Nanowire Photoelectrochemistry.

Recent applications of photoelectrochemistry at the semiconductor/liquid interface provide a renewable route of mimicking natural photosynthesis and yielding chemicals from sunlight, water, and air. Nanowires, defined as one-dimensional nanostructures, exhibit multiple unique features for photoelectrochemical applications and promise better performance as compared to their bulk counterparts. This article reviews the use of semiconductor nanowires in photoelectrochemistry. After introducing fundamental concepts essential to understanding nanowires and photoelectrochemistry, the review considers answers to the following questions: (1) How can we interface semiconductor nanowires with other building blocks for enhanced photoelectrochemical responses? (2) How are nanowires utilized for photoelectrochemical half reactions? (3) What are the techniques that allow us to obtain fundamental insights of photoelectrochemistry at single-nanowire level? (4) What are the design strategies for an integrated nanosystem that mimics a closed cycle in artificial photosynthesis? This framework should help readers evaluate the salient features of nanowires for photoelectrochemical applications, promoting the sustainable development of solar-powered chemical plants that will benefit our society in the long run

L dwarfs detection from SDSS images using improved Faster R-CNN

We present a data-driven approach to automatically detect L dwarfs from Sloan Digital Sky Survey(SDSS) images using an improved Faster R-CNN framework based on deep learning. The established L dwarf automatic detection (LDAD) model distinguishes L dwarfs from other celestial objects and backgrounds in SDSS field images by learning the features of 387 SDSS images containing L dwarfs. Applying the LDAD model to the SDSS images containing 93 labeled L dwarfs in the test set, we successfully detected 83 known L dwarfs with a recall rate of 89.25% for known L dwarfs. Several techniques are implemented in the LDAD model to improve its detection performance for L dwarfs,including the deep residual network and the feature pyramid network. As a result, the LDAD model outperforms the model of the original Faster R-CNN, whose recall rate of known L dwarfs is 80.65% for the same test set. The LDAD model was applied to detect L dwarfs from a larger validation set including 843 labeled L dwarfs, resulting in a recall rate of 94.42% for known L dwarfs. The newly identified candidates include L dwarfs, late M and T dwarfs, which were estimated from color (i-z) and spectral type relation. The contamination rates for the test candidates and validation candidates are 8.60% and 9.27%, respectively. The detection results indicate that our model is effective to search for L dwarfs from astronomical images.Comment: 12 pages, 10 figures, accepted to be published in A

Excitation-wavelength-dependent small polaron trapping of photoexcited carriers in α-Fe_2O_3

Small polaron formation is known to limit ground-state mobilities in metal oxide photocatalysts. However, the role of small polaron formation in the photoexcited state and how this affects the photoconversion efficiency has yet to be determined. Here, transient femtosecond extreme-ultraviolet measurements suggest that small polaron localization is responsible for the ultrafast trapping of photoexcited carriers in haematite (α-Fe_2O_3). Small polaron formation is evidenced by a sub-100 fs splitting of the Fe 3p core orbitals in the Fe M_(2,3) edge. The small polaron formation kinetics reproduces the triple-exponential relaxation frequently attributed to trap states. However, the measured spectral signature resembles only the spectral predictions of a small polaron and not the pre-edge features expected for mid-gap trap states. The small polaron formation probability, hopping radius and lifetime varies with excitation wavelength, decreasing with increasing energy in the t_(2g) conduction band. The excitation-wavelength-dependent localization of carriers by small polaron formation is potentially a limiting factor in haematite’s photoconversion efficiency

Excitation-wavelength-dependent small polaron trapping of photoexcited carriers in α-Fe_2O_3

Small polaron formation is known to limit ground-state mobilities in metal oxide photocatalysts. However, the role of small polaron formation in the photoexcited state and how this affects the photoconversion efficiency has yet to be determined. Here, transient femtosecond extreme-ultraviolet measurements suggest that small polaron localization is responsible for the ultrafast trapping of photoexcited carriers in haematite (α-Fe_2O_3). Small polaron formation is evidenced by a sub-100 fs splitting of the Fe 3p core orbitals in the Fe M_(2,3) edge. The small polaron formation kinetics reproduces the triple-exponential relaxation frequently attributed to trap states. However, the measured spectral signature resembles only the spectral predictions of a small polaron and not the pre-edge features expected for mid-gap trap states. The small polaron formation probability, hopping radius and lifetime varies with excitation wavelength, decreasing with increasing energy in the t_(2g) conduction band. The excitation-wavelength-dependent localization of carriers by small polaron formation is potentially a limiting factor in haematite’s photoconversion efficiency

Solution processed low power organic field-effect transistor bio-chemical sensor of high transconductance efficiency

Developing organic field-effect transistor (OFET) biosensors for customizable detection of biomarkers for many diseases would provide a low-cost and convenient tool for both biological studies and clinical diagnosis. In this work, design principles of the OFET transducer for biosensors were derived to relate the signal-to-noise ratio (SNR) to the device-performance parameters. Steep subthreshold swing (SS), proper threshold voltage (Vth), good-enough bias-stress stability, and mechanical durability are shown to be the key prerequisites for realizing OFET bio-sensors of high transconductance efficiency (gm/ID) for large SNR. Combining a low trap-density channel and a high-k/low-k gate dielectric layer, low-temperature (<100 °C) solution-processed flexible OFETs can meet the performance requirements to maximize the gm/ID. An extended gate-structure OFET biosensor was further implemented for label-free detection of miR-21, achieving a detection limit below 10 pM with high selectivity at a low operation voltage (<1 V)

Silicon nanowires for solar-to-fuel conversion

Photoelectrochemistry is one of several promising approaches for the realization of efficient solar-to-fuel conversion. Recent work has shown that photoelectrodes made of semiconductor nanowires can have better photoelectrochemical (PEC) performance than their planar counterparts for several reasons including the enhanced light absorption efficiency, release of the high requirement on the minority carrier diffusion length and providing much more catalytic sites for the electrochemical oxidation/reduction reactions to happen. Owing to its earth-abundance, biocompatibility, suitable band structure and stability in aqueous condition, Si nanowire is widely considered as a promising photocathode candidate. Though Si nanowire has been studied for both solar hydrogen evolution and CO2 reduction, our understanding on this photocathode is still not comprehensive, from both fundamental and practical perspectives. Under this context, the subject of my graduate focuses on investigating the properties, understanding the benefits and improving the efficiency of Si nanowire photocathode.Although much effort has been focused on studying Si nanowire arrays, inhomogeneity in the geometry, doping, defects and catalyst loading present in such arrays can obscure the link between these properties and the nanowires’ PEC performance; correlating the performance with the specific properties of individual wire is difficult because of ensemble averaging. Here, we show that a single-nanowire-based photoelectrode platform can be used to reliably probe the current-voltage (I-V) characteristics of individual nanowires. We found that the photovoltage output of ensemble array samples can be limited by poorly performing individual wires, which highlights the importance of improving the nanowire homogeneity within an array. Furthermore, this platform allows the flux of photo-generated electrons to be quantified as a function of the lengths and diameters of individual nanowires, and the flux over the entire nanowire surface (7-30 electrons/ (nm2∙s)) is found to be significantly reduced as compared to that of a planar analogue (~1,200 electrons/ (nm2∙s)). Such characterization of the photo-generated carrier flux at the semiconductor/electrolyte interface is essential for designing nanowire photoelectrodes that match the activity of their loaded electrocatalysts.Based on the information obtained from single-nanowire photoelectrode, we moved forward to develop approaches to improving the energy conversion efficiency of Si nanowire photocathode. First, we demonstrate the resonant absorption effect of Si nanowire photoelectrode. Strongly dependent on the nanowire’s diameter, such resonant effect provides guidance to design proper nanowire geometry for maximized light absorption. Second, we try to use Cu to replace Au for the vapor-liquid-solid (VLS) Si nanowire growth. It shows that Cu-catalyzed VLS Si nanowire photocathode outperforms the Au-catalyzed counterpart. Such result highlights the importance of improving the material’s quality, especially avoiding the metal contamination, to realize efficient nanowire-based solar-to-fuel conversion. Third, we use a commercial chemical vapor deposition (CVD) system for wafer-scale Si nanowire growth. Attributed to the capability of precisely-controlled in-situ boron doping, the CVD yields Si nanowire arrays with decent PEC performance. Our approach opens up the opportunities for scale-up production of high-quality Si nanowire photocathode. Recently the inorganic/microorganism hybrid systems have attracted a lot of interests in the field of microbial electrosynthesis and artificial photosynthesis. However, the electron transfer pathway from electrode to microorganism is still elusive. With Si nanowire/Ni/S. Ovata hybrids as a model system, the last part of my graduate research used sophisticated electrochemical methods to investigate the cathodic electron transfer mechanism in the bacteria-catalyzing CO2-reducing process. The Tafel plot on biotic condition yields fast kinetics (low Tafel slope) at lower over-potential region and slow kinetics (high Tafel slope) at higher over-potential region. Comparison with the abiotic Tafel plot suggests that H2-mediated electron transfer dominates at higher over-potential. The charge transfer resistance extracted from the EIS measurement is consistent with the information obtained from the Tafel plot. The comparison between Ni-based hybrids and Pt-based hybrids system indicate that Ni plays an important role in such kinetics transition. At lower over-potential, the Ni species is oxidized into Ni(OH)2, which is proposed here to bind with the conductive protein complexes on the membrane of S. Ovata bacteria. Such binding would induce the direct electron transfer from Si cathode to the intracellular environment and thus facilitate the kinetics. Our results provide the guidance to design the efficient bio-inorganic interface in the field of microbial electrosynthesis and artificial photosynthesis

Fixed-Time Synchronization for Different Dimensional Complex Network Systems with Unknown Parameters via Adaptive Control

This article is related to the issue of fixed-time synchronization of different dimensional complex network systems with unknown parameters. Two suitable adaptive controllers and dynamic parameter estimations are proposed such that the complex network driving and response systems can be synchronized in the settling time. Based on fixed-time control theory and Lyapunov functional method, novel sufficient conditions are provided to guarantee the synchronization within the fixed times, and the settling times are explicitly evaluated, which are independent of the initial synchronization errors. Finally, a numerical example is given to illustrate the effectiveness of the proposed control algorithms

Fe₃O₄–Graphite Composites as a Microwave Absorber with Bimodal Microwave Absorption

Microwave absorption in the low-frequency region is a major challenge in the development of carbon-based absorbers. Fe3O4–graphite composites with both low-frequency region and high-frequency region absorption were prepared through a facile solvothermal method. The electromagnetic properties and impedance matching characteristics of the samples were regulated by changing the dosage of graphite. Interestingly, an excellent bimodal microwave absorption (MA) performance was obtained when the molar ratio of iron and graphite was 3:10 (Fe3O4–2PG). With the optimal matching thickness of 4 mm, the Fe3O4–2PG sample shows good performances with respect to the effective absorption bandwidth of 3.3 GHz; the minimum reflection loss (RLmin) in C-band (4–8 GHz) is −40.6 dB, and its RLmin in Ku-band (12–18 GHz) is −29.82 dB. The good bimodal MA performance of Fe3O4–2PG could be attributed to the synergistic effects and interfacial polarization between Fe3O4 nanoparticles and graphite. Furthermore, Fe3O4–2PG and Fe3O4–1PG have electromagnetic absorption peaks in both the C-band and Ku-band, which broaden the absorption band of electromagnetic waves, which is beneficial to solve the problem of 5 G electromagnetic radiation. Therefore, the research results have special significance for the radiation interference of 5 G technology and the shielding absorption of C-band radar waves

Nanowire Photoelectrochemistry.

Recent applications of photoelectrochemistry at the semiconductor/liquid interface provide a renewable route of mimicking natural photosynthesis and yielding chemicals from sunlight, water, and air. Nanowires, defined as one-dimensional nanostructures, exhibit multiple unique features for photoelectrochemical applications and promise better performance as compared to their bulk counterparts. This article reviews the use of semiconductor nanowires in photoelectrochemistry. After introducing fundamental concepts essential to understanding nanowires and photoelectrochemistry, the review considers answers to the following questions: (1) How can we interface semiconductor nanowires with other building blocks for enhanced photoelectrochemical responses? (2) How are nanowires utilized for photoelectrochemical half reactions? (3) What are the techniques that allow us to obtain fundamental insights of photoelectrochemistry at single-nanowire level? (4) What are the design strategies for an integrated nanosystem that mimics a closed cycle in artificial photosynthesis? This framework should help readers evaluate the salient features of nanowires for photoelectrochemical applications, promoting the sustainable development of solar-powered chemical plants that will benefit our society in the long run