Graphene and other two-dimensional materials, such as transition metal dichalcogenides, have rapidly established themselves as intriguing building blocks for optoelectronic applications, with a strong focus on various photodetection platforms The versatility of these material systems enables their application in areas including ultrafast and ultrasensitive detection of light in the ultraviolet, visible, infrared and terahertz frequency ranges These detectors can be integrated with other photonic components based on the same material, as well as with silicon photonic and electronic technologies Here, we provide an overview and evaluation of state-of-the-art photodetectors based on graphene, other two-dimensional materials, and hybrid systems based on the combination of different two-dimensional crystals or of two-dimensional crystals and other (nano)materials, such as plasmonic nanoparticles, semiconductors, quantum dots, or their integration with (silicon) waveguides

http://www-g.eng.cam.ac.uk/nms/publications/pdf/nnano.2014.215.pdf

Photodetectors based on graphene, other two-dimensional materials and hybrid systems

Interactions between particles in quantum many-body systems can lead to collective behavior described by hydrodynamics. One such system is the electron-hole plasma in graphene near the charge-neutrality point, which can form a strongly coupled Dirac fluid. This charge-neutral plasma of quasi-relativistic fermions is expected to exhibit a substantial enhancement of the thermal conductivity, thanks to decoupling of charge and heat currents within hydrodynamics. Employing high-sensitivity Johnson noise thermometry, we report an order of magnitude increase in the thermal conductivity and the breakdown of the Wiedemann-Franz law in the thermally populated charge-neutral plasma in graphene. This result is a signature of the Dirac fluid and constitutes direct evidence of collective motion in a quantum electronic fluid.

https://science.sciencemag.org/content/sci/351/6277/1058.full.pdf

Observation of the Dirac fluid and the breakdown of the Wiedemann-Franz law in graphene

When intense light interacts with an atomic gas, recollision between an ionizing electron and its parent ion creates high-order harmonics of the fundamental laser frequency. This sub-cycle effect generates coherent soft X-rays and attosecond pulses, and provides a means to image molecular orbitals. Recently, high harmonics have been generated from bulk crystals, but what mechanism dominates the emission remains uncertain. To resolve this issue, we adapt measurement methods from gas-phase research to solid zinc oxide driven by mid-infrared laser fields of 0.25 volts per angstrom. We find that when we alter the generation process with a second-harmonic beam, the modified harmonic spectrum bears the signature of a generalized recollision between an electron and its associated hole. In addition, we find that solid-state high harmonics are perturbed by fields so weak that they are present in conventional electronic circuits, thus opening a route to integrate electronics with attosecond and high-harmonic technology. Future experiments will permit the band structure of a solid to be tomographically reconstructed.

/pdf/linking-high-harmonics-from-gases-and-solids-4rtfjbqvr5.pdf

Linking high harmonics from gases and solids

Multifunctional energy storage and conversion devices that incorporate novel features and functions in intelligent and interactive modes, represent a radical advance in consumer products, such as wearable electronics, healthcare devices, artificial intelligence, electric vehicles, smart household, and space satellites, etc. Here, smart energy devices are defined to be energy devices that are responsive to changes in configurational integrity, voltage, mechanical deformation, light, and temperature, called self-healability, electrochromism, shape memory, photodetection, and thermal responsivity. Advisable materials, device designs, and performances are crucial for the development of energy electronics endowed with these smart functions. Integrating these smart functions in energy storage and conversion devices gives rise to great challenges from the viewpoint of both understanding the fundamental mechanisms and practical implementation. Current state-of-art examples of these smart multifunctional energy devices, pertinent to materials, fabrication strategies, and performances, are highlighted. In addition, current challenges and potential solutions from materials synthesis to device performances are discussed. Finally, some important directions in this fast developing field are considered to further expand their application.

Multifunctional Energy Storage and Conversion Devices

The optical properties of graphene are made unique by the linear band structure and the vanishing density of states at the Dirac point. It has been proposed that even in the absence of a bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing at arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by impact ionization has been discussed in the context of light harvesting applications. However, all of these effects are difficult to test quantitatively by measuring the transient optical properties alone, as these only indirectly reflect the energy- and momentum-dependent carrier distributions. Here, we use time- and angle-resolved photoemission spectroscopy with femtosecond extreme-ultraviolet pulses to directly probe the non-equilibrium response of Dirac electrons near the K-point of the Brillouin zone. In lightly hole-doped epitaxial graphene samples, we explore excitation in the mid- and near-infrared, both below and above the minimum photon energy for direct interband transitions. Whereas excitation in the mid-infrared results only in heating of the equilibrium carrier distribution, interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible. However, in neither excitation regime do we find any indication of carrier multiplication, questioning the applicability of graphene for light harvesting.

/pdf/snapshots-of-non-equilibrium-dirac-carrier-distributions-in-2j8jkcsg1p.pdf

Snapshots of non-equilibrium Dirac carrier distributions in graphene

The ultrafast dynamics of excited carriers in graphene is closely linked to the Dirac spectrum and plays a central role for many electronic and optoelectronic applications. Harvesting energy from excited electron-hole pairs, for instance, is only possible if these pairs can be separated before they lose energy to vibrations, merely heating the lattice. Until now, the hot carrier dynamics in graphene could only be accessed indirectly. Here, we present a dynamical view on the Dirac cone by time-and angle-resolved photoemission spectroscopy. This allows us to show the quasi-instant thermalization of the electron gas to a temperature of approximate to 2000 K, to determine the time-resolved carrier density, and to disentangle the subsequent decay into excitations of optical phonons and acoustic phonons (directly and via supercollisions).

Direct View of Hot Carrier Dynamics in Graphene

Bilayer graphene is a highly promising material for electronic and optoelectronic applications since it is supporting massive Dirac fermions with a tunable band gap. However, no consistent picture of the gap’s effect on the optical and transport behavior has emerged so far, and it has been proposed that the insulating nature of the gap could be compromised by unavoidable structural defects, by topological in-gap states, or that the electronic structure could be altogether changed by many-body effects. Here, we directly follow the excited carriers in bilayer graphene on a femtosecond time scale, using ultrafast time- and angle-resolved photoemission. We find a behavior consistent with a single-particle band gap. Compared to monolayer graphene, the existence of this band gap leads to an increased carrier lifetime in the minimum of the lowest conduction band. This is in sharp contrast to the second substate of the conduction band, in which the excited electrons decay through fast, phonon-assisted interband transitions.

/pdf/ultrafast-dynamics-of-massive-dirac-fermions-in-bilayer-3wry7t3r4n.pdf

Ultrafast Dynamics of Massive Dirac Fermions in Bilayer Graphene

The doping of quasi-freestanding graphene (QFG) on H-terminated, Si-face 6H-, 4H-, and 3C-SiC is studied by angle-resolved photoelectron spectroscopy close to the Dirac point. Using semi-insulating as well as n-type doped substrates we shed light on the contributions to the charge carrier density in QFG caused by (i) the spontaneous polarization of the substrate, and (ii) the band alignment between the substrate and the graphene layer. In this way we provide quantitative support for the previously suggested model of polarization doping of graphene on SiC (Ristein et al 2012 Phys. Rev. Lett. 108 246104).

/pdf/polarization-doping-of-graphene-on-silicon-carbide-58qxo0dlah.pdf

Polarization doping of graphene on silicon carbide

The work function and electronic structure of epitaxial graphene as well as of quasi-freestanding graphene multilayer samples were studied by Kelvin probe and angle resolved photoelectron spectroscopy. The work function converges towards the value of graphite as the number of layers is increased. Thereby, n-type doped epitaxial graphene layers have a work function lower than graphite and p-type doped quasi-freestanding graphene layers exhibit a work function higher than graphite. We explain the behaviour by the filling of the -bands due to substrate interactions.

/pdf/work-function-of-graphene-multilayers-on-sic-0001-3b0lgor3yq.pdf

Christian Raidel

Papers

Direct View of Hot Carrier Dynamics in Graphene

Ultrafast Dynamics of Massive Dirac Fermions in Bilayer Graphene

Polarization doping of graphene on silicon carbide

Work function of graphene multilayers on SiC(0001)