Vapor --
Solid Growth of High Optical Quality MoS2 Monolayers with Near-Unity Valley Polarization

Vapor-Solid Growth of High OpticalQuality MoS2Monolayers withNear-Unity Valley Polarization Sanfeng Wu，ts Chunming Huang，s Grant Aivazian， Jason S. Ross，David H. Cobden， andXiaodong Xut.*.* 'Department of Physics and "Department of Material Science and Engineering， University of Washington， Seattle， Washington 98195， United States. These authorscontributed equally to this work. lransition metal dichalcogenides MX2(M =Mo， W； X = S， Se， etc.) havelayered structures with van der Waalsinteractions between the layers. Mono-layers of such materials were first obtainedby the mechanical exfoliation techniquetypically used for graphene. Subsequentinvestigation has shown that these two-dimensional (2D) semiconductors1-3exhi-bit unique properties， such as a transitionfrom an indirect band gap in the bulk to adirect band gap at monolayer thicknesses，34massive Dirac-like behavior of the electrons，excellent field-effect transistor performanceat room temperature， and completely tun-able 2D excitonic effects.’ Recently， these monolayers have alsobeen suggested as good candidates for therealization of valley-based electronics.5.8-10In monolayer MoSz， there are two energy-degenerate Dirac valleys at the corners of thehexagonal Brillouin zone. 10 The Berry curva-ture and magnetic moments of electrons as-sociated with different valleys have oppositesign and are linked to measurable quantities which can distinguish the valleys， such ask-resolved optical dichroism， offering the pos-sibility of manipulating and utilizing the valleydegree of freedom.Valley polarization hasbeen demonstrated in MoS2 monolayers bycircularly polarized light excitation，-10 andelectrical control of it has been reported inbilayer samples. Progress thus far has relied mainly onmechanically exfoliated samples wherescaling for device applications 14 is probablyimpossible. Recent attempts to develop morescalable techniques include exfoliation inliquids，215，16 hydrothermal synthesis，epitaxygrowth using graphene，8 and soft sulfur-ization.19.20 However， these methods are noteasily integrated with device fabrication.Chem-ical vapor deposition has also been exploredusing a Mo film (or MoOs powder2) andsulfur powder as the reactants， yielding mono-layers of MoS2 on 300 nm SiOz/Si substratecompatible with device fabrication.21，22 It hasyet to be proven though that such mono-layers have sufficient quality for investigatingvalley-related physics. Intervalley scattering enhanced by defects and impurities can reduce ordestroy the valley polarization， as evident from thedisparate degrees of polarization reported by differ-ent groups.8-10，13，23 A high degree of valley polariza-tion is required for valley physics and is also a hallmarkof crystal quality. Here we introduce a new and straightforward meth-od for obtaining high optical quality monolayer MoS2via a vapor-solid (VS) growth mechanism.4 Up to400 um’monolayer flakes with triangular shape aredirectly produced on insulating substrates such asSiO2， sapphire， and glass， without using any catalysts.The growth procedure is simple physical vapor trans-port， using a MoS2 powder source and Ar carrier gas(details are given in Figure1 and Methods)， similar tothe procedure used for growing BizSes topologicalinsulator nanoplates.24 Using polarization-resolvedphotoluminescence (PL)，1 we observe valley polariza-tion approaching near-unity at low temperature (30 K)and 35% at room temperature. This observation de-monstrates that these monolayers are of high qualityand are suitable for valley physics and applications. RESULTS AND DISCUSSION The resulting MoS2 monolayers are characterized byoptical microscopy (OM， Zeiss Axio Imager A1)， atomicforce microscopy (AFM， Veeco Dimension 3100)， scan-ning electron microscopy (SEM， FEl Sirion)， and micro-Raman spectroscopy (Renishaw inVia Raman Microscope).Figure 2 is a typical SEM image of a sample grown on SiOz/Si. The crystallites have lateral dimensions up to 25 um andare approximately equilateral triangles (see Figure 2 inset).This is consistent with the triangular symmetry of mono-layer MoS2 (Figure 1c). It suggests that each is a singlecrystal without extended defects or grain boundaries；25.26the facets are then the most slow-growing or stablesymmetry-equivalent crystal planes-it remains to beestablished whether these are the "zigzag" or the“arm-chair" edges. Therefore， another advantage over exfolia-tion techniques is that the crystal axes can be immediatelyidentified by inspection. Optical and AFM Characterization. Figure 3a-c showsoptical microscope images of growth on sapphire，glass， and 300 nm SiOz/Si substrates， respectively.The color contrast of all of the larger crystallites isuniform； moreover， for those on SiO2/Si (Figure 3c)， it isidentical to that of exfoliated monolayers on the samesubstrate. These facts strongly indicate that they aremonolayers.32The growth on sapphire is much dens-er than that on both SiO2 and glass， but on all of thesubstrates， nucleation appears to be random， as wasfound for VS growth of topological insulators.24Smaller(<2 um)， thicker crystallites are also present， especiallyon SiO2. We speculate that the growth kinetics are suchthat a monolayer is favored and grows rapidly if thenucleating crystal is aligned suitably with the substrate；otherwise， more three-dimensional growth occurs. The Figure 1. (a) Growth setup and conditions. (b) Cartoonindicating the structure of the triangular monolayer crystal-lites. (c) Structure of monolayer MoS2. Figure 2. Scanning electron microscope image of triangularMoS2 monolayer crystallites grown on a 300 nm SiO2/Sisubstrate. The inset shows the 60° corners of a selectedcrystallite with a clean surface. Figure 3. Optical microscope images of MoS2 crystallitesgrown on (a) sapphire，(b) glass， and (c)SiOz/Si. Scale bar is10 um. A typical triangular crystallite is further character-ized by(d) AFM image with 3 um scale bar. The inset plot isthe height profile along the black line shown in the image，demonstrating its monolayer thickness. monolayer thickness is confirmed by atomic forcemicroscopy (AFM).3，6，28 Figure 3d shows an AFM imageof one crystallite on SiO2， revealing a flat， uniformsurface. A line cut along the red line (Figure 3e) shows 一E> Figure 4. (a) Raman spectra of monolayer MoS2 grown on different substrates， excited by a 514.5 nm laser. For comparison，the spectra from a mechanically exfoliated monolayer and a bulk MoS2 crystal are also shown. (b，c) Intensity maps of the twoRaman modes， excited by a 532 nm laser line from a typical crystallite.(d，e) Corresponding peak position maps. an apparent thickness of ~0.75 nm on the SiOz/Sisubstrate， consistent with previous measurements ofmonolayers.46 Similar measurements on sapphire sub-strates are shown in the Supporting Information. Raman Characterization. The samples were also stu-died by Raman spectroscopy. Typical Raman spectrafrom the triangular crystallites grown on differentsubstrates， as well as from exfoliated monolayer andbulk MoS2， using a 514.5 nm excitation laser，are shownin Figure 4. We observe both of the Raman modes(E2g and AA11gg) expected for monolayer MoS2.34，28，29 The laxatEza peak is at 386 cmfor the SiOz substrate， 384 cm-for sapphire， and 385 cm- for glass. The A1g peak is at405 cm-for SiO2 and sapphire and 404 cm-for glass.The peak separations are 19， 21， and 19 cm-1， respec-tively. All of these numbers agree well with the ex-foliated monolayer sample. In order to investigate theuniformity of the grown monolayer sample， we alsoperformed scanning Raman measurements (excited bya 532 nm laser line). Intensity and peak position mapsfor a triangular crystallite are shown in Figure 4b-e.Note that the different excitation energy leads to adifferent intensity ratio between the two peaks. Thepeak separation resulting from the map is 22 ±1.5 cm-1. It clearly demonstrates that this entire crys-tallite is a uniform monolayer. Optical Valley-Selective Effect. To investigate the po-tential of these monolayer crystallites for 2D optoelec-tronics and valley-related device applications， weperformed polarization-resolved PL.8，9，13 Circularly po-larized PL measurements can identify valley polariza-tion in monolayer MoS2 created by appropriate opticalpumping： the +K and -K valleys are selectively excitedbyoorolight， respectively， as indicated Figure 5a. 10Due to the large k-space separation of the valleys，intervalley scattering is suppressed and the valley re-laxation time is longer than the electron-hole recom-bination time. Emission from a given valley is alsocircularly polarized， and the degree to which the PLhas the same helicity as the incident light thereforereflects the degree of valley polarization. Large valleypolarization provides evidence for good sample quality，as impurities and defects in the crystal will cause inter-valley scattering even at low temperature. In our measurements， a 632 nm He-Ne laser beamis circularly polarized by a quarter-wave plate (QWP)and focused at normal incidence onto the monolayersample held in a cryostat. The PL signal is selectivelydetected for botho and o polarization using thesetup described in ref 13. The laser spot size is about2 um with an intensity of~150 W/cm2. We define thedegree of PL polarization， which reflects the valley Figure 5. (a) Brillouin zone and K-point band edges inmonolayer MoSz indicating the optical selection rule. (b)o(black) ando (red) components of.. the PL signal for ourmonolayer MoSz on a SiOz/Si substrate，excited by o* laserlight at 632 nm wavelength at 30 K. (c) Degree of PLpolarization vs photon energy， calculated from (b). (d，e)Corresponding observations on a sample grown on a sap-phire substrate. The polarization approaches unity on bothsubstrates. polarization， as8，9，13 n=(PL(o*)-PL(o))/(PL(o*) +PL(o)). For a substrate temperature of 30 K， the PL spectrafor o* excitation are shown in Figure 5b，d for mono-layer crystallites on SiOz and sapphire substrates. Theresults with o excitation are similar (see SupportingInformation). The cutoff in the spectra at ~1.92 eV isdue to the notch filter placed in the collection opticalpath for blocking the laser light. The sharp spikessuperimposed on the spectra are Raman scatteringpeaks. The spectra show only a single emission peak at~1.9 eV in SiO2 substrate， in contrast with reports onexfoliated samples3 where a second broad impuritypeak is present at~1.77 eV. The absence of an impuritypeak is powerful evidence of excellent crystal quality. The PL signal is highly o*-polarized for both sub-strates. Reported degrees of valley polarization at lowtemperatures from mechanically exfoliated mono-layers in the literature vary widely：30，50，1°80，13andup to 100% on boron nitride substrate， showing that METHODS A MoS2 powder source (Alfa Aesar， 99% purity) in an aluminaboat is placed in the center of a horizontal quartz tube furnace(CARBOLITE 12/600 1200C tube furnace with 1 in. tube diameter)，asillustrated in Figure 1. The insulating substrate (either 300 nm SiOz/Si，(0001) sapphire， or normal glass) is cleaned in acetone， isopropylalcohol， and deionized water and is placed downstream far from theoven center in a cooler zone (at ~650℃ during growth). The tube is 0.8 F 0.4 0.0 1.6 1.7 1.8 1.9 d 0.8 = 0.4 0.0 .1.6 1.7 1.8 1.9 Photon Energy (eV) Figure6. (a)o andocomponents of the PL for monolayeron SiOz/Si substrate， and (b) degree of PL circular polariza-tion vs photon energy at room temperature. (c，d) Similarmeasurements on a sapphiresubstrate. Up to ~35% polar-ization is observed on both substrates. intervalley scattering is very sensitive to sample details.The degree of polarization in our monolayers is plottedin Figure 5c，e for both SiO2 and sapphire substrates. Wesee near-unity polarization on SiO2 and more than 95%on sapphire， with the polarization decreasing at lowerphoton energies as in previous reports. Interestingly， the PL polarization is substantial evenat room temperature，approaching a maximum of 35%at ~1.92 eV on both substrates， as shown in Figure 6.Intervalley scattering increases with temperature dueto enhanced phonon populations， resulting in thedecrease of the valley polarization and usually makingit vanish at room temperature，' although recently，23there has been a report of 40% of valley polarization at300 K from a mechanical exfoliation sample. Thus ourVS grown samples are as good as the highest opticalquality samples obtained by mechanical exfoliation. CoNCLUSION In summary， we report a simple method for growinghigh optical quality monolayer MoS directly on var-ious insulating substrates，which should facilitate de-vice fabrication without the need for a transfer process.The absence of impurity luminescence and the sub-stantial room temperature polarization imply excellentcrystal quality and the potential for optoelectronicapplications without the need for low temperatures.The technique could also be applicable to other TMDCs. initially pumped to a base pressure of 20 mTorr and flushed with theArcarrier gas (~20 sccm) repeatedly at room temperature to removeoxygen contamination. With the carrier gas flowing and the pressuremaintained at ~20 Torr， the furnace temperature is then increased to~900℃(~35C/min) and held there for 15-20 min before beingallowed to cool naturally (see Supporting Information). Conflict of Interest： The authors declare no competingfinancial interest. Acknowledgment. This work was mainly supported by theU.S. Department of Energy， Office of Basic Energy Sciences，Division of Materials Sciences and Engineering (Awards DE-SC0008145 and DE-SC0002197). G.A. was supported by DARPAN66001-11-1-4124. Supporting Information Available：Temperature profile of thegrowth， AFM characterization on the sapphire substrate， andcomplementary data for the PL polarization. This material isavailable free of charge via the Internet at http：//pubs.acs.org. REFERENCES AND NOTES ( 1 . N ovoselov， K. S.；Jiang，D.；Schedin， F.； Booth， T.J； Khotkevich， V. V.； Morozov， S. V.； Geim ， A. K . Two-Dimensional Atomic C rystals. Proc. Natl. Acad. Sci. U.S.A. 2005， 102，10451-10453. ) ( 2.( C oleman，J. N.；Lotya，M.；O'Neill， A.； Bergin， S. D.； King，P. J.； Khan， U . ； Young， K.； Gaucher， A.； De， S.；Smit h ， R. J. ； et al. T wo-Dimensional Nanosheets Produced by Liquid Exfolia- t ion o f Layered M aterials. Science 2011， 331， 568. ) ( 3 . M ak， K. F . ； L ee， C .； Hone，J.； S han， J.； Heinz，T . 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