Quantum Shell

(c) PL spectra for the ensemble (black) and a single dot at various times (colors). ..... to (i) sustained on episodes (τon) of rapid laser absorptio...
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14229

2008, 112, 14229–14232 Published on Web 08/21/2008

Single Dot Spectroscopy of Two-Color Quantum Dot/Quantum Shell Nanostructures Eva A. Dias,† Amy F. Grimes,‡ Douglas S. English,*,‡,§ and Patanjali Kambhampati*,† Department of Chemistry, McGill UniVersity, Montreal, QC, H3A 2K6, Canada, and Department of Chemistry, UniVersity of Maryland, College Park, Maryland 20742 ReceiVed: July 25, 2008; ReVised Manuscript ReceiVed: August 14, 2008

Single dot spectroscopy is performed on two-color CdSe/ZnS/CdSe core/barrier/shell nanostructures. Unlike quantum dots cores, these systems have two phases with which to emit and ultimately examine for blinking analysis. These particles are brighter than conventional quantum dots and also show the photoluminescence (PL) intensity and energy fluctuations characteristic of quantum dots. Single dot spectral diffusion analysis yields no measureable energy shift correlation between the core and the shell on the 200 ms time scale. In contrast, the single dot PL from the CdSe shell has narrower linewidths than the CdSe core, indicating differences in its spectral diffusion on shorter timescales. Spectroscopic experiments on single quantum dots have been under intense investigation both for practical application of imaging,1,2 as well as for the fundamental issue of the physical origin of blinking.3-16 In all cases, the quantum dots emitted a single band of photoluminescence (PL) based upon an emissive core. These materials were initially quantum dot cores (e.g., CdSe), subsequently followed by permutations of shell materials, such as CdS or ZnS. The shell material provides increased photostability of the core and allows chemical surface modification for imaging applications. Even greater functionality can be achieved through the presence of two-color emission in a coupled quantum dot system. Two emissive phases should have promise for increased brightness and photoluminescence on times in imaging applications. From a photophysical perspective, there will be additional observables to facilitate mechanistic investigations of the blinking process. Recently, Peng and co-workers developed a novel CdSe/ZnS/ CdSe core/barrier shell system for which there are two emissive phases.17 In this system, one can epitaxially grow several monolayers of different shell materials. The key observation was that both CdSe phases emitted, and the PL of each phase could be controlled by thickness of the ZnS shell, suggesting some form of interaction. Subsequent experiments by our group suggested that the two CdSe phases were coupled via tunneling through the ZnS barrier.18 In addition, we found that the outer CdSe shell served as a light harvester, increasing the effective brightness of the core PL. This structure was subsequently used toward white light emission19 and spin dynamics.20 Focusing on the emissive properties, the CdSe shell makes the CdSe core brighter, without needing to change the core size.18 Increasing the size of the core would increase the absorption cross section, but would also change the PL wavelength. With this system, one can in principle obtain * To whom correspondence should be addressed. E-mail: (P.K.) [email protected]; (D.S.E.) [email protected]. † McGill University. ‡ University of Maryland. § Current Address: Department of Chemistry, Wichita State University, Wichita, Kansas, 67260-0051.

10.1021/jp806621q CCC: $40.75

brighter CdSe cores at any wavelength. Doing so suggests an obvious benefit to imaging applications. However, the size of the particle may have influence on its location in a biological environment, which is a topic unto itself.21 The less obvious and more interesting issue is the extent to which the two phases undergo correlated PL blinking. Completely uncorrelated PL would yield a fluorophore which is simply on longer, an additional improvement for imaging. However, any presence of correlations in blinking could yield valuable information on the physical mechanism underlying blinking in colloidal quantum dots. In this letter, we report on spectrally resolved single dot spectroscopy on the CdSe/ZnS/CdSe core/barrier/shell system. The single dot PL trajectories of either phase are consistent with prior works on CdSe/ZnS core/shell systems in terms of the relevant observables: intensity trajectories, spectral diffusion,4,10 and correlation between energy shifts and line width.10 There are two new observations which are made possible by the presence of two emissive phases. First, the PL energies show no measurable correlation on the 200 ms time scale of this experiment. Second, the single dot linewidths of the CdSe shell PL are significantly smaller than the PL from the CdSe core. CdSe nanocrystals were prepared using the alternate precursor method developed by Peng and co-workers.22 The layered CdSe/ ZnS/CdSe systems were prepared using the modified successive ionic layer adsorption and reaction (SILAR) method developed by Peng and co-workers.17,22,23 This system was recently reproduced by our group and Rogach et al.18,19 For this study three monolayers (ML) of ZnS were added, followed by three ML of CdSe. The size of the CdSe core used for the single dot experiments was R ) 1.6 nm. The size was estimated using published sizing curves.24 From these cores, the core/shell/shell structure was grown with 3 ML of ZnS and 3 ML of CdSe for a total R ∼ 3.7 nm. The single dot experiments were conducted in air on a freshly cleaned glass coverslip using a home-built confocal microscope. A dilute solution of QDs was deposited from toluene by spin coating to give a sparse covering of single dots which were  2008 American Chemical Society

14230 J. Phys. Chem. C, Vol. 112, No. 37, 2008

Letters

Figure 1. (a) Ensemble absorption spectrum of CdSe core quantum dots and CdSe/ZnS/CdSe core/barrier shells normalized to the band edge absorption for the core. (b) Ensemble PL spectrum of CdSe core quantum dots and CdSe/ZnS/CdSe core/barrier shells normalized to the core PL. (c) PL spectra for the ensemble (black) and a single dot at various times (colors).

easily resolved with a focused probe beam of FWHM ) 350 nm. Excitation was at 488 nm. The presence of single dots is confirmed by observing discreet one-step cessation of emission to the background. The microscope used was previously described.25-27 The data were acquired by either separating the photoluminescence (PL) into two channels with a dichroic beam splitter and detecting with avalanche photodiodes (APD) at 1 ms binning time, or by spectrally resolving PL using a monochromator and a charge coupled device (CCD) at 200 ms binning time. Details of the synthesis and microscopy are in the Supporting Information. Figure 1 shows representative ensemble and single dot spectra. The ensemble spectra in Figure 1a,b have been discussed in detail by Battaglia et al.17 and Dias et al.,18 and will not be further discussed here. Figure 1c compares the ensemble PL spectra to the single dot spectra. The single dot spectra were selected from a time sequence of spectra acquired from the same dot. The peaks in the single dot spectra do not necessarily overlap with the ensemble spectra due to inhomogeneities in each of the three phases. This time series clearly shows the dramatic spectral changes observed for a single dot. Figure 2 shows the single dot intensity trajectories, illustrating the standard blinking/flickering behavior characteristic of single dots. A common approach to single dot PL experiments is to perform wavelength integrated experiments. Doing so improves signal-to-noise, but more importantly enables short binning times. The effect of binning time on the trajectories has been previously discussed by Yang and co-workers.11 In the case of this two-color system, some wavelength resolution is necessary in order to distinguish the phases. The intensity trajectories shown in Figure 2a were acquired using dichroic mirrors and filters to isolate the PL from each phase. With a 1 ms bin time, the blinking/flickering behavior typical of a CdSe/ZnS dot is observed. The CdSe shell trajectories (not shown) are similar, although not identical. This approach is not ideal, since the PL spectra of the two phases are sufficiently closely spaced as to allow leakage between the two channels. Correction for channel

Figure 2. Single dot transients showing intensity fluctuations. (a) Single wavelength transients measured for the CdSe core PL at 1 ms binning time. (b) The same data up-binned to 200 ms for comparison to the spectral measurements. (c) Integrated area of the CdSe core PL by spectral measurements with 200 ms binning time. (d) Integrated area of the CdSe shell PL by spectral measurements with 200 ms binning time.

leakage is impractical since spectral diffusion will create an unknown time varying leakage function. Hence, fully wavelength resolved measurements offer cleaner spectral analysis at the cost of time resolution (200 ms versus 1 ms). Figure 2b shows how the intensity trajectories acquired at 1 ms bin would appear when up-binned to the 200 ms binning time used for the fully wavelength-resolved experiments. The fully wavelength-resolved experiments are shown in Figure 2c,d. In these wavelength resolved trajectories, the individual spectra were fitted to two Gaussians in order to extract a set of time dependent parameters (intensity, width, energy) for each emitting phase. The data in Figure 2c,d shows the integrated spectral area for each phase. These trajectories are consistent with the up-binned data in Figure 2b. The intensity trajectories of this system are similar to the previous single dot experiments on CdSe/ZnS.4,6-8,10,12,16,28,29 Insight into the photophysics of single dot PL has been obtained from the phenomenology of spectral diffusion. At low temperature, one sees small amplitude diffusion with a subensemble of large amplitude jumps which follow an on-off-on event.4,5,28 These experiments by Bawendi and co-workers were done at 10 K. Similar results were obtained at 300 K by

Letters

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Figure 3. Correlation of energy shifts with linewidths. (a) Energy and line width trajectory for the core PL. (b) Correlation between energy shift from the maximum and line width for the core. (c) Energy and line width trajectory for the shell PL. (d) Correlation between energy shift from maximum and line width for the shell.

Mulvaney and co-workers, who additionally showed that the magnitude of the energy shift correlated with the line width.10 Figure 3a shows the energy and line width trajectory of the CdSe core, while Figure 3b shows a correlation plot between line width and the energy difference from the maximum. Similar behavior is observed for the CdSe shell phase in Figure 3c,d. The correlation between energy difference and line width reproduces the recent results by Mulvaney and co-workers.10 The data in Figures 1-3 show that the single dot spectroscopy of a given phase of this core/barrier/shell system is consistent with the prior works on quantum dot cores. The first new observation is that there may be times when the CdSe core is on and the CdSe shell is off and vice versa (Figure 2). Elimination of the wavelength resolution would thereby reduce the dark periods of the quantum dot which would be beneficial for imaging applications in conjunction with the increased brightness of the core.18,19 Prior work by Bawendi and co-workers4 showed correlation between the net energy shifts and on/off/on events. Those experiments at 10 K and 100 ms binning showed two Gaussian distributions of energy shifts: a small amplitude distribution due to spectral diffusion and a large amplitude distribution related to the blinking events. Similar results were observed by Mulvaney and co-workers10 at 300 K and 3 s binning. They showed how the distribution was insensitive to the dielectric environment. The energy shift distributions have provided insight into the relation between spectral diffusion and blinking,4,5,10,28 as well as suggesting the location of charge trapping during dark periods.10 As a first step toward investigating correlation between phases, we present net energy shift histograms in Figure 4 for each emitting phase and for the ratio of the core/shell. The energy shifts reported here and in the prior works,4,10 correspond to the net energy shift from point to point. The histograms here include the spectral jumps which accompanied an on/off/on series. The energy shifts for the core/shell ratio were obtained by computing the ratio of the energies, computing the net shifts

Figure 4. Evaluating correlation in net consecutive energy shifts (Ei+1 - Ei) between the core and the shell. (a) Net energy shift histograms at 200 ms binning time for the core, (b) shell, and (c) core/shell ratio. (d) The core histogram up-binned to 3 s.

of the ratio, and converting the ratio shifts to energy units by multiplying by the mean of the core and shell energies. The data for the core and the shell PL shows energy shift distributions with a width σ ∼ 7 meV. This distribution is wider than the Bawendi work which was at 10 K,4 as compared to 300 K here. This distribution is also wider than the Mulvaney work,10 which used 3 s binning time as opposed to the 200 ms bins used here. Up-binning these data (Figure 4d) recovers histograms similar to those of Mulvaney and co-workers.10 The energy shift ratio (core/shell) may be used as a probe of correlation in spectral diffusion. If the width (1/e) of an individual phase is σ, complete anticorrelation would yield a width of 2σ, zero correlation would yield 2 σ, and complete correlation would yield a delta function (limited by instrumental noise). The data in Figure 4c shows no measurable correlation in the spectral diffusion on the 200 ms time scale, since the width of the net shifts in the ratio is precisely 2 larger than the mean of the core and shell widths. The presence or absence of measureable correlation is clearly related to the time scale of the measurement. In other words, the width of the ratio histogram should depend upon binning time, reflecting the cross-correlation function of the two phases. The 200 ms binning time in the spectrally resolved measurements will miss the early time dynamics in the core/shell crosscorrelation. The presence of unresolved fluctuations is indicated by the single dot linewidths, Figure 3. The linewidths (1/e) of the core are 50-70 meV, whereas the shell linewidths are 35-45 meV. In contrast, the ensemble measurements yield a core line width of 70 meV and a shell line width of 110 meV. The differences between the ensemble and single dot line width suggests that the shells have more inhomogeneous broadening than the core. The surprising result is that the shells have narrower single dot PL linewidths than the cores. Given the time scale of the

14232 J. Phys. Chem. C, Vol. 112, No. 37, 2008 measurement, the single dot linewidths reflect the magnitude and time scale of the spectral diffusion.5,28 We cannot at present distinguish magnitude from time scale, but it is clear from the linewidths that the shell does not undergo the same spectral diffusion as the core on timescales faster than 200 ms. One might anticipate that time scale is a key issue in light of ultrafast exciton relaxation dynamics in quantum dots,30-33 as well as the competing processes in these core/barrier/shell systems.18 We briefly review the current phenomenology and pictures of quantum dot blinking3-15,28 to provide context for these results. The simple observation of blinking is generally assigned to an Auger type ionization process.4-6,8,28,29,34,35 However, the details of the mechanism remain controversial. Generally, it is believed that ejection or trapping of a carrier leads to changes in the level structure and oscillator strength of the intrinsic quantized core transitions.7,8,28,36,37 This proposition is consistent with observations of fluctuating coupling to polar optical phonons28,38 and correlation between line width and energy shifts.10,28 However, it is not clear whether it is the electron or hole which gets trapped, and furthermore where it gets trapped.28 If the carrier were ejected to the surrounding matrix, there should be sensitivity to the dielectric constant of the matrix. Recent experiments by Mulvaney are inconsistent with this supposition, suggesting that the carrier gets trapped at the CdSe/ZnS interface.10 Furthermore, both single dot PL14 and femtosecond experiments39,40 suggest that holes are dominantly trapped at the interface. Isolated analysis of either the core or the shell yields observations consistent with the prior literature. The presence of two emissive phases presents the opportunity to investigate cross-correlation between phases, for example, spectral diffusion. We find no measurable spectral diffusion correlation on the 200 ms time scale of the experiment. Yet, the single dot linewidths suggest that the CdSe core and the CdSe shell do not undergo identical spectral diffusion. In summary, these preliminary single dot experiments on the CdSe/ZnS/CdSe core/barrier/shell system suggest its value in microscopy and blinking studies. First, the material is brighter than standard CdSe/ZnS core/shell dots due to the outer CdSe shell.18,19 Second, the core and shell do not necessarily blink in concert, yielding a system which is on a longer fraction of time. Third, the presence of two emissive phases increases the number of observables, facilitating additional information which may be valuable for studies of blinking mechanisms. These initial experiments suggest that the core and shell undergo spectral diffusion which is uncorrelated on the 200 ms time scale, and distinct on faster timescales. Acknowledgment. Financial support from CFI, NSERC, FQRNT, and McGill University is acknowledged. This work was partially supported in part by a grant from the Center for Nano-Manufacturing & Metrology at UMD to D.S.E. E.A.D. acknowledges fellowship support from NSERC-PGS. We thank the McGill University Center for Self-Assembled Chemical Structures for use of their facilities. We acknowledge helpful discussions with G. Cosa, P. W. Wiseman, and C. D. Heyes. Supporting Information Available: Synthetic procedures and single dot PL experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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