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1.
J Phys Chem Lett ; : 5177-5183, 2020 Jun 18.
Artigo em Inglês | MEDLINE | ID: mdl-32298584

RESUMO

Density functional theory calculations were performed for the electronic and the ferroelectric properties of the bulk and the monolayer benzylammonium lead-halide (BA2PbCl4). Our calculations indicate that both the bulk and monolayer systems display a band gap of ∼3.3 eV (HSE06+SOC) and a spontaneous polarization of ∼5.4 µC/cm2. The similar physical properties of bulk and monolayer systems suggest a strong decoupling among the layers in this hybrid organic-inorganic perovskite. Both the ferroelectricity, through associated structure distortion, and the spin-orbit coupling, through splitting induced in the electronic bands, significantly influence the band gaps. Most importantly, we found for the first time in a two-dimensional hybrid organic-inorganic class of material, a peculiar spin texture topology such as a unidirectional spin-orbit field, which may lead to a protection against spin decoherence.

2.
Inorg Chem ; 58(22): 14939-14980, 2019 Nov 18.
Artigo em Inglês | MEDLINE | ID: mdl-31668070

RESUMO

Nanostructured materials are essential building blocks for the fabrication of new devices for energy harvesting/storage, sensing, catalysis, magnetic, and optoelectronic applications. However, because of the increase of technological needs, it is essential to identify new functional materials and improve the properties of existing ones. The objective of this Viewpoint is to examine the state of the art of atomic-scale simulative and experimental protocols aimed to the design of novel functional nanostructured materials, and to present new perspectives in the relative fields. This is the result of the debates of Symposium I "Atomic-scale design protocols towards energy, electronic, catalysis, and sensing applications", which took place within the 2018 European Materials Research Society fall meeting.

3.
J Chem Phys ; 151(12): 124704, 2019 Sep 28.
Artigo em Inglês | MEDLINE | ID: mdl-31575169

RESUMO

Density functional theory calculations have been performed for the structural, electronic, magnetic, and ferroelectric properties of a mixed-valence Fe(ii)-Fe(iii) formate framework [NH2(CH3)2][FeiiiFeii(HCOO)6]. Recent experiments report a spontaneous electric polarization, and our calculations are in agreement with the reported experimental value. Furthermore, we shed light onto the microscopic mechanism leading to the observed value, as well as on how to possibly enhance the polarization. The interplay between charge ordering, dipolar ordering of DMA+ cations, and the induced structural distortions suggest new interesting directions to explore in these complex multifunctional hybrid perovskites.

4.
J Phys Chem Lett ; 10(6): 1319-1324, 2019 Mar 21.
Artigo em Inglês | MEDLINE | ID: mdl-30776247

RESUMO

Biological ferroelectric materials have great potential in biosensing and disease diagnosis and treatment. Glycine crystals form the simplest bioferroelectric materials, and here we investigate the polarizations of its ß- and γ-phases. Using density functional theory, we predict that glycine crystals can develop polarizations  even larger than those of conventional inorganic ferroelectrics. Further, using systematic molecular dynamics simulations utilizing polarized crystal charges, we predict the Curie temperature of γ-glycine to be 630 K, with a required coercive field to switch its polarization states of 1 V·nm-1, consistent with experimental evidence. This work sheds light on the microscopic mechanism of electric dipole ordering in biomaterials, helping in the material design of novel bioferroelectrics.


Assuntos
Glicina/química , Cristalização , Eletricidade , Simulação de Dinâmica Molecular , Teoria Quântica , Temperatura
5.
Adv Mater ; 31(5): e1804629, 2019 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-30516849

RESUMO

Recently, an emergent layered material Td -WTe2 was explored for its novel electron-hole overlapping band structure and anisotropic inplane crystal structure. Here, the photoresponse of mechanically exfoliated WTe2 flakes is investigated. A large anomalous current decrease for visible (514.5 nm), and mid- and far-infrared (3.8 and 10.6 µm) laser irradiation is observed, which can be attributed to light-induced surface bandgap opening from the first-principles calculations. The photocurrent and responsivity can be as large as 40 µA and 250 A W-1 for a 3.8 µm laser at 77 K. Furthermore, the WTe2 anomalous photocurrent matches its in-plane crystal structure and exhibits light polarization dependence, maximal for linear laser polarization along the W atom chain a direction and minimal for the perpendicular b direction, with the anisotropic ratio of 4.9. Consistently, first-principles calculations confirm the angle-dependent bandgap opening of WTe2 under polarized light irradiation. The anomalous and polarization-sensitive photoresponses suggest that linearly polarized light can significantly tune the WTe2 surface electronic structure, providing a potential approach to detect polarized and broadband lights up to far infrared range.

6.
Sci Rep ; 8(1): 7436, 2018 May 09.
Artigo em Inglês | MEDLINE | ID: mdl-29743631

RESUMO

Spin-valley and electronic band topological properties have been extensively explored in quantum material science, yet their coexistence has rarely been realized in stoichiometric two-dimensional (2D) materials. We theoretically predict the quantum spin Hall effect (QSHE) in the hydrofluorinated bismuth (Bi2HF) nanosheet where the hydrogen (H) and fluorine (F) atoms are functionalized on opposite sides of bismuth (Bi) atomic monolayer. Such Bi2HF nanosheet is found to be a 2D topological insulator with a giant band gap of 0.97 eV which might host room temperature QSHE. The atomistic structure of Bi2HF nanosheet is noncentrosymmetric and the spontaneous polarization arises from the hydrofluorinated morphology. The phonon spectrum and ab initio molecular dynamic (AIMD) calculations reveal that the proposed Bi2HF nanosheet is dynamically and thermally stable. The inversion symmetry breaking together with spin-orbit coupling (SOC) leads to the coupling between spin and valley in Bi2HF nanosheet. The emerging valley-dependent properties and the interplay between intrinsic dipole and SOC are investigated using first-principles calculations combined with an effective Hamiltonian model. The topological invariant of the Bi2HF nanosheet is confirmed by using Wilson loop method and the calculated helical metallic edge states are shown to host QSHE. The Bi2HF nanosheet is therefore a promising platform to realize room temperature QSHE and valley spintronics.

7.
J Chem Phys ; 146(22): 224702, 2017 Jun 14.
Artigo em Inglês | MEDLINE | ID: mdl-29166069

RESUMO

The effect of the SCN- ion on the structural, electronic, optical, and mechanical properties of the layered (MA)2Pb(SCN)2I2 (MA=CH3NH3+) perovskite is investigated by using first-principles calculations. Our results suggest that the introduction of SCN- ions at the apical positions gives rise to shorter Pb-S bond lengths, more distorted octahedra, and more hydrogen bonds, which have important effects on the electronic, optical, mechanical, and piezoelectric properties in (MA)2Pb(SCN)2I2. Furthermore, a strong relativistic Rashba splitting is induced due to the breaking of the inversion symmetry, which helps to suppress the carrier recombination and enhance the carrier lifetime. The analysis of mechanical properties reveals that the incorporation of SCN- ions is beneficial to strengthen Young's modulus of the perovskite materials and it enhances the piezoelectric properties. Our investigation suggests that doping SCN- ions into the perovskite materials could be a promising strategy to improve the stability and mechanical properties of organic-inorganic hybrid halide perovskite compounds.

8.
IUCrJ ; 4(Pt 5): 598-603, 2017 Sep 01.
Artigo em Inglês | MEDLINE | ID: mdl-28989715

RESUMO

Spin reorientation is a magnetic phase transition in which rotation of the magnetization vector with respect to the crystallographic axes occurs upon a change in the temperature or magnetic field. For example, SmFeO3 shows a magnetization rotation from the c axis above 480 K to the a axis below 450 K, known as the Γ4 → Γ2 transition. This work reports the successful synthesis of the new single-crystal perovskite SmFe0.75Mn0.25O3 and finds interesting spin reorientations above and below room temperature. In addition to the spin reorientation of the Γ4 → Γ2 magnetic phase transition observed at around TSR2 = 382 K, a new spin reorientation, Γ2 → Γ1, was seen at around TSR1 = 212 K due to Mn doping, which could not be observed in the parent rare earth perovskite compound. This unexpected spin configuration has complete antiferromagnetic order without any canting-induced weak ferromagnetic moment, resulting in zero magnetization in the low-temperature regime. M-T and M-H measurements have been made to study the temperature and magnetic-field dependence of the observed spin reorientation transitions.

9.
Inorg Chem ; 56(16): 9474-9485, 2017 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-28782949

RESUMO

The nontrivial aspects of electron structure in lanthanide complexes, considering ligand field (LF) and exchange coupling effects, have been investigated by means of density functional theory (DFT) calculations, taking as a prototypic case study a series of binuclear complexes [LCu(O2COMe)Ln(thd)2], where L2- = N,N'-2,2-dimethyl-propylene-di(3-methoxy-salicylidene-iminato) and Ln = Tb, Lu, and Gd. Particular attention has been devoted to the Cu-Tb complex, which shows a quasi-degenerate nonrelativistic ground state. Challenging the limits of density functional theory (DFT), we devised a practical route to obtain different convergent solutions, permuting the starting guess orbitals in a manner resembling the run of the ß electron formally originating from the f8 configuration of the Tb(III) over seven molecular orbitals (MOs) with predominant f-type character. Although the obtained states cannot be claimed as the DFT computed split of the 7F multiplet, the results are yet interesting numeric experiments, relevant for the ligand field effects. We also performed broken symmetry (BS) DFT estimation of exchange coupling in the Cu-Gd system, using different settings, with Gaussian-type and plane-wave bases, finding a good match with the coupling parameter from experimental data. We also caught BS-type states for each of the mentioned series of different states emulated for the Cu-Tb complex, finding almost equal exchange coupling parameters throughout the seven LF-like configurations, the magnitude of the J parameter being comparable with those of the Cu-Gd system.

10.
J Am Chem Soc ; 139(37): 12883-12886, 2017 09 20.
Artigo em Inglês | MEDLINE | ID: mdl-28853870

RESUMO

Hybrid organic-inorganic compounds attract a lot of interest for their flexible structures and multifunctional properties. For example, they can have coexisting magnetism and ferroelectricity whose possible coupling gives rise to magnetoelectricity. Here using first-principles computations, we show that, in a perovskite metal-organic framework (MOF), the magnetic and electric orders are further coupled to optical excitations, leading to an Electric tuning of the Magneto-Optical Kerr effect (EMOKE). Moreover, the Kerr angle can be switched by reversal of both ferroelectric and magnetic polarization only. The interplay between the Kerr angle and the organic-inorganic components of MOFs offers surprising unprecedented tools for engineering MOKE in complex compounds. Note that this work may be relevant to acentric magnetic systems in general, e.g., multiferroics.

11.
Inorg Chem ; 56(1): 197-207, 2017 Jan 03.
Artigo em Inglês | MEDLINE | ID: mdl-27935298

RESUMO

A study of the magnetic structure of the [NH2(CH3)2]n[FeIIIMII(HCOO)6]n niccolite-like compounds, with MII = CoII (2) and MnII (3) ions, has been carried out using neutron diffraction and compared with the previously reported FeII-containing compound (1). The inclusion of two different metallic atoms into the niccolite-like structure framework leads to the formation of isostructural compounds with very different magnetic behaviors due to the compensation or not of the different spins involved in each lattice. Below TN, the magnetic order in these compounds varies from ferrimagnetic behavior for 1 and 2 to an antiferromagnetic behavior with a weak spin canting for 3. Structure refinements of 2 and 3 at low temperature (45 K) have been carried out combining synchrotron X-ray and high-resolution neutron diffraction in a multipattern approach. The magnetic structures have been determined from the difference patterns between the neutron data in the paramagnetic and the magnetically ordered regions. These difference patterns have been analyzed using a simulated annealing protocol and symmetry analysis techniques. The obtained magnetic structures have been further rationalized by means of ab initio DFT calculations. The direction of the magnetic moment of each compound has been determined. The easy axis of the MII for compound 1 (FeII) is along the c axis; for compound 2 (CoII), the moments are mainly within the ab plane; finally, for compound 3 (MnII), the calculations show that the moments have components both in the ab plane and along the c axis.

12.
Inorg Chem ; 56(1): 33-41, 2017 Jan 03.
Artigo em Inglês | MEDLINE | ID: mdl-27626290

RESUMO

High-quality single crystals of perovskite-like (CH3NH3)3Bi2I9 hybrids have been synthesized, using a layered-solution crystal-growth technique. The large dielectric constant is strongly affected by the polar ordering of its constituents. Progressive dipolar ordering of the methylammonium cation upon cooling below 300 K gradually converts the hexagonal structure (space group P63/mmc) into a monoclinic phase (C2/c) at 160 K. A well-pronounced, ferrielectric phase transition at 143 K is governed by in-plane ordering of the bismuth lone pair that breaks inversion symmetry and results in a polar phase (space group P21). The dielectric constant is markedly higher in the C2/c phase above this transition. Here, the bismuth lone pair is disordered in-plane, allowing the polarizability to be substantially enhanced. Density functional theory calculations estimate a large ferroelectric polarization of 7.94 µC/cm2 along the polar axis in the P21 phase. The calculated polarization has almost equal contributions of the methylammonium and Bi3+ lone pair, which are fairly decoupled.

13.
Sci Rep ; 6: 37529, 2016 11 25.
Artigo em Inglês | MEDLINE | ID: mdl-27886220

RESUMO

RFeO3 orthoferrites, where R is a rare-earth ion of the lanthanide series, are attracting attention mostly because of their promising fast spin dynamics. The magnetic properties of these materials seem to crucially depend on whether the magnetizations of the R and Fe ions' weak ferromagnetic (WFM) components are parallel or antiparallel to each other. Here, we report an extensive investigation of a high-quality DyFeO3 single crystal in which the induced Dy3+ magnetization (FDy) has a natural tendency to be antiparallel to Fe3+ sublattice magnetization (FFe) within a large temperature window. Moreover, we find that specific variations of temperature and applied magnetic fields allow us to make FDy parallel to FFe, or force a spin-flip transition in FFe, among other effects. We found three different magnetic states that respond to temperature and magnetic fields, i.e. linear versus constant or, alternatively, presenting either behavior depending on the history of the sample. An original magnetic field-versus-temperature phase diagram is constructed to indicate the region of stability of the different magnetic phases, and to reveal the precise conditions yielding sudden spin switching and reversals. Knowledge of such a phase diagram is of potential importance to applications in spintronics and magnetic devices.

14.
Inorg Chem ; 55(20): 10337-10342, 2016 Oct 17.
Artigo em Inglês | MEDLINE | ID: mdl-27676140

RESUMO

A displacive-type mechanism, which accounts for the occurrence of ferroelectricity in most inorganic ferroelectrics, is rarely found in molecule-based ferroelectrics. Its role is often covered by the predominant order-disorder one. Herein, we report a lone-pair-electron-driven displacive-type ferroelectric organic-inorganic hybrid compound, [H2dmdap][SbCl5] (1; dmdap = N,N-dimethyl-1,3-diaminopropane). The structure of 1 features a typical zigzag chain of [SbCl5]∞ containing cis-connected anionic octahedra. The compound undergoes a second-order paraelectric-ferroelectric phase transition at 143 K (P21/c ↔ Pc) with a saturation polarization of 1.36 µC·cm-2 and a coercive field of 3.5 kV·cm-1 at 119 K. Theoretical study discloses the ferroelectricity mainly originating from the relative displacements of the Sb and Cl ions in the crystal lattice, which are driven by the 5s2 lone-pair electrons of the SbIII center. Furthermore, on the basis of analysis, possible routes are suggested to enhance ferroelectric polarization in this class of compounds.

15.
Phys Rev Lett ; 117(7): 076401, 2016 Aug 12.
Artigo em Inglês | MEDLINE | ID: mdl-27563977

RESUMO

By means of density functional theory based calculations, we study the role of spin-orbit coupling in the new family of ABC hyperferroelectrics [Garrity, Rabe, and Vanderbilt Phys. Rev. Lett. 112, 127601 (2014)]. We unveil an extremely rich physics strongly linked to ferroelectric properties, ranging from the electric control of bulk Rashba effect to the existence of a three-dimensional topological insulator phase, with concomitant topological surface states even in the ultrathin film limit. Moreover, we predict that the topological transition, as induced by alloying, is followed by a Weyl semimetal phase of finite concentration extension, which is robust against disorder, putting forward hyperferroelectrics as promising candidates for spin-orbitronic applications.

16.
Sci Rep ; 6: 28618, 2016 06 28.
Artigo em Inglês | MEDLINE | ID: mdl-27350083

RESUMO

The development of high efficiency perovskite solar cells has sparked a multitude of measurements on the optical properties of these materials. For the most studied methylammonium(MA)PbI3 perovskite, a large range (6-55 meV) of exciton binding energies has been reported by various experiments. The existence of excitons at room temperature is unclear. For the MAPbX3 perovskites we report on relativistic Bethe-Salpeter Equation calculations (GW-BSE). This method is capable to directly calculate excitonic properties from first-principles. At low temperatures it predicts exciton binding energies in agreement with the reported 'large' values. For MAPbI3, phonon modes present in this frequency range have a negligible contribution to the ionic screening. By calculating the polarization in time from finite temperature molecular dynamics, we show that at room temperature this does not change. We therefore exclude ionic screening as an explanation for the experimentally observed reduction of the exciton binding energy at room temperature and argue in favor of the formation of polarons.

17.
Chemistry ; 22(23): 7863-70, 2016 06 01.
Artigo em Inglês | MEDLINE | ID: mdl-27072487

RESUMO

The perovskite azido compound [(CH3 )4 N][Mn(N3 )3 ], which undergoes a first-order phase change at Tt =310 K with an associated magnetic bistability, was revisited in the search for additional ferroic orders. The driving force for such structural transition is multifold and involves a peculiar cooperative rotation of the [MnN6 ] octahedral as well as order/disorder and off-center shifts of the [(CH3 )4 N](+) cations and bridging azide ligands, which also bend and change their coordination mode. According to DFT calculations the latter two give rise to the appearance of electric dipoles in the low-temperature (LT) polymorph, the polarization of which nevertheless cancels out due to their antiparallel alignment in the crystal. The conversion of this antiferroelectric phase to the paraelectric phase could be responsible for the experimental dielectric anomaly detected at 310 K. Additionally, the structural change involves a ferroelastic phase transition, whereby the LT polymorph exhibits an unusual and anisotropic thermal behavior. Hence, [(CH3 )4 N][Mn(N3 )3 ] is a singular material in which three ferroic orders coexist even above room temperature.

18.
Dalton Trans ; 45(6): 2574-83, 2016 Feb 14.
Artigo em Inglês | MEDLINE | ID: mdl-26725595

RESUMO

We report the synthesis, single crystal X-ray diffraction, and thermal, dielectric, Raman and infrared studies of a novel heterometallic formate [C2H5NH3][Na0.5Fe0.5(HCOO)3] (EtANaFe). The thermal studies show that EtANaFe undergoes a second-order phase transition at about 360 K. X-ray diffraction data revealed that the high-temperature structure is monoclinic, space group P2(1)/n, with dynamically disordered ethylammonium (EtA(+)) cations. EtANaFe possesses a polar low-temperature structure with the space group Pn and, in principle, is ferroelectric below 360 K. Dielectric data show that the reciprocal of the real part of dielectric permittivity above and below the phase transition temperature follows the Curie-Weiss, as expected for a ferroelectric phase transition. Based on theoretical calculations, we estimated the polarization as (0.2, 0, 0.8) µC cm(-2), i.e., lying within the ac plane. The obtained data also indicate that the driving force of the phase transition is ordering of EtA(+) cations. However, this ordering is accompanied by significant distortion of the metal formate framework.

19.
J Phys Chem Lett ; 6(22): 4553-9, 2015 Nov 19.
Artigo em Inglês | MEDLINE | ID: mdl-26512946

RESUMO

Metal-organic frameworks (MOFs) are hybrid crystalline compounds comprised of an extended ordered network made up of organic molecules, organic linkers and metal cations. In particular, MOFs with the same topology as inorganic perovskites have been shown to possess interesting properties, e.g., coexistence of ferroelectric and magnetic ordering. Using first-principles density functional theory, we have investigated the effect of strain on the compounds C(NH2)3Cr(HCOO)3 and (CH3CH2NH3)Mn(HCOO)3. Here, we show that compressive strain can substantially increase the ferroelectric polarization by more than 300%, and we discuss the mechanism involved in the strain enhancement of polarization. Our study highlights the complex interplay between strain and organic cations' dipoles and put forward the possibility of tuning of ferroelectric polarization through appropriate thin film growing.

20.
J Phys Chem Lett ; 6(12): 2223-31, 2015 Jun 18.
Artigo em Inglês | MEDLINE | ID: mdl-26266595

RESUMO

Ferroelectricity in halide perovskites currently represents a crucial issue, as it may have an important role for the enhancement of solar cells efficiency. Simulations of ferroelectric properties based on density functional theory are conceptually more demanding compared with "conventional" inorganic ferroelectrics due to the presence of both organic and inorganic components in the same compound. Here we present a detailed study focused on the prototypical CH3NH3PbI3 perovskite. By using density functional theory combined with symmetry mode analysis, we disentangle the contributions of the methylammonium cations and the role of the inorganic framework, therefore suggesting possible routes to enhance the polarization in this compound. Our estimate of the polarization for the tetragonal phase at low temperature is ∼4.42 µC/cm(2), which is substantially lower than that of traditional perovskite oxides.

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