Microwave shock motivating the Sr substitution of 2D porous GdFeO_(3) perovskite for highly active oxygen evolution

The incorporation of partial A-site substitution in perovskite oxides represents a promising strategy for precisely controlling the electronic configuration and enhancing its intrinsic catalytic activity.Conventional methods for A-site substitution typically involve prolonged high-temperature processes.While these processes promote the development of unique nanostructures with highly exposed active sites,they often result in the uncontrolled configuration of introduced elements.Herein,we present a novel approach for synthesizing two-dimensional(2D)porous GdFeO_(3) perovskite with A-site strontium(Sr)substitution utilizing microwave shock method.This technique enables precise control of the Sr content and simultaneous construction of 2D porous structures in one step,capitalizing on the advantages of rapid heating and cooling(temperature~1100 K,rate~70 K s^(-1)).The active sites of this oxygen-rich defect structure can be clearly revealed through the simulation of the electronic configuration and the comprehensive analysis of the crystal structure.For electrocatalytic oxygen evolution reaction application,the synthesized 2D porous Gd_(0.8)Sr_(0.2)FeO_(3) electrocatalyst exhibits an exceptional overpotential of 294 mV at a current density of 10 mA cm^(-2)and a small Tafel slope of 55.85 mV dec^(-1)in alkaline electrolytes.This study offers a fresh perspective on designing crystal configurations and the construction of nanostructures in perovskite.

Two/Quasi-two-dimensional perovskite-based heterostructures:construction,properties and applications

Two-dimensional(2D)/quasi-2D organic-inorganic halide perovskites are regarded as naturally formed multiple quantum wells with inorganic layers isolated by long organic chains,which exhibit layered structure,large exciton binding energy,strong nonlinear optical effect,tunable bandgap via changing the layer number or chemical composition,improved environmental stability,and excellent optoelectronic properties.The extensive choice of long organic chains endows 2D/quasi-2D perovskites with tunable electron-phonon coupling strength,chirality,or ferroelectricity properties.In particular,the layered nature of 2D/quasi-2D perovskites allows us to exfoliate them to thin plates to integrate with other materials to form heterostructures,the fundamental structural units for optoelectronic devices,which would greatly extend the functionalities in view of the diversity of 2D/quasi-2D perovskites.In this paper,the recent achievements of 2D/quasi-2D perovskite-based heterostructures are reviewed.First,the structure and physical properties of 2D/quasi-2D perovskites are introduced.We then discuss the construction and characterizations of 2D/quasi-2D perovskite-based heterostructures and highlight the prominent optical properties of the constructed heterostructures.Further,the potential applications of 2D/quasi-2D perovskite-based heterostructures in photovoltaic devices,light emitting devices,photodetectors/phototransistors,and valleytronic devices are demonstrated.Finally,we summarize the current challenges and propose further research directions in the field of 2D/quasi-2D perovskite-based heterostructures.

Influence of Organic Cations on the Crystal and Electronic Structures of Two-dimensional Lead Iodide Perovskites

The crystal structures and electronic structures(including band gap,project density of states,partial charge density,effective mass and electron localization function)of the 2D lead iodide perovskites hybrids with different organic spacer cations of 4-fluorophenylethanaminium(4F-PEA^(+)),ethanolamine(EA^(+)),thienylethylamine(TEA^(+))were investigated using first-principles calculations.It was found the higher dipole moment,the stronger the hydrogen bonding between the organic amino and iodide in the inorganic layer,and the larger the[PbI_(6)]^(4-)octahedral distortions in these crystal structure.Further quantifying the degree of the distortions using OctaDist software showed that the distortion of adjacent[PbI_(6)]^(4-)octahedra had a decisive effect on the band gap.Specifically,the greater deviation of Pb-I-Pb bond angles from 180°,together with the larger distortion of multiple[PbI_(6)]^(4-)octahedron resulted in a wider band gap,which was verified by calculated band gap using different DFT methods.The results outlined the relationships of hydrogen bonding,ocathedra distortion and band structure in 2D perovskites,highlighting the importance of the cations on the structural tuning and optoelectronic properties.

Phase Regulation and Defect Passivation Enabled by Phosphoryl Chloride Molecules for Efficient Quasi‑2D Perovskite Light‑Emitting Diodes

Quasi-2D perovskites have attracted tremendous interest for application as lightemission layers in light-emitting diodes(LEDs).However,the heterogeneous n phase and non-uniform distribution still severely limit the further development of quasi-2D perovskite LEDs(Pero-LEDs).Meanwhile,the increased defect density caused by the reduced dimension and grain size induces non-radiative recombination and further deteriorates the device performance.Here,we found that a series of molecules containing phosphoryl chloride functional groups have noticeable enhancement effects on the device performance of quasi-2D Pero-LEDs.Then,we studied the modification mechanism by focusing on the bis(2-oxo-3-oxazolidinyl)phosphinic chloride(BOPCl).It is concluded that the BOPCl can not only regulate the phase distribution by decreasing the crystallization rate but also remain in the grain boundaries and passivate the defects.As a result,the corresponding quasi-2D Pero-LEDs obtained a maximum external quantum efficiency(EQE_(max))of 20.82%and an average EQE(EQE_(ave))of around 20%on the optimal 50 devices,proving excellent reproducibility.Our work provides a new selection of molecular types for regulating the crystallization and passivating the defects of quasi-2D perovskite films.

Magnetic and electronic properties of bulk and two-dimensional FeBi_(2)Te_(4):A first-principles study

Layered magnetic materials,such as MnBi_(2)Te_(4),have drawn much attention owing to their potential for realizing twodimensional(2D)magnetism and possible topological states.Recently,FeBi_(2)Te_(4),which is isostructural to MnBi_(2)Te_(4),has been synthesized in experiments,but its detailed magnetic ordering and band topology have not been clearly understood yet.Here,based on first-principles calculations,we investigate the magnetic and electronic properties of FeBi_(2)Te_(4)in bulk and 2D forms.We show that different from MnBi_(2)Te_(4),the magnetic ground states of bulk,single-layer,and bilayer FeBi_(2)Te_(4)all favor a 120°noncollinear antiferromagnetic ordering,and they are topologically trivial narrow-gap semiconductors.For the bilayer case,we find that a quantum anomalous Hall effect with a unit Chern number is realized in the ferromagnetic state,which may be achieved in experiment by an external magnetic field or by magnetic proximity coupling.Our work clarifies the physical properties of the new material system of FeBi_(2)Te_(4)and reveals it as a potential platform for studying magnetic frustration down to 2D limit as well as quantum anomalous Hall effect.

Boosting efficiency and stability of 2D alternating cation perovskite solar cells via rational surface-modification: Marked passivation efficacy of anion

Two-dimensional(2D) alternating cation(ACI) perovskite surface defects,especially dominant iodine vacancies(V_Ⅰ) and undercoordinated Pb^(2+),limit the performance of perovskite solar cells(PVSCs).To address the issue,1-butyl-3-methylimidazolium trifluoro-methane-sulfonate(BMIMOTF) and its iodide counterpart(BMIMI) are utilized to modify the perovskite surface respectively.We find that BMIMI can change the perovskite surface,whereas BMIMOTF shows a nondestructive and more effective defect passivation,giving significantly reduced defect density and suppressed charge-carrier nonradiative recombination.This mainly attributes to the marked passivation efficacy of OTF-anion on V_Ⅰ and undercoordinated Pb^(2+),rather than BMIMI^(+) cation.Benefiting from the rational surface-modification of BMMIMOTF,the films exhibit an optimized energy level alignment,enhanced hydrophobicity and suppressed ion migration.Consequently,the BMIMOTF-modified devices achieve an impressive efficiency of 21.38% with a record open-circuit voltage of 1.195 V,which is among the best efficiencies reported for 2D PVSCs,and display greatly enhanced humidity and thermal stability.

Review on engineering two-dimensional nanomaterials for promoting efficiency and stability of perovskite solar cells

Perovskite solar cell(PSC) has gradually shown its great superiority in photovoltaic filed to compete commercial solar cells owing to its great advantages, such as high efficiency and low fabrication cost. On the way towards commercialization, great efforts have been achieved by accelerating charge extraction and reducing carrier recombination. Recently, two-dimensional(2 D) layered materials have attracted increasing interests for application in PSCs due to their distinctive chemical and physical properties, such as high carrier mobility and tunable bandgap, which greatly determines the perovskite film growth kinetics, carrier transfer and stability of PSCs. Therefore, with the aim to better understand their recent development and application in PSC, in this review, the emerging 2D materials beyond graphene as charge transport layers, buffer layers and additives in perovskite film for enhancing the efficiency and stability of PSCs are summarized. However, there are still some crucial challenges to be addressed for commercialization. Finally, the challenges and prospects of these 2D nanomaterials for application in PSCs are further proposed for future development.

Small molecule interfacial cross-linker for highly efficient two-dimensional perovskite solar cells

The nonradiative recombination of charge carriers at the hole transport layer(HTL)/perovskite interface generally induces remarkable performance loss of the inverted two-dimensional perovskite solar cells(2D PSCs). Herein, a cross-linkable small molecule of 2-mercaptoimidazole(2-MI) was introduced into the nickel oxide(NiO_(x))/2D perovskite interface. Experiments have confirmed the formation of Ni-N covalent bond by N atom in the 2-MI and Ni in the NiO_(x) and the coordinating between S atom of 2-MI and under-coordinated Pb^(2+) near to the NiO_(x)/perovskite interface, which contributes to creating a crosslinking between NiO_(x)/perovskite interface to restrain charge carrier recombination and enhance the extraction of hole carriers at the interface. Besides, the 2-MI modification layer is also beneficial for promoting the crystallinity of 2D perovskite. Consequently, the inverted 2D PSCs with 2-MI modification achieved the best power conversion efficiency of 15%. This paves a route to acquire highly efficient 2D PSCs by constructing a cross-linking at the NiO_(x)HTL/2D perovskite interface.

Waveguide-integrated optical modulators with two-dimensional materials

Waveguide-integrated optical modulators are indispensable for on-chip optical interconnects and optical computing.To cope with the ever-increasing amount of data being generated and consumed,ultrafast waveguide-integrated optical modulators with low energy consumption are highly demanded.In recent years,two-dimensional(2D)materials have attracted a lot of attention and have provided tremendous opportunities for the development of high-performance waveguide-integrated optical modulators because of their extraordinary optoelectronic properties and versatile compatibility.This paper reviews the state-of-the-art waveguide-integrated optical modulators with 2D materials,providing researchers with the developing trends in the field and allowing them to identify existing challenges and promising potential solutions.First,the concept and fundamental mechanisms of optical modulation with 2D materials are summarized.Second,a review of waveguide-integrated optical modulators employing electro-optic,all-optic,and thermo-optic effects is provided.Finally,the challenges and perspectives of waveguide-integrated modulators with 2D materials are discussed.

Two-dimensional plane strain consolidation of unsaturated soils considering the depth-dependent stress

In practical engineering,the total vertical stress in the soil layer is not constant due to stress diffusion,and varies with time and depth.Therefore,the purpose of this paper is to investigate the effect of stress diffusion on the two-dimensional(2D)plane strain consolidation properties of unsaturated soils when the stress varies with time and depth.A series of semi-analytical solutions in terms of excess pore air and water pressures and settlement for 2D plane strain consolidation of unsaturated soils can be derived with the joint use of Laplace transform and Fourier sine series expansion.Then,the inverse Laplace transform of the semi-analytical solution is given in the time domain using a self-programmed code based on Crump’s method.The reliability of the obtained solutions is proved by the degeneration.Finally,the 2D plots of excess pore pressures and the curves of settlement varying with time,considering different physical parameters of unsaturated soil stratum and depth-dependent stress,are depicted and analyzed to study the 2D plane strain consolidation properties of unsaturated soils subjected to the depthdependent stress.

Numerical study on THz radiation of two-dimensional plasmon resonance of GaN HEMT array

The Ga N high electron mobility transistor(HEMT)has been considered as a potential terahertz(THz)radiation source,yet the low radiation power level restricts their applications.The HEMT array is thought to improve the coupling efficiency between two-dimensional(2D)plasmons and THz radiation.In this work,we investigate the plasma oscillation,electromagnetic radiation,and the integration characteristics of Ga N HEMT targeting at a high THz radiation power source.The quantitative radiation power and directivity are obtained for integrated Ga N HEMT array with different array periods and element numbers.With the same initial plasma oscillation phase among the HEMT units,the radiation power of the two-element HEMT array can achieve 4 times as the single HEMT radiation power when the array period is shorter than 1/8electromagnetic wavelength.In addition,the radiation power of the HEMT array varies almost linearly with the element number,the smaller array period can lead to the greater radiation power.It shows that increasing the array period could narrow the main radiated lobe width while weaken the radiation power.Increasing the element number can improve both the radiation directivity and power.We also synchronize the plasma wave phases in the HEMT array by adopting an external Gaussian plane wave with central frequency the same as the plasmon resonant frequency,which solves the problem of the radiation power reduction caused by the asynchronous plasma oscillation phases among the elements.The study of the radiation power amplification of the one-dimensional(1D)Ga N HEMT array provides useful guidance for the research of compact high-power solid-state terahertz sources.

Green's functions of two-dimensional piezoelectric quasicrystal in half-space and bimaterials

In this paper,we obtain Green’s functions of two-dimensional(2D)piezoelectric quasicrystal(PQC)in half-space and bimaterials.Based on the elastic theory of QCs,the Stroh formalism is used to derive the general solutions of displacements and stresses.Then,we obtain the analytical solutions of half-space and bimaterial Green’s functions.Besides,the interfacial Green’s function for bimaterials is also obtained in the analytical form.Before numerical studies,a comparative study is carried out to validate the present solutions.Typical numerical examples are performed to investigate the effects of multi-physics loadings such as the line force,the line dislocation,the line charge,and the phason line force.As a result,the coupling effect among the phonon field,the phason field,and the electric field is prominent,and the butterfly-shaped contours are characteristic in 2D PQCs.In addition,the changes of material parameters cause variations in physical quantities to a certain degree.

Recent progress of the optoelectronic properties of 2D Ruddlesden-Popper perovskites

Two-dimensional(2 D) hybrid organic-inorganic perovskites have recently attracted attention due to their layered nature, naturally formed quantum well structure, large exciton binding energy and especially better long-term environmental stability compared with their three-dimensional(3 D) counterparts. In this report, we present a brief overview of the recent progress of the optoelectronic applications in 2 D perovskites. The layer number dependent physical properties of 2 D perovskites will first be introduced and then the different synthetic approaches to achieve 2 D perovskites with different morphologies will be discussed. The optical, optoelectronic properties and self-trapped states in 2 D perovskites will be described, which are indispensable for designing the new device structures with novel functionalities and improving the device performance. Subsequently, a brief summary of the advantages and the current research status of the 2 D perovskite-based heterostructures will be illustrated.Finally, a perspective of 2 D perovskite materials is given toward their material synthesis and novel device applications.

A guide to use fluorinated aromatic bulky cations for stable and high-performance 2D/3D perovskite solar cells:The more fluorination the better?

While serious stability issues impede the commercialization of perovskite solar cells(PSCs),two-dime nsional(2D)perovskites based on fluorinated bulky cations have emerged as more intrinsically stable materials.However,the influence of fluorination degree of the bulky aromatic cation on the per-formance of resulting PSCs has not been scrutinized.Here,2D perovskites(FxPEA)_(2)PbI_(4)(x=1,2,3,5)are grown in situ on the surface of the three-dime nsion al(3D)perovskite and dem on strate effective passivation of the surface defects of 3D perovskite.The power conversion efficiency(PCE)of the optimized devices were boosted from 20.75%for the control device to 21.09%,22.06%,22.74%and 21.86%for 2D/3D devices treated with 4-fluorophenethylamine iodide,3,5-difluorophenylethylamine iodide,2,4,5-trifluoroethylphenylethylamine iodide,and 1,2,3,4,5-pentafluorophenylethylamine iodide,respectively.We firstly reported two unexplored RP-type layered perovskites with F_(2)PEAI and F_(3)PEAI as bulky cations.The combined experimental and theoretical analysis revealed the reasons behind the various morphology,device performances,dynamic behavior,and humidity stability.The best performing F_(5)PEAI-treated device retaining 95.0%of its initial PCE under ambient atmosphere(with RH of 60%±5%)without encapsulation for 300 h storage.This work provides useful guidance for selecting fluorinated bulky cations with different molecular electronic properties,which will play an essential role in further improving the performance/stability of PSCs for the sake of further commercialization.

Enhanced efficiency and stability of perovskite solar cells by 2D perovskite vapor-assisted interface optimization

Organic–inorganic perovskites solar cells(PSCs)have attracted great attention due to their rapid progress in power conversion efficiency(PCE).However,there is still an enormous challenge to achieve both high efficiency and stability devices as the decomposition of perovskite materials under humid and light conditions.Herein,we demonstrate that high efficiency and stability of PSCs can be obtained by the reaction of three-dimensional(3D)perovskite with 1,4-butanediamine iodide(BEAI2)vapor.The incorporation of BEAI2 intensively promotes the crystallization of perovskite film with large grain size(~500 nm).Further characterization reveals that the post-treatment perovskite film delivered low interface trap density with long carrier lifetime(>200 ns),long carrier diffusion length(>600 nm)and large carrier mobility(>1.5 cm^2 V-1S-1).Solar cells employing such post-treatment films demonstrated 19.58%PCE without hysteresis.Moreover,the post-treatment devices can retain over 90%original efficiencies stored under ambient atmospheric conditions and exhibit better stability under 85℃and continuous illumination as a two-dimensional(2D)perovskite thin layer is formed on the surface/or at the grain boundaries of 3D perovskite.This study offers an effective way to obtain PSCs with high efficiency and stability.

Formamidinium-incorporated Dion-Jacobson phase 2D perovskites for highly efficient and stable photovoltaics

Dion-Jacobson phase two-dimensional(DJ 2D)perovskites,recently attracting considerable interests,exhibit excellent environmental stability and structural tunability,but their solar cells still offer unsatisfactory power conversion efficiencies(PCEs).Herein,we develop DJ 2D perovskites employing formamidinium(FA+)as a ternary cation in the perovskite cages((PDA)(FA)x(MA)3-xPb4 I13,χ=0,0.15,0.3 and 0.6,PDA=1,3-propanediammonium)for highly efficient and stable perovskite solar cells(PSCs).We found that the DJ 2D perovskite with a 10%FA+fraction presents improved crystallinity,preferred vertical orientation,and longer charge carrier lifetime compared to that without FA+doping.As a result,the FAdoped DJ 2D PSCs exhibit a champion PCE of 14.74%with superior device stability.The unencapsulated devices sustain over 92%of its initial PCE after storage at a constant relative humidity(RH)of 65%for 6000 h,90%by heat at 85℃in air for 800 h,and 94%under 1-sun illumination for 5000 h.These findings demonstrate that the incorporation of FA cation into the DJ 2D perovskite is a promising strategy to develop highly efficient and stable DJ 2D PSCs.

Efficient Two‑Dimensional Perovskite Solar Cells Realized by Incorporation of Ti_(3)C_(2)T_(x) MXene as Nano‑Dopants

Two-dimensional(2D)perovskites solar cells(PSCs)have attracted considerable attention owing to their excellent stability against humidity;however,some imperfectness of 2D perovskites,such as poor crystallinity,disordered orientation,and inferior charge transport still limit the power conversion efficiency(PCE)of 2D PSCs.In this work,2D Ti3C2Tx MXene nanosheets with high electrical conductivity and mobility were employed as a nanosized additive to prepare 2D Ruddlesden–Popper perovskite films.The PCE of solar cells was increased from 13.69(without additive)to 15.71%after incorporating the Ti_(3)C_(2)T_(x) nanosheets with an optimized concentration.This improved performance is attributed to the enhanced crystallinity,orientation,and passivated trap states in the 3D phase that result in accelerated charge transfer process in vertical direction.More importantly,the unencapsulated cells exhibited excellent stability under ambient conditions with 55±5%relative humidity.

Phenylfluorenamine-functionalized poly(N-vinylcarbazole)s as dopant-free polymer hole-transporting materials for inverted quasi-2D perovskite solar cells

In order to improve the efficiency and stability of inverted three-dimensional(3D) or quasi-2D perovskite solar cells(PSCs) for future commercialization, exploring high efficient dopant-free polymer holetransporting materials(HTMs) is still desired and meaningful. One simple and efficient way to achieve high performance dopant-free HTMs is to synthesize novel non-conjugated side-chain polymers via rational molecular design. In this work, N-(4-methoxyphenyl)-9,9-dimethyl-9H-fluoren-2-amine(FMeNPh) groups are introduced into the poly(N-vinylcarbazole)(PVK) side chains to afford two nonconjugated polymers PVCz-DFMeNPh and PVCz-FMeNPh as dopant-free HTMs in inverted quasi-2D PSCs. Benefited from the flexible properties of polyethylene backbone and excellent optoelectronic natures of FMeNPh side-chain groups, PVCz-DFMeNPh with more FMeNPh units exhibited excellent thermal stability, well-matched energy levels and improved charge mobility as compared to PTAA and PVCzFMeNPh. Moreover, the morphologies investigation of quasi-2D perovskite on PVCz-DFMeNPh shows more compact and homogeneous perovskite films than those on PTAA and PVCz-FMeNPh. As a result,the dopant-free PVCz-DFMeNPh based inverted quasi-2D PSCs deliver power conversion efficiency(PCE) up to 18.44% as well as negligible hysteresis and favorable long-term stability, which represents as excellent performance reported to date for inverted quasi-2D PSCs. The results demonstrate the great potentials of constructing non-conjugated side-chain polymer HTMs based on phenylfluorenamine-func tionalized PVK for the development of high efficient and stable inverted 3D or quasi-2D PSCs.

Phenylformamidinium-enabled quasi-2D Ruddlesden-Popper perovskite solar cells with improved stability

Two-dimensional(2D)/quasi-2D perovskite solar cells(PSCs)incorporating organic spacer cations exhibit appealing ambient stability in comparison with their 3D analogs.Most reported organic spacer cations are based on ammonium,whereas formamidinium(FA^(+))has been seldom applied despite that FA has been extensively used in high-efficiency 3D PSCs.Herein,a novel FA-based organic spacer cation,4-chloro-phenylformamidinium(CPFA^(+)),is applied in quasi-2D Ruddlesden-Popper(RP)PSCs for the first time,and methylammonium chloride(MACl)is employed to promote crystal growth and orientation of perovskite film,resulting in high power conversion efficiency(PCE)with improved stability.Upon incorporating CPFA+organic spacer cation and MACl additive,high-quality quasi-2D CPFA_(2)MA_(n-1)Pb_(n)(I_(0.857)Cl_(0.143))_(3n+1)(n=9)perovskite film forms,exhibiting improved crystal orientation,reduced trap state density,prolonged carrier lifetime and optimized energy level alignment.Consequently,the CPFA_(2)MA_(n-1)Pb_(n)(I_(0.857)Cl_(0.143))_(3n+1)(n=9)quasi-2D RP PSC devices deliver a highest PCE of 14.78%.Moreover,the un-encapsulated CPFA-based quasi-2D RP PSC devices maintain~80%of its original PCE after exceeding 2000 h storage under ambient condition,whereas the 3D MAPb I3counterparts retain only~45%of its original PCE.Thus,the ambient stability of quasi-2D RP PSC devices is improved obviously relative to its 3D MAPb I3counterpart.

Low-dimensional phases engineering for improving the emission efficiency and stability of quasi-2D perovskite films

The two-dimensional(2 D) Ruddlesden–Popper-type perovskites, possessing tunable bandgap, narrow light emission,strong quantum confinement effect, as well as a simple preparation method, are identified as a new generation of candidate materials for efficient light-emitting diodes. However, the preparation of high-quality quasi-2 D perovskite films is still a challenge currently, such as the severe mixing of phases and a high density of defects within the films, impeding the further promotion of device performance. Here, we prepared the quasi-2 D PEA_(2) MA_(n-1) Pbn Br_(3 n+1) perovskite films by a modified spin-coating method, and the phases with large bandgap were effectively suppressed by the vacuum evaporation treatment. We systematically investigated the optical properties and stability of the optimized films, and the photoluminescence(PL) quantum yield of the treated films was enhanced from 23% to 45%. We also studied the emission mechanisms by temperature-dependent PL spectra. Moreover, the stability of films against moisture, ultraviolet light, and heat was also greatly improved.