Junfeng Han

The Bi2Se3 family is widely regarded as the prototypical three-dimensional topological insulator, which is an ideal platform for investigating topological quantum phenomena such as the quantum anomalous Hall effect and Majorana fermions. Further application requires high-quality film growth with cost-effective and stable methods. In this work, we developed the evaporation technique to grow high-quality Bi2Se3 films and performed systematic transport studies on Bi2Se3 films with distinct magnetic surface modifications. Variable temperature-dependent resistance measurements revealed metallic conducting behavior. The magnetoresistive response was characterized by zero-field resistance minimum at low temperatures of 2 K. This behavior could be attributed to the dominance of the weak antilocalization effect. The phase coherence length at 2 K was around 300 nm. Hall measurements demonstrated that the carrier concentration of 7.45 × 1018 cm−3 at 2 K and the mobility is as high as 955 cm2/(V s), consistent with a single-carrier transport mechanism, with quantum interference dominated by topological surface states. The spin-momentum-locked nature of topological surface states endows them with magnetic sensitivity. A significant change in the quantum transport behavior was observed in Bi2Se3 thin-film samples with Cr atoms deposited on the film surface. This work demonstrated the quality of Bi2Se3 films and its potential for developing spintronic logic devices and topological electronic devices.

Bi4Br4, an emerging quasi-1D topological insulator, exhibits rich topological phases and great potential for low-power electronics and topological quantum computing. High-quality, oriented Bi4Br4 films are essential for exploring low-dimensional topological physics and developing functional devices. However, the growth of such films is highly challenging due to their sensitivity to growth parameters, and current synthesis techniques can only yield randomly oriented or multidirectional Bi4Br4 nanostructures, which impedes the study of intrinsic anisotropy and device integration. Here, we demonstrate a molecular beam epitaxy approach using anisotropic WTe2 as an epitaxial template to achieve oriented Bi4Br4 nanoribbon arrays with controlled thickness. Through systematic optimization of growth temperature, source rate ratio and annealing conditions, we established the optimal window for high-quality nanoribbon synthesis. The resulting nanostructures exhibit well-defined morphologies and sharp edges, with a thickness of 2–9 nm. This work establishes a viable route for the oriented growth of Bi4Br4 and provides an ideal platform for investigating topological edge transport, anisotropic phenomena, and the design of topological quantum devices.

Hexavalent chromium ions in industrial wastewater can bring serious pollution to the environment and a variety of harms to human health. It is necessary to develop a novel technique to determine the heavy ions. In this work, we designed a special machine to enhance the production of microfluidic paper based analysis devices (μPADs, ∼1000 μPADs/h), which could be used to determine the industrial wastewater. We optimized the diphenyl carbamide solution and the volume of liquid to be tested. Finally, we established a fitting curve of the standard Cr concentration and color intensity from the μPADs. We carried out outside experiments in an industrial park, which needed a great amount of stable and repeatable devices for the test. In the concentration range of 250-600 mg/L, 74% of the measurement results had a relative error of less than 20%; in the concentration range of 50-250 mg/L, 72% of the measurement results had a relative error of less than 20%. Our results demonstrated that the accuracy and repeatability satisfied the requirements for wastewater detection, which could be extended to industrial application.

In the present study, a topological semimetal NiTe₂-based electrochemical biosensor was designed and fabricated, leveraging the material's inherent topological surface state and conductive bulk properties. The NiTe₂ electrode was fabricated via mechanical exfoliation from a high-quality NiTe₂ single crystal. Owing to its robust layered structure and unique Dirac surface states, the topological semimetal NiTe₂ facilitates rapid electron transfer at the electrode surface, thereby enhancing the sensor's performance. The developed biosensor showed a linear response range for DNA concentrations spanning from 10^-15 to 10^-7 M, with a detection limit as low as 10^-16 M. In contrast, its detection response toward RNA was more sensitive, covering a concentration range of 10^-18 to 10^-15 M. Furthermore, this sensor was employed for the detection of synthetic SARS-CoV-2 pseudovirus at a concentration of 1000 copies/mL. The results demonstrated that the sensor could effectively differentiate between negative and positive samples, exhibiting excellent sensitivity, specificity, and stability. Consequently, the NiTe₂-based biosensor possesses significant potential for application in the clinical diagnosis of SARS-CoV-2 pathogens and other acute infectious diseases.

Interferon-gamma (IFN-γ), an essential inflammatory cytokine, is intricately associated with a variety of fatal diseases as a key early biomarker. In this work, we designed and constructed an electrochemical aptasensor based on topological insulator Bi₂Se₃ sheets. Micron-scale Bi₂Se₃ sheets were prepared by electrochemical exfoliation from single crystals to make electrodes of the aptasensors. The unique and robust Dirac surface states of Bi₂Se₃ could enhance the charge transfer efficiency of the solid-liquid interface, improving the performance of the aptasensors. The developed aptasensor exhibits a linear response to IFN-γ concentration in the range of 1-100 pg/mL with a detection limit as low as 0.6 pg/mL, enabling it to meet the clinical requirements. The performance of the aptasensors also shows excellent stability and selectivity. Furthermore, the aptasensor was applied to human serum detection and was comparable in performance to the clinical standard enzyme-linked immunosorbent assay technique. Our work indicates that the aptasensor based on Bi₂Se₃ sheets has great potential for application in the clinical detection of IFN-γ and other possible biomarkers.

Nickel (II) as one of the primary categories of heavy metals can lead to serious health problems if achieving the critical levels in the water. Thus, it is vital to propose a stable, reliable, and economical approach for detecting Ni ions. The microfluidic paper-based analytical devices (µPADs) are potential candidates for the detection of water quality parameters including pH, heavy ions, nitrite and so on. However, it suffers from a huge error caused by the environment and artificial mistakes. In this study, we proposed an improved technique route to increase the stability and reliability of microfluidic paper-based analytical devices. The main technique points include a stable light source, a matched camera, improved reliability of the devices, and effective calculated methods. Finally, we established 15 standard curves that could be used to detect nickel ions and obtained uniform colorimetric results with reliability and repeatability. With those improvements, the relative errors for the five types of real water samples from the Zhongshan industrial parks were reduced to 0.26%, 14.78%, 24.20%, 50.29% and 3.53%, respectively. These results were conducive to exploring this technique for the detection of nickel ions in wastewater from the Zhongshan industrial parks. The results demonstrated that the above technique route is promising for the detection of other heavy metal ions in industrial effluent.

Bi₂Se₃ is a typical topological material with the gapless surface state, which has many potential applications. In this work, high‐quality Bi₂Se₃ thin films are prepared by co‐evaporating bismuth and selenium on (100) silicon substrate, followed by in‐situ annealing process. The key parameters are optimized, including growth temperature and the flux ratio of Bi and Se. The crystal orientation and quality of the thin films are characterized by X‐ray diffraction, Raman spectroscopy, X‐ray photoelectron spectroscopy, and atomic force microscopy. Furthermore, Bi₂Se₃ thin films are employed as the working electrode of the biosensor, and their sensitive detection of SARS‐CoV‐2 characteristic gene fragments in the COVID‐19 epidemic is investigated. The cyclic voltammetry, square wave voltammetry, and electrochemical impedance spectroscopy demonstrate that the DNA biosensor is successfully constructed based on Bi₂Se₃ thin films. This study develops a method of the large‐scale homogeneous Bi₂Se₃ thin films growth by thermal evaporation technology, which has great application potential in electrochemical biosensors.

In this work, vertically aligned heterophase TiO₂ nanocone structures (TNCs) are successfully synthesized via designing a glycerol-assisted hydrothermal approach. Significantly, a remarkably high photodegradation rate is achieved.