Oxide Monolayer Research Articles

Second Harmonic Generation in Janus Transition Metal Chalcogenide Oxide Monolayers: A First-Principles Investigation.

Due to the unique optical responses induced by vertical atomic asymmetry inside a monolayer, two-dimensional Janus structures have been conceived as promising building blocks for nanoscale optical devices. In this paper, second harmonic generation (SHG) in Janus transition metal chalcogenide oxide monolayers is systematically investigated by the first-principles calculations. Second-order nonlinear susceptibilities are theoretically determined for Janus MXO (M = Mo/W, X = S/Se/Te) monolayers. The calculated values are comparable in magnitude with Janus MoSSe monolayer. X-M-O symmetry breaking leads to non-zero components in vertical direction, compared with the non-Janus structure. Focusing on the SHG induced by incident light at 1064 nm, polarization-dependent responses of six Janus MXO monolayers are demonstrated. The symmetry of p-polarization changes from six-fold to three-fold with acute incidence angle. Moreover, the effects of biaxial strain on band structures and SHG are further investigated, taking MoSO as an exemplary case. We expect these results to bring in recipes for designing nonlinear optical devices based on Janus transition metal chalcogenide oxide monolayers.

First-principles outlook of two-dimensional B3O3 monolayer as an anode material for non-lithium ion (K+, Ca2+, and Al3+) batteries

The rapid development of high-capacity anodes for non-lithium ion batteries (NLIBs) has attracted significant attention in the scientific community towards removing fossil fuels. Herein, first-principles calculations based on density functional theory (DFT) were utilized to look into the applicability of the hexagonal boron oxide (B3O3) monolayer as an anode material for non-lithium ion (K+, Ca2+, and Al3+) batteries. The K+, Ca2+, and Al3+ ions are preferentially adsorbed on the large hollow site of B3O3 monolayer with binding energies of −53.77, −170.55, and −620.85 kcal mol−1, respectively. It was found the adsorption energies generally decrease by increasing the metal ion concentration. The theoretical storage capacities were predicted to be 790.33, 1185.50, and 1778.25 mAh g−1 for K+, Ca2+, and Al3+ ions, respectively. Furthermore, the diffusion energy barrier of a single metal ion was very low (0.22/0.39, 0.12/0.35, and 0.17/0.05 eV for K+, Ca2+, and Al3+ ions, respectively). The findings suggested that the B3O3 monolayer can be used in designing high-performance boron-containing nanostructures for NLIBs.

Stability and mixing behavior of vanadium-iron oxide monolayers on Pt(111) and Ru(0001) substrates

Cation mixing is a well-recognized means to obtain oxides of desired functionality with predetermined structure and stoichiometry, which yet has been only little analyzed at the nanoscale. In this context, we present a comparative analysis of the stability and mixing properties of O-poor and O-rich two-dimensional V–Fe oxides grown on Pt(111) and Ru(0001) surfaces, with the aim of gaining an insight into the role of substrate and oxygen conditions on the accessible Fe contents. We find that due to the high oxygen affinity of the Ru substrate, the mixed O-rich layers are highly stable while the stability of O-poor layers is limited to inaccessibly oxygen-poor environments. In contrast, on the Pt surface, O-poor and O-rich layers coexist with, however, a much lower Fe content in the O-rich phase. We show that cationic mixing (formation of mixed V–Fe pairs) is favored in all considered systems. It results from local cation–cation interactions, reinforced by a site effect in O-rich layers on the Ru substrate. In O-rich layers on Pt, Fe–Fe repulsion is so large that it precludes the possibility of substantial Fe content. These findings highlight the subtle interplay between structural effects, oxygen chemical potential, and substrate characteristics (work function and affinity towards oxygen), which governs the mixing of complex 2D oxide phases on metallic substrates.

Electric-field control of reversible electronic and magnetic transitions in two-dimensional oxide monolayer magnets

Atomically thin oxide magnetic materials are highly desirable due to the promising potential to integrate two-dimensional (2D) magnets into next-generation spintronics. Therefore, 2D oxide magnetism is expected to be effectively tuned by the magnetic and electrical fields, holding prospective for future low-dissipation electronic devices. However, the electric-field control of 2D oxide monolayer magnetism has rarely been reported. Here, we present the realization of 2D monolayer magnetism in oxide (SrRuO3)1/(SrTiO3)N (N = 1, 3) superlattices that shows an efficient and reversible phase transition through electric-field controlled proton (H+) evolution. By using ionic liquid gating to modulate the proton concentration in (SrRuO3)1/(SrTiO3)1 superlattice, an electric-field induced metal-insulator transition was observed, along with gradually suppressed magnetic ordering and modulated magnetic anisotropy. Theoretical analysis reveals that proton intercalation plays a crucial role in both electronic and magnetic phase transitions. Strikingly, SrTiO3 layers can act as a proton sieve, which have a significant influence on proton evolution. Our work stimulates the tuning functionality of 2D oxide monolayer magnetism by voltage control, providing potential for future energy-efficient electronics.

Magnetic transitions of hydrogenated H x CrO2 (x = 0–2) monolayer from a ferromagnetic half-metal to antiferromagnetic insulator

Two-dimensional (2D) transition metal oxide monolayers are currently attracting great interest in materials research due to their versatility and tunable electronic and magnetic properties. In this study, we report the prediction of magnetic phase changes in H x CrO2 (0 ⩽ x ⩽ 2) monolayer on the basis of first-principles calculations. As the H adsorption concentration x increases from 0 to 0.75, H x CrO2 monolayer transforms from a ferromagnetic (FM) half-metal to a small-gap FM insulator. When x = 1.00 and 1.25, it behaves as a bipolar antiferromagnetic (AFM) insulator, and eventually becomes an AFM insulator as x increases further up to 2.00. The results suggest that the magnetic properties of CrO2 monolayer can be effectively controlled by hydrogenation, and that H x CrO2 monolayers have the potential for realizing tunable 2D magnetic materials. Our results provide a comprehensive understanding of the hydrogenated 2D transition metal CrO2 and provide a research method that can be used as a reference for the hydrogenation of other similar 2D materials.

Spherical aluminum oxide nanoparticle synthesis and monolayer film assembly

Spherical aluminum oxide nanoparticle synthesis and monolayer film assembly

Structural Reconstruction Modulated Physical Properties of Titanium Oxide at the Monolayer Limit

To suppress surface dangling bonds, monolayer oxides derived from non-layered bulks usually undergo a pronounced structural reconstruction. It remains challenging to resolve these structural reconstructions and the induced distinct modulation of intrinsic properties. In this study, the structural reconstruction-modulated electronic, polaronic, and exitonic properties of a non-layered oxide at the monolayer limit are unraveled. Based on first-principles calculations and tight-binding simulations for a stable titanium dioxide (TiO2) monolayer, we show that its distinct surface Kagome sublattices host a topologically nontrivial flat band at the valence band edge. The strong electron–hole interaction in this monolayer oxide gives rise to a large exciton binding energy of around 2.49 eV. Interestingly, the monolayer TiO2 also exhibits strong electron–lattice coupling, which favors the formation of small electron polarons and thus greatly reduces its band gap energy into the visible light range. This work could be useful to understand the structural reconstruction-induced modulation of exotic physical and chemical properties for a broad range of non-layered oxides at the monolayer limit.

Efficient tunability of size and optical properties of reduced graphene oxide-ZnO composite nanocrystallites on solid substrates

Growth of ultra-thin luminescent reduced graphene oxide (rGO)-zinc oxide (ZnO) composite nanostructures was accomplished by Langmuir-Blodgett (LB) transfer of GO-Zn composite layers onto solid substrates and subsequent oxidation. Layer structure of LB transferred composites and oxidation procedure determine the size distribution, density, structural order, optical quality of grown ZnO and the extent of GO reduction. Prolonged oxidation (120 min) of composite monolayers produce ZnO nanocrystallites nearly deprived of defects and exhibiting substantially enhanced band edge emission. Prior anchoring of Zn2+ ions on GO layers and oxidation facilitate controlled growth of ZnO nanocrystallites and nanoclusters (20–100 nm), offer tunability of absorption edge (325–355 nm) and assists in amendment of surface defects/band edge emission (376–384 nm). Conductivity of precursor and oxidized composite layers was mapped by recording STM images as well as STS spectra and, the obtained differential conductivity spectra substantiate the changes in GO conductivity and provide range of bandgap values of grown ZnO nanocrystallites (3.1–3.4 eV). The multiple GO-Zn compositing procedures and optimum oxidation conditions introduced in present work facilitate development of rGO-ZnO composite nanostructures with high optical, crystalline quality and conductive characteristics, which pave path for fabrication of such nanostructures for optoelectronic device applications.

High durability and stability of 2D nanofluidic devices for long-term single-molecule sensing

Nanopores in two-dimensional (2D) membranes hold immense potential in single-molecule sensing, osmotic power generation, and information storage. Recent advances in 2D nanopores, especially on single-layer MoS2, focus on the scalable growth and manufacturing of nanopore devices. However, there still remains a bottleneck in controlling the nanopore stability in atomically thin membranes. Here, we evaluate the major factors responsible for the instability of the monolayer MoS2 nanopores. We identify chemical oxidation and delamination of monolayers from their underlying substrates as the major reasons for the instability of MoS2 nanopores. Surface modification of the substrate and reducing the oxygen from the measurement solution improves nanopore stability and dramatically increases their shelf-life. Understanding nanopore growth and stability can provide insights into controlling the pore size, shape and can enable long-term measurements with a high signal-to-noise ratio and engineering durable nanopore devices.

Oxygen Storage by Tin Oxide Monolayers on Pt3Sn(111)

The high performance of platinum–tin catalysts for oxidation reactions has been linked to the formation of tin oxides at the metal surface, but little is known about the structure of these oxides or the chemical behavior that determines their catalytic properties. We show here how surface oxides on Pt3Sn(111) incorporate oxygen at the metal interface, which may be subsequently removed by reaction with CO. The storage mechanism, where oxygen uptake occurs without loss of interfacial Pt–Sn bonds, is enabled by the peculiar asymmetrical coordination state of Sn2+. O atoms are bound at pocket sites in the 2D oxide sheet between these outward-buckled Sn atoms and metallic Sn in the alloy surface below.

Negative piezoelectricity and enhanced electrical conductivity at the interfaces of two-dimensional dialkali oxide and chalcogenide monolayers

The negative piezoelectric coefficient in ferroelectric polymers has been widely studied. However, the same in the two-dimensional (2D) materials is scarcely reported, which stresses the need for its thorough investigation. Here, the 2D van der Waals (vdW) heterobilayers (vdWHs) of the dialkali metal monochalcogenide ${M}_{2}X$ ($M=\mathrm{Na}$, K, Rb, or Cs; and $X=\mathrm{O}$, S, Se, or Te) monolayers show negative out-of-plane piezoelectricity. Out of all the explored heterobilayers of the family, the maximum piezoelectric strain coefficient in the out-of-plane direction ${d}_{33}=\ensuremath{-}39\phantom{\rule{0.28em}{0ex}}\mathrm{pm}\phantom{\rule{0.28em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}$ is found in the ${\mathrm{Na}}_{2}\mathrm{Te}$/${\mathrm{Cs}}_{2}\mathrm{S}$ system. This strong piezoelectric coefficient arises from the notable electrostatic potential energy difference and band offset between the constituent monolayers. The small out-of-plane Young's modulus ($Y\ensuremath{\sim}11\phantom{\rule{0.28em}{0ex}}\mathrm{N}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}\mathbf{1}}$), originating from the weak vdW interlayer bonding relative to the stronger intralayer bonding, accounts for the anomalous negative piezoelectricity in the heterostructure. The combination of minute out-of-plane ($Y=\ensuremath{\sim}11\phantom{\rule{0.28em}{0ex}}\mathrm{N}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}\mathbf{1}}$) and in-plane ($Y=\ensuremath{\sim}23\phantom{\rule{0.28em}{0ex}}\mathrm{N}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}\mathbf{1}}$) Young's moduli with small bending modulus (8.83 eV) and negative piezoelectric coefficient underlines the suitability of the proposed vdWH for stretchable and flexible piezotronic devices. Moreover, external perturbations are found to modify the properties of the system. The vertical compressive strain widens the band gap by elevating the conduction band minimum in the heterostructure. An externally applied electric field of low strength ($\ensuremath{-}0.33$ and $+0.31\phantom{\rule{0.28em}{0ex}}\mathrm{V}/\AA{}$) is found to change the semiconducting heterostructure to metallic upon the closure of the band gap, which is attributed to the dropping of nearly free electron gas (NFEG) states to the Fermi level. A significant enhancement in electrical conductivity has been brought up by the NFEG states upon reaching the Fermi level, which inspires the use of the heterostructure in a nonresistive charge carrier transport application in electronics.

Dodecagonal Zinc Oxide ( d - ZnO ) Monolayer for Water Desalination and Detection of Toxic Gases

Nanoporous materials have attracted great interest because of their variety of applications in nanodevices, such as gas storage, low-density magnetic storage, energy storage, supercapacitors, catalysis, membranes, etc. The most common purpose of using nanoporous materials is to make a material much lighter while preserving or improving the high structural stability of these compounds. In this work, we propose a two-dimensional dodecagonal zinc oxide ($d$-$\mathrm{Zn}\mathrm{O}$) monolayer via first-principles calculations based on density-functional theory (DFT). Our extensive analysis shows that this semiconducting porous $d$-$\mathrm{Zn}\mathrm{O}$ material is mechanically, dynamically, and thermally stable and suitable for various applications, such as water membrane and gas detection at room temperature and above. We study the water permeability and ${\mathrm{Na}}^{+}$ and ${\mathrm{Cl}}^{\ensuremath{-}}$ ions' rejection of $d$-$\mathrm{Zn}\mathrm{O}$ material via conducting DFT and molecular dynamics (MD) simulations. Our simulations show that the energy barrier of the water molecule and ${\mathrm{Na}}^{+}/{\mathrm{Cl}}^{\ensuremath{-}}$ ions passing through the porous $d$-$\mathrm{Zn}\mathrm{O}$ structure is low and high, respectively. In addition, MD calculations show that the water permeability performance of $d$-$\mathrm{Zn}\mathrm{O}$ material is high enough to use this material for water desalination applications. For further investigations, the detection of some selected gases ($\mathrm{CO}$, $\mathrm{SO}$, $\mathrm{NO}$, ${\mathrm{CO}}_{2}$, ${\mathrm{SO}}_{2}$, and ${\mathrm{NO}}_{2}$) are investigated on $d$-$\mathrm{Zn}\mathrm{O}$ and find that ${\mathrm{NO}}_{2}$, and ${\mathrm{SO}}_{2}$ would preferentially be detected on the $d$-$\mathrm{Zn}\mathrm{O}$ substrate due to their high adsorption energy values as compared to physisorption of $\mathrm{CO}$, $\mathrm{NO}$, $\mathrm{SO}$, and ${\mathrm{CO}}_{2}$ molecules on the $d$-$\mathrm{Zn}\mathrm{O}$ surface.

Crystallization and melting of polymer chains on graphene and graphene oxide.

This study employs all-atomistic (AA) molecular dynamics (MD) simulations to investigate the crystallization and melting behavior of polar and nonpolar polymer chains on monolayers of graphene and graphene oxide (GO). Polyvinyl alcohol (PVA) and polyethylene (PE) are used as representative polar and nonpolar polymers, respectively. A modified order parameter is introduced to quantify the degree of two-dimensional (2D) crystallization of polymer chains. Our results show that PVA and PE chains exhibit significantly different crystallization behavior. PVA chains tend to form a more rounded, denser, and folded-stemmed lamellar structure, while PE chains tend to form an elongated straight pattern. The presence of oxidation groups on the GO substrate reduces the crystallinity of both PVA and PE chains, which is derived from the analysis of modified order parameter. Meanwhile, the crystallization patterns of polymer chains are influenced by the percentage, chemical components, and distribution of the oxidation groups. In addition, our study reveals that 2D crystalized polymer chains exhibit different melting behavior depending on their polarity. PVA chains exhibit a more molecular weight-dependent melting temperature than PE chains, which have a lower melting temperature and are relatively insensitive to molecular weight. These findings highlight the critical role of substrate and chain polarity in the crystallization and melting of polymer chains. Overall, our study provides valuable insights into the design of graphene-based polymer heterostructures and composites with tailored properties.

Tuning quantum capacitance in 2D graphene electrodes: the role of defects and charge carrier concentration

Electrochemical oxidation of a graphene monolayer results in a G-band shift in the micro-Raman spectrum and an increase in the quantum capacitance, resulting from an increase in the number and density of defects and charge carrier concentration in the graphene structure.

<i>In situ</i> Monitoring of the Oxidation of Unsaturated Lipid Monolayers under Low-level Ozone by HD-SFG Spectroscopy

肺胞の表面を覆う肺サーファクタントは，主成分である脂質の両親媒性により，呼吸機能の維持に重要な役割を果たしている。最近，この肺サーファクタントに含まれる不飽和脂質が空気中に含まれる程度の極低濃度オゾンによっても酸化されることを示唆する結果が報告されている。極低濃度オゾンの呼吸器への影響を明らかにする上で分子レベルでの反応機構を明らかにすることは重要であるが，技術的な難しさのためにその詳細はこれまでに十分には明らかになっていない。和周波発生（SFG）分光法は，2次の非線形光学効果に基づき分子の界面構造に非常に敏感な分光法である。特に，SFG光の干渉信号を検出するヘテロダイン検出は，従来の光強度測定と比較してより多くの情報を得ることが可能である。本稿では，ヘテロダイン検出SFG分光法の原理と極低濃度オゾン環境下における不飽和脂質単分子膜の酸化反応のその場測定への適用例について紹介する。

First-principles investigation on structural and optoelectronic properties of buckled SnO monolayer for effective solar energy scavenging

Two-dimensional (2D) materials have gathered considerable attention for next-generation optoelectronic devices owing to their intriguing optical and electronic properties. The semiconductor SnO has shown potential for flexible electronics, optoelectronics, gas sensing, and Li-ion battery applications due to its indirect bandgap, and high carrier mobility. In the present work, we investigate the physical properties of a buckled tin oxide (SnO) monolayer using density functional theory (DFT) calculations. The structural analysis reveals that buckled SnO monolayer is energetically stable and exhibits the hexagonal crystal structure. Phonon dispersion analysis depicts the dynamical stability of the buckled SnO monolayer and obtained the quadratic behavior of flexural acoustic modes (ZA) near the zone center. Further, the electronic property calculation shows that the buckled SnO is an indirect semiconductor with a bandgap of 1.71 eV and the transparent behavior of the material is disclosed by analyzing the optical parameters. Moreover, the computed solar power conversion efficiency of 16.76% for buckled SnO monolayer demonstrates the potential as a light absorber in the field of solar energy scavenging.

Theoretical insights into the two-dimensional gallium oxide monolayer for adsorption and gas sensing of C4F7N decomposition products

A new descriptor w is provided to explain the different gas sensitivity. Displacement of local atoms and an electron localization–delocalization transition caused by biaxial strain can regulate the gas sensing behavior.

Two-Dimensional Multiferroics with Intrinsic Magnetoelectric Coupling in A-Site Ordered Perovskite Monolayers

The magnetoelectric coupling effect in multiferroics provides a route to realize the control of magnetism by electric field. Here, we demonstrate the coexistence and coupling of ferroelectricity and ferromagnetism in designed A-site ordered perovskite oxide monolayers by combining symmetry analysis and first-principles calculation. These monolayers all exhibit a layered ordering and tilt distortion, and some of them exhibit rotation or Jahn-Teller distortion simultaneously, leading to the emergence of in-plane ferroelectricity. The Mn-based monolayers exhibit robust ferromagnetism, while some monolayers tend to form E-type spin order due to the splitting of the nearest-neighbor exchange interactions. Whether polarization reversal can lead to magnetization reversal depends on the mode of ferroelectric switching, that is, only the ferroelectric switching that reversing the tilt distortion can lead to magnetization reversal. This work demonstrates the feasibility of controlling the direction of magnetization by electric field in the monolayer limit of perovskites.

Hydroxyl-functionalized ZnO monolayers for optoelectronic devices: Atomic structures and electronic properties

Two-dimensional materials have been extensively investigated both experimentally and theoretically due to their appealing physical/chemical properties. Zinc oxide (ZnO) monolayers, as typical two-dimensional materials, have also aroused wide concern. However, the improper bandgap or nonmagnetic limits their applications in optoelectronic or spintronic devices. Herein, based on first-principles calculations, the effects of individual hydroxyl adsorption and hydroxyl group functionalization on the atomic and electronic structures of ZnO monolayers are investigated. The results demonstrated that the most stable structure for individual hydroxyl on the ZnO monolayer is the one in which the hydroxyl is adsorbed on the top of the hollow site with an adsorption energy of −1.448 eV. Furthermore, hydroxyl could easily aggregate on ZnO monolayer with increasing of hydroxyl coverage and form epoxy groups. Upon hydroxyl group functionalization, ZnO monolayers exhibit mechanical and thermal stability. Moreover, OH–ZnO and ZnO–OH monolayers exhibit magnetic semiconductors, while OH–ZnO–OH monolayers are nonmagnetic semiconductor. Additionally, the band gaps of ZnO monolayer with hydroxyl group functionalization could be effectively modulated via applying the strain. This study opens a new avenue for preparing potential high-performance ZnO-based optoelectronic devices.

In situ investigation of the oxidation of a phospholipid monolayer by reactive oxygen species

The oxidation of membrane lipids has been widely studied for several decades owing to its significance in biological systems. However, despite its damaging physiological impact and its known role in many diseases, relatively little is understood about the specific structural consequences of oxidative action, particularly in vivo. In this work, a combination of sum-frequency generation spectroscopy, surface tensiometry, and surface-selective infrared spectroscopies are used to gain deeper insight into the oxidation of phospholipids by reactive oxygen species generated in situ. Oxidation is achieved by employing the Fenton reaction to convert physiological levels of H2O2 into OH and HO2 radicals in proximity to the headgroups of lipid monolayers at the air-water interface. By temporally monitoring the surface tension and spectroscopic changes at the interface as the oxidation proceeds, the impact of oxidation on the structure, conformation, and intermolecular interactions within the membrane has been revealed.