arXiv subjects

Farhad Khoeini

Publications and source records attributed to Farhad Khoeini.

At least 19 recordsLinked to original sources

arXiv2024

Phosphorene Junctions as a Platform for Spin-Selective Quantum Dots in Next-Generation Devices

The impact of vacancies on spin-resolved electronic properties of quantum dots (QDs) in phosphorene-based junctions, are investigated numerically. Regardless of the crystal orientation, a phosphorene nanoribbon (PNR) containing a monovacancy is found to exhibit a topological quasi-flat band that emerges within the band gap. The electronic properties of QDs, including spatial confinement and energy level distribution, can be strongly tuned by controlling the topological structure of the QDs and by applying electric fields. Additionally, these QDs exhibit remarkable spin-selective properties under a ferromagnetic exchange field, enabling the manipulation of QD features. This opens up the potential for novel applications such as quantum computing, magnetic sensing, spin-based light emission.

cond-mat.mes-hall↗

arXiv2024

Band engineering of phosphorene/graphene van der Waals nanoribbons toward high-efficiency thermoelectric devices

Vertical integration of dissimilar layered materials in a so-called van der Waals (vdW) heterostructure (HS) has emerged as a useful tool to engineer band alignments and interfaces. In this paper, we investigate thermoelectric currents in a phosphorene/graphene vdW nanoribbon consisting of an armchair graphene nanoribbon (AGNR) stacked on an armchair phosphorene nanoribbon (APNR). We focus on the currents driven by a temperature difference between the leads in a two-probe junction. In contrast to pristine AGNRs and APNRs, such an armchair-edged HS can provide several nano-amperes of the current at room temperature, without any external field. External electric fields modify the electronic band structure and are able to induce a type-I to type-II band alignment transition and a direct-to-indirect band gap transition. Biasing the APNR/AGNR by an external electric field is found to strongly increase the thermally induced current and also control the direction of current flow at moderate temperatures. These results are important for potential applications of the APNR/AGNR vdW HS in flexible electronics and thermoelectric devices.

cond-mat.mes-hall↗

arXiv2024

Flexible thermoelectrics in crossed graphene/hBN composites

Nanostructures exhibit unusual properties due to the dominance of quantum mechanical effects. In addition, the geometry of a nanostructure can have a strong influence on its physical properties. Using the tight-binding (TB) and force-constant (FC) approaches with the help of the non-equilibrium Green's function (NEGF) method, the transport and thermoelectric properties of cross-shaped (X-shaped) composite heterostructures are studied in two cases: Mixed graphene and h-BN (HETX-CBN) and all graphene (HETX-C) cross-shaped structures. Our numerical results show that an X-shaped structure helps to manipulate its electronic and phononic properties. The transport energy gap can be tuned in the range of ~0.8 eV by changing one arm width. Due to the drastic decrease in the electronic conductance of HETX-CBN and the dominance of the phononic thermal conductance, the ZT performance is degraded despite the high S value (in the order of meV). However, HETX-C has better ZT performance due to better electronic conductance and lower phononic/electronic thermal ratio, it can enhance the ZT ~2.5 times compared to that of zigzag graphene nanoribbon. The thermoelectric properties of the system can be tuned by controlling the size of the arms of the device and the type of its atoms.

cond-mat.mtrl-sci↗

arXiv2023

Insights into electronic and transport properties of phosphorene nanorings in two perpendicular directions: Effects of circular and elliptical external potentials

In this work, we study the electronic and transport properties of phosphorene nanorings in two perpendicular directions (zigzag and armchair directions) in the presence of zigzag metallic source and drain leads. Our results are based on the non-equilibrium Green's function (NEGF) method and a five-parameter tight-binding (TB) approach. We investigate how system parameters affect the electronic transport. These parameters include the radius of the rings, the width of the leads and the external potential. Our results show that for all configurations studied, a transport energy gap exists whose width can be tuned by the width of the leads and the radius of the nanoring. The transmission function of wider leads shows more sensitivity to the variation of the inner radius due to higher electronic states that can respond to smaller changes in the scattering region. In addition, the transport along the armchair direction is more susceptible to tuning than the transport along the zigzag direction. The effects of external potentials on the conductance are more pronounced than the geometrical parameters. In particular the circular potential of the amplitude of 0.1 eV can widen the transport gap by about ~0.35 eV.

cond-mat.mes-hall↗

arXiv2022

Electronic transport and thermoelectric properties of phosphorene nanodisk under an electric field

The Seebeck coefficient is an important quantity in determining the thermoelectric efficiency of a material. Phosphorene is a two-dimensional material with a puckered structure, which makes its properties anisotropic. In this work, a phosphorene nanodisk (PDisk) with a radius of 3.1 nm connected to two zigzag phosphorene nanoribbons is studied, numerically, by the tight-binding (TB) and non-equilibrium Greens function (NEGF) methods in the presence of transverse and perpendicular electric fields. Our results show that the change of the structure from a zigzag ribbon form to a disk one creates an energy gap in the structure, so that for a typical nanodisk with a radius of 3.1 nm, the size of the energy gap is 3.88 eV. Besides, with this change, the maximum Seebeck coefficient increases from 1.54 to 2.03 mV/K. Furthermore, we can control the electron transmission and Seebeck coefficients with the help of the electric fields. The numerical results show that with the increase of the electric field, the transmission coefficient decreases, and the Seebeck coefficient changes. The effect of a perpendicular electric field on the Seebeck coefficient is weaker than a transverse electric field. For an applied transverse electric field of 0.3 V/nm, the maximum Seebeck coefficient enhances to 2.09 mV/K.

cond-mat.mtrl-sci↗

arXiv2022

Tuning Phononic and Electronic Contributions of Thermoelectric in defected S-Shape Graphene Nanoribbons

Thermoelectrics as a way to use waste heat, is essential in electronic industries, but its low performance at operational temperatures makes it inappropriate in practical applications. Tailoring graphene can change its properties. In this work, we are interested in studying the transport properties of S-shape graphene structures with the single vacancy (SV) and double vacancy (DV) models. The structures are composed of a chiral part, which is an armchair graphene nanoribbon, and two zigzag graphene ribbons. We investigate the changes in the figure of merit by means of the Seebeck coefficient, electronic conductance, and electronic and phononic conductances with the vacancies in different device sizes. The transport properties of the system are studied by using the non-equilibrium Greens function method, so that the related Hamiltonians (dynamical matrices) are obtained from the tight-binding (force constant) model. The maximum figure of merit (ZT) obtains for the DVs in all lengths. Physical properties of such a system can be tuned by controlling various parameters such as the location and the type of the defects, and the device size. Our findings show that lengthening the structure can reduce phononic contribution, and single vacancies than double vacancies can better distinguish between electronic thermal conductance behavior and electronic conductance one. Namely, vacancy engineering can significantly increase thermoelectric performance. In the large devices, the SVs can increase the ZT up to 2.5 times.

cond-mat.mes-hall↗

arXiv2021

Thermal transport in two-dimensional C3N/C2N superlattices: A molecular dynamics approach

Nanostructured superlattices have been the focus of many researchers due to their physical and manipulatable properties. They aim to find promising materials for new electronic and thermoelectric devices. In the present study, we investigate the thermal conductivity of two-dimensional (2D) C3N/ C2N superlattices using non-equilibrium molecular dynamics. We analyze the dependence of thermal conductivity on the total length, temperature, and the temperature difference between thermal baths for the superlattices. The minimum thermal conductivity and the phonon mean free path at a superlattice period of 5.2 nm are 23.2W/m.K and 24.7 nm, respectively. Our results show that at a specific total length, as the period increases, the number of interfaces decreases, thus the total thermal resistance decreases, and the effective thermal conductivity of the system increases. We found that at long lengths (L_x >80 nm), the high-frequency and low-wavelength phonons are scattered throughout the interfaces, while at short lengths, there is a wave interference that reduces the thermal conductivity. The combination of these two effects, i.e., the wave interference and the interface scattering, is the reason for the existence of a minimum thermal conductivity in superlattices.

cond-mat.mtrl-sci↗

arXiv2021

Vacancy tuned thermoelectric properties and high spin filtering performance in graphene/silicene heterostructures

The main contribution of this paper is to study the spin caloritronic effects in defected graphene/silicene nanoribbon (GSNR) junctions. Each step-like GSNR is subjected to the ferromagnetic exchange and local external electric fields, and their responses are determined using the nonequilibrium Greens function (NEGF) approach. To further study the thermoelectric (TE) properties of the GSNRs, three defect arrangements of divacancies (DVs) are also considered for a larger system, and their responses are re-evaluated. The results demonstrate that the defected GSNRs with the DVs can provide an almost perfect thermal spin filtering effect (SFE), and spin switching. A negative differential thermoelectric resistance (NDTR) effect and high spin polarization efficiency (SPE) larger than 99.99 percent are obtained. The system with the DV defects can show a large spin-dependent Seebeck coefficient, equal to 1.2 mV/K, which is relatively large and acceptable. Appropriate thermal and electronic properties of the GSNRs can also be obtained by tuning up the DV orientation in the device region. Accordingly, the step-like GSNRs can be employed to produce high efficiency spin caloritronic devices with various features in practical applications.

cond-mat.mes-hall↗

arXiv2021

Effect of graphene and carbon-nitride nanofillers on the thermal transport properties of polymer nanocomposites: A combined molecular dynamics and finite element study

Low thermal conductivity of polymers, which is one of the considerable drawbacks of commonly used composite structures, has been the focus of many researchers aiming to achieve high-performance polymer-based nanocomposites through the inclusion of highly thermally conductive fillers inside the polymer matrices. Thus, in the present study, a multiscale scheme using non-equilibrium molecular dynamics (NEMD) and finite element (FE) method is developed to explore the impact of different nano-sized fillers (carbon-nitride and graphene) on the effective thermal conductivity of polyethylene-based nanocomposites. We show that the thermal conductivity of amorphous polyethylene at room temperature using the reactive bond order (REBO) interatomic potential is nearly 0.36 0.05W/mK. Also, the atomistic results predict that, compared to the C3N and graphene nanosheets, the C2N nanofilm presents a much stronger interfacial thermal conductance (ITC) with polyethylene. Furthermore, the results indicate that the effective thermal conductivity values of C2N-polyethylene, C3N-polyethylene, and graphene-polyethylene nanocomposite, at constant volume fractions of 1%, are about 0.47, 0.56, and 0.74 W/mK, respectively. In other words, the results of our models reveal that the thermal conductivity of fillers is the dominant factor that defines the effective thermal conductivity of nanocomposites.

cond-mat.mtrl-sci↗

arXiv2020

Pure spin current and negative differential electric resistance in hybrid systems

This paper aims to investigate the spin dependent transport properties in hybrid nanoribbons, using the non equilibrium Greens function method. The effects of temperature gradient between the left and right leads, the ferromagnetic exchange field, and the local external electric fields are also included. The results show that the spin up and spin down currents are produced and flow in opposite directions with almost equal magnitudes. This evidently shows that the carrier transport is dominated by the thermal spin current. Whereas the thermal electron current is almost disappeared.

cond-mat.mes-hall↗

arXiv2020

Highly tunable charge transport in defective graphene nanoribbons under external local forces and constraints: A hybrid computational study

In this paper, we propose a combined modeling of molecular mechanics (MM) and the tight-binding (TB) approach, which enables us to study the effect of factors such as external local forces, constraints, and vacancy defects on electronic transport properties of nanomaterials. Nanostructures selected in this work are armchair graphene nanoribbons (AGNRs). According to this method, the nanostructure is modeled as a frame, and the beam element is applied for illustrating the covalent interatomic interactions in bonds. In our calculations, the terms of torsional, stretching, and bending energies are considered. The selected pristine nanoribbon is a metal, and the purpose of this study is to find the effects of mechanical loading, the vacancy defects and their positions on the electrical conductance of the structure. We observe that the presence of vacancy defects in the structure leads to the opening of an energy gap, which changes the phase of the nanostructure from metal to the semiconductor. We find that with increasing the number of point defects, the energy gap size of the strained system grows. Besides, increasing the magnitude of the local force reduces the conductance, and the energy gap of the system. By changing parameters such as the number of point defects and magnitude local forces, the transport gap of the system can be controlled. The results of this research may be useful in the design of nanoelectromechanical systems.

physics.comp-ph↗

arXiv2020

Tunable electronic properties and electric-field-induced phase transition in phosphorene/graphene heterostructures

The shortcomings of mono-component systems, e.g., the gapless nature of graphene, the lack of air-stability in phosphorene, etc. have drawn great attention toward stacked materials expected to show interesting electronic and optical properties. Using the tight-binding approach and the Green's function method, we investigate the electronic properties of armchair-edged lateral phosphorene/graphene heterostructures, which are either semiconductor/semiconductor or semiconductor/metal heterostructures, depending on the width of graphene ribbon. It is found that the system is narrow-gapped, and the bandgap can be modulated by tuning the size of the domains. Besides, the analysis of the bandgap variation against the width of the component phosphorene ribbon indicates that, in semiconductor/metal heterostructure, phosphorene ribbon does not induce any electronic state near the Fermi level, suggesting that the suppressed electron transport should be attributed to the hole transfer across the interface. Furthermore, we show that the transverse electric field can significantly diversify the electronic behavior of the heterostructure, i.e., the heterostructure undergoes the semiconductor-metal phase transition. Moreover, tuning the transverse electric field yields an intriguing possibility that the system can undergo a topological phase transition from a band insulator to a topological insulator.

cond-mat.mes-hall↗

arXiv2020

Heat transfer in strained twin graphene: A non-equilibrium molecular dynamics simulation

In this work, we study the thermal energy transport properties of twin graphene, which has been introduced recently as a new two-dimensional carbon nano structure. The thermal conductivity is investigated using non-equilibrium molecular dynamics simulation. We examine the effects of the length, temperature, and also the uni axial strain along with both armchair and zigzag directions. We found that the conductivity increases with growing the system length, while that slightly decreases with increasing the mean temperature of the system. Moreover, it is shown that the applied strain up to 0.02 will increase the thermal conductivity, and in the interval 0.02-0.06, it has a decreasing trend which can be used for tuning the thermal properties. Finally, the phonon density of states is investigated to study the behavior of thermal conductivity, fundamentally. We can control the thermal properties of the system with changing parameters such as strain. Our results may be important in the design of cooling electronic devices and thermal circuits.

physics.comp-ph↗

arXiv2020

Fulleryne, a new member of the carbon cages family

In this study, based on density functional theory (DFT), we propose a new branch of pseudo-fullerenes which contain triple bonds with sp hybridization. We should call these new nanostructures fullerynes, according to IUPAC. We present four samples with the chemical formula of C4nHn, and structures derived from fullerenes. We compare the structural and electronic properties of these structures with those of two common fullerene and fullerene systems. The calculated electron affinities of the sampled fullerynes are negative, and much smaller than those of fullerenes, so they should be chemically more stable than fullerenes. Although fulleranes also exhibit higher chemical stability than fullerynes, but pentagon or hexagon of the fullerane structures cannot pass ions and molecules. Applications of fullerynes can be included in the storage of ions and gases at the nanoscale. On the other hand, they can also be used as cathode/anode electrodes in lithium-ion batteries.

cond-mat.mtrl-sci↗

arXiv2019

Thermal rectification and interface thermal resistance in hybrid pillared-graphene and graphene: A molecular dynamics approach

In this study, we investigate the thermal rectification and thermal resistance in the hybrid pillared-graphene and graphene (PGG) system. This is done through the classical molecular dynamics simulation (MD) and also with a continuum model. At first, the thermal conductivity of both pillared-graphene and graphene is calculated employing MD simulation and Fourier low. Our results show that the thermal conductivity of the pillared-graphene is much smaller than the graphene by an order of magnitude. Next, by applying positive and negative temperature gradients along the longitudinal direction of PGG, the thermal rectification is examined. The MD results indicate that for the lengths in the range of 36 to 86nm, the thermal rectification remains almost constant (~3-5%). We have also studied the phonon density of states (DOS) on both sides of the interface of PGG. The DOS curves show that there is phonon scattering at low frequencies (acoustic mode) that depends on the imposed temperature gradient direction in the system. Therefore, we can introduce the PGG as a promising thermal rectifier at room temperature. Furthermore, in the following of this work, we also explore the temperature distribution over the PGG by using the continuum model. The results that obtained from the continuum model predict the MD results such as the temperature distribution in the upper half layer and lower full layer graphene, the temperature gap and also the thermal resistance at the interface.

cond-mat.mtrl-sci↗

arXiv2019

Thermal conductivity and thermal rectification of nanoporous graphene: A molecular dynamics simulation

Using non-equilibrium molecular dynamics (NEMD) simulation, we study thermal properties of the so-called nanoporous graphene (NPG) sheet which contains a series of nanoporous in an ordered way and was synthesized recently (Science 360 (2018), 199). The dependence of thermal conductivity on sample size, edge chirality, and porosity concentration are investigated. Our results indicate that the thermal conductivity of NPG is about two orders smaller compared with of pristine graphene. Therefore this sheet can be used as a thermoelectric material. Also, the porosity concentration helps us to tune the thermal conductivity. Moreover, the results show that the thermal conductivity increases with growing sample length due to ballistic transport. On the other hand, along the armchair direction, the thermal conductivity is larger than zigzag direction. We also examined the thermal properties of the interface of NPG and graphene. The temperature drops significantly through the interface leading to the thermal resistance. The thermal resistance changes with imposed heat flux direction, and this difference cause significantly large thermal rectification factor, and heat current prefers one direction to another. Besides, to investigate those quantities fundamentally, we study the phonon density of states and scattering of them.

physics.comp-ph↗

arXiv2019

Thermal transport in silicene nanotubes: Effects of length, grain boundary and strain

Thermal transport behavior in silicene nanotubes has become more important due to the application of these promising nanostructures in the engineering of next-generation nanoelectronic devices. We apply non-equilibrium molecular dynamics (NEMD) simulations to study the thermal conductivity of silicene nanotubes with different lengths and diameters. We further explore the effects of grain boundary, strain, vacancy defect, and temperature in the range of 300-700 K on the thermal conductivity. Our results indicate that the thermal conductivity varies with the length approximately in the range of 24-34 W/m.K but exhibits insensitivity to the diameter and chirality. Besides, silicene nanotubes consisting of the grain boundary exhibit nearly 30% lower thermal conductivity compared with pristine ones. We discuss the underlying mechanism for the conductivity suppression of the system consisting of the grain boundary by calculating the phonon power spectral density. We find that by increasing the defect concentration and temperature, the thermal conductivity of the system decreases desirably. Moreover, for strained nanotubes, we observe unexpected changes in the thermal conductivity, so that the conductivity first increases significantly with tensile strain and then starts to decrease. The maximum thermal conductivity for the armchair and zigzag edge tubes appears at the strains about 3% and 5%, respectively, which is about 28% more than that of the unstrained structure.

physics.comp-ph↗

arXiv2019

Vesicle-like structure of lipid-based nanoparticles as drug delivery system revealed by molecular dynamics simulations

Lipid-based drug delivery systems are considered as promising vehicles for hydrophobic drug compounds. Lipid distribution within the droplet can affect drug loading capacity in these carriers. It is extremely challenging to determine the nanostructure within these carriers through the implementation of the direct experimental methods due to the ultrafine size. Therefore, coarse grained molecular dynamics (MD) simulation was utilized to model different lipid-based nanoparticles of the diameter about 12 nm including solid lipid nanoparticles (SLN), nanoemulsion (NE), and nanostructured lipid carriers (NLC), and the organization of the lipids within the carriers was explored. The aforementioned nanoparticles consisted of stearic acid, oleic acid as lipids, and sodium dodecyl sulfate (SDS) as a surfactant in water medium. Furthermore, the impact of solid to liquid mass ratio on the lipid distribution within the lipid matrix was investigated regarding the NLC simulations. We observed the vesicle-like structure for all the investigated systems in which the hydrophilic moieties of the lipids and surfactant organized a semi-bilayer fold into the droplet and the hydrophobic tails accumulated among them. It is worth mentioning although SDS as a harsh surfactant, which is a special case, was expected to be present in the surface of the droplet, it penetrated into the lipids. Our results showed remarkable entrapped water beads inside the droplet in the form of one or more cavities along the internal layer of the head groups which was surrounded by lipid head groups. It was also reported that in the building structure of the nanoemulsion and SLN, in the central parts of the droplets, lipids were denser than the case of NLCs. Our results indicated that, in the case of NLC simulations, the lipid distribution within the lipid matrix was insensitive to the mass fraction of solid to liquid lipids.

physics.comp-ph↗