5.2 Microscopy and Spectroscopy
145
Fig. 5.9 PL of a WS 2 /WSe 2 heterobilayer on hBN recorded with a streak camera with picoseconds
time resolution. Here, an off-resonant excitation at 2.7 eV at an irradiance of 54 µJ cm −2 was
used. This false-colour logarithmically-scaled intensity contour diagram shows TRPL data from
four energy ranges stitched together (from dark to bright: minimum to maximum counts, white:
background). For the peaks representing intralayer excitons of the two TMDC materials, a fast
decay is seen on the time scale of a few ps (WSe 2 about twice as slow as WS 2 ). The slightly shorter
instrument response time for the used setting amounts to 2 ps. After the initial picoseconds, the
WSe 2 peak exhibits a red-shift and the decay in this spectral region continues with a longer time
constant. Top panel: Temporally-integrated spectrum obtained from the time–energy spectrum. Side
panel: PL transients were extracted by spectral integration (indicated as colour-coded shaded area in
the top panel), that is 1.42–1.49 eV (interlayer excitons, pink line), 1.60–1.76 eV (WSe 2 and lowerenergetic moiré states, thick red line), 1.77–1.86 eV (higher-energetic moiré states, orange line),
and 1.95–2.06 eV (WS 2 , thick green line). Apparently, the low-energy interlayer population builds
up after about 50 ps, being electronically ‘fed’ by higher energy states, and decays slowly on the
100-ps time scale, whereas the spectrally centered so-called bilayer moiré states decay even much
slower. The spectrally fully-integrated signal is shown as black transient (thick line) for comparison.
Transient counts are in semilogarithmic representation normalised to the energy integration range.
Data obtained by M. Shah and L.M. Schneider in the author’s team, for the bilayer system studied
time-integrated in [2]
can deliver the time constant τ for the emission process starting at t 0 , which commonly coincides for ultrafast excitation with the arrival of the pump pulse. Typically,
picosecond- and sub-picosecond-pulsed lasers with synchronized repetition rate of
typically 80 MHz (corresponding to a pulse separation of 12.5 ns) are employed for
such time-resolved detection scheme. For further details, the interested reader is
referred to the guide on the webpage of a renowned company, which produces scientific streak camera systems [19]. Note that the evaluation of the data may require some
background, noise or artifact correction, as well as the consideration of wavelengthdependent responsivity of the detection apparatus. Moreover, for decay times in the
ns range, the backsweep artifact should be taken into account, which typically reveals
itself in the recordings as a signal preceeding the excitation pulse.
145
Fig. 5.9 PL of a WS 2 /WSe 2 heterobilayer on hBN recorded with a streak camera with picoseconds
time resolution. Here, an off-resonant excitation at 2.7 eV at an irradiance of 54 µJ cm −2 was
used. This false-colour logarithmically-scaled intensity contour diagram shows TRPL data from
four energy ranges stitched together (from dark to bright: minimum to maximum counts, white:
background). For the peaks representing intralayer excitons of the two TMDC materials, a fast
decay is seen on the time scale of a few ps (WSe 2 about twice as slow as WS 2 ). The slightly shorter
instrument response time for the used setting amounts to 2 ps. After the initial picoseconds, the
WSe 2 peak exhibits a red-shift and the decay in this spectral region continues with a longer time
constant. Top panel: Temporally-integrated spectrum obtained from the time–energy spectrum. Side
panel: PL transients were extracted by spectral integration (indicated as colour-coded shaded area in
the top panel), that is 1.42–1.49 eV (interlayer excitons, pink line), 1.60–1.76 eV (WSe 2 and lowerenergetic moiré states, thick red line), 1.77–1.86 eV (higher-energetic moiré states, orange line),
and 1.95–2.06 eV (WS 2 , thick green line). Apparently, the low-energy interlayer population builds
up after about 50 ps, being electronically ‘fed’ by higher energy states, and decays slowly on the
100-ps time scale, whereas the spectrally centered so-called bilayer moiré states decay even much
slower. The spectrally fully-integrated signal is shown as black transient (thick line) for comparison.
Transient counts are in semilogarithmic representation normalised to the energy integration range.
Data obtained by M. Shah and L.M. Schneider in the author’s team, for the bilayer system studied
time-integrated in [2]
can deliver the time constant τ for the emission process starting at t 0 , which commonly coincides for ultrafast excitation with the arrival of the pump pulse. Typically,
picosecond- and sub-picosecond-pulsed lasers with synchronized repetition rate of
typically 80 MHz (corresponding to a pulse separation of 12.5 ns) are employed for
such time-resolved detection scheme. For further details, the interested reader is
referred to the guide on the webpage of a renowned company, which produces scientific streak camera systems [19]. Note that the evaluation of the data may require some
background, noise or artifact correction, as well as the consideration of wavelengthdependent responsivity of the detection apparatus. Moreover, for decay times in the
ns range, the backsweep artifact should be taken into account, which typically reveals
itself in the recordings as a signal preceeding the excitation pulse.