Sensitive Raman Probe of Electronic Interactions between Monolayer Graphene and Substrate under Electrochemical Potential Control

Abstract

In situ electrochemical Raman spectroscopic measurements of defect-free monolayer graphene on various substrates were performed under electrochemical potential control. The G and 2D Raman band wavenumbers (ω G , ω 2D ) of graphene were found to depend upon the electrochemical potential, i.e., the charge density of graphene. The values of ω G and ω 2D also varied depending on the choice of substrates. On metal substrates where graphene was synthesized by chemical vapor deposition, a strong blue shift of ω 2D was induced, which could not account for the strain and charge doping. We attributed the blue shift of ω 2D to a change in the electronic properties of graphene induced by distinct electronic interactions with the metal substrates. To explain the unique characteristics in the Raman spectrum of graphene on various substrates, a novel mechanism is proposed considering reduction of the Fermi velocity in graphene owing to dielectric screening from the metal substrates.

Oxygen intercalation in PVD graphene grown on copper substrates: A decoupling approach

“…The 2D band frequency (ω 2D ) has been correlated with strain in graphene layers. However, its absence in the spectra corresponding to the as-grown sample prevents its comparison with the O-intercalated sample by the use of ω 2 D vs ω G vectors, often employed to evaluate both strain and charge density in graphene layers [ 60 , 61 ]. After oxygen intercalation, ω 2D appears at an average value of 2718 cm -1 , corresponding to a blue-shift of > 28 cm -1 with respect to free-standing graphene [ 62 ].…”

“…Previous reports of large blue-shifts in ω 2D , between 15 and 40 cm -1 , in CVD-grown graphene on Cu foil [ 55 , 63 , 64 ] and on single crystal Cu(100) and Cu(111) [ 20 , 65 ] have been attributed mainly to compressive strain. Since every band in the Raman spectrum is affected by strain [ 61 ], the average ω D and ω G values of 1373 and 1608 cm -1 , respectively observed in the as-grown sample, also represent a significant blue-shift with respect to free-standing graphene and are coherent with compressive strain. Regarding the G band, in the as-grown sample a broad band profile is observed with ω G ~ 1608 cm -1 , albeit with some contribution due to the D’ band.…”

“…Albeit the G peak frequency is nearer to that of an unperturbed graphene layer, the shifted 2D requires rationalization. It is important to note that various studies have suggested that the values of ω G and ω 2D cannot be explained solely by charge doping and strain [ 67 ], and that variations in the Fermi velocity can alter the ω 2 D without affecting the ω G [ 61 , 68 ]. Finally, previous studies on the oxidation of graphene on Cu surfaces have attributed red-shifts in ω D , ω G and ω 2 D to strain release after oxidation, as mentioned previously, which augment with the growth of the oxide underlayer [ 20 , 48 ] producing corrugation and wrinkling in the graphene sheet [ 69 ].…”

Growth and Characterization of Graphene Layers on Different Kinds of Copper Surfaces

“…Recently it was shown for graphene grown on Cu that in ω G -ω 2D space the correlative shift of ω G and ω 2D can be decomposed into a purely strain-induced shift [12] and a shift due to the fractional change in the effective Fermi velocity provided the graphene layers are doping-free [27]. As there is increasing evidence in the literature that the doping of graphene on Cu foil is negligible and thermally activated [28,29], we can apply a similar approach and show in Figure 4b the (ω G , ω 2D ) values from our measurements on the three samples. As can be appreciated from Figure 4a,b, the entire surface of the "fresh" sample displays a low-intensity graphene signal characteristic for strong coupling to the Cu substrate, with typical frequency values grouping around 1589 cm −1 for ω G and around 2660 cm −1 for ω 2D .…”

Precise Control of Nanoscale Interface for Efficient Electrochemical Reactions

“…[55][56][57] To understand the molecular behavior strongly interacted with the plasmonic field at the well-defined nanostructure surface, we attempted to combine SERS measurements with the electrochemical potential control method. [58][59][60][61][62] As mentioned above, the electrochemical potential scan can easily change not only the molecular orientation via the surface charge but also the resonance effect. As one example, we have conducted electrochemical SERS measurements using bi-analyte solution containing 4'4-bipyride and 2'2-bipyride.…”