Electronic Spectra of Graphene in Far- and Deep-Ultraviolet Region: Attenuated Total Reflection Spectroscopy and Quantum Chemical Calculation Study

Abstract

We measured the electronic spectra of graphene nanostructures (flakes and platelets) extending into the far-ultraviolet (FUV) region by attenuated total reflection far- and deep-ultraviolet (ATR–FUV–DUV) spectroscopy in the region of 2.76–8.55 eV (450–145 nm). The major absorption of graphene appears in the DUV region (4.7 eV), as already reported; however, we observed a new peak in the FUV region, visible clearly in the case of flakes at 7.5–7.7 eV (165–161 nm) and less pronounced in the spectrum of the platelets at 6.6–6.7 eV (188–185 nm). Graphene flakes (thickness 1–2 nm; sub-micrometers of side dimension) and nanoplatelets (thickness 6–8 nm; several micrometers of side dimension) give notably different ATR absorbance spectra in the spectral region studied. This discrepancy is reduced upon applying mechanical pressure on the samples. These observations can evidence that the morphology as well as electronic structure of graphene can be manifested in the FUV–DUV region. Quantum chemical calculations were applied to several molecular models incorporating the expected principal structural features of graphene nanostructures. On the basis of the results of time-dependent density functional theory and Zerner’s intermediate neglect of differential overlap (ZINDO) calculations, it was possible to consistently reproduce the experimental spectral variations in terms of both band positions and intensities. The spectral differences result from the differences in the die area, ordering and the number of layers, and structural factors which separate nanoflakes and nanoplatelets. These results provide insights into the probable origins of the spectral variability of graphene nanostructures as well as the molecular orbitals involved in a FUV π–π* transition of graphene nanostructures.

ATR-far-ultraviolet spectroscopy: a challenge to new σ chemistry

“…In the reviewed study, the ZINDO/s method was used to obtain relative excitation energies, which were in good agreement with the observed peaks in the experimental spectra, with additional linear scaling of the calculated vertical excitation energies 54,107 In the case of ATR-FUV-DUV study of graphene nanostructures, wavelength scaling yielded a remarkably good agreement with experimental data, as demonstrated in Fig. 21.…”

“…108,110,111 The examination of these experimental spectra with aid from quantum chemical calculations study also revealed a dependence of the 165 nm peak on the graphene morphology. 107 The presence of the DUV peak in the spectra of graphene and other nanocarbon structures has been previously documented, but its reported position has varied widely, ranging from 227-260 nm (4.65-5.46 eV). 112,113 The debate surrounding the case of graphite highlights the need for continued experimental and computational investigations into the specifics of FUV-DUV absorption characteristics in carbon nanostructures.…”

“…[114][115][116] ZINDO/s approach was also applied for the purpose of ATR-FUV-DUV study of carbon nanostructures, with additional evaluation of its reliability compared with TD-FT calculations performed on a limited scale, for selected models that captured all significant structural features that were meaningful for the examined systems. 107 A comparison with TD-DFT was conducted for smaller models representing key structural features of graphene nanoflakes, and both methods produced comparable theoretical FUV-DUV spectra, as shown in Fig. 20.…”

“…[104][105][106] The examination of electronic and structural properties of graphene nanostructures, as well as their composites with polymers, has formed a remarkable breakthrough enabled by ATR-FUV-DUV spectroscopy. 71,107 In particular, the investigation of the FUV-DUV spectra of graphene nanoparticles delivered a novel perspective on the high-energy electronic transitions of carbon nanostructures, 107 complementing and extending beyond the previously available knowledge on graphite spectra in the high-energy region. 108 Graphene-based nanostructures, including flakes and platelets, offer an economical substrate for the production of nanocomposites with polymers, enhancing their material properties.…”

“…71,109 However, conventional electronic absorption spectroscopy is severely limited when examining such samples. 107 Little was known on the high-energy electronic transitions of graphene nanostructures, particularly in the FUV-DUV region, and scarce data about their absorption in wavelengths below 200 nm (above 6.2 eV) was prior available. The study of graphene nanoflakes (with a surface area of 300 m 2 g À1 ) and nanoplatelets (6-8 nm thick, 5 mm wide) by ATR-FUV-DUV spectroscopy revealed a previously unknown FUV peak of low intensity at approximately 165 nm (7.5 eV), in addition to the strong FUV band at about 265 nm (4.65 eV) that was already well-known.…”