A self-limited atomic layer deposition of WS2 based on the

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A self-limited atomic layer deposition of WS2 based on the chemisorption and reduction of bis(t-butylimido)bis(dimethylamino) complexes Yanlin Wu, Muhammad Hamid Raza, Yen-Chun Chen, Patrick Amsalem, Sebastian Wahl, Kai Skrodczky, Xiaomin Xu, Kapil Shyam Lokare, Medet Zhukush, Pooja Gaval, Norbert Koch, Elsje Alessandra Quadrelli, and Nicola Pinna Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03921 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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Chemistry of Materials

A self-limited atomic layer deposition of WS2 based on the chemisorption and reduction of bis(t-butylimido)bis(dimethylamino) complexes Yanlin Wu,a Muhammad Hamid Raza,a Yen-Chun Chen,a Patrick Amsalem,b Sebastian Wahl,a Kai Skrodczky,a Xiaomin Xu,b Kapil Shyam Lokare,a Medet Zhukush,c Pooja Gaval,c Norbert Koch,b Elsje Alessandra Quadrelli,c Nicola Pinnaa* Institut für Chemie and IRIS Adlershof Humboldt-Universität zu Berlin Brook-Taylor-Str. 2, 12489 Berlin, Germany a

Institut für Physik and IRIS Adlershof, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 6, 12489 Berlin, Germany b

Université de Lyon, Institut de Chimie de Lyon UMR 5265 (CNRS – CPE Lyon – Université Lyon 1) Bâtiment 308 F, 43 Boulevard du 11 Novembre 1918, 69616 Villeurbanne, France c

*E-mail: [email protected] Abstract A novel self-terminating chemical approach for the deposition of WS2 by atomic layer deposition based on chemisorption of bis(t-butylimido)bis(dimethylamino)tungsten(VI) followed by sulfurization by H2S is reported. A broad spectrum of reaction parameters including temperatures of the reaction chamber and the precursor and durations of every atomic layer deposition (ALD) step are investigated and optimized to reach a high growth per cycle of 1.7 Å and a high quality of the deposited thin films. The self-terminating behaviour of this reaction is determined by the variation of the dose of the precursors. Surface and bulk sensitive techniques prove that highly pure and welldefined WS2 layers can be synthesized by ALD. Imaging methods show that WS2 grows as platelets with a thickness of 6-10 nm and diameter of 30 nm, which do not vary dramatically with the number of ALD cycles. A low deposition temperature process followed by a post annealing under H2S is also investigated in order to produce a conformal WS2 film. Finally, a reaction mechanism could be proposed by studying the chemisorption of bis(t-butylimido)bis(dimethylamino)tungsten(VI) onto silica, and the thermal and chemical reactivity of chemisorbed species by small molecule analyses.

Introduction Several transition metal dichalcogenides (TMDs) formed by chalcogenide ions (E) and transition metals (M, generally group IV-VII) exhibit layered crystalline structures. Each layer is made of ME6 trigonal prisms or octahedrons which share edges with each other. The layers are kept together by weak Van der Waals interactions between the chalcogenide ions. The crystal structure varies from the hexagonal P63/mmc for molybdenite (MoS2) and tungstenite (WS2) to the orthorhombic structure Pmn21, as for MoTe2. Due to such a layered structure, MoS2 and WS2 can be employed as dry lubricants at higher temperatures compared to graphite.1 Most interestingly, monolayer or few monolayers TMDs exhibit unique electrical and optical properties due to quantum confinement and surface effects that arise from the transition of an indirect bandgap to a direct bandgap semiconductor.2 This provides tunable electronic properties and low dimensionalities and strongly bounded excitons.3,4 Therefore the TMDs are now being considered for a variety of technological applications in optoelectronics, including solar cells, photo-detectors, light-emitting diodes, and 1 ACS Paragon Plus Environment

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photo-transistors.5-10 The van der Waals interactions between neighboring layers and the large specific surface area make 2D TMDs attractive candidates for energy storage (e.g. supercapacitors and batteries) and sensing applications.11-13 Especially, the large surface-to-volume ratio improves the sensitivity, selectivity and low power consumption of TMDs based sensors significantly.14,15 MoS2 received the most attention because of its relatively easy preparation.16,17 WS2,18,19 MoSe2,20,21 and WSe222,23 also gained interest due to their related structures and similarly interesting chemical and optoelectronic properties. Especially, WS2 exhibits superior optical properties and larger spin-orbit coupling,24 such as tunable bandgap between 2.41 and 3.11 eV25 and single sharp photoluminescence peak26 compared to MoS2. However, the lack of a well-developed synthesis procedure with finely controlled WS2 thin film thickness, crystallite size, and spatial homogeneity has limited their technological applications. There are a number of reports making use of chemical vapor deposition (CVD) to prepare WS2 monolayers.27-29 One possibility is using WO3 and sulfur as precursors for growing WS2 directly onto a substrate.28 A second possibility consists on depositing a layer of WOx in the first step and then reducing and sulfurizing this layer to WS2.29 The main challenge of CVD-based approaches lies in the difficulty in controlling the layer thickness precisely and producing extended and single crystalline flakes. As a finely tunable gas phase deposition method, atomic layer deposition (ALD) has attracted a wide attention.30-34 Indeed, ALD allows for a better control of the thin film thickness at the Ångström scale. A possible strategy to prepare WS2 by ALD consists of the deposition of WOx using known ALD processes and then transform it into WS2 by treatment with sulfur. However, an incomplete reduction and sulfurization process can influence the optical and electrical properties due to the presence of residual oxygen or the formation of W(VI) species. Therefore, the development of an ALD process for the deposition of highly pure WS2 will be a crucial step for the synthesis of large area and high quality WS2 layers. Using H2S as sulfur source in ALD is an option which has been used for the synthesis of metal sulfide materials.35,36 Different metal complexes have been used as precursors for ALD processes. 37,38,39 Metal amides are a class of highly reactive metal complexes which are often used as metal precursors in ALD processes. For example, tris(dimethylamino)antimony (Sb(NMe2)3),40,41 tetra(dimethylamino)molybdenum (Mo(NMe2)4)42,43 have been used as metal precursors for the ALD metal sulfides. However, Mo(NMe2)4 is an air-sensitive compound which rapidly decomposes under air close to room temperature even within the timescale of a deposition process. Bis(tbutylimido)bis(dimethylamino) metal complexes (metal=Cr(VI), W(VI), Mo(VI)), might be interesting for ALD processes of metal chalcogenides. Indeed, bis(t-butylimido)bis(dimethylamino) complexes exhibit a slightly lower reactivity than tetra-amides and require a longer reaction time, but exhibit a higher temperature stability.44 There are few reports using bis(t-butylimido)bis(dimethylamino)tungsten(VI) (BTBMW) and ozone or water as oxygen sources for WO3 ALD45 at high deposition temperatures (above 300°C) and with long water exposure/reaction times (over 60 s). However, up to now this class of metal complexes has not yet been recognized as effective precursors for the ALD of metal sulfides.46 In this work, we introduce a new ALD process of tungsten sulfide based on bis(t-butylimido)bis(dimethylamino) tungsten(VI) using H2S as sulfur source. The self-limited character of this process is ascertained, the ALD parameters are optimized and a possible reaction mechanism is proposed based on the study of the reaction of the tungsten(VI) precursor onto silica. Experimental Section 2 ACS Paragon Plus Environment

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All the depositions and pretreatments with O3 and H2O have been carried out in a home-made hot wall reactor with DP-series pneumatic valves from Swagelok. Bis(t-butylimido)bis(dimethylamino)tungsten(VI) (BTBMW) (Strem) and H2S (Air liquide) are used as precursors. Attention, H2S is a highly toxic and flammable gas. The tungsten precursor was kept in a Swagelok stainless steel bottle and degassed before use. A stream of ozone was obtained as a mixture with dioxygen in 10% nominal concentration from a generator by OzoneLab. Si-wafer (Siegert Wafer), silica, SiO2 (Aerosil® Degussa, 200 m2.g-1), carbon nanotubes (CNTs) (PR-24, Pyrograf Products, Inc.) and sapphire (Siegert Wafer) are used as substrates for different characterization methods. Oxygen, nitrogen, nitric acid and NaOH are used as received. CNTs substrate preparation: The treatment of CNTs performed prior to the ALD was reported previously.47 200 mg CNTs were dispersed in 100 mL concentrated nitric acid in a round bottom flask, and the dispersion was refluxed at 105°C under stirring for 6 h. The CNTs were washed with distilled water by re-dispersion and centrifugation cycles until the solution exhibited a neutral pH. The oxidized CNTs were then collected and dried at 80°C under vacuum overnight. 10 mg of oxidized CNTs were dispersed in 2 mL ethanol by ultra-sonication for 30 minutes. The dispersion was dropcasted on an aluminum foil cleaned with acetone and dried under a nitrogen flow. Preparation of silica-500 (SiO2-500): Silica, conditioned as a loose powder, was compacted with distilled water, calcined at 200°C in air for 1 h and partially dehydroxylated at 500°C under high vacuum (10-5torr) for 16 h at 3°C/min to give SiO2-500 . The resulting material has a silanol density of 1.4 OH/nm2.48 O3/H2O pretreatment. To activate the Si wafer and sapphire surface, a pretreatment with ozone and water was established at 300°C before the ALD process. 20 ozone pulses were first carried out. This step was followed by 20 water pulses. Pulse time, exposure time and purge time for both ozone and water treatment processes were 0.2 s, 15 s and 15 s, respectively. WS2 ALD. Bis(t-butylimido)bis(dimethylamino)tungsten(VI) (BTBMW) was kept at 80°C and H2S at room temperature with a chamber entrance pressure of 400 mbar if otherwise not mentioned. Pulse time, exposure time and purge time were set to 1.5 s, 15 s and 30 s for BTBMW and 0.2, 60, 30 for H2S as standard if not specially mentioned in the text. The pneumatic valves and the deliver lines were kept at 130°C. The deposition temperature was kept at 300°C in order to deposit high quality WS2. However, all parameters were varied to study this ALD reaction and optimize the deposition conditions and are described in detail in the results section. Characterization. The film thickness was characterized by spectroscopic ellipsometry on Si wafer. Data were collected from 370 to 1000 nm under 70° incidence angle with a SENpro spectroscopic ellipsometer from Sentech. Fits were performed using the model and software SpectraRay 4 provided with the instrument which is calculated from optical index and layer roughness. All the deposited layers are fitted with the same parameter. Although, the as fitted results are not the absolute layer thickness, these numbers are the relative values, which also have physical meaning and still can be used to compare the layer thickness of materials deposited under different conditions and for various number of cycles. Therefore, the layer thickness and the growth per cycle in this manuscript are not the absolute values, but these relative values can give an overview of the deposition and allow us to study the growth of the WS2 on planar substrates. The morphology and structure were characterized by a Philips CM200 LaB6 transmission electron microscope (TEM) operated at 200 kV. High resolution transmission electron microscopy (HRTEM) images were obtained using a FEI Talos F200S operated at 200kV scanning/transmission electron microscope 3 ACS Paragon Plus Environment


(S/TEM). The samples for X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were drop casted oxidized CNTs (dispersed in ethanol by ultra-sonication) deposited onto an aluminum foil cleaned with acetone and subsequently dried under a nitrogen flow. XPS was performed in an ultrahigh vacuum chamber (base pressure 5.10-10 mbar) using a JEOL JPS-9030 set-up comprising a photoelectron spectrometer hemispherical energy analyzer and a monochromatic Al Kα (hν = 1486.6 eV) X-ray source. The XPS measurements were performed with an energy resolution of 0.7 eV as determined on a polycrystalline Ag 3d core level. A STOE MP X-ray diffractometer (XRD) operated at 40 kV, 100 mA with Molybdenum Kα radiation (λ = 0.7094 Å) was used for structural analysis. The 2θ range was set from 2° to 52°. Grazing incidence X-ray diffraction (GIXRD) was measured with a Siwafer after 350 ALD cycles with an incident angle of 1.5°. Raman spectra were acquired with a confocal microscope (XploRA, Horiba Ltd.)-based Raman spectrometer using a 532 nm laser as the excitation wavelength. The 520 cm-1 phonon mode from the silicon substrate was used for calibration. AFM topography images were obtained by using a Bruker Icon SFM and CONTV-A cantilevers. Raman and AFM measurements were performed in ambient condition. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) spectra were recorded on a Nicolet 6700-FT spectrometer using a cell equipped with CaF2 window. Typically, 64 scans were accumulated for each spectrum (resolution 4 cm-1). Solution NMR spectra were recorded on BRUKER AVANCE 300 spectrometer (1H: 300.1 MHz, 13C: 75.4 MHz). Chemical shifts are given in ppm (h) relative to TMS (tetramethylsilane). Spectra were recorded using the residual peak of the deuterated solvent as internal standard. Gas phase analysis was performed on a HP 5890 gas chromatograph, equipped with a flame ionization detector (FID) and a KCl/Al2O3 on fused silica column (50 m × 0.32 mm). A spherical syringe joint was used to extract 300μL of gaseous product into a gas syringe and was injected into GC.

Reaction of BTBMW with silica. A 500 mL closed tubular reactor was loaded in an argon-filled glovebox with two vessels containing silica powder (ca. 300 mg) and bis(tbutylimido)bis(dimethylamino)-tungsten(VI) (100 µl), placed under static vacuum for 30 min in an oven pre-heated at 60°C and cooled back at room temperature. The volatiles were then collected by condensation into a Young-valve NMR tube containing C6D6 and analyzed by 1H-NMR spectroscopy. The yellow powder was treated under dynamic high vacuum (10-5 torr) and analyzed by DRIFTS. A first aliquot of the yellow powder was exposed to water vapor and the resulting volatiles condensed into a C6D6 filled Young-valve NMR tube and analyzed by 1H-NMR spectroscopy. A second aliquot of the yellow powder was analyzed by thermogravimetric analysis (1°C/min heating ramp, under N2). The remaining powder was heated to 300°C (10°C/min heating ramp and 20 min heating at 300°C) under dynamic or static vacuum and IR spectra of the resulting powders were recorded. Following the thermal treatment under static vacuum, the head space of the reactor was collected and analyzed by GC-MS. The red powder was further exposed to water vapor and the volatiles condensed into a C6D6 filled Young-valve NMR tube and analyzed by 1H-NMR spectroscopy and GC-MS.

Results and Discussion Bis(t-butylimido)bis(dimethylamino)tungsten(VI) (BTBMW) has been reported to react with water at high temperatures (above 300°C) for the ALD of WO3. High deposition temperatures were necessary to obtain a substantial growth per cycle.45 Therefore, we also started developing the WS2 ALD process at high temperature (300°C). The pulse, exposure and purge times were set to 1.5 s, 15 s and 30 s for BTBMW and 0.2 s, 15 s and 30 s for H2S, respectively. These parameters were chosen due to 4 ACS Paragon Plus Environment

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the experience acquired in our group by using the same self-made reactor for other ALD reactions. However, no deposition was observed. Considering the available reports on the use of BTBMW for the WO3 ALD, stating that an extremely long water exposure time was necessary due to the slow kinetic process,49 a long H2S exposure time was carried out following two different procedures. One consists of four pulses and exposures (0.2 s and 15 s respectively), and the other in only one short H2S pulse followed by a long exposure (0.2 s and 60 s respectively). Both methods exhibit the same growth per cycle (GPC) of approx. 1.7 Å/cycle. These results show that similarly as for the deposition of WO3, the co-reactant (H2S) reaction step is also a slow kinetic process even at 300°C, thus a long exposure to H2S is necessary. Additionally, these results prove that the GPC stays constant with different amounts of H2S, suggesting that the sulfurization reaction is self-terminated. The selftermination behavior was also confirmed for the tungsten precursor. The pulse time was varied between 0.5 s and 2 s for BTBMW. The GPC first increased with the increasing pulse time of BTBMW and remained stable when the pulsing time was longer than 1 s (Figure 1a). In order to prove that the partial pressure of BTBMW is enough to saturate the reaction chamber, a four time pulses and exposure of BTBMW (1.5 s/30 s) was carried out. The GPC remained constant at 1.7 Å. The fact that i) the GPC as a function of the pulse time and the number of pulses of BTBMW is constant, as well as ii) the GPC with the variation of the number of pulses of H2S, proves that our ALD process exhibits the required self-termination behavior. Finally, a long exposure time of BTBMW (60 s) showed also a GPC of 1.7 Å precluding the thermal decomposition of the tungsten precursor and a CVD component on the WS2 ALD process at 300°C. This study allowed us to set the standard ALD parameters for our reactor. The precursors pulses/exposures/purging times will be from now on kept constant at 1.5 s/15 s/30 s and 0.2 s/60 s/30 s for BTBMW and H2S, respectively.

Figure 1 (a) Surface saturation determination of WS2 ALD reaction. The GPC saturates at a certain precursor dosage, after which it remains constant, reflecting the self-limiting characteristic of the ALD process. The pulse durations of BTBMW are varied between 0.5 and 2 s while the pulse of H2S was kept at 0.2 s. The red dotted line is a least-squares fit. (b) Thin film thickness after 100 ALD cycles at different deposition temperatures characterized by spectroscopic ellipsometry.

The influence of the O3/H2O pretreatment on the GPC was determined by comparing two Si wafers with and without pretreatment. The reactor was kept at 300°C during the pretreatment. The thickness of WS2 layer after 200 ALD cycles with and without pretreatment was 33 nm (GPC = 1.7 Å) and 1.7 nm (GPC = 0.085 Å), respectively. The pretreatment increases the GPC significantly, most 5 ACS Paragon Plus Environment


likely because it increases the concentration of the surface-active sites. The reaction temperature was then optimized between 100°C and 325°C. The GPC was determined by spectroscopic ellipsometry using a Si wafer as substrate and the quality of the deposited layers was characterized by XPS and Raman spectroscopy. The results from ellipsometry (Figure 1b) shows that at very low reaction temperatures, the GPC was significantly high (~3 Å/cycle), which suggests that at low temperature condensation of the precursor onto the substrate surface occurs, leading to a CVD-like process. Between 200°C and 300°C the GPC increases with the increase of the deposition temperature. Above 300°C the GPC stays constant. The ellipsometry results provide information on the film thickness, but cannot provide information on chemical composition and crystallinity of the deposited material. XPS study: The W 4f and S 2p core levels measured by XPS are presented In Figure 2a for different reaction temperatures. The W 4f spectra consist of a doublet due to 4f 7/2 and 5/2 spin-orbit components, the two peaks being separated by 2.15 eV. At temperatures above 250°C, it shifts to lower BE (32.1-32.3 eV). The latter observed range of BE typically corresponds to W(IV) in WS2.49,50 In contrast, at lower temperatures, here ranging from 100°C up to 200°C, the 4f 7/2 component is found at a binding energy (BE) of 33.3 eV. The W 4f BE for the samples grown at lower temperature are slightly shifted comparing to the BE of WS2. This BE is intermediate between that of W(IV) in WS2 and that of W(VI) as in WO3 (ca. 35.6 eV BE), which suggests non-completely reduced W species at low reaction temperature. Figure 2b displays the temperature-dependent S 2p spectra whose doublet due to 3/2 and 1/2 spin-orbit components is separated by 1.15 eV. At low as well as at high reaction temperature, the S 2p3/2 peak maximum is found at 162 eV BE and is only observed at slightly lower BE (161.8 eV BE) for the 250°C sample. This range of binding energy is consistent with the literature values as determined for S2- in WS2.49,50 Additionally, at low deposition temperature a low N 1s contribution at approx. 400 eV is observed. As expected, nitrogen contribution disappears with the increase of the deposition temperature (Figure S2).

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Figure 2. XPS of W 4f (a) and S 2p (b) core levels for WS2 deposited on CNTs at different temperatures. The dashed line is a guide to the eye for showing the energy shift of the core levels.

Figure 3. Deconvolution of the W 4f (a) and S 2p (b) spectra of WS2 on CNTs deposited at 250°C and 300°C. The XPS analysis includes W4+, W6+ and S2- contributions.

Generally the full widths at half maximum (FWHM) of the core level spectra are excellent indicators of the sample quality because they can usually be correlated to chemical purity and presence of defects. The lineshape of both core levels at low temperature (at 100°C, 125°C and 200°C) is very broad with a FWHM of ca. 1.5-1.6 eV (Figure 2). Notably, the spectral width for both W and S becomes sharper upon increasing the deposition temperature. This is revealed in more detail by fitting the W 4f and S 2p spectra for the samples processed at 250°C and 300°C (Figure 3). The fits of the films grown at 300°C provide a FWHM of 0.8 eV for both the W and S core levels. Such width, together with the determined BE for W 4f and S 2p, compares very well with previous XPS studies of 7 ACS Paragon Plus Environment


WS2 grown by other methods and likely indicates the formation of a well-defined, possibly (poly-)crystalline, WS2 film.49-51 A FWHM of 1.2-1.3 eV is determined for the WS2 film deposited at 250°C, suggesting that this reaction temperature leads to films of lower quality, both in term of chemical homogeneity and crystallinity. In addition, the XPS signal at ca. 35.6 eV BE assigned to W(VI) is clearly observed for the sample produced at 250°C. The W(VI) contribution likely arises from the incomplete reduction of the tungsten precursor at low reaction temperature (see below). When the temperature is increased to 300°C, this signal decreases close to the detection limit further demonstrating the improved quality of the sample deposited at 300°C. Raman study: The formation of WS2 on CNTs is further studied by Raman spectroscopy in the spectral region between 300 cm-1 and 500 cm-1, which is usually used to unambiguously assess the formation of WS2. The Raman spectra of the samples grown at different deposition temperatures are compared to a reference multilayer CVD-grown WS2 flakes. Multilayer WS2 flakes were synthesized with a tube furnace using sulfur and WO3 powder as the precursors52 (Figure 4). In agreement with the literature, the CVD produced WS2 spectrum exhibits two strong peaks centered at 352 cm-1 and 419 cm-1, which constitute the vibrational fingerprint of crystalline WS2.53,54 The first peak features a double structure which can be ascribed to both the 2LA(M) longitudinal acoustic mode at the M point (352 cm-1) and E12g (Γ) (355 cm-1) in plane phonon mode while the second peak (419 cm-1) corresponds to the A1g(Γ) out-of-plane mode.53,54 For the samples produced by ALD at low temperature, both of these features are missing demonstrating that WS2 has not formed under these reaction conditions. At 250°C, WS2 related features can be observed but are relatively faint and broad compared to the reference CVD WS2 spectrum. Only at high deposition temperature, e.g. 300°C, the Raman spectrum displays the WS2 phonon modes with spectral features comparably sharp and intense similarly to the reference sample, thereby proving the formation of well-defined WS2 film. Noteworthy, this conclusion is in good agreement with the XPS results. Therefore, considering the results from ellipsometry, XPS and Raman, the reaction temperature in the following will be kept at 300°C as standard to produce high growth rate, highly pure and well-defined WS2.

Figure 4. Raman spectra of WS2 deposited on CNTs by ALD at different temperatures. The spectrum at bottom corresponds to a multilayer WS2 film grown by CVD.52


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The GPC was systematically determined in terms of thickness vs. number of cycles (Figure 5) with the data collected from spectroscopic ellipsometry on Si/SiO2 wafers. Figure 5a shows the original data from spectroscopic ellipsometry in which the curves shift to low Δ (phase shift of the polarized light) with the increase of the number of ALD cycles. The shift between 0 and 50 cycles is smaller compared to higher number of ALD cycles. This could be caused by the non-saturated growth at the beginning of the deposition due to the initial low surface-active site concentration or the different growth modes during the deposition procedure. The growth is highly linear (R2 > 0.99) with a high growth per cycle of 1.7 Å (Figure 5b). The intercept on x-axis at 40 cycles proves that there is indeed an inhibition on the nucleation of WS2 at the beginning of the deposition.

Figure 5 (a) Spectroscopic ellipsometry graphs (Delta Δ(λ)) of Si/SiO2 wafers for various numbers of ALD cycles (0, 50, 100, 150, 200, 250, 300 and 350), evidencing the systematic film thickness increase. (b) Growth curve determined from the ellipsometry data, evidencing the linear behavior with a growth per cycle of 1.7 Å. The experimental uncertainty on film thickness is ±3 nm due to the error of the measurement of ellipsometry and the fitting model.



Figure 6 High resolution transmission electron micrographs of WS2/MW-CNT deposited at 300°C after 50 (a and e), 200 (b and f) and 350 (c d and g) ALD cycles. HR-TEM micrograph of the edge of a coated MWCNT (d). The inset shows the power spectrum of the region selected by the black circle. The scale bars are the same for a, b and c and the scale bars of e, f and g are the same. The highlighted parts in e, f and g are the walls of CNTs.

Transmission electron microscopy, powder X-ray diffraction and atomic force microscopy studies: The morphology of WS2 deposited onto CNTs was investigated by high resolution transmission electron microscopy (HRTEM) (Figure 6). Our ALD process leads to platelet shaped WS2 particles 10 ACS Paragon Plus Environment

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grown on the CNTs. After 50 ALD cycles, the WS2 platelets are approx. 30 nm in diameter and 10 nm in thickness. However, the surface is not completely covered and the platelets are not connected with each other (Figure 6a and e). After 200 cycles the CNTs are fully coated with WS2 platelets (Figure 6b and f) forming a continuous layer of WS2 on the CNTs. After 350 cycles the CNTs surface is completely covered with a WS2 film, but the deposited layer is not as smooth as the one with 200 ALD cycles. In high magnification images (Figure 6e, f, and g), it is clearly showed that the walls of CNTs (highlighted in green) are coated both in the inside and the outside. Especially, a low number of ALD cycles, the WS2 platelets mainly grow along the CNTs walls (Figure 6e and f). However, after 350 ALD cycles, the platelets do not only grow along the CNTs walls to form a continuous layer, but also grow vertically (Figure 6c and g). Additionally, it is also confirmed that the thickness of the WS2 coating increases with the increase of the number of cycles. However, the size of the platelets does not increase linearly with the number of ALD cycles. These results show that new WS2 platelets nucleate during the whole ALD process even on the WS2 surface from the previous deposition cycles leading to an increase of the roughness of the coating with the increase of ALD cycles, especially due to some platelets growing perpendicularly to the substrate surface. At higher magnification, a single crystallite, presenting the (002) planes perpendicular to the electron beam, is selected (black circle in Figure 6d) and its power spectrum (Insets of Figure 6d) shows the characteristic reflections of the tungstenite structure aligned along the [111] zone axis (COD 96-900-9146). In Figure 6d, the (011) lattice planes are marked by the red area and the corresponding spots in the power spectrum are highlighted by red circles. All in all, HRTEM studies demonstrate a conformal coating of the CNTs by tungstenite particles consisting of platelets with thicknesses not linearly dependent on the number of ALD cycles.

Figure 7. X-ray diffraction (XRD) patterns (Mo Kα) of CNTs coated with 50 (cyan) and 200 (red) WS2 ALD cycles. Asterisks mark the reflections of the CNTs. The patterns of WS2 from COD 96-900-9146 (dark red columns).

The structure is confirmed by X-ray diffraction studies on WS2 grown on CNTs. The high surface area substrate allows for depositing a large amount of WS2 platelets onto the high surface area of the CNTs, leading to a strong intensity of the reflections in the XRD pattern. In Figure 7, the reflections at 11 ACS Paragon Plus Environment


approx. 12° (marked with an asterisk), are due to the CNTs. The peaks at 6.4° (002), 14.8° (100), 15.15° (101), 16.18° (012), 19.6° (006), 25.78° (110) and 26.24° (008) are characteristic of the WS2 (COD 96900-9146) and are observed in both patterns. As expected, an increased amount of deposited material leads to sharper and more intense peaks (cf. also the relative ratio between CNTs and WS2 reflections). Most of the reflections of WS2 are also observed from the grazing incidence X-ray diffraction (GIXRD) experiments on a Si-wafer coated with 350 ALD cycles (Figure S3). The bandgap of the crystalline WS2 by ALD is characterized by UV/Vis spectroscopy (Figure S4). The bandgap is approx. 1.85-2.0 eV55 which is similar as published values for thin layers of WS2.56,57 Finally, the surface of the WS2 films is characterized by atomic force microscopy (AFM) from samples deposited on sapphire with different number of ALD cycles. Also in this case WS2 platelets having approx. 30 nm in diameter are observed (Figure S5). These results are in good agreement with the electron microscopy study. Indeed, as in the TEM study on CNTs, the 50 ALD cycles sample shows that the substrate was not fully covered with WS2 platelets and the surface of the substrate was still observed as demonstrated also by the line profile (Figure S5a,b and c). On the other hand, the image of the sample with 350 ALD cycles shows a complete covered surface with WS2 platelets with a higher roughness (Figure S5d, e and f). Finally, the line profiles of both samples display that the thickness of the platelets is between 6 and 10 nm which is in good agreement with HRTEM studies on the CNTs. TEM and AFM studies showed that under the standard deposition condition at 300°C the deposition leads to a non-conformal film made of crystalline nanoplatelets. In order to avoid the crystallization during the ALD process and to obtain a conformal coating, the deposition was carried out at low temperature (125°C) and was followed by a post-deposition treatment under H2S at 300°C to complete the sulfurization process. The AFM images and their line profiles display smooth layers both as deposited and after annealing at 300°C under H2S (Figure S6). The TEM images of CNTs coated with 50 ALD cycles of WS2 at 125°C after annealing at 300°C under H2S show also a continuous and smooth WS2 coating both on the inside and outside of the CNTs (Figure S7). The highly crystalline nanoplatelets which are deposited at 300 °C are replaced by randomly oriented pseudo-amorphous particles forming a conformal film. This observation is in good agreement with what was previously discussed by Ritala et al. 58

Proposed growth mechanisms: The growth mechanism of WS2 from alternating pulses of W(VI) precursor and H2S at 300°C has to imply several complex transformations, including also the reduction of the metal center. In addition to the numerous mechanism-related experimental evidence presented so far, we investigated the surface chemistry between the W(VI) precursor BTBMW and 3D beads of non-porous silica, a known pertinent model of ALD–grown MoS2 monolayers on silicon-based wafers.48 The first step of the ALD reaction was modeled by exposing silica previously dehydroxylated at 500°C (SiO2-500) to the vapor pressure of bis(tbutylimido)bis(dimethylamino)tungsten(VI) at 60°C. The low temperature was chosen to allow the observation of a well-behaved (if any) chemisorption in the absence of other thermally-induced phenomena. Indeed, a more complex reorganization is expected to occur at higher temperature in the ALD process presented here as indicated by the XPS evidence discussed above, where the concomitance of W(VI) and W(IV) species is observed below 250°C, while temperatures above 250°C induce reduction of all centers to the (+IV) oxidation state. 12 ACS Paragon Plus Environment

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The volatile byproducts of the gas-solid reaction between BTBMW vapors and SiO2-500 surface were condensed in C6D6 and analyzed by 1H-NMR spectroscopy and GC-MS, respectively. The 1H-NMR spectrum showed NHMe2 as the unique byproduct of the chemisorption step at 60°C. (1H-NMR (C6D6) δ = 2.13 ppm). The resulting tungsten-containing silica-based solid was then treated under vacuum (to remove physisorbed precursors and/or dimethylamine, if any) and exposed to water vapor. The collections of the hydrolysis volatiles in C6D6 led to the identification of HNMe2 and H2NtBu (1H-NMR (C6D6) δ = 0.95 ppm) in a 1:2 ratio as the sole by-products of the hydrolysis reaction, in agreement with the grafting reaction reported in eq. 1. The monitoring of the reaction by IR spectroscopy is consistent with equation 1 since the almost complete disappearance of the 3747 cm-1 band (indicating the almost quantitative reaction of surface silanols) and the concomitant appearance of intense bands between 2780-2970 cm-1 (due to the alkyl moieties of the surface species) are observed (Figure 8). NMe2 t

BuN

W

NtBu

NMe2

ButN

OH O

NMe2 W

O NtBu Si O O O

Si O O

(1)

SiO2-500 NHMe2

These results are in agreement with step (a) of the ALD mechanism proposed in Figure 9. The further conversion of monometallic W(VI) surface species to W(IV) intermediates leading to WS2 is certainly a complex combination of thermally-induced re-arrangements as well as H2S induced reactions, possibly involving H2S-mediated reduction, which complicates the establishment of a growth mechanism as a succession of separate and molecularly well-defined steps. We will represent this complex and still ill-defined set of chemical transformations as route (b) in the proposed mechanistic scheme reported in Figure 9. The silica-based tungsten containing solid, with the proposed surface species (≡SiO)W(NMe2)(=NtBu)2 complex according to reaction 1, was heated to 300°C and the volatile compounds analyzed by 1HNMR spectroscopy and ESI-MS (Figures S10-S12). The mass analysis shows the [M-H]+ peaks for tertbutylamine and dimethylamine (at m/z = 72 and 46 amu, respectively); a further peak at m/z = 88 amu, compatible with methyltertbutyl amine, is also observed. The 1H NMR spectrum shows no strong majority species among the amines. At this point, it is not yet clear how the amines are formed because a source of protons is needed. Noteworthy, no isobutene was observed during heating, which excludes the thermal decomposition of the chemisorbed precursor via a ß-hydride elimination as it was suggested by Becker et al. for the ALD of WN from BTBMW and ammonia.59 At the same time, the amount of amines released by thermal heating is only a small fraction of the amido and imido ligands present in solid (≡SiO)W(NMe2)(=NtBu)2. Indeed, the DRIFT spectra before and after heating to 300°C show small relative changes (fine structure and intensity of the bands) between the 2780-2970 cm-1 region, characteristic of C-H stretching vibrations, and the other regions which are expected to be unaffected by the thermal treatment (cf. the region between 2100 and 1700 cm-1, characteristic for the silica framework deformation modes, and the region around 3747 13 ACS Paragon Plus Environment


cm-1, characteristics for residual surface silanols) (Figure 8). These findings are also confirmed by the hydrolysis of the surface species after heating at 300°C showing both the release of dimethylamine and tert-butylamine. Overall, the thermal study on the silica-supported (≡SiO)W(NMe2)(=NtBu)2 shows that under non-hydrolytic conditions only a fraction of the ligands are released. Finally, taking also into account that the ALD process is self-limited (see above) we can exclude a growth mechanism based on the thermal decomposition of the chemisorbed monomers.

Figure 8. DRIFT spectra of starting SiO2-500 (black), BTBMW adsorbed on SiO2-500 at 60°C (red), and after heating at 300°C under dynamic (green) and static (blue) vacuum.

As discussed above, an ozone-water-pretreatment is necessary to decrease the distance between OH-surface species for the ALD of WS2 onto SiO2-passivated silicon wafers. Without this pretreatment, almost no deposition was observed. This observation denotes the need to obtain chemisorbed W(NMe2)(=NtBu)2 species which are close enough to initiate the nucleation of the ALD film. Tungsten centers close enough to react with each other, either during a surface reorganization step or during the reaction with H2S, is a prerequisite. Indeed, it is plausible that a dimerization of the surface (SiO)W(NMe2)(=NtBu)2 species would take place possibly forming W-N(tBu)-W bridges involving a reduction of the W(VI) to W(IV) prior to the H2S pulse, or that the reduction of W(VI) takes place during the H2S pulse following a concerted mechanism between more than one W center accompanied by the elimination of HNMe2 and H2NtBu leading to the formation of WS2 (Figure 9b). However, additional studies such as in situ XPS and FT-IR in an ALD reactor would be needed to discriminate between the two hypotheses and will be presented in a following up publication.


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OH

OH

Silica NMe2 t

BuN

W

NtBu

NMe2

(eq. 1)

HNMe2

NMe2 t

NMe2

BuN W NtBu tBuN W NtBu O

O

H 2S

HS

SH

S

W O

W S

O

(c)

H 2S (b) SH

SH

Silica NMe2 t

NMe2

BuN W NtBu tBuN W NtBu S

S NMe2

(a)

t

BuN

W

NtBu

NMe2

HNMe2

Figure 9 Proposed reaction steps in one cycle of WS2 ALD from BTBMW and H2S. a) Chemisorption: surface hydroxyl groups cleave W―NMe2 bonds of the tungsten complex; b) sulfurization and reduction: the dimethylamino groups and the t-butylimido are removed by H2S accompanied by the reduction of W(VI) to W(IV); c) completion of the cycle: W―SH groups are left as surface active site for the next ALD cycle.

Finally, following a reviewer’s suggestion we checked the catalytic activity of CNTs coated with WS2 towards the electrochemical hydrogen evolution reaction (HER). The overpotential at 10 mA cm‒2 was 0.26 V (Figure S8), which is similar to the one reported in the case of graphene oxide coated with WS2.60



Conclusions A novel atomic layer deposition (ALD) process of WS2 from stable and commercially available bis(tbutylimido)bis(dimethylamino)-tungsten(VI) (BTBMW) is developed. A broad spectrum of deposition parameters including the reaction temperature, the precursor temperature, the duration of every ALD step, as well as the influence of the substrate surface activation are investigated. The selfterminating behavior of this reaction is also investigated by variations of the pulse duration of the precursors and exposure times. The growth per cycle is approximatively 1.7 Å under optimized deposition conditions. The chemical, structural and morphological properties of the WS2 are studied by combining a range of analytical techniques. XPS studies demonstrate the absence of W(VI) (35.5 eV) and the presence of intense and sharp W(IV) (32.3 eV) contributions, proving the high purity of the deposited WS2 and the complete reduction of W(VI) to W(IV) during the ALD reaction at 300°C. The two well defined stokes contributions centered at 352 cm-1 and 420 cm-1 in the Raman spectra are the fingerprint of crystalline WS2. Phase contrast transmission electron microscopy studies demonstrate the deposition of a rough film made of highly crystalline WS2 platelets. If the deposition is carried out at low temperature it is possible to obtain a conformal coating WS2 upon treatment with H2S at 300°C. A reaction mechanism could be proposed by studying the chemisorption of bis(t-butylimido)bis(dimethylamino)tungsten(VI) onto silica, and the thermal and chemical reactivity of chemisorbed species by 1H-NMR spectroscopy, GC-MS and FT-IR spectroscopy. All in all, the novel ALD process introduced in this article provides an easy access to nanostructured transition metal dichalcogenides and can certainly be extended to other materials subjected to the availability of similar transition metals imido-amino complexes. Acknowledgements The authors thank Dr. Matthias Karg for the discussion of chemical reaction mechanism and Dr. Soohyung Park for XPS measurements. P. A. and N. K. thank the DFG (AM/419-1; SFB951) for funding. X. Xu acknowledges the Alexander von Humboldt Foundation for funding. Muhammad Hamid Raza acknowledges the University of the Punjab, Lahore, Pakistan for the PhD allowance.

Supporting Information. Determination of the growth per cycle, XPS of N 1s, Grazing incidence X-ray diffraction, UV/Vis absorption spectra, AFM images, High resolution transmission electron micrographs, hydrogen evolution linear swap voltammetry, 1H-NMR spectra, ESI-MS.


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A self-limited atomic layer deposition of WS2 based on the

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