1. Realization of Air-Stable Encapsulated Vanadium
Tetracyanoethylene
• Context & motivation
• Encapsulation Process
• Does the process damage
the film?
• Does it protect the film?
• Implications
I. H. Froning1, M. Harberts1, Y. Lu2, H. Yu1, A. J. Epstein 1,2, and E. Johnston-Halperin1
1Department of Physics, The Ohio State University, Columbus, OH 43210-1117, USA
2Department of Chemistry, The Ohio State University, Columbus, OH 43210-1173, USA
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1 cm
1 cm
Grant DMR1207243
2. Organic, magnetic devices can do more if
they are air stable
• Many organic materials are air-sensitive (ours included)
Air stability would allow for broader use
• A multiplicity of encapsulation strategies has been
developed for air-sensitive OLEDs, OPVs, and OFETs
Flexible, air-stable devices exist in all of these areas1,2,3,4
Long-range ordering could make encapsulation of magnetic
materials more difficult
• Why not try existing encapsulation techniques on
organic magnets (and see if it works)?
1. Chwang, Rothman, Mao, et al., Appl. Phys. Lett. 83, 413 (2003).
2. Lungenschmied, Dennler, Neugebauer, et al., Sol. Energy Mater. Sol. Cells 91, 379 (2007).
3. Sekitani, Zschieschang, Klauk, and Someya, Nat. Mater. 9, 1015 (2010).
4. Kaltenbrunner, Sekitani, Reeder, Yokota, et al., Nature 499, 458 (2013).
3. V[TCNE]x is an interesting material, and so
we’d like to encapsulate it
It’s a room-temperature
organic ferrimagnet…
• Demonstrated potential for
use in spin valves5,6 &
microwave electronics7
• Straightforward to make8
• TC near 600 K
…but it’s air sensitive
• Begins degrading immediately
after air-exposure
Loses opacity as it does
• Must seal samples in order to
perform experiments
26 Hours0 Hours
T=300 K
H=100 Oe
5. Yoo, Chen et al., Nat. Mater. 9, 638 (2010).
6. Li, C.-Y. Kao et al., Adv. Mater. 23, 3382 (2011).
7. Yu, Harberts et al., Appl. Phys. Lett. 105, 012407 (2014).
8. Harberts, Lu, Yu, et al., in press, J. Vis. Exp.
4. Encapsulation process is straightforward
Two Questions:
• Does it interfere with
the film’s bulk
magnetic properties?
• If not, does it protect
the film’s bulk
magnetic properties?• Apply Ossila E131
Encapsulation Epoxy to
film with syringe
• Put cover slide on top
• Illuminate for 1 hour
5. QUESTION 1: does this interfere with
V[TCNE]x’s bulk properties?
FTIR
• No new bonds in bulk
SQUID Magnetometry
• Magnetic moment not
suppressed
• Coercivity unchanged Suggests
minimal interaction at epoxy-
V[TCNE]x interface
Compare bare samples to encapsulated samples without
exposing either to atmosphere
Conclusion: bulk magnetic properties are largely unaffected
Transmission(a.u.)
Wavenumber (cm-1)
T=300 K
6. QUESTION 2: does this preserve V[TCNE]x’s
bulk magnetic properties?
• Coercivity and saturation
are robust
• Remanence drops more
rapidly
Monitor encapsulated sample over time. How long is it
stable?
Conclusion: The overall lifetime is increased, but can we
characterize the decay process more clearly?
-70 0 70
0
-40
40
H (kOe)
M(emu/cm3)
T=300 K
7. What is the decay process?
• Remanence goes to 0
by 710 hours (1 month)
• TC & saturation are
more robust
TC above 300 K at 710
hours
Our understanding:
loss of long range order,
then loss of spins
Conclusion: The encapsulated sample takes 1 month to degrade
completely but different parameters decay at different rates
8. Conclusions
• Encapsulated V[TCNE]x is
air-stable for 2 to 4 weeks,
depending on what
parameters are important
to a given measurement
Can perform measurements
under atmospheric
conditions!
• This is the first
encapsulation method we
tried
• Points to a promising
future for organic
magnetic devices!
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Editor's Notes
Thank you for the introduction. My goal, over the next 10 minutes, will be to demonstrate why and how we encapsulate this promising organic-based magnet, before answering two questions: 1st, does the encapsulation process damage the film; and 2nd, does it protect the film? I will end by briefly discussing the implications of this work for the field. As a non-quantitative preview though, the top row here shows a bare film of VTCNE on the day of growth, and 1 month later, whereas the second row shows the same thing for an encapsulated film.
The organic magnets my lab works are air-sensitive. And that’s a problem not just because it makes working with these materials difficult for us. It’s a problem because we want collaborators in academia to use these materials to study magnetism. We want industry to use them in the kind of low-cost, chemically tunable spintronics devices and flexible, high frequency magnetoelectronics applications that we know they are often well suited for. A lot of that becomes much easier if these materials are air-stable.
It is encouraging to remember, however, that many other organic electronics are also air-sensitive. This has led to a multiplicity of encapsulation strategies for OLEDs, OPVs, and OFETs. Flexible, air-stable devices have been demonstrated in all three of these areas. Now, there is no reason to expect that, just because a strategy works on an OLED, it will work on a magnetic material too. In particular, if an OLED degrades at its surface, that only affects charge transport at the surface. Magnetic ordering, on the other hand, is a long range effect, and so if the surface is damaged, that damage could be reflected in the bulk magnetic properties of the material.
Still, if applying these techniques to organic magnets makes them more air-stable, that provides a great opportunity to explore these materials in new regimes. So there’s not much to lose by trying.
The magnet we work with most often is VTCNE, which is an interesting material with a notable limitation. Let’s start with why it’s interesting.
First, it’s a thin-film organic-based ferrimagnet, with demonstrated potential for use in spintronics and microwave electronics applications. It’s also relatively straightforward to grow via chemical vapor deposition at low temperatures and pressures. Most importantly for the purposes of this discussion, though, it has a Curie temperature near 600 Kelvin, so it’s magnetic properties are already fairly stable with respect to temperature. This makes it a good candidate for device applications, but…
…It’s air-sensitive! It begins to degrade immediately after being exposed to air, which means that we cannot perform measurements on samples without first sealing them in specialized assemblies, specific to each measurement we want to do. And that works – we’ve had success doing it that way for years – but it’s cumbersome. And it makes broader applications of the material more difficult. So. Can we make it air-stable, and if so, for how long?
The encapsulation strategy we used is straightforward. We purchased a UV-cured epoxy, intended for use with organic materials, applied several droplets of it to the top of our film, pressed down on top of that with a cover slide, and illuminated with white light for one hour to cure it. And… that’s it. The easy part is done: now we need to figure out whether this works, and to do that, we need to answer two questions:
1st, does the epoxy interfere with the film’s bulk magnetic properties? And 2nd, does it actually protect those properties against degradation?
So, what does this epoxy do to the VTCNE film? Does it interact with it? Kill the magnetic moment of the sample?
To address this, we compared unprotected VTCNE films to encapsulated films using two methods: Fourier Transform Infrared Spectroscopy, and SQUID magnetometry – without exposing any samples to the atmosphere.
On this slide everything in red is a bare film, and everything in blue is encapsulated. The top curve here shows the results of an FTIR measurement on bare VTCNE, with the cyano bond highlighted. The bottom curve is encapsulated VTCNE, with the background signal of the epoxy we applied subtracted – this should show us whether the epoxy has broken the bond between the cyano group and vanadium ion in the bulk of the material. We see no evidence of new bonds, although this does not preclude surface interactions. This promising, but what we really want to know is whether encapsulation affects the magnetic moment of the sample, and for that we turn to magnetometry measurements.
The figure on the right shows the volume-normalized, room temperature M vs H plots of bare and encapsulated VTCNE at high field to examine the effects of encapsulation on the overall magnetization of the films. We see that that the saturation magnetization of the encapsulated film (in blue) is not suppressed by the presence of the epoxy.
Next, let’s look at low field effects. Here is the same figure, zoomed in to the region of the hysteresis loop and normalized by remanence rather than volume. This normalization lets us examine any changes in the shape of the loop, independent of overall sample magnetization: what we’re asking here is whether the magnetic behavior of the encapsulated film is markedly different from that of a bare film. The shapes are remarkably similar, and the coercivity is unchanged by the presence of the epoxy. We note that coercivity is an extrinsic parameter that should be sensitive to interactions at the epoxy-VTCNE interface. Thus, we have indirect evidence that these interactions are minimal…
..and we conclude that the bulk magnetic properties of the VTCNE films are largely unaffected by this encapsulation process.
Now, this is promising, but, remember that the point of encapsulating these samples is to increase their in-air lifetime. Have we achieved that?
To address that 2nd question, we performed more SQUID magnetometry measurements on another encapsulated VTCNE film, exposed to air over the course of one month.
Here, none of the curves are normalized by remanence – only by volume – but once again the main figure is a zoomed-in picture of the hysteresis loop, and the inset shows the full field sweep. We see that the coercivity in the main figure and saturation magnetization in the inset are unchanged until 340 hours, or about 2 weeks, and the saturation is still nonzero after 710 hours, or 1 month, which is a significant improvement over their usual lifetime of 10s of minutes. The remanence drops more rapidly however, which suggests a more complicated decay process.
So this figure tells us that the film is still magnetic after 1 month, but perhaps we can see the decay process more clearly by extracting some magnetic parameters from this data and plotting them over time.
Here, we plot three magnetic parameters of the encapsulated sample from the last slide as a function of exposure time, on a semi-log scale. Those parameters are Curie temperature, saturation magnetization, and remanence. We see that after 1 month, the remanence has dropped to nearly 0, but TC and Ms decay more slowly. We believe this signifies that as the sample decays there is a loss of long-range order before there is a loss of spins. This is in accordance with our understanding of what makes encapsulation of a magnetic material different from OLED encapsulation: surface degradation in an OLED only affects the material locally, whereas a magnetic ordering parameter like remanence is globally sensitive to these surface effects and therefore likely to be less robust against degradation.
However, the fact that the Curie temperature is still above room temperature after 1 month means that, even if magnets are harder to protect than OLEDs, this encapsulation strategy has increased the overall lifetime of the sample from 10s of minutes to 1 month.
That’s a major improvement. Depending on what magnetic parameters are important to a given measurement, this means that encapsulated VTCNE is now air-stable for 2 to 4 weeks. That’s immediately useful: if we have a measurement we’d like to perform on a VTCNE film that takes, say, a few days to run, and requires the sample to be exposed to air for that whole time, we can do that now. Previously, if that measurement took even an hour, we had to seal the sample within a specialized assembly to protect it from the ambient environment.
I’d also like to point out that this is the first encapsulation method we tried. Different methods that work with other organic electronics may work on organic magnets as well. The takeaway here is that there is almost certainly room for improvement.
Which brings me to my last, most important point. Organic magnets have not found their way into consumer electronics the way OLEDs have. There are a number of reasons for this, but lack of air-stability is a critical constraint – a device with an in-air lifetime of 20 minutes is just not that useful in most contexts. The work I’ve presented here demonstrates a result that has been immediately useful in our own lab, but it also points to a promising future of air-stable, organic-based, magnetic devices.
So thank you for your time and your attention, and for more details, we have a paper in review. But, for now, I’d like to open it up for a couple minutes of questions.
