Dynamic Modification of Fermi Energy in Single-Layer Graphene by Photoinduced Electron Transfer from Carbon Dots

Abstract

Graphene (Gr)—a single layer of two-dimensional sp2 carbon atoms—and Carbon Dots (CDs)—a novel class of carbon nanoparticles—are two outstanding nanomaterials, renowned for their peculiar properties: Gr for its excellent charge-transport, and CDs for their impressive emission properties. Such features, coupled with a strong sensitivity to the environment, originate the interest in bringing together these two nanomaterials in order to combine their complementary properties. In this work, the investigation of a solid-phase composite of CDs deposited on Gr is reported. The CD emission efficiency is reduced by the contact of Gr. At the same time, the Raman analysis of Gr demonstrates the increase of Fermi energy when it is in contact with CDs under certain conditions. The interaction between CDs and Gr is modeled in terms of an electron-transfer from photoexcited CDs to Gr, wherein an electron is first transferred from the carbon core to the surface states of CDs, and from there to Gr. There, the accumulated electrons determine a dynamical n-doping effect modulated by photoexcitation. The CD–graphene interaction unveiled herein is a step forward in the understanding of the mutual influence between carbon-based nanomaterials, with potential prospects in light conversion applications.

Micro-photoluminescence of Carbon Dots Deposited on Twisted Double-Layer Graphene Grown by Chemical Vapor Deposition

“…Looking at Figure d, we notice that the BLs Raman spectra have a notable PL band centered at around 2.32 eV, which is due to the presence of CDs on top of the graphene substrate. As mentioned above, the emission of CDs is affected by the substrate they are deposited on. ,, In fact, for the spectra acquired on BL graphene, the integrated μPL area (from 1120 to 2980 cm –1 ) is about 10 times more intense compared to the case of SL graphene (black line in Figure d). Figure e shows the μPL map associated with the spectra recorded within the area of Figure a.…”

“…A large PL band centered at about 2.36 eV characterizes CD/SiO 2 spectrum (red curve), while the CD/SL spectrum (blue curve) exhibits the characteristic G and 2D graphene Raman bands superimposed to a background CD PL band centered at ∼2.32 eV. , It is worth to note that the sample of graphene features a low-intensity D band both in the naked and in the CD-covered region, underlying that the quality of graphene is not modified by the deposition. The strong PL contribution dominates the interaction between CDs and light on the SiO 2 surface . On the other side, it is not surprising to find that the intensity of the PL band is significantly quenched when the CDs are deposited on graphene.…”

“…The structural and emission properties of CDs deposited on SL graphene were previously investigated. , It has been shown that the PL quenching of CDs deposited on SL graphene can be explained invoking the interaction between the surface states of CDs, responsible of the emission process, , and the band structure of graphene . Briefly, thanks to the close contact between the excited surface states of CDs and the SL graphene conduction band, photoelectrons, generated by the absorption process at energies greater than 2 eV, can be transferred from CDs to SL graphene …”

“…The strong PL contribution dominates the interaction between CDs and light on the SiO 2 surface. 60 On the other side, it is not surprising to find that the intensity of the PL band is significantly quenched when the CDs are deposited on graphene. This is in line with previous studies, where the quenching effect of CD PL due to graphene was observed by exciting the CDs/SL samples with a laser wavelength of 532 nm (2.33 eV).…”

“…This is in line with previous studies, where the quenching effect of CD PL due to graphene was observed by exciting the CDs/SL samples with a laser wavelength of 532 nm (2.33 eV). 17,45,60 In order to emphasize this effect, a line profile measured along the dashed lines in Figure 2a−c is reported in Figure 2e− g. Here, in correspondence of the area covered by graphene, CD PL undergoes a strong intensity reduction if compared to the μPL level observed on the region uncovered by graphene, that is, the SiO 2 substrate (Figure 2f), as also confirmed by the line profile of A 2D (Figure 2g), increasing as the PL is dropping, and the topography height profile acquired with AFM (Figure 2e), which allowed us to measure an SL graphene thickness of 1 nm when lying on SiO 2 /Si. The presence of many double layers in the commercial CVD graphene analyzed induced us to carry out a more detailed μPL spectroscopy investigation on the emission properties of CDs interacting with BL graphene (CDs/BL).…”