Two‐dimensional (2D) organic–inorganic hybrid perovskite (OIHP) ferroelectrics have attracted widespread interest in the field of optoelectronics due to the combination of excellent semiconducting and ferroelectric properties. The Curie temperature (TC), below which ferroelectricity exists, is a crucial parameter for ferroelectrics. However, the lack of research on TC tuning of 2D OIHP ferroelectrics hinders their further progress. Here, through incorporating ethylammonium (EA) as cage‐confined rotators, we obtained two 2D OIHP ferroelectrics, (IBA)2(EA)Pb2Br7 (2L; IBA=isobutylammonium), and (IBA)2(EA)2Pb3Br10 (3L). Intriguingly, TC is successfully tuned from 326 K (2L) to 370 K (3L) with increasing layer thickness. Structural and computational analyses suggest that the improvement of TC is due to the higher phase‐transition energy barrier triggered by the cage‐confined EA rotators with increased layer thickness. This work suggests that EA is an effective “cage‐confined rotator” to rationally design high‐TC 2D OIHP ferroelectrics.

Electrocaloric effect driven by electric fields displays great potential in realizing highly efficient solid-state refrigeration. Nevertheless, most known electrocaloric materials exhibit relatively poor cooling performance near room temperature, which hinders their further applications. The emerging family of hybrid perovskite ferroelectrics, which exhibits superior structural diversity, large heat exchange and broad property tenability, offers an ideal platform. Herein, we report an exceptionally large electrocaloric effect near room temperature in a designed hybrid perovskite ferroelectric [(CH3)2CHCH2NH3]2PbCl4, which exhibits a sharp first-order phase transition at 302 K, superior spontaneous polarization (>4.8 μC/cm2) and relatively small coercive field (<15 kV/cm). Strikingly, a large isothermal entropy change ΔS of 25.64 J/kg/K and adiabatic temperature change ΔT of 11.06 K under a small electric field ΔE of 29.7 kV/cm at room temperature are achieved, with giant electrocaloric strengths of isothermal ΔS/ΔE of 0.86 J·cm/kg/K/kV and adiabatic ΔT/ΔE of 370 mK·cm/kV, which is larger than those of traditional ferroelectrics. This work presents a general approach to the design of hybrid perovskite ferroelectrics, as well as provides a family of candidate materials with potentially prominent electrocaloric performance for room temperature solid-state refrigeration.

Due to the breakthrough development of layered hybrid perovskites, the multilayered hybrid double perovskites have emerged as outstanding semiconducting materials owing to their environmental friendliness and superior stability. Despite recent booming advances, the realization of above-room temperature ferroelectricity in this fascinating family remains a huge challenge. Herein, when the molecular design strategy of aromatic cation alloying is applied, an above-room temperature “green” bilayered hybrid double perovskite photoferroelectric, (C6H5CH2NH3)2CsAgBiBr7 (BCAB), is successfully developed with a notable saturation polarization of 10.5 μC·cm–2 and high-Curie temperature (T c ∼ 483 K). Strikingly, such a T c achieves a new record in multilayered hybrid perovskite ferroelectrics, which extends the ferroelectric working temperature to a high level. Further computational investigation reveals that the high-T c originated from the high phase-transition energy barrier switched by the rotation of the aromatic cation in the confined environment of the inorganic layers. In addition, benefiting from the attractive polarization and remarkable photoelectric properties, a bulk photovoltaic effect (BPVE) with a prominent zero-bias photocurrent (2.5 μA·cm–2) is achieved. As far as we know, such a high-T c multilayered hybrid double perovskite ferroelectric is unprecedented, which sheds light on the rational design of an environmental photoferroelectric for high performance photoelectric devices.

Atomic layer deposition (ALD) is a novel deposition technique for constructing uniform, conformal, and ultrathin films in microelectronics, catalysis, energy storage, and conversion. The possible reaction pathways for the uncatalyzed and catalyzed ALD of silicon dioxide (SiO 2 ) using SiCl 4 and H 2 O have been investigated by density functional theory (DFT) calculations, combining static transition state searches with Born−Oppenheimer molecular dynamics (BOMD) simulations. In stepwise pathways of the uncatalyzed SiO 2 ALD reaction, the rate-determining step is the Si−O bond formation accompanied by the rotation of SiCl 4 with the activation free energy of 23.8 kcal/mol. The introduction of Lewis-base catalyst, pyridine or NH 3 , can reduce the activation free energy to 6.8 or 2.7 kcal/mol. The low energy barrier and flexible pentacoordinated intermediate facilitate the surface pseudorotation (SPR) pathway, which is similar to Berry pseudorotation (BPR) pathway of the trigonal bipyramid (TBP) molecules, such as Fe(CO) 5 , SiCl 5 − , and PF 5 . The catalyzed reaction may undergo multistep pathways, including adsorption of precursor, axial addition, surface pseudorotation, axial elimination, and desorption of byproduct steps.With one ligand pivot linked to the surface, the catalyzed reaction possesses three possible rotation modes. Through the lowbarrier pseudorotation transition states, the axial angle changes from near 180°to 120°and the equatorial angle changes from 120°to near 180°, indicating the pairwise exchange of axial and equatorial ligands. The generality of Berry and surface pseudorotations with the characterized TBP topology exhibits the common fluxional behavior in pentacoordinated compounds containing main-group and metal elements. Useful information can be provided for ALD fabrication of various functional materials.

Ab initio molecular dynamics (AIMD) simulations are employed to investigate the chemical mechanism underlying the Ni-catalyzed transformation of amorphous carbon (a-C) into graphene in the rapid thermal processing (RTP) experiment to directly grow graphene on various dielectric surfaces via the evaporation of surplus Ni and C at 1100 °C (below the melting point of bulk Ni). It is found that the a-C-to-graphene transformation entails the metal-induced crystallization and layer exchange mechanism, rather than the conventional dissolution/precipitation mechanism typically involved in Ni-catalyzed chemical vapor deposition (CVD) growth of graphene. The multi-layer graphene can be tuned by changing the relative thicknesses of deposited a-C and Ni thin films. Our AIMD simulations suggest that the easy evaporation of surplus Ni with excess C is likely attributed to the formation of a viscous-liquid-like Ni-C solution within the temperature range of 900-1800 K and to the faster diffusion of C atoms than that of Ni atoms above 600 K. Even at room temperature, sp(3)-C atoms in a-C are quickly converted to sp(2)-C atoms in the course of the simulation, and the graphitic C formation can occur at low temperature. When the temperature is as high as 1200 K, the grown graphitic structures reversely dissolve into Ni. Because the rate of temperature increase is considerably faster in the AIMD simulations than in realistic experiments, defects in the grown graphitic structures are kinetically trapped. In this kinetic growth stage, the carbon structures grown from sp(3)-carbon or from sp(2)-carbon exhibit marked differences.