Optical Absorption Efficiency Research Articles

An ultra-flexible plasmonic metamaterial film for efficient omnidirectional and broadband optical absorption

An omnidirectional and broadband optical absorber has long been pursued for its wide application in optics, sensing and energy fields. The recent development of flexible and non-planar optoelectronic devices, however, poses a great challenge to fabricate an optical absorber with excellent mechanical flexibility. Here, based on a facile solution method, we demonstrate an ultra-flexible plasmonic metamaterial film (PMF), which is a composite of gold nanoparticles (Au NPs) and aramid nanofibers, to achieve omnidirectional and broadband optical absorption. Due to the comprehensive contributions of the anti-reflection effect of the PMF surface, localized surface plasmon resonances of the Au NPs, and non-resonant decay of light inside the nanocomposite, the PMF exhibits highly efficient omnidirectional and broadband absorption at visible and near-infrared frequencies. In addition, it also presents exceptional mechanical and fast collective light-heating properties, which makes it promising to be applied on flexible and non-planar photo-thermal devices.

Nanocrystalline CeB<sub>6</sub> and SmB<sub>6</sub> powder prepared by evaporative condensation method and their visible light transparency

In the present work, the nanocrystalline CeB<sub>6</sub> and SmB<sub>6</sub> powder are successfully prepared by evaporative condensation method. The phase composition, grain morphology, microstructure and optical absorption properties for each of the prepared powders are studied systematically. The results show that the main phase of nanocrystalline CeB<sub>6</sub> powder and SmB<sub>6</sub> powder are both composed of CaB6-type cubic structure with space group of <i>Pm</i>-3<i>m</i>. The scanning electron microscope results show that the synthesized CeB<sub>6</sub> and SmB<sub>6</sub> nanoparticles display an spherical morphology with an average grain size of 50 nm. The high resolution transmission electron microscopy observation results show that there exist many intrinsic crystal defects in nanocrystalline SmB<sub>6</sub>, such as lattice distortions or edge dislocations, due to the high volatility characteristic of Sm atom in the condensation (crystallization) process. The optical absorption results show that the absorption valley of nanocrystalline CeB<sub>6</sub> and SmB<sub>6</sub> are respectively located at 599 nm and 632 nm, indicating the high transparency characteristic of visible light. To further qualitatively explain the difference in optical absorption mechanism between CeB<sub>6</sub> and SmB<sub>6</sub>, the first principle calculations are employed to calculate their band structures, densities of states, optical absorption energy, and plasma resonance frequency energy. The calculation results show that there is an electron band crossing the Fermi energy for both CeB<sub>6</sub> and SmB<sub>6</sub>, indicating their typical conductor behaviors. The upmost valence band of CeB<sub>6</sub> and SmB<sub>6</sub> are composed of B-2p and B-2s states, and their bottommost conduction bands are mainly composed of Ce-4f, Ce-5d, Sm-4f, Sm-5d, B-2p and B-2s states. In addition, the volume plasma of carrier electrons can be described in the electron energy-loss function. The peak position in the low energy region of the loss function corresponds to the relevant plasma frequency. As a result, the calculated low energy loss function of CeB<sub>6</sub> and SmB<sub>6</sub> are 1.96 eV and 1.5 eV, respectively. Moreover, the calculated absorption valley of CeB<sub>6</sub> and SmB<sub>6</sub> respectively appear at 639 nm and 800 nm, which are in good accordance with the experimental results. Therefore, as an efficient optical absorption materials, the nanocrystalline CeB<sub>6</sub> and SmB<sub>6</sub> should open the way to extending the optical applications of rare-earth hexaborides.

Physical manipulation of ultrathin-film optical interference for super absorption and two-dimensional heterojunction photoconversion**Project supported by the National Natural Science Foundation of China (Grant Nos. 61675142 and 61875143), the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20140359), the Natural Science Research Project of the Higher Educational Institutions of Jiangsu Province, China (Grant No. 17KJA480004), the Postgraduate Research & Practice

Ultrathin optical interference in a system composed of absorbing material and metal reflector has attracted extensive attention due to its potential application in realizing highly efficient optical absorption by using extremely thin semiconductor material. In this paper, we study the physics behind the high absorption of ultrathin film from the viewpoint of destructive interference and admittance matching, particularly addressing the phase evolution by light propagation and interface reflection. The physical manipulations of the ultrathin interference effect by controlling the substrate material and semiconductor material/thickness are examined. We introduce typical two-dimensional materials — i.e., MoS2 and WSe2 — as the absorbing layer with thickness below 10 nm, which exhibits ∼ 90% absorption in a large range of incident angle (0° ∼ 70°). According to the ultrathin interference mechanism, we propose the ultrathin (< 20 nm) MoS2/WSe2 heterojunction for photovoltaic application and carefully examine the detailed optoelectronic responses by coupled multiphysics simulation. By comparing the same cells on SiO2 substrate, both the short-circuit current density (up to 20 mA/cm2) and the photoelectric conversion efficiency (up to 9.5%) are found to be increased by ∼ 200%.

Fabrication of a semi-transparent thin-film Sb2Se3 solar cell

Semi-transparent solar cells have been well developed in recent years, and can potentially be applied in many areas. Sb2Se3 has also received an increasing amount of attention because of its excellent optoelectronic properties. In order to combine these advantages, we attempted to fabricate a semi-transparent thin-film Sb2Se3 solar cell which meanwhile maintains the high efficiency. The effect of thickness of Sb2Se3 thin film on the optical and electrical properties were investigated. The optical absorption and power conversion efficiency were improved for Sb2Se3 thin film solar cells as the increase of the thickness of Sb2Se3 thin film, however, the obtained 120-nm-thick Sb2Se3 film was completely opaque. The optimized device was achieved with the thickness of 80 nm which exhibits the semi-transparent property and almost the same efficiency as the 120 nm Sb2Se3 thin film solar cell. This work demonstrates that the fabricated device has potential for applications in small wearable devices and may be extended to large-area glass-face applications.

InSe Monolayer: Promising Cocatalyst of g-C3N4 for Water Splitting under Visible Light

Graphitic carbon nitride (g-C3N4) is considered to be a promising material for water splitting and has been intensively investigated in recent years. However, its commercial application is limited due to the low efficiencies of optical absorption and electron–hole separation. Herein, based on the first-principle simulations, we predict that strain tuned g-C3N4/InSe and bilayer g-C3N4/InSe hetrostructures can significantly promote the photocatalytic activity of isolated g-C3N4. Our results demonstrate that the proposed heterostructures possess direct bandgaps, tunable electronic properties, type-II band alignment, and efficient optical absorption. In addition, using the nonequilibrium Green’s function method within the framework of density functional theory (NEGF-DFT), the photoinduced current of the composites is evaluated. Surprisingly, the predicted photocurrent under the visible-light radiation is up to 2.9 μA/mm2 in our proposed heterostructures, which is far superior to other g-C3N4-based composites....

Ultrathin-Film Titania Photocatalyst on Nanocavity for CO2 Reduction with Boosted Catalytic Efficiencies.

Photocatalytic CO2 reduction with water to hydrocarbons represents a viable and sustainable process toward greenhouse gas reduction and fuel/chemical production. Development of more efficient catalysts is the key to mitigate the limits in photocatalytic processes. Here, a novel ultrathin‐film photocatalytic light absorber (UFPLA) with TiO2 films to design efficient photocatalytic CO2 conversion processes is created. The UFPLA structure conquers the intrinsic trade‐off between optical absorption and charge carrier extraction efficiency, that is, a solar absorber should be thick enough to absorb majority of the light allowable by its bandgap but thin enough to allow charge carrier extraction for reactions. The as‐obtained structures significantly improve TiO2 photocatalytic activity and selectivity to oxygenated hydrocarbons than the benchmark photocatalyst (Aeroxide P25). Remarkably, UFPLAs with 2‐nm‐thick TiO2 films result in hydrocarbon formation rates of 0.967 mmol g−1 h−1, corresponding to 1145 times higher activity than Aeroxide P25. This observation is confirmed by femtosecond transient absorption spectroscopic experiments where longer charge carrier lifetimes are recorded for the thinner films. The current work demonstrates a powerful strategy to control light absorption and catalysis in CO2 conversion and, therefore, creates new and transformative ways of converting solar energy and greenhouse gas to alcohol fuels/chemicals.

A DFT study of electronic structures and optical properties of nickel, nitrogen doped rutile TiO2

The first-principle method based on the density functional theory (DFT) was used to calculate the electronic structure and optical properties of pure, N-doped, Ni-doped and Ni/N co-doped rutile TiO2 systems. The band structures, electronic density of states and spectral absorption coefficients were calculated after geometrically optimizing the above TiO2 systems. The results show that there are impurity energy levels appear within the band gaps of Ni-, N-doped systems, which provided by N-2p orbit and Ni-3d orbit; meanwhile, more complicated impurity energy levels appear within the band gaps of N/Ni co-doped rutile TiO2. The co-doped rutile TiO2 has a better optical absorption efficiency and stronger absorption cut-off wavelength red-shift. Which can be caused by the hybridization of Ni-3d orbit and N-2p orbit provides local impurity levels, so that electrons transition occurred from the valence band maximum to the impurity levels and also from the impurity levels to the bottom of conduction band, which contributes to the electron transition and enhances the absorption performance of rutile TiO2.

Magnetically amplified photothermal therapies and multimodal imaging with magneto-plasmonic nanodomes

Nanotherapies require new ways for controlling and improving the delivery of the therapeutic agents to the site of action to maximize their efficacy and minimize the side effects. This control is particularly relevant in photothermal treatments to reduce the required light intensity and amount of injected nanoparticles, and to minimize necrotic cell deaths. Here we present a novel concept for multifunctional nanobiomedical agents: magneto-plasmonic (MP) nanodomes for magnetically guided and amplified photothermal therapies and as contrast agents for multimodal imaging. The MP nanodomes are composed of a Fe/Au bilayer semi-shell deposited on a 100nm diameter fluorescent polystyrene nanosphere, which gather a unique combination of straightforward functionalization, high colloidal stability, very strong ferromagnetic behavior and intense optical absorption efficiency in the near infrared. We show that the photothermal conversion efficiency of the Fe/Au nanodomes with high Fe ratios is substantially larger than pure plasmonic Au nanodomes and the state-of-art plasmonic nanoheaters, i.e. Au nanorods and nanoshells, by merging strong optical absorption, minimized scattering and low optical anisotropy. Remarkably, the effective magnetophoretic concentration of the Fe/Au nanodomes at the illumination region enables large local increase of the optically induced temperature rise. The Fe semishell also provides very intense T2 contrast in nuclear magnetic resonance, which is at least 15-fold larger per particle than commercial iron oxide contrast agents. Moreover, the fluorescent polystyrene nanosphere and the Au semishell integrate valuable fluorescent and X-ray contrasts, respectively, which we have used to assess the nanodomes internalization by cancer cells. The MP nanodomes are nontoxic to cells even in the case of magnetophoretic local enrichment with initially high particle concentration (100μg/mL). Remarkably, we demonstrate amplified local photothermal treatments by the magnetic enrichment of the nanodomes at the illumination region, which enables reaching nearly 100% reduction of cell viability with low particle concentration (10μg/mL) and mild NIR laser intensity (5W/cm2). These results highlight the high potential of MP nanodomes for magnetically guided and amplified photothermal therapies.

Hydrothermal synthesis of nanostructured CuS for broadband efficient optical absorption and high-performance photo-thermal conversion

Solar photo-thermal conversion is the simplest way to utilize solar energy by using nanofluid as the working medium. In this paper, we aim to investigate the broadband optical absorption and high-performance photo-thermal conversion via CuS nanofluids to realize the efficient solar energy utilization. Nanostructured CuS nanospheres and nanotubes were successfully synthesized by using simple hydrothermal reaction. The crystal structures, element compositions and surface properties of the obtained CuS nanostructures were characterized. And the formation mechanisms of the nanostructured CuS were discussed. The optical absorption and photo-thermal conversion performances of the CuS nanofluids based on water were studied. The UV/Vis/NIR absorption spectra tests show that the CuS nanofluids prepared have strong optical absorption in both visible light and near infrared light regions. The photo-thermal conversion results indicate that the CuS nanofluids possess good photo-thermal conversion performances, and the maximum temperature of the nanofluid reaches about 72.15 ℃ only with the volume fraction of 0.02% related to the nanotube nanofluid. Moreover, at the same volume fractions, the nanotube nanofluids show better photo-thermal conversion performances than those of the nanosphere nanofluids. This implies a simple approach to enhance photo-thermal conversion for efficient solar energy utilization.

Construction of an attapulgite intercalated mesoporous g-C3N4 with enhanced photocatalytic activity for antibiotic degradation

A novel intercalation structure g-C3N4 photocatalyst is successfully fabricated by eutectic technique and calcination method, it shows a better degradation efficiency to tetracycline under the visible light. Moreover, the as-prepared photocatalysts are characterized by a series of means, such as PL, FL spectra, EIS, ESR etc. The improved photocatalytic performance could be attributed to the intercalation effect and conductivity of attapulgite (ATP), which enhanced the optical absorption and separation efficiency of photogenerated electron-holes (decay lifetime is 4.3 ns). Moreover, radical trapping experiments and ESR measurement indicated the superoxide radical and photogenerated holes are the major activity species on removing tetracycline in g-C3N4@ATP. The degradation mechanism and the degradation intermediate products are proposed to better understand the reaction process. Additionally, the work provides a new insight for constructing other high performance, low-cost and intercalation structure photocatalyst for wastewater treatment.

Surface plasmon effect in electrodeposited diamond-like carbon films for photovoltaic application

Diamond-like carbon (DLC) films and nanocrystalline silver particles containing diamond-like carbon (DLC:Ag) films were electrodeposited on n-type silicon substrate (n-Si) to prepare n-Si/DLC and n-Si/DLC:Ag heterostructures for photovoltaic (PV) applications. Surface plasmon resonance (SPR) effect in this cell structure and its overall performance have been studied in terms of morphology, optical absorption, current-voltage characteristics, capacitance-voltage characteristics, band diagram and external quantum efficiency measurements. Localized surface plasmon resonance effect of silver nanoparticles (Ag NPs) in n-Si/DLC:Ag PV structure exhibited an enhancement of ∼28% in short circuit current density (JSC), which improved the overall efficiency of the heterostructures.

Composition-Dependent Functionality of Copper Vanadate Photoanodes.

To understand functional roles of constituent elements in ternary metal oxide photoanodes, essential photoelectrochemical (PEC) properties are systematically analyzed on a series of copper vanadate compounds with different Cu:V elemental ratios. Homogeneous and highly continuous thin films of β-Cu2V2O7, γ-Cu3V2O8, Cu11V6O26, and Cu5V2O10 are grown via reactive co-sputtering and their performance characteristics for the light-driven oxygen evolution reaction are evaluated. All four compounds have similar bandgaps in the range of 1.83-2.03 eV, though Cu-rich phases exhibit stronger optical absorption and higher charge separation efficiencies. Transient photocurrent analysis reveals a reduction of surface catalytic activity with increasing Cu:V elemental ratio due to competitive charge recombination at Cu-related surface states. This comprehensive analysis of PEC functionalities-including photon absorption, carrier separation, and heterogeneous charge transfer-informs strategies for improving PEC activity in the copper vanadate materials system and provides insights that may aid discovery, design, and engineering of new photoelectrode materials.

Synthesis and Photo‐Thermal Conversion Properties of Hierarchical Titanium Nitride Nanotube Mesh for Solar Water Evaporation

Hierarchical TiN nanotube mesh (HTNM) is prepared by calcining TiO2 nanotube mesh (TONM) in NH3 flow where the TONM is fabricated by electrochemical anodic oxidation of Ti mesh. Calcining temperature affects not only phase composition but also morphology of the HTNM, which further affects its optical absorption and solar water evaporation conversion efficiency (η). The η can reach to 85.36% under 2.5 kW m−2 simulated solar irradiation because HTNM efficiently utilizes solar light. Moreover, it can be reutilized because of its stabilities of the phase composition and nanostructure during the measurements of solar water evaporation performance.

First-principles calculations of the electronic properties of SiC-based bilayer and trilayer heterostructures.

Recently, van der Waals (vdW) two-dimensional heterostructures have attracted great attention. The combination structures demonstrate unique properties that individual layers do not possess, which foretell promising future applications. Here, we investigate the structural and electronic properties of SiC/graphene, SiC/MoS2, and graphene/SiC/MoS2 vdW heterostructures using first-principles calculations. The SiC/graphene interface forms a p-type Schottky contact, which can be turned into an n-type Schottky contact by applying an external electric field. Moreover, a transition from a Schottky to an Ohmic contact at the interface can be triggered by varying the interlayer distance or applying an external electric field. The SiC/MoS2 interface forms a type-II heterostructure, in which the recombination of photoexcited charges will be greatly suppressed. The transition from type-II to type-III band alignment can be realized in the SiC/MoS2 heterostructure by applying a biaxial strain. This heterostructure also shows excellent optical absorption abilities in the visible and far-infrared range, which merits its application as a photocatalyst. The trilayer heterostructure exhibits a tunable Schottky barrier with different stacking patterns and the assembled graphene could act as a protective encapsulating layer on SiC/MoS2. The results show that graphene and MoS2 can tune and improve the electronic performance of SiC and demonstrate the promising application of SiC-based heterostructures for nanoelectronics and nanophotonics.

Magnetic field modulated SERS enhancement of CoPt hollow nanoparticles with sizes below 10 nm.

It is well known that Pt shows much weaker plasmonic effects in the visible spectrum when compared to Au, Ag and Cu. Therefore, the realization of efficient optical absorption by Pt nanoparticles with sizes below 10 nm in the visible spectrum remains a challenge. One possible way to enhance the optical absorption is to prepare Pt-based bimetallic magnetic nanoparticles. Furthermore, if an external magnetic field is applied, the synergistic effect of both electric and magnetic fields may provide sufficient SERS enhancement. In this paper, CoPt hollow nanoparticles (NPs) with sizes below 10 nm and ultrathin shells (∼2 nm) were synthesized in solution, at room temperature. The NPs have high surface-to-volume ratios and excellent structural stability. The hollow NPs exhibited enhanced light absorption characterized by surface-enhanced Raman scattering (SERS) with 4-mercaptobenzoic acid (4-MBA) as tip molecules. It is noted that the SERS enhancement of these alloyed NPs can be tuned by using an external magnetic field. A synergistic optical effect between these hollow NPs and the Ag film substrate is obtained. Hence, CoPt hollow NPs show promise as SERS substrates and potential for other applications in optical enhancement.

Effect of Co-60 gamma radiation on optical, dielectric and mechanical properties of strontium L-ascorbate hexahydrate NLO crystal

A potentially useful nonlinear optical semi-organic single crystal of strontium L-ascorbate hexahydrate (SLAH) was grown by solution growth slow evaporation technique at room temperature. The grown crystal is semi transparent, yellowish in color with monoclinic crystal system having space group P21 and is stable up to 198°C. Further, SLAH crystals were irradiated with gamma rays produced by 60Co with different doses of 10 KGy, 30 KGy and 50 KGy at room temperature and then studied the effect of gamma-rays on dielectric properties, optical absorption, microhardness and SHG efficiency. The absorption study reveals that the absorbance of the grown crystal is appeared to be low throughout the visible region with a lower cutoff wavelength of 277nm and these parameters are not affected upon gamma irradiation. The luminescence intensity of the crystal is also not affected by the irradiation. There is noticeable changes were observed in dielectric properties and hardness of the materials for different doses of gamma irradiation. The second harmonic generation (SHG) efficiency of the grown crystal is 0.54 times that of the KDP crystal and is decreased moderately by increasing the dosage of gamma irradiation.

CH3NH3PbI3/GeSe bilayer heterojunction solar cell with high performance

Perovskite (CH3NH3PbI3) solar cells have made significant advances recently. In this paper, we propose a bilayer heterojunction solar cell comprised of a perovskite layer combining with a IV-VI group semiconductor layer, which can give a conversion efficiency even higher than the conventional perovskite solar cell. Such a scheme uses a property that the semiconductor layer with a direct band gap can be better in absorption of long wavelength light and is complementary to the perovskite layer. We studied the semiconducting layers such as GeSe, SnSe, GeS, and SnS, respectively, and found that GeSe is the best, where the optical absorption efficiency in the perovskite/GeSe solar cell is dramatically increased. It turns out that the short circuit current density is enhanced 100% and the power conversion efficiency is promoted 42.7% (to a high value of 23.77%) larger than that in a solar cell with only single perovskite layer. The power conversion efficiency can be further promoted so long as the fill factor and open-circuit voltage are improved. This strategy opens a new way on developing the solar cells with high performance and practical applications.

A Novel Iris Mechanism for Solar Thermal Receivers

Variable aperture mechanisms are being used in many fields including medicine, electronics, fluid mechanics, and optics. The main design characteristics of these aperture concepts are the use of multiple blades regulating aperture area and consequently the incoming medium flow. Manufacturing complexities primarily depend on the concept geometry, material, and the process application requirements. Design of a variable aperture demands meticulous methodology and careful consideration of the application field. This paper provides an in-depth methodology on the design of a novel iris mechanism for temperature control in high temperature solar thermal receivers and solar reactors. Such methodology can be used as a guideline for iris mechanisms implemented in other applications as well as in design of different apparatuses exposed to high temperature. Optical simulations in present study have been performed to demonstrate enhanced performance of the iris mechanism over conventional Venetian blind shutter serving as optical attenuators in concentrating solar power systems. Results showed that optical absorption efficiency is improved by 14% while reradiation loss through the aperture is reduced by 2.3% when the iris mechanism is used. Correlation for adaptive control of aperture area was found through computational surface area measurement. Experimental testing with a 7 kW solar simulator at different power levels demonstrated the performance of the mechanism to maintain stable temperature under variable flux.

Au nanoparticle-decorated silicon pyramids for plasmon-enhanced hot electron near-infrared photodetection

The heterojunction between metal and silicon (Si) is an attractive route to extend the response of Si-based photodiodes into the near-infrared (NIR) region, so-called Schottky barrier diodes. Photons absorbed into a metallic nanostructure excite the surface plasmon resonances (SPRs), which can be damped non-radiatively through the creation of hot electrons. Unfortunately, the quantum efficiency of hot electron detectors remains low due to low optical absorption and poor electron injection efficiency. In this study, we propose an efficient and low-cost plasmonic hot electron NIR photodetector based on a Au nanoparticle (Au NP)-decorated Si pyramid Schottky junction. The large-area and lithography-free photodetector is realized by using an anisotropic chemical wet etching and rapid thermal annealing (RTA) of a thin Au film. We experimentally demonstrate that these hot electron detectors have broad photoresponsivity spectra in the NIR region of 1200–1475 nm, with a low dark current on the order of 10−5 A cm−2. The observed responsivities enable these devices to be competitive with other reported Si-based NIR hot electron photodetectors using perfectly periodic nanostructures. The improved performance is attributed to the pyramid surface which can enhance light trapping and the localized electric field, and the nano-sized Au NPs which are beneficial for the tunneling of hot electrons. The simple and large-area preparation processes make them suitable for large-scale thermophotovoltaic cell and low-cost NIR detection applications.

Optical Absorption of Thin Film Solar Cells with Hybrid Arranged Bottom Grating

Crystalline silicon thin film solar cells with hybrid arranged bottom grating are proposed. Optical absorption efficiency and photocurrent density are calculated to get optimized bottom grating parameters. Compared with mono arranged Ag grating or Al-doped zinc-oxide grating, hybrid arranged bottom grating could couple more near-infrared region lights into the active absorber layer. Optical absorption enhancement profiles are plotted for monolayer grating solar cells with four different bottom grating arrangements, which agree with dispersion characteristics well. The absorption enhancement profiles illustrate the mechanism of the more coupling of near-infrared lights. Electrical modeling is considered in the end and it is found that hybrid arranged bottom grating’s thin film solar cell outperforms the thin film solar cells with mono arranged bottom gratings evidently.