Vanadium selenoether and selenolate complexes, potential single-source precursors for CVD of VSe2 thin films

Article abstract of DOI:10.1039/b817903d

Reactions of VCl₄ with one mol equiv. of L-L (L-L = MeSe(CH ₂)₂SeMe, MeSe(CH₂)₃SeMe, ⁿBuSe(CH₂)₂SeⁿBu) in anhydrous CH₂Cl₂ solution at

Full text of DOI:10.1039/b817903d

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Vanadium selenoether and selenolate complexes, potential single-source
precursors for CVD of VSe2 thin films
Andrew L. Hector,* Marek Jura, William Levason, Stuart D. Reid
and Gillian Reid
Received (in Durham, UK) 13th October 2008, Accepted 20th November 2008
First published as an Advance Article on the web 11th December 2008
DOI: 10.1039/b817903d
Reactions of VCl4 with one mol equiv. of L–L (L–L = MeSe(CH2)2SeMe, MeSe(CH2)3SeMe,
nBuSe(CH2)2SenBu) in anhydrous CH2Cl2 solution at room temperature give [VCl4(L–L)] as very
moisture-sensitive dark purple solids. Using VCl4 and excess selenoether in gently refluxing
CH2Cl2 leads to reduction to [VCl3(L–L)] (L–L as above and o-C6H4(CH2SeMe)2), while VCl4
reacts with excess SeMe2 at room temperature to give [VCl3(SeMe2)2]. All new complexes were
characterised by microanalysis, IR and UV/visible spectroscopy and magnetic measurements.
Reaction of [(Cp)2VCl2] with two mol equiv. of LiSetBu in anhydrous thf gives the VIV selenolate
complex [(Cp)2V(SetBu)2] as a very moisture-sensitive brown solid. The new complexes have been
investigated as possible reagents for deposition of vanadium selenide. While low pressure chemical
vapour deposition (LPCVD) experiments showed that the diselenoether complexes were not
sufficiently volatile for VSe2 deposition, [VCl3(SeMe2)2] gives very thin deposits of VSe2. LPCVD
studies on [(Cp)2V(SetBu)2] at 600 1C produce thicker black films of VSe2. In all cases EDX
measurements show that the films are Se deficient.
soft neutral ligands, such as the heavier chalcogenoethers, are
relatively little explored.11 We have previously characterised
Introduction
Vanadium diselenide, VSe2, is a member of the family of
diselenoether (e.g. MeSe(CH2)2SeMe, MeSe(CH2)3SeMe,
layered chalcogenides, ME2 (M = Ti, Zr, Hf, Nb, Mo etc.,
o-C6H4(SeMe)2, PhSe(CH2)2SePh, (L–L)) complexes of
titanium(IV)12 including the six-coordinate [TiX4(L–L)] (X =
E = S or Se), which are based upon strongly bound E–M–E
layers, stacked with weak interlayer EÁ Á ÁE interactions. As a
Cl or Br), and the six- and eight-coordinate zirconium(IV) and
hafnium(IV)13 complexes [MCl4{MeSe(CH2)2SeMe}] and
result, these materials have important applications including
solid state lubricants, battery cathodes, in solar energy cells,
[MCl4{MeSe(CH2)2SeMe}2]. The Zr and Hf complexes are
and in sensors. Vanadium diselenide, which is usually metal
poorly soluble in non-coordinating solvents, whilst the
rich V1+xSe2, has received considerable attention, both
diselenoethers are displaced by solvents containing nitrogen
in terms of intercalation reactions,1,2 and its highly
or oxygen donor functions, and all are exceedingly moisture
unusual charge density wave properties.3,4 Although chemical
sensitive. Very little is known about selenoether complexes of
vapour (I2) transport has been used to prepare single
vanadium—the only examples in the literature are substituted
crystals of VSe2,5 the first successful use of chemical vapour
carbonyl derivatives.11 The vanadium(V) oxide halides
deposition (CVD) approaches to make films of VSe2
VOX3 and VO2X (X = F or Cl) are immediately reduced by
and related selenides have been reported only recently.6,7
SeMe2 or MeSe(CH2)2SeMe.14 There appear to be no
Atmospheric pressure (AP)CVD of matt black films, with
vanadium halide examples with lower oxidation states,
although thioether complexes of VII and VIII are well
composition close to VSe2, was achieved using the dual source
precursors [V(NMe2)4] and SetBu2.6 Single-source low
established,15–17 and we have recently characterised VIV
pressure (LP)CVD of titanium diselenide films has been
examples including [VCl4{RS(CH2)2SR}] and the tetrameric
demonstrated using [TiCl4(Se2Et2)2], [TiCl4(SeEt2)2],8 or
[Ti{o-C6H4(CH2SeMe)2}],9 whilst ZrSe2 and HfSe2 films have
i
[{VOCl2{RS(CH2)2SR}}4] (R = Me, Et, Pr etc.).18
been obtained very recently by us via LPCVD from
[(Cp)2M(SetBu)2] (M = Zr or Hf).10 We report here attempts
to extend our recent studies9,10 to the production of VSe2 films,
Experimental
and describe the study of some vanadium selenoether
and selenolate ligand chemistry and LPCVD with selected
complexes.
All reactions were conducted under rigorously anhydrous
conditions and under dry nitrogen using standard vacuum
line, Schlenk and glove-box techniques. A fresh bottle of VCl4
(Aldrich) was used as received (VCl4 is a brown liquid which
slowly loses Cl2 on standing and deposits solid VCl3).
[(Cp)2VCl2] was obtained from Aldrich. Solvents were dried
by distillation from CaH2 (CH2Cl2 or MeCN) or Na/benzo-
phenone ketyl (thf, hexane and diethyl ether). The selenoethers
The early transition metal halides in their higher oxidation
states are hard, oxophilic Lewis acids and their complexes with
School of Chemistry, University of Southampton, Southampton,
UK SO17 1BJ. E-mail: A.L.Hector@soton.ac.uk
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RSe(CH2)2SeR (R = Me, nBu), MeSe(CH2)3SeMe, and
o-C6H4(CH2SeMe)2 were made by literature methods,19,20
dried and stored over molecular sieves. SeMe2 was obtained
from Aldrich and was stored over molecular sieves. Physical
measurements were made as previously14,18 and micro-
analytical data were obtained from the microanalytical
laboratories of Strathclyde University or Medac Ltd. All
measurements were made on freshly prepared samples.
solution lightened quickly and rapidly deposited a lilac
precipitate, which was separated after ca. 10 h by filtration,
and dried in vacuo. Yield: 0.32 g, 73%. Calc. for
C10H22Cl3Se2VÁCH2Cl2 (542.4): C, 24.4; H, 4.5. Found: C,
23.8; H, 4.4%. IR (Nujol) [cmÀ1]: n = 337(sh), 320(vs,br),
270(sh) (V–Cl). UV/Vis (d.r. in BaSO4): Emax [cmÀ1] = 11 500,
17 500, 20 000(sh), 29 400(sh). meff (298 K) = 2.64 mB.
[VCl3(SeMe2)2]. VCl4 (0.19 g, 1.0 mmol) was dissolved in
CH2Cl2 (20 mL) and SeMe2 (0.33g, 3.0 mmol) in CH2Cl2
(5 mL) added slowly. A dark solution formed immediately,
which then paled on stirring and deposited a pink solid. This
was filtered off after ca. 15 min and dried in vacuo. Yield:
0.28 g, 68%. Calc. for C4H12Cl3Se2V (375.4): C, 12.8; H, 3.2.
Found: C, 12.7; H, 3.2%. IR (Nujol) [cmÀ1]: n = 393(s)
(V–Cl). UV/Vis (d.r. in BaSO4): Emax [cmÀ1] = 6000, 7200,
12 500, 16 400, 27 000(sh). meff (298 K) = 2.65 mB.
Preparations
[VCl4{MeSe(CH2)2SeMe}]. VCl4 (0.19 g, 1.0 mmol) was
dissolved in CH2Cl2 (15 mL). MeSe(CH2)2SeMe (0.22 g,
1.0 mmol) in CH2Cl2 (10 mL) was added slowly producing a
very dark solution and precipitate. Concentration in vacuo to
ca. 5 mL, followed by filtration produced a dark purple
powder, which was dried in vacuo. Yield: 0.36 g, 88%. Calc.
for C4H10Cl4Se2V (408.8): C, 11.8; H, 2.5. Found: C, 12.1; H,
2.6%. IR (Nujol) [cmÀ1]: n = 363(sh), 300(vs), 290(sh) (V–Cl).
UV/Vis (diffuse reflectance in BaSO4): Emax [cmÀ1] = 17 850,
20 400, 29 000. meff (298 K) = 1.76 mB.
Larger quantities were obtained by scaling up the reaction
(Â3) without problems.
[(Cp)2V(SetBu)2]. To a frozen (77 K) suspension of Se
powder (0.65 g, 8.3 mmol) in thf (30 mL) was added dropwise
tBuLi (7.3 mL, 1.7 M in pentane). Upon warming to room
temperature the solid dissolved to give a colourless solution.
After a further 1 h of stirring, the resulting solution was added
to a stirred suspension of [(Cp)2VCl2] (1.0 g, 4.0 mmol) in thf
(20 mL) to give a brown mixture which was stirred at room
temperature for 2 h. The volatiles were removed at reduced
pressure, the brown solid extracted with Et2O (50 mL) and
filtered through Celite. The precipitate was washed with Et2O
(3 Â 10 mL), and the Et2O washings were combined and
concentrated, ca. 10 mL. The brown solid which deposited was
separated by decantation and dried in vacuo. Yield: 0.11 g,
25%. Further solid was obtained by evaporating the filtrate.
The complex is extremely readily hydrolysed. Chemical tests
showed it to contain significant amounts of residual LiCl,
carried through in the ether extraction and leading to a poor fit
of the microanalytical data on several samples. Attempts at
recrystallisation and sublimation led to decomposition.
UV/Vis (d.r. in BaSO4): Emax [cmÀ1] = 13 300, 22 320, 26 050.
The following compounds were made similarly.
[VCl4{MeSe(CH2)3SeMe}]. Dark purple powder. Yield:
85%. Calc. for C5H12Cl4Se2V (442.8): C, 14.2; H, 2.9. Found:
C, 13.7; H, 3.0%. IR (Nujol) [cmÀ1]: n = 363(sh) 337(vs,br),
328(sh) (V–Cl). UV/Vis (d.r. in BaSO4): Emax [cmÀ1] = 17 700,
20 500, 28 600. meff (298 K) = 1.70 mB.
[VCl4{nBuSe(CH2)2SenBu}]. Dark purple powder. Yield:
81%. Calc. for C10H22Cl4Se2V (493.0): C, 24.4; H, 4.5. Found:
C, 24.4; H, 4.1%. IR (Nujol) [cmÀ1]: n = 360(s), 300(vs)
(V–Cl). UV/Vis (d.r. in BaSO4): Emax [cmÀ1] = 18 200, 20 400,
28 500(sh). meff (298 K) = 1.75 mB.
[VCl3{MeSe(CH2)3SeMe}]. VCl4 (0.19 g, 1.0 mmol) was
dissolved in CH2Cl2 (15 mL) and MeSe(CH2)3SeMe (0.46 g,
2.0 mmol) in CH2Cl2 (10 mL) was added slowly producing a
very dark solution and precipitate. The solution was stirred
under gentle reflux for 1 h, when it slowly became paler.
Concentration in vacuo to ca. 5 mL produced a pale lilac
solid, which was filtered off and dried in vacuo. Yield: 0.31 g,
81%. Calc. for C5H12Cl3Se2V (385.5): C, 15.6; H, 2.6. Found:
C, 15.5; H, 2.9%. IR (Nujol) [cmÀ1]: n = 363(br), 270(m)
(V–Cl). UV/Vis (d.r. in BaSO4): Emax [cmÀ1] = 11 500, 17 480,
30 150. meff (298 K) = 2.50 mB.
Deposition and film characterisation
LPCVD experiments were carried out on selected samples in a
40 cm silica tube (B8 mm I.D.) sealed at one end. Approxi-
mately 100 mg of the precursor was placed at the sealed end
followed by cut silica tiles (15 Â 6 Â 1 mm) along most of the
length of the tube. The tiles were cleaned with acid piranha
etch and dried thoroughly prior to use. The entire assembly
was placed in a 30 cm tube furnace with the part of the tube
containing the precursor protruding from the other side, and
the tube was evacuated to ca. 0.05 mbar. This vacuum was
maintained throughout the deposition. The tube furnace was
heated to 500 or 600 1C then the tube was slowly moved back
through the furnace until the precursor showed signs of
sublimation. Heating was continued for 1 h, then the furnace
was allowed to cool under nitrogen and the silica tube was
transferred to the glove box.
[VCl3{nBuSe(CH2)2SenBu}]. VCl4 (0.19 g, 1.0 mmol) was
n
dissolved in CH2Cl2 (15 mL) and BuSe(CH2)2SenBu (0.44 g,
2.0 mmol) in CH2Cl2 (10 mL) was added, and the mixture was
gently refluxed for 1 h. The initial dark precipitate slowly
lightened in colour. The solution was cooled to room tem-
perature, filtered, the precipitate was washed with CH2Cl2
(5 mL) and dried in vacuo. Dull lilac powder. Yield: 0.33 g,
72%. Calc. for C10H22Cl3Se2V (457.5): C, 26.3; H, 4.9. Found:
C, 26.4; H, 4.7%. IR (Nujol) [cmÀ1]: n = 355(s), 300(s),
270(m,sh) (V–Cl). UV/Vis (d.r. in BaSO4): Emax [cmÀ1] =
11 760, 17 900, 27 800(sh), 30 500. meff (298 K) = 2.66 mB.
[VCl3{o-C6H4(CH2SeMe)2}]. VCl4 (0.19 g, 1.0 mmol) was
dissolved in CH2Cl2 (15 mL) and o-C6H4(CH2SeMe)2 (0.58 g,
2.0 mmol) in CH2Cl2 (10 mL) added. The initially dark
Coated tiles were generally black in colour. Coatings were
usually found to be very thin and so the thickest coatings were
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chosen for characterisation. The films were also soft, being
easily scratched by metal tools as expected for these selenides,6
but were well adhered to the substrate surface. Powder X-ray
diffraction data were collected with a Bruker D8 Advance with
GADDS diffractometer using Cu-Ka1 radiation and an
incident beam at a 51 angle to the substrate surface. Typically
data were collected for 6–10 h. Scanning electron microscopy
(SEM) used a Jeol JSM5910, with gold or carbon coated
samples. Standardless quantitative energy dispersive X-ray
(EDX) analyses used an Oxford Instruments Inca 300 probe.
suitable crystals to confirm the structures, but there seems little
doubt that they are closely analogous to the Group 4
complexes.12,13 The diffuse reflectance UV/visible spectra are
dominated by two intense overlapping transitions at B20 500
and 18 000 cmÀ1, which we assign as Cl(p) - V(t2g) and
Se(p) - V(t2g), respectively. There is no band evident that is
assignable to the d–d transition T2g - Eg expected in this
region; it is clearly obscured by the charge transfer bands. The
alternative assignment of the observed bands as the d–d
transitions, with splitting caused by the lower than Oh
symmetry, is ruled out by the absence of other bands
o25 000 cmÀ1 which could be the LMCT transitions. In the
TiIV analogues Se(p) - Ti(t2g) are B21 000 cmÀ1 12 and the
Se(p) - V(t2g) are expected to be at significantly lower energy
in the more easily reduced VIV complexes.
2
2
Results and discussion
Following our previously described routes to MSe2 (M = Ti,
Zr and Hf) thin films,9,10 we have investigated similar
approaches to VSe2 via single-source precursor compounds
based upon VIII and VIV complexes involving neutral
selenoether and anionic selenolate ligands. This led to the
preparation and characterisation of the first complexes of VCl4
and VCl3 with selenoether ligands and synthesis of a VIV
selenolate. Deposition studies based upon selected compounds
are described later.
[VCl3(SeMe2)2] and [VCl3(L–L)] complexes. As indicated
above, the [VCl4(L–L)] complexes are easily reduced by
refluxing the preparative solutions, giving pale lilac solids.
We found that a 1 : 2 VCl4 : L–L ratio gave pure samples of
these lilac solids, [VCl3(L–L)] (L–L = MeSe(CH2)3SeMe,
nBuSe(CH2)2SenBu or o-C6H4(CH2SeMe)2), which were
insoluble in chlorocarbons or MeCN and decomposed by
alcohols. The VIII complexes are also moisture sensitive,
turning blue-green in air, although much less readily hydro-
lysed than the VIV analogues above.
Synthesis and characterisation
[VCl4(L–L)] complexes. The reaction of VCl4 with
MeSe(CH2)nSeMe (n = 2 or 3) in a 1 : 1 molar ratio, in
anhydrous CH2Cl2 solution at room temperature produced
very dark purple powders formulated on the basis of micro-
analyses, spectroscopic and magnetic measurements as
[VCl4{MeSe(CH2)nSeMe}]. The complexes are extremely
readily hydrolysed in solution and the solids fume in air, and
change colour due to hydrolysis. The isolated complexes are
insoluble in chlorocarbons or toluene and reduced by MeCN,
thf or acetone. Poor solubility in chlorocarbons and hydrolytic
instability is also characteristic of Ti, and especially Zr and Hf
analogues,12,13 but the easy reduction of the VIV complexes is
not found in Group 4 analogues. In an attempt to improve
solubility, the complex of the n-butyl substituted
diselenoether, nBuSe(CH2)2SenBu, was prepared, but this also
gave a poorly soluble complex. The reaction of VCl4 with
o-C6H4(CH2SeMe)2 gave a dull lilac product directly, which
was identified as a VIII complex (below). Similar species were
produced on refluxing the reaction mixtures containing VCl4
and 2 mol equiv. of RSe(CH2)2SeR (R = Me or nBu) in
CH2Cl2. The reactions of VCl4 with the monodentate SeMe2
gave the VIII complex directly (below), with no evidence that
VIV species could be isolated. This dependence of the ability to
form VIV complexes on the selenoether denticity and archi-
tecture is generally similar to that found with thioethers,18
although the selenoether complexes appear less stable,
reflecting the increasing softness and greater reducing power
of the selenium donors.
As expected the diffuse reflectance UV/visible spectra are
quite different to those of the VIV complexes, showing weak
features at B11 500, 17 500 and a strong, broad transition at
B30 000 cmÀ1. The latter is assigned as a charge transfer band,
but the weak features are assigned as the two d–d transitions
3
of VIII in an octahedral environment, specifically T1g(F) -
3
3
3T2g(F) and T1g(F) - T1g(P).17,21,22 The complexes have
room temperature magnetic moments of B2.6 mB, confirming
the presence of d2 VIII. The IR spectra show only weak
selenoether bands above 400 cmÀ1, and several overlapping
strong bands in the region 400–300 cmÀ1, clearly in terminal
V–Cl stretching modes. A weaker band B270 cmÀ1 is tenta-
tively assigned as a V–Cl bridging mode. The IR spectra
and the UV/visible thus lead to a likely assignment as six-
coordinate chloride-bridged vanadium(III) complexes. In
contrast, VCl4 and SeMe2 react in CH2Cl2 solution, irrespec-
tive of the reagent ratios used, to form pink [VCl3(SeMe2)2].
This complex exhibits a broad V–Cl stretching vibration in the
far IR spectrum at 393 cmÀ1
, and diffuse reflectance
UV/visible transitions at 6000, 7200, 12 500, 16 400,
27 000(sh). The UV/visible data are very similar to those
reported for the VIII thioether complexes [VCl3(SR2)2]
(R = Me, Et etc.),15,16 which are believed to have trigonal
bipyramidal structures.
[(Cp)2V(SetBu)2]. The use of the tert-butyl selenolate follows
from our previous work on Group 4 analogues,10 which were
shown to have sufficient volatility to function as CVD
precursors for MSe2 (M = Ti, Zr, Hf). The VIV selenolate
complex was obtained from the reaction of [(Cp)2VCl2] with
two equiv. of LiSetBu in anhydrous thf solution. After
removal of the thf, the residue was extracted with diethyl
ether and filtered. Concentration of the filtrate gave the
The three [VCl4(L–L)] (L–L = MeSe(CH2)nSeMe (n = 2 or 3),
nBuSe(CH2)2SenBu) are paramagnetic with meff B 1.7 mB and
exhibit several very strong features in the far IR spectra in the
range 370–300 cmÀ1 assigned as terminal n(V–Cl) modes of
cis-octahedral species (group theory predicts four IR active
modes: 2A1 + B1 + B2). Despite many attempts, their
instability and poor solubility have prevented the growth of
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Fig. 1 X-Ray diffraction pattern of a VSe2 film deposited at 600 1C
from [(Cp)2V(SetBu)2]. Miller indices are labelled according to VSe2
25
Fig. 2 SEM image of a VSe2 film deposited at 600 1C from
ꢀ
(P3m1, 1T).
[(Cp)2V(SetBu)2].
product as a paramagnetic, extremely moisture-sensitive
orange-brown powder. It usually retained some LiCl from
the preparation (confirmed by a flame test and a silver nitrate
spot test), but since the complex is unstable in solution,
attempts to purify it further by recrystallisation resulted in
considerable decomposition, hence further purification was
not attempted. Importantly, the deposition studies described
below showed no chloride impurity in the films produced. The
only prior reports of VIV selenolates are [(Cp)2V(SePh2)2]23
and [(Cp**)2V(L–L)] (Cp** = C5H4Me, C5H4Et; L–L =
various heterocyclic diselenolates).24
Se (again with some Si and O evident from the substrate),
indicating a Se-deficient composition of VSe1.55. A weak carbon
peak was also observed—by analogy with the films grown from
[(Cp)2M(SetBu)2] (M = Ti, Zr, Hf)10 a carbon content of B15%
can be expected in these films. The sample produced at 500 1C
was poorly diffracting, but with a long collection time the broad
100, 011, 102, 110 and 111 reflections of the same VSe2 phase were
all visible. SEM showed less dense coverage of the tile than at
600 1C and EDX a composition of VSe1.4
.
A previous study of the APCVD of VSe2 from V(NMe2)4
and SetBu2 precursors found deposition of close to stoichio-
metric material with platelet crystallites growing perpendicular
to the substrate.6 Using [(Cp)2V(SetBu)2] by LPCVD the
growth mode is the same but the films are Se deficient, either
due to volatilisation of Se in the vacuum or a less favourable
decomposition pathway.
Deposition studies
In all of the examples attempted, incomplete sublimation of the
vanadium selenoether and selenolate precursors was observed.
Significant amounts of residue remained once the deposition
was complete and the cold walls of the silica tube at the other
end were coated with black and red deposits of elemental Se.
The bidentate selenoether complexes did not produce films.
However, LPCVD using [VCl3(SeMe2)2] at 600 1C resulted in
thin black films on the silica tiles through the furnace hot zone.
SEM measurements showed low density growth of small
crystallites, many of which were hexagonal plates with their
faces parallel to the surface of the substrate. EDX measure-
ments confirmed the presence of V and Se (V : Se = 1 : 0.9),
together with Si and O from the substrate and a weak carbon
peak. The PXD pattern was weak, but showed peaks assigned
Conclusions
The first series of VIV and VIII selenoether complexes has been
obtained and characterised. Deposition studies (LPCVD)
show that, in general, these have insufficient volatility to serve
as LPCVD precursors, although some deposition was achieved
from [VCl3(SeMe2)2]. This contrasts with previous work on
similar TiCl4 complexes.8,9 [(Cp)2V(SetBu)2], although both
moisture-sensitive and easily reduced, does produce black,
crystalline films of VSe2 via LPCVD. Modifications of the
co-ligands in vanadium selenolates may further improve them
as precursors for VSe2 deposition via AACVD or precursor
injection techniques.
25
ꢀ
to VSe2 with the typical P3m1 (1T) structure. Lowering the
temperature to 500 1C did not improve the films.
Using [(Cp)2V(SetBu)2] resulted in films at 500 and 600 1C by
LPCVD. Those produced at 600 1C were visibly thicker and
diffracted well. PXD patterns identified the films as VSe2, Fig. 1,
and the lattice parameters were refined as a = 3.387(19) and
c = 5.849(25) A. Previous reports show the lattice parameters
for bulk VSe2 to be a = 3.355–3.358 and c = 6.107–6.134 A, with
selenium deficiency causing a to increase and c to decrease (e.g.
3.431 and 5.972 A for VSe1.16).25 SEM measurements showed
even growth of B2 mm platelets perpendicular to the substrate
surface, Fig. 2. This growth mode is typical of metal dichalco-
genide films due to growth occurring along the a and b axes of
these layered compounds.6–10 EDX measurements showed V and
Acknowledgements
The authors thank EPSRC and the Royal Society (University
Research Fellowship, ALH) for funding.
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