Newsletter - August 14, 2024

Smart Material Solutions continues to thrive as a collaborative Advanced Manufacturing R&D company, with support from the US Army, NASA, and industry partners. While our research covers a wide range of applications, we’re approaching the commercialization stage on a solar enhancement film and a display product under development for an industry customer! 

We also continue to conduct advanced research in the interest of national defense. Over the past six months, we’ve had the pleasure of meeting with staffers for our state’s US senators and hearing direct validation of the need for domestic advanced manufacturing technologies from a three-star general in the US Army. 

We are committed to commercializing these applications so we can at long last see them in use on the field and in homes and classrooms across the world!

Prototype Solar Panels

Our highly collaborative Army-funded work to create enhancement films for solar panels has been a resounding technical success. For this work, SMS created a seamless drum mold of a novel hierarchical nano and micropattern that captures light. Then MicroContinuum, Inc. (Watertown, MA) replicated that mold into more than two hundred linear feet of polymer encapsulation film, which Powerfilm Solar (Ames, IA) incorporated into their commercially available flexible solar panels.

The films we’ve created are especially effective for applications where the panel is not directly facing the sun, see below:

Now, we’re iterating with additional features to improve durability to both abrasion and long-duration UV exposure. Solar is a tough market to penetrate, but with a 2-10% increase in power over the course of a day, our value add is real.

NASA and Army SBIR Updates

Our NASA project to reduce dust adhesion for lunar applications has entered its third year. There continues to be significant interest, and SMS is under a no-cost extension to continue the work with University of Texas, Austin.

The Army work to create plasmonic metasurfaces roll-to-roll has also progressed as the SBIR Sequential Phase 2 project nears the end of year 1. The team, including MicroContinuum Inc. and University of Delaware, is on track to produce demo surfaces in 2025.

Visit to the US Senate

SMS has joined with thousands of other US companies to lobby congress this year. We believe that companies should be able to deduct R&D expenses from income, as they could from 1954-2022. Nearly all of congress agrees, but politics are extremely complex. Stephen visited the Legislative Aides for both of North Carolina’s US Senators - Thom Tillis and Ted Budd - to make the case of a small advanced manufacturing research company in their state.

 

Upcoming Conference 

  • SPIE Optics and Photonics Conference, San Diego (August 19-23) - Stephen and Lauren will attend.

  • ASPE Conference, Houston (November 4-8) - Stephen will attend.

  • Photonics West, San Francisco (January 25-30)

Recent Conferences 

  • DEFTEC Federal Technology Symposium, outside Fort Liberty (August 6) - Stephen and Nicky attended, and Nicky presented in a “shark tank” style pitch. We heard about the need for a strengthened US manufacturing base directly from a three-star Army general!

  • Nanoimprint and Nanoprint Technologies (NNT) / NIL Industrial Day, Lund, Sweden (June 24-27) - Stephen attended.

  • Radtech Conference, Orlando, Florida (May 20-22) - Robin attended, presented, and was awarded a conference scholarship as a winner of the RadLaunch startup competition. 

  • International Soft Matter Conference, Raleigh, NC (July 29 - August 2) - Robin attended and presented a poster.

Blogs

Check out our new blog posts on FIB machining of diamond and Lenticular Lenses for security features, as well as a gallery of cool SEM images from our work!

Nanoscale Machining with a Focused Ion Beam

Machinists shape parts using tools like milling machines, which cut away material with a spinning bit like the one shown in Figure 1A. Milling on the nanoscale requires a nanoscale tool like a focused ion beam (FIB), which can be as small as a few nanometers in diameter. When an ion beam strikes a surface, the incoming ions knock out atoms, removing material in the beam’s path (Figure 1B).

Figure 1: Illustration of milling with a milling machine (A) and FIB milling on the nanoscale (B).

Figure 2: Electron microscopy image of examples of FIB milling and deposition.

The FIB-milling process can also be modified to deposit material instead of removing it, enabling FIB deposition of complex 3D nanostructures in a process that is similar to some 3D-printing techniques (Figure 2). The nanoscale patterns created with FIB milling (also known as FIB machining) and FIB deposition have applications ranging from solar panels to antimicrobial surfaces.

FIB Milling

FIB milling uses an ion beam to remove material in its path. The ion beam often consists of gallium ions, but other ions including xenon, argon, nitrogen, and oxygen can also be used.

Figure 3: FIB milling process viewed from the atomic scale.

Let’s consider what happens at the atomic scale when a beam of gallium ions hits a piece of gold. As illustrated in Figure 3, gallium ions that collide with the gold will knock out some of the gold atoms in a process known as sputtering. A fraction of the sputtered gold atoms may then redeposit on another part of the gold piece. Some of the gallium ions will rebound away from the gold surface, while others become implanted inside the gold. The end result is the removal of gold atoms from the place of impact.

The more time the ion beam is held at a particular position, the more material it will remove from that position. Thus, scientists can create complex nanostructures by controlling how long the beam sits over each position of a sample. This is unlike a milling machine, that effectively subtracts the exact geometry of the cutting tool from the workpiece within one revolution of the tool, then stops cutting, no matter how long the tool dwells.

Further, a FIB removes more material near the beam center and less material at the beam edges. This occurs because a FIB contains more ions near the center of the beam and fewer ions near the beam edges as illustrated in Figure 4.

Figure 4: Plot illustrating distribution of ions in a FIB (left), and illustration of the cross section of a material that has been FIB milled at one position (right).

Another complexity arises from the dependence of sputtering efficiency on incidence angle. Figure 5 shows how sputtering can be inefficient when the FIB is perpendicular to a surface, because the sputtered ions must exit the sample in the opposite direction of the incoming ion beam. Tilting the sample will increase sputtering efficiency since sputtered ions do not have to change direction so drastically to leave the sample. This leads to an angle-dependence of the sputtering yield, or the number of sputter atoms per incoming ion. A surface that has been nanostructured with a FIB beam will have a variety of angles between the local surface and the incoming FIB beam, meaning that different points in a material will sputter different amounts of material when exposed to the same ion dose. Precise control over the shape of a FIB-milled nanostructure therefore requires compensation for both the angle-dependent sputtering efficiency and the shape of the ion beam.

Figure 5: Angle dependence of sputter yield. Inserts illustrate sputtering with a FIB that is perpendicular to the sputtered surface and incident on a tilted surface.

FIB Deposition

A FIB can also be used to deposit material if certain precursor molecules are introduced into the system. Properly chosen molecules will adsorb onto the surface of the material. When the FIB scans over these adsorbed molecules, it will drive a chemical reaction that solidifies these materials as illustrated in Figure 6. Materials that are commonly deposited by FIB include platinum, diamond-like carbon, tungsten, and silica. FIB-deposition can be used to repair circuits in microchips, protect underlying features, or to make cool-looking nanostructures like those in Figure 7. Although this blog discusses FIB-deposition techniques, electron beams like those in electron microscopes can also drive chemical reactions that solidify precursor molecules.

Figure 6: During FIB deposition, a FIB drives a chemical reaction that solidifies precursors introduced into the system.

Figure 7: Examples of FIB-deposited nanostructures including nanoscale models of a bacteriophage and the leaning tower of Pisa. Nanostructures created by Reo Kometani and Sunao Ishihara.

FIB Patterning at Smart Material Solutions

Smart Material Solutions often uses a FIB to nanopattern the diamonds it uses for its patented nanocoining process to create seamless cylindrical molds for R2R nanoimprint lithography. Figure 8 shows some of SMS’s FIB-milled nanostructures for a variety of applications including dust-mitigating surfaces, light-trapping films to increase the efficiency of solar panels, and anti-reflective motheye films. One of our employees, Nichole Scott, even used the FIB to engrave the side of her engagement ring as shown in the video in Figure 9.

Figure 8: Some of the nanostructures FIB-milled into diamond at North Carolina State University’s Analytical Instrumentation Facility (AIF).

Figure 9: Video showing FIB-patterned engagement ring.

Seeing Double: Shifting Images with Micro Optics

Figure 1: Lenticular Pokémon card.

As someone who grew up in the early aughts, Pokémon was an essential part of my childhood. But I will confess I cared less about which cards could help me beat my brother and placed more value on which cards looked the coolest, so it should come as no surprise that the metallic cards and hologram collectibles were my favorites. The holograms, primarily stickers and coins, showed shifting images as you tilted them and were my first introduction to lenticular arrays (Figure 1).

These holograms make use of lenticular lenses and composite images (Figures 2 & 3) to create simple optical illusions like transformations, animations, and forced perspective.

Figure 2: Optical microscope images of a lenticular lens array (right) and the printed background (left). Taken from a lenticular Pokémon card.

Figure 3: SEM image of the microlens array on a lenticular Pokémon card.

Lenticular Lenses

In its simplest form, a lenticular print consists of an array of cylindrical lenses placed on top of a composite image. The lenses and image are aligned to create the shifting effect. When viewed from one angle, the lenses direct one portion of the composite image to the viewer. As the viewing angle shifts, a different region of the background image is projected to the viewer from the lens, creating the illusion of transformation or motion as the observer shifts their perspective (Figure 4).

 

Figure 4. As the angle of the lens relative to the viewer shifts, the portion of the background projected to the viewer also shifts.

 

Evolution

As the resolution limit of nanoprinting and nanoimprinting has improved, the complexity of optical effects that can be achieved has too. Patents for lenticular lenses date back to the late 1800’s and early 1900’s, when inventors were attempting to create a way to look at stereoscopic images without the need for glasses, but it didn’t become popular until the 1950’s and 1960’s when the introduction of injection molding and roll-to-roll processing made it possible to mass produce lenticular images. It was primarily used for novelty items and advertisements, including ‘I like Ike’ campaign buttons in the 1952 election (Figure 5).

 

Figure 5: I like Ike campaign buttons (Image source).

 

As printing and imprinting abilities improved, more images could be stitched together to simulate more realistic motion, as demonstrated by photicular books. The technology also advanced to incorporate spherical lenses - or fly’s eye arrays - which operate like cylindrical lenses but work in two directions and therefore generate more realistic depth and movement effects. By creating more complex images, with processes that are harder to duplicate without specialized technology, these once novel illusions can have a powerful impact on security and other advanced applications.

Anti-counterfeit

What began as a novel 2D effect in post cards and children’s toys as evolved beyond the 1D lenticular lens into a 2D micro lens array creating interactive security features strong enough to protect the most valuable denomination of US currency.

Microlens arrays are powerful anti-counterfeiting measures because they are difficult to reproduce and can quickly reveal if a good is genuine without the need for additional technology. If the lens array and the background image are designed to easily delaminate, micro-optic security features can also reveal if goods have been tampered with, since the micro-optic effect requires careful alignment of lens array and background image.

Counterfeit goods can pose serious risks to consumers, especially in industries like pharmaceuticals where fake medications can have harmful effects. Counterfeiting also has serious implications for currency, since mass amounts of counterfeit money has the potential to generate inflation and damage economic stability and ultimately weaken the public’s trust in their currency. The US $100 bill has a microlens stripe on it which features ortho-parallactic motion - meaning that the animation motion moves perpendicular to the motion of the bill. This is achieved with a fly’s eye array that allows for bi-directional effects (Figure 6).

Figure 6: SEM images of the microlens stripe on a $100 bill.

Packaging and XR

Other application areas have also benefited from the advanced manufacturing ability, for example packaging and advertisements have come a long way from the simple, dual-image prints utilized in the 1950’s. Companies like Fathom Optics can create packaging with advanced motion and depth effects to grab customers’ attention and enhance brand recognition. The advanced printing techniques can also be utilized to create tamper evident devices that reveal when packaging has been opened, similar to those employed in the anti-counterfeit space.

Microlens arrays have also been used in the display space, for outdated products like 3D displays as well as up-and-coming technologies like AR and VR headsets. While they are no longer in use, 3D TVs were once seen as the future of television and made use of lenticular lenses to create the perception of depth without the need for the viewer to be wearing glasses. AR and VR goggles may make use of fly’s eye 3D arrays on top of microdisplays in an effort to make goggles more efficient and compact to improve the wearability and usability of the devices.

SMS and Microlens Arrays

SMS, with our patented nanocoining process, currently specializes in microlenses that are even smaller than those needed for the applications above - less than 10 micrometers in pitch (Figure 7). In doing so, we hope to help power the next generation of microlens applications!

Figure 7: SEM images of an indented mold (left) and its replica (right).

Acknowledgements:

Thank you to Caleb Meredith and Ben Bleiman for lending your expertise.

The SEM images were taken at the North Carolina State University Analytical Instrumentation Facility.

SEM Gallery: The Art of Nanoscale Research

We take thousands of scanning electron micrographs every year to document, characterize, and evaluate our surfaces. This collection contains our favorite images from the year, highlighting patterns from our ongoing research initiatives as well as the natural sources that help inspire us.

Developing patterns

A look at some patterns currently being developed

500nm bumps indented into a copper mold

400nm features replicated into a UV-cured polymer

5µm features replicated into a UV-cured polymer

Interwoven indents created in a nickel mold

Multi-scale features replicated into a UV-cured polymer

Replica Defects

As shown by the edge of this replica, the defects stand out in otherwise uniform patterns.

Areas where resist didn't fill, creating voids in the replica

A thin portion of resist resulting in a lack of feature fill (left) and a scratch in the resist creating loose polymer gumdrops (right)

Wrinkles and bubbles in the replicated polymer

Occasionally hard particles in the mold will damage the indenting tool, leading to missing features

Insects

These small organisms often possess nanopatterns that help them survive. Scientists study them to enhance their understanding of nanostructures.

 

The moth-eye effect gets its name from the hierarchical structures that can be found on the top of a moth's eye. These bio-inspired surfaces can be used to supress reflections on optics or increase transmission through solar panels.

Comparison of a textured moth's eye (left) and the smooth eye of a green june beetle (right).

Biocidal nanopillars seen on a cicada wing (top) and the scales from a moth's wing (bottom).

These images were taken by Lauren Micklow, Nicky Scott, Brenna Tryon, and Malcolm Shumel at the North Carolina State University Analytical Instrumentation Facility using an FEI Quanta 3D DualBeam SEM/FIB or a Helios 5 Hydra DualBeam SEM/pFIB.

Newsletter - October 3, 2023

Thanks to all for continuing to follow our journey here at Smart Material Solutions! If you’re a potential industrial partner, now is the ideal time to start pilot scale work with us. Our patterning capabilities are better than ever, and we have more than two years of funding runway through non-dilutive R&D contracts with NASA and the Army. We’ve also added key skill sets to our team here at SMS.

As always, we appreciate your support, feedback, and curiosity. Please don’t hesitate to reach out!

Seamless Molds for R2R NIL

Over the past year, we’ve increased the range of our feature patterning capabilities. We can now make features on a pitch from 200 nm to 8 microns with aspect ratios (feature height / pitch) of up to 0.7. We can make seamless cylindrical molds up to 160 mm with extremely low defect rates and oftentimes precision shape control. Mold materials include nickel, copper, aluminum, and even UV transparent glass! All of our molds are available with adhesion reducing fluorination coatings.

The images below show recent work creating nanoscale, microscale, hierarchical, and multiscale features on large, seamless drum molds.

 
 

Welcome to Robin McDonald and Sidney Cox!

We’re excited to welcome our newest full time team member, Robin McDonald. Robin is a talented materials scientist who graduated at the top of her class at Caltech. We’ve also added part time consulting with Dr. Sidney Cox, a physical chemist with three decades of experience at Dupont and Tethis. We now have four full-time and two part-time staff, three active university partnerships, and two funded industrial sub-contractors helping us take the benefits of metamaterials and nanostructured surfaces to the industrial scale!

New Army SBIR and STTR Awards

SMS recently signed two new contracts with the Army, securing a 24 month funding runway - our longest ever! The first is a Phase II SBIR to roll-to-roll (R2R) fabricate thin-film solar panels with light-trapping, anti-fouling coatings in collaboration with MicroContinuum and PowerFilm. The second contract, a Sequential Phase II STTR, will allow us to collaborate with MicroContinuum and the University of Delaware to R2R fabricate a series of plasmonic metamaterial films with advanced optical properties.

Conference Schedule

We will be attending and presenting at several conferences this fall. We look forward to meeting with you there!

  • NNT - Nanoimprint and Nanoprint Technologies - (Boston, MA, October 9-11). Lauren Micklow will be giving a talk at 11:20 am on Wednesday.

  • ASPE - American Society of Precision Engineering - (Boston, MA, November 12-17)

  • Carolina Science Symposium (Raleigh, NC, November 17)

Blogs

Check out our new blog posts on electrolytic and electroless coatings and microscopy!

Electrolytic and Electroless Coatings

Metallic Coatings

What do galvanized screws, pennies, and white gold jewelry have in common? They all have a coating that enhances their performance. These coatings, or outer shells, are often applied as a cheap and easy way to alter a material's properties. They can be made up of a variety of materials including polymers, ceramics, and metals depending on the application and desired properties. Changing and optimizing these properties can result in high-performance materials perfect for applications where ultra-high strength, ultralight weight, or corrosion resistance is required. One of the most well known examples of a metallic coating is galvanized steel. Steel is a common material used in many structural applications due to its high strength and ease of production, but is extremely prone to rusting in everyday conditions. Galvanizing the steel, or applying a thin coating of zinc, prevents it from rusting, creating a material with high strength and corrosion resistance that is still easy to produce. Two ways to apply a metallic coating are electroplating and electroless plating, each of which produce coatings with unique material properties.

Electrolytic Metallic Coatings

Figure 1: Photo of a sanded penny showing the zinc core below the electrolytic copper coating.

Electroplating uses electricity to plate dissolved metal ions onto the surface of a material. One of the best examples of this process is the penny. Contrary to popular belief, the copper penny is not solid copper, but a piece of zinc that has been electroplated with copper. It would be very expensive to make solid copper pennies, costing much more than a penny is worth. By using zinc-coated copper, the value of the materials in the coin is closer to the value of the penny. Figure 1 shows a penny that has been sanded to reveal the zinc core inside.

Fun Fact: While pennies are copper-coated zinc, nickels are nickel-covered copper created by pressing together different metallic layers.

The basic concept of electroplating is shown in Figure 2, using the penny as an example. A zinc disk and a copper electrode are suspended in an electrolyte solution that contains dissolved copper ions. The zinc disc and copper electrode are then connected to a power source, such as a battery, that pulls electrons from the copper electrode, giving it a positive charge, and pushes them to the zinc disc, giving it a negative charge. As the battery pulls electrons away, copper atoms on the surface of the electrode become copper ions with a 2+ charge, causing them to dissolve into the electrolyte. On the other side of the battery, the negatively charged zinc attracts the positive copper ions to itself. When a copper ion in the electrolyte reaches the zinc disc, the disc can give it two electrons to replace its two missing electrons, thus turning the ion back into a copper atom on the surface of the zinc. These processes occur simultaneously, with the copper electrode constantly replacing the ions in solution that are plated onto the zinc. As more and more copper atoms build up on the zinc, a copper coating is created, giving the penny the distinct color that we all know. Similar processes exist for electroplating other metals including nickel, gold, and chromium.

 

Figure 2: Illustraiton of electrolytic copper coating.

 

Fun Fact: Chrome on cars is actually metal (usually steel or zinc) that is electroplated with copper, then nickel, and finally chromium.

These electroplated coatings are particularly good at increasing the conductivity and heat resistance of the base metal, making it good for a variety applications beyond currency. However, electrolytic coatings often contain issues with uniformity and have increased thickness of coatings around edges and corners because of natural variation in the way that electrons are distributed over surfaces. Additionally, areas of the part closer to or in direct view of the plating metal electrode will be plated more easily, while holes or the backside of parts may not be coated well at all, an effect known as shading. Applications of electroplating include machinery parts, jewelry, and phone parts due to the increased durability and lifetime of the part as well as the relatively low cost and fast rate of plating.

Electroless Metallic Coatings 

As its name suggests, electroless plating involves metal coating without the application of electric current. Instead, heat is used to activate chemical reactions that transfer electrons between chemicals in the bath and ions of the coating metal. While electroless coatings are less common than electrolytic coatings, they have many applications in high-performance applications, such as specialty kitchen knives, printed circuit boards, aircraft parts, and jewelry. Printed circuit boards, which often feature layers of electroless nickel and electroless palladium, are perhaps the most common application and can be found in many electronic devices. 

Figure 3: Penny before and after a thin electroless nickel coating. The penny was coated for 35 second, which resulted in a 250nm-thick coating or 2000 nickel atoms.

In order to explain the basic concept of electroless coatings, let's go back to the copper penny and coat it with an electroless nickel layer, as we did in our lab (Figure 3). Placing a penny in an electroless nickel solution will cause nickel to coat the surface due to the reaction of positively charged nickel ions in the solution with electrons to create nickel metal, as illustrated in Figure 4. At the same time, another reaction will produce the necessary electrons on the same surface. The exact chemical reaction that creates electrons depends on the chemistry and composition of the bath, which must be closely monitored to achieve a well-adhered and uniform coating. The electron-producing reaction also causes other atoms to plate alongside the nickel, creating a nickel alloy. The electroless bath used to coat the penny in Figure 3 results in an alloy that is 6-9% phosphorous and 91-94% nickel. For these reasons, the bath is usually expensive to make and difficult to keep running, which explains why electroless coatings are less common in industry than their electrolytic counterparts. 

 

Figure 4: Illustration of electroless nickel plating.

 

Figure 5: Elemental EDS map of the cross section of the nickel-coated penny with zinc, copper, and nickel shown in magenta, green, and red, respectively.

Figure 5 shows the layers of the nickel-coated copper penny from Figure 3. The penny was cut in half, exposing the cross section. Then scanning electron microscopy (SEM) images were taken at NC State University’s Analytical Instrumentation Facility (AIF) and a technique called electron dispersive spectroscopy (EDS) was used to create an elemental map. The map makes it easy to see the zinc base (magenta), the electrolytic copper layer (green), and finally the very thin electroless nickel layer (red) that was coated in our lab.

One of the most important benefits of electroless coatings over electrolytic coatings is that they are significantly more uniform. This uniformity happens because there is no dependence on electrical charges or distance from the metal electrode. These coatings also provide more corrosion protection and durability compared to electrolytic coatings. Finally, an electroless coating does not require a part to be conductive and with the right chemistry can coat polymers in addition to a variety of metals.

Metallic Coatings at SMS

Smart Material Solutions often uses contractor-supplied metal coatings to change the hardness and the surface energy of nanopatterned metal drum molds. Metals that are naturally hard are difficult to indent, requiring more force which is more likely to break the diamond die. On the other hand, metals that are naturally soft may not hold up to roll-to-roll applications. Also, even though copper molds are easier to indent because they are softer, copper is a metal with a large surface energy, meaning that it is sticky and polymers tend to adhere to it instead of peeling off to form a replica of the mold. To balance all of these concerns, SMS is currently working on applying extremely thin nickel-phosphorus coatings onto nanopatterned copper molds using an electroless process. The complex chemistry of creating and maintaining electroless nickel baths can be simplified by purchasing one of many commercially available electroless nickel kits. 

Figure 6: Copper cylinder before (a) and after (b) coating with 100 nm of nickel.

Joshua Murray and Brenna Tryon, explored the chemistry and practical challenges of electroless nickel coating during their summer internship at SMS. They started with a kit from Caswell, Inc. known as One-Plate®. One example of the success using Caswell’s kit is shown in Figure 6, where a 2-inch diameter copper cylinder was coated with around 100 nanometers of nickel. The coating is thin enough to not substantially change the shape of the nanopattern, which can be seen by the presence of the diffraction pattern in both pictures. These metallic coatings allow us to further optimize the material properties of our metal molds, which allows for easier production of nanopatterned films and coatings.

Microscopy: The Limits of Light in the Nanoworld

Looking into the Nanoworld

I’m going to ask the question that bothered me to no end while I was in grade school. If the 100x objective will allow me to see this amoeba, why can’t we just put on a 1,000,000x objective so I can see its individual atoms?! It turns out, no matter how petulant a nerd I was, there was a very good reason that my 6th grade teacher couldn’t produce a live view of a proton on the benchtop visible microscope.

It’s not that my school’s budget didn’t include a line for the 1,000,000x objective. There is a fundamental limit to how small of a thing you can see with visible light. This limit exists because light, by its nature, is a wave with a given wavelength. Roughly speaking, you cannot see anything that is smaller than half the wavelength you’re using to observe (see the last section for details). In the case of humans, our eyes can see light that ranges between 380 nm (violet) and 750 nm (red), known as the “visible band.” This means, no matter how much we zoom in, the human eye is not going to see anything smaller than 200 nm, even with the best visible light microscopes.

Defying the Limits of the Human Eye

To look smaller, you need to reduce the wavelength of the light or wave you’re using to observe. Figure 1 shows the range of waves on the electromagnetic spectrum, including the visible light band between 380 and 750 nm and common objects overlaid for scale. Longer wavelengths include infrared light that is used in night vision and thermal imaging, microwaves that can heat your food, and radio waves, while shorter wavelengths include ultraviolet light, X-rays, and gamma rays.

Figure 1. Electromagnetic spectrum.

Figure 2 zooms in on the wavelength ranges used by thermal imaging, night vision, and visible light, along with the range of features sizes that SMS can make and use to manipulate the electromagnetic spectrum.

 

Figure 2. Subset of the electromagnetic spectrum.

 

Viewing objects that are smaller than visible wavelengths can be done by either slowing down visible light to reduce its wavelength or by using shorter wavelengths to begin with. However, using wavelengths beyond the visible spectrum becomes understandably tricky, because in order for us to see what the shorter wavelength light sees, we need to sense it with a detector then convert it back into a ‘false image’, typically by a computer. The common types of microscopy are described below, and their viewing ranges are overlaid on Figure 1 as well.

Figure 3. Optical microscopy image showing an onion’s cell walls.

Optical Microscopy, also known as light microscopy or visible microscopy, is the most commonly used form of microscopy. This simple, low-cost method uses visible light to observe specimens and provides valuable insights into their structures. Techniques like brightfield and darkfield microscopy offer versatility in analyzing a wide range of samples from cells, tissues, and microorganisms to microstructures in engineered materials (Figure 3). However, optical microscopes cannot resolve nanoscale objects that are substantially smaller than the visible light wavelengths they use to probe the sample.

Oil Immersion Microscopy is an extension of visible microscopy that enhances resolution by making the observations in a droplet of oil, where the speed and therefore the wavelength of ‘visible’ light is reduced. To achieve this, a special immersion oil is placed between the objective and the sample. However, this requires careful handling methods and cleanup, and the observation often destroys or contaminates your sample.

Scanning Electron Microscopy (SEM) is a powerful technique that uses a focused beam of electrons to obtain high-resolution images of a sample's surface. Much like a photon of light, electrons also have a wavelength that depends on their energy. The energy of electrons in a SEM allows them to probe nanoscale features that are much smaller than what is observable with optical microscopy (Figure 4). By scanning the surface with the electron beam, SEM creates detailed images, revealing the topography and surface features of the specimen with exceptional depth of field (Figure 5). Most traditional SEMs require electrically conductive samples and vacuum conditions that can affect the sample's natural state of biological samples, but specially designed environmental SEMs can image non-conductive and even wet samples in a gaseous environment.

Figure 4: Sub-micron details of a Blue Morpho butterfly wing.

Figure 5: Wide view SEM of a Blue Morpho butterfly wing showing depth of field.

Figure 6. TEM of GaP nanowire growth using a Cu seed taken by Tianyi Hu for his article in Small Structures.

Transmission Electron Microscopy (TEM) uses a beam of high-energy electrons that passes through thin sections of a sample to create a highly detailed image of the specimen's internal features. TEM enables scientists to visualize fine details such as individual atoms and lattice structures like those seen in the GaP nanowire and Cu seed in Figure 6, but the technique requires specialized sample preparation like thin sectioning, which can be time-consuming and introduce artifacts. Additionally, high vacuum conditions can pose limitations for certain types of samples.

Figure 7. AFM of polymer nanofeatures created by SMS with dimensions labeled in microns.

Atomic Force Microscopy (AFM) is different, as it does not use light to ‘probe’ the surface being measured. Instead, AFM measures the interaction forces between a sharp probe and a sample's surface. Since the probe point can have a radius as small as a few nanometers, it can resolve extremely fine features. As the probe scans the surface, it records the variations in forces, creating a 3D topographical map that can have atomic-scale resolution (Figure 7). However, AFM operates at a slower speed compared to other techniques and may experience challenges in imaging soft or fragile samples due to probe-sample interactions.

Microscopy in Action at SMS

At Smart Material Solutions, we use microscopy every day to measure the functional structured surfaces we fabricate. These surfaces may consist of features that are 5 µm across and therefore observable, with some level of detail, using a visible microscope. Or they may consist of features smaller than 200 nm that require an SEM or AFM to be resolved at all. Figures 8 and 9 show arrays of similarly shaped features with center-to-center distances of 300 nm and 5 µm, respectively, as observed by several different microscopy techniques. Interestingly, the 300 nm features become visible with oil immersion, in the top right image!

Figure 8. A 300nm mold imaged using visible microscopy (top left), oil immersion microscopy (top right), SEM (bottom left) and AFM (bottom right). For the visible and oil immersion micrographs, the imaging wavelength is overlaid on a magnified view.

Figure 9. A 5µm mold imaged using visible microscopy (top left), oil immersion microscopy (top right), SEM (bottom left) and AFM (bottom right).

Taking Advantage of the Diffraction Limit

In microscopy, the diffraction limit is the minimum resolvable distance, originally defined by Ernst Abbe as:

D =  
λ
2*n*sin(θ)
−  
λ
2*NA

Where n is the index of refraction of the medium, e.g. air, and theta is the half angle of the light converging from the objective. n*sin(theta) is known as the numerical aperture, NA, of the optical system, which can be up to ~1.6 in modern, high-end visible microscopes.

This limit is almost always a nuisance that must be overcome with a more complex and expensive instrument. However, at SMS, we specialize in using this effect to our advantage. For example, leveraging the moth-eye effect, we make features so small that they appear invisible to the human eye, but nevertheless reduce reflections, mitigate dust adhesion, or help kill or shed microbes. Further, we’ve begun building multilayer structures comprised of different materials - arranged in sub-wavelength arrays - that exhibit properties unattainable by any naturally occurring materials. These are known as metamaterials, and they’re the future.   

Newsletter - April 5, 2023

Smart Material Solutions continues to grow! In the last nine months, we’ve made a significant breakthrough in our efforts to fabricate large-area metamaterials for the Army, demonstrated a unique structure to enhance the performance of flexible solar panels, and found our work on the cover of ACS Applied Materials and Interfaces! 

Our industrial partnerships are expanding alongside our federally funded research, and we’re steadily growing our team of employees and partners along with our portfolio of IP. In the past year, a greater percentage of our funding has come from the private sector than ever before. 

Metamaterial fabrication enabled by cylindrical photomasks

Metamaterial fabrication involves structuring a material to give it valuable properties beyond those of the bulk material. To this end, we recently fabricated a seamless, cylindrical photomask for roll-to-roll (R2R) photolithography to create plasmonic metamaterials with nanoscale patterns that give rise to extraordinary optical properties.

The images below show a photo of a 6-inch diameter seamlessly patterned cylindrical photomask (A), an optical microscopy image with light transmitting through the photomask’s holes (B), and an SEM image of a polymer replica of the mold’s surface (C). SMS partner MicroContinuum, Inc. will soon use this mold to pattern a plasmonic metamaterial absorber film for our Phase II SBIR project with the Army. 

The US Army SBIR office invited our team to submit a Sequential Phase II STTR proposal to continue this exciting work. We proposed creating a series of large-area infrared metamaterials for testing with MicroContinuum and Professor Mark Mirotznik’s group at the University of Delaware.

Self-cleaning, light-trapping nanopatterned coatings for solar panels

As winners of the Army’s XTech Clean Tech competition, we were awarded a Phase I SBIR to add light-trapping, self-cleaning coatings to solar panels. Our unique hierarchical pattern combines nanoscale moth-eye features and microlenses to both increase power output of the solar panel and prevent the buildup of dust, dirt, and grime that can block incoming light. 

The figure below shows SEM of the hierarchical pattern (A), SEM of a seam between a smooth (left) and a hierarchical (right) pattern with substantially more dust adhered to the smooth film (B), and the short-circuit current of two prototype solar cells in outdoor conditions. The solar cell with a hierarchical coating (left) produces more current than the cell without a coating (right). We’re now working on a Phase II SBIR proposal with MicroContinuum, Inc. and PowerFilm, Inc.

Cover article on dust-mitigating surfaces

Our NASA-funded work on dust-mitigating surfaces with Professor Chih-Hao Chang’s group at UT Austin was recently featured in a cover article in ACS Applied Materials & Interfaces. This paper discusses how our nanopatterns can decrease the adhesion of Lunar dust by over 90% compared to smooth surfaces of the same material.

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