Confinement effects on optical phonons in spherical, rod and tetrapod-

tetrapod-shaped nanocrystals detected by Raman spectroscopy 151

B.7 Confinement effects on optical phonons in

spherical, rod- and tetrapod-shaped nanocrystals

detected by Raman spectroscopy

C. Nobile, S. Kudera, A. Fiore, L. Carbone, G. Chilla, T. Kipp, D. Heit- mann, Cingolani, L. Manna, R. Krahne

Phys. stat. sol. (a) 204(2), pp. 483–486, 2007

Spherical, rod- and tetrapod shaped CdSe nanocrystals are investigated by Raman spectroscopy and the longitudinal-optical and surface optical phonons are observed. We find that the position of the longitudinal-optical phonon slightly red-shifts with decreasing diameter, whereas the position of the surface optical phonon depends significantly on diameter and length of the rods or the tetrapod arms.

phys. stat. sol. (a) 204, No. 2, 483– 486 (2007) / DOI 10.1002/pssa.200673223

Paper

Confinement effects on optical phonons in spherical, rod-, and tetrapod-shaped nanocrystals detected

by Raman spectroscopy

Concetta Nobile1, Stefan Kudera1, Angela Fiore1, Luigi Carbone1, Gerwin Chilla2, Tobias Kipp2, Detlef Heitmann2, Roberto Cingolani1, Liberato Manna1,

and Roman Krahne*, 1

1_{National Nanotechnology Laboratory of CNR-INFM, Via per Arnesano km5, 73100 Lecce, Italy} 2_{Institute of Applied Physics, University of Hamburg, 20355 Hamburg, Germany}

Received 30 July 2006, revised 7 November 2006, accepted 13 November 2006 Published online 6 February 2007

PACS 63.22.+m, 71.35.Ji, 73.22.–f, 78.30.Fs, 78,67.Bf

Spherical, rod- and tetrapod shaped CdSe nanocrystals are investigated by Raman spectroscopy and the longitudinal-optical and surface optical phonons are observed. We find that the position of the longitudinal-optical phonon slightly red-shifts with decreasing diameter, whereas the position of the surface optical phonon depends significantly on diameter and length of the rods or the tetrapod arms.

Chemical synthesis has achieved remarkable control over the size and shape of colloidal nanocrystals leading to spheres [1], rods [2–4], and branched nanostructures [5–7]. The shape of colloidal nanocrystals has strong effects on their optical and electronic properties, for example, in spherical nanocrystals the electronic bandgap, the resonant Stokes shift, and the optical phonon excitation energies depend on the nanocrystal diameter. Rod-shaped nanocrystals provide a uniaxial symmetry that leads to laterally and longitudinally confined vibronic excitations [8–10]. Tetrapods represent an even more complex geometry where four arms branch out from a central core that results in novel electronic and optical properties [11–13]. Optical phonons in nanocrystal spheres and rods have been studied experimentally and theoretically [14 – 18]. In spherical nanocrystals the confinement leads to a red-shift and broadening of the longitudinal-optical (LO) phonon mode [19], and to the observation of surface-optical (SO) phonons [20, 21]. Phonon excitations in nanorods and tetrapods can be treated in a nanowire picture [8, 9, 13, 22 – 25] where the confinement depends on the length and diameter of the nanocrystal structure.

Colloidal CdSe nanospheres, nanorods, and tetrapods in wurtzite crystalline structure were fabricated by chemical synthesis in a hot mixture of surfactants following a published approach [4, 5, 7, 26]. The unique axis of the wurtzite crystalline structure enables anisotropic growth due to the different chemical reactivity and growth rate of the lateral and longitudinal crystal facets. The transmission electron microscopy images of the three samples with different shapes that will be investigated in the following are shown in Fig. 1. The Raman experiments were performed using a diode laser at 532 nm wavelength that was coupled into an optical microscope setup and detected by a triple Raman spectrometer (Dilor XY) and a charge coupled device camera. The nanocrystals were casted from solution onto a Si substrate, dried under nitrogen flow and mounted into an optical cryostat.

*_{Corresponding author: e-mail:}_{[email protected]}

484 C. Nobile et al.: Confinement effects on optical phonons

Fig. 1 Transmission electron microscopy images of CdSe (a) dots with 8 nm diameter, (b) rods with 5/25 nm diameter/length and (c) tetrapods with 6/40 nm arm diameter and length.

Figure 2 shows the absorption and luminescence spectra of the three samples that are displayed in Fig. 1, recorded at room temperature and in solution. The well defined level structure in the dots leads to a series of distinct peaks in the absorption spectrum. In rods and tetrapods the more complicated electronic structure results in overlapping peaks in their absortion spectra. The fluorescence peaks show a blue-shift with decreasing diameter of the nanocrystals, confirming that the diameter is the dominant parameter for the optical excitations. The fluorescence peaks are red shifted from the lowest absorption peaks by the Stokes shift, which increases with decreasing nanocrystal diameter and rising nanocrystal shape complexity. The particular geometry of the tetrapods leads to a double peak structure in their fluorescence [12, 27]. Here we observe a large peak at 2 eV that results from recombination of the exciton ground state and a small peak at the high energy side at 2.2 eV peak that we believe originates from recombination of the first excited exciton state.

Figure 3 shows Raman spectra of the dots, tetrapods and rods recorded at T = 13 K with a power den- sity of 1 mW/µm2. We observe an asymmetric excitation and its higher scattering orders (not shown) for

Fig. 2 (online colour at: www.pss-a.com) Fluorescence (solid lines) and absorption spectra (dashed lines) of the dot, rod, and tetrapod samples under investigation. The fluorescence of the tetrapods shows a second peak (at 2.2 eV), whereas in the case of dots and rods only a single peak is observed. The green vertical line marks the laser excitation energy for the Raman experiments.

B.7. Confinement effects on optical phonons in spherical, rod- and

phys. stat. sol. (a) 204, No. 2 (2007) 485

Paper

all nanocrystal shapes. The high energy peaks of this excitation can be attributed to the LO phonon scattering. The Lorentzian fits to the data reveal well defined peaks at 215 cm–1 for dots, 214 cm–1 for tetrapods and 213 cm–1 for rods showing a trend that the LO phonon shifts to lower energy with decreasing nanocrystal diameter, as it was observed in experiments on spheres with different diameters. The low energy side of the excitation can be fitted with peaks at 203 cm–1 for dots, 196 cm–1 for rods, and for tetrapods with a double peak located at 185 and 200 cm–1. The low energy peak values can be modeled by the SO phonon energies for the respective geometries. To calculate the SO phonon energies of spherical nanocrystals we apply the dielectric continuum model of Ruppin and Englman [28]

2 2 0 M SO TO M ( 1) / ; 1, 2, 3... , ( 1) / l l _l l l ε ε ω ω ε• ε + + = = + +

where _ε_M = 1 (surrounding medium dielectric constant) for vacuum, _ε_∞ and _ε₀ are the bulk CdSe high frequency and static dielectric constants, respectively, and ω_TO = 173 cm–1 is the frequency of the trans- verse-optical phonon. This approach yields for the SO phonon ground mode (l = 1) of CdSe spheres

ωSO = 205 cm–1, in good agreement with the low energy peak of the dot sample located at 203 cm–1.

Fig. 3 (online colour at: www.pss-a.com) Raman spectra of nanocrystals with different shape. We observe the LO phonon modes at 215 cm–1 for dots, 213 cm–1 for rods and 214 cm–1 for tetrapods, showing a trend that a decreasing nanocrystal diameter leads to an increasing red shift of the LO phonon frequency. The SO phonon modes at the low energy side of the LO phonon peak are sensitive to the aspect ratio of the nanocrystal shape. The thin solid lines show best Lor- entzian fits to the data.

486 C. Nobile et al.: Confinement effects on optical phonons

For the rods and tetrapods we follow the approach of Gupta and coworkers [9] for surface optical (SO) phonons in nanowires [13]: 2 2 2 SO TO M ; / 2 , ( ) p x q d f x ϖ ω ω ε• ε = + = ◊ +

where ϖ_p2=ε_• (ω_LO2 -ω_TO2 ) is the screened ion-plasma frequency, f x( ) ( ( ) ( ))/( ( )= I x K x₀ ₁ I x K x₁ ₀( )) (with I and K Bessel functions), and d is the rod (or tetrapod arm) diameter. We can take the length of the rods (or the tetrapod arms) as the longitudinal symmetry breaking mechanism that leads to q=2πl. For the tetrapod samples the SO phonon is calculated at ω_SO = 185 cm–1 (5/40 nm diameter/length), and for the rods ω_SO = 195 cm–1 (5/22 nm diameter/length), also in very good agreement with our data. The sig- nal that we observe in the tetrapods in the regime between the SO and the LO phonon could be due to higher harmonics of the surface modes, or could originate from LO or TO phonon mode splittings due to the wire geometry [13].

In conclusion, we measured the LO and SO phonons in CdSe nanocrystals with different shapes. We find that the LO phonon frequency depends dominantly on the nanocrystal diamenter, whereas the SO phonon is also sensitive on the length of the nanocrystal rods.

Acknowledgements The authors gratefully acknowledge the support by the SA-NANO European project (Con- tract No. STRP013698), by the MIUR-FIRB and MIUR 297 (Contract No. 13587) projects.

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B.7. Confinement effects on optical phonons in spherical, rod- and

156 Appendix B. Tetrapods

B.8 Confined Optical Phonon Modes in Aligned

Nanorod Arrays Detected by Resonant Inelastic

Light Scattering

C. Nobile, V.A. Fonoberov, S. Kudera, A. Della Torre, A. Ruffino, G. Chilla, T. Kipp, D. Heitmann, L. Manna, R. Cingolani, A.A. Balandin, R. Krahne

Nano Letters 7(2), pp. 476–479, 2007

We investigated the optical phonon excitations of laterally aligned nanorod arrays by resonant Raman scattering. We observed a strong suppression of the surface-optical phonon modes in the closely packed aligned arrays and a small asymmetry in the longitudinal-optical phonon peak with respect to the rod alignment orientation. These observations can be explained by the spatial distribution of the potential of the different phonon modes derived from the first principles calculations.

Confined Optical Phonon Modes in

Aligned Nanorod Arrays Detected by

Resonant Inelastic Light Scattering

Concetta Nobile,†_{Vladimir A. Fonoberov,}‡_{Stefan Kudera,}†_{Antonio Della Torre,}†

Antonio Ruffino,†_{Gerwin Chilla,}§_{Tobias Kipp,}§_{Detlef Heitmann,}§

Liberato Manna,†_{Roberto Cingolani,}†_{Alexander A. Balandin,}‡_and

Roman Krahne*,†

National Nanotechnology Laboratory of CNR-INFM, Via per Arnesano, 73100 Lecce, Italy, Department of Electrical Engineering, UniVersity of CaliforniasRiVerside, RiVerside, California 92521, Institute of Applied Physics, UniVersity of Hamburg, 20355 Hamburg, Germany

Received December 2, 2006; Revised Manuscript Received January 10, 2007

ABSTRACT

We investigated the optical phonon excitations of laterally aligned nanorod arrays by resonant Raman scattering. We observed a strong suppression of the surface-optical phonon modes in the closely packed aligned arrays and a small asymmetry in the longitudinal-optical phonon peak with respect to the rod alignment orientation. These observations can be explained by the spatial distribution of the potential of the different phonon modes derived from the first principles calculations.

The chemical synthesis of colloidal semiconductor nanorods allows one to control the length and diameter and thereby provides the possibility to tune their optical and electronic properties. For this reason, nanorods have emerged as a very interesting material for both fundamental studies1,2_{and for} optical3_{and electronic applications.}4_{In particular, the uniaxial} symmetry in shape and crystal structure of wurtzite nanorods has strong effects on their electronic level structure and optical excitations.5-8 _{This results, for example, in the} emission of linearly polarized light from CdSe nanorods.9 Spectral hole-burning experiments on CdSe/ZnS nanorods have revealed a difference in decoherence rate for rods and spherical nanocrystals by investigating the zero-phonon line line width.10 _{Raman spectroscopy has proven to be a} powerful tool for the investigation of lattice vibrations,11,12 and confined optical phonon excitations in nanorods and nanowires have been studied experimentally13-19_{and were} described by different theoretical models.20-25 _Recently, efforts toward the self-assembly of nanorods and nanowires into ordered arrays, for example, by electric field mediated alignment, have been reported.26-29_{This technique can be} exploited to realize ordered arrays of nanorods on the scale

of some microns, which is comparable with the spatial resolution of optical spectroscopy and thereby enables optical spectroscopy studies of orientation-dependent properties on large ensembles of nanorods.

In this letter, we present a detailed comparison between experiments and first principles theory on optical phonon excitations in CdSe nanorods. We report Raman experiments that probe the optical lattice vibrations in ordered arrays of CdSe nanorods, which were assembled on a substrate surface by electric-field-induced alignment. We found that the lateral packing of nanorods into dense arrays leads to the suppression of the surface optical phonon modes. In the LO phonon peak, we observe a fine structure that depends on the relative orientation of the nanorods with respect to the incident and detected light polarization. We compare the experimental data with first principles calculations on corresponding nanostructures, which reveal the symmetry of the phonon potentials of the Raman active modes and provide a qualitative explanation of the experimental spectra.

Colloidal wurtzite CdSe nanorods were fabricated by chemical synthesis in a hot mixture of surfactants following a published approach.30_{The nanorods samples were inspected} by transmission electron microscope imaging, and fluorescence and absorption spectra were recorded at room temperature in solution (see Supporting Information). For random orientation experiments, the nanorods were drop-casted from solution on a Si substrate and dried by nitrogen flow.

* Corresponding author. E-mail: [email protected]. Telephone: +_{39 0832 298 209. Fax:} +_{39 0832 298 238.}

†_{National Nanotechnology Laboratory of CNR-INFM.}

‡_{Department of Electrical Engineering, University of California} s

Riverside.

§_{Institute of Applied Physics, University of Hamburg.}

NANO LETTERS

2007 Vol. 7, No. 2

476-479

Published on Web 01/23/2007

In document Kudera, Stefan (2007): Formation of Colloidal Semiconductor Nanocrystals: The Aspect of Nucleation. Dissertation, LMU München: Fakultät für Physik (Page 165-171)