Dipole-Aligned Energy Transfer between Excitons ... - ACS Publications

Letter Cite This: ACS Photonics XXXX, XXX, XXX-XXX

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Dipole-Aligned Energy Transfer between Excitons in TwoDimensional Transition Metal Dichalcogenide and Organic Semiconductor Jie Gu,†,‡ Xiao Liu,§ Erh-chen Lin,∥ Yi-Hsien Lee,∥ Stephen R. Forrest,§,⊥ and Vinod M. Menon*,†,‡ †

Department of Physics, City College of the City University of New York (CUNY), New York, New York 10031, United States Department of Physics, Graduate Center of the City University of New York (CUNY), New York, New York 10016, United States § Department of Electrical Engineering and Computer Science and ⊥Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, United States ∥ Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan ‡

S Supporting Information *

ABSTRACT: Efficient Fö rster resonant energy transfer is observed between excitons in a two-dimensional (2D) monolayer of the transition metal dichalcogenide, MoSe2, and an 2 nm thick layer of the organic material, 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). The exciton transition dipoles are horizontally aligned, enabling efficient energy transfer between these dissimilar materials. Energy transfer is observed using timeresolved and steady state photoluminescence and photoluminescence excitation spectroscopy. Time-resolved measurements show a reduction in the donor (PTCDA) lifetime, and steady state emission experiments show a decrease in donor and an increase in acceptor (MoSe2) emission. Photoluminescence excitation spectra show a spectral dependence of the energy transfer process, with a maximum efficiency at the absorption maximum of the donor. The planar dipole orientation is determined using Fourier space imaging. The efficient energy transfer from low mobility organic materials to higher mobility 2D semiconductors along with their extremely large oscillator strengths presents an attractive platform for developing high efficiency energy harvesting systems that cover a wide spectral range. KEYWORDS: Förster resonance energy transfer, 2D material, Fourier imaging

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on the symmetry points in energy-momentum dispersion relationships, making them suitable for valleytronics.17,18 These interesting materials show promise as a platform for next generation optoelectronic devices such as thin film field effect transistors, solar cells, light emitters, and integrated circuits.19−21 Despite these characteristics, some critical issues need to be addressed before TMDs can be used in practical optoelectronic devices. One is their low absorption per layer despite a large oscillator strength. This shortcoming can be mitigated by using an efficient light absorber such as an organic layer that efficiently transfers energy to the high mobility TMDs. Such hybrid systems can enjoy broadband absorption along with efficient energy harvesting. 22 The nonradiative Fö rster resonance energy transfer (FRET)23 occurs via dipole coupling between donor and acceptor molecules. The parameters that determine the efficiency of nonradiative energy transfer include

rganic−inorganic hybrid heterostructures that synergistically combine the distinct optoelectronic and mechanical properties of two contacting materials systems is an attractive platform for realizing enhanced optoelectronic device performance as well as for observing emergent properties such as nonlinear optical response.1−3 Prior work on hybrid heterostructures has primarily relied on coupling of organic semiconductors with III−V or II−VI semiconductors having very small exciton binding energies (10−60 meV), or to colloidal quantum dots (QD) to realize either rapid energy transfer or the formation of hybrid excitonic states.2,4−11 The recent introduction of two-dimensional transition metal dichalcogenides (TMDs) presents a new class of inorganic materials with very large exciton binding energies (0.1−0.5 eV) and a mechanical compatibility with organic optoelectronics. Transition metal dichalcogenides have demonstrated high mobility12−14 as well as an indirect-to-direct band gap transition when their thickness decreases from bulk to a single monolayer.15,16 The reduced screening arising from the two dimensionality of the TMDs results in enhanced exciton binding energy and oscillator strength. In addition, their broken inversion symmetries result in spin selection rules depending © XXXX American Chemical Society

Special Issue: Strong Coupling of Molecules to Cavities Received: July 5, 2017 Published: October 10, 2017 A

DOI: 10.1021/acsphotonics.7b00730 ACS Photonics XXXX, XXX, XXX−XXX

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Figure 1. (a) Top: MoSe2 monolayer; Middle: Structural molecular formula of PTCDA; Bottom: schematic of the hybrid sample. (b) Schematic of the set up for Fourier space imaging. (c) Fourier space image of MoSe2 (1), PTCDA (2), the calculated emission pattern of an in-plane dipole (3), and an out-of-plane dipole (4). All the wave vectors are scaled with respect to individual material’s emission wave vector in vacuum, k0. The arrow indicates the direction of the polarizer.

the spectral overlap between emission of the donor and absorption of the acceptor, the physical distance between the donor and acceptor, dimensionality of the materials, and relative orientation of the donor and acceptor dipole moments.24 There have been reports of energy transfer to 2D TMDs using colloidal QDs as donors25−28 where a decrease in the lifetime of the donor QD provided evidence of energy transfer. More importantly, the dipole orientation of the donors and acceptors could not be controlled. Here we demonstrate energy transfer between in-plane dipoles of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) and 2D MoSe2. To exploit the unique advantages of both materials−larger absorption in organic molecules and higher mobility in 2D TMDs, the energy transfer should occur from the organic PTCDA to the 2D MoSe2. By stacking a monolayer MoSe2 on top of 2 nm thick film of PTCDA, we observe the direct quenching of photoluminescence (PL) in PTCDA as well as the increase in PL of MoSe2. The PL lifetime of PTCDA is found to decrease in the presence of the MoSe2, while that for MoSe2 increases. This change is explained using a rate equation model. Finally, PL excitation spectra showed an increase in steady state PL of MoSe2 when excited at the absorption maximum of PTCDA. The structures of MoSe2 and PTCDA, along with a schematic of the heterojunction are shown in Figure 1a. A thin layer PTCDA (2 nm) was grown on a glass slide (150 μm thick) by vacuum thermal evaporation in a system with base pressure of 2 × 10−7 Torr. Monolayer MoSe2 was grown on an SiO2/Si substrate by chemical vapor deposition (CVD) and then dry-transferred on top of the PTCDA film.29

Samples were excited by a 500 fs pulsed laser (Toptica, FemtoFiber pro TVIS with repetition rate 80 MHz) at a wavelength of 510 nm. The beam was focused to a ∼2 μm diameter spot via a 50× objective. The PL spectrum was collected by the same objective and coupled to a monochromator (Princeton Instruments Acton SpectraPro SP-2500) with a 1024 × 1024 CCD camera (Princeton Instruments 1024 PIXIS). Time-resolved PL was measured by a time correlated single photon counting (TCSPC) detector. The excited state dipole orientation of PTCDA and MoSe2 were measured using Fourier plane imaging by confocal microscopy shown in Figure 1b. An oil immersion objective (Olympus MPLAPON100XO) with numerical aperture 1.4 was used to ensure a large collection angle. Different dipole orientations (with respect to the glass substrate surface) resulted in distinguishable orthogonal image patterns in the Fourier plane of the objective.24,30 The monochromator was maintained in the reflection geometry so that the two orthogonal momentum directions parallel to the sample surface could be simultaneously detected by the CCD camera. Different band-pass filters (670 ± 5 nm for PTCDA and 790 ± 5 nm for MoSe2) were placed in front of the entrance slit of the monochromator to reduce chromatic aberration of the Fourier plane image. The absorption of MoSe2 and PTCDA were obtained using differential reflectivity carried out under white light illumination. The Fourier plane images of the emission intensity for both MoSe2 and PTCDA in Figure 1c show that the excited state dipoles are horizontally aligned. These patterns are similar to that from a calculation of an in-plane dipole shown in Figure 1c B


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Figure 2. (a) Differential reflectivity of PTCDA (blue triangles) and MoSe2 (red circles), and PL of PTCDA (black squares). The intensities are normalized. (b) Photoluminescence of PTCDA (black squares), MoSe2 (blue triangles), and the hybrid sample (red circles).

Figure 3. (a) Top: time-resolved photoluminescence (PL) of PTCDA with (red) and without (black) MoSe2 on its surface; Bottom: time-resolved PL of PTCDA with (red) and without (black) MoSe2 in the presence of a 10 nm thick Al2O3 spacer. (b) Time-resolved PL of MoSe2 on a bare substrate (black squares) and PTCDA (red circles).

acceptor, an experiment was carried out by growing a 10 nm Al2O3 spacer via atomic layer deposition (ALD) at 80 °C on top of PTCDA prior to the MoSe2 transfer. The PTCDA lifetime was measured with and without the MoSe2 in the presence of the spacer. No discernible change in lifetime is observed in the presence of the acceptor, indicating that FRET is suppressed by the presence of the 10 nm thick spacer. The MoSe2 exciton lifetime was found to increase in the hybrid structure compared to that for MoSe2 only. In the hybrid sample, PTCDA undergoes FRET to MoSe2. The MoSe2 exciton itself has a much shorter lifetime (Figure 3b, black curve) compared to that for PTCDA, but the excited state of MoSe2 continues to be populated by FRET, which results in an increase in the effective PL lifetime of MoSe2 (Figure 3b, red curve). This effect can be quantitatively explained using the following rate equations:

(3). As a comparison, the calculated pattern of an out-of-plane dipole is shown in Figure 1c (4). In Figure 2a, the absorption peaks of A, B, and C excitons in MoSe2 and excitons in PTCDA are shown. The PL spectrum of PTCDA shown as the black curve in Figure 2a indicates overlap with the B exciton absorption in MoSe2, which enables FRET from PTCDA to MoSe2. The PL spectra of PTCDA and MoSe2 are compared to that of the hybrid sample. The PL spectrum from the same MoSe2 island was measured before and after transferring to the PTCDA. All the measurements were done under a laser power of 1.28 μW/mm2. Figure 2b shows the PL spectrum of PTCDA, MoSe2, and the hybrid sample. The PL intensity of PTCDA is quenched while that of MoSe2 increases in the hybrid sample. The FRET efficiency, η = 1 − IDA/ID = 30%. Here IDA and ID are the spectrally integrated donor emission intensities with and without the acceptor layer on the PTCDA surface, respectively. This value is smaller than the theoretically predicted value of 47% (see Supporting Information). This is attributed to disorder at the interface, possible formation of charge transfer states and finite spectral overlap between the donor and the acceptor. The exciton dynamics of PTCDA, MoSe2, and the hybrid sample are shown in Figure 3. Band pass filters were used (670 ± 5 nm for PTCDA and 790 ± 5 nm for MoSe2) to obtain the lifetimes of the corresponding materials. For PTCDA, a biexponential fit yields τ1 = 80 ps and τ2 = 521 ps, while for the hybrid sample, τ1 = 56 ps and τ2 = 383 ps. This 30% decrease of PTCDA exciton lifetime in the hybrid sample is consistent with the FRET efficiency calculated from the PL intensity quenching data. Since the FRET efficiency decreases with the sixth power of the distance between donor and

⎧ dNp Np ⎪ =− − τp ⎪ dt ⎨ ⎪ dNm N =− m + ⎪ d τm t ⎩

Np τfret Np τfret

(1)

Here, Np and Nm are the exciton populations of PTCDA and MoSe2, respectively. τp and τm are the lifetimes of PTCDA and MoSe2 excitons, respectively, and τfret is the energy transfer time constant. Solution of eq 1 yields the exciton population in ∼ τp × τfret τ = is the modified PTCDA of N = N 0e−t / τp . Here, ∼ p

p

p

τp + τfret

PTCDA exciton lifetime in hybrid sample. Therefore, C


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⎛ Np0∼ τpτm Np0∼ τpτm ⎞ −t / τ −t / ∼ τp ⎜Nm0 − ⎟e m + e ⎜ ⎟ ∼ τfret(∼ τp − τm) τ τ − τ ( ) fret p m ⎠ ⎝

between organic molecules and inorganic 2D TMDs provide a platform to develop enhanced light harvesting systems, as well as thin film optoelectronic devices.

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(2)

We can see that the effective PL lifetime of MoSe2 is directly τp in the hybrid related to the modified PTCDA lifetime, ∼ sample, which is much longer than the MoSe2 lifetime τm. To further substantiate the existence of energy transfer, we carried out photoluminescence excitation (PLE) experiments where the excitation laser wavelength is tuned across the absorption spectrum of PTCDA at constant power (1.28 μw/ mm2) while monitoring the emission intensity of MoSe2 at a wavelength of 787 nm corresponding to its emission maximum. A MoSe2 monolayer transferred to a bare substrate was used as a control. The same MoSe2 sample was then transferred onto PTCDA for PLE measurements. Similar measurements were also carried out on the hybrid sample with the 10 nm Al2O3 spacer. Results of the PLE experiments are shown in Figure 4

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b00730. FRET efficiency calculation; Figures S1 and S2, distance dependent efficiency, and supporting references (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Stephen R. Forrest: 0000-0003-0131-1903 Vinod M. Menon: 0000-0002-9725-6445 Author Contributions

J.G. and X.L. carried out the experiments and analysis, E.L. and Y.L. were involved in the MoSe2 growth, S.R.F. and V.M.M worked on the analysis and supervised the project. All authors contributed to the writing of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation, Division of Materials Research, Award No. 1410249 (CCNY), 1411604 (Michigan). Work at NTHU was supported through Asian Office of Aerospace Research and Development (FA2386-16-1-4009,) and Ministry of Science and Technology PR China (MoST 105-2112-M-007-032-MY3 and MoST 1052119-M-007-027).

Figure 4. Photoluminescence excitation spectral intensity of MoSe2 at a wavelength of 787 nm under different excitation wavelengths. The data are scaled so that the intensities at excitation wavelength of 590 nm are the same for all the samples. The hybrid PTCDA/MoSe2 samples with (blue triangles) and without (red circles) a 10 nm thick Al2O3 spacer between the semiconductors is compared to the MoSe2 sample (black square). The red and violet are the guide lines.

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REFERENCES

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