Oxygen Termination Research Articles

Hybrid MXene-Graphene/Hexagonal Boron Nitride Structures: Electronic and Molecular Adsorption Properties.

Recent advances in experimental techniques allow for the fabrication of hybrid structures. Here, we study the electronic and molecular adsorption properties of the graphene (G)/hexagonal boron nitride (h-BN)-MXenes (MoC) hybrid nanosheets. We use first-principles calculations to explore the structure and electronic properties of the hybrid structures of G-2H-MoC and h-BN-2H-MoC with two different oxygen terminations of the MoC surface. The embedding of G or h-BN patches creates structural defects at the patch-MoC border and adds new states in the vicinity of the Fermi energy. Since this can be utilized for molecular adsorption and/or sensing, we investigate the ability of the G-M-O1 and BN-M-O1 hybrid structures to adsorb twelve molecules. Generally, the adsorption on the hybrid systems is significantly higher than on the pristine systems, except for N and H, which are weakly adsorbed on all systems. We find that OH, NO, NO, and SO are chemisorbed on the hybrid systems. COOH may be chemisorbed, or it may dissociate depending on its location at the edge between the G/h-BN and the MXene. NH is chemisorbed/physisorbed on the BN/G-M-O1 systems. CO, HS, CO, and CH are physisorbed on the hybrid systems. Our results indicate that the studied hybrid systems can be used for molecular filtration/sensing and catalysis.

Synthesis and in situ sulfidation of molybdenum carbide MXene using fluorine-free etchant for electrocatalytic hydrogen evolution reactions

Synthesizing MXenes from Mn+1AXn (MAX) phases using hazardous hydrogen fluoride is a common and effective method. However, fluorine termination on the basal planes and edges of the resulting MXenes is undesirable for the electrocatalytic hydrogen evolution reaction (HER), while oxygen (O), hydroxyl (OH), and sulfur (S) termination favors this reaction. Herein, we unveil a simple fluorine-free exfoliation and two-step vulcanization method for synthesizing molybdenum sulfide-modified molybdenum carbide (MoS2/Mo2CTx MXene, T = OH, O, S) for the HER in alkaline medium. Microwave-assisted hydrothermal treatment of the MAX phase (Mo3AlC2) with sodium hydroxide-sodium sulfide as an etching solution and thioacetamide as a source of sulfide ions enabled the selective dissolution of the aluminum (Al) layer and sulfidation of the surface Mo atoms to form amorphous MoS2. Thus, the vulcanization of Mo2CTx MXene resulted in the formation of MoS2/Mo2CTx MXene. The MoS2 formed on the surface of Mo2CTx provided enhanced stability by preventing oxidation. MoS2/Mo2CTx exhibited enhanced electrocatalytic activity toward the HER, mainly due to the O, OH, and amorphous MoS2 functionalities. The MoS2 sites on the surface exhibited an overpotential of 110 ± 7 mV at a current density of 10 mA cm−2 as a result of enhanced charge transfer and mass transfer. Thus, the sulfidation method demonstrated herein is capable of producing amorphous MoS2 structures on Mo2CTx MXene, which could be applied for the surface modification of other molybdenum-based nanomaterials or electrocatalysts to improve stability and enhance electrocatalytic activity.

A Peculiar Binding Characterization of DNA (RNA) Nucleobases at MoOS-Based Janus Biosensor: Dissimilar Facets Role on Selectivity and Sensitivity.

Distinctive properties of Janus monolayer have drawn much interest in biotechnology applications. For this purpose, it has explored theoretically all sensing possibilities of nucleobases molecules (DNA/RNA) by Janus MoOS monolayer on both oxygen and sulfur terminations by means of rigorous first–principles calculation. Indeed, differences in interaction energy between nucleobases indicate that a monolayer can be used for DNA sequencing. Exothermic interaction energy range for DNA/RNA molecules with both oxygen and sulfur sides of the Janus MoOS surfaces have been found to range between (0.61–0.91 eV), and (0.63–0.88 eV), respectively, and the binding distances indicate that these molecules bind to both facets by physisorption. The exchange of weak electronic charges between the MoOS monolayer and the nucleobases molecules has been studied by means of Hirshfeld-I charge analysis. It has been observed that the introduction of DNA/RNA nucleobases molecules alters the electronic properties of both oxygen and sulfur atomic layers of the Janus MoOS complex systems as determined by plotting the 3D Kohn–Sham frontier orbitals. A good correlation has been found between the interaction energy, van der Waals energy, Hirshfeld-I, and d–band center as a function of the nucleobase’s affinity, and the interaction energy, suggesting adsorption dominated by van der Waals interactions driven by molybdenum d–orbital. Moreover, the lowering in the adsorption energy leads to an active interaction of the DNA/RNA with the surfaces, accordingly its conduct to shorter the recovery time. The selectivity of the biosensor modulation device has illustrated a significant sensitivity for the nucleobases on both the oxygen and sulfur layer sides of the MoOS monolayer. This finding reveals that apart from graphene, dichalcogenides–Janus transition metal may also be adequate for identifying DNA/RNA bases in applied biotechnology.

Adsorption of H2S on AgO(001) surface: A density‐functional theory investigation

The adsorption of the harmful H2S molecule on the AgO(001) surface was investigated using the GGA + PBE and GGA + U methods. Firstly, we have calculated the band structure of AgO. The GGA + PBE functional predicts that the AgO system is metallic, while the GGA + U calculation shows that AgO is semimetallic with a small band gap. Secondly, we have studied the interaction between the H2S molecule and the (001) surface of AgO with oxygen (O) or silver (Ag) termination. Adsorption energies, structural parameters, projected density of states (PDOS), and difference in electron density distribution have determined. Our calculations show that the favorable adsorption site of H2S on AgO(001) surface is dependent of the number of layers of the AgO(001) slab, the exchange correlation functional (GGA + PBE or GGA + U), and the termination used.

Strain engineering of electronic structure and mechanical switch device for edge modified Net-Y nanoribbons

Net-Y is a new two-dimensional carbon structure, which has attracted research interest recently. Here, we study the relevant AB-type ribbons with edge modification, focusing on their strain controlling effects on their electronic structure and device characteristics. Intrinsic ribbons are metallic, but hydrogen or oxygen termination can transform them into semiconductors. Applying strain can effectively control the band gap size, resulting in a transition from an indirect band gap to a smaller direct band gap under appropriate strain, favorably to light absorbing. Strain can also change the work function of ribbons, especially for compressive strains, the work function is lowered significantly, which is beneficial to the improving of the field emission behaviors of ribbons. The analysis demonstrates that the change in band gap size is closely related to the variation of bonding and non-bonding composition between atoms with strain, while the change of work function is due to the variation of the attraction force and repulsion force between atoms upon strain. More interestingly, the strain can significantly regulates the <i>I</i>-<i>V</i> characteristic of device based on related ribbons. Therefore, a strain-gated mechanical switch with a very high current switching ratio <i>I</i><sub>on</sub>/<i>I</i><sub>off</sub> can be obtained by making it reversibly work between the “on” and “off” states with stretching and compressing ribbons, which is of great significance in developing the logic circuits for flexible wearable electronic devices.

Effect of oxygen termination on the interaction of first row transition metals with M2C MXenes and the feasibility of single-atom catalysts

A systematic density functional theory study to investigate the stability of single-atom catalysts (SACs) over a series of O-terminated MXenes with the stoichiometry of M2CO2.

Oxygenation of Diamond Surfaces via Hummer’s Method

Oxygen bonded with diamond surfaces impacts important properties such as electrical conductivity, Schottky barrier height, field emission, and chemical reactivity. Though processes such as thermal, hydrogen plasma, etc., are efficient in oxidizing the hydrogen-terminated diamond surfaces, the oxidation of pristine diamond surfaces through wet chemical treatments is still in its infancy. Herein, we investigated the efficacy of Hummer’s method, one of the most celebrated chemical oxidation procedures to convert graphite to graphene oxide, to oxidize the pristine diamond surfaces. We attempted to oxidize both microcrystalline diamond powders and polycrystalline diamond wafers. Due to the presence of an acidic oxidative environment and the formation of strong oxidizing agents such as Mn2O7 and MnO3+ during the course of the reaction, Hummer’s method is found to be very effective in oxidizing the pristine diamond surfaces. The degree of oxygen termination is validated through various spectroscopic and surface probe measurements. Microcrystalline diamond powder is more prone to oxidation to polycrystalline diamond wafers due to excess surface area, and many facets with different dangling bond densities are exposed to the oxidizing medium. The experimental observations are endorsed through molecular dynamics simulations.

Ohmic and Schottky contacts of hydrogenated and oxygenated boron-doped single-crystal diamond with hill-like polycrystalline grains* *Project supported by the Key-Area Research and Development Program of Guangdong Province (Grant No. 2020B0101690001) and the National Natural Science Foundation of China (NSFC) (Grant No. 51972135).

Hill-like polycrystalline diamond grains (HPDGs) randomly emerged on a heavy boron-doped p+ single-crystal diamond (SCD) film by prolonging the growth duration of the chemical vapor deposition process. The Raman spectral results confirm that a relatively higher boron concentration (∼ 1.1 × 1021 cm−3) is detected on the HPDG with respect to the SCD region (∼ 5.4 × 1020 cm−3). It demonstrates that the Au/SCD interface can be modulated from ohmic to Schottky contact by varying the surface from hydrogen to oxygen termination. The current–voltage curve between two HPDGs is nearly linear with either oxygen or hydrogen termination, which means that the HPDGs provide a leakage path to form an ohmic contact. There are obvious rectification characteristics between oxygen-terminated HPDGs and SCD based on the difference in boron doping levels in those regions. The results reveal that the highly boron-doped HPDGs grown in SCD can be adopted as ohmic electrodes for Hall measurement and electronic devices.

Exploring the Potentials of Ti3CiN2-iTx (i = 0, 1, 2)-MXene for Anode Materials of High-Performance Sodium-Ion Batteries.

Two-dimensional (2D) MXenes, including carbides, nitrides, and carbonitrides MXene, have been proved to be a possible candidate as anode materials of sodium-ion batteries. This paper focuses on the electronic properties and the electrochemical performance of nitrides MXene. First, density functional theory simulations were utilized to disclose the geometric structure and electronic properties, Na diffusion path, and storage behaviors of titanium carbonitrides Ti3CNTx, nitrides MXene Ti3N2Tx, and carbides MXene Ti3C2Tx with oxygen terminations, predicting the more excellent performance of Ti3N2O2 than Ti3C2O2. Also, then the structure characterization and electrochemical performance experiments of Ti3C2Tx and Ti3CNTx were conducted to verify the theoretical predictions and test the cycling performances. The superior performance of Ti3N2O2 originates from the stronger connection of O-Ti-N than that of O-Ti-C, resulting in the stackings of Ti3N2O2 being tighter and the interlayer spacings being larger than that of Ti3C2O2, which is advantageous to sodiation and desodiation. The capacity of Ti3CNTx increased again to 145 mAh/g after 35 cycles at a current density of 20 mA/g, which demonstrated a better rate performance than Ti3C2Tx corroborated by the diffusion barriers of the theoretical calculation results. Ti3CNTx exhibits a good cycling performance of 110 mAh/g (≈60% of the initial value) after 200 cycles, which is better than that of 87 mAh/g (≈51% of the initial value) of Ti3C2Tx. It is worth noting that all these performances ensure that nitride MXene is more suitable as the anode material of Na-ion batteries than carbide MXene. These findings are conducive to expanding the MXene family and promoting their application in energy storage applications.

Achieving high capacitance from porous boron-doped diamond by tuning the surface termination

Porous boron-doped diamond (PBDD) electrode with a high potential window, excellent stability, controllable conductivity, enhanced specific area, and tunable surface termination is a promising electrode material for the candidate. The surface termination of the PBDD electrode is easily tuned by the cyclic voltammetry (CV) scanning process with a positive potential, and an oxygen-terminated (OT) surface is achieved. The OT PBDD electrode exhibits a peak specific capacitance of 20.31 mF cm −2 (38.3% increase compared with the HT PBDD electrode, at 100 mV s −1 ) when 2.4 V (vs. Ag/AgCl) potential is applied, and it further comes to 36.74 mF cm −2 (81.8% increase than the HT PBDD electrode, at 10 mV s −1 ) with a 2.7 V (vs. Ag/AgCl) polarization potential. As the polarization voltage further increases to 3.0 V (vs. Ag/AgCl) or higher, the specific capacitance decrease. The OT PBDD electrode also keeps capacitance retention of 94.8% after 6000 CV cycles at 200 mV s −1 . Moreover, we built up a symmetric device using the OT PBDD as the electrode, which exhibits good electrochemical performance, such as wide potential, high energy density (15.1 Wh kg −1 ), and high power density (4.05 kW kg −1 ). • The PBDD is synthesized via regrowth of diamond nanoparticles in an Ar atmosphere. • The PBDD electrode achieves an oxygen-terminated (OT) surface by anodic polarization. • The OT PBDD device's energy density increases from 7.37 to 15.1 Wh kg −1 after polarization. • The OT PBDD device has the same potential window as the hydrogen-terminated PBDD.

Enhanced electrochemical capacitance of nitrogen-doped ultrananocrystalline diamond through oxygen treatment

The electrochemical capacitance of nitrogen-doped ultrananocrystalline diamond (N-UNCD) can be dramatically increased by treating the surface with an RF-oxygen plasma. Such treated surfaces display excellent properties for use as electrodes in neural stimulation and recording. In the present work, we elucidate the origins of this phenomenon by investigating the effects of different methods of oxygen termination. We found that the increase in electrochemical capacitance is dependent on the details of the method used for oxygen termination. Whilst N-UNCD subjected to UV/ozone treatment, oxygen plasma treatment, and furnace annealing in oxygen gas all displayed increased surface capacitance, the highest capacitance was exhibited by the oxygen annealed sample, with which we achieved ~ 3 orders of magnitude increase in the electrochemical capacitance as compared to the as-grown sample. The maximum recorded capacitance was 3746 ± 132 µF cm−2, which is substantially greater than previously reported N-UNCD electrodes’ electrochemical capacitance (1070 µF cm−2, W. Tong. et al, 2016). Our findings point to the presence of sub-surface solid state capacitance which contributes significantly to the observed electrochemical capacitance of the oxygen terminated N-UNCD electrodes. When combined with the favourable biocompatibility and inertness of the N-UNCD, our approach may provide a route towards the development of advanced neural sensing and stimulating electrodes.

Running-in behavior of a H-DLC/Al2O3 pair at the nanoscale

Diamond-like carbon (DLC) film has been developed as an extremely effective lubricant to reduce energy dissipation; however, most films should undergo running-in to achieve a super-low friction state. In this study, the running-in behaviors of an H-DLC/Al2O3 pair were investigated through a controllable single-asperity contact study using an atomic force microscope. This study presents direct evidence that illustrates the role of transfer layer formation and oxide layer removal in the friction reduction during running-in. After 200 sliding cycles, a thin transfer layer was formed on the Al2O3 tip. Compared with a clean tip, this modified tip showed a significantly lower adhesion force and friction force on the original H-DLC film, which confirmed the contribution of the transfer layer formation in the friction reduction during running-in. It was also found that the friction coefficient of the H-DLC/Al2O3 pair decreased linearly as the oxygen concentration of the H-DLC substrate surface decreased. This phenomenon can be explained by a change in the contact surface from an oxygen termination with strong hydrogen bond interactions to a hydrogen termination with weak van der Waals interactions. These results provide new insights that quantitatively reveal the running-in mechanism at the nanoscale, which may help with the design optimization of DLC films for different environmental applications.

Influence of temperature on the electrochemical window of boron doped diamond: a comparison of commercially available electrodes

This work compares the electrochemical windows of polished and unpolished boron doped diamond (BDD) electrodes with hydrogen and oxygen terminations at a series of temperatures up to 125 °C. The experiment was run at 5 bar pressure to avoid complications due to bubble formation. An alternative method for determining the electrochemical window is compared to the most commonly used method, which defines the window at an arbitrary current density cut-off (Jcut-off) value. This arbitrary method is heavily influenced by the mass transport of the electrolyte and cannot be used to compare electrodes across literature where different Jcut-off values have been used. A linear fit method is described which is less affected by the experimental conditions in a given measurement system. This enables a more accurate comparison of the relative electrochemical window from various diamond electrode types from reported results. Through comparison of polished and unpolished BDD electrodes, with hydrogen and oxygen surface terminations, it is determined that the electrochemical window of BDD electrodes narrows as temperature increases; activation energies are reported.

Oxidized Si terminated diamond and its MOSFET operation with SiO2 gate insulator

During selective epitaxial growth of diamond through SiO2 masks, silicon terminations were formed on a diamond surface by replacing oxygen terminations under the masks. The high temperature of selective growth and its reductive atmosphere possibly allowed Si atoms in SiO2 to interact with the diamond surface, resulting in silicon terminated diamond (C–Si diamond) composed of a monolayer or thin multi-layers of carbon and silicon bonds on diamond. Diamond metal oxide semiconductor field effect transistors (MOSFETs), with a C–Si diamond channel and selectively grown undoped or heavily boron-doped (p+) source/drain (S/D) layers, have been fabricated. Both the MOSFETs with undoped and p+ S/D exhibited enhancement mode (normally off) FET characteristics. The drain current (IDS) of the undoped device reached −17 mA/mm with threshold voltage (VT) −19 V; the p+ device attained a high IDS −165 mA/mm with a VT of −6 V being one of the best normally off diamond FETs. Transmission electron microscopy and energy dispersive x-ray spectroscopy confirmed the presence of C–Si diamond under the SiO2 masking area. The field effect mobility and interface state density at the C–Si/SiO2 (220 nm)/Al2O3 (100 nm) MOS capacitor are 102 cm2 V−1 s−1 and 4.6 × 1012 cm−2 eV−1, respectively. The MOSFET operation of C–Si diamond provides an alternative approach for diamond.

Stern layers on RuO2 (100) and (110) in electrolyte: Surface X-ray scattering studies

Abstract Electrochemical Stern layers are observed on the surfaces of RuO2 single crystals in 0.1 M CsF electrolyte. The Stern layers formed at the interfaces of RuO2 (110) and (100) are compared to the previously reported Stern layer on Pt(111) [Liu et al., J. Phys. Chem. Lett., 9 (2018) 1265]. While the Cs+ density profiles at the potentials close to hydrogen evolution reactions are similar, the hydration layers intervening the surface and the Cs+ layer are significantly denser on RuO2 surfaces than that on Pt(111) surface, reflecting the oxygen termination of RuO2 surfaces. The overall similarities between Stern layers on ruthenium surfaces and platinum surface suggest the universal presence of Stern layers in all well-defined solid-electrolyte interfaces.

Interface defect engineering for high-performance MOSFETs with novel carrier mobility model: Theory and experimental verification

As the conventional hydrogen-termination method has a limited ability to improve the interface quality between SiO2 and its Si substrate, an alternative termination method to reduce the influence of interface states is necessary. Interface engineering using first-principles calculations to suppress the influence of interface states is proposed based on the findings that silicon with dangling bonds is their primary origin. First-principles calculations indicate that the interface states can be terminated with oxygen when incorporated into the SiO2/Si interface without additional oxidation, which generates other interface states from an appropriate oxygen-anneal process. It is experimentally shown that such an oxygen termination can be realized in slow and low-temperature annealing, and the oxygen-termination method is a promising alternative for hydrogen termination. The stronger Si–O bond introduced from the oxygen termination compared with the Si–H bonds from hydrogen termination ensures a better interface quality. As one oxygen atom terminates two silicon atoms, the oxygen-termination method can efficiently suppress the number of interface defects compared with hydrogen and fluorine termination. The mobility degradation due to the interface states was improved more from oxygen termination than from hydrogen termination because the strength of Coulomb scattering due to Si–O dipoles is reduced from the heavier oxygen mass. Theoretical predictions were verified using experiments, indicating that the oxygen-termination method under appropriately optimized annealing conditions (speed and temperature) is a promising candidate to improve the interface quality by reducing the influence of interface states.

Diamond/γ-alumina band offset determination by XPS

γ-Alumina is a promising candidate for fabricating the gate of the diamond metal oxide semiconductor field effect transistor based on oxygen termination due to its high bandgap of 6.7 eV and high static dielectric constant of 9. Besides these properties, having a sufficient barrier for holes is mandatory to avoid carriers leakage through the gate. However, the band offset of the diamond/alumina heterojunction can be affected by the alumina crystallinity and interface bonds, which depend on multiple factors such as deposition and annealing temperature or diamond surface treatment prior to deposition.In this work, the heterojunction of atomic layer deposited alumina and (1 0 0) p-diamond is studied using X-ray photoelectron spectroscopy (XPS). Transmission electron microscopy studies reveal that the deposited alumina layer is 35 nm thick and present the gamma phase. The valence band offset between diamond and γ-alumina is evaluated on a single sample with a new methodology based on an ion etching XPS depth profile. The obtained value for the valence band offset of diamond and γ-alumina is 3.4 eV.

Electrochemical reduction of nitrate on boron-doped diamond electrodes: Effects of surface termination and boron-doping level

This study is among the first to systematically study the electrochemical reduction of nitrate on boron-doped diamond (BDD) films with different surface terminations and boron-doping levels. The highest nitrate reduction efficiency was 48% and the highest selectivity in the production of nitrogen gas was 44.5%, which were achieved using a BDD electrode with a hydrogen-terminated surface and a B/C ratio of 1.0%. C–H bonds served as the anchor points for attracting NO3− anions close to the electrode surface, and thus accelerating the formation of NO3−(ads). Compared to oxygen termination, hydrogen-terminated BDD exhibited higher electrochemical reactivity for reducing nitrate, resulting from the formation of shallow acceptor states and small interfacial band bending. The hydrophobicity of the hydrogen-terminated BDD inhibited water electrolysis and the subsequent adsorption of atomic hydrogen, leading to increased selectivity in the production of nitrogen gas. A BDD electrode with a boron-doping level of 1.0% increased the density of acceptor states, thereby enhancing the conductivity and promoting the formation of C–H bonds after the cathodic reduction pretreatment leading to the direct reduction of nitrate.

Enhanced and switchable silicon-vacancy photoluminescence in air-annealed nanocrystalline diamond films

Silicon vacancy (SiV) centers in CVD-deposited nanodiamond particles or films always exhibit a photoluminescent (PL)-quenching behavior. Herein, in order to overcome this issue, air annealing of nanocrystalline diamond (NCD) films with uniform doping of Si atoms is employed to improve the PL emission efficiency of SiV centers. Without severe mass loss in diamond, SiV centers exhibit an increased PL emission intensity at 738 nm with an increased annealing temperature and time. In comparison to the untreated film, the maximum PL enhancement reaches about 200 and 1473 folds when the NCD films are annealed at 600 °C for 60 min and 700 °C for 20 min, respectively. Such giant increase in SiV PL emission is mainly attributed to the transition of diamond surface from hydrogen to oxygen termination together with the optimized crystalline quality of the annealed films, as confirmed by the HRTEM, Raman and XPS measurements. A schematic model based on the theory of surface band bending is proposed to elucidate the mechanism of the SiV PL enhancement in oxygen-terminated NCD films: a near-surface light trap layer formed in hydrogen-terminated NCD is converted during the oxidation process, leading to the additional collection of SiV PL emission from the inner layer.

Schottky barrier diode fabricated on oxygen-terminated diamond using a selective growth approach

A Schottky barrier diode (SBD) fabricated on oxygen-terminated diamond using a selective growth approach is demonstrated. Patterned tungsten (W) film was used as the mask, and ohmic contact was formed between W and diamond during the selective growth process. After growth, the selective epitaxial diamond surface was treated by UV-ozone to achieve oxygen termination. Then, Zr/Ni/Au Schottky electrode was patterned on the selective grown diamond surface. The I-V characteristics indicate that the SBD is p-type, and a rectification ratio of more than 5 orders of magnitude at ±20 V at room temperature is observed. The acceptor might be introduced by W incorporated into diamond during selective growth process, and its concentration was determined to be 2.7 × 1014 cm−3 from C−2-V curve. The reverse breakdown voltage of the SBD is 1316 V, corresponding to a breakdown electrical field of 6.3 MV/cm. The reverse current-voltage characteristics show that the UV ozone treatment can favorably suppress reverse leakage current.