Metal-insulator-semiconductor photoelectrodes for enhanced photoelectrochemical water splitting.

Photoelectrochemical (PEC) water splitting provides a scalable and integrated platform to harness renewable solar energy for green hydrogen production. The practical implementation of PEC systems hinges on addressing three critical challenges: enhancing energy conversion efficiency, ensuring long-term stability, and achieving economic viability. Metal-insulator-semiconductor (MIS) heterojunction photoelectrodes have gained significant attention over the last decade for their ability to efficiently segregate photogenerated carriers and mitigate corrosion-induced semiconductor degradation. This review discusses the structural composition and interfacial intricacies of MIS photoelectrodes tailored for PEC water splitting. The application of MIS heterostructures across various semiconductor light-absorbing layers, including traditional photovoltaic-grade semiconductors, metal oxides, and emerging materials, is presented first. Subsequently, this review elucidates the reaction mechanisms and respective merits of vacuum and non-vacuum deposition techniques in the fabrication of the insulator layers. In the context of the metal layers, this review extends beyond the conventional scope, not only by introducing metal-based cocatalysts, but also by exploring the latest advancements in molecular and single-atom catalysts integrated within MIS photoelectrodes. Furthermore, a systematic summary of carrier transfer mechanisms and interface design principles of MIS photoelectrodes is presented, which are pivotal for optimizing energy band alignment and enhancing solar-to-chemical conversion efficiency within the PEC system. Finally, this review explores innovative derivative configurations of MIS photoelectrodes, including back-illuminated MIS photoelectrodes, inverted MIS photoelectrodes, tandem MIS photoelectrodes, and monolithically integrated wireless MIS photoelectrodes. These novel architectures address the limitations of traditional MIS structures by effectively coupling different functional modules, minimizing optical and ohmic losses, and mitigating recombination losses.

Inorganic perovskite-based active multifunctional integrated photonic devices.

The development of highly efficient active integrated photonic circuits is crucial for advancing information and computing science. Lead halide perovskite semiconductors, with their exceptional optoelectronic properties, offer a promising platform for such devices. In this study, active micro multifunctional photonic devices were fabricated on monocrystalline CsPbBr3 perovskite thin films using a top-down etching technique with focused ion beams. The etched microwire exhibited a high-quality micro laser that could serve as a light source for integrated devices, facilitating angle-dependent effective propagation between coupled perovskite-microwire waveguides. Employing this strategy, multiple perovskite-based active integrated photonic devices were realized for the first time. These devices included a micro beam splitter that coherently separated lasing signals, an X-coupler performing transfer matrix functions with two distinguishable light sources, and a Mach-Zehnder interferometer manipulating the splitting and coalescence of coherent light beams. These results provide a proof-of-concept for active integrated functionalized photonic devices based on perovskite semiconductors, representing a promising avenue for practical applications in integrated optical chips.

Realization of tunable-performance in atomic layer deposited Hf-doped In2O3 thin film transistor via oxygen vacancy modulation.

Due to the limitation of inherent ultra-high electron concentration, the electrical properties of In2O3 resemble those of conductors rather than semiconductors prior to special treatment. In this study, the effect of various annealing treatments on the microstructure, optical properties, and oxygen vacancies of the films and transistors is systematically investigated. Our finding reveals a progressive crystallization trend in the films with increasing annealing temperature. In addition, a higher annealing temperature is also associated with the reduction in the concentration of oxygen vacancies, as well as an elevation in both optical transmittance and optical bandgap. Furthermore, with the implementation of annealing process, the devices gradually transform from no pronounced gate control to exhibit with excellent gate control and electrical performances. The atomic layer deposited Hf-doped In2O3 thin film transistor annealed at 250 °C exhibits optimal electrical properties, with a field-effect mobility of 18.65 cm2 V-1 s-1, a subthreshold swing of 0.18 V/dec, and an Ion/Ioff ratio of 2.76 × 106. The results indicate that the impact of varying annealing temperatures can be attributed to the modulation of oxygen vacancies within the films. This work serves as a complementary study for the existing post-treatment of oxide films and provides a reliable reference for utilization of the annealing process in practical applications.

Tunable-performance all-oxide structure field-effect transistor based atomic layer deposited Hf-doped In2O3 thin films.

The relocation of peripheral transistors from the front-end-of-line (FEOL) to the back-end-of-line (BEOL) in fabrication processes is of significant interest, as it allows for the introduction of novel functionality in the BEOL while providing additional die area in the FEOL. Oxide semiconductor-based transistors serve as attractive candidates for BEOL. Within these categories, In2O3 material is particularly notable; nonetheless, the excessive intrinsic carrier concentration poses a limitation on its broader applicability. Herein, the deposition of Hf-doped In2O3 (IHO) films via atomic layer deposition for the first time demonstrates an effective method for tuning the intrinsic carrier concentration, where the doping concentration plays a critical role in determine the properties of IHO films and all-oxide structure transistors with Au-free process. The all-oxide transistors with In2O3: HfO2 ratio of 10:1 exhibited optimal electrical properties, including high on-current with 249 µA, field-effect mobility of 13.4 cm2 V-1 s-1, and on/off ratio exceeding 106, and also achieved excellent stability under long time positive bias stress and negative bias stress. These findings suggest that this study not only introduces a straightforward and efficient approach to improve the properties of In2O3 material and transistors, but as well paves the way for development of all-oxide transistors and their integration into BEOL technology.

Modulated wafer-scale WS2 films based on atomic-layer-deposition for various device applications.

Tungsten disulfide (WS2) is promising for potential applications in transistors and gas sensors due to its high mobility and high adsorption of gas molecules onto edge sites. This work comprehensively studied the deposition temperature, growth mechanism, annealing conditions, and Nb doping of WS2 to prepare high-quality wafer-scale N- and P-type WS2 films by atomic layer deposition (ALD). It shows that the deposition and annealing temperature greatly influence the electronic properties and crystallinity of WS2, and insufficient annealing will seriously reduce the switch ratio and on-state current of the field effect transistors (FETs). Besides, the morphologies and carrier types of WS2 films can be controlled by adjusting the processes of ALD. The obtained WS2 films and the films with vertical structures were used to fabricate FETs and gas sensors, respectively. Among them, the Ion/Ioff ratio of N- and P-type WS2 FETs is 105 and 102, respectively, and the response of N- and P-type gas sensors is 14% and 42% under 50 ppm NH3 at room temperature, respectively. We have successfully demonstrated a controllable ALD process to modify the morphology and doping behavior of WS2 films with various device functionalities based on acquisitive characteristics.

Optoelectronic Devices of Large-Scale Transferred All-Inorganic Lead Halide Perovskite Thin Films.

We report the large-scale transfer process for monocrystalline CsPbBr3 thin films prepared by chemical vapor deposition (CVD) with excellent optical properties and stability. The transfer process is robust, simple, and effective, in which CsPbBr3 thin films could be transferred to several substrates and effectively avoid chemical or physical fabrication processes to damage the perovskite surface. Moreover, the transfer process endows CsPbBr3 and substrates with atomically clean and electronically flat interfaces. We utilize this transfer process to realize several optoelectronic devices, including a photonic laser with a threshold of 61 µJ/cm2, a photodetector with a responsivity of 2.4 A/W, and a transistor with a hole mobility of 11.47 cm2 V-1 s-1. High device performances mainly originate from low defects of high-quality single-crystal perovskite and seamless contact between CsPbBr3 and target substrates. The large-scale nondestructive transfer process provides promising opportunities for optoelectronic applications based on monocrystalline perovskites.

Post-annealing optimization of the heteroepitaxial La-doped SrSnO3 integrated on silicon via ALD.

Wide band gap (WBG) alkaline-earth stannate transparent oxide semiconductors (TOSs) have attracted increasing attention in recent years for their high carrier mobility and outstanding optoelectronic properties, and have been applied widely in various devices, such as flat-panel displays. Most alkaline-earth stannates are grown by molecular beam epitaxy (MBE); there are some intractable issues with the tin source including the volatility with SnO and Sn sources and the decomposition of the SnO2 source. In contrast, atomic layer deposition (ALD) serves as an ideal technique for the growth of complex stannate perovskites with precise stoichiometry control and tunable thickness at the atomic scale. Herein, we report the La-SrSnO3/BaTiO3 perovskite heterostructure heterogeneously integrated on Si (001), which uses ALD-grown La-doped SrSnO3 (LSSO) as a channel material and MBE-grown BaTiO3 (BTO) as a dielectric material. The reflective high-energy electron diffraction and X-ray diffraction results indicate the crystallinity of each epitaxial layer with a full width at half maximum (FWHM) of 0.62°. In situ X-ray photoelectron spectroscopy results confirm that there was no Sn0 state in ALD-deposited LSSO. Besides, we report a strategy for the post-treatment of LSSO/BTO perovskite heterostructures by controlling the oxygen annealing temperature and time, with a maximum oxide capacitance Cox of 0.31 µF cm-2 and a minimum low-frequency dispersion for the devices with 7 h oxygen annealing at 400 °C. The enhancement of capacitance properties is primarily attributed to a decrease of oxygen vacancies in the films and interface defects in the heterostructure interfaces during an additional ex situ excess oxygen annealing. This work expands current optimization methods for reducing defects in epitaxial LSSO/BTO perovskite heterostructures and shows that excess oxygen annealing is a powerful tool for enhancing the capacitance properties of LSSO/BTO heterostructures.

Wafer-Scale Synthesis of WS2 Films with In Situ Controllable p-Type Doping by Atomic Layer Deposition.

Wafer-scale synthesis of p-type TMD films is critical for its commercialization in next-generation electro/optoelectronics. In this work, wafer-scale intrinsic n-type WS2 films and in situ Nb-doped p-type WS2 films were synthesized through atomic layer deposition (ALD) on 8-inch α-Al2O3/Si wafers, 2-inch sapphire, and 1 cm2 GaN substrate pieces. The Nb doping concentration was precisely controlled by altering cycle number of Nb precursor and activated by postannealing. WS2 n-FETs and Nb-doped p-FETs with different Nb concentrations have been fabricated using CMOS-compatible processes. X-ray photoelectron spectroscopy, Raman spectroscopy, and Hall measurements confirmed the effective substitutional doping with Nb. The on/off ratio and electron mobility of WS2 n-FET are as high as 105 and 6.85 cm2 V-1 s-1, respectively. In WS2 p-FET with 15-cycle Nb doping, the on/off ratio and hole mobility are 10 and 0.016 cm2 V-1 s-1, respectively. The p-n structure based on n- and p- type WS2 films was proved with a 104 rectifying ratio. The realization of controllable in situ Nb-doped WS2 films paved a way for fabricating wafer-scale complementary WS2 FETs.

Epigenomics of human CD8 T cell differentiation and aging.

The efficacy of the adaptive immune response declines dramatically with age, but the cell-intrinsic mechanisms driving immune aging in humans remain poorly understood. Immune aging is characterized by a loss of self-renewing naïve cells and the accumulation of differentiated but dysfunctional cells within the CD8 T cell compartment. Using ATAC-seq, we inferred the transcription factor binding activities correlated with naive and central and effector memory CD8 T cell states in young adults. Integrating our results with RNA-seq, we identified transcription networks associated with CD8 T cell differentiation, with prominent roles implicated for BATF, ETS1, Eomes, and Sp1. Extending our analysis to aged humans, we found that the differences between the memory and naive subsets were largely preserved across age, but that naive and central memory cells from older individuals exhibited a shift toward more differentiated patterns of chromatin openness. Additionally, aged naive cells displayed a loss in chromatin accessibility at gene promoters, largely associated with a decrease in NRF1 binding. This shift was implicated in a marked drop-off in the ability of the aged naive cells to transcribe respiratory chain genes, which may explain the reduced capacity of oxidative phosphorylation in older naïve cells. Our findings identify BATF- and NRF1-driven gene regulation as potential targets for delaying CD8 T cell aging and restoring function.

Direct Growth of Al2O3 on Black Phosphorus by Plasma-Enhanced Atomic Layer Deposition.

Growing high-quality and uniform dielectric on black phosphorus is challenging since it is easy to react with O2 or H2O in ambient. In this work, we have directly grown Al2O3 on BP using plasma-enhanced atomic layer deposition (PEALD). The surface roughness of BP with covered Al2O3 film can reduce significantly, which is due to the removal of oxidized bubble in BP surface by oxygen plasma. It was also found there is an interfacial layer of PO x in between amorphous Al2O3 film and crystallized BP, which is verified by both X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) measurements. By increasing temperature, the PO x can be converted into fully oxidized P2O5.

Mechanisms shaping the naïve T cell repertoire in the elderly - thymic involution or peripheral homeostatic proliferation?

The ability of the human immune system to repel infections is drastically diminished with age. Elderly individuals are more susceptible to new threats and are less able to control endogenous infections. The thymus, which is the sole source of new T cells, has been proposed as a target for regenerative efforts to improve immune competence, as thymic activity is dramatically reduced after puberty. In this review, we review the role of the thymus in the maintenance of T cell homeostasis throughout life and contrast the differences in mice and humans. We propose that in humans, lack of thymic T cell generation does not explain a decline in T cell receptor diversity nor would thymic rejuvenation restore diversity. Initial studies using next generation sequencing are beginning to establish lower boundaries of T cell receptor diversity. With increasing sequencing depth and the development of new statistical models, we are now in the position to test this model and to assess the impact of age on T cell diversity and clonality.

Gene loops enhance transcriptional directionality.

Eukaryotic genomes are extensively transcribed, forming both messenger RNAs (mRNAs) and noncoding RNAs (ncRNAs). ncRNAs made by RNA polymerase II often initiate from bidirectional promoters (nucleosome-depleted chromatin) that synthesize mRNA and ncRNA in opposite directions. We demonstrate that, by adopting a gene-loop conformation, actively transcribed mRNA encoding genes restrict divergent transcription of ncRNAs. Because gene-loop formation depends on a protein factor (Ssu72) that coassociates with both the promoter and the terminator, the inactivation of Ssu72 leads to increased synthesis of promoter-associated divergent ncRNAs, referred to as Ssu72-restricted transcripts (SRTs). Similarly, inactivation of individual gene loops by gene mutation enhances SRT synthesis. We demonstrate that gene-loop conformation enforces transcriptional directionality on otherwise bidirectional promoters.

Emerging Views on the CTD Code.

The C-terminal domain (CTD) of RNA polymerase II (Pol II) consists of conserved heptapeptide repeats that function as a binding platform for different protein complexes involved in transcription, RNA processing, export, and chromatin remodeling. The CTD repeats are subject to sequential waves of posttranslational modifications during specific stages of the transcription cycle. These patterned modifications have led to the postulation of the "CTD code" hypothesis, where stage-specific patterns define a spatiotemporal code that is recognized by the appropriate interacting partners. Here, we highlight the role of CTD modifications in directing transcription initiation, elongation, and termination. We examine the major readers, writers, and erasers of the CTD code and examine the relevance of describing patterns of posttranslational modifications as a "code." Finally, we discuss major questions regarding the function of the newly discovered CTD modifications and the fundamental insights into transcription regulation that will necessarily emerge upon addressing those challenges.

Ssu72 phosphatase-dependent erasure of phospho-Ser7 marks on the RNA polymerase II C-terminal domain is essential for viability and transcription termination.

The C-terminal domain (CTD) of the largest subunit of RNA polymerase II (Pol II) serves an important role in coordinating stage-specific recruitment and release of cellular machines during transcription. Dynamic placement and removal of phosphorylation marks on different residues of a repeating heptapeptide (YSPTSPS) of the CTD underlies the engagement of relevant cellular machinery. Whereas sequential placement of phosphorylation marks is well explored, genome-wide engagement of phosphatases that remove these CTD marks is poorly understood. In particular, identifying the enzyme that erases phospho-Ser7 (Ser7-P) marks is especially important, because we find that substituting this residue with a glutamate, a phospho-mimic, is lethal. Our observations implicate Ssu72 as a Ser7-P phosphatase. We report that removal of all phospho-CTD marks during transcription termination is mechanistically coupled. An inability to remove these marks prevents Pol II from terminating efficiently and will likely impede subsequent assembly into the pre-initiation complex.

Chemical-genomic dissection of the CTD code.

Sequential modifications of the RNA polymerase II (Pol II) C-terminal domain (CTD) coordinate the stage-specific association and release of cellular machines during transcription. Here we examine the genome-wide distributions of the 'early' (phospho-Ser5 (Ser5-P)), 'mid' (Ser7-P) and 'late' (Ser2-P) CTD marks. We identify gene class-specific patterns and find widespread co-occurrence of the CTD marks. Contrary to its role in 3'-processing of noncoding RNA, the Ser7-P marks are placed early and retained until transcription termination at all Pol II-dependent genes. Chemical-genomic analysis reveals that the promoter-distal Ser7-P marks are not remnants of early phosphorylation but are placed anew by the CTD kinase Bur1. Consistent with the ability of Bur1 to facilitate transcription elongation and suppress cryptic transcription, high levels of Ser7-P are observed at highly transcribed genes. We propose that Ser7-P could facilitate elongation and suppress cryptic transcription.

TFIIH kinase places bivalent marks on the carboxy-terminal domain of RNA polymerase II.

Posttranslational modifications of the carboxy-terminal domain (CTD) of the largest subunit of RNA polymerase II (Pol II) specify a molecular recognition code that is deciphered by proteins involved in RNA biogenesis. The CTD is comprised of a repeating heptapeptide (Y(1)S(2)P(3)T(4)S(5)P(6)S(7)). Recently, phosphorylation of serine 7 was shown to be important for cotranscriptional processing of two snRNAs in mammalian cells. Here we report that Kin28/Cdk7, a subunit of the evolutionarily conserved TFIIH complex, is a Ser7 kinase. The ability of Kin28/Cdk7 to phosphorylate Ser7 is particularly surprising because this kinase functions at promoters of protein-coding genes, rather than being restricted to promoter-distal regions of snRNA genes. Kin28/Cdk7 is also known to phosphorylate Ser5 residues of the CTD at gene promoters. Taken together, our results implicate the TFIIH kinase in placing bivalent Ser5 and Ser7 marks early in gene transcription. These bivalent CTD marks, in concert with cues within nascent transcripts, specify the cotranscriptional engagement of the relevant RNA processing machinery.