On-Wire Design of Axial Periodic Halide Perovskite Superlattices for High-Performance Photodetection.

Precise synthesis of all-inorganic lead halide perovskite nanowire heterostructures and superlattices with designable modulation of chemical compositions is essential for tailoring their optoelectronic properties. Nevertheless, controllable synthesis of perovskite nanostructure heterostructures remains challenging and underexplored to date. Here, we report a rational strategy for wafer-scale synthesis of one-dimensional periodic CsPbCl3/CsPbI3 superlattices. We show that the highly parallel array of halide perovskite nanowires can be prepared roughly as horizontally guided growth on an M-plane sapphire. A periodic patterning of the sapphire substrate enables position-selective ion exchange to obtain highly periodic CsPbCl3/CsPbI3 nanowire superlattices. This patterning is further confirmed by micro-photoluminescence investigations, which show that two separate band-edge emission peaks appear at the interface of a CsPbCl3/CsPbI3 heterojunction. Additionally, compared with the pure CsPbCl3 nanowires, photodetectors fabricated using these periodic heterostructure nanowires exhibit superior photoelectric performance, namely, high ION/IOFF ratio (104), higher responsivity (49 A/W), and higher detectivity (1.51 × 1013 Jones). Moreover, a spatially resolved visible image sensor based on periodic nanowire superlattices is demonstrated with good imaging capability, suggesting promising application prospects in future photoelectronic imaging systems. All these results based on the periodic CsPbCl3/CsPbI3 nanowire superlattices provides an attractive material platform for integrated perovskite devices and circuits.

Mask-inspired moisture-transmitting and durable thermochromic perovskite smart windows.

Thermochromic perovskite smart windows (TPWs) are a cutting-edge energy-efficient window technology. However, like most perovskite-based devices, humidity-related degradation limits their widespread application. Herein, inspired by the structure of medical masks, a unique triple-layer thermochromic perovskite window (MTPW) that enable sufficient water vapor transmission to trigger the thermochromism but effectively repel detrimental water and moisture to extend its lifespan is developed. The MTPW demonstrates superhydrophobicity and maintains a solar modulation ability above 20% during a 45-day aging test, with a decay rate 37 times lower than that of a pristine TPW. It can also immobilize lead ions and significantly reduce lead leakage by 66 times. Furthermore, a significant haze reduction from 90% to 30% is achieved, overcoming the blurriness problem of TPWs. Benefiting from the improved optical performance, extended lifespan, suppressed lead leakage, and facile fabrication, the MTPW pushes forward the wide applications of smart windows in green buildings.

Optoelectronic properties and ultrafast carrier dynamics of copper iodide thin films.

As a promising high mobility p-type wide bandgap semiconductor, copper iodide has received increasing attention in recent years. However, the defect physics/evolution are still controversial, and particularly the ultrafast carrier and exciton dynamics in copper iodide has rarely been investigated. Here, we study these fundamental properties for copper iodide thin films by a synergistic approach employing a combination of analytical techniques. Steady-state photoluminescence spectra reveal that the emission at ~420 nm arises from the recombination of electrons with neutral copper vacancies. The photogenerated carrier density dependent ultrafast physical processes are elucidated with using the femtosecond transient absorption spectroscopy. Both the effects of hot-phonon bottleneck and the Auger heating significantly slow down the cooling rate of hot-carriers in the case of high excitation density. The effect of defects on the carrier recombination and the two-photon induced ultrafast carrier dynamics are also investigated. These findings are crucial to the optoelectronic applications of copper iodide.

Suppressing Nickel Oxide/Perovskite Interface Redox Reaction and Defects for Highly Performed and Stable Inverted Perovskite Solar Cells.

The inorganic hole transport layer of nickel oxide (NiOx ) has shown highly efficient, low-cost, and scalable in perovskite photovoltaics. However, redox reactions at the interface between NiOx and perovskites limit their commercialization. In this study, ABABr (4-(2-Aminoethyl) benzoic acid bromide) between the NiOx and different perovskite layers to address the issues has been introduced. How the ABABr interacts with NiOx and perovskites is experimentally and theoretically investigated. These results show that the ABABr molecule chemically reacts with the NiOx via electrostatic attraction on one side, whereas on the other side, it forms a strong hydrogen bond via the NH3 + group with perovskites layers, thus directly diminishing the redox reaction between the NiOx and perovskites layers and passivating the layer surfaces. Additionally, the ABABr interface modification leads to significant improvements in perovskite film morphology, crystallization, and band alignment. The perovskites solar cells (PSCs) based on an ABABr interface modification show power conversion efficiency (PCE) improvement by over 13% and maintain over 90% of its PCE after continuous operation at maximum power point for over 500 h. The work not only contributes to the development of novel interlayers for stable PSCs but also to the understanding of how to prevent interface redox reactions.

Room-Temperature Broad-Wavelength-Tunable Single-Mode Lasing from Alloyed CdS1-xSex Nanotripods.

Wavelength-tunable semiconductor nanolasers have attracted tremendous attention for their tunable emissions and robust stability, bringing possibilities for various applications, including nanophotonic circuits, solid-state white-light sources, wavelength-converted devices, and on-chip optical communications. Here, we report on the demonstration of broadband-tunable, single-mode nanolasers based on high-quality alloyed single crystalline CdS1-xSex (x = 0-1) nanotripods with well-formed facets fabricated using a conventional CVD approach. Microstructural characterization and optical investigations reveal that these structures are crystalline with composition-tunable CdS1-xSex alloys. Microphotoluminescence spectra and mapping of these nanotripods exhibit emissions with continuous wavelengths from 509 to 712 nm, further demonstrating that the CdS1-xSex alloys have tunable bandgaps due to the composition gradient. Additionally, under a pulse laser illumination, room-temperature single-mode lasing is clearly observed from these nanotripods cavities, which shows almost identical emission lines with a high-quality factor of ∼1231. More importantly, wavelength continuously tunable nanolasers from 520 to 738 nm are successfully constructed using these bandgap gradient nanotripods. The capability to fabricate high-quality tunable nanolasers represents a significant step toward high-integration optical circuits and photonics communications.

On-wire bandgap engineering via a magnetic-pulled CVD approach and optoelectronic applications of one-dimensional nanostructures.

Since the emergence of one-dimensional nanostructures, in particular the bandgap-graded semiconductor nanowires/ribbons or heterostructures, lots of attentions have been devoted to unraveling their intriguing properties and finding applications for future developments in optical communications and integrated optoelectronic devices. In particular, the ability to modulate the bandgap along a single nanostructure greatly enhances their functionalities in optoelectronics, and hence these studies are essential to pave the way for future high-integrated devices and circuits. Herein, we focus on a brief review on recent advances about the synthesis through a magnetic-pulled chemical vapor deposition approach, crystal structure and the unique optical and electronic properties of on-nanostructures semiconductors, including axial nanowire heterostructures, asymmetrical/symmetric bandgap gradient nanowires, lateral heterostructure nanoribbons, lateral bandgap graded ribbons. Moreover, recent developments in applications using low-dimensional bandgap modulated structures, especially in bandgap-graded nanowires and heterostructures, are summarized, including multicolor lasers, waveguides, white-light sources, photodetectors, and spectrometers, where the main strategies and unique features are addressed. Finally, future outlook and perspectives for the current challenges and the future opportunities of one-dimensional nanostructures with bandgap engineering are discussed to provide a roadmap future development in the field.

Near-Infrared-Activated Thermochromic Perovskite Smart Windows.

Perovskite-based thermochromic smart windows that can change color have attracted much interest. However, the high transition temperature (>45 °C in air) hinders their practical application. Herein, a near-infrared (NIR) activated thermochromic perovskite window that enables reversible transition cycles at room temperature is proposed. Under natural sunlight (>700 W m-2 ), it efficiently harvests 78% NIR light to trigger the thermochromism of perovskites, blocking the heat gain from both the visible and NIR light. Meanwhile, it also exhibits a low mid-infrared emissivity of <0.3, suppressing thermal radiation to the indoor environment. A field test demonstrates that this smart window can reduce the indoor temperature by 8 °C compared to a normal glass window at noon. The near-room-temperature color change, multispectral thermal management, outstanding energy-saving ability, and climate adaptability, and solution-based process of this window make it unique and promising for real applications.

Doping limitation due to self-compensation by native defects in In-doped rocksalt CdxZn1-xO.

Cadmium oxide (CdO)-ZnO alloys (CdxZn1-xO) exhibit a transformation from the wurtzite to the rocksalt (RS) phase at a CdO composition of ∼70% with a drastic change in the band gap and electrical properties. RS-CdxZn1-xO alloys (x> 0.7) are particularly interesting for transparent conductor applications due to their wide band gap and high electron mobility. In this work, we synthesized RS-CdxZn1-xO alloys doped with different concentrations of In dopants and evaluated their electrical and optical properties. Experimental results are analyzed in terms of the amphoteric native defect model and compared directly to defect formation energies obtained by hybrid density functional theory (DFT) calculations. A saturation in electron concentration of ∼7 × 1020 cm-3accompanied by a rapid drop in electron mobility is observed for the RS-CdxZn1-xO films with 0.7 ⩽x< 1 when the In dopant concentration [In] is larger than 3%. Hybrid DFT calculations confirm that the formation energy of metal vacancy acceptor defects is significantly lower in RS-CdxZn1-xO than in CdO, and hence limits the free carrier concentration. Mobility calculations reveal that due to the strong compensation by native defects, RS-CdxZn1-xO alloys exhibit a compensation ratio of >0.7 for films withx< 0.8. As a consequence of the compensation by native defects, in heavily doped RS-CdxZn1-xO carrier-induced band filling effect is limited. Furthermore, the much lower mobility of the RS-CdxZn1-xO alloys also results in a higher resistivity and reduced transmittance in the near infra-red region (λ > 1100 nm), making the material not suitable as transparent conductors for full spectrum photovoltaics.

Effects of oxygen flow ratio and thermal annealing on defect evolution of aluminum doped zinc oxide thin films by reactive DC magnetron sputtering.

Al doped ZnO (AZO) is a promising transparent conducting oxide to replace the expensive Sn doped In2O3(ITO). Understanding the formation and evolution of defects in AZO is essential for its further improvement. Here, we synthesize transparent conducting AZO thin films by reactive DC magnetron sputtering. The effects of oxygen flow ratio as well as the rapid thermal annealing (RTA) in different conditions on their structural and optoelectrical properties were investigated by a variety of analytical techniques. We find that AZO thin films grown in O-rich conditions exhibit inferior optoelectrical performance as compared with those grown in Zn-rich conditions, possibly due to the formation of excessive native acceptor defects and/or secondary phases (e.g. Al2O3). Temperature-dependent Hall measurements indicate that mobilities of these highly degenerate AZO films withN> 1020 cm-3are primarily limited by ionized and neutral impurities, while films with relatively lowN∼ 1019 cm-3exhibit a temperature-activated mobility owing to the grain-barrier scattering. AsNincreases, the optical band gap of AZO thin film increases as a result of Burstein-Moss shift and band gap narrowing. RTA treatments under appropriate conditions (i.e. at 500 °C for 60 s in Ar) can further improve the electrical properties of AZO thin film, with low resistivity of ∼6.2 × 10-4Ω cm achieved, while RTA at high temperature with longer time can lead to the formation of substantial sub-gap defect states and thus lowers the electron mobility. X-ray photoelectron spectroscopy provides further evidence on the variation of Al (Zn) content at the surface of AZO thin films with different processing conditions.

Flexibility of Room-Temperature-Synthesized Amorphous CdO-In2O3 Alloy Films and Their Application as Transparent Conductors in Solar Cells.

Due to their low-temperature deposition, high mobility (>10 cm2/V·s), and electrical conductivity, amorphous ionic oxide semiconductors (AIOSs) have received much attention for their applications in flexible and/or organic electro-optical devices. Here, we report on a study of the flexibility of CdO-In2O3 alloy thin films, deposited on a polyethylene terephthalate (PET) substrate by radio frequency magnetron sputtering at room temperature. Cd1-xInxO1+δ alloys with the composition of x > 0.6 are amorphous, exhibiting a high electron mobility of 40-50 cm2/V·s, a low resistivity of ∼3 × 10-4 Ω·cm, and high transmittance over a wide spectral window of 350 to >1600 nm. The flexibility of both crystalline and amorphous Cd1-xInxO1+δ films on the PET substrate was investigated by measuring their electrical resistivity after both compressive and tensile bending with a range of bending radii and repeated bending cycles. Under both compressive and tensile bending with Rb = 16.5 mm, no significant degradation was observed for both the crystalline and amorphous films up to 300 bending cycles. For a smaller bending radius, the amorphous film shows much less electrical degradation than the crystalline films under compressive bending due to less film delamination at the bending sites. On the other hand, for a small bending radius (<16 mm), both crystalline and amorphous films degrade after repeated tensile bending, most likely due to the development of microcracks in the films. To demonstrate the application of amorphous Cd1-xInxO1+δ alloy in photovoltaics, we fabricated perovskite and bulk-heterojunction organic solar cells (OSCs) on glass and flexible PET utilizing amorphous Cd1-xInxO1+δ layers as transparent electrodes. The organic-inorganic hybrid perovskite solar cells (PSCs) exhibit a power conversion efficiency (PCE) of ∼11 to 12% under both front and back illumination, demonstrating good bifacial performance with bifaciality factor >90%. The OSCs fabricated on an amorphous Cd1-xInxO1+δ-coated flexible PET substrate achieve a promising PCE of 12.06%. Our results strongly suggest the technological potentials of amorphous Cd1-xInxO1+δ as a reliable and effective transparent conducting material for flexible and organic optoelectronic devices.

Two-Step Chemical Vapor Deposition-Synthesized Lead-Free All-Inorganic Cs3Sb2Br9 Perovskite Microplates for Optoelectronic Applications.

Lead-based halide perovskites (APbX3, where A = organic or inorganic cation, X = Cl, Br, I) are suitable materials for many optoelectronic devices due to their many attractive properties. However, the concern of lead toxicity and the poor ambient and operational stability of the organic cation group greatly limit their practical utilization. Therefore, there has recently been great interest in lead-free, environment-friendly all-inorganic halide perovskites (IHPs). Sb and Sn are common species suggested to replace Pb for Pb-free IHPs. However, the large difference in the melting points of the precursor materials (e.g., CsBr and SbBr3 precursors for Cs3Sb2Br9) makes the chemical vapor deposition (CVD) growth of high-quality Pb-free IHPs a very challenging task. In this work, we developed a two-step CVD method to overcome this challenge and successfully synthesized Pb-free Cs3Sb2Br9 perovskite microplates. Cs3Sb2Br9 microplates ∼25 µm in size with the exciton absorption peak at ∼2.8 eV and a band gap of ∼2.85 eV were obtained. The microplates have a smooth hexagonal morphology and show a large Stokes shift of ∼450 meV and exciton binding energy of ∼200 meV. To demonstrate the applications of these microplates in optoelectronics, simple photoconductive devices were fabricated. These photodetectors exhibit a current on/off ratio of 2.36 × 102, a responsivity of 36.9 mA/W, and a detectivity of 1.0 × 1010 Jones with a fast response of rise and decay time of 61.5 and 24 ms, respectively, upon 450 nm photon irradiation. Finally, the Cs3Sb2Br9 microplates also show good stability in ambient air without encapsulation. These results demonstrate that the 2-step CVD process is an effective approach to synthesize high-quality all-inorganic lead-free Cs3Sb2Br9 perovskite microplates that have the potential for future high-performance optoelectronic device applications.

Self-Densified Optically Transparent VO2 Thermochromic Wood Film for Smart Windows.

Optically transparent wood has emerged as a promising glazing material. Thanks to the high optical transmittance, strong mechanical properties, and excellent thermal insulation capability of transparent wood, it offers a potential alternative to glass for window applications. Recently, thermo-, electro-, and photochromic transparent woods that dynamically modulate light transmittance have been investigated to improve building energy efficiency. However, it remains challenging to widely replace windows with transparent wood because of its poor weather resistance. In this study, an environment-friendly thermochromic transparent wood film (TTWF) with thermal switching of transmittance is proposed and demonstrated. To achieve thermochromism, the bleached wood is impregnated with the vanadium dioxide (VO2)/polyvinyl alcohol composite. Due to the self-densification of cellulose microfibrils during the evaporation of solvents, the transparent wood is in the form of thin films, which can be attached on the inner face of a window to protect it from severe weather conditions, making the installation convenient and low-cost. Furthermore, the surface of VO2-TTWF is modified by octadecyltrichlorosilane to enhance the waterproof ability and achieve self-cleaning and antidust functions. The proposed VO2-TTWF shows great potential for application in energy-efficient buildings using sustainable materials with advanced optical properties (i.e., Tlum = 50.5%, ΔTsol = 3.4%, and haze = 70%) that are mechanically robust (i.e., σ = 130.6 MPa along the wood growth direction), have low-thermal conductivity (i.e., K = 0.29 W m-1 K-1 along the perpendicular direction to the wood fibers), and demonstrate hydrophobic self-cleaning and antidust functions (i.e., contact angle: 121.9°). An experiment, using a model house, showed that the VO2-TTWF attached on the inner face of the window could significantly reduce the indoor air temperature by 33.9 °C compared with a bare glass panel, proving that VO2-TTWF has potential to be applied as a new-generation energy-efficient material for smart windows.

Bio-inspired TiO2 nano-cone antireflection layer for the optical performance improvement of VO2 thermochromic smart windows.

Vanadium dioxide (VO2) is a promising material for thermochromic glazing. However, VO2 thermochromic smart windows suffer from several problems that prevent commercialization: low luminous transmittance (Tlum) and low solar modulation ability (ΔTsol). The solution to these problems can be sought from nature where the evolution of various species has enabled them to survive. Investigations into the morphology of moths eyes has shown that their unique nanostructures provide an excellent antireflection optical layer that helps moths sharply capture the light in each wavelength from a wide angle. Inspired by this mechanism, a VO2 thermochromic smart window coated with a TiO2 antireflection layer with a novel nano-cone structure, is presented in this study to achieve high Tlum and ΔTsol. Optimization for the key structure parameters is summarized based on the FDTD numerical simulations. The optimized structure exhibits a Tlum of 55.4% with ΔTsol of 11.3%, an improvement of about 39% and 72% respectively compared to the VO2 window without an antireflection layer. Furthermore, wide-angle antireflection and polarization independence are also demonstrated by this nano-cone coating. This work provides an alternative method to enhance the optical performance of VO2 smart windows.

Electronically Controlled Chemical Stability of Compound Semiconductor Surfaces.

Effects of a humid environment on the degradation of semiconductors were studied to understand the role of the surface charge on material stability. Two distinctly different semiconductors with the Fermi level stabilization energy EFS located inside the conduction band (CdO) and valence band (SnTe) were selected, and effects of an exposure to 85 °C and 85% relative humidity conditions on their electrical properties were investigated. Undoped CdO films with bulk Fermi level EF below EFS and positively charged surface are very unstable. The stability greatly improves with doping when EF shifts above EFS, and the surface becomes negatively charged. This charge-controlled reactivity is further confirmed by the superior stability of undoped p-type SnTe with EF above EFS. These distinct reactivities are explained by the surface attracting either the reactive OH- or passivating H+ ions. The present results have important implications for understanding the interaction of semiconductor surfaces with water or, in general, ionic solutions.

Three-dimensional band structure and surface electron accumulation of rs-CdxZn1-xO studied by angle-resolved photoemission spectroscopy.

Three-dimensional band structure of rock-salt (rs) CdxZn1-xO (x = 1.0, 0.83, and 0.60) have been determined by angle-resolved photoemission spectroscopy (ARPES) using synchrotron radiation. Valence-band features shift to higher binding energy with Zn content, while the conduction band position does not depend strongly on Zn content. An increase of the indirect band gap with Zn-doping is larger than that of the direct band gap, reflecting a weaker hybridization between Zn 3d and O 2p than that between Cd 4d and O 2p. Two-dimensional electronic states due to the quantization along surface normal direction are formed in the surface accumulation layer and show non-parabolic dispersions. Binding energy of the quantized two-dimensional state is well reproduced using an accumulation potential with the observed surface band bending and the characteristic width of about 30 Å.

Room-Temperature-Synthesized High-Mobility Transparent Amorphous CdO-Ga2O3 Alloys with Widely Tunable Electronic Bands.

In this work, we have synthesized Cd1-xGaxO1+δ alloy thin films at room temperature over the entire composition range by radio frequency magnetron sputtering. We found that alloy films with high Ga contents of x > 0.3 are amorphous. Amorphous Cd1-xGaxO1+δ alloys in the composition range of 0.3 < x < 0.5 exhibit a high electron mobility of 10-20 cm2 V-1 s-1 with a resistivity in the range of 10-2 to high 10-4 Ω cm range. The resistivity of the amorphous alloys can also be controlled over 5 orders of magnitude from 7 × 10-4 to 77 Ω cm by controlling the oxygen stoichiometry. Over the entire composition range, these crystalline and amorphous alloys have a large tunable intrinsic band gap range of 2.2-4.8 eV as well as a conduction band minimum range of 5.8-4.5 eV below the vacuum level. Our results suggest that amorphous Cd1-xGaxO1+δ alloy films with 0.3 < x < 0.4 have favorable optoelectronic properties as transparent conductors on flexible and/or organic substrates, whereas the band edges and electrical conductivity of films with 0.3 < x < 0.7 can be manipulated for transparent thin-film transistors as well as electron transport layers.

Intermixing studies in GaN1-xSbx highly mismatched alloys.

GaN1-xSbx with x∼5%-7% is a highly mismatched alloy predicted to have favorable properties for application as an electrode in a photoelectrochemical cell for solar water splitting. In this study, we grew GaN1-xSbx under conditions intended to induce phase segregation. Prior experiments with the similar alloy GaN1-xAsx, the tendency of Sb to surfact, and the low growth temperatures needed to incorporate Sb all suggested that GaN1-xSbx alloys would likely exhibit phase segregation. We found that, except for very high Sb compositions, this was not the case and that instead interdiffusion dominated. Characteristics measured by optical absorption were similar to intentionally grown bulk alloys for the same composition. Furthermore, the alloys produced by this method maintained crystallinity for very high Sb compositions and allowed higher overall Sb compositions. This method may allow higher temperature growth while still achieving needed Sb compositions for solar water splitting applications.

Formation of Nanoscale Composites of Compound Semiconductors Driven by Charge Transfer.

Composites are a class of materials that are formed by mixing two or more components. These materials often have new functional properties compared to their constituent materials. Traditionally composites are formed by self-assembly due to structural dissimilarities or by engineering different layers or structures in the material. Here we report the synthesis of a uniform and stoichiometric composite of CdO and SnTe with a novel nanocomposite structure stabilized by the dissimilarity of the electronic band structure of the constituent materials. The composite has interesting electronic properties which range from highly n-type in CdO to semi-insulating in the intermediate composition range to highly p-type in SnTe. This can be explained by the overlap of the conduction and valence band of the constituent compounds. Ultimately, our work identifies a new class of composite semiconductors in which nanoscale self-organization is driven and stabilized by charge transfer between constituent materials.

On-Nanowire Axial Heterojunction Design for High-Performance Photodetectors.

We report the growth of high-quality CdS/CdSxSe1-x axial heterostructure nanowires (NWHs) via a temperature-controlled chemical vapor deposition method. Microstructural characterizations revealed that these NWHs have a single-crystalline structure with abrupt heterojunctions. Local photoluminescence and mapping near the heterojunctions show only two separated narrow band-edge emission bands from the two different adjacent semiconductors, further demonstrating the high-quality of these heterostructures. Moreover, the photodetector based on the single NWH shows a performance (higher responsivity (1.18 × 10(2) A/W), faster response speed (rise ∼68 µs, decay ∼137 µs), higher Ion/Ioff ratio (10(5)), higher EQE (3.1 × 10(4) %), and broader detection range (350-650 nm)) at room temperature superior to that of photodetectors based on single band gap nanostructures. This work suggests a much simpler route to achieve superior NWHs for applications in optoelectronic devices.

Mechanistic insights into chemical and photochemical transformations of bismuth vanadate photoanodes.

Artificial photosynthesis relies on the availability of semiconductors that are chemically stable and can efficiently capture solar energy. Although metal oxide semiconductors have been investigated for their promise to resist oxidative attack, materials in this class can suffer from chemical and photochemical instability. Here we present a methodology for evaluating corrosion mechanisms and apply it to bismuth vanadate, a state-of-the-art photoanode. Analysis of changing morphology and composition under solar water splitting conditions reveals chemical instabilities that are not predicted from thermodynamic considerations of stable solid oxide phases, as represented by the Pourbaix diagram for the system. Computational modelling indicates that photoexcited charge carriers accumulated at the surface destabilize the lattice, and that self-passivation by formation of a chemically stable surface phase is kinetically hindered. Although chemical stability of metal oxides cannot be assumed, insight into corrosion mechanisms aids development of protection strategies and discovery of semiconductors with improved stability.