Charge-Sensitive Surface Optical Phonon in CdS Quantum Dots

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Charge-Sensitive Surface Optical Phonon in CdS Quantum Dots Studied by Resonant Raman Spectroscopy Yongkuan Wu, Shaoqing Jin, Yun Ye, Shengyang Wang, Zhaochi Feng,* and Can Li* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, P.O. Box 110, Dalian 116023, China S Supporting Information *

ABSTRACT: The probe of surface properties of quantum dots (QDs) is of great importance as the surface properties of QDs greatly influence their performance in photoelectronic devices and photocatalysis. In this work, resonant Raman spectroscopy was used to study the surface optical (SO) phonon of CdS QDs. It was found that the intensity of SO phonon peak of CdS QDs increased as the size of the QDs decreased, but the dielectric constant of the surrounding media, which showed striking impacts on SO phonon peak in previous publications, has no visible influence in our experiments. Meanwhile, the intensity of SO phonon peak changes obviously when the surface ligands of CdS QDs or the excitation power/wavelength is changed. Besides, the loading of platinum catalyst seriously weakens the intensity of SO phonon peak. These experimental results can be interpreted with a charge-sensitive surface optical phonon model.

reaction method reported elsewhere.25−28 After washing, the CdS and CdS/ZnS QDs were dispersed into n-hexane directly or dispersed into water through a phase transfer process, and its surface was passivated with different ligands (thioglycolic acid (TGA), mercaptopropionic acid (MPA), and mercaptohexanoic acid (MHA), respectively).27−29 if not specified, the CdS QDs used in the experiments were dispersed in water and with TGA as surface ligand. The UV−vis absorption spectra were recorded on a Cary 5000 spectrometer (Varian Inc.). Resonance Raman spectra were recorded on a home assembled UV Raman spectrograph with spectral resolution of 2 cm−1. A continuously tunable laser between 388 and 464 nm was used as an exciting source for the resonance Raman spectroscopy. The platinum catalyst was loaded as follows: chloroplatinic acid aqueous solution was added into the CdS QDs solution, and after tens of minutes of UV laser irradiation, platinum catalyst formed on the surface of CdS QDs through photoreduction.28,30

1. INTRODUCTION As the size of semiconductor materials is on the order of their exciton Bohr radius, their optical and electronic properties are quite different from those of the bulk due to the quantum confinement effect and surface effect, and these small nanoparticles are termed quantum dots (QDs).1,2 As the size of QDs decreases, the surface/volume ratio increases rapidly, and the surface effect becomes more pronounced.3,4 Thus, the probe of surface properties of QDs is of great importance. Surface-related vibration mode (surface optical (SO) phonon), which can be detected by Raman spectroscopy, is sensitive to surface properties and offers a direct way to probe the surface properties. Previous publications focused on the influence of the size of nanoparticles and the dielectric constant of surrounding media on the SO phonon peak.5−23 As we know, except for the size of QDs and the dielectric constant of surrounding media, the actual situation in QDs is complicated, including the charge distribution, the carrier dynamics, the surface defects, ligands, etc. Moreover, to the best of our knowledge, the influence of charge distribution, carrier dynamics, and surface properties of QDs on the SO phonon has been scarcely reported. In this paper, CdS QDs were chosen as model material, and the influences of size, surface, excitation power, excitation wavelength, and loading of platinum catalyst on SO phonon were investigated by resonant Raman spectroscopy; a chargesensitive surface optical phonon model was proposed.

3. RESULTS AND DISCUSSION Figure 1a shows the UV−vis absorption spectra of the CdS QDs, where the shift of the first absorption band indicates that the size of CdS QDs is in the quantum confinement range. Estimated from the published empirical equation,31 the diameters of the four CdS QDs samples are 3.2, 4.0, 5.2, and 6.3 nm, respectively. For convenience, we used the estimated diameter of CdS QDs to label the four samples. Three distinct

2. EXPERIMENTAL SECTION The CdS QDs were prepared according to the procedure reported by Yu et al.24 The CdS/ZnS core/shell QDs were prepared according to the typical ionic layer adsorption and © XXXX American Chemical Society

Received: September 29, 2014 Revised: November 15, 2014

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features can be observed in the Raman spectra of these four samples: the first-order longitudinal optical (1LO) phonon peak at ∼300 cm−1, the second-order longitudinal optical (2LO) phonon peak at ∼600 cm−1, and the broad shoulder peak below 1LO phonon peak (Figure 1b). According to the literature,5−14,32−40 the broad shoulder peak below 1LO phonon peak is attributed to SO phonon. In Figure 1b, we can see that the position of SO phonon peak shifts as the size of CdS QDs is decreased, and the cause has been discussed in the literature.9,34 Besides, the intensity of SO phonon peak increases as the size of CdS QDs is decreased. Previous research found that the growth of protective shell on the core of QDs is an effective solution to reduce the surface defects and enhance the photoluminescence efficiency.28,41 Here, ZnS molecular layers were added onto the surface of CdS QDs as a protective shell, and the influence of core/shell structure on the SO phonon was detected. After four monolayers of ZnS were added onto the surface of CdS QDs, the red shift of the first absorption band is obvious (Figure 2a), and this indicates that the quantum confinement effect weakens after ZnS shell is added. From the Raman spectra (Figure 2b), a 9 cm−1 shift for the 1LO phonon peak is observed after ZnS shell is added, which originates from the difference of lattice constant for CdS and ZnS4, whereas the variation of SO phonon peak is small when ZnS shell is present. It was reported that the dielectric constant of surrounding media has significance influence on the SO phonon peak,5,8,10−14 so we started out considering the weak dependence of SO phonon peak on the ZnS shell may be due to the small difference of the relative dielectric constant for bulk CdS and ZnS (CdS: 11.6, ZnS: 8.2). But the following experiments seem not to support this explanation. When we changed the surrounding media from water to methanol, ethanol, and n-

Figure 1. UV−vis absorption spectra (a) and Raman spectra (b) of CdS QDs with different sizes. All the Raman spectra were excited by 392 nm laser. The intensity of 1LO phonon peak was normalized.

Figure 2. Influence of ZnS shell and surface ligands on the UV−vis absorption spectra (a, c) and Raman spectra (b, d) of CdS QDs. The Raman spectra of (b) and (d) were excited by 390 and 398 nm wavelength laser light, respectively, and the intensity of 1LO phonon peak was normalized. B

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Figure 3. Raman spectra of CdS QDs with different excitation power (a) (excited by 410 nm laser) and different wavelength (b). The intensity of 1LO phonon peak was normalized.

trapped carriers may play a role in breaking the momentum conservation constraints or enhance the allowable SO phonon scattering through the surface potential. Thus, at higher excitation power, more surface-trapped carriers result in the stronger SO phonon peak, indicating that the SO phonon peak of CdS QDs is charge-sensitive. To further understand this phenomenon, the influence of excitation wavelength on the intensity of SO phonon peak of CdS QDs was investigated. From Figure 3b, we can see that the intensity of SO phonon peak varies when the excitation wavelength is changed; that is, the intensity of SO phonon peak is excitation-wavelength dependent. A similar result has been reported recently.43 As we know, the excitation wavelength can influence the carrier dynamics of QDs.44,51,53 At different excitation wavelength, the energy of photogenerated carriers is different, and this may lead to the fact that the photogenerated carriers of QDs may relax through different paths. Thus, at different excitation wavelengths, the number of surface-trapped carriers may be different. As discussed above, the surfacetrapped carriers can influence the observable SO phonon scattering of CdS QDs; thus, the observed phenomenon that the intensity of SO phonon peak is excitation-wavelength dependent can be interpreted. Among the three data points shown here, we can see that the intensity of SO phonon peak is strongest at 428 nm excitation and weakest at 464 nm excitation. According to all the recorded data (not shown here), the profile of the intensity of SO phonon peak is an analogous parabola in our excitation wavelength range (388−464 nm). So for CdS QDs, there is an optimal excitation wavelength under which the amount of surface-trapped carriers is maximal and the intensity of SO phonon peak is strongest. Figure 4 shows that the intensity of SO phonon peak of CdS QDs decreases distinctly after platinum catalyst is loaded on the surface of CdS QDs. Our previous research demonstrated that the photogenerated electrons could be transferred from CdS QDs to platinum catalyst due to the higher Fermi level of CdS.28,30 Thus, the loading of platinum catalyst can decrease the surface-trapped carriers (electrons) of CdS QDs, which then brings about the decrease of the intensity of SO phonon peak. But there also is another interpretation for the decrease of the intensity of SO phonon peak. When metal platinum was loaded onto the surface of CdS QDs, the electric potential generated from SO phonon will be interrupted at the interface of platinum/CdS, and the SO phonon scattering may be suppressed.52 From our results shown above, we suggest that the amount of surface-trapped carriers should be taken into account in the size-dependence SO phonon peak intensity (shown in Figure

hexane, the relative dielectric constant for those media was ∼80.4, 32.7, 24.3, and 1.9, respectively; under such huge variation of dielectric constant of the surrounding media, no visible variation of the SO phonon peak of CdS QDs was observed (shown in Figure s1). Thus, for the CdS QDs, the dielectric constant of the surrounding media has less influence on the SO phonon peak, and the small variation of the SO phonon peak after ZnS shell is added may be due to the size effect or other factors. Surface ligand is another important factor that can influence the photoelectric property of QDs.40,42 It was reported that the SO phonon peak of QDs was not sensitive to ligand exchange.40,43 Here, we used TGA, MPA, and MHA as the surface ligands of CdS QDs, and the main difference among the three kinds of surface ligands is the length of molecular chain. From Figure 2c, we can see a small red shift of the first absorption band when the length of molecular chain is increased. However, Raman spectra (Figure 2d) show that the intensity of SO phonon peak increases obviously when the length of molecular chain is increased and demonstrate that the SO phonon peak is sensitive to ligand exchange. Until now, we can find that the surface ligand of CdS QDs rather than the dielectric constant of surrounding media has important influence on the SO phonon. We also examined whether the properties of excitation light have influence on the SO phonon peak of CdS QDs. From Figure 3a, we can see that the intensity of SO phonon peak is power-dependent; as the excitation power is increased from 4 to 28 to 240 mW, the intensity of SO phonon peak increases obviously. This experimental phenomenon is reversible, and thus we can exclude the influence of laser-induced structural change. The power-dependence of SO phonon peak intensity may give us new insight into the properties of SO phonon. Time-resolved spectroscopy demonstrated that the relaxation dynamics of photogenerated carriers in QDs are powerdependent.11,44−51 Combined with our results, the SO phonon peak intensity of CdS QDs may be related to the carrier dynamics. At higher excitation power, more photogenerated carriers are generated, and more electrons or holes may be trapped within and at the surfaces of CdS QDs. Since the SO phonon is a surface-related vibration mode, we supposed that the surface-trapped carriers might play an important role in the Raman scattering of SO phonons. At a perfect surface, SO phonon scattering cannot be observed due to the momentum conservation constraints, but the existence of impurity or surface asymmetry may break the momentum conservation constraints and enable the observation of SO phonon scattering.12−14,21,52 In our case, we infer that the surfaceC

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ASSOCIATED CONTENT

S Supporting Information *

Raman spectra of CdS QDs obtained using different solvents. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 86-411-84379070. Fax: 86411-84694447. (C.L.) *E-mail: [email protected]. (Z.F.) Notes

The authors declare no competing financial interest.

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Figure 4. Raman spectra of CdS QDs before (black line) and after (red line) platinum catalyst was loaded. The Raman spectra were excited by 398 nm laser, and the intensity of 1LO phonon peak was normalized.

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1). As the size of QDs decreases, the surface/volume ratio increases rapidly, and the possibility for the carriers to be trapped at the surface is increased too. So the phenomenon that the intensity of SO phonon peak increases as the size of CdS QDs is decreased may be because more carriers are trapped by the surface for smaller CdS QDs. Figure 2 revealed that the surface properties (core/shell, ligands) of CdS QDs have certain influence on the intensity of SO phonon peak for the CdS QDs. As mentioned above, the surface of QDs can influence the charge distribution and the dynamics of photogenerated carriers. For the case of different surface ligands, it is considered that the longer molecular chain can obstruct the photogenerated carriers from diffusing into the surrounding media; thus, more carriers would accumulate at the surface of CdS QDs that gave rise to the increase of SO phonon peak intensity. According to above results and discussion, we proposed a charge-sensitive surface optical phonon model. In this model, the surface-trapped carriers play a big role in the Raman scattering of SO phonon. Since SO phonon is a surface-related vibration mode, the electric potential around the surface can affect the phonon scattering process, whereas the surfacetrapped carriers can modify the surface potential; thus, the fact that the intensity of SO phonon peak of CdS QDs has close relationship with the amount of surface-trapped carriers is reasonable. Although the microcosmic mechanism is not clear now, our study will be continued.

4. CONCLUSION The surface vibration mode (surface optical phonon) of CdS QDs was studied using resonance Raman spectroscopy. It was found that the intensity of SO phonon peak of CdS QDs increased as the size of the QDs decreased. Meanwhile, the intensity of SO phonon peak changed obviously when the surface ligands of CdS QDs or the excitation power/wavelength changed. Besides, the loading of platinum catalyst seriously weakened the intensity of SO phonon peak. These experimental results imply that the intensity of SO phonon peak of CdS QDs has close relationship with the surfacetrapped carriers and can be interpreted with a charge-sensitive surface optical phonon model. D

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