J. Phys. Chem. 1996, 100, 20021-20026
20021
CdS:Mn Nanocrystals in Transparent Xerogel Matrices: Synthesis and Luminescence Properties G. Counio, S. Esnouf, T. Gacoin, and J.-P. Boilot* Laboratoire de Physique de la Matie` re Condense´ e, EÄ cole Polytechnique, Centre National de la Recherche Scientifique, URA D-1254, 91128 Palaiseau Cedex, France ReceiVed: July 1, 1996; In Final Form: October 8, 1996X
Mn2+-doped CdS nanocrystals (1-2 nm in size) dispersed in organic-inorganic silica xerogels are prepared by combining a controlled precipitation of nanocrystals in inverted micelles, a separation as a pure capped cluster powder, and a dispersion in hydrolyzed silicon alkoxide. ESR study identified two Mn2+ sites, one bulklike, and another one which is attributed to ions located near the surface and which represents the main contribution. The average number of Mn2+ per particle is in the 0.2-0.8 range. The luminescence is characteristic of a Mn2+ internal transition, with energy and lifetime as in bulk materials. This bright emission (quantum yield of 7%) corresponds to an energy transfer from surface trapped carriers to Mn2+ ions.
Introduction It is now well-known that electronic properties of a semiconductor having its dimension reduced to a few nanometers undergo a drastic change. This progressive transition from bulk to molecular-like behavior has been the subject of extensive investigations and is now well understood in term of quantum confinment.1 A lot of work has been dedicated to the studying of the optical properties of these systems in which a large thirdorder susceptibility was expected.2 Most of them were achieved on II-VI compounds (ZnS, CdS, CdSe), whose synthesis as monodisperse nanoparticles is well controlled. These works have made clear the major influence of the surface of the nanoparticles which cannot be considered only as an excised fragment of the bulk material. We have assisted more recently to the emergence of another field of interest based on the luminescence properties of the nanocrystals which may find application in electroluminescent displays.3 Bulk II-VI semiconductors doped with transition metal ions may be of special interest since they are already wellknown as good luminophores, especially in the system CdZnS: Mn.4 The synthesis of those compounds as nanoparticles may be achieved with the main purpose of studying the effect of quantum confinment on the luminescence properties. The incorporation of Mn2+ ions in II-VI nanocrystals has been the subject of only a few investigations almost exclusively on the ZnMnS system.5 There is indeed a major difficulty in obtaining well-characterized doped nanoparticles, since we consider a size range for which the distribution, the surface state, and the chemical composition of the particles drastically affect their physical properties. All of these parameters may be precisely defined in the bulk materials but can only be found as distributions in an assembly of small particles. This paper is dedicated to the synthesis and characterization of Mn2+-doped CdS nanoparticles trapped in sol-gel silica matrices. A first experimental part describes the synthesis of the materials, following a general process we used in the case of pure CdS nanocrystals.6 The second part describes the ESR characterization of Mn2+ ions in term of concentration and spatial location within the CdS aggregates. Finally, we compare the luminescence properties of the material with bulk CdS:Mn and undoped CdS nanoparticles. This leads us to give an X
Abstract published in AdVance ACS Abstracts, November 15, 1996.
S0022-3654(96)01937-5 CCC: $12.00
interpretation of the recombination process responsible for the luminescence of the Mn2+ ion in the CdS:Mn nanoaggregates. Experimental Section The process we used for the synthesis of the CdS:Mn/silica nanocomposite is schematically described in Figure 1. In a first step, the particles are synthesized by controlled precipitation in inverted micelles. Then, pyridine is added to the microemulsion allowing the separation of the particles as a powder. Finally, this powder is dispersed in a solution of silicon alkoxide which provides, after inorganic polymerization, the final composite material. The controlled precipitation of the CdS:Mn nanoparticles is achieved in inverted micelles from the water/AOT/heptane ternary system, where AOT stands for the bis(2-ethylhexyl) sulfosuccinate sodium salt surfactant molecule. In all experiments, the AOT and water concentrations were taken equal to 0.5 and 2.5 mol‚L-1, respectively. A first solution is prepared, containing the cadmium and manganese salts dissolved in water droplets. This solution is then added to the same volume of a similar solution containing sodium sulfide. In all experiments, the initial concentrations of Cd2+, Mn2+, and S2- dissolved in the water pools were taken respectively equal to 0.2, 0.12, and 0.38 mol‚L-1. After a few seconds, a yellow color appears within the solution, attesting for the formation of particles. Nevertheless, we found that, at this step, the precipitation reaction is not complete due to an increase of the apparent solubility of the sulfides because of the complexation of the Cd2+ and Mn2+ ions by the sulfonate groups of the AOT polar heads. The addition of pyridine to the solution (1.4 × 10-2 mol‚L-1) was found to improve the sulfide precipitation yield through a mechanism which is not yet well understood. At the same time, the pyridine molecule complexes the surface of the nanoparticles which are thus no more stable in the inverted micelles. This allows the separation of the sulfides as a powder of pyridinecapped nanoparticles. This powder is washed with large amounts of petroleum ether to eliminate residual AOT, and the nanocrystals are finally dispersed in pure pyridine, giving a transparent yellow-orange solution. The average size of the nanoclusters was determined by comparing the shift of the UV-visible absorption band (recorded at room temperature with a Shimadzu UV-160A spec© 1996 American Chemical Society
20022 J. Phys. Chem., Vol. 100, No. 51, 1996
Counio et al.
Figure 1. Schematic representation of the process used for the preparation of CdS:Mn nanocrystal-doped silica xerogels.
of fractal silica clusters which percolation makes the solution turns into a gel (sol-gel transition). CdS:Mn nanoaggregates are incorporated by adding a suspension of nanocrystals in pyridine just before the sol-gel transition. After the transition, the careful drying of the gel at ambient temperature gives the final material either as bulk disks a few centimeters in diameter or as thin films previously deposited on various substrates (glass, ITO, etc.). ESR Characterization of CdS:Mn Nanocrystals
Figure 2. High-resolution transmission electron microscopy micrograph of CdS:Mn nanoparticles.
trophotometer) with the correlation proposed by Wang et al.7 For the same experimental conditions of synthesis, the particle average size is found to fluctuate between 1.2 and 2.4 nm, supposing that the correlation between the electronic gap and the size of the nanocrystals is not affected by the presence of Mn2+ ions. This assumption requires slightly doped-CdS nanoparticles as further confirmed by ESR experiments. Structural characterization was performed by high-resolution transmission electronic microscopy (Philips EM 430 ST microscope working at 300 kV). A cubic blende structure was observed for nanoclusters of 2 nm in size (Figure 2). Even if this differs from the stable wurtzite-like structure which can usually be seen in the bulk material, it is consistent with what has been previously observed in the case of undoped CdS nanocrystals.6 The incorporation of the particles in a transparent silica matrix is achieved using the oportunities given by the sol-gel chemistry. In a typical experiment, 20 mL of a 2.7 mol‚L-1 ethanolic solution of an alkoxide precursor is hydrolyzed by adding 2.9 mL of pH 2.5 water. The alkoxide we used in this work is the methyltriethoxysilane or MTEOS [CH3Si(OEt)3]. This choice is justified by our previous work which showed that, compared to the tetraethoxysilane [Si(OEt)4], the presence of a nonhydrolyzable alkyl group provides matrices with good optical and mechanical quality, with a better stability of the aggregates toward photodegradation. Hydrolysis and further condensation of the alkoxide molecules lead to the formation
ESR experiments were performed to determine the Mn2+ concentration and also to probe the local atomic and electronic structure of Mn in the nanocrystals. The main point is to know if the finite size influences the electronic properties of the manganese ions.8 1. Experimental Section. X-band ESR spectra between 4.2 and 300 K were collected on a BRUKER ER200D instrument equipped with an OXFORD continuous flow cryostat. The Q-band experiments were performed at room temperature with a BRUKER ESP 300E instrument. Most of the ESR experiments were realized on nanocrystal powders. It was checked that no significant change is observed in the ESR spectra for nanoclusters dispersed in a solvent or in a gel matrix. 2. Background. The Mn2+ ion has five electrons in its d-orbitals. In a cubic host such as zinc blende CdS, the d-orbitals are no more degenerated and split into two groups of orbitals, each of which is occupied by one electron. This leads to a 6A1 ground state with a total spin S ) 5/2. The theory of electron paramagnetic resonance of Mn2+ in various fields has already been largely developed.9 The spin Hamiltonian which describes the spin multiplet of the ground state for an isolated Mn2+ ion in a host single crystal is given by
[
1 H ) geµBH0‚S + a Sx4 + Sy4 + Sz4 6 S(S + 1) 1 + S‚A‚I S(S + 1)(3S2 + 3S - 1) + D Sz2 5 3
] (
)
with S ) 5/2 and I ) 5/2. ge and µB are respectively the Lande´ factor and the Bohr magneton. The first term of the Hamiltonian is the Zeeman term. The two following terms describe the zeromagnetic-field splitting: the second one corresponds to plain cubic symmetry, and the third one corresponds to the axial term of the crystal field. In the case of the cubic symmetry, this
CdS:Mn Nanocrystals
J. Phys. Chem., Vol. 100, No. 51, 1996 20023
Figure 3. Absorption spectra of CdS:Mn nanocrystals dispersed in pyridine. The average diameters are respectively (a) 2.4 and (b) 1.6 nm.
third term generally arises from disorder such as strain fluctuation or defects. Finally, the last term is the hyperfine structure term. For systems such as Mn2+ which has no unpaired s-electrons, no hyperfine interaction is expected to exist. The origin of the large hyperfine constant actually observed is that the exchange interaction between d-electrons and core s-electrons is spin orientation dependent. This causes a polarization of the s2 configuration. In the case of Mn2+, the hyperfine interaction reduces itself to the isotropic part and the spectrum is split into 2I + 1 evenly spaced lines. The isotropic hyperfine coupling constant is given by10
1 8π Aiso ) ge µBgn µn ∑(|ψisv (0)|2 - |ψisV (0)|2) 3 2S i This theory accounts quantitatively for the observed hyperfine structure of Mn2+ in purely ionic crystals. But, the main point is that Aiso is extremely sensitive to chemical environments and the overall electronic wave function. For instance, in host crystals such as cadmium chalcogenides, the measured value of Aiso is much less than in purely ionic solids and depends on the nature of the anion (S, Se, or Te) resulting from the partial covalent character of the bonding. 3. Results. 3.1. Mn Location and Concentration. Figure 4a,b shows the ESR spectra of a representative sample taken at room temperature and at 4.2 K, respectively. A detailed analysis of these spectra allows distinguishment of four contributions: Signal I: The spectrum is composed of six intense lines and of intermediate weaker bands. These resonance lines correspond respectively to allowed (∆mI ) 0) and forbidden (∆mI ) (1) hyperfine transitions between the Zeeman sublevels (mS ) (1/2). The nuclear hyperfine splitting can be directly determined, Aiso ≈ 64.5 × 10-4 cm-1 (i.e., 6.9 mT). This value is very close to the one measured in bulk CdS (Aiso ) (65.3 ( 0.4) × 10-4 cm-1).11 It is characteristic of Mn2+ ions in a tetrahedral crystal field. Signal II: It is a broad slightly dissymmetric signal that is clearly seen on the ESR spectra at 4.2 K. Saturation experiments show that the broadening of the line is inhomogeneous. The shape of the signal is showed in Figure 4c. This spectrum was recorded at 4.2 K and at high power level, with signal I almost completely saturated. Its shape and line width, which is about 5 times 7 mT, are characteristic of Mn2+ ions tetrahedrally coordinated in a disordered material. This signal
Figure 4. ESR spectra of CdS:Mn nanocrystals. Microwave frequency ν ) 9.43 GHz. (a) Spectrum recorded at room temperature and microwave power P ) 25 mW. (b) Spectrum recorded at 4.2 K and P ) 0.25 mW. (c) Spectrum recorded at 4.2 K and P ) 250 mW. The dashed line was obtained by subtracting the Curie-behaving signal (b) from signal (a).
would then correspond to Mn2+ ions which are located inside the nanocrystals, but in strongly distorted tetrahedral sites. Signal III: The intensity of this broad Lorentzian shaped signal (dashed line in Figure 4a) increases with the initial Mn2+ concentration. It shows an antiferromagnetic behavior because it almost disappears when the sample is cooled to 4.2 K. This signal is characteristic of Mn2+ ions in exchange interaction and certainly originates from a phase containing a large proportion of MnS, a byproduct of the synthesis. Signal IV: Isolated manganese ions in an octahedral field (Aiso ≈ 90 × 10-4 cm-1). The intensity is always low and not related to the initial Mn2+ concentration. This shows that these ions are located outside the nanoparticles. This signal is better resolved on the Q-band spectrum (see Figure 5), indicating ligand field fluctuations. It corresponds to Mn2+ ions either unreacted or adsorbed at the surface of the nanocrystals. Eight samples were prepared using the same experimental conditions. In each case, we carefully compared the intensity of the two first signals to a standard of copper sulfate pentahydrate polycrystals so as to deduce the number of manganese ions inside the sample. We performed a doublenumerical integration to obtain the intensity of signal II. Special care was taken to subtract the baseline correctly. The CdS weight fraction in the sample was obtained from the optical absorption of a known amount of sample dispersed in pyridine. A known solution of CdS particles with the same average size was used as a reference. The Mn:Cd atomic ratios do not significantly depend on the size of the particles and are respectively 10-4 and 4 × 10-3 for the two ESR contributions (I and II). However, the average number of manganese per nanocrystal directly depends on the size and varies from 0.2 to 0.8 for nanocrystals of respectively 1.2 and 2.4 nm. 3.2. Atomic and Electronic Structure of Mn2+ inside the
20024 J. Phys. Chem., Vol. 100, No. 51, 1996
Figure 5. ESR spectra of CdS:Mn nanocrystals recorded at room temperature and at microwave frequency ν ) 33.9 GHz.
Nanocrystals. We now focus our attention on signals I and II which represent manganese ions located in tetrahedral sites. 3.2.1. Signal I: We notice that, even if the structure of nanocrystals is zinc blende, Mn2+ ions do not occupy cubic sites, as purely cubic centers have zero probability of forbidden transition. In fact, the spectrum is similar to those observed in strained single-crystal or polycrystalline materials, suggesting disorder in the nanocrystals. Its main characteristics are as follows: (1) the noncentral transitions (mS * +1/2) are unresolved; (2) the lines are symmetric showing a rather isotropic spectrum; (3) the intensities of the different allowed transitions are different and the transitions at high field are broader. To estimate the spin Hamiltonian parameters, the transition probabilities are calculated with Bir’s method.12 To take into account the disorder and contrarily to Allen’s approach, we do not suppose the axial zero field splitting parameter D to be unique and we integrate over a distribution P(D) of spin Hamiltonian parameters which corresponds to a crystal field distribution. Finally
I5/2/I3/2 ≈ 1 +
4096 M2 15 H 2 0
ImI is the intensity of the hyperfine line corresponding to a given nuclear quantum number mI and M2 is the second-order moment of the distribution P(D). This distribution is supposed Gaussian. If D can take positive and negative values, the ratio I5/2/I3/2 does not explicitly depend on the average value of P(D) and consequently M2 ) ∆D2, where ∆D ≈ 84 × 10-4 cm-1 represents the half-width at half-height of the distribution. The thermal treatment is a well-known technique to improve the crystallinity. Nanocrystals were heated at 160 °C for 6 h in ethylpyridine in order to check the influence of atomic defects or dislocations on the crystal field distribution. We observed a significant decrease of the line width, but the relative intensity ratios of the allowed transitions remained the same. This shows that the disorder does not mainly arise from usual defects. We are led to conclude that the effects of lower symmetry and of crystal field distribution are due to the finite size of the clusters which are so small that the cubic symmetry is always broken and the value of the crystal field varies even inside a nanocrystal. 3.2.2. Signal II: In such disordered or polycrystalline materials, noncentral transitions are always unresolved due to a broad zero field splitting parameters distribution. Therefore, signal
Counio et al. II is clearly an envelope of central transitions. Dipolar interactions and crystal field distribution are the two usual origins for the broadening of the ESR signal of transition metal ions. For instance, in bulk ZnS powders or thin films structures and for concentrations near 1%, an unresolved line is superimposed to the axial Mn2+ spectrum.13 This line was attributed to regions with high manganese concentrations. Besides, in disordered materials such as glass or frozen solutions, the disorder is modeled by a distribution of spin Hamiltonian parameters. However, the situation seems to be different in the case of Mn-doped CdS nanoaggregates for the following reasons: First, we observed that the average number of manganese per aggregate is always lower than one, then intracluster dipolar interactions appear highly improbable. Besides, the similarity between ESR spectra performed on nanocrystals either assembled in a powder or dispersed in a solution or a gel shows that intercluster interactions are negligible. Second, the Q-band spectrum is very close to the X-band spectrum (see Figure 5). As we note no significant broadening or narrowing as a function of the frequency, distribution of crystal field and g anisotropy are then not sufficient to explain the broadening of signal II. Then, we can conclude that the main source of broadening for the signal II is due neither to dipolar interaction nor to a distribution of crystal field. This suggests a distribution of hyperfine interaction which could arise from isolated Mn2+ ions located inside the nanocrystals, but near the surface. In fact, if a manganese ion is close to the surface, then its electronic density will undergo a drastic change. Therefore, fluctuations in the covalence of the Mn-S bond will give rise a distribution of the hyperfine interaction. We have now to figure out what the range of this interaction is. In other words, we have to determine how many shells of nearest neighbors a manganese ion needs to exhibit the same properties as in the bulk material and contribute to the signal I in the ESR spectrum. Obviously the first sulfur anionic shell must be complete, but this is not sufficient; the covalence of the Mn-S bond must be bulklike. This last condition can be violated near the surface, where the coordination of sulfur atoms can be incomplete. However, our experiments permit fixing an upper limit of the number of atomic shells around a manganese ion “near the surface” which contributes to the ESR signal II. Indeed signal I is still observed even for very small clusters (average diameter 1.2 nm). By using the model developed by Lippens and Lannoo,14 we find that these clusters are composed at most of four shells. We can deduce that the Mn2+ ions “near the surface” are located inside the two or three last atomic layers of the nanocrystal. These results can be used to provide information on the homogeneity of the manganese distribution in nanocrystals. We are able to estimate the theoretical ratio of the number of cations “near the surface” and “in the core” of the nanocrystal. It is equal to 7 for a 1.2 nm diameter cluster and very close to one for a 2.4 nm mean diameter, to be compared with an experimental ratio of about 40, whatever the mean diameter is. Besides, the high intensity of signal II, regardless of the size, allows us to conclude that it cannot only originate from the smallest nanoclusters, even if they would be selectively doped with Mn2+ ions. Subsequently, the Mn2+ distribution inside the nanoaggregates is extremely heterogeneous and the majority of manganese ions are located near the surface. This explanation is consistent with the fact that the largest proportion of manganese ions only precipitates after the introduction of the pyridine.
CdS:Mn Nanocrystals
Figure 6. Luminescence and excitation spectra of CdS:Mn particles in the final xerogel. Emission peak maximum is at 2.16 eV and excitation peak maximum at 3.04 eV (the decrease of the excitation intensity at high energy is due to a competitive absorption from the gel matrix).
J. Phys. Chem., Vol. 100, No. 51, 1996 20025
Figure 7. Luminescence and phosphorescence spectra of a colloidal solution of CdS:Mn nanocrystals at room temperature: luminescence before (a) and after (b) adding a small amount of methylviologen; phosphorescence before (c) and after (d) adding a small amount of methylviologen. Phosphorescence spectra were recorded 2 ms after excitation.
Photoluminescent Properties Photoluminescence and excitation spectra were recorded at room temperature (Hitachi F4500 spectrophotometer) on doped nanocrystals dispersed in pyridine and in silica xerogels. Spectra recorded on a representative sample are shown in Figure 6. The photoluminescence excitation spectrum follows the absorption of the sample and does not significantly change with the emission wavelength. The photoluminescence spectrum excited at 2.95 eV is dominated by a yellow band peaking at 2.16 eV, characteristic of the Mn2+ internal 4T1 f 6A1 transition.15 Nevertheless, previous time-resolved experiments have clearly shown that this contribution is surimposed to a weaker broad band which is attributed to the surface recombination commonly observed in the case of undoped CdS nanoparticles.16 The two signals are well separated since the radiative lifetime of the Mn2+ emission was found to be 1.7 ms, whereas it is on the microsecond time scale for surface recombination.17 It is not yet clear if both signals originate from the same nanocrystals through a competitive mechanism or if they arise from two populations of clusters such as the Mn2+-doped and the undoped nanocrystals. We note that the radiative lifetime of the Mn2+ emission is nearly the same as in bulk materials,18 opposite to the shorter lifetimes observed by Bhargava et al. on Mn2+-doped ZnS nanocrystals.5 Quantum yield evaluations were made by comparing the integrated intensity of the luminescence of our samples with those of standards of known efficiency. These standards were a Rhodamine 6G solution in acetone for CdS:Mn in pyridine solution and an erbium-doped fluoride glass in the case of xerogels. Typical quantum efficiencies in pyridine were about 1.3%. This figure goes up to about 7% upon the incorporation of the nanocrystals in a methyltriethoxysilane-based xerogel. This enhancement of the photoluminescence quantum yield upon the incorporation of the aggregates in hybrid organic-inorganic gels has already been reported in the case of pure CdS nanocrystals. It seems to be related to a weak interaction between the guest nanoclusters and this kind of host organicinorganic gel matrix.19 In any case, this behavior points out an influence of the surface of the nanoparticles on the Mn2+ emission. This is confirmed by classical quenching experiments20 performed by addition of methylviologen to a colloidal solution of CdS:Mn nanoparticles. This oxidizer is well-known to act as a scavenger
which captures electrons trapped at the surface of the nanocrystals. Figure 7 clearly shows that the methylviologen quenches the Mn2+ emission as well as the surface recombination emission. This indicates a possible energy transfer between the surface-trapped excited carriers and the Mn2+ ions. In bulk Mn2+-doped II-VI semiconductor, it is generally admitted that a strong luminescence requires the presence of so-called sensitizers such as Cl, In, Ga, Ag, and Cu.21 The Mn2+ emission is thought to occur after a resonant energy transfer from defect centers associated with these impurities. Concerning Mn2+-doped nanocrystals, we suggest that the role played in the bulk by the sensitizers is now played by the surface of the nanoparticles. The Mn2+ emission is then assumed to happen after an energy transfer from excited carriers trapped at the surface to Mn2+ ions. The efficiency of the transfer depends upon two parameters: the distance between the two centers and the spectral overlapping between the Mn2+ energy levels and the emission from surface recombination. In any case, the transfer can only occur through exchange interaction, since the optical transitions from ground to triplet states of Mn2+ are spin-forbidden electric dipole transitions. The comparison between the position of the five first absorption peaks of a Mn2+ ion in a tetrahedral ligand field (from 2.38 to 3.27 eV)18 and the luminescence spectrum of undoped CdS nanoclusters (from 1.77 to 2.76 eV)19 shows that the spectral overlapping is very small. In this configuration (exchange interaction and weak spectral matching), Mn2+ ions and sensitizing centers on the surface must be very close (
