Applications In Nanodevices Research Articles

Microscale spectroscopic mapping of defect evolution and filling in large-area growth of monolayer MoS2

Defects in monolayer transition metal dichalcogenides are known to be problematic in that they usually deteriorate optical and electronic properties, thus acting as a constraint in nanodevice applications. In this regard, understanding defect-related strain and charge distributions is intriguing to identify the inhomogeneous energy landscape of two-dimensional materials. Herein, simultaneous Raman and photoluminescence results unveil how strain and charge carriers evolve over the large-area growth of monolayer MoS2. Unexpectedly, spatially-resolved correlation analysis between the in-plane and out-of-plane phonon frequencies of MoS2 reveals that both compressive strain and charge carriers increase as the morphology evolves from a triangular flake to a partially coalesced film and, finally, to a fully covered film, indicating that the number of sulfur vacancies increases with surface coverage. Theoretically calculated formation energies of sulfur vacancies support the evolution scenarios with respect to morphological variations. Photoluminescence mappings of excitons and trions provide further quantitative information on electron concentrations that are consistent with Raman correlation data. Notably, chemical treatment of defective MoS2 with a p-type dopant leads to a relaxation of compressive strain and a decrease in excess electrons, where in particular, the p-doping effects are more prominent in the fully covered film region than in the partially coalesced region.

Self-Sustained Oscillation of Electrothermally Responsive Liquid Crystal Elastomer Film in Steady-State Circuits.

Self-excited oscillations have the advantages of absorbing energy from a stable environment and Self-control; therefore, Self-excited motion patterns have broader applications in micro devices, autonomous robots, sensors and energy-generating devices. In this paper, a Self-sustained curling liquid crystal elastomer (LCE) film-mass system is proposed on the basis of electrothermally responsive materials, which can realize Self-oscillation under a steady-state current. Based on the contact model and dynamic LCE model, a nonlinear dynamics model of LCE film in steady-state circuits is developed and numerical calculations are carried out using the Runge-Kutta method. Through numerical calculations, it is demonstrated that LCE film-mass systems have two motion patterns in steady-state circuits: namely, a Self-oscillation pattern and a stationary pattern. Self-sustained curling of LCE film originates from the fact that the energy absorbed by the system exceeds the energy dissipated due to the damping effect. In addition, the critical conditions for triggering Self-oscillation and the effects of several key dimensionless system parameters on the amplitude and period of Self-oscillation are investigated in detail. Calculation results show that the height of electrolyte solution, gravitational acceleration, elastic modulus of LCE film, limit temperature, curvature coefficient, thermal shrinkage coefficient and damping factor all have a modulating effect on the amplitude and period of Self-oscillation. This research may deepen the understanding of Self-excited oscillation, with promising applications in energy harvesting, power generation, monitoring, soft robotics, medical devices, and micro and nano devices.

Electric Field‐Induced Phase Transition on HPX6 (X = C, Si, Ge, Sn) Monolayers

Herein, the electronic, thermoelectric, and optical properties of semimetallic HPX6 (X = C, Si, Ge, Sn) monolayers are systematically studied under the influence of external electric field in the framework of density functional theory. A band tuning has been achieved in these structures by the application of an external electric field of appropriate strength. It is predicted that Dirac cone splitting is nearly proportional to the external electric field strength. The modulation of electric properties induced by external field can alter the position of chemical potential in the band diagram and brings significant improvement in thermoelectric responses. The application of an external electric field significantly modulates the optical properties. The electric field‐induced HPX6 system provides better thermoelectric and optical response for nanodevice applications.

Study Using the Brus Equation to Examine How Quantum Confinement Energy Affects the Optical Characteristics of Cadmium Sulfide and Zinc Selenide

In this paper, the Brus equation was used to study the effects of quantum confinement energy on the optical properties of material with quantum dot (QD). It was found that the wavelength and energy gap depend on the quantum confinement energy. With the increase in the energy of quantum confinement, the energy gap increases and the absorption wavelength decreases and shifts toward the UV region. This large blue shift observed in CdS and ZnSe QDs could be exploited for the potential applications as smart materials in photonic nanodevice and various other applications.

Exploring isotropic tendency for the blue membrane containing wild-type bacteriorhodopsin

Bacteriorhodopsin of purple membrane has wide potential applications in bioelectronics and biophotonic nanodevices. Upon acidification, it turns blue and upon further acidification by HCl, it retains its purple color. The acid-induced structural changes might be correlated to its crystalline structure, which might be mediated by lipids of purple membrane. Therefore, the present study aims at revealing the acidic pH dependence of anisotropic properties of bacteriorhodopsin. The electric impedance has been measured for parallel- and perpendicular-oriented purple membrane, in addition to the randomly-oriented one in the acidic pH range. The results have showed that the electric anisotropy is proportional to the color transitions occurred at low pH with consistent pKa values. It has found that the bacteriorhodopsin, upon turning into blue form, tends to be isotropic within narrow pH region around 2.55, whereas it preserves its anisotropy in its purple form. It is noteworthy that several mutants of bacteriorhodopsin that resemble its blue form became attractive in technical applications such as real-time holographic interferometry and optical data storage. Accordingly, such isotropic tendency might implicate bacteriorhodopsin for further potential technical applications.

Mechanical properties of 2D metal–organic and covalent–organic frameworks with non trivial topological band dispersion

Using density functional theory (DFT), we investigate mechanical properties of a few 2D metal–organic frameworks (MOFs) and covalent–organic frameworks (COFs) having Dirac and flat bands. These porous materials have become a subject of great captivation because of their physical stability, distinctive structural characteristics and large surface to volume ratio. The inherent porosity of these frameworks gives rise to many fascinating and occasionally surprising phenomena, which makes them potential candidates for technological applications. For reliable usage of MOFs/COFs in functional nanodevice and practical application, it is quite imperative to investigate their mechanical properties. Thus, herein a particular attention is paid to study elastic deformation of few 2D MOFs and COFs having non trivial topological band dispersion in the regime with linear dependency of stress upon strain. Specially, we consider different types of deformation and find all the components of elastic tensor from the stress–strain and energy–strain curves. These findings may provide useful information to fabricate the MOFs/COFs based devices by lowering the number of experiments.

Investigation of Giant Nonlinearity in a Plasmonic Metasurface with Epsilon-Near-Zero Film

Plasmonic metamaterials can exhibit a variety of physical optical properties that offer extraordinary nonlinear conversion efficiency for ultra-compact nanodevice applications. Furthermore, the optical-rectification effect from the plasmonic nonlinear metasurfaces (NLMSs) can be used as a compact source of deep-subwavelength thickness to radiate broadband terahertz (THz) signals. Meanwhile, a novel dual-mode metasurface consisting of a split-ring resonator (SRR) array and an epsilon-near-zero (ENZ) layer was presented to boost the THz conversion efficiency further. In this paper, to explore the mechanism of THz generation from plasmonic NLMSs, the Maxwell-hydrodynamic multiphysics model is adopted to investigate complex linear and intrinsic nonlinear dynamics in plasmonics. We solve the multiphysics model using the finite-difference time-domain (FDTD) method, and the numerical results demonstrate the physical mechanism of the THz generation processes which cannot be observed in our previous experiments directly. The proposed method reveals a new approach for developing new types of high-conversion-efficiency nonlinear nanodevices.

Theoretical study on the thermal transport and its tunability of a-plane trilayer GaN

Two-dimensional (2D) a-plane gallium nitride, a non-layered 2D material, has promising applications in photoelectric nanodevices due to its direct band bandgap. Herein, employing molecular dynamics simulations, we studied the thermal transport properties of a-plane trilayer GaN, and the temperature, together with strain modulation on the thermal conductivity of the system. The a-plane trilayer GaN shows anisotropic thermal conductivity with 70.22 Wm−1K−1 and 41.81 Wm−1K−1 along zigzag- and armchair- directions respectively at room temperature when extrapolated to infinite size. In addition, the thermal conductivity of trilayer GaN exhibits decreasing trend in response to the increase of temperature. The thermal conductivity decreases monotonically with the increased compressive uniaxial and biaxial strain, while it shows an up-then-down trend under tensile strain. The tunability of thermal conductivity under biaxial strain is much larger than that of uniaxial strain. The phonon density of states is further investigated to understand the behavior of thermal conductivity. The tunability of the system thermal conductivity will expand its applications in thermal management and nanodevices.

Ultrahigh Carrier Mobility in Two-Dimensional IV-VI Semiconductors for Photocatalytic Water Splitting.

Two-dimensional materials have been developed as novel photovoltaic and photocatalytic devices because of their excellent properties. In this work, four δ-IV-VI monolayers, GeS, GeSe, SiS and SiSe, are investigated as semiconductors with desirable bandgaps using the first-principles method. These δ-IV-VI monolayers exhibit exceptional toughness; in particular, the yield strength of the GeSe monolayer has no obvious deterioration at 30% strain. Interestingly, the GeSe monolayer also possesses ultrahigh electron mobility along the x direction of approximately 32,507 cm2·V-1·s-1, which is much higher than that of the other δ-IV-VI monolayers. Moreover, the calculated capacity for hydrogen evolution reaction of these δ-IV-VI monolayers further implies their potential for applications in photovoltaic and nano-devices.

Mechanical behavior of monolayer MoS2 films with arrayed dislocation defects

Compared with single and randomly distributed defects, arrayed defects exist widely in the film structure and can be designed artificially to achieve various specific functions. However, the mechanical behavior of transition metal dihalides with dislocation array defects is unclear; this limits their application in new flexible nanodevices. In this study, the strength and fracture properties of monolayer MoS2 films containing 5|7 (S5|7 and Mo5|7) dislocation defects under uniaxial tension were studied by molecular dynamics simulation. The results show that the number of defects, adjacent spacing, and alignment angle affect the critical strain and tensile strength of the monolayer MoS2 films. These results provide a basis for further understanding the mechanical behavior of monolayer MoS2 thin films and provide a theoretical reference for the application of nano-electronic devices based on MoS2.

Ultrahigh current and ultralow power dissipation of Janus monolayer IIIA-VIA Ga2XY MOSFETs

With the advances of two-dimensional semiconductors, it is promising for the miniaturization of highly integrated circuits. However, the high leakage current, small on-state current and unnecessary power consumption hinder the development of high-performance nanodevices. Here, we design sub-5-nm double-gate monolayer Ga2XY (X, Y = S, Se and Te) metal–oxidesemiconductor field-effect transistors. For all the designed devices, the on-state current, intrinsic delay time and power delay product can meet the standard of the International Technology Roadmap for Semiconductor in 2028. Additionally, the devices based on large bandgap materials have strong inhibition effect on leakage current. In particular, the Ga2SSe device has optimized subthreshold swing as low as 60.7 mV/dec and Ion of 5082 μA/μm (about 6 times higher than the high-performance requirements). The extremely low energy-delay product of 3.64 × 10−31 Js/μm is obtained for the Ga2SeTe device. Therefore, monolayer Ga2XY can be expected for the application of next-generation high-performance electronic nanodevices.

Photoluminescence, electrical and mechanical properties of ultra-long single crystalline Al4O4C nanowires

Ultra-long single-crystalline orthorhombic Al4O4C nanowires (NWs) were synthesized via annealing a mixture of Ta, Al, and graphite powders at 1500 °C for 1 h in a flowing argon atmosphere based on the vapor-liquid-solid (VSL) mechanism. The molten Al-Ta droplets formed by adding Ta resulted in its growth along the [101] direction. Therefore, highly crystalline Al4O4C NWs possessed rough surfaces, a high aspect ratio (∼2200) with diameters ranging from 100 nm to 500 nm and lengths up to hundreds of micrometers (660 μm). They exhibited a decreased bandgap of 2.63 eV and broad emission bands in yellow/near-infrared light region under the excitation of 532 nm, which was originated by Ar doping and surficial defects. Furthermore, the single nanowire electrode devices were fabricated using the FIB-SEM technique to accurately measure the electrical and mechanical properties. Al4O4C NWs possessed the intrinsic resistivity of 202.61 Ω·m, Young's modulus of 93.89 ± 13.34 GPa, and yield strength of 4.08 ± 0.51 GPa, and therefore it is potential for nanodevice applications.

Angular resolved ultra-low frequency Raman spectroscopy of few-layer black phosphorous

Few-layer black phosphorus (FLBP) exhibits valuable mechanical, optical, and electronic properties for potential applications in nanodevices, ranging from insulators to superconductors. In particular, their band structure and optical properties are closely related to layer numbers, stacking sequences, and twist angles. However, the mechanism behind these factors is not fully understood, as it is quite difficult to obtain stacking details using non-destructive probes. This article provides direct experimental evidence of the weak interlayer van der Waals (vdW) interaction of FLBP using a state-of-the-art, angular resolved, ultra-low frequency Raman spectroscopy system. A modified linear-chain model is presented to evaluate the layer number dependence on the extremely low-frequency breathing modes (Ag3 and Ag4). The polarization analysis illustrates that most bi-layer BP samples prefer the AB stacking sequence. Furthermore, the stacking angle of bi-layer BP was assigned based on its minimum intensity of Ag3 and Ag4 modes. This investigation on angular resolved extremely low-frequency Raman spectra sheds light on a new approach for analyzing the stacking details of FLBP, which is of great significance for developing novel FLBP-based nanodevices.

Two-dimensional T[formula omitted]-phase MA[formula omitted]N[formula omitted] (M=Mo/W, A=Si/Ge) nanosheets: First-principles insights into the structural stability, electronic property and catalytic performance for hydrogen evolution reaction

For the emerging MA2Z4 materials, most researches focus on their perfect trigonal-prismatic (H-phase) and octahedral (T-phase) geometries, while the distorted octahedral (T′-phase) one is rarely studied. In this work, we perform a first-principles study on the T′-phase MA2N4 (M=Mo/W, A=Si/Ge) nanosheets to investigate their structural, mechanical, electronic and catalytic properties. As a metastable phase, the T′-MA2N4 nanosheets still possess sufficient stability from the energetic, dynamical, thermal and mechanical points of view. Unlike the semiconducting H-phase counterparts, the T′-MA2N4 nanosheets display a semimetallic or metallic behaviour depending on the lattice constants. Interestingly, these T′-MA2N4 nanosheets exhibit highly efficient catalytic performance for hydrogen evolution reaction (HER). The Gibbs free energy of hydrogen adsorption on the T′-MoGe2N4 nanosheet is close to zero, which is even superior to the well-known Pt catalyst. Furthermore, all the surface N atoms behave as active sites, giving rise to excellent basal HER activity. For these MA2N4 systems, the T′-phase will become more favourable than the H-phase via a combined effect of electron injection and tensile strain. Our study demonstrates that the T′-phase MA2N4 nanosheets possess peculiar electronic properties and promising HER activity, opening up potential applications in nano-devices and renewable energy.

Two-dimensional half-metallicity and fully spin-polarized topological fermions in monolayer EuOBr

Two-dimensional (2D) half-metal and topological states have been the current research focus in condensed matter physics. Herein, we report a novel 2D material named EuOBr monolayer, which can simultaneously show 2D half-metal and topological fermions. This material shows a metallic state in the spin-up channel but a large insulating gap of 4.38 eV in the spin-down channel. In the conducting spin channel, the EuOBr monolayer shows the coexistence of Weyl points and nodal-lines near the Fermi level. These nodal-lines are classified by type-I, hybrid, closed, and open nodal-lines. The symmetry analysis suggests these nodal-lines are protected by the mirror symmetry, which cannot be broken even spin–orbit coupling is included because the ground magnetization direction in the material is out-of-plane [001]. The topological fermions in the EuOBr monolayer are fully spin-polarized, which can be meaningful for future applications in topological spintronic nano-devices.

Structural diversity and optoelectronic properties of chemical modification pentagonal quantum dots

A first-principle study of the structural diversity and optoelectronic properties of the small penta-graphene quantum dots (PGQDs) has been performed. The stability and optoelectronic properties of the PGQDs are investigated under the effect of chemical modifications. PGQDs are edge functionalized by non-metallic atoms (Si, P, O, F) such as identical edge termination (Si-PGQD, P-PGQD, O-PGQD, F-PGQD) and alternate edge termination (Si-O-PGQD, H-P-PGQD). Further, H-PGQDs are also doped and co-doped with B and P atoms. All studied structures are stable with strong electronic quantization and exhibit semiconducting or metallic properties depending on the termination, doping elements and their site. Absorption peaks in the visible region were not observed for hydrogen passivation PGQDs. However, some absorption peaks appear in this region for edge-passivated. In addition, there are dramatic changes in the electronic properties of B, P, BP-doped PGQDs to give peak shifts to the visible region from the ultraviolet region of the pure sample due to hybridization effects. The enhanced reactivity, controllable electronic properties of edge passivation, and doping make PGQDs ideal for new nanodevice applications.

Accessible light-controlled knockdown of cell-free protein synthesis using phosphorothioate-caged antisense oligonucleotides

Controlling cell-free expression of a gene to protein with non-invasive stimuli is vital to the future application of DNA nanodevices and synthetic cells. However, little emphasis has been placed on developing light-controlled ‘off’ switches for cell-free expression. Light-activated antisense oligonucleotides have been developed to induce gene knockdown in living cells; however, they are complicated to synthesise and have not been tested in cell-free systems. Developing simple, accessible methods to produce light-activated antisense oligonucleotides will be crucial for allowing their application in cell-free biology and biotechnology. Here, we report a mild, one-step method for selectively attaching commercially-available photoremovable protecting groups, photocages, onto phosphorothioate linkages of antisense oligonucleotides. Using this photocaging method, upon illumination, the original phosphorothioate antisense oligonucleotide is reformed. Photocaged antisense oligonucleotides, containing mixed phosphorothioate and phosphate backbones, showed a drastic reduction in duplex formation and RNase H activity, which was recovered upon illumination. We then demonstrated that these photocaged antisense oligonucleotides can be used to knock down cell-free protein synthesis using light. This simple and accessible technology will have future applications in light-controlled biological logic gates and regulating the activity of synthetic cells.

Ultrahigh energy density solid state supercapacitor based on metal halide perovskite nanocrystal electrodes: Real-life applications

Inorganic lead halide perovskite nanocrystals have now become an emerging material for modern nanodevice applications. But, the huge toxicity of lead to the ecosystem has limited its applications in modern technology. In this view, a large organic cation-based metal halide perovskite may be considered the most efficient supercapacitor electrode material. Here, a new type and high molecular organic cations based low-dimensional metal halide perovskite (LDMHP) nanocrystals (NCs) are synthesized by a chemical process and their performances as supercapacitor electrode is tested. An excellent charge storage capacity and especially the Tin (Sn)-based perovskite NCs showed a very high specific capacitance and energy density of ~1536 Fg−1 and ~213 Whkg−1 at a current density of 2.0 Ag−1, respectively. The calculated variable parameter (b) value from cyclic voltammetry showed that the total capacity of the Sn-based perovskite NCs electrode is controlled by capacitor-like behaviour. The Sn NCs also found to have a very high DC dielectric constant at room temperature, possibly responsible for their superior supercapacitor performance. A solid-state supercapacitor based on synthesized NCs was used for real-life applications of supplying sufficient power to light emitting diodes (LEDs) for a long time.

Hydrogenated cove-edge aluminum nitride nanoribbons for ultrascaled resonant tunneling diode applications: a computational DFT study

While synthesizing quasi-one-dimensional nanoribbons, there is a finite probability that edges have cove-edge defects. This paper focuses on the structural, electronic, and transport properties of cove-edge aluminum nitride nanoribbons (AlNNR) using density functional theory and the non-equilibrium Green’s function (NEGF) method. The cove-edge AlNNRs are thermodynamically stable and exhibit metallic behavior. Interestingly, the calculated current–voltage characteristics of the cove-edge AlNNR-based nanodevices show negative differential resistance (NDR). The H-AlN-Cove nanodevice exhibits high peak-to-valley current ratio (PVCR) of the order of 107. The calculated PVCR of the H-AlN-Cove nanodevice is 106 times higher than that of the silicene nanoribbon (SiNR) and graphene nanoribbon (GNR), and 104 times higher than that of the phosphorene nanoribbon (PNR) and arsenene nanoribbons (ANR)-based devices respectively. The NDR feature with high PVCR provides a prospect for the cove-edge AlNNR in nanodevice applications.

In-plane momentum-dependent spin texture originating from surface symmetry breaking in the two-dimensional van der Waals ferromagnet CrGeTe3

Investigating electronic structures of two-dimensional van der Waals magnets is of great significance for the application in low-power nanodevices. The two-dimensional van der Waals ferromagnet ${\mathrm{CrGeTe}}_{3}$ has attracted much attention with chemically stable properties and relatively high Curie temperature. Here, we investigate the in-plane spin texture of ${\mathrm{CrGeTe}}_{3}$ in the ferromagnetic state by first-principles calculations, which reveal a helical-type spin texture that is attributed to surface symmetry breaking instead of spin-layer locking effect. Such spin texture is also supported by our spin-resolved photoemission spectroscopy experimental data. Our results provide a platform for exploring next-generation spintronic devices.