Polarization independent grating in a GaN-on-sapphire photonic integrated circuit.

In this work, we report the realization of a polarization-insensitive grating coupler, single-mode waveguide, and ring resonator in the GaN-on-sapphire platform. We provide a detailed demonstration of the material characterization, device simulation, and experimental results. We achieve a grating coupler efficiency of -5.2 dB/coupler with a 1 dB and 3 dB bandwidth of 40 nm and 80 nm, respectively. We measure a single-mode waveguide loss of -6 dB/cm. The losses measured here are the lowest in a GaN-on-sapphire photonic circuit. This demonstration provides opportunities for the development of on-chip linear and non-linear optical processes using the GaN-on-sapphire platform. To the best of our knowledge, this is the first demonstration of an integrated photonic device using a GaN HEMT stack with 2D electron gas.

Spontaneous Time-Reversal Symmetry Breaking at Individual Grain Boundaries in Graphene.

Graphene grain boundaries (GBs) have attracted interest for their ability to host nearly dispersionless electronic bands and magnetic instabilities. Here, we employ quantum transport and universal conductance fluctuation measurements to experimentally demonstrate a spontaneous breaking of time-reversal symmetry across individual GBs of chemical vapor deposited graphene. While quantum transport across the GBs indicate spin-scattering-induced dephasing and hence formation of local magnetic moments, below T≲4 K we observe complete lifting of time-reversal symmetry at high carrier densities (n≳5×10^{12} cm^{-2}) and low temperature (T≲2 K). An unprecedented thirtyfold reduction in the universal conductance fluctuation magnitude with increasing doping density further supports the possibility of an emergent frozen magnetic state at the GBs. Our experimental results suggest that realistic GBs of graphene can be a promising resource for new electronic phases and spin-based applications.

Magnitude and Origin of Electrical Noise at Individual Grain Boundaries in Graphene.

Grain boundaries (GBs) are undesired in large area layered 2D materials as they degrade the device quality and their electronic performance. Here we show that the grain boundaries in graphene which induce additional scattering of carriers in the conduction channel also act as an additional and strong source of electrical noise especially at the room temperature. From graphene field effect transistors consisting of single GB, we find that the electrical noise across the graphene GBs can be nearly 10 000 times larger than the noise from equivalent dimensions in single crystalline graphene. At high carrier densities (n), the noise magnitude across the GBs decreases as ∝1/n, suggesting Hooge-type mobility fluctuations, whereas at low n close to the Dirac point, the noise magnitude could be quantitatively described by the fluctuations in the number of propagating modes across the GB.

Making consistent contacts to graphene: effect of architecture and growth induced defects.

The effects of contact architecture, graphene defect density and metal-semiconductor work function difference on the resistivity of metal-graphene contacts have been investigated. An architecture with metal on the bottom of graphene is found to yield resistivities that are lower, by a factor of four, and most consistent as compared to metal on top of graphene. Growth defects in graphene film were found to further reduce resistivity by a factor of two. Using a combination of method and metal used, the contact resistivity of graphene has been decreased by a factor of 10 to 1200 ± 250 [Formula: see text] using palladium as the contact metal. While the improved consistency is due to the metal being able to contact uncontaminated graphene in the metal on the bottom architecture, lower contact resistivities observed on defective graphene with the same metal are attributed to the increased number of modes of quantum transport in the channel.

Giant electrostriction-like response from defective non-ferroelectric epitaxial BaTiO3 integrated on Si (100).

Lead-free, silicon compatible materials showing large electromechanical responses comparable to, or better than conventional relaxor ferroelectrics, are desirable for various nanoelectromechanical devices and applications. Defect-engineered electrostriction has recently been gaining popularity to obtain enhanced electromechanical responses at sub 100 Hz frequencies. Here, we report record values of electrostrictive strain coefficients (M31) at frequencies as large as 5 kHz (1.04×10-14 m2/V2 at 1 kHz, and 3.87×10-15 m2/V2 at 5 kHz) using A-site and oxygen-deficient barium titanate thin-films, epitaxially integrated onto Si. The effect is robust and retained upon cycling upto 6 million times. Our perovskite films are non-ferroelectric, exhibit a different symmetry compared to stoichiometric BaTiO3 and are characterized by twin boundaries and nano polar-like regions. We show that the dielectric relaxation arising from the defect-induced features correlates well with the observed giant electrostriction-like response. These films show large coefficient of thermal expansion (2.36 × 10-5/K), which along with the giant M31 implies a considerable increase in the lattice anharmonicity induced by the defects. Our work provides a crucial step forward towards formulating guidelines to engineer large electromechanical responses even at higher frequencies in lead-free thin films.

Effect of Heat Treatment on Creep Deformation and Fracture Properties for a Coarse-Grained Inconel 718 Manufactured by Directed Energy Deposition.

The creep properties of a laser-directed energy deposition (L-DED) technique manufactured Inconel 718 (IN718) was investigated at 650 °C/700 MPa. Microstructure and creep properties of L-DED IN718 samples were tailored by various post heat treatments involving homogenization heat treatment with temperature ranging from 1080 to 1180 °C + double aging and hot isostatic pressing (HIP). Microstructural changes and their influence on the creep behavior and fracture mechanism were observed and discussed. The results show that L-DED sample heat treated by a simple double aging exhibits a 49% increase in creep lifetime tr and a comparable creep elongation ɛf when compared to the wrought material, due to the reserved coarse dislocation cell substructure from the L-DED process. The loss of dislocation cell structure and the coarsening of grains at higher temperature of heat treatments contributes to a shorter tr, εf, but faster ε̇min (minimum creep rate). The present work demonstrates that a simultaneous improvement of creep strength and creep elongation can be achieved in the case of a coarse-grained L-DED IN718 by a double aging treatment which can preserve both the strengthening precipitates and an appropriate size of dislocation cells.

Realizing avalanche criticality in neuromorphic networks on a 2D hBN platform.

Networks and systems which exhibit brain-like behavior can analyze information from intrinsically noisy and unstructured data with very low power consumption. Such characteristics arise due to the critical nature and complex interconnectivity of the brain and its neuronal network. We demonstrate a system comprising of multilayer hexagonal boron nitride (hBN) films contacted with silver (Ag), which can uniquely host two different self-assembled networks, which are self-organized at criticality (SOC). This system shows bipolar resistive switching between the high resistance state (HRS) and the low resistance state (LRS). In the HRS, Ag clusters (nodes) intercalate in the van der Waals gaps of hBN forming a network of tunnel junctions, whereas the LRS contains a network of Ag filaments. The temporal avalanche dynamics in both these states exhibit power-law scaling, long-range temporal correlation, and SOC. These networks can be tuned from one to another with voltage as a control parameter. For the first time, two different neural networks are realized in a single CMOS compatible, 2D material platform.

Elevated Temperature Baseplate Effect on Microstructure, Mechanical Properties, and Thermal Stress Evaluation by Numerical Simulation for Austenite Stainless Steel 316L Fabricated by Directed Energy Deposition.

In the present study, the effect of material deposition at the elevated temperature baseplate on the microstructure and mechanical properties was investigated and correlated to the unique thermal history by using numerical simulation. Numerical results agreed well with the experimental results of microstructure and mechanical properties. Numerical results revealed a significant decrease in temperature gradient and a 40% decrease in thermal stress due to material deposition on the elevated temperature baseplate. The reduced thermal stress and temperature gradient resulted in coarser grain features, which in turn led to a decrease in hardness and tensile strength, especially for the bottom region near the baseplate. Meanwhile, no significant effect could be found for ductility. In addition, an elevated temperature baseplate promoted less heterogeneity in hardness and tensile properties along the building direction. The current work demonstrates a collective and direct understanding of the baseplate preheating effect on thermal stress, microstructure and mechanical properties and their correlations, which is believed beneficial for the better utilization of baseplate preheating positive effects.

The effect of strain on effective Duffing nonlinearity in the CVD-MoS2 resonator.

We demonstrate all electrical measurements on NEMS devices fabricated using CVD grown monolayer MoS2. The as-grown monolayer film of MoS2 on top of the SiO2/Si wafer is processed to fabricate arrays and individual NEMS devices without the complex pick and transfer techniques associated with graphene. The electromechanical properties of the devices are on par with those fabricated using the exfoliation method. The frequency response of these devices is then used as a probe to estimate the linear thermal expansion coefficient of the material and evaluate the effect of strain on the effective Duffing nonlinearity in the devices.

Reversible defect engineering in graphene grain boundaries.

Research efforts in large area graphene synthesis have been focused on increasing grain size. Here, it is shown that, beyond 1 µm grain size, grain boundary engineering determines the electronic properties of the monolayer. It is established by chemical vapor deposition experiments and first-principle calculations that there is a thermodynamic correlation between the vapor phase chemistry and carbon potential at grain boundaries and triple junctions. As a result, boundary formation can be controlled, and well-formed boundaries can be intentionally made defective, reversibly. In 100 µm long channels this aspect is demonstrated by reversibly changing room temperature electronic mobilities from 1000 to 20,000 cm2 V-1 s-1. Water permeation experiments show that changes are localized to grain boundaries. Electron microscopy is further used to correlate the global vapor phase conditions and the boundary defect types. Such thermodynamic control is essential to enable consistent growth and control of two-dimensional layer properties over large areas.

Thickness Dependent Parasitic Channel Formation at AlN/Si Interfaces.

The performance of GaN-on-Silicon electronic devices is severely degraded by the presence of a parasitic conduction pathway at the nitride-substrate interface which contributes to switching losses and lower breakdown voltages. The physical nature of such a parasitic channel and its properties are however, not well understood. We report on a pronounced thickness dependence of the parasitic channel formation at AlN/Si interfaces due to increased surface acceptor densities at the interface in silicon. The origin of these surface acceptors is analyzed using secondary ion mass spectroscopy measurements and traced to thermal acceptor formation due to Si-O-N complexes. Low-temperature (5 K) magneto-resistance (MR) data reveals a transition from positive to negative MR with increasing AlN film thickness indicating the presence of an inversion layer of electrons which also contributes to parasitic channel formation but whose contribution is secondary at room temperatures.

Bright-field Nanoscopy: Visualizing Nano-structures with Localized Optical Contrast Using a Conventional Microscope.

Most methods for optical visualization beyond the diffraction limit rely on fluorescence emission by molecular tags. Here, we report a method for visualization of nanostructures down to a few nanometers using a conventional bright-field microscope without requiring additional molecular tags such as fluorophores. The technique, Bright-field Nanoscopy, is based on the strong thickness dependent color of ultra-thin germanium on an optically thick gold film. We demonstrate the visualization of grain boundaries in chemical vapour deposited single layer graphene and the detection of single 40 nm Ag nanoparticles. We estimate a size detection limit of about 2 nm using this technique. In addition to visualizing nano-structures, this technique can be used to probe fluid phenomena at the nanoscale, such as transport through 2D membranes. We estimated the water transport rate through a 1 nm thick polymer film using this technique, as an illustration. Further, the technique can also be extended to study the transport of specific ions in the solution. It is anticipated that this technique will find use in applications ranging from single-nanoparticles resolved sensing to studying nanoscale fluid-solid interface phenomena.

Million-Fold Decrease in Polymer Moisture Permeability by a Graphene Monolayer.

Flexible, transparent, and moisture-impermeable materials are critical for packaging applications in electronic, food, and pharmaceutical industries. Here, we report that a single graphene layer embedded in a flexible polymer reduces its water vapor transmission rate (WVTR) by up to a million-fold. Large-area, transparent, graphene-embedded polymers (GEPs) with a WVTR as low as 10(-6) g m(-2) day(-1) are demonstrated. Monolayered graphene, synthesized by chemical vapor deposition, has been transferred onto the polymer substrate directly by a very simple and scalable melt casting process to fabricate the GEPs. The performances of the encapsulated organic photovoltaic (OPV) devices do not vary even after subjecting the GEPs to cyclic bending for 1000 cycles. Accelerated aging studies of working OPV devices encapsulated in the GEPs show a 50% lifetime of equivalent to 1 000 000 min, which satisfies the requirements of organic electronics.

Graphene on paper: a simple, low-cost chemical sensing platform.

Graphene layers have been transferred directly on to paper without any intermediate layers to yield G-paper. Resistive gas sensors have been fabricated using strips of G-paper. These sensors achieved a remarkable lower limit of detection of ∼300 parts per trillion (ppt) for NO2, which is comparable to or better than those from other paper-based sensors. Ultraviolet exposure was found to dramatically reduce the recovery time and improve response times. G-paper sensors are also found to be robust against minor strain, which was also found to increase sensitivity. G-paper is expected to enable a simple and inexpensive low-cost flexible graphene platform.

Ultrahigh field enhancement and photoresponse in atomically separated arrays of plasmonic dimers.

Combining oblique angle deposition with standard graphene transfer protocols, two planar arrays of metal nanoparticles are fabricated that are vertically separated by atomic dimensions, corresponding precisely to the thickness of a single layer of graphene, i.e., 0.34 nm. Upon illumination of light at an appropriate wavelength, the local electromagnetic field at the junction of the dimers can be increased dramatically, thereby resulting in the most sensitive graphene-plasmonic hybrid photodetector reported to date.

A predictive approach to CVD of crystalline layers of TMDs: the case of MoS2.

Layered transition metal dichalcogenides (TMDs), such as MoS2, are candidate materials for next generation 2-D electronic and optoelectronic devices. The ability to grow uniform, crystalline, atomic layers over large areas is the key to developing such technology. We report a chemical vapor deposition (CVD) technique which yields n-layered MoS2 on a variety of substrates. A generic approach suitable to all TMDs, involving thermodynamic modeling to identify the appropriate CVD process window, and quantitative control of the vapor phase supersaturation, is demonstrated. All reactant sources in our method are outside the growth chamber, a significant improvement over vapor-based methods for atomic layers reported to date. The as-deposited layers are p-type, due to Mo deficiency, with field effect and Hall hole mobilities of up to 2.4 cm(2) V(-1) s(-1) and 44 cm(2) V(-1) s(-1) respectively. These are among the best reported yet for CVD MoS2.

A model for managing delirious older inpatients.

Although multiple models of care exist to prevent the development of delirium in hospitalized patients, models for the management of patients for whom delirium is unpreventable or who already have delirium on admission to the hospital are needed. This article describes the development, management, and economics of a new model of care for patients with delirium, called the Delirium Room (DR). The DR is a specialized 4-bed unit that provides 24-hour intensive nursing care and is completely free of physical restraints. Another important feature of the 4-bed DR is that it is an integral part of a 22-bed acute care for the elderly (ACE) unit. As such, patients in the DR benefit from features of the ACE unit: a change in the physical environment of the medical floor to promote mobility and function and discourage bedrest, comprehensive geriatric care that identifies and addresses problems that can lead to a decline in function, and use of a daily multidisciplinary team meeting. This article also presents descriptive data on a group of delirious patients managed in the DR with the intention of giving baseline data for other ACE units that are considering opening a DR or for future prospective studies in this area.

Diagnostic utility of heat stable alkaline phosphatase in hypertensive disorders of pregnancy.

BACKGROUND: Hypertensive disorders in pregnancy (HDP) complicate 3-10% of all pregnancies. Though there are several biochemical parameters which aid in predicting hypertension of pregnancy, human placental alkaline phosphatase (PLAP), synthesized in placenta during pregnancy by placental syncytiotrophoblast, assumes diagnostic relevance. The purpose of this study was to compare the total alkaline phosphatase (ALP) and heat stable placental alkaline phosphatase (PLAP) levels in the serum of normotensive and hypertensive disorders of pregnancy and to evaluate the clinical utility of ALP and PLAP as a reliable, sensitive, specific and economical biochemical marker of HDP. MATERIALS AND METHODS: This was a case control study, carried out on pregnant women with hypertension, of south Indian population. Study included pregnant women, 60 patients with hypertension and 60 controls. Biochemical assays were carried out by the IFCC approved procedures based on spectrophotometric method and using fully automated random access chemistry analyser. Data was compared by using student t-test. ROC was drawn to find out optimum cut off for ALP, PLAP and PLAP/ALP ratio in HDP. Pearson's correlation was performed to ascertain the association among markers. RESULTS: Serum total ALP, PLAP and PLAP/ALP ratio levels were significantly higher in hypertensive pregnant women when compared to controls (p<0.05). There was significant correlation among ALP, PLAP and DBP. ROC analysis of ALP (169.5), PLAP (69) and PLAP/ALP (0.44) ratios showed optimum cut-offs in diagnosis of hypertension in pregnancy. CONCLUSION: Serum heat stable ALP isoenzyme and PLAP/ALP ratio could be useful adjuvant markers in diagnosis of HDP in association with other relevant and economically viable biochemical tests.

Association Between Waist Circumference and Serum Triacylglcerol Status in Type 2 Diabetes Mellitus (T2DM): A Study Performed on a Rural Population in Puducherry.

INTRODUCTION: India is on the verge of being the diabetic capital of the world, owing to the increasing prevalence of diabetes in India. The aim of this study was to find the association between anthropometry and dyslipidaemia, with a particular reference to the triacylglycerol levels, besides suggesting a better anthropometric variable. METHOD: A group of eighty diabetic patients took part in the study. The Waist Circumference (WC), the waist to hip ratio, the serum triacylglycerol (TAG) levels and the glycated haemoglobin (HbA1c) levels were measured and studied. RESULTS: We found out that there was a significant association between the WC and the TAG levels (p-value = 0.030), between high TAG levels and high HbA1c values (p-value = 0.038) with greater accuracy, in favour of the waist circumference. CONCLUSION: This depicted the fact that a simple measuring tape which is used in measuring the WC, could prove to be a useful and a cost-effective diagnostic approach for monitoring the metabolic complications of type 2 DM.