Growth Control of Quasi-two-dimensional Perovskites:Structure-dependent Exciton and Charge Behavior

While three-dimensional perovskites have high defect tolerance and an adjustable bandgap,their charges tend to be free rather than forming excitons,making them unsuitable for use in efficient light-emitting diodes(LEDs).Rather,quasi-two-dimensional(Q-2D)perovskites offer high photoluminescence quantum yield along with the advantages of bulk perovskites,making them ideal for high-performance LEDs.In Q-2D perovskites,the structure(which includes factors like crystal orientation,phase distribution,and layer thickness)directly influences how excitons and charge carriers behave within the material.Growth control techniques,such as varying the synthesis conditions or employing methods,allow for fine-tuning the structural characteristics of these materials,which in turn affect exciton dynamics and charge transport.This review starts with a description of the basic properties of Q-2D perovskites,examines crystal growth in solution,explains how structure affects energy transfer behavior,and concludes with future directions for Q-2D perovskite LEDs.By understanding and optimizing the structure-dependent behavior,researchers can better control exciton dynamics and charge transport,which are crucial for enhancing the performance of optoelectronic devices like solar cells and LEDs.

Coexistence of Quasi-two-dimensional Superconductivity and Tunable Kondo Lattice in a van der Waals Superconductor

Realization of Kondo lattice in superconducting van der Waals materials not only provides a unique opportunity for tuning the Kondo lattice behavior by electrical gating or intercalation,but also is helpful for further understanding the heavy fermion superconductivity.Here we report a low-temperature and vector-magneticfield scanning tunneling microscopy and spectroscopy study on a superconducting compound(4Hb-TaS_(2))with alternate stacking of 1T-TaS_(2)and 1H-TaS_(2)layers.We observe the quasi-two-dimensional superconductivity in the 1H-TaS_(2)layer with anisotropic response to the in-plane and out-of-plane magnetic fields.In the 1T-TaS_(2)layer,we detect the Kondo resonance peak that results from the Kondo screening of the unpaired electrons in the Star-of-David clusters.We also find that the intensity of the Kondo resonance peak is sensitive to its relative position with the Fermi level,and it can be significantly enhanced when it is further shifted towards the Fermi level by evaporating Pb atoms onto the 1T-TaS_(2)surface.Our results not only are important for fully understanding the electronic properties of 4Hb-TaS_(2),but also pave the way for creating tunable Kondo lattice in the superconducting van der Waals materials.

Predicted High Thermoelectric Performance of Quasi-Two-Dimensional Compound GeAs Using First-Principles Calculations

The electronic structure of binary quasi-two-dimensional GeAs is investigated using first-principles calculations, and it is found that the anisotropic structure of the layered compound GeAs brings about the anisotropy of the transport properties. Meanwhile, the band structure of GeAs exhibits a relatively large dispersion near the valence-band maximum in the Z –V direction while it is rather flat in the Z –Γ direction, which is highly desirable for good thermoelectric performance. The calculated partial charge density distribution also reveals that GeAs possesses anisotropic electrical conductivity. Based on the semi-classical Boltzmann transport theory, the anisotropic transport properties are observed, and the optimal doping concentrations are estimated. The temperature dependence transport properties of p-type GeAs are compared with the experimental data in good agreement, and the theoretical figure-of-merit ZT has been predicted as well.

Quasi-two-dimensionalβ-Ga2O3 field effect transistors with large drain current density and low contact resistance via controlled formation of interfacial oxygen vacancies

Quasi-two-dimensi on al(2D)β-Ga2O3 is a rediscovered metal-oxide semiconductor with an ultra-wide bandgap of 4.6-4.9eV.It has been reported to be a promising material for next-generation power and radio frequency electronics.Field effect transistors(FETs)that can switch at high voltage are key compone nts in power and radio frequency devices,and reliable Ohmic con tacts are essential for high FET performance.However,obtaining low contact resistance onβ-Ga2O3 FETs is difficult since reactions betweenβ-Ga2O3 and metal contacts are not fully understood.Herein,we experimentally demonstrate the importanee of reactions at the metal/β-Ga2O3 interface and the corresponding effects of these reactions on FET performance.When Ti is employed as the metal contact,annealing ofβ-Ga2O3 FETs in argon can effectively transform Schottky contacts into Ohmic contacts and permit a large drain current density of-3.1 mA//μm.The contact resistance(Rcontact)between the Ti electrodes andβ-Ga2O3 decreased from^430 to^0.387Ω·mm after annealing.X-ray photoelectron spectroscopy(XPS)confirmed the formation of oxygen vacancies at the Ti/β-Ga2O3 interface after annealing,which is believed to cause the improved FET performance.The results of this study pave the way for greater application ofβ-Ga2O3 in electronics.

Effect of Chloride Ion Concentrations on Luminescence Peak Blue Shift of Light-Emitting Diode Using Anti-Solvent Extraction of Quasi-Two-Dimensional Perovskite

In recent years,Perovskite Light-Emitting Diodes(PeLEDs)have received considerable attention in academia.However,with the development of PeLEDs,commercial applications of full-color PeLED technology are largely limited by the progress of blue-emitting devices,due to the uncontrollably accurate composition,unstable properties,and low luminance.In this article,we add Cesium chloride(CsCl)to the quasi-two-dimensional(quasi-2D)perovskite precursor solution and achieve the relatively blue shifts of PeLED emission peak by introducing chloride ions for photoluminescence(PL)and electroluminescence(EL).We also found that the introduction of chlorine ions can make quasi-2D perovskite films thinner with smoother surface of 0.408 nm.It is interesting that the EL peaks and intensities of PeLED are adjustable under different driving voltages in high concentration chlorine-added perovskite devices,and different processes of photo-excited,photo-quenched,and photo-excited occur sequentially with the increasing driving voltage.Our work provides a path for demonstrating full-color screens in the future.

Recent progress in quasi-two-dimensional and quantum dot perovskite light-emitting diodes harnessing the diverse effects of ligands:A review

Halide perovskites have been extensively studied for use as light-emitting diodes(LEDs)in next-generation displays due to their beneficial characteristics,including their high color purity and wide color gamut.Halide perovskites can be categorized into four representative structures:three-dimensional(3D)bulk,two-dimensional(2D),quasi-two-dimensional(quasi-2D),and quantum dot(QD).Recently,excellent advances in the performance of perovskite LEDs(PeLEDs),especially those with quasi-2D and QD architectures,have been demonstrated with the incorporation of organic chain ligands.Ligands can both modify the structure of PeLEDs,such as forming multi-quantum wells in quasi-2D PeLEDs and essential passivation layers in QD PeLEDs,and also enhance their optical performance.The appropriate use of ligands in PeLEDs can thus lead to greater luminescence,current efficiency,power efficiency,and external quantum efficiency.In this review,the principal roles of ligands in quasi-2D and QD PeLEDs are systematically summarized.Furthermore,current limitations and future perspectives are discussed in detail.

Electron mobility in the linear region of an AlGaN/AlN/GaN heterostructure field-effect transistor

We simulate the current-voltage （I-V） characteristics of AlGaN/AlN/GaN heterostructure field-effect transistors （HFETs） with different gate lengths using the quasi-two-dimensional （quasi-2D） model. The calculation results obtained using the modified mobility model are found to accord well with the experimental data. By analyzing the variation of the electron mobility for the two-dimensional electron gas （213EG） with the electric field in the linear region of the AlGaN/AlN/GaN HFET I-V output characteristics, it is found that the polarization Coulomb field scattering still plays an important role in the electron mobility of AlGaN/AlN/GaN HFETs at the higher drain voltage and channel electric field. As drain voltage and channel electric field increase, the 2DEG density reduces and the polarization Coulomb field scattering increases, as a result, the 2DEG electron mobility decreases.

钾氨电子化合物中的准二维自旋-派尔斯相变

Electron-phonon interactions and electron-electron correlations represent two crucial facets of condensed matter physics.For instance,in a half-filled spin-1/2 anti-ferromagnetic chain,the lattice dimerization induced by electron-nucleus interaction can be intensified by onsite Coulomb repulsion,resulting in a spin-Peierls state.Through first-principles calculations and crystal structure prediction methods,we have identified that under mild pressures,potassium and ammonia can form stable compounds:R3m K(NH_(3))_(2),Pm3 m K(NH_(3))_(2),and Cm K_(2)(NH_(3))_(3).Our predictions suggest that the R3 m K(NH_(3))_(2)exhibits electride characteristics,marked by the formation of interstitial anionic electrons(IAEs)in the interlayer space.These IAEs are arranged in quasi-two-dimensional triangular arrays.With increasing pressure,the electronic van-Hove singularity shifts toward the Fermi level,resulting in an augmented density of states and the onset of both Peierls and magnetic instabilities.Analyzing these instabilities,we determine that the ground state of the R3 m K(NH_(3))_(2)is the dimerized P2_(1)/m phase with zigzag-type anti-ferromagnetic IAEs.This state can be described by the triangular-lattice antiferromagnetic Heisenberg model with modulated magnetic interactions.Furthermore,we unveil the coexistence and positive interplay between magnetic and Peierls instability,constituting a scenario of spin-Peierls instability unprecedented in realistic 2D materials,particularly involving IAEs.This work provides valuable insights into the coupling of IAEs with the adjacent lattice and their spin correlations in quantum materials.

基于准二维钙钛矿的高稳定电阻随机存储器

电阻式随机存储器(RRAM)一直被视为新兴存储器技术中具有挑战性的替代品.近年来,人们发现基于有机-无机杂化卤化物钙钛矿的RRAM具有优异的存储特性.本研究利用简单的一步旋涂策略,在空气中不使用反溶剂,制备出了具有准二维钙钛矿活性层的Al/(PEA)_(2)-MAPb_(2)I_(3)Br_(4)/SnO_(2)/氧化铟锡(ITO)非易失性RRAM,该器件表现出了出色且高度稳定的双极性电阻切换特性.此外,该器件表现出约为10~4的开/关比,低SET电压(约0.8 V),并且具有相对稳定的耐久性(大于1000次循环).高阻态和低阻态的传导机制分别是空间电荷限流传导和欧姆传导.此外,由于苯乙胺分子的疏水性,该器件在40%湿度下放置90天依然表现出显著的阻变特性.因此,这种高性能、稳定的RRAM为未来存储器的商业化提供了可能性.

Epitaxial quasi-2D/3D hybrid perovskite heterojunctions for photodetector with enhanced detectivity and stability

Although numerous metal halide perovskite materials have been investigated in the field of optoelectronic,the development of perovskite heterojunctions with exotic structures is still rare.Herein,we report the epitaxial growth of quasi-two-dimensional(Q-2D)perovskites on methylammonium lead iodide(MAPbI_(3))single crystals to form perovskite heterojunctions with interfacial bonding.The MAPbI_(3)adjacent to epitaxial Q-2D perovskite shows blue shifted photoluminescence with shortened lifetime,which becomes significant with the reduced layer number of the Q-2D perovskites.Our findings suggest the presence of an interfacial strain gradient leading to enhanced photocarrier separation.Accordingly,compared to the MAPbI_(3)single crystal detector,the BA_(2)MAPb_(2)I_(7)/MAPbI_(3)(BA:n-butylamine)heterojunction-based photodetector demonstrates a bandpass detecting property and exhibits 5 times enhanced external quantum efficiency and 83 times enhanced specific detectivity(D*=3.26×10^(11)Jones).Remarkably,the unencapsulated BA_(2)MAPb_(2)I_(7)/MAPbI_(3)heterojunction is stable in ambient condition for>300 days.The Q-2D/3D heterojunction shows suppressed ion inter-diffusion due to the presence of Q-2D phase.

Onset of the Meissner effect at 65 K in Fe Se thin film grown on Nb-doped Sr TiO_3 substrate

We report the Meissner effect studies on an Fe Se thin film grown on Nb-doped Sr Ti O3 substrate by molecular beam epitaxy. Two-coil mutual inductance measurement clearly demonstrates the onset of diamagnetic screening at 65 K, which is consistent with the gap opening temperature determined by previous angle-resolved photoemission spectroscopy results. The applied magnetic field causes a broadening of the superconducting transition near the onset temperature, which is the typical behavior for quasi-two-dimensional superconductors. Our results provide direct evidence that Fe Se thin film grown on Nb-doped Sr Ti O3 substrate has an onset TC* 65 K,which is the highest among all iron-based superconductors discovered so far.

Numerical Study on Flow Structure of a Shallow Laminar Round Jet

The evolution mechanism and characteristics of the submerged laminar round jet in a viscous homogenous shallow water layer are investigated through computational modeling. The laminar mode is used to solve the Navier-Stokes equations. In order to visualize the formation and evolution of the flow pattern, the volume of fluid(VOF) method is adopted to simulate the free surface of the water layer below the air and to trace the jet fluid. The results show that the jet forms a class of quasi-two-dimensional(Q2D) vortex structures in the ambient fluid with unequal influence from the bottom wall and free surface. The time dependence of three parameters,defined for the flow pattern as jet length, spiral radius and pattern length, is investigated quantitatively in their non-dimensional forms. Three different Reynolds numbers and two injection durations are further considered to discuss their influence on the flow pattern.