Encapsulated Nanoparticles Produced by Pulsed Laser Ablation of MoS2-Te Composite Target J. J. Hu,* Jeffrey S. Zabinski, John E. Bultman, Jeffrey H. Sanders, and Andrey A. Voevodin
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2603–2605
Materials and Manufacturing Directorate, Air Force Research Laboratory (AFRL/MLBT), Wright-Patterson Air Force Base, Dayton, Ohio 45433-7750 ReceiVed August 28, 2007; ReVised Manuscript ReceiVed February 14, 2008
ABSTRACT: Encapsulated nanoparticles were produced by the pulsed laser ablation of MoS2-Te composite targets in a high vacuum chamber. Transmission electron microscope and X-ray energy dispersive spectrometer measurements showed that core-like Mo-rich nanoparticles were encapsulated with shell-like Te-rich materials. Layer structures of the hexagonal MoTe2 phase were formed on the nanoparticle surface that was linked to the diffusion-controlled migration and crystallization. The mechanism of nanoparticle syntheses was discussed in relationship with the pulsed laser ablation of MoS2-Te target materials, the formation of Mo-rich cores, and the growth of layer-structured MoTe2 shells. Hexagonal MX2 phase (M ) Mo, W, Nb, Ta; X ) S, Se, Te) metal dichalcogenides are composed of two parallel sandwiched X-M-X atomic layers, which form the layered structures stacking along the c-axis. The atoms within each X-M-X layer have strong covalent bonding, but two adjacent layers are bonded to each other with weak van der Waals force. Folding of X-M-X atomic sheets can result in the formation of inorganic fullerene-like (I.F.) materials such as nanoparticles, nanotubes and polyhedral structures, which exhibit advanced properties in a wide range of applications. The I.F. nanoparticles of MoS2 and WS2 were first synthesized by solid-gas reactions and used as solid lubricant additives for oil.1–4 WS2 nanotubes were also produced and showed significantly higher resistance than that of carbon nanotubes in shock-wave impact tests.5 Near-monodisperse nanofibers and nanotubules of MoS2 were prepared by template synthesis and considered for use as hydrodesulfurization catalysts and battery cathodes.6 Open-ended MoS2 nanotubes were produced by a solid-gas reaction and used for catalytic conversion of CO + H2 to CH4 + H2O.7 Polycrystalline MoS2 nanoribbon arrays were made by electrochemical/chemical synthesis and studied for electronic properties.8 The syntheses of I.F.-MX2 nanoparticles had been performed by the arc discharge in high-pressure nitrogen and in water,9–11 and by ablating MX2 targets using pulsed excimer laser radiation.12–15 The reactive laser ablation method had been employed to synthesize the nanotubes and fullerene-like nanostructures of lamellar NiCl2 compounds.16 Recently, new lamellar TaS2 and WS2 nano materials were produced by laser ablation in addition to their nanoparticles and nanotubes.17,18 From the multiple techniques explored for I.F. material syntheses, the laser ablation method offers a continuity of nanoparticle productions with a unique possibility for making composite I.F. structures by adjusting target material compositions. In this study, we used powder-based MoS2-Te materials as the target to produce composite nanoparticles by pulsed laser ablation in a high vacuum chamber. The microstructure and chemistry of the produced nanoparticles were investigated and the growth mechanism was discussed in detail. A Lambda Physik COMPex 205 KrF excimer laser was used to provide a pulsed beam of UV radiation of 248 nm wavelength, 20 ns duration, 50 Hz rate, and 400 mJ energy. Beam steering was done using a computer controlled mirror system, which permits the laser beam to randomly strike the target over the ablation area. Laser targets were fabricated by mixing MoS2 and Te powders at a 75:25 weight ratio, and were cold pressed into compact disks of * To whom correspondence
[email protected].
should
be
addressed.
E-mail:
25.4 mm diameter by 6.4 mm thickness. Plasma plumes were produced by the target material sublimation and ionization under the focused laser beam pulse with an instantaneous power density of approximate 0.5 GWt/cm2. The ablated materials were deposited as films on 440C steel substrates of 25.4 mm diameter, and the growth was terminated at a film thickness of 2-4 µm. Both target and substrate were rotated to ensure the deposition uniformity. All depositions were performed at room temperature in a UHV compatible chamber, which was evacuated by a turbo pump to 10-8 Torr background pressure. The produced film topography was investigated using a Leica 360 scanning electron microscope (SEM), which was equipped with a field-emission-gun (FEG) and was operated at an accelerating voltage of 25 kV. The samples collected on carbon grids were analyzed using a Philips CM200-FEG transmission electron microscope (TEM) operated at 200 kV. A Philips CompuStage double tilt specimen holder with low X-ray background was used. The probe size of incident electron beams was adjustable from 25 nm down to 1 nm for chemical microanalyses using a NORAN X-ray energy dispersive spectrometer (EDS) installed on the TEM. EDS elemental mapping was carried out by electron beam scan. The SEM image in Figure 1a shows the granular surface of the as-deposited films consisted of round-shaped nanoparticles in aggregations. The nanoparticle structure in the films was confirmed by TEM observations, as shown in Figure 1b. The atomic concentrations of Mo, S and Te were measured for the film bulk and for the individual nanoparticles by EDS, respectively, and the composition results were presented by the histograms in Figure 1c. The nanoparticles, as indicated with white circles in Figure 1b, had a significantly higher content of Mo and lower contents of S and Te in comparison to the films. The nanoparticles were moderately Mo-rich, followed by S and then by Te in atomic concentrations. The difference in chemical compositions between the film bulk and individual nanoparticles suggested that there is an intermediate material between the nanoparticles, which has the composition with a reduced Mo content. Figure 2 shows a TEM image taken from the nanoparticles with the corresponding EDS elemental mapping. The minimum and maximum diameters of the observed nanoparticles were measured between 60 and 210 nm (Figure 2a). The round-shaped nanoparticles were mostly enwrapped with some irregularly shaped materials, which formed a surrounding shell and had a brighter image contrast in comparison to the nanoparticle cores, as shown in Figures 1b and 2a. Figures 2b, 2c and 2d are elemental mapping results for Mo, S and Te, respectively. It was shown that the corelike nanoparticles were composed of Mo and S compositions (see
10.1021/cg7008144 CCC: $40.75 2008 American Chemical Society Published on Web 07/18/2008
2604 Crystal Growth & Design, Vol. 8, No. 8, 2008
Figure 1. (a) SEM image showing the granular film surface where round-shaped nanoparticles formed in aggregations. (b) TEM image taken from the nanoparticles in the film. (c) EDS chemical analysis of the as-deposited film and nanoparticle that show Mo, S and Te compositions in histograms.
Figure 2. TEM image of the encapsulated nanoparticles with different sizes (a), and EDS elemental mapping images taken from the same sample for the chemical compositions of (b) Mo, (c) S, and (d) Te.
Figures 2b and 2c), and the atomic concentration of Mo is higher than S (see Figure 1c). The shell-like materials around nanoparticles were mainly composed of Te (see Figure 2d). Therefore, the nanoparticle formation was likely accompanied with the segregation of the laser ablated MoS2-Te source materials leading to the formation of Mo-rich nanoparticle cores and Te-rich surrounding shells. High-resolution TEM images were taken from the nanoparticles and exhibited some fringes of the layer structures encapsulating the cores, as shown in Figure 3. The fringe spacing was measured as 0.70 nm that approximately equals the lattice spacing between two adjacent Te-Mo-Te layers - a half of the c parameter of hexagonal 2H-MoTe2 crystals (a ) b ) 0.35144 nm, c ) 1.3884 nm). This suggests that the laminar growth along 2H-MoTe2 (002) basal planes may occur at the interfaces between core-like Morich nanoparticles and shell-like Te-rich materials. Tilting the specimen with respect to the incident electron beam in a TEM can produce the projection of nanoparticles at a different
Communications
Figure 3. High-resolution TEM image of the encapsulated nanoparticle showing the 2H-MoTe2 (002) fringes on the surface of the spherical nanoparticle with surrounding shell material.
Figure 4. (a, b) TEM images of the encapsulated nanoparticle taken at 0° and 20° tilt. (c, d) High-resolution TEM images showing the 2HMoTe2 (002) fringes on the spherical nanoparticle surface taken at 0° and 20° tilt.
angle. Figures 4a and 4b are the images taken from the same nanoparticle without specimen tilt and with a tilt of 20°, respectively. No obvious geometrical change happened to the roundshaped nanoparticle cores, while a significant elongation was created in the projection of the Te-rich surrounding material showing in brighter image contrast than the core. It implies that the core has a nearly spherical shape in comparison to the irregular surrounding material. Figures 4c and 4d are the high-resolution TEM images taken from the nanoparticle surface without specimen tilt and with a tilt of 20°, respectively. Fringes of 0.70 nm spacing were projected from 2H-MoTe2 (002) basal planes and were visible in both tilting conditions. Therefore, the 2H-MoTe2 laminates were conformably produced on the spherical surface of nanoparticles, which was then followed by Te-rich surrounding materials. The mechanism of producing encapsulated nanoparticles by pulsed laser ablation in this study may have involved three
Communications processes. The first process referred to the interaction between pulsed laser radiation and source material. In the pulsed laser ablation, the incident laser pulse energy is known to cause a transient superheating of the target surface that results in both sublimation and melting at the laser strike location. Thus, the liquid droplets of target materials could be expected to form because of melting and boiling.19 The droplets were expelled from the target by two potential forces: (1) volume change on melting followed by thermal expansion of the liquid;20 (2) recoil pressure of the sublimated gas exerted by the shock wave of the laser ablation plume adiabatically expanding the target surface in vaccum.21 Ions, neutrals, atomic clusters and particulates are the predominant components of the plume. Local spot melting and plume photodecomposition at the target vicinity during the laser pulse duration can result in the loss of sulfur in the ejected clusters and particulates because it needs less heat to vaporize in comparison to molybdenum and tellurium.22 The plume expansion was less constrained by the low background pressure that increased the loss of volatile sulfur. This can explain the significant high Mo content in the nanoparticle cores (see Figure 1c). Sen et al. reported an excess amount of W metal encapsulated within the WS2 cage, which was produced by laser ablation of pressed WS2 target under argon flow.14 They believe that the sulfur loss gave rise to the metallic W content in nanoparticle cores. In addition, I.F.-MoS2 nanoparticles filled with high concentration Mo cores were demonstrated to form in a water-media arc synthesis.11 These works support the above suggested mechanism for the formation of Mo-rich nanoparticle cores. Additionally, low vapor pressure Mo may condense first and then reacts with the hot S and Te vapor as it condenses onto the Mo nuclei resulting in the formation of dichalcogenide shells on a spherical Mo-rich core. Tellurium has a large diffusion coefficient and high mobility at elevated temperatures. Thermodynamic migration of Te content because of laser-induced heating can possibly be a reason for the shell-like Te-rich materials aggregation on the surface of core-like Mo-rich nanoparticles (see Figure 2). In the hot plume, the Mo-rich nanoparticles are available as metallic templates for reaction with Te through a diffusion-controlled process on their surfaces. As a result, the crystallization of 2H-MoTe2 phase occurred in the immediate vicinity of Mo-rich nanoparticles and formed the layer structures on their surfaces (see Figure 3). Meanwhile, the laser ablation heating is highly transient because the hot plume exists for less than 10 µs, which may interrupt further crystallization of the 2H-MoTe2 phase so as to produce a Te-rich encapsulating shell. The produced composite nanoparticle materials can offer some interesting mechanical, electronic and optical property combinations due to the tellurium coating on nanoparticle cores. For example, the encapsulation can protect the nanoparticles from oxidation in humid air during sliding friction tests. Tribological properties of the films consisting of the MoS2-Te composite nanoparticles are currently being explored over a wide range of environments. An extensively long wear life was provided by the composite nanoparticle films in humid air tribotests in comparison to pure MoS2 films. These preliminary results are very encouraging and will be explored further. In conclusion, the encapsulated composite nanoparticles were produced by the pulsed laser ablation of MoS2-Te composite materials. TEM and EDS measurements showed that the round-
Crystal Growth & Design, Vol. 8, No. 8, 2008 2605 shaped Mo-rich nanoparticles were encapsulated with shell-like Terich materials. At the Mo-rich core/Te-rich shell interface, a conformal crystalline growth of laminar 2H-MoTe2 was demonstrated by the lattice images of 2H-MoTe2 (002) basal planes projected from a tilted specimen. The mechanism of the composite nanoparticle syntheses was proposed, involving the interaction between pulsed laser radiation and target source material, the photodecomposition of ejected clusters and particulates with sulfur loss and Mo-rich nanoparticle formation, the thermodynamic migration and aggregation of tellurium as a shell around the nanoparticle core, and the laminar 2H-MoTe2 growth at the Morich core/Te-rich shell interface.
Acknowledgment. The Air Force Office of Scientific Research is gratefully acknowledged for financial support. Supporting Information Available: X-ray diffraction and EDS elemental mapping for the nanoparticle. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) Tenne, R.; Margulis, L.; Genut, M.; Hodes, G. Nature 1992, 360, 444. (2) Margulis, L.; Salitra, G.; Tenne, R.; Talianker, M. Nature 1993, 365, 113. (3) Feldman, Y.; Frey, G. L.; Homyonfer, M.; Lyakhovitskaya, V.; Margulis, L.; Cohen, H.; Hodes, G; Hutchison, J. L.; Tenne, R. J. Am. Chem. Soc. 1996, 118, 5362. (4) Rapoport, L.; Bilik, Yu.; Feldman, Y.; Homyonfer, M.; Cohen, S. R.; Tenne, R. Nature 1997, 387, 791. (5) Zhu, Y. Q.; Sekine, T.; Brigatti, K. S.; Firth, S.; Tenne, R.; Rosentsveig, R.; Kroto, H. W.; Walton, D. R. M. J. Am. Chem. Soc. 2003, 125, 1329. (6) Zelenski, C. M.; Dorhout, P. K. J. Am. Chem. Soc. 1998, 120, 734. (7) Chen, J.; Li, S. -L.; Xu, Q.; Tanaka, K. Chem. Commun. 2002, 1722. (8) Li, Q.; Newberg, J. T.; Walter, E. C.; Hemminger, J. C.; Penner, R. M. Nano Lett. 2004, 4, 277. (9) Chhowalla, M.; Amaratunga, G. A. J. Nature 2000, 407, 164. (10) Sano, N.; Wang, H.; Chhowalla, M.; Alexandrou, I.; Amaratunga, G. A. J.; Naito, M.; Kanki, T. Chem. Phys. Lett. 2003, 368, 331. (11) Hu, J. J.; Bultman, J. E.; Zabinski, J. S. Tribol. Lett. 2004, 17, 543. (12) Parilla, P. A.; Dillon, A. C.; Jones, K. M.; Riker, G.; Schulz, D. L.; Ginley, D. S.; Heben, M. J. Nature 1999, 397, 114. (13) Parilla, P. A.; Dillon, A. C.; Parkinson, B. A.; Jones, K. M.; Alleman, J.; Riker, G.; Ginley, D. S.; Heben, M. J. J. Phys. Chem. B 2004, 108, 6197. (14) Sen, R.; Govindaraj, A.; Suenaga, K.; Suzuki, S.; Kataura, H.; Iijima, S.; Achiba, Y. Chem. Phys. Lett. 2001, 340, 242. (15) Schuffenhauer, C.; Parkinson, B. A.; Jin-Phillipp, N. Y.; Joly-Pottuz, L.; Martin, J. -M.; Popovitz-Biro, R.; Tenne, R. Small 2005, 1, 1100. (16) Hacohen, Y. R.; Popovitz-Biro, R.; Prior, Y.; Gemming, S.; Seifert, G.; Tenne, R. Phys. Chem. Chem. Phys. 2003, 5, 1644. (17) Chick, K. Y.; Nath, M.; Parkinson, B. A. J. Mater. Res. 2006, 21, 1243. (18) Hu, J. J.; Zabinski, J. S.; Sanders, J. H.; Bultman, J. E.; Voevodin, A. A. J. Phys. Chem. B 2006, 110, 8914. (19) Ready, J. F. Appl. Phys. Lett. 1963, 3, 11. (20) Kelly, R.; Cuomo, J. J.; Leary, P. A.; Rothenberg, J. E.; Braren, B. E.; Aliotta, C. F. Nucl. Instrum. Methods Phys. Res. 1985, B9, 329. (21) Batanov, V. A.; Fedorov, V. B. JETP Lett. 1973, 17, 247. (22) Eryu, O.; Murakami, K.; Masuda, K. Appl. Phys. Lett. 1989, 54, 2716.
CG7008144
