Structural Origin of the Midgap Electronic States and the Urbach Tail in Pnictogen-Chalcogenide Glasses

Zinc metal complexes synthesized by a green method as a new approach to alter the structural and optical characteristics of PVA: new field for polymer composite fabrication with controlled optical band gap

The current study employed a novel approach to design polymer composites with modified structural and declined optical band gaps. The results obtained in the present work for polymer composites can be considered an original method to make a new field for research based on green chemistry. Natural dyes extracted from green tea were mixed with hydrated zinc acetate (Zn(CH3COO)2·2H2O) to produce a metal complex. FTIR results comprehensively established the formation of the Zn-metal complex. The interaction among various components of PVA : Zn-metal complex composite was investigated using FTIR spectroscopy. The non-existence of anion bands of acetate in the Zn-metal complex spectrum confirms the formation of the Zn-metal complex. XRD analysis reveals that the Zn-metal complex improves the amorphous phase of the PVA-based composites. The absorption edge of the doped films shifted towards the lower photon energies. Optical dielectric properties were used to determine N/m*, ε ∞, τ, μ opt, ω p, and ρ opt; the W-D model was used to calculate E d, E o and n o parameters. The optical dielectric loss parameter was used to determine the optical band gap while the Tauc model was employed to identify various types of electron transitions. The optical energy band gap was 6.05 eV for clean PVA while it decreased to 1 eV for PVA inserted with the Zn-metal complex. The increase in Urbach energy from 0.26 eV to 0.45 eV is an evidence of the boost of amorphous phases in PVA : Zn-metal complex composites. The nonlinear refractive index and the first-order and second-order nonlinear optical susceptibilities were determined. The value of E o obtained from the W-D model closely matches the optical energy band gap obtained from the Tauc model, which indicates the precision of the analysis in the present study. The increase in SELF and VELF in the composite films establishes that new energy states assigned to the added Zn-metal complex amplify the probability of light-matter interaction.

Black graphitic carbon nitride nanosheets with mid-gap states realizing highly efficient near-infrared photo-thermal conversion for photoacoustic imaging

Developing metal-free photo-thermal transduction nanoagents (PTNAs) with high conversion efficiency addresses the balance between superior photothermal performance and good biocompatibility in the field of bio-applications of PTNAs. Herein, we highlight the bandgap-engineered black graphitic carbon nitride nanosheets (denoted as B-g-C₃N₄) as a novel metal-free PTNA with high conversion efficiency (62% at 808 nm) for photoacoustic imaging. The B-g-C₃N₄ absorbed infrared light with a narrowed bandgap and electronic states within the band (known as mid-gap states) due to the synergistic effect of sulfur incorporation, nitrogen vacancies and the porous structure. Notably, the mid-gap states mediated the non-irradiative recombination of electrons and holes, bringing about energy dissipated as phonons. Owing to the high conversion efficiency as well as superior biocompatibility, HeLa cells incubated with B-g-C₃N₄ can be ablated under 808 nm light illumination. Furthermore, the B-g-C₃N₄ realized cross-sectional multispectral optoacoustic tomography (MSOT) imaging of the U14-tumor-bearing mouse. This work expands the catalogue of highly efficient metal-free PTNAs, showing great promise in biological applications.

Temperature-driven narrowing of the insulating gap as a precursor of the insulator-to-metal transition: Implications for the electronic structure of solids

We present a microscopic picture rationalizing the surprisingly steep decrease in the bandgap with temperature in insulators, crystalline or otherwise. The gap narrowing largely results from fluctuations of long-wavelength optical phonons-when the latter are present-or their disordered analogs if the material is amorphous. We elaborate on this notion to show that possibly with the exception of weakly bound solids made of closed-shell electronic configurations, the existence of an insulating gap or pseudogap in a periodic solid implies that optical phonons must be present, too. This means that in an insulating solid, the primitive cell must have at least two atoms and/or that a charge density wave is present, with the possible exception of weakly bonded solids such as rare-gas or ferromagnetic Wigner crystals. As a corollary, a (periodic) elemental solid held together by nonclosed shell interactions and whose primitive unit contains only one atom will ordinarily be a metal, consistent with observation. Consequences of the present picture for Wigner solids are discussed. A simple field theory of the metal-insulator transition is constructed that directly ties long-wavelength optical vibrations with fluctuations of an order parameter for the metal-insulator transition. The order parameter is shown to have at least two components, yet no Goldstone mode arises as a result of the transition.