Molecular Doping and Band-Gap Opening of Bilayer Graphene

Abstract

The ability to induce an energy band gap in bilayer graphene is an important development in graphene science and opens up potential applications in electronics and photonics. Here we report the emergence of permanent electronic and optical band gaps in bilayer graphene upon adsorption of  electron containing molecules. Adsorption of n-or p-type dopant molecules on one layer results in an asymmetric charge distribution between the top and bottom layers and in the formation of an energy gap. The resultant band gap scales linearly with induced carrier density though a slight asymmetry is found between n-type dopants, where the band gap varies as 47 meV/10 13 cm -2 , and p-type dopants where it varies as 40 meV/10 13 cm -2 .Decamethylcobaltocene (DMC, n-type) and 3,6-difluoro-2,5,7,7,8,8-hexacyano-quinodimethane (F2-HCNQ, p-type) are found to be the best molecules at inducing the largest electronic band gaps up to 0.15 eV. Optical adsorption transitions in the 2.8 -4 m region of the spectrum can result between states that are not Pauli blocked. Comparison is made between the band gaps calculated from adsorbate-induced electric fields and from average displacement fields found in dual gate bilayer graphene devices. A key advantage of using molecular adsorption with  electron containing molecules is the high binding energy can induce a permanent band gap and

Band gap modulation of bilayer graphene by single and dual molecular doping: A van der Waals density-functional study

“…The corresponding electron and hole effective masses are calculated to be 0.045m 0 and 0.056m 0 . These values agree very well with previous DFT estimates [15], but overestimate slightly experimental ones [46]: 0.041 and 0.036m 0 , respectively. The bands in green correspond to the two next levels.…”

“…They are very similar to the monolayer case, with very small energy differences between adsorption sites. Moreover the charge transfers from the different configurations are almost equal, as reported in a previous study [15]. Thus only structures, which minimize the total energy, are taken into consideration in the following.…”

“…3c indicate that TTF's adsorption doesn't affect electronic structure of BLG except an upshift of Fermi level with an E D value of 251 meV. This is in contradiction with the reported band gap of 80 meV [15]. One could first attribute this discrepancy to a concentration effect, since we have 1 TTF molecule per 98 C atoms, when 72 C atoms were used in Samuels' work.…”

“…Therefore, considerable efforts have been made to open band gaps in MLG or BLG. Both theoretical and experimental studies have demonstrated that a band gap can appear in the Bernal-stacking (AB-stacking) BLG, by breaking inversion symmetry [9][10][11][12][13][14][15], by applying an external electric field for instance [16][17][18], by breaking in-plane symmetry [19], by applying different homogeneous strains to the two layers [20] or by varying the interlayer spacing [21]. Molecular doping is also an alternative by provoking asymmetric charge distribution in graphene layers.…”

Tuning the charge carriers in epitaxial graphene on SiC(0001) from electron to hole via molecular doping with C60F48

“…Importantly, not only is it clear that this quantitative model is quite accurate in predicting the observed doping behaviour of F4-TCNQ and C 60 F 48 deposited onto epitaxial graphene, it should be equally applicable to other tailored electron affinity organic molecules, representing a powerful framework within which doping behaviour can be predicted. Our successful description of the doping process differs significantly from that given previously in terms of independent particle band structure calculations 30,31 in that it takes the highly correlated nature of molecular orbitals and the temperature dependence of doping efficiency properly into account.…”