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Analytica Chimica Acta 568 (2006) 2840

Review

Metal oxide nano-crystals for gas sensing

Elisabetta Comini

SENSOR Lab, CNR-INFM, Brescia University, via valotti 9, 25133 Brescia, Italy

Received 8 August 2005; received in revised form 21 October 2005; accepted 25 October 2005

Available online 1 December 2005

Abstract

This review article is focused on the description of metal oxide single crystalline nanostructures used for gas sensing. Metal oxide nano-

wires are crystalline structures with precise chemical composition, surface terminations, and dislocation-defect free. Their nanosized dimension

generate properties that can be significantly different from their coarse-grained polycrystalline counterpart. Surface effects appear because of themagnification in the specific surface of nanostructures, leading to an enhancement of the properties related to that, such as catalytic activity or

surface adsorption. Properties that are basic phenomenon underlying solid-state gas sensors.

Their use as gas-sensing materials should reduce instabilities, suffered from their polycrystalline counterpart, associated with grain coalescence

and drift in electrical properties. High degree of crystallinity and atomic sharp terminations make them very promising for better understanding of

sensing principles and for development of a new generation of gas sensors. These sensing nano-crystals can be used as resistors, in FET based or

optical based gas sensors. The gas experiments presented confirm good sensing properties, the possibility to use dopants and catalyser such in thin

film gas sensors and the real integration in low power consumption transducers of single crystalline nanobelts prove the feasibility of large scale

manufacturing of well-organized sensor arrays based on different nanostructures. Nevertheless, a greater control in the growth is required for an

application in commercial systems, together with a thorough understanding of the growth mechanism that can lead to a control in nano-wires size

and size distributions, shape, crystal structure and atomic termination.

2005 Elsevier B.V. All rights reserved.

Keywords: Nano crystals; Gas sensors; FET; PL; Nanowires

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2. Deposition techniques and growth mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3. Working principle of metal oxide gas sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4. Measurements methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.1. DC conductimetric gas sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.1.1. Tin oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4.1.2. Zinc oxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.1.3. Other metal oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

4.2. FET based gas sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

4.2.1. Indium oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364.2.2. Tin oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4.2.3. Zinc oxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.3. PL based gas sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

5. Other fields of application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Tel.: +390303715706; fax: +390302091271.E-mail address: [email protected].

0003-2670/$ see front matter 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.aca.2005.10.069

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E. Comini / Analytica Chimica Acta 568 (2006) 2840 29

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

1. Introduction

Metal oxides represent an assorted and appealing class of

materials which properties cover the entire range from metals to

semiconductors and insulators and almost all aspects of mate-

rial science and physics in areas including superconductivity

and magnetism. In the field of chemical sensing, for more than

five decades it has been known that the electrical conductivity

of semiconductors varies with the composition of the gas atmo-

sphere surrounding them.

Gassensors have a great influence in many areas such as envi-

ronmental monitoring, domestic safety, public security, automo-

tive applications, air conditioning in airplanes, spacecrafts and

houses, sensors networks. Due to this huge application rangethe need of cheap, small, low power consuming and reliable

solid state gas sensors, has grown over the years and triggered

a huge research worldwide to overcome metal oxide sensors

drawbacks, summed up in improving the well known 3S: Sen-

sitivity, Selectivity and Stability.

The sensing properties of semiconductor metal oxide in form

of thin or thick films other than SnO2, like TiO2, WO3, ZnO,

Fe2O3 and In2O3, have been studied as well as the benefits from

the addition of noble metals: Pd, Pt, Au, Ag, in improving selec-

tivity and stability. In 1991 Yamazoe [1] showed that reduction

of crystallite size caused a huge improvement in sensor perfor-

mance. In a low grain size metal oxide almost all the carriersare 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 den-

sity, produced by targetgasesspecies, hasa great effect on sensor

conductance. The challenge became to prepare materials with

small crystallize size which were stable when operated at high

temperature for long periods.

From the preparation side, first generation devices were pre-

pared by thick film technology starting from powders. Since

sensor performance depends on percolation path of electrons

through intergranular regions, by varying small details in the

preparation process, each sensor differed slightly in its initial

characteristics. Therefore the materials fabrication processeshave been improved towards thin film technology, a more auto-

mated production method that offers higher reproducibility and

compatibility with Si technology, by physical and chemical

vapour deposition. However, the technological improvement

went along with a reduction of sensing performances due to

a lower porosity of the prepared devices. Both thin and thick

films electrical properties drift due to grain coalescence, poros-

ity modification and grain-boundary alteration. These effects

become more critical because the metal oxide layers must be

kept at a relatively high temperature in order to guarantee the

reversibility of chemical reactions at surface. Thus, several solu-

tions have been put forward to stabilize the nanostructure, e.g.

addition of a foreign element [2] or phase [3]. An unexpected

step forward has been the successful preparation of stable singlecrystal quasi-one-dimensional semiconducting oxides nanos-

tructures (so called nano-belts, nano-wires or nano-ribbons) by

simply evaporating the desired commercial metal oxide powders

at high temperatures [4,5]. Their crystallinity assures improved

stability and the nanosized lateral dimension the good sensing

properties. Their peculiar characteristics and size effects make

them interesting both for fundamental studies and for potential

nano-device applications, leading to a third generation of metal

oxide gas sensors.

This review article is focused on the description of metal

oxide single crystalline nanostructure used for gas sensing.

Subject treatment will start with presenting the preparation tech-niques and their development and improvements, pointing out

the steps critical for applications in real environments. Further-

more an outlook on other possible intriguing and new applica-

tions of metal oxide single crystals will be presented.

2. Deposition techniques and growth mechanisms

Nanocrystalline materials can be classified into different

categories depending on the number of dimensions that are

nanosized (with dimensions lower than 100 nm); a possible clas-

sification is zero dimensional for clusters, mono dimensional for

nano-wires and two dimensional for films.Numerous one-dimensional oxide nanostructures with useful

properties, compositions, and morphologies have recently been

fabricated using so-called bottom-up synthetic routes. Some of

these structures could not have been created easily or economi-

cally using top-down technologies.

A nomenclature for these peculiar structures has not been

well established. In the literature a lot of different names has

been used, like whiskers, fibers, fibrils, nanotubules, nanocable

etc. A few classes of these new nanostructures with potential

as sensing devices are summarized schematically in Fig. 1. The

geometrical shapes can be tubes, cages, cylindrical wires, rods,

nails, cables, belts, sheets and even more complex morphologies.

When developing 1D nanocrystals the most importantrequirements are dimensions and morphology control, uni-

formity and crystalline properties. In order to obtain one-

dimensional structures the growth musthave a preferential direc-

tion with a faster growth rate. In the past years the number

of synthesis techniques has grown exponentially. The evapo-

ration method will be extensively described since it is one of the

promising and most explored in the recent papers and also one

of the cheapest for single crystal production.

The main advantage of the vapour phase process is the possi-

bility to produce different type of materials in an easy way and

with cheap deposition systems [7]. The source material has to be

evaporated and then transported and condensed on a substrate. A

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30 E. Comini / Analytica Chimica Acta 568 (2006) 2840

Fig. 1. Different 1D metal oxide nanostructures, from top right: nanowire,

core-shell structure, nanotubule, nanobelt, dendrite, hierarchical nanostructure,

nanorod, nanoring, nanocomb.

typical experimental set-up consists of a vacuum-sealed tubularfurnace with an inlet for the carrier gas and an output connected

to a vacuum pump as reported in Fig. 2. The growth chamber has

to be designed in order to obtain the proper temperature gradient

necessary for 1D nanostructures preparation. The metal oxide

powder is placed in the higher temperature region and the gas

carrier transports the evaporated oxide towards the substrates

where it condenses. Other than direct heating for the formation

of 1D nanostructure, the source can be evaporated using pulsed

laser ablation. This kind of deposition set-up is quite similar

to the one reported in Fig. 2, with an additional window that

allows laser incidence on source material. The uses of laser to

produce evaporation of the oxide source allows lower opera-tion temperatures, furthermore the substrate temperature can be

varied independently from the source one. The drawback is the

higher cost of such deposition system.

The growth process can be summarize in some steps: dif-

fusion of growth species towards substrate surface, adsorp-

tion/desorption mechanism of molecules on substrate surface,

diffusion of adsorbed species on substrate surface since a growth

site is reached, aggregation of adsorbed species to form the crys-

tal and diffusion and evacuation of possible by products [8].

Some of these growth steps can limit the crystal growth rate.

If the concentration of the growth species is low then the limiting

Fig. 2. Experimental set-up for vapour phase deposition of 1D structures with

an heating furnace, source material (metal, metal oxide), vacuum sealed tube

(quartz, alumina), inlet for gas carriers (Ar, O2, H2), valves and vacuum pump.

step is the adsorption mechanism, the growth rate is controlled

by the condensation rate, which is proportional to the vapour

pressure of the growth species:

J= P02mkT

(1)

where is the accommodation coefficient and sigma is the

super-saturation of the growth species in the gas phase. When

the growth species concentration is high, the limiting factor is

the surface growth step. As the vapour pressure increases the

defect formation probability increases and furthermore possible

secondary nucleation site occur losing the mono-crystallinity.

Growth mechanism can be classified as: vaporsolid (VS),

vaporliquidsolid (VLS) and solutionliquidsolid (SLS).

The VS growth takes place when the nano-wire crystalliza-

tion originates from direct condensation from vapour phase

without the use of a catalyser. At the beginnings the growth was

attributed to the presence of lattice defects, but when defects-

free nano-wires were observed this explanation cannot be any

longer accepted. Another peculiar effect registered was a nano-wire growth rate higher than the calculated condensation rate

from the vapour phase. A possible explanation proposed is that

all non-wire faces adsorb the molecules that then diffuse on the

principal growth surface of the wire.

In VLS both liquid and solid phase contribute to the process;

in general the liquid phase is a liquid metal cluster or catalyser

that acts as a favourite site for adsorption of the vapour phase

and, when the super saturation is reached, acts as a favourite site

for crystallization. This is an old growth mechanism proposed in

1964 by Wagner. In order to obtain this process there are some

requirements to fulfil, such as: catalyser has to allow the forma-

tion of a liquidsolution with thematerialthat hasto be deposited,the catalyser vapour pressure must be low, the catalyser has to

be chemical inert. For metal oxide nano-wires the metal itself

can act as a catalyzer. The growth species in the vapour phase

diffuses in the liquid drop of catalyser, when the concentration

in the drop is too high the growth species precipitate and form

the nano-wire. The liquid drop of the catalyser is a preferential

growth site because all the adsorbed atoms are captured and are

not able to desorb. This induces a higher growth rate of the VLS

with respect to the VS growth process, but a drawback is the

possible presence of the catalyser at the top of the grown struc-

ture. An advantage is an enhanced control in nano-wire diameter

induced by the control in catalyser dispersion and dimension.

In general high temperatures are required for VLS growth ofnano-wires, an alternative method is the SLS. This method is

similar to VLS, but the source material is a solution, SLS can

operate at lower temperatures than VLS. Indeed the produced

nano-wires are in general polycrystalline and for this reason are

not so valuable for gas sensing applications.

In the vapour phase deposition, the control of super satura-

tion condition is the key factor to control the morphology of

the deposition products. A low super-saturation is required for

1D nanostructure, a medium super-saturation for bulk crystals,

while at high super-saturation powders are formed by homoge-

neous nucleation in the vapour phase. The dimensions of the 1D

structures can be controlledvarying parameters suchas pressure,

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Fig. 3. Secondary electron images of different Inidium oxide structures obtained by a vapour phase deposition depending on substrates temperature: from 1100 to

800 C wires with lateral dimensions of the order of microns, 3D crystalline structures, nano-wires.

temperature and substrates. The effective value of super satura-

tion is a difficult number to evaluate in the deposition chamber

without studies of all the thermodynamics variables inside the

deposition chamber. A key point is that the local variations of

the thermo-dynamic variable such as pressure temperature and

concentrations are not known along the furnace.

Among the studies on 1D nanostructures, several investiga-

tions have been devoted to tin and zinc oxide nano-wires/nano-

belts and different approaches to their synthesis have been

developed [913]. At the present, vapour phase processesseem to be the most promising ones, due to their simplicity

and low costs. Some works drew the attention on the VLS

(vapourliquidsolid) mechanism, through which thermal evap-

oration techniques can favour a fast growth below 1000 C(crystal growth by thermal evaporation of SnO2 powders usually

occurs at temperatures higher than 1300 C).We report in detail an example of vapour phase deposition

procedure for the preparation of indium oxide nano-wires/nano-

belts. The evaporation temperatures of these oxides are quite

high then a tubular furnace with maximum heating temperatures

higher than thousands degree centigrade, together with alumina

tube, low vacuum pumping system has to be used (see Fig. 2).

The set-up can be designed to allow a direct (from source mate-

rial to substrates) or reversed (from substrates to, source) flux.

The direct flux is used during the deposition process while the

reversed flux is used during the temperature gradient in order

to avoid the formation of uncontrolled nanostructures. Special

kind of boats for source and for substrates in alumina can be

used in order to obtain a quite selective deposition. The temper-

ature of the furnace, the pressure, the flux and the gases have

to be optimized, to obtain 1D and not 3D structures, for each

different apparatus and are not directly transferable form one to

others apparatus. Cleanness of all the alumina parts used during

the deposition is a key factor for the deposition of reproducible

structures, for this reason strict procedures before every depo-sition have to be followed. This problem is more important in

depositions from evaporation of oxides powders since the high

temperature requires the use of alumina and there is no easy

wet cleaning process for oxide removal over alumina, a possible

way is the use of higher temperatures to induce evaporation of

the oxide deposited over tubes, substrates and source holders. In

the particular case of indium oxide, that has a cubic structure,

the deposition condition are more critical due to the difficul-

ties in producing anisotropic growth from such a symmetric

structure. Different morphologies can be obtained depending

on the temperature of the substrates, from 1100 to 800 C wires

with lateral dimensions of the order of microns, 3D crystalline

structures, nano-wires can be prepared as reported in Fig. 3.

The high degree of crystallinity is confirmed by HRTEM, the

nanowire section is constant through all its length without any

defect and with atomically sharp terminations. As it has been

already pointed out the high degree of crystallinity and the con-

trol in the surface termination is a key factor for producing and

understanding stable and reliable gas sensors.

Vapor phase deposition is not the only technique used for the

preparation of 1D structures. For example 1D nanostructures

synthesis can be obtained also by templates. The template canserve as a container, the nanostructure grows within or around

it; the shape of 1D nanostructure is complementary to the one

of the chosen template. This technique provides a good control

of the uniformity and on the dimension (owing a good control

on pores dimensions and distribution), however the number of

nano-wires that can be produced are limited by the template and

the template removal can cause damage to 1D structures.

3. Working principle of metal oxide gas sensors

Conductimetric metal oxide gas sensors rely on changes of

electrical conductivity due to the interaction with the surround-ing atmosphere. The normal operation temperature of metal

oxide gas sensors is in general within the range between 200

and500 C where conduction is electronic andoxygen vacanciesare doubly ionized. At higher temperatures, oxygen vacancies

mobility become appreciable and the mechanism of conduc-

tion become mixed ionic-electronic. Metal oxide gas sensors,

to avoid long term changes, should be operated at temperatures

low enough so that appreciable bulk variation never occurs and

high enough so that gas reactions occur in a time on the order

of the desired response time. Clearly when dealing with single

crystals synthesised at temperatures higher than the operating

temperatures of the sensors there should not be instability prob-

lems caused by structural changes or coalescence.When a metal oxide is operated in the semiconducting tem-

perature range, the charge transfer process induced by surface

reactions determines its resistance. In single crystal-based gas

sensors the current flows parallel to the surface and is modulated

by the surface reactions like the channel of a Field Effect Tran-

sistor by the gate voltage. When the channel is fully depleted,

carriers thermally activated from surface states are responsible

for conduction.

At the interface between metal and metal oxide, necessary to

connect the nanowire to the outside world, metal semiconductor

junction will appear, and also this junction can play a role in the

sensing mechanism. In the case of single crystalline layers the

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contact resistance is more important since it is in series to the

semiconductor resistance while for polycrystalline material it is

connected to a large number of resistances.

In the case of a one-dimensional wire along the main axis the

treatment can be done using the Poisson equation in cylindrical

coordinates. When dealing with wire with lateral dimensions

of the order of hundreds of nanometer, gas adsorption creates

a surface depletion layer, the conducting channel thickness is

reduced. The mobility dependence on surface coverage can be

neglected because electron diffusionlength (about1 nm)is much

shorter than the diameter (tens of nanometres). The dependence

of the conductance on the density of occupied surface states is

linear, that is very weak compared to the case of polycrystalline

materials.

Electrical transport changes when the thickness of the wire is

small like in nano-wires with lateral dimensions low compared

to the debye lenght, the space charge region extends through

all the wire cross section and all electrons are trapped in sur-

face states. The bands shape can be parabolic or flat as for small

grain polycrystalline materials, with the difference that currentflows parallel instead that perpendicular to the surface like in a

pinched-off FET channel. Density of carriers is thermally acti-

vated from surface states into the conduction band. When charge

density is reduced due to surface reactions with ionosorbed oxy-

gen, large variations of carrier concentrations, and therefore of

conductivity, are produced by the transition from strongly acti-

vated to not activated carrier density [58].

The sensing mechanism in metal oxide gas sensors is related

to ionosorption of species over their surfaces. The most impor-

tant ionosorbed species when operating in ambient air are oxy-

gen and water. In the temperature range between 100 and 500C

oxygen ionosorbs over metal oxide, for example, in a molecu-lar (O2) and atomic form (O) [14], since O2 has a lower

activation energy it is dominating up to about 200 C, at highertemperature the O form dominates. For some reducing gases,gas detection is related to the reactions between the species to

be detected and ionosorbed surface oxygen. When a reducing

gas like CO comes into contact with the surface, the following

reactions may take place:

COgas COads (2)

COads+Oads CO2,gas+ e (3)

These consume ionosorbed oxygen and in turn change theelectrical conductance of metal oxide. The overall effect is a

change of the density of ionosorbed oxygen that is detected

as an increase of sensor conductance. Direct adsorption is also

proposed for the gaseous specieslike strongly electronegative

NO2whose effect is to decrease sensor conductance:

NO2,gas NO2,ads (4)

e + NO2,ads NO2,ads (5)

The occupation of surface states, which are much deeper in

the band-gap than oxygens, increases the surface potential and

reduces the overall sensor conductance.

4. Measurements methods

Metal oxide 1D structures can be configured, to produce con-

ductimetric gas sensors, in FET devices or in normal resistor

configuration with single or multiple nano-wires in one device.

In both configurations 1D structures can be grown directly on

transducer or transferred to it after the deposition process on a

more suitable substrate. Obviously not all the transducers are

suited for depositions at high temperatures and the presence of

metals can cause a catalytic growth. If the growth is made on the

final device transducer, there will be an ensemble of nano-wires

on allthe substratesurface anda selective removal have to be per-

formed if a single wire sensor is desired. This is of course not an

easy task when dealing with metal oxide structures due to their

resistance to corrosive environments. After selective removal,

the metal contacts have to be deposited on the nano-wires.

If the deposition is performed on substrates other than the

transducer, the nano-wire can be dispersed in a solvent and

transferred by drop-coating on the complete transducer (with

metallization pads). Then in order to improve the mechani-cal stability and electrical contacts, thin metallizations can be

deposited by focused ion beam (FIB) or electron beam deposi-

tion (EBD) between the nano-wires and the pre-existing pads.

In single nano-wire configurations a nano-manipulator can be

used for transferring nano-wire in the desired position on the

transducer. Of coursesome of these procedures described arenot

ready for an implementation for large-scale production. Contact-

ing issue remains still one of the biggest open problems when

dealing with 1D nanostructures.

A different possibility to measure a gas sensing activity is

monitoring the optical luminescence properties instead of the

electrical ones, in such case there is no problem relating withcontacts or manipulation, but the final device cannot be as cheap

as the one based on electrical properties monitoring and the

sensing mechanism is not yet completely understood.

The research on gas sensing properties of nano-crystals is

still not as developed as the one on the preparation of 1D nanos-

tructure, this is also related to the difficulties that we have just

exposed like the contacting problems for electrical measure-

ments.The articles recently publishedwill be reported following

the measurement method used, i.e. DC conductimetric, FET or

optical gas sensors (Table 1).

4.1. DC conductimetric gas sensors

The easiest way to transduce the gas sensor is by simply mea-

suring the DC resistance of the sensing element as a function of

the surrounding atmosphere. The transducer has to provide at

least two electrical contacts on the metal oxide to measure con-

ductance changes and a heating system in order to maintain the

metal oxide at the suitable operation temperature that in gen-

eral is of the order of hundreds degrees centigrade. The simplest

transducer is a bulk insulating substrate with electrical contacts

on one side of the substrate and a heater on the backside.

As it has been already pointed out only some of the manifold

metal oxide nanocrystals produced were tested with particular

regard to their electrical properties in controlled atmosphere for

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Fig. 4. Layout of two terminal resistive measurements with interdigitated contacts on the bottom of metal oxide nanowires in a single wire and multiple wires

configuration.

gas sensing applications. Table 1 reports a list of the metal oxide

nano-crystal gas sensors that have been found in the literature

together with the target gas chosen. In the field of metal oxide

conductimetric gas sensors tin oxide is by far one of the moststudied and also one of the few that has been commercialized,

in form of thick films, due to its better performances in terms of

sensitivity and stability compared to other oxides. In the nano-

Table 1

Listof publishedstudies on gassensingpropertiesof 1D nanostructures, together

with publication year metal oxide analysed, measurement method and target

species studied

Year Metal oxide Measurements

method

Target species Reference

2002 SnO2 DC, M CO, NO2 ethanol [5]

SnO2 FET, S NO2 [34]

2003 TiO2 DC, M H2, CO, NH3, O2 [26]

In2O3 FET, S NH3, NO2 [29]

In2O3 FET, S NH3 [31]

2004 Cd:ZnO DC, M RH [21]

ZnO DC, M Ethanol [22]

In2O3 FET, S, M NO2 [32]

SnO2 FET, S CO, N2, O2 [36]

ZnO FET, S O2 [38]

In2O3 DC, M Ethanol [19]

2005 ZnO DC, M RH [20]

ZnO DC, M CO, H2S, HCHO,

NH3

[23]

ZnO DC, M H2 [24]V2O5 DC, M 1-Butylamine,

toluene, propanol

[25]

In2O3 FET, S Lipo protein [33]

SnO2 + Pd FET, S O2 [37]

SnO2 DC, M CO, NO2, ethanol [15]

Ru:SnO2 DC, M [16]

SnO2 + CuO DC, M H2S [17]

ZnO FET, S NO2, NH3 [40]

SnO2 PL, M NO2, NH3, CO [44]

SnO2 DC, S DMMP [18]

MoO3 DC, M CO, ethanol [54]

WO3 DC, M NO2 [55]

M: multiple nanowire, S: single nanowire, DC: DC resistive configuration, FET:

FET based gas sensors, PL: PL based gas sensors.

crystals case instead Zinc oxide is one of the most studied, this is

due to the easiness in preparing nano-wires and multiple intrigu-

ing nanostructure, and furthermore to the biocompatibility of

this oxide that make it promising also for medical and in vivoapplications.

4.1.1. Tin oxide

Tin oxide nanobelts based gas sensor was the first one with

a simple DC-resistive measurement presented in 2002 [5]. The

transducer fabrication, a platinum interdigitated electrode struc-

ture, was made using sputtering technique with shadow masking

on alumina substrate. Then, a bunchof nanobelts was transferred

onto the electrodes for electric conductance measurements, the

gases tested were CO, NO2 and ethanol that are important for

environmental applications, for breath analyser and food quality

control. CO and ethanol were found to increase the conductiv-ity, that is common for an n-type semiconductor such as tin

oxide, while the opposite behaviour was registered for NO2, as

explained in the previous section.

Fig. 5 reports an example of variation of the current flowing

through the nanobelts configured in a DC resistor configuration,

as depicted in Fig. 4, towards ethanol and NO2 at 300C with

50% RH using synthetic air as a carrier gas in order to repro-

duce a real environment. Theconductance value in air wasstable

during all the operation time and there was no poisoning of the

Fig. 5. Kinetic response of a SnO2 nanowires based sensor conductance pro-

ducedby stepsin ethanol andNO2 concentrationswith50% RHand ata working

temperature of 300 C.

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sensor due to gas exposures. The sensor dynamics were limited

by the filling time of the chamber. The authors performed also

measurements as a function of the nanobelts-nanowires densi-

ties and reported a dependency of the gas sensing properties

on the densities of the meshes, suggesting that in a DC con-

figuration with multiple wires also the potential barriers, that

can appear due to wire to wire contact, could be important for

sensing mechanism [15].

Another work published on tin oxide nanowires in this DC

configuration is reporting the effect on Ru doping obtained by

simultaneous evaporation of oxides [16]. The gas tested was

nitrogen dioxide, but the concentration, used (50 ppm) was very

high compared to the threshold for environmental monitoring,

but still it is interesting to see how the Ru doping could increase

NO2 response.

Another work on SnO2 1D nanostructure reports the effect of

catalysts on nanowires sensing properties [17]. Nanobelts and

nanowires were mixed with a CuO 4% mol solution. This cata-

lystwas chosensince it isknownin thick and thinfilm toenhance

the sensing properties towards H2S of SnO2 (an effect ascribedto the pn junction formation). The increase in H2S response

was confirmed in 1D nanostructure, the detection limit reached

was lower than 3 ppm. Of coursethe addition of a polycrystalline

material reduces the advantages of the use of single crystalline

metal oxide as sensing layer.

Yu et al. recently published a contribution on tin oxide

nanobelts proving their integration with micro-machined sub-

strate, which is crucial if a real application is envisaged,

and showing their sensitivity to nerve agent, an application

of increasing interest for security reasons [18]. SnO2 single

nanobelt have been transferred on a silicon microsystem device

fabricated with the top-down approach. The resistance of thenanobelt increased of 5% when 78 ppb of dimethyl methylphos-

phonate (DMMP) a nerve agent simulant (Fig. 7).

Furthermore the sensor poisoning, present in some measure-

ments, was attributed to the contacts failure and eliminated with

the deposition of good ohmic electrical contacts on the nanobelt.

Theintegration of top down and bottom up approaches prove the

feasibility of large scale manufacturing of well organized sensor

arrays based on different nanostructures.

4.1.2. Zinc oxide

As far as zinc oxide nano-crystals are concerned, gas sensing

properties have been studied towards CO, H2S, HCHO, NH3,

H2, ethanol and RH.Zang et al. reported variations in the electrical properties

due to humidity changes at room temperature [20]. Nanorods

and nanowires have been deposited on a Si oxidized substrate

with interdigitated Pt electrods on the top and their electri-

cal properties have been tested at room temperature in pres-

ence of different relative humidity values. The resistance of 1D

ZnO nanostructures decreased as RH value was increased, the

change of resistance of nanowires were relatively linear with

RH from 12 to 97%, with a total variation of about 4 orders of

magnitude. The response of nanorods based sensor was lower

and the authors ascribed this difference to more homogeneous

nanowires morphology and to smaller lateral nanowires dimen-

sions that increased their higher specific surface area. The gas

test measurements were repeated at 25 C every 5 days for 1month and the variations in the resistance were less than 3% for

theentire RHrangetested, this is of coursean encouraging result,

for a room temperature working sensor. Other tests on humidity

resistancevariation have been reported in [21] forCddopedZnO

nanowires. The results showed a variation of 3 orders of mag-

nitude to 95% RH that is a worse performance compared to the

one reported in [20]. In the same year this group reported about

ethanol sensing properties of ZnO nanowires [22]. This work

is interesting because nanowires were dispersed on a micro-

machined substrate complete with Pt interdigitated contacts and

heater. The final sensor showed good ethanol sensing proper-

ties and the detection limit was lower than 1ppm of ethanol, no

saturation effect was registered for concentration till 200 ppm,

response and recovery times were quite fast, of the orders of

seconds. The detection limit is lower than the limit needed for

breath analyser and the fast dynamics are compatible with a real

application also if response to possible interfering gases still

have to be investigated.A more complete gas characterization on the sensing prop-

erties of ZnO nanorods was reported by Jiaquiang et al. [23].

The nanorods were deposited on alumina tubes with two gold

contacts at the end, a heating wire was inserted in the tube to

operate in the temperature range 100500 C. ZnO nanorodsshowed a predominant sensitivity to alcohols compared to other

gases tested such as CO, H2S, HCHO, NH3. Concentrations of

ethanol as small as 10 ppmwere detected at 330Cwithresponseand recovery times lower than 10 s, the detection limit was not

estimated but also in this case it should be compatible with the

afore mentioned application.

Wang et al. suggested application of nanorods as hydro-gen sensors in [24]. ZnO nanorods were characterized towards

hydrogen at room temperature. This study reported the response

of bare ZnO and Pd coated ZnO nanorods, the deposition was

performed over a glass substrate with pre-existing Au contacts,

and Pd deposition was performed with sputtering. The nanorods

resulted covered for the 70% with a rms roughness of 8 nm and

a catalytic effect was obtained. A strong increase in the response

to hydrogen was measured, especially at low concentration lev-

els. Furthermore no response to oxygen variations have been

registered at room temperature, recovery time transients were

faster than 20 s upon removal of H2.

4.1.3. Other metal oxidesOther than tin and zinc oxide also other oxides were inves-

tigated in DC resistor configuration, such as indium, vanadium,

molybdenum, tungsten and titanium oxide, but only sporadic

works were reported.

Indium oxide nanowires have been tested towards ethanol

by Xiangfeng et al. [19]. A mixture of In2O3 nanowire and

polyvinyl alcohol solution was coated on alumina tubes with

two gold contacts at the end, a heating wire was inserted in

the tube to operate in the temperature range 100500 C. Theresistance of the nanowires was monitored in presence of air,

ethanol and other gases. The highest response was obtained

with ethanol, the detection limit was estimated to be equal to

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Fig. 6. Layout of three terminal FET with a metal oxide single nanowire or multiple metal oxide nanowires acting as channel.

or lower than 100 ppm and response and recovery times were

in the second range.Raible et al. [25] report about a V2O5 nanofiber based sensor.

Vanadia nanofibers were deposited on silicon substrate and the

resistance of the nanofibers was measured at room temperature

in presence of different concentration of 1-butylamine. The esti-

mated detection limit was 30 ppb and the same sensor resulted

less sensitive to ammonia and quite insensitive to vapour of

toluene and 1-propanol. The response to 1-butylamine increased

linearly with relative humidity and also the type of contacting

changed the response characteristics. The response and recovery

times were quite long due to the low operation temperature, but

the good response to amines is promising for the development

of sensors for healthcare applications like diagnosis of uremia,

cancer, or for food freshness analyses.

In 2005, a work on molybdenum oxide nanorods gas sensing

propertieshas beenpublished[54]. The nanorods were deposited

on alumina substrates and then equipped with Pt heater and

IDC contacts in order to investigate their gas sensing properties.

MoO3 nanorods were characterized by high response to ethanol

and CO at temperatures in the range of hundreds degrees centi-

grade. The response of thin films with the same structure was

comparatively studied and nanorods based sensors resulted one

order of magnitude more sensitive of their 3D counterpart. The

authors ascribed this peculiarity to the high surface to volume

ratio due to the intrinsic morphology of nanorods and to the very

reduced lateral dimensions of these nanorods.Sawicka et al.presented in 2005 [55] nitrogen dioxide sensing

properties of tungsten oxide nanowires prepared with electro-

spinning. The effect of processing parameters variation was

studied and a comparison with thin films prepared by solgel

was presented. WO3 nanowires showed better NO2 gas sensing

performances compared to solgel processed films. Nanowires

exhibit improved gas sensitivity, faster response and lower gas

detection limit that the solgel based elements. These results

were attributed to the increase in the surface area of these struc-

tures.

All these examples reported were related to single crystals

in form of nanowire, nanorods or nanobelts, while, compared

Fig. 7. Response of the as-assembled nanobelt sensor to 78 and 53 ppb DMMP

balanced with air when the nanobelt temperature was 500 C. The voltageapplied to the nanobelt was 1.5 V. Reprinted with permission from Appl. Phys.

Lett. 86 (2005) 063101. Copyright 2005. American Institute of Physics. M 81.

to the large amount of literature on gas sensing properties of

carbon nanotubes, only little attention was put in the studies of

gas sensing properties of tubular structures. For example Vargh-

ese et al. [26] studied the hydrogen sensing properties of titania

nano-tubes prepared by anodization. The tests were performed

in nitrogen atmosphere and 1% H2 produced a variation of 3

orders of magnitude in the resistance of the array of titania nano-

tubes, the response increased with temperature and the response

time was 23 min. As the diameter of the pore decreased thesensing performance increased. The sensors showed high selec-

tivity compared to CO and NH3, but showed a high response to

oxygen.

4.2. FET based gas sensors

Nanowires can be configured in a FET structure with a 3

terminal configuration as represented in Fig. 6. In such way

the Fermi level within the band gap of the nanowire can be

varied and used to control surface process electronically. The

nanowire acts as a conductive channel that joins source and

drain electrode. Tuning of the metal oxide properties in a FET

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configuration has been reported in references [27,26]. Investi-

gations on SnO2 and ZnO have been performed while oper-

ating in air, in vacuum and in presence of low oxygen and

nitrogen concentration in vacuum [27]. Two methods were

used to produce the FET structure, the first one consisted in

nanowires dispersion in ethanol by ultrasonication and dry-

ing on a silicon dioxide-silicon substrate, and then in elec-

trodes deposition by electron beam lithography. The second

procedure consisted in deposition of dispersed nano-wires on

predefined electrode arrays. This latter approach led to very

resistive contacts. These FET devices showed good switch-

ing rations between ON and OFF states. The channel conduc-

tance and the threshold were sensitive to the gaseous surround-

ing atmospheres. Furthermore a big influence was found on

nanowires pre-treatment, if an annealing in vacuum was per-

formed the nano-wire showed a very high electron density, the

presence of oxygen of course caused oxygen ionosorption and

depletion of electrons on the nano-wire surface, shifting the

threshold potential to more positive values. A strong depen-

dence on the channel length of the switching ratio was foundand for lengths higher than 100 nm no valuable switching was

recorded.

Specific works devoted to the gas sensing properties of metal

oxides nanowires in a FET configuration were performed on tin,

indium and zinc oxide (Table 1), the device can be tested in a

single wire or a multiple wire FET as depicted in Fig. 6.

4.2.1. Indium oxide

Li et al. [29] reports an electrical characterization of a sin-

gle indium oxide nanowire with dimensions of the order of ten

nanometers towards ammonia and nitrogen dioxide in presence

of argon at room temperature. The concentration used were veryhigh compared to the range interesting for environmental appli-

cations and the use of argon instead of air of course does not

match in general with a real application, but the measurements

showed a huge variation that can foresee good results also in

normal environment as reported by other groups. A pinch off

effect was reported, and also a negative resistance effect, the

authors ascribed this effect to a redistribution of the electron

density in the conduction channel closer to the nanowire surface

due to the bias. In the same reference a study of the influence on

the sensing properties of the gate voltage was performed, and

the results were promising for a possible control on the sensi-

tivity towards gases in a FET structure. This electro-absoption

effect, of course, is not only related to 1D structure, but it hasbeen reported also in thin films in [30] the key factor that dis-

tinguish 1D structures from thin films is the effect that small

variation in surface coverage can cause in the conducting prop-

erties. The same group reported in [31] the gate-screening effect

at high NH3 concentrations and ascribed it to adsorbed NH3molecules working as charge traps. Furthermore a change of

conductance in opposite directions was observed with different

nanowire sensors and the authors suggest that this response is

caused by various doping concentrations in the semiconducting

In2O3 nano-wires, pointing out that a strict control in the depo-

sition process and post-treatment procedures has to be achieved

to produce reliable sensors.

In 2004 the same authors reported the detection of NO2 down

to ppblevels for thefirst time with metal oxide nano-wiretransis-

tors [32]. Two types of devices were studied one based on single

and one on multiple In2O3 nano-wires operating at room tem-

perature. While single In2O3 nano-wire devices exhibited strong

gate dependence and nice transistor behaviour, sensors based on

multiple In2O3 nano-wires displayed numerous advantages in

terms of greater reliability, high sensitivity down to ppb range,

and an easier fabrication. Of course operating at room tempera-

ture reaction dynamics are very slow and in the time cycle used

the steady state value was not reached; furthermore to recover

the initial conductance value UV light exposure was used to

desorb nitrogen dioxide molecules previously adsorbed on the

metal oxide surface.

In 2005, Tang et al. published interesting results on the

response to low density lipoprotein of metal oxide nanowires

and carbon nanotubes [33]. In2O3 nanowire and carbon nan-

otube transistors were used to study the chemical gating effect

of low-density lipoproteins. The adsorption of lipoproteins on

these two different surfaces revealed a tenfold higher adsorp-tion on carbon nanotubes than on In2O3 nanowires because of

hydrophobic/hydrophilic interactions. In2O3 nanowire transis-

tors exhibited higher conductance accompanied by a negative

shift of the threshold voltage, the nanotube transistors showed

lower conductance after the exposure. This is attributed to the

complementarydoping typeof In2O3 nanowires (n type) andcar-

bon nanotubes (p type). These experiments are important since

they open up new possibility for developing sensors also for

biological quantities.

4.2.2. Tin oxide

Law et al. [34] analysed room temperature sensing propertiesof a single crystalline tin oxide nanowire sensor towards nitro-

gen dioxide. NO2 chemisorb strongly on SnO2 surface, and at

room temperature desorption is not complete when the NO2 is

removed. The use of UV light to activate this process, that has

proven to be effective with thin films [35], was used on this

single nanowire and led to a positive effect on both adsorption

and desorption process. In the dark oxygen adsorbs on the sur-

face capturing electrons form the semiconductor and creates a

depletion layer, while upon UV exposure photo-generated holes

migrate to the surface and recombine with electrons releasing

oxygen ions, with a depletion layer decrease and an increase

in conductance. The detection limit suggested was 210 ppm of

nitrogen dioxide that is not suited for environmental applicationfor example, but the response could be increased with the use

of dopants or catalysers. The response and recovery were quite

fast also if working at room temperature; the advantages of these

sensors are the low power consumption due to room tempera-

ture operation and potential stability due to the single crystalline

nature of these structures.

Other experiments have been presented by Zhang et al. [36]

on tin oxide single nanowire sensor in a FET structure; the

atmospheres tested were pure nitrogen, nitrogen and oxygen,

nitrogen, oxygen and CO. The experimental observations sug-

gest that oxygen adsorptiondesorption rate and CO oxidation

can be changed by varying the electron density in the nano-

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Fig. 8. (a)Schematic view of theformation of electron depleted regions beneath

andin theimmediate vicinity of twoPd nanoparticles.(b) Response of a pristine

(dashed line) and Pd-functionalized (solid line) nanostructure to sequential oxy-

gen and hydrogen pulses at 473 K (top pane) and 543K (bottom). Reprinted

with permission from Nanoletters (2005) 5 (4) 667673. Copyright 2005

[37].

wire through the change in potential. It is interesting to note

how oxygen coverage was controlled by gate voltage, at 6 Vno oxygen adsorption was registered, but still a response to CO

was recorded and this was attributed to the interaction with lat-

tice oxygen. Gate potential increase can enhance CO sensing

properties, while above a threshold value the CO sensing prop-

erties are not related to gate potential. Such a tuneable sensor

can ease the application of gas sensor in real environment where

more than one gas species are present simultaneously. This tun-

ability due to the electro absorptive effect is easy obtainable in

a FET structure.

In 2005, Kolmakov et al. [37] studied the effect of catalysts

in tin oxide single wire FET structures. The sensing capabilitiesof SnO2 single nano-wires and nano-belts in a FET configu-

ration before and after functionalization with Pd catalyst were

reported. The catalyst deposition was performed in the same

chamber used for gas tests in order to have a direct compari-

son on the same sensor structure. A change in the conductance

was recorded in the early stage of Pd deposition indicating the

formation of barrier type junction. The improvement in the sens-

ing performance after catalysation was ascribed to a combined

effect of spill-over of atomic oxygen formed catalytically on Pd

clusters and migrating on SnO2 surface and to the back spill-

over effect in which weakly bound molecular oxygen migrates

to Pd clusters and are catalytically dissociated (Fig. 8).

All these studies on single wire structures are important for

the intrinsic results and also for the possibilities to correlate the

experiments with theoretical studies performed on the interac-

tion of specific surface termination with simple molecules like

CO or NO2 [57].

4.2.3. Zinc oxideFurther works in a FET configuration have been reported

on zinc oxide nanostructures. Fan et al. [38] reported oxygen

adsorption on the nanowire surface. It was shown a consider-

able variation of electrical properties of the single crystal ZnO

nanowire upon oxygen introduction. Furthermore an interesting

study of the response to oxygen as a function of the nanowire

dimensions was reported, evidencing an increase in the response

as the nanowire radius decreases. The same group in [39] char-

acterized the electrical properties of ZnO nanowire field effect

transistors with scanning probe microscopy. The potential drop

at Schottky barrier contact wasanalysed, theconductive SPMtip

was used as a movable local gate in order to change the electrical

properties of FET device. Finally, gas-sensing properties of ZnOsingle nanowire FET were tested towards NO2 and NH3 at room

temperature [40]. As in the previous reported investigations, the

electrical field applied to the back gate electrode influenced the

sensitivity. A strong field was used to refresh the sensor after

molecules adsorption at room temperature, the negative voltage

pulse produced a complete recovery of the conductance value to

the initial level before NO2 exposure. The negative gate poten-

tial depleted electron in nano-wire and reduced the number of

electrons available at the vacancy sites, the hole migration to the

surface instead led to a discharge of gas molecules. Furthermore

if a dipole moment is associated with the adsorbed molecule the

gate field can induce a repulsive field weakening the bonding.Some of the electrical measurement on nanowires gassensors

were carried out in ideal atmospheres, but clearly, if a gas sens-

ing device has to be produced, more realistic condition have to

be explored, air has to be used as a carrier gas and the effect of

interfering gases and humidity have to be taken into account

in the right concentration range. Nevertheless the presented

experiments show good sensing properties, the possibility to use

dopants and catalyser such in the thin film gas sensors and the

real integration in low power consumption transducers of single

crystalline nanobelts. Theintegration of top down and bottom up

approaches prove the feasibility of large scale manufacturing of

well-organized sensor arrays based on different nanostructures.

4.3. PL based gas sensors

A new and yet not well-explored possible detection method

is based on photo-luminescence quenching. It is well known that

these 1D metal oxide nanostructure such as zinc oxide, indium

oxide and tin oxide show a visible emission when excited in

the UV range. Most of the works are related to ZnO due to its

manifold optical properties that can be exploited in different

application such as lasing. Peculiar effects are attributed to the

decrease in the lateral dimensions of these single crystals struc-

tures, for example the onset of visible PL spectra of high energy

gap metal oxides and a progressive increase of the green light

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emissionintensitywas registeredas the wiresdiameterdecreases

[41]. Quantum size effects and increase of surface, rich in oxy-

gen vacancies, to volume ratio influence the electrical properties

of the metal oxide as the studies presented in the previous sec-

tions prove, but also the optical properties can change with gas

exposure.

Manystudies reported the onsetof visible photoluminescense

in nanocrystalsbut only few reported thephotoluminescenseas a

function of the surrounding atmosphere. The onset of visible PL

in tin oxide fishbone like nanoribbons has been recently showed

[42] and luminescence properties of SnO2 nanoparticles studied

[43], reporting broad PL optical bands from tin oxide nanobelts

in the visible range from 400 to 600 nm. Only Faglia et al. [44]

published on the quenching in the visible photoluminescence

(PL) of tin oxide nanobelts due to the introduction of nitrogen

dioxide at ppm level in a fast (time scale order of seconds) and

reversible way.

The examined species were NO2 (300 ppb10 ppm), NH3(50 ppm) and CO (101000 ppm) in dry and humid synthetic

air and normal ambient pressure conditions. The response washighly selective towards humidity and other polluting species

like CO and NH3. The authors believed that adsorbed gaseous

species, that create surface states, could quench PL by creating

competitive non-radiative paths. A comparison between con-

ductimetric and PL response suggests that the two responses are

attributed to different adsorption processes [56].

The sensing mechanism underneath a photoluminescense

based gas sensor is not completely understood; nevertheless

it proves the possibility to produce a sensing device based on

simultaneous monitoring of both resistance and luminescence

in order to exploit all the different informations obtained from

metal oxide nanowires interaction with gases.

5. Other fields of application

There are also many other applications where these nano-

crystals could be exploited including: nano-electronics, func-

tional nano-structured materials, novel probe microscopy. To

realize these and other proposed use, 1D nanostructure will

however require an understanding of fundamental chemistry

and physics undergoing preparation and materials proper-

ties. Some of these manifold possibilities will be describe

hereafter.

Electronic circuit built with nanowires or nanotubes can be

a possible way to maintain the miniaturization trend that hasnow featured a slow down due to intrinsic limit in lithographic

processes. Park et al. [45] prove the ability of single crystal to

be used in logic circuits. The application of metal oxide in real

electronic circuits was limited by the ability to produce epitaxial

or crystalline films and also by difficulties in controlling metal

oxide junction characteristics. However single crystalline oxide

nanostructures seem to solve part of these limits. High perfor-

mance MESFETs and logic gate devices have been produced

using electron beam deposition and metal evaporation to pro-

duce ohmic contacts on ZnO nanorods. Furthermore nano-wire

can be used also in nano-electronic in order to go beyond fab-

rication limit determined by top-down approaches. Prototype

devices have been prepared with bottom-up approach trough

self-assembly such as FET, pn junctions, etc [46].

The extremely high photoconductivity effect makes 1D

nanowires also good candidates for highly sensitive electrical

switching. For example Kind et al. [47] report that the conduc-

tance of single ZnO nano-wire was extremely sensitive to UV

light exposure, the light induced transition between insulator

to semiconductor allows them to reversibly switch the nano-

wire between ON and OFF state rapidly, the time scale was

supposed to be reduced to the microsecond scale with proper

optimisations. The switch was also wavelength selective, i.e.

there is no photo-response with wavelengths outside the UV

range, since the photons cannot be absorbed, the cut-off for

ZnO was about 385 nm. Another possible application that has

been demonstrated is information storage, obtained by Yun et

al. [48] with ferroelectric BaTiO3 nano-wires. Non volatile elec-

tric polarization was reproducibly induced and manipulated on

nano-wires, these 10 nm nano-wires retained ferro-electricity

with a coercive field for reversal polarization of about 7 kV/cm

and with a retention time for the induced polarization exceeding5 days.

Also if realization and feasibility of electronic circuits and

non volatile memories with single crystal metal oxide nanos-

tructure is a big step towards nano-electronics of course there is

still much work on this topic to get closer to large-scale fabrica-

tion.

Nanowires might also be the basis of high definition TV,

the electrons needed for cathode ray guns within televisions

screen can be pulled from tips of nanowires and the sharp

ends of these structures allow the use of lower level electric

fields. Li et al. [49] report about the possibility in electron emit-

ter and flat panel display applications with ZnO nano-needles.Needle tips diameters were in a range of 2050 nm. The nano-

needles were single crystals growing along the [0 0 1] direction

and exhibiting multiple tip surface perturbations, just 13 nm

in dimension. Field-emission measurements on the prepared

nanostructures showed fairly low turn-on and threshold fields

of 2.5 and 4.0 V/mm, respectively. The nano-size perturbations

on the nano-needle tips were assumed to cause such excellent

field-emission performance.

Photonic crystals can be produced with an array of catalyst

patterned with electron beam lithography used as a template

for the growth of ordered nanowires structures by VLS growth

mechanism. The optical and electrical properties can be tailored

with the catalyst template characteristics, using light emittingnano-junctions.

Nanowires can also be used for lasing and as guide of light

in the same way as optical fibers. The ones with faceted flat

ends can serve as intrinsic optical resonant cavities to generate

coherent light. Huang et al. [50] observed room temperature UV

lasing with ZnO nanorods array deposited on sapphire substrate

with VLS technique. These properties of ZnO have already been

reported in thin films and disordered particles [51]. Due to the

difference of refractive index of ZnO, sapphire and air nano-

rods ends can serve as mirrors for the optical cavity without

cleavage and etching. The pump used was Ng:YAG laser, once

the power exceeded the treshold value of 40 kW cm2

a sharp

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Fig. 9. Schematic drawing of a possible array of cantilevers coated with different materials and SEM image of a ZnO nanocomb.

peak was registered in the emission spectrum, with width 50

times smaller than the spontaneous emission.Finally 1D nanostructures can be used also as novel probes

for SPM, the resolution of AFM is determined by size and shape

of the probe tip used for imaging, carbon nanotubes have been

used at the end of Si tips [52], but of course also metal oxide

1D structure can be used as tips in order to enhance the res-

olution of the lateral information acquired as reported in ref.

[53]. Furthermore metal oxides are ideal candidates for can-

tilever applications due to their defect free crystallinity and to

their excellent mechanical properties. The reduced dimensions

of nanobelts increase the cantilever sensitivity, for example a

nanocomb can be viewed as an array of nanocantilever and with

dip pen each cantilever can be coated with different catalysers

or polymers (Fig. 9) producing an array of microns dimensions.

6. Conclusions

A survey of preparation techniques, physical and chemical

properties, and performances of 1D nanocrystals metal oxide

gas sensors has been presented. The anisotropic growth required

by the 1D nanocrystals can be achieved in different conditions,

due to the crystallographic structure of the substrate, to the con-

finement obtained by a template, controlling the supersaturation

and other parameters. We have focused our attention to the tech-

niques used for the preparation of most of single crystalline

gas sensor, i.e. vapour phase process. Considerable efforts andprogress have been made in aspects of the development and

testing of 1D metal oxides gas sensors. The discovery of gas

sensing properties of single crystal nanostructure comparable or

even better than their thin film counterpart triggered the atten-

tion of the gas sensor research community. Of course not all

the problems of metal oxide gas sensor can be solved with the

use of single crystalline materials, but at least some of them.

The greater surface to volume ratio, the better stoichiometry

and greater level of crystallinity compared to polycrystalline

oxides, which reduce instability associated with hopping and

coalescence, make newly developed quasi mono-dimensional

semiconducting oxide very promising for better understanding

of sensing principles and development of a new generation of

sensors. The selectivity of course still remains a concern formetal oxide based gas sensor. This may be improved by fab-

ricating sensor arrays using several different nanobelts, or by

functionaliztion of their surfaces. The use of a single crystalline

structure with lateral dimensions of less than hundreds nanome-

ters allows the fabrication of an array of sensors in a chip with

lateral dimensions of the order of microns.

Still a greater control in the growth is required for an appli-

cation in commercial systems, together with a thorough under-

standing of the growth mechanism that can lead to a control

in nano-wires size and size distributions, shape, crystal struc-

ture and atomic termination. A great attention has to be paid to

problems like the electrical contacts and nano-manipulation that

allow production and integration of sensors.

Progress both in synthesisand manipulation of 1D nanostruc-

tures is growing fast and with it the range of materials prepared

and the applications foreseen. The future developments will be

improving quickly and in an unpredictable way.

Acknowledgments

The author wants to thank first of all the member of the

SENSOR lab group in Brescia, which she belong to, and fur-

thermore the European Commission that funded her research

in the nanostructures field with the project NMP4-CT-2003-001528 Nanostructured solid-state gas sensors with superior

performance (NANOS4).

References

[1] N. Yamazoe, Sens. Actuators B 5 (1991) 7.

[2] N. Bonini, M.C. Carotta, V. Guidi, C. Malagu, G. Martinelli, L. Paglia-

longa, M. Sacerdoti, Sens. Actuators B 68 (2000) 274.

[3] E. Comini, G. Sberveglieri, M. Ferroni, V. Guidi, C. Frigeri, D.

Boscarino, J. Mater. Res. 16 (2001) 1559.

[4] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947.

[5] E. Comini, G. Faglia, G. Sberveglieri, Z. Pan, Z.L. Wang, Appl. Phys.

Lett. 81 (2002) 1869.

7/27/2019 [22]Sensing

13/13


[7] Y. Zhang, N. Wang, S. Gao, R. He, S. Miao, J. Liu, J. Zhu, X. Zhang,

Chem. Mater. 14 (2002) 3564.

[8] G. Cao, Nanostructures and Nanomaterials, World Scientific Publishing,

Singapore, 2004, p. 110.

[9] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947.

[10] A. Kolmakov, Y. Zhang, G. Cheng, M. Moskovits, Adv. Mater. 15 (2003)

997.

[11] J. Hu, Y. Bando, Q. Liu, D. Golberg, Adv. Funct. Mater. 13 (2003) 493.

[12] Y. Chen, X. Cui, K. Zhang, D. Pan, S. Zhang, B. Wang, J.G. How,Chem. Phys. Lett. 369 (2003) 16.

[13] H.Y. Dang, J. Wang, S.S. Fan, Nanotechnology 14 (2003) 738.

[14] N. Barsan, U. Weimar, J. Electroceram. 7 (2001) 143.

[15] E. Comini, E. Faglia, G. Sberveglieri, D. Calestani, M. Zha, L. Zanotti,

to appear on Sens. Actuators B (2005).

[16] N.S. Ramgir, I.S. Mulla, K.P. Vijayamohanan, Sens. Actuators B 107

(2005) 708.

[17] X. Kong, Y. Li, Sens. Actuators B 105 (2005) 449.

[18] C. Yu, Q. Hao, S. Saha, L. Shi, X. Kong, Z.L. Wang, App. Phys. Lett.

86 (2005) 063101.

[19] C. Xiangfeng, W. Caihong, J. Dongli, Z. Chenmou, Chem. Phys. Lett.

399 (2004) 461.

[20] Y. Zhang, K. Yu, D. Jang, Z. Zhu, H. Geng, L. Lou, App. Sur. Sci. 242

(2005) 212.

[21] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, X.G. Gao, J.P. Li,

App. Phys. Lett. 84 (2004) 3085.

[22] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin,

App. Phys. Lett. 84 (2004) 3654.

[23] X. Jiaquiang, C. Yuping, L. Yadong, S. Jianian, J. Mater. Sci. 40 (2005)

2919.

[24] H.T. Wang, B.S. Kang, F. Ren, L.C. Tien, P.W. Sadik, D.P. Norton, S.J.

Pearton, J. Lin, App. Phys. Lett. 86 (2005) 243503.

[25] I. Raible, M. Burghard, U. Schlecht, A. Yasuda, T. Vossmeyer, Sens.

Actuators B 106 (2005) 730.

[26] O.K. Varghese, D. Gong, M. Paulose, K.G. Ong, G.A. Crimes, Sens.

Actuators B 93 (2003) 388.

[27] M.S. Arnold, P. Avuoris, Z.W. Pan, Z.L. Wang, J. Phys. Chem. B 107

(2003) 659.

[29] C. Li, D.H. Zhang, X.L. Liu, S. Han, T. Tang, J. Han, C. Zhou, Appl.

Phys. Lett. 82 (2003) 1613.

[30] K. Schamagl, M. Bogner, A. Fhucs, R. Winter, T. Doll, I. Eisele, Sens.

Actuators B 57 (1999) 35.

[31] D. Zhang, C. Li, X. Liu, S. Han, T. Tang, C. Zhou, Appl. Phys. Lett.

83 (2003) 1845.

[32] D. Zhang, C. Li, Z. Liu, S. Han, T. Tang, B. Lei, C. Zhou, Nano Lett.

4 (2004) 1919.

[33] T. Tang, X.L. Liu, C. Li, B. Lei, D. Zhang, M. Rouhanizadeh, T. Hsiai,

C.W. Zhou, App. Phys. Lett. 86 (2005) 103903.

[34] M. Law, H. Kind, B. Messer, F. Kim, P. Yang, Angew. Chem. Int. Ed.

41 (2002) 2405.

[35] E. Comini, G. Faglia, G. Sberveglieri, Sens. Actuators B 78 (2001) 73.

[36] Y. Zhang, A. Kolmakov, S. Chretien, H. Metiu, M. Moskovitz, Nanolet-

ters 4 (2004) 403.[37] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovitz,

Nanoletters 5 (2005) 667.

[38] Z. Fan, D. Wang, P. Chan, W. Tseng, J.G. Lu, App. Phys. Lett. 84

(2004) 5925.

[39] Z. Fan, J.G. Lu, Appl. Phys. Lett. 86 (2005) 032111.

[40] Z. Fan, J.G. Lu, Appl. Phys. Lett. 86 (2005) 123510.

[41] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater.

13 (2001) 113.

[42] J. Hu, Y. Bando, D. Golberg, Chem. Phys. Lett. 372 (2003) 758.

[43] F. Gu, W.S.H.C. Song, M. Lu, Y. Qi, G. Zhou, D. Xu, D. Yuan, Chem.

Phys. Lett. 372 (2003) 451.

[44] G. Faglia, C. Baratto, G. Sberveglieri, M. Zha, A. Zappettini, App. Phys.

Lett. 86 (2005) 011923.

[45] W.I. Park, J.S. Kim, G.C. Yi, H.J. Lee, Adv. Mater. 17 (2005) 1393.

[46] Y. Huang, X. Duan, Q. Wei, C.M. Lieber, Science 291 (2001) 630.

[47] H. Kind, H. Yan, M. Law, B. Messer, P. Yang, Adv. Mater. 14 (2002)

18.

[48] W.S. Yun, J.J. Urban, Q. Gu, H. Park, Nano Lett. 2 (2002) 447.

[49] Y.B. Li, Y. Bando, D. Golberg, App. Phys. Lett. 84 (2004) 3603.

[50] M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R.

Russo, P. Yang, Science 292 (2001) 1897.

[51] Z.K. Tang, G.L. Wong, P. Yu, H. Kind, A. Ohtomo, H. Koinuma, Y.

Segawa, Appl. Phys. Lett. 72 (1998) 3270.

[52] H. Dai, J.H. Hafner, A.G. Rinzler, D.T. Colbert, R.E. Smalley, Nature

384 (1996) 147.

[53] W. Hughes, Z.L. Wang, Appl. Phys. Lett. 82 (2003) 2886.

[54] E. Comini, L. Yubao, Y. Brando, G. Sberveglieri, Chem. Phys. Lett. 407

(2005) 368.

[55] K.M. Sawicka, A.K. Prasad, P.I. Gouma, Sens. Lett. 3 (2005) 1.

[56] C. Baratto, E. Comini, G. Faglia, G. Sberveglieri, M. Zha, A. Zappettini,

Sens. Actuators B 109 (2005) 2.

[57] A. Maiti, J.A. Rodriguez, M. Law, P. Kung, J.R. McKinney, P. Yang,

Nanoletters 3 (2003) 1025.

[58] G. Faglia, unpublished work.

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