Insertion of the Liquid Crystal 5CB into Monovacancy Graphene.

Interfacial interactions between liquid crystal (LC) and two-dimensional (2D) materials provide a platform to facilitate novel optical and electronic material properties. These interactions are uniquely sensitive to the local energy landscape of the atomically thick 2D surface, which can be strongly influenced by defects that are introduced, either by design or as a byproduct of fabrication processes. Herein, we present density functional theory (DFT) calculations of the LC mesogen 4-cyan-4'-pentylbiphenyl (5CB) on graphene in the presence of a monovacancy (MV-G). We find that the monovacancy strengthens the binding of 5CB in the planar alignment and that the structure is lower in energy than the corresponding homeotropic structure. However, if the molecule is able to approach the monovacancy homeotropically, 5CB undergoes a chemical reaction, releasing 4.5 eV in the process. This reaction follows a step-by-step process gradually adding bonds, inserting the 5CB cyano group into MV-G. We conclude that this irreversible insertion reaction is likely spontaneous, potentially providing a new avenue for controlling both LC behavior and graphene properties.

Synthesis and Structure of Sn14Cl6(CH2SiMe3)12: Toward Nanoclusters of 4-Coordinate α-Sn.

Orange crystals of a Sn14 cluster have been isolated in up to 22% yield from a reaction between Me3SiCH2SnCl3, SnCl4, and LiAlH4. The structure determined by single crystal X-ray diffraction shows three unique Sn atoms in a 6:6:2 ratio, with all Sn atoms 4-coordinate, similar to the tetrahedral bonding in elemental gray Sn. The solid state 117Sn MAS NMR spectrum shows the three types of distinct Sn atoms in the expected 3:3:1 intensity ratio with respective chemical shifts of 87.9, -66.6, and -607.1 ppm relative to Me4Sn. The chemical shift of the two Sn atoms without ligands (bonded only to Sn), at -607.1 ppm, is the most upfield, and is the closest to the chemical shift, reported here, of bulk gray tin (-910 ppm). First-principles density functional theory calculations of the chemical shielding tensors corroborate this assignment. While the core coordination is distorted from the ideal tetrahedral arrangement in the diamond structure of gray tin, this Sn14 cluster, as the largest reported cluster with all 4-coordinate Sn, represents a major incremental step toward being able to prepare atomically precise nanoparticles of gray tin.

Hydrolysis of Dimethyl Methylphosphonate by the Cyclic Tetramer of Zirconium Hydroxide.

We present hybrid density functional theory (DFT) calculations of hydrolysis of dimethyl methylphosphonate (DMMP) by the cyclic tetramer of zirconium hydroxide [Zr4(OH)16]. Various binding configurations of DMMP and its hydrolysis products on the tetramer as well as transition structures connecting them were explored using structure optimizations based on multiple, randomly selected initial structures. We find that DMMP can bind to the tetramer through the phosphoryl O, forming either a strong hydrogen bond to a bridging hydroxyl or a coordinate bond to a coordinatively unsaturated Zr atom. The resulting hydrogen-bonded complexes and Lewis adducts have similar energies. We also find that hydrolysis of a P-OCH3 bond can occur either via an addition-elimination mechanism involving a same-site terminal hydroxyl or direct interchange between a terminal hydroxyl and a methoxy group of DMMP. The computed activation and reaction enthalpies show that the addition-elimination is both kinetically and thermodynamically favored over the direct interchange. Our findings support recent observations of the reactivity of amorphous zirconium hydroxide toward phosphonate esters including chemical warfare agents.

Triangular lattice exciton model.

We present a minimalistic equilateral triangular lattice model showing explicitly that the two-dimensional hydrogen model for excitons breaks down for excitons in semiconducting monolayer transition-metal dichalcogenides due to lattice effects and that these excitons are neither Wannier nor Frenkel excitons but rather span an intermediate regime. The model is formulated on sparse form in direct space, allowing it to be solved with great computational efficiency.

Confinement, transport gap, and valley polarization in graphene from two parallel decorated line defects.

Quantum transport calculations show that a transport gap approximately E(g) = 2hv(F)/W can be engineered in graphene using two parallel transport barriers, separated by W, extended along the zigzag direction. The barriers, modeled by chemically decorated observed line defects, create confinement and resonance bands tracing the bands in zigzag nanoribbons. The resonance bands terminate at the dimensional crossover, where the states become boundary-localized, leaving the transport gap. The structure also allows for nearly perfect valley polarization.

Demonstration of the Effectiveness of the Cascaded Variational Quantum Eigensolver Using the Jastrow Ansatz for Molecular Calculations.

We demonstrate how the cascaded variational quantum eigensolver (CVQE) can be applied to study molecular systems for the family of Jastrow ansatzes. Specifically, we applied CVQE to the water molecule. We find that CVQE has a number of advantages. In particular, our results show that CVQE requires 2 to 3 orders of magnitude fewer quantum computing (QC) executions than VQE for the water molecule. Furthermore, our results indicate that CVQE might provide some robustness against two-qubit gate errors given that the number of CNOT gates used in our calculation was ∼300 and the errors in the QC calculations are still comparable to those obtained by VQE.

Chemically isolated graphene nanoribbons reversibly formed in fluorographene using polymer nanowire masks.

We demonstrated the fabrication of graphene nanoribbons (GNRs) as narrow as 35 nm created using scanning probe lithography to deposit a polymer mask(1-3) and then fluorinating the sample to isolate the masked graphene from the surrounding wide band gap fluorographene. The polymer protected the GNR from atmospheric adsorbates while the adjacent fluorographene stably p-doped the GNRs which had electron mobilities of ∼2700 cm2/(V·s). Chemical isolation of the GNR enabled resetting the device to nearly pristine graphene.

Edges bring new dimension to graphene nanoribbons.

Chemistry at the edges of saturated graphene nanoribbons can cause ribbons to leave the plane and form three-dimensional helical structures. Calculations, based on density functional theory and enabled by adopting helical symmetry, show that F-terminated armchair ribbons are intrinsically twisted in helices, unlike flat H-terminated strips. Twisting ribbons of either termination couple the conduction and valence bands, resulting in band gap modulation. This electromechanical response could be exploited in switches and sensor applications.

Conductance anisotropy in epitaxial graphene sheets generated by substrate interactions.

We present the first microscopic transport study of epitaxial graphene on SiC using an ultrahigh vacuum four-probe scanning tunneling microscope. Anisotropic conductivity is observed that is caused by the interaction between the graphene and the underlying substrate. These results can be explained by a model where charge buildup at the step edges leads to local scattering of charge carriers. This highlights the importance of considering substrate effects in proposed devices that utilize nanoscale patterning of graphene on electrically isolated substrates.

Structural and theoretical studies of 4-chloro-2-methyl-6-oxo-3,6-dideuteropyrimidin-1-ium chloride (d 6).

The title compound, C5D6ClN2O+·Cl-, crystallizes in the ortho-rhom-bic space group, Pbcm, and consists of a 4-chloro-2-methyl-6-oxo-3,6-di-hydro-pyrimidin-1-ium cation and a chloride anion where both moieties lie on a crystallographic mirror. The cation is disordered and was refined as two equivalent forms with occupancies of 0.750 (4)/0.250 (4), while the chloride anion is triply disordered with occupancies of 0.774 (12), 0.12 (2), and 0.11 (2). Unusually, the bond angles around the C=O unit range from 127.2 (6) to 115.2 (3)° and similar angles have been found in other structures containing a 6-oxo-3,6-di-hydro-pyrimidin-1-ium cation, including the monclinic polymorph of the title compound, which crystallizes in the monoclinic space group P21/c [Kawai et al. (1973 ▸). Cryst. Struct. Comm. 2, 663-666]. The cations and anions pack into sheets in the ab plane linked by N-H⋯Cl hydrogen bonds as well as C-H⋯O and Cl⋯O inter-actions. In graph-set notation, these form R 3 3(11) and R 3 2(9) rings. Theoretical calculations seem to indicate that the reason for the unusual angles at the sp 2 C is the electrostatic inter-action between the oxygen atom and the adjacent N-H hydrogen.

Synthesis, structure, and theoretical studies of a calcium complex of a unique dianion derived from 1-methyl-pyrrolidin-2-one.

The title compound, catena-poly[[tetra-kis-(1-methyl-pyrrolidin-2-one-κO)calcium(II)]-µ-(E)-1,1'-dimethyl-2,2'-dioxo-1,1',2,2'-tetra-hydro-[3,3'-bipyrrolyl-idene]-5,5'-bis-(thiol-ato)-κ2 O:O'], [Ca(C10H8N2O2S2)(C5H9NO)4] n , 1, crystallizes in the triclinic space group P . The crystal studied was twinned by non-merohedry via two different twofold operations, about the normals to (001) and (10), giving four twin domains with refined occupancies of 0.412 (4), 0.366 (4), 0.055 (1), 0.167 (4). The Ca atoms are located on centers of inversion. Each Ca is surrounded by four 1-methyl-pyrrolidin-2-one (NMP) ligands and coordinated through one of the two O atoms to two (E)-1,1'-dimethyl-2,2'-dioxo-1,1',2,2'-tetra-hydro-[3,3'-bipyrrolyl-idene]-5,5'-bis-(thiol-ate), [C10H8N2O2S2]2-, dianions (abbreviation: DMTBT). This dianion thus facilitates the formation of a 1-D polymer, which propagates in the [011] direction. These ribbons are linked by inter-molecular C-H⋯S inter-actions. Each Ca atom is in an octa-hedral CaO6 six-coordinate environment with Ca-O bond lengths ranging from 2.308 (6) to 2.341 (6) Å, cis bond angles ranging from 88.2 (2) to 91.8 (2)° and the trans angles all 180° due to the Ca atoms being located on centers of inversion. Theoretical calculations were carried out using density functional theory (DFT) and the results showed that although the central DMTBT dianion is planar there is likely some resonance across the central bond between both aza-pentyl rings, but this is not sufficient to establish a ring current. The calculated UV-vis spectrum shows a peak at 625 nm, which accounts for the deep blue-purple color of solutions of the complex.

Analysis of Correlated Dynamics in the Grotthuss Mechanism of Proton Diffusion.

Using a large set of ab initio molecular dynamics trajectories we demonstrate that the mechanistic details of aqueous proton diffusion are insensitive to finite size effects. Furthermore, we show how correlation in the proton hopping direction is related to the presolvation of the hydronium ion. Specifically, we observe a dependence of the probability for the excess proton to return to its previous hydronium ion on whether that hydronium ion was accepting a hydrogen bond from a fourth water molecule at the time the excess proton left. The dynamics of this fourth water molecule was previously linked to the net displacement of the proton, and our analysis shows that this connection is due to the changes in the hopping probability that we calculate. Additionally, we show how our simulated dynamics with correlations that imply a faster time scale are compatible with recent spectroscopy results that point to a slower hopping time scale by looking closely at which proton transitions are being taken into consideration. Finally, we show that the correlation in proton hopping directions is not strongly influenced by interactions among hydronium ions.

Thermotropic liquid crystal (5CB) on two-dimensional materials.

We present ground-state electronic properties of the liquid crystal 4-cyano-4^{'}-pentylbiphenyl (5CB) on the two-dimensional materials monolayer graphene, hexagonal boron nitride, and phosphorene. Our density functional theory results show that the physisorption is robust on all surfaces with the strongest binding of 5CB on phosphorene. All surfaces exhibit flexural distortion, especially monolayer graphene and hexagonal boron nitride. While we find type-I alignment for all three substrates, meaning the Fermi level of the system is in the HOMO-LUMO gap of 5CB, the band structures are qualitatively different. Unlike for graphene and phosphorene, the HOMO-LUMO of 5CB appear as localized states within the band gap of boron nitride. In addition, we find that the valence band for boron nitride is sensitive to the orientation of 5CB relative to the surface. The qualitatively different band structures demonstrate the importance of substrate selection for tailoring the electronic and optoelectronic properties of nematic liquid crystals on two-dimensional materials.

Correlated dynamics in aqueous proton diffusion.

The aqueous proton displays an anomalously large diffusion coefficient that is up to 7 times that of similarly sized cations. There is general consensus that the proton achieves its high diffusion through the Grotthuss mechanism, whereby protons hop from one molecule to the next. A main assumption concerning the extraction of the timescale of the Grotthuss mechanism from experimental results has been that, on average, there is an equal probability for the proton to hop to any of its neighboring water molecules. Herein, we present ab initio simulations that show this assumption is not generally valid. Specifically, we observe that there is an increased probability for the proton to revert back to its previous location. These correlations indicate that the interpretation of the experimental results need to be re-examined and suggest that the timescale of the Grotthuss mechanism is significantly shorter than was previously thought.

Using dark states for exciton storage in transition-metal dichalcogenides.

We explore the possibility of storing excitons in excitonic dark states in monolayer semiconducting transition-metal dichalcogenides. In addition to being optically inactive, these dark states require the electron and hole to be spatially separated, thus inhibiting electron/hole recombination and allowing exciton lifetimes to be extended. Based on an atomistic exciton model, we derive transition matrix elements and an approximate selection rule showing that excitons could be transitioned into and out of dark states using a pulsed infrared laser. For illustration, we also present exciton population scenarios based on a population analysis for different recombination decay constants. Longer exciton lifetimes could make these materials candidates for applications in energy management and quantum information processing.

Hidden one-electron interactions in carbon nanotubes revealed in graphene nanostrips.

Many single-wall carbon nanotube (SWNT) properties near the Fermi level were successfully predicted using a nearest-neighbor tight-binding model characterized by a single parameter, V1. We show however that this model fails for armchair-edge graphene nanostrips due to interactions directly across hexagons. These same interactions are found largely hidden in the description of SWNTs, where they renormalize V1 leaving previous nearest-neighbor model SWNT results largely intact while resolving a long-standing puzzle regarding the magnitude of V1.

Ballistic transport in graphene nanostrips in the presence of disorder: importance of edge effects.

Stimulated by recent advances in isolating graphene and similarities to single-wall carbon nanotubes, simulations were performed to assess the effects of static disorder on the conductance of metallic armchair- and zigzag-edge graphene nanostrips. Both strip types were found to have outstanding ballistic transport properties in the presence of a substrate-induced disorder. However, only the zigzag-edge strips retain these properties in the presence of irregular edges, making them better initial synthetic targets for ballistic device applications.

Graphene nanostrip digital memory device.

In equilibrium, graphene nanostrips, with hydrogens sp2-bonded to carbons along their zigzag edges, are expected to exhibit a spin-polarized ground state. However, in the presence of a ballistic current, we find that there exists a voltage range over which both spin-polarized and spin-unpolarized nanostrip states are stable. These states can represent a bit in a binary memory device that could be switched through the applied bias and read by measuring the current through the nanostrip.