Graphene-enhanced Raman scattering on single layer and bilayers of pristine and hydrogenated graphene

Abstract

Graphene-enhanced Raman scattering (GeRS) on isotopically labelled bilayer and a single layer of pristine and partially hydrogenated graphene has been studied. the hydrogenated graphene sample showed a change in relative intensities of Raman bands of Rhodamine 6 G (R6G) with different vibrational energies deposited on a single layer and bilayer graphene. the change corresponds qualitatively to different doping of graphene in both areas. Pristine graphene sample exhibited no difference in doping nor relative intensities of R6G Raman peaks in the single layer and bilayer areas. Therefore, it was concluded that strain and strain inhomogeneities do not affect the GERS. Because of analyzing relative intensities of selected peaks of the R6G probe molecules, it is possible to obtain these results without determining the enhancement factor and without assuming homogeneous coverage of the molecules. furthermore, we tested the approach on copper phtalocyanine molecules. Enhancement of the signal in spectroscopy has crucial importance for detection and study of a low amount of species. For Raman spectroscopy, the surface-enhanced Raman scattering (SERS) technique is widely used 1 enabling even single-molecule detection 2. One of the restrictions of this approach is the limited stability of the metals that are needed to achieve signal enhancement 3. Recently, it was observed that graphene itself can provide significant enhancement of the Raman signal and the so-called graphene-enhanced Raman scattering (GERS) 4-6 was established. Note that both SERS and GERS can contribute to the molecular Raman signal together 7. Apart from the relatively good chemical stability of graphene, it was also found that graphene quenches photoluminescence 8 , which is important for practical experiments. The GERS was observed for various molecules 9-11 and also for other 2D materials, which were employed as active substrate 12. Furthermore, it was shown that the enhancement can be tuned by changing the Fermi energy of graphene (modified by the electrical field effect) 13 , by substitutional doping with heteroatoms 14,15 , or by chemical functionalization 16,17. More detailed studies demonstrated that the enhancement is also a function of the phonon energy of the specific vibration and also laser excitation energy 10. The observed effects were rationalized by a simple theoretical approach taking into account several different resonance processes 18. It was already shown previously that different doping of graphene induced by functionalization leads to a change in the relative intensities of individual GERS peaks of the probe molecules 16. The overall GERS enhancement depends on the laser excitation energy, lowest unoccupied molecular orbital (LUMO), and highest occupied molecular orbital (HOMO) energies of the probe molecules, the vibrational energy of the actual molecular Raman mode, and on the Fermi energy of graphene 18. Specifically:

Toward Graphene‐Enhanced Spectroelectrochemical Sensors

“…Material Synthesis: As reported previously, SLG was grown via the standard CVD method. [19] In brief, a copper foil was at 1000 °C for 30 min under a 50 sccm hydrogen flow. Subsequently, CH 4 (1 sccm) was introduced into the chamber for 20 min.…”

“…Partially hydrogenated graphene samples were prepared in a high‐pressure autoclave as described in an earlier study. [ 19 ] The autoclave was flushed several times with hydrogen to remove the air. Then, the autoclave was filled with hydrogen at a pressure of 5 bar.…”

“…Quite recently, we have shown different GERS enhancements utilizing chemically modified graphene, which have different levels of doping. [ 19 ] However, graphene possesses a tunable work function with the application of external potential. [ 20 ] Utilizing this property, an electrochemical GERS sensor has huge potential to circumvent the rigid energy‐specific basis of the enhancement.…”

Emerging graphene-based sensors for the detection of food adulterants and toxicants – A review

“…Additionally, graphene enhanced LSPR devices have been modelled, fabricated and characterized, but there is limited published data on implementation in bio-sensing. Graphene has also been incorporated into SERS, a 2020 paper by Valeš (Valeš et al, 2020) on Graphene-enhanced Raman scattering (GERS) studied mono and bilayer pristine graphene partially hydrogenated to enhance the Raman signal whilst improving the quench photoluminescence simultaneously. This work highlights the change of Raman signal bands of rhodamine 6 G (R6G) when using hydrogenated and pristine graphene.…”

Ultrasensitive Detection and In Situ Imaging of Analytes on Graphene Oxide Analogues Using Enhanced Raman Spectroscopy

“…The aforementioned properties of graphene thus eliminate the problems of irreproducibility and spectral instability that are commonly encountered in SERS . Additionally the chemical enhancement in GERS can be fine-tuned by controlling the interaction distance between the analyte and graphene, the applied-voltage dependent Fermi-level shift of graphene, , the type of chemical doping, − the size of the graphitic domains and the molecular structure and energy levels of the analyte …”