Anisotropic Mobility Research Articles

Designing an isoelectronic counterpart to group III-V semiconductors: A novel honeycomb BeCN2 monolayer with ultra-high mobility toward solar driven overall water splitting and bifunctional oxygen evolution/reduction reaction

Like group III-V materials, which have obviously broadened the scope of group IV family and enriched the semiconducting members, a lightest representative of group II−IV−V2 family, BeCN2 monolayer, as an isoelectronic counterpart to group III-V semiconductor has for long remained unsolved for its applications in HER/OER/ORR, although previous first-principles investigations have touched the configuration and existence toward multiple crystalline forms of BeCN2. Using density functional theory calculations, the stable BeCN2 monolayer is predicted to be a direct bandgap semiconductor with high ultraviolet light absorption, achieving photocatalytic overall water-splitting within full pH, whose photogenerated electrons/holes will provide an external potential to obviously reduce the overpotential of HER and even make it achieve a spontaneous OER within pH 0–14. Meanwhile, it presents significantly anisotropic and high carrier mobilities (104 order) with preferred electron/hole transport along the x/y-direction, providing rich and required charge sources for HER/OER/ORR. After TM-decoration, the catalytic activities are greatly enhanced, achieved by the low HER and OER/ORR overpotentials of 0.05/0.07 V for V/Co@BeCN2 and 0.79/0.43 V for Ni@BeCN2, respectively. Plus, under pH 0–14, the OER overpotential of Ni@BeCN2 can be completely overcome by the external potential provided by photogenerated holes, achieving bifunctional catalysis for OER and ORR. Particularly, the Ni-d orbital is evidently hybridized with the O-p orbital for OH*Ni@BeCN2, so that the initial lone electrons located at the anti-bonding orbital pair up can populate the downward bonding orbital, allowing the species *OH to be moderately adsorbed on Ni@BeCN2, and thus redound the performance of OER/ORR.

Tuneable Anisotropic Plasmonics with Shape-Symmetric Conducting Polymer Nanoantennas.

A wide range of nanophotonic applications rely on polarization-dependent plasmonic resonances, which usually requires metallic nanostructures that have anisotropic shape. This work demonstrates polarization-dependent plasmonic resonances instead by breaking symmetry via material permittivity. The study shows that molecular alignment of a conducting polymer can lead to a material with polarization-dependent plasma frequency and corresponding in-plane hyperbolic permittivity region. This result is not expected based only on anisotropic charge mobility but implies that also the effective mass of the charge carriers becomes anisotropic upon polymer alignment. This unique feature is used to demonstrate circularly symmetric nanoantennas that provide different plasmonic resonances parallel and perpendicular to the alignment direction. The nanoantennas are further tuneable via the redox state of the polymer. Importantly, polymer alignment could blueshift the plasma wavelength and resonances by several hundreds of nanometers, forming a novel approach toward reaching the ultimate goal of redox-tunable conducting polymer nanoantennas for visible light.

Electronic, Magnetic, and Optical Tunability of SiP Monolayers via 3d Transition‐Metal Doping

Herein, the electronic, magnetic, and optical properties of direct‐band semiconducting SiP monolayers (MLs) doped with 3d transition‐metal (TM) atoms are investigated using first‐principles calculations. The results indicate that TM (TM = Sc, Ti, Co, Ni, Cu, and Zn)‐doped SiP MLs are nonmagnetic, while TM (TM = V, Cr, Mn, Fe)‐doped ones exhibit prominent magnetic moments. Specifically, V‐, Cr‐, and Fe‐doped SiP MLs are magnetic semiconductors and Mn‐doped SiP ML is a spin gapless semiconductor, where the contributions of the TM atoms to magnetic moments are dominant and the contributions of the nearest Si and P atoms are either small or negligible. Furthermore, the TM (TM = V, Cr, Mn, Fe) doping can enhance the carrier mobility in certain directions and result in highly in‐plane anisotropic mobility behaviors. In addition, the TM (TM = V, Cr, Mn, Fe) doping can not only broaden the optical absorption range of SiP ML to the infrared region, but also enhance the optical absorption coefficients of SiP ML in the visible region. These results provide insights into the TM doping effect on the electronic, spintronic, and optical properties of SiP MLs and may be useful for related applications based on SiP MLs.

A DNA turbine powered by a transmembrane potential across a nanopore

Rotary motors play key roles in energy transduction, from macroscale windmills to nanoscale turbines such as ATP synthase in cells. Despite our abilities to construct engines at many scales, developing functional synthetic turbines at the nanoscale has remained challenging. Here, we experimentally demonstrate rationally designed nanoscale DNA origami turbines with three chiral blades. These DNA nanoturbines are 24–27 nm in height and diameter and can utilize transmembrane electrochemical potentials across nanopores to drive DNA bundles into sustained unidirectional rotations of up to 10 revolutions s−1. The rotation direction is set by the designed chirality of the turbine. All-atom molecular dynamics simulations show how hydrodynamic flows drive this turbine. At high salt concentrations, the rotation direction of turbines with the same chirality is reversed, which is explained by a change in the anisotropy of the electrophoretic mobility. Our artificial turbines operate autonomously in physiological conditions, converting energy from naturally abundant electrochemical potentials into mechanical work. The results open new possibilities for engineering active robotics at the nanoscale.

Two-dimensional β-noble-transition-metal chalcogenide: novel highly stable semiconductors with manifold outstanding optoelectronic properties and strong in-plane anisotropy.

In this work, five two-dimensional (2D) noble-transition-metal chalcogenide (NTMC) semiconductors, namely β-NX (N = Au, Ag; X = S, Se, Te), were designed and predicted by first-principles simulations. Structurally, the monolayer β-NX materials have good energetic, mechanical, dynamical, and thermal stability. They contain two inequivalent noble-transition-metal atoms in the unit cell, and the N-X bond comprises a partial ionic bond and a partial covalent bond. Regarding the electronic properties, the β-NX materials are indirect-band-gap semiconductors with appropriate band-gap values. They have tiny electron effective masses. The hole effective masses exhibit significant differences in different directions, indicating strongly anisotropic hole mobility. In addition, the coexistence of linear and square-planar channels means that the diffusion and transport of carriers should be anisotropic. In terms of optical properties, the β-NX materials show high absorption coefficients. The absorption and reflection characteristics reveal strong anisotropy in different directions. Therefore, the β-NX materials are indirect-band-gap semiconductors with good stability, high absorption coefficients, and strong mechanical, electronic, transport, and optical anisotropy. In the future, they could have great potential as 2D semiconductors in nano-electronics and nano-optoelectronics.

Silica-water superstructure and one-dimensional superionic conduit in Earth's mantle.

Water in Earth's deep interior is predicted to be hydroxyl (OH-) stored in nominally anhydrous minerals, profoundly modulating both structure and dynamics of Earth's mantle. Here, we use a high-dimensional neuro-network potential and machine learning algorithm to investigate the weight percent water incorporation in stishovite, a main constituent of the subducted oceanic crust. We found that stishovite and water prefer forming medium- to long-range ordered superstructures, featuring one-dimensional (1D) water channels. Synthesizing single crystals of hydrous stishovite, we verified the ordering of OH- groups in the water channels through optical and nuclear magnetic resonance spectroscopy and found an average H-H distance of 2.05(3) Å, confirming simulation results. Upon heating, H atoms were predicted to behave fluid-like inside the channels, leading to an exotic 1D superionic state. Water-bearing stishovite could feature high ionic mobility and strong electrical anisotropy, manifesting as electrical heterogeneity in Earth's mantle.

Kagome-like BiP3 Monolayer: An Emerging Quasi-Direct Auxetic Semiconductor Coupled with High Anisotropic Mobility toward Visible-Light-Driven Photoelectrocatalytic pH-Robust Overall Water-Splitting.

Two-dimensional (2D) Janus materials exhibit an outstanding potential that can meet the rigorous requirements of photocatalytic water splitting resulting from their unique atomic arrangement. However, these materials are quite scarce. Through ab initio density functional theory calculations, we introduce a kagome topology into the honeycomb lattice of blue phosphorene using phosphorus and bismuth atoms to build a hybrid honeycomb-like kagome lattice, realized by a hitherto unknown kagome-like Janus-like BiP3 monolayer with robust stability. Excitingly, the out-of-plane asymmetry benefiting from kagome and honeycomb topologies gives rise to a significantly negative out-of-plane Poisson's ratio and an obvious built-in electric field pointing from the sublayer of the P atom to the sublayer of the Bi atom. In conjunction with the investigations that encompass semiconducting properties, such as a quasi-direct gap, suitable band-edge positions, effective visible-light absorption, and high carrier mobility, the BiP3 monolayer achieves overall water splitting at pH 0-14 regardless of strain. Moreover, this intrinsic electric field provides a sufficient photogenerated carrier driving force for water splitting. The bare BiP3 comprises P and Bi atoms that function as catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) active sites, respectively. Upon exposure to light, the reaction of water into H2 and O2 can be observed across a pH range of 0-14. Meanwhile, by designing a transition-metal single-atom catalyst (TM@BiP3), our investigations have shown that embedding a single TM on BiP3 is a feasible route to improving the HER/OER activity by reducing the overpotentials to -0.039 and 0.58 eV for Mo and Os atoms, respectively. In this case, the positive value of the external potential acts as a sufficient OER driving force, i.e., in the light environment, the Os@BiP3 system can promote water molecules spontaneously oxidized into O2 at pH 0-14.

Modulating electronic properties of β-Ga2O3 by strain engineering

β-Ga2O3 is a promising material for the development of next-generation power electronic and optoelectronic devices due to its exceptional properties, including ultrawide bandgap and thermodynamic stability. Strain engineering has emerged as a powerful method to modulate the physical properties of materials and has been widely employed in semiconductor devices to enhance their performance and functionality. Our study focuses on the effects of strain engineering on the electronic properties of β-Ga2O3. Using density functional theory, we calculated the band structures and electron effective masses of β-Ga2O3 under different strain states. Our investigation revealed that strain manipulation can induce an indirect-direct bandgap transition. Strain can also lead to changes in effective masses and anisotropy of electron mobility. Our calculations provide important insights into the potential of strain engineering as a powerful tool for modulating the electronic properties of β-Ga2O3, with important implications for practical device applications.

The evolution of grain boundary energy in textured and untextured Ca‐doped alumina during grain growth

AbstractThe role of anisotropic grain boundary energy in grain growth is investigated using textured microstructures that contain a high proportion of special grain boundaries. Textured and untextured Ca‐doped alumina was prepared by slip casting inside and outside a high magnetic field, respectively. At 1600°C, the textured microstructure exhibits faster growth than the untextured microstructure and its population of low‐angle boundaries increases. Atomic force microscopy (AFM) is employed to measure the geometry of thermal grooves to assess the relative grain boundary energy of these systems before and after growth. In the textured microstructure, the grain boundary energy distribution narrows and shifts to a lower average energy. Conversely, the energy distribution broadens for the untextured microstructure as it grows and exhibits abnormal grain growth. Further analysis of the boundary networks neighboring abnormal grains reveals an energy incentive that facilitates their growth. These results suggest that coarsening is not the only dominant grain growth mechanism and that the system can lower its energy effectively by replacing high energy boundaries with those of low energy. The faster growth of lower energy boundaries suggests that isotropic simulations do not adequately account for anisotropic grain growth mechanisms or anisotropic mobility.

Negative Poisson's ratio and anisotropic carrier mobility in ternary Janus Si2XY (X/Y= S, Se, Te): First-principles prediction

In this Letter, the structural, mechanical, electronic, and transport properties of two-dimensional (2D) ternary Janus Si2XY (X/Y= S, Se, Te) monolayers are studied based on the calculations using first-principles density functional theory. All three structures are found as direct semiconductors with moderated bandgap energies and good stabilities for experimental synthesis. The transport properties are also examined by calculating the carrier mobilities. We find that the carrier mobilities of all three monolayers are anisotropic not only between the electrons and holes but also between the two transport directions. The Si2SSe monolayer exhibited the highest electron mobility of 897.66 cm2 V−1 s−1 in the x axis. Amazingly, our calculations reveal that the Si2XY monolayers are auxetic materials with negative Poisson's ratio along both x and y axes. Particularly, the Si2SSe has the largest negative Poisson's ratio value of –0.131 in the x direction. These obtained results open more prospects for advanced applications of these materials in electronics, optoelectronics, and nanomechanics.

Experimental and Theoretical Study on Anisotropic Electron Mobility in 4H‐SiC

Electron mobility parallel to the c‐axis in 4H‐SiC is experimentally determined by Hall effect measurements over wide donor density and temperature ranges (6 × 1014–3 × 1018 cm−3 and 140–600 K), and it is compared with that perpendicular to the c‐axis obtained for the same conditions. Empirical equations for the mobility along both directions are determined as functions of donor density and temperature, which contribute to the simulation and designing of SiC devices. The origin of the mobility anisotropy is discussed, focusing on the electron effective mass anisotropy. For a precise analysis, taking into account the effect of electrons at a higher energy region than the conduction band bottom, an average electron effective mass considering the energy distribution is theoretically calculated from the band structure of SiC. As a result, it is clarified that the electron mobility anisotropy including its temperature dependence is explained by the average electron effective mass anisotropy.

Theoretical investigation of the electronic structure and thermoelectric performance of 2D GeSb2Te4 and GeBi2Te4

Thermoelectric technology is attractive for waste heat recovery and clean energy; it is highly desired as a potential power supply. In this paper, we investigated the stability, elastic, electronic structures and thermoelectric properties of unexplored 2D materials, GeSb2Te4 and GeBi2Te4, which deliver high stability, acceptable cleavage energies (0.32–0.33 J/m2), and narrow band gaps of 0.80 and 0.69 eV. We also revealed that the septuple layers possess anisotropic electron and hole mobilities of about 1593.71/791.50 and 522.04/240.52 cm2/Vs, which result in high conductivities of 107–108 S/m, as well as desirable thermoelectric power factors of 28.08–70.382 mW/Km2. Besides, owing to the low group velocities and strong dissipative scattering for low-lying phonons, the materials inherit the low lattice thermal conductivities of 1.51 and 0.41 W/mK. As a result, their thermoelectric figure of merit reaches 1.60 and 1.70 at 300 K, and rises further to 3.80 and 4.16 at 700 K. Together, 2D GeSb2Te4 and GeBi2Te4 are suitable candidates for low-medium temperature thermoelectric application.

Suppression of 1/f noise in graphene due to anisotropic mobility fluctuations induced by impurity motion

Low frequency resistance variations due to mobility fluctuations is one of the key factors of 1/f noise in metallic conductors. According to theory, such noise in a two-dimensional (2D) device can be suppressed to zero at small magnetic fields, implying important technological benefits for low noise 2D devices. In this work, we provide evidence of anisotropic mobility fluctuations by demonstrating a strong field-induced suppression of noise in a high-mobility graphene Corbino disk, even though the device displays only a tiny amount of 1/f noise inherently. The suppression of the 1/f noise depends on charge density, showing less non-uniform mobility fluctuations away from the Dirac point with charge puddles. We model our results using an approach based on impurity clustering dynamics and find our results consistent with the 1/f noise induced by scattering of carriers on mobile impurities forming clusters.

Quaternary 2D monolayer Cu2Cl2Se2Hg2: anisotropic carrier mobility and tunable bandgap for transistor and photocatalytic applications

Despite the advantages of quaternary two-dimensional (2D) materials, fewer studies have been done on them than binary 2D materials. Calculations of quaternary 2D monolayer Cu2Cl2Se2Hg2 based on density functional theory and Green’s function surface analysis provide insights into its structural, dynamic, and thermal stability. This material has a direct band gap of 0.91/2.0 eV (Perdew–Burke–Ernzerhof/Heyd–Scuseria–Ernzerhof) and demonstrates anisotropic carrier mobility. The electron mobility in the a direction is 1.2 × 103 cm2 V−1 s−1, which is significantly higher than the hole mobility of 0.48 × 103 cm2 V−1 s−1. In the b direction, the electron mobility is 1.01 × 103 cm2 V−1 s−1 and is 8.9 times larger than the hole mobility of 0.11 × 103cm2 V−1 s−1. The light absorption coefficients of Cu2Cl2Se2Hg2 are 1.0 × 105 cm−1 and 2.5 × 105 cm−1 in the visible and ultraviolet ranges, respectively. Uniaxial strain leads to an anisotropic alteration in the band gap and band edge position. By manipulating the strain direction and level in Cu2Cl2Se2Hg2, it is possible to increase the current ON/OFF ratio for field-effect transistors (FETs) and to facilitate photocatalytic water splitting through a redox reaction. The research reveals that Cu2Cl2Se2Hg2, a 2D monolayer in the quaternary form, has promising capabilities as an alternative for creating crystal-oriented FETs and photocatalytic water splitting systems.

Single-layer ZnGaInS4: Desirable bandgap and high carrier separation efficiency for optoelectronics

Two-dimensional optoelectronic materials with desirable bandgaps and high carrier separation efficiency are in urgent need. Herein, single-layer ZnGaInS4 is designed and thoroughly studied by first-principles calculations. Our calculations reveal the excellent stability of single-layer ZnGaInS4, and the small cleavage energy demonstrates the feasibility to exfoliate single-layer ZnGaInS4 from bulk counterparts. At the HSE06 level, single-layer ZnGaInS4 has a moderate bandgap of 2.05 eV, and its valence band maximum and conduction band minimum are spatially separated, thus impeding the electron-hole pair formation and enhancing the photoelectronic performance. In the visible region, single-layer ZnGaInS4 exhibits an optical absorption coefficient of ∼ 105 cm−1. Notably, single-layer ZnGaInS4 possesses high electron carrier mobility and low hole carrier mobility, further affirming the effective carrier separation. Another attractive characteristic is that single-layer ZnGaInS4 has anisotropic hole mobility. The desirable bandgap and high separation efficiency of generated carriers, together with abundant optical absorption, promise single-layer ZnGaInS4 is a hopeful candidate in atomic-thick optoelectronic devices.

Mn-X (X = F, Cl, Br, I) Co-Doped GeSe Monolayers: Stabilities and Electronic, Spintronic and Optical Properties.

GeSe monolayer (ML) has recently attracted much interest due to its unique structure and excellent physical properties that can be effectively tuned through single doping of various elements. However, the co-doping effects on GeSe ML are rarely studied. In this study, the structures and physical properties of Mn-X (X = F, Cl, Br, I) co-doped GeSe MLs are investigated by using first-principle calculations. The formation energy and phonon disspersion analyses reveal the stability of Mn-Cl and Mn-Br co-doped GeSe MLs and instability of Mn-F and Mn-I co-doped GeSe MLs. The stable Mn-X (X = Cl, Br) co-doped GeSe MLs exhibit complex bonding structures with respect to Mn-doped GeSe ML. More importantly, Mn-Cl and Mn-Br co-doping can not only tune magnetic properties, but also change the electronic properties of GeSe MLs, which makes Mn-X co-doped GeSe MLs indirect band semiconductors with anisotropic large carrier mobility and asymmetric spin-dependent band structures. Furthermore, Mn-X (X = Cl, Br) co-doped GeSe MLs show weakened in-plane optical absorption and reflection in the visible band. Our results may be useful for electronic, spintronic and optical applications based on Mn-X co-doped GeSe MLs.

Increasing crystal plane orientation ratio of n-type Bi2WO6 to enhancing surface photovoltage due to higher hole mobility

Enhancing photoelectric sensitivity of semiconductors is meaningful to develop novel photoelectric materials but still challenging for a pristine semiconductor. Here we increase the crystal plane orientation ratio for higher hole mobility to enhance the surface photovoltage of n-type Bi2WO6 depending on the anisotropy of hole mobility. A strategy of hydrothermal synthesis and post-heating is adopted to prepare Bi2WO6. The diffraction peak relative intensity of (200) crystal plane in Bi2WO6 increases with the post-heating temperature from 200 to 400 °C and then decreases. The surface photovoltage of nano-sheet Bi2WO6 is enhanced to the maximum and then decreased with the diffraction peak change of (200) crystal plane. Theory calculation shows the hole effective mass of the crystal with (200) crystal plane is smaller than that of (113), indicating the higher hole mobility in (200) plane which contributes to the enhanced photovoltaic response. The investigation paves a way to enhance the photovoltaic properties of semiconductors by increasing the hole mobility depending on surface photovoltaic technology facilely.

Molten Salts‐Driven Discovery of a Polar Mixed‐Anion 3D Framework at the Nanoscale: Zn4Si2O7Cl2, Charge Transport and Photoelectrocatalytic Water Splitting

AbstractMixed‐anion compounds widen the chemical space of attainable materials compared to single anionic compounds, but the exploration of their structural diversity is limited by common synthetic paths. Especially, oxychlorides rely mainly on layered structures, which suffer from low stability during photo(electro)catalytic processes. Herein we report a strategy to design a new polar 3D tetrahedral framework with composition Zn4Si2O7Cl2. We use a molten salt medium to enable low temperature crystallization of nanowires of this new compound, by relying on tetrahedral building units present in the melt to build the connectivity of the oxychloride. These units are combined with silicon‐based connectors from a non‐oxidic Zintl phase to enable precise tuning of the oxygen content. This structure brings high chemical and thermal stability, as well as strongly anisotropic hole mobility along the polar axis. These features, associated with the ability to adjust the transport properties by doping, enable to tune water splitting properties for photoelectrocatalytic H2 evolution and water oxidation. This work then paves the way to a new family of mixed‐anion solids

Molten Salts-Driven Discovery of a Polar Mixed-Anion 3D framework at the nanoscale: Zn4Si2O7Cl2, Charge Transport and Photoelectrocatalytic Water Splitting.

Mixed-anion compounds widen the chemical space of attainable materials compared to single anionic compounds, but the exploration of their structural diversity is limited by common synthetic paths. Especially, oxychlorides rely mainly on layered structures, which suffer from low stability during photo(electro)catalytic processes. Herein we report a strategy to design a new polar 3D tetrahedral framework with composition Zn4Si2O7Cl2. We use a molten salt medium to enable low temperature crystallization of nanowires of this new compound, by relying on tetrahedral building units present in the melt to build the connectivity of the oxychloride. These units are combined with silicon-based connectors from a non-oxidic Zintl phase to enable precise tuning of the oxygen content. This structure brings high chemical and thermal stability, as well as strongly anisotropic hole mobility along the polar axis. These features, associated with the ability to adjust the transport properties by doping, enable to tune water splitting properties for photoelectrocatalytic H2 evolution and water oxidation. This work then paves the way to a new family of mixed-anion solids.

Theoretical insight on the charge transport properties: The formation of "head-to-tail" and "head-to-head" stacking of asymmetric aryl anthracene derivatives.

Organic semiconductors (OSCs) are widely used in flexible display, renewable energy, and biosensors, owing to their unique solid-state physical and optoelectronic properties. Among the abundant crystal library of OSCs, asymmetric aryl anthracene derivatives have irreplaceable advantages due to the interplay between their distinct π-conjugated geometry and molecular stacking as well as efficient light emission and charge transport properties that can be simultaneously utilized. However, the poor crystal stacking patterns of most asymmetric molecules limit their utility as excellent OSCs. Thus, it is crucial to clarify the structural features that enable the extremely ordered stacking and favorable electronic structure of asymmetric anthracene derivatives to become high-performance OSCs. This contribution investigates the charge transport properties of a series of asymmetric aryl anthracene derivatives to reveal the modulation factors of the molecular stacking modes and to explore the structural factors, which are beneficial to charge transport. The analysis demonstrated that the vinyl-linker facilitated the injection of hole carriers, and the alkynyl-linker effectively reduces the reorganization energy. Importantly, the linear polarizability and permanent dipole moment of a single molecule play a vital regulation to molecular stacking modes and the transfer integral of the dimer. The "head-to-head stacking" motif shows a compact stacking pattern and the maximum 2D anisotropic mobility more than 10cm2V-1s-1. These findings sharpen our understanding of the charge transport properties in asymmetric organic semiconductors and are essential for developing a diverse range of high-performance OSC materials.