Table of contents

In this letter, we propose the design of a frequency scanning antenna (FSA) based on a hybrid mode of a quasi-TEM mode and a spoof surface plasmon polaritons (SSPPs) mode. The two modes are hybridized using a partially grounded comb-shape microstrip (CSM) structure, a combination of conventional microstrip line and the comb-like structure, the former of which supports propagation of quasi-TEM mode while the latter SSPPs mode. The CSM modulated by periodical radiating elements can achieve wide-angle frequency scanning antennas. Since the dispersion curve of the hybrid mode on CSM lies below that of pure microstrip lines, a larger phase shift can be achieved within the same propagation length than for pure microstrip lines. This is favorable for wide-angle scanning in a narrower bandwidth, so as to take up less spectrum resource. Moreover, dispersion of the hybrid mode can be engineered so that linear frequency scanning can be realized approximately. As an example, an X-band FSA was designed, fabricated and measured. The simulated and measured results agree well, and show that the prototype can achieve  ±60° scanning in 8.5–12.5 GHz. In particular, the scanning angles within [−40°, +40°] are linearly proportional to frequency in 9.0–12.0 GHz, with an average gain of about 14 dB and a half-power beam-width of 6°. Moreover, due to the usage of radiating elements with smoothly-varying strip-widths, the open stopband is suppressed and there is no degradation for radiation at the broadside. This work provides an alternative method of designing wide-angle FSAs that may find applications in spectral analysis, direction of arrival estimation, etc.

Among ultrathin layered van der Waals materials hexagonal boron nitride has attracted considerable attention. The van der Waals character of its surface together with its insulating properties makes it an ideal substrate for the fabrication of high quality organic crystalline thin films with minimum disturbance from the substrate. hBN can either be used as interface layer decoupling the adsorbed species from the metallic or dielectric support or multilayer hBN can directly serve as an inert, weakly interacting substrate. The adsorbed species and resulting thin organic crystallites can then be considered as almost free standing enabling access to their intrinsic properties. Technologically, this means that organic thin film transistors—ultimately only limited by the intrinsic properties of the organic crystals—can be realized.

The efficient decoupling of the adsorbed organic molecules from the support also provides the opportunity to gain new insights in fundamental aspects of intermolecular interactions, self-assembly, chemical reactions, or electronic properties which might otherwise be inaccessible due to the strong adsorbate-substrate interactions.

In this review, we provide an overview on the adsorption, assembly, growth and properties of organic molecular films from sub monolayer coverages to crystalline thin films on hBN substrates mainly obtained by mechanical exfoliation.

The ability to convert low-energy quanta into a quantum of higher energy is critical for a variety of applications, including photovoltaics, volumetric display, bioimaging, multiplexing sensing, super-resolution imaging, optogenetics, and potentially many others. Although the processes of second harmonic generation and multiphoton (or two-photon) absorption can be used to generate photon upconversion, lanthanide-doped upconversion nanocrystals have emerged as an attractive alternative for nonlinear upconversion of near-infrared light with pump intensities several orders of magnitude lower than required by conventional nonlinear crystals. Over the past five years, considerable efforts have been made to tune the photoluminescence of upconversion nanocrystals, and significant progress has been achieved. In this review, we focus on manipulation of the wavelength, emission intensity and lifetime of upconversion nanocrystals. Here, we outline the fundamental principle for the upconversion phenomenon, review the current experimental state-of-the-art for controlling photon upconversion in lanthanide-doped nanocrystals and highlight the prospects for multifunctional upconversion nanocrystals currently in development.

Field emission based microplasma actuators generate highly non-neutral surface discharges that can be used to heat, pump, and mix the flow through microchannels and offer an innovative solution to the problems associated with microcombustion. They provide a constant source of heat to counter the large heat loss through the combustor surface, they aid in flow transport at low Reynolds numbers without the use of moving parts, and they provide a constant supply of radicals to promote chain branching reactions. In this work, we present two actuator concepts for the generation of field emission microplasma, one with offset electrodes and the other with planar electrodes. They operate at input voltages in the 275–325 V range at a frequency of 1 GHz which is found to be the most suitable value for flow enhancement. The momentum and energy imparted by the charged particles to the neutrals as modeled by 2D particle-in-cell with Monte Carlo collisions are applied to actuate flow in microchannels using 2D computational fluid dynamics modeling. The planar electrode configuration is found to be more suitable for the purpose of heating, igniting and mixing the flow, as well as improving its residence time through a 10 mm long microcombustor. The combustion of hydrogen and air with the help of 4 such actuators, each with a power consumption of 47.5 mW cm⁻¹, generates power with an efficiency of 90.5%. Such microcombustors can be applied to all battery based systems requiring micropower generation with the the ultimate goal of 'generating power on a chip'.

Electrically induced resistive switching resulting from ionic transport and electrochemical redox reactions is promising for future generation non-volatile memory devices and artificial neural computing. The key ingredient for the highly efficient neural computing in this context is a memristor, which is a special type of a resistive two-terminal element whose electrical properties depend on not only the state of the element but also how the state has been achieved in its history. Memristor characteristics are demonstrated in a bilayer junction of Al and a Bi–Cu–S alloy utilizing electrically reversible generation of an insulating interface. The high resistance due to the interface layer drops abruptly by orders of magnitude when the barrier is annihilated electrochemically under a bias. The barrier is made to be robust by applying a reverse bias, giving rise to a controllable memory effect on the switching phenomenon. The switching mechanism based on the manipulation of a barrier, which is complementary to conventional bridging conductive filaments, will open the way for new functionalities as device elements.

The active photonics based on the two-dimensional material graphene have attracted a great deal of interest for developing tunable and compact optical devices with high efficiency. Here, we integrate graphene into the Fano-resonant all-dielectric metasurfaces consisting of silicon split resonators, and systematically investigate the strong interaction between graphene and the highly localized hot spots inside feed gaps in the near infrared regime. The numerical results show that the integrated graphene can substantially reduce the Fano resonance due to the coupling effect between the intrinsic absorption of graphene with enhanced electric field in the localized hotspot. With the manipulation of the surface conductivity via varying Fermi level and the layer number of graphene, the Fano resonance strength obtains a significant modulation and is even switched off. This work provides a great degree of freedom to tailor light-matter interaction at the nanoscale and opens up avenues for actively tunable and integrated nanophotonic device applications, such as optical biosensing, slow light and enhanced nonlinear effects.

We study the structural, electronic and transport property of bilayer blue phosphorus (BBP) by using the first-principles. Our results show that the band gap can be adjusted by different stacking structures of the BBP. We simulate the functional device based on AA-, AB- and AC-stacking BBP and the transport characteristics of the current-voltage curve with nonlinear competitive behavior are investigated. Of the three devices, AA stacking BBP has the highest conductivity. Under special bias, the currents of AB- and AC-stacking devices produce interesting competitive behavior. The transport characteristics behaviors of the BBP can be explained by the band structure, transport spectrum and molecular projected self-consistent Hamiltonian. We can control the change of current by adjusting the different contact modes of the BBP. The BBP with interesting electronic and transport properties are expected to have potential applications in nanoelectronics.

In this paper, we report the study of the frequency-dependent plasmonic enhancement of a circular disk nano-optical antenna array and the photo-response of the optical antenna enhanced photodetector at different frequencies using a femtosecond (fs) laser frequency comb. A fs-laser frequency comb can provide hundreds of evenly spaced harmonic frequencies and thus allows simultaneous measurement of the plasmonic optical antenna enhancement effect at these harmonic frequencies. This offers a highly efficient frequency-dependent measurement approach compared to the conventional method of modulating of a c.w. laser, which measures the frequency response at each frequency. The impulse response of the circular disk nano-optical antenna array and the electric-field (E-field) distribution profile are simulated under a fs laser illumination. The light intensity spectrum is simulated and verified to have uniform intensities on the harmonic frequencies within the  ±5 GHz frequency range. The photocurrent densities in different regions of a GaAs p-i-n photodetector are analyzed together with their frequency dependence at the harmonic frequencies of the fs laser frequency comb with a repetition rate of MHz. A circular disk nano-optical antenna array enhanced GaAs p-i-n photodetector was fabricated and measured using a fs laser frequency comb with the same repetition rate. The nano-optical antenna can provide ~20 dB enhancement for the harmonic frequencies and extend the detector cut-off frequency from 2.4 GHz to 4.2 GHz.

Solution processed metal oxide thin-film transistors (MO-TFTs) have gained momentum in recent years and opened new horizons for their extended applications in thin-film optoelectronics. Unfortunately, the relatively high processing temperature has become the main limitation for their commercial application in flexible electronics. In this work, ultrashort femtosecond (fs) laser pulses were used for the annealing of printed InGaZnO TFT arrays with low temperature and fast processing. The high transient intensity and negligible heat-affected zone is beneficial to the effective conversion of precursor to metal-oxide lattices, which was also verified by the x-ray photoelectron spectroscopy analysis. Moreover, for the first time, based on optical engineering method, dielectric mirrors (DMs) is proposed to realize reflection of photon irradiation and protect the underlying polymer substrate. TFTs on polyethylene naphthalate (PEN) substrate with polymer insulator exhibited a mobility of 4.24 cm² V⁻¹ s⁻¹, along with good mechanical stability. The mechanism of DMs for the improvement of the device performance was systematically investigated based on laser-matter interaction, optical structure and photon transport. DMs can reduce laser-induced pressure and variation of dielectric properties in PEN substrate resulting from the impact ionization due to a strong nonlinear multi-photon absorption. Our work clearly demonstrated that flexible MO-TFTs can be simply fabricated with fs-laser annealing. More importantly, this novel method is applicable for a wide range of MO and polymer substrates, by solving the incompatibility of solution processed MO and polymer substrates due to high processing temperature. Therefore, the proposed method showed tremendous potential to be applicable in flexible electronics such as sensors, display and circuits.

A first-principles study is presented of the phonons in Be-IV-N₂ compounds, with IV  =  (Si,Ge). These compounds are closely related to wurtzite BN by replacing the group III (B) by group II (Be) and group IV (Si or Ge). The phonon frequencies at the Brillouin zone center are used to predict the corresponding infrared and Raman spectra. The phonon frequencies, oscillator strengths, Born effective charges, and dielectric constants on which these spectra are based are provided.

We perform nonadiabatic molecular dynamics to investigate the relaxation of the excited electrons and holes in g-C₃N₄/MoS₂ heterojunctions. The results indicate that the hot carriers relaxation dynamics strongly depend on the nonadiabatic coupling, band gap and pure-dephasing time. We choose hot electrons and hot holes excited to different energy states with similar excess energies ΔE  ≈  0.42 eV, and hot electrons and hot holes relax to lower energy states with the time scales 0.57 ps and 0.23 ps. Stronger nonadiabatic coupling and slower pure-dephasing time accelerate the hot holes transfer. In general, it is found that the hot holes decay faster than hot electrons, because the couplings between the holes states in the valence band are stronger than the electrons states in the conduction band.

In this work, a novel composite material of semiconducting single-walled carbon nanotubes/nickel oxide (semi-SWCNT/NiO_x) is well designed to act as the channel layer in solution-processed p-type thin-film transistors (TFTs). The construction of the semi-SWCNT/NiO_x matrix is expected to effectively enhance the mobility of TFTs because the SWCNT can replace the parts of the NiO_x system and provide fast tracks for carrier transport. Compared to the mobility of 0.63 cm² V⁻¹ s⁻¹ in pure NiO_x TFT, the 1.0 wt.% SWCNT/NiO_x TFT shows an excellent mobility of 3.26 cm² V⁻¹ s⁻¹ with the SiO₂ insulator. Furthermore, the solution-processed ZrO₂ dielectric is employed to further enhance the mobility of the SWCNT/NiO_x TFT. The mobility of the 1.0 wt.% SWCNT/NiO_x TFT based on the ZrO₂ dielectric is 6.58 cm² V⁻¹ s⁻¹, which is nearly ten times that of the pure NiO_x TFTs on a SiO₂ insulator. The transmission line model (TLM) can further demonstrate that the channel resistance and contact resistance of the device is dramatically reduced with SWCNT incorporated. The results suggest that the SWCNT/NiO_x TFTs are promising for the development of low-cost and transparent electronic applications.

Plasma actuators have shown their ability for different applications within the active flow control and heat transfer fields. These simple devices are inexpensive, present robustness, low weight and are fully electronic. However, they still present some debilities in terms of durability and maximum induced flow velocity. To overcome these issues, during the last years, a few different configurations of dielectric barrier discharge plasma actuators have been proposed but, always making use of dielectric barrier discharge layers of constant thickness. For the first time, the present study herein introduces a new plasma actuator configuration that makes use of a stair-shaped dielectric layer, which aims to increase the induced flow velocity and improve the mechanical efficiency without compromising the durability of the device. In this new concept, instead of using a dielectric layer with constant thickness, the thickness of the dielectric layer decreases along the covered electrode width, which leads to an increase of the plasma discharge extension and an increase of the maximum induced flow velocity. Several actuators based on a stair-shaped dielectric layer were experimentally tested and compared with the conventional actuators. The results demonstrated that the stair-shaped actuators allow us to obtain a plasma discharge longer. It was found that by implementing a stair-shaped dielectric layer with a slope angle, in a 1.92 mm thickness actuator, it is possible to increase the maximum induced flow velocity in about 32% by consuming 36% less power. These results lead to a mechanical efficiency five times bigger than the obtained with conventional constant thickness actuators.

Plasma activated water (PAW) containing reactive oxygen and nitrogen species (RONS) is of great interest for applications in the agricultural and food industries, where processing methods with large capacities and high energy-yields are required. In this work, we studied the differences between seven types of discharge schemes in terms of the production rates and concentration ratios of RONS using deionized water and tap water in comparison. We used N₂ and air as the feed gas with a variable admixture of water vapour. When O₂ was incorporated into the reaction system, the major products in the PAW became and , while small amounts of H₂O₂ and were detected only in O₂ poor situations with water vapour. Of the major products, the condition of whether or becomes predominant is dependent on the extent of successive oxidation reactions from NO to NO₂ and NO₃ and the competing rates between gas phase reactions and dissolutions into water. In our tested discharge schemes, those with volumetric glow-like discharge structures produced rich PAW, while those with spark discharge structures over the water surface or in water were favourable for the production of rich PAW. In particular, a discharge scheme with a wire-in-nozzle structure operated in a spark-mode with a bubbling gas flow yielded PAW with the highest concentration, more than 70%, at a high energy-yield of 2 g kWh⁻¹. In the storage period, was converted into due to ionic reactions in aqueous solution, but the buffer action of tap water was observed to suppress the conversion for a fairly long period.

I study the influence of transverse electric fields on the interfacial forces between a graphene layer and a carbon nanotube tip by means of atomistic simulations, in which a Gaussian regularized charge-dipole potential is combined with classical force fields. A significant effect of the field-induced electric charge on the normal force is observed. The normal pressure is found to be sensitive to the presence of a transverse electric field, while the friction force remains relatively invariant for the here-used field intensities. The contact can even be turned to have a negative coefficient of friction in a constant-distance scenario when the field strength reaches a critical value, which increases with decreasing tip-surface distance. These results shed light on how the frictional properties of nanomaterials can be controlled via applied electric fields.

The abrupt metal insulator transition in VO₂ is attracting considerable interest from both fundamental and applicative angles. We report on DC I–V characteristics measured on VO₂ single crystals in the two-probe configuration at several ambient temperatures below the insulator–metal (I–M) transition. The insulator-mixed-metal-insulator transition is induced by Joule heating above ambient temperature in the range of negative differential resistivity (NDR). In this range the stability of I(V) is governed by the load resistance R_L. Steady state I(V) is obtained for R_L  >  |dV/dI|_max in the NDR regime. For R_L  <  |dV/dI|_max there is switching between initial and final steady states associated with peaks in the Joule power, that are higher the lower R_L is. The peaks caused by steep switching are superfluous and damaging the samples. On the other hand, the large R_L needed for steady state is the main power consumer in the circuit at high currents. The present work is motivated by the need to avoid damaging switching in the NDR regime while reducing the power consumption in the circuit. Large resistance modulation can be obtained under steady state conditions with reduced power consumption by increasing the ambient temperature of the device above room temperature. Under steady state conditions, the transition to the mixed metal-insulator state is smooth and is followed closely by appearance of sliding domains.

Conventional acoustic focusing requires an array of actuators or waveguides to form a complex wavefront, resulting in a high cost or bulky size. In this paper, an original gradient Helmholtz-resonator (HR)-based acoustic metasurface (AMS) is presented. The phase shift of AMS units can be precisely controlled over the full phase range and continuously tuned by varying the slit width. The transmission efficiency of the AMSs is relatively high, benefitting from the impedance matching. Several typical situations in acoustic focusing with different focusing parameters are realized by the designed AMSs. The results of the finite element method demonstrate that moving the position of the focal point or changing the incident angle can be realized by tuning the slit width distribution. A further analysis indicates that the discrete resolution is considerably fine, as a result of the suitable deep-subwavelength parameters of the AMS and the high accuracy of the phase shift of each unit. The acoustic intensities at the focal point reach 12.3 to 17.6 times that of the incident plane wave, owing to the high transmission efficiency. Due to these significant advantages, the designed gradient HR-based AMS is able to offer a tuneable acoustic lens in medical sonography, localized heating, nondestructive flaw detection and particle trapping.

In this article, a textile-based metamaterial with broadband microwave absorption was developed using a screen printing technique. The metamaterial has over 90% absorption from 7.39 GHz to 18 GHz. The metamaterial consists of a top layer of the printed structure of commercial conductive inks on various kinds of clothes, which is separated from a conductive ground plane with flexible dielectric foam of 3.2 mm thickness. The metamaterial absorber was simulated using ANSYS HFSS software for various thicknesses of the printed ink. It was observed that the absorption band varies with variation in printed thickness, and the optimized printed thickness was found to be about 50 µm. With the increase in printed thickness, the absorption shifts from broadband to narrow band. To achieve the optimum thickness in fabrication, statistically designed experiments were conducted to study the variation of printed thickness and width with different kinds of clothes and substrates (FR4, plain weave cotton cloth, and twill weave cotton cloth), mesh number of the screen (50–110) and the number of passes (1–3). Substrate material and the number of passes were found to be the most significant factors that affect the printed width resolution and thickness. Rigid copper foil and printed cloth could both be used as the ground plane. A complete, flexible absorber was fabricated using printed cloth as the ground plane. The microwave response (absorption) of all the fabricated absorbers was measured and found to be in agreement (more than 90%) with the simulation. Further, the fabricated absorber on the cloth substrate was also made hydrophobic by treating it with polydimethylsiloxane.

The presented research introduces an innovative bottom-up approach, involving synthesis, characterization and application of SnO₂ nanomaterials in advanced technologies. SnO₂ nanosheets are synthesized with a hydrothermal method and screen-printed on a flexible substrate with interdigitated electrodes. The obtained measurements reveal that the synthesized material has a rutile crystal structure with preferable (1 1 0) orientation and oxygen-defects, unique morphology and good electric conductivity. Multifunctional performance is evaluated for gas and ultraviolet A (UVA; 365 nm) light sensing. The designed sensor shows a better response to ethanol in comparison to 2-propanol and acetone, indicating the selectivity feature among the investigated volatile organic compounds. Photocurrent measurements reveal a good photoconversion rate, suitable for UVA monitoring.

In recent years, the requirement for high-performance and lower-energy-consuming nano/micro devices has triggered widespread studies in low-dimensional materials with better interfacial thermal conduction. Here, we report the interfacial thermal resistance between single-layer transition metal dichalcogenides (MoX₂ (X  =  S or Se)) and their dielectric substrate, combining the differential 3ω method and finite element simulation. The MoX₂ samples are directly grown on SiO₂ substrate by chemical vapor deposition (CVD) to reduce interfacial thermal resistance due to conformal interface between MoX₂ and the substrate. We observe that the interfacial thermal resistance of MoS₂/oxide and MoSe₂/oxide reaches ~4.76  ×  10⁻⁸ m² KW⁻¹ and ~4.95  ×  10⁻⁷ m² KW⁻¹, one order of magnitude smaller than that in CVD-transferred or Scotch-tape samples, due to better interface with fewer voids and less roughness. The larger interfacial thermal resistance in MoSe₂ than in MoS₂ is believed to result from larger mismatch of atomic mass on the two sides of the interface. Our results indicate that the interfacial thermal resistance can be managed by improving the combination between MoX₂ and its dielectric substrate to enhance the thermal transport across their interface.

In this paper, we investigate transmission properties of dextrose (D)-(+)-glucose solution with concentration variations using a reusable coplanar waveguide (CPW) device in the frequency ranging from 1 GHz to 10 GHz. In the present experiment, three concentrations of glucose solution, 0.2, 0.4, and 0.6 g ml⁻¹, are measured on the proposed CPW device, and these results are also compared with that of deionized water as a control solution. From the comparative results, as glucose concentration increases, transmission properties are gradually degraded due to the reduction of penetration depth as well as the increase of electric resistance in the observed frequency region. As a result, it can be noted that the concentration variation of D-(+)-glucose solution has a significant effect on the transmission properties related to the microwave circuit, i.e. electric resistance, and field, i.e. penetration depth. In terms of the microwave circuit and field, we expect that our finding can provide fundamental sensing properties for highly sensitive glucose detection via a non-invasive and non-contact technique.

Microcantilever sensors are ultrasensitive devices with miniaturized size and fast responding capability. However, they are generally designed to operate in a very narrow frequency range, which has limited their applications. We present here an all-optical technique that enables efficient actuation and rapid tuning of the vibration behaviors of microcantilevers. In particular, the resonant frequency of a microcantilever beam can be continuously tuned remotely by a laser induced photothermal effect to cover a large range with boosted vibration amplitude, beneficial for improving the detection threshold for force and mass. The underlying principles for photothermal actuation and elastic nonlinearity are discussed in detail. A verification experiment proves its potentials in detecting acoustic waves with frequencies far from the initial resonant frequency of the microcantilever.

It is often difficult to implement complex microscopy systems without spherical aberration. Herein, we developed a novel, robust, three-dimensional (3D), bifocal plane, single-particle tracking technique, based on a dual-objective fluorescent reflection system with spherical aberration (DOFR–SA). It can simultaneously image a pair of focused and defocused planes containing fluorescent particles with a single camera instead of splitting photons into two channels. Based on the 3D DOFR–SA, the desired position accuracy along the z-axis was achieved without compromising the precisions of the (x, y ) positions, even with limited number of photons from a single molecule. Accordingly, this method was applied to fluorescent particle tracking in biofluids and living cells with high-spatial and temporal precisions.

Solution-processable hybrid perovskite solar cells offer potential in the photovoltaic field due to their low-cost fabrication and high efficiency. However, an undesirable current–voltage (J-V) hysteresis hampers the applications of perovskite solar cells. In particular, for the inverted device, the understandings for J-V hysteresis origination are not uniform, and the inverted hysteresis phenomenon has been further complicated the hysteresis behavior. In this report, an external bias precondition method is adopted to unveil the origin of the inverted hysteresis. The results indicate that the extents of inverted hysteresis are very much dependent on the bias direction of the precondition. To further unveil the effect of the precondition on inverted hysteresis, the microscopic J-V hysteresis was also observed by using conductive atomic force microscopy (c-AFM) measurements. The results indicate that ion migration and accumulation slowly built up at the grain boundaries of the perovskite film when repeating the scan using c-AFM. Furthermore, the transient characteristics based on capacity-frequency plots and open-circuit voltage decay further identify the presence of difference for the different bias preconditions on the device, because the different bias precondition could induce different directions of ions migration and accumulation. These observations are surprising; it can be further identified that the inverted hysteresis originates from the ionic migration and accumulation, and the grain boundary is as the channel of ionic migration and accumulation. Therefore, the grain boundary plays an important role on the hysteresis effect, and preparation of large-grain or single-crystal perovskite films is the way to reduce the hysteresis effect.