Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Two dimensional materials”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Direct measurement of ferroelectric polarization in a tunable semimetal

Abstract Ferroelectricity, the electrostatic counterpart to ferromagnetism, has long been thought to be incompatible with metallicity due to screening of electric dipoles and external electric fields by itinerant charges. Recent measurements, however, demonstrated signatures of ferroelectric switching in the electrical conductance of bilayers and trilayers of WTe 2 , a semimetallic transition metal dichalcogenide with broken inversion symmetry. An especially promising aspect of this system is that the density of electrons and holes can be continuously tuned by an external gate voltage. This degree of freedom enables measurement of the spontaneous polarization as free carriers are added to the system. Here we employ capacitive sensing in dual-gated mesoscopic devices of bilayer WTe 2 to directly measure the spontaneous polarization in the metallic state and quantify the effect of free carriers on the polarization in the conduction and valence bands, separately. We compare our results to a low-energy model for the electronic bands and identify the layer-polarized states that contribute to transport and polarization simultaneously. Bilayer WTe 2 is thus shown to be a fully tunable ferroelectric metal and an ideal platform for exploring polar ordering, ferroelectric transitions, and applications in the presence of free carriers.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Spin/valley pumping of resident electrons in WSe2 and WS2 monolayers

Abstract Monolayers of transition metal dichalcogenides are ideal materials to control both spin and valley degrees of freedom either electrically or optically. Nevertheless, optical excitation mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here we demonstrate a very efficient spin/valley optical pumping of resident electrons in n-doped WSe 2 and WS 2 monolayers. We observe that, using a continuous wave laser and appropriate doping and excitation densities, negative trion doublet lines exhibit circular polarization of opposite sign and the photoluminescence intensity of the triplet trion is more than four times larger with circular excitation than with linear excitation. We interpret our results as a consequence of a large dynamic polarization of resident electrons using circular light.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Strain fields in twisted bilayer graphene

Van der Waals heteroepitaxy allows deterministic control over lattice mismatch or azimuthal orientation between atomic layers to produce long-wavelength superlattices. The resulting electronic phases depend critically on the superlattice periodicity and localized structural deformations that introduce disorder and strain. In this study we used Bragg interferometry to capture atomic displacement fields in twisted bilayer graphene with twist angles < 2°. Nanoscale spatial fluctuations in twist angle and uniaxial heterostrain were statistically evaluated, revealing the prevalence of short-range disorder in moiré heterostructures. By quantitatively mapping strain tensor fields, we uncovered two regimes of structural relaxation and disentangled the electronic contributions of constituent rotation modes. Further, we found that applied heterostrain accumulates anisotropically in saddle-point regions, generating distinctive striped strain phases. Our results establish the reconstruction mechanics underpinning the twist-angle-dependent electronic behaviour of twisted bilayer graphene and provide a framework for directly visualizing structural relaxation, disorder and strain in moiré materials.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Tuning colour centres at a twisted hexagonal boron nitride interface

The colour centre platform holds promise for quantum technologies, and hexagonal boron nitride has attracted attention due to the high brightness and stability, optically addressable spin states and wide wavelength coverage discovered in its emitters. However, its application is hindered by the typically random defect distribution and complex mesoscopic environment. In this study, employing cathodoluminescence, we demonstrate on-demand activation and control of colour centre emission at the twisted interface of two hexagonal boron nitride flakes. Further, we show that colour centre emission brightness can be enhanced by two orders of magnitude by tuning the twist angle. Additionally, by applying an external voltage, nearly 100% brightness modulation is achieved. Our ab initio GW and GW plus Bethe–Salpeter equation calculations suggest that the emission is correlated to nitrogen vacancies and that a twist-induced moiré potential facilitates electron–hole recombination. This mechanism is further exploited to draw nanoscale colour centre patterns using electron beams.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Local atomic stacking and symmetry in twisted graphene trilayers

Moiré superlattices formed from twisting trilayers of graphene are an ideal model for studying electronic correlation, and offer several advantages over bilayer analogues, including more robust and tunable superconductivity and a wide range of twist angles associated with flat band formation. Atomic reconstruction, which strongly impacts the electronic structure of twisted graphene structures, has been suggested to play a major role in the relative versatility of superconductivity in trilayers. Here, we exploit an inteferometric 4D-STEM approach to image a wide range of trilayer graphene structures. Here, our results unveil a considerably different model for moiré lattice relaxation in trilayers than that proposed from previous measurements, informing a thorough understanding of how reconstruction modulates the atomic stacking symmetries crucial for establishing superconductivity and other correlated phases in twisted graphene trilayers.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Bilayer Wigner crystals in a transition metal dichalcogenide heterostructure

One of the first theoretically predicted manifestations of strong interactions in many-electron systems was the Wigner crystal, in which electrons crystallize into a regular lattice. The crystal can melt via either thermal or quantum fluctuations. Quantum melting of the Wigner crystal is predicted to produce exotic intermediate phases and quantum magnetism because of the intricate interplay of Coulomb interactions and kinetic energy. However, studying two-dimensional Wigner crystals in the quantum regime has often required a strong magnetic field or a moiré superlattice potential, thus limiting access to the full phase diagram of the interacting electron liquid. Here we report the observation of bilayer Wigner crystals without magnetic fields or moiré potentials in an atomically thin transition metal dichalcogenide heterostructure, which consists of two MoSe 2 monolayers separated by hexagonal boron nitride. We observe optical signatures of robust correlated insulating states at symmetric (1:1) and asymmetric (3:1, 4:1 and 7:1) electron doping of the two MoSe 2 layers at cryogenic temperatures. Here, we attribute these features to bilayer Wigner crystals composed of two interlocked commensurate triangular electron lattices, stabilized by inter-layer interaction. The Wigner crystal phases are remarkably stable, and undergo quantum and thermal melting transitions at electron densities of up to 6 × 10 12 per square centimetre and at temperatures of up to about 40 kelvin. Our results demonstrate that an atomically thin heterostructure is a highly tunable platform for realizing many-body electronic states and probing their liquid–solid and magnetic quantum phase transitions.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Gate-tunable ReS{sub 2}/MoTe{sub 2} heterojunction with high-performance photodetection

Van der Waals heterojunctions based on atomically thin two-dimensional (2D) materials have attracted numerous attention for their special scientific research value and promising applications in photoelectric and micro-nano electronic devices. Especially, the carrier generation, separation, and extraction process in 2D materials can be easily modulated by external field, which may facilitate some multifunctional electronics and optoelectronics. In this paper, we report a unique type-II band alignment ReS{sub 2}/MoTe{sub 2} heterojunction with rectification inversion due to the fact the bottom few-layer MoTe{sub 2} can be easily tuned from p-type to n-type state through the applied back-gate voltage. Then we study photodetection properties of ReS{sub 2}/MoTe{sub 2} heterojunction, a relatively fast photoresponse time of 109 μs and a considerable photoresponsivity of 0.34 AW{sup −1} for 520 nm at room temperature show great potential in photodetection. Our studies of ReS{sub 2}/MoTe{sub 2} heterojunction with rectification inversion and high-performance photodetection will facilitate the development of electronics and optoelectronics based on atomically-thin heterojunctions.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

An Inverse-Designed Nanophotonic Interface for Excitons in Atomically Thin Materials

Efficient nanophotonic devices are essential for applications in quantum networking, optical information processing, sensing, and nonlinear optics. Extensive research efforts have focused on integrating two-dimensional (2D) materials into photonic structures, but this integration is often limited by size and material quality. Here, we use hexagonal boron nitride (hBN), a benchmark choice for encapsulating atomically thin materials, as a waveguiding layer while simultaneously improving the optical quality of the embedded films. When combined with a photonic inverse design, it becomes a complete nanophotonic platform to interface with optically active 2D materials. Grating couplers and low-loss waveguides provide optical interfacing and routing, tunable cavities provide a large exciton-photon coupling to transition metal dichalcogenide (TMD) monolayers through Purcell enhancement, and metasurfaces enable the efficient detection of TMD dark excitons. This work paves the way for advanced 2D-material nanophotonic structures for classical and quantum nonlinear optics.

2D materials↗
DOE OSTI2016

What can molecular simulation do for global warming?

Carbon capture is necessary to reduce CO 2 emissions from burning fossil fuels, which has led to global warming. Molecular simulations offer chemical insights and design principles for new separation media and for understanding the separation process. In this review, we summarize recent applications of simulation methods from ab initio and density functional theory to classical molecular dynamics and Grand canonical Monte Carlo in understanding ionic liquids and porous carbonaceous materials for CO 2 separation, especially the postcombustion CO 2 / N 2 separation. We highlight design and simulation of the porous two‐dimensional ( 2D ) materials as the highly selective membranes for CO 2 separation. Simulated structure–property relationships for the materials are discussed in connection to the corresponding chemisorption, physisorption, or membrane process. In chemisorption, the focus is on reducing the heat of reaction with CO 2 ; in physisorption, the key is to increase the binding strength via CO 2 ‐philic groups; in membrane process, the key is to increase solubility for ionic‐liquid membranes and to control pore size for 2D materials. Challenges and opportunities for simulating emerging materials are also discussed. WIREs Comput Mol Sci 2016, 6:173–197. doi: 10.1002/wcms.1241 This article is categorized under: Structure and Mechanism > Computational Materials Science Electronic Structure Theory > Density Functional Theory Molecular and Statistical Mechanics > Molecular Interactions

Tian, Ziqi↗
DOE OSTI2024

Deviatoric stress-induced metallization, layer reconstruction and collapse of van der Waals bonded zirconium disulfide

In contrast to two-dimensional (2D) monolayer materials, van der Waals layered transition metal dichalcogenides exhibit rich polymorphism, making them promising candidates for novel superconductor, topological insulators and electrochemical catalysts. Here, we highlight the role of hydrostatic pressure on the evolution of electronic and crystal structures of layered ZrS2. Under deviatoric stress, our electrical experiments demonstrate a semiconductor-to-metal transition above 30.2 GPa, while quasi-hydrostatic compression postponed the metallization to 38.9 GPa. Both X-ray diffraction and Raman results reveal structural phase transitions different from those under hydrostatic pressure. Under deviatoric stress, ZrS2 rearranges the original ZrS6 octahedra into ZrS8 cuboids at 5.5 GPa, in which the unique cuboids coordination of Zr atoms is thermodynamically metastable. The structure collapses to a partially disordered phase at 17.4 GPa. These complex phase transitions present the importance of deviatoric stress on the highly tunable electronic properties of ZrS2 with possible implications for optoelectronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Alternative route for electrochemical ammonia synthesis by reduction of nitrate on copper nanosheets

We propose an efficient and clean avenue for ammonia synthesis, via electroreduction of nitrate which could be obtained from industrial wastewater, domestic sewage, sodium nitrate ore, and nitrification of bacteria and electrochemical oxidation of nitrogen, which addresses the water pollution issues and simultaneously upgrades the nitrate to high-value ammonia. At a low overpotential of -0.15 V versus RHE, Cu nanosheets achieved an ammonia formation rate of 390.1 μg mg -1 Cu h -1 and a Faradaic efficiency of 99.7%, attributed to suppression of the HER activity and apparent improvement of the rate of rate-determining step on Cu (111). Such an ammonia formation rate is more than two orders of magnitude higher than electrochemical nitrogen reduction reaction to ammonia. This work not only develops a powerful strategy to the rational design of robust and efficient catalysts by crystal facet engineering, but also provides an alternative route for electrochemical ammonia synthesis by reduction of nitrate.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Nanolatticed Architecture Mitigates Damage in Shark Egg Cases

Structural versatility and multifunctionality of biological materials have resulted in countless bioinspired strategies seeking to emulate the properties of nature. The nanostructured egg case of swell sharks is one of the toughest permeable membranes known and, thus, presents itself as a model system for materials where the conflicting properties, strength and porosity, are desirable. The egg case possesses an intricately ordered structure that is designed to protect delicate embryos from the external environment while enabling respiratory and metabolic exchange, achieving a tactical balance between conflicting properties. Herein, structural analyses revealed an enabling nanolattice architecture that constitutes a Bouligand-like nanoribbon hierarchical assembly. Three distinct hierarchical architectural adaptations enhance egg case survival: Bouligand-like organization for in-plane isotropic reinforcement, noncylindrical nanoribbons maximizing interfacial stress distribution, and highly ordered nanolattices enabling permeability and lattice-governed toughening mechanisms. Furthermore, these discoveries provide fundamental insights for the improvement of multifunctional membranes, fiber-reinforced soft composites, and mechanical metamaterials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2017

Ultralow-loss polaritons in isotopically pure boron nitride

Conventional optical components are limited to size scales much larger than the wavelength of light, as changes to the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultrathin, so-called ‘flat’ optical components that beget abrupt changes in these properties over distances significantly shorter than the free-space wavelength. Although high optical losses still plague many approaches, phonon polariton (PhP) materials have demonstrated long lifetimes for sub-diffractional modes in comparison to plasmon-polariton-based nanophotonics. We experimentally observe a threefold improvement in polariton lifetime through isotopic enrichment of hexagonal boron nitride (hBN). Commensurate increases in the polariton propagation length are demonstrated via direct imaging of polaritonic standing waves by means of infrared nano-optics. Lastly, our results provide the foundation for a materials-growth-directed approach aimed at realizing the loss control necessary for the development of PhP-based nanophotonic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Tuning of the electronic and vibrational properties of epitaxial MoS 2 through He-ion beam modification

Abstract Atomically thin transition metal dichalcogenides (TMDs), like MoS 2 with high carrier mobilities and tunable electron dispersions, are unique active material candidates for next generation opto-electronic devices. Previous studies on ion irradiation show great potential applications when applied to two-dimensional (2D) materials, yet have been limited to micron size exfoliated flakes or smaller. To demonstrate the scalability of this method for industrial applications, we report the application of relatively low power (50 keV) 4 He + ion irradiation towards tuning the optoelectronic properties of an epitaxially grown continuous film of MoS 2 at the wafer scale, and demonstrate that precise manipulation of atomistic defects can be achieved in TMD films using ion implanters. The effect of 4 He + ion fluence on the PL and Raman signatures of the irradiated film provides new insights into the type and concentration of defects formed in the MoS 2 lattice, which are quantified through ion beam analysis. PL and Raman spectroscopy indicate that point defects are generated without causing disruption to the underlying lattice structure of the 2D films and hence, this technique can prove to be an effective way to achieve defect-mediated control over the opto-electronic properties of MoS 2 and other 2D materials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2020

Strongly adhesive dry transfer technique for van der Waals heterostructure

That one can stack van der Waals materials with atomically sharp interfaces has provided a new material platform of constructing heterostructures. The technical challenge of mechanical stacking is picking up the exfoliated atomically thin materials after mechanical exfoliation without chemical and mechanical degradation. Chemically inert hexagonal boron nitride (hBN) has been widely used for encapsulating and picking up vdW materials. However, due to the relatively weak adhesion of hBN, assembling vdW heterostructures based on hBN has been limited. Here, we report a new dry transfer technique. We used two vdW semiconductors (ZnPS 3 and CrPS 4 ) to pick up and encapsulate layers for vdW heterostructures, which otherwise are known to be hard to fabricate. By combining with optimized polycaprolactone (PCL) providing strong adhesion, we demonstrated various vertical heterostructure devices, including quasi-2D superconducting NbSe 2 Josephson junctions with atomically clean interface. The versatility of the PCL-based vdW stacking method provides a new route for assembling complex 2D vdW materials without interfacial degradation.

36 MATERIALS SCIENCE↗
DOE OSTI2015

High-Responsivity Graphene–Boron Nitride Photodetector and Autocorrelator in a Silicon Photonic Integrated Circuit

Graphene and other two-dimensional (2D) materials have emerged as promising materials for broadband and ultrafast photodetection and optical modulation. These optoelectronic capabilities can augment complementary metal–oxide–semiconductor (CMOS) devices for high-speed and low-power optical interconnects. We demonstrate an on-chip ultrafast photodetector based on a two-dimensional heterostructure consisting of high-quality graphene encapsulated in hexagonal boron nitride. Coupled to the optical mode of a silicon waveguide, this 2D heterostructure-based photodetector exhibits a maximum responsivity of 0.36 A/W and high-speed operation with a 3 dB cutoff at 42 GHz. From photocurrent measurements as a function of the top-gate and source-drain voltages, we conclude that the photoresponse is consistent with hot electron mediated effects. At moderate peak powers above 50 mW, we observe a saturating photocurrent consistent with the mechanisms of electron–phonon supercollision cooling. This nonlinear photoresponse enables optical on-chip autocorrelation measurements with picosecond-scale timing resolution and exceptionally low peak powers.

77 NANOSCIENCE AND NANOTECHNOLOGY↗