DOI: 10.1007/s00339-007-3978-9 Appl. Phys. A 88, 45–48 (2007) Materials Science & Processing Applied Physics A e. comini ✉ g. faglia m. ferroni g. sberveglieri Gas sensing properties of zinc oxide nanostructures prepared by thermal evaporation SENSOR CNR-INFM, Department of Chemistry and Physics, Brescia University, via Valotti 9, 25133 Brescia, Italy Received: 28 September 2005/Accepted: 13 January 2007 Published online: 18 April 2007 • © Springer-Verlag 2007 ABSTRACT Progress has been achieved in the synthesis, struc- tural characterization and physical properties investigation of nanostructures. We have focused our attention on zinc oxide nanostructures. We report on the growth of ZnO nanostructures using vapour phase technique. We have synthesized, depending on the growth conditions, different nanostructures such as wires and combs of zinc oxide. ZnO nanowires electrical properties have been characterised in presence of different gases, the re- sults highlight remarkable response to acetone and ethanol with detection limits lower than 1 ppm. PACS 73.63.Bd; 74.78.Na 1 Introduction In the last few years, there has been growing re- search on the investigation of nanostructures. Progress has been achieved in the synthesis, structural characterization and physical properties investigation of nanostructures. These ma- terials, due to their peculiar characteristics and size effects, often show novel physical properties compared to those of the bulk. These potentialities can be exploited both for funda- mental study and for potential nanodevice applications. The potential of these innovative structures is being exploited also for gas-sensing applications. Among them newly developed metal oxide nanobelts and nanorings [1] are potential candi- dates for fabrication of nanoscale devices. Their extraordinary sensing properties have been recently shown for ultra sensi- tive gas [2, 3] and DNA detection [4, 5]. The finite size of the metal oxide wires confines the electrons wave function, lead- ing to quantized energy levels and to a huge modification of the transport and optical properties of the material. The hugely enhanced surface/volume ratio augments the role of surface states in the sensor response. The deposition of metal oxide semiconductor nanostruc- tures has been made by vapour phase technique from metal powders. The deposition technique is very simple and cheap, and the size and shape can be easily controlled. The as- synthesized oxide nanostructure are pure, structurally uni- form, and single crystalline. The controlled pressure of the inert atmosphere and the gradient of temperature within the ✉ Fax: +390302091271, E-mail: comini@tflab.ing.unibs.it furnace allow condensation and nucleation of the nanostruc- tures downstream the gas flow. Such a peculiar thermody- namic condition promotes formation of nanosized 1-D struc- tures, fulfilling the essential requirements for highly sensitive molecular detection. Electron microscopy has been exten- sively used to investigate morphology and structure. 2 Experimental 2.1 Deposition procedure Nanocrystalline materials can be classified into dif- ferent categories depending on the number of dimensions that are nanostructured (with dimensions lower than 100 nm); a possible classification is zero-dimensional for clusters, mono-dimensional for nano-wires and two-dimensional for films. Numerous one-dimensional oxide nanostructures with useful properties, compositions, and morphologies have re- cently been fabricated using so-called bottom-up synthetic routes, we will describe a vapor phase deposition procedure. The growth of ZnO nano-wires was carried out in a ho- rizontal tube furnace by vapor transport process; zinc powder was preferred to the oxide powder since it allows evapora- tion at lower temperatures. The deposition conditions were tailored in order to promote formation of 1D-nanostructures, through changing evaporation temperature, carrier gas and its flux. For the deposition of zinc oxide nanowires, the optimal deposition conditions require an evaporation temperature of 600 ◦ C at ambient pressure with 500 sccm of Ar as carrier, a background oxygen content of 21% and a relative humid- ity of 20% at 20 ◦ C. The alumina substrates were placed at a distance ranging from 3 to 5 mm from the source material. The deposition of Mo:ZnO nanocombs was performed using zinc powders and a Mo foil as a substrate and as a cru- cible for the Zn powder. With the same deposition condi- tions (i.e., an evaporation temperature of 600 ◦ C at ambi- ent pressure with 500 sccm of Ar as carrier), we have ob- tained, at a distance of 10 mm approximately from the powder, nanocombs of zinc oxide. 2.2 Morphological and structural characterisation Morphological investigation of the growth mech- anism was carried out by LEO 1525 SEM equipped with field emission gun and in-lens secondary electrons detector. The enhanced performance of the system allows for the obser-