Effect of Mn2+ Doping on the Photoluminescence of Hybrid One-Dimensional Lead Halide Post-Perovskites.

Although organic-inorganic hybrid one-dimensional (1D) lead halide postperovskites (LHPPs) have been reported to show white luminescence and tunable photoluminescence quantum yield (PLQY), their structure-property relationships are not fully understood. Here, we used Mn2+ to test the doping effect on the luminescence of two 1D-LHPPs compounds, namely, {TETA[Pb2Br6]}n 1 and {TETA[Pb2Cl6]}n 2, where TETA = triethylenetetrammonium. We found the pristine compounds show yellowish (551 nm) and bluish (447 nm) emission for 1 and 2, respectively, nanosecond excitation lifetimes (4.17 ns for 1 and 2.29 ns for 2) and low PLQYs (4.65 and 3.57% for 1 and 2, respectively). By fine-doping the Mn2+ ions to ca. 8% the PLQYs for 1 and 2 are maximized to 24 and 25% for 1 and 2, respectively. Upon the increasing Mn2+ dopant, the emission wavelengths can also vary gradually from 551 to 615 nm and from 447 to 660 nm for 1 and 2, respectively, covering almost the whole visible-light range, and the excitation lifetimes are enhanced to microseconds (0.77 µs for 1 and 0.39 µs for 2), owing to the more spin-forbidden d-d transition (4T1-6A1) component from the Mn2+ ions present in the photoluminescence spectra. Moreover, these Mn2+-doped 1D-LHPPs demonstrate high structural and optical stability in humid and high-temperature environments. Hence, such doped materials can be fabricated into a UV-pumped white light-emitting diode, rendering the potential application for solid-state lighting and display systems.

Out-of-plane Emission Dipole of Second Harmonic Generation in Odd- and Even-layered vdWs Janus Nb3SeI7.

Integration of atomically thin nonlinear optical (NLO) devices demands an out-of-plane (OP) emission dipole of second harmonic generation (SHG) to enhance the spontaneous emission for nanophotonics. However, the research on van der Waals (vdWs) materials with an OP emission dipole of SHG is still in its infancy. Here, by coupling back focal plane (BFP) imaging with numerical simulations and density functional theory (DFT) calculations, we demonstrate that vdWs Janus Nb3SeI7, ranging from bulk to the monolayer limit, exhibits a dominant OP emission dipole of SHG owing to the breaking of the OP symmetry. Explicitly, even-layered Nb3SeI7 with C6v symmetry is predicted to exhibit a pure OP emission dipole attributed to the only second-order susceptibility coefficient χzxx. Meanwhile, although odd-layered Nb3SeI7 with C3v symmetry has both OP and IP dipole components (χzxx and χyyy), the value of χzxx is 1 order of magnitude greater than that of χyyy, leading to an approximate OP emission dipole of SHG. Moreover, the crystal symmetry and OP emission dipole can be preserved under hydrostatic pressure, accompanied by the enhanced χzxx and the resulting 3-fold increase in SHG intensity. The reported stable OP dipole in 2D vdWs Nb3SeI7 can facilitate the rapid development of chip-integrated NLO devices.

Strong Sliding Ferroelectricity and Interlayer Sliding Controllable Spintronic Effect in Two-Dimensional HgI2 Layers.

Exploration of two-dimensional (2D) sliding ferroelectric (FE) materials with experimentally detectable ferroelectricity and value-added novel functionalities is highly sought for the development of 2D "slidetronics". Herein, based on first-principles calculations, we identify the synthesizable van der Waals (vdW) layered crystals HgX2 (X = Br and I) as a new class of 2D sliding ferroelectrics. Both HgBr2 and HgI2 in 2D multilayered forms adopt the preferential stacking sequence, leading to room temperature stable out-of-plane (vertical) ferroelectricity that can be reversed via the sliding of adjacent monolayers. Owing to strong interlayer coupling and interfacial charge rearrangement, 2D HgI2 layers possess strong sliding ferroelectricity up to 0.16 µC/cm2, readily detectable in experiment. Moreover, robust sliding ferroelectricity and interlayer sliding controllable Rashba spin texture of FE-HgI2 layers enable potential applications as 2D spintronic devices such that the electric control of electron spin detection can be realized at the 2D regime.

Fine Tuning Ag(I)-Sb(III) Hybrid Iodides for Light Detection.

Lead-free hybrid double perovskite iodides (HDPIs) have piqued increasing research interest due to their environmental friendliness and high stability. However, such antimony-based HDPIs with strong photocurrent response are currently very limited. Here, we successfully design and construct five Ag(I)-Sb(III)-based HDPIs using two types of cyclic aliphatic amines as A-site templates. Interestingly, these Ag(I)-Sb(III) HDPIs exhibit relatively narrow band gaps, preferred orientation, and high stability after being processed into thin films on the indium tin oxide (ITO) substrate. Notably, under illuminations of a xenon lamp, all HDPIs exhibit considerable photocurrent responses, reaching a maximum difference of 17 µA·cm-2 for ASI 1, which is the highest among lead-free halogen-based organic-inorganic hybrid compounds to date. Combining the considerable photocurrents and the high stability, the optoelectronic applications of two-dimensional Ag(I)-Sb(III) HDPIs can be expected.

Two-dimensional semiconducting Cu(I)/Sb(III) bimetallic hybrid iodides with a double perovskite structure and photocurrent response.

Stable lead-free hybrid halide double perovskites have sparked widespread interest as a new kind of photoelectric material. Herein, for the first time, we successfully incorporated copper(I) and antimony(III) into two two-dimensional (2D) hybrid bimetallic double perovskite iodides, namely (NH3C6H11)4CuSbI8·H2O (CuSbI-1) and (NH3C6H10NH3)2CuSbI8·0.5H2O (CuSbI-2), using cyclohexylamine and 1,4-cyclohexanediamine as organic components. The band gaps for CuSbI-1 and CuSbI-2 were determined to be 2.22(2) eV and 2.21(2) eV, respectively. Furthermore, these two layered perovskites were readily dissolved in an organic solvent (1 mL DMF can dissolve 1 g sample for each compound) and could form smooth, pinhole-free, and uniform thin films through a facile spin-coating method. Photocurrent experiments with xenon lamp irradiation revealed the obvious photoelectric responses for both 2D double perovskites. The ratio of the photocurrent to the dark current (Ilight/Idark) for CuSbI-1 and CuSbI-2 is about 23 and 10, respectively, further suggesting their potential to be applied as light harvesters or light detectors. More importantly, these 2D double perovskite iodides show high moisture and thermal stabilities, indicating their potential for optoelectronic applications.

Two-Dimensional Ferroelasticity and Domain-Wall Flexoelectricity in HgX2 (X = Br or I) Monolayers.

Electromechanical phenomena in two-dimensional (2D) materials can be related to sizable electric polarizations and switchable spontaneous ferroelasticity, allowing them to be used as miniaturized electronic and memory devices. Even in a parent centrosymmetric (nonpolar) ferroelastic (FE) material, non-zero polarization can be produced around the FE domain wall, owing to the strain-gradient-induced flexoelectricity. Compared with the negligibly weak flexoelectric effect in bulk compounds, significant electric polarizations can be expected in 2D FE materials that sustain a large elastic strain and a strain gradient. Using first-principles calculations, we predict that spontaneous 2D ferroelasticity and domain-wall flexoelectricity can be simultaneously realized in synthetic HgX2 (X = Br or I) monolayers. The FE phase renders three oriented variants, which form FE domain walls with a large strain gradient and the associated domain-wall flexoelectric polarizations. Our thermodynamic stability analysis and kinetic barrier simulations allow us to manipulate the domain-wall flexoelectricity via applied mechanical stress, thereby enabling future electromechanical applications in nanoelectronics.

InBi: A Ferroelastic Monolayer with Strain Tunable Spin-Orbit Dirac Points and Carrier Self-Doping Effect.

Semimetallic two-dimensional (2D) Dirac materials beyond graphene, especially 2D materials with robust Dirac points against the spin-orbit coupling (SOC), are still highly sought. Herein, we theoretically demonstrate the InBi monolayer as a long-sought 2D Dirac material whose exotic Dirac Fermionic states cannot be gapped out by SOC. The InBi monolayer with the litharge crystal structure possesses not only 4-fold band degeneracy, linear energy dispersion, and ultrahigh Fermi velocity in the order of 105 m/s, but also spontaneous ferroelasticity that can lead to the orthorhombic lattice deformation and semimetallic electronic structure. Specifically, the symmetry protected spin-orbit Dirac points in 2D InBi are located at the Brillouin Zone (BZ) boundary and near the Fermi level in energy. More importantly, with coexisting spin-orbit Dirac points and spontaneous ferroelasticity, the InBi monolayer exhibits an additional advantage for engineering Dirac Fermionic states by ferroelastic (FE) strain. Energy levels of Dirac points are strongly coupled to FE strain, and the semimetallic electronic structure of the InBi monolayer is also susceptible to the FE strain induced carrier self-doping effect. Depending on the strain orientation within the InBi monolayer, electron and hole Fermi pockets will develop along the two planar directions, leading to the characteristic transport coefficients (as evidenced by our transport simulations based on Boltzmann formalism) for future experimental detection. FE strain tunable Dirac Fermionic states together with the carrier self-doping effect will benefit future development of ultrathin electronic devices with both high carrier mobility and controllable charge conductivities.

Two-dimensional ferroelasticity and ferroelastic strain controllable anisotropic transport properties in CuTe monolayer.

Two-dimensional ferroelastic (2D-FE) materials where FE strain originates from the lattice deformation associated with spontaneous FE phase transition, hold great promise as miniaturized shape-memory devices. Moreover, the structural anisotropy within the low-symmetry 2D-FE materials can usually lead to intrinsic anisotropy in their electronic or transport properties as well. As a result, the strong coupling of FE strain with the anisotropic electronic structure or electric-/thermoelectric-transport will largely extend the functionality and device applications for 2D-FE materials. In the current work, after performing comprehensive first-principles calculations in combination with transport simulations based on the Boltzmann formalism, we identify the experimentally synthesizable CuTe monolayer as a new 2D-FE material whose anisotropic electric- and thermoelectric-transport properties can be effectively manipulated by FE strain. Typically, CuTe monolayers that can be potentially exfoliated from the synthesized van der Waals (vdW) layered CuTe bulk are predicted to exhibit the room temperature stable ferroelasticity and large axial FE strain (up to 18.4%) created by the in-plane orthorhombic lattice deformation. Owing to the planar orientation dependent metallic vs. nearly semiconducting electronic structure, highly anisotropic electric conductivity and thermopower coefficient can be obtained along the two planar principal axes of the CuTe monolayer. To simulate the more realistic experimental scenarios, coherent formation of FE domain walls and domain-wall motion assisted FE switching have also been evaluated in CuTe multi-domain configurations. Based on the transverse thermoelectric effect inherent in anisotropic CuTe monolayers, the schematic model for obtaining the FE strain controllable electric current within CuTe multi-domain configurations has been proposed, which can be verified experimentally.

An ultrathin two-dimensional vertical ferroelectric tunneling junction based on CuInP2S6 monolayer.

Ferroelectric (FE) materials, especially ABO3 FE perovskite oxides, have been extensively studied for their important applications in memory devices, electronics and sensors. However, the integration of FE perovskite oxides into miniaturized memory and electronic devices has been impeded by the critical thickness limitation, as out-of-plane ferroelectricity in most FE perovskite oxides will disappear when the oxide thin film thickness is below a critical value. On the other side, CuInP2S6 (CIPS) nano-flake, a prototypical two-dimensional (2D) FE material, has recently been demonstrated to display stable out-of-plane ferroelectricity at the atomic layer thickness by experiment, which offers a new candidate for developing FE devices in the 2D nanoscale regime. Herein, after investigation of the structural and ferroelectric properties of 2D CIPS layers, especially the interactions between out-of-plane polarization and the corresponding depolarization field using first-principles calculations, we reveal that out-of-plane ferroelectricity can even persist in the CIPS monolayer, which is only 3.4 Å in thickness. Moreover, in order to explore the potential application of 2D FE CIPS layers as minimized FE devices, we design an ultrathin ferroelectric tunneling junction (FTJ) composed of a graphene/CIPS monolayer/graphene vertical van der Waals (vdW) heterostructure. Our transport simulations based on the non-equilibrium Green's function formalism predict that such an ultrathin FTJ device can still exhibit the typical tunneling electroresistance (TER) effect, where tunneling current strongly depends on the direction of FE polarization. Our work not only elucidates the origin of stable out-of-plane ferroelectricity appearing in 2D CIPS layers, but also demonstrates the practical application of a CIPS based 2D FTJ as a miniaturized, multi-functional and low-power consumption memory device for modern electronics.

Two-Dimensional Silver(I)-Dithiocarboxylate Coordination Polymer Exhibiting Strong Near-Infrared Photothermal Effect.

Materials that demonstrate near-infrared (NIR) absorption and can simultaneously convert the electromagnetic irradiation into heat are promising for photothermal therapy. Traditionally, such a material is either pure inorganic, such as CuS, Ag2S, and carbon nanotube, or pure organic, such as polyaniline, polypyrrole, and conjugated polymers. Here we show that strong NIR photothermal effect can also be achieved in inorganic-organic hybrid coordination polymers (CPs) or metal-organic frameworks (MOFs). Our strategy is to construct CPs with inorganic Ag-S components that are interlinked by the organic ligands into a higher-dimensional hybrid network. Interestingly, the two resulting CPs, [Ag(Py-4-CSS)] n 1 and [Ag2(Py-4-CSS)(Py-4-CSSS)] n 2 (Py-4-CSS = pyridine-4-dithiocarboxylate; Py-4-CSSS = pyridine-4-perthiocarboxylate), show disparate structures due to the varied coordination mode of the pyridine group. For 1, the N atom coordinates to the Ag+ center and forms a two-dimensional square framework, while for 2, such a Ag-N bond is disconnected and forms only a one-dimensional structure. Interestingly, this difference leads to the distinct absorption properties in the NIR region. Under 800 nm radiation, the temperature of 1 can rise up to 24.5 °C in 3 min with photothermal conversion efficiency of 22.1%, which is about 2× that of pure inorganic Ag2S material and among the highest compared to various known inorganic materials, for example, Au nanoshells (13%), nanorods (21%), and Cu2- xSe nanocrystals (22%) irradiated with 800 nm light, while for 2, the NIR absorption is absent. This result first demonstrates that the inorganic-organic hybrid approach can be applied to construct superior NIR photothermal materials, but the control of the structure is vital. Here the coordinating nitrogen atoms in 1 are conceived to be critical in promoting the charge transfer between the dithiocarboxylate ligands. To elucidate the response to NIR irradiation of 1, we measured the heat capacity and dielectric constant of 1 and also performed density functional theory calculations. Significantly, the large dielectric constant and flat energy bands indicates 1 is much easier to be polarized and has a high electron effective mass. Thus, unlike the pure inorganic material, such as Ag2S, in which electron and hole can quantum mechanically combine to give off light, the joint-force of organic ligands in 1 effectively enhances polaronic recombination into heat.

A-Site Ordered Double Perovskite CaMnTi2O6 as a Multifunctional Piezoelectric and Ferroelectric-Photovoltaic Material.

The double perovskite CaMnTi2O6, is a rare A-site ordered perovskite oxide that exhibits a sizable ferroelectric polarization and relatively high Curie temperature. Using first-principles calculations combined with detailed symmetry analyses, we identify the origin of the ferroelectricity in CaMnTi2O6. We further explore the material properties of CaMnTi2O6, including its ferroelectric polarization, dielectric and piezoelectric responses, magnetic order, electronic structure, and optical absorption coefficient. It is found that CaMnTi2O6 exhibits room-temperature-stable ferroelectricity and moderate piezoelectric responses. Moreover, CaMnTi2O6 is predicted to have a semiconducting energy band gap similar to that of BiFeO3, and its band gap can further be tuned via distortions of the planar Mn-O bond lengths. CaMnTi2O6 exemplifies a new class of single-phase semiconducting ferroelectric perovskites for potential applications in ferroelectric photovoltaic solar cells.

Interplay of Cation Ordering and Ferroelectricity in Perovskite Tin Iodides: Designing a Polar Halide Perovskite for Photovoltaic Applications.

Owing to its ideal semiconducting band gap and good carrier-transport properties, the fully inorganic perovskite CsSnI3 has been proposed as a visible-light absorber for photovoltaic (PV) applications. However, compared to the organic-inorganic lead halide perovskite CH3NH3PbI3, CsSnI3 solar cells display very low energy conversion efficiency. In this work, we propose a potential route to improve the PV properties of CsSnI3. Using first-principles calculations, we examine the crystal structures and electronic properties of CsSnI3, including its structural polymorphs. Next, we purposefully order Cs and Rb cations on the A site to create the double perovskite (CsRb)Sn2I6. We find that a stable ferroelectric polarization arises from the nontrivial coupling between polar displacements and octahedral rotations of the SnI6 network. These ferroelectric double perovskites are predicted to have energy band gaps and carrier effective masses similar to those of CsSnI3. More importantly, unlike nonpolar CsSnI3, the electric polarization present in ferroelectric (CsRb)Sn2I6 can effectively separate the photoexcited carriers, leading to novel ferroelectric PV materials with potentially enhanced energy conversion efficiency.

Lithium-Boron (Li-B) Monolayers: First-Principles Cluster Expansion and Possible Two-Dimensional Superconductivity.

Recent works demonstrated that the superconductivity at two-dimensional (2-D) can be achieved in Li-decorated graphene (Nature Phys. 2012, 8, 131 and Proc. Natl. Acad. Sci. 2015, 112, 11795). Inspired by the progress made in graphene, we predict by using the first-principles calculations that Li-incorporated B monolayers (Li-B monolayers) can be alternative 2-D superconductors. First-principles cluster expansion approach was used to evaluate the structural diversity and energetic stability of the 2-D Li-B monolayers by treating them as ternary Lix⬡yB1-x-y pseudoalloys (⬡ refers to B hexagonal hole). After thoroughly exploring the Li-B configuration space, several well-ordered and stable Li-B monolayers were identified. Detailed analyses regarding the electronic structures and lattice dynamics properties of the predicted Li-B monolayers were performed. Compared with the non-superconducting pure B-sheet, some predicted Li-B monolayers can exhibit the phonon-mediated superconducting properties above the liquid helium temperature.

Engineering the field emission properties of graphene film by gas adsorbates.

We study the effect of different gas adsorbates (N2, O2, and CO2) on the field emission (FE) properties of graphene film. Our experimental measurements show that, except for N2, adsorption of O2 and CO2 reduces the FE current of the graphene film. Moreover, the measured FE hysteresis loop is almost quenched after adsorbing O2 or CO2. The ab initio calculations quantify the experimental results by predicting a larger work function and smaller FE current for the graphene system after O2 or CO2 adsorption. In contrast, the change of FE properties in N2 adsorbed graphene is negligible. A "depolarization electric field model" is then proposed to explain the observed FE hysteresis behavior.

Perovskite oxides for visible-light-absorbing ferroelectric and photovoltaic materials.

Ferroelectrics have recently attracted attention as a candidate class of materials for use in photovoltaic devices, and for the coupling of light absorption with other functional properties. In these materials, the strong inversion symmetry breaking that is due to spontaneous electric polarization promotes the desirable separation of photo-excited carriers and allows voltages higher than the bandgap, which may enable efficiencies beyond the maximum possible in a conventional p-n junction solar cell. Ferroelectric oxides are also stable in a wide range of mechanical, chemical and thermal conditions and can be fabricated using low-cost methods such as sol-gel thin-film deposition and sputtering. Recent work has shown how a decrease in ferroelectric layer thickness and judicious engineering of domain structures and ferroelectric-electrode interfaces can greatly increase the current harvested from ferroelectric absorber materials, increasing the power conversion efficiency from about 10(-4) to about 0.5 per cent. Further improvements in photovoltaic efficiency have been inhibited by the wide bandgaps (2.7-4 electronvolts) of ferroelectric oxides, which allow the use of only 8-20 per cent of the solar spectrum. Here we describe a family of single-phase solid oxide solutions made from low-cost and non-toxic elements using conventional solid-state methods: [KNbO3]1 - x[BaNi1/2Nb1/2O3 - δ]x (KBNNO). These oxides exhibit both ferroelectricity and a wide variation of direct bandgaps in the range 1.1-3.8 electronvolts. In particular, the x = 0.1 composition is polar at room temperature, has a direct bandgap of 1.39 electronvolts and has a photocurrent density approximately 50 times larger than that of the classic ferroelectric (Pb,La)(Zr,Ti)O3 material. The ability of KBNNO to absorb three to six times more solar energy than the current ferroelectric materials suggests a route to viable ferroelectric semiconductor-based cells for solar energy conversion and other applications.

A nanoscale shape memory oxide.

Stimulus-responsive shape-memory materials have attracted tremendous research interests recently, with much effort focused on improving their mechanical actuation. Driven by the needs of nanoelectromechanical devices, materials with large mechanical strain, particularly at nanoscale level, are therefore desired. Here we report on the discovery of a large shape-memory effect in bismuth ferrite at the nanoscale. A maximum strain of up to ~14% and a large volumetric work density of ~600±90 J cm(-3) can be achieved in association with a martensitic-like phase transformation. With a single step, control of the phase transformation by thermal activation or electric field has been reversibly achieved without the assistance of external recovery stress. Although aspects such as hysteresis, microcracking and so on have to be taken into consideration for real devices, the large shape-memory effect in this oxide surpasses most alloys and, therefore, demonstrates itself as an extraordinary material for potential use in state-of-art nanosystems.

Noncovalent functionalization of BN nanotubes with perylene derivative molecules: an ab initio study.

The noncovalent functionalization of boron nitride nanotubes (BNNTs) with perylene-derived molecules has been reported experimentally [Wang et al. J. Am. Chem. Soc. 2008, 130, 8144]. Here we study the structural and electronic properties for the perylene-derivative functionalized BNNTs using first-principles calculations. Our calculations highlight the electronic structure modifications of BNNT through the noncovalent functionalization and demonstrate that van der Waals interactions between the adsorbed perylene derivatives and host BN layers facilitate the functionalization. We also provide an explanation for the red-shift of optical adsorption bands observed in experiment and discuss improvements in theoretical calculations of noncovalently functionalized BNNTs.

The nature of radiative transitions in o-doped boron nitride nanotubes.

Recent measurements on cathodoluminescences spectra of natural and isotope-substituted boron nitride nanotubes (BNNTs) surprisingly suggest the existence of pronounced radiative transitions in BN tubes, which are possibly induced by the oxygen substitutional impurities of the samples. [Han, W. Q. et al. Nano Lett. 2008, 8, 491] However, the structural pattern of the O-doped BN tube is unknown, as a result, how does the substitutional impurity in BNNT contribute to the observed radiative transitions is still a puzzle. Using first-principle calculations, we predict a novel, stable O-doped BNNT configuration. Such a structure contains one B(3)O(6) group, which is similar to the structural unit of boron oxide. Our calculations demonstrate that this type of O substitutional impurity can result in some donor-like gap states in the electronic structure and lead to the significant changes on the optical properties of BNNTs. The vibrational properties of the O-doped BNNT and boron oxide are also investigated. Our work elucidates the origins for experimental findings and provides a strong theoretical evidence on the existence of O substitutional impurity-induced radiative transitions in BNNT systems.