Thin film Co[TCNE]2 and VyCo1-y[TCNE] 2 magnetic materials

Article abstract of DOI:10.1016/j.jmmm.2012.02.037

A chemical vapor deposition method has been developed for the synthesis of both solvent-free Co[TCNE]₂ and V_yCo_{1- y}[TCNE]₂ thin films. Both materials have been previously synthesized by solution metho

Full text of DOI:10.1016/j.jmmm.2012.02.037

Journal of Magnetism and Magnetic Materials 324 (2012) 2218–2223
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Thin film Co[TCNE]2 and VyCo1ꢀy[TCNE]2 magnetic materials
Preston K. Erickson, Joel S. Miller n
Department of Chemistry, University of Utah, Salt Lake City, UT 84112, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A chemical vapor deposition method has been developed for the synthesis of both solvent-free
Co[TCNE]2 and VyCo1ꢀy[TCNE]2 thin films. Both materials have been previously synthesized by solution
methods, but contain solvent. The Co[TCNE]2 thin films were characterized by infrared spectroscopy
and magnetic studies, and albeit solvent-free were determined to be similar to the analogous solution-
prepared samples as they are paramagnetic with slight antiferromagnetic coupling. In contrast to the
solution-based synthesis, VyCo1ꢀy-[TCNE]2 showed no dependence of coercive field based on the
composition of the films, even though infrared spectroscopic data indicates formation of a solid-
solution thin film, and not a physical mixture.
Received 18 October 2011
Received in revised form
3 February 2012
Available online 28 February 2012
Keywords:
Magnetic film
CVD
Organic-based magnet
& 2012 Elsevier B.V. All rights reserved.
1. Introduction
films were relatively air stable and could be handled in the air,
albeit for a short time. Nonetheless, Parylene coatings were
Thin magnetic films are needed for many studies and applica-
tions, and organic-based magnets [1–4] are well suited for their
development due to magnetic ordering temperatures, Tc, well
above room temperature [5–9]. Organic-based magnets enable
modulation of their properties via organic synthesis methods,
compatibility with polymers for composites, transparency, low
temperature processibility, and conductivity ranging from insu-
lating to semiconducting behavior etc. [10].
The room temperature V[TCNE]x ꢁ zCH2Cl2 (Tcꢂ400 K; xꢂ2;
zꢂ0.5; TCNE¼tetracyanoethylene) organic-based magnet can be
prepared from the reaction of TCNE and V(CO)6 [11] or V(C6H6)2
[12]. Likewise, solid solution VyM1ꢀy(TCNE)x ꢁ zCH2Cl2 (M¼Fe
[13], Co [14], Ni [15]; Tco400 K; xꢂ2; zꢂ0.5;o1oyo0) mag-
nets have been also been prepared. Use of solvents, however, is
problematic as thin films cannot be fabricated.
Thin films of the room temperature V[TCNE]x magnet have
been prepared via low temperature chemical vapor deposition
(CVD) of TCNE and V(CO)6 [16] while M[TCNE]x (M¼V [17,18], Cr
[17], Fe [19,20], Co [20], Nb [17], Mo [17]) have been reported via
physical/chemical vapor deposition of TCNE and non-metal
carbonyls.
These thin films eliminate the adverse effect of the solvent and
were grown from the gas phase reaction of TCNE and V(CO)6.
Films were easily deposited on glass, Teflons, alkali halide, metal,
Si wafer, and amorphous carbon substrates, and magnetically
order at room temperature. Unexpectedly, unlike V[TCNE]x ꢁ zS,
which decomposed at the diffusion rate of oxygen [4], V[TCNE]x
developed to enhance the stability towards air [21]. V[TCNE]x
films in addition to being magnetically ordered exhibit high dc
electrical conductivity (10ꢀ2 S cmꢀ1) at room temperature [22],
and its electrons in valence and conducting bands are fully spin
polarized. Furthermore, it is
a half semiconducting magnet
[18,23–25] with linear and quadratic –magnetoresistance with
applied field (H) below and above Tc, respectively [26]. Thus, with
low spin orbit coupling (longer spin coherence time and spin-flip
length) this magnet is an appealing material for future genera-
tions of magnetic applications, e.g., spintronics [27].
With the success in making V[TCNE]x thin films, we have sought
to make other M[TCNE]x thin films. M¼Co is the most attractive due
to the availability Co2(CO)8, which has a similar stability with
respect to V(CO)6, and is neither far more stable, as occurs for
Cr(CO)6, Mn2(CO)10, and Fe(CO)5, nor far less stable, as occurs for
Ni(CO)4. The physical chemical deposition of Co(TCNE)x has been
reported, but its IR and magnetic properties were not discussed [20].
In addition, we have targeted making mixed-metal thin films of
VyCo1ꢀy(TCNE)2 composition. As observed for VyCo1ꢀy(TCNE)2 ꢁ
zCH2Cl2, increasing the Co content should increase the coercivity
without deleteriously affecting Tc [14]. Hence, herein we report the
fabrication of thin films of Co(TCNE)2, and report preliminary results
on making VyCo1ꢀy(TCNE)2 thin films.
2. Results and discussion
Using the CVD apparatus described in Fig. 1, the ideal tem-
peratures were determined to be 45 1C for TCNE and 10 1C for
Co2(CO)8 for its deposition to react and form Co[TCNE]2 films.
These films were brown and non-lustrous in appearance and
n Corresponding author. Tel.: þ1 801 585 5455; fax: þ1 801 581 8433.
E-mail address: jsmiller@chem.utah.edu (J.S. Miller).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2012.02.037

P.K. Erickson, J.S. Miller / Journal of Magnetism and Magnetic Materials 324 (2012) 2218–2223
2219
Fig. 1. CVD apparatus for Co[TCNE]2 films (a), and part 14b (b); (top view) used for mixed-metal TCNE formation.
2300
2250
2200
2150
2100
2050
2000
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber, cm-1
Fig. 2. IR spectrum of a CVD deposited Co[TCNE]2 film (blue), and made form solution (black). (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of this article.)
uniform across the substrate [28]. Hence, atypical for CVD, one
source had to be heated, while to the other cooled. The preferred
N2 flow rate is 120 SCCM for TCNE, and 100 SCCM for Co2(CO)8.
Neither the run time for the depositions, nor the amount of
starting material had any significant effect on the resulting
amount of film. As discussed, film deposition was interrupted
before either starting material was exhausted in all films.
CꢃN stretch (nCN) region, which exhibits characteristic absorp-
tions based on the different ionic character of TCNE, and how it is
coordinated to other atoms [29]. The IR absorption peaks at 2222
and 2171 cmꢀ1, as well as the shoulder at 2110 cmꢀ1 match data
from the solution-prepared samples that have been reported in
the literature, Fig. 2 [14]. Like the solution made material, the
ꢄ
composition of the film may be (a) ꢀCoII{[TCNE] ꢀ}2 with an S¼3/2
ꢄ
Films deposited on KBr plates were used in infrared spectro-
scopy (IR), Fig. 2. The key IR absorptions for TCNE occurs in the
CoII and two S¼1/2 [TCNE]
spin sites, (b) CoII{[TC-
NE] ꢀ}{[C4(CN)8]}1/2 with an S¼3/2 CoII and one S¼1/2 [TCNE]
ꢄ
ꢄꢀ

2220
P.K. Erickson, J.S. Miller / Journal of Magnetism and Magnetic Materials 324 (2012) 2218–2223
spin sites, or (c) CoII{[C4(CN)8]} with only an S¼3/2 CoII spin site.
An analysis of the IR data does not distinguish among these
possibilities.
After nine days in a glove box the spectra did not show any
noticeable changes. However, after two hours in air, the shoulder
at 2110 cmꢀ1 began to disappear, and the 2225 and 2172 cmꢀ1
absorption shift to 2222 and 2177 cmꢀ1, respectively. Further-
more, a new absorption 1385 cmꢀ1 appears indicating a rearran-
gement forming
structural rearrangements occurs upon exposure to air.
The magnetic susceptibility, , of the Co[TCNE]2 films were
measured between 5 and 300 K, and plotted as
T(T) and wꢀ1(T).
p
-[TCNE]22ꢀ [29,30]. Hence, decomposition and
w
w
The films possessed some ferromagnetic cobalt impurity that is
expected from thermal decomposition of Co2(CO)8. As noted in
the experimental section, this was determined to be 475 ppm Co,
and the
in Fig. 3. The 300 K value of
T(T) remains constant until ꢂ50 K when it decreases in value.
The anomaly at ꢂ50 K is attributed to an instrumental artifact
w
T(T) and wꢀ1(T) data were corrected for this, and plotted
T is 3.3 emu K/mol, and upon cooling
w
w
Fig. 4. Non-lustrous and uniform V0.86Co0.14[TCNE]2 film.
arising from oxygen. The room-temperature wT exceeds the spin
only expectation for an S¼3/2 CoII spin (1.875 emu K/mol), or it
ꢄ
with one (2.25 emu K/mol) or two S¼1/2 [TCNE] - (2.625 emu K/mol)
that might be present. Since CoII is anisotropic its Lande g value
´
can range from 2 to 5 [31]; hence, the aforementioned formula-
tions a, b, and c cannot be distinguished.
Above 5 K, both
expression, p(T
w
T(T) and wꢀ1(T) can be fit to the Curie–Weiss
w
ꢀ y) y¼ ꢀ7.5 K for the
ꢀ1, Eq. ((1)), and
aforementioned a, b, and c formulations, i.e., one anisotropic
(gCo42 [31]) S¼3/2 CoII spin, and zero, one, or two isotropic
ꢄ
ꢀ
(g¼2) S¼1/2 [TCNE]
spin sites. The fits are essentially indis-
tinguishable, but have gCo¼2.70, 2.55, and 2.40, respectively, for
ꢄ
ꢀ
zero, one, or two isotropic (g¼2) S¼1/2 [TCNE]
spin sites.
Hence, Co(TCNE)2 films are paramagnetic, with
y
o0 indicating
antiferromagnetic coupling, and no evidence for magnetic order-
ing, as reported for the solution made material [14], is present.
VyCo1ꢀy[TCNE]2 films were successfully deposited in the
apparatus with the modifications discussed above, with both
metal carbonyls at an optimal temperature of 17 1C. All other
conditions were the same. The films appear black, as opposed to
brown for the Co[TCNE]2 films, and are also non-lustrous, Fig. 4.
The IR spectra show nCN absorptions at 2212, 2160, and
2100 cmꢀ1 consistent with formation of a solid solution [12]
rather than a physical mixture of V[TCNE]x and Co[TCNE]2 (Fig. 5).
EDS data show that a range of compositions was obtained from
film to film, even though deposition parameters were kept as
Fig. 5. IR spectra in the nCN region for VyCo1ꢀy(TCNE)2 (y¼1, 0.83, 0.55, 0).
Table 1
Compositions of VyCo1ꢀy[TCNE]2 Films by EDS.
Film % of V and Co in
VyCo1ꢀy[TCNE]2 on KBr
Errora % of V and Co in
Errora
VyCo1ꢀy[TCNE]2 on glass
1
2
V: 54 Co: 46
73
72
–
V: 32 Co: 68
V: 36 Co: 64
V: 72 Co: 28
V: 36 Co: 64
V: 56 Co: 44
75
75
V: 83 Co: 17
3
No data available
V: 87 Co: 13
75
4
73
–
712
710
–
5
No data available
V: 56 Co: 44
6
714 No data available
7
V: 75 Co: 25
74
71
–
V: 50 Co: 50
V: 45 Co: 55
V: 53 Co: 47
V: 80 Co: 20
722
711
716
79
8
V: 52 Co: 48
9
No data available
V: 80 Co: 20
10
78
a
Error reported as one standard deviation of available composition data.
consistent as possible (Table 1). The nominal compositions of
the films made and studied are V0.83Co0.17(TCNE)2 and V0.55Co0.45
(TCNE)2, as ascertained from the IR [12] and EDX data. Composi-
tion, however, also varied between the material deposited on the
KBr and glass substrates, and often a composition gradient was
observed on a specific substrate. These latter two effects are
attributed to the geometry of the deposition zone, as the vapors
have different reaction rates and have different paths to each
substrate (Fig. 6). The compositional variations are a result of the
CVD, and no mechanism for eliminating compositional variance
has been identified.
Fig. 3. .
fit to Eq. (1). The anomaly at ꢂ50 K is an instrumental artifact.
wT(T) ( ꢁ ) and w
-1(T) (~) for Co[TCNE]2 taken at 1000 Oe applied field, and

P.K. Erickson, J.S. Miller / Journal of Magnetism and Magnetic Materials 324 (2012) 2218–2223
2221
2500
2000
1500
1000
500
0
0
50
100
150
200
250
300
350
400
Fig. 6. Schematic view of disposition of the exit port for the metal carbonyls and
Temperature, T, K
the position of the glass slide, and KBr pellet substrates.
Fig. 8. Temperature dependent magnetization for V0.36Co0.64(TCNE)2 showing the
Tc of 340 K.
3000
2000
1000
0
-1000
-2000
-3000
-200
-150
-100
-50
0
50
100
150
200
Fig. 7. T(T) and w
1(T) for V0.36Co0.64(TCNE)2 taken at 1000 Oe applied field.
Field, H, Oe
Fig. 9. 5-K magnetic field dependent magnetization for V0.36Co0.64(TCNE)2 show-
The
w
(T) for several VyCo1ꢀy(TCNE)2 films were measured
ing a hysteresis.
between 5 and 300 K, and plotted as
w
T(T) and wꢀ1(T), e.g.,
Fig. 7. Again the films had ꢂ500 ppm cobalt impurities that is
expected from Co2(CO)8 decomposition, and the data was cor-
rected for this. The temperature dependent magnetization, M(T),
indicates magnetic ordering with for example a Tc of 340 K from
the temperature for which M(T)-0, e.g., Fig. 8. The field depen-
dent magnetization, M(H), show for example, a coercive field of
ꢂ10 Oe, and remnant magnetization of ꢂ200 emu Oe/mol, Fig. 9.
The IR and critical temperature indicate formation solid-solu-
tion VyCo1ꢀy[TCNE]2 thin-films, and an increase in coercivity
in these mixed-metal TCNE films is expected. Based on the
VyCo1ꢀy(TCNE)2 (yꢂ0.36) solid-solution prepared from solution,
a coercivity of ꢂ200 Oe is expected, but this is not observed. None
of the films had a coercivity greater than 10 Oe for any composition
suggesting the necessity of the occluded solvent to achieve
enhanced coercivity, and this is under further investigation.
hysteresis with a coercivity o10 Oe, which is substantially lower
than that observed for VyCo1ꢀy(TCNE)2 ꢁ zCH2Cl2 suggesting that
solvent is needed to stabilize enhanced coercivity. Thin magnetic
films of VyM1ꢀy(TCNE)2 (M¼Ni, Fe) as well as of ternary
VxMyM0z(TCNE)2 (MaM0 ¼Co, Ni, Fe) compositions should be
achievable based on modification of the work reported herein.
The ability to use low temperature processing to produce thin
magnetic films on a wide variety of substrates flexible provides
the opportunity to prepare a wide range of structures, e.g.,
patterned and multilayered, of potential importance for scientific
study (e.g., the role of shape anisotropy) and for technology (e.g.,
magnetic shielding and magnetic memories).
4. Experimental section
3. Conclusions
Starting Materials. Crude tetracyanoethylene (TCNE) and cobalt
carbonyl, Co2(CO)8, were obtained commercially (TCI America
and STREM, respectively) and were sublimed before use. The TCNE
was sublimed under static vacuum (1.070.25 Torr) in a water-
cooled sublimator at 60 1C. Sublimation of ꢂ15 g of TCNE took
approximately three days. Co2(CO)8 was sublimed in an ice-cooled
sublimator under static vacuum of (1.070.25 Torr) at room
Using a low-temperature (To55 1C) chemical vapor deposition
(CVD) methodology, thin films of organic-based magnetic materials
of Co(TCNE)2 and VyCo1ꢀy(TCNE)2 compositions were prepared and
studied. Magnetic ordering above room temperature was observed
for the latter material. Solvent-free VyCo1ꢀy(TCNE)2 exhibited

2222
P.K. Erickson, J.S. Miller / Journal of Magnetism and Magnetic Materials 324 (2012) 2218–2223
temperature for approximately 7 h. Care was taken to make sure
the Co2(CO)8 was never exposed to air by sealing the sublimator
under vacuum inside a nitrogen-filled glove box before sublima-
tion. Furthermore, the sublimator seal was not broken until the
Co2(CO)8 was collected in the glove box. Co2(CO)8 was stored in a
freezer at ꢀ20 1C to protect it from thermal degradation. V(CO)6
was prepared by reacting 6 g of [Et4N][V(CO)6] with 40 g of H3PO4
according to the literature preparation [32].
the stoichiometry; thus, one component was exhausted, and the
other could possibly deposit on top of the film as a contaminant.
Hence, the CVD was interrupted before either material was
exhausted to maintain the stoichiometry of the film, and there
was no need to accurately measure the starting materials. Typically,
50 to 75 mg of Co2(CO)8 and ꢂ100 mg of TCNE were used in the
CVD apparatus, and this would deposit ꢂ1.570.5 mg of film on the
glass cover slide over ꢂ4 h of deposition.
Chemical vapor deposition apparatus. The chemical vapor
deposition (CVD) apparatus was based on an earlier version
[16]. The glassware was tested rigorously both outside and inside
the glove box in which all depositions would occur to confirm
that appropriate vacuum levels, flow rates, and temperatures
were attainable. The final apparatus for depositing single-metal
TCNE films is shown in Fig. 1(a).
The CVD of the VyCo1ꢀy[TCNE]2 films followed almost the same
procedure as the Co[TCNE]2 films, with the exception of the
temperature at which the V(CO)6 and Co2(CO)8 were held, which
was increased to 17 1C to increase the rate of sublimation for the
Co2(CO)8. This was done with the intent of introducing more cobalt
into the films to increase the coercivity of the films, as discussed.
Characterization. Infrared spectroscopy data [400–4000ꢀ1
(71 cmꢀ1)] were collected on a Bruker Tensor 37 Infrared Spectro-
meter. KBr was used as the substrate for all IR measurements. Each
film was measured after deposition was complete. To measure the
effect of time and air exposure on the film, one sample was measured
multiple times up to a week after deposition, and then exposed to air
and measured several more times.
Magnetic data (5–300 K) were collected on a Quantum Design
MPMS 5T SQUID magnetometer, as previously described [33]. The
films deposited on glass cover slides, were used in the magnetic
measurements. The film-covered glass slides were packed into
gelatin capsules during measurements. The films possessed some
ferromagnetic cobalt impurity. The presence and amount of Co
The glass CVD tube was ꢂ50 cm long and had a diameter of
3.3 cm. The inside tube had a diameter of 1 cm and met in the
middle with a 1.27 cm gap in between them. A resistive heater
(Watlow Thinband C/NSTB1G3J1-A12) was wrapped around the
outer chamber over the TCNE boat, and a thermocouple (OMEGA
Chromel Alumel Type K) attached to the outside of the glass near
this heater monitored and controlled its temperature via a
temperature controller (Red Lion Model T-16 Temperature/Pro-
cess Controller). The T-shaped glass holder for the Co2(CO)8 was
submerged in a silicon oil bath mounted on a Peltier cooling unit,
which uses the thermoelectric effect to achieve adequately low
temperatures. Another thermocouple (OMEGA Chromel Alumel
Type K) rested in the silicon oil to monitor and control that
temperature as well. Substrates for film deposition were mounted
in a square glass tube (2 ꢅ 2 cm) and centered in the deposition
chamber for maximum exposure to the deposited film. A hose
was connected to an Edwards E2M5 two-stage vacuum pump
outside the glove box, which pulled nitrogen from the box
through the deposition chamber and out the vacuum line. This
created both the necessary nitrogen flow to carry the vapors to
the center of the chamber as well as a sufficient vacuum level
inside the chamber with a minimum value of 10 Torr.
For VyCo1ꢀy[TCNE]2 thin-films, the apparatus in Fig. 1(b) was
adequate except for the need to introduce two metal sources at
once. In preliminary depositions, both Co2(CO)8 and V(CO)6 were
mixed together and included in the same substrate, but both
metals mirrored out, coating the tube, and no film was deposited.
To resolve this a smaller, concentric tube has been added along
with a second input, allowing Co2(CO)8 to pass down the outer
section and V(CO)6 to pass down the central tube, not meeting
until the point of deposition (Fig. 1(b); thus, alleviating metal
mirroring. Both holders were cooled in the same silicon oil bath to
preserve consistency with the single-metal TCNE film method.
Chemical Vapor Deposition. TCNE (ca 100 mg) was placed in a
small glass boat on the left side of the chamber, and Co2(CO)8 (ca
50 mg) was placed in a T-shaped glass boat on the right, Fig. 1. The
TCNE was heated to 45 to 55 1C, ensuring sublimation inside the
chamber, and the Co2(CO)8 cooled to 10 1C to maintain an accep-
table rate of sublimation. Nitrogen flow (measured in standard cubic
centimeters per minute, SCCM) transported the sublimed TCNE at
120–150 SCCM, and the Co2(CO)8 at 100 SCCM to the center of the
tube where they reacted and deposited onto the substrates. Micro-
scope cover slides (VWR micro cover glass) (ꢂ1.9 cm) were placed
in the substrate holder. Potassium bromide (KBr) plates were used
as substrates for infrared spectroscopy characterization in all films.
Since the film forming reaction is:
was determined from a as a Honda plot [34], i.e.,
w
or M/H vs. Hꢀ1
using the relationship of ppm-Co¼106
Dw/DH
ꢀ1/163 emu/g-Co.
This gives an upper limit of 834 ppm Co for the reported film.
However, when this amount of cobalt was taken into account in
the data workup, it overcorrected and gave a negative
wT(T)
value. Manual adjustment of T(T) levels off with a value of 475
w
and 500 ppm cobalt for Co[TCNE]2 and V0.36Co0.64[TCNE]2 films,
respectively.
For the VyCo1ꢀy[TCNE]2 films, a scanning electron microscope
(SEM) (Hitachi 5-3000N) equipped with energy dispersive X-ray
spectroscopy (EDS) (EDAX PV7746) was used to gather composi-
tion data, focusing on the fractions of vanadium and cobalt in
the films.
Acknowledgements
We appreciate the continued partial support by the Depart-
ment of Energy Division of Material Science (Grant Nos. DE-FG03-
93ER45504 and DE-FG02-01ER45931).
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not provide better understanding of the magnetic behavior, since it could be
related to magnetic impurities.
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