Image: George Nazin is developing scanning probe methods to image electronics states in materials. [Oregon]
From defects in carbon nanotubes to molecular fluorescence, Professor George Nazin has used STM to image the unimaginable.
Earlier this year, Professor George Nazin and colleagues from the University of Oregon unveiled their cryogen-free scanning tunneling microscope, built to image carbon nanotubes and other materials at atomic resolution.
The instrument follows more than a decade of dedicated STM-related research, but for Nazin, this is just the beginning.
Already his team has pinpointed defects in carbon nanotubes that impede electron flow and imaged charge traps in nanocrystals that deplete the efficiency of next-generation solar cells.
Nazin now intends to image more and more nanomaterials, research that is set to shape the electronic transistors and photovoltaics devices of tomorrow. So where did it all start?
Nazin has always loved Physics. Growing up in Troitsk, a small town just outside Moscow, he was immersed in the subject from an early age.
"I grew up in a somewhat unusual place that was primarily research-oriented and home to a lot of scientists and physicists," he says. "My father is a physicist and many of the people around me as I was growing up were physicists, so physics was a natural thing to do; it was just a question of what kind of physics."
Passion for physics
Theoretical physics was Nazin's first choice, with the young researcher graduating with a Master of Science degree from the Moscow Institute of Physics and Technology that focused on theoretical and computational physics in 1999.
In his words, this branch of the subject was 'great' but a move to the US for his PhD at the University of California Irvine brought new opportunities.
As Nazin explains, he had been studying the theory behind scanning tunnelling microscopy as part of his Masters and knew of a UC Irvine laboratory, headed up by Professor Wilson Ho, that was putting the theory into practise.
"I realised that there were a lot of exciting opportunities to do real cutting-edge experiments that would be very interesting," he says.
"[At Moscow] we just didn't have the kind of infrastructure that existed at Irvine... but my theory gave me some unique insights into the kind of experiment I could do, and how this experiment could be carried out," he adds.
And so Nazin spent the next seven years using STM in single-molecule optical spectroscopy, to study the physics behind individual molecules.
Professor George Nazin and his researchers have built a novel cryogen-free STM to push back the boundaries of atomic resolution imaging. [Oregon University]
Working with a variable-temperature STM, built by Ho's research team in the late 1990s for atomic-scale research, Nazin and fellow researchers started probing the atomic structure and local electronic properties of molecules in unprecedented detail.
As Nazin puts it: "Our experiments with molecules were very interactive. We could make these molecules move, change shape, break them apart as well as synthesise molecules from individual atoms or smaller molecules. This was simply incredible to me and I couldn't get enough of it."
Come 2003, Nazin, Ho and colleague Xiaohui Qiu had detected the fluorescence of a single porphyrin molecule, confirming the feasibility of fluorescence spectroscopy with STM.
And later that year, the team used the same STM to assemble a metal-molecule-metal junction, probing the atomic structure and local electronic properties with breathtaking resolution.
Both breakthroughs were published in Science, each set the scene for future nanodevice design, and as Nazin says: "These were experiments that you couldn't read about in a book or even a research paper, we were studying molecular behaviour in this way for the first time."
During his time at UC Irvine, Nazin also got his first taste for building microscopes.
Working alongside colleagues from Ho's group, he spent two years constructing a STM specifically for atomic-scale physics and optical spectroscopy that now resides with Professor Ho and world-renowned chemist, Professor Vartkess Apkarian at the Center for Chemistry at the Space-Time Limit, UC Irvine.
As time passed, Nazin wanted to start applying his research more. "I wanted to explore things more at the device level, and learn how to make devices and characterise them," he says. "Bridging the gap between atomic characterisation and understanding a device is a challenge and I could do both."
So the researcher took up a research position at Brookhaven National Laboratory in 2007, having been awarded a Goldhaber Distinguished Fellowship from BNL, designed to support promising young scientists.
Here he studied the internal electronic structure of graphene devices using scanning photocurrent microscopy. The instrument was based on an integrated confocal optical microscope-superconducting magnet set-up he had constructed.
With fellowship complete, Nazin headed west across the US to the Department of Chemistry and Biochemistry, University of Oregon, to take the position of Assistant Professor of Physical Chemistry.
As he says: "I felt there were a lot of opportunities for academic and industrial collaboration here. We have excellent nanocharacterisation and nanofabrication facilities and I wanted to apply my techniques of imaging electronic states in different materials."
Since joining Oregon, Nazin has built up his now seven-strong research team from scratch. Some of his students have already made the move into industry, namely Intel, as the end application of Nazin's research is now clearly semiconductors.
Crucially, in the last few years, Nazin and his team have constructed a liquid helium-free, cryogenic ultrahigh vacuum STM to image carbon nanotubes and semiconductors at atomic resolution.
Nazin built the UHV STM with colleagues over three years for highly detailed, atomic-scale spectroscopic investigations. [Nazin]
Using a closed-cycle cryostat, based on a design widely used in magnetic resonance imaging, helium gas is recirculated, rather than using a continuous supply of liquid helium, to cool the instrument to temperatures as low as 16 K.
For Nazin, building a helium-free STM is a real bonus as operating costs are much lower, and the instrument can be operated pretty much continuously.
"I'd spent a long time thinking about the design, and calculations led me to believe that the cryostat and STM scanner should be able to work together," he says. "But now we see this instrument is orders of magnitude better in many ways than what we had at Irvine, so we hope to achieve even more detailed results on different systems using this."
STM image of nanotube: George Nazin uses STM-based spectroscopy to understand the electronic and excitonic states of carbon nanotubes. [Nazin et al]
Nazin reckons, for example, the spatial drift of his STM comes in at around a quarter of an angstrom an hour, some ten times better than today's high performance instruments.
"We can stay on a single atom for hours without drifting off, which means we can map the electronic states of nanomaterials over large areas, which is critical to STM spectroscopy," he explains.
"We had the courage to build it, and try it and now it's worked," he adds. "Building an instrument can be daunting but you have to keep the faith and know that eventually everything works out."
And with carbon nanotubes and nanocrystals with photovoltaic potential already characterised, Nazin and his team intend to continue their nanotube research as well as image a wider variety of nanocrystals and quantum dots.
Indeed, a key aim is to work out how quantum dots can be synthesised to minimise surface defects and optimise electronic properties.
"We want to focus on the science right now and show exactly what we can do with our new instrument," says Nazin.
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