Photodetectors are electronic devices which are able to sense light. They are present in everyday tools such as mobile phones, televisions, remote controls, cars and much more. For example they are used to sense the amount of light which illuminates the screen of a mobile to tune its luminosity accordingly. These kind of devices are able to record the presence and the amount (intensity) of light.
There is another class of photodetectors which are also able to sense the position of a light spot which is illuminating them. One example is the sensor of a digital camera, where each single pixel represents a coordinate in the sensor. However, this type of sensor is only able to discriminate between two pixels and the ultimate resolution depends on the distance between the two pixels.
True position-sensitive detectors are able to provide a continuous signal which is proportional to the position of a light spot (for example a laser) impinging on their surface. Such detectors are routinely used in many scientific instruments for positioning and motion control (as an example they are used in atomic-force microscopes). Their use in future technologies, such as flexible and wearable electronics, or in the “Internet of Things”, could bring new functionalities to everyday objects. However, current position-sensitive photodetectors are bulky, not flexible and energy-demanding.
Graphene has been extensively used to realise different photodetectors which could be embedded into future flexible and wearable technologies. However, so far, no such detectors have been demonstrated which can also sense position.
In the University of Exeter we developed for the first time such detector based on iron-chloride intercalated graphene (FeCl3-FLG), also known as GraphExeter. By using hexagonal domains of graphene grown by atmospheric pressure chemical vapour deposition (APCVD), we were able to realise a device (see figure above) which presents a photo-response which depends on the position of the laser illumination. Not only, this response has a four-fold symmetry and it is linear with respect to position, two of the main characteristics of position-sensitive photodetectors.
The image on the left shows such response. In the top panel a map of the photo-response as a function of position is shown, with a clear symmetry between the two contacts used (in red). The extracted data show the liner response.
The reason for such behaviour lies in the distribution of doping due to the peculiar geometry of the intercalation of FeCl3, which creates areas of higher and lower doping which act as photo-active junctions. This is shown in the figure above and highlighted by the green areas.
This work can pave the way towards the realisation of novel wearable sensors with enhanced position-sensitive capabilities.
The full paper is available in open-access at the following link: De Sanctis, A. et al. “Functionalised hexagonal-domain graphene for position-sensitive photodetectors”, Nanotechnology 28, 124004 (2017).