32
2 Entering a Two-Dimensional Materials World
Gapless Nature Used for Long-Wavelength Detectors
The gapless nature of graphene bears another important advantage when compared
to conventional semiconductors as it comes to long-wavelength detection, for which
typically a lack of suitable small-gap materials prevents efficient and sensitive photodetection. Recent advances regarding graphene-based far-IR detectors are briefly
highlighted here as well. Due to different physical phenomena, different detection
mechanisms such as a photovoltaic, thermoelectric or bolometric one are enabled
by graphene. Thus, the implementation of graphene-based IR and THz photodetectors has been pursued [13, 20, 196]. To overcome limitations imposed by the weak
absorption from graphene’s interband transitions, many efforts were devoted to the
enhancement of the absorption by utilizing microcavities [197], employing antenna
coupling [198], patterning graphene into periodic metamaterials [199], and including
plasmonic nanostructures on top of the graphene [200]. In fact, plasmonic nanoparticles can even be used in combination with conventional THz antenna materials to
boost their performance [201].
2.4 Layered Systems Based on Monolayer Semiconductors
Owing to the transition from indirect to direct semiconductor towards the monolayer
regime, TMDCs have received considerable attention as possible building blocks for
miniaturised device concepts and energy-efficient optoelectronics. Naturally, vdW
epitaxy becomes attractive with regard to the achievement of TMDC heterostructure
assembly and the exploitation of the benefits when returning to the 3D world from
the 2D plane.
2.4.1 Physics of Transition-Metal Dichalcogenide
Heterostructures
Currently, research activities regarding heterostructure fabrication and studies are
thriving. A plethora of publications deals with the role of monolayer stacking principles (see for instance [56, 91]) as well as optical and electronic properties of 2D
heterostructures—prominently with eyes towards interlayer excitons [40, 202–206]
for possible observation of collective behaviour such as condensation and superfluidity [207–209]. Optoelectronic heterostructure properties have been studied in dependence of the band-alignment [93–96], interlayer band-hybridisation [210], dielectric
changes [40, 88, 211], phase-space mismatch [212] (generally relevant for vdW
stacks, e.g. for twisted graphene bilayers [213]), lifetime modifications [40], moiré
potential landscapes [101, 102, 214], twist angles [215], and stacking symmetry
[216–219].
2 Entering a Two-Dimensional Materials World
Gapless Nature Used for Long-Wavelength Detectors
The gapless nature of graphene bears another important advantage when compared
to conventional semiconductors as it comes to long-wavelength detection, for which
typically a lack of suitable small-gap materials prevents efficient and sensitive photodetection. Recent advances regarding graphene-based far-IR detectors are briefly
highlighted here as well. Due to different physical phenomena, different detection
mechanisms such as a photovoltaic, thermoelectric or bolometric one are enabled
by graphene. Thus, the implementation of graphene-based IR and THz photodetectors has been pursued [13, 20, 196]. To overcome limitations imposed by the weak
absorption from graphene’s interband transitions, many efforts were devoted to the
enhancement of the absorption by utilizing microcavities [197], employing antenna
coupling [198], patterning graphene into periodic metamaterials [199], and including
plasmonic nanostructures on top of the graphene [200]. In fact, plasmonic nanoparticles can even be used in combination with conventional THz antenna materials to
boost their performance [201].
2.4 Layered Systems Based on Monolayer Semiconductors
Owing to the transition from indirect to direct semiconductor towards the monolayer
regime, TMDCs have received considerable attention as possible building blocks for
miniaturised device concepts and energy-efficient optoelectronics. Naturally, vdW
epitaxy becomes attractive with regard to the achievement of TMDC heterostructure
assembly and the exploitation of the benefits when returning to the 3D world from
the 2D plane.
2.4.1 Physics of Transition-Metal Dichalcogenide
Heterostructures
Currently, research activities regarding heterostructure fabrication and studies are
thriving. A plethora of publications deals with the role of monolayer stacking principles (see for instance [56, 91]) as well as optical and electronic properties of 2D
heterostructures—prominently with eyes towards interlayer excitons [40, 202–206]
for possible observation of collective behaviour such as condensation and superfluidity [207–209]. Optoelectronic heterostructure properties have been studied in dependence of the band-alignment [93–96], interlayer band-hybridisation [210], dielectric
changes [40, 88, 211], phase-space mismatch [212] (generally relevant for vdW
stacks, e.g. for twisted graphene bilayers [213]), lifetime modifications [40], moiré
potential landscapes [101, 102, 214], twist angles [215], and stacking symmetry
[216–219].