Ifat Jahangir

Negative Refraction with Superior Transmission in Graphene-Hexagonal Boron Nitride (hBN) Multilayer Hyper Crystal

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.

Dual-channel microcantilever heaters for volatile organic compound detection and mixture analysis

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.

Back gated FETs fabricated by large-area, transfer-free growth of a few layer MoS2 with high electron mobility

We demonstrate a transfer-free method for producing 3–5 monolayers, large area MoS2 by pre-oxidation of metallic Mo. The growth temperature was reduced, eliminating free sulfur-induced degradation of the SiO2 gate dielectric in strong accumulation, which suppressed the leakage current at VGS=−3 V by a factor of ≥108, when compared to a single step direct sulfidation method. Back-gated field effect transistors with an accumulation electron mobility of  >80 cm2/Vs, an on/off ratio of  >105, and a subthreshold swing of 84 mV/dec from this MoS2 represent the state-of-the-art on SiO2. In accumulation, current saturation was attributed to charge control rather than velocity saturation. The hysteresis-free transistor characteristics were stable up to a temperature of 500 K.

III-V Nitride based triangular microcantilever heater for selective detection of organic vapors at low temperatures

Detection of volatile organic compounds (VOCs), which are widely used in industrial processes and household products, is very important due to significant health hazards associated with them.1 The common techniques used for detecting VOCs often suffer from one or more of the following issues - high power consumption, limited selectivity, complicated functionalization technique and expensive characterization tools.2-7 In this work, we present an unfunctionalized AlGaN/GaN heterostructure based triangular microcantilever heater (TMH) sensor, which has been utilized to perform selective detection of a wide range of diluted VOCs below their auto-ignition temperature based on responses that are strongly correlated with their latent heat and dipole moment.

Modeling the performance limits of novel microcantilever heaters for volatile organic compound detection

We present a theoretical model estimating the performance limits of novel AlGaN/GaN heterostructure based microcantilever heater sensors to perform advanced volatile organic compound (VOC) detection and mixture analysis. Operating without any specific surface functionalization or treatment; these devices utilize the strong surface polarization of AlGaN as well as the unique device geometries, to perform selective detection of analytes based on their latent heat of evaporation and molecular dipole moment over a wide concentration range. The presented model incorporates heat transfer, Joule heating, thermal expansion and evaporative heat loss mechanisms, to predict device behaviors such as temperature profiles and sensing performance limits under various steady-state and transient test conditions. In addition, the versatility of the proposed model enables us to successfully predict the capability of the device to perform mixture analysis, and provides guidelines to further optimize the device properties to achieve a limit of detection in sub-ppm concentration.

Richardson constant and electrostatics in transfer-free CVD grown few-layer MoS2/graphene barristor with Schottky barrier modulation >0.6eV

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 Richardsons 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.

Optimization of variable stiffness composite plates with cut-outs subjected to compression, tension and shear using an adjoint formulation

This paper presents an improved and extended version of a framework for the design of variable stiffness fiber composite panels developed by the authors. The framework supports the design of panels subjected to multiple load cases, each case a combination of compression or tension and shear. The framework consists of a finite element (FE) solver, an optimizer, a novel approach to relate design variables to the stiffness matrix in the FE module, constraint evaluation modules for manufacturing and buckling constraints and a postprocessor that translates the theoretical optimal result from the optimizer into discrete tow paths for each ply including a cut and restart function. The formulation of the design variables using a manufacturing mesh separate from the FE mesh limits the number of design variables while preserving smoothness of the solution and allows easy specification of manufacturing constraints enforced by the envisioned fiber steering process, for example the minimum course radius to prevent tow buckling. The framework is intended for inclusion in an MDO based aircraft wing weight estimation tool in which it is combined with aerodynamic analysis and optimization. Results obtained with the framework show the structural benefit of using variable stiffness also in case of multiple load cases. The design variable formulation and the adjoint based sensitivity analysis lead to acceptable calculation time while preserving accuracy and smoothness of the solution. Separation of optimizer and tow path planner allows multiple practical interpretations of the theoretical optimization result. This preserves the influence of the manufacturing engineer on the practical panel lay-up and enables the user to control overlaps and gaps using cut-and-restart functionality.

Tunable graphene/Indium Nitride heterostructure diode sensor

In this work, a graphene/InN thin film heterostructure based diode sensor is demonstrated. InN thin film is grown on GaN/sapphire substrate, and CVD grown graphene is then transferred on the device to make a graphene/InN diode structure. Electrical characterization of the device shows good rectifying behavior across the graphene/InN heterojunction. Preliminary experiments with trace amount of water and acetone vapors and NO2 gas show highly promising results. It is observed that this sensor offers better sensitivity than simple graphene or InN based conductometric sensors, primarily because of the presence of a tunable Schottky barrier formed between graphene and InN that can be modulated by different analyte gas molecules.

InN nanowires based multi-modal environmental sensors

High quality InN nanowires (NWs) have been investigated for sensing applications. The NWs were synthesized using a novel approach that enhanced the cracking of NH3 precursor and dramatically improving the growth rate and material quality. Field effect transistors (FETs) based on these high quality InN NWs showed excellent electrical characteristics and good average mobility (270 cm2/Vs). Preliminary sensing results for dilute acetone, diluted water vapor and NO2 at various concentrations are very encouraging. Because of high carrier density and mobility, these NWs are also good candidates for field emission based sensing. During the field emission experiments, sharp change in NW current was observed when supplied voltage increased to 2 V for ~1μm distance between NW and plate, demonstrating the suitability of these NWs for gaseous detection based on the field emission method, and for multimodal sensing in combination with FET based sensing method.

High Temperature AlGaN/GaN Membrane Based Pressure Sensors

A highly sensitive Gallium Nitride (GaN) diaphragm based micro-scale pressure sensor with an AlGaN/GaN heterostructure field effect transistor (HFET) deflection transducer has been designed and fabricated for high temperature applications. The performance of the pressure sensor was studied over a pressure range of 20 kPa, which resulted in an ultra-high sensitivity of ~0.76%/kPa, with a signal-to-noise ratio as high as 16 dB, when biased optimally in the subthreshold region. A high gauge factor of 260 was determined from strain distribution in the sensor membrane obtained from finite element simulations. A repeatable sensor performance was observed over multiple pressure cycles up to a temperature of 200 °C.