Structure of Amorphous Selenium: Small Ring, Big Controversy.

Selenium (Se) discovered in 1817 belongs to the family of chalcogens. Surprisingly, despite the long history of over two centuries and the chemical simplicity of Se, the structure of amorphous Se (a-Se) remains controversial to date regarding the dominance of chains versus rings. Here, we find that vapor-deposited a-Se is composed of disordered rings rather than chains in melt-quenched a-Se. We further reveal that the main origin of this controversy is the facile transition of rings to chains arising from the inherent instability of rings. This transition can be inadvertently triggered by certain characterization techniques themselves containing above-bandgap illumination (above 2.1 eV) or heating (above 50 °C). We finally build a roadmap for obtaining accurate Raman spectra by using above-bandgap excitation lasers with low photon flux (below 1017 phs m-2 s-1) and below-bandgap excitation lasers measured at low temperatures (below -40 °C) to minimize the photoexcitation- and heat-induced ring-to-chain transitions.

Coupled photothermal vortices for capture, sorting, and transportation of particles.

Optofluidic techniques have evolved as a prospering strategy for microparticle manipulation via fluid. Unfortunately, there is still a lack of manipulation with simple preparation, easy operation, and multifunctional integration. In this Letter, we present an optofluidic device based on a graphite oxide (GO)-coated dual-fiber structure for multifunctional particle manipulation. By changing the optical power and the relative distance of the fibers, the system can excite thermal fluidic vortices with three inter-coupled states, namely uncoupled, partially coupled and completely coupled states, and therefore can realize capture, sorting, and transportation of the target particles. We conduct a numerical analysis of the whole system, and the results are consistent with the experimental phenomena. This versatile device can be utilized to manipulate target particles in complex microscopic material populations with the advantages of flexible operation, user-friendly control, and low cost.

Size Dependence of Gold Nanorods for Efficient and Rapid Photothermal Therapy.

In recent years, gold nanomaterials have become a hot topic in photothermal tumor therapy due to their unique surface plasmon resonance characteristics. The effectiveness of photothermal therapy is highly dependent on the shape and size of gold nanoparticles. In this work, we investigate the photothermal therapeutic effects of four different sizes of gold nanorods (GNRs). The results show that the uptake of short GNRs with aspect ratios 3.3-3.5 by cells is higher than that of GNRs with aspect ratios 4-5.5. Using a laser with single pulse energy as low as 28 pJ laser for 20 s can induce the death of liver cancer cells co-cultured with short GNRs. Long GNRs required twice the energy to achieve the same therapeutic effect. The dual-temperature model is used to simulate the photothermal response of intracellular clusters irradiated by a laser. It is found that small GNRs are easier to compact because of their morphological characteristics, and the electromagnetic coupling between GNRs is better, which increases the internal field enhancement, resulting in higher local temperature. Compared with a single GNR, GNR clusters are less dependent on polarization and wavelength, which is more conducive to the flexible selection of excitation laser sources.

Lanthanide-Like Contraction Enables the Fabrication of High-Purity Selenium Films for Efficient Indoor Photovoltaics.

The lanthanide contraction involves a reduction in atomic radius among f-block elements below the expected level. A similar contraction is observed in group-16 elements. The atomic radius of Se (117 pm) is slightly larger than that of S (104 pm) arising from the presence of d electrons, compared to the significant increase in atomic radius from O (73 pm) to S. This lanthanide-like contraction contributes to Se's robust oxidative resistance. Here we report a selective oxidation strategy utilizing Se's strong antioxidative property to remove coexisting narrow-bandgap Te impurities from Se feedstocks. This strategy selectively oxidizes volatile Te impurities into involatile TeO2 that remains in the evaporation source, while only volatile Se deposits onto the substrate during the thermal-evaporation deposition process. This enables the fabrication of high-purity Se films possessing a wide bandgap of 1.88 eV, ideally suited to the optimal bandgap for indoor photovoltaics (IPVs). The resulting Se photovoltaics exhibit an efficiency of 20.1% under 1000-lux indoor illumination, outperforming market-dominant amorphous silicon and all types of lead-free perovskite IPVs. Unencapsulated Se devices show no efficiency degradation after 20,000 hours of storage in ambient atmosphere.

Regioselective Multisite Atomic-Chlorine Passivation Enables Efficient and Stable Perovskite Solar Cells.

Passivating defects using organic halide salts, especially chlorides, is an effective method to improve power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) arising from the stronger Pb-Cl bonding than Pb-I and Pb-Br bonding. However, Cl- anions with a small radius are prone to incorporation into the perovskite lattice that distorts the lead halide octahedron, degrading the photovoltaic performance. Here, we substitute atomic-Cl-containing organic molecules for widely used ionic-Cl salts, which not only retain the efficient passivation by Cl but also prevent the incorporation of Cl into the bulk lattice, benefiting from the strong covalent bonding between Cl atoms and organic frameworks. We find that only when the distance of Cl atoms in single molecules matches well with the distance of halide ions in perovskites can such a configuration maximize the defect passivation. We thereby optimize the molecular configuration to enable multiple Cl atoms in an optimal spatial position to maximize their binding with surface defects. The resulting PSCs achieve a certified PCE of 25.02%, among the highest PCEs for PSCs, and retain 90% of their initial PCE after 500 h of continuous operation.

Microfiber-directed reversible assembly of Au nanoparticles for SERS detection of pollutants.

Surface-enhanced Raman scattering (SERS) spectroscopy has attracted tremendous interest as a highly sensitive label-free tool to detect pollutants in aqueous environments. However, the high cost and poor reusability of conventional SERS substrates restrict their further applications in rapid and reproducible pollutant detection. Here, we report a reliable optical manipulation method to achieve rapid photothermal self-assembly of Au nanoparticles (AuNPs) in water within 30 s by a tapered optical fiber, which is utilized for highly sensitive SERS substrate preparation. The results show that the SERS substrate achieves low detection limits of 10-9 mol/L with an enhancement factor (EF) of 106 for chemical pollutants solutions, including thiram, pyrene, and rhodamine 6G. The SERS enhancement effect based on assembled AuNPs was more than 20 times that based on a gold colloid solution. As a result, the smart reversible assembly of AuNPs exhibits switchable plasmonic coupling for tuning SERS activity, which is promising for the application of SERS-based sensors and environmental pollutant detection.

PVA-SM microstructure enhanced ratiometric fluorescence probe for formaldehyde detection in solution and gas.

Formaldehyde (FA) is one of the most common pollutants, which has tremendous harm to humans and environment. In this work, 4-amino-3-pentene-2-one (Fluoral-p) and SiO2 coated quantum dot (QD@SiO2) were combined to implement a new ratiometric fluorescence probe QD@SiO2-Fluoral-p for FA detection. In addition, by utilization of polyvinyl alcohol (PVA) and SiO2 microsphere (SM), a kind of PVA-SM microstructure was assembled with QD@SiO2-Fluoral-p to composite a signal enhanced sensing film. The QD@SiO2-Fluoral-p exhibited good response to 0-400 mg/L FA solution and an enhancement around 15 folds was realized after introducing PVA-SM. In both situations, the probe showed linear relationship to FA concentration (CFA), with detection limits of 14 and 0.5 mg/L, respectively. Also, the sensing film showed a good linear response to FA gas in the range of 0 to 2 ppm, with a detection limit 0.03 ppm. As a result, the PVA-SM enhanced ratiometric fluorescence probe features high sensitivity, low detection limit, good selectivity, as well as portable, which can serve as a useful tool for investigating FA in solution and gas at room temperature.

Photothermal microfluidic-assisted self-cleaning effect for a highly reusable SERS sensor.

The synergistic integration of optofluidic and surface enhanced Raman scattering (SERS) sensing is a new analytical technique that provides a number of unique characteristics for enhancing the sensing performance and simplifying the design of microsystems. Here, we propose a reusable optofluidic SERS sensor by integrating Au nanoisland substrate (AuNIS)-coated fiber into a microfluidic chip. Through both systematic experimental and theoretical analysis, the sensor enables efficient self-cleaning based on its optical-to-heat-hydrodynamic energy conversion property. Besides, the sensor exhibits the instrument detection limit down to 10-13mol/L and enhancement factor of 106 for Rhodamine 6G. Our optofluidic SERS sensor with such a photothermal microfluidic-assisted self-cleaning method has the advantages of portability, simple operation, and high cleaning efficiency, which will provide a new, to the best of our knowledge, concept and approach for cost-effective and reusable sensors.

Vortices-interaction-induced microstreaming for the pump-free separation of particles.

Microfluidic techniques have emerged as promising strategies for a wide variety of synthetic or biological sorting. Unfortunately, there is still a lack of sorting with automatic and handy operation. In contrast to passively generated vortices, the thermocapillary vortices produced by temperature gradient have the advantages of flexible manipulation, stable strength, and simple integration. In this Letter, we present a device used for the pump-free separation of particles through vortices interaction without external fluidic control systems required for the majority of existing devices. Specifically, the device induces a different flow type upon the actuation of optical power, and the flow functions, such as simultaneous pumping and sorting, agree with stimulation results very well. More importantly, our developed sorting device can achieve separations by means of tunable cutoff diameter size. Therefore, this versatile device can be utilized to sort complex samples with the advantages of portability, user-friendly control, and automation.

Generation and manipulation of oil-in-water micro-droplets by confined thermocapillary microvortices.

Optofluidic manipulation of droplets is critical in droplet-based microfluidic systems for chemistry, biology, and medicine. Here, we reported a thermocapillary microvortices-based manipulation platform for controlling oil-in-water droplets through integrating a photothermal waveguide into a microfluidic chip. The sizes and shapes of the droplets can be controlled by adjusting optical power or positions of the water-oil interface. Here, teardrop-shaped droplets, which can encapsulate and accumulate mesoscopic matters easily, were generated when the water-oil interface and the channel boundaries approached the photothermal waveguide center simultaneously. The results showed that the thermocapillary microvortices have good controllability of droplet positions, droplet volumes, and encapsulated-particle distribution and thus it will be a powerful droplet manipulation strategy for microreactors and microcapsules.

Strain-engineering the in-plane electrical anisotropy of GeSe monolayers.

As a representative in-plane anisotropic two-dimensional (2D) material, germanium monoselenide (GeSe) has attracted considerable attention recently due to its various in-plane anisotropic material properties originating from the low symmetry of a puckered honeycomb structure. Although there have been plenty of reports on the in-plane anisotropic vibrational, electrical and optical properties of GeSe, the strain effect on those appealing anisotropies is still under exploration. Here we report a systematic first-principles computational investigation of strain-engineering of the anisotropic electronic properties of GeSe monolayers. We found that the anisotropic ratio of the effective mass and mobility of charge carriers (electrons and holes) of GeSe along two principle axes can be controlled by using simple strain conditions. Notably, the preferred conducting direction of GeSe can be even rotated by 90° under an appropriate uniaxial strain (>5%). Such effective strain modulation of the electronic anisotropy of GeSe monolayers provides them abundant opportunities for future mechanical-electronic devices.

Investigation of Oxygen Passivation for High-Performance All-Inorganic Perovskite Solar Cells.

Defect passivation using oxygen has been identified as an efficient and convenient approach to suppress nonradiative recombination and improve the photovoltaic performance of hybrid organic-inorganic halide perovskites (HHPs). However, oxygen can seriously undermine the chemical stability of HHPs due to the reaction of superoxide with protonated organic cations such as CH3NH3+ and [(NH2)2CH]+, thus hindering the deep understanding of how oxygen affects their defect properties. Here we substitute free-proton inorganic Cs+ for organic moiety to avoid the negative effect of oxygen and then systematically investigate the oxygen passivation mechanism in all-inorganic halide perovskites (IHPs) from theory to experiment. We find that, in contrast to conventional oxygen molecule passivation just through physisorption on the surface of perovskites, the oxygen atom can provide a better passivation effect due to its stronger interaction with perovskites. The key point to achieve O-passivated perovskites rather than O2 is the dry-air processing condition, which can dissociate the O2 into O during the annealing process. O-passivated IHP solar cells exhibit enhanced power conversion efficiency (PCE) and better air stability than O2-passivated cells. These results not only provide deep insights into the passivation effect of oxygen on perovskites but also demonstrate the great potential of IHPs for high photovoltaic performance with simplified ambient processing.

Miniaturized optical fiber tweezers for cell separation by optical force.

In advanced biomedicine and microfluidics, there is a strong desire to sort and manipulate various cells and bacteria based on miniaturized microfluidic chips. Here, by integrating fiber tweezers into a T-type microfluidic channel, we report an optofluidic chip to selectively trap Escherichia coli in human blood solution based on different sizes and shapes. Furthermore, we simulate the trapping and pushing regions of other cells and bacteria, including rod-shaped bacteria, sphere-shaped bacteria, and cancer cells based on finite-difference analysis. With the advantages of controllability, low optical power, and compact construction, the strategy may be possibly applied in the fields of optical separation, cell transportation, and water quality analysis.

Air-Stable In-Plane Anisotropic GeSe2 for Highly Polarization-Sensitive Photodetection in Short Wave Region.

In-plane anisotropic layered materials such as black phosphorus (BP) have emerged as an important class of two-dimensional (2D) materials that bring a new dimension to the properties of 2D materials, hence providing a wide range of opportunities for developing conceptually new device applications. However, all of recently reported anisotropic 2D materials are relatively narrow-bandgap semiconductors (<2 eV), and there has been no report about this type of materials with wide bandgap, restricting the relevant applications such as polarization-sensitive photodetection in short wave region. Here we present a new member of the family, germanium diselenide (GeSe2) with a wide bandgap of 2.74 eV, and systematically investigate the in-plane anisotropic structural, vibrational, electrical, and optical properties from theory to experiment. Photodetectors based on GeSe2 exhibit a highly polarization-sensitive photoresponse in short wave region due to the optical absorption anisotropy induced by in-plane anisotropy in crystal structure. Furthermore, exfoliated GeSe2 flakes show an outstanding stability in ambient air which originates from the high activation energy of oxygen chemisorption on GeSe2 (2.12 eV) through our theoretical calculations, about three times higher than that of BP (0.71 eV). Such unique in-plane anisotropy and wide bandgap, together with high air stability, make GeSe2 a promising candidate for future 2D optoelectronic applications in short wave region.

High throughput trapping and arrangement of biological cells using self-assembled optical tweezer.

Lately, a fiber-based optical tweezer that traps and arranges the micro/nano-particles is crucial in practical applications, because such a device can trap the biological samples and drive them to the designated position in a microfluidic system or vessel without harming them. Here, we report a new type of fiber optical tweezer, which can trap and arrange erythrocytes. It is prepared by coating graphene on the cross section of a microfiber. Our results demonstrate that thermal-gradient-induced natural convection flow and thermophoresis can trap the erythrocytes under low incident power, and the optical scattering force can arrange them precisely under higher incident power. The proposed optical tweezer has high flexibility, easy fabrication, and high integration with lab-on-a-chip, and shows considerable potential for application in various fields, such as biophysics, biochemistry, and life sciences.

Metal exsolution from perovskite-based anodes in solid oxide fuel cells.

Solid oxide fuel cells (SOFCs) are highly efficient and environmentally friendly devices for converting fuel into electrical energy. In this regard, metal nanoparticles (NPs) loaded onto the anode oxide play a crucial role due to their exceptional catalytic activity. NPs synthesized through exsolution exhibit excellent dispersion and stability, garnering significant attention for comprehending the exsolution process mechanism and consequently improving synthesis effectiveness. This review presents recent advancements in the exsolution process, focusing on the influence of oxygen vacancies, A-site defects, lattice strain, and phase transformation on the variation of the octahedral crystal field in perovskites. Moreover, we offer insights into future research directions to further enhance our understanding of the mechanism and achieve significant exsolution of NPs on perovskites.

Unusual defect properties of the one-dimensional photovoltaic semiconductor selenium.

Defect tolerance is crucial in photovoltaic absorbers. Here we report that trigonal selenium (t-Se) exhibits a perovskite-like antibonding valence band maximum arising from Se p-p coupling. This results in the shallow nature of dominant Se vacancy defects. We further reveal the unique defect self-healing characteristic of Se intrinsic point defects.

Density Functional Theory Provides Insights into ß-SnSe Monolayers as a Highly Sensitive and Recoverable Ozone Sensing Material.

This study explores the potential of ß-SnSe monolayers as a promising material for ozone (O3) sensing using density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) method. The adsorption characteristics of O3 molecules on the ß-SnSe monolayer surface were thoroughly investigated, including adsorption energy, band structure, density of states (DOSs), differential charge density, and Bader charge analysis. Post-adsorption, hybridization energy levels were introduced into the system, leading to a reduced band gap and increased electrical conductivity. A robust charge exchange between O3 and the ß-SnSe monolayer was observed, indicative of chemisorption. Recovery time calculations also revealed that the ß-SnSe monolayer could be reused after O3 adsorption. The sensitivity of the ß-SnSe monolayer to O3 was quantitatively evaluated through current-voltage characteristic simulations, revealing an extraordinary sensitivity of 1817.57% at a bias voltage of 1.2 V. This sensitivity surpasses that of other two-dimensional materials such as graphene oxide. This comprehensive investigation demonstrates the exceptional potential of ß-SnSe monolayers as a highly sensitive, recoverable, and environmentally friendly O3 sensing material.

First-Principles Insights into Highly Sensitive and Reusable MoS2 Monolayers for Heavy Metal Detection.

This study explores the potential of MoS2 monolayers as heavy metal sensors for As, Cd, Hg, and Pb using density functional theory (DFT) and Non-Equilibrium Green's Function (NEGF) simulations. Our findings reveal that As and Pb adsorption significantly alters the surface structure and electronic properties of MoS2, introducing impurity levels and reducing the band gap. Conversely, Cd and Hg exhibit weaker interactions with the MoS2 surface. The MoS2 monolayer sensors demonstrate exceptional sensitivity for all four target heavy metals, with values reaching 126,452.28% for As, 1862.67% for Cd, 427.71% for Hg, and 83,438.90% for Pb. Additionally, the sensors demonstrate selectivity for As and Pb through distinct response peaks at specific bias voltages. As and Pb adsorption also induces magnetism in the MoS2 system, potentially enabling magnetic sensing applications. The MoS2 monolayer's moderate adsorption energy facilitates rapid sensor recovery at room temperature for As, Hg, and Cd. Notably, Pb recovery time can be significantly reduced at elevated temperatures, highlighting the reusability of the sensor. These results underscore the potential of MoS2 monolayers as highly sensitive, selective, and regenerable sensors for real-time heavy metal detection.

Sustainable Recycling of Selenium-Based Optoelectronic Devices.

Selenium (Se), the world's oldest optoelectronic material, has been widely applied in various optoelectronic devices such as commercial X-ray flat-panel detectors and photovoltaics. However, despite the rare and widely-dispersed nature of Se element, a sustainable recycling of Se and other valuable materials from spent Se-based devices has not been developed so far. Here a sustainable strategy is reported that makes use of the significantly higher vapor pressure of volatile Se compared to other functional layers to recycle all of them from end-of-life Se-based devices through a closed-space evaporation process, utilizing Se photovoltaic devices as a case study. This strategy results in high recycling yields of ≈ 98% for Se and 100% for other functional materials including valuable gold electrodes and glass/FTO/TiO2 substrates. The refabricated photovoltaic devices based on these recycled materials achieve an efficiency of 12.33% under 1000-lux indoor illumination, comparable to devices fabricated using commercially sourced materials and surpassing the current indoor photovoltaic industry standard of amorphous silicon cells.