Photo-Thermoelectric Conversion Using Black Silicon with Enhanced Light Trapping Performance far beyond the Band Edge Absorption

The Quest for Two-Dimensional MBenes: From Structural Evolution to Solar-Driven Hybrid Systems for Water-Fuel-Energy Generation and Phototherapy

The field of 2D materials has advanced significantly with the emergence of MBenes, a new material derived from the MAX phases family, a novel class of materials that originates from the MAX phases family. Herein, this article explores the unique characteristics and morphological variations of MBenes, offering a comprehensive overview of their structural evolution. First, the discussion explores the evolutionary period of 2D MBenes associated with the several techniques for synthesizing, modifying, and characterizing MBenes to tailor their structure and enhance their functionality. The focus then shifts to the defect chemistry of MBenes, electronic, catalytic, and photothermal properties which play a crucial role in designing multifunctional solar-driven hybrid systems. Second, the recent advancements and potentials of 2D MBenes in solar-driven hybrid systems e.g. photo-electro catalysis, hybrid solar evaporators for freshwater and thermoelectric generators, and phototherapy, emphasizing their crucial significance in tackling energy and environmental issues, are explored. The study further explores the fundamental principles that regulate the improved photocatalytic and photothermal characteristics of MBenes, highlighting their promise for effective utilization of solar energy and remediation of the environment. The study also thoroughly assesses MBenes' scalability, stability, and cost effectiveness in solar-driven systems. Current insights and future directions allow researchers to utilize MBenes for sustainable and varied applications. This review regarding MBenes will be valuable to early researchers intrigued with synthesizing and utilizing 2D materials for solar-powered water-energy-fuel and phototherapy systems.

Flexible Janus Black Silicon Photothermal Conversion Membranes for Highly Efficient Solar-Driven Interfacial Water Purification

Photothermal conversion materials are critical in the development of solar-driven interfacial evaporation techniques; however, achieving a high energy conversion efficiency remains challenging owing to the high cost and instability of light-absorbing materials, in addition to the difficulties of simultaneously improving light absorption while suppressing heat loss. A black silicon (Si) powder with a porous structure was prepared by chemical etching of a low-cost commercial micron-sized Al-Si alloy, and a flexible Janus black Si photothermal conversion membrane was constructed. The partially broken spherical particles and porous structure obtained after etching enhanced the refraction of light from the Si powder, imparting the prepared membrane with an average light absorption rate of 95.95% over the solar spectrum. Evaporation from the membrane increased the intermediate water content and reduced the equivalent evaporation enthalpy. The thermal conduction loss was inhibited through a one-dimensional water transport structure, and the membrane achieved a water evaporation rate of 2.17 kg m-2 h-1 and a photothermal efficiency of 94.95% under 1 sun illumination. Benefiting from the broadband absorption and high photothermal efficiency of black Si powder, surface modification of hydrophobic polydimethylsiloxane, and directional salt-out structure design, the evaporation rate of the Janus black Si membrane-based system in a 10% NaCl solution was maintained >2.10 kg m-2 h-1 after 7 days of continuous evaporation cycles. The removal rate of metal ions from simulated seawater and from practical wastewater containing complex heavy metals reached >99.9%, indicating the promising potential of black Si membrane for application in solar-driven interfacial water purification.

Evaluating the Field Emission Properties of N-Type Black Silicon Cold Cathodes Based on a Three-Dimensional Model

Black silicon (BS), a nanostructured silicon surface containing highly roughened surface morphology, has recently emerged as a promising candidate for field emission (FE) cathodes in novel electron sources due to its huge number of sharp tips with ease of large-scale fabrication and controllable geometrical shapes. However, evaluating the FE performance of BS-based nanostructures with high accuracy is still a challenge due to the increasing complexity in the surface morphology. Here, we demonstrate a 3D modeling methodology to fully characterize highly disordered BS-based field emitters randomly distributed on a roughened nonflat surface. We fabricated BS cathode samples with different morphological features to demonstrate the validity of this method. We utilize parametrized scanning electron microscopy images that provide high-precision morphology details, successfully describing the electric field distribution in field emitters and linking the theoretical analysis with the measured FE property of the complex nanostructures with high precision. The 3D model developed here reveals a relationship between the field emission performance and the density of the cones, successfully reproducing the classical relationship between current density J and electric field E (J-E curve). The proposed modeling approach is expected to offer a powerful tool to accurately describe the field emission properties of large-scale, disordered nano cold cathodes, thus serving as a guide for the design and application of BS as a field electron emission material.

p-n hybrid bulk heterojunction enables enhanced photothermoelectric performance with UV-Vis-NIR light

Infrared light accounts for the vast majority of natural light energy, however, the challenge of converting infrared light directly into electricity is too difficult. The photothermoelectric (PTE) effect (connecting the photothermal (PT) and thermoelectric (TE) effects) provides a feasible solution for the indirect conversion of infrared light into electrical energy. Therefore, it is of great significance to actively seek and explore materials with good PT and TE performance to fully harvest infrared light energy. Here, we prepared an organic-inorganic hybrid bulk heterojunction film by combining poly(3,4-ethylene-dioxythiophene):polystyrenesulphonate (PEDOT:PSS) and ZnO nanowires (ZnO-NWs). This common composite strategy is able to utilize the ultra-wide spectrum ranging from ultraviolet-visible (UV-Vis) to near-infrared (NIR) light to realize light-to-electricity conversion based on the PTE effect. ZnO-NWs can not only increase the Seebeck coefficient of PEDOT:PSS, but also enhance the absorption of the hybrid film under the NIR light. Thereby, the enhancement of the photothermal-induced voltage was achieved due to the separation of generated electron-hole pairs in the built-in electric field induced by a photothermal gradient. This study provides a new suggestion for improving the PTE performance of the material and making better use of solar energy.

Evaluating the Optical Response of Heavily Decorated Black Silicon Based on a Realistic 3D Modeling Methodology

Combining black silicon (BS), a nanostructured silicon containing highly roughened surface morphology with plasmonic materials, is becoming an attractive approach for greatly enhancing light-matter interactions with promising applications of sensing and light harvesting. However, precisely describing the optical response of a heavily decorated BS structure is still challenging due to the increasing complexity in surface morphology and plasmon hybridization. Here, we propose and fully characterize BS-based multistacked nanostructures with randomly distributed nanoparticles on the highly roughened nonflat surface. We demonstrate a realistic 3D modeling methodology based on parametrized scanning electron microscopy images that provides high-precision morphology details, successfully linking the theoretical analysis with experimental optical response of the complex nanostructures. Far-field calculations very nicely reproduce experimental reflectance spectra, revealing the dependency of light trapping on the thickness of the conformal reflector and the atop nanoparticle size. Near-field analysis clearly identifies three types of stochastic "hotspots". Their contribution to the overall field enhancement is shown to be very much sensitive to the nanoscale surface morphology. The simulated near-field property is then used to examine the measured surface-enhanced Raman scattering (SERS) response on the multistacked structures. The present modeling approach combined with spectroscopic characterizations is expected to offer a powerful tool for the precise description of the optical response of other large-scale highly disordered realistic 3D systems.

Unraveling the Mechanism of Maskless Nanopatterning of Black Silicon by CF4/H2 Plasma Reactive-Ion Etching

The process of deep texturization of the crystalline silicon surface is intimately related to its promising diverse applications, such as bactericidal surfaces for integrated lab-on-chip devices and absorptive optical layers (black silicon-BSi). Surface structuring by a maskless texturization appeals as a cost-effective approach, which is up-scalable for large-area production. In the case of silicon, it occurs by means of reactive plasma processes (RIE-reactive-ion etching) using fluorocarbon CF₄ and H₂ as reaction gases, leading to self-assembled cylindrical and pyramidal nanopillars. The mechanism of silicon erosion has been widely studied and described as it is for the masked RIE process. However, the onset of the erosion and the reaction kinetics leading to defined maskless patterning have not been unraveled to date. In this work, we specifically tackle this issue by analyzing the results of three different RIE recipes, specifically designed for the purpose. The mechanism of surface self-nanopatterning is revealed by deeply investigating the physical chemistry of the etching process at the nanoscale and the evolution of surface morphology. We monitored the progress in surface patterning and the composition of the etching plasma at different times during the RIE process. We confirm that nanopattering issues from a net erosion, as contributed by chemical etching, physical sputtering, and by the synergistic plasma effect. We propose a qualitative model to explain the onset, the evolution, and the stopping of the process. As the RIE process is started, a high density of surface defects is initially created at the free silicon surface by energetic ion sputtering. Contextually, a polymeric overlayer is synthesized on the Si surface, as thick as 5 nm on average, and self-aggregates into nanoclusters. The latter phenomenon can be explained by considering that the initial creation of surface defects increases the activation energy for surface diffusion of deposited CF and CF₂ species and prevents them from aggregating into a continuous Volmer-Weber polymeric film. The clusterization of the polymer provides the self-masking effect since the beginning, which eventually triggers surface patterning. Once started, the maskless texturing proceeds in analogy with the masked case, that is, by combined chemical etching and ion sputtering, and ceases because of the loss of ion energy. In the case of CF₄/H₂ RIE processes at 10% of H₂ and by supplying 200 W of RF power for 20 min, nanopillars of 200 nm in height and 100 nm in width were formed. We therefore propose that a precise assessment of surface defect formation and density in dependence on the initial RIE process parameters can be the key to open a full control of outcomes of maskless patterning.

Constructing A/B-Side Heterogeneous Asynchronous Structure with Ag2Se Layers and Bushy-like PPy toward High-Performance Flexible Photo-Thermoelectric Generators

The enthusiasm for environmental energy harvesting has triggered a boom in research on photo-thermoelectric generators (PTEGs), and the relevant applications are mainly focused on self-energy supply sensors owing to the limitations of their output performances. For this purpose, high-output hierarchical heterogeneous PTEGs were constructed by assembling separately optimized thermoelectric (TE) and photothermal (PT) layers. The pressure and temperature conditions of Ag2Se films during the pressing process were first explored, and the sample with the optimal performance and least defects was selected as the TE layer. At the same time, different morphologies of polypyrrole (PPy) PT layers were electrochemically synthesized. It is found that the three-dimensional structure of Bushy-PPy could effectively improve the light absorption and thus enhance the PT conversion performance. The final assembled PTEG can produce an output voltage of -9.03 mV and an output power of 3.53 μW under the irradiation of a near-infrared light source of 300 mW cm-2 without a cooling source, and it can also achieve considerable output power under visible light irradiation of different intensities. Combining its high retentions of electrical conductivity (99%) and output performance (97%) after 1000 bending-tension cycles, it is proven to be a promising next-generation wearable flexible energy harvesting device.

Omnidirectional and broadband photon harvesting in self-organized Ge columnar nanovoids

Highly porous Germanium surfaces with uniformly distributed columnar nanovoid structures are fabricated over a large area (wafer scale) by large fluence Sn⁺irradiation through a thin silicon nitride layer. The latter represents a one-step highly reproducible approach with no material loss to strongly increase photon harvesting into a semiconductor active layer by exploiting the moth-eye antireflection effect. The ion implantation through the nitride cap layer allows fabricating porous nanostructures with high aspect ratio, which can be tailored by varying ion fluence. By comparing the reflectivity of nanoporous Ge films with a flat reference we demonstrate a strong and omnidirectional reduction in the optical reflectivity by a factor of 96% in the selected spectral regions around 960 nm and by a factor of 67.1% averaged over the broad spectral range from 350 to 1800 nm. Such highly anti-reflective nanostructured Ge films prepared over large-areas with a self-organized maskless approach have the potential to impact real world applications aiming at energy harvesting.

Effect of synthesis time on plasmonic properties of Ag dendritic nanoforests

The effects of synthesis time on the plasmonic properties of Ag dendritic nanoforests on Si substrate (Ag-DNF/Si) samples synthesized through the fluoride-assisted galvanic replacement reaction were investigated. The Ag-DNF/Si samples were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, reflection spectroscopy, X-ray diffraction and surface-enhanced Raman spectroscopy (SERS). The prolonged reaction time led to the growth of an Ag-DNF layer and etched Si hole array. SEM images and variations in the fractal dimension index indicated that complex-structure, feather-like leaves became coral-like branches between 30 and 60 min of synthesis. The morphological variation during the growth of the Ag DNFs resulted in different optical responses to light illumination, especially those of light harvest and energy transformation. The sample achieved the most desirable light-to-heat conversion efficiency and SERS response with a 30 min growth time. A longer synthesis time or thicker Ag-DNF layer on the Si substrate did not have superior plasmonic properties.

Black Silver: Three-Dimensional Ag Hybrid Plasmonic Nanostructures with Strong Photon Coupling for Scalable Photothermoelectric Power Generation

The conversion of solar energy into electric power has been extensively studied, for example, by photovoltaics. However, photo-thermoelectric (P-TE) conversion as an effective solar-to-electricity conversion process is less studied. Here, we present an efficient full-solar-spectrum plasmonic absorber for scalable P-TE conversion based on a simple integration of light absorber and commercial thermoelectric modules. Our developed light absorber of silica-silver hybrid structures achieves an average absorption of 99.4% in the wavelength range from 200 to 2500 nm, which covers over 98% solar energy in this range. It thus appears fully matte black and is named black silver. The light absorber includes a hierarchical structure with Ag nanoparticles attached on three-dimensional SiO₂ nanostructures, resulting in ultrahigh absorption. Strong localized surface plasmon resonance hybridization together with multiple scattering causes the perfect light absorption. Using the black silver as a light absorber for P-TE power generation, it can achieve a peak voltage density as high as 82.5 V m^-2 under a solar intensity of 100 mW cm^-2, which is large enough to power numerous electronic devices. By assembling 20 thermoelectric modules in series, we test their possibility of practical application, and they can also achieve an average voltage density of 70.66 V m^-2. Our work opens up a promising technology that facilitates high-efficiency and scalable solar energy conversion via the P-TE effect.

Tailoring Patterned Visible-Light Scattering by Silicon Photonic Crystals

Searching for the relationship between the nanostructure and optical properties has always been exciting the researchers in the field of optics (linear optics as well as non-linear optics), energy harvesting (anti-reflective Si solar cells, perovskite solar cells, ..., etc.), and industry (anti-reflection coating on car windows, sunglasses, etc.). In this work, we present an approach for nanostructuring the silicon substrate to silicon photonic crystals. By precisely controlling the etching time and etching path after using nanoimprint lithography, ordered arrays of inverted Si nanopyramids and Si nanopillars with good homogeneity, uniform surface roughness, high reproducibility of pattern transfer, and a controllable aspect ratio are prepared. Experimental investigation of the optical properties indicates that the reflections of these Si nanostructures are mainly determined by the aspect ratio as well as the period of nanostructures. Furthermore, we have experimentally observed visible-light scattering (V-LS) patterns on inverted Si nanopyramids and Si nanopillars, and their corresponding patterns can be precisely controlled by the patterned nanostructures. The V-LS pattern, background, and "ghost peaks" on the angle-resolved scattering results are caused by constructive interference, destructive interference, and the interference situation between both. This controllable nanopatterning on crystalline Si substrates with precisely tunable optical properties shows great potential for applications in many fields, for example, optics, electronics, and energy.

Organic/Inorganic Hybrid Boosting Energy Harvesting Based on the Photothermoelectric Effect

Attracted by the capability of light to heat and electricity conversion, the photothermoelectric (PTE) effect has drawn great attention in the field of energy conversion and self-powered electronics. However, it still requires effective strategies to convert electricity from light based on the corresponding photothermoelectric generator. Herein, considering the broad photoresponse and large Seebeck effect of tellurium nanowires (Te NWs) as well as the high electrical conductivity of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), PEDOT:PSS/Te NW hybrid thin films were fabricated to enhance the conversion efficiency by the photothermoelectric effect with respect to single thermoelectric performance. A detailed comparison has been achieved between the photothermoelectric and thermoelectric properties induced by light illumination and heating plates through current-voltage (I-V) transport, respectively. PEDOT:PSS/Te NW hybrid films also show an enhanced photothermal harvesting compared to pure PEDOT:PSS. A photothermoelectric device was assembled based on the as-fabricated PEDOT:PSS/Te NW hybrid films with 90 wt% Te NWs and achieved a competitive output power density with good stability, which may provide insights into improving solar energy harvesting-based photothermoelectric conversion by organic/inorganic hybrids.

Photothermal Effects and Heat Conduction in Nanogranular Silicon Films

We present results on the photothermal (PT) and heat conductive properties of nanogranular silicon (Si) films synthesized by evaporation of colloidal droplets (drop-casting) of 100 ± 50 nm-sized crystalline Si nanoparticles (NP) deposited on glass substrates. Simulations of the absorbed light intensity and photo-induced temperature distribution across the Si NP films were carried out by using the Finite difference time domain (FDTD) and finite element mesh (FEM) modeling and the obtained data were compared with the local temperatures measured by micro-Raman spectroscopy and then was used for determining the heat conductivities k in the films of various thicknesses. The cubic-to-hexagonal phase transition in Si NP films caused by laser-induced heating was found to be heavily influenced by the film thickness and heat-conductive properties of glass substrate, on which the films were deposited. The k values in drop-casted Si nanogranular films were found to be in the range of lowest k of other types of nanostructurely voided Si films due to enhanced phonon scattering across inherently voided topology, weak NP-NP and NP-substrate interface bonding within nanogranular Si films.