Solar cells based on organometallic halide perovskite absorber layers are emerging as a high-performance photovoltaic technology. Using highly sensitive photothermal deflection and photocurrent spectroscopy, we measure the absorption spectrum of CH3NH3PbI3 perovskite thin films at room temperature. We find a high absorption coefficient with particularly sharp onset. Below the bandgap, the absorption is exponential over more than four decades with an Urbach energy as small as 15 meV, which suggests a well-ordered microstructure. No deep states are found down to the detection limit of ∼1 cm–1. These results confirm the excellent electronic properties of perovskite thin films, enabling the very high open-circuit voltages reported for perovskite solar cells. Following intentional moisture ingress, we find that the absorption at photon energies below 2.4 eV is strongly reduced, pointing to a compositional change of the material.

Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance

We developed two-step solution-phase reactions to form hybrid materials of Mn3O4 nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Mn3O4 nanoparticles grown selectively on RGO sheets over free particle growth in solution allowed for the electrically insulating Mn3O4 nanoparticles wired up to a current collector through the underlying conducting graphene network. The Mn3O4 nanoparticles formed on RGO show a high specific capacity up to ~900mAh/g near its theoretical capacity with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn3O4 nanoparticles grown atop. The Mn3O4/RGO hybrid could be a promising candidate material for high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for design and synthesis of battery electrodes based on highly insulating materials.

Mn3O4-Graphene Hybrid as a High Capacity Anode Material for Lithium Ion Batteries

Crystal facet engineering of semiconductor photocatalysts: motivations, advances and unique properties

Rational design of core-shell Co@C microspheres for high-performance microwave absorption

Composites of magnetic metal nanoparticles and carbon materials are highly desirable for high-performance microwave absorbers due to their compatible dielectric loss and magnetic loss abilities. In this article, novel nanocomposites, Fe/C nanocubes, have been successfully prepared through an in situ route from a metal–organic framework, Prussian blue, by controlled high-temperature pyrolysis. The resultant nanocubes are actually composed of a cubic framework of amorphous carbon and uniformly dispersed core–shell Fe@graphitic carbon nanoparticles. Within the studied pyrolysis temperature range (600–700 °C), the porous structure, iron content, magnetic properties, and graphitization degree of the Fe/C nanocubes can be well modulated. Particularly, the improved carbon graphitization degree, both in amorphous frameworks and graphitic shells, results in enhanced complex permittivity and dielectric loss properties. The homogeneous chemical composition and microstructure stimulate the formation of multiple dielectric resonances by regularizing various polarizations. The synergistic effect of dielectric loss, magnetic loss, matched impedance, and dielectric resonances accounts for the improved microwave absorption properties of the Fe/C nanocubes. The absorption bands of the optimum one obtained at 650 °C are superior to most composites ever reported. By considering the good chemical homogeneity and microwave absorption, we believe that the as-fabricated Fe/C nanocubes will be promising candidates as highly effective microwave absorbers.

Metal organic framework-derived Fe/C nanocubes toward efficient microwave absorption

Effects of particle size on the magnetic and microwave absorption properties of carbon-coated nickel nanocapsules

Co3O4/C nanocapsules with onion-like carbon shells as anode material for lithium ion batteries

Flower-like FeNi@C nanocomposites self-assembled by FeNi nanosheets and flocculent carbon were synthesized by the hydrothermal method. The electromagnetic parameters of FeNi@C–paraffin composites were measured at 0.03–18 GHz. Dual dielectric relaxations were observed in the composite system due to the cooperative consequence of the FeNi–C interfaces and dielectric carbon. The strong magnetic loss is from the coexistence of natural resonance and exchange resonance. For the 2.0 mm thickness layer, an optimal reflection loss (RL) of −46.7 dB is observed at 3.17 GHz, whereas the absorbent with a thickness of 1.3 mm has an RL of −32.78 dB at 13.78 GHz. The excellent microwave absorption abilities result from the synergy of dielectric and magnetic losses and quarter-wavelength cancellation.

Enhanced microwave absorption of flower-like FeNi@C nanocomposites by dual dielectric relaxation and multiple magnetic resonance

Electromagnetic ( EM) wave absorption properties of the paraffin-Fe(Mn)/ferrite nanocapsule composite ( with 40 wt% nanocapsules) were investigated in the 2-18 GHz frequency range. The Fe(Mn)/ferrite nanocapsules synthesized by arc-discharging have a core-shell structure with cores 10-30 nm in diameter and shells about 5 nm in thickness. The reflection loss (RL) calculated from the measured permittivity and permeability was found to exceed -20 dB in the 13-15 GHz range for a thickness of about 1.75-1.95 mm. The optimal RL is -28.2 dB at about 14 GHz with 1.84mm thickness. RL values exceeding -10 dB were obtained for the absorber of 1.7mm thickness in the range 12.7-17.1 GHz, which almost covered the whole Ku-band (12.4-18 GHz). The excellent EM wave absorption properties of the Fe(Mn)/ferrite nanocapsules significantly depend on the core-shell microstructure and the match of dielectric and magnetic losses.

Xianguo Liu

Papers

Effects of particle size on the magnetic and microwave absorption properties of carbon-coated nickel nanocapsules

Co3O4/C nanocapsules with onion-like carbon shells as anode material for lithium ion batteries

Enhanced microwave absorption of flower-like FeNi@C nanocomposites by dual dielectric relaxation and multiple magnetic resonance

Microwave-absorption properties of Fe(Mn)/ferrite nanocapsules

Magnetic and Microwave-absorption Properties of Graphite-coated (Fe, Ni) Nanocapsules