Directed Energy Transfer from Monolayer WS2 to Near-Infrared Emitting PbS-CdS Quantum Dots

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.

Keywords: energy transfer; lead sulfide−cadmium sulfide; quantum dot; transition metal dichalcogenide; tungsten disulfide; two-dimensional; zero-dimensional.

Figure 1

(a) Cartoon illustrating heterostructure sample fabrication process (1–6) and (b) initial PL characterization via 50× objective. (c) Monolayer WS₂ normalized absorption (light blue circles with solid dark blue line as a guide for the eye) and PL (dashed dark blue line). (d) Colloidal PbS–CdS normalized absorption (black solid line) and PL (black dashed line) spectra. (e) PL spectra of WS₂–PbS–CdS 2D-QD heterostructure (red) and PbS–CdS film on bare glass substrate (black) measured with 514.5 nm continuous wave laser at 80.2 W/cm².

Figure 2

(a) Optical micrograph of a WS₂ flake (left) showing monolayer (red dotted outline), multilayers (blue outline), and bulk crystal (black outline) with corresponding confocal NIR PL map of QD emission from the heterostructure (right) measured with a 514.5 nm continuous wave laser at 80.2 W/cm². Right-hand side scale bar represents 50 μm. (b) QD PL spectra from heterostructure (red) and bare substrate (black) taken from points marked “x” in (a), right-hand side. Green dashed lines represent single Gaussian peak fits. (c) Normalized PLE spectra of heterostructure (red) and QD (control) obtained via scanning wavelengths about the WS₂ “A” exciton (616 nm) and detecting QD PL (900 nm). PLE spectra normalized by the average signal between 670 and 700 nm. (d) Normalized “subtract” (red) signal derived via subtraction of QD PLE signal from heterostructure PLE signal in (b) and overlapped with typical WS₂ absorption spectrum (blue circles). (e) Estimated contribution to QD PL (PL_ctr) by the WS₂ monolayer as a function of excitation wavelength with peak value of 58% at 616 nm (∼2.0 eV).

Figure 3

(a) Low fluence time-resolved WS₂ PL decay signals from pristine (blue) and heterostructure (red) samples measured with 509 nm pulsed excitation at 0.01 μJ/cm². Exponential decay fits are shown as dotted black lines. (b) Time-resolved WS₂ PL decay fluence series from pristine (blue) and heterostructure (red) samples. Pristine WS₂ PL decay signals show general increase in lifetime as a function of pump fluence due to exciton trapping. All WS₂ PL in the heterostructure signal quenched below IRF (gray dash-dotted line) due to fast exciton transfer.

Figure 4

QD TRPL decay spectra of heterostructure (red) and bare substrate (black) measured with 509 nm pulsed excitation at 0.5 MHz. The early time signal in heterostructure PL decay convoluted with IRF confirms that ET phenomenon faster than resolution of TCSPC setup available for this study.

Figure 5

(a) Scatter plots of monolayer WS₂ (visible) PL integrals and the corresponding PL peak wavelengths extracted from spatial PL maps of the sample in pristine (blue) and heterostructure (red) form. PL measured with 514 nm continuous wave laser excitation at 80.2 W cm^–2 intensity. (b) WS₂ PL spectra of an example of the monolayer in pristine (blue) and heterostructure (red) cases. Spectra are deconvoluted with Gaussian peaks which represent the neutral exciton (dashed lines) and a lower-energy species X₂ (dotted lines). (c) TRPL decay spectra of pristine (blue) and heterostructure (red), measured with 509 nm excitation at 63.4 W cm^–2 intensity. Black dashed lines represent decay fits. IRF given by gray dot-dash line. (d) Energy level diagram illustrating radiative exciton pathways in pristine WS₂ (left-hand side) and in the heterostructure. Blue arrows represent initial excitation; orange arrows represent WS₂ excitons, and red arrows represent down-shifted excitons that recombine at lower energy in the PbS–CdS QD.