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 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 contradistinction 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 phonon modes 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
W
e report here on an observation of enhanced Raman intensities of phonon modes of semiconductor nanostructures 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 magnitude in the Raman intensity for adsorbed species when compared to that expected from the same number of nonadsorbed molecules. Although SERS has been mostly restricted to the investigation 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 Enhancement 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 semiconductor 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 enhancement 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 r 2011 American Chemical Society
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 the nanoparticle 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 empty molecular orbitals may result. It is the interband transitions in the semiconductor as well as the moleculeQD charge-transfer transitions that are implicated in the surfaceinduced 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. However, 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 Published: March 04, 2011 671
dx.doi.org/10.1021/jz2001562 | J. Phys. Chem. Lett. 2011, 2, 671–674
The Journal of Physical Chemistry Letters
LETTER
Figure 1. SEM images of anodized TiO2 before annealing (left) and after annealing (right); both at 30 000 magnification.
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 Liþ increases the intensities of the phonon modes. Excitation is at 514.5 nm.
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 decreasing electron affinity, and this is taken to be indicative of chargetransfer resonance contribution to the enhancement. In separate experiments, we have also observed a similar effect in TiO2 QDs of size 7.2 nm15 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 wavelength 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 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 Adsorbatesa adsorbate none
intensity Eg(1)
EF
EA (eV)
3632
4-MBA
81 540
22
2.79
PATP 4-MPy
128 800 162 200
35 45
1.87 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.
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 (
