Acknowledgments ReferencesIOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICSJ. Phys. D: Appl. Phys. 41 (2008) 162001 (3pp) doi:10.1088/0022-3727/41/16/162001FAST TRACK COMMUNICATIONNano-structured TiO2film fabricated atroom temperature and its acousticpropertiesJie Zhu1, Wenwu Cao1, Bei Jiang1, D S Zhang2, H Zheng3, Q Zhou3andK K Shung31Material Research Institute, The Pennsylvania State University, University Park, PA 16802, USA2Chemat Technology, Inc., Northridge, CA 91324, USA3Department of Biomedical Engineering, NIH Resource on Medical Ultrasonic Transducer TechnologyUniversity of Southern California, Los Angeles, CA 90089, USAE-mail: [email protected] 29 April 2008, in final form 27 June 2008Published 24 July 2008Onlineatstacks.iop.org/JPhysD/41/162001AbstractNano-structured TiO2thin film has been successfully fabricated at room temperature. Using aquarter wavelength characterization method, we have measured the acoustic impedance of thisporous film, which can be adjusted from 5.3 to 7.19 Mrayl by curing it at differenttemperatures. The uniform microstructure and easy fabrication at room temperature make thismaterial an excellent candidate for matching layers of ultra-high frequency ultrasonic imagingtransducers.(Some figures in this article are in colour only in the electronic version)Matching layers are crucial components of ultrasonic probesfor medical imaging. Without proper matching layers,large acoustic impedance mismatch between piezoelectrictransducer and the imaging body will result in poor imagingquality due to reflection at the interface [1]. For singlefrequency transducers, such reflection greatly reduces thepower efficiency and the sensitivity of the transducer. Fora broadband transducer, such reflected signals could producea long ringdown if the backing does not absorb the reflectedenergy so that the resolution will be greatly reduced. For asingle frequency wave propagating in an infinite medium, aquarter wavelength matching layer in between the piezoelectricand imaging medium can resolve this problem. Theoretically,if the piezoelectric is also infinite or in the case when thefinite transducer has a lossy backing, the acoustic impedance(Z = v × ρ) of the matching layer is the geometric mean ofthe piezoelectric material (Zc) and the imaging medium (Zm),i.e. Z =√ZcZm. For air backed transducers, the optimizedacoustic impedance of the matching layer for a broadbandtransducer is Z = Z1/3cZ2/3m[2, 3]. When two matching layersare used, one of them needs to have a lower acoustic impedancethan the geometric mean, while the other needs to have a higheracoustic impedance than the geometric mean. In general,the acoustic impedance of matching layers is in the range of3–15 Mrayl. Unfortunately, there are no natural materials thatcan meet such requirements, therefore, solid particle/polymercomposites are commonly used as matching layer materialsin practice [4]. For ultra-high frequency transducers (centrefrequency >50 MHz), the thickness of the quarter wavelengthmatching layers is less than 10 µm. This demands that theparticle size in the composite be much less than 0.5 µm in orderfor the composite to behave like a single phase material withminimal wave scattering. High acoustic impedance requireshigher volume loading of solid particles, but it is impossibleto load high volume ultra-fine powders into the polymerwithout introducing air bubbles. Therefore, all ultra-highfrequency transducers currently used or under developmentuse unfilled polymers as matching layer because of the lackof desired matching layer material [5]. This problem hindersthe development of ultra-high frequency ultrasonic imaging0022-3727/08/162001+03$30.00 1 © 2008 IOP Publishing Ltd Printed in the UKJ. Phys. D: Appl. Phys. 41 (2008) 162001 Fast Track Communicationtechnology. In a previous work, we reported the developmentof SiO2nano-composite using the sol–gel technique [6], whichcan be made to have good uniformity and adjustable acousticimpedance between 2.2 and 5.7 Mrayl. Although a big stepforward, the acoustic impedance of the SiO2nano-compositeis still not high enough, particularly for broadband transducersfor which the acoustic impedance of one of the matchinglayers is more than 8 Mrayl. In addition, the sol–gel film isdifficult to lift off the substrate and the spin-coating method isnot very convenient to use. It would be desirable to simplifythe fabrication process and lower the processing temperatureso that the matching layers can be directly coated onto thetransducer without causing depoling of the piezoelectric piece.Nano-size titanium dioxide (TiO2) is one of the mostimportant materials being studied in recent years byresearchers for many practical applications [7, 8]. However,there were no reports on its elastic properties or acousticproperties to date. Compared with SiO2, the acousticimpedance of TiO2is more than twice (∼20 Mrayl); therefore,composites made of TiO2nano-particles should have a higheracoustic impedance than SiO2nano-composites.From research on dye-sensitized solar cells [9], we foundthat TiO2nano-particles can be made into porous nanostructureby adding a small amount of TiiVtetraisopropoxide (TTIP)diluted with ethanol solution. The TiO2nano-particles werepurchased from Degussa, Germany, which contains 30% rutileand 70% anatase. TiO2: TTIP : ethanol are mixed at the molarratio of 1.0 : 0.05 : 6.0. The mixture paste was dispersedwell using an ultrasonic horn (UPS400, Hielscer-UltrasonicTechnology) for 30 min. The mixture paste was coated ontoa glass plate substrate by the doctor-blade technique. Thethickness of the film is about 8 µm. The ethanol evaporatedat room temperature and the TTIP formed amorphous TiO2tobond the TiO2nano-particles together. The amorphous TiO2bonds can crystallize if the sample is annealed at a temperature>400◦C. However, for matching layer applications, theacoustic impedance is the key parameter while crystallizationis not necessary.The fabricated films have a similar morphology tothose fabricated by the traditional method at much highertemperatures [9]. From the micrographs of scanningelectron microscopy (SEM) shown in figure 1, homogeneouslydistributed TiO2particles are well bonded together with theparticle size distribution mainly in the range 25–40 nm. Theporous structure is uniform in all the three dimensions.In order to precisely measure the acoustic impedanceof such thin films, the through-transmission