Illustration of the interference between light from the quantum dot (black sphere) and radiation from the mirror dipole (black sphere on the wire). This interference will slightly distort the perceived location of the diffraction spot as imaged on a black screen at the top. The distortion is different depending on whether the quantum dot dipole is oriented perpendicular (red) or parallel (blue) to the wire surface, a difference that can be visualized by imaging the diffraction spot along different polarizations. Credit: Ropp

The 2014 chemistry Nobel Prize recognized important microscopy research that enabled greatly improved spatial resolution. This innovation, resulting in nanometer resolution, was made possible by making the source (the emitter) of the illumination quite small and by moving it quite close to the object being imaged. One problem with this approach is that in such proximity, the emitter and object can interact with each other, blurring the resulting image. Now, a new JQI study has shown how to sharpen nanoscale microscopy (nanoscopy) even more by better locating the exact position of the light source.

Diffraction limit

Traditional microscopy is limited by the diffraction of light around objects. That is, when a light wave from the source strikes the object, the wave will scatter somewhat. This scattering limits the spatial resolution of a conventional microscope to no better than about one-half the wavelength of the light being used. For visible light, diffraction limits the resolution to no be better than a few hundred nanometers.

How then, can microscopy using visible light attain a resolution down to several nanometers? By using tiny light sources that are no larger than a few nanometers in diameter. Examples of these types of light sources are fluorescent molecules, nanoparticles, and quantum dots. The JQI work uses quantum dots which are tiny crystals of a semiconductor material that can emit single photons of light. If such tiny light sources are close enough to the object meant to be mapped or imaged, nanometer-scale features can be resolved. This type of microscopy, called "Super-resolution imaging," surmounts the standard diffraction limit.

Image-dipole distortions

JQI fellow Edo Waks and his colleagues have performed nanoscopic mappings of the electromagnetic field profile around silver nano-wires by positioning quantum dots (the emitter) nearby. (Previous work: http://phys.org/news/2013-02-quantum-dots-probe-nanowires.html ). They discovered that sub-wavelength imaging suffered from a fundamental problem, namely that an "image dipole" induced in the surface of the nanowire was distorting knowledge of the quantum dot's true position. This uncertainty in the position of the quantum dot translates directly into a distortion of theelectromagnetic field measurement of the object.

The distortion results from the fact that an electric charge positioned near a metallic surface will produce just such an electric field as if a ghostly negative charge were located as far beneath the surface as the original charge is above it. This is analogous to the image you see when looking at yourself in a mirror; the mirror object appears to be as far behind the mirror as you are in front. The quantum dot does not have a net electrical charge but it does have a net electrical dipole, a slight displacement of positive and negative charge within the dot.

Thus when the dot approaches the wire, the wire develops an "image" electrical dipole whose emission can interfere with the dot's own emission. Since the measured light from the dot is the substance of the imaging process, the presence of light coming from the "image dipole" can interfere with light coming directly from the dot. This distorts the perceived position of the dot by an amount which is 10 times higher than the expected spatial accuracy of the imaging technique (as if the nanowire were acting as a sort of funhouse mirror).

The JQI experiment successfully measured the image-dipole effect and properly showed that it can be corrected under appropriate circumstances. The resulting work provides a more accurate map of the electromagnetic fields surrounding the nanowire.

The JQI scientists published their results in the journal Nature Communications.

Lead author Chad Ropp (now a postdoctoral fellow at the University of California, Berkeley) says that the main goal of the experiment was to produce better super-resolution imaging: "Any time you use a nanoscale emitter to perform super-resolution imaging near a metal or high-dielectric structure image-dipole effects can cause errors. Because these effects can distort the measurement of the nano-emitter's position they are important to consider for any type of super-resolved imaging that performs spatial mapping."

"Historically scientists have assumed negligible errors in the accuracy of super-resolved imaging," says Ropp. "What we are showing here is that there are indeed substantial inaccuracies and we describe a procedure on how to correct for them."

Lithium deposited on the platinum anode at the beginning (top), during (middle) and end (bottom) of the second cycle. Residual “dead lithium” can be seen on and around the anode.

Used in everything from electric vehicles to laptop computers, the lithium battery is ubiquitous, but it is not well understood at the atomic scale. To see what happens on the nanoscale, scientists at DOE's Joint Center for Energy Storage Research (JCESR) designed and implemented a small device, known as an operando electrochemical stage. Using this stage inside a state-of-the-art aberration-corrected transmission electron microscope they can take nanoscale-resolution pictures of lithium ions as they are deposited on or dissolve off of an electrode while the battery runs.

With the new stage, scientists can directly image changes as they occur. The new images allow precise measurements and descriptions of what happens inside the battery. This information is vital to control performance- and safety-limiting processes. Now, scientists can rapidly visualize and test new pairings of electrodes and electrolytes (see Battery 101). The new stage will help quickly sort through options for longer lasting, safer batteries.

Moving beyond the current industry-standard lithium-ion battery has been difficult. In lithium-air and other designs, interactions at the electrode-electrolyte interfaces affect the battery's performance and safety. To understand the reactions, scientists at the Pacific Northwest National Laboratory, as part of JCESR, created an operando electrochemical stage. Using it in an aberration-corrected scanning transmission electron microscope, scientists can now chemically image the interface between the platinum anode and the electrolyte during the battery operation.

The imaging method highlights solid lithium metal, uniquely identifying it from the components that make up the protective solid-electrolyte interphase layer. Using these images and standard electrochemical data, scientists can quantify, at the nanoscale, the amount of lithium that ends up irreversibly deposited after each charge/discharge cycle. This means they can view dendrites—the microscopic thorns that cause batteries to fail—as they form.

The technique also shows the growth of the solid-electrolyte interphase layer, which wraps around and protects the anode. The layer is formed as a result of the electrolyte breaking down. In their studies, the team found that extended battery cycling leads to lithium growing beneath the layer—the genesis of the dendrites that have implications for battery safety and performance.

This new imaging tool opens up possibilities to rapidly visualize and test electrode/electrolyte pairings for new battery systems. These systems could allow electric cars to travel great distances between charges. Also, one day, such systems could store energy from wind and solar stations, making the intermittent energy available when needed.

Battery 101

Most of the rechargeable batteries used today are lithium-ion batteries, which have two electrodes: one that's positively charged and contains lithium and another, negative one that's typically made of graphite. Electricity is generated when electrons flow through a wire that connects the two. To control the electrons, positively charged lithium atoms shuffle from one electrode to the other through another path: the electrolyte solution in which the electrodes sit. But graphite has a low energy storagecapacity, limiting the amount of energy a lithium-ion battery can provide smart phones and electric vehicles.

When lithium-based rechargeable batteries were first developed in the 1970s, researchers used lithium for the negative electrode, which is also known as an anode. Lithium was chosen because it has ten times more energy storage capacity than graphite. Problem was, the lithium-carrying electrolyte reacted with the lithium anode. This caused microscopic lithium dendrites to grow and led the early batteries to fail.

Many have tweaked rechargeable batteries over the years in an attempt to resolve the dendrite problem. In the early 1990s, researchers switched to other materials such as graphite for the anode. More recently, scientists have also coated the anode with a protective layer, while others have created electrolyte additives. Some solutions eliminated dendrites, but also resulted in impractical batteries with little power. Other methods only slowed, but didn't stop, the fiber's growth.

Using liquid phase TEM, Alivisatos et al. were able to track gold nanorods in real time. Credit: American Chemical Society

A candid photo can reveal much more about the mood of a party than a stiff, posed picture. The same might be true for molecules, according to researchers. In a report appearing in the journal ACS Central Science, they report use of a newly developed method that can take a candid snapshot of how molecules really move in vitro and in cells. This information could help resolve some controversial claims about how nanocrystals assemble.

Paul Alivisatos and colleagues note that microscopy is often limited by how samples are prepared. Currently, the most powerful microscopes require samples to be dried under a vacuum. That freezes molecules in one place, wherever they were when they were dried. But many materials behave very differently when they're in liquid, like when those molecules are in a living cell. Some molecules can move freely, whereas others have more limited mobility. Optical microscopy is a good way to investigate such things at the microscale level, but until recently, it hasn't been ideal for smaller objects like nanoparticles.

The researchers used the newly developed technique of liquid-phase transmission electron microscopy to visualize and track gold nanorods—which could be used in cancer therapy— in real time. The nanorods assembled differently, depending on whether they were in liquid or dried. Rather than focus on the details in a given assembly, the team parsed large amounts of data to monitor the positions of each nanoparticle. That gave them a quantitative understanding of previously hidden factors involved in nanocrystal assembly. They say that such data could help researchers more fully understand how nanoparticles assemble, a process shrouded in controversy, and how molecules move within living cells.

This image depicts a new system capable of detecting defects and networks of nanostructures below the surface of layered nanocomposites gaining commercial use. From left to right, an atomic force microscope image of the surface of a nanocomposite, wormlike carbon nanotubes below the surface and a graphic of the working instrument. Credit: Purdue University

Layered nanocomposites containing tiny structures mixed into a polymer matrix are gaining commercial use, but their complex nature can hide defects that affect performance.

Now researchers have developed a system capable of detecting such defects using a "Kelvin probe" scanning method with an atomic force microscope. The ability to look below the surface of nanocomposites represents a potential new quality-control tool for industry. 

"This is important for anything having polymers that contain small structures, including photovoltaics for solar cells, organic conducting devices for flexible electronics, battery materials and so on," said Arvind Raman, the Robert V. Adams Professor of Mechanical Engineering and associate dean for the Global Engineering Program at Purdue University.

Nanocomposites are layered materials containing various structures such as carbon nanotubes, ultrathin sheets of carbon called graphene, gold nanoparticles and graphite nanofibers, mixed into a polymer matrix.

"We need a tool that permits us to see how these nano-objects are distributed within a polymer matrix," Raman said. "You may look at the whole film and say, 'Well, it isn't performing as advertised,' but you don't know why. This allows you to see under the surface in a non-destructive manner."

Findings appeared in the February issue of ACS Nano, published by the American Chemical Society. The paper was authored by doctoral student Octavio Alejandro Castañeda-Uribe, from Universidad de los Andes (Uniandes) in Colombia; Ronald Reifenberger, a Purdue professor of physics; Raman; and Alba Avila, an associate professor in the Electrical and Electronic Department at Uniandes who is affiliated with the microelectronics center (CMUA) there.

The Kelvin probe method has been used to map electrical charge on the surfaces of materials. However, now researchers have discovered that the method can be used to look below the surface, detecting three-dimensional networks of nanostructures embedded deep inside the polymer matrix.

"This allows us to correlate these networks with the multifunctional properties of nanocomposites," Avila said.

An atomic force microscope uses a tiny vibrating probe called a cantilever to yield information about materials and surfaces on the scale of nanometers, or billionths of a meter. The instrument enables scientists to "see" objects far smaller than possible using light microscopes. In Kelvin probe scanning an alternating current is applied to the sample being studied, causing the probe to vibrate at a certain frequency, and then a direct current is applied to the probe, partially nullifying the effect of the alternating current.

"You nullify the main frequency, but it turns out there is a second frequency that is not nulled," Raman said. "You kind of mute the main signal, but there is a higher tone that remains in the cantilever, and that higher tone is very sensitive to what's underneath the surface."

The new findings identify precisely how deeply and through how many layers the method can probe into a material. Researchers developed computational methods and an experimental technique making the tool possible.

"If the nanocomposite doesn't work well, you have to be able to look inside," Raman said. "You have to do quality control at the nanoscale."

Nanotubes and other nanostructures should ideally be well distributed throughout the nanocomposite, forming a continuous network. However, the structures tend to clump together instead, hindering performance.

"So, now we can see where they are clumping together and where they are not because you can see below the surface without destroying the sample," he said.

The method also allows researchers to determine the orientation, connectivity and size distribution, or the particle-to-particle variation of size, which is important for quality control.

Images created with the method show wormlike carbon nanotubes below the surface of a composite. The researchers systematically added layers and showed that the method is capable of detecting structures down to a depth of about 400 nanometers.

Purdue worked with researchers at Uniandes in Bogotá through the Colombia-Purdue Institute, which fosters partnerships among Purdue and institutions in Colombia, including universities, companies, government ministries and nongovernmental organizations.

"It's a good example of how you bring international teams together to get something really good accomplished," Raman said.

Uniandes researchers were involved in processing the nanocomposite films and also developing the experimental technique. Nanocomposite film processing and development of the atomic force microscope experimental technique were carried out by a team at Purdue's Birck Nanotechnology Center. The computations were carried out at Uniandes.

"This collaboration made it possible to provide research training and access to facilities at both universities for advanced research focused on exploring depth-detection limits of characterization techniques based on atomic force microscopy," Avila said. "These limits are needed to confidently detect, characterize, and quantify the location of the nanomaterial networks within a polymer matrix, allowing 3-D image reconstruction of nanocomposites and a more reliable prediction, estimation and correlation of the properties of nanocomposites."