Layer Control of WSe2 via Selective SurfaceLayer OxidationZhen Li,† Sisi Yang,‡ Rohan Dhall,† Ewa Kosmowska,§ Haotian Shi,∥ Ioannis Chatzakis,†

and Stephen B. Cronin*,†,‡,∥

Departments of †Electrical Engineering, ‡Physics and Astronomy, and ∥Chemistry, University of Southern California, Los Angeles,California 90089, United States§XEI Scientific, Redwood City, California 94063, United States

*S Supporting Information

ABSTRACT: We report Raman and photoluminescencespectra of mono- and few-layer WSe2 and MoSe2 takenbefore and after exposure to a remote oxygen plasma. Forbilayer and trilayer WSe2, we observe an increase in thephotoluminescence intensity and a blue shift of thephotoluminescence peak positions after oxygen plasmatreatment. The photoluminescence spectra of trilayer WSe2exhibit features of a bilayer after oxygen plasma treatment.Bilayer WSe2 exhibits features of a monolayer, and thephotoluminescence of monolayer WSe2 is completelyabsent after the oxygen plasma treatment. These changes are observed consistently in more than 20 flakes. Themechanism of the changes observed in the photoluminescence spectra of WSe2 is due to the selective oxidation of thetopmost layer. As a result, N-layer WSe2 is reduced to N−1 layers. Raman spectra and AFM images taken from the WSe2flakes before and after the oxygen treatment corroborate these findings. Because of the low kinetic energy of the oxygenradicals in the remote oxygen plasma, the oxidation is self-limiting. By varying the process duration from 1 to 10 min, weconfirmed that the oxidation will only affect the topmost layer of the WSe2 flakes. X-ray photoelectron spectroscopy showsthat the surface layer WOx of the sample can be removed by a quick dip in KOH solution. Therefore, this techniqueprovides a promising way of controlling the thickness of WSe2 layer by layer.

KEYWORDS: dichalcogenide, WSe2, MoSe2, photoluminescence, oxygen plasma, oxidation

Two-dimensional transition metal dichalcogenides(TMDCs), such as MoS2, WSe2, and MoSe2, havedemonstrated great optical properties and the potential

for future applications in electronic and optoelectronic devices.Various devices based on TMDCs have been demonstrated,including field-effect transistors (FETs),1 photodetectors,2,3 andlight-emitting diodes (LEDs).4−6 One of the unique opticalproperties of these TMDCs is the indirect-to-direct band gaptransition that occurs when the thickness of these materials ischanged from bulk to a monolayer flake. Monolayer TMDCsare direct band gap semiconductors exhibiting strong photo-luminescence (PL), and few-layer TMDCs are indirectsemiconductors, in which the luminescent properties arestrongly suppressed.7−9 Because of the two-dimensional natureand high surface-to-volume ratio of TMDCs, surface states andinteraction with the substrate are particularly important in thesetwo-dimensional material systems. Previously, various passiva-tion methods have been used to improve both the carriermobility and luminescence efficiency of TMDCs, includingatomic layer deposition (ALD),1 hexagonal boron nitrideencapsulation,10−13 and immersion in ionic liquid solutions.14,15

Yamamoto et al. studied the selective oxidation of WSe2 afterexposure to O3 at 100 °C for a few hours.16 While theformation of a uniform surface oxide is advantageous byproviding protection from environmental effects and providinga good seed layer for ALD of high-k dielectrics, this previousozone treatment also resulted in a decrease in the PL intensity,indicating damage to the underlying layers of the material.Low-power, remote oxygen plasmas are known to remove

hydrocarbons from delicate sample surfaces that are susceptibleto damage by the highly energetic ions in the plasma.17,18 Inour previous work,19 we reported oxygen-plasma-induced layerdecoupling of few-layer MoS2 and an associated enhancementin the PL intensity. In the work presented here, we investigatethe effect of this gentle oxygen plasma treatment on transitionmetal selenides, WSe2 and MoSe2. Unlike the sulfide, ahomogeneous single-layer oxidation is grown on the surfaceof the WSe2 flakes after the treatment. Compared to O3

Received: April 12, 2016Accepted: July 8, 2016

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exposure,16 the remote oxygen plasma treatment presents aneasier and more controllable approach, which causes lessdamage to the WSe2 flakes. In total, we compare 27 WSe2 flakesand 15 MoSe2 flakes before and after the treatment, withdifferent plasma exposure times ranging from 1 to 10 min. ThePL and Raman spectra show very consistently that only the firstlayer on the surface of the WSe2 flake is oxidized during theplasma treatment. The oxygen ions do not penetrate furtherinto the flake and oxidize the underlying layers, even withlonger exposure times. The PL intensity of WSe2 and MoSe2after the treatment is increased by up to 6-fold, furtherindicating that the underlying layers of WSe2 are not affectedsignificantly by oxygen plasma. To confirm the oxidation of theWSe2 samples, we performed X-ray photoelectron spectroscopy(XPS) of WSe2 before and after the oxygen plasma treatment.In this experiment, we start by exfoliating WSe2 and MoSe2

flakes onto Si substrates with 300 nm oxide. Based on thecontrast observed in an optical microscope, we select differentWSe2 samples ranging from one monolayer to a few layers inthickness. Raman and photoluminescence spectroscopy areused to confirm the layer number of a given flake.20−22 Afterfinishing the characterization of the pristine samples, weperformed oxygen plasma treatment with an Evactron plasmadecontaminator designed by XEI Scientific, Inc. In typicalplasma cleaners (i.e., ashers), the sample is bombarded withions carrying significant kinetic energy, resulting in a“sputtering” mechanism that is detrimental to these delicate2D-layered materials. Remote plasma cleaners, on the otherhand, rely mainly on the chemical reactivity of the oxygenradicals. Here, the sample is placed 10 cm away from theplasma. By the time the oxygen radicals diffuse to the sample,they have lost most of their kinetic energy but retain theirchemical reactivity.Figure 1a shows optical microscope images of bilayer and

trilayer WSe2 flakes deposited on the Si/SiO2 substrate takenbefore and after the oxygen plasma treatment. As previousstudies have shown,21,23 the bulk 1B2g Raman mode at 310cm−1 does not exist in monolayer WSe2 and is only observed inmultilayer and bulk WSe2. The Raman spectra of bilayer WSe2taken before and after oxygen plasma treatment (plotted inFigure 1b) show that this Raman mode disappears after thetreatment. In addition, monolayer WSe2 only exhibits a singlePL peak around 1.63 eV, while bilayer WSe2 has two PL peaksat lower energies, 1.53 and 1.61 eV.20 Here, both the Raman(Figure 1b) and PL spectra (Figure 1c) before and after oxygenplasma indicate that the bilayer WSe2 flake is converted intomonolayer WSe2, and the flake is changed from an indirect to adirect band gap semiconductor. As a result, there is a 5-foldincrease in the PL intensity. Besides surface oxidation, there area few other mechanisms that can cause a bilayer-to-monolayertransition and the disappearance of the bulk 1B2g Raman mode.For example, interlayer decoupling can be induced by oxygenintercalation, as we observed in few-layer MoS2.

19 In order toconfirm that oxidation occurs in WSe2, we further investigatePL, Raman, and XPS spectra of monolayer, trilayer, and few-layer WSe2 flakes before and after oxygen plasma treatment.Figure 2a,b shows the Raman spectra of monolayer and

trilayer WSe2 flakes before and after the oxygen treatment. Formonolayer WSe2, there is no

1B2g Raman mode at 310 cm−1. Ifthe oxygen plasma introduces oxygen intercalation between theWSe2 layers, the trilayer WSe2 would also behave like amonolayer after the plasma treatment. However, the 310 cm−1

peak remains in the Raman spectra of trilayer WSe2 after the

treatment. In the trilayer WSe2, there are also two main PLpeaks. The higher-energy peak remains at a position similar tothat of bilayer WSe2 (1.61 eV), while the lower-energy peak isobserved at 1.47 eV before oxygen plasma treatment.20 Afterthe treatment, the PL peaks shift to 1.51 and 1.61 eV,corresponding to bilayer emission, as shown in Figure 2d. Formonolayer WSe2, on the other hand, no Raman or PL signalcan be observed (Figure 2c) after the oxygen plasma treatment.The absence of Raman peaks indicates that the monolayer hasbeen mostly oxidized.As mentioned above, in typical plasma cleaners used in

semiconductor processing, the sample is bombarded with ionsthat may cause physical damage and thinning effects. To verifythat the WSe2 monolayer is not “etched” away but is insteadoxidized after the remote oxygen plasma, atomic forcemicroscope (AFM) images are taken from WSe2 flakes beforeand after treatment, as shown in Figure 3. From the AFMimages of monolayer and bilayer pristine and oxidized WSe2flakes, we found that the thickness of WSe2 flakes remains thesame after the treatment. Although both Raman and PL spectraidentified the WSe2 sample in Figure 3a as monolayer beforetreatment, the thickness measured from the AFM image is

Figure 1. (a) Optical images of bilayer and trilayer WSe2 flakesbefore and after the oxygen plasma treatment. The scale bar is 5μm. (b) Raman spectra of the bilayer WSe2 flake taken before andafter the oxygen plasma treatment. The inset shows the bulk 1B2gRaman mode of WSe2 at 310 cm

−1, indicating that the bilayer WSe2flake is converted into a monolayer after the oxygen plasmatreatment. (c) Photoluminescence spectra of WSe2 before and afterthe oxygen plasma treatment, also indicating a bilayer-to-monolayer transition.

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thicker than theoretical thickness of a monolayer WSe2, 0.7 nm.This may be caused by the differences in gradients of theattractive forces and lateral forces on the material and the

substrate, which has been reported on both graphene and MoS2before.24−27 The fact that there is no observable decrease in thethickness of monolayer and bilayer WSe2 samples before and

Figure 2. Raman spectra of the (a) monolayer and (b) trilayer WSe2 flakes taken before and after the oxygen plasma treatment. Inset: Ramanmode of WSe2 at 310 cm−1, which is not exhibited in monolayer WSe2. Photoluminescence spectra of (c) monolayer and (d) trilayer WSe2before and after the oxygen plasma treatment. The two PL peaks of trilayer WSe2 at 1.47 and 1.61 eV shift to 1.53 and 1.61 eV, correspondingto bilayer WSe2. Monolayer WSe2 is oxidized completely, and no PL signal is observed after the plasma treatment.

Figure 3. Optical and AFM images of (a) monolayer and (c) bilayer before and after the oxygen plasma treatment. The scale bar in the AFMimages is 1 μm. Height profiles of WSe2 (b) monolayer and (d) bilayer flakes before and after the treatment, along the white dashed lines in(a) and (c), respectively, show that the process does not change the thickness of the flake by a significant amount.

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after treatment suggests that the topmost WSe2 layer is notremoved due to physical bombardment. In addition, the AFMimages show that the oxide film is uniform and does not changethe surface topology (i.e., roughness) of the material ascompared to other dielectric layers deposited, for example, byALD.28

RESULTS AND DISCUSSIONAmong the 27 WSe2 flakes that were compared before and afterthe oxygen plasma treatment, 11 out of 11 WSe2 bilayer flakesbecame monolayer according to Raman and PL results. For all11 of these flakes, the 310 cm−1 Raman peak disappeared, andthe PL spectra changed from two peaks to one peak centeredaround 1.63 eV. Seven out of seven trilayer WSe2 flakes wereconverted to bilayers, and five out of six monolayer WSe2 flakeswere completely oxidized, exhibiting no PL signal after plasmatreatment. The oxygen plasma treatment had very little effecton the three remaining thicker WSe2 flakes (more than fivelayers) because the properties of N-layer and N−1-layer WSe2are very similar when N is a large number. Figure 4a−d shows

histograms of the PL intensity and peak position of bilayerWSe2 before and after oxygen plasma treatment. The PLspectra of bilayer WSe2 flakes can be fitted with two peaks, oneat 1.53 eV and the other at 1.61 eV. We used the lower-energyPL peak to compare with the “monolayer” PL peak aftertreatment. Due to the indirect-to-direct band gap transition, thePL peak is blue-shifted and the PL intensity is increased. Asimilar analysis for trilayer WSe2 samples can be found inFigure S4 in the Supporting Information. An average blue shiftof 40 meV of the lower-energy PL peak is observed in thetrilayer WSe2 samples after the oxygen plasma treatment,consistent with bilayer emission. These statistics show veryreliably that bilayer WSe2 flakes are reduced to monolayers, andtrilayer WSe2 flakes become bilayers. Figure 4e shows thenormalized PL spectra of bilayer WSe2 samples before and afterthe oxygen plasma treatment. For all samples, the PL peak afterthe oxygen plasma treatment is blue-shifted to 1.63 eVconsistently, and the PL intensity is increased for all samplesexcept one. To examine the time dependence of the oxygenplasma treatment, we varied the duration of the plasma from 1

Figure 4. Histograms of bilayer WSe2 PL peak position (the lower-energy peak) (a) before and (b) after the oxygen plasma treatment. (c)Shift in PL peak position before and after treatment. (d) PL intensity increase due the indirect-to-direct band gap transition. (e) PL spectranormalized by the PL peak intensity at 1.63 eV of 11 bilayer WSe2 flakes taken before and after the oxygen plasma treatment.

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to 10 min. The Raman, PL, and XPS results show no significantdifference in these two cases (see Figure S5), indicating that theoxidation is surface-saturated on the topmost layer, and thediffusion rate of excited oxygen radicals into the underlyingWSe2 layers is much smaller compared to the reaction ratebetween the oxygen and the surface layer. As a result, theprocess is self-limited. More detailed information, including PLand Raman spectra measured from monolayer, bilayer, andtrilayer flakes, can be found in the Supporting Information.Since only the top WSe2 layer is affected by the oxygen

plasma treatment, the oxide is amorphous rather thancrystalline. As a result, no WO2 or WO3 Raman peaks areobserved from the oxidation layer. Instead, we performed XPSof WSe2 before and after oxygen plasma exposure. Figure 5

shows XPS spectra of the W4f core level of an exfoliated WSe2sample before and after oxygen plasma treatment. The XPSspectrum of as-exfoliated WSe2 shows the 4f7/2 and 4f5/2features at 31.7 and 33.7 eV, corresponding to selenide-bondedtungsten.29 After the oxygen treatment, the XPS spectrummeasured from the same sample shows the emergence ofweaker doublet peaks at 35.0 and 37.1 eV, in addition to thestronger selenide-bonded tungsten peaks at 31.7 and 33.7 eV.

The larger binding energy doublet corresponds to 4f7/2 and4f5/2 tungsten oxide (WOx with x ≤ 3).30−32 To confirm thatthere is only surface oxidation, the WSe2 sample was quicklydipped into a 1 mol/L KOH solution for less than 3 s toremove the surface oxide. After the dip in this base, the WOxdoublet peaks disappear, suggesting that the surface oxide layeris removed by the KOH solution.Similar effects of the remote oxygen plasma treatment are

also observed in MoSe2 flakes. Figure 6 shows Raman and PLspectra of a MoSe2 flake before and after the oxygen plasmatreatment. The intensity of the Raman peak near 360 cm−1 isreduced after the treatment, which can be assigned to the B2gmode.20 As with WSe2, the B2g mode is only observed in few-layer MoSe2 flakes but is absent in monolayer MoSe2. As shownin Figure 6, the disappearance of the B2g Raman peak of theMoSe2 after the treatment suggests a bilayer-to-monolayertransition. A blue shift in the PL peak from 1.52 to 1.57 eV isobserved after the oxygen plasma treatment. Also, the PLintensity is increased by more than 3-fold. This plasmatreatment will not affect few-layer MoSe2 flakes thicker thanfive layers. Additional PL spectra of MoSe2 flakes taken beforeand after the oxygen plasma treatment are included in FigureS6 in the Supporting Information. A consistent blue shift of thePL peak position and increase of the PL intensity is observedfrom MoSe2 samples after the oxygen plasma treatment. Theseresults imply that the selective surface oxidation by remoteoxygen plasma may be applied to other transition metalselenide materials and, in general, may serve as a self-limitingoxidation technique for other TMDCs.

CONCLUSION

In summary, we demonstrate that a remote oxygen plasmatreatment provides a self-limiting oxidation of the surface layerof WSe2 and MoSe2. Consequently, monolayer WSe2 samplesare oxidized completely, bilayer WSe2 flakes become mono-layer, and, in general, N-layer WSe2 is reduced to N−1 layers, asestablished by Raman and PL spectra measured before and afterthe plasma treatment process. The PL intensity measured from“oxidized” bilayer WSe2 is increased by as much as 6× and iscomparable to pristine monolayer WSe2, which indicates thatthe remote oxygen radicals do not penetrate beyond thetopmost layer. It is somewhat surprising that the oxygen radicalchemistry is different for transition metal selenides (WSe2,MoSe2) and sulfides (MoS2), even though they come from thesame column of the periodic chart. Compared to other layer-

Figure 5. XPS spectra of tungsten 4f core level of the WSe2exfoliated sample before and after the oxygen plasma treatmentand after a quick dip in 1 mol/L KOH solution to remove thesurface oxide. The 31.7 and 33.7 eV peaks correspond to the WSe2.After the oxygen plasma treatment, two additional peaks with largerbinding energies appear at 35.0 and 37.1 eV, corresponding to 4f7/2and 4f5/2 tungsten oxide (WOx with x ≤ 3). After a quick dip in 1mol/L KOH solution, the same sample is measured again, and nooxide peaks are observed.

Figure 6. (a) Raman spectra of the bilayer MoSe2 flake taken before and after the oxygen plasma treatment. The inset shows the B2g Ramanmode of MoSe2 around 360 cm−1. (b) Photoluminescence spectra of MoSe2 before and after the oxygen plasma treatment. The PL peak blueshifts from 1.52 to 1.57 eV.

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by-layer thinning methods of graphene and TMDCs,33−35 theremote oxygen plasma treatment relies on chemical oxidationrather than physical bombardment. The kinetic energy of theoxygen radicals is very low, which makes it hard for them topenetrate the top layer oxide, resulting in a self-limiting process.This treatment will only oxidize one layer of WSe2, even whenwe increase the treatment duration to 10 min, and the oxidecan later be removed by KOH solution. The surface oxide layermay be advantageous for electronic devices, providingprotection from environmental effects and as an effective holeinjection layer to WSe2.

36 In addition, this surface will likelyserve as a good seed layer for atomic layer deposition of high-kdielectrics to improve the ALD/TMDC interface and thequality of the dielectric film.28,37

METHODSPreparation of Single- and Few-Layer TMDC Films.

Monolayer and few-layer WSe2 and MoSe2 films were exfoliatedfrom bulk WSe2 (HQ Graphene) and MoSe2 (2Dsemiconductors Inc.)onto oxidized Si substrates containing lithographically defined gridmarkers by the widely used “Scotch tape” method.26

Raman and Photoluminescence Spectroscopy. Raman andphotoluminescence spectra were taken using a Renishaw InViaspectrometer with a 532 nm laser (10 μW) focused through a 100×objective lens. Raman and PL spectra were collected at roomtemperature under ambient conditions. Optical images of the laserspot positions were recorded to ensure that the spectra were measuredat the same location of the flake before and after the oxygen plasmatreatment.Oxygen Plasma Treatment. WSe2 and MoSe2 samples were

exposed to a remote oxygen plasma for 2−5 min at a pressure of 200mTorr using ambient air. The plasma power was set to 15 W. Despitethe presence of nitrogen in the gas mixture, the plasma itself mainlyconsisted of oxygen radicals since the N2 molecules have a muchhigher bond strength. The samples were placed at a distance of 10 cmfrom the plasma source to avoid direct bombardment from the highlyenergetic oxygen ions.X-ray Photoelectron Spectroscopy. As-exfoliated, after oxygen

treatment, and after a KOH dip, WSe2 samples were investigated usinga Kratos Axis Ultra DLD (X-ray photoelectron spectrometer). Theinstrument has a typical sampling depth of 2−3 nm. The z-height ofthe samples was focused using a green laser spot (rough alignment),and the O 1s signal was counted (fine alignment) to make sure thesamples were focused by the X-ray. The binding energy range weexplored for tungsten was from 40 to 23 eV, and ten 60 s sweeps werecollected.

ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsnano.6b02488.

PL spectra of monolayer and trilayer WSe2, Ramanspectra of bilayer WSe2, statistical analysis of PL peakshifts of bilayer and trilayer WSe2 flakes, and PL spectraof MoSe2 flakes (PDF)

AUTHOR INFORMATIONCorresponding Author*E-mail: scronin@usc.edu.NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENTSThis research was supported by Department of Energy (DOE)Award No. DE-FG02-07ER46376 (Z.L.) and NSF Award No.

1402906. (S.Y.). The authors thank Dr. Marcos Pimenta foruseful insight and discussions.

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ACS Nano Article

DOI: 10.1021/acsnano.6b02488ACS Nano XXXX, XXX, XXX−XXX

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