Seeded growth of highly crystalline molybdenum disulphide monolayers at controlled locations

Abstract

Monolayer transition metal dichalcogenides are materials with an atomic structure complementary to graphene but diverse properties, including direct energy bandgaps, which makes them intriguing candidates for optoelectronic devices. Various approaches have been demonstrated for the growth of molybdenum disulphide (MoS₂ ) on insulating substrates, but to date, growth of isolated crystalline flakes has been demonstrated at random locations only. Here we use patterned seeds of molybdenum source material to grow flakes of MoS₂ at predetermined locations with micrometre-scale resolution. MoS₂ flakes are predominantly monolayers with high material quality, as confirmed by atomic force microscopy, transmission electron microscopy and Raman and photoluminescence spectroscopy. As the monolayer flakes are isolated at predetermined locations, transistor fabrication requires only a single lithographic step. Device measurements exhibit carrier mobility and on/off ratio that exceed 10 cm² V^-1 s^-1 and 10⁶ , respectively. The technique provides a path for in-depth physical analysis of monolayer MoS₂ and fabrication of MoS₂ -based integrated circuits.

ARTICLE

Received 16 Jun 2014 | Accepted 12 Dec 2014 | Published 28 Jan 2015

Gang Hee Han1,2, Nicholas J. Kybert1, Carl H. Naylor1, Bum Su Lee3, Jinglei Ping1, Joo Hee Park3, Jisoo Kang4, Si Young Lee2, Young Hee Lee2, Ritesh Agarwal3 & A.T. Charlie Johnson1,3,4

Monolayer transition metal dichalcogenides are materials with an atomic structure complementary to graphene but diverse properties, including direct energy bandgaps, which makes them intriguing candidates for optoelectronic devices. Various approaches have been demonstrated for the growth of molybdenum disulphide (MoS2) on insulating substrates, but to date, growth of isolated crystalline akes has been demonstrated at random locations only.

Here we use patterned seeds of molybdenum source material to grow akes of MoS2 at

predetermined locations with micrometre-scale resolution. MoS2 akes are predominantly monolayers with high material quality, as conrmed by atomic force microscopy, transmission electron microscopy and Raman and photoluminescence spectroscopy. As the mono-layer akes are isolated at predetermined locations, transistor fabrication requires only a single lithographic step. Device measurements exhibit carrier mobility and on/off ratio that exceed 10 cm2 V 1 s 1 and 106, respectively. The technique provides a path for in-depth physical analysis of monolayer MoS2 and fabrication of MoS2-based integrated circuits.

DOI: 10.1038/ncomms7128

Seeded growth of highly crystalline molybdenum disulphide monolayers at controlled locations

1 Department of Physics and Astronomy, University of Pennsylvania, 209S 33rd Street, Philadelphia, Pennsylvania 19104 6396, USA. 2 Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Sungkyunkwan University, Suwon 440 746, South Korea. 3 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 6272, USA. 4 Nano/Bio Interface Center, University of Pennsylvania, Philadelphia, Pennsylvania 19104 6272, USA. Correspondence and requests for materials should be addressed to G.H.H. (email: mailto:[email protected]

Web End [email protected] ) or to A.T.C.J. (email: mailto:[email protected]

Web End [email protected] ).

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The presence of an intrinsic direct energy bandgap in molybdenum disulphide (MoS2) monolayers makes this two-dimensional material of both fundamental and

technological interest110. Several synthetic methods have been reported, including sulphiding of Mo thin lm11, and annealing of ammonium tetrathiomolybdate, which has both Mo and S components12. One promising technique is the growth of MoS2 akes by chemical vapour deposition (CVD) of molybdenum trioxide (MoO3) and solid sulphur, which can yield highly crystalline, predominantly monolayer material1317. However, this method typically leads to growth of MoS2 akes in uncontrolled locations on the substrate, which limits its utility for many applications. Control over the position of MoS2 akes on oxidized silicon has been reported through the use of stamping18 and masking19 techniques, but neither of these allowed for the creation of large (tens of micrometres) monolayer domains at predetermined locations with micrometre-scale resolution. Substrate patterning has been used to control nucleation and growth of crystalline akes of two-dimensional topological insulator materials on mica20, but it remains unclear whether this approach can be generalized to MoS2 on oxidized silicon. Seeding techniques that allow growth at dened locations are well established for graphene21,22, and equivalent approaches for MoS2 synthesis are highly desirable.

Here we report the use of lithographically patterned islands of MoO3 or ammonium heptamolybdate (AHM) as seed material for the growth of crystalline MoS2 monolayers at predened locations on oxidized silicon substrates. MoS2 grown by this approach shows many favourable characteristics, for example, high crystallinity; excellent optical properties, including a direct bandgap, as indicated by photoluminescence (PL) and reection measurements; and carrier mobility ca 10 cm2 V 1 s 1. Because the method enables control over the location of discrete MoS2 akes, integration into device structures is straightforward using optical lithography, with no need for an etching step. Although some applications may require further study to achieve control over the precise shape and size of the MoS2 akes, the methodology presented here opens a path towards scalable production of high-quality devices and circuits based on MoS2 and other transition metal dichalcogenides.

ResultsGrowth process. Figure 1 is a schematic of the MoS2 growth process (see the Methods section and Supplementary Fig. 1 for

details). The process begins with patterning of an array of square windows, typically 510 mm on a side, by e-beam or optical lithography. Next, molybdenum-containing seed material is deposited into the windows, either by thermal evaporation of MoO3 or by spin-casting of a saturated solution of AHM in water.

The AHM solution collects in the square wells but there is almost no deposition of AHM on the resist layer, which is not wet by the solution. Spin coating of AHM has advantages of speed, simplicity and requiring no equipment beyond a standard spin coater. The resist and unwanted molybdenum source material are removed in a standard lift-off step.

At this point, the molybdenum-containing seed particle has a shape that depends upon the relative humidity and other details of the deposition process. To improve the uniformity of the seed shape and subsequent MoS2 growth, an aggregation step is performed (see Supplementary Figs 2 and 3) where the sample temperature is reduced to below the dew point using a Peltier cooler. Owing to the hygroscopic nature of MoO3 and AHM, water collects on the surface of the seed, which is transformed into a spherical-cap shape. This reduces the surface area of the seed particles and makes them more uniform, leading to more efcient and reproducible MoS2 growth. This step ensures that the seed particle is sufciently thick to not be consumed by evaporation during the heat-up phase of the growth process. This step is essential when AHM is used as the moblydenum source, as it is quite thin when initially deposited (B50 nm, see

Supplementary Fig. 2). In contrast, the aggregation step improves the growth reproducibility but is not required when evaporation is used to deposit a MoO3 seed of thickness approximately 150 nm or greater. The aggregation step is followed by an annealing step at 300 C in a nitrogen atmosphere, which converts AHM to MoO3 (refs 23,24).

The next step in the process is chemical treatment of the SiO2 surface, which has been shown to facilitate growth of MoS2. For

example, it was reported that MoS2 growth was promoted when the SiO2 surface was coated with materials that contain carbon ring structures that persist at the MoS2 growth temperature, for example, perylene-3,4,9,10-tetracarboxylicdianhydride, perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt13 or reduced graphene oxide13,16. These materials promote monolayer growth by increasing the surface adhesive energy relative to the adatom cohesive energy17. We achieved excellent results by rst preparing the surface with an oxygen plasma treatment and then spin-casting onto the substrate an aqueous solution of 0.11% sodium cholate, which contains multiple carbon ring structures.

Bead

AHM or MoO3

Resist

Resist lift-off

S sublimation

MoS2 growth

MoS2

Figure 1 | Schematic of the growth process for molybdenum disulphide using patterned molybdenum sources. An array of square wells is patterned in a conventional lithographic resist. The wells are lled by spin coating with ammonium heptamolybdate ((NH4)6Mo7O24 4H2O, AHM) solution or

thermal evaporation of molybdenum trioxide (MoO3). The resist is removed in a lift-off process. Water droplets are condensed onto the AMH or MoO3 by reducing the sample temperature to below the dew point using a Peltier cooler. The AMH or MoO3 is solubilized and forms spherical beads upon drying. MoS2 akes are then synthesized by sublimation of solid S in a nitrogen environment in a tube furnace.

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For MoS2 growth, the sample was placed in the middle of the furnace, and 2050 mg of solid sulphur was placed at the upstream end. The entire growth process, including temperature ramp, growth and cooling, was conducted under a nitrogen ow of 700 s.c.c.m. The use of patterned molybdenum source material resulted in the growth of an array of triangular/spike-shaped akes of monolayer MoS2 (Figs 2a and 3a,b,d). The akes comprised multiple crystalline domains of MoS2, as analysed below (see Fig. 4). The majority of the ake region consisted of monolayer MoS2, with small regions of multilayer MoS2 and/or residual seed material observed at the centre of the ake (Fig. 3d). It was found that the formation of multilayer regions could be suppressed by optimizing the dimensions of the growth seed (see Supplementary Fig. 4). The structure is consistent with a model where growth is initiated at multiple points on the MoO3 seed to produce MoS2 crystallites that eventually merge into a larger, faceted structure. For additional data in support of this picture, see Supplementary Fig. 5. Although the sizes and locations of the akes were primarily determined by the array of the growth seeds, it was consistently observed that akes that grew at the edge of the array were markedly smaller than those that grew in the middle. This is presumed to reect diffusion of source material as the furnace temperature was increased that led to shrinkage of seed particles on the edge of the array. Particles in the centre of the array are more replenished than those at the edges because of the larger number of neighbouring particles. Once sulphur sublimation and transport into the furnace occurs, the larger source particles at the centre of the array grow larger MoS2 akes than the smaller source particles at the edges. In some experiments, small, triangular, presumably single-crystal MoS2 akes were observed to grow in the regions between the MoO3 seeds (data not shown). This occurred during growths that were at higher temperature compared with the samples shown in Figs 2a and 3a, and where sulphur was introduced at a later time.

Presumably, these conditions allowed for evaporation of Mo from the growth seed, which then reacted with the injected sulphur to form MoS2 crystallites in regions between the seeds.

Structural and optical property measurements. To demonstrate the versatility of the growth technique, we synthesized a set of monolayer MoS2 akes to spell out the acronym for the Nano-Bio

Interface Center at the University of Pennsylvania, NBIC@ UPENN (Fig. 3a). The centre-to-centre spacing between adjacent seeds was ca 15 mm. Atomic force microscopy (AFM) was used to conrm that the height of a typical ake was 0.7 nm (Fig. 3b, inset; additional line scan data are provided in Supplementary Fig. 6. AFM images of other representative monolayer akes of MoS2 are provided in Supplementary Fig. 7), as expected for monolayer MoS2 (refs 1,13,15,25,26). The Raman spectrum of the monolayer region of the akes (Fig. 3c) showed the expected peak spacing of 19 cm 1 between the E2g and A1g modes. Raman mapping (Fig. 3e,f) was used to conrm that the Raman spectrum was uniform over the monolayer region as well as distinct from the spectra associated with multilayer regions and the substrate2527 (Maps of Raman peak positions are also shown in Supplementary Fig 8.). We performed PL and reection measurements to conrm that the existence of a direct bandgap, which is known to be characteristic of monolayer MoS2 (refs 2831). Peak features associated with the A (1.83 eV in

PL and 1.91 eV in reection measurement) and B (1.97 eV in PL and 2.07 eV in reection measurement) excitons were also observed (Fig. 3f,g), conrming that the MoS2 grown by the seeded CVD process shows the same characteristics as monolayer material fabricated by mechanical exfoliation32.

Sample crystallinity was further characterized with bright-eld imaging and selected area electron diffraction (SAED) measurements conducted in the transmission electron microscope (TEM).

Zone 2 Zone 3

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Figure 2 | Growth of molybdenum disulphide. (a) 5 5 array of akes of monolayer MoS2 grown by chemical vapour deposition. Scale bar is 50 mm.

Different growth zones, with different average size of the akes (see b), are indicated by outlining: dark blue (zone 1), blue (zone 2) and light blue (zone 3). (b) Areas of individual akes for zones 1, 2 and 3 in a. Large (5001300 mm2) akes of crystalline, monolayer MoS2 (with small multilayer regions) were grown from the seed particles. The size of the akes is largest in zone 1 and smallest in zone 3. (c) Schematic illustrating the proposed mechanism underlying the observation that the growth process results in larger MoS2 akes in the interior of the array and smaller akes towards the edges.

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Figure 3 | AFM and optical analysis of MoS2 akes. (a) Optical micrograph of MoS2 akes grown at locations that form NBIC @ UPENN. Scale bar, 100 mm. (b) Selected area (dashed square in a) at higher magnication. Scale bar, 20 mm. Inset: AFM image of selected MoS2 ake (dashed square). Scan image is 10 10 mm2. (c) Raman signal from the ake shows the E2g and A1g peaks with a spacing (19 cm 1) characteristic of monolayer MoS2. (d) Optical

image of MoS2 ake from the array in Fig. 2a. Scale bar, 10 mm. (e,f) 2D Raman maps taken from the ake shown in d (dashed line) for the E2g mode (e) and A1g mode (f). Scan images are 10 10 mm2. (g) Photoluminescence (PL) spectrum for the ake shows strong emission associated with the A and B excition

at 1.83 eV and 1.97 eV, respectively. Strong PL is characteristic of monolayer MoS2 and is absent in multilayer lms. (h) Differential reectance spectrum showing the A and B exciton features.

A gold grid was rst patterned onto a MoS2 ake to prevent tearing of the ake when it was transferred onto a TEM grid covered by a carbon membrane with 1-mm diameter holes (Fig. 4a and Supplementary Fig. 9). To minimize damage to the sample, imaging and SAED were conducted using an accelerating voltage of 80 kV. The TEM bright-eld images are shown in Fig. 4a,b. Triangular shaped multilayer MoS2 akes were observed very close to the seed particle, possibly due to limited diffusion of the source during the growth time. Further from the seed, ower-petal-like monolayer MoS2 regions were observed (dashed square in Fig. 4a,b; see Supplementary Fig. 10 for the raw image used to create Fig. 4b and an atomic resolution image of a monolayer MoS2 ake). The crystalline structure of the

MoS2 was investigated by SAED. Using the intensities of the rst- and/or second-order diffraction spots33,34, the ower-petal-like structures were identied as monolayer MoS2 (see

Supplementary Fig. 11). Individual single-crystal MoS2 domains, labelled AD in Fig. 4b, were distinguished by the orientations of the diffraction spots in SAED (Fig. 4cf). The data indicate that each domain is well crystallized and their orientations are uncorrelated.

Fabrication and measurement of electronic devices. An etch step is typically used to dene individual devices from a continuous layer of material, which creates the risk of degrading a two-dimensional material as well as added processing cost. In contrast, the patterned growth approach to mono-layer MoS2 outlined here has the advantage of enabling direct fabrication of multiple devices (Fig 5af), without the need for etching. To demonstrate process compatibility with conventional manufacturing, an array of MoO3 growth seeds was patterned by e-beam lithography, and MoS2 monolayer akes were synthesized as described above. As the MoS2 akes were grown at known, pre-patterned locations, it would be straightforward to align an electrode pattern to the MoS2 akes, with no need for a separate etch step. After fabrication of the electrodes, the sample was annealed at 150 C at 10 5 Torr for 1 h. Figure 5g shows the IdsVg characteristics for a set of devices prepared using this approach, measured at a pressure of 40 mTorr. Device mobilities were between 8.2 and11.4 cm2 V 1 s 1 and the on/off ratios were in excess 106 comparable to earlier best reports for exfoliated MoS2 monolayers14,15,32,33.

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a b

DiscussionIn summary, we have developed a novel patterned growth method for akes of highly crystalline monolayer MoS2, where molybdenum-containing growth seeds (AHM or MoO3) are

dened using conventional lithographic methods. This technique should be generalizable to the growth of any layered transition metal dichalcogenide, and it creates a pathway to controllably tailor the size and position of the material. Control of growth parameters enables synthesis of high quality, crystalline, mono-layer MoS2 with electrical and optical properties comparable to exfoliated material. The process enables straightforward fabrication of integrated circuits without the need for a separate etch step. The work represents an important advance towards scalable production of high-quality layered transition metal dichalcogenide-based devices and circuits. An ongoing challenge to the eld is patterned growth of monolayer, single-crystalline domain MoS2 at predened locations.

Methods

Preparation of patterned molybdenum sources. Electron beam or photo-lithography was used to prepare 5 5 arrays of 5-mm square windows. A 7 7

array of 3-mm square windows was used for the experiment that resulted in the growth of exclusively monolayer MoS2 akes (see Supplementary Fig. 4). Molybdenum-containing source material was then deposited by either spin coating (1,000 r.p.m. for 30 s, followed by 3,000 r.p.m. for 10 s) with a saturated solution of AHM (product number A674, Fisher Scientic) in deionized water or thermal evaporation of 150-nm-thick molybdenum trioxide (MoO3, product number 203815, Sigma-Aldrich). The resist was removed using acetone followed by isopropyl alcohol, and the sample was then blown dry with compressed nitrogen gas. An aggregation step was then performed to obtain regular beads of the source material. Although monitoring the sample under an optical microscope, the substrate was cooled until the hygroscopic AHM or MoO3 absorbed water, forming droplets at the seed locations. As the temperature was increased back to room temperature, the water evaporated, leaving behind rounded, uniform, beads of AHM or MoO3 as shown in Supplementary Fig. 2. Samples were then annealed at 300 C for 20 min in a ow of 500 s.c.c.m. of nitrogen gas (99.999% purity). To promote MoS2 growth, the sample was treated with oxygen plasma (65 W,300 mTorr, 2 min) and then spin-coated (3,000 r.p.m., 1 min) with a surfactant solution (0.11% solution of sodium cholate in deionized water).

Growth process. Samples with patterned growth seeds were placed into a 1-inch CVD furnace with a 25-mg solid sulphur source (part number 213292, Sigma-Aldrich) at the upstream end. The furnace temperature was increased to 750 C at a ramp rate of 70 C min 1 under a nitrogen (99.999%) ow of 700 s.c.c.m. The temperature was held constant for 20 min during the MoS2 growth phase, and the system was then rapidly cooled to room temperature. It was noted that the sulphur source started to melt when the temperature in the middle of the furnace was ca 650 C.

D

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Seed Au frame

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Figure 4 | Transmission electron micrographs and electron diffraction data. (a) Monolayer MoS2 regions are coloured grey, light blue, red and purple (top-bottom) according to their crystallographic orientation. The black regions are parts of a gold frame used to prevent tearing during the transfer process (see main text and Supplementary Fig. 6). Triangular shaped multilayer MoS2 akes were formed in the seed region, indicated by the yellow arrow. (b) Bright-eld (BF) image from the square region outlined by a dashed white line in a. Scale bars in g a and b are 5 mm and 3 mm, respectively. (cf) Electron diffraction data from the regions labelled AD in b.

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Figure 5 | Device fabrication and IV measurement. (a) Optical micrograph of devices that was fabricated from MoS2 grown by the patterned seed technique. Scale bar, 100 mm. (bf) Higher magnication images of the rectangular region indicated in a. Scale bar, 20 mm. (g) IdsVg characteristics for the devices in bf with 1 V Vds. Inset: IdsVds characteristics at several back gate voltages.

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Device fabrication and currentvoltage measurements. After growth of monolayer MoS2 akes from patterned growth seeds, electron beam lithography was used to dene an electrode pattern that was aligned to the growth sites. The electrode metallization consisted of 1 nm Cr and 30 nm Au deposited by e-beam evaporation. After metallization, devices were annealed for 60 min at 150 C at a pressure of 10 5 Torr. Currentvoltage measurements were conducted at room temperature under a vacuum of 40 mTorr.

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Acknowledgements

This work was supported by the National Science Foundation Accelerating Innovation in Research Program AIR ENG-1312202 and the Nano/Bio Interface Center NSF NSEC DMR08-32802. B.S.L., J.H.P. and R.A. were supported by the National Institutes of Health through the NIH Directors New Innovator Award Program, 1-DP2-7251-01 and the US Army Research Ofce under Grant No. W911NF-11-1-0024. R.A. and A.T.C.J. acknowledge Seed Project support from the LRSM, NSF MRSEC DMR-1120901. G.H.H., S.Y.L. and Y.H.L. acknowledge IBS-R011-D1 (Institute for Basic Science). We acknowledge use of facilities associated with the Nano/Bio Interface Center (Raman spectroscopy, AFM) and the Nanoscale Characterization Facility (TEM) at the University of Pennsylvania. The aberration-corrected TEM image presented in Supplementary Fig. 9 was obtained by Zhengqing John Qi and Julio A. Rodrguez-Manzo (Drndic group, University of Pennsylvania), using an aberration-corrected transmission electron microscope at the Center for Functional Nanomaterials, Brookhaven National Laboratory.

Author contributions

A.T.C.J. directed the research; G.H.H. proposed and designed the experiment; G.H.H. and C.H.N. carried out MoS2 growth. Electrical measurements were performed by N.J.K.,

C.H.N. and G.H.H., and also by S.Y.L. under supervision of Y.H.L. N.J.K. and G.H.H. performed AFM and Raman characterization; J.P., J.K., and G.H.H. performed sample preparation and characterization for TEM experiments; B.S.L. and J.H.P. (under the supervision of R.A.) carried out photoluminescence and reection measurements. G.H.H., N.J.K., C.H.N. and A.T.C.J. wrote the manuscript, with input and approval from all the authors.

Additional information

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Competing nancial interests: The authors declare no competing nancial interests.

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How to cite this article: Han, G. H. et al. Seeded growth of highly crystalline molybdenum disulphide monolayers at controlled locations. Nat. Commun. 6:6128 doi: 10.1038/ncomms7128 (2015).

6 NATURE COMMUNICATIONS | 6:6128 | DOI: 10.1038/ncomms7128 | http://www.nature.com/naturecommunications

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