Assessment of the Thermal Conductivity of BN-C Nanostructures

Chemical and structural diversity present in hexagonal boron nitride ((h-BN) and graphene hybrid nanostructures provide new avenues for tuning various properties for their technological applications. In this paper we investigate the variation of thermal conductivity ($\kappa$) of hybrid graphene/h-BN nanostructures: stripe superlattices and BN (graphene) dots embedded in graphene (BN) are investigated using equilibrium molecular dynamics. To simulate these systems, we have parameterized a Tersoff type interaction potential to reproduce the ab initio energetics of the B-C and N-C bonds for studying the various interfaces that emerge in these hybrid nanostructures. We demonstrate that both the details of the interface, including energetic stability and shape, as well as the spacing of the interfaces in the material exert strong control on the thermal conductivity of these systems. For stripe superlattices, we find that zigzag configured interfaces produce a higher $\kappa$ in the direction parallel to the interface than the armchair configuration, while the perpendicular conductivity is less prone to the details of the interface and is limited by the $\kappa$ of h-BN. Additionally, the embedded dot structures, having mixed zigzag and armchair interfaces, affects the thermal transport properties more strongly than superlattices. Though dot radius appears to have little effect on the magnitude of reduction, we find that dot concentration (50% yielding the greatest reduction) and composition (embedded graphene dots showing larger reduction that h-BN dot) have a significant effect

Equilibrium Limit of Boundary Scattering in Carbon Nanostructures: Molecular Dynamics Calculations of Thermal Transport

It is widely known that graphene and many of its derivative nanostructures have exceedingly high reported thermal conductivities (up to 4000 W/mK at 300 K). Such attractive thermal properties beg the use of these structures in practical devices; however, to implement these materials while preserving transport quality, the influence of structure on thermal conductivity should be thoroughly understood. For graphene nanostructures, having average phonon mean free paths on the order of one micron, a primary concern is how size influences the potential for heat conduction. To investigate this, we employ a novel technique to evaluate the lattice thermal conductivity from the Green-Kubo relations and equilibrium molecular dynamics in systems where phonon-boundary scattering dominates heat flow. Specifically, the thermal conductivities of graphene nanoribbons and carbon nanotubes are calculated in sizes up to 3 microns, and the relative influence of boundary scattering on thermal transport is determined to be dominant at sizes less than 1 micron, after which the thermal transport largely depends on the quality of the nanostructure interface. The method is also extended to carbon nanostructures (fullerenes) where phonon confinement, as opposed to boundary scattering, dominates, and general trends related to the influence of curvature on thermal transport in these materials are discussed

Modeling and optimization of Tunnel-FET architectures exploiting carrier gas dimensionality

The semiconductor industry, governed by the Moore's law, has achieved the almost unbelievable feat of exponentially increasing performance while lowering the costs for years. The main enabler for this achievement has been the scaling of the CMOS transistor that allowed the manufacturers to pack more and more functionality into the same chip area. However, it is now widely agreed that the happy days of scaling are well over and we are about to reach the physical limits of the CMOS concept. One major, insurmountable limit of CMOS is the so-called thermionic emission limit which dictates that the switching slope of the transistor cannot go below 60mV/dec at room temperature. This makes it impossible to scale down the supply voltage for CMOS transistor without dramatically increasing the static power consumption. To address this issue, a novel transistor concept called Tunnel FET (TFET) which utilizes the quantum mechanical band-to-band tunneling (BTBT) has been proposed. TFETs possess the potential to overcome the thermionic emission limit and therefore allow for low supply voltage operation. This thesis aims at investigating the performance of TFETs with alternative architectures exploiting quantized carrier gases through quantum mechanical simulations. To this end, 1D and 2D self-consistent Schrödinger-Poisson solvers with closed boundaries are developed along with the phonon-assisted and direct BTBT models implemented as a post-processing step. Moreover, we propose an efficient method to incorporate the quantization along the transverse direction which enables us to simulate different dimensionality combinations. The implemented models are calibrated against experimental and more fundamental quantum mechanical simulation methods such as k.p and tight-binding NEGF using tunneling diode structures. Using these tools, we simulate an advanced TFET architecture called electron-hole bilayer TFET (EHBTFET) which exploits BTBT between 2D electron and hole gases electrostatically induced by two separate oppositely biased gates. The subband-to-subband tunneling is first analyzed with the 1D simulator where the device working principle is demonstrated. Then, non-idealities of the EHBTFET operation such as the lateral tunneling and corner effects are investigated using the 2D simulator. The origin of the lateral leakage and techniques to reduce it are analyzed in detail. A parameter space analysis of the EHBTFET is performed by simulating a wide range of channel materials, channel thickness and oxide thicknesses. Our results indicate the possibility of having 2D-2D and 3D-3D tunneling for the EHBTFET, depending on the parameters chosen. A novel digital logic scheme utilizing the independent biasing property of the EHBTFET n- and p-gates is proposed and verified through quantum-corrected TCAD simulations. The performance benchmarking against a 28nm FD-SOI CMOS technology is performed as well. The results indicate that the EHBTFET logic can outperform the CMOS counterpart in the low supply voltage (subthreshold) regime, where it can offer significantly higher drive current due to its steep switching slope. We also compare the different dimensionality cases and highlight important differences between the face and edge tunneling devices in terms of their dependence on the device parameters (channel material, channel thickness and EOT)

Bibliometric analysis of refugee effects to economic development

The global refugee crisis presents significant challenges to economic development, especially in developing countries and regions that host large numbers of refugees. This study provides a comprehensive bibliometric analysis of research conducted on the effects of refugees on economic development over the past three decades. The study utilized the Web of Science (WoS) database to identify and analyze scholarly publications related to the topic of economic impact of refugees. The analysis focused on trends, patterns, and key themes in the literature. The results indicate a growing body of research on the topic, with particular emphasis on the effects of refugees on food aid, market outcomes, labor markets, and macroeconomic performance. The study examines the economic impact of refugees to countries, specifically their effects on labor markets, housing, food prices, and employment sectors. The study employs bibliometric analysis to systematically review and synthesize existing research, providing a comprehensive overview of the current state of knowledge on the subject. The findings enhance comprehension of the economic consequences of refugee movements and can guide policy and practice in the realm of economic development and refugee integration

Economic Voting and Media Influence in a Competitive Authoritarian Setting: Evidence from Turkey

It is generally assumed that individuals take national economic performance into account while voting. But the question of how perceptions about the economy may be influenced by partisan media remains understudied. Analyzing survey data from Turkey with various robust analysis techniques we demonstrate that reliance on pro government media as a news source makes voters’ economic perceptions significantly more favorable, which in turn increases the likelihood of incumbent vote. In addition, we demonstrate that the audience of pro-government media are more likely to display “sociotropic overestimation”—thinking that the national economy has done better compared to their own household experience; and “counterfactual rationalization”—thinking, regardless of how they view actual economic performance, that it could be worse under alternative leadership. The results suggest that when the economy is manifestly deteriorating, authoritarian incumbents may try to use media influence to convince the electorate that the status quo is better than the alternatives

The modified post-earthquake damage assessment methodology for TCIP (TCIP-DAM-2020)

Chapter 5Post-Earthquake damage assessment has always been one of the major challenges that both engineers and authorities face after disastrous earthquakes all around the world. Considering the number of buildings in need of inspection and the insufficient number of qualified inspectors, the availability of a thorough, quantitative and rapidly applicable damage assessment methodology is vitally important after such events. At the beginning of the new millennia, an assessment system satisfying these needs was developed for the Turkish Catastrophe Insurance Pool (TCIP, known as DASK in Turkey) to evaluate the damages in reinforced concrete (RC) and masonry structures. Since its enforcement, this assessment method has been successfully used after several earthquakes that took place in Turkey, such as 2011 Van Earthquake, 2011 Kutahya Earthquake, 2019 Istanbul Earthquake and 2020 Elazig Earthquake to decide the future of damaged structures to be either ‘repaired’ or ‘demolished’.Scopus - Affiliation ID: 6010507

Influence of disorder on thermal transport properties of boron nitride nanostructures

WOS: 000306992800004The impact of isotopes on thermal transport in boron nitride nanotubes (BNNTs) and boron nitride white graphene is systematically studied via molecular dynamic simulations. By varying the concentration of the B-10 isotope in these materials, we find that thermal conductivity ranges from 340 to 500W/m(-1) K-1, closely agreeing with experimental observations for isotopically pure and natural (19.9% B-10) BNNTs. Further, we investigate the interplay between dimension and isotope disorder in several C-based materials. Our results show a general trend of decreasing influence of isotope disorder with dimension of these materials.NSF [DMR 0844082]; AFRL; ARO; ONR; US Department of Energy; Scientific and Technological Research Council of Turkey (TUBITAK)We acknowledge support from NSF (Grant No. DMR 0844082) to the International Institute of Materials for Energy Conversion at Texas A&M University as well as AFRL. Parts of the computations were carried out by the Laboratory of Computational Engineering of Nanomaterials, supported by ARO, ONR, and US Department of Energy grants. We would also like to thank the Supercomputing Center of Texas A&M University for a generous time allocation for this project. C.S. acknowledges the support from The Scientific and Technological Research Council of Turkey (TUBITAK) toward his research at Anadolu University

The Electron\u2013Hole Bilayer TFET: Dimensionality Effects and Optimization

An extensive parameter analysis is performed on the electron-hole bilayer tunnel field-effect transistor (EHBTFET) using a 1-D effective mass Schr\uf6dinger-Poisson solver with corrections for band non-parabolicity considering thin InAs, In0.53Ga0.47As, Ge, Si0.5Ge0.5, and Si films. It is found that depending on the channel material and channel thickness, the EHBTFET can operate either as a 2-D-2-D or 3-D-3-D tunneling device. InAs offers the highest I ON, whereas for the Si and Si0.5Ge0.5 EHBTFETs, significant current levels cannot be achieved within a reasonable voltage range. The general trends are explained through an analytical model that shows close agreement with the numerical results

A review of selected topics in physics based modeling for tunnel field-effect transistors

The research field on tunnel-FETs (TFETs) has been rapidly developing in the last ten years, driven by the quest for a new electronic switch operating at a supply voltage well below 1 V and thus delivering substantial improvements in the energy efficiency of integrated circuits. This paper reviews several aspects related to physics based modeling in TFETs, and shows how the description of these transistors implies a remarkable innovation and poses new challenges compared to conventional MOSFETs. A hierarchy of numerical models exist for TFETs covering a wide range of predictive capabilities and computational complexities. We start by reviewing seminal contributions on direct and indirect band-to-band tunneling (BTBT) modeling in semiconductors, from which most TCAD models have been actually derived. Then we move to the features and limitations of TCAD models themselves and to the discussion of what we define non-self-consistent quantum models, where BTBT is computed with rigorous quantum-mechanical models starting from frozen potential profiles and closed-boundary Schr\uf6dinger equation problems. We will then address models that solve the open-boundary Schr\uf6dinger equation problem, based either on the non-equilibrium Green's function NEGF or on the quantum-transmitting-boundary formalism, and show how the computational burden of these models may vary in a wide range depending on the Hamiltonian employed in the calculations. A specific section is devoted to TFETs based on 2D crystals and van der Waals hetero-structures. The main goal of this paper is to provide the reader with an introduction to the most important physics based models for TFETs, and with a possible guidance to the wide and rapidly developing literature in this exciting research field