36
2 Entering a Two-Dimensional Materials World
Band Gap Engineering
An incredible effort is going on worldwide to harvest the yield from 2D heterostructuring and the anticipated “band-gap engineering” (for applications) using the existing and known materials at ones disposal. A few of these outcomes are highlighted
in a number of timely reviews [7, 56, 98, 205, 239]. One of such heterostructuring endeavours is reflected by one of the author’s projects (DFG RA2841/5-1), in
which 2D semiconductor type-I and type-II heterostructures incorporating MoTe 2
are primarily investigated.
Harnessing Post-TMDCs—Golden GaTe Example
Novel nanomaterials commonly are based on existing bulk crystalline systems, which
have been later rediscovered in the nanoscale as interesting candidates for optoelectronics with 2D materials. Similar to TMDCs, group-III monochalcogenides exhibit
interesting properties as layered materials. One example is GaTe, which remarkably
behaves oppositely to TMDCs when thinned down to a monolayer. It becomes an
indirect semiconductor as monolayer due to a change in the crystal structure (transition from a monoclinic to a hexagonal phase) [240], whereas as few-layer system
it features a direct gap [241]. Thereby, it offers direct-gap emission for a wide range
of thicknesses. When decorated with gold nanoparticles, which efficiently grow on
facets of multi-layered GaTe 2D crystals due to the defect density on the material
surface, GaTe has been shown to act as a very sensitive probe for certain aromatic
molecules and could become a promising candidate for surface-enhanced Raman
spectroscopy [242].
Twist Angle and Moiré Superlattices
Nevertheless, the world of heterostructure studies with 2D materials is far more rich
than the mere variability of material combinations. Twist angles are known to play a
major role and promise unprecedented properties from monolayer–monolayer stacks.
While phase-space mismatch introduced by an interlayer twist has a direct impact on
charge-transfer processes [212], more changes to the system arise from moiré potential landscapes originating from the lattice mismatch and/or twist angles between
adjacent layers [99, 100, 107–110]. Thereby, periodic nanoislands are formed acting as perfect superlattices of quantum dots with supercell symmetry points affecting both interlayer spacings and energy scales locally [101, 102]. Thus, the recently
reported moiré exciton states unravel interesting optical and electronic properties
of bilayer systems [102–105, 214]. Current signatures for testbed heterostructures
stacked and probed in Marburg indicate strong oscillator strengths of new excitonic
species in possible moiré landscapes via optical absorption spectra [228] and very
long PL lifetimes (see Figs. 5.11 and 5.9, respectively), triggering further investigations. So far, the observation of energy-split bilayer inter- and intralayer excitons has
emerged as a spectral fingerprint for the formation of such superlattices as discussed
in the literature.
High-Symmetry Bilayer Configurations
In addition to arbitrary twist angles, high-symmetry stacking configurations [19,
243] open up new possibilities to alter the electronic band structure of homobilayers
2 Entering a Two-Dimensional Materials World
Band Gap Engineering
An incredible effort is going on worldwide to harvest the yield from 2D heterostructuring and the anticipated “band-gap engineering” (for applications) using the existing and known materials at ones disposal. A few of these outcomes are highlighted
in a number of timely reviews [7, 56, 98, 205, 239]. One of such heterostructuring endeavours is reflected by one of the author’s projects (DFG RA2841/5-1), in
which 2D semiconductor type-I and type-II heterostructures incorporating MoTe 2
are primarily investigated.
Harnessing Post-TMDCs—Golden GaTe Example
Novel nanomaterials commonly are based on existing bulk crystalline systems, which
have been later rediscovered in the nanoscale as interesting candidates for optoelectronics with 2D materials. Similar to TMDCs, group-III monochalcogenides exhibit
interesting properties as layered materials. One example is GaTe, which remarkably
behaves oppositely to TMDCs when thinned down to a monolayer. It becomes an
indirect semiconductor as monolayer due to a change in the crystal structure (transition from a monoclinic to a hexagonal phase) [240], whereas as few-layer system
it features a direct gap [241]. Thereby, it offers direct-gap emission for a wide range
of thicknesses. When decorated with gold nanoparticles, which efficiently grow on
facets of multi-layered GaTe 2D crystals due to the defect density on the material
surface, GaTe has been shown to act as a very sensitive probe for certain aromatic
molecules and could become a promising candidate for surface-enhanced Raman
spectroscopy [242].
Twist Angle and Moiré Superlattices
Nevertheless, the world of heterostructure studies with 2D materials is far more rich
than the mere variability of material combinations. Twist angles are known to play a
major role and promise unprecedented properties from monolayer–monolayer stacks.
While phase-space mismatch introduced by an interlayer twist has a direct impact on
charge-transfer processes [212], more changes to the system arise from moiré potential landscapes originating from the lattice mismatch and/or twist angles between
adjacent layers [99, 100, 107–110]. Thereby, periodic nanoislands are formed acting as perfect superlattices of quantum dots with supercell symmetry points affecting both interlayer spacings and energy scales locally [101, 102]. Thus, the recently
reported moiré exciton states unravel interesting optical and electronic properties
of bilayer systems [102–105, 214]. Current signatures for testbed heterostructures
stacked and probed in Marburg indicate strong oscillator strengths of new excitonic
species in possible moiré landscapes via optical absorption spectra [228] and very
long PL lifetimes (see Figs. 5.11 and 5.9, respectively), triggering further investigations. So far, the observation of energy-split bilayer inter- and intralayer excitons has
emerged as a spectral fingerprint for the formation of such superlattices as discussed
in the literature.
High-Symmetry Bilayer Configurations
In addition to arbitrary twist angles, high-symmetry stacking configurations [19,
243] open up new possibilities to alter the electronic band structure of homobilayers