Graphene, a thin sheet of carbon atoms only one atom thick, has been used to realise a new photodetector (i.e. an electronic component used to sense and measure light) which is promising for some special applications. In the university of Exeter, me and my colleagues used a chemically-modified form of graphene, known as Iron Chloride-intercalated few-layer graphene, to realise such detector. The material is formed by sheets of graphene in between which a layer of iron chloride (FeCl3) molecules are intercalated and it was first studied in the group of Prof. Saverio Russo in 2012 (Adv. Materials 2012).
In this new work, published in Science Advances in May 2017, we have managed to modify the structure of this material using a laser, in order to create a junction which is able to sense light (see also my previous post on position-sensitive photodetectors). The exposure to intense laser light allows to displace the FeCl3 molecules in order to change the doping induced in the graphene, thus realising a so-called p-p’ junction. The technique is similar to a painting, where the graphene is the “canvas” and the laser the “brush”. The difference is that the “ink” is already in the canvas and it is “burned” by the laser in order to “draw” the circuit. Upon exposure to light, the “engraved” junction generates a current current which is measured between the two contacts of the device.
Several parameters determine the performance of a photodetector, the most important are (1) the responsivity, which is the amount of current measured for a given amount of light, (2) the speed, that is how fast light can “flash” and be detected as separate flashes, (3) the noise level, which is related to the faintest amount of light that can be detected and (4) the saturation power, which represents the maximum amount of light which can be detected. All these parameters can be combined to extract a figure called “Linear Dynamic Rage” or LDR. This parameter is very important as it defines the range of light intensities which can be detected by the device and also determines how many different shades of a particular colour can be discriminated. It is a very important parameter, for example, for camera sensors as it determines the fidelity with which a picture can reproduce reality.
Our new photodetector shows a LDR which is 4500 times larger than any other graphene-based photodetector so far reported. Such extraordinary LDR makes this photodetector suitable for high-definition imaging. Furthermore, being made of graphene, which is flexible and transparent, it could be implemented into future flexible electronics technologies including smart textiles and electronic clothing.At the same time, the ability of this material to sustain very large illumination powers makes it suitable to be used in harsh environments where normal photodetectors would fail, such as inside nuclear reactors.
This extraordinary LDR comes from the fundamental physics behind the light sensing ability of our intercalated graphene, which we unveiled for the first time. We discovered, in fact, that the mechanism behind it is fundamentally different from the one found in normal graphene. In the latter case, the photo-response is due to the photo-thermoelectric effect, that is the presence of a potential difference due to a difference in temperature. In the case of our device, instead, we found that the effect is closer to what happens in a conventional solar panel, where light is directly converted into electricity. Such difference turns out to be fundamental for the performance of the device.
Many properties of FeCl3 intercalated graphene have been discovered in the past few years, however it seems that this extraordinary material is still full of surprises!
You can read the paper at the following link (Open Access): De Sanctis, A. et al. “Extraordinary linear dynamic range in laser-defined functionalized graphene photodetectors”, Science Advances 3, e1602617.