J. Phys. Chem. C 2007, 111, 579-585
579
Superradiance in GaSe Nanoparticle Aggregates Karoly Mogyorosi and David F. Kelley* UniVersity of CaliforniasMerced, Merced, California 95344 ReceiVed: August 6, 2006; In Final Form: September 26, 2006
Transient absorption results are presented on two types of GaSe nanoparticle aggregates. The results indicate that both types of aggregates exhibit larger emission oscillator strengths than the corresponding monomers, that is, superradiance. One type of GaSe nanoparticle is synthesized by conventional methods, and the other is synthesized at room temperature from very small GaSe particle nuclei. The latter type of particles form very strongly interacting aggregates, presumably because they have fewer edge-binding ligands. In both cases, the extent of superradiance is inferred from the stimulated emission band in the transient absorption spectrum. Oscillator strength ratios (n-aggregate/n monomers) are 1.5 and 1.8-2.0 for the conventional and cold-synthesized aggregates, respectively. These ratios can be quantitatively understood in terms of an electrostatic coupling model and random inhomogeneous broadening in the aggregates.
Introduction J-aggregates of organic dyes are well known and have been extensively studied, both experimentally and theoretically.1-3 The oscillators on individual dye molecules interact coherently to form delocalized states, some having very low and others very high oscillator strengths. In the case of J-aggregates, the transition dipoles align such that the states having the largest oscillator strengths are at the lowest energies. Due to motional narrowing, very little vibronic activity is observed in the absorption and fluorescence spectra of these aggregates. Thus, the fluorescence spectra of organic J-aggregates are typically spectrally sharp and intense and have short fluorescence lifetimes. There is a thermal distribution of the occupied states, and because the states having the largest oscillator strengths are at the lowest energies, the fluorescence is particularly intense at low temperature.4,5 The formation of J-aggregates is usually thought of exclusively with organic dyes, but can occur in any set of coupled radiative oscillators. The formation of “nanoparticle J-aggregates” has recently been reported for GaSe nanoparticles in concentrated solutions.6,7 GaSe is a layered semiconductor, having highly anisotropic physical, electronic, and optical properties. The bulk material has an indirect band gap at about 2.1 eV and a direct transition at Γ at slightly higher energy.8-13 GaSe nanoparticles consist of single tetra-layer disks; Se-Ga-Ga-Se. Particles ranging in diameter from 2.5 to 12 nm have been synthesized.14,15 The size-dependent static spectroscopy is reasonably well understood. The largest particles exhibit a z-axis quantum confinement of about 4000 cm-1. Smaller particles exhibit the same z-axis quantum confinement, in addition to quantum confinement in the x,y plane. We have recently shown that these particles stack to form locally one-dimensional aggregates at high concentrations in room temperature solutions.6,7 The lowest energy transition is z-polarized, and in this geometry, the oscillators on adjacent particles interact to form J-aggregates. This suggests that relaxed excited states should exhibit superradiance, just as in the case of organic J-aggregates. However, the photophysics of these * Corresponding author. E-mail:
[email protected].
nanoparticles is complicated. Following excitation, they undergo a variety of relaxation and carrier trapping processes.15-17 Because of the complicated photophysics, radiative lifetimes are difficult to determine accurately from emission measurements. However, oscillator strengths can be easily determined by time-resolved stimulated emission measurements. The extent of stimulated emission is assessed from time- and wavelengthresolved absorption change measurements following band gap excitation: a transient absorption difference (TA) spectrum. We have recently reported these spectra in the context of assigning the size-dependent electron and hole intraband transitions.15,18 The TA spectra of GaSe nanoparticles are dominated by two features: a positive absorption in the 540 - 750 nm region and an apparent negative absorption at shorter wavelengths. The positive absorption is due to hole and/or electron intraband transitions. The negative absorption is due to a combination of intraband absorptions and (dominantly) stimulated emission. Absolute measures of the extent of stimulated emission are difficult to make. However, the amplitude of the intraband absorptions should be independent of the extent of aggregation. Thus, the extent of stimulated emission may be determined using the amount of further red transient absorption as an internal standard. In this paper, we analyze the transient absorption and stimulated emission of approximately 5.6 nm, relatively monodisperse GaSe nanoparticles. We have also synthesized somewhat smaller (4 - 5 nm) GaSe nanoparticles that have fewer edge ligands and are therefore considerably stronger interacting than those produced by the conventional synthesis. Not surprisingly, aggregates of these particles also give more intense superradiance. In both cases, we compare the observed extent of superradiance with that calculated from a Monte Carlo method that models the aggregate as a linear array of inhomogeneously broadened, coupled oscillators. Experimental Section Three different GaSe nanoparticle samples were synthesized and used in the studies presented here. The first sample was prepared as described in our previous publications.7,15 The sample used here was synthesized using five particle growth
10.1021/jp0650533 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/10/2006
580 J. Phys. Chem. C, Vol. 111, No. 2, 2007
Mogyorosi and Kelley
Figure 2. Absorption spectra of the ultrafiltration methanol solution (solid line), the solute phase, rescaled to correspond to a volume of 50 mL (dashed line) and the diluted retentate phase (dotted line). Also shown is the absorption spectrum of the UFA sample scaled to correspond to a final volume of 50 mL and measured in a 10 mm cell (large dots/line).
Figure 1. (A) Absorption spectra of × 5 butanol diluted (solid line) and concentrated (dotted line) monodisperse 5.6 nm conventional (S5) particles, cold-synthesized 3.9 nm (UFA) (line/open dots) and 4.6 nm (UFB) (line/filled dots) particles. (B) Emission spectra of butanol diluted (solid line) and concentrated (dotted line) monodisperse 5.6 nm conventional (S5) particles, cold-synthesized 3.9 nm (UFA) (line/open dots) and 4.6 nm (UFB) (line/filled dots) particles.
cycles and is therefore referred to as a “conventional” S5 sample. Its absorption and emission spectra are shown in Figure 1 and are very similar to those previously published for other conventional samples of similar sized particles.7,15 The spectroscopic results presented here were obtained on a sample having an average particle diameter of 5.6 nm. The other two samples were prepared as follows. First, two precursor solutions were made: a solution of trioctylphosphine selenium (TOPSe) made from 6.0 mL of trioctylphosphine (TOP) with 0.7110 g of Se (99.999%, Alfa Aesar) and a solution of GaMe3 made from 1.02 mL of GaMe3 (99%, Strem) with 6.0 mL of TOP. In all cases, the TOP (90%, Sigma-Aldrich) was vacuum-distilled, methanol-extracted (the methanol was 99.8%, Sigma-Aldrich, distilled from sodium under argon), and vacuum-distilled again. The reaction mixture begins with a solution of 0.3925 g of Se dissolved in TOP (8.9 mL) in the reaction flask at room temperature and under an argon atmosphere. This was followed by addition of 0.7 mL of GaMe3 precursor solution. The reaction mixture is heated to 280 °C in 13 min, cooled to 268 °C, and kept at this temperature for 90 min. The reaction mixture is subsequently rapidly cooled to about 50-80 °C, followed by injection of 0.63 mL of each of the TOPSe and GaMe3/TOP precursor solutions and reheating to 268 °C. The temperature is maintained at 268 °C for 90 min and then cooled to 80 °C. As the reaction proceeds the clear yellow dispersion becomes opalescent. The precipitate is separated from the sample by centrifugation and the supernatant injected into the cleaned flask along with another 0.63 mL of each of the precursor solutions. The entire heat-react-coolcentrifuge cycle was repeated two more times with the addition of 1.25 mL of the TOPSe and GaMe3 precursor solutions to yield a conventional TOPO-free S3 sample. A 5 mL aliquot of
this solution was extracted with 20 mL of dry methanol under nitrogen. The methanol extracts some fraction of the particles, a small amount of TOP, and small GaSe nuclei also formed in the synthesis. The methanol phase is diluted to a final volume of 50 mL with dry, distilled methanol and ultrafiltrated in a Millipore solvent resistant stirred cell with a 1000 Da regenerated cellulose membrane (Millipore, PLAC type). Approximately 90% of the total volume is filtered through the membrane and collected in a nitrogen-purged flask. This is referred to as the “solute phase”. The retentate phase (∼5 mL) was diluted with methanol to its original volume of 50 mL. Figure 2 shows the absorption spectra of the methanol solution, the solute phase (rescaled to correspond to a volume of 50 mL), and the diluted retentate phase. The starting methanol solution and the retentate phase show a slight yellow color, and the solute phase shows basically no color. Comparison of the starting solution and the retentate phase spectra reveals that there is an absorbance decrease below 430 nm, which becomes larger at shorter wavelengths. This indicates that the larger particles extracted to the methanol were mainly retained there and only the very small ones (
