Intrinsic magnetic topological insulators offer low disorder and large magnetic bandgaps for robust magnetic topological phases operating at higher temperatures. By controlling the layer thickness, emergent phenomena such as the quantum anomalous Hall (QAH) effect and axion insulator phases have been realized. These observations occur at temperatures significantly lower than the Néel temperature of bulk MnBi 2 Te 4 , and measurement of the magnetic energy gap at the Dirac point in ultra-thin MnBi 2 Te 4 has yet to be achieved. Critical to achieving the promise of this system is a direct measurement of the layer-dependent energy gap and verification of a temperature-dependent topological phase transition from large bandgap QAH insulator to a gapless TI paramagnetic phase. Here we utilize temperature-dependent angle-resolved photoemission spectroscopy to study epitaxial ultra-thin MnBi 2 Te 4 . We directly observe a layer-dependent crossover from a 2D ferromagnetic insulator with a bandgap greater than 780 meV in one septuple layer (1 SL) to a QAH insulator with a large energy gap (>70 meV) at 8 K in 3 and 5 SL MnBi 2 Te 4 . The QAH gap is confirmed to be magnetic in origin, as it becomes gapless with increasing temperature above 8 K.

This work presents the enhanced electrochemiluminescence (ECL) of a newly prepared nanosilver-carbon nanodot (Ag-C-dot) composite and its application for the sensitive detection of sulfide (S(2-)) ions. The Ag-C-dot composite was easily prepared by adding silver nitrate into C-dot colloids through an alkaline reduction. The obtained Ag-C-dots were characterized by UV-vis spectra, fluorescence spectra and transmission electron microscopy. The electrochemical and ECL behaviors of the Ag-C-dot composite were investigated by cyclic voltammetry. Moreover, a simple label-free method to detect S(2-) ions with a high selectivity and sensitivity has been developed based on the ECL of the Ag-C-dot composite in aqueous media. The sensing mechanism could be due to the strong and specific interaction between the S(2-) ions and the Ag atoms/ions on the surface of the Ag-C-dot composite, which dramatically affects the resulting ECL of the Ag-C-dot composite. The linear response to detect S(2-) ions ranges from 0.05 to 100 μM with a detection limit of 0.027 μM (~1 ppb). This work indicates that the Ag-C-dot nanocomposite possesses potential applications for environment sensing.

In this study, we report a metal-ion-assisted precipitation etching strategy that can be used to manipulate the optical properties associated with the assembling of sulfur quantum dots (S dots) using copper ions. Transmission electron microscopy confirmed that the S dots were mainly distributed within 50−80 nm and that they exhibited an ambiguous boundary. After the post-synthetic Cu 2+ -assisted modification was completed, the assisted precipitation-etching S dots (APE-S dots) were observed to exhibit a relatively clear boundary with a high fluorescence (FL) quantum yield (QY) of 32.8%. Simultaneously, the Fourier transform infrared radiation, X-ray photoelectron spectra, and timeresolved FL decay spectra were used to illustrate the improvement in the FL QY of the APE-S dots.