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Method to create colloidal diamonds developed

The colloidal diamond has been a dream of researchers since the 1990s. These structures — stable, self-assembled formations of miniscule materials — have the potential to make light waves as useful as electrons in computing, and hold promise for a host of other applications. But while the idea of colloidal diamonds was developed decades ago, no one was able to reliably produce the structures. Until now.

Researchers led by David Pine, professor of chemical and biomolecular engineering at the NYU Tandon School of Engineering and professor of physics at NYU, have devised a new process for the reliable self-assembly of colloids in a diamond formation that could lead to cheap, scalable fabrication of such structures. The discovery, detailed in “Colloidal Diamond,” appearing in the September 24 issue of Nature, could open the door to highly efficient optical circuits leading to advances in optical computers and lasers, light filters that are more reliable and cheaper to produce than ever before, and much more.

Pine and his colleagues, including lead author Mingxin He, a postdoctoral researcher in the Department of Physics at NYU, and corresponding author Stefano Sacanna, associate professor of chemistry at NYU, have been studying colloids and the possible ways they can be structured for decades. These materials, made up of spheres hundreds of times smaller than the diameter of a human hair, can be arranged in different crystalline shapes depending on how the spheres are linked to one another. Each colloid attaches to another using strands of DNA glued to surfaces of the colloids that function as a kind of molecular Velcro. When colloids collide with each other in a liquid bath, the DNA snags and the colloids are linked. Depending on where the DNA is attached to the colloid, they can spontaneously create complex structures.

This process has been used to create strings of colloids and even colloids in a cubic formation. But these structures did not produce the Holy Grail of photonics — a band gap for visible light. Much as a semiconductor filters out electrons in a circuit, a band gap filters out certain wavelengths of light. Filtering light in this way can be reliably achieved by colloids if they are arranged in a diamond formation, a process deemed too difficult and expensive to perform at commercial scale.

“There’s been a great desire among engineers to make a diamond structure,” said Pine. “Most researchers had given up on it, to tell you the truth — we may be the only group in the world who is still working on this. So I think the publication of the paper will come as something of a surprise to the community.”

The investigators, including Etienne Ducrot, a former postdoc at NYU Tandon, now at the Centre de Recherche Paul Pascal — CNRS, Pessac, France; and Gi-Ra Yi of Sungkyunkwan University, Suwon, South Korea, discovered that they could use a steric interlock mechanism that would spontaneously produce the necessary staggered bonds to make this structure possible. When these pyramidal colloids approached each other, they linked in the necessary orientation to generate a diamond formation. Rather than going through the painstaking and expensive process of building these structures through the use of nanomachines, this mechanism allows the colloids to structure themselves without the need for outside interference. Furthermore, the diamond structures are stable, even when the liquid they form in is removed.

The discovery was made because He, a graduate student at NYU Tandon at the time, noticed an unusual feature of the colloids he was synthesizing in a pyramidal formation. He and his colleagues drew out all of the ways these structures could be linked. When they happened upon a particular interlinked structure, they realized they had hit upon the proper method. “After creating all these models, we saw immediately that we had created diamonds,” said He.

“Dr. Pine’s long-sought demonstration of the first self-assembled colloidal diamond lattices will unlock new research and development opportunities for important Department of Defense technologies which could benefit from 3D photonic crystals,” said Dr. Evan Runnerstrom, program manager, Army Research Office (ARO), an element of the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory.

He explained that potential future advances include applications for high-efficiency lasers with reduced weight and energy demands for precision sensors and directed energy systems; and precise control of light for 3D integrated photonic circuits or optical signature management.

“I am thrilled with this result because it wonderfully illustrates a central goal of ARO’s Materials Design Program — to support high-risk, high-reward research that unlocks bottom-up routes to creating extraordinary materials that were previously impossible to make.”

The team, which also includes John Gales, a graduate student in physics at NYU, and Zhe Gong, a postdoc at the University of Pennsylvania, formerly a graduate student in chemistry at NYU, are now focused on seeing how these colloidal diamonds can be used in a practical setting. They are already creating materials using their new structures that can filter out optical wavelengths in order to prove their usefulness in future technologies.

This research was supported by the US Army Research Office under award number W911NF-17-1-0328. Additional funding was provided by the National Science Foundation under award number DMR-1610788.

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Physicists develop basic principles for mini-labs on chips

Colloidal particles have become increasingly important for research as vehicles of biochemical agents. In future, it will be possible to study their behaviour much more efficiently than before by placing them on a magnetised chip. A research team from the University of Bayreuth reports on these new findings in the journal Nature Communications. The scientists have discovered that colloidal rods can be moved on a chip quickly, precisely, and in different directions, almost like chess pieces. A pre-programmed magnetic field even enables these controlled movements to occur simultaneously.

For the recently published study, the research team, led by Prof. Dr. Thomas Fischer, Professor of Experimental Physics at the University of Bayreuth, worked closely with partners at the University of Pozn√°n and the University of Kassel. To begin with, individual spherical colloidal particles constituted the building blocks for rods of different lengths. These particles were assembled in such a way as to allow the rods to move in different directions on a magnetised chip like upright chess figures — as if by magic, but in fact determined by the characteristics of the magnetic field.

In a further step, the scientists succeeded in eliciting individual movements in various directions simultaneously. The critical factor here was the “programming” of the magnetic field with the aid of a mathematical code, which in encrypted form, outlines all the movements to be performed by the figures. When these movements are carried out simultaneously, they take up to one tenth of the time needed if they are carried out one after the other like the moves on a chessboard.

“The simultaneity of differently directed movements makes research into colloidal particles and their dynamics much more efficient,” says Adrian Ernst, doctoral student in the Bayreuth research team and co-author of the publication. “Miniaturised laboratories on small chips measuring just a few centimetres in size are being used more and more in basic physics research to gain insights into the properties and dynamics of materials. Our new research results reinforce this trend. Because colloidal particles are in many cases very well suited as vehicles for active substances, our research results could be of particular benefit to biomedicine and biotechnology,” says Mahla Mirzaee-Kakhki, first author and Bayreuth doctoral student.

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Colloidal quantum dot laser diodes are just around the corner

Los Alamos scientists have incorporated meticulously engineered colloidal quantum dots into a new type of light emitting diodes (LEDs) containing an integrated optical resonator, which allows them to function as lasers. These novel, dual-function devices clear the path towards versatile, manufacturing-friendly laser diodes. The technology can potentially revolutionize numerous fields from photonics and optoelectronics to chemical sensing and medical diagnostics.

“This latest breakthrough along with other recent advances in quantum dot chemistry and device engineering that we have achieved suggest that laser diodes assembled from solution may soon become a reality,” said Victor Klimov, head of the quantum dot group at Los Alamos National Laboratory. “Quantum dot displays and television sets are already available as commercial products. The colloidal quantum dot lasers seem to be next in line.”

Colloidal quantum dot lasers can be manufactured using cheaper, simpler methods than modern semiconductor laser diodes that require sophisticated, vacuum-based, layer-by-layer deposition techniques. Solution-processable lasers can be produced in less-challenging lab and factory conditions, and could lead to devices that would benefit a number of emerging fields including integrated photonic circuits, optical circuitry, lab-on-a-chip platforms, and wearable devices.

For the past two decades, the Los Alamos quantum dot team has been working on fundamental and applied aspects of lasing devices based on semiconductor nanocrystals prepared via colloidal chemistry. These particles, also known as colloidal quantum dots, can be easily processed from their native solution environment to create various optical, electronic, and optoelectronic devices. Furthermore, they can be ‘size-tuned’ for lasing applications to produce colors not accessible with existing semiconductor laser diodes.

In a paper published today in Nature Communications, the Los Alamos researchers successfully resolved several challenges on the path to commercially viable colloidal quantum dot technology. In particular they demonstrated an operational LED, which also functioned as an optically-pumped, low-threshold laser. To achieve these behaviors, they incorporated an optical resonator directly into the LED architecture without obstructing charge-carrier flows into the quantum dot emitting layer. Further, by carefully designing the structure of their multilayered device, they could achieve good confinement of the emitted light within the ultrathin quantum dot medium on the order of 50 nanometers across. This is key to obtaining the lasing effect and, at the same time, allowing for efficient excitation of the quantum dots by the electrical current. The final ingredient of this successful demonstration was unique, home-made quantum dots perfected for lasing applications per recipes developed by the Los Alamos team over the years of research into the chemistry and physics of these nanostructures.

Presently, the Los Alamos scientists are tackling the remaining challenge, which is boosting the current density to levels sufficient for obtaining so-called ‘population inversion’ — the regime when the quantum dot active medium turns into a light amplifier.

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Charge change: How electric forces vary in colloids

Colloidal suspensions heterogenous mixtures of particles with diameters of about 2-500 nanometers, which are permanently suspended in a second phase, usually a liquid. Owing to the small particle size of the suspended material, a colloid does not separate into its characteristic components even if allowed to remain undisturbed, nor can the suspended material be separated through filtration. Colloids are distinguished from other types of mixtures by several important distinctive properties, one of which is the electrokinetic force in colloidal suspensions, also known as the “zeta potential.”

To explore zeta potential, we must first understand what a “slipping surface” is. A slipping surface is an “electrical double layer” that forms on the surface of any object when it is exposed to a fluid. This double layer consists of one layer of charges that adhere to the surface of the object as a result of chemical interactions, and a second layer of opposite charges that are attracted to the first layer. Due to the attraction between these two layers of opposite “ions” or charges, an electric potential is created, and this is the zeta potential. The zeta potential occurs in double layers on the surface of particles suspended in colloids as well.

Prof Hiroyuki Ohshima of Tokyo University of Science has been a lifelong theoretical researcher of electrokinetic phenomena such as the movement of colloidal particles in an electric field and electrostatic interactions between colloidal particles. He has recently summarized some of the major findings in his field in a review published in the journal Advances in Colloid and Interface Science. He asserts the importance of zeta potential in colloidal surface chemistry. According to him, “the dispersion stability of colloidal particles, which is one of the most important issues in colloid surface chemistry, greatly depends on the zeta potential of the particles.”

Zeta potential is calculated based on the electrophoretic mobility of the particles. Until now, the no-slip boundary condition of the fluid, which assumes that the fluid will have zero velocity relative to the boundary, has been applied when calculating the zeta potential. However, while this condition is applicable to particles with a hydrophilic (“water-loving”) surface, it cannot be applied to particles with a hydrophobic (“water-shy”) surface. In this case, the Navier boundary condition, which considers the relative velocity of the fluid, is applied.

In the Navier boundary condition, the effect of the hydrodynamic slip is characterized by the slipping length. When the surface is hydrophilic, the slipping length is considered to be zero, and it progressively increases with the increase in hydrophobicity of the surface, where the molecules of the particle surface weakly interact with the molecules in the surrounding phase so that liquid slip occurs. In accordance, an infinitely large slipping length theoretically corresponds with a completely hydrophobic surface. From this information, theoretical calculations show that electrophoretic mobility and sedimentation potential increase with increasing slipping length.

According to Prof Ohshima, what is more interesting is that if we accept the possibility of the presence of a slipping surface on a spherical solid colloidal particle, we can observe that the electrokinetic properties of this solid particle will be hydrodynamically similar to those of a liquid drop.

These findings highlight the importance of reconsidering how the electrokinetic properties of hydrophilic and hydrophobic surfaces vary and showcase how they affect the dynamics of colloidal suspensions. Prof Ohshima concludes, “We have constructed a general theory describing various electrokinetic phenomena of particles with a sliding surface. By applying this theory, we could expect a more accurate evaluation of zeta potential and colloidal particle dispersion stability in the future.”

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Scientists discover ‘electron equivalents’ in colloidal systems

Scientists find unusual behaviors in colloidal crystals.

Atoms have a positively charged center surrounded by a cloud of negatively charged particles. This type of arrangement, it turns out, can also occur at a more macroscopic level, giving new insights into the nature of how materials form and interact.

In a new study from the U.S. Department of Energy’s (DOE) Argonne National Laboratory, scientists have examined the internal structure of a material called a colloidal crystal, which consists of a highly ordered array of larger and smaller particles interspersed in regular arrangements. A greater knowledge of how colloidal crystals are structured and behave could help scientists determine the applications to which they are best suited, like photonics.

In pioneering research outlined in a recent issue of Science, scientists tethered smaller particles to larger ones using DNA, allowing them to determine how the smaller particles filled in the regions surrounding the larger ones. When using particles as small as 1.4 nanometers — extremely small for colloidal particles — scientists observed an exciting effect: The small particles roamed around regularly ordered larger particles instead of remaining locked in an ordered fashion.

Because of this behavior, the colloidal crystals could be designed to lead to a variety of new technologies in the field of optics, catalysis, and drug delivery. The small particles have the potential to act as messengers, carrying other molecules, electric current or information from one end of a crystal to another.

“The smaller particles essentially act like a glue that holds the larger particle arrangement together,” said Argonne X-ray physicist and study author Byeongdu Lee. “With only a few beads of glue, the best position to place them is on the corners between the larger particles. If you add more glue beads, they would overflow to the edges.”

The small particles that sit on the corners tend to stay still — a configuration Lee called localization. The additional particles that are on the edges have more freedom of movement, becoming delocalized. By being tethered to larger particles and with the ability to be both localized and delocalized, the small particles act as “electron equivalents” in the crystal structure. The delocalization of small particles, which the authors called metallicity, had not been observed so far in colloidal particle assemblies.

Additionally, since the small particles delocalize in part, the effect creates a material that challenges most traditional definitions of a crystal, according to Lee.

“Normally, when you change the composition of a crystal, the structure changes as well,” he said. “Here, you can have a material that is able to maintain its overall structure with different proportions of its components.”

To image the structure of the colloidal crystals, Lee and his colleagues used the high-brightness X-ray beams provided by Argonne’s Advanced Photon Source (APS), a DOE Office of Science User Facility. The APS offered a key advantage in that it allowed the scientists to observe the structure of the crystal directly in solution. “This system is only stable in solution, once it dries, the structure deforms,” Lee said.

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