3D Localized Trions in Monolayer WSe2 in a Charge Tunable van der

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3D localized trions in monolayer WSe2 in a charge tunable van der Waals heterostructure Chitraleema Chakraborty, Liangyu Qiu, Kumarasiri Konthasinghe, Arunabh Mukherjee, Sajal Dhara, and Nick Vamivakas Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b05409 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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3D localized trions in monolayer WSe2 in a charge tunable van der Waals heterostructure Chitraleema Chakraborty,†,‡ Liangyu Qiu,¶,‡ Kumarasiri Konthasinghe,¶,‡ Arunabh Mukherjee,¶,‡ Sajal Dhara,¶,‡,§ and Nick Vamivakas∗,¶,k,†,‡ †Materials Science, University of Rochester, Rochester, NY 14627, USA ‡Center for Coherence and Quantum Optics, University of Rochester, Rochester, NY 14627, USA ¶Institute of Optics, University of Rochester, Rochester, NY 14627, USA §Department of Physics, Indian Institute of Technology, Kharagpur, 721302, India kDepartment of Physics, University of Rochester, Rochester, NY 14627, USA E-mail: [email protected] Abstract Monolayer transition metal dichalcogenides (TMDCs) have recently emerged as a host material for localized optically active quantum emitters that generate single photons 1–5 . Here, we investigate fully localized excitons and trions from such TMDC quantum emitters embedded in a van der Waals heterostucture. We use direct electrostatic doping through the vertical heterostructure device assembly to generate quantum confined trions. Distinct spectral jumps as a function of applied voltage bias, and excitation power dependent charging, demonstrate the observation of the two different excitonic complexes. We also observe a reduction of the intervalley electron-hole exchange interaction in the confined trion due to the addition of an extra electron which is manifested by a decrease in its fine structure splitting. We further confirm this decrease

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of exchange interaction for the case of the charged states by a comparative study of the circular polarization resolved photoluminescence from individual excitonic states. The valley polarization selection rules inherited by the localized trions will provide a pathway towards realizing a localized spin-valley-photon interface.

Keywords localized trion, monolayer tungsten diselenide, quantum dot, van der Waals heterostructure, transition metal dichalcogenide The operation of electronic devices is made possible by the electron’s charge. Recent advances in nanoscience have created the opportunity to leverage the electron’s spin in future technologies. 6,7 It is envisioned that spin can serve as a store of information 8,9 or provide a vehicle for high resolution sensing. 10,11 Of particular interest is electron spin in quantum confined nanomaterials that support optical transitions. In the previous, photons link spin ground states and trion excited states. 12 In this work, we present the observation of 3D confined trions in a van der Waals heterostructure. A single layer of tungsten diselenide hosts quantum dot (QD) like emitters attributed to defects 1–5,13 or strain gradients 14,15 and we demonstrate voltage controlled QD charging enhancing the valley polarization of the localized trion complex. From here forward, we use the term QD to refer to these emitters. The observation of 3D confined trion in a van der Waals heterostructure will present new opportunities for quantum photonics and provide an interesting environment to study confined spin-valley physics. The emergence of TMDCs as materials for nano-optoelectronics has focused attention on the existence and stability of their neutral and charged excitons. 16 Interesting in TMDCs is the symmetry mediated strong photon polarization selection rule that interfaces the two inequivalent K points of the Brillouin zone with distinct circular polarization orientations. As a result it has been proselytized that the valley index may provide a novel information bearing degree of freedom 17 and its optical manipulation has been explored. Important for 2


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single spin studies in TMDCs is that photon energy and polarization can selectively excite specific spin ground state/trion excited state combinations. 16 The recent discovery of localized QD in single layer TMDCs has catalyzed investigations into 3D localized exciton resonances in these materials. 18–20 As a result there has been intense interest as to whether it may be possible to controllably charge the TMDC-QDs. Such controlled charging would engender the QDs with stable spin ground states that could be utilized as a qubit or nanoscale sensor. Further, the band edge Bloch states in the K and -K valleys has been used to describe the bound states 28 and under appropriate conditions, the QD can inherit the strong polarization selection rules of the host material providing a robust spin-valley-photon interface. Previously, a complete recovery of valley polarization was demonstrated in neutral excitons using magnetic field where Zeeman interaction is utilized to overcome the intervalley exchange interaction. 2–4 In this work, we demonstrate that controllably charging individual QDs in a van der Waals heterostructure can also result in a partial recovery of the valley polarization. Pronounced spectral shifts in the voltage-dependent photoluminescence signal provide evidence of charging. The polarization of photons emitted by the TMDC QD further confirms the formation of trions. The observation, creation and voltage control of quantum confined trions in our device is the central result of the manuscript. Our device (see Fig. 1a), is a van der Waals heterostructure 21 assembled by a complete dry transfer technique using PDMS stamp. 22 It consists of a tungsten diselenide (WSe2 ) layer, which naturally hosts individual QDs, 1 sandwiched by insulating hexagonal boron nitride layers and capped on the top and bottom by few layer graphene flakes that serve as electrodes (Fig S1 for I-V characteristics). The substrate used is a standard doped silicon with 300 nm oxide layer on which gold electrodes are lithographically patterned. All measurements are performed in the attodry 1000 optical cryostat at 4K. Laser excitation energy is 1.77 eV. We use electrostatic doping through the top and bottom graphene electrode to generate charged 3D confined excitons. As illustrated in Fig. 1b, dependent on the applied voltage, the QD

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Figure 1: Device architecture. a, A schematic representation of the van der Waals heterostructure that creates a hetereojunction device for electrostatic charging of monolayer WSe2 hosting individual quantum dots. FLG = few layer graphene flake, h-BN = hexagonal boron nitride, 1L WSe2 = Monolayer tungsten diselenide. Voltage is applied simultaneously across WSe2 and top and bottom graphene via Vt and Vb. b, Formation of the trion (X− ) in the device by capture of an extra electron leads to a suppression of the intervalley mixing from the electron-hole exchange interaction giving rise to valley polarized emission (top). For neutral excitons (X) the intervalley electron-hole exchange interaction mixes the two valley states giving rise to linearly polarized emission (middle) which is also inherited by the biexciton feature (XX, bottom). c, Voltage dependent photoluminescence demonstrating a distinct spectral jump between two emission lines. Voltage (V) is set such that V=Vt=Vb. The higher voltage plateau above VX− (indicated by dashed red line) originates from a localized trion state (X− ) whereas the two lower voltage plateau are from the neutral exciton (X) and biexciton (XX) emission. The linecuts in the inset are taken above (10 V) and below (5 V) VX− .

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Figure 2: Photoluminescence analysis. a, Power dependent PL intensity for the exciton (X) and the biexciton (XX) lines, b, Power dependence of the exciton and trion voltage plateaus for 3 different powers. Starting from top: 13 µW, 5 µW and 0.15 µW. The red dotted vertical lines represent the voltage at which the trion to exciton crossover occurs at each power. c, Voltage threshold for exciton to trion crossover indicating a linear dependence (red dotted line) on excitation power. stabilizes either a localized neutral exciton and biexciton or a negatively charged exciton a trion - excited state. To identify QD emission and characterize their charging state the sample is raster scanned to locate spectrally narrow (100 µeV - 400 µeV ) emission peaks red shifted from the delocalized neutral excitons in WSe2 that emit at ∼1.75 eV. 1 The emitters are located mostly near the edges or wrinkles in the flakes. After locating the emitter, the spectral content of the PL as a function of voltage applied across the device is recorded. An exemplary data set that shows QD charging and photoluminescence (PL) voltage plateaus for each emitter species is presented in Fig. 1c. The neutral exciton (X) and the biexciton (XX) share a common voltage plateau as expected. With increasing voltage, appearance of a singly charged trion (X− ) is observed. The bottom panel of Fig. 1c are linecuts at two different voltages also indicating the presence of valley mixed exciton states for the neutral exciton, biexciton and the trion, exhibited by the splitting in the emission lines commonly referred to as the fine structure splitting (FSS). We focus on the singly charged exciton for the rest of the manuscript. The observed behavior is characteristic of all emitters that have been measured (see Supplementary Information for more examples). The PL intensity of both the exciton and

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the biexciton reduces as the trion appears, whose intensity increases with increasing voltage. The PL spectral jump and the distinct PL voltage plateaus are indications of trion formation and has been observed in similar diode based indium arsenide quantum dot devices. 23 From the PL map the difference in the exciton and trion emission energy is around 9 meV which is a measure of the trion’s binding energy (EX − EX − ). The average binding energy from the measured emitters is 8 meV with a standard deviation of 4 meV. Binding energy of the charged complexes can fluctuate due to randomness of the environment. 24 We further confirm the different excitonic complexes by measuring PL intensities as a function of excitation power. Figure 2a presents power dependent intensity of the exciton and biexciton line. The intensity of the biexciton feature increases faster as a function of power compared to the neutral exciton. Fitting these lines to a power law (I = A + B.I n ) yields a coefficient of n=0.64 for the exciton and n=1.31 for the biexciton confirming the sub and super-linear behavior as expected from the exciton and biexciton respectively 20 (See supplementary Fig S2 for additional data relevant to biexciton formation). The influence of the laser power on the charging process is presented in Fig. 2b. PL voltage profiles for 3 different excitation powers is shown. As the pump power increases (from bottom to top panel), the photogeneration of negative trions becomes more efficient. This also results in reduction of the threshold voltage required for the charging process in response to the additional built-in field caused by excess photogenerated carriers [Fig. 2(c)]. 25 In our data, this is exhibited by the voltage at which a crossover from exciton to trion occurs. The lower effective mass of electrons in WSe2 26 results in a preference for photogenerated negatively charged trion formation. 27 The polarization of the emitted photons also carries information regarding the QD’s charging state. For polarization resolved measurements, the QD is excited with circularly polarized light and the σ − and σ + polarized PL is analyzed in Fig.3a and b. The polarization is studied at 3V and at 15V. The polarization resolved spectra indicate the split states of the trion preserves the polarization selection rules 28 (Fig. 3b) where the lower energy is

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Figure 3: Polarization resolved data : a, b, σ − and σ + emission spectra of the exciton (trion) at 3V (15V). The solid (dotted) line represents σ − (σ + ) detection. c, Evolution of the emission intensity in the circular detection basis as a function of voltage mapped for the lower energy peak of the exciton (in black circles) and trion (red triangle). Solid (open) legends represent σ − (σ + ) polarized emission intensity. Excitation laser is σ − polarized. σ + polarized whereas the higher energy peak is σ − polarized. However the exciton peaks do not prefer any circular polarization states (Fig. 3a) exhibiting a predominantly linear polarization (Fig. S2). The previous is examined as a function of applied voltage in Fig. 3c. In the data the solid circles (triangles) correspond to intensity of σ − emission for the lower energy peak of the exciton (trion) whereas the open circles (triangles) correspond to σ + emission. Throughout the charging plateau, the studied trion peak preserves the σ − polarization. The different charge configurations of the excited state provides a window into understanding the observed polarization contrast. In Fig. 4a the DoCP is plotted for each of the exciton and trion voltage plateaus. For the neutral exciton (X, black circles), the DoCP is negligible. When the ground state (|0i) of a localized spin-valley emitter under the influence of anisotropic intervalley electron-hole exchange interaction is optically excited, an exciton with spin-valley mixed state is formed represented by |+i and |−i. In this case, electron-hole exchange rate is comparable to or faster the radiative recombination rate of the individual valley exciton [as observed for case of the delocalized excitons 29 ] giving rise to a mixture of σ − and σ + state. Next, we consider the trion that exhibits a fixed degree of circular polarization and averages at 44 percent as indicated by the red horizontal line (Fig. 4a). Non-zero DoCP throughout the trion voltage plateau is not unexpected as even the deloclalized valley trions 7


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have a slower depolarization rate than the valley excitons. 29 For the case of these localized emitters, the finite DoCP across the plateau is due to a reduction of the electron-hole exchange interaction due to addition of an extra electron which is also confirmed by a reduction of the FSS when compared to the neutral exciton (Fig. 4b) (Additional data on polarization and magnetic field response is in supplementary Fig. S3-S4). The FSS and DoCP remains constant along the charging plateau of both the exciton and trion. a

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Figure 4: Valley polarization vs electric field. a, Degree of circular polarization of neutral exciton (X, black) and trion (X− , red) as a function of voltage. A scatter in the DoCP is due to fluctuations in intensity due to local environment of the emitter. b, Fine structure splitting as a function of voltage for the observed neutral exciton (black) and trions (red) generated from fit to Lorentzian function. Error bar indicates the resolution of the spectrometer. c, Schematics of energy levels of the neutral exciton and trion emission with x− fine structure splitting ∆x0 e−h and ∆e−h respectively due to electron-hole exchange interaction in an anisotropic potential. X represents the neutral exciton states with linearly polarized emission from the split states represented by |−i and |+i. X− represents the trion states with an extra electron (|ei) occupying the ground state causing a reduction in the electron-hole ∼ ∼ exchange interaction ∆x− e−h and circularly polarized split states |−i and |+i. To understand the possible valley depolarization mechanisms for the trion, consider the ground state which is occupied by an extra electron (|ei). Optical excitation generates a localized trion along with a fine structure splitting. However now, there is a reduction in the 8


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intervalley electron-hole exchange interaction (∆e−h ) compared to the neutral exciton due to the addition of the extra electron (Fig. 4c). In InAs quantum dots the addition of an extra electron has shown to completely suppress the fine structure splitting due to the formation of a spin singlet state. 30,31 However, complete reduction of the intervalley exchange interaction is not observed for the case of the localized trions in these emitters. Prior studies on the delocalized trion states in monolayer WSe2 have confirmed that exchange interactions can also affect the trion. 29,32,33 A fine structure splitting results from electron-hole and electronelectron interactions which splits the intervalley and intravalley trion states. Depolarization in the trion can take place by exchange interaction or by a combination of exchange along with single particle spin flip. 29 Spin flip processes are generally slower reducing the net depolarization rate, resulting in an increase in polarization of the trions. Thus for the case of the localized trion in WSe2 one observes a trion fine structure due to the electronelectron and electron-hole interactions. However, addition of the extra electron should also participate in suppressing the electron-hole exchange interaction. These two competing processes can alter the observed fine structure splitting and polarization of the localized trions when compared to the localized neutral excitons. Additionally, it has been described that charging a quantum dot with a single electron may not guarantee complete suppression of the electron-hole exchange interaction. 30 This is possible for the case of quantum dots with shallow confinement where redistribution of wave function of the electron pair can lead to non-zero local spin density for the excited trion state. The previous carrier redistribution mitigating singlet formation is likely as the large FSS in the confined neutral excitons is the result of strong coulomb interactions inherited from the monolayer TMDCs. In conclusion, we have demonstrated the controlled charging of TMDC-QDs in a van der Waals heterostructure. A reduction in the fine structure splitting along with a finite degree of circular polarization is observed for the case of the charged emitter. Stable, and optically accessible individual ground state spins are critical for the application of van der Waals heterostructure based QDs in quantum photonics and metrology. Understanding the

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interplay between both valley and spin lifetimes and coherences is a natural next step. We also expect coherent optical control of the spin ground states to be possible via the excited state trions not only providing a pathway for all-optical information processing, 34,35 but also opening new directions in solid-state quantum optics studies.

Acknowledgement This work was supported by NSF EFRI EFMA-1542707, NSF CAREER DMR 1553788, AFOSR FA9550-16-1-0020 and the Leonard Mandel Faculty Fellowship in Quantum Optics.

Supporting Information Available The supplementary information provides the following: Device, Idenitification of neutral exciton and biexciton, Charging and polarization in QD exhibiting split emission lines, Magnetic field response of the emitters exhibiting a split emission line. This material is available free of charge via the Internet at http://pubs.acs.org/.

Competing financial interests The authors declare no competing financial interests.

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(25) Seidl, S.; Kroner, M.; Dalgarno, P. A.; Högele, A.; Smith, J. M.; Ediger, M.; Gerardot, B. D.; Garcia, J. M.; Petroff, P. M.; Karrai, K.; Warburton, R. J. Phys. Rev. B 2005, 72, 195339. (26) Aivazian, G.; Gong, Z.; Jones, A. M.; Chu, R.-L.; Yan, J.; Mandrus, D. G.; Zhang, C.; Cobden, D.; Yao, W.; Xu, X. Nat. Phys. 2015, 11, 148–152. (27) Tang, J.; Cao, S.; Gao, Y.; Sun, Y.; Geng, W.; Williams, D. A.; Jin, K.; Xu, X. Appl. Phys. Lett. 2014, 105, 041109. (28) Wu, Y.; Tong, Q.; Liu, G.-B.; Yu, H.; Yao, W. Phys. Rev. B 2016, 93, 045313. (29) Singh, A.; Tran, K.; Kolarczik, M.; Seifert, J.; Wang, Y.; Hao, K.; Pleskot, D.; Gabor, N. M.; Helmrich, S.; Owschimikow, N.; Woggon, U.; Li, X. Phys. Rev. Lett. 2016, 117, 257402. (30) Bayer, M.; Ortner, G.; Stern, O.; Kuther, A.; Gorbunov, A. A.; Forchel, A.; Hawrylak, P.; Fafard, S.; Hinzer, K.; Reinecke, T. L.; Walck, S. N.; Reithmaier, J. P.; Klopf, F.; Schäfer, F. Phys. Rev. B 2002, 65, 195315. (31) Bracker, A. S.; Stinaff, E. A.; Gammon, D.; Ware, M. E.; Tischler, J. G.; Shabaev, A.; Efros, A. L.; Park, D.; Gershoni, D.; Korenev, V. L.; Merkulov, I. A. Phys. Rev. Lett. 2005, 94, 047402. (32) Plechinger, G.; Nagler, P.; Arora, A.; Schmidt, R.; Chernikov, A.; del ÃĄguila, A. G.; Christianen, P. C.; Bratschitsch, R.; SchÃĳller, C.; Korn, T. Nat. Commun. 2016, 7, 12715. (33) Courtade, E. et al. Phys. Rev. B 2017, 96 . (34) Press, D.; Ladd, T. D.; Zhang, B.; Yamamoto, Y. Nature 2008, 456, 218–221. (35) Kim, J.; Hong, X.; Jin, C.; Shi, S.-F.; Chang, C.-Y. S.; Chiu, M.-H.; Li, L.-J.; Wang, F. Science 2014, 346, 1205–1208. 13


Nano Letters

for TOC only 20

X-

15

Vt

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 14

Vb

10 5 0

XX

X V = Vx-

-5 1.68 1.69 Energy (eV)

14


3D Localized Trions in Monolayer WSe2 in a Charge Tunable van der

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