Browsing by Author "Karppinen, Maarit, Academy Professor, Aalto University, Department of Chemistry, Finland"

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Surface Functionalization by Atomic Layer Deposited Binary Oxide Thin Films
(2013) Malm, Jari
School of Chemical Technology | Doctoral dissertation (article-based)
The materials of today have intriguing properties. The mastering of phenomena at the nanometer range often forms the basis for the understanding of novel materials and their functional properties. In this thesis three materials - zinc oxide (ZnO), titanium dioxide (TiO2) and tungsten trioxide (WO3) - in the form of thin films less than 100 nanometers in thickness are being studied from the surface functionalization point of view. The film application method used was atomic layer deposition (ALD). First, the low-temperature deposition features of ZnO were studied. The deposition could be performed at as low as room temperature. Below 70 ºC the hexagonal wurzite structure of ZnO oriented along the c axis, and above this temperature orientation along the a axis was also observed. The crystallinity improved in post-deposition annealings at 400–600 ºC in argon and oxygen atmospheres while keeping the original preferential orientation unchanged. ZnO was also used to study the tailoring of a natural template, the wing surface of the cicada (Pomponia Intermedia) insect. The nanoscale pillar structure of the cicada wing could be area-selectively coated with a 100 nm thick ZnO film in the interstitial space between the pillars. By applying a thin aluminum oxide seed layer prior to ZnO deposition the wing nanostructure could be uniformly coated. Furthermore, the water wettability of the cicada wing was studied. The originally superhydrophobic wing surface was coated with ZnO while keeping the original superhydrophobicity almost intact. When exposed to ultraviolet (UV) light the surface was successfully turned hydrophilic and back to hydrophobic under storage in dark. The tunable wettability phenomenon was used to demonstrate directing fluid flows on planar, ZnO-coated quartz surfaces utilizing hydrophilic patterns irradiated on a hydrophobic surface by UV laser. The reversible patterning may find use in microfluidic and lab-on-a-chip devices. TiO2 is a common ALD film material capable of being deposited at low temperatures. Here its possibilities were demonstrated by depositing a TiO2 layer on nanofibrillated cellulose (NFC) template, and after removal of the template using the formed TiO2 nanotube network in a dropcast form as a humidity sensor. The usable relative humidity range was 40–80 % where the dropcast sensor layer gave repeatable resistive and capacitive response. Finally a new ALD process of WO3 films from W(CO)6 and O3 was introduced. The ALD temperature window was observed at 195–205 ºC with a deposition rate of 0.23 nm/cycle. This process provides a straightforward option for the ALD of WO3 as opposed to the methods based on in situ generated oxyfluoride intermediates published earlier.