Growth, structure and electrical conduction of WO 3 nanorods M. Gillet a, * , R. Delamare a , E. Gillet b a Universite ´ Paul Ce ´ zanne, Aix-Marseille III, Faculte ´ des Sciences et Techniques, 52 Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France b Department of Electronics and Vacuum physics,Faculty of Mathematics and Physics, Charles University, V Holesovickach 2, 180 00 Prague, Czech Republic Available online 20 July 2007 Abstract We present a very simple method to obtain tungsten trioxide nanorods. The nanorods are epitaxially grown on a mica substrate in low supersaturation conditions. Investigations of morphology, crystallographic structure and chemical composition of the nanorods allow us to propose a growth model in which the potassium ions of the substrate play a major role inducing the one-dimensional structure. The nanorod growth is initiated by the formation of a hexagonal tungsten bronze (HTB) epitaxially oriented on the mica. By using a conductive atomic force microscopy technique, we characterise the electrical conduction of WO 3 networks. # 2007 Elsevier B.V. All rights reserved. PACS : 81.07 Bc; 61.46Hk Keywords: Tungsten oxide; Nanorods; Epitaxial growth; Conductive nanostructure network 1. Introduction In the recent past years, tungsten trioxide has attracted attention as candidate for chemical semiconductor-based sensors. The mechanism of the electrical conductivity change of the oxide surface under gas exposure is understood in term of adsorption–desorption reactions involving surface oxygen vacancies. Consequently, the sensing response of oxide films is highly dependent on their surface structure and morphology. A lot of sensing tests towards various gas molecules where carried out on WO 3 polycrystalline thin films [1–3], they evidenced that the sensing response steeply increases when the grain size decreases. In more recent studies tungsten nanostructures (nanowires, nanobelts and nanorods) were investigated [4–15] and some of them were tested as chemical sensing material. Due to their wide surface to volume ratio and to their small dimensions compared to the Debye length,they promise to have a high sensitivity and to be good candidate for future chemical sensors working at low temperature and even at room temperature. Of special interest are the synthesis and the structural and electrical characterisation of such one-dimen- sional WO 3 nanostructures which is the aim of the present paper. 2. Experimental procedure Tungsten oxide nanorods are synthesized by vapour deposition on a mica cleavage in a low supersaturation regime [16]. The vapour source is a WO 3 thin film (10 nm thick) heated in atmospheric pressure at a temperature T 1 = 590 8C, the sublimated species are condensed on the mica substrate located at 3 mm above the vapour source and maintained at T 2 = 360 8C. After cooling at room temperature the deposits are examined by atomic force microscopy (AFM) in taping mode and then taken off their substrate by a carbon replica for observation in selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). The chemical composition of the nanorods is determined by Energy dispersive X-ray spectroscopy (EDX). Conductive AFM (CAFM) investigates the electrical conduction of WO 3 nanorods. The measurements are carried out in air using the Digital Instruments microscope ‘‘Nano- scope III’’ equipped with a conductive tip operating in contact mode. The nanorods are partially embedded in a gold thin film acting as a grounded electrode and the tip as a second mobile www.elsevier.com/locate/apsusc Applied Surface Science 254 (2007) 270–273 * Corresponding author. Tel.: +33 04 9166 1460. E-mail address: marcel.gillet@l2mp.fr (M. Gillet). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.07.124 electrode providing two terminal electrical measurements. This technique allows obtaining simultaneously a classical topo- graphic image of the nanorod and a representation of the special current distribution. It is also possible to record I(V) characteristics curve in single point mode. 3. Results 3.1. Structure and composition of the WO 3 nanorods The Fig. 1a and b are typical images acquired on tungsten oxide nanorods in taping AFM and CAFM mode, respectively. The topographical image of Fig. 1a shows that the nanorods are organised into a network with two preferential growth directions at 608 that suggests an epitaxial orientation in accordance with the six fold symmetry of the (0 0 0 1) mica surface. For deposition time varying between 30 and 90 min the nanorods dimensions lies in the 1–30 mm, 10–200 nm, 1– 50 nm, ranges for length, width and thickness, respectively. Generally, the thickness and width of the nanorods depend on the deposition time that does not influence their length and density. The image in Fig. 2 presents the thickness profile in of nanorod by mean of cross-sections according to the transversal AB and longitudinal CD profiles. The investigated thicknesses, correspond to one, two or several monolayers of oxide if one assumes that the lattice constant c value of the WO 3 monoclinic structure represents the thickness of one monolayer. These observations suggest a layer by layer growth mode. In addition to the rods, there is evidence for the growth of 3-D aggregates, their density is inversely proportional to the density of the rods. This evidences that the formation of the nanorods results of a competition between the both 1-D and 3-D growth processes. Figs. 3 and 4 illustrate the structure investigations carried out on two different thickness nanorods by SAED. The Fig. 3 is the electron diffraction pattern of a nanorod with a thickness e %1.9 nm It exhibits a rectangular basic cell from which we Fig. 1. Topographic image (left) and electrical (right) images simultaneously obtained on tungsten oxide nanorods. On the electrical image we have reported the resistance values measured on somes points of the nanorod network. Fig. 2. HRTEM image (a) and cross-sections of tungsten oxide nanorods; (b) cross-section along the line AB; (c) cross-section along the line CD. M. Gillet et al. / Applied Surface Science 254 (2007) 270–273 271 deduce interatomic distances d 1 = 0.62 nm and d 2 = 0.38 nm corresponding to d(1 0 0) = 0.634 nm and d(0 0 2) = 0.831 nm of the WO 3 hexagonal lattice (a = b = 0.73 nm and c = 0.77 nm). The HRTEM image (not shown) exhibits a rectangular unit mesh with dimensions of 0.625 and 0.383 nm corresponding to the atomic distances in the (1 0 0) plane of the hexagonal structure. The nanorod surface is parallel to the (1 0 0) plan and the length direction lies in the [0 0 1] axis. The Fig. 4 depicts the SAED pattern obtained on a nanorod 7 nm thick, it indicates that in this case the nanorod has a monoclinic single crystalline structure with lattice parameters: a = 0.77 nm, b = 0.75 nm, c = 0.73 nm, b = 90 and with a (0 0 1) plane parallel to the surface The length direction lies in the [0 1 0] axis. In the HRTEM image (not shown) two sets of parallel fringes are visible, with spacing of 0.38 and 0.37 nm in accordance with the (0 0 2) and the (0 2 0) planes of the monoclinic structure. These results indicate that the very thin nanorods (one to four monolayers) have a hexagonal structure which transforms in the monoclinic structure for thicker nanorods. The chemical composition of the nanorods was analysed by EDX. The EDX spectra evidence that the nanorod contained potassium in addition to tungsten and oxygen atoms. However, the relative concentration of potassium decreases when the thickness of the nanorod increases; this result suggests that the detected potassium is concentrated in the first monolayers due to the growth of a thin layer of tungsten bronze (KxWO 3 ). In effect, the mica surface is potassium terminated so that hexagonal tungsten bronze HTB can nucleate and grow on the substrate. The small misfit ( f = 0.5) in the [0 1 0] direction of the growing HTB favours the growth in this direction. 3.2. Growth model Considering the results relative both to the structure and to the composition we propose the following growth model for WO 3 nanorods on the mica substrate: In a first step one layer thick HTB nanorods are formed on the mica surface. The HTB nanorods are epitaxially oriented on the substrate, the length direction corresponds to the best accommodation of the HTB on the mica. The second step concerns the growth of some monolayers of hexagonal WO 3 which perfectly matches with the underlying HTB. Finally, the nanorod grows in thickness by deposition of WO 3 monoclinic on the top of the hexagonal WO 3 . In this last step the hexagonal phase can be transform into a monoclinic one by a topotactic transformation [17]. 4. Electrical conduction Fig. 5 illustrates a current–voltage measurement obtained on a nanorod by ramping the bias voltage from V tip = À3to+3V. The amplitude of the I (V) characteristics strongly depends on Fig. 3. Electron diffraction pattern of a tungsten oxide nanorod (thickness e = 1.9 nm). The basic rectangular cell of the hexagonal structure is shown. Fig. 4. Electron diffraction pattern of a tungsten oxide nanorod (thickness e = 7 nm). The basic square cell of the monoclinic structure is shown. Fig. 5. Current–voltage characteristics obtained on a nanorod by ramping the bias voltage from V tip = À3to+3V. M. Gillet et al. / Applied Surface Science 254 (2007) 270–273272 the value of the nanostructure resistance. The shape of the curve was elucidated in terms of electrical contacts [18]. In such a two-probe method, the electrical contacts play an important role, in particular the contact between the AFM tip and the nanorod. Fig. 1b shows an electrical image obtained on a net of nanorods. We have reported the resistance values for some points on the nanorods, the measurements are evidently affected by the tip contact, however, they prove that the nanorods are well electrically connected each other’s. 5. Summary Tungsten oxide nanorods have been epitaxially grown on a mica substrate using a very simple vapour–solid growth process. The WO 3 vapour source is heated at a low temperature as compared to the high temperatures generally used in similar nanorod synthesises. It results that the growth proceeds in low sursaturation conditions. The investigations of the morphology, structure and chemical composition allow us to propose a growth model that involves in the formation of a very thin epitaxial hexagonal tungsten bronze compound on the potassium terminated surface of the mica. The accommodation, with a small misfit, of the potassium ions of the mica lattice with the potassium sites in the HTB induces a fast growth towards one direction giving a rod shape nanostructure. The further growth proceeds by the formation layer by layer of a hexagonal and monoclinic tungsten trioxide successively. The conductive atomic force microscopy is well suitable to investigate the electrical conduction of such nanostructures allowing either to obtain simultaneously the topographic image and the spatial current distribution or to record the current– voltage characteristics in a given point of the nanorod. 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Structure and composition of the WO 3 nanorods The Fig. 1a and b are typical images acquired on tungsten oxide nanorods in taping AFM and CAFM