title: Molecular selectivity of graphene-enhanced Raman scattering.
abstract: Graphene-enhanced Raman scattering (GERS) is a recently discovered Raman enhancement phenomenon that uses graphene as the substrate for Raman enhancement and can produce clean and reproducible Raman signals of molecules with increased signal intensity. Compared to conventional Raman enhancement techniques, such as surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS), in which the Raman enhancement is essentially due to the electromagnetic mechanism, GERS mainly relies on a chemical mechanism and therefore shows unique molecular selectivity. In this paper, we report graphene-enhanced Raman scattering of a variety of different molecules with different molecular properties. We report a strong molecular selectivity for the GERS effect with enhancement factors varying by as much as 2 orders of magnitude for different molecules. Selection rules are discussed with reference to two main features of the molecule, namely its molecular energy levels and molecular structures. In particular, the enhancement factor involving molecular energy levels requires the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies to be within a suitable range with respect to graphene's Fermi level, and this enhancement effect can be explained by the time-dependent perturbation theory of Raman scattering. The enhancement factor involving the choice of molecular structures indicates that molecular symmetry and substituents similar to that of the graphene structure are found to be favorable for GERS enhancement. The effectiveness of these factors can be explained by group theory and the charge-transfer interaction between molecules and graphene. Both factors, involving the molecular energy levels and structural symmetry of the molecules, suggest that a remarkable GERS enhancement requires strong molecule-graphene coupling and thus effective charge transfer between the molecules and graphene. These conclusions are further experimentally supported by the change of the UV-visible absorption spectra of molecules when in contact with graphene and these conclusions are theoretically corroborated by first-principles calculations. These research findings are important for gaining fundamental insights into the graphene-molecule interaction and the chemical mechanism in Raman enhancement, as well as for advancing the role of such understanding both in guiding chemical and molecule detection applications and in medical and biological technology developments.
authors.affiliation: †Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
authors.forename: Shengxi
authors.initials: S
authors.lastname: Huang
authors.affiliation: †Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
authors.forename: Xi
authors.initials: X
authors.lastname: Ling
authors.affiliation: ∥Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, United States.
authors.forename: Liangbo
authors.initials: L
authors.lastname: Liang
authors.affiliation: †Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
authors.forename: Yi
authors.initials: Y
authors.lastname: Song
authors.affiliation: †Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
authors.forename: Wenjing
authors.initials: W
authors.lastname: Fang
authors.affiliation: §Center for Nanochemistry, Beijing National Laboratory for Molecular Sciences, Key Laboratory for the Physics and Chemistry of Nanodevices, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People's Republic of China.
authors.forename: Jin
authors.initials: J
authors.lastname: Zhang
authors.affiliation: †Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
authors.forename: Jing
authors.initials: J
authors.lastname: Kong
authors.affiliation: ∥Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, United States.
authors.forename: Vincent
authors.initials: V
authors.lastname: Meunier
authors.affiliation: †Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.|⊥Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
authors.forename: Mildred S
authors.initials: MS
authors.lastname: Dresselhaus
keywords: Graphene-enhanced Raman scattering; chemical enhancement; graphene−molecule interaction; molecular energy level; molecular structure