High Light Absorption Research Articles

A dual-functional hydrogel for efficient water purification: Integrating solar interfacial evaporation with fenton reaction

Solar interfacial evaporation is a potential technology to produce clean water due to its simplicity and being driven by renewable clean energy, but it still requires further development to break through the bottleneck of removing volatile organic compounds (VOCs), especially in wastewater treatment. Herein, we proposed a dual-functional hydrogel evaporator that coupled solar interfacial evaporation with Fenton reaction to simultaneously remove VOCs and non-volatile pollutants from water with low energy consumption and high efficiency. The evaporator was composed with β-FeOOH and polydopamine (PDA) on an electrospun nanofibrous hydrogel. Arising from the PDA with excellent photothermal properties, the evaporator revealed a high light absorption characteristics (∼90%) and photothermal efficiency (83.4%), which ensured a favorable evaporation rate of 1.70 kg m-2 h-1 under one solar irradiation. More importantly, benefited from the coupled Fenton reaction, the VOCs removal rate of β-FeOOH@PDA/polyvinyl alcohol nanofibrous hydrogel (β-FeOOH@PPNH) reached 95.8%, which was 6.5 times than that of sole solar interfacial evaporation (14.8%). In addition, the evaporator exhibited an outstanding non-volatile pollutant removal capability and stable removal performance for organic pollutants over a long period of operation. The prepared β-FeOOH@PPNH evaporator provides a promising idea for simultaneous removal of non-volatile pollutants and volatile pollutants performance in long-term water purification.

Comprehensive device simulation of barium silicide solar cells with diverse ETMs and HTMs

The photovoltaic performances of n-i-p or inverted p-i-n BaSi2 homojunction solar cells are limited by the parasitic high light absorption of n-BaSi2 or p-BaSi2 window layer. In this work, diverse electron transport materials (ETMs) and hole transport materials (HTMs) are employed instead of BaSi2 as window layers to promote the performances of BaSi2 solar cells, and the effects of different ETMs and HTMs were simulated and comprehensively studied. For n-ETM/i-BaSi2/p-BaSi2 solar cells, the solar cell with ZnO exhibits the highest efficiency of 28.21%. As increasing the thicknesses of ETM layer from 5[Formula: see text]nm to 100[Formula: see text]nm, the efficiency for n-BaSi2/i-BaSi2/p-BaSi2 solar cell decreases seriously from 24.52% to 15.99%, while the solar cells with wide band gap ETMs including SnO2, TiO2 and ZnO show neglectable change in performances. The performances n-ETM/i-BaSi2/p-BaSi2 solar cells are affected by donor like defects more than acceptor like defects in i-BaSi2. For p-HTM/i-BaSi2/n-BaSi2 solar cells, employing Cu2O, CuI, CuSCN or NiO as HTM layer produces evident valence band discrepancies at HTM/i-BaSi2 interfaces. Heavy doping in HTM layer is necessary desired to overcome the energy band barriers. As increasing the thicknesses of HTM layer from 5 nm to 100 nm, the efficiency for p-BaSi2/i-BaSi2/n-BaSi2 solar cell also decreases from 24.52% to 15.99%, while the solar cells with wide band gap HTMs including CuI, CuSCN and NiO show neglectable change, and the efficiency for p-Cu2O/i-BaSi2/n-BaSi2 solar cells decreases slightly from 25.86% to 24.88%. The performances of p-HTM/i-BaSi2/n-BaSi2 solar cells are affected by acceptor like defects more than donor like defects in i-BaSi2.

Enhancements on oxygen vacancy and light absorption of alkaline-etched BiVO4 for photoelectrochemical water oxidation

Designing the favorable morphology with large surface area and abundant oxygen vacancies for BiVO4 can enhance its electron diffusion lengths and improve its photocatalytic ability toward photoelectrochemical water oxidation. The ex-situ alkaline etching process is reported to be facile for enhancing the light absorption and oxygen vacancy of BiVO4, but the systematic study of alkaline etching on the photocatalytic ability of BiVO4 is rare. In this study, alkaline-etched BiVO4 is firstly fabricated using a hydrothermal process with different durations as the photocatalyst of water oxidation. The BiVO4 etched by alkaline for longer durations presents rougher surface, more oxygen vacancies and higher light absorption. The larger carrier densities and the smaller charge-transfer resistances are also achieved for the alkaline etched BiVO4. The largest photocurrent density of 2.38 mA/cm2 is achieved at 1.23 V versus reversible hydrogen electrode in the Na2SO4 electrolyte for BiVO4 etched by alkaline for 45 min, while the pristine BiVO4 electrode only shows a photocurrent density of 0.59 mA/cm2. The photocurrent retention of 81% is also obtained for the alkaline etched BiVO4 electrode after continuous illumination for 6000 s. The results imply the feasibility of applying the alkaline etching process on refining the photoelectrochemical catalytic ability of BiVO4.

Computational prediction of two-dimensional o-Al2N2 under applied strain for boosting the photocatalytic hydrogen evolution reaction performance

Photocatalytic water splitting for clean hydrogen fuel production provides a promising approach to solve the energy and environmental issues. Recently, two-dimensional (2D) photocatalysts have attracted growing interest owing to their short carrier diffusion path, abundant active sites and large surface area. This study explores the photocatalytic performance of 2D orthorhombic dialuminum dinitride (o-Al2N2) using density functional theory. The computational results show that the o-Al2N2 monolayer has a semiconductor character with indirect and moderate bandgap. Moreover, this system exhibits high light absorption in the visible region, referring to its high capacity for harvesting sunlight. Meanwhile, under neutral pH, the band edge positions are suitable to straddle water redox potentials and the hydrogen evolution reaction is energetically favorable to allow hydrogen production on the surface of 2D o-Al2N2 compound. More importantly, the photocatalytic activity of o-Al2N2 monolayer is significantly improved under slight biaxial compressive strain. Therefore, our findings suggest that the o-Al2N2 nanomaterial is a highly efficient 2D photocatalyst for hydrogen production via water splitting under neutral pH.

In Situ Encapsulation of Graphene Quantum Dots in Highly Stable Porphyrin Metal-Organic Frameworks for Efficient Photocatalytic CO2 Reduction.

Photocatalytic CO2 reduction to valuable hydrocarbon solar fuel is of great significance but still challenging. Strong CO2 enrichment ability and easily adjustable structures make metal-organic frameworks (MOFs) potential photocatalysts for CO2 conversion. Even though pure MOFs have the potential for photoreduction of CO2, the efficiency is still quite low due to rapid photogenerated electron-hole recombination and other drawbacks. In this work, graphene quantum dots (GQDs) were in situ encapsulated into highly stable MOFs via a solvothermal method for this challenging task. The GQDs@PCN-222 with encapsulated GQDs showed similar Powder X-ray Diffraction (PXRD) patterns to PCN-222, indicating the retained structure. The porous structure was also retained with a Brunauer-Emmett-Teller (BET) surface area of 2066 m2/g. After incorporation of GQDs, the shape of GQDs@PCN-222 particles remained, as revealed by the scanning electron microscope (SEM). As most of the GQDs were covered by thick PCN-222, it was hard to observe those GQDs using a transmission electron microscope (TEM) and a high-resolution transmission electron microscope (HRTEM) directly, the treatment of digested GQDs@PCN-222 particles by immersion in a 1 mM aqueous KOH solution can make the incorporated GQDs visible in TEM and HRTEM. The linker, deep purple porphyrins, make MOFs a highly visible light harvester up to 800 nm. The introduction of GQDs inside PCN-222 can effectively promote the spatial separation of the photogenerated electron-hole pairs during the photocatalytic process, which was proved by the transient photocurrent plot and photoluminescence emission spectra. Compared with pure PCN-222, the obtained GQDs@PCN-222 displayed dramatically enhanced CO production derived from CO2 photoreduction with 147.8 μmol/g/h in a 10 h period under visible light irradiation with triethanolamine (TEOA) as a sacrificial agent. This study demonstrated that the combination of GQDs and high light absorption MOFs provides a new platform for photocatalytic CO2 reduction.

Excitation-Dependent High-Lying Excitonic Exchange via Interlayer Energy Transfer from Lower-to-Higher Bandgap 2D Material.

High light absorption (∼15%) and strong photoluminescence (PL) emission in monolayer (1L) transition metal dichalcogenides (TMDs) make them ideal candidates for optoelectronic device applications. Competing interlayer charge transfer (CT) and energy transfer (ET) processes control the photocarrier relaxation pathways in TMD heterostructures (HSs). In TMDs, long-distance ET can survive up to several tens of nm, unlike the CT process. Our experiment shows that an efficient ET occurs from the 1Ls WSe2-to-MoS2 with an interlayer hexagonal boron nitride (hBN), due to the resonant overlapping of the high-lying excitonic states between the two TMDs, resulting in enhanced HS MoS2 PL emission. This type of unconventional ET from the lower-to-higher optical bandgap material is not typical in the TMD HSs. With increasing temperature, the ET process becomes weaker due to the increased electron-phonon scattering, destroying the enhanced MoS2 emission. Our work provides new insight into the long-distance ET process and its effect on the photocarrier relaxation pathways.

Highly efficient carbonization of nanocellulose to biocarbon aerogels with ultrahigh light absorption efficiency and evaporation rate as bifunctional solar/electric driven steam generator for water purification

Solar photo-thermal materials can be used to prepare fresh water in a green, economical and environmentally friendly way and have great potential for applications in solving the world's freshwater crisis. Here, by carbonizing nanocellulose aerogels, the obtained cellulose nanofiber crosslinked vinyltrimethoxysilane carbon aerogels (CVCAs) have a reticulated three-dimensional structure, high electrical conductivity (0.79 S/m) and low thermal conductivity(0.32 W/mK). Polypyrrole (PPy) coated on CVCAs (CVCAs-P) imparted its hydrophilicity and excellent electrical conductivity (1.95 S/m). CVCAs-P showed excellent performance in the field of interfacial evaporation due to its three-dimensional reticular structure, and low thermal conductivity, which gave a high light absorption (94%–97%) and an apparent water evaporation performance of 1.54 kg m−2 h−1 under 1 sun. Moreover, CVCAs-P with good electrical conductivity can be used for electro-thermal conversion using an external power supply, allowing for electrically driven water evaporation at night. This work solves the problem of conventional interfacial evaporation materials being overly dependent on sunlight.

Emission and optical characteristics of brown carbon in size-segregated particles from three types of Chinese ships

Brown carbon (BrC) is one of the important light absorption substances that have high light absorption ability under short wavelength light. However, limit studies have focused on the BrC emission from ships. In this study, size-segregated particulate matters (PM) were collected from three different types of ships, light absorption characteristics and size distribution of methanol-soluble BrC and water-soluble BrC in PM from ship exhausts were investigated. Results showed that four-stroke low-power diesel fishing boat (4-LDF) had the highest mass concentrations of methanol-soluble organic carbon (MSOC) and water-soluble organic carbon (WSOC), followed by 2-stroke high-power heavy-fuel-oil vessel (2-HHV), and four-stroke high-power marine-diesel vessel (4-HMV). While 2-HHV had obviously higher light absorption coefficients of methanol-soluble BrC (Abs365,M) and water-soluble BrC (Abs365,W) in unit weight of PM than the other two types of ships. The tested ships presented comparable or higher absorption efficiency of BrC in water extracts (MAE365,W) compared with other BrC emission sources. Majority of BrC was concentrated in fine particles, and the particle size distributions of both Abs365,M and Abs365,W showed bimodal patterns, peaking at 0.43–0.65 µm and 4.7–5.8 µm, respectively. However, different particle size distributions were found for MAE365,M between diesel and heavy fuel oil ships. Besides, different wavelength dependence in particles with different size were also detected. Ship exhaust could be confirmed as a non-ignorable BrC emission source, and complex influencing factor could affect the light absorption characteristics of ship emissions. Particle size should also be considered when light absorption ability of BrC was evaluated.

Cuprous oxide core-shell heterostructure facilely encapsulated by cadmium metal organic frameworks for enhanced photocatalytic hydrogen generation

Light absorption and separation of photo-generated electrons and holes are important to photocatalytic hydrogen evolution of a photocatalyst. Herein, a special metal organic framework (MOFs) was fabricated by encapsulating of 2-methylimidazole and cadmium nitrate on the silica coated cuprous oxide (Cu2O/SiO2/CdIF). The structure, morphology, composition, and valence state of the obtained materials were analyzed. It was demonstrated that the core-shell Cu2O/SiO2/CdIF heterostructure was achieved. Also, the photocatalytic activity and stability of H2 generation over the Cu2O/SiO2/CdIF nanocomposite were investigated. Interestingly, the Cu2O/SiO2/CdIF nanocomposite exhibited high activity which was hardly changed after five recycles. By means of Raman spectra, solid diffuse reflection UV–vis spectra, photoelectric response, and interfacial resistance, it was shown that higher light absorption, efficient charge separation and strong synergistic effect among the components in the heterostructure were responsible to the enhanced performance in the Cu2O/SiO2/CdIF nanocomposite.

Visible light-assisted photocatalytic degradation of methylene blue by 1D/2D CdxOy clusters based on metal-organic frameworks without H2O2

Metal-organic frameworks (MOFs) photocatalysts, which contain metal oxo-clusters and organic chromophore linkers, have high visible light absorption utilization, large porosity, specific surface area, effective photogenerated electron and hole separation capabilities. Herein, two types of CdxOy cluster complexes, namely 1D CdxOy cluster Cd-mBDC (mBDC: 1,3-benzenedicarboxylic acid) and 2D CdxOy cluster Cd-2-NH2-mBDC (2-NH2-mBDC: 2-aminoisophthalic acid) were successfully synthesized, and their photocatalytic activity were evaluated by photocatalytic degradation of methylene blue (MB) under visible light without H2O2. The 2D CdxOy cluster of Cd-2-NH2-mBDC can be simplified as a two-dimensional mesh (3.82)(32.82) topology by considering the Cd ions as connected nodes. In addition, conditional experiments showed that the clusters photocatalyst could achieve the best catalytic efficiency under initial dye concentration of 10 mg/L and pH value of 7. Compared with Cd-mBDC, the introduction of amino auxochrome in Cd-2-NH2-mBDC broadened the absorption range and improved the visible light utilization, and the rigid 2D cluster structure facilitated the transfer of photogenerated electrons and enhanced stability. For Cd-2-NH2-mBDC, the photodegradation efficiency of MB could reach 88.44% within 180 min (0.121 min−1) and basically unchanged during five-cycles degradation process. Therefore, Cd-2-NH2-mBDC displays excellent photocatalytic activity and stability for degradation of MB under visible light. Furthermore, the possible mechanism of photocatalytic degradation properties has been also deduced.

86‐5: Late‐News Paper: Green‐ and Red‐Emitting Perovskite Nanocrystal Inks for Color Conversion Display Technologies

Perovskite nanocrystals (PeNCs) are promising color converters due to their high blue light absorption and narrow‐band emission. This paper reports on the bright and stable PeNCs inks for patterned green/red color conversion pixels. Inks designed to stabilize PeNCs have a shelf life of more than one month, and UV‐cured PeNCs films exhibit high color conversion efficiency and practical durability to blue light and heat. Our PeNCs inks are capable of sufficient color conversion even at high pixel resolutions and will be a viable solution for full‐color OLED and micro‐LED displays.

Optimization of Efficient Perovskite-Si Hybrid Tandem Solar Cells

Perovskite-silicon tandem solar cells have attracted much attention to photovoltaic community because of their high efficiency via easy fabrication methods and availability of precursor material abundant in nature. The properties of both perovskite and silicon meet ideal solar cell standards such as high light absorption potential, long carrier diffusion length and fast charge separation process. Semi-transparent solar cell with widely tunable band gap of perovskite material is compatible with silicon solar cell for tandem structures. A perovskite-silicon tandem solar cell four terminal configuration optimization was performed through numerical simulation. The optimized four terminal perovskite-silicon tandem solar cell performances was investigated by varying the thickness of top and bottom solar cell absorber layers, defect density of the absorber layer, and temperature. Perovskite-silicon tandem solar cell showed better photovoltaic performance under constant illumination condition. A high performance mechanically attached four terminal (4-T) perovskite-silicon tandem solar cell has total power conversion efficiency (PCE) of 34.88% by optimized parameters through simulation. It has shown 37% efficiency with matched current of 23.71mA/cm2. These numerical simulation results are provided the parameter values for further experimental assignment.

Tree-Inspired Aerogel with a Radial and Centrosymmetric Structure for Efficient Solar-Powered Water Purification.

Solar-powered water purification is one of the promising choices for clean water production. However, it remains challenging to develop aerogel solar evaporators that simultaneously possess enhanced light-to-heat conversion, optimal thermal management, and salt crystal deposition inhibition. Herein, to address this challenge, we have developed a 3D chitosan-reduced graphene oxide/polypyrrole (CS-RGO/PPy) aerogel vaporizer with a vertical and radially aligned structure through a directional freezing process, inspired by the featured structure of conifers. The radially porous walls and vertically arranged channels within the 3D aerogel were able to facilitate high light absorption, localizing converted heat, rapid water transport, and self-salt discharge. Under 1 sun irradiation, the aerogel vaporizer displayed an improved light absorption characteristic of 95% and a high-rate evaporation (∼3.19 kg m-2 h-1) that achieved continuous freshwater from the saturated brine production without solid salt crystallization. Besides achieving seawater desalination, the obtained aerogel could purify organic wastewater and emulsions through solar distillation with high-rate continuous water production.

An ecofriendly and efficient wood-based polyoxovanadate solar evaporation generator

Currently, freshwater scarcity is a global challenge, and developing low-cost methods for purifying, desalinating, and distilling seawater is crucial. Natural wood, owing to its advantages of high hydrophilicity, low density, microporous channels, and low thermal conductivity, is widely considered a vehicle for solar evaporation. However, the majority of the reported wood-based solar evaporation generators have been economically inefficient, with limited evaporation rates under low-power solar irradiation. Herein, we developed an ecofriendly and efficient wood-based solar evaporation generator comprising a material with excellent photothermal conversion efficiency, polyoxovanadate, loaded on the wood surface with a natural channel structure. The solar evaporation generator exhibited high light absorption capacity (∼98%) in the wide wavelength range of 200–1200 nm, rapid water transport through the wood channels, minimal heat dissipation due to the adiabaticity of the wood, and reduced evaporation enthalpy. Therefore, the overall evaporation efficiency was increased, and a high evaporation rate of 2.23 kg m−2 h−1 and solar-vapor efficiency of 90% under 1 Sun was observed. This environment-friendly, low-cost, and high-efficiency evaporation generator has enormous potential for practical applications in solar desalination and water purification.

Biocomposite Cryogels for Photothermal Decontamination of Water.

An effective and sustainable approach to deal with the scarcity of freshwater is interfacial solar-driven evaporation. Nonetheless, some serious challenges for photothermal materials still need to be considered, such as long-term stability in harsh environments, eco-friendly materials, and cost-effective and simple fabrication processes. Keeping these points in mind, we present a multifunctional silver-coated vegetable waste biocomposite cryogel that not only exhibits high porosity and enhanced wettability and stability but also possesses high light absorption and low thermal conductivity favorable for heat localization, solar steam generation, and efficient photothermal conversion efficiency. The achieved solar evaporation rate is 1.17 kg m-2 h-1 with a solar-to-vapor conversion efficiency of 81.11% under 1 Sun irradiation. The developed material is able to effectively desalinate artificial seawater and decontaminate synthetic wastewater (e.g., water containing dye molecules and mercury ions) with an efficiency of >99%. Most importantly, the composite cryogel presents antifouling properties, and in particular, salt antifouling ability and anti-biofouling properties. Thus, the numerous functionalities of the biocomposite cryogel make it a cost-effective promising device for prolonged water decontamination processes.

Pinhole-free 2D Ruddlesden–Popper perovskite layer with close packed large crystalline grains, suitable for optoelectronic applications

Here, we achieved pinhole-free 2D Ruddlesden–Popper Perovskite (RPP) BA2PbI4 layers with close packed crystalline grains with dimension of about 30 × 30 µm2, which have been demonstrated to be favorable for optoelectronic applications, such as fast response RPP-based metal/semiconductor/metal photodetectors. We explored affecting parameters in hot casting of BA2PbI4 layers, and proved that oxygen plasma treatment prior to hot casting plays a significant role to achieve high quality close packed polycrystalline RPP layers at lower hot cast temperatures. Moreover, we demonstrate that crystal growth of 2D BA2PbI4 can be dominantly controlled by the rate of solvent evaporation through substrate temperature or rotational speed, while molarity of the prepared RPP/DMF precursor is the dominant factor that determines the RPP layer thickness, and can affect the spectral response of the realized photodetector. Benefiting from the high light absorption and inherent chemical stability of 2D RPP layers, we achieved high responsivity and stability, and fast response photodetection from perovskite active layer. We achieved a fast photoresponse with rise and fall times of 189 µs and 300 µs, and the maximum responsivity of 119 mA/W and detectivity of 2.15 × 108 Jones in response to illumination wavelength of 450 nm. The presented polycrystalline RPP-based photodetector benefits from a simple and low-cost fabrication process, suitable for large area production on glass substrate, a good stability and responsivity, and a promising fast photoresponse, even around that of exfoliated single crystal RPP-based counterparts. However, it is well known that exfoliation methods suffer from poor repeatability and scalability, which make them incompatible with mass production and large area applications.

Piezo‐Phototronic Effect‐Induced Self‐Powered Broadband Photodetectors using Environmentally Stable α‐CsPbI3 Perovskite Nanocrystals

AbstractUtilizing the remarkably high light absorption and long carrier diffusion length of inorganic perovskites, superior performance self‐powered broadband photodetectors using the vertical heterojunction of cesium lead iodide (CsPbI3) nanocrystals (NCs) and zinc oxide (ZnO) films on a flexible platform are reported. Here, the pure cubic or α‐phase of as‐synthesized CsPbI3 NCs facilitate the novelty of the present work in terms of their enhanced structural stability suitable for optical applications. The α‐CsPbI3/ZnO device exhibits superior performance with photoresponsivity of ≈8.2 AW−1, ON/OFF ratio of ≈2.4 × 104, and specific detectivity up to ≈1.4 × 1012 Jones. Furthermore, the piezo‐phototronic effect of ZnO has been exploited to enhance the device performance by utilizing the compressive strain‐induced piezoelectric charges for modulating the generation, separation, and transportation of charge carriers. By introducing an approximate −0.042% compressive strain in the hybrid heterostructure, the enhancement of photocurrent, spectral responsivity, and specific detectivity of the detector by ≈619%, ≈536%, and ≈506%, respectively, under visible light illumination has been successfully achieved. This work not only presents an inventive method for improving the performance of perovskite photodetectors through interface engineering, but it also provides a thorough understanding of the piezo‐phototronic effect on optoelectronic devices.

Highly Transparent Carbon Nanofibrous Membranes Inspired by Dragonfly Wings.

Carbon nanofibrous membrane (CNFM) materials are commonly black and opaque, and their poor optical performance severely limits their application in emerging fields, such as electronic skin, wearable devices, and environmental technologies. However, it is extremely difficult for carbon nanofibrous membranes to achieve high light transmittance owing to their complex fibrous structures and high light absorption. Few researchers have studied transparent carbon nanofibrous membrane (TCNFM) materials. In the current study, a biomimetic TCNFM inspired by dragonfly wings is fabricated using electrospinning technologies and a self-designed patterned substrate, with the aim to construct a differential electric field. Compared with the disordered CNFM, the resultant TCNFM yields an approximately 18-fold higher light transmittance. The freestanding TCNFMs also exhibit high porosities (>90%), good flexibility, and good mechanical properties. The mechanism by which the TCNFMs achieve high transparency and reduce light absorption is also elucidated. In addition, the TCNFMs display a high PM0.3 removal efficiency (>90%), low air resistance (<100 Pa), and good conductive properties, including a low resistivity (<0.37 Ω·cm).

Highly sensitive photodetector based on laser-generated graphene with 3D heterogeneous multiscale porous structures

Graphene holds great promise in optoelectronics because of its high broadband light absorption across a wide range of wavelengths. Despite low optical absorption, various techniques have been devised to address this challenge. This study presents a new highly sensitive, photodetector based on three-dimensional (3D) porous graphene derived from fluorinated polyimide (fPI-3DPG). This material is produced via a laser photothermal process and features micro- and nanopore structures that increase the light absorbing area and support optically resonant structures. The resulting fPI-3DPG photodetectors display high photoresponsivity of 4.4 mA W−1 in the visible light range, an ultrahigh signal-to-noise ratio of ∼208, and a noise equivalent power of 0.54 × 10−11 W Hz−1/2. These photodetectors are also mechanically durable, surviving 10,000 cycles of bending and twisting. The laser photothermal method used in this study enables fast, cost effective production of wearable and flexible optical sensors for imaging and spectroscopy applications, making this 3D graphene a promising functional material for sensing technologies.

Janus-functionalization induced magnetism and improved optoelectronic properties in two-dimension silicene and germanene: insights from first-principles calculations

In this paper, the structural stability, optoelectronic and magnetic properties of silicene and germanene monolayers Janus-functionalized simultaneously with hydrogen and alkali metal atoms (Li and Na) are investigated systematically by using first-principles calculations. The calculated results of the ab initio molecular dynamics simulations and cohesive energies indicate that all functionalized cases have good stability. Meanwhile, the calculated band structures show that all functionalized cases retain the Dirac cone. Particularly, the cases of HSiLi and HGeLi show metallic nature but retain semiconducting characteristics. Besides, the above two cases can present obvious magnetic behavior and their magnetic moments are mainly originated by the p states of Li atom. The metallic property and weak magnetic character can also be found in the case of HGeNa. While the case of HSiNa exhibits the nonmagnetic semiconducting property with a indirect band gap of 0.42 eV calculated by the HSE06 hybrid functional. It is also found that the optical absorption in the visible region of silicene and germanene can be effectively improved by Janus-functionalization. Specifically, a high optical absorption of visible light in an order of 4.5 × 105 cm−1 can be observed in the case of HSiNa. Furthermore, in the visible region, the reflection coefficients of all functionalized cases can also be enhanced. These results demonstrate the feasibility of the Janus-functionalization method to modulate the optoelectronic and magnetic properties of silicene and germanene, expanding their potential applications in the fields of spintronics and optoelectronics.