2D Heterostructures

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Letter

Multiple Exciton Harvesting at 0D/2D Heterostructures Aamir Mushtaq, Supriya Ghosh, Abdus Salam Sarkar, and Suman Kalyan Pal ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00544 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on July 30, 2017

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ACS Energy Letters

Multiple Exciton Harvesting at 0D/2D Heterostructures

Aamir Mushtaq, Supriya Ghosh, Abdus Salam Sarkar and Suman Kalyan Pal School of Basic Sciences and Advanced Material Research Center, Indian Institute of Technology Mandi, Kamand 175005, H.P, India. *

Corresponding Author: Tel.: +91 1905 267040; Fax: +91 1905 267009; E-mail: [email protected]

Abstract Heterostructures of zero/two-dimensional (0D/2D) materials, especially quantum dots (QDs)/nanosheets (NSs) have attracted several attentions for extracting photogenerated electrons/holes. Herein, we report the dissociation of exciton at the heterojunction of CdSe (cadmium selenide) QD and MoS2 (molybdenum disulfide) nanosheet utilizing steady-state and time-resolved spectroscopic techniques. Quasi type II semiconductor like band energy alignment of the 0D/2D heterojunction facilitates exciton breaking via hole transfer from QD to MoS2. Furthermore, we demonstrate the extraction of two holes from doubly excited QDs (created via high power exciation) following the dissociation of biexciton at the 0D/2D interface. This work is expected to provide a new concept of exploiting multiple exciton generation in quantum dot sensitized solar cells by harvesting multiple carriers.

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Colloidal semiconducting quantum dots (0D-QDs) is a class of materials that exhibit high absorption cross-sections, broad tunability of band gaps and high quantum efficiencies.1,2 These unique electronic properties make them ideal materials for photovoltaic,3-6 photodetectors,7-9 and light-emitting (LEDs) devices.10,11 Multiple exciton generation (MEG) or carrier multiplication (CM) is a process where several excitons are generated as a result of the absorption of a high energy photon, typically higher than double the band gap, in QDs.12,13 Efficient MEG was observed in many QDs: PbSe,14,15 PbS,14,16,17 Si,17,18 CdSe,18-20 and InAs21 by many groups using different techniques. Successful exploitation of CM could improve the efficiency of quantum dot sensitized solar cells (QDSSCs).12 Klimov22 has theoretically shown that an ideal CM yield in QDSSC can produce a power conversion efficiency exceeding 44%, which is much higher than the Shockley-Queisser limit (∼33%) for a single junction photovoltaics.23 In fact successful utilization of MEG using metal oxides (MOs) in QDSSCs has been demonstrated in several studies.24,25 Two-dimensional (2D) materials, on the other hand, have attracted incredible interest due to their intriguing electrical, optical and mechanical properties26-30 since the exfoliation of graphene in 2004. 2D transition metal dichalcogenides (2D TMDCs) having chemical formula MX2, where the transitional metal atom (M) is sandwiched between two chalcogen (X) atoms, show unique optoelectronic properties including a non-zero direct or indirect band gap.31,32 Among all TMDCs, MoS2 has particular importance in solar cell as n-type material because of large absorption coefficient33-36 and direct nature of the band gap in monolayer.3739

Layered MoS2 sheets find applications in photovoltaic and photodetector devices due to

high carrier mobility.40-42 As a matter of fact, Coulomb interaction in these materials is significantly enhanced43 because of charge carrier confinement within the 2D plane.44


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Recently, van der Walls heterojunctions of 2D TMDCs have shown to be useful for the dissociation of excitons via fast interfacial charge transfer.45-47 In the heterojunction between WS2 and MoS2, hole transfer was observed to occur in ultrafast timescale from MoS2 to WS2.45 While, the dissociation of excitons was reported to takes place in the heterostructures of WSe2/MoS2 via ultrafast electron transfer from monolayer WSe2 to MoS2.47 It was also noticed that both electron and hole transfers can occur at the MoS2/MoSe2 interface in opposite direction.45 The type II semiconductor like band energy alignment in these 2D/2D heterostructures facilitates the formation of an indirect exciton (IX), where the electron is located in one 2D material and the hole in other 2D material.45,46 Nowadays, researchers are looking for combining 0D and 2D materials into one nano-heterostructure, which may provide additional benefits. Prins et al.48 reported the enhancement of the efficiency of the Förster resonance energy transfer (FRET) at the 0D/2D heterojunction of cadmium selenide/cadmium zinc sulfide (CdSe/CdZnS) core/shell QD and MoS2 with the decrease in layer number of MoS2. In the recent past, Boulesbaa et al.1 have reported both electron and hole transfer at the 0D/2D interface of CdSe and monolayer WS2 leading to the formation of hybrid exciton. In order to take the advantage of MEG, multiple carriers have to be extracted by dissociating multiple excitons. Although, generation of multiple excitation in QDs has been extensively studied,12,13,18,49 very limited effort was made to collect multiple carriers. A donor-acceptor interface has to be created for the extraction of multiple carriers via charge (electron/hole) transfer. Zidek et al.50 reported harvesting of multiple exciton from CdSe QDs via electron transfer at the interface with ZnO. However, breaking of multiple excitons efficiently by creating a suitable semiconductor junction is still a challenge. In recent times, 0D/2D heterojunctions have shown promise to dissociate excitons. In this work, we not only demonstrate the dissociation of single exciton, but also biexciton at the 0D/2D heterojunction 3 ACS Paragon Plus Environment

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of CdSe QD and MoS2. Our steady-state spectroscopic and femtosecond transient absorption (TA) studies reveal that exciton created in the QD breaks at the 0D/2D heterojunction following hole transfer from QD to MoS2. In addition, we mimic MEG (by creating multiple excitons in a single QD) in QDs by using high intensity excitation (not high energy photon). Comparison of TA kinetics at different excitation intensities suggests the feasibility of biexciton breaking at QD/MoS2 interface. We find that biexciton dissociates in two steps: injection of first hole in tens of picoseconds followed by injection of the second hole in hundreds of picoseconds from QD to MoS2. 0D and 2D materials. We first prepare CdSe QDs (0D) and MoS2 nanosheets (2D) via chemical methods (see Supporting Information for detail synthesis procedure). The morphology of 2D-TMD, MoS2 nanosheets was investigated by TEM and AFM. While TEM and HR-TEM images indicate the formation of sheets having average size ~200 nm, SAED pattern confirms hexagonal crystal structure of MoS2 (Figure 1a, b and c). Corresponding fast Fourier transform (FFT) image (Figure 1d) infers highly crystalline MoS2 structure with lattice spacing of 2.7 Ȧ.51 MoS2 nanosheets are clearly visible in the SEM images (Figure 1e). When CdSe QDs are mixed with MoS2, QDs get adsorbed to the surface of MoS2 nanosheets forming 0D/2D heterostructures (Figure 1e). AFM studies also identified MoS2 nanosheets of few hundred nanometers of dimension (supporting information Figure S1). The thickness of the MoS2 sheets was estimated from AFM height profile and found to be 3.5 nm (supporting information Figure S1). It has been reported that the thickness of single layer MoS2 sheets vary from 0.65 to 1.2 nm.34,52 Therefore, synthesized MoS2 nanosheets are expected to be of few layers (four-layer). Furthermore, Raman spectroscopy is used to characterize the MoS2 sheets and find the number of layers in each sheet. Raman spectrum (supporting information Figure S2) depicts the characteristic in-plane (E12g) and out of plane (A1g) modes of vibration of MoS2 at 384 and 407.5 cm-1, respectively. The wave number 4 ACS Paragon Plus Environment

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difference (∆ω=ω(A1g) - ω(E12g)) between the Raman modes (E12g and A1g) of MoS2 was found to be 23.5 cm-1. According to previous reports, such value of ∆ω corresponds to MoS2 nanosheets having four layers.35,53 Optical properties. Absorption spectroscopy was employed to confirm the formation of CdSe QDs. Figure 2a depicts the absorption spectrum of colloidal CdSe nanocrystals with characteristics excitonic peak at 2.34 eV. The size of the CdSe nanocrystals was estimated from the absorption onset by utilizing the relation54,55 d = (1.6122 10 ) - (2.6575 10 ) + (1.6242 10) - (0.4277) 41.57 Where, d is diameter of the QDs and is the absorption onset in nm.


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Figure 1. Morphology of 2D MoS2 nanosheets: (a) TEM images, (b) high resolution TEM (HR-TEM) images, (c) SAED pattern, (d) fast Fourier transform (FFT) filtered atomic resolution of the selected area in (b), (e) SEM images. (f) SEM images showing the adsorption of QDs on MoS2 nanosheets. From the above equation, we obtained that the size of nanocrystals is 2.9 nm. As the radius is much less than the Bohr exciton radius (5.4 nm) in bulk CdSe, we can call the synthesized particles QDs. The absorption spectrum of MoS2 nanosheets in isopropyl alcohol is shown in figure 2a. It exhibits two characteristic excitonic peaks: one around 2.03 eV and the other at 1.84 eV.38 These peaks are associated with the inter band excitonic transitions corresponding to the energy splitting from the valence band spin-orbit coupling at the K point of the Brillouin zone.38,56,57 The characteristic peaks of CdSe QDs and MoS2 nanosheets remain unaltered in the absorption spectrum (Figure 2a) of their mixture suggesting negligible interaction between them in the ground state. Photoluminescence (PL) quenching and hole transfer. In order to learn about the interaction between CdSe QDs and MoS2 in the excited state it is very important to measure the PL spectrum of QDs in the absence and presence of 2D sheets. Figure 2b shows the PL spectrum of colloidal QDs without and with MoS2 sheets after exciting at 2.8 eV. Bare CdSe QDs exhibit PL band around 2.34 eV. A small Stokes shift of the PL band suggests that the PL in CdSe QDs originates mainly from excitonic recombination. The contribution of the defect emission that is supposed to be more red-shifted with respect to the absorption band is very slim. The excitonic emission (intensity) of CdSe QDs is found to be greatly quenched in the presence of MoS2. As QDs and 2D sheets do not interact in the ground state the quenching is not due to a static process like complex formation in the ground state. The PL quenching is most probably the results of an additional deexcitation channel, e.g., charge or


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energy transfer. In order to understand whether energy transfer is occurring between CdSe QD and MoS2, we monitored the emission from MoS2. PL from MoS2 sheets is not detectable in normal fluorometer, but red PL is observed following excitation of the lowest absorption band (~640 nm) using a 639 nm laser in a confocal microscope (Figure 2c). MoS2 sheets also emit red PL following excitation with a 401 nm laser light (Figure 2d). However, negligible red emission was detected from the mixture of CdSe QDs and MoS2 sheets at 401 nm excitation (Figure 2e). As both CdSe and MoS2 absorb light at this excitation wavelength, energy transfer from CdSe to MoS2 must results into PL enhancement, but reduction of MoS2 PL was noticed. Therefore, it is clear from these observations that the quenching of the PL of CdSe is not due to the energy transfer from excited CdSe QD to MoS2 nanosheet. On the other hand, almost zero PL from MoS2 in the mixture could be rationalized by its low concentration (in the mixture) which absorbs small amount of light and hence emits unnoticeable PL. We examine energetic positions of the conduction band (CB) and the valence band (VB) of 0D and 2D materials to check the possibility of charge transfer. The energy of the VB of CdSe QD that is independent of QD size and type of ligand is -6.09 eV.6,58 The CB energy is obtained by adding the measured band gap for CdSe QD with the energy of the VB. Similarly, the energy of the CB of MoS2 sheets is estimated from the band gap and the position of the VB (-5.6 eV).59,60 Energy band alignment for this QD/2D heterojunction based on these energy values is shown in figure 2f. It is apparent from figure 2f that CdSe QD and MoS2 nanosheet form a quasi type II heterojunction. The energy offset between the two VBs is enough to provide activation energy for breaking the excitons. Therefore, the excitons that are generated in the CdSe QDs dissociate at the 0D/2D interface via hole transfer resulting the quenching of the QD PL.


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Figure 2. (a) Absorption spectrum of MoS2 nanosheets, CdSe QDs and CdSe-MoS2 nanosheets, (b) PL properties of CdSe QDs and CdSe-MoS2 following excitation at 2.8 eV. Confocal image of only MoS2 at excitation wavelengths (c) 639 nm, (d) 401 nm and (e) MoS2 nanosheets in presence of CdSe QDs at excitation wavelength 401 nm. (f) Energy level diagram showing the possibility of hole transfer from photoexcited QD to MoS2 sheets.

Dissociation of exciton as well as biexciton. Femtosecond transient absorption (TA) spectroscopy was used to understand the exciton dissociation mechanism at the 0D/2D interface including the hole transfer dynamics. We have measured TA spectra in the visible range of CdSe QDs without and with MoS2 nanosheets following 2.58 eV excitation (laser fluence 78 µJ/cm2/pulse). Figure 3a and b depict measured TA spectra at different time delays. TA spectra of CdSe QDs show a single negative absorption band with maximum at 2.34 eV. As CdSe QDs has excitonic absorption band at 2.34 eV, the band in the TA spectra could safely be assigned to the ground state bleach. This bleach band may arise due to the 8 ACS Paragon Plus Environment

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depletion of 1S exciton1 through the excitation of electrons from the VB to the CB of the QD (i.e., by electron state filling1) by pump light. It is clear from Figure 3a that TA signal grows by 0.5 ps and then undergoes a fast recovery followed by a slower one. In the presence of MoS2, TA spectra of CdSe QDs also show ground state bleach band at 2.34 eV (Figure 3b). Although the rise of TA signal is similar to that of bare QDs, recovery is faster in the presence of MoS2.

Figure 3. TA spectra of CdSe QDs without (a) and with (b) MoS2 nanosheets following 2.58 eV excitation (laser fluence 78µJ/cm2/pulse) at different pump-probe delays. (c) TA kinetics of CdSe and CdSe-MoS2 probing at 2.43 eV. (d) TA kinetics of bare QDs at 2.43 eV measured at different pump intensities and normalized on the long lived decay. Inset: Difference between TA kinetics for high and low pump intensities with single-exponential decay fitting.


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The faster recovery of the bleach signal could be attributed to the breaking of exciton via hole transfer from the VB of the QD to the VB of MoS2 nanosheets. TA kinetics near the bleach spectral maximum is then analyzed to explain the transfer dynamics. For bare QDs, TA kinetics (at 2.43 and 2.38 eV, Figure 3c and S3) can be described by bi-exponential decay with a short (~1-2 ps) and a long (~14-18 ns) time component (Table1). The initial fast bleach recovery is attributed to the Auger processes, which is in agreement with previous reports.13,61 On the other hand, in the presence of MoS2, kinetic traces become significantly faster (Figure 3c and S3). This is a consequence of the additional quenching root - the dissociation of excitons via hole transfer from the QD to MoS2. The injection of hole from QDs is a complex process because of system inhomogeneity and hence the exact calculation of injection rate is tricky. However, an estimate of the hole transfer rate can be made by computing the average lifetimes (τ) of QDs and QD-MoS2 systems using the following relation.50

=

!

−

(1)

!

We obtain an hole injection rate of 1.65 × 109 s-1(injection time ~600 ps) from equation 1. Obtained hole transfer rate is consistent with the literature reports. Recently, Zheng et al.58 reported hole transfer from MPA (3-mercaptopropionic acid) capped CdSe QD to NiO in the timescale of ~50 ps. The reason for the one order slower rate in 0D/2D heterostructure could be the presence of long TOP ligands, which reduce electronic coupling between CdSe QD and MoS2 nanosheet. Nonetheless, even slower (timescale ~few ns) hole transfer rates have been reported for other QD systems.54,62


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Table 1. Fitting results of the TA kinetics at 2.43 eV (excitation intensity 78 µJ/cm2) Sample

#$ ps (A1)

#% ns (A2)

#&'( ns

CdSe QDs

1 (14%)

16.5 (86 %)

14.7

CdSe-MoS2

3.7 (26 %)

0.78 (74 %)

0.58

At low excitation power, the number of excitons in each QD is expected to be small and the corresponding studies describe the extraction of one hole from QD to MoS2 via dissociation of a single exciton. It is worth noting that as MEG in CdSe QDs via impact ionization has already been extensively studied,18-20 we focus onto collection of multiple carriers via dissociation of multiple excitons. This is the reason we do not aim MEG by excitation with high energy photon, rather mimic MEG by directly populating the CB of the QDs with high excitation power (intensity). TA spectra of both QD and QD-MoS2 have measured at a higher pump fluence of 211 µJ/cm2/pulse to study multiple exciton harvesting and presented in figure S4. In both the cases, negative absorption signals are observed corresponding to the bleach of 1S excitonic band. The bleach recovery for only QD is appeared to be faster than the low energy excitation. In fact multiexcitons are formed in the QD at higher pump intensity and recombine stepwise (through higher order Auger recombination) to a single exciton, which decays on a longer (nanosecond) timescale. Therefore, one can extract the contribution of Auger recombination by calculating the difference between TA kinetics for different intensities after normalizing at long time corresponding to the single excitonic decay (Figure 3d). The difference in normalized TA kinetics exhibits an initial fast decay due to higher order Auger recombination and a major slow single-exponential decay corresponding to biexcitonic Auger recombination (Inset of 11 ACS Paragon Plus Environment

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Figure 3d). Single-exponential fitting of the differential data results into a biexcitonic Auger recombination time of 120 ps.

(b) (b)

Figure 4. (a) TA kinetics of CdSe-MoS2 acquired at 2.43 eV for different pump intensities. Inset: Difference between TA kinetics for high and low pump intensities with singleexponential decay fitting. (b) Schematic representation of the extraction of two holes from QD to MoS2 nanosheet. Next we examine how the multiexcitonic dynamics in the QD gets influenced by MoS2. Figure 4a and S5 show the comparison of the TA kinetics of CdSe-MoS2 at low and high excitation intensities at 2.43 eV and 2.38 eV, respectively. It is clear from the figures that the TA kinetics at higher intensity decays faster than the low intensity signal. We calculate the difference between two TA kinetics after normalizing at long time (Inset of Figure 4a). The difference signal shows a single-exponential feature with a lifetime value 42 ps. This decay dynamics is not due to biexcitonic Auger recombination as the lifetime is much less than the biexciton recombination time obtained for bare QDs. Observed fast decay component at high excitation intensity could be attributed to hole injection from QD to MoS2. To summarize the obtained picture, from a doubly excited QD, first a hole is transferred to MoS2 within 42 ps. After the injection of first hole a trion (two electrons and a hole) that can recombine through Auger process remains in the QD. According to the


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previous report the lifetime of a trion is more than 8 times of the biexciton Auger recombination time.63 The trion lifetime of our CdSe QD is much higher than the injection time 600 of a single hole. As the hole injection dominates over trion recombination, the injection of the second hole can be observed. Multiple exciton generation (MEG) creates on an average two excitons per QD. Therefore, harvesting of multiple exciton is possible using CdSe QD/MoS2 heterostructure. In conclusion, our results reveal that a single exciton can break at the 0D/2D interface via hole transfer from QD to MoS2. The hole injection time is estimated to be 600 ps from TA kinetics measurements. Furthermore, extraction of two holes from a QD in which biexciton is formed at higher excitation intensity is favorable in CdSe QD/MoS2 heterostructures. In that case hole transfer occurs in steps: first hole injects within 42 ps followed by slow injection of the second one. The exploitation of MEG effect by harvesting multiple holes is therefore a feasible scenario. Acknowledgements This work is supported by the COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH (CSIR), Government of India under Grant No. 03(1325)/14/EMR-II. AM and ASS are thankful to IIT Mandi for his fellowship and Advanced Materials Research Centre for the experimental facilities. Supporting Information Experimental methods, AFM images of MoS2 nanosheets, Raman spectrum of MoS2 nanosheets, TA kinectics of CdSe QDs and MoS2 probing at 2.38 eV with low pump power, TA spectra of CdSe QDs without and with MoS2 at high pump power and comparison of TA kinetics of CdSe-MoS2 acquired at 2.38 eV for different pump powers..


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