Current Applied Physics 10 (2010) 636–641

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Current Applied Physics

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A facile thermal evaporation route for large-area synthesis of tin oxide nanowires:Characterizations and their use for liquid petroleum gas sensor

Nguyen Van Hieu a,*, Le Thi Ngoc Loan b, Nguyen Duc Khoang a,b,c,d, Nguyen Tuan Minh a,Do Thanh Viet a, Do Cong Minh a, Tran Trung c, Nguyen Duc Chien d

a International Training Institute for Materials Science (ITIMS), Hanoi University of Technology, Viet Namb Experimental and Practical Center, Quy Nhon University, Viet Namc Faculty of Environment and Chemistry, Hung Yen University of Technology and Education, Viet Namd Institute of Engineering Physics (IEP), Hanoi University of Technology (HUT), Viet Nam

a r t i c l e i n f o

Article history:Received 11 April 2009Received in revised form 1 August 2009Accepted 14 August 2009Available online 20 August 2009

PACS:61.46.Km82.45.Yz81.07.-b

Keywords:NanowiresTin oxideGas sensor

1567-1739/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.cap.2009.08.008

* Corresponding author. Address: International TraScience (ITIMS), Hanoi University of Technology (HUHanoi, Viet Nam. Tel.: +84 4 8680787; fax: +84 4 869

E-mail addresses: hieu@itims.edu.vn, hieunv-itims

a b s t r a c t

In this paper, a very simple procedure was presented for the reproducible synthesis of large-area SnO2

nanowires (NWs) on a silicon substrate by evaporating Sn powders at temperatures of 700, 750, and800 �C. As-obtained SnO2 NWs were characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy. Theyrevealed that the morphology of the NWs is affected by growth temperature and the SnO2 NWs are sin-gle-crystalline tetragonal. The band gap of the NWs is in the range of 4.2–4.3 eV as determined from UV/visible absorption. The NWs show stable photoluminescence with an emission peak centered at around620 nm at room-temperature. The sensors fabricated from the SnO2 NWs synthesized at 700 �C exhibitedgood response to LPG (liquefied petroleum gas) at an operating temperature of 400 �C.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Quasi one-dimensional (Q1D) semiconductor metal oxide(SMO) nanostructures have attracted increasing attention in theconstruction of nanodevices ranging from (opto-) electronic de-vices to chemical sensors since the successful synthesis of varioussemiconductor metal oxide nanobelts through a simple method re-ported by Wang et al. [1]. These structures with high aspect ratio(i.e., size confinement in two coordinates) offer better crystallinity,higher integration density, and lower power consumption. In addi-tion, they demonstrate superior sensitivity to surface chemicalprocesses due to their large surface-to-volume ratio and smalldiameter comparable to Debye length (a measure of field penetra-tion into the bulk) [2,3]. A comprehensive review of state-of-the-art research activities focusing on the chemical sensors made ofQ1D nanostructures has been done by several authors [3–5].Although many different Q1D nanostructures of SMO such as

ll rights reserved.

ining Institute for MaterialsT), No. 1, Dai Co Viet Road,2963.

@mail.hut.edu.vn (N.V. Hieu).

SnO2, ZnO, In2O3, and TiO2 have been investigated for gas-sensingapplications, researchers have paid greater attention to those sen-sors based on SnO2 NWs. This is because their counterparts such asthick films, porous pellets, and thin films are versatile in theircapability to sense a variety of gases [6], and are commerciallyavailable. The excellent sensing properties of SnO2 Q1D nanostruc-tures have been demonstrated for the detection of various gases[7–11]. The selectivity and sensitivity of the SnO2 NW sensor canbe significantly enhanced by doping it with other metal oxides[12–14] or by functionalizing it with catalytically active materials[15,16]. Recently, SnO2 NWs material has been used to develop anelectronic nose [17], and a single NWs field effect transistor devicehas been realized for sensing pH [18]. There is a huge applicationsof SnO2 NWs. Therefore, the effective synthesis methods and char-acterizations of their fundamental properties need to be fully ex-plored. In almost all previous works, SnO2 NWs were generallysynthesized at temperatures higher than 850 �C, and their funda-mental properties have intensively been characterized by variousmethods [19–21]. Optical measurements such as PL and UV/visiblespectroscopy are very useful for the determination of the structure,defects, and impurities in NWs [20,21]. It can be recognized thatthe synthesis of SnO2 NWs at lower temperatures (�700 �C) has

N.V. Hieu et al. / Current Applied Physics 10 (2010) 636–641 637

a significant meaning because it is more compatible with micro-electronic technology for device fabrication, providing a prospec-tive platform for constructing nanodevices based on SnO2 NWs.In this paper, we present the successful synthesis of SnO2 nano-wires (NWs) on Si substrate using a simple physical vapor-deposi-tion method at 700, 750, and 800 �C. The synthesis method iscompletely different from the most of previously reported process.As-synthesized SnO2 NWs have been characterized in-depth toconfirm that the obtained NWs are similar with the SnO2 NWs syn-thesized at a high temperature (950 �C) in our previous works [19–21]. Additionally, the LPG (liquefied petroleum gas) sensing prop-erties of the SnO2 NWs material have been studied for the firsttime. A good performance of SnO2 NWs sensors has been obtainedwith an operating temperature of 400 �C.

2. Experimental

The SnO2 NWs were grown in a quartz tube located in a hori-zontal furnace with a sharp temperature gradient (Lingdberg/BlueM, Model: TF55030A, USA). Both ends of the quartz tube weresealed with rubber O-rings. The ultimate vacuum for this configu-ration was �5 � 10�3 Torr. The carrier gas-line (Ar) and O2 gas-linewere connected to the left end of the quartz tube, and their flowrate was modulated by a digital mass-flow-control system (Aal-borg, Model: GFC17S-VALD2-A0200, USA). The right end of thequartz tube was connected to a rotary pump through a needlevalve in order to maintain the desired pressure in the tube. PureSn powder (Merck, 99.8%) was placed in an alumina boat as evap-oration source. The substrates with a previously deposited Au cat-alyst layer (thickness: �10 nm) were placed approximately 2–3 cmfrom the source on both sides (up-stream and down-stream) asindicated in a previous work [24]. The growth process was dividedinto two steps. Initially, the quartz tube was evacuated to 10�2 Torrand purged several times with Ar gas (99.999%). Subsequently, the

Fig. 1. (a) Thermal evaporation set-up; (b) Optical microscope image of SnO2 nanowir(sample A), 750 �C (sample B), and 800 �C (sample C).

quartz tube was evacuated to 10�2 Torr again, and the furnacetemperature was increased from room-temperature to 700 �C(sample A) or 750 �C (sample B) or 800 �C (sample C) in 30 min.It should be noted that Ar gas-flow was not introduced during thisstep. This is completely different from many previous reports onsynthesizing SnO2 NWs by thermal evaporation. After the furnacetemperature reached the synthesized temperatures, oxygen gaswas added to the quartz tube at a flow rate of 0.3 sccm, and thegrowth process was maintained for another 30 min. During theO2 addition step, the pressure inside the tube was in the range of0.5–5 Torr. The as-synthesized SnO2 NWs were analyzed by fieldemission scanning electron microscopy (FE-SEM, 4800, Hitachi, Ja-pan), transmission electron microscopy (TEM, JEM-100CX), and Ra-man and X-ray diffraction (XRD, Philips Xpert Pro) with CuKaradiation generated at a voltage of 40 kV as source. UV/visibleabsorption measurements were carried out on the SnO2 nanowiresusing a spectrophotometer (Shimadzu UV-2450, Japan). The photo-luminescence (PL) spectrum at room-temperature were acquiredfrom 360 to 910 nm using a 325 nm He–Cd laser.

For gas-sensing characterization, the as-synthesized SnO2 NWsat 700 �C were dispersed in ethanol and subsequently deposited onthe pre-fabricated interdigitated electrode substrate using a micro-pipette. The flow-through technique was employed for the sensorcharacterizations. The as-fabricated sensors were measured attemperatures of 350, 400, and 450 �C and LPG concentrations of500, 1000, 2000, and 4000 ppm.

3. Results and discussion

The SnO2 NWs products synthesized at temperatures of 700 �C(sample A), 750 �C (sample B), and 800 �C (sample C) and obtainedform the down-stream and the up-stream (see Fig. 1a) are shownin Fig. 1b. It can be seen that the growth products from both sidesseem to be very similar. The morphology of the NWs of the both

e substrates placed at down-stream and up-stream of the source grown at 700 �C

638 N.V. Hieu et al. / Current Applied Physics 10 (2010) 636–641

sides was actually characterized and we have found out that theyare very similar for a certain synthesized temperature (data notshow). This can be attributed to the fact that the carrier gas wasnot used during the NWs growth process. As usual the synthesisprocess of SnO2 NWs by thermal evaporation technique as previ-ously reported [1,8,15,26] was used the Ar gas-flow as carriesgas. Accordingly, the NWs product was only obtained at thedown-stream. It can be recognized that the morphology and phasestructure of the NWs depend on various processing parameterssuch as source materials, temperature, pressure and carrier gas-flow rate. So our synthesized process without using a carrier gascould be much simple to optimize the growth process in compar-ison with the previous works [1,8,15,26]. Actually, the growth pro-cess in the current work was previously used to synthesize theSnO2 NWs at higher temperature (950 �C) with using the sourcematerial of SnO [24]. In comparison, the NW synthesized at thehigher temperature have relatively large diameter (not show).For comparison the morphology of samples A, B, and C, the FE-SEM and TEM characterization results are presented in Fig. 2. Uni-form SnO2 NWs with homogeneous entanglement were producedon a very large-area on the substrates for samples A, B, and C.The diameter of the SnO2 NWs ranged from 50 to 150 nm, andthe lengths ranged from 50–150 lm, which are very similar forthe three samples. It has been found that the sample A has betterhomogeneity with the smooth and uniform NWs along the fiberaxis, and their diameter is also relatively smaller than that of theother NWs sample (B and C). This suggests that the process synthe-sis at the temperature of 700 �C seems to be a promising material

Fig. 2. FE-SEM and TEM characterization of samples A, B, and C; (A1), (B1), and (C1) are im(B3), and (C3) images are by TEM.

for the construction of nanodevices. This is an interested finding,because many works have been reported in literature that theSnO2 NWs were successfully synthesized at the temperature rangeof 750–1200 �C [13,5,24,26].

Fig. 2(A3), 2(B3), and 2(C3) shows the NWs with a catalyst par-ticle on their tip for the three samples. These catalyst particleswere not easily found in the FE-SEM image because the NWs weretoo long. The growth mechanism of SnO2 NWs in the present workcould be explained on the basis of the vapor–liquid–solid (VLS)mechanism that has been reported by Wagner and Ellis for the firsttime [25]. Briefly, the NWs growth mechanism in our experimentcan be described as follows. Sn vapor, which comes from the Snpowder source, is naturally spread out by thermal diffusion overboth substrates placed at the up-stream and down-stream, andcondensed again on the substrates, forming Sn–Au alloyed dropletsby reacting with the Au particles. At the same time, these alloyeddroplets can provide the energetically favored sites for the adsorp-tion of Sn vapor. Subsequently, the oxygen introduced in the tubereacts with the liquid Sn in the droplets to form SnO2 NWs. Thisalso results in the fact that the NW products obtained from theup-stream and down-stream are very similar.

Fig. 3 shows the XRD patterns of samples A, B, and C. The XRDpatterns of the NWs samples are all attributed to the tetragonal ru-tile structure, which agrees well with the reported data from theJCPDS card (77-0450). The Raman spectra of Samples A, B, and Cas shown in Fig. 4 are to further determine the characteristic ofthe NWs. In Fig. 4, three fundamental Raman scattering peaks at475, 633, and 774 cm�1 are observed for the three samples. The

ages by FE-SEM; (A2), (B2), and (C2) are FE-SEM images at higher magnification; (A3),

20 25 30 35 40 45 50 55 60 65 70 75

Sample C

Sample B

Sample A

(800oC)

(750oC)

(202

)

(301

)(1

12)

(310

)

(002

)(2

20)

(211

)

(111

)(200

)(1

01)

Inte

nsity

(a.u

.)

2θ (degree)

(110

)(700oC)

Fig. 3. XRD patterns of the synthesized SnO2 NWs from samples A, B, and C.

400 500 600 700 800 900

2000

4000

6000

8000

10000

Sample C

Sample B

(800 oC)

(750 oC)

(700 oC)

774

633

476

Inte

snis

ty (a

.u)

Wave number (cm-1)

Sample A

Fig. 4. Raman scattering spectrum of the SnO2 NWs from sample A, B, and C.

250 300 350 400 450 500 550 600 6500

1

2

3

4

5

6

Sample B Sample C(700oC) (800 oC)(750 oC)Sample A

Abs

orbt

ance

(a.u

)

Wavelength (nm)

3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8

0

100

200

300

400

500

600 Sample A Sample B Sample C

(αhν

)2

Photon energy (eV)

Fig. 5. UV/visible absorption spectra of the SnO2 NWs from samples A, B, and C.

N.V. Hieu et al. / Current Applied Physics 10 (2010) 636–641 639

peak at 475 cm�1 can be assigned to Eg, the peak at 633 cm�1 canbe attributed to the A1g mode, and the peak at the 774 cm�1 can beidentified as the B2g mode. These peaks indicate the typical featureof the SnO2 NWs’ Rutile-like structure. Our Raman results are con-sistent with previously reported data [18,19]. The high-energy shiftof an absorption edge is generally expected for nanocrystallinematerials.

To confirm this, absorption spectra were acquired from theSnO2 nanowires from samples A, B, and C, and the results are pre-sented in Fig. 5. The optical transition of the SnO2 crystals is wellknown to be a direct type. In this case, the absorption coefficienta is expressed as a(hm)1 (hm�Eg)1/2/hm. Plots of (a(hm))2 versushm can be derived from the absorption data in Fig. 5 as shown inthe inset of Fig. 5. The intercept of the tangent to the plot gives agood approximation of the band gap energy of the direct bandgap materials. The band gaps for samples A, B, and C are around4.2–4.3 eV. These values are larger than that of bulk SnO2

(3.62 eV). The quantum size effect could be a plausible explanationfor this observation [20]. This also agrees with the Raman charac-terization result from the current work. Although the band gap ofsamples A, B, and C are not much different, we can somehow ob-serve that Eg (Sample A) > Eg (Sample B) > Eg (sample C). This isto further confirm that the NWs diameter of sample A character-ized by FE-SEM and TEM is relatively smaller than those of samplesB and C.

It has been well known that the optical properties of a semicon-ductor are related to both intrinsic and extrinsic effects and the PLis a suitable technique to determine crystalline quality and thepresence of impurities in materials, as well as exciton fine struc-tures [19–21]. For these reasons, the room-temperature PL spectraof the SnO2 NWs from samples A, B, and C are characterized andshown in Fig. 6. It can be seen that a very strong peak located atyellow emission around 620 nm is observed for the three samples.

The emission peak at 620 nm (2.00 eV) is smaller than the bandgap width of 4.2–4.3 eV of the SnO2 NWs as determined from UV/visible spectroscopy. So the visible emission peaks cannot be as-cribed to the direct recombination of a conduction electron inthe Sn 4d band and a hole in the O 2p valence band. It is wellknown that the semiconductor behavior of SnO2 is attributed tothe presence of oxygen vacancies, which is also crucial to theiroptical properties [20,19]. Therefore, the emission peak at�620 nm is believed to originate from the luminescence centersformed by tin interstitials or dangling bonds in the SnO2 NWs.The oxygen vacancies with high density interact with interfacialtin and leads to the formation of a considerable amount of trapped

300 400 500 600 700 800 900 10000.0

2.0k

4.0k

6.0k

8.0k

10.0k

12.0k

14.0k

16.0k

18.0k

Sample B

Inte

nsity

(a.u

.)

Emission wavelength (nm)

700oC

750oC800oC

Sample A

Sample C

Fig. 6. Room-temperature PL spectrum of the SnO2 NWs from samples A, B, and C.

-75.00µ

-50.00µ

-25.00µ

0.00

25.00µ

50.00µ

75.00µ

Cur

rent

(A) Troom~26oC

(c)

-40.0µ

-20.0µ

0.0

20.0µ

40.0µ

60.0µ

(e)

(d)

T=100oC

-9 -6 -3 0 3 6 9

-1.8m

-1.2m

-600.0µ

0.0

600.0µ

1.2m

1.8m

Cur

rent

(A)

Voltage (V)

T=200oC

-9 -6 -3 0 3 6 9-2.0m

-1.5m

-1.0m

-500.0µ

0.0

500.0µ

1.0m

1.5m

2.0m(f)

T=400oC

Voltage (V)

T=300oC

-75.00µ

-50.00µ

-25.00µ

0.00

25.00µ

50.00µ

75.00µ

Troom~26oC

-40.0µ

-20.0µ

0.0

20.0µ

40.0µ

60.0µ

T=100oC

-9 -6 -3 0 3 6 9

-1.8m

-1.2m

-600.0µ

0.0

600.0µ

1.2m

1.8m

T=200oC

-9 -6 -3 0 3 6 9-2.0m

-1.5m

-1.0m

-500.0µ

0.0

500.0µ

1.0m

1.5m

2.0m

T=400oC

T=300oC

(a) (b)

Fig. 7. Pre-fabricated electrodes (a), SnO2 NW gas sensors (b) imaged by an opticalmicroscope, and I–V characteristics of the sensors measured at different temper-atures of 26 �C (c), 100 �C (d), 200 �C (e), and 400 �C (f).

140 160 180 200 220 240 260 2800.0

20.0k

40.0k

60.0k

80.0k

100.0k

120.0k

4000 ppm2000 ppm

1000 ppm

500 ppm

R (Ω

)

Time (s)

Operating temp: 400oCResponse ~10 s

LPG

Air

Recovery ~12 s

Fig. 8. Response characteristic of the SnO2 NW sensor measured at various LPGconcentrations and at the temperature of 400 �C.

Table 1Brief summary of results reported on semiconductor LPG sensor.

Sensor type LPG(ppm)

OperationT(�C)

Response References

SnO2 NWs 500 400 Ra/Rg � 2.2 This workSnO2 thick film 10,000 350 (Ra�Rg)/Rg � 0.93 [25]SnO2 thick film 200 300 (Ra�Rg)/Rg � 0.7 [26]SnO2 thin film 800 400 (Ra�Rg)/Rg � 1.38 [27]SnO2 thick film 1000 350 (Gg�Ga)/Ga � 3.68 [28]SnO2 thin film 1000 345 (Ra�Rg)/Rg � 0.1 [29]

0 500 1000 1500 2000 2500 3000 3500 4000 4500

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Resp

onse

(Rai

r/Rga

s)

LPG (ppm)

450 0C 400 0C 300 0C

Fig. 9. Response as a function of LPG concentration at different operatingtemperatures.

640 N.V. Hieu et al. / Current Applied Physics 10 (2010) 636–641

states within the band gap, giving rise to a high PL intensity atroom-temperature [19–23].

To carry out gas-sensing characterization, the SnO2 NWs syn-thesized at the temperature of 700 �C (sample A) are used for gassensor fabrication. The interdigitated electrodes before and afterthe NWs deposition are shown in Fig. 7a and b, respectively. Ohmicbehavior is very important to gas-sensing properties because thesensitivity of the gas sensor is affected by contact resistance.

Hence, current–voltage (I–V) is first characterized. Fig. 7c, d, e,and f represent the current–voltage (I–V) characteristics of thegas sensor in air at a temperature of 26 �C (room-temperature),100, 200, and 400 �C. The (I–V) curve of the as-fabricated gas sensorshows good ohmic behavior at an operating temperature higherthan 200 �C. This points out that not only the metal–semiconduc-tor junction between the Au contact layer and the SnO2 NWs butalso the semiconductor–semiconductor junction between theSnO2 NWs is ohmic for an operating temperature higher than200 �C.

Fig. 8 shows the responses of the SnO2 NW sensors under expo-sure to 500, 1000, 2000, and 4000 ppm of LPG at 400 �C. The 90%response time for gas exposure (t90%(air-to-gas)) and that for recovery(t90%(gas-to-air)) calculated from the resistance–time data are shownin Fig. 8. The t90%(air-to-gas) values in the sensing of 500, 1000, 2000,and 4000 ppm LPG are around 10 s, while the t90%(gas-to-air) valuesare around 12 s.

The corresponding response from the present work was calcu-lated (Ra/Rg) to be 1.7, 2.2, 3.1, and 4.3. A brief review of the resultson semiconductor LPG sensors as reported by different groups issummarized in Table 1 for comparison. It can be seen that theNWs SnO2 sensor has a relatively higher response to LPG thanthe SnO2 sensors based on nanoparticles thin/thick films as de-picted in the Table 1. It should be noted that the current NWs sen-sor responses to LPG are not really high in comparison with Pd-, Pt-, RuO2-doped SnO2 nanoparticles films [28–30]. However, The NWssensors can be significantly improved by doping or simply func-tionalizing catalyst materials as demonstrated in our previouswork [26]. The surface functionalization of the NWs sensor withdifferent catalytic materials could be an efficient method to devel-op novel gas sensor and gas sensor array for the detection of multi-gases [15].

N.V. Hieu et al. / Current Applied Physics 10 (2010) 636–641 641

The NWs sensor response as a function of LPG concentration foroperating temperatures of 300, 400, and 450 �C are shown in Fig. 9.It can be seen that the response increases linearly with an increasein LPG concentration. This would be convenient for practical appli-cation. Additionally, the optimized operating temperature for thedetection of LPG seems to be around 400 �C.

4. Conclusion

We have demonstrated that single-crystalline SnO2 NWs can besuccessfully prepared on silicon substrates through the simplethermal evaporation of Sn powder at temperatures of 700, 750,and 850 �C. A reproducibility and a very large-scale of the NWsare obtained for gas sensor application. The morphology of theSnO2 NWs is relatively affected by the growth temperature. Themicrostructures of the as-obtained NWs were intensively charac-terized by various methods. The as-synthesized NWs at 700 �Cwere used to fabricate the gas sensor through the drop-coatingmethod. The as-fabricated SnO2 NW sensors exhibited good re-sponse to LPG at 400 �C. However, their sensitivity and selectivitycan be improved further by surface catalytic doping or functional-ization, or plasma treatment.

Acknowledgments

This work was supported by the application-oriented basic re-search program (2009-2012, Code: 05/09/HÐ-DTÐL).

References

[1] W. Pan, Z.R. Dai, Z.L. Wang, Nanobelts of semiconducting oxide, Science 291(2001) 1947–1949.

[2] J.G. Lu, P. Chang, Z. Fan, Quasi-one-dimensional metal oxide materials –synthesis, properties and applications, Mater. Sci. Eng. R 52 (2006) 49–91.

[3] A. Kolmakov, M. Moskovits, Chemical sensing and catalysis by one-dimensional metal–oxide nanostructures, Annu. Rev. Mater. Res. 34 (2004)150–180.

[4] X.-J. Huang, Y.-K. Choi, Chemical sensors based on nanostructured materials,Sens. Actuators B (2006) 150–180.

[5] Elisabetta Comini, Metal oxide nano-crystals for gas sensing, Anal. Chim. Acta568 (2006) 28–40.

[6] N. Yamazoe, Toward innovations of gas sensor technology, Sens. Actuators B108 (2005) 2–14.

[7] Y.J. Chen, X.Y. Xue, Y.G. Wang, T.H. Wang, Synthesis and ethanol sensingcharacteristics of single crystalline SnO2 nanorods, Appl. Phys. Lett. 87 (2005)2335101–2335103.

[8] E. Comini, G. Faglia, G. Sberveglieri, Z. Pan, Z.L. Wang, Stable and highlysensitive gas sensors based on semiconducting oxide nanobelts, Appl. Phys.Lett. 81 (2002) 1869–1871.

[9] E. Comini, G. Faglia, G. Sberveglieri, D. Calestani, L. Zanotti, M. Zha, Tin oxidenanobelts electrical and sensing properties, Sens. Actuators B 111–112 (2005)2–6.

[10] A. Kolmakov, Y. Zhang, G. Cheng, M. Moskovits, Detection of CO and O2 usingtin oxide nanowires sensors, Adv. Mater. 15 (2003) 997–1000.

[11] G. Sberveglieri, C. Baratto, E. Comini, G. Faglia, M. Ferroni, A. Ponzoni, A.Vomiero, Synthesis and characterization of semiconducting nanowires for gassensing, Sens. Actuators B 121 (2007) 208–213.

[12] Q. Wan, T.H. Wang, Single-crystalline Sb-doped SnO2 nanowires: synthesis andgas sensor application, Chem. Commun. (2005) 3841–3843.

[13] X.Y. Xue, Y.J. Chen, Y.G. Liu, S.L. Shi, Y.G. Wang, T.H. Wang, Synthesis andethanol sensing properties of indium-doped tin oxide nanowires, Appl. Phys.Lett. 88 (2006) 201907/1–201907/3.

[14] N.S. Ramgir, I.S. Mulla, K.P. Vijayamohanan, A room temperature nitric oxidesensor actualized from Ru-doped SnO2 nanowires, Sens. Actuators B 107(2005) 708–715.

[15] A. Kolmakov, D.O. Klenov, Y. Lilach, S. Stemmer, M. Moskovits, Enhanced gassensing by individual SnO2 nanowires and nanobelts functionalized with Pdcatalyst particles, Nano Lett. 5 (2005) 667–673.

[16] L.H. Qian, K. Wang, Y. Li, H.T. Fang, Q.H. Lu, X.L. Ma, CO sensor based on Au-decorated SnO2 nanobelt, Mater. Chem. Phys. 100 (2006) 82–84.

[17] V. Sysoev, J. Goschnick, T. Schneider, E. Strelcov, A. Kolmakov, A gradientmicroarray electronic nose based on percolating SnO2 nanowire sensingelements, Nano Lett. 7 (2007) 3182–3188.

[18] Yi Cheng, P. Xiong, C.S. Yun, G.F. Strouse, J.P. Zheng, R. Yang, Z.L. Wang,Mechanism and Optimization of pH Sensing Using SnO2 Nanobelt Field EffectTransistors, Nano Lett. 8 (2008) 4179–4184.

[19] W. Wang, C. Xu, G. Wang, Y. Liu, C. Zheng, Synthesis and Raman scatteringstudy of rutile SnO2 nanowires, J. Appl. Phys. 92 (2002) 2740–2742.

[20] S. Luo, P.K. Chu, W. Liu, M. Zhang, C. Lin, Origin of low-temperaturephotoluminescence from SnO2 nanowires fabricated by thermal evaporationand annealed in different ambients, Appl. Lett. 88 (2006) 183112–183113.

[21] S. Luo, J. Fan, W. Liu, M. Zhang, Z. Song, C. Lin, X. Wu, P. Chu, Synthesis and low-temperature photoluminescence properties of SnO2 nanowires and nanoblets,Nanotechnology 17 (2006) 1695–1699.

[22] A. Kar, M.A. Stroscio, M. Dutta, J. Kumari, M. Meyyappan, Observation ofultraviolet emission and effect of surface states on the luminescence from tinoxide nanowires, Appl. Lett. 94 (2009) 101905–101913.

[23] H.T. Chen, S.J. Xiong, X.L. Wu, J. Zhu, J.C. Shen, P.K. Chu, Tin oxide nanoribonswith vacancy structures in luminescence-sensitive oxygen sensing, Nano Lett.9 (2009) 1926–1931.

[24] N.V. Hieu, Highly reproducible synthesis of very large-scale tin oxidenanowires used for screen-printed gas sensor, Sens. Actuator B, in press.

[25] R.S. Wagner, W.C. Ellis, Vapor–liquid–solid mechanism of single crystalgrowth, Appl. Phys. Lett. 4 (1964) 89–90.

[26] N.V. Hieu, H.-R. Kim, B.-K. Ju, J.-H. Lee, Enhanced performance of SnO2

nanowires ethanol sensor by functionalizing with La2O3, Sens. Actuators B 133(2008) 228–234.

[27] A.R. Phani, S. Manorama, V.J. Rao, Preparation, characterization and electricalproperties of SnO2 based liquid petroleum gas sensor, Mater. Chem. Phys. 58(1999) 101–108.

[28] M.V. Vaishampayan, R.G. Deshmukh, I.S. Mulla, Influence of Pd doping onmorphology and LPG response of SnO2, Sens. Actuators 131 (2008) 665–672.

[29] M.H.M. Reddy, A.N. Chandorkar, E-beam deposited SnO2, Pt–SnO2 and Pd–SnO2 thin film LPG, Thin Solid Films 349 (1999) 260–265.

[30] M.S. Wagh, G.H. Jain, D.R. Patil, S.A. Patil, L.A. Patil, Surface customization ofSnO2 thick films using RuO2 as a surfactant for the LPG response, Sens.Actuators B 122 (2007) 357–364.