Manufacturing Bits: July 22

Skateboarding on 2D materials
Two-dimensional materials are gaining steam in the R&D labs. The 2D materials include graphene, boron nitride (BN) and the transition-metal dichalcogenides (TMDs). These materials are attractive candidates for futuristic field-effect transistors (FETs).

But researchers must gain more insight into these materials in order to understand their properties. For example, researchers must discover how and why these materials bend and break under stress. As it turns out, the atoms in various 2D materials are prone to stress at certain points. Researchers equated the findings to a skateboarder falling off a skateboard.

Using supercomputers at the U.S. Department of Energy’s Brookhaven National Laboratory, Columbia University mapped out the transformations and breaking points of various 2D materials, such as BN, graphene, graphane and molybdenum disulfide.

Top and side views of soft mode instabilities in strained monolayer materials. In graphene, boron nitride, and graphane the backbone distorts towards isolated six-atom rings, while molybdenum disulfide undergoes a distinct distortion towards trigonal pyramidal coordination. (Source: Brookhaven)

Researchers used a mathematical framework called density functional theory (DFT). This is a computational quantum mechanical modeling method to investigate the electronic structure of systems. With a supercomputer, the DFT calculations revealed the characteristics of 2D materials. They also determined whether they acted as metals, semiconductors, or insulators under strain.

In 2D materials, atoms vibrate in place. The different vibrational states or modes dictate many of the mechanical properties of the materials. When the hexagonal structures of these materials are strained, the vibrating atoms slip free and form new structures as the materials break, according to researchers.

“Here we show that, as in graphene, a soft mode occurs at the K point in BN, graphane, and MoS2, while not in silicone,” according to researchers in an abstract. “The transition is first order in all cases except graphene. In BN and graphane the soft mode corresponds to a Kekulé-like distortion similar to that of graphene, while MoS2 has a distinct distortion. The phase transitions for BN, graphane, and MoS2 are not associated with the opening of a band gap, which indicates that Fermi surface nesting is not the driving force.”

On Brookhaven’s Web site, Columbia University Ph.D. candidate Eric Isaacs added: “Imagine a skateboarder in a half-pipe. Normally, the skater glides back and forth but remains centered over the bottom. But if we twist and deform that half-pipe enough, the skateboarder rolls out and never returns—that’s like this soft mode where the vibrating atoms move away from their positions in the lattice.”

In turn, researchers gained insight into controlling the strain of these materials. “This work is immediately useful to a large community of researchers excited to learn about and exploit graphene and its cousins,” Isaacs said.

Brighter LEDs
LEDs are a hot topic. The technology is paving the way for solid-state lighting in cars, homes and businesses.

A blue diode chip combined with yellow phosphor is a common way to fabricate today’s white LEDs. But due to the lack of red light in the mix, it is difficult to make warm white LEDs with a high color rendering index and a low correlated color temperature, according to researchers from the Chinese Academy of Sciences and National Taiwan University.

To address the challenge, a red phosphor can be added to the LED. Typically, red phosphors are based on rare earth materials, such as Eu2+ or Ce3+ doped oxynitride compounds. But these compounds are costly and have low luminous efficacies, according to researchers.

So, the ideal red phosphor must include narrow absorption and emission bands. Researchers from the Chinese Academy of Sciences and National Taiwan University have devised an efficient non-rare-earth red phosphor for warm white LEDs.

Researchers have developed a cation exchange method for synthesizing Mn4+ ion activated fluoride red phosphors. The phosphors exhibit an intense excitation band at ~460 nm with a bandwidth of ~50 nm, according to researchers. The red emissions are centered at ~630 nm, thereby rendering a higher LER in warm white LEDs as compared to oxynitride red phosphors.

“Currently, it remains a challenge to synthesize these phosphors with high photoluminescence quantum yields through a convenient chemical route,” according to researchers. “Herein we propose a general but convenient strategy based on efficient cation exchange reaction, which had been originally regarded only effective in synthesizing nano-sized materials before, for the synthesis of Mn4+-activated fluoride microcrystals such as K2TiF6, K2SiF6, NaGdF4 and NaYF4.”

Researchers achieved a photoluminescence quantum yield as high as 98% for K2TiF6:Mn4+. “By employing it as red phosphor, we fabricate a high-performance white LED with low correlated color temperature (3,556 K), high-colour-rendering index (Ra=81) and luminous efficacy of 116 lm W−1,” according to researchers.

Looking for nanoparticle partners
Sandia National Laboratorieshas devised a technology to synthesize titanium-dioxide (TiO2) nanoparticles for use in solar cells and LEDs. Now, the research lab is seeking partners that can help commercialize the technology.

TiO2 nanoparticles are promising. They can be used as fillers to tune the refractive index of anti-reflective coatings on signs and optical encapsulants for LEDs, solar cells and other optical devices.

But TiO2 nanoparticles are difficult and expensive to make. Current production methods for TiO2 require high-temperature processing or costly surfactants. In contrast, Sandia has devised low-cost materials that are small, uniform and don’t clump.

The method produces nanoparticles roughly 5nm in diameter. As a result, there is little light scattering with the technology. “Embodiments of this invention use titanium isopropoxide as the titanium precursor and isopropanol as both the solvent and ligand for ligand-stabilized brookite-phase titania,” according to researchers. “Isopropanol molecules serve as the ligands interacting with the titania surfaces that stabilize the titania nanoparticles. The isopropanol ligands can be exchanged with other alcohols and other ligands during or after the nanoparticle formation reaction.”