Laser ablation process

Molecular Studies on Laser Ablation Processes of Polymeric Materials by Time-Resolved Luminescence Spectroscopy... 

In 1998, U.S. Department of Energy (DOE) researchers estimated that using the F2 Associates, Inc., laser ablation process would cost 9.92/ft for a mobile robotic unit, and 6.77/tf if a hand-held unit was used. This estimate was for the D D of paint 1 mil thick (D189031, p. vii). This compared favorably with conventional D D technologies. Details of this estimate are summarized in Table 1. 

Figure 2.13 b) Schematic diagram of photon-solid interaction by the laser ablation process. 

Recently, the VLS growth method has been extended beyond the gas-phase reaction to synthesis of Si nanowires in Si-containing solvent (Holmes et al, 2000). In this case 2.5-nm Au nanocrystals were dispersed in supercritical hexane with a silicon precursor (e.g., diphenylsilane) under a pressure of 200-270 bar at 500°C, at which temperature the diphenylsilane decomposes to Si atoms. The Au nanocrystals serve as seeds for the Si nanowire growth, because they form an alloy with Si, which is in equilibrium with pure Si. It is suggested that the Si atoms would dissolve in the Au crystals until the saturation point is reached then they are expelled from the particle to form a nanowire with a diameter similar to the catalyst particle. This method has an advantage over the laser-ablated Si nanowire in that the nanowire diameter can be well controlled by the Au particle size, whereas liquid metal droplets produced by the laser ablation process tend to exhibit a much broader size distribution. With this approach, highly crystalline Si nanowires with diameters ranging from 4 nm to 5 nm have been produced by Holmes et al. (2000). The crystal orientation of these Si nanowires can be controlled by the reaction pressure. 

The utilily of PLD for Ihin film synlhesis is due in large part to Ihe unique characteristics of Ihe laser ablation process. As indicated above, laser ablation is a nonequilibrium process that enables stoichiometric evaporation of elements from a target source. In addition, it is also possible to control the energy of evaporated species in PLD, and thus control film growlh on Ihe subslrale surface. The underlying basis for these features of laser ablation and their utihty in thin film synthesis are described below. 

With any laser ablation process, the interaction of the laser with the sample, and hence the amount of material ablated can vary significantly, depending on the sample matrix, color and condition of the surface and the thickness of any coating. This normally necessitates use of an internal standard approach to the analysis, where the intensity of the analyte element is ratioed to the line from another element in the sample whose concentration is known. One potential problem with this approach however, is that one element may be preferentially ablated in preference... 

R. Sen, Y. Ohtsuka, T. Ishigaki, D. Kasuya, S. Suzuki, H. Kataura, and Y. Achiba, Time period for the growth of single-wall carbon nanotubes in the laser ablation process evidence from gas dynamic studies and time resolved imaging Chem. Phys. Lett. 332,467-473 (2000). 

Besides spatial deflection of the laser beam, temporal pulse shaping can be applied in order to influence the efficiency and quality of the ablation process during laser beam drilling. Figure 7 shows an example for the influence of temporal pulse shaping on the laser ablation process. 

The physics behind laser ablation is much more complicated than as explained above. Three characteristic timescales are involved to define the nature of laser interaction with a metallic material [1], They are the electron cooling time lattice heating time Tj, and laser pulse duration tl. For a nanosecond pulse laser, tL 3> Xi. the process is predominantly laser heating. For a picosecond pulse laser, x femtosecond pulse laser, the process is exclusively a laser ablation process. Laser ablation of semiconductor and dielectric materials involves different mechanisms [2]. 

The surface characteristics of a microfluidic channel are very important in determining the flow in electrokinetically driven systems. In electrokinetically driven systems, the bulk flow is created by movement of the mobile diffuse layer near the channel wall/solution interface that is termed electroosmotic flow (EOF). The EOF is dependent on the surface of the microchannel walls. Roberts et al. demonstrated the generation of EOF on laser-ablated polymer substrates for the first time, using the parallel processing mode with a photomask and an ArF excimer laser at 193 nm [17]. A variety of polymer substrates such as polystyrene, polycarbonate, cellulose acetate, and poly(ethylene terephthalate) (PET) were ablated to fabricate microfluidic channels. The laser ablation process alters the surface chemistry of the machined regions and produced negatively charged. 

Scott CD, Arepalli S, Nikolaev P, Smalley RE. Growth mechanisms for single-wall carbon nanotubes in a laser-ablation process. Appl Phys A 2001 72 573-80. 

Except for initially producing a sheath/core SiOz/Si nanowhisker, the laser ablation process parallels the metal catalyzed chemical vapor deposition process (Chapter 2.2.3). In this process, the Si that is desired is generated by chemical vapor deposition and dissolved in molten metal droplets, e.g., Au or Fe. The molten alloy droplets, e.g., SIAu, which result in this process sequence, give rise to the grovrth of single crystal Si micro-whiskers by a similar overall VLS phase transformation. 

The laser ablation process has been demonstrated so far only for Si and Ge nanowires [74], but it is clear that it is a new generic tool for growing crystalline nanowires. Thus, it should be possible to make nanowires or nanowhiskers of SiC, GaAs, Bi2Te3 and BN in this way and perhaps, in the presence of atomic hydrogen, even diamond nanowires [74]. 

Steps 1 and 2 are the laser ablation process and steps 3 and 4 are the ESl-MS analytical finish. 

The matrix is selected to absorb most of the energy few (if any) electronic states of the analyte (the polymer) are excited directly. However, since the analyte is intimately mixed with the matrix, the analyte is carried into the gas phase as a consequence of the phase transformation in the matrix. The analsd, the matrix molecules, clusters of matrix molecules, cations and cation clusters, as well as various combinations of each of these species have been detected in the plnme from this laser ablation process (21-23). 

Time period for the growth of single-wall carbon nanotubes in the laser ablation process evidence from gas dynamic studies and time resolved imaging. Chemical Physics Letters, 332,467-473. 

Hirata, T. and Miyazaki, Z. (2007) Highspeed camera imaging for laser ablation process for further reliable elemental analysis using inductively coupled plasma-mass spectrometry. Anal Chem., 79, 147-152. 

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