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Monolayer hexagonal boron nitride (hBN) has been widely considered a fundamental building block for 2D heterostructures and devices. However, the controlled and scalable synthesis of hBN and its 2D heterostructures has remained a daunting challenge. Here, an hBN/graphene (hBN/G) interface-mediated growth process for the controlled synthesis of high-quality monolayer hBN is proposed and further demonstrated. It is discovered that the in-plane hBN/G interface can be precisely controlled, enabling the scalable epitaxy of unidirectional monolayer hBN on graphene, which exhibits a uniform moiré superlattice consistent with single-domain hBN, aligned to the underlying graphene lattice. Furthermore, it is identified that the deep-ultraviolet emission at 6.12 eV stems from the 1s-exciton state of monolayer hBN with a giant renormalized direct bandgap on graphene. This work provides a viable path for the controlled synthesis of ultraclean, wafer-scale, atomically ordered 2D quantum materials, as well as the fabrication of 2D quantum electronic and optoelectronic devices.

Significance Despite a century of scientific fascination with nacre’s periodic mesostructure, there remains a fundamental question of whether long-range order exists. In this work, the stochastic growth that leads to mesoscale order in the nacreous pearl is revealed by quantifying its structural coherence across entire pearl specimens. We find that mollusks strike a balance between preserving translational symmetry and minimizing thickness variation of layers by creating a paracrystal with medium-range order. Self-correcting growth processes allow pearls to quickly attenuate disorder, accommodate topological defects in tablet structure, and maintain order throughout a fluctuating external environment. These observations were made possible by characterizing the entire structure of Akoya “keshi” pearls (∼3 mm) at high resolution (, A pearl’s distinguished beauty and toughness are attributable to the periodic stacking of aragonite tablets known as nacre. Nacre has naturally occurring mesoscale periodicity that remarkably arises in the absence of discrete translational symmetry. Gleaning the inspiring biomineral design of a pearl requires quantifying its structural coherence and understanding the stochastic processes that influence formation. By characterizing the entire structure of pearls (∼3 mm) in a cross-section at high resolution, we show that nacre has medium-range mesoscale periodicity. Self-correcting growth mechanisms actively remedy disorder and topological defects of the tablets and act as a countervailing process to long-range disorder. Nacre has a correlation length of roughly 16 tablets (∼5.5 µm) despite persistent fluctuations and topological defects. For longer distances (>25 tablets, ∼8.5 µm), the frequency spectrum of nacre tablets follows f−1.5 behavior, suggesting that growth is coupled to external stochastic processes—a universality found across disparate natural phenomena, which now includes pearls.

Producing hydrogen by unassisted solar water splitting is one essential step to make direct solar fuel conversion a viable energy source. To date, however, there has been no demonstration of stable...

Group III-nitride semiconductors exhibit many ideal characteristics for solar water splitting, including a tunable energy bandgap across nearly the entire solar spectrum and suitable band edge positions for water oxidation and proton reduction under visible and near-infrared light irradiation. To date, however, the best reported energy conversion efficiency for III-nitride semiconductor photocathodes is still below 1%. Here we report on the demonstration of a relatively efficient p-type In0.42Ga0.58N photocathode, which is monolithically integrated on an n-type nonplanar Si wafer through a GaN nanowire tunnel junction. The open pillar design, together with the nonplanar Si wafer can significantly maximize light trapping, whereas the tunnel junction reduces the interfacial resistance and enhances the extraction of photo-generated electrons. In addition, photodeposited Pt nanoparticles on InGaN nanowire surfaces significantly improve the cathodic performance. The nanowire photocathode exhibits a photocurrent density of 12.3 mA cm−2 at 0 V vs. RHE and an onset potential of 0.79 V vs. RHE under AM 1.5 G one-sun illumination. The maximum applied bias photon-to-current efficiency reaches 4% at ~0.52 V vs. RHE, which is one order of magnitude higher than the previously reported values for III-nitride photocathodes. Significantly, no performance degradation was measured for over 30 h solar water splitting with a steady photocurrent density ~12 mA cm−2 without using any extra surface protection, which is attributed to the spontaneous formation of N-terminated surfaces of InGaN nanowires to protect against photocorrosion.

We have investigated the nature of Al0.56Ga0.44N/Al0.62Ga0.38N multiquantum wells grown by plasma-assisted molecular beam epitaxy for application in deep-ultraviolet light emitters. Excitation and temperature-dependent and time-resolved photoluminescence measurements and transmission and reflectance spectroscopy have been complemented by high-angle annular dark field scanning transmission electron microscopy. The 3 nm quantum wells are characterized by interface roughness having a height of 0.3–1 nm and the maximum value is in excellent agreement with values obtained from calculations done to analyze the measured photoluminescence lineshape. The radiative lifetime increases with temperature, suggesting the role of electron-hole scattering to cool photoexcited carriers to the ground state of the quantum wells.

Rutile compounds have exotic functional properties that can be applied for various electronic applications; however, the limited availability of epitaxial substrates has restricted the study of rutile thin films to a limited range of lattice parameters. Here, rutile GeO2 is demonstrated as a new rutile substrate with lattice parameters of [Formula: see text] and [Formula: see text]. Rutile GeO2 single crystals up to 4 mm in size are grown by the flux method. X-ray diffraction reveals high crystallinity with a rocking curve having a full width half-maximum of 0.0572°. After mechanical polishing, a surface roughness of less than 0.1 nm was obtained, and reflection high-energy electron diffraction shows a crystalline surface. Finally, epitaxial growth of (110)-oriented TiO2 thin films on GeO2 substrates was demonstrated using molecular beam epitaxy. Templated by rutile GeO2 substrates, our findings open the possibility of stabilizing new rutile thin films and strain states for the tuning of physical properties.

A pearl's distinguished beauty and toughness are attributable to the periodic stacking of aragonite tablets known as nacre. Nacre has naturally occurring mesoscale periodicity that remarkably arises in the absence of discrete translational symmetry. Gleaning the inspiring biomineral design of a pearl requires quantifying its structural coherence and understanding the stochastic processes that influence formation. By characterizing the entire structure of pearls (~3 mm) in cross-section at high resolution, we show nacre has medium-range mesoscale periodicity. Self-correcting growth mechanisms actively remedy disorder and topological defects of the tablets and act as a countervailing process to long-range disorder. Nacre has a correlation length of roughly 16 tablets (~5.5 um) despite persistent fluctuations and topological defects. For longer distances (> 25 tablets, ~8.5 um), the frequency spectrum of nacre tablets follows f^(-1.5) behavior suggesting growth is coupled to external stochastic processes-a universality found across disparate natural phenomena which now includes pearls., 19 pages, 4 figures, PNAS 118(42)

Compelling evidence suggests distinct correlated electron behavior may exist only in clean 2D materials such as 1T-TaS2. Unfortunately, experiment and theory suggest that extrinsic disorder in free standing 2D layers disrupts correlation-driven quantum behavior. Here we demonstrate a route to realizing fragile 2D quantum states through endotaxial polytype engineering of van der Waals materials. The true isolation of 2D charge density waves (CDWs) between metallic layers stabilizes commensurate long-range order and lifts the coupling between neighboring CDW layers to restore mirror symmetries via interlayer CDW twinning. The twinned-commensurate charge density wave (tC-CDW) reported herein has a single metal–insulator phase transition at ~350 K as measured structurally and electronically. Fast in-situ transmission electron microscopy and scanned nanobeam diffraction map the formation of tC-CDWs. This work introduces endotaxial polytype engineering of van der Waals materials to access latent 2D ground states distinct from conventional 2D fabrication., Correlated quantum states in free-standing two-dimensional materials are susceptible to defects and thermal disorder. Here, the authors demonstrate two-dimensional ordered charge density wave states above room temperature in clean interleaved polytype heterostructures of a van der Waals material.

The causes for the efficiency droop of AlGaN UV LEDs will be investigated. Auger recombination coefficients of AlGaN multiple quantum wells with different Al composition have been measured. These results have been used to design efficient AlGaN LEDs with emission wavelength lower than 255 nm.

We have demonstrated a well-ordered In-rich single crystalline InGaN nanoridge array grown on GaN/sapphire substrate using the integration of top-down etching and bottom-up molecular beam epitaxy. During the initial growth of InGaN on a patterned GaN/sapphire substrate, a (1011) r-plane predominantly forms, suppressing the growth in [1011] crystal direction and resulting in a triangular InGaN nanoprism. As the growth proceeds further, a narrow (∼50 nm) single-crystal fin-shaped InGaN nanoridge forms atop the InGaN nanoprism structure. The resulting narrow fin-shaped InGaN nanoridge structure shows extremely strong photoluminescence (PL) intensity with a center wavelength at 524–560 nm and narrow distribution compared to the epitaxially grown planar InGaN layer or InGaN nanowire. High-resolution scanning transmission electron microscopy (STEM) combined with an energy-dispersive X-ray (EDS) map reveals that a sharply faceted single-crystal InGaN nanoridge (∼50 nm width) forms along the top of each InGaN n...

The performance of overall solar water splitting has been largely limited by the half-reaction of water oxidation. Here, we report a 1.7 eV bandgap InGaN nanowire photoanode for efficient solar water oxidation. It produces a low onset potential of 0.1 V versus a reversible hydrogen electrode (RHE) and a high photocurrent density of 5.2 mA/cm2 at a potential as low as 0.6 V versus RHE. The photoanode yields a half-cell solar energy conversion efficiency up to 3.6%, a record for a single-photon photoanode to our knowledge. Furthermore, in the presence of hole scavengers, the photocurrent density of the InGaN photoanode reaches 21.2 mA/cm2 at 1.23 V versus RHE, which approaches the theoretical limit for a 1.7 eV InGaN absorber. The InGaN nanowire photoanode may serve as an ideal top cell in a photoelectrochemical tandem device when stacked with a 0.9–1.2 eV bandgap bottom cell, which can potentially deliver solar-to-hydrogen efficiency over 25%.

Entropy-stabilized oxides possess a large configurational entropy that allows for the unique ability to include typically immiscible concentrations of species in different configurations. Particularly in oxides, where the physical behavior is strongly correlated to stereochemistry and electronic structure, entropic stabilization creates a unique platform to tailor the interplay of extreme structural and chemical disorder to realize unprecedented functionalities. Here, we control stereochemically driven structural disorder in single crystalline, rocksalt, (MgCoNiCuZn)O-type entropy-stabilized oxides through the incorporation of ${\mathrm{Cu}}^{2+}$ cations. We harness the disorder to tune the degree of glassiness in the antiferromagnetic structure. Structural distortions driven by the Jahn-Teller effect lead to a difference in valence on the Co cation sites, which extends to dilution and disorder of the magnetic lattice. A spin glass model reveals that the fractional spin ordering of the magnetic lattice can be tuned by \ensuremath{\sim}65%. These findings demonstrate entropy-stabilization as a tool for control of functional phenomena.

We present experimental results confirming extreme quantum confinement in GaN/AlxGa1-xN (x = 0.65 and 1.0) nanowire and planar heterostructures, where the GaN layer thickness is of the order of a monolayer. The results were obtained from temperature- and excitation-dependent and time-resolved photoluminescence measurements. In the GaN/AlN nanowire heterostructure array sample, the measured emission peak at 300 K is ∼5.18-5.28 eV. This is in excellent agreement with the calculated optical gap of 5.23 eV and 160-260 meV below the calculated electronic gap of 5.44 eV, suggesting that the observed emission is excitonic in nature with an exciton binding energy of ∼160-260 meV. Similarly, in the monolayer GaN/Al0.65Ga0.35N planar heterostructure, the measured emission peak at 300 K is 4.785 eV and in good agreement with the calculated optical gap of 4.68 eV and 95 meV below the calculated electronic gap of 4.88 eV. The estimated exciton binding energy is 95 meV and in close agreement with our theoretical calculations. Excitation-dependent and time-resolved photoluminescence data support the presence of excitonic transitions. Our results indicate that deep-ultraviolet excitonic light sources and microcavity devices can be realized with heterostructures incorporating monolayer-thick GaN.

Tunnel-injected deep ultraviolet LEDs using a ∼2.5 nm GaN tunnel junction, with emission peak at 265 nm have been demonstrated with a maximum external quantum efficiency reaching ∼8% and wall plug efficiency reaching ∼4.5%.

We have studied the design, epitaxy, and performance characteristics of deep ultraviolet (UV) AlGaN light emitting diodes (LEDs). By combining the tunnel junction and polarization-engineered AlGaN electron blocking layer, a maximum external quantum efficiency and wall-plug efficiency of 0.35% and 0.21%, respectively, were measured for devices operating at ∼245 nm, which are over one order of magnitude higher than previously reported tunnel junction devices at this wavelength. Severe efficiency droop, however, was measured at very low current densities (∼0.25 A/cm2), which, together with the transverse magnetic (TM) polarized emission, is identified to be the primary limiting factors for the device performance. Detailed electrical and optical analysis further shows that the observed efficiency droop is largely due to an electrical effect instead of an optical phenomenon. Our studies suggest that AlGaN deep UV LEDs with efficiency comparable to InGaN blue-emitting quantum wells can be potentially achieved if issues related to electron overflow and TM polarized emission are effectively addressed.

The combination of soft nanoscale organic components with inorganic nanograins hierarchically designed by natural organisms results in highly ductile structural materials that can withstand mechanical impact and exhibit high resilience on the macro- and nano-scale. Our investigation of nacre deformation reveals the underlying nanomechanics that govern the structural resilience and absorption of mechanical energy. Using high-resolution scanning/transmission electron microscopy (S/TEM) combined with in situ indentation, we observe nanoscale recovery of heavily deformed nacre that restores its mechanical strength on external stimuli up to 80% of its yield strength. Under compression, nacre undergoes deformation of nanograins and non-destructive locking across organic interfaces such that adjacent inorganic tablets structurally join. The locked tablets respond to strain as a continuous material, yet the organic boundaries between them still restrict crack propagation. Remarkably, the completely locked interface recovers its original morphology without any noticeable deformation after compressive contact stresses as large as 1.2 GPa., 4 figures

We report on the van der Waals epitaxy of high-quality single-crystalline AlN and the demonstration of AlGaN tunnel junction deep-ultraviolet light-emitting diodes directly on graphene, which were achieved by using plasma-assisted molecular beam epitaxy. It is observed that the substrate/template beneath graphene plays a critical role in governing the initial AlN nucleation. In situ reflection high energy electron diffraction and detailed scanning transmission electron microscopy studies confirm the epitaxial registry of the AlN epilayer with the underlying template. Detailed studies further suggest that the large-scale parallel epitaxial relationship for the AlN epilayer grown on graphene with the underlying template is driven by the strong surface electrostatic potential of AlN. The realization of high-quality AlN by van der Waals epitaxy is further confirmed through the demonstration of AlGaN deep-ultraviolet light-emitting diodes operating at ∼260 nm, which exhibit a maximum external quantum efficiency of 4% for an unpackaged device. This work provides a viable path for the van der Waals epitaxy of ultra-wide bandgap semiconductors, providing a path to achieve high performance deep-ultraviolet photonic and optoelectronic devices that were previously difficult.

AlGaN is the material of choice for high-efficiency deep UV light sources, which is the only alternative technology to replace mercury lamps for water purification and disinfection. At present, however, AlGaN-based mid- and deep UV LEDs exhibit very low efficiency. Here, we report a detailed investigation of the epitaxy and characterization of LEDs utilizing an AlGaN/GaN/AlGaN tunnel junction structure, operating at ∼ 265 nm , which have the potential to break the efficiency bottleneck of deep UV photonics. A thin GaN layer was incorporated between p + and n + -AlGaN to reduce the tunneling barrier. By optimizing the thickness of the GaN layer and thickness of the top n -AlGaN contact layer, we demonstrate AlGaN deep UV LEDs with a maximum external quantum efficiency of 11% and wall-plug efficiency of 7.6% for direct on-wafer measurement. It is also observed that the devices exhibit severe efficiency droop under low current densities, which is explained by the low hole mobility, due to the hole hopping conduction in the Mg impurity band and the resulting electron overflow.

Though enzyme-linked immunosorbent assay is widely used in laboratory medicine, it has limitations due to interactions between enzymes and secondary antibodies. Here, we propose a new cardiac assay scheme for detecting troponin I antigens based on La(1– x )Sr x MnO3 ( $x=0.10$ , 0.15, and 0.25) nanoparticles (NPs). These NPs are synthesized via a modified polyol process followed by heat treatment. An increase in $x$ induces changes in crystal structure and increases both Curie temperature and saturation magnetization. The NPs with $x=0.15$ exhibit a Curie temperature close to the human body temperature. The limit of detection is found to be 0.78 ng/mL. We also confirm that LaSrMnO NPs are reusable after heating the assay plate.

We report a simple approach to synthesize ZnO–Ag composite nanocrystals with different ZnO/Ag ratios via a nanoemulsion process. The morphology and structure of the nanocrystals are investigated by X-ray diffraction and transmission electron microscopy, showing the high crystallinity of the nanocrystals with narrow size distributions. The corresponding optical and magnetic properties are analyzed with photoluminescence spectroscopy, ultraviolet–visible spectrometry and physical property measurement system in detail. The ensuing magnetism is interpreted in terms of nanosizing and other effects. Such ZnO–Ag nanocrystals may be further explored for biomedical applications and spintronic devices with interesting optical and magnetic properties.