表面等离激元效应Semiconductor Phonon Modes

Published: March 04, 2011 r 2011 American Chemical Society 671 dx.doi.org/10.1021/jz2001562 | J. Phys. Chem. Lett. 2011, 2, 671–674 LETTER pubs.acs.org/JPCL Enhanced Raman Spectroscopy of Nanostructured Semiconductor Phonon Modes Stephen Ma,†,‡,§ Richard Livingstone,† Bing Zhao,‡ and John R. Lombardi*,† †Department of Chemistry and Department of Chemical Engineering, The City College of New York, New York, New York 10031, United States ‡State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, People’s Republic of China, College of Chemistry We report here on an observation of enhanced Ramanintensities of phonon modes of semiconductor nanostruc- tures induced by adsorption of molecular adducts. In some ways, this resembles surface-enhanced Raman spectroscopy (SERS), which is characterized by an increase of many orders of magni- tude in the Raman intensity for adsorbed species when compared to that expected from the same number of nonadsorbed mole- cules. Although SERS has been mostly restricted to the investiga- tion of molecules adsorbed on metallic surfaces, we have begun to examine the use of this effect to provide insight into the nature of interaction between semiconductor quantum dots (QDs) and species adsorbed on them. A recent discovery by one of our collaborators1 shows that surface-enhanced Raman scattering can directly probe the adsorption of molecules on InAs/GaAs QD nanostructures. We have since extended that result in this laboratory by observation of enhanced Raman intensity in several molecules on self-assembled CdSe/CdMgZnSe QDs.2,3 En- hancement factors on the order of at least 103 were observed in the former system, while in the latter, they were as high as 105. Surface enhancement has since been observed for molecules on semiconductor nanoparticles in colloidal suspensions such as CdS,4 ZnS,5 ZnO,6 CuO7 CdTe,8 TiO2, 9,10 and PbS.11 One advantage of studying semiconductor systems in comparison to metals is that there are numerous additional parameters that may be readily controlled, such as band gap, exciton Bohr radius, phonon coupling strength, barrier confinement, and surface morphology. By tailoring the specific properties of semiconduc- tor QDs, we expect to be able to determine the optimum conditions needed to obtain the largest enhancement factors. It is of considerable interest to examine Raman enhancement on semiconductors because it is apparent that the cause of the enhance- ment in semiconductors must be quite different than that in metals. In metal SERS, the location of the surface plasmon resonance is crucial to the effect because it often coincides with the optical laser frequency. For semiconductors, where plasmon resonances tend to be in the infrared, such resonances are much less likely to contribute to the enhancement. When the dimension of the semiconductor becomes comparable to the size of the exciton Bohr radius, the valence and conduction bands are narrowed in spherical QDs, resembling atomic levels. Exciton-like interband transitions between these levels are responsible for much of the spectroscopy, both absorption and emission in these systems. The resulting levels depend on the nature of the confinement of thenanoparticle and are therefore dependent on both the size and the nature of the surrounding media. If the surrounding material is an adsorbed molecule, charge-transfer transitions from filled molecular orbitals to the conduction band or from the valence band to emptymolecular orbitalsmay result. It is the interband transitions in the semiconductor as well as the molecule- QD charge-transfer transitions that are implicated in the surface- induced enhancement of the Raman signal in semiconductors.3 Until now, all of the attention has been focused on the enhancement of the Raman intensities of the adsorbate. How- ever, in the course of this work, we have observed that the phonon bands of several of the semiconductor QDs are also enhanced. We report here on some of these observations, namely, both nanostructured and QDs for TiO2 and QDs for ZnO and PbS. In each of these cases, we observe enhancement of the intensity of optical phonon modes when compared with the bare semiconductor system under the same conditions. To our knowledge, there has only been one similar report on CdTe nanocrystals12 in which pyridine, used as an adsorbate, enhanced the Raman signal of the LO phonon modes. We have noted enhancement of one of these modes previously in CdTe in ref 8. In all of the following, spectra were taken at several points in the sample and also from several different samples to ensure reproducibility and uniformity of results. Received: February 1, 2011 Accepted: February 28, 2011 ABSTRACT: We report the observation of enhanced Raman intensity of the phonon modes of nanosized semiconductor particles induced by adsorption of various molecules. This is in contra- distinction to surface-enhanced Raman spectroscopy (SERS), in which the Raman lines of an adsorbate are enhanced by proximity to either a metal or semiconductor nanoparticle. We report on enhancements of phononmodes in nanostructured and quantum dots of TiO2, as well as quantum dots of ZnO and PbS. Because plasmon resonances in semiconductor systems are far in the IR, it is likely that some combination of interband and charge-transfer resonances is responsible for the observed enhancement. SECTION: Surfaces, Interfaces, Catalysis 672 dx.doi.org/10.1021/jz2001562 |J. Phys. Chem. Lett. 2011, 2, 671–674 The Journal of Physical Chemistry Letters LETTER In Figure 1, we show SEM images of nanostructured TiO2 produced by electrochemical anodization of a Ti foil in a solution of ethylene glycol and NH4F following the procedure of Meng et al.13 In Figure 1a, it can be seen that numerous cavities have been produced on the order of several hundred nanometers, and in Figure 1b, after annealing at 450 �C for 3 h, pore sizes on the order of 50 nm have been produced in fairly regular arrays. In Figure 2, we show the region of the phonon bands (<1000 cm-1) of the annealed nanostructured TiO2 with various adsorbed molecules (p-aminothiophenol (PATP), 4-mercapto- pyridine (4-MPy), and 4-mercaptobenzoic acid (4-MBA)) com- pared with bare TiO2. The excitation wavelength was 514.5 nm. The longitudinal optical (LO) phonon modes of several symme- tries (E1, A1g, and B1g) characteristic of the anatase phase are shown, and it can be seen that their intensities are considerably enhanced in the presence of an adsorbate. No spectra were observed for the unannealed samples. We may estimate the enhancement factor, at least for the Eg(1) line, by noting that the intensity of that line in the bare TiO2 spectrum is weak but measurable. We can then take the ratio of intensities of the lines with molecules adsorbed to that of the bare TiO2 spectrum as the enhancement factor. This is shown in Table 1, along with the electron affinities (EA) of the adsorbates.14 Enhancement factors of 22-45 were obtained. It can be seen that the enhancement factor increases with decreas- ing electron affinity, and this is taken to be indicative of charge- transfer resonance contribution to the enhancement. In separate experiments, we have also observed a similar effect in TiO2 QDs of size 7.2 nm 15 with a dye molecule (N-719 Dye: CAS #: 207347-46-4; (Bu4N)2-[Ru(dcbpyH)2(NCS)2]), similar to a system used for solar cell research. See ref 15 for details of sample preparation. By careful sonication and washing, we removed excess dye from the sample and believe that the coverage is a monolayer or less. In this case, we utilized additives of t-butylpyridine and Liþ, which have been shown16 to increase charge transfer from the dye to the conduction band of the TiO2. In this experiment, the intensity observed under the same conditions without the additives in the phonon region was negligible. Also, without the dye but only Liþ or t-butylpyridine, no measurable enhancement was observed. In Figure 3, we display the effect on the phonon modes of TiO2 QDs that have been coated with the dye as a sensitizer. The excitation wave- length was 514.5 nm. This dye allows resonant absorption of visible light and photoinduced charge transfer of an electron from the dye to the conduction band of the TiO2 QD. Increasing the concentration of t-butylpyridine and Liþ has the effect of increasing the intensity of the phonon modes in a manner similar to the experiments shown in Figure 2. Both experiments separately indicate that charge transfer is a very likely cause of the enhancement observed. In Figure 4, we show the Raman intensities of the phonon modes of ZnO QDs of diameter 27.7 nm, with 4-MPy adsorbed. See ref 6 for details of sample preparation and extinction spectra. Various modes of symmetry A1 and E2 are clearly seen to be enhanced. The assignments are taken from Damen et al.17 and recent work by Cusco et al.18 In this figure, we show the Figure 1. SEM images of anodized TiO2 before annealing (left) and after annealing (right); both at 30 000� magnification. Figure 2. Raman intensities in nanostructured TiO2 phonon modes, enhanced by various adsorbates. Excitation is at 514.5 nm. Table 1. Intensities (in arbitrary units) of the Eg(1) Phonon Line of TiO2 with Various Adsorbates a adsorbate intensity Eg(1) EF EA (eV) none 3632 4-MBA 81 540 22 2.79 PATP 128 800 35 1.87 4-MPy 162 200 45 1.46 a See Figure 2. In column 3, we display the enhancement factor (EF), and in the last column, we give the electron affinity (EA) of the adsorbate. Figure 3. Raman intensities of phonon modes of TiO2 nanoparticles with black dye (N-719 Dye: CAS #: 207347-46-4; (Bu4N)2-[Ru- (dcbpyH)2(NCS)2]) adsorbed. Addition of t-butylpyridine and Li þ increases the intensities of the phonon modes. Excitation is at 514.5 nm. 673 dx.doi.org/10.1021/jz2001562 |J. Phys. Chem. Lett. 2011, 2, 671–674 The Journal of Physical Chemistry Letters LETTER excitation wavelength dependence at 488, 632, and 514 nm. Bare QDs show no measurable signal under the same conditions. There is considerably larger enhancement at 632 nm, indicating a possible resonance in this region, although detailed excitation profiles are needed to verify the exact location of the resonance. In ref 6, the extinction spectrum of 4-MPy on ZnO indicates a broad absorption in the visible, but it is not sufficiently distinct for direct comparison with the excitation profiles. Because the band gap resonance for ZnO QDs is in the UV, it is likely that a charge-transfer resonance is responsible for this enhancement. The direct interband transitions are out of the reach of the laser excitation energy used. Charge-transfer transitions have previ- ously been identified in ZnO QDs with adsorbed species,19,20 including 4-MPy, by observing size-dependent resonances. These resonances occur at approximately the same particle size as is used in this experiment, so that it is quite likely that a similar mechanism is responsible for these observations as well. In Figure 5, we show the Raman spectrum of 4-MPy adsorbed on PbS QDs (8.2 nm diameter). See ref 11 for details of sample preparation. It can be seen that the LOmode at 220 cm-1 as well as the band assigned21,22 as a two-phonon (opticalþ acoustical) mode at 268 cm-1 is strongly enhanced, even compared to the enhancement of the molecular lines.11 The phonon modes are not observed at all on the bare QDs under the same conditions. In previous work on 4-MPy on PbS QDs, we showed that a size- dependent resonance was due to a charge-transfer transition between the valence band of the semiconductor and the lowest- lying empty molecular orbital of the 4-MPy. The resonance occurs at approximately the same particle size as that used here. However, interband contributions to the resonance Raman spectrum cannot be ruled out3 because there is sufficient energy in the excitation laser to achieve such resonance. In PbS, the bulk band gap is 0.4 eV, well below the excitation energies used here. The overtones 2LO and 3LO are very weakly enhanced if at all. This relatively weak overtone spectrum is most likely due to the reduction of electrical-vibrational coupling, which has been observed to decrease with decreasing particle dimension.23 This is clearly a surface phenomenon because the molecules are adsorbed on the surface. Additionally, for QDs, there is a relatively high ratio of surface atoms to interior atoms, at least when compared to the bulk. Thus, we are most likely observing enhancement of the surface phonons. In bulk Rama spectra, surface phonon modes are often shifted slightly from the bulk modes and are usually weaker and broader, often obscured by the bulk24 modes. Note, however, that QDs consist largely of surface atoms, and the effect of surface adsorbates is such that these phonons are most likely enhanced more that the bulk phonons, while the bulk modes will be weaker. The latter are barely seen in our experiments without the adsorbate. The shifts are too small to be distinguished with relatively broad, room- temperature line widths, so that further low-temperature studies are planned, which will result in narrower lines. Furthermore, it is probable that the enhancement is caused by the samemechanism as that responsible for the enhancement of the molecular lines, that is, interband transitions in the semiconductor nanosystem and/or charge-transfer resonances between the molecule and semiconductor, as discussed above. However, this remains to be proven. Further experiments along these lines would involve careful excitation profiles and comparison with those of the molecular Raman profiles as well as the absorption/emission spectrum of the semiconductor system. If the Raman profiles of the phonon modes match the charge-transfer band of the molecules, it would provide compelling evidence of charge- transfer contributions. On the other hand, matching with the absorption spectrum of the nanostructures would indicate in- stead the contribution of interband transitions. Similar results would be expected by varying the nanoparticle size and observing size-dependent resonances. Other evidence can be gained from systematic variation of the ionization potential (IP) or the electron affinity (EA) of the adsorbate or by varying the semiconductor band gap. ’AUTHOR INFORMATION Corresponding Author *E-mail: lombardi@sci.ccny.cuny.edu. Notes §Some of this research was carried out while one of us (S.M.) was a visiting scholar at Jilin University. He is also a member of the Figure 5. Enhancement of phonon modes of PbS QDs (8.2 nm diameter) with adsorbed 4-MPy. The excitation wavelength is 514.5 nm. See ref 11 for a detailed discussion of the 4-MPy region of the spectrum as a function of quantum confinement. Figure 4. Raman intensities of phonon modes of ZnO QDs (27.7 nm diameter) with 4-MPy adsorbed. Excitationwavelengths of 488, 632, and 514 nm are shown. 674 dx.doi.org/10.1021/jz2001562 |J. Phys. Chem. Lett. 2011, 2, 671–674 The Journal of Physical Chemistry Letters LETTER Macaulay Honors College of The City College of New York, class of 2011. ’ACKNOWLEDGMENT This research was supported by the NSFC (20573041, 20773044) of P. R. China, the Program for Changjiang Scholars and Innovative Research Team in University (IRT0422), the Program for New Century Excellent Talents in University, and the 111 Project (B06009). This project was supported by Award No. 2006-DN-BX-K034 awarded by the National Institute of Justice, Office of Justice Programs, U.S. Department of Justice. The opinions, findings, and conclusions or recommendations expressed in this publication/program/exhibition are those of the author(s) and do not necessarily reflect those of the Depart- ment of Justice. ’REFERENCES (1) Quagliano, L. G. 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