JOURNAL OF RAMAN SPECTROSCOPY J. Raman Spectrosc. 2003; 34: 100–103 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jrs.959 Raman spectroscopy of II–VI semiconductor nanostructures: CdS quantum dots B. Schreder, 1 C. Dem, 1 M. Schmitt, 1 A. Materny, 1† W. Kiefer, 1* U. Winkler 2 and E. Umbach 2 1 Institut f ¨ ur Physikalische Chemie der Universit ¨ at W ¨ urzburg, Am Hubland, D-97074 W ¨ urzburg, Germany 2 Experimentelle Physik II, Universit ¨ at W ¨ urzburg, Am Hubland, D-97074 W ¨ urzburg, Germany Received 6 June 2002; Accepted 10 October 2002 Information about confinement effects and dot–matrix interactions of CdS nanoparticles was obtained from resonance Raman spectroscopy. The quantum dots had diameters of 3 and 5 nm and were prepared with and without organic spacer groups. It was found that the spacer improves the quality of the nanocrystallites. No phonon confinement shift could be observed even for the small quantum dots. The linewidths of the overtone series point to a mechanism of vibrational relaxation which is dominated by the decay of the LO phonons into acoustic phonons. Copyright  2003 John Wiley & Sons, Ltd. KEYWORDS: semiconductors; nanostructures; quantum dots; cadmium sulfide INTRODUCTION The development of faster and smaller opto-electronic devices has resulted in remarkable progress in electronics, data processing and communication techniques. The search for materials well suited for applications in these areas finally led to interest in semiconductor nanostructures such as quantum dots (QDs). This is mainly due to the change of their optical properties for particle diameters smaller than the Bohr exciton radius. The so-called ‘size quantization effect’ makes semiconductor nanoparticles interesting materials for opto-electronic applications. 1,2 The most common method of preparing the QDs is the controlled precipitation of small crystallites in a saturated solution or in a glass matrix. 3–12 By additional sintering of these samples, the QD diameters can be varied as a function of temperature and duration of the annealing process 13 – 16 . The resulting QDs show a non- uniform distribution within the matrix and an approximate Gaussian distribution of particle sizes. 11,12 In order to achieve high concentrations of QDs, aggregation has to be prevented. For this purpose, organic molecules are added which serve as spacers. 5,14 This may result in a change of the properties of the QDs, e.g. caused by strain. Additionally, the matrix is in most cases amorphous. Therefore, phonons are confined within the nanocrystals. L Correspondence to: W. Kiefer, Institut f ¨ ur Physikalische Chemie der Universit¨ at W ¨ urzburg, Am Hubland, D-97074 W ¨ urzburg, Germany. E-mail: wolfgang.kiefer@mail.uni-wuerzburg.de † Present address: School of Engineering and Science, International University Bremen, P.O. Box 750 561, D-28725 Bremen, Germany Contract/grant sponsor: Deutsche Forschungsgemeinschaft. Contract/grant sponsor: Fonds der Chemischen Industrie. In this paper, we present the results of Raman spectro- scopic investigations of CdS QDs with and without spacer groups. Owing to the small scattering volume of these QDs, Raman spectra can only be obtained under resonance con- ditions. 17 Information can be gained about confinement effects and also dot–matrix interactions, e.g. resulting in strain-induced shifts of the LO phonon bands. EXPERIMENTAL The synthesis of the CdS QDs has been described in Refs 18 and 19. The three samples investigated will be denoted CdS5 (mean diameter ¾5 nm, no spacer, yellow–orange appearance), CdS5n (mean diameter ¾5 nm, spacer, yellow appearance) and CdS3n (mean diameter ¾3 nm, spacer, white–yellow appearance). The locations of the shoulder in the corresponding absorption spectra (not shown) were used to estimate the average particle size referring to tight- binding calculations, which is a well established method. 20 The nanoparticle solutions were placed on object slides; the solvent was then rapidly evaporated. For the Raman investigations, the samples were cooled to about 10 K. The resonance Raman spectra were recorded using a micro-Raman setup described elsewhere in more detail. 21 For Raman excitation, radiation of different wavelengths from an argon ion laser was used. RESULTS AND DISCUSSION The bandgap energy E was determined theoretically as a function of the QD diameter d using the following simplified Copyright  2003 John Wiley & Sons, Ltd.