Glucose-derived water-soluble crystalline graphene quantum dots (GQDs) with an average diameter as small as 1.65 nm (∼5 layers) were prepared by a facile microwave-assisted hydrothermal method. The GQDs exhibits deep ultraviolet (DUV) emission of 4.1 eV, which is the shortest emission wavelength among all the solution-based QDs. The GQDs exhibit typical excitation wavelength-dependent properties as expected in carbon-based quantum dots. However, the emission wavelength is independent of the size of the GQDs. The unique optical properties of the GQDs are attributed to the self-passivated layer on the surface of the GQDs as revealed by electron energy loss spectroscopy. The photoluminescence quantum yields of the GQDs were determined to be 7-11%. The GQDs are capable of converting blue light into white light when the GQDs are coated onto a blue light emitting diode.

Molybdenum disulphide (MoS2), which is a typical semiconductor from the family of layered transition metal dichalcogenides (TMDs), is an attractive material for optoelectronic and photodetection applications because of its tunable bandgap and high quantum luminescence efficiency. Although a high photoresponsivity of 880–2000 AW−1 and photogain up to 5000 have been demonstrated in MoS2-based photodetectors, the light absorption and gain mechanisms are two fundamental issues preventing these materials from further improvement. In addition, it is still debated whether monolayer or multilayer MoS2 could deliver better performance. Here, we demonstrate a photoresponsivity of approximately 104 AW−1 and a photogain of approximately 107 electrons per photon in an n-n heterostructure photodetector that consists of a multilayer MoS2 thin film covered with a thin layer of graphene quantum dots (GQDs). The enhanced light-matter interaction results from effective charge transfer and the re-absorption of photons, leading to enhanced light absorption and the creation of electron-hole pairs. It is feasible to scale up the device and obtain a fast response, thus making it one step closer to practical applications.

Experimental processPreparation of water-soluble GQDs: The water-soluble GQDs were prepared by microwave-assisted hydrothermal pyrolysis method. The glucose (Sigma-Aldrich, > 99.5%) was dissolved in deionized water with the concentration of ~11 wt%. The solutions were tapped to the glass bottle with tightened cover. The glass bottle was heated with a conventional microwave oven (Galanz P70B17L-T1) at a power of 595 W for a specific time (1, 3, 5, 7 and 9 min). Followed by the microwave irradiation, the GQD solutions were cooled down to room temperature under air ambient without further purification.Preparation of GQD/agar composite: 10 mg of agar flakes (Chancevon Agar) was inserted into 2 mL of the GQD solution at 85°C under vortex mixing. The mixture was well stirred and heated until it became homogeneous with the solution.Subsequently the mixture was poured into a 0.8 cm diameter glass tube mound and was cooled down to room temperature under ambient atmosphere.

The emissive properties of both doped and non-doped graphene quantum dots (GQDs) with sizes ranging from 3-11 nm were analyzed at the single particle level. Both doped and nondoped GQDs are a composite of particles exhibiting green, red, or NIR fluorescence on excitation at 488, 561, and 640 nm, respectively. Nitrogen doped GQDs (N-GQDs) with diameters ranging from 3.4 to 5.2 nm show a larger proportion of particles with NIR emission as compared to non-doped particles. and the fluorescence intermittency seen in single GQD particles. While ms to sec time scale blinking was regularly observed for red emitting non-doped GQDs, nitrogen doping significantly reduced blinking. Both doped and non-doped particles also exhibit moderate size dependent photophysical properties.

Polyaniline-functionalized graphene quantum dots (PANI-GQD) and pristine graphene quantum dots (GQDs) were utilized for optoelectronic devices. The PANI-GQD based photodetector exhibited higher responsivity which is about an order of magnitude at 405 nm and 7 folds at 532 nm as compared to GQD-based photodetectors. The improved photoresponse is attributed to the enhanced interconnection of GQD by island-like polymer matrices, which facilitate carrier transport within the polymer matrices. The optically tunable current-voltage (I-V) hysteresis of PANI-GQD was also demonstrated. The hysteresis magnifies progressively with light intensity at a scan range of ±1 V. Both GQD and PANI-GQD devices change from positive to negative photocurrent when the bias reaches 4 V. Photogenerated carriers are excited to the trapping states in GQDs with increased bias. The trapped charges interact with charges injected from the electrodes which results in a net decrease of free charge carriers and a negative photocurrent. The photocurrent switching phenomenon in GQD and PANI-GQD devices may open up novel applications in optoelectronics.