A way with atoms


Rebecca Pool

Tuesday, May 7, 2019 - 14:15
Image: Professor Philip Moriarty has pushed back the boundaries of scanning probe microscopy while sharing his enthusiasm for Physics, far and wide.
Microscopist and physicist, Professor Philip Moriarty, wants to build a 'matter compiler'. Having spent the best part of two decades prodding, pulling and picking up individual atoms with scanning probe microscopes, he now intends to fulfil sci-fi prophecies and use the instruments to automatically build 3D objects, an atom at a time.
"We'd like to have a few buttons on the front panel of a scanning tunnelling microscope or an atomic force microscope that will control the atomic structure of the probe tip," says Moriarty. "You'd push the button and walk away while the microscope optimises the tip for imaging, spectroscopy and manipulation."
"Ultimately we could use this to 3D print atoms - it's a long way off but that's what we're aiming for," he adds.
Right now, Moriarty, Professor of Physics at the UK-based Nottingham University, is amongst a handful of researchers using modified atomic force microscopy to interrogate and manipulate molecules, atoms, and even single chemical bonds.
Having spent years developing dynamic force microscopy, he can harness the chemomechanical forces between a sharp tip and a sample to, say, map force fields for hydrogen-bonded molecules or image variations in the binding energy of fullerene molecules, with the best of them.
But there's more to Moriarty than scanning probe microscopy and mechanochemistry.
Professor Philip Moriarty's love of Physics is accompanied by an over-riding enthusiasm to convey the subject to the general public, sometimes with the help of heavy metal and his guitar.
While quite literally pushing the boundaries of atom arrangement, he's written a book on how to explain quantum physics with heavy metal, won award after award for public engagement, and alongside Physics and Astronomy colleagues at Nottingham, notched up more than 70 million views on YouTube with physics videos (see 'A passion for communication'). So what drives the prolific Professor Philip Moriarty?
In his words, he wasn't a 'model' student. Distracted by music and being in a band, he failed the third year of his four-year degree in Applied Physics, from Dublin City University.
Still, after repeating that year and finishing his final year, he got a 2.I, and has never looked back.
"If I hadn't failed I'd have drifted through with a third class degree and wouldn't have been able to do a PhD, so this was the best thing that could have happened to me," he reflects. "And in that fourth year I worked pretty hard, got my teeth into a project on computerised tomography - CT scanning for medical applications - and this all pulled my marks up."
With degree in hand, Moriarty joined forces with Professor Greg Hughes, from Physical Sciences at Dublin City University, for his PhD, and discovered scanning probe microscopy.
First steps
It was 1990, IBM researchers were already arranging individual atoms on a surface with a scanning tunnelling microscope tip, and Moriarty was fascinated.
So, in tandem with photoelectron spectroscopy analysis at the, now defunct, Daresbury synchrotron, the young researcher started to use scanning probe microscopy to investigate sulphur-passivated semiconductor surfaces.
"Initially the project was very much about calibrating the SPM probe, and I am no metrologist," he laughs. "But eventually the research mutated and became more about imaging these semiconductor surfaces at atomic resolution... and combined with the synchrotron work, it was a really interesting PhD."
Importantly, as Moriarty highlights, Professor Greg Hughes was an inspirational mentor, and, as he jokes, they almost exclusively communicated in swear words.
"He was enthusiastic and hands-on, and all of his students had ownership of their projects," he says. "Yet he didn't drop us in the deep end, we had a gradual process of learning and he was a good friend throughout... he was pretty much the ideal supervisor."
In 1994 and post-PhD, Moriarty moved across to the Department of Physics at the University of Nottingham, to take up postdoctoral research with Professor Peter Beton.
This time, moving atoms and molecules dominated Moriarty's research, with the young researcher using the tip of a STM to position and manipulate individual C60 molecules and related fullerenes on a silicon surface at room temperature.
A dynamic force microscope image acquired using a qPlus sensor of a 2D assembly of hydrogen bonding molecules (NTCDI) on a suitably passivated silicon surface. [Taken from A. Sweetman et al., Nature Comms., 5, 3931 (2014).] Note that the features between the molecules, while at exactly the positions one would expect for hydrogen bonds, are almost certainly artefacts arising from relaxation of the apex of the tip.
A lectureship swiftly followed and come 1997, Moriarty found himself building a research group. In his words, he was not 'as strategic and targeted' as he could have been, casting the net wide in his search for research funds.
"Many new lecturers are in the same position - you're scrambling around to get funds and applying to as many pots as possible," he says. "Looking back, it is important to focus and not to get torn in too many directions... there is pressure to quickly set up your group but really don't try to chase every funding possibility."
At the same time, Moriarty and his researchers were trying to build their first instrument; an ultra high vacuum photon emission scanning tunnelling microscope.
With this, they intended to examine how light could be emitted from fullerene molecules on silicon substrates, and then use the instrument to optically probe the magnetic properties of transition metal clusters.
Success ensued, but Moriarty vowed to never build a microscope from scratch again.
"Unless you are carrying out a very unusual experiment that requires a novel microscope, I would say it is a false economy to build your own," he says. "A PhD researcher can spend a lot of time building an instrument and then has to still get sufficient data to get that doctorate."
"But a student that can come in on a commercial instrument will hit the ground running, so this is the imbalance," he adds.
Come 2001, Moriarty's interest in molecule and atom assembly was growing. With fellow researchers, he had already developed an ultra-high vacuum hybrid scanning near-field optical microscopy-scanning tunnelling microscopy to study the vibrational and electronic properties of carbon nanotubes and fullerenes.
But at the same time, he was also looking more closely at atomic force microscopy, and how it could be used to study self-organisation in nanoparticle arrays, including thiol-passivated Au nanoparticles on silicon.
AFM image of the influence of substrate wettability on pattern formation in de-wetting nanofluids; the nanofluid was a solution of thiol-passivated gold nanoparticles. The square region, which is 4 microns on a side, was created by oxidation of hydrogen-passivated silicon with the AFM tip and is hydrophilic as compared to the surrounding hydrophobic region. [Dr. Matthew Blunt, now at University College London.  See CP Martin et al., Phys. Rev. Lett. 99 116103 (2007) for more details.]
"In terms of non-equilibrium physics this was so interesting," says Moriarty. "For example, instead of using temperature or heat to drive the evolution of a nanoparticle assembly, we found that we could use the AFM itself, in tapping mode, to drive the system, towards equilibrium."
Automating molecule manipulation
Moriarty's fascination with scanning probe microscopy continued. Then in 2008 and by this time holding the position of Professor of Physics at Nottingham, he won a mighty £1.7 million grant from the UK-based Engineering and Physical Sciences Research Council to take single molecule manipulation to a new level.
Having repeatedly demonstrated atom manipulation, he and colleagues were to now develop the necessary protocols towards automated atom-by-atom assembly of 3D structures using dynamic force microscopy, also known as non-contact ATM.
Within three years they had made a significant first step by creating an atomic toggle switch based on a silicon dimer.
Here, they would position the probe tip of their DFM just above a dimer atom, so that the force between the atom and tip would pull the atom upwards, altering the dimer's bond angle and flipping its state.
The universal “horns” symbol of the heavy metal community written by moving single silver atoms on an Ag(111) surface using the tip of an STM. Ag is a heavy metal. [Thanks to Dr. Adam Sweetman, University of Leeds and a fellow metal fan, for his help in constructing the atomic horns.]
Research continued apace, with the researchers pushing the limits of DFM ever-further.
Each DFM imaging breakthrough has nudged Moriarty closer to the much-coveted matter compiler. 
More recently, the Professor has won further funds to explore mechanochemistry at the single bond limit, and is also setting up a new high magnetic field scanning probe microscope, funded by the EPSRC’s Strategic Equipment Fund, as a national facility to measure atomic structure, chemical forces, electronic properties and magnetic behaviour.
Right now, Moriarty and colleagues, from Nottingham and the University of Utrecht in The Netherlands, are training machine learning algorithms to characterise atomic-resolution images from the new instrument.
The work builds on past collaboration with Computer Scientist, Professor Natalio Krasnogor from the University of Newcastle, UK, on evolutionary algorithms for molecular self-assembly, but now involves ensembles of convolutional neural networks.
"We need to train the AFM to recognise when it has a good or bad image and this is the first step towards this," highlights Moriarty. "We've already reached the point where [the model] can distinguish between atoms, atomic roads and dimers, so it's all working pretty well."
Moriarty jokes that at least by retirement-age he might well have pushed 3D printing all the way down to the atomic level, which is quite a comeback from that undergraduate failure that still haunts him. So what advice would he give to the would-be researchers of tomorrow?
"It has been hard work and grind but tenacity is key," he says. "Sometimes the students performing at the top of the class in exams are not the same students that do well in experimental physics."
"What has driven me is this huge interest in Physics," he adds. "That plus being extremely stubborn to the point of belligerence, and that is what has always got me through."
Professor Philip Moriarty:
A passion for communication
Philip Moriarty's love of Physics is accompanied by an over-riding enthusiasm to convey the subject to the general public.
As he puts it: "They money that funds the work that we do comes from the public purse so there is an obligation to explain and justify what we are doing," he says. "But equally, I totally enjoy doing this."
Indeed, a host of projects has ensued. In 2008, he and like-minded researchers joined forces with artists and sculptors, Tom Grimsey and Theo Kaccoufa, to create large-scale kinetic sculptures that mimicked nanoparticle system dynamics.
Around the same time Moriarty, along with a number of his colleagues at Nottingham, started to work with with video journalist, Brady Haran, on the Sixty Symbols Project to produce a series of videos, designed to improve public understanding of science.
Involvement with Maths and Computer Science YouTube series, Numberphile and Computerphile, as well as his book, 'When the Uncertainty Principle Goes to 11: Or How to Explain Quantum Physics with Heavy Metal' have followed. And no doubt, yet more will come.
"The more people that we can get into science and critical thinking in this 'post-truth' era, the better, but what I’d also like to dispel is that perception of 'physics is too difficult for me'," says Moriarty. "I want to show people that no, you don't need to be 'uber-smart' to do his, what it really takes is an interest, enthusiasm and tenacity."


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