First principles study of thermoelectric performance in pristine and binary alloyed monolayers of noble metals

In search of novel thermoelectric materials, we have investigated the thermoelectric performance of freestanding pristine and binary alloyed monolayers of noble metals (viz. Au, Ag, Cu, Pt) in honeycomb morphology using first principles methods based on density functional theory (DFT) and the non-equilibrium Green's function (NEGF) approach. The requisite electron transmission function is calculated using the TranSIESTA code - a module of the SIESTA package, while its phonon counterpart is obtained using the Phonons module of the python package along with the NEGF utility - PHtrans. Transport is explored along both the armchair (ac) and zigzag (zz) directions. Due to greater transverse length of the unit cell, electron and phonon transport is found to be more in the zz-direction. Among the pristine monolayers, Pt is found to exhibit ferromagnetism as well as the highest thermoelectric efficiency ZT ∼ 8 with an external bias of μ = 0.795 eV. However, alloying brings in a characteristic energy gap in the phononic transmission function, which diminishes the (room-temperature) relative phononic conductance to less than 5% in Pt-containing alloys. Also, monolayers of AuAg and AuCu turn semiconducting, with the former yielding a high Seebeck coefficient of 1264.96 μV K^-1 at μ ∼ -0.055 eV. Moreover, they exhibit an enhanced ZT near the edges of the band gap, with ZT_max as high as 3.74 and 3.05, respectively. Interestingly, the Pt-containing alloyed monolayers also show a ferromagnetic character and hence, a spin-dependent Seebeck effect. In the CuPt monolayer, the bias μ can be tuned to attain an appreciable charge figure of merit Z_cT = 4.80 at μ = 0.58 eV, while a decent spin figure of merit Z_sT = 1.43 at μ = 0.56 eV. Instead, the pristine Pt monolayer shows a much better Z_sT = 5.45 for a bias of μ = 0.785 eV. Nevertheless, the predicted ZT remains smaller than the corresponding alloyed atomic chains in double zigzag topology.

Quantum Sensing of Thermoelectric Power in Low-Dimensional Materials

Thermoelectric power, has been extensively studied in low-dimensional materials where quantum confinement and spin textures can largely modulate thermopower generation. In addition to classical and macroscopic values, thermopower also varies locally over a wide range of length scales, and is fundamentally linked to electron wave functions and phonon propagation. Various experimental methods for the quantum sensing of localized thermopower have been suggested, particularly based on scanning probe microscopy. Here, critical advances in the quantum sensing of thermopower are introduced, from the atomic to the several-hundred-nanometer scales, including the unique role of low-dimensionality, defects, spins, and relativistic effects for optimized power generation. Investigating the microscopic nature of thermopower in quantum materials can provide insights useful for the design of advanced materials for future thermoelectric applications. Quantum sensing techniques for thermopower can pave the way to practical and novel energy devices for a sustainable society.

Tuning the thermoelectric response of silicene nanoribbons with vacancies

In this work, we present a thorough study of the thermoelectric properties of silicene nanoribbons in the presence of a random distribution of atomic vacancies. By using a linear approach within the Landauer formalism, we calculate phonon and electron thermal conductances, the electric conductance, the Seebeck coefficient and the figure of merit of the nanoribbons. We found a sizable reduction of the phonon thermal conductance as a function of the vacancy concentration over a wide range of temperature. At the same time, the electric properties are not severely deteriorated, leading to an overall remarkable thermoelectric efficiency. We conclude that the incorporation of vacancies paves the way for designing better and more efficient nanoscale thermoelectric devices.

Thermoelectric properties of graphene-like nanoribbon studied from the perspective of symmetry

We have studied the charge and spin thermopower systematically in a ferromagnetic junction of graphene-like zigzag nanoribbon modified by two on-site disorders in the tight-binding model. Symmetries of the transmission spectra and geometry configuration of the two disorders are important factors in determining the thermoelectric properties of the system. Conditions to achieve pure charge and pure spin thermopower are discussed from the perspective of symmetry. Symmetry breaking is required sometimes to obtain large figure of merit. The type and strength of the disorders can be used to further manipulate the spin polarization of thermal current. Disorders inside nanoribbon instead of on edge can then be used to finely tune the performance of the junction. The results may have great application value in designing thermoelectric devices.

Largely enhanced thermoelectric effect and pure spin current in silicene-based devices under hydrogen modification

Based on the density functional theory and nonequilibrium Green's function methods, we launch a systematic study of the magnetic properties and thermoelectric effects in silicene-based devices constructed by using zigzag silicene nanoribbons (ZSiNRs). By modulating the adsorption site, it is found that the ground state of ZSiNRs varies from an antiferromagnetic state to a ferromagnetic state. Meanwhile, a spin-degenerate semiconductor evolves into a spin semiconductor. The spin and charge thermoelectric figure of merits have an almost equal value of about 60 in the narrow device, which originates from the spin-dependent conductance dips and high spin-filtering effects. Moreover, a thermally-driven pure spin current in the silicene-based devices is obtained in the absence of the gate voltage, and its magnitude is effectively enhanced as the device width increases. Our results suggest that the silicene-based devices have very good prospects for spin caloritronics.

Thermal Transport in Phosphorene

Phosphorene, a novel elemental 2D semiconductor, possesses fascinating chemical and physical properties which are distinctively different from other 2D materials. The rapidly growing applications of phosphorene in nano/optoelectronics and thermoelectrics call for comprehensive studies of thermal transport properties. In this Review, based on the theoretical and experimental progresses, the thermal transport properties of single-layer phosphorene, multilayer phosphorene (nanofilms), and bulk black phosphorus are summarized to give a general view of the overall thermal conductivity trend from single-layer to bulk form. The mechanism underlying the discrepancy in the reported thermal conductivity of phosphorene is discussed by reviewing the effect of different functionals and cutoff distances on the thermal transport evaluations. This Review then provides fundamental insight into the thermal transport in phosphorene by reviewing the role of resonant bonding in driving giant phonon anharmonicity and long-range interactions. In addition, the extrinsic thermal conductivity of phosphorene is reviewed by discussing the effects of strain and substrate, together with phosphorene based heterostructures and nanoribbons. This Review summarizes the progress of thermal transport in phosphorene from both theoretical calculations and experimental measurements, which would be of significance to the design and development of efficient phosphorene based nanoelectronics.

Recent progress in 2D group-VA semiconductors: from theory to experiment

This review provides recent theoretical and experimental progress in the fundamental properties, electronic modulations, fabrications and applications of 2D group-VA materials.

Enhancement of thermoelectric efficiency by quantum interference effects in trilayer silicene flakes

In recent years, the enhancement of thermoelectric efficiencies has been accomplished in nanoscale systems by making use of quantum effects. We exploit the presence of quantum interference phenomena such as bound states in the continuum and Fano antiresonances in trilayer silicene flakes to produce sharp changes in the electronic transmission of the system. By applying symmetric gate voltages the thermoelectric properties can be tuned and, for particular flake lengths, a great enhancement of the figure of merit can be achieved. We show that the most favorable configurations are those in which the electronic transmission is dominated by the coupling of bound states to the continuum, tuned by an external gate.

Defect-sensitive performance of silicene sheets under uniaxial tension: mechanical properties, electronic structures and failure behavior

As a silicon analog of graphene, silicene has attracted considerable attention due to its unique physical and chemical properties.

Design of spin-Seebeck diode with spin semiconductors

We report a new design of spin-Seebeck diode using two-dimensional spin semiconductors such as sawtooth-like (ST) silicence nanoribbons (SiNRs), to generate unidirectional spin currents with a temperature gradient. ST SiNRs have subbands with opposite spins across the Fermi level and hence the flow of thermally excited carriers may produce a net spin current but not charge current. Moreover, we found that even-width ST SiNRs display a remarkable negative differential thermoelectric resistance due to a charge-current compensation mechanism. In contrast, odd-width ST SiNRs manifest features of a thermoelectric diode and can be used to produce both charge and spin currents with temperature gradient. These findings can be extended to other spin semiconductors and open the door for designs of new materials and spin caloritronic devices.

A theoretical review on electronic, magnetic and optical properties of silicene

Inspired by the success of graphene, various two dimensional (2D) structures in free standing (FS) (hypothetical) form and on different substrates have been proposed recently. Silicene, a silicon counterpart of graphene, is predicted to possess massless Dirac fermions and to exhibit an experimentally accessible quantum spin Hall effect. Since the effective spin-orbit interaction is quite significant compared to graphene, buckling in silicene opens a gap of 1.55 meV at the Dirac point. This band gap can be further tailored by applying in plane stress, an external electric field, chemical functionalization and defects. In this topical theoretical review, we would like to explore the electronic, magnetic and optical properties, including Raman spectroscopy of various important derivatives of monolayer and bilayer silicene (BLS) with different adatoms (doping). The magnetic properties can be tailored by chemical functionalization, such as hydrogenation and introducing vacancy into the pristine planar silicene. Apart from some universal features of optical absorption present in all these 2D materials, the study on reflectivity modulation with doping (Al and P) concentration in silicene has indicated the emergence of some strong peaks having the robust characteristic of a doped reflective surface for both polarizations of the electromagnetic (EM) field. Besides this, attempts will be made to understand the electronic properties of silicene from some simple tight-binding Hamiltonian. We also point out the importance of shape dependence and optical anisotropy properties in silicene nanodisks and establish that a zigzag trigonal possesses the maximum magnetic moment. We also suggest future directions to be explored to make the synthesis of silicene and its various derivatives viable for verification of theoretical predictions. Although this is a fairly new route, the results obtained so far from experimental and theoretical studies in understanding silicene have shown enough significant promising features to open a new direction in the silicon industry, silicon based nano-structures in spintronics and in opto-electronic devices.

Electronic Structures of Silicene Nanoribbons: Two-Edge-Chemistry Modification and First-Principles Study

In this paper, we investigate the structural and electronic properties of zigzag silicene nanoribbons (ZSiNRs) with edge-chemistry modified by H, F, OH, and O, using the ab initio density functional theory method and local spin-density approximation. Three kinds of spin polarized configurations are considered: nonspin polarization (NM), ferromagnetic spin coupling for all electrons (FM), ferromagnetic ordering along each edge, and antiparallel spin orientation between the two edges (AFM). The H, F, and OH groups modified 8-ZSiNRs have the AFM ground state. The directly edge oxidized (O1) ZSiNRs yield the same energy and band structure for NM, FM, and AFM configurations, owning to the same s p (2) hybridization. And replacing the Si atoms on the two edges with O atoms (O2) yields FM ground state. The edge-chemistry-modified ZSiNRs all exhibit metallic band structures. And the modifications introduce special edge state strongly localized at the Si atoms in the edge, except for the O1 form. The modification of the zigzag edges of silicene nanoribbons is a key issue to apply the silicene into the field effect transistors (FETs) and gives more necessity to better understand the experimental findings.

Crystal orbital studies on the 1D silic-diyne nanoribbons and nanotubes

This work presents crystal orbital studies on novel one-dimensional (1D) nanoscale materials derived from a Si-diyne sheet, based on the density functional theory. The two-dimensional (2D) Si-diyne layer is observed to be carbo-merized silicene, with a similar structure to graphdiyne. The 2D Si-diyne and its 1D ribbons and tubes, of different size and chirality, have been addressed systematically. The low dimensional Si-diyne materials studied exhibit relatively high stability, according to phonon-frequency calculations and molecular dynamics simulations. With comparable diameters, the Si-diyne tubes have lower strain energies than silicene and silicon carbide nanotubes. The Si-diyne layer and its 1D derivatives are all semiconductors, regardless of the size and chirality of the strips and tubes. In addition, the band gaps of the 1D Si-diyne nanoribbons and nanotubes with different chirality, always monotonically decrease as their sizes increases. A quantitative relationship between the band gap and the size of the ribbons and tubes was obtained. The mobility of charge carriers for the 1D Si-diyne structures was also investigated. It was found that both hole and electron mobility of the ribbons and tubes exhibit linear increase with increasing size. The electrons have greater mobility than the holes for each strip and tube. In addition, the mechanical properties of the Si-diyne nanostructures were also investigated by calculation of the Young's modulus and the Poisson's ratio.

Zigzag nanoribbons of two-dimensional silicene-like crystals: magnetic, topological and thermoelectric properties

The effects of electron-electron and spin-orbit interactions on the ground-state magnetic configuration and on the corresponding thermoelectric and spin thermoelectric properties in zigzag nanoribbons of two-dimensional hexagonal crystals are analysed theoretically. The thermoelectric properties of quasi-stable magnetic states are also considered. Of particular interest is the influence of Coulomb and spin-orbit interactions on the topological edge states and on the transition between the topological insulator and conventional gap insulator states. It is shown that the interplay of both interactions also has a significant impact on the transport and thermoelectric characteristics of the nanoribbons. The spin-orbit interaction also determines the in-plane magnetic easy axis. The thermoelectric properties of nanoribbons with in-plane magnetic moments are compared to those of nanoribbons with edge magnetic moments oriented perpendicularly to their plane. Nanoribbons with ferromagnetic alignment of the edge moments are shown to reveal spin thermoelectricity in addition to the conventional one.

Symmetry-dependent spin-charge transport and thermopower through a ZSiNR-based FM/normal/FM junction

We investigate the spin-dependent transport and spin thermopower for a zigzag silicene nanoribbon (ZSiNR) with two ends covered by ferromagnets (FMs) under the modulation of a perpendicular electric field, where we take the 6- and 7-ZSiNR to exemplify the effect of the even- and odd-N ZSiNRs, respectively. By using the nonequilibrium Green's function approach, it is demonstrated that a ZSiNR-based FM/normal/FM junction still shows an interesting symmetry-dependent property although the σ mirror plane is absent for any ZSiNR due to the buckled structure of silicene. The junction with even- or odd-N ZSiNR has very different transport and thermopower behavior, which is attributed to the different parity of π and [Formula: see text] band wavefunctions under the c 2 symmetry operation with respect to the centre axis between two edges, and is linked to the unique symmetry of the band structure for the ribbon. As a result, the magnetoresistance (MR) for the 6-ZSiNR junction with a 100% plateau around zero electron energy is observed, but the plateau is absent for the 7-ZSiNR one. In addition, the spin thermopower also displays the even-odd behaviour. The 6-ZSiNR junction is found to possess superior thermospin performance compared with the 7-ZSiNR one, and its spin thermopower can be improved by one order of magnitude in the absence of an electric field. As the strength of the field increases, the spin thermopower for the 6-ZSiNR junction dramatically decreases, while it notably enhances for the 7-ZSiNR one. Interestingly, the spin thermopower for both junctions is strongly dependent on the strength of magnetisation in FM, and it can be very pronounced with a maximum absolute value of 200 μV K(-1)by the optimisation of the parameters. However, with the increase in temperature, the spin thermopower for the 6-ZSiNR junction decreases, but the situation for the 7-ZSiNR one is opposite. Finally, the spin figure of merit for the 6-ZSiNR junction is found to be four orders of magnitude larger than that for the 7-ZSiNR one. This even-odd effect is common for N-SiNR, and the result can be regarded as an advance in the understanding of the characteristics of silicene and may be valuable for experimentally designing spin valve and heat spintronic devices based on silicene.

Molecular Dynamics Simulations of Thermal Conductivity of Germanene Nanoribbons (GeNR) with Armchair and Zigzag Chirality

Germanene, an allotrope of germanium which is a two dimensional material withsp2hybridization, has almost the same properties with graphene except for its buckled structure. In this study, germanium nanoribbon (GeNR) is use for it is still a new material for nanoscale level of research. In this paper, we investigate the effect of chirality on the thermal conductivity of zigzag GeNR (ZGeNR) and armchair GeNR (AGeNR) chiralities using equilibrium molecular dynamics with varied lengths at fixed temperature and varied temperatures at fixed length. The simulations were carried out in Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) using Tersoff potential for the Ge-Ge interactions. The thermal conductivity is calculated using Green-Kubo method. It is found that the chirality can affect the thermal conductivity of GeNR. Our results show that thermal conductivity of AGeNR is higher than ZGeNR in both increasing temperatures and lengths similar to the thermal conductivity behavior obtained in silicene nanoribbons [Int. J. Mech. Mater. Des. 9 (2013) 105].

Enhanced thermoelectric efficiency in ferromagnetic silicene nanoribbons terminated with hydrogen atoms

Using ab initio methods we calculate thermoelectric and spin thermoelectric properties of silicene nanoribbons with bare, mono-hydrogenated and di-hydrogenated edges. Asymmetric structures, in which one edge is either bare or di-hydrogenated while the other edge is mono-hydrogenated (0H-1H and 2H-1H nanoribbons), have a ferromagnetic ground state and display remarkable conventional and spin thermoelectric properties. Strong enhancement of the thermoelectric efficiency, both conventional and spin ones, results from a very specific band structure of such nanoribbons, where one spin channel is blocked due to an energy gap while the other spin channel is highly conductive. In turn, 0H-2H and 2H-2H nanoribbons (with one edge being either bare or di-hydrogenated and the other edge being di-hydrogenated) are antiferromagnetic in the ground state. Accordingly, the corresponding spin channels are equivalent, and only conventional thermoelectric effects can occur in these nanoribbons.

Enhanced thermoelectric efficiency of porous silicene nanoribbons

There is a critical need to attain new sustainable materials for direct upgrade of waste heat to electrical energy via the thermoelectric effect. Here we demonstrate that the thermoelectric performance of silicene nanoribbons can be improved dramatically by introducing nanopores and tuning the Fermi energy. We predict that values of electronic thermoelectric figure of merit ZT_e up to 160 are achievable, provided the Fermi energy is located approximately 100 meV above the charge neutrality point. Including the effect of phonons yields a value for the full figure of merit of ZT = 3.5. Furthermore the sign of the thermopower S can be varied with achievable values as high as S = +/− 500 μV/K. As a method of tuning the Fermi energy, we analyse the effect of doping the silicene with either a strong electron donor (TTF) or a strong electron acceptor (TCNQ) and demonstrate that adsorbed layers of the former increases ZT_e to a value of 3.1, which is insensitive to temperature over the range 100 K – 400 K. This combination of a high, temperature-insensitive ZT_e, and the ability to choose the sign of the thermopower identifies nanoporous silicene as an ideal thermoelectric material with the potential for unprecedented performance.

Elemental analogues of graphene: silicene, germanene, stanene, and phosphorene

The fascinating electronic and optoelectronic properties of free-standing graphene has led to the exploration of alternative two-dimensional materials that can be easily integrated with current generation of electronic technologies. In contrast to 2D oxide and dichalcogenides, elemental 2D analogues of graphene, which include monolayer silicon (silicene), are fast emerging as promising alternatives, with predictions of high degree of integration with existing technologies. This article reviews this emerging class of 2D elemental materials - silicene, germanene, stanene, and phosphorene--with emphasis on fundamental properties and synthesis techniques. The need for further investigations to establish controlled synthesis techniques and the viability of such elemental 2D materials is highlighted. Future prospects harnessing the ability to manipulate the electronic structure of these materials for nano- and opto-electronic applications are identified.

Spin effects in thermoelectric phenomena in SiC nanoribbons

Using ab initio methods we calculate the thermoelectric and spin thermoelectric properties of zigzag SiC nanoribbons, asymmetrically terminated with hydrogen. Such nanoribbons display a ferromagnetic ground state, with edge magnetic moments oriented in parallel. Both thermopower and spin thermopower have been determined as a function of chemical potential and temperature. To find the thermoelectric efficiency, the total heat conductance has been calculated, i.e. the electronic and phonon contributions. Numerical results for SiC nanoribbons are compared with those for graphene and silicene ones.

Boron nitride zigzag nanoribbons: optimal thermoelectric systems

Conventional and spin related thermoelectric effects in zigzag boron nitride nanoribbons are studied theoretically within the Density Functional Theory (DFT) approach.

Phosphorene nanoribbon as a promising candidate for thermoelectric applications

In this work, the electronic properties of phosphorene nanoribbons with different width and edge configurations are studied by using density functional theory. It is found that the armchair phosphorene nanoribbons are semiconducting while the zigzag nanoribbons are metallic. The band gaps of armchair nanoribbons decrease monotonically with increasing ribbon width. By passivating the edge phosphorus atoms with hydrogen, the zigzag series also become semiconducting, while the armchair series exhibit a larger band gap than their pristine counterpart. The electronic transport properties of these phosphorene nanoribbons are then investigated using Boltzmann theory and relaxation time approximation. We find that all the semiconducting nanoribbons exhibit very large values of Seebeck coefficient and can be further enhanced by hydrogen passivation at the edge. Taking pristine armchair nanoribbons and hydrogen-passivated zigzag naoribbons with width N = 7, 8, 9 as examples, we calculate the lattice thermal conductivity with the help of phonon Boltzmann transport equation and evaluate the width-dependent thermoelectric performance. Due to significantly enhanced Seebeck coefficient and decreased thermal conductivity, we find that at least one type of phosphorene nanoribbons can be optimized to exhibit very high figure of merit (ZT values) at room temperature, which suggests their appealing thermoelectric applications.

Temperature-controlled giant thermal magnetoresistance behaviors in doped zigzag-edged silicene nanoribbons

Based on the nonequilibrium Green's function (NEGF) method combined with density functional theory (DFT), we investigate the spin-dependent thermoelectric transport properties of zigzag-edged silicene nanoribbons (ZSiNRs) doped by an Al–P bonded pair at different edge positions.