Chemical vapour deposition of rhenium disulfide and rhenium-doped molybdenum disulfide thin films using single-source precursors

Polycrystalline thin films of rhenium disulfide (ReS2) and the alloys Mo1 xRexS2 (0 r x r 0.06) have been deposited by aerosol-assisted chemical vapour deposition (AA-CVD) using [Re(m-SPr)3(S Pr)6] (1) and [Mo(S2CNEt2)4] (2) in different molar ratios at 475 1C. The deposited films were characterised by p-XRD, SEM, and ICP-OE, Raman, and EDX spectroscopies. The p-XRD patterns of the films deposited from (1)


Introduction
Two-dimensional transition metal dichalcogenides (TMDCs) which have the general formula MX 2 , including molybdenum disulfide (MoS 2 ), tungsten disulfide (WS 2 ), and tungsten diselenide (WSe 2 ) have attracted interest because of their optoelectronic properties in monolayer form [1][2][3] with useful carrier mobilities and mechanical flexibility. [4][5][6][7][8][9] MoS 2 , in particular, has attracted considerable attention because of its potential applications in hydrogen storage, solid lubricants, capacitors, and electrochemical devices. [10][11][12][13][14] MoS 2 can also be used as a catalyst (e.g. in hydrogen evolution reactions) due to its high-energy crystal edges. 15 The structure of bulk MoS 2 is akin to that of graphite in terms of a repeating layer structure, 16 held together by non-covalent interactions. 2H-MoS 2 bulk shows a change from an indirect bandgap to a direct bandgap as the 1H-MoS 2 monolayer form is approached, 15 as per MoSe 2 , WSe 2 , and WS 2 . 17,18 MoS 2 thin films have been synthesised by aerosol assisted chemical vapour deposition (AA-CVD) using metal dithiocarbamate-based precursors. 19 Thin films of MoS 2 doped with Re have been previously synthesised by using spray pyrolysis. 20 We and others have recently developed a general approach to doping MoS 2 with transition metal cations such as chromium based on AA-CVD. 21, 22 Rhenium disulfide (ReS 2 ) has a direct bandgap which remains as such in the monolayer form unlike most other TMDCs. Few, if any, changes are observed in the Raman spectrum of monolayer ReS 2 compared to bulk and few-layer variants. 23 Transition metal doped ReS 2 has been reported. 24 Single crystals of Mo-doped ReSe 2 have been synthesised by chemical vapour transport method with bromine as a transport agent. 25 The growth of ReS 2 monolayers has been achieved using CVD at 450 1C, the as-deposited ReS 2 is an n-type semiconductor. 26 Colloidal ReS 2 nanoparticles have also been prepared. 27 Two-dimensional nanosheets of ReS 2 have been synthesised by lithium intercalation, and have unique photocatalytic properties which can potentially be superior to other twodimensional materials. 28 Large area deposition of ReS 2 sheets with good crystallinity has been achieved using simple CVD, from ReO 3 treated with elemental sulfur vapour at 500 1C. 29 Rhenium-doped MoS 2 can potentially be used as a model for immobilization of radioactive technetium-99 ( 99 Tc). 30 Technetium-99 is very mobile in water hence there is concern about its release into the environment. In 2008 the production of 99 Tc was close to 290 metric tonnes worldwide. 31 Technetium-99 is produced from uranium fission, and is present in nature at a very low level. 32 99 Tc has a long half-life (ca. 2 Â 10 5 years), and it forms about 6% of the fission product from uranium. 33,34 The relatively short-lived isotope 99m Tc is a widely used radionuclide in nuclear medicine. 35 The ionic radii of Tc(IV), Re(IV) and Mo(IV) are 0.65, 0.63 and 0.65 Å respectively, whilst the Shannon-Prewitt crystal radii of Tc(IV), Re(IV) and Mo(IV) are also similar, 0.79, 0.77 and 0.79 Å respectively. 36 The Gibbs free energy change associated with the formation of the M(IV) oxidation state from elemental rhenium and technetium are DG = 1.3F and 1.1F kJ mol À1 respectively, where F is the Faraday constant. 37 Hence from these data it may be surmised that isovalent substitution of Re into MoS 2 may be performed without significant induced strain in the lattice and minimal free energy penalty from disruption of the host lattice and that rhenium(IV) is a good model for technetium(IV). Entrainment of Re into host structures such as iron phosphate glasses is also known 38 In this paper we report the deposition of Mo 1Àx Re x S 2 (0 r x r 0.06) alloyed thin films using AA-CVD from Mo and Re single source precursors. Molybdenite exists usually as the 2H stacked (ABA) polytype (P6 3 mmc). 39,40 Rhenium disulfide has a different layered structure that is classified in the P% 1 space group, 41 of much lower symmetry than most transition metal dichalcogenides which seems to be driven by the participation of each d 3 -Re ion bonding to three others in the same layer which results in the formation of Re 4 parallelograms, with vertices linked in a linear fashion throughout the sheet. The Re-based parallelograms dictate the relative displacements of the sulfur atoms in the hexagonal close packed arrays, and due to this, the sulfur atoms in the layer are rippled on the sheet surface, in contrast to MoS 2 , where the sulfur layers appear smooth. At low levels of rhenium incorporation we find that the parent molybdenite structure holds due to the aforementioned similarities in crystal radii and the thermodynamic similarities in their +4 oxidation state. Significantly, this makes Re-doped MoS 2 a potentially useful model system for understanding the thermodynamic stability of 99 Tc in a host molybdenum disulfide lattice which will be of particular interest in nuclear chemistry for storage capability-building using inert materials that do not leach radioactive material. This is the first example of single source precursors being used to produce such materials.

Experimental section
All reactions were carried out under dry nitrogen atmosphere using standard Schlenk techniques. Solvents were purchased from Sigma-Aldrich or Fisher and used without further purification. Reagents were purchased from Sigma-Aldrich.

Instrumentation
Elemental analysis was performed by the University of Manchester micro-analytical laboratory. A Seiko SSC/S200 model was used for TGA measurements with a heating rate of 10 1C min À1 under nitrogen. Scanning electron microscopy (SEM) was performed in secondary electron mode with a Zeiss Ultra55 microscope with an accelerating voltage of 10 kV. Energy-dispersive X-ray (EDX) spectroscopy was performed on the same system at an accelerating voltage of 30 kV using an Oxford Instruments INCA pentaFETx3 detector. Transmission electron microscopy (TEM) was performed on a FEI Tecnai G2 operating at 200 kV. Nuclear magnetic resonance (NMR) spectra were recorded using a 400 MHz Bruker spectrometer. A Bruker D8 AXE diffractometer was used to record p-XRD patterns, equipped with a Cu-K a source (1.5406 Å). Thin films were scanned between 10 and 801 with step size 0.021 and a dwell time of 3 s.

Synthesis of rhenium nonachloride (Re 3 Cl 9 )
The apparatus was dried by heat gun in a stream of dry nitrogen, and then was moved with the stopcock closed into a glove bag. Rhenium pentachloride (ReCl 5 , 2.35 g) was added into the tube and moved from the glove bag and clamped, with a moderate stream of N 2 . The tube was heated in a slow Bunsen burner flame, after that the ReCl 5 started to melt and the vapour was under reflux. Brown fumes were observed to be liberated. Heating was stopped during the reaction at regular intervals to harvest the crystals which were formed on the wall of tube. This process was repeated until no more brown fumes were liberated and only solid remained. 42 The apparatus was allowed to cool, and the tube sealed off and moved to a nitrogen filled glove bag to transfer the product, a black-red solid, into an ampoule. Yield: 72%. Anal. calc. for Re 3
Synthesis of rhenium iso-propylthiolate Re 3 (l-SPr i ) 3 (SPr i ) 6 , (1) This compound was prepared using the method of Cohen and co-workers. 44 Briefly, Re 3 Cl 9 (THF) 3 (1.7 g, 1.5 mmol) was dissolved in of THF (140 mL). Sodium isopropylthiolate (1.7 g, 17.8 mmol) was added to the red solution of Re 3 Cl 9 (THF) 3 . This mixture was refluxed for 48 h after which the THF was removed under reduced pressure. The resulting solid was then extracted with hexane (5 Â 20 mL); the combined organic phase was filtered through Celite, the solvent was removed by reduced pressure then dried by vacuum oven overnight. Yield: 55%. Anal. calc. for Re 3 S 9 C 27 H 63 (%) C, 26.28; H, 5.14; found (%) C, 25

Synthesis and characterisation of the rhenium and molybdenum precursors
Re(m-S i Pr) 3 (S i Pr) 6 (1) and Mo(S 2 CNEt 2 ) 4 (2) were synthesised by literature procedures. 44,45 All characterisation data matched with that previously reported. Complex (1) was a dark red powder, whilst complex (2) appeared as dark purple crystals. Both precursors were freely soluble in THF. Thermogravimetric analysis (TGA) of complexes (1) and (2) was used to determine the thermal decomposition range for both complexes (Fig. 1). Precursor (1) decomposed in four steps in the temperature ranges of; 40-90 1C, 140-210 1C, and 215-360 1C, followed by a slower decomposition process in the temperature range 810-1000 1C and complete decomposition occurred after a 10 min. hold at 1000 1C. The remaining mass of residue (62%) suggests the formation of a rhenium sulfide (calc. 61%) (Fig. 1). Precursor (2), had a TGA profile consistent with that previously reported, 22 showing four decomposition steps in the range 0-600 1C with a final residue (29%) close to that of MoS 2 (calc. 24%).

Deposition and characterization of ReS 2 thin films
Deposition of ReS 2 films was performed using AA-CVD. The reactor temperature for deposition of ReS 2 , MoS 2 , and Re-doped MoS 2 was chosen based on thermogravimetric analysis results (TGA, vide supra). Varying molar ratios of precursor (1) and (2) were used in a THF aerosol to achieve doped thin films which were deposited at 475 1C. The films were studied by inductively coupled plasma optical emission spectroscopy (ICP-OES), powder X-ray diffraction (p-XRD), scanning electron microscopy (SEM) and Raman and energy dispersive X-ray (EDX) spectroscopies. The p-XRD pattern of rhenium disulfide thin films deposited at 475 1C (Fig. 2) revealed two major reflections at 2y = 14.371, which is assigned to the (001) plane of 1T-ReS 2 , and at 2y = 27.891 corresponding to the (111) plane. Thus the ReS 2 produced by AACVD has an apparent preferred orientation in the (001) plane.
Raman spectroscopy of the films revealed vibrational modes that can be linked to ReS 2 films. E g modes which we ascribe to in-plane vibrations of the Re atoms in ReS 2 , and A g modes corresponding to the out-of-plane vibrations of Re atoms (Fig. 3). The morphological features of thin films of rhenium sulfide were investigated by scanning electron microscopy (SEM), which revealed particles with teardrop morphology (Fig. 4a and b). Bright field transmission electron microscopy (TEM) reveals that this material is flake-like in appearance at the nanoscale ( Fig. 4c-g),  which is consistent with the layered structure of ReS 2 . Selected area electron diffraction (SAED) patterns taken from these flakes show that they are highly crystalline (Fig. 4h).
Elemental analysis of the rhenium sulfide thin films by energy dispersive X-ray (EDX) spectroscopy. Thin films of ReS 2 grown from Re(m-S i Pr) 3 (S i Pr) 6 at 475 1C analysed as follows: Re 30%, S 70% giving a composition of ReS 2.3 . Molybdenum sulfide was also grown under the same conditions; SEM and EDX and Raman spectroscopies were consistent with that previously reported for this material (ESI †). 19 Deposition and characterization of Mo-doped ReS 2 thin films Rhenium-alloyed thin films of molybdenum disulfide (Mo 1Àx Re x S 2 where 0 r x r 0.06) were synthesised by AA-CVD at 475 1C. EDX spectroscopy and ICP-OES confirmed that the rhenium dopant was successfully entrained into MoS 2 thin films (Fig. 5). In all cases, the incorporation of Re into MoS 2 is inefficient and the amount of Re found by EDX and ICP-OES in the films is only ca. 10% of that available Re in the aerosol feed.
The p-XRD patterns of Mo 1Àx Re x S 2 (0 r x r 0.06) for the ratios 1.79 and 1.80 mol% show four reflections at 2y = 14.3, 33.2, 39.5, and 58.71 which can be assigned to the (002), (100), (103), and (110) reflections respectively. There was no significant change with the incorporation of 1.79 and 1.80 mol% of Re on the pXRD patterns. However, on increasing the level of Re to more than 1.80 mol%, a change in the appearance of the diffraction patterns was observed; the (002) reflection became weak and broadened (Fig. 6). These changes in the powder pattern can be attributed to increasing Re mol%, which affects the stacking layers of the S-Mo-S layers in the basal plane, 47 which has previously been observed for Cr-doped MoS 2 . 21 Bright field TEM of the doped thin films (ESI †) reveal that increasing the level of Re in the films does indeed cause a shift to material with a more amorphous appearance. This could potentially be caused the formation of intra-layer Re-Re bonds that locally disrupt the structure by S displacement (vide supra). The d-spacing for the (002) plane of the MoS 2 film was 6.23 Å which is larger than that reported by Schoenfeld et al. 40 Indeed, in our and other reports of MoS 2 synthesised by AACVD from dithiocarbamato molybdenum(IV) precursors the observed spacings of 6.25 and 6.20 Å, 19,21 suggest that MoS 2 produced by AA-CVD is lattice expanded in the [002] direction as compared to bulk material. The rhenium doped MoS 2 displays a monotonic increase in the d-spacing that is linear with doping, suggesting that doping of Re affords a linear lattice expansion in the [002] direction of the crystal. If the literature value of d(002) for bulk molybdenite is used, 46 the trend becomes linear. We tentatively suggest that the effect of doping is pseudo-Vegardian.  Raman spectroscopy was used to study Mo 1Àx Re x S 2 (0 r x r 0.06) films. The MoS 2 film had two main bands observed at 407.6 cm À1 and 380.6 cm À1 corresponding to the A 1g and E 2g optical phonon modes (ESI †). The Raman spectra for the Re-doped MoS 2 films are shown in Fig. 7a. Significant changes in Raman spectra occurred at 3.6 mol% Re with the appearance of a band which could correspond to the A 1g mode for ReS 2 . Dependence of the Raman shift of the E 1 2g and A 1g optical modes of Mo 1Àx Re x S 2 on the amount of Re-dopant is shown in Fig. 7b. The difference in the magnitude of the shifts observed for both bands gives insight into the manner of doping. 48 The A 1g mode, which consists of the vibrational displacement of sulfur atoms only is only slightly shifted from ca. 407 cm À1 to ca. 406 cm À1 . On the other hand, the E 1 2g mode, which involves the vibration of metal and sulfur atoms in a layer is significantly shifted from ca. 382 cm À1 to ca. 376 cm À1 . This is consistent with the substitutional doping of the heavier Re atoms into the Mo layer.
The surface morphology of the Mo 1Àx Re x S 2 (0 r x r 0.06) films deposited by AA-CVD at 475 1C were investigated by SEM. Different morphologies were observed on changing the amount of rhenium; MoS 2 had a lamellar morphology, but Re-doped MoS 2 1.79% gave clusters. Increasing the Re to 3.60% gave feather-like crystals which were also observed for material with 6.25% Re (Fig. 8). Representative elemental mapping of the Re-doped MoS 2 thin film at 1.80% revealed that rhenium is evenly distributed in the film (Fig. 8). This was found in all alloyed films, suggesting that the isovalent substitution of Re(IV) for Mo(IV) was homogeneous under the conditions employed in this study.

Conclusions
Thin films of Mo 1Àx Re x S 2 (0 r x r 0.06) were deposited by AA-CVD from the single-source precursors Re(m-S i Pr) 3 (S i Pr) 6 (1) and Mo(S 2 CNEt 2 ) 4 (2). The Re-doped MoS 2 thin films were deposited by using different molar ratios of (1) and (2). The morphology of thin films as investigated by SEM changes as the doping of MoS 2 with rhenium is increased The p-XRD patterns  for the Re-doped MoS 2 shows systematic variations in the peak shape, intensity, and the position of the (002) planes as the rhenium content varies. The d(002) spacing in the alloys increases in a pseudo-Vegardian manner. EDX mapping of films demonstrated the spatial homogeneity of the doping. The MoS 2 alloy with Re is a promising as a model system for the stability of 99 Tc in the host molybdenum disulfide crystal due to crystal radii and thermodynamic similarities between Re(IV) and Tc(IV). The materials could also be exfoliated either by liquid or mechanical means to make novel 2D materials; we are currently investigating this possibility.