Anal. Chem. 1995,67,528-534
Determination of Relevant Size Parameters for Sonicated and Unsonicated Nanocrystalline GaAs Particles Carolynne R. S. Hagan, Shreyas 5. Kher, Lara I. Halaoui, Rlchard L. Wells,*lt and Louis A. Coury, Jr.*,* Department of Chemistry, Box 90346, Duke University, Dutham, North Carolina 27708-0346
Nanocrystalline GaAs was characterized using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), multipoint Brunauer-EmmettTeller (BET) surface area analysis, scanuing tunneling microscopy(STM),scanning electron microscopy(SEM), and W/vis spectrophotometry. Each of these techniques was evaluated for the determination of particle size. Comparable results were obtained from XRD, HRTEM, BET, and STM. W/vis absorption thresholds were consistent with quantum con6nement &&, but the magnitude of the blue shift in the absorption edge was greater than expectedfor particles of this size. Exposure of the particles to high-intensity, 20-lrHz ultrasound resulted in deaggregabion of particleswithout signi6cantly affecting the crystalline regions responsible for quantum confinement. In a recent review, Bohn and Harris cited the importance of analytical chemistry in the semiconductor industry with emphasis on the determination of impurities and dopant concentrations in semiconductors,two of the most important aspects which govern electronic and optoelectronicproperties.’ As technology advances in this industry, attention has also been directed toward quantization effects in nanocrystalline semiconductors.2 Such quantum size effects occur when the physical dimension of a semiconductor particle approaches that of the bulk exciton diameter (i.e., the physical distance separating the electron-hole pair in the bulk material).3 In such cases, the electron in the semiconductor particle is theoretically treated as a particle in a box, and discrete energy levels arise within the semiconductor that are further apart than those in the bulk, giving rise to a larger apparent band gap.4 The increase in band gap is more pronounced as the particle size decreases! Such particles are often referred to as “quantum dots” or “zero-dimensional”materials to distinguish them from quantum structures of higher dimensionality, such as quantum wells and nanowires.5 The ability to create semiconductors with tunable band gaps could lead to important advances in many fields. The area of solar energy conversion is likely to benefit, as a much broader region + Corresponding author for materials synthesis. + Corresponding author for materials characterization. (1) Bohn, P. W.; Hanis, T. D. Anal. Chem. 1990, 62, 767A-777A (2) Steigemald, M. L.;Brus, L. E.Acc. Chem. Res. 1990,23, 183-188. (3) Brus, L. E. J Chem. Phys. 1983, 80, 4403. (4) Weller, H. Adu. Muter. 1993, 5, 88. (5) Reed, M. A. Sci. Am. 1993, 268, 118-123.
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of the solar spectrum could be exploited for energy storage and retrieval. Aside from the promise of tailoring the band gap of semiconductorsby controlling particle size, other useful features are predicted for quantum dots. Diminished recombination rates relative to films are expected due to the lack of grain boundaries and uniform absorption characteristics for arrays of dots. This feature has important consequences for the construction of more efficient photoelectrochemical devices.6 Also, high nonlinear optical susceptibilities are anticipated for quantum dots (viz., unusual absorptivity and refractive index changes due to electronhole pair formation) and are of interest for use in information processing arrays.’ All of these research objectives are critically dependent upon the precise and accurate measurement of particle sizes ranging from tens to hundreds of angstroms in diameter. To date, the vast majority of quantum dot studies have focused on 11-VI semiconductors (e.g., CdS, CdSe). After the 1989 discovery by Wells and cc-workers of a solution synthetic route to GaAs? however, several groups have undertaken studies of GaAs and other 111-V quantum dots prepared by this method or with modificationsof itg4 Kher and Wells have recently reported an even more facile solution synthesis which is general for 111-V nanocrystalline semiconductors.12 GaAs is a technologically important direct band gap semiconductor, second only to Si in its commercial uses; thus, study of GaAs in quantum dot form will undoubtedly be an active research area in the coming years. Recently, results from high-resolution transmission electron microscopy (HRTEM), small angle X-ray scattering (SAXS), and low-frequency inelastic Raman scattering (LOFIRS) were compared for the determination of particle size of Cd(S,Se) quantum dots in g1a~ses.l~ The work reported here compares additional techniques which are also currently being used to determine nanocrystalline dimensions. The techniques of HRTEM, X-ray diffraction 0 ) , W/vis spectroscopy, Brunauer-Emmett(6) Meyer, G. J.; Searson, P. C. Electrochem. SOC.Intetfuce 1993, 2, 23-27. (7) Garmire, E. Phys. Toduy 1994, 47, 42-48. (8) (a) Wells, R L.; Pit&, C. G.; McPhail, A T.; Purdy, A P.; Shatieezad, S.; Hallock, R B. Muter. Res. SOC.Symp. Proc. 1989,I31,45.6)Wells, R L.; Pitt,C. G.; McPhail,A T.; Purdy, G P.; Shatieezad, S.; Hallock, R B. Chem. Mater. 1989, I , 4. (9) Olshavsky, M. A; Goldstein,A N.; Alivisatos,A P.J. Am. Chem. Soc. 1990, 112, 9438-9439. (10) Uchida, H.; Curtis, C. J.; N o d , A J.J. Phys. Chem. 1991,95,5382-5384. (11) Butler, L.;Redmond, G.; Fitztnaurice, D.J.Phys. Chem. 1 9 9 3 , 9 7 , 1075010755 and references therein. (12) (a) Kher, S. S.; Wells, R L. US. Patent Appl. 08/189, 232, filed on January 31, 1994. 6)Kher, S. S.;Wells, R L.. Chem. Muter. 1994, 6, 2056-2062. (c) Kher, S. S.;Wells, R L. Mater. Res. Symp. Proc. 1994, 351,293. (13) Champagnon, B.; Andrianasolo, B.; Ramos, A; Gandais, M.; AUais, M.; Benoit, J. P. J. Appl. Phys. 1993, 73, 2775-2780 and references therein.
0003-2700/95/0367-0528$9.00/0 0 1995 American Chemical Society
Teller (BET) surface area measurements, and scanning tunneling microscopy (STM) will be explored as methods for particle size determination of nanocrystalline GaAs. In addition to the determination of particle size, the effect of high-intensity, low-frequency ultrasound on nanocrystalline GaAs has been investigated. Sonication has previously been demonstrated to alter particle size.14 Suslick and co-workers found that malleable metals aggregateI4as a result of the high temperatures (5000 IQ , pressures (500 atm) ,15J6and fluid velocities (100 m/s) l7 associated with acoustic cavitation. In contrast, brittle materials exhibiting a laminar structure (e.g., TaSj tend to break apart when sonicated.'* The effects of ultrasound on nanocrystalline GaAs slurries with respect to particle size, extent of aggregation, and crystalline domain size will be discussed. EXPERIMENTAL SECTION
Synthesis. Nanocrystalline GaAs was prepared by a new onepot synthesis recently developed by Kher and Wells.12 The new method consists of in situ synthesis of (Na/IQ& from sodium potassium alloy (99.95%,Strem Chemicals) and oxygen-depleted arsenic powder (99.9999%, Atomergic Chemetals) in refluxing toluene. Subsequently, GaC13 solution (99.999%, Strem) in dry, distilled diglyme was added to (Na/IQ& in toluene, and refluxing the reaction mixture afforded gallium arsenide nanocrystals. Instrumental Considerations. Sonication was performed on slurries containing 80 mg of GaAs and 30 mL of propylene carbonate (Aldrich) with a 2@kHz, 475W Heat Systems highintensity probe sonicator using an accessory microtip oscillating at 48 pm (peak-to-peak) for 1 h. All sonication studies were performed in a jacketed temperature-controlled cell (Heat Systems) using a Brinkmann Lauda recirculating bath containing ethylene glycol cooled to -18 "C. Solvent was removed under low pressure at about 250 "C in a vacuum oven. For XRD analysis, GaAs was mounted on a glass microscope slide using double-sided adhesive tape. Measurements were obtained with a Phillips XRG 3000 X-ray diffractometerequipped with a single crystal graphite monochromator,using Cu Ka (1= 1.5418A) radiation, and data were digitally stored on an IBM PC. Lattice images were obtained with a TOPCON EM002B HRTEM located at the Analytical Instrumentation Facility at North Carolina State University (NCSU) working at a 2WkV accelerating voltage. Samples were prepared by depositing drops of either pentane or ethanol suspensions onto 1500 mesh copper grids (Structure Probe Inc.) without supporting films. SEMs were recorded on a JEOL 600 Field Emission SEM, also located at the Analytical Instrumentation Facility at NCSU. Ethanol or pentane suspensions of GaAs were deposited directly onto polished aluminum SEM stages. BET measurements were performed with a Quantasorb BET surface area analyzer manufactured by Quantachrome. Samples were analyzed in a microcell 200 (Quantachrome) and were outgassed for at least 5 h at a temperature of at least 300 "C. GaAs powder was extracted eight times with pentane (Aldrich) in a rigorous attempt to remove residual propylene carbonate from the sonicated sample and was dried in a vacuum oven overnight at 100 "C prior to the outgassing procedure. (14)Suslick, K S.; Casadonte, D. J.; Doktycz, S. J. Chem. Muter. 1989, 1, 6. (15)Suslick, K S.Science 1990,247,1439. (16)Suslick, K S. Sci. Am. 1989,260,80. (17)Suslick, K S.;Doktycz, S. J. Adv. Sonochem. 1990, I , 137-230. (18)Suslick, K S.; Green, M. L. H.; Thompson, M. E.; Chatakondu, K J . Chem. Soc., Chem. Commun. 1987,900.
STM images were obtained with a Burleigh ARIS2200E STM equipped with a mechanically prepared Pt/Ir tip. GaAs ethanol suspensions were deposited onto planar, polycrystalline gold electrodes (AAI-ABTech) and imaged in air. All electrochemistrywas performed using a BAS 1WB potentiostat interfaced to an IBMcompatible 80386 computer. Gold, glassy carbon, and platinum disk working electrodes as well as Ag/AgCl reference electrodes were purchased from BAS. A Pt wire (Aldrich) served as auxiliary electrode. Supporting electrolytes were prepared from either KC1 (Mallinckrodt) or phosphate buffer at pH = 7 (Fisher). All suspensions of GaAs were prepared from the dried, annealed powder in the designated solvent by sonicating in a Branson 1200 bench-top ultrasonic cleaning bath for less than 1 min to disperse the particles. Filtration experiments were performed using Swin-Lok Filter Holders (Nuclepore) and polycarbonate capillary pore membranes with pore sizes of 800,500, 200, 100, 50, 30, and 15 nm (Nuclepore). UV/vis measurements were performed on suspensions of GaAs prepared in ethanol (Aldrich) or deionized water purified with a Barnstead NANOpure ultrapure water system, which exhibited a minimum resistivity of 18.0 MBcm. Measurements were acquired with a Hewlett-Packard Model 8452A diode array spectrophotometer and quartz cuvettes having a path length of 1 cm. Care was taken to acquire the spectra immediately after the suspensions were formed, as prolonged exposure to water has been reported to contribute to the growth of oxide 1a~ers.l~ For GaAs suspended in water for 2 days, the presence of gallium oxide was verified by W/vis through comparison with the spectrum of pure GazO3 (Aldrich) in water. The presence of arsenic oxide on GaAs exposed to water was verified electrochemically through comparisons of cyclic voltammograms (CVs) of GaAs suspensions before and after exposure to water with CVs of pure A s 2 0 3 (Aldrich) in water. RESULTS AND DISCUSSION
XRD. After synthesis, the material was first characterized by powder X-ray diffraction to identify the crystalline phase. XRD involves the direct application of X-rays of known wavelength onto a powder and the subsequent reflection of those X-rays by lattice planes of the crystal at an angle 28 with respect to the X-ray beam.20 Crystalline regions smaller than about 200 nm are known to exhibit difhaction peaks that are signifmntly broadened with respect to those observed for large crystallites. This broadening effect has been described in a quantitative way in the Scherrer equation,2O
where B is the broadening of the diffraction peak associated with a particular (hkl) lattice plane (full width at half-height) expressed in units of 28; K is a constant in general equal to 1;L is the average length of the crystallite; 1 is the wavelength of the incident X-rays; and 8 is half the reflected angle. The XRD pattern in Figure 1 shows the (lll),(220), (331), and (400)reflections of GaAs, and (19)Liliental-Weber, Z.;Wilmsen, C . W.; G i b , K M.; Kirchner, P. D.; Baker, J. M.;Woodall,J. M.I. Appl. Phys 1990,67,1863-1867and references therein. (20) Hug, H. P.; Alexander, L E. X-Ray Diffraction Proceduresfor Polycrystalline and Amorphous Muterink John Wiley and Sons: New York, 1974;pp 687690 and references therein.
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28 (degrees) Figure 1. XRD of unsonicated GaAs using Cu K a = 1.5148 A. Data were smoothed using a 5 point moving average prior to line shape analysis.
the corresponding d-spacings matched well with reported Calculation of particle size based on this broadening using the (111) reflection yields a value of 12.1 nm for K = 1. The reproducibility of the XRD measurement was determined by obtaining three powder patterns on three different days. The standard deviation of these measurements was only 0.1 nm; however, variance in baseline determination can yield an uncertainty in particle size of up to 2 nm. The XRD powder pattern of the nanocrystalline GaAs which was exposed to 1 h of high intensity ultrasound in propylene carbonate was also obtained. The line shape analysis yielded a particle size of 12 nm as well. This result suggests that the extreme temperatures and pressures associated with sonication under these conditions do not affect the crystalline regions of the sample to a significant degree. HRTEM. High-resolution TEM requires transmission of electrons through the sample in order to obtain an image. Since samples must be very thin (
