Spin seebeck effect and thermal colossal magnetoresistance in graphene nanoribbon heterojunction

Spin-thermoelectric properties and giant tunneling magnetoresistance of boron-substituted graphene nanoribbon: a first principle study

In this study we have designed a spin caloritronic device based on boron doped armchair graphene nanoribbons (B2-7AGNR). In presence of ferromagnetic (FM) graphitic-carbon nitride (g-C₄N₃) electrodes the spin-thermoelectric features of the device, both for FM and antiferromagnetic (AFM) states, are studied using first principle calculations. The spin polarized transmission peaks and the presence of density of states near the Fermi level indicate that the system have large spin-thermoelectric figure of merit. In addition, it is observed that the system has a large tunneling magnetoresistance due to the difference in total current between FM and AFM configurations. Further studies reveal that the spin component of the Seebeck coefficient of the device is much higher than the other zigzag and armchair nanoribbons. When the spin magnetic moments of the electrodes are aligned in parallel manner, spin-thermoelectric figure of merit of the system becomes significantly high. It has also been found that on decreasing temperature the efficiency of the device increases. As a whole, the numerical results show thatg-C₄N₃-B2-7AGNR-g-C₄N₃system in FM configuration is an efficient low temperature thermoelectric device.

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 Green's 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% are obtained. The system with the DV defects can show a large spin-dependent Seebeck coefficient, equal to S_s ⁓ 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.

The Strain-Tuned Spin Seebeck Effect, Spin Polarization, and Giant Magnetoresistance of a Graphene Nanobubble in Zigzag Graphene Nanoribbons

By using first-principle calculations combined with the non-equilibrium Green's function approach, we studied the spin caloritronic properties of zigzag graphene nanoribbons with a nanobubble at the edge (NB-ZGNRs). The thermal spin-polarized currents can be induced by a temperature difference, and the spin Seebeck effect is found in the nanoribbon. The spin polarization, magnetoresistance, and Seebeck coefficients are discussed, which are strongly affected and can be tuned by the geometrical strain. Moreover, some novel spin caloritronic devices are designed, such as a device that generates bidirectional perfect spin currents and thermally induced giant magnetoresistances. Our results open up the possibility of tuning the spin caloritronic properties of the NB-ZGNR-based devices by changing the elastic strain on the graphene nanobubble.

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.

Investigation of room temperature ferromagnetism in transition metal doped BiFeO3

Spintronic functionality in ferromagnetic materials is a next-generation technique, to be used in data storage, high-frequency communications, and logic devices with minimum energy consumption. Ultra-low energy consumption in high-speed logic devices can be envisioned by inducing ferromagnetic behavior into room temperature multiferroic materials. However, there is a scarcity of room temperature multiferroic materials which have a definite spin degree of freedom. To fully exploit these technological challenges, we introduce the induced ferromagnetism in bismuth ferrite (BiFeO₃, BFO) by doping transition metal (Cr, Ni, Co) elements. Our investigation initiates with the experimental study on chemically synthesized BiFe_(1-x)M _x O₃ samples where x  =  0.0625 (6.25%) and M  =  Cr, Ni and Co. Experimental findings are verified by theoretical simulation using density functional theory (DFT  +  U) and gauge including projector augmented wave (GIPAW) based calculation. All the experimental studies are done at room temperature while the theoretical verification using DFT is carried to understand the underlying mechanism behind the magnetic behavior of doped BiFeO₃. It is done by optimizing the structural parameters comparable to the room temperature values. Microstructural and magnetic properties are studied using x-ray diffraction (XRD), transmission electron microscopy (TEM) and Vibrating sample magnetometer (VSM). All these experimental studies confirm the structural changes and induced ferromagnetism with doping. X-ray photoelectron spectroscopy (XPS) verified the reason behind this ferromagnetic property on the basis of oxygen vacancy content. Electron paramagnetic resonance (EPR) spectroscopy shows the tuning of Δg values due to enhanced magnetization. The density of states (DOS) calculations were performed on BFO (band-gap 1.89 eV) after structural optimization using DFT  +  U method, confirm our experimental findings. Magnetic moment values change drastically with doping elements (M), i.e. almost negligible for BFO (antiferromagnetic) to maximum (2.85 μ _B/f.u.) for Ni-doped sample. We also compute the EPR g-tensor using GIPAW method to confirm the tuning of Δg values due to enhanced magnetization. These results can highlight the impact and importance of suitable transition element doping to induce the room temperature ferromagnetism in BiFeO₃.

Designing a highly efficient graphene quantum spin heat engine

We design a quantum spin heat engine using spin polarized ballistic modes generated in a strained graphene monolayer doped with a magnetic impurity. We observe remarkably large efficiency and large thermoelectric figure of merit both for the charge as well as spin variants of the quantum heat engine. This suggests the use of this device as a highly efficient quantum heat engine for charge as well as spin based transport. Further, a comparison is drawn between the device characteristics of a graphene spin heat engine against a quantum spin Hall heat engine. The reason being edge modes because of their origin should give much better performance. In this respect we observe our graphene based spin heat engine can almost match the performance characteristics of a quantum spin Hall heat engine. Finally, we show that a pure spin current can be transported in our device in absence of any charge current.

The spin-dependent Seebeck effect and the charge and spin figure of merit in a hybrid structure of single-walled carbon nanotubes and zigzag-edge graphene nanoribbons

A hybrid structure of carbon nanotubes and graphene nanoribbons was predicted and synthesized (Y. Li et al., Nat. Nanotechnol., 2012, 7, 394-400; P. Lou, J. Phys. Chem. C, 2014, 118, 4475-4482). Herein, using the non-equilibrium Green's function (NEGF) combined with density functional theory (DFT), the thermal spin transport properties and the figure of merit (a material constant proportional to the efficiency of a thermoelectric couple made with the material) of a composite of single-walled carbon nanotubes and zigzag-edge graphene nanoribbons, labeled (6,6)SWCNT/n-ZGNR, are investigated for n = 1, 2, 3, and 8. The results manifest that spin-dependent currents with opposite flow directions were generated when a temperature gradient was applied between two electrodes, indicating the occurrence of the spin-dependent Seebeck effect (SDSE). Remarkably, when n = 3, the charge current is equal to zero, meaning that a perfect SDSE is observed. Moreover, a pure spin-dependent Seebeck diode (SDSD) effect can be observed. Finally, we notice that the device presents an n-type characteristic when n = 1, while the device has a p-type feature when n = 2. In particular, the spin-up thermopower is equal to the spin-down thermopower when n = 3; as a consequence, the charge thermopower is equal to zero, further demonstrating that a perfect SDSE is generated. These discoveries indicate that the (6,6)SWCNT/n-ZGNR is a promising candidate for spin caloritronics devices.

Metal-free magnetism, spin-dependent Seebeck effect, and spin-Seebeck diode effect in armchair graphene nanoribbons

Metal-free magnetism and spin caloritronics are at the forefront of condensed-matter physics. Here, the electronic structures and thermal spin-dependent transport properties of armchair graphene nanoribbons (N-AGNRs), where N is the ribbon width (N = 5-23), are systematically studied. The results show that the indirect band gaps exhibit not only oscillatory behavior but also periodic characteristics with E _3p  > E_3p+1 > E_3p+2 (E _3p , E_3p+1 and E_3p+2 are the band gaps energy) for a certain integer p, with increasing AGNR width. The magnetic ground states are ferromagnetic (FM) with a Curie temperatures (T _C ) above room temperature. Furthermore, the spin-up and spin-down currents with opposite directions, generated by a temperature gradient, are almost symmetrical, indicating the appearance of the perfect spin-dependent Seebeck effect (SDSE). Moreover, thermally driven spin currents through the nanodevices induced the spin-Seebeck diode (SSD) effect. Our calculation results indicated that AGNRs can be applied in thermal spin nanodevices.

Multiple thermal spin transport performances of graphene nanoribbon heterojuction co-doped with Nitrogen and Boron

Graphene nanoribbon is a popular material in spintronics owing to its unique electronic properties. Here, we propose a novel spin caloritronics device based on zigzag graphene nanoribbon (ZGNR), which is a heterojunction consisting of a pure single-hydrogen-terminated ZGNR and one doped with nitrogen and boron. Using the density functional theory combined with the non-equilibrium Green’s function, we investigate the thermal spin transport properties of the heterojunction under different magnetic configurations only by a temperature gradient without an external gate or bias voltage. Our results indicate that thermally-induced spin polarized currents can be tuned by switching the magnetic configurations, resulting in a perfect thermal colossal magnetoresistance effect. The heterojunctions with different magnetic configurations exhibit a variety of excellent transport characteristics, including the spin-Seebeck effect, the spin-filtering effect, the temperature switching effect, the negative differential thermal resistance effect and the spin-Seebeck diode feature, which makes the heterojunction a promising candidate for high-efficiently multifunctional spin caloritronic applications.

The spin-dependent transport properties of zigzag α-graphyne nanoribbons and new device design

By performing first-principle quantum transport calculations, we studied the electronic and transport properties of zigzag α-graphyne nanoribbons in different magnetic configurations. We designed the device based on zigzag α-graphyne nanoribbon and studied the spin-dependent transport properties, whose current-voltage curves show obvious spin-polarization and conductance plateaus. The interesting transport behaviours can be explained by the transport spectra under different magnetic configurations, which basically depends on the symmetry matching of the electrodes’ bandstructures. Simultaneously, spin Seebeck effect is also found in the device. Thus, according to the transport behaviours, zigzag α-graphyne nanoribbons can be used as a dual spin filter diode, a molecule signal converter and a spin caloritronics device, which indicates that α-graphyne is a promising candidate for the future application in spintronics.

Multiple helical configuration and quantity threshold of graphene nanoribbons inside a single-walled carbon nanotube

Molecular dynamics simulation has been carried out to explore the configuration and quantity threshold of multiple graphene nanoribbons (GNRs) in single-walled carbon nanotube (SWCNT). The simulation results showed that several GNRs tangled together to form a perfect spiral structure to maximize the π-π stacking area when filling inside SWCNT. The formation of multiple helical configuration is influenced by the combined effect of structure stability, initial arrangement and tube space, meanwhile its forming time is related to helical angle. The simulated threshold of GNRs in SWCNT decreases with GNR width but increases with SWCNT diameter, and two formulas have come up in this study to estimate the quantity threshold for GNRs. It has been found that multilayered graphite is hard to be stripped in SWCNT because the special helical configuration with incompletely separated GNRs is metastable. This work provides a possibility to control the configuration of GNR@SWCNT.

Thermoelectric effects in graphene nanostructures

The thermoelectric properties of graphene and graphene nanostructures have recently attracted significant attention from the physics and engineering communities. In fundamental physics, the analysis of Seebeck and Nernst effects is very useful in elucidating some details of the electronic band structure of graphene that cannot be probed by conductance measurements alone, due in particular to the ambipolar nature of this gapless material. For applications in thermoelectric energy conversion, graphene has two major disadvantages. It is gapless, which leads to a small Seebeck coefficient due to the opposite contributions of electrons and holes, and it is an excellent thermal conductor. The thermoelectric figure of merit ZT of a two-dimensional (2D) graphene sheet is thus very limited. However, many works have demonstrated recently that appropriate nanostructuring and bandgap engineering of graphene can concomitantly strongly reduce the lattice thermal conductance and enhance the Seebeck coefficient without dramatically degrading the electronic conductance. Hence, in various graphene nanostructures, ZT has been predicted to be high enough to make them attractive for energy conversion. In this article, we review the main results obtained experimentally and theoretically on the thermoelectric properties of graphene and its nanostructures, emphasizing the physical effects that govern these properties. Beyond pure graphene structures, we discuss also the thermoelectric properties of some hybrid graphene structures, as graphane, layered carbon allotropes such as graphynes and graphdiynes, and graphene/hexagonal boron nitride heterostructures which offer new opportunities. Finally, we briefly review the recent activities on other atomically thin 2D semiconductors with finite bandgap, i.e. dichalcogenides and phosphorene, which have attracted great attention for various kinds of applications, including thermoelectrics.

Control of electronic transport in nanohole defective zigzag graphene nanoribbon by means of side alkene chain

Spin-dependent transport properties can be modulated by the parity of the side alkene chain in defective ZGNR junctions.

Geometric and electronic properties of edge-decorated graphene nanoribbons

Edge-decorated graphene nanoribbons are investigated with the density functional theory; they reveal three stable geometric structures. The first type is a tubular structure formed by the covalent bonds of decorating boron or nitrogen atoms. The second one consists of curved nanoribbons created by the dipole-dipole interactions between two edges when decorated with Be, Mg, or Al atoms. The final structure is a flat nanoribbon produced due to the repulsive force between two edges; most decorated structures belong to this type. Various decorating atoms, different curvature angles, and the zigzag edge structure are reflected in the electronic properties, magnetic properties, and bonding configurations. Most of the resulting structures are conductors with relatively high free carrier densities, whereas a few are semiconductors due to the zigzag-edge-induced anti-ferromagnetism.

Nearly perfect spin filter, spin valve and negative differential resistance effects in a Fe4-based single-molecule junction

The spin-polarized transport in a single-molecule magnet Fe₄ sandwiched between two gold electrodes is studied, using nonequilibrium Green's functions in combination with the density-functional theory. We predict that the device possesses spin filter effect (SFE), spin valve effect (SVE), and negative differential resistance (NDR) behavior. Moreover, we also find that the appropriate chemical ligand, coupling the single molecule to leads, is a key factor for manipulating spin-dependent transport. The device containing the methyl ligand behaves as a nearly perfect spin filter with efficiency approaching 100%, and the transport is dominated by transmission through the Fe₄ metal center. However, in the case of phenyl ligand, the spin filter effect seems to be reduced, but the spin valve effect is significantly enhanced with a large magnetoresistance ratio, reaching 1800%. This may be attributed to the blocking effect of the phenyl ligands in mediating transport. Our findings suggest that such a multifunctional molecular device, possessing SVE, NDR and high SFE simultaneously, would be an excellent candidate for spintronics of molecular devices.

Ultrahigh spin thermopower and pure spin current in a single-molecule magnet

Using the non-equilibrium Green's function (NEGF) formalism within the sequential regime, we studied ultrahigh spin thermopower and pure spin current in single-molecule magnet(SMM), which is attached to nonmagnetic metal wires with spin bias and angle (θ) between the easy axis of SMM and the spin orientation in the electrodes. A pure spin current can be generated by tuning the gate voltage and temperature difference with finite spin bias and the arbitrary angle except of . In the linear regime, large thermopower can be obtained by modifying V_g and the angles (θ). These results are useful in fabricating and advantaging SMM devices based on spin caloritronics.

Perfect spin-filter, spin-valve, switching and negative differential resistance in an organic molecular device with graphene leads

A multiple-effect organic molecular device for spintronics is proposed by performing first-principle quantum transport calculations.

The transport properties and new device design: the case of 6,6,12-graphyne nanoribbons

By performing first-principle quantum transport calculations, we studied the transport properties of three kinds of 6,6,12-graphyne nanoribbons with different edges and different cutting directions. The nanoribbon with zigzag edges shows metallic properties and the spin-polarized currents show an obvious negative differential resistance effect, the other two nanoribbons terminated by a phenyl ring are semiconductors and spin-unpolarized. We also designed several nanowire devices based on these 6,6,12-graphyne nanoribbons, such as rectifier, spin filter diode, spin FET and spin caloritronics devices. These results indicate that 6,6,12-graphyne is a potential candidate for spintronics and spin caloritronics.