Electronic Modulation in Site-Selective Occupation of Quasi-2D Triangular-Lattice Cs2CuCl4-xBrx Perovskite Probed by Surface-Sensitive Characterization.

A controllable electronic manipulation in a frustrated magnetic system such as solution-based two-dimensional (2D) all-inorganic perovskites offers a possible route for their integrations with electronic and magnetic devices for their advanced applications. Here, we perform element-specific investigations of an emergent class of quasi-2D all-inorganic perovskites Cs2CuCl4-xBrx with (0 ≤ x ≤ 4) using a combination of synchrotron-radiation photoelectron techniques. Surface- and element-sensitive X-ray absorption spectroscopy spectra of Cu L2,3 edges indicate strong electronic transition that is largely influenced by their halogen content at room temperature. This implies that site-selective occupation largely dominates the electronic transition across the unoccupied states of these series since chlorine atoms possess a stronger electronegative character than bromine atoms. Moreover, the implication of halogen site is reflected in the valence band of Cl-rich copper perovskite in which the valence band edge is closer to Fermi energy (EF) than that of the Br-rich compound. Furthermore, X-ray magnetic circular dichroism spectra of mixed ratio and Br-rich compounds exhibit antiferromagnetism at room temperature. These site-specific magnetic-spectroscopic results are corroborated by density functional theory calculations. The strong electronic modulation and the local magnetic spectroscopy results in these solution-based and low-temperature-growth materials will pave the way toward energy- and cost-efficient perovskite devices.

Current Oscillations and Intermittent Emission Near an Electrode Interface in a Hybrid Organic-Inorganic Perovskite Single Crystal.

Hybrid organic-inorganic lead perovskites have a great potential in optoelectronic device applications because of their high stability, narrow band emission, and strong luminescence. Single crystals with few defects are the best candidates to disclose a variety of interesting and important properties for light-emitting devices. Here, we investigate a single-crystalline CH3NH3PbBr3 perovskite for its transport and electroluminescence properties. A simple fabrication method was used to obtain a 10 ± 2 µm channel between two gold wire electrodes, which showed bright intermittent electroluminescence near the interface of one wire after cooling down with a constant biasing voltage. The active region of the perovskite single crystal was pristine, well isolated from surroundings through fabrication to the characterization process. Our presented sample provided an ideal condition to study bulk ionic-electronic properties of hybrid halide perovskites. At constant 6 V bias, the current through the sample shows temperature-dependent oscillation with Arrhenius behavior, suggesting a thermally activated process. The light emission from the sample experiences an intermittent emission rate once every 26 ± 6 min. Here, we envisage that the current oscillations and intermittent emission are caused by ion-mediated negative differential resistance and conductive filament formation, respectively. The latter observation inspires future applications of the material from neuromorphic computing to the development of electroluminescence devices.

Chemical and Bandgap Engineering in Monolayer Hexagonal Boron Nitride.

Monolayer hexagonal boron nitride (h-BN) possesses a wide bandgap of ~6 eV. Trimming down the bandgap is technically attractive, yet poses remarkable challenges in chemistry. One strategy is to topological reform the h-BN's hexagonal structure, which involves defects or grain boundaries (GBs) engineering in the basal plane. The other way is to invite foreign atoms, such as carbon, to forge bizarre hybrid structures like hetero-junctions or semiconducting h-BNC materials. Here we successfully developed a general chemical method to synthesize these different h-BN derivatives, showcasing how the chemical structure can be manipulated with or without a graphene precursor, and the bandgap be tuned to ~2 eV, only one third of the pristine one's.

Pristine Graphene Electrode in Hydrogen Evolution Reaction.

Graphene, the sp2 carbonaceous two-dimensional (2D) material, is gaining more attention in recent electrochemical studies. However, this atomic thick electrode usually suffers with surface contamination and poor electrochemical endurance. To overcome the drawbacks, we developed a PMMA-assisted, flipped transfer method to fabricate the graphene electrode with pristine surface and prolonged lifetime in hydrogen evolution reaction (HER). The HER performances of the single-layer graphene (SLG) were evaluated on various insulating and conductive substrates, including SiO2, polymers, SLG, highly oriented pyrolytic graphite (HOPG), and copper. The parallel Tafel slopes of SLG, bilayer graphene (BLG), and HOPG suggest they share the same electrochemical activities deriving from the sp2 carbon basal plane. Moreover, the atomic barriers, both for SLG and the single-layer h-BN (SLBN), are semitransparent in HER for the underneath copper, providing a new perspective for the 2D materials to protect and couple with the other electrochemical catalysts.

A tough and high-performance transparent electrode from a scalable and transfer-free method.

Conductive metal films are patterned into transparent metal nanowire networks by using electrospun fibers as a mask. Both the transmittance and sheet resistance (6 Ω/□ at 83% transmittance and 24 Ω/□ at 92% transmittance) of the metal nanowire-based electrode out-perform commercial indium doped tin oxide (ITO) electrodes. The metal nanowire-based transparent electrodes were fabricated on both rigid glass and flexible polyethylene terephthalate (PET) substrates. In addition to state of art performance, the transparent electrodes also exhibit outstanding toughness. They can withstand repeated scotch tape peeling and various bending tests. The method for making the metal nanowire is scalable, and a touch screen on flexible substrate is demonstrated.