quasi - one dimensional metal oxide semiconductors

play between bulk and surface properties and processes in MOX nanowires sensors together with theirdevelopment as real world sensing platforms.Q1D metal oxide nanostructures have several

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Quasi-one dimensional metal oxide semiconductors:

Preparation, characterization and application as chemical sensors

E Comini, C Baratto, G Faglia, M Ferroni, A Vomiero, G Sberveglieri*

SENSOR Lab, CNR-INFM, Dipartimento di Chimica e Fisica per l’Ingegneria e per i Materiali, Brescia University,

via Valotti 9, 25133 Brescia, Italy

In particular, metal oxides (MOX) are attracting an increasinginterest for both fundamental and applied science MOX Q1D arecrystalline structures with well-defined chemical composition,surface terminations, free from dislocation and other extendeddefects In addition, nanowires may exhibit physical propertieswhich are significantly different from their coarse-grained poly-crystalline counterpart because of their nanosized dimensions.Surface effects dominate due to the increase of their specific sur-face, which leads to the enhancement of the surface related prop-erties, such as catalytic activity or surface adsorption: keyproperties for superior chemical sensors production

High degree of crystallinity and atomic sharp terminations makenanowires very promising for the development of a new genera-tion of gas sensors reducing instabilities, typical in polycrystallinesystems, associated with grain coalescence and drift in electricalproperties These sensitive nanocrystals may be used as resistors,and in FET based or optical based gas sensors

This article presents an up-to-date review of Q1D metal oxidematerials research for gas sensors application, due to the greatresearch effort in the ﬁeld it could not cover all the interesting works

* Corresponding author Tel.: +39 030 3715771; fax: +39 030 2091271.

E-mail address: sbervegl@sensor.ing.unibs.it (G Sberveglieri).

URL: http://sensor.ing.unibs.it (G Sberveglieri).

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reported, the ones that, according to the authors, are going to contrib-ute to this ﬁeld’s further development were selected and described

Contents

1 Introduction 2

2 Deposition techniques and growth mechanisms 3

2.1 Vapor phase growth 4

2.1.1 Vapor–liquid–solid mechanism 5

2.1.2 Vapor–solid mechanism 9

2.2 Solution phase growth 10

2.2.1 Template-assisted synthesis 11

2.2.2 Template-free methods 12

3 Vertical and horizontal alignment techniques 13

3.1 Electric field alignment 18

3.2 Nanomanipulation 21

4 Doping of quasi 1D metal oxide nanostructures 22

5 Preparation of quasi 1D metal oxide heterostructures 24

6 Applications of metal oxide nanostructures 30

6.1 Metal oxide gas sensors 30

6.1.1 Surface adsorption 30

6.1.2 Detection through surface reactions 31

6.1.3 DC resistance transduction 31

6.1.4 Conductometric gas sensors 31

6.1.5 Single nanowire transistor (SNT) based gas sensors 45

6.1.6 PL based gas sensors 50

6.2 Other application fields 56

6.2.1 Lasers 56

6.2.2 Solar cells 57

6.2.3 Field emitters 57

6.2.4 Li-ion batteries 59

6.2.5 Single nanowire transistors for biosensing 59

7 Conclusions 59

Acknowledgements 60

References 60

1 Introduction

The increasing concerns with pollution on health and safety stress the need of monitoring all as-pects of the environment in real time, and in turn led to a tremendous effort in terms of research and funding for the development of sensors devoted to several applications[1–9]

As far as chemical sensing is concerned, it has been known, from more than five decades, that the electrical conductivity of metal oxides semiconductors varies with the composition of the surrounding gas atmosphere The sensing properties of semiconductor metal oxides in form of thin or thick films other than SnO2, like TiO2, WO3, ZnO, Fe2O3and In2O3, have been studied as well as the benefits from the addition of noble metals – Pd, Pt, Au, Ag – in improving selectivity and stability

In 1991 Yamazoe showed that reduction of crystallite size went along with a signiﬁcant increase in sensor performance[10] In a nanosized grain metal oxide almost all the carriers are trapped in surface states and only a few thermal activated carriers are available for conduction In this conﬁguration the transition from activated to strongly not activated carrier density, produced by target gases species, has a huge effect on sensor conductance Thus, the technological challenge moved to the fabrication

of materials with small crystallize size which maintained their stability over long-term operation at high temperature A huge variety of devices have been developed mainly by an empirical approach

2 E Comini et al / Progress in Materials Science 54 (2009) 1–67

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play between bulk and surface properties and processes in MOX nanowires sensors together with theirdevelopment as real world sensing platforms.

Q1D metal oxide nanostructures have several advantages with respect to traditional thin- and thickﬁlm sensors such as very large surface-to-volume ratio, dimensions comparable to the extension ofsurface charge region, superior stability owing to the high crystallinity[11], relatively simple prepa-ration methods that allow large-scale production[14], possible functionalization of their surface with

a target-specific receptor species[190], modulation of their operating temperature to select the propergas semiconductor reactions, catalyst deposition over the surface for promotion or inhibition of spe-cific reactions and finally the possibility of field-effect transistors (FET) configuration that allows theuse of gate potential controlling the sensitivity and selectivity[188]

Preparation and performances of these emerging nanosized structures have been reviewed by anumber of authors[12–15], but this research ﬁeld is growing so fast that there is still the need of areview focused on sensing applications

This review article is focused on the description of metal oxide single crystalline Q1D tures used for gas-sensing application, speciﬁcally on the promising approaches that are going to con-tribute to the further development of this ﬁeld The overview will start from presenting the fabricationtechniques and the growth mechanisms, focusing on their development and improvements, andpointing out the steps critical for application in real environments Then the application as chemicalsensors will be addressed Furthermore an outlook on other possible new applications of metal oxidesingle crystalline nanowires will be presented

nanostruc-2 Deposition techniques and growth mechanisms

Nanocrystalline materials can be classified into different categories depending on the number ofdimensions that are nanostructured (with dimensions lower than 100 nm); we will follow one ofthe possible classification: i.e zero dimensional for clusters, mono dimensional for nanowires andtwo dimensional for films

There are two different approaches to the production of 1D structures: top-down and bottom uptechnologies

The ﬁrst one is based on standard micro fabrication methods with deposition, etching and ion beammilling on planar substrates in order to reduce the lateral dimensions of the ﬁlms to the nanometersize Electron beam, focused ion beam, X-ray lithography, nano-imprinting and scanning probemicroscopy techniques can be used for the selective removal processes The advantages are the use

of the well developed technology of semiconductor industry and the ability to work on planar faces, while disadvantages are their extremely elevated costs and preparation times

sur-In the top-down approach highly ordered nanowires can be obtained[16–19], but at the momentthis technology does not fulﬁl the industrial requirements for the production of low cost and largenumbers of devices Furthermore the 1D nanostructures produced with these techniques are in gen-eral not single-crystalline

The second approach, bottom-up, consists of the assembly of molecular building blocks or chemicalsynthesis by vapor phase transport, electrochemical deposition, solution-based techniques or tem-plate growth Its advantages are the high purity of the nanocrystalline materials produced, their small

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diameters, the low cost of the experimental set ups together with the possibility to easily vary theintentional doping and the possible formation of junctions The main disadvantage regards their inte-gration on planar substrates for the exploitation of their useful properties, for example transfer andcontacting on transducers can be troublesome.

The bottom-up approach allows low cost fabrication although it could be very difﬁcult to get themwell arranged and patterned[20]

Furthermore more control and insight into the growth process must be achieved for their fruitfulintegration in functional devices

The most promising approach to produce functional nanowires will be the combination of the twopreparation technologies

This review article will be focused on the bottom-up techniques for the preparation of 1D crystal nanostructures

single-Numerous one-dimensional oxide nanostructures with useful properties, compositions, and phologies have recently been fabricated using bottom-up synthetic routes Some of these structurescould not have been created easily or economically using top-down technologies

mor-A nomenclature for these peculiar structures has not been well established In the literature a lot ofdifferent names have been used, like whiskers, fibers, fibrils, nanotubules, nanocable, etc The defini-tion of these 1D nanostructures is not well established A few classes of these new nanostructures withpotential as sensing devices are summarized schematically inFig 1 The geometrical shapes can betubes, cages, cylindrical wires, rods, nails, cables, belts, sheets and even more complex morphologies.When developing 1D nanocrystals the most important requirements are dimensions and morphol-ogy control, uniformity and crystalline properties In order to obtain one-dimensional structures apreferential growth direction with a faster growth rate must exists Achieving 1D growth in systemswith a isotropic atomic bonding requires a break in the symmetry during the growth and not just stop-ping the growth process at an early stage (0 and 2D)

In the past years the number of synthesis techniques has grown exponentially We can divide thesegrowth mechanisms in different categories, ﬁrst of all catalyst-free and catalyst assisted proceduresand then we can distinguish between vapor and solution phase growth As far as metal oxides are con-cerned the most used procedure is the vapor phase one But solution phase growth techniques provide

a more ﬂexible synthesis process with even lower production costs

There are different growth mechanism depending on the presence of a catalyst, i.e vapor–liquid–solid (VLS), solution–liquid–solid (SLS) or vapor–solid (VS) process

2.1 Vapor phase growth

The vapor phase approach was used in the early 60’ for the preparation of micrometer-size kers These whiskers were prepared either by simple physical sublimation of the source material or

whis-Fig 1 Schematic drawing of some of the possible morphologies: (a) nanowire, (b) core–shell nanowire, (c) nanotube, (d) nanobelt, (e) hierarchical structure, (f) nanorod and (g) nanoring.

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phase precursor, the liquid catalyst droplet, and the solid crystalline product (Fig 5).

VLS in the last decades was one of the most important methods for preparing 1D structures, it ispromising as a scalable, economical and controllable growth of different materials (oxide, semicon-ductors, .) Understanding the growth dynamics is important to have a greater control in the nano-wires shape, diameter and for a selective growth

In general the presence of a metal particle, of size comparable to the nanowire, at its apex leads tothe conclusion that the growth mechanism followed the vapor–liquid–solid (VLS) process, but thisdoes not determine the phase of the catalyst during growth

In most of the catalytic growths, nanowires have uniform diameters The section can be rounded orpolygonal with atomically sharp lateral terminations The growth process takes some dead time, astarting period before the real growth begins, this was experimented also for vapor phase processes[22]

The catalytic particles can be formed by vapor phase and/or surface diffusion transport or be ited from the evaporation of a colloidal solution or by deposition of a thin ﬁlm onto the substrate Ifthe metal does not wet the substrate, it will form clusters as the result of Volmer–Weber growth[23]

depos-or when the substrate is kept at the high temperatures required fdepos-or the growth process, the onset ofOstwald ripening[24]will lead to a distribution of cluster sizes In some cases the catalyst clustersthat initiate the NWs growth can also be formed at the initial deposition step; for example when car-bothermal reduction is used to generate a volatile metal that is transported from a carrier gas and thencondense on the substrate

Sometimes the catalyst may undergo other processes before becoming active for the growth ofnanowires after its formation or deposition A mixture of the growth compound and the metal might

be more active for the NW formation than the pure metal catalyst, and may be required to form analloy, a true eutectic or some solid/liquid solution In this case, saturation of the catalytic particle withthe growth material or the formation of the proper composition may explain the dead time period be-fore growth The incorporation of a signiﬁcant amount of growth material into the catalytic particle isexpected to change the volume and, in turn, the diameter of the catalyst from its initial value with achange in the NW section Consequently Ostwald ripening and incorporation of growth material con-tribute in changing the size of the catalytic particles

A constant section NWs growth may correspond to a condensation on the catalyst surface and fusion and segregation at the interface between catalyst and nanowire When the condensation andincorporation is occurring only on the catalyst and not onto the NW sides, a constant catalyst sectionresults in a constant nanowire section

dif-The dimensions of the catalyst clusters can determine the NW section either by direct matching ofthe size or by mechanism involving the catalyst curvature in which strain and lattice matching areimportant The NW section will decrease and eventually the growth process will end if the catalyst

is consumed or evaporates during the growth, or when the material is no longer supplied, or if thetemperature is reduced below a critical value necessary for the growth process Temperature is akey factor in determining processes such as dissociative adsorption, surface diffusion, bulk diffusionthrough the catalyst, solubility and thermodynamic stability of certain phases

The catalyst cluster can offer a higher sticking coefﬁcient, but the difference in sticking coefﬁcientsalone cannot account for the NWs growth process Further considerations must be performed to ex-plain the preferential incorporation at the interface between nanowire and catalyst For example

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the catalytic particle can lower the energy barrier for the incorporation of new material at the growthinterface compared to the one needed for nucleation of an island on a sidewall or on the substrate.Adsorption occurs from the ﬂuid (gaseous, liquid or supercritical) phase, it can be molecular or dis-sociative and may occur on nanowire, catalyst, or substrate The catalyst can activate the growth with

a sticking coefﬁcient higher on its surface and vanishing elsewhere After the adsorption there is theadatoms diffusion onto or into the catalyst, across the substrate, or on the NWs lateral sides In order

to have the unidirectional growth, the last two processes must be rapid and avoid secondarynucleation

The nanowires can grow from the top or the bottom of the catalyst cluster and as reported inFig 2,

a catalyst cluster can give rise to single or multiple nanowires growth The catalyst can be found at thebottom or top of the nanowire In single NW growth there is a one-to-one correspondence betweencatalyst and nanowires In single wire growth control over the nanowire diameter should be obtainedcontrolling the catalyst radius While in multiple nanowires growth the section must be related toother factors such as the curvature of the growth interface and lattice matching between the catalyticparticle and the nanowire

Regardless of the phase of the catalyst, the major requirement is the mobility of the growth rial that can allow reaching the growth interface with a low probability of nucleation in sites otherthan the nanowire–catalyst interface

mate-The growth activation energy can be related to activated adsorption or with surface or bulk sion The essential role of the catalyst appears to be lowering the activation energy of nucleation at theinterface There is a substantial barrier associated with the formation of the critical nucleation cluster

diffu-at a random position on the substrdiffu-ate or nanowire according to classical nuclediffu-ation theory If the cdiffu-at-alyst can lower the nucleation barrier at the particle/nanowire interface, then growth may only occur

cat-Fig 2 The processes that occur during catalytic growth (a) In root growth, the particle stays at the bottom of the nanowire (b)

In ﬂoat growth, the particle remains at the top of the nanowire (c) In multiple prong growth, more than one nanowire grows from one particle and the nanowires must necessarily have a smaller radius than the particle (d) In single-prong growth, one nanowire corresponds to one particle One of the surest signs of this mode is that the particle and nanowire have very similar

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The effects of size on the growth kinetics of nanowires by the vapor–liquid–solid mechanism wereaddressed from the theoretical point of view in[27] The dependences of the growth rate and the acti-vation energy of crystallization on size were given quantitatively The obtained theoretical resultsshowed that the smaller the nanowire radius, the slower the growth rate, and the activation energy

of crystallization increases with decreasing radius of the nanowire These theoretical predictions are

in agreement with the experimental cases However, this conclusion depends on the growth tions[28]since the extent of supersaturation within the catalyst depends on the temperature andgas-phase composition Transitions from smaller diameters having lower growth rates to smallerdiameter having higher growth rates can occur as temperature and gas-phase composition arechanged

condi-Although it is commonly believed that in the VLS process, the size of the catalyst particles mines the NWs width, this is not true for all deposition conditions Experimental studies on ZnONWs growth on Al0.5Ga0.5N substrate conﬁrm that this rule only applies when the catalyst particlesare reasonably small (<40 nm)[29]

deter-A linear relationship between the density of the nanowires and the thickness of the catalyst layerwas found, therefore catalyst thickness control could be a very simple and effective way to achievedensity control of aligned nanowires over a large surface area To reveal why the density varies, butthe width remains constant, the wetting behavior of a gold layer on the substrate was investigatedwhen heated to the growth temperature

The results classiﬁed the growth processes into three categories: separated dots initiated growth,continuous layer initiated growth, and scattered particle initiated growth Because of the wetting sit-uation between the melted catalyst droplet and the substrate, more energy favorable sites were cre-ated for nanowire growth with thinner catalyst layers Moreover, when the catalyst layer wassufﬁciently thick, a continuous ZnO network would be deposited simultaneously at the bottom ofthe nanowires (Fig 3)

Another important process controlling the cluster dimensions, that is in general forgotten, is thethermodynamic limit for the minimum radius of the metal liquid clusters at high temperature

rmin¼ 2rLVVL=RT ln s

where rLVis the liquid vapor surface free energy, VLis the molar volume of liquid, and s is the vaporphase supersaturation

Furthermore as well as the equilibrium vapor pressure of a solid surface also the solubility depends

on the surface curvature As the size are reduced the solubility increases, as a result higher uration in the vapor phase has to be created Higher supersaturation may promote lateral growth onthe NWs side or homogeneous nucleation in the gas phase

supersat-A procedure for controlling the radial and axial dimensions of SnO2NWs has been presented in[30,31]by combining VLS approach with molecule-based chemical vapor deposition The synthesiswas based on the decomposition of discrete molecular species, which allows growing nanowires atlow temperatures with a precise control over their diameter and length The precursor chemistrywas chosen to facilitate the stripping of organic ligands and to achieve complete decomposition that

is critical for maintaining the gas phase supersaturation necessary for 1D growth Axial and radialdimensions of the NWs were varied by adjusting the precursor feedstock, deposition temperature,and catalyst size

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Despite the success of all these growth procedures, there have been just few comparative studies oncatalysts and substrates inﬂuence Such studies are valuable because both the catalyst and substrate playimportant roles in NWs structure and properties In reference[32], a case study of ZnO nanowire growthwas performed; four different catalysts and substrates of different materials, structure, and crystal ori-entation were investigated It was found that the growth depends on the choice of surface catalysts, e.g.for the Fe catalysts, the growth of ZnO nanowires may occur via a vapor–solid process, while, for the case

of Au, Ag, and Ni catalysts, the vapor–liquid–solid process usually dominates the wire growth more differences in growth were also closely related to the differences in materials properties of thesewires, including the degree of nanowire alignment on substrates and the atomic composition ratio of Zn/

Further-O, as well as the relative intensity of the oxygen vacancy-related emission in PL spectra

The use of different catalysts provides the versatility of growth for one-dimensional ZnO structures with different ranges of parameters such as diameters, areal densities, and aspect ratios

nano-Fig 3 (a) Variation of density (left-hand vertical axis) and width (right-hand vertical axis) of the aligned ZnO nanowires with the thickness of gold catalyst layer Inset: Top-view SEM image of the aligned ZnO nanowires used for density calculation, the scale bar represents 200 nm (b, c) TEM images of ZnO nanowires catalyzed by 1 and 8 nm gold layers, respectively Inset: Selected area electron diffraction pattern recorded from a nanowire indicated by the circle in image c catalyzed by the corresponding gold layers Reprinted with permission from [29]

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This works suggested that, compared to noble-metal catalysts, growth using transition metal catalystsoccurs at a relatively faster rate and therefore typically yields thicker wires with higher aspect ratio.However, the NWs have more oxygen vacancies affecting other properties, such as electrical transportand surface chemistry.

Few in situ studies were performed especially regarding the Si NWs growth Si nanowires growth

by the vapor–liquid–solid mechanism was monitored using real time in situ ultra high vacuum mission electron microscopy[33]

trans-A growth rate independent of wire diameter was found Showing that the irreversible, kineticallylimited, dissociative adsorption of disilane directly on the catalyst surface was the unique rate-limit-ing step (Fig 4)

The growth rates were independent of wire diameter, and increased linearly with pressure Fromthe growth rate measurements, the reactive sticking probabilities of Si2H6 at the droplet surfaceand at the wire sidewall was determined A novel dependence of growth rate on wire taper, whichwas attributed to the deposition of excess Si from the shrinking droplets, was observed

Many open questions still remain regarding the different experimental evidences on VLS growth,top or bottom catalyst cluster, single or multiple nanowires growth, the relation between the catalystand nanowire size, the possibility of an auto-catalyzed growth But, regardless of the catalyst phase(either liquid or solid) the growth dynamics does not change and catalytic growth still appears to

be the most powerful method for producing 1D nanostructures

2.1.2 Vapor–solid mechanism

The VS growth takes place when the nanowire crystallization originates from the direct tion from the vapor phase without the use of a catalyser At the beginnings the growth was attributed

condensa-to the presence of lattice defects, but when defects-free nanowires were observed this explanation

Fig 4 (a) Representative bright field TEM images of a Si wire acquired at four successive times during deposition White arrows highlight a reference point on the wire sidewall (b) Length L (open squares) and diameter d (solid circles) of the same wire as a function of t The straight line is a least-squares fit to the first 1200 s Reprinted Fig 2 with permission from [33] http:// link.aps.org/abstract/PRL/v96/e096105

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cannot be any longer accepted Another peculiar effect registered was a nanowire growth rate higherthan the calculated condensation rate from the vapor phase A possible interpretation is that all thefaces of the nanowire adsorb the molecules that afterwards diffuse on the principal growth surface

of the wire

VS process occurs in many catalyst-free growth processes Quite a few experimental and ical works have proposed that the minimization of surface free energy primarily governs the VS pro-cess Under high temperature condition, source materials are vaporized and then directly condensed

theoret-on the substrate placed in the low temperature regitheoret-on Once the ctheoret-ondensatitheoret-on process happens, theinitially condensed molecules form seed crystals serving as the nucleation sites As a result, they facil-itate directional growth to minimize the surface energy

This self-catalytic growth associated with many thermodynamic parameters is a rather cated process that needs quantitative modelling[34] It was reported for indium oxide, In2O3wireswere synthesized through thermal evaporation of pure In2O3powders and the effect of substrate seed-ing was studied for controlling density distribution and lateral dimensions of the wires The wires ex-hibit uniform section, atomically sharp lateral facets, and pyramidal termination, typical of a VSgrowth mechanism assisted by oxidized nanocrystalline seeds

compli-Other growth conditions have been reported, for example ZnO NWs were synthesized by a VP cess using a thin ﬁlm (10 nm) of tin as catalyst Carbothermal reduction was used to reduce the sourcetemperature needed for the vapor phase production The tip of the NWs resulted without the catalystand was attributed to VS process[35]

pro-Many report also the NWs production by simple oxidation of the metal composing the metal oxide[36], for example[37]report the growth of CuO NWs from copper foils oxidized in wet air at temper-atures between 300 and 800 °C Within the temperature range of 400–700 °C, the nanowires formedhave two different morphologies, curved and straight, with diameters between 50 and 400 nm andlengths between 1 and 15lm The growth behavior was explained in terms of kinetics involvingshort-circuit diffusion, the strength of the nanowires, and the thickness ratio of the oxide scale andthe metal The formation kinetic of CuO nanowires was governed by the short-circuit diffusion ofatoms or ions during the reaction The deformation of thin oxide scale under thermal stresses may alsocontribute to the formation of curved nanowires

The vapor–solid VS growth was attributed also for a two-step high-temperature, catalyst-free,physical evaporation of tungsten oxide NWs[38] The procedure consisted in heating tungsten pow-der, during the heating an oxide layer might be formed on the metal surface; the oxide then can evap-orate and redeposit on the substrate surface, forming one dimensional nanostructures Alternatively,the tungsten metal was vaporized ﬁrst; a subsequent oxidation during the deposition on the substratemay also form the nanostructure

2.2 Solution phase growth

Growth of nanowires, nanorods and nanoneedles in solution phase has been successfully achieved.These growth methods usually require ambient temperature so that complexity and cost of fabricationare considerably reduced To develop strategies that can guide and conﬁne the growth direction toform Q1D nanostructures, researchers have used a number of approaches that may be grouped intotemplate-assisted and template-free methods

The solution-based catalyzed-growth mechanism is similar to the previously described VLSmechanism, in this case a nanometer-scale metallic droplet catalyze the precursors decompositionand crystalline nanowire growth The variants of VLS growth in solutions SLS and supercriticalﬂuid–liquid–solid (SFLS) growths provide nanowire solubility, control over surface ligation, andsmaller diameters But the VLS growth in general produce nanowires of the best crystallinequality

There exist strong indications that the catalyst droplets in the SLS, as well as in VLS, mechanismsplay a catalytic role in precursor decomposition, in addition to catalyze the NWs growth (Fig 6) Theearly VLS literature claimed such a role on the basis of various experimental observations[21,39,40],including that VLS crystal growth typically occurs at temperatures several hundreds of degrees lowerthan epitaxial ﬁlm growth from the same precursors

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Thus, the droplets perform a dual role as ideally rough surfaces for precursor adsorption anddecomposition and as a crystallization solvent supporting semiconductor crystal-lattice formationand, hence, wire growth.

As well as for VLS, melting points, solvating abilities, and reactivities are the important criteria forselecting the potential SLS catalyst materials Moreover, at least one of the components of the productsemiconductor phase must have ﬁnite, but limited solubility in the catalyst material, so that highsupersaturations can be achieved Finally, the catalyst should not react with or form a solid solutionwith the target semiconductor phase (unless the catalyst material is the same as one of the constituentelements of the semiconductor)

2.2.1 Template-assisted synthesis

Anodization growth technique is a well-established process to growth of oxide coatings and tecting layers on metal surfaces since the past century Only in the last decades it was used for thepreparation of porous ﬁlms that later were used also for the production of nanowires and nanotubes[41–43]

pro-The most used porous material is alumina and in general the starting metallic material is ium[44]

alumin-There are several references reporting on this template-synthesis strategy for nanofabrication such

as Hulteen and Martin[45]considered as one of the pioneer groups in this subject, particularly infunctional nanowire arrays fabrication

Fig 6 Schematic representation of the solution–liquid–solid growth of nanowires Precursors in the liquid phase react to form the nanowire.

Fig 5 Schematic representation of vapor–liquid–solid growth of nanowires The catalyst is in the liquid phase and precursors can adsorb and condense to form the nanowire.

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The same experimental procedure is recently being used for the preparation of nanowires andnanotubes of other materials such as TiO2[46].

The advantages of anodization and electroplating processes for 1D nanostructures production arelow costs, repeatability and potential compatibility with silicon technologies which make these nano-structure synthesis procedures interesting, one of the main disadvantages is the poor crystallinity ofthe produced NWs Control in nanowire dimensions and the morphology of the ordered arrays can beachieved Because the diameter of these nano-channels and the inter-channel distance are easily con-trolled by the anodization voltage, it provides a convenient way to manipulate the aspect ratio and thearea density of Q1D nanostructures

The use of two anodization processes to obtain hexagonal close packed highly ordered nanoporousalumina membranes was reported by Masuda and Fukuda’s[47] After this report different procedures

to obtain porous templates have been presented for the production of 1D nanostructures orderedarrays

With the use of a periodic structured template, such as anodic aluminium oxide (AAO), molecularsieves, and polymer membranes, 1D nanostructures can be prepared thanks to the conﬁnement effect

or the porous template

The arrays grow with ordered hexagonal cells with central pores parallel to each other and with asymmetry axis perpendicularly oriented to the substrate surfaces The most used procedure to obtainfunctionalized AAO is electrochemical growth technique [48–51] In 2003 great advances wereachieved by using a combination of nano-imprint and lithographic techniques with a subsequentanodization process of aluminium metal and other metallic or semiconductor substrates[52,53].Q1D nanostructures can be deposited into the pores using electrodeposition or sol–gel depositionmethods

In the latter case, as the first step, colloidal (sol) suspension of the desired particles is preparedfrom the solution of precursor molecules, afterwards an AAO template is immersed into the sol sus-pension, so that the sol can aggregate on the AAO template surface Deposition time must be carefullychosen in order to allow the sol particles to fill the template pores and form 1D nanostructures Thefinal step consists in a thermal treatment to remove the gel

Finally also the 1D nanostructures can be used as templates for the physical conﬁnement of thegrowth, for example the synthesis of LaCoO3nanowires using carbon nanotubes (CNT) as templatehas been reported[54] The precursor solution for the nanowires was obtained by dissolving La(NO3)3

and Co(NO3)2in water CNT were dispersed in the precursor solution by sonication and stirring Thenthe solution was centrifugated, dried and ﬁnally calcinated to remove the CNT template This synthesis

is a combination of solution and templates methods, but without the use of electrochemical deposition.Despite of its simplicity template based growth is characterized by the production of polycrystal-line nanowires that can limit their potential for both fundamental studies and applications

2.2.2 Template-free methods

A big research effort has been reserved to the template-free methods for the deposition of 1D structures in liquid environment; the most important procedures are surfactant assisted, sonochem-ical, hydrothermal, organometallic methods and electrospinning

nano-The use of a surfactant can promote the anisotropic crystal growth required for the production of1D crystals The anisotropic growth is in general performed in a three phases system, oil, surfactantand aqueous phase These surfactants conﬁne the crystal growth as in microreactors Key points arethe selection of precursor and surfactants, and parameters such as temperature, pH value, and reac-tants concentration

Different surfactants were proposed depending on the material that is addressed, for example oleicacid, hexylphosphonic acid (HPA), tetradecylphosphonic acid (TDPA), trioctylphosphine oxide (TOPO),and trioctylphosphine (TOP) Surfactant-assisted methods is a trial-and-error procedure and requiresmuch effort to select the appropriate capping agents and reaction environment[55–57]

In the sonochemical method, instead ultrasonic wave are exploited to modify the crystal growthchanging the reaction conditions, by acoustic stirring

During sonication bubbles are formed in the aqueous solution, they grow and then collapse In suchenvironments it’s easy to reach extreme reaction conditions (temperature greater than 5000 K, pres-

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proaches[61–64]but other oxides were prepared such as CuO[65], ceria[66]and titania[67].The preparation of crystalline manganese oxide nanorods is reported in[68], consisting in an easyone-pot hydrothermal treatment of commercial granular/bulky MnO2in ammonia solution Post-cal-cination treatment of the hydrothermal products was not necessary and no organic solvent wasneeded in the process.

The synthesis of Cu2O nanowires was achieved by the reduction of cupric acetate with o-anisidine,pyrrole, or 2,5-dimethoxyaniline under hydrothermal conditions[69] The diameter and morphology

of Cu2O nanowires can be easily tuned by the choice of reductant type and synthetic temperature

It has just been published in[70]the growth of ZnO nanowires aligned to Si and C coated substrateswith the appropriate choice of process conditions without the use of a ZnO thin ﬁlm

Another interesting approach for the formation of nanorods is the organometallic method presented

in[71,72] It has been used for the synthesis, at room temperature, of homogeneous ZnO tures of isotropic or rod shape Organometallic complexes have been used as precursors to overcomethe problem of interaction of ionic species with the particles growth process[72] The synthesis of me-tal nanoparticles takes advantage of the reactivity of metal-organic precursors to CO, H2, UV irradia-tion or heat treatment The method uses both the exothermic reaction of the organometallic precursorwith water to produce crystalline zinc oxide and a kinetic control of the decomposition by long-alkyl-chain amine ligands in the presence or absence of additional solvents to control size and morphology.Quantitative yields of ZnO nanostructures is reported The mechanism of particle growth involvesmass transport of zinc atoms, the amine ligand playing a fundamental role in the process and remain-ing coordinated to the particles throughout the synthesis The metal oxide products were dissolved incommon organic solvents forming clean and clear luminescent solutions that could easily be depos-ited on various surfaces as monolayers or thick layers

nanostruc-Finally another template-free synthesis process is electrospinning Electrospinning uses an electricalcharge to form a mat of fine fibers It may be considered as an electrospray process A high voltageinduces the formation of a liquid jet In electrospinning, a solid fiber is generated as the electrifiedjet is continuously stretched due to the electrostatic repulsions between the surface charges andthe evaporation of solvent Whipping due to a bending instability in the electrified jet and concomi-tant evaporation of solvent (and, in some cases reaction of the materials in the jet with the environ-ment) allow this jet to be stretched to nanometer-scale diameters The elongation by bendinginstability results in the fabrication of uniform fibers with nanometer-scale diameters

The ﬁrst patent that described the operation of electrospinning appeared in 1934, when Formalasdisclosed an apparatus for producing polymer ﬁlaments by taking advantage of the electrostatic repul-sions between surface charges[73]

A number of oxides that include Al2O3, CuO, NiO, TiO2, SiO2, V2O5, ZnO, Co3O4, Nb2O5, MoO3, andMgTiO3have been fabricated as ﬁbrous structures[74–83]

3 Vertical and horizontal alignment techniques

The assembly of nanowires into ordered arrays is critical to the realization of integrated electronicarchitectures Methodologies for development of large-scale hierarchical organization of nanowire ar-rays have been recently developed[84]

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One of the possible strategies to avoid complex and time expensive manipulation of single wires forsuitable alignment is the epitaxial growth of single-crystalline nanowires onto lattice-matched sub-strates, where the most of the nanowires grow along a precise direction with respect to the substrateplane Such techniques can allow direct integration of single nanowires and nanowire arrays intoeffective devices The epitaxial growth on lattice-matched substrates plays a key role in the orderedassembling of nanowires Frequently used substrates are crystalline silicon [85,86] and sapphire[87]as well as yttrium stabilized zirconia (YSZ)[88]and GaN[89,90] Typically, catalytic seeds of no-ble metals (Pt and Au) are dispersed either randomly or in ordered arrays onto the substrate, leading

to nanowires growth according to the VLS mechanism The regular alignment of nanowires being tained via VLS growth assisted by noble-metal catalyst[91]or through the deposition of a buffer layerpreliminary to the growth of the nanowires[92]

ob-It has been recently reported the possibility of industrial scale patterning using nanolithographictechniques[93] This indication suggests the effective possibility of deserving patterned substratesfor large-scale applications, and foresees industrial use of nanowires

The experimental techniques to measure the relative orientation between the grown tures and the substrate basically rely on the diffraction of X-rays[94]or electrons[95], which allowrespectively high sampling capability over large sample areas or investigating the crystalline arrange-ment at a small scale[96]

nanostruc-The direct integration of In2O3nanowires grown using a bottom-up approach has been applied byMeyyappan and co-workers[87]to obtain a vertical ﬁeld-effect transistor (Fig 7) Optical sapphire (a-

Al2O3) has been used for the growth of the nanowires, due to the good lattice matching with the [0 0 1]axis of cubic In2O3(lattice constant a = 10.1 Å, JCPDS 89-4595)

With respect to the traditional VLS mechanism, in which single crystalline nanowire grows fromthe catalytically active seed, dynamic simultaneous nucleation and epitaxial growth events can occurdue to the presence of single-crystal substrate, driven by competitive growth mechanisms: namelynucleation, 2D buffer layer formation over the substrate, nanowire elongation and nanothreadformation

Epitaxial growth and characterization of perfectly aligned and three-dimensionally branched ITOnanowire arrays with a controlled crystallographic growth direction has been exploited[97] Yt-trium-stabilized zirconia (YSZ) has been used as the substrate because YSZ, with a ﬂuorite crystal

Fig 7 FE-SEM micrographs of vertical In 2 O 3 nanowire arrays on single crystalline optical a-sapphire substrates (a) A 45° perspective view of an array of nanowires on a-sapphire The inset shows a schematic (not to scale) of a typical nanowire with

an aspect ratio a/b 4 (b) A zoom-in 45° perspective view of the nanowires showing orthogonal directionality, nanosized Au catalytic heads, and nanothread cladding along the square core of the nanowires (c) A 5° pperspective view of the array, revealing uniform growth over the surface of the substrate and the rectangular footprint of the pyramidal base The inset shows

a zoom-in top view of a square columnar nanowire with its pyramidal base Scale bars: 2lm, 0.25lm, and 1lm for A, B, and C

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crystal of rutile structure with four {1 1 0} peripheral surfaces structure and h0 0 1i growth direction.The cross sectional dimensions of the tubes increased exponentially with the temperature at whichthey were grown, with an activation energy for tube growth of about 0.44 eV The tubules were found

to grow according to a self-catalyzed, direct VS process, where most new material is incorporated intothe bottom parts of each tubule through surface diffusion The simple control of the temperaturedetermines the lateral dimensions of the condensation products Initially, grains of the condensed bulklayer are randomly oriented As condensation continues, grains with energetically favorable crystallo-graphic planes will preferentially grow larger, while grains with energetically unfavorable surfacesgradually shrink

Many studies pointed out the attention on the heteroepitaxial growth of ZnO nanostructures ofvarious shapes over different substrates

Aligned growth of ZnO nanorods has been successfully achieved via a VLS process, with the use ofgold[99,100]and tin[101]as catalysts

A combined technique based on epitaxial growth and substrate template has been investigated.The self-assembly-based mask technique and the surface epitaxial approach were combined to growlarge-area hexagonal arrays of aligned ZnO nanorods[102] The synthesis used a gold catalyst tem-plate produced by a self-assembled monolayer (SAM) of submicron polystyrene spheres (Fig 8) andguided VLS growth on a single-crystal sapphire ð2 1 1 0Þ substrate (Fig 9) The nanorod grows along[0 0 0 1] direction and its side surfaces are deﬁned by f2 1 1 0g The collective optical properties ofthe aligned ZnO nanorods were investigated using PL It was shown that luminescence was emittedmainly along the axis of the ZnO nanorods, indicating that collective properties can be tailored underproper orientation of nanowire arrays

Aligned ZnO nanowire/nanorod arrays following a predesigned pattern and feature with controlledsite, shape, distribution, and orientation were also obtained by Wang and co-workers[103] The tech-

Fig 8 Gold pattern produced by an evaporation technique using the shadow provided by the monolayer of self-assembled

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nique relies on the integration of atomic force microscopy (AFM) nanomachining with catalyticallyactivated VLS growth Single-crystal sapphire (1 1 2 0) substrate was chosen to grow ZnO nanorods.Si(1 0 0) substrate was also proven effective for aligned growth of ZnO quasi 1D nanostructures[104] Well-aligned ZnO nanorod and nanobolt arrays were synthesized on p-type Si(1 0 0) substrates

by a simple physical vapor deposition method (Fig 10) The effect of the atmosphere on the shape ofthe condensation products was investigated: the switch from Ar to air during the growth causes thechange from nanorods into nanobolts completely The element ratio of Zn to O in the vapor phase dur-ing the growth discriminates the formation of nanorods and nanobolts Photoluminescence (PL) spec-tra indicate a strong emission peak around 3.26 eV attributed to exciton-related emission and anotherpeak at 2.48 eV related to defects In case of the nanobolts, the peak at 2.48 is weaker as compared tothe nanorods

Aside to vertical alignment, horizontal alignment of nanowires via heteroepitaxial growth is beingexplored First attempts are reported for lateral orientated growth of In2O3nanowire (NW) and nano-rod (NR) arrays on (0 0 1) and (1 1 1) surfaces of Si substrates[105].The lateral self-aligned In2O3nano-wire and nanorod arrays on Si can offer some unique advantages for fabricating parallel nanodevicesthat can be integrated directly with silicon technology

Evidence of epitaxial growth have been recorded mainly in systems where metal catalysts havebeen used for promoting the formation of nanowires Indeed, the VLS mechanism and the formation

Fig 9 (a) Low-magniﬁcation top-view SEM image of aligned ZnO nanorods grown onto a honeycomb catalyst pattern (b) Side view of the aligned ZnO nanorods at an angle of 30° (c, d) Top and a 30° view of aligned ZnO nanorods, where the hexagonal pattern is apparent (d) Aligned ZnO nanorods at the edge of the growth pattern Reprinted with permission from Ref [102]

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of a nucleation center are expected to magnify the interaction between the crystalline substrate andthe nucleating nanowire[106] Nagashima et al report about MgO nanowires prepared through Aucatalyst[107], while ZnO on Si via Pt catalyst was described by[108] Patterned and epitaxial struc-tures were investigated by[103,109].

As far as catalyst-free preparation is concerned, Baxter et al reports that the orientation of ZnOnanowires prepared on sapphire is governed by epitaxial relations[110].Fig 11highlights the parallelorientation and dense arrangement of ZnO nanowires, which were produced in absence of metal cat-alysts It turned out that the epitaxy is favored when the substrate is oriented normal to the a-plane, asthe [0 0 0 1] direction of ZnO is matched by the ½1 1 2 0 direction of sapphire and vice versa, accordingalso to the ﬁndings of Chen et al.[111]

The epitaxial growth of ZnO nanowires on c-plane of sapphire was investigated by[112], as well asthe orientation of ZnO on m-plane oriented sapphire[113].Fig 12shows the nanowires inclined inopposite directions at about 30° This arrangement was preserved over a wide pressure range forthe deposition, resulting in different aspect ratio for the nanowires

The buffer layer is often homogenous to the composition of the nanowires, a notable example beingreported by Wan et al.[88,114], where vertically aligned tin-doped indium oxide (ITO) nanowires over

a ITO buffer layer were obtained by thermal evaporation on lattice-matched[100]YSZ substrate

In some cases an heterogeneous buffer layer has been used to implement a functional properties inaddition to the promotion of epitaxial growth: In fact, TiN performed as a good electrode and diffusionbarrier material Lin et al.[92]developed a ZnO (nanowires)/TiN (buffer layer)/Si (substrate), wherethe following lattice distance matching has been determined: ½1 2 1 0ZnOk½1 1 1TiNk½0 1 1Si and

½0 0 0 1ZnOk½1 1 1TiNk½1 1 1Si TiN features a lattice mismatch of about 8% with respect to ZnO and vored a Volmer–Weber growth mechanism

fa-Fig 10 (a) SEM image of the aligned ZnO nanorods, (b) magniﬁed SEM image of the aligned ZnO nanorods, (c) SEM image of the aligned ZnO nanobolts, (d) top view of the hexagonal structure of the nanotips, (e) single ZnO nanobolt, and (f) ZnO nanobolts directly grown on a p-type Si(1 0 0) substrate Reprinted with permission from Ref [104]

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Template alignment of quasi 1D nanostructures can be achieved by suitable application of plate-assisted growth, as described in Section 2.2.1.

tem-3.1 Electric ﬁeld alignment

In principle dielectrophoresis (DEP), which is the electrokinetic motion of dielectrically polarizedmaterials in non-uniform electric ﬁelds, could be a powerful tool for self alignment of nanowires into

a well deﬁned space region, in which an external electric ﬁeld is induced, without mechanical anipulation techniques

nanom-DEP has been shown to be capable of aligning metallic nanostructures, like carbon nanotubes andgold nanowires directly between electrodes [115,116] Effective manipulation of gold nanowires[117], synthesized by template electrodeposition in porous aluminium oxide membranes, was ob-tained The dielectrophoretic force was modelled on the basis of the interaction between the alternat-ing applied ﬁeld and the induced dipole moment of gold nanowires, and allowed the electricalcharacterization of the nanowires (seeFig 13)

Very recently dielectrophoresis was applied also for manipulating semiconducting and oxide wires to obtain prototype devices

nano-Fig 11 SEM view of ZnO nanowires grown on a-plane sapphire The nanowires grow perpendicular to the substrate and with nearly perfect rotational alignment with respect to the substrate lattice Scale bar corresponds to 100 nm Reprinted from Ref [110] , Copyright (2005), with permission from Elsevier.

Fig 12 Tilted view (30°) SEM images of ZnO nanowire samples grown by high-pressure pulsed laser deposition with an oxygen ﬂow of (a) 0 sccm, (b) 20 sccm and (c) 35 sccm Reprinted from Ref [113] , with permission from IOP Publishing.

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Positive ac DEP has been used to align CdSe semiconductor nanowires near patterned trodesFig 14 [118] The induced dipole of the wires is proportional to their conductivity, due to theirlarge geometric aspect ratio.

microelec-AC dielectrophoretic manipulation was applied for fabrication of nanosensors based on tin oxidenanobelts[119] Positive and negative DEP was used for the assembly of a nanodevice, which con-sisted of SnO2nanobelts attached to castellated gold electrodes deﬁned on a glass substrate, and cov-ered by a microchannel (Fig 15)

Controlled assembly of ZnO nanowires was carried out using DEP[120] A structure similar to aﬁeld-effect transistor with two isolated top electrodes comprising the source and drain and a lowersubstrate electrode as the gate was used for the dielectrophoresis-based assembly of zinc oxide nano-wires The geometry of the electrodes as well as the magnitude and frequency of the applied electric

Fig 14 Dielectrophoretically aligned CdSe NWs using an ac electric field (10 V) with electrodes separated by a 20lm gap [(a) and (b)] (1 MHz) Resulting bright field image after alignment and epifluorescence image taken at t = 50 s during the alignment, respectively [(c) and (d)] (10 kHz) Resulting alignment in 190 s, under illumination (100 W/cm 2

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ﬁeld was proven signiﬁcantly affecting the density of the nanowires assembled between the trodes (Fig 16).

elec-Dielectrophoretic alignment was employed to design simple structures such as Schottky diodesformed across Au electrodes using ZnO nanobelts and nanowires[121] The formation of the Schottkydiodes is suggested due to the asymmetric contacts formed in the dielectrophoresis aligning process.The detailed IV characteristics of the Schottky diodes have been investigated at low temperatures.Dielectrophoretic integration of nanodevices with CMOS VLSI circuitry[122]was also achieved, en-abling the creation of integrated circuits that include readout, signal processing and communicationcircuitry The nanostructures have been manipulated using dielectrophoretic forces, allowing theirindividual assembly and characterization (seeFig 17)

ZnO nanowire-based UV photosensor have been fabricated and characterized using DEP[123] ZnOnanowires, which were synthesized by nanoparticle-assisted pulsed laser deposition, were suspended

Fig 15 SnO 2 nanobelts bridging DEP electrode gaps during positive dielectrophoresis Smaller nanobelts are sticking out of the electrodes due to the action of the electric ﬁeld Reprinted from Ref [119] , with permission from Copyright 2005 Elsevier.

Fig 16 SEM images of assembled ZnO nanowires between electrodes with different gap distances An ac voltage of 10V pp at a frequency of 20 MHz was applied to the right electrode relative to the left grounded electrode (a) The left four ﬁngers are all part of the left electrode and the right four ﬁngers are all part of the right electrode The four gap distances are 2, 4, 6, and 10lm respectively and are enlarged in (b)–(e) The scale bars are 10 mm in (a) and 2lm in (b)–(e) Reprinted with permission from Ref [120]

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in ethanol, and were trapped in the microelectrode gap where the electric ﬁeld became higher Thetrapped ZnO nanowires were aligned along the electric ﬁeld line and bridged the electrode gap.3.2 Nanomanipulation

The manipulation of nanostructures has been pursued in the last years to probe individual wire-like structures and to fabricate prototypes of electronic micro-sized devices, when direct integra-tion of the growth of nanowires into a functional substrate has not been achieved

nano-Various approaches for manipulation of metal oxide nanowires have been proposed, includingmechanical[124], electrostatic[125], and dielectrophoretic[121]methods Alignment of single nano-wires or assembling of several identical nanostructures is obtained under vacuum condition or in li-quid or standard atmospheric environment, most of the manipulation being performed in associationwith highly-resolved imaging techniques such as scanning microscopy (SPM and SEM) and even trans-mission electron microscopy (TEM) [126,127] The methodology for manipulation of metal oxidenanostructures has largely beneﬁted from the previous efforts expended for the characterization ofcarbon nanotubes and nanostructures[128,129]

Fig 17 SEM photograph of assembly sites initially electrically isolated from the readout circuitry Assembled nanowires are clamped and connected to the readout circuit using a post-assembly e-beam lithography and metallization step Reprinted from Ref [122]

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Manipulation has been proved as a powerful technique tool for studying the intrinsic transportproperties of nanowires, and to provide an experimental proof for innovative functional concepts[130].

The electrical characterization of metal oxide nanowires has been performed via two-contacts orfour-contacts methods, where the conductive paths are provided by either patterned structures[112], piezo-actuated probes, or metallic deposits obtained by focused ion beam[131] These ap-proaches allow the measurement of physical quantities such as free-electron concentration, electricalmobility, and conductivity in single tin oxide nanowires, accounting for the contribution of the metal-lic contact, as reported in Ref.[131]

Mechanical manipulation of oxidic structures has been performed in air through AFM[132]for icon oxide helical structures (seeFig 18) and has been also implemented in electron microscopes.Recently, nanoindentation has been introduced to determine the hardness and elastic modulus ofzinc or tantalum oxide nanowires, and to determine the Young modulus of several nanostructures[133–135]

sil-4 Doping of quasi 1D metal oxide nanostructures

Doping of nanowires is pursued to the controlled modiﬁcation of the characteristics of the wires, in terms of morphological features as well as electrical or optical properties Differently fromheterogeneous systems formed by metallic catalytic particles and metal-oxide nanowires, the intro-duction of dopants assumes preservation of the crystalline structure for the nanowire and avoids for-mation of precipitates, segregation phenomena, or nucleation of second phases

The presence of dopants may introduce a distorsion of the lattice and guide the growth of the wires to a speciﬁc crystallographic direction[136] As an example, Fan et al reported about the change

nano-in morphology and crystallnano-ine habit nano-in ZnO nanostructures nano-in consequence of nano-introduction of nano-indium.Fig 19summarizes the proposed model that suggest a change in the nucleation behavior of ZnO at thesolid–liquid interface upon supersaturation and oxidation of Zn

The quantitative description of the correlation between the atomic structure and the properties ofthe nanostructure makes the investigation of dopant dispersion a challenging task Indeed, the smalldimension of the nanowires requires both spatial resolution and chemical sensitivity[137]; therefore

a reliable determination of dopant dispersion may be achieved through a complementary approach,where the spatial distribution of dopants is associated to the measurement of the electrical activation

of ionized atoms For these reasons, electron microscopy, scanning probe microscopy, X-ray and chrotron radiation diffraction techniques are associated to photoluminescence spectroscopy and elec-trical transport measurements Elemental analysis was also carried out by Rahm et al using particleinduced X-ray emission, Rutherford backscattering spectroscopy, and Q-band electron spin resonance

syn-Fig 19 Schematic model of the growth processes for ZnO nanowire and nanobelt (dimensions not to scale) The main difference is that the [0 0 0 1]-axial nanowire (a) grows upwards on the top (0 0 0 1) plane of a ZnO columnar/pyramidal nuclei, whereas the h1 1 2 0i nanobelt (b) grows mainly from the side faces of a quasi-hexagonal indium doped ZnO pad Reprinted from

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remarkable achievement The inherent difﬁculty in producing stable p-type behavior and the effect

of P on the photoemission of ZnO has been discussed by Shan et al.[142]

The papers of Xiang et al., and Shan et al.[141,142]also introduce the issue of the doping ology The concentration of oxygen vacancies can be controlled through variation of the oxygen con-centration in the Ar/O2 gas carrier during the synthesis of nanowires, and similar results can beobtained via post-synthesis treatment in reducing atmosphere[140]

method-Dopant addition is basically carried out by modifying the composition of the precursor in the oration–condensation process, despite the limited capability to manage the amount of dopant even-tually introduced in the nanowires The signiﬁcant difference between the elemental ratio in the

evap-Fig 20 (left) SEM image of P-doped ZnO nanowires, showing that the nanowires have uniform diameter (about 55 nm) The marker corresponds to 200 nm (center) X-ray diffraction pattern of ZnO nanowires with no peaks associated to second phases

or clusters (right) Excitonic peaks of PL spectra at 10 K of n-type (green line), as-grown (red line), and annealed (blue line)

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precursor material and the composition obtained for the nanowires marks a critical issue of the oration–condensation approach, that is discussed by Nguyen et al.[143] Liquid solution synthesesand hydrothermal methods are promising for good reproducibility, controlled stoichiometry, and pre-cise doping of the nanowires (as discussed in Section2.2).

evap-Post-synthesis treatments in controlled conditions of atmosphere and temperature allow an proved dopant addition, as the temperature activated diffusion of dopants into the nanostructuremay be tailored

im-The effort of ZnO doping with V-group elements such as As, P, and N has been documented[144,145,139,146], and also doping with Tm, Yb, and Eu using ion implantation and post annealingwas reported in the literature[147]

Similarly to ZnO nanowires, SnO2and In2O3nanowires were doped with Sb, Ta and other elements,which could favorably substitute the cation in the nanowire crystalline[148–150] In addition, thepreparation of nanowires of the ternary system of In-stabilized tin oxide has been investigated[143].Wan et al., together with Dattoli et al highlight the effect of donor dopants on the electrical prop-erties of tin oxide nanowires[148,149]: an increased mobility in excess of 100 cm2/V s was statisti-cally determined from ﬁeld-effect transistor (FET) devices as shown in Fig 21, and a metallicbehavior associated to as low a electrical resistivity as 4 104Xcm have been recorded though elec-trical transport measurements

5 Preparation of quasi 1D metal oxide heterostructures

The creation of heterostructures is being strongly investigated in the last years in order to exploitthe functional properties arising from the junction of different materials and/or the effect of hierarchi-cal organization of 1D nanostructures InFig 22the typical shapes of heterostructures fabricated up tonow are reported (dendritic growth, superlattice in a single nanowire, polycrystals coalescence on asingle backbone, core–shell geometry), each shape enabling exploitation of different functional prop-erties of these innovative structures Typically VLS and VS growth mechanisms can be combined forenabling or inhibiting predeﬁned growth directions and modiﬁcations of the crystalline assembly

As an instance, combined VLS–VLS growth under sequential seeding of the catalyst leads to formation

of dendritic shapes; while VLS–VS process can lead to formation of core–shell structures, or decoration

of nanowires with small crystals

III–V heterostructures have been created since 2002 by Yang and co-workers[151] The laser tion process was applied as a programmable pulsed vapor source, allowing a block-by-block growth of

abla-Fig 21 I ds –V ds curves for back-gated Ta-doped SnO 2 nanowire FET devices fabricated on silicon substrates and operated with

V gs ranging between 6 and 4 V in 2 V steps from top to bottom Reprinted with permission from Ref [149]

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the nanowire with a well-deﬁned compositional proﬁle along the wire axis via CVD process Si/SiGesuperlattices in a single-crystalline nanowire have been obtained (Fig 23).

Dendritic nanowires growth mediated by a self-assembled catalyst was also exploited for tion of InAs structures[152]

produc-Typically, heterostructures of metal oxides are more difﬁcult to be synthesized with respect to III–Vsemiconductors, and in fact the block-by-block mechanism was never reported for oxides

Fig 23 (a) SEM image of the heterostructured nanowire array on Si (1 1 1) substrate The scale bar is 1lm The inset shows the tip of one nanowire The scale bar is 100 nm (b) STEM image of two nanowires in bright ﬁeld mode The scale bar is 500 nm (c) EDS spectrum of the Ge rich region on Si/SiGe superlattice nanowires (d) Line proﬁle of EDS signal from Si and Ge components along the nanowire growth axis The experiments were carried out on a Philip CM200 TEM operated at 200 keV Reprinted from Ref [151]

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The ﬁrst attempt for creation of heterostructures was based on spatially controlled doping of singlenanowires during NWs growth Selective doping of single oxide ZnO nanowire was carried out for cre-ation of nano-junctions by introducing the dopant in vertically grown single-crystalline ZnO nano-wires A section of the nanowires was doped with aluminium as donor during crystal nucleation,resulting in n-n+ junction (Fig 24)[153].

Heterostructures of one single material (ZnO) of different shapes have been obtained: verticallyaligned 2D and 1D ZnO nanostructures have been grown on electrically conducting, highly orientedpyrolytic graphite (HOPG) and on insulating (1 1 2 0) sapphire substrates[154] It was demonstratedthat the simple parameter of time duration of the growth process discriminates the formation of epi-taxially oriented 1D structures at the junction of the nanowalls As in most of the growth processes ofheterostructures, key role is played by the catalyst In this case nanowire growth at the junction ofnanowalls is the result of the interplay between the dynamic wetting of gold and its thermally acti-vated surface diffusion (seeFig 25)

One further process for obtaining heterostructures starting from a single material is the sequentialoxidation of metal nanowires Arrays of metal–metal oxide core–shell nanowires and single-crystal-line metal oxide nanotubes have been obtained[155] The process is based on the kinetic control ofthe conversion of single-crystalline Bi nanowires to Bi–Bi2O3core–shell nanowires via a multistep,slow oxidation method, and then on the control of their further conversion to a single-crystalline

Bi2O3nanotube array via fast oxidation

Only very recently complex structures of different oxides have been obtained by properly ing VLS and VS condensation: core–shell,[156,157]longitudinal,[157] branched heterostructures,[88,158]decorated nanowires[159] Epitaxial relationship is typically found between the crystal-lat-tice of the nanowire acting as the backbone in dendritic growth (or as the core in the core–shell geom-etry) and the second material, which is applied in the second step of condensation

combin-Various mechanisms have been set-up for production of core–shell heterojunctions

ZnO–Al2O3and ZnO–TiO2core–shell nanowires have been synthesized using a two-step process(Fig 26)[160] First, ZnO nanowires were grown in aqueous solution using a seeded growth process.Then atomic layer deposition (ALD) was applied to cover each nanowire with a thin layer of amor-phous Al2O3or TiO2 The TiO2ﬁlm was amorphous for thickness below 5 nm, while anatase polycrys-tals were detected for thickness above 5 nm

Similar technique was applied for creation of indium tin oxide (ITO) nanowires coated with TiO2

[161] Catalyst-free transport-and-condensation method was applied for nanowires growth, using a

Fig 24 I–V characteristics of single ZnO vertical nanowires with a junction between intrinsically doped and Al-doped regions (n–n+ junction) as probed by STM tip Characteristics from a number of single nanowire junctions are presented in the ﬁgure showing the degree of reproducibility Broken line is a ﬁt to the general empirical equation Inset shows I–V’s from intrinsically doped (pristine) single nanowires Reprinted with permission from Ref [153]

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Fig 25 Zinc oxide nanowalls and nanowires (A) SEM image of quasi-3D ZnO nanostructures grown on a sapphire using 4 to

5 nm Au thin ﬁlm as the catalyst The inset shows a SEM perspective view (B) 2D ZnO nanowalls on a sapphire with a height

5lm (C) An array of free-standing 1D ZnO nanowires on a HOPG substrate using 15 Å Au ultrathin ﬁlm as the catalyst (D) Schematic illustration showing the growth mechanism of ZnO nanowalls and nanowires Reprinted from Ref [154] , with permission form AAAS.

Fig 26 ZnO–Al 2 O 3 core–shell nanowires (a) Low-magniﬁcation image of a wire that has been cleaved in two Scale bar, 50 nm (b) Electron diffraction pattern of the same wire Only ZnO spots are present (c) EDS elemental proﬁle along the dashed line in

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pulsed laser ablation for creation of the volatile species[162] Nanocrystalline TiO2thin ﬁlm was thendeposited by RF sputtering to obtain the core–shell structure.

Homojunction has been obtained between b-Ga2O3nanowires and nanocrystals via vapor phaseepitaxy (VPE): VLS and VS growth mechanism are simultaneously present, the ﬁrst driving wire elon-gation, the second being responsible for nanocrystal nucleation[163] A similar mechanism, namelythe oriented attachment, was investigated for obtaining the formation of heterostructures in aSnO2–TiO2system[164]

A non-equilibrium synthesis technique has been developed to produce novel transition metal oxidesingle-crystal nanowires, including YBa2Cu3O6.66, La0.67Ca0.33MnO3, PbZr0.58Ti0.42O3, and Fe3O4[156].Vertically aligned single-crystalline MgO nanowires, grown via VLS method, were applied as tem-plates for epitaxial deposition of the desired transition metal oxides using pulsed laser deposition,and led to core–shell nanowires (Fig 27)

Radial and longitudinal nanosized In2O3–SnO2 heterostructures were produced by sequentialtransport and condensation steps (Fig 28) Radially shaped heterostructures were achieved via VS–

VS condensation, while VLS–VLS steps led to formation of longitudinal structures, based on the lytic activity of the gold cluster during the different condensation steps Lattice matching between thecore In2O3nanowire and the external SnO2was demonstrated in the radial heterostructures, similar toRef.[156] However, the lateral dimensions of the crystalline domains of the second phase are limited

cata-by mechanisms of energy reduction, which lead to formation of polycrystals

Sequential seeding of active catalyst was successfully applied for obtaining the growth of branchednanowire structures, in which semiconducting In2O3nanowire arrays with variable densities weregrown epitaxially on metallic ITO nanowire backbones (Fig 29)[88]

Branched 1D heterostructures have been synthesized by growing vanadium oxide nanostructures

on SnO2backbones A sequential two-step strategy allowed formation of hierarchical structures, in

Fig 27 MgO nanowire templates were also used for PZT and Fe 3 O 4 core–shell nanowire synthesis (a) TEM image of a PZT nanowire The catalyst can be seen at the end of the nanowire Inset: HRTEM image of the MgO/PZT core–shell nanowire at the sample tilt angle of 45 degrees (b) SAED pattern of a MgO/PZT nanowire Red arrows inside are PZT diffraction dots and blue arrows outside are MgO diffraction dots (c) TEM image of an Fe 3 O 4 nanowire (d) SAED pattern of a MgO/ Fe 3 O 4 core–shell nanowire Red and blue arrows indicate Fe 3 O 4 and MgO diffraction patterns, respectively The weak and unmarked spots come

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which a heteroepitaxial growth of vanadium oxide on tin oxide was revealed Shaping of line vanadium oxide was possible in form of buds or nanorods by varying the mass ﬂow of the vana-dium precursors.

nanocrystal-Various ZnO nanostructures, such as nanobelts, nanorods, and nanowires, have been grown on synthesized SnO2nanobelts via a simple thermal evaporation of Zn powders, without using any cat-alysts, producing various heterostructures [165] The evaporation temperature is the critical

pre-Fig 29 (a, b) Schematics of the growth processes of branched In 2 O 3 nanowires, showing the deposition of Au catalysts on ITO backbones (a) and the subsequent growth of In 2 O 3 nanowire branches in a second VLS growth process (b) (c, d) SEM images of branched In 2 O 3 nanowires grown on ITO nanowire backbones The thickness of the Au catalyst is 10 nm in (c) and 2 nm in (d) Scale bars: 500 nm Reprinted from Ref [88]

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experimental parameter for the formation of different morphologies of these nanostructures temperature photoluminescence spectra of the heterostructures show that the relative intensity ofultraviolet emission to the green band can be tuned by controlling the morphologies and sizes of thesecondary-grown 1D ZnO nanostructures, suggesting that the nano-heterostructures of these nano-structures grown on SnO2nanobelts may have potential applications in nano-optoelectronic devices.

Room-6 Applications of metal oxide nanostructures

6.1 Metal oxide gas sensors

Metal oxides semiconductors are normally high gap metal oxides in which the semiconductingbehavior arises from deviation of stoichiometry[166] They should always be regarded as compen-sated semiconductor: cation vacancies are acceptors, yielding holes and negative charged vacancies,shallow states made up of oxygen vacancies acts as n-type donors, since the bonding electrons onthe adjacent cation are easily removed and donated to the conduction band[167]

The termination of the periodic structure of a semiconductor at its free surface may form localized electronic states within the semiconductor bandgap and/or a double layer of charge, known

surface-as a surface dipole

The appearance of surface-localized acceptor states in n-type semiconductors induces chargetransfer between bulk and surface in order to establish thermal equilibrium between the two Thecharge transfer results in a non-neutral region (with a non-zero electric field) in the semiconductorbulk, usually referred to as the surface space charge region (SCR)[168] This region depleted of major-ity carriers extends approximately a few Debye Lengths LD¼ ffiffiffiffiffiffiffiffi

ekT

q 2 Nd

qinto the bulk; typical values of LD

for tin oxide ranges from 130 to 10 nm when temperature changes from 400 to 700 K

In addition to surface states, another important phenomenon associated with a semiconductor face is the surface dipole[169] An adsorbate layer may result in a surface dipole, the magnitude ofwhich depends on the ionicity of the adsorbate–substrate bond The surface dipole manifests itself

sur-as a step in the electric potential at the surface because the potential changes abruptly over severalmonolayers This is in contrast to the macroscopic dipole created by the surface states and surface SCR

As for conduction, metal oxide gas sensors are generally operated in air in the temperature rangebetween 500 and 800 K where conduction is electronic and oxygen vacancies are doubly ionized andﬁxed In quasi 1D gas sensors the current ﬂows parallel to the surface When the nanowire is fullydepleted, carriers thermally activated from surface states are responsible for conduction Indeedwhen considering nanowires bundles, the conduction mechanism is dominated by the inherentintercrystalline boundaries at nanowires connections – like in polycrystalline samples – rather than

by the intracrystalline characteristics; the intergranular contact provides most of the sample tance[170]

resis-The metal semiconductor junction that forms at the interface between the layer and the contactscan play a role in gas detection, enhanced by the fact that the metal used for the contact acts also as acatalyst The contact resistance is more important for single nanowires since it is in series to the semi-conductor resistance that for bundles where it is connected to a large number of resistances.6.1.1 Surface adsorption

The ﬁrst step of association of gas species with a solid surface is physisorption, afterwards thephysisorbed species can be chemisorbed if they exchange electrons with the semiconductor surface,becoming chemisorbed[171] When the adsorbate acts as a surface state capturing an electron or

an hole chemisorption is often named ionosorption

Physisorption is a slightly exothermic process characterized by high coverage at low temperatureand a low coverage at high temperature If the partial pressure is very low, Henry’s Law applies and theamount physisorbed is simply proportional to the partial pressure In contrast of physisorption, that is

a slightly exothermic, inactivated process, chemisorption and desorption are activated processes Theactivation energies can be supplied either thermally or by a non-equilibrium process such asillumination

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occupied states.

6.1.2 Detection through surface reactions

The process of gas detection is intimately related to the reactions between the species to be tected and ionosorbed surface oxygen In the temperature range between 400 and 800 K oxygen ion-osorbs over SnO2and other oxides in a molecular (O

de-2) and atomic form (O)[173]; when a reducinggas like CO comes into contact with the surface, it oxidizes to CO2by reacting with ionosorbed oxygen,releasing electrons from surface states to the conduction band The overall effect at equilibrium isshrinking of the density of ionosorbed oxygen, i.e occupied surface acceptor states Indeed surfacereactions are still debated: CO sensing, for example, could take place through reaction with hydroxylgroups, producing atomic hydrogen that recombines with oxygen lattice and releases a free electron[174]and even by direct adsorption as CO+[175]

Direct adsorption is also proposed for the gaseous species –like strongly electronegative NO

2

whose effect is to decrease sensor conductance eþ NO2;ads¡ NO2;ads The occupation of surface states,which are much deeper in the bandgap than oxygen’s, increases the surface potential and reduces theoverall sensor conductance

An important ubiquitous species that ionosorbs over MOX surfaces is water[173] The tion of water onto oxide from air can be very strong, forming an ‘‘hydroxylated surface”, where the

chemisorp-OHion is bounded to the cation and the H+ion to the oxide anion The overall effect of water vapor

is to increase the surface conductance, although the surface reactions are still debated and manymechanisms have been proposed[176,172,177]

6.1.3 DC resistance transduction

The easiest measurable physical quantity that can be transduced is the sensor conductance in DCconditions In laboratory tests it is normally measured by a voltamperometric technique at constantbias while in commercial gas sensors the layer is usually inserted inside a voltage divider

The sensor response towards a target gas concentration for metal oxide semiconductor sensors isusually deﬁned as the (relative) change of conductance, with the obvious relation

repre-6.1.4 Conductometric gas sensors

Starting from the ﬁrst paper that pointed out the feasibility to use bundles of nanowires as gas sors[179]there is plenty of literature regarding gas-sensing with conductometric nanowires The aim

sen-of this section is to give a critic review sen-of the work presented in literature about conductometric gas

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sensors The topic will be divided in section regarding different materials used for nanostructure aration and gas sensing.

prep-As shown inTable 1, the majority of the works found in literature is based on bundles of tures In this case, many of these nanostructures are contacted and their electrical parameters esti-mated However, only some mean values of these parameters are determined due to the dispersionexisting among the contacted nanostructures and the grain boundaries among them Where possible,

nanostruc-we will report separately papers which presents measurements on bundles of nanowires from that on

References SnO 2 Ethanol, CO, NO 2 Mesh of nanobelts 200 wide 20–40 thick 200–400 [179]

SnO 2 CO, relative humidity Single nanowire 25, 70 200–295 [184–186]

SnO 2 NO 2 Single nanobelt 80–120 wide 10–30 thick UV activated [188]

ZnO Ethanol, methane,

2-butanone,

triethylamine,

isopropanol

ZnO Ethanol, H 2 S, HCHO,

ZnO–Pt H 2 , ethanol Paste of nanorods,

nanowires, nanotubes

20–30 60–100 ZnO–He +

WO 2.72 NH 3 , Ethanol, NO 2 Mesh of nanorods 4 50–200-UV

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surface reactions with gaseous species Photochemical activation has also been used.

Since many authors did not follow a correct procedure for gas testing, it is quite difﬁcult to compareresults from different groups A constant ﬂux of gaseous species at ambient pressure should be em-ployed instead of vacuum ambient with injection of desired amount of gases The widespread use

of adhesion agent to realize a gas-sensing paste of nanowires leaves some doubts about the inﬂuence

of the agent on gas sensing measurements

6.1.4.1 Tin dioxide

6.1.4.1.1 Multiple nanowires Comini et al.[179]were the ﬁrst to present their results on nanobelts

of SnO2with a rectangular cross section (200 nm large-20/40 nm thick) in a ribbon-like morphology,prepared by a thermal evaporation of oxide powders under controlled conditions without the pres-ence of a catalyst

After deposition, a bunch of nanobelts was placed onto the interdigitated Pt electrodes to measuretheir electric conductance They obtained a good response to gaseous polluting species like CO and

NO2for environmental applications, as well as to ethanol ð250 ppm;S

G¼ 41:6) for breath analyzersand food control applications All measurements were done at 400 °C working temperature for reduc-ing gases and 200 °C for oxidizing gases in humid air (30% RH) (seeFig 30)

They have shown the experimental feasibility of fabricating nanosized sensors using the integrity

of individual nanobelts, even if the size of the nanobelts is quite big in order to have a complete tion of the conduction path Moreover the conduction should be governed by the barrier between thebelts contacting each other

deple-The same group reported later the use of mesh of SnO2nanowires for ozone sensing at 400 °C ofvery low concentration of ozone (70 ppb)[180]

Fig 30 Response of bunch of SnO 2 nanobelts at 400 °C working temperature towards 250 ppm of ethanol [179] Reprint

Tiêu đề	Quasi-one Dimensional Metal Oxide Semiconductors
Tác giả	E. Comini, C. Baratto, G. Faglia, M. Ferroni, A. Vomiero, G. Sberveglieri
Trường học	Brescia University
Chuyên ngành	Materials Science
Thể loại	review article
Năm xuất bản	2009
Thành phố	Brescia

Định dạng
Số trang	67
Dung lượng	3,72 MB

Tài liệu tham khảo	Loại	Chi tiết
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