Modification of the National Weather Service Distributed Hydrologic Model for subsurface water exchanges between grids

To account for spatial variability of precipitation, as well as basin physiographic properties, the National Weather Service (NWS) has developed a distributed version of its hydrologic component, termed the Hydrology Laboratory-Research Distributed Hydrologic Model (HL-RDHM). Because channels are the only source of water exchange between neighboring computational elements, the absence of such exchange has been identified as a weakness in the model. The primary objective of this paper is to modify the model structure to account for subsurface water exchanges without dramatically altering the conceptual framework of the water balance module. The subsurface exchanges are established by partitioning the slow response components released from the lower layer storages into two parts: the first part involves the grid's conceptual channel, while the second is added to the lower layer storages of the downstream pixel. Realizing the deficiency of the water balance module to locate the lower zone layers in sufficient depths, a complementary study is conducted to test the feasibility of further improvement in the modified model by equally shifting downward the lower zone layers of all pixels over the basin. The Baron Fork at Eldon, Oklahoma, is chosen as the test basin. Ten years of grid-based multisensor precipitation data are used to investigate the effects of the modification, plus shifting the lower zone layers on model performance. The results show that the modified-shifted HL-RDHM can markedly improve the streamflow simulations at the interior point, as well as very high peak-flow simulations at the basin's outlet. Copyright 2011 by the American Geophysical Union

Simulation study of Fermi level depinning in metal-MoS2 contacts

We used Density Functional Theory (DFT) to study the Fermi level pinning and Schottky barrier height in metal-MoS2 contacts. We showed that the Fermi level de-pinning could be attained by controlling the distance between the metal and MoS2. In particular, with proper buffer layers and the use of back-gated structures, the Schottky barrier height can be practically zeroed in some metal-MoS2 stacks, which is important to attain Ohmic contacts

Ohmic Behavior in Metal Contacts to n/p-Type Transition-Metal Dichalcogenides: Schottky versus Tunneling Barrier Trade-off

High contact resistance (RC) between 3D metallic conductors and single-layer 2D semiconductors poses major challenges toward their integration in nanoscale electronic devices. While in experiments the large RC values can be partly due to defects, ab initio simulations suggest that, even in defect-free structures, the interaction between metal and semiconductor orbitals can induce gap states that pin the Fermi level in the semiconductor band gap, increase the Schottky barrier height (SBH), and thus degrade the contact resistance. In this paper, we investigate, by using an in-house-developed ab initio transport methodology that combines density functional theory and nonequilibrium Green’s function (NEGF) transport calculations, the physical properties and electrical resistance of several options for n-type top metal contacts to monolayer MoS2, even in the presence of buffer layers, and for p-type contacts to monolayer WSe2. The delicate interplay between the SBH and tunneling barrier thickness is quantitatively analyzed, confirming the excellent properties of the Bi-MoS2 system as an n-type ohmic contact. Moreover, simulation results supported by literature experiments suggest that the Au-WSe2 system is a promising candidate for p-type ohmic contacts. Finally, our analysis also reveals that a small modulation of a few angstroms of the distance between the (semi)metal and the transition-metal dichalcogenide (TMD) leads to large variations of RC. This could help to explain the scattering of RC values experimentally reported in the literature because different metal deposition techniques can result in small changes of the metal-to-TMD distance besides affecting the density of possible defects

DFT study of graphene doping due to metal contacts

The experimental results of Metal\u2013graphene (M\u2013G) contact resistance (RC) have been investigated in\u2013depth by means of Density Functional Theory (DFT). The simulations allowed us to build a consistent picture explaining the RC dependence on the metal contact materials employed in this work and on the applied back\u2013gate voltage. In this respect, the M\u2013G distance is paramount in determining the RC behavior

Negative capacitance field-effect transistor based on a two-dimensional ferroelectric

Negative capacitance field effect transistors (NCFETs) based on ferroelectric materials have been the focus of intensive research activities because of their relatively small sub-Threshold swing. This work proposes and presents a comprehensive study of a NCFET based on few-layer alpha-In2 Se3 as the ferroelectric in order to reduce the sub-Threshold swing through voltage amplification effect. By employing first principles electronic structure calculations, the Landau constants of mono and few-layer alpha-In2 Se3 are extracted which were utilized for analyzing the characteristics of a NCFET with a monolayer MoS2 as the channel material. Sub-Threshold swings in the range of sim 27-59 mV/dec were achieved for few-layer alpha-In2 Se3 that can be further improved by increasing the thickness of the ferroelectric layer and by using a thinner or high-kappa insulate layer

Engineering of metal-MoS2 contacts to overcome Fermi level pinning

Fermi level pinning (FLP) in metal-MoS2 contacts induces large Schottky barrier heights which in turn results in large contact resistances. In this work, we made use of Density Functional Theory (DFT) to study the origin of FLP in MoS2 contacts with a variety of metals. We also reported how the Fermi level de-pinning could be attained by controlling the distance between the metal and MoS2. In this respect, the metal-MoS2 contacts can be engineered by means of the insertion of proper buffer layers and the use of back-gated structures. This results in a practically zeroed Schottky barrier heights for some specific metal-MoS2 stacks, which it is crucial to attain Ohmic contacts with low series resistances

Development of a Fluorinated Graphene-Based Resistive Humidity Sensor

This work presents the development of novel fluorinated graphene (FG)-based resistive humidity sensor. The humidity sensor was fabricated by drop-casting FG suspension, as the humidity sensing material, on silver (Ag)-based interdigitated electrodes (IDEs). The silver-based IDEs were screen printed on a flexible polyimide substrate. The FG suspension was synthesized by uniform dispersion of FG in isopropyl alcohol (IPA), using the ultra-sonication process. The resistive response of the fabricated humidity sensors towards varying relative humidity (RH) levels was investigated, when the RH was varied from 20% to 80%, in steps of 10%, and at a temperature of 24 \ub0C. A relative resistance change of 13.3% was observed when the RH was changed from 20% to 80%, with a sensitivity of 0.22%/%RH for the FG-based humidity sensor. Response time and recovery time of 82 s and 125 s, respectively, was obtained for the fabricated sensor. In addition, the effect of varying operating temperatures on the response of the fabricated humidity sensors was investigated. The average temperature coefficient of resistance of sensors was obtained as approximately -0.3%/\ub0C. A linear relation between the temperature and the relative resistance change of sensors was observed. Further, first-principles study, employing density functional theory calculations, was performed to investigate interactions between the fluorine atom and graphene substrate, as well as humidity sensing behavior of the FG. DFT calculations showed that hydrogen atoms of the water molecule move towards the fluorine atom of the FG during the relaxation process, confirming the hydrogen bonding between FG and water molecules. The Eads of -0.50 eV was calculated for the adsorption of water molecule on the FG, demonstrating the strong humidity sensing property of the FG. The results demonstrate that FG, a highly stable derivative of graphene, is a potential material for humidity sensing applications

Titanium Carbide MXene as NH3 Sensor: Realistic First-Principles Study

This work presents a more realistic study on the potential of titanium carbide MXene (Ti3C2Tx) for gas sensing, by employing first principle calculations. The effects of different ratios of different functional groups on the adsorption of NH3, NO, NO2, N2O, CO, CO2, CH4, and H2S gas molecules on Ti3C2Tx were analyzed. The results indicated that Ti3C2Tx is considerably more sensitive to NH3, among the studied gas molecules, with a charge transfer of -0.098 e and an adsorption energy of -0.36 eV. By analyzing the electrostatic surface potential (ESP) and the projected density of states (PDOS), important physical and mechanical properties that determine the strength and nature of gas-substrate interactions were investigated, and also, the significant role of electrostatic effects on the charge transfer mechanism was revealed. Further, the Bader charge analysis for the closest oxygen and fluorine atoms to NH3 molecule showed that oxygen atoms have 60% to 180% larger charge transfer than fluorine atoms, supporting that Ti3C2Tx substrate with a relatively lower ratio of fluorine surface terminations has a stronger interaction with NH3 gas molecules. The calculations show that in the presence of water molecules, approximately 90% smaller charge transfer between NH3 molecule and the Ti3C2Tx occurs. Ab initio molecular dynamics simulations (AIMD) were also carried out to evaluate the thermal stabilities of Mxenes. The comprehensive study presented in this work provides insights and paves the way for realizing sensitive NH3 sensors based on Ti3C2Tx that can be tuned by the ratio of surface termination groups