28
2 Entering a Two-Dimensional Materials World
stants between about 2 and 33 [144]. Their obtained peak position changes indicated
binding energy modifications for A and B excitons.
Nevertheless, no significant wavelength shifts for suspended monolayers (initially
surrounded by vacuum from both sides) due to a change of the dielectric function
of the environment were reported by Rigosi et al.—an effect attributed to band-gap
renormalisation simultaneous to the change of binding energy [153].
It was also shown that the PL efficiency in the form of intensity changes could
be varied with the help of substrate interactions engineering according to Yu et al.
[86], hinting at the role of doping; and effects of the substrate type and the material–
substrate bonding on high-temperature behaviour of monolayer WS 2 were discussed
by Su et al. [154].
In addition, magneto-optical measurements on monolayer WSe 2 concerning the
dielectric environment (on mainly exfoliated monolayer samples, that were 2 out of
3 environment settings) by Stier et al. showed a diamagnetic shift of the monolayer
resonance revealing a change of the deduced mean exciton radius
5 by 33% [49].
Correspondingly, binding energy changes were evidenced with altered dielectric
environments (also see [88]).
Shortly after, from the comprehensive and comparative work of Lippert et al.,
which was in preparation in parallel to aforementioned studies in that important time
period of excitonic studies in monolayers, it could be concluded that there is a complex interplay between local strain, dielectric environment and doping [87]. Therefor
PL, time-resolved PL and Raman measurements were performed—both at room temperature and 10 K at the example of an archetype monolayer semiconductor, that is
WSe 2 . Moreover, biexcitons were only observable—at cryogenic temperatures—for
low-doping and low-strain samples (i.e. with low trion/exciton ratio and low Raman
shifts for a certain mode, respectively) (see [87]). In addition, spectral signatures
showed that reduced inhomogeneous-broadening and low dephasing support biexciton formation (i.e. a narrow linewidth and pronounced degree of circular polarisation,
respectively). It was also shown that room-temperature species exhibited a linearity
factor common for bound
6 and uncommon for free excitonic species, which were
obtained at low temperatures (below 100 K, also see temperature behaviour of the
linearity factor in [40]). Follow-up exciton–exciton annihilation studies using timeresolved PL also gave hints at the Mott density and exciton Bohr radius modifications
through the dielectric environment [38].
Thus, these studies inspired further detailed investigations concerning substrate
and environment related optoelectronic properties of 2D materials and supported the
tailoring of application oriented 2D systems. An example how the substrate landscape
5 Unfortunately, the reference sample of that study comprised CVD-grown monolayer material,
being well known to be inferior/different in quality than their exfoliated counterparts, until very
recently.
6 Here, ‘bound’ is not referring to Coulomb binding, but meaning bound in the sense of localised
(also see examples from 2D research in [28, 155]). Later, the higher-temperature species obtained
between about 100 K and room temperature had been viewed in the quasi-particle picture as excitons
in a polarisation cloud with phonons, i.e. forming polarons. This was analysed and discussed in
[53] and understood as one reason for the vanishing of optical dispersion above 100 K in [52].
2 Entering a Two-Dimensional Materials World
stants between about 2 and 33 [144]. Their obtained peak position changes indicated
binding energy modifications for A and B excitons.
Nevertheless, no significant wavelength shifts for suspended monolayers (initially
surrounded by vacuum from both sides) due to a change of the dielectric function
of the environment were reported by Rigosi et al.—an effect attributed to band-gap
renormalisation simultaneous to the change of binding energy [153].
It was also shown that the PL efficiency in the form of intensity changes could
be varied with the help of substrate interactions engineering according to Yu et al.
[86], hinting at the role of doping; and effects of the substrate type and the material–
substrate bonding on high-temperature behaviour of monolayer WS 2 were discussed
by Su et al. [154].
In addition, magneto-optical measurements on monolayer WSe 2 concerning the
dielectric environment (on mainly exfoliated monolayer samples, that were 2 out of
3 environment settings) by Stier et al. showed a diamagnetic shift of the monolayer
resonance revealing a change of the deduced mean exciton radius
5 by 33% [49].
Correspondingly, binding energy changes were evidenced with altered dielectric
environments (also see [88]).
Shortly after, from the comprehensive and comparative work of Lippert et al.,
which was in preparation in parallel to aforementioned studies in that important time
period of excitonic studies in monolayers, it could be concluded that there is a complex interplay between local strain, dielectric environment and doping [87]. Therefor
PL, time-resolved PL and Raman measurements were performed—both at room temperature and 10 K at the example of an archetype monolayer semiconductor, that is
WSe 2 . Moreover, biexcitons were only observable—at cryogenic temperatures—for
low-doping and low-strain samples (i.e. with low trion/exciton ratio and low Raman
shifts for a certain mode, respectively) (see [87]). In addition, spectral signatures
showed that reduced inhomogeneous-broadening and low dephasing support biexciton formation (i.e. a narrow linewidth and pronounced degree of circular polarisation,
respectively). It was also shown that room-temperature species exhibited a linearity
factor common for bound
6 and uncommon for free excitonic species, which were
obtained at low temperatures (below 100 K, also see temperature behaviour of the
linearity factor in [40]). Follow-up exciton–exciton annihilation studies using timeresolved PL also gave hints at the Mott density and exciton Bohr radius modifications
through the dielectric environment [38].
Thus, these studies inspired further detailed investigations concerning substrate
and environment related optoelectronic properties of 2D materials and supported the
tailoring of application oriented 2D systems. An example how the substrate landscape
5 Unfortunately, the reference sample of that study comprised CVD-grown monolayer material,
being well known to be inferior/different in quality than their exfoliated counterparts, until very
recently.
6 Here, ‘bound’ is not referring to Coulomb binding, but meaning bound in the sense of localised
(also see examples from 2D research in [28, 155]). Later, the higher-temperature species obtained
between about 100 K and room temperature had been viewed in the quasi-particle picture as excitons
in a polarisation cloud with phonons, i.e. forming polarons. This was analysed and discussed in
[53] and understood as one reason for the vanishing of optical dispersion above 100 K in [52].