Shell Nanocrystals


Aug 9, 2013 - Semiconductor core–shell nanocrystals (NCs) have greatly improved luminescent properties including better resistance to photobleaching...
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Raman- and IR-Active Phonons in CdSe/CdS Core/Shell Nanocrystals in the Presence of Interface Alloying and Strain Volodymyr M. Dzhagan,*,† Mykhailo Ya. Valakh,† Alexander G. Milekhin,‡ Nikolay A. Yeryukov,‡ Dietrich R.T. Zahn,§ Elsa Cassette,∥ Thomas Pons,∥ and Benoit Dubertret∥ †

V.E. Lashkaryov Institute of Semiconductor Physics of the National Academy of Sciences of Ukraine, 45 Nauky av., 03028 Kyiv, Ukraine ‡ A.V. Rzhanov Institute of Semiconductor Physics, Lavrentieva av., 630090 Novosibirsk, Russia § Semiconductor Physics, Chemnitz University of Technology, 60 Reichenhainer str., D-09107 Chemnitz, Germany ∥ Laboratoire de Physique et d’Etude des Matériaux, CNRS, ESPCI, 75005 Paris, France ABSTRACT: Semiconductor core−shell nanocrystals (NCs) have greatly improved luminescent properties including better resistance to photobleaching and ligand exchange. It was suggested that compound alloying at the core/shell interface could play an important role in obtaining bright and stable NCs. Here, we investigate the interface composition and strain evolution in spherical and dot-in-plate CdSe/CdS nanocrystals with shell thickness ranging from 1 to 3 nm, using a combination of Raman and infrared spectroscopy. A slower rate of strain accumulation in the core is observed for dot-in-plate nanocrystals and is linked to the anisotropic shape of the plate-like shell. We resolved the respective contributions of the core, shell, and alloyed interface and observed a drastic change of the shell-related Raman feature with the appearance of the bulk-like optical phonon in the shell thicker than 1 nm. The average composition of the alloy interface is estimated using the frequencies of the alloy modes. Because of the high crystallinity of the samples, up to fourth-order optical phonon processes are observed and analyzed. This work confirms the presence of an alloyed interface in core/shell CdSe/CdS structures of different geometries and establishes a precise roadmap for its quantitative analysis using vibrational spectroscopy.



INTRODUCTION Semiconductor core−shell nanocrystals (NCs) have greatly improved luminescent properties including higher quantum yields,1 better resistance to photobleaching,2 more robust fluorescence upon ligand exchange,3 and transfer into water4 and, in the case of thick shells, strongly suppressed Auger recombination5,6 leading to quasicomplete absence of photoluminescence (PL) emission blinking at the single particle level.7−9 The synthesis of a blinking-free semiconductor core/ shell nanocrystal with 100% quantum yield is a goal that has motivated fundamental research for several decades, and this was recently reached but at cryogenic temperatures only.10 It was suggested that compound alloying at the interface of the core and the shell could play an important role in the suppression of Auger recombination11 and thus is a crucial parameter to control the synthesis of bright and stable NCs. A precise technique for the analysis of the interface of core/shell structures is thus needed. The first observations of interfacial alloying in colloidal core−shell NCs were performed using Raman spectroscopy.12−14 For thick-shell NCs, a recent study by Garcia-Santamaria et al.15 used fluorescence line narrowing experiments to observe phonon replica linked to the CdSe and CdS longitudinal optical (LO) phonons and the CdSe−CdS mixed overtone, which was interpreted as a signature of an alloying of the CdSe/CdS interface. This attribution is © 2013 American Chemical Society

ambiguous since the CdSe−CdS mixed overtone could also result from second-order processes involving phonons of CdS and CdSe. A more recent study16 used Raman spectroscopy on CdSe/CdS NCs with various thicknesses and identified a band around 205 cm−1 as characteristics of interface alloying between the core and the shell, an interpretation similar to the one performed earlier on CdSe/ZnS.12,14 The work16 confirmed that Raman spectroscopy is indeed a well-suited tool to analyze in detail the interfacial alloying in core/shell structures; however, it was limited to thin-shell NCs with a spherical shell, and some features of the Raman spectra were not interpreted. In a recent study of thin-shell CdSe/CdS NCs formed from magic-sized spherical CdSe clusters,17 a modification of the phonon confinement model was proposed, supposed to account for the effects of interface alloying. However, due to the strong phonon confinement in the latter ultrasmall (diameter of only 1.36 nm) NCs and inhomogeneity of shell thickness, no Raman bands of the alloy layer were resolved. Here, we studied the dependence of the interface alloying and strain evolution of 3 nm spherical core NCs embedded in Received: May 12, 2013 Revised: August 2, 2013 Published: August 9, 2013 18225

dx.doi.org/10.1021/jp4046808 | J. Phys. Chem. C 2013, 117, 18225−18233

The Journal of Physical Chemistry C

Article

Figure 1. PL excitation (plain line) and PL (dotted line) of the CdSe core. Spherical (a) and dots-in-plate (b) CdSe/CdS core/shell NCs with 0, 3, 6, and 9 monolayers of CdS.

the CdS shell with two different shapes: spherical or plate-like. Apart from the novelty and the interesting optical properties of the dot-in-plates structures,18 one motivation to compare core/ shell materials with shells of different shape is to evaluate the influence of strain inhomogeneity on the interdiffusion at the interface of two lattice mismatched semiconductors.19,20 In this study, we succeeded to identify both the CdSe- and CdS-like alloyed interface-related vibrations and to relate the higherorder Raman features at 480−490 cm−1 as well as in the range of 700−1100 cm−1 with certain combinations of first-order optical modes of the core, the shell, and the alloyed interface. We also propose a method based on the frequency of the alloyed phonon modes to extract the average composition of the alloy interface.

Figure 2. TEM image of the spherical CdSe/CdS NCs with 9 CdS MLs and an average diameter of 8.6 nm (a) and dots-in-plate with average lateral size of 3.2 nm and average thickness of 2.1 nm (b). The drawings in (b) illustrate two orientations of the dots-in-plate.



EXPERIMENTAL METHODS Synthesis of the CdSe Core and CdSe/CdS Core/Shell NCs. Spherical CdSe wurtzite nanocrystals with a 3 nm diameter were synthesized as reported in the work of Cassette et al.18 Briefly, a mixture of selenium precursors and oleylamine was injected in a solution of cadmium oleate, octadecene, and trioctylphosphine oxide heated at 300 °C. These nanocrystals are perfectly spherical with the first absorption maximum at 550 nm and an excitonic PL emission maximum at 570 nm (Figure 1a) with a full width at half-maximum of 25−26 nm being characteristic for a good NC size homogeneity. Both the spherical and dot-in-plate CdSe/CdS core/shell structures (Figure 2a) were synthesized as reported previously.18 Briefly, the CdS shell is grown layer by layer using sequential addition of the sulfur and of the cadmium precursors with 15 min in between each injection. The final shape of the nanocrystals, spherical of disk-shaped, is controlled with the temperature used for the shell growth, the preparation of the cadmium precursor, as well as the precursor injected for the first monolayer. For the spherical CdSe/CdS core/shell structures, three samples with shell thicknesses corresponding to 0.9 nm (∼3 CdS monolayers (MLs)), 1.9 nm (6 CdS MLs), and 2.7 nm (9 CdS MLs) were prepared (Table 1). These samples will be referred to in the text as S3, S6, and S9. For the dot-in-plate CdSe/CdS core/shell structures (Figure 2b), three samples with shell thicknesses corresponding to 1.1 nm (∼3 CdS MLs), 2.1 nm (6 CdS MLs), and 3.1 nm (9 CdS MLs) were prepared

Table 1. Shell Thicknesses of the Core/Shell NC Samples Investigated spherical NCs

dot-in-plate NCs

sample

shell thickness, nm

sample

shell thickness (t1/t2a), nm

S3 S6 S9

0.9 1.9 2.7

P3 P6 P9

0.3/1.1 0.6/2.1 1.2/3.1

a

t1 is the shell thickness on the top and bottom facets of the plate (i.e., in the direction perpendicular to the plane of the dot-in-plate structure), and t2 is the shell thickness on its side surface.

(Table 1). These samples will be referred to in the text as P3, P6, and P9. Raman and IR Measurements. Raman spectra were excited with the 476 nm line of an Ar+-ion laser and recorded with a Dilor XY triple monochromator equipped with a Peltiercooled CCD detector. The spectral resolution was ∼2 cm−1. Measurements were performed at temperature of 35 K provided by a close-cycle helium cryostat (Oxford Instruments). IR reflection spectra were recorded at near-normal and off-normal incidence (angles of light incidence are Θ = 15° and Θ = 75°, respectively) using a IR Fourier transform spectrometer Vertex 80v in the spectral range of the optical vibrations 90−600 cm−1 of the crystal lattices in the NPs. IR reflectance spectra were taken at room temperature (RT) using nonpolarized light. The resolution was 2 cm−1 over the whole spectral range. The number of scans was 300. NCs were 18226

dx.doi.org/10.1021/jp4046808 | J. Phys. Chem. C 2013, 117, 18225−18233

The Journal of Physical Chemistry C

Article

phonons, either via combined optical-acoustical modes or via scattering of the exciton to acoustic phonons at the initial stage of the exciton relaxation. This hypothesis has recently been further explored in the work on ZnTe nanorods,30 with the most favorable mechanism assumed to be exciton scattering on acoustic phonons followed by LO phonon emission (nLO in the general case, n = 0, 1, 2, ..., as the HFS is observed for LO overtones (Figure 3)). There is yet a third possibility for the HFS. It could be related to the presence of undercoordinated surface bonds. Ab initio calculations of the phonon density of states (DOS) for CdSe31 and other nanostructures32,33 reveal a DOS-related structure above the bulk LO phonon frequency, which was assigned to the one phonon DOS induced by reconstruction of undercoordinated surface bonds.32,33 This interpretation is in good agreement with the appearance of the HFS only in the relatively small (