Ultrathin Layer Research Articles

Hydrogen Sensing via Heterolytic H2 Activation at Room Temperature by Atomic Layer Deposited Ceria.

Ultrathin atomic layer deposited ceria films (< 20 nm) are capable of H2 heterolytic activation at room temperature, undergoing a significant reduction regardless of the absolute pressure, as measured under in-situ conditions by near ambient pressure X-ray photoelectron spectroscopy. ALD-ceria can gradually reduce as a function of H2 concentration under H2/O2 environments, especially for diluted mixtures below 10%. At room temperature, this reduction is limited to the surface region, where the hydroxylation of the ceria surface induces a charge transfer towards the ceria matrix, reducing Ce4+ cations to Ce3+. Thus, ALD-ceria replicates the expected sensing mechanism of metal oxides at low temperatures without using any noble metal decorating the oxide surface to enhance H2 dissociation. The intrinsic defects of the ALD deposit seem to play a crucial role since the post-annealing process capable of healing these defects leads to decreased film reactivity. The sensing behavior was successfully demonstrated in sensor test structures by resistance changes towards low concentrations of H2 at low operating temperatures without using noble metals. These promising results call for combining ALD-ceria with more conductive metal oxides, taking advantage of the charge transfer at the interface and thus modifying the depletion layer formed at the heterojunction.

Anchorable Polymers Enabling Ultra-Thin and Robust Hole-Transporting Layers for High-Efficiency Inverted Perovskite Solar Cells.

Currently, the development of polymeric hole-transporting materials (HTMs) lags behind that of small-molecule HTMs in inverted perovskite solar cells (PSCs). A critical challenge is that conventional polymeric HTMs are incapable of forming ultra-thin and conformal coatings like self-assembly monolayers (SAMs), especially for substrates with rough surface morphology. Herein, we address this challenge by designing anchorable polymeric HTMs (CP1 to CP5). Specifically, coordinative pyridyl groups are introduced as side-chains on poly-triarylamine (PTAA) backbone with varied contents by copolymerization method, resulting in chemical interactions between polymeric HTMs and substrates. The strong interaction allows them to be processed into ultra-thin, uniform, and robust hole-transporting layers through employing low-concentration solutions (0.1 mg mL-1, vs. 2.0-5.0 mg mL-1 for conventional PTAA), greatly decreasing charge transport losses. Moreover, upon systematically tuning the pyridyl substitution ratio, the energy levels, surface wetting, solution processability, and defect passivation capability of such anchorable HTMs are simultaneously optimized. Based on the optimal CP4, we achieved highly efficient inverted PSCs with power conversion efficiencies (PCEs) up to 26.21 %, which is on par with state-of-the-art SAM-based inverted PSCs. Furthermore, these devices exhibit enhanced stabilities under repeated current-voltage scans and reverse bias ageing compared with SAM-based devices.

Highly Reversible Anode-Free Lithium Metal Batteries Enabled by Porous Organic Cages with Subnano Lithiophilic Triangular Windows.

The widespread application of anode-free lithium metal batteries (AFLMBs) is hindered by the severe dendrite growth and side reactions due to the poor reversibility of Li plating/stripping. Herein, our study introduces an ultrathin interphase layer of covalent cage 3 (CC3) for highly reversible AFLMBs. The subnano triangular windows in CC3 serve as a Li+ sieve to accelerate Li+ desolvation and transport kinetics, inhibit electrolyte decomposition, and form LiF- and Li3N-rich solid-electrolyte interphases. Moreover, the lithiophilic backbone of CC3 homogenizes Li+ distribution and deposition with mitigated dendrite growth. Thus, CC3 promotes Li plating/stripping kinetics and reversibility, achieving an ultralong stability over 8000 h of the Cu@CC3 electrode. Furthermore, practical Cu@CC3/LiFePO4 AFLMBs deliver a capacity retention (66%) over 600 cycles. This work emphasizes the effectiveness of CC3 to regulate the Li plating/stripping behavior, demonstrating the application potential of porous organic cages for enhancing the cycle life of AFLMBs.

Interfacial Synergism in NiMoO4/FeOOH Heterostructure for Enhanced Alkaline Oxygen Evolution and Urea Oxidation.

FeOOH with excellent catalytic properties for oxygen evolution and also considered to be a true active site has attracted great interest in recent years. However, the intrinsic low conductivity limits its catalytic performance. Herein, a one-dimensional core-shell NiMoO4/FeOOH heterojunction with high OER activity and stability was developed. At current densities of 10 and 100 mA cm-2, low overpotentials of 194 and 266 mV are need to drive oxygen evolution, meanwhile, the electrode exhibits high catalytic kinetics with small Tafel slopes of 53.4 mV dec-1 and excellent stability over 100 hours. Further analysis showed that the ultrathin FeOOH layer (~5 nm) uniformly covered the surface of NiMoO4 nanorods, acting as an active species and facilitating the surface change transfer to the interior. The internal NiMoO4 cores, on the other hand, provides reliable electron transmission as a highly conductive medium, and can effectively overcome the low conductivity of FeOOH. DFT calculations further manifest the strong electronic interactions between NiMoO4 and FeOOH species. The NiMoO4 core serves as mass transport of active materials is beneficial to tune the adsorption energy of OH- on the surface of electrocatalysts.

Hierarchical-Porous Hollow Nitrogen-Doped Carbon-Supported Pt Alloy Catalysts with a Controllable Triheterointerface for Methanol Electrooxidation.

Carbon-supported Pt-based catalysts are the most effective catalysts for direct methanol fuel cells (DMFCs). However, challenges such as high Pt loading, cost, and susceptibility to CO poisoning severely hinder the development of DMFCs. In this paper, CoFe2O4@polymer@ZIF-67 is prepared successfully through sequential solution polymerization and in situ growth with modified CoFe2O4 as the core. Subsequently, a hierarchical-porous hollow nitrogen-doped carbon-confined controllable triheterointerface catalyst, PtCoFe-CoFeOx@N-HHCS, was successfully prepared via a strategy involving high-temperature-induced phase migration and in situ chemical replacement. Under the optimal conditions, the mass activity of PtCoFe-CoFeOx@N-HHCS reached 1054 mA mgPt-1, which is 4.1 and 2.1 times higher than those of commercial Pt/C and commercial PtRu/C, respectively. The peak potential of the CO electrooxidation of the PtCoFe-CoFeOx@N-HHCS shifts negatively by 70 mV compared with commercial Pt/C. The high methanol oxidation performance is attributed to the highly dispersed triheterointerface, hierarchical-porous hollow structure, and nitrogen-doped ultrathin carbon layer. The highly dispersed triheterointerface of PtCoFe-CoFeOx@N-HHCS promotes the release of Pt and enhances the electron transfer rate through interfacial interaction, significantly improving the catalyst activity. The confinement effect of the nitrogen-doped ultrathin carbon layer prevents Pt dissolution and enhances the stability of the catalyst. The hierarchical-porous hollow structure provides rapid mass transfer channels for the methanol oxidation reaction, enhancing the reaction rate. The synergistic effect of multiple approaches endows PtCoFe-CoFeOx@N-HHCS with good methanol oxidation performance. This work provides important prospects for preparing highly active, stable, and low-loading Pt catalysts.

An ultrathin conformal layer of dry-coated Li+-conductive glass–ceramic for single-crystalline nickel-rich cathode towards all-solid-state lithium batteries

All-solid-state lithium batteries pairing nickel-rich oxide cathodes with nonflammable solid electrolytes deliver greatly combined safety and high-energy–density merits. However, the oxide cathodes possess high oxidability and easily oxidize solid electrolytes, such as sulfides and polymers, leading to great interfacial resistance and rapid performance degradation. Herein, by employing single-crystalline LiNi0.83Co0.11Mn0.06O2 (SCNCM) as typical oxide cathode, we propose a Li+-conductive glass–ceramic phosphate (GCP) as an ultrathin conformal layer using a cost-effective dry processing strategy. The GCP conformal layer possesses triple appealing advantages: (i) GCP melts at a relatively low temperature (∼790 °C) and spreads out to improve the coating coverage, effectively blocking the interfacial oxidation of solid electrolyte by high-oxidative cathode. (ii) The melted GCP adheres tightly to the cathode surface, alleviating the delamination of coating layer on the cathode during long-term cycling. (iii) GCP possesses excellent Li+ conductivity of 0.29 mS cm−1, facilitating the charge transfer and Li+ diffusion processes. These functions contribute to diminishing interfacial resistance increases across cycling, improving longterm cycling stability of SCNCM cathodes. Consequently, the SCNCM@GCP shows excellent interfacial stability with sulfide solid electrolyte, possessing an outstanding capacity retention of 86.1 % after 100 cycles with a high initial specific discharge capacity of 161.9mAh/g at 0.2C (72.4 % and 130.2mAh/g for SCNCM).

Electrochemical and Structural Properties of Ultrathin Atomic Layer Deposition Coatings for Corrosion Resistance on Aluminum alloy.

Electrochemical and Structural Properties of Ultrathin Atomic Layer Deposition Coatings for Corrosion Resistance on Aluminum alloy.

Evaluation of performance and reliability of TFT devices with ultra-thin HfTiO dielectric layer deposited by plasma enhanced atomic layer deposition

Evaluation of performance and reliability of TFT devices with ultra-thin HfTiO dielectric layer deposited by plasma enhanced atomic layer deposition

Towards a long-term stable MAPbBr3 single crystal-based photoconductor with a high on/off ratio and detectivity.

Photodetectors based on lead halide perovskites often show excellent performance but poor stability. Herein, we demonstrate a photodetector based on MAPbBr3 single crystals passivated with an ultrathin layer of PbSO4, which shows superior detectivity and on/off ratios compared to the control device due to the combined effect of lower surface traps, reduced recombination and low dark current. In addition, the device retained ∼56% of its initial D* with an impressive on/off ratio of ∼801 after one year compared to ∼22% of D* and an on/off ratio of ∼6 of the control device.

Mitigating interfacial carrier crowding by an ultrathin LiF interlayer towards efficient and stable perovskite light-emitting diodes

Quasi-two-dimensional (quasi-2D) perovskite-based light-emitting diodes (PeLEDs) have attracted intensive attention due to their high quantum yields, tunable emission wavelengths, and solution-processing capability, showing great potential in next-generation display and lighting applications. However, further performance enhancement in PeLEDs is severely limited by the uncontrolled transfer of charge carriers under bias, leading to crowding of interfacial carriers and severe efficiency roll-off. Herein, we insert an ultra-thin dielectric buffer layer of lithium fluoride (LiF) into the electron transport layer (ETL) to regulate the transfer dynamics of electrons and passivate the interfacial defects simultaneously. The dielectric LiF interlayer can effectively reduce the efficiency roll-off in PeLEDs by improving the charge balance through preventing the overwhelming injection of electrons. Moreover, the fluoride anions from LiF can passivate the surface defects of the perovskite film, enhancing the radiative recombination. As a result, the LiF interlayer-assisted quasi-2D PeLED presents an outstanding external quantum efficiency (EQE) of 24.03% and a maximum brightness of 30 845 cd m−2. The operational stability of the device is also extended, with a half-lifetime (T50) of 71.28 min (at an initial luminance of 1 000 cd m−2), which is 7.4-fold longer than that for the control device.Graphical

Artificial Control of Giant Converse Magnetoelectric Effect in Spintronic Multiferroic Heterostructure.

To develop voltage-controlled magnetization switching technologies for spintronics applications, a highly (422)-oriented Co2FeSi layer on top of the piezoelectric PMN-PT(011) is experimentally demonstrated by inserting a vanadium (V) ultra-thin layer. The strength of the growth-induced magnetic anisotropy of the (422)-oriented Co2FeSi layers can be artificially controlled by tuning the thicknesses of the inserted V and the grown Co2FeSi layers. As a result, a giant converse magnetoelectric effect (over 10-5 s m-1) and a non-volatile binary state at zero electric field are simultaneously achieved in the (422)-oriented Co2FeSi/V/PMN-PT(011) multiferroic heterostructure. This study leads to a way toward magnetoresistive random-access-memory (MRAM) with a low power writingtechnology.

Band alignment characterizations of grafted GaAs/(2¯01) Ga2O3 heterojunction via x-ray photoelectron spectroscopy

Research on gallium oxide (Ga2O3) has accelerated due to its exceptional properties, including an ultrawide bandgap, native substrate availability, and n-type doping capability. However, significant challenges remain, particularly in achieving effective p-type doping, which hinders the development of Ga2O3-based bipolar devices like heterojunction bipolar transistors (HBTs). To address this, we propose integrating mature III–V materials, specifically n-AlGaAs/p-GaAs as the emitter (E) and base (B) layers, with n-Ga2O3 as the collector (C) to form III–V/Ga2O3 n–p–n HBT. This hetero-material integration could be achieved using advanced semiconductor grafting techniques that could create arbitrary lattice-mismatched heterojunctions by introducing an ultrathin dielectric interfacial layer. This study focused on revealing the band alignment at the base–collector (B–C) junction using a n-Ga2O3(2¯01) orientated substrate combined with p-GaAs for potential HBT applications. We discovered a type-II band alignment between p-GaAs and Ga2O3(2¯01), with the p-GaAs conduction band approximately 0.614 eV higher than that of Ga2O3(2¯01). This staggered alignment allows for direct and efficient electron transport from the p-GaAs base to the n-Ga2O3 collector, avoiding the electron blocking issues present in p-GaAs/Ga2O3 (010) heterojunctions. Additionally, our study suggests the potentially existing type-II alignment between the (2¯01) and (010) Ga2O3 interfaces, highlighting the orientation-dependent band offsets. These findings are pivotal for developing high-performance Ga2O3-based HBTs, leveraging the strengths of Ga2O3 and well-established semiconductor materials to drive advancements in high-power electronics.

Low-temperature fabrication of amorphous carbon films as a universal template for remote epitaxy

Recently, remote epitaxy has been explored for the fabrication of freestanding semiconductor membranes and substrate re-use. For remote epitaxy a thin 2D material layer is either manually transferred to a substrate or grown directly on a substrate at high temperature, thus limiting the process scalability or the choice of substrates. Here, we report on the low-temperature deposition (300 °C) of ultrathin sp2-hybridized 2D amorphous carbon layers with roughness ≤0.3 nm on III-V semiconductor substrates by plasma-enhanced chemical vapor deposition as a universal template for remote epitaxy. We present growth and detailed characterization of 2D amorphous carbon layers on various host substrates and their subsequent remote epitaxial overgrowth by solid-source molecular beam epitaxy. We observe that a low-temperature nucleation step is favorable for nucleation of III-V material growth on amorphous carbon coated substrates. Under optimized preparation conditions, we obtain high-quality, single-crystalline GaAs, cubic-AlN, cubic-GaN and InxGa1−xAs layers on GaAs, 3C-SiC and InP carbon-coated (001)-oriented substrates. Our results demonstrate a universal template fabrication process for remote epitaxy.

Analysis of Optical Thin-films: Towards Lower Reflectivity for High Performance Solar Cells and Modern Photonic Devices Applications

Recently, optical thin-films with lower reflectivity have attracted much interest for their suitability in high performance thin-film solar cells and various modern photonics devices, such as electronic display panels touchscreens, smart optical glass windows, spectacles frames, super-compact camera lenses, laser systems and optical fiber communications since lowering reflectivity coating improves the device performances. However, obtaining reduced reflectance from this arrangement remains challenging issue. As the film optical properties, such as the absorbance, reflection and transmission of particular wavelength of electromagnetic radiation can be carefully controlled by optimizing thin-film fabrication materials as well as structures, there is a lot of research scope in optimizing device reflectivity by assessing various film- and substrate materials as well as their thicknesses. Therefore, in this study, the reflectance performances of optical thin-films were characterized for obtaining lower reflectivity for various types of modern photonics applications. To obtain this, three novel optoelectronic materials InGaAs, CdTe and CsPbBr&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; for film layer&amp;lt;sub&amp;gt;, &amp;lt;/sub&amp;gt;three widely used substrate materials glass, Al&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;3 &amp;lt;/sub&amp;gt;and steel as well as various thicknesses of film layer were evaluated. Reflectance studied of the thin-films for the three film materials have been clarified that CsPbBr&amp;lt;sub&amp;gt;3 &amp;lt;/sub&amp;gt;is the best among these three film materials to be used for reducing the light reflection of the thin-film. Lower reflectivity of thin-films on glass substrate suggested that glass is better than both Al&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;3 &amp;lt;/sub&amp;gt;and steel as substrate in high efficiency thin-film solar cells and various photonics devices. In addition, evaluation of reflectance for various film thicknesses showed that ultra-thin film layer is superior for reducing the reflection of solar energy by thin-film structure. We have therefore proposed that thin-film with the combination of CsPbBr&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; based ultra-thin film layer on glass substrate would be one of the best possible solutions for reducing reflectivity of solar cells and various photonics devices, thereby for possibly increasing the performance efficiency. This research result would be very beneficial for the development of renewable energy and photonics based nanotechnology, thereby play a significant role for reducing global energy crisis and green-house gas emission concurrently and sustainably in the modern world.

Flexible Piezoresistive Film Pressure Sensor Based on Double-Sided Microstructure Sensing Layer.

Flexible thin-film pressure sensors have garnered significant attention due to their applications in industrial inspection and human-computer interactions. However, due to their ultra-thin structure, these sensors often exhibit lower performance, including a narrow pressure response range and low sensitivity, which constrains their further application. The most commonly used microstructure fabrication methods are challenging to apply to ultra-thin functional layers and may compromise the structural stability of the sensors. In this study, we present a novel design of a film pressure sensor with a double-sided microstructure sensing layer by adopting the template method. By incorporating the double-sided microstructures, the proposed thin-film pressure sensor can simultaneously achieve a high sensitivity value of 5.5 kPa-1 and a wide range of 140 kPa, while maintaining a short response time of 120 ms and a low detection limit. This flexible film pressure sensor demonstrates considerable potential for distributed pressure sensing and industrial pressure monitoring applications.

Brownmillerite-Type Ca2FeCoO5 Ultrathin Layers as Anodic Catalysts for Water Electrolysis

Brownmillerite-Type Ca₂FeCoO₅ Ultrathin Layers as Anodic Catalysts for Water Electrolysis

Ultra-thin p-AlGaN insert layer for enhancing the electrical performance of AlGaN-based deep-ultraviolet light-emitting diodes

A composite p-contact structure is proposed to enhance the electrical performance of AlGaN-based deep-ultraviolet light-emitting diodes (DUV-LEDs). With the insertion of a 1 nm-thick p-AlGaN layer prior to the contact layer, the operating voltage at 100 mA is significantly reduced by 0.2–0.3 V in 277 nm DUV-LEDs, leading to a maximum wall-plug efficiency of 10.7%. It is experimentally demonstrated that the reduction of operating voltage is mainly attributed to the lower specific contact resistivity of p-contact. The improvement mechanism of contact property is further explored by the theoretical calculations of energy band, where the hole accumulation in the contact layer, owing to the greater valence band offset induced by the p-AlGaN insert one, is speculated to promote the tunneling at the metal/semiconductor interface. The composite p-contact structure in this study is compatible with the present commercial DUV-LED epitaxial structure, enabling it to promote further development of this field.

A Liquid Metal Balloon for the Exfoliation of an Ultrathin and Uniform Gallium Oxide Layer.

We report the exfoliation of ultrathin gallium oxide (Ga2O3) films from liquid metal balloons, formed by injecting air into droplets of eutectic gallium-indium alloy (eGaIn). These Ga2O3 films enable the selective adsorption of carbon nanotubes (CNTs) dispersed in water, resulting in the formation of a dense, percolating CNT network on their surface. The self-assembled CNT network on Ga2O3 provides a versatile platform for device fabrication. As an example application, we fabricated a chemiresistive gas sensor for detecting simulants of chemical warfare agents (CWAs), including diisopropyl methylphosphonate (DIMP), dimethyl methylphosphonate (DMMP), and triethyl phosphate (TEP). The sensor exhibited reversible responses, high sensitivity, and low limits of detection (13 ppb for DIMP, 28 ppb for DMMP, and 53 ppb for TEP). These findings highlight the potential of Ga2O3 films derived from liquid metal balloons for integrating CNTs into functional electronic devices.

Interface Engineering of MOF Nanosheets for Accelerated Redox Kinetics in Lithium-Sulfur Batteries.

Modifying the separator is considered as an effective strategy for achieving high performance lithium-sulfur (Li-S) batteries. However, most modification layers are excessively thick, with catalytic active sites primarily located within the material's interior. This configuration severely impacts Li+ transport and the efficient catalytic conversion of polysulfides. Therefore, there is an urgent need to develop a multifunctional separator that integrates ultrathin design, catalytic activity, and ion sieving capabilities. Herein, we successfully linked TCPP(Ni) as a secondary ligand with Zr-BTB nanosheets to create an ultra-thin separator modification layer (Zr-TCPP(Ni)) with efficient ion sieving and catalytic properties. The resultant multifunctional separators provide robust ion sieving capabilities that promote rapid Li+ transport and intercept polysulfides shuttling. Therefore, The Zr-TCPP(Ni)@PP cell maintains 70.0 % of its initial capacity after 600 cycles at a high rate of 3 C, while achieving an impressive areal capacity of 4.55 mA h cm-2 even with high sulfur content of 80 wt% at 0.5 C. This work provides valuable insights for rational design of MOF interface engineering in high energy density Li-S batteries.

Transition Metal Chalcogenides/Nonconjugated Polymer Artificial Photosystems for Photoredox Catalysis.

Surface charge transfer doping (SCTD) has been established as an efficient strategy to achieve strong electronic coupling interactions between semiconductors and dopants, which lead to highly efficient electron transport over semiconductors. Herein, we report a facile, easily accessible, and effective SCTD strategy to exquisitely modulate the interfacial charge transfer over transition metal chalcogenides (TMCs: CdS, Zn0.5Cd0.5S, CdIn2S4, and ZnIn2S4) through surface modification with a nonconjugated polymer, poly(dimethyldiallylammonium chloride) (PDDA). We provide evidence that PDDA, as a surface electron transfer acceptor, can be used to enable rapid, directional, and tunable charge transfer along with an optimal charge lifetime over TMCs in photoredox catalysis because of the high-efficiency electron-trapping property of quaternary ammonium functional groups in the molecular structure of PDDA. The thus-assembled PDDA-encapsulated TMC composite artificial photosystems demonstrate significantly enhanced and versatile photoredox catalytic activities toward visible-light-responsive photocatalytic reduction of aromatic nitro compounds, photocatalytic oxidation of aromatic alcohols, and photocatalytic H2 production, wherein the ultrathin PDDA layer accelerates the interfacial charge transport and separation rate over TMC substrates. Moreover, it was evidenced that such an interface engineering strategy is general for a collection of TMCs. Our work will provide conceptual insights into nonconjugated polymer-based artificial photosystems for optimizing solar energy utilization.