In this article, we have theoretically investigated the performance of graphene-hexagonal Boron Nitride (hBN) multilayer structure (hyper crystal) to demonstrate all angle negative refraction along with superior transmission. hBN, one of the latest natural hyperbolic materials, can be a very strong contender to form a hyper crystal with graphene due to its excellence as a graphene-compatible substrate. Although bare hBN can exhibit negative refraction, the transmission is generally low due to its high reflectivity. Whereas due to graphene’s 2D nature and metallic characteristics in the frequency range where hBN behaves as a type-I hyperbolic material, we have found graphene-hBN hyper-crystals to exhibit all angle negative refraction with superior transmission. Interestingly, superior transmission from the whole structure can be fully controlled by the tunability of graphene without hampering the negative refraction originated mainly from hBN. We have also presented an effective medium description of the hyper crystal in the low-k limit and validated the proposed theory analytically and with full wave simulations. Along with the current extensive research on hybridization of graphene plasmon polaritons with (hyperbolic) hBN phonon polaritons, this work might have some substantial impact on this field of research and can be very useful in applications such as hyper-lensing.
We demonstrate a large area MoS2/graphene barristor, using a transfer-free method for producing 3–5 monolayer (ML) thick MoS2. The gate-controlled diodes show good rectification, with an ON/OFF ratio of ∼103. The temperature dependent back-gated study reveals Richardson's coefficient to be 80.3 ± 18.4 A/cm2/K and a mean electron effective mass of (0.66 ± 0.15)m0. Capacitance and current based measurements show the effective barrier height to vary over a large range of 0.24–0.91 eV due to incomplete field screening through the thin MoS2. Finally, we show that this barristor shows significant visible photoresponse, scaling with the Schottky barrier height. A response time of ∼10 s suggests that photoconductive gain is present in this device, resulting in high external quantum efficiency.
a b s t r a c tWe report on the unique detection of dilute volatile organic compounds below their auto-ignition temperature using a novel AlGaN/GaN heterostructure based triangular microcantilever heater. With a low input power of 12 mW, the microcantilever heater was found to reach a maximum temperature of 330 • C at the tip, which was verified through infrared microscopy and Raman spectroscopy. Unique threshold voltages were observed for various organic analytes with different functional groups, beyond which the heater current started to increase sharply in presence of analytes, which correlated strongly with their latent heat of evaporation. Dilute vapors with concentrations as low as 50 parts per million (ppm) could be detected selectively with a noise limited resolution down to 5 ppm. On the other hand, the magnitude of change of current for a fixed applied voltage was found to be dependent on the molecular dipole moment of the analytes, which can likely be attributed to the strong surface polarization of AlGaN. A simple circuit model has been proposed to explain the observations. Heat transfer and Joule heating simulations were performed using finite element method to model the electro-thermal characteristics of the microcantilever heater, which were in good agreement with the experimental observations.
We report on novel microcantilever heater sensors with separate AlGaN/GaN heterostructure based heater and sensor channels to perform advanced volatile organic compound (VOC) detection and mixture analysis. Operating without any surface functionalization or treatment, these microcantilevers utilize the strong surface polarization of AlGaN, as well as the unique heater and sensor channel geometries, to perform selective detection of analytes based on their latent heat of evaporation and molecular dipole moment over a wide concentration range with sub-ppm detection limit. The dual-channel microcantilevers have demonstrated much superior sensing behavior compared to the single-channel ones, with the capability to not only identify individual VOCs with much higher specificity, but also uniquely detect them in a generic multi-component mixture of VOCs. In addition, utilizing two different dual channel configurations and sensing modalities, we have been able to quantitatively determine individual analyte concentration in a VOC mixture. An algorithm for complete mixture analysis, with unique identification of components and accurate determination of their concentration, has been presented based on simultaneous operation of an array of these microcantilever heaters in multiple sensing modalities.
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