Meng-Qiu Long

Electronic Structure and Carrier Mobility in Graphdiyne Sheet and Nanoribbons: Theoretical Predictions

Using density functional theory coupled with Boltzmann transport equation with relaxation time approximation, we investigate the electronic structure and predict the charge mobility for a new carbon allotrope, the graphdiyne for both the sheet and nanoribbons. It is shown that the graphdiyne sheet is a semiconductor with a band gap of 0.46 eV. The calculated in-plane intrinsic electron mobility can reach the order of 10(5) cm(2)/(V s) at room temperature, while the hole mobility is about an order of magnitude lower.

First-principles prediction of charge mobility in carbon and organic nanomaterials

We summarize our recent progresses in developing first-principles methods for predicting the intrinsic charge mobility in carbon and organic nanomaterials, within the framework of Boltzmann transport theory and relaxation time approximation. The electron-phonon couplings are described by Bardeen and Shockleys deformation potential theory, namely delocalized electrons scattered by longitudinal acoustic phonons as modeled by uniform lattice dilation. We have applied such methodology to calculating the charge carrier mobilities of graphene and graphdiyne, both sheets and nanoribbons, as well as closely packed organic crystals. The intrinsic charge carrier mobilities for graphene sheet and naphthalene are calculated to be 3 × 10(5) and ∼60 cm(2) V(-1) s(-1) respectively at room temperature, in reasonable agreement with previous studies. We also present some new theoretical results for the recently discovered organic electronic materials, diacene-fused thienothiophenes, for which the charge carrier mobilities are predicted to be around 100 cm(2) V(-1) s(-1).

Theoretical predictions of size-dependent carrier mobility and polarity in graphene.

First-principles density functional theory coupled with deformation potential calculations indicate a strong width-dependent carrier mobility: for an armchair graphene ribbon whose width (i.e., number of carbons along the edge) is N = 3k, the room-temperature electron mobility is calculated to be approximately 10(6) cm(2) V(-1) s(-1) and the hole mobility approximately 10(4) cm(2) V(-1) s(-1), while for N = 3k + 1 or 3k + 2, the hole mobility is calculated to be 4-8 x 10(5) cm(2) V(-1) s(-1) and the electron mobility approximately 10(4) cm(2) V(-1) s(-1). Such alternating behavior is absent in zigzag-type graphene.

Effect of length and size of heterojunction on the transport properties of carbon-nanotube devices

By applying nonequilibrium Green’s functions in combination with the density-functional theory, we investigate the electronic transport properties of molecular junctions constructed by the mirror symmetrical straight carbon-nanotube heterojunctions. The results show that the length and size of heterojunction play an important role in the electronic transport properties of these systems. The negative differential resistance behavior can be observed in such devices with certain length and size of heterojunction. A mechanism is suggested for the negative differential resistance behavior.

Negative differential resistance induced by intermolecular interaction in a bimolecular device

Using nonequilibrium Green’s functions in combination with the density-functional theory, we study the electronic transport properties of the molecular device constructed by two cofacial oligo(phenylene ethynylene) molecules and gold electrodes. The results show that negative differential resistance can be observed when the intermolecular distance closes to a certain value. We propose that a combination of the splitting of the molecular orbitals due to the intermolecular interaction and the change of the coupling between the molecules and the electrodes at different biases might be responsible for the negative differential resistance behavior.

Negative differential resistance behaviors in porphyrin molecular junctions modulated with side groups

By applying nonequilibrium Green’s functions in combination with the density-functional theory, we investigate the electronic transport properties of molecular junctions constructed by the porphyrin molecule with donor or acceptor side groups. The results show that the side groups play important role on the electron transport properties. Negative differential resistance (NDR) is observed in such devices. Especially for the molecule with electron-donating group (−NH2), two NDR appear at different bias voltage regions, and the origins for both NDR behavior are different. A mechanism is proposed for the NDR behavior.

First-principles investigation of organic semiconductors for thermoelectric applications

First-principles band structure calculations coupled with the Boltzmann transport theory are used to study the thermoelectric properties in pentacene and rubrene crystals. In the constant relaxation time and rigid band approximations, the electronic contribution to the Seebeck coefficient is obtained. The absolute value of Seebeck coefficient and its temperature and carrier density dependences are in quantitative agreement with the recent field-effect-modulated measurement. The dimensionless thermoelectric figure of merit is further evaluated based on the calculated transport coefficients and experimental parameters. The peak values of figure of merit in pentacene fall in the range of 0.8-1.1, which are close to those of the best bulk thermoelectric materials. Our investigations show that organic semiconductors can be potentially good thermoelectric materials for near-room-temperature applications.

Effect of intertube interaction on the transport properties of a carbon double-nanotube device

By applying nonequilibrium Green’s functions and first-principles calculations, we investigate the transport behaviors of the bitube device with two single-walled nanotubes attached to metal electrodes. The results show that the intertube interactions play an important role in the conducting behavior of these systems. By adjusting the intertube distance and the orientational order, namely changing the magnitude of the intertube interactions, a different transport behavior can be observed in the system.

EFFECTS OF STRUCTURAL AND CENTRAL METAL IONS MODIFICATION ON THE ELECTRONIC TRANSPORT PROPERTIES OF PORPHYRIN MOLECULAR JUNCTIONS

We investigate the electronic transport properties of molecular devices constructed by porphyrin modulated with central metal ions by applying the elastic scattering Greens function theory approach in combination with the hybrid density functional theory. The effect of the porphyrin center metal ions and the twisting of the middle benzene rings on the electronic transport properties of the molecular devices is considered in detail. The results show that the the center metal ions can enhance the coupling of the molecule and electrode, while the twisting of the middle benzene rings is of a opposite effect.