Synthesis and properties of MoCl4 complexes with thio- and seleno-ethers and their use for chemical vapour deposition of MoSe2 and MoS2 films

Article abstract of DOI:10.1039/c7dt04352j

Treatment of trans-[MoCl₄(MeCN)₂] with L (L = Me₂S, Me₂Se, THT, 1/2MeSCH₂CH₂SMe) in CH₂Cl₂ solution, or reaction of MoCl₅ with excess L′ (L′ = ⁿB

Full text of DOI:10.1039/c7dt04352j

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Synthesis and properties of MoCl4 complexes with
thio- and seleno-ethers and their use for chemical
vapour deposition of MoSe2 and MoS2 films†
Cite this: DOI: 10.1039/c7dt04352j
Yao-Pang Chang, Andrew L. Hector,
Joshua Whittam
William Levason,
Gillian Reid * and
Treatment of trans-[MoCl4(MeCN)2] with L (L = Me2S, Me2Se, THT, 1MeSCH2CH2SMe) in CH2Cl2 solution,
2
i
2
or reaction of MoCl5 with excess L’ (L’
=
nBu2S, nBu2Se, 1MeSCH2CH2SMe, 1 PrSCH2CH2SiPr,
2
21MeSCH2CH2CH2SMe, 1MeSeCH2CH2CH2SeMe) in MeCN, produces the Mo(IV) complexes, [MoCl4(L)2]
2
and [MoCl4(L’)2], respectively, in good yield. The new complexes have been characterised by IR and UV-
vis spectroscopy, elemental analysis and magnetic measurements, whilst crystal structure analyses of
i
trans-[MoCl4(Me2S)2], cis-[MoCl4{RS(CH2)2SR}] (R = Me, Pr) and cis-[MoCl4{MeS(CH2)3SMe}] confirmed
their identities and distorted octahedral geometries. The potential of [MoCl4(nBu2E)2] (E = S, Se) as the
first examples of molybdenum halide derived single source CVD precursors for the growth of MoE2 thin
films was first probed by TGA, which showed multi-step decomposition processes, with the masses of the
final residues consistent with MoSe2 (E = Se) and MoCl4 (E = S), respectively. Low pressure CVD experi-
ments conducted at 750 °C (E = S) and 525 °C (E = Se) gave silvery and golden yellow films, respectively.
Grazing incidence and in plane XRD data confirmed these to be 2H-MoE2, whilst the texture of the MoSe2
was investigated using pole figure measurements. SEM and EDX data, optical and Raman data on the films
are also reported.
Received 18th November 2017,
Accepted 15th December 2017
DOI: 10.1039/c7dt04352j
rsc.li/dalton
are being actively developed.8,9 The most common methods of
forming MoS2 or MoSe2 thin films have been exfoliation from
Introduction
The coordination chemistry of molybdenum extends over nine bulk samples, or heating MoO3 (or sometimes molybdenum
formal oxidation states (−II to +VI) and has been intensively metal) with S or Se in a hydrogen/inert gas atmosphere, to
studied over the last 50 years.1–3 Over this period, in addition form the desired MoE2 (E = S or Se) 2D films on an appropriate
to exploring the basic chemistry, much effort has been devoted substrate.10,11–13 Thermal decomposition of [NH4]2[MoS4] in a
to studying the binding and reduction of dinitrogen by molyb- hydrogen atmosphere has also recently been reported.13
denum complexes,2,4 and in modelling the active sites of
Chalcogenoether complexes of the early d-block metal
molybdoenzymes.4,5 In the last 10 years the study of early halides can function as single source CVD precursors for some
d-block dichalcogenides, including MoS2 and MoSe2, has ME2 (for example, M = Ti, V or Nb; E = S or Se).14,15 Chemical
become the focus of much effort. Layered early transition vapour deposition from molybdenum halide complexes has
metal dichalcogenides, ME2 (M = Nb, Ta, V, W, etc.; E = S, Se or rarely been used to form molybdenum dichalcogenides, although
Te), are inorganic analogues of graphene, and their properties MoSe2 films were produced by dual source atmospheric pressure
can be tuned by varying the metal and the chalcogen. chemical vapour deposition (CVD) using MoCl5 and either Et2Se
Production of the materials as thin films maximises the an- or nBu2Se.16 The tetrathiolate complex, [Mo(StBu)4], has also been
isotropy of their magnetic or electronic properties and thus used to deposit MoS2 films by CVD.17
methods to deposit 2D layers are of particular interest
Although there are a few early reports of MoCl4 complexes
currently.6–8 Applications of these materials in optoelectronics, with mono-, di- and poly-thioethers and thiamacrocycles,1,2,18
spintronics, sensors, electrocatalysts and magnetic materials the only recent work is the detailed characterisation, including
the X-ray crystal structures, of trans-[MoCl4(Et2S)2]19 and trans-
[MoCl4(THT)2] (THT = tetrahydrothiophene).20
Here we report the preparation, spectroscopic and struc-
tural characterisation of a series of new complexes of molyb-
Chemistry, University of Southampton, Southampton SO17 1BJ, UK.
E-mail: G.Reid@soton.ac.uk
†Electronic supplementary information (ESI) available. CCDC 1557208–1557213.
denum(IV) chloride with thio- and seleno-ether ligands and the
For ESI and crystallographic data in CIF or other electronic format see DOI:
evaluation of a subset of these as single source low pressure
10.1039/c7dt04352j
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CVD reagents for the deposition of the corresponding MoE2 changed to orange. The solution was taken to dryness in vacuo,
thin films.
to produce a dark orange solid. Yield: 296 mg, 78%. Required
for C8H16Cl4MoS2 (414.0): C, 23.20; H, 3.89. Found: C, 23.28;
H, 3.73%. IR (Nujol/cm−1): 338 (Mo–Cl). UV/vis (diffuse reflec-
tance)/cm−1: 22 700, 19 400, 16 600 (sh).
Experimental
trans-[MoCl4(Me2S)2]. MoCl5 (200 mg, 0.75 mmol) was dis-
solved in CH2Cl2 (15 mL). Anhydrous Me2S (1 mL) was then
added with stirring for 30 min and gave a red-orange solution.
The solution was filtered and evaporated in vacuo. n-Hexane
(10 mL) was added to wash the solid and then removed with a
syringe. The dark orange solid was dried in vacuo. Yield:
206 mg, 76%. Required for C4H12Cl4MoS2 (360.02): C, 13.27;
H, 3.34. Found: C, 13.36; H, 3.48%. IR (Nujol/cm−1): 332 (Mo–
Cl). UV/vis (diffuse reflectance)/cm−1: 22 000, 19 400, 16 800
(sh). μeff: 2.22 B.M (298 K).
Orange crystals of trans-[MoCl4(Me2S)2] grew by allowing a
CH2Cl2 solution to evaporate slowly in a N2 atmosphere. A few
orange-red crystals were also obtained by recrystallisation of
crude [MoCl4(Me2S)2] from CH2Cl2; these were found to be
[Me2SCH2SMe][MoCl5(Me2S)] from an X-ray structure
determination.
Alternative method: MoCl5 (150 mg, 0.55 mmol) was dis-
solved in MeCN (10 mL) with stirring to give a dark brown
solution. The solution was stirred for another 10 min and
taken to dryness to produce a dark brown solid. The solid was
suspended in CH2Cl2 (10 mL) before adding Me2S (3 mL). The
suspended solid dissolved immediately after the Me2S was
added in to give a red-brown solution. The solution was stirred
for another 10 min and then pumped to dryness, leaving a
dark orange solid. The product was spectroscopically identical
with the product made from MoCl5 in CH2Cl2.
Syntheses were performed by using standard Schlenk and
glove-box techniques under a dry N2 atmosphere. MoCl5 was
obtained from Sigma-Aldrich and stored in
a glovebox.
Tetrahydrothiophene, Me2S and nBu2S were purchased from
Sigma-Aldrich and Me2Se from Strem. Solvents were dried by
distillation from CaH2 (CH2Cl2, MeCN), Na (Et2O) or Na/benzo-
phenone ketyl (n-hexane). Dichalcogenoethers MeE(CH2)nEMe
i
n
(E = S, Se; n = 2, 3), PrS(CH2)2SiPr and Bu2Se were prepared
via literature methods.21,22
Infrared spectra were recorded using
a PerkinElmer
Spectrum 100 spectrometer in the range 4000–200 cm−1, and
samples prepared as Nujol mulls between CsI plates. UV/
visible spectra were recorded as powdered solids, using the
diffuse reflectance attachment of a PerkinElmer 750S spectro-
meter. Microanalyses on new compounds were undertaken by
London Metropolitan University. Thermogravimetric analysis
(TGA) used a Netzsch TG209 F1 Libra analyser under a flow of
argon at 65 mL min−1, contained within a dry, nitrogen
purged glovebox. The temperature was increased at a rate of
10 °C min−1
.
Single crystal X-ray experimental
Data collections used a Rigaku AFC12 goniometer equipped
with an enhanced sensitivity (HG) Saturn724+ detector
mounted at the window of an FR-E+ SuperBright molybdenum
(λ = 0.71073) rotating anode generator with VHF Varimax
optics (70 micron focus) with the crystal held at 100 K (N2 cryo-
stream). Structure solution and refinement were performed
trans-[MoCl4(nBu2S)2]. MoCl5 (205 mg, 0.75 mmol) was dis-
solved in CH2Cl2 (10 mL). A solution of nBu2S (274 mg,
2.5 mmol) in CH2Cl2 (ca. 3 mL) was then added with stirring
for 30 min and gave a red-orange solution. The solution was
filtered and evaporated to dryness in vacuo. The residue was
washed with n-hexane (10 mL), and the dark orange oil dried
in vacuo. Yield: 250 mg, 62%. Required for C16H36Cl4MoS2
(526.31): C, 36.51; H, 6.13. Found: C, 36.15; H, 6.72%. IR
(Nujol/cm−1): 334, 305sh (Mo–Cl). UV/vis (diffuse reflectance)/
cm−1: 41 700, 20 900, 19 100, 18 500 (sh).
using
SHELX(S/L)97
and
SHELX-2014/7
and
were
straightforward.23–25 H atoms were added and refined using a
riding model. The X-ray data collection details are in Table S1
(ESI†). CCDC reference numbers in CIF format are
[MoCl4(NCMe)2] 1557208; [MoCl4(Me2S)2] 557209; [MoCl4
(MeSCH2CH2SMe)] 557210; [MoCl4(iPrSCH2CH2SiPr)] 1557211;
[MoCl4(MeSCH2CH2CH2SMe)] 1557212; [MoCl5(Me2S)][Me2S
CH2SMe] 1557213.†
Alternative method: MoCl5 (136 mg, 0.5 mmol) was dis-
solved in MeCN (10 mL) with stirring for 30 min to give a dark
brown solution. The solution taken to dryness in vacuo and left
Complex synthesis
trans-[MoCl4(MeCN)2]. Was made by a modified literature a dark brown solid. CH2Cl2 (15 mL) was added, followed by a
method.26 MoCl5 (270 mg, 1.0 mmol) was dissolved in MeCN solution of nBu2S (0.4 mL, 2.0 mmol) in CH2Cl2 (2 mL). The
(20 mL) with stirring for 30 min to give a dark brown solution. dark solution changed to red-orange immediately. The solu-
The solution was evaporated in vacuo. The resulting dark tion was then stirred for 15 min then evaporated in vacuo to
brown solid was washed by n-hexane (5 mL), the washings dec- afford a dark orange oil. The product was spectroscopically
anted off, and the solid dried in vacuo. Yield: 249 mg, 78%. identical to that made from MoCl5 in CH2Cl2.
Required for C4H6Cl4MoN2 (319.56): C, 15.02; H, 1.89; N, 8.76.
Found: C, 14.84; H, 1.81; N, 8.61%. IR (Nujol/cm−1): 2314, pended in CH2Cl2 (15 mL) and Me2Se (196 mg. 1.96 mmol)
2283 (MeCN), 335 (Mo–Cl). was added with stirring for 30 min and produced an orange-
trans-[MoCl4(Me2Se)2]. MoCl5 (205 mg, 0.75 mmol) was sus-
trans-[MoCl4(THT)2]. MoCl5 (250 mg, 0.915 mmol) was dis- red solution. The solution was filtered and taken to dryness
solved in CH2Cl2 (10 mL). THT (0.26 mL, 2.97 mmol) was then in vacuo. The solid was washed with n-hexane (10 mL), the
added with stirring for 30 min during which the solution washings removed with a syringe, and the residual dark orange
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powder dried in vacuo. Yield: 283 mg, 77%. Required for and solid formed immediately. The solution was stirred for
C4H12Cl4MoSe2 (455.81): C, 10.54; H, 2.65. Found: C, 10.69; H, 30 min and then evaporated in vacuo. The solid was washed
2.43%. IR (Nujol/cm−1): 306 (Mo–Cl). μeff: 2.59 B.M. UV/vis with Et2O (5 mL), the Et2O removed via a syringe, and the solid
(diffuse reflectance)/cm−1: 37 000, 33 300, 26 800, 21 800, 19 000. was dried in vacuo. Orange powder. Yield: 160 mg, 77%.
Alternative method: [MoCl4(MeCN)2] (100 mg, 0.31 mmol) Required for C8H18Cl4MoS2 (416.11): C, 23.09; H, 4.36. Found:
was dissolved in 10 mL CH2Cl2 with stirring. A solution of C, 23.90; H, 4.56%. IR (Nujol/cm−1): 371sh, 350, 309 (Mo–Cl).
Me2Se (73 mg, 0.65 mmol) and CH2Cl2 (5 mL) was then added. UV/vis (diffuse reflectance)/cm−1: 41 300, 26 800, 25 000 (sh),
The solution was stirred for 1 h. resulting in a red-orange solu- 21 000, 18 700 (sh). μeff: 2.18 B.M. Orange red crystals grew by
tion. Evaporation in vacuo left a dark orange powder. Yield: allowing a CH2Cl2 solution to evaporate slowly under N2.
99 mg, 70%. Required for C4H12Cl4MoSe2 (455.81): C, 10.54;
H, 2.65. Found: C, 10.61; H, 2.54%. The product was spectro- was suspended in CH2Cl2 (20 mL) and
scopically identical to that made from MoCl5. MeSCH2CH2CH2SMe (109 mg, 0.8 mmol) in CH2Cl2 (ca. 3 mL)
[MoCl4(MeSCH2CH2CH2SMe)]. MoCl5 (205 mg, 0.75 mmol)
a
solution of
trans-[MoCl4(nBu2Se)2]. MoCl5 (136 mg, 0.5 mmol) was dis- was added, and stirred overnight to give an orange solution
solved in MeCN (15 mL) with stirring for 30 min to give a dark and some orange solid. The solution was removed by a
brown solution. The solution was taken to dryness in vacuo syringe, and the solid washed with n-hexane (10 mL). The solid
and left a dark brown solid. CH2Cl2 (25 mL) was added, fol- was subsequently dried in vacuo. Yield: 83 mg, 30%. Required
lowed by a solution of nBu2Se (193 mg, 1.0 mmol) in CH2Cl2 for C5H12Cl4MoS2 (374.03): C, 16.06; H, 3.23. Found: C, 16.22;
(2 mL). The dark solution changed to red-orange immediately. H, 3.35%. IR (Nujol/cm−1): 362, 342, 327 (Mo–Cl). UV/vis
The solution was then stirred for 15 min then evaporated (diffuse reflectance)/cm−1: 40 500, 22 070, 19 500. μeff: 2.21
in vacuo to afford a dark red-orange oil. Yield: 292 mg, 94%. B.M. Orange crystals grew by allowing a CH2Cl2 solution to
Required for C16H36Cl4MoSe2 (624.13): C, 30.79; H, 5.81. evaporate under N2.
Found: C, 30.23; H, 6.00%. IR (Nujol/cm−1): 342s, 305 (sh)
(Mo–Cl). UV/vis (diffuse reflectance)/cm−1: 40 000, 32 000, [MoCl4(iPrSCH2CH2SiPr)] and obtained as a light orange
[MoCl4(MeSeCH2CH2CH2SeMe)]. Was made as described for
28 000, 22 900, 21 000.
powder. Yield: 59%. Required for C5H12Cl4MoSe2 (467.82): C,
[MoCl4(MeSCH2CH2SMe)]. MoCl5 (205 mg, 0.75 mmol) was 12.84; H, 2.59. Found: C, 13.01; H, 2.57%. IR (Nujol/cm−1):
dissolved in CH2Cl2 (10 mL). A solution of MeSCH2CH2SMe 332, 308 (Mo–Cl). UV/vis (diffuse reflectance)/cm−1: 41 100,
(92 mg, 0.75 mmol) in CH2Cl2 (ca. 3 mL) was added and the 26 800, 21 000, 18 700.
mixture stirred overnight. A deep orange solution and a brown
LPCVD of MoS2 films using [MoCl4(nBu2S)2]. The precursor
solid formed. More CH2Cl2 (10 mL) was added and the solu- (190 mg) dissolved in CH2Cl2 (1 mL) was loaded into the
tion was filtered. The filtrate was evaporated to dryness in closed end of a silica tube in an N2 purged glove box, and the
vacuo. n-Hexane (10 mL) was added to wash the solid and sub- solvent removed. Silica substrates (ca. 1 × 8 × 20 mm3) were
sequently removed by a syringe. The solid was dried in vacuo to then loaded in the tube and placed end-to-end. The tube was
give a dark orange solid. Yield: 157 mg, 58%. IR (Nujol/cm−1): set in a furnace so that the substrates were in the heated zone
356, 319, 293 (Mo–Cl). UV/vis (diffuse reflectance)/cm−1
:
and the precursor was ca. 2 cm away from the start of the
41 500, 27 300, 21 000, 19 600. μeff: 2.66 B.M. Orange crystals heated zone. The tube was evacuated to 0.1 mm Hg, and the
grew on allowing a CH2Cl2 solution to evaporate in a N2 furnace was heated to 750 °C and left for 10 minutes to allow
atmosphere.
the temperature to stabilise. The tube was gradually moved
Alternative method: MoCl5 (136 mg, 0.5 mmol) was dis- into the hot zone until evaporation of the precursor began to
solved in MeCN (10 mL) with stirring for 10 min to give a dark occur. The position was then maintained until the all the pre-
brown solution. The solution taken to dryness in vacuo, cursor had evaporated. A yellow/orange film was observed at
CH2Cl2 (10 mL) was added, followed by a solution of the opposite end of the tube (S). After ca. 30 min the tube was
MeSCH2CH2SMe (180 mg, 1.5 mmol) in CH2Cl2 (2 mL). The cooled to room temperature and the tiles were unloaded at
dark solution changed to light orange-green immediately and ambient temperature in a dinitrogen atmosphere. Silver films
deposited a brown solid, which was filtered off. The brown were observed on the first two substrates, which corresponds
solid was dried in vacuo and washed with Et2O (ca. 5 mL), then to 735 °C (determined by temperature profiling). The silver
extracted with CH2Cl2 (ca. 5 m). The resulting solution was films are identical by grazing incidence and in-plane XRD and
again taken to dryness in vacuo. Yield: 60 mg, 33%. Required SEM/EDX analysis.
for C4H10Cl4MoS2·0.1Et2O (367.4): C, 14.38; H, 3.02. Found: C,
14.38; H, 3.02% (sample contained ca. 10% Et2O from wash sition procedure and with ca. 50 mg of precursor complex.
solvent; this was identified via 1H NMR spectroscopy).
Depositions were undertaken with the furnace temperature set
MoSe2 films using [MoCl4(nBu2Se)2]. Using the same depo-
[MoCl4(iPrSCH2CH2SiPr)]. MoCl5 (136 mg, 0.5 mmol) was at 400, 450, 500 and 500 °C. A red (Se) film was observed at the
dissolved in CH3CN (10 mL) forming dark brown solution. The far end of the tube and all experiments produced golden/
solution was stirred for 10 min and then evaporated in vacuo, brown films on the substrates. Temperature profiling for the
giving dark brown solid. The solid was dissolved in CH2Cl2 best MoSe2 films confirmed that the actual deposition temp-
i
(15 mL) and a solution of PrSCH2CH2SiPr (270 mg, 1.5 mmol) erature was 535 °C. Using larger precursor quantities (ca.
in CH2Cl2 (2 mL) was added. The colour changed to orange 200 mg) led to thicker films being formed.
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Film characterisation
X-Ray diffraction (XRD) patterns were collected in grazing inci-
dence mode (θ1 = 1°) or in-plane mode (θ1 = 0.5°, 2θχ scan with
the detector scanning in the film plane) using a Rigaku
SmartLab diffractometer (Cu-Kα, λ = 1.5418 Å) with parallel
X-ray beam and a DTex Ultra 250 1D detector. Phase matching,
lattice parameter calculations (MoS2) and Le Bail fitting
(MoSe2) used the PDXL2 software package27 and diffraction
patterns from ICSD.28 Scanning electron microscopy (SEM)
was performed on samples at an accelerating voltage of 10 to
15 kV using a JEOL JSM6500 or a Philips XL30 ESEM. Film
thicknesses were measured by fracturing the substrate and
gold sputtering the edge in order to control charging of the
insulating silica surface. Energy dispersive X-ray (EDX) analysis
data were obtained with an Oxford INCA x-act X-ray detector
(JSM6500) or Thermofisher Ultradry NSS 3 (XL30). Raman
Fig. 1 The structure of one of the two crystallographically independent
cations and molybdenum anions in [Me2SCH2SMe][MoCl5(Me2S)]
showing the atom numbering scheme and with ellipsoids drawn at the
50% probability level. The other cation and anion are very similar.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (°): Mo1–Cl4 = 2.3251(17), Mo1–Cl1 = 2.3509(17), Mo1–Cl3 =
2.3633(18), Mo1–Cl5 = 2.3847(18), Mo1–Cl2 = 2.3885(17), Mo1–S1 =
2.5538(19), (cis) Cl–Mo–Cl = 88.10(6)–92.87(7), Cl–Mo1–S1 = 83.14(6)–
91.74(6).
spectra were collected using
a Renishaw InVia Raman
Microscope with a 100 mW He–Ne 785 nm Laser. Optical
spectra were obtained in transmission mode using
PerkinElmer 750S UV-visible spectrometer.
a
crystal structure determination as the sulfonium salt,
[Me2SCH2SMe][MoCl5(Me2S)] (Fig. 1). This cation has also
been identified as a product of reaction of MoCl5 with
DMSO;32 in the latter case it was obtained with an [MoOCl4]−
anion.32
Results and discussion
Two routes were used to prepare the MoCl4 complexes
(Scheme 1), either substitution of the MeCN ligands in trans-
[MoCl4(MeCN)2] with the chalcogenoether ligand in CH2Cl2
solution or reaction of MoCl5 with excess ligand in CH2Cl2.
[MoCl4(RCN)2] (R = Me, Et, nPr) are well known starting
materials29–31 and an X-ray crystal structure determination of
the complex with R = Me showed it to be the trans isomer (see
Fig. S1, ESI†). The crystals are isomorphous with trans-
[WCl4(MeCN)2].31 In the preparations using MoCl5, the chalco-
gen ligand reduces the molybdenum centre, being halogenated
in the process, and careful washing is needed to remove the
by-products. A few crystals of a minor by-product from the syn-
thesis of [MoCl4(Me2S)2] from MoCl5 were identified by a
The isolated, coordinatively saturated MoCl4L2 complexes
are mildly hydrolytically sensitive. The crystal structures were
determined for trans-[MoCl4(Me2S)], cis-[MoCl4{RS(CH2)2SR}]
i
(R = Me, Pr) and cis-[MoCl4{MeS(CH2)3SMe}] and show the
expected six-coordinate geometries (Fig. 2–4), with the chelate
bite of the dithioethers responsible for the deviations from
regular octahedral geometry. The bond lengths are similar to
those in related Mo(IV) complexes.19,20,33
The complexes are paramagnetic with μeff in the range
2.0–2.7 B.M. The values are lower than the predicted spin-only
value for a d2 ion (2.82 B.M.) due to the significant spin–orbit
coupling, as expected for 4d elements,34 and are similar to
Fig. 2 The structure of the centrosymmetric [MoCl4(Me2S)2] showing
the atom numbering scheme and with ellipsoids drawn at the 50% prob-
ability level. Hydrogen atoms are omitted for clarity. Symmetry oper-
ation: i = −x, −y, −z + 1. Selected bond lengths (Å) and angles (°): Mo1–
Cl1 = 2.3457(5), Mo1–Cl2 = 2.3323(5), Mo1–S1 = 2.5297(6); Cl1–Mo1–
Cl2 = 89.39(2), Cl1i–Mo1–Cl2i = 90.61(2), Cl1–Mo1–S1i = 88.65(2), Cl1i–
Mo1–S1i = 91.35(2), Cl2–Mo1–S1 = 90.51(2), Cl2i–Mo1–S1 = 89.49(2).
Scheme 1 The preparative routes to the MoCl4 chalcogenoether
complexes.
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Fig. 3 (a) The structure of [MoCl4(MeSCH2CH2SMe)] showing the atom numbering scheme and with ellipsoids drawn at the 50% probability level.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Mo1–Cl3 = 2.247(3), Mo1–Cl1 = 2.305(3), Mo1–Cl2 = 2.305(3),
Mo1–Cl4 = 2.339(3), Mo1–S1 = 2.519(3), Mo1–S2 = 2.591(3), Cl3–Mo1–Cl1 = 97.61(12), Cl3–Mo1–Cl2 = 98.42(13), Cl3–Mo1–Cl4 = 96.83(11), Cl1–
Mo1–Cl4 = 92.47(10), Cl2–Mo1–Cl4 = 92.95(10), Cl3–Mo1–S1 = 87.75(11), Cl1–Mo1–S1 = 82.18(10), Cl2–Mo1–S1 = 91.07(10), Cl1–Mo1–S2 =
85.34(10), Cl2–Mo1–S2 = 77.66(11), Cl4–Mo1–S2 = 92.13(10), S1–Mo1–S2 = 83.62(9). (b) The structure of [MoCl4(iPrSCH2CH2SiPr)] showing the atom
numbering scheme and with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (°): Mo1–Cl1 = 2.3203(7), Mo1–Cl2 = 2.3295(7), Mo1–Cl3 = 2.3135(7), Mo1–Cl4 = 2.3016(7), Mo–1S1 = 2.5872(7), Mo1–S2 = 2.5730(7); Cl2–
Mo1–Cl4 = 93.37(3), Cl4–Mo1–Cl3 = 97.30(3), Cl4–Mo1–Cl1 = 96.60(3), Cl3–Mo1–Cl2 = 96.57(3), Cl1–Mo1–Cl2 = 96.14(3), Cl4–Mo1–S2 = 92.81(3),
Cl3–Mo1–S2 = 79.16(2), Cl1–Mo1–S2 = 86.63(2), Cl3–Mo1–S1 = 87.01(2), Cl1–Mo1–S1 = 78.40(2), Cl2–Mo1–S1 = 89.61(2), S2–Mo1–S1 = 84.52(2).
Fig. 4 The structure of [MoCl4(MeSCH2CH2CH2SMe)] showing the
atom numbering scheme and with ellipsoids drawn at the 50% prob-
ability level. Hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles (°): Mo1–Cl3 = 2.2686(6), Mo1–Cl1 = 2.3143(6),
Fig. 5 TGA of [MoCl4(nBu2E)2] (E = S or Se) obtained under flowing Ar
Mo1–Cl4 = 2.3380(6), Mo1–Cl2 = 2.3486(6), Mo1–S1 = 2.5282(6), Mo1–
(65 mL min−1) at ambient pressure with a heating rate of 10 °C min−1
.
S2 = 2.5390(6), Cl3–Mo1–Cl1 = 98.40(2), Cl1–Mo1–Cl4 = 97.61(2),Cl3–
Mo1–Cl2 = 96.29(2), Cl1–Mo1–Cl2 = 92.82(2), Cl4–Mo1–Cl2 = 97.58(2),
Cl3–Mo–1S1 87.41(2), Cl1–Mo1–S1 83.61(2), Cl4–Mo1–S1 =
=
=
79.79(2), Cl3–Mo1–S2 = 86.13(2), Cl4–Mo1–S2 = 78.20(2), Cl2–Mo1–S2 =
85.94(2), S1–Mo1–S2 = 97.370(19).
gravimetric analysis (TGA). [MoCl4(nBu2Se)2] underwent three
mass loss steps at 75–125 °C, 180–200 °C and 200–280 °C,
leaving a residual mass of ca. 39.4%, which gradually reduced
further as the temperature was raised to 600 °C (Fig. 5). This
those observed in related complexes.18–20,29,33 The solid state
UV/visible spectra exhibit two overlapping bands in the range
18 000–22 000 cm−1, which can be assigned in approximate Oh
suggests
a complex decomposition pathway with MoSe2
(40.7%) or MoCl4 (38%) as the final residue. [MoCl4(nBu2S)2]
also shows three mass loss steps at 25–160 °C, 170–200 °C and
210–300 °C, leaving a residual mass of ca. 44.9%, which under-
goes a further small mass loss (ca. 5%) on heating to 600 °C
(Fig. 5). The residual mass is higher than that expected for
MoS2 (30%), but similar to that expected for MoCl4 (44.7%);
3
3
symmetry as the d–d transitions, T1g
3T2g,35 with ill-defined charge transfer transitions evident at
>30 000 cm−1
→ →
3T1g(P) and T1g
.
CVD of molybdenum dichalcogenide films
The thermal decomposition of the [MoCl4(nBu2E)2] complexes some sublimation of MoCl4 at high temperature would be
at atmospheric pressure was first probed using thermo- expected; we note that [NbCl4(nBu2S)2] left NbCl4 upon heating
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in vacuo.36 It is important to note that the TGA experiments molybdenum metal that oxidised post-deposition. Parkin and
are performed at atmosphere pressure under flowing co-workers previously deposited films containing a mixture of
1
argon, and hence very different from LPCVD conditions. the 2H- and 3R-type MoSe2 films using dual source atmos-
However, TGA does provide a guide to the volatility of the pheric pressure CVD,16 but we are not aware of any other
precursors.
reports of single phase MoSe2 films from CVD from molyb-
CVD test experiments focussed on those complexes bearing denum chloride derived precursors.
nBu substituents since, unlike Me substituents, these tend to
The grazing incidence diffraction patterns of the MoSe2
be promote elimination of the organic by-products, most likely films are dominated by the 002 reflection, suggesting <00l>
via β-hydride elimination reactions. CVD onto silica tiles using alignment of the crystallites. These layered materials typically
ca. 30–70 mg of the [MoCl4(nBu2Se)2] precursor resulted in grow as platelet-shaped crystallites with the ab crystallographic
deposition of a reflective golden film on tiles positioned in the axes in the plane of the crystallites, so this would suggest
hottest part of the furnace at 400–550 °C. Grazing incidence platelets lying flat on the substrate. An in-plane XRD measure-
XRD (Fig. 6) showed these films to be the 2H form of MoSe2 ment (Fig. 6) supressed the 002 reflection and enhanced the
(P63/mmc). Lattice parameters determined from a Le Bail fit to 100 and 110, supporting this conclusion. SEM images (Fig. 7)
the pattern are a = 3.2666(10) and c = 13.170(16) Å (Rwp = 1.8%, showed mainly the edges of platelets suggesting they were
Rp = 1.3%; see ESI Fig. S2† for fit). The lattice parameters standing on edge rather than lying flat, but imaging the edge
listed in ICSD for this phase are in the ranges a = 3.288–3.290 of a cross-section showed a dense, flat film with thickness of
and c = 12.90–12.94 Å.28 The films are air and moisture stable around 130 nm underlying these orthogonally oriented plate-
(over many months), but easily scratched with a metal spatula. lets. A pole figure measurement (Fig. 8) taken with 2θ = 13.54°
With deposition temperatures higher than 550 °C, MoO2 thin (002 reflection) exhibits a single and sharp peak (FWHM ∼10°)
films were recovered, presumably resulting from deposition of with α = 90°, consistent with the vast majority of crystallites
adopting the <00l> orientation. The pole figure with 2θ =
56.10° (110 reflection), exhibits a ring with α close to zero (the
edge of the peak is cut off as the measurement cannot be
made right at the substrate plane), also consistent with the
<00l> preferred orientation. Unfortunately, the pole figure at
the 103 reflection position was featureless. This may suggest
stacking errors reducing its intensity.
Energy dispersive X-ray (EDX) analysis data showed signifi-
cant Si and O from the substrate, in addition to peaks due to
Mo and Se, indicating that the films are thin. There was no evi-
dence for any residual Cl in the films. The Mo : Se ratio was
quantified as 1 : 1.9, consistent within error with the formation
of MoSe2.
The crystallite size in the MoSe2 film produced at 550 °C
was calculated from the grazing incidence XRD data using the
Williamson–Hall method. The 6.6(9) nm size that resulted is
much smaller than the largest dimensions of the platelets
observed in Fig. 7 (ca. 100 nm), but the observed preferred
orientation shows the bulk of the material to be contained
within the dense underlying film. The diffraction crystallite
size appears to be dominated by this part of the sample. Using
a larger amount of precursor (ca. 200 mg) resulted in thicker
Fig. 6 Grazing incidence XRD (GIXRD, incidence angle = 1°) and in-
plane XRD (IPXRD, incidence angle = 0.5°) from MoSe2 thin films de-
posited by CVD from [MoCl4(nBu2Se)2] at temperatures as shown. The
broad feature at 2θ ∼ 22° is from the SiO2 substrate. The calculated
pattern for bulk polycrystalline 2H-MoSe2 (P63/mmc) with Miller indices
is shown for comparison.28
Fig. 7 SEM images of MoSe2 films produced by CVD using ∼70 mg [MoCl4(nBu2Se)2] at 550 °C. Top view (left) and cross-section (right).
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Fig. 10 Grazing incidence (incidence angle = 1°) and in-plane (inci-
dence angle = 0.5°) XRD patterns from the MoS2 thin film deposited
from [MoCl4(nBu2S)2] at 750 °C, and the simulated XRD pattern from
bulk 2H-MoS2.28 The broad feature at 2θ ∼ 22° is from the SiO2
substrate.
Fig. 8 002 (top) and 110 (bottom) pole figure measurements shown as
2D plots (left) and integrated plots with α (right) for MoSe2 thin films
produced by CVD using ∼70 mg [MoCl4(nBu2Se)2] at 550 °C.
described above, CVD on SiO2 substrates at 750 °C resulted in
very thin yellow films. These were too thin to generate any
X-ray diffraction in grazing incidence or in plane measure-
ment, or to image effectively in the SEM, but did produce
films, but the optical quality of the films became worse. SEM
images (ESI, Fig. S3†) showed that some crystallites continued
to grow preferentially and thus the film thickness was less con-
sistent over the area of the film.
42
optical spectra consistent with MoS2 (ESI, Fig. S5†). At lower
temperatures no deposition occurred. However, by using larger
amounts of [MoCl4(nBu2S)2] (200 mg) at 750 °C, silver films
were obtained. Grazing incidence XRD (Fig. 10) showed
similar features in these films to those seen for MoSe2 in
Fig. 7, but with much weaker intensities. The pattern was
dominated by the interlayer spacing peak (the 002 reflection),
which gives no information on the exact phase of MoS2 pro-
duced. However, the in-plane XRD pattern contained stronger
100, 103 and 110 reflections, confirming that, like MoSe2,
MoS2 adopted the 2H structure. The lattice parameters were
refined as a = 3.13(5) and c = 13.7(8) Å, although there is sig-
nificant uncertainty in this result due to the weak, broad
peaks. Lattice parameters in ICSD are in the range a =
3.14–3.16 Å and c = 12.29–12.53 Å.28 The suppression of the
002 reflection in the in-plane pattern suggests <00l> preferred
orientation like that observed in MoSe2, but unfortunately, the
scattering from these films was not strong enough for pole
figure measurements. The Raman spectrum recorded from the
MoS2 film collected using 785 nm excitation showed weak
bands at ca. 373 and 406 cm−1, assigned as the E2g and A1g
vibrational modes in 2H-MoS2.37,40,41,43–45
The Raman spectrum from the MoSe2 film was collected
using 785 nm excitation and shows three peaks at 140, 241
and 290 cm−1 assigned to the E1g, A1g and E2g vibration modes
of 2H-MoSe2, respectively (Fig. 9),16,37–40 whilst peaks at ca.
317, 455 and 595 cm−1 can be attributed to contribution from
acoustic phonons to the Raman scattering spectrum.41 The
optical spectra (ESI, Fig. S4†) showed the expected excitonic
transition features around 700 nm.42
When using similar amounts of [MoCl4(nBu2S)2] (ca.
30–70 mg) to that used in most of the MoSe2 depositions
SEM images confirm that the orthogonally oriented crystal-
lites on the MoS2 film are very small, but that the underlying
dense layer has a similar thickness to that found for MoSe2
(∼150 nm, Fig. 11). The weak and broad diffraction data
suggest the crystallite sizes may also be smaller in the dense
layer. Due to a close overlap between the Mo Lα (2.293 keV)
and S Kα (2.307 keV) fluorescence peaks, it was not possible to
obtain the Mo : S ratio from the EDX data. However, these did
confirm the presence of both elements and there is no evi-
dence for any residual Cl in the films.
Fig. 9 Raman spectrum of the MoSe2 film deposited by LPCVD from
[MoCl4(nBu2Se)2] at 550 °C.
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Fig. 11 SEM images of MoS2 films produced by CVD using ∼200 mg [MoCl4(nBu2S)2] at 750 °C.
2 C. G. Young, in Comprehensive Coordination Chemistry II,
ed. J. A. McCleverty and T. J. Meyer, Elsevier, Oxford, 2004,
Vol. 4, Section 4.7.
Conclusions
The synthesis and characterisation of a new series of well-
defined, distorted octahedral Mo(IV) chloride complexes with
mono- and bi-dentate thioether and selenoether ligands has
been demonstrated, with single crystal structures determined
for several representative examples. TGA analysis of
[MoCl4(nBu2S)2] and [MoCl4(nBu2Se)2] suggested they may be
sufficiently volatile to be used as sources of MoS2 and MoSe2,
respectively. This was borne out by LPCVD experiments which
produced silver (2H-MoS2) and golden yellow (2H-MoSe2) thin
films. The identities of the films, crystallite sizes and mor-
phologies were determined via XRD, SEM/EDX, Raman and
optical measurements. The successful identification of single
source CVD precursors for their deposition is an important devel-
opment given the very high level of interest in thin films of these
materials currently and their range of potential applications.
3 E. I. Stiefel, Prog. Inorg. Chem., 1977, 22, 1.
4 M. Hidai and Y. Mizobe, in Metal Ions in Biological Systems,
ed. A. Sigel and H. Sigel, Marcel Dekker, NY, 2002, vol. 39,
p. 121.
5 Molybdenum and Tungsten; their role in biological processes,
Metal ions in biological systems, ed. A. Sigel and H. Sigel,
Marcel Dekker, NY, 2002, vol. 39.
6 M. Chhowalla, Z. Liu and H. Zhang, Chem. Soc. Rev., 2015,
44, 2584, (Eds. of themed issue on early transition metal
dichalcogenides).
7 M. Chhowalla, H. S. Shin, G. Eda, L.-J. Li, K. P. Loh and
H. Zhang, Nat. Chem., 2013, 5, 263; J. Liu and X.-W. Liu,
Adv. Mater., 2012, 24, 4097.
8 J. R. Brent, N. Savjani and P. O’Brien, Prog. Mater. Sci.,
2017, 89, 411.
9 Q. Xiang, J. Yu and M. Jaroniec, J. Am. Chem. Soc., 2012,
134, 6575; K. Lee, R. Gatensby, N. McEvoy, T. Hallam and
G. S. Duesberg, Adv. Mater., 2013, 25, 6699; Z. Yan,
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Conflicts of interest
There are no conflicts to declare.
10 J. S. Rhyee, J. Kwon, P. Dak, J. H. Kim, S. M. Kim, J. Park,
Y. K. Hong, W. G. Song, I. Omkaram, M. A. Alam and
S. Kim, Adv. Mater., 2016, 28, 2316; Y.-H. Chang, W. Zhang,
Y. Zhu, Y. Han, J. Pu, J.-K. Chang, W.-T. Hsu, J.-K. Huang,
C.-L. Hsu, M.-H. Chiu, T. Takenobu, H. Li, C.-I. Wu,
W.-H. Chang, A. T. S. Wee and L.-J. Li, ACS Nano, 2014, 8,
8582; X. Wang, Y. Gong, G. Shi, W. L. Chow, K. Keyshar,
G. Ye, R. Vajtai, J. Lou, Z. Liu, E. Ringe, B. K. Tay and
P. M. Ajayan, ACS Nano, 2014, 8, 5125.
Acknowledgements
We thank the EPSRC for support (through EP/K00509X/1, EP/
K009877/1 and EP/M508147/1) and the University of
Southampton for a VC Scholarship (to Y.-P. C).
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