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Langmuir 2002, 18, 1490-1493
Solid Solution of Cd1-yMnyS Nanocrystals L. Levy,† D. Ingert,† N. Feltin,† V. Briois,‡ and M. P. Pileni*,† Laboratoire LM2N, ESA 7070, Universite P et M Curie (Paris VI), BP 52, 4 Place Jussieu, 75231 Paris Cedex 05, France, and LURE, Universite´ Paris-Sud, BP34, 91898 Orsay Cedex, France Received August 2, 2001. In Final Form: December 4, 2001 Formation of a solid solution of Cd1-yMnyS nanocrystals is demonstrated using EXAFS (extended X-ray absorption fine structure).
I. Introduction Diluted magnetic semiconductors (DMSs) II-VI are ternary alloys in which cations of a nonmagnetic AIIBVI semiconductor matrix (like CdS, CdTe, ZnSe, etc.) are randomly substituted by magnetic ions (Mn2+, Fe2+, Co2+). DMS bulk materials have been extensively studied in the past few years by several groups.1 They exhibit a number of interesting magnetooptical effects such as a giant Faraday rotation,2 large Zeeman splitting of carriers,3 and bound magnetic polarons.4 The presence of localized magnetic ions in a semiconductor alloy leads to a hybridization of the Mn 3d states with the sp-band states. These exchange interactions play a dominating role in the magnetic properties of DMSs4-6 and alter the manganese ion composition dependence of the band gap.7 In addition, there appears to be a growing interest in nanosized DMSs in the semiconductor scientist community.8-13 In our previous papers, we demonstrated that colloidal self-assemblies make it possible to synthesize Cd1-yMnyS nanosized particles and to independently control size and composition, y.14-16 A progressive control of the nanocrystal average diameter (from 1.8 to 4 nm) and composition (from y ) 0.006 to 0.36) has been obtained. As in bulk materials, EPR (electron paramagnetic resonance) and photoluminescence (PL) measurements show that Mn2+ ions in the * To whom correspondence should be addressed. † Universite P et M Curie. ‡ Universite ´ Paris-Sud. (1) Furdyna, J. K. J. Appl. Phys. 1991, 64, 6114. (2) Gaj, J. A.; Proc. 15th Internat. Conf. Physics of Semiconductors. J. Phys. Soc. Jpn. 1980, 49, suppl. A, 797. (3) Bastard, G.; Rigaux, C.; Mycielski, A. Phys. Status Solidi B 1977, 79, 585. (4) Dietl, T.; Spalek, J. Phys. Rev. Lett. 1982, 48, 355. (5) Bastard, G.; Rigaux, C.; Mycielski, A. Phys. Status Solidi B 1977, 79, 585. (6) Kacman, P. Semicond. Sci. Technol. 2001, 16, R25. (7) Holden, T. M.; Dolling, G.; Sears, V. F.; Furdyna, J. K.; Giriat, W. Phys. Rev. B 1982, 26, 5074. (8) Bhargava, R. N. J. Lumin. 1997, 72-74, 46. (9) Bandaranayake, R. J.; Lin, J. Y.; Jiang, H. X.; Sorensen, C. M. J. Magn. Magn. Mater. 1997, 169, 289. (10) Yu, I.; Isobe, T.; Senna, M. J. Phys. Chem. Solids 1996, 57, 373. (11) Oka, Y.; Yanata, K. J. Lumin. 1996, 70, 35. (12) Counio, G.; Esnouf, S.; Gacoin, T.; Boilot, J. P. J. Phys. Chem. 1996, 100, 20021. (13) Norris, D. J.; Yao, N.; Charnock, F. T.; Kennedy, T. A. Nano Lett. 2001, 1 (1), 3. (14) Levy, L.; Hochepied, J. F.; Pileni, M. P. J. Phys. Chem. 1996, 100, 18322. (15) Levy, L.; Feltin, N.; Ingert, D.; Pileni, M. P. J. Phys. Chem. B 1997, 101, 9153. (16) Levy, L.; Ingert, D.; Feltin, N.; Pileni, M. P. Adv. Mater. 1998, 10, 53.
CdS matrix on the nanoscale range are located in a tetrahedral site.17,18 However, the Mn2+-Mn2+ interactions are markedly enhanced compared to those in the bulk material.17,18 A fundamental question is whether there is a random or a nonrandom substitution of the Mn2+ cations in a CdS matrix. To answer this, a local and selective probe is needed to obtain a greater knowledge of electronic states. In this study, X-ray absorption fine-structure (EXAFS) experiments make it possible to investigate the local structure and it is shown that a solid solution is formed for any Cd1-yMnyS nanocrystal composition. II. Experimental Section Materials. Sodium di(ethyl-2-hexyl) sulfosuccinate, Na(AOT), was from Sigma, and sodium sulfide (Na2S) was from Janssen. Isooctane was obtained from Fluka, ethanol from Prolabo, and heptane and dodecanethiol from Merck. Cadmium and manganese di(ethyl-2-hexyl) sulfosuccinate [Cd(AOT)2 and Mn(AOT)2] were synthesized in our laboratory as described previously.19 Apparatus. Energy-dispersive spectrometry (EDS) measurements were made with a Link AN 10,000. A JEOL (100 kV) model JEM 100CX II was used for transmission electron microscopy (TEM) and electron diffraction. The mean diameter, Dm, and the standard deviation, σm, were derived from an average number of 500 particles. EXAFS measurements were carried out at LURE, the French synchrotron radiation laboratory, using the DCI storage ring (300 mA, 1.856 GeV, λc ) 3.7 Å) on the D44 EXAFS station. At the Cd edge (26711 eV), the measurements were carried out in transmission mode using two ionization chambers filled with krypton. X-rays were monochromatized using a Ge(400) double crystal. For each sample, data acquisitions were repeated seven times. The data collection at the Mn edge (6539 eV) was done by fluorescence using a Si(Li) monoelement solid-state detector. A Si(311) double-crystal monochromator was used, and harmonic contamination was avoided by using a special mirror device.20
III. Synthesis and Characterization Cd1-yMnyS nanoparticles are synthesized in water in oil droplets stabilized by a monolayer of surfactant.21 This colloidal dispersion is usually called reverse micelles. The surfactant used is sodium bis(2-ethylhexyl) sulfosuccinate, (17) Feltin, N.; Levy, L.; Ingert, D.; Pileni, M. P. J. Phys. Chem. B 1999, 103, 4. (18) Feltin, N.; Levy, L.; Ingert, D.; Pileni, M. P. Adv. Mater. 1999, 11, 398. (19) Petit, C.; Lixon, P.; Pileni, M. P. Langmuir 1991, 7, 2620. (20) Sainctavit, P.; Petiau, J.; Manceau, A.; Rivallant, R.; Belakhovsky, M.; Renaud, G. Nucl. Instrum. Methods Phys. Res. 1988, A273, 423. (21) Pileni, M. P. Structure and reactivity in reverse micelles; Elsevier: Amsterdam, 1989.
10.1021/la011232u CCC: $22.00 © 2002 American Chemical Society Published on Web 01/26/2002
Solid Solution of Cd1-yMnyS Nanocrystals
called Na(AOT). Coprecipitation takes place in the droplets on mixing two micellar solutions having the same water content, w ) [H2O]/[AOT]: 0.1 M Na(AOT) containing S2- ions; and a mixed micellar solution containing Cd(AOT)2, Mn(AOT)2, and Na(AOT). An excess of sulfur ions is used in the syntheses (x ) ([Cd2+] + [Mn2+])/[S2-] ) 1/2), at various ratios of Mn(AOT)2 and Cd(AOT)2. The diameter of the water droplet varies linearly, from 0.5 to 18 nm,21 with the relative water/surfactant concentration ratio. The particle size is controlled with reverse micelle water content: the size of coated particles increases with increasing the droplet size in which the synthesis was made. By changing the ratio of Mn(AOT)2 and Cd(AOT)2 used for the synthesis, the manganese composition within particles varies. With this procedure, size and composition of nanocrystals are controlled independently.14 Details are given in refs 15 and 16. The average sizes are determined from histogram plots derived from an average number of 500 particles observed by TEM. The average particle sizes and their distributions are 1.8 nm (16%), 3.2 nm (15%), and 4 nm (10%). The presence of Mn2+ in a tetrahedral environment in the CdS matrix is confirmed by EPR spectroscopy.17,18 Mn2+ replaces a fraction of the Cd2+ within the structure. The extracted particles are analyzed by EDS, and the Cd1-yMnyS composition is deduced by averaging data taken from several areas of the carbon grid. The obtained average compositions, y, are 0, 0.05, 0.1, and 0.2. The nanocrystal structure determined from electron diffraction is zinc blende, whereas it is wurtzite in the bulk phase. Similar structural changes have been observed with IIVI semiconductor nanocrystals.22,23 The average lattice parameter is constant (a0 ) 5.83); the slight variation of this value with nanocrystal dimensions is neglected with respect to experimental error. No diffractograms corresponding to MnS or MnO are observed. IV. EXAFS Background The absorption spectra are obtained by averaging seven experimental spectra for manganese and cadmium K edges by a cumulative approach:
Langmuir, Vol. 18, No. 5, 2002 1491
the k3χ(k) weighted data before Fourier transforming from k ) 3.8 to 14 Å-1 at the Cd and Mn K edges. The expression used for simulating the EXAFS signal is
χ(k) ) S02 k
∑i
Ni
e-2k σi e-2Ri/λ(k) fi(π,κ) sin[2kRi + φi(k)] 2
2
Ri2
with
k)
x
8π2me (E - E0) h2
The mean free path for excited electrons, λ(k), is related to a Γ parameter through the equation λ(k) ) k/Γ; S02 is a many-body amplitude factor set to unity for all samples. Retrodiffusion amplitude fi(π,k) and dephasing Φi(k) were calculated from Mackale’s tables or extracted from reference experiments with models such as CdS and pure micrometer size particles from Aldrich. For this extraction, we take the interatomic distance and coordination numbers of the bulk phase, and the Γ parameter is set equal to 0. For the use of tabulated functions, the Γ parameter was first determined using the model compounds and fixed at the value thus obtained for fitting. The remaining free parameters Ri, Ni, σi, and E0 are the interatomic distance, the coordination number, the Debye-Waller factor for a given shell, and the ionization energy of the considered absorption edge (which fix the origin of the kinetic energy of the emitted electron), respectively. Only the best fit selected with χ(k) k3 weighting was retained. The first neighbor numbers are determined within about 10% precision, and the first neighbor distances were determined within an uncertainty of about 0.01 Å. V. Results and Discussion
where µ(E), µ1(E), and µ0(E) are the observed absorption, the atomic absorption, and the pre-edge background, respectively. The normalization which includes the preedge background last removal is obtained using the Lengeler-Eisenberger formula.26 The atomic absorption coefficient, µ1(E), is approximated by using a fifth-degree polynomial expression. E0 is taken at the beginning of the absorption slope. A Kaiser window (τ ) 3.5) is applied to
Fourier transforms of cadmium K edge EXAFS signals of CdS and Cd0.8Mn0.2S nanocrystals having 3.2 nm average diameter are compared to those of the CdS bulk phase (Figure 1). With nanocrystals, the first coordination sphere is observed in all the spectra, whereas the second one is observed for the bulk phase with a low intensity at 300K. Similar behavior has been observed27,28 with a second coordination in bulk material and not with nanocrystals. The first coordination shell is filtered, and the best curve-fitting is obtained with the characteristic parameters of the nanocrystals such as the interatomic distance, R ) 2.52 Å, the coordination number, N ) 4, the Debye-Waller factor, σ ) 0.08 Å, and Γ ) 0.03 Å-2. Figure 2 shows such fits for 3.2 nm Cd1-yMnyS nanocrystals at various compositions (y ) 0 and y ) 0.2) and bulk CdS. The distance of the first neighbors (4 sulfur atoms) from the absorbing atom (Cd, i.e., 2.52 Å) is the same in the bulk phase as in nanocrystals. This distance is independent of the amount of manganese included in the CdS matrix as is usually observed for a solid solution.29 Figure 3 shows Fourier transforms of Cd0.9Mn0.1S nanocrystals for various average sizes (Dm ) 1.8, 3.2, and
(22) Cizeron, J.; Pileni, M. P. J. Phys. Chem. 1995, 99, 17410. (23) Vogel, W.; Urban, J.; Kundu, M.; Kulkarni, S. K. Langmuir 1997, 13, 827. (24) Mathan, N.; Prouset, E.; Husson, E.; Dexpert, H. J. Phys.: Condens. Matter 1993, 5, 1291. (25) Michalowicz, A. Socie´te´ franc¸ aise de chimie: Paris, 1991; p 102. (26) Lengeler, B.; Eisenberger, P. Phys. Rev. B 1980, 21, 4507.
(27) Rockenberger, J.; Tro¨ger, L.; Kornowski, A.; Vossmeyer, T.; Eychmu¨ller, A.; Feldhaus, J.; Weller, H. J. Phys. Chem. B 1997, 101, 2691. (28) O’Day, P. A.; Ebert, M.; Carroll, S. A.; Waychunas, G. A.; Bird, D. K.; Neuhoff, P. Stanford’s activity report, Proposal 2325Mp. (29) Happo, N.; Sato, H.; Mihara, T.; Mimura, K.; Hosokawa, S.; Ueda, Y.; Taniguchi, M. J. Phys.: Condens. Matter 1996, 8, 4315.
∑I0/∑I) µ(E) ) (∑I/∑I0)
µ(E) ) ln(
for transmission experiments for fluorescence experiments
The EXAFS signal extraction, deglitching, background removal, normalization, Fourier transformation, filtering, back transforming, and fitting procedures were the same as those used in refs 24 and 25. The EXAFS signal χ(E) is
χ(E) ) [µ(E) - µ1(E)]/[µ1(E) - µ0(E)]
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Figure 1. EXAFS signal Fourier transforms at Cd K edge for (a) bulk CdS, (b) 3.2 nm CdS nanoparticles, and (c) 3.2 nm Cd0.8Mn0.2S nanoparticles.
Figure 2. Experimental k3χ (black squares) compared with the filtered first-shell function based on parameters for mainpeak fitting (full line) for (a) bulk CdS, (b) 3.2 nm CdS nanoparticles, and (c) 3.2 nm Cd0.8Mn0.2S nanoparticles.
4 nm) at the manganese K edge. The second (next nearest neighbors, NNN) and first coordination spheres (nearest neighbors, NN) are observable for 3.2 and 4 nm samples. Conversely, only the first shell is observed with nanocrystals having 1.8 nm average diameter. Appearance of the second coordination sphere with increasing the nanocrystal size is explained in terms of a decrease in the disorder and an increase in the crystallinity with increasing the particle size. With 1.8 nm nanocrystals, the distance distribution of NNN atoms around Mn is highly
Levy et al.
Figure 3. EXAFS signal Fourier transforms at Mn K edge for (a) 1.8 nm, (b) 3.2 nm, and (c) 4 nm Cd0.9Mn0.1S nanoparticles.
Figure 4. Experimental k3χ (black squares) compared with the filtered first-shell function (full line) for (a) 1.8 nm, (b) 3.2 nm, and (c) 4 nm Cd0.9Mn0.1S nanoparticles and with the secondshell function for (d) 3.2 nm and (e) 4 nm Cd0.9Mn0.1S nanoparticles. The parameters for main-peak fitting are listed in Table 1.
disordered. Hence, by decreasing the particle size, the disorder in the CdS matrix increases. This is explained by a decrease in the crystallinity. This is confirmed by electron diffraction patterns where the diffraction rings are diffuse. This change in the disorder and crystallinity with increasing the particle size is confirmed by the Fourier
Solid Solution of Cd1-yMnyS Nanocrystals
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Table 1. Local Structure around Mn in 1.8, 3.2, and 4 nm Cd0.9Mn0.1S Nanoparticles Obtained from Curve-Fitting the First and Second Main Peaks in the Fourier Transformed Dataa first layer Dm (nm)
N1
σ1 (Å)
Γ1 (Å-2)
second layer R1 (Å)
∆E1 (eV)
N2
σ2 (Å)
Γ2 (Å-2)
R2 (Å)
∆E2 (eV)
Mn-thio 0.6 0.6 0.6
0.00 0.05 0.00
0.01 0.01 0.01
2.60 2.60 2.60
0.3 0.3 0.3
Mn-Mn 1.8 1.8
0.09 0.02
0.02 0.05
3.80 3.77
0.0 0.0
First Shell 1.8 3.2 4 MnSbulk
Mn-S 3.5 3.5 3.5 4
3.2 4
Mn-Cd 11.2 10.2
0.11 0.09 0.08
0.01 0.01 0.01
2.38 2.38 2.38 2.30
0.3 0.3 0.3 Second Shell
0.4 0.2
0.02 0.05
4.20 4.20
0.0 0.0
a N is the number of atoms in the nth layer, σ is the Debye-Waller factor, R is the mean distance between the absorbing atom and n n n the first shell, and ∆En is the difference between the theoretical and actual zero kinetic energy values.
transform spectrum where an increase in the first two peak intensities with increasing the particle size is observed. Filtered functions are extracted from various coordination spheres (only for 3.2 and 4 nm samples), and the best curve-fittings are shown in Figure 4. The parameters used in the calculations, where two layers are taken into account, are given in Table 1. The first layer is exclusively composed of sulfur atoms with the greater part (85%) located at 2.38 Å from the Mn atoms. In the second layer, a small amount of sulfur (15%) is located at 2.6 Å taking into account the Mn-thiododecane bound on the surface. The filter extracted from the second coordination sphere could be fitted by a mixed layer of cadmium and manganese with the stoichiometry imposed by the alloy Cd0.9Mn0.1S (NCd ) 10.5 and NMn ) 1.8). The ratio, yEXAFS ) 0.15, given by calculation is not the exact value (y ) 0.1). This difference is attributed to estimated Γ and ∆E0 but could be considered as a correct qualitative result. This good agreement between the calculated number of cadmium and manganese atoms with
composition y strongly supports formation of a solid solution. The distances found for Cd and Mn atoms are around 4.2 and 3.8 Å, respectively. The data obtained from the second coordination sphere confirms formation of a solid solution. VI. Conclusion In this paper, it has been demonstrated that four sulfur atoms, similar to that observed in the CdS bulk phase, surround each cadmium in Cd1-yMnyS. At the manganese edge, the second coordination sphere is fitted by a mixed layer of cadmium and manganese with nearly the right stoichiometry, y. This indicates formation of a solid solution. Furthermore, the second coordination sphere is observed for the largest nanocrystals indicating that for any composition the crystallinity of the nanoparticles increases with increasing the particle size. LA011232U
