III-Nitride Nanocrystal Based Green and Ultraviolet Optoelectronics

Extensive research efforts have been devoted to III-nitride based solid-state lighting since the first demonstration of high-brightness GaN-based blue light emitting diodes (LEDs). Over the past decade, the performance of GaN-based LEDs including external quantum efficiency (EQE), wall-plug efficiency, output power and lifetime has been improved significantly while the cost of GaN substrate has been reduced drastically. Although the development of blue and near ultraviolet (UV) LED is mature, achieving equally excellent performance in other wavelengths based on III-nitrides is still challenging. Especially, the significant efficiency droop in the green wavelength, known as “green gap” and the extremely low EQE in the UV regime, known as “UV threshold”, have become two most urgent issues. Green LEDs emit light that is most sensitive to human eye, implicating its importance in a variety of applications such as screen- and projection-based displays. UV light sources have a variety of applications including water and air purification, sterilization/disinfection of medical tools, medical diagnostics, phototherapy, sensing, which make solid-state deep UV (DUV) light sources with compactness, low operating power and long lifetime highly desirable. The deterioration of performance with green LEDs originates from increased indium content of the active region, which could degrade material quality and increase quantum confined Stark effect due to the high polarization fields in c-plane InGaN/GaN quantum wells (QWs). Meanwhile, limiting factors in III-nitride UV LEDs include low internal quantum efficiency due to large densities of dislocations, poor carrier injection efficiency and low light extraction efficiency. In this dissertation, we have investigated the molecular beam epitaxial growth, structural characterization, and electrical and optical properties of low-dimensional III-nitride nanocrystals as potential solutions to above-mentioned issues. Through a combination of theoretical calculation and experimental investigation, we show that defects formation in AlN could be precisely controlled under N-rich epitaxy condition. With further optimized p-type doping, AlN nanowire-based LEDs emitting at 210 nm were fabricated. We report DUV excitonic LEDs with the incorporation of monolayer GaN with emission wavelengths of ~238 nm, and exhibit suppressed Auger recombination, negligible efficiency droop and a small turn on voltage ~5 V. To enhance the light extraction efficiency of AlGaN nanowires grown on Si substrate, we demonstrated epitaxy of AlGaN nanowires on Al coated Si(001) substrate wherein Al film functions as a UV light reflective layer to enhance the light extraction efficiency. AlGaN nanowire-based DUV LEDs on Al film were successfully grown and fabricated and measured with a turn-on voltage of 7 V and an electroluminescence emission at 288 nm. Green-emitting InGaN/GaN nanowire LEDs on Si(001) substrate were demonstrated, wherein the active region and p-contact layer consist of InGaN/GaN disks-in-nanowires and Mg-doped GaN epilayers. The incorporation of planar p-GaN layer significantly reduces the fabrication complexity of nanowire-based devices and improves the robustness of electrical connection, leading to a more stable device operation. We also demonstrated micrometer scale InGaN photonic nanocrystal green LEDs with ultra-stable operation. The emission features a wavelength of ~548 nm and a spectral linewidth of ~4 nm, which is nearly five to ten times narrower than that of conventional InGaN QW LEDs in this wavelength range. Significantly, the device performance, in terms of the emission peak and spectral linewidth, is nearly invariant with injection current. Work presented in this thesis provides a new approach for achieving high-performance green and DUV LEDs by using III-nitride nanostructures.PHDElectrical and Computer EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/163122/1/ypwu_1.pd

Anisotropic Interfacial Force Field for Interfaces of Water with Hexagonal Boron Nitride

This study introduces an anisotropic interfacial potential that provides an accurate description of the van der Waals (vdW) interactions between water and hexagonal boron nitride (h-BN) at their interface. Benchmarked against the strongly constrained and appropriately normed (SCAN) functional, the developed force field demonstrates remarkable consistency with reference data sets, including binding energy curves and sliding potential energy surfaces for various configurations involving a water molecule adsorbed atop the h-BN surface. These findings highlight the significant improvement achieved by the developed force field in empirically describing the anisotropic vdW interactions of the water/h-BN heterointerfaces. Utilizing this anisotropic force field, molecular dynamics simulations demonstrate that atomically-flat pristine h-BN exhibits inherent hydrophobicity. However, when atomic-step surface roughness is introduced, the wettability of h-BN undergoes a significant change, leading to a hydrophilic nature. The calculated water contact angle (WCA) for the roughened h-BN surface is approximately 64{\deg}, which closely aligns with experimental WCA values ranging from 52{\deg} to 67{\deg}. These findings indicate the high probability of the presence of atomic steps on the surfaces of experimental h-BN samples, emphasizing the need for further experimental verification. The development of the anisotropic interfacial force field for accurately describing interactions at the water/h-BN heterointerfaces is a significant advancement in accurately simulating the wettability of two-dimensional (2D) materials, offering a reliable tool for studying the dynamic and transport properties of water at these interfaces, with implications for materials science and nanotechnology.Comment: 22 pages, 5 figure

Recurrent Temporal Revision Graph Networks

Temporal graphs offer more accurate modeling of many real-world scenarios than static graphs. However, neighbor aggregation, a critical building block of graph networks, for temporal graphs, is currently straightforwardly extended from that of static graphs. It can be computationally expensive when involving all historical neighbors during such aggregation. In practice, typically only a subset of the most recent neighbors are involved. However, such subsampling leads to incomplete and biased neighbor information. To address this limitation, we propose a novel framework for temporal neighbor aggregation that uses the recurrent neural network with node-wise hidden states to integrate information from all historical neighbors for each node to acquire the complete neighbor information. We demonstrate the superior theoretical expressiveness of the proposed framework as well as its state-of-the-art performance in real-world applications. Notably, it achieves a significant +9.6% improvement on averaged precision in a real-world Ecommerce dataset over existing methods on 2-layer models

Submicron full- color LED pixels for microdisplays and micro- LED main displays

We demonstrate a bottom- up approach to the construction of micro- LEDs as small as 150 nm in lateral dimension. Molecular beam epitaxy (MBE) is used to fabricate such nanostructured LEDs from InGaN, from the blue to red regions of the spectrum, providing a single material set useful for an entire RGB display.We demonstrate a bottom- up approach to the construction of micro- LEDs as small as 150 nm in lateral dimension. Molecular beam epitaxy (MBE) is used to fabricate such nanostructured LEDs from InGaN, from the blue to red regions of the spectrum, providing a single material set useful for an entire RGB display. We then consider collective effects of arrays of such LEDs.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155468/1/jsid899_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/155468/2/jsid899.pd

Boron nitride nanosheets/PNIPAM hydrogels with improved thermo-responsive performance

Thermo-responsive hydrogel is an important smart material. However, its slow thermal response rate limits the scope of its applications. Boron nitride nanosheet-reinforced thermos-responsive hydrogels, which can be controlled by heating, were fabricated by in situ polymerization of N-isopropylacrylamide in the presence of boron nitride nanosheets. The hydrogels exhibit excellent thermo-responsiveness and much enhanced thermal response rate than that of pure poly(N-isopropylacrylamide) hydrogels. Interestingly, the hydrogels can be driven to move in aqueous solution by heating. Importantly, the composite hydrogel is hydrophilic at a temperature below lower critical solution temperature (LCST), while it is hydrophobic at a temperature above LCST. Therefore, it can be used for quick absorption and release of dyes and oils from water. All these properties demonstrate the potential of hydrogel composites for water purification and treatment

Controlling Defect Formation of Nanoscale AlN: Toward Efficient Current Conduction of Ultrawide‐Bandgap Semiconductors

Ultrawide‐bandgap semiconductors such as AlN, BN, and diamond hold tremendous promise for high‐efficiency deep‐ultraviolet optoelectronics and high‐power/frequency electronics, but their practical application has been limited by poor current conduction. Through a combined theoretical and experimental study, it is shown that a critical challenge can be addressed for AlN nanostructures by using N‐rich epitaxy. Under N‐rich conditions, the p‐type Al‐substitutional Mg‐dopant formation energy is significantly reduced by 2 eV, whereas the formation energy for N‐vacancy related compensating defects is increased by ≈3 eV, both of which are essential to achieve high hole concentrations of AlN. Detailed analysis of the current−voltage characteristics of AlN p‐i‐n diodes suggests that current conduction is dominated by hole‐carrier tunneling at room temperature, which is directly related to the activation energy of Mg dopants. At high Mg concentrations, the dispersion of Mg acceptor energy levels leads to drastically reduced activation energy for a portion of Mg dopants, evidenced by the small tunneling energy of 67 meV, which explains the efficient current conduction and the very small turn‐on voltage (≈5 V) for the diodes made of nanoscale AlN. This work shows that nanostructures can overcome the dopability challenges of ultrawide‐bandgap semiconductors and significantly increase the efficiency of devices.Controlled defects formation and efficient current conduction of nanoscale AlN are realized. Under N‐rich epitaxy conditions, the formation energy for N‐vacancy related compensating defects is increased by nearly 3 eV, eliminating donor‐like compensating defects. Meanwhile, the p‐type Al‐substitutional Mg‐dopant formation energy is reduced by 2 eV, significantly enhancing Mg‐dopant incorporation and reducing hole carrier tunneling barrier.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/162750/3/aelm202000337-sup-0001-SuppMat.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/162750/2/aelm202000337_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/162750/1/aelm202000337.pd

Resveratrol Regulates Mitochondrial Biogenesis and Fission/Fusion to Attenuate Rotenone-Induced Neurotoxicity

It has been confirmed that mitochondrial impairment may underlie both sporadic and familial Parkinson’s disease (PD). Mitochondrial fission/fusion and biogenesis are key processes in regulating mitochondrial homeostasis. Therefore, we explored whether the protective effect of resveratrol in rotenone-induced neurotoxicity was associated with mitochondrial fission/fusion and biogenesis. The results showed that resveratrol could not only promote mitochondrial mass and DNA copy number but also improve mitochondrial homeostasis and neuron function in rats and PC12 cells damaged by rotenone. We also observed effects with alterations in proteins known to regulate mitochondrial fission/fusion and biogenesis in rotenone-induced neurotoxicity. Therefore, our findings suggest that resveratrol may prevent rotenone-induced neurotoxicity through regulating mitochondrial fission/fusion and biogenesis

Spectroscopic super-resolution fluorescence cell imaging using ultra-small Ge quantum dots

QMUL/CSC scholarship (2011611045); BBSRC grant (BB/J001473/1)