The development of next-generation power electronics demands ultrawide-bandgap semiconductors that can operate under extreme conditions. While alpha-phase gallium oxide (alpha-Ga2O3) exhibits exceptional properties, its practical application is hindered by intrinsically low thermal conductivity (8.0-11.6 W/m center dot K) and limited conduction band offsets (CBOs) with conventional dielectrics. In this study, we demonstrate the successful epitaxial growth of beryllium oxide (BeO) on alpha-Ga2O3 using plasma-enhanced atomic layer deposition, which provides a large CBO and suggests potential advantages in interfacial thermal properties due to the high thermal conductivity of BeO. Structural analyses using transmission electron microscopy and X-ray diffraction verified that BeO grew as a single crystal with atomically sharp interfaces, enabled by hexagonal symmetry matching. Furthermore, the band alignment was identified as type-I, with a CBO of 2.7 eV. These results indicate that BeO is a promising dielectric for alpha-Ga2O3 devices.

Addressing the persistent challenges of heterostructure integration and low thermal conductivity in β-Ga2O3 devices, we report the epitaxial growth of single-crystalline BeO thin films on (2¯ 0 1) β-Ga2O3 substrates via plasma-enhanced atomic layer deposition. X-ray diffraction confirms the epitaxial growth of wurtzite (0 0 0 2) BeO films with well-defined out-of-plane and in-plane alignments, despite the dissimilar crystal structures. High-resolution transmission electron microscopy and fast Fourier transform analyses reveal a sharp BeO/ β-Ga2O3 interface and coherent crystallographic alignment. X-ray photoelectron spectroscopy demonstrates type-II band alignment with a valence-band offset of –0.5 eV and a conduction-band offset of 4.4 eV, distinct from the type-I alignment observed in non-epitaxial BeO/(0 0 1) β-Ga2O3 heterostructures. Electrical characterization shows that a Mo/(0 0 0 2) BeO/(2¯ 0 1) β-Ga2O3 MOS capacitor exhibits an exceptionally low leakage current density of 5.92 × 10-10A cm−2 at 1 MVcm−1 and minimal frequency dispersion in capacitance–voltage data, underscoring the excellent insulating properties and interface quality of epitaxial BeO films. This new route for epitaxial BeO growth on (2¯ 0 1) β-Ga2O3 holds strong promise for next-generation β-Ga2O3 heterostructure-based power electronics and ultraviolet optoelectronic devices.

While the ice-templating method has emerged as a powerful approach for fabricating complex architectures of various materials, the underlying mechanism has yet to be fully elucidated. Here, it's shown that dissolved solutes drive nonplanar ice crystal growth and generate interfacial boundaries where polymerization reactions can be space-confined. The ice crystal growth fronts are captured using in situ optical microscopy and observed to exhibit a sheath-like appearance, a result that is rationalized using constitutional supercooling theory. Building on these results, two-dimensional (2D) polymer sheets of poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole (PPy), and polyaniline (PANI) with relatively high crystallinities are realized when micelles of sodium dodecyl sulfate are used to confine the oxidative polymerizations of the constituent monomers within the boundaries of the corresponding ice crystals. Supercapacitors (SCs) fabricated with the 2D PEDOT, PPy, and PANI sheets are found to exhibit high areal capacitances of 242.2, 640.9, and 648.2 mF cm-2 at 0.2 mA cm-2, respectively. Moreover, the SCs display remarkable cycling stabilities, i.e., 98.4%, 72.3%, and 81.6% of the respective initial capacitance values after 10 000 cycles at 10 mA cm-2. These results advance the mechanistic understanding of the ice-templating technique and expand its potential utility in the rational design of hierarchically structured materials.

Silicon (Si) is a promising anode material that is used in lithium-ion batteries (LIBs) due to its high theoretical specific capacity (3579 mAh g-1). However, the practical use of Si-based anodes has been challenged by the material's low intrinsic electrical conductivity and severe volume changes that occur during the lithiation/ delithiation process. Here, we report that a Si/carbon composite constructed from Si nanobeads, 1D cellulosenanofiber-derived carbon (CC), and 2D graphene can overcome the aforementioned challenges. The composite (designated as Si@G-CC) is prepared by wrapping functionalized Si nanobeads with graphene oxide followed by mixing with cellulose nanofibers and then subjecting the mixture to an elevated temperature. The 1D CC bridges neighboring Si nanobeads and prevents 2D graphene sheets from restacking, while the intertwined carbon network provides continuous conductive pathways. Collectively, these features effectively buffer the Si from the volume changes that occur during lithiation/delithiation cycling. In half-cells, a Si@G-CC electrode is found to exhibit outstanding electrochemical performance in terms of specific capacity (3.20 Ah g- 1 at 0.36 A g- 1), rate capability (0.88 Ah g-1 at 7.20 A g-1), and cycling stability (0.97 Ah g-1 at 0.36 A g- 1 after 100 cycles). Full cells that are assembled with Si@G-CC as the anode and LiNi0.8Co0.1Mn0.1O2 as the cathode also exhibit remarkable cycling stability (i.e., 76.3 mAh g-1 based on the cathode mass after 100 cycles at 0.1C). The findings obtained in this study provide new approaches for using novel carbon structures to control the dimensional changes that are experienced by high-capacity electrodes used in contemporary energy-related technologies, while enhancing the electrochemical performance.

Zinc-air batteries are promising devices for use in safe and cost-effective energy storage applications. However, their practical utility is hindered by sluggish oxygen evolution and oxygen reduction reactions. We show that hybridizing electronically coupled Co single atoms and a CoPd alloy within a three-dimensional macroporous N-doped carbon (CoPd/Co@3D NC) enables high-performance N2H4-assisted zinc-air batteries, where the oxygen evolution reaction is replaced with the thermodynamically more favorable hydrazine oxidation reaction. Experimental and theoretical data reveal that electronic coupling between the Co single atoms and the CoPd alloy mutually modifies their electronic structures, which, in turn, weakens oxygen adsorption on the Co single atoms and enhances N2H4 adsorption on the Pd sites. The electronic-coupling effect accelerates the rate-limiting steps of the oxygen evolution and hydrazine oxidation reactions. Consequently, N2H4-assisted zinc-air batteries employing CoPd/Co@3D NC exhibit a small charging/discharging voltage gap of 0.52 V at 100 mA cm-2, a peak specific power of 151 mW cm-2, and a specific capacity of 810 mAh g-1 at 10 mA cm-2; these values are competitive when compared to those exhibited by conventional zinc-air batteries.

Beryllium oxide (BeO) has attracted significant attention as a high-k gate dielectric for advanced Si CMOS devices because of its intrinsically wide bandgap, high dielectric constant, and high thermal conductivity. However, atomic layer deposition (ALD) of BeO remains limited by incomplete ligand-exchange reactions, resulting in residual impurities and defect formation that degrade the intrinsic properties of BeO films. To address this limitation, we introduce an atomic layer annealing (ALA) process incorporating an in-cycle plasma step into each ALD cycle. The plasma exposure supplies additional surface energy without increasing the process temperature, inducing an atomic-scale annealing effect. Consequently, the ALA-grown BeO films exhibited a bandgap of 9.7 eV and a large conduction band offset of 5.6 eV with Si. This widened bandgap also implies high transparency in the ultraviolet region and strong bonding characteristics, potentially enabling UV-transparent passivation. When integrated as gate dielectrics in Si MOS capacitors, the films showed an exceptionally low leakage current density of 3.66 × 10−10 A cm−2 at − 1 MV cm−1. These results demonstrate that ALA enables high-quality BeO with enhanced electrical insulating properties, highlighting its potential as an ultrawide-bandgap high-k dielectric for Si CMOS technologies.

Addressing the persistent challenges of heterostructure integration and low thermal conductivity in (3-Ga2O3 devices, we report the epitaxial growth of single-crystalline BeO thin films on (201) (3-Ga2O3 substrates via plasma-enhanced atomic layer deposition. X-ray diffraction confirms the epitaxial growth of wurtzite (0002) BeO films with well-defined out-of-plane and in-plane alignments, despite the dissimilar crystal structures. Highresolution transmission electron microscopy and fast Fourier transform analyses reveal a sharp BeO/(3-Ga2O3 interface and coherent crystallographic alignment. X-ray photoelectron spectroscopy demonstrates type-II band alignment with a valence band offset of -0.5 eV and a conduction band offset of 4.4 eV, distinct from the type-I alignment observed in non-epitaxial BeO/(001) (3-Ga2O3 heterostructures. Electrical characterization shows that a Mo/(0002) BeO/(201) (3-Ga2O3 MOS capacitor exhibits an exceptionally low leakage current density of 5.92 x 10-10 A cm-2 at 1 MVcm- 1 and minimal frequency dispersion in capacitance-voltage data, underscoring the excellent insulating properties and interface quality of epitaxial BeO films. This new route for epitaxial BeO growth on (201) (3-Ga2O3 holds strong promise for next-generation (3-Ga2O3 heterostructure-based power electronics and ultraviolet optoelectronic devices.

The electronic spin state of metal atomic catalysts is pivotal in determining their catalytic activities. However, such catalysts that are prepared via conventional synthetic approaches often suffer from poorly defined coordination environments, which create challenges in precisely controlling their electronic spin configurations. Herein, we show that a CoFe dual-atom catalyst featuring coordinatively asymmetric FeN2 and CoN3 sites with a fixed coupling distance can be synthesized using a conjugated microporous polymer (designated as CoFe-CMP) and then demonstrate that the catalyst promotes the key reactions underlying both charging and discharge processes of sulfur cathodes. Experimental and theoretical data reveal that the Fe atoms adopt a high-spin state (S = 3/2) and the Co atoms assume a low-spin state (S = 1/2). The former configuration enhances electronic coupling with polysulfides, while the latter promotes the diffusion of Li atoms that are released upon Li2S degradation. Collectively, these processes facilitate the interconversion between polysulfides and Li2S, which are critical for optimizing lithium-sulfur (Li-S) battery operation. Li-S cells containing CoFe-CMP as the sulfur host exhibit outstanding performance in terms of specific capacity (1487 mAh g-1 at 0.1 C), rate capacity (676.3 mAh g-1 at 5 C), and cycling stability (a specific capacity of 499.2 mAh g-1 is measured after 300 cycles at 0.2 C). This research provides a general methodology for tuning the electronic spin states of metal atomic catalysts, as well as guidance for adapting these catalysts for use in other applications.

Wurtzite structured materials have been widely studied for piezoelectric nanogenerators (PENGs) due to their effective piezoelectric response. Among them, beryllium oxide (BeO) has high thermal conductivity (similar to 370 W/m & centerdot;K at 300 K) and chemical stability makes it a promising candidate. However, the potential of BeO has been underexplored owing to the challenges in its nanoscale synthesis. In this study, we experimentally demonstrate for the first time the piezoelectric properties of BeO nanowires (NWs) and films. BeO NW arrays were synthesized via atomic layer deposition within anodic aluminum oxide templates. Structural and compositional analyses confirmed BeO NW formation, and piezoresponse force microscopy revealed an effective piezoelectric response with a longitudinal piezoelectric constant (d33) of 15.8 pm/V for NW, compared to 4.8 pm/V for thin films. Additionally, the BeO NW-based PENG achieved a maximum output voltage of similar to 26.1 V and a maximum power density of similar to 0.32 & micro;W/cm2, which significantly exceed those of thin films (similar to 12.0 V and similar to 0.09 & micro;W/cm2, respectively) and are comparable to those of conventional wurtzite material-based systems. These findings establish BeO as an experimentally validated wurtzite-type piezoelectric material and demonstrate its potential for energy harvesting, self-powered sensors, and reliable operation in harsh environments where thermal and chemical stability are essential.

Organic two-dimensional (2D) materials hold great potential in a broad range of applications. However, their practical utility is constrained by limited structural diversity and a lack of synthetic strategies. Herein, we report that simply stirring immiscible solutions of poly(propylene glycol)bis(2-aminopropyl ether) and 1,3,5-benzenetricarbonyl trichloride results in a dynamic microinterfacial polymerization that affords novel 2D polymer sheets (designated as PEO-BTA). These sheets can be transformed to Na-ion conducting materials via treatment with sodium hydride, followed by incorporation of a prototypical MOF, HKUST-1, to obtain composite sheets (MOF@PEO-BTA-Na) that retain the structural integrity of the original 2D polymer sheets. The 2D composite sheets can be assembled into self-supporting membranes and used as a quasi-solid-state electrolyte (QSSE) with a remarkably high ionic conductivity value of 2.80 × 10-3 S cm-1 and a Na+ transference number of 0.95. Consequently, the QSSE facilitates uniform Na plating in Na//Na and Na//Cu cells. Na//NaTi2(PO4)3 cells containing MOF@PEO-BTA-Na QSSE exhibit a high initial specific capacity (129.1 mAh g-1 at 0.5 C), superior rate capability (60.0 mAh g-1 at 20 C), and high-capacity retention (92% after 1000 cycles at 1 C). This work establishes a new, scalable approach for synthesizing 2D organic sheets with promising applications in energy-related areas.

We report the design, synthesis, and study of poly(methylene)s with benzoxazine groups attached to every repeat unit of the main polymer chain. Diazoacetate monomers functionalized with pendant benzoxazine groups were first prepared and characterized. The length of the spacer between the diazoacetate and the pendant benzoxazine proved to be a critical design factor. When the spacer was relatively short (i.e., one methylene unit), the corresponding polymer underwent premature decomposition by emitting gaseous carbon dioxide at elevated temperatures. The use of longer (i.e., ethylene) spacers effectively circumvented premature decomposition. Each monomer was transformed into its respective poly(methylene) using a C1 polymerization methodology that employed a Pd-based catalyst. A poly(vinylene) analogue, which features benzoxazine groups attached to every other carbon unit of the main chain, and a bis(benzoxazine) that is commonly used to prepare thermosets were also prepared as controls. The structures of the monomers and polymers were elucidated using NMR and Fourier transform infrared (FT-IR) spectroscopy, and the molecular weights of the polymers were determined using size exclusion chromatography (SEC). Exposure of the benzoxazine-functionalized polymers or the bis(benzoxazine) to elevated temperatures, typically >200 degrees C, afforded mechanically robust, cross-linked resins. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) were used to characterize the thermal and physical properties of the cured materials. The thermally cured poly(methylene)s exhibited higher cross-link densities (25,937 mol m(-3)) than those derived from the poly(vinylene) analogue (12,904 mol m(-3)) or the bis(benzoxazine) (3513 mol m(-3)). The cured poly(methylene)s also exhibited a higher storage modulus (329.8 MPa) than the controls (145.8 and 44.6 MPa, respectively). Although the control thermosets were stiffer at room temperature (5.4-5.5 GPa vs 4.4 GPa), the higher cross-link density of the cured C1 polymers provides superior high-temperature performance and thermal stability. These findings demonstrate how incorporating benzoxazines into every repeat unit of a polymer backbone effectively increases cross-link density and enables the development of robust, high-performance thermosets with potential utility in advanced structural and electronic applications.

We demonstrate a new approach for Beryllium Oxide (BeO) film deposition using a discrete feeding method (DFM) to achieve low-temperature thermal atomic layer deposition (ThALD). Typically, BeO film deposition using ThALD at low temperatures leads to a low density and high carbon impurities, which degrade the electrical properties. To overcome this issue, the DFM was applied to successfully deposit BeO films at 150 °C, resulting in a significant improvement in the electrical performance compared to that obtained with the conventional ThALD process. Although previous studies on the DFM have focused on improving the growth properties of thin films and demonstrating the advantages of the method, this is the first study to investigate the temperature-dependent behavior of films grown using the DFM in ALD. At 150 °C, the film density of BeO grown by discrete feeding thermal ALD (DF-ThALD) was 2.95 g/cm3, which is higher than that of films grown by the conventional ThALD process. The leakage current density was reduced to 3.7 × 10⁻⁶ A/cm2 at -1 MV/cm, representing a reduction of approximately five orders of magnitude. Hence, the results of this study highlight the effectiveness of the DFM in improving the quality of BeO films in low-temperature ThALD processes.

Atomic layer deposition-grown beryllium oxide (BeO) is gaining attention as a dielectric material that can minimize device power consumption because of its high dielectric constant, high thermal conductivity, and low leakage current enabled by its wide bandgap energy. In this study, the impact of BeO dielectrics on InSnZnO (ITZO) thin-film transistors (TFTs) was investigated, revealing that adding a hafnium dioxide (HfO2) layer can enhance electrical performance and bias stress reliability. Time-of-flight secondary-ion mass spectrometry and X-ray photoelectron spectroscopy confirmed that the single-BeO dielectric-based ITZO TFTs exhibited a low mobility of 27.6 cm2/V·s due to Be migration and demonstrated abnormal threshold voltage (VTH) shifts under bias stress. Conversely, the HfO2 20 nm/BeO hetero-dielectric ITZO TFTs exhibited a high mobility of 76.6 cm2/V·s and enhanced abnormal VTH shift characteristics. Therefore, these results demonstrate that our high-performance HfO2/BeO hetero-dielectric-based ITZO TFTs could be utilized in back-end-of-line devices for monolithic three-dimensional memory technologies.

In this study, we fabricated high-quality beryllium oxide (BeO) films using discrete-feeding plasma-enhanced atomic layer deposition (DF-PEALD). BeO has exceptionally high thermal conductivity (330 W/m-K), a large bandgap energy, and a high dielectric constant, making it an optimal dielectric that can solve the thermal problems caused by the miniaturization of transistors. In atomic layer deposition (ALD), the physically adsorbed precursors and byproducts present during precursor injection act as a barrier to the full saturation of the substrate surface. The discrete feeding method (DFM) is a process that divides the precursor feeding and purge steps into several individual units without changing the overall process. This mitigates the effects of in-process screening and suppresses the formation of interfacial layers and carbon impurities. The grain size of the BeO fabricated using DF-PEALD was found to be 31.3 % higher than that of BeO fabricated using plasma-enhanced atomic layer deposition (PEALD) without the DFM, and its density (3.01 g/cm3) more closely matched that of the bulk. The dielectric constant of the BeO films fabricated using DF-PEALD was 8.8, the bandgap energy was 8.1 eV, and the leakage current density was 1.21 x 10-9 A/cm2 at -1 MV/cm.

Beryllium oxide (BeO) has exceptionally high thermal conductivity (330 W m(-1)K-1), a large bandgap energy, and a high dielectric constant, making it an optimal dielectric for high-power devices. However, its direct application on 4H-SiC is hindered by interfacial carbon-cluster formation during high-temperature annealing, primarily due to the decomposition of 4H-SiC. In this study, a SiO2 interlayer is introduced between BeO and 4H-SiC using plasma-enhanced chemical-vapor deposition to address these challenges. Electrical measurements reveal that the BeO/SiO2/4H-SiC stack exhibits a reduced leakage-current density, an enhanced breakdown field (>7.5 MV cm(-1)), and a smaller capacitance-voltage hysteresis compared with direct BeO deposition owing to reduced interface defects. Band-alignment analysis shows an increased conduction-band offset between BeO/4H-SiC, potentially contributing to improved carrier confinement. The interface trap density (Dit) is reduced by two orders of magnitude, indicating an improved interface quality owing to the presence of the SiO2 interlayer. The SiO2 interlayer significantly improves interface quality, reduces leakage current, and enhances the breakdown field of the BeO/4H-SiC system. These results suggest that interfacial engineering using a SiO2 interlayer can be an effective approach for improving the electrical reliability of high-temperature dielectric stacks on 4H-SiC.

The polysulfide shuttle effect and the growth of Li dendrites are detrimental to the practical use of lithium-sulfur (Li-S) batteries. Here, we show that these challenges can be eliminated with hexamethylphosphoramide (HMPA) as a co-solvent. HMPA shows strong coordination interaction with Li+ and thus preferentially remains in the inner sheath of the Li+ solvation structure. The solvation structure not only minimizes the solubility of poly-sulfides but also generates a LiF-rich solid-electrolyte interphase, which are responsible to the suppression of polysulfide shuttle effect and the elimination of Li dendrites, respectively. As a result, HMPA-based electrolytes enable Li//Li symmetric cells to exhibit long cycling lifetimes of up to 600 h at 5 mA cm-2 along with significantly reduced polarization. Moreover, Li-S cells containing the HMPA-based electrolyte also display marked enhancement in cycling stability (e.g., 619.6 mAh g-1 after 300 cycles at 0.2 C). These results are superior to those measured for control cells that contain a conventional electrolyte. The concepts and method described herein may be extended to benefit other types of energy storage devices that use active metals as anodes.

The synthesis and study of an imide-annulated N-heterocyclic carbene are described. The electrophilic nature of the carbene was assessed through analysis of its chalcogen adducts and Ir carbonyl complexes. When subjected to UV irradiation, the carbene underwent cycloaddition with benzene and other arenes to form seven-membered rings with 1,2-disubstitution patterns. The structures of these products were conclusively determined through a series of single-crystal X-ray analyses and other analytical techniques. A mechanism that proceeds through Büchner-type ring expansion followed by isomerization and is facilitated by the unique electronic structure of the carbene is proposed.

The catalytic activities displayed by single-atom catalysts (SACs) depend on the coordination structure. SACs supported on carbon materials often adopt saturated coordination structures with uneven distributions because they require high-temperature conditions during synthesis. Herein, bisnitrogen-chelated Co SACs that are coordinatively unsaturated are prepared by integrating a Co complex into a conjugated microporous polymer (CMP-CoN2). Compared with saturated analogues, i.e., tetranitrogen-chelated Co SACs (denoted as CMP-CoN4), CMP-CoN2 exhibits higher electrocatalytic activity in polysulfide conversions due to an enhanced hybridization between the 3d orbitals of the Co atoms and the 3p orbitals of the S atoms in the polysulfide. As a result, sulfur cathodes prepared with CoN2 deliver outstanding performance metrics, including a high specific capacity (1393 mA h g-1 at 0.1 C), a superior rate capacity (673.2 mA h g-1 at 6 C), and a low capacity decay rate (of only 0.045% per cycle at 2 C over 1000 cycles). They also outperform sulfur cathodes that contain CMP-CoN4 or CMPs that are devoid of Co SACs. This work reveals how the catalytic activity displayed by SACs is affected by their coordination structures, and the rules that underpin the structure-activity relationship may be extended to designing electrocatalysts for use in other applications.

We demonstrated a new method for the low-temperature solid-phase crystallization (SPC) of germanium on beryllium oxide (BeO) films for monolithic 3D (M3D) integration. Using a wurtzite crystal BeO film, known for its high thermal conductivity of 370 W/m-k at 300 K and covalent bonding characteristics, as the underlying layer, we crystallized Ge at a reduced temperature of 410 degrees C. For the Ge-on-BeO, the formation of larger grains was consistently promoted at annealing temperatures of 410-500 degrees C, with a notable 125 % increase in the grain size at 500 degrees C compared to that at the SiO2 underlayer. The polycrystalline Ge layers crystallized on the BeO retained their tensile strain, as confirmed by the Raman spectra. Furthermore, its optical bandgap of similar to 1.36 eV and average roughness of 0.847 nm at an annealing temperature of 450 degrees C make it more suitable for an M3D upper channel layer than Ge-on-SiO2. A comprehensive experimental analysis confirmed the enhanced crystallinity, stability, and channel properties of poly-Ge layers crystallized on BeO. Hence, this study developed a new method for the low-temperature SPC process and highlighted the potential of BeO as a crystallization-assistance thermal-management material for next-generation 3D integrated technology.

In an effort to target polypeptides at nonterminal sites, we screened the binding of the synthetic receptor cucurbit[8]uril (Q8) to a small library of tetrapeptides, each containing a nonterminal dipeptide binding site. The resulting leads were characterized in detail using a combination of isothermal titration calorimetry, 1H NMR spectroscopy, electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS), and X-ray crystallography. The equilibrium dissociation constant values determined for the binding of Q8 to nonterminal dipeptide sites Lys-Phe (KF) and Phe-Lys (FK) were 60 and 86 nm, respectively. These are to the best of our knowledge the highest affinities reported to date for any synthetic receptor targeting a nonterminal site on an unmodified peptide. A 0.79 Å resolution crystal structure was obtained for the complex of Q8 with the peptide Gly-Gly-Leu-Tyr-Gly-Gly-Gly (GGLYGGG) and reveals structural details of the pair-inclusion motif. The molecular basis for recognition is established to be the inclusion of the side chains of Leu and Tyr residues, as well as an extensive network of hydrogen bonds between the peptide backbone, the carbonyl oxygens of Q8, and proximal water molecules. In addition, the crystal structure reveals that Q8 induces a type II β-turn. The sequence-selectivity, high affinity, reversibility, and detailed structural characterization of this system should facilitate the development of applications involving ligand-induced polypeptide folding.