Table of contents

The micro-scale patterns in graphene on Si/SiO₂ substrate were fabricated using ultrashort 515 nm laser pulses. For both picosecond and femtosecond laser pulses two competitive processes, based on photo-thermal (ablation) and photochemical (oxidation/etching) effects, were observed. The laser-induced etching of graphene starts just below the threshold energy of graphene ablation: 1.7 nJ per 280 fs pulse and 2.7 µJ per 30 ps pulse. Whilst etching is not sensitive to thermal properties of graphene and provides uniform patterns, the ablation, in contrast, is highly affected by defects in the graphene structure like wrinkles and bilayer islands. The mechanisms of ultrafast laser interaction with graphene are discussed.

Developing inexpensive and simple DNA sequencing methods capable of detecting entire genomes in short periods of time could revolutionize the world of medicine and technology. It will also lead to major advances in our understanding of fundamental biological processes. It has been shown that nanopores have the ability of single-molecule sensing of various biological molecules rapidly and at a low cost. This has stimulated significant experimental efforts in developing DNA sequencing techniques by utilizing biological and artificial nanopores. In this review, we discuss recent progress in the nanopore sequencing field with a focus on the nature of nanopores and on sensing mechanisms during the translocation. Current challenges and alternative methods are also discussed.

In this article the doping of the oxidic compound semiconductor ZnO is reviewed with special emphasis on n-type doping. ZnO naturally exhibits n-type conductivity, which is used in the application of highly doped n-type ZnO as a transparent electrode, for instance in thin film solar cells. For prospective application of ZnO in other electronic devices (LEDs, UV photodetectors or power devices) p-type doping is required, which has been reported only minimally. Highly n-type doped ZnO can be prepared by doping with the group IIIB elements B, Al, Ga, and In, which act as shallow donors according to the simple hydrogen-like substitutional donor model of Bethe (1942 Theory of the Boundary Layer of Crystal Rectifiers (Boston, MA: MIT Rad Lab.)).

Group IIIA elements (Sc, Y, La etc) are also known to act as shallow donors in ZnO, similarly explainable by the shallow donor model of Bethe. Some reports showed that even group IVA (Ti, Zr, Hf) and IVB (Si, Ge) elements can be used to prepare highly doped ZnO films—which, however, can no longer be explained by the simple hydrogen-like substitutional donor model. More probably, these elements form defect complexes that act as shallow donors in ZnO.

On the other hand, group V elements on oxygen lattice sites (N, P, As, and Sb), which were viewed for a long time as typical shallow acceptors, behave instead as deep acceptors, preventing high hole concentrations in ZnO at room temperature. Also, 'self'-compensation, i.e. the formation of a large number of intrinsic donors at high acceptor concentrations seems to counteract the p-type doping of ZnO.

At donor concentrations above about 10²⁰ cm⁻³, the electrical activation of the dopant elements is often less than 100%, especially in polycrystalline thin films. Reasons for the electrical deactivation of the dopant atoms are (i) the formation of dopant-defect complexes, (ii) the compensation of the electrons by acceptors (O_i, V_Zn) or (iii) the formation of secondary phases, for instance Al₂O₃, Ga₂O₃ etc. The strong influence of the different deposition methods and annealing conditions on the doping of ZnO is discussed.

This review shows that, though it is one of the best-investigated oxide compound semiconductors over many decades, understanding of the details of the doping properties and mechanisms of zinc oxide is still in its infancy.

Based on this review, prospective research opportunities are devised.

Most battery positive electrodes operate with a 3d transition-metal (TM) reaction centre. A direct and quantitative probe of the TM states upon electrochemical cycling is valuable for understanding the detailed cycling mechanism and charge diffusion in the electrodes, which is related with many practical parameters of a battery. This review includes a comprehensive summary of our recent demonstrations of five different types of quantitative analysis of the TM states in battery electrodes based on soft x-ray absorption spectroscopy and multiplet calculations. In LiFePO₄, a system of a well-known two-phase transformation type, the TM redox could be strictly determined through a simple linear combination of the two end-members. In Mn-based compounds, the Mn states could also be quantitatively evaluated, but a set of reference spectra with all the three possible Mn valences needs to be deliberately selected and considered in the fitting. Although the fluorescence signals suffer the self-absorption distortion, the multiplet calculations could consider the distortion effect, which allows a quantitative determination of the overall Ni oxidation state in the bulk. With the aid of multiplet calculations, one could also achieve a quasi-quantitative analysis of the Co redox evolution in LiCoO₂ based on the energy position of the spectroscopic peak. The benefit of multiplet calculations is more important for studying electrode materials with TMs of mixed spin states, as exemplified by the quantitative analysis of the mixed spin Na_2−xFe₂(CN)₆ system. At the end, we showcase that such quantitative analysis could provide valuable information for optimizing the electrochemical performance of Na_0.44MnO₂ electrodes for Na-ion batteries. The methodology summarized in this review could be extended to other energy application systems with TM redox centre for detailed analysis, for example, fuel cell and catalytic materials.

We report the engineering of the polar magnetooptical (MO) Kerr effect in perpendicularly magnetized L1₀–MnAl epitaxial films with remarkably tuned magnetization, strain, and structural disorder by varying substrate temperature (T_s) during molecular-beam epitaxy growth. The Kerr rotation was enhanced by a factor of up to 5 with T_s increasing from 150 to 350 °C as a direct consequence of the improvement of the magnetization. A similar remarkable tuning effect was also observed on the Kerr ellipticity and the magnitude of the complex Kerr angle, while the phase of the complex Kerr angle appears to be independent of the magnetization. The combination of the good semiconductor compatibility, the moderate coercivity of 0.3–8.2 kOe, the tunable polar MO Kerr effect of up to ~0.034°, and giant spin precession frequencies of up to ~180 GHz makes L1₀–MnAl films a very interesting MO material. Our results give insights into both the microscopic mechanisms of the MO Kerr effect in L1₀–MnAl alloys and their scientific and technological application potential in the emerging spintronics and ultrafast MO modulators.

The energy efficiency of the spin Hall effects (SHE) can be enhanced if the electrical conductivity is decreased without sacrificing the spin Hall conductivity. The resistivity of Pt films can be increased to 150–300 µΩ · cm by mesoscopic lateral confinement, thereby decreasing the conductivity. The SHE and inverse spin Hall effects (ISHE) in these mesoscopic Pt films are explored at 10 K by using the nonlocal spin injection/detection method. All relevant physical quantities are determined in situ on the same substrate, and a quantitative approach is developed to characterize all processes effectively. Extensive measurements with various Pt thickness values reveal an upper limit for the Pt spin diffusion length: ${{\lambda}_{\text{pt}}}$  ⩽  0.8 nm. The average product of ${{\lambda}_{\text{pt}}}$ and the Pt spin Hall angle ${{\alpha}_{\text{H}}}$ is substantial: ${{\alpha}_{\text{H}}}{{\lambda}_{\text{pt}}}$  =  (0.142  ±  0.040) nm for 4 nm thick Pt, though a gradual decrease is observed at larger Pt thickness. The results suggest enhanced spin Hall effects in resistive mesoscopic Pt films.

Fe_3−xMn_xO₄/p-Si heterostructures (x  =  0, 0.25, and 0.5) were prepared using pulse laser deposition to explore their magneto-electric transport characteristics. All the heterostructures exhibit a rectifying property and junction magnetoresistance of 90% (x  =  0), 117% (x  =  0.25) and 120% (x  =  0.5) at room temperature (300 K), low bias voltage (0 to  −4 V) and low magnetic field (<1 T). Significantly, the sign (positive or negative) of junction magnetoresistance depends on the range of bias voltage for all heterostructures, but for a particular range of voltage, the sign inversion (positive to negative and vice versa) of junction magnetoresistance is observed in the heterostructure of Mn substituted Fe₃O₄ (Fe_3−xMn_xO₄) compared to the virgin (Fe₃O₄) one. The enhancement of junction magnetoresistance and its sign inversion upon Mn substitution in Fe₃O₄ are assigned to the enhancement of magnetization and the spin filtering at the junction of the heterostructures. The electronic band structure of the Fe₃O₄/SiO₂/p-Si heterostructure and the p-type degenerate semiconducting feature of Mn-substituted Fe_3−xMn_xO₄ films are considered to explain the results.

The effect of ferromagnetic layer thickness on the temperature-dependent stray-field-induced coupling mechanism is investigated in perpendicular pseudo-spin-valves based on [Ni/Co]₅/Cu/Co-[Ni/Co]_n (n  =  2, 3, 4, and 5). Experimental observations show that as n increases from 2 to 4, the difference in coercivity and anisotropy between the two ([Ni/Co]₅ or bottom-layer, and [Ni/Co]_n or top-layer) layers increases and the room temperature coupling strength decreases. The coupling then increases for n  =  5, as the coercivity difference shrinks and anisotropy decreases. At reduced temperature, the layers start to decouple at a temperature, which increases with n from 2 to 4 and decreases for n  =  5 via a stray-field domain-replication mechanism. Our results are useful to control the coupling in pseudo-spin-valves for practical applications in magnetoresistive devices.

Amorphous carbon thin films were fabricated on a SiO₂ substrate using a chemical vapor deposition (CVD) technique. The structural and surface morphology of the films were analyzed by Raman spectrometry and high resolution transmission electron microscopy (HRTEM), respectively. The atomic ratio % of C(sp²) and C(sp³) bonds was estimated using x-ray photoelectron spectroscopy (XPS). The film shows angular magnetoresistance (MR) of 18% and 1.6% at 2 K and room temperature respectively. The mechanism of this angular MR was discussed by use of a grain boundary scattering (GBS) model.

A series of (Y,Lu)₃Al₅O₁₂:Ce³⁺ (YLuAG:Ce) solid solutions were synthesized via the solid-state reaction route. The phosphors are all of the cubic garnet crystal structures confirmed by x-ray diffraction (XRD). The internal quantum efficiency and emission intensity of the phosphors can be enhanced by increasing the Lu³⁺ content in the host lattice, along with a blue shift of the emission peak. In addition, the blue shift of the emission peak correlates very well with the lattice contraction. Intense light-emitting diodes (LEDs) are successfully fabricated based on the YLuAG:Ce phosphors and 450 nm blue Ga(In)N chips. The luminous efficiency of YLuAG:Ce phosphors converted LEDs increases with elevating Lu³⁺ concentration. The results indicate that Ce³⁺ doped YLuAG solid solutions, especially LuAG:Ce phosphor is a promising green phosphor for solid-state lighting.

Surface plasmon polaritons have been proposed in the architectures of several solar cells as a way to enhance light collection and thus to increase their efficiency. Here, Ag nanoparticles (NPs) are embedded in a SiON antireflective layer using an electroless technique. The plasmonic effects are modeled and observed experimentally for NPs 5 to 200 nm in size. The systematic comparison of scattering and extinction efficiencies computed as a function of the NPs and surrounding medium properties allows establishing engineering rules, validated by the experimental measurements. The fact that Ag NPs larger than 30 nm mainly contribute to light scattering and therefore to optical path enlargement (green–red light), whereas those smaller than 15 nm absorb light by light trapping (blue–green), is demonstrated and physically explained. A physical barrier making it impossible to shift the dominant resonance beyond 650 nm is pointed out.

Anisotropic optical polarization of AlGaN has been one of the major challenges responsible for the poor efficiency of AlGaN-based ultraviolet light emitting diodes (UV LEDs). In this work, we experimentally investigated the effect of internal strain on the optical polarization of AlGaN epilayers which were pseudomorphically grown on Al_xGa_1−xN templates with Al composition changing from 0.1 to 0.42. High-resolution x-ray diffraction and reciprocal space mapping were conducted to determine the crystal quality and strain status. Polarization-dependent photoluminescence (PL) measurement was performed to study the degree of polarization (DOP) of light emission from lateral facet of the AlGaN epilayer. The result showed that the DOP increased from  −0.69 to  −0.24 with the in-plane strain changing from tensile status (1.19%) to compressive status (−0.70%) and it exhibited a strong dependence of the DOP on the strain. These results demonstrated that the compressive in-plane strain could facilitate TE mode emission from AlGaN, which providing a potential way to enhance the surface light emission of AlGaN-based UV LEDs via strain management of the active region.

A photoswitched single-molecule junction, a stable and reversible single-molecule electrical switch, has been successfully prepared by means of molecular engineering (2016 Science352 1443). In this work we use a first-principles computational approach to investigate the spin valve effect of an azobenzene-based spin optoelectronic device. Our results demonstrate that the magnetoresistive ratio of the spin optoelectronic device is only about 65% when the azobenzene is in cis configuration, which is a low performance for practical applications. However, the magnetoresistive ratio of the device can be enhanced to about 2775% when the cis configuration of the azobenzene is changed into the trans configuration by applying a pulse of light. As a consequence, photoexcitation provides an effective way to obtain a high-performance spin optoelectronic device.

We present in this study how DC bias voltage impacts on the fluxes of reactive species on the skin tissue by means of a plasma–tissue interaction model. The DC bias voltage inputs less than 2% of the total discharge power, and hence it has little influence on the whole plasma characteritics including the volume-averaged densities of reactive species and the heating effect. However, it pushes the plasma bulk towards the skin surface, which significantly changes the local plasma characteristics in the vicinity of the skin surface, and in consequence remarkably enhances the flux densities of reactive species on the skin tissue. With the consideration of plasma dosage and heat damage on the skin tissue, DC bias voltage is a better approach compared with the common approach of increasing the plasma power. Since the DC voltage is easy to apply on the human body, it is a promising approach for use in clincial applications.

This paper reports physical characteristics of water surface discharges. Discharges were produced by metal needle-to-water surface geometry, with the needle electrode driven by 47 kV (FWHM) positive voltage pulses of 2 µs duration. Propagation of discharges along the water surface was confined between glass plates with 2 mm separation. This allowed generation of highly reproducible 634 mm-long plasma filaments. Experiments were performed using different atmospheres: air, N₂, and O₂, each at atmospheric pressure. Time- and spatially-resolved spectroscopic measurements revealed that early spectra of discharges in air and nitrogen atmospheres were dominated by N₂ 2nd positive system. N₂ radiation disappeared after approx. 150 ns, replaced by emissions from atomic hydrogen. Spectra of discharges in O₂ atmosphere were dominated by emissions from atomic oxygen. Time- and spatially-resolved emission spectra were used to determine temperatures in plasma. Atomic hydrogen emissions showed excitation temperature of discharges in air to be about 2  ×  10⁴ K. Electron number densities determined by Stark broadening of the hydrogen H_β line reached a maximum value of ~10¹⁸ cm⁻³ just after plasma initiation. Electron number densities and temperatures depended only slightly on distance from needle electrode, indicating formation of high conductivity leader channels. Direct observation of discharges by high speed camera showed that the average leader head propagation speed was 412 km · s⁻¹, which is substantially higher value than that observed in experiments with shorter streamers driven by lower voltages.

Measurements of transport coefficients of electrons in a scanning drift tube apparatus are reported for different gases: argon, synthetic air, methane and deuterium. The experimental system allows the spatio-temporal development of the electron swarms ('swarm maps') to be recorded and this information, when compared with the profiles predicted by theory, makes it possible to determine the 'time-of-flight' transport coefficients: the bulk drift velocity, the longitudinal diffusion coefficient and the effective ionization coefficient, in a well-defined way. From these data, the effective Townsend ionization coefficient is determined as well. The swarm maps provide, additionally, direct, unambiguous information about the hydrodynamic/non-hydrodynamic regimes of the swarms, aiding the selection of the proper regions applicable for the determination of the transport coefficients.

We report the first instance of time-resolved imaging of surface streamers in air propagating on the surface of titanium dioxide (TiO₂) and alumina (γ-Al₂O₃) beads at ambient temperature and atmospheric pressure. The propagation velocity of primary streamers was found to be dependent primarily on the applied voltage and the type of catalyst. The presence of Ag nanoparticles enhanced the propagation velocity of primary streamers in both TiO₂ and γ-Al₂O₃. Some of the primary streamers passed through a partial discharge, which resulted in enhanced discharge intensity. Through successive steps, the partial discharge served as a staging point for primary streamers, and promoted their propagation toward the next catalyst bead. For a given configuration and catalyst, the velocity of the primary streamer was largely influenced by applied voltage and catalyst type. For a mesh-to-mesh reactor with Ag/ TiO₂ catalyst, the primary streamer reached about 660 km s⁻¹. Secondary streamers occurred with much slower velocities after the primary streamer had disappeared. In contrast to primary streamers, secondary streamer velocities were almost completely independent of the applied voltage on both TiO₂ (150  ±  50 km s⁻¹) and γ-Al₂O₃ (70  ±  10 km s⁻¹). Detailed time-resolved imaging data on surface streamers can provide important insight into understanding and modeling plasma-catalysis, which can accelerate the progress of research and development in this area.

In this paper, we present the investigation realized on an experimental setup that simulates an arc column subjected to the transient phase of a lightning current waveform in laboratory conditions. Optical emission spectroscopy is employed to assess space- and time-resolved properties of this high current pulsed arc. Different current peak levels are utilised in this work, ranging from 10 kA to 100 kA, with a peak time around 15 µs. Ionic lines of nitrogen and oxygen are used to determine the radial profiles of temperature and electron density of the arc channel over time from 2 µs to 36 µs. A combination of 192 N II and O II lines is considered in the calculation of the bound–bound contribution of the absorption coefficient of the plasma channel. Calculations of the optical thickness showed that self-absorption of these ionic lines in the arc column is important. To obtain temperature and electron density profiles in the arc, we solved the radiative transfer equation across the channel under an axisymmetric assumption and considering the channel formed by uniform concentric layers. For the 100 kA current peak level, the temperature reaches more than 38 000 K and the electron density reaches 5  ×  10¹⁸ cm⁻³. The pressure inside the channel is calculated using the air plasma composition at local thermodynamic equilibrium, and reaches 45 bar. The results are discussed and utilised to estimate the electrical conductivity of the arc channel.

Plasma redistribution in a symmetric microchannel-cavity hybrid structure device has been investigated by modulating the applied electric field strength. The device array has been operated in 200 Torr of argon, driven by a 20 kHz bipolar waveform. With the existence of the intervening microchannel between microcavities, several stable modes of operation of the microplasma have been observed, including cavity mode, hybrid mode and channel mode. Transition between the modes occurs with modulation of the applied voltage from 800 to 1100 V. The characteristics of microplasma propagation in different modes are investigated and the propagation speed along diagonal direction of the device in cavity mode, hybrid and channel mode are calculated to be ~48, ~29 and ~32 km s⁻¹, respectively. Nonhomogeneous electric field strength distribution and plasma interaction have been discussed to explain these experimental results. Emission intensity and propagation speed differences in the cavity mode between the polarities of the applied voltage are interpreted through spatially resolved measurements of the emission profile in a partial channel-cavity array.

This paper reports excellent electrical properties in polypropylene grafted with maleic anhydride (PP-g-MAH) and a related mechanism of the enhanced electrical properties. The chemical structure of PP-g-MAH was analyzed and its effect on space charge accumulation, electrical breakdown strength and DC conductivity was studied. Compared with pure PP, the PP-g-MAH exhibits remarkably suppressed space charge accumulation, enhanced electrical breakdown strength and reduced conduction current. The mechanism enhancing the electrical properties was studied by measuring the trap level distribution. It can be explained that abundant deep traps are introduced in PP-g-MAH with the introduction of polar groups in MAH, which reduces the charge mobility and raises the charge injection barrier so as to suppress space charge accumulation. This investigation would contribute to propose a new material modification strategy for designing high-voltage direct current insulation material in addition to the inclusion of nanoparticles.

Although thermal cloak has been studied extensively, the specific discussions on the proper experimental conditions to successfully observe the thermal cloaking effect are lacking. In this study, we focus on exploring the proper experimental conditions for 2D thermal cloaking demonstration. A mathematical model is established and detailed discussions are presented based on the model. The proper experimental conditions are suggested and verified with finite element simulations.

In this paper, we directly visualized the charges distributed in one-atom-thick reduced graphene oxide (rGO) sheet induced by adjacent charged rGO using a sample-charged mode scanning polarization force microscopy. We found that electron carriers could be attracted to one side of the rGO sheet and leave holes on the other side. The induced charges were distributed inhomogeneously; that is, contrary to earlier reports, the free carrier concentration was neither distributed on the ends, nor distributed uniformly on the whole rGO sheet. When the surrounding rGO sheets were injected with electrostatic charges, the motion of the charge carriers happened in the target-neutral rGO sheet simultaneously. The charges induced in the rGO sheet by isolated charges on adjacent rGO sheets decayed rapidly with the increasing of their separated distance. In addition, fine control of the distribution of the induced charges in a single rGO sheet could be realized through placing more isolated charges in the surrounding areas. These findings suggest a feasible and precise strategy for the modulation and design of local-charge-sensitive functional graphene-based systems.

A simplified and novel theoretical model for coplanar interdigital electroadhesives has been presented in this paper. The model has been verified based on a mechatronic and reconfigurable testing platform, and a repeatable testing procedure. The theoretical results have shown that, for interdigital electroadhesive pads to achieve the maximum electroadhesive forces on non-conductive substrates, there is an optimum electrode width/space between electrodes (width/space) ratio, approximately 1.8. On conductive substrates, however, the width/space ratio should be as large as possible. The 2D electrostatic simulation results have shown that, the optimum ratio is significantly affected by the existence of the air gap and substrate thickness variation. A novel analysis of the force between the electroadhesive pad and the substrate has highlighted the inappropriateness to derive the normal forces by the division of the measured shear forces and the friction coefficients. In addition, the electroadhesive forces obtained in a 5 d period in an ambient environment have highlighted the importance of controlling the environment when testing the pads to validate the models. Based on the confident experimental platform and procedure, the results obtained have validated the theoretical results. The results are useful insights for the investigation into environmentally stable and optimized electroadhesives.

We used ab initio molecular orbital (MO) calculations to study the differences in the intermediate structures and the electronic states involved in the adsorption of O₂ onto 13-atom metal clusters of Pt, Cu, and Au. Additionally, the conditions required for the electrocatalytic oxygen reduction reaction (ORR) on the Pt, Cu, and Au clusters were investigated and discussed. The intermediates involved in O₂ adsorption onto Pt, Cu, and Au were found to be (Pt–O)–(Pt–O), Cu–O, and Au–O₂, respectively. The differences in the O₂ adsorption intermediates is explained on the basis of our analysis of the projected density of state (PDOS) area of the new MOs produced from a mixture of the 2pπ^* orbitals of O₂ and the d orbitals of the metal clusters. The formation of the (Pt–O)–(Pt–O) intermediate after the adsorption of O₂ onto the Pt cluster is attributed to the emergence of an antibonding orbital above the Fermi level. Thus, this electronic state can lead to the decomposition and desorption of O₂ molecules, thereby promoting the high-activity level of ORR. For the Cu cluster, a new antibonding orbital was observed below the Fermi level. Moreover, the Cu cluster surface can only promote O₂ decomposition and not O₂ desorption due to the formation of copper oxides. For the Au cluster, no new MOs related to 2pπ^* orbitals of O₂ appeared because O₂ was molecularly adsorbed, implying that the Au cluster is an inefficient ORR catalyst.

With the great demand in the applications of flexible electronics, the methods leading to improvements in the electrical and mechanical performance have been widely investigated. In this work, we firstly prepared a hybrid composite ink using Ag nanoparticles and graphene. Then, a hot-press sintering process was deployed to obtain the desired electrical tracks which could be applied in flexible electronics. We have systematically investigated the effects of sintering time, pressure and temperature, as well as the different percentage of weight (wt%) of graphene for the electrical and mechanical performance of sintered electrical tracks. We achieved reasonably low electrical resistivity at low sintering temperature (120 °C). Specifically, the resistivity reaches 6.19  ×  10⁻⁸ Ω · m which is just 3.87 times higher than the value of bulk silver. Additionally, the prepared hybrid composite ink obtained better electrical reliability against bending test comparing with Ag nanoparticle ink. Finally, the optimal wt% of graphene and potential effect to the electrical and mechanical performance were also investigated.