Single-molecule Surface Enhanced Raman Scattering and nano-plasmonics

The intensity of the light scattered from the rough metallic surface is strongly enhanced, making possible the unprecedentedly sensitive observation of the molecules adsorbed on the surface by conventional spectroscopic tools - down to the single-molecule level. The associated phenomenon of Surface-Enhanced Raman Scattering (SERS) is widely applicable in science and technology, especially in the fields of analytical spectroscopy, where the single molecule detection level is needed, and nano-plasmonic optics, where the molecule is used as an efficient light sensor operating far beyond the diffraction limit. Furthermore, the strong signal enhancement opens the doors for the observation of different physical and chemical processes on the single-molecule level.

Our research interests involve several related aspects:

Nano-plasmonic optics

Surface plasmon resonances (SPR) and SERS are two phenomena that intrinsically go together [1]. SPR are excited on the surfaces of high roughness that provide sufficient coupling between the excitation light and the plasmon modes. The particular surfaces of our interest are small aggregates of spherical metallic (gold and silver) nano-particles (NPs) of 10-100 nm in diameter. Owing to their SPR, such NPs have very high absorption in the visible region. When illuminated with laser light, the incident electromagnetic field is strongly enhanced in the near vicinity of the NPs, sometimes also described by the field of the dipole moment induced in the metal.
Control of light properties on the nano-scale via the control of the NP aggregates geometry (e.g. dimers and trimers) is one of our research subjects. Recently, managing of the light polarization was demonstrated in our lab [2], utilizing a molecule adsorbed in the gap between the NPs (hot spots) to detect the light.
Another ongoing project is the construction of a nano-lens: an adjustable nano-device, made of metallic NPs, which allows a systematic study of the properties of a molecule located in a hot-spot [3,4], as well as the construction of the precise NP geometry required to manipulate various light properties, and much more….

Polarization response

Polarization response of a nanoparticle trimer (see Ref. 2)
(A) SEM image of a trimer. A red arrow indicates the position of the molecule that leads to the best agreement between experiment and calculation.
(B) Normalized RS intensity at 555 nm (black squares) and 583 nm (red circles) as a function of the angle of rotation of the incident polarization. The intensities at both wavelengths show approximately the same profile, but the maximal intensity is observed at 75°, which does not match any pair of nanoparticles in the trimer. The green line is the result of a calculation assuming that the molecule is situated at the junction with a gap of 1 nm marked with red arrow in the SEM image, and the corresponding geometrical parameters from the SEM image R1 = 44 nm, R2 = 35 nm, and R3 = 28 nm as the input.
(C) Depolarization ratio (ρ) measured at 555 nm (black squares) and 583 nm (red circles). Depolarization profiles are wavelength dependent in this case, and are aligned differently than the intensity profiles. The black and red lines show the result of calculations at the two wavelengths, assuming that the molecule is situated at the junction marked with the red arrow in SEM image.
(D) SEM image of a second trimer. A red arrow indicates the position of the molecule that leads to the best agreement between experiment and calculation.
(E) Normalized RS intensity at 555 nm (black squares) and 583 nm (red circles) as a function of the angle of rotation of the incident polarization. As in B, the intensity profile does not peak along the direction of the axis connecting particles 2 and 3.
(F) Depolarization ratio (ρ) measured at 555 nm (black squares) and 583 nm (red circles). As in C, depolarization profiles are wavelength-dependent, and are aligned differently than the intensity profiles. The black and red lines show the result of calculations at the two wavelengths,
assuming that the molecule is situated at the junction marked with red arrow in the SEM image. 

Charge transfer interaction

Some of the details of the SERS mechanism are still not fully understood even after ~30 years of research. These involve charge transfer (CT) processes from the metallic surface to the adsorbed molecule and vice versa. The existence of CT is suggested by experimental observations that cannot be predicted by the electromagnetic mechanism of SERS alone [5]. The CT states originate from mixing between LUMO (HOMO) molecular orbitals and the conduction band of the metal [6]. Recently, fluctuations of the metal work-function (which can modulate the CT process) were associated with SERS signal fluctuations and sampled by control of the surface diffusion rates in various environments [7]. Study of the CT interaction on the single-molecule level, allowed by SERS, is one subject of the research conducted in our lab.

Our optical set-up includes laser-illuminated (CW and ps/fs) inverted microscope operated in the epi-illumination mode and coupled to a spectrograph equipped with back-illuminated CCD camera. Spherical nano-particles are deposited from a colloidal solution on the ITO slide, which acts also as a working electrode of an electrochemical cell used to tune the Fermi level of the metal into the resonance with the molecular states [5].

References:

  1. Kneip, K., Moskovits, M., Kneip, H. (Eds.) SERS – Physics and Applications. Topics in Appl. Phys. 103, Springer, Berlin, (2006).
  2. Shegai, T., et al. Managing light polarization via plasmon-molecule interactions within an asymmetric metal nano-particle trimer, Proc. Natl. Acad. Sci. USA. 105, 16448, (2008).
  3. Lukatsky, D. B., Haran, G., Safran, S. A. Slow fluctuations in enhanced Raman scattering and surface roughness relaxation, Phys. Rev. E 67, 062402, (2003).
  4. Weiss, A., Haran, G. Time-Dependent Single-Molecule Raman Scattering as a Probe of Surface Dynamics, J. Phys. Chem. B 105, 12348 (2001).
  5. Shegai, T., Vaskevich A., Rubinstein I., Haran G. Raman Spectroelectrochemistry of molecules within individual electromagnetic hot spots, J. Am. Chem. Soc. 131(40), 2009.
  6. Lombardi, J. R., Birke, R. L. A unified approach to SERS, J. Phys. Chem. C 112, 5605 (2008).
  7. Haran, G. Single Molecule Raman Spectroscopy and Local Work Function Fluctuations, Isr. J. Chem. 44, 385 (2004).