Enhanced Fluorescence Intermittency in Mn-Doped Single ZnSe

Department of Physics, and Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701. J. Phys. Chem. C , 2008, 11...
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J. Phys. Chem. C 2008, 112, 20200–20205

Enhanced Fluorescence Intermittency in Mn-Doped Single ZnSe Quantum Dots Yanpeng Zhang,*,† Chenli Gan,† Javed Muhammad,† David Battaglia,‡ Xiaogang Peng,‡ and Min Xiao*,† Department of Physics, and Department of Chemistry and Biochemistry, UniVersity of Arkansas, FayetteVille, Arkansas 72701 ReceiVed: June 29, 2008; ReVised Manuscript ReceiVed: October 28, 2008

We report detailed experimental studies of enhanced fluorescence intermittency in Mn-doped single ZnSe quantum dots. The measured intermittency statistics and its dependences on the excitation intensity and dot size indicate that the enhanced blinking might be due to the strong coupling between the multiple Mn2+ ions within a single nanocrystal. The measured statistical results are compared with a diffusion-controlled electrontransfer model, which gives the diffusion rates for the “on” and “off” time distributions. Fluorescence intermittency (or blinking) is a universal behavior of single colloidal semiconductor quantum dots (QDs), which has been extensively studied in many colloidal nanocrystals (NCs), such as CdSe, CdTe, InP, and CdSe/ZnS QDs, in the past 10 years.1-19 Typically, the intermittency happens between two distinct on/off states and can be considered as twostate blinking with simple on/off behavior. Several different mechanisms (such as due to a charging discharging,1,2 a phenomenological charge tunneling model,3,4 and a no longlived charge trap model 17-19) have been proposed to explain these observed blinking behaviors and to calculate the on/off time probability distributions. The correlation between the spectral diffusion and the fluorescence intermittency, where a QD changes in time between the charge-separated dark state and the neutral bright state, has been experimentally established.10,15 Also, the second-order intensity correlation, based on these three- and four-level models, has been used to compare with the experimentally measured results.14,16 Recently, enhanced fluorescence intermittency in CdSe/ZnS QD clusters has been observed,9 which shows sharp and abrupt on/off behaviors due to electronic coupling between neighboring QDs within short distance. With such enhanced blinking, the on/off time probabilities show broad multistate distributions deviating significantly from the clear two-state (neutral-“on” and charged-“off”) distribution as always observed in the single QD case.12,13 The newly developed high quality Mn- and Cu-doped ZnSe d-dots 20-24 have several superior properties comparing to the traditional semiconductor QDs,25-29 such as reduced selfquenching due to large Stokes shift, greatly suppressed host emission, and improved stabilities over thermal, chemical, and photochemical disturbances.21 Such improved d-dots do not contain any heavy metal ions and can be used to advance practical applications in biomedical labels,23 LEDs,30 and lasers.31 In a Mn-doped single ZnSe NC synthesized by the nucleationdoping method, there is a MnSe crystal core and a ZnSe shell with some Mn2+ ions diffused into the ZnSe lattices.21-23 This system behaves as ions in a crystal field, as shown in Figure 1(a). The diameters of the MnSe core and the ZnSe shell, as well as the number of Mn2+ ions diffused in the ZnSe shell, * To whom correspondence should be addressed. † Department of Physics. ‡ Department of Chemistry and Biochemistry.

can all be controlled in the synthesis process and determined by the ion diffusion model.21,22 In this work, we experimentally investigate the blinking behavior of an individual Mn-doped ZnSe NC. The observed blinking signals show the signature of enhanced fluorescence intermittency as observed in closely packed QD clusters,9 indicating collective emission from multiple Mn2+ ions within the single d-dot possibly due to coupling between them. It has been well established that the fluorescence of Mn-doped ZnSe NCs at a wavelength around 580 nm is from the transition 4T1to 6 A1 of the Mn2+ ions in the crystal field of the ZnSe NC. The excitation light is absorbed by the host ZnSe QD (valence band (VB) to conduction band (CB), as shown in Figure 1(b)), it nonradiatively decays quickly (in ps25) to the upper level of the Mn2+ ions (4T1), which decays slowly (∼0.2 ms 28,32) to the lower state (6A1). Since there are multiple (closely packed) Mn2+ ions in each d-dot, there can be multiple excitations of Mn2+ ions, especially when the excitation intensity is high (more carriers are excited to the CB). The transition between 4T1 and 6 A1 in Mn2+ ions gives the “bright” state. However, the charge separations in the host ZnSe can cause the transition (tunneling) from the bright state into the charged “dark” states |D〉 and |D/〉,17-19 as shown in Figure 1(c). This four-state model (two neutral bright states CB and VB, and two charge-separated dark states |D/〉 and |D〉) has been successfully used to explain the blinking behaviors in single semiconductor QDs17-19 and can also be used to qualitatively describe the current Mn-doped QD system when the bright states are replaced by 4T1 and 6A1 for the Mn2+ ions. By carefully measuring the statistics of the on/ off blinking behaviours in the Mn-doped ZnSe NCs, especially for different d-dot sizes and excitation powers, we believe that the enhanced fluorescence intermittency might be caused by the coupled Mn2+ ions (in the ZnSe crystal field) within the individual d-dot. Here, the enhancement is defined as relative to the case without coupling between Mn ions. The measured on/off probability distributions for the d-dots of different sizes and different excitation powers are compared with the diffusioncontrolled electron-transfer (DCET) model,17-19 and good agreements are obtained. Understanding the mechanism for this enhanced blinking phenomenon may help to develop new electro-optic and biological applications for such ion-doped d-dots on the single QD level.

10.1021/jp805855m CCC: $40.75  2008 American Chemical Society Published on Web 12/04/2008

Mn-Doped Single ZnSe Quantum Dots

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Figure 1. The mechanism diagrams for the enhanced blinking of an isolated d-dot with closely packed multi-ions: (a) Structure model for the Mn-doped ZnSe NC with a MnSe core and a ZnSe shell with some diffused Mn2+ ions; (b) Schematic representation of electron-hole recombinations of multi-ions in a d-dot; (c) Blinking menchansim (four-state light-driven spectral diffusion and fluorescence blinking model17) of an isolated d-dot with a doped ion. The electronic coupling (or electron transfer) between states 4T1 and |D〉 could result in the blinking behavior of a d-dot. The W and A are the photoexcitation rate and the nonradiative Auger decay rate, respectively.17

The Mn-doped ZnSe NCs are synthesized by using the nucleation-doping method.21,22 Dopant and host precursors are first mixed during the nucleation. After nucleation, the reaction condition is tuned to be sufficiently mild to make the dopant precursors inactive, and the growth of the host becomes the only procedure, which overcoats the dopants. The concentrations of the dopants in the nuclei can be tuned by varying the precursor ratio. The nucleation-doping structure can be considered as a combination of doped NCs and core/shell NCs (Figure 1(a)). There are about 6∼9 Mn2+ ions in each d-dot diffusion region in our samples (calculated by the ion diffusion model21-23), with different overall diameters of 3.5, 5, and 7 nm, respectively.21,22 All d-dots have the same MnSe core of 1.5 nm in diameter and different ZnSe overcoating layer thicknesses. The small MnSe core has an emission wavelength shorter than the excitation wavelength (488 nm) and will not contribute to the detected fluorescence (near 580 nm) in the experiments. An amount of Mn2+ ions will diffuse into the ZnSe layer during the annealing process in the synthesis, forming a diffusion region as shown in Figure 1(a). The diffusion region is a little thicker for the larger NCs following the ion diffusion model. The overall numbers of Mn2+ ions are the same for these Mn/ZnSe d-dots with different diameters. Ions in each individual d-dot are clustered together in close proximity (Figure 1(a)) for enhanced blinking to occur. It is similar to the enhanced blinking behavior in the closely packed QD clusters,9 where the enhanced blinking is due to dot-dot coupling. Samples for fluorescence intermittency measurements were prepared by spin coating dilute solutions of the NCs in toluene with 1% polymethyl methacrylate (or in pure toluene) onto quartz coverslips. The substrates were examined to make sure there was no dominating background luminescence prior to the sample deposition. The d-dot density on the substrate was controlled by changing concentration of the d-dots in the solution before spin coating. The experiment was performed by using a standard far-field confocal microscopic technique.33,34 The 488 nm line of an argon-ion laser was used as the excitation source. A highmagnification microscope objective (200×, NA ) 0.7) was used to excite the sample and collect the fluorescence which was then sent to the spectrometer and detected by PMT. Two holographic notch filters and one colored glass filter were used to remove any residual 488 nm light from entering the PMT. A controlled x-y-z piezoelectric translation stage scans the sample under a diffraction-limited laser excitation spot. By carefully controlling the density of the d-dots on the sample, the d-dot density can be less than one dot per micrometer square, as shown in Figure 2(a). To isolate the fluorescence from an

individual d-dot, the piezoelectric stage was electronically adjusted to maximize emission from the selected single NC. The system has been calibrated by using single CdSe/ZnS QDs to ensure the bright spots as single dots. Then, the slit of the spectrometer was narrowed down to select only one dot to be within the field of view (Figure 2(b)). Also, the emission spectrum of the single d-dot emission at 300 K and at an excitation intensity of 350 W/cm2 is recorded by PMT with an integration time of 10 s/nm and shown in Figure 2(c) (the asymmetric broad shoulders on both sides of the spectrum mainly result from the spectrometer with the wider slit width), which includes some small spectral diffusion (near the sharp peak at the center of the spectrum) as observed in most of single colloidal QDs.10,15,17 Figure 2(d) presents the spectrum of single Mn/ZnSe d-dot with the narrower slit width and 1 s integration time. The fluorescence measurements for the intermittency were taken at the fixed wavelength of around 580 nm with only one dot to be within the field of view. Figure 3 shows fluorescence intensity trajectories of two different size d-dots with different excitation intensities. Figure 3(a) presents the blinking signal for the larger d-dot (7.0 nm in diameter) at a low excitation intensity (25 W/cm2). The rare jumping events indicate very low excitation probability of the Mn2+ ions in the d-dot. As the excitation intensity increases to 150 W/cm2 (Figure 3(b)) and 350 W/cm2 (Figure 3(c)) for the same d-dot, the jumping events increase substantially, indicating a higher level of excitation for the Mn2+ ions in the d-dot. The broad multistate distribution 12,13 at the higher excitation intensity (Figure 3(c), right figure) could be considered as the signature of enhanced intermittency9 possibly due to coupling between different ions, similar to the case with coupled QDs in clusters.9 Although the exact distance between ions in this d-dot (7.0 nm in diameter) cannot be determined, we assume that Mn2+ ions are uniformly distributed in the diffusion region of the ZnSe shell (Figure 1(a)) and the average distance between Mn ions can be estimated to be about 1.3 nm (both the diffusion layer thickness and the number of Mn ions in the diffusion region can be calculated by the ion diffusion model21-23). When the same measurements (same excitation intensities) are repeated with a smaller d-dot (3.5 nm in diameter, with an estimated average distance between ions to be about 1.0 nm), the differences in blinking trajectories are quite dramatic, as shown in Figure 3(d)-(f). For the same excitation intensity, the jumping events are much more for the smaller d-dot, with a broader intensity distribution, which indicates stronger electronic coupling between the Mn2+ ions in the d-dot due to closer proximity. This is consistent with the enhanced intermittency of QD clusters observed in ref 9.

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Figure 2. PL images of single Mn/ZnSe d-dots (a) with a fully opened entrance slit of the spectrometer and (b) with a narrower slit for measuring the blinking of an individual d-dot, which are taken at room sample temperature with a CCD integration time of 10 s. (c) and (d) The spectra of a single Mn/ZnSe d-dot with the wide and narrow slit widths, with the 10 and 1 s integration times, respectively. There are three spectra in (d) taken at different times with a narrow slit.

Figure 3. Fluorescence trajectory segments (left) and the corresponding photon count histograms (right) with excitation intensities of 25 W/cm2 ((a) and (d)), 150 W/cm2 ((b) and (e)), and 350 W/cm2 ((c) and (f)) for 7.0 nm ((a), (b), and (c)) and 3.5 nm ((d), (e), and (f)) isolated Mn/ZnSe d-dots, respectively.

In the four-state DCET model 17-19 for semiconductor colloidal QDs, when time t is larger than the critical time constant tc,i (in ms) but shorter than the effective diffusion time constant τi (1∼50 s), the on and off probability distribution functions can be written as

Pon/off(t) ≈ at-mexp(-Γon/offt)

(1)

where a ) tc,i/π/2, i ) “on” or “off”. Γi is proportional to 1/τi, and τon and τoff are the diffusion correlation times for the bright state 4T1 and dark state |D〉, respectively. From the previous study of temperature-dependence on time behavior, τon is expected to have a much smaller value at 300 K.17-19 The power-law exponent m is calculated to be 3/2 for intermittency in normal diffusion case.17 Our measurement time range (between 0.01 and 1s) for the blinking behavior just falls in the applicable time scales of eq 1. Equation 1 predicts a breakdown of the power-law behavior with a bending tail for the on-time events (determined by Γon with sensitive dependences on excitation light intensity, QD size,

and temperature) and eventually for the off-time events as well at much longer time.17-19 Specifically, the bending tail is shown to be more clear for the bright period and less obvious for the dark period, which is consistent with the experimental observations using semiconductor QDs.17-19 As compared to the fluorescence decay time (∼0.2 ms) of the bright state,28,32 Auger relaxation (∼30 ps) in the dark state is much faster,35 and τoff could be substantially longer, which leads to a smaller bending. Our experimental observations indicate that such DCET model also holds for the current d-dots, and we determine the powerlaw exponent m and the parameters Γon/off by fitting our experimentally measured data with eq 1. We first present the results of measured on and off time distributions for the Mn-doped ZnSe NCs with 5.0 nm diameter at room temperature. The excitation intensity is set at 350 W/cm2. Figure 4 shows the measured results for the on and off time distributions together with the fits to eq 1 (solid lines). The fitting parameters are m ) 1.4(3), Γon ) 1.0 s-1 for the on time distribution (Figure 4(a)), and m ) 1.6(4), Γoff ) 0.02 s-1

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Figure 4. The log-log plots of the time dependence of Pon/off(t) for both on (a) and off (b) events from experimental data of 5.0 nm Mn/ZnSe d-dot at 300 K and excitation intensity of 350 W/cm2. The data (square point) are fitted to Pon/off(t) ≈ at-mexp (-Γon/offt) (solid line) and an exponential function (short dashed line); the fitted parameters are shown in the plots. The vertical arrow in (a) corresponds to the truncation point where the power-law behavior begins to end.

Figure 5. Experimental probability density distributions of (a) the on times for excitation intensities of 25 W/cm2 (square), 150 W/cm2 (circle), 350 W/cm2 (triangle), and 600 W/cm2 (reverse triangle); the arrows indicate the truncation points for each curves, respectively. (b) The power dependences of Γon for d-dots with diameters of 3.5, 5.0, and 7.0 nm, respectively.

for the off time distribution (Figure 4(b)), respectively. The vertical arrow in Figure 4(a) corresponds to the truncation point where the power-law behavior (Pon/off˜at-m) is estimated to end. This is consistent with the DCET model with a much less obvious bending tail for the off period (Figure 4(b)).17-19 We also fitted our data with other functions, such as a simple exponential (dashed lines), in Figure 4 and found that the fits to other functions are definitely not as good as to eq 1, which is a strong support for using the DCET model. Next, the results of on-time probability for different excitation intensities are presented in Figure 5. As one can see from Figure 5(a), the bending tail starts earlier for higher excitation intensity due to faster diffusion time for the bright state 4T1 as predicted in the DCET model. To obtain consistent fitting parameter Γon, the power-law exponent m is kept fixed in these fitted curves. The dependences of Γon for different dot sizes as a function of excitation power are given in Figure 5(b). The relation between the size (or cross section) of the dots and the excitation intensity is not a simple one. It depends on the region of the excitation intensity. As one can see that at low excitation intensities (I < 300 W/cm2), smaller dots have a larger diffusion rate than the larger dots. At high excitation intensities (I > 500 W/cm2), the effect is opposite (i.e., smaller dots have smaller diffusion rate). At the middle intensity region (300 W/cm2 < I < 500 W/cm2), the situation is quite complicated. Obviously, a simple cross section effect due to different dot sizes cannot explain such behavior. The displacement of the potential surface of state 4T1 due to a randomly oriented local electric field can result in the spectral diffusion.10,15,36 However, these local field random changes could be created by the localized charge reorganization in the d-dot off process.10

For the same number of Mn2+ ions in each dot, when the d-dot size changes, the average ion-ion distance and therefore the electronic and dipole couplings, as well as the diffusion correlation times for the bright state 4T1 and dark state |D〉, will be modified. These factors will significantly change the diffuse rates Γon and Γoff. We experimentally measured the on and off time distributions of the Mn-doped d-dots for three different sizes (diameters 3.5, 5.0, and 7.0 nm, respectively) and present the results in Figure 6. As one can see from Figure 6(a), the bending tail of the on time probability distribution starts at shorter time for the larger d-dot comparing to the smaller d-dot. Similar behavior appears for the off time distribution, except with less dramatic drops for different sizes. Notice that the best fit for the power-law exponent m for the off time distribution is larger than the on time distribution value. At high excitation intensities, the fitted Γon values (with fixed m ) 1.4(5)) for different d-dot sizes are given in the right side of Figure 5(b), which shows an obvious increase as d-dot size increases. Other than the change of the coupling strength between ions in the d-dot due to size change and the diffusion time for the bright state 4T1, another effect of size change is the effective absorption area for the excitation light. For a given excitation intensity, larger d-dot will have a larger effective absorption cross section and therefore a large excitation probability, same as increasing excitation intensity, which agrees well with the measured dependence of probability density distribution on the excitation intensity for the fixed d-dot size (Figure 5(a)). The data presented in Figure 3 were for low and intermediate excitation intensity regions (I < 350 W/cm2). The changes of multistate blinking behaviors, as shown in Figure 3, due to the change of d-dot size (Figure 3(d)s(f) vs Figure 3(a)s(c)) cannot be explained

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Figure 6. The log-log plots of the time dependence of Pon/off(t) for both (a) on and (b) off events from experimental data of Mn/ZnSe d-dots at 300 K and 600 W/cm2 with different dot sizes of 3.5 nm (square), 5.0 nm (circle), and 7.0 nm (triangle). The arrows in (a) indicate the truncation points for the probability distributions for the d-dots of different dot sizes.

by the simple change of absorption cross section (due to size change), as can be easily seen in Figure 5(b). The four-state DCET model17-19 was developed for explaining the fluorescence intermittency of the single colloidal semiconductor QD. Therefore, although Γon is a good measure of the diffusion correlation time in a single semiconductor QD with a two-state blinking behavior, it cannot capture all the features in the multistate blinking events, as in the case of a NC doped with multiple ions. More quantitative works, both theoretical and experimental, are needed to fully understand and describe the multistate blinking behaviors. If samples with better controlled number of ions doped in a NC (such as one ion per NC and two ions per NC, etc.) could be synthesized and the distance between ions could be precisely determined and controlled, then better controlled experiments could be performed. However, without such ideal experimental samples and systems, our current study can serve as an important step toward the understanding of such interesting ion-doped NCs and can stimulate more researchers, both experimentalists and theorists, to work in this direction. The Mn-dopped ZnSe NCs have unique optical properties as compared to the standard semiconductor QDs. The emission near 580 nm is associated with the transition 4T1-6A1 of the Mn2+ ions in the crystal field of the ZnSe NC. The luminescent transition from the lowest-excited level 4T1 (spin 3/2) to the ground state (spin 5/2) is spin forbidden, but the weak spin-orbital interaction makes this transition slightly allowed. Since the spin-orbit interaction is very weak, the oscillator strength is very small, which leads to the luminescence lifetime being very long (∼millisecond time scale).28,32 Another important feature in this d-dot system is the efficient energy transfer from conduction band of the host semiconductor NC to the 4T1 state of the Mn2+ ion, which greatly suppresses the fluorescence from the host QD (ZnSe). When many Mn2+ ions exist in the crystal field of ZnSe and with small distance, efficient couplings between these ions can become important and will greatly affect the light emitting behaviors of such d-dot system.9,37-40 At this time, the exact coupling mechanism between the ions in the host crystal field is not known. Therefore, studying the single d-dot intermittency, in combination with appropriate modeling, can provide a good way to probe such internal coupling in the d-dots. Although some properties of such d-dots are similar to the standard semiconductor QDs, the current d-dot system is more complicated and interesting with some unique features.21-23 Further studies are needed to uncover the exact mechanism of how electrons are transferred in such d-dots and how these ions are electronically coupled or whether the emissions of ions can be radiatively coupled within the single d-dot. The measured

blinking is accompanied with the spectral diffusion. Strong evidence for the correlation between the blinking events with spectral diffusion has been established in several studies.10,17,36 Figures 4-6 and eq 1 suggest that there exist the intrinsically strong correlation between the blinking events and spectral diffusion, but the mechanism of such correlation is not fully understood in such ion-doped NCs and needs further investigation. In summary, we experimentally studied the enhanced fluorescence intermittency of individual Mn-doped ZnSe NCs. The blinking enhancement is possibly due to the couplings between the multiple Mn2+ ions residing inside a single d-dot, similar to the enhanced intermittency due to closely packed QD clusters.9 Further experimental and theoretical studies are needed to confirm such coupling between the doped Mn ions and uncover the exact coupling mechanism. Both excitation intensity dependence and size dependence of the on and off time distributions for such enhanced blinking behaviors were measured and compared with the four-state diffusion-controlled electron-transfer model developed in ref 17 Studies of such intermittency behaviors of an individual d-dot can be very important for the potential applications of such unique light emitters in biological labeling, LEDs, lasers, and other optelectronic devices. Acknowledgment. We acknowledge funding support from the NSF/ MRSEC (DMR-0520550), Army Research office (W911NF-05-1-0353), and Arkansas Science and Technology Authority. References and Notes (1) Nirmal, M.; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J. J.; Trautman, J. K.; Harris, T. D.; Brus, L. E. Nature 1996, 383, 802. (2) Efros, A. L.; Rosen, M. Phys. ReV. Lett. 1997, 78, 1110. (3) Kuno, M.; Fromm, D. P.; Hamann, H. F.; Gallagher, A.; Nesbitt, D. J. J. Chem. Phys. 2000, 112, 3117. (4) Shimizu, K. T.; Woo, W. K.; Fisher, B. R.; Eisler, H. J.; Bawendi, M. G. Phys. ReV. Lett. 2002, 89, 117401. (5) Kuno, M.; Fromm, D. P.; Johnson, S. T.; Gallagher, A.; Nesbitt, D. J. Phys. ReV. B 2003, 67, 125304. (6) Shimizu, K. T.; Neuhauser, R. G.; Leatherdale, C. A.; Empedocles, S. A.; Woo, W. K.; Bawendi, M. G. Phys. ReV. B 2001, 63, 205316. (7) Kuno, M.; Fromm, D. P.; Hamann, H. F.; Gallagher, A.; Nesbitt, D. J. J. Chem. Phys. 2001, 115, 1028. (8) Hohng, S.; Ha, T. J. Am. Chem. Soc. 2004, 126, 1324. (9) Yu, M.; Orden, A. V. Phys. ReV. Lett. 2006, 97, 237402. (10) Neuhauser, R. G.; Shimizu, K. T.; Woo, W. K.; Empedocles, S. A.; Bawendi, M. G. Phys. ReV. Lett. 2000, 85, 3301. (11) Banin, U.; Bruchez, M.; Alivisatos, A. P.; Ha, T.; Weiss, S.; Chemla, D. S. J. Chem. Phys. 2000, 112, 3117. (12) Zhang, K.; Chang, H.; Fu, A.; Alivisatos, A. P.; Yang, H. Nano Lett. 2006, 6, 843.

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