Acoustic Phonon Contributions to the Emission Spectrum of Single

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J. Phys. Chem. C 2008, 112, 1878-1884

Acoustic Phonon Contributions to the Emission Spectrum of Single CdSe Nanocrystals Mark J. Ferne´ e,* Brad N. Littleton, Steven Cooper, and Halina Rubinsztein-Dunlop Centre for Quantum Computer Technology, School of Physical Sciences, The UniVersity of Queensland, Queensland, 4072, Australia

Daniel E. Go´ mez and Paul Mulvaney School of Chemistry, The UniVersity of Melbourne, ParkVille, Victoria, 3010, Australia ReceiVed: October 12, 2007; In Final Form: NoVember 6, 2007

Narrow zero-phonon emission lines are observed in single CdSe/CdZnS core/shell colloidal nanocrystals over a range of cryogenic temperatures up to 40 K. These nanocrystals display dramaticaly improved spectral stability enabling the observation of acoustic phonon sidebands accompanying most zero-phonon lines. A discrete phonon mode is attributed to the electron coupling to the l ) 0 acoustic breathing mode via the deformation coupling. The Huang-Rhys parameter, Sac, for this interaction is found to vary from 0.0016 to 0.09, demonstrating a wide dispersion in exciton-phonon coupling between different nanocrystals. Indeed, we observe single nanocrystals in which all acoustic phonon sidebands vanish, in close agreement with theoretical predictions that there should be negligible acoustic phonon coupling in an ideal spherical CdSe nanocrystal. Such nanocrystals are virtually decoupled from their environment, which is potentially useful for quantum technologies, such as single photon sources and quantum computing. In general, the ability to detect and quantify phonon interactions within single nanocrystals will provide significant insight into energy relaxation and dephasing processes in these systems.

Introduction Semiconductor nanocrystals (NCs), otherwise known as quantum dots (QDs), should ideally behave like artificial atoms. The ability to engineer single atom-like emitters in the solidstate is extremely attractive for quantum technologies such as single photon sources1-4 and is an extremely active area of research.5-7 To date, the best performance in terms of spectrally and temporally stable emission has been from QDs grown by molecular beam epitaxy, a traditional solid-state approach. These self-assembled QDs (SAQDs) represent the current state-ofthe-art in QD-based single photon sources.2-4 Alternatively, single colloidal NCs have also been proposed for use as single photon sources, following the detection of anti-bunching.8,9 These QDs are grown using wet chemical methods and result in individual NCs suspended in a liquid solvent. This provides an interesting alternative to SAQDs, as individual NCs can be manipulated and positioned into a wide range of different environments. However, the spectral properties of colloidal NCs make them far less suitable for use as single photon sources. Properties such as blinking,10-13 spectral diffusion,13-17 and bleaching18 dramatically limit the potential uses. All of these properties are linked to interactions with the surface,19 which is relatively close to the charge carriers, in contrast to SAQDs. This presents a technical challenge. Nevertheless, considerable improvements in various spectral properties have recently been reported.20,21 Acoustic phonon scattering is the main cause of dephasing and decoherence of electronic excited states in the solid-state at cryogenic temperatures.22-25 The detection of acoustic phonon sidebands in single SAQD spectra has facilitated quantification * Corresponding author. Ph: +61 7 3365 3425; fax: +61 7 3365 1242; e-mail: [email protected].

of the phonon coupling strength, elucidating dephasing and line broadening mechanisms.26-28 Ideally, low acoustic phonon coupling is desirable for quantum technologies in order to reduce decoherence in the excited state. Phonon-free spectral line shapes have been found from single CdSe SAQDs,29,30 suggesting that CdSe QDs may be useful for such purposes. Theoretically, a low coupling strength has been predicted for CdSe QDs.31 However, single CdSe NCs were shown to exhibit strong spectral diffusion that dominated the spectral line shape.10,14 This property has meant that single NC spectroscopy has been unsuitable for detecting acoustic phonon interactions. Therefore ensemble techniques have been employed to study the interaction of the CdSe NCs with acoustic phonons, such as Raman scattering,32 fluorescence line narrowing,33 spectral hole burning,34 photon echo,35 and pump-probe measurements.36 Here we show that CdSe/CdZnS graded shell NCs can exhibit narrow emission spectra over a range of cryogenic temperatures, with improved spectral stability resembling behavior more commonly associated with SAQDs. We observe a zero-phonon spectral line with a width at or close to the instrument resolution limit. This narrow feature allows us to confidently assign a broad pedestal to acoustic phonon interactions with the excitonic state. This interpretation is supported by the observation of discrete sidebands, which are well-described using a Huang-Rhys phonon model. We find there is a wide variation in acoustic phonon coupling. Most dramatically, however, we observe for the first time that there are CdSe NCs with narrow spectral lines that exhibit no detectable acoustic phonon coupling over a range of cryogenic temperatures. Experimental Procedures The CdSe/CdZnS NCs used throughout this study were sourced from a commercial supplier. Two different emission

10.1021/jp709939c CCC: $40.75 © 2008 American Chemical Society Published on Web 01/19/2008

Zero-Phonon Emission Lines in CdSe NCs

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Figure 1. Low-temperature spectra (3.1 K) of single CdSe/CdZnS NCs (ITK655) pumped with an irradiance of 8 W/cm2 with a single frame integration time of 10 s. (a) A single frame of the time series showing narrow and broad features. Two spectra are circled and labeled A and B. (b) The time series of the two spectra, A and B, indicating the level of spectral diffusion. Spectrum B also exhibits a long blinking-off period. (c) A spectral average of time series A obtained by first aligning the individual spectral peaks before computing the average. A single spectral frame is also included for comparison. An asterisk is placed atop a peak that is attributed to excitonic fine structure. (d) A histogram of peak positions indicating that maximum spectral excursion is on the order of 1 meV. (e,f) The corresponding spectral average and peak histogram for spectrum B.

wavelengths in the red part of the spectrum were chosen, encompassing a large variation in core sizes (Invitrogen ITK605, ITK655 with core radii of approximately 3 and 5 nm, respectively20,21). These particular NCs are based on a CdSe core encapsulated by an epitaxially deposited shell that transitions from CdS to ZnS.18 This technique allows for thicker shells of better quality as a result of superior lattice strain relief, resulting in luminescence quantum yields greater than 80%.18,37 Apart from the higher quantum yields, other photophysical properties such as single NC blinking appear to be relatively unchanged at room temperature. Samples were prepared by diluting the stock solution with decane to a concentration of 10-11 M and spin coating on a crystal quartz substrate. The NCs were deposited directly on the quartz substrate without a supporting polymer matrix. The sample was loaded into a continuous flow liquid He cryostat (Oxford Microstat), and the sample chamber was evacuated using a turbo pump. The sample could be cooled to temperatures approaching 3 K. A 532 nm DPSS laser was used to provide wide field illumination of the sample using an epifluorescence geometry. The signal was collected by a long working distance objective (Nikon, NA 0.7 100×), filtered to remove the excitation line and passed to a 300 mm imaging spectrometer (Acton 300i) with a resolution of 80 µeV. Spectra were recorded using an EMCCD camera (Andor iXon) allowing relatively short exposures and high frame rates. Results and Discussion A typical low-temperature (3.1 K) CCD spectrum is shown in Figure 1a. Spectra from a number of individual NC’s can be seen. A common feature is the presence of a relatively bright narrow feature sitting atop a broader pedestal. We examine two of the spectra labeled A and B. The first (A) represents a class of NCs that display extremely narrow spectral lines (which we label the zero-phonon line (ZPL)) with a barely detectable pedestal component, whereas B represents a broader ZPL as

well as a brighter broader pedestal component. A series of 200 successive frames was recorded, and the spectral time series is shown in Figure 1b for both NC A and NC B. Here we see that spectral diffusion causes the ZPLs to slowly shift over time (we also note that NC B is “off” for most of the time series, as a direct consequence of photoluminescence blinking10,11). This relatively slow spectral diffusion allows us to track and align the individual spectra relative to the ZPL in order to perform averages, and yields much better signal-to-noise ratios in the spectra. In Figure 1c, we see the result of this ZPL-alignment for NC A. A spectrum from a single frame is shown underneath the 200 frame average. The average reveals a narrow ZPL (280 µeV full width at half-maximum (fwhm)) sitting atop a broader pedestal. A histogram of ZPL peak positions is shown in Figure 1d, revealing a spectral meander over slightly more than 1 meV. Figure 1e,f shows the ZPL-aligned average (∼500 µeV fwhm), single frame spectrum, and peak position histogram obtained for NC B. For NC B, the pedestal is actually visible in every frame of the time series after the initial on-time. The correlation of the pedestal with the spectral wander implies that it results from an intrinsic NC process and not an artifact of spectral diffusion. We observe narrow (sub-millielectronvolt fwhm) ZPLs sitting atop broader pedestals in both NC samples over all temperatures studied. In Figure 2 we show a series of spectra from different individual NCs covering the temperature range used in our study. Each spectrum has been aligned to the narrow ZPL peak. We see that narrow spectral peaks are still observed in some NCs at 40 K. This is true for both NC samples used in this study. In general, most ZPL’s sit atop a broad pedestal, and the size of the broad pedestals varies widely between NCs. Some of the spectra also display clear sidebands, which will be investigated further. Unambiguous identification of the different components of the spectra is aided by using spectral diffusion time series

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Ferne´e et al. observation of sidebands on many different NCs with nearly identical sideband energy spacings over a range of temperatures. The combination of a broad pedestal along with discrete sidebands can be explained by considering the types of acoustic phonon interactions within a CdSe NC. Although small NCs have a discrete acoustic phonon spectrum, the density of modes can still be quite high.23,31 For spherical nanoparticles, both spheroidal and torsional modes are possible. These modes can couple to the exciton via either a piezo-electric interaction or a deformation potential interaction.22,23 While the deformation potential interaction is dominant for smaller nanoparticles, both interactions need to be considered for larger nanoparticles. The piezoelectric coupling involves both spheroidal and torsional acoustic modes coupled to both the electron and the hole. This is likely to result in broad spectral wings due to the relatively high density of modes. However, if we consider only the deformation potential, the coupling can be simplified, as we will show below. The Hamiltonian representing linear exciton-phonon coupling (in a two-level approximation) is given by23

H)

lmj ∑ lmj

(

bˆ †lmjbˆ lmj +

)

1 2

+

E0|φE〉〈φE| +

Mlmj(bˆ lmj + bˆ †lmj)|φE〉〈φE| ∑ lmj

(1)

where the sum is over the different angular momentum mode indices associated with bosonic annihilation and creation operators, bˆ lmj and bˆ †lmj for phonons with energy lmj, E0 is the exciton energy, Mlmj is the exciton-phonon coupling constant, and |φE〉 ) |φe〉 X |φh〉 is the exciton wave function comprising both electron and hole wave functions. For deformation potential coupling, the exciton-phonon coupling constant has the relatively simple form23

Mlmj ) - hjplmj (De〈φe|jl(hjre)Ylm(Ωe)|φe〉 + Dh〈φh|jl(hjrh)Ylm(Ωh)|φh〉) (2) Figure 2. Spectra exhibiting narrow ZPLs obtained from a range of single CdSe/CdZnS NCs over the range of temperatures studied. The left column corresponds to NCs with a room-temperature emission centered at 605 nm (sample ITK605), and the right column represents NCs with a room-temperature emission at 655 nm (sample ITK655). All spectra are centered on the narrow ZPL, revealing well-defined sidebands.

correlations.38,39 Emission that is intrinsic to a single NC (such as emission related to excitonic recombination) should spectrally shift in unison with the narrow central emission peak. Therefore, by collecting long time series, we are able to comfortably assign spectral features to the intrinsic behavior of a single NC. This is most clearly observed in Figure 3a, where a relatively complex spectrum is observed to shift in lockstep with a bright central peak. After first aligning the ZPLs and then averaging, a spectrum with a high signal-to-noise ratio is obtained. Here we see that the ZPL is flanked by two small peaks. These features are clearly observed in the time series, and so cannot be attributed to an artifact of spectral diffusion itself. The consistency with which we obtain a narrow central peak sitting atop a broader pedestal, along with the prefect correlation with spectral diffusion wandering, strongly suggests that we are in fact observing acoustic phonon sidebands, similar to those observed in SAQDs22-24 and predicted theoretically.26-28 The discrete sidebands observed in Figure 3b should then correspond to a confined phonon mode. This is confirmed through our

where the first and second terms correspond to deformation coupling to the electron and hole respectively. Thus De,h are the deformation coupling constants for the electron and hole, jl(hjre,h) are spherical Bessel functions, and Ylm(Ωe,h) are the spherical harmonics. The coefficients hj and plmj are specified by Takagahara.23 If we now consider coupling to the electron wavefunction of the band edge exciton, the coupling integral can be readily evaluated using the expression for the electron wavefunction of the band-edge exciton from Efros et al.40 Evaluation of the angular integrals reveals that the electron wavefunction only couples to the l ) 0 breathing mode of the NC. The more complex CdSe hole fine structure, allows deformation coupling to other acoustic phonon modes; however, this coupling should be far smaller than that of the electron due to the weak confinement of the hole in large CdSe-based NCs.41 Therefore the discrete sidebands are a direct consequence of the deformation coupling to the electron. We model the deformation coupling mechanism as follows: we treat the NC as an isolated optomechanical system, which evolves according to the Hamiltonian, eq 1, but we drop the summation indices as we only consider coupling to a single phonon mode. Both photons and phonons are coupled into and out of the system via bath operators defined as C ˆ ex ) xγex |0〉〈φex|, C ˆ P ) xΓP|φex〉〈0|, and C ˆ ac ) xγac bˆ , where γex and γac are the exciton and phonon decay rates, respectively, and

Zero-Phonon Emission Lines in CdSe NCs

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Figure 3. Low-temperature spectra (20 K) of single CdSe/CdZnS NCs (ITK655) pumped with an irradiance of 340 W/cm2 with a single frame integration time of 2 s. (a) A time series plot of a narrow spectral feature with discrete sidebands. (b) A spectral average of the last 40 s of the time series following alignment of the central peak position. The spectrum displays discrete sidebands. A phonon coupling model provides a good fit to the spectrum. (c) The output from the phonon coupling model as described in the text. This output is then convolved with a Gaussian broadening function and added to a Gaussian pedestal in order to provide the fit in panel b. The acoustic phonon energy, ac, simulation temperature, T, and Huang-Rhys coupling parameter, Sac, as derived from the model, are also indicated in the box.

Figure 4. (a) Spectra obtained from different single NCs (ITK655), each exhibiting discrete phonon sidebands. The dashed red line is the model fit, and the model parameters are indicated in the accompanying box. The substrate temperature is also indicated in the upper right corner of each spectrum. (b) Spectra obtained from a single NC (ITK605) exhibiting discrete sidebands as well as an additional spectral feature at two different temperatures. A model fit is also included that only includes the main ZPL and phonon sidebands. The parameters used in the model are indicated in the accompanying boxes.

ΓP is the incoherent pumping rate. An appropriate master equation can be written as follows:

nac + 1 i (2C ˆ acFˆ C Fˆ˘ ) - [H, Fˆ ] + ˆ †ac - C ˆ †acC ˆ acFˆ - Fˆ C ˆ †acC ˆ ac) + p 2p nac 1 (2C ˆ †acFˆ C (2C ˆ exFˆ C ˆ ac - C ˆ acC ˆ †acFˆ - Fˆ C ˆ acC ˆ †ac) + ˆ †ex 2p 2p 1 C ˆ †exC (2C ˆ †PFˆ C ˆ exFˆ - Fˆ C ˆ †exC ˆ ex) + ˆP - C ˆ PC ˆ †PFˆ - Fˆ C ˆ PC ˆ †P) (3) 2p where Fˆ is the reduced density operator for the nano-optical system represented by the Hamiltonian in eq 1. As is usual, the exciton couples to a zero temperature photon reservoir, whereas the low-energy acoustic phonons must couple to a finite temperature phonon reservoir. An incoherent pump term is also included to simulate continuous wave (cw) laser excitation. The average number of phonons of energy  in the reservoir at a temperature T is given by nac + (e/kBT - 1)-1. Operation in a steady-state (cw excitation) regime requires numerically solving the master equation as an exponential series, so that the two-time correlation function can be calculated. The power spectrum is then found using the Wiener-Khintchine theorem. A broad phonon pedestal is added to both sides of the ZPL in order to aid the fit. The use of an additive pedestal is justified by the observation of spectra comprised solely of a pedestal sans discrete sidebands, such as that shown in Figures 1e and 2. This simulates an additional coupling process such as the piezoelectric coupling.23 Finally, in order to account for a finite instrument resolution and spectral diffusion-induced broadening, the power spectrum is convolved with either a Lorentzian or Gaussian function (the choice being determined by the quality of the fit). This model has the advantage of including an additional broadening mechanism through the phonon damping term.

Strong phonon damping naturally results in broadening of the ZPL, and so should be considered in exciton-phonon interactions.24 The exact nature of the damping depends on the environment. Coupling to additional fine structure levels,24 external vibration modes, mass loading effects, and external trap states42 can be included via this damping term. We obtain excellent fits with the data using this model (as evidenced in Figure 3b). Even though there are a large number of parameters that must be fit, determination of the phonon energy, coupling energy, and local temperature are possible, as these parameters are approximately independent under the conditions described in the experiment. The underlying spectrum obtained from the solution of the master equation is shown in Figure 3c along with the determined acoustic phonon energy and Huang-Rhys parameter, Sac. This fit falls within a range of reasonable parameters that provide good fits for all of our data. This assumes a sufficiently low pump power so that the system is allowed to relax completely between excitation events, which approximates the experimental conditions used throughout these experiments. Thus we determine the l ) 0 acoustic breathing mode energy, ac, to be approximately 1.3 meV for NCs with a band-edge exciton energy near 2 eV, and 1.8 meV for NCs with a band-edge energy near 2.1 eV (determined at 3 K). We also estimate a Huang-Rhys parameter for the deformation coupling to the l ) 0 phonon in the range of Sac ) 0.016 to 0.09 as determined by the range of fits at different temperatures and different NCs shown in Figure 4a. This coupling is up to 2 orders of magnitude stronger than what has been predicted for purely core CdSe NCs.31 This discrepancy may indicate that the surface passivating shell has a marked effect on the phonon coupling or that the surface itself is involved in the phonon interaction. The observation of discrete sidebands also shows that the acoustic modes are underdamped in this case.

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Ferne´e et al.

Figure 5. Low-temperature spectra (3.2 K) of single CdSe/CdZnS NCs (ITK605) pumped with an irradiance of 90 W/cm2 with a single frame integration time of 10 s. (a) Time series of a single NC spectrum with no discernible sidebands. (b) An average spectrum obtained following the peak alignment procedure. A fit using the phonon coupling model is also indicated. A Lorentzian convolution with an fwhm of 280 µeV is used. The Huang-Rhys parameter represents the largest value that shows no detectable sidebands. A single spectrum from the time series is also included below. Nearly half of the light falls on a single pixel, indicating a line width at or below the resolution limit of 80µeV. (c) Two narrow spectra obtained from the same NC at two different temperatures with a pump irradiance of 28 W/cm2. It is apparent that there is negligible phonon coupling even at these elevated temperatures. The apparent broadening of the 20 K spectral line is most likely attributable to increasing spectral diffusion at elevated temperatures.

The anti-Stokes sideband is found to persist to the coldest temperatures used in this study (such as that in Figure 1c), contradicting the predictions of the model. This anomaly is most likely an indication of localized heating due to hot carrier relaxation, as we are exciting far above the band-edge. The sideband could be simulated by raising the temperature parameter from 3 K to approximately 20 K, where the thermal energy is close to the l ) 0 acoustic phonon energy. In general, all the fits required a simulation temperature higher than the substrate temperature, thus supporting the hypothesis of laser-induced heating of the sample. In Figure 4b we show a temperature series obtained from the smaller NC sample along with the best fit obtained by the model. In this case we find that the anti-Stokes sideband is not adequately fit by the model. However, on the high-energy side, we also see an additional spectral line that is separated from the main ZPL by approximately twice the l ) 0 acoustic phonon energy. This additional spectral line is thermally populated, indicating its dependency on the main ZPL. Therefore, it is possible that the anti-Stokes sideband is resonantly enhanced in this case. This type of behavior is under further investigation. These data verify that we have clearly and unambiguously identified the acoustic phonon contributions to the single NC spectra, and isolated the deformation potential component, from which we derive the exciton-acoustic phonon coupling strength, Sac. However, for the smaller NCs studied (Invitrogen ITK605), we find a sub-ensemble (approximately 10% of the observed NCs) that possesses neither a phonon pedestal nor discrete sidebands. An example of these phonon-free spectra is shown in the time series of Figure 5a and the associated averaged spectrum in 5b. This single spectral feature is also well described by the model of eq 3, providing, in this case, an upper limit of the Huang-Rhys parameter of Sac ) 0.002. In contrast to the results presented so far, this value of Sac is in the range of values predicted for CdSe NCs31 using the theory of Takagahara.22,23 A single spectrum extracted from the time series is also shown in Figure 5b. An important feature of this spectrum is that it

Figure 6. Spectra of single CdSe/CdZnS NCs (ITK605) pumped with an irradiance of 28 W/cm2. (a) An average spectrum showing a single narrow peak (obtained with a 10 s integration time per spectrum). An arrow indicates the energy region where an LO phonon is expected. An asterisk indicates an additional peak that is correlated to the main peak throughout the time series. (b) An average spectrum (obtained with a 5 s integration time per spectrum) obtained from the same sample used in panel a, showing a single narrow peak with associated acoustic phonon sidebands as well as a clearly identifiable LO phonon replica. An asterisk identifies an additional peak attributed to exciton fine structure. The spectrum in panel a corresponds to a freshly prepared sample, whereas the spectrum in panel b was obtained the following day after one additional thermal cycle to room temperature (the sample remained in the cryostat under vacuum).

displays a resolution-limited line width (i.e., fwhm approximately 80 µeV) in spite of the fact that it was acquired over a 10 s integration time. This line width is smaller than that obtained from the averaged spectrum, which is a clear indication that spectral diffusion causes some of the broadening observed. However, the observation of resolution-limited line widths under moderate excitation powers (i.e., 90 W/cm2) implies that, for this particular sub-ensemble, spectral diffusion effects are highly suppressed. As phonon coupling should be independent of temperature, we would expect such narrow lines to persist to higher temperatures. We find this is indeed the case, as can be seen in the temperature series shown in Figure 5c. Narrow, acoustic phonon-free spectra are one example of the variation in acoustic phonon coupling that we observe in single NC spectra. We also find a similar variation in optical phonon coupling. At cryogenic temperatures, most single CdSe NC spectra exhibit both a primary peak (the ZPL) accompanied by a smaller peak separated by one longitudinal optical (LO) phonon energy, which corresponds to the simultaneous emission of both a photon and an optical phonon. This is often termed a phonon echo. In Figure 6a we show a single narrow spectral feature that shows no detectable LO phonon echo, whereas in Figure 6b a different NC exhibits both acoustic phonon sidebands as well as a strong LO phonon signature. The two

Zero-Phonon Emission Lines in CdSe NCs spectra were obtained from the same sample, the only difference being the age of the sample. Figure 6a was obtained directly after deposition, while 6b was obtained the following day after one additional heating and cooling cycle of the cryostat. The LO signals were easily detected in the 1 day aged sample, but not in the fresh sample. In general, we find that neither sample studied exhibits a detectable LO phonon echo. Spectra devoid of detectable LO phonon echoes have also been observed in CdSe nanorods37,38 as well as CdSe SAQDs.29,30 In general, we observe a large variation in the spectral width and size of the phonon pedestal. The observation of a narrow ZPL in conjunction with correlations with spectral diffusion enables us to assign the broad pedestal to acoustic phonon interactions with confidence. A large variation in individual NC spectra is representative of both NC samples used in this study. Therefore the acoustic phonon interaction must be extremely variable. Examples of this variation are shown in Figures 1, 2, 4, 5, and 6. We note that optical phonon sidebands are generally not observed in single NC spectra from either sample except as noted above. This then suggests that the origin of both acoustic phonon coupling and optical phonon coupling is largely determined by extrinsic influences, such as surface effects and other morphological influences. The ability to detect acoustic phonon interactions thus provides an important new gauge of NC quality that can be used for improving synthetic routes. Finally, we have not yet attempted to attribute the ZPL emission to any state within the CdSe NC fine structure. At cryogenic temperatures, emission from spherical NCs is commonly observed from the F ) ( 2 ground state,37 and this is our tentative assignment. However, we commonly observe additional fine-structure emission from these NCs, which suggests that, for at least some NCs, the ZPL emission may come from a “bright” exciton state, similar to what has been found for CdSe SAQDs.29 Investigations are underway to conclusively identify the emitting state. Conclusion We find that the latest generation of CdSe/CdZnS core/graded shell NCs exhibit dramatically enhanced spectral stability at cryogenic temperatures as evidenced by the observation of narrow ZPLs at all temperatures studied. The narrow ZPLs are used to identify clear acoustic phonon sidebands among the underlying spectral diffusion. A large variation in acoustic phonon coupling is found between individual NCs. Discrete sidebands are observed in some NCs and identified as the breathing mode coupling to the electron via the deformation potential. Importantly, within the range of coupling strengths, there exists a sub-ensemble that exhibits minimal coupling to acoustic phonons. In this case, an upper limit in the coupling strength is estimated and is found to agree with a theoretical prediction for an ideal spherical CdSe NC. Such NCs exhibit minimal coupling to their solid-state environment and are therefore a better realization of an artificial atom, which may be valuable for quantum technologies. Phonon scattering from electronic states are fundamental processes responsible for energy relaxation and decoherence in NCs and are also implicated in spin relaxation. We have shown that it is now possible to study the acoustic phonon interaction in single NCs at cryogenic temperatures. While these results seem remarkable in the light of previous studies of single colloidal NCs,10,14-16 we find that the spectral properties are now converging on those already observed in CdSe SAQDs.29,30 This presents the exciting prospect that the colloidal synthesis may be further refined in order to replicate the spectral properties

J. Phys. Chem. C, Vol. 112, No. 6, 2008 1883 observed in SAQDs, which, in many senses, represent the stateof-the-art for technological applications. Acknowledgment. M.J.F. and B.L. would like to acknowledge the support of the Australian Research Council. P.M. thanks the ARC for support through Grant FF 01451486. References and Notes (1) Kiraz, A.; Atature, M.; Imamoglu, A. Phys. ReV. A 2004, 69, 032305. (2) Santori, C.; Fattal, D.; Vuckovic, J.; Solomon, G. S.; Yamamoto, Y. Nature 2002, 419, 594-7. (3) Stevenson, R. M.; Young, R. J.; Atkinson, P.; Cooper, K.; Ritchie, D. A.; Shields, A. J. Nature 2006, 439, 179-82. (4) Laurent, S.; Varoutsis, S.; Le Gratiet, L.; Lemaitre, A.; Sagnes, I.; Raineri, F.; Levenson, A.; Robert-Philip, I.; Abram, I. Appl. Phys. Lett. 2005, 87, 163107. (5) Akimov, I. A.; Andrews, J. T.; Henneberger, F. Phys. ReV. Lett. 2006, 96, 067401. (6) Yoshie, T.; Scherer, A.; Hendrickson, J.; Khitrova, G.; Gibbs, H. M.; Rupper, G.; Ell, C.; Shchekin, O. B.; Deppe, D. G. Nature 2004, 432, 200-3. (7) Reithmaier, J. P.; Sek, G.; Loffler, A.; Hofmann, C.; Kuhn, S.; Reitzenstein, S.; Keldysh, L. V.; Kulakovskii, V. D.; Reinecke, T. L.; Forchel, A. Nature 2004, 432, 197-9. (8) Michler, P.; Imamoglu, A.; Mason, M. D.; Carson, P. J.; Strouse, G. F.; Buratto, S. K. Nature 2000, 406, 968-70. (9) Brokmann, X.; Giacobino, E.; Dahan, M.; Hermier, J. P. Appl. Phys. Lett. 2004, 85 (5), 712-4. (10) Empedocles, S. A.; Norris, D. J.; Bawendi, M. G. Phys. ReV. Lett. 1996, 77 (18), 3873-6. (11) Shimizu, K. T.; Neuhauser, R. G.; Leatherdale, C. A.; Empedocles, S. A.; Woo, W. K.; Bawendi, M. G. Phys. ReV. B 2001, 63, 205316. (12) Chung, I.; Bawendi, M. G. Phys. ReV. B 2004, 70, 165304. (13) Neuhauser, R. G.; Shimizu, K. T.; Woo, W. K.; Empedocles, S. A.; Bawendi, M. G. Phys. ReV. Lett. 2000, 85 (15), 3301-4. (14) Empedocles, S. A.; Bawendi, M. G. J. Phys. Chem. B 1999, 103, 1826-30. (15) Empedocles, S. A.; Bawendi, M. G. Science 1997, 278, 2114-7. (16) Muller, J.; Lupton, J. M.; Rogach, A. L.; Feldmann, J.; Talapin, D. V.; Weller, H. Phys. ReV. B 2005, 72, 205339. (17) Go´mez, D. E.; van Embden, J.; Mulvaney, P. Appl. Phys. Lett. 2006, 88, 154106. (18) Xie, R.; Li, J.; Basche, T.; Mews, A. J. Am. Chem. Soc. 2005, 127, 7480-8. (19) Hohng, S.; Ha, T. J. Am. Chem. Soc. 2004, 126, 1324-5. (20) Fisher, B.; Caruge, J. M.; Zehnder, D.; Bawendi, M. G. Phys. ReV. Lett. 2005, 94, 087403. (21) Fisher, B.; Caruge, J.-M.; Chan, Y.-T.; Halpert, J.; Bawendi, M. G. Chem. Phys. 2005, 318, 71-81. (22) Takagahara, T. Phys. ReV. Lett. 1993, 71 (21), 3577-80. (23) Takagahara, T. J. Lumin. 1996, 70, 129-143. (24) Muljarov, E. A.; Zimmermann, R. Phys. ReV. Lett. 2004, 93, 237401. (25) Palinginis, P.; Wang, H.; Goupalov, S. V.; Citrin, D. S.; Dobrowolska, M.; Furdyna, J. K. Phys. ReV. B 2004, 70, 073302. (26) Peter, E.; Hours, J.; Senellart, P.; Vasanelli, A.; Cavanna, A.; Bloch, J.; Gerard, J. M. Phys. ReV. B 2004, 69, 041307(R). (27) Favero, I.; Cassabois, G.; Ferreira, R.; Darson, D.; Voisin, C.; Tignon, J.; Delalande, C.; Bastard, G.; Roussignol, Ph. Phys. ReV. B 2003, 68, 233301. (28) Besombes, L.; Kheng, K.; Marsal, L.; Mariette, H. Phys. ReV. B 2001, 63, 155307. (29) Graham, T. C. M.; Curran, A.; Tang, X.; Morrod, J. K.; Prior, K. A.; Warburton, R. J. Phys. Status Solidi B 2006, 243 (4), 782-6. (30) Hundt, A.; Flissikowski, T.; Lowisch, M.; Rabe, M.; Henneberger, F. Phys. Status Solidi B 2001, 224 (1), 159-63. (31) Salvadore, M. R.; Graham, M. W.; Scholes, G. D. J. Chem. Phys. 2006, 125, 184709. (32) Saviot, L.; Champagnon, B.; Duval, E.; Kudriavtsev, I. A.; Ekimov, A. I. J. Non-Cryst. Solids 1996, 197, 238-46. (33) Woggon, U.; Gindele, F.; Wind, O.; Klingshirn, C. Phys. ReV. B 1996, 54 (3), 1506-9. (34) Palinginis, P.; Tavenner, S.; Lonergan, M.; Wang, H. Phys. ReV. B 2003, 67, 201307(R). (35) Takemoto, K.; Hyun, B.-R.; Masumoto, Y. J. Lumin. 2000, 8789, 485-7. (36) Sun, D. H.; Wittenberg, J. S.; Banin, U.; Alivisatos, A. P. J. Phys. Chem B 2006, 110, 19884-90. (37) Le Thomas, N.; Herz, E.; Schops, O.; Woggon, U. Phys. ReV. Lett. 2005, 94, 016803.

1884 J. Phys. Chem. C, Vol. 112, No. 6, 2008 (38) Le Thomas, N.; Allione, M.; Fedutik, Y.; Woggon, U.; Artemyev, M. V.; Ustinovich, E. A. Appl. Phys. Lett. 2006, 94, 016803. (39) Talapin, D. V.; Mekis, I.; Gotzinger, S.; Kornowski, A.; Benson, O.; Weller, H. J. Phys. Chem. B 2004, 108, 18826-31. (40) Efros, Al. L.; Rosen, M.; Kuno, M.; Nirmal, M.; Norris, D. J.; Bawendi, M. Phys. ReV. B 1996, 54 (7), 4843-56.

Ferne´e et al. (41) Ekimov, A. I.; Hache, F.; Schanne-Klein, M. C.; Ricard, D.; Flytzanis, C.; Kudryavtsev, I. A.; Yazeva, T. V.; Rodina, A. V.; Efros, A. L. J. Opt. Soc. Am. B 1993, 10 (1), 100-7. (42) Favero, I.; Berthelot, A.; Cassabois, G.; Voisin, C.; Delalande, C.; Roussignol, Ph.; Ferriera, R.; Gerard, J. M. Phys. ReV. B 2007, 75, 073308.