High thermoelectric figure of merit in nonplanar graphene nanoribbons with periodic divacancies.

GeTe Thermoelectrics

Thermoelectric materials with crystal-amorphicity duality induced by large atomic size mismatch

By performing extensive density functional theory calculations combined with non‐equilibrium Green's function technique, we predict the rhombic porous carbon nitride nanoribbon (rPCNNR) and the vertical rPCNNR junction exhibiting high thermoelectric figure of merit (ZT) values of 0.57 and 2.1 at room temperature respectively. Theoretical results reveal that the ZT value of rPCNNR is significantly larger than that of armchair graphene nanoribbon with the almost same width (~0.035) due to the large Seebeck coefficients and the significantly decreased thermal conductance of rPCNNR, where the phonon states are blocked by the built‐in porous structure and rhombic edge in rPCNNR. The ZT value is further enhanced to be 2.1 in the vertical rPCNNR junction, which is achieved by the synergy effect between the dramatically suppressed thermal conductance in in‐plane direction due to the weak van der Waals interaction between two rPCNNRs, the almost unchanged Seebeck coefficients, and the good electron conductivity provided by the strong overlapping of delocalized VB‐ and CB‐derived states in the scattering region. These presented findings highlight rPCNNR as a promising candidate in building flexible devices with high thermoelectric performance.

Enhanced thermoelectric figure of merit in graphene nanoribbons by creating a distortion and transition-metal doping

Molecular dynamics simulations of armchair graphene nanoribbons and zigzag graphene nanoribbons with different sizes were performed at room temperature. Double vacancy defects were introduced in each graphene nanoribbon at its center or at its edge. The effect of defect on the mechanical behavior was studied by comparing the stress–strain response and the fracture toughness of each pair of pristine and defective graphene nanoribbon. Results show that the effect of vacancies in zigzag graphene nanoribbon is more profound than in armchair graphene nanoribbon. Also, the effect of double vacancy defect on the ultimate failure stress is greater in zigzag graphene nanoribbons than in armchair graphene nanoribbon due to bond orientation with respect to loading direction. Strength reduction can be as high as 17.5% in armchair graphene nanoribbon with no significant difference between single and double vacancies, while for zigzag graphene nanoribbon, the strength reduction is up to 30% for single vacancy and 43% for double vacancy defects. It is observed that for zigzag graphene nanoribbon with double vacancy at the edge, the direction of the failure plane is oriented at ±30° with respect to the loading direction while it is always perpendicular to the direction of loading in armchair graphene nanoribbon. Results have been verified through studying the fracture toughness parameters in each case as well.

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.

We investigate by first-principles calculation the thermal, electrical and thermoelectric properties of adenine and porphine molecules connected to a zigzag graphene nanoribbon (ZGNR). The results show that the phonon thermal conductance is reduced drastically in both porphine and adenine. However, their electron transport properties have different characteristics. Porphine–symmetric ZGNR exhibits different electron transport properties at negative and positive energies, leading to mutation of the transmission curves at an energy of 0 eV and a high thermoelectric figure of merit. Furthermore, the thermoelectric effect in adenine can also be greatly enhanced with the help of a carbon atom chain at the edge.

We study the thermoelectric properties of devices made of two partially overlapped graphene sheets. As a consequence of the weak van der Waals interactions between graphene layers, it is shown that the phonon conductance in these junctions is strongly reduced compared to that of single graphene layer structures. In contrast, their electrical conductance is more weakly affected. We hence demonstrate that the thermoelectric figure of merit can reach values higher than 1 at room temperature in graphene materials having the advantage of offering either a bandgap or a conduction gap. We consider in particular the cases of stacks of armchair graphene nanoribbons, misoriented graphene nanoribbons and graphene nanomeshes. For the latter device, a figure of merit of 1.8 is obtained at room temperature and reaches even 3.2 at 600K. The vertical design of graphene layers thus appears as an efficient way to achieve high thermoelectric efficiency in graphene devices.

The unraveling process of armchair and zigzag graphene nanoribbons (GNRs) was studied with molecular dynamics simulations using the Adaptive Intermolecular Reactive Empirical Bond Order Potential for carbon–carbon bond. Simulations were performed at 300°K, with GNR length and width varying from 2.5 nm to 15 nm in 2.5 nm increments. In these simulations, the unraveling of the GNRs was started from two positions; the corner or the middle of the top side. Force–displacement relationship was analyzed for the terminal atom of the unraveling chains. For armchair GNRs (AGNRs) that were unraveled from the corner, the force required for the onset of the unraveling is in the range of 4.279–5.045 eV/Å, and the observed failure force in the carbon chain is in the range of 5.553–5.963 eV/Å. Unraveling will not happen when AGNRs are unraveled from the middle, and zigzag GNRs (ZGNRs) are unraveled either from corner or middle. For the latter cases, the bond between the terminal atom and GNR sheet breaks under the stretching force, and only one carbon atom can be pulled out from the GNR sheet. The size effect of width and length on the unraveling process was also studied. Simulations show that size has a trivial effect on unraveling. Comparison between unraveling of AGNRs and ZGNRs indicates that AGNRs are perfect structure to produce Monatomic Carbon Chains, while ZGNRs are more stable and are good candidate for graphene nanodevices that are free from unraveling disintegration.

Improving the thermoelectric performance of designed graphene nanoribbons is a key stage in the production of thermoelectric nanodevices with many applications. The chemical doping allows armchair graphene nanoribbons (AGNRs) to exhibit controllable thermoelectric characteristics. Here, we use density functional theory-based tight-binding (DFTB) coupled with the non-equilibrium Green's function (NEGF) to study the electronic and thermoelectric properties of AGNR with boron nitride (BN) dimers at room temperature. Changing the concentrations (from 4.17 % (BN)1-structure to 12.5 % (BN)3-structure) and geometrical pattern (ortho, meta, and para form) of BN dimers in the graphene nanoribbons may have a significant effect on the thermoelectric (TE) properties. Our results show that the TE properties of AGNR depend not only on the amount of BN dimers but also on the arrangement of the BN dimers in the AGNR. The thermoelectric figure of merit (ZT) of nanoribbons at room temperature has improved from less than 0.7 to more than 2. These results could be used as an indicator to design nanodevices that have good TE applications.

10 - Two-dimensional (2D) thermoelectric materials

(GeTe)80(Ag0.8Sb1.2Te2.2)20 thermoelectric bulks were fabricated with melt spinning technique. High thermoelectric figure of merit, ZT of 1.78 was achieved at 778 K due to the existence of large density of in-situ nanostructures. The thermal stability of these alloys were highly concerned for practical applications. The thermoelectric properties were measured after annealing these samples under the maximum working temperature of 833 K for 100 h. And the thermoelectric property variation was analyzed by comparing with that of the samples before annealing. It was noticed that after annealing, the electrical conductivity increased and Seebeck coefficient decreased correspondingly. However, most of the nanostructure features could survive the long time annealing process, which suggested that the major phonon scattering mechanism remained, therefore, the phonon thermal conductivity kept consistent. Finally, the ZT values over the entire temperature range were almost the same before and after annealing, which proved the high thermal stability of these bulks.

The thermoelectric properties of monolayer Tl2O are studied using first-principles calculations with all involved electrical and thermal transport properties calculated in the parameter-free frameworks. It is found that monolayer Tl2O possesses remarkably high thermoelectric figure of merit, zT, due to its ultralow lattice thermal conductivity and fairly good power factor. The room temperature zT can be as high as 1.4 and 1.2 for n- and p-type systems, respectively, whereas the maximum zT values can reach up to 5.3 and 4.2 as the temperature increases to 800 K. In addition, it is clarified that the mobilities of monolayer Tl2O are orders of magnitude smaller than previous estimation from simplified semiempirical models. The room temperature electron and hole mobilities are only about 56 and 11cm2V−1s−1, respectively, due to the heavy effective mass along with strong polar optical phonons coupling scattering. Nonetheless, the intrinsically ultrahigh zT from entire first-principles calculations stimulate that the further experimental verification and exploration for practical application are worthwhile.

Designing an n-type thermoelectric material with a high thermoelectric figure of merit at near room temperature is extremely challenging. Generally, pristine Ag2Se reveals unusually low thermal conductivity along with a high electrical conductivity and Seebeck coefficient, which leads to high thermoelectric performance (n-type) at room temperature. Herein, we report a pseudo-ternary phase (Ag2Se0.5Te0.25S0.25) that exhibits significantly high thermoelectric performance (zT ∼ 2.1) even at 400 K. First-principles calculation reveals that the Rashba type of spin-dependent band spitting, which originates due to sulfur and tellurium substitution, helps to improve the thermopower magnitude. We also show that the intrinsic carrier mobility is not only controlled by the carrier effective mass but is substantially limited by longitudinal acoustic and optical phonon modes, which is an extension of the deformation potential theory. Locally off-center sulfur atoms, together with the increase in configurational entropy via substitution of Te and S atoms in Ag2Se, lead to a drastic reduction in the lattice thermal conductivity (klat ∼ 0.34 W m-1 K-1 at 400 K). The Rashba effect coupled with the configurational entropy synergistically results in a high thermoelectric figure of merit in the n-type thermoelectric material working in the near-room-temperature regime.

Aspect ratio effect on shear modulus and ultimate shear strength of graphene nanoribbons