Correction to "Metallic Germanium (111) Slab Structures".

[This corrects the article DOI: 10.1021/acsomega.3c03191.].

Lead-free europium and ytterbium perovskites.

Halide perovskite is a material with fantastic properties that could substantially impact next-generation optoelectronics. However, toxic Pb cations in the most excellent and stable perovskites have raised environmental concerns and hindered their commercialization. A new lead-free perovskite material needs to be discovered and address this issue. This research predicts that MAEuCl3 and MAYbCl3 are dynamic stable structures with wide band gap properties, making them promising lead-free perovskites. The study discusses these two new perovskites' XRD, electrical conductivity, and carrier mobility. Based on the properties of Eu2+ and Yb2+ perovskites, the iodide-based variant is suitable for device operation within the temperature range of 400 K to 600 K. The bromide-based perovskite is appropriate for temperatures ranging from 500 K to 700 K. In comparison, the chloride-based perovskite can be used for device operation in the higher temperature range of 800 K to 1100 K. Additionally, this research identifies the dynamic stability issue of lanthanide cations in halide perovskites, providing a reference for future lead-free perovskite applications.

Metallic Germanium (111) Slab Structures.

Prior research has indicated that the surface electron conductivity of Ge (111) wafers surpasses that of Ge (100) and Ge (110) wafers. This disparity has been ascribed to the variations in bond length, geometry, and frontier orbital electron energy distribution across different surface planes. The ab initio molecular dynamics (AIMD) simulation is used for the thermal stability of the Ge (111) slabs with different thicknesses and has provided new knowledge of its potential applications. To delve deeper into the properties of Ge (111) surfaces, we executed calculations for one- and two-layer Ge (111) surface slabs. The electrical conductivities of these slabs at room temperature were determined to be 966081.89 and 760157.03 Ω-1 m-1, respectively, with a unit cell conductivity of 1.96 Ω-1 m-1. These findings align with actual experimental data. Notably, the electrical conductivity of the single-layer Ge (111) surface exceeded that of intrinsic Ge by 100,000 times, heralding intriguing potential for including Ge surfaces in future device applications.

Stabile fluoro-benzene-based spacer for lead-free Dion-Jacobson perovskites.

Two-dimensional perovskite materials have been investigated as potential candidates for next-generation-wide band gap devices and lead-based perovskites are the most common materials within two-and three-dimensional structures due to their superior optoelectronic properties. Nevertheless, the stability and toxic element issues are the two significant shortcomings of device commercialization. The fluoro-benzene-based divalent ammonium spacer cations and replacing Zn2+ with Pb2+ will improve the two-dimensional perovskite stability. These stable lead-free wide band gap two-dimensional structures have better carrier mobility at high-temperature regions. Therefore, lead-free two-dimensional perovskites might be suitable for higher temperatures optoelectronic applications.

Expression of concern: Surface-dependent band structure variations and bond deviations of GaN.

Expression of concern for 'Surface-dependent band structure variations and bond deviations of GaN' by Chih-Shan Tan et al., Phys. Chem. Chem. Phys., 2022, 24, 9135-9140, https://doi.org/10.1039/D2CP00100D.

Optoelectronic Properties Prediction of Lead-Free Methylammonium Alkaline-Earth Perovskite Based on DFT Calculations.

Dynamical stability plays an essential role in phase transition and structure, and it could be a fundamental method of discovering new lead-free perovskite materials. The perovskite materials are well-known for their excellent optoelectronic properties, but the lead element inside could be a hindrance to future development. This research is trying to predict the promising cation candidates in the high-temperature application for lead-free perovskite materials from the replacement of lead in MAPbCl3 (MA = methylammonium) with alkaline-earth cations. The alkaline-earth cations are of a stable positive divalent sort, which is the same as Pb, and most of them are abundant in nature. Therefore, by improving the dynamical stability, the Mg2+, Ca2+, and Sr2+ cations replacement of lead ions could stabilize the perovskite structure by decreasing the imaginary part of phonon density of states. Finally, the density functional theory results show that the MACaCl3 could be a dynamic stable lead-free methylammonium perovskite material with an ultrawide band gap (5.96 eV).

Surface-dependent band structure variations and bond deviations of GaN.

Density functional theory (DFT) calculations on a tunable number of GaN (0001) planes give an invariant band structure, density of states (DOS) diagram, and band gap of the GaN unit cell. Dissimilar band structures and DOS diagrams are obtained for 1, 3, 5, 7, and 9 layers of GaN (101̄0) planes, but the same band structure as that of the (0001) plane returns for 2, 4, 6, and 8 (101̄0) planes. Furthermore, 1 to 4 layers of GaN (101̄1) planes exhibit dissimilar band structures, but the GaN unit cell band structure is obtained for 5 (101̄1) planes. While there are no changes to the Ga-N bond length and bond geometry for the (0001) planes, the (101̄0) planes present bond length variation and bond distortion with odd numbers of layers. Bond length and bond direction deviations are also obtained for 1 to 4 (101̄1) planes. These results suggest that slight structural deviations may be present near the GaN surface to produce facet-dependent properties, and such atomic position deviations in the surface layer can be observed in various semiconductors.

Density Functional Theory Study of Metallic Silicon (111) Plane Structures.

The band structure on the surface might be influenced by the abruptly ended periodic structure and change the physical properties of the semiconductor. By using the density functional theory, this research also demonstrates that the Si unit cell has the calculated room-temperature electrical conductivity as 4.01 × 10-6 (Ω-1 cm-1), similar to the experimental result. Thus, the Si(111) plane structures are calculated, and we found out that the one-layer and two-layer plane structures have the theoretical room-temperature electrical conductivities as 129.68 (Ω-1 cm-1) and 547.80 (Ω-1 cm-1), respectively. In addition, the results reveal that the conduction band and valance band of the Si(111) one-layer and two-layer structures will connect on the ⟨111⟩ direction, mainly contributed by Si 3p orbitals. Thus, the band structure at the ⟨111⟩ direction on the Si(111) surface has variation and increases the electrical conductivity to 7 to 8 orders compared to the intrinsic Si and offers new surface science and surface engineering concepts for future applications.

Transition Metal Ions in Methylammonium Chloride Perovskites.

Organic-inorganic perovskite materials have become star materials for future wide band gap optoelectronics due to their excellent optical and electrical properties. However, the lead ions inside perovskites have become a crucial environmental issue in the commercialization of wide band gap perovskite devices . This research tries to find the structure and properties of lead-free perovskite materials by screening Sn2+ and transition-metal ions to replace Pb2+ within the methylammonium (MA)-based chloride perovskite and find out a new two-dimensional structure of MA-based transition-metal ion chlorides. Overall, MAZnCl3 may be a potential ultraviolet-C luminescent material with a stable two-dimensional structure with a wide band gap of 5.64 eV, which is suitable for ultraviolet-C luminescence applications.

Current Rectification and Photo-Responsive Current Achieved through Interfacial Facet Control of Cu2O-Si Wafer Heterojunctions.

Conductive atomic force microscopy (C-AFM) was employed to perform conductivity measurements on a facet-specific Cu2O cube, octahedron, and rhombic dodecahedron and intrinsic Si {100}, {111}, and {110} wafers. Similar I-V curves to those recorded previously using a nanomanipulator were obtained with the exception of high conductivity for the Si {110} wafer. Next, I-V curves of different Cu2O-Si heterostructures were evaluated. Among the nine possible arrangements, Cu2O octahedron/Si {100} wafer and Cu2O octahedron/Si {110} wafer combinations show good current rectification behaviors. Under white light illumination, Cu2O cube/Si {110} wafer and Cu2O rhombic dodecahedron/Si {111} wafer combinations exhibit the largest degrees of photocurrent, so such interfacial plane-controlled semiconductor heterojunctions with light sensitivity can be applied to make photodetectors. Adjusted band diagrams are presented highlighting different interfacial band bending situations to facilitate or inhibit current flow for different Cu2O-Si junctions. More importantly, the observation of clear current-rectifying effects produced at the semiconductor heterojunctions with properly selected contacting faces or planes implies that novel field-effect transistors (FETs) can be fabricated using this design strategy, which should integrate well with current chip manufacturing processes.

Pentafluoropyridine functionalized novel heteroatom-doped with hierarchical porous 3D cross-linked graphene for supercapacitor applications.

The fabrication with high energy density and superior electrical/electrochemical properties of hierarchical porous 3D cross-linked graphene-based supercapacitors is one of the most urgent challenges for developing high-power energy supplies. We facilely synthesized a simple, eco-friendly, cost-effective heteroatoms (nitrogen, phosphorus, and fluorine) co-doped graphene oxide (NPFG) reduced by hydrothermal functionalization and freeze-drying approach with high specific surface areas and hierarchical pore structures. The effect of different heteroatoms doping on the energy storage performance of the synthesized reduced graphene oxide is investigated extensively. The electrochemical analysis performed in a three-electrode system via cyclic voltammetry (CV), galvanostatic charging-discharging (GCD), and electrochemical impedance spectroscopy (EIS) demonstrates that the nitrogen, phosphorous, and fluorine co-doped graphene (NPFG-0.3) synthesized with the optimum amount of pentafluoropyridine and phytic acid (PA) exhibits a notably enhanced specific capacitance (319 F g-1 at 0.5 A g-1), good rate capability, short relaxation time constant (τ = 28.4 ms), and higher diffusion coefficient of electrolytic cations (Dk+ = 8.8261 × 10-9 cm2 s-1) in 6 M KOH aqueous electrolyte. The density functional theory (DFT) calculation result indicates that the N, F, and P atomic replacement within the rGO model could increase the energy value (G T) from -673.79 eV to -643.26 eV, demonstrating how the atomic level energy could improve the electrochemical reactivity with the electrolyte. The improved performance of NPFG-0.3 over NFG, PG, and pure rGO is mainly ascribed to the fast-kinetic process owing to the well-balanced electron/ion transport phenomenon. A symmetric coin cell supercapacitor device fabricated using NPFG-0.3 as the anode and cathode material with 6 M KOH aqueous electrolyte exhibits maximum specific energy of 38 W h kg-1, a maximum specific power of 716 W kg-1, and ∼88.2% capacitance retention after 10 000 cycles. The facile synthesis approach and promising electrochemical results suggest this synthesized NPFG-0.3 material has high potential for future supercapacitor application.

Accelerated discovery of CO2 electrocatalysts using active machine learning.

The rapid increase in global energy demand and the need to replace carbon dioxide (CO2)-emitting fossil fuels with renewable sources have driven interest in chemical storage of intermittent solar and wind energy1,2. Particularly attractive is the electrochemical reduction of CO2 to chemical feedstocks, which uses both CO2 and renewable energy3-8. Copper has been the predominant electrocatalyst for this reaction when aiming for more valuable multi-carbon products9-16, and process improvements have been particularly notable when targeting ethylene. However, the energy efficiency and productivity (current density) achieved so far still fall below the values required to produce ethylene at cost-competitive prices. Here we describe Cu-Al electrocatalysts, identified using density functional theory calculations in combination with active machine learning, that efficiently reduce CO2 to ethylene with the highest Faradaic efficiency reported so far. This Faradaic efficiency of over 80 per cent (compared to about 66 per cent for pure Cu) is achieved at a current density of 400 milliamperes per square centimetre (at 1.5 volts versus a reversible hydrogen electrode) and a cathodic-side (half-cell) ethylene power conversion efficiency of 55 ± 2 per cent at 150 milliamperes per square centimetre. We perform computational studies that suggest that the Cu-Al alloys provide multiple sites and surface orientations with near-optimal CO binding for both efficient and selective CO2 reduction17. Furthermore, in situ X-ray absorption measurements reveal that Cu and Al enable a favourable Cu coordination environment that enhances C-C dimerization. These findings illustrate the value of computation and machine learning in guiding the experimental exploration of multi-metallic systems that go beyond the limitations of conventional single-metal electrocatalysts.

Heterogeneous Supersaturation in Mixed Perovskites.

Thin-film solar cells based on hybrid lead halide perovskites have achieved certified power conversion efficiencies exceeding 24%, approaching those of crystalline silicon. This motivates deeper studies of the mechanisms that determine their performance. Twin defect sites have been proposed as a source of traps in perovskites, yet their origin and influence on photovoltaic performance remain unclear. It is found that twin defects-observed herein via both transmission electron microscopy and X-ray diffraction-are correlated with the amount of antisolvent added to the perovskite and that twin defects in the highest-performing perovskite photovoltaics are suppressed. Heterogeneous supersaturation nucleation is discussed as a contributor to efficient perovskite-based optoelectronic devices.

Enhanced Nitrate-to-Ammonia Activity on Copper-Nickel Alloys via Tuning of Intermediate Adsorption.

Electrochemical conversion of nitrate (NO3-) into ammonia (NH3) recycles nitrogen and offers a route to the production of NH3, which is more valuable than dinitrogen gas. However, today's development of NO3- electroreduction remains hindered by the lack of a mechanistic picture of how catalyst structure may be tuned to enhance catalytic activity. Here we demonstrate enhanced NO3- reduction reaction (NO3-RR) performance on Cu50Ni50 alloy catalysts, including a 0.12 V upshift in the half-wave potential and a 6-fold increase in activity compared to those obtained with pure Cu at 0 V vs reversible hydrogen electrode (RHE). Ni alloying enables tuning of the Cu d-band center and modulates the adsorption energies of intermediates such as *NO3-, *NO2, and *NH2. Using density functional theory calculations, we identify a NO3-RR-to-NH3 pathway and offer an adsorption energy-activity relationship for the CuNi alloy system. This correlation between catalyst electronic structure and NO3-RR activity offers a design platform for further development of NO3-RR catalysts.

Facet-Dependent Surface Trap States and Carrier Lifetimes of Silicon.

The facet-dependent electrical conductivity properties of silicon wafers result from significant band structure differences and variations in bond length, bond geometry, and frontier orbital electron distribution between the metal-like and semiconducting planes of silicon. To further understand the emergence of conductivity facet effects, electrochemical impedance measurements were conducted on intrinsic Si {100}, {110}, and {111} wafers. The attempt-to-escape frequency, obtained from temperature-dependent capacitance versus applied frequency curves, and other parameters derived from typical semiconductor property measurements were used to construct a diagram of the trap energy level (Et) and the amount of trap states Nt(Et). The trap states are located 0.61-0.72 eV above the silicon conduction band. Compared to {100} and {110} wafers, Si {111} wafer shows far less densities of trap states over the range of -0.2 to 2 V. Since these trap states inhibit direct electron excitation to the conduction band, the {111} wafer having much fewer trap states presents the best electrical conductivity property. Impedance data also provide facet-specific carrier lifetimes. The {111} surface gives consistently the lowest carrier lifetime, which reflects its high electrical conductivity. Lastly, ultraviolet photoelectron spectra and diffuse reflectance spectra were taken to obtain Schottky barriers between Ag and contacting Si wafers. The most conductive {111} surface presenting the largest Schottky barrier means the degrees of surface band bending used to explain facet-dependent electrical behaviors cannot be reliably attained this way.

Molecular tuning of CO2-to-ethylene conversion.

The electrocatalytic reduction of carbon dioxide, powered by renewable electricity, to produce valuable fuels and feedstocks provides a sustainable and carbon-neutral approach to the storage of energy produced by intermittent renewable sources1. However, the highly selective generation of economically desirable products such as ethylene from the carbon dioxide reduction reaction (CO2RR) remains a challenge2. Tuning the stabilities of intermediates to favour a desired reaction pathway can improve selectivity3-5, and this has recently been explored for the reaction on copper by controlling morphology6, grain boundaries7, facets8, oxidation state9 and dopants10. Unfortunately, the Faradaic efficiency for ethylene is still low in neutral media (60 per cent at a partial current density of 7 milliamperes per square centimetre in the best catalyst reported so far9), resulting in a low energy efficiency. Here we present a molecular tuning strategy-the functionalization of the surface of electrocatalysts with organic molecules-that stabilizes intermediates for more selective CO2RR to ethylene. Using electrochemical, operando/in situ spectroscopic and computational studies, we investigate the influence of a library of molecules, derived by electro-dimerization of arylpyridiniums11, adsorbed on copper. We find that the adhered molecules improve the stabilization of an 'atop-bound' CO intermediate (that is, an intermediate bound to a single copper atom), thereby favouring further reduction to ethylene. As a result of this strategy, we report the CO2RR to ethylene with a Faradaic efficiency of 72 per cent at a partial current density of 230 milliamperes per square centimetre in a liquid-electrolyte flow cell in a neutral medium. We report stable ethylene electrosynthesis for 190 hours in a system based on a membrane-electrode assembly that provides a full-cell energy efficiency of 20 per cent. We anticipate that this may be generalized to enable molecular strategies to complement heterogeneous catalysts by stabilizing intermediates through local molecular tuning.

Efficient upgrading of CO to C3 fuel using asymmetric C-C coupling active sites.

The electroreduction of C1 feedgas to high-energy-density fuels provides an attractive avenue to the storage of renewable electricity. Much progress has been made to improve selectivity to C1 and C2 products, however, the selectivity to desirable high-energy-density C3 products remains relatively low. We reason that C3 electrosynthesis relies on a higher-order reaction pathway that requires the formation of multiple carbon-carbon (C-C) bonds, and thus pursue a strategy explicitly designed to couple C2 with C1 intermediates. We develop an approach wherein neighboring copper atoms having distinct electronic structures interact with two adsorbates to catalyze an asymmetric reaction. We achieve a record n-propanol Faradaic efficiency (FE) of (33 ± 1)% with a conversion rate of (4.5 ± 0.1) mA cm-2, and a record n-propanol cathodic energy conversion efficiency (EEcathodic half-cell) of 21%. The FE and EEcathodic half-cell represent a 1.3× improvement relative to previously-published CO-to-n-propanol electroreduction reports.

Lattice anchoring stabilizes solution-processed semiconductors.

The stability of solution-processed semiconductors remains an important area for improvement on their path to wider deployment. Inorganic caesium lead halide perovskites have a bandgap well suited to tandem solar cells1 but suffer from an undesired phase transition near room temperature2. Colloidal quantum dots (CQDs) are structurally robust materials prized for their size-tunable bandgap3; however, they also require further advances in stability because they are prone to aggregation and surface oxidization at high temperatures as a consequence of incomplete surface passivation4,5. Here we report 'lattice-anchored' hybrid materials that combine caesium lead halide perovskites with lead chalcogenide CQDs, in which lattice matching between the two materials contributes to a stability exceeding that of the constituents. We find that CQDs keep the perovskite in its desired cubic phase, suppressing the transition to the undesired lattice-mismatched phases. The stability of the CQD-anchored perovskite in air is enhanced by an order of magnitude compared with pristine perovskite, and the material remains stable for more than six months at ambient conditions (25 degrees Celsius and about 30 per cent humidity) and more than five hours at 200 degrees Celsius. The perovskite prevents oxidation of the CQD surfaces and reduces the agglomeration of the nanoparticles at 100 degrees Celsius by a factor of five compared with CQD controls. The matrix-protected CQDs show a photoluminescence quantum efficiency of 30 per cent for a CQD solid emitting at infrared wavelengths. The lattice-anchored CQD:perovskite solid exhibits a doubling in charge carrier mobility as a result of a reduced energy barrier for carrier hopping compared with the pure CQD solid. These benefits have potential uses in solution-processed optoelectronic devices.

Density Functional Theory Calculations Revealing Metal-like Band Structures and Work Function Variation for Ultrathin Gallium Arsenide (111) Surface Layers.

Density functional theory (DFT) calculations have been performed on tunable numbers of gallium arsenide (100), (110), and (111) planes for their electron density of states (DOS) plots and the corresponding band diagrams. The GaAs (100) and (110) planes show the same semiconducting band structure with tunable plane layers and a band gap of 1.35 eV around the Fermi level. In contrast, metal-like band structures are obtained with a continuous band structure around the Fermi level for 1, 2, 4, 5, 7, and 8 layers of GaAs (111) planes. For 3, 6, and 9 GaAs (111) planes, the same semiconducting band structure as seen in the (100) and (110) planes returns. The results suggest the GaAs {111} face should be more electrically conductive than its {100} and {110} faces, due to the merged conduction band and valence band. GaAs (100) and (110) planes give a fixed work function, but the (111) planes have variable work function values that are smaller than that obtained for the (100) and (110) planes. Furthermore, bond length, bond geometry, and frontier orbital electron number and energy distribution show notable differences between the metal-like and semiconducting plane cases, so the emergence of plane-dependent electronic properties have quantum mechanical origin at the orbital level. GaAs should possess similar facet-dependent electronic properties to those of Si and Ge.

Contactless measurements of photocarrier transport properties in perovskite single crystals.

The remarkable properties of metal halide perovskites arising from their impressive charge carrier diffusion lengths have led to rapid advances in solution-processed optoelectronics. Unfortunately, diffusion lengths reported in perovskite single crystals have ranged widely - from 3 µm to 3 mm - for ostensibly similar materials. Here we report a contactless method to measure the carrier mobility and further extract the diffusion length: our approach avoids both the effects of contact resistance and those of high electric field. We vary the density of quenchers - epitaxially included within perovskite single crystals - and report the dependence of excited state lifetime in the perovskite on inter-quencher spacing. Our results are repeatable and self-consistent (i.e. they agree on diffusion length for many different quencher concentrations) to within ± 6%. Using this method, we obtain a diffusion length in metal-halide perovskites of 2.6 µm ± 0.1 µm.