Researcher developing (non) linear nano-optical methods to characterize heterogenous interfaces

The Great Barrier Reef covers 133,000 square miles off the coast of Queensland, Australia, in the Coral Sea. It is the largest living structure in the world, formed by tiny creatures called polyps, with nearly endless heterogeneous peaks, dips, micro basins, and other rich topological features.

Now, take that and shrink it down to 10 nanometers. That’s what life looks like to Patrick El-Khoury, a senior research scientist at Pacific Northwest National Laboratory in Richland, Washington.

El-Khoury spends his time examining heterogeneous interfaces on the nanoscale, where chemical, physical, and biological properties change over tiny length scales.

His job is to conduct basic chemical physics research on heterogeneous interfaces encountered in the chemical, biological, and energy sciences. To do that, he predominantly uses Tip-Enhanced Raman Spectroscopy (TERS), which allows him to see and identify (bio) molecular and (bio) material systems with few-nanometer precision. He also adapts and actively develops novel nano-imaging and nano-spectroscopy approaches that can be used to visualize matter over the length (nanometers) and time scales (femtoseconds) of relevance to chemical transformations.

“You look at these nano-basins, or local environments, and they each feature unique physical and chemical properties,” he said. “It makes the local properties of nearby (bio)molecular systems even more complex than they naturally are.”

El-Khoury sees heterogeneity to be the rule rather than the exception. The properties of most realistic interfaces encountered in biology, chemistry, and materials sciences vary over the nanoscale. This is a far cry from the idealized systems that are described in textbooks. Embracing and understanding this exquisite complexity is pre-requisite to the development of more efficient functional materials that can be deployed, e.g., in modern photovoltaic and optoelectronic devices.

“We image things with very high spatial resolution, and because we are using different tools of optical spectroscopy such as Raman scattering, we’re also able to identifying them. In effect, you have a nanoscopic chemical eye that can tell you where the stuff is and what it is.”

The high spatial resolution that is attainable with the techniques used by El-Khoury necessitates that the signals are nascent from no more than a handful of molecules. In this regime of ultrasensitive optical spectroscopy, the rules of the game are different.

“Part of our work involves using well-known molecules to characterize the complex local environments in which they reside. In TERS, this means looking at different properties of nano-localized optical fields with ultrahigh spatial resolution. This requires a detailed fundamental understanding of the operative physics in TERS.”

Biology and energy researchers are often interested in processes taking place at solid/liquid interfaces. This poses a challenge from the measurement science perspective, since chemical visualization of such interfaces under ambient laboratory conditions is not trivial with existing technologies.

“You have methods that yield nanometer spatial resolution with no chemical information. You also have methods that have been developed for about a century now that give you ample chemical information and selectivity, but that lack the required spatial resolution. This is particularly true when the goal is to visualize biological and energy processes as they happen, or in situ / in operando.”

Though there is tremendous value in conducting fundamental research in controlled environments – after all, that’s where much of our knowledge of physics has stemmed from, at some point, this idealized knowledge doesn't transfer over to real life because of the above-mentioned nanoscale heterogeneity, particularly at solid-liquid interfaces.

“The best example I can give you is that of a catalyst,” he said. “You have a catalytic surface that operates at a certain efficiency. The question then becomes why isn’t it 100 percent efficient? Answering that question requires finding and characterizing the so-called active sites and comparing them to inactive catalytic sites, which are generally much more prevalent in a device. Do active sites feature unique properties compared to the rest of the surface? Do successful catalytic conversion events occur at topographically distinct sites? Is the unique topography accompanied by unique physical (e.g. electric fields) and chemical (e.g. oxidation states) properties? These are the kind of questions that need to be asked. Answering these questions with an eye on the nanoscale requires multimodal nano-topographic and nano-chemical characterization tools, such as TERS.“

That, he describes, is seeing something that’s (in)efficient, like a solar cell, zooming in and trying to ask the basic question of why it is or isn’t it working at specific sites. The fundamental chemical physics studies that he performs are meant to inform device makers that, for example, have trouble with local oxidation sites in particular devices.

El-Khoury also explained how he is constantly deploying and developing novel nano-spectroscopic tools by combining different optical spectroscopy methods with tools of nano-optics  

“The Raman-based approach can be generalized if you have an optical setup that is flexible– and enough lasers of course. At this point, we successfully performed not just nano-Raman, but also nano-PL, nano-extinction, and multimodal nonlinear optical measurements like nano-CARS, nano-SHG/SFG, and nano-TPPL. Some of these are published, others are on the way out.”

El-Khoury ended by stressing that biological/energy science/material science applications of these unique nano-optical technologies are important, but it is equally important to further develop the existing basic understanding of the optical signatures of (bio)molecules and (bio)materials on the nanoscale.

“I’m not the guy that makes wafer-scale devices. But I can probably generate enough useful information about existing and prototypical devices to inform — or just annoy device scientists.”

He also emphasized the importance of not taking anything for granted, e.g., spectroscopic selection rules on the nanoscale. He joked that “rules are meant to be broken” in referring to the differences between conventional/nano-optical measurements that track many/few molecules.


Patrick Z. El Khoury in his lab

El-Khoury takes advantage of several HORIBA instruments, including every AFM-optical system offered by the company, the EvoNano. That said, his optical setups are highly customized to enable multimodal linear and nonlinear optical nano-imaging and nano-spectroscopy. The combination of nano-optical measurements that can be performed in a single optical setup that he calls “our own trios” is unique to his PNNL lab. His group also uses the LabRam HR Evolution for routine micro-Raman and ultra-low frequency Raman measurements. All his setups are described in recent publications from his group.

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