2294
J. Phys. Chem. C 2008, 112, 2294-2297
Deposition of Ordered Arrays of Gold and Platinum Nanoparticles with an Adjustable Particle Size and Interparticle Spacing Using Supercritical CO2 Alexander B. Smetana,†,‡ Joanna Shaofen Wang,†,‡ John J. Boeckl,‡ Gail J. Brown,‡ and Chien M. Wai*,† Department of Chemistry, UniVersity of Idaho, Moscow, Idaho 83844, and Air Force Research Lab, Materials and Manufacturing Directorate, WPAFB, Dayton, Ohio 45433-7707 ReceiVed: October 9, 2007; In Final Form: NoVember 25, 2007
Gold and platinum nanoparticles were prepared using a reverse micelle technique, creating products with several particle sizes. The stabilized metal nanoparticles can be deposited over long-range distances forming ordered arrays using supercritical carbon dioxide to remove the colloid solvent. The use of supercritical carbon dioxide creates uniformly deposited particle layers and can be removed without disturbing the precipitated particles. We also show that these nanoparticles can be deposited efficiently in nanometer trenches, which is not possible using conventional particle deposition by liquid evaporation.
Introduction Since nanomaterials have been envisioned, their enormous potential for technological advancement has been studied by scientists and entrepreneurs alike. However, to fully utilize their properties, these materials must be deposited by a controlled and reproducible means. Successful synthetic strategies for small spherical particles of a metallic and semiconductor nature have been developed by numerous laboratories in recent times.1-5 While fundamentally fascinating on an individual basis, these particles require deposition in large two-dimensional superlattice arrays to expose their full potential as candidates for sensors and extremely high-density data storage devices.6-11 Furthermore, when crystals of these nanoparticles can be grown in three dimensions, the possibility of photonic materials becomes a reality.12,13 The most stable and uniform spherical nanoparticles being researched are coated with an organic molecule that serves as a chemical and physical stabilizer for the highly reactive core materials. The most commonly used ligand is 1-dodecanethiol.1 Conveniently, if the size distribution of a colloid of spherical nanoparticles is sufficiently narrow (approximately less than 10%), these particles will self-assemble into larger structures when removed from solution.14 As with the more familiar case of crystallization of molecules, the quality of the nanoparticle crystal formation can be improved by careful manipulation of the environmental parameters. Nanoparticle deposition from a liquid phase has already been demonstrated for the creation of large ordered two- and threedimensional superlattices.15-17 This method of deposition from a liquid phase, however, has several drawbacks. Fast deposition leads to imperfect ordering and sometimes large faults in the crystal order.18 In addition, several drying patterns are often observed, giving incomplete and unpredictable ordering of the particles. To compensate, higher boiling point solvents can be added to the colloid, which enables the particles to form more * Corresponding author. E-mail:
[email protected]; tel.: (208) 885-6787; fax: (208) 885-6173. † University of Idaho. ‡ Air Force Research Lab.
perfect crystals.19 The consequences are an increased time scale for crystal formation and a large residual amount of heavy organics that need to be removed from the sample. Some deposition methods have been found that give beautiful large geometric crystals of nanoparticles, but the time of formation can be on the order of weeks.20 Special substrates can also be used to encourage uniform deposition such as highly ordered pyrolytic graphite (HOPG), molybdenum disulfide, Teflon, and mica supports.21,22 Other techniques exist to create superlattices of nanoparticles, for example, Mayya et al. conducted the growth of superlattices with particles trapped at an interface between a soluble and an insoluble phase.23 Here, we set out to create 2- and 3-D superlattices by deposition from solutions containing stabilized noble metal nanoparticles without evaporation but rather by removal of the solvent using supercritical carbon dioxide (Sc-CO2). Supercritical carbon dioxide is capable of removing the solvent without disturbing the superlattices due to its near zero surface tension. We show that this is an effective and relatively fast nanoparticle deposition method for making wide-area ordered arrays of nanoparticles. We combined this supercritical fluid deposition technique with a versatile reverse micelle synthesis method in which we can carefully tailor the particle size, type of the metal nanoparticle core, and nature of the chemical stabilizer surrounding the nanoparticles.24 Self-assembly of the protected nanoparticles when deposited from supercritical carbon dioxide leads to the formation of wide-area ordered arrays of monodispersive metal nanoparticles with interparticle spacing controllable by the nature of the chemical stabilizer. Substances in the supercritical state have displayed a remarkable penetrating ability into solids. We show that by using supercritical fluid carbon dioxide, we can deposit nanoparticles into small trenches milled into silicon wafers with the potential of being used as charge storage devices. This deposition method results in more uniform arrays than those produced by evaporation from the liquid phase. Experimental Procedures Gold or platinum nanoparticles were prepared by a reverse micelle-templated synthesis that we modified and is described
10.1021/jp7098703 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/29/2008
Deposition of Nanoparticles Using Supercritical CO2 in the literature.24 Briefly, the metal nanoparticles were formed by the chemical reduction of 0.3 M HAuCl4 or 0.3 M Na2PtCl4. Reduction occurs in AOT (sodium bis(2-ethylhexyl)sulfosuccinate) reverse micelles prepared in hexane in a 1:1 molar ratio of metal oxidation state to reducing agent. By controlling the dimension of the water core (W value) and the temperature, different sizes of metal nanoparticles can be synthesized using this method. A micellar solution of reducing agent was added dropwise over a period of 1 min to a similar solution of metal ions. The reduction was quenched by adding a stabilizing agent to coat the bare metal particles: 100 µL in the Au reactions and 25 µL for Pt. Decanethiol, dodecanethiol, and pentadecanethiol were used as stabilizing agents to control the interparticle distances in the superlattice similar to the approach reported by other laboratories.21,25,26 The particles were precipitated with ethanol and washed 2 times before redispersion in 2 mL of toluene. All measurements including particle size and interparticle distances for Au and Pt nanoparticles were measured using Matrox Inspector Interactive Imaging Software. Supercritical carbon dioxide precipitation of the nanoparticles was carried out inside a 17 mL chamber of a stainless steel high-pressure apparatus. The cell was contained inside a conventional box oven for temperature control and connected to an ISCO syringe pump to introduce CO2 and control the pressure. A small vial was placed in the cell containing a carbon coated TEM grid or a piece of silicon wafer with 200 µL of the metal colloid synthesized as mentioned previously. The apparatus was sealed and slowly charged with 60 atm of CO2 at room temperature over a period of 120 min. The camber was then brought to a pressure of 70 atm and closed with respect to the CO2 pump. The entire apparatus was brought to 40 °C to bring the carbon dioxide in the reaction chamber into the supercritical phase. During this heating, the pressure inside the chamber rose to roughly 100 atm. Then, the ISCO pump was used to raise the pressure up to a final pressure of 120 atm. During this period, the carbon dioxide mixes with the toluene, changing its polarity and causing precipitation of the nanoparticles. The high-pressure apparatus was left at this condition of 40 °C and 120 atm for 20 min to ensure an equilibrium state for the one-phase Sc-CO2/toluene solution. Finally, the chamber was vented slowly (∼0.2 mL/min) into a graduated cylinder placed in a fume hood removing toluene and CO2 and leaving the gold nanoparticles on the bottom of the vial. When vented CO2/toluene was trapped in an organic solvent, no detectable amount of gold nanoparticles was found in the trap solution. The apparatus was then brought back to room temperature and disassembled, and the TEM grid or silicon wafer was removed. To visually observe the precipitation, a second high-pressure apparatus equipped with a quartz window on either side was also used with the same procedure described previously. This allowed direct observation of the nanoparticle precipitation and progress of the reaction. In a separate experiment, two glass vessels were placed inside the chamber on separate ends. One contained gold nanoparticles solvated in toluene, while the other was empty. A single TEM grid was placed on the bottom of both vessels. During initial pressurization to 60 atm, the volume of the toluene solvent containing the dissolved nanoparticles visually increased. Soon afterward, the colloid changed color, indicating precipitation of the particles. This precipitation most likely occurs due to a gas-antisolvent (GAS) mechanism as described by Liu et al. where liquid CO2 swells the toluene solvent and begins to alter the polarity of the organic solvent, which becomes unfavorable for gold nanoparticle stabilization in the colloid.27 The nanoparticles precipitate uniformly and
J. Phys. Chem. C, Vol. 112, No. 7, 2008 2295
Figure 1. TEM images of Au nanoparticles of different sizes capped with dodecane thiol and deposited using Sc-CO2. Particles in panel a are 4.9 ( 0.4 nm, while particles in panel b are 6.1 ( 0.5 nm in diameter. An EDS spectrum of the gold particles is shown below the TEM images.
settle onto the bottom of the vial and coat the TEM grid. Toluene was removed with carbon dioxide in the supercritical phase, leaving the nanoparticles and grids behind. The TEM grid that was placed in the empty glass vessel was observed to show a very sparse coverage of nanoparticles indicative of a very spare solubility of ligand coated nanoparticles in supercritical CO2. To test the deposition of Au nanoparticles in small trenches, 3 in. (111) oriented Si wafers phosphorus doped (n-type) to 2-6 Ω cm were used in all experiments. Trenches were transferred into the substrate material Si via an FEI Strata DB235 SEM/FIB (focused ion beam) with a Magnum ion column. The Ga+ ion source was operated at 30 keV and 2.2 µA with a 10 pA aperture restricting the ion flux impinging on the sample surface. Automated software enabled stage positioning control to achieve pattern sizes ranging from 20 to 75 nm in diameter and 5 to 100 nm in depth. Pattern depth and size were calibrated using in situ SEM imaging in both plane and cross-section modes. Cross-sections were fabricated using the FIB with a capping layer of Pt deposited to preserve the surface. Results and Discussion Now that several successful synthetic strategies have been developed, deposition will be a key aspect in integrating nanomaterials into potential devices. While superlattices on flat substrates are useful for many applications, being able to deposit particles inside structures with a high uniformity may be more representative of the industry’s needs. We have shown that our synthetic method produces Au particles with a good dispersity over a size range of 2.2-6.6 nm by controlling the rate of micelle reactions with temperature regulation.24 Here, we extend this synthesis to include ligands of various lengths to increase their utility in functional devices and to test their ability to be deposited in a useful and predictable manner. Representative TEM images of Au particle depositions from Sc-CO2 with differently sized nanoparticles are shown in Figure 1a,b. The particles in each image are capped with dodecanethiol and are deposited in an identical manner. The sole difference in the particles is the Au core size, which is 4.9 ( 0.4 and 6.1 ( 0.5 nm, respectively. The deposition proves to be reproducible and effective for Au particles regardless of their size. An EDS spectrum is provided, confirming the identity of the particles as gold.
2296 J. Phys. Chem. C, Vol. 112, No. 7, 2008
Smetana et al.
Figure 4. TEM images of Au nanoparticle arrays using: (a) decanethiol C10H21SH and (b) pentadecanethiol C15H31SH.
Figure 2. TEM images (a and b) of Pt nanoparticles with an average diameter of 3.6 ( 0.4 nm deposited using Sc-CO2. A typical EDS spectrum of Pt nanoparticles with peaks corresponding to Pt particles, copper TEM grid, carbon film, and thiol molecules is shown the TEM images.
Figure 3. TEM images of 3.6 nm Au nanoparticles after deposition using Sc-CO2. The image displays long-range ordering of nearly monodisperse nanoparticles.
We also attempted to extend the synthesis to platinum particles as well as gold. Several experimental parameters were altered to accommodate the different chemical nature of Pt. The most monodisperse particles were formed using a temperature bath at 30 °C, a large W value of 16, and the use of the stronger NaBH4 as a reducing agent. We show that the Pt particles deposit similarly to Au irrespective of the metal core as the ligands dictate the overall solubility of the particles. Pt particles capped with dodecanethiol have a diameter of 3.6 ( 0.4 nm and are shown in Figure 2a, b (TEM), and c (EDS). The TEM images in Figure 3 show that the Au nanoparticle deposition is very uniform over long ranges. A careful comparison was made between the interparticle distance in the superlattices recovered from the high-pressure apparatus and the interparticle distance in the superlattices recovered from traditional benchtop evaporation from toluene. It was found that the high pressure during precipitation had little effect on the interparticle distance and that the values were within acceptable limits as compared to those dried from toluene on the benchtop.
This is likely due to nearly complete interdigitation of the alkane chains in the superlattice in both preparations. We intended to use the particles as electrical devices where their size and interparticle distance will be critical to their performance. To this end, we prepared several nanoparticle products, and all were deposited successfully using the ScCO2 method outlined previously. The monodispersive Au nanoparticles in Figure 4 are capped with C10H21SH and C15H31SH. The particles were spherical with an average diameter of 4.5 ( 0.4 nm for decanethiol and 6.1 ( 0.5 nm for pentadecanethiol particles with a standard deviation approximately less than 9%. The interparticle distance was measured by subtracting the average particle size from the center-to-center distance for C10H21SH and C15H31SH for values of 1.7 ( 0.7 and 2.2 ( 0.4 nm, respectively. According to Bain et al.,28 the ligand length L (units of nm) can be approximated by L ) 0.25 + 0.127n, where n is the number of carbon atoms in the alkyl chain.25,29 This equation gives values of 1.52 and 2.16 nm for single decanethiol (C10H21SH) and pentadecanethiol (C15H31SH) molecules, respectively. Extensive interdigitation would yield values much less than 2 times this length. Our observed interparticle distances of 1.7 nm for decanethiol and 2.2 nm for pentadecanethiol are close to near complete interdigitation. The interparticle distances observed by this study are similar to the values reported by other laboratories.21,25 The average value for interparticle distances between two adjacent Pt nanoparticles coated in dodecanethiol was measured to be 2.4 ( 0.4 nm for a close packed area. The higher value suggests a less extensive interdigitation between the Pt nanoparticles than between the gold counterparts. This may be caused by excess organic material dissolved in the superlattice or less efficient ordering resulting from the smaller sized particles. The true utility of Sc-CO2 as a particle deposition method is its great penetrating ability. Particles precipitated from this medium should be able to be placed in nanometer sized areas. To illustrate this, nanometer trenches were milled into silicon wafers. By means of the same procedure we used to deposit uniform Au nanoparticles on the flat carbon coated TEM grids, we produced particle filled nanoscale trenches in the surface of (111) oriented silicon wafers. The trenches were milled and imaged using an FEI Strata DB235 focused ion beam (FIB) with a 30 keV Ga+ ion beam. The as-milled wells are shown in the electron micrograph of Figure 5a. Benchtop deposition by placing the as-milled Si wafer in the bottom of a glass vial filled with the Au colloid followed by evaporation of the solvent is shown in Figure 5b. The particles show inhomogeneous and incomplete precipitation that is similar to results obtained by Cui et al. for particles smaller than 10 nm.30 When an as-milled Si wafer was used in the supercritical fluid cell in place of the carbon coated TEM grid, a uniform Au superlattice was deposited into the nanoscale trenches. The penetration into the trenches by the particles is
Deposition of Nanoparticles Using Supercritical CO2
J. Phys. Chem. C, Vol. 112, No. 7, 2008 2297 Acknowledgment. The authors are thankful for the support from AFOSR (FA9550-06-1-0526). References and Notes
Figure 5. (a) As-milled trenches in (111) silicon wafer, (b) deposition of Au nanoparticles by benchtop evaporation from toluene, (c) deposition of Au nanoparticles by Sc-CO2 removal of the toluene solvent, (d) deposition of Au nanoparticles in deeper trenches, and (e) deposition of Au nanoparticles in a shallow trench.
complete and tightly packs the available space. This can be seen visually in Figure 5c. Here, we demonstrate that our microfabrication procedure can deposit metal nanoparticles into nanoscale structures. For shallow trenches, the formation of a 2-D Au superlattice in the trenches is possible as shown in Figure 5e. Deeper trenches may contain more than one layer of particles as shown in Figure 5d, but we need to investigate this further. Future work will involve measuring the electronic response of the particle filled trenches with respect to the particle size, interparticle distance, and trench dimensions. Conclusion In conclusion, the TEM images show that nanoparticle deposition via a high-pressure chamber using Sc-CO2 can produce very ordered superlattices over large areas without disruption as in conventional liquid evaporation. These gold superlattices can be quite easily tailored with respect to the particle size and interparticle distances using our synthetic method24 and precipitation procedure given here. This demonstrates a procedure that can be used to create reliable nanoparticle arrays for electronic and sensor applications without the uncertainties involved in conventional solvent evaporation where nonuniformities exist over large areas due to surface tension effects. Going a step further, we also demonstrated the ability to deposit nanoparticles into nanoscale trenches in the surface of a (111) silicon wafer using the penetrating properties of Sc-CO2.
(1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (2) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1993, 97, 7061. (3) Jana, N. R.; Gearheart, L.; Murphy, C. J. AdV. Mater. 2001, 13, 1389. (4) Garcia-Martinez, J. C.; Scott, R. W. J.; Crooks, R. M. J. Am. Chem. Soc. 2003, 125, 11190. (5) Li, M.; Schnablegger, H.; Mann, S. Nature (London, U.K.) 1999, 402, 393. (6) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (7) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (8) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science (Washington, DC, U.S.) 1997, 277, 1078. (9) Sampaio, J. F.; Beverly, K. C.; Heath, J. R. J. Phys. Chem. B 2001, 105, 8797. (10) Wang, L. Y.; Shi, X.; Kariuki, N. N.; Schadt, M.; Wang, G. R.; Rendeng, Q.; Choi, J.; Luo, J.; Lu, S.; Zhong, C.-J. J. Am. Chem. Soc. 2007, 129, 2161. (11) Chan, G. H.; Zhao, J.; Hicks, E. M.; Schatz, G. C.; Van Duyne, R. P. Nano Lett. 2007, 7, 1947. (12) Yablonovitch, E. Phys. ReV. Lett. 1987, 58, 2059. (13) Blanco, A.; Chomski, E.; Grabtchak, S.; Ibisate, M.; John, S.; Leonard, S. W.; Lopez, C.; Meseguer, F.; Miguez, H.; Mondia, J. P.; Ozin, G. A.; Toader, O.; van Driel, H. M. Nature (London, U.K.) 2000, 405, 437. (14) Stoeva, S.; Klabunde, K. J.; Sorensen, C. M.; Dragieva, I. J. Am. Chem. Soc. 2002, 124, 2305. (15) Harfenist, S. A.; Wang, Z. L. J. Phys. Chem. B 1999, 103, 4342. (16) Lin, X. M.; Jaeger, H. M.; Sorensen, C. M.; Klabunde, K. J. J. Phys. Chem. B 2001, 105, 3353. (17) Stoeva, S. I.; Prasad, B. L. V.; Sitharaman, U.; Stoimenov, P.; Zaikovske, V.; Sorensen, C. M.; Klabunde, K. J. J. Phys. Chem. B 2003, 107, 7441. (18) Shah, P. S.; Novick, B. J.; Hwang, H. S.; Lim, K. T.; Carbonell, R. G.; Johnston, K. P.; Korgel, B. A. Nano Lett. 2003, 3, 1671. (19) Narayanan, S.; Wang, J.; Lin, X.-M. Phys. ReV. Lett. 2004, 93, 135503. (20) Yao, H.; Minami, T.; Hori, A.; Koma, M.; Kimura, K. J. Phys. Chem. B 2006, 110, 14040. (21) Motte, L.; Lacaze, E.; Maillard, M.; Pileni, M. P. Langmuir 2000, 16, 3803. (22) Martin, J. E.; Wilcoxon, J. P.; Odinek, J.; Provencio, P. J. Phys. Chem. B 2000, 104, 9475. (23) Mayya, K. S.; Sastry, M. Langmuir 1999, 15, 1902. (24) Smetana, A. B.; Wang, J. S.; Wai, C. M.; Boeckl, J.; Brown, G. Langmuir 2007, 23, 10429. (25) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Langmuir 2002, 18, 7515. (26) Motte, L.; Pileni, M. P. J. Phys. Chem. B 1998, 102, 4104. (27) Liu, J.; Anand, M.; Roberts, C. B. Langmuir 2006, 22, 3964. (28) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (29) Du, W.; Qian, X.; Ma, X.; Gong, Q.; Cao, H.; Yin, J. Chem.sEur. J. 2007, 13, 3241. (30) Cui, Y.; Bjork, M. T.; Liddle, J. A.; Sonnichsen, C.; Boussert, B.; Alivisatos, A. P. Nano Lett. 2004, 4, 1093.
