**2. Photodetection mechanisms in 2D materials**

Generally, photocurrent generation mechanisms are divided into three categories, viz. photovoltaic effect, photo-thermoelectric effect, and photo-bolometric effect. In the photovoltaic effect, a built-in electric field results in the separation of the electrons and holes. This built-in electric field may be generated due to a Schottky barrier at the metal–semiconductor interface. Photodetectors working under this mechanism are called photodiodes. In the photo-thermoelectric effect, a non-uniform light source is used. This light source leads to non-uniform heating of the channel, resulting in a temperature gradient within the channel. Due to this temperature gradient, carriers move from the high-temperature region to the low-temperature region. The migration of the carriers leads to their accumulation in the low-temperature region, which results in a potential. The photo-bolometric effect is based on uniform heating of the material under illumination. This uniform heating results in a change in the resistivity of the material. This effect is directly proportional to the variation of the material's conductivity and the increment in temperature caused by light irradiation. In contrast to the photo-thermoelectric effect, the photo-bolometric effect does not drive the current but only changes the intensity of the current under external bias and illumination.Another unique mechanism observed in optoelectronic devices like photodetectors is internal photoemission (IPE). IPE involves photoinjection of electrons from an emitter/source (metal or semiconductor) into the conduction band of a collector/drain (semiconductor or insulator) in a BJT/FET. The holes are photoinjected into the valence band of the collector/drain and is called as hole photoemission [24]. In IPE, an optical excitation of electrons in the metal to an energy above the Schottky barrier is involved. These excited electrons are then transported to the conduction band of the semiconductor. The Initial theory of IPE was proposed by Fowler [25, 26]. However, this theory does not take the thickness of the Schottky

metal layer into consideration. Over the years the original theory of IPE has been refined largely resulting in much better assessment of the performance of the devices based on this effect [27, 28].
