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Kacper Oreszczuk, Wojciech Pacuski, Aleksander Rodek, Mateusz Raczyński, Tomasz Kazimierczuk, Karol Nogajewski, [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), Marek Potemski, Piotr Kossacki

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[Short excitonic lifetimes of MoSe<sub>2</sub> monolayers grown by molecular beam epitaxy on the hexagonal boron nitride](https://mdr.nims.go.jp/datasets/8de1f976-35d0-403c-8634-7e53fcf31633)

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Short excitonic lifetimes of MoSe2 monolayers grown by molecular beam epitaxy on the hexagonal boron nitride2D Materials      PAPER • OPEN ACCESSShort excitonic lifetimes of MoSe2 monolayersgrown by molecular beam epitaxy on thehexagonal boron nitrideTo cite this article: Kacper Oreszczuk et al 2024 2D Mater. 11 025029 View the article online for updates and enhancements.You may also likeFirst-principles study of metals, metalloidsand halogens doped monolayer MoSe2 totune its electronic propertiesQurat Ul Ain Asif, Hamayl Asim, AzeemGhulam Nabi et al.-Single- and Few-Layers MoSe2Nanoflowers: Synthesis, Characterization,and Their PiezoresponseMei Hsuan Wu and Jyh Ming Wu-Thermal transport properties of MoS2 andMoSe2 monolayersAli Kandemir, Haluk Yapicioglu, AlperKinaci et al.-This content was downloaded from IP address 144.213.253.16 on 16/03/2024 at 01:20https://doi.org/10.1088/2053-1583/ad3135/article/10.1088/1402-4896/acf2cc/article/10.1088/1402-4896/acf2cc/article/10.1088/1402-4896/acf2cc/article/10.1088/1402-4896/acf2cc/article/10.1088/1402-4896/acf2cc/article/10.1149/MA2016-01/25/1288/article/10.1149/MA2016-01/25/1288/article/10.1149/MA2016-01/25/1288/article/10.1088/0957-4484/27/5/055703/article/10.1088/0957-4484/27/5/055703/article/10.1088/0957-4484/27/5/055703/article/10.1088/0957-4484/27/5/055703/article/10.1088/0957-4484/27/5/055703/article/10.1088/0957-4484/27/5/0557032D Mater. 11 (2024) 025029 https://doi.org/10.1088/2053-1583/ad3135OPEN ACCESSRECEIVED24 October 2023REVISED27 January 2024ACCEPTED FOR PUBLICATION7 March 2024PUBLISHED15 March 2024Original Content fromthis work may be usedunder the terms of theCreative CommonsAttribution 4.0 licence.Any further distributionof this work mustmaintain attribution tothe author(s) and the titleof the work, journalcitation and DOI.PAPERShort excitonic lifetimes of MoSe2 monolayers grown by molecularbeam epitaxy on the hexagonal boron nitrideKacper Oreszczuk1,∗, Wojciech Pacuski1, Aleksander Rodek1, Mateusz Raczyński1,Tomasz Kazimierczuk1, Karol Nogajewski1, Takashi Taniguchi2, Kenji Watanabe3, Marek Potemski1,4,5and Piotr Kossacki11 Institute of Experimental Physics, Faculty of Physics, University of Warsaw, Pasteura 5, Warsaw 02-093, Poland2 Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan3 Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan4 Laboratoire National des Champs Magnétiques Intenses, UPR 3228, CNRS, EMFL, Univ; Grenoble Alpes, Grenoble 38042, France5 CENTERA Labs, Institute of High Pressure Physics, PAS, Warsaw 01-142, Poland∗ Author to whom any correspondence should be addressed.E-mail: Kacper.Oreszczuk@fuw.edu.plKeywords:molecular beam epitaxy, MoSe2, transition metal dichalcogenides, time resolved photoluminescence,excitation correlation spectroscopy, excitonic lifetimesSupplementary material for this article is available onlineAbstractWe present a time-resolved optical study of recently developed narrow-line MoSe2 monolayersgrown on hexagonal boron nitride with means of molecular beam epitaxy. We find that thephotoluminescence decay times are significantly shorter than in the case of the exfoliated samples,even below one picosecond. Such a short timescale requires measurements with better resolutionthan achievable with a streak camera. Therefore, we employ an excitation correlation spectroscopypump-probe technique. This approach allows us to identify two distinct non-radiativerecombination channels attributed to lattice imperfections. The first channel is active at heliumtemperatures. It reduces the lifetime of the neutral exciton to below one picosecond. The secondchannel becomes active at elevated temperatures, further shortening the lifetimes of both neutraland charged exciton. The high effectiveness of both radiative and non-radiative recombinationmakes epitaxial MoSe2 a promising material for ultrafast optoelectronics.1. IntroductionMonolayers of transition metal dichalcogenides(TMDs) draw a lot of attention as semiconduct-ing materials with robust optical properties, strongCoulomb interaction [1–3] and an optically access-ible valley degree of freedom[4–7]. The extraordinaryproperties of TMDs make them promising candid-ates for applications in photonics and optoelectron-ics. Fundamental research on such systems usuallyrequired using exfoliated or mechanically transferredlayers, as they provided the best optical properties, inparticular: narrow linewidths of excitonic transitions[8–11]. The exfoliation process, however, suffersfrom limited scalability and large inhomogeneity,hindering the perspectives of potential commercialapplications.Epitaxial techniques, such as molecular beamepitaxy (MBE), are advantageous as they eliminatethe aforementioned drawbacks: they provide repro-ducible, wafer-scale, homogeneous layers [12–14].However, for a long time, epitaxial techniquessuffered from significant spectral broadening, whichhindered the observation of various interestingoptical effects. Fortunately, a recent report [15] hasaddressed this issue by implementing slow-growth-rate epitaxy on an atomically flat hexagonal boronnitride substrate. This novel approach has enabled thegrowth of excitonic narrow line MBE MoSe2 mono-layers, thereby opening up possibilities for advanced,time-resolved optical research, as demonstrated inthis manuscript.Short excitonic lifetimes and relaxation timesobserved in monolayers of semiconducting TMDs[16–21] are promising for applications in nonlinearoptics [22, 23] that may now become viable due tothe development of scalable growth process of highoptical quality material [15, 24]. Possible applications© 2024 The Author(s). Published by IOP Publishing Ltdhttps://doi.org/10.1088/2053-1583/ad3135https://crossmark.crossref.org/dialog/?doi=10.1088/2053-1583/ad3135&domain=pdf&date_stamp=2024-3-15https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://orcid.org/0000-0001-8830-0005https://orcid.org/0000-0001-8329-5278https://orcid.org/0000-0002-0263-3122https://orcid.org/0000-0003-0443-1943https://orcid.org/0000-0001-6545-4167https://orcid.org/0000-0003-3701-8119https://orcid.org/0000-0002-7558-1044mailto:Kacper.Oreszczuk@fuw.edu.plhttps://doi.org/10.1088/2053-1583/ad31352D Mater. 11 (2024) 025029 K Oreszczuk et alinclude higher harmonic generation [25, 26], THzwave generation [27, 28], parametric amplification[29] or nanoscale light sources [29, 30]. Such pro-cesses require control over the characteristic times ofthe excitonic relaxation processes. In this work, weemploy various experimental approaches to investig-ate the excitonic dynamics in epitaxially grownMoSe2monolayer quantitatively, and we observe relaxation,which is even faster than in the case of exfoliatedTMDlayers.2. MethodsThe epitaxial growth of studied MoSe2 was madepossible by employing the hexagonal boron nitridesubstrate. Thin hBN flakes obtained by mechanicalexfoliation were deposited on a silicon substrate witha layer of silicon oxide with a thickness of 90 nm.Prepared substrates were placed in the chamber ofthe molecular beam epitaxy machine, where MoSe2was grown over the entire surface of the sample(figures 1(b) and (c)). The growth was performedat the temperature of 300 ◦C and was followed withhigh-temperature annealing in the temperature of750 ◦C. Such a procedure allowed us to obtain a high-quality monolayer with relatively small areas whereMoSe2 is discontinuous or takes the form of a bilayer(figures 1(b) and (c)). Two samples were grown inseparate growth processes for experiments describedin this work. The sample used in the first part ofthis work (figures 1–4) was grown with a very slowgrowth rate (10 hMl−1), while the second sample(figures 5 and 6) was grown with a moderate growthrate (25minMl−1). More details of the growth pro-cess are described in [15, 31].Optical measurements were carried out in ahelium bath cryostat. Samples were placed in agaseous helium atmosphere at the temperature of 5 K,unless noted otherwise. The laser beam was focusedby a single aspheric lens (numerical aperture= 0.68)to a spot smaller than 2µm. The sample wasmountedon the x–y–z piezoelectric stage inside the cryostat.Time-resolved measurements were performedwith a pulsedOPO laser tuned at 610nm (2030meV).The repetition rate was 75.7MHz with a half-maximum pulse width of 160 fs. The streak camerameasurements were performed with averaged laserpower equal to 100µW.A large part of the time-resolved measurementswas performed with excitation correlation spectro-scopy (ECS) technique [32, 33]. In ECS measure-ment, the time-integrated PL spectrum is acquiredunder the excitation from two identical laser pulsetrains. The total PL intensity of excitonic peaks isprobed for different delays between the pulse trains.ECS measurement is effective if the dependence ofthe PL intensity on the excitation power is not lin-ear. We consider the simplified model of saturationbehavior related to the limited poll of the unoccupiedexcitonic states. Under such a picture, the number ofexcitons created with individual laser pulse is propor-tional to the number of unoccupied excitonic statesat the time of the impact. The PL intensity reaches aminimum when the laser pulses overlap temporarily.If the pulses are separated, the PL intensity rises expo-nentially with the delay between pulses, asymptotic-ally reaching its large-separation limit. The character-istic time of this exponential behavior is assumed tobe exactly equal to the population lifetime of the stud-ied excitonic state.Other causes of the sublinearity at the excita-tion mechanism may include, for example, the Augerrecombination. In this case, the ECS signal decay isgenerally faster than the population decay. The dif-ference, however, would only be significant in theregime of the very high excitation powers, wherethe higher-order components in the ECS transientemerge. See supplementary infomation for a moredetailed exploration of the scope and applicabilityof the ECS approach in the presence of the Augerrecombination.Regardless of the exact mechanism behind theECS transients, we interpret the ECS characteristictimes as related to the respective excitonic lifetimes.We estimate that in thiswork, the population lifetimesare underestimated by no more than 20%, even if theAuger recombination was the only source of the sub-linearity at the excitation mechanism.ECS measurements were carried out with twoidentical laser pulse trains, each at the average laserpower of 120µW . The time separation betweenpulses was tuned with a delay line.3. ResultsThe photoluminescence decay transients of the neut-ral and negatively charged excitons (X, CX)measureddirectly with a streak camera are presented in figure 2.The measurement reveals the short main compon-ent of the photoluminescence decay, followed by theweaker long-lived signal. The lifetimes of the shortcomponents are significantly smaller compared to theexfoliated samples presented in literature [34, 35]and in the supplementary information of this work.The typical photoluminescence lifetime in exfoliatedMoSe2 monolayers is between 2 ps and 10 ps for theneutral exciton and between 5 ps and 100 ps for thecharged exciton.To quantify the observed decay times, we fit thebiexponential decay:I(t) =Aτexp(− tτ)+Bτlongexp(− tτlong). (1)The fits were performed in convolution with theimpulse response function of the experimental setup,which was approximated with a Gaussian transientcorresponding to the experimentally obtained laserpulse profile.22D Mater. 11 (2024) 025029 K Oreszczuk et alFigure 1. (a) Normalized PL spectra of the MBE MoSe2 monolayer on hBN flake under the pulsed excitation with 610nm(2030meV), 120µW laser. The shaded regions span the spectral integration range for X and CX peaks during the time-resolvedmeasurements. (b) Scheme of the MoSe2 monolayer grown on Si/SiO2 substrate partially covered with hBN flakes. (c)Photograph of the example hBN flake covered with MoSe2 monolayer.Figure 2. Time-resolved PL of neutral (open symbols) and charged (closed symbols) exciton peaks obtained with the streakcamera, at T= 5K for epitaxial MoSe2 monolayer grown on hBN. Solid black lines: fit of the double-exponential decay of chargedexciton convoluted with impulse response function approximated by Gaussian. The impulse Response Function of theexperimental setup, as measured on the laser pulse, is presented as a grey area.The lifetime τ of the short-lived component of thePL signal is below the resolution of the streak camera,allowing only for its course estimations. We observethe lifetime of X resonance to be shorter than 1ps,and the lifetime of the CX resonance being in theorder of 1.3ps. Weak, long-lived components of thelifetime of 10–20 ps may be related to excitons loc-alized on charge fluctuations, nonuniform strain, ordefects. Such localizing centers may be more abund-ant in MBE-grown samples considering their richermicroscopic structure [15, 36]. The long componentis spectrally correlated with the neutral and chargeexciton photoluminescence signal and is character-ized by a common timescale for both excitonic peaks,which can be a fingerprint of a long-lived reservoirhigher in the relaxation ladder exposed in conditionsof non-resonant excitation.Quantitative investigation of processes in subpi-cosecond timescale requires an experimental methodcapable of higher temporal resolution than providedby a streak camera. Hence, we employ the ECS tech-nique, which provides a time resolution comparableto the width of the laser pulse. The result of theexample ECS measurement is presented in figure 3.The PL intensity from the MoSe2 is integrated overthe spectral ranges of X and CX resonances (See:figure 1(a) and probed with different time delaysbetween two laser pulse trains. Similarly, as in the caseof the StreakCamerameasurements, two-exponentialdecay of the signal is observed. The laser autocorrela-tion function of FWHM= 260 ps (corresponding tothe pulse duration of 180 ps) was used as an impulseresponse function during fitting. The characteristictimes of the fast component of the ECS signal are32D Mater. 11 (2024) 025029 K Oreszczuk et alFigure 3. CS signal of neutral and charged exciton peaks at the temperature of 5 K, measurd for epitaxial MoSe2 monolayer grownon hBN. PL intensity is normalized to the limit of infinite pulse separation. Inset: ECS signal acquired under two differentexcitation powers. The long-lived component was subtracted for clarity. Values of 60µW and 120µW relate to the power of theindividual pulse train.Figure 4. Comparison of (a) neutral and (b) charged exciton ECS signal decay times (solid markers) with their total PL intensity(open markers, slightly shifted for clarity) presented as a function of temperature. The vertical scale of PL Intensity wasnormalized on each panel to match the ECS signal decay times on the opposite vertical axis. Lines—fits of the Arrhenius equationwith activation energy parameters: Ea = 20meV (solid line), Ea = 15meV, Ea = 30meV (dashed lines).equal to 0.72ps for X resonance, and 1.77 ps for CXresonance (figure 3). These values are in quantitat-ive agreement with the direct measurements of thePL decay time. We observe little dependence on theexcitation power (inset in figure 3), which proves thatobserved ECS transients are directly related to theexciton lifetimes. No fingerprints of the carrier relax-ation between the optical excitation and PL emis-sion energies were observed. We estimate that suchrelaxation processes occur in timescales shorter than200 ps. See supplementary infomation, chapter 4 foran extended discussion of the role of delayed excitonformation.Measured excitonic lifetimes were similar forsamples grown in different conditions and regardlessof their degree of degradation in environmental con-ditions (supplementary information, figure 2). Thissuggests that the nature of the extraordinarily shortexcitonic lifetimes is directly related to the epitaxialgrowth process. Chalcogenide vacancies are the mostprevalent defect site in exfoliated samples [37]. Non-equilibrium epitaxial growth in high vacuum furtherincreases the prevalence of the selenium vacancies[36], which may be the dominant contributor to theobserved rapid decay.Having established a suitable tool that allows forprobing of the short excitonic lifetimes, we invest-igate the temperature’s effects on the recombinationdynamics. The increase of the temperature results infurther shortening of the decay time obtained forX and CX peak (figures 4(a) and (b)). The short-ening is followed by a proportional decrease of thetotal intensity of the luminescence signal. The long-lived component can be neglected, as it composesunder 10% of the total photoluminescence intensity(figure 2).The correlation between the lifetime andthe total intensity evidences strong thermally-activated non-radiative recombination channels. Wemodel the excitonic recombination rates with the42D Mater. 11 (2024) 025029 K Oreszczuk et allow-temperature component τ−10 and additional,thermally activated component described by theArrhenius equation:τ−1 = τ−10 +Aexp−EakBT. (2)We fit the Arrhenius equation to the experimentaldata in figures 4(a) and (b) and obtain the activa-tion energy of the non-radiative processes equal toEa = 20+10−5 meV. The same value of Ea was used forX and CX fits in figures 4(a) and (b).The microscopic structure of the monolayer ischaracterized by abundant gaps or bilayer fragments[15].With the relatively high point defect density [36]and the average distance to the monolayer edge in theballpark of tens of nanometers, the transport effectscan be relevant to the efficiency of the recombina-tion channels. The diffusion coefficient and the meanfree time are relatively high inmonolayerMoSe2 [38],allowing efficient transport over tens of nanomet-ers in subpicosecond timescale. The decrease of theexcitonic lifetime in elevated temperatures may resultfrom increased carrier mobility towards the mono-layer edges or areas of increased point defect density.In this interpretation, the activation energy of Ea =20+10−5 meV is related to the amplitude of the potentialfluctuations in the sample plane. The linewidths ofthe excitonic transitions are significantly smaller thanthe activation energy, which indicates a high degreeof exciton localization.Next, we investigate the recombination processesat low temperatures below the activation threshold.The low-temperature excitonic recombination rateτ−10 can be described as a sum of the radiative (τ−1γ )and non-radiative (τ−1NR ) terms:τ−10 = τ−1γ + τ−1NR . (3)The radiative decay rate is expected to dependstrongly on the thickness of the hBN bottom layerdue to the Purcell effect [35]. Assuming that thenon-radiative effects remain unchanged regardless ofthe photonic environment, it is possible to determ-ine whether radiative or non-radiative channelsare a dominant recombination mechanism at lowtemperatures.Figure 5(a) shows the reflectivity spectra of fourMoSe2 monolayers grown simultaneously on dif-ferent hBN flakes. The thickness of the flakes wasobtained from the reflectivity measurements in abroad VIS-NIR range. The spectra were fitted withthe dispersion-corrected refractive indexes of the hBNand SiO2 (90 nm) layers. Having determined thethicknesses of the hBN, we calculated the Purcellfactors for the excitonic emission. Two selected spotswere characterized by the low Purcell factor of 0.26and 0.30, while the other two exhibited high values of0.93 and 1.21. The Purcell factor Fp was calculated inlinewith equations (S35)–(S38) in the supplementaryinformation of the related work of Fang et al [35].The time-resolved profiles of the ECS signal wereacquired at each studied spot (figures 5(b)–(e)). Wefind that the dynamics of the ECS signal are not sig-nificantly dependent on the calculated Purcell factor.The calculated decay times of X and CX peaks wereequal to about 0.65 ps and 2.0 ps, respectively, similarto the results obtained on the other sample at the tem-perature of 5 K. Any variations between spots of dif-ferent Purcell factors were confined within the exper-imental uncertainty, which suggests a dominant roleof the non-radiative excitation channels even at lowtemperatures.The dependence of the decay times of the Xand CX peaks on the Purcell Factor is presented onfigure 6. Based on the acquired data we provide estim-ations on the radiative and non-radiative recombin-ation terms. Lower boundary estimation of the char-acteristic time of the non-radiative processes can beobtained under the assumption of negligible (but suf-ficient to observe the PL signal) relative efficiency ofthe radiative processes. We estimate the τXNR to begreater than 0.5 ps and τCXNR to be greater than 1.6 ps.In the alternate edge case of relatively strong radiativerecombination, we estimate the upper limit of the τXNRto be 1.1 ps and the upper limit of the τCXNR to be 2.7 ps.Presented estimations result in the radiative recom-bination times in all cases equal to 1.25 ps or greaterfor the neutral exciton and 8 ps or greater for thecharged exciton (at the Purcell factor of FP = 1). Theabove ranges result in upper limits of quantum effi-ciency of photoluminescence of 0.47 for X and 0.25for CX (at FP = 1).4. DiscussionWe show that MBE-grown MoSe2 gives stable andhigh optical quality photoluminescence that facilit-ates the application of advanced time-resolved spec-troscopy techniques. The observed excitonic lifetimesremain similar in different spots on the sample as wellas on two independently grown samples, proving uni-formity, reliability, and the repeatability of the growthprocess. The excellent performance of our samplesdraws closer to the perspective of substituting exfoli-ated flakes with an industrially scalable alternative ina broad range of potential applications, particularlyafter the use of epitaxial hBN substrate [24].We observe that the PL decay times of MBE-grown monolayers are significantly shorter than inthe typical exfoliated samples. Utilizing the ECS tech-nique, we identified two independent non-radiativerecombination channels responsible for the short-ening of the excitonic lifetimes. One in low tem-peratures was identified by probing monolayers inphotonic environments described by different PurcellFactors. The non-radiative recombination at low tem-peratures dominates over the radiative channel, and52D Mater. 11 (2024) 025029 K Oreszczuk et alFigure 5. (a) Solid lines: reflectivity spectra of the MoSe2 monolayer grown on the hBN flakes of varying thickness. Dashed lines:simulation result used for calculation of the gBN thickness. (b) Schematic representation of the sample structure (c) PredictedPurcell Factor at different thicknesses of the hBN spacer. (d)–(g) Time-resolved measurements of the ECS signal decay for X andCX on spots on the sample with low (d)–(e) and high (f)–(g) predicted values of the Purcell factor.Figure 6. Characteristic decay times of neutral (a) and charged (b) exciton on the spots with different Purcell factors. Gray lines:fits for low and high estimates of the characteristic time of the non-radiative recombination channels.can be attributed to the high density of seleniumvacancies resulting from the epitaxial growth pro-cess. It brings the neutral exciton lifetime from severalpicoseconds down to about 0.5–1.0 ps, pushing thequantum efficiency of emission below 50%. Anothernon-radiative process activates at elevated temper-atures, possibly due to the increased carrier mobil-ity, further halving the observed decay times. For62D Mater. 11 (2024) 025029 K Oreszczuk et almost materials, a short nonradiative time comparedto longer radiative lifetimes would typically result invery efficient luminescence quenching. However, inthe case of TMDs with fast radiative times, such asMoSe2, luminescence can still be measured and util-ized in ultra-fast optoelectronics, even in the presenceof rapid radiative recombination.Data availability statementThe data that support the findings of this study areopenly available at the following URL/DOI: https://doi.org/10.6084/m9.figshare.25093712.AcknowledgmentThis work was supported by National Science Centre,Poland, under Projects 2021/41/N/ST3/04240 and2021/41/B/ST3/04183. K W and T T acknowledgesupport from the JSPS KAKENHI (Grant Numbers21H05233 and 23H02052) and World PremierInternational Research Center Initiative (WPI),MEXT, Japan. The work has also been supportedby the EUGraphene Flagship project. The Polish par-ticipation in the EuropeanMagnetic Field Laboratory(EMFL) is supported by the DIR/WK/2018/07 grantfrom the Ministry of Science and Higher Education,Poland. We express our gratitude to Rafał Bożekfor measuring the AFM images presented in theSupplementary Information.Author contributionsK O, A R, T K, and M R conducted photolumines-cence and reflectivity measurements. A R analyzedthe reflectivity measurements, and K O analyzed allother experimental data. W P and K N prepared thesamples. T T and K W produced hBN used in thesamples. M P and P K conceived and supervised theproject. K O led the preparation of the manuscript,while other authors actively contributed.Conflict of interestThe authors declare no competing interests.ORCID iDsKacper Oreszczuk https://orcid.org/0000-0001-8830-0005Wojciech Pacuski https://orcid.org/0000-0001-8329-5278Aleksander Rodek https://orcid.org/0000-0002-0263-3122Mateusz Raczyński https://orcid.org/0000-0003-0443-1943Tomasz Kazimierczuk https://orcid.org/0000-0001-6545-4167Kenji Watanabe https://orcid.org/0000-0003-3701-8119Piotr Kossacki https://orcid.org/0000-0002-7558-1044References[1] You Y, Zhang X-X, Berkelbach T C, Hybertsen M S,Reichman D R and Heinz T F 2015 Observation ofbiexcitons in monolayer WSe2 Nat. Phys. 11 477[2] He K, Kumar N, Zhao L, Wang Z, Mak K F, Zhao H andShan J 2014 Tightly bound excitons in monolayer WSe2Phys. Rev. Lett. 113 026803[3] Ross J S et al 2013 Electrical control of neutral and chargedexcitons in a monolayer semiconductor Nat. 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Introduction 2. Methods 3. Results 4. Discussion References