And here I am again, with another paper just published in Angewandte Chemie. This work hasn’t been an emotional rollercoaster like the
“hemilabile ligand for molecular electronics” that I described in August last year, but it has been an absolute mammoth of data
acquisition and analysis, performed with incredible perseverance and diligence by soon-to-be-Dr. Chuanli Wu.
It all started with a small pot of money I was given in 2018 for the synthesis of some cluster-containing molecular wires. The idea was to use these funds to gain some skills in small, atomically defined cluster preparation and purification (somewhat different from the chemistry I did up to then), and to use these to collect single-cluster conductance data that could underpin my fellowship applications. One of the reviewers for my Future Leaders Fellowship application (alas, unsuccessful) raised doubts on the stability of clusters under an intense electrical field, such as the one that is experienced by a molecular junction, and I wanted to show that these can be extremely resilient species. After the funds were released, I went back to the bench and prepared a few different classes of clusters – from pure metallic materials containing copper, silver or ruthenium, to metal chalcogenides and metal oxides. The latter, perhaps better known as polyoxometalates, interested me for their electrochemical behaviour – some of the larger ones have multiple, well-defined oxidation states, and their cyclic voltammograms (CV) can be oddly fascinating (see here on the right for an example from a Keggin polyoxoanion) as all the different charge states give very well-defined reversible redox processes and therefore many pleasant waves in the CV!
For a cluster to be used efficiently in molecular junctions it needs to be terminated with aurophilic groups, which can form coordination bonds with two nanoelectrodes and close the circuit. This somewhat limits the range of polyoxometalates I could use, as their functionalisation with organic ligands is not exactly straightforward. I started by synthesising a Lindqvist polyoxovanadate (see XRD structure on the left) and I measured its conductance – it proved to be quite poorly conductive, and a bit too close to the noise level of our instruments at the time to be studied in more detail, but it gave me the initial data I needed for my fellowship applications, which I’m sure helped me greatly in securing the Royal Society URF position. That it was so insulating wasn’t surprising – most bulk metal oxides are good insulators, and the Lindqvist anion proved to be no exception. By then, however, I really wanted to find a redox-active cluster that could study at the single-molecule level in an electrochemical environment. A literature search turned up a small polyoxomolybdate, having Anderson-Evans structure, that could be functionalised in an optimal way, with the two organic ligands coordinated to opposite sides of a MnO6 octahedron. The synthesis looked straightforward on paper, but it took me a few weeks before I had a sample pure enough for characterisation and crystal growing – which was good: all useful skills I had to learn if I wanted to work with more of these cluster materials.
Just as I was finishing preparing these compounds, Chuanli, who was visiting our laboratory at the time, had also finished collecting all the data she needed for a project she was working on – the study of the in-situ formation of hydrogen-bonding chains during break-junction experiments (published a few months ago in Nanoscale). Before this work, she had worked quite extensively on the in-situ electrochemistry of NiO, in a project that was very challenging but, alas, resulted only in more questions than answers. Nickel is indeed a weird metal. While partially unsuccessful, she gained a lot of experience in the use of the electrochemical scanning tunnelling microscope, and she decided to take on the challenge of measuring the polyoxometalate single-molecule conductance under electrochemical potential control. The idea here was to probe the charge transport properties of the polyoxometalate in its various electrochemical states. Initial cyclic voltammetry showed us we could both oxidise and reduce the Anderson-Evans cluster from its natural -3 state, to either -2 or -4. And, as usual, our problems started. The polyoxomolybdate wasn’t that soluble in the solvents we regularly use in molecular electronics studies, and the necessity of an ionic environment (an electrolyte) further complicated the issue. Chuanli started screening a few solvents where we knew we could get an electrochemical response – acetonitrile, propylene carbonate, DMF, etc. – but each of them presented an issue in the single-molecule measurements we wanted to perform. They either resulted in too much background noise, dissolved the electrodes coating, prevented formation of junctions or gave an unreliable electrochemical potential (a problem in most non-aqueous electrochemical studies).
At that time, Xiaohang Qiao (a PhD student working with us) was studying the use of deep eutectic mixtures in molecular electronics. These are excellent solvents, they provide an ionic environment ideally suited to electrochemistry, and showed good promise for STM work. So Chuanli and Xiaohang teamed up and started studying the polyoxometalate in a deep eutectic mixture of choline chloride and urea. This started to give us quite good results: the polyoxomolybdate was soluble, we could form junctions in the electrochemical environment (made of 4 electrodes, two Au as source/drain and two Pt as counter/reference as in the figure here), and we could see a difference in the conductance as the potential was made more negative and the polyoxometalate charge state was changed from -3 to -4. Furthermore, the presence of many chloride ions allowed us to use a chloridised silver wire as reference electrode, with excellent stability. Unfortunately, they then had issues at positive potentials: the excess chloride ions that gave us the benefit of a stable reference electrode also caused the Au electrodes to oxidise and dissolve before we could access the -2 state in this setup.
We then remembered that a few years ago our lab published a series of papers in JACS (I helped out on the third in the series) on the use of ionic liquids in single-molecule electronics. They had everything we were looking for: powerful solvents with a wide electrochemical window and a simple Pt wire can be used as reference electrode with excellent stability – thinking back, I don’t really know why we didn’t use these from the beginning.
Ah yes, ionic liquids are quite a nightmare to purify. And expensive. Very expensive.
Ionic liquids tend to pick up water quite quickly – when they’re wet, the electrochemical window narrows down a lot, and we needed all of it, or at least 2.5 V. Also, last year we weren’t exactly swimming in cash. Luckily, we had about 50-75 mL of 1-butyl-3-methylimidazolium trifluoromethanesulfonate in our glove box, leftover from our previous studies – while it sat there for more than 4 years, it couldn’t have gone too bad in an inert, dry atmosphere. We cleaned it up, testing its electrochemistry until it was pure (and it took some time!), and then Chuanli started collecting data. This setup worked like a charm!
As you can read in the paper, using an ionic liquid allowed us to probe conductance over 2.7 V of electrochemical potential, where we could measure the polyoxometalate in all three states (-2, -3 and -4). We found a rather unusual response – as the cluster was oxidised or reduced from its native -3 state, conductance suddenly dropped by one order of magnitude. From our data, it looked like the mechanism of charge transport was phase-coherent tunnelling, and the polyoxometalate in different charge states presented a different barrier to charge tunnelling. While not a novel phenomenon (see examples here, here and here), it was pretty exciting to find out such a behaviour in a polyoxometalate. To further strengthen our claim, we did some modelling, which required quite a lot of data analysis (good thing I had an M.2 NVMe solid state disk with a write speed of ~1.9 GB/s or this would have taken years), and some more experiments, which you can read about in the paper (that is open access and free to read to everyone).
What I think is staggering about this project is the sheer amount of work performed by Chuanli. In the paper, we present data from approximately 100000 (yes, one hundred thousand!) single-molecule devices she fabricated under electrochemical control, and an additional 20000 devices fabricated under source-drain bias modulation. But this is just the tip of the iceberg. There are many “preliminary measurements” in other solvents (the ones that didn’t really worked as we wanted), and from the sheer size of her “POM” folder (sitting on my back-up disk at 11.3 GB zipped at ~20% rate) I estimate at least another 200000 junctions were fabricated for this project. And I’m being conservative!
Anyway, the publishing process was refreshingly quick and straightforward - so this story ends in a quite anticlimatic fashion. We had an initial desk rejection from another journal, but after submitting to Angewandte Chemie we received very good feedback from the reviewers – they asked for a single control experiment which we hadn’t done. We did it, it worked as expected, and we finally got this beast published.
I hope you’ll enjoy reading about our work as much as I enjoyed writing about it – honestly, with experimental data as good as the one Chuanli collected, the paper almost wrote itself.
Many thanks to soon-to-be-Dr. Chuanli Wu for for keeping up with my continuous “MORE DATA!” requests for many months, and to Prof. Richard J. Nichols and Prof. Simon J. Higgins for their help in putting this together. Edited on 26/05/2020 to correct a few typos. You never proofread enough!
So, I'll start this column by writing about this paper here, that we recently published in Angewandte Chemie.
As you'll see, this is a nice story with failures, puzzlement and despair, but also great teamwork and sustained effort. It follows a quite oftenly encountered scientific path: we started
looking into some compounds to test one particular theory (quantum interference) but instead, we found something
completely different (mechanoresistance) but equally exciting, that can also help
us to understand some earlier puzzling results we (and other research groups around the world) had obtained. In some ways, it is also a manifestation of
"Why spend a day in the library when you can learn the same thing by working in the laboratory for a month?".
The bulk of this work was performed whilst I was working as a PDRA for Prof. Richard Nichols,
and I was training one of Prof. Simon J. Higgins (my PhD supervisor) students in the group, now Dr. Nicolo' Ferri.
He picked up a project I left incomplete at the end of my PhD (late 2014-early 2015) as I hadn't been able to synthesise
one of the key compounds, a fused bithiophene with a bridging carbonyl function and methyl thioether contact groups for binding to gold electrodes.
The target was quite interesting because it had a very narrow bandgap and theory had predicted some odd quantum interference effects from the carbonyl,
that would result in unusually high (or unusually low, depending on which theoretician we listened to...) conductance.
Following a few recipes for apparently similar compounds I found in the literature just gave a black sticky mess,
and after a few disappointing attempts, I started my PDRA working on molecule/semiconductor interfaces and I completely forgot about the whole project.
Just to explain what kind of chemist Nicolo' is, he not only willingly accepted this work that gave me only pain and disappointment,
but actually managed to synthesise this key compound, grew beautiful (and big) shiny purple crystals, got the SCXRD that confirmed the structure,
did some more synthetic work, and performed all the measurements that you can find in the above paper.
Now, for the organikers out there, we initially tried to synthesise this in a quite unpleasant sequence, where all steps involved either something toxic, something smelly or something pyrophoric (well, it's just n-BuLi, but still...)! Starting from cheap-ish 2-bromothiophene he had to do: