Morphological control of Au and Co Nanostructures by pulsed laser irradiation

Abstract

Because nano-material science is one of the most rapidly growing field, understanding physical properties nano-materials as well as nano-scaled phenomena become very important. Laser and x-ray are the most common tools applied for nano-material science researches. Laser pulses with short pulse duration can induce morphological changes of nano-structures when experiments are performed in variety of liquid or gas media. X-ray is optimal analyzing tools including X-ray diffraction (XRD) which is widely applied for analyzing crystalline structure nano-materials. In this thesis, we would like to talk about three topics related to morphological and structural changes on nano-Co and nano-Au materials by nano-second (ns) laser pulses. The first topic is about formation of CoO nano-walls (NWs) by the reaction of O2-and Co2+ which are produced by laser induced dissociation of water and Co thin film. For 10.5 nm Co thin film, ion exchange resulted CoO NWs formation in the hole region where Co was not abundant. Regions abundant with Co formed CoO nano-particles (NPs) by final result of grooving dewetting process. Reduction of sapphire to Al was also observed. However, thinner Co film (4.8 nm) only produced CoO NWs by full dissociation of Co under water without any spinodal dewetting process. In air condition, irradiation of 10 nm Co thin film finally ended up with Co-CoO core-shell structured NPs. Formation of CoO cap resulted remarkable wetting with contact angle 30.7°. Mean NPs size was increased with number of irradiated pulses. In vacuum condition, 10 nm thick Co film finally ended up with Co NP which had γ-alumina surrounding ridge and thinner (5 nm) amorphous alumina layer than previous sample in air condition. Sapphire substrate surface was momentarily molten by laser pulse induced photothermal heating of Co NPs. Capillary force created 15 nm high ridge around Co NP to make wetting phenomena with contact angle 71.3°. The second topic is about difference of morphological evolution for Au NPs and Co NPs when they are irradiated with ns laser pulses. Faceted Au NPs (100 ~ 450 nm in size) were rounded and fragmented into smaller NPs. Fragmentation was occurred with three stages; decrease of main particle size, growth of surrounding particles, and growth of distant particles with depletion of main and surrounding particles. Fluence as low as 0.22 J/cm2 gave irregular particle shape when less than five pulses were irradiated, whereas specimen irradiated with more than 10 pulses resulted hemi-ellipsoid shape. Prolonged irradiation mainly left small particles less than 20 nm size. Total volume of Au NPs, and the diffraction intensity of the Au (111) Bragg peak were decreased with increasing fluence. Au NPs irradiated with fluence 0.28 J/cm2 or higher had right-shifted bi-component XRD peak, whereas fluence 0.22 J/cm2 or less did not induce any peak shift. On the other hand, irradiated Co NPs went through evaporation process rather than fragmentation. Total volume decrease was observed as well as abrupt depression of XRD signal at first 2-pulse irradiation. Irradiation by more pulses created left-shifted bi-component Bragg peaks. In final topic, we observe the evidence for photothermal explosion of Au NPs and go for the laser irradiation on mono-disperse Au NPs (100 nm and 40 nm size) by ns laser pulses. We hypothesize the correlation between the explosive behavior of Au NPs and optical absorption coefficient ratio calculated by Rayleigh and Mie theory. Irradiation was performed by single pulse with energy 5 ~ 27.5 mJ (62 ~ 340 mJ/cm2). For 100 nm Au NPs, number percentage of exploded particles was increased in the pulse energy range between 10 ~ 20 mJ with mean slope 7.20. For 40 nm Au NPs, it was increased when the pulse energy was higher than 15 mJ with mean slope 2.01. The mean slope or exploded percentage increase ratio between 40 nm and 100 nm Au NPs was approximately 3.58 which fitted into theoretically calculated absorption ratio 3.85. XRD measurement described Au (111) Bragg peak signal for Au NPs irradiated with corressponding laser pulse energies. Higher the laser pulse energy, weaker the integrated intensity. The experimental results provide insight how optical absorption is closely related to photothermal explosion effects.