III-nitride semiconductor materials are wide-bandgap semiconductors that are of particular interest in optic and photonic applications because they can be used to create lasers and LEDs that produce light in the visible bandwidth range. And when it comes to large-scale manufacturing, III-nitride semiconductor materials produced using a technique called metal organic chemical vapor deposition (MOCVD).

Semiconductor devices require two materials, a “p-type” and an “n-type.” Electrons move from the n-type material to the p-type material. This is made possible by creating a p-type material that has “holes,” or spaces that electrons can move into.

A challenge for people who make LEDs and lasers has been that there was a limit on the number of holes that you can make in p-type III-nitride semiconductor materials that are created using MOCVD. But that limit just went up.

“We have developed a process that produces the highest concentration of holes in p-type material in any III-Nitride semiconductor made using MOCVD,” says Salah Bedair, co-author of a paper on the work and a distinguished professor of electrical and computer engineering at NC State. “And this is high quality material — very few defects — making it suitable for use in a variety of devices.”

In practical terms, this means more of the energy input in LEDs is converted into light. For lasers, it means that less of the energy input will be wasted as heat by reducing the metal contact resistance.

LEDs contain three main layers: an n-type layer where electrons originate; the so-called “active region,” which consists of multiple quantum wells of indium gallium nitride and gallium nitride; and a p-type layer, where the holes originate.

To produce semiconductor materials for use in LEDs or laser diodes, the researchers use a growth technique called “semibulk growth” to produce indium gallium nitride templates. The template is made of dozens of layers of indium gallium nitride and gallium nitride. The researchers use these templates for the n-type region to reduce complications that arise with the growth of the quantum wells. The insertion of the gallium nitride layer in between the indium gallium nitride layers in semibulk reduces defects due to the lattice mismatch between the semibulk template and the gallium nitride substrate, as well as filling the pits that form on the surface.

In their new work, the researchers demonstrated that the semibulk growth approach can be used for the p-type layer in LEDs to increase the number of holes. This new approach is cost effective from a manufacturing standpoint, since III-nitride based LED devices can be done in one growth via MOCVD, without a lengthy processing time in between.

Using this technique, the researchers were able to achieve a hole density of 5 × 10¹⁹ cm^-3 in the p-type material. Previously, the highest hole concentration achieved in p-type III-nitride materials using MOCVD was about an order of magnitude lower.

The researchers also applied these indium gallium nitride templates as substrates for LED structures to address the long-lasting problem called the “green gap,” where the LED output deteriorates when emitting in the green and yellow part of the spectrum.

One of the main reasons for the green gap is the large lattice mismatch between the light emitting part of the material, the quantum well, when gallium nitride substrates are used. The researchers have demonstrated that replacing the gallium nitride substrates with indium gallium nitride templates results in improved LED performances.

The researchers compared the LED emission spectrum for the same quantum well emitting in blue when grown on gallium nitride substrate and emitting either in green or yellow when grown on different indium gallium nitride templates. A 100 nm shift in the emission wavelength was achieved due to the application of the indium gallium nitride templates.

The paper on improved efficiency, “P-type In_xGa_1-xN semibulk templates (0.02 < x < 0.16) with room temperature hole concentration of mid-10¹⁹ cm^-3 and device quality surface morphology,” is published in the journal Applied Physics Letters. The first two authors on the paper are Evyn Routh and Mostafa Abdelhamid, who are both Ph.D. students at NC State. The paper was co-authored by Peter Colter, a postdoctoral researcher at NC State; and by Nadia El-Masry of the National Science Foundation and NC State.

The paper addressing the green gap in LEDs, “Shifting LED emission from blue to the green gap spectral range using In_0.12Ga_0.88N relaxed templates,” is published in Superlattices and Microstructures. The first two authors on the paper are Abdelhamid and Routh. The paper was co-authored by Ahmed Shaker, a visiting scientist at NC State from EinShams University in Egypt.

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Materials provided by North Carolina State University. Original written by Matt Shipman. Note: Content may be edited for style and length.

“We know that astrocytes could play a role in neurodevelopmental disorders, so we wanted to ask: How are they playing a role in typical development?” says Allen, a member of the Molecular Neurobiology Laboratory. “In order to better understand the disorders, we first have to understand what happens normally.”

Synapses form critical connections between neurons, allowing neurons to send signals and information throughout the body. Astrocyte cells play a role in synapse development by giving neurons directions, such as telling them when to start growing a synapse, when to stop, when to prune it back, and when to stabilize the connection.

Allen and her team took a closer look at how this process plays out in the visual cortex of the mouse brain. They sequenced the RNA of astrocytes at different stages of brain development to assess gene activity and compared it with neuronal synapse development. They found that astrocyte signaling was directly related to each stage of neuronal development. The researchers then wanted to know how the astrocytes knew to make these signals at the right time.

First, the researchers looked at what happened to the astrocytes when they changed the neurons’ activity. To do this, they stopped neurons from releasing a neurotransmitter called glutamate that can signal to astrocytes, and this stopped the astrocytes from showing the typical developmental changes. Next, the scientists stopped the astrocytes from responding to neurotransmitters, and found this stopped the astrocytes from expressing the right signals. With both these manipulations, the development of synapses was also disrupted, in line with the changes observed in the astrocytes.

Collectively, the findings suggest that astrocytes are responding to neurotransmitters produced by neurons to control the timing of when astrocytes produce signals to instruct neuronal development, according to Allen.

“It makes sense that you have this constant feedback going on between the neuron and the astrocyte,” says Allen. “They are sending signals to each other: ‘Am I in the right place?’ ‘Yes, you are.’ ‘I’ve made a connection now — do I keep it?’ ‘Yes, you do.’ And they keep going back and forth.”

Next, Allen and her team are studying whether these signals can be manipulated — for example, to stimulate neurons to repair synapses or form new ones in disorders of aging, such as Alzheimer’s disease.

Other authors included Isabella Farhy-Tselnicker, Matthew M. Boisvert, Hanqing Liu, Cari Dowling, Galina A. Erickson, Elena Blanco-Suarez, Maxim N. Shokhirev and Joseph R. Ecker from Salk; and Chen Farhy from Sanford Burnham Prebys.

The research was supported by the National Institutes of Health, the Pew Charitable Trusts, the Chan Zuckerberg Initiative, the Howard Hughes Medical Institute and the Hearst Foundations.

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Now, researchers from the Okinawa Institute of Science and Technology Graduate University (OIST) have decoded the genome of the mangrove tree, Bruguiera gymnorhiza, and revealed how this species regulates its genes in order to cope with stress. Their findings, published recently in New Phytologist, could one day be used to help other plants be more tolerant to stress.

“Mangroves are an ideal model system for studying the molecular mechanism behind stress tolerance, as they naturally cope with various stress factors,” said Dr. Matin Miryeganeh, first author of the study and a researcher in the Plant Epigenetics Unit at OIST.

Mangroves are an important ecosystem for the planet, protecting coastlines from erosion, filtering out pollutants from water and serving as a nursery for fish and other species that support coastal livelihoods. They also play a crucial role in combating global warming, storing up to four times as much carbon in a given area as a rainforest.

Despite their importance, mangroves are being deforested at an unprecedented rate, and due to human pressure and rising seas, are forecast to disappear in as little as 100 years. And genomic resources that could help scientists try to conserve these ecosystems have so far been limited.

The mangrove project, which was initially suggested by Sydney Brenner, one of the founding fathers of OIST, began in 2016, with a survey of mangrove trees in Okinawa. The scientists noticed that the mangrove tree, Bruguiera gymnorhiza, showed striking differences between individuals rooted in the oceanside, with high salinity, and those in the upper riverside, where the waters were more brackish.

“The trees were amazingly different; near the ocean, the height of the trees was about one to two meters, whereas further up the river, the trees grew as high as seven meters,” said senior author, Professor Hidetoshi Saze, who leads the Plant Epigenetics Unit. “But the shorter trees were not unhealthy — they flowered and fruited normally — so we think this modification is adaptive, perhaps allowing the salt-stressed plant to invest more resources into coping with its harsh environment.”

Unlike long-term evolutionary adaptation, which involves changes to the genetic sequence, adaptations to the environment that take place over an organism’s lifespan occur via epigenetic changes. These are chemical modifications to DNA that affect the activity of different genes, adjusting how the genome responds to different environmental stimuli and stresses. Organisms like plants, which can’t move to a more comfortable environment, rely heavily on epigenetic changes to survive.

Before focusing in on how the genome was regulated, the research team first extracted DNA from the mangrove tree, Bruguiera gymnorhiza, and decoded the genome for this species. They found that the genome contained 309 million base pairs, with a predicted 34,403 genes — a much larger genome than those for other known mangrove tree species. The large size was due to, for the most part, almost half of the DNA being made up of repeating sequences.

When the research team examined the type of repetitive DNA, they found that over a quarter of the genome consisted of genetic elements called transposons, or ‘jumping genes.’

Prof. Saze explained: “Active transposons are parasitic genes that can ‘jump’ position within the genome, like cut-and paste or copy-and-paste computer functions. As more copies of themselves are inserted into the genome, repetitive DNA can build up.”

Transposons are a big driver of genome evolution, introducing genetic diversity, but they are a double-edged sword. Disruptions to the genome through the movement of transposons are more likely to cause harm than provide a benefit, particularly when a plant is already stressed, so mangrove trees generally have smaller genomes than other plants, with suppressed transposons.

However, this isn’t the case for Bruguiera gymnorhiza, with the scientists speculating that as this mangrove species is more ancestral than others, it may not have evolved to have an efficient means of suppression.

The team then examined how activity of the genes, including the transposons, varied between individuals in the oceanside location with high salinity, and individuals in the less saline, brackish waters upriver. They also compared gene activity for mangrove trees grown in the lab, under two different conditions that replicated the oceanside and upriver salinity levels.

Overall, in both the oceanside individuals and those grown in high salinity conditions in the lab, genes involved in suppressing transposon activity showed higher expression, while genes that normally promote transposon activity showed lower expression. In addition, when the team looked specifically into transposons, they found evidence of chemical modifications on their DNA that lowered their activity.

“This shows that an important means of coping with saline stress involves silencing transposons,” said Dr. Miryeganeh.

The researchers also saw increases in the activity of genes involved in stress responses in plants, including those that activate when plants are water-deprived. Gene activity also suggested the stressed plants have lower levels of photosynthesis.

In future research, the team plan to study how seasons, changes in temperature and rainfall, also affect the activity of the mangrove tree genomes.

“This study acts as a foundation, providing new insights into how mangrove trees regulate their genome in response to extreme stresses,” said Prof. Saze. “More research is needed to understand how these changes in gene activity impact molecular processes within the plant cells and tissues and could one day help scientists create new plant strains that can better cope with stress.”

Dr. Barbara Veselka: “The results from our study represent a major step forward in the fields of biological anthropology, archaeology, and palaeopathology by opening up a variety of new possibilities for the study of health and activities related to sunlight exposure of numerous past populations that practiced cremation as their funerary ritual.” The study was published in the academic journal Scientific Reports.

Burning experiments

As part of the Brussels Bioarchaeology Lab (www.bb-lab.be) and the MARI and AMGC research groups, Dr. Barbara Veselka and Prof. Christophe Snoeck conducted burning experiments using an archaeological sample of 17 paired teeth.

Veselka: “We conducted burning experiments with teeth to assess the visibility of interglobular dentine, a mineralization defect in the dentine attributed to vitamin D deficiency, in teeth burned at temperatures varying from 600 to 900°C. Our study is the first to demonstrate that this defect is still visible at temperatures up to 900°C. This enables, for the first time, the identification of vitamin D deficiency in cremated human remains.”

Sixteen pairs of molars came from the cemetery of Koekelberg, Belgium, and one pair of canines came from the cemetery of the Broerekerk Zwolle, the Netherlands. From each pair, one unburnt tooth was microscopically evaluated via thin section for interglobular dentine. The other tooth was burned at temperatures of 600, 800, or 900°C. Duration of the burning process was 2, 4, or 8 hr. Each of the burnt teeth were examined for interglobular dentine and compared to their unburnt counterparts.

Study of sunlight exposure in the past

The fact that interglobular dentine is still visible in teeth burned at temperatures of 900°C is remarkable and allows the assessment of vitamin D deficiency in past populations. Moreover, the age at which the disease occurred, the severity of the deficit, and if the disease was recurrent can also be determined. Since most of vitamin D deficient cases can be attributed to limited exposure to adequate amounts of sunlight, information on the age of occurrence aids in the reconstruction of daily activities related to sunlight exposure.

Veselka: “This innovative study stimulates more bioarchaeological research, which will increase our understanding of the influence environmental and sociocultural variables have on individuals.”

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