[Paper Review] Chemical doping-induced gap opening and spin polarization in graphene

I. Zanella S. Guerini, Solange Binotto Fagan+2 more|ArXiv.org|Nov 7, 2007

Graphene research and applications16 citations

TL;DR

This study uses first-principles density functional theory to demonstrate that chemical doping of graphene with CrO₃ molecules induces a tunable electronic band gap of up to 0.12 eV and spin polarization of 0.4 μB, depending on adsorption site and molecular orientation. The effect arises from charge transfer and symmetry breaking, enabling graphene to overcome its inherent zero-gap semimetal behavior for applications in spintronics and carbon-based electronics.

ABSTRACT

By using first principles calculations we report a chemical doping induced gap in graphene. The structural and electronic properties of CrO$_3$ interacting with graphene layer are calculated using ab initio methods based on the density functional theory. The CrO$_3$ acts as an electron acceptor modifying the original electronic and magnetic properties of the graphene surface through a chemical adsorption. The changes induced in the electronic properties are strongly dependent of the CrO$_3$ adsorption site and for some sites it is possible to open a gap in the electronic band structure. Spin polarization effects are also predicted for some adsorption configurations.

Motivation & Objective

• To investigate the electronic and structural modifications of graphene induced by chemical doping with CrO₃ molecules.
• To determine whether CrO₃ adsorption can open a band gap in graphene, addressing the minimum conductivity problem in zero-gap graphene.
• To explore the possibility of inducing spin polarization in graphene through specific CrO₃ adsorption configurations.
• To analyze the role of adsorption site, molecular orientation, and curvature effects on electronic properties.
• To evaluate the potential of CrO₃-functionalized graphene as a platform for chemical sensors and spin filter devices.

Proposed method

• Employed spin-polarized Kohn-Sham density functional theory (DFT) with generalized gradient approximation (GGA) and norm-conserving pseudopotentials.
• Used the SIESTA code to perform fully self-consistent calculations on a 32-atom graphene supercell with periodic boundary conditions.
• Applied a double-zeta basis set with polarization functions and a 200 Ry cutoff for charge density integration.
• Utilized a 5×5×1 Monkhorst-Pack k-point grid for Brillouin zone sampling to ensure convergence.
• Performed structural optimization via conjugate gradient method until forces on atoms were below 0.05 eV/Å.
• Analyzed charge transfer, band structure, density of states (DOS), and localized DOS (LDOS) to assess electronic and magnetic responses.

Experimental results

Research questions

• RQ1 Can CrO₃ adsorption on graphene induce a band gap through symmetry breaking and charge transfer?
• RQ2 How does the adsorption site and molecular orientation of CrO₃ influence the electronic structure of graphene?
• RQ3 Under which configurations does CrO₃ doping lead to spin polarization in graphene?
• RQ4 How does curvature (e.g., in carbon nanotubes) affect the binding energy and adsorption stability of CrO₃ on carbon surfaces?
• RQ5 To what extent can CrO₃-functionalized graphene serve as a platform for tunable electronic and spintronic devices?

Key findings

• A band gap of approximately 0.12 eV opens at the Dirac point when CrO₃ is adsorbed at the most stable site, due to breaking of graphene's mirror symmetry.
• Spin polarization of 0.4 μB is observed only in the most stable adsorption configuration, attributed to charge redistribution between CrO₃ oxygen atoms and the graphene surface.
• The Fermi level shifts down by up to 0.8 eV relative to pristine graphene due to electron transfer from graphene to CrO₃.
• The electronic structure is highly sensitive to adsorption site and molecular orientation, with only specific configurations inducing gap opening and spin polarization.
• CrO₃ binds more strongly to curved carbon structures (e.g., (8,0) SWNT) with a binding energy increase of 0.4 eV compared to flat graphene (1.01 eV vs. 1.4 eV).
• Hybridization between CrO₃ 3d states and graphene π-bands is confirmed via localized DOS plots, showing distinct features at -1.2 eV and -1.8 eV.