Evaluation of Charge and Energy Storage in Molecular Nanocapacitors

Abstract

A model is presented herein for the evaluation of stored charge and energy in molecular-scale capacitors composed of parallel nanosheets. In this model, the nanocapacitor is exposed to an external electric field, and the charging process is considered as a three-stage mechanism, including isolated, exposed, and frozen stages, where each stage possesses its own Hamiltonian and wavefunction. In this way, the third stage’s Hamiltonian is the same as that of the first stage, while its wavefunction is frozen to that of the second stage, and consequently, stored energy can be calculated as the expectation value of second stage’s wavefunction with respect to the first stage’s Hamiltonian. Electron density is then integrated over half-space, i.e., the space separated by a virtual plane located at the middle and parallel to electrodes, to reveal stored charge on nanosheets. The formalism is applied to two parallel hexagonal graphene flakes as nanocapacitor’s electrodes, and results are compared with experimental values of similar systems.

Asymmetric electronic deformation in graphene molecular capacitors

“…Three point groups ( D 2h , D 3h , or D 6h ) for graphene flake with two sizes at four different voltages (1, 2, 3, and 4 V) are considered. Figure 2 displays six graphene flakes and their labels used throughout this work and Figure 3 shows schematic structure of two parallel graphene flakes, where CC and CH bond lengths and angles are frozen close to their optimized values 1.41, 1.07 Å and 120°, respectively, and their interflake distances are frozen to 3.5 Å, near to experimental values for graphene–graphene distance (3.34–3.36 Å) [77], theoretical values depending on stacking (2.99–4.59 Å) [78], and used value for separation distance in graphene molecular capacitors [60, 79]. Figure 4.…”

“…In the past several decades, theoretical chemistry has played an important role in prediction of electronic properties required for a single molecule to function as an electronic component, such as unimolecular rectifier [55]. In 1974, Aviram and Ratner published a ground breaking paper on molecular rectifiers, proposing a simple method for creating true unimolecular rectifiers using organic molecules [56–60]. A new methodology, introduced by Mandado et al, directly obtains electric conductance from conducting channels created by the bias voltage's with the help of mixing of the occupied and virtual orbital spaces [30, 57, 60].…”

“…In 1974, Aviram and Ratner published a ground breaking paper on molecular rectifiers, proposing a simple method for creating true unimolecular rectifiers using organic molecules [56–60]. A new methodology, introduced by Mandado et al, directly obtains electric conductance from conducting channels created by the bias voltage's with the help of mixing of the occupied and virtual orbital spaces [30, 57, 60]. This methodology has been endorsed for analysis of electron transport in molecular chains, as shown by the experimental results and nonequilibrium Green's function (NEGF) results [56].…”

“…One of the major topics in the field of computational molecular electronics is analyzing charge distribution and its response to external electric field and several techniques are developed to analyze how these two components react to the applied voltage [10, 63, 64]. Also, electronic deformation density analysis is conducted for many chemical systems by present author including intermolecular and steric interaction analysis [65, 66], capacitance of bilayer graphene flakes [30, 60], and interatomic interaction [67]. In this respect, it has been shown that electronic deformations density can be used to analyze response of charge density to applied voltage [10].…”