Showing papers by "Mildred S. Dresselhaus published in 2017"

Subthreshold swing improvement in MoS2 transistors by the negative-capacitance effect in a ferroelectric Al-doped-HfO2/HfO2 gate dielectric stack.

Abstract: Obtaining a subthreshold swing (SS) below the thermionic limit of 60 mV dec−1 by exploiting the negative-capacitance (NC) effect in ferroelectric (FE) materials is a novel effective technique to allow the reduction of the supply voltage and power consumption in field effect transistors (FETs). At the same time, two-dimensional layered semiconductors, such as molybdenum disulfide (MoS2), have been shown to be promising candidates to replace silicon MOSFETs in sub-5 nm-channel technology nodes. In this paper, we demonstrate NC MoS2 FETs by incorporating a ferroelectric Al-doped HfO2 (Al : HfO2), a technologically compatible material, in the FET gate stack. Al : HfO2 thin films were deposited on Si wafers by atomic layer deposition. Voltage amplification up to 1.25 times was observed in a FE bilayer stack of Al : HfO2/HfO2 with a Ni metallic intermediate layer. The minimum SS (SSmin) of the NC-MoS2 FET built on the FE bilayer improved to 57 mV dec−1 at room temperature, compared with SSmin = 67 mV dec−1 for the MoS2 FET with only HfO2 as a gate dielectric.

Bottom-up Design of Three-Dimensional Carbon-Honeycomb with Superb Specific Strength and High Thermal Conductivity

Abstract: Low-dimensional carbon allotropes, from fullerenes, carbon nanotubes, to graphene, have been broadly explored due to their outstanding and special properties. However, there exist significant challenges in retaining such properties of basic building blocks when scaling them up to three-dimensional materials and structures for many technological applications. Here we show theoretically the atomistic structure of a stable three-dimensional carbon honeycomb (C-honeycomb) structure with superb mechanical and thermal properties. A combination of sp2 bonding in the wall and sp3 bonding in the triple junction of C-honeycomb is the key to retain the stability of C-honeycomb. The specific strength could be the best in structural carbon materials, and this strength remains at a high level but tunable with different cell sizes. C-honeycomb is also found to have a very high thermal conductivity, for example, >100 W/mK along the axis of the hexagonal cell with a density only ∼0.4 g/cm3. Because of the low density and ...

Hot Electron Transistor with van der Waals Base-Collector Heterojunction and High-Performance GaN Emitter

Abstract: Single layer graphene is an ideal material for the base layer of hot electron transistors (HETs) for potential terahertz (THz) applications. The ultrathin body and exceptionally long mean free path maximizes the probability for ballistic transport across the base of the HET. We demonstrate for the first time the operation of a high-performance HET using a graphene/WSe2 van der Waals (vdW) heterostructure as a base-collector barrier. The resulting device with a GaN/AlN heterojunction as emitter, exhibits a current density of 50 A/cm2, direct current gain above 3 and 75% injection efficiency, which are record values among graphene-base HETs. These results not only provide a scheme to overcome the limitations of graphene-base HETs toward THz operation but are also the first demonstration of a GaN/vdW heterostructure in HETs, revealing the potential for novel electronic and optoelectronic applications.

Role of Molecular Sieves in the CVD Synthesis of Large-Area 2D MoTe2

Abstract: The synthesis of high-quality 2D MoTe2 with a desired phase on SiO2/Si substrate is crucial to its diverse applications. A side reaction of Te with the substrate Si leading to SiTe and Si2Te3 tends to happen during growth, resulting in the failure to obtain MoTe2. It has been found that molecular sieves can adsorb the silicon telluride byproducts and eliminate the influence of the side reaction during the chemical vapor deposition synthesis of MoTe2. With the help of molecular sieves, few-layer 1T′ MoTe2 can be grown from the MoOx precursor. Pure 1T′ MoTe2 and 2H MoTe2 regions in centimeter-sized areas synthesized on the same piece of SiO2/Si substrate can be obtained by using an overlapped geometry. The strategy provides a new method to controllably synthesize MoTe2 with desired phases and can be generalizable to the synthesis of other tellurium-based layered materials.

Nonperturbative Quantum Nature of the Dislocation-Phonon Interaction.

Abstract: Despite the long history of dislocation–phonon interaction studies, there are many problems that have not been fully resolved during this development. These include an incompatibility between a perturbative approach and the long-range nature of a dislocation, the relation between static and dynamic scattering, and their capability of dealing with thermal transport phenomena for bulk material only. Here by utilizing a fully quantized dislocation field, which we called a “dislon”, a phonon interacting with a dislocation is renormalized as a quasi-phonon, with shifted quasi-phonon energy, and accompanied by a finite quasi-phonon lifetime, which are reducible to classical results. A series of outstanding legacy issues including those above can be directly explained within this unified phonon renormalization approach. For instance, a renormalized phonon naturally resolves the decade-long debate between dynamic and static dislocation–phonon scattering approaches, as two limiting cases. In particular, at nanosca...

Nitrogen-doped porous carbon monoliths from polyacrylonitrile (PAN) and carbon nanotubes as electrodes for supercapacitors

Abstract: Nitrogen-doped porous activated carbon monoliths (NDP-ACMs) have long been the most desirable materials for supercapacitors. Unique to the conventional template based Lewis acid/base activation methods, herein, we report on a simple yet practicable novel approach to production of the three-dimensional NDP-ACMs (3D-NDP-ACMs). Polyacrylonitrile (PAN) contained carbon nanotubes (CNTs), being pre-dispersed into a tubular level of dispersions, were used as the starting material and the 3D-NDP-ACMs were obtained via a template-free process. First, a continuous mesoporous PAN/CNT based 3D monolith was established by using a template-free temperature-induced phase separation (TTPS). Second, a nitrogen-doped 3D-ACM with a surface area of 613.8 m2/g and a pore volume 0.366 cm3/g was obtained. A typical supercapacitor with our 3D-NDP-ACMs as the functioning electrodes gave a specific capacitance stabilized at 216 F/g even after 3000 cycles, demonstrating the advantageous performance of the PAN/CNT based 3D-NDP-ACMs.

Corrigendum: Nitrogen-doped porous carbon monoliths from polyacrylonitrile (PAN) and carbon nanotubes as electrodes for supercapacitors.

Abstract: Nitrogen-doped porous activated carbon monoliths (NDP-ACMs) have long been the most desirable materials for supercapacitors. Unique to the conventional template based Lewis acid/base activation methods, herein, we report on a simple yet practicable novel approach to production of the three-dimensional NDP-ACMs (3D-NDP-ACMs). Polyacrylonitrile (PAN) contained carbon nanotubes (CNTs), being pre-dispersed into a tubular level of dispersions, were used as the starting material and the 3D-NDP-ACMs were obtained via a template-free process. First, a continuous mesoporous PAN/CNT based 3D monolith was established by using a template-free temperature-induced phase separation (TTPS). Second, a nitrogen-doped 3D-ACM with a surface area of 613.8 m2/g and a pore volume 0.366 cm3/g was obtained. A typical supercapacitor with our 3D-NDP-ACMs as the functioning electrodes gave a specific capacitance stabilized at 216 F/g even after 3000 cycles, demonstrating the advantageous performance of the PAN/CNT based 3D-NDP-ACMs.

Sensitive Phonon-Based Probe for Structure Identification of 1T′ MoTe2

Abstract: In this work, by combining transmission electron microscopy and polarized Raman spectroscopy for the 1T′ MoTe2 flakes with different thicknesses, we found that the polarization dependence of Raman intensity is given as a function of excitation laser wavelength, phonon symmetry, and phonon frequency, but has weak dependence on the flake thickness from few-layer to multilayer. In addition, the frequency of Raman peaks and the relative Raman intensity are sensitive to flake thickness, which manifests Raman spectroscopy as an effective probe for thickness of 1T′ MoTe2. Our work demonstrates that polarized Raman spectroscopy is a powerful and nondestructive method to quickly identify the crystal structure and thickness of 1T′ MoTe2 simultaneously, which opens up opportunities for the in situ probe of anisotropic properties and broad applications of this novel material.

Electron energy can oscillate near a crystal dislocation

Abstract: Crystal dislocations govern the plastic mechanical properties of materials but also affect the electrical and optical properties. However, a fundamental and quantitative quantum field theory of a dislocation has remained undiscovered for decades. Here we present an exactly-solvable one-dimensional quantum field theory of a dislocation, for both edge and screw dislocations in an isotropic medium, by introducing a new quasiparticle which we have called the 'dislon'. The electron-dislocation relaxation time can then be studied directly from the electron self-energy calculation, which is reducible to classical results. In addition, we predict that the electron energy will experience an oscillation pattern near a dislocation. Compared with the electron density's Friedel oscillation, such an oscillation is intrinsically different since it exists even with only single electron is present. With our approach, the effect of dislocations on materials' non-mechanical properties can be studied at a full quantum field theoretical level.

Strengthening of Ceramic-based Artificial Nacre via Synergistic Interactions of 1D Vanadium Pentoxide and 2D Graphene Oxide Building Blocks.

Abstract: Nature has evolved hierarchical structures of hybrid materials with excellent mechanical properties. Inspired by nacre’s architecture, a ternary nanostructured composite has been developed, wherein stacked lamellas of 1D vanadium pentoxide nanofibres, intercalated with water molecules, are complemented by 2D graphene oxide (GO) nanosheets. The components self-assemble at low temperature into hierarchically arranged, highly flexible ceramic-based papers. The papers’ mechanical properties are found to be strongly influenced by the amount of the integrated GO phase. Nanoindentation tests reveal an out-of-plane decrease in Young’s modulus with increasing GO content. Furthermore, nanotensile tests reveal that the ceramic-based papers with 0.5 wt% GO show superior in-plane mechanical performance, compared to papers with higher GO contents as well as to pristine V2O5 and GO papers. Remarkably, the performance is preserved even after stretching the composite material for 100 nanotensile test cycles. The good mechanical stability and unique combination of stiffness and flexibility enable this material to memorize its micro- and macroscopic shape after repeated mechanical deformations. These findings provide useful guidelines for the development of bioinspired, multifunctional systems whose hierarchical structure imparts tailored mechanical properties and cycling stability, which is essential for applications such as actuators or flexible electrodes for advanced energy storage.

Enhanced Raman scattering on functionalized graphene substrates

Abstract: The graphene-enhanced Raman scattering of Rhodamine 6G molecules on pristine, fluorinated and 4-nitrophenyl functionalized graphene substrates was studied. The uniformity of the Raman signal enhancement was studied by making large Raman maps. The relative enhancement of the Raman signal is demonstrated to be dependent on the functional groups, which was rationalized by the different doping levels of pristine, fluorinated and 4-nitrophenyl functionalized graphene substrates. The impact of the Fermi energy of graphene and the energy of molecular vibrations was considered together for the first time in order to explain the Raman signal enhancement. This approach enables to understand the enhancement of the Raman signal without any assumption about the uniformity of the molecules on the graphene surface and without a necessity to know the enhancement factor. The agreement between the theory and the experimental data was further demonstrated for varying excitation energy.

Tailoring Superconductivity with Quantum Dislocations

Abstract: Despite the established knowledge that crystal dislocations can affect a material’s superconducting properties, the exact mechanism of the electron-dislocation interaction in a dislocated superconductor has long been missing. Being a type of defect, dislocations are expected to decrease a material’s superconducting transition temperature (Tc) by breaking the coherence. Yet experimentally, even in isotropic type I superconductors, dislocations can either decrease, increase, or have little influence on Tc. These experimental findings have yet to be understood. Although the anisotropic pairing in dirty superconductors has explained impurity-induced Tc reduction, no quantitative agreement has been reached in the case a dislocation given its complexity. In this study, by generalizing the one-dimensional quantized dislocation field to three dimensions, we reveal that there are indeed two distinct types of electron-dislocation interactions. Besides the usual electron-dislocation potential scattering, there is an...

Exploring Low Internal Reorganization Energies for Silicene Nanoclusters

Abstract: High-performance materials rely on small reorganization energies to facilitate both charge separation and charge transport. Here, we performed DFT calculations to predict small reorganization energies of rectangular silicene nanoclusters with hydrogen-passivated edges denoted by H-SiNC. We observe that across all geometries, H-SiNCs feature large electron affinities and highly stabilized anionic states, indicating their potential as n-type materials. Our findings suggest that fine-tuning the size of H-SiNCs along the zigzag and armchair directions may permit the design of novel n-type electronic materials and spinctronics devices that incorporate both high electron affinities and very low internal reorganization energies.

Detection of Multiconfigurational States of Hydrogen-Passivated Silicene Nanoclusters.

Abstract: Utilizing density functional theory (DFT) and a complete active space self-consistent field (CASSCF) approach,we study the electronic properties of rectangular silicene nano clusters with hydrogen passivated edges denoted by H-SiNCs (nz,na), with nz and na representing the zigzag and armchair directions, respectively. The results show that in the nz direction, the H-SiNCs prefer to be in a singlet (S = 0) ground state for nz > na. However, a transition from a singlet (S = 0) to a triplet (S = 1) ground state is revealed for na > nz. Through the calculated Raman spectrum, the S = 0 and S = 1 ground states can be observed by the E2g (G) and A (D) Raman modes. Furthermore, H-SiNC clusters are shown to have HOMO–LUMO (HL) energy gaps, which decrease as a function of na and nz for S = 0 and S = 1 states. The H-SiNC with a S = 1 ground state can be potentially used for silicene-based spintronic devices.

Designing a Double-Pole Nanoscale Relay Based on a Carbon Nanotube: A Theoretical Study

Abstract: With low leakage current, and thus low power consumption and high ON/OFF ratio, nanoelectromechanical switches offer a promising basis for next-generation electronics. They also tolerate radiation, temperature variations, and external electric fields, which makes them suitable for extreme environments, as in space exploration or nuclear disaster recovery. Considering the unique elastic and electrical properties of carbon nanotubes (CNTs), the authors present analytical expressions for the characteristic ``pull-in'' and ``pull-back'' voltages of double-pole CNT nanorelays, plus a generic model and design rules for such devices.

Designing a Double-Pole Nanoscale Relay Based on a Carbon Nanotube: A Theoretical Study

Abstract: With low leakage current, and thus low power consumption and high ON/OFF ratio, nanoelectromechanical switches offer a promising basis for next-generation electronics. They also tolerate radiation, temperature variations, and external electric fields, which makes them suitable for extreme environments, as in space exploration or nuclear disaster recovery. Considering the unique elastic and electrical properties of carbon nanotubes (CNTs), the authors present analytical expressions for the characteristic ``pull-in'' and ``pull-back'' voltages of double-pole CNT nanorelays, plus a generic model and design rules for such devices.

Electron energy can oscillate near a crystal dislocation

Abstract: United States. Department of Energy. Office of Science. Solid-State Solar Thermal Energy Conversion Center (Award DE-SC0001299/DE-FG02-09ER46577)