Samia Subrina

Dimensional crossover of thermal transport in few-layer graphene

Graphene, in addition to its unique electronic and optical properties, reveals unusually high thermal conductivity. The fact that the thermal conductivity of large enough graphene sheets should be higher than that of basal planes of bulk graphite was predicted theoretically by Klemens. However, the exact mechanisms behind the drastic alteration of a materials intrinsic ability to conduct heat as its dimensionality changes from two to three dimensions remain elusive. The recent availability of high-quality few-layer graphene (FLG) materials allowed us to study dimensional crossover experimentally. Here we show that the room-temperature thermal conductivity changes from approximately 2,800 to approximately 1,300 W m(-1) K(-1) as the number of atomic planes in FLG increases from 2 to 4. We explained the observed evolution from two dimensions to bulk by the cross-plane coupling of the low-energy phonons and changes in the phonon Umklapp scattering. The obtained results shed light on heat conduction in low-dimensional materials and may open up FLG applications in thermal management of nanoelectronics.

Heat Removal in Silicon-on-Insulator Integrated Circuits With Graphene Lateral Heat Spreaders

Graphene was recently proposed as a material for heat removal owing to its extremely high thermal conductivity. We simulated heat propagation in silicon-on-insulator (SOI) circuits with and without graphene lateral heat spreaders. Numerical solutions of the heat-propagation equations were obtained using the finite-element method. The analysis was focused on the prototype SOI circuits with the metal-oxide-semiconductor field-effect transistors. It was found that the incorporation of graphene or few-layer graphene (FLG) layers with proper heat sinks can substantially lower the temperature of the localized hot spots. The maximum temperature in the transistor channels was studied as function of graphenes thermal conductivity and the thickness of FLG. The developed model and obtained results are important for the design of graphene heat spreaders and interconnects.

Reduced thermal resistance of the silicon-synthetic diamond composite substrates at elevated temperatures

The authors report results of experimental investigation of thermal conductivity of synthetic diamond-silicon composite substrates. Although composite substrates are more thermally resistive than silicon at room temperature they outperform conventional wafers at elevated temperatures owing to different thermal conductivity dependence on temperature. The crossover point is reached near ∼360 K and can be made even lower by tuning the polycrystalline-grain size, film thickness, and interface quality. The reduction of thermal resistance of composite wafers at temperatures, typical for operation of electronic chips, may lead to better thermal management and new phonon-engineered methods for the electron mobility enhancement.

Impact of vacancies on the thermal conductivity of graphene nanoribbons: A molecular dynamics simulation study

Equilibrium molecular dynamics simulation using 2nd generation Reactive Bond Order interatomic potential has been performed to model the thermal transport of nanometer sized zigzag defected graphene nanoribbons (GNRs) containing several types of vacancies. We have investigated the thermal conductivity of defected GNRs as a function of vacancy concentration within a range of 0.5% to 5% and temperature ranging from 300K to 600K, along with a comparative analysis of those for pristine GNRs. We find that, a vacancy concentration of 0.5% leads to over 90% reduction in the thermal conductivity of GNRs. At low defect concentration, the decay rate is faster but ceases gradually at higher defect concentration. With the increasing temperature, thermal conductivity of defected GNRs decreases but shows less variation in comparison with that of pristine GNRs at higher temperatures. Such comprehensive study on several vacancy type defects in GNRs can provide further insight to tune up the thermal transport characterist...

Thermal transport in graphene/stanene hetero-bilayer nanostructures with vacancies: an equilibrium molecular dynamics study

In this study, we have performed equilibrium molecular dynamics simulations to model the thermal transport in nanometer sized graphene/stanene hetero-bilayer structures. Our simulations include the computation of thermal conductivity of pristine as well as defected structures containing several types of vacancies namely point vacancy, bi-vacancy and edge-vacancy. The room temperature thermal conductivity of the pristine 10 nm × 3 nm graphene/stanene hetero-bilayer is estimated to be 127.2 ± 13.8 W m−1 K−1. We have studied the impact of temperature and width of the sample on thermal transport in both pristine and defected nanoribbons. Thermal conductivity is found to decrease with the increasing temperature while it tends to increase with the increasing width. Furthermore, we have investigated the thermal conductivity of defected bilayers as a function of vacancy concentration within a range of 0.5% to 2% and compared those for pristine structures. A vacancy concentration of 2% leads to 50–70% reduction in the thermal conductivity of the pristine bilayer nanoribbons. Such a study provides a good insight into the optimization and control of thermal transport characteristics of the low dimensional graphene/stanene nanostructure based thermal and nanoelectronic devices.

Electronic structure of bilayer graphene physisorbed on metal substrates

Graphene-metal interfaces have recently become popular for graphene growth and for making contacts in numerous thermal and photo-electronic devices. A number of studies have already been made to investigate the interfacial properties when single layer graphene is grown on metal substrates. In this study, we consider the physisorption of bilayer graphene on metals and find a significant bandgap opening which is otherwise absent in the single layer case. This gap arises from the asymmetry in the bilayer due to the charge transfer process at the interface. This charge transfer also causes doping in the bilayer graphene and a corresponding shift in the Fermi level. In this work, we present a thorough investigation into the induced bandgap and Fermi level shift when bilayer graphene is adsorbed on Cu, Al, Ag, Pt, and Au(111) surfaces first by reporting their values from Density Functional Theory (DFT) studies with Local Density Approximation functional used for exchange-correlation energy. Next, to obtain an e...

Heat Transport in Graphene Interconnect Networks With Graphene Lateral Heat Spreaders

We simulated heat propagation in the integrated graphene heat spreaders within the interconnect hierarchy. In the considered design, the graphene layers perform the dual functions of interconnects and heat spreaders. We investigated Joule heating effects within the chip with graphene interconnect networks and heat spreaders. Numerical solutions for direct current and heat propagation equations were found using the finite-element method. The simulation results showed that the use of graphene as interconnects as well as heat spreaders lowers the maximum temperature of the chip. The maximum temperature of the chip was studied as a function of the interconnect current and thickness of few-layer graphene. Our results are important for design of graphene-based thermal and electrical interconnect networks in the next generations of integrated circuits and 3-D electronics.

Characterization of thermal and mechanical properties of stanene nanoribbons: a molecular dynamics study

Stanene, a buckled honeycomb structure of monolayer tin, has several intriguing electrical and thermoelectrical applications that closely depend on its thermal, mechanical, and electrical properties. However, thermal and mechanical characterizations of stanene nanoribbons (STNRs) have not yet been comprehensively investigated. In this study, we have performed an equilibrium molecular dynamics simulation to characterize the thermal and mechanical properties of STNRs using the modified embedded-atom method potential. The room temperature thermal conductivities of pristine 10 nm × 3 nm zigzag and armchair stanene nanoribbon were estimated to be 0.95 ± 0.024 W m−1 K−1 and 0.89 ± 0.026 W m−1 K−1, respectively. We also studied the thermal conductivity as a function of temperature and width of the ribbon. The thermal conductivity was found to decrease with increasing temperature, whereas it tends to increase with increasing width for both configurations. In all cases, the zigzag STNR exhibited a higher thermal conductivity than its armchair counterpart did. Furthermore, our study includes an investigation of the thermal transport in defected STNRs. For a defect concentration of ∼1.5%, the thermal conductivity of defected stanene nanoribbon experiences a reduction of approximately 30–50%, whereas a ∼70–90% reduction was observed at a vacancy concentration of ∼5% for various types of defects. Finally, the stress–strain behavior of STNRs with varying width was analyzed using uniaxial loading. Zigzag STNRs were found to have higher fracture strength than their armchair counterparts. Moreover, with increasing width, both fracture strain and fracture stress of armchair STNRs were found to show small variations compared with their zigzag counterparts. This study provides insights for tuning the thermo-mechanical characteristics of stanene-based nanostructures for thermal management and possible applications as thermoelectrics.

Thermal transport characterization of hexagonal boron nitride nanoribbons using molecular dynamics simulation

Due to similar atomic bonding and electronic structure to graphene, hexagonal boron nitride (h-BN) has broad application prospects such as the design of next generation energy efficient nano-electronic devices. Practical design and efficient performance of these devices based on h-BN nanostructures would require proper thermal characterization of h-BN nanostructures. Hence, in this study we have performed equilibrium molecular dynamics (EMD) simulation using an optimized Tersoff-type interatomic potential to model the thermal transport of nanometer sized zigzag hexagonal boron nitride nanoribbons (h-BNNRs). We have investigated the thermal conductivity of h-BNNRs as a function of temperature, length and width. Thermal conductivity of h-BNNRs shows strong temperature dependence. With increasing width, thermal conductivity increases while an opposite pattern is observed with the increase in length. Our study on h-BNNRs shows considerably lower thermal conductivity compared to GNRs. To elucidate these aspect...

A two dimensional analytical model of drain to source current and subthreshold slope of a triple material double gate MOSFET

A two dimensional analytical model of drain to source current as well as subthreshold slope of a triple material double gate MOSFET has been developed in this work. Basic drift-diffusion equation has been used to derive the drain to source current model. An expression of pinch-off voltage has been derived for modeling drain current in saturation region. The current in the device has been studied as function of drain to source voltage and gate voltage as well. In the work, leakage current in zero gate bias condition has also been presented. Variations in subthreshold slope of the device owing to change in device parameters have been lifted up. Our model results have been verified with the simulation data obtained by using a professional numerical device simulator.