J. Phys. Chem. C 2007, 111, 4643-4647
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Investigations of Laser Evaporation in Ambient Pressure Helium with Ultrafast Hard X-ray Pulses Fang Shan, Rhiannon Porter, Neal Cheng, Daniel J. Masiel, and Ting Guo* Department of Chemistry, UniVersity of California, One Shields AVenue, DaVis, California 95616 ReceiVed: October 5, 2006; In Final Form: January 28, 2007
High-spatial and -temporal resolution direct imaging of the plumes created in laser evaporation in ambient pressure helium was achieved for the first time with an ultrafast hard X-ray source. Picosecond and nanosecond laser pulses were used to evaporate a thin metal film in ∼1 atm of helium. The plumes, which contained atoms, ions, and possibly nanoparticles, were imaged with hard X-ray pulses emitted from a tungsten rod ultrafast hard X-ray apparatus. No evaporation was observed in the first nanosecond or so after irradiation with either picosecond or nanosecond ablation pulses. Approximately 10 µm thick plumes were clearly observed at ∼100 nanoseconds after laser irradiation. The corresponding slow speed of the plumes was attributed mainly to the high-pressure background gas. Attempts were made to identify the composition of these plumes with a dual laser pulse evaporation-ultrafast X-ray pulse imaging scheme.
Introduction Laser vaporization has been extensively studied and employed in various applications since the invention of the laser. These applications include ablation and processing of high-melting temperature materials, generation of clusters, chemical syntheses of nanomaterials and thin films, and the production of superconductors.1-14 Recently, laser vaporization has been used to make fullerenes and carbon nanotubes, semiconductor nanoparticles, and silicon-based nanowires.13-17 Synthesis of new materials via laser vaporization relies on reactions between evaporated species, which may contain atoms, ions, and small nanoclusters, and the background gases. The chemical dynamics of the evaporated materials has been investigated with timeresolved optical spectroscopy, and the products deposited on the surface of various substrates have been carefully examined with electron microscopes.11,13,18,19 For example, fluorescence from carbon dimers, trimers, and clusters and other fluorescent species have been studied and used to infer plasma dynamics on nanosecond or longer timescales.18 X-ray absorption spectroscopy (XAS) and especially timeresolved XAS has been developed to study various chemical dynamics.20-23 A distinct advantage of XAS is that XAS methods can be used to obtain chemical information such as the size, structure, and oxidation state of the ablated materials. Furthermore, X-ray spectra may be more easily simulated theoretically because they are more pertinent to the configuration of core electrons and therefore the location of atoms.24 In addition, XAS can also follow the mass of the ablated materials instead of just the fluorescent materials. This is advantageous because most nanoparticle catalysts mentioned above may be nonfluorescent. To perform these investigations, ultrafast X-ray sources such as table-top laser-driven electron X-ray sources (LEXS) can be employed.25 These table-top X-ray sources normally emit X-rays from very small volumes, making them suitable to detect fast motions with high-spatial resolution. These X-ray sources have been used to investigate the plasma * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: (530) 754-5283. Fax: (530) 752-8995.
dynamics of the laser ablation of Al (absorption edge at ∼1.3 keV) and Si (1.8 keV) targets.26-30 Most of these studies are conducted in vacuum, and it has been speculated that the evaporated materials are usually in the form of plasmas composed of ions, atoms, and possibly very small clusters.18,31-33 In contrast, most chemical applications mentioned above are carried out in a high-pressure background gas such as He or Ar. For example, high-purity single-walled carbon nanotubes (SWNTs) have been successfully made with laser vaporization of a mixture of graphite and metal (Fe, Ni, or Co) powders in a few hundred Torr of Ar gas.16 High-quality semiconductor nanowires have also been synthesized with laser vaporization of a mixture of gold and silicon powder under similar conditions.17 To study catalytic materials in the examples mentioned above, X-ray sources operated in the higher-energy region (810 keV or higher) are needed. Another direct benefit of using hard X-rays is that these X-rays are attenuated less by the background gas and other materials created in laser vaporization. To date, neither optical nor X-ray methods have been employed to investigate laser vaporization in high pressure (∼1 atm) gas. A deep understanding of these catalytic growth processes may benefit many scientific fields, including petroleum and energy research, combustion, catalysis, inorganic chemistry, and organometallic chemistry. Laser-vaporized species reacting in a levitated and isolated physical state without contact with any substrates also represents a unique chemical reaction environment that is worth investigating. In this report, we wish to show that hard ultrafast X-rays can be used to image laser-generated plasmas in high-pressure background gas. In combination with other recently developed methods such as ultrafast selected energy X-ray absorption spectroscopy (USEXAS),34 it is then possible to investigate the catalytic processes of the growth of many nanomaterials. In the near future, we wish to prove that pulsed X-ray analytical tools are capable of providing the necessary chemical information for understanding the dynamics of those important processes described above.
10.1021/jp066559o CCC: $37.00 © 2007 American Chemical Society Published on Web 03/07/2007
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Figure 1. Schematic diagrams of the terawatt laser and ultrafast tungsten target X-ray apparatus (top panel). Laser beams 1 (532 nm) and 2 (800 nm) are used to vaporize the metal tape. The 532 nm nanosecond pulses are delayed in two different ways, as shown in panels A (in air) and B (in fiber). These delay lines can generate 100-200 ns delays. Panel C shows a delay line for the 800 nm picosecond pulses. This delay line can reach only ∼1 ns. A dual-pulse experimental arrangement (panel D) is shown in which both the 532 and 800 nm beams are used to vaporize the tape target. The laser burn profiles for the beams delivered to the target through each delay line are shown in each panel.
Experimental Details Figure 1 shows the schematics of the overall experimental setup. It includes a laser system, a thin-film target for laser evaporation in a tape chamber, and an ultrafast laser-driven electron X-ray source with an X-ray detection CCD (top panel). Figure 1 also shows three optical delay lines for single optical pulse vaporization experiments (panels A-C) and a dual-pulse laser evaporation setup (panel 1D). The ultrafast driving laser and the X-ray source were described elsewhere.35,36 The thinfilm tape target was based on a commercial magnetic tape drive. The composition of the metal films was 70% Co, 20% Ti, and 10% Fe, which were purchased and used with any further treatment. The thickness of the metal film was ∼3 µm, determined with a scanning electron microscope (SEM). Labview programs were used to control the CCD, X-ray target, film target, and the shutter for the ultrafast laser. The film target was installed in a helium chamber, and the tape moved at 150 mm/s. A constraint on the dimension of the laser beams on the metal films was that the X-ray absorption by plumes had to be strong enough to produce high contrast images. This means that the X-ray path length in the plumes had to be long enough. We estimated that this distance to be 1 mm. Because the lateral dimension of the plumes should be very close to that
of the laser beam at early stages of evaporation, the laser beam size along the X-ray path should be at least 1 mm. Two different kinds of laser were used to vaporize the metal thin films on the tape. One was a beam split from a nanosecond pump laser (second harmonic from a Q-switched Nd:YAG laser at 532 nm and 6 ns pulse duration) used for pumping the regenerative amplifier. This laser beam was introduced to the target via either a normal optical delay line (1A) or an optical fiber delay line (1B). The normal optical delay line produced ∼170 ns delay with respect to the X-ray probe pulse, which was caused by the intrinsic delay for the 800 nm light in the regenerative amplifier that was used to produce the probing X-ray pulses. When delay 1A was used, the laser beam was of higher quality and was focused with a cylindrical lens to a 50 µm × 5 mm spot. When a long 600 µm diameter multimode optical fiber was used to delay the 532 nm light outside the regenerative amplifier through large NA imaging lenses, the delay was adjustable from -50 ns to +180 ns. The coupling efficiency was ∼60%, and the power density on the film ranged from 200 to 300 mJ/cm2. When delay 1B was used, regular spherical lenses were used to image the fiber core onto the film. The focal spot size at the film was 1 mm in diameter.
Investigation of Laser Evaporation in Ambient Pressure The second evaporation laser used here was 10% of the 800 nm light produced from the terawatt laser (Figure 1C). The other 90% was used to generate ultrafast X-rays. The pulse duration of this pump light was set to be a few picoseconds by adjusting the grating separation of the compressor. The focal spot size at the tape target was 150 µm × 4.5 mm. Figure 1D shows the dual-pulse experimental setup. In this case, both the 800 and 532 nm lasers going through regular optical delay lines 1A and 1C were introduced to the sample via a dichroic mirror and then through the cylindrical lens. The two beams were aligned so that they overlapped in space at the film target. The 532 nm light was ∼170 ns ahead (fixed) of the 800 nm light, and the delay for the 800 nm pulses was adjustable from -100 to +900 ps with respect to X-ray pulses. Burn patterns of the thin films from two pump laser beams are shown in Figure 1A-D. When the plumes were imaged with ultrafast X-ray pulses, the CCD was 93 cm away from the source and the tape target was 19 cm away from the source. This gave rise to a magnification factor of 4.9×. However, the true plume size could not be inferred directly from the images on the CCD camera using this magnification factor if the X-ray source is not a point source. Under such conditions, it is necessary to use ray tracing to model the imaging process to obtain accurate X-ray source and plume size. To do so, we first imaged standard objects of small features with very sharp edges; the width of the edges (less than 1 µm) was much less than that of the X-ray source size. We then derived the X-ray source size by varying the X-ray source size in the ray tracing program so that the edges in the simulated images were the same as those measured with the X-ray source. The calibration target was standard transmission electron microscope (TEM) grids. The width of the grid bars are 10, 37, and 55 µm, and the edges were much sharper. The thickness of these grids varied from 17 to 20 µm. Using this procedure, we were able to determine the X-ray source size and then calibrate the plume size. Several hundred images were recorded and then added to obtain the final images. No special image processing methods were used. The data acquisition time for each image was ∼0.2 s. Other methods were used, although no improvement was observed. X-ray energy spectra were obtained from images of short acquisition times (0.2 s or less) using a single-photon counting method. This method counted only those X-rays which interacted with single pixels in the CCD (not excitation to adjacent pixels) in an image, and only those images with 15% or more of such single-pixel excitation were used. Results A typical X-ray spectrum from the source is given in Figure 2. The grating separation was set a few millimeters away from that producing the shortest optical pulses. The duration of the optical pulses was a few picoseconds at this grating separation. Under this condition, no super-hard X-rays (>100 keV) were observed. The X-ray spectrum contained mostly hard X-rays from 2 to 11 keV, and the most intense lines were tungsten L lines. The low-energy cutoff was caused by the CCD. These hard X-rays were used to image the plumes. Figure 3A and B shows the results of the calibration using three TEM grids, as well as the simulation results (3C). The features in all three grids were clearly resolved, indicating that the size of the X-ray source was comparable to the width of the grids. To accurately determine the size of the X-ray source, it is necessary to use the slopes of the edges of the images, as shown in Figure 3B. The width for opaqueness changing from 10 to 90% in these images was 2 pixels when the objects are
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Figure 2. X-ray spectrum acquired with the CCD in the single photon counting mode. L and M lines of tungsten are visible, in addition to bremsstrahlung background between 2 and 6 keV.
Figure 3. X-ray images of three TEM grids (panel A) and line plots of the cross sections of the bars in the grids (panel B). The simulated line scans of an opaque object imaged with three different X-ray source sizes of 10, 20, and 30 µm are also shown (panel C).
fully resolved, as indicated in the 55 and 37 µm cases. This slope would require the source size be ∼20 µm so that the simulated width was close to 2 pixels for the 10 to 90% transition, as shown in Figure 3C; 10 or 30 µm source sizes clearly produced different transition widths. Figure 3B also shows that the image contrast already deteriorated for the 10 µm grid because the X-ray source size was evidently larger than the bar width of the grid. This calibration process is important because otherwise it would be impossible to directly determine the dimension of the plumes for nonpoint X-ray sources. As a result, the dimension of any objects could only be obtained via simulation rather than being measured directly from an image. When the 800 nm picosecond pump laser was used, the optical delay line 1C could only reach -100 to +900 ps delay. The absolute measurement error of time zero (0 ps delay) was (30 ps. No plumes were observed within this delay range, suggesting that the plume did not develop until maybe many nanoseconds after laser irradiation. Because the spatial resolution of our X-ray imaging system was very high, on the order of microns, we concluded that it would take many nanoseconds for the plumes to develop in a high-pressure background gas. Figure 4 shows the images of the plumes generated from the irradiation of the 532 nm laser at -50 ns with +140 to +180 ns delays. These delays were obtained with the fiber delay line 1B. As shown, no plumes were visible before time zero (-50 ns), but a plume was clearly seen at +140 ns. The height (diameter) of the plume was estimated to be 1 mm, similar to
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Figure 4. X-ray images of laser-produced plumes at different delays (optical fiber delay line shown in 1B). The laser beam size was 1 mm diameter on the target. The plume grew larger and thicker at a later delay of 180 ns than 140 ns. The thick horizontal lines indicate the position where maximum evaporation occurred, and vertical lines guide the position of the tape (black line) and the center of the plumes (red line) moving away from the tape.
Figure 5. X-ray images of laser-produced plumes at different delays (delay line shown in shown in 1A). The laser beam size was 5 mm × 50 µm on the target. The plumes are shown at the center of the circles, which are drawn to assist the visualization of the plumes.
the beam diameter of the excitation laser. A thick line is drawn near the center of the evaporation, and vertical lines are drawn to locate the center of the plumes (red for +140 and +180 ns), as well as the tapes (black vertical lines). From the image simulation, the thickness of the plume was determined to be 10 µm at the +180 ns delay. Plume thickness was slightly smaller at a delay of +140 ns than that at one of +180 ns. The edge of the plume was closer to the target than the center part, indicating that the speed of the plume depends on the laser irradiation intensity. Figure 4 also shows an image of the plume dynamics without the optical pump. On the basis of the thicknesses of the plumes at these three different delays, the speed of the plumes was estimated to be 100 m/s. Figure 5 shows images at different time delays obtained with the regular delay line 1A. As mentioned earlier, this delay line can only provide delays from +157 to +178 ns. Because the beam quality was higher when the light did not need to go through the multimode optical fibers, the pump light can be focused with a cylindrical lens. This produced a well-defined beam on the film, as shown in Figure 1A. The thickness of the plume increased as a function of delay, reaching ∼10 µm at 178 ns. Circles are drawn to locate the plumes. Again the speed of the plume can be estimated to be 100 m/s based on these images. The vertical heights of the plumes measured with the X-ray imaging were simulated and determined to be 60 µm, slightly greater than that of the optical pump size. This increase may be caused by the slight tilting of the pump light on the film. Figure 6 shows the results of the dual-pulse experiments. The overlap between the two pump beams were verified indepen-
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Figure 6. X-ray images of picosecond laser vaporization of plumes generated with nanosecond laser pulses. The picosecond laser pulse was delay ∼170 ns with respect to the nanosecond ablation pulse, and the delay shown here is that between the picosecond vaporization pulse and X-ray probe pulse. Circles are drawn to show the locations of the plumes.
dently with an optical microscope (as shown in Figure 1D). The dual-pulse measurement was designed to test whether the plume at 170 ns after the creation of the plume contained a large amount of nanoparticles. If it does, then the second laser pulse (picoseond pulse duration) should be able to re-evaporate these nanoparticles into atoms. Because the atoms coming from these nanoparticles may possess much higher kinetic energy and thus travel at a much greater speed, the plume may weaken or even disappear upon irradiation of intense picosecond laser pulses. This process has never been observed, although fast-moving atomic species after laser vaporization of bulk target were observed.30 As shown in Figure 6, the results seemed to suggest that the plume at +900 ps delay was only slightly weaker than that at -100 ps. If this were true, then the plume must contain some nanoparticles, which were re-evaporated by the second picosecond pulse. However, the difference between two plumes at these two delays was too small to draw any conclusions. Further experimental data are needed. Discussion The speed of the plumes traveling in the He gas measured with ultrafast X-ray pulses was slower than that obtained with the optical measurements. However, as we stated earlier that most of the optical measurements were performed in a vacuum or at very low background pressure (
