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In this work, we, for the first time, observed the remarkable thermoelectric properties of a few high-quality PtSe2 layered films fabricated by a post selenization of Pt thin films. An excellent po...

Oxygen release and irreversible cation migration are the main causes of voltage fade in Li-rich transition metal oxide cathode. But their correlation is not very clear and voltage decay is still a bottleneck. Herein, we modulate the oxygen anionic redox chemistry by constructing Li2ZrO3 slabs into Li2MnO3 domain in Li1.21Ni0.28Mn0.51O2, which induces the lattice strain, tunes the chemical environment for redox-active oxygen and enlarges the gap between metallic and anionic bands. This modulation expands the region in which lattice oxygen contributes capacity by oxidation to oxygen holes and relieves the charge transfer from anionic band to antibonding metal–oxygen band under a deep delithiation. This restrains cation reduction, metal–oxygen bond fracture, and the formation of localized O2 molecule, which fundamentally inhibits lattice oxygen escape and cation migration. The modulated cathode demonstrates a low voltage decay rate (0.45 millivolt per cycle) and a long cyclic stability., Voltage fade is a critical issue for Li-rich transition metal oxide cathode. Here, the authors modulate the oxygen anionic redox chemistry and enlarges the gap between metallic and anionic states by constructing Li2ZrO3 slabs into Li2MnO3 domain in Li1.21Ni0.28Mn0.51O2 which fundamentally suppresses the voltage decay.

Anionic redox chemistry endows Li-rich layered oxide cathode with high specific capacity, but it also causes some critical issues such as voltage decay, structure degradation, and irreversible oxyg...

Tuning the anionic redox chemistry (O2− → O2n−) activity and reversibility by crystal and/or electronic modulation is essential for Li-rich oxide cathode materials. Herein, we report a facile strategy to improve the activity and reversibility of both anionic and cationic redox by integrating oxygen vacancies and the spinel phase. The initial specific capacity (216.1 mA h g−1vs. 316.3 mA h g−1), coulombic efficiency (80% vs. 94.8%), long-term cycling stability (1000 cycles at 5C) and voltage decay have all been greatly improved due to the largely suppressed irreversible oxygen release. The underlying modulation mechanism has been unraveled. Firstly, the introduction of oxygen vacancies decreases the covalency of TM–O and the density of states of the O 2p band, which mitigates the irreversible oxygen release during oxygen redox. Secondly, the spinel phase integration induced by oxygen vacancies not only improves the Li-ion conductivity and the rate capability due to its 3D Li+ channel and the expanded Li layer but also enhances the structural stability. Thirdly, the first-principles calculations indicate that the increase of delocalized electrons around the transition metal also intensifies the MnO6 octahedral distortion and the inactive Mn-ions are partially activated during the first cycle and participate in the charge compensation. This study sheds some new light on designing high-performance Li-rich layered oxide cathode materials by regulating the anionic and cationic redox with the incorporation of oxygen vacancies and the spinel phase.

Germanium tellurides and its pseudo binary compounds offer interesting properties that are important in thermoelectric and phase-change applications. Despite being a class of materials under scrutiny since its discovery, unique properties and functionalities kept on emerging in recent years. In this work, we observed another unique property of Ge-Sb-Te (GST) thin film that can be beneficial in its development for thermoelectric application. A rapid heating and quenching process of the GST film resulted in a metastable rock-salt cubic structure, exhibiting a unique electronic-transition-like behavior. Above the transition temperature at 150 °C, we observed a temperature-induced band modulation, corroborated with changes in its effective mass and valence band position that leads to favorable electronic and thermoelectric properties. Charge transfer between Sb and Te occurred, accompanied by a distorted cubic-to-cubic structural change. The interplay of the electronic and lattice structure born out of the co...

This work demonstrates the first example of interfacial manipulation in a hybrid photocatalyst based on poly(3-hexylthiophene-2,5-diyl) (P3HT) nanoparticle and graphene oxide (GO) bulk heterojunctions to efficiently reduce CO2 into selective industrial hydrocarbons under gas-phase reaction and visible-light illumination. High selectivity of chemical products (methanol and acetaldehyde) was observed. Moreover, the hybrid photocatalyst’s solar-to-fuel conversion efficiency was 13.5 times higher than that of pure GO. The increased production yield stems from the co-catalytic and sensitizing role of P3HT in the hybrid system due to its ability to extend light absorption to the visible range and improve interfacial charge transfer to GO. The hybrid P3HT-GO formed a type II heterojunction, and its static and dynamic exciton behaviors were examined using fluorescence spectroscopy and exciton lifetime mapping. A reduced fluorescence decay time was observed by interfacial manipulation for improved dispersion, indicating a more efficient charge transfer from the excited P3HT to GO. Thus, the conducting polymer nanoparticles, 2D nanocarbon, have demonstrated superior performance as a metal-free, non-toxic, low-cost, and scalable heterogeneous photocatalyst for CO2 reduction to solar fuel, a solid–gas system.

Layered MoS2 is considered as one of the most promising two-dimensional photocatalytic materials for hydrogen evolution and water splitting; however, the electronic structure at the MoS2-liquid interface is so far insufficiently resolved. Measuring and understanding the band offset at the surfaces of MoS2 are crucial for understanding catalytic reactions and to achieve further improvements in performance. Herein, the heterogeneous charge transfer behavior of MoS2 flakes of various layer numbers and sizes is addressed with high spatial resolution in organic solutions using the ferrocene/ferrocenium (Fc/Fc+) redox pair as a probe in near-field scanning electrochemical microscopy, i.e. in close nm probe-sample proximity. Redox mapping reveals an area and layer dependent reactivity for MoS2 with a detailed insight into the local processes as band offset and confinement of the faradaic current obtained. In combination with additional characterization methods, we deduce a band alignment occurring at the liquid-solid interface., Here, high-resolution atomic force microscopy and scanning electrochemical microscopy are used to investigate the electron transfer behaviour of layered MoS2 flakes in organic solutions, offering insights on the electronic band alignment at the solid-liquid interface.

Nonnoble metal catalysts are low-cost alternatives to Pt for the oxygen reduction reactions (ORRs), which have been studied for various applications in electrocatalytic systems. Among them, transition metal complexes, characterized by a redox-active single-metal-atom with biomimetic ligands, such as pyrolyzed cobalt–nitrogen–carbon (Co–Nx/C), have attracted considerable attention. Therefore, we reported the ORR mechanism of pyrolyzed Vitamin B12 using operando X-ray absorption spectroscopy coupled with electrochemical impedance spectroscopy, which enables operando monitoring of the oxygen binding site on the metal center. Our results revealed the preferential adsorption of oxygen at the Co2+ center, with end-on coordination forming a Co2+-oxo species. Furthermore, the charge transfer mechanism between the catalyst and reactant enables further Co–O species formation. These experimental findings, corroborated with first-principle calculations, provide insight into metal active-site geometry and structural evolution during ORR, which could be used for developing material design strategies for high-performance electrocatalysts for fuel cell applications., Understanding active-site geometry and structural evolution during electrocatalysis is important for further development. Here the authors use operando X-ray absorption spectroscopy combined with electrochemical impedance spectroscopy to investigate single atom catalysts derived from Vitamin B12.

In this paper, thermoelectric performance of the ZnO thin films were investigated via doping (Gallium - Ga and Indium - In) and post-thermal treatment. Various kinds of defects, such as zinc (Zn) vacancies, Ga substitute for Zn, zinc interstitials, oxygen vacancies, oxygen interstitials, oxygen antisite, were induced in the ZnO thin films by doping with Ga, In and post-thermal treatment. Annealed thin films have better thermoelectric performance than unannealed thin films. The figure of merit ZT values at 300 °C were 0.114 and 0.186 for the single Ga-doped ZnO (GZO@500) and dually In, Ga doped ZnO (IGZO@500) thin films annealed at 500 °C, respectively. Higher concentration of zinc interstitial led the GZO@500 thin film to have larger power factor and larger total thermal conductivity, resulting in lower ZT than the IGZO@500 thin film. To the best of our knowledge, the ZT value of 0.186 at 300 °C obtained from our IGZO@500 thin films is the highest ever reported for n-type ZnO based thin films.

Applications of lithium ion battery have been hampered by a lack of ideal anode materials in terms of capacity and stability. The emergence of metal chalcogenide as a candidate material has reinvigorated the search of a low cost and high capacity material system. However, debate about the underlying mechanisms and overall appraisal of its usage in lithium ion battery system remains. Here, a comprehensive study on the energy storage mechanism of copper zinc tin sulfide (CZTS) nanowalls possessing ultrahigh rate capability (500 mAh g−1 charged within 60 s) is reported. Structural evolutions along with the accompanying changes in the oxidation state upon charge/discharge were monitored by ex-situ X-ray diffraction and X-ray photoelectron spectroscopy. During lithiation, lithium ion reacted with CZTS to form lithium sulfides. At the same time, a sequential conversion reactions of copper, zinc and tin sulfides enabled the CZTS nanowalls to achieve excellent electrochemical performance (1400 mAh g−1 at a current density of 1000 mA g−1 over 400 cycles). Multi-element metal chalcogenides in conjunction with an adhesion-enhancing seed layer and a rational nanostructure design hold the key to such ultrahigh capacity and stable anode materials for next generation energy storage devices.

We investigated the effect of single and multidopants on the thermoelectrical properties of host ZnO films. Incorporation of the single dopant Ga in the ZnO films improved the conductivity and mobility but lowered the Seebeck coefficient. Dual Ga- and In-doped ZnO thin films show slightly decreased electrical conductivity but improved Seebeck coefficient. The variation of thermoelectric properties is discussed in terms of film crystallinity, which is subject to the dopants' radius. Small amounts of In dopants with a large radius may introduce localized regions in the host film, affecting the thermoelectric properties. Consequently, a 1.5 times increase in power factor, three times reduction in thermal conductivity, and 5-fold enhancement in the figure of merit ZT have been achieved at 110 °C. The results also indicate that the balanced control of both electron and lattice thermal conductivities through dopant selection are necessary to attain low total thermal conductivity.

The performance of the recently commissioned spectrometer PEAXIS for resonant inelastic soft X-ray scattering (RIXS) and X-ray photoelectron spectroscopy (XPS) and its hosting beamline U41-PEAXIS at the BESSY II synchrotron are characterized. The beamline provides linearly polarized light from 180 eV - 1600 eV allowing for RIXS measurements in the range of 200 eV - 1200 eV. The monochromator optics can be operated in different configurations for the benefit of either high flux, providing up to $10^{12}$ photons/s within the focal spot at the sample, or high energy resolution with a full width at half maximum of, Comment: 12 pages, 14 figures

In this work, we prepare H and Ga co-doped ZnO (HGZO) films by a sputtering method with over 80% transparency in 400–1100 nm wavelength range and a sheet resistance as low as 3.9 Ω/sq. Hydrogen is recognized as a key element to enhance transmittance and conductivity. Spectroscopic evidence has indicated that hydrogen assists in substituting Ga3+ for Zn2+ (GaZn) in ZnO lattice; it produces more effective GaZn donors and increases electron concentration. We propose for the first time based on interstitial and substitutional hydrogen configurations that the interactions of Zn atom with its neighboring hydrogen atoms can weaken Zn–O bonds in ZnO lattice, and thus facilitate the substitution of Ga3+ for Zn2+. Furthermore, hydrogen can improve the quality of crystalline grains by lowering point-defect density such as Ga, Zn interstitials, Zn, and O vacancies, which strongly enhance electron mobility in HGZO films. The highest mobility of 48.6 cm2/Vs was obtained in the best-performing film.

A high-voltage LiCoO2 cathode material has been increasingly studied due to its high capacity; however, the structural instability of this material results in poor cycling performance at high voltages, restricting its application. We use a controlled Mg doping strategy to modulate the crystal and electronic structure of the material and unravel the different effects of Mg occupation at Li and Co sites on the structure and electrochemical performance of LiCoO2. Mg substitution at Li sites provides much better electrochemical performance than Mg substitution at Co sites. Compared with bare LiCoO2, the substitution of Mg for Li and Co significantly enhances the capacity retention from 0.5% to 58.6% and 85.6% (500 cycles at 5C), respectively. Mg substitution at Li sites provides a “pillar’ to stabilize the layered structure and increases the interlayer spacing (I(LiO2)) to reduce the energy barrier for Li+ migration. The stress and strain on the crystal structure caused by the substantial expansion and contraction during cycling are alleviated, while the stability of oxygen in the Li0.96Mg0.04CoO2 sample is enhanced; additionally, the destruction of the CoO6 octahedron is also significantly inhibited, all of which confirm the increase in stabilization due to the Mg substitution in LiCoO2. This study offers some insights on the distinct effects of the same dopant at different crystal sites, which is instructive to develop a precisely controlled doping strategy.

Phase-change memory materials such as the pseudobinary GeTe-Sb2Te3 compounds have recently gained attention for their good thermoelectric properties, which can be used for power-generation/cooling applications. In this work, GeTe-rich Ge–Sb–Te thin films deposited using a radio-frequency magnetron sputtering method readily exhibit the metastable face-centered cubic (FCC) phase at room temperature. This is in stark contrast to its bulk form, which only transforms to its FCC phase after a transition temperature of around 350 °C. Based on previous works, the FCC phase contributes to the superior thermoelectric properties of this material system. In this study, by decreasing the working deposition pressure, the preferred orientation of (200) plane is observed that translates to improved carrier mobility. Moreover, increasing the annealing temperature has been shown to decrease the carrier concentration due to Te deficiency, leading to a significant improvement in the Seebeck coefficient of the film. By combining these effects, an optimized thermoelectric power factor (21 μW/cm K2) was obtained at an operating temperature of 350 °C.

We present a systematic study in investigating the orientation characteristics of pentacene molecules grown on graphene substrates using polarized Raman spectroscopy. The substrate-induced orientation alignment of pentacene can be well distinguished through the polarized Raman spectra. Interestingly, we found that the nature of polycrystalline graphene not only provides efficient route to control molecular orientation, but also acts as an excellent template allowing conjugated molecules to stack accordingly. The relative orientation of the well-aligned pentacene molecules and the nearby graphene domains exhibits several preferred angles due to atomic interactions. This unique feature is further examined and verified by single domain graphene. Furthermore, polarized Raman spectroscopy contains abundant information allowing us to analyze the ordering level of pentacene films with various thicknesses, which provides insightful perspectives of manipulating molecular orientations with graphene and spatial orga...

We synthesized a bismuth-rich bismuth sulfide (Bi2+xS3) to improve the electrical conductivity. A percolated composite was obtained by mixing Bi2+xS3 with commercial Bi2S3 so that the other thermoelectric parameters were not compromised. The Bi2+xS3 acted as a conductive percolating channel within the Bi2S3 matrix. This percolation approach retained the high Seebeck coefficient of the Bi2S3 matrix while improving the electrical conductivity. A dimensionless figure of merit (zT) up to five times that of the pure Bi2S3 sample was achieved.

Manipulating the nature of defects and carrier dynamics in kesterite Cu2ZnSnS4 (CZTS) solar cells is challenging because of the complex behavior of defects. Among the various strategies used to reduce the defect levels, sodium-incorporated CZTS absorbers are effective and beneficial for defect passivation. In this study, we proposed a bifacial sodium-incorporated treatment (BSIT) in CZTS for the control of defect levels. The defect energy levels of the absorber measured by admittance spectroscopy decrease from 263 to 112 meV with increasing Na contents. In addition, impedance measurements of the sample after the BSIT showed an improved carrier dynamics with a prolonged minority carrier lifetime from 0.9 to 1.8 μs. This suggests that the post treatment of NaF diffused from the top and bottom surfaces of the CZTS absorbers is beneficial for carrier transport behaviors. Our results demonstrated that by manipulating the sodium content in the BSIT, defect passivation and effective reduction of carrier recombination can be attained, resulting in enhanced CZTS device performance from 4.1% to 5.6%.

Germanium antimony telluride (GeSbTe or GST), a popular material in optical and non-volatile memory devices, attracted renewed attention due to its potential for thermoelectric applications. In this study, we have employed a two-stage engineering process to enhance the thermoelectric properties of GeTe-rich GeSbTe. First, we introduced vacancy into the material by modifying the germanium content without disrupting the crystal structure. This influenced the electronic properties of the GeTe-rich GeSbTe and improved the overall dimensionless figure of merit, zT, from 0.7 to 1.1. Second, we rapidly cooled the material after annealing, further enhancing the zT value from 1.1 to 1.48 – one of the highest values reported for this material. In-depth studies suggest that disorder in the crystal structure was created via rapid cooling enhanced phonon scattering and effectively reduced the thermal conductivity, which, in turn, enhanced the thermoelectric performance.

Current research has demonstrated the combination of the high theoretical capacity offered by silicon nanoparticles (Si NPs) with graphene-based matrices in order to produce a high energy density and stable lithium ion battery system. However, Si NPs do not mix well with graphene oxide aqueous suspensions and thus create severe segregation of the materials. In this letter, we propose a simple, cost-effective and commercially-viable solvent exchange process to improve the interaction of the Si NPs in poor solvent environments such as in aqueous media. Using N-methylpyrrolidone (NMP) to disperse the Si NPs and followed by a solvent exchange process, we are able to improve the dispersion and stability of Si NPs in aqueous graphene oxide aqueous suspension. Consequently, this also improves the output and the stability of the lithium ion battery using the aforementioned composite system.

Transporting buffer layers are important components of polymer-based organic photovoltaic devices. In this study, we have investigated the effects of the oxidation state in copper oxide based buffer layer in conjunction to its role in device performance. We have shown that variation in the oxidation state affects the band alignment and built-in voltage of the device, therefore leading to variation in device performance. Specifically, the fully oxidized copper oxide buffer layer has a valence band position at 5.12 eV, much closer to the highest occupied molecular orbital of poly(3-hexylthiophene-2,5-diyl) (P3HT) (∼5.2 eV), giving a best fill factor and efficiency at 57% and 4.06%, respectively. Lastly, we also demonstrate significant enhancement in device stability, with power conversion efficiency maintained at 75% of the original value even after 40 days, and propose a strategy for recovering the device performance based on the observed property of the oxide buffer layer.

Li2CuO2 is a typical charge transfer insulator with CuO2 chains that are composed of edge-shared CuO4 plaquettes. The existence of oxygen vacancies for single crystals prepared under various oxygen partial pressures has been confirmed by the chemical and thermogravimetric analyses. The puzzling discovery of extra magnetic moment near the oxygen site by earlier neutron scattering studies has been verified by a thorough Curie–Weiss law analysis of spin susceptibilities, and resolved quantitatively with a molecular orbital model of edge-sharing CuO2 chains containing oxygen vacancies.

A low-cost and green method of producing nanocrystalline silicon thin films is presented. Using a magnesiothermic reduction process, we have successfully converted the surface of soda lime glass directly into silicon thin film. Furthermore, by varying reaction time, the amount of silicon produced in thin film form (or layer thickness) can be controlled. The nanocrystalline silicon thin films on glass were characterized using scanning electron microscopy, energy dispersive spectroscopy and Raman spectroscopy. Finally, the optical properties of the thin films derived at different reaction times were also measured. The band gaps of the synthesized thin films were within the range of 2.3–2.5 eV.