Functional Metal Oxide Materials Deposited by Inkjet Printing Technique

Abstract: This thesis presents a comprehensive study of the intrinsic room temperature ferromagnetism in 85 to 100nm thin films produced by in situ deposition of industrially important pristine MgO and ZnO by inkjet printing. It is suggested that the observed long range magnetic order, the so-called do magnetism in these oxides arises from cation vacancies. Wide range scanned XPS studies rule out any other source but of intrinsic nature in this first report on inkjetted oxide films. (Accepted for publication in APL 2010; and JPC Letters 2010)The functional properties of the above oxide films have been exploited by inkjet patterning devices based on UV-sensing, photoconductivity and field-effect transistors with attractive features. Development of devices and prototypes is an area of growing importance among future technologies. Complementary to photolithography, inkjet printing is increasingly considered a cost-effective and flexible microfabrication method to structure functional materials. The ease of mass fabrication and the inherent flexibility of inkjet technology make it a suitable method for the manufacture of microsystems and components. The results presented in this dissertation summarize recent achievements in this relatively new technology for the development of miniaturized devices:1)      RTFM in pure and Mn-doped ZnO thin films: (IEEE Trans. Magnetics 2010) Tailored RTFM in Mn-doped ZnO thin films synthesized by inkjet printing has been realized. Highly c-axis oriented 80 to 400 nm thin films are obtained with a magnetic moment as large as 2.1 ?B/Mn2+. X-ray absorption and emission spectra measurements at the O K edge and the Mn L edge suggest strong p-d hybridization between the Mn2+ and O2-. Furthermore, Mn L edge XAS indicates the emergence of Mn3+/Mn4+ mixed valence states for films annealed above 500 °C that leads to suppression of ferromagnetic ordering.2)      RTFM in MgO thin films (JPC Letters 2010) Solution processed homogeneous (200) oriented MgO ~85 nm thin films show room temperature ferromagnetism with a saturation magnetization MS as high as ~0.63 emu/g. X-ray photoelectron spectroscopy investigations show the absence of any contamination while the Mg 2p, and O 1s spectra indicate the role of defect structure at the Mg site. By controlling the pH values of the precursors the concentration of the defects can be varied and hence to tune the magnetization.3)      Photoconductivity of pure and Al-doped ZnO thin films: (2009 MRS proceedings). Pure and Al-doped ZnO, 120-300 nm thin films are found to exhibit a transmittance above 85-90% in the visible wavelength range. The electrical conductivity of Al-doped ZnO thin films is found to be larger by two orders of magnitude than that for pure ZnO films while the photoconductivity is found to increase by about three orders of magnitude under UV irradiation.4)      The photosensitivity of CaS-composited ZnO thin films: (JPC Letters 2010) We have engineered a 3 orders of magnitude enhancement of the ultraviolet photoresponse of ZnO thin films, fabricated by inkjet printing, and then capped with CdS nanoparticles by dip coating. As a consequence, the decay time of the photoresponse is reduced to about 4 ms. Capping with CdS not only suppresses the detrimental passivation layer of ZnO thin films, but also generates an interfacial carrier transport layer to reduce the probability of carrier recombination.5)      Indium-doped Zinc oxide field effect transistors: (Materials Letters 2010) In darkness the In-doped zinc oxide field effect transistor (IZO-FET) exhibits a saturation current level of about 10 ?A, an incremental mobility as high as 8 cm2 V-1 s-1, and a current on/off ratio of 104 ~105. When illuminated by 363 nm, 1.7 mW/cm2 UV light, the IZO-FET displays a photocurrent of 2 mA, and a darkness current of ~20 nA at an optimized gate voltage of -2V. The device is effectively turned on in about 5 ms and off in 10 ms.?