Nanostructured materials laser ablation

The last problem of this series concerns femtosecond laser ablation from gold nanoparticles [87]. In this process, solid material transforms into a volatile phase initiated by rapid deposition of energy. This ablation is nonthermal in nature. Material ejection is induced by the enhancement of the electric field close to the curved nanoparticle surface. This ablation is achievable for laser excitation powers far below the onset of general catastrophic material deterioration, such as plasma formation or laser-induced explosive boiling. Anisotropy in the ablation pattern was observed. It coincides with a reduction of the surface barrier from water vaporization and particle melting. This effect limits any high-power manipulation of nanostructured surfaces such as surface-enhanced Raman measurements or plasmonics with femtosecond pulses. 

Further modification of the above nanostructures is useful for obtaining new functional materials. Thirdly, we apply the dopant-induced laser ablation technique to site-selectively doped thin diblock copolymer films with spheres (sea-island), cylinders (hole-network), and wormlike structures on the nanoscale [19, 20]. When the dye-doped component parts are ablated away by laser light, the films are modified selectively. Concerning the laser ablation of diblock copolymer films, Lengl et al. carried out the excimer laser ablation of diblock copolymer monolayer films, forming spherical micelles loaded with an Au salt to obtain metallic Au nanodots [21]. They used the laser ablation to remove the polymer matrix. In our experiment, however, the laser ablation is used to remove one component of block copolymers. Thereby, we can expect to obtain new functional materials with novel nanostmctures. 

As aforementioned, diblock copolymer films have a wide variety of nanosized microphase separation structures such as spheres, cylinders, and lamellae. As described in the above subsection, photofunctional chromophores were able to be doped site-selectively into the nanoscale microdomain structures of the diblock copolymer films, resulting in nanoscale surface morphological change of the doped films. The further modification of the nanostructures is useful for obtaining new functional materials. Hence, in order to create further surface morphological change of the nanoscale microdomain structures, dopant-induced laser ablation is applied to the site-selectively doped diblock polymer films. 

The top-down approach starts with a bulk material and attempts to break it down into nanoscaled materials through physical methods. Hence, most of these techniques are really forms of fabrication rather than synthesis. For nanostructured bulk phases, including powders, the common methods are milling, devitrification of metallic glass, and severe plastic deformation. For nanocrystalline thin films (films with nanosized crystallites), methods include thermal vaporization (under high vacuum), laser ablation, and sputtering (thermal plasma), all of which were... 

Rare earth silicates exhibit potential applications as stable luminescent materials for phosphors, scintillators, and detectors. Silica and silicon substrates are frequently used for thin films fabrication, and their nanostructures including monodisperse sphere, NWs are also reliable templates and substrates. However, the composition, structure, and phase of rare earth silicates are rather complex, for example, there are many phases like silicate R2SiOs, disilicate R2Si207 (A-type, tetragonal), hexagonal Rx(Si04)602 oxyapatite, etc. The controlled synthesis of single-phase rare earth silicate nanomateriais can only be reached with precisely controlled experimental conditions. A number of heat treatment based routes, such as solid state reaction of rare earth oxides with silica/silicon substrate, sol-gel methods, and combustion method, as well as physical routes like pulsed laser ablation, have been applied to prepare various rare earth silicate powders and films. The optical properties of rare earth silicate nanocrystalline films and powders have been studied. 

Unfortunately, current S3mthesis techniques, such as chemical vapor deposition, arc discharge, laser ablation, or detonation, typically lead to a mixture of various nanostructures, amorphous carbon, and catalyst particles rather than a particular nanostructure with defined properties, thus limiting the number of potential applications [1]. Even if pure materials were available, the size-dependence of most nanomaterial properties requires a high structural selectivity. In order to fully exploit the great potential of carbon nanostmctures, one needs to provide purification procedures that allow for a selective separation of carbon nanostructures, and methods which enable size control and surface functionalization. 

Laser ablation has been broadly applied and developed for the synthesis of diverse nanomaterials [21,22], In this approach, an incident laser pulse penetrates into the surface of the material within a certain penetration depth. Electrons are removed from the bulk and the irradiated surface is then heated up and vaporized. At a high enough laser flux, the material is converted to plasma. Consequently, the large pressure difference between the laser produced initial seed plasma and ambient atmosphere causes a rapid expansion of the plasma plume and then it cools down. The plasma species will nucleate and grow into desirable nanostructures, either on a substrate or in a cool Hquid medium [21]. 

Nanophase materials in which the average grain size, phases or crystallites are in the nanometer regime have recently been the focus of intense research effort (1-3). This interest has developed due to their superior properties compared to conventional materials which have particle sizes on the order of a micron (4-5). Nanostructured materials have traditionally been prepared by a variety of techniques which include physical methods such as gas-phase condensation, metal evaporation, spray pyrolysis, laser ablation and plasma synthesis (6-12). Chemical methods to synthesize such materials have frequently been used due to the better control of the stoichiometry in the end-product, the molecular level mixing of the constituent phases and the feasibility of low cost bulk production of these materials. Various chemical... 

Novel ideas have been proposed in laser ablation of materials to generate nanoparticles used in nanoelectronics, production of polymer semiconductor composites for development of non-linear optics for waveguides, molecular and nanostructure self-assembly techniques, high-performance catalysts, control of nanoparticles resulted from combustion and plasma processes, and special sensors applied in chemical plants and the environment. 

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