A multiscale study of electronic structure and quantum transport in C_6n²H_6n-based graphene quantum dots

Abstract

We implement a bottom-up multiscale approach for the modeling of defect localization in C 6n 2 H6n islands, i.e. graphene quantum dots with a hexagonal symmetry, by means of density functional and semiempirical approaches. Using the ab initio calculations as a reference, we recognize the theoretical framework under which semiempirical methods describe adequately the electronic structure of the studied systems and thereon proceed to the calculation of quantum transport within the non-equilibrium Green's function formalism. The computational data reveal an impurity-like behavior of vacancies in these clusters and evidence the role of parameterization even within the same semiempirical context. In terms of conduction, failure to capture the proper chemical aspects in the presence of generic local alterations of the ideal atomic structure results in an improper description of the transport features. As an example, we show wavefunction localization phenomena induced by the presence of vacancies and discuss the importance of their modeling for the conduction characteristics of the studied structures.

Lack of universal conductance features in disordered graphene nanoribbons

“…However, using the following calibration eV eV t d d 10 , 9 . 1 = = ε (i.e a rather large value of the on-site energy) the states associated to V-type defects in the TB model recover the features of those calculated with the ab-initio scheme [18].…”

“…[15]. First principle calculation of V-type defects [17,18] shows that they deviate from the typical behavior of the ∞ → l ε scatter center (in particular these calculations do not evidence zero modes for isolated V). However, using the following calibration eV eV t d d 10 , 9 .…”

Conductance distribution in doped and defected graphene nanoribbons

“…A particular consequence of the symmetry of the zero mode should be an on/off switching of the low bias conductance in contacted graphene quantum dots depending on the contact's location. 12 Ab initio calculations have excluded such a behavior since states around the charge neutrality points do not show zero-energy mode symmetries. Moreover, opposite to the impurity case and due to the bond reconstructions, the vacancy presence significantly alters also the off-diagonal sector of the Fock matrix projected on the p z orbitals.…”

“…As a consequence, in order to obtain a reliable and effective calibration of the V-type defect in a nearestneighbor tight-binding Hamiltonian, the on-site and hopping terms relative to the site occupied by the defects has been tuned in order to obtain states around the charge neutrality point which reproduce the correct symmetries ͑and the correct conductance features including the lacking of the cited on/off switch͒ of the ab initio calculations. 12 The tuned values are i = 10 eV and t ij = 1.9 eV ͑to be compared to the 2.7 eV values of the perfect lattice͒.…”

Transport and noise properties of graphene-based transistors revealed through atomistic modelling

“…We compute mobility through computation of resistance on an ensemble of about 100 nanoribbon segments with different actual distribution of non-idealities as where L is the segment length, Q i and R ch,i are the total charge and the resistance of the i-th simulated GNR segment, respectively, R B is the resistance of a ballistic segment in the same bias conditions and δ ε denotes the statistical average on the ensemble. We model defects by computing the on-site energy and the hopping parameter of the Hamiltonian from DFT cal-culations [6], and edge-disordered ribbons by randomly removing atoms on the two outermost dimer lines. For ionized impurities in the dielectric layer, we assume a point charge approximation.…”