Surface-enhanced Raman scattering (SERS) is a powerful optical technique of ultrasensitive analysis and single-molecule detection. Currently, this method allows solving the most difficult analytical problems from identifying trace amounts of explosives to high-precision imaging of cancer cells. The role of SERS spectroscopy and its sub-technique, tip-enhanced Raman scattering (TERS) spectroscopy, is growing every year. This is evident from Scopus statistic, namely a number of publications for 2010-2014 is 5837 whereas the related value for 2015-2019 is 8665. Due to expanding application of SERS spectroscopy in chemical analysis and bioimaging, elucidation of the fundamental principles responsible for origin of optical signal is important issue. It is generally accepted that SERS phenomenon has the contributions from two mechanisms: electromagnetic and chemical. The electromagnetic mechanism is responsible for enhancement of incident and scattered light due to high local electromagnetic fields near a nanostructured metal surface. The chemical mechanism consists in the change of molecular polarization due to its interaction with metal surface. The chemical contribution to SERS is most significant when resonance electron transition occurs between the Fermi level of metal substrate and the molecular orbital of the adsorbed molecule. Recently, a number of different experimental and theoretical groups have tried to propose a unified model for formation of the SERS signal. Description of chemical mechanism of SERS is most challenging and controversial. But this mechanism is responsible for unique features of SERS spectroscopy such as sensitivity to molecule orientation and selective band enhancements. The proposed project will be focused on the theoretical and experimental elucidation of a role of charge transfer in SERS spectra arising, especially in resonance conditions conducing to surface enhanced resonance Raman scattering (SERRS). The results obtained will contribute to increasing the accuracy of SERS computational prediction and to explanation of the surface selection rules that will give more reliable interpretation of experimental spectra in various fields of SERS application.
A complex approach including advanced theoretical calculations and high quality experimental measurements will be used in the project. Theoretical part will be performed in SUT under Dr. Z. Jamshidi supervision. Announced advanced theoretical approach consists in using the time-dependent approach to the polarizability tensor introduced by Heller. In this approach, the excited wave functions are treated as a dynamic wave packet of the ground-state wave functions, propagating on the excited-state potential energy surface (PES). Herein, we consider the independent mode displaced harmonic oscillator (IMDHO) model for the excited potential energy surface and the excited-state gradient approximation to obtain the final formula for the polarizability tensor. Herein, the dimensionless displacements represent the dimensionless origin shift between the equilibrium minima of the ground and excited potential energy curves. Moreover, these calculations can be extended to include overtones and combination bands in the final prediction. All these calculations will be done by ORCA and ADF packages.
Experimental part of the project will be carried out under Dr. E. Solovyeva supervision in SPBU. SERS spectra will be measured using Ag nanoparticles solution synthesized by reduction of silver nitrate with sodium borohydride (Creighton method). Ag nanoparticles prepared by this method are well-proven as reproducible and stable SERS substrates giving a high enhancement of the Raman signal. SERS spectra will be acquired under various conditions that may turn on or turn off a charge transfer mechanism, namely, at different excitation wavelengths, at different surface coverage and at different ionic compositions of background solution. SERS measurements will be accompanied by UV-vis absorption study to trace the feasibility to charge transfer and the wavelengths of its occurrence.
4,4'-derivative of stilbene and diphenylacetylene will be the objects of the investigation in the theoretical and experimental parts simultaneously. In the theoretical calculations, their complexes with polyatomic Ag clusters will be used as a model system. In the experimental works, these molecules will be directly adsorbed on Ag nanoparticle surface.
The сhoiсe of 4,4'-derivative of stilbene and diphenylacetylene is made of the following reasons. i) high adsorption affinity to the metal surface; ii) capability to the resonance Raman scattering when overtones and combination bands are well-manifested; iii) the molecules have an inversion center that is important for revealing of surface selection rules. Altogether, 4,4'-derivative of stilbene and diphenylacetylene are ideal candidates for the comparison of calculated and experimental data in view of elicitation of charge transfer contribution to SERS spectra.
The synergism of the theoretical approach and experimental methodology imparts enormous advantages to the proposed study.
As for theoretical modeling and experimental SERS studies, both research groups have a massive background that can be traced in their recent publications.
Dr. Jamshidi research group focuses on computational spectroscopy and especially theoretical investigation of surface-enhanced Raman scattering (SERS) since 2012. They study the chemical mechanism of SERS to find out the unified formula that explains surface selection rules. Her group performs theoretical calculations to model the important factors that influence the pattern of signals, and then by comparing the model with experimental observations try to develop and progress theoretical approach toward a complete and accurate simulation. In this regard, collaborating with experimental group is a great opportunity to assay the theoretical approach.
The relevant studies have been supported by Iranian National Science Foundation (INSF), Holland Research School of Molecular Chemistry (HRSMC), Chemistry and Chemical Research Center of Iran (CCERCI), and Sharif University of Technology (SUT).
[1] Theoretical simulation of surface‐enhanced resonance Raman spectroscopy of cytosine and its tautomers R Sharafdini, M Mohammadpour, S Ramazani, Z Jamshidi, Journal of Raman Spectroscopy 51 (1), 55-65, 2019.
[2] Charge-transfer surface-enhanced resonance Raman spectra of benzene-like derivative compounds under the effect of an external electric field S Ashtari-Jafari, MH Khodabandeh, Z Jamshidi Physical Chemistry Chemical Physics 21 (43), 23996-24006, 2019.
[3] Elucidation of charge-transfer SERS selection rules by considering the excited state properties and the role of electrode potential, M. Mohammadpour, MH Khodabandeh, L. Visscher and Z. Jamshidi, Physical Chemistry Chemical Physics 19 (11), 7833-7843, 2017.
[4] Theoretical investigation of the weak interactions of rare gas atoms with silver clusters by resonance Raman spectroscopy modeling S. Yasrebi, Z. Jamshidi, International Journal of Quantum Chemistry 117 (15), e25389, 2017.
[5] Comparative assessment of density functional methods for evaluating essential parameters to simulate SERS spectra within the excited state energy gradient approximation M. Mohammadpour and Z. Jamshidi The Journal of chemical physics 144 (19), 194302, 2016.

Dr. Solovyeva performs experimental research in the field of SERS spectroscopy for more than 10 years. Her group master perfectly a number of techniques for SERS substrates synthesis and sample preparation together with spectroscopic measurements. Dr. Solovyeva has wide experience of study of chemical factors contribution to the SERS signal. This topic was partly considered in her PhD dissertation [1]. Recently, Dr. Solovyeva has been developing two directions: i) SERS study of biologically relevant compounds [2-3], and ii) synthesis and optical investigation of highly effective plasmonic substrates with “hot spots” [4-5]. Deep insight into molecule-surface interactions has been reached for many systems. The relevant studies have been supported by Russian Science Foundation (№17-73-10209), Russian Foundation of Basic Research (№20-33-70034) and Saint-Petersburg City Government grants.
[1] Solovyeva, E. The role of chemical factors in the formation of a surface-enhanced Raman signal on the example of pyridine and acridine molecules adsorbed on a silver surface, PhD dissertation, Saint-Petersburg state university (2011) p. 95.
[2] E. V. Solovyeva, E. Borisov, Demonstration of Physical and Analytical Features of Surface-Enhanced Raman Scattering by Analysis of Folic Acid in Commercial Tablets, Journal of Chemical Education (2020) just accepted
[3] E. V. Solovyeva, A. Rakhimbekova, Y. V. Lanchuk, L. A. Myund, A. S. Denisova, SERS investigation of neocuproine adsorption on silver: influence of electrode potential on methyl groups, Journal of Raman Spectroscopy 49 (2018) 207–214. https://doi.org/10.1002/jrs.5265
[4] E.V. Solovyeva, E. V. Ubyivovk, A. S. Denisova, Effect of diaminostilbene as a molecular linker on Ag nanoparticles: SERS study of aggregation and interparticle hot spots in various environments, Coll. Surf. A 528 (2018) 542-548. DOI: 10.1016/j.colsurfa.2017.11.040
[5] O. V. Odintsova, A. N. Smirnov, E. V. Solovyeva, Plasmonic nanoparticles modified by dimercaptostilbene for metamaterials, Proc. SPIE 11025 (2019) 1102512 doi: 10.1117/12.2520783
The project will result in valuable experimental and theoretical data about surface-enhanced Raman scattering of a number of stilbene and diphenylacetylene compounds. The experimental results will include: Raman and SERS spectra of 4,4'-derivative of stilbene and diphenylacetylene adsorbed on Ag nanoparticles, their concentration dependencies and excitation profiles; UV-vis absorption spectra of the same systems. The theoretical results will include: simulation of Raman and SERS spectra for 4,4'-derivative of stilbene, based on excited-state gradient approximation, upon changing the energy of incident light to be in resonance or off resonance with charge-transfer states; improvement of our theoretical approach to find out overtones and combination bands; calculation of the experimental excited-state dimensionless displacement and explaination of the selective enhancement and de-enhancement of modes by comparing to the theoretical values.
Based on the experimental and calculated data, the applied theoretical approach will be validated and effect of charge transfer on the pattern and relative intensity of SERS spectra will be desribed. Finally, we believe that this collaboration between Dr. Zahra Jamshidi and Dr. Elena Solovyeva will provide valuable data for further theoretical studies on the charge-transfer SERS mechanism and assay theoretical simulation and also will improve the theoretical method in order to propose and complete surface selection rules.
The results of the project will contribute to teaching process. Dr. Zahra Jamshidi will give invited lecture about the resonance Raman scattering and chemical mechanism of SERS as a part of teaching course «Modern methods of spectroscopy based on signal enhancement» (course number 051090) realizing by Dr. Solovyeva within the master’s program 04.04.01 «Chemistry» at Chemistry Institute of SPBU. Supposedly, the lecture will be carried out in on-line format.
Altogether, the following project outcomes are expected:
- Publication of at least 2 papers in high-level international journals (Scopus or WoS indexed)
- Attending 1 international conference
- Organizing one research workshop (live or on-line format)
- Invited lecture
- Co-advising one joint PhD project
To continue collaboration between research groups of Dr. Jamshidi and Dr. Solovyeva, we plan to apply in 2021 on funding support from the join program between Iran National Science Foundation (INSF) and Russian Foundation for Basic Research (RFBR) (http://insf.org/en/news/34/insf-rfbr-2020-call-deadline-extended).