inorganic nanowires

Theseinclude: i the use of the anisotropic crystallographic structure of the solid tofacilitate 1D nanowire growth; ii the introduction of a solid–liquid interface; iiiuse of templates w

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Inorganicnanowires

Chemistry and Physics of Materials Unit and CSIR Centre of Excellence in Chemistry, Jawaharlal Nehru

Centre for Advanced Scientiﬁc Research, Jakkur P.O., Bangalore 560 064, India

Abstract

Since the discovery of carbon nanotubes, there has been great interest in the synthesis and characterization of other one-dimensional materials A variety of inorganic materials have been prepared in the form of nanowires with a diameter of a few nm and lengths going up

to several microns In order to produce the nanowires, both vapor-growth and solution-growth processes have been made use of Besides physical methods, such as thermal evapor-ation and laser ablevapor-ation, chemical methods including solvothermal, hydrothermal and car-bothermal reactions have been employed for their synthesis In this article, we describe the synthesis, structure and properties of nanowires of various inorganic materials, which include elements, oxides, nitrides, carbides and chalcogenides Wherever possible, we have also included the relevant information on related one-dimensional materials, such as nano-belts

Keywords: Nanostructures; Nanowires; Nanorods; One-dimensional materials

Contents

1 Introduction 7

2 Syntheticstrategies 8

2.1 Vapor phase growth of nanowires 8

2.1.1 Vapor–liquid–solid growth 8

2.1.2 Oxide-assisted growth 11

2.1.3 Vapor–solid growth 12

2.1.4 Carbothermal reactions 12

2.2 Solution based growth of nanowires 13

2.2.1 Highly anisotropic crystal structures 14

Corresponding author Tel.: +91-80-846-2760; fax: +91-80-856-3075.

E-mail address: cnrrao@jncasr.ac.in (C.N.R Rao).

doi:10.1016/j.progsolidstchem.2003.08.001

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2.2.2 Template-based synthesis 14

2.2.3 Solution–liquid–solid process 16

2.2.4 Solvothermal synthesis 16

2.3 Growth control and integration 16

3 Elemental nanowires 18

3.1 Silicon 18

3.2 Germanium 24

3.3 Boron 26

3.4 In, Sn and Pb 28

3.5 Sb and Bi 28

3.6 Se and Te 29

3.7 Compound semiconductors 31

3.8 Gold 32

3.9 Silver 34

3.10 Iron 37

3.11 Cobalt 38

3.12 Nickel 41

3.13 Copper 43

3.14 Other metals and alloys 45

4 Oxide nanowires 47

4.1 MgO 47

4.2 Al2O3 50

4.3 Ga2O3 54

4.4 In2O3 58

4.5 SnO2 61

4.6 Sb2O3and Sb2O5 64

4.7 SiO2 64

4.8 GeO2 68

4.9 TiO2 70

4.10 MnO2and Mn3O4 72

4.11 CuxO 74

4.12 ZnO 78

4.13 V2O5 83

4.14 WOx 83

4.15 Other binary oxides 83

4.16 Ternary and quarternary oxides 86

5 Nitride nanowires 88

5.1 BN 88

5.2 AlN 91

5.3 GaN 94

5.4 InN 102

5.5 Si3N4and Si2N2O 105

C.N.R Rao et al / Progress in Solid State Chemistry 31 (2003) 5–147

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1 Introduction

Ever since the discovery of carbon nanotubes by Iijima[1], there has been great interest in the synthesis and characterization of other one-dimensional (1D) struc-tures Nanowires, nanorods and nanobelts constitute an important class of 1D nanostructures, which provide models to study the relationship between electrical transport, optical and other properties with dimensionality and size conﬁnement The inorganic nanowires can also act as active components in devices as revealed

by recent investigations In the last 3–4 years, a variety of inorganic materials nanowires has been synthesized and characterized Thus, nanowires of elements, oxides, nitrides, carbides and chalcogenides, have been generated by employing various strategies One of the crucial factors in the synthesis of nanowires is the control of composition, size and crystallinity Among the methods employed, some are based on vapor phase techniques, while others are solution techniques Compared to physical methods such as nanolithography and other patterning tech-niques, chemical methods have been more versatile and eﬀective in the synthesis of these nanowires Thus, techniques involving chemical vapor deposition (CVD), precursor decomposition, as well as solvothermal, hydrothermal and carbothermal

6 Metal carbide nanowires 109

6.1 Carbides of Al and B 109

6.2 SiC 110

6.3 TiC 114

7 Metal chalcogenide nanowires 115

7.1 CdS 115

7.2 CdSe 117

7.3 PbS and PbSe 119

7.4 Bismuth chalcogenides 120

7.5 Ti,Zr, Hf sulﬁdes 120

7.6 CuS and CuSe 121

7.7 ZnS and ZnSe 123

7.8 Ag2Se and NiS 126

7.9 NbS2and NbSe2 126

7.10 Other chalcogenides 126

8 Other semiconductor nanowires 128

8.1 GaAs 128

8.2 InP 130

8.3 GaP 131

9 Miscellaneous nanowires 132

10 Concluding remarks 133

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methods have been widely employed Several physical methods, especially scopic techniques such as scanning electron microscopy (SEM), transmissionelectron microscopy (TEM), scanning tunneling microscopy (STM) and atomic forcemicroscopy (AFM) are commonly used to characterize nanowires There are afew surveys of nanowires [2,3] and of nanotubes [4,5] in the literature In thisarticle, we present a comprehensive and up-to-date review of the various families

micro-of inorganicnanowires wherein we discuss their synthesis along with theirproperties Wherever possible, we have also indicated potential applications

2 Synthetic strategies

An important aspect of the 1D structures relates to their crystallization [6],wherein the evolution of a solid from a vapor, a liquid, or a solid phase involvesnucleation and growth As the concentration of the building units (atoms, ions, ormolecules) of a solid becomes suﬃciently high, they aggregate into small nuclei orclusters through homogeneous nucleation These clusters serve as seeds for furthergrowth to form larger clusters Several synthetic strategies have been developed for1D nanowires with diﬀerent levels of control over the growth parameters Theseinclude: (i) the use of the anisotropic crystallographic structure of the solid tofacilitate 1D nanowire growth; (ii) the introduction of a solid–liquid interface; (iii)use of templates (with 1D morphologies) to direct the formation of nanowires; (iv)supersaturation control to modify the growth habit of a seed; (v) use of cappingagents to kinetically control the growth rates of the various facets of a seed; and(vi) self-assembly of zero-dimensional (0D) nanostructures They are convenientlycategorized into (a) growth in the vapor phase; and (b) solution-based growth.2.1 Vapor phase growth of nanowires

Vapor phase growth is extensively used for producing nanowires Starting withthe simple evaporation technique in an appropriate atmosphere to produce elemen-tal or oxide nanowires, vapor–liquid–solid, vapor–solid and other processes arealso made use of:

2.1.1 Vapor–liquid–solid growth

The growth of nanowires via a gas phase reaction involving the vapor–liquid–solid (VLS) process has been widely studied Wagner [6], during his studies ofgrowth of large single-crystalline whiskers, proposed in 1960s, a mechanism for thegrowth via gas phase reaction involving the so-called vapor–liquid–solid process

He studied the growth of mm-sized Si whiskers in the presence of Au particles.According to this mechanism, the anisotropic crystal growth is promoted by thepresence of the liquid alloy/solid interface This mechanism has been widelyaccepted and applied for understanding the growth of various nanowires includingthose of Si and Ge among others The growth of Ge nanowires using Au clusters

as a solvent at high temperature is explained based on the Ge-Au phase diagramshown in Fig 1 Ge and Au form a liquid alloy when the temperature is higherthan the eutectic point (363 v

C) as shown in Fig 1(a-I) The liquid surface has a

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large accommodation coeﬃcient and is therefore a preferred deposition site for theincoming Ge vapor After the liquid alloy becomes supersaturated with Ge, pre-cipitation of the Ge nanowire occurs at the solid-liquid interface (Fig 1(a-II–III).Until recently, the only evidence that nanowires grew by this mechanism was thepresence of alloy droplets at the tips of the nanowires Wu et al [7]have reportedreal-time observations of Ge nanowire growth in an in situ high-temperature TEM,which demonstrate the validity of the VLS growth mechanism Their experimental

Fig 1 (a) Schematic illustration of vapor-solid growth mechanism including three stages (I) alloying, (II) nucleation and (III) axial growth Three stages are projected onto the coventional Au-Ge phase diagram; (b) shows the compositional and phase evolution during the nanowire growth process (Wu and Yang [7] ).

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observations suggest that there are three growth stages: metal alloying, crystalnucleation and axial growth (Fig 2).

Fig 2(a)–(f) shows a sequence of TEM images during the in situ growth of a Genanowire Three stages, I–III, are clearly identiﬁed (I), Alloying process, (Fig 2(a)–(c)): The maximum temperature that could be attained in the system was 900v

C,

up to which the Au clusters remain in the solid state in the absence of Ge vapor.With increasing amount of Ge vapor condensation and dissolution, Ge and Auform an alloy and liquefy The volume of the alloy droplet increases and theelemental contrast decreases, while the alloy composition crosses sequentially, fromleft to right, a biphasicregion (solid Au and Au/Ge liquid alloy) and a single-phase region (liquid) An isothermal line in the Au-Ge phase diagram (Fig 1(b))shows the alloying process (II), Nucleation, (Fig 2(d)–(e)): As the concentration

of Ge increases in the Au-Ge alloy droplet, the process of nucleation of the wire begins Knowing the alloy volume change, it is estimated that the nucleationgenerally occurs at a Ge weight percentage of 50–60% (III), Axial growth,(Fig 2(d)–(f)): Once the Ge nanocrystal nucleates at the liquid/solid interface, fur-ther condensation/dissolution of the Ge vapor into the system increases theamount of Ge precipitation from the alloy The incoming Ge vapors diﬀuse andcondense at the solid/liquid interface, thus suppressing secondary nucleationevents The interface is then pushed forward (or backward) to form nanowires(Fig 2(f)) This study conﬁrms the validity of the VLS growth mechanism at thenanometer scale

nano-Since the diameter of the nanowires is determined by the diameter of the catalystparticles, this method provides an eﬃcient means to obtain uniform-sized nano-wires Also, with the knowledge of the phase diagram of the reacting species, the

Fig 2 In situ TEM images recorded during the process of nanowire growth (a) Au nanoclusters in solid state at 500 v

C; (b) alloying initiated at 800 v

C, at this stage Au exists mostly in solid state; (c) liquid Au/Ge alloy; (d) the nucleation of Ge nanocrystal on the alloy surface; (e) Ge nanocrystal elon- gates with further Ge condensation and eventually forms a wire (f) (Wu and Yang [7] ).

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growth temperature can be set in between the eutectic point and the melting point

of the material Physical methods, such as laser ablation or thermal evaporation, aswell as chemical methods such as chemical vapor deposition can be used to gener-ate the reactant species in vapor form, required for the nanowire growth Catalystparticles can be sputtered onto the substrates or metal nanoparticles prepared bysolution-based routes used as the catalysts An advantage of this route is that pat-terned deposition of catalyst particles yields patterned nanowires Using thisgrowth mechanism, nanowires of materials including elements, oxides, carbides,phosphides, etc., have been successfully obtained, as detailed in the forthcomingsections

2.1.2 Oxide-assisted growth

In contrast to the well-established VLS growth, Lee and co-workers [8,9] haveproposed a nanowire growth mechanism called the oxide-assisted growth mech-anism No metal catalyst is required for the synthesis of nanowires by this means.Based on their experimental observations, these workers ﬁnd that the growth of Sinanowires is greatly enhanced when SiO2-containing Si powder targets were used.Limited quantities of Si nanowires were obtained even with a target made of pure

Si powder (99.995%)

Lee et al propose that the growth of the Si nanowires is assisted by the Si oxide,where the SixO (x > 1) vapor generated by thermal evaporation or laser ablationplays the key role Nucleation of the nanoparticles is assumed to occur on the sub-strate as shown in eqs (1) and (2)

These decompositions result in the precipitation of Si nanoparticles, which act asthe nuclei of the silicon nanowires covered by shells of silicon oxide The precipi-tation, nucleation and growth of the nanowires occur in the area near the cold ﬁn-ger, suggesting that the temperature gradient provides the external driving force forthe formation and growth of the nanowires

Fig 3(a)–(c) show the TEM images of the formation of nanowire nuclei at theinitial stages Fig 3(a) shows Si nanoparticles covered by an amorphous siliconoxide layer The nanoparticles that are isolated, with the growth directions normal

to the substrate surface, exhibit the fastest growth The tip of the Si crystalline corecontains a high concentration of defects, as marked by arrows inFig 3(c).Fig 4

shows a schematic of the nanowire growth by this mechanism The growth of thesilicon nanowires is determined by four factors: (1) catalytic eﬀect of the SixO(x > 1) layer on the nanowire tips; (2) retardation of the lateral growth of nano-wires by the SiO2component in the shells, formed by the decomposition of SiO; (3)stacking faults along the nanowire growth direction of <112>, which normallycontain easy-moving 1/6[112] and nonmoving 1/3[111] partial dislocations, andmicro-twins present at the tip areas causing fast growth of Si nanowires and (4) the{111} surfaces, which have the lowest surface among the Si surfaces, playing animportant role in nucleation and growth, since the energy of the system is reduced

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signiﬁcantly when the {111} surfaces are parallel to the axis of the nanowires Thelast two factors ensure that only the nuclei that have their <112> direction parallel

to the growth direction grow fast (Fig 4(b))

2.1.3 Vapor–solid growth

The vapor–solid (VS) method for whisker growth also holds for the growth of1D nanomaterials [6] In this process, evaporation, chemical reduction or gaseousreaction ﬁrst generates the vapor The vapor is subsequently transported and con-densed onto a substrate The VS method has been used to prepare whiskers ofoxide, as well as metals with micrometer diameters It is, therefore, possible to syn-thesize the 1D nanostructures using the VS process if one can control thenucleation and the subsequent growth process Using the VS method, nanowires ofthe oxides of Zn, Sn, In, Cd, Mg, Ga and Al have been obtained

2.1.4 Carbothermal reactions

Nanowires of a variety of oxides, nitrides and carbides can be synthesized bycarbothermal reactions For example, carbon (activated carbon or carbon nano-tubes) in mixture with an oxide produces sub-oxidic vapor species which reactswith C, O2, N2or NH3to produce the desired nanowires Thus, heating a mixture

of Ga2O3and carbon in N2or NH3produces GaN nanowires Carbothermal tions generally involve the following steps:

reac-metal oxideþ C ! metal suboxide þ CO

Fig 3 TEM micrographs of (a) Si nanowire nuclei formed on the Mo grid and (b), (c) initial growth stages of the nanowires (Lee et al [8] ).

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metal suboxideþ O2! metal oxide nanowires

metal suboxideþ NH3! metal nitride nanowires þ CO þ H2

metal suboxideþ N2! metal nitride nanowires þ CO

metal suboxideþ C ! metal carbide nanowires þ CO

The ﬁrst step normally involves the formation of a metal suboxide by the reaction ofthe metal oxide with carbon Depending on the desired product, the suboxide heated

in the presence of O2, NH3, N2or C yields oxide, nitride or carbide nanowires.2.2 Solution-based growth of nanowires

This syntheticstrategy for nanowires makes use of anisotropicgrowth dictated

by the crystallographic structure of the solid material, or conﬁned and directed bytemplates, or kinetically controlled by supersaturation, or by the use of appropriatecapping agent

Fig 4 Schematic describing the nucleation and growth mechanism of Si nanowires The parallel lines indicate the [112] orientation (a) Si oxide vapor is deposited ﬁrst and forms the matrix within which the

Si nanoparticles are precipitated (b) Nanoparticles in a preferred orientation grow fast and form wires Nanoparticles with nonpreferred orientations may form chains of nanoparticles (Lee et al [8] ).

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nano-2.2.1 Highly anisotropic crystal structures

Solid materials such as polysulphurnitride, (SN)x, grow into 1D nanostructures,the habit being determined by the anisotropicbonding in the structure [10,11].Other materials, such as selenium [12,13], tellurium [14] and molybdenumchalcogenides[15]are easily obtained as nanowires due to anisotropicbonding, whichdictates the crystallization to occur along the c-axis, favoring the stronger cova-lent bonds over the relatively weak van der Waals forces between the chains.2.2.2 Template-based synthesis

Template-directed synthesis represents a convenient and versatile method forgenerating 1D nanostructures In this technique, the template serves as a scaﬀoldagainst which other materials with similar morphologies are synthesized That is,the in situ generated material is shaped into a nanostructure with a morphologycomplementary to that of the template The templates could be nanoscale channelswithin mesoporous materials, porous alumina and polycarbonate membranes Thenanoscale channels are ﬁlled using, the solution, the sol-gel or the electrochemicalmethod The nanowires so produced are released from the templates by removal ofthe host matrix[16] Unlike the polymer membranes fabricated by track etching,anodic alumina membranes (AAMs) containing a hexagonally packed 2D array ofcylindrical pores with a uniform size are prepared using anodization of aluminiumfoils in an acidic medium (Fig 5) Several materials have been fabricated intonanowires using AAMs in the templating process The various inorganic materialsinclude Au, Ag, Pt, TiO2, MnO2, ZnO, SnO2, In2O3, CdS, CdSe, CdTe, electro-nically conducting polymers such as polypyrole, poly(3-methylthiophene) and poly-aniline, as well as carbon nanotubules The only drawback of this method is that it

is diﬃcult to obtain materials that are single-crystalline

Fig 5 TEM micrograph of an anodic alumina membrane (AAM) (Zheng et al [16c] ) C.N.R Rao et al / Progress in Solid State Chemistry 31 (2003) 5–147

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Besides alumina and polymer membranes with high surface areas and uniformpore sizes, mesoporous silica has been successfully used as a template for the syn-thesis of polymer and inorganicnanowires Mesophase structures self-assembledfrom surfactants (Fig 6) provide another class of useful and versatile templates forgenerating 1D nanostructures in relatively large quantities It is well known that atcritical micellar concentration (CMC) surfactant molecules spontaneously organizeinto rod-shaped micelles[17] These anisotropic structures can be used immediately

as soft templates to promote the formation of nanorods when coupled with priate chemical or electrochemical reaction The surfactant needs to be selectivelyremoved to collect the nanorods/nanowires Based on this principle, nanowires ofCuS, CuSe, CdS, CdSe, ZnS and ZnSe have been grown, by using surfactants such

appro-as Na-AOT and Triton X of known concentrations[18,19]

Nanowires themselves can be used as templates to generate the nanowires ofother materials The template may be coated to the nanowire (physical) formingcoaxial nanocables [20], or it might react with the nanowires forming a newmaterial[21] In the physical methods (solution or sol-gel coating), surfaces of thenanowires are directly coated with conformal sheaths made of a diﬀerent material

to form coaxial nanocables Subsequent dissolution of the original nanowires leads

to nanotubes of the coated materials The sol-gel coating method is a generic route

to synthesize co-axial nanocables that may contain electrically conductive metalcores and insulating sheaths

Govindaraj et al.[22]have demonstrated that a variety of metal nanowires of 1–1.4 nm diameter can be readily prepared by ﬁlling SWNTs, opened by acid treat-ment Nanowires of Au, Pt, Pd and Ag have been synthesized by employing sealed-tube reactions, as well as solution methods In addition, incorporation of thin lay-ers of metals in the intertubular space of the SWNT bundles has been observed(see Section 3.8 for details)

Fig 6 Schematic illustration showing the formation of nanowires by templating against mesostructures which are self-assembled from surfactant molecules (a) Formation of cylindrical micelle; (b) formation

of the desired material in the aqueous phase encapsulated by the cylindrical micelle; (c) removal of the surfactant molecule with an appropriate solvent (or by calcination) to obtain an individual nanowire (Xia et al [17a] ).

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2.2.3 Solution–liquid–solid process

Buhro and coworkers [23] have developed a low temperature solution–liquid–solid (SLS) method for the synthesis of crystalline nanowires of III–V semi-conductors[24] In a typical procedure, a metal (e.g In, Sn, Bi) with a low meltingpoint is used as a catalyst, and the desired material generated through thedecomposition of organometallic precursors Nanowhiskers of InP, InAs andGaAs have been prepared by low temperature (203 v

C) solution phase reactions.The schematic illustration inFig 7clearly shows the growth of nanowires or whis-kers through the SLS method The products obtained are generally single-crystal-line

Korgel et al.[25]have used the supercritical ﬂuid–liquid–solid (SFLS) method tosynthesize bulk quantities of defect-free silicon and germanium nanowires, details

of which are presented later in the article

In addition to these solution routes to elemental III–V semiconductor nanowires,

it has been reported recently that by exploiting the selective capping capacities ofmixed surfactants, it is possible to extend the synthesis of the II–VI semiconductornanocrystals to that of semiconductor nanorods[26], a version of nanowires withrelatively shorter aspect ratios

2.2.4 Solvothermal synthesis

Solvothermal methodology is extensively employed as a solution route to duce semiconductor nanowires and nanorods In this process, a solvent is mixedwith metal precursors and crystal growth regulating or templating agents, such asamines This solution mixture is placed in an autoclave maintained at relativelyhigh temperatures and pressures to carry out the crystal growth and the assemblyprocess The methodology is quite versatile and has enabled the synthesis of crys-talline nanowires of semiconductors and other materials

pro-2.3 Growth control and integration

A signiﬁcant challenge in the chemical synthesis of nanowires is how to ally control the nanostructure assemblies so that their size, dimensionality, inter-faces and their 2D and 3D superstructures can be tailor-made towards desired

ration-Fig 7 Schematic illustration showing the growth of nanowire through the solution–liquid–solid (SLS) mechanism which is similar to the vapor–liquid–solid (VLS) process (Trentler et al [23] ).

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functionality Many physical and thermodynamic properties are dent Several groups of workers have synthesized uniform-sized nanowires by theVLS process using clusters with narrow size distributions.

diameter-depen-Controlling the growth orientation is important for the applications of wires By applying the conventional epitaxial crystal growth technique to the VLSprocess, a vapor–liquid–solid epitaxy technique has been developed for the con-trolled synthesis of nanowire arrays Nanowires generally have preferred growthdirections For example, zinc oxide nanowires prefer to grow along their c-axis,that is along the <001> direction [27,28] Also, Si nanowires grow along the

nano-<111> direction when grown by the VLS growth process, but can be made togrow along the <112> or the <110> direction by the oxide-assisted growth mech-anism

It is clear from the VLS nanowire growth mechanism that the initial positions of

Au clusters or Au thin ﬁlms control the positions of the nanowires By creatingdesired patterns of Au using a lithographictechnique, it is possible to grow ZnOnanowires of the same designed pattern since they grow vertically only from theregion coated with Au and form the designed patterns of ZnO nanowire arrays

[27,28] Similarly, networks of nanowires with the precise placement of individualnanowires on substrates with the desired conﬁguration is achieved by the surfacepatterning strategy[27,28]

Integration of nanowire building blocks into complex functional networks in acontrolled fashion is a major challenge The direct one-step growth process hasbeen used [27,28] In this process, the nanowires, grown by the VLS method, arepatterned on substrates by selectively depositing in catalyst particles Another way

is to place the nanowire building blocks together into the functional structure todevelop a hierarchical assembly By using a simple dubbed microﬂuidic-assisted

Fig 8 Schematic illustration of the microﬂuidic-assisted nanowire integration process for nanowire face patterning (Wu et al [30b] ).

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sur-nanowire integration process, wherein the sur-nanowire solution/suspension is ﬁlled inthe microchannels formed between poly(dimethylsiloxane) (PDMS) micromouldand a ﬂat Si substrate, followed by the evaporation of the solvent, nanowire sur-face patterning and alignment has been achieved [29,30] A schematic illustration

of the microﬂuidic-assisted nanowire integration process is shown in Fig 8 TheLangmuir Blodgett technique has also been used to obtain aligned, high-densitynanowire assemblies[31]

3 Elemental nanowires

3.1 Silicon

Silicon nanowires (SiNWs) have been prepared by a variety of methods, whichinclude physical evaporation of the metal at one end and chemical vapor depo-sition (CVD) at the other The methods employ SiOx and other precursors as sili-

Fig 9 (a) TEM image of the SiNWs with an average diameter of around 15 nm The inset shows the SAED (b) TEM of the SiNWs after etching the outer oxide layer in dilute HF (Yu et al [32] ).

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con sources The ﬁrst report of the synthesis of SiNWs by thermal evaporation was

by Yu et al [32], who sublimed a hot-pressed Si powder target mixed with Fe at

1200 v

C in flowing Ar gas at a pressure of ~100 Torr Using this simple method,they could obtain SiNWs with a diameter of ~15 nm and length varying from a fewtens to hundreds of microns as shown in Fig 9(a) The inset in the figure shows aselected-area electron diffraction (SAED) pattern, which is similar to that of bulksilicon The nanowires were sheathed by an amorphous oxide layer of about 2 nm,which could be etched out by treatment with a dilute HF solution A TEM image

of the nanowires after this treatment is shown inFig 9(b) By varying the ambientpressure between 150 to 600 Torr [33], the diameters of the nanowires was con-trolled The average size of the nanowires increases with the increasing gas press-ure By using Fe-patterned Si substrates and employing thermal evaporation, thenanowires can be positioned[34] The silicon substrates are patterned with a 5 nm-thick Fe ﬁlm by electron beam evaporation and lithography and the SiNWs selec-tively grown onto them By heating pure Si powder at 1373 K under Ar ﬂow onto

a quartz substrate coated with Fe(NO3)3, it has been possible to obtain Si and SiOx

(x¼ 1 to 2) nanostructures [35] The products obtained include ﬁst-capped SiOx

fibers (Si core), tree-like SiOxnanofibers and tadpole-like SiOxnanofibers

The vapor–liquid–solid (VLS) method, involving the use of liquid–metal solventswith low solubility for Si and other elemental semiconductor materials, has beensuccessful in producing SiNWs in large quantities by a low temperature route[36].SiNWs with a uniform diameter of ~6 nm were synthesized using Ga as the moltensolvent at temperatures below 400 vC in a hydrogen plasma Defect-free SiNWswith diameters in the range of 4–5 nm and lengths of several microns were synthe-sized using a supercritical ﬂuid solution-phase approach wherein alkanethiol-coated Au nanocrystals (2.5 nm in diameter) were used as seeds to direct the one-dimensional crystallization of Si in a solvent heated and pressurized above its criti-cal point[25] The reaction pressure controlled the orientation of the nanowires.Application of a voltage between a Si substrate and an Au STM tip [37] pro-duces SiNWs The most common technique, however, is laser ablation By thismethod, high-purity, crystalline nanowires are obtained in high yields [38] Thesehave diameters ranging from 3 to 43 nm with lengths extending up to a few hun-dred microns TEM studies show them to possess a high density of structuraldefects, which may play a role in the formation of the SiNWs and in the determi-nation of the morphology [39] The diameters of the nanowires synthesized usinglaser ablation change with the ambient gas[40] Thus, nanowires with diﬀerent dia-meters have been synthesized in the presence of He, Ar (5% H2) and N2

Laser ablation has been combined with the VLS method to good eﬀect to thesize semiconductor nanowires[41] In this process, laser ablation is employed toprepare nanometric catalyst clusters that deﬁne the size of the Si/Ge nanowiresproduced by the VLS growth In Fig 10(a), we show a TEM image of SiNWsobtained by the ablation of a Si0.9Fe0.1target at 1200v

syn-C, with diameters of ~10 nmand lengths above 1 lm The presence of the catalyst particles at the ends of nano-wires suggests that they grow by the VLS mechanism An oxide layer, as evidencedfrom the TEM image inFig 10(b), sheaths the nanowires The inset shows that the

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nanowires are single-crystalline The HREM image in Fig 10(c) reveals that thenanowires grow along the [111] direction The use of targets of Si mixed with SiO2

appears to enhance the formation and growth of SiNWs obtained by laser ablation

[9,42] SiO2plays a more important role than the metal in the laser ablation thesis of SiNWs To describe the formation of SiNWs by laser ablation, a cluster-solidmechanism has been proposed [43] In the growth process, an amorphousmatrix is deposited from the oxide vapor and subsequent phase separation in thematrix leads to the formation of nanowires with a single-crystalline Si core and anoxide sheath Due to the oxide sheath, the core grows only in one dimension

syn-Thermal evaporation of a mixture of Si and SiO2yields SiNWs[44] The wires consist of a polycrystalline Si core with a high density of defects and a siliconoxide shell Highly oriented, long SiNWs are obtained in large yields on ﬂat siliconsubstrates by the thermal evaporation of SiO [45] The SEM images in Fig 11,reveal the aligned nature of the nanowires, with the length of the individual nano-

nano-Fig 10 (a) A TEM image of silicon nanowires by the laser ablation of a Si 0.9 Fe 0.1 target Scale bar, 100

nm (b) TEM image of a single silicon nanowire showing the crystalline core (dark) and the amorphous SiO x sheath (light) Scale bar 10 nm Inset shows the SAED pattern (c) HREM image of the crystalline

Si core and amorphous SiO x sheath The (111) planes (black arrows) with a spacing of 0.31 nm are oriented perpendicular to the growth direction (white arrow) (Morales and Lieber [41] ).

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wires extending up to 1.5–2 mm SiNWs have also been synthesized by the thermalevaporation of SiO powders without any metal catalyst [46] These have beengrown from particles and the growth mechanism examined The substrate tempera-ture is crucial for controlling the diameter of the nanowires, as well as themorphologies resulting from thermal evaporation of SiO powders mixed with 0–1%

Fe [47] Ultraﬁne SiNWs of diameters between 1 and 5 nm, sheathed with a SiO2outer layer of 10–20 nm, were synthesized by oxide-assisted growth via the dis-proportionation of thermally evaporated SiO using a zeolite template[48] The zeo-lite restricts the growth of the nanowires laterally and supplies the oxide to formthe outer sheath

SiNWs have been grown on Si(111) by the VLS process using silane as the Sisource and Au as the mediating solvent [49] The wires so obtained were singlecrystalline exhibiting growth defects, such as bends and kinks Using well-deﬁned

Au nanoclusters as catalysts for 1-D growth via the VLS mechanism, SiNWs havebeen synthesized using SiH4as the Si source [50] The diameters of the nanowiresobtained are similar to those of the catalytic Au clusters Amorphous SiNWs (10–

50 nm diameters) have been obtained with Au-Pd co-deposited Si oxide substrates

by thermal CVD using SiH4gas at 800v

C[51] SiNWs are produced by the Ti alyzed decomposition of SiH4in diﬀerent atmospheres, such as H2and N2[52].Dimensionally ordered SiNWs are formed within mesoporous silica using asupercritical ﬂuid solution-phase technique[53] The mesoporous silica matrix pro-vides a means of producing a high density of stable, well ordered arrays of SiNWs.Ordered SiNWs arrays have been prepared on Si wafers without the use of a tem-

cat-Fig 11 (a)–(d) SEM images of oriented SiNWs at diﬀerent magniﬁcations (Shi et al [45] ).

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plate in an aqueous HF solution containig silver nitrate near room temperature

[54]

A silicon wire has been fabricated on a SIMOX (separation by implanted gen) wafer[55] SIMOX is a SOI (silicon on insulator) fabrication technique due toits good homogeneity of the thin silicon ﬁlm on the buried SiO2 The size of thewire was controlled by electron beam lithography, the thickness of the top Si layer,and ﬁnal oxidation SiO2sheathed crystalline SiNWs are generated from a heatedSi–SiO2 mixture [56] The nanowires grow along <111> and are found to bevirtually defect-free Synthesis of NiSi2/Si and CoSi2/Si has been demonstrated onthe surface of bare SiNWs using metal vapor vacuum arc implantation[57] Nano-wires of ScSi2, ErSi2, DySi2 and GdSi2 have also been grown on Si (001) sub-strates, with widths and heights in the ranges 3–11 nm and 0.2–3 nm, respectively

oxy-[58] Detailed study of the structural and electronic properties of Gd disilicidenanowires on Si(100) have been made using STM and STS [59] Free-standingDySi2nanowires have been formed on Si (001) by self-assembly[60]

Various physical methods have been used to characterize SiNWs SiNWs, whenexcited with green light, emit red light due to the recombination of the electron-hole pairs across the band gap Yu et al.[32]obtained SiNWs that emit stable bluelight unrelated to quantum conﬁnement, which they attributed to the presence ofthe amorphous silicon oxide over-coating layer Li et al [49] obtained a strongemission at ~720 nm for nanowires with diameters <5 nm Zhang et al [40]

obtained diﬀerent photoluminescence (PL) emissions centered at 624 nm (1.99 eV)and 783 (1.58 eV) depending on the synthetic conditions used, which they attrib-uted to quantum size eﬀects in the thin SiNWs

Raman spectra of SiNWs match those predicted by the quantum conﬁnementmodel for Si microcrystals [61] However, the sizes predicted do not match thoseobserved in TEM, possibly because the SiNWs are composed of smaller Si grains

If the size of the grains is taken into account, better agreement is obtained

Doped SiNWs of n- and p-types have been prepared by introducing B or Pdopants during the growth of SiNWs by laser ablation[62] It is possible to heavilydope SiNWs and approach the metallic regime Doping of SiNWs by Li has beencarried out by an electrochemical insertion method at room temperature[63] Thecrystalline structure of the SiNWs, investigated by HREM, was graduallydestroyed with increasing Li+ ion dose Ma et al [64] have performed STM andSTS measurements on B-doped and undoped SiNWs The STM images (Fig 12(a)–(d)) showed the presence of nanoparticle chains and nanowires in the B-dopedSiNWs sample, while STS measurements showed an enhancement in the electricalconductance due to boron doping B- and P-doped SiNWs were used as buildingblocks to assemble three types of semiconducting nanodevices [65] Passive diodestructures consisting of crossed p- and n-type nanowires exhibit rectifying transportsimilar to planar p-junctions Active bipolar transistors, consisting of heavily andlightly n-doped nanowires crossing a common p-type wire base, exhibit commonbase and emitter current gains as large as 0.94 and 0.16, respectively Doped nano-wires have been used to assemble complementary inverter-like structures

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Catalytic growth of metal–semiconductor junctions between carbon nanotubesand SiNWs has been reported [66] The junctions exhibit reproducible rectifyingbehavior and could act as building blocks for nanoelectronics Room-temperatureCoulomb blockade eﬀects and the inﬂuence of a capacitively coupled gate on thetransport properties of conducting silicon wires have been studied[67].

Transport measurements have been carried out on 15–35 nm diameter SiNWsgrown using SiH4CVD via Au/Zn particle-nucleated VLS growth at 440 v

C[68].The eﬀect of both Al and Ti/Au contacts to the wires were investigated Thermaltreatment of the fabricated devices resulted in better electrical contacts andincreased the nanowires conductance by as much as 104 Using these SiNWs, sev-eral types of devices including crossed nanowire devices, 4- and 6-terminal devices,and 3-terminal (gate) devices were fabricated [69] The resistivity could be variedfrom >105 Xcm to ~103 Xcm based on the nature of the electrical contact(Schottky or Ohmic) and the doping levels

A supercritical ﬂuid inclusion-phase technique has been developed to embedSiNWs within the pores of mesoporous silica[70] These nanocrystalline materials

Fig 12 STM images of individual SiNWs: (a) undoped SiNW The inset shows the image of an removed, H-terminated SiNW; (b) a B-doped nanoparticle chain; (c) a B-doped nanowire; and (d) boron-induced reconstruction of SiNW (Ma et al [64] ).

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oxide-display intense room temperature UV and visible PL spectra, the wavelengthdepending on the diameter of the nanowires.

Intramolecular junctions (IMJs) in SiNWs have been formed from a singlegrowth process[71] STM shows IMJs formed by fusing together two straight wiresegments at an angle of 30v

, which repeats itself in a regular pattern across thenanowire

Chemical sensitivity of SiNW bundles has been studied[72] Upon exposure to

NH3gas and water vapor, the electrical resistance of the HF-etched SiNWs relative

to the non-etched SiNWs decreases at room temperature This phenomenon serves

as the basis for a new sensor

3.2 Germanium

Germanium nanowires (GeNWs) with diameters in the 10–100 nm range havebeen synthesized via the VLS method, using Au clusters as catalysts in a sealed-tube chemical vapor transport system [73] Melting and recrystallization processes

of individual nanowires have been observed by recording the TEM images, whileheating the nanowires The growth and nucleation of individual nanowires weremonitored within a high-temperature TEM when Ge was evaporated into mono-disperse Au clusters[7], to demonstrate the validity of the VLS growth mechanism

at the nanometer scale The three well-deﬁned stages discussed earlier in Section 2could be clearly identiﬁed during the process: metal alloying, crystal nucleation andaxial growth A mixture of Geþ GeI4, when sublimed on to Au-coated Si sub-strates, produces single-crystalline GeNWs with diameters less than 30 nm[74].Single-crystalline GeNWs are obtained in high yields by CVD of GeH4 at 275v

C with Au nanocrystals as seed particles[75] The SEM image inFig 13(a) showsthe nanowires to have diameters of ~25 nm and lengths up to tens of lm TheHREM image and the electron diﬀraction pattern inFig 13(b) show the nanowires

to be single-crystalline The nanowires form by the VLS growth mechanism, as denced by the presence of catalyst particles at the ends of the nanowires

evi-GeNWs with 10–150 nm diameter and lengths of several microns were grown incyclohexane heated and pressurized above its critical point [76] Alkanethiol-pro-tected Au nanocrystals 2.5–6.5 nm in diameter were used to seed the formation ofthe wires, which occurs through a solution–liquid–solid mechanism A supercriticalﬂuid solution-phase method has also been demonstrated for the synthesis ofGeNWs within the pores of an ordered mesoporous material [77] Diphe-nylgermane was decomposed in hexane at 773 K and 375 bar in the presence ofmesoporous silica Reduction of GeCl4and phenylGeCl3by Na metal in an alkanesolvent at elevated temperature and pressure produces GeNWs with diameters inthe range of 7–30 nm and length upto 10 lm[78]

High-vacuum electron beam evaporation has been used to synthesize Ge arrays on N+-type Si(100) and Si3N4using Ti as catalyst [79,80] The surface mor-phology of Ti nanocrystal catalyst and Ge cone-arrays was investigated GeNWs,consisting of a crystalline Ge core and an amorphous GeO2sheath, have been pro-duced by the laser ablation of a mixture of Ge and GeO [81] The crystalline Ge

cone-C.N.R Rao et al / Progress in Solid State Chemistry 31 (2003) 5–147

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core lies in the axial [211] direction and is terminated by the {111} facets on thesurface Photoluminescence and Raman scattering measurements have been repor-ted on Ge wires formed by self-assembly on Si(113) substrate[82] The samples aregrown at 500v

C by solid-source molecular beam epitaxy

Thermal evaporation of Ge powder at 950 v

C onto Au nanoparticles at 500v

Cproduces GeNWs[83] The diameters of the nanowires depend on the diameters ofthe catalyst nanoparticles used Temperature-dependent I–V characteristics of asingle GeNW with a diameter of 120 nm is shown inFig 14(a) An AFM image isshown in Fig 14(b) Transport measurements indicate that the wires are heavilydoped during the growth process The data can be explained by the thermal ﬂuctu-ation tunneling conduction model

Fig 13 (a) SEM image of GeNWs synthesized by CVD at 275 v

C on a SiO 2 /Si substrate The inset shows an AFM image of Au nanoclusters on the substrate recorded prior to CVD (b) HREM of a single GeNW Inset shows the SAED pattern (Wang and Dai [75] ).

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Fig 14 (a) I–V curves of Ge nanowires at diﬀerent temperatures (b) AFM image of a Ge nanowire device (Gu et al [83] ).

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amorphous BNWs [85] Magnetron sputtering using a B target in an Ar phere also produces ordered BNW arrays [86] Featherlike BNWs arranged inlarge-scale arrays with multiple Y- or T-nanojunctions are produced by using

atmos-RF magnetron sputtering in an Ar atmosphere [87] The target used in this casewas a mixture of Bþ B2O3 powders

BNWs are obtained by the laser ablation of B targets at high temperatures[88].The nanowires have diameters ranging from 30 to 60 nm with lengths of several

Fig 15 SEM images of the boron nanowire arrays grown on Si substrates (a) A low-magniﬁcation image showing that the nanowire arrays grew uniformly on the substrate over large areas The arrow- head shows the root part of the nanowire arrays, which was exposed by peeling operations (b) Cross- sectional image showing that the nanowire arrays grew perpendicularly to the substrate surface (c) High-magniﬁcation SEM image showing that most of the B nanowire tips have a platform-shaped morphology with a diameter of 60-80 nm (Cao et al [84] ).

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tens of microns Laser ablation of a B pellet in a furnace produces B nanobelts thatare rectangular in cross-section with a width-to-thickness ratio of about 5, severaltens of nm to about 150 nm in width and several lm to mm in length[89] Theseare well crystallized in a tetragonal structure and have a 2–4 nm thick amorphoussheath.

B2H6in Ar passed over NiB at 1100v

C yields crystalline BNWs[90] These haddiameters in the range of 20 to 200 nm and lengths of several microns The nano-wires were semi-conducting and have properties akin to those of elemental boron.MgB2 nanowires with diameters between 50 and 400 nm are prepared by thereaction of BNWs with Mg vapor [91] These nanowires exhibited a super-conducting transition temperature of ~33 K

3.4 In, Sn and Pb

The growth of In on a Si(001) 2 n nanostructured surface has been gated by in situ STM[92] The deposited In atoms predominantly occupy the nor-mal 2 1 dimer-row structure, and develop into an uniform array of In nanowires.Long chain amines have been used as templates for the synthesis of In nano-whiskers from InCp (Cp=C5H5 )[93] These workers also extended the strategy tosynthesize nanowires of In3Sn

investi-b-Sn nanowires surrounded by graphiticmaterial, with diameters 100 nm andlengths of2 lm, are produced by the passing of a current between graphite rodsimmersed in a molten mixture of LiCl and SnCl2 under Ar at 600 v

C [94] longed electron beam irradiation of the nanowires leads to axial growth, re-orien-tation and dynamictransformations

Pro-Pb nanowire arrays have been fabricated in an AAM, by anodization of a pure

Al foil and subsequent electrodeposition of Pb[95] The nanowires are talline with an average diameter of 40 nm The nanowire arrays embedded in theAAM can only transmit polarized light vertical to the wires

single-crys-3.5 Sb and Bi

Single-crystalline Sb nanowire arrays are obtained by pulsed electrodeposition inAAM [96] Fig 16(a) shows a ﬁeld emission SEM image of an array after theAAM was partially etched Fig 16(b) shows the degree of ﬁlling of the template.The nanowires have diameters of ~40 nm as can be seen from Fig 16(c) Asrevealed by the XRD pattern in Fig 16(d), the nanowires grow along the ½1120direction

Bi nanowires (BiNWs) can be extruded at room temperature from the surfaces

of freshly grown composite thin ﬁlms consisting of Bi and chrome-nitride[97] Thenanowires have diameters ranging from 30 to 200 nm and lengths up to several

mm Highly oriented hexagonal arrays of parallel Ni and Bi nanowires with meters ~50 nm and lengths up to 50 lm were synthesized by electrodeposition[98]

dia-A hexagonally close-packed nanochannel anodized alumina ﬁlm was used as thedeposition template

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Single-crystal BiNWs are formed inside SWNTs by capillary filling [99] Bi isintroduced as a gas, solution or solid, the solution phase process being the mostefficient method Infrared absorption experiments and theoretically calculationshave been performed on BiNWs[100] Experimentally obtained absorption spectravalidate quantum confinement in the BiNWs.

Mak-Fig 17(c) and (d) Optical properties of the nanowires, as well as their conductivity, have been studied In aqueous solution, Se molecules produced from

photo-Fig 16 (a) A typical field emission SEM image showing the general morphology of the Sb nanowire array (b) A field emission SEM image showing the degree of filling of the template and the height vari- ation of the nanowires (c) A TEM image of the Sn nanowires and (d) XRD pattern of the Sb nanowire array; the sole diffraction peak indicates the same orientation of all the nanowires (Zhang et al [96] ).

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the decomposition of selenodigluthathiones continually stack on previously formeda-monoclinic Se nanoparticles along the [001] direction, gradually producing a-monoclinic Se nanowires[102].

Se nanowires are produced by the reduction of selenate (SeO4 ) by the proteincytochrome C3[103] Nanoparticles of Se precipitate from an aqueous solution atroom temperature, followed by spontaneous self-assembling into nanowires Trig-onal Se nanowires have been converted into single-crystalline nanowires of Ag2Se

by reacting with aqueous AgNO3solution at room temperature[21] In this tory, Se nanorods have been prepared at room temperature using Se powder andNaBH4 in an aqueous medium [104] Se initially reacts with NaBH4 to giveNaHSe, which in turn can be used as an aqueous Se2 source [105] The NaHSesolution, within a few hours, undergoes disproportionation to give a red dispersion

labora-of a-Se particles On aging this solution for 1 week, t-Se nanorods with dimensions

of ~100 nm and several microns in length can be obtained Under solvothermalconditions, this chemical procedure yields nanobelts and other nanostructures.Single-crystal Te nanorods with diameters of 14 nm and lengths 300 nm wereprepared through the reduction of [TeS4]2 by SO3

using the surfactant sodiumdodecyl benzenesulfonate [106] Adjusting the concentration of the reactants per-mits control over the diameter of the nanorods The nucleation and growth pro-

Fig 17 (a), (b) SEM images of t-Se nanowires that are ~100 nm in diameter, prepared by reﬂuxing the solution at 110 v

C Some a-Se colloidal nanoparticles are still present in the sample (c), (d) SEM images

of t-Se nanowires with diameters of ~800 nm, prepared by reﬂuxing at 130 v

C The solvent used was ethylene glycol instead of water (Gates et al [13] ).

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cesses are understood in terms of the solid–solution–solid transformation and factant-assisted growth mechanism A solution-phase approach has been used tosynthesize nanorods of Se/Te alloys[107] The lateral dimensions of the nanowirescould be controlled in the range of 50–250 nm by changing the nucleation process.3.7 Compound semiconductors

sur-Using molecular beam epitaxy, a lattice of InAs nanowires that spontaneouslyorganize in three dimensions within an InAs/GaSb superlattice has been obtained

[108] The periodicnanowires are ~10 nm high, 120 nm wide and several lm longalong [110], with face-centered cubic-like vertical ordering within the superlattice.The appearance of InAs quantum-wire like morphology on an AlInAs buﬀer layergrown by MBE in nominal InP (001) surfaces has also been investigated[109].Electron transport properties of Bi1-xSbxnanowires (0 x 0:3) have been mea-sured[110] Such nanowire arrays were synthesized by the pressure injection of themolten alloys into anodicalumina thin ﬁlms[111] The resistivity of 65 nm Bi1-xSbx

nanowires shows complex variations with x and temperature, exhibiting to-semiconductor transitions[112]

semimetal-Fig 18 TEM image of gold nanorods after one round of puriﬁcation (Busbee et al [113] ).

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to the formation of highly oriented, ﬂat Au sheets and ribbon-like nanocrystalsbound to the monolayer[114] This one-step process constrains the growth of the

Au nanocrystals to within the plane of the Langmuir monolayers Recently, liquid–liquid interfaces have been used to make ﬁlms of Au and other metals by takingthe metal organicprecursors in the organiclayer and the reducing agent in theaqueous layer[115]

Single-crystalline and highly ordered Au nanowire arrays are electrochemicallyfabricated within hexagonally close-packed nanochannel alumina templates withpore diameters of 35–100 nm[116] AAMs containing 200 nm diameter pores havealso been replicated electrochemically with Au and Ag to make free-standing nano-wires several microns in length [117] The I–V characteristics of the nanowiresshow current rectifying behavior An electric-ﬁeld assisted assembly has beendescribed to position individual nanowires suspended in a dielectric mediumbetween two electrodes deﬁned lithographically on a silica substrate [118] Thisapproach has facilitated rapid electrical characterization of nanowires of 350- and70-nm diameter, having room-temperature resistivities of ~2.9 and 4:5 106 X

cm Porous polycarbonate membranes can be employed for the synthesis of Aunanowires, as well [119] Atomic force microscopy has enabled to obtain the I–Vcharacteristics Au nanowires and nanotubes have been prepared via electrolessdeposition of Au onto the pore walls of a porous polymericmembrane [120] Thepores in the support membrane, a commercial available nanoporous polycarbonateﬁlter with cylindrical pores, acts as a template for the nanostructures Based on thedeposition time, nanotubes or nanowires are obtained Nanowires synthesizedusing porous alumina templates have been assembled using DNA [121] Oligonu-cleotides were adsorbed as monolayer coatings on these wires through Au-thiollinkage Duplexes are formed between the strands on the nanowires and on Au-coated glass slides bound the two surfaces together Tris(hydromethyl)phosphine-capped Au nanoparticles which bind to DNA have been used to synthesize Aunanowires[122] The particles bound to DNA are immobilized on silicon, followed

by electroplating, to obtain nanowires of 30–40 nm diameter and low conductivity(compared to bulk gold) Biological molecules, such as histidine-rich peptides, havealso been used as templates to synthesize Au nanowires [123] Self-assembly ofdodecanthiol-capped Au nanoparticles in a matrix of dipalmitoylphosphatidylcho-line at the air/water interface yield continuous Au nanowires resembling a molecu-lar electronic circuit board[124]

Carbon nanotubes are eﬀectively used as templates for the self-assembly andthermal processing of Au nanowires[125] Nanowires of several metals such as Au,

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Ag, Pt and Pd have been produced in the capillaries of single-walled carbon tubes (SWNTs) by means of sealed tube reactions[22] The Au nanowires had dia-meters between 1–1.4 nm and lengths of 15–70 nm in Fig 19, we show TEMimages of the Au nanowires inside SWNTs obtained by this route These are sin-gle-crystalline with a spacing of ~0.23 nm, corresponding to the (111) planes Theymelt around 350v

nano-C, giving nanoparticles as products

Helical, multi-shell Au nanowires are formed under UHV conditions by electronbeam thinning technique [126] By irradiating thin organicﬁlms containing Au or

Ag nanoparticles with a series of ultrashort laser pulses, wire arrays containingmore than 30 nearly parallel wires several lm in length are obtained [127] Theindividual nanowires are between 100 and 200 nm wide with equal line space ratiossigniﬁcantly smaller than the laser wavelength used Au nanowires on insulatingsubstrates have been fabricated by ﬁeld-induced mass transport using an oscillatingAu-coated AFM tip[128]

On excitation of the Au nanorods with an intense pulsed laser, they change theirshape [129] The ﬁnal products depend on the energy of the laser pulse, as well as

on the width The shape transformations are followed by changes in the plasmon

Fig 19 (a) TEM image of Au nanowires inside SWNTs obtained by a sealed tube reaction Inset shows the corresponding SAED (b) HREM of the Au nanowires showing them to be single-crystalline (Govin- daraj et al [22] ).

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absorption and TEM The visualization of 2D confined suspensions of Au ods has been reported[130] and their diffusion coefficients determined by particletracking measurements The diffusion coefficient is affected by the surface chemistry

nanor-of the nanowires and the substrate and by the dimensions nanor-of the nanowires Spincoating of thiol-passivated Au nanoparticles onto silicon produces nanostructuredcellular networks [131] On annealing between 500 to 600 K, the cellular mor-phology of the nanocrystalline foam is preserved A nano-replication method bywhich the preparation of an artiﬁcial template for Au nanowire self-assembly hasbeen demonstrated[132] The optical response of a nanowire grating as a function

of the geometry of the nanowire cross-section, the grating constant and the wire material has been investigated[133] The nanowire gratings were produced byelectron beam lithography

nano-Electron transport properties of Au nanowires were investigated experimentallyand also based on theoretical analysis [134] Resistivities of polycrystalline Aunanowires have been measured at some locations connected through small grainboundaries[135] These nanowires were obtained by high-resolution microcontactprinting of a self-assembled monolayer as a resist, followed by selective etching.The electron relaxation dynamics and the thermal cooling of Au nanorods andtruncated tetrahedra in air and water have been studied, after excitation with fem-tosecond laser pulses [136] The possibility of using Au nanowires as buildingblocks for self-assembling logic and memory circuits has been discussed[137] Highaspect ratio wires, with diameters 15–350 nm containing ‘‘stripes’’ of differentmetals, semiconductors, colloid/polymer multilayers, and self-assembled mono-layers have been made By using the distinct surface chemistry of different stripes,nanowires can be selectively derivatized and positioned on patterned surfaces TheI–V curves of single nanowires can be measured Nanowire conductors, rectifiers,switches and photoconductors have been characterized A method of connectingnanowires consisting of ligand stabilized Au55 clusters with metal arrays prepared

by using metal evaporation through a mask of monodisperse latex beads has beendeveloped[138]

3.9 Silver

Using seed-mediated growth in micellar media, silver nanorods and nanowireshave been prepared [139] In the ﬁrst step, Ag nanoparticles of ~4 nm diameterwere prepared by the citrate route Silver nitrate was then reduced in the presence

of the Ag seeds, CTAB, NaOH and ascorbic acid The rods, wires and sphereswere separated by centrifugation In Fig 20, we show a TEM image of the Agnanowires (AgNWs) obtained by this method These have lengths between 1 and 4

lm with an aspect ratio 50–350 AgNWs with sizes of ~35 300 nm have beensynthesized in a lamellar liquid crystalline alignment of oleate vesicles by UVirradiation under ambient conditions [140] Passivation of oleate amphiphiles onthe surface of Ag nanoparticles is utilized for the nucleation and oriented growth

of AgNWs A self-seeded process has been used to obtain AgNWs, in whichAgNO and poly(vinylpyrrolidone) (PVP) solutions were simultaneously injected

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into reﬂuxing ethylene glycol through a two-channel syringe pump[141] Ag particles formed initially serve as seeds for the subsequent nanowire growth Apolymer-surfactant-assisted-sandwiched reduction route, in which the sandwichorganometallic compound acts as a reducing agent, permits the formation ofelemental Ag along a certain axis in the xy-plane[142] The curl and extension ofpolyethylene glycol in a mixed solvent provide microemulsions for conﬁning the1D growth A soft, solution-phase approach has been used to synthesize large-scalequantities of bicrystalline AgNWs with diameters in the 30–40 nm range andlengths of ~50 lm [143] Pt nanoparticles act as seeds for nanowire growth in thepresence of PVP Transport property measurements at room temperature indicatethe nanowires to be electrically continuous with a conductivity of 0:8 105 S=cm.

nano-A reduction method has been used to synthesize long, straight and continuousAgNWs by using nanocrystalline AgBr and a developer (containing N-methyl-paminophenol, citric acid, AgNO3 solution and water) as precursors, in the pres-ence of gelatin [144] UV irradiation of AgNO3has been used for the preparation

of single-crystal Ag nanorods and dendritic supramolecular nanostructures atroom temperature using polyvinyl alcohol as the protecting agent [145] Ag wires

of 0.4 nm diameter and micrometer length were grown inside the pores of assembled calix[4]hydroquinone nanotubes by electro-/photochemical redox reac-

self-Fig 20 TEM images of Ag nanowires obtained using the procedure outlined in [139] (Jana et al [139] ).

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tion[146] The wires are stable in aqueous environments and occur as coherentlyoriented 3D arrays of ultrahigh density Chain-like nanocrystals of Ag, as well of

Bi2Te3were synthesized via a solvothermal process in the presence of minetetraacetic acid disodium salt or PVP at 180vC[147] The Bi2Te3nanocrystalswere sheet-rods with an ordered array of Bi2Te3 nanosheets, and the Ag chainsconsisted of Ag nanoparticles Double-hydrophobic block copolymers have beenused to synthesize AgNWs in aqueous solutions without the use of UV irradiation

ethylenedia-or electrochemistry[148] The fabrication of crystalline AgNW arrays by deposition from a reverse hexagonal liquid–crystalline phase containing 1D aque-ous channels has been demonstrated[149] A high electric ﬁeld applied during theelectrodeposition helps to align the liquid–crystalline phase

electro-Single-walled carbon nanotubes have been filled with AgBr-AgCl without ing significant chemical or thermal damage Exposure to light or electronic beamresults in the reduction the silver halide to give continuous metallic silver nano-wires within the tubules[150] InFig 21(a) and (b), we show a structural represen-tation and HREM image of an empty unfilled SWNT, respectively Fig 21(c)shows a HREM image of the SWNT filled with a KCl-UCl4 eutectic mixture.Shown inFig 21(d) is a HREM image of a large diameter (~3.8 nm) SWNT filledwith 17 layers of silver metal The initial filling medium in this case was AgCl AHREM image of the product of SWNT filling with AgBr within a bundle of(10,10) SWNTs is shown in Fig 21(e) The paths of empty and filled SWNTs are

caus-Fig 21 (a) Schematic representation of a SWNT (b) HREM image showing an empty SWNT The defect region is shown by the arrow (c) SWNT filled with a KCl-UCl 4 eutectic mixture (d) Wide capillary SWNT filled with Ag metal formed by capillary insertion of AgCl followed by photolytic decomposition The indicated d-spacing correspond to the (020) lattice planes of Ag metal (e) Incorporation of AgBr into an SWNT bundle Filled SWNTs are indicated by light arrows and unfilled by dark arrows (Sloan et al [150] ).

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indicated by dark and light arrows, respectively Zeolites have also been used tosynthesize AgNWs [151] The Ag+ ions are exchanged with zeolites and treatedwith a solution of AgNO3at 260vC to further introduce Ag+ions into the zeolitepores On exposure to the electron beam, AgNWs are formed Highly orderedAgNW arrays have been obtained by pulsed electrodeposition in self-orderedporous alumina templates [152] In Fig 22, we show SEM images of the silver-ﬁl-led alumina membrane Electrochemical plating into monodomain porous aluminatemplates also yields AgNWs with a high aspect ratio[153] The nanowires have alength of greater than 30 lm with diameters in the 180–400 nm range Scanningtunneling microscopy has been used to create nanometer-scale pits on Ag ﬁlmsgrown on Cu(100) substrates [154] Atomicresolution images show that the Agstructure is intact within these pits.

Plasmon resonance of AgNWs with a nonregular cross-section on the 20–50 nmdiameter range has been investigated [155] Atomicarrangements and quantumconductance of AgNWs generated by mechanical elongation have been analyzed

[156] The structural behavior of the nanowires has been employed to interpretquantum conductance data The quantum transport properties of ultrathin silvernanowires have also been theoretically investigated [157] A method to synthesizeAg/SiO2coaxial nanocables with thicknesses in 2–100 nm range and lengths up to

~50 lm has been developed [20] These nanostructures were prepared by coatingbicrystalline AgNWs with sheaths of silica through a sol-gel process

3.10 Iron

The fabrication of large arrays of oriented ferromagnetic iron nanorods by ous chemical growth, without the use of a template or surfactant and withoutapplying electric or magnetic ﬁelds, has been reported [158] The method involvesthe growth of b-ironoxyhydroxide nanorods from an aqueous ferricchloride sol-ution onto substrates followed by reduction in a hydrogen atmosphere at mild tem-peratures Fe nanowires (FeNWs) have been grown in a mesoporous silica matrix

aque-Fig 22 SEM images of silver-ﬁlled alumina membranes (a) Top view of an unthinned sample; (b) top view of the same sample ~200 nm underneath the initial surface; and (c) side view of a fracture (Sauer

et al [152] ).

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by the decomposition of Fe(CO)5 by UV radiation [159] Magneticsusceptibilitymeasurements indicate superparamagnetic properties of these samples Local spin-density-functional theory has been applied to describe the structural and magneticproperties of FeNWs consisting of chains of single atoms [160] An unsupportedisolated wire is shown to be unstable with respect to both dimerization and bend-ing The preferred positions of wires grown on stepped Cu(11n) surfaces are theinner corner sites of the steps Fe-ﬁlled BN nanocables with intermediate C layers(introduced to overcome the wetting restriction of a BN surface) have been fabri-cated[161] Phase separation between C and BN was recognized using EELS andHREM.

FeNWs encapsulated in aligned carbon nanotube bundles have been synthesized

by a simple route [162] The method is based on the pyrolysis of a ferrocene–hydrocarbon mixture where aligned CNTs are obtained, containing a good pro-portion of FeNWs encapsulated in the nanotubes Shown in Fig 23(a) are thealigned bundles of CNTs TEM observations reveal that FeNWs are encapsulatedinside the nanotubes as can be seen fromFig 23(b) and (c) SAED patterns of thenanowires show them to be single-crystalline Magnetization measurements onthese nanowires show Barkhausen jumps, similar to those observed with amorph-ous iron nanowires A typical hysteresis curve with a Ms value of 24 emu/g, exhi-biting such features is shown in Fig 24(a) The jumps with 5 emu/g steps inmagnetization arise from the magnetization reversal in the encapsulated iron nano-wires or depinning of large domains on increase of the magnetization ﬁeld On rep-etition of the magnetization measurement on the sample, the shape of hysteresiscurve showed slight changes as can be seen fromFig 24(b) The observed change

in the shape of the hysteresis curve demonstrates the instability of the magneticprocesses Magnetization reversal in two-dimensional arrays of parallel ferromag-neticFeNWs embedded in nanoporous alumina templates has also been examined

[163] By combining bulk magnetization measurements with ﬁeld-dependent surements, the macroscopic hysteresis loop has been decomposed in terms of theirreversible magnetization response of individual nanowires

mea-CNTs have been used as nanoreactors for boriding FeNWs[164] Fe0.5B wires have been synthesized via boriding FeNWs encapsulated in CNTs

nano-3.11 Cobalt

Ultrahigh-density arrays of nanopores with high aspect ratios can be synthesizedusing the self-assembled morphology of asymmetric block copolymers [165] Elec-trodeposition enables the formation of vertical arrays of Co nanowires (CoNWs)with densities in the excess of 1:9 1011 wires=cm2 Polyaniline nanotubules sup-ported within alumina membranes have been used as templates for the synthesis ofCoNWs[166] SEM images of the polyaniline nanotubules and polyaniline nanotu-bules ﬁlled with CoNWs are shown in Fig 25 The alumina layer is removed bydissolution in NaOH solution Fig 26 shows two hysteresis loops of the cobaltnanowire arrays with the membrane support, when the ﬁeld is applied parallel andperpendicular to the nanowires The array exhibits uniaxial magnetic anisotropy

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with the easy axis parallel to the nanowires: the magnetization, being perpendicular

to the membrane The coercivities obtained for the nanowires too are enhanced ascompared to bulk Co Electrodeposition into the pores of polycarbonate mem-branes also yields CoNW arrays[167] A magneticﬁeld parallel or perpendicular tothe membrane plane applied during deposition controls the wire growth Magneticproperties of an ordered array of concentric composite nanostructures of ZrO2

nanotubules and CoNWs containing continuous and uniform CoNWs in the ZrO2

nanotubules have been reported[168] The ZrO2envelope protects the metal wires from oxidation and corrosion

nano-Fig 23 (a) SEM image of aligned carbon nanotubes obtained from the pyrolysis of ferrocene-acetylene mixture at 1100 v

C; (b) and (c) TEM images of the iron nanowires encapsulated inside the nanotubes Inset shows the SAED of a nanowire (Satishkumar et al [162] ).

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Using STM, the growth of Co onto Cu(110) surfaces can be altered by ﬁrst minating the Cu(110) surface with an atomic N-induced (2 3) structure [169].Growth of Co on such surfaces results in ordered arrays of CoNWs Magnetotran-sport measurements on single CoNWs, fabricated by electrodeposition in nanoporousmembranes and contacted by an electron beam lithographic technique showinhomogeneity of the magnetization reversal processes along the same wire [170].Magnetoresistance (MR) of single CoNWs, prepared by electron beam lithogra-phy, shows anisotropy due to the MR reversal process [171] Monte Carlo simu-lations suggest magnetization distribution during the reversal process, revealingdiﬀerent mechanisms depending on the width of the wire.

ter-Fe1-xCox(0 x 1) nanowires have been self-assembled by electrodeposition inporous alumina ﬁlms and magneticstudies performed on them[172] Ordered fer-

Fig 24 (a) Hysteresis loop showing multiple steps due to the magnetization reversal in interacting iron nanowires encapsulated in carbon nanotube bundles (b) Hysteresis loop obtained with the same sample

as in (a) on repeating the experiment (Satishkumar et al [162] ).

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romagnetic-nonmagnetic alloys (Co-Cu, Co-Ag, Fe-Ag) nanowire arrays embedded

in the nanochannels of anodic alumina membranes have also been fabricated byelectrodeposition and their magnetic properties investigated[173]

3.12 Nickel

Ni nanowires (NiNWs) of ~4 nm diameter are formed in CNTs with an innertubule diameter of 20 nm[174] Carbon was initially deposited into the channels ofporous AAM followed by MOCVD of nickelocene The templates were dissolved

in NaOH to get NiNWs in the CNTs.Fig 27shows TEM images of the CNT/Ni

Fig 25 SEM images of (a) polyaniline nanotubules and (b) polyaniline nanotubules ﬁlled with Co nanowires obtained after dissolution of alumina (Cao et al [166] ).

Fig 26 Hysteresis loop of a supported array of Co nanowires varied for various angles between the applied magneticﬁeld and the membrane plane (Cao et al [166] ).

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NW composites The magnetic behavior of 100 nm periodic arrays of NiNWsembedded in an ordered alumina pore matrix has been characterized[175] Reduc-ing the diameter of the nanowires from 55 to 30 nm, while keeping the interwiredistance constant leads to increasing coercive ﬁelds (from 600 to 1200 Oe) and reman-ence (from 30 to 100%).

The synthesis of Ni nanorods has been accomplished by a solution-based route

by using hexadecylamine as the shape controlling agent [176] Increasing the

con-Fig 27 Bright-ﬁeld TEM images at diﬀerent areas for carbon tubes/nickel nanowire composites pared by MOCVD (Pradhan et al [174] ).

pre-C.N.R Rao et al / Progress in Solid State Chemistry 31 (2003) 5–147

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centration of the amine favors the formation of nanorods, nearly monodisperse,probably through the speciﬁc coordination of the amine.

Polarization-dependent scattering of light has been reported from homogeneous,multi-segment Ag, Au and NiNWs [177] The metallicnanowires were preparedwithin a polycarbonate membrane template by a combination of electroplating (Auand Ni) and electroless (Ag) growth processes The magnetic alignment of fluor-escent NiNWs has been demonstrated [178] NiNWs were prepared by electro-chemical growth in alumina templates and functionalized with luminescentporphyrins after removal of the template The response of the nanowires to mag-netic fields was quantified using video-microscopy

Ni and Co wires of diameters 35–500 nm can be electrodeposited into the drical pores of track-etched polymer membranes and their magnetic propertiesstudied [179] The study shows intrinsicdiﬀerences between the magnetizationreversal mechanism in the two systems Static and dynamic aspects of the magneti-zation reversal in nanowire arrays of Fe, Co and Ni produced by electrodeposition

cylin-in porous AAM templates have been examcylin-ined along with the magnetizationproperties[180]

NiNW arrays with a diamond-shaped cross-section are produced in nanoporoussingle mica crystal membranes by electrodeposition[181] The wires have diameters

of ~120 nm and lengths of 5 lm Magnetization anisotropy is formed in the wiresdue to the quasi-one-dimensional shape and the diamond cross-section

Magnetic nanocontacts are obtained by a templating method involving the trodeposition of Ni within the pores of track-etched polymer membranes [182] Atroom temperature, the electrical conductance shows quantization steps in units of

elec-e2/h, as expected of ferromagnetic metals without spin degeneracy

Inelasticlight scattering has enabled a study of the dynamicproperties of form 2D arrays of NiNWs [183] Polycrystalline Ni wires, with diameters of ~100

uni-nm and resistance up to 20 X, have been prepared by the controlled etching ofmicroscopic wires, and the MR properties studied[184]

Highly ordered composite Ni-Cu nanowire arrays have been electrodepositedinto the pores of anodic alumina templates characterized by various techniques

[185]

3.13 Copper

Adelung et al.[186]report that the deposition of Cu onto layered crystals (VSe2)induces the formation of self-assembled nanowire networks, which are stable underambient conditions They also ﬁnd the Cu deposition to induce the formation ofnanotunnels on the layered-crystal surface High aspect ratio Cu nanowires encap-sulated in poly(dimethylsiloxane) are obtained under mild conditions, by the sol-vent-free reduction of solid CuCl by (Me3Si)4Si in vapor[187] The nanowires grow

by the VS mechanism Cu metal has been deposited onto DNA forming like structures that are ~3 nm high[188] DNA is ﬁrst aligned on a Si substrate fol-lowed by treatment with Cu(NO3)2when Cu2+ions get associated with the DNA

nanowire-On reduction by ascorbic acid, a metallic Cu sheath is formed around the DNA

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Ultra-high density arrays of Cu and Ni nanowires have been synthesized insideordered porous inorganic materials through a Pd-catalyzed electroless depositionprocess[189] Pd nanoparticles were introduced into the pores of mesoporous SBA

15 followed by the electroless deposition of Ni and Cu to yield nanowires insidethe channels of the mesoporous material A template synthesis of large-scale Y-junction Cu nanowires has been reported [190] A Y-shaped nanochannel porousanodicalumina template is ﬁrst prepared by the anodization of aluminium inwhich Cu is electrodeposited to form the Y-junction metal nanowires TypicalSEM images shown in Fig 28 reveal Y-junction Cu nanowires fabricated by thismethod

Fig 28 SEM images of Y-junction copper nanowires: (a) top view and (b) cross-sectional view (Gao

et al [190] ).

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Tiêu đề	Inorganic nanowires
Tác giả	C.N.R. Rao, F.L. Deepak, Gautam Gundiah, A. Govindaraj
Trường học	Jawaharlal Nehru Centre for Advanced Scientific Research
Chuyên ngành	Chemistry and Physics of Materials
Thể loại	Progress in Solid State Chemistry
Năm xuất bản	2003
Thành phố	Bangalore

Định dạng
Số trang	143
Dung lượng	7,73 MB