Applications In Nanodevices Research Articles

Visualization of edge-modulated charge-density-wave orders in monolayer transition-metal-dichalcogenide metal

In two-dimensional materials with the many-body quantum states, edges become especially significant for realizing a host of physical phenomena and for potential applications in nanodevices. Here, we report the successful construction of ultra-flat monolayer 1H-phase niobium diselenide (NbSe2) with atomically sharp zigzag edges. Our scanning tunneling microscopy and spectroscopy measurements reveal that such zigzag edges hold intriguing one-dimensional edge states. Moreover, we observe an obvious energy-dependent charge-density-wave (CDW) modulation near the edge, highlighting the significant edge-CDW interference interactions. Our findings provide a comprehensive study of tunable structural and electronic properties at the edges in monolayer NbSe2. More importantly, the edge-CDW interference model can be feasible for other CDW metals, suggesting a promising direction of extending desired edge functionalities.

Production of Large‐Area Nucleus‐Free Single‐Crystal Graphene‐Mesh Metamaterials with Zigzag Edges

In addition to conventional monolayer or bilayer graphene films, graphene-mesh metamaterials have attracted considerable research attention within the scientific community owing to their unique physical and optical properties. Currently, most graphene-mesh metamaterials are fabricated using common lithography techniques on exfoliated graphene flakes, which require the deposition and removal of chemicals during fabrication. This process may introduce contamination or doping, thereby limiting their production size and application in nanodevices. Herein, the controlled production of wafer-scale high-quality single-crystal nucleus-free graphene-mesh metamaterial films with zigzag edges is demonstrated. The 13 C-isotopic labeling graphene-growth approach, large-area Raman mapping techniques, and a uniquely designed high-voltage localized-space air-ionization etching method are utilized to directly remove the graphene nuclei. Subsequently, a hydrogen-assisted anisotropic etching process is employed for transforming irregular edges into zigzag edges within the hexagonal-shaped holes, producing a large-scale single-crystal high-quality graphene-mesh metamaterial film on a Cu(111) substrate. The carrier mobilities of the fabricated field-effect transistors on the as-produced films are measured. The findings of this study enable the large-scale production of high-quality low-dimensional graphene-mesh metamaterials and provide insights for the application of integrated circuits based on graphene and other 2D metamaterials.

Research on the auxetic behavior and mechanical properties of periodically rotating graphene nanostructures

Abstract Negative Poisson’s ratio (auxetic) material is one of the most widely studied metamaterials, and recent attempts have been made to discover auxeticity in graphene-based and related carbon-based materials. However, it is shown that negative Poisson’s ratio effect requires special conditions, such as high temperature. Achieving negative Poisson’s ratio effect under large strain at ambient conditions is the key to graphene materials in nano-device applications. In order to discover the auxetic properties of nanostructures under large strain, this article proposes periodically rotating graphene nanostructures (PRGNs) which are the combination of graphene and rotating rigid unit structures. Poisson’s ratio, Young’s modulus, and damage mechanism of PRGNs are investigated using molecular dynamics simulation. It can be possible to conclude that PRGNs can also exhibit auxetic behavior, and their negative Poisson’s ratio effect can be maintained even at large strains (ε ∼ 0.1). Poisson’s ratio can be regulated by adjusting the value of the geometry parameters of the graphene sheets (GSs), which comprise the PRGNs, and changed from negative to positive and from positive to negative. Also, the influences of the structural size of GSs and the connection angle between GSs on the mechanical properties are explored, which will provide a theoretical basis for the preparation and performance optimization of GSs and the nano-auxetic properties of materials.

Modifications of optical, structural, chemical and morphological properties of molybdenum disulfide (MoS2) sputtered thin films under high dose gamma radiation

Molybdenum disulfide (MoS2) is considered to be a promising member of Transition metal dichalcogenides (TMDCs) displays significant physical and chemical peculiarities that make it promising candidate for extensive applications in nano and micro optoelectronic device applications, energy harvesting and light and gas detection devices. In the present work, we illustrated the influence of gamma ray irradiation on optical, morphological, structural and electronic peculiarities of 2D MoS2 thin films. 2D MoS2 thin films were grown by DC sputtering technique. The synthesized thin films samples were exposed to gamma radiation with 1500 kGy dose. There are significant structural and morphological alterations occur in thin films of MoS2 after exposure to gamma irradiation. From the absorption spectra Urbach energy, indirect optical band gap energy, absorption edge, and extinction coefficient were calculated. Alterations that occur in optical, structural, morphological and electronic parameters after the exposure of gamma irradiation have been corresponding with radiation instigated compositional and structural defects. The peculiarities of virgin and gamma exposed MoS2 thin films with 1500 kGy gamma-ray dose from Co-60 source were studied by UV spectrophotometer, X-ray diffraction (XRD), Rutherford Backscattering Spectroscopy (RBS), X-Ray photoelectron spectroscopy (XPS), Photoluminescence (PL) spectroscopy, Atomic Force Microscopy (AFM), Fourier-transform infrared spectroscopy (FTIR).

First-principles study of two-dimensional MoN2X2Y2 (X=B[formula omitted]In, Y=N[formula omitted]Te) nanosheets: The III–VI analogues of MoSi2N4 with peculiar electronic and magnetic properties

The recent discovery of MoSi2N4 nanosheet boosts researches on the layered MA2Z4 materials. Utilizing first-principles calculations, we investigate the isoelectronic and isostructural analogues of MoSi2N4, i.e. group III–VI MoN2X2Y2 (X=B∼In, Y=N∼Te) nanosheets. Nine III–VI XY combinations are revealed to form stable MoN2X2Y2 systems with robust dynamical, thermal and energetic stability. They can be indirect-gap semiconductors or metals depending on the XY compositions. There is a linear relationship between the gap size and lattice constant of semiconducting MoN2X2Y2 systems. More interestingly, a Mexican-hat-like band dispersion appears in the top valence band of MoN2X2Y2 (XY=AlO, GaO, InO) nanosheets, for which the hole doping can induce Stoner ferromagnetism and convert the systems into half-metals. For the van der Waals heterostructures formed by MoN2X2Y2 (XY=BS, AlO) and MoSi2N4 nanosheets, they exhibit versatile and strain-tunable band alignments, including the straddling-gap type-I, staggered-gap type-II, and broken-gap type-III ones. Our study demonstrates that the composition of surface layers is a new degree to manipulate the electronic structure of MA2Z4-based materials and it will bring tunable electronic and magnetic behaviours for the electrics, spintronics, and nano-device applications.

Revisiting the Rate-Dependent Mechanical Response of Typical Silicon Structures via Molecular Dynamics

Strain rate is a critical parameter in the mechanical application of nano-devices. A comparative atomistic study on both perfect monocrystalline silicon crystal and silicon nanowire was performed to investigate how the strain rate affects the mechanical response of these silicon structures. Using a rate response model, the strain rate sensitivity and the critical strain rate of two structures were given. The rate-dependent dislocation activities in the fracture process were also discussed, from which the dislocation nucleation and motion were found to play an important role in the low strain rate deformations. Finally, through the comparison of five equivalent stresses, the von Mises stress was verified as a robust yield criterion of the two silicon structures under the strain rate effects.

First-Principles Study of Electronic Properties of Substitutionally Doped Monolayer SnP3.

SnP3 has a great prospect in electronic and thermoelectric device applications due to its moderate band gap, high carrier mobility, absorption coefficients, and dynamical and chemical stability. Doping in two-dimensional semiconductors is likely to display various anomalous behaviors when compared to doping in bulk semiconductors due to the significant electron confinement effect. By introducing foreign atoms from group III to VI, we can successfully modify the electronic properties of two-dimensional SnP3. The interaction mechanism between the dopants and atoms nearby is also different from the type of doped atom. Both Sn7BP24 and Sn7NP24 systems are indirect bandgap semiconductors, while the Sn7AlP24, Sn7GaP24, Sn7PP24, and Sn7AsP24 systems are metallic due to the contribution of doped atoms intersecting the Fermi level. For all substitutionally doped 2D SnP3 systems considered here, all metallic systems are nonmagnetic states. In addition, monolayer Sn7XP24 and Sn8P23Y may have long-range and local magnetic moments, respectively, because of the degree of hybridization between the dopant and its adjacent atoms. The results complement theoretical knowledge and reveal prospective applications of SnP3-based electrical nanodevices for the future.

Laser modification of Au–CuO–Au structures for improved electrical and electro-optical properties

CuO nanomaterials are one of the metal-oxides that received extensive investigations in recent years due to their versatility for applications in high-performance nano-devices. Tailoring the device performance through the engineering of properties in the CuO nanomaterials thus attracted lots of effort. In this paper, we show that nanosecond (ns) laser irradiation is effective in improving the electrical and optoelectrical properties in the copper oxide nanowires (CuO NWs). We find that ns laser irradiation can achieve joining between CuO NWs and interdigital gold electrodes. Meanwhile, the concentration and type of point defects in CuO can be controlled by ns laser irradiation as well. An increase in the concentration of defect centers, together with a reduction in the potential energy barrier at the Au/CuO interfaces due to laser irradiation increases electrical conductivity and enhances photo-conductivity. We demonstrate that the enhanced electrical and photo-conductivity achieved through ns laser irradiation can be beneficial for applications such as resistive switching and photo-detection.

First principles prediction of the stable MgFCl Janus monolayer with tunable properties

Herein, we introduce a new Janus monolayer, namely MgFCl, where its structural, electronic, and optical properties are systematically investigated by means of first-principles calculations. It is demonstrated that MgFCl single layer is structurally and dynamically stable. The electronic band structure indicates a wide Γ − Γ direct gap of 4.32(5.61) eV as determined by PBE(HSE06) functional. Consequently it exhibits a good absorption in the far and extreme ultraviolet region. In addition, the physical properties tuning via external strain and alkaline earths substitution is also studied. Results show a strong strain-dependence of the energy gap, however the direct gap nature is retained. The alkaline earths incorporation induces a strong structural local distortion due to the atomic size difference. A band gap reduction of 0.19 eV is caused by Ca atom, while the Sr and Ba atoms induce insignificant change of this important parameter. Calculated spectra suggest that the MgFCl monolayer absorption may be extended to a wider energy range and enhanced by applying proper strain and alloying. Results presented herein may recommend new two-dimensional (2D) materials for applications in optoelectronic nano devices, specifically, those working under high energy conditions. • MgFCl Janus monolayer exhibits good stability. • MgFCl Janus monolayer is a direct gap insulator with band gap of 5.61 eV. • The external strain can tune effectively the electronic structure. • Ca incorporation reduces the energy gap, while Sr and Ba presence induces insignificant change. • Optical properties of MgFCl monolayer can be enhanced by either external strains or alkaline earth substitution.

Tunable band gaps and optical absorption properties of bent MoS2 nanoribbons

The large tunability of band gaps and optical absorptions of armchair MoS2 nanoribbons of different widths under bending is studied using density functional theory and many-body perturbation GW and Bethe–Salpeter equation approaches. We find that there are three critical bending curvatures, and the non-edge and edge band gaps generally show a non-monotonic trend with bending. The non-degenerate edge gap splits show an oscillating feature with ribbon width n, with a period Delta n=3, due to quantum confinement effects. The complex strain patterns on the bent nanoribbons control the varying features of band structures and band gaps that result in varying exciton formations and optical properties. The binding energy and the spin singlet–triplet split of the exciton forming the lowest absorption peak generally decrease with bending curvatures. The large tunability of optical properties of bent MoS2 nanoribbons is promising and will find applications in tunable optoelectronic nanodevices.

Multiferroic laminated composites with interfacial imperfections and the nonlocal effect

In this study, we investigate multiferroic magneto-electro-elastic (MEE) laminated plates with interfacial imperfections by using nonlocal theory. The multilayered MEE plate is considered a simply supported rectangular plate subjected to appropriate boundary conditions. Generalized membrane-type and generalized linear-spring-type imperfect interfaces are considered. We extend the pseudo-Stroh formalism and propagator matrix method to derive analytical solutions for an orthotropic and multilayered MEE plate with interfacial imperfections and the nonlocal effect. The key step is first to observe that the equilibrium equations and interface conditions are equivalent to those in the classical MEE multilayered problem. Next, we derive the relationship between nonlocal and classical tractions. The derived solutions are then applied to BaTiO3−CoFe2O4 sandwich plates. Numerical calculations show that interfacial imperfections and the nonlocal effect can be used to tune the field distribution and ME effect. The findings can provide insights for the application and design of various nanodevices.

Controlling magnetic-semiconductor properties of the Si- and Al-doped blue phosphorene monolayer

Doping has been widely employed as an efficient method to diversify a materials properties. In this work, the structural, magnetic, and electronic properties of pristine aluminum (Al)-, and silicon (Si)-doped blue phosphorene monolayer are investigated using first-principles calculations. Pristine monolayer is a non-magnetic, wide gap, semiconductor with a band gap of 1.81 eV. The 1Si-doped system is a ferromagnetic semiconductor. However, the magnetism is turned off when increasing the dopant composition with small Si–Si distance. Further separating the dopants recovers, step by step, the magnetic properties and an antiferromagnetic (AFM)-ferromagnetic (FM) state transition will take place at large dopants separation. In contrast, Al doping retains the non-magnetic semiconductor behavior of blue phosphorene. However, significant energy gap reduction is achieved, where this parameter exhibits a strong dependence on the dopant concentration and doping configuration. Such control may also induce the indirect-direct gap transition. Our results introduce prospective two-dimensional (2D) materials for application in spintronic and optoelectronic nano devices, which can be realized and stabilized in experiments as suggested by the calculated formation and cohesive energies.

Polariton-induced transparency in hybrid 2D-material hetero-nanostructure with multi-functions

In this work, we theoretically investigate the polariton-induced transparency (PoIT) phenomenon based on a hetero-nanostructure that possesses hybrid two-dimensional (2D) materials. The hetero-nanostructure consists of periodic hexagonal boron nitride (hBN) and graphene nanoribbons that are separated by a dielectric spacer. Consequently, the near-field coupling between hyperbolic phonon-polaritonic resonances on hBN ribbons and localized surface plasmon resonances on graphene ribbons leads to a transparent window in the mid-infrared region, where strong phase dispersion as well as slow light effect exist. Such hybrid-polariton induced transparency is dominated by electric dipole, and possesses reciprocity. It can be passively tailored by changing structural parameters, and actively tuned by shifting graphene Fermi level. Meanwhile, it is insensitive to the incident angle within a wider range, and rotating polarization angle of the incident wave leads to the excitation of edge phonon polaritons, which enriches the adjustability of this heterostructure. Furthermore, it can serve as a mid-infrared sensor to detect surrounding refractive index change, gas concentration, and molecular fingerprints. And introducing a metal reflector can easily turn it to be a wide-angle dual-band light absorber. Finally, we investigate the near-omnidirectional PoIT and polarization-dependent PoIT in the extended bilayer metasurface. This work has potential mid-infrared multifunctional applications in low-dimensional hybrid-polaritonic nanodevices for phase manipulation, slow light, sensing, and absorption.

Design and fabrication of bimetallic plasmonic colloids through cold nanowelding.

The integration of Au and Ag into nanoalloys has emerged as an intriguing strategy to further tailor and boost the plasmonic properties of optical substrates. Conventional approaches for fabricating these materials via chemical reductions of metal salts in solution suffer from some limitations, such as the possibility of retaining the original morphology of the monometallic substrate. Spontaneous nanowelding at room temperature has emerged as an alternative route to tailor Au/Ag nanomaterials. Herein, we perform a thorough study on the cold-welding process of silver nanoparticles onto gold substrates to gain a better understanding of the role of different variables in enabling the formation of well-defined bimetallic structures that retain the original gold substrate morphology. To this end, we systematically varied the size of silver nanoparticles, dimensions and geometries of gold substrates, solvent polarity and structural nature of the polymeric coating. A wide range of optical and microscopy techniques have been used to provide a complementary and detailed description of the nanowelding process. We believe this extensive study will provide valuable insights into the optimal design and engineering of bimetallic plasmonic Ag/Au structures for application in nanodevices.

Recent advances in two-dimensional van der Waals magnets

Two-dimensional (2D) magnets have evoked tremendous interest within the research community due to their fascinating features and novel mechanisms, as well as their potential applications in magnetic nanodevices. In this review, state-of-the-art research into the exploration of 2D magnets from the perspective of their magnetic interaction and order mechanisms is discussed. The properties of these magnets can be effectively modulated by varying the external parameters, such as the charge carrier doping, thickness effect, pressure and strain. The potential applications of heterostructures of these 2D magnets in terms of the interlayer coupling strength are reviewed, and the challenges and outlook for this field are proposed.

Thermoelectric, spin-dependent optical and quantum transport properties of 2D half-metallic Co2Se3.

In this paper, the thermoelectric, spin-dependent optical and quantum transport properties of a two-dimensional (2D) Co2Se3 monolayer are investigated using first principles calculations. The stability of the Co2Se3 monolayer is confirmed by energy-cohesive calculations and phonon dispersion analysis. According to electronic and magnetic calculations, Co2Se3 is a Dirac half-metal (DHM) with several Dirac cones around the Fermi level in the spin-down channel and exhibits intrinsic ferromagnetism with a high Curie temperature. Investigation of the thermoelectric properties shows that the Co2Se3 monolayer with ZT ∼ 1 at low temperature (T < 300 K) to medium temperature (500 < T < 700 K) can be introduced as a suitable candidate for thermoelectric applications. Spin-dependent optical calculations indicate that the maximum reflection and absorption for 2D Co2Se3 are in the energy ranges of 0-1 and 12-8 eV, respectively, so this structure is a suitable wave reflector in the infrared range (IR) and a good absorbent in the far-ultraviolet (FUV) range. On the other hand, the unique feature of Co2Se3 encouraged us to study the spin-dependent transport properties. A spin-down current as large as 27(30) μA passing through in the x(y)-direction, while the spin-up current passing through the scattering region up to 1.2 (1) V is approximately zero, induces 100% spin filtering properties in the Co2Se3 monolayer, so this structure has great potential for application in spintronic nano-devices.

Polymorphic Ga2S3 nanowires: phase-controlled growth and crystal structure calculations.

The polymorphism of nanostructures is of paramount importance for many promising applications in high-performance nanodevices. We report the chemical vapor deposition synthesis of Ga2S3 nanowires (NWs) that show the consecutive phase transitions of monoclinic (M) → hexagonal (H) → wurtzite (W) → zinc blende (C) when lowering the growth temperature from 850 to 600 °C. At the highest temperature, single-crystalline NWs were grown in the thermodynamically stable M phase. Two types of H phase exhibited 1.8 nm periodic superlattice structures owing to the distinctively ordered Ga sites. They consisted of three rotational variants of the M phase along the growth direction ([001]M = [0001]H/W) but with different sequences in the variants. The phases shared the same crystallographic axis within the NWs, producing novel core–shell structures to illustrate the phase evolution. The relative stabilities of these phases were predicted using density functional theory calculations, and the results support the successive phase evolution. Photodetector devices based on the p-type M and H phase Ga2S3 NWs showed excellent UV photoresponse performance.

Epitaxial growth of structure-tunable ZnO/ZnS core/shell nanowire arrays using HfO2 as the buffer layer.

Synthesis of high-quality ZnO/ZnS heterostructures with tunable phase and controlled structures is in high demand due to their adjustable band gap and efficient electron-hole pair separation. In this report, for the first time, remote heteroepitaxy of single-crystalline ZnO/ZnS core/shell nanowire arrays has been realized using amorphous HfO2 as the buffer layer. Zinc blende or wurtzite ZnS epilayer can be efficiently fabricated under the same thermal deposition condition by adjusting the buffer layer thickness, even among the same batch of products, respectively. Structural characterization reveals "(01-10)ZnOwz//(2-20)ZnSZB, [0001]ZnOWZ//[001]ZnSZB" and "(01-10)ZnOWZ//(01-10)ZnSWZ, [0002]ZnOWZ//[0002]ZnSWZ" epitaxial relationships between the core and the shell, respectively. The cathodoluminescence measurement demonstrates that the tuning of the optical properties can be accomplished by preparing a heterostructure with HfO2, in which a strong green emission increases at the expense of the quenching of UV emission. In addition, the core/shell heterostructure based Schottky diode exhibits an asymmetrical rectifying behavior and an outstanding photo-electronic switching-effect. We believe that the aforementioned results could provide fundamental insights for epitaxial growth of structure-tunable ZnO/ZnS heterostructures on the nanoscale. Furthermore, this promising route buffered by the high-k material can broaden the options for fabricating heterojunctions and promote their application in photoelectric nanodevices.

Theoretical prediction of Janus PdXO (X = S, Se, Te) monolayers: structural, electronic, and transport properties.

Due to the broken vertical symmetry, the Janus material possesses many extraordinary physico-chemical and mechanical properties that cannot be found in original symmetric materials. In this paper, we study in detail the structural, electronic, and transport properties of 1T Janus PdXO monolayers (X = S, Se, Te) by means of density functional theory. PdXO monolayers are observed to be stable based on the analysis of the vibrational characteristics and molecular dynamics simulations. All three PdXO structures exhibit semiconducting characteristics with indirect bandgap based on evaluations with hybrid functional Heyd–Scuseria–Ernzerhof (HSE06). The influences of the spin–orbit coupling (SOC) on the band diagram of PdXO are strong. Particularly, when the SOC is included, PdTeO is calculated to be metallic by the HSE06+SOC approach. With high electron mobility, Janus PdXO structures have good potential for applications in future nanodevices.

Discriminating sensing of explosive molecules using graphene-boron nitride-graphene heteronanosheets.

Since the synthesis of graphene-boron nitride heterostructures, their interesting electronic properties have attracted huge attention for real-world nanodevice applications. In this work, we combined density functional theory (DFT) with a Green's function approach to examine the potential of graphene-boron nitride-graphene heteronanosheets (h-NSHs) for discriminating single molecule sensing. Our result demonstrates that the graphene-boron nitride-graphene (h-NSHs) can be used for discriminate sensing of the 2,4-dinitrotoluene (DNT), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), pentaerythritol tetranitrate (PENT), and 2,4,6-trinitrotoluene (TNT) molecules. We demonstrate that as the length of the BN region increases, the sensitivity of the heteronanosheets to the presence of these explosive substances increases.