Opening an Electrical Band Gap of Bilayer Graphene with Molecular Doping

Abstract

The opening of an electrical band gap in graphene is crucial for its application for logic circuits. Recent studies have shown that an energy gap in Bernal-stacked bilayer graphene can be generated by applying an electric displacement field. Molecular doping has also been proposed to open the electrical gap of bilayer graphene by breaking either in-plane symmetry or inversion symmetry; however, no direct observation of an electrical gap has been reported. Here we discover that the organic molecule triazine is able to form a uniform thin coating on the top surface of a bilayer graphene, which efficiently blocks the accessible doping sites and prevents ambient p-doping on the top layer. The charge distribution asymmetry between the top and bottom layers can then be enhanced simply by increasing the p-doping from oxygen/moisture to the bottom layer. The on/off current ratio for a bottom-gated bilayer transistor operated in ambient condition is improved by at least 1 order of magnitude. The estimated electrical band gap is up to ∼111 meV at room temperature. The observed electrical band gap dependence on the hole-carrier density increase agrees well with the recent density-functional theory calculations. This research provides a simple method to obtain a graphene bilayer transistor with a moderate on/off current ratio, which can be stably operated in air without the need to use an additional top gate.

Molecular Doping and Band-Gap Opening of Bilayer Graphene

“…Zhang et al measured a variation of 70 meV/10 13 cm -2 using different concentrations of oxygen/moisture and a protective layer of triazine. 27 We believe that the asymmetric behaviour of the band gap with carrier concentration is due to the asymmetric shape of the conduction and valence bands due to non-zero  4 hopping. It is interesting to note that we find the ratio of hole to electron effective masses found in undoped BLG to be 1.15, which is similar to the inverse of the ratio of the variation of band gap with carrier concentration for p-type and n-type dopants (1.18).…”

Carbon-Based Metal-Free Electrocatalysis for Energy Conversion, Energy Storage, and Environmental Protection

“…In addition to the heteroatom-doping-induced intramolecular charge transfer or spin redistribution that boosts electrocatalytic activities of carbon-based catalysts described above, charge-transfer doping graphitic all-carbon carbon nanomaterials with electron acceptor(s) or donor(s) can induce intermolecular charge transfer to impart catalytic activities to heteroatom-free carbon catalysts [106][107][108]. Much like heteroatom-doping-induced intramolecular charge redistribution to facilitate ORR processes, the physical adsorption of certain (macro)molecules onto all-carbon CNTs [107] or graphene sheets [108] can cause intermolecular charge transfers to induce ORR electrocatalytic activities similar to those of commercial Pt/C catalysts.…”

Electron-state engineering of bilayer graphene by ionic molecules

“…Indeed, many theoretical and experimental studies have designed and synthesized several derivatives of graphene having a large band-gap, such as graphene nanoribbon, 5,6 graphene nanomesh, 7 functionalized graphene, 8 bilayer graphene under an external electric field, 9-11 or molecular-doped bilayer graphene. [12][13][14][15] However, these methods have several disadvantages related to cost, performance, accessibility, and controllability. Therefore, there continues to exist a need to develop a simple and less expensive method for synthesizing the semiconducting graphene.…”

“…The gap value is comparable to those obtained in bilayer graphene under extremely high external electric field [9][10][11] or that with doped molecules. [12][13][14][15] In this structure, the top of the valence band and the bottom of the conduction band possess their p-state nature and are localized on the bilayer graphene. Therefore, bilayer graphene sandwiched by this cation-anion pair exhibits the characteristic features of an intrinsic semiconductor.…”

“…This fact highlights the great advantage of using ionic molecules in a sandwich structure as opposed to using other molecules adsorbed on bilayer graphene for realizing the semiconducting graphene. [12][13][14][15] In these doping procedures, the states of the guest molecules usually lie in the gap of the bilayer graphene which could affect the performance of graphene-based devices. 13 We explore the physical origin of the moderate band-gap emerging in bilayer graphene sandwiched by a cation-anion pair by investigating the electrostatic potential profile along the direction normal to the graphene layers.…”