J. Phys. Chem. B 2001, 105, 10197-10202
10197
Luminescence Quantum Efficiency of Nanocrystalline ZnS:Mn2+. 1. Surface Passivation and Mn2+ Concentration Ageeth A. Bol* and Andries Meijerink Debye Institute, Physics and Chemistry of Condensed Matter, Utrecht UniVersity, P.O. Box 80 000, 3508 TA Utrecht, The Netherlands ReceiVed: February 27, 2001; In Final Form: June 26, 2001
The luminescence quantum efficiency of nanocrystalline ZnS:Mn2+ is studied to provide a better understanding on how the quantum efficiency is influenced by the Mn2+ concentration, the nature of the passivating polymer, and the synthesis conditions. The results show an increase of the luminescence quantum efficiency with the Mn2+ concentration in the nanocrystals for very low Mn2+ concentrations. Between 0.3 and 1.5 at. % Mn2+ the increase in quantum efficiency levels off, to reach an almost constant level between 1.5 and 5.6 at. % Mn2+. Up to a concentration of 5.6 at. %, no concentration quenching is observed. The influence of the nature of the passivating polymer is investigated by comparing the luminescence quantum efficiencies for nanoparticles coated with poly(vinylbutyral) (PVB), poly(vinyl alcohol) (PVA), methacrylic acid (MA), or sodium polyphosphate (PP) or without a passivating polymer. For the presently used synthesis method (in water), the highest quantum efficiencies (around 4%) are obtained for nanocrystalline ZnS:Mn2+ capped with PP. Nanoparticles synthesized in a nitrogen atmosphere have higher quantum yields than nanoparticles made in ambient air. In general, large variations in luminescence properties are observed due to unintentional variations in the synthesis conditions. For research on the luminescence properties and quantum efficiencies of nanocrystalline ZnS:Mn2+, it is very important to check the reproducibility of results, to standardize synthesis conditions, and to measure absolute quantum efficiencies rather than relative changes in luminescence intensity.
1. Introduction Semiconductor nanoparticles with dimensions smaller than or in the order of the size of the bulk exciton show unique optical properties, which depend strongly on the size.1-4 These properties have stimulated great interest in semiconductor nanoparticles both from a fundamental and from an applied point of view. The change of the energy level structure can be explained by strong confinement of the charge carriers in all three dimensions. Besides the size quantization, surface effects can strongly influence the optical properties of these nanoparticles. A frequently cited publication about doped nanocrystalline semiconductors appeared in 1994. Bhargava5 reported that ZnS nanoparticles doped with luminescent ions (Mn2+) exhibit a high luminescence quantum efficiency (18%) and a luminescence lifetime shortening of the Mn2+ emission (from milliseconds to nanoseconds) due to size quantization. Recently, papers have appeared in which the observed lifetime shortening was shown to be based on misinterpretation of results.6,7 Although there is no million-fold decrease in the Mn2+ lifetime, the paper of Bhargava has triggered many groups to study the luminescence properties of nanocrystalline semiconductors doped with luminescent ions, and since 1994 a stillincreasing number of papers have appeared on doped nanocrystals. In addition to publications on the lifetime of the orange emission of nanocrystalline ZnS:Mn2+, many articles have reported on factors influencing the quantum efficiency of these nanoparticles. The luminescence quantum efficiency is important * Corresponding author. Fax: +31 30 253 2403. E-mail: a.a.bol@ phys.uu.nl.
for the potential use of nanocrystalline ZnS:Mn2+ in lightemitting devices where a high luminescence quantum efficiency is required. Several researchers8-13 have found that the quantum efficiency of nanocrystalline ZnS:Mn2+ is dependent on the Mn2+ concentration. Koshravi et al.8 observed that the luminescence of nanocrystalline ZnS:Mn2+ increased with increasing Mn2+ concentration. At a concentration of 0.12 at. % Mn2+ relative to Zn2+ a maximum was reached. Above this concentration the luminescence intensity fell off rapidly. Later, Sooklal and co-workers9 observed a maximum at a Mn2+ concentration of 2 at. % present in the reaction vessel. The actual amount of Mn2+ incorporated in the nanoparticles was not reported, but is undoubtedly much lower than 2 at. %. Leeb et al.10 published that maximum quantum efficiency was reached at a Mn2+ content of 1 at. % relative to Zn2+. They explained the optimum by concentration quenching. Concentration quenching involves resonant transfer of electronic excitation energy from the initially absorbing ion to another identical ion and, after a number of energy transfer steps, to a quenching site (for example, a defect).14 Finally, Malik and co-workers11 found an optimum Mn2+ concentration of 0.72 at. %. Furthermore, a few papers have appeared wherein an increase in luminescence intensity was noticed with increasing Mn2+ concentration.12,13 However, no decrease in luminescence intensity was reported at higher Mn2+ concentration (above 0.47 at. % (incorporated)12 or 1.5 at. % (in starting solution)13) in these articles. Besides reports on the influence of the Mn2+ concentration on the luminescence quantum efficiency of nanocrystalline ZnS: Mn2+, numerous papers have appeared on the influence of the capping polymers on the luminescence quantum efficiency, e.g.,
10.1021/jp0107560 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/28/2001
10198 J. Phys. Chem. B, Vol. 105, No. 42, 2001 refs 12, 13, and 15-18. Each paper reports on one particular passivator (e.g., acrylic acid (AA), methacrylic acid (MA), poly(methyl methacrylate) (PMMA), dodecylbenzenesulfonic acid sodium salt (DBS), 3-methacryloxypropyltrimethoxysilane (MPTS), mixture of poly(oxyethylene)5nonylphenol (NP-5) and poly(oxyethylene)9nonylphenol (NP-9)) and mostly only changes in luminescence intensity are reported and not changes in absolute quantum efficiencies. Due to the absence of information on absolute quantum efficiencies in these publications, it is very difficult to compare the various results. One of the main problems in understanding variations in the quantum efficiency of nanoparticle luminescence is the large surface area and the role of (surface) defects in the luminescence process. As a result, the luminescence quantum efficiency is extremely sensitive to the synthesis conditions. The defect chemistry of inorganic solids is strongly influenced by small (intentional or unintentional) changes in synthesis procedures. Therefore, it is not certain that by changing one parameter all the other parameters that could influence the luminescence efficiency stay constant. Large variations in quantum efficiency may therefore be caused by changes in other parameters than the intentionally varied parameter. Hence, if one is studying the role of one parameter, one has to be extremely careful in keeping all other conditions the same. It is our impression that this has not always been the case. For example, in some studies on the influence of the Mn2+ concentration on the luminescence efficiency, a single sample with much higher quantum efficiency than other samples is found.8 The Mn2+ concentration of this sample is then identified as the optimum concentration, and the lower intensity at higher Mn2+ concentrations is explained by concentration quenching. Probably, the large differences reported for optimum concentrations of Mn2+ are related to accidentally obtained high quantum efficiencies for a certain ZnS:Mn2+ sample and are not related to the Mn2+ concentration or concentration quenching. In this study a systematic investigation of the influence of the Mn2+ concentration and passivating polymer on the luminescence quantum efficiency of nanocrystalline ZnS:Mn2+ is reported. In addition, the influence of the atmosphere during the synthesis was investigated. Great care was taken to keep all the parameters, except the one varied, the same. The Mn2+ concentration was varied and the nanoparticles were passivated with several polymers. Methacrylic acid (MA), sodium polyphosphate (PP), poly(vinylbutyral) (PVB), and poly(vinyl alcohol) (PVA) were used as capping agents. Samples without passivating layer were also prepared. Some syntheses were done both in air and in nitrogen. The properties of the various samples were compared. Absolute quantum efficiencies were measured as well. In a subsequent paper the variation of the luminescence quantum efficiency due to postsynthesis UV irradiation is discussed.19 2. Experimental Section The synthesis route followed to make nanocrystalline ZnS: Mn2+ resembles standard methods for synthesis of nanocrystalline II-VI semiconductors. The method used for the synthesis of nanocrystalline ZnS:Mn2+ coated with PP is very similar to the one described by Yu et al.,20 except that the synthesis was done in water instead of methanol. To study the influence of the Mn2+ concentration on the luminescence quantum efficiency, three series of ZnS:Mn2+ samples were made in which the Mn2+ concentration was varied. To provide an optimum reproducibility, in each of the series the samples were prepared at the
Bol and Meijerink same time and from the same stock solutions, keeping all reaction conditions the same. To synthesize ZnS:Mn2+ nanoparticles with different Mn2+ concentrations, 10 mL of 1 M Zn(CH3COO)2‚2H2O and 0, 0.5, 0.8, 1.1, 1.5, 2.0, 3.0, or 5.0 mL of 1 M Mn(CH3COO)2‚4H2O (series 1), 0, 0.5, 1, 1.5, 2, 3, 5, 7.5, or 10 mL of 0.1 M Mn(CH3COO)2‚4H2O (series 2), 0.75, 1, 1.5, 2.0, 3.0, 5.0, or 7.5 mL of 1 M Mn(CH3COO)2‚4H2O (series 3) were added to an aqueous solution of 10 g of Na(PO3)n (Aldrich, 96%, n ∼ 10). The total volume after the addition was 90 mL. After about 10 min of stirring, 10 mL of a 1 M Na2S‚9H2O solution was injected into the solution. Immediately after the injection of the Na2S solution a turbid white fluid was obtained. Then the particles were centrifuged, rinsed with distilled water and ethanol, and dried in a vacuum. The same method was applied to passivate nanoparticles with MA and PVA. Passivation with MA (Aldrich) was carried out with 10 mL of methacrylic acid instead of 10 g of Na(PO3)n. The PVA-coated nanoparticles were made using 4.4 g of PVA (Aldrich, MW 85 000-146 000). To dissolve all the PVA, the mixture of PVA and distilled water was heated to its boiling point and allowed to cool. The particles without passivating polymer were made utilizing the same synthesis route, without polymer. The synthesis of the nanoparticles coated with PVB was carried out in absolute ethanol, as PVB does not dissolve in water. A 14.2 g sample of PVB (Acros, MW 36 000) was dissolved in absolute ethanol. To obtain a 1 M Zn(CH3COO)2‚ 2H2O solution and a 0.01 M Mn(CH3COO)2‚4H2O solution in ethanol, the ethanol was slightly acidified (pH 5) with concentrated HNO3. A 10 mL volume of 1 M Zn(CH3COO)2‚2H2O in absolute ethanol and 2-20 mL of 0.01 M Mn(CH3COO)2‚4H2O were added to the PVB solution. The total volume after the addition was 85 mL. Because of the low solubility of Na2S in ethanol, 15 mL of a 1 M solution of Na2S in ethanol (not all the Na2S has dissolved) was injected instead of 10 mL to compensate for the only partly dissolved Na2S. X-ray powder diffraction patterns of the nanoparticles were obtained with a Phillips PW 1729 X-ray generator with Cu KR radiation (λ ) 1.542 Å). From the line width the particle diameter was calculated using the Scherrer formula.21 A PerkinElmer Optima-3000 was used to determine the chemical composition of the samples by inductively coupled plasma analysis (ICP). Emission and excitation spectra were recorded on a SPEX Fluorolog spectrofluorometer Model F2002, equipped with two double-grating 0.22 m monochromators (SPEX 1680) and a 450 W xenon lamp as a excitation source. The emission was detected with a cooled Hamamatsu R928 photomultiplier. Emission spectra were corrected for the sensitivity of the photomultiplier tube, and excitation spectra were corrected for the intensity of the xenon lamp. The quantum efficiencies of the various samples were calculated using the lamp phosphor BaMgAl10O17:Eu2+ (BAM, commercial phosphor purchased from Philips). This phosphor has a known quantum efficiency of about 90% for an excitation wavelength of 300 nm.14 The emission spectra (λexc ) 300 nm) of both BAM and the sample with unknown quantum efficiency were recorded under the same circumstances. To ensure the same packing density, the sample and the phospor were weighed beforehand. The quantum efficiency was calculated with eq 1, assuming that BAM and the sample have about the same absorbance (∼100% according to diffuse reflection measurements) at an excitation wavelength of 300 nm:
Luminescence Quantum Efficiency of ZnS:Mn2+. 1
QEsample )
∫Isample × QEBAM ∫IBAM
J. Phys. Chem. B, Vol. 105, No. 42, 2001 10199
(1)
where QEsample ) quantum efficiency of the sample at λexc ) 300 nm QEBAM ) quantum efficiency of BAM at λexc ) 300 nm (90%) ∫Isample ) integrated emission intensity of the sample at λexc ) 300 nm ∫IBAM ) integrated emission intensity of BAM at λexc ) 300 nm The quantum efficiency calculated with this formula gives a reasonable estimate of the actual quantum efficiency (error estimated to be about 10%), and it provides a good way to compare the absolute quantum efficiencies of the various samples. 3. Results and Discussion 3.1. Influence of the Mn2+ Concentration. To study the influence of the Mn2+ concentration on the quantum efficiency of nanocrystalline ZnS:Mn2+, great care must be taken to keep the reaction conditions constant while varying the Mn2+ concentration. Therefore, three series of eight samples with different Mn2+ concentrations were made. The samples within one series were prepared simultaneously, keeping all the reaction conditions, except the Mn2+ concentration, constant. PP was used as passivating polymer. With the ICP technique the amounts of Zn, Mn, and S in the samples were determined. From these results the Mn/Zn atomic ratio and the Zn/S atomic ratio were calculated. Counio et al.22 reported that thorough rinsing of samples of nanocrystalline CdS:Mn2+ with water of pH 2.5 was necessary to measure the Mn2+ amount which was really inside the nanoparticles, washing away the Mn2+ ions at the surface of the nanoparticles. This was checked for some of the present samples, but no difference in Mn2+ content was found with ICP analysis before and after rinsing. Therefore, no additional rinsing step with acidified water was performed before the ICP analysis. In Table 1 the Mn/Zn ratio used in the synthesis (at. %), the Mn/Zn ratio determined by ICP (at. %), and the total luminescence quantum efficiency for the samples of the three series are shown. The Zn/S ratio for the samples of series 1 is 1.1, while the Zn/S ratio for series 2 and 3 is 0.7. The Zn/S ratio in the reaction vessel for the three series was 1. The average particle diameter (determined with XRD) was 4.0 nm for the samples of series 1, 2.8 nm for the samples of series 2, and 3.8 nm for the samples of series 3. Within a series the Zn/S ratio and the average particle size is constant within the uncertainty of the measurements. The Zn/S ratio and the average particle size are independent of the Mn2+ concentration. The differences between the series indicate that unintentional changes in the reaction conditions occur when samples are not made simultaneously. It can be observed in Table 1 that by increasing the amount of Mn2+ precursor the concentration Mn2+ incorporated in the nanoparticles increases as well. The Mn2+ concentration incorporated in the nanoparticles is however far less than the Mn2+ concentration present during the synthesis. The maximum Mn/ Zn ratio incorporated was 5.6 at. % (for 75 at. % Mn2+ in the solution (series 3)). This concentration of 5.6 at. % corresponds to approximately 40 Mn2+ ions per nanoparticle (diameter 3.8 nm). From Table 1 it is clear that the relation between the Mn2+
Figure 1. Emission spectra measured under 300 nm excitation of nanocrystalline ZnS:Mn2+ with Mn2+ concentrations incorporated in the nanoparticles of 0, 0.26, 0.34, 0.38, 0.78, and 1.08 at. % (series 1).
TABLE 1: Mn2+ Concentration Used in the Synthesis and Determined by ICP (at. %), and the Luminescence Quantum Efficiency of the Various Samples of Nanocrystalline ZnS:Mn2+
series
Mn2+ concn (at. % rel to Zn) used in synth
detd by ICP
lumin quantum eff (%)
1 1 1 1 1 1 1 1
0 5.0 8.0 11 15 20 30 50
0 0.26 0.34 0.82 0.38 0.50 0.50 1.08
0.5 2.8 3.0 3.7 3.2 3.4 3.3 3.5
2 2 2 2 2 2 2 2
0 0.5 1.0 1.5 2.0 3.0 5.0 7.5
0 0.08 0.17 0.25 0.37 0.68 1.06 1.49
0.5 0.8 1.3 1.5 2.1 2.7 3.4 3.8
3 3 3 3 3 3 3
7.5 10 15 20 30 50 75
0.84 1.22 1.47 1.88 3.08 4.32 5.59
1.5 2.1 1.9 2.2 1.9 1.7 1.7
concentration used in the synthesis and the Mn2+ concentration incorporated in the nanoparticles is quite linear within one series. However, sometimes deviations from this behavior are observed. For example, the sample with a relative Mn2+ precursor concentration of 11 at. % of series 1 shows a large deviation from the other samples. This sample has a relatively high Mn2+ concentration (0.82 at. %) incorporated in the ZnS nanoparticles compared to the samples with a Mn2+ precursor concentration of 8 and 15 at. % (respectively 0.34 and 0.38 at. %). This illustrates that the preparation of nanocrystalline semiconductors is very sensitive to very small deviations in the reaction conditions, even if the samples are prepared simultaneously. Figure 1 shows some emission spectra (λexc ) 300 nm) for samples of series 1. Nanocrystalline ZnS:Mn2+ shows a weak purple/blue emission (420 nm) and an orange emission (590 nm) under 300 nm excitation. The blue emission can be assigned
10200 J. Phys. Chem. B, Vol. 105, No. 42, 2001
Figure 2. Quantum efficiency (%) of the total luminescence of nanocrystalline ZnS:Mn2+ as a function of the Mn/Zn ratio (at. %) incorporated in the ZnS:Mn2+ nanoparticles measured at an excitation wavelength of 300 nm for the three independent series. The nanoparticles were passivated with PP.
to a defect-related emission of the ZnS host, and the orange emission can be attributed to the 4T1-6A1 transition of the Mn2+ ion. The sample without Mn2+ only shows the blue ZnS-related emission. As soon as Mn2+ is incorporated in the ZnS nanoparticles, the intensity of the blue emission decreases and the Mn2+ emission comes up, since the energy transfer between the ZnS host and the Mn2+ impurity is very efficient. With an increasing concentration of Mn2+ incorporated in the nanoparticle, the Mn2+ emission intensity increases. Another noticeable observation is a slight shift of the Mn2+ emission to the red with increasing Mn2+ concentration. A red shift in the emission at higher Mn2+ concentrations can be explained by pair formation. Due to magnetic interactions between neighboring Mn2+ ions, the emission of magnetically coupled (ferromagnetic or antiferromagnetic) pairs is usually observed to be red shifted by 100-400 cm-1.23,24 In Figure 2 the total luminescence quantum efficiency (%) for the samples of the three series is plotted as a function of the Mn/Zn ratio (at. %) in the nanoparticles determined by ICP analysis. The importance of reproducibility is evident from the differences between the three series, which were not synthesized at the same time. The same trend is found, however, for the three independent series: for very low concentrations the luminescence quantum efficiency increases with increasing Mn2+ concentration. Above a Mn2+ concentration of about 1.5 at. % the total quantum efficiency remains approximately constant and varies within a range of 0.2% around an average quantum efficiency of 1.8% for series 3. This variation is due to errors in the measurements and small variations in the luminescence efficiency, not related to the Mn2+ concentration. In contrast to earlier publications,8-11 a decrease of the luminescence intensity at high Mn2+ concentrations was not observed. Previous reports stated that concentration quenching was observed above Mn2+ concentrations of 0.12,8 0.72,11 and 1 at. % Mn10 incorporated in the nanoparticle. Probably, the earlier reported decrease in quantum efficiency with increasing Mn2+ content was due not to the increase in Mn2+ concentration, but to other factors which influence the quantum efficiency (e.g., particle size deviation, creation of defects) or based on one sample with a (accidentally) higher efficiency. The absence of concentration quenching for Mn2+ concentrations up to 5.6 at. % is in line with what is expected.
Bol and Meijerink Concentration quenching is due to energy migration over a donor sublattice to traps, which are present in a low concentration. Efficient energy migration via an array of donors to traps requires high donor concentrations since otherwise migration is limited to very short distances and the traps will not be reached. In the case of Mn2+ the transition involved in the energy migration is the parity- and spin-forbidden 6A1 T 4T1 transition within the 3d5 configuration of the Mn2+ ion. Due to the forbidden character of the transition, energy transfer between Mn2+ neighbors via dipole-dipole interaction is not efficient and only transfer between nearest neighbor Mn2+ ions is expected.25 As a result, the Mn2+ concentration at which energy migration is expected to be observed is high (around 10 at. %) and the percolation point is not reached until Mn2+ concentrations are as high as 25 at. %. The situation is very similar to concentration quenching in concentrated systems of rare earth ions (involving equally forbidden transitions with millisecond lifetimes).26,27 Concentration quenching for commercial luminescent materials based on Mn2+ or rare earth ions has been extensively studied, and no concentration quenching is observed below concentrations of 10 at. % of the luminescent ion.24,28 The presently observed absence of concentration quenching for Mn concentrations up to 5.6 at. % is in line with these results. To illustrate that it is very unlikely to observe concentration quenching by energy migration over Mn2+ ions at low concentrations, one may consider a ZnS particle with a diameter of 4 nm (containing about 700 Zn2+ sites) and a Mn2+ concentration of 1 at. %. On average, these particles contain seven Mn2+ ions and the percentage of Mn2+ ions with one Mn2+ neighbor is 9.5%, which means that 90.5% of the Mn2+ ions are isolated and have only nearest Zn2+ neighbors.29 Clearly, in this situation no efficient energy migration over an array of many Mn2+ ions can be expected. For particles with an average particle diameter of 4 nm and 0.1 at. % of Mn2+ incorporated, all Mn2+ ions are isolated, while for an Mn2+ concentration of 5.6 at. % 46% of the Mn2+ ions are isolated (i.e., have no Mn2+ neighbor). Although concentration quenching due to energy migration is not expected in Mn2+ phosphors below concentrations of about 10 at. % Mn2+, in bulk ZnS:Mn2+ a decrease in the light output has been observed at Mn2+ concentrations above typically 1 at. % (again different concentrations are mentioned in different papers); see, e.g., refs 30 and 31. Also here energy migration over the Mn2+ sublattice has been suggested and even modeled.32 In the model efficient energy transfer between Mn2+ ions via dipole-dipole interaction over very large distances has to be assumed. As was argued above, this is not expected for the strongly forbidden transitions on Mn2+ and is not in line with research on concentration quenching for Mn2+ in other systems (no quenching below 10 at. % Mn2+). Although it is not the purpose of this paper to explain the reduced efficiency of bulk ZnS:Mn2+ at Mn2+ concentrations above concentrations as low as 1 at. %, a possible explanation is that at higher Mn2+ concentrations some Mn is present as Mn3+ or Mn4+ which will act as very efficient quenchers of the orange Mn2+ emission due to the low position of the charge-transfer absorption band of these ions in a sulfide. 3.2. Influence of the Passivating Polymer. To investigate the influence of the passivating polymer on the luminescence quantum efficiency, samples were passivated with poly(vinylbutyral) (PVB), methacrylic acid (MA), poly(vinyl alcohol) (PVA), and sodium polyphosphate (PP). Also, samples without passivating polymer were synthesized. First of all, the amount of Mn2+ solution used in the synthesis was varied, keeping the passivating polymer constant. This time the samples were not
Luminescence Quantum Efficiency of ZnS:Mn2+. 1
J. Phys. Chem. B, Vol. 105, No. 42, 2001 10201
TABLE 2: Quantum Efficiency, Average Diameter, and Mn/Zn Ratio Incorporated and Used in the Synthesis for the Samples with the Highest Quantum Efficiency for Different Capping Polymers polymer
Mn/Zn ratio (at. %) used in synth
incorporated
av diam (nm)
quantum eff (%)
PVB PVA without MA PP
10 1.5 1.5 1.5 0.7
2.38 0.96 0.48 0.35 0.13
3.2 3.7 4.3 4.1 3.0
3.5 2.7 1.6 1.0 4.5
prepared simultaneously. It was noticed that in a series of increasing Mn2+ precursor concentrations sometimes a sample shows a dramatic deviation from the trend in quantum efficiencies (both higher and lower values). We feel that even small deviations in, for example, temperature, pH, or a new bottle of polymer, can already cause large variations in the luminescence quantum efficiency. This can be explained by the fact that nanoparticles have a large surface-to-volume ratio. Therefore, radiative decay competes in a great extent with nonradiative decay at the surface of the particles. Small deviations in reaction conditions can change the surface states of the nanoparticles, which have a large influence on the luminescence intensity. Despite the large variations in quantum efficiencies, some general trends were observed. Unpassivated samples usually have low quantum efficiencies (typically 0.3%), although sometimes a quantum efficiency above 1% is obtained. Surface passivation with MA results in quantum efficiencies of typically 1%. For PVB the measured quantum efficiencies vary strongly and values between 1 and 3% are found. For PVA values of 2-3% are obtained. In the presently used synthesis method, the best results were obtained for sodium polyphosphate with quantum efficiencies around 4%. It should be stressed that for a different synthesis method (for example in a different solvent or at a different temperature) the results are expected to be different. It is well-known that high quantum efficiencies can be obtained using MA as passivating polymer in toluene.5 For each polymer, the sample with the highest quantum efficiency (not automatically the sample prepared with the largest amount of Mn2+) was analyzed using the ICP technique. Table 2 shows the results of the ICP analysis, the average particle diameter (determined with XRD21), and the luminescence quantum efficiencies for these samples. Again, it is found that the amount of Mn2+ incorporated in the nanoparticles is much less than the Mn2+ precursor concentration used for the synthesis of the samples. Furthermore, it turned out that the percentage of the Mn2+ precursor concentration that was incorporated in the nanoparticles is very dependent on the polymer used to passivate the nanoparticles. For the synthesis of the samples coated with PVA and MA and the sample without passivating polymer, the same Mn2+ precursor concentration was used (1.5 at. %), but the amount of Mn2+ incorporated in the ZnS ranged from 0.35 at. % for the MA-coated sample to 0.96 at. % for the sample passivated with PVA. Hence, by altering the passivating polymer also the amount of Mn2+ incorporated in the nanoparticles is influenced. The average particle diameter varies between 3 and 4.5 nm. If no passivating polymer is present, the average particle size is larger than in the presence of a capping polymer. This is in line with the idea that the particles grow while surrounded by the polymer. The stronger the polymer coordinates, the slower the particles grow and the smaller the average particle size will be. The data in Table 2 suggest an inverse relation between the particle size and the quantum efficiency: the smaller particle
sizes show the higher quantum efficiencies. A dependence of the quantum efficiency on particle size has been reported before and ascribed to quantum size effects.5 Even though it is not likely that quantum size effects (when defined as changes in the electronic structure of the semiconductor particle) are responsible for increased quantum efficiency in smaller particles,33 it can be expected that smaller particles show higher quantum efficiencies. For small particles the probability that a particle contains no quenching centers (and thus a high quantum efficiency) increases as the particle becomes smaller. For example, very high quantum efficiencies (over 50%) have been reported for ZnS capped CdSe nanocrystals.34 Such high quantum efficiencies can only be obtained in nanocrystalline samples, where many “perfect” particles with a quantum efficiency close to unity can be formed. Because of the small size, the probability for defect-free crystallites is high. In larger semiconductor structures there will always be defects, and because of the high mobility of the optically excited free electrons and holes in a bulk semiconductor, nonradiative recombination at defect sites occurs. In nanocrystalline semiconductor particles the charge carriers are confined in the volume of the nanocrystal, and if there is no nonradiative recombination center (such as a defect) in the nanocrystal, the nanocrystal will have a high luminescence quantum efficiency. These effects will also occur in nanocrystalline ZnS:Mn2+. If nonradiative recombination at the surface is inhibited by a proper choice of polymer that passivates surface states, the highest quantum efficiencies are expected for the smaller ZnS:Mn2+ nanocrystals. To increase the quantum efficiency further, there are many synthesis parameters that can be varied, such as temperature, solvent, pH, and concentrations. Also a postsynthesis treatment by ultraviolet radiation is known to significantly increase the quantum yield.12,16,19,35 In view of the high quantum efficiencies achieved for other nanocrystalline semiconductors,34,36 it can be expected that the quantum efficiency on nanocrystalline ZnS: Mn2+ can be much higher than the highest values reported before now.5 3.3. Influence of the Atmosphere. To study the influence of the atmosphere during the synthesis, nanoparticles passivated with MA and PP and without passivating polymer have been synthesized in air and in a glovebox filled with nitrogen. Extreme care was taken to keep the reaction conditions, other than the atmosphere, constant (e.g., same stock solutions, same Mn2+ precursor concentration). It is observed that the yield of the products prepared in the glovebox is higher than in air. In addition, the samples made in air, except the PP-coated sample, are slightly colored. The MA-coated sample was slightly yellow, while the sample without passivating polymer was faint pink. The samples made in the glovebox (N2 atmosphere) are white. This indicates that, in the presence of oxygen, side reactions take place that form colored species. Both an oxidation reaction of the (organic) polymer and the ZnS may be involved in the coloration. For ZnS extensive research has been done in the past to unravel the photooxidation reactions that result in coloration of ZnS-based white pigments.37-39 It is not within the scope of this paper to understand the slight coloration of the ZnS:Mn2+ for the synthesis in air. The average particle sizes of the samples prepared in nitrogen and air were the same. In Table 3 the absolute quantum efficiencies for the samples made in nitrogen and in air are shown. From the table it is clear that the luminescence quantum efficiencies for the samples made in the glovebox are higher than those for the samples made in air.
10202 J. Phys. Chem. B, Vol. 105, No. 42, 2001 TABLE 3: Total Luminescence Quantum Efficiency of Nanocrystalline ZnS:Mn2+ Passivated with MA and PP and without Passivating Polymer Prepared in Nitrogen or Air polymer
ambient atmos
quantum eff (%)
MA MA PP PP without without
N2 air N2 air N2 air
0.7 0.5 3.7 1.4 2.9 1.6
4. Conclusions The influences of various factors on the luminescence quantum efficiency of Mn2+-doped ZnS nanocrystals have been investigated. Variation of the Mn2+ concentration shows an increase of the quantum efficiency with increasing Mn2+ concentrations at very low Mn2+ concentrations. Contrary to previous publications, we show and explain that concentration quenching does not occur up to the highest Mn2+ concentrations that have been incorporated (about 5.6 at. %). A clear influence of the type of passivating polymer (PVA, PVB, MA, PP, or none) and the atmosphere (nitrogen or air) is observed. The highest quantum efficiencies (around 4%) are obtained using polyphosphate (PP) in a nitrogen atmosphere. We also observe large variations in quantum efficiencies due to unintentional variations in the synthesis conditions. This shows that it is very important to test reproducibility and measure absolute quantum efficiencies in this field of research. Acknowledgment. The financial support of Philips Lighting NV is gratefully acknowledged. We thank Ms Helen de Waard for performing the ICP analysis. References and Notes (1) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1985, 82, 552. (2) Brus, L. J. Phys. Chem. 1986, 90, 2555. (3) Henglein, A. Chem. ReV. 1989, 89, 1861. (4) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525. (5) Bhargava, R. N.; Gallagher, D. Phys. ReV. Lett. 1994, 72, 416. (6) Murase, N.; Jagannathan, R.; Kanematsu, Y.; Watanabe, M.; Kurita, A.; Hirata, K.; Yazawa, T.; Kushida, T. J. Phys. Chem. B 1999, 103, 754. (7) Bol, A. A.; Meijerink, A. Phys. ReV. B 1998, 58, R15997.
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