Ultra-low power threshold for laser induced changes in optical properties of 2D Molybdenum dichalcogenides
Fabian Cadiz, Cedric Robert, Gang Wang, Wilson Kong, Xi Fan, Mark Blei, Delphine Lagarde, Maxime Gay, Marco Manca, Takashi Taniguchi, Kenji Watanabe, Thierry Amand, Xavier Marie, Pierre Renucci, Sefaattin Tongay, Bernhard Urbaszek
UUltra-low power threshold for laser induced changes in opticalproperties of 2D Molybdenum dichalcogenides
Fabian Cadiz , Cedric Robert , Gang Wang , Wilson Kong , Xi Fan , Mark Blei , DelphineLagarde , Maxime Gay , Marco Manca , Takashi Taniguchi , Kenji Watanabe , ThierryAmand , Xavier Marie , Pierre Renucci , Sefaattin Tongay , and Bernhard Urbaszek Universit´e de Toulouse, INSA-CNRS-UPS,LPCNO, 135 Av. Rangueil, 31077 Toulouse, France School for Engineering of Matter, Transport and Energy,Arizona State University, Tempe, Arizona 85287, USA Advanced Materials Laboratory, National Institute forMaterials Science, Tsukuba, Ibaraki 305-0044, Japan
Abstract
The optical response of traditional semiconductors depends on the laser excitation power usedin experiments. For two-dimensional (2D) semiconductors, laser excitation effects are anticipatedto be vastly different due to complexity added by their ultimate thinness, high surface to volumeratio, and laser-membrane interaction effects. We show in this article that under laser excitationthe optical properties of 2D materials undergo irreversible changes. Most surprisingly these effectstake place even at low steady state excitation, which is commonly thought to be non-intrusive. Inlow temperature photoluminescence (PL) we show for monolayer (ML) MoSe samples grown bydifferent techniques that laser treatment increases significantly the trion (i.e. charged exciton)contribution to the emission compared to the neutral exciton emission. Comparison betweensamples exfoliated onto different substrates shows that laser induced doping is more efficient forML MoSe on SiO /Si compared to h-BN and gold. For ML MoS we show that exposure tolaser radiation with an average power in the µ W/ µ m range does not just increase the trion-to-exciton PL emission ratio, but may result in the irreversible disappearance of the neutral excitonPL emission and a shift of the main PL peak to lower energy. a r X i v : . [ c ond - m a t . m t r l - s c i ] J un IG. 1:
MoSe monolayer samples (a) Schematic of chemical vapor phase transport (VPT)set-up used for MoSe bulk sample growth (b) Optical microscope image of monolayer MoSe (reddotted outline) exfoliated from VPT grown bulk onto SiO /Si. (c) Raman spectra for MoSe Tidoped (top panel) and not intentionally doped (bottom panel) confirm monolayer thickness bycomparison with [21] (d) Atomic force microscopy show monolayer step-height.
Introduction.—
Atomically thin layers of Van der Waals bonded materials open up newpossibilities for physics and chemistry on the nanoscale and for new applications in electron-ics and photonics [1–7]. Here the group-VI Transition metal dichalcogenides (TMDs) of theform MX , where M= M o, W and X=
S, Se , are of particular interest: These indirect semi-conductors in bulk form become direct semiconductors when thinned down to one monolayer(ML), which makes them especially attractive for optoelectronics [8–12].The optical properties of TMD MLs are most commonly probed in optical spectroscopy:A laser of suitable energy will create an electron-hole pair (exciton) and subsequently thephotoluminescence (PL) emission is monitored. This simple technique led to the discoveryof ML MoS having a direct bandgap [13–15]. The optical properties are governed by neutraland charged excitons (trions). There are different physical origins of the resident carriersprobed in optical spectroscopy: intrinsic dopants, molecules on the ML surface, carrierstrapped at the ML-substrate interface [16–20]. Here we show that the excitation laser itselfcan have profound impact on the optical properties, in particular on the doping and emissionfrom localized states.We show that this a general phenomenon for MoX MLs by systematically comparingMoSe ML exfoliated from ultra-pure vapor phase transport (VPT) grown samples with2ommercial samples. In ML MoSe the sharp exciton emission lines at cryogenic temper-atures serve as a sensitive probe of the trion-to-neutral exciton PL emission ratio, whichindicates presence/absence of additional carriers. As the Ti-doped sample exfoliated fromVPT grown bulk is presumably p-type (confirmed in Hall measurements on bulk) an in-crease of the trion contribution is consistent with adding holes. We then show that in thesesMoSe sample systems the effect of laser doping is clearest on SiO substrates and muchweaker in samples exfoliated onto h-BN and Au. Laser induced doping is also studied in MLMoS . Here exposure to pulsed lasers leads to irreversible changes in the emission spectrum:the neutral exciton signature is lost in PL and the main PL emission is shifted in energybelow the initial trion emission. This laser engineering of optical properties for ML MoS isdemonstrated at T=4 K and at room temperature. MoSe and MoS monolayers exfoliated from different bulk samples.— Chemical VPT was used to grow MoSe and Ti-doped MoSe crystals. Molybdenum(99.9999%), Selenium shots (99.9999%) and I , which acted as a transport agent, were sealedin a quartz tube at 5 × − Torr vacuum. The precursors (hot end) were kept at 1085 ◦ C ina 3-zone horizontal furnace while maintaining a 55 ◦ C temperature difference on the cold end( ≈ ◦ C) to initiate nucleation and growth [22], see schematic in Fig. 1a. To incorporateTi into MoSe , the temperature difference between the two ends of the tube varied from55 to 65 ◦ C. For comparison bulk MoSe crystals were purchased from 2D semiconductors.The MoS bulk was also supplied by 2D semiconductors. Using a dry-stamping technique[23] MLs from different bulk sources were deposited on either SiO /Si, h-BN [24] or goldsubstrates. Monolayer thickness was confirmed by several techniques: in optical contrastmeasurements (Fig. 1b), in Raman spectroscopy [21] (Fig. 1c) and atomic force microscopy(Fig. 1d). The thickness of MoSe MLs measures ∼ g ) peak of MoSe softens from bulk (blue dashed line) to ML (red dashed line) as shownin Fig. 1c due to much reduced restoring forces acting on individual ML sheets. Optical spectroscopy techniques.—
A purpose-build micro-PL set-up is used torecord the PL spectra in the temperature range T = 4 −
300 K [25]. The sample is placedon 3-axis stepper motors to control the sample position with nm precision inside the low-vibration closed cycle He cryostat. MLs were excited with a linearly polarized cw laser (532nm or 633 nm wavelength) or with 1 . IG. 2:
Low temperature PL of monolayer MoSe (a) PL spectrum of MoSe ML (exfoliatedfrom a VPT-grown bulk) at T=8 K for a 40 nW cw excitation (633 nm) before and after beingexposed to a power of 40 µ W at 633 nm during 4 minutes. After exposure, the trion intensityincreases. The inset shows that the trion’s dissociation energy increases as a function of the T/Xratio, which is a signature of laser-induced doping of the ML. (b) Same as (a) for a Ti-dopedMoSe ML (exfoliated from a VPT-grown bulk). (c) Same as (a) for MoSe ML exfoliated froma commercial (2D Semiconductors) bulk crystal. All samples were mechanically exfoliated on 90nm-thick SiO layer on top of a Si substrate. excitation spot diameter is diffraction limited ≤ µ m, i.e considerably smaller than the MLsize of typically ∼ µ m × µ m. The PL emission is dispersed in a spectrometer (f=50 cm)and detected with a liquid nitrogen cooled Si-CCD back-illuminated deep-depletion camera. Effect of Laser radiation on optical properties of MoSe MLs .— Changes ofthe optical emission of ML TMDs as a function of laser excitation power have been reportedmany times in the literature. Here two different scenarios have to be distinguished. On theone hand, creating more carriers (excitons) will induce eventually interactions between freeand localized excitons, trions and resident carriers and possibly result in biexciton formation[27–35]. On the other hand, laser excitation can physically induce non-reversible changesof the ML sample and therefore its optical emission [16, 36–41]. For intermediate laserpower ranges all these physical processes might occur simultaneously, which might explainthe wide range of values reported for the neutral and charged exciton PL emission energyin the literature especially for MoS and WS .4 IG. 3:
Time evolution and substrate effects for laser treatment of ML MoSe (a) Time-evolution of the PL spectrum at T = 8 K of a VPT-grown MoSe ML exfoliated onto a SiO /Sisubstrate while being exposed to a cw excitation (532 nm) at 230 µ W. (b) T and X integratedintensity as a function of time for the conditions of panel (a), revealing doping of the ML on atimescale of several minutes. (c)Ti-doped VPT-grown MoSe ML exfoliated onto few-layer h-BN.Time-evolution of the PL spectrum at T = 4 K while being exposed to a cw excitation (633 nm)at 40 µ W during 4 minutes. Only a very slight increase of the T emission is observed. (d) Time-evolution of the PL spectrum at T = 4 K of a commercial MoSe ML exfoliated onto a gold layer(different colours for different times as in panel (a)). The ML is exposed to a cw excitation (633nm) at 50 µ W during 10 minutes. No significant change of the spectrum is observed.
Our target is to distinguish between these two scenarios. A simple approach is toperform power dependent measurements several times to verify if the laser radiation in-duced irreversible changes to the spectra. For this we start each experiment with pristine,as-exfoliated flakes that we probe at very low laser power (nW/ µ m ). We start with lowtemperature measurements at T=10 K in vacuum (10 − mbar) on MoSe samples grownunder controlled conditions and exfoliated onto different substrates for comparison.5anel (a) of Fig.2 shows the PL spectrum using an extremely low excitation power of40 nW (black curve) of ML MoSe exfoliated onto SiO /Si from a VPT grown bulk sam-ple. The PL spectrum of the pristine sample is dominated by strong and spectrally narrow(FWHM ≈ ML samples [42–44]. The PL FWHM for both transitions of just a few meVare among the best reported in the literature and confirm the excellent sample quality of thepristine flakes. After this initial low power measurement, the laser excitation power at thissample position is increased to 40 µ W and kept constant during 4 minutes (no measurableevolution is detected for longer times). Directly afterwards, the laser power is lowered to40 nW, to compare with the black curve before laser treatment, and the PL response ismeasured (red curve). Remarkably, the trion-to-neutral exciton PL emission intensity ratio
T /X has significantly changed, indicating strong doping as a result of the laser treatment.Shown in the inset is the trion’s dissociation energy, defined as the difference between theemission energy of the neutral exciton ( E X ) and the trion ( E T ), as a function of the T /X
PL intensity ratio. In a simple picture, the trion’s dissociation energy can be written as [45]: E X − E T = E F + E BT where E TB is the trion binding energy (typically ∼
25 meV) and E F is the Fermi level withrespect to the bottom of the conduction band for electrons, with respect to the top of thevalence band for holes, respectively. The observed linear increase of E X − E T when T /X increases is a signature of an increase of the Fermi level, i.e., of the doping of the ML. Panels(b) and (c) of Fig.2 show the laser-induced doping of Ti-doped VPT grown MoSe MLs andcommercial MoSe MLs, respectively. For the different sample sets compared in Fig. 1a,band c, exposure to a 40 µ W excitation during 4 minutes produces an irreversible change inthe
T /X spectral weight. We have detected irreversible changes of T/X in vacuum even forpowers as low as 1 µ W/ µ m . Please note that in many optical spectroscopy measurementsof TMDs MLs the excitation densities are much larger than the ones used in this study.Although the doping due to the laser treatment is clearly visible in all panels in Fig. 2,the nature of the doping (n- or p-type) still needs to be determined. Here the results on theTi-doped VPT sample give helpful indications in Fig. 2b: Hall conductivity measurementson the bulk sample before exfoliation indicate p-doping - we therefore assume that the trion6 IG. 4: cw Laser treatment of ML MoS (a) commercial MoS ML exfoliated onto a SiO /Sisubstrate. Time evolution of the PL spectrum while being exposed to a cw excitation 532 nm at60 µ W, revealing X, T and localized exciton emission features which evolve over several minutes.(b) T, X and localized emission integrated intensity as a function of time for the conditions ofpanel (a). Both the X and the localized exciton emission decrease as a function of time, whilethe T gains in intensity. (c) Trion’s dissociation energy as a function of the T/X ratio for a MoS ML. The different values of
T /X have been obtained by exposing the ML to different laser powersat different positions on the same ML flake. (d) PL spectrum at 0 . µ W cw excitation (532 nm)measured after laser treatment of a MoS ML, revealing high T emission (black curve). Also shownis the differential reflectivity spectrum (red curve) in which the main absorption is dominated bythe X resonance at 1 .
96 eV, the additional transition at ≈ B ) [13]. in the pristine sample when deposited on SiO is positively charged. The T/X ratio increasesgradually as the laser power is increased, which would be consistent with extra holes beingcreated by the laser treatment. Dynamics of Laser treatment in ML MoSe .— The dynamics of this laser dopingis shown in panel (a) of Fig. 3 in real-time for a commercial MoSe ML exposed to a cwexcitation at 230 µ W. The evolution of the X and T integrated PL intensity as a function of7ime is shown in panel (b), where it can be seen that the X intensity presents a first rapiddecrease (within seconds) followed by a second slower decrease in a timescale of several min-utes. The trion evolution, in contrast, is marked by an increase on similar timescales. Lasertreatment for longer than 4 minutes did not result in any further, measurable evolution ofthe PL spectrum. The total PL intensity (Trion + exciton) is decreasing in Fig. 3b as morecarriers are added, which might be due to the complex interplay between optically brightand dark state of the trion and exciton [46–48].
Results for ML MoSe on different substrates .— In order to check the applica-bility of this laser-induced doping of MoSe MLs for different device geometries, we alsoexfoliated MoSe MLs onto a few-layer h-BN film [24] and also a 50 nm- thick gold film.When deposited onto h-BN, only a very small increase of the T emission was observed after4 minutes exposure to a cw laser at 40 µ W, as shown in panel (c) of Fig. 3. When depositedon top of a gold layer, no effect of the laser treatment was observed even after 10 minutesof exposure to a cw laser at 40 µ W. In our experiments the MLs exfoliated onto SiO /Sishowed by far the strongest impact of laser radiation on the optical emission properties. Treatment of ML MoS with cw Lasers .— The drastic changes of the emissionproperties of MoSe MLs as a function of laser power raise the question if similar effects canbe observed in other materials. Below we show our systematic study on ML MoS whichconfirms the strong impact of the excitation laser on the local doping in the layer. Panel (a)of Fig. 4 shows the time-evolution of the PL signal from MoS MLs at T=8 K when exposedto a cw excitation at 60 µ W at 532 nm. Three distinct PL emission peaks are observed,associated to the X (1 .
96 eV),T (1 .
93 eV), and spectrally broad localized exciton emission[49], which peaks approximately at 1 .
85 eV. Please note that under laser illumination, boththe X and the localized emission decrease, whereas the trion increases (panel (b) of Fig.4).Again, the dynamics is characterized by a fast and a slow component, of several seconds andseveral minutes, respectively. Also in this case the total PL intensity decrease as for MLMoSe in Fig. 3b.By exposing different regions of the MoS ML to different laser powers, it is possibleto control the
T /X ratio, and therefore, the doping of the ML. This is demonstrated inpanel (c) of Fig.4, in which increasing the laser power between 10 µ W and 200 µ W allows totune the
T /X ratio by almost one order of magnitude. In these experiments we have useddifferent laser power exposure on different sample spots to control the T/X ratio locally. As8
IG. 5: pulsed Laser treatment of ML MoS (a) PL spectrum of a MoS ML at T = 10 K asa function of average power for a pulsed (1 . average power/ µ m .(b) Same as (a) but at T= 300 K in vacuum. (c) PL spectrum at T = 10 Kand 0 . µ W excitation before (black curve) and after (red curve) exposure at 24 . µ W for severalminutes. (d) same as (c) but at T = 300 K in vacuum. The hysteresis after laser exposure is alsovisible, with a redshift of the spectrum by ∼
40 meV when the PL is presumably dominated bythe trion. shown for MoSe MLs in Fig. 2a, an increase of the trion’s spectral weight is accompaniedby an increase of the trion’s dissociation energy. In panel (d) of Fig.4, the PL of a MoS ML after laser treatment probed subsequently with 0 . µ W excitation reveals high dopingas inferred from the strong T emission (black curve). By illuminating the same region ofthe ML with an halogen lamp focused into a spot of ∼ µ m, the differential reflectivity ofthis doped region is obtained. The spectrum plotted in Fig. 4d (red curve) indicates thatthe X transition is still the dominant absorption mechanism and that no significant shift ofthe X transition is observed with respect to undoped regions. This suggests that no signifi-9ant band-gap renormalization is induced by the laser treatment. The same conclusion canbe drawn for the MoSe MLs studied, as the X peak PL energy (the optical band gap) inFig. 2a,b and c does not change after the laser treatment.
Treatment of ML MoS with pulsed Lasers .— Finally, we demonstrate that afurther increase of the laser excitation power completely quenches the X emission in MoS MLs, as is shown in panel (a) of Fig. 5. For these more extreme experiments, a pulsed exci-tation with 80 MHz repetition frequency at 400 nm was used, in contrast to all experimentsshown so far carried out with weak cw sources that can be found for example in commer-cial PL/Raman systems. At the lowest average power used of 0.1 µ W/ µ m (peak powerof 600 µ W/ µ m ), the PL spectrum presents X,T, and localized exciton emission. Increasingthe average excitation power up to ∼ µ W/ µ m results in a complete disappearance of theX line and a dramatic reduction of the localized exciton emission. Only a broad peak about20 meV below the T energy is observed, accompanied by a small shoulder at lower energies.The 2 PL spectra of ML MoS before and after laser treatment are totally different. Whendecreasing the average excitation power back to 0.1 µ W/ µ m , a dramatic hysteresis of thePL spectrum is observed, as illustrated by panel (c) of Fig.5, similar to experiments usingelevated laser power in ML WS [29, 37]. Please note that in many studies, the broad peakat ∼ . , and that valley-polarization experiments have been often performed with HeNe laser excitation at 1.96 eV,which corresponds to a perfectly-resonant excitation of the neutral exciton transition. Thesefindings demonstrate that for pulsed excitations, a time-averaged power of only a few µ Wis enough to change the MoS MLs optical properties in a non-reversible way. This hasto be taken into account when analysing the complex physics probed in time-resolved PLmeasurements [26, 50], pump-probe [51–54] and Kerr rotations experiments [55, 56], whichare often carried out in this excitation power regime.Many experiments on MoS with pulsed or cw excitation are carried out at room temper-ature [57]. We demonstrate the effect of the excitation laser on the optical spectrum also atT=300 K in vacuum conditions, as shown in panel (b) of Fig.5. A clear redshift of the PLpeak position is observed and also a hysteresis of the PL at the lowest power used is evident(panel (d) of Fig.5). Discussion. — There are several physical processes that can contribute to the modifi-cation of the Trion-to-neutral exciton PL emission ratio due to laser treatment. Behind this10ie the different physical origins of excess carriers coming from doping of the TMD material,charges trapped at the ML-substrate interface and molecules on the ML surface.One possibility is that the laser induced changes are purely electronic i.e. due to opticalionization of impurities. These effects can have lifetimes from fractions of second to days[58]. These physical processes were initially uncovered by Staebler and Wronski in hydro-genated amorphous SiO [59]. One possibility is that the additional charges are opticallycreated from defects in SiO and are subsequently transferred to the TMD ML. In addition,charges trapped at defects could be optically activated in the TMD ML itself, as suggestedby photoconductivity measurements [60]. A charge transfer from SiO to the TMD mono-layer could be suppressed by insertion of an h-BN layer [61], which might explain the absenceof optical doping for this particular sample in Fig. 3c.Another possibility is that local heating results in defect formation/modification, as sug-gested by our substrate dependent studies. In Fig 4 we see that the laser treatment doesnot modify the emission for ML MoSe exfoliated onto h-BN or gold. Their thermal con-ductivities are much larger than that of SiO , as κ (SiO2-amorphous) ≈ − κ (h-BN) ≈
300 W/m.K [62] and κ (Au) ≈
300 W/m.K [63]. This may indicate that thelaser-induced doping of TMD MLs is thermally driven. In this case, the contact betweenthe ML and a good thermal conductor may facilitate heat dissipation and as a consequenceprevent the laser-induced doping of the ML. As the neutral exciton transition energy didnot shift measurably for ML MoSe (Fig. 3a) and ML MoS (Fig. 4a) during laser treatmentat T=4 K, strong heating effects are not detected in our experiments. Thermal conductivityis not the only difference between the substrate materials. For example, the atomic flatnessof h-BN can also influence charge trapping processes and defect creation/propagation in theML [64, 65].Although the exact microscopic mechanisms still need to be understood, there are severalimportant practical implications coming from our experiments on the modification of theoptical properties of 2D materials following exposure to laser radiation. For thorough opti-cal studies MoX ML samples should be investigated at very low laser power i.e. nW/ µ m .When investigating power dependence, hysteresis effects are very likely to occur if the max-imum laser power used is too high. Our laser treatment technique could be used to locallypattern doped regions in the 2D crystal, possibly by using patterned substrates (SiO /Siversus h-BN). Our work shows that the neutral exciton emission of ML MoS exfoliated on11iO is at 1.96 eV, i.e. excitation with a HeNe laser is resonant with the neutral excitontransition. This will results in sharp, intense Stokes-lines from resonant Raman scatter-ing [66, 67] superimposed on the PL signal. The interplay between laser treatment andsuper-acid treatment might help in the future to identify how these two techniques influencedifferent type of defects [49, 57]. Experiments probing pristine samples that need high laserpower to generate enough signal will be difficult to compare with low power measurementssuch as white light reflectivity, as the optical properties will be modified. Acknowledgements. — We thank ANR MoS2ValleyControl, ERC Grant No. 306719and ITN SpinNANO for financial support. X.M. also acknowledges the Institut Universitairede France. F.C. and P.R. thank the grant NEXT No ANR-10-LABX-0037 in the frameworkof the Programme des Investissements d’Avenir”. K.W. and T.T. acknowledge supportfrom the Elemental Strategy Initiative conducted by the MEXT, Japan and a Grant-in-Aidfor Scientific Research on Innovative Areas ”Science of Atomic Layers” from JSPS. S.T.acknowledges support from National Science Foundation (DMR-1552220) and thanks INSAToulouse for a visiting Professorship grant. [1] K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, andA. K. Geim, Proc. Natl Acad. Sci. USA , 10451 (2005).[2] A. K. Geim and I. V. Grigorieva, Nature , 419 (2013).[3] O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, and A. Kis, Nature Nanotechnology , 497501 (2013).[4] M. Chhowalla, H. S. Shin, G. Eda, L.-J. Li, K. P. Loh, and H. Zhang, Nature chemistry ,263 (2013).[5] H. Li, J. Wu, Z. Yin, and H. Zhang, Accounts of chemical research , 1067 (2014).[6] X. Yu, M. S. Pr´evot, N. Guijarro, and K. Sivula, Nature communications , 7596 (2015).[7] A. Castellanos-Gomez, Nature Photonics , 202 (2016).[8] A. Pospischil, M. M. Furchi, and T. Mueller, Nature nanotechnology , 257 (2014).[9] K. F. Mak and J. Shan, Nature Photonics , 216 (2016).[10] R. S. Sundaram, M. Engel, A. Lombardo, R. Krupke, A. C. Ferrari, P. Avouris, and M. Steiner,Nano Letters , 1416 (2013).
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