Giant Optical Second Harmonic Generation in Two-Dimensional Multiferroics

Nonlinear optical properties of materials such as second and higher order harmonic generation and electro-optic effect play pivotal roles in lasers, frequency conversion, electro-optic modulators, switches, and so forth. The strength of nonlinear optical responses highly depends on intrinsic crystal symmetry, transition dipole moments, specific optical excitation, and local environment. Using first-principles electronic structure theory, here we predict giant second harmonic generation (SHG) in recently discovered two-dimensional (2D) ferroelectric–ferroelastic multiferroics–group IV monochalcogenides (i.e., GeSe, GeS, SnSe, and SnS). Remarkably, the strength of SHG susceptibility in GeSe and SnSe monolayers is more than 1 order of magnitude higher than that in monolayer MoS₂, and 2 orders of magnitude higher than that in monolayer hexagonal BN. Their extraordinary SHG is dominated by the large residual of two opposite intraband contributions in the SHG susceptibility. More importantly, the SHG polarization anisotropy is strongly correlated with the intrinsic ferroelastic and ferroelectric orders in group IV monochalcogenide monolayers. Our present findings provide a microscopic understanding of the large SHG susceptibility in 2D group IV monochalcogenide multiferroics from first-principles theory and open up a variety of new avenues for 2D ferroelectrics, multiferroics, and nonlinear optoelectronics, for example, realizing active electrical/optical/mechanical switching of ferroic orders in 2D multiferroics and in situ ultrafast optical characterization of local atomistic and electronic structures using noncontact noninvasive optical SHG techniques

Tunable Exciton Funnel Using Moiré Superlattice in Twisted van der Waals Bilayer

A spatially varying bandgap drives exciton motion and can be used to funnel energy within a solid (Nat. Photonics 2012, 6, 866−872). This bandgap modulation can be created by composition variation (traditional heterojunction), elastic strain, or in the work shown next, by a small twist between two identical semiconducting atomic sheets, creating an internal stacking translation <i><b>u</b></i>(<i><b>r</b></i>) that varies gently with position <i><b>r</b></i> and controls the local bandgap <i>E</i>_g(<i><b>u</b></i>(<i><b>r</b></i>)). Recently synthesized carbon/boron nitride (Nat. Nanotechnol. 2013, 8, 119) and phosphorene (Nat. Nanotechnol. 2014, 9, 372) may be used to construct this twisted semiconductor bilayer that may be regarded as an in-plane crystal but an out-of-plane molecule, which could be useful in solar energy harvesting and electroluminescence. Here, by first-principles methods, we compute the bandgap map and delineate its material and geometric sensitivities. <i>E</i>_g(<i><b>u</b></i>(<i><b>r</b></i>)) is predicted to have multiple local minima (“funnel centers”) due to secondary or even tertiary periodic structures in-plane, leading to a hitherto unreported pattern of multiple “exciton flow basins”. A compressive strain or electric field will further enhance <i>E</i>_g-contrast in different regions of the pseudoheterostructure so as to absorb or emit even broader spectrum of light

Ripplocations in van der Waals Layers

Dislocations are topological line defects in three-dimensional crystals. Same-sign dislocations repel according to Frank’s rule |<b>b</b>₁ + <b>b</b>₂|² > |<b>b</b>₁|² + |<b>b</b>₂|². This rule is broken for dislocations in van der Waals (vdW) layers, which possess crystallographic Burgers vector as ordinary dislocations but feature “surface ripples” due to the ease of bending and weak vdW adhesion of the atomic layers. We term these line defects “ripplocations” in accordance to their dual “surface ripple” and “crystallographic dislocation” characters. Unlike conventional ripples on noncrystalline (vacuum, amorphous, or fluid) substrates, ripplocations tend to be very straight, narrow, and crystallographically oriented. The self-energy of surface ripplocations scales sublinearly with |<b>b</b>|, indicating that same-sign ripplocations attract and tend to merge, opposite to conventional dislocations. Using in situ transmission electron microscopy, we directly observed ripplocation generation and motion when few-layer MoS₂ films were lithiated or mechanically processed. Being a new subclass of elementary defects, ripplocations are expected to be important in the processing and defect engineering of vdW layers

Strain-Engineering of Band Gaps in Piezoelectric Boron Nitride Nanoribbons

Two-dimensional atomic sheets such as graphene and boron nitride monolayers represent a new class of nanostructured materials for a variety of applications. However, the intrinsic electronic structure of graphene and h-BN atomic sheets limits their direct application in electronic devices. By first-principles density functional theory calculations we demonstrate that band gap of zigzag BN nanoribbons can be significantly tuned under uniaxial tensile strain. The unexpected sensitivity of band gap results from reduced orbital hybridization upon elastic strain. Furthermore, sizable dipole moment and piezoelectric effect are found in these ribbons owing to structural asymmetry and hydrogen passivation. This will offer new opportunities to optimize two-dimensional nanoribbons for applications such as electronic, piezoelectric, photovoltaic, and opto-electronic devices

In Situ Observation of Random Solid Solution Zone in LiFePO₄ Electrode

Nanostructured LiFePO₄ (LFP) electrodes have attracted great interest in the Li-ion battery field. Recently there have been debates on the presence and role of metastable phases during lithiation/delithiation, originating from the apparent high rate capability of LFP batteries despite poor electronic/ionic conductivities of bulk LFP and FePO₄ (FP) phases. Here we report a potentiostatic in situ transmission electron microscopy (TEM) study of LFP electrode kinetics during delithiation. Using in situ high-resolution TEM, a Li-sublattice disordered solid solution zone (SSZ) is observed to form quickly and reach 10–25 nm × 20–40 nm in size, different from the sharp LFP|FP interface observed under other conditions. This 20 nm scale SSZ is quite stable and persists for hundreds of seconds at room temperature during our experiments. In contrast to the nanoscopically sharp LFP|FP interface, the wider SSZ seen here contains no dislocations, so reduced fatigue and enhanced cycle life can be expected along with enhanced rate capability. Our findings suggest that the disordered SSZ could dominate phase transformation behavior at nonequilibrium condition when high current/voltage is applied; for larger particles, the SSZ could still be important as it provides out-of-equilibrium but atomically wide avenues for Li⁺/e^– transport

Experimental validation of the Network model.

<p>Solid line indicates the viscosity of BLJ liquid calculated by the Network model. Symbols are experimental data on fragile glass formers <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0017909#pone.0017909-Angell1" target="_blank">[3]</a>.</p

Viscosity computed using the Network model expression, <b>Eq. (16)</b>, with the four TSP trajectories shown in <b>Fig. 2</b> as input.

<p>Results for each trajectory are denoted by a different symbol, squares for trajectory I, triangle for II, inverted triangles for III, and circles for trajectory IV. Solid curve is a spline fit to all the calculated viscosities.</p

Data used in the Network model calculation.

<p>(a) Average inherent structure (IS) energy of BLJ liquid as a function of temperature, (b) Distributions of IS energies at four temperatures, 1.0, 0.5, 0.4, and 0.3, (c) Four TSP trajectories initialized at different energy minima (right panel).</p

Comparison of an effective temperature-dependent activation barrier obtained from experimental data (symbols) [<b>4</b>] with similarly reduced results of the Network model (solid curve).

<p>The value of <i>T</i>* is 0.63 for the Network model.</p

ATF4-CHOP pathway and related apoptotic pathway inhibition by LPS preconditioning after IR.

<p>(A) Western-assisted analysis of ATF4, CHOP, Cleaved Caspase-12, Cleaved Caspase-3 and β-Actin. Representative of three experiments. (B) Relative quantities of protein of ATF4, CHOP, Cleaved Caspase-12, Cleaved Caspase-3 to β-Actin, Mean±SD, **P<0.001 versus sham group; ^##P<0.001 versus IR group; ^#P<0.05 versus IR group. (C) immunohistochmistry analysis of CHOP: (a) sham group; (b) IR group and (c) LPS PC+IR group. Positive cells were quantified in six high-power fields (400×), and expressed as percentages of positive cells among total cells. Mean±SD,**P<0.001 versus sham group; ^##P<0.001 versus IR group.</p