Prediction of a Novel Electromechanical Response in Polar Polymers with Rigid Backbones: Contrasting Furan-Derived Nanothreads to Poly(Vinylidene Fluoride).

Syn furan nanothreads have all oxygen atoms arranged on one side of the thread backbone; these polar threads present intriguing opportunities in electromechanical response owing to their rigid ladder-like backbone. We retrained a C/H/O reactive force field to simulate their response to external electric field for both end-anchored individual threads and bulk nanothread crystals, contrasting the results to those for poly(vinylidene fluoride) (PVDF) polymer. Whereas the field induces a length-independent torque in PVDF through backbone rotation about σ bonds, furan-derived nanothreads generate a length-dependent torque by progressively twisting their rigid backbone. This mode of response couples the rotational history of the electric field to axial tension in the anchored thread. In simulations of densely packed syn furan nanothread crystals without anchors, the crystals pole in a field (∼3 GV/m at 300 K) similar to that seen in simulations of PVDF, suggesting that crystals of polar nanothreads can be ferroelectric.

Confined Monolayer Ag As a Large Gap 2D Semiconductor and Its Momentum Resolved Excited States.

2D materials have intriguing quantum phenomena that are distinctively different from their bulk counterparts. Recently, epitaxially synthesized wafer-scale 2D metals, composed of elemental atoms, are attracting attention not only for their potential applications but also for exotic quantum effects such as superconductivity. By mapping momentum-resolved electronic states using time-resolved and angle-resolved photoemission spectroscopy (ARPES), we reveal that monolayer Ag confined between bilayer graphene and SiC is a large gap (>1 eV) 2D semiconductor, consistent with ab initio GW calculations. The measured valence band dispersion matches the GW quasiparticle band structure. However, the conduction band dispersion shows an anomalously large effective mass of 2.4 m0. Possible mechanisms for this large enhancement in the "apparent mass" are discussed.

Theoretical Studies of Furan and Thiophene Nanothreads: Structures, Cycloaddition Barriers, and Activation Volumes.

This theoretical study examines the formation, structure, and stability of two of the most ordered nanothreads produced yet, those derived from furan and thiophene. The energetic consequences and activation barriers of the first two steps of oligomerization via a Diels-Alder mechanism were examined. The ca. 20 GPa difference in the synthetic pressures (lower for furan) is explainable in terms of the greater loss of aromaticity by the thiophene. The effects of pressure on the reaction profiles, operating through a volume decrease along the reaction coordinate, are illustrated. The interesting option of polymerization proceeding in one or two directions opens up the possibility of polymers with opposing, cumulative dipole moments. The computed activation volumes are consistently more negative for furan, in accordance with the lower onset pressure of furan polymerization. The energetics of three ordered polymer structures were examined. The syn polymer, with all O/S atoms on the same side, if not allowed to distort, is at a high energy relative to the other two due to the O/S lone pair repulsion, understandably greater for S than for O at the 2.8/2.6 Å separation. Set free, the syn isomers curve or arch in two- or three-dimensional (helical) ways, whose energetics are traced in detail. The syn polymer can also stabilize itself by twisting into zig-zag or helical energy minima. The release of strain in a linear thread as the pressure is relaxed to 1 atm, with consequent thread curving, is a likely mechanism for the observed loss of the crystalline order in the polymer as it is returned to ambient pressure.

Photochemically Mediated Polymerization of Molecular Furan and Pyridine: Synthesis of Nanothreads at Reduced Pressures.

Nanothreads are emerging one-dimensional sp3-hybridized materials with high predicted tensile strength and a tunable band gap. They can be synthesized by compressing aromatic or nonaromatic small molecules to pressures ranging from 15-30 GPa. Recently, new avenues are being sought that reduce the pressure required to afford nanothreads; the focus has been placed on the polymerization of molecules with reduced aromaticity, favorable stacking, and/or the use of higher reaction temperatures. Herein, we report the photochemically mediated polymerization of pyridine and furan aromatic precursors, which achieves nanothread formation at reduced pressures. In the case of pyridine, it was found that a combination of slow compression/decompression with broadband UV light exposure yielded a crystalline product featuring a six-fold diffraction pattern with similar interplanar spacings to previously synthesized pyridine-derived nanothreads at a reduced pressure. When furan is compressed to 8 GPa and exposed to broadband UV light, a crystalline solid is recovered that similarly demonstrates X-ray diffraction with an interplanar spacing akin to that of the high-pressure synthesized furan-derived nanothreads. Our method realizes a 1.9-fold reduction in the maximum pressure required to afford furan-derived nanothreads and a 1.4-fold reduction in pressure required for pyridine-derived nanothreads. Density functional theory and multiconfigurational wavefunction-based computations were used to understand the photochemical activation of furan and subsequent cascade thermal cycloadditions. The reduction of the onset pressure is caused by an initial [4+4] cycloaddition followed by increasingly facile thermal [4+2]-cycloadditions during polymerization.

Shape Entropy of a Reconfigurable Ising Surface.

Disclinations in a 2D sheet create regions of Gaussian curvature whose inversion produces a reconfigurable surface with many distinct metastable shapes, as shown by molecular dynamics of a disclinated graphene monolayer. This material has a near-Gaussian "density of shapes" and an effectively antiferromagnetic interaction between adjacent cones. A∼10 nm patch has hundreds of distinct metastable shapes with tunable stability and topography on the size scale of biomolecules. As every conical disclination provides an Ising-like degree of freedom, we call this technique "Isigami."

Nonstoichiometric Salt Intercalation as a Means to Stabilize Alkali Doping of 2D Materials.

Although doping with alkali atoms is a powerful technique for introducing charge carriers into physical systems, the resulting charge-transfer systems are generally not air stable. Here we describe computationally a strategy towards increasing the stability of alkali-doped materials that employs stoichiometrically unbalanced salt crystals with excess cations (which could be deposited during, e.g., in situ gating) to achieve doping levels similar to those attained by pure alkali metal doping. The crystalline interior of the salt crystal acts as a template to stabilize the excess dopant atoms against oxidation and deintercalation, which otherwise would be highly favorable. We characterize this doping method for graphene, NbSe_{2}, and Bi_{2}Se_{3} and its effect on direct-to-indirect band gap transitions, 2D superconductivity, and thermoelectric performance. Salt intercalation should be generally applicable to systems which can accommodate this "ionic crystal" doping (and particularly favorable when geometrical packing constraints favor nonstoichiometry).

Illuminating Invisible Grain Boundaries in Coalesced Single-Orientation WS2 Monolayer Films.

Engineering atomic-scale defects is crucial for realizing wafer-scale, single-crystalline transition metal dichalcogenide monolayers for electronic devices. However, connecting atomic-scale defects to larger morphologies poses a significant challenge. Using electron microscopy and ReaxFF reactive force field-based molecular dynamics simulations, we provide insights into WS2 crystal growth mechanisms, providing a direct link between synthetic conditions and microstructure. Dark-field TEM imaging of coalesced monolayer WS2 films illuminates defect arrays that atomic-resolution STEM imaging identifies as translational grain boundaries. Electron diffraction and high-resolution imaging reveal that the films have nearly a single orientation with imperfectly stitched domains that tilt out-of-plane when released from the substrate. Imaging and ReaxFF simulations uncover two types of translational mismatch, and we discuss their origin related to relatively fast growth rates. Statistical analysis of >1300 facets demonstrates that microstructural features are constructed from nanometer-scale building blocks, describing the system across sub-Ångstrom to multimicrometer length scales.

Perfect and Defective 13C-Furan-Derived Nanothreads from Modest-Pressure Synthesis Analyzed by 13C NMR.

The molecular structure of nanothreads produced by the slow compression of 13C4-furan was studied by advanced solid-state NMR. Spectral editing showed that >95% of carbon atoms were bonded to one hydrogen (C-H) and that there were 2-4% CH2, 0.6% C═O, and <0.3% CH3 groups. Alkenes accounted for 18% of the CH moieties, while trapped, unreacted furan made up 7%. Two-dimensional (2D) 13C-13C and 1H-13C NMR identified 12% of all carbon in asymmetric O-CH═CH-CH-CH- and 24% in symmetric O-CH-CH═CH-CH- rings. While the former represented defects or chain ends, some of the latter appeared to form repeating thread segments. Around 10% of carbon atoms were found in highly ordered, fully saturated nanothread segments. Unusually slow 13C spin-exchange with sites outside the perfect thread segments documented a length of at least 14 bonds; the small width of the perfect-thread signals also implied a fairly long, regular structure. Carbons in the perfect threads underwent relatively slow spin-lattice relaxation, indicating slow spin exchange with other threads and smaller amplitude motions. Through partial inversion recovery, the signals of the perfect threads were observed and analyzed selectively. Previously considered syn-threads with four different C-H bond orientations were ruled out by centerband-only detection of exchange NMR, which was, on the contrary, consistent with anti-threads. The observed 13C chemical shifts were matched well by quantum-chemical calculations for anti-threads but not for more complex structures like syn/anti-threads. These observations represent the first direct determination of the atomic-level structure of fully saturated nanothreads.

Atomically thin half-van der Waals metals enabled by confinement heteroepitaxy.

Atomically thin two-dimensional (2D) metals may be key ingredients in next-generation quantum and optoelectronic devices. However, 2D metals must be stabilized against environmental degradation and integrated into heterostructure devices at the wafer scale. The high-energy interface between silicon carbide and epitaxial graphene provides an intriguing framework for stabilizing a diverse range of 2D metals. Here we demonstrate large-area, environmentally stable, single-crystal 2D gallium, indium and tin that are stabilized at the interface of epitaxial graphene and silicon carbide. The 2D metals are covalently bonded to SiC below but present a non-bonded interface to the graphene overlayer; that is, they are 'half van der Waals' metals with strong internal gradients in bonding character. These non-centrosymmetric 2D metals offer compelling opportunities for superconducting devices, topological phenomena and advanced optoelectronic properties. For example, the reported 2D Ga is a superconductor that combines six strongly coupled Ga-derived electron pockets with a large nearly free-electron Fermi surface that closely approaches the Dirac points of the graphene overlayer.

Field-Tunable Interactions and Frustration in Underlayer-Mediated Artificial Spin Ice.

Artificial spin ice systems have opened experimental windows into a range of model magnetic systems through the control of interactions among nanomagnet moments. This control has previously been enabled by altering the nanomagnet size and the geometry of their placement. Here we demonstrate that the interactions in artificial spin ice can be further controlled by including a soft ferromagnetic underlayer below the moments. Such a substrate also breaks the symmetry in the array when magnetized, introducing a directional component to the correlations. Using spatially resolved magneto-optical Kerr effect microscopy to image the demagnetized ground states, we show that the correlation of the demagnetized states depends on the direction of the underlayer magnetization. Further, the relative interaction strength of nearest and next-nearest neighbors varies significantly with the array geometry. We exploit this feature to induce frustration in an inherently unfrustrated square lattice geometry, demonstrating new possibilities for effective geometries in two-dimensional nanomagnetic systems.