J. Phys. Chem. B 1997, 101, 8887-8891
8887
Solid Solution of CdyZn1-yS Nanosized Particles: Photophysical Properties J. Cizeron and M. P. Pileni* Laboratoire SRSI, URA CNRS 1662, UniVersite´ P et M Curie, (Paris VI), BP 52, 4 Place Jussieu, 75231 Paris Cedex 05, France, and CEA-CE Saclay, DRECAM-SCM, 91191 Gif sur YVette, Cedex, France ReceiVed: April 18, 1997; In Final Form: August 5, 1997X
Optical properties of CdyZn1-yS nanoparticles differing by their size and composition are presented. The fluorescence quantum yield increases with increasing particle size and composition (y). The photoluminescence, PL, spectra are due to defect states and follow the quantum size effect. A red-shift in the fluorescence spectrum with increasing cadmium composition and particle size is observed which is attributed to a transition from the valence band to a deeply trapped hole. Both absorption and fluorescence spectra in various media and at temperatures from 77 to 300 K are presented.
I. Introduction Optical properties of II-VI nanosemiconductors have been widely studied from experimental1-5 and theoretical viewpoints.6-10 These studies give a good understanding of the quantum size effect. It is certain that surface states play an important role in fluorescence properties,11-13 but due to the many unknown parameters related to the surface structure, it is difficult to describe their influence. Few studies of these optical properties involving ternary compounds are available.14-17 In our previous paper, CdyZn1-yS nanoparticles were synthesized by using reverse micelles. Control of the particle size and composition, y, was obtained.18 Here, we report the photophysical properties of CdyZn1-yS nanosized particles. II. Experimental Section II.1. Materials. Isooctane and sodium bis(2-ethylhexyl)sulfosuccinate were from Fluka and Sigma, respectively. Pyridine, sodium sulfate, heptane, dodecanethiol, and benzenethiol were from Merck. All these products were used without further purification. Syntheses of zinc and cadmium bis(2-ethylhexyl)sulfosuccinate, Zn(AOT)2, and Cd(AOT)2 have been described previously.19 II.2. Synthesis and Extraction of Nanosized Particles. Colloidal CdyZn1-yS particles were prepared by mixing three micellar solutions having the same water content {w ) [H2O]/ [AOT]}: one contains sulfide ions {Na2S} and the two others Zn(AOT)2 and Cd(AOT)2 separately. The ratio x ) ([Cd2+] + [Zn2+])/[S2-] is fixed at 1. Due to the dynamic properties of reverse micelles, the reaction occurs in a few seconds. The overall (2-ethylhexyl)sulfosuccinate concentration is kept constant at 0.1 M. Immediately after mixing the micellar solutions, CdyZn1-yS nanoparticles are formed. Addition of either benzenethiol, dodecanethiol, or thiophenol induces a chemical reaction at the interface between cations and thio derivatives20 which prevents the particle growth. The coated particles are either kept in reverse micelles or extracted from them by alcohol addition. They are redispersed in nonaqueous solvents with formation of an optically clear solution. The syntheses were performed at water contents of 2.5, 5, and 10, which corresponds to 2, 2.3, * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, September 15, 1997.
S1089-5647(97)01357-6 CCC: $14.00
and 2.9 nm average particle sizes, respectively. As demonstrated in the previous paper,18 this technique allows either varying the composition, y, keeping the size constant or keeping the composition constant and varying the particle size. The composition, y, is determined by energy dispersion spectroscopy, EDS. Electron diffraction patterns show a zinc blende structure even for CdS particles whereas the CdS bulk phase is wurtzite as observed for other II-VI semiconductors.21,22 II.3. Equipment. Absorption spectra were recorded on a Cary IE spectrometer. Photoluminescence spectra were recorded with a Spex Fluorolog spectrometer with a T geometry. The cryoscopic equipment was an Oxford DN1704 liquid nitrogen cryostat with temperature regulation between 77 and 300 K. The samples were first cooled at 77 K and then slowly heated with plateaus at various temperatures in order to obtain absorption and luminescence spectra. II.4. Determination of the Relative Photoluminescence Quantum Yield. The photoluminescence, PL, is recorded in wavenumbers. Because zero PL was not reached, the values were obtained by extrapolation assuming PL followed a Gaussian curve. The total spectrum area was measured, and the highest yield is estimated to be 100. III. Results and Discussions Absorption spectra were obtained for bare CdyZn1-yS nanosized particle and those coated by various thio derivatives. These particles were either trapped in micellar solution or dispersed in a bulk solvent. Luminescence spectra were obtained for particles coated by dodecanethiol. At room temperature, the CdyZn1-yS luminescence properties are similar in micellar solution and for particles dispersed in bulk solvent. The variation of luminescence with temperature was obtained for coated particles dispersed in isopentane/methylcyclohexane (3:1 v/v). III.1. Variation of the Absorption Spectra with Size, Composition, and Various Coatings. Figure 1 shows the absorption spectra of coated CdyZn1-yS nanoparticles differing by their size and composition. For a given composition, the absorption spectrum is red-shifted with increasing particle size. This has been well established and is attributed to a quantum size effect: the diameter of the particles approaches the excitonic diameter and their electronic properties change.1-5 At a fixed size, the red-shift in the absorption spectrum with increasing composition, y (Figure 1), is due to changes in the solid-phase © 1997 American Chemical Society
8888 J. Phys. Chem. B, Vol. 101, No. 44, 1997
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Figure 1. Absoption spectra of CdyZn1-yS nanoparticles differing by their sizes and compositions. Composition: y, 0 (A, B, C); y, 0.25 (D, E, F); y, 0.5 (G, H, I); y, 0.75 (J, K, L); y, 1 (M, N, O). Average diameter: d, 2 nm (A, D, G, J, M); d, 2.3 nm (B, E, H, K, N); and d, 2.9 nm (C, F, I, L, O).
TABLE 1: Variation of the Energy Bandgap, Eg, in eV with Particle Size and Composition D (nm) y (%)
2
2.3
2.9
0 25 50 75 100
4.20 3.60 3.32 3.16 3.10
4.16 3.46 3.22 3.06 2.99
3.97 3.26 2.97 2.76 2.67
composition.18 The energy bandgap, determined from the absorption onset, smoothly increases with increasing composition from ZnS to CdS (Table 1). Similar behavior was observed for the bandgap variation of bulk CdyZn1-yS23 and for mixed nanoparticles.14-16 The coated CdyZn1-yS absorption spectra do not change compared to those observed in micellar solutions. No changes either with the various thio derivatives used to coat the particles (dodecanethiol and benzenethiol) nor with the solvent used to disperse the coated particles (pyridine, heptane, isopentane/ methylcyclohexane(3:1 v/v) are observed. These results are rather surprising compared to those published previously, where the absorption spectra of CdS particles vary with the charge and chemical composition of the particle surface.2,26,27 Our data could be explained by the fact that the charge and the surface composition of CdyZn1-yS nanosized particles remain unchanged with increasing particle size and composition, y. This assumption is supported by the following. (i) In a previous paper,24 we deduced the coverage of CdyZn1-yS nanosized particles differing by size and composition. This estimate was made by taking into account the number of sulfurs per cadmium atom determined by EDS and from simulation of the nanosized particles with a zinc blende structure. The excess of sulfur atoms was attributed to the coverage. It remains at 20% with particles differing by their size and composition and the various thio derivatives.
Figure 2. PL spectra of CdyZn1-yS nanoparticles differing by their sizes and compositions. Composition: y, 0 (A, B, C); y, 0.25 (D, E, F); y, 0.5 (G, H, I); y, 0.75 (J, K, L); y, 1 (M, N, O). Average diameter: d, 2 nm (A, D, G, J, M); d, 2.3 nm (B, E, H, K, N); and d, 2.9 nm (C, F, I, L, O).
(ii) The control of the size and composition is obtained by slight differences in the synthetic mode (changing the number of water per surfactant molecules). This cannot markedly perturb the surface composition. (iii) From the literature we know that the particle charge changes with the pH of the aqueous solution25 in which the synthesis is performed. In reverse micelles, the pH cannot be evaluated. However the slight increase in the number of water molecules per surfactant needed to obtain a size control does not drastically change the number of protons in the water pool. Thus it can be concluded that because CdyZn1-yS nanoparticles retain the same charge and same surface composition, their absorption spectra are not sensitive to the various coverages. No obvious explanation can be given by the fact that the absorption spectra do not change with the polarity of the bulk solvent. III.2. Photoluminescence Spectra. At room temperature, photoluminescence, PL, spectra of CdyZn1-yS particles coated with thiododecane are broad and red-shifted compared to the absorption onset (Figure 2). They are attributed to deep traps, as it has been observed with CdS nanoparticles.13,28-31 The PL spectra remain the same on changing the excitation wavelength, indicating a low size distribution. At a fixed composition, y, the relative fluorescence quantum yield increases with increasing particle size. This is attributed to the strong confinement of the charge carriers, which favors the nonradiative de-excitation at the surface. The increase in the particle diameter induces a decrease in the confinement and favors the radiative processes. Table 2 shows an increase in the relative PL quantum yield with increasing composition and that it reaches a maximum
Solid Solution of CdyZn1-yS Nanosized Particles
J. Phys. Chem. B, Vol. 101, No. 44, 1997 8889
TABLE 2: Variation of the Relative PL Quantum Yield with Composition, y, and Particle Size D (nm) y (%)
2
2.3
2.9
25 50 75 100
12 3 11 12
4 23 25 40
12 40 100 71
TABLE 3: Variation of the Energy Difference between the Energy Bandgap and PL Maximum (in eV) with Particle Size and Composition D (nm) y (%)
2
2.3
2.9
0 25 50 75 100
1.02 0.54 0.73 0.73 0.76
1.02 0.74 0.79 0.81 0.80
0.95 1.05 0.85 0.78 0.83
TABLE 4: Differences in the ∆E and Energy Bandgap, Eg, with Particle Size at Various Compositions (Obtained with Different Particle Sizes (2, 2.3, and 2.9 nm) y (%) δ∆E ∆E2,3(nm) - ∆E2(nm) (eV) δEg, Eg2(nm) - Eg2.3(nm) (eV) δ∆E, ∆E2,9(nm) - ∆E2(nm) (eV) δEg, Eg2(nm) - Eg2.9(nm) (eV)
0
25
50
75
100
0 0.04 -0.07 0.23
0.2 0.14 0.51 0.34
0.06 0.10 0.12 0.35
0.08 0.10 0.05 0.40
0.04 0.11 0.07 0.43
around 75%. This could be due either to the increase in the number of defect states or to a decrease in the nonradiative yield of CdyZn1-yS particles when cadmium composition increases. III.3. Variation of the Energy Difference between Absorption Onset and Maximum Fluorescence. The PL maximum is shifted to lower energies with increasing either the particle size or cadmium composition. The derived energy difference between absorption onset and fluorescence maximum, ∆E, is given in Table 3. III.3.1. For y Value Equal or Up to 50%. The ∆E variation (Table 3) with particle size is small compared to the energy bandgap (Table 1). Table 4 shows a large difference between the variation of ∆E between two particle sizes, δ∆E, and that of the bandgap for the same sizes, δEg. This is more pronounced when the difference in size is larger. With increasing particle size, the δ∆E value is small compared to that obtained from the energy bandgap variation δEg. This indicates that δ∆E is a small fraction of the extra energy due to confinement. We can then conclude that the fluorescence process follows the quantum size effect observed from the absorption. Because the CdyZn1-yS structure remains unchanged and retains the zinc blende phase and the surface is unchanged, it can be assumed that, at a given composition, the energy level of a given trap site does not markedly change with size. The charge carriers involved in this process are either deeply trapped holes or electrons (Figure 3). When the charge carriers involved are deeply trapped holes, the PL corresponds to a transition from the conduction band to the hope trap state (Figure 3A). If the trap carriers are electrons, the PL takes place from the state due to sulfur vacancies to the valence band (Figure 3B). This transition is not very sensitive to the quantum size effect.30 To differentiate between these two mechanisms, we have to take into account several factors: The fraction of extra kinetic energy due to confinement carried by electrons can be estimated from the effective mass of the electron, me, and hole, mh, {mh/(mh + me)}. With CdS, there is a relatively large difference in these
effective masses and the fraction of extra kinetic energy is rather high, 75%. This indicates that the recombination of an electron with a trapped hole is more sensitive to size effects than that of a trapped electron with a hole. For a transition arising from trapped electrons, the variation of ∆E with particle size, δ∆E, would be similar to that of the energy bandgap, δEg. We would expect a large variation of ∆E with the particle size, which is found to be rather constant (Table 3). From this, it can be concluded that the charge carriers cannot be attributed to sulfur vacancies. On the other hand, various data are in favor of charge carriers involving deeply trapped holes (Figure 3A). (i) One is the small change in ∆E with particle size. Furthermore, this variation in ∆E is smaller for cadmium-rich particles (Table 3) having a lower electron to hole effective mass ratio. This is in good agreement with the hypothesis of a confined electron with a hole trap recombination. (ii) For CdS particles, the sulfur level perturbed by a cadmium vacancy is found at 0.84 eV from the valence band.32 This value is in rather good agreement with the ∆E given in Table 3. This is confirmed (see below) by the unchanged ∆E values with temperature which remain around 0.8 eV, (Table 5). (iii) The small change in the ∆E value with composition can be explained by the presence of the CdS structure trap state, which does not change with y. This is supported by the fact that the local environment (at the nearest neighbors) is unchanged with both composition and size,24 although the data extracted from EXAFS measurements are less sensitive to minor defects than those obtained by luminescence spectroscopy. III.3.2. At y ) 25%. Table 4 shows a large variation of δ∆E with size. The variation of the energy bandgap, δEg, with size is smaller than that observed for ∆E (Table 3). Thus, the δ∆E variation with size is higher than the quantum confinement energy variation. This could be attributed to a change in the hole-trapping sites with composition due to the low cadmium composition. The variation of ∆E with temperature is quite large (see below Table 5) and confirms a change in the trapping sites in the particles. III.3.3. For ZnS Particles. ∆E does not change with particle size and remains close to 1 eV. This can be attributed to other trap sites. The fact that ∆E varies slowly should be related to a much less pronounced confinement regime for ZnS particles for these sizes than for cadmium-rich particles. III.4. Comparison of Absorption and PLE Spectra. The absorption and photoluminescence excitation spectra, PLE, are compared. Figure 4 shows a PLE red-shift compared to the absorption spectra for the lowest particle diameters, whereas the shift is negligible for larger particles. The red-shift in the PLE spectra compared to the absorption (Figure 4) observed for the lower particle size is attributed to a drastic drop in the PL quantum yield with a decrease in particle size. In fact, the relative PL quantum yield markedly decreases with decreasing particle size (Table 2). Hence, the PL cannot take into account emission due to the smallest particles, whereas the absorption spectrum does. The increase in the size distribution with decreasing particle size cannot be retained because (i) The fluorescence spectra remain the same on changing the excitation wavelength, indicating a low size distribution; (ii) no differences in the size distribution are observed in the histograms; (iii) in most nanoparticle syntheses using this technique,33 the size distribution increases with particle size. From this, it is concluded that the change in the excitation fluorescence spectra compared to the absorption is due to the marked decrease in the fluorescence quantum yield with
8890 J. Phys. Chem. B, Vol. 101, No. 44, 1997
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Figure 3. Fluorescence mechanism schemes involving traps for holes (A) or electron (B).
TABLE 5. Variation of the Energy Difference (eV) between Absorption Onset and Fluorescence Maximum with Composition and Temperature (* Excitation Energy, nm). the Particle Diameters Are 2.5 nm y % (*) T (K)
25%(320)
50%(360)
75%(380)
100%(400)
77 100 150 200 225 250 275 300
0.79 0.77 0.81 0.85 0.86 0.90 0.91 0.95
0.77 0.76 0.79 0.77 0.76 0.79 0.79 0.80
0.82 0.79 0.86 0.82 0.81 0.84 0.84 0.85
0.77 0.80 0.81 0.79 0.80 0.82 0.82
Figure 5. Variation of the maximum fluorescence intensity with temperature and with various particles diameters, d (nm), and composition, y. Average diameters: (A, C, F) 2 nm; (B, D, E, and G) 2.3 nm. Composition: (A, B) y ) 25%; (C, D) y ) 50%, (E) y ) 75%; and (F, G) y ) 100%. Excitation wavelengths: (A, B) 300 nm, 0; 320 nm, 4, (C) 320 nm, 0; 340 nm, 4, (D, F) 340 nm, 0; and 360 nm, 4, (E) 360 nm, 0; 380 nm, 4, (G) 380 nm, 0, 400 nm, 4.
Figure 4. Comparison of absorption and PLE spectra for mixed composition particles with y ) 0.5 and y ) 0.75 for three different sizes.
decreasing particle size. The PLE does not take into account the presence of the smallest particles, whereas absorption does. III.5. Variation of the Absorption and Luminescence Spectra with Temperature. The absorption, PL, and PLE spectra were recorded in the temperature range 77-300 K. As
expected, a slight blue-shift in the absorption and PLE spectra is observed. This is attributed to the relative position of the conduction and valence bands due to dilatation of the lattice with increasing temperature and to electron-lattice interactions.34 Figure 5 shows the variation of the maximum fluorescence intensity with temperature. Above 100 K a large increase in the fluorescence intensity is observed with decreasing temperature, whereas below this it remains unchanged. Table 5 gives the variation of ∆E (energy difference between absorption onset and fluorescence maximum) with temperature and composition. On decreasing the temperature from 300 to 77 K, the PL spectra remain broad and red-shifted compared to the absorption. This confirms that the fluorescence is due to defect states. No direct fluorescence is observed; this could be related to the following. The energy bandgap of these compounds is high. In the case of CdSe, direct fluorescence is observed for particles as small as 2 nm. While for CdS, with a larger bandgap, the direct fluorescence is observed for larger particles (3 nm diameter).13 In the present experiments, the particle bandgap is, at least,
Solid Solution of CdyZn1-yS Nanosized Particles higher than that of pure CdS and their diameters are below 3 nm. Thus, CdyZn1-yS bandgaps are probably too high to induce a direct transition. The fact that below a given particle size the direct fluorescence is no longer observed can be attributed to the confinement effect. Therefore, in very small particles (diameter less than exciton bohr radius) the exciton does not have to migrate to reach the surface, so that the charge carriers are easily trapped by surface defects in these structures.9 The particle size increase decreases the confinement and then decreases the probability for a carrier to be trapped before direct recombination. Below 100 K, the maximum fluorescence intensity does not vary with temperature and with particle size. This could be due to multiphonon nonradiative electron transfer30,35 and not to quantized vibration modes as suggested in ref 36 because of the insensitivity of the plateau temperature to size variation. IV. Conclusion Absorption spectra studies of CdyZn1-yS particles show the quantum size effect for various compositions. This size effect is more pronounced for cadmium-rich particles because of the lower charge carrier effective mass. The CdyZn1-yS absorption spectra are not affected by changing the surrounding media by heptane and pyridine. The PL spectra are due to trapping states. These roughly behave like an absorption transition for cadmium-rich particles. The energy difference between maximal fluorescence and absorption onset is not nearly so size dependent for cadmiumrich particles. It is explained as a radiative recombination involving a hole trap with an electron in confined states. The fluorescence intensity variation with the size indicates the importance of the particle surface for nonradiative deexcitation. For CdyZn1-yS nanoparticles differing by their sizes and compositions, the progressive increase in the fluorescence intensity with decreasing temperature reaches a plateau around 130 K. Due to a nonexponential shape, it is not possible to extract an apparent activation energy for a nonradiative process. Similarly, the energy difference between absorption onset and maximum fluorescence does not change with temperature for cadmium-rich particles, whereas it varies for cadmium-poor particles. This confirms that defect states change with particle composition. References and Notes (1) Brus, L. E. J. Phys. Chem. 1983, 79, 5566; 1984, 80, 4403; 1986, 90, 2555.
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