Clip from NanoXpo 2018: Hui Zheng

Hui Zheng's research aims to make airplanes safer in the future. Zheng is a nanoengineering Ph.D. student in Professor Shyue Ping Ong's Materials Virtual Lab at UC San Diego. Using DFT calculations, Zheng is finding ways to re-engineer materials -- such as those found in the fan blades of airplane engines -- to make them stronger and resistant to cracking.

Zheng describes her project in this video, taken at NanoXpo 2018:

Poster title: "Role of Zr in Strengthening MoSi2 Grain Boundaries from DFT Calculations"

NanoXpo is an annual event held by the Graduate Society of Nanoengineers to showcase graduate research in the UC San Diego Department of NanoEngineering.

Gallium Nitride ‘Tangoes’ with Silicon to Overcome Nature’s Material Limitations

Gallium nitride (GaN) is a material that is used for radio and satellite communications in civil and military applications and in solid-state lighting such as LED bulbs. Researchers are also exploring GaN for use in high power applications such as power grids and electric vehicles. The market for GaN power devices is expected to reach $2.6 billion dollars by 2022. However, GaN is not an earth abundant material and only recently, small diameter GaN substrates have started to become available. Researchers have been growing GaN on foreign substrates for almost 5 decades, but the quality of the grown materials is compromised, especially on the standard microelectronics substrate, silicon (Si), which is over 1000 times cheaper than GaN substrates. The origin of the problem is a classical one: high quality material deposition is usually carried out near 1,000 degrees Celsius, but when dissimilar materials are cooled down to room temperature, their contraction can be disproportionate, resulting in the formation of cracks and material failure. This is exactly what happens when GaN is grown on Si. And because the crack severity depends on the thickness of the layers, the thickest pure and semiconductive GaN layer that can be grown on Si is 4.5 micrometers thick — too thin to provide good use of GaN for high power (kilovolt-scale) applications which require much thicker layers (10 microns or more). 

Scanning electron microscopy image of 

crack-free GaN on Si (19 μm thick at center).

Now researchers at the Integrated Electronics and Biointerfaces Group at UC San Diego led by electrical engineering professor Shadi Dayeh have solved this classical problem of thermal mismatches in the growth of dissimilar materials. In an article published on Aug. 21 in Advanced Materials, they combined fundamental crystal properties of GaN and geometrical effects to deflect strain from the crystal planes that usually crack under stress to the surface facets that can freely expand and contract in response to stress. By doing so, they were able to grow crack-free 19-micron-thick layers of GaN on Si — thicker than what’s needed for high-power applications. In the resulting structures, both GaN and Si had exposed surfaces to enable them to move, twist or “tango” together without cracking despite their thermal mismatch. 

Electrical engineering professor Shadi Dayeh (left) and 

Ph.D. graduate student Atsunori Tanaka (right) 

near the GaN MOCVD facility in the Qualcomm Institute 

at UC San Diego.

Thick layers also allowed the crystal defects — threading dislocations — to reduce from commonly achieved 10⁸ – 10⁹ per centimeter squared on Si to 10⁷. And with the high material quality, Dayeh and his team demonstrated the first vertical GaN switches on Si. “This is the result of nearly four years of diligent efforts by graduate student Atsunori Tanaka, who learned and quickly excelled in the GaN metal organic chemical vapor deposition here at UC San Diego,” said Dayeh. “Our graduate students go through a full cycle of rigorous training in all aspects in electronic materials and devices and are prepared to tackle the greatest challenges in this area. A group of very talented students including Atsunori Tanaka, Woojin Choi, who fabricated the vertical switches, and Renjie Chen, who did the electron microscopy, have teamed up to complete the research,” Dayeh continued. Based on this work, Dayeh received funding in July from the National Science Foundation to realize a monolithically integrated GaN power converter on Si.

The growth, device fabrication and characterization were performed at UC San Diego and the electron microscopy was performed at the Center for Integrated Nanotechnologies (CINT), a Department of Energy Office of Basic Science user facility that provides access to top-of-the-line equipment under a user proposal system.

Engineer demonstrates technique for targeting RNA inside living cells

Dave Nelles

When he’s not surfing in Mexico or listening to electronic music, Dave Nelles is busy tinkering – inside living cells!

Growing up, Nelles always knew he wanted to develop technology, but was intrigued by the complexity and diversity of processes in biology.

“Biology is on the verge of becoming a predictive and quantitative pursuit,” says Nelles. “Compared to fields like physics where we have many good models of natural phenomena, biology in general is less mature. One reason for this is a lack of tools to measure and alter specific components of living cells.”

Motivated by this gap, Nelles focused his graduate work in materials science and engineering on technologies to measure and alter a fundamental biological molecule: RNA. Inside cells, DNA is transcribed into messenger RNA (mRNA), which is subsequently translated into protein.

Proteins are the building blocks of life – many functions that take place inside of a cell are made possible by proteins.

“In many diseases, the processing of mRNA is dysfunctional, meaning that the protein that is encoded for by that RNA will not be made correctly, or at all,” said Nelles.

In molecular biology, there’s a technique called CRISPR-Cas9 that is used to modify DNA and has the potential to cure a range of genetic diseases. Nelles and his collaborators have been able to demonstrate that CRISPR-Cas9 can not only bind to DNA, but also to RNA.  This approach is described in a paper published on March 17^th in the journal Cell.

Nelles explains, “Just as CRISPR-Cas9 is making genetic engineering accessible to any scientist with access to basic equipment, RNA-targeted Cas9 may support countless other efforts for studying the role of RNA processing in disease or for identifying drugs that reverse defects in RNA processing.”

In collaboration with Mitchell O’Connell in the lab of Jennifer Doudna at the University of California, Berkeley, Nelles tagged Cas9 with a fluorescent protein and targeted various RNAs to track their movement inside living cells.

“This work is the first example, to our knowledge, of targeting RNA in living cells with CRISPR-Cas9,” said senior author Gene Yeo, PhD, associate professor of Cellular and Molecular Medicine. “Our current work focuses on tracking the movement of RNA inside the cell, but future developments could enable researchers to measure other RNA features or advance therapeutic approaches to correct disease-causing RNA behaviors.”

“For many experiments involving RNA tracking, the cells need to be dead or the targeted RNA must be genetically modified in order for the RNA to be detectable,” said Nelles. “Our experiments were done inside living cells with unmodified RNAs, which has many advantages – for example, we were able to observe RNA being transported to stress granules over time.“

Stress granules are accumulations of RNA and protein in a cell and their formation has been linked to neurodegenerative diseases. Nelles and his team hope that providing a way to track these RNAs will assist with new drug development.

After graduating this Spring, Nelles will be continuing his work as a postdoc at UC San Diego.

Want to learn more about other projects at the Jacobs School? Register to attend Research Expo on April 14, 2016.

Minerals, Metals, and Materials Society to Honor Professor Marc A. Meyers with Symposium

The Minerals Metals and Materials Society (TMS) will honor Marc A. Meyers, a materials science professor at the UC San Diego Jacobs School of Engineering at their annual meeting in February 2014 through a special symposium on Dynamic Behavior of Materials. (Meyers is affiliated with Mechanical & Aerospace Engineering, and NanoEngineering at the Jacobs School.)

Symposium organizers: Naresh Thadhani, Georgia Institute of Technology and George Thompson Gray, Los Alamos National Laboratory

Sponsorship: TMS/ASM: Mechanical Behavior of Materials Committee

Abstract submission is open until July 1. A description of the symposium from the website of TMS2014 is below.

The dynamic behavior of materials encompasses a broad range of phenomena associated with extreme environment and with relevance to technological applications in military and civilian sectors. The field of dynamic behavior of materials comprises diverse phenomena such as deformation, fracture, fragmentation, shear localization, damage dissipation, chemical reactions under extreme conditions, and processing (combustion synthesis; shock compaction; explosive welding and fabrication; shock and shear synthesis of novel materials). It has evolved considerably in the past twenty years and is now at a stage where its significance to all classes of materials including metals, ceramics, polymers, and composites is becoming relevant.

It is recognized today, as evidenced by the contributions herein, that materials aspects are of utmost importance in extreme dynamic loading events. The macro mechanical and physical processes that govern the phenomena manifest themselves at the microstructural level, by dazzling complexity of defect configurations and effects. Nevertheless, these processes/mechanisms can be quantitatively treated on the basis of accumulated knowledge. We are entering an exciting stage where our capabilities, from continuum and molecular dynamics computations, enable realistic predictions of materials performances and are starting to guide not only the design process but also our further micromechanical understanding of deformation processes at every level, including the basic dislocation mechanisms. The multiple technologies applications of this field include crashworthiness, machining, and important military effects of armor and projectile designs, ballistic penetrations, and explosive dynamics leading in general to the design of conventional and nuclear weapons. Applications in the medical field are also becoming important, with recent developments aimed at understanding traumatic brain injury and drug delivery. The dynamic behavior of materials during processing, including during compaction, synthesis, welding, forming, etc., is also of considerable importance. The symposium organizers hope that, through the publications of the symposium articles, the materials community will become more exposed to this research field.