Xiaomei Lü

Size effect of multiferroics is important for its potential applications in new type miniaturized multifunctional devices and thus has been widely studied. However, is there special size effect in the materials with spiral modulated spin structure (such as BiFeO3)? It is still an issue to be investigated. In this report, structural, magnetic and magnetoelectric coupling properties are investigated for sol-gel prepared BiFeO3 nanoparticles with various sizes. It is found that a structural anomaly arises for the particles with size close to the 62 nm period of the spiral modulated spin structure, which induces an obviously increased ferromagnetism. In addition, large magnetoelectric coupling effect is observed in 62 nm BiFeO3 nanoparticles. Our result provides another insight into the size effect of BiFeO3, and also a clue to the magnetic structure at nanoscale.

Polycrystalline Bi1−xNdxFeO3 (x=0–0.15) thin films were prepared on (111) Pt∕Ti∕SiO2∕Si substrates via metal organic deposition method. The effect of Nd dopant on the structural, electric, and magnetic properties was studied. It was found that the ferroelectric polarization and saturation magnetization of the films were enhanced by appropriate Nd doping due to the structural distortion and the suppressed cycloidal spin structure. Meanwhile, Nd-doped BiFeO3 thin films exhibited magnetic anisotropy because of the magnetocrystalline anisotropy.

Materials for photosensitized oxygen activation are extremely important for a suite of photodynamic applications in biomedical, analytical, and energy sectors. Carbon-based photosensitizers are attractive for their low cost and high stability, but most of them such as fullerene and graphene quantum dots suffer from low efficiency, and the rational design of carbon-based photosensitizers remains a challenge. Given the similar chemical origin of phosphorescence and photosensitization, we herein synthesized a series of nitrogen-doped carbon dots (C-dots) and confirmed that their photo-oxidation activity correlated with their phosphorescence quantum yields, providing a direction for the rational designing of such materials. Compared to other carbon nanomaterials and molecular photosensitizers, these C-dots have the highest activity, and they can finish oxidation reactions in a few seconds. The excellent photosensitized oxygen activation makes these water-soluble C-dots a promising oxidase-mimicking nanozyme for photodynamic antimicrobial chemotherapy and other applications.

A long-standing challenge in nanozyme catalysis is low activity at physiological pH, especially for oxidase- and peroxidase-mimicking nanozymes. We herein communicate that Mn(II) can promote catalysis at neutral pH for carbon dots (C-dots) as a photo-oxidase nanozyme. The C-dots produce singlet oxygen upon light irradiation to oxidize Mn(II) to Mn(III), which is confirmed by a suite of spectroscopic evidence. The in situ produced Mn(III) acts as a mediator, analogous to mediators in electrochemistry to enhance electron transfer. None of the other divalent metal ions show such an effect, allowing the selective detection of Mn(II) down to 5 nM. EDTA further enhances the activity by stabilizing the highly active Mn(III), producing an intense blue color by oxidizing 3,3',5,5'-tetramethylbenzidine (TMB) in just 10 s. Finally, this reaction was used to evaluate antioxidants. With this method, more analytical and biomedical applications of nanozymes can be exploited at neutral pH, and it may inspire other strategies to overcome the pH limitation in nanozyme catalysis.

Near-infrared (NIR) lasers are key components for applications, such as telecommunication, spectroscopy, display, and biomedical tissue imaging. Inorganic III-V semiconductor (GaAs) NIR lasers have achieved great successes but require expensive and sophisticated device fabrication techniques. Organic semiconductors exhibit chemically tunable optoelectronic properties together with self-assembling features that are well suitable for low-temperature solution processing. Major blocks in realizing NIR organic lasing include low stimulated emission of narrow-bandgap molecules due to fast nonradiative decay and exciton-exciton annihilation, which is considered as a main loss channel of population inversion for organic lasers under high carrier densities. Here we designed and synthesized the small organic molecule (E)-3-(4-(di-p-tolylamino)phenyl)-1-(1-hydroxynaphthalen-2-yl)prop-2-en-1-one (DPHP) with amphiphilic nature, which elaborately self-assembles into micrometer-sized hemispheres that simultaneously serves as the NIR emission medium with a photoluminescence quantum efficiency of ∼15.2%, and the high-Q (∼1.4 × 10(3)) whispering gallery mode microcavity. Moreover, the radiative rate of DPHP hemispheres is enhanced up to ∼1.98 × 10(9) s(-1) on account of the exciton-vibrational coupling in the solid state with the J-type molecular-coupling component, and meanwhile the exciton-exciton annihilation process is eliminated. As a result, NIR lasing with a low threshold of ∼610 nJ/cm(2) is achieved in the single DPHP hemisphere at room temperature. Our demonstration is a major step toward incorporating the organic coherent light sources into the compact optoelectronic devices at NIR wavelengths.