Neng Li

Heteroatom-doped carbon dots: synthesis, characterization, properties, photoluminescence mechanism and biological applications

Heteroatom-doped carbon dots (CDs), due to their excellent photoluminescence (PL) properties, attracted widespread attention recently and demonstrated immense promise for diverse applications, particularly for biological applications. The objective of this feature article is to provide a comprehensive overview of the recent progress in the research and development of heteroatom-doped CDs and a detailed description of the influence of single or co-doping heteroatoms on their PL behavior. The most recent understanding and critical insights into the PL mechanism of heteroatom-doped CDs are also highlighted. Moreover, potential bio-related applications of heteroatom-doped CDs in biosensing, bioimaging, and theranostics are also reviewed. This state-of-the-art review will provide a platform for understanding the intricate details of heteroatom-doped CDs, a summary of the latest progress in the field, and related applications in biology and is expected to inspire further developments in this exciting class of materials.

Interface Bonds Determined Gas-Sensing of SnO2-SnS2 Hybrids to Ammonia at Room Temperature.

Unique gas-sensing properties of semiconducting hybrids that are mainly related to the heterogeneous interfaces have been considerably reported. However, the effect of heterogeneous interfaces on the gas-sensing properties is still unclear, which hinders the development of semiconducting hybrids in gas-sensing applications. In this work, SnO2-SnS2 hybrids were synthesized by the oxidation of SnS2 at 300 °C with different times and exhibited high response to NH3 at room temperature. With the increasing oxidation time, the relative concentration of interfacial Sn bonds, O-Sn-S, among the total Sn species of the SnO2-SnS2 hybrids increased first and then decreased. Interestingly, it can be found that the response of SnO2-SnS2 hybrids to NH3 at room temperature exhibited a strong dependence on the interfacial bonds. With more chemical bonds at the interface, the lower interface state density and the higher charge density of SnO2 led to more chemisorbed oxygen, resulting in a high response to NH3. Our results revealed the real roles of the heterogeneous interface in gas-sensing properties of hybrids and the importance of the interfacial bonds, which offers guidance for the material design to develop hybrid-based sensors.

Synthesis, mechanistic investigation, and application of photoluminescent sulfur and nitrogen co-doped carbon dots

Heteroatom doped carbon dots (CDs) with consummate photoluminescence quantum yield (PLQY) are of great interest in various applications such as trace element detection, biomolecule markers, and chemical sensing. However, due to the low doping efficiency of the reaction, a high precursor ratio is routinely used to obtain CDs with considerable photoluminescence quantum yield (PLQY). In this contribution, we report a single-step hydrothermal method having the highest doping efficiency to synthesize sulfur and nitrogen co-doped semi-crystalline carbon dots (S,N-CDs) with superior quantum yield (QY). Here, the unprecedented doping efficiency of the reaction enables an order of magnitude reduction in the starting precursor ratio, in comparison with previous reports. Moreover, for the first time, complementary theoretical and comprehensive spectroscopic techniques were employed to derive deep insight into the photoluminescence mechanism and the shifting of specific energy levels in doped CDs was identified as the reason behind the enhanced photoluminescence of doped CDs. The PLQY and luminescent characteristics of the S,N-CDs can be tuned by controlling the precursor molar ratio, the extent of surface oxidation, and chemical status of S in the CDs. While most methods report high PLQY for amorphous CDs, our technique produces semi-crystalline CDs with more than 55% QY. Another unique attribute of the S,N-CDs is the high monodispersity and defined surface chemistry and the resultant highly robust excitation-independent luminescence that is stable over a broad range of pH values and in an extremely reactive environment. The detailed structural and chemical investigations using spectroscopic and microscopic techniques combining molecular simulation revealed that the superior PLQY and luminescence of S,N-CDs are due to the heteroatom directed, oxidized carbon-based surface passivation. The remarkable, robust fluorescence properties of S,N-CDs were applied for the ultra-trace detection of Hg2+ with a detection limit of 100 pM. The novel insights into the photoluminescence of doped CDs, the robust luminescence, and enhanced doping reaction efficiency reported here are envisaged to drive transformative changes in the design of CDs with peerless properties for futuristic applications.

Directional Heat Dissipation across the Interface in Anatase–Rutile Nanocomposites

Understanding the structures and properties of interfaces in (nano-)composites helps to reveal their important influence on reactivity and overall performance. TiO2 is a technologically important material, and anatase/rutile TiO2 composites have been shown to display enhanced photocatalytic performance over pure anatase or rutile TiO2. This has been attributed to a synergistic effect between the two phases, but the origin of this effect as well as the structure of the interface has not been established. Using Raman spectroscopy, here we provide evidence of distinct differences in the thermal properties of the anatase and rutile moieties in the composite, with anatase becoming effectively much warmer than the rutile phase under laser irradiation. With the help of first-principles calculations, we analyze the atomic structure and unique electronic properties of the composite and infer possible reasons for the directional heat dissipation across the interface.

Preserving the half-metallicity at the surfaces of rocksalt CaN and SrN and the interfaces of CaN/InN and SrN/GaP: a density functional study.

Recent theoretical studies indicate that metastable rocksalt CaN, SrN, and BaN exhibit half-metallic ferromagnetism (Volnianska and Boguslawski 2007 Phys. Rev. B 75 224418; Gao et al 2008 Phys. Lett. A 372 1512), and further experiments confirm the existence of self-assembled metastable CaN nanostructures (Liu et al 2008 Surf. Sci. 602 1844). We here use the first-principles method based on density functional theory to investigate the structural, electronic, and magnetic properties of the (111) surfaces of CaN and SrN and the interfaces of CaN/InN(111) and SrN/GaP(111). The surface stability from the calculated surface energy indicates that the N-terminated (111) surface is more stable than the Ca (Sr)-terminated (111) surface in the N-rich environment. For CaN and SrN, both anion- and cation-terminated (111) surfaces preserve the half-metallic characteristics of the bulk. Interfacial studies indicate that the half-metallicity of bulk CaN is retained in two of the four possible configurations of the CaN/InN(111) interface, while for the interface of SrN/GaP(111) only one interfacial configuration shows half-metallicity. Furthermore, we assess the interfacial adhesive strength for all the possible different configurations of the interfaces studied here by calculating the interface adhesion energies.

Promising prospects for 2D d2–d4 M3C2 transition metal carbides (MXenes) in N2 capture and conversion into ammonia

Density functional theory investigations of M3C2 transition metal carbides from the d2, d3, and d4 series suggest promising N2 capture behaviour, displaying spontaneous chemisorption energies that are larger than those for the capture of CO2 and H2O in d3 and d4 MXenes. The chemisorbed N2 becomes activated, promoting its catalytic conversion into NH3. The first proton–electron transfer is found to be the rate-determining step for the whole process, with an activation barrier of only 0.64 eV vs. SHE for V3C2.

Photocatalytic fixation of nitrogen to ammonia: state-of-the-art advancements and future prospects

The burgeoning development of ammonia (NH3) synthesis technology addresses the urgency of food intake required to sustain the population growth of the last 100 years. To date, NH3 has mostly been synthesized by the Haber–Bosch process in industry. Under the ever-increasing pressure of the fossil fuel depletion crisis and anthropogenic global climate change with continuous CO2 emission in the 21st century, research targeting the synthesis of NH3 under mild conditions in a sustainable and environment friendly manner is vigorous and thriving. Therefore, the focus of this review is the state-of-the-art engineering of efficient photocatalysts for dinitrogen (N2) fixation toward NH3 synthesis. Strenuous efforts have been devoted to modifying the intrinsic properties of semiconductors (i.e. poor electron transport, rapid electron–hole recombination and sluggish reaction kinetics), including nanoarchitecture design, crystal facet engineering, doping and heterostructuring. Herein, this review provides insights into the most recent advancements in understanding the charge carrier kinetics of photocatalysts with respect to charge transfer, migration and separation, which are of fundamental significance to photocatalytic N2 fixation. Subsequently, the challenges, outlooks and future prospects at the forefront of this research platform are presented. As such, it is anticipated that this review will shed new light on photocatalytic N2 fixation and NH3 synthesis and will also provide a blueprint for further investigations and momentous breakthroughs in next-generation catalyst design.

Highly fluorescent Zn-doped carbon dots as Fenton reaction-based bio-sensors: an integrative experimental–theoretical consideration

Heteroatom doped carbon dots (CDs), with high photoluminescence quantum yield (PLQY), are of keen interest in various applications such as chemical sensors, bio-imaging, electronics, and photovoltaics. Zinc, an important element assisting the electron-transfer process and an essential trace element for cells, is a promising metal dopant for CDs, which could potentially lead to multifunctional CDs. In this contribution, we report a single-step, high efficiency, hydrothermal method to synthesize Zn-doped carbon dots (Zn-CDs) with a superior PLQY. The PLQY and luminescence characteristic of Zn-CDs can be tuned by controlling the precursor ratio, and the surface oxidation in the CDs. Though a few studies have reported metal doped CDs with good PLQY, the as prepared Zn-Cds in the present method exhibited a PLQY up to 32.3%. To the best of our knowledge, there is no report regarding the facile preparation of single metal-doped CDs with a QY more than 30%. Another unique attribute of the Zn-CDs is the high monodispersity and the resultant highly robust excitation-independent luminescence that is stable over a broad range of pH values. Spectroscopic investigations indicated that the superior PLQY and luminescence of Zn-CDs are due to the heteroatom directed, oxidized carbon-based surface passivation. Furthermore, we developed a novel and sensitive biosensor for the detection of hydrogen peroxide and glucose leveraging the robust fluorescence properties of Zn-CDs. Under optimal conditions, Zn-CDs demonstrated high sensitivity and response to hydrogen peroxide and glucose over a wide range of concentrations, with a linear range of 10-80 μM and 5-100 μM, respectively, indicating their great potential as a fluorescent probe for chemical sensing.

2D MoS2/polyaniline heterostructures with enlarged interlayer spacing for superior lithium and sodium storage

The exploitation of high-capacity and long-life MoS2-based materials is highly important for developing lithium ion batteries (LIBs) and sodium ion batteries (SIBs). Herein, we demonstrate the confined synthesis of 2D MoS2/polyaniline (MoS2/PANI) nanosheet heterostructures with well-defined interfaces, in which the interlayer distance of MoS2 is greatly enlarged from 0.62 nm to 1.08 nm. The introduction of such a big interlayer distance for efficient Li+/Na+ storage has never been demonstrated before. The unique MoS2/PANI nanosheets can address well the key challenges of traditional MoS2 anode materials related to low conductivity particularly in the vertical direction, easy restacking/aggregation, large volumetric change and sluggish Li+/Na+ diffusion kinetics in the interlamination. Consequently, they deliver a high reversible capacity, superior rate capability and long cycle life for both LIBs and SIBs. A state-of-the-art ab initio molecular dynamics (AIMD) simulation also reveals that MoS2/PANI nanosheets with enlarged interlayer spacing possess a remarkably improved Li+/Na+ diffusion mobility compared to pristine MoS2 nanosheets. The present material design concept opens new directions for finding efficient LIBs/SIBs anodes with high capacity, rate capability and stability.

Sequential coupling transport for the dark current of quantum dots-in-well infrared photodetectors

The dark current characteristics and temperature dependence for quantum dot infrared photodetectors have been investigated by comparing the dark current activation energies between two samples with identical structure of the dots-in-well in nanoscale but different microscale n-i-n environments. A sequential coupling transport mechanism for the dark current between the nanoscale and the microscale processes is proposed. The dark current is determined by the additive mode of two activation energies: Ea,micro from the built-in potential in the microscale and Ea,nano related to the thermally assisted tunneling in nanoscale. The activation energies Ea,micro and Ea,nano decrease exponentially and linearly with increasing applied electric field, respectively.