired compound. Since the process involves chemical reactions that are carried out at moderate temperatures and pressures, the oxides are normally precipitated as single crystal particles. Also, the products have a higher degree of purity and homogeneity and should contain fewer structural defects than those obtained by conventional processes. Wang et al. have recently demonstrated the synthesis of nanorods/nanowires and nanosheets of rare earth compounds, hydroxides and fluorides by hydrothermal

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method. Subsequent dehydration, sulfidation and fluoridation could be adopted to obtain rare earth oxide, oxysulphide and oxyhalide nanostructures. By tuning the control factors such as pH, temperature and concentration during precipitation – hydrothermal process, it is possible to get anisotropic growth of materials [93]. Many other oxide nanowires by hydrothermal methods are also reported; MnO2 [94], V2O5 [95], potassium titanate K2Ti6O13 [96] and single crystalline nanowires of barium doped rare earth manganite (La0.5Ba0.5MnO3) [97]. BaTiO3 nanotubes arrays are also prepared by hydrothermal method [98].

7. Mechanism of 1D nanostructures of layered materials In many of the solution based redox synthetic routes and also in some surfactant assemblies, the formation of 1D nanostructures is through the formation of 2D nano sheets, which subsequently roll up to form nanotubes or nanowires. This has motivated the concept of synthesizing 1D nanostructures from artificial lamellar structures [99]. 7.1. Artificial lamellar structures Although, many oxides may have layer structures, not all of them can be transformed into 1D nanostructures, partly because of the strong interaction between the layers. Therefore, the synthesis of many oxide 1D nanostructures are through the preparation of lamellar structures. The method is based on self assembly of inorganic precursors at the template-solution interface using organic molecules as structure directing agents. The interaction between organic molecule and inorganic precursor could be coordinative interaction, electrostatic interaction or even hydrogen bonding. Under a suitable condition, interlayer interaction of lamellar intercalates could diminish from the edges. Then the rolling up of the layers into tubules would take place. The use of this method to prepare 1D nanostructures is very well demonstrated in case of V2O5, MnO2, WO3
x and LnOH etc. Under hydrothermal, salvothermal or sonochemical conditions, lamellar structures form, which roll up to yield nanotubules or nanowires. These surfactant assisted or surfactant free solution methods are attractive for large scale production of nanowires as they offer the advantages of low cost, simple apparatus and low temperature preparations. Further investigation has to be focused on quality of the nanowires and on means of obtaining well-aligned uniform array of nanowires with uniform morphology and perfect crystallinity. In this regard, we emphasize that the chemical physical template route is worth perusing as it has the potential to yield ordered array of single crystalline nanowires within templates.

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8. Template-free solution method for ordered nanowire array Ordered array of ZnO nanowires on various substrates including plastic and glass can be obtained by low temperature (< 100 -C) solution growth technique [100 – 102]. In a typical reaction, an aqueous solution of Zinc nitrate and HMT (hexamethyltetramine) is prepared with a concentration of 0.03 mol/L of both the solutes so that there is no precipitation at room temperature. The seed coated (ZnO nanoparticles) substrates are dipped within the solution and the beaker and heated at 90 -C for several hours. The schematic and flow chart of the procedure to prepare ZnO nanowires is depicted in Fig. 6. The underlying principle of this method is the controlled precipitation of the desired phase from the solution containing metal ions. During precipitation, the formation of a solid phase in the solution should start when the ionic product (IP) exceeds the solubility product (K ps), which depends on the temperature and pH of the solution. As the temperature is raised, HMT decomposes into formaldehyde and ammonia increasing the pH of the solution and inducing precipitation of ZnO/Zn(OH)2. NH3 also reduces the concentration of Zn2+ ion by producing complex ions of the type, Zn(NH3 )n 2+ (n = 1 – 4 the most stable coordination number), which avoids the spontaneous precipitation. For nanowire growth, nucleation of ZnO on the substrate is preferred, rather than within the solution. This can be achieved by introducing substrates pre coated with ZnO nanoparticles, which act as nucleation centres. This synthesis route which allows kinetic growth of ZnO

Fig. 7. Scanning electron micrograph of ZnO nanowires on (A) seeded and (B) bare Si wafer.

leads to oriented growth of nanowires all along the z-axis. Fig. 7(A) and (B) shows the scanning electron micrograph of ZnO nanowires on seeded and bare Si nanowires.

Fig. 6. The process chart and schematic picture of oriented ZnO nanowire growth on substrates.

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The most critical parameters to achieve oriented growth are, 1. Controlling the solubility of the precursors and the degree of supersaturation so that massive precipitation is not the dominant reaction, i.e., keeping the reaction temperature low and reducing the precursor concentration low. 2. Reducing the interfacial energy between the substrate and the particle by functionalizing the substrate, i.e., by introducing a large number of nuclei (seeds) of the desired material on the substrate. 3. Ensuring the kinetic growth of oriented nanostructures. This technique of template free low temperature synthesis to obtain ordered array of nanowires on any substrate is a very powerful method and need to be explored for many more materials systems.

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now getting ready to move to the Industry. The features of the nanowire synthesis method desired by an industrial manufacturer: (1) Low cost of precursor materials and the infrastructure for materials processing (2) Good understanding and affordable control of process parameters leading to minimum variation in the product properties (3) Easy scalability (4) Environmentally benign precursor materials (5) Easy integrability into existing device structures (6) Compatibility with the device fabrication enviroment (7) Absence of volatile and toxic byproducts during the synthesis and (8) The synthesis should involve minimum human intervention and be capable of complete automation. The onus is on us, nanowire researchers, to focus our research to ensure that our synthesis methods meet the above mentioned requirements. This and only this would accelerate the bridging of the gap between the science and technology of nanowires.

References 9. Nanowires technology The industrial development of nanostructured products is broadly based on three core technologies (i) rational design and fabrication of high-quality nanostructures (ii) flexible assembly of high-performance nanostructures into devices and (iii) precise engineering of the unique properties arising from quantum confinement effects. We have listed the websites of a few leading manufacturers [103]. IBM leads the owners of Nanotechnology patents with 2092 patents followed by Xerox (1039) and 3M (809) and the University of California (540) deserves special mention being the only academia in the top twenty list [104].

10. Summary Nanowire fabrication methods based on chemical solution processing emerge as clear winners by virtue of their good stoichiometry control, morphology control, lowcost infrastructural requirements and easy scalability. However the challenge is to understand the chemistry at the nanoscale and tailor the morphology by controlling the reaction kinetics. The most promising chemical processing approach would use a combination of physical and chemical templates. Large-scale implementation of chemical solution processing approaches is greatly facilitated by commercial availability of readymade templates (as in the case of AAO or polymeric membranes). Development of methods aimed at in situ growth of templates on functional materials would help us to overcome various challenges associated with handling nanowires and thereby accelerate the integration of nanowires into devices. Chemical solution processing methods of fabricating nanowires that are so popular and familiar in our labs are

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