1 Experimental Methods: a) Materials: Cadmium oxide (CdO) powder, Zinc oxide (ZnO), Trioctylphosphine (T OP ), Selenium powder, Sulphur, Oleic acid (OA), ODE, Chloroform and Toluene, were obtained from Sigma-Aldrich (Germany) for the synthesis of QDs. Dodecanethiol (DDT ), HAuCl 4 , N aBH 4 granules, N aOH, Hexene also procured from Sigma-Aldrich and Acetone,hexane, HCL from HDFCL for AuNP synthesis. Deionized water (18.2 Megaohms-cm, Milipore) was used for all Langmuir-Blodgett (LB) film transfer. b) Synthesis and characterization of Quantum Dot: The CdSe-ZnS core-shell quantum dots are prepared traditional hot-injection synthesis of CdSe cores followed by slow growth of a ZnS shell from solution. CdO and ZnO are mixed and dissolved in OA and ODE and the mixture is heated up to 310 deg C to get clear solution and then inject required amount of TOP-S and TOP-Se stock solution in the three neck flux containing hot Cadmium based growth solution. After injection, we maintain a constant temperature of 300 deg C for desired amount of time to get proper size of QD. This quantum dot has Cadmium selenide (CdSe) spherical core and it is covered by Zinc-Selenide (ZnS) shell. By keeping the temperature constant at 300 degree for 10 min, the formed QD has diameter around 6.7 nm and Photoluminescence emission at wavelength 570 nm. c)Synthesis and characterization of Gold Nano Praticle: Gold nanoparticles (AuNP) are made in water by borohydride reduction and phase-transferred to hexane. This dodecanethiolate-protected gold nanoparticles synthesis is a fast process and does not require a cleaning step. The stock solution of 50 mM gold chloride was made by dissolving HAuCl 4 − 3H 2 O with the same molar concentration of HCl in a glass vial and the aqueous stock solution of 50 mM borohydride was made by dissolving N aBH 4 granules with the same molar amount of N aOH. Now 100 µL of the AuCl 4 solution added with water and later injected 300 µL of the borohydride solution to that while stirring the mixture on a mechanical shaker for uniform * murugesh@iisc.ac.in † basu@iisc.ac.in
Hybrid devices consisting of graphene or transition metal dichalcogenides (TMDs) and semiconductor quantum dots (QDs) were widely studied for potential photodetector and photovoltaic applications, while for photodetector applications, high internal quantum efficiency (IQE) is required for photovoltaic applications and enhanced carrier diffusion length is also desirable. Here, we reported the electrical measurements on hybrid field-effect optoelectronic devices consisting of compact QD monolayer at controlled separations from single-layer graphene, and the structure is characterized by high IQE and large enhancement of minority carrier diffusion length. While the IQE ranges from 10.2% to 18.2% depending on QD-graphene separation, d s, the carrier diffusion length, L D, estimated from scanning photocurrent microscopy (SPCM) measurements, could be enhanced by a factor of 5–8 as compared to that of pristine graphene. IQE and L D could be tuned by varying back gate voltage and controlling the extent of charge separation from the proximal QD layer due to photoexcitation. The obtained IQE values were remarkably high, considering that only a single QD layer was used, and the parameters could be further enhanced in such devices significantly by stacking multiple layers of QDs. Our results could have significant implications for utilizing these hybrid devices as photodetectors and active photovoltaic materials with high efficiency.
Despite the many fascinating discoveries of fundamental significance and device applications involving graphene, one area that has been lacking is graphene-based displays and emissive devices. Since graphene by itself has weak and wavelength-independent absorption and no emission in the visible range, such devices must rely on synergistic combination with other highly sensitive optical materials such as quantum dots. However, the well-known strong nonradiative energy transfer between emitters and quantum dots and graphene makes it impossible to create such devices due to strong emission quenching. Here we report the first demonstration of enhanced photoluminescence of quantum dots in close proximity to graphene field effect transistor devices, which are electrically and spectrally tunable. The enhanced emission originates from super-radiance between closely packed quantum dots placed close to single-layer graphene, which overcomes the strong nonradiative quenching observed earlier. Finite difference time domain simulations shed light on the regime in which such effects are likely to dominate. Our work opens up new avenues for research on novel displays, lasers, and emissive devices involving graphene−quantum dot hybrids as well as to study fundamental aspects of electrically tunable light−matter interactions at the nanoscale.
The combination of semiconductor quantum dots (QD) and single-layer graphene (SLG) can lead to the formation of optoelectronic devices with enhanced sensitivity and can have extensive applications in the field of the photodetector and photovoltaics. The optical properties of the resultant hybrid material are controlled by the interplay of energy transfer between QDs and charge transfer between the QDs and SLG. By studying the steady-state and time-resolved photoluminescence spectroscopy of hybrid QD−SLG devices, we observe a subtle interplay of short-and long-range energy transfer between cadmium selenide (CdSe) QDs in a compact monolayer solid film placed in close proximity to an SLG and the charge transfer from the QD solid to SLG. At larger separation, δ, between the compact monolayer QD and SLG, the emission properties are dominated by mutual energy transfer between the QDs. At relatively smaller separation the emission from QDs, which is strongly quenched, is dominated by charge transfer between QDs and SLG. In addition, we are also able to tune the relative strength of energy and charge transfer by electrostatic doping through the back gate voltage, which provides a novel pathway to tune emission properties of these devices for possible applications as photodetectors, in photovoltaics, and for sensing.
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