Applications In Nanodevices Research Articles

Perpendicular magnetic anisotropy in half-metallic thin-film Co2CrAl

Magnetocrystalline anisotropy (MCA) is one of the key parameters investigated in spin-based electronics (spintronics), e.g. for memory applications. Here, we employ first-principles calculations to study MCA in thin film full Heusler alloy Co2CrAl. This material was studied in the past, and has been reported to exhibit half-metallic electronic structure in bulk geometry. In our recent work, we showed that it retains a 100% spin-polarization in thin-film geometry, at CrAl atomic surface termination. Here, we show that the same termination results in a perpendicular magnetic anisotropy, while Co surface termination not only destroys the half-metallicity, but also results in in-plane magnetization orientation. In addition, for films thicker than around 20 nm the contribution from magnetic shape anisotropy may become decisive, resulting in in-plane magnetization orientation. To the best of our knowledge, this is one of the first reports of half-metallic thin-film surfaces with perpendicular magnetic anisotropy. This result may be of interest for potential nano-device applications, and may stimulate a further experimental study of this and similar materials.

Huge stretchability and reversibility of helical graphenes using molecular dynamics simulations and simplified theoretical models

Helical nanostructures, like nanosprings, made from carbon-based materials have broad promising applications in mechanical nanodevices and energy-absorbing materials. However, it is a big challenge to reveal the mechanism of their excellent mechanical properties due to the lack of theoretical models. Here we report the huge stretchability and reversibility of helical graphenes (HGs) using molecular dynamics (MD) simulations. Simplified theoretical models based on interlayer van der Waals (vdW) interactions are developed to predict the tensile properties of different-size HGs in three elastic stages including the initial deformation, stable delamination and elastic deformation. During the tensile process, the vdW interactions between adjacent free coils play a key role in the initial deformation and delamination stages, while the tensile behavior is dominated by the stretching of carbon-carbon bonds in the elastic deformation stage. The obtained theoretical results agree well with those from present MD simulations. Meanwhile, both the critical force in the initial deformation stage and the critical elastic force in the elastic deformation stage decrease with increasing temperature. In particular, the mechanical behavior of HGs under cyclic loading shows obvious hysteresis loops, which produce large energy dissipation due to the recovery of interlayer vdW interactions. These findings should be of great help towards understanding the stability and reliability of these helical structures, and giving an insight to design a new generation of nanodevices.

Developing Cu pore-filling percentage in hard anodized anodic aluminum oxide templates with large diameters

This study aims to overcome a major challenge that is reaching high pore-filling percentage (FP) in the fabrication of high aspect ratio (HAR) nanowires (NWs) grown electrochemically in large diameter porous templates. By ultrathinning the barrier layer of hard anodized anodic aluminum oxide (AAO) templates with large pore diameters (Dp) of 110 and 180 nm, while also filling its branched sections with FeCoNi, the development of Cu FP was investigated for different Cu solution concentration (MCu = 0.05–0.3 M) in the fabrication of HAR Cu NW arrays using a pulsed electrochemical deposition technique under optimized parameters. At Dp = 180 nm, field-emission scanning electron microscopic investigations revealed that, increasing MCu increases the corresponding FP up to 62%, resulting in highly uniform NW arrays. Structural properties were investigated by X-ray diffraction analysis, indicating that different phase percentages of Cu and Cu2O are formed depending on Dp of AAO and MCu. The resulting HAR Cu NWs were also released from the hard templates to obtain free-standing NW arrays with a length of 70 μm, which may find potential use for large-scale nanodevice applications.

Electrochemical Delamination of Ultralarge Few‐Layer Black Phosphorus with a Hydrogen‐Free Intercalation Mechanism

Due to strong interlayer interaction and ease of oxidation issues of black phosphorus (BP), the domain size of artificial synthesized few-layer black phosphorus (FL-BP) crystals is often below 10µm, which extremely limits its further applications in large-area thin-film devices and integrated circuits. Herein, a hydrogen-free electrochemical delamination strategy through weak Lewis acid intercalation enabled exfoliation is developed to produce ultralarge FL-BP single-crystalline domains with high quality. The interaction between the weak Lewis acid tetra-n-butylammonium acetate (CH3 COOTBA) and P atoms promotes the average domain size of FL-BP crystal up to 77.6± 15.0µm and the largest domain size is found to be as large as 119µm. The presence of H+ and H2 O is found to sharply decrease the size of as-exfoliated FL-BP flakes. The electronic transport measurements show that the delaminated FL-BP crystals exhibit a high hole mobility of 76 cm2 V-1 s-1 and an on/off ratio of 103 at 298 K. A broadband photoresponse from 532 to 1850nm with ultrahigh responsivity is achieved. This work provides a scalable, simple, and low-cost approach for large-area BP films that meet industrial requirements for nanodevices applications.

Applications of Photo-Responsive Molecules in Nanopore-based Devices

Abstract In biochemistry, it is crucial to explore the conformation, response to external environment and interaction rules of biomacromolecules and small molecules for understanding the correlation of the structures and properties among organic matters. However, it is a challenge for the traditional techniques to observe the conformation heterogeneity and interpret the interaction mechanism of the biomolecules in living cells. As the emerged the third generation single-molecule gene sequencing technology, nanopore is used to monitor the conformation change of the molecules in situ and therefore demonstrate good applications in single-molecule sensing and nucleic acid and protein sequencing. To further improve the detection resolution and precision, the combined photoelectric detection that can achieve higher accuracy via introducing optical-responsive molecules is adopted. In view of the extensive applications and development status of nanopore technique in single-molecule conformation resolution, this paper summarizes the research progress of the nanopore devices and the design and applications of some typical photo-responsive molecules in nanopore sensing, including azobenzenes and their derivatives (Azos), spiropyrane (SP) and diarylethene (DAE). Light regulation is a simple and effective approach for the monitoring of the molecular structures, and the further integration with nanopore devices exhibits great potential in single-molecule detection meanwhile sheds light on the design and applications of multi-responsive nano-devices.

Atomic-Scale Studies of Overlapping Grain Boundaries between Parallel and Quasi-Parallel Grains in Low-Symmetry Monolayer ReS2

Summary Grain boundaries (GBs) are significant microstructures that dominate properties of polycrystalline two-dimensional (2D) materials. Low-symmetry rhenium disulfide monolayers provide an ideal platform to investigate diverse configurations of GBs and their orientation-dependent characteristics. Here, we utilize the preferential deposition of platinum-based nanoparticles on grain edges to rapidly locate nanoscale-wide overlapping GBs during microscopic-scale observation. Atomic-resolution investigations by aberration-corrected annular dark-field scanning transmission electron microscopy reveal diverse overlapping GBs constructed by parallel grains with transversal displacement. They prefer to align along several primary lattice directions with different interlayer atomic registries. Calculations show that the electronic band structures of overlapping GBs are strongly direction dependent, suggesting their potential in tunable electronic and photonic devices. Incommensurate overlapping GBs formed by quasi-parallel grains with ultrasmall twist angles are also observed. These results unveil the rich polymorphism of GBs and new opportunities to tailor properties of anisotropic 2D materials via GB engineering.

Two-dimensional ZnI2 monolayer as a photocatalyst for water splitting and improvement its electronic and optical properties by strains

We investigate the electronic and optical properties of zinc iodide (ZnI 2 ) under the effect of the biaxial strains. It has been emphasized that ZnI 2 monolayer is stable based on the molecular dynamics simulations, phonon dispersion curve and binding energy calculations. In the equilibrium state, the results revealed that the ZnI 2 monolayer is a semiconductor with the indirect bandgap value of 2.018 eV/2.94 eV using PBE/HSE06 methods. In addition, the ZnI 2 monolayer has appropriate band-edge positions for the oxidation and reduction reactions of water splitting at pH = 0. Besides, it is disclosed that the energy bandgap varies extremely with the influence of biaxial strain reaching about 1.631 eV with the compressive strain of −6%. Generally, the change in the bandgap is higher with the compressive strain state than that with the tensile strain state. However, all energy gaps are located in the visible region and distributed in wide range regions which can have extensive nanodevices applications. Most importantly, a remarkable enhancement in the optical properties can be perceived under the strain effect. Precisely, the absorption coefficient achieves a maximum value of 16.6 × 10 4 cm −1 at −6% within the ultraviolet range. Finally, our perusal highlights that the effect of the biaxial strain on the ZnI 2 monolayer presents effectual guidance to design highly effective optoelectronic materials for nanoscale devices. • ZnI 2 monolayer is semiconductor with a bandgap of 2.018/2.94 eV using PBE/HSE06 methods. • The energy bandgap is tuning considerably under the biaxial strain effect. • Tensile strain results in significant red-shifting in the absorption edge. • The ZnI 2 monolayer can act as a high-efficiency photocatalyst for water splitting. • Control of the electronic and optical properties is beneficial in nanoelectronic applications.

Enhanced magnetic properties of Fe3+ doped ZnS nanocrystals via low temperature co-precipitation: spintronic and nano-device applications

The present work emphasises synthesis and characterization of undoped and Fe3+ doped ZnS nanocrystals. The prepared samples are synthesized by simple co-precipitation technique. The recorded XRD pattern confirms the cubic structure of prepared samples and its estimated average crystallite size is 3–3.5 nm range, and also evaluated micro-structural parameters. SEM images of prepared samples exhibit granular structure with spherical particles and non uniform in shape, size where as Fe3+ doped sample is more agglomerated. TEM images of undoped sample depicted rod like structures where as Fe3+ doped sample exhibits spherical shaped structures. The energy bands obtained in UV–vis region from optical absorption spectrum are recognized as characterstic bands of Fe3+ ions and also evaluated the crystal field, inter electronic parameters (Dq, B and C). EPR spectrum of Fe3+ doped sample exhibited a strong signal at g = 2.136. Photoluminescence spectrum of undoped sample is witnessed by strong blue emission band at 421 nm where as Fe3+ doped ZnS has an additional weak peak observed at 609 nm (4T2 → 6A1). The evaluated CCT values are 2568 K for ZnS and 3079 K for Fe3+ doped ZnS. VSM analysis reveals the observation of enhanced magnetism for Fe3+ doped ZnS nanocrystals compared to undoped sample ZnS. Thermal analysis signifies that the prepared samples are stable upto 888 °C (ZnS) and 993 °C (Fe3+ doped ZnS). The prepared materials may suitable for the various device applications in nanoscale magnetic, optical-spintronic devices and led to fabricate DMS materials for new trending applications.

Structural stability, magneto-electronic properties, and tuning effects for transition metal-doped net-Y nanoribbons

Low-dimensional carbon allotropes have attracted much attention due to their great potential for applications in future electronic and magnetic devices. Recently, new graphene-like nanoribbons consisting of four-, six-, and eight-membered carbon rings have been synthesized successfully, and a related two-dimensional structure net-Y was subsequently proposed. Here, we report theoretical investigations on the structural stability and magneto-electronic properties for the net-Y armchair nanoribbons doped with low-concentration transition metal (TM) atoms (Mn, Ni, Fe, Co, and V), especially focusing on their electric-magnetic and mechano-magnetic coupling effects. The calculated binding energy and molecular dynamics simulation confirm that all these ribbons hold a high stability. Particularly, by TM-doping, the intrinsic non-magnetic semiconducting ribbon can be converted to a bipolar magnetic semiconductor (BMS), a half metal (HM), or a spin-degenerate magnetic semiconductor (MSC), presenting rich magnetic features. Meanwhile, this magnetism is flexibly tunable by the transverse electric field or stretched strain. For example, a low electric field can transform the ribbon into the HM phase, and the mechanic strain can realize a continuous magnetic phase transition among BMS, magnetic metal (MS), MSC, HM, and half semiconductor. These findings indicate that the TM-doped net-Y ribbon possesses favorable magnetism, which might have promising applications in magnetic nanodevices.

Ion-beam-etching based lift-off for reliable patterning of dense and inverse metallic nanostructures towards 10-nm scale

Reliable fabrication of metallic patterns at nanoscale is of great interest for various practical applications in nanodevices. Electron-beam-lithography based lift-off is the most often used method to define exquisite metal nanostructures at the 10-nm scale. In a common wet lift-off process, the metal material on top is lifted-off and washed away together with the sacrificial resist layer below. However, the wet lift-off process is sometimes ultrasonic agitation and/or heating-dependent and limited by the probable remained debris problem. In this work, we present an alternative lift-off process to obtain metallic patterns via removing the unwanted parts by using ion beam etching. We demonstrated that this ion-beam-etching based lift-off process is effective for isolated exposed negative-tone resist to fabricate nanoholes and nanoslits array and for dense networked exposed resist to fabricate high-density packed nanostructures. SERS measurements were conducted to demonstrate the molecular detection capability of the nanostructures with tiny gaps fabricated by this ion-beam-etching based lift-off process.

Structure and electronic properties of single-walled C3N nanotubes

Abstract One-dimensional nanotubes have become an indispensable ideal candidate material for nano-device applications due to their excellent and unique electronic, mechanical, and thermal properties. By the first-principles method of density functional theory, we have theoretically investigated the structural stability, electronic properties, carrier mobility, and Poisson's ratio of C3N single-walled nanotubes (C3NSWNT). We find that C3NSWNT is stable and the ground state of the system is non-magnetic. The electronic properties and carrier mobilities of C3NSWNT can be adjusted by diameter and edge engineering. The electron mobility of (n,n) armchair C3NSWNT (A-C3NSWNT) is lower than that of (n,0) zigzag C3NSWNT (Z-C3NSWNT), but the hole mobility of (n,n) A-C3NSWNT is higher than that of (n,0) Z-C3NSWNT. Moreover, both A-C3NSWNT and Z-C3NSWNT can transfer from semiconductor to metal by tuning the electric field, and Z-C3NSWNT is more sensitive to the applied electric field than A-C3NSWNT due to smaller energy gap. But only A-C3NSWNT can transfer from semiconductor to metal by tuning strain, and be more suitable to the application in nano electromechanical switching devices. These research results may provide some theoretical support for the potential application and development of nanoelectronic devices based on C3NSWNT.

Outstanding thermo-mechanical properties of graphene-like B3C3 and C3N3

Experimentally fabricated two-dimensional (2D) carbon-based nanomaterials have received significant attention because of their ultra-high physical properties, in recent years. In this manuscript, the thermal conductivity (TC) and mechanical response of 2D B3C3 and C3N3 structures are studied in detail, using molecular dynamics (MD) simulations. Their superior mechanical properties (Young’s modulus, ultimate tensile strength and failure strain) and TC make an excellent candidate for various applications of nanodevices. The mechanical properties of these 2D structures are also examined at five various temperatures up to 900 K along with the different loading directions and various strain rates from 107 to 109 s−1. MD results demonstrate that the mechanical properties of these 2D structures gradually decrease as temperature increases, due to the weakening effect of high temprerature. Additionally, when the strain rate increases, it is revealed that the mechanical properties show an increasing trend. Furthermore, at 300 K, the failure processes of these 2D structures are studied. MD simulations results demonstrate that these structures show brittle failure mechanism. On the other hand, various types of structural defects occurs during the production process and so these defects affect the physical properties of these structures adversely. Accordingly, the effects of various atom types, such as, N, B and C, vacancy defects on the mechanical properties and TC of these structures were investigated. The existence of vacancy defects in these structures reduces the TC and mechanical properties significantly by increasing the concentrations of defects. Finally, non-equilibrium MD simulations results indicate exceptionally high TC values of these structures.

Strain enhanced inplane ferroelectricity in few layer SnS

Two dimensional (2D) materials such as graphene and transition metal dichalcogenide (TMDC) like MoS2, WTe2 have brought widespread attention as their novel 2D confined properties and applications in nano devices. Among them, SnS draws people’s attention as the rise of inplane ferroelectricity study. Using first principle DFT calculation, we find the structure shrinking more flatten when monolayer SnS isolated from its bulk matrix. Our result shows that monolayer SnS is much flexible with elastic modulus about 4.4 GPa. The size of the band gap can be tuned from 0.06eV to 0.11 eV by strain. Under positive armchair strain/negative zigzag strain there is even a band gap transition between direct and indirect, which can be applied in PL control. And the born effective charge for strained monolayer SnS can be tuned up to 10% by 5% strain, indicating the great tunability in monolayer SnS. Our results pave the way for further study of controlling the electronic and ferroelectric properties in monolayer SnS and similar inplane FE 2D materials.

Synthesis of Atomically Thin 1T‐TaSe2 with a Strongly Enhanced Charge‐Density‐Wave Order

AbstractBulk 1T‐TaSe2 as a charge‐density‐wave (CDW) conductor is of special interest for CDW‐based nanodevice applications because of its high CDW transition temperature. Reduced dimensionality of the strongly correlated material is expected to result in significantly different collective properties. However, the growth of atomically thin 1T‐TaSe2 crystals remains elusive, thus hampering studies of dimensionality effects on the CDW of the material. Herein, chemical vapor deposition (CVD) of atomically thin TaSe2 crystals is reported with controlled 1T phase. Scanning transmission electron microscopy suggests the high crystallinity and the formation of CDW superlattice in the ultrathin 1T‐TaSe2 crystals. The commensurate–incommensurate CDW transition temperature of the grown 1T‐TaSe2 increases with decreasing film thickness and reaches a value of 570 K in a 3 nm thick layer, which is 97 K higher than that of previously reported bulk 1T‐TaSe2. This work enables the exploration of collective phenomena of 1T‐TaSe2 in the 2D limit, as well as offers the possibility of utilizing the high‐temperature CDW films in ultrathin phase‐change devices.

Design and Simulation Analysis of Vertical Double-Gate MOSFET (VDGM) Structure for Nano-Device Application

Design and simulation analysis of vertical MOSFET structure with double gate structure on each side of insulating pillar for nanodevice applications is presented. The body doping effect on vertical channel for channel length, L g = 50nm and analyzing its effect towards such small devices was successfully performed. The analysis continued with the comparative investigation of device performance with conventional planar MOSFET as scaling L g down to 50nm. The final part evaluates the innovative design of incorporating dielectric pocket (DP) on top of vertical MOSFET turret with comprehensive device performance analysis as compared to standard vertical MOSFET in nanoscale realm. An optimized body doping for enhanced performance of vertical MOSFET was revealed. The vicinity of DP near the drain end is found to reduce the charge sharing effects between source and drain that gives better gate control of the depletion region for short channel effect (SCE) suppression in nanodevice structure.

Various challenges in realizing spin-gapless semiconductivity in Ti2CoSi

Spin-gapless semiconductors are recently discovered class of materials that behave as an insulator for one spin channel and as a zero-gap semiconductor for the opposite spin. Here, we show from first-principle calculations that one such material Ti2CoSi predicted to exhibit spin-gapless semiconductivity has an energetically close non-spin-polarized phase. In particular, we show that the regular Heusler phase of this material is non-magnetic, while the inverted Heusler phase is nearly spin-gapless semiconducting, with a very small energy difference of ≈0.1 eV per 16-atom cell, in favor of the regular Heusler structure. Moreover, we also show that a 100% spin polarization in inverted Heusler phase is detrimentally affected by the emergence of surface states in thin-film geometry. These results need to be taken into account for realistic implementations of this and similar materials in nano-device applications, which rely on highly spin-polarized current in thin-film geometry.

A Novel Hyperbolic Two-Dimensional Carbon Material with an In-Plane Negative Poisson's Ratio Behavior and Low-Gap Semiconductor Characteristics.

Poisson’s ratio is one of the fundamental features of materials, reflecting the transverse strain response to the applied certain axial strain. The materials with a negative Poisson’s ratio, also known as auxetic materials, are rare in nature and attract lots of research interest because of their unusual mechanical behavior and extensive applications in mechanical nanodevices. Here, we proposed and studied a novel hyperbolic two-dimensional graphene-like structure (GLS) showing a negative Poisson’s ratio behavior by first-principles calculation. The thermodynamic, dynamic, lattice dynamic, and mechanical stabilities of the GLS were carefully studied. In addition, we also explored the electronic structure, mechanical characteristics, and optical-electronic characteristics. The GLS not only displays a negative Poisson’s ratio in certain directions but also shows low-gap semiconductor characteristics and superior electronic conductivity. It is a potential sunscreen material because of the outstanding reflection and absorption for ultraviolet and infrared light.

Observation of anomalous phase transition and band gap shrinkage in zinc germanate nanorods

This work reports the high temperature anomalous phase transition with high dielectric constant and low band gap in pure zinc germanate (Zn2GeO4) nanorods. Crystal structure of Zn2GeO4 nanorods were analyzed by X-ray powder diffraction and Raman studies. Electron microscopy reveals that nanorods have average diameter and length of 30 nm and 100 nm, respectively. UV–ViS study indicates low band gap of 3.96 eV. The observed dielectric constant is 40 times higher than bulk Zn2GeO4. High dielectric constant of ~772 and phase transition was observed around 400 °C. In the low and high temperature range, activation energy were ~0.33 and 1.34 eV. High dielectric constant and shrinkage in bang gap with temperature phase transition of Zn2GeO4 nanorods makes it the potential candidate for nano-optoelectronic and nanodevices applications in near future.

Tuning the Electronic and Optical Properties of Sc2CF2 MXene Monolayer Using Biaxial Strain

The family of two-dimensional (2D) transition-metal carbides and nitrides, known as MXenes, has attracted substantial attention in science and technology. The electronic structure, optical properties, and bandgap tuning under compressive and tensile strain (CTS) of 2D Sc2CF2 are investigated herein using density functional theory calculations. The results reveal that the bandgap of 2D Sc2CF2 can be tuned by applying CTS. The covalent bonding between the atoms become stronger under compressive strain. To investigate the optical properties, the absorption spectrum and percentage reflectivity of these structures along the x and z directions are calculated using the real and imaginary parts of the dielectric function. Upon applying 2%, 4%, and 6% compressive strain, the number of absorption peaks increases in the visible region along the x direction. These tunable electronic and optical properties of semiconducting 2D Sc2CF2 make it a candidate for the design of optoelectronic devices and in nanodevice applications.

Tuning spin filtering and spin rectifying behaviors of zigzag silicon carbon nanoribbons by edge dual-hydrogenation

Abstract Using non-equilibrium Green's function combined with density functional theory, we investigate spin-resolved transport properties of zigzag silicon carbon nanoribbons (zSiCNRs) with the different edge hydrogenations. The dual-hydrogenation on edge C or Si atoms all can break the magnetic degeneracy of zSiCNRs with mono-hydrogenation, and change it from the initial metallicity to halfmetallic behavior under the ferromagnetic state. Under the parallel magnetic configuration, the dual-hydrogenation on edge C or Si atoms of zSiCNR devices can induce and reverse a prefect spin filtering behavior with nearly 100% spin filtering efficiency. Under the anti-parallel magnetic configuration, the dual-hydrogenation on edge C or Si atoms can enhance the spin rectifying behavior of zSiCNR devices obviously with the substantial increases in corresponding spin rectifying ratios of α-spin or β-spin currents. The above findings are very useful for its functional application in silicon carbon spin-dependent nanodevices.