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At least 1,153 records · Page 64
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Stacking-dependent interlayer coupling in trilayer MoS 2 with broken inversion symmetry

The stacking configuration in few-layer two-dimensional (2D) materials results in different structural symmetries and layer-to-layer interactions, and hence it provides a very useful parameter for tuning their electronic properties. For example, ABA-stacking trilayer graphene remains semimetallic similar to that of monolayer, while ABC-stacking is predicted to be a tunable band gap semiconductor under an external electric field. Such stacking dependence resulting from many-body interactions has recently been the focus of intense research activities. Here we demonstrate that few-layer MoS 2 samples grown by chemical vapor deposition with different stacking configurations (AA, AB for bilayer; AAB, ABB, ABA, AAA for trilayer) exhibit distinct coupling phenomena in both photoluminescence and Raman spectra. By means of ultralow-frequency (ULF) Raman spectroscopy, we demonstrate that the evolution of interlayer interaction with various stacking configurations correlates strongly with layer-breathing mode (LBM) vibrations. Our ab initio calculations reveal that the layer-dependent properties arise from both the spin–orbit coupling (SOC) and interlayer coupling in different structural symmetries. Lastly, such detailed understanding provides useful guidance for future spintronics fabrication using various stacked few-layer MoS 2 blocks.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Dynamic Carrier Modulation via Nonlinear Acoustoelectric Transport in van der Waals Heterostructures

Dynamically manipulating carriers in van der Waals heterostructures could enable solid-state quantum simulators with tunable lattice parameters. A key requirement is the formation of deep potential wells to reliably trap excitations. Here, we report the observation of nonlinear acoustoelectric transport and dynamic carrier modulation in boron nitride-encapsulated graphene devices coupled to intense surface acoustic waves (SAWs) on LiNbO 3 substrates. SAWs generate strong acoustoelectric current densities ( J AE ), transitioning from linear to nonlinear regimes with increasing SAW intensity. In the nonlinear regime, periodic carrier (electrons, holes, or their mixtures) stripes emerge. Using counter-propagating SAWs, we create standing SAWs (SSAWs) to dynamically manipulate charge distributions without static gates. The saturation of J AE , attenuation transitions, and tunable resistance peaks confirms strong carrier localization. Finally, these results establish SAWs as a powerful tool for controlling carrier dynamics in two-dimensional (2D) materials, paving the way for the development of time-dependent quantum systems and acoustic lattices for quantum simulation.

2D materials↗
DOE OSTI2017

Sequence of Silicon Monolayer Structures Grown on a Ru Surface: from a Herringbone Structure to Silicene

Silicon-based two-dimensional (2D) materials are uniquely suited for integration in Si-based electronics. Silicene, an analogue of graphene, was recently fabricated on several substrates and was used to make a field-effect transistor. Here, we report that when Ru(0001) is used as a substrate, a range of distinct monolayer silicon structures forms, evolving toward silicene with increasing Si coverage. Low Si coverage produces a herringbone structure, a hitherto undiscovered 2D phase of silicon. With increasing Si coverage, herringbone elbows evolve into silicene-like honeycomb stripes under tension, resulting in a herringbone-honeycomb 2D superlattice. At even higher coverage, the honeycomb stripes widen and merge coherently to form silicene in registry with the substrate. Scanning tunneling microscopy (STM) was used to image the structures. The structural stability and electronic properties of the Si 2D structures, the interaction between the Si 2D structures and the Ru substrate, and the evolution of the distinct monolayer Si structures were elucidated by density functional theory (DFT) calculations. This work paves the way for further investigations of monolayer Si structures, the corresponding growth mechanisms, and possible functionalization by impurities.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Plasmonic Hot-Carrier Generation and Catalysis in Ti 3 C 2 O 2 from Real-Time TDDFT Simulations

Photoinduced hot electrons are central to plasmon-driven catalysis. Atomically thin Ti 3 C 2 O 2 , with high carrier density and broad optical absorption, offers a promising platform for plasmon-driven reactions. However, comprehensive investigations of its plasmon resonance, hot-carrier generation, and plasmonic catalytic performance remain limited. In this work, real-time time-dependent density functional theory (rt-TDDFT) was employed to study Ti 3 C 2 O 2 ’s plasmon excitation and hot-carrier generation from nonradiative plasmon damping. The temporal evolution of the dipole moment reveals plasmon resonance in Ti 3 C 2 O 2 , followed by strong plasmon damping that redistributes the stored energy to generate hot carriers. Ti 3 C 2 O 2 with low oxygen vacancy concentration (O v -Ti 3 C 2 O 2 ) exhibits plasmonic behavior resembling the pristine surface, and the plasmon-generated hot electrons can markedly reduce the dissociation barrier of CO 2 at the oxygen vacancy. These findings provide fundamental insights into the plasmonic properties of Ti 3 C 2 O 2 and how they drive its catalytic performance in surface reactions, which is valuable for advancing plasmon-driven catalysis.

CO2 reduction↗
DOE OSTI2017

Dynamic Phase Engineering of Bendable Transition Metal Dichalcogenide Monolayers

Current interest in two-dimensional (2D) materials is driven in part by the ability to dramatically alter their optoelectronic properties through strain and phase engineering. A combination of these approaches can be applied in quasi-2D transition metal dichalcogenide (TMD) monolayers to induce displacive structural transformations between semiconducting (H) and metallic/semimetallic (T') phases. We classify such transformations in Group VI TMDs, and formulate a multiscale, first-principles-informed modeling framework to describe evolution of microstructural domain morphologies in elastically bendable 2D monolayers. We demonstrate that morphology and mechanical response can be controlled via application of strain either uniformly or through local probes to generate functionally patterned conductive T' domains. Such systems form dynamically programmable electromechanical 2D materials, capable of rapid local switching between domains with qualitatively different transport properties. Finally, this enables dynamic “drawing” of localized conducting regions in an otherwise semiconducting TMD monolayer, opening several interesting device-relevant functionalities such as the ability to dynamically “rewire” a device in real time.

2D materials↗
DOE OSTI2017

Direct Four-Probe Measurement of Grain-Boundary Resistivity and Mobility in Millimeter-Sized Graphene

Grain boundaries (GBs) in polycrystalline graphene scatter charge carriers, which reduces carrier mobility and limits graphene applications in high-speed electronics. Here we report the extraction of the resistivity of GBs and the effect of GBs on carrier mobility by direct four-probe measurements on millimeter-sized graphene bicrystals grown by chemical vapor deposition (CVD). To extract the GB resistivity and carrier mobility from direct four-probe intragrain and intergrain measurements, an electronically equivalent extended 2D GB region is defined based on Ohm’s law. Measurements on seven representative GBs find that the maximum resistivities are in the range of several kΩ·μm to more than 100 kΩ·μm. Furthermore, the mobility in these defective regions is reduced to 0.4–5.9‰ of the mobility of single-crystal, pristine graphene. Similarly, the effect of wrinkles on carrier transport can also be derived. We report the present approach provides a reliable way to directly probe charge-carrier scattering at GBs and can be further applied to evaluate the GB effect of other two-dimensional polycrystalline materials, such as transition-metal dichalcogenides (TMDCs).

36 MATERIALS SCIENCE↗
DOE OSTI2018

Persistent Charge-Density-Wave Order in Single-Layer TaSe 2

Understanding the collective electronic phases of two-dimensional (2D) materials requires a comprehensive investigation of the relation between lattice symmetry and electronic structure1-3. Single layers of transition metal dichalcogenides (TMDs) provide a material platform to study collective states such as charge density wave (CDW) and superconductivity4-7. By combining angle-resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy/spectroscopy (STM/STS), and density functional theory (DFT) calculations, we present the electronic characterization of single-layer 1H-TaSe2 grown by molecular beam epitaxy (MBE). We demonstrate that 3×3 CDW order persists despite distinct changes in the low energy electronic structure highlighted by a reduction in the number of bands crossing the Fermi energy (EF) and corresponding modification of Fermi surface (FS) topology. Enhanced spin orbit coupling and lattice distortion in the single-layer limit play a crucial role in the formation of CDW order. Our findings provide a deeper understanding of the nature of CDW order in the 2D limit.

2D materials↗
DOE OSTI2018

Electron-Beam-Induced Synthesis of Hexagonal 1 H -MoSe 2 from Square β-FeSe Decorated with Mo Adatoms

Two-dimensional (2D) materials have generated interest in the scientific community because of the advanced electronic applications they might offer. Powerful electron beam microscopes have been used not only to evaluate the structures of these materials but also to manipulate them by forming vacancies, nanofragments, and nanowires or joining nanoislands together. In this work, we show that the electron beam in a scanning transmission electron microscope (STEM) can be used in yet another way: to mediate the synthesis of 2D 1H-MoSe 2 from Mo-decorated 2D β-FeSe and simultaneously image the process on the atomic scale. This is quite remarkable given the different crystal structures of the reactant (square lattice β-FeSe) and the product (hexagonal lattice 1H-MoSe 2 ). Herein, the feasibility of the transformation was first explored by theoretical calculations that predicted that the reaction is exothermic. Furthermore, a theoretical reaction path to forming a stable 1H-MoSe 2 nucleation kernel within pure β-FeSe was found, demonstrating that the pertinent energy barriers are smaller than the energy supplied by the STEM electron beam.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Superconductor-Insulator Transitions in Exfoliated Bi 2 Sr 2 CaCu 2 O 8+δ Flakes

The recently developed solid electrolyte not only allows for substantial charge carrier modulation, which is crucial for realizing emergent quantum phenomena, but also offers a facile platform to integrate with surface sensitive probes, such as tunneling spectroscopy. Here, we apply this unique technique to address the superconductor-insulator transition (SIT) in mechanically exfoliated Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO) by transport and tunneling measurements. Back-gating via the solid electrolyte engenders intercalation of lithium ions into BSCCO, inducing a reversible SIT. Through tunneling spectroscopy across a graphite (G)/BSCCO heterojunction, we further observe that a symmetric pseudo-gap persists deep in the insulating regime, indicative of a percolative transition. The combination of the field effect modulation and spectroscopic study is applicable to a large variety of two-dimensional (2D) materials for gaining deeper insights into the many-body physics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

MoS 2 Liquid Cell Electron Microscopy Through Clean and Fast Polymer-Free MoS 2 Transfer

Two dimensional (2D) materials have found various applications because of their unique physical properties. For example, graphene has been used as the electron transparent membrane for liquid cell transmission electron microscopy (TEM) due to its high mechanical strength and flexibility, single-atom thickness, chemical inertness, etc. Here, we report using 2D MoS 2 as a functional substrate as well as the membrane window for liquid cell TEM, which is enabled by our facile and polymer-free MoS 2 transfer process. This provides the opportunity to investigate the growth of Pt nanocrystals on MoS 2 substrates, which elucidates the formation mechanisms of such heterostructured 2D materials. We find that Pt nanocrystals formed in MoS 2 liquid cells have a strong tendency to align their crystal lattice with that of MoS 2 , suggesting a van der Waals epitaxial relationship. Importantly, we can study its impact on the kinetics of the nanocrystal formation. Here, the development of MoS 2 liquid cells will allow further study of various liquid phenomena on MoS 2 , and the polymer-free MoS 2 transfer process will be implemented in a wide range of applications.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Quasi-Ballistic Thermal Transport Across MoS 2 Thin Films

Layered two-dimensional (2D) materials have highly anisotropic thermal properties between the in-plane and cross-plane directions. Conventionally, it is thought that cross-plane thermal conductivities (κ z ) are low, and therefore c -axis phonon mean free paths (MFPs) are small. Here, we measure κ z across MoS 2 films of varying thickness (20–240 nm) and uncover evidence of very long c -axis phonon MFPs at room temperature in these layered semiconductors. Experimental data obtained using time-domain thermoreflectance (TDTR) are in good agreement with first-principles density functional theory (DFT). These calculations suggest that ~50% of the heat is carried by phonons with MFP > 200 nm, exceeding kinetic theory estimates by nearly 2 orders of magnitude. Because of quasi-ballistic effects, the κ z of nanometer-thin films of MoS 2 scales with their thickness and the volumetric thermal resistance asymptotes to a nonzero value, ~10 m 2 K GW –1 . This contributes as much as 30% to the total thermal resistance of a 20 nm thick film, the rest being limited by thermal interface resistance with the SiO 2 substrate and top-side aluminum transducer. Furthermore, these findings are essential for understanding heat flow across nanometer-thin films of MoS 2 for optoelectronic and thermoelectric applications.

42 ENGINEERING↗
DOE OSTI2019

Nonlinear Mode Coupling and One-to-One Internal Resonances in a Monolayer WS 2 Nanoresonator

Nanomechanical resonators make exquisite force sensors due to their small footprint, low dissipation, and high frequencies. Because the lowest resolvable force is limited by ambient thermal noise, resonators are either operated at cryogenic temperatures or coupled to a high-finesse optical or microwave cavity to reach sub aN Hz -1/2 . sensitivity. Here, we show that operating a monolayer WS 2 nanoresonator in the strongly nonlinear regime can lead to comparable force sensitivities at room temperature. Cavity interferometry was used to transduce the nonlinear response of the nanoresonator, which was characterized by multiple pairs of 1:1 internal resonance. Some of the modes exhibited exotic line shapes due to the appearance of Hopf bifurcations, where the bifurcation frequency varied linearly with the driving force and forms the basis of the advanced sensing modality. The modality is less sensitive to the measurement bandwidth, limited only by the intrinsic frequency fluctuations, and therefore, advantageous in the detection of weak incoherent forces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Tailored Plasmons in Pentacene/Graphene Heterostructures with Interlayer Electron Transfer

van der Waals (vdW) heterostructures, which are produced by the precise assemblies of varieties of two-dimensional (2D) materials, have demonstrated many novel properties and functionalities. Here we report a nanoplasmonic study of vdW heterostructures that were produced by depositing ordered molecular layers of pentacene on top of graphene. We find through nanoinfrared (IR) imaging that surface plasmons formed due to the collective oscillations of Dirac Fermions in graphene are highly sensitive to the adjacent pentacene layers. In particular, the plasmon wavelength declines systematically but nonlinearly with increasing pentacene thickness. Further analysis and density functional theory (DFT) calculations indicate that the observed peculiar thickness dependence is mainly due to the tunneling-type electron transfer from pentacene to graphene. Here, our work unveils a new method for tailoring graphene plasmons and deepens our understanding of the intriguing nano-optical phenomena due to interlayer couplings in novel vdW heterostructures.

42 ENGINEERING↗
DOE OSTI2022

Penta-SiCN: A Highly Auxetic Monolayer

We report the negative Poisson’s (NPR) ratio in a two-dimensional (2D) material is a counterintuitive mechanical property that facilitates the development of nanoscale devices with sophisticated functionality. Inspired by the peculiar buckled lower-symmetric, trilayered geometry of pentagonal monolayers, we theoretically predict penta-SiCN, a ternary auxetic metallic monolayer with highly tunable NPR. The penta-SiCN is structurally, thermally, dynamically, and mechanically stable, and sustainable at and beyond room temperature with experimental feasibility. It possesses nontrivial geometrical and mechanical isotropy and relatively moderate thickness. Remarkably, the shorter and quasi sp3-hybridized C–N bond and the rigidity against the strain allow the monolayer to possess a high value of NPR (-0.136), even higher than that of black phosphorene, extendable up to -0.639 by 4% of biaxial stretching. On the other hand, the 2D Young’s modulus of 129.88 N/m decreases to 41.34 N/m at equivalent stretching, indicating relative softening and flexibility. Interestingly, a buckled-to-planar phase transition is identified at 10% biaxial strain before it suffers the fracture at 16%. Additionally, the strong optical anisotropy, absorbance (up to 6.51 x 105 cm -1 ), and presence of plasmon frequency demonstrate its potential application in optomechanical and plasmonics.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Role of Heterointerface in Lithium-Induced Phase Transition in T d -WTe 2 Nanoflakes

A new polytype of WTe 2 with a bandgap has been recently discovered through the intercalation of lithium into the van der Waals gaps of T d -WTe 2 . Here, we report the effects of reduced thicknesses and heterointerfaces on the intercalation-induced phase transition in WTe 2 . Using in situ Raman spectroscopy during the electrochemical lithiation of WTe 2 flakes as a function of flake thickness, we observe that additional electrochemical energy is required for the phase transition of WTe 2 from the T d phase to the new lithiated T d ' phase, going from 0.8 V of the applied electrochemical voltage for a thick flake to 0.5 V and 0.3 V for 7- and 5-layered samples, respectively. We ascribe this suppression of the phase transition to the interfacial interaction between the nanoflake and SiO 2 /Si substrate, which plays an increasing role as the sample thickness is reduced. The suppressed kinetics of the phase transition can be mitigated by placing the WTe 2 flake on a hexagonal boron nitride (hBN) flake, which facilitates the release of the in-plane strain induced by the phase transition. Importantly, our study underscores the significance of interfacial effects in modulating phase transitions in two-dimensional (2D) materials, suggesting heterogeneous transition pathways, as well as interfacial engineering to control these phase transitions.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Inhibition of AlF3·3H2O Impurity Formation in Ti3C2Tx MXene Synthesis under a Unique CoFx/HCl Etching Environment

MXenes, most commonly transition metal carbides, are a family of two-dimensional (2D) materials with promising potential in, among other applications, supercapacitors and batteries. MXenes are synthesized by etching of aluminum or gallium layers in its parent MAX phase directly by HF or by HF in situ formation using a fluoride salt and strong acid. A commonly undesired byproduct of MXene synthesis is AlF3·3H2O. To relieve MXenes from AlF3·3H2O impurity, it is important to elucidate the factors that drive its formation. Here, we dually deduce the conditions that lead to AlF3·3H2O formation while exploring etching with cobalt fluorides (CoF2/CoF3). Previously uncharacterized, etching with cobalt fluorides offers a forthright method to etch MXenes while intercalating cobalt cations. The influence of this etching environment and AlF3·3H2O formation on MXene’s structure, morphology, and surface bonding is investigated. Ionic strength of solution used for etching is found to be a critical driving factor in the formation of AlF3·3H2O impurity formation. Specifically, when the ionic strength falls between ~8.5 and 10 M, AlF3 complexation is stable. As a result, Ti3C2Tx MXene phase with AlF3·3H2O impurity is obtained. In contrast, near-pure MXene is the only solid state product when I is smaller than 8.5 M or larger than 10 M. Hence, high purity MXene phase can be synthesized by subtle compositional tuning to manipulate the ionic strength of the etching environment.

Cockerham, Cody B.↗
DOE OSTI2023

Role of Curvature in Stabilizing Boron-Doped Nanocorrugated Graphene

Boron-doped carbon nanostructures have attracted great interest recently because of their remarkable electrocatalytic performance comparable to or better than that of conventional metal catalysts. In a previous work (Carbon 123, 605 (2017)), we reported that along with significant performance improvement, B doping enhances the oxidation resistance of few-layer graphene (FLG) that provides increased structural stability for intermediate-temperature fuel-cell electrodes. In general, detailed characterization of the atomic and electronic structure transformations that occur in B-doped carbon nanostructures during fuel-cell operation is lacking. In this work, we use aberration-corrected scanning transmission electron microscopy, nanobeam electron diffraction, and electron energy-loss spectroscopy (EELS) to characterize the atomic and electronic structures of B-doped FLG before and after fuel-cell operation. These data point to the nanoscale corrugation of B-doped FLGs as the key factor responsible for increased stability and high corrosion resistance. In conclusion, the similarity of the 1s to π* and σ* transition features in the B K-edge EELS to those in B-doped carbon nanotubes provides an estimate for the curvature of nanocorrugation in B-FLG.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Raman Digital Twin of Monolayer Janus Transition Metal Dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) are a key class of two-dimensional (2D) materials with broad technological potential. Their Janus counterparts exhibit unique properties due to broken out-of-plane symmetry and further enrich the functionalities of TMDs. However, experimental synthesis and identification of Janus TMDs remain challenging. It is thus highly desirable to have a rapid, simple, and in situ characterization technique to monitor, in real time, the conversion process from the parent to Janus structure. Raman spectroscopy stands out for such a task as it is a powerful, nondestructive, and very commonly used tool to characterize 2D materials both in situ and ex situ. To realize the full potential of Raman spectroscopy on rapid characterization of Janus TMDs, we present a computational “Raman digital twin” library for various monolayer Janus TMDs in both 2H and Td phases. We focus on group-6 TMDs: MoS 2 , WS 2 , MoSe 2 , WSe 2 , MoTe 2 , WTe 2 and their Janus variants: MoSSe, MoSTe, MoSeTe, WSSe, WSTe, and WSeTe. Using first-principles density functional theory (DFT), we calculate their vibrational properties and predict distinct Raman fingerprints. These phonon and Raman signatures reflect each material’s structural symmetry and atomic composition, enabling clear identification via Raman spectroscopy. Our theoretical work supports experimental efforts by providing benchmarks for material identification, structural analysis, and quality control. In conclusion, the computational library expedites the discovery and development of Janus 2D materials, facilitating tighter integration between theoretical predictions and experimental validation.

Chalcogenides↗