Three-dimensional Counterparts Research Articles

Strain engineering of electronic and magnetic properties of double-transition metal ferromagnetic semiconductor MXenes

Strain engineering appears as an effective way to modulate physical and chemical properties of two-dimensional (2D) materials. In contrast to their three-dimensional counterparts, 2D materials can withstand high strain before rapture, which promises unique opportunities to control and tune their electronic, optical, and magnetic properties. Recently predicted Hf2MnC2O2 and Hf2VC2O2 double transition metal ferromagnetic semiconductor MXenes show robust ferromagnetic ground state with high Curie temperature. In this study, we investigated the structural, electronic, and magnetic properties of those 2D materials under the biaxial strain using density functional theory. Both strain free monolayers are indirect bandgap semiconductors. Strain engineering can be exploited to turn semiconductor monolayers into metallic or semi-metallic ones depending on the size and type of the applied strain. For instance, a semiconductor to metal transition occurs at −3% compression and 8% tension in Hf2MnC2O2 and also at −2% compression and 9% tension in Hf2VC2O2. Electron and hole effective masses are able to be tuned significantly. The ferromagnetic phase becomes stronger (weaker) as compared to the anti-ferromagnetic phase of both types of monolayers by applying the biaxial tensile (compressive) strain. Our calculations indicated that the Curie temperature (TC) is highly sensitive to the size and type of strain. TC increases (decreases) with the tensile (compressive) strain. While TC is 444 K at a compressive strain of 4%, it becomes 1577 K at a tensile strain of 8% for Hf2MnC2O2.

Viscous resistance in drop coalescence

Hydrodynamics of drop coalescence has been studied theoretically and numerically by solving the Navier Stokes equation considering a single fluid after the minimum bridge formation. Many experiments have been performed to document bridge growth over time with the use of high speed videography and electrical methods. However, internal fluid motion during coalescence has not been extensively studied, in part due to the spherical shape of the drops. This work observed overall fluid motion (except at the site of early coalescence) using particle image velocimetry for two-dimensional (sandwiched drop) coalescence. Fluid motion inside the bulk drops is inertial, and governing fluid flow in the bridge region is one dimensional. At the merging interface, incoming liquids join and coflow in the perpendicular direction. These observations were extended to a three-dimensional counterpart, and a scaling law was developed that was validated through experimentation. While flow in the bulk drops is inertial, the dominant resistance comes through a viscous effect in the merging interface region and at the lesser extent in the bridge region. Early dynamics of drop coalescence is dominated by the Ohnesorge number (Oh), and later dynamics are dependent on how drops are bounded.

INCORPORATING THREE-DIMENSIONAL BAROCLINIC PROCESSES FOR ACCURATE DEPTH-INTEGRATED COASTAL CIRCULATION MODELLING

Depth-integrated coastal circulation models have proven to provide high-fidelity solutions to storm surge, tides, tsunamis, and those with wave-coupling (Westerink et al. 2008; Titov & Synolakis 1998). They are able to provide high horizontal resolution, and stable, high-order and low-diffusion numerical schemes making them often more useful than their three-dimensional counterparts despite more assumptions on the physics. However, the assumptions of no baroclinicity means that some processes such as steric effects, barotropic energy to baroclinic tidal energy conversion, and major baroclinic current systems cannot be directly modelled, creating limitations. This is directly problematic for example in storm surge forecasting systems where an offset between measured and predicted hydrographs may be present due to seasonal heating and cooling, changes to nearshore stratification and to the transport rates of major current systems and fresh water outflows.
 
 This study presents a process coupling paradigm to incorporate three-dimensional baroclinic effects into a depth-integrated model from a widely used and available baroclinic model, HYCOM (http://hycom.org). The depth-integrated finite-element model, ADCIRC (Westerink et al. 1992) is able to provide high-resolution to the coastal area, and has proven to be extremely accurate for storm surge modelling in e.g. the Gulf of Mexico (Westerink et al. 2008). However, along the Gulf of Mexico and east-coast of USA there are noticeable seasonal and inter-annual trends in the coastal sea level making forecasting more challenging than hindcasting. We present the effects of applying the information provided by HYCOM to ADCIRC in order to improve predicted hydrographs throughout the year of 2012 and in particular during Hurricane Sandy.

Methylammonium acetate as an additive to improve performance and eliminate J-V hysteresis in 2D homologous organic-inorganic perovskite solar cells

Films of two-dimensional (2D) perovskite (PEA)2(MA)n-1PbnI3n+1 (PEA = phenylethylammonium, MA = methylammonium) series (n = 1, 2, 3) show high stability in contrast to their three-dimensional counterparts. Development of an effective means to fabricate 2D perovskite solar cells which can achieve a better power conversion efficiency (PCE) and have less current density-voltage (J-V) hysteresis, is crucial to boost their applications. We present a one-step solution process to fabricate the 2D perovskite (PEA)2(MA)2Pb3I10 as light absorbers in solar cells by adding methylammonium acetate (MAAc) in a precursor solution. With an appropriate molar ratio of MAAc, the 2D perovskite films show a smoother surface, higher coverage, and better crystallinity, resulting in an optimal PCE of 5.7%, and importantly, no J-V hysteresis.

Properties of spoof plasmon in thin structures.

Spoof surface plasmon polariton (SSPP) is an exotic electromagnetic state that confines light at a subwavelength scale at a design-specific frequency. It has been known for a while that spoof plasmon mode can exist in planar, thin structures with dispersion properties similar to that of its wide three-dimensional structure counterpart. We, however, have shown that spoof plasmons in thin structures possess some unique properties that remain unexplored. Our analysis reveals that the field interior to SSPP waveguide can achieve an exceptional hyperbolic spatial dependence, which can explain why spoof plasma resonance incurs red-shift with the reduction of the waveguide thickness, whereas common wisdom suggests frequency blue-shift of a resonant structure with its size reduction. In addition, we show that strong confinement can be achieved over a wide band in thin spoof plasmon structure, ranging from the spoof plasma frequency up to a lower frequency considerably away from the resonant point. The nature of lateral confinement in thin SSPP structures may enable interesting applications involving fast modulation rate due to enhanced sensitivity of optical modes without compromising modal confinement.

PIN(2)-monopole Floer homology and the Rokhlin invariant

We show that the bar version of the$\text{Pin}(2)$-monopole Floer homology of a three-manifold$Y$equipped with a self-conjugate spin$^{c}$structure$\mathfrak{s}$is determined by the triple cup product of$Y$together with the Rokhlin invariants of the spin structures inducing$\mathfrak{s}$. This is a manifestation of mod $2$index theory and can be interpreted as a three-dimensional counterpart of Atiyah’s classical results regarding spin structures on Riemann surfaces.

Ti3C2Tx MXene Catalyzed Ethylbenzene Dehydrogenation: Active Sites and Mechanism Exploration from both Experimental and Theoretical Aspects

Two-dimensional materials (such as graphene, MXenes) usually perform remarkably well as catalysts in comparison with their three-dimensional counterparts. In this work, Ti3AlC2-derived Ti3C2Tx MXene (T = O, OH, F) was prepared and employed in the direct dehydrogenation of ethylbenzene. A 92 μmol m–2 h–1 reactivity and a 40 h stability with almost no deactivation were observed, which is much better than those of the previously reported nanocarbon catalysts. The graphene-like layered structure and the C–Ti–O termination groups induced by the preparation process were thought to account for the good catalytic performance of Ti3C2Tx MXene from both experimental and computational perspectives. The discovery expands the application of two-dimensional MXenes and provides more choices in exploring high-activity catalysts for hydrocarbon dehydrogenation reactions.

Two-Step Growth of 2D Organic-Inorganic Perovskite Microplates and Arrays for Functional Optoelectronics.

Two-dimensional (2D) perovskites have recently attracted intensive interest for their great stability against moisture, oxygen, and illumination compared with their three-dimensional (3D) counterparts. However, their incompatibility with a typical lithography process makes it difficult to fabricate integrated device arrays and extract basic optical and electronic parameters from individual devices. Here, we develop a combination of solution synthesis and a gas-solid-phase intercalation strategy to achieve hexagonal-shaped 2D perovskite microplates and arrays for functional optoelectronics. The 2D perovskite microplates were achieved by first synthesizing the lead iodide (PbI2) microplates from an aqueous solution and then following with intercalation via the vapor transport method. This method further allows us to synthesize arrays of 2D perovskite microplates and create individual 2D perovskite microplate-based photodetectors. In particular, chlorine (Cl) can be efficiently incorporated into the microplates, resulting in significantly improved performance of the 2D perovskite microplate-based photodetectors.

Zero-Dimensional Organic-Inorganic Perovskite Variant: Transition between Molecular and Solid Crystal.

Low-dimensional organic-inorganic halide perovskites (OIHPs) have attracted intense interest recently for photovoltaic applications, due to their markedly high chemical stability as compared to the widely studied three-dimensional (3D) counterparts. However, low-dimensional OIHPs usually give much lower device performance than the 3D OIHPs. In particular, for the zero-dimensional (0D) OIHPs, it is believed that the strong intrinsic quantum-confinement effects can lead to extremely low carrier motility, which can severely limit the photovoltaic performance. Herein, we predict a new family of 0D perovskite variants that, surprisingly, exhibit outstanding optoelectronic properties. We show that the "atypical" carrier mobility of these new 0D perovskites is attributed to the strong electronic interaction between neighboring octahedrons in the crystal. These findings also suggest a new materials design strategy for resolving the low-performance issue commonly associated with the low-dimensional OIHPs for photovoltaic applications.

Molecular Rh(III) and Ir(III) Catalysts Immobilized on Bipyridine-Based Covalent Triazine Frameworks for the Hydrogenation of CO2 to Formate

The catalytic reactivity of molecular Rh(III)/Ir(III) catalysts immobilized on two- and three-dimensional Bipyridine-based Covalent Triazine Frameworks (bpy-CTF) for the hydrogenation of CO2 to formate has been described. The heterogenized Ir complex demonstrated superior catalytic efficiency over its Rh counterpart. The Ir catalyst immobilized on two-dimensional bpy-CTF showed an improved turnover frequency and turnover number compared to its three-dimensional counterpart. The two-dimensional Ir catalyst produced a maximum formate concentration of 1.8 M and maintained its catalytic efficiency over five consecutive runs with an average of 92% in each cycle. The reduced activity after recycling was studied by density functional theory calculations, and a plausible leaching pathway along with a rational catalyst design guidance have been proposed.

Wavefront manipulation by acoustic metasurfaces: from physics and applications

AbstractMolding the wavefront of acoustic waves into the desired shape is of paramount significance in acoustics, which however are usually constrained by the acoustical response of naturally available materials. The emergence of acoustic metamaterials built by assembling artificial subwavelength elements provides distinct response to acoustic waves unattainable in nature. More recently, acoustic metasurfaces, a class of metamaterials with a reduced dimensionality, empower new physics and lead to extended functionalities different from their three-dimensional counterparts, enabling controlling, transmitted or reflected acoustic waves in ways that were not possible before. In this review paper, we present a comprehensive view of this rapidly growing research field by introducing the basic concepts of acoustic metasurfaces and the recent developments that have occurred over the past few years. We review the interesting properties of acoustic metasurfaces and their important functionalities of wavefront manipulation, followed by an outlook for promising future directions and potential practical applications.

Cross-plane coherent acoustic phonons in two-dimensional organic-inorganic hybrid perovskites

Two-dimensional Ruddlesden–Popper organic–inorganic hybrid layered perovskites (2D RPs) are solution-grown semiconductors with prospective applications in next-generation optoelectronics. The heat-carrying, low-energy acoustic phonons, which are important for heat management of 2D RP-based devices, have remained unexplored. Here we report on the generation and propagation of coherent longitudinal acoustic phonons along the cross-plane direction of 2D RPs, following separate characterizations of below-bandgap refractive indices. Through experiments on single crystals of systematically varied perovskite layer thickness, we demonstrate significant reduction in both group velocity and propagation length of acoustic phonons in 2D RPs as compared to the three-dimensional methylammonium lead iodide counterpart. As borne out by a minimal coarse-grained model, these vibrational properties arise from a large acoustic impedance mismatch between the alternating layers of perovskite sheets and bulky organic cations. Our results inform on thermal transport in highly impedance-mismatched crystal sub-lattices and provide insights towards design of materials that exhibit highly anisotropic thermal dissipation properties.

Novel Series of Quasi-2D Ruddlesden-Popper Perovskites Based on Short-Chained Spacer Cation for Enhanced Photodetection.

Quasi two-dimensional (2D) layered organic-inorganic perovskite materials (e.g., (BA)2(MA) n-1Pb nI3 n+1; BA = butylamine; MA = methylamine) have recently attracted wide attention because of their superior moisture stability as compared with three-dimensional counterparts. Inevitably, hydrophobic yet insulating long-chained organic cations improve the stability at the cost of hindering charge transport, leading to the unsatisfied performance of subsequently fabricated devices. Here, we reported the synthesis of quasi-2D ( iBA)2(MA) n-1Pb nI3 n+1 perovskites, where the relatively pure-phase ( iBA)2PbI4 and ( iBA)2MA3Pb4I13 films can be obtained. Because of the shorter-branched chain of iBA as compared with that of its linear equivalent ( n-butylamine, BA), the resulting ( iBA)2(MA) n-1Pb nI3 n+1 perovskites exhibit much enhanced photodetection properties without sacrificing their excellent stability. Through hot-casting, the optimized ( iBA)2(MA) n-1Pb nI3 n+1 perovskite films with n = 4 give the significantly improved crystallinity, demonstrating the high responsivity of 117.09 mA/W, large on-off ratio of 4.0 × 102, and fast response speed (rise and decay time of 16 and 15 ms, respectively). These figure-of-merits are comparable or even better than those of state-of-the-art quasi-2D perovskite-based photodetectors reported to date. Our work not only paves a practical way for future perovskite photodetector fabrication via modulation of their intrinsic material properties but also provides a direction for further performance enhancement of other perovskite optoelectronics.

Layers for red luminescence

Perovskite Materials Two-dimensional organic-inorganic perovskites differ from their three-dimensional counterparts by having a quantum well structure that can alter their luminescent properties. Typically, these materials are formed by replacing A-site cations such as cesium with larger organic cations. Nazarenko et al. found that lead halide with two smaller cations, formamide (FA) and guanidinium (G)—FAGPbI4—exhibited red photoluminescence at room temperature and was stable up to 255°C. The material is also photoconductive. J. Am. Chem. Soc. 10.1021/jacs.8b00194 (2018).

Observation of enhanced hot phonon bottleneck effect in 2D perovskites

We thoroughly investigated the carrier-phonon relaxation process in 2D halide perovskites with the general formula of (BA)2(MA)n-1PbnI3n+1, where n = 2, 3, and 4, by femtosecond transient absorption spectroscopy. A significant enhancement of the hot phonon bottleneck effect is observed in the natural multiple quantum well with n = 2 and 3. Specifically, the sample with n = 3 shows a 1000 ps hot carrier relaxation time to reach room temperature, which is 10 times longer compared with its three-dimensional counterpart. We believe that both the organic cation and quantum confinement effect are responsible for this phenomenon. The acoustic phonon cannot propagate due to the decrease in group velocity caused by the confinement effect in such a quantum well structure. In addition, the confined acoustic phonons can up-convert to optical phonons due to the presence of a “hybrid phonon” induced by the organic cation. This result suggests a promising way to obtain long-live hot carrier materials.

Manipulating the one-dimensional topological edge state of Bi bilayer nanoribbons via magnetic orientation and electric field

Despite the superiority of two-dimensional (2D) topological insulators (TIs) over their three-dimensional (3D) counterparts in various aspects and the essential distinction between them in structural symmetry, the variation of the topological one-dimensional (1D) edge states upon magnetic interaction and their application for spintronic devices have not been sufficiently illuminated. Here, we reveal that 1D edge states of 2D TIs have a unique magnetic response never observed in 2D surface states of 3D TIs, and using this exotic nature we propose a way to utilize the spin-polarized channel for spintronic applications. We investigate the effects of width and magnetic decoration on the 1D topological edge state of Bi bilayer nanoribbons (BNRs). Through the Zak phase, we find that the zero-energy states are enforced at the magnetic domain boundaries in the Cr-decorated BNR and directly examine their robustness using short-range magnetic domain structures. We also demonstrate that 1D edge states of BNRs can be selectively and reversibly controlled by the combination of magnetic reorientation and electric field without compromising their structural integrity. Our work provides a fundamental understanding of 1D topological edge states and shows the opportunity of using these features in spintronic devices.

Two-Dimensional Oxides: Recent Progress in Nanosheets

Abstract Two-dimensional (2D) materials have been widely investigated for the last few years, introducing nanosheets and ultrathin films. The often superior electrical, optical and mechanical properties in contrast to their three-dimensional (3D) bulk counterparts offer a promising field of opportunities. Especially new research fields for already existing and novel applications are opened by downsizing and improving the materials at the same time. Some of the most promising application fields are namely supercapacitors, electrochromic devices, (bio-) chemical sensors, photovoltaic devices, thermoelectrics, (photo-) catalysts and membranes. The role of oxides in this field of materials deserves a closer look due to their availability, durability and further advantages. Here, recent progress in oxidic nanosheets is highlighted and the benefit of 2D oxides for applications discussed in-depth. Therefore, different synthesis techniques and microstructures are compared more closely.

Enhanced Thermal Stability in Perovskite Solar Cells by Assembling 2D/3D Stacking Structures.

Two-dimensional (2D) perovskites have been shown to be more stable than their three-dimensional (3D) counterparts due to the protection of the organic ligands. Herein a method is introduced to form 2D/3D stacking structures by the reaction of 3D perovskite with n-Butylamine (BA). Different from regular treatment with n-Butylammonium iodide (BAI) where 2D perovskite with various layers form, the reaction of BA with MAPbI3 only produce (BA)2PbI4, which has better protection due to more organic ligands in (BA)2PbI4 than the mixture of 2D perovskites. Compared to BAI treatment, BA treatment results in smoother 2D perovskite layer on 3D perovskites with a better coverage. The photovoltaic devices with 2D/3D stacking structures show much improved stability in comparison to their 3D counterparts when subjected to heat stress tests. Moreover, the conversion of defective surface into 2D layers also induces passivation of the 3D perovskites resulting in an enhanced efficiency.

Universal statistics of vortex tangles in three-dimensional random waves

The tangled nodal lines (wave vortices) in random, three-dimensional wavefields are studied as an exemplar of a fractal loop soup. Their statistics are a three-dimensional counterpart to the characteristic random behaviour of nodal domains in quantum chaos, but in three dimensions the filaments can wind around one another to give distinctly different large scale behaviours. By tracing numerically the structure of the vortices, their conformations are shown to follow recent analytical predictions for random vortex tangles with periodic boundaries, where the local disorder of the model ‘averages out’ to produce large scale power law scaling relations whose universality classes do not depend on the local physics. These results explain previous numerical measurements in terms of an explicit effect of the periodic boundaries, where the statistics of the vortices are strongly affected by the large scale connectedness of the system even at arbitrarily high energies. The statistics are investigated primarily for static (monochromatic) wavefields, but the analytical results are further shown to directly describe the reconnection statistics of vortices evolving in certain dynamic systems, or occurring during random perturbations of the static configuration.

Charge-Carrier Dynamics and Crystalline Texture of Layered Ruddlesden–Popper Hybrid Lead Iodide Perovskite Thin Films

Solution-processable organic metal halide Ruddlesden–Popper phases have shown promise in optoelectronics because of their efficiencies in solar cells along with increased material stability relative to their three-dimensional counterparts (CH3NH3PbI3). Here, we study the layered material butylammonium methylammonium lead iodide (C4H9NH3)2(CH3NH3)n−1PbnI3n+1 for values of n ranging from 1 to 4. Thin films cast from solution show a gradual change in the crystalline texture of the two-dimensional lead iodide layers from being parallel to the substrate to perpendicular with increasing n. Contactless time-resolved microwave conductivity measurements show that the average recombination rate order increases with n and that the yield–mobility products and carrier lifetimes of these thin films are much lower than that of CH3NH3PbI3, along with increased higher-order recombination rate constants.