Research case study: Novel opto-electronic devices based on graphene plasmonics

Atomically thin materials are enabling a new paradigm in ultra-sensitive light detection by leveraging on a unique selection of properties, thus allowing an expansion in the range of potential applications. For instance, these materials offer the ultimate lightweight solution for the creation of space-bound photodetectors, or wearable optoelectronic devices.

So far, the operational bandwidth of atomically thin heterostructure photodetectors is typically below 1 Hz, due to the presence of defects and disorder, which is too slow for many practical applications. Faster operational speeds have been demonstrated but these require the use of large gate voltage pulses, an extremely impractical strategy in highly integrated architectures.

To circumvent this limitation, we have encapsulated heterostructure photodetectors of tungsten disulphide (WS2) and graphene in an ionic polymer (see figure 1). In such a structure, light is primarily absorbed in the WS2 generating electron-hole pairs with one charge carrier transferred to graphene whilst the other remains trapped within WS2.

Owing to the ultra-high conductivity of graphene, a gain mechanism is possible whereby this charge carrier is recirculated multiple times before recombination with the trapped charge. Therefore, for a single photon the number of charge carriers extracted at the electrode can exceed 106 allowing ultra-sensitive light detection across a broad spectral range (figure 2a).

Unique to our work, we demonstrate that the efficient electrostatic screening of charged impurities and defects, by encapsulation of the device in an ionic polymer, results in photodetection response times more than 103 times faster than previously reported, without the need for large gate voltage pulses (figure 2b).

The combination of both high sensitivity to light and fast response times make these photodetectors suitable for video-frame-rate imaging applications and as a result the creation of atomically thin cameras can be envisioned.

Figure 1. 3D illustration of the WS2-graphene photodetector with electrical connections included.

Figure 2. (a) Responsivity, R, as a function of incident photon energy, E, revealing spectral range of WS2-graphene photodetector. (b) Normalised photocurrent signal, Ipc/Ipc0, as a function of light modulation frequency, f. Inset eye diagram acquired at 2.9 kbit/s.