Multiphonon Raman spectroscopy properties and Raman mapping of 2D van der Waals solids: graphene and beyond

Abstract

Strong in-plane bonding (covalent) and weak van der Waals (vdW) interplanar interactions characterize a number of layered solids, as epitomized by graphite. The advent of graphene (Gr), individual atomic two-dimensional (2D) layers, isolated from mineral graphite via micromechanical exfoliation enabled the ability to pick, place and stack of arbitrary compositions. Moreover, this discovery implicated an access to other 2D vdW solids beyond graphene and artificially stacking atomic layers forming heterostructures/superlattices. Raman spectroscopy (RS) is a fast reliable non-destructive analytical tool and an integral part for lattice dynamical structural characterization of crystalline solids at nanoscale, revealing not only the collective atomic/molecular motions but also localized vibrations/modes and specifically used to determine the number of graphene layers and of other 2D vdW solids. We present Raman spectroscopy in first-, second-and higher-order vibrational modes involving 3 and 4 phonons (overtones and combination) and mapping of graphene (mono-, bi-, tri-and few-) layers, semiconducting transition metal dichalcogenides (TMDs) [molybdenum disulfide (MoS 2 ) and tungsten disulfide (WS 2 )] and wide bandgap hexagonal boron nitride (h-BN) dispersed monolayers, revealing various molecular vibrations and structural quality/disorder. First-and higherorder phonon modes are observed and analyzed in terms of Raman intensity (spatial inhomogeneity or thickness variation), band position (intrinsic mechanical strain) and intensity ratio (structural disorder) as a function of graphene layer (n). An empirical relation for G band position with n is corroborated. All of the higher order modes are observed to upshift almost linearly with n, betraying the underlying interlayer vdW interactions. These findings exemplify the evolution of structural parameters in layered materials in changing from 3-to 2-or low-dimensional regime. The results are presented in view of applications of graphene by itself and in combination that help better understanding of physical and electronic properties for nano-/optoelectronics.

Functionalized Graphene–Polyoxometalate Nanodots Assembly as “Organic–Inorganic” Hybrid Supercapacitors and Insights into Electrode/Electrolyte Interfacial Processes

“…For instance, four prominent bands namely, D (disorder-induced in-plane A 1g zone edge mode), G (in-plane E 2g vibration mode of sp 2 hybridized carbon atoms), 2D (second-order D) and D+G (combination) bands occurring at approximately 1335, 1600, 2660 and 2925 cm −1 , respectively are characteristic bands for rGO. The G and 2D peak positions, the shape of 2D peak, the peak intensity, D to G intensity ratio (I D /I G ), the G to 2D peak intensity (I G /I 2D ) ratio determine number of layers, type and density of defects, degree of disorder, sp 2 -bonded carbon domains size, stacking order and electronic properties of edges for graphene [61]. The shift in G band position can also be used to determine the extent of reduction of graphene oxide (GO) to rGO.…”

An Insight into the Phase Transformation of WS2 upon Fluorination

“…The lower modes are highly active in FWS 2 , as indicated by the Raman maps and the Raman spectra in Figure c,d and Figure S2 in the Supporting Information. The pristine WS 2 belonging to the 2H phase predominantly displays the E 2g (Γ) and A 1g (Γ) modes from the in‐plane and out of plane vibrations respectively . On fluorination, these modes remain relatively unchanged although many lower order peaks seem to appear.…”

Polyurethane-graphene nanocomposite foams with enhanced thermal insulating properties

“…Several characteristic peaks can be observed: at 1344 cm -1 the D band, at 1590 cm -1 the G band, at 2688 cm -1 the 2D band; moreover, also the not very common D + G band at 2923 cm -1 can be seen. [13][14][15][16][17] Considering the position of the 2D band, it can be stated that the graphene used is a few-layer graphene; the presence of several chemical and/or physical defects can be inferred from the high D band absorbance. Also, atomic force microscopy analysis on graphene sample (Fig.…”