Direct broadband radiation detectors most often employ the temperature change of the small energy absorber as an indicator of the radiation. The smaller the heat capacitance of the absorber, the higher are the temperature change and the output signal of the detector. The two dimensional materials have very small volume and, hence, - a vanishingly small heat capacitance. The change of the detector temperature is then expected to be high. In the extreme case of graphene, the electrons in graphene are moreover decoupled from the lattice (phonons), making the thermal capacitance even smaller. It was found for instance that electrons in graphene can be several hundred kelvin hotter than the lattice by applying a dc current.
Figure 1. A schematic of thermoelectric radiation detector. The antenna converts the external THz radiation into a current through graphene. The current results in the Joule heating and increased electronic temperature, which is read out by measuring the thermoelectric voltage across a p-n junction created in graphene by external local doping.
There are several effects that can be used to convert the changes in temperature to electrical readout signal of a detector. In common bolometers, it is the temperature dependent electrical resistivity that is used for this purpose. Unfortunately, the graphene resistivity does not change with temperature much, unless a bandgap is somehow induced in graphene (e.g. in nano-scale structures or because of many defects). Because such a bandgap is usually small, significant changes of resistance are expected only at low temperature.
However, the thermopower in graphene is relatively high at all temperatures, making it a very convenient tool for reading out the temperature raise in graphene due to the incoming radiation. We realized this concept focusing the radiation onto a pn junction in graphene. The pn junction was created by using a split top gate. In a clever design shown in the Figure 2, the antennas, which focus the radiation served also as the top gates. In this design, the radiation couples to graphene capacitively. It also assured that the maximum temperature increase occurred exactly at the most sensitive part of the structure, the pn junction. The great advantage of this design is also the relaxed requirements on the quality of the contact between graphene and the readout electrodes. The responsivity of up to 700 V/W was measured in such detectors.
Figure 2. Radiation detector with a capacitive coupling of antennas to graphene and thermoelectric readout. (a) The lumped element model of the device. A p-n junction is created by applying the DC voltages, V1 and V2, to the top gates, thereby forming an intrinsic thermocouple in the graphene. The antenna parts, AG1 and AG2, are coupled to the graphene through the distributed capacitances, also serving as top gates. The TEP signal is read out as the voltage between S1 and S2. (b) The spatial distribution of the real- and imaginary part of the AC current in the graphene (dashed blue- and dotted orange curves, respectively). The square of the current (i.e. power) is shown as a solid green curve. It increases towards the pn-junction (dashed arrow). (c) A magnified central part of the device. The scale bar is 15 µm. (d) An overview of the device
The simplicity of these devices prompts for making large focal plane arrays of such detectors. If placed on a flexible substrate, the arrays would enable designs of an “eye” for the radiation spectral range that the human eye is not sensitive to. Other 2D materials like phosphorene will be tried out as well.
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