Switchable Polar Nanotexture in Nanolaminates HfO2 -ZrO2 for Ultrafast Logic-in-Memory Operations.

Nontrivial topological polar textures in ferroelectric materials, including vortices, skyrmions, and others, have the potential to develop ultrafast, high-density, reliable multilevel memory storage and conceptually innovative processing units, even beyond the limit of binary storage of 180° aligned polar materials. However, the realization of switchable polar textures at room temperature in ferroelectric materials integrated directly into silicon using a straightforward large area fabrication technique and effectively utilizing it to design multilevel programable memory and processing units has not yet been demonstrated. Here, utilizing vector piezoresponse force and conductive atomic force microscopy, microscopic evidence of the electric field switchable polar nanotexture is provided at room temperature in HfO2 -ZrO2 nanolaminates grown directly onto silicon using an atomic layer deposition technique. Additionally, a two-terminal Au/nanolaminates/Si ferroelectric tunnel junction is designed, which shows ultrafast (≈83 ns) nonvolatile multilevel current switching with high on/off ratio (>106 ), long-term durability (>4000 s), and giant tunnel electroresistance (108 %). Furthermore, 14 Boolean logic operations are tested utilizing a single device as a proof-of-concept for reconfigurable logic-in-memory processing. The results offer a potential approach to "processing with polar textures" and addressing the challenges of developing high-performance multilevel in-memory processing technology by virtue of its fundamentally distinct mechanism of operation.

Remarkable Electrical Conductivity Increase and Pure Metallic Properties from Semiconducting Colloidal Nanocrystals by Cation Exchange for Solution-Processable Optoelectronic Applications.

The authors report a strategic approach to achieve metallic properties from semiconducting CuFeS colloidal nanocrystal (NC) solids through cation exchange method. An unprecedentedly high electrical conductivity is realized by the efficient generation of charge carriers onto a semiconducting CuS NC template via minimal Fe exchange. An electrical conductivity exceeding 10 500 S cm-1 (13 400 S cm-1 at 2 K) and a sheet resistance of 17 Ω/sq at room temperature, which are among the highest values for solution-processable semiconducting NCs, are achieved successfully from bornite-phase CuFeS NC films possessing 10% Fe atom. The temperature dependence of the corresponding films exhibits pure metallic characteristics. Highly conducting NCs are demonstrated for a thermoelectric layer exhibiting a high power factor over 1.2 mW m-1 K-2 at room temperature, electrical wires for switching on light emitting diods (LEDs), and source-drain electrodes for p- and n-type organic field-effect transistors. Ambient stability, eco-friendly composition, and solution-processability further validate their sustainable and practical applicability. The present study provides a simple but very effective method for significantly increasing charge carrier concentrations in semiconducting colloidal NCs to achieve metallic properties, which is applicable to various optoelectronic devices.

Site-Specific Regulated Memristors via Electron-Beam-Induced Functionalization of HfO2.

Emerging nonvolatile resistive switching, also known as the memristor, works with a distinct concept that relies mainly on the change in the composition of the active materials, rather than to store the charge. Particularly for oxide-based memristors, the switching is often governed by the random and unpredicted temporal/spatial migration of oxygen defects, resulting in possessing limitations in terms of control over conduction channel formation and inability to regulate hysteresis loop opening. Therefore, site specific dynamic control of defect concentration in the active materials can offer a unique opportunity to realize on-demand regulation of memory storage and artificial intelligence capabilities. Here, high-performance, site-specific spatially scalable memristor devices are fabricated by stabilizing the conduction channel via manipulation of oxygen defects using electron-beam irradiation. Specifically, the memristors exhibit highly stable and electron-beam dose-regulated multilevel analog hysteresis loop opening with adjustable switching ratios even higher than 104 . Additionally, broad modulation of neural activities, including short- and long-term plasticity, paired-pulse facilitation, spike-timing-dependent plasticity, and dynamic multipattern memory processing, are demonstrated. The work opens a new possibility to regulate the resistive switching behavior and control mimicking of neural activities, providing a hitherto unseen tunability in two-terminal oxide-based memristors.

Low-temperature n-type doping of insulating ultrathin amorphous indium oxide using H plasma-assisted annealing.

Low-temperature process compatibility is a key factor in successfully constructing additional functional circuits on top of pre-existing circuitry without corrupting characteristics thereof, a technique that typically requires die-to-die (wafer-to-wafer) stacking and interconnecting. And against thermal annealing, which is mandatory and is possible only globally for activating amorphous oxide semiconductors, the selective control of electrical characteristics of the oxide thin-films for integrated circuit applications is challenging. Here, a low-temperature process that enables n-type doping of the designed region of insulating In2O3thin-film is demonstrated. A short hydrogen plasma treatment followed by low-temperature annealing is used to increase interstitial and substitutional hydrogen associated bond states creating shallow donor levels in the insulating In2O3surface to transform the thin-film into an n-type semiconductor. As a result, an In2O3thin-film transistor with a high on/off current ratio (>108), a field-effect mobility of 3.8 cm2V-1s-1, and a threshold voltage of ∼3.0 V has been developed. Compared to performing just thermal annealing, the H-plasma assisted annealing process resulted in an n-type In2O3thin-film transistor showing similar characteristics, while the processing time was reduced by ∼1/3 and the plasma-untreated area still remained insulating. With further development, the hydrogen plasma doping process may make possible a monolithic planar process technology for amorphous oxide semiconductors.

A novel sensor based on WO3·0.33H2O nanorods modified electrode for the detection and degradation of herbicide, carbendazim.

In the present-day scenario, it is necessary to establish more flexible, effective and selective analytical methods that are easy to operate and less expensive. Cyclic voltammetry (CV) can be a useful technique to assess minute quantity of pollutants and in this work, an effort has been made to detect the trace quantification from the environmental samples. Herein, electrochemical sensor was fabricated using tungsten oxide nanorod (WO3·0.33H2O) for sensitive detection of fungicide, carbendazim (CBZ). Under optimal conditions, while studying the effect of pH on peak current, the highest peak current was observed at pH 4.2. The degradation of CBZ followed the mixed diffusion-adsorption controlled and quasi-reversible processess at the WO3·0.33H2O/GC electrode surface. Using WO3·0.33H2O/GCE sensor in SWV provided the lowest limit of detection (LOD) and limit of quantification (LOQ) values of 2.21 × 10-8 M and 7.37 × 10-8 M, respectively over the concentration ranges of 1.0 × 10-7 M to 2.5 × 10-4 M. The proposed method demonstrates potential applicability of the fabricated sensor for soil and water samples analysis in the management of creating a benign environment.

Tunable polaron-induced coloration of tungsten oxide via a multi-step control of the physicochemical property for the detection of gaseous F.

The tunable polaron effect of amorphous tungsten oxide on FTO substrates has been used to detect fluorine in the gas phase via photochemical and gasochromic reactions. By combining photochemical (UV exposure under an H2 atomsphere) and gasochromic (XeF2 exposure) reactions, the detection of gaseous fluorine using amorphous tungsten oxide is described. The effective hydrogenation of WO3 was achieved using UV/H2 exposure to prepare hydrogenated tungsten oxide (H-WO3-x) upon activating the strong polaron-coupling to infrared (IR) light to decrease IR transmission from 70 to 20% at 1000 nm wavelength. This is explained by creation of W 5d unpaired electrons excited by band-edge defect states or W5+ states. The H-WO3-x lattice structure was maintained as an amorphous structure and found to have hydrogen-associated shallow- and oxygen vacancy-associated deep-trap levels with a moderate enhancement of the n-type characteristic. The gasochromic reaction takes place within tens of seconds at room temperature upon exposure to XeF2 gas leading to atomic F insertion. Fluorine, which is one of the most electronegative materials, is combined with the W5+ and W6+ in H-WO3-x to remove H to form volatile HF vapor and the formation of W-F bonds. The global incorporation of fluorine effectively turns H-WO3-x into F-WO3-x structures and deactivates the polaron-IR coupling (IR transmission change from 20 to 70%) since all the band-edge defect states are passivated upon F insertion with a strong n-doping effect. Therefore, this approach, entirely processed at room temperature, is highly applicable to fluorine detecting sensors and devices utilizing the polaron-IR coupling effect.

Flexo-Pyrophotronic Effect Modulated Giant Near Infrared Photoresponse from VO2 -Based Heterojunction for Optical Communication.

The flexoelectric phenomenon, which occurs when materials undergo mechanical deformation and cause strain gradients and a related spontaneous electric polarization field, can result in wide variety of energy- and cost-saving mechano-opto-electronics, such as night vision, communication, and security. However, accurate sensing of weak intensities under self-powered conditions with stable photocurrent and rapid temporal response remains essential despite the challenges related to having suitable band alignment and high junction quality. Taking use of the flexoelectric phenomena, it is shown that a centrosymmetric VO2 -based heterojunction exhibits a self-powered (i.e., 0 V), infrared (λ = 940 nm) photoresponse. Specifically, the device shows giant current modulation (103 %), good responsivity of >2.4 mA W-1 , reasonable specific detectivity of ≈1010 Jones, and a fast response speed of 0.5 ms, even at the nanoscale modulation. Through manipulation of the applied inhomogeneous force, the sensitivity of the infrared response is enhanced (> 640%). Ultrafast night optical communication like Morse code distress (SOS) signal sensing and high-performing obstacle sensors with potential impact alarms are created as proof-of-concept applications. These findings validate the potential of emerging mechanoelectrical coupling for a wide variety of novel applications, including mechanoptical switches, photovoltaics, sensors, and autonomous vehicles, which require tunable optoelectronic performance.

Transformative Multifunction Deep Ultraviolet Photodetectors for On-Demand Applications: From Fast Optical Communication to Tunable In-Sensor Photocurrent Integration.

The strategic utilization of photodetectors' transient response could open new frontiers from free-space optical communication to the emerging field of neuromorphic optoelectronics. Contrarily, while communication requires a fast response, neuromorphic applications benefit from a slow and integrative transient photocurrent. By integrating these functionalities in a single device, this study unveils a photodetector with tunable responses, bridging the gap between optical communication and neuromorphic sensing and creating a versatile platform with on-demand applications. Particularly, a Ga2O3-based photodetector was designed, exhibiting a photocurrent on/off ratio close to 104, high responsivity of 0.43 A/W, and detectivity 1.22 × 1013 Jones under deep ultraviolet illumination (λ ∼ 260 nm). The photodetector demonstrates transient time-dependent on operational voltage, ranging from 10-4 to 0.2 s. The underlying mechanism is attributed to the voltage-dependent balance between photocarrier generation and defect-related recombination, as revealed by electrostatic force microscopy. Additionally, we have demonstrated potential applications, including digital Morse code interpretation, tunable integration of optical inputs within the sensor, one-time readouts, and effective analog Morse code reading. Furthermore, the effectiveness of input information recognition using analog integration, even with anomalies, was demonstrated. This work establishes a versatile approach for tunable in-sensor optical processing, potentially useful for a wide range of applications, from free-space optical communication to neuromorphic sensing.

A Single-Pixel Event Photoactive Device for Real-Time, In-Sensor Spatiotemporal Optical Information Processing.

The increasing demand for energy-efficient, sophisticated optical sensing technologies in various applications, from machine vision to optical communication, highlights the necessity for innovations in spatiotemporal information sensing and processing at a nearly single-pixel scale. Traditional methods, including multi-pixel photodetector arrays and event-based camera systems, often fail to provide rapid, real-time detection and processing of dynamic events within the sensor. This shortcoming is particularly notable in handling high-dimensional spatiotemporal data, where the dependency on sequential data input and external processing tools leads to latency, reduced throughput, and heightened energy consumption, thereby impeding real-time parallel data processing capabilities. Here, a carrier-selective, single-pixel, position-sensitive planar photoactive device that integrates spatiotemporal event sensing with inherent short-term memory capabilities is introduced. The proof-of-concept single-pixel event photoactive device enables in-sensor spatiotemporal parallel optical information processing, efficiently managing multibit (>4 bit) data simultaneously and facilitating ultrafast (≈0.4 µs) recognition of input patterns with low energy consumption (25 femtojoules). Additionally, by adjusting the operating speed from continuous to pulsed light illumination, the sensor array can detect trajectories and absolute position of events, offering in-sensor optical flow detection. This single-pixel event photodetector marks significant advancement toward developing compact, energy-efficient, ultrafast sensors suitable for a wide range of in sensor-based photonic applications.

A new insight into vacancy modulation in lead-doped tungsten oxide nonarchitect for photoelectrochemical water splitting: An experimental and density functional theory approach.

In this study, the impact of lead (Pb) doping on the photoelectrochemical (PEC) water splitting performance of tungsten oxide (WO3) photoanodes was investigated through a combination of experimental and theoretical approaches. Pb-doped WO3 nanostructured thin films were synthesized hydrothermally, and extensive characterizations were conducted to study their morphologies, band edge, optical and photoelectrochemical properties. Pb-doped WO3 exhibited efficient carrier density and charge separations by reducing the charge transfer resistance. The 0.96 at% Pb doping shows a record photocurrent of âˆ¼ 1.49 mAcm-2 and âˆ¼ 3.44 mAcm-2 (with the hole scavenger) at 1.23 V vs. RHE besides yielding a high charge separation and Faradaic efficiencies of âˆ¼ 86 % and > 90 %, respectively. A shift in the Fermi level towards the conduction band was also observed upon the Pb doping. Additionally, density functional theory (DFT) simulations demonstrated the changes in the density of states and bandgap upon Pb doping, exhibiting favorable changes in the surface and bulk properties of WO3.

Picoampere Dark Current and Electro-Opto-Coupled Sub-to-Super-linear Response from Mott-Transition Enabled Infrared Photodetector for Near-Sensor Vision Processing.

Light-intensity selective superlinear photodetectors with ultralow dark current can provide an essential breakthrough for the development of high-performing near-sensor vision processing. However, the development of near-sensor vision processing is not only conceptually important for device operation (given that sensors naturally exhibit linear/sublinear responses), but also essential to get rid of the massive amount of data generated during object sensing and classification with noisy inputs. Therefore, achieving the giant superlinear photoresponse while maintaining the picoampere leakage current, irrespective of the measurement bias, is one of the most challenging tasks. Here, Mott material (vanadium dioxide) and silicon-based integrated infrared photodetectors are developed that show giant superlinear photoresponse (exponent >18) and ultralow dark current of 4.46 pA. Specifically, the device demonstrates an electro-opto-coupled insulator-to-metal transition, which leads to outstanding photocurrent on/off ratio (>106 ), a high responsivity (>1 mA W-1 ), and excellent detectivity (>1012  Jones), while maintaining response speed (τr  = 6 µs and τf  = 10 µs). Further, intensity-selective near-sensor processing is demonstrated and night vision pattern reorganization even with noisy inputs is exhibited. This research will pave the way for the creation of high-performance photodetectors with potential uses, such as in night vision, pattern recognition, and neuromorphic processing.

Room-Temperature Quantum Diodes with Dynamic Memory for Neural Logic Operations.

The pursuit of high-performance, next-generation nanoelectronics is fundamentally reliant on exploiting quantum phenomena such as tunneling at room temperature. However, quantum tunneling and memory dynamics are governed by two conflicting parameters: the presence or absence of defects. Therefore, the integration of both attributes within a single device presents substantial challenges. Nevertheless, successful integration has the potential to prompt crucial breakthroughs by emulating biobrain-like dynamics, in turn enabling sophisticated in-material neural logic operations. In this work, we demonstrate that a conformal nanolaminate HfO2/ZrO2 structure on silicon enables high-performing (>106 s) Fowler-Nordheim tunneling at room temperature. In addition, the device exhibits unipolar dynamic hysteresis loop opening (on/off ratio >102) with high endurance (>104 cycles) along with negative differential resistance, which is attributed to the collective ferroelectric and capacitive effects and is utilized to emulate synaptic functions. Further, proof-of-concept logic gates based on voltage-dependent plasticity and time-domain were developed using a single device, offering in-material neural-like data processing. These findings pave the way for the realization of high-performing and scalability tunneling devices in advanced nanoelectronics, which mark a promising route toward the development of next-generation, fundamental neural logic computing systems.

Enhanced photoactivity towards bismuth vanadate water splitting through tantalum doping: An experimental and density functional theory study.

The activation of hole trap states in bismuth vanadate (BiVO4) is considered an effective strategy to enhance the photoelectrochemical (PEC) water-splitting activity. Herein, we propose a theoretical and experimental study of tantalum (Ta) doping to BiVO4 leading to the introduction of hole trap states for the enhanced PEC activity. The doping of Ta is found to alter the structural and chemical surroundings via displacement of vanadium (V) atoms that cause distortions in the lattice via the formation of hole trap states. A significant enhancement of photocurrent to ∼4.2 mA cm-2 was recorded attributing to the effective charge separation of efficiency of ∼96.7 %. Furthermore, the doping of Ta in the BiVO4 lattice offers improved charge transport in bulk and decreased charge transfer resistance at the electrolyte interface. The Ta-doped BiVO4 displays the effective production of hydrogen (H2) and oxygen (O2) under AM 1.5 G illumination with a faradaic efficiency of 90 %. Moreover, the density functional theory (DFT) study confirms the decrease in optical band gap and the activation of hole trap states below the conduction band (CB) with a contribution of Ta towards both valence and CB that increases the charge separation and majority charge carrier density, respectively. The findings of this work propose that the displacement of V sites with Ta atoms in BiVO4 photoanodes is an efficient approach for enhanced PEC activity.

Surface plasmon-driven hot electron flow probed with metal-semiconductor nanodiodes.

A continuous flow of hot electrons that are not at thermal equilibrium with the surrounding metal atoms is generated by the absorption of photons. Here we show that hot electron flow generated on a gold thin film by photon absorption (or internal photoemission) is amplified by localized surface plasmon resonance. This was achieved by direct measurement of photocurrent on a chemically modified gold thin film of metal-semiconductor (TiO(2)) Schottky diodes. The short-circuit photocurrent obtained with low-energy photons is consistent with Fowler's law, confirming the presence of hot electron flows. The morphology of the metal thin film was modified to a connected gold island structure after heating such that it exhibits surface plasmon. Photocurrent and optical measurements on the connected island structures revealed the presence of a localized surface plasmon at 550 ± 20 nm. The results indicate an intrinsic correlation between the hot electron flow generated by internal photoemission and localized surface plasmon resonance.

Generation of highly n-type titanium oxide using plasma fluorine insertion.

True n-type doping of titanium oxide without formation of midgap states would expand the use of metal oxides for charge-based devices. We demonstrate that plasma-assisted fluorine insertion passivates defect states and that fluorine acts as an n-type donor in titanium oxide. This enabled us to modify the Fermi level and transport properties of titanium oxide outside the limits of O vacancy doping. The origin of the electronic structure modification is explained by ab initio calculation.

High-Performing Self-Powered Photosensing and Reconfigurable Pyro-photoelectric Memory with Ferroelectric Hafnium Oxide.

With highly diverse multifunctional properties, hafnium oxide (HfO2 ) has attracted considerable attention not only because of its potential to address fundamental questions about material behaviors, but also its potential for applied perspectives like ferroelectric memory, transistors, and pyroelectric sensors. However, effective harvesting of the pyro-photoelectric effect of HfO2 to develop high-performing self-biased photosensors and electric writable and optical readable memory has yet to be developed. Here, a proof-of-concept HfO2 -based self-powered and ultrafast (response time ≈ 60 µs) infrared pyroelectric sensor with a responsivity of up to 68 µA W-1 is developed. In particular, temporal infrared light illumination induced surface heating and, in turn, change in spontaneous polarization are attributed to robust pyro-photocurrent generation. Further, controllable suspension and reestablishment of the self-biased pyro-photocurrent response with a short electric pulse are demonstrated, which offers a conceptually new kind of photoreadable memory. Potentially, the novel approach opens a new avenue for designing on-demand pyro-phototronic response over a desired area and offers the opportunity to utilize it for various applications, including memory storage, neuromorphic vision sensors, classification, and emergency alert systems.

Adaptive Memory and In Materia Reinforcement Learning Enabled by Flexoelectric-like Response from Ultrathin HfO2.

Reinforcement learning (RL) is a mathematical framework of neural learning by trial and error that revolutionized the field of artificial intelligence. However, until now, RL has been implemented in algorithms with the compatibly of traditional complementary metal-oxide-semiconductor-based von Neumann digital platforms, which thus limits performance in terms of latency, fault tolerance, and robustness. Here, we demonstrate that nanocolumnar (∼12 nm) HfO2 structures can be used as building blocks to conduct the RL within the material by combining its stress-adjustable charge transport and memory functions. Specifically, HfO2 nanostructures grown by the sputtering method exhibit self-assembled vertical nanocolumnar structures that generate voltage depending on the impact of stress under self-biased conditions. The observed results are attributed to the flexoelectric-like response of HfO2. Further, multilevel current (>10-3 A) modulation with touch and controlled suspension (∼10-12 A) with a short electric pulse (100 ms) were demonstrated, yielding a proof-of-concept memory with an on/off ratio greater than 109. Utilizing multipattern dynamic memory and tactile sensing, RL was implemented to successfully solve a maze game using an array of 6 × 4. This work could pave the way to do RL within materials for a variety of applications such as memory storage, neuromorphic sensors, smart robots, and human-machine interaction systems.

Ultrahigh-Speed In-Memory Electronics Enabled by Proximity-Oxidation-Evolved Metal Oxide Redox Transistors.

The pursuit of a universal device that combines nonvolatile multilevel storage, ultrafast writing/erasing speed, nondestructive readout, and embedded processing with low power consumption demands the development of innovative architectures. Although thin-film transistors and redox-based resistive-switching devices have independently been proven to be ideal building blocks for data processing and storage, it is still difficult to achieve both well-controlled multilevel memory and high-precision ultrafast processing in a single unit, even though this is essential for the large-scale hardware implementation of in-memory computing. In this work, an ultrafast (≈42 ns) and programable redox thin-film transistor (ReTFT) memory made of a proximity-oxidation-grown TiO2 layer is developed, which has on/off ratio of 105 , nonvolatile multilevel analog storage with a long retention time, strong durability, and high reliability. Utilizing the proof-of-concept ReTFTs, circuits capable of performing fundamental NOT, AND, and OR operations with reconfigurable logic-in-memory processing are developed. Further, on-demand signal memory-processing operations, like multi-terminal addressable memory, learning, pattern recognition, and classification, are explored for prospective application in neuromorphic hardware. This device, which operates on a fundamentally different mechanism, presents an alternate solution to the problems associated with the creation of high-performing in-memory processing technology.

Modulation of electrical properties in MoTe2 by XeF2-mediated surface oxidation.

Transition metal dichalcogenides (TMDs) are promising candidates for the semiconductor industry owing to their superior electrical properties. Their surface oxidation is of interest because their electrical properties can be easily modulated by an oxidized layer on top of them. Here, we demonstrate the XeF2-mediated surface oxidation of 2H-MoTe2 (alpha phase MoTe2). MoTe2 exposed to XeF2 gas forms a thin and uniform oxidized layer (∼2.5 nm-thick MoO x ) on MoTe2 regardless of the exposure time (within ∼120 s) due to the passivation effect and simultaneous etching. We used the oxidized layer for contacts between the metal and MoTe2, which help reduce the contact resistance by overcoming the Fermi level pinning effect by the direct metal deposition process. The MoTe2 field-effect transistors (FETs) with a MoO x interlayer exhibited two orders of magnitude higher field-effect hole mobility of 6.31 cm2 V-1 s-1 with a high on/off current ratio of ∼105 than that of the MoTe2 device with conventional metal contacts (0.07 cm2 V-1 s-1). Our work shows a straightforward and effective method for forming a thin oxide layer for MoTe2 devices, applicable for 2D electronics.

An Artificial Mechano-Nociceptor with Mott Transition.

Intelligent touch sensing is now becoming an essential part of various human-machine interactions and communication, including in touchpads, autonomous vehicles, and smart robotics. Usually, sensing of physical objects is enabled by applied force/pressure sensors; however, reported conventional tactile devices are not able to differentiate sharp and blunt objects, although sharp objects can cause unavoidable damage. Therefore, it is central issue to implement electronic devices that can classify sense of touch and simultaneously generate pain signals to avoid further potential damage from sharp objects. Here, concept of force-enabled nociceptive behavior is proposed and demonstrated using vanadium oxide-based artificial receptors. Specifically, versatile criteria of bio-nociceptor like threshold, relaxation, no adaptation, allodynia, and hyperalgesia behaviors are triggered by pointed force, but the device does not mimic any of these by the force applied by blunt objects; thus, the proposed device classifies the intent of touch. Further, supported by finite element simulation, the nanoscale dynamic is unambiguously revealed by conductive atomic force microscopy and results are attributed to the point force-triggered Mott transition, as also confirmed by temperature-dependent measurements. The reported features open a new avenue for developing mechano-nociceptors, which enable a high-level of artificial intelligence within the device to classify physical touch.