146
5 Optical Measurement Techniques
Typically, streak camera measurements are employed to investigate charge-carrier,
quasi-particle, fluid or relaxation dynamics in various types of quantum structures (cf.
[20–24]), or quasi-particle lifetimes such as polariton decay from microcavities (see
for instance [25, 26], and the Supporting Information section of [27]). Picosecondresolved transient PL can for instance provide insights into the different effects on
charge-transfer processes in organic semiconductor heterosystems [28] and TMDC
stacks [5], or the influence of growth parameters on quantum structures [29, 30].
Streak camera traces for monolayer WSe 2 and heterostructures of it were for
instance studied in [5, 31] in order to measure the very fast temporal luminescence decays of 2D excitons and density-dependent exciton–exciton annihilation
4
in this material system for different environmental configurations (substrate- or
hBN-supported, hBN-capped or hBN-encapsulated monolayers, and monolayer–
monolayer heterostructures on hBN). Thereby, an effective time resolution of 3 ps
was given which cannot be easily achieved with other means.
Even the possible impact of different heterostructure twist angles on the interlayer
coupling situation can be deduced from spectrally-resolved lifetime measurements
(cf. [5, 32]). Similar streak camera measurements have been performed for such
samples regarding high-charge-carrier-density effects and dynamics on different substrates, and the influence of hBN as well as the bilayer stacking order on the spectral
and dynamics properties of 2D WSe 2 emission [33]. Recently, works by the author
and co-workers have indicated by streak camera TRPL data that interlayer excitons
and possible moiré features in stacked monolayer–monolayer TMDC heterolayers
(cf. [2]) with nearly aligned/anti-aligned twist angle (estimation was only possible
modulo 60
◦ ) feature very long decay times on the ns scale (see Fig. 5.9). This is in
contrast to the ultrafast picoseconds decay of the intralayer counterparts in the same
structure.
Alternatively, time-correlated single-photon counting with ultrafast APDs
5 can be
used to build histograms of detection events with a time-tagging mode based on the
measured delay between start and stop signal. Conveniently, a pulse from the excitation laser provides the start signal via a photodiode (or an electronic trigger signal
is used) and the emitted sample photon detected by the APD delivers the stop time.
Transient PL recorded with the time-tagging mode is assembled through a plot of
photon counts as a function of delay time. The temporal resolution is limited by the
instrument response function of the photodetector, typically ranging between tens of
picoseconds to hundreds of picoseconds, whereas the histogram resolution depends
4 The exciton–exciton annihilation rate was modelled in [5] using a rate equation model
d N
dt =
G(t) −
N
τ − ξ N 2 , with source term G(t), linear decay term with time constant (low-density PL
lifetime) τ and quadratic term with annihilation rate ξ . Two possibilities for modelling were pursued in [5] (also see references therein for details about the models): (1) exciton diffusion, (2)
Förster transfer (long-range dipole–dipole interactions). The former one is intuitive, with diffusion
constant, mean-free path and mean exciton size (Bohr radius) relevant for possible annihilation processes due to short distance interactions (collision of excitons), the latter one with Förster radius is
considered a long-range annihilation mechanism, reminiscent of the situation for exciton–polaritons.
5 Acronym for avalanche photodetectors.
5 Optical Measurement Techniques
Typically, streak camera measurements are employed to investigate charge-carrier,
quasi-particle, fluid or relaxation dynamics in various types of quantum structures (cf.
[20–24]), or quasi-particle lifetimes such as polariton decay from microcavities (see
for instance [25, 26], and the Supporting Information section of [27]). Picosecondresolved transient PL can for instance provide insights into the different effects on
charge-transfer processes in organic semiconductor heterosystems [28] and TMDC
stacks [5], or the influence of growth parameters on quantum structures [29, 30].
Streak camera traces for monolayer WSe 2 and heterostructures of it were for
instance studied in [5, 31] in order to measure the very fast temporal luminescence decays of 2D excitons and density-dependent exciton–exciton annihilation
4
in this material system for different environmental configurations (substrate- or
hBN-supported, hBN-capped or hBN-encapsulated monolayers, and monolayer–
monolayer heterostructures on hBN). Thereby, an effective time resolution of 3 ps
was given which cannot be easily achieved with other means.
Even the possible impact of different heterostructure twist angles on the interlayer
coupling situation can be deduced from spectrally-resolved lifetime measurements
(cf. [5, 32]). Similar streak camera measurements have been performed for such
samples regarding high-charge-carrier-density effects and dynamics on different substrates, and the influence of hBN as well as the bilayer stacking order on the spectral
and dynamics properties of 2D WSe 2 emission [33]. Recently, works by the author
and co-workers have indicated by streak camera TRPL data that interlayer excitons
and possible moiré features in stacked monolayer–monolayer TMDC heterolayers
(cf. [2]) with nearly aligned/anti-aligned twist angle (estimation was only possible
modulo 60
◦ ) feature very long decay times on the ns scale (see Fig. 5.9). This is in
contrast to the ultrafast picoseconds decay of the intralayer counterparts in the same
structure.
Alternatively, time-correlated single-photon counting with ultrafast APDs
5 can be
used to build histograms of detection events with a time-tagging mode based on the
measured delay between start and stop signal. Conveniently, a pulse from the excitation laser provides the start signal via a photodiode (or an electronic trigger signal
is used) and the emitted sample photon detected by the APD delivers the stop time.
Transient PL recorded with the time-tagging mode is assembled through a plot of
photon counts as a function of delay time. The temporal resolution is limited by the
instrument response function of the photodetector, typically ranging between tens of
picoseconds to hundreds of picoseconds, whereas the histogram resolution depends
4 The exciton–exciton annihilation rate was modelled in [5] using a rate equation model
d N
dt =
G(t) −
N
τ − ξ N 2 , with source term G(t), linear decay term with time constant (low-density PL
lifetime) τ and quadratic term with annihilation rate ξ . Two possibilities for modelling were pursued in [5] (also see references therein for details about the models): (1) exciton diffusion, (2)
Förster transfer (long-range dipole–dipole interactions). The former one is intuitive, with diffusion
constant, mean-free path and mean exciton size (Bohr radius) relevant for possible annihilation processes due to short distance interactions (collision of excitons), the latter one with Förster radius is
considered a long-range annihilation mechanism, reminiscent of the situation for exciton–polaritons.
5 Acronym for avalanche photodetectors.