In the emerging era of Internet of Things (IoT), power sources for wireless sensor nodes in conjunction with efficient and secure wireless data transfer are required. Energy harvesting technologies are promising solution toward meeting the requirements for sustainable power sources for the IoT. In this review, we focus on approaches for harvesting stray vibrations and magnetic field due to their abundance in the environment. Piezoelectric materials and piezoelectric–magnetostrictive [magnetoelectric (ME)] composites can be used to harvest vibration and magnetic field, respectively. Currently, such harvesters use modified lead zirconate titanate (or lead-based) piezoelectric materials and ME composites. However, environmental concerns and government regulations require the development of a suitable lead-free replacement for lead-based piezoelectric materials. In the past decade, several lead-free piezoelectric compositions have been developed and demonstrated with promising piezoelectric response. This paper reviews the significant results reported on lead-free piezoelectric materials with respect to high-density energy harvesting, covering novel processing techniques for improving the piezoelectric response and temperature stability. The review of the state-of-the-art studies on vibration and magnetic field harvesting is provided and the results are used to discuss various strategies for designing high-performance energy harvesting devices.

For one-dimensional nanomaterials, the performances are strongly related to the diameters, lengths, morphologies, and structures, implying that it is of great significance to understand the related growth mechanisms and thus to achieve the desired nanostructures. Thermal oxidation of copper has been widely used to fabricate CuO nanowires (NWs), whereas the growth mechanism still remains controversial in spite of the extensive investigations. Therefore, this review aims to offer a critical discussion about the growth mechanisms. First, the effects of different growth conditions on the growth of CuO NWs are introduced for basic understanding. Subsequently, the proposed mechanisms in different literature studies, i.e., the vapor–solid, self-catalyzed growth, stress-induced growth, stress grain boundary (GB) diffusion, and oxygen concentration gradient, are discussed and summarized. It seems that the combination of “stress GB diffusion” and “oxygen concentration gradient” mechanisms could be relevant for the growth of CuO NWs via thermal oxidation of copper.

Flexibility is a key parameter of device mechanical robustness. The most profound challenge for the realization of flexible electronics is associated with the relatively low flexibility of power sources. In this article, two kinds of energy applications, which have gained increasing attention in the field of flexibility in recent years, are introduced: the lithium-ion batteries and the supercapacitors. We overview the latest progresses in flexible materials and manufacturing technology. The performances of the energy devices based on flexible materials are introduced. The advantages and disadvantages of different manufacturing processes are discussed systematically. We then focus on current technical difficulties and future prospects of research in flexibility.

For fabricating photovoltaic BaSi2 films with controlled carrier density and suppressed oxidation by thermal evaporation, the mechanism determining the film composition from incongruently evaporated BaSi2 must be elucidated. We investigated the effects of source premelting, substrate temperature, and thickness on the structural and electrical properties of evaporated BaSi2 films. It is found by room-temperature deposition that the vapor composition continuously changes from being Ba-rich to being Si-rich. Source premelting suppresses the deposition of Ba-rich vapor. Deposition at 600–700 °C shows that BaSi2 forms through the mutual diffusion of Ba and Si, followed by surface oxidation by residual gas. Surface oxidation can be suppressed by a-axis-oriented growth. By changing the film thickness, the optimum thickness to obtain homogeneous films with suppressed oxidation is revealed. Sufficient diffusion leads to high film resistivities and low electron densities, which demonstrates a close relationship between the film composition and the electrical properties.

Graphene possesses exceptional mechanical, electrical, and thermal properties that stand out for numerous applications in materials and energy-related areas. The growing demand to produce high-quality large-scale graphene films inexpensively remains a challenge. The work presented in this paper emphasizes a straightforward method of producing high-quality graphene films using cellulose as the starting materials. We demonstrate the synthesis of defect-free graphene films (as thin as ∼10 layers) on substrates up to 7 cm2 in area. Graphitic films were characterized using Infrared Raman, energy-dispersive X-ray spectroscopy, X-ray diffraction (XRD), scanning electron microcopy SEM, and high-resolution transmission electron microscopy (HRTEM). Our XRD, Raman, and HRTEM studies indicated that the synthetic temperature was critical in the synthesis of high-quality graphene films using cellulose as the carbon source material. Systematic studies revealed that defect-free large area graphitic films were produced at a synthetic temperature of ∼900 °C. The Raman D band peak intensity decreased for the samples synthesized at higher temperature but was absent for the samples prepared at 900 °C. Both the HRTEM and selected area electron diffraction confirm the highly ordered arrangement of carbon atoms in the sample matrix. The measured distance between lattice fringes was 0.335 nm, which matches with the literature reported fringe distance for the high-quality graphene. The XRD spectrum of the thin graphitic samples synthesized at 900 °C displayed a sharp diffraction peak 2θ–26.5° characteristic of highly crystalline defect-free graphene. Functional photodetector and photovoltaic (PV) devices were fabricated using graphitic films. The graphitic films were used as one of the electrodes for the PV devices yielded a power conversion efficiency of ∼1%. Our synthetic method can be potentially used for producing high-quality free-standing graphene films inexpensively at large-scale.

In this study, we report on the mechanical cleavage of conductive metal-based aluminum diboride (AlB2) flakes. The cleavage resulted in a highly single crystalline 2D material and had an atomically flat and smooth surface as shown by atomic force microscopy (AFM) and secondary ion mass spectrometry. Nanoindentation and AFM imaging of freshly cleaved specimens revealed sub-nm roughness and 30% improvement in the nanomechanical properties as compared to the as-grown AlB2 flakes. Once exposed to ambient air, the cleaved AlB2 flakes formed a superficial oxidation layer of less than 1 nm thickness within 5 min. Owing to the smooth surface, ultra-thin and stable oxide layer, and the excellent mechanical and electrical characteristics of AlB2, the cleaved flakes present an ideal 2D material for emerging applications in microfabrication such as the growth of epitaxial thin films. To prove the sub-nm surface characteristics of cleaved AlB2, a 10-nm thick TiO2 film was deposited on a freshly cleaved AlB2 using atomic layer deposition. Surface roughness and compositional consistency of this film were compared with a control sample deposited on Si. The TiO2 film on AlB2 showed a distinct thin interface layer with fewer defects than TiO2 on Si and superior flatness.

This research introduces a novel single-step approach to improve the absorbency under load (AUL) of agricultural superabsorbent polymers (SAPs) made of acrylamide. Crosslinked (acrylamide–potassium acrylate–acrylic acid) terpolymers were successfully prepared and modified via a transamidation (amide exchange) reaction. The surface of the polymer particles was treated using polyamine modifiers (i.e., diethylenetriamine and polyethyleneimine) in the presence or absence of AlCl3 as a catalyst. The modification reaction was confirmed via spectral, morphological, and rheological studies. The process variables including time and temperature, the modifier type and amount, and the catalyst concentration were found to affect the polymer swelling properties. The swelling capacity of the control and treated SAPs were determined in deionized water and saline. The AUL in saline, as a key swelling property of SAPs, was also determined. The AUL of the PEI-treated samples (19.82–24.8 g/g) was higher than those of the control (17.7 g/g) and the DETA-treated SAPs (18.3–23.5 g/g). In conclusion, the transamidation effectively improved the AUL of the terpolymer superabsorbents by about 25%.

The valence states, the distribution of Co ions, and defect structures in the Co-doped ITO films with Co concentrations of 5–13 at.% were examined by X-ray absorption spectroscopy (XAS) at Co, K, and L-edges. The structural analyses and ab initio calculations reveal that the Co atoms are substantially incorporated into the ITO lattice and form cobalt–vacancy complexes, while partial formation of Co0 species is observed for all the films. The analyses of Co–K edge XAS reveal that the Co–O bond length RCo–O is shortened and the corresponding Debye–Waller factor (σ2) obviously increases with Co doping, implying the relaxation of oxygen environment around the substitutional Co ions. The qualitative fitting of Co L3-edge XAS further confirms the coexistence of Co0 and Co2+ in the films. The Co atoms mainly occupy the substitutional sites of In2O3 lattices with the metallic Co clusters being about 20–43 at.% for the 5, 7, and 8.5 at.% Co-doped ITO films. However, a significant fraction (∼57 at.%) of metallic Co clusters is found in the 13 at.% Co-doped ITO film.

The microstructure of In(As,Bi)/GaAs heterostructures grown by low-temperature molecular beam epitaxy with special attention to the interfaces was studied by scanning/transmission electron microscopy, energy dispersive X-ray microanalysis, and X-ray diffraction and reflectivity. Two samples grown at similar conditions with and without the presence of the Bi-contained layer, formed at 350 °C, are considered. These samples were jointly analyzed to clarify Bi influence on the crystal structure. Two types of QD-like clusters at the GaAs/In(As,Bi) interface were found. The first type exhibited a zinc blend crystal structure, which is typical for A3B5 semiconductors. The second type adopted a tetragonal PbO crystal structure and was found in different orientations. The joint analysis by electron microscopy and X-ray methods demonstrated that the incorporation of Bi atoms into the InAs layer leads to the strain relaxation at the interface in the growth direction. According to electron microscopy data, this strain release is more pronounced around the clusters of the second type.

7-Decyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10) is a soluble organic semiconductor that can afford high mobility organic thin-film transistors (OTFTs). The material exhibits inherent high layered crystallinity due to the formation of bilayer-type layered-herringbone packing that involves nearly independent π-electron core layers and alkyl-chain layers within the crystals. Here, we discuss that the bottom-gate/top-contact OTFTs composed of single-crystalline Ph-BTBT-C10 channel layers exhibit noticeable effects in the device characteristics caused by the highly insulating nature of the alkyl-chain layers. Notable layer-number (n) dependence was observed in the nonlinear current–voltage characteristics and the device mobility (2–14 cm2/Vs, with TFT ideality factor 15–46%, mainly due to large threshold voltages), which can be clearly ascribed to the tunneling-based interlayer access resistance across the alkyl-chain layers. The gated-four-probe measurements of single-crystalline OTFTs also revealed quite high mobility more than 40 cm2/Vs along the channel semiconducting layer, whereas highly insulating effects due to the alkyl-chain layers were also apparent as the large hysteresis in the gate-off states of OTFTs. We discuss the whole features of the tunneling-based access resistance in the device operations of single-crystalline OTFTs, on the basis of comparison between experimental results and model simulations.

Phenol red dyed bis thiourea cadmium acetate (BTCA) crystals of ∼30 × 10 × 6 mm dimension have been grown for the first time using the slow evaporation solution technique. Diffuse reflectance measurements show absorption bands at 363 and 563 nm in the doped crystal. Optical energy gap was calculated to be 4–5 eV. Photoluminescence spectra were recorded using 320 nm excitation source. The chemical etching study was done and etch pit density was found to be reduced from 4.5 × 103/cm2 (pure) to 3.0 × 102/cm2 (dyed). Mechanical strength is increased from 74.1 kg/mm2 for pure to 94.7 kg/mm2 for dyed crystals. The enriched properties of BTCA in the presence of dye suggest that the dyed crystals will be more applicable compared to pure crystals.

Two series of binuclear metal phthalocyanine complexes M2(PcTN)2Nap and M2(PcTA)2Nap (M = Mn2+, Fe2+, Co2+, Cu2+) were designed and synthesized through the liquid solvent method and amination reaction. Elemental analysis, IR, and UV-vis spectroscopy were applied to characterize the compounds. To evaluate their catalytic performance, all the compounds were respectively added into the electrolyte of Li/SOCl2 battery systems as well as three-electrode systems for cyclic voltammetry (CV) measurements. The research studies indicate that the average discharge voltage and discharge time of the battery could be effectively enhanced by 0.2440 V and 810.7 s when compared with the battery in the absence of the compounds. As for capacities of the batteries containing catalysts, they were also found to have an improvement of 51.78–91.62%. Among the effects of diverse metal ions on the catalytic performance of phthalocyanines, the complexes whose center metal ions were Mn2+ or Co2+ exhibited relatively high catalytic performance. Meanwhile, combined with experimental results of CV analyses, the suggested catalytic mechanism of binuclear phthalocyanines for catalyzing Li/SOCl2 batteries had been proposed.

Flower-like AgI/Bi5O7I hybrid photocatalysts were fabricated via a hydrothermal method and the subsequent heating process with AgI/Bi4O5I2 as the intermediate. X-ray powder diffraction, Raman, X-ray photoelectron spectroscopy, diffuse reflectance spectra, scanning electron microscopy, transmission electron microscopy, photoluminescence, and electrochemical methods were used to reveal the structure, elemental content, morphology, and charge separation capabilities of the as-prepared samples. The photocatalytic test showed that the AgI/Bi5O7I composites own much higher photoactivity than pure AgI and Bi5O7I. Based on the result of XPS analysis, the composite is believed to be the Ag/AgI/Bi5O7I system. Due to the suitable band potentials of AgI and Bi5O7I, the ternary system can form a heterojunction structure which works in a Z-scheme mechanism with Ag nanoparticles as the transfer media. The guided charge transfer in the composite prolongs the life time of charge carriers and eventually leads to the high photocatalytic activity of AgI/Bi5O7I. Additionally, the flower-like structure of the composite also contributes to the photocatalytic reaction.

The objective of the study was to modify the external surface of commercially produced polyethylene (PE) tubes made by Balton, Poland, to improve their hydrophilic properties. The process was conducted in a new dielectric barrier discharge reactor. The carrier gases were argon and air, whereas carbon dioxide and hydrogen were the doping gases. The influence of the gas composition in the plasma chamber on the surface free energy (SFE) of PE tubes was investigated. For the gas composition 50 vol% of Ar + 50 vol% of CO2, the highest value of the SFE (53.4 mJ/m2) was obtained. It means an increase in SFE approx. 17% as compared to the unmodified sample (46.0 mJ/m2). Fourier-transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR) results indicates that on the surface of the tubes, carboxyl, carbonyl, and hydroxyl groups were formed. Those oxygen-containing groups could be responsible for the increase of the hydrophilic effect. The O/C ratio on the surface, measured by the X-ray photoelectron spectroscopy method, was three times higher in the case of the modified samples than in those which were not subjected to plasma treatment.

Palladium (Pd) and gold (Au) nanoparticles (NPs) hybridized on two types of carbon supports, graphene and granular activated carbon (GAC), were shown to be promising catalysts for the sustainable hydrodehalogenation of aqueous trichloroethylene (TCE). These catalysts are capable of degrading TCE more rapidly than commercial Pd-on-GAC catalysts. The catalysts were synthesized at room temperature without the use of any environmentally unfriendly chemicals. Pd was chosen for its catalytic potency to break down TCE, while Au acts as a strong promoter of the catalytic activity of Pd. The results indicate that both graphene and GAC are favorable supports for the NPs due to high surface-to-volume ratios, unique surface properties, and the prevention of NP aggregation. The properties of NP catalysts were characterized using electron microscopy and spectroscopy techniques. The TCE degradation results indicate that the GAC-supported catalysts have a higher rate of TCE removal than the commercial Pd-on-GAC catalyst, and the degradation rate is greatly increased when using graphene-supported samples.

A series of Ba-modified MnCeOx/TiO2 catalysts were prepared by a wet impregnation method and tested for selective catalytic reduction (SCR) of NO with NH3 at low temperature. The results showed that Ba additives obviously improved the catalytic performance of the MnCeOx/TiO2 catalyst for NH3-SCR, and the BaMnCeOx/TiO2 catalyst with 3 wt% BaO exhibited the optimal catalytic performance. Moreover, the introduction of Ba also improved the resistances toward water vapor and SO2 of catalysts. The N2 adsorption, H2-TPR, and X-ray photoelectron spectroscopy results showed that the addition of Ba increased the specific surface area, redox properties, and concentrations of surface Mn4+ and chemisorbed oxygen of catalysts. Furthermore, NH3-TPD and NO-TPD were used to investigate the absorption of NH3 and NO on the catalyst. The results revealed that although the introduction of Ba significantly promoted the adsorption of NO, it also inhibited the adsorption of NH3. Consequently, the catalytic performance of MnCeOx/TiO2 was greatly improved with the Ba additives.

The topological insulator/superconductor heterostructure is one of the most promising platforms to create and manipulate Majorana bound states. Here, we used molecular beam epitaxy to grow high-quality (Bi0.5Sb0.5)2Te3 films on Nb surfaces. To promote proper (Bi0.5Sb0.5)2Te3 film nucleation in the early growth stage, we developed a two-step growth method. Bi, Sb, and Te clusters were first evaporated at a low temperature of 180 °C, which is below the typical growth temperature and then annealed to form a crystalized passivation layer. Second, a standard (Bi0.5Sb0.5)2Te3 film was grown under the normal deposition temperature of 280 °C. We used reflection high-energy electron diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction to further characterize the (Bi0.5Sb0.5)2Te3 film and passivation layer quality. Finally, the top Nb film was laid down by magnetron sputtering at room temperature. The hetero-Nb/epitaxial (Bi0.5Sb0.5)2Te3/Nb stacks were further fabricated into micro-Josephson junctions and showed clear Josephson currents demonstrating an excellent material quality.

In this paper, Sm3+-doped silicate glasses containing AgNO3 were obtained by the common melting quenching method. Influence of AgNO3 concentration on the absorption and emission characteristics of Sm3+ were systematically investigated. With the increase of AgNO3 content from 0 to 3.0 wt%, the ultraviolet region absorption edge shows a slight blue-shift from 275 to 260 nm. Exciting by 255 nm, the visible emission intensity of Sm3+-doped silicate glass containing 0.5 wt% AgNO3 was about 31 times stronger than that of Sm3+ singly doped silicate glass. Fluorescence decay curves for the visible emission followed double exponential decay. Two fluorescence lifetimes were obtained, one was about 7–20 μs which was comparable with the lifetimes of 350 nm emission which derived from Ag+, another was about 2 ms which was comparable with that of the visible emission from Sm3+ excited by 401 nm. Thus, the significant enhancement visible emission of Sm3+ excited by 255 nm can be ascribed to the energy transfer from Ag+ to Sm3+.