This paper investigates link-by-link channel-coded PNC (physical layer network coding), in which a critical process at the relay is to transform the superimposed channel-coded packets received from ...the two end nodes (plus noise), Y 3 = X 1 + X 2 +W 3 , to the network-coded combination of the source packets, S 1 oplus S 2 . This is in contrast to the traditional multiple-access problem, in which the goal is to obtain both S 1 and S 2 explicitly at the relay node. Trying to obtain S 1 and S 2 explicitly is an overkill if we are only interested in S1oplusS 2 . In this paper, we refer to the transformation Y 3 rarr S 1 oplus S 2 as the channel-decoding- network-coding process (CNC) in that it involves both channel decoding and network coding operations. This paper shows that if we adopt the repeat accumulate (RA) channel code at the two end nodes, then there is a compatible decoder at the relay that can perform the transformation Y 3 rarr S 1 oplusS 2 efficiently. Specifically, we redesign the belief propagation decoding algorithm of the RA code for traditional point-to-point channel to suit the need of the PNC multiple-access channel. Simulation results show that our new scheme outperforms the previously proposed schemes significantly in terms of BER without added complexity.
2D phosphorene, arsenene, antimonene, and bismuthene, as a fast‐growing family of 2D monoelemental materials, have attracted enormous interest in the scientific community owing to their intriguing ...structures and extraordinary electronic properties. Tuning the monoelemental crystals into bielemental ones between group‐VA elements is able to preserve their advantages of unique structures, modulate their properties, and further expand their multifunctional applications. Herein, a review of the historical work is provided for both theoretical predictions and experimental advances of 2D V‐V binary materials. Their various intriguing electronic properties are discussed, including band structure, carrier mobility, Rashba effect, and topological state. An emphasis is also given to their progress in fabricated approaches and potential applications. Finally, a detailed presentation on the opportunities and challenges in the future development of 2D V‐V binary materials is given.
2D V‐V binary materials, as the internal combination of group‐VA elements (N, P, As, Sb, Bi), have become a popular research topic. Through the interaction among charge, orbital, lattice, and spin degrees of freedom, the favorable and superior properties in 2D V‐V binary materials can be further modulated, extending their application in novel electronic, optoelectronic, and energy devices.
The typical two‐dimensional (2D) semiconductors MoS2, MoSe2, WS2, WSe2 and black phosphorus have garnered tremendous interest for their unique electronic, optical, and chemical properties. However, ...all 2D semiconductors reported thus far feature band gaps that are smaller than 2.0 eV, which has greatly restricted their applications, especially in optoelectronic devices with photoresponse in the blue and UV range. Novel 2D mono‐elemental semiconductors, namely monolayered arsenene and antimonene, with wide band gaps and high stability were now developed based on first‐principles calculations. Interestingly, although As and Sb are typically semimetals in the bulk, they are transformed into indirect semiconductors with band gaps of 2.49 and 2.28 eV when thinned to one atomic layer. Significantly, under small biaxial strain, these materials were transformed from indirect into direct band‐gap semiconductors. Such dramatic changes in the electronic structure could pave the way for transistors with high on/off ratios, optoelectronic devices working under blue or UV light, and mechanical sensors based on new 2D crystals.
Unlike black phosphorus, both arsenic and antimony are typical semimetals in their natural, layered bulk state. However, monolayered arsenene and antimonene are indirect wide‐band‐gap semiconductors, and under strain, they become direct band‐gap semiconductors. Owing to these band‐gap transitions, these materials could find applications in nano‐ and optoelectronic devices.
Developing efficient catalysts for nitrogen fixation is becoming increasingly important but is still challenging due to the lack of robust design criteria for tackling the activity and selectivity ...problems, especially for electrochemical nitrogen reduction reaction (NRR). Herein, by means of large-scale density functional theory (DFT) computations, we reported a descriptor-based design principle to explore the large composition space of two-dimensional (2D) biatom catalysts (BACs), namely, metal dimers supported on 2D expanded phthalocyanine (M2-Pc or MM′-Pc), toward the NRR at the acid conditions. We sampled both homonuclear (M2-Pc) and heteronuclear (MM′-Pc) BACs and constructed the activity map of BACs by using N2H* adsorption energy as the activity descriptor, which reduces the number of promising catalyst candidates from over 900 to less than 100. This strategy allowed us to readily identify 3 homonuclear and 28 heteronuclear BACs, which could break the metal-based activity benchmark toward the efficient NRR. Particularly, using the free energy difference of H* and N2H* as a selectivity descriptor, we screened out five systems, including Ti2-Pc, V2-Pc, TiV-Pc, VCr-Pc, and VTa-Pc, which exhibit a strong capability of suppressing the competitive hydrogen evolution reaction (HER) with favorable limiting potential of −0.75, −0.39, −0.74, −0.85, and −0.47 V, respectively. This work not only broadens the possibility of discovering more efficient BACs toward N2 fixation but also provides a feasible strategy for rational design of NRR electrocatalysts and helps pave the way to fast screening and design of efficient BACs for the NRR and other electrochemical reactions.
Recently, Kovalenko and co‐workers and Li and co‐workers developed CsPbX3 (X = Cl, Br, I) inorganic perovskite quantum dots (IPQDs), which exhibited ultrahigh photoluminescence (PL) quantum yields ...(QYs), low‐threshold lasing, and multicolor electroluminescence. However, the usual synthesis needs high temperature, inert gas protection, and localized injection operation, which are severely against applications. Moreover, the so unexpectedly high QYs are very confusing. Here, for the first time, the IPQDs' room‐temperature (RT) synthesis, superior PL, underlying origins and potentials in lighting and displays are reported. The synthesis is designed according to supersaturated recrystallization (SR), which is operated at RT, within few seconds, free from inert gas and injection operation. Although formed at RT, IPQDs' PLs have QYs of 80%, 95%, 70%, and FWHMs of 35, 20, and 18 nm for red, green, and blue emissions. As to the origins, the observed 40 meV exciton binding energy, halogen self‐passivation effect, and CsPbX3@X quantum‐well band alignment are proposed to guarantee the excitons generation and high‐rate radiative recombination at RT. Moreover, such superior optical merits endow them with promising potentials in lighting and displays, which are primarily demonstrated by the white light‐emitting diodes with tunable color temperature and wide color gamut.
A room‐temperature supersaturated recrystallization method is developed to rapidly synthesize all‐inorganic halide perovskite QDs with blue, green, and red luminescent quantum yields of 70%–95% and line‐widths less than 35 nm. The origins of the optical superiority are proposed to be the observed 40 meV exciton binding energy, surface self‐passivation effect, and quantum‐well band alignment. Such superior optical merits endow them with promising potentials in healthy lighting and wide‐color‐gamut displays, which are primarily demonstrated by the color‐temperature‐tunable white light‐emitting diodes.
Early diagnosis of gear transmission has been a significant challenge, because gear faults occur primarily at microstructure or even material level but their effects can only be observed indirectly ...at a system level. The performance of a gear fault diagnosis system depends significantly on the features extracted and the classifier subsequently applied. Traditionally, fault-related features are extracted and identified based on domain expertise through data preprocessing which are system-specific and may not be easily generalized. On the other hand, although recently the deep neural networks based approaches featuring adaptive feature extractions and inherent classifications have attracted attention, they usually require a substantial set of training data. Aiming at tackling these issues, this paper presents a deep convolutional neural network-based transfer learning approach. The proposed transfer learning architecture consists of two parts; the first part is constructed with a pre-trained deep neural network that serves to extract the features automatically from the input, and the second part is a fully connected stage to classify the features that needs to be trained using gear fault experimental data. Case analyses using experimental data from a benchmark gear system indicate that the proposed approach not only entertains preprocessing free adaptive feature extractions, but also requires only a small set of training data.
On-site production of hydrogen peroxide (H2O2) using electrochemical methods could be more efficient than the current industrial process. However, due to the existence of scaling relations for the ...adsorption of reaction intermediates, there is a long established trade-off between the activity and selectivity of the catalysts, as the enhancement of catalytic activity is typically accompanied by a four-electron O2 reduction reaction (ORR), leading to the reduced selectivity for the H2O2 production. Herein, by means of density functional theory (DFT) computations, we reported the feasibility of several classes of important and representative experimentally achievable single-atom catalysts (SACs) toward two-electron ORR, paying attention to their stability, selectivity, and activity at the acidic medium. Starting from 210 two-dimensional (2D) SACs, we demonstrated that 31 SACs have the potential to break the metal-based scaling relations and simultaneously achieve high activity and selectivity toward H2O2 production and screened out 7 SACs with higher activity than the PtHg4 in acidic media. Especially, a noble metal-free SAC, namely, a single Zn atom centered phthalocyanine (Zn@Pc-N4), has a remarkable activity improvement with a small overpotential of 0.15 V. Moreover, using multivariable analysis and machine-learning techniques, we provided a comprehensive understanding of the underlying origin of the selectivity and activity of SACs and unveiled the intrinsic correlations between structure and catalytic performance. This work may pave a way to the design and discovery of more promising materials for H2O2 production.
Authentication is a key requirement for secure communications in modern wireless systems. Compared with the conventional authentication at the upper layer using a cryptographic tool, authentication ...at the physical layer has many advantages, including enhanced security through the introduction of uncertainty to adversaries and increased efficiency and compatibility through the avoidance of operations at the upper layer, particularly in heterogeneous coexistence environments, e.g., 5G wireless systems. In this paper, we investigate authentication at the physical layer under time-varying fading channels. Conventional authentication schemes at the physical layer operate poorly under fast fading or frequency selective fading channels. Furthermore, conventional schemes require additional complicated preprocessing such as channel estimation and message symbol recovery through demodulation and decoding. This paper proposes a new blind authentication scheme at the physical layer that combines the techniques of blind known interference cancellation (BKIC) and differential processing to implement authentication without requiring any of the above-described preprocessing. The proposed scheme utilizes both the smoothing (SM) technique and belief propagation (BP) technique to achieve BKIC through the distinct blind authentication schemes with a superimposed tag over pilots (BSUPs) referred to, respectively, as BSUP-SM and BSUP-BP. The proposed scheme not only effectively suppresses the deteriorate effect of fading channels without any additional preprocessing but is also covert to unaware users, robust to interference, and secure for identity verification. The tradeoffs of using the proposed system with respect to various goals are discussed and analyzed. The performance of the BSUP-SM scheme depends on the channel fading and the length of the pilot cluster, while the performance of the BSUP-BP scheme is not sensitive to the above factors but depends instead on the quantization step used. The BSUP-BP scheme works well in fast fading channels, even in frequency selective fading channels.
The stability and optoelectronic device performance of perovskite quantum dots (Pe‐QDs) are severely limited by present ligand strategies since these ligands exhibit a highly dynamic binding state, ...resulting in serious complications in QD purification and storage. Here, a “Br‐equivalent” ligand strategy is developed in which the proposed strong ionic sulfonate heads, for example, benzenesulfonic acid, can firmly bind to the exposed Pb ions to form a steady binding state, and can also effectively eliminate the exciton trapping probability due to bromide vacancies. From these two aspects, the sulfonate heads play a similar role as natural Br ions in a perfect perovskite lattice. Using this approach, high photoluminescence quantum yield (PL QY) > 90% is facilely achieved without the need for amine‐related ligands. Furthermore, the prepared PL QYs are well maintained after eight purification cycles, more than five months of storage, and high‐flux photo‐irradiation. This is the first report of high and versatile stabilities of Pe‐QD, which should enable their improved application in lighting, displays, and biologic imaging.
A new equivalent ligand strategy with a strong ionic sulfonate head is demonstrated and the purification and storage problems of perovskite nanocrystals are overcome. Both theoretical and experimental results prove the elimination of nonradiative recombination and high quantum efficiency are maintained throughout purification, storage, and irradiation.
Solar energy-assisted water oxidative hydrogen peroxide (H2O2) production on an anode combined with H2 production on a cathode increases the value of solar water splitting, but the challenge of the ...dominant oxidative product, O2, needs to be overcome. Here, we report a SnO2–x overlayer coated BiVO4 photoanode, which demonstrates the great ability to near-completely suppress O2 evolution for photoelectrochemical (PEC) H2O oxidative H2O2 evolution. Based on the surface hole accumulation measured by surface photovoltage, downward quasi-hole Fermi energy at the photoanode/electrolyte interface and thermodynamic Gibbs free energy between 2-electron and 4-electron competitive reactions, we are able to consider the photoinduced holes of BiVO4 that migrate to the SnO2–x overlayer kinetically favor H2O2 evolution with great selectivity by reduced band bending. The formation of H2O2 may be mediated by the formation of hydroxyl radicals (OH·), from 1-electron water oxidation reactions, as evidenced by spin-trapping electron paramagnetic resonance (EPR) studies conducted herein. In addition to the H2O oxidative H2O2 evolution from PEC water splitting, the SnO2–x /BiVO4 photoanode can also inhibit H2O2 decomposition into O2 under either electrocatalysis or photocatalysis conditions for continuous H2O2 accumulation. Overall, the SnO2–x /BiVO4 photoanode achieves a Faraday efficiency (FE) of over 86% for H2O2 generation in a wide potential region (0.6–2.1 V vs reversible hydrogen electrode (RHE)) and an H2O2 evolution rate averaging 0.825 μmol/min/cm2 at 1.23 V vs RHE under AM 1.5 illumination, corresponding to a solar to H2O2 efficiency of ∼5.6%; this performance surpasses almost all previous solar energy-assisted H2O2 evolution performances. Because of the simultaneous production of H2O2 and H2 by solar water splitting in the PEC cells, our results highlight a potentially greener and more cost-effective approach for “solar-to-fuel” conversion.
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