Layer-Dependent Photoinduced Electron Transfer in 0D–2D Lead

Jun 21, 2019 - We show that tunability in electron-transfer rate can be achieved following ... of 2.58 nm estimated by transmission electron microscop...
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Layer-Dependent Photoinduced Electron Transfer in 0D−2D Lead Sulfide/Cadmium Sulfide−Layered Molybdenum Disulfide Hybrids Downloaded via UNIV OF SOUTHERN INDIANA on July 24, 2019 at 08:07:54 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Jia-Shiang Chen,†,‡ Mingxing Li,† Qin Wu,† Eduard Fron,§ Xiao Tong,† and Mircea Cotlet*,† †

Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States, Department of Materials Science and Chemical Engineering, Stony Brook University, Stony Brook, New York 11794, United States, § Department of Chemistry, Katholieke Universiteit Leuven, 3001 Leuven, Belgium ‡

S Supporting Information *

ABSTRACT: We demonstrate layer-dependent electron transfer between core/shell PbS/CdS quantum dots (QDs) and layered MoS2 via energy band gap engineering of both the donor (QDs) and the acceptor (MoS2) components. We do this by (i) changing the size of the QD or (ii) by changing the number of layers of MoS2, and each of these approaches alters the band gap and/or the donor−acceptor separation distance, thus providing a means of tuning the charge-transfer rate. We find the charge-transfer rate to be maximal for QDs of smallest size and for QDs combined with a 5-layer MoS2 or thicker. We model this layer-dependent charge-transfer rate with a theoretical model derived from Marcus theory previously applied to nonadiabatic electron transfer in weakly coupled systems by considering the QD transferring photogenerated electrons to noninteracting monolayers within a few layers of MoS2. KEYWORDS: 0D−2D semiconductor hybrid, transition-metal dichalcogenides, electron transfer, photoluminescence, scanning photocurrent microscopy, band gap engineering, 2D van der Waals

H

transfer, or energy transfer, depending on the foreseen application of the hybrid. QDs are typically band gap engineered throughout changing their size and/or composition, while TMDs can be tuned via the number of stacked monolayers and/or the TMD’s composition.14 To date, 0D− 2D QD−TMDs have been demonstrated by combining CdSe, PbS, and perovskite CsPbX3 QDs with single and few layers of MoS2 and SnS2.1−4,6 Layered MoS2 is a TMD of particular interest because it exhibits a clear transition from indirect band gap semiconductor at bulk to direct band gap semiconductor at monolayer, with the monolayer exhibiting strong photoluminescence (PL).15,16 Combined with PbS QDs absorbing and emitting in the near-infrared, the resulting PbS−MoS2 hybrid becomes a highly sensitive photodetector for nearinfrared light.1

ybrid nanostructures formed from colloidal semiconductor quantum dots (QDs, or 0D nanomaterials) and layered transition-metal dichalcogenides (TMDs, or 2D semiconductor materials), also known as 0D−2D QD−TMD nanostructures, have risen as superior hybrid nanostructures over pristine 2D TMDs when utilized in optoelectronic applications like solar photovoltaics, solid-state lighting, or photodetectors.1−10 In these hybrids, the QD component provides most of the light-harvesting function through a large absorption cross section which can span a spectral range from ultraviolet to visible and up to nearinfrared, depending on the QD’s material composition and size.11,12 The atomically thin TMD component provides high in-plane carrier mobility, which is crucial for photodetector applications.13 It also exhibits an extremely large surface-tovolume ratio, which results in the accommodation of a large population of QDs, usually in the form of a monolayer. Within a 0D−2D hybrid, both QD and TMD components can be band gap engineered to obtain a desired overlap architecture which may promote a particular interfacial interaction, charge © 2019 American Chemical Society

Received: June 4, 2019 Accepted: June 21, 2019 Published: June 21, 2019 8461

DOI: 10.1021/acsnano.9b04367 ACS Nano 2019, 13, 8461−8468

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Figure 1. Designed 0D−2D QD−MoS2 hybrids with tunable photoinduced charge transfer. (a) Optical absorption and photoluminescence (PL) emission spectra of MoS2 flakes dispersed in water and of PbS/CdS QDs dissolved in toluene. PL peaks for PbS/CdS QDs are located at 900 nm (red), 955 nm (purple), and 1010 nm (orange). (b) Energy levels for the 900 nm emitting PbS/CdS QD with respect to vacuum level as estimated from UV-photoelectron spectroscopy (UPS) and absorption spectroscopy (see Figure S5 and related description in the Supporting Information for details) (WF, work function; EF, Fermi level; Eg, energy band gap; CB, conduction band; VB, valence band). (c and e) Energy band diagrams for monolayer, bilayer, trilayer, and bulk MoS2 and for PbS/CdS QDs of various sizes and emission colors. CB and VB edge values for MoS2 are from refs 14, 18, and 19, and those for PbS/CdS QD955 and QD1010 are estimated based on their band gap energy and adjusted to the values measured for PbS/CdS QD900 according to ref 20. (d and f) Concept drawing of 0D−2D PbS/CdS QD−MoS2 hybrids showing band gap engineering of the acceptor (MoS2) and donor (QD) in an attempt to tune the charge-transfer rate. In panel d, PbS/CdS QDs of 900 nm emission are deposited on 1, 2, and 3 layers of MoS2, and a stronger electron transfer is observed with increased number of MoS2 layers. In panel f, bilayer MoS2 is combined with PbS/CdS QDs with PL peaks at 900, 955, and 1010 nm, respectively, with electron transfer becoming stronger with decreased QD size.

picture of electron transfer between a point donor (QD) and a surface (2D) acceptor (layered MoS2) which we derive from Marcus’s theory for nonadiabatic electron transfer, and we show that this model fits rather well our experimental data of layer-dependent electron transfer.

In the present work, we investigate the interfacial interaction between photoexcited PbS QDs with single and few layers of MoS2 while part of a 0D−2D hybrid, with an emphasis on the experimental identification of the mechanism dominating the interfacial interaction between the two semiconducting components. Because PbS QDs and layered MoS2 form a type II heterojunction (Figure 1c,e), we expect the interfacial interaction to be dominated by photoinduced charge (electron) transport from photoexcited QDs to layered MoS2, and we demonstrate the presence of this phenomenon by a combination of laser-induced time-resolved optical (photoluminescence, PL)2 and photocurrent imaging measurements.17 We also explore the ability to tune the strength (rate) of this interfacial interaction. We show that tunability in electron-transfer rate can be achieved following band gap engineering of (i) MoS2 flake thickness throughout a series of hybrids with varying number of layers (Figure 1c,d), from monolayer to bulk (>10 layers), and (ii) by band gap engineering of core/shell PbS/CdS QDs by changing the QDs core size (Figure 1e,f). We provide the mechanistic

RESULTS AND DISCUSSION In a first attempt, we combined core-only PbS QDs emitting at 900 nm with a few layers of MoS2. Figure S1 in the Supporting Information shows complete quenching of the photoluminescence emitted by PbS QDs deposited on the MoS2 flake, following 475 nm optical excitation in a confocal PL microscope. From a PL lifetime of 476 ns on a SiO2/Si substrate, the emission is reduced to 0.095 ns, which is the response time (instrument response function) of the confocal PL instrument. This suggests that PL quenching of core-only PbS QDs by layered MoS2 proceeds with an efficiency of >99.99% and that the dynamics of photoinduced electron transfer is faster than the response time of our confocal PL microscope and therefore is unable to be analyzed by our 8462

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Figure 2. (a and b) Optical bright-field images of exfoliated MoS2 flakes on SiO2/Si. (c and d) Atomic force microscopy (AFM) images of MoS2 flakes from panels a and b. Insets: AFM height profiles measured along indicated red arrows, providing thicknesses for monolayer (≈1 nm), bilayer (≈1.8 nm), three-layer (≈2.8 nm), and four-layer (≈3.8 nm) MoS2. (e and f) Confocal PL lifetime images of PbS/CdS QD− MoS2 hybrids, showing significant PL quenching for QDs deposited on MoS2 (located inside dashed blue perimeters) vs QDs deposited on SiO2/Si (located outside blue perimeters).

Figure 3. (a) PL decays from PbS/CdS QDs (PL@900 nm) deposited on MoS2 flakes with different number of layers. (0L, SiO2/Si substrate; 1L, monolayer; 2L, bilayer MoS2; etc.) (b) Electron-transfer rate vs number of MoS2 layers. Fit according to Marcus theory using eq 3 (see also the Supporting Information) where multiple layers of MoS2 are treated as independent acceptors for the QD.

Figure 2c,d; insets show AFM profiles of layer thickness) and by Raman microspectroscopy (Figure S2). Confocal absorption spectra of monolayer and few-layer MoS2 are shown in Figure S3. For layer-dependent electron-transfer studies, we utilized core/shell PbS/CdS QDs emitting at 900 nm (Figure 1a,b) with an average size (diameter) of 2.58 nm estimated by transmission electron microscopy (TEM, Figure S4a). To enable improved electronic coupling between QDs and TMDs, we exchanged the oleic acid surface ligands with 1, 2ethanedithiol (EDT) via a layer-by-layer process.1 This type of short ligand has been shown to enable stronger coupling between PbS/CdS QDs and MoS2, leading to enhanced charge transfer.25,26 The band gap alignment for various MoS2 layers and PbS/CdS QDs emitting at 900 nm are shown in Figure 1c, and the values for the valence band (VB) maxima and the conduction band (CB) minima of this particular QD shown in

instrument. To bypass this issue, we decided to utilize core/ shell PbS/CdS QDs with a type I energy band alignment where the CdS shell provides improved passivation leading to increased PL emission, but it also acts as a tunneling barrier for the photogenerated electron moving from the PbS core onto the layered MoS2, thus slowing down the electron-transfer process.21 This leads to some preservation of the PL signal from the QDs interacting with layered MoS2 and in turn allows probing of electron-transfer dynamics by confocal PL microscopy.22−24 MoS2 flakes with varying thicknesses, from a single layer to a few (2, 3, 4, 5) layers and to bulk (>10 layers), were obtained by mechanical exfoliation on SiO2/Si substrates from bulk crystals (HQ Graphene, Netherlands). Examples of such flakes are given in Figure 2a,b (optical bright field images), and their thicknesses were confirmed by atomic force microscopy (AFM, 8463

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Table 1. Average PL Lifetime and Calculated Electron-Transfer Rate of PbS/CdS QDs Deposited on MoS2 with a Varying Number of Layers MoS2 layers

PL lifetime of QD900 (μs)

0L 1L 2L 3L 4L 5L bulk

± ± ± ± ± ± ±

2.01 1.74 1.62 1.59 1.49 1.43 1.43

0.020 0.015 0.026 0.007 0.035 0.106 0.108

kET (×105 s−1)

PL lifetime of QD955 (μs) a

0.77 1.19 1.31 1.73 2.02 2.02

± ± ± ± ± ±

0.050 0.101 0.027 0.160 0.258 0.274

0.944 0.881 0.863 0.841

kET (×105 s−1)

b

/ 0.938 ± 0.002 ± 0.01 ± 0.002

PL lifetime of QD1010 (μs) a

0.74 ± 0.026 0.92 ± 0.142 1.28 ± 0.035

kET (×105 s−1)

b

0.596 / 0.566 0.581 ± 0.006 0.541 ± 0.007 0.56 ± 0.009

0.41 ± 0.198 0.79 ± 0.024 1.08 ± 0.29

a

These PL lifetime values belong to QDs deposited on SiO2/Si and assembled with 1L and 3L MoS2. These values were used to calculate kET. These PL lifetime values belong to QDs deposited on SiO2/Si and assembled with 2L MoS2. While not significant, these differences in PL lifetimes arise from measurements performed at different times (over one month) with QDs from the same batch. b

with τQDs−MoS2 and τQDs PL lifetimes of PbS/CdS QDs on MoS2 (donor−acceptor hybrid) and on the SiO2/Si substrate (donor-only), respectively. As seen in Figure 3b, the rate for electron transfer, kET, increases with the increase in the number of MoS2 layers, up to five-layers, from which it saturates, having similar value as for the bulk (>10 layers) MoS2. We attribute the enhancement of electron-transfer rate to the strengthened driving force, resulting from the increased energy offset (ΔECB) between donor and acceptor components as shown in the energy diagram from Figure 1c. Confocal FLIM does not necessarily prove that the PL quenching of the QDs by MoS2 has indeed happened because of transfer of photogenerated electrons from the photoexcited PbS core to MoS2. Nonradiative energy transfer can have a similar signature as photoinduced electron transfer with respect to the PL quenching of QDs in the presence of MoS2, although we have ruled out this possibility because of the lack of overlap between the absorption spectrum of MoS2 and the emission of PbS/CdS QDs (Figure 1a). However, having introduced the CdS shell (∼0.3 nm thick) as a tunneling barrier for photoinduced electron transfer in order to detect the dynamics of this process via photoluminescence measurements, this could in principle slow down the electron transfer in favor of nonradiative energy transfer, even in the presence of a minimal spectral overlap of the donor−acceptor components of the hybrid. To verify that this is not the case, we have performed additional experiments to indubitably confirm the presence of electron transfer in these PbS/CdS QD−MoS2 hybrids, and they are discussed below. We recently introduced scanning photocurrent microscopy (SPCM) as an imaging method capable of discriminating between interfacial charge transfer and nonradiative energy transfer in 0D−2D QD−TMD hybrids by following their optoelectronic signature under light exposure.17 In brief, for a QD−TMD hybrid which exhibits interfacial nonradiative energy transfer, when the hybrid is incorporated in a fieldeffect transistor (FET) operated at zero bias voltage and under light illumination, excitons (bound electron−hole pairs) generated in QDs are transferred to MoS2 via nonradiative energy transfer. An increase in light intensity will produce more excitons in the QDs, which get transferred to MoS2 and increase the charge carrier density in the TMD and consequently the photogenerated current in the hybrid FET. For a QD−TMD hybrid exhibiting interfacial charge transfer, when incorporated in a FET which is operated at zero bias voltage and exposed to light, for every photogenerated charge transferred from the QD to MoS2, an opposite photogenerated charge remains in the QD’s core; in the case of PbS, this will be

Figure 1b were obtained from ultraviolet photoelectron spectroscopy (UPS) and absorption spectroscopy measurements (see Figures S5 and S6), while those of MoS2 were taken from literature.14,18,19 As shown in Figure 1c,e, PbS/CdS QDs and layered MoS2 form a type-II heterojunction, which favors photoinduced electron transfer from photoexcited QDs to MoS2, with the electron transfer expected to strengthen with the increased number of MoS2 layers because of an increase in electron-transfer driving force. At the same time nonradiative energy transfer from photoexcited QDs to MoS2 can be ruled out as a possible quenching mechanism because of the lack of spectral overlap between the absorption spectrum of MoS2 and the PL spectrum of core/shell PbS/CdS QDs (Figures 1a and S3).27,28 Confocal fluorescence lifetime microscopy (FLIM) images shown in Figure 2e,f were recorded with 475 nm optical excitation, and they evidence significant PL quenching for QDs deposited on MoS2 flakes (areas inside dashed blue colored perimeters) versus QDs deposited on a SiO2/Si substrate (outside dashed blue colored area). It is noteworthy that the thicker the MoS2 flake is, the stronger the PL quenching of QDs becomes, which is expected given the enlargement of the energy band offset, ΔECB, between the CB of the donor (QD) and acceptor (MoS2) components (Figure 1c). Diffraction limited, spatially resolved PL decays shown in Figure 3a were recorded from various points on the MoS2 flakes following 475 nm laser excitation, and they correspond to QDs deposited on a SiO2/Si substrate, on monolayer, bilayer, and so on (Figure 3a, 0L, 1L, 2L, etc., respectively). For clarity, these decays have been normalized and offset. The values of the PL lifetimes for QDs reduce with the increase in the number of MoS2 layers, which is consistent with the previous observation from FLIM images (Figure 2e,f), suggesting again increased PL quenching with added number of MoS2 layers. The average PL lifetime (τav) of PbS/CdS QDs decreases from 2.01 ± 0.02 μs on a SiO2/Si substrate, to 1.74 ± 0.015 μs on monolayer MoS2, and down to 1.43 ± 0.106 μs for five-layer MoS2 (Figure 3a and Table 1). A similar value of 1.43 ± 0.108 μs is detected for bulk MoS2 (>10 layers), and this indicates a saturation of the rate for electron transfer from five layers and up. Average PL lifetimes were calculated from biexponential tail fits with the PL lifetimes being amplitude averaged. With the detected PL lifetimes from Table 1, we calculated electron-transfer rates versus number of MoS2 layers (Figure 3b) according to kET =

1 1 − τQD−MoS2 τQD

(1) 8464

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Figure 4. (a) Schematic illustration of a back-gated MoS2 FET on SiO2/Si. (b) Optical bright-field image of a 3L MoS2 FET device. (c and d) Spatially resolved reflection image and scanning photocurrent microscopy (SPCM) image of pristine 3L MoS2 FET. (S and D denote source and drain contacts, respectively.) (e) Change in photocurrent vs excitation power measured in the PbS/CdS QDs−MoS2 FET. See text for the definition of ΔIph/Iph(MoS2).

laser power as found in Figure 4e, that is, to show that coreonly PbS and layered MoS2 interact also by interfacial electron transfer. (See Figure S7, Supporting Information) It is noteworthy that the photogating effect in the case of coreonly PbS QD−3L MoS2 hybrid FET becomes stronger compared to the core/shell PbS/CdS QD-3L MoS2 hybrid, i.e., ΔIph/Iph(MoS2) decreases from 1 to 0.078 for core-only QDs when power increases while for core/shell QD-based hybrid FET ΔIph/Iph(MoS2) decreases from 0.33 to 0.045. This is expected in view of the enhanced electron transfer in the case of core-only PbS-MoS2 hybrid versus core/shell PbS/CdS QD−MoS2 hybrid, the later hybrid having a CdS shell which acts as a tunneling barrier for electron transfer. The kinetics of photoinduced electron transfer between PbS/CdS QDs and layered MoS2 can be understood in view of the Marcus theory of electron transfer, a model which has been previously applied to various donor−acceptor systems such as organic photovoltaics, dye-sensitized solar cells, and QDhybrids.25,29−33 The nonadiabatic electron transfer within the weak electronic coupling can be well described by this theoretical model.34 In the case of our hybrid, the CdS shell passivating the core PbS sets the donor (PbS core) and acceptor (MoS2) in a weak coupling regime. In this model, the rate of electron transfer can be expressed as a function of the driving force (Gibbs free energy, −ΔG°), the nuclear reorganization energy (λ), and the electronic coupling strength (V).

a hole. The remaining hole will generate a positive top gate potential, which will affect the Fermi level of MoS2 and therefore the photogenerated current in the FET.6,7 For example, if the light illumination intensity is increased, the positive top gate potential increases and pushes up the local Fermi level of MoS2, resulting in an increase in band bending at the FET electrodes, and this will hinder the collection of electrons, thus reducing the photogenerated current. Therefore, monitoring the photogenerated current at either source or drain electrodes versus the applied illumination power will allow unequivocal discrimination between the two interfacial processes. To demonstrate that electron transfer is the main interfacial interaction between PbS/CdS and layered MoS2, we fabricated an FET device based on a 3-layer (3L) MoS2 (Figure 4a,b) and with PbS/CdS QDs on top of MoS2. Spatially resolved reflection and photocurrent images for this particular device are shown in the same Figure 4. Illumination of the pristine MoS2 FET (488 nm laser) produces positive photocurrents at the source electrode (red region in Figure 4d) and negative photocurrents at the drain electrode (blue). Figure 4e shows the photocurrent change (ΔIph/Iph(MoS2)) versus laser power following the addition of the PbS/CdS QDs to the pristine MoS2 FET, with ΔIph = Iph(QD−MoS2) − Iph(MoS2), with Iph(QD−MoS2) and Iph(MoS2) peak photocurrents measured from SPCM near the source electrode of the QD−3L MoS2 hybrid FET and of pristine 3L MoS2 FET, respectively. The trend in Figure 4e shows that ΔIph/Iph(MoS2) decreases with increased laser power, similar to the CdSe QD−MoS2 hybrid FET, a system we previously investigated and demonstrated to exhibit interfacial charge (hole) transfer.17 We also performed SPCM measurements for a core-only PbS QD−3L MoS2 FET (Figure S7) to observe a similar trend for ΔIph/Iph(MoS2) versus

2π 2ijj 1 yzz V j z ℏ jjk 4πλkBT zz{

1/2

kET =

ÉÑ ÅÄÅ Å (λ + ΔG°)2 ÑÑÑ ÑÑ expÅÅÅÅ− ÅÅÇ 4λkBT ÑÑÑÖ

(2)

where ℏ is the reduced Planck constant, kB the Boltzmann constant, and T the temperature. The driving force is set by the 8465

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ACS Nano difference between the LUMO (or CB) levels of the donor and acceptor components. The electronic coupling strength, V, in general reduces exponentially with distance,35,36 so it can be described as a simple exponential distance-dependent decay function (see details in the Supporting Information). We consider each TMD layer as an independent acceptor, and as such, for a multilayer acceptor, kET will be a summation of electron-transfer rates of the QD with each MoS2 monolayer i y ∑ [k 0 exp(−d(x))] jjj 1 zzzzz 1 k 4πλkBT { ÄÅ ÉÑ 2 ÅÅ (λ + ΔG°(x)) ÑÑ ÑÑ expÅÅÅÅ− ÑÑ ÅÅÇ 4λkBT ÑÑÖ

2π kET(x) = ℏ

x

2j j

1/2

(3)

with x-number of MoS2 monolayers on which a QD rests. The electronic coupling strength (V) and the driving force will also be layer dependent, V(x) ∼ k0 exp(−d(x)), with d the distance between the QD and MoS2 layers and k0 a constant, while ΔG° will change with added layers because of the change in CB of MoS2 (see Figure 1c and the Supporting Information). As the number of MoS2 layers increases, −ΔG° increases and, as such, the rate of electron transfer also increases (see Figure 3b, ascendant region in the beginning of the curve at small number of layers). Nevertheless, the relatively small increase in driving force combined with the increase in donor−acceptor separation distance in thick MoS2 results in saturation of electron-transfer rate, that is, after five layers, the rate experiences no enhancement with any added monolayer. A fit according to eq 3 is shown in Figure 3b. We further demonstrate the ability to control the electrontransfer rate through band gap engineering of QDs as part of the hybrid. We chose PbS/CdS QDs with varying core size and similar CdS shell thickness totaling a QD diameter of 2.58 nm (QD with PL peak at 900 nm), 2.97 nm (PL peak at 955 nm), and 3.56 nm (PL peak at 1010 nm) (Figure 1a, PL spectra; Figure S4, TEM data) and combined each of the three QDs with 1L, 2L and 3L MoS2. Diffraction-limited PL decays are shown in Figure S8, with the corresponding PL lifetimes and calculated electron-transfer rates shown in Table 1. The electron-transfer rate versus QD size dependency is shown in Figure 5, demonstrating our ability to tune the rate for electron transfer through band gap engineering of the QD. It is noteworthy that when decreasing the QD size, the donor− acceptor energy band offset, ΔECB, increases, and this leads to an increase in the driving force and the rate for electron transfer (see Figures 1e,f and 5).

Figure 5. Electron-transfer rate vs QD size for PbS/CdS QD− MoS2 hybrids with different number of MoS2 layers: 1L (black), 2L (red), and 3L (blue). The size of QD900 is 2.58 nm, QD955 is 2.97 nm, and QD1010 is 3.56 nm.

donor−acceptor systems where individual MoS2 layers were treated as noninteracting layers when transferring with QDs and found this theory to fit well our experimental data. In light of a previous work by Lin et al., thickness-dependent charge transport properties of monolayer and few-layer MoS2 FETs have been studied, and it was shown that the maximum mobility was estimated between five layers and ten layers, with experimental value of ∼70 cm2 V−1 s−1 (at room temperature) for a 5-layer MoS2 FET.37 Combined with our observation of the strongest electron transfer in QD−TMD hybrids, 5-layer MoS2 provides an excellent platform for designing chargetransfer hybrid devices of 2D-MoS2 with QDs. Therefore, the trade-off between the carrier mobility and the charge-transfer rate indicates that a well-performing hybrid in terms of electron transfer will have a core-only donor PbS QD of smallest possible size achievable through colloidal synthesis and a few (5) layers MoS2 absorption counterpart, rather than one monolayer.

METHODS Materials and 0D−2D Hybrid Fabrication. PbS core-only and PbS/CdS core/shell QDs were purchased from Nano Optical Materials, United States. The bulk crystal of MoS2 was purchased from HQ-Graphene, Netherlands, and exfoliated into few layers and a monolayer on SiO2 (300 nm)/Si substrates by the modified mechanical exfoliation method.38 QDs were diluted (59 nmoles) by toluene and deposited onto MoS2 flakes by spin-coating via a layer-bylayer process,1 including the ligand exchange process by using 1, 2ethanedithiol (EDT, 2% in acetonitrile) in an environmentally controlled argon glovebox. The thickness of the MoS2 layer was characterized by atomic force microscopy (Park NX20 AFM) and by Raman microspectroscopy (WiTec Alpha300). Time-Resolved Confocal PL Microscopy. A scanning confocal inverted microscope has been used in combination with a laser excitation source of 475 nm and 400 kHz repetition rate (Mai Tai Spectra-Physics) using an average power of 30 nW at the sample, which was prepared in the glovebox and transferred to a sealed holder. FLIM images were acquired with a near-infrared-enhanced singlephoton counting avalanche photodiode (SPCM-NIR, Excelitas Technologies) coupled to a time-analyzer (PicoHarp 300, PicoQuant) by sending the collected PL signal through a 60×, 0.9 NA air lens (Olympus) with PL optically filtered through a dichroic mirror (Di801) and a band-pass filter (Semrock, 935/170), and spatially

CONCLUSIONS In summary, we demonstrated interfacial electron transfer in core-only PbS QD−layered MoS2 and core/shell PbS/CdS QD−layered MoS2 hybrids with a combination of timeresolved confocal and photocurrent imaging microscopies. We also demonstrated tunability of electron-transfer rate through band gap engineering of either donor (QD size) or acceptor (number of MoS2 layers) components in core/shell PbS/CdS QD−layered MoS2 hybrids. We found the rate for electron transfer to be maximal for the smallest sized QDs and to exhibit a layer-number dependency, increasing with added layers, up to 5-layer MoS2 from which a saturation occurs, with a saturation rate similar in value to that of QDs transferring to bulk MoS2. We modeled the layer dependency of electrontransfer rate in view of the Marcus theory for weakly coupled 8466

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ACS Nano filtered by a 75 μm pinhole. Data acquisition and analysis were performed by the SymPhoTime 64 software (PicoQuant, Germany). Absorption and PL Measurements. MoS2 bulk crystal was sonicated in water, and the solution was measured by a PerkinElmer UV−vis Lambda 25 for the absorption spectra. The absorption spectra of 2D-MoS2 on cover glasses from one layer to five layers were measured by a Nikon Eclipse Ti2 inverted microscope equipped with an Acton 500 spectrograph and a Princeton PIXIS 100 CCD camera (NT&C, Germany). PL and absorption spectra of PbS/CdS QDs (in toluene) were recorded by a Horiba Nanolog PL spectrometer equipped with a Symphony II Linear InGaAs detector and by a UV− vis Lambda 35 spectrophotometer, respectively. Device Fabrication and SPCM Measurement. We fabricated the FET device based on three-layer MoS2 on SiO2/Si and source− drain contacts (Au/Ti, 50/5 nm) by electron-beam lithography and electron-beam deposition in a cleanroom. The MoS2−FET device was then tested by a Signatone probe station and connected to a breadboard via wire bonding for the SPCM measurement. A homebuilt SPCM is based on an inverted Olympus IX 81 microscope coupled with optical excitation of 488 nm solid state laser (Coherent Sapphire 100) and a low-noise current amplifier (Femto DLPCA), converted into voltage by a lock-in amplifier (Stanford Research 830) and read by a SoftDB scanning probe microscope controller (for more details, see ref 17). Ultraviolet Photoelectron Spectroscopy. The sample, PbS/ CdS QD900, was ligand exchanged to EDT and deposited on a Si wafer via the layer-by-layer process (repeated to ensure a thick film to minimize contributions from Si substrate) and was conducted in an ultrahigh vacuum (UHV) system with base pressure of 2 × 10−10 Torr equipped with a SPECS Phoibos 100, MCD-5 hemispherical energy analyzer, and an ultraviolet source UVS 10/35.

program is administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by ORAU under Contract Number DE-SC0014664. All opinions expressed in this paper are the author’s and do not necessarily reflect the policies and views of DOE, ORAU, or ORISE.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b04367. PL decays of core-only PbS QDs, confocal absorption and Raman spectra of MoS2 flakes, TEM images and size histograms of PbS/CdS QDs, UPS and absorption spectra of PbS/CdS QDs, calculation of electron-transfer rates for QD−MoS2 hybrids, SPCM data of the PbS core-only FET hybrid, and PL decays of PbS/CdS QDs on one- to three-layer MoS2 flakes (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Jia-Shiang Chen: 0000-0003-1612-3008 Qin Wu: 0000-0001-6350-6672 Eduard Fron: 0000-0003-2260-0798 Mircea Cotlet: 0000-0002-5024-3540 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory (BNL), which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program. The SCGSR 8467

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