J. Phys. Chem. 1995,99, 7754-7759
7754
Synthesis and Characterization of InP, Gap, and GaInP2 Quantum Dots 0. I. Mikik,* J. R. Sprague, C. J. Curtis,* K. M. Jones, J. L Machol, and A. J. Nozik* National Renewable Energy Laboratory, 161 7 Cole Blvd., Golden, Colorado 80401
H. Giessen, B. Fluegel, G. Mohs, and N. Peyghambarian Optical Sciences Center, University of Arizona, Tucson, Arizona 85721 Received: October 6, 1994; In Final Form: January 26, 1995@
Quantum dots (QDs) of InP, Gap, and GaInP2 with diameters ranging from 20 to 65 A were synthesized as well-crystallized nanoparticles with bulk zinc blende structure. The synthesis of InP, GaP, and GaInP2 QDs was achieved by heating appropriate organometallic precursors with stabilizers in high boiling solvents for several days to produce QDs which can be dissolved in nonpolar organic solvents, forming transparent colloidal QD dispersions. The high sample quality of the InP and GaP QDs results in excitonic features in the absorption spectra. Ternary QDs of GaInPz were synthesized with a well-crystallized zinc blende structure and lattice spacing between InP and GaP. The QDs were characterized by TEM, powder x-ray diffraction, steady state optical absorption and photoluminescence spectroscopy, transient photoluminescence spectroscopy, and fs to ps pump-probe absorption (Le., hole-burning) spectroscopy.
Introduction The unique size-dependent, optical, photocatalytic, and nonlinear optical properties of colloidal nanocrystalline semiconductor particles (called quantum dots (QDs)) continue to attract considerable interest.] Excellent progress has been made in the preparation and characterization of QDs made from IIVI compounds, oxides (Ti, In, Cu, W) and iodides (Hg, Pb, Bi); the preparation and properties of Si QDs have also been reported.2 However, the preparation of high-quality QDs of 111-V semiconductorshas proven to be pr~blematic.~-'~ Here, we report the synthesis and properties of excellent quality binary 111-V InP and GaP QDs, and the temary 111-V QD, GaInPz. It is interesting to compare the properties of InP and GaP QDs because the former is a direct gap semiconductor, while the latter is an indirect gap semiconductor; the effects of size quantization on the optical properties of the indirect semiconductor GaP are explored here. GaInP2 is of interest because its band gap depends on the degree of atomic ordering in the material, and the effect of quantization on this behavior has not been investigated. The preparation of well-crystallized QDs of 111-V compounds requires high temperature, and the relevant solution chemistry is not well-developed. At high temperatures, quantum dots tend to aggregate irreversibly, and the resulting precipitate cannot be redissolved to form colloidal solutions; also, at high temperatures the particles tend to grow and lose their quantum confinement. However, using high-boiling solvents ( t > 300 "C) and certain stabilizers, we have recently succeeded in preparing well-crystallized InP QDs" with diameters -25 A. In this work, we report on additional characterization studies of InP QDs, and the successful synthesis of colloidal dispersions of QDs of crystalline GaP and GaInP2 ranging in diameter from 20 to 65 A.
Experimental Section General. All organic solvents were dried and distilled under nitrogen before use. Chloroindium oxalate and chlorogallium oxalate complexes that are used in the QD synthesis were @
Abstract published in Advance ACS Abstracts, May 1, 1995.
prepared by mixing anhydrous Inch or GaC13 with Na2C204 in CH3CN in a 1:1 molar ratio, and heating the solution of 70 "C under N2 for 15 h. The resulting suspension at white material was filtered and the solvent removed from the clear filtrate to give the chlorometal oxalate complex. The complex appears to contain two metal atoms per oxalate and three chloride ligands per metal atom, but the mode of oxalate binding and the number of CH3CN ligands present have not yet been established. All synthetic procedures were carried out using standard airless techniques. Synthesis of Colloidal InP QDs. Quantum dots of InP were synthesized by mixing the chloroindium oxalate complex and P(SiMe3)3 in a molar ratio of 1n:P of 1.6:l in CH3CN at room temperature to form a soluble orange InP precursor species, and then heating this precursor solution with a mixture of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP). Use of TOPO/TOP as a colloidal stabilizer was based on the recent work of Murray, Noms, and BawendiI2who demonstrated that TOPOROP could control the growth and the size distribution of CdSe QDs. If InC13 is used as the indium source, a solution is formed which does not yield crystalline InP at 320 "C. Three different particle size QDs were prepared as follows: colloid (a) was formed by heating 0.21 g of chloroindium oxalate with 0.13 g of P(SiMe3)3 in 5 g of TOPO and 5 g of TOP at 270 "C for 3 days; QD colloids (b) and (c) were prepared by heating 0.42 g of chloroindium oxalate with 0.26 g of P(SiMe& in 0.2 g of TOPO and 2 g of TOP for 3 days at 270 and 300 "C, respectively. After heating, the InP colloids were precipitated with CH30H, and the resulting powder was washed with 200 mL of CH3OH. The InP QD powder can then be completely redissolved in toluene to form the QD colloid. Synthesis of Colloidal GaP QDs. Quantum dots of GaP were synthesized by mixing GaCl3 (or the chlorogallium oxalate complex) and P(SiMe3)3 in a molar ratio of Ga:P of 1:l in toluene at room temperature to form a GaP precursor species and then heating this precursor in a high-boiling solvent. Wells et al.I3 first synthesized and characterized the yellow GaP precursor, [C12GaP(SiMe3)2]2, that is formed from GaC13 and P(SiMe3)3. The GaP precursor solution is added to a toluene solution of TOPO, stirred for 1 h, and then transferred to a high-
0022-365419512099-7754$09.00/0 0 1995 American Chemical Society
InP, GaP, and GaInP2 Quantum Dots boiling solvent; toluene was removed under vacuum and the resulting transparent viscous solution was slowly heated. The colloidal 30 8, GaP QDs reported here were prepared by first mixing 0.18 g of GaC13 with 0.25 g of P(SiMe3)s in 0.1 g of TOPO and 2 g of TOP and heating to 270-320 “C. At this stage the GaP is amorphous. To prepare crystallized QDs, the amorphous material is mixed with 10 g of TOPO (or 5 g of triphenylphosphine) and heated at 360 “C for 3 days. The GaP QDs were precipitated with CH30H, separated, washed with CH30H, and redissolved in toluene; when triphenylphosphine or tris(2-diphenylphosphinoethy1)phosphine was used instead of TOPO, the QD precipitate only partially dissolved in toluene. Synthesis of Colloidal GaInP2 QDs. Quantum dots of GaInP2 were synthesized by mixing chlorogallium oxalate and chloroindium oxalate complexes and P(SiMe3)3 in the molar ratio of Ga:In:P of 1:1:2.6 in toluene at room temperature, followed by heating. The specific conditions for the 25 8, QDs were to heat 0.19 g of chlorogallium oxalate and 0.23 g of chloroindium oxalate with 0.52 g of P(SiMe3)3 in 0.1 g of TOPO and 0.4 g of tris(2-diphenylphospinoethy1)phosphine at 400 “C for 3 days. GaInP2 QDs were separated, purified with CH3OH, and partially redissolved in a 0.1% solution of TOPO in toluene. The larger, more crystalline 65 8, sample was obtained by heating a flask containing the isolated material in a yellow flame, and redissolving in a similar manner. Optical Characterization and Time-Resolved Spectroscopy. Optical absorption spectra were collected at room temperature using a Cary 5E W-VIS -NIR spectrophotometer with 0.2-cm quartz cuvettes. Samples were prepared by dispersing washed QDs in toluene. Photoluminescence spectra were obtained at room temperature using a SPEX Fluorolog-2 spectrometer. Emission lifetimes were determined by timecorrelated single photon counting. A cavity-dumped synchronously-pumped dye laser (Spectra-Physics 3500) operating at 585 nm provided pump pulses of 10 ps. A Hamamatsu microchannel plate detector provided a typical instrument response function of 70 ps. The lifetimes were estimated by fitting the decay using a sum of exponentials as an indicator of the decay time. Femtosecond laser pump-probe (Le., hole buming) experiments were performed at the Optical Sciences Center at the University of Arizona. The laser system is composed of a colliding-pulse mode-locked laser and a six-pass dye amplifier pumped by a tightly folded resonator (TFR) Nd:YLF laser operating at 1000 Hz and 527 nm. The output of this amplifier pumps on ethylene glycol jet to produce a chirped 1-ps continuum pulse from 450 to 700 nm. The continuum pulse is split with one part becoming the probe pulse while the other part is passed through an interference filter producing a spectrally narrow (-10 nm bandwidth) pulse that is amplified in a second six-pass amplifier pumped by another TFR Nd:YLF laser operating at 1000 Hz and 527 nm. The 520 nm sample (see Figure 3) was excited by a 115 fs, 420 nJ pump pulse focused to a 45 p m diameter spot, producing a power density of 230 GW/cm2. 13 nJ/pulse or 7 GW/cm2 was used to excite the 590 nm sample. The central wavelength was adjusted by angle tuning an interference filter. 230 GW/cm2 produced some transient signals lasting a few hundred femtoseconds in the blank solvent, while 7 GW/cm2 produced no such transients. At this power density no transient signals were recorded. The pump pulse can be optically delayed over a 500ps range and is typically 1000 times more intense than the probe pulse. Normally, the optical delay of the pump pulse with respect to the probe pulse is fixed for a given experimental run. The pump and the probe pulse, which are orthogonally polarized, cross at an angle in the sample, and the difference spectrum of
J. Phys. Chem., Vol. 99, No. 19, 1995 7755 the probe pulse with the pump pulse on and off is recorded with an optical multichannel analyzer. Approximately 130 nm of the probe pulse spectrum is recorded simultaneously. After the difference spectrum is recorded, the optical delay of the pump pulse is moved to a new value and the data acquisition process is repeated. Because the probe is a 1-ps chirped pulse (red wavelengths are on the leading edge of the pulse while blue wavelengths are on the trailing edge), one must correct for the apparent delay in the transient absorption signal of the blue part of the spectrum with respect to the red by “dechirping” the data. All of the pump-probe data presented in this paper have been “dechirped” except for the data at time delays of 2 ps and longer which do not require this correction. As a further point of clarification, zero time delay refers to the peak of the probe pulse overlapping with the peak of the pump pulse. Because the pump pulse is shorter by a factor of almost 10 than the probe pulse, time delays between these pulses over a 1-ps range will result in some temporal overlap within the sample. The designation of zero time delay is therefore arbitrary. Time delay values in the figure captions are relative time delays between pulses for all the data in that figure. A .time delay of -5 ps means that the probe pulse passed through the sample 5 ps ahead of the pump pulse. Since the repetition rate of the laser is 1000 Hz, the effective time delay in this case is 1 ms. The optical density was adjusted so that the absorption of the pump pulse would be maximized while maintaining an optically transparent solution. If the solution becomes too concentrated, the particles start to precipitate and cause the sample to appear cloudy, scattering the laser light. This latter situation was completely avoided. All optical experiments were performed at room temperature to avoid precipitating the QDs. X-ray Powder Diffraction. Powder X-ray diffraction spectra were collected on a Rigaku 300 Rotaflex diffractometer operating in the Bragg configuration using Cu K a radiation. Samples for X-ray diffraction were prepared from -50 to 100 mg of thoroughly washed and dried nanocrystalline powder. Transmission Electron Microscopy. A Phillips CM-30 electron microscope operating at 200 kV was used for transmission electron microscopy (TEM). Imaging was carried out in bright field with an objective aperture selected to permit lattice imaging of the (1 11) zinc blend plane. Samples were prepared by depositing colloidal solutions on grids using a glass nebulizer.
Results and Discussion InP QDs. Different particle sizes of InF’ QDs can be obtained by changing the concentration of the orange precursor in the TOPO/TOP solution or by changing the temperature at which the solution is heated. The duration of heating also improves the QD crystallinity. The precursor has a high decomposition temperature (>270 “C); this is advantageous for the formation of InP quantum dots because the rate of QD formation is controlled by the rate of decomposition of the precursor. This slow process leads to InP QDs with a narrow size distribution. Figure 1 shows the absorption spectra of InP QD colloids with different particle diameters; bulk InP is black with a band gap of 1.35 eV (918 nm). The colloidal solutions with different particle sizes range in color from orange to brown. The brown colloid has the largest particle diameter of 46 8,;the other sizes are 35 8, and 26 8,. TOPO used for the preparation of the QD colloids forms a complex with In3+. However, this complex absorbs below 350 nm and does not interfere with the absorption spectra of the InP colloids. Heating TOPO above 150 “C also results in a very small degree of unstructured background absorption that is less than 1% of the absorbance of the QD samples at the excitonic peaks and can be ignored.
MiCiC et al.
7756 J. Phys. Chem., Vol. 99, No. 19, I995 2.5
2:
20 2.0
,
I
A
15
10
05
0.0
00 4
700
800
500
500
800
600
BOO
700
Wavelenglh (nm)
Wavelengths (nm)
Figure 1. Absorption spectra for In! QD colloids with different diameters: (a) 26 A; (b) 35 A; (c) 46 A.
Figure 3. Spectra for two InP QD colloids used in pump-probe experiments, designated as 520 nm and 590 nm. 0 08
d
eH
s:
z
005
004 003 002
.Q
000 -001
640
620
600
580
560
540
520
500
Wavelength [nm] IO
20
30
40
50
BO
70
2Q
Figure 2. X-ray ditfraction pattern for dried InP QDs colloids with diameters of (a) 26 A, (b) 35 A, and (c) 46 A compared with the peak positions of bulk zinc blende InP (d).
Figure 2 shows the X-ray diffraction pattems of the 26, 35, and 46 8, InP QD particles formed into a film by drying the colloids. The peaks assigned to diffraction from the ( l l l ) , (220), and (311) planes of crystalline zinc blende bulk InP are seen at 28 of 26.2 f 0.2", 46.3 f 0.2", and 51.7 f 0.2", respectively. The mean particle diameter was estimated from the Debye-Scherer formula. These diameters are in agreement with the values obtained from transmission electron microscopy and from SAXS data. In the absence of the TOPO stabilizer, the particles grow large and the sharp peaks of bulk InP are obtained. The shape and size distribution of the InP QDs were determined by TEM. TEM pictures of InP preparations with TOPO that were only heated to 220 "C for 3 days do not show the formation of either amorphous or crystalline InP. Upon heating to 240 "C for 3 days the formation of nanoparticles is evident, but the product is primarily amorphous as judged from electron diffraction. However, when the preparation is heated at 270 "C for 3 days, electron diffraction pattems show the (1 1l), (220), and (311) planes of zinc blende InP. The photoluminescence (PL) for InP QD preparations showed two emission bands: one band in the visible (400-600 nm) and a second band above 800 nm. The measured PL lifetime was found to be -10 ns for the visible band and 2500 ns for the near-IR band. However, it was found that TOPO itself emits strongly in the visible after it had been heated at 260 "C for 4 h; hence we
Figure 4. Pump-probe experiments for InP QDs showing the differential absorbance (-AA) spectra at room temperature taken every 100 fs after time zero for the 590 nm InP QD colloid pumped at 560 nm. Positive values on the figure correspond to bleaching, which becomes saturated after about 250-300 fs. Time zero is defined in the text in the Experimental Section; the curve labeled (a) is at time zero. could not establish the intrinsic visible PL properties of our TOPO-capped quantum dots. We note that an unusual feature of the PL from TOPO that was heat-treated by itself at 270 "C is that the positions of the emission peaks vary linearly with the excitation frequency over visible excitation ranging from about 2 to 3 eV; these emission peaks were red-shifted from the excitation energy by 0.4 to 0.5 eV. The near-IR PL with a lifetime >500 ns can be attributed to the InP QDs; the large Stokes shift and the > 500 ns lifetime are believed to be caused by trapping and subsequent emission from the trap states. The absorption spectra of the two InP QD samples used in the pump-probe experiments are shown in Figure 3; they are labeled by the approximate wavelength of their excitonic peaks. Three types of time-resolved pump-probe experiments were recorded for both samples. First, short time delays with 50 fs steps were used to determine the dynamics of the onset of the transient bleach signal. Second, longer time delays from 2 to 200 ps were used to observe the decay of the transient bleach signal; and third, the pumping wavelength was changed to see if any shift in the transient spectrum occurred with pumping wavelength. Figure 4 shows the femtosecond transient absorption (bleaching) behavior for the 590 nm QD sample in 100 fs steps after being pumped at 560 nm. Clearly, the onset of the bleach (burned hole) is very fast reaching its maximum in 250-300 fs.
IIP, GaP, and GaInP2 Quantum Dots
-0.02
J. Phys. Chem., Vol. 99, No. 19, 1995 7757
; 640
620
600
580
560
540
520
500
Wavelength [nm]
Figure 5. Pump-probe experiments for the 520 nm InP QD sample (pumped at 553 nm) with longer time delays (from 2 to 200 ps): (a) at 2 ps, (b) at 10 ps; (c) at 50 ps: (d) at 100 ps; (e) at 200 ps. Curve f is at -5 ps (equivalent to f l ms; see text).
a
0.16
7
~
amorphous (270%)
'..
bulk direct
bulk indirect 559 nm (2.22eV)
',446 nm (2.78eW
.. '....R"U!y.!??"C!
I
,____,,, I
Wavelengths (nm)
Figure 7. Optical absorption spectra at room temperature of GaP QDS
4
-0.02
640
620
600
580
560
540
520
heated at different temperatures and dissolved in toluene; also shown (dotted line) is the absorption spectrum of the GaP precursor {[ClzGaP(SiMe&]z}. The direct and indirect band gaps of bulk GaP are indicated by the vertical line markers on the abscissa.
500
Wavelength [nm]
b 04
640
620
600
580
560
540
520
500
Wavelength [nm]
Figure 6. Pump-probe experiments showing differential absorbance
(-AA)spectra as a function of the pump wavelength:. (a) 520 nm InP QD colloid: (b) 590 nm InP QD sample. The pump wavelengths are indicated by the arrows along the wavelength axes. The spectra are shown after a time delay of 2 ps.
Figure 5 shows the transient absorption signal for the 520 nm sample (after being pumped at 553 nm) using longer time delays from 2 to 200 ps. The signal appears to decay about 30% over this time range. With a time delay of 1 ms we could still observe that the transient signal had not completely decayed. Finally, we changed the pump wavelength and monitored the transient absorption as a function of excitation energy. In Figure 6a we show the transient bleaching signal for the 520 nm QD sample for three different pump wavelengths that are all to the red of the excitonic absorbance peak; the burned hole is always
10
20
30
40
50
60
70
2 0
Figure 8. X-ray diffraction pattem for dried GaP QD colloids heated at (a) 270 "C, (b) 320 "C, (c) 370 "C, and (d) 400 "C, and compared with the peak positions of bulk zinc blende GaP (e).
blue-shifted with respect to the pump energy and the peak moves slightly bluer with bluer excitation. The results for the 590 nm QD samples are shown in Figure 6b. In Figure 6b we see that pumping relatively far to the blue of the excitonic peak for the
MiCiC et al.
7758 J. Phys. Chem., Vol. 99, No. 19, 1995
20
30
50
40
60
70
20
Figure 9. X-ray diffraction pattems for QD colloids of GaInP2 (65 A), GaP (30 A), and InP (35 A).
590 nm QD sample produces a bleached peak that is red-shifted with respect to the pump, but the peak of this bumed hole does not move much when the wavelength of the pump is changed over a small range. If the 590 nm sample is pumped to the red of the excitonic peak, then the bumed hole is blue-shifted and moves slightly with changing pump wavelength. These results are the first example of pump-probe experiments and hole buming in InP quantum dots. Pump-probe results for GaAs have been reported, but they showed a 200 ps risetime for photobleaching followed by a slower decay.6 The nearinstantaneous bleaching of the Inp quantum dot absorption signal
Figure 10. Absorption spectra for 25
A GaInP2 colloids;
is consistent with QD state filling; this has also been observed in 11-VI QD ~amp1es.I~ The position of the bumed hole as a function of the pump wavelength can yield information on whether the optical transitions in QDs are homogeneously or inhomogeneously b r ~ a d e n e d . ' From ~ Figures 3 and 4 we see that pumping to the blue (at 560 nm) of the exciton peak for the 590 nm QD results in a red shift of the bumed hole (peak about 570 nm). From Figures 3 and 5 we see that pumping to the red (at 553 nm) of the exciton peak for the 520 nm QD results in a blue shift of the bumed hole (peak about 530 nm). The same effects are seen in Figure 6. These results indicate that the transitions in the 520 nm and 590 nm samples are homogeneously broadened since only such a system could show bumed holes that are blue-shifted when pumping to the red of the excitonic peak and red-shifted holes when pumping to the blue of the excitonic peak. The independence of the bumed-hole peak position with pump wavelength that is shown in Figure 6b is also consistent with homogeneous broadening. The small movement of the red-shifted peaks of the burned holes with pump wavelengths (Figure 6a) suggests some degree of inhomogeneous line broadening is also present in this sample. We conclude, therefore, that both homogeneous and inhomogeneous broadening affect the experimental line widths, but that the latter has a smaller influence. The very long time for the bleaching recovery for InP QDs at room temperature is intriguing; this effect could be caused by very slow hot electron relaxation in QDs5,I6 However, further work is necessary to distinguish between long decay times due to slowed carrier cooling and those due to other effects, such as traps. GaP Quantum Dots. The reaction of GaCl3 with P(SiMe33)3 yields the well-defined pale yellow precursor, [C12GaP(SiMe3)2]2,I3which dissolves in TOP/TOPO/toluene to form a transparent yellow solution. Chlorogallium oxalate also forms a precursor upon reaction with P(SiMe33)3; both precursors begin to absorb light at about 450 nm. Figure 7 shows the absorption spectra of the precursor [C12GaP(SiMes)& and GaP QDs produced from this precursor by heating at 270, 310, 370, and 400 "C.
insert shows that the colloid exhibits a direct band gap of 2.7 eV.
InP, GaP, and GaInP2 Quantum Dots The mean particle diameters of our GaP QD preparations were estimated from the line broadening of their X-ray diffraction pattems and from TEM. Figure 8 shows the X-ray diffraction pattems for GaP QD samples that were heated at different temperatures. For diffraction pattern d, the temperature was 400 "C and the size estimated to be about 30 A; for diffraction pattem c, the temperature was 370 "C and the size estimated to be about 20 A. The diffraction patterns for the lower temperature preparations (a and b) were too broad and ill-defined to permit an estimate of the particle size. The absorption spectrum of the 30 8, diameter GaP QD colloid (heated at 400 "C) shown in Figure 7 exhibits a shoulder at 420 nm (2.95 eV) and a shallow tail that extends out to about 650 nm (1.91 eV). For 20 %, diameter GaP QDs (heated at 370 "C) the shoulder is at 390 nm (3.17 eV) and the tail extends to about 550 nm. Bulk GaP is an indirect semiconductor with an indirect band gap of 2.22 eV (559 nm) and a direct band gap of 2.78 eV (446 nm). Theoretical calculations by Rama Krishna and Friesner" on GaP QDs show that the increase of the indirect band gap with decreasing QD size is much less pronounced than that for the direct gap; for 30 A diameter GaP QDs the direct and indirect band gaps are predicted to be 3.35 eV and 2.4 eV, respectively. Below 30 A the direct band gap is predicted to decrease with decreasing size while the indirect band gap continues to increase. As a result, GaP is expected to undergo a transition from an indirect semiconductor to a direct semiconductor below about 20 A. In Figure 7, we attribute the steep absorption and shoulder at 420 nm to a direct transition in the GaP QDs; the shallow tail region above 500 nm is attributed to the indirect transition. In Figure 7 the position of our assigned direct transition at 2.95 eV is less than that predicted by theory (3.35 eV) for 30 8, GaP QDs;I7 the reason for this discrepancy is not currently understood. Also, it should be noted that the absorption tail extends below the indirect band gap of bulk GaP. The origin of this subgap absorption is also not understood at the present time; it could be caused either by a high density of subgap states in the GaP QDs, by impurities created by the high decomposition temperature, or by Urbach-type band-tailing produced by unintentional doping in the QDs.Is We note that such subgap absorption below the band gap was also observed in GaP nanocrystals that were prepared in zeolite cavities by the gas phase reaction of trimethylgallium and phosphine at temperatures above 225 O C 8 This latter result implies that the subgap absorption in GaP QDs is related to intrinsic behavior, and is not caused by synthetic byproducts or impurities. GaInPz Quantum Dots. Colloidal GaInP2 QDS were synthesized that had mean particle diameters of 25 8, and 65 A; the X-ray diffraction pattern for the 65 8, sample is shown in Figure 9 along with that of the GaP and InP QDs. As expected for GaInP2, the lattice spacings of these QDs is approximately the average of that for GaP and InP. The ternary Ga-In-P system forms solid solutions which can exhibit direct band gaps ranging from 1.7 to 2.2 eV, depending upon composition and growth temperat~re.'~-*~ At the composition G%.~IQ~P, the structure can be either atomically ordered or disordered (random a l l o ~ ) ; ' ~the- ~band ~ gap is direct, but it can range from about 1.8 to 2.0 eV, depending upon the degree of atomic ordering. Although one issue of interest is how size quantization will affect the dependence of band gap on atomic ordering, we cannot at the present time specify the degree of atomic order in our GaInPz QDs. The absorption spectrum of the 25 8, GaInP2 QDs is shown in Figure 10. An estimate of the direct band gap of the GaInP2
J. Phys. Chem., Vol. 99, No. 19, 1995 7759 QDs from a plot of the square of the absorbance times photon energy versus photon energy (see insert of Figure 10) indicates a value of about 2.7 eV; this value is blue-shifted from the bulk value of 1.8-2.0, as expected for QDs. No excitonic structure is observed in Figure 10; this may be caused by a sufficiently wide size distribution which could easily mask excitonic peaks in QDS.~-IO
Acknowledgment. It is a great pleasure to dedicate this article to Mostafa A. El-Sayed. His enormous positive influence on The Joumal of Physical Chemistry, as well as in his own fields of research, is greatly appreciated by the physical chemistry community. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences. O.I.M. and C.J.C. received partial support from the NREL DDF Fund. H. Giessen, B. Fluegel, G. Mohs, and N. Peyghambarian were supported by NSF and AFOSRBMDO. References and Notes (1) For comprehensive reviews see: (a) Bawendi, M. G.; Steigerwald, M. L.; BNS, L. E. Annu. Rev. Phys. Chem. 1990,41,477. Brus, L. Appl. Phys. 1991, A53, 465. (b) Yoffe, A. D. Adv. Phys. 1993, 42, 173. (c) Steigenvald, M. L.; Brus, L. E. Annu. Rev. Mater. Sci. 1989, 19, 471. (d) Henglein, A. Chem. Rev. 1989, 89, 1861. Henglein, A. Top Curr. Chem. 1988, 143. (e) Kamat, P. V. In Kinetics and Catalysis in Microheterogeneous Systems; Graetzel, M., Kalyansendaram, Eds.; Marcel Decker: New York, 1991; p 376. (f)Special Issue on Quantum Dots, Isr. J. Chem. 1993, 33(1). (g) Peterson, M.; Nozik, A. J. In Photoelectrochemisty and Photovoltaics of Layered Semiconductor; Aruchamy, A,, Ed.; Kluwer: Boston, 1992; p 297. (h) Shiana, J. J.; Goldstein, A. N.; Alivisatos, A. P. J. Chem. Phys. 1990, 92, 3232. (i) Wang, Y.; Henon, Y. J. Phys. Chem. 1991,95, 525. (2) Bms, L. J. Phys. Chem. 1994, 98, 3575. (3) Byme, E. K.; Parkanyi, L.; Theopold, K. H. Science 1988, 241, 332. (4) Olshavsky, M. A.; Goldstein, A. N.; Alivisatos, A. P. J . Am. Chem. SOC. 1990, 112, 9438. ( 5 ) Uchida, H.; Curtis, C. J.; Nozik, A. J. J. Phys. Chem. 1991, 95, 5383. (6) Uchida, H.; Curtis, C. J.; Kamat, P. V.; Jones, K. M.; Nozik, A. J. J. Phys. Chem. 1992, 96, 1156. (7) Butler, L.; Redmond, G.; Fitzmaurice, D. J . Phys. Chem. 1993, 97, 10750. (8) MacDougall, J. E.; Eckert, H.; Stucky, G. D.; Herron, N.; Wang, Y.; Moller, K.; Bein, T.; Cox, D. J . Am. Chem. SOC. 1989, 111, 8006. (9) Douglas, T.; Theopold, K. H. Inorg. Chem. 1991, 30; 594. (10) Uchida, H.; Ogata, T.; Yoneyama, H. Chem. Phys. Lett. 1990,173, 103. (1 1) MiCiC, 0. I.; Sprague, J. R.; Curtis, C. J.; Jones, K. M.; Nozik, A. J. J. Phvs. Chem. 1994. 98, 4966. (12) ~Murray,C. B.; Noms, D. J.; Bawendi, M. G. J . Am. Chem. SOC. 1993, 115, 8706. (13) Aubuchon, S. R.; McPhail, A. T.; Wells, R. L.; Giambra, J. A,; Bowser, J. R. Chem. Mafer. 1994, 6, 82. Wells, R. L.; Self, M. F.; McPhail, A. T.; Auuchon, S. R.; Wandenberg, R. C.; Jasinski, J. P. Organometallics 1993, 12, 2832. (14) Deleted in proof. (15) Peyghambarian, N.; Fluegel, B.; H u h , D.; Migus, A,; Joeffre, M.; Antonetti, A.; Koch, S. W.; Lindberg, M. IEEEJ. Quant. Elec. 1989,25(12), 2516. (16) Benisty, H.; Sotomayor-Torres, C. M.; Weisbuch, C. Phys. Rev. E 1991, 44, 10945. (17) Rama Krishna, M. V.; Friesner, R. A. J. Chem. Phys. 1991, 95, 525. (18) Pankove. J. I. Ootical Processes in Semiconductors: Dover: New York, '1971; p 43. (19) DeLong, M. C.; Ohlsen, W. D.; Viohl, I.; Taylor, P. C.; and Olson, J. M. J. Aoul. Phvs. 1991. 70. 2780. (20) w i i , S.-H.; Zunger, A. Phys. E 1989, 39, 3279. Wei, S.-H.; Ferreira, L. G.; Zunger, A. Phys. Rev. E 1990,41,8240. Froyen, S.; Zunger, A. Phys. Rev. Lett. 1991, 66, 2132. (21) Mascarenhas, A.; Olson, J. M. Phys. Rev. E 1990,41,9947. Homer, G. S.; Mascarenhas, A.; Froyen, S.; Alonso, R. G.; Bertness, K. A.; Olson, J. M. Phys. Rev. B 1993.47, 4041. Olson, J. M.; Kurtz, S. R.; Kibbler, A. E.; Faine, P.Appl. Phys. Lett. 1990, 56, 623. JP942736M
