Reversible Surface Research Articles

(Invited) Metallization for Advanced Semiconductor Technology Nodes: Wet-Chemical Deposition and Etching, Characterization, and (Semi) Damascene Approaches

To ensure semiconductor technology scaling to continue for the decades to come, the industry focuses on identifying alternative metals (elemental or alloys), that can replace copper in the back end of line (BEOL) integration scheme, for which the metal is used as main conductor for most of the metallization levels. Recently, the semi-damascene process is being developed, in which the interconnect metal is first deposited and then patterned using a dry etching process (‘direct metal etch’), in a fashion closely resembling the traditional Al metallization. One of the advantages is that any metal that can be deposited as a blanket thin film (either by physical vapor deposition (PVD), electrochemical deposition (ECD), electroless deposition (ELD), chemical vapour deposition (CVD), or atomic layer deposition (ALD)) can subsequently be patterned, provided that the critical requirement for dry etching of ‘volatilization’ of the metal is fulfilled. Furthermore, it allows the integration of materials for which no ECD, ELD, CVD or ALD process is available to fill damascene trenches.In this contribution, intrinsic material properties (bulk resistivity, electromigration, ...) and composition (surface versus bulk composition as well as surface oxide) of a wide range of metals and alloys of interest to the BEOL will be compared. An overview of wet-chemical deposition approaches of a few selected metals (Cu and Rh) will be given. For ELD, the focus will be on discussing the mechanistic understanding of the intrinsically complex process, for which the stability and reactivity of the reducing agent is affected by solution pH and composition as well as the presence of oxygen. For ECD, results will be discussed for Rh and Ni, for which an electrochemical quartz crystal microbalance (EQCM) was used to quantify the amount of material deposited and interpret the various features detected in cyclic voltammograms. Metal ion solution speciation as well as stability, of importance for both deposition methods, has been studied using UV-vis spectroscopy and electrochemical characterization (linear sweep and cyclic voltammetry). Deposits were analysed morphologically using transmission electron microscopy and the chemical composition was assessed by elemental mapping.During process integration, the surface-chemical composition of metals / alloys needs to be carefully controlled, as this can, for instance, critically impact metal line resistance or result in wet etching or corrosion. A few example cases in which this is relevant will be presented, e.g. the reversible surface oxidation and reduction of Cu was studied using EQCM, the surface chemistry of NiAl was characterized using electrochemical measurements, and the reversible surface nitridation of a model metal (Mo) is demonstrated as a method to protect against unwanted metal dissolution.

Electrosorption of alkaline earth metal ions onto activated carbon as affected by pH and applied potential (pE)

The electrosorption of alkaline earth metal ions from aqueous solution was studied using a graphite supported activated carbon electrode (NSA@G). Zeta potential measurement revealed a low pHzpc of 3.0 for the NSA electrode, suggesting a negatively charged L-type carbon surface. The electrosorption behavior of Ca2+ ion followed the Langmuir adsorption isotherm and pseudo-first-order rate law. The initial Ca2+ ion concentration, solution pH, and applied potential affected the electrosorption capacity. Results showed that both reversible surface charge (regulated by the potential determining ions, i.e., H+ and OH−, or pH) and polarizable surface charge (controlled by the applied potential, or pE) contributed to the overall Ca2+ ion removal process. The contribution of reversible and polarizable surface charge varied with pH and pE, respectively. Specifically, the reversible surface charge played a more significant role in Ca2+ electrosorption at high pH value and low pE (at 60–83 % of the total Ca2+ uptake), while the polarizable surface charge dominated at low pH and high pE (at 60–62 % of the total Ca2+ uptake). Factors, such as ionic radius, hydration ratio, and hydration enthalpy, significantly affected the electrosorption capacity of divalent alkaline earth metals, i.e., Ca2+, Mg2+, Sr2+, and Ba2+, over NSA@G electrode. NoveltyThis work elucidated the mechanisms of electrode charging and Ca2+ ion uptake via electrosorption. Previous research often attributed ion electrosorption capacity solely to the surface charge derived from a polarizable electrode. In addition to polarizable surface charge, which is controlled by the applied potential (or pE), this study demonstrated that reversible surface charge, regulated by the potential determining ions, i.e., H+ and OH− ions (or pH), also played a significant role in total Ca2+ ion removal. The novelty of this work lies in quantifying the contribution of reversible and polarizable surface charge to overall Ca2+ ion electrosorption. Furthermore, this study investigated the factors affecting the electrosorption behavior of alkaline earth metals, i.e., Mg2+, Ca2+, Sr2+, and Ba2+. A rational approach to predicting electrosorption performance is also proposed, using capacitance characterization from cyclic voltammetry measurements based on Lipmann's electrocapillarity theory.

High-energy Mn2V2O7//C asymmetric supercapacitors in aqueous/organic hybrid electrolytes

The demand for advanced energy storage devices, such as high-energy density supercapacitors, has spurred efforts to develop new environmentally friendly and efficient electrode materials and electrolyte solutions for sustainable development. A prominent strategy is the fabrication of high-voltage and energy-density asymmetric supercapacitors (AS). In this study, dimanganese divanadate (Mn2V2O7, MVO) particles were synthesized via a hydrothermal process and used as pseudocapacitive electrodes for AS. The pseudocapacitive MVO electrode offered synergistic advantages of reversible surface or near-surface Faradaic reactions for charge storage. Another approach to address the challenges of limited energy density in supercapacitors was modifying the electrolyte solution to widen the operating voltage window and increase energy density. A hybrid electrolyte solution of 5 M sodium perchlorate (NaClO4) in a mixture of (2:1) volume ratio of acetonitrile (AN) and water (H2O) was introduced to expand the voltage window in the AS. The fabricated AS with MVO as a pseudocapacitive electrode and activated carbon as a double-layer capacitive electrode operated in a voltage window > 2.0 V with the hybrid electrolyte. The AS device exhibited a capacitance of 156.9 F g−1 at 0.1 A g−1 and a high energy density of 87.2 Wh kg−1, surpassing the carbon-based symmetric supercapacitor (58.9 Wh kg−1). Furthermore, the pseudocapacitive supercapacitor showed exceptional long-term stability over 50,000 cycles. Structural and surface characterization of the MVO electrode after CV testing validated that the charge storage mechanism in Mn2V2O7//C was a surface-controlled process. The performance of the AS pouch cell revealed the great promise of the pseudocapacitive electrode in practical applications.

Self-limited and reversible surface hydration of Na2Fe(SO4)2 cathodes for long-cycle-life Na-ion batteries

Air stability is a crucial factor in the practical application of a battery material, as it profoundly affects the material's preparation, storage, and electrode fabrication processes. Sodium iron sulfate cathodes, despite their attractive attributes in cost and electrochemical performance, are widely believed to be unstable upon air exposure because of the sulfate group. Here we report remarkable air-stability of the Na2Fe(SO4)2-based (NFS) cathodes (minimal decay in 20 % RH air for 60 days, 91.9 % capacity retention after 3500 cycles in half cells) and their outstanding cycle performance in practically relevant pouch-type full cells (∼100 Wh kg-1 specific energy, >1000 cycle-life). Although the NFS cathodes do react with moisture H2O to produce Na2Fe(SO4)2⋅4H2O but the hydration is spatially confined at the NFS particles’ surface and not propagating into their bulk. Further, the structural changes are reversible when the surface-hydrated NFS particles are heated in the typical electrode vacuum-drying process, avoiding extra treatment and additional cost. This work reveals the promising properties of the NFS cathode materials towards high-performance and sustainable Na-ion batteries.

Illuminating Biomimetic Nanochannels: Unveiling Macroscopic Anticounterfeiting and Photoswitchable Ion Conductivity via Polymer Tailoring.

Artificial photomodulated channels represent a significant advancement toward practical photogated systems because of their remote noncontact stimulation. Ion transport behaviors in artificial photomodulated channels, however, still require further investigation, especially in multiple nanochannels that closely resemble biological structures. Herein, we present the design and development of photoswitchable ion nanochannels inspired by natural channelrhodopsins (ChRs), utilizing photoresponsive polymers grafted anodic aluminum oxide (AAO) membranes. Our approach integrates spiropyran (SP) as photoresponsive molecules into nanochannels through surface-initiated atom transfer radical polymerization (SI-ATRP), creating a responsive system that modulates ionic conductivity and hydrophilicity in response to light stimuli. A key design feature is the reversible ring-opening photoisomerization of spiropyran groups under UV irradiation. This transformation, observable at the molecular level and macroscopically, allows the surface inside the nanochannels to switch between hydrophobic and hydrophilic states, thus efficiently modulating ion transport via changing water wetting behaviors. The patternable and erasable polySP-grafted AAO, based on a controllable and reversible photochromic effect, also shows potential applications in anticounterfeiting. This study pioneers achieving macroscopic anticounterfeiting and photoinduced photoswitching through reversible surface chemistry and expands the application of polymer-grafted structures in multiple nanochannels.

Anion Storing Boron Nitride Hybrid Nanosheets for High-Performance Dual Ion and Zinc Alkaline Full Cells.

Hexagonal boron nitride (BN), a well-known member of 2D materials, has a structure similar to graphene and is often referred to as white graphene. Despite its unique physical and chemical properties for energy storage applications, there have been very few studies on how BN stores anion carriers. Herein, the hybrid architecture and anion storage mechanism of BN nanosheets for high-performance hybrid energy storage full cells based on dual-ion and Zinc (Zn) alkaline systems is demonstrated. The chemical bonding between BN and reduced graphene oxide (rGO) is attributed to the formation of the heterointerface, which facilitates the charge transfer kinetics during an OH storing process. Based on the reversible surface redox reaction of BN and rGO hybrid (BN@rGO) confirmed by computational and spectroscopic analyses, the BN@rGO electrode is applied to both Na and OH dual-ion and Zn alkaline full cells. In the dual-ion system, Ti3C2‖BN@rGO full cells extended the operating voltage range up to 1.7V, delivering a cell capacity of 49.4mAhg-1 at 1000mAg-1 and retaining 80% of its initial capacity after 40000 cycles. In the Zn alkaline system, Zn‖BN@rGO full cells achieved a cell capacity of 58.1mAhg-1 at 1000mAg-1 and retained 80% capacity over 90000 cycles.

Direct Observation of Reversible Surface Potential in Pseudocapacitive Electrochromic Films by Kelvin Probe Force Microscopy

High-resolution topography and local potential images together are very powerful tools for understanding the local environment of nanomaterials. In electrochemistry, there are significant changes within the electrode materials upon charging and discharging. However, the surface changes for pseudocapacitive materials in repetitive electrochemical cycling are easily being overlooked. Thanks to the advances in Atomic Force Microscopy, it becomes feasible to visualize the electrical changes in surface upon charge transfer with Kelvin Probe Force Microscopy (KPFM) equipped with a conductive tip. We report the KPFM surface potential images and profiles of NiO electrochromic films upon bleaching (charging) and coloration (discharging). The measured surface potential is about 60 times higher in bleaching state upon charging at 20mC/cm2. What is interesting is that the potential profile shows identical topographic features with the height profile of surface morphology. The surface particles further demonstrate shape entanglement in one direction (from ~100nm to ~140nm) and shrinkage in the orthogonal direction, suggesting the charge-induced electrical stress. According to the pseudocapacitive features in the cyclic voltammetry and electrochemical impedance spectroscopy, the surface-dominant redox behavior is within expectation. Though, this dynamic surface potential change is not well studied but important, and associated with work function fluctuations and various kinds of nanoscale charge-sensitive properties, especially for efficient electrochromic supercapacitor energy storage.

Membraneless Electrochemical Synthesis Strategy toward Nitrate-to-Ammonia Conversion.

Electroreduction of nitrate (NO3RR) to ammonia in membraneless electrolyzers is of great significance for reducing the cost and saving energy consumption. However, severe chemical crossover with side reactions makes it challenging to achieve ideal electrolysis. Herein, we propose a general strategy for efficient membraneless ammonia synthesis by screening NO3RR catalysts with inferior oxygen reduction activity and matching the counter electrode (CE) with good oxygen evolution activity while blocking anodic ammonia oxidation. Consequently, screening the available Co-Co system, the membraneless NO3--to-NH3 conversion performance was significantly higher than H-type cells using costly proton-exchange membranes. At 200 mA cm-2, the full-cell voltage of the membraneless system (∼2.5 V) is 4 V lower than that of the membrane system (∼6.5 V), and the savings are 61.4 kW h (or 56.9%) per 1 kg NH3 produced. A well-designed pulse process, inducing reversible surface reconstruction that in situ generates and restores the active Co(III) species at the working electrode and forms favorable Co3O4/CoOOH at the CE, further significantly improves NO3--to-NH3 conversion and blocks side reactions. A maximum NH3 yield rate of 1500.9 μmol cm-2 h-1 was achieved at -0.9 V (Faraday efficiency 92.6%). This pulse-coupled membraneless strategy provides new insights into design complex electrochemical synthesis.

ON-OFF Control of Marangoni Self-propulsion via A Supra-amphiphile Fuel and Switch.

Marangoni self-propulsion refers to motion of liquid or solid driven by a surface tension gradient, and has applications in soft robots/devices, cargo delivery, self-assembly etc. However, two problems remain to be addressed for motion control (e.g., ON-OFF) with conventional surfactants as Marangoni fuel: (1) limited motion lifetime due to saturated interfacial adsorption of surfactants; (2) in- situ motion stop is difficult once Marangoni flows are triggered. Instead of covalent surfactants, supra-amphiphiles with hydrophilic and hydrophobic parts linked noncovalently, hold promise to solve these problems owing to its dynamic and reversible surface activity responsively. Here, we propose a new concept of 'supra-amphiphile fuel and switch' based on the facile synthesis of disodium-4-azobenzene-amino-1,3-benzenedisulfonate (DABS) linked by a Schiff base, which has amphiphilicity for self-propulsion, hydrolyzes timely to avoid saturated adsorption, and provides pH-responsive control over ON-OFF motion. The self-propulsion lifetime is extended by 50-fold with DABS and motion control is achieved. The mechanism is revealed with coupled interface chemistry involving two competitive processes of interfacial adsorption and hydrolysis of DABS based on both experiments and simulation. The concept of 'supra-amphiphile fuel and switch' provides an active solution to prolong and control Marangoni self-propulsive devices for the advance of intelligent material systems.

ON–OFF Control of Marangoni Self‐propulsion via A Supra‐amphiphile Fuel and Switch

AbstractMarangoni self‐propulsion refers to motion of liquid or solid driven by a surface tension gradient, and has applications in soft robots/devices, cargo delivery, self‐assembly etc. However, two problems remain to be addressed for motion control (e.g., ON–OFF) with conventional surfactants as Marangoni fuel: (1) limited motion lifetime due to saturated interfacial adsorption of surfactants; (2) in‐ situ motion stop is difficult once Marangoni flows are triggered. Instead of covalent surfactants, supra‐amphiphiles with hydrophilic and hydrophobic parts linked noncovalently, hold promise to solve these problems owing to its dynamic and reversible surface activity responsively. Here, we propose a new concept of ‘supra‐amphiphile fuel and switch’ based on the facile synthesis of disodium‐4‐azobenzene‐amino‐1,3‐benzenedisulfonate (DABS) linked by a Schiff base, which has amphiphilicity for self‐propulsion, hydrolyzes timely to avoid saturated adsorption, and provides pH‐responsive control over ON‐OFF motion. The self‐propulsion lifetime is extended by 50‐fold with DABS and motion control is achieved. The mechanism is revealed with coupled interface chemistry involving two competitive processes of interfacial adsorption and hydrolysis of DABS based on both experiments and simulation. The concept of ‘supra‐amphiphile fuel and switch’ provides an active solution to prolong and control Marangoni self‐propulsive devices for the advance of intelligent material systems.

Reversible surface reconstruction of bifunctional NiCu/FeNi2S4 heterostructure catalyst for overall water splitting

The synergistic effects of bifunctional catalysts in both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) must be better understood. Herein, the 3D hierarchical heterostructure consisting of NiCu surface layer assembled on the FeNi2S4 (or Ni-Fe layered-double-hydroxide, NiFe-LDH) is constructed on the nickel foam (NF) and the bifunctional catalytic effects of HER/OER are investigated in an alkaline solution. Although the high OER activity of NiFe-LDH and HER activity of NiCu/NF, NiCu/FeNi2S4 heterostructures is demonstrated by the small overpotentials of HER/OER (306 mV at a current density of 10 mA cm-2), in comparison with NiCu/NF (315 mV) and NiCu/NiFe-LDH (357 mV). An ultra-low cell voltage of 1.54 V is accomplished in overall water splitting. The high-performance bifunctional catalytic effects can be attributed to the synergistic effects of reversible surface reconstruction in the presence of S element, enabling the formation of the NiO/Ni interface at the low potential for HER and high oxidation states (Ni2+δ) at the high potential for OER. The results provide insights into the design of bifunctional electrocatalysts with high activity.

Reversible Surface Engineering of Cellulose Elementary Fibrils: From Ultralong Nanocelluloses to Advanced Cellulosic Materials.

Cellulose nanofibrils (CNFs) are supramolecular assemblies of cellulose chains that provide outstanding mechanical support and structural functions for cellulosic organisms. However, traditional chemical pretreatments and mechanical defibrillation of natural cellulose produce irreversible surface functionalization and adverse effects of morphology of the CNFs, respectively, which limit the utilization of CNFs in nanoassembly and surface functionalization. Herein, this work presents a facile and energetically efficient surface engineering strategy to completely exfoliate cellulose elementary fibrils from various bioresources, which provides CNFs with ultrahigh aspect ratios (≈1400) and reversible surface. During the mild process of swelling and esterification, the crystallinity and the morphology of the elementary fibrils are retained, resulting in high yields (98%) with low energy consumption (12.4kJ g-1). In particular, on the CNF surface, the surface hydroxyl groups are restored by removal of the carboxyl moieties via saponification, which offers a significant opportunity for reconstitution of stronger hydrogen bonding interfaces. Therefore, the resultant CNFs can be used as sustainable building blocks for construction of multidimensional advanced cellulosic materials, e.g., 1D filaments, 2D films, and 3D aerogels. The proposed surface engineering strategy provides a new platform for fully utilizing the characteristics of the cellulose elementary fibrils in the development of high-performance cellulosic materials.

Heterogeneous Reactions of Phenol on Different Components of Mineral Dust Aerosol: Formation of Oxidized Organic and Nitro-Phenolic Compounds.

Phenol, a common semi-volatile compound associated with different emissions including from plants and biomass burning, as well as anthropogenic emissions and its derivatives, are important components of secondary organic aerosols (SOAs). Gas and aqueous phase reactions of phenol, in the presence of photochemical drivers, are fairly well understood. However, despite observations showing aromatic content within SOA size and mass increases during dust episodes, the heterogeneous reactions of phenol with mineral dusts are poorly understood. In the current study, surface reactions of phenol at the gas/solid interface with different components of mineral dust including SiO2, α-Fe2O3, and TiO2 have been investigated. Whereas reversible surface adsorption of phenol occurs on SiO2 surfaces, for both α-Fe2O3 and TiO2 surfaces, phenol reacts to form a wide range of OH substituted aromatic products. For α-Fe2O3 surfaces that have been nitrated by gas-phase reactions of nitric acid prior to exposure to phenol, unique compounds form on the surface including nitro-phenolic compounds. Moreover, additional surface chemistry was observed when adsorbed nitro-phenolic products were exposed to gas-phase SO2 as a result of the formation of adsorbed nitrite from nitrate redox chemistry with adsorbed SO2. Overall, this study reveals the extensive chemistry as well as the complexity of reactions of prevalent organic compounds leading to the formation of SOA on mineral surfaces.

Gradient Pores Enhance Charge Storage Density of Carbonaceous Cathodes for Zn-Ion Capacitor.

Engineering carbonaceous cathode materials with adequately accessible active sites is crucial for unleashing their charge storage potential. Herein, activated meso-microporous shell carbon (MMSC-A)nanofibers are constructed to enhance the zinc ion storage density by forming a gradient-pore structure. A dominating pore size of 0.86nm is tailored to cater for the solvated [Zn(H2 O)6 ]2+ . Moreover, these gradient porous nanofibers feature rapid ion/electron dual conduction pathways and offer abundant active surfaces with high affinity to electrolyte. When employed in Zn-ion capacitors (ZICs), the electrode delivers significantly enhanced capacity (257 mAh g-1 ), energy density (200Wh kg-1 at 78W kg-1 ), and cyclic stability (95% retention after 10000 cycles) compared to nonactivated carbon nanofibers electrode. A series of in situ characterization techniques unveil that the improved Zn2+ storage capability stems from size compatibility between the pores and [Zn(H2 O)6 ]2+ , the co-adsorption of Zn2+ , H+ , and SO4 2- , as well as reversible surface chemical interaction. This work presents an effective method to engineering meso-microporous carbon materials toward high energy-density storage, and also offers insights into the Zn2+ storage mechanism in such gradient-pore structures.

Carbon-based nanotube and graphene thermal stable materials generated via surface oxidation and chemical modification

We have developed a facile approach to undertake chemical modifications of carbon nanotubes (CNT) and of graphite/graphene via oxidation and surface chemical modification to superhydrophobic nanomaterials. Chemical silane is functionalized to these oxide nanowires forms of carbon-oxygen-silicon chemical bonds via the liquid chemical-vapor-deposition (CVD) technique. After a correlation test of combinations of heating at a set temperature then measuring the surface water contact angle, these two types of materials have been generated showing both having very high thermally stable superhydrophobic nanomaterials. Both of modified CNT and graphene can maintain a superhydrophobic surface at extremely high temperatures, up to 550 and 600 °C, generated from graphene oxide (GO) and surface oxidized carbon nanotubes (CNTO), respectively. Over the critical switchable temperature, the surface switches from superhydrophobic to superhydrophilic immediately; it can be reversed to superhydrophobic by re-coating silane, thereby showing a reversible surface property. The superhydrophobic carbon nanomaterials can rapidly absorb oil either under water or from the surface water. Gaussian DFT calculations for the dissociation energies via different pathways confirm that this system is quite stable, consistent with the thermal test results.

Solid-State Redox-Active Pseudocapacitor with Improved Performance at High Temperature

With the emergence of environmental issues such as global warming as well as our new demand for efficient energy storage systems (EES) that can power portable electronics; batteries and supercapacitors are gaining more attention in the latest literature. Batteries can provide high energy density and lower power densities (e.g. for LIB, 1kW.kg-1 vs. 150-200 Wh.kg-1), while supercapacitors are high power devices with low energy densities (e.g. for most commercial devices ⁓ 10 kW.kg-1 vs. 5-10 Wh.kg-1). In order to couple high energy and high power densities of both devices, pseudocapacitance was introduced in 1991 by Conway 1 using metal oxide. Pseudocapacitance is a fast and reversible surface faradaic process with performance similar to double layer capacitors but higher energy densities closer to that of batteries. Aside from metal oxides, conductive polymers also demonstrate pseudocapacitance. In this regards, the use of conductive polymers as nontoxic, abundant, low cost and sustainable organic material is on the rise. Nevertheless, the inherent issue related to instability of the conductive polymer especially at high degree of oxidation had restricted their application in EES. However, redox active polymers (RAPs) with non-conjugated backbone and redox active pendant group demonstrates improved performance and structural diversity with more distinct redox potentials. 2 Amongst them, quinone-containing polymers with their high theoretical capacities, facile kinetics and tunable redox potential are at the top of the list. Particularly, catechol, a bioinspired ortho-quinone based polymer has gain popularity following a work by Detrembleur et al. 3 However; the solubility of the organic materials in organic solvents hinders their application in EES. To this end, using solid-state electrolyte can help alleviate the issue. Nevertheless, solid-state design stimulate sluggish kinetics and reduce electrode/electrolyte interfacial area, a component required to achieve high pseudocapacitance. Consequently, nano-structuring the redox polymer can help improve the contact resistance as well as increasing the surface area. In addition, the nanoparticles structure will promote more distinct redox processes. 4 In this study, we demonstrate for the first time, a prototype pouch cell based on all-solid-state organic hybrid supercapacitor bearing catechol redox active moieties. Using emulsion polymerization, catechol based RAP nanoparticles of size ranging from 50 to 150 nm were synthesized 4 and together with activated carbon was used as working electrode. Block-co-polymers based solid electrolyte was employed as electrolyte as well as binder for both cathode and anode. Using this design, discharge capacities of ⁓ 10 and 60 mAh/g at room temperature and at 50 °C were obtained, respectively.

(Invited) Atomic Imaging and Device Characterization of Molecularly Thin 2D Hybrid Perovskites

2D Organic-Inorganic Hybrid perovskites (OIHPs) possess alternating layers of organic molecules and inorganic quantum wells. By tuning the interaction between organic and inorganic layers, very interesting properties can be introduced. The successful isolation of hybrid perovskite monolayers with clean and flat surfaces are particularly suitable for atomic structure characterization and optoelectronic device fabrication. In this talk, I will present recent results on the excitonic properties and atomic structure of molecularly thin OIHPs1,2. We found that a reversible shift in excitonic energies can be induced upon laser irradiation in large-sized 2D hybrid perovskite monolayers which is attributed to the reversible structural reorientation of the surface BA cations in the easily deformable lattice3. The photodetection performance of monolayer, bilayer and thicker OIHPs were compared. The internal quantum efficiency was measured to be 34% for a monolayer (BA)2(MA)3Pb4I13 and 19% for the bulk crystal. In addition, 2D heterostructures of molecularly thin OIHPs/graphene were constructed which exhibited a lower barrier than gold for carrier injection, enabling applications in field effect transistors (FETs)4. Electron tunneling occurs across the interface of organic molecular layers on 2D perovskite and graphene, while photoinduced charge transfer occurs at femtosecond timescale (~50 fs).Resolving the atomic structure of 2D hybrid perovskite is a challenging task because it is easily damaged by electron beam. Here we use scanning tunneling microscopy (STM) & Qplus AFM to directly visualize surface octahedral tilt in freshly exfoliated 2D Ruddlesden-Popper perovskites (RPPs)5. The experimentally determined octahedral tilts from n = 1 to n = 4 RPPs from STM images are found to agree very well with out-of-plane surface octahedral tilts predicted by density functional theory calculations. The surface-enhanced octahedral tilt is correlated to excitonic redshift observed in photoluminescence (PL) and promotes Rashba spin splitting for n > 1. Our studies demonstrate a protocol to investigate structure-property correlations in low dimensional 2D OIHPs, which will pave the way to better material design and device performance.References Leng, W. Fu, Y. Liu, M. Chhowalla & KP Loh* “From Bulk to Molecularly Thin Hybrid Perovskites” Nature Reviews Materials , 5, 482-500 (2020).Zhao, KP Loh, K. Leng* “Organic-inorganic hybrid perovskites and their heterostructures” Matter , 5, 4153-4169 (2022).Leng, I. Abdelwaha, I. Verzhbitskiy, M. Telychko, L. Chu, W. Fu, X. Chi, N. Guo, Z. Chen, Z. Chen, C. Zhang, Q. Xu, J. Lu, M. Chhowalla, G. Eda, KP Loh* “Molecularly thin two-dimensional hybrid perovskites with tunable optoelectronic properties due to reversible surface relaxation” Nature Materials , 17, 908-914 (2018).Leng, L. Wang, Y. Shao, I. Abdelwahab, G. Grinblat, I. Verzhbitskiy, Y. Cai, X. Chi, W. Fu, P. Song, G. Eda, S. A. Maier & KP Loh* “Electron Tunneling at the Molecularly Thin 2D Perovskite and Graphene Van der Waals Interface” Nature Communications , 11, 5483 (2020).Y Shao, W. Gao, H. Yan, R. Li, I. Abdelwahab, L. Zhuang, W. Fu, SP Lau, SF YU, Y. Cai, KP Loh, K. Leng* “Unlocking Surface Octahedral Tilt in Two-dimensional Ruddlesden-Popper Perovskites” Nature Communications , 13, 138 (2022). AcknowledgementK.L. would like to acknowledge the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. PolyU15305221 for GRF project funded in 2021/22 Exercise and Project No. PolyU25305222 for ECS project funded in 2022/23 Exercise).

Effect of the Initial Orientation of a Working Tool on the Stress–Strain State during Reversible Surface Plastic Deformation

Effect of the Initial Orientation of a Working Tool on the Stress–Strain State during Reversible Surface Plastic Deformation

Reversible surface modification of pan-based carbon fibers by a ferrocene-based surfactant

Reversible surface modification of pan-based carbon fibers by a ferrocene-based surfactant

Plasma-induced reversible surface modification and its impact on oxygen heterogeneous recombination

A novel model is developed for atomic oxygen surface kinetics in silica-like walls, introducing a plasma-induced surface modification, which may impact intermediate pressure plasma reactors. The model is the first to reproduce experimental measurements in an oxygen glow discharge operating in the pressure range between 0.27 mbar (0.2 Torr) and 4 mbar (3 Torr), showing a decrease with pressure of the O recombination probability on Pyrex between 0.27 mbar and 1 mbar. The numerical simulations suggest that a modification is induced by the production and destruction of metastable chemisorption sites at the surface. As such, the Langmuir–Hinshelwood (L-H) and Eley-Rideal (E-R) recombination mechanisms take place involving not only physisorption and stable chemisorption sites, but also metastable chemisorption sites, produced by the impact of fast O2 ions and neutrals. The presence of metastable sites can be reversed by increasing the plasma pressure.