title: Ultrasensitive molecular sensor using N-doped graphene through enhanced Raman scattering.
abstract: As a novel and efficient surface analysis technique, graphene-enhanced Raman scattering (GERS) has attracted increasing research attention in recent years. In particular, chemically doped graphene exhibits improved GERS effects when compared with pristine graphene for certain dyes, and it can be used to efficiently detect trace amounts of molecules. However, the GERS mechanism remains an open question. We present a comprehensive study on the GERS effect of pristine graphene and nitrogen-doped graphene. By controlling nitrogen doping, the Fermi level (E F) of graphene shifts, and if this shift aligns with the lowest unoccupied molecular orbital (LUMO) of a molecule, charge transfer is enhanced, thus significantly amplifying the molecule's vibrational Raman modes. We confirmed these findings using different organic fluorescent molecules: rhodamine B, crystal violet, and methylene blue. The Raman signals from these dye molecules can be detected even for concentrations as low as 10(-11) M, thus providing outstanding molecular sensing capabilities. To explain our results, these nitrogen-doped graphene-molecule systems were modeled using dispersion-corrected density functional theory. Furthermore, we demonstrated that it is possible to determine the gaps between the highest occupied and the lowest unoccupied molecular orbitals (HOMO-LUMO) of different molecules when different laser excitations are used. Our simulated Raman spectra of the molecules also suggest that the measured Raman shifts come from the dyes that have an extra electron. This work demonstrates that nitrogen-doped graphene has enormous potential as a substrate when detecting low concentrations of molecules and could also allow for an effective identification of their HOMO-LUMO gaps.
authors.affiliation: Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: Simin
authors.initials: S
authors.lastname: Feng
authors.affiliation: Instituto de Física, Universidade de São Paulo, São Paulo, SP 05508-090, Brazil.
authors.forename: Maria Cristina
authors.identifier: 0000-0003-3657-8560
authors.initials: MC
authors.lastname: Dos Santos
authors.affiliation: Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA.; Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais 30123-970, Brazil.
authors.forename: Bruno R
authors.initials: BR
authors.lastname: Carvalho
authors.affiliation: Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China.
authors.forename: Ruitao
authors.initials: R
authors.lastname: Lv
authors.affiliation: Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou 215123, P. R. China.
authors.forename: Qing
authors.initials: Q
authors.lastname: Li
authors.affiliation: Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: Kazunori
authors.initials: K
authors.lastname: Fujisawa
authors.affiliation: Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: Ana Laura
authors.initials: AL
authors.lastname: Elías
authors.affiliation: Department of Material Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: Yu
authors.initials: Y
authors.lastname: Lei
authors.affiliation: Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: Nestor
authors.initials: N
authors.lastname: Perea-López
authors.affiliation: Institute of Carbon Science and Technology, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan.
authors.forename: Morinobu
authors.initials: M
authors.lastname: Endo
authors.affiliation: School of Physics, Huazhong University of Science and Technology, Wuhan, Hubei 430074, P. R. China.
authors.forename: Minghu
authors.initials: M
authors.lastname: Pan
authors.affiliation: Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais 30123-970, Brazil.
authors.forename: Marcos A
authors.initials: MA
authors.lastname: Pimenta
authors.affiliation: Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA.; Department of Material Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA.; Institute of Carbon Science and Technology, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan.; Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: Mauricio
authors.initials: M
authors.lastname: Terrones
mesh_terms: D005840:Gentian Violet / Q000032:analysis; D006108:Graphite / Q000737:chemistry*; D057230:Limit of Detection; D008751:Methylene Blue / Q000032:analysis; D018625:Microscopy, Atomic Force; D009584:Nitrogen / Q000737:chemistry*; D056951:Photoelectron Spectroscopy; D011789:Quantum Theory; D012235:Rhodamines / Q000032:analysis; D013056:Spectrophotometry, Ultraviolet; D013059:Spectrum Analysis, Raman
chemical_list: D012235:Rhodamines; D006108:Graphite; D005840:Gentian Violet; C029773:rhodamine B; D009584:Nitrogen; D008751:Methylene Blue
keywords: GERS; N-doped graphene; SERS; charge transfer; enhanced raman spectroscopy; homo-lumo gap; sensor