Spectroscopic photoemission and low-energy electron microscopy studies of the surface and electronic structure of two-dimensional materials
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99 GENERAL AND MISCELLANEOUS↗
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.
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Dynamics of ballistic photocurrents driven by Coulomb scattering in a two-dimensional material
First-principles real-time time-dependent density-functional theory (rt-TDDFT) calculations reveal the existence of ballistic photocurrents generated by Coulomb scattering, which has not previously been considered as a mechanism for the bulk photovoltaic effect. With monolayer GeS as an example, it is predicted that ballistic currents can be comparable to shift currents under experimentally accessible conditions.
2-dimensional systems↗
Topological Exact Flat Bands in Two-Dimensional Materials under Periodic Strain
Here we study flat bands and their topology in 2D materials with quadratic band crossing points under periodic strain. In contrast to Dirac points in graphene, where strain acts as a vector potential, strain for quadratic band crossing points serves as a director potential with angular momentum . We prove that when the strengths of the strain fields hit certain “magic” values, exact flat bands with emerge at charge neutrality point in the chiral limit, in strong analogy to magic angle twisted-bilayer graphene. These flat bands have ideal quantum geometry for the realization of fractional Chern insulators, and they are always fragile topological. The number of flat bands can be doubled for certain point group, and the interacting Hamiltonian is exactly solvable at integer fillings. We further demonstrate the stability of these flat bands against deviations from the chiral limit, and discuss possible realization in 2D materials.
36 MATERIALS SCIENCE↗
Autocorrected off-axis holography of two-dimensional materials
Not Available
Kern, Felix (ORCID:000000018199127X)↗
Out-of-plane ferroelectricity and robust magnetoelectricity in quasi-two-dimensional materials
Thin-film ferroelectrics have been pursued for capacitive and nonvolatile memory devices. They rely on polarizations that are oriented in an out-of-plane direction to facilitate integration and addressability with complementary metal-oxide semiconductor architectures. The internal depolarization field, however, formed by surface charges can suppress the out-of-plane polarization in ultrathin ferroelectric films that could otherwise exhibit lower coercive fields and operate with lower power. Here, we unveil stabilization of a polar longitudinal optical (LO) mode in the n = 2 Ruddlesden–Popper family that produces out-of-plane ferroelectricity, persists under open-circuit boundary conditions, and is distinct from hyperferroelectricity. Our first-principles calculations show the stabilization of the LO mode is ubiquitous in chalcogenides and halides and relies on anharmonic trilinear mode coupling. We further show that the out-of-plane ferroelectricity can be predicted with a crystallographic tolerance factor, and we use these insights to design a room-temperature multiferroic with strong magnetoelectric coupling suitable for magneto-electric spin-orbit transistors.
36 MATERIALS SCIENCE↗
Opportunities and Challenges for Quantum Emitters in One and Two Dimensional Materials
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Material Science↗
Innovative Proton Conductive Membranes Based on Two-Dimensional Materials
The team of Oak Ridge National Laboratory, New Mexico State University, and General Graphene Corporation staff will develop and commercialize paradigm-shifting low-cost, high-performing, proton-conductive membranes made from abundant and durable 2D materials. Proposed membranes are expected to be an order of magnitude better in performance and cost compared with the current state-of-the-art formulation, Nafion, and thus are expected to eliminate huge roadblocks in proliferation of fuel cells and flow batteries. The major goal for this work is to develop a fundamentally new learning curve for novel proton exchange membranes needed for various applications. The new technology will overcome the shortcomings of the current state-of-the-art membranes that are based upon Nafion polymer by offering higher proton conductivities, the ability to operate at much lower external humidity levels and higher temperatures (>120°C), and negligible fuel and oxidant crossover through the membrane.
36 MATERIALS SCIENCE↗
A Study of Phonon Modes of Magnetic Two-dimensional Materials using Optical Spectroscopy
Raman spectroscopy measuring phonon vibration modes of MnBi2nTe3n+1 (n=1,2) showed abnormal changes in linewidths of MnBi2Te4. Out-of-plane force constant was also estimated via Davydov splitting of A1g mode (136 cm-1) of MnBi4Te7.
Kang, Jin Ho↗
Two-dimensional material based layer transfer: challenges and opportunities
For future of electronics such as bioelectronics, 3D integrated electronics, and bendable electronics, needs for flexibility and stackability of electronic products have substantially grown up. However, conventional wafer-based single-crystalline semiconductors cannot catch up with such trends because they are bound to the thick rigid wafers such that they are neither flexible nor stackable. Although polymer-based organic electronic materials are more compatible as they are mechanically complaint and less costly than inorganic counterparts, their electronic/photonic performance is substantially inferior to that of single-crystalline inorganic materials. For the past few decades, many different strategies to mitigate such performance-mechanical compliance dilemma by developing methods to obtain cheap, flexible, stackable, single-crystalline inorganic systems. In today’s talk, I will discuss about various strategies to realize such a dream electronic system and how these strategies unlock new ways of manufacturing advanced electronic systems. One of the most promising approaches is to fabricate freestanding ultrathin membranes from existing functional material wafers by peeling the active layers from the top. I will overview all freestanding membrane methods and especially dive deeply into 2D materials-based layer transfer (2DLT) technique that has been recently emerging as one of the very promising options. In the 2DLT process, one can produce single-crystalline freestanding membranes from 3D materials by performing remote epitaxy or van der Waals epitaxy on 2D materials followed by peeling active single-crystalline films from the 2D materials. I will discuss about the challenges and opportunities of this technology. In addition, I will present unprecedented artificial heterostructures that can be enabled by stacking those freestanding 3D material membranes.
KIM, JEEHWAN↗
Proton selective membranes based on two dimensional materials
Proton conductive membrane includes a proton selective layer of 80-100% carbon with sp2 hybridization having a thickness of 0.3-100 nm, with 0-20% of hydrogen, oxygen, nitrogen and sp3 carbon; wherein the sp2 carbon is in a form of graphene-like material; the proton selective layer having a plurality of pores formed by any of 7, 8, 9 or 10 sp2 carbon cycles or a combination thereof, with the pores having an effective diameter of up to 0.6 nm; an ionomeric polymer layer on the proton selective layer. Total thickness of the proton conductive membrane is less than 50 microns. The ionomeric polymer is PFSA (perfluorinated sulfonic acid), PVP (polyvinylpyrrolidone) or PVA (poly vinyl alcohol) with iodide or bromide counterion dissolved inside. The graphene-like material is CVD graphene or reduced graphene oxide (rGO). A D to G Raman band ratio of the membrane is more than 0.1.
Smirnov, Sergei↗
Method of manufacturing proton selective membranes based on two dimensional materials
Proton conductive membrane includes a proton selective layer of 80-100% carbon with sp2 hybridization having a thickness of 0.3-100 nm, with 0-20% of hydrogen, oxygen, nitrogen and sp3 carbon; wherein the sp2 carbon is in a form of graphene-like material; the proton selective layer having a plurality of pores formed by any of 7, 8, 9 or 10 sp2 carbon cycles or a combination thereof, with the pores having an effective diameter of up to 0.6 nm; an ionomeric polymer layer on the proton selective layer. Total thickness of the proton conductive membrane is less than 50 microns. The ionomeric polymer is PFSA (perfluorinated sulfonic acid), PVP (polyvinylpyrrolidone) or PVA (poly vinyl alcohol) with iodide or bromide counterion dissolved inside. The graphene-like material is CVD graphene or reduced graphene oxide (rGO). A D to G Raman band ratio of the membrane is more than 0.1.
Smirnov, Sergei↗
Two-dimensional multifunctional materials from endohedral fullerenes
A multifunctional two-dimensional (2D) material is theoretically predicted based on systematic ab initio calculations and model simulations for the honeycomb lattice of endohedral fullerene W@C 28 molecules. It has structural bistability, ferroelectricity, multiple magnetic phases, and excellent valley characters and can be easily functionalized by the proximity effect with magnetic isolators such as MnTiO 3 . Furthermore, we may also manipulate the valley Hall and spin transport properties by selectively switching a few W@C 28 molecules to the metastable phase. These conclusions contribute to integrating different functions in a single 2D material for technological innovations.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Data mining and computational screening of Rashba-Dresselhaus splitting and optoelectronic properties in two-dimensional perovskite materials
Recent developments highlighting the promise of two-dimensional perovskites have vastly increased the compositional search space in the perovskite family. This presents a great opportunity for the realization of highly performant devices and practical challenges associated with the identification of candidate materials. High-fidelity computational screening offers great value in this regard. In this study, we carry out a multiscale computational workflow, generating a dataset of two-dimensional perovskites in the Dion-Jacobson and Ruddlesden-Popper phases. Our dataset comprises ten B-site cations, four halogens, and over 20 organic cations across over 2000 materials. We compute electronic properties, thermoelectric performance, and numerous geometric characteristics. Furthermore, we introduce a framework for the high-throughput computation of Rashba-Dresselhaus splitting. Finally, we use this dataset to train machine learning models for the accurate prediction of band gaps, candidate Rashba-Dresselhaus materials, and partial charges. The work presented herein can aid future investigations of two-dimensional perovskites with targeted applications in mind.
14 SOLAR ENERGY↗
Tuning energy barriers by doping 2D group-IV monochalcogenides
Structural degeneracies underpin the ferroic behavior of anisotropic next-generation two-dimensional materials and lead to peculiar two-dimensional structural transformations under external fields, charge doping, and/or temperature. The most direct indicator of the ease of these transformations is an elastic energy barrier, defined as the energy difference between the (degenerate) structural ground state unit cell and a unit cell with an increased structural symmetry. Proximity of a two-dimensional material to a bulk substrate can affect the magnitude of the critical fields and/or temperature at which these transformations occur, with the first effect being a relative charge transfer, which could trigger a structural quantum phase transition. With this physical picture in mind, we report the effect of modest charge doping (within -0.2 and +0.2 electrons per unit cell) on the elastic energy barrier J s of ferroelastic black phosphorene and nine ferroelectric/ferroelastic monochalcogenide monolayers. J s is the energy needed to create a Pnm2 1 →P4/nmm two-dimensional structural transformation, and it is sensitive to the orbital character of the electronic charge added or removed. Similar to the effect on the elastic energy barrier of ferroelastic SnO monolayers, group-IV monochalcogenide monolayers show a tunable elastic energy barrier for similar amounts of doping, and a decrease (increase) of J s can be engineered under a modest hole (electron) doping of not more than one-tenth of an electron or a hole per atom.
36 MATERIALS SCIENCE↗
Thin film CZTSSe photovoltaic device
A method for forming a back contact on an absorber layer in a photovoltaic device includes forming a two dimensional material on a first substrate. An absorber layer including Cu—Zn—Sn—S(Se) (CZTSSe) is grown over the first substrate on the two dimensional material. A buffer layer is grown on the absorber layer on a side opposite the two dimensional material. The absorber layer is exfoliated from the two dimensional material to remove the first substrate from a backside of the absorber layer opposite the buffer layer. A back contact is deposited on the absorber layer.
14 SOLAR ENERGY↗
Atomic-scale manipulation of polar domain boundaries in monolayer ferroelectric In 2 Se 3
Domain boundaries have been intensively investigated in bulk ferroelectric materials and two-dimensional materials. Many methods such as electrical, mechanical and optical approaches have been utilized to probe and manipulate domain boundaries. So far most research focuses on the initial and final states of domain boundaries before and after manipulation, while the microscopic understanding of the evolution of domain boundaries remains elusive. In this paper, we report controllable manipulation of the domain boundaries in two-dimensional ferroelectric In 2 Se 3 with atomic precision using scanning tunneling microscopy. We show that the movements of the domain boundaries can be driven by the electric field from a scanning tunneling microscope tip and proceed by the collective shifting of atoms at the domain boundaries. Our density functional theory calculations reveal the energy path and evolution of the domain boundary movement. The results provide deep insight into domain boundaries in two-dimensional ferroelectric materials and will inspire inventive applications of these materials.
36 MATERIALS SCIENCE↗
Mechanical properties of freestanding few-layer graphene/boron nitride/polymer heterostacks investigated with local and non-local techniques
van der Waals two-dimensional materials and heterostructures combined with polymer films continue to attract research attention to elucidate their functionality and potential applications. This study presents the fabrication and mechanical testing of 2D material heterostacks, consisting of few-layer boron nitride and graphene heterostructures synthesized via chemical vapor deposition, capped with a polymethyl methacrylate layer and suspended across ∼200 μm wide trenches using a combined wet–dry transfer method. The mechanical characterization of the heterostacks was performed using two independent approaches: (a) non-local testing with a custom-built tensile testing platform and (b) local load–displacement testing employing atomic force microscopy probes, complemented by finite element simulations. Both approaches provided new results, which are in good agreement with each other. Overall, our findings offer new insights into a combined load capacity in complex multi-material two-dimensional systems, and can contribute to advancing micro and nano-scale device designs and implementations.
Lespasio, Marcus↗
Noncollinear phases in moiré magnets
Significance Moiré patterns, which result when two or more two-dimensional materials with incommensurate or rotated lattices are layered together, create controllable electronic bands that have recently been shown to induce a tremendous wealth of physical phenomena. Here we initiate the theoretical study of moiré patterns’ influence on magnetic states of localized spins. We construct a general formalism using continuum field theory and present thorough analyses in twisted bilayers of antiferromagnets and also ferromagnets, which are within the experimental reach of Van der Waals materials. Two-dimensional magnets, well known for their strong intrinsic spin fluctuations, may serve as a new platform for moiré effects and open the door to a large class of novel phenomena that were once unimaginable.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗