The emergence of quantum capacitance in epitaxial graphene

Abstract

We found an intrinsic redistribution of charge arises between epitaxial graphene, which has intrinsically n-type doping, and an undoped substrate. In particular, we studied in detail epitaxial graphene layers thermally elaborated on C-terminated $4H$-$SiC$ ($4H$-$SiC$ ($000{\bar{1}}$)). We have investigated the charge distribution in graphene-substrate systems using Raman spectroscopy. The influence of the substrate plasmons on the longitudinal optical phonons of the $SiC$ substrates has been detected. The associated charge redistribution reveals the formation of a capacitance between the graphene and the substrate. Thus, we give for the first time direct evidence that the excess negative charge in epitaxial monolayer graphene could be self-compensated by the $SiC$ substrate without initial doping. This induced a previously unseen redistribution of the charge-carrier density at the substrate-graphene interface. There a quantum capacitor appears, without resorting to any intentional external doping, as is fundamentally required for epitaxial graphene. Although we have determined the electric field existing inside the capacitor and revealed the presence of a minigap ($\approx 4.3meV$) for epitaxial graphene on $4H$-$SiC$ face terminated carbon, it remains small in comparison to that obtained for graphene on face terminated $Si$. The fundamental electronic properties found here in graphene on $SiC$ substrates may be important for developing the next generation of quantum technologies and electronic/plasmonic devices.Comment: 26 pages, 8 figures, available online as uncorrected proof, Journal of Materials Chemistry C (2016

Efficient green emission from edge states in graphene perforated by nitrogen plasma treatment

“…It is important that the topmost level, which is clearly recognized in the original BLG sample, is absent in the BLG sample treated with N 2 plasma. Thus, from the present AFM study we can conclude that (i) the presence of nanopockets of intercalated water or other solvents trapped between the adjacent CVD graphene sheets during wet-transfer to a SiO 2 /Si substrate leads to the pronounced rippling effects on the upper original BLG sheet and (ii) the nitrogen plasma treatment (with the plasma strength 0.34 mbar and 20 W) with the exposure time of 90 s leads to etching of the ripples, nanodomes, and bubbles [34][35][36][37]. At the same time, according to our AFM study, the applied oxygen plasma conditions (with the plasma strength 0.06 mbar and 20 W) with the exposure time of 90 s lead to weaker etching effect of the upper BLG sheet at room temperature (see figure S2(a-c)).…”

“…Here, we concentrate our attention on the morphology aspects in plasma treatment of graphene, which have not yet been fully explored. It is well known that freely suspended graphene has "intrinsic" ripples, nanodomes, and bubbles [33][34][35][36][37][38][39]. The existing microscopic corrugations have nanosized lateral dimensions and a height displacement of about 0.7 to 1 nm.…”

“…Other nitrogen doping methods include N-doping of graphene, for example, through electrothermal reactions with NH 3 [30], by performing the arc discharge of graphite electrodes in the presence of H 2 , He, and NH 3 [31], by controlled electrochemical deposition [32], etc Here, we concentrate our attention on the morphology aspects in plasma treatment of graphene, which have not yet been fully explored. It is well known that freely suspended graphene has 'intrinsic' ripples, nanodomes, and bubbles [33][34][35][36][37][38][39]. The existing microscopic corrugations have nanosized lateral dimensions and a height displacement of about 0.7 to 1 nm.…”

Morphological imperfections of epitaxial graphene: from a hindrance to the generation of new photo-responses in the visible domain

“…Conventionally, an I G /I 2D ratio of <0.5 corresponds to single layer graphene (n = 1), a ratio of 1.5 relates to four-layer graphene (n = 4) and a ratio of 2 denotes five-layer graphene (n = 5). 30,31 Here, the I G /I 2D ratio varies between 0.2-4. Various graphene flakes were located and mainly single (n = 1), four-(n = 4) and five-(n = 5) layer coverage was found (see, Fig.…”

“…However, a negligible amount of graphite flakes (I G /I 2D = 2.5, n > 5) were also located. 31,32 From the mapping intensities the graphene flakes appear to be randomly distributed. Nevertheless, the graphene quality in each flake is highly homogeneous.…”