Nonpolar Substrates Research Articles

Aluminum in Frustrated Lewis Pair Chemistry.

This review article describes the development of the use of aluminum compounds in the chemistry of frustrated Lewis pairs (FLPs) over the last 14 years. It also discusses the synthesis, reactivity and catalytic applications of intermolecular, intramolecular and so-called hidden FLPs with phosphorus, nitrogen and carbon Lewis bases. The intrinsically higher acidity of aluminum compounds compared to their boron analogs opens up different reaction pathways. The results are presented in a more or less chronological order. It is shown that Al FLPs react with a variety of polar and non-polar substrates and form both stable adducts and reversibly activate bonds. Consequently, some catalytic applications of the title compounds were presented such as dimerization of alkynes, hydrogenation of tert-butyl ethylene and imines, C-F bond activation, reduction of CO2, dehydrogenation of amine borane and transfer of ammonia. In addition, various Al FLPs were used as initiators in polymerization reactions.

Structural and tribological studies on the interaction of porcine gastric mucin with non- and cationic-modified β-lactoglobulins

β-lactoglobulin (BLG) is the major whey protein with negative charges at neutral pH in aqueous media. Thus, the interaction with mucins, the major polyanionic component of mucus, is very weak due to the electrostatic repulsion between them. The present study postulates that cationization of BLG molecules may reverse the interaction characteristics between BLG and mucin from repulsive to associative. To this end, cationic-modified BLGs were prepared by grafting positively charged ethylenediamine (EDA) moieties into the negatively charged carboxyl groups on the aspartic and glutamic acid residues and compared with non-modified BLG upon mixing with porcine gastric mucin (PGM). To characterize the structural and conformational features of PGM, non/cationized BLGs, and their mixtures, various spectroscopic approaches, including zeta potential, dynamic light scattering (DLS), and circular dichroism (CD) spectroscopy were employed. Importantly, we have taken surface adsorption with optical waveguide lightmode spectroscopy (OWLS), and tribological properties with pin-on-disk tribometry at the sliding interface as the key approaches to determine the interaction nature between them as mixing PGM with polycations can lead to synergistic lubrication at the nonpolar substrate in neutral aqueous media as a result of an electrostatic association. All the spectroscopic studies and a substantial improvement in lubricity collectively supported a tenacious and associative interaction between PGM and cationized BLGs, but not between PGM and non-modified BLG. This study demonstrates a unique and successful approach to intensify the interaction between BLG and mucins, which is meaningful for a broad range of disciplines, including food science, macromolecular interactions, and biolubrication etc.

Transparent PDMS Surfaces with Covalently Attached Lubricants for Enhanced Anti-adhesion Performance.

Liquid-like surfaces featuring slippery, omniphobic, covalently attached liquids (SOCALs) reduce unwanted adhesion by providing a molecularly smooth and slippery surface arising from the high mobility of the liquid chains. Such SOCALs are commonly prepared on hard substrates, such as glass, wafers, or metal oxides, despite the importance of nonpolar elastomeric substrates, such as polydimethylsiloxane (PDMS) in anti-fouling or nonstick applications. Compared to polar elastomers, hydrophobic PDMS elastomer activation and covalent functionalization are significantly more challenging, as PDMS tends to display fast hydrophobic recovery upon activation as well as superficial cracking. Through the extraction of excess PDMS oligomers and fine-tuning of plasma activation parameters, homogeneously functionalized PDMS with fluorinated polysiloxane brushes could be obtained while at the same time reducing crack formation. Polymer brush mobility was increased through the addition of a smaller molecular silane linker to exhibit enhanced dewetting properties and reduced substrate swelling compared to functionalizations featuring hydrocarbon functionalities. Linear polymer brushes were verified by thermogravimetric analysis. The optical properties of PDMS remained unaffected by the activation in high-frequency plasma but were impacted by low-frequency plasma. Drastic decreases in solid adhesion of not just complex contaminants but even ice could be shown in horizontal push tests, demonstrating the potential of SOCAL-functionalized PDMS surfaces for improved nonstick applications.

Saltwater-Induced Rapid Gelation of Photoredox-Responsive Mucomimetic Hydrogels.

Shear-thinning hydrogels represent an important class of injectable soft materials that are often used in a wide range of biomedical applications. Creation of new shear-thinning materials often requires that factors such as viscosity, injection rate/force, and needle gauge be evaluated to achieve efficient delivery, while simultaneously protecting potentially sensitive cargo. Here, a new approach to establishing shear-thinning hydrogels is reported where a host-guest cross-linked network initially remains soluble in deionized water but is kinetically trapped as a viscous hydrogel once exposed to saltwater. The shear-thinning properties of the hydrogel is then "switched on" in response to heating or exposure to visible light. These hydrogels consist of polynorbornene-based bottlebrush copolymers with porphyrin- and oligoviologen-containing side chains that are cross-linked through the reversible formation of β-cyclodextrin-adamantane inclusion complexes. The resultant viscous hydrogels display broad adhesive properties across polar and nonpolar substrates, mimicking that of natural mucous and thus making it easier to distribute onto a wide range of surfaces. Additional control over the hydrogel's mechanical properties (storage/loss moduli) and performance (adhesion) is achieved post-injection using a low-energy (blue light) photoinduced electron-transfer process. This work envisions these injectable copolymers and multimodal hydrogels can serve as versatile next-generation biomaterials capable of light-based mechanical manipulation post-injection.

Silane Effects on Adhesion Enhancement of 2K Polyurethane Adhesives.

With excellent properties such as great flexibility, outstanding chemical resistance, and superb mechanical strength, two-part polyurethane (2K PU) adhesives have been widely applied in many applications, including those in transportation and construction. Despite the extensive use, their adhesion to nonpolar polymer substrates still needs to be improved and has been widely studied. The incorporation of silane molecules and the use of plasma treatment on substrate surfaces are two popular methods to increase the adhesion of 2K PU adhesives, but their detailed adhesion enhancement mechanisms are still largely unknown. In this research, sum frequency generation (SFG) vibrational spectroscopy was used to probe the influence of added or coated silanes on the interfacial structure at the buried polypropylene (PP)/2K PU adhesive interface in situ. How plasma treatment on PP could improve adhesion was also investigated. To achieve maximum adhesion, two methods to involve silanes were studied. In the first method, silanes were directly mixed with the 2K PU adhesive before use. In the second method, silane molecules were spin-coated onto the PP substrate before the PU adhesive applied. It was found that the first method could not improve the 2K PU adhesion to PP, while the second method could substantially enhance such adhesion. SFG studies demonstrated that with the second method silane molecules chemically reacted at the interface to connect PP and 2K PU adhesive to improve the adhesion. With the first method, silane molecules could not effectively diffuse to the interface to enhance adhesion. In this research, plasma treatment was also found to be a useful method to improve the adhesion of the 2K PU adhesive to nonpolar polymer materials.

Solvent Engineering for Nonpolar Substrate Glycosylation Catalyzed by the UDP-Glucose-Dependent Glycosyltransferase UGT71E5: Intensification of the Synthesis of 15-Hydroxy Cinmethylin β-d-Glucoside.

Sugar nucleotide-dependent glycosyltransferases are powerful catalysts of the glycosylation of natural products and xenobiotics. The low solubility of the aglycone substrate often limits the synthetic efficiency of the transformation catalyzed. Here, we explored different approaches of solvent engineering for reaction intensification of β-glycosylation of 15HCM (a C15-hydroxylated, plant detoxification metabolite of the herbicide cinmethylin) catalyzed by safflower UGT71E5 using UDP-glucose as the donor substrate. Use of a cosolvent (DMSO, ethanol, and acetonitrile; ≤50 vol %) or a water-immiscible solvent (n-dodecane, n-heptane, n-hexane, and 1-hexene) was ineffective due to enzyme activity and stability, both impaired ≥10-fold compared to a pure aqueous solvent. Complexation in 2-hydroxypropyl-β-cyclodextrin enabled dissolution of 50 mM 15HCM while retaining the UGT71E5 activity (∼0.32 U/mg) and stability. Using UDP-glucose recycling, 15HCM was converted completely, and 15HCM β-d-glucoside was isolated in 90% yield (∼150 mg). Collectively, this study highlights the requirement for a mild, enzyme-compatible strategy for aglycone solubility enhancement in glycosyltransferase catalysis applied to glycoside synthesis.

A water-based adhesive with polar recognition both for TPO substrate and WPU coating

Thermoplastic polyolefin (TPO) is widely used in automotive interior materials, but it has long been faced with the challenge of weak adhesion between water-based polyurethane (WPU) coating and substrate. Herein, chlorinated polypropylene (CPP) was grafted by styrene (St) firstly, then a water-based CPP adhesives with polar recognition both for non-polar TPO substrate and polar WPU coating were prepared via phase conversion methods, and the adhesives properties as well as the adhesive mechanism for different polar substrates were investigated. The introduction of polystyrene side-chain has improved the recognition ability of CPP adhesive to non-polar TPO substrate and the interaction force between interfaces. Compared with unmodified CPP adhesive, T-peeling strength of modified one has increased by 35 % and reached 24.2 N/cm, meeting the adhesive requirements of TPO as an automobile interior material. Adhesive mechanism study reveals that polar mutual recognition CPP adhesive plays a bridging role, one end recognizes the non-polar TPO substrate and another end recognizes the polar WPU surface coating, and forms a strong bond between the guest and the host through their respective interfacial force.

Paraoxonase 1 and atherosclerosis.

Paraoxonase 1 (PON1), residing almost exclusively on HDL, was discovered because of its hydrolytic activity towards organophosphates. Subsequently, it was also found to hydrolyse a wide range of substrates, including lactones and lipid hydroperoxides. PON1 is critical for the capacity of HDL to protect LDL and outer cell membranes against harmful oxidative modification, but this activity depends on its location within the hydrophobic lipid domains of HDL. It does not prevent conjugated diene formation, but directs lipid peroxidation products derived from these to become harmless carboxylic acids rather than aldehydes which might adduct to apolipoprotein B. Serum PON1 is inversely related to the incidence of new atherosclerotic cardiovascular disease (ASCVD) events, particularly in diabetes and established ASCVD. Its serum activity is frequently discordant with that of HDL cholesterol. PON1 activity is diminished in dyslipidaemia, diabetes, and inflammatory disease. Polymorphisms, most notably Q192R, can affect activity towards some substrates, but not towards phenyl acetate. Gene ablation or over-expression of human PON1 in rodent models is associated with increased and decreased atherosclerosis susceptibility respectively. PON1 antioxidant activity is enhanced by apolipoprotein AI and lecithin:cholesterol acyl transferase and diminished by apolipoprotein AII, serum amyloid A, and myeloperoxidase. PON1 loses this activity when separated from its lipid environment. Information about its structure has been obtained from water soluble mutants created by directed evolution. Such recombinant PON1 may, however, lose the capacity to hydrolyse non-polar substrates. Whilst nutrition and pre-existing lipid modifying drugs can influence PON1 activity there is a cogent need for more specific PON1-raising medication to be developed.

High-Performance Integrated rGO-[Polymeric Ionic Liquid] [Heteropolyanions] for Catalytic Degradation of Azo Dyes

Polymeric ionic liquid (such as poly[ViEtIm]Br)-modified reduced graphene oxide (rGO), rGO-poly[ViEtIm]Br, was nominated as an open carrier to construct a degradation platform. The large specific surface of rGO together with the anion-exchange property of poly[ViEtIm]Br terminals led to the wide growth of heteropolyanions (like [PW12O40]3-, [PMo12O40]3-, and [SiW12O40]4-), thus assembling the integrated catalyst rGO-poly[ViEtIm][heteropolyanions]. The grafted poly[ViEtIm]Br provided an anchor point to interlink the polar heteropolyanions and the nonpolar rGO substrate, endowing this graphene-based catalyst with excellent dispersibility. The adequate exposure of heteropolyanions further promoted the decolorization capability during the degradation procedure. Morphology, structure, and properties of materials were confirmed and monitored via transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-vis) spectroscopy, etc. rGO-poly[ViEtIm][PW12O40] was selected as the optimal catalyst with degradation efficiency toward methyl orange reaching 98.7% in 3 h. In addition, the excellent structural stability of the catalyst improved the decolorization efficiency, which reached 95% after recycling five times.

3D Printable One‐Part Carbon Nanotube‐Elastomer Ink for Health Monitoring Applications

Abstract3D printing of conductive elastomers is a promising route to personalized health monitoring applications due to its flexibility and biocompatibility. Here, a one‐part, highly conductive, flexible, stretchable, 3D printable carbon nanotube (CNT)‐silicone composite is developed and thoroughly characterized. The one‐part nature of the inks: i) enables printing without prior mixing and cures under ambient conditions; ii) allows direct dispensing at ≈100 µm resolution printability on nonpolar and polar substrates; iii) forms both self‐supporting and high‐aspect‐ratio structures, key aspects in additive biomanufacturing that eliminate the need for sacrificial layers; and iv) lends efficient, reproducible, and highly sensitive responses to various tensile and compressive stimuli. The high electrical and thermal conductivity of the CNT‐silicone composite is further extended to facilitate use as a flexible and stretchable heating element, with applications in body temperature regulation, water distillation, and dual temperature sensing and Joule heating. Overall, the facile fabrication of this composite points to excellent synergy with direct ink writing and can be used to prepare patient‐specific wearable electronics for motion detection and cardiac and respiratory monitoring devices and toward advanced personal health tracking and bionic skin applications.

Nanorod-interlayered thin film composite membranes for ultrafast nanofiltration

Customizing high-performance nanofiltration membranes that can break the trade-off effect is critical to the development of membrane separation. Building an intermediate layer between the support and the selective layer to accelerate water transport is considered to be an effective solution to this issue. However, the synthesis of interlayered thin-film composite membranes (i-TFCs) on non-polar substrates with large pores and high porosity remains challenging. Here, we report a high-performance i-TFC membrane with FeOOH nanorods as interlayers. The porous and robust nanorods greatly increased the water transport paths and the mechanical strength of the membrane. Under the high pressure of 20 bar, the i-TFC membrane still remained a high salt rejection above 96 % while showing a high pure water permeance up to 48.9 L·m−2·h−1·bar−1. Furthermore, the presence of nanorods enhanced the hydrophilicity and compactness of the substrate, leading to the formation of a 23-nm thick polyamide layer without nanorod structures. The versatility of the strategy was also demonstrated with three different hydrophobic substrates. This work will thus provide further insights into the development of high-performance membranes for molecular separations.

Polar and Non-Polar Zn1−xMgxO:Sb Grown by MBE

The article presents a systematic study of Sb-doped Zn1-xMgxO layers, with various concentrations of Mg, that were successfully grown by plasma-assisted MBE on polar a- and c-oriented and non-polar r-oriented sapphire substrates. X-ray diffraction confirmed the polar c-orientation of alloys grown on c-and a-oriented sapphire and non-polar structures grown on r-oriented substrates. A uniform depth distribution of the Sb dopant at level of 2 × 1020 cm-3 was determined by SIMS measurements. Raman spectroscopy revealed the presence of Sb-related modes in all samples. It also showed that Mg alloying reduces the compressive strain associated with Sb doping in ZnO. XPS analysis indicates that the chemical state of Sb atoms in ZnMgO is 3+, suggesting a substitutional position of SbZn, probably associated with two VZn vacancies. Luminescence and transmission spectra were measured to determine the band gaps of the Zn1-xMgxO layers. The band gap energies extracted from the transmittance measurements differ slightly for the a, c, and r substrate orientations, and the differences increase with increasing Mg content, despite identical growth conditions. The differences between the energy gaps, determined from transmission and PL peaks, are closely correlated with the Stokes shift and increase with the Mg content in the analyzed series of ZnMgO layers.

One-Pot Synthesis of UPy-Functionalized Nanocellulose under Mechanochemical Synergy for High-Performance Epoxy Nanocomposites.

The high strength, high specific surface area, excellent biocompatibility, and degradability of nanocellulose (NCC) make it a potential reinforcing phase for composite materials. However, the polyhydroxyl property of NCC renders it prone to self-aggregation and it has weak interfacial compatibility with non-polar substrates, limiting its enhancement performance for composite materials. Therefore, based on the high reactivity of NCC, the chemical modification of NCC to introduce functional groups is the basis for effectively reducing its self-aggregation, improving its interfacial compatibility with the polymer matrix, and creating nanocellulose-based functional materials. The existing functional modifications of NCC have limitations; they require cumbersome steps, generate low yields, and are environmentally unfriendly. Herein, ureido-pyrimidinone (UPy) was introduced to NCC through a sustainable and high-efficiency avenue formed by the mechanochemical synergy of microwaves and ultrasonication. The obtained UPy-modified nanocellulose (NCC-UPy) exhibited a rod-like shape, with a length of 200–300 nm and a width of 20–30 nm, which presented oriented and stable dispersion in an aqueous medium, and the zeta potential reached −40 mV. Moreover, NCC-UPy had good thermostability (>350 °C) and high crystallinity (82.5%) within the crystal type of cellulose I. Using the as-prepared NCC-UPy as a molecular bridge, it was organically combined with epoxy resin through multiple hydrogen bonds to construct a nanocomposite membrane with superior mechanical strength and thermal stability. The results revealed that NCC-UPy dispersed uniformly in the epoxy matrix without aggregating and that the interfacial compatibility was good, leading to an 87% increase in the tensile strength of the formed nanocomposite membrane when 0.5 wt.% NCC-UPy was loaded. It was proved that NCC-UPy had remarkable reinforcing potential and effective stress transfer capacity for composites. Consequently, this study may open the door to the development of a one-pot green approach for undertaking the functional modification of NCC, and it is of great significance for the development of NCC-based nanocomposites.

The Structural Evolution of Semipolar (11-22) Plane AlN Tem-Plate on m-Plane Sapphire Prepared by Sputtering and High Temperature Annealing.

In this work, the epitaxial semipolar (11–22) AlN was prepared on nonpolar m-sapphire substrate by combining sputtering and high-temperature annealing. According to our systematic measurements and analysis from XRD, Raman spectra, and AFM, the evolution of crystalline structure and morphology was investigated upon increasing AlN thickness and annealing duration. The annealing operation intensively resets the lattice and improves the crystalline quality. By varying the film thickness, the contribution from the AlN-sapphire interface on crystalline quality and lattice parameters during the annealing process was investigated, and its contribution was found to be not so obvious when the thickness increased from 300 nm to 1000 nm. When the annealing was performed under durations from 1 to 5 h, the crystalline quality was found unchanged; meanwhile, the evolution of morphology was pronounced, and it means the crystalline reorganization happens prior to morphology reset. Finally, the annealing treatment enabled a zig-zag morphology on the AlN template along the sapphire [0001] direction in the plane, which potentially affects the subsequent device epitaxy process. Therefore, our results act as important experience for the semipolar nitride semiconductor laser device preparation, particularly for the epitaxy of microcavity structure through providing the crystalline evolution.

Two-dimensional confinement of excitons at the interface in nonpolar MgZnO/ZnO heterostructures

We report on the polarity-dependent excitonic emission at the interface in MgZnO/ZnO heterostructures grown on both polar and nonpolar ZnO single-crystal substrates. Structural and morphological analyses confirm that the heterostructures grow homoepitaxially on nonpolar $m$-plane and $a$-plane crystal substrates. The nonpolar heterostructures are investigated by depth-resolved cathodoluminescence spectroscopy revealing an excitonic interface emission centered at 3.12 eV, identified as the $H$-band signature of indirect excitons. These results reveal the formation of robust indirect excitons confined at the MgZnO/ZnO interface due to the presence of an epitaxially strained interfacial layer near the interface in the MgZnO, which is confirmed by high-order x-ray diffraction. It is also found that the $H$-band emission originates from the self-absorption of the ZnO free exciton emission in the proximity of the interface. Temperature-dependent studies of the $H$ band yields a thermal activation energy of 47 meV for the indirect exciton in the nonpolar heterojunctions. This activation energy is the largest among all free and bound excitons in ZnO and is much higher than the values found in AlGaN/GaN and MgZnO/ZnO/sapphire hetrostructures. Our findings indicate that the optical properties of nonpolar heterostructures are strongly influenced by the piezoelectric effect.

Olefin-Surface Interactions: A Key Activity Parameter in Silica-Supported Olefin Metathesis Catalysts.

Molecularly defined and classical heterogeneous Mo-based metathesis catalysts are shown to display distinct and unexpected reactivity patterns for the metathesis of long-chain α-olefins at low temperatures (<100 °C). Catalysts based on supported Mo oxo species, whether prepared via wet impregnation or surface organometallic chemistry (SOMC), exhibit strong activity dependencies on the α-olefin chain length, with slower reaction rates for longer substrate chain lengths. In contrast, molecular and supported Mo alkylidenes are highly active and do not display such dramatic dependence on the chain length. State-of-the-art two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy analyses of postmetathesis catalysts, complemented by Fourier transform infrared (FT-IR) spectroscopy and molecular dynamics calculations, evidence that the activity decrease observed for supported Mo oxo catalysts relates to the strong adsorption of internal olefin metathesis products because of interactions with surface Si–OH groups. Overall, this study shows that in addition to the nature and the number of active sites, the metathesis rates and the overall catalytic performance depend on product desorption, even in the liquid phase with nonpolar substrates. This study further highlights the role of the support and active site composition and dynamics on activity as well as the need for considering adsorption in catalyst design.

Atomic-Scale Mechanism of Spontaneous Polarity Inversion in AlN on Nonpolar Sapphire Substrate Grown by MOCVD.

The performance of nitride devices is strongly affected by their polarity. Understanding the polarity determination and evolution mechanism of polar wurtzite nitrides on nonpolar substrates is therefore critically important. This work confirms that the polarity of AlN on sapphire prepared by metal-organic chemical vapor deposition is not inherited from the nitrides/sapphire interface as widely accepted, instead, experiences a spontaneous polarity inversion during the growth. It is found that at the initial growth stage, the interface favors the nitrogen-polarity, rather than the widely accepted metal-polarity or randomly coexisting. However, the polarity subsequently converts into the metal-polar situation, at first locally then expanding into the whole area, driven by the anisotropy of surface energies, which results in universally existing inherent inverse grain boundaries. Furthermore, vertical two-dimensional electron accumulation originating from the lattice symmetry breaking at the inverse grain boundary is first revealed. This work identifies another cause of high-density defects in nitride epilayers, besides lattice mismatch induced dislocations. These findings also offer new insights into atomic structure and determination mechanism of polarity in nitrides, providing clues for its manipulation toward the novel hetero-polarity devices.

Enhanced catalytic performance of palladium nanoparticles in MOFs by channel engineering

The embedding of metal nanoparticles in MOFs is highly relevant in catalysis research. The MOF matrix prevents the NMP agglomeration. Furthermore, the MOF can easily be functionalized to design an optimal chemical environment for the benefit of a catalytic reaction. We report on a series of [email protected] materials, namely [email protected] and [email protected]nip, consisting of the structural prototype CuBTC (= [Cu3BTC2]; BTC = benzene-1,3,5-tricarboxylate). Pd NPs were incorporated by rapid “bottle-around-the-ship” encapsulation. Regulation of the microenvironment around the Pd NPs by using alkoxy-functionalized fragmented linkers H2Cnip (n = 3, 6, 10) allows to adjust the hydrophobicity. These modifications significantly improve the catalytic activity for alkene hydrogenation compared with [email protected] Our work suggests that the “channel engineering,” i.e., the introduction of hydrophobic alkyl chains to the MOF linkers, increases the interactions with non-polar substrates, leading to a facilitated substrate diffusion in the host, which is an efficient way to optimize the [email protected] catalysts.

Two-Inch Aluminum Nitride (AIN) Single Crystal Growth for Commercial Applications

Aluminum Nitride (AlN) is an Ultra-wide Bandgap (UWBG) semiconductor that attracts significant interest for its potential application for high power switches, high power density, high frequency and high operating temperature for rf. Currently, AlN substrates are being used for ultraviolet (UV) light emitters at wavelengths shorter than 300nm including UVC Light Emitting Diodes (UVC LED) as well as laser diodes. Successful commercialization of the aluminum nitride as an UWBG substrate depends on several factors such as substrate availability, size, quality, and cost. Here we report on the current status of the bulk growth of AlN single-crystals as well as their subsequent processing for making substrates.The AlN crystals are grown using a proprietary physical vapor transport (PVT) technique where a sublimation-recondensation approach is utilized. This method shows very high potential in terms of growing large (> 2-in in diameter) bulk AlN crystals. However, some challenges have yet to be resolved en route to 100 mm diameter crystals. The crystal growth is carried out in a Tungsten crucible that contains the AlN seed and the source material. The AlN source material sublimes and decomposes into Aluminum atoms and Nitrogen molecules. These species are then transported to the seed where they form AlN and incorporate into the growing crystal along the c-axis. Aluminum nitride is used as a native seed utilizing the Al-polar face. Increasing the crystal dimensions demands careful tailoring of thermal gradients to lower the thermally induced stresses and achieve high crystalline quality. Currently, AlN single crystals with diameters > 2 inch are grown using the method described above and larger diameters are possible with appropriate furnace design. Large bulk crystals could be beneficial for production of non-polar and semi-polar substrates which are used to reduce the internal electric fields due to polarization.The AlN crystals are sliced into c-face oriented wafers and subsequently polished. Several substrate parameters are tracked such as the wafer bow, thickness variation, surface roughness, and sub-surface damage. The two-inch AlN wafers have a radius of curvature ≥ 20 m, RMS ≤ 5 Å, and zero sub-surface damage. The crystalline quality is measured by X-ray diffractometry (XRD) rocking curves, etch pit density, and photo-elastic birefringence pattern (cross-polar imaging). Narrow rocking curves for both symmetric and asymmetric reflections with full width at half maximum (FWHM) of 6 - 10 arcsec are measured across the whole the wafer area. The AlN wafers are transparent in the UV region. The graph shows a 52-point map of the absorption coefficient values 8 – 10 cm-1 at 265nm wavelength for a 2-in AlN substrate. The very low UV absorption is linked to the material purity measured by Secondary Ion Mass Spectrometry (SIMS) and Glow Discharge Mass Spectrometry (GDMS). Excellent material purity ensures achievement of high thermal conductivity, a crucial parameter for an UWBG substrate. We measure the thermal conductivity using a Xenon flash method conforming to the ASTM E1461-01 or ASTM E1461-13 standards. Using this method directly on 2-inch AlN substrate we obtain room temperature thermal conductivity 293 W/m*K at room temperature (RT) and with very high slope of about -1.6 for the temperature range from RT to 300°C suggesting minimal phonon scattering.AlN substrates made using the above described methods are used to fabricate high performance, long lifetime UVC LEDs for air purification, water disinfection, surface cleaning, and environmental sensing. High quality pseudomorphic epitaxial layers are grown using Metal-Organic Chemical Vapor Deposition (MOCVD) process to form the device structure. Commercially available WD product (Klaran™) achieve germicidal power outputs that exceed 80 mW with drive currents up to 700 mA at a nominal wavelength of 265 nm. The devices feature long lifetimes exceeding 6000 hours at L50B10 as well as a wide radiation pattern with a viewing angle of 130°. Figure 1

Nanoscale structural investigation of Zn1–x Mg x O alloy films on polar and nonpolar ZnO substrates with different Mg contents* *Project supported by the National Natural Science Foundation of China (Grant No. 11804050).

Zn1–x Mg x O alloy films are important deep ultraviolet photoelectric materials. In this work, we used plasma-assisted molecular beam epitaxy to prepare Zn1–x Mg x O films with different magnesium contents on polar (0001) and nonpolar () ZnO substrates. The nanoscale structural features of the grown alloy films as well as the interfaces were investigated. It was observed that the cubic phases of the alloy films emerged when the Mg content reached 20% and 37% for the alloy films grown on the (0001) and () ZnO substrates, respectively. High-resolution transmission electron microscopy images revealed cubic phases without visible hexagonal phases for the alloy films with more than 70% magnesium, and the cubic phases exhibited three-fold and two-fold rotations for the alloy films on the polar (0001) and nonpolar () ZnO substrates, respectively. This work aims to provide references for monitoring the Zn1–x Mg x O film structure with respect to different substrate orientations.