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NO2OPTICAL SENSING INZNONANOSTRUCTURES

A.Cret, A. Taurino, D. Valerini, C. Martucci,F. Quaranta, M. Lomascolo, P. Siciliano and R. Rella

Institute for Microelectronics and MicrosystemsIMM-CNR,Via per Monteroni, 73100 Lecce, Italy

arianna.creti@le.imm.cnr.it

Abstract The optical response at room temperature of ZnO

nanostructures, grown by controlled vapour phase deposition,

to low concentrations of NO2 gas mixed in dry-air has been

investigated. In particular the quenching in the emission

signal, due to the interactions between oxidizing gas molecules

and sample surface, has been monitored in excitonic and defect

band, as a function of NO2 gas concentration. The dynamic

responses and the resulting calibration curves, obtained for

each emission band, have been compared. Furthermore

responses of nanorods with different size and nanostructures

with different exotic shape have been discussed in order toclarify the role of the morphology and/or dimensions and

structural quality. In particular we have correlated the growth

conditions and the resulting structural and optical properties

of the samples, highlighting their role in the optical sensing

response.

I. INTRODUCTIONZinc oxide results material of great interest in electronic

and optoelectronic applications for UV light emitting diodes,lasing media and sensing. In particular ZnO micro/nanostructures of high quality and different shape have beentaken in investigation for high-performance gas sensors, due

to their good properties such as sensitivity, rapid responseand fast recovery. Notwithstanding the electricalmeasurements monitoring in sensing applications is widelyexploited, at present few papers report on optical sensing inZnO material [1,2].

In our work, we explore the possibility to detect thepresence of NO2 gas at room temperature, by measuring thePL quenching/restoring in presence /absence of gas. Thehigh quality samples allows us to exploit the optical responseto gas, at room temperature, both for excitonic (FE) anddefect (D) emission band, rather than the only defect one asusually reported. It results that the defect band shows asmaller response than the FE band one, in the all investigatedsamples.

We have grown high quality ZnO nanostructures ondifferent substrates and with different shapes by means ofvapor phase deposition (VPD). The high crystal quality hasbeen deduced by Photoluminescence (PL) at low temperature(7K) and X-ray diffraction (XRD) measurements.

The ZnO nanostructures have been used as activematerial in optical sensing measurements of NO2 gas in dryair. In particular the changes induced by the gas moleculesin FE and D emission band intensity have beendiscriminated.

II. EXPERIMENTALZnO nanostructures investigated in this paper have beengrown on indium tin oxide (ITO) and sapphire substrates,by VPD growth technique, in a conventional furnace withhorizontal quartz tube (25 mm inner diameter). Zn 5N puregranular source was placed in a tungsten crucible andinserted in the furnace at room temperature. The substratewas placed in front of / or next to the crucible, varying thedistance. Ar 5N was used as carrier gas with a flux of 1Nl/min. The temperature ramp started after five minutes ofgas flux. The furnace was heated up to 600C in 10 min,maintained at this temperature during the deposition timeand then slowly cooled down to room temperature.The ITO and sapphire substrates were preliminarily coatedwith a 3 nm gold film deposited by e-beam evaporation.A JEOL 6500F Scanning Electron Microscope was used to

analyze the surface morphology of the samples and X-raydiffraction measurements were performed in the -2geometry by using the CuK excitation line (=1.5402 )of a Rigaku diffractometer.Photoluminescence (PL) measurements at low temperature(7K), have been carried out by He-Cd laser (325 nm), at lowpump intensity ( ~ 5 meV), and dispersed by an 0.3 m focallength monocromator TRIAX 320 equipped with a cooledGaAs photomultiplier operating in photon-counts mode.Finally, for the sensing measurements, the samples wereplaced inside an aluminum sealed chamber in which theywere exposed to the detectable gas or dry air. The gasmixing station consists of a mass flow controller (Brooks

Instruments 5850S) equipped with three mass flowmeters/controller (operating in the range 2.5-50 ml/min) anda system of teflon pipelines and switching valves. All theoptical sensing measurements were performed at room

1-4244-2581-5/08/$20.00 2008 IEEE 309 IEEE SENSORS 2008 Conference

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Figure 1: SEM image of the ZnO nanostructures of sample 1# (a), 2# (b),3# (3) and 4# (d).

temperature. In this case the PL, carried out by He-Cd laser(325 nm), has been dispersed by an 0.1 m focal lengthAvantes spectrometer mod. MC 2000. The procedure fortesting the optical gas sensing properties of thenanostructured ZnO films, provides that the sample has tobe preliminary expose to a flow of dry air, in order toacquire the baseline for the successive analysis of the opticalresponse to the NO2 gas. The total flow was kept constant at50 ml/min. The variation in the integrated PL signal of thesample under exposure to NO2 gas, coming from defect andexcitonic bands, was recorded during a minimum period oftime (10min) necessary to reach an almost equilibriumsignal, in the response phase and in the restoring process, inorder to analyse also the reversibility of the interactionprocesses.

III. RESULTS AND DISCUSSIONSIn this work we have investigated four different samples:sample 1# and 2#, grown on ITO substrate and sample 3#and 4#, grown on sapphire substrate. The sample 1#, hasbeen obtained with the substrate placed in front of thecrucible at a distance of 22 mm and maintaining the furnacetemperature for 30 min. As shown by the SEM image inFig. 1 a), it consists of a poor density of nanorods randomlydispersed on the surface of the substrate. The nanorods havean hexagonal section with a tapered tip and an averagediameter of (130 20) nm. The sample 2# has been alsoobtained with the ITO substrate placed in front of the

crucible, but at a reduced distance of 10 mm andmaintaining the furnace temperature at 600C for 60 min.The same growth conditions were used for sample 3#. Bothsamples exhibit nanorods with a shape similar to sample 1#,but in this case a very high density has been achieved (seeFig. 1b) and 1c)). Furthermore the nanorods of sample 3#have a significantly larger diameter and a much wider sizedispersion (ranging between 250 and 500 nm), than the oneof sample 2#, which averaging diameter results to be

Figure 2: XRD spectra of sample 1# (a), 2# (b), 3# (c) and 4# (d).

(120 20) nm, complarable with sample 1#. In particularthe samples 2# and 3# grew on a ZnO bulk layer of fewmicrons thickness.Finally, placing the substrate next to the crucible andmaintaining the same experimental growth conditions ofsample 2# and 3#, nanostructures of exotic shape (Fig.1 d)have been obtained on sapphire substrate.The crystal quality of the samples has been investigated bymeans X-ray diffraction (XRD) and photoluminescencemeasurements at low temperature (7K). Fig. 2 show thetheta-2theta x-ray diffraction spectra obtained respectivelyfrom sample 1# - 4#.The XRD spectrum of sample 1# (Fig.2) is dominated bytwo main peaks, namely the 100 and 002 of the ZnO

wurtzite structure. The presence of these peaks demonstratesthe misorientation of the nanorods with respect to the z axisof the wurtzite structure, as expected from the SEM image.The other three spectra show a very intense 002 peakindicating a good structural quality of the relevant ZnOfilms. Nevertheless, some differences can be observed: thespectrum of sample 2# evidences secondary peaks of higherintensity in comparison with the other two samples,suggesting a worse structural quality of the sample. Thisdemonstrates that the crystallinity of the substrate influencesthe structural properties of the ZnO films.Finally the comparison between sample 3# and sample 4#shows that the intensity of the sapphire peaks is quite

different for the two samples, being comparable with theintensity of the 002 ZnO main peak in the case of sample 3#and almost half of the 002 peak for sample 4#. This wouldindicate a higher thickness of the deposited film in the caseof the substrate placed next to the crucible.Fig. 3 shows the low temperature (7K) PL spectra of theinvestigated samples. In the all spectra the signal rangesbetween the visible range (green-orange) and the UV-region. In particular the visible signal, due to defect

b)

a)

d)

c)1m)

1m) 1m)

1m)

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Figure 3: Comparison between PL spectra of samples 1# - 4#, detected atlow temperature (7K).

emission band, attributed usually to oxygen vacancies(green emission) and interstitial oxygen (orange emission)[3], is smaller (3 order of magnitude) than the UV signal,due to the band-edge emission of the ZnO. This indicatesthe high structural quality of the samples.A detailed analysis of excitonic band spectra shows a morestructured spectrum in the sample 3#, where besides thetypical bound exciton D0X and the free exciton FXAtransitions, FXB transition [4] and also the first excitedstates FXAB [4] are visible, confirming the better crystalquality of the sample 3#, in accordance with the x-rayspectra.In order to exploit the variation in the optical properties ofnanostructures in optical sensing applications, we have

performed optical sensing test on different samples 1# - 4#.The comparison between their responses, allows us to studythe role of different features (dimensions, morphologicalshape and crystal quality) of the sample in the surface-gasinteraction mechanism and, correlating the features to thegrowth conditions, to improve the sample preparation.The sensing measurement is based on the variation of thePL emission due to gas/surface interaction (adsorption).ZnO nanostructures result able to detect NO2 by PLquenching measurements because the oxidizing gasinteracting with the optical sensing surface, act as electrontrapper, causing the decrease of charge-carriers which canrecombine radiatively, resulting in a reduced PL signal. [5].

In our samples, due to high optical quality, we havemonitored the optical sensing response due to the defectband (usually reported) and of the excitonic band.

The dynamic response of the analysed samples, due to thequenching of the PL intensity during the interactionprocesses with the gas molecules, in the two emission bandshas been measured. The response signal is detected fordifferent gas concentration values ranging from 20 ppm to115 ppm. We observe that in the case of low gas

Figure 4: Typical dynamics responses due to the quenching of PL intensityduring the interaction with NO2 gas molecules relative to the excitonic anddefect band emission of sample 3#

concentration (20 ppm), 10 minutes are need to reaches anequilibrium value of the response and to restore the signal.The dynamic response of sample 1# in the two emissionband (not shown) is linear only at low concentration and itshows evident saturation effect at high gas concentration (70ppm, see also Fig. 5). The measurements performed onsample 2# and 3# show a linear response as a function ofgas concentration, as shown in the Fig. 4 where only theresponse of sample 3# is reported as an example. Thecalibration curves of sample 2# and 3# are almost the same.The different diameter doesnt affect the interactionmechanism with the gas molecules. The response of sample2# and 3# are larger than the 1# one in the case of excitonicband before the signal saturation (see Fig. 5), but it resultscomparable in the case of defect one. It is important to notethat the excitonic band shows a larger PL reduction than thedefect band in all the investigated samples, and its responseincreases as the crystal quality increases, as expected. Thesample with exotic shape 4#, shows a sensing responsecomparable to the one of sample 2# and 3#, but nosaturation effect of signal is present either at high gasconcentration. In fact the sample 4#, shows a constantresponse in both band (of about 10 % and 4% in excitonicand defect band respectively) and a restoring signalcomparable to the baseline, in all cycle, in spite of manycycles at gas concentration of 115 pmm. Nevertheless in thiscase anomalous effect after some NO2 exposure cycles

appears. In the sample 4# the signal increases with respectto the baseline. This behaviour is not clear, furtherinvestigation has to be carried out particularly on exoticshape samples. Repeatability and reproducibility in theoptical sensing response in fact has been obtained only inZnO nanowires. Preliminary study on different samples withmorphology comparable to the one of sample 4#, showscontrasting results. Nevertheless in the case of sample 4#,

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Figure 5: Comparison between the calibration curves of sample 1# and 3#

we can attribute its larger stability at high gas concentration

to the higher thickness of the film and to the larger surfaceto volume ratio.Calibration curves are reported in Fig. 4. In particular weshow the excitonic and defect band responses of sample 1#and 3#, as a function of NO2 gas concentration. Thesaturation effect in both band of sample 1# is clearlyevident. On the contrary, the sample 3# (or 2#) shows alinear behaviour in the investigated concentration range. Inthe Fig. 5 the response is calculated by considering thevariation in the PL signal in dry air (PLdry-air) and in thepresence of NO2 gas (PLNO2) respectively, by using therelation R/R=PLdry-air - PLNO2/PLdry-air. An highersensitivity is evidenced in the case of excitonic band of the

sample 3# respect to the defect band.

IV. CONCLUSIONSIn this paper we have investigated the optical sensingproperties of ZnO nanostructures. In particular we have

correlated the sensing response to NO2 gas molecules to theshape and to the crystal quality of the samples. It results thatregular shape (exagonal nanowires) and high crystal qualityensures a high response to gas sensing. The excitonic bandshows a larger response than the defect one, in all samples,but the larger response, the larger drift with respect thebaseline value. The interaction processes with the gas in factresults irreversible, particularly at high gas concentration,namely the quenching of emission signal is not restorable.This effect could be overcome in the case of thicker films,which result more stable. The optical sensing measurementsof ZnO nanostructures with exotic shape in fact resultconstant in both band even if the sample is exposed at highgas concentration for more time. The role of thicknessand/or exotic shape has to be clarified.

V. ACKNOWLEDGEMENTWe thank Flavio Casino and Giovanni Montagna for theirtechnical help.

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improving the performance of ZnO gas sensors Sens. Actuators Bvol. 114, pag. 301-307, 2006.

[2] G. Sberveglieri, C. Baratto, E. Comini, G. Faglia, M. Ferroni, A.Ponzoni and A. Vomiero, Synthesis and characterization ofsemiconducting nanowires for gas sensing, Sens. Actuators B vol.121, pag. 208-213, 2007.

[3] Z.W.Liu, C.K. Ong, T.Yu and Z.X. Shen, Catalyst-free pulsed-laser-deposited ZnO nanorods and their room temperaturephotoluminescence properties Appl. Phys. Lett. 88,pag. 053110-13,2006 and references therein.

[4] A. Tsukazaki, A. Ohtomo, M. Kawasaki, T. Makino, C.H. Chia, Y.Segawa and H. Koinuma, Emission from the higher-order excitonsin ZnO films grown by laser molecular-beam epitaxy , Appl. Phys.Lett. 84, 3858-61, 2004

[5] A Bismuto, S Lettieri, P Maddalena, C Baratto, E Comini, G Faglia,G Sberveglieri and L Zanotti, Room-temperature gas sensing basedon visible photoluminescence properties of metal oxide nanobelts , J.Opt. A: Pure Appl. Opt. 8, 585-

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