Shell Nanorods - Nano

Sasha Tavenner-Kruger, Young-Shin Park, Mark Lonergan, Ulrike Woggon, and Hailin Wang*. Department of Physics and Department of Chemistry, University ...
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NANO LETTERS

Zero-Phonon Linewidth in CdSe/ZnS Core/Shell Nanorods

2006 Vol. 6, No. 9 2154-2157

Sasha Tavenner-Kruger,† Young-Shin Park,† Mark Lonergan,‡ Ulrike Woggon,§ and Hailin Wang*,† Department of Physics and Department of Chemistry, UniVersity of Oregon, Eugene, Oregon 97403, and Fachbereich Physik, UniVersitaet Dortmund, Otto-Hahn-Strasse 4, 44227 Dortmund, Germany Received June 6, 2006; Revised Manuscript Received July 17, 2006

ABSTRACT High-resolution spectral hole-burning studies of CdSe/ZnS core/shell nanorods reveal a sharp zero-phonon line, with a line width dependent on the measurement time scale. The zero-phonon line width is attributed to contributions from radiative decay, spectral diffusion induced by surface electric field fluctuations, and phonon-assisted migration of excitons localized in the nanorods. A decoherence rate as small as 4.5 GHz has been observed, when the effects of spectral diffusion are suppressed in the spectral hole-burning measurement. Comparison between zero-phonon line widths in nanorods and spherical nanocrystals also elucidates important differences in the decoherence process between the one- and zero-dimensional nanostructures.

The optical absorption spectra of a semiconductor nanocrystal feature a zero-phonon line (ZPL) along with optical and acoustic phonon sidebands that arise from phonon-assisted transitions. The ZPL line width or the homogeneous line width reflects the fundamental dynamic interaction between the exciton and the surrounding environment, including coupling to lattice vibrations and to the electromagnetic vacuum. In the ideal limit that coupling to the electromagnetic vacuum is the only decoherence mechanism, the homogeneous line width is determined by the radiative lifetime. Nearly lifetime-limited homogeneous line widths have been observed in self-assembled quantum dots grown by molecular beam epitaxy.1 Although comparable homogeneous line widths have been observed in spherical nanocrystals, because of the relatively small radiative decay rate in the nanocrystals, these line widths still far exceed the expected radiative line width.2 Optical properties of nanorods (NRs) differ significantly from those of spherical nanocrystals. The one-dimensional excitons in the NRs feature much faster radiative decay than that of spherical nanocrystals,3,4 raising the prospect that lifetime-limited homogeneous line width can be achieved in the one-dimensional NRs. Compared with spherical nanocrystals, NRs also feature highly polarized photoluminescence, higher quantum efficiency, greater Stokes shift, faster carrier relaxation, and reduced Auger processes.5-9 Many * Corresponding author. E-mail: [email protected]. † Department of Physics, University of Oregon. ‡ Department of Chemistry, University of Oregon. § Fachbereich Physik, Universitaet Dortmund. 10.1021/nl0613003 CCC: $33.50 Published on Web 08/01/2006

© 2006 American Chemical Society

of these properties have also been attributed to the onedimensional electronic energy structure of the NRs.3 A number of spectroscopic techniques have been used for the measurement of the ZPL line width in semiconductor nanocrystals. Single-nanocrystal photoluminescence (PL) studies eliminate the inhomogeneous broadening. The PL studies, however, are limited in spectral resolution and are also complicated by rapid spectral diffusion induced by surface electric field fluctuations inherent in nanocrystals. Transient nonlinear optical spectroscopy such as photon echoes requires relatively high excitation laser intensity, which can lead to additional decoherence processes. A highly effective approach is high-resolution spectral hole-burning (SHB). The SHB can be carried out at excitation levels lower than 1 W/cm2 and can feature spectral resolution better than 0.01 µeV. In addition, effects of spectral diffusion can be suppressed in SHB by performing the measurement at high modulation frequencies. Earlier SHB studies in spherical CdSe/ZnS core/shell nanocrystals have revealed a homogeneous line width as small as 6 µeV,2 which corresponds to a decoherence rate of 0.75 GHz and is comparable to that obtained in self-assembled CdSe quantum dots.10 In this paper, we report experimental studies of the ZPL line width in CdSe/ZnS core/shell NRs using high-resolution SHB. The ZPL line width obtained depends strongly on the modulation frequency used in the SHB measurement, reflecting the effects of spectral diffusion occurring at relatively slow time scales. By suppressing the contribution from the spectral diffusion, we have obtained ZPL line widths corresponding to decoherence rates as small as 4.5 GHz.

Figure 1. SHB response of CdSe/ZnS nanorods at T ) 8 K and Ω ) 3 kHz, with Ipump ) 3 W/cm2 and Iprobe ) 1 W/cm2. The inset shows the PL spectrum of the sample. The arrow in the inset indicates the spectral position of the pump beam used for the SHB measurements.

Comparison between ZPL line widths in NRs and spherical nanocrystals suggests relaxation processes in NRs that do not occur in the zero-dimensional nanocrystals and points to potential avenues for further reducing the ZPL line width in NRs toward the radiative limit. The CdSe/ZnS core/shell NRs were synthesized according to a chemical precipitation method developed earlier.11 The NRs were grown at high temperature in a solution of trioctylphosphine oxide (TOPO) and hexylphosphonic acid, which promotes growth of the CdSe nanocrystal along the c axis. The growth of the ZnS capping layer was preceded by a partial ligand exchange of hexadecylamine for TOPO. For transmission measurements, the NRs were dispersed in a thin film of polystyrene deposited on a sapphire disk. Note that residual phosphonic acid leads to a milky appearance of the polystyrene-nanorod thin film, which causes excessive optical scattering. Numerous purification steps were thus undertaken to remove the final traces of phosphonic acid from the NR solution. The NRs were characterized by absorption, PL, and transmission electron microscopy (TEM). Here, we present experimental results from NRs that have an average aspect ratio of 9:1 (21 nm in average length and 2.4 nm in average diameter). For SHB studies, the change in the transmission of a probe beam induced by an orthogonally polarized pump beam was measured with lock-in detection. Two tunable ring dye lasers were used as the pump and probe, respectively. The intensity of the pump beam was modulated at frequencies up to 2 MHz using an acousto-optic modulator. For measurements at low temperature, the sample was mounted in the coldfinger of a helium cryostat. Figure 1 shows a differential transmission spectrum obtained at T ) 8 K. The nonlinear response was measured as a function of the detuning between the pump and probe. The pump wavelength was fixed at 590 nm, as indicated in the PL spectrum shown in the inset of Figure 1. The intensity of the pump and probe beams are 3 and 1 W/cm2, respectively. The modulation frequency used for lock-in detection is 1 kHz. The SHB response shown in Figure 1 features a sharp resonance at zero pump-probe detuning, Nano Lett., Vol. 6, No. 9, 2006

Figure 2. Decoherence rate vs modulation frequency for nanorods (squares) and spherical nanocrystals (triangles) at T ) 8 K and with Ipump ) 3 W/cm2 and Iprobe ) 1 W/cm2.

along with a much broader pedestal. Theoretically, interband optical absorption in nanocrystals is characterized by a manifold of zero-phonon and photon-assisted transitions. The sharp resonance in the SHB response arises when the pump and probe couple to the zero-phonon transition. The broad background is due to the coupling of the pump and probe to different transitions in the absorption manifold. This assignment is further confirmed by the observation that the relative contribution of the sharp resonance to the overall SHB response decreases with increasing temperature (not shown). Note that SHB responses in spherical CdSe/ZnS nanocrystals obtained in earlier studies reveal sharp acoustic phonon sidebands arising from confined acoustic phonon modes that have a relatively long lifetime and can couple strongly to the optical transitions.2 In comparison, there is no clear evidence for the presence of these sharp phonon sidebands in SHB responses for NRs. For a third-order nonlinear optical process, the line width of the sharp SHB resonance is given by twice the homogeneous line width or four times the intrinsic decoherence rate of the zero-phonon transition. This is no longer true if the optical transition frequency fluctuates slowly as a function of time. These spectral diffusion processes lead to a SHB line width that depends strongly on the duration of the measurement. In typical single-nanocrystal PL measurements, the fluctuation in the nanocrystal transition frequency can far exceed the intrinsic homogeneous line width. Effects of the spectral diffusion on the ZPL line width, however, can be eliminated or reduced significantly if the duration of the measurement is short compared with the time scale for the spectral fluctuation. For SHB with lock-in detection, the measurement duration is set by the modulation period of the pump intensity. Figure 2 shows the decoherence rate obtained from the SHB response as a function of the modulation frequency, Ω. The observed decoherence rate decreases rapidly with increasing modulation frequency, indicating that shortening the measurement time scale significantly reduces effects of spectral diffusion on the ZPL line width measurement. The signal-to-noise ratio of the SHB response, however, deteriorates at high modulation frequency, limiting the measurement to Ω below a few megahertz. The SHB line width also depends strongly on the level of optical excitations. Figure 3a shows the decoherence rate 2155

Figure 3. (a) Decoherence rate vs pump beam intensity at T ) 8 K and Ω ) 1 kHz and with Iprobe ) 1 W/cm2. (b) Decoherence rate vs temperature at Ω ) 1 kHz and with Ipump ) 3 W/cm2 and Iprobe ) 0.3 W/cm2.

obtained from the SHB response as a function of the pump intensity (Ω ) 1 kHz). As shown in Figure 3a, the decoherence rate increases significantly with increasing pump intensity. This power broadening can result from saturation beyond the third-order limit and can also result from intensity-dependent spectral diffusion processes. At the lowest intensity for the SHB measurements (∼1 W/cm2), which is significantly smaller than typical intensities (∼10 W/cm2) used for single-nanocrystal PL studies, the decoherence rate becomes nearly independent of the excitation level. Figure 3b shows the decoherence rate obtained from the SHB response as a function of the temperature. The data were obtained at Ω ) 1 kHz and at a pump and probe intensity of 3 and 0.3 W/cm2, respectively. Because the relative contribution of the ZPL to the SHB response decreases with temperature, deteriorating signal-to-noise ratio at relatively high temperatures has prevented us from obtaining data at temperatures above 25 K. Nevertheless, Figure 3b shows clearly that the decoherence rate increases quickly with temperature, indicating a significant contribution to decoherence from electron-phonon interactions. For comparison, Figure 2 also plots the data obtained from an earlier experimental study on SHB from spherical CdSe/ ZnS core/shell nanocrystals with an average diameter of 9 nm.2 As shown in Figure 2, the decoherence rate of the spherical nanocrystals decreases from 4 GHz to less than 1 GHz, as the modulation frequency increases from 1 kHz to 1 MHz. In contrast, under similar conditions, the decoherence rate of the NRs decreases from 8 GHz (33 µeV) to 4.5 GHz (19 µeV). For measurements at a relatively small modulation frequency (Ω ≈ 1 kHz), the contribution to the SHB line width from spectral diffusion is thus of the same order of magnitude for both NRs and spherical nanocrystals. The relative contribution from spectral diffusion to the SHB line width, however, is considerably smaller for NRs than that for spherical nanocrystals, as shown in Figure 2. These results are consistent with an earlier single-NR PL study, where the NR line width is limited by the spectral resolution of the measurement and the PL resonance from a single NR is relatively stable within the spectral resolution of the PL measurement.4 Note that although SHB measurements performed at high modulation frequencies avoid contributions from spectral diffusion occurring at slower time scales, 2156

residual spectral diffusion occurring at faster time scales can still lead to additional broadening of the measured ZPL line width. In this regard, decoherence rates obtained at high modulation frequencies can only be viewed as an upper limit to the intrinsic decoherence rates. Decoherence mechanisms in semiconductor nanocrystals still remain poorly understood. For the spherical nanocrystals, decoherence rates obtained at high modulation frequencies are comparable to those obtained in self-assembled quantum dots2,10 but still far exceed the expected radiative decay rate (∼0.015 GHz) of the corresponding optical transition. The measured ZPL line width thus arises primarily from coupling to lattice vibrations as well as from residual spectral diffusion occurring at faster time scales. How electron-phonon interactions contribute to the line width of the ZPL, however, is still a subject of considerable debate.12 In comparison to spherical nanocrystals, one-dimensional excitons in NRs feature much faster radiative decay processes.3 A radiative lifetime on the order of 0.8 ns, which corresponds to a radiative decay rate of 0.2 GHz and is more than 1 order of magnitude shorter than that in spherical nanocrystals, has been observed in CdSe/ZnS core/shell NRs.4 The ZPL line width obtained in the NRs, however, is far from being radiative-limited and is also considerably greater than that for the spherical nanocrystals. The onedimensional nature of NRs leads to additional relaxation processes that do not occur in the zero-dimensional nanocrystals. Specifically, fluctuations in the potential for onedimensional excitons can lead to localization of the excitons in different parts of the NR. Migrations of excitons among different localization sites with local potential minima can take place via absorption or emission of acoustic phonons, contributing to the ZPL line width. The phonon-assisted migration process persists and can be highly effective even at very low temperature, as shown in earlier studies of localized excitons in GaAs/AlGaAs quantum wells.13 Potential fluctuations in NRs can arise from a variety of mechanisms, such as surface or interface roughness, nonmonotonic variations of the NR thickness, and NR bending. Furthermore, compared with excitons in spherical nanocrystals, the one-dimensional excitons in NRs are also sensitive to fluctuations in the dielectric confinement provided by the difference in the dielectric constants between the NR core and the surrounding medium.3 It should be pointed out that the ZnS overcoating of the NRs can also significantly affect the ZPL line width. The ZnS shell in principle defines a better potential well for electrons and holes than a dielectric matrix (polymer) or a solvent. In addition, the ZnS shell reduces the defect density at the interface, leading to improved quantum efficiency and reduced spectral diffusion.11 Both of these improvements are important for the success of the SHB measurement. Nevertheless, nonuniformity in the ZnS shell leads to fluctuations in the confinement potential, including fluctuations in the dielectric confinement. In summary, high-resolution SHB studies in CdSe/ZnS NRs show a sharp ZPL along with a broad background associated with acoustic phonon sidebands. By suppressing Nano Lett., Vol. 6, No. 9, 2006

the effects of spectral diffusion in the nonlinear optical measurements, we have obtained a decoherence rate as small as 4.5 GHz for the NRs. A comparison between the ZPL line widths in NRs and spherical nanocrystals suggests phonon-assisted migration of excitons localized in the nanorods as an important decoherence mechanism for the NRs. Because of the much faster radiative decay rate in NRs, the NRs have a better prospect than spherical nanocrystals for attaining a homogeneous line width limited by the radiative lifetime. Further reducing the homogeneous line width in NRs, however, will require a much improved interface and/or surface, including a more uniform overcoating for the NRs. We hope that our work will lead to further studies on improving NR synthesis as well as stimulate further theoretical efforts on understanding intrinsic decoherence processes in semiconductor nanorods. Acknowledgment. We thank Phedon Palinginis for technical assistance and for many helpful discussions. This work was supported by NSF under grant No. DMR0201784, by a NSF IGERT award (S.T.-K.), and by DARPA UPR.

Nano Lett., Vol. 6, No. 9, 2006

References (1) Borri, P.; Langbein, W.; Schneider, S.; Woggon, U.; Sellin, R. L.; Ouyang, D.; Bimberg, D. Phys. ReV. Lett. 2001, 87, 157401. (2) (a) Palinginis, P.; Wang, H. Appl. Phys. Lett. 2001, 78, 1541. (b) Palinginis, P.; Tavenner, S.; Lonergan, M.; Wang, H. Phys. ReV. B 2003, 67, 201307. (3) Shabaev, A.; Efros, Al. L. Nano Lett. 2004, 4, 1821. (4) Le Thomas, N.; Herz, E.; Scho¨ps, O.; Woggon, U.; Artymyev, M. V. Phys. ReV. Lett. 2005, 94, 016803. (5) Peng, X. G.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (6) Hu, J.; Li, L.; Yang, W.; Manna, L.; Wang, L.; Alivisatos, A. P. Science 2001, 292, 2060. (7) Chen, X.; Nazzal, A.; Goorskey, D.; Xiao, M.; Peng, Z. A.; Peng, X. Phys. ReV. B 2001, 64, 245304. (8) Mohamed, M. B.; Burda, C.; El-Sayed, M. A. Nano Lett. 2001, 1, 589. (9) Htoon, H.; Hollingsworth, J. A.; Dickerson, R.; Klimov, V. I. Phys. ReV. Lett. 2003, 91, 227401. (10) Palinginis, P.; Wang, H.; Goupalov, S. V.; Citrin, D. S.; Dobrowolska, M.; Furdyna, J. K. Phys. ReV. B. 2004, 70, 073302. (11) Mokari, T.; Banin, U. Chem. Mater. 2003, 15, 3955. (12) Muljarov, E. A.; Takagahara, T.; Zimmermann, R. Phys. ReV. Lett. 2005, 95, 177405. (13) Takagahara, T. Phys. ReV. B 1985, 32, 7013.

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