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. Winkler2 and E. Umbach2 1 2
¨ Wurzburg, Institut fur Am Hubland, D-97074 Wurzburg, Germany ¨ Physikalische Chemie der Universitat ¨ ¨ ¨ Wurzburg, Experimentelle Physik II, Universitat Am Hubland, D-97074 Wurzburg, 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 process13 – 16 . The resulting QDs show a nonuniform 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. Ł Correspondence to: W. Kiefer, Institut fur ¨ Physikalische Chemie der Universit¨at Wurzburg, Am Hubland, D-97074 Wurzburg, ¨ ¨ Germany. E-mail:
[email protected] † 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 spectroscopic 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 conditions. 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 tightbinding 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.
CdS Quantumdots
equation:22 2¯h2 2 d2
1 1 C Ł mŁe mh
3.536e2 εd
1
where d is the particle diameter, mŁe and mŁh are the electron and hole effective masses, respectively ε is the dielectric constant. Figure 1 shows the result of the calculation of the bandgap energy E as a function of particle diameter. The bandgap energy for CdS bulk is 2.58 eV (480.6 nm) at 10 K. It becomes obvious that an appreciable rise in energy with decreasing diameter d can be found observed only for d < 10 nm. For QDs with diameter, 5 nm the resonance maximum can be expected for an excitation wavelength of about 462 nm; for 3 nm the maximum lies at about 440 nm. In Fig. 2, Raman spectra of the three samples, CdS5, CdS5n and CdS3n, are displayed. The spectral region of the Raman spectra shown contains the 1LO phonon band of
Figure 1. Calculation of the bandgap energy of CdS nanocrystals as a function of particle diameter using Eqn (1).
Relative Raman Intensity
CdS3n 3 nm, with spacer
CdS5n 5 nm, with spacer CdS5 5 nm, without spacer
CdS5 CdS5n
nD1
nD2
nD3
nD4
12.9 9.6
24.6 16.1
— 24.5
— 36.5
the samples. The excitation wavelengths were 457.9 nm for CdS3n and 488 nm for CdS and CdS5n. According to the results shown in Fig. 1, these excitations are at the edge of the absorption band. However, for CdS5 and CdS5n very intense signals including overtone progressions could be observed. Additionally, the half-widths of the Raman lines were found to be clearly smaller for the samples (CdS5n) (see Table 1) with spacer. The broadening of the half-widths compared with the bulk material could have two reasons: (1) the size distribution of the QD and the presence of defects, and (2) the contribution of phonon confinement. The effect of confinement is small; confinement would give a larger asymmetry of the bands. For CdS5n the overtones up to n D 4 were analyzed, whereas for CdS5 a progression up to n D 2 emerged, which hints at a weak contribution of defects. The corresponding Raman spectra are shown in Fig. 3. These results indicate a better crystal quality for QDs with spacer considering the half-widths and the overall intensity. Shiang et al. pointed out a connection between the half-widths of the overtone lines and the mechanism of the vibrational relaxation. 23 In accordance with this model a relaxation mechanism dominated by the decay of LO phonons into acoustic phonons can be concluded for our samples. A dephasing mechanism would result in a square dependence between half-widths and order of the phonon bands. Figure 4 shows Raman spectra obtained from the CdS3n sample for different excitation wavelengths. The LO band
5 nm with spacer
Relative Raman Intensity
E D
Table 1. Full widths at half-maximum (cm1 ) of the nLO phonon bands of CdS5 and CdS5n
230
630
1030
1430
5 nm with spacer
5 nm without spacer
250 270 290 310 330 350 370
230 280 330 380 430 480 530 580 630 680
Wavenumber / cm−1
Wavenumber/cm−1
Figure 2. Raman spectra of the CdS nanocrystals for excitation wavelengths of 488.0 nm (d D 5 nm) and 457.9 nm (d D 3 nm).
Copyright 2003 John Wiley & Sons, Ltd.
Figure 3. Raman spectra of CdS5 and CdS5n for an excitation wavelength of 488.0 nm. The inset shows the overtone progression of CdS5n.
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were synthesized with and without organic spacer groups. From a simple model calculation, the size dependence of the bandgap energy was determined. Using this information, Raman spectra of the different samples were taken using different excitation wavelengths. From these spectra, we conclude that the QDs have a fairly good crystal quality, which becomes worse with the preparation of the crystallites without spacer. An enhanced surface contribution for smaller QDs results in broader LO phonon lines for the 3 nm sample. No red shift of the phonons due to a phonon confinement could be observed. From the linewidths of the observed overtones, a statement can be made about the mechanism of vibrational relaxation. A domination of the decay of the LO phonons into acoustic phonons can be concluded. Figure 4. Raman spectra of CdS3n with different excitation wavelengths.
appears only as weak signal and is considerably broader than the signal found for the 5nm QDs. The latter may point to a worse crystal quality (defects) compared with CdS5 and CdS5n, but most likely is due to the larger contribution of the surface. Another reason for the line broadening could be the simultaneous contribution of QDs with different sizes. The signal intensity increases for higher excitation energies and an overtone progression can be detected. Additionally, a shift of the LO phonon by 4 cm1 to lower wavenumbers can be observed on going from 457.9 to 434 nm. No further shift is observed for the excitation wavelengths 431.8 and 428.7 nm. Therefore, this shift cannot be assigned to phonon confinement. Similar observations have been reported by other groups.15,24 There, a change of the size distribution of the crystallites was assumed as a reason for the phonon shifts; the different excitation wavelengths selectively enhance contributions from QDs with certain diameters. The phonon confinement only contributes to a slightly asymmetric lineshape, which results from the relaxation of the k D 0 conservation momentum rule. According to Eqn (1), the 457.9 nm wavelength should enhance the contribution of crystallites with a diameter of 3.8 nm, whereas the wavelengths 428.7, 431.8 and 434.3 nm should selectively enhance the contributions of crystallites with diameters in the 2–2.2 nm range, The downshift could be due to this size decrease. There are no systematic changes when the excitation wavelength is changed from 434.3 to 428.7 nm. However, the bandgap change is small (about 40 meV) and the line broadening is large.
CONCLUSION We have presented and discussed results obtained from resonance Raman spectroscopic investigations of CdS nanoparticles. The QDs considered had diameters of 3 and 5 nm and
Copyright 2003 John Wiley & Sons, Ltd.
Acknowledgements We gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 410, Teilprojekte C3, C5) and the Fonds der Chemischen Industrie.
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