Ahsan Uddin

Functionalized graphene/silicon chemi-diode H2 sensor with tunable sensitivity

A reverse bias tunable Pd- and Pt-functionalized graphene/Si heterostructure Schottky diode H2 sensor has been demonstrated. Compared to the graphene chemiresistor sensor, the chemi-diode sensor offers more than one order of magnitude higher sensitivity as the molecular adsorption induced Schottky barrier height change causes the heterojunction current to vary exponentially in reverse bias. The reverse bias operation also enables low power consumption, as well as modulation of the atomically thin graphenes Fermi level, leading to tunable sensitivity and detection of H₂ down to the sub-ppm range.

Impact identification in sandwich structures using solitary wave-supporting granular crystal sensors

A new diagnostic method to identify the location and magnitude of external impact on a sandwich structure using granular crystal sensors was studied. The granular crystal sensors are composed of one-dimensional chains of spherical particles that are inserted in a thick core of the sandwich structure. Given an external impact, the embedded sensors generate compact-supported, highly nonlinear solitary waves resulting from the dispersive and nonlinear characteristics of granular crystals. In this study, the propagating mechanism of highly nonlinear solitary waves in relation to various impact conditions was investigated. Particularly, it was reported that the flight time and magnitude of solitary waves are highly sensitive to the location and amplitude of impact. By analyzing measured solitary waves, the striker’s impact location and drop height was successfully predicted nondestructively. It was found that the diagnostic results are in agreement with the numerical simulations obtained from a combined spectr...

Impact induced solitary wave propagation through a woodpile structure

In this paper, we investigate solitary wave propagation through a one-dimensional woodpile structure excited by low and high velocity impact. Woodpile structures are a sub-class of granular metamaterial, which supports propagation of nonlinear waves. Hertz contact law governs the behavior of the solitary wave propagation through the granular media. Towards an experimental study, a woodpile structure was fabricated by orthogonally stacking cylindrical rods. A shock tube facility has been developed to launch an impactor on the woodpile structure at a velocity of 30 m s−1. Embedded granular chain sensors were fabricated to study the behavior of the solitary wave. The impact induced stress wave is studied to investigate solitary wave parameters, i.e. contact force, contact time, and solitary wave velocity. With the aid of the experimental setup, numerical simulations, and a theoretical solution based on the long wavelength approximation, formation of the solitary wave in the woodpile structure is validated to a reasonable degree of accuracy. The nondispersive and compact supported solitary waves traveling at sonic wave velocity offer unique properties that could be leveraged for application in nondestructive testing and structural health monitoring.

Probabilistic contact force model for low velocity impact on honeycomb structure

Abstract The analysis and design of honeycomb sandwich structures involve quantification of dynamic contact force, local indentation of the structure when subjected to impact. However, the estimation of contact force is challenging as it involves wide uncertainties arising from initial conditions, geometrical and material parameters. Due to complex dynamic failure modes, the analytical solution fails to accurately predict the contact force. Therefore, the current study aims to propose a probabilistic model that can predict peak dynamic contact force when honeycomb structures are subjected to low velocity impacts. In the initial phase of this study, material and geometric parameters that could influence the response are studied in detail. The probabilistic model is developed based on these parameters and provides an estimate of peak contact force for impact energies within the defined limits. To develop the probabilistic model, we resorted to numerical simulations. It is shown that the numerical results are converging with low velocity impact experiments. Extensive finite element simulations are conducted at selective design points to generate a representative data for calibrating the probabilistic model. Once calibrated the probabilistic model circumvents any further dependency on conducting expensive FE simulations or experiments and expresses the contact force using simple explicit equation. In this paper, the emphasis is laid on the developing a probabilistic framework which could account for uncertainties involved in the estimate of structural response. The proposed model helps in designing robust honeycomb composites for application in aerospace structures.

Improvement in electrical properties of CVD graphene on low temperature pulsed laser deposited boron nitride on SiO 2 /Si substrate

Graphene has been extensively researched over the past decade due to its outstanding electrical, optical and mechanical properties. Since charge carriers in graphene are confined within one atomic layer thickness, their transport properties are easily influenced by the surrounding medium. Recently, significant enhancement in the transport properties of graphene has been observed as it forms layered heterostructure with hexagonal Boron Nitride (hBN), which offers an inert surface, high surface optical phonon modes for heat dissipation and a nearly lattice matched structure [1]. In general, the methods for synthesizing hBN require either high growth or high annealing temperature (~1000 °C) [2-3]. Pulsed laser deposition (PLD) offers an attractive alternative to overcome the high temperature requirement by increasing the excitations of deposited atoms and extending resonance time of the energetic species presence at the condensation surface. This facilitates low temperature growth of amorphous BN (a-BN) [4] which can then be phase transformed to polycrystalline hBN by low temperature annealing [5]. Here, we are reporting the electrical characterization of chemical vapor deposition (CVD) graphene on 5 and 30 nm BN grown on SiO2/Si substrate initially by PLD at 200 °C, and annealing at 400 °C for transformation to polycrystalline hBN. As synthesized BN was found to improve the electrical properties of graphene by significantly enhancing mobility, reducing carrier inhomogeneity and lowering extrinsic doping compared to graphene transferred on SiO2/Si substrate.

High gain bipolar photo-transistor operation in graphene/SiC Schottky interfaces: The role of minority carriers

We propose a new class of semiconductor transistor devices based on graphene/SiC and graphene/Si Schottky junctions that have the potential to be transformative. By using the graphene as collector/emitter in a bipolar transistor (BJT) and not as a channel material, there is relaxation of the tolerances in graphene thickness and quality, simplifying growth, device design and fabrication. This also enables the exploitation of engineered defects in thicker (2-5ML) graphene films for flexible electronics, currently not being considered, as well controlled uniform defects are preferred to localized random defect clusters. We will discuss an SiF4 based growth method that enables temperature programmed defect engineering. We will discuss the use of electron-beam induced current (EBIC) to characterize these materials. Based on recent results at our lab, a graphene/SiC Schottky junction behaves as a collector (GC) and an emitter (GE) in a BJT with common emitter gain, β>50, measured under phototransistor operation mode. The transparent graphene Schottky collector/emitter junction enables opto-electronic applications, minimizes series resistance in the device due to the thin graphene layer, and also minimizes charge storage time (diffusion capacitance), enabling high speed operation. Furthermore, the observation of β>50 with a GE-BJT demonstrates that significant minority carrier injection occurs in these Schottky junctions, contrary to what is commonly assumed. The injection of minority carriers has the ability to induce conductivity modulation in the underlying semiconductor, reducing overall device resistance. The role of minority carriers in Schottky Junctions will be discussed.

Tunable graphene/Si chemi-diode H 2 sensor: Comparison between Pd and Pt functionalization, and effect of illumination

Tunable graphene/Si heterostructure Schottky diode H2 sensor with two different metal (Pd and Pt) functionalization has been demonstrated. In reverse bias, the molecular adsorption induced Schottky barrier height change causes the heterojunction current to vary exponentially which results in more than an order of higher sensitivity of the chemi-diode sensor compared to the graphene chemiresistor sensor. Tunable sensitivity of the sensor has been achieved through graphene Fermi level modulation by varying the reverse bias magnitude. Operation in reverse bias also enabled low power consumption.

Pt-functionalized graphene/Si heterostructure for hydrogen sensing

In this work, H2 sensing by graphene/Si Schottky diode with Pt functionalization has been demonstrated. Both p-and n-Si/graphene Schottky devices showed conductivity change in presence of H2 molecules in ambient conditions at room temperature. Successful detection of 10 ppm H2 in air has been demonstrated. Performance enhancement compared to the Pt-functionalized just graphene based sensor is validated by the response obtained from both device types fabricated on the same chip. Seven fold increase in sensitivity in H2 sensing has been obtained for graphene/Si devices compared to the simple graphene sensor in ambient condition.

Gas sensing by graphene/silicon hetrostructure

Graphene, a two-dimensional material with a very high charge carrier concentration, is ideal for sensing chemical species based upon charge exchange. The sensitivity of graphene is shown to improve many folds by using graphene/semiconductor heterostructure. A new amperometric chemical sensing paradigm based upon transport across graphene/p-Si Schottky diode under reverse bias is demonstrated in this work. The reported very high sensitivity of graphene/p-Si heterostructure is in direct agreement with small change in Schottky barrier height due to molecular adsorption on graphene causing large change in reverse saturation current due to exponential dependence of later on the former.