Hierarchical Fe3 O4 hollow spheres constructed by nanosheets are obtained from solvothermally synthesized Fe-glycerate hollow spheres. With the unique structural features, these hierarchical Fe3 O4 hollow spheres exhibit excellent electrochemical lithium-storage performance.

Selenium has attracted intensive attention as a promising material candidate for future optoelectronic applications. However, selenium has a strong tendency to grow into nanowire forms due to its anisotropic atomic structure, which has largely hindered the exploration of its potential applications. In this work, using a physical vapor deposition method, we have demonstrated the synthesis of large-size, high-quality 2D selenium nanosheets, and the minimum thickness of which could be as thin as 5 nm. The Se nanosheet exhibits a strong in-plane anisotropic property, which is determined by angle-resolved Raman spectroscopy. Back-gating field-effect transistors (FETs) based on Se nanosheet exhibit p-type transport behaviors with on-state current density around 20 mA/mm at Vds=3V. Four-terminal field-effect devices are also fabricated to evaluate the intrinsic hole mobility of selenium nanosheet, and the value is determined to be 0.26 cm 2 V -1 s -1 at 300 K. The selenium nanosheet phototransistors show an excellent photoresponsivity up to 263 A/W, with the rise time of 0.1s and fall time of 0.12s. These results suggest that crystal selenium as a 2D form of 1D van der Waals solid, opens more feasibility to explore device applications.

Lead-free double perovskite nanocrystals (NCs) have emerged as a new category of materials that hold the potential for overcoming the instability and toxicity issues of lead-based counterparts. Doping chemistry represents a unique avenue toward tuning and optimizing the intrinsic optical and electronic properties of semiconductor materials. In this study, we report the first example of doping Yb 3+ ions into lead-free double perovskite Cs 2 AgBiX 6 (X = Cl − , Br − ) NCs via a hot injection method. The doping of Yb 3+ endows the double perovskite NCs with a newly emerged near-infrared emission band (sensitized from the NC hosts) in addition to their intrinsic trap-related visible photoluminescence. By controlling the Yb-doping concentration, the dual emission profiles and photon relaxation dynamics of the double perovskite NCs can be systematically tuned. Furthermore, we have successfully inserted divalent Mn 2+ ions in Cs 2 AgBiCl 6 NCs and observed emergence of dopant emission. Our work illustrates an effective and facile route toward modifying and optimizing optical properties of double perovskite Cs 2 AgBiX 6 (X = Cl − , Br − ) NCs with an indirect bandgap nature, which can broaden a range of their potential applications in optoelectronic devices.

2D transition metal dichalcogenide (TMD) materials have been recognized as active platforms for surface‐enhanced Raman spectroscopy (SERS). Here, the effect of crystal structure (phase) transition is shown, which leads to altered electronic structures of TMD materials, on the Raman enhancement. Using thermally evaporated copper phthalocyanine, solution soaked rhodamine 6G, and crystal violet as typical probe molecules, it is found that a phase transition from 2H‐ to 1T‐phase can significantly increase the Raman enhancement effect on MoX2 (X = S, Se) monolayers through a predominantly chemical mechanism. First‐principle density functional theory calculations indicate that the significant enhancement of the Raman signals on metallic 1T‐MoX2 can be attributed to the facilitated electron transfer from the Fermi energy level of metallic 1T‐MoX2 to the highest occupied molecular orbital level of the probe molecules, which is more efficient than the process from the top of valence band of semiconducting 2H‐MoX2. This study not only reveals the origin of the Raman enhancement and identifies 1T‐MoSe2 and 1T‐MoS2 as potential Raman enhancement substrates, but also paves the way for designing new 2D SERS substrates via phase‐transition engineering.

Theranostic agents for magnetic resonance imaging (MRI) guided photothermal therapy have attracted intensive interest in cancer diagnosis and treatment. However, the development of biocompatible theranostic agents with high photothermal conversion efficiency and good MRI contrast effect remains a challenge. Herein, PEGylated Mn2+-chelated polydopamine (PMPDA) nanoparticles were successfully developed as novel theranostic agents for simultaneous MRI signal enhancement and photothermal ablation of cancer cells, based on intrinsic manganese-chelating properties and strong near-infrared absorption of polydopamine nanomaterials. The obtained PMPDA nanoparticles showed significant MRI signal enhancement for both in vitro and in vivo imaging. Highly effective photothermal ablation of HeLa cells exposed to PMPDA nanoparticles was then achieved upon laser irradiation for 10 min. Furthermore, the excellent biocompatibility of PMPDA nanoparticles, because of the use of Mn2+ ions as diagnostic agents and biocompatible polydopamine as photothermal agents, was confirmed by a standard MTT assay. Therefore, the developed PMPDA nanoparticles could be used as a promising theranostic agent for MRI-guided photothermal therapy of cancer cells.