Thermoelectric Properties Of Graphene Nanoribbons Research Articles

Tuning the thermoelectric properties of graphene nanoribbons by vacancy defect with Ge-doping

In this work, the influence of vacancy defects and germanium (Ge)-doping on structural stability, electronic and thermoelectric (TE) characteristics of armchair graphene nanoribbons (AGNRs) have been studied by density functional theory (DFT) based tight-binding coupled with nonequilibrium green function (NEGF) calculations. Three concentrations of Ge impurities, single, two, and three, are identified at different sites with vacancy defects. As a result, the DFT calculations suggest that the defected AGNRs with three Ge impurities are structurally favorable configurations due to the lowest required cohesive energy. At low doping ratio, small bandgap for defected AGNRs is predicted leading to high Seebeck coefficient and figure of merit. In contrast, at high doping ratio, low Seebeck coefficient and figure of merit are noticed. The results show that the TE properties of AGNRs do not only depend on the Ge concentrations with vacancy defect but also depend on the geometrical pattern of Ge impurities. As a result, exploiting the electronic and thermoelectric properties of AGNRs to create nanostructure, which can be used in many important applications for nanoelectronics and spintronics.

Retraction Note to: The Effects of Percent and Position of Nitrogen Atoms on Electronic and Thermoelectric Properties of Graphene Nanoribbons

Retraction Note to: The Effects of Percent and Position of Nitrogen Atoms on Electronic and Thermoelectric Properties of Graphene Nanoribbons

Optimal design of thermoelectric properties of graphene nanoribbons with 5-7 ring defects based on Bayesian algorithm

Owing to the huge degree of freedom of structure, the optimal design of thermoelectric conversion performance of defective graphene nanoribbons is one of the difficulties in the field of materials research. In this paper, the thermoelectric properties of graphene nanoribbons with 5-7 ring defects are optimized by using non-equilibrium Green's function combined with Bayesian algorithm.The results show that the Bayesian algorithm is effective and advantageous in the search of graphene nanoribbons with 5-7 ring defects with high thermoelectric conversion efficiency. It is found that the single configuration with the best thermoelectric conversion performance can be quickly and accurately searched from 32896 candidate structures by using Bayesian algorithm. Even in the least efficient round of optimization, only 1495 candidate structures (about 4.54% of all candidate structures) need to be calculated to find the best configuration. It is also found that the thermoelectric value <i>ZT</i> (about 1.13) of the optimal configuration of 5-7 ring defective graphene nanoribbons (21.162 and 1.23 nm in length and width, respectively) at room temperature is nearly one order of magnitude higher than that of the perfect graphene nanoribbons (about 0.14). This is mainly due to the fact that the 5-7 ring defects effectively inhibit the electron thermal conductivity of the system, which makes the maximum balance between the weakening effect of the power factor and the inhibiting effect of the thermal conductivity (positive effect). The results of this study provide a new feasible scheme for designing and fabricating the graphene nanoribbon thermoelectric devices with excellent thermoelectric conversion efficiencies.

Electrical control of the spin-Seebeck coefficient in graphene nanoribbons with asymmetric zigzag edge extensions.

We investigate the effect of the electric field on the spin-dependent thermoelectric properties of graphene nanoribbons with asymmetric zigzag edge extensions. The Hubbard Hamiltonian predicts spin-semiconducting behavior with localized band structures due to the magnetic properties of the zigzag edge extensions. Applying a temperature gradient induces thermal spin-dependent currents, pure spin currents, and large spin-Seebeck coefficients, which are similar to other graphene-based structures. Considering the effect of the electric field sensitively reduces the spin gap and finally the threshold temperature. Moreover, the electric field slightly widens the localized band structures around the Fermi energy and could induce ten times more spin current in these nanoribbons.

Theoretical Study on the Thermoelectric Properties of Porous Armchair Graphene Nanoribbons

Thermoelectric properties of porous graphene nanoribbons (GNRs) have been explored for a range of pore dimensions in order to achieve a high performance one-dimensional nanoscale thermoelectric device. In this paper, study has been done to observe the effect of different nanoporous shapes and their associated positions on the thermoelectric properties of GNRs. The aim of this work is to study the effect of various circular, triangular, rectangular and rhombus shape dimensions so as to tune the pore to its optimal dimension that would enhance the overall thermoelectric efficiency. Further, the effect of passivation of pore edges has been studied for all shapes so as to observe its effect on thermoelectric performance. Also, the effect of temperature dependence on thermoelectric efficiency has been studied. Ballistic transport regime and semi empirical method using Huckel basis set is used to obtain the electrical properties while the tersoff potential is used for the phononic system.

Thermoelectric properties of polycrystalline graphene nanoribbons

Thermoelectric materials, which can convert heat energy into electric energy and also from electric energy into heat energy, have aroused widespread interest of both theoretical and technological researches recently. Graphene is a typical two-dimensional carbon nanomaterial and regarded as a competitive candidate for the next-generation micro/nano-devices. Unfortunately, graphene is an inefficient thermoelectric material due to the extremely high thermal conductivity. To overcome this drawback, exploring an effective way to improve the thermoelectric performance is of critical importance. In this paper, using the nonequilibrium Green’s function approach, we systematically investigate the effects of grain boundary on the thermoelectric properties of graphene nanoribbons. The results show that owing to the existence of grain boundary, the phonons and electrons encounter great scatterings when they transmit through the polycrystalline graphene nanoribbons. These scatterings cause the phononic and electronic transmission coefficient to decrease dramatically, and thus leading the thermal conductance (including both electron and phonon parts) of graphene nanoribbons to be evidently suppressed. Meanwhile, such scatterings induce more intense transmission peaks and pits in the electronic transmission spectrum of polycrystalline graphene nanoribbons. Generally, the Seebeck coefficient depends on the derivative of electronic transmission coefficient. The larger the logarithmic derivative of transmission, the higher the Seebeck coefficient can be obtained. Therefore Seebeck coefficient is improved obviously in the polycrystalline graphene nanoribbons. Based on such two positive effects, the thermoelectric performance of polycrystalline graphene nanoribbons is significantly enhanced. At room temperature, the thermoelectric figure of merit of polycrystalline graphene nanoribbons can approach to 0.3, which is about 6 times larger than that of pristine graphene nanoribbon (figure of merit is about 0.05). It is also found that the quantity of grain boundaries and length of system can further improve the thermoelectric properties of the polycrystalline graphene nanoribbons, while the width of system has a limited influence on it. This is because the quantity of grain boundaries and length of polycrystalline graphene nanoribbons can give rise to more intense phonon and electron scatterings and further decreasing of thermal conductance and enhancement of Seebeck coefficient. The results presented in this paper demonstrate that polycrystalline structure is indeed an effective way to improve the thermoelectric conversion efficiency of graphene nanoribbons, and provide a theoretical guideline for designing and preparing thermoelectric devices based on graphene nanoribbons.

Enhanced Thermoelectric Properties of Armchair Graphene Nanoribbons with Pore Passivation

There is a need to discover efficient thermoelectric materials that can generate electricity from waste heat and could play an important role in a global sustainable energy solution. Graphene Nanoribbons have been explored for a range of pore dimensions in order to achieve better thermoelectric performance. In this paper, we investigate the thermoelectric properties of porous armchair graphene nanoribbons by introducing hydrogen atoms as passivators at the pore surfaces. The aim of this work is to study the influence of pore passivation on the thermoelectric parameters as a function of pore geometry so as to open the possibility for an optimal pore engineering which can significantly improve the thermoelectric efficiency. The results show that the phonon thermal conductivity has a very little dependence on the pore edge passivation. An improvement in thermoelectric figure of merit is achieved due to the increased values of the power factor with consistent values of thermalconductivity. The unique thermoelectric properties of graphene nanoribbons with pore passivation suggest their great potentials for nanoscale thermoelectric applications. Within ballistic transport regime, semi-empirical extended Huckel method has been used for electrical properties while Tersoff potential has been employed for phononic calculations.

Thermoelectric properties of graphene nanoribbons with surface roughness

We theoretically investigate the ballistic thermoelectric performance of graphene nanoribbons with surface roughness using the nonequilibrium Green's function method. The results show that the surface roughness could dramatically reduce the thermal conductance of graphene nanoribbons, and thus lead to the boosting of thermoelectric performance of graphene (the figure of merit can be as high as 3.7 at room temperature). Meanwhile, the electron transport properties of different edged rough graphene nanoribbons exhibit distinctive anisotropic behaviors, i.e., the thermal power of armchair edged nanoribbons significantly increases, while that of zigzag edged remains nearly unchanged, which is mainly attributed to the edge effect. The findings presented in this paper qualify surface roughness as an efficient approach to enhance the thermoelectric performance of graphene nanoribbons.

High thermoelectric and electronic performance in graphene nanoribbons by isotope and vacancy engineering

We study the effects of isotope doping and vacancies on the electronic, phononic and thermoelectric properties of graphene nano-ribbons by means of atomistic simulation based on semi-empirical Tight Binding and Force Constant models combined with the Non-Equilibrium Green’s Function formalism of transport. We demonstrate that from the phononic perspective, the impact of isotope doping on transmission is very dependent on the phonon frequency and particularly strong at high frequency, while vacancies wield their influence on the whole frequency range. From the electronic point of view, if placed at dimer line positions m = 3i + 1, 3i + 2, vacancies induce a strong reduction of the electrical conductance while vacancies at positions m = 3i act as isotopes, i.e. they are transparent to electrons and do not affect the electronic properties. The thermoelectric study is conducted by combining both electronic and phononic properties. It demonstrates the possibility of a remarkable enhancement of figure of merit ZT. In the presence of both types of defects, ZT can be enhanced significantly from 0.26 in pristine ribbons to values larger than 2.5 for ribbons of width M = 5.

Electron and phonon transport in twisted graphene nanoribbons

We theoretically study the electronic, thermal and thermoelectric properties of graphene nanoribbons under torsional deformations. The modelling follows a nonequilibrium Green’s function approach in the ballistic transport regime, describing the electrical and phononic properties through ab initio density functional theory and empirical interatomic potentials, respectively. We consider two different types of deformations, a continuous twist of a given angle applied to the nanoribbon, and two consecutive twists applied in opposite angular directions. The numerical results are carefully analysed in terms of spatially-resolved electron eigenchannels, polarization-dependent phonon transmission and thermoelectric figure-of-merit.

The effect of electron–phonon interaction on the thermoelectric properties of defect zigzag nanoribbons

Thermoelectric properties of graphene nanoribbons with periodic edge vacancies and antidot lattice are investigated. The electron-phonon interaction is taken into account in the framework of the Hubbard-Holstein model with the use of the Lang-Firsov unitary transformation scheme. The electron transmission function, the thermopower and the thermoelectric figure of merit are calculated. We have found that the electron-phonon interaction causes a decrease in the peak values of the thermoelectric figure of merit and the shift of the peak positions closer to the center of the bandgap. The effects are more pronounced for the secondary peaks that appear in the structures with periodic antidot.

RETRACTED ARTICLE: The Effects of Percent and Position of Nitrogen Atoms on Electronic and Thermoelectric Properties of Graphene Nanoribbons

Graphene-based nanostructures exhibit electronic properties that are not present in extended graphene. Graphene nanoribbons (GNRs) are expected to display extraordinary properties in the form of nanostructures. The effect of percent and position of nitrogen atoms on electronic and thermoelectric properties of a GNR is studied using Landauer approach and density functional theory. For this purpose the density of States, electronic current and thermal current have been calculated. Moreover, an analytical model for the thermo-conductance of the nanosized junction in two-dimensional graphene nanosystems developed. The results show that increasing of nitrogen atoms, increases the splitting of p-orbitals as well as band gap at Fermi level. Also the presence of nitrogen impurities is shown to yield resonant backscattering, whose features are strongly dependent on the position of the dopants. It is demonstrated that increasing N concentration decrease the thermal conductivity due to multi-scattering. In addition I–V characteristics exhibit robust nonlinear behaviors, which are strongly dependent on the position and theconcentration of the nitrogen atoms.

Enhanced thermoelectric performance of graphene nanoribbon-based devices

There have been numerous theoretical studies on exciting thermoelectric properties of graphene nano-ribbons (GNRs); however, most of these studies are mainly based on simulations. In this work, we measure and characterize the thermoelectric properties of GNRs and compare the results with theoretical predictions. Our experimental results verify that nano-structuring and patterning graphene into nano-ribbons significantly enhance its thermoelectric power, confirming previous predictions. Although patterning results in lower conductance (G), the overall power factor (S2G) increases for nanoribbons. We demonstrate that edge roughness plays an important role in achieving such an enhanced performance and support it through first principles simulations. We show that uncontrolled edge roughness, which is considered detrimental in GNR-based electronic devices, leads to enhanced thermoelectric performance of GNR-based thermoelectric devices. The result validates previously reported theoretical studies of GNRs and demonstrates the potential of GNRs for the realization of highly efficient thermoelectric devices.

Spectacular enhancement of thermoelectric phenomena in chemically synthesized graphene nanoribbons with substitution atoms.

We analyze theoretically the transport and thermoelectric properties of graphene nanoribbons of a specific geometry, which have been synthesized recently from polymers [Cai, et al., Nature, 2011, 466, 470]. When such nanoribbons are modified at one of the two edges by Al or N substitutions, they acquire a ferromagnetic moment localized at the modified edge. We present numerical results on the electronic structure and thermoelectric properties (including also spin thermoelectricity) of the modified nanoribbons. The results show that such nanoribbons can display large thermoelectric efficiency in certain regions of chemical potential, where the corresponding electric and spin figures of merit achieve unusually large values. The enhancement of thermoelectric efficiency follows from a reduced phonon heat conductance of the nanoribbons and from their peculiar electronic band structure. Thus, such nanoribbons are promising for practical applications in nanoelectronic and spintronic devices.

High Performance Graphene Nano-ribbon Thermoelectric Devices by Incorporation and Dimensional Tuning of Nanopores.

Thermoelectric properties of Graphene nano-ribbons (GNRs) with nanopores (NPs) are explored for a range of pore dimensions in order to achieve a high performance two-dimensional nano-scale thermoelectric device. We reduce thermal conductivity of GNRs by introducing pores in them in order to enhance their thermoelectric performance. The electrical properties (Seebeck coefficient and conductivity) of the device usually degrade with pore inclusion; however, we tune the pore to its optimal dimension in order to minimize this degradation, enhancing the overall thermoelectric performance (high ZT value) of our device. We observe that the side channel width plays an important role to achieve optimal performance while the effect of pore length is less pronounced. This result is consistent with the fact that electronic conduction in GNRs is dominated along its edges. Ballistic transport regime is assumed and a semi-empirical method using Huckel basis set is used to obtain the electrical properties, while the phononic system is characterized by Tersoff empirical potential model. The proposed device structure has potential applications as a nanoscale local cooler and as a thermoelectric power generator.

Enhancing the thermoelectric figure of merit in engineered graphene nanoribbons.

We demonstrate that thermoelectric properties of graphene nanoribbons can be dramatically improved by introducing nanopores. In monolayer graphene, this increases the electronic thermoelectric figure of merit ZTe from 0.01 to 0.5. The largest values of ZTe are found when a nanopore is introduced into bilayer graphene, such that the current flows from one layer to the other via the inner surface of the pore, for which values as high as ZTe = 2.45 are obtained. All thermoelectric properties can be further enhanced by tuning the Fermi energy of the leads.

Graphene nanoribbon thermopower as a tool for molecular spectroscopy

In this work, we study thermoelectric properties of graphene nanoribbons with side-attached organic molecules. By adopting a single-band tight binding Hamiltonian and the Green's function formalism, we calculated the transmission and Seebeck coefficients for different hybrid systems. The corresponding thermopower profiles exhibit a series of sharp peaks at the eigenenergies of the isolated molecule indicating that the system can be proposed as a molecular thermo-device. We have studied the effects of the temperature on the thermoelectric response, and considered random configurations of molecule distributions, in different disorder regimes. We have found that the main features of the thermopower are robust under temperature and disorder.

Thermoelectric properties of hexagonal graphene quantum dots

By using the atomistic nonequilibrium Greenʼs function method, we investigate the thermoelectric properties of graphene nanoribbons in the presence of two constrictions (or hexagonal graphene quantum dots). With decreasing widths of the constrictions, the thermal conductance of the nanoribbon can be reduced largely while S2Ge (S is the Seebeck coefficient and Ge is the electronic conductance) remains still high as compared with the results of the pristine nanoribbon. Thus, the thermoelectric figure of merit ZT can be enhanced largely. In fact, in the presence of narrowest constrictions the ZT values of the zigzag quantum dots can exceed one at room temperature, while the ZT values of the armchair quantum dots may be close to one, depending on the size of the dot.

A theoretical study on thermoelectric properties of graphene nanoribbons

We investigate the thermoelectric properties of graphene nanoribbons (GNRs) by solving atomistic electron and phonon transport equations in the nonequilibrium Green’s function formalism. The dependence of thermopower on temperature and chemical potential is compared to that of graphene, which shows the important role of quasi-one-dimensional geometry in determining the thermoelectric properties of a GNR. The edge roughness and lattice vacancy are found to increase the thermopower but decrease the thermoelectric ZT factor because the decrease in the electronic conductance outweighs the decrease in the thermal conductance and the increase in the thermopower.