156
5 Optical Measurement Techniques
Fig. 5.12 Example input-output diagram showing PL intensity as a function of the irradiance for
the emission from a CVD monolayer grown on a sapphire substrate. The linearity factors α for
the exciton (X) and trion (X*) are given in the low- and high-density regimes (L/H) according
to the pseudo-linear fit of the double-logarithmically plotted data (dashed/dotted lines). The estimated carrier excitation density is given on the top axis, with an arrow indicating the Mott density.
Reproduced with permission. [31] Copyright 2018 Springer Nature
Analysing the linearity factor α of an emitter according to
I = P
α
(5.3)
(see Fig. 5.12), i.e. the proportionality of measured intensity I to excitation power
P, can be helpful to shed light on the nature of the optical transition. Chargecarrier-density-dependent PL signal for different species can provide a hint regarding qualitatively different dynamics and types of resonances. Typically, input–output
measurements are plotted with double-logarithmic scale to indicate the exponent via
the slope. A linear regime indicates free excitons [7, 77, 78], a sublinear behaviour
indicates defect or bound states [77]. However, this type of experiment is not unambiguous. For instance, biexcitons and plasma share the same predicted linearity factor
[73, 79].
For monolayer WSe 2 , also hBN-encapsulated monolayers, PL measurements of
the linearity factor as a function of temperature indicate a transition from a linear to
a sublinear regime by a temperature-dependent decrease of the linearity factor above
100 K [5]. A similar temperature-dependent PL experiment focused on linewidth
evaluation showed that at temperatures below 100 K, the linewidth changes linearly,
but above 100 K, an exponential increase of the linewidth with temperature was
obtained [80, 81]. Such behaviour has been attributed to the formation of phonon
sidebands (PSBs) [80, 82, 83], obtained for both bright and dark excitonic species
5 Optical Measurement Techniques
Fig. 5.12 Example input-output diagram showing PL intensity as a function of the irradiance for
the emission from a CVD monolayer grown on a sapphire substrate. The linearity factors α for
the exciton (X) and trion (X*) are given in the low- and high-density regimes (L/H) according
to the pseudo-linear fit of the double-logarithmically plotted data (dashed/dotted lines). The estimated carrier excitation density is given on the top axis, with an arrow indicating the Mott density.
Reproduced with permission. [31] Copyright 2018 Springer Nature
Analysing the linearity factor α of an emitter according to
I = P
α
(5.3)
(see Fig. 5.12), i.e. the proportionality of measured intensity I to excitation power
P, can be helpful to shed light on the nature of the optical transition. Chargecarrier-density-dependent PL signal for different species can provide a hint regarding qualitatively different dynamics and types of resonances. Typically, input–output
measurements are plotted with double-logarithmic scale to indicate the exponent via
the slope. A linear regime indicates free excitons [7, 77, 78], a sublinear behaviour
indicates defect or bound states [77]. However, this type of experiment is not unambiguous. For instance, biexcitons and plasma share the same predicted linearity factor
[73, 79].
For monolayer WSe 2 , also hBN-encapsulated monolayers, PL measurements of
the linearity factor as a function of temperature indicate a transition from a linear to
a sublinear regime by a temperature-dependent decrease of the linearity factor above
100 K [5]. A similar temperature-dependent PL experiment focused on linewidth
evaluation showed that at temperatures below 100 K, the linewidth changes linearly,
but above 100 K, an exponential increase of the linewidth with temperature was
obtained [80, 81]. Such behaviour has been attributed to the formation of phonon
sidebands (PSBs) [80, 82, 83], obtained for both bright and dark excitonic species