4128
J. Phys. Chem. B 2001, 105, 4128-4132
Formation and Distinctive Decay Times of Surface- and Lattice-Bound Mn2+ Impurity Luminescence in ZnS Nanoparticles Jae Hun Chung, Chil Seong Ah, and Du-Jeon Jang* School of Chemistry and Molecular Engineering, Seoul National UniVersity, Seoul 151-742, Korea ReceiVed: July 27, 2000; In Final Form: February 14, 2001
The Mn2+ impurity luminescence decay of Mn2+-doped ZnS nanoparticles has been clearly distinguished from ZnS host emission decay by measuring transient absorption and emission kinetic profiles in the picosecond-millisecond time domain. The Mn2+ luminescence for the doped sample shows two decay components of 0.18 and 2 ms. The fast one is attributed to surface-bound Mn2+ impurities and the slow one to lattice-bound Mn2+ impurities. In comparison, the slowest component of ZnS host emission decays within 20 ns. Moreover, excitation-energy transfer from ZnS host to Mn2+ impurity is also measured to occur on the time scale of 700 ps.
Introduction Nanoparticles have a variety of unique spectroscopic, electronic, and chemical properties that germinate from their quantum-confinement effects and high surface/volume ratios.1-6 In particular, the optical properties of nanocrystalline metals3,7-9 and semiconductors1,2,10-12 are of great interest in both basic and applied researches. The optical properties of transition- or lanthanide-metal ion-doped semiconductor nanoparticles have been extensively investigated recently.13-16 Mn2+-doped ZnS nanoparticles especially have been extensively studied with interest,17-32 as their efficient and characteristic luminescence might lead to a new class of luminescent materials in diverse applications such as displays, sensors, and lasers.11,14,29,30,33 It is well-known21-23 that while undoped ZnS nanoparticles emit in the blue upon UV excitation, Mn2+-doped ones yield an orange emission band in addition to the blue band with significantly reduced emission intensity. The orange emission is known to result from the 4T1 f 6A1 transition of the Mn2+ impurity excited via energy transfer from the ZnS host.18,19,21-23 The short-decay time of Mn2+ luminescence, together with its high quantum efficiency, is particularly important for applications where saturation limits the light output of a luminescent material. However, there has been a controversy over the decay kinetics of the Mn2+ impurity luminescence.19-23 As the wave functions of conduction electrons in ZnS are d-orbital-like,16 a strong hybridization between the s-p states of the ZnS host and the d states of Mn2+ impurity was suggested21,22 to occur in Mn2+doped ZnS nanoparticles, resulting in a fast energy transfer from the ZnS host to Mn2+ impurity. In addition, the spin-forbidden 4T f 6A transition of the Mn2+ impurity should be less 1 1 forbidden. Thus the hybridization would bring in a short decay time of the nanosecond range for the d-d transition of Mn2+.21,22 A few other groups23,24 also have reported the lifetime shortening of Mn2+ emission owing to quantum confinement effects. However, some other groups report the lifetime of Mn2+ luminescence to be in the millisecond range19,20,32 while attributing the fast decay component in the nanosecond range * To whom correspondence should be addressed. Telephone: +82-2875-6624. Fax: +82-2-889-1568. E-mail:
[email protected].
to ZnS defect emission.20 Furthermore, the spectral shift of Mn2+ emission expected in the case of hybridization has not been observed experimentally so far. In this paper, by showing transient absorption as well as emission kinetic profiles, we are able to clarify that the origin of the fast component of Mn2+ impurity luminescence in ZnS nanoparticles has two decay components of 0.18 and 2 ms. The fast and slow ones are ascribed to exterior and interior Mn2+ impurities, respectively. We also show that the energy transfer from ZnS host to Mn2+ impurity takes place in a time interval of 700 ps. Experimental Section Materials. Na2S‚9H2O, Zn(NO3)2‚6H2O, and Mn(NO3)2‚ 6H2O were used as purchased from the Aldrich Chemical (Milwaukee, WI). For the preparation of undoped ZnS nanoparticles dispersed in water (free sample), 30 mL of 2-mM Na2S‚ 9H2O was added to 30 mL of 2-mM Zn(NO3)2‚6H2O aqueous solution, which was preadjusted to pH 10.3. For the synthesis of 2% (in mole) Mn2+-doped ZnS nanoparticles suspended in water (doped sample), 30 mL of 2-mM Na2S‚9H2O aqueous solution was added to 30-mL aqueous basic (pH 10.3) solution of 2-mM Zn(NO3)2‚6H2O and 40-µM Mn(NO3)2‚6H2O with stirring. 2% Mn2+-doped and ZnS-passivated ZnS nanoparticles (doped and passivated sample) were prepared by adding 2.5 mL of 40-mM Zn(NO3)2‚6H2O aqueous solution and 2.5 mL of 40-mM Na2S‚9H2O aqueous solution to 10 mL of the 2% Mn2+-doped sample at pH 10.3. The average diameters of free and doped nanoparticles were estimated to be about 6 nm by using a transmission electron microscope (JEOL, JEM2000). Picosecond Measurements. An actively/passively modelocked Nd:YAG laser (Quantel, YG501) was used for all kinetic measurements. Picosecond kinetic profiles were detected with a 10-ps streak camera (Hamamatsu, C2830), which was attached with a CCD detector (Princeton Instruments, RTE128H). The emission light was collected from the front surface of the sample after excitation by the fourth harmonic (266 nm) laser pulses of 25-ps duration for the luminescence kinetic measurements. For transient-absorption kinetic profiles, transient absorption produced by 266-nm excitation pulses was probed with the
10.1021/jp002692j CCC: $20.00 © 2001 American Chemical Society Published on Web 04/12/2001
Mn2+ Impurity Luminescence in ZnS Nanoparticles
J. Phys. Chem. B, Vol. 105, No. 19, 2001 4129
Figure 1. ZnS-luminescence decay kinetic profiles, monitored at 400 nm (a) and 600 nm (b) and shown in three different time ranges of undoped (solid line) and 2% Mn2+-doped (dotted line) ZnS nanoparticles suspended in water.
fluorescence from a laser dye of rhodamine 6G, excited with the pulses split off from the sample-excitation pulses.34 The wavelength of the sample emission as well as the absorptionprobe light was separated with combined band-pass filters. Kinetic profiles measured at different optical decays were connected to make a long-time window profile. Kinetic time constants were extracted by fitting measured kinetic profiles to computer-simulated kinetic curves convoluted with the instrumental temporal response functions. 4T -State Measurements of Mn2+. The luminescence decay 1 kinetic profile of the 4T1 state of Mn2+ impurity was obtained by detecting emission from a laser-excited doped sample with an intensified CCD (Princeton Instruments, ICCD576G) of 2-ns gating resolution attached to a 0.5-m spectrometer (Acton Research, Spectrapro-500). The transient-absorption kinetic profile of the 4T1 state of Mn2+ impurity was obtained by monitoring the intensity change of the probe beam from a 300-W Xe lamp (Schoeffel, LPS255), passing through a laser-excited sample, with a photomultiplier tube (Hamamatsu, R928) connected to a 200-MHz oscilloscope (Tektronix, TDS350). Results and Discussion The emission decay kinetics of 2% Mn2+-doped ZnS nanoparticles at 400 nm, where ZnS host emission shows about the maximum, is quite different from that of the Mn2+-free sample in the short-time domains of Figure 1a. However, they become similar to each other in the long-time range. Each of them shows multiple decay components. Since the dynamic range (the total window time divided by the temporal resolution) of a streak camera is quite small, emission kinetics with multiple decay components have to be measured at various streak rates. Thus, emission decay from each sample at a given wavelength is shown in three different time windows. Furthermore, kinetic profiles measured in the 10-ns time window at different optical delays were connected
TABLE 1: ZnS Luminescence Decay Constants Deconvoluted from the Kinetic Profiles in Figure 1 sample
λem (nm)
free
400
doped
400
free
600
doped
600
decay time (ns) 0.03(83 ( 2%)a + 0.7(10 ( 2%) + 3(6 ( 4%) + 20(1 ( 0.3%) 0.03(53 ( 14%) + 0.7(19 ( 7%) + 3(27 ( 17%) + 20(1 ( 0.6%) 0.03(74 ( 4%) + 0.7(15 ( 5%) + 3(10 ( 8%) + 20(1 ( 0.4%) 0.03(61 ( 8%) + 0.7(25 ( 4%) + 3(13 ( 9%) + 20(1 ( 0.4%)
a Intensity percentage of each component at zero time (given in a parenthesis), in which the error is the average deviation.
to make the profiles of 40-ns window time, with the temporal response of the 10-ns time window. All the emission kinetic profiles, measured with different streak rates at a certain wavelength for a certain sample, should be fitted to a single function convoluted with the respectively measured temporal response functions. After iterative fitting, a quadruple exponential decay is found to be the simplest exponential function that fits to every kinetic profile measured at various streak rates. The kinetic changes with collection wavelength and Mn2+ doping may vary the time constants and the relative amplitudes as well as the number of the components. Clearly, more than four components are too many to be deconvoluted from kinetic profiles measured with a streak camera with a short dynamic range. Additionally, deconvolution with the variation of both time constants and relative amplitudes for four components are also ambiguous. Thus, the time constants of the quadruple decays were fixed, and the relative amplitudes were varied to fit all the kinetic profiles (Table 1). Fixing the amplitudes and varying the time constants did not give satisfactory results in fitting all the profiles.
4130 J. Phys. Chem. B, Vol. 105, No. 19, 2001
Chung et al.
Figure 2. Picosecond transient-absorption kinetic profiles at 580 nm of undoped (solid line) and 2% Mn2+-doped (dotted line) ZnS nanoparticles in water.
TABLE 2: Time Constants from the Transient Absorption Kinetic Profiles in Figure 2 sample
rise time (ns)
decay time (ns)
free doped
instant(43%) + 3.0(57%) instant(76%) + 0.7(24%)
20 ∞
All the emission profiles of the Mn2+-free sample at 400 nm were fitted with four distinguishable decay components of 30 ps, 700 ps, 3 ns, and 20 ns. (Although the time of 30 ps is similar to the instrumental-response time, it is not due to scattering because excitation light was completely filtered out.) The 30-ps time is considered as the time for the electrons in the conduction band to be ensnared into shallow trap sites. Generally, the electrons in the conduction band of a semiconductor decay to the lowest state of conduction band, and then they are caught in shallow trap sites. The confined electrons also move into deeper trap sites consecutively at progressively slower rates, finally recombining with the holes.35 The 700-ps and 3-ns times are assigned to different trapping times, and the 20-ns constant is the electron-hole recombination time at the deepest trap site among the sites that can be temporally distinguished with our method. The relative amplitude of the 30-ps component is significantly smaller in the doped sample than in the free sample. This indicates that the presence of Mn2+ impurities with different energy levels slows down fast energyflow processes in ZnS, such as the fast trapping process of the 30-ps component. However, it is interesting to note that the relative amplitudes of the 700-ps and 3-ns decay components are significantly larger in the doped sample than in the free sample. Together with the transient absorption rise components of 700 ps and 3 ns in Figure 2, the increase of the 700-ps emission decay component in the doped sample suggests that the slow trapping process of 3 ns is quenched by energy-transfer process to Mn2+ impurities occurring on the time scale of 700 ps (vide infra; time constants given in Table 2). The emission decay kinetic profiles at 600 nm (Figure 1b), where Mn2+ impurity emission shows about the maximum, are similar to those at 400 nm, although the 600-nm profiles decay more slowly overall than the profiles at 400 nm. At both wavelengths, the overall emission of the doped sample decays more slowly, especially in the short-time range, than that of the free sample. The presence of Mn2+ impurities with significantly different energy levels slows down the relaxation of ZnS-excitation energy on the short-time scale of 30 ps. On the other hand, the presence of Mn2+ impurities with lower energy levels quenches ZnS emission via energy transfer to Mn2+ impurities in the long-time range. In particular, at 600 nm, where the energy levels of ZnS defects overlap better with
those of Mn2+ impurities than at 400 nm, the kinetic curves of the doped sample become faster in the long-time range than those of the free sample owing to energy transfer. The main conclusion resulting from Figure 1 is that the emission at 600 nm, as well as that at 400 nm, within several tens of nanoseconds is not attributable to Mn2+ impurities at all, although its kinetics changes with the presence of Mn2+. This emission arises from the conduction band, trap sites, or defects of ZnS. If any emission component results from the excited state of Mn2+ impurity, it should have a longer lifetime than those of ZnS trap sites or ZnS defects, and its fractional amplitude at 400 nm should be considerably smaller than that at 600 nm. Furthermore, the fractional amplitude for the 600 nm region of the undoped sample should be negligibly smaller than that of the Mn2+-doped sample. None of the emission decay components of the Mn2+-doped sample in Figure 1 satisfies the abovedescribed conditions of Mn2+ emission. These results suggest that, on the nanosecond-time scale (Figure 1), all the emission components of the Mn2+-doped sample are emitted from ZnS host rather than from Mn2+ impurity. Even the emission collected at 600 nm is simply a long-wavelength tail of ZnS emission with extremely weak intensity. The slowest decay component of 20 ns is attributed to defect-related ZnS emission as well. Whereas the emission decay kinetics of the doped sample is similar to that of the free sample on the time scale of several tens of nanoseconds (Figure 1), the transient-absorption kinetics of the two samples are extremely different from each other (Figure 2). Although the transient absorption of Mn2+-free ZnS nanoparticles decays with a time constant of 20 ns, that of the doped sample does not decay at all in the time window of Figure 2. It is quite interesting to note that the transient-absorption decay time of the free sample is the same as the time constant of the slowest emission decay component of the same sample. This again indicates that the 20-ns emission decay component of Mn2+-doped ZnS nanoparticles and that of Mn2+-free ones result from defect-related ZnS emission rather than from Mn2+ impurity luminescence. The leveled-off transient-absorption decay kinetics of the doped sample suggests that the electrons trapped into the excited 4T1 state of the Mn2+ impurity do not decay in the nanosecond time window of Figure 2. A portion (24%) of the transient absorption in the doped samples rises in a time scale of 0.7 ns, while the rest of it rises instantly. The 0.7-ns process is assigned to the 4T1-state formation of the Mn2+ impurity. This process is assigned to energy transfer from the deep trap sites of ZnS host to the Mn2+ impurity. The slowrise component of 3 ns in the Mn2+-free particles is due to the formation of the deep trapping sites. Along these lines, 3 ns is a very slow trapping time into the deepest trap sites such as ZnS surface defect sites. The energy transfer to Mn2+ is suggested to quench a significant portion of the very slow trapping process to give birth to the 700-ps rise component in the doped sample. The further absorption increase implies that the electrons in the 4T1 state of Mn2+ impurities as well as in the experimentally distinguishable deepest trap sites of ZnS have a larger extinction coefficient than those at the shallow or deep trap sites of ZnS or those at the valence band of ZnS. As ZnS surface defect sites with a lower symmetry are expected to have a larger extinction coefficient than those of ZnS shallow or deep trap sites, a further absorption rise suggests that 3 ns is the slowest trapping time into ZnS surface defect sites. As the 4T1 state of Mn2+ is populated on the time scale of 700 ps via energy transfer from excited ZnS trap sites, the overall transient absorption increases, although the 4T1 state does not emit within
Mn2+ Impurity Luminescence in ZnS Nanoparticles
Figure 3. Transient-absorption (at 460 nm; dotted) and emission (at 600 nm; circled) decay kinetic profiles of the 4T1 state of Mn2+ impurity in 2% Mn2+-doped ZnS nanoparticles dispersed in water. The crossed profile shows the emission decay kinetics of the 2% Mn2+-doped and ZnS-passivated sample.
TABLE 3: Decay Constants of the 4T1 State of Mn2+, Extracted from Figure 3 profile
sample
monitored method
decay time (ms)
dotted circled crossed
doped doped doped and passivated
transient absorption emission emission
0.18(66%) + 2(34%) 0.18(72%) + 2(28%) 2(100%)
several tens of nanoseconds. This is explained with the selection rules that absorption from the 4T1 state to upper states is allowed while emission from the 4T1 to the 6A1 state is forbidden. Figure 3 shows that the relaxation of excited Mn2+ impurity in ZnS nanoparticles takes a time scale of milliseconds rather than nanoseconds (decay constants given in Table 3). For the undoped sample, neither transient absorption nor emission could be observed at all on the millisecond time scale. The transient absorption and the emission of Mn2+ impurity in the doped sample decay with dual decay components of 0.18 and 2 ms. However, the Mn2+ emission in the Mn2+-doped and ZnSpassivated sample shows a single-exponential decay of 2 ms. As all the Mn2+ impurities are located inside the ZnS lattice, only the slow component is observed in the doped and passivated sample. Thus we assign the fast-decay time of 0.18 ms to the 4T1 lifetime of surface-bound (exterior) Mn2+ impurity and the slow one of 2 ms to the 4T1 lifetime of lattice-bound (interior) Mn2+ impurity. Our unpublished experimental results performed with Mn2+-passivated ZnS nanoparticles also support this assignment. (The Mn2+-passivated sample shows only a fast component as all the Mn2+ impurities are located at the surface. However, not only the lifetime but also the spectrum of Mn2+ luminescence from the capped sample, where Mn2+ has broad energy bands, are significantly different from those of the fast decaying Mn2+ luminescence from the doped sample, where Mn2+ has discrete energy levels.) The significantly different binding environment makes the oscillator strength of the exterior Mn2+ impurity substantially higher than that of the interior Mn2+ impurity. The relatively shorter lifetime and large fractional amplitude of the exterior Mn2+ ion result from the relatively significant increase of its oscillator strength. The fact that the Mn2+-luminescence lifetime is almost independent of temperature variations20,30 suggests that the 4T1 state of the Mn2+ impurity is mainly quenched radiatively. We have found that the variation in the average particle size in the nanometer range effectively changes the relative abundance of the exterior Mn2+. No observable change in Mn2+ luminescence with change in the average particle size also supports that the lifetime shortening of the exterior Mn2+
J. Phys. Chem. B, Vol. 105, No. 19, 2001 4131 results from its increased radiative relaxation rate rather than its increased nonradiative relaxation rate. A simple calculation indicates that about 25% of Mn2+ impurities are located at the surface of 2% Mn2+-doped ZnS nanoparticles with the average diameter of 6 nm. Considering that the exterior Mn2+ impurity of 25% relative abundance brings in the average transition amplitude percentage of 69%, we can estimate that an exterior Mn2+ impurity has a transition strength about 7 times larger than that of an interior Mn2+ impurity. The lifetime of the exterior Mn2+ moieties is, thus, expected to be smaller than that of the interior ones by 7 times. Our observed lifetime of the fast component is smaller by 11 times than that of the slow one. Considering our experimental errors, the augmentation effect of the transition strength resulting from being at the surface agrees well in magnitude with the shortening effect of the lifetime. Combining the enhancement effects of transition strength and relaxation rate, we can state roughly that the oscillator strength of an exterior Mn2+ ion is larger by a factor of 1 order of magnitude over that of an interior Mn2+ ion. It has been speculated19,20 that the observed shorter-decay component of Mn2+ impurities arises from Mn2+ impurity located at the surface of nanoparticles, although extremely different decay times of nanosecond and millisecond scales have been contradictorily reported.21-24 Our results also show that the exterior Mn2+ impurity have certainly a shorter luminescence lifetime than the interior ones. However, the shortening factor is just 1 order of magnitude. The observation of the 20-ns emission decay component from the both Mn2+-doped and Mn2+-free samples (Figure 1) indicates that all the nanosecond luminescence decay components result from defect-related ZnS host rather than from the Mn2+ impurity. We have shown with Figure 2 that ZnS-excitation energy transfers to Mn2+ on the time scale of 700 ps. This somewhat slow-transfer time rather denies the speculation21-24 that there is a strong hybridization between the s-p states of ZnS and the d states of Mn2+ impurity. We have also verified that the luminescence of the Mn2+ impurities doped in ZnS nanoparticles decays on the time scale of milliseconds (Figure 3). This also refutes the speculation21-24 that the strong lifetime shortening results from quantum confinement effects.21-24 We attribute the lifetime shortening of exterior Mn2+ impurity by 10 times mainly to the enhanced spin-orbit coupling and vibronic coupling with the vibrational motions of water molecules or anions coordinated to the surfacebound Mn2+ impurities. The lower binding symmetry is also considered to contribute to the transition-strength enhancement of the exterior Mn2+ impurity, since the 4T1-6A1 transition of the interior Mn2+ is forbidden by the parity and spin selection rules. As the average size of the nanoparticles decreases, the fraction of the exterior Mn2+ impurity increases so that the overall apparent lifetime of Mn2+ luminescence decreases. Despite the fact that the size decrease of the host ZnS nanoparticles shortens the overall Mn2+-luminescence lifetime, neither a strong hybridization between ZnS and Mn2+ nor a strong lifetime shortening of Mn2+ emission occurs in Mn2+doped ZnS nanoparticles. Contrary to our expectations, our results thus indicate that the Mn2+-doped ZnS nanoparticles are not a new promising class of luminescent materials, as suggested previously.21-23 In summary, we have elucidated experimentally that all the nanosecond luminescence decay components of Mn2+-doped ZnS nanoparticles are emitted from defect-related ZnS host rather than from Mn2+ impurity. Mn2+ impurity emission decays with dual time constants of 0.18 and 2 ms. The fast component results from surface-bound Mn2+ impurity, while the slow one
4132 J. Phys. Chem. B, Vol. 105, No. 19, 2001 from lattice-bound Mn2+ impurity. The overall lifetime decrease with particle-size reduction is due to the fractional increase of surface-bound Mn2+ impurity rather than to quantum confinement effects. Increased spin-orbit coupling and vibronic coupling with the vibrational motions of coordinated species and lowered binding symmetry are considered to result in the transition-strength increase of surface-bound Mn2+ impurities, which shortens their luminescence lifetime. The energy transfer time of 700 ps from ZnS host to Mn2+ impurity indicates that the hypothesis of a strong (s,p,d) hybridization21-23 between ZnS and Mn2+ is not plausible. Acknowledgment. The Korea Research Foundation (KRF2000-015-DP0193) supported this work. D.J.J. and J.H.C. also acknowledge the Center for Molecular Catalysis and the Brain Korea 21 Program, respectively. We thank the Equipment Joint Use Program of the Korea Basic Science Institute as well. References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (3) Alvarez, M. M.; Khoury, J. T.; Gregory Schaaff, T.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (4) Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95, 525. (5) Norris, D. J.; Efros, Al. L.; Rosen, M.; Bawandi. M. G. Phys. ReV. B 1996, 53, 16347. (6) Mic´ic´, O. I.; Cheong, H. M.; Fu, H.; Zunger, A.; Sprague, J. R.; Mascarenhas, A.; Nozik, A. J. J. Phys. Chem. B 1997, 101, 4904. (7) Ah, C. S.; Han, H. S.; Kim, K.; Jang, D.-J. J. Phys. Chem. B 2000, 104, 8153. (8) Ah, C. S.; Han, H. S.; Kim, K.; Jang, D.-J. Pure Appl. Chem. 2000, 72, 91. (9) Ah, C. S.; Han, H. S.; Kim, K.; Jang, D.-J. Mol. Cryst. Liq. Cryst. 1999, 337, 209. (10) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (11) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016. (12) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013.
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