Synthesis and characterization of semiconducting nanowires for gas sensing

Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học

Sensors and Actuators B 121 (2007) 208–213

Synthesis and characterization of semiconducting

nanowires for gas sensing

G Sberveglieri∗, C Baratto, E Comini, G Faglia, M Ferroni,

A Ponzoni, A Vomiero

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

Brescia University, via Valotti 9, 25133 Brescia, Italy

Available online 27 October 2006

Abstract

Quasi one-dimensional nanostructures of semiconducting metal oxides are promising for the development of nano-devices Tin, indium, and zinc oxides were produced in form of single-crystalline nanowires through condensation from vapor phase Such a growth occurs in controlled thermo-dynamical condition and size reduction effects on the electrical and optical response to gases have been exploited Preparation, microstructural, and electrical characterization of nanowires are presented and the peculiarities of these innovative structures are highlighted

© 2006 Elsevier B.V All rights reserved

Keywords: Nanowires; SnO2; In2 O3; ZnO; Ozone

1 Introduction

A new generation of nanostructures has been recently

pro-duced and has attracted the interest of a wide research

com-munity[1] These fascinating quasi one-dimensional

nanostruc-tures, namely nanowires, nanorods, and nanobelts, exhibit a

single-crystalline arrangement and feature unusual electrical and

optical properties, which arise from size reduction or quantum

confinement as crystal size is comparable to the wavelength of

the electronic wave-function[2]

Presently, the synthesis of nanowires of

semiconduct-ing metal oxides (MOX) is based on thermal

decomposi-tion of precursor powders followed by vapor–solid (VS) or

vapor–liquid–solid (VLS) growth [3] Such a growth in

con-trolled thermodynamic condition appears highly promising for

nanostructure fabrication, due to its simplicity and low cost with

respect to the technology of silicon processing and to other

top-down approaches

A potential application of nanowires is gas sensing, which

MOX are widely employed for Tin-, indium-, and zinc-oxide

nanowires may constitute the building blocks for a novel class

of nano-devices Some authors of the present work demonstrated

first the gas sensing properties of SnO2nanowires[4] Indeed,

∗Corresponding author.

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

nanowires may overcome some typical limitations of sensing layers based on polycrystalline nanostructures In general, the fabrication of polycrystalline sensing layers is directed to con-trol diffusion phenomena, which greatly influence the structural and electrical properties and contribute either positively or nega-tively to the long-term stability Most of the techniques employed for conventional synthesis (sol–gel, condensation from liquid

or gas phase, chemical or physical vapor deposition) require thermal treatment[5–8] Calcination, firing, or annealing defini-tively stabilizes stoichiometry, crystalline phase, and determines the other non-equilibrium characteristics such as porosity, inter-faces, and defects Unfortunately, thermal treatment promotes grain coarsening and causes degradation of the functionality by suppressing the surface-to-volume ratio[9]

Differently, newly developed quasi one-dimensional nanos-tructures envisage long durability owing to their exceptionally high degree of crystallinity[10] The transverse dimension of nanowires may result even smaller than the Debye length asso-ciated to the surface space-charge region and in such condition the detection efficiency of gas molecules adsorbed at surface may reach very high value[11] This extraordinary sensing potential has been recently demonstrated for operation in liquid envi-ronment or at room temperature[12–14] Among the possible applications in the field of bio-nanotechnology, sensitive DNA and protein detection are presently under investigation[15] This paper summarizes the preparation and characterization

of tin, zinc and indium oxide nanowires The electrical and

0925-4005/$ – see front matter © 2006 Elsevier B.V All rights reserved.

doi: 10.1016/j.snb.2006.09.049

Table 1

Basic operating parameters for nanowire growth from vapor condensation

Nanowires Precursor Decomposition

temperature ( ◦C) Substratetemperature (◦C) Duration (min) Pressure (mbar) Substrate Catalyst

optical properties of nanowires were investigated with particular

regard to gas sensing behavior

2 Experimental

The growth of MOX nanowires from vapor phase is based on

the evaporation–condensation technique [10] The oxide

pre-cursor powder is placed at the center of an alumina tube and

then temperature is raised above the limit of decomposition for

the oxide (from 600◦C for zinc oxide to 1500◦C for indium

oxide) [16] A controlled flow of inert gas (usually argon) is

maintained during decomposition and the overall pressure

mea-sures hundreds of mbar The temperature gradient downstream

the gas flow promotes condensation of cations on clean alumina

substrates and allows interaction with the residual oxygen The

peculiar thermodynamic conditions promote growth of

nano-sized one-dimensional structures instead of equi-axed grains

Fig 1shows the nucleation of indium oxide nanowires as

achieved by the evaporation–condensation process The SEM

image shows the crystal habit for the nanowires: the section

appears to be squared and the apex of the wires is tapered In

Fig 1 Nucleation of indium oxide nanowires over polycrystalline alumina.

general, no epitaxial relationship between the orientation of the wire and the alumina grains has been observed Control over the direction of growth as well as pattering of the substrates may be achieved by assisting the growth mechanism through dispersion

of catalysts[17]

By varying the operating conditions, nanostructures can be produced with different length and shapes [18] During tem-perature transients, the argon flow is reversed in order to pre-vent uncontrolled condensation.Table 1summarizes the basic operating parameters for production of SnO2, ZnO, and In2O3

nanowires Despite the deposition technique is relatively simple, cleanness of the alumina tubes and purity of the atmosphere are the key factor for the reproducibility of deposition

3 Microstructural characterization

Scanning and transmission electron microscopy (SEM and TEM) have been carried out in order to determine the degree

of homogeneity and crystalline arrangement High-resolution TEM imaging is useful for investigation of the termination of the nanowire lateral sides and apex Electron diffraction (ED) and analysis of zero-order and higher-order Laue-zones allows precise determination of unit cell and space group

Incoherent imaging techniques such as STEM with the High-Angle-Annular-Dark-Field detector (STEM-HAADF) were used for the investigation of the shape of the nanowires and impurities and local variations in the composition (Z-contrast)

The nanowires prepared featured a very high aspect ratio

as the length exceeds several microns and the width is smaller than 100 nm As shown inFig 2a, the length and width of the nanowire measure 25.3␮m and about 50 nm, respectively The length and flexibility allows nano-manipulation for removal and positioning over Si-based substrates for functional characteriza-tion (seeFig 2b) High-resolution TEM and electron diffraction showed that the wire is single crystalline, with atomically sharp termination of lateral sides Measured Bragg reflections and the whole symmetry of the ED pattern (seeFig 2c) agree with the

direction of the electron beam is parallel to the [0 1 0] zone-axis

of the reciprocal lattice and the nanowire grows along to the [1 0 0] direction

210 G Sberveglieri et al / Sensors and Actuators B 121 (2007) 208–213

Fig 2 Characteristics of SnO2 nanowires: (a) low-magnification TEM image of a very long SnO2 nanowire, (b) removal of nanowires from the alumina substrate through manipulators for structural and electrical characterization, (c) ED pattern of nanowire, (d) STEM-HAADF image of a nanowire, and (e) linescan of the HAADF signal (solid line) and numerical fit of the shape of the nanowire (dashed line).

As both composition and phase can be considered uniform for

the crystalline SnO2 nanowire; STEM-HAADF directly

visu-alizes variations in the projected thickness Fig 2d shows a

STEM-HAADF image of a SnO2 nanowire, about 45 nm in

width: the contrast of the wire is not constant along its section,

indicating a variation of thickness The thickness fitted from

HADDF profile measures 48± 0.2 nm (see Fig 2e), based on

the approximation of a circular section of the wire A

width-to-thickness ratio very close to 1 may thus be considered The

asymmetry of the line profile with respect to the circular fit curve

indicates that the shape of the wire section is more likely to be a

regular polyhedron, as it is expected for a wire with crystalline

habit and crystal facets as lateral sides

3.2 In 2 O 3 nanowires

Synthesis of indium oxide nanowires is difficult because of

the high temperature required for decomposition of the

pre-cursor oxide[18] In addition, In2O3usually crystallizes in a

highly symmetric cubic structure, and the thermodynamic

con-ditions required for producing anisotropic growth are critical

to achieve

The SEM image, presented inFig 3a, shows two nanowires

of indium oxide The wires are capable to bend because of their very small transverse dimension TEM analysis (see Fig 3b and c) highlighted that the nanowires are single crystalline ED

determined that the crystalline phase for the nanowire is Ia−3 body-centered cubic In2O3and that the growth direction is par-allel to the [1 0 0] direction

3.3 ZnO nanowires

ZnO nanowires may be produced at relatively low decom-position temperature (seeTable 1); the size and shape of the obtained nanowires is however sensitive to the condensation condition.Fig 4shows that ZnO nanowires smaller than 10 nm

in width can be produced The capability to control the lateral dimension of the nanowires will allow the systematic investiga-tion of size reducinvestiga-tion effects on the electrical and gas sensing behavior of ZnO nanowires

Fig 3 Characteristics of In3O2 nanowires: (a) SEM image of In3O2 nanowires, (b) TEM image of nanowire 70 nm in width, and (c) ED pattern from the nanowire.

Fig 4 Characteristics of ZnO nanowires: (a–c) variation of the size for the ZnO nanowires for different growth conditions, (d) TEM image of ZnO crystalline nanowire, (e) high-resolution TEM image of the hexagonal nanowire lattice, and (f) digital diffractogram and sketch of the indexed Bragg reflections.

TEM observation confirms the regular crystalline

arrange-ment for the nanowires No evidences of extended crystal defects

governing the growth have been recorded The high-resolution

TEM image and the corresponding digital diffractogram indicate

that the lattice symmetry is hexagonal and that the longitudinal

direction of growth is parallel to the c-axis of the crystal unit

cell

4 Electrical characterization

For the electrical characterization, the electrical Pt contacts

were deposited by sputtering, while a Pt heating meander is

realized on the opposite side of the substrate

Gas sensing characterization was carried out by

volt-amperometric technique; the sensors were biased by 1 V and the

electrical current was measured by a picoammeter The

refer-ence atmosphere of synthetic air was maintained at the constant

condition of 0.3 l/min flow, 20◦C temperature, and 50% relative

humidity

Nanowires were tested towards ozone generated by a UV

lamp discharge Its concentration was measured at the chamber

outlet by a detector based on the wet chemical Brewer–Milford

principle

Fig 5shows that SnO2nanowire and In2O3nanowires exhibit

good response towards ozone together with an appreciable

capa-bility to distinguish among different ozone concentrations The

complete recovery of the baseline value after ozone injection

indicates that no poisoning effects occurred, as is sometimes

encountered for conventional MOX-based sensors in sensing of

oxidizing species[19]

ZnO nanowires exhibit low response to ozone By

observ-ing the dynamic of response, three processes with different

time constant can be observed: a quick decrease in conductance

occurred after the ozone injection and is followed by a

conduc-tance increase; finally a very slow process prevented the sensor

response from reaching a steady-state value even after 1 h from

ozone injection Despite this phenomenon, the response keeps

reversible

The high response of the nanowires can be attributed

to their small lateral dimension Indeed, when the lateral dimensions of the nanowire are sufficiently reduced, then the nanowire can be completely depleted and the response to gases increases[20]

Fig 6shows the ozone sensing capability of SnO2and In2O3

nanowires as a function of the operating temperature ZnO nanowires are not reported because of their slow response The highest response is obtained for an operating temperature of

400◦C for both the samples.

5 Optical characterization

Photoluminescence (PL) spectroscopy was performed over a wide temperature and wavelength range for the purpose of inves-tigating the effect of adsorbed gases on the optical properties of zinc oxide nanowires

As visible in Fig 7, the PL in the visible and ultra-violet region of the light spectrum is quenched by 12 ppm of NO2 The

Fig 5 Variation of current as function of ozone concentration for (a) SnO2 nanowires operated at 400 ◦C, (b) ZnO nanowires operated at 350◦C, and (c) In2 O3 nanowires operated at 400 ◦C The reference atmosphere is synthetic air

at 20 ◦C and 50% relative humidity.

212 G Sberveglieri et al / Sensors and Actuators B 121 (2007) 208–213

Fig 6 Response of the nanowires of SnO2 (solid line), and In3O2 (dashed line)

as a function of the operating temperature Ozone concentration is 280 ppb.

Fig 7 Spectrum of photoluminescence at room temperature for ZnO nanowires

in dry air (open squares), 20 min after NO2 introduction (open triangles) and

20 min after dry air restoration (open circles).

effect is fast (time scale order of seconds) and fully reversible

The amplitude of quenching achieves its maximum at room

temperature and the influence of humidity and other reducing

gases is negligible This feature could be interesting for

applica-tion of nanowires as a selective optical sensor working at room

temperature

6 Concluding remarks

Nanowires of semiconducting MOX can be effectively

pro-duced through evaporation–condensation process Control over

the size of the nanowires is achieved by proper modification

of the operating conditions Nanowires of SnO2, In2O3 and

ZnO have been produced in their stable and common crystalline

phase

The high degree of crystallinity and the small lateral

dimen-sion of these quasi 1D nanostructures open the perspective of a

new class of stable nano-devices for gas sensing

Acknowledgements

Financial support from European Union and MIUR is

gratefully acknowledged: “Nanostructured solid-state gas

sen-sors with superior performance-NANOS4” STREP project no.

001528 “Nanostructured semiconductors for chemical

sens-ing” PRIN project 2004 “Quasi mono dimensional nanosensors for label free ultra sensitive biological detection” PRIN project

2005

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Biographies

G Sberveglieri was born on 17 July 1947 and received his degree in physics cum

laude from the University of Parma (Italy), where he started in 1971 his research activities on the preparation of semiconducting thin film solar cells He is now the director of the CNR, INFM Sensor Laboratory ( http://sensor.ing.unibs.it )

at Brescia University where more than 20 researchers are working In 1988 he established the Gas Sensor Lab, mainly devoted to the preparation and char-acterization of thin film chemical sensors based on nanostructured metal oxide semiconductors and, since the mid 1990s, to the area of electronic noses In

1994, he was appointed full professor in physics He is referee of many inter-national journals and associate editor of IEEE Sensor Journal and has acted

as chairman in several Conferences on Materials Science and on Sensors He

has been the general chairman of IMCS11th (11th International Meeting on

Chemical Sensors) and he is the chair of the Steering Committee of the IMCS

series Conference During 30 years of scientific activity he published more than

250 papers in international journals; he presented more than 250 oral

commu-nications to international congresses (12 plenary talks and 45 invited talks).

He also is an evaluator of European Union, in the area of nanoscience and

nanomaterials, and the coordinator of the EU Project NANOS4 (nanostructured

solid-state gas sensors with superior performance) and several Italian projects on

gas sensors.

C Baratto was born in Brescia in 1972 and has received the degree in applied

physics at the University of Parma in 1997 In 1998 she started her collaboration

with the Sensor Lab and in 2002 she received the PhD degree Now she works

as a researcher at the Sensor Lab Research topics are study and development of

innovative gas sensors (metal oxide thin films and nanobelts, porous silicon,

car-bon nanotubes) Main activities are thin film deposition by magnetron sputtering,

electrical characterization of gas sensor, optical characterization of gas sensor

(photoluminescence, reflectance and surface photovoltage measurements).

E Comini was born on 21 November 1972 and she received her degree in physics

at Pisa University in 1996 She is presently working on chemical sensors She

received her PhD in material science at the University of Brescia She is now an

assistant professor at the University of Brescia.

G Faglia received an MS degree from the Polytechnic of Milan in 1991 with

a thesis on gas sensors In 1992, he has been appointed as a researcher by

the Thin Film Lab at the University of Brescia He is involved in the study of

the interactions between gases and semiconductor surfaces and in gas sensors electrical characterization In 1996, he has received the PhD degree by discussing

a thesis on semiconductor gas sensors In 2000, he has been appointed associate professor in experimental physics at University of Brescia During his career Guido Faglia has published more than 80 articles on International Journals with referee.

M Ferroni received his PhD degree in physics at the University of Ferrara in

1998, and became researcher at the University of Brescia in 2004 His mean research activity concerns the characterization of nanostructured metal oxides

by means of transmission and scanning electron microscopy Presently, Matteo Ferroni is in charge of the high-resolution scanning electron microscopy facility

at the CNR-INFM SENSOR laboratory in Brescia.

A Ponzoni was born in 1976 He received the degree in physics from the

Uni-versity of Parma in 2000 In 2006, he received the PhD degree in material engineering from the University of Brescia with a thesis on nanostructured metal oxides for gas sensing applications His main activity regards synthesis and elec-trical characterization of metal oxides for gas sensing applications Presently,

he is researcher at the CNR-INFM Sensor Lab, Brescia.

A Vomiero received his degree in physics at the University of Padova in 1999,

and his PhD in electronic engineering at the University of Trento in 2003 His main activities deal with the synthesis of thin films and nanostructured materials

by the means of PVD techniques and the application of low energy nuclear techniques to materials science Presently, he is researcher at the CNR-INFM SENSOR Lab, Brescia.