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Register a new userPeculiarities of the SERS Spectra of 4,4-Bipyridine Molecule in a Single Molecule Detection Regime
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The paper briefly describes main statements of the theory of the SERS spectra with regards to the single molecule regime, when the enhancement achieves the values . Analysis of the spectra of 4,4-bypyridine, obtained on the dimer lattice of sharp nanoparticles points out that the observed enhancement is caused exclusively by a strong quadrupole light-molecule interaction, which manifests in the presence of lines, caused by vibrations with the unit irreducible representations of the and symmetry groups, which apparently describe the symmetry properties of the molecule. The study of the spectra, obtained by Tip enhanced spectroscopy demonstrates that the strong quadrupole light-molecule interaction still plays a leading role, however the strong dipole interaction still manifests in the existence of very weak forbidden lines. This result apparently is associated with another experimental geometry .
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The SEHRS and SERS spactra of 4,4 - Bipyridine are analyzed on the base of the Dipole-Quadrupole theory for two possible geometries of the molecule. It is demonstrated that there appear strong lines caused by vibrations transforming after a unit irreducible representation both for the geometry with D2 and D2h symmetry groups, which may probably describe the symmetry properties of the molecule. Appearance of these lines is associated with a strong quadrupole light-molecule interaction, which arises in nano size rregions of sharp roughness of the metal. In addition, there are the lines caused by contributions from both the vibrations transforming after the unit irredicible representations A or Ag and the representations B1 or B1u, respectively, which describe transformational properties of the Ez component of the dipole moment, which is perpendicular to the surface for both geometries. This result is associated with a specific geometry of the molecule, when the indicated vibrations can be nearly degenerated and can not be resolved by the SEHRS and SERS spectra analysis. This issue is in a full compliance with the results of the SEHRS and SERS Dipole-Quadrupole theory.
The SERS spectrum of hydroquinone, adsorbed on nanoparticles of titanium dioxide is analyzed. It is pointed out that the enhancement is stronger for larger mean size of nanoparticles that is in an agreement with the electrostatic approximation. In addition it is found that there are the lines, which are forbidden in usual Raman spectra. Along with this there is the enhancement, caused both by the normal and tangential components of the electric field. This result is in agreement with the theory of SERS on semiconductor and dielectric substrates. Discovery of the forbidden lines indicates sufficiently large role of the strong quadrupole light-molecule interaction in such a system.
The SEHRS spectrum of 4,4 Bipyridine is analyzed on the base of the Dipole Quadrupole theory. It is demonstrated that there appear strong lines caused by vibrations transforming after a unit irreducible representation of the D2 symmetry group. which is most probably describes the symmetry properties of the molecule. These lines are nearly forbidden for the molecule, adsorbed on rough metal surface. Appearance of these lines is associated with a strong quadrupole light molecule interaction, which exists in this system. In addition, there are lines, caused by contributions from both the vibrations transforming after the unit irreducible representation A and the representation B1, which describes transformational properties of the dz component of the dipole moment, which is perpendicular to the surface. This result is associated with the specific geometry of the molecule, when the indicated vibrations can be nearly degenerated and cannot be resolved by the SEHRS spectra analysis. Analysis of the SEHRS spectra for the possible geometry of the molecule with the D2h symmetry group leads to similar results. This issue is in a full coincidence with the results of the SEHRS Dipole Quadrupole theory.
Inspired by recent measurements of forces and conductances of bipyridine nano-junctions, we have performed density functional theory calculations of structure and electron transport in a bipyridine molecule attached between gold electrodes for seven different contact geometries. The calculations show that both the bonding force and the conductance are sensitive to the surface structure, and that both properties are in good agreement with experiment for contact geometries characterized by intermediate coordination of the metal atoms corresponding to a stepped surface. The conductance is mediated by the lowest unoccupied molecular orbital, which can be illustrated by a quantitative comparison with a one-level model. Implications for the interpretation of the experimentally determined force and conductance distributions are discussed.
Surface enhanced Raman scattering (SERS) is optically sensitive and chemically specific to detect single molecule spectroscopic signatures. Facilitating this capability in optically-trapped nanoparticles at low laser power remains a significant challenge. In this letter, we show single molecule SERS signatures in reversible assemblies of trapped plasmonic nanoparticles using a single laser excitation (633 nm). Importantly, this trap is facilitated by the thermoplasmonic field of a single gold nanoparticle dropcasted on a glass surface. We employ bi-analyte SERS technique to ascertain the single molecule statistical signatures, and identify the critical parameters of the thermoplasmonic tweezer that provide this sensitivity. Furthermore, we show the utility of this low power ($approx$0.1 mW/$mu$m^2) tweezer platform to trap single gold nanoparticle and transport assembly of nanoparticles. Given that our configuration is based on a dropcasted gold nanoparticle, we envisage its utility to create reconfigurable plasmonic metafluids in physiological and catalytic environments, and can be potentially adapted as an in-vivo plasmonic tweezer.
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