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5 Optical Measurement Techniques
linear or circular polarisation is given by ρ = (I Co − I Contra )/(I Co + I Contra ). Generally, the polarisation-filtering behaviour of the optics and the monochromator should
be taken into account for such measurements.
5.3.3 Photocurrent Measurements
Photocurrent spectroscopy can provide additional information about semiconducting behaviour for materials through measurements of the electrical response as a
function of optical irradiation. For instance, this method was applied in the last century to characterise bulk TMDC crystals [94]. It is also widely used in the field of
phototransistor studies, including recent works with graphene or ultrathin TMDCs
(see for instance [95, 96] and references therein) or photodetecting devices involving
both 2D material types (e.g., in [97]).
The use of a monochromator behind a thermal white-light source (a wavelength
sweep) can provide a simple means of wavelength tunable photoexcitation of the
target material placed for instance in a specially-prepared microscope setup with
contacting needles. The photoinduced current (measured with external bias applied)
as a function of the photon energy allowed for instance the characterisation of the
photoexcitation wavelength (peak in the photocurrent spectrum) for a new material
with semiconductor properties in [98] (further details on the method are summarised
in the Supporting Information sections). The setup employed for this experiment was
placed in a nitrogen-filled box to minimise exposure to moisture and oxygen.
In this context, also current–voltage (I/V) characteristics can shed light on electrical properties when comparing for instance white-light exposed samples with their
unexcited case in a dark environment. As a reference system, a prominent optoelectronic material with a direct band gap such as GaAs can be employed. Such studies
for instance added to the characterisation of a new chemical compound K 2 Hg 2 Se 3
developed by the Dehnen group in Marburg with interesting optoelectronic, photophysical, and thermoelectric properties, which was thereby identified as photoconducting material with a direct band gap around 1.4 eV [98]. Similar experiments also
led to the characterisation of a new compound from the family of organic–inorganic
hybrid materials based on a porphyrin diacid with interesting colour-change effect
[99], synthesised and explored in the Heine group in Marburg.
In addition to I/V curves, typical optoelectronic characterisation furthermore
involves the on/off behaviour of a photoconducting material. This can demonstrate
the reversibility of the light-induced effects, as well as provide information on the
recovery timescales and possible switching times.
5 Optical Measurement Techniques
linear or circular polarisation is given by ρ = (I Co − I Contra )/(I Co + I Contra ). Generally, the polarisation-filtering behaviour of the optics and the monochromator should
be taken into account for such measurements.
5.3.3 Photocurrent Measurements
Photocurrent spectroscopy can provide additional information about semiconducting behaviour for materials through measurements of the electrical response as a
function of optical irradiation. For instance, this method was applied in the last century to characterise bulk TMDC crystals [94]. It is also widely used in the field of
phototransistor studies, including recent works with graphene or ultrathin TMDCs
(see for instance [95, 96] and references therein) or photodetecting devices involving
both 2D material types (e.g., in [97]).
The use of a monochromator behind a thermal white-light source (a wavelength
sweep) can provide a simple means of wavelength tunable photoexcitation of the
target material placed for instance in a specially-prepared microscope setup with
contacting needles. The photoinduced current (measured with external bias applied)
as a function of the photon energy allowed for instance the characterisation of the
photoexcitation wavelength (peak in the photocurrent spectrum) for a new material
with semiconductor properties in [98] (further details on the method are summarised
in the Supporting Information sections). The setup employed for this experiment was
placed in a nitrogen-filled box to minimise exposure to moisture and oxygen.
In this context, also current–voltage (I/V) characteristics can shed light on electrical properties when comparing for instance white-light exposed samples with their
unexcited case in a dark environment. As a reference system, a prominent optoelectronic material with a direct band gap such as GaAs can be employed. Such studies
for instance added to the characterisation of a new chemical compound K 2 Hg 2 Se 3
developed by the Dehnen group in Marburg with interesting optoelectronic, photophysical, and thermoelectric properties, which was thereby identified as photoconducting material with a direct band gap around 1.4 eV [98]. Similar experiments also
led to the characterisation of a new compound from the family of organic–inorganic
hybrid materials based on a porphyrin diacid with interesting colour-change effect
[99], synthesised and explored in the Heine group in Marburg.
In addition to I/V curves, typical optoelectronic characterisation furthermore
involves the on/off behaviour of a photoconducting material. This can demonstrate
the reversibility of the light-induced effects, as well as provide information on the
recovery timescales and possible switching times.