Nano-objects of desire: Assembling ordered nanostructures in 3-D

A schematic of the programmable assembly of 3-D ordered nanostructures from material voxels that can carry inorganic or organic nanoparticles with different functions, such as light emitters and absorbers, proteins, and enzymes with chemical activity. Material voxels are fabricated from DNA and nano-objects of different kinds, and their assembly is guided by the voxel design and DNA-programmable interactions. Credit: Brookhaven National Laboratory

Scientists have developed a platform for assembling nanosized material components, or “nano-objects,” of very different types—inorganic or organic—into desired 3-D structures. Though self-assembly (SA) has successfully been used to organize nanomaterials of several kinds, the process has been extremely system-specific, generating different structures based on the intrinsic properties of the materials. As reported in a paper published today in Nature Materials, their new DNA-programmable nanofabrication platform can be applied to organize a variety of 3-D materials in the same prescribed ways at the nanoscale (billionths of a meter), where unique optical, chemical, and other properties emerge.

“One of the major reasons why SA is not a technique of choice for practical applications is that the same SA process cannot be applied across a broad range of materials to create identical 3-D ordered arrays from different nanocomponents,” explained corresponding author Oleg Gang, leader of the Soft and Bio Nanomaterials Group at the Center for Functional Nanomaterials (CFN)—a U.S. Department of Energy (DOE) Office of Science User Facility at Brookhaven National Laboratory—and a professor of Chemical Engineering and of Applied Physics and Materials Science at Columbia Engineering. “Here, we decoupled the SA process from material properties by designing rigid polyhedral DNA frames that can encapsulate various inorganic or organic nano-objects, including metals, semiconductors, and even proteins and enzymes.”

The scientists engineered synthetic DNA frames in the shape of a cube, octahedron, and tetrahedron. Inside the frames are DNA “arms” that only nano-objects with the complementary DNA sequence can bind to. These material voxels—the integration of the DNA frame and nano-object—are the building blocks from which macro-scale 3-D structures can be made. The frames connect to each other regardless of what kind of nano-object is inside (or not) according to the complementary sequences they are encoded with at their vertices. Depending on their shape, frames have a different number of vertices and thus form entirely different structures. Any nano-objects hosted inside the frames take on that specific frame structure.

Schematic of the platform for assembling 3-D lattices from inorganic and organic nano-objects with DNA frames shaped as a tetrahedron (top row), octahedron (middle row), and cube (bottom row). The frame valence is determined by the vertices and corresponds to the number of connections (bonds) and how these connections are organized relative to one another. For example, the tetrahedral frame has a valence of four. The resulting 3-D lattices are based on the shape of the DNA frame–tetrahedral frames assemble into diamond structures, octahedral into simple cubic, and cubic into body-centered cubic–regardless of which nano-object (if any) is inside the frame. Credit: Nature Materials

To demonstrate their assembly approach, the scientists selected metallic (gold) and semiconducting (cadmium selenide) nanoparticles and a bacterial protein (streptavidin) as the inorganic and organic nano-objects to be placed inside the DNA frames. First, they confirmed the integrity of the DNA frames and formation of material voxels by imaging with electron microscopes at the CFN Electron Microscopy Facility and the Van Andel Institute, which has a suite of instruments that operate at cryogenic temperatures for biological samples. They then probed the 3-D lattice structures at the Coherent Hard X-ray Scattering and Complex Materials Scattering beamlines of the National Synchrotron Light Source II (NSLS-II)—another DOE Office of Science User Facility at Brookhaven Lab. Columbia Engineering Bykhovsky Professor of Chemical Engineering Sanat Kumar and his group performed computational modeling revealing that the experimentally observed lattice structures (based on the X-ray scattering patterns) were the most thermodynamically stable ones that the material voxels could form.

“These material voxels allow us to begin to use ideas derived from atoms (and molecules) and the crystals that they form, and port this vast knowledge and database to systems of interest at the nanoscale,” explained Kumar.

Gang’s students at Columbia then demonstrated how the assembly platform could be used to drive the organization of two different kinds of materials with chemical and optical functions. In one case, they co-assembled two enzymes, creating 3-D arrays with a high packing density. Though the enzymes remained chemically unchanged, they showed about a fourfold increase in enzymatic activity. These “nanoreactors” could be used to manipulate cascade reactions and enable the fabrication of chemically active materials. For the optical material demonstration, they mixed two different colors of quantum dots—tiny nanocrystals that are being used to make television displays with high color saturation and brightness. Images captured with a fluorescence microscope showed that the formed lattice maintained color purity below the diffraction limit (wavelength) of light; this property could allow for significant resolution improvement in various display and optical communication technologies.

“We need to rethink how materials can be formed and how they function,” said Gang. “Material redesign may not be necessary; simply packaging existing materials in new ways could enhance their properties. Potentially, our platform could be an enabling technology ‘beyond 3-D printing manufacturing’ to control materials at much smaller scales and with greater material variety and designed compositions. Using the same approach to form 3-D lattices from desired nano-objects of different material classes, integrating those that would otherwise be considered incompatible, could revolutionize nanomanufacturing.”

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Nanoscale sculpturing leads to unusual packing of nanocubes

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More information: Ordered three-dimensional nanomaterials using DNA-prescribed and valence-controlled material voxels, Nature Materials (2020). DOI: 10.1038/s41563-019-0550-x , https://nature.com/articles/s41563-019-0550-x

Journal information: Nature Materials

Provided by Brookhaven National Laboratory

Brookhaven National Laboratory – Searching for More Cost Efficient Catalysts for Hydrogen Fuel Cells – Illuminating Nanoparticle Growth With X-Rays

Brookhaven Lab scientists Mingyuan Ge, Iradwikanari Waluyo, and Adrian Hunt are pictured left to right at the IOS beamline, where they studied the growth pathway of an efficient catalyst for hydrogen fuel cells. Credit: Brookhaven National Laboratory

Hydrogen fuel cells are a promising technology for producing clean and renewable energy, but the cost and activity of their cathode materials is a major challenge for commercialization. Many fuel cells require expensive platinum-based catalysts–substances that initiate and speed up chemical reactions–to help convert renewable fuels into electrical energy. To make hydrogen fuel cells commercially viable, scientists are searching for more affordable catalysts that provide the same efficiency as pure platinum.

“Like a battery, hydrogen fuel cells convert stored chemical energy into electricity. The difference is that you’re using a replenishable fuel so, in principle, that ‘battery’ would last forever,” said Adrian Hunt, a scientist at the National Synchrotron Light Source II(NSLS-II), a U.S. Department of Energy (DOE) Office of Science User Facility at DOE’s Brookhaven National Laboratory. “Finding a cheap and effective catalyst for hydrogen fuel cells is basically the holy grail for making this technology more feasible.”

Taking part in this worldwide search for fuel cell cathode materials, researchers at the University of Akron developed a new method of synthesizing catalysts from a combination of metals–platinum and nickel–that form octahedral (eight-sided) shaped nanoparticles. While scientists have identified this catalyst as one of the most efficient replacements for pure platinum, they have not fully understood why it grows in an octahedral shape. To better understand the growth process, the researchers at the University of Akron collaborated with multiple institutions, including Brookhaven and its NSLS-II.

Schematic diagram of the oxygen reduction reaction (reduction of O2 into H2O) on the Pt(110) surface of the PtPb/Pt nanoplates, with purple representing Pt atoms and orange representing Pb atoms. Credit: Brookhaven National Laboratory

“Understanding how the faceted catalyst is formed plays a key role in establishing its structure-property correlation and designing a better catalyst,” said Zhenmeng Peng, principal investigator of the catalysis lab at the University of Akron. “The growth process case for the platinum-nickel system is quite sophisticated, so we collaborated with several experienced groups to address the challenges. The cutting-edge techniques at Brookhaven National Lab were of great help to study this research topic.”

Using the ultrabright x-rays at NSLS-II and the advanced capabilities of NSLS-II’s In situ and Operando Soft X-ray Spectroscopy (IOS) beamline, the researchers revealed the chemical characterization of the catalyst’s growth pathway in real time. Their findings are published in Nature Communications.

“We used a research technique called ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) to study the surface composition and chemical state of the metals in the nanoparticles during the growth reaction,” said Iradwikanari Waluyo, lead scientist at IOS and a co-corresponding author of the research paper. “In this technique, we direct x-rays at a sample, which causes electrons to be released. By analyzing the energy of these electrons, we are able to distinguish the chemical elements in the sample, as well as their chemical and oxidation states.”

Hunt, who is also an author on the paper, added, “It is similar to the way sunlight interacts with our clothing. Sunlight is roughly yellow, but once it hits a person’s shirt, you can tell whether the shirt is blue, red, or green.”

Rather than colors, the scientists were identifying chemical information on the surface of the catalyst and comparing it to its interior. They discovered that, during the growth reaction, metallic platinum forms first and becomes the core of the nanoparticles. Then, when the reaction reaches a slightly higher temperature, platinum helps form metallic nickel, which later segregates to the surface of the nanoparticle. In the final stages of growth, the surface becomes roughly an equal mixture of the two metals. This interesting synergistic effect between platinum and nickel plays a significant role in the development of the nanoparticle’s octahedral shape, as well as its reactivity.

“The nice thing about these findings is that nickel is a cheap material, whereas platinum is expensive,” Hunt said. “So, if the nickel on the surface of the nanoparticle is catalyzing the reaction, and these nanoparticles are still more active than platinum by itself, then hopefully, with more research, we can figure out the minimum amount of platinum to add and still get the high activity, creating a more cost-effective catalyst.”

The findings depended on the advanced capabilities of IOS, where the researchers were able to run the experiments at gas pressures higher than what is usually possible in conventional XPS experiments.

“At IOS, we were able to follow changes in the composition and chemical state of the nanoparticles in real time during the real growth conditions,” said Waluyo.

Additional x-ray and electron imaging studies completed at the Advanced Photon Source (APS) at DOE’s Argonne National Laboratory–another DOE Office of Science User Facility–and University of California-Irvine, respectively, complemented the work at NSLS-II.

“This fundamental work highlights the significant role of segregated nickel in forming the octahedral-shaped catalyst. We have achieved more insight into shape control of catalyst nanoparticles,” Peng said. “Our next step is to study catalytic properties of the faceted nanoparticles to understand the structure-property correlation.”

Brookhaven National Lab: The rapid self-assembly of nanoscale patterns for next-generation materials: From Electronics and Computing to Energy and Medicine

The ability to quickly generate ultra-small, well-ordered nanopatterns over large areas on material surfaces is critical to the fabrication of next-generation technologies in many industries, from electronics and computing to energy and medicine. For example, patterned media, in which data are stored in periodic arrays of magnetic pillars or bars, could significantly improve the storage density of hard disk drives.

Scientists can coax thin films of self-assembling materials called block copolymers—chains of chemically distinct macromolecules (polymer “blocks”) linked together—into desired nanoscale patterns through heating (annealing) them on a substrate. However, defective structures that deviate from the regular pattern emerge early on during self-assembly.

Materials scientist Gregory Doerk preparing a sample for electron microscopy at Brookhaven Lab’s Center for Functional Nanomaterials. The scanning electron microscope image on the computer screen shows a cross-sectional view of line …more

The presence of these defects inhibits the use of block copolymers in the nanopatterning of technologies that require a nearly perfect ordering—such as magnetic media, computer chips, antireflective surfaces, and medical diagnostic devices. With continued annealing, the block copolymer patterns can reconfigure to remove the imperfections, but this process is exceedingly slow. The polymer blocks do not readily mix with each other, so they must overcome an extremely large energy barrier to reconfigure.

Adding small things with a big impact

Now, scientists from the Center for Functional Nanomaterials (CFN)—a U.S. Department of Energy (DOE) Office of Science User Facility at Brookhaven National Laboratory—have come up with a way to massively speed up the ordering process. They blended a line-forming block copolymer with significantly smaller polymer chains made of only one type of molecule (homopolymers) from each of the two constituent blocks. The electron microscopy images they took after annealing the films for only a few minutes show that the addition of these two smaller homopolymers dramatically increases the size of well-ordered line-pattern areas, or “grains.”

As shown in the illustration, a block copolymer consists of different molecule chains (red and blue) linked together; a homopolymer chain consists of identical molecules (red or blue). In this study, scientists blended a block copolymer …more

“Without the homopolymers, the same block copolymer cannot produce grains with these sizes,” said CFN materials scientist Gregory Doerk, who led the work, which was published online in an ACS Nano paper on December 1. “Blending in homopolymers that are less than one-tenth of the size of the block copolymer greatly accelerates the ordering process. In the resulting line patterns, there is a constant spacing between each of the lines, and the same directions of line-pattern orientations—for example, vertical or horizontal—persist over longer distances.”

Doerk and coauthor Kevin Yager, leader of the Electronic Nanomaterials Group at CFN, used image analysis software to calculate the grain size and repeat spacing of the line patterns.

While blending different concentrations of homopolymer to determine how much was needed to achieve the accelerated ordering, they discovered that the ordering sped up as more homopolymer was added. But too much homopolymer actually resulted in disordered patterns.

The scanning electron microscope images taken after thermal annealing at around 480 degrees Fahrenheit for five minutes show that the block copolymer/homopolymer blend generates a line pattern with a significantly higher degree of …more

“The homopolymers accelerate the self-assembly process because they are small enough to uniformly distribute throughout their respective polymer blocks,” said Doerk. “Their presence weakens the interface between the two blocks, lowering the energy barrier associated with the block copolymer reconfiguring to remove the defects. But if the interface is weakened too much through the addition of too much homopolymer, then the blocks will mix together, resulting in a completely disordered phase.”

Guiding the self-assembly of useful nanopatterns in minutes

To demonstrate how the rapid ordering in the blended system could accelerate the self-assembly of well-aligned nanopatterns over large areas, Doerk and Yager used line-pattern templates they had previously prepared through photolithography. Used to build almost all of today’s digital devices, photolithography involves projecting light through a mask (a plate containing the desired pattern) that is positioned over a wafer (usually made of silicon) coated with a light-sensitive material. This template can then be used to direct the self-assembly of block copolymers, which fill in the spaces between the template guides. In this case, after only two minutes of annealing, the polymer blend self-assembles into lines that are aligned across these gaps. However, after the same annealing time, the unblended block copolymer self-assembles into a mostly unaligned pattern with many defects between the gaps.

The unblended block copolymer aligns well close to the template guides (“sidewalls”), but this alignment degrades further in, as evident by the appearance of the fingerprint-like pattern in the center of the scanning electron microscope …more

“The width of the gaps is more than 80 times the repeat spacing, so the fact that we got this degree of alignment with our polymer blend is really exciting because it means we can use templates with huge gaps, created with very low-resolution lithography,” said Doerk. “Typically, expensive high-resolution lithography equipment is needed to align block copolymer patterns over this large of an area.”

For these patterns to be useful for many nanopatterning applications, they often need to be transferred to other more robust materials that can withstand harsh manufacturing processes—for example, etching, which removes layers from silicon wafer surfaces to create integrated circuits or make the surfaces antireflective. In this study, the scientists converted the nanopatterns into a metal-oxide replica. Through chemical etching, they then transferred the replica pattern into a silicon dioxide layer on a silicon wafer, achieving clearly defined line patterns.

Doerk suspects that blending homopolymers with other block copolymers will similarly yield accelerated assembly, and he is interested in studying blended polymers that self-assemble into more complicated patterns. The x-ray scattering capabilities at the National Synchrotron Light Source II—another DOE Office of Science User Facility at Brookhaven—could provide the structural information needed to conduct such studies.

A scanning electron microscope image showing a cross-sectional view of the line patterns transferred into a silicon dioxide layer. Credit: Brookhaven National Laboratory

“We have introduced a very simple and easily controlled way of immensely accelerating self-assembly,” concluded Doerk. “Our approach should substantially reduce the number of defects, helping to meet the demands of the semiconductor industry. At CFN, it opens up possibilities for us to use block copolymer self-assembly to make some of the new functional materials that we envision.”

 Explore further: Self-assembling polymers provide thin nanowire template

More information: Gregory S. Doerk et al. Rapid Ordering in “Wet Brush” Block Copolymer/Homopolymer Ternary Blends, ACS Nano (2017). DOI: 10.1021/acsnano.7b06154

Journal reference: ACS Nano  

Provided by: Brookhaven National Laboratory

 

DOE – Brookhaven: Smarter self-assembly opens new pathways for nanotechnology – Essential to Fully Exploit the Nanoscale for ‘Next Generation’ Electronic Devices

To continue advancing, next-generation electronic devices must fully exploit the nanoscale, where materials span just billionths of a meter. But balancing complexity, precision, and manufacturing scalability on such fantastically small scales is inevitably difficult. Fortunately, some nanomaterials can be coaxed into snapping themselves into desired formations-a process called self-assembly.

Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have just developed a way to direct the self-assembly of multiple molecular patterns within a single material, producing new nanoscale architectures. The results were published in the journal Nature Communications.

“This is a significant conceptual leap in self-assembly,” said Brookhaven Lab physicist Aaron Stein, lead author on the study. “In the past, we were limited to a single emergent pattern, but this technique breaks that barrier with relative ease. This is significant for basic research, certainly, but it could also change the way we design and manufacture electronics.”

Microchips, for example, use meticulously patterned templates to produce the nanoscale structures that process and store information. Through self-assembly, however, these structures can spontaneously form without that exhaustive preliminary patterning. And now, self-assembly can generate multiple distinct patterns-greatly increasing the complexity of nanostructures that can be formed in a single step.

“This technique fits quite easily into existing microchip fabrication workflows,” said study coauthor Kevin Yager, also a Brookhaven physicist. “It’s exciting to make a fundamental discovery that could one day find its way into our computers.”

The experimental work was conducted entirely at Brookhaven Lab’s Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility, leveraging in-house expertise and instrumentation.

Electron beam lithography is used to adjust the spacing and thickness of line patterns etched onto a template (lower layer). These patterns drive a self-assembling block copolymer (top layer) to locally form different types of patterns, …more

 

Cooking up organized complexity

The collaboration used block copolymers-chains of two distinct molecules linked together-because of their intrinsic ability to self-assemble.

“As powerful as self-assembly is, we suspected that guiding the process would enhance it to create truly ‘responsive’ self-assembly,” said study coauthor Greg Doerk of Brookhaven. “That’s exactly where we pushed it.”

To guide self-assembly, scientists create precise but simple substrate templates. Using a method called electron beam lithography-Stein’s specialty-they etch patterns thousands of times thinner than a human hair on the template surface. They then add a solution containing a set of block copolymers onto the template, spin the substrate to create a thin coating, and “bake” it all in an oven to kick the molecules into formation. Thermal energy drives interaction between the block copolymers and the template, setting the final configuration-in this instance, parallel lines or dots in a grid.

“In conventional self-assembly, the final nanostructures follow the template’s guiding lines, but are of a single pattern type,” Stein said. “But that all just changed.”

Brookhaven National Laboratory Center for Functional Nanomaterials researchers Gwen Wright and Aaron Stein are at the electron beam lithography writer in the CFN cleanroom. Credit: Brookhaven National Laboratory

Lines and dots, living together

The collaboration had previously discovered that mixing together different block copolymers allowed multiple, co-existing line and dot nanostructures to form.

“We had discovered an exciting phenomenon, but couldn’t select which morphology would emerge,” Yager said. But then the team found that tweaking the substrate changed the structures that emerged. By simply adjusting the spacing and thickness of the lithographic line patterns-easy to fabricate using modern tools-the self-assembling blocks can be locally converted into ultra-thin lines, or high-density arrays of nano-dots.

“We realized that combining our self-assembling materials with nanofabricated guides gave us that elusive control. And, of course, these new geometries are achieved on an incredibly small scale,” said Yager.

“In essence,” said Stein, “we’ve created ‘smart’ templates for nanomaterial self-assembly. How far we can push the technique remains to be seen, but it opens some very promising pathways.”

Gwen Wright, another CFN coauthor, added, “Many nano-fabrication labs should be able to do this tomorrow with their in-house tools-the trick was discovering it was even possible.”

The scientists plan to increase the sophistication of the process, using more complex materials in order to move toward more device-like architectures.

“The ongoing and open collaboration within the CFN made this possible,” said Charles Black, director of the CFN. “We had experts in self-assembly, electron beam lithography, and even electron microscopy to characterize the materials, all under one roof, all pushing the limits of nanoscience.”

Explore further: Copolymers block out new approaches to microelectronics at NIST

More information: A. Stein et al, Selective directed self-assembly of coexisting morphologies using block copolymer blends, Nature Communications (2016). DOI: 10.1038/ncomms12366

Journal reference: Nature Communications

 

Brookhaven National Laboratory: Smarter self-assembly opens new pathways for nanotechnology ~ “Potentially Changing the way we design and manufacture electronics.”

To continue advancing, next-generation electronic devices must fully exploit the nanoscale, where materials span just billionths of a meter. But balancing complexity, precision, and manufacturing scalability on such fantastically small scales is inevitably difficult. Fortunately, some nanomaterials can be coaxed into snapping themselves into desired formations—a process called self-assembly.

Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have just developed a way to direct the self-assembly of multiple molecular patterns within a single material, producing new nanoscale architectures. The results were published in the journal Nature Communications (“Selective directed self-assembly of coexisting morphologies using block copolymer blends”).

Figure 1 Figure 1: Electron beam lithography is used to adjust the spacing and thickness of line patterns etched onto a template (lower layer). These patterns drive a self-assembling block copolymer (top layer) to locally form different types of patterns, depending on the underlying template. Thus, a single material can be coaxed into forming distinct nanopatterns for example, lines or dots ‹ in close proximity. These mixed-configuration materials could lead to new applications in microelectronics.

“This is a significant conceptual leap in self-assembly,” said Brookhaven Lab physicist Aaron Stein, lead author on the study. “In the past, we were limited to a single emergent pattern, but this technique breaks that barrier with relative ease. This is significant for basic research, certainly, but it could also change the way we design and manufacture electronics.”

Microchips, for example, use meticulously patterned templates to produce the nanoscale structures that process and store information. Through self-assembly, however, these structures can spontaneously form without that exhaustive preliminary patterning. And now, self-assembly can generate multiple distinct patterns—greatly increasing the complexity of nanostructures that can be formed in a single step.

“This technique fits quite easily into existing microchip fabrication workflows,” said study coauthor Kevin Yager, also a Brookhaven physicist. “It’s exciting to make a fundamental discovery that could one day find its way into our computers.”

The experimental work was conducted entirely at Brookhaven Lab’s Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility, leveraging in-house expertise and instrumentation.

Cooking up organized complexity

The collaboration used block copolymers—chains of two distinct molecules linked together—because of their intrinsic ability to self-assemble.

“As powerful as self-assembly is, we suspected that guiding the process would enhance it to create truly ‘responsive’ self-assembly,” said study coauthor Greg Doerk of Brookhaven. “That’s exactly where we pushed it.”

To guide self-assembly, scientists create precise but simple substrate templates. Using a method called electron beam lithography—Stein’s specialty—they etch patterns thousands of times thinner than a human hair on the template surface. They then add a solution containing a set of block copolymers onto the template, spin the substrate to create a thin coating, and “bake” it all in an oven to kick the molecules into formation. Thermal energy drives interaction between the block copolymers and the template, setting the final configuration—in this instance, parallel lines or dots in a grid.

“In conventional self-assembly, the final nanostructures follow the template’s guiding lines, but are of a single pattern type,” Stein said. “But that all just changed.”

Lines and dots, living together

The collaboration had previously discovered that mixing together different block copolymers allowed multiple, co-existing line and dot nanostructures to form.

“We had discovered an exciting phenomenon, but couldn’t select which morphology would emerge,” Yager said. But then the team found that tweaking the substrate changed the structures that emerged. By simply adjusting the spacing and thickness of the lithographic line patterns—easy to fabricate using modern tools—the self-assembling blocks can be locally converted into ultra-thin lines, or high-density arrays of nano-dots.

“We realized that combining our self-assembling materials with nanofabricated guides gave us that elusive control. And, of course, these new geometries are achieved on an incredibly small scale,” said Yager.

“In essence,” said Stein, “we’ve created ‘smart’ templates for nanomaterial self-assembly. How far we can push the technique remains to be seen, but it opens some very promising pathways.”

Gwen Wright, another CFN coauthor, added, “Many nano-fabrication labs should be able to do this tomorrow with their in-house tools—the trick was discovering it was even possible.”

The scientists plan to increase the sophistication of the process, using more complex materials in order to move toward more device-like architectures.

“The ongoing and open collaboration within the CFN made this possible,” said Charles Black, director of the CFN. “We had experts in self-assembly, electron beam lithography, and even electron microscopy to characterize the materials, all under one roof, all pushing the limits of nanoscience.”

Source: Brookhaven National Laboratory

Read more: Smarter self-assembly opens new pathways for nanotechnology

Quantum Dots: Enhancing Light-to-Current Conversion: Better Semiconductors, Solar Cells and Photdetectors

Single nanocrystal spectroscopy identifies the interaction between zero-dimensional CdSe/ZnS nano crystals (quantum dots) and two-dimensional layered tin disulfide as a non-radiative energy transfer, whose strength increases with increasing …more

Harnessing the power of the sun and creating light-harvesting or light-sensing devices requires a material that both absorbs light efficiently and converts the energy to highly mobile electrical current. Finding the ideal mix of properties in a single material is a challenge, so scientists have been experimenting with ways to combine different materials to create “hybrids” with enhanced features.

In two just-published papers, scientists from the U.S. Department of Energy’s Brookhaven National Laboratory, Stony Brook University, and the University of Nebraska describe one such approach that combines the excellent light-harvesting properties of quantum dots with the tunable electrical conductivity of a layered tin disulfide semiconductor.

The hybrid material exhibited enhanced light-harvesting properties through the absorption of light by the quantum dots and their energy transfer to tin disulfide, both in laboratory tests and when incorporated into electronic devices. The research paves the way for using these materials in optoelectronic applications such as energy-harvesting photovoltaics, light sensors, and light emitting diodes (LEDs).

According to Mircea Cotlet, the physical chemist who led this work at Brookhaven Lab’s Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility, “Two-dimensional metal dichalcogenides like tin disulfide have some promising properties for solar energy conversion and photodetector applications, including a high surface-to-volume aspect ratio. But no semiconducting material has it all. These materials are very thin and they are poor light absorbers. So we were trying to mix them with other nanomaterials like light-absorbing quantum dots to improve their performance through energy transfer.”

One paper, just published in the journal ACS Nano, describes a fundamental study of the hybrid quantum dot/tin disulfide material by itself. The work analyzes how light excites the quantum dots (made of a cadmium selenide core surrounded by a zinc sulfide shell), which then transfer the absorbed energy to layers of nearby tin disulfide.

“We have come up with an interesting approach to discriminate energy transfer from charge transfer, two common types of interactions promoted by light in such hybrids,” said Prahlad Routh, a graduate student from Stony Brook University working with Cotlet and co-first author of the ACS Nano paper. “We do this using single nanocrystal spectroscopy to look at how individual quantum dots blink when interacting with sheet-like tin disulfide. This straightforward method can assess whether components in such semiconducting hybrids interact either by energy or by charge transfer.”

The researchers found that the rate for non-radiative energy transfer from individual quantum dots to tin disulfide increases with an increasing number of tin disulfide layers. But performance in laboratory tests isn’t enough to prove the merits of potential new materials. So the scientists incorporated the hybrid material into an electronic device, a photo-field-effect-transistor, a type of photon detector commonly used for light sensing applications.

As described in a paper published online March 24 in Applied Physics Letters, the hybrid material dramatically enhanced the performance of the photo-field-effect transistors-resulting in a photocurrent response (conversion of light to electric current) that was 500 percent better than transistors made with the tin disulfide material alone.

“This kind of energy transfer is a key process that enables photosynthesis in nature,” said Chang-Yong Nam, a materials scientist at Center for Functional Nanomaterials and co-corresponding author of the APL paper. “Researchers have been trying to emulate this principle in light-harvesting electrical devices, but it has been difficult particularly for new material systems such as the tin disulfide we studied. Our device demonstrates the performance benefits realized by using both energy transfer processes and new low-dimensional materials.”

Cotlet concludes, “The idea of ‘doping’ two-dimensional layered materials with quantum dots to enhance their light absorbing properties shows promise for designing better solar cells and photodetectors.”

Explore further: Small size enhances charge transfer in quantum dots

More information: Yuan Huang et al. Hybrid quantum dot-tin disulfide field-effect transistors with improved photocurrent and spectral responsivity, Applied Physics Letters (2016). DOI: 10.1063/1.4944781

Huidong Zang et al. Nonradiative Energy Transfer from Individual CdSe/ZnS Quantum Dots to Single-Layer and Few-Layer Tin Disulfide, ACS Nano (2016). DOI: 10.1021/acsnano.6b01538

Journal reference: ACS Nano Applied Physics Letters

Brookhaven National Laboratory: Self-Assembled Nanotextures Create Anti-Reflective Surface: Applications for Solar Cells and Military Radar Camouflage

Reducing the amount of sunlight that bounces off the surface of solar cells helps maximize the conversion of the sun’s rays to electricity, so manufacturers use coatings to cut down on reflections. Now scientists at the U.S. Department of Energy’s Brookhaven National Laboratory show that etching a nanoscale texture onto the silicon material itself creates an antireflective surface that works as well as state-of-the-art thin-film multilayer coatings.

Their method, described in the journal Nature Communications and submitted for patent protection, has potential for streamlining silicon solar cell production and reducing manufacturing costs. The approach may find additional applications in reducing glare from windows, providing radar camouflage for military equipment, and increasing the brightness of light-emitting diodes.

“For antireflection applications, the idea is to prevent light or radio waves from bouncing at interfaces between materials,” said physicist Charles Black, who led the research at Brookhaven Lab’s Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility.

Preventing reflections requires controlling an abrupt change in “refractive index,” a property that affects how waves such as light propagate through a material. This occurs at the interface where two materials with very different refractive indices meet, for example at the interface between air and silicon. Adding a coating with an intermediate refractive index at the interface eases the transition between materials and reduces the reflection, Black explained.

A closeup shows how the nanotextured square of silicon completely blocks reflection compared with the surrounding silicon wafer.

Credit: Brookhaven National Laboratory

[Click to enlarge image]

“The issue with using such coatings for solar cells,” he said, “is that we’d prefer to fully capture every color of the light spectrum within the device, and we’d like to capture the light irrespective of the direction it comes from. But each color of light couples best with a different antireflection coating, and each coating is optimized for light coming from a particular direction. So you deal with these issues by using multiple antireflection layers. We were interested in looking for a better way.”

For inspiration, the scientists turned to a well-known example of an antireflective surface in nature, the eyes of common moths. The surfaces of their compound eyes have textured patterns made of many tiny “posts,” each smaller than the wavelengths of light. This textured surface improves moths’ nighttime vision, and also prevents the “deer in the headlights” reflecting glow that might allow predators to detect them.

“We set out to recreate moth eye patterns in silicon at even smaller sizes using methods of nanotechnology,” said Atikur Rahman, a postdoctoral fellow working with Black at the CFN and first author of the study.

The scientists started by coating the top surface of a silicon solar cell with a polymer material called a “block copolymer,” which can be made to self-organize into an ordered surface pattern with dimensions measuring only tens of nanometers. The self-assembled pattern served as a template for forming posts in the solar cell like those in the moth eye using a plasma of reactive gases-a technique commonly used in the manufacture of semiconductor electronic circuits.

The resulting surface nanotexture served to gradually change the refractive index to drastically cut down on reflection of many wavelengths of light simultaneously, regardless of the direction of light impinging on the solar cell.

“Adding these nanotextures turned the normally shiny silicon surface absolutely black,” Rahman said.

Solar cells textured in this way outperform those coated with a single antireflective film by about 20 percent, and bring light into the device as well as the best multi-layer-coatings used in the industry.

“We are working to understand whether there are economic advantages to assembling silicon solar cells using our method, compared to other, established processes in the industry,” Black said.

Hidden layer explains better-than-expected performance

One intriguing aspect of the study was that the scientists achieved the antireflective performance by creating nanoposts only half as tall as the required height predicted by a mathematical model describing the effect. So they called upon the expertise of colleagues at the CFN and other Brookhaven scientists to help sort out the mystery.

“This is a powerful advantage of doing research at the CFN-both for us and for academic and industrial researchers coming to use our facilities,” Black said. “We have all these experts around who can help you solve your problems.”

Using a combination of computational modeling, electron microscopy, and surface science, the team deduced that a thin layer of silicon oxide similar to what typically forms when silicon is exposed to air seemed to be having an outsized effect.

“On a flat surface, this layer is so thin that its effect is minimal,” explained Matt Eisaman of Brookhaven’s Sustainable Energy Technologies Department and a professor at Stony Brook University. “But on the nanopatterned surface, with the thin oxide layer surrounding all sides of the nanotexture, the oxide can have a larger effect because it makes up a significant portion of the nanotextured material.”

Said Black, “This ‘hidden’ layer was the key to the extra boost in performance.”

The scientists are now interested in developing their self-assembly based method of nanotexture patterning for other materials, including glass and plastic, for antiglare windows and coatings for solar panels.

This research was supported by the DOE Office of Science.

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The above story is based on materials provided by Brookhaven National Laboratory. Note: Materials may be edited for content and length.

Small Particles, Big Findings

Scientists collaborate to maximize energy gains from tiny nanoparticles

“We are developing a new ‘micro-reactor’ that enables us to explore many aspects of catalytic function using multiple approaches at Brookhaven’s National Synchrotron Light Source (NSLS), the soon-to-be-completed NSLS-II, and the Center for Functional Nanomaterials (CFN),” said Stach, who works at the CFN.  “This approach lets us understand multiple aspects of how catalysts work so that we can tweak their design to improve their function. This work could lead to big gains in energy efficiency and cost savings for industrial processes.”  – Anatoly Frenkel, Yeshiva Univerity

Sometimes big change comes from small beginnings. That’s especially true in the research of Anatoly Frenkel, a professor of physics at Yeshiva University, who is working to reinvent the way we use and produce energy by unlocking the potential of some of the world’s tiniest structures: nanoparticles.

“The nanoparticle is the smallest unit in most novel materials, and all of its properties are linked in one way or another to its structure,” said Frenkel. “If we can understand that connection, we can derive much more information about how it can be used for catalysis, energy, and other purposes.”

“This work could lead to big gains in energy efficiency and cost savings for industrial processes.”

— Eric Stach, CFN

Frenkel is collaborating with materials scientist Eric Stach and others at the U.S. Department of Energy’s Brookhaven National Laboratory to develop new ways to study how nanoparticles behave in catalysts—the “kick-starters” of chemical reactions that convert fuels to useable forms of energy and transform raw materials to industrial products.

“We are developing a new ‘micro-reactor’ that enables us to explore many aspects of catalytic function using multiple approaches at Brookhaven’s National Synchrotron Light Source (NSLS), the soon-to-be-completed NSLS-II, and the Center for Functional Nanomaterials (CFN),” said Stach, who works at the CFN.  “This approach lets us understand multiple aspects of how catalysts work so that we can tweak their design to improve their function. This work could lead to big gains in energy efficiency and cost savings for industrial processes.”

High-tech tools for science

Until now, the methods for understanding catalytic properties could only be used one at a time, with the catalyst ending up in a different state for each of the experiments. This made it difficult to compare information obtained using the different instruments. The new micro-reactor will employ multiple techniques—microscopy, spectroscopy, and diffraction—to examine different properties of catalysts simultaneously under operating conditions. By keeping particles in the same structural and dynamic state under the same reaction conditions, the micro-reactor will give scientists a much better sense of how they function.

This high-resolution transmission electron micrograph taken at the CFN reveals the arrangement of cerium oxide nanoparticles (bright angular “slashes” at the bottom of the image) supported on a titania substrate (background)‹a combination being explored as a catalyst for splitting water molecules to release hydrogen as fuel and for other energy-transformation reactions.

 

“These developments have resulted from the combination of unique facilities available at Brookhaven,” said Frenkel. “By working closely with Eric, we realized that there was a way to make both x-ray and electron-based methods work in a truly complementary fashion.

Each technique has strengths, Stach explained. “At the NSLS, using powerful beams of x-rays, we can tell how the entire group of nanoparticles behaves, while electron microscopy at the CFN lets us see the atomic structure of each nanoparticle.  By having both of these views of the catalysts we can more clearly understand the relationship between catalyst structure and function.”

Said Frenkel, “It was very satisfying for us to conduct the first tests with the reactor at each facility and receive positive results. I am particularly grateful to Ryan Tappero, the scientist who runs NSLS beamline X27A, for his expert help with x-ray data acquisition.”

Frenkel has had an ongoing collaboration with scientists at Brookhaven. Last year, with post-doctoral research associate Qi Wang, Frenkel and Stach measured properties of nanoparticles using the x-rays produced by the NSLS as well as atomic-scale imaging with electrons at the CFN. As reported in a paper published in the Journal of the American Chemical Society earlier this year, they discovered that rather than changing completely from one state to another at a certain temperature and size, as had been previously believed, there is a transition zone between states when particles are changing forms.

“This is of significance fundamentally because until now, the structures were known to merely change from one form to another—they were never envisioned to coexist in different forms,” Frenkel said. “With our information we can explain why catalysts often don’t work as expected and how to improve them.”

Training for young scientists

Anatoly Frenkel of Yeshiva University with students from Stern College for Women at the National Synchrotron Light Source at Brookhaven National Laboratory.

 

The collaboration also offers opportunities for students to experience the challenges of research, giving them access to the world-class tools at Brookhaven. Frenkel’s undergraduate students at Yeshiva University’s Stern College for Women help with measurements, data analysis, and interpretation, and many have already accompanied him to Brookhaven to assist in his work using NSLS and other cutting-edge instruments.

“I’m giving them firsthand experience about what a researcher’s life is like early on as they conduct first-rate research,” said Frenkel. “This experience opens doors to any field they want to be in.”

Alyssa Lerner, a pre-engineering major who has been working with Frenkel at Brookhaven, said the research “has helped me develop skills like computational analysis and critical thinking, which are essential in any scientific field. The hands-on experimental experience has given me a better understanding of how the scientific community operates, helping me make more informed career-related choices as I continue to advance my education.”

Pairing up students and mentors to advance education and making use of complementary imaging techniques to enhance energy efficiency—just two of the positive outcomes of this successful collaboration.

“By bringing together multiple complementary techniques to illuminate the same process we’re going to understand how nanomaterials work,” Frenkel said. “Ultimately, this research will create a better way of using, storing, and converting energy.”

The CFN and NSLS facilities at Brookhaven Lab are supported by the Department of Energy’s Office of Science. The collaborative work of Frenkel and Stach is funded by the Office of Science and Brookhaven’s Laboratory Directed Research and Development program.

The Center for Functional Nanomaterials is one of five DOE Nanoscale Science Research Centers (NSRCs), national user facilities for interdisciplinary research at the nanoscale, supported by the DOE Office of Science.  Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative.  The NSRCs are located at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge and Sandia and Los Alamos National Laboratories.  For more information about the DOE NSRCs, please visit http://science.energy.gov.

The National Synchrotron Light Source (NSLS) provides intense beams of infrared, ultraviolet, and x-ray light for basic and applied research in physics, chemistry, medicine, geophysics, and environmental and materials sciences.  Supported by the Office of Basic Energy Sciences within the U.S. Department of Energy, the NSLS is one of the world’s most widely used scientific facilities. For more information, visit http://www.nsls.bnl.gov.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov

This story incorporates content from a piece by Perel Skier on the Yeshiva University news blog.

DOE: Mixing Nanoparticles to Make “Multifunctional” Materials

Posted: Oct 20, 2013

Mixing nanoparticles to make multifunctional materials

(Nanowerk News) Scientists at the U.S. Department of Energy‘s Brookhaven National Laboratory have developed a general approach for combining different types of nanoparticles to produce large-scale composite materials.

The technique, described in a paper published online by Nature Nanotechnology on October 20, 2013 (“A general strategy for the DNA-mediated self-assembly of functional nanoparticles into heterogeneous systems”), opens many opportunities for mixing and matching particles with different magnetic, optical, or chemical properties to form new, multifunctional materials or materials with enhanced performance for a wide range of potential applications.

The approach takes advantage of the attractive pairing of complementary strands of synthetic DNA-based on the molecule that carries the genetic code in its sequence of matched bases known by the letters A, T, G, and C.

After coating the nanoparticles with a chemically standardized “construction platform” and adding extender molecules to which DNA can easily bind, the scientists attach complementary lab-designed DNA strands to the two different kinds of nanoparticles they want to link up. The natural pairing of the matching strands then “self-assembles” the particles into a three-dimensional array consisting of billions of particles. Varying the length of the DNA linkers, their surface density on particles, and other factors gives scientists the ability to control and optimize different types of newly formed materials and their properties.

DNA linkers allow different kinds of nanoparticles to self-assemble and form relatively large-scale nanocomposite arrays. This approach allows for mixing and matching components for the design of multifunctional materials.

“Our study demonstrates that DNA-driven assembly methods enable the by-design creation of large-scale ‘superlattice’ nanocomposites from a broad range of nanocomponents now available-including magnetic, catalytic, and fluorescent nanoparticles,” said Brookhaven physicist Oleg Gang, who led the research at the Lab’s Center for Functional Nanomaterials (CFN). “This advance builds on our previous work with simpler systems, where we demonstrated that pairing nanoparticles with different functions can affect the individual particles’ performance, and it offers routes for the fabrication of new materials with combined, enhanced, or even brand new functions.”

Future applications could include quantum dots whose glowing fluorescence can be controlled by an external magnetic field for new kinds of switches or sensors; gold nanoparticles that synergistically enhance the brightness of quantum dots’ fluorescent glow; or catalytic nanomaterials that absorb the “poisons” that normally degrade their performance, Gang said.

“Modern nano-synthesis methods provide scientists with diverse types of nanoparticles from a wide range of atomic elements,” said Yugang Zhang, first author of the paper. “With our approach, scientists can explore pairings of these particles in a rational way.”

Pairing up dissimilar particles presents many challenges the scientists investigated in the work leading to this paper. To understand the fundamental aspects of various newly formed materials they used a wide range of techniques, including x-ray scattering studies at Brookhaven’s National Synchrotron Light Source (NSLS) and spectroscopy and electron microcopy at the CFN.

For example, the scientists explored the effect of particle shape. “In principle, differently shaped particles don’t want to coexist in one lattice,” said Gang. “They either tend to separate into different phases like oil and water refusing to mix or form disordered structures.”

The scientists discovered that DNA not only helps the particles mix, but it can also improve order for such systems when a thicker DNA shell around the particles is used.

They also investigated how the DNA-pairing mechanism and other intrinsic physical forces, such as magnetic attraction among particles, might compete during the assembly process.

For example, magnetic particles tend to clump to form aggregates that can hinder the binding of DNA from another type of particle. “We show that shorter DNA strands are more effective at competing against magnetic attraction,” Gang said.

For the particular composite of gold and magnetic nanoparticles they created, the scientists discovered that applying an external magnetic field could “switch” the material’s phase and affect the ordering of the particles.

“This was just a demonstration that it can be done, but it could have an application-perhaps magnetic switches, or materials that might be able to change shape on demand,” said Zhang.

DNA linkers allow different kinds of nanoparticles to self-assemble and form  relatively large-scale nanocomposite arrays. This approach allows for mixing and  matching components for the design of multifunctional materials.

The third fundamental factor the scientists explored was how the particles were ordered in the superlattice arrays: Does one type of particle always occupy the same position relative to the other type-like boys and girls sitting in alternating seats in a movie theater-or are they interspersed more randomly?

“This is what we call a compositional order, which is important for example for quantum dots because their optical properties-e.g., their ability to glow-depend on how many gold nanoparticles are in the surrounding environment,” said Gang. “If you have compositional disorder, the optical properties would be different.” In the experiments, increasing the thickness of the soft DNA shells around the particles increased compositional disorder.

These fundamental principles give scientists a framework for designing new materials. The specific conditions required for a particular application will be dependent on the particles being used, Zhang emphasized, but the general assembly approach would be the same.

Said Gang, “We can vary the lengths of the DNA strands to change the distance between particles from about 10 nanometers to under 100 nanometers-which is important for applications because many optical, magnetic, and other properties of nanoparticles depend on the positioning at this scale. We are excited by the avenues this research opens up in terms of future directions for engineering novel classes of materials that exploit collective effects and multifunctionality.”

Source: Brookhaven National Laboratory

 

Read more: http://www.nanowerk.com/news2/newsid=32807.php#ixzz2iImpDZ6p

Polymer Solar Cells Employing Förster Resonance Energy Transfer

What is the Scientific Achievement?

Next generation solar panels could yield substantially lower costs per kilowatt-hour with this technological development.

 

Two crucial tasks exist for realizing high-efficiency polymer solar cells: increasing the range of the spectral absorption of light and efficiently harvesting photo-generated excitons.  In this work, Förster resonance energy transfer (FRET)-based heterojunction polymer solar cells that incorporate squaraine dye (SQ) were fabricated and investigated.

 

The high absorbance of squaraine in the near-infrared region broadens the spectral absorption of the solar cells and assists in developing an ordered nano-morphology for enhanced charge transport.  Femtosecond spectroscopic studies revealed highly efficient (up to 96%) excitation energy transfer from poly(3-hexylthiophene), also known as P3HT, to squaraine occurring on a picosecond timescale.

A 38% increase in power conversion efficiency was realized to reach 4.5%; this finding suggests that this system has improved exciton migration over long distances.  This architecture transcends traditional multiblend systems, allowing multiple donor materials with separate spectral responses to work synergistically, thereby enabling an improvement in light absorption and conversion.  This discovery opens up a new avenue for the development of high-efficiency polymer solar cells.

Why Does This Matter?

A new energy transfer mechanism has been exploited for the first time, allowing significantly more efficient energy harvesting in P3HT/dye solar cells compared to P3HT-alone solar cells.  Also, broadening the light absorption spectrum into the near-infrared region and developing nanoscale parts to the solar cell has improved the device.

Allowing different light-absorbing materials to work synergistically has led to well-ordered polymer networks without post-processing.

Energy level diagram of the components of the ternary blend solar cell highlighting pathways for charge generation.

 

What Are the Specifics?

• CFN Capability: CFN’s Advanced Optical Spectroscopy & Microscopy Facility was used to understand the energy conversion mechanism and rate of electronic transfer between the dye and polymer in the solar cells.
• The use of squaraine dye and FRET of charge carriers improved the efficiency of polymer solar cells.  Femtosecond spectroscopic studies revealed highly efficient excitation energy transfer from P3HT to SQ occurring on a picosecond timescale.  This suggested that this system has improved exciton migration over long distances.
• For the first time, FRET was exploited to enhance exciton harvesting in polymer bulk heterojunction solar cells.

Reference

Jing-Shun Huang¹, Tenghooi Goh¹, Xiaokai Li¹, Matthew Y. Sfeir², Elizabeth A. Bielinski³, Stephanie Tomasulo⁴, Minjoo L. Lee⁴, Nilay Hazari³, and André D. Taylor¹, Polymer bulk heterojunction solar cells employing Förster resonance energy transfer, Nature Photonics 7, 479-485 (2013).

1. Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, USA
2. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA
3. Department of Chemistry, Yale University, New Haven, Connecticut 06511, USA
4. Department of Electrical Engineering, Yale University, New Haven, Connecticut 06511, USA

Acknowledgment of Support

This work was supported primarily by the SOLAR program of the National Science Foundation (NSF; DMR-0934520) and the Yale Climate and Energy Institute. A.D.T. acknowledges support from a NSF-CAREER award (CBET-0954985) and NASA (CT Space Grant Consortium). Research was carried out in part at the Centre for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the US Department of Energy, Office of Basic Energy Sciences (contract no. DE-AC02-98CH10886). The authors thank C. Schmuttenmaer, E. Yan and S. Wang for informative discussions.

Related Links

• Nature Photonics: Polymer bulk heterojunction solar cells employing Förster resonance energy transfer