Confined Optical Vibrations in CdTe Quantum Dots and Clusters

Abstract

Absorption of far-infrared (FIR) radiation and Raman scattering in CdTe nano-crystals (spherical quantum dots (QDs) of several nanometers in size) and Cd 54 Te 32 (SCH 2 CH 2 OH) 8À 32 clusters prepared by colloidal synthesis were studied. Raman spectra of both QDs and clusters are dominated by a peak situated slightly below the bulk CdTe LO phonon frequency. Experimental FIR transmittance spectra of films containing CdTe clusters show a broad band between the bulk TO and LO phonon frequencies, while there is only a single dip at the Frö hlich frequency observed for larger QDs. These observations are in agreement with the results of our numerical lattice dynamics calculations and previously published theoretical results based on a continuous model. It is suggested that the lattice dynamics of very small clusters is similar to that of large crystallites in the sense that there still exists the dispersion of optical vibration frequencies within the LO-TO band of bulk material.Introduction II-VI semiconductor nanocrystals embedded in glass were the first quantum dots (QDs) to be studied [1,2]. Since that time, they have been produced by different techniques and extensively investigated. Wet chemistry methods have shown the ability to produce colloidal solutions of nearly spherical crystalline nanoparticles of various II-VI semiconductors with controllable size [3][4][5][6]. From such a solution, one can deposit films containing QDs embedded in a matrix or matrix-free (MF) films of near closely-packed QDs onto different substrates. In particular, it has become possible to produce extremelly small semiconductor clusters of some ''magic" sizes, for example, Cd 54 Te 32 (SCH 2 CH 2 OH) 8À 32 particles [7] (below referred to as Cd 54 Te clusters). While the structure of QD electronic states is well understood based on the effective mass (EM) theory and (for smaller crystallites) numerical calculations, phonons confined in QDs (commonly referred to as ''confined phonons") received much less attention until the last few years [8][9][10][11][12]. It has been shown theoretically [10, 11] that mechanical confinement indeed becomes important both for far-infrared (FIR) and Raman-active phonons in the limit of small QD size. Recently [12], we demonstrated the existence of spatially quantized dipolar modes for CdSe QDs of about 2 nm in diameter. Their frequencies were successfully calculated using the approach proposed in [10], which consists in solving the coupled continuous equation of motion and the Poisson equation. The validity of this semi-analytical model is, of course, a nice fact. However, it is unclear if it can be applied (if only qualitatively) to even smaller QDs and clusters. In other words, the question is whether bulk phonon dispersion curve parameters and TO-LO splitting have any relation to the vibrational properties of the clus-

Structural and optical properties of CdTe thin film: A detailed investigation using optical absorption, XRD, and Raman spectroscopies

“…q % 150:0 cm À1 , which plays the role of the Fr€ ohlich mode [47]. The phonon mode for the present CdTe powder is centered at about 152.3 cm À1 , which is close to the Fr€ ohlich mode frequency, as seen in Fig.…”

“…, the X‐critical‐point LO‐phonon wave numbers of CdTe bulk are 4.03, 4.86, and 4.92 THz, which equals 134.3, 162.0, and 164.0 cm −1 , respectively. The resultant of the 134.3 and 164.0 cm −1 wave numbers gives a single peak centered at a wave number of , which plays the role of the Fröhlich mode . The phonon mode for the present CdTe powder is centered at about 152.3 cm −1 , which is close to the Fröhlich mode frequency, as seen in Fig.…”

Raman spectroscopy of optical phonons confined in semiconductor quantum dots and nanocrystals

“…33, we assume that the expression in square brackets in Eqn (22) can be approximated by [. 33, we assume that the expression in square brackets in Eqn (22) can be approximated by [.…”

Phonon Spectra of Small Colloidal II-VI Semiconductor Nanocrystals

“…From the viewpoint of the model based on bulk dispersion curves, the strong spatial confinement along with lattice distortion due to the reconstruction induced by a domination of surface atoms can lead to the situation when only phonons (TO and LO) close to (bulk) zone boundary exist in NCs. These ZE phonons manifest themselves in Raman spectrum as a single broad feature because of a very small TO-LO splitting near International Journal of Spectroscopy 5 the zone boundary, as has been inferred previously for the description of IR phonon spectra of Cd 56 Te 32 clusters [43].In conclusion, the resonant Raman spectra of several kinds of II-VI NCs have been analyzed. It is shown that similar set of phonon modes can be derived from fitting the spectra of high enough quality.…”