fabrication of nanowires of multicomponent oxides review of recent advances

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 un ique 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 nanomateri als researc h [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 throu gh rational design and intelligent synthesis
methods has motivated researchers to envision radical
innovations and enabled technologists to implement novel
applications. There are numerous challeng es 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 synthes is of nanowires of multi-
component materials. The focus of our review is on
solution-based chemical processing methods and template-
directed synthesis of nanow ires. 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 impor-
tant 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-
0928-4931/$ - see front matter D 2005 Published by Elsevier B.V.
doi:10.1016/j.msec.2005.06.054
* Corresponding author. Tel.: +91 80 23608653; fax: +91 80 23602602.
E-mail address: (K.S. Shankar).

Materials Science and Engineering C 25 (2005) 738 – 751
www.elsevier.com/locate/msec
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 one-
dimensional systems arise from their unique chemistry and
physics [2]. Recent demonstration of ballistic electron trans -
port in many metallic nanowires [3], size controlled semi-
metal to semiconductor transition in bismuth nanowires [4],
electrically controllable UV lasing from a single zinc oxide
nanowire [5], size dependent excitation or emission for pho-
toluminescence 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. Re-
cently 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 proportion-

ately significant. Keeping in mind, the multi-faceted func-
tional properties of these materials that include elect ronic
and ionic conductivity, superconductivity, ferroelectricity,
piezoelectricity, optical non-linear ity 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 multi-
component oxides.
3. Introduction to synthesis of nanowires
The synthe sis o f 1D nanostructure s in general a nd
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 achie-
ving 1D growth in materials, where atomic bonding is re-
latively 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 proportion-
ately in multicomponent materials as we need to achieve the
desired stoichiometry within the nanodimensions.
Attempts to break the growth symmetry either phy sically
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
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 dis-
cussed 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 nano-
structures, 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, Al
2
O
3
, ZnO,
SnO
2
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.
K.S. Shankar, A.K. Raychaudhuri / Materials Science and Engineering C 25 (2005) 738 –751 739
From Table 1 we find that there are relatively few
multicomponent materials w hose 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 nanostruct ures.
4.1. Vapor phase growth
Vapor phase synth esis 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
Table 1
Oxide nanowires applications and synthesis (this is not an exhaustive, but a representative list)
Material Method Properties and applications Reference
MgO VS High melting point (2400 -C) and high heat capacity—functional composite
as a reinforcement agents and pinning centers
[105]
Cu
2
O Vapor phase Direct bandgap semiconductor (2 eV)—conversion of optical, electrical and
chemical energy
[106]
Surfactant-assisted [107]
SiO
2

VLS Optical wave guiding [108]
CVD [109]
Laser ablation [110]
Ga
2
O
3
Vapor phase Wide band gap semiconductor (4.9 eV)—blue light emission and gas sensing,
catalytic converter
[111]
Molten phase [112]
Al
2
O
3
Vapor phase High temperature—insulation [113]
Etching of AAO template Applications [114]
In
2
O
3
Vapor phase Transparent conducting oxide—solar cells, LEDs, gas detector [115]
Electrodeposition– oxidation UV light detector [50]
Laser ablation Flash memory [116]
Polyol [76]
SnO
2
Vapor phase Gas sensors, solar cells [19,20]
Flux growth [117]
Electrodepositon– oxidation [51]

Microemulsion [83]
MnO
2
Hydrothermal Electrodes for Li ion batteries [94]
Sonochemical [88]
Sb
2
O
3
Microemulsion High-efficiency flame-retardant—synergist in plastics, paints, adhesives, and
textile back coatings
[118]
TiO
2
Sol – gel Pollution control, molecular detection, tips for nano probe [58]
Anodic oxidative hydrolysis [53,54]
Cathodically induced sol – gel [73]
Electrophoretic [70]
Hydrothermal [82]
Polyol [76]
V
2
O
5
Sol – gel Electrochromic Fsmart_ windows [58]
Surfactant-assisted [95]
WOx Surfactant-assisted Electrochromic Fsmart_ windows [99]
ZnO Electrodeposition– oxidation 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
[52]
Microemulsion– hydrothermal [85]
Low temperature precipitation [100 – 102]
BaTiO
3
Sol – gel template based hydrothermal Ferroelectric, piezoelectric—FERAM, temperature sensors, DRAM capacitors [86,98]
PZT Sol – gel template-based Ferroelectric, piezoelectric— FERAM, pressure sensors, actuators, micromotors,
acousto-optic modulators, accelerometers, displacement sensors for AFMs,
IR bolometers, photoacoustic gas sensors, sonar transducers, etc.
[71]
LiNiO
2
Sol – gel template-based High energy electrochemical storage—nano form – longer life – >1400 cycles [60]
LiMn
2
O
4
[61]
LiCoO
2
[62]
LiNi
0.5
Co
0.5
O
2
[63]

La
1Àx
Ca
x
MnO
3
Sol – gel template based Magnetoresistance—magnetic field sensors, [65,66]
MRAM [98]
La
1Àx
Sr
x
MnO
3
Hydrothermal
La
1 À x
Ba
x
MnO
3
Hydrothermal
K.S. Shankar, A.K. Raychaudhuri / Materials Science and Engineering C 25 (2005) 738 – 751740
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 gold-
nanocluster-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 v ery 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 nano-
structures [32–34]. A typical template contains very
small cylindrical pores. Nanowires can be fabricated by
filling the pores with the desired material and crystalliz-
ing them.
4.2.1.1. Physical and chemical templates. In template-
assisted synthesis of nanostructu res, 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
Fig. 1. Schematic illustration of different methods used for nanowire synthesis; (A) VLS method, (B) Sol – gel synthesis, (C) Electrodeposition and (D)
Surfactant assisted.
K.S. Shankar, A.K. Raychaudhuri / Materials Science and Engineering C 25 (2005) 738 – 751 741
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. An o-

dized 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 result-
ing 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 varie d from 10 up to 200
nm with a pore density in the range of 10
9
–10
11
pores/cm
2
.
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. Oth er membran es. Another class of po rous
templates commonly used for nanowire synthesis are those
fabricated by chemically etching particle tracks originating
from ion bombardment [41], such as track-etched polycar-
bonate 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]. Commer-
cial 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, electro-
deposition or capillary-rise.
5.1. Pressure injectio n
The pressure injection technique is often employed for
fabricating highly crystalline na nowires from a low-
melting 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 pressur e inject ion tech niqu e usually
possess high crystallinity and a preferred crystal orientation
along the wire axis. Not suitable for metal oxides because
of their high melting point.
Fig. 2. Schematic illustration of the different templates used in nanowire
synthesis; (A) AAO membrane, (B) Copolymer template and (C) Micelle
soft templates.
K.S. Shankar, A.K. Raychaudhuri / Materials Science and Engineering C 25 (2005) 738 – 751742
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 In
2
O
3
[50],SnO
2
[51] and ZnO

[52] nanowires.
Advantages of utilizing this method are that it is cost-
effective 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 crystal-
line nanowires are obtained: TiO
2
nanowires by anodic
oxidative hydrolysis of acidic aqueous TiCl
3
solutions
followed by annealing [53,54 ]. Zu et al. have reported the
growth of CdTe single crystalline n anowires by electro-
deposition [55].
5.3. Sol –gel deposition
Sol–gel processing has evolved into a general and
powerful approach to prepare highly stoichiometric nano-
crystalline materials and has proved to be a very good
method to prepare nanocrystalline materials of multi-
component oxides [56]. Sol– gel synthesis combined with
template-aided synthesis and electrodeposition (electro-
phoretic 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 preci pitation
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
involve hydrolysis, i.e., substitution of alkoxy ligands by
hydroxylated species XOH as follows:
MðORÞ
z
þ y XOH !½MðORÞ
z
À
y
ðOXÞ
y
!MO
x
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 compl ex oxide. There are
several ways of avoidi ng homocondensation and achieve a
homogeneous mixture of multicomponents at the molecular
level. Polymer precur sor 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]. Poly-
meric 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 processi ng. Examples
include TiO
2
,V
2

O
5
,WO
3
, ZnO [58],Ga
2
O
3
,In
2
O
3
[59].
In-template nanowires of LiNiO
2
[60], LiMn
2
O
4
[61],
LiCoO
2
[62], and LiNi
0.5
Co
0.5
O
2
[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 c omplex oxi des of the kind
lanthanum calcium manganese oxide [65,66] and lantha-
num stro ntium man ganese 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 T
c
(T
c
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. T
c
ehancement arises
mainly from the hardening of the Jahn–Teller (JT) phonon
K.S. Shankar, A.K. Raychaudhuri / Materials Science and Engineering C 25 (2005) 738 – 751 743
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 T
c
enhance-
ment. Manganite nanowires with enhanced T

c
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 electrostati-
cally 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 mangan-
ites [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 metho d 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-cat ion chain along the pore axis and
the a ttachment of cations to only at specific sites along the
chain limit the number of nuclei formed along a given cross
section. The fewer number o f 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
Fig. 3. Schematic illustration and the flow chart of the procedure used in nanowire synthesis of manganites by sol – gel template-aided sythesis.
K.S. Shankar, A.K. Raychaudhuri / Materials Science and Engineering C 25 (2005) 738 – 751744
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
BaTiO
3
,TiO
2
, SiO
2
, Lead Zirconium Titanate (PZT), and
Sr
2
Nb
2
O
7
[69–72] are prepared by this method.
Fig. 5. Schematic illustration of filling of AAO membranes with PEG-cation complex and subsequent nanowire growth.
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.
K.S. Shankar, A.K. Raychaudhuri / Materials Science and Engineering C 25 (2005) 738 – 751 745
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 nano-

wires. 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. More-
over, 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
Ti O
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, micro-
emulsion or soft template methods like surfactant assem-
blies and micelles come handy.
6. Template-free solution based methods
6.1. Polyol method
Polyol process involves boiling metal precursors or
salts in e thylene 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 TiO

2
,SnO
2
,In
2
O
3
, 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 crystal-
lized 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 th e
anisotro pic 1D growth of nanocrystals. Solution phase
synthetic routes have been optimized to produce mono-
dispersed quantum dots, i.e., zero-dimensional isotropic
nanocrystals [77]. Surfactants are necessary in this case to
stabilize the interfaces of the nan oparticles 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 sys tem consists of an oil phase, a
surfactant phase and an aqueous ph ase. It is basically a
thermodynamically stable isotropic dispersion of an aque-
ous 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 nan owires of TiO
2
[82]. Microemulsion method
is also used to prepare nanowires of SnO
2
[83]. Even
nanowires of complex polyoxometalate of the kind
Ag
4
SiW
12
O
40
are prepared using microemulsion technique
consisting of ethanol and AOT (sodium bis- (2-ethyexyl-

sulfosuccinate) [84]. Zhang et al. have used microemulsion
mediated hydrothermal process to prepare nanowires of
ZnO [85].
Single crystalline BaTiO
3
and SrTiO
3
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 BaTiO
3
nanowires, an excess of
H
2
O
2
was added at 1 00 -C to heptadecane solution
containing a 10:1 molar ratio of BaTi[OCH(CH
3
)
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
À 1
.
It is proposed that the anisotropic growth takes place
most likely due to precursor decomposition and crystalliza-
tion 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. Moreo ver, there are too many param-
eters 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
K.S. Shankar, A.K. Raychaudhuri / Materials Science and Engineering C 25 (2005) 738 – 751746
and guides the growth direction and does not serve as a
microreactor. The behavior of su rfactant 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 mor e 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 MnO
2
[88],Fe
2
O
3
[89], and V
2
O
5
[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 Bi
2
S
3
nanorods by

microwave irradiation of formaldehyde solution of bismuth
nitrate and thiorea through the formation of bismuth thiorea
complexes [91]. Nanostructures of CuS including nano-
tubules 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 solvotherm al 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
method. Subsequent dehydration, sulfidation and fluori-
dation 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; MnO
2
[94],V
2
O
5
[95], potassium titanate
K
2
Ti
6
O
13
[96] and single crystalline nanowires of barium
doped rare earth manganite (La
0.5
Ba
0.5
MnO
3
) [97].
BaTiO
3
nanotubes arrays are also prepared by hydro-
thermal 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 synthesiz-
ing 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 nano-
structures are through the preparation of lamellar struc-
tures. The method is based on self assembly of inorganic
precursors at the template-sol ution 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 dimin-
ish 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 V
2
O
5
, MnO
2
,WO
3Àx
and LnOH etc. Under
hydrothermal, salvothermal or sonochemical conditions,
lamellar structures form, which roll up to yield nano-

tubules or nanowires.
These surfactant assisted or surfactant free solution
methods are attractive for large scale production of nano-
wires as they offer the advantages of low cost, simple
apparatus and low temperature preparations. Further inves-
tigation has to be focused on quality of the nanowires and
on means of obtai nin g well-aligne d u nif orm array o f
nanowires with uniform morphology and perfect crystal-
linity. 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 nano-
wires within templates.
K.S. Shankar, A.K. Raychaudhuri / Materials Science and Engineering C 25 (2005) 738 – 751 747
8. Template-free solution method for ordered nanowire
array
Ordered array of ZnO nanowires on various substrates
includingplasticandglasscanbeobtainedbylow
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 prin ciple 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. NH
3
also reduces
the concentration of Zn
2+
ion by producing complex ions
of the type, Zn(NH
3
)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
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.
Fig. 7. Scanning electron micrograph of ZnO nanowires on (A) seeded and
(B) bare Si wafer.
K.S. Shankar, A.K. Raychaudhuri / Materials Science and Engineering C 25 (2005) 738 – 751748

The most critical parameters to achieve oriented growth
are,
1. Controlli ng 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 concentra-
tion low.
2. Reducing the interfaci al 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 nanostructu res.
This technique of template free low temperature syn-
thesis to obtain ordered array of nanowires on any substrate
is a very powerful method and need to be explored for many
more materials systems.
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, low-
cost 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 wo uld use a combinatio n of physical and
chemical templates.
Large-scale implementation of chemical solution pro-
cessing approaches is greatly facilitated by commercial
availability of readymade templates (as in the case of AAO
or polymeric membranes). Developm ent 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
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 mat erials processing (2) Good under-
standing 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 struc-
tures (6) Compatibility with the device fabrication enviro-
ment (7) Absence of volatile and toxic byproducts during
the synthesis and (8) The synthesis should involve mini-
mum 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
[1] Z.L. Wang (Ed.), Nanowires and Nanobelts—Materials, Properties
and Devices, Kluwer Academic Publishers, Boston, 2003, Two
volumes.
[2] J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435.
[3] D.A. Wharam, T.J. Thornton, R. Newbury, M. Pepper, H. Ahmed,
J.E.F. Frost, D.G. Hasko, D.C. Peacock, D.A. Ritchie, G.A.C. Jones,
J. Phys. C. Solid State Phys. 21 (1988) L209.
[4] Y.M. Lin, O. Rabin, S.B. Cronin, J.Y. Ying, M.S. Dresselhaus, Appl.
Phys. Lett. 81 (2002) 2403.
[5] M. Huang, S. Mao, H. Freik, H. Yan, Y. Wu, H. Kind, E. Weber, R.
Russo, P. Yang, Science 292 (2001) 1897.
[6] M.S. Gudikson, J. Wang, C.M. Lieber, J. Phys. Chem., B 106 (2002)
4036.
[7] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Science 293 (2001) 1289.
[8] L.A. Bauer, N.S. Birenbaum, G.J. Meyer, J. Mater. Chem. 14 (2004)
517.
[9] F.J. Himpsel, T. Jung, A. Kirakosian, J.L. Lin, D.Y. Petrovych, H.
Rausher, J. Viernow, MRS Bull. 24 (8) (1999) 20.
[10] H. Dai, E.W. Wong, Y.Z. Lu, S. Fan, C.M. Lieber, Nature 375 (1995)
769.
[11] C.R. Martin, Science 266 (1994) 1961.
[12] C.N.R. Rao, A. Muller, A.K. Cheetham (Eds.), The Chemistry of
Nanomaterials: Synthesis, Properties and Applications, Wiley-VCH,
2004, p. 688.

[13] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89.
[14] T.J. Trentler, K.M. Hickman, S.C. Goel, A.M. Viano, P.C. Gibbons,
W.E. Buhro, Science 270 (1995) 1791.
[15] P. Yang, C.M. Lieber, Science 273 (1996) 1836.
[16] P. Yang, C.M. Lieber, J. Mater. Res. 12 (1997) 2172.
[17] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim,
H. Yan, Adv. Mater. 15 (5) (2003) 353.
[18] C.N.R. Rao, F.L. Deepak, G. Gundaiah, A. Govindaraj, Prog. Solid
State Chem. 31 (2003) 1.
[19] Yang, Lieber, Science 273 (1996) 1836.
[20] Pan, Wang, et al., Science 291 (2001) 1947.
[21] M.S. Arnold, Ph. Avouris, Z.W. Pan, Z.L. Wang, J. Phys. Chem., B
107 (2003) 659.
[22] M.S. Gudikson, L.J. Lauhon, J. Wang, D. Smith, C.M. Lieber, Nature
415 (2002) 617.
[23] X. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 409 (2001)
66.
[24] Y. Cui, C.M. Lieber, Science (2001) 851.
K.S. Shankar, A.K. Raychaudhuri / Materials Science and Engineering C 25 (2005) 738 – 751 749
[25] Y. Huang, X. Duan, Y. Cui, L. Lauhon, K. Kim, C.M. Lieber, Science
294 (2001) 1313.
[26] Y. Zhang, N. Wang, S. Guo, R. He, S. Miao, J. Liu, J. Zhu, X. Zhang,
Chem. Mater. 14 (2002) 3564.
[27] H.Y. Dang, J. Wang, S.S. Fan, Nanotechnology 14 (2003) 738.
[28] J.K.N. Mbindyo, B.D. Reiss, B.R. Martin, C.D. Keating, M.J. Natan,
T.E. Mallouk, Adv. Mater. 13 (2001) 249.
[29] J. Heremans, C.M. Thrush, Y M. Lin, S. Cronin, Z. Zhang, M.S.
Dresselhaus, J.F. Mansfeld, Phys. Rev., B 61 (2000) 2921.
[30] G.S. Cheng, L.D. Zhang, S.H. Chen, Y. Li, L. Li, X.G. Zhu, Y. Zhu,
G.T. Fei, Y.Q. Mao, J. Mater. Res. 15 (2000) 347.

[31] A.D. Berry, R.J. Tonucci, M. Fatemi, Appl. Phys. Lett. 69 (1996)
2846.
[32] G.A. Ozin, Adv. Mater. 4 (1992) 612.
[33] R.J. Tonucci, B.J. Justus, A.J. Campillo, C.E. Ford, Science 258
(1992) 783.
[34] J.Y. Ying, Sci. Spectra 18 (1999) 56.
[35] J.W. Diggle, T.C. Downie, C.W. Goulding, Chem. Rev. 69 (1969)
365.
[36] F. Li, L. Zhang, R.M. Metzger, Chem. Mater. 10 (1998) 2470.
[38] H. Chik, J.M. Xu, Mater. Sci. Eng., R Rep. 43 (2004) 103.
[39] J. Li, C. Papadopoulos, J.M. Xu, Nature 402 (1999) 253.
[40] S Z. Chu, K. Wada, S. Inoue, S I. Todoroki, Chem. Mater. 14
(2002) 266.
[41] E. Ferain, R. Legras, Nucl. Instrum. Methods Phys. Res., B 84 (1993)
331.
[42] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge,
K.D. Schmitt, C.T W. Chu, D.H. Olson, E.W. Sheppard, S.B.
McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114
(1992) 10834.
[43] C G. Wu, T. Bein, Science 264 (1994) 1757.
[44] T. Liu, C. Burger, B. Chu, Prog. Polym. Sci. 28 (2003) 5.
[45] M. Lazzari, M.A. Lopez-Quintela, Adv. Mater. 15 (2003) 19.
[46] E. Braun, Y. Eichen, U. Sivan, G. Ben-Yoseph, Nature 391 (1998)
775.
[47] R. Bashir, Mater. Today (2000 Nov/Dec) 30.
[48] M.S. Dresselhaus, Y M. Lin, O. Rabin, A. Jorio, A.G.S. Filho, M.A.
Pimenta, R. Saito, G.G. Samsonidze, G. Dresselhaus, Mater. Sci.
Eng., C 1008 (2002) 1.
[49] C. Schonenberger, B.M.I. van der Zande, L.G.J. Fokkinink, M.
Henny, C. Schmid, M. Kruger, A. Bachtold, R. Huber, H. Birk, U.

Staufer, J. Phys. Chem., B 10 (1997) 5497.
[50] M.J. Zheng, L.D. Zhang, G.H. Li, X.Y. Zhang, X.F. Wang, Appl.
Phys. Lett. 79 (2001) 839.
[51] M. Zheng, G. Li, X. Zhang, S. Huang, Y. Lei, L. Zhang, Chem.
Mater. 13 (2001) 3859.
[52] Y. Li, G.W. Meng, L.D. Zhang, F. Phillip, Appl. Phys. Lett. 76
(2000) 2011.
[53] X.Y. Zhang, L.D. Zhang, W. Chen, G.W. Meng, M.J. Zheng, L.X.
Zhao, Chem. Mater. 13 (2001) 2511.
[54] X. Zhang, B. Yao, L. Zhao, C. Liang, L. Zhang, Y. Mao, J.
Electrochem. Soc. 148 (2001).
[55] D.S. Xu, Y.G. Guo, D.P. Yu, G.L. Guo, Y.Q. Tang, J. Mater. Res. 17
(2002) 1711.
[56] C.J. Brinker, G.W. Scherer, Sol– Gel Science: the Physics and
Chemistry of Sol– Gel Processing, Academic Press, San Diego,
1990.
[57] Li, H. Zhang, F. Chi, S. Li, B. Xu, M. Zhao, Mater. Sci. Eng., B 18
(1993) 209.
[58] B.B. Lakshmi, C.J. Patrisi, C.M. Martin, Chem. Mater. 9 (1997)
2544.
[59] B. Cheng, E.T. Samulski, J. Mater. Chem. 11 (2001) 2901.
[60] Y. Zhou, J. Huang, C. Shen, H. Li, Mater. Sci. Eng., A 335 (2002)
260.
[61] Y. Zhou, C. Shen, J. Huang, H. Li, Mater. Sci. Eng., B 95 (2002) 77.
[62] Y. Zhou, C. Shen, H. Li, Solid State Ionics 146 (2002) 81.
[63] Y. Zhou, H. Li, J. Mater. Sci. 37 (2002) 5261.
[64] H. Xu, D.H. Qin, Z. Yang, H.L. Li, Mater. Chem. Phys. 80 (2003)
524.
[65] X.Y. Ma, H. Zhang, J. Xu, J. Niu, Q. Yang, J. Sha, D. Yang, Chem.
Phys. Lett. 363 (2002) 579.

[66] Shantha Shankar, Sohini Kar, G.N. Subbanna, A.K. Raychaudhuri,
Appl. Phys. Lett. 84 (6) (2004) 993.
[67] K. Shantha Shankar and A.K. Raychaudhuri, submitted to Nano-
technology (2004).
[68] Y. Lei, L.D. Zhang, G.W. Meng, G.H. Li, X.Y. Zhang, C.H.
Liang, W. Chen, X.Y. Wang, Appl. Phys. Lett. 78 (8)
(2001).
[69] S.J. Limmer, S. Seraji, Y. Wu, T.P. Chou, C. Nguyen, G.Z. Cao, Adv.
Funct. Mater. 12 (2002) 59.
[70] S. Seraji, Y. Wu, N.E.J. Larson, M.J. Forbess, S.J. Limmer, T.P.
Chou, G.Z. Cao, Adv. Mater. 12 (2000) 1421.
[71] S.J. Limmer, S. Seraji, Y. Wu, M.J. Forbess, T.P. Chou, C. Nguyen,
G.Z. Cao, Adv. Mater. 13 (2001) 1269.
[72] K. Baba, O. Nishizato, A. Takamura, M. Katsube, J. Ceram. Soc.
Jpn., Int. Ed. 101 (1993) 1370.
[73] Z. Maio, D. Xu, J. Ouyang, G. Guo, X. Zhao, Y. Tang, Nano Lett. 2
(2002) 717.
[74] Z.R. Dai, Z.W. Pan, Z.L. Wang, Adv. Funct. Mater. 13 (2003) 9;
Y. Chen, X. Cui, K. Zhang, D. Pan, S. Zhang, B. Wang, J.G. Hou,
Chem. Phys. Lett. 369 (2003) 16;
J.K. Jian, X.L. Chen, W.J. Wang, L. Dai, Y.P. Xu, Appl. Phys., A 76
(2003) 291;
P. Nguyen, H.T. Ng, J. Kong, A.M. Cassell, R. Quinn, J. Li, J. Han,
M. McNeil, M. Meyyappan, Nano Lett. 3 (2003) 925.
[75] Y. Sun, B. Gates, B. Mayers, Y. Xia, Nano Lett. 2 (2002) 165.
[76] X. Jiang, Y. Wang, T. Herricks, Y. Xia, J. Mater. Chem. 14 (4) (2004)
695.
[77] X. Peng, J. Wickham, A.P. Alivisatos, J. Am. Chem. Soc. 120 (1998)
5343.
[78] L. Manna, E.C. Scher, A.P. Alivisatos, J. Am. Chem. Soc. 122 (2000)

12700.
[79] M. Chen, Y. Xie, J. Lu, Y.J. Xiong, S.Y. Zhang, Y.T. Qian, X.M. Liu,
J. Mater. Chem. 12 (2002) 748.
[80] L. Yan, Y.D. Li, Z.X. Deng, J. Zhangand, X.M. Sun, Int. J. Inorg.
Mater. 3 (2001) 633.
[82] G.L. Li, G.H. Wang, J.M. Hong, J. Mater. Res. 14 (8) (1999) 3346
(Patent No : ZL 98 1 11329.X, 1/9/2002).
[83] Y.K. Liu, C.L. Zhang, W.Z. Wang, C.R. Ying, G.H. Wang, Adv.
Mater. 13 (24) (2001) 1883.
[84] Z. Kang, E. Wang, M. Jiang, S. Lian, Nanotechnology 15 (2004)
55.
[85] J. Zhang, L. Sun, H. Pan, C. Liao, C. Yan, New J. Chem. 26 (2002)
33.
[86] M. Cao, C. Hu, E.J. Wang, Am. Chem. Soc. 125 (2003) 11196;
J.J. Urban, J.E. Spanier, L. Ouyang, W.S. Yun, H. Park, Adv. Mater.
15 (2003) 423.
[87] K.S. Suslick (Ed.), Ultrasound: its Chemical, Physical and Biological
Effect, VCH, Weinheim, 1988.
[88] IMLB 12 Meeting, * 2004 The Electrochemical Society K-H Kim
and K-B Kim.
[89] R. Vijaya Kumar, Yu Koltypin, X.N. Xu, Y. Yeshurun, A. Gedanken,
I. Felner, J. Appl. Phys. 89 (11) (2001)1988.
[90] J. Muster, G.T. Kim, V. Krstic, J.G. Park, Y.W. Park, S. Roth, M.
Burghard, Adv. Mater. 12 (2000) 420.
[91] X H. Liao, H. Wang, J J. Zhu, H Y. Chen, Mater. Res. Bull. 36
(2001) 2339.
[92] Y. Ni, H. Liu, F. Wang, G. Yin, J. Hong, X. Ma, Z. Xu, Appl. Phys.,
A 339 (2003) 2336.
[93] X. Wang, Y. Li, Chem. Eur. J. 9 (2003) 5627.
[94] Wang, Y.D. Li, J. Am. Chem. Soc. 124 (2002) 2884.

[95] J. Mater. Res. 17 (8) (2002) 1981.
[96] G.H. Du, Q. Chen, P.D. Han, Y. Yu, L.M. Peng, Phys. Rev., B 67
(2003) 035323.
K.S. Shankar, A.K. Raychaudhuri / Materials Science and Engineering C 25 (2005) 738 – 751750
[97] D. Zhu, H. Zhu, Y.H. Zhang, J. Phys. Condens. Mater. 14 (2002)
519;
D. Zhu, H. Zhu, Y.H. Zhang, Appl. Phys. Lett. 80 (9) (2002) 1634.
[98] N.P. Padture, X. Wei, J. Am. Ceram. Soc. (2003).
[99] X. Chen, X.M. Sun, Y.D. Li, Inorg. Chem. 41 (2002) 4524.
[100] Z.R. Tian, J.A. Voigt, J. Liu, B. Mckenzie, M.J. Mcdermott, M.J.
Mcdermott, M.A. Rodriguez, H. Konishi, H. Xu, Nat. Mater. 2
(2003) 821.
[101] L.E. Greene, M. Law, J. Goldberger, F. Kim, J.C. Johnson, Y. Zhang,
R.J. Saykally, P.D. Yang, Ang. Chemie 42 (26) (2003) 3031.
[102] L. Vayssieres, Adv. Mater. 15 (5) (2003) 464.
[103] http://www .nanosysinc.com/, http://www.
nano-lab.com/, />companies/0,2012,00.html.
[104] R.A. Bleeker, L.M. Troilo, D.P. Ciminello, Mater. Today (2004
February) 44.
[105] Y. Yin, G. Zhang, Y. Xia, Adv. Funct Mater. 12 (2002) 293;
R. Ma, Y. Bondo, Chem. Phys. Lett. 370 (2003) 770.
[106] X. Jiang, T. Herricks, Y. Xia, Nano Lett. 2 (2002) 1333.
[107] W. Wang, Y. Zhan, G. Wang, Chem. Commun. (2001) 727.
[108] Z.Q. Liu, S.S. Xie, L.F. Sun, D.S. Tang, W.Y. Zhou, C.Y. Wang,
W. Liu, Y.B. Li, X.P. Zou, G.J. Wang, Mater. Res. 16 (2001)
683.
[109] B. Zheng, Y. Wu, P. Yang, J. Liu, Adv. Mater. 14 (2002) 122.
[110] D.P. Yu, Q.L. Hang, Y. Ding, H.Z. Zhang, Z.G. Bai, J.J. Wang, Y.H.
Zou, W. Qian, G.C. Xiong, S.Q. Feng, Appl. Phys. Lett. 73 (1998)
3076.

[111] X.C. Wu, W.H. Song, W.D. Huang, M.H. Pu, B. Zhao, Y.P. Sun, J.J.
Du, Chem.Phys. Lett. 328 (2000) 5.
[112] S. Sharma, M.K. Sunkara, J. Am. Chem. Soc. 124 (2002)
12288.
[113] V. Valcarcel, A. Souto, F. Guitian, Adv. Mater. 10 (1998)
138.
[114] Z.L. Xiao, C.Y. Han, U. Welp, H.H. Wang, W.K. Kwok, G.A.
Willing, J.M. Hiller, R.E. Cook, D.J. Miller, G.W. Crabtree, Nano
Lett. 2 (2002) 1293.
[115] X.S. Peng, Y.W. Wang, J. Zhang, X.F. Wang, L.X. Zhao, G.W. Meng,
L.D. Zhang, Appl. Phys., A 74 (2001) 1.
[116] C. Li, D. Zhang, S. Han, X. Liu, T. Tang, C. Zhou, Adv. Mater. 15
(2003) 143.
[117] Z.R. Dai, Z.W. Pan, X.L. Wang, Solid State Commun. 118 (2001)
351.
[118] L. Guo, Z. Wu, T. Liu, W. Wang, H. Zhu, Chem. Phys. Lett. 318
(2000) 49.
K.S. Shankar, A.K. Raychaudhuri / Materials Science and Engineering C 25 (2005) 738 – 751 751