Power/Performance Bits: Jan. 25

Nanoscale 3D optics
Researchers at Rice University and University of Houston are using 3D printing to build nanostructures of silica for micro-scale electronic, mechanical, and photonic devices.

“It’s very tough to make complicated, three-dimensional geometries with traditional photolithography techniques,” said Jun Lou, a professor of materials science and nanoengineering at Rice. “It’s also not very ‘green’ because it requires a lot of chemicals and a lot of steps. And even with all that effort, some structures are impossible to make with those methods. In principle, we can print arbitrary 3D shapes, which could be very interesting for making exotic photonic devices. That’s what we’re trying to demonstrate.”

The team used a two-photon polymerization process to print structures with lines only several hundred nanometers wide, smaller than the wavelength of light. Lasers “write” the lines by prompting the ink to absorb two photons, initiating free-radical polymerization of the material.

Delicate structures printed by materials scientists at Rice University as seen in microscope images. Sintering turns them into either glass or cristobalite. (Credit: Nanomaterials, Nanomechanics and Nanodevices Lab at Rice University)

“Normal polymerization involves polymer monomers and photoinitiators, molecules that absorb light and generate free radicals,” said Boyu Zhang, a Rice graduate student. “In our process, the photoinitiators absorb two photons at the same time, which requires a lot of energy. Only a very small peak of this energy causes polymerization, and that in only a very tiny space. That’s why this process allows us to go beyond the diffraction limit of light.”

To create the ink, the researchers used resins containing nanospheres of silicon dioxide doped with polyethylene glycol to make them soluble.

After printing, the structure is solidified through high-temperature sintering, which eliminates all the polymer from the product, leaving amorphous glass or polycrystalline cristobalite. “When heated, the material goes through phases from glass to crystal, and the higher the temperature, the more ordered the crystals become,” Lou said.

The lab also demonstrated doping the material with various rare earth salts to make the products photoluminescent, an important property for optical applications. The lab’s next goal is to refine the process to achieve sub-10 nanometer resolution.

Simple spatial light modulator
Researchers from Harvard University and University of Washington developed a simple spatial light modulator made from gold electrodes covered by a thin film of electro-optical material that changes its optical properties in response to electric signals.

“This simple spatial light modulator is a bridge between the realms of optics and electronics,” said Cristina Benea-Chelmus, a postdoctoral fellow at Harvard School of Engineering and Applied Sciences (SEAS). “When you interface optics with electronics, you can use the entire backbone of electronics that has been developed to open up new functionalities in optics.”

The researchers used electro-optic materials that changes its refractive index when an electrical signal is applied. By dividing the material into pixels, the researchers could control the intensity of light in each pixel separately with interlocking electrodes. With only a small amount of power, the device could dramatically change the intensity of light at each pixel and can efficiently modulate light across the visible spectrum.

“We consider our work to mark the beginning of an up-and-coming field of hybrid organic-nanostructured electro-optics with broad applications in imaging, remote control, environmental monitoring, adaptive optics and laser ranging,” said Federico Capasso, professor of applied physics and senior research fellow in electrical engineering at Harvard.

The researchers have so far demonstrated the new spatial light modulators for image projection and remote sensing by single-pixel imaging, and plan to work on commercializing the technology.

Twisting 2D TMDs
Researchers from Pennsylvania State University, Harvard University, Massachusetts Institute of Technology, and Rutgers University investigated how controlling twist angles in a particular type of bilayer 2D material used in optoelectronic devices could strengthen the intrinsic electric charge that exists between the two layers.

The researchers worked with regular transition metal dichalcogenides (TMD) 2D materials and Janus TMDs, a class of 2D materials named after the Roman god of duality, Janus. These bilayer 2D materials have an interaction between layers known as a van der Waals interlayer coupling that leads to a charge transfer, a process important to the functionality of electronic devices. The charge transfer for both sides of conventional TMDs is the same due to each side having the same type of atoms. In the case of Janus TMD materials, the atoms on each side of the material are different types, leading to varied charge transfer when each side is in contact with other 2D materials.

“In our study, the two types of atoms on each side of the Janus TMD material were sulfur and selenium,” said Shengxi Huang, assistant professor of electrical engineering and biomedical engineering at Penn State. “Because they are different, there can be a charge separation or charge imbalance for the top and bottom side. It creates a vertically directed intrinsic electric field that is very different from conventional 2D materials.”

Previously, the researchers worked to understand whether this intrinsic electric field would impact adjacent 2D materials when they are layered. They found that the coupling is stronger in the Janus 2D materials than traditional 2D materials, due to the asymmetric charge caused by the different types of atoms on each side.

This time, they manually stacked two types of material layers, Janus TMD and regular 2D materials, which caused random angles depending on how they were stacked. But when they tuned the angles of how each layer was stacked to specific degrees, they made an interesting finding. If the triangle-shaped materials are twisted to stack at a zero-degree angle, when they are perfectly aligned, or at a 60-degree angle, when they are the exact opposite of perfect alignment, they found the couplings to be much stronger than at random angles. In addition, they also found the interlayer coupling is stronger when the Janus TMD is layered on the conventional TMD with the same type of element.

“The main finding was that for this same sulfur/sulfur interface, the interlayer coupling is much stronger than the sulfur/selenium interface,” Huang said. “And this is because of the charge distribution related to the dipole direction in these atoms. This means there can be an effective charge transfer between the two layers. Based on our calculation, the separation, meaning the distance between the interlayers, is much smaller, so that shows there’s a stronger coupling.”

To make the observations, the team used low-frequency Raman spectroscopy and photoluminescence spectroscopy.

“These new material abilities can affect lots of applications, ranging from optoelectronics to electronic devices to catalytic abilities in electrochemical devices such as batteries,” Huang said. “These devices are all over in our everyday lives, such as lighting, electronics, appliances and batteries.”

“People outside our field could benefit from our study,” said Kunyan Zhang, doctoral candidate in electrical engineering at Penn State. “Tuning this kind of interior coupling using the interface with twist angles was not studied before. Those findings may be striking for others in the 2D field whose work does not involve Janus TMDs.”