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Materials with in-plane electrical anisotropy have great potential for designing artificial synaptic devices. However, natural materials with strong intrinsic in-plane electrical anisotropy are rare. We introduce a simple strategy to produce extremely large electrical anisotropy via grating gating of a semiconductor two-dimensional electron gas (2DEG) of AlGaN/GaN. We show that periodically modulated electric potential in the 2DEG induces in-plane electrical anisotropy, which is significantly enhanced in a magnetic field, leading to an ultra large electrical anisotropy. This is induced by a giant positive magnetoresistance and a giant negative magnetoresistance under two orthogonally oriented in-plane current flows, respectively. This giant electrical anisotropy is in situ tunable by tailoring both the grating gate voltage and the magnetic field. Our semiconductor device with controllable giant electrical anisotropy will stimulate new device applications, such as multi-terminal memtransistors and bionic synapses.

The interplay between topology and valley degree of freedom has attracted much interest because it can realize new phenomena and applications. Here, based on first-principles calculations, we demonstrate intrinsically valley-polarized quantum anomalous Hall effect in monolayer ferrovalley material: Janus VSiGeN4, of which the edge states are chiral-spin-valley locking. Besides, a small tensile or compressive strain can drive phase transition in the material from valley-polarized quantum anomalous Hall state to half-valley-metal state. With the increase of the strain, the material turns into ferrovalley semiconductor with valley anomalous Hall effect. The origin of phase transition is sequent band inversion of V d orbital at K valley. Moreover, we find that phase transition causes the sign reversal of Berry curvature and induces different polarized light absorption in different valley states. Our work provides an ideal material platform for practical applications and experimental exploration of the interplay between topology, spintronics, and valleytronics., Comment: 8 pages, 6 figures

Two-dimensional (2D) ferromagnetic materials with high Curie temperatures (TC) and tunable physical properties are crucial to the development of nanoscale spintronics devices. Here, we investigate the newly synthesized iodinene using first-principles calculations. Our results show that doping carriers in monolayer and bilayer iodinene can easily introduce itinerant ferromagnetism due to a flatband structure near the Fermi level, and that the associated TC is higher than room temperature. Moreover, we find that a structural phase transition can be achieved through the application of moderate tensile strain for both monolayer and bilayer iodinene. The magnetic moment, Curie temperature, band structure, carrier effective mass, and optical absorption can be changed significantly through this phase transition, and the transition can also lead to a magnetic phase transition with an appropriate doping concentration. Our work provides a feasible approach for designing 2D magnetic materials with potential for application in microelectronics devices.

Two-dimensional (2D) GaInS3 nanosheets are found to exhibit thermal and structural stabilities, good oxidation resistance, and tunable and layer-dependent electronic properties from first-principles calculations. Remarkably, the nanosheets with arbitrary thickness possess robust in-plane piezoelectricity without the odd-even effect commonly observed in other 2D piezoelectric materials, which is attributed to the retention of noncentrosymmetry resulting from their homogeneous and direct stacking patterns. The piezoelectric stress coefficient e 11 3 D of the nanosheets is about 0.23 C/m2, almost independent of the numbers of atomic layers of 2D GaInS3. The stability in piezoelectricity and the high carrier mobility of 2D GaInS3 nanosheets could endow them with promising application prospects in nanoelectronic and nanoelectromechanical devices.

Based on the different propagation characteristics of surface plasmon polaritons (SPPs) excited on different metallic surfaces and on the tendency of SPPs to the region in which SPPs possess lower phase velocity, we design a kind of metal heterowaveguide for nanometric focusing of light. Finite-difference time-domain simulations demonstrate that, by converting light into SPPs, such heterowaveguides constructed with both Ag and Al can focus incident light into about a 20nm×30nm (λ∕27×λ∕18≈0.002λ2) domain with higher than 15% coupling efficiency.

GeSn-on-insulator (GeSnOI) on Silicon (Si) substrate was realized using direct wafer bonding technique. This process involves the growth of Ge 1- xSnx layer on a first Si (001) substrate (donor wafer) followed by the deposition of SiO2 on Ge 1- xSnx, the bonding of the donor wafer to a second Si (001) substrate (handle wafer), and removal of the Si donor wafer. The GeSnOI material quality is investigated using high-resolution transmission electron microscopy, high-resolution X-ray diffraction (HRXRD), atomic-force microscopy, Raman spectroscopy, and spectroscopic ellipsometry. The Ge 1- xSnx layer on GeSnOI substrate has a surface roughness of 1.90 nm, which is higher than that of the original Ge 1- xSnx epilayer before transfer (surface roughness is 0528 nm). The compressive strain of the Ge 1- xSnx film in the GeSnOI is as low as 0.10% as confirmed using HRXRD and Raman spectroscopy. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

We report on the in situ engineering of hybrid Co-picene magnetic structures with variable spin coupling using a low-temperature scanning tunneling microscope. Single picene molecules adsorbed on Au(111) are manipulated to accommodate individual Co atoms one by one, forming stable artificial hybrid structures with magnetism introduced by the Co atoms. By monitoring the evolution of the Kondo effect at each site of Co atom, we found that the picene molecule plays an important role in tuning the spin coupling between individual Co atoms, which is confirmed by theoretical calculations based on the density-functional theory. Our findings indicate that the hybrid metal-molecule structures with variable spin coupling on surfaces can be artificially constructed in a controlled manner.

We report the measurements of single-electron tunneling (SET) spectra for individual and coupled ultrasmall palladium nanoparticles using scanning tunneling microscopy. Extra fine structures, in addition to the Coulomb blockade and staircases in current–voltage curves, was observed for individual Pd particles of about 2 nm in diameter, which is attributed to the interplay of the SET effect and the effects of discrete energy levels due to significant quantum confinement effects in nanoparticles. The origination of the negative differential resistance effect in series-coupled Pd particles is also attributed to the discreteness of energy levels of both the coupled Pd particles.

Well-ordered striped structures are developed from alkanethiol self-assembled monolayers on an Au(111) surface following well-controlled annealing processes. We demonstrate here that such regular concave–convex molecular structures can be used as the templates for growing one-dimensional molecular and atomic chains. By depositing C60 molecules onto the striped surface, C60 bimolecular chains are self-assembled. Due to the breaking of C–S bonds under certain conditions, residual S atoms can form a S monoatomic chain between two adjacent stripe pairs of thoroughly lying-down molecules. Possible growth mechanisms are discussed.

Negative differential-resistance (NDR) molecular device is realized involving two C60 molecules, one is adsorbed on the tip of a scanning tunneling microscope and the other is on the surface of the hexanethiol self-assembled monolayer. The narrow local density of states features near the Fermi energy of the C60 molecules lead to the obvious NDR effect. Such controllable tunneling structure and the associated known electronic states ensure the stability and reproducibility of the NDR device.

By using scanning tunneling microscopy/spectroscopy, we have studied the current–voltage characteristics of two-dimensional (2D) Au clusters, thermally deposited on self-assembled alkanethiol monolayer. The curves display Coulomb blockade and staircase with asymmetric behavior. The measured zero conductance gap as a function of cluster size is in excellent agreement with classical model calculations, in which the 2D Au island is treated as metallic in the planar direction but nonmetallic in the normal direction.

We have succeeded in synthesizing PrFe11V1−xTix (x=0.2–1) compound and their nitrides with the ThMn12-type structure. The phase formation and magnetic properties have been investigated by x-ray diffraction, differential thermometric analysis, and magnetic measurement. The stable temperature range of the 1-12 phase for PrFe11V1−xTix alloys has been determined as a function of Ti content. PrFe11V compounds with the ThMn12-type structure do not exist and PrFe11Ti compounds with the TnMn12-type structure are obtained by annealing in a narrow temperature range between 1303 and 1383 K. Furthermore, 1-12 phase with the ThMn12-type structure can be obtained at lower temperature and wider temperature range with decreasing Ti content x (0.2⩽x⩽1). PrFe11V1−xTixNy with x=0.2–1 has a Tc of about 730–785 K, Ba larger than 8 T and Ms in the range 144–148 emu/g. These intrinsic magnetic properties are highly favorable for permanent magnet applications. As a preliminary, an intrinsic coercivity of 5.4 kOe is obtained for ...

Adding salts into polymer solution has been found to modulate the fiber structure and significantly improve the solution spinnability in electrospinning. However, the mechanisms have not been fully understood. This work adopted molecular dynamics method to investigate the dynamic behavior of poly(ethylene oxide) (PEO)/water droplet with or without dissolved NaCl salt under high-voltage electric field. Our simulation results agreed with the previous experimental reports well. We observed that some daughter droplets detach from the mother droplet due to the ions evaporation and hydration effect, which significantly accelerates the water evaporation and hence improves the solution spinnability. We also observed that some sodium ions are always coordinated with the ether oxygen group in the PEO chain. When these ions are accelerated by the electric field, the PEO chain segments follow the motion of the ions, inevitably stretching the chain and improving the fiber morphology.

Near-resonant second-harmonic generation (SHG) in c-axis oriented ZnO nanorods is studied under the femtosecond laser with wavelength from 780 nm to 810 nm. A highly efficient SHG is obtained, which is attributed to the d131 component of the second-order nonlinear susceptibility. The largest d131 value is estimated to be 10.2 pm/V at the pumping wavelength of 800 nm, which indicates a large SHG response of the c-axis oriented ZnO nanorods in the near-resonant region. Theoretical calculation based on finite-difference time-domain simulation suggests a four-fold local-field enhancement of the SHG.

We present the fabrication and scanning tunneling microscopy (STM) characterization of suspended monolayer graphene (SMG) on periodic Si nanostructure. Monolayer graphene (MG) was grown on Cu foils by chemical vapor deposition (CVD) and transferred onto a Si substrate with etched array of periodic nanopillars, obtaining partly suspended MG. Low-temperature STM characterization was performed on the suspension area of the MG with atomic resolution images obtained. The scanning tunneling spectroscopy of SMG shows a nonlinear behavior near the Fermi level (EF), which is attributed to the Dirac cone reshaped by electron-electron interaction.

We demonstrate that the tunable molecule-substrate interaction offers possibility to realize a single cobalt phthalocyanine (CoPc) molecule rectifier. CoPc molecule shows a prominent rectifying effect at about −0.30 eV when a monolayer graphene (MG) is intercalated between CoPc and Ru(0001) substrate. First-principles calculations show that CoPc molecule couples with MG/Ru(0001) substrate mainly through the dz2 orbital of Co atom. The tunneling between CoPc molecule and MG/Ru(0001) substrate is mainly intermediated by the Co-dz2 orbital locating at −0.35 eV. The resonant tunneling through this single orbital gives rise to the abrupt current enhancement and hence the rectifying effect.

In this letter, we theoretically investigate the coupling of far-infrared surface plasmon polaritons (SPPs) between spatially separated graphene sheets. By using the finite-difference frequency-domain method, we numerically illustrate the SPP propagation and modulation in the graphene sheets. The coupling of SPPs is employed to design zero insertion loss optical splitters, 1 × 2 digital optical spatial switches, and ultra-compact Mach-Zehnder interferometers with the arm length far below the diffraction limit. The study provides an effective way in designing graphene based high-speed and ultra-compact optoelectronic devices.

Hybrid Si/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solar cells based on nanowire array structures with radial and axial junctions are prepared and investigated. Compared to axial junction cells, radial junction cells exhibit much higher external quantum efficiency for long wavelength light due to improved separation of photogenerated charge carriers, resulting in more than 10 times higher power conversion efficiency ( PCE ). By tuning the height of nanowire array for the hybrid radial junction cells and investigating the external quantum efficiency and open circuit voltage decay, we identify that inefficient carrier collection due to interfacial recombination is the major limiting factor for the hybrid radial junction cell performance.

An Er/Yb silicate strip loaded waveguide was fabricated for optical amplification purpose. A 2.4-μm-wide SiO2 strip was deposited on top of the Er/Yb silicate active layer. Experiment data showed a 5.5 dB signal enhancement in a 7.8-mm-long waveguide pumped by a laser of 372 mW at 1480 nm. The signal is not saturated and can be further enhanced by increasing pumping power and decreasing waveguide loss. The strong red light emission at 660 nm was also observed due to excited state absorption and Yb3+ participated energy transfer upconversion processes.

By using a metal-insulator-metal waveguide with temporally oscillating core width, the single frequency of incident surface plasmon polaritons (SPPs) could be converted to many discrete frequencies with equivalent intervals. The conversion efficiency is affected by the waveguide structure and also the amplitude and frequency of the oscillation. Finite-difference time-domain simulations revealed that the SPP frequency shift behavior is analogous to the spatial light diffraction in optical waveguide arrays. The discrete frequencies of SPPs can further be controlled by varying the initial phase of the waveguide core width oscillation. The results are explained analytically using the coupled mode theory.

International audience; In contrast to earlier classical studies that analyze the surface Bloch modes supported by metallic gratings composed of slits as collective surface-plasmon-polaritons (SPPs) resonances (or poles) of the entire periodic problem, we study the normalized rate of SPPs that are locally launched on every individual ridge of metallic lamellar gratings. With this "microscopic" description at the unit-cell level, we further explain how these individual SPPs constructively interfere to build up the classical collective resonances. The approach, which combines analytical treatments and fully-vectorial computations, shines new light on an important and classical phenomenon of grating diffraction

We present our study on the Si(111)-3×3-Ag surface using scanning tunneling microscopy/spectroscopy. The results reveal that the well defined localized surface-state bands S2/S3 in the surface with lightly doped Si substrate play an important role in electron transport. The relative wide space charge layer beneath the surface interplays with the localized surface states, thus leading to the effect of the negative differential resistance.

We experimentally studied the enhanced optical transmission of perforated silver films in the middle-infrared region. The optical transmission properties of the samples with classical inductive frequency selective surface structures are also affected by both the periodicity and the aperture shape. Our results indicate that the physical operating mechanisms are the same for the plasmon crystal and the frequency selective surface, and both surface plasmon polaritons due to the periodicity and waveguide modes that are present in single holes have effect in the optical transmission enhancement.

We reported here the experimental results of the metal film perforated with circular and cross-dipole apertures. The transmission spectrum of the sample with a complex unit cell is substantially different from that of the corresponding sample with a simple unit cell. The localized surface plasmon polaritons are suggested to be involved into the enhanced transmission.

The authors propose and numerically demonstrate a planar metal heterostructure for guiding surface plasmon polaritons (SPPs). By the analysis of the effective index method and finite-difference time-domain numerical simulations, they reveal that the planar metal heterostructure created by inlaying a Ag film, on which SPPs show a lower phase velocity, into an Al film and coating them with a uniform dielectric layer can strongly confine SPPs in the Ag region of the waveguide with 171×33nm2 cross section and 1.76μm propagation length. The planar metal heterostructures provide a way for constructing various nanoscale counterparts of conventional planar integrated devices such as splitters, resonators, sensors, optical switches, logical circuit, etc.

The propagation properties of surface plasmon polaritons (SPPs) along metal heterowaveguide (MHWG)-constructed bending nanowaveguides are investigated by finite-difference time-domain simulation. The results reveal that SPPs can pass through 90° sharp bending MHWGs with higher than 90% propagation efficiency. Further numerical demonstrations of the bending MHWGs in splitters and Mach-Zehnder interferometers imply that MHWGs may find significant applications in highly integrated nanophotonic networks.

The authors report experimental results of optical transmission through metal films perforated with two periodic arrays of subwavelength holes in the middle-infrared region. Multiple transmission bands can be achieved by such single-layer structured metal film, and their locations were controlled independently by the periodicities of the individual hole arrays. The coupling between surface plasmon polaritons in the cascaded structure is also studied.

The authors theoretically demonstrate a plasmonic waveguide ring resonator (WRR) working at terahertz frequencies considering that surface plasmon polaritons (SPPs) can effectively propagate on bended plasmonic waveguides with dielectric claddings possessing high refractive index. A free spectral region of 110GHz has been achieved by decreasing ring radius of the WRR, which could be much smaller than the operating wavelength. By introducing dielectric and plasmonic waveguides at both sides of the resonant ring, respectively, SPPs can be efficiently excited at the output of the WRR through mode coupling. The plasmonic WRRs may find great applications in the integration of terahertz generation and control platform.

Light cannot be strongly confined in two-dimensional (2D) slab photonic crystal waveguides (PCWs) when the slab thickness is less than half a wavelength. By using the finite-difference time-domain method, we theoretically demonstrate that two metal plates clad 2D PCWs (MCPCWs) constructed by sandwiching a PCW into two parallel metal plates can well compress light field in the vertical direction of PCW plane. A method for improving the propagation efficiency of MCPCWs is suggested and demonstrated numerically. Our result implies that MCPCWs may provide another method to create planar PC devices on metallic surfaces for highly integrated optical circuits.

By modulating the dielectric properties of metallic surfaces with different metal materials, we theoretically proposed and numerically demonstrated that a nanoslit surrounded by metallic heterostructures constructed with metals Al and Ag can realize directional beaming of light. The beaming is due to the electromagnetic surface resonances modulated by the dielectric properties of the output metallic surface. The structure would be interesting for a variety of optical devices and elements for nanophotonics.

Metal heterostructures constructed surface plasmon polaritons (SPPs) Bragg reflectors and nanocavities on flat metallic surfaces are proposed and demonstrated numerically. A metal heterowaveguide structured by alternately stacking two kinds of metal gap waveguides (MGWs) shows periodically effective refraction index modulation to SPPs and produces SPP propagation on flat metallic surfaces a band gap in certain frequencies, known as plasmonic band gap, in which SPP propagation is forbidden. Changing the width of one MGW in the heterowaveguide, a SPP nanocavity with high quality factor can be created. Our results imply a broad possibility of constructed SPP-based Bragg reflectors, emitter, and filters, etc., on flat metallic surfaces for planar nanometeric photonic networks.