J. Phys. Chem. C 2008, 112, 1865-1870
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Orientation-Dependent Raman Spectroscopy of Single Wurtzite CdS Nanowires H. M. Fan,*,†,‡ X. F. Fan,† Z. H. Ni,†,‡ Z. X. Shen,*,† Y. P. Feng,‡ and B. S. Zou§ DiVision of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological UniVersity, Singapore 637371, Department of Physics, 2 Science DriVe 3, National UniVersity of Singapore, Singapore 117542, and Micro-Nano Technologies Research Center, Hunan UniVersity, Changsha, China 410082 ReceiVed: October 3, 2007; In Final Form: NoVember 3, 2007
Orientation-dependent Raman measurements on single wurtzite CdS nanowires with an average diameter of 60 nm have been performed. The resonance Raman spectra of nanowires are compared with those of bulk ribbon. Different from the bulk sample, the intensity of most Raman bands of the nanowires are polarized along the longtitudinal axis because of the shape-induced depolarization effect, which is in agreement with the predication based on a simple dielectric contrast model and experimental results of polarized photoluminescence. The detailed angular dependencies of various Raman bands of the nanowires show that LO features and their overtones are affected strongly by the anisotropic geometry of the nanowires while the EH2 band shows a distinctive angular dependence, which reflects the interplay of crystal symmetry, electronic resonance, and shape effect in relative Raman intensity. These results indicate that the anisotropic shape of CdS nanowires is a dominant factor in determining their optical properties.
Introduction One-dimensional nanostructures such as nanowires and nanotubes have attracted much attention because their anisotropic geometries result in unique physical properties and bring great potential applications in nanolasers,1 thermal conductors,2 superconducting quantum interference,3 photonics integration,4 and magnetic domain-wall logic.5 In particular, the effect due to their high aspect ratios would result in significant changes in the optical properties of isolated semiconductor nanowires compared with that of corresponding nanocrystals and bulk materials.6 Raman spectroscopy has been extended to measure the phonon scattering of a single semiconductor nanowire, and it has become the most promising technique for the characterization of crystal structure, phonon behaviors, electronic states, and composition of nanowires.7,8 Orientation-dependent Raman spectroscopy, in particular, is of great interest for the investigation of the fundamental physical process of single nanowires and nanotubes. Duesberg et al.9 have performed orientationdependent Raman measurements of isolated carbon nanotubes. The results indicate that the shape effect can overwhelm the selection rules. And hence angular dependencies of all bands follow a cos2(θ) law because of the highly anisotropic shape of the nanowires and the large dielectric contrast between the nanowire and surroundings. A similar strong polarization was also observed for all Raman bands of a single monoclinic CuO nanowire (∼100 nm).10 However, polarization of Raman bands should depend on the interplay and competition among crystal symmetry, electronic resonance effect, and shape effect; they indeed conspire it. Recent work on polarization anisotropy in Raman scattering of * Corresponding authors. E-mail: H. M. Fan
[email protected]; Z. X. Shen
[email protected]. † Nanyang Technological University. ‡ National University of Singapore. § Hunan University.
cubic-SiC (cubic) single nanowires by Fre´chette et al.11 indicated that polarization anisotropy of the SiC nanowire is enhanced significantly by the crystal structure. Such interplay between crystal symmetry and shape effect also occurs in orientationdependent photoluminescence (PL) of the nanowires.12,13 For a single-crystal semiconductor nanowire with a defined growth direction and a fixed polarization plane of incident light, both angular-dependent (due to the shape effect) and angularindependent Raman bands can be expected. Livneh et al.14 have reported detailed experimental and theoretical Raman investigations of single wurtzite GaN nanowires (∼150 nm in diameter) grown in the b a and b c directions, respectively, where b a and b c are the lattice vectors of wurtzite GaN. Unfortunately, the diameters of their samples are too large to consider the shape effect. Therefore, a Raman spectroscopy investigation on single semiconductor nanowire with diameters sufficiently below the wavelength of incident light is necessary to gain insight into the fundamental physical processes of how the highly anisotropic shape affects the angular dependencies of various Raman bands, as well as applied interests alike. As a typical wurtzite structure semiconductor, CdS nanowires can be used as photonic circuit elements15 and electrically driven nanoscale lasers.16 Recent progress in the study of the stimulated emission of a single CdS nanowire17 and nanoribbon18 revealed phonon-assisted lasing at room temperature. In this content, it is of interest to explore the phonon behaviors in a single CdS nanowire and to correlate them with the structures and optical properties of the nanowires. Furthermore, single-crystal nanowires in the wurtzite structure exhibit various symmetrydependent bands with different angular dependencies. The existence of a large number of wurtzite semiconductor nanowires and a group of very closely related materials in the zinc-blende structure makes orientation-dependent Raman investigation of such system very attractive. To date, orientation-dependent Raman spectroscopy has not been carried out for a single CdS nanowire. In this work, we report an orientation-dependent
10.1021/jp7096839 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/17/2008
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Fan et al.
Raman spectroscopy study on spatially separated single CdS nanowires with diameters down to 60 nm at room temperature. The angular dependencies of various bands are derived from the polarized resonance Raman spectra. The experimental data of the nanowires are compared with that of bulk ribbon as well as prediction based on a simple dielectric contrast model.
spread of ∼10%. X-ray diffraction patterns (XRD) and transmission electron microscopy (TEM) images show that the single-crystal CdS nanowires adopted the hexagonal wurtzite structure and is grown along the [001] direction (c b axis) as shown in Figure 1b and d. For comparison, similar measurements were carried out on an individual wurtzite CdS ribbon with a rectangular cross section of around 1.2 µm × 0.6 µm and a length of more than 100 µm, placed on a piece of silicon substrate. The ribbon has a growth direction along [001], with (21h0) and (010) side surfaces, which were confirmed by TEM as shown in Figure 1c. The absorption spectra of our sample at room temperature were obtained by using a UV-visible spectrophotometer (Shimadzu, UV-1700), with the sample dispersed in ethanol solution. Polarized and orientation-dependent Raman measurements were conducted with a confocal microscope (Renishaw InVia System) in conjunction with a high-resolution piezoelectric stage. A 100 × (0.90 NA) microscope objective was used to focus the laser beam and collect the scattered light. The excitation line is 532 nm with a power density of ∼105 W/cm2 and a spot size of ∼ 0.7 µm in a typical experiment. To perform the orientation-dependent Raman measurement, we inserted a half-wave retardation plate before the microscope objective lens. The twofold rotation of the polarization is equivalent to an effective rotation of the nanowire on the substrate. The nanowire and ribbon were aligned so that the b c axis of the nanowire matched the z direction, where the propagation direction of the incident laser is assigned as the y direction in our backscattering experimental geometry as shown in Figure 2c. The Raman spectra with a 514 nm laser line were carried out using JobinYvon T64000 Raman system. The Raman images were obtained by using a WITEC CRM200 Raman system.
Experimental Section
Results and Discussion
The CdS nanowires were synthesized by a simple hydrothermal method19 and dispersed onto Ti-coated Si (Si/Ti) substrates for Raman measurements. The scanning electron microscope (SEM) image shown in Figure 1a reveals that the CdS nanowires have typical lengths in the range of several to several tens of micrometers and average diameters of around 60 nm with a
The hexagonal wurtzite structured CdS nanowires belong to 4 the space group C6V . According to factor group analysis,20 the Raman active modes are 1A1 + 1E1 + 2E2 (EH2 and EL2 ), while 2B2 modes are silent. For the A1 branch the phonon polarization is in the z direction, whereas for the doubly degenerate E1 and E2 branches the phonon polarizations are in the xy plane.
Figure 1. Morphology and structure of single-crystal CdS nanowires. (a) SEM image of CdS nanowires. (b) TEM image of a CdS nanowire. The inset shows the SAED pattern. (c) TEM image of CdS ribbon. The insets are the SAED pattern and scheme of the ribbon. (d) XRD patterns of CdS nanowires.
Figure 2. (a) Polarized Raman spectra of a single nanowire and bulk ribbon in Y(ZZ)Y h and Y(XX)Y h configurations at room temperature. The inset is the optical image of the measured CdS nanowire, the scale bar is 5 µm. (b) The absorption of CdS nanowires and bulk ribbons. The position of 532 nm excitation energy for the resonance Raman measurements is marked by an arrow. (c) The schematic setup of the orientation-dependent Raman measurements. e´ denotes incident light, and the arrow denotes the polarization direction of the light.
Orientation-Dependent Raman Spectroscopy
Figure 3. Polarized Raman spectra of single CdS nanowires with different excitation laser energies.
Because the wurtzite structure is noncentrosymmetric, both A1 and E1 modes split into longitudinal optical (LO) and transverse optical (TO) components. Raman spectra under the polarization configurationsY(ZZ)Y h , Y(XX)Y h are shown in Figure 2a. Three first-order Raman peaks are observed. The weak peaks at 236 and 255 cm-1 are assigned to A1 (TO) and EH2 respectively, whereas the peaks at 303 and 604 cm-1 are assigned to LO and 2LO phonon. EL2 is absent in our experiment. The peaks at 214, 326, 350, and 371 are assigned to multiphonon scattering, which has been reported in bulk CdS.21 The peak at 521 cm-1 arises from the Si substrate. For both the nanowire and bulk ribbon, the appearance of the LO phonon scattering peak near 303 cm-1 suggests the breaking of selection rules because of electronic resonance; and they do not reflect pure A1 or E1 modes. Because the electrostatic forces dominate over the anisotropy forces in the Raman effects of ionic crystal CdS,22 the frequency of A1 (LO) is very close to that of E1 (LO). As a result, no frequency shift of the LO peak was observed in our two configurations, which agrees well with results of Routkevitch et al.23 The resonance Raman scattering, especially multiphonon Raman scattering of CdS, is complicated and sensitive to temperature, excitation energy, and surrounding media.24,25 Therefore, an appropriate excitation energy must be used in order to gain maximum spectroscopic information and significant resonance enhancement in the Raman study of a single nanowire.26 The absorption spectra of the CdS nanowires are shown in Figure 2b where the excitation energy for Raman measurements is indicated by the arrow. The absorption peak
J. Phys. Chem. C, Vol. 112, No. 6, 2008 1867 of the CdS nanowires is located at about 2.50 eV, which shows a slight blueshift compared with that of the bulk ribbons (∼2.46 eV). This slight blueshift could not be attributed to the quantum confinement effect because the average diameter of the nanowires is much larger than its exciton Bohr radius (2.16 nm); the possibility of the dielectric confinement effect or surface dipole-dipole interaction may be responsible for this band gap reconstruction. However, such a small shift in band gap would give rise to a large change in the Raman intensity of the resonance Raman spectroscopy. For example, Raman peaks were clearly observable with an excitation light of 514 nm for the CdS nanowires. Under the same conditions, the intensities of all Raman peaks weakened and a strong PL background appeared for bulk ribbon.26 Several multiphonon features are observed in our measurements because the resonance excitation energy used (532 nm laser) is lower than that of the absorption band. Figure 3 shows the polarized Raman spectra of the nanowire excited by different laser energies. Clearly, the multiphonon band at 214 cm-1 is invisible under a 514 nm excitation laser. Orientation-dependent polarized Raman spectroscopy of a single isolated CdS nanowire was performed to understand the shape effect. As shown in Figure 4a, the intensities of all Raman bands, except that of EH2 , of the nanowire show a minimum when the polarization of the incident light is perpendicular to the nanowire axis. This is in clear contrast to that of the CdS bulk ribbon shown in Figure 4b. Because the intensity of spontaneous Raman scattering is proportional to the square of the local internal field,8 the weak Raman intensity is attributed to the reduced local internal field inside the nanowire when the incident field is perpendicular to axis of the nanowire. This shape-induced reduction of local internal field (depolarization effect) can be explained by applying a simple dielectric contrast model based on classical electromagnetic theory to an infinite dielectric cylinder. It can be shown easily that the local internal field is hardly affected when the polarization of the incident field is parallel to the axis of the cylinder. However, if the polarization of the incident filed is perpendicular to the axis of the cylinder, then the local internal field, E⊥, is attenuated and is given by E⊥ ) (20)/( + 0)Ee,6 where Ee is the electric field of the incident light, and (0) is the dielectric constant of the cylinder (vacuum). In contrast, the Raman peak of EH2 becomes stronger when the polarization of the incident light is perpendicular to the nanowire axis (see Figure 2), implying that its polarization anisotropy was not affected by the shape effect. This depolarization effect has been further confirmed by polarized photoluminescence (PL) spectra of single CdS nano-
Figure 4. Orientation-dependent resonance Raman spectroscopy of (a) a single nanowire and (b) a bulk ribbon excited by a 532 nm laser. The 521 cm-1 peaks in part a correspond to the Si substrate. θ denotes the angle between the long axis of the nanowire (also the b c axis) and the laser polarization.
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Figure 5. Polarized PL spectra of (a) single CdS nanowire and (b) bulk ribbon excited by a 325 nm laser. (c) The dielectric contrast model of polarization anisotropy of the CdS nanowire.
wires. Figure 5 shows the PL spectra of a single nanowire and the ribbon at room temperature with the polarization of the excitation laser parallel and perpendicular to their the longitudinal axes. The polarized PL spectra of the nanowire show giant polarization anisotropy, and the intensities of the PL bands show a minimum when the excitation laser is perpendicular to the long axis of the wire, which is opposite to that of the ribbon. A similar result has been reported in the CdSe nanowire.13 Notice that the polarized PL of bulk ribbon is caused by select rules in the wurtzite structure.27 The measured polarization ratio F ) (I|| - I⊥)/(I|| + I⊥), with I|| and I⊥ being the intensity of PL peak in the directions parallel and perpendicular to the nanowire axis, respectively, is ∼0.80. On the basis of the above model, we calculate the internal electric field of the CdS nanowire for parallel and perpendicular polarization incident fields using the dielectric constant (5.3) of bulk CdS as shown in Figure 5c. The calculation polarization ratio is 0.82, which is in agreement with our experimental value. Figure 6 shows the angular dependencies of Raman scattering intensities of various bands observed. The circles represent data obtained from the bulk ribbon, and triangles denote those from the nanowire. According to the dielectric contrast model, the angular dependence of the Raman bands due to the shape effect can be described simply by I (θ) ∝ cos2 (θ) Ee2, where θ is the angle between the polarization vector and long axis of the nanowire. We have fitted the above expression to the measured Raman intensities of the nanowires, and the results are shown using solid lines in Figure 4a-c. As expected, the angular
Fan et al. dependencies of LO, 2LO, and the multiphonon band (214 cm-1) of the nanowire are consistent with the theoretical curve, indicating that the shape effect is the dominant factor of these Raman bands. These results of the nanowire have shown significantly different polarization behaviors from the corresponding bulk ribbon. Theoretical calculations of the relative Raman intensity of the LO peak and its overtone for bulk ribbon are very difficult. Though the LO mode can be comparable in intensity to the corresponding TO mode, their angular dependencies differ greatly because of the presence of strong electronic resonance. Our experimental angular dependence of LO for bulk ribbon is basically identical to that of A1(LO) in GaN nanowires.14 The multiphonon mode at 214 cm-1 shows a similar angular dependence as the LO mode for both nanowire and bulk ribbon, implying that the symmetry of the multiphonon mode is similar to that of the LO mode. This observation supports the assignment of Tell et al.28 that the multiphonon mode is predominantly of A1 symmetry, but in disagreement with the E2 symmetry assigned by Poulet and Mathieu.29 The angular dependence of EH2 (255 cm-1) is different from that of LO and the multiphonon as shown in Figure 4d. The theoretical angular dependence of the EH2 can be obtained easily from crystal symmetry, I ∝ sin4(θ)Ee2. Hence, the intensity-angle curve of the nanowire should exhibit independence from the shape effect. As can be seen in Figure 4d, the angular dependence for the ribbon (circle) where b a is parallel to the Y direction agrees well with the theoretical prediction (solid line). However, it deviates somewhat from the theoretical curve to experimental data for the nanowire. The intensity is unexpectedly high when θ is 50° and 130°, and very low intensities are observed near 90°. Furthermore, the width and frequency of the peak also vary with the angle θ. All of this indicates that the peak may not be a pure EH2 band. The possible presence of zinc-blende content or a* directed nanowire could be excluded based on TEM and XRD results. The background noise that arose from the substrate is also negligible because the measured Raman background of the substrate is very smooth. One possible reason for this anomalous broadness and enhancement of the EH2 band at 50° and 130° is the presence of a quasiphonon formed by the mixing of A1 (236 cm-1) and E1 bands (∼241 cm-1, observed in the XZ configuration).23 The quasiphonon appears because of the deviation of the crystal b a axis from the normal of the substrate and may result in a weak signal in the spectroscopic region. If not treated carefully, then it will lead to an error in curve fitting, that is, fitting the spectroscopic peak to a single Lorentzian function. Although we have observed the quasiphonon peak near 242 cm-1 for bulk ribbon by varying the angle R, the peak near 255 cm-1 of the nanowire is too weak to distinguish those two modes (quasiphonon and EH2 ). Nonetheless, this band shows an explicit angular dependence that is significantly different from the polarized LO and multiphonon bands because of the components of the polarizability tensor for the E2 mode vanish along the axis of the wire. Because the intensity of the Raman peak depends strongly on the orientation of the nanowires, here we demonstrate interesting polarization-dependent Raman images. Figure 7a shows an optical image of the CdS nanowires. Two nanowires are presented, and their orientations are almost vertical to each other. Raman images were obtained using the intensity of the 2LO bands because it can be recorded easily in a fast scanning process due to the strong Raman scattering signal. Figure 7b and c shows the Raman images of the vertical and horizontal laser polarization. According to our above experimental result,
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Figure 6. Angular dependencies of the normalized intensities of various Raman bands of a CdS nanowire (triangle) and ribbon (circle) as a function of the angle θ. The corresponding peaks are (a) 303 cm-1, (b) 604 cm-1, (c) 214 cm-1, and (d) 255 cm-1. The solid lines in a-c (best fit to the form of I(θ) ) c1*cos2(θ) + c2) show that the polarization Raman follow the angular dependence predicted based on the shape effect. The solid line in part d is obtained by fitting data of the ribbon by the form I ∝ sin4(θ) according to the prediction of crystal symmetry.
Figure 7. Polarization-dependent Raman images of CdS nanowires. (a) The optical image of CdS nanowires. (b and c) Vertical and horizontal polarization Raman scanning images.
1870 J. Phys. Chem. C, Vol. 112, No. 6, 2008 the nanowire whose orientation is perpendicular to the polarization of the excitation laser nearly disappears in the Raman images, whereas the nanowire whose orientation is parallel to the polarization of the excitation laser can be observed clearly. Conclusions The orientation-dependent Raman measurements of individual wurtzite CdS nanowires with an average diameter of about 60 nm have been performed at room temperature. By comparing with that of the CdS bulk ribbon and prediction based on a theoretical model, the orientation-dependent resonance Raman spectra of the nanowire reveal that the angular dependencies of LO, 2LO, and multiphonon (214 cm-1) bands are affected strongly by a highly anisotropic shape, whereas that of the EH2 band is independent because of the crystal symmetry of the nanowire. The opposite Raman polarization of these bands observed in a single CdS nanowire may find potential applications in nanophotonics. The polarization-dependent Raman images are also obtained; this observation is consistent with our analysis of the orientation dependence of single CdS nanowire. These conclusions are valid as long as the diameter of the nanowires is sufficiently below the wavelength of incident light. Despite the crystal symmetry, the resonance effect and shape effect will contribute to the Raman intensity of the CdS nanowire; the experimental data manifest that the shape effect plays a dominant role in determining the optical properties of single nanowires. These results may be useful in understanding Raman spectroscopy and images of single semiconductor nanowires with wurtzite structures, such as ZnO, CdSe, and so forth. Acknowledgment. The project was partly supported by the Ministry of Education (MOE) Academic Research Fund under Grants R-144-000-209-112 and R-144-000-121-112. References and Notes (1) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897.
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