Optical complex materials offer unprecedented opportunity to engineer fundamental band dispersion, which enables novel optoelectronic functionality and devices. Exploration of the photonic Dirac cone at the center of momentum space has inspired an exceptional characteristic of zero index, which is similar to zero effective mass in Fermionic Dirac systems. Such all-dielectric zero-index photonic crystals provide an in-plane mechanism such that the energy of the propagating waves can be well confined along the chip direction. A straightforward example is to achieve the anomalous focusing effect without longitudinal spherical aberration when the size of the zero-index lens is large enough. Here, we designed and fabricated a prototype of a zero-refractive-index lens by using a large-area silicon nanopillar array with a plane-concave profile. The near-zero refractive index was quantitatively measured near 1550 nm through the anomalous focusing effect, predictable by effective medium theory. The zero-index lens was also demonstrated to have ultralow longitudinal spherical aberration. Such an integrated-circuit-compatible device provides a new route to integrate all-silicon zero-index materials into optical communication, sensing, and modulation and to study fundamental physics in the emergent fields of topological photonics and valley photonics.Keywords: low aberration; metamaterials; photonic crystal; silicon photonics; zero index;

A featured “vapor transportation” assembly technique was developed to attain layer-by-layer stacking continuous graphene oxide (GO) films on both flat and concavo-concave surfaces. Few-layer (layer number < 10) GO sheets were “evaporated” (carried by water vapor) from the water-dispersed GO suspension and smoothly/uniformly tiled on the substrate surface. We have found evidence of the influence of the deposition time and substrate–liquid separation on the film thickness. A model was proposed for interpreting the assembly process. It was found that a current conditioning would induce a reduction of the GO surface and form an Ohmic contact between the GO–metal interfaces. Accordingly, an actively modulated GO cold cathode was fabricated by locally depositing continuous GO sheets on the drain electrode of a metal-oxide-semiconductor field effect transistor (MOSFET). The field emission current of the GO cathode can be precisely controlled by the MOSFET gate voltage (VGS). A current modulation range from 1 × 10−10 A to 6.9 × 10−6 A (4 orders of magnitude) was achieved by tuning the VGS from 0.812 V to 1.728 V. Due to the self-acting positive feedback of the MOSFET, the emission current fluctuation was dramatically reduced from 57.4% (non-control) to 3.4% (controlled). Furthermore, the integrated GO cathode was employed for a lab-prototype display pixel application demonstrating the active modulation of the phosphor luminance, i.e. from 0.01 cd m−2 to 34.18 cd m−2.

The laser-induced doping and fine patterning of luminescent ZnS nanospheres is reported. Monodispersive ZnS nanospheres were synthesized in a large scale (gram-quantity) by a low temperature (70 °C) aqueous method following a self-aggregation and size-limiting process. The nanospheres were in a narrow (60–80 nm) diameter distribution and were formed by aggregated nanocrystals (∼4 nm in diameter). Laser-induced doping to obtain luminescent patterns was realized by performing a laser irradiation (wavelength: 1064 nm) on the films of nanospheres mixed with dopant precursors (i.e. MnCl2 or AlCl3). Patterns on both the flat and highly curved substrates were fabricated. Fine luminescent microarrays with sizes of ∼15 μm were fabricated by further performing a laser-induced “peel-off” on the doped films. Single-element (Mn) doping and dual-element (Al–Cl) co-doping were demonstrated. Strong photoluminescence spectra centered at ∼590 nm and ∼513 nm were attained from the Mn doped and Al–Cl co-doped patterns, respectively, while the corresponding cathodoluminescence revealed a slight blue shift. The formation of a donor–acceptor pair was proposed to interpret the luminescence intensity enhancement of the featured Al–Cl co-doped ZnS. This work provides an approach to produce fine luminescent patterns for application in high resolution displays, multi-color light emitting diodes and X-ray imaging.

Precisely-controlled fabrication of single ZnO nanoemitter arrays and their possible application in low energy parallel electron beam exposure are reported. A well defined polymethyl methacrylate (PMMA) nanohole template was employed for local solution-phase growth of single ZnO nanoemitter arrays. Chlorine plasma etching for surface smoothing and pulsed-laser illumination in nitrogen for nitrogen doping were performed, which can significantly enhance the electron emission and improve the emitter-to-emitter uniformity in performance. Mechanisms responsible for the field emission enhancing effect are proposed. Low voltage (368 V) e-beam exposure was performed by using a ZnO nanoemitter array and a periodical hole pattern (0.72–1.26 μm in diameter) was produced on a thin (25 nm) PMMA. The work demonstrates the feasibility of utilizing single ZnO nano-field emitter arrays for low voltage parallel electron beam lithography.

We report the effect of carbon–oxygen atomic ratio (C/O ratio) on the field emission properties of the chemically reduced few-layer graphite oxide (GO). The field emission properties are found to be a non-monotonic function of the C/O ratio in a wide range of 2.06–14.80. Samples with C/O ratio of 6.98 show the lowest turn-on (1.80 MV/m), threshold fields (5.15 MV/m) and much higher current density (44.08 mA/cm2 at 9.00 MV/m). Long-time (10 h) current stability test of the GO at a high current density (∼13 mA/cm2) resulted in the reduction of the GO. The samples with field emission induced reduction show the same non-monotonic effect of the C/O ratio on the field emission properties as that of the chemically reduced GOs. The average current fluctuation of the GO is higher than that of the reduced GO, which is due to the oxygen desorption during the electron emission. The effect of the carbon–oxygen bonds on the surface potential barrier of the reduced GO edges is proposed in detail for interpreting the experimental observations.

Due to its difficulty, experimental measurement of field emission from a single-layer graphene has not been reported, although field emission from a two-dimensional (2D) regime has been an attractive topic. The open surface and sharp edge of graphene are beneficial for field electron emission. A 2D geometrical effect, such as massless Dirac fermion, can lead to new mechanisms in field emission. Here, we report our findings from in situ field electron emission characterization on an individual singe-layer graphene and the understanding of the related mechanism. The measurement of field emission from the edges was done using a microanode probe equipped in a scanning electron microscope. We show that repeatable stable field emission current can be obtained after a careful conditioning process. This enables us to examine experimentally the typical features of the field emission from a 2D regime. We plot current versus applied field data, respectively, in ln(I/E3/2) ∼ 1/E and ln(I/E3) ∼ 1/E2 coordinates, which have recently been proposed for field emission from graphene in high- and low-field regimes. It is observed that the plots all exhibit an upward bending feature, revealing that the field emission processes undergo from a low- to high-field transition. We discuss with theoretical analysis the physical mechanism responsible for the new phenomena.Keywords: 2D regime; field emission; line current density; microanode probe; single-layer graphene; upward bending FN plot

We report on how to tune the density of zinc oxide (ZnO) nanowire arrays in a wide range (more than 5 orders of magnitude) in a low temperature (80 °C) solution-phase growth process. A model based on the coexistence of nanowire growth and etching of the ultrathin (<3.5 nm) ZnO seed-layer was proposed to explain the effect. It was demonstrated that when the seed-layer thickness changes from 1.5 to 3.5 nm, the nanowire density increases from 6.8 × 104 to 2.6 × 1010 cm−2. This significant variation of density with the seed-layer thickness was found to happen only when the layer thickness is within a few nanometers. If it is too thick (>3.5 nm), the variation is very narrow, and if it is too thin (<1.5 nm), no nanowires can grow. In addition, the density variation was accompanied by the change of both diameter and height of the nanowires, which leads to a change in aspect ratio. Both changes in density and aspect ratio were found to obviously affect the field emission characteristics. It was demonstrated that optimal conditions can be found to grow ZnO nanowire films with better field emission characteristics.

Three-dimensional (3D) six-fold symmetry ZnO sub-microstructures were prepared by using a low temperature (80 °C) solution method. The 3D structure has unique configuration, i.e., a support core with six symmetric branches located on its middle. It was found that the structure can only be produced in a critical basicity condition, i.e., pH=11.8±0.2. The formation mechanism of the 3D structure involves the enhancing effect on the growth of ZnO [0 0 0 1] direction and the secondary nucleation process. The 3D sub-microstructure is likely to be candidate as building block for nano-electromechanical system (NEMS) and nanomachines, i.e., act as a gear.

The laser-induced doping and fine patterning of luminescent ZnS nanospheres is reported. Monodispersive ZnS nanospheres were synthesized in a large scale (gram-quantity) by a low temperature (70 °C) aqueous method following a self-aggregation and size-limiting process. The nanospheres were in a narrow (60–80 nm) diameter distribution and were formed by aggregated nanocrystals (∼4 nm in diameter). Laser-induced doping to obtain luminescent patterns was realized by performing a laser irradiation (wavelength: 1064 nm) on the films of nanospheres mixed with dopant precursors (i.e. MnCl2 or AlCl3). Patterns on both the flat and highly curved substrates were fabricated. Fine luminescent microarrays with sizes of ∼15 μm were fabricated by further performing a laser-induced “peel-off” on the doped films. Single-element (Mn) doping and dual-element (Al–Cl) co-doping were demonstrated. Strong photoluminescence spectra centered at ∼590 nm and ∼513 nm were attained from the Mn doped and Al–Cl co-doped patterns, respectively, while the corresponding cathodoluminescence revealed a slight blue shift. The formation of a donor–acceptor pair was proposed to interpret the luminescence intensity enhancement of the featured Al–Cl co-doped ZnS. This work provides an approach to produce fine luminescent patterns for application in high resolution displays, multi-color light emitting diodes and X-ray imaging.