Laser Ablation Method for Synthesis of Nanomaterials
W. Cao
Skyspring Nanomaterials, Inc.
http://www.ssnano.com, wcao@ssnano.com
Since the discovery of laser decades ago, laser has been intensively used and studied for various applications including laser ablation. Even though the first experimental paper about laser ablation was reported as early as 1963, laser ablation was not employed for synthesizing nanomaterials with the purpose for gas sensing until mid 1990s.
The principle of laser ablation has been described in several papers. Laser ablation means the removal of material from a surface by means of laser irradiation. The term “laser ablation” is used to emphasize the nonequilibrium vapor/plasma conditions created at the surface by intense laser pulse, to distinguish from “laser evaporation,” which is heating and evaporation of material in condition of thermodynamic equilibrium. A typical schematic diagram of laser ablation is shown in the following figure. Briefly, there are two essential parts in the laser ablation device, a pulsed laser (CO2 laser, Nd-YAG laser, ArF excimer laser, or XeCl excimer laser) and an ablation chamber. The high power of the laser beam induces large light absorption on the surface of target, which makes temperature of the absorbing material increase rapidly. As a result, the material on the surface of target vaporizes into laser plume. In some cases, the vaporized materials condensate into cluster and particle without any chemical reaction. In some other cases, the vaporized material reacts with introduced reactants to form new materials. The condensed particle will be either deposited on a substrate or collected through a filter system consisting of a glass fiber mesh. Then, the collected nanoparticle can be coated on a substrate through drop-coating or screen-printing process.
Williams and Coles prepared nanocrystalline SnO2 by a laser ablation technique for detection of CO, H2, and CH4. Their studies revealed that the gaseous atmosphere in which the condensation of the laser-ablated SnO2 occurs has a significant influence on the size of the nanoparticles generated. The use of Ar at the pressure of 1 mbar to replace the standard conditions employed in air at 1 bar led to a decrease in SnO2 grain size to 8 nm. Furthermore, by shortening the laser pulse from the customary 20 ms to 30 ns employinga XeCl excimer laser, a further reduction in the grain size was achieved. Their gas sensors based on nanocrystalline SnO2 powders prepared by laser ablation and gas-phase condensation route offered enhanced sensitivity to CO, H2, and CH4 compared with the materials prepared by conventional methods. Hu and his co-workers prepared nanocrystalline SnO2 thin film using a SnO2 target and a metallic Sn target respectively for C2H5OH detection. Their results demonstrated that the oxidation of Sn into SnO2 depends strongly on the substrate temperature. Oxidation of Sn into SnO2 proceeds mainly on the substrate surface instead of in the ablation plume during the condensation of Sn species onto the substrate, even if the ambient oxygen pressure reaches 100–150 Pa. Recently, Starke and Coles reported their gas sensors prepared using laser ablated nanocrystalline metal oxides. They found that SnO2 and In2O3 are capable of detecting ozone at concentrations well below 100 ppb with response times of less than one minute. Pt doped SnO2 and, particularly, In2O3 show some cross sensitivity to NO and NO2. WO3 shows sensing properties superior to these two materials in terms of selectivity and response time but regrettably does not exhibit such high sensitivity. Their CO sensor is highly sensitive to single-figure ppm concentration with a resolution down to 1 ppm. These studies demonstrate that the laser ablated nanostructured metal oxides can greatly enhance the sensing performance of gas sensors.
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