Browsing by Author "Ali, Saima"
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Item Aluminum oxide/titanium dioxide nanolaminates grown by atomic layer deposition: Growth and mechanical properties(2017-01-01) Ylivaara, Oili M E; Kilpi, Lauri; Liu, Xuwen; Sintonen, Sakari; Ali, Saima; Laitinen, Mikko; Julin, Jaakko; Haimi, Eero; Sajavaara, Timo; Lipsanen, Harri; Hannula, Simo-Pekka; Ronkainen, Helena; Puurunen, Riikka; Department of Chemistry and Materials Science; Department of Materials Science and Engineering; Department of Micro and Nanosciences; Department of Chemical and Metallurgical Engineering; Department of Electronics and Nanoengineering; Harri Lipsanen Group; VTT Technical Research Centre of Finland; University of Jyväskylä; Department of Materials Science and EngineeringAtomic layer deposition (ALD) is based on self-limiting surface reactions. This and cyclic process enable the growth of conformal thin films with precise thickness control and sharp interfaces. A multilayered thin film, which is nanolaminate, can be grown using ALD with tuneable electrical and optical properties to be exploited, for example, in the microelectromechanical systems. In this work, the tunability of the residual stress, adhesion, and mechanical properties of the ALD nanolaminates composed of aluminum oxide (Al2O3) and titanium dioxide (TiO2) films on silicon were explored as a function of growth temperature (110-300 °C), film thickness (20-300 nm), bilayer thickness (0.1-100 nm), and TiO2 content (0%-100%). Al2O3 was grown from Me3Al and H2O, and TiO2 from TiCl4 and H2O. According to wafer curvature measurements, Al2O3/TiO2 nanolaminates were under tensile stress; bilayer thickness and growth temperature were the major parameters affecting the stress; the residual stress decreased with increasing bilayer thickness and ALD temperature. Hardness increased with increasing ALD temperature and decreased with increasing TiO2 fraction. Contact modulus remained approximately stable. The adhesion of the nanolaminate film was good on silicon.Item Characterization of atomic layer deposition films by x-ray scattering and atomic force microscopy(2012) Ali, Saima; Sintonen, Sakari; Mikro- ja nanotekniikan laitos; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Lipsanen, HarriAtomic Layer Deposition (ALD) is a thin film deposition technique that has received a lot of attention in many fields of science and industry due to its sequential, self-limiting reactions. Although it is slow method, it allows uniform, high density films with conformal depositions on 3D high aspect ratio structures. ALD allows thickness control of the film down to sub nanometre range. For more understanding and future applications, certain properties like accurate thickness, density and roughness of film is needed to get characterized. In this work ALD deposited aluminium oxide, titanium dioxide, aluminium nitride and a nanolaminate sample are characterized by X-ray Diffraction (XRD), X-ray Reflectivity (XRR) and Atomic Force Microscopy (AFM) methods. The density, thickness and roughness of films are measured by XRR method. Grazing Incidence X-ray Diffraction (GIXRD) is used to examine the crystal nature of the deposited films. There were no peaks found for the amorphous films. The crystallite size and phases of crystalline films are determined by this method. AFM technique was used to see the morphology of the films. AFM gives the roughness of the films as well which was giving quiet similar results to that of roughness values found by XRR. These results give a better understanding of ALD films deposited at different temperature conditions and with different thickness values.Item Control of the size of silver nanoparticles and release of silver in heat treated SiO2-Ag composite powders(2018-01-05) Granbohm, Henrika; Larismaa, Juha; Ali, Saima; Johansson, Leena Sisko; Hannula, Simo Pekka; Department of Chemistry and Materials Science; Department of Bioproducts and Biosystems; Aalto Nanofab; Advanced and functional Materials; Bio-based Colloids and MaterialsThe growth of silver nanoparticles, the activation energy for silver particle growth, and the release of silver species in heat treated SiO2-Ag composite powders are investigated. The silver particle growth is controlled by heat treatment for 75 min of the as-synthesized SiO2-Ag composite powder at 300-800 °C. During heat treatment the mean size of the Ag particles increases from 10 nm up to 61 nm with increasing temperature, however, the particle size distribution widens and the mean size increases with increasing heat treatment temperature. Based on X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) studies, silver particles are crystalline and in a metallic state after annealing in all SiO2-Ag composite powders. The growth of Ag particles is suggested to take place via diffusion and Ostwald ripening. The activation energy for particle growth was determined as 0.14 eV. The dissolution of silver in aqueous solutions from the SiO2-Ag composites heat treated, at 300 °C, 600 °C, and 700 °C, was investigated by varying pH and temperature. The dissolution was reduced in all conditions with increasing silver particle size, i.e., when the total surface area of Ag particles is reduced. It is suggested that the dissolution of silver from the composite powders can conveniently be adjusted by controlling the Ag particle size by the heat treatment of the composite powder.Item Effect of Morphology and Crystal Structure on the Thermal Conductivity of Titania Nanotubes(2018-01-01) Ali, Saima; Orell, Olli; Kanerva, Mikko; Hannula, Simo Pekka; Department of Chemistry and Materials Science; Tampere University of TechnologyTitania nanotubes (TNTs) with different morphology and crystal structure are prepared by chemical processing and rapid breakdown anodization (RBA) methods. The nanotubes are studied in terms of thermal conductivity. The TNTs with variable wall thickness below 30 nm have significantly reduced thermal conductivity than bulk titania, due to the phonon confinement, smaller phonon mean free path, and enhanced phonon boundary scattering. The amorphous nanotubes (TNTAmor) have comparatively thicker walls than both crystalline nanotubes. The TNTAmor has a thermal conductivity of 0.98 W m−1 K−1, which is slightly less than the thermal conductivity of crystalline anatase nanotubes (TNTA; 1.07 W m−1 K−1). However, the titania nanotubes with mixed structure (TNTA,T) and the smallest dimensions have the lowest thermal conductivity of 0.75 W m−1 K−1, probably due to the phonon confinement. The experimental results are compared with the theoretical study considering the size confinement effect with different wall dimensionsof TNTs and surface scattering. The results agree well with the surface roughness factor (p) of 0.26 for TNTA,T, 0.18 for TNTA, and 0.65 for TNTAmor, indicating diffusive phonon scattering and rougher surfaces for TNTA. Interestingly, the present results together with those presented in literature suggest that thermal conductivity reduction with respect to the wall thickness occurs also for the amorphous nanotubes. This is ascribed to the role of propagons in the thermal transport of disordered structures.Item Mechanical and optical properties of as-grown and thermally annealed titanium dioxide from titanium tetrachloride and water by atomic layer deposition(Elsevier Science, 2021-08-31) Ylivaara, Oili M.E.; Langner, Andreas; Liu, Xuwen; Schneider, Dieter; Julin, Jaakko; Arstila, Kai; Sintonen, Sakari; Ali, Saima; Lipsanen, Harri; Sajavaara, Timo; Hannula, Simo-Pekka; Puurunen, Riikka L.; Department of Chemical and Metallurgical Engineering; Department of Electronics and Nanoengineering; Department of Chemistry and Materials Science; Harri Lipsanen Group; Advanced and functional Materials; Catalysis; VTT Technical Research Centre of Finland; Fraunhofer Institute for Material and Beam Technology; University of JyväskyläThe use of thin-films made by atomic layer deposition (ALD) is increasing in the field of optical sensing. ALD TiO2 has been widely characterized for its physical and optical properties, but systematic information about the influence of thermal history to optical and mechanical properties of the film is lacking. Optical applications require planar surface and tunability of the refractive index and residual stress. In addition, mechanical properties such as elastic modulus and film hardness influence the performance of the layer, especially, when optics is integrated with microelectromechanical systems. In this work, optical properties, density, elemental analysis, residual stress, elastic modulus and hardness of as-grown ALD TiO2 thin films on silicon were studied at temperature range from 80 to 350 °C and influence of post-ALD thermal annealing was studied on films annealed up to 900 °C. ALD TiO2 films were under tensile stress in the scale of hundreds of MPa. The stress depended both on the ALD temperature and film thickness in a complex way, and onset of crystallization increased the residual stress. Films grown at 110 and 300 °C were able to withstand post-ALD annealing at 420 °C without major change in residual stress, refractive index or extinction coefficient. Elastic modulus and hardness increased upon crystallization with increasing ALD temperature. The results presented here help to improve the design of the optical devices by choosing films with desired optical properties, and further help to design the post-ALD thermal budget so that films maintain their desired features.Item Processing and properties of titania nanostructures(Aalto University, 2018) Ali, Saima; Kemian ja materiaalitieteen laitos; Department of Chemistry and Materials Science; Advanced and Functional Materials; Kemian tekniikan korkeakoulu; School of Chemical Technology; Hannula, Simo-Pekka, Prof., Aalto University, Department of Chemistry and Materials Science, FinlandProperties of nanostructured materials differ vastly from those of bulk materials and modifications of the nanostructures may be used to develop novel functional materials with unique properties. This thesis focuses on the preparation of titania nanostructures with different crystal structures and morphologies to investigate their thermal conductivity and photocatalytic properties.Template-free synthesis methods, such as chemical processing and rapid breakdown anodization (RBA), have been used for the preparation of titania nanotubes (TNTs) and the nanolaminate thin films have been deposited by atomic layer deposition. Thermal conductivity of the nanostructures with different dimensions, crystallinity and phase structure is investigated. Both as-prepared and annealed TNTs are also tested for the photocatalytic degradation of organic pollutants using model dyes. The TNTs synthesized by chemical processing are multiwalled, open-ended, and have a wall thickness of 4-5 nm with mixed anatase/titanate crystal structure, while the TNTs prepared by RBA are single-walled with one end open and the other end closed. Amorphous TNTs with a wall thickness of 15-30 nm are obtained using an organic electrolyte and crystalline TNTs with a wall thickness of 7-12 nm are prepared by an aqueous electrolyte. When annealed at higher temperatures the TNTs diffuse to nanorods with a modified crystal structure and chemical composition. The wall thickness is seen to have a clear influence on the thermal conductivity of the crystalline TNTs, which is reduced by decreasing the wall thickness. The thermal conductivity of amorphous TNTs is slightly lower than that of the crystalline nanotube and comparison with the literature values reveal the impact of wall dimensions on the net thermal conductivity, also in case of amorphous TNTs. The thermal conductivity of amorphous Al2O3/TiO2 nanolaminates is lower than that of titania thin films. It is found that the thermal conductivity decreases by increasing the interface density, revealing the influence of non-negligible Kapitza resistance on the overall thermal conductivity in amorphous nanolaminates. Of the chemically processed TNTs, the as-synthesized TNTs are the most efficient catalysts under the UV radiation due to a higher specific surface area and a large number of hydroxyl groups on the surface. However, the TNTs prepared by RBA aqueous electrolyte show a complete decolorization of dyes under the solar irradiation. As-prepared TNTs and TNTs annealed at 250 and 450 oC are found to be the most efficient catalysts. The number of reactive surface sites, band gap, specific surface area, photocatalytic mechanism and crystal structure of the TNTs are all seen to influence the overall photocatalytic efficiency. The findings presented in the thesis also support the understanding of thermal properties of titania nanostructures for number of potential applications.Item Review Article : Recommended reading list of early publications on atomic layer deposition - Outcome of the "Virtual Project on the History of ALD"(2017-01-01) Ahvenniemi, Esko; Akbashev, Andrew R.; Ali, Saima; Bechelany, Mikhael; Berdova, Maria; Boyadjiev, Stefan; Cameron, David C.; Chen, Rong; Chubarov, Mikhail; Cremers, Veronique; Devi, Anjana; Drozd, Viktor; Elnikova, Liliya; Gottardi, Gloria; Grigoras, Kestutis; Hausmann, Dennis M.; Hwang, Cheol Seong; Jen, Shih Hui; Kallio, Tanja; Kanervo, Jaana; Khmelnitskiy, Ivan; Kim, Do Han; Klibanov, Lev; Koshtyal, Yury; Krause, A. Outi I.; Kuhs, Jakob; Kärkkänen, Irina; Kääriäinen, Marja Leena; Kääriäinen, Tommi; Lamagna, Luca; Łapicki, Adam A.; Leskelä, Markku; Lipsanen, Harri; Lyytinen, Jussi; Malkov, Anatoly; Malygin, Anatoly; Mennad, Abdelkader; Militzer, Christian; Molarius, Jyrki; Norek, Małgorzata; Özgit-Akgün, Çaǧla; Panov, Mikhail; Pedersen, Henrik; Piallat, Fabien; Popov, Georgi; Puurunen, Riikka L.; Rampelberg, Geert; Ras, Robin H A; Rauwel, Erwan; Roozeboom, Fred; Sajavaara, Timo; Salami, Hossein; Savin, Hele; Schneider, Nathanaelle; Seidel, Thomas E.; Sundqvist, Jonas; Suyatin, Dmitry B.; Törndahl, Tobias; Van Ommen, J. Ruud; Wiemer, Claudia; Ylivaara, Oili M E; Yurkevich, Oksana; Department of Chemistry and Materials Science; Department of Electronics and Nanoengineering; School services, CHEM; School common, CHEM; Department of Chemistry; Department of Materials Science and Engineering; Department of Micro and Nanosciences; Department of Chemical and Metallurgical Engineering; Department of Applied Physics; Soft Matter and Wetting; Harri Lipsanen Group; Hele Savin Group; Stanford University; Centre National de la Recherche Scientifique (CNRS); Bulgarian Academy of Sciences; Masaryk University; Huazhong University of Science and Technology; Université Grenoble Alpes; Ghent University; Ruhr University Bochum; St. Petersburg State University; Alikhanov Institute for Theoretical and Experimental Physics; Fondazione Bruno Kessler; Lam Research Corporation; Seoul National University; Global Foundries, Inc.; St. Petersburg Electrotechnical University; Massachusetts Institute of Technology; Techinsights; Ioffe Institute; Sentech Instruments GmbH; NovaldMedical Ltd Oy; STMicroelectronics; Seagate Technology; University of Helsinki; St. Petersburg State Institute of Technology; Renewable Energy Development Center; Chemnitz University of Technology; Summa Semiconductor Oy; Military University of Technology Warsaw; ASELSAN Inc.; Linköping University; KOBUS; Tallinn University of Technology; Netherlands Organisation for Applied Scientific Research; University of Jyväskylä; University of Maryland, College Park; Institut de recherche et développement sur l’énergie photovoltaïque; Seitek50; Fraunhofer Institute for Ceramic Technologies and Systems; Lund University; Uppsala University; Delft University of Technology; National Research Council of Italy; Immanuel Kant Baltic Federal University; University of Twente; VTT Technical Research Centre of FinlandAtomic layer deposition (ALD), a gas-phase thin film deposition technique based on repeated, self-terminating gas-solid reactions, has become the method of choice in semiconductor manufacturing and many other technological areas for depositing thin conformal inorganic material layers for various applications. ALD has been discovered and developed independently, at least twice, under different names: atomic layer epitaxy (ALE) and molecular layering. ALE, dating back to 1974 in Finland, has been commonly known as the origin of ALD, while work done since the 1960s in the Soviet Union under the name "molecular layering" (and sometimes other names) has remained much less known. The virtual project on the history of ALD (VPHA) is a volunteer-based effort with open participation, set up to make the early days of ALD more transparent. In VPHA, started in July 2013, the target is to list, read and comment on all early ALD academic and patent literature up to 1986. VPHA has resulted in two essays and several presentations at international conferences. This paper, based on a poster presentation at the 16th International Conference on Atomic Layer Deposition in Dublin, Ireland, 2016, presents a recommended reading list of early ALD publications, created collectively by the VPHA participants through voting. The list contains 22 publications from Finland, Japan, Soviet Union, United Kingdom, and United States. Up to now, a balanced overview regarding the early history of ALD has been missing; the current list is an attempt to remedy this deficiency.Item Thermomechanical properties of aluminum oxide thin films made by atomic layer deposition(American Institute of Physics, 2022-12-01) Ylivaara, Oili M.E.; Langner, Andreas; Ek, Satu; Malm, Jari; Julin, Jaakko; Laitinen, Mikko; Ali, Saima; Sintonen, Sakari; Lipsanen, Harri; Sajavaara, Timo; Puurunen, Riikka L.; Department of Electronics and Nanoengineering; Department of Chemical and Metallurgical Engineering; Harri Lipsanen Group; Catalysis; VTT Technical Research Centre of Finland; Picosun Oy; University of JyväskyläIn microelectromechanical system devices, thin films experience thermal processing at temperatures some cases exceeding the growth or deposition temperature of the film. In the case of the thin film grown by atomic layer deposition (ALD) at relatively low temperatures, post-ALD thermal processing or high device operation temperature might cause performance issues at device level or even device failure. In this work, residual stress and the role of intrinsic stress in ALD Al2O3 films grown from Me3Al and H2O, O3, or O2 (plasma ALD) were studied via post-ALD thermal processing. Thermal expansion coefficient was determined using thermal cycling and the double substrate method. For some samples, post-ALD thermal annealing was done in nitrogen at 300, 450, 700, or 900 °C. Selected samples were also studied for crystallinity, composition, and optical properties. Samples that were thermally annealed at 900 °C had increased residual stress value (1400-1600 MPa) upon formation of denser Al2O3 phase. The thermal expansion coefficient varied somewhat between Al2O3 made using different oxygen precursors. For thermal-Al2O3, intrinsic stress decreased with increasing growth temperature. ALD Al2O3 grown with plasma process had the lowest intrinsic stress. The results show that ALD Al2O3 grown at 200 and 300 °C is suitable for applications, where films are exposed to post-ALD thermal processing even at temperature of 700 °C without a major change in optical properties or residual stress.Item Titania nanotubes prepared by rapid breakdown anodization for photocatalytic decolorization of organic dyes under UV and natural solar light(2018-06-14) Ali, Saima; Granbohm, Henrika; Lahtinen, Jouko; Hannula, Simo-Pekka; Department of Chemistry and Materials Science; Surface Science; Advanced and functional Materials; Department of Applied PhysicsTitania nanotube (TNT) powder was prepared by rapid breakdown anodization (RBA) in a perchloric acid electrolyte. The photocatalytic efficiency of the as-prepared and powders annealed at temperatures between 250 and 550 °C was tested under UV and natural sunlight irradiation by decolorization of both anionic and cationic organic dyes, i.e., methyl orange (MO) and rhodamine B (RhB), as model pollutants. The tubular structure of the nanotubes was retained up to 250 °C, while at 350 °C and above, the nanotubes transformed into nanorods and nanoparticles. Depending on the annealing temperature, the TNTs consist of anatase, mixed anatase/brookite, or anatase/rutile phases. The bandgap of the as-prepared nanotubes is 3.04 eV, and it shifts towards the visible light region upon annealing. The X-ray photoelectron spectroscopy (XPS) results show the presence of titania and impurities including chlorine on the surface of the TNTs. The atomic ratio of Ti/O remains unchanged for the annealed TNTs, but the concentration of chlorine decreases with temperature. The photoluminescence (PL) indicate high electron-hole recombination for the as-prepared TNTs, probably due to the residual impurities, low crystallinity, and vacancies in the structure, while the highest photocurrent was observed for the TNT sample annealed at 450 °C. The TNTs induce a small degradation of the dyes under UV light; however, contrary to previous reports, complete decolorization of dyes is observed under sunlight. All TNT samples showed higher decolorization rates under sunlight irradiation than under UV light. The highest reaction rate for the TNT samples was obtained for the as-prepared TNT powder sample under sunlight using RhB (κ1 = 1.29 h−1). This is attributed to the bandgap, specific surface area and the crystal structure of the nanotubes. The as-prepared TNTs performed most efficiently for decolorization of RhB and outperformed the reference anatase powder under sunlight irradiation. This could be attributed to the abundance of reactive sites, higher specific surface area, and degradation mechanism of RhB. These RBA TNT photocatalyst powders demonstrate a more efficient use of the sunlight spectrum, making them viable for environmental remediation.Item X-ray reflectivity characterization of atomic layer deposition Al2O3/TiO2 nanolaminates with ultrathin bilayers(American Vacuum Society, 2014) Sintonen, Sakari; Ali, Saima; Ylivaara, Oili M. E.; Puurunen, Riikka L.; Lipsanen, Harri; Mikro- ja nanotekniikan laitos; Department of Micro and Nanosciences; Sähkötekniikan korkeakoulu; School of Electrical EngineeringNanolaminate structures have many prospective uses in mechanical, electrical, and optical applications due to the wide selection of materials and precise control over layer thicknesses. In this work, ultrathin Al2O3/TiO2 nanolaminate structures deposited by atomic layer deposition from Me3Al, TiCl4, and H2O precursors with intended bilayer thicknesses ranging from 0.1 to 50 nm were characterized by x-ray reflectivity (XRR) measurements. The measurements were simulated to obtain values for thickness, density, and roughness of constituting layers. XRR analysis shows that the individual layers within the nanolaminate remain discrete for bilayers as thin as 0.8 nm. Further reduction in bilayer thickness produces a composite of the two materials.
