**5. Optoelectronic memories**

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

channel, yielding improved on–off ratio in detector [47, 49].

made with high sensitivity based on the photoconductive gain and vertical photovoltaic effects. The photodetection gain originates from the separation of electron– hole pairs at the heterostructure interface, with one kind of carrier accumulated in the 2D high mobility channel, therefore yielding amplified photoconductive gains by the ratio of injected charges compared to the inherent carrier concentration in 2D channel [47]. A representative example is PbS quantum dot (QD)-sensitized graphene, in which the QDs and 2D surface are coupled by vdW interaction

(**Figure 5a**) [48]. Upon illumination, holes are injected into graphene and transport there with dramatically increased mobility compared to QDs that have large amount of grain boundaries and surface states. In this way, ultrahigh responsivity >107

has been demonstrated in such hybrid photodetectors. Notably, based on the gatemodulated Fermi level in graphene, the charge injection from PbS QDs to graphene can be extensively tailored. As indicated in **Figure 5b**, the attained responsivity is sensitive to the applied gate bias; under *V*g = 4 V, the photoresponse gain is tuned even to zero by eliminating the interfacial charge transfer. Such widely tuned gain is potentially useful for intentionally selected sensitivity levels for a detector. However, due to the zero-bandgap nature of graphene, hybrid detectors with graphene as the channel exhibit large dark current and low detectivity. Alternatively, other 2D semiconductors, such as MoS2 and WSe2, have been also explored as the

In addition to colloidal quantum dots, 2D heterostructures based on vertically stacked 2D layers can also make up phototransistors. A narrow bandgap semiconductor can be stacked on another 2D material for extended photodetection spectra. As illustrated in **Figure 5c**, BP is stacked on a WSe2 channel [17]. The photoexcited carriers in BP by near-infrared photons are separated by the type II interface, with

*Phototransistors based on various heterostructures. (a) The schematic of PbS quantum dots sensitized graphene for infrared photodetection; (b) the back-gate-modulated responsivity of the hybrid photodetector, reproduced with permission from Ref. [48], Copyright 2012 Nature Publishing Group. dependence of the responsivity on the different wavelength. (c) Configuration of a vertically stacked BP/WSe2 heterostructure and (d) its wavelength-dependent gain and detectivity, reproduced with permission from Ref. [17], Copyright 2017 Elsevier Ltd. (d) Detectivity of various photodetector versus wavelength of the incident laser. (e) Illustration of the organic/inorganic vdW heterostructured phototransistor based on ZnPc-decorated MoS2 and (f) its photoresponse behavior, which is greatly improved compared to photoconductors that suffer persistent photoconductance, reproduced with permission from Ref. [16], Copyright 2018 American Chemical Society.*

A/W

**156**

**Figure 5.**

Optoelectronic memory can transform incident optical signals into stored electric charges [52]. Considering the light program signals can be free from interferences, the optoelectronic memories are particularly attractive for realizing high-throughput data storage, e.g., in parallel computing [53]. A typical optoelectronic memory is consisted of light sensing part and charge storage component, which could be feasibly realized using multilayered 2D stacking. Compared to the conventional 3D counterparts, the 2D devices have the advantages in having high on–off ratio by the ultrathin channel, the conductance of which can be feasibly modulated via slight amount of trapped charges. According to the charge trapping mechanism, in the following we describe two kinds of optoelectronic memories, based on, respectively, the charge trapping in (i) defect energy states or (ii) float gates.
