Hot-Carrier Seebeck Effect: Diffusion and Remote Detection of Hot Carriers in Graphene

Spectrally Tunable Ultrafast Long Wave Infrared Detection at Room Temperature

Room-temperature longwave infrared (LWIR) detectors are preferred over cryogenically cooled solutions due to the cost effectiveness and ease of operation. The performance of present uncooled LWIR detectors such as microbolometers, is limited by reduced sensitivity, slow response time, and the lack of dynamic spectral tunability. Here, we present a graphene-based efficient room-temperature LWIR detector with high detectivity and fast response time utilizing its tunable optical and electronic characteristics. The inherent weak light absorption is enhanced by Dirac plasmons on the patterned graphene coupled to an optical cavity. The absorbed energy is converted into photovoltage by the Seebeck effect with an asymmetric carrier generation environment. Further, dynamic spectral tunability in the 8-12 μm LWIR band is achieved by electrostatic gating. The proposed detection platform paves the path to a fresh generation of uncooled graphene-based LWIR photodetectors for wide ranging applications such as molecular sensing, medical diagnostics, military, security and space.

Van der Waals heterostructures for spintronics and opto-spintronics

The large variety of 2D materials and their co-integration in van der Waals heterostructures enable innovative device engineering. In addition, their atomically thin nature promotes the design of artificial materials by proximity effects that originate from short-range interactions. Such a designer approach is particularly compelling for spintronics, which typically harnesses functionalities from thin layers of magnetic and non-magnetic materials and the interfaces between them. Here we provide an overview of recent progress in 2D spintronics and opto-spintronics using van der Waals heterostructures. After an introduction to the forefront of spin transport research, we highlight the unique spin-related phenomena arising from spin-orbit and magnetic proximity effects. We further describe the ability to create multifunctional hybrid heterostructures based on van der Waals materials, combining spin, valley and excitonic degrees of freedom. We end with an outlook on perspectives and challenges for the design and production of ultracompact all-2D spin devices and their potential applications in conventional and quantum technologies.

Hot carriers in graphene - fundamentals and applications

Hot charge carriers in graphene exhibit fascinating physical phenomena, whose understanding has improved greatly over the past decade. They have distinctly different physical properties compared to, for example, hot carriers in conventional metals. This is predominantly the result of graphene's linear energy-momentum dispersion, its phonon properties, its all-interface character, and the tunability of its carrier density down to very small values, and from electron- to hole-doping. Since a few years, we have witnessed an increasing interest in technological applications enabled by hot carriers in graphene. Of particular interest are optical and optoelectronic applications, where hot carriers are used to detect (photodetection), convert (nonlinear photonics), or emit (luminescence) light. Graphene-enabled systems in these application areas could find widespread use and have a disruptive impact, for example in the field of data communication, high-frequency electronics, and industrial quality control. The aim of this review is to provide an overview of the most relevant physics and working principles that are relevant for applications exploiting hot carriers in graphene.

Regulating the interface defect of TiO2/Ag2O nanoheterojunction and its effect on photogenerated carrier dynamics

In this paper, the defects of TiO₂/Ag₂O nanoheterojunctions are regulated to evaluate the effect of the interface defects on carrier trapping and recombination dynamics by time-resolved photoluminescence spectroscopy (TRPL) and time-resolved terahertz (THZ) spectroscopy. TRPL spectra reveal that interface defects can act as a recombination center and have an accelerative effect on the recombination process of photogenerated carriers under ultraviolet light. Moreover, THZ spectroscopy results demonstrate that interface defects can effectively trap electrons and expedite the Auger recombination. Furthermore, the influence of interface defects on the photocarrier dynamics of TiO₂/Ag₂O nanoheterojunctions was comprehensively analyzed, providing a valuable experimental reference for the regulation and application of interface defect-fabricated nanoheterojunctions.

Photo thermal effect graphene detector featuring 105 Gbit s-1 NRZ and 120 Gbit s-1 PAM4 direct detection

One of the main challenges of next generation optical communication is to increase the available bandwidth while reducing the size, cost and power consumption of photonic integrated circuits. Graphene has been recently proposed to be integrated with silicon photonics to meet these goals because of its high mobility, fast carrier dynamics and ultra-broadband optical properties. We focus on graphene photodetectors for high speed datacom and telecom applications based on the photo-thermo-electric effect, allowing for direct optical power to voltage conversion, zero dark current, and ultra-fast operation. We report on a chemical vapour deposition graphene photodetector based on the photo-thermoelectric effect, integrated on a silicon waveguide, providing frequency response >65 GHz and optimized to be interfaced to a 50 Ω voltage amplifier for direct voltage amplification. We demonstrate a system test leading to direct detection of 105 Gbit s^-1 non-return to zero and 120 Gbit s^-1 4-level pulse amplitude modulation optical signals.

Misorientation-Controlled Cross-Plane Thermoelectricity in Twisted Bilayer Graphene

The introduction of "twist" or relative rotation between two atomically thin van der Waals membranes gives rise to periodic moiré potential, leading to a substantial alteration of the band structure of the planar assembly. While most of the recent experiments primarily focus on the electronic-band hybridization by probing in-plane transport properties, here we report out-of-plane thermoelectric measurements across the van der Waals gap in twisted bilayer graphene, which exhibits an interplay of twist-dependent interlayer electronic and phononic hybridization. We show that at large twist angles, the thermopower is entirely driven by a novel phonon-drag effect at subnanometer scale, while the electronic component of the thermopower is recovered only when the misorientation between the layers is reduced to <6°. Our experiment shows that cross-plane thermoelectricity at low angles is exceptionally sensitive to the nature of band dispersion and may provide fundamental insights into the coherence of electronic states in twisted bilayer graphene.

The Thermal, Electrical and ThermoelectricProperties of Graphene Nanomaterials

Graphene, as a typical two-dimensional nanometer material, has shown its unique application potential in electrical characteristics, thermal properties, and thermoelectric properties by virtue of its novel electronic structure. The field of traditional material modification mainly changes or enhances certain properties of materials by mixing a variety of materials (to form a heterostructure) and doping. For graphene as well, this paper specifically discusses the use of traditional modification methods to improve graphene’s electrical and thermoelectrical properties. More deeply, since graphene is an atomic-level thin film material, its shape and edge conformation (zigzag boundary and armchair boundary) have a great impact on performance. Therefore, this paper reviews the graphene modification field in recent years. Through the change in the shape of graphene, the change in the boundary structure configuration, the doping of other atoms, and the formation of a heterostructure, the electrical, thermal, and thermoelectric properties of graphene change, resulting in broader applications in more fields. Through studies of graphene’s electrical, thermal, and thermoelectric properties in recent years, progress has been made not only in experimental testing, but also in theoretical calculation. These aspects of graphene are reviewed in this paper.

Plasmon induced thermoelectric effect in graphene

Graphene has emerged as a promising material for optoelectronics due to its potential for ultrafast and broad-band photodetection. The photoresponse of graphene junctions is characterized by two competing photocurrent generation mechanisms: a conventional photovoltaic effect and a more dominant hot-carrier-assisted photothermoelectric (PTE) effect. The PTE effect is understood to rely on variations in the Seebeck coefficient through the graphene doping profile. A second PTE effect can occur across a homogeneous graphene channel in the presence of an electronic temperature gradient. Here, we study the latter effect facilitated by strongly localised plasmonic heating of graphene carriers in the presence of nanostructured electrical contacts resulting in electronic temperatures of the order of 2000 K. At certain conditions, the plasmon-induced PTE photocurrent contribution can be isolated. In this regime, the device effectively operates as a sensitive electronic thermometer and as such represents an enabling technology for development of hot carrier based plasmonic devices.

The photoresponse of graphene-based photodetectors is dominated by photovoltaic and photothermoelectric effects. Here, the authors leverage strongly localised plasmonic heating of graphene carriers to detect a second photothermoelectric effect occurring across a homogeneous channel in the presence of an electronic temperature gradient.

Armchair graphene nanoribbons with giant spin thermoelectric efficiency

Spin-caloritronic effects in armchair graphene nanoribbons (AGNRs) with various ribbon widths and periodic structural defects in the form of triangular antidots were systematically studied. Our results showed that by engineering defects in AGNRs, one could not only reduce the phononic thermal conductance for enhancing the thermoelectric efficiency, but also induce a ferromagnetic ground state. Interestingly, AGNRs with triangular antidots exhibit spin-semiconducting behavior with a tunable spin gap and a narrow spin-polarized band around the Fermi level. Therefore, AGNRs with antidots exhibit spin-up and spin-down currents with opposite flow directions under a temperature gradient, and they also exhibit a giant spin Seebeck coefficient () and spin figure of merit () that are much larger than those of zigzag GNRs. Finally, these results pave the way towards the application of defective AGNRs in spin-caloritronic devices operating at room temperature with a giant spin thermoelectric efficiency.

Thermoelectric spin voltage in graphene

In recent years, new spin-dependent thermal effects have been discovered in ferromagnets, stimulating a growing interest in spin caloritronics, a field that exploits the interaction between spin and heat currents ^1,2 . Amongst the most intriguing phenomena is the spin Seebeck effect ^3-5 , in which a thermal gradient gives rise to spin currents that are detected through the inverse spin Hall effect ^6-8 . Non-magnetic materials such as graphene are also relevant for spin caloritronics, thanks to efficient spin transport ^9-11 , energy-dependent carrier mobility and unique density of states ^12,13 . Here, we propose and demonstrate that a carrier thermal gradient in a graphene lateral spin valve can lead to a large increase of the spin voltage near to the graphene charge neutrality point. Such an increase results from a thermoelectric spin voltage, which is analogous to the voltage in a thermocouple and that can be enhanced by the presence of hot carriers generated by an applied current ^14-17 . These results could prove crucial to drive graphene spintronic devices and, in particular, to sustain pure spin signals with thermal gradients and to tune the remote spin accumulation by varying the spin-injection bias.

Photothermoelectric Effects and Large Photovoltages in Plasmonic Au Nanowires with Nanogaps

Nanostructured metals subject to local optical interrogation can generate open-circuit photovoltages potentially useful for energy conversion and photodetection. We report a study of the photovoltage as a function of illumination position in single-metal Au nanowires and nanowires with nanogaps formed by electromigration. We use a laser scanning microscope to locally heat the metal nanostructures via excitation of a local plasmon resonance and direct absorption. In nanowires without nanogaps, where charge transport is diffusive, we observe voltage distributions consistent with thermoelectricity, with the local Seebeck coefficient depending on the width of the nanowire. In the nanowires with nanogaps, where charge transport is by tunneling, we observe large photovoltages up to tens of mV, with magnitude, polarization dependence, and spatial localization that follow the plasmon resonance in the nanogap. This is consistent with a model of photocurrent across the nanogap carried by the nonequilibrium, "hot" carriers generated upon plasmon excitation.

Direct electronic measurement of Peltier cooling and heating in graphene

Thermoelectric effects allow the generation of electrical power from waste heat and the electrical control of cooling and heating. Remarkably, these effects are also highly sensitive to the asymmetry in the density of states around the Fermi energy and can therefore be exploited as probes of distortions in the electronic structure at the nanoscale. Here we consider two-dimensional graphene as an excellent nanoscale carbon material for exploring the interaction between electronic and thermal transport phenomena, by presenting a direct and quantitative measurement of the Peltier component to electronic cooling and heating in graphene. Thanks to an architecture including nanoscale thermometers, we detected Peltier component modulation of up to 15 mK for currents of 20 μA at room temperature and observed a full reversal between Peltier cooling and heating for electron and hole regimes. This fundamental thermodynamic property is a complementary tool for the study of nanoscale thermoelectric transport in two-dimensional materials.

Determination of the spin-lifetime anisotropy in graphene using oblique spin precession

We determine the spin-lifetime anisotropy of spin-polarized carriers in graphene. In contrast to prior approaches, our method does not require large out-of-plane magnetic fields and thus it is reliable for both low- and high-carrier densities. We first determine the in-plane spin lifetime by conventional spin precession measurements with magnetic fields perpendicular to the graphene plane. Then, to evaluate the out-of-plane spin lifetime, we implement spin precession measurements under oblique magnetic fields that generate an out-of-plane spin population. We find that the spin-lifetime anisotropy of graphene on silicon oxide is independent of carrier density and temperature down to 150 K, and much weaker than previously reported. Indeed, within the experimental uncertainty, the spin relaxation is isotropic. Altogether with the gate dependence of the spin lifetime, this indicates that the spin relaxation is driven by magnetic impurities or random spin-orbit or gauge fields.

Effects of Dephasing on Spin Lifetime in Ballistic Spin-Orbit Materials

We theoretically investigate spin dynamics in spin-orbit-coupled materials. In the ballistic limit, the spin lifetime is dictated by dephasing that arises from energy broadening plus a nonuniform spin precession. For the case of clean graphene, we find a strong anisotropy with spin lifetimes that can be short even for modest energy scales, on the order of a few ns. These results offer deeper insight into the nature of spin dynamics in graphene, and are also applicable to the investigation of other systems where spin-orbit coupling plays an important role.