EVIDENCE OF SERS EFFECT FOR SEMICONDUCTOR SURFACES Lucia G. Quagliano Consiglio Nazionale delle Ricerche (CNR) Institute for Nanostructured Materials (ISMN), Area della Ricerca di Roma, P.O. Box 010- 00016 Monterotondo Scalo, Roma (Italy) E-mail: lucia.quagliano@mlib.cnr.it. We present an extension of Surface Enhanced Raman Scattering (SERS) effect to semiconductor materials. Until now SERS experiments were essentially restricted to adsorbates on rough metallic surfaces, especially noble and alkali metals [1-5]. Our results indicate that SERS could be a useful method for identifying species adsorbed on semiconductor materials and providing information on the interaction adsorbate-semiconductor. As matter of fact normal Raman spectroscopy does not have the sensitivity to detect adsorbates at semiconductor surfaces. The work reported here is a follow-up of our previous work [6-7] and demonstrates that SERS enhancements are obtained on semiconductors. Using pyridine as test molecule we observed surface-enhanced Raman scattering on various kinds of GaAs surfaces: coated with nanometer-sized Ag particles, subjected to a chemical etching process and nanostructured as in quantum dots structures. A surface roughness appears to be necessary on our semiconductor materials for SERS activation. Pyridine (py) was chosen for this study since it is the most studied SERS system and therefore can be used as a model system. SERS spectra of pyridine molecules show a new vibrational band, probably due to a new bond between adsorbed species and GaAs, and significant relative intensity changes compared with the Raman spectrum of pyridine free in solution. Since it is extremely difficult to eliminate the possibility that adsorbates are bound to the Ag rather than to the semiconductor surface, it was important to obtain SERS spectra directly from the bare semiconductors without Ag film. For this purpose in addition to "Ag overlayer" method by which GaAs surfaces are coated with a silver film consisting of well isolated Ag particles we have used nanostructures, in particular self-organized InAs/GaAs quantum dots, and a procedure based on the use of a chemical etching. Our studies show that SERS spectra of py molecules adsorbed on quantum dots as well as on etched GaAs surface are similar to that obtained from the surface coated with an Ag island film. This demostrates that the presence of Ag film on the semiconductor surface did not significantly change the Raman spectra of the adsorbed molecules. These results prove that by using the Ag-overlayer technique the SERS signal arises from molecules directly adsorbed to semiconductor surface and not from the Ag film itself. In conclusion, these studies show that by using SERS spectroscopy it is possibile to detect and characterize species adsorbed onto semiconductor surfaces. SERS investigation at surfaces and interfaces can be an important contribution to the analysis of semiconductor devices. Therefore, this extension of SERS spectroscopy to semiconductor materials is important for developing SERS as a new surface sensitive probe in material science. This work is significant also from the SERS point of view because shows that semiconductors are intrinsically SERS active. It could provide an opportunity to contribute to a better uderstanding of the enhancement process costituting the SERS effect. Acknowledgements The author thanks Dr. J-M Gerard for providing the self-assembled quantum dots. She also acknowledges Dr. B. Jusserand for useful discussion and collaboration. Part of this work was supported by the CNR-CNRS project. 1. R. K. Chang and T. E. Furtak, Surface Enhanced Raman Scattering, Plenum, New York, 1982. 2. A. Otto, in Light Scattering in Solids IV, edited by M. Cardona and G. Guentherodt, Topic in Applied Optics, Springer, Heidelberg, 1984, p.289. 3. R. P. Van Duyne, Chemical and Biochemical Applications of Lasers, edited by C. B. Moore, Academic Press, New York, 1979, Vol. 4, p.101. 4. M. Moskovits, Rev. Mod. Phys. 57, 783 (1985). 5. Z. Q. Tian, Internet Journal of vibrational spectroscopy [www.ijvs.com] 4, 2, 2 (2000). 6. L. G. Quagliano, B. Jusserand and D. Orani, J. of Raman Spectroscopy 29, 721 (1998). 7. L. G. Quagliano, Internet Journal of vibrational spectroscopy [www.ijvs.com] 4, 2, 3 (2000).
EVIDENCE OF SERS EFFECT FOR SEMICONDUCTOR SURFACES
Quagliano Lucia g
2002
Abstract
EVIDENCE OF SERS EFFECT FOR SEMICONDUCTOR SURFACES Lucia G. Quagliano Consiglio Nazionale delle Ricerche (CNR) Institute for Nanostructured Materials (ISMN), Area della Ricerca di Roma, P.O. Box 010- 00016 Monterotondo Scalo, Roma (Italy) E-mail: lucia.quagliano@mlib.cnr.it. We present an extension of Surface Enhanced Raman Scattering (SERS) effect to semiconductor materials. Until now SERS experiments were essentially restricted to adsorbates on rough metallic surfaces, especially noble and alkali metals [1-5]. Our results indicate that SERS could be a useful method for identifying species adsorbed on semiconductor materials and providing information on the interaction adsorbate-semiconductor. As matter of fact normal Raman spectroscopy does not have the sensitivity to detect adsorbates at semiconductor surfaces. The work reported here is a follow-up of our previous work [6-7] and demonstrates that SERS enhancements are obtained on semiconductors. Using pyridine as test molecule we observed surface-enhanced Raman scattering on various kinds of GaAs surfaces: coated with nanometer-sized Ag particles, subjected to a chemical etching process and nanostructured as in quantum dots structures. A surface roughness appears to be necessary on our semiconductor materials for SERS activation. Pyridine (py) was chosen for this study since it is the most studied SERS system and therefore can be used as a model system. SERS spectra of pyridine molecules show a new vibrational band, probably due to a new bond between adsorbed species and GaAs, and significant relative intensity changes compared with the Raman spectrum of pyridine free in solution. Since it is extremely difficult to eliminate the possibility that adsorbates are bound to the Ag rather than to the semiconductor surface, it was important to obtain SERS spectra directly from the bare semiconductors without Ag film. For this purpose in addition to "Ag overlayer" method by which GaAs surfaces are coated with a silver film consisting of well isolated Ag particles we have used nanostructures, in particular self-organized InAs/GaAs quantum dots, and a procedure based on the use of a chemical etching. Our studies show that SERS spectra of py molecules adsorbed on quantum dots as well as on etched GaAs surface are similar to that obtained from the surface coated with an Ag island film. This demostrates that the presence of Ag film on the semiconductor surface did not significantly change the Raman spectra of the adsorbed molecules. These results prove that by using the Ag-overlayer technique the SERS signal arises from molecules directly adsorbed to semiconductor surface and not from the Ag film itself. In conclusion, these studies show that by using SERS spectroscopy it is possibile to detect and characterize species adsorbed onto semiconductor surfaces. SERS investigation at surfaces and interfaces can be an important contribution to the analysis of semiconductor devices. Therefore, this extension of SERS spectroscopy to semiconductor materials is important for developing SERS as a new surface sensitive probe in material science. This work is significant also from the SERS point of view because shows that semiconductors are intrinsically SERS active. It could provide an opportunity to contribute to a better uderstanding of the enhancement process costituting the SERS effect. Acknowledgements The author thanks Dr. J-M Gerard for providing the self-assembled quantum dots. She also acknowledges Dr. B. Jusserand for useful discussion and collaboration. Part of this work was supported by the CNR-CNRS project. 1. R. K. Chang and T. E. Furtak, Surface Enhanced Raman Scattering, Plenum, New York, 1982. 2. A. Otto, in Light Scattering in Solids IV, edited by M. Cardona and G. Guentherodt, Topic in Applied Optics, Springer, Heidelberg, 1984, p.289. 3. R. P. Van Duyne, Chemical and Biochemical Applications of Lasers, edited by C. B. Moore, Academic Press, New York, 1979, Vol. 4, p.101. 4. M. Moskovits, Rev. Mod. Phys. 57, 783 (1985). 5. Z. Q. Tian, Internet Journal of vibrational spectroscopy [www.ijvs.com] 4, 2, 2 (2000). 6. L. G. Quagliano, B. Jusserand and D. Orani, J. of Raman Spectroscopy 29, 721 (1998). 7. L. G. Quagliano, Internet Journal of vibrational spectroscopy [www.ijvs.com] 4, 2, 3 (2000).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
