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dots. The fluorescence peak is red shifted from the lowest absorption peak by the Stokes shift,15, 16 which increases with decreasing nanocrystal size. The tetrapods (Fig. 2c–2d) display absorption spectra with overlapping peaks that ori- ginate from the more complex electronic structure.13 In particular, the luminescence signal of larger tetrapods (TP2) consists of a double-peak structure that can be clearly re- solved at low temperatures (Fig. 2e), whereas the dots show only a single peak with slight asymmetric broadening on the low-energy shoulder. For the low-temperature measure- ments, the solution containing the nanocrystals was drop- casted onto the surface of a silicon substrate, the solvent was allowed to evaporate under soft nitrogen flow, and the samples were mounted in an optical cryostat.
The Raman experiments were performed using a tunable Ti-Sa laser (700–850 nm) and a semiconductor diode laser (532 nm wavelength). The excitation light (30 mW laser power) was focused onto the samples on a spot with a 50-
mm diameter, and the signal was collected by an achro- matic lens and detected by a triple Raman spectrometer (DILOR XY) and a charge-coupled device (CCD) camera. Figure 3a shows Raman spectra of large (D1: 6 nm dia- meter) and small (D2: 4 nm diameter) dots and of two tetrapod samples with different arm lengths (TP1: ap- proximately 3 nm arm length and diameter; TP2: 30 nm arm length, 7 nm arm diameter) at T¼15 K with a laser excitation energy of 2.33 eV, which is well above the band gap of the nanocrystals (see dashed vertical line in Fig. 2). At this excitation energy, we observe the LO phonon at 170 cm1and its second scattering order at 340 cm1. The LO phonon excitation is slightly red shifted and broadened with respect to the bulk value (172 cm1), as expected for
nanocrystal dots.19, 25 In the spectra of the spherical nanocrystals, we can identify the SO phonon at 147 cm1on the low-energy side of the LO phonon. The SO phonon energy can be calculated as the sphere-surface ground mode using the dielectric continuum model of Ruppin and Englman27:
x2 SO¼x 2 TO e0þ2em e1þ2em
which yields 149 cm1 as SO phonon energy ifem (sur-
rounding medium dielectric constant) is taken as 6.5 (average of vacuum and Si).e?ande0are the bulk CdTe high-frequency and static dielectric constants, respec- tively, andoTOis the frequency of the transverse-optical
phonon.28For a sample with a high density of nanocrystals (as is the case in our experiments), em can be assumed
larger (em¼10), which reduces the theoretical value for the
SO phonon to 147 cm1, leading to even better agreement with our experimental value.
In Figure 3b, we plot the Raman spectra of dots and tetrapods measured with a laser excitation energy on the red edge of their respective first absorption peak atT¼15 K (see dotted vertical line in Fig. 2). In this energy range, the exciting laser energy is selectively exciting classes of Fig. 1. Transmission electron microscopy images of nanocrystals
with different shapes: (a) spherical nanocrystals with 4-nm diameter; (b) tetrapod-shape nanocrystals with very short arms (arm length of ap- proximately 3 nm is comparable to arm diameter); (c) tetrapods with well- developed arms (arm length and diameter are 30 and 7 nm, respectively).
Fig. 2. (a)–(d) Fluorescence (solid lines) and absorption (dotted lines) spectra of spherical and tetrapod-shaped CdTe nanocrystals at room tem- perature; (e) photoluminescence of large (D1) and small (D2) nanocrystal dots and tetrapods (TP2) at low temperature (T¼15 K). The tetrapod luminescence of TP2 shows a double peak, whereas on spherical nano- crystals only a single peak is observed. For the low-temperature mea- surements, the nanocrystals were drop-casted on a Si substrate. The dotted lines show the laser excitation energies for the Raman scattering ex- periments reported in Fig. 3.
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Krahne et al. Optical Phonon Scattering in Nanocrystals
J. Nanoelectron. Optoelectron. 1, 104–107,2006 105
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nanocrystals that absorb at the same energy. For the large dots D2, we observe broad peaks at 177 cm1(full width at half maximum [FWHM]¼23 cm1) and 347 cm1 (FWHM¼20 cm1) that are blue shifted with respect to the LO phonon energy of 170 cm1found in Figure 2a. These broad peaks are reminiscent of phonon replicas that were reported in FLN experiments on nanocrystal dots,15, 16in which the nanocrystals are also excited with the laser energy on the red edge of the first absorption peak. In FLN spectra, the energy difference between the excitation laser and the position of thenth LO phonon replica consists of the re- sonant Stokes shift (*8 cm1for nanocrystal dots with 6- nm diameter) plusntimes the LO phonon energy.
The phonon spectrum of very short tetrapods TP1 (arm length comparable to arm diameter) also shows broad phonon replica (at 180 and 353 cm1) that are blue shifted from the bulk phonon energy. This energy difference of the LO phonon replica in Figure 3b to the LO phonon excitation in Figure 3a of 10 cm1can be regarded as the resonant Stokes shift and is slightly larger than the Stokes shift observed in the case of the large dots D2 due to shape
asymmetry. Otherwise, sample TP1 can be treated as a deformed dot in which the carriers are mostly localized in and near the core.
The LO phonon excitations of tetrapods with arm lengths much larger than their arm diameter14that are observed with laser energies at the band edge differ significantly from the signal obtained from spherical nanocrystals. The spec- trum of sample TP2 in Figure 3b shows a sharp peak at 173.5 cm1and a broad signal at its low-energy shoulder that originate from phonon excitations in the TP arms. These phonon excitations that are observed in larger tetra- pods14are dominated by the shape of the tetrapod arms and can be modeled in a nanowire picture.9, 11
To gain more insight into the difference of the phonon excitations of dots and tetrapods at laser energies near the band gap, we discuss the scattering processes for the pho- non excitations. In Raman scattering with laser energies above the band gap, an incoming photon is absorbed in- stantaneously by the nanocrystal into a virtual or real ex- citon state. The exciton transfers part of its energy to the crystal lattice, creating a phonon, and recombines under photon emission.
Figure 4a illustrates the phonon-scattering process in FLN experiments, in which the lowest optically allowed state, the bright exciton, is excited by the incoming laser light. Then, the bright state relaxes nonradiatively into the dark state (mediated by acoustic phonons), and the dark state recombines by LO phonon-assisted transitions.16 In this scattering process, the LO phonon replica line shape is dominated by inhomogeneous broadening of the excited class of nanocrystals, leading to a FWHM of 14 cm1. The key difference between spherical nanocrystals and tetra- pods is that in tetrapods the electron and hole wave func- tions of first excited exciton states and the ground exciton state have a different spatial distribution.13, 29In particular, the electrons and holes of the first excited states are lo- calized in the arms, which leads to stronger coupling to the phonons and to an increased radiative recombination probability (illustrated in Fig. 4b). We think that the pho- non replicas that originate from ground exciton state tran- sitions are suppressed in tetrapods due to the localization of the carriers. This interpretation is supported by the fact that the main contribution of the phonon excitation comes from the arm regions (if only by the ratio of arms to core volume). In the sample TP1, the arms are too short (see Fig. 1b) to have a significant impact on the phonon ex- citations, whereas in TP2 and larger tetrapods the phonon spectrum is dominated by the arms.
In conclusion, we have measured the optical phonon ex- citations of spherical and tetrapod-shape nanocrystals with laser energies above the band gap and on the red edge of their absorption spectrum. For tetrapods with well-defined arms, we find that the phonon-assisted transitions from the exciton ground state are suppressed, and we observe the LO phonon excitation with narrow line width. We propose the carrier localization of the first excited exciton states in the tetrapod Fig. 3. Raman spectra of small (blue: 4 nm diameter) and large (red: 6 nm
diameter) nanocrystal dots and tetrapods (TP1, green; TP2, magenta) at
T¼15 K. (a) Laser excitation energy above the band gap; (b) laser ex- citation energy on the low-energy side of their photoluminescence signal. The peak labeled A1 in (a) at 128 cm1
originates most likely from tellurium formed by photocrystallization, and the sharp excitation in (b) at the LO bulk frequency, 172 cm1, originates from CdTe aggregates formed by laser illumination. The spectra have been shifted vertically for clarity, and the vertical gray line is a guide to the eye indicating the bulk CdTe LO phonon energy.
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106 J. Nanoelectron. Optoelectron. 1, 104–107,2006
B.6. Shape Dependence of the Scattering Processes of Optical Phonons in
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arm as the origin for the absence of broad phonon replicas, which are observed for spherical nanocrystals at such laser excitation energies.
Acknowledgments: We gratefully acknowledge the support by the SA-NANO European project (contract STRP013698), by the MIUR-FIRB and MIUR 297 (contract 13587) projects, and by the German Science Foundation through SFB 508.
References
1. A. P. Alivisatos.Science,271, 933–937 (1996).
2. B. Alperson, I., Rubinstein, and G. Hodes.Phys. Rev. B,6308, 081303 (2001).
3. S. Kan, T. Mokari, E. Rothenberg, and U. Banin.Nat. Mater.,2, 155– 158 (2003).
4.J. T. Hu, L. S. Li, W. D. Yang, L. Manna, L. W. Wang, and A. P. Alivisatos.Science,292, 2060–2063 (2001).
5.L. Manna, E. C. Scher, L. S. Li, and A. P. Alivisatos.J. Am. Chem. Soc.,124, 7136–7145 (2002).
6.L. Manna, D. J. Milliron, A. Meisel, E. C. Scher, and A. P. Alivisatos.
Nat. Mater.,2, 382–385 (2003).
7.K. A. Dick, K. Deppert, M. W. Larsson, T. Martensson, W. Seifert, L. R. Wallenberg, and L. Samuelson.Nat. Mater.,3, 380–384 (2004). 8.L. Carbone, S. Kudera, E. Carlino, W. J. Parak, C. Giannini, R. Cingolani, and L. Manna.J. Am. Chem. Soc.,128, 748–755 (2006). 9.G. D. Mahan, R. Gupta, Q. Xiong, C. K. Adu, and P. C. Eklund.Phys.
Rev. B,68, 73402 (2003).
10. R. Gupta, Q. Xiong, G. D. Mahan, and P. C. Eklund.Nano Lett.,3, 1745–1750 (2003).
11. Q. H. Xiong, J. G. Wang, O. Reese, L. C. L. Y. Voon, and P. C. Eklund.Nano Lett.,4, 1991–1996 (2004).
12. Y. Cui, U. Banin, M. T. Bjork, and A. P. Alivisatos.Nano Lett.,5, 1519–1523 (2005).
13. D. Tarı`, M. De Giorgi, F. Della Sala, L. Carbone, R. Krahne, L. Manna, R. Cingolani, S. Kudera, and W. J. Parak.Appl. Phys. Lett.,87, 224101 (2005).
14. R. Krahne, G. Chilla, C. Schu¨ller, L. Carbone, S. Kudera, G. Man- narini, L. Manna, D. Heitmann, and R. Cingolani.Nano Lett.,6(3), 478–482 (2006).
15. M. Nirmal, D. J. Norris, M. Kuno, M. G. Bawendi, A. Efros, and M. Rosen.Phys. Rev. Lett.,75, 3728–3731 (1995).
16. A. L. Efros, M. Rosen, M. Kuno, M. Nirmal, D. J. Norris, and M. Bawendi.Phys. Rev. B Condens. Matter,54, 4843–4856 (1996). 17. V. Klimov., Semiconductor and Metal Nanocrystals, Marcel Dekker,
New York (2004).
18. E. Roca, C. Tralleroginer, and M. Cardona.Phys. Rev. B,49, 13704– 13711 (1994).
19. M. P. Chamberlain, C. Tralleroginer, and M. Cardona.Phys. Rev., B,
51, 1680–1693 (1995).
20. P. Yu and M. Cardona. Fundamentals of Semiconductors, 2nd ed., Springer, New York (1999).
21. J. J. Shiang, A. N. Goldstein, and A. P. Alivisatos.J. Chem. Phys.,92, 3232–3233 (1990).
22. J. J. Shiang, S. H. Risbud, and A. P. Alivisatos.J. Chem. Phys.,98, 8432–8442 (1993).
23. K. A. Alim, V. A. Fonoberov, and A. A. Balandin.Appl. Phys. Lett.,
86, 053103 (2005).
24. A. Roy and A. K. Sood.Phys. Rev. B,53, 12127–12132 (1996). 25. A. M. dePaula, L. C. Barbosa, C. H. B. Cruz, O. L. Alves, J. A.
Sanjurjo, and C. L. Cesar.Appl. Phys. Lett.,69, 357–359 (1996). 26. L. Manna, E. C. Scher, and A. P. Alivisatos.J. Cluster Sci.,13, 521–
532 (2002).
27. R. Ruppin and R. Englman.Rep. Prog. Phys.33, 149 (1970). 28. Landolt-Bo¨rnstein—Group III. Condensed Matter, II-VI and I-VII
Compounds, Volume 41, Subvolume B. Springer, Berlin/Heidelberg (1999).
29. J. B. Li and L. W. Wang.Nano Lett.,3, 1357–1363 (2003). Fig. 4. Schematics of the scattering processes for LO phonons. (a) In
spherical nanocrystals, the excitation on the red edge of the luminescence peak probes the bright exciton state, followed by a relaxation into the dark exciton state. The dark state recombines by phonon-assisted emission. The relaxation of the excited state leads to inhomogeneous broadening of the LO phonon replica; (b) in tetrapods, the electrons and holes of the first excited states are localized in the arms and therefore are spatially sepa- rated from the carriers in the exciton ground state in the core. This leads to an increased radiative recombination probability, assisted by phonon scattering, of the first excited states.
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