Abstract
Heterostructures of two-dimensional (2D) transition metal dichalcogenides (TMDs) and inorganic semiconducting zero-dimensional (0D) quantum dots (QDs) offer useful charge and energy transfer pathways, which could form the basis of future optoelectronic devices. To date, most have focused on charge transfer and energy transfer from QDs to TMDs, that is, from 0D to 2D. Here, we present a study of the energy transfer process from a 2D to 0D material, specifically exploring energy transfer from monolayer tungsten disulfide (WS2) to near-infrared emitting lead sulfide–cadmium sulfide (PbS–CdS) QDs. The high absorption cross section of WS2 in the visible region combined with the potentially high photoluminescence (PL) efficiency of PbS QD systems makes this an interesting donor–acceptor system that can effectively use the WS2 as an antenna and the QD as a tunable emitter, in this case, downshifting the emission energy over hundreds of millielectron volts. We study the energy transfer process using photoluminescence excitation and PL microscopy and show that 58% of the QD PL arises due to energy transfer from the WS2. Time-resolved photoluminescence microscopy studies show that the energy transfer process is faster than the intrinsic PL quenching by trap states in the WS2, thus allowing for efficient energy transfer. Our results establish that QDs could be used as tunable and high PL efficiency emitters to modify the emission properties of TMDs. Such TMD-QD heterostructures could have applications in light-emitting technologies or artificial light-harvesting systems or be used to read out the state of TMD devices optically in various logic and computing applications.
Highlights
Heterostructures of two-dimensional (2D) transition metal dichalcogenides (TMDs) and inorganic semiconducting zero-dimensional (0D) quantum dots (QDs) offer useful charge and energy transfer pathways, which could form the basis of future optoelectronic devices
We present a down-shifting heterostructure system, where monolayer tungsten disulfide acts as an antenna from which optically generated excitons are funneled to lowerenergy lead sulfide−cadmium sulfide (PbS−CdS) near-infrared (NIR) QD emitters
Following monolayer WS2 exfoliation, a single QD layer was deposited on the sample surface using a conventional layer-by-layer method:[40,41] A linker layer of 1,3-benzenedithiol (BDT) was first deposited via spin-coating to ensure strong adhesion of QDs on the sample surface; a low concentration (0.5 mg mL−1) of oleic acid (OA)-capped PbS−CdS QDs was spun onto the sample; and excess nanocrystal and ligand material was rinsed off by spin-coating toluene, leaving a single layer of QD film
Summary
Heterostructures of two-dimensional (2D) transition metal dichalcogenides (TMDs) and inorganic semiconducting zero-dimensional (0D) quantum dots (QDs) offer useful charge and energy transfer pathways, which could form the basis of future optoelectronic devices. A number of monolayer TMDs such as tungsten disulfide (WS2) have a direct optical gap.[5] This property compounded with high absorption coefficients, high carrier mobilities,[5] and potentially high photoluminescence quantum efficiency[9−11] (PLQE) promises great potential for their application in optoelectronic devices, namely, photodetectors, light-emitting diodes (LEDs), and photovoltaics (PV).[12] The reduced dielectric screening in the monolayer limit compared to that of their bulk counterparts gives rise to tightly bound electron− hole pairs (i.e., excitons) with binding energies on the order of hundreds of millielectronvolts at room temperature.[13,14] As a consequence, monolayer TMDs provide a convenient medium to study diverse excitonic species that arise via exciton−exciton or exciton−charge interaction.[13,15−17] Alternatively, these tightly bound excitons can be funneled to other fluorescent media, where they recombine radiatively at lower energy, tuning the emission properties of TMD excitons. Between QDs and monolayer TMDs for applications in photodetectors[24−31] and phototransistors.[32,33] To date, studies on the energy transfer in 2D-QD heterostructures for lightharvesting and light-sensing applications have mainly focused on 0D→2D exciton transfer where monolayer TMDs or graphene are used as efficient exciton sinks to which optically or electrically generated excitons from QD emitters are nonradiatively transferred.[22,30,34−39]
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