Measuring Local Electric Fields and Local Charge Densities at Electrode Surfaces Using Graphene-Enhanced Raman Spectroscopy (GERS)-Based Stark-Shifts

Abstract

We report spectroscopic measurements of the local electric fields and local charge densities at electrode surfaces using graphene-enhanced Raman spectroscopy (GERS) based on the Stark-shifts of surface-bound molecules and the G band frequency shift in graphene. Here, monolayer graphene is used as the working electrode in a three-terminal potentiostat while Raman spectra are collected in situ under applied electrochemical potentials using a water immersion lens. First, a thin layer (1 Å) of copper­(II) phthalocyanine (CuPc) molecules are deposited on monolayer graphene by thermal evaporation. GERS spectra are then taken in an aqueous solution as a function of the applied electrochemical potential. The shifts in vibrational frequencies of the graphene G band and CuPc are obtained simultaneously and correlated. The upshifts in the G band Raman mode are used to determine the free carrier density in the graphene sheet under these applied potentials. Of the three dominant peaks in the Raman spectra of CuPc (i.e., 1531, 1450, and 1340 cm–1), only the 1531 cm–1 peak exhibits Stark-shifts and can, thus, be used to report the local electric field strength at the electrode surface under electrochemical working conditions. Between applied electrochemical potentials from −0.8 V to 0.8 V vs NHE, the free carrier density in the graphene electrode spans a range from −4 × 1012 cm–2 to 2 × 1012 cm–2. Corresponding Stark-shifts in the CuPc peak around 1531 cm–1 are observed up to 1.0 cm–1 over a range of electric field strengths between −3.78 × 106 and 1.85 × 106 V/cm. Slightly larger Stark-shifts are observed in a 1 M KCl solution, compared to those observed in DI water, as expected based on the higher ion concentration of the electrolyte. Based on our data, we determine the Stark shift tuning rate to be 0.178 cm–1/ (106 V/cm), which is relatively small due to the planar nature of the CuPc molecule, which largely lies perpendicular to the electric field at this electrode surface. Computational simulations using density functional theory (DFT) predict similar Stark shifts and provide a detailed atomistic picture of the electric field-induced perturbations to the surface-bound CuPc molecules.

Two Damping Mechanisms in the Chemically Enhanced Raman Scattering on Graphene: DFT Study

“…First, we would like to address the frequencies shifts. We know that it can be due to the Stark effect 15 or due to the charging of the molecule. 27 Since our molecule is planar and all four vibrational modes have in-plane geometries, the Stark effect should not be dominating.…”

“…22,23 However, it also means that the enhancement is mainly chemical, opening a great opportunity for modeling the interfacial CE mechanism in Raman spectroscopy. 15 GERS experiments are typically done with resonant molecules, like porphyrins, strong resonant Raman cross sections of which compensate for the lack of plasmonic enhancement on graphene. 24 The molecule of our choice for this work, TCNQ, is convenient from the computational point of view and common in graphene functionalization studies, but practical GERS measurements with this molecule would be challenging.…”

Ultra-High-Speed Dynamics in Surface-Enhanced Raman Scattering

“…16,17 In fact, there are even reports of highly sensitive "SERS" from nonplasmonic structures. 18,19 In those cases, the enhancement is driven by resonances that involve new electronic transitions available in the nanoenvironment. While chemical and molecular resonance mechanisms can be extremely relevant for certain systems, 15 here we will restrict the discussion to plasmonic and hotspot-driven SERS, since it provides the bulk of the enhancement in a typical experiment with noble metal nanostructures.…”

“…The concept of plasmonic hotspots is central to interpreting the SERS effect and provides an understanding of SIFs in the context of a purely electromagnetic enhancement mechanism . It is also widely accepted that “chemical mechanisms” and resonance Raman effects also play a role in enhancing the Raman response from adsorbed molecules. , In fact, there are even reports of highly sensitive “SERS” from nonplasmonic structures. , In those cases, the enhancement is driven by resonances that involve new electronic transitions available in the nanoenvironment. While chemical and molecular resonance mechanisms can be extremely relevant for certain systems, here we will restrict the discussion to plasmonic and hotspot-driven SERS, since it provides the bulk of the enhancement in a typical experiment with noble metal nanostructures.…”

Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy for Probing Riboflavin on Graphene

“…Nevertheless, resonance Raman spectroscopy provides information on the structure of graphene skeleton [ 12 ]; usually, no direct information on the structure of molecular groups covalently attached to the carbon matrix or adsorbed compounds can be acquired. In some cases, adsorbed organic molecules can be probed by graphene-enhanced Raman spectroscopy (GERS) [ 15 , 16 , 17 , 18 , 19 , 20 , 21 ]. In GERS the dominant Raman signal enhancement mechanism is chemical enhancement due to charge transfer excitation [ 17 , 21 ].…”