Eur. Phys. J. D 24, 277–282 (2003) DOI: 10.1140/epjd/e2003-00189-2 T HE EUROPEAN P HYSICAL JOURNAL D Supersonic cluster beam deposition of nanostructured titania E. Barborini 1 , I.N. Kholmanov 1 , A.M. Conti 1 , P. Piseri 1 , S. Vinati 1 , P. Milani 1, a , and C. Ducati 2 1 INFM, Dipartimento di Fisica, Universit`a di Milano, Via Celoria 16, 20133 Milano, Italy 2 Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 IPZ, UK Received 10 September 2002 Published online 3 July 2003 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2003 Abstract. Nanostructured titanium dioxide films have been deposited by supersonic cluster beam depo- sition (CBD). Nanoparticles are produced by a pulsed microplasma cluster source (PMCS) and selected by aerodynamic separation effects. The as-deposited film is a complex mixture where amorphous material coexists, at the nanoscale, with anatase and rutile crystal phases. The nanocrystalline fraction of the film is characterized by crystal size ranging from 100 nm to less than 5 nm. We have characterized the film structure by transmission electron microscopy, Raman spectromicroscopy, X-ray diffraction, and UV-visible spectroscopy showing that correlations exist between cluster size and film properties. In particular if very small clusters are deposited, the film shows a predominant rutile phase whereas larger clusters form films with mainly anatase structure. Our observations suggest that phonon confinement effects are responsible for a significant shift and broadening observed for the Raman peaks. In addition, optical gap tuning is provided by mass selection: large clusters assembling generates a film with 3.22 eV optical gap, while smallest clusters 3.52 eV. PACS. 77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc. – 81.05.Zx New materials: theory, design, and fabrication – 78.40.-q Absorption and reflection spectra: visible and ultraviolet – 63.22.+m Phonons or vibrational states in low-dimensional structures and nanoscale materials 1 Introduction Titanium dioxide is one of the most promising semicon- ductor for applications in microelectronic devices, sensors, photocatalysis, optical coatings, energy production and storage [1–6]. TiO 2 has three polymorphic forms: anatase, rutile, and brookite. Anatase is less stable than rutile and transforms to rutile after annealing at high tempera- tures [5,7,8]. The physico-chemical properties of TiO 2 are different for different polymorphs and are also influenced by the nanostructure. The control of the crystalline structure at the nanoscale is thus very important for applications. For example anatase and rutile have different electronic and optical properties that can be tailored by varying the size distribution of the particles in nanosized titania [9]. In anatase nanoparticles, down to diameters of roughly 8 nm, broadening and shifting of Raman lines have been observed and attributed to a combination of phonon con- finement, deviation from stoichiometry, mechanical stress, and influence of processing conditions [3,10–12]. To clearly distinguish between different effects and to control the fi- nal material properties one should be able to tailor the nanostructure of the material and the crystalline struc- ture of the nanoscale building blocks. a e-mail: pmilani@mi.infn.it Several techniques for the synthesis of nanophase tita- nium dioxide have been developed: sol-gel processing [13], chemical vapor deposition [14], hydrolysis process [15], col- loidal chemistry method [16]. In most cases as-prepared nanostructured titanium dioxide films have an amorphous or amorphous-crystalline polytype structure. In order to obtain more stable phases with well-defined crystalline structure the as-deposited samples usually undergo ther- mal treatments. In general, material processing by thermal treatment leads to phase transitions, stoichoimetry varia- tions, oxidation and densification which consequently af- fect the optical, electrical and other physical properties of the material [17–19]. In addition, annealing process signifi- cantly changes morphology and texture of the sample [20]. Here we show that nanocrystalline TiO 2 films can be efficiently produced by low energy supersonic cluster beam deposition (CBD). By exploiting the high intensity and mass separation effects typical of supersonic expansions it is possible to deposit TiO 2 films with controlled nanos- tructure. We show that ultrasmall clusters have a different crystalline structure (anatase or rutile) depending on their dimensions and that it is possible to produce films with controlled nanostructure by selecting the clusters prior to deposition. Analogously, optical band gap of the films can be tuned by selecting precursors with the opportune size. The effects of annealing process on crystal phases and on surface morphology are also presented.