chemical sensing and catalysis by 1-d metal oxide nanostructures

8/12/2019 Chemical Sensing and Catalysis by 1-d Metal Oxide Nanostructures

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Annu. Rev. Mater. Res. 2004. 34:15180doi: 10.1146/annurev.matsci.34.040203.112141

Copyright c 2004 by Annual Reviews. All rights reservedFirst published online as a Review in Advance on March 17, 2004

CHEMICALSENSING AND CATALYSISBY ONE-DIMENSIONAL METAL-OXIDENANOSTRUCTURES

Andrei Kolmakov and Martin MoskovitsDepartment of Chemistry and Biochemistry, University of California,

Santa Barbara, email: [email protected], [email protected]

Key Words one-dimensional nanostructures, sensors, catalysis

Abstract Metal-oxide nanowires can function as sensitive and selective chemicalor biological sensors, which could potentially be massively multiplexed in devices ofsmall size. The active nanowire sensor element in such devices can be configured eitheras resistors whose conductance is altered by charge-transfer processes occurring at theirsurfaces or as field-effect transistors whose properties can be controlled by applying anappropriate potential onto its gate. Functionalizing the surface of these entities offers

yet another avenue for expanding their sensing capability. In turn, because chargeexchange between an adsorbate and the nanowire can change the electron densityin the nanowire, modifying the nanowires carrier density by external means, suchas applying a potential to the gate, could modify its surface chemical properties andperhaps change the rate and selectivity of catalytic processes occurring at its surface.Although research on the use of metal-oxide nanowires as sensors is still in early stages,several encouraging experiments have been reported that are interesting in their ownright and indicative of a promising future.

INTRODUCTION

Chemical and biological sensors have a profound influence in the areas of personal

safety, public security, medical diagnosis, detection of environmental toxins, semi-

conductor processing, agriculture, and the automotive and aerospace industries

(14 and references therein). The past few decades have seen the development of

a multitude of simple, robust, solid-state sensors whose operation is based on the

transduction of the binding of an analyte at the active surface of the sensor to a

measurable signal that most often is a change in the resistance, capacitance, or

temperature of the active element.

The evolution of gas sensors closely parallels developments in microelectronics

in that the architecture of sensing elements is influenced by design trends in planar

electronics, and one of the major goals of the field is to design nano-sensors that

could be easily integrated with modern electronic fabrication technologies. For

byHokkaidoUniversityon06/09/09.Forpersonaluseonly.


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152 KOLMAKOV MOSKOVITS

Figure 1 A cartoon of a nanowire-based electronic nose. The nanowire surfaces are

functionalized with molecule-selective receptors. The operation is based on molecular

selective bonding, signal transduction, and odor detection through complex pattern

recognition.

example, the current goal is to replace the large arrays of macroscopic individual

gas sensors used for many years for multicomponent analysis, each having its as-

sociated electrodes,filters, heating elements, and temperature detection, with an

electronic nose embodied in a single device that integrates the sensing and signal

processing functions in one chip (58). Multicomponent gas analysis with these

devices is accomplished by pattern recognition analogous to odor identification

by highly evolved organisms (Figure 1) (911). By increasing the sensitivity, the

selectivity, the number of sensing elements, and the power of the pattern recogni-

tion algorithms, one can envision a potent device that can detect minute quantities

(ultimately one molecule) of an explosive, biohazard, toxin, or environmentally

sensitive substance against a complex and changing background, then signal an

alert or takeintelligentaction. However, this requires an increase in the sensitiv-

ity and selectivity of active sensor elements despite the loss of active area and the

increased proximity of neighboring individual sensing elements as the individual

components are miniaturized. Recent progress in materials science and the many

new sensing paradigms originating out of nanoscience and technology, particu-

larly from bottom-up fabrication, makes one optimistic that these goals are within

reach.

Metal oxides possess a broad range of electronic, chemical, and physical prop-

erties that are often highly sensitive to changes in their chemical environment.

Because of these properties, metal oxides have been widely studied, and most

commercial sensors are based on appropriately structured and doped oxides.

Nevertheless, much new science awaits discovery, and novel fabrication strate-

gies remain to be explored in this class of materials by using strategies based



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NANOWIRES, SENSORS, AND CATALYSTS 153

on nanoscience and technology. Traditional sensor fabrication methods make use

of pristine or doped metal oxides configured as single crystals, thin and thick

films, ceramics, and powders through a variety of detection and transduction prin-

ciples, based on the semiconducting, ionic conducting, photoconducting, piezo-electric, pyroelectric, and luminescence properties of metal oxides (4, 1214).

Chemical and biological sensors having nanostructured metal oxides and espe-

cially metal-oxide nanowires benefit from the comprehensive understanding that

exists of the physical and chemical properties of their macroscopic counterparts

(15).

This review is limited primarily to semiconducting devices with quasi-one-

dimensional nanostructures such as nanowires and nanobelts. Likewise, we restrict

ourselves to two related device configurations: conductometric elements and field-

effect transistors. A few issues relating to real-world sensors and sensor arrays arealso discussed.

Numerous quasi-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 economically using top-down technologies. A few classes of these new

nanostructures with potential as sensing devices are summarized schematically in

Figure 2. These achievements in oxide one-dimensional nanostructure synthesis

and characterization were recently reviewed by Xia et al. (16) and others elsewhere

(1719). Much work has also been published on the use of carbon nanotubes,individually or as arrays, as sensors (2025). Although we do not refer to this

work (which has also been thoroughly reviewed) (2630), the great progress

made to date in understanding the electronic properties of carbon nanotubes, their

Figure 2 A schematic summary of the kinds of quasi-one-dimensional metal-

oxide nanostructures already reported (see reviews 16, 17). (A) nanowires and

nanorods; (B) core-shell structures with metallic inner core, semiconductor, or

metal-oxide; (C) nanotubules/nanopipes and hollow nanorods; (D) heterostructures;

(E) nanobelts/nanoribbons; (F) nanotapes, (G) dendrites, (H) hierarchical nanostruc-

tures; (I) nanosphere assembly; (J) nanosprings.



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reactivity toward gases, photochemical properties, junction effects, and perfor-

mance when configured as transistors certainly informs the discussion of all quasi-

one-dimensional systems. We therefore acknowledge the great debt we owe to that

literature in establishing and clarifying many of the key questions pertaining toquasi-one-dimensional nanostructures.

The properties of bulk semiconducting oxides have been extensively studied

and documented. Not so those of quasi-one-dimensional oxide nanostructures (i.e.,

systems with diameters below 100 nm), which are expected to possess novel

characteristics for the following reasons:

(a) A large surface-to-volume ratio means that a significant fraction of the

atoms (or molecules) in such systems are surface atoms that can participate

in surface reactions.

(b) The Debye length D (a measure of the field penetration into the bulk)

for most semiconducting oxide nanowires is comparable to their radius

over a wide temperature and doping range, which causes their electronic

properties to be strongly influenced by processes at their surface. As a result,

one can envision situations in which a nanowires conductivity could vary

from a fully nonconductive state to a highly conductive state entirely on the

basis of the chemistry transpiring at its surface. This could result in better

sensitivity and selectivity. For example, sensitivities up to 105-fold greater

than those of comparable solid film devices have already been reportedfor sensors on the basis of individual In2O3nanowires (31). The signal-to-

noise ratio obtained indicates that 103 molecules can be reliably detected

on a 3-m-long device. By shortening the conductive channel length to

30 nm, the adsorption of as few as 10 molecules could, in principle, be

detected.

(c) The average time it takes photo-excited carriers to diffuse from the interior

of an oxide nanowire to its surface (10121010 s) is greatly reduced

with respect to electron-to-hole recombination times (109108 s). This

implies that surface photoinduced redox reactions (Figure 3) with quan-tum yields close to unity are routinely possible on nanowires (assuming

reactants reach the surfaces rapidly enough and interfacial charge transfer

rates are not limiting). The rapid diffusion rate of electrons and holes to the

surface of a nanostructure provides another opportunity as well. The recov-

ery and response times of conductometric sensors are determined by the

adsorption-desorption kinetics that depends on the operation temperature.

The increased electron and hole diffusion rate to the surface of the nano-

device allows the analyte to be rapidly photo-desorbed from the surface

(a few seconds) even at room at temperature.(d) Semiconducting oxide nanowires are usually stoichiometrically better de-

fined and have a greater level of crystallinity than the multigranular oxides

currently used in sensors, potentially reducing the instability associated

with percolation or hopping conduction.



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Figure 3 A summary of a few of the electronic, chemi-

cal, and optical processes occurring on metal oxides that

can benefit from reduction in size to the nanometer range.

(e) Nanowires are easily configurable as field-effect transistors (FETs) and

potentially integratable with conventional devices and device fabrication

techniques. Configured as a three-terminal FET, the position of the Fermi

level within the bandgap of the nanowire could be varied and thus used to

alter and control surface processes electronically.

(f) Finally, as the diameter of the nanowire is reduced, or as its materials prop-

erties are modulated either along its radial or axial direction, one can expect

to see the onset of progressively more significant quantum effects (32).

Surface Reactions on One-Dimensional Oxides,Gas Sensing, and Catalysis

The exploration of the metal-oxide nanostructures as a platform for chemical sens-

ing is a recent event. Yang and coworkers fabricated and tested the performance

of individual SnO2 single-crystal nanoribbons configured as four-probe conduc-

tometric chemical sensors both with and without concurrent UV irradiation (33).

Photoinduced desorption of the analyte can lead to rapid detection and reversible

operation of a sensor even at room temperature. A detection limit 3 ppm and re-sponse/recovery times of the order of seconds were achieved for NO2. Comparing

the performance of the ohmic nanoribbon sensors with those that showed rectifi-

cation led the authors to conclude that the nanoribbons themselves dominate the

photo-chemical response and not the phenomena occurring at the Schottky barriers.



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Figure 4 Top: TEM, HRTEM, SEM images of an individual SnO2nanoribbon: (A)low magnification, (B) atomically resolved, and (C) deposited on previously prepared

Au electrodes.Bottom: The conductance response of the nanoribbon to NO2pulses in

air with simultaneous 365 nm irradiation (after Law et al. 33).

A wide array of potentially useful one-dimensional metal-oxide nanostructures,

including nanobelts, were synthesized and characterized in Wangs group (19, 34)

and in other laboratories (see 16, 17 and references therein). Comini et al. (35)

configured groups of the SnO2 nanobelts between platinum interdigitated elec-

trodes and assessed their behavior at 300400

C under a constantflux of syntheticair. The nanobelt sensors showed excellent sensitivity toward CO, ethanol, and

NO2. NO2could be detected down to a few parts per billion.

Individual SnO2and ZnO2single-crystalline nanobelts (30300 nm width and

1030 nm thickness) (34) were configured as FETs and studied by Arnold et al.



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(36). The electrical properties of these individual nanobelts in vacuum, in air,

and under oxygen, as a function of thermal treatment, suggested that the oxygen

adsorption and desorption dynamics depends sensitively on the concentration of

surface oxygen vacancies, which, in turn, affect the electron density in the nanobelt.CdO nanowires, nanobelts, and nanowhiskers are prospective active elements

for LEDs and lasers from nanostructures. The Zhou group (37) showed that

in vacuum, as-prepared CdO nanowires have a carrier concentration of

1.3 1020 cm3 arising from oxygen vacancies and interstitial Cd. Temperature-

dependent conductance measurements indicate an activated process with Ea 13.3

meV at high temperature, switching to tunneling conductance below 30 K. The

conductance of single nanowires exposed to 200 ppm of NO2(an oxidizing gas)

at room temperature dropped by 30%.

Kolmakov et al. used nanoporous alumina as a template for synthesizing arraysof parallel Sn nanowires, which were converted to polycrystalline SnO2 nanowires

of controlled composition and size (38). Conductance measurements on these in-

dividual nanowires were carried out in inert, oxidizing, and reducing environments

in the temperature range25300C (39). At high temperatures and under an inert

or reducing ambient, the nanowires behaved as highly doped semiconductors or

quasi-metals with high conductances that depended weakly on temperature. When

exposed to oxygen, the nanowires were transformed to weakly doped semiconduc-

tors with a high conductance activation energy. The switching between the high

and low conductance states of the nanowires was fully reversible at all tempera-tures. Configured as a CO sensor, a detection limit ofa few 100 ppm for CO

in dry air and at 300C was measured with these SnO2 nanowires, with sensor

response times of30 s.

The above observations can be largely accounted for in terms of mechanisms

developed over many years to explain the function of polycrystalline metal-oxide

gas sensors (4043). This mechanism is outlined below, using SnO2 nanowires

in the presence of oxygen (an electron acceptor) and CO (an electron donor) as a

model system for oxide semiconductor systems more generally. Specific departures

from this general picture are pointed out for individual cases and for other surfaceadsorbate molecules when necessary.

The surface of stoichiometric tin oxide (a large bandgap semiconductor) is rel-

atively inert. Even moderate annealing in vacuum, or under an inert or reducing

atmosphere, causes some of the surface oxygen atoms to desorb, leaving behind

oxygen vacancy sites (Figure 5). Likewise, exposure to UV results in oxygen pho-

todesorption (or of other surface species) even at low temperatures. Essentially, all

experiments carried out to date on metal-oxide nanowires (or other nanostructures)

indicate that the role of oxygen vacancies dominates their electronic properties

along much the same lines as they do in bulk systems. Each vacancy results in theformation of afilled (donor) intragap state lying just below the conduction band

edge (Figure 5c). The energy interval between these states (or at least some) and

the conduction band is small enough that a large fraction of the electrons in the

donor states is ionized even at low temperatures, thus converting the material into



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Figure 5 (a) Stoichiometric SnO2 (110) surface, (b) partially reduced SnO2 with

missing bridging oxygens. Molecular oxygen binds to the vacancy sites as an electron

acceptor. CO molecules react with preadsorbed oxygens. Electrons are released back

to the nanowire [a,bafter Kohl (14) with modifications]. (c) Oxygen vacancies makeSnO2into ann-type semiconductor. (d) When the Debye length is comparable to the

radius of the nanowire, adsorption of electron acceptors shifts the position of the Fermi

level away from the conduction band.

an n-type semiconductor. At a given temperature the conductance of the nanowire,

G=R2en/L, is determined by the equilibrium conditions determining the rel-

ative concentrations of (singly or doubly) ionized surface vacancy states, which

determine the electron concentration in the bulk of the material. (Surface defects

can also migrate into the interior, resulting in bulk defects that are clearly much less

responsive to surface processes, and their low diffusion constant implies that they

are normally not important participants in the materials sensing action, which re-

quires a response time faster than the inverse diffusion rate. However, bulk defects

do contribute to a sensors long-term stability.)

The conductance of SnO2changes rapidly with gas adsorption as a result of a

(usually) multistep process wherein the first step is the adsorption of a molecule

(for example, with O2, which might dissociate into two surface oxygen ions af-

ter chemisorption), with a consequent molecule-to-SnO2charge transfer (or vice

versa). With oxygen as the adsorbate, the aforementioned surface vacancies are par-

tially repopulated, which results in ionized (ionosorbed) surface oxygen of the gen-

eral formOS. The resulting (equilibrium) surface oxygen coverage, , depends on

the oxygen partial pressure and the system temperature through the temperature-

dependent adsorption/desorption rate constants, kads/des, on the concentration of



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itinerant electrons, n, and the concentration of unoccupied chemisorption (vacancy)

sites,Ns.

2 Ogas

2 + e

+ Ns Os

kad s Ns n p/2O2

= kdes

[where, = {1,2} accounts for the charge and molecular or atomic nature of thechemisorbed oxygen (44)]. In forming ionosorbed oxygen, electrons become lo-

calized on the adsorbate, creating a 30100-nm-thick, electron-deficient surface

layer corresponding approximately to the Debye length for SnO2(in the temper-

ature range 300500 K), which results in band bending in the surface region of

bulk samples. For 10100 nm diameter nanowires, the charge-depletion layer en-compasses the entire nanowire, resulting in so-calledflat-band conditions wherein

the relative position of the Fermi level shifts away from the conduction band edge

not only at the surface but throughout the nanowire (Figure 5d). Ultimately, a new

kinetic equilibrium among the free electrons and the neutral and ionized vacancies

is re-established. Under these nearly flat-band conditions at moderate tempera-

tures and for electron momenta directed radially, electrons can reach the surface

of the nanowire with essentially no interference from the low electrostatic barrier.

As a result, the electrons become distributed homogeneously throughout the entire

volume of the nanowire. Accordingly, the charge conservation condition simplifiesto

Ns =R

2 (n nm ),

where nm is the density of itinerant electrons remaining in the nanowire after

exposure to the adsorbate. The accompanying electron depletion n = 2Ns/R

results in a significant drop in conductance:

G =R2e

L 2Ns

R ,

and the corresponding depopulation of the shallow donor states results in an in-

crease in activation energy (39). [We neglect the dependence of the mobility on

the surface coverage, a reasonable approximation at small bias voltages and when

the electron diffusion length (1 nm) is much smaller than the diameter of the

nanowire (50 nm) (45)].

Upon adsorbing a reducing gas such as CO, the following surface reaction takes

place with the ionoadsorbed oxygen

COgas + OS COgas

2 + e,

which results in the reformation of the adsorption (defect) sites and the redonation

of electrons to the SnO2(Figure 5b). It can be shown that underflat-band condi-

tions the increase in electron concentration, nC O p

+1

CO , and therefore in the



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conductivity of the nanowire, GCO e nCO(T) (T), increases mono-

tonically with CO partial pressure (44). This was confirmed experimentally on

nanowires assumingO (, = {1}) to be the dominant reactive surface species

(39). The foregoing simple mechanism is able to account for the operation oftin-oxide nanowire sensors under ideal ambients consisting of dry oxygen and a

combustible gas such as CO. In a real-world environment, a large array of other

molecules (chief among them, water) complicates the picture. Surface hydrox-

yls and hydrocarbons can temporarily or permanently react with adsorption sites

modifying or adding to the possible reaction pathways.

A consequence of being able to shift the position of the Fermi level of the oxide

nanowire by applying an externalfield or by doping the nanowire is the possibility

of controlling molecular adsorption onto its surface (resulting in the oscillation of

the adsorbate between an electron donor and acceptor). An interesting instance ofthis was reported recently (46) with In2O3nanowires exposed to NH3. For nano-

wires with a low density of oxygen vacancies (corresponding to a Fermi level

lower in energy within the bandgap), the adsorbate behaved as an electron donor,

causing the resistivity of the nanowire to increase upon exposure to ammonia.

With a higher oxygen vacancy density (the Fermi level nearer to the lower edge of

the conduction band), the NH3behaved as an acceptor, quenching the nanowires

conductance (Figure 6).

Single Nanowire FETsThe architecture of a typical nanowire-based FET is shown in Figure 7. The

nanowire acts as a conducting channel that joins a source and drain electrode. The

entire assembly rests on a thin oxidefilm, which, itself, lies on top of a conducting

(in this casep-type Si) gate electrode. (This is a so-called back gate configuration.

A top gate can also be deposited on the nanowire as an alternative.) Tuning a

nanowires properties by configuring it as the conductive channel of a FET was

Figure 6 Alternating donor (right plot) versus acceptor (left plot) behavior of

NH3adsorbate as a function of the doping level of an In 2O3nanowire (taken from

Zhang et al. 46).



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Figure 7 A schematic traces the response of the nanowires conductance

to the charge state of the adsorbed molecules [adapted with permission from

Nano Lett. 2002. Copyright Am. Chem. Soc. (49)].

recently reported with single metal-oxide nanowires and nanobelts (36, 47). The

pioneering effort in this regard is from Lieber, who has also reported the opera-

tion of nanowire sensors in an aqueous medium (48). This FET device consists

of an individual Si nanowire acting as the conductive channel whose thin, native

oxide skin, used as the gate oxide, is functionalized with target-specific receptors.

These receptors change their charge state when bonded to their target species. The

layer of molecular or ionic receptors essentially acts as a polarized gate electrode

modifying the carrier density inside the Si nanowire and, therefore, its conduc-

tance. By terminating the surface with 3-aminopropyltriethoxysilane, calmodulin,or biotin receptors, the nanowire was used, respectively, as a pH monitor, a Ca 2+

ion concentration detector, and a sensor for a variety of biomolecules. pH sens-

ing resulted from the change in the charge state of the amine as it gained or lost

protons in response to the pH of the surrounding medium. A similar approach was



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used by this group to create bistable switches (49). By covering a semiconducting

nanowire with an oxide layer of variable thickness (Figure 7, top panel) and then

bonding cobalt phthalocyanine, which is capable of existing in two or more redox

states to the oxide, the layer of molecules acted as a virtual gate electrode alteringthe carrier concentration in the nanowire by changing the redox state of the cobalt

phthalocyanine (Figure 7a,b). These molecules bonded to the oxide could also be

made to accept or donate electrons either by varying the potential applied to a

global back gate or by pulsing the bias voltage of the nanowire.

Although the primary goal of this particular study was to create a nanoscale

logic device, the results demonstrate the subtle interplay between the chemistry

at the surface of the oxide and the nanowire conductance. Sweeping the voltage

from negative to positive values then back again often shows significant hysteresis

and other memory effects. These effects normally arise from trapping of chargesat various interface (and other) sites, frequently residing there for a long time, or

(in the absence of heating or irradiation with light) indefinitely. This also leads to

a better appreciation of the variety of effects that a nanowire sensor must contend

with when operating in a real-world environment.

Arnold et al. (36) succeeded in creating working FETs out of pristine, individual

SnO2 and ZnO2 single-crystalline nanobelts and investigated their operation in

air, in vacuum, and after admitting low concentrations of oxygen and nitrogen

in vacuum chamber. The performance of the devices as a function of thermal

pretreatment in air and in vacuum was also reported. Nanobelts with conductingchannels as short as 100 nmand as longas 6m were used. These devices exhibited

excellent switching ratios (the resistance ratios between the ON and OFF states)

up to 106 and electron mobilities as high as 125 cm2 V1 s1 at room temperature

in air. The channel conductance and the threshold voltage (defined as the value of

the gate potential required to turn the device on) of the devices were found to be

sensitive to the gas environment and thermal pretreatment.

For example, nanobelts annealed in vacuum were found to have such high

electron densities that they could not be gated at reasonable values of the gate

voltage. However, exposure to even low concentrations of oxygen dramaticallydepleted the electron density in the nanobelt and shifted its threshold potential

toward more positive Vgvalues, as previously indicated should be the case for an

n-type semiconductor. The switching ratio of the nanobelt FET was also found to

be strongly dependent on the channel length. For channel lengths in the range of

100 to 500 nm, no significant modulation of the conductance with gate voltage

was observed, implying some, as yet poorly understood size dependence on the

performance of nanowire-based FET devices.

Zhou and coworkers (31) characterized and explored the room-temperature

electronic properties of individual In2O3 nanowires (10 nm diameter) configuredas FET sensors. IDS(VDS) measured in atmospheres consisting of trace amounts

of NO2or NH3 mixed in Ar showed significant conductance decreases when the

target gas was introduced (Figure 8). Both trace gases behaved as oxidizers. Apart

from the dramatic increase of the resistance, exposure to oxidizing gases induced



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Figure 8 Top: IDS(VDS) measured in pure Ar before and after 1% NH3 is admitted

at room temperature. Note different left and right scales. Bottom: the corresponding

I(VG) (after Li et al. 31).

positive shifts in the threshold voltage to values exceeding 15 V, consistent with an

appreciable reduction in the carrier density inside the nanowire. The dynamic rangeof the resistivity change, Rgas IN/Rinert, due to 100 ppm of NO2, was measured to be

106, an impressive increase. Concentrations as low as 500 ppb and response times

of the order of a few seconds (with 100 ppm of NO2) were reported. Recovery of

the nanowires original conductance following the normally irreversible adsorption

of the target molecule was accomplished using UV irradiation. At high values

of VDS, IDS is often observed to saturate, occasionally with portions of the I(V)

curve showing negative differential resistance (Figure 8 top panel). The observed

current saturation at high-bias voltage (the so-called pinch-off effect) is typical

for FETs. It results from the electron depletion near the drain electrode when itis at a high potential. The negative differential resistance, on the other hand, is an

intriguing observation. The authors tentatively ascribed this effect to bias-induced

redistribution of electron density in the conduction channel closer to the nanowire

surface, where increased scattering probability degrades the electron mobility.



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One can see from the above discussion that setting the gate potential at an

appropriate value increases the sensitivity S = Ranalyte IN/Rambientof a nanowire

sensor configured as a FET. As an example, for the In2O3 nanowire sensor shown in

Figure 8, setting Vgto 10 V results in a value of S 2 for 1% of NH3, whereasunder the same conditions the sensitivity of the device increases 105-fold at Vg =

30 V. This electronic control over sensitivity is one of the major promising

characteristics of nanowire-based FET sensors, especially when they are incor-

porated into arrays in their eventual real-world applications. The question arises:

Can one control the selectivity of a FET sensor using the same general approach?

If so, a more subtle, and potentially chemically more interesting effect might be

observable. Because many target gases are detected through a catalytic reaction

(for example, CO detection is carried out using the catalytic oxidation of CO to

CO2 on the surface of SnO2), gate control of sensor selectivity implies controlof surface reactivity, including catalysis, by changing the potential applied to the

gate. Recent experiments on the oxidation of CO on SnO2nanowires suggest that

this is indeed possible (50, 51). This effect is not unique with nanowires. Indeed,

such concepts are integral aspects of the electronic theory of catalysis and gas

sensing by semiconductors (52). The effect of an electrostatic field on the sur-

face chemistry has also been reported for thinfilm semiconductors and FET-based

chemical sensors fabricated by traditional technologies (5357). The difference

between what one expects with bulk systems and with nanowires, however, is

one of extent. The combination of factors such as the comparability of the Debyelength and nanowire radius, the small number of carriers present in the nanowire

(typically105 electrons), the high surface-to-bulk ratio, and its small capacitance

suggests that a relatively small number of adsorbed molecules can alter the car-

rier concentration significantly. Reciprocally, the removal or addition of a small

number of electrons can alter its surface chemistry measurably. For example, be-

cause chemisorbed oxygen intermediates are required in the catalytic oxidation of

CO, and a small number of electrons is required to create these adsorbed oxygen

species, the catalytic oxidation of CO should then be significantly controllable

using relatively small values of the gate voltage (Figure 9).The source-drain current, ISD, of a SnO2nanowire configured as a back-gated

FET was measured at constant VSD as a function of the gate-to-source voltage

underflowing gas with various partial pressures of nitrogen, oxygen, and CO.

(Figure 9, top panel). Baseline values of IDSwere established through prolonged

exposure of the system to dry N2 while maintaining the system at the selected

gate potential and temperature. At timet1, 10 sccm of oxygen gas were mixed into

the 100 sccm nitrogen flow. This was followed at time t2 by the addition of CO

(5 sccm) into the gas flowing into the cell. The steady-state value of the source-

drain conductance in the dry nitrogen atmosphere, GN2, decreases monotonicallyand significantly as the gate potential becomes more negative and increases a

little and then saturates at positive values of the gate potential (Figure 9, bot-

tom panel). This behavior is expected. SnO2 is an n-type semiconductor; hence

a negative gate voltage will cause the electron density in the nanowire to de-

crease. Also plotted in Figure 9 is the conductance decrease, Gox y , following



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Figure 9 (a) Response of a SnO2 nanowires current to the sudden addition of oxygen

to the flowing nitrogen gas at time t1 followed by the addition of CO at time t2 at

various values of the gate potentials at 553 K. GN2is the steady-state current in a dry

nitrogen environment; GO2(CO), the values of the conductance decrease (increase)

when O2 (CO) gas is sequentially admitted into the gas cell. (b) The reactivity ofoxygen, Goxy, and CO, GCO, as a function of gate voltage as measured by the

total change in conductance determined from the response curves shown in (a). The

nanowire conductance GN2under dry nitrogen is included for comparison. Also shown

is the extent of reaction of CO through a putative second-reaction channel that does

not involve ionosorbed oxygens as a reagent (51).



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exposure to oxygen and after steady state is once again achieved. Interestingly,

Gox y follows the gate dependence of the conductance closely. At sufficiently

negative values of the gate potential, Gox y drops to zero (Figure 9, bottom

panel). If one assumes that the reduction in electron density is proportional tothe coverage of ionosorbed oxygen, oxy(see above), the reduction ofGox y to

zero implies that at these values of the gate potential, ionosorption no longer takes

place. (Of course, one cannot be sure that physisorbed oxygen is absent from

the surface.) The CO-induced conductance increase ofGCO (which we asso-

ciate with the catalytic oxidation of CO at the tin oxide surface) begins almost

immediately upon the introduction of CO and achieves steady state slowly. The

latter is not a monotonic function of the gate potential but shows a maximum for

gate voltage values in the range 2 to 0 (Figure 9, bottom panel). Interestingly,

the addition of CO to the gasflowing over the nanowire increases its conductanceeven at Vg =6 V, when no oxygen ionosorption takes place. This behavior can be

reconciled if one assumes that there are at least two CO reaction channels

(Figure 10). The first channel (B in Figure 10) is the chemical reaction of CO

with the preadsorbed (inosorbed) oxygens, which depends on the availability of

free electrons and should therefore depend on gate voltage. The second channel

(Ain Figure 10) is believed to be the interaction of the CO molecule with lattice

oxygen (or with some other species) that donates electrons back to the nanowire.

This reaction channel, which is assumed to be independent of the pre-existing elec-

tron density, will not be affected by altering the value of the gate potential. If thissimple model describing the two-step processoxygen chemisorption followed

by CO oxidationis assumed to apply, one can determine from the experimental

curves shown in Figure 9 (bottom panel) the gatepotential dependence of the

equilibrium coverage of ionosorbed oxygen before [1(Vg )] and after CO is ad-

mitted into N2 + O2mixture [2(Vg)]. Using this model, the two coverage values

can be obtained from the experimental data as Gox y (Vg ) = C [1(Vg )] and

Gco(Vg ) AC [2(Vg )1(Vg )]. The results are shown in the Figure 10 (the

constantArepresents here the gate-independent reaction channel). According to

this treatment, the major effect of admitting CO is to shift the equilibrium oxygencoverage to lower values at any given value ofVg. At negative values ofVg, the

combined effect of the low-electron density and reaction with CO totally elimi-

nates ionosorbed oxygen from the surface of the nanowire (triangles in Figure 10

bottom). When electrons (and therefore ionosorbed oxygens) become plentiful as

Vgbecomes large and positive, the role of CO decreases and the equilibrium oxy-

gen coverage at high-gate potential values is no longer sensitive to the presence of

CO. This phenomenon can therefore account for the amonotonic dependence of

Gco(Vg ) (Figure 9). It is important to note that the drop in the value ofGco(Vg )

does not mean that the CO oxidation process is slowing down but rather indicatesthe limitations in associating conductance values, proportionally, with surface cov-

erage without taking into account changes in the surface-chemical mechanism.

The above two-channel model describing CO oxidation on the tin dioxide

nanowire has an interesting implication for nanowires as sensor (or, reciprocally,



17/32


Figure 10 Top: Schematic diagram of the major electron donor-acceptor reaction

channels: (A) interaction of CO molecule with lattice oxygens (or some other acceptor

site); (B) the same with ionosorbed oxygens; (C) oxygen ionosorption (the reverse,i.e., desorption process, is not shown here). For simplicity, we imply OS to be the

dominant chemisorbed species, although both single atom and diatomic species are

known to exist on the surface under appropriate conditions. Bottom: The calculated

equilibrium relative oxygen coverage as a function of gate potential before (squares)

and after (triangles) CO gas is admitted (51).

as a CO oxidation catalyst). Because the first channel depends directly on the

ionosorbed oxygen coverage, increasing this coverage by applying a large positive

potential on the gate results in increased sensitivity toward CO. At low (nega-tive) gate potentials, the reaction mechanism is independent of the ionosorbed

oxygen, causing the CO-sensing mechanism to switch to another mode. This il-

lustrates the prospect of a highly gate-tunable sensor (or nano-catalyst) whose

sensitivity (reactivity) and selectivity is tunable using the gate potential as a



18/32


surface-chemistry determining parameter. The nanowire FET then becomes, in

essence, a sensor (nano-reactor) with electronically controllable selectivity and

sensitivity (reactivity).

PHOTOCHEMICAL PROPERTIES OF INDIVIDUALMETAL-OXIDE NANOWIRES

The photochemical and photophysical properties of metal-oxide nanoparticles

and materials fabricated from them have been studied extensively, especially

in the context of solar energy conversion. In a similar vein, small diameter,

quasi-one-dimensional oxide nanostructures are promising photocatalysts on ac-

count of the efficient migration of electrons and holes to the nanostructure surfacewhere they can participate in chemical reactions before recombining. Although

these sorts of applications of oxide quasi-one-dimensional nanostructures are in

their infancy, a few compelling studies have already appeared. Dai and coworkers

recently reported molecular photodesorption from single-walled carbon nanotubes

and its dramatic influence on the electronic properties of a FET fabricated from

it (58). Yang and coworkers observed a record six-order-of-magnitude increase

in conductance when ZnO nanowires were exposed to 0.3 mWcm2 of 365 nm

UV light (whose photon energy just exceeds the materials bandgap cut off) (59).

Similar results were reported for SnO2nanoribbons (33) whose sensing properties

were investigated under the influence of photoexcitation. The photoresponse of

the nanowire and its associated time constants were found to depend sensitively

on the nature of the gas environment. On the basis of this observation, the au-

thors suggested that the photoresponse of nanoribbons and nanowires depended

on two contributions: (a) direct excitation of electron-hole pairs, which produce

a photocurrent that is influenced by the bias, and (b) photoinduced desorption of

ionosorbed species through a photochemical reaction of the form h+ + A A0

(A = O, O2, NO2 etc.), which eliminates the adsorbate-induced gating of the

nanoribbon thereby increasing the conductance of the nanowire. These effects are

exploitable in low-temperature sensing where many molecules stick irreversibly

to oxide surfaces. For example, room temperature detection of NO2at concentra-

tions as low as 3 ppm and with response/recovery times of the order of seconds

was reported with concurrent UV illumination. Although comparable sensitivity

was reported for thinfilm semiconducting oxide sensors at room temperature (and

under UV illumination) (60, 61), their recovery times are of the order of hours.

Avouris and coworkers reported similar observations on ZnO nanobelt FETs

(36). On shutting off the 350 nm excitation, the current decay kinetics showed

two characteristic time constants: a fast process, which was likely due to electron

recombination, and a slow decay process, which was ascribed to the readsorption

of the (electron acceptor) molecules.

On illuminating an In2O3 NW in air at room temperature with 3 mW/cm2 of

254 nm UV, Zhou et al. (62) observed a 104-fold increase in conductance

(Figure 11 top) The negative shift of the threshold voltage of the nanowire



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Figure 11 Top: Photoresponse of the current through an In2O3nanowire to photoex-

citation, in turn, with 365 and 254 nm radiation. Bottom: Two NO2-sensing cycles:

(a) 254 nm light is ON; (b) light is OFF and Arflux is ON; (c) 0.1% NO2 is ON;

(d) NO2is OFF; (e) Ar is OFF; (f) 254 nm light is ON followed with send cycle (from

Zhang et al. 62).

configured as a FET upon UV exposure suggested an release of electrons into its

conduction band with electron concentration increases1.8 nm1 and7.7 nm1

of nanowire length for 365 nm and 254 nm light, respectively. The measured

transconductance of the FET indicated not only an increased electron

density but also 7-fold and 80-fold respective increases in mobility for the two



20/32


wavelengths used. (The increased electron mobilities might be due to photon-

assisted electron excitation out of traps.) Because all measurements were per-

formed in air, the results were explained in terms of the conventional model wherein

increases in conductance are linked to photoinduced quenching of O

2 and OH

electron acceptors and the photogeneration of electron-hole pairs. The desorption

of adsorbed NO2(an electron acceptor) was also studied (Figure 11, bottom). The

same group investigated the infrared photodetection properties of CdO nanowires

(37). Promising results were obtained with pulsed 60 mWcm2 of 950 nm light at

1.2 K, which caused the photoconductance to switch with an on/off ratio of 8.6.

[CdO is an indirect bandgap semiconductor (Eindirectg 0.55 eV).]

OXIDE NANOWIRES IN A REAL-WORLD ENVIRONMENT

Most nanowire sensor studies are carried out in idealized atmospheres. Although

studies with nanowire sensors are rather recent, it is not too early to consider

some of the challenges associated with fabricating sensing devices and carrying

out electron transport measurements under more realistic conditions (63). Nano-

device fabrication includes a number of steps (such as the nanowire growth, ap-

plication and removal of a photoresist, deposition of electrodes) where the surface

(and bulk) of the nanowires and the surrounding area are exposed to reactive

species. In addition, in an ambient environment the nanostructure surface or itsjunctions can adsorb reactive species capable of altering its device performance.

This likely accounts for the significant scatter in the electron transport values,

the sensing behavior, and the performance as a FET reported on closely similar

systems by different groups. Even presumably pristine individual nanostructures

manifest memory effects (that is, electron transport that apparently depends on

the order in which measurements are carried out) or hysteresis effects in the

IDS(VDS) or IDS(VG) measurements (63). In an attempt to reduce these effects,

several empirical cleaning procedures and measurement protocols have been pro-

posed, which usually include preannealing the nanowire device in oxygen or invacuum, UV irradiation, plasma or ozone surface cleaning, the use of initial high-

current pulses, and so on. The origin of the current instabilities observed with oxide

nanowires fall into three groups: (a) contacts effects, (b) adsorbed contaminants,

and (c) impurities in or on the support layer (which is often SiO2) in the proximity

of the nanostructure. As was shown for carbon nanotubes, Schottky barriers formed

at the carbon/metal contacts, and their change in properties with gate potential or

as a result of gas adsorption can dominate the measured transport properties of the

device (30). Using a reactive metal (e.g., Ti) underlayer in forming Ti/Au contacts

and isolating the contacts from the gaseous environment by covering them with aninert layer reduces but does not fully eliminate these effects. This long-standing

problem was recently revisited for macroscopic sensors where the effect of the

applied potential, the influence of the ambient on the state of the Schottky barrier,

and the role of poisoning catalytic reactions occurring at the contact electrodes



21/32


Figure 12 (a) The response of the source-drain conductance (IDS)ofaSnO2 nanowire

resulting from the slow dissipation of accumulated surface charge in the vicinity of

the nanowire, which acts as a parasitic gate. The charge was induced by the impo-

sition of a negative potential Vg on the back-gate electrode (A. Kolmakov, Y. Lilach

& M. Moskovits, unpublished data); (b) Surface potential map image of a biased

nanowire connecting twoelectrodes, obtained using AFM. The halo-likeareas along the

nanowires are likely from induced surface charge. (G. Cheng, K. Jones & M. Moskovits,

unpublished data).

were discussed (44). No corresponding systematic studies have been done so farfor oxide nanowires where such effects are expected to be at least as signi-

ficant as in macroscopic systems. The contamination of the nanowire surface

or the gate oxide in the proximity of the nanowire, with water or certain other

molecules, can act as a (poorly controlled) external gate and alter the measured

conductance so that it shows memory or hysteresis effects, which can take a long

time to dissipate in the absence of treatments for discharging the trapped charges

using UV irradiation or thermal treatments. This issue was recently explored with

carbon nanotube FETs (63). Memory effects whose characteristics depend criti-

cally on how the nanowires were fabricated, stored, and operated are also oftenobserved with metal-oxide nanostructures. For example, Figure 12a shows the time

evolution of the source-drain current through a SnO2 nanowire FET following a

sudden change in the gate potential (from large negative values to zero) under dry

N2 and at 300 C (A. Kolmakov, Y. Lilach & M. Moskovits, unpublished data).

The large observed decrease in IDSfollowed by a slow decay is likely due to the

accumulation of positive surface charge in the proximity of the nanowire (or on its

surface) when the gate was negatively biased. When the gate is suddenly grounded,

this surface charge does not dissipate immediately but acts toward the nanowire

as an effective positive gate. That is, the electron density in the SnO 2 nanowireremains high as long as these surface charges remain in place, adding their influ-

ence to that of the potential applied to the gate. When the initial gate voltage is less

negative (see second cycle in Figure 12a), less charge is induced, resulting in a

diminished transient current response and one of shorter duration. These induced



22/32


23/32


Figure 13 Left top: Cartoon of a planar nanowire array sensor with individually

addressable (multiple-nanowire) sensing elements based on PAO template synthesis.

Bottom: an intermediate step in the fabrication of a planar nanowire array sensor before

oxidation of the Sn nanowires (G. Cheng, A. Kolmakov & M. Moskovits, unpublished

data).Right top: the sensing performance of an individual SnO2nanowire (39).

the underlying aluminum layer, and etching down the oxide matrix to expose the

nanowire tips at both ends, a system of patterned metal electrodes is

lithographically deposited top and bottom to act both as addressable pads for

electrically contacting the array of wires and as the anchoring structure that main-

tains the integrity of the nanowire device (Figure 13, left top graph). Much of the

alumina matrix is then removed by wet etching, leaving behind a planar array of a

large number of nanowires (Figure 13, bottom), most of which are fully exposed

to the ambient medium (G. Cheng, A. Kolmakov & M. Moskovits, unpublished

data). A typical device fabricated in this manner would contain hundreds of indi-

vidually functionalized cells of nanowires that can be interrogated using cross-bar

electrodes. The bulk composition can be tuned during the nanostructure growth

(for example via changing reactants). Sub-micrometer cell sizes and distances

over which the functionalization varies are routinely possible. This has resulted in

multisensor arrays with a large number of individual sensors in an area a few

microns across, which creates a so-called electronic nose with thousands of indi-

vidually readable receptors.



24/32


Arrays of nanotubules were also successfully grown inside the channels of

porous anodic oxide films (73, 74). Such devices have the added advantage that

the gas orfluid carrying the analyte can flow through the interior of the nanotubules,

potentially improving the devices detectivity (or if used as a catalyst, its catalyticefficiency). Moreover, the supporting oxide matrix need not be removed for the

sensing surface to be able to contact the ambient carrying the analyte. In principle,

large areas could be covered with such nanostructures.

Because titanium metal is one of the metals capable of growing porous ox-

ides, the Ti nanoporous template itself can be used as a sensor platform. This

was successfully reported recently by Egashiras and Grimess groups, who fab-

ricated planar array sensors based on titania nanotubules and the nanostructured

porous titania itself (7577). Arrays of parallel titania nanotubules with diame-

ters in the range 20100 nm were obtained by anodizing Ti foil in hydrofluoricacid solution (Figure 14, top left). As with alumina, control over the diameter

Figure 14 TiO2 nanotubules (top left) and planar sensor (top right) based

on self-organization of oxide during the anodization of Ti metal foils.Bottom:

The sensing performance at 290C toward H2 (after Varghese et al. 77).



25/32


and length of the nanotubules was achieved by varying the anodization voltage

and time. The stoichiometry and crystallinity were improved by postannealing

at high temperatures in oxygen. Impedance measurements were carried out by

depositing two platinum electrodes, as shown in Figure 14 (top right). Recordvalues for hydrogen sensitivity with TiO2 were reported for this nanotubule ar-

ray sensor (Figure 14, bottom). The superior sensitivity was explained in terms

Figure 15 Silica helices: (a) pillars, (b) zig-zag/chevron structures (d) grown using

ballistic glancing angle deposition (GLAD) (79). (c) Top view of the pillar structure

grown on a lithographically preseeded support (80). (e) The response of a humidity

sensor determined by SiO2 pillars fabricated in this manner (84).



26/32


27/32


ACKNOWLEDGMENTS

We thank Drs. Y. Zhang, G. Cheng, and Y. Lilach for their crucial contribution to

this work and Profs. H. Metiu and E. McFarland for helpful discussions and for

loaning us some equipment. This work was supported by a DURINT grant from

AFOSR. Extensive use of the MRL Central Facilities at UCSB was supported by

the National Science Foundation under award No. DMR96-32,716.

TheAnnual Review of Materials Researchis online at

http://matsci.annualreviews.org

LITERATURE CITED

1. Gopel W, Hesse J, Zemel JN. 1995. Sen-

sors: A Comprehensive Survey, pp. 17.

New York: VCH

2. Moseley PT, Tofield BC, eds. 1987.Solid-

State Gas Sensors. Bristol/Philadelphia:

Hilger

3. Sberveglieri G, ed. 1992. Gas Sensors:

Principles, Operation and Developments.

Boston: Kluwer

4. Mandelis A, Christofides C. 1993.Physics,Chemistry and Technology of Solid State

Gas Sensor Devices. New York: Wiley-

Interscience

5. Gopel W. 1991. Chemical sensing,

molecular electronics and nanotechno-

logyinterface technologies down to

the molecular scale. Sensors Actuators B

4:721

6. Gopel W. 1996. Ultimate limits in the

miniaturization of chemical sensors. Sen-sors Actuators A56:83102

7. Hagleitner C, Hierlemann A, Lange D,

Kummer A, Kerness N, et al. 2001. Smart

single-chip gas sensor microsystem. Na-

ture414:29396

8. Semancik S, Cavicchi RE, Wheeler MC,

Tiffany JE, Poirier GE, et al. 2001. Micro-

hotplate platforms for chemical sensor re-

search.Sensors Actuators B77:57991

9. Gopel W. 1996. Nanosensors and molec-ular recognition. Microelect. Eng. 32:75

110

10. Albert KJ, Lewis NS, Schauer CL, Sotz-

ing GA, Stitzel SE, et al. 2000. Cross-

reactive chemical sensor arrays.Chem. Rev.

100:2595626

11. Pearce TC, Schiffman SS, Nagle HT, Gard-

ner JW, eds. 2003.Handbook of Machine

Olfaction: Electronic Nose Technology.

Weinheim: Wiley-VCH

12. Moseley PT. 1992. Materials selection for

semiconductor gas sensors. Sensors Actu-

ators B6:14956

13. White NM, Turner JD. 1997. Thick-filmsensors: past, present and future.Meas. Sci.

Technol.8:120

14. Kohl D. 2001. Function and applications of

gas sensors.J. Phys. D34:R12549

15. Henrich VE, Cox PA. 1996. Surface Sci-

ence of Metal Oxides. Cambridge/New

York: Cambridge

16. Xia YN, Yang PD, Sun YG, Wu YY,

Mayers B, et al. 2003. One-dimensional

nanostructures: synthesis, characterization,and applications. Adv. Mater. 15:353

89

17. Wang ZL. 2003.Nanowires and Nanobelts:

Materials, Properties and Devices. Dor-

drecht/London/New York: Kluwer

18. Bandyopadhyay S, Nalwa HS, eds. 2002.

Quantum Dots and Nanowires. Stevenson

Ranch, CA: American Scientific

19. Dai ZR, Pan ZW, Wang ZL. 2003. Novel

nanostructures of functional oxides synthe-sized by thermal evaporation.Adv. Funct.

Mater.13:924

20. Kong J, Franklin NR, Zhou CW, Chap-

line MG, Peng S, et al. 2000. Nanotube



28/32


molecular wires as chemical sensors.Sci-

ence287:62225

21. Collins PG, Bradley K, Ishigami M, Zettl

A. 2000. Extreme oxygen sensitivity of

electronic properties of carbon nanotubes.

Science287:18014

22. Varghese OK, Kichambre PD, Gong D,

Ong KG, Dickey EC, Grimes CA. 2001.

Gas sensing characteristics of multi-wall

carbon nanotubes. Sensors Actuators B

81:3241

23. Kong J, Chapline MG, Dai HJ. 2001. Func-

tionalized carbon nanotubes for molecular

hydrogen sensors.Adv. Mater.13:13848624. Dai LM, Soundarrajan P, Kim T. 2002. Sen-

sors and sensor arrays based on conjugated

polymers and carbon nanotubes.Pure Appl.

Chem.74:175372

25. Pengfei QF, Vermesh O, Grecu M, Javey A,

Wang O, et al. 2003. Toward large arrays

of multiplex functionalized carbon nan-

otube sensors for highly sensitive and selec-

tive molecular detection.Nano Lett. 3:347

5126. Lieber CM. 1998. One-dimensional nanos-

tructures: chemistry, physics and applica-

tions.Solid State Commun.107:60716

27. Cohen ML. 2001. Nanotubes, nanoscience,

and nanotechnology. Mater. Sci. Eng. C

15:111

28. Ajayan PM, Zhou OZ. 2001. Applications

of carbon nanotubes. See. Ref. 29, pp. 391

425

29. Dresselhaus MS, Dresselhaus G, AvourisP, eds. 2001. Carbon Nanotubes: Synthe-

sis, Structure, Properties and Applications.

New York: Springer Verlag

30. Avouris P. 2002. Molecular electronics

with carbon nanotubes. Acc. Chem. Res.

35:102634

31. Li C, Zhang DH, Liu XL, Han S, Tang T,

et al. 2003. In2O3 nanowires as chemical

sensors.Appl. Phys. Lett.82:161315

32. Park WI, Yi GC, Kim M, Pennycook

SJ. 2003. Quantum confinement observed

in ZnO/ZnMgO nanorod heterostructures.

Adv. Mater.15:52629

33. Law M, Kind H, Messer B, Kim F,

Yang PD. 2002. Photochemical sensing of

NO2with SnO2nanoribbon nanosensors at

room temperature.Angew. Chem. Int. Ed.

41:24058

34. Pan ZW, Dai ZR, Wang ZL. 2001.

Nanobelts of semiconducting oxides. Sci-

ence291:194749

35. Comini E, Faglia G, Sberveglieri G, Pan

ZW, Wang ZL. 2002. Stable and highly sen-

sitive gas sensors based on semiconducting

oxide nanobelts.Appl. Phys. Lett. 81:1869

71

36. Arnold MS, Avouris P, Pan ZW, Wang

ZL. 2003. Field-effect transistors based onsingle semiconducting oxide nanobelts. J.

Phys. Chem. B107:65963

37. Liu X, Li C, Han S, Han J, Zhou CW.

2003. Synthesis and electronic transport

studies of CdO nanoneedles. Appl. Phys.

Lett.82:195052

38. Kolmakov A, Zhang Y, Moskovits M.

2003. Topotactic thermal oxidation of

Sn nanowires: intermediate suboxides and

coreshell metastable structures. NanoLett.3:112529

39. Kolmakov A, Zhang Y, Cheng G,

Moskovits M. 2003. Detection of CO and

oxygen using tin oxide nanowire sensors.

Adv. Mater.15:9971000

40. Zemel JN. 1988. Theoretical description of

gas-film interaction on SnOx. Thin Solid

Films163:189202

41. Gopel W. 1989. Solid-state chemical

sensorsatomistic models and researchtrends.Sensors Actuators16:16793

42. Kohl D. 1989. Surface processes in the de-

tection of reducing gases with SnO2-based

devices.Sensors Actuators18:71113

43. Barsan N, Grigorovici R, Ionescu R,

Motronea M, Vancu A. 1989. Mecha-

nism of gas-detection in polycrystalline

thick-film SnO2 sensors. Thin Solid Films

171:5363

44. Barsan N, Weimar U. 2001. Conduction

model of metal oxide gas sensors. J. Elec-

troceram. 7:14367

45. Barsan N. 1994. Conduction models

in gas-sensing SnO2 layersgrain-size



29/32


effects and ambient atmosphere influence.

Sensors Actuators B17:24146

46. Zhang DJ, Li C, Liu XL, Han S, Tang T,

Zhou CW. 2003. Doping dependent NH3

sensing of indium oxide nanowires.Appl.

Phys. Lett.83:184547

47. Li C, Zhang DH, Han S, Liu XL, Tang

T, Zhou CW. 2003. Diameter-controlled

growth of single-crystalline In2O3 nano-

wires and their electronic properties.Adv.

Mater.15:14345

48. Cui Y, Wei QQ, Park HK, Lieber CM. 2001.

Nanowire nanosensors for highly sensitive

and selective detection of biological andchemical species.Science293:128992

49. Duan XF, Huang Y, Lieber CM. 2002. Non-

volatile memory and programmable logic

from molecule-gated nanowires.Nano Lett.

2:48790

50. Zhang Y, Kolmakov A, Metiu HSC,

Moskovits M. 2004. Control of catalytic

reactions at the surface of a metal oxide

nanowire by manipulating electron density

inside it.Nano Lett.In press51. Zhang Y, Kolmakov A, Lilach Y, Cheng G,

Metiu H, Moskovits M. 2004. Electronic

control of the chemistry and catalysis at the

surface of a tin oxide nanowire.J. Chem.

Phys.To be submitted

52. Wolkenstein T. 1991. Electronic Pro-

cesses on Semiconductor Surfaces During

Chemisorption: New York: Kluwer

53. Keier NP, Mikheeva EP, Usoltsev LG.

1968. Field effect method used in studyinghow catalytic dehydration of isopropyl al-

cohol on TiO2films is influenced by Fermi

level.Dokl. Akad. Nauk SSSR 182:13032

54. Hoenig SA, Lane JR. 1968. Chemisorption

of oxygen on zinc oxideeffect of a dc

electricfield.Surf. Sci.11:16366

55. Hellmich W, Muller G, Bosch-Von Braun-

muhl C, Doll T, Eisele I. 1997. Field-effect-

induced gas sensitivity changes in metal

oxides. Sensors Actuators B 43:132

39

56. Kunishima YN, Miyayama M, Yanagida H.

1996. Effects of the external electricfield

from a substrate on Cl2 gas adsorption on

SnO2thinfilms.Appl. Phys. Lett. 69:632

34

57. Scharnagl K, Bogner M, Fuchs A, Winter

R, Doll T, Eisele I. 1999. Enhanced room

temperature gas sensing with metal oxides

by means of the electroadsorptive effect in

hybrid suspended gate FET.Sensors Actu-

ators B57:3538

58. Chen RJ, Franklin NR, Kong J, Cao J,

Tombler TW, et al. 2001. Molecular pho-

todesorption from single-walled carbon

nanotubes.Appl. Phys. Lett.79:225860

59. Kind H, Yan HQ, Messer B, Law M, Yang

PD. 2002. Nanowire ultraviolet photode-tectors and optical switches. Adv. Mater.

14:15861

60. Comini E, Cristalli A, Faglia G, Sberveg-

lieri G. 2000. Light enhanced gas sensing

properties of indium oxide and tin dioxide

sensors.Sensors Actuators B65:26063

61. Comini E, Faglia G, Sberveglieri G. 2001.

UV light activation of tin oxide thin films

for NO2sensing at low temperatures.Sen-

sors Actuators B78:737762. Zhang D, Li C, Han S, Liu X, Tang T, et al.

2003. Ultraviolet photodetection properties

of indium oxide nanowires.Appl. Phys. A

77:16366

63. Kim W, Javey A, Vermesh O, Wang O, Li

YM, Dai HJ. 2003. Hysteresis caused by

water molecules in carbon nanotube field-

effect transistors.Nano Lett.3:19398

64. Whang D, Jin S, Wu Y, Lieber CM. 2003.

Large-scale hierarchical organization ofnanowire arrays for integrated nanosys-

tems.Nano Lett.3:125559

65. Lieber CM. 2003. Nanoscale science and

technology: building a big future from

small things.MRS Bull.28:48691

66. Qi P, Vermesh O, Grecu M, Javey A, Wang

Q, Dai H. 2003. Toward large arrays of

multiplex functionalized carbon nanotube

sensors for highly sensitive and selective

molecular detection.Nano Lett.3:34751

67. Li FY, Zhang L, Metzger RM. 1998. On the

growth of highly ordered pores in anodized

aluminum oxide. Chem. Mater. 10:2470

80



30/32


68. Lakshmi BB, Patrissi CJ, Martin CR. 1997.

Sol-gel template synthesis of semiconduc-

tor oxide micro- and nanostructures.Chem.

Mater.9:254450

69. Nicewarner-Pena SR, Freeman RG, Reiss

BD, He L, Pena DJ, et al. 2001. Submi-

crometer metallic barcodes. Science 294:

13741

70. Routkevitch D, Tager AA, Haruyama J, Al-

mawlawi D, Moskovits M, Xu JM. 1996.

Nonlithographic nano-wire arrays: fabri-

cation, physics, and device applications.

IEEE Trans. Electron Devices 43:164658

71. Metzger RM, Konovalov VV, Sun M, Xu T,Zangari G, et al. 2000. Magnetic nanowires

in hexagonally ordered pores of alumina.

IEEE Trans. Magn.36:3035

72. Kovtyukhova NI, Martin BR, Mbindyo

JKN, Mallouk TE, Cabassi M, Mayer TS.

2002. Layer-by-layer self-assembly strat-

egy for template synthesis of nanoscale de-

vices.Mater. Sci. Eng. C19:25562

73. Lakshmi BB, Dorhout PK, Martin CR.

1997. Sol-gel template synthesis of semi-conductor nanostructures.Chem. Mater.9:

85762

74. Michailowski A, Al Mawlawi D, Cheng

GS, Moskovits M. 2001. Highly regu-

lar anatase nanotubule arrays fabricated in

porous anodic templates. Chem. Phys. Lett.

349:15

75. Varghese OK, Gong D, Paulose M, Ong

KG, Grimes CA. 2003. Hydrogen sensing

using titanium nanotubes. Sensors Actua-tors B93:33844

76. Iwanaga T, Hyodo T, Shimizu Y, Egashira

M. 2003. H2 sensing properties and mecha-

nism of anodically oxidized TiO2film con-

tacted with Pd electrode. Sensors Actuators

B93:51925

77. Varghese OK, Grimes CA. 2003. Metal

oxide nanoarchitectures for enviromental

sensing. J. Nanosci. Nanotechn. 3:277

93

78. Sit JC, Vick D, Robbie K, Brett MJ.

1999. Thinfilm microstructure control us-

ing glancing angle deposition by sputter-

ing.J. Mater. Res.14:119799

79. Smy T, Vick D, Brett MJ, Dew SK, Wu AT,

et al. 2000. Three-dimensional simulation

offilm microstructure produced by glanc-

ing angle deposition.J. Vac. Sci. Technol. A18:250712

80. Dick B, Brett MJ, Smy T. 2003. Controlled

growth of periodic pillars by glancing angle

deposition.J. Vac. Sci. Technol. B 21:2328

81. Kennedy SR, Brett MJ, Toader O, John S.

2002. Fabrication of tetragonal square spi-

ral photonic crystals.Nano Lett.2:5962

82. Wu AT, Seto M, Brett MJ. 1999. Capaci-

tive SiO humidity sensors with novel mi-

crostructures.Sensors Mater.11:49350583. Wu AT, Brett MJ. 2001. Sensing humid-

ity using nanostructured SiO posts: mecha-

nism and optimization.Sensors Mater.13:

399431

84. Harris KD, Huizinga A, Brett MJ. 2002.

High-speed porous thinfilm humidity sen-

sors.Electrochem. Solid State Lett.5:H27

29

85. Dohnalek Z, Kimmel GA, McCready DE,

Young JS, Dohnalkova A, et al. 2002.Structural and chemical characterization of

aligned crystalline nanoporous MgOfilms

grown via reactive ballistic deposition. J.

Phys. Chem. B106:352629



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Annual Review of Materials Research

Volume 34, 2004

CONTENTS

QUANTUMDOTOPTO-ELECTRONICDEVICES,P. Bhattacharya, S. Ghosh,

and A.D. Stiff-Roberts 1

SYNTHESISROUTES FORLARGEVOLUMES OFNANOPARTICLES,

Ombretta Masala and Ram Seshadri 41

SEMICONDUCTORNANOWIRES ANDNANOTUBES,Matt Law,Joshua Goldberger, and Peidong Yang 83

SIMULATIONS OFDNA-NANOTUBEINTERACTIONS,Huajian Gao

and Yong Kong 123

CHEMICALSENSING ANDCATALYSIS BYONE-DIMENSIONAL

METAL-OXIDENANOSTRUCTURES,Andrei Kolmakov

and Martin Moskovits 151

SELF-ASSEMBLED SEMICONDUCTORQUANTUMDOTS: FUNDAMENTAL

PHYSICS ANDDEVICEAPPLICATIONS,M.S. Skolnick

and D.J. Mowbray 181THERMAL TRANSPORT INNANOFLUIDS,J.A. Eastman, S.R. Phillpot,

S.U.S. Choi, and P. Keblinski 219

UNUSUALPROPERTIES ANDSTRUCTURE OFCARBONNANOTUBES,

M.S. Dresselhaus, G. Dresselhaus, and A. Jorio 247

MODELING ANDSIMULATION OFBIOMATERIALS,Antonio Redondo

and Richard LeSar 279

BIONANOMECHANICALSYSTEMS,Jacob J. Schmidt

and Carlo D. Montemagno 315UNCONVENTIONAL NANOFABRICATION,Byron D. Gates, Qiaobing Xu,

J. Christopher Love, Daniel B. Wolfe, and George M. Whitesides 339

MATERIALSASSEMBLY ANDFORMATIONUSINGENGINEERED

POLYPEPTIDES,Mehmet Sarikaya, Candan Tamerler,

Daniel T. Schwartz, and Francois Baneyx 373

vii

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on06/09/09.Forpersonaluseonly

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32/32

viii CONTENTS

INDEXES

Subject Index 409

Cumulative Index of Contributing Authors, Volumes 3034 443

Cumulative Index of Chapter Titles, Volumes 3034 445

ERRATA

An online log of corrections toAnnual Review of Materials Research

chapters may be found at http://matsci.annualreviews.org/errata.shtml

byHokkaidoUniversity

on06/09/09.Forpersonaluseonly

.

chemical sensing and catalysis by 1-d metal oxide nanostructures

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