Single-Source Precursor for Tungsten Dichalcogenide Thin Films

Apr 12, 2017 - Aerosol-assisted chemical vapor deposition of WS 2 from the single source precursor WS(S 2 )(S 2 CNEt 2 ) 2. Nathaniel E. Richey , Chan...
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A Single Source Precursor for Tungsten Dichalcogenide Thin Films: Mo1-xWxS2 (0 # x # 1) Alloys by AerosolAssisted Chemical Vapor Deposition (AACVD) Aleksander A. Tedstone, Edward A Lewis, Nicky Savjani, Xiang Li Zhong, Sarah J. Haigh, Paul O'Brien, and David J. Lewis Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b05271 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 13, 2017

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GRAPHICAL ABSTRACT FOR TOC 254x190mm (96 x 96 DPI)

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A Single Source Precursor for Tungsten Dichalcogenide Thin Films: Mo1-xWxS2 (0 ≤ x ≤ 1) Alloys by Aerosol-Assisted Chemical Vapor Deposition (AACVD) Aleksander A. Tedstone,1 Edward A. Lewis,2 Nicky Savjani,1 Xiang Li Zhong,2 Sarah J. Haigh2, Paul O’Brien*1,2 and David J. Lewis.2,* 1 2

School of Chemistry, University of Manchester, Oxford Road, M13 9PL, United Kingdom. School of Materials, University of Manchester, Oxford Road, M13 9PL, United Kingdom.

ABSTRACT: The coordination complex WS3L2 (where L = S2CN(CH2CH3)2 ) can be used to deposit tungsten disulfide (WS2) thin films by aerosol-assisted chemical vapour deposition (AACVD). When WS3L2 is used in conjunction with the previously reported precursor, MoL4 which produces molybdenum disulfide (MoS2) by AACVD, alloyed thin films of the type Mo1xWxS2 are produced. The W/Mo ratio can be controlled by changing the relative concentrations of precursors in the carrier aerosol, allowing straightforward manipulation of the optical properties of the material and exquisite control of the final film composition.

Metal chalcogenides are useful semiconductor materials for a range of applications.1 The transition metal dichalcogenides (TMDCs) molybdenum disulfide (MoS2) and tungsten disulfide (WS2) in particular have been used in photovoltaic devices,2 electronic devices3 and biosensing using field-effect transistors.4 MoS2 has been used as a catalyst in hydrogen evolution reactions,5 and it has been shown that the performance of such catalysts can be improved by doping the material with other transition metal ions.6 Both bulk MoS2 and WS2 possess indirect band gaps of around 1.3 and 1.4 eV respectively but with relatively low carrier mobilities e.g for MoS2 µn ≈ 100 cm2 Vs-1. Interestingly, they both adopt isostructural van der Waals layer motifs and when exfoliated to a single monolayer, the band gaps of MoS2 and WS2 increase to 1.6 and 1.7 eV respectively due to quantum confinement in the c crystallographic axis direction,7 and the transition becomes allowed and direct in nature.8

Figure 1. Molecular precursor complexes synthesised and used in this study for chemical vapour deposition of Mo1-xWxS2 (0 ≤ x ≤ 1) thin films.

Band gap engineering of semiconductors is extremely important to optimise the absorption energy for a given application. Band gap tuning of TMDCs has been demonstrated by approaches including chemical treatment, dimension tuning, defect engineering and dielectric screening.9 Alloying or doping of the TMDC allows precise band gap tuning over a wider energy range.10, 11 Moreover, the metal centres in many TMDC materials adopt the same trigonal-prismatic or hexagonal coordination geometies and the crystal unit cells have similar lattice dimensions, allowing relatively strain-free inclusion of heteroatom dopants within the framework,12, 13 and as such there is little thermodynamic penalty to the formation of stable, ternary materials. The latter have composition-dependant band gaps and potentially improved performance in applications compared to the parent binary compounds.10 Band gap tuning of TMDCs via doping has been demonstrated in bulk TMDC crystals14-16 as well as 2-dimensional materials.17 The introduction of dilute magnetic properties into nanosheets has been predicted 18 and is being explored experimentally, yet is hampered by 3D vs. 2D coordination preferences of the magnetic dopants e.g. Mn2+, Fe2+ as well as the influence of the substrate, as-reported by Robinson and co-workers,19 and this still remains a challenge.6 Aerosol-assisted chemical vapour deposition (AACVD) is an inexpensive ambient pressure CVD technique which can produce thin films of metal chalcogenide semiconductors.20 We have recently reported the synthesis of metal chalcogenide materials for photovoltaic, optoelectronic and tribological applications using AACVD including MoS2,21 tin sulfide (SnS),22, 23 copper zinc tin sulfide (CZTS),24 cadmium sulfide (CdS)25 and pyrite (FeS2).26 We have also shown using a dual-source precursor approach that it is possible to alloy transition metal cations such as chromium into the MoS2,27 leading to morphological and crystallographic changes which may improve mechanical properties.28 Outside of metal chalcogenide materials, we have recently explored the use of AACVD to produce inorganic-organic perovskite materials that may be of importance in low-cost thin film photovoltaic materials.29 CVD is an inherently scalable process, and recently there have been significant reports of the chemical vapour deposition of MoS2 over areas greater than 1000 mm2.30 The discrete molecular precursors which are often used in the AACVD approach offer many advantages including preclusion of pre-reaction and control over product stoichiometry as-described in the review by Malik et al.31 Alloying the materials MoS2 and WS2 should allow access to a continuous range of band gaps between those of the parent materials, and the suppression of defect states that reduce carrier life-

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Figure 2. EDX spectroscopy and representative plan-view secondary-electron (SE) SEM (8 kV) images of Mo1-xWxS2 (0 ≤ x ≤ 1) thin films deposited by AACVD at 450 °C using WS3L2 and MoL4 single-source precursors. (a) Relationship between available at% tungsten in precursor feed and the amount of tungsten found in Mo1-xWxS2 (0 ≤ x ≤ 1) thin films thin films by quantitative EDX spectroscopy. (b) SE SEM image of an MoS2 thin film (x= 0). (c) SE SEM image of an Mo0.7W0.3S2 thin film (x = 0.3). (d) SE SEM image of an Mo0.5W0.5S2 thin film (x = 0.5). (e) SE SEM image of an Mo0.4W0.6S2 thin film (x = 0.6). (f) SE SEM image of a WS2 thin film (x = 1). All scale bars represent 4 µm.

times, as-demonstrated by Yang et al.32 A major barrier to realisation of TMDC based optoelectronic devices is the short lifetimes of charge carriers generated in the material due to chalcogen vacancies, an effect that can be mitigated by use of the alloy Mo0.5W0.5S2, yielding faster response times and higher responsivity in the devices fabricated with this material.33 In this paper we report the alloying of MoS2 with W by AACVD using WS3L2 and MoL4 molecular precursors (where L = S2CN(CH2CH3)2 see Figure 1 for structures) to produce Mo1-xWxS2 polycrystalline thin films over the range 0 ≤ x ≤ 1. To the best of our knowledge, it is the first time that a single-source tungsten precursor has been used for the synthesis of WS2 thin films by a CVD approach.

The synthesis of the novel air-stable single source precursor WS3L2 was achieved by the reaction of [NH4]2WS4 and sodium diethyldithiocarbamate in an acidic medium, to produce WS3L2 as a dark green powder, accompanied by a small quantity of the tungsten dimer W2S4L4, which was isolated and analysed separately. WS3L2 can be easily removed by extraction from crude WS3L2 using acetone. The monomeric tungsten precursor was deemed pure by elemental analyses. Other analytical data were in agreement with the literature.34-38 Peaks corresponding to protonated WS3L2 as well as its acetonitrile adduct were dominant in the electrospray mass spectrum: ES-TOF+ m/z: 576.94 (33 %, [M+H]+), 660.04 (100 % [MeCN·M+H]+). The isotope pattern of

Figure 3. Optical analyses of Mo1-xWxS2 (0 ≤ x ≤ 1) thin films deposited by AACVD at 450 °C using WS3L2 and MoL4 single-source precursors. (a) Raman spectra of Mo1≤ x ≤ 1) thin films. (b) Shifts of the E2g (I, II) and A1g (III) bands of Mo1-xWxS2 (0 ≤ x ≤ 1) thin films as a function of mole fraction (x) of tungsten as-found by EDX spectroscopy. The evolutions of bands I, II and III are fitted to linear functions in all cases. (c) UV-Vis-NIR spectra of Mo1-xWxS2 (0 ≤ x ≤ 1). xWxS2 (0

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the complex from mass spectrometry confirms the inclusion of only one tungsten centre. WS3L2 is readily soluble in a range of organic solvents (such as dichloromethane, tetrahydrofuran, toluene, and acetonitrile) but it is insoluble in alcohols and non-polar hydrocarbons such as hexane. It is currently unclear what exactly the mechanism of formation is for WS3L2, and though investigations are underway to ascertain how this complex is formed, and we tentatively suggest that the heteroleptic sulfur ligands arise from partial decomposition of the dithiocarbamate precursors in the acidic reaction media.39 All synthetic techniques and associated analytical data are detailed in the Supporting Information. Thermogravimetric analysis (TGA) can be used to analyse the weight loss of a compound when heated under an inert atmosphere and thus infer the decomposition products produced during chemical vapour deposition. Our results matched well with those previously reported for MoL4. 40 After the final decomposition step at around 400 °C, the residue is comprised mainly of MoS2. TGA at a heating rate of 10 °C min-1 of WS3L2 revealed a four step decomposition with inflection points for each step at 173, 206, 330 and 402 °C, corresponding to weight losses of 5%, 20%, 11% and 13%, with a total aggregate weight loss of 49% (Supporting Information), which is attributed to decomposition of the dithiocarbamate ligands.40-43 The calculated weight loss for the formation of pure WS2 is 57%. We tentatively ascribe the 8% discrepancy in the weight loss to residues from the dithiocarbamate ligands, as has been observed previously in the TGA analysis transition metal dithiocarbamate complexes by T.J. Marks et al.40 This is confirmed by further heating to 1000 °C which reduces the percentage weight remaining to exactly that expected for the formation of WS2 (Supporting Information). Thin films of WS2, MoS2 and Mo1-xWxS2 (0 < x < 1) alloys were deposited on glass substrates by aerosol-assisted chemical vapour deposition (AACVD). Solutions of either WS3L2 or MoL4 in THF were used as the feeds for the compounds WS2 and MoS2, whilst for Mo1-xWxS2 (0 < x < 1) a mixture of both precursors in THF was used, keeping the total no of moles of metal constant with respect to the deposition of the end compounds. Precursor solutions were nebulised and the vapour carried by a stream of argon gas into the furnace and thermally decomposed on the substrate. The TGA analysis of both precursors performed a priori (vide supra) was used to select a furnace temperature of 450 °C in which decomposition of both precursors occurred simultaneously. All films produced by AACVD were matte grey to black in colour. Powder X-ray diffraction measurements (Supporting information) suggest that the crystals in the films have preferred orientation in the (002) basal plane. Measurement of the lattice constant c did not vary significantly across the compositional variations made. This is most likely due to convolution of the effect of alloying with lattice strain imparted by grain size variations in the samples. Stylus profilometry of scratched films revealed thicknesses in the range 300 – 1000 nm, which was supported by side-on SEM images on fractured films (Supporting Information). Energy dispersive X-ray (EDX) spectroscopy of the Mo1-xWxS2 (0 ≤ x ≤ 1) thin films deposited by AACVD at 450 °C using mixtures of WS3L2 and MoL4 demonstrated a linear relationship between the available tungsten in the feed solution and the amount of tungsten found in the Mo1-xWxS2 (0 ≤ x ≤ 1) alloyed thin films (Figure 3 a). Only the elements W and S were found in films generated from the decomposition of WS3L2 alone, with ca. 28 at.% W found in the film, close to the theoretical value of 33 at.% W in WS2. Similarly, AACVD using the MoL4 precursor produced solely MoS2 thin films. By using both precursors it was possible to access the alloys Mo1-xWxS2 with x = 0.3, 0.5 and 0.6,

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and if one includes the end compounds MoS2 (x = 0) and WS2 (x = 1 ), as described above, it is clear that a full series of films ranging from pure MoS2 to pure WS2 with alloys in between could be accessed by AACVD. Secondary-electron scanning electron microscopy (SEM) was used to investigate the surface morphology of Mo1-xWxS2 (0 ≤ x ≤ 1) thin films. Secondary electron SEM images of MoS2 and WS2 films (Figures 2b and 2f) revealed a lamellar-like appearance for both films, consistent with the preferred orientation observed in the (002) plane from powder X-ray diffraction measurements performed on films. The lamellar features observed in WS2 films were, in general, larger than those observed in the MoS2 film. This lamellar appearance was also observed in all alloyed films (Figure 2c,d,e) Interestingly, the films with lower amounts of tungsten incorporated (Mo0.5W0.5S2 and Mo0.7W0.3S2) had areas where nanowires had grown (Supporting Information), the composition of these nanowires was found to be the same as that of the bulk film by EDX spectroscopy. Hence, alloying of MoS2 with tungsten, under these conditions, could potentially produce structures with high surface areas that may be useful for catalytic applications. Raman spectroscopy was used to confirm the formation of MoS2 and WS2 (Figure 3a). The Raman spectrum of the film produced by AACVD from WS3L2 at 450 °C possess two major bands at 355 and 420 cm-1, corresponding to the E2g and A1g optical phonons of WS2, undoubtedly confirming its formation from the single source precursor. Similarly, the Raman spectrum of the film produced by AACVD from MoL4 at 450 °C gives two bands at 383 and 408 cm-1, which can be assigned to the E2g and A1g optical phonon modes respectively, agreeing with our previous work with this precursor.27 Raman spectroscopy was also used to investigate Mo1-xWxS2 (x = 0.3, 0.5 and 0.6) thin films deposited by AACVD from mixtures of WS3L2 and MoL4 at 450 °C. Alloyed thin films display a single broad band A1g phonon, alongside two phonon bands of E2g symmetry. The dependence of the Raman shift for the three prominent bands in all films (denoted with Roman numerals I, II and III) was plotted as a function of W content (mole fraction x) as found by EDX spectroscopy (Figure 3 b). The A1g phonon displays one-mode behaviour, as only the sulfur atoms are displaced during this lattice vibration, and hence is common to both WS2 and MoS2, as well as the Mo1-xWxS2 (x = 0.3, 0.5 and 0.6) alloys in between. On the other hand, the E2g phonon is found to exhibit two-mode type behaviour, primarily due to the fact that metal centres are also involved in this lattice vibration, and the greater atomic mass of the tungsten atom compared with the molybdenum atoms ensures that they modes can be easily resolved in the Raman spectra. The shifts in the bands are fully consistent with the observations made by Huang and coworkers for Mo1-xWxS2 (0 ≤ x ≤ 1).14 Additionally, UV-Vis-NIR absorbance spectroscopy demonstrated a progressive and monotonic red shift in absorbance onset in going from WS2 (ca. 850 nm) through Mo1-xWxS2 (0 < x < 1) alloys and finally MoS2 (ca. 950 nm). This is consistent with tuning of the optical band gap of the materials by alloying, and is consistent with the magnitude of the indirect bandgaps found by Kam and Parkinson for the end compounds.44 High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) of NMP-exfoliated flakes revealed highly crystalline few layer sheets with a hexagonal crystal structure, as revealed by Fourier transforms of the images (Figure 4). This observation corroborates well with the results of p-XRD and Raman spectroscopy, which were consistent with the 2H-MoS2 structure, preserved at all dopant levels and in both pure MoS2 and pure WS2 as synthesised by AACVD. Previous work on MoS2 generated by the method presented here displays the same struc-

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tural features when analysed by HAADF-STEM.27 Energy dispersive X-ray (EDX) spectrum imaging shows a uniform distribution of Mo and W throughout the material, indicating an alloyed material rather than a segregated structure. Quantification of Mo:W ratios was performed using the Cliff-Lorimer approach, 4 flakes from each sample were analysed giving the following compositions: W0.3Mo0.7S2, 29.4 ± 1.9 at.% W; W0.5Mo0.5S2, 56.7 ± 6.2 at.% W; W0.6Mo0.4S2, 74.3 ± 4.3 at.% W. The elemental homogeneity of films was also apparent using EDX elemental mapping of Mo, W and S at 10 µm scale in the SEM (Supporting Information). Inductively coupled plasma optical emission spectroscopy (ICPOES) was also used for the analysis of tungsten and molybdenum ratios and agreed closely with the ratios found by EDX spectroscopy (see Figure S3), indicating that the two chosen precursors

SEM. Raman spectroscopy shows that the A1g band of the films displays single mode behaviour, whilst the E2g band displays dual mode behaviour due to metal alloying. Atomic resolution HAADF STEM images with EDX spectroscopy which coupled to EDX spectrum mapping at the nanoscale and microscale reveals homogeneous alloying, and we conclude that the control over alloying is quite exquisite using this approach. ASSOCIATED CONTENT

Supporting Information Supporting Information available: synthetic procedures and characterization, instrumentation, description of AACVD apparatus, characterization of reaction by-product, TGA of WS3L2, SEM images of Mo1xWxS2 nanowires, full X-ray powder patterns of Mo1-xWxS2 (0 ≤ x ≤ 1) thin films deposited by AACVD at 450 °C using WS3L2 and MoL4 single-source precursors, full EDX spectra and spectrum mapping of thin films at the nanoscale and microscale, and film thickness measurement by stylus profilometry. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author *[email protected] *paul.o’[email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT

Figure 4. (a) HAADF-STEM image of a flake taken from the W0.5Mo0.5S2 sample. (b), (c), and (d) Elemental maps extracted from EDX spectrum image showing distributions of S, Mo and W respectively, demonstrating homogeneous distribution of Mo and W. (e) Atomic resolution HAADF-STEM image of WS2 flake. The Fourier transform is inset in the top right corner, the flake is viewed down the [001] zone axis and that spots can be indexed to the {100} and {110} families of planes. (f) Enlarged region [indicated by dashed box in (e)] is shown for clarity, the lattice spacing of (100) planes is measured as ~0.27 nm. (g) Target vs. Measured concentrations of tungsten dopant concentration, by STEM-EDX and ICP-OES. Atomic resolution imaging was performed on all samples, crystalline flakes with the expected hexagonal crystals structure were found in all cases. Spectrum imaging of all samples showed similarly homogeneous alloying; these additional spectrum images and their corresponding summed spectra can be found in the supporting information.

are well-matched for this dual source route to alloyed Mo1-xWxS2 films via AACVD. The linear relationship between at.% W in the CVD feed with at.% W found in the Mo1-xWxS2 (0 ≤ x ≤ 1) product ensures that stoichiometry can be controlled across the whole range of alloys explored. Conclusions We have synthesised a range of thin solid films of formula Mo1≤ x ≤ 1) by AACVD using a dual precursor approach. To the best of our knowledge, this is the first instance of the chemical vapour deposition of WS2 from a single-source precursor. We have characterised the films using EDX spectroscopy and SE

xWxS2 (0

N.S. and P.O.B. thank the Parker Family for funding. Some of the equipment used in this study were provided by the Engineering and Physical Sciences Research Council (Core Capability in Chemistry, EPSRC grant number EP/K039547/1). SJH and EAL thank the Defense Threat Reduction Agency (grant HDTRA1-121-0013) and EPSRC (grants EP/K016946/1, EP/L01548X/1 EP/M010619/1 and EP/G03737X/1). All research data supporting this publication are presented within this publication. REFERENCES 1. Lokhande, C. D., Chemical Deposition of Metal Chalcogenide Thin Films. Mater. Chem. Phys. 1991, 27, 1-43. 2. Tsai, M.-L.; Su, S.-H.; Chang, J.-K.; Tsai, D.-S.; Chen, C.-H.; Wu, C.-I.; Li, L.-J.; Chen, L.-J.; He, J.-H., Monolayer MoS2 Heterojunction Solar Cells. ACS Nano 2014, 8, 8317-8322. 3. Sanne, A.; Ghosh, R.; Rai, A.; Yogeesh, M. N.; Shin, S. H.; Sharma, A.; Jarvis, K.; Mathew, L.; Rao, R.; Akinwande, D.; Banerjee, S., Radio Frequency Transistors and Circuits Based on CVD MoS2. Nano Lett. 2015, 15, 5039–5045. 4. Sarkar, D.; Liu, W.; Xie, X.; Anselmo, A. C.; Mitragotri, S.; Banerjee, K., MoS2 Field-Effect Transistor for Next-Generation LabelFree Biosensors. ACS Nano 2014, 8, 3992-4003. 5. Hinnemann, B.; Moses, P. G.; Bonde, J.; Jorgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Norskov, J. K., Biomimetic hydrogen evolution: MoS2 nanoparticles as catalyst for hydrogen evolution. J. Am. Chem. Soc. 2005, 127, 5308-5309. 6. Tedstone, A. A.; Lewis, D. J.; O’Brien, P., Synthesis, Properties, and Applications of Transition Metal-Doped Layered Transition Metal Dichalcogenides. Chem. Mater. 2016, 28, 1965-1974. 7. Heine, T., Transition Metal Chalcogenides: Ultrathin Inorganic Materials with Tunable Electronic Properties. Acc. Chem. Res. 2015, 48, 65-72. 8. Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F., Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805.

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