Ziao Tian

Deterministic Self-Rolling of Ultrathin Nanocrystalline Diamond Nanomembranes for 3D Tubular/Helical Architecture

Nanocrystalline diamond nanomembranes with thinning-reduced flexural rigidities can be shaped into various 3D mesostructures, such as tubes, jagged ribbons, nested tubes, helices, and nested rings. Microscale helical diamond architectures are formed by controlled debonding in agreement with finite-element simulation results. Rolled-up diamond tubular microcavities exhibit pronounced defect-related photoluminescence with whispering-gallery-mode resonance.

Nanogranular SiO2 proton gated silicon layer transistor mimicking biological synapses

Silicon on insulator (SOI)-based transistors gated by nanogranular SiO2 proton conducting electrolytes were fabricated to mimic synapse behaviors. This SOI-based device has both top proton gate and bottom buried oxide gate. Electrical transfer properties of top proton gate show hysteresis curves different from those of bottom gate, and therefore, excitatory post-synaptic current and paired pulse facilitation (PPF) behavior of biological synapses are mimicked. Moreover, we noticed that PPF index can be effectively tuned by the spike interval applied on the top proton gate. Synaptic behaviors and functions, like short-term memory, and its properties are also experimentally demonstrated in our device. Such SOI-based electronic synapses are promising for building neuromorphic systems.

Plasmon-Matter Interactions in Optoelectronic Metamaterials with Negative Refractive Index

Optoelectronic metamaterials composed of nanoscale metallic structures and semiconductor quantum structures constitute a powerful platform to explore light-matter interaction and new devices. In this work, we numerically study an optoelectronically coupled metamaterial consisting of metallic double fishnet (DF) layers and semiconductor quantum well (QW) spacing layer. When the electronic intersubband transition (ISBT) in the QW coincides with the plasmonic resonances of the DF structure, the plasmon-matter interaction (PMI) can modify the optical properties considerably. In case of the ISBT-matching localized surface plasmons (LSP), i.e., fQWu2009=u2009fLSP, the polarization-selection-rule forbidden ISBT absorption can be enabled due to the nonnegligible Ezu2009field distributions while the retrieved optical constants remain almost unchanged. However, when the gap surface plasmons (GSP) are matched, i.e., fQWu2009=u2009fGSP, PMI exhibits a clear anti-crossing behavior implying strong coupling effects between ISBT and GSP resonance and formation of intersubband polaritons. The effective optical constants are therefore modulated appreciably. The large difference between GSP and LSP can be attributed to their distinctive resonance qualities (Q-factors) and polarization conversion ratios (99.28xa0% for GSP and 1.54xa0% for LSP) from the transverse electric (TE) to transverse magnetic (TM) mode. Our results provide insight into the physical mechanism of PMI in nanoscale semiconductor-plasmon hybrid systems and suggest an alternative means in tunable negative refractive index (NRI) applications.

Stimuli-responsive and on-chip nanomembrane micro-rolls for enhanced macroscopic visual hydrogen detection

Palladium nanomembranes roll into microscale actuators and their active array responses upon hydrogen stimuli within seconds. Nanomembrane rolling offers advanced three-dimensional (3D) mesostructures in electronics, optics, and biomedical applications. We demonstrate a high-density and on-chip array of rolled-up nanomembrane actuators with stimuli-responsive function based on the volume expansion of palladium in hydrogen milieu. The uniform stimuli-responsive behavior of high-density nanomembrane rolls leads to huge macroscopic visual detection with more than 50% transmittance change under optimization of micropattern design. The reversible shape changing between rolled and flat (unrolled) statuses can be well explained on the basis of the elastic mechanical model. The strain change in the palladium layer during hydrogen absorption and desorption produces a marked change in the diameter of nanomembrane rolls. We found that a functional palladium layer established an external compressive strain after hydrogen stimuli and thus also reduced the rolls’ diameters. The large area of the nanomembrane roll array performs excellent nonelectrical hydrogen detection, with response and recovery speeds within seconds. Our work suggests a new strategy to integrate high-density 3D mesoscale architectures into functional devices and systems.

Reconfigurable Vanadium Dioxide Nanomembranes and Microtubes with Controllable Phase Transition Temperatures

Two additional structural forms, free-standing nanomembranes and microtubes, are reported and added to the vanadium dioxide (VO2) material family. Free-standing VO2 nanomembranes were fabricated by precisely thinning as-grown VO2 thin films and etching away the sacrificial layer underneath. VO2 microtubes with a range of controllable diameters were rolled-up from the VO2 nanomembranes. When a VO2 nanomembrane is rolled-up into a microtubular structure, a significant compressive strain is generated and accommodated therein, which decreases the phase transition temperature of the VO2 material. The magnitude of the compressive strain is determined by the curvature of the VO2 microtube, which can be rationally and accurately designed by controlling the tube diameter during the rolling-up fabrication process. The VO2 microtube rolling-up process presents a novel way to controllably tune the phase transition temperature of VO2 materials over a wide range toward practical applications. Furthermore, the rolling-up process is reversible. A VO2 microtube can be transformed back into a nanomembrane by introducing an external strain. Because of its tunable phase transition temperature and reversible shape transformation, the VO2 nanomembrane-microtube structure is promising for device applications. As an example application, a tubular microactuator device with low driving energy but large displacement is demonstrated at various triggering temperatures.

Anisotropic Rolling and Controlled Chirality of Nanocrystalline Diamond Nanomembranes toward Biomimetic Helical Frameworks

Future advances in materials will be aided by improved dimensional control in fabrication of 3D hierarchical structures. Self-rolling technology provides additional degrees of freedom in 3D design by enabling an arbitrary rolling direction with controllable curvature. Here, we demonstrate that deterministic helical structures with variable rolling directions can be formed through releasing a strained nanomembrane patterned in a utility knife shape. The asymmetry of the membrane shape provides anisotropic driving force generated by the disparity between the etching rates along different sides in this asymmetric shape. A transient finite element method (FEM) model of diagonal rolling is established to analyze the relationships among geometries, elastic properties, and boundary conditions. On the basis of this model, a diamond-based helical framework consisting of two or three helical segments has been fabricated to mimic the shapes of natural plants. Further experiment has been done to extend this approach to other materials and material combinations, such as MoSe2/Cr, Cr/Pt, and VO2. To demonstrate the possible application accessible by our technology to new fields, VO2-based helical microscale actuation has been demonstrated with photocontrollable bending in a selected region, as well as morphable and recognizable helix. This study offers a new way to construct helical mesostructures that combine special properties of the advanced materials, thus possess novel features and potential applications.

Artificial neuron synapse transistor based on silicon nanomembrane on plastic substrate

Silicon nanomembrane (SiNM) transistors gated by chitosan membrane were fabricated on plastic substrate to mimic synapse behaviors. The device has both a bottom proton gate (BG) and multiple side gates (SG). Electrical transfer properties of BG show hysteresis curves different from those of typical SiO 2 gate dielectric. Synaptic behaviors and functions by linear accumulation and release of protons have been mimicked on this device:excitatory post-synaptic current (EPSC) and paired pulse facilitation behavior of biological synapses were mimicked and the paired-pulse facilitation index could be effectively tuned by the spike interval applied on the BG. Synaptic behaviors and functions, including short-term memory and long-term memory, were also experimentally demonstrated in BG mode. Meanwhile, spiking logic operation and logic modulation were realized in SG mode.

The Effect of Substrate Temperatures on CdS Film Prepared by Thermal Evaporation

A suitable deposition method of CdS is necessary for the high performance CIGS(Cu(In0.7Ga0.3)Se2.2) solar cells. In this paper, CdS films were deposited onto glass substrates at the substrate temperture of 50°C、100°C、150°C by thermal evaporation, the effect of the temperature were presented. CdS film deposited at substrate temperture of 150°C was annealed at 150°C for 30min. All films were characterized for their morphology, structure and optical property using scanning electron microscope(SEM), X-ray diffractometer(XRD) and UV–VIS–IR transmittance respectively.The quantum efficiency of the fabricated solar cells with annealed CdS buffer layer was also enhanced at short wavelength. This new method leads to the improved performance of CIGS solar cells and also simplify the whole fabrication technology.

Infrared tubular microcavity based on rolled-up GeSn/Ge nanomembranes

Germanium-Tin (GeSn) alloys have attracted great amounts of attention as these group IV semiconductors present direct band-gap behavior with high Sn content and are compatible with current complementary metal oxide semiconductor technology. In this work, three dimensional tubular GeSn/Ge micro-resonators with a diameter of around 7.3 μm were demonstrated by rolling up GeSn nanomembranes (NM) grown on a Ge-on-insulator wafer via molecular beam epitaxy. The microstructural properties of the resonators were carefully investigated and the strain distributions of the rolled-up GeSn/Ge microcavities along the radial direction were studied by utilizing micro-Raman spectroscopy with different excitation laser wavelengths. The values of the strains calculated from Raman shifts agree well with the theoretical prediction. Coupled with fiber tapers, as-fabricated devices present a high quality factor of up to 800 in the transmission spectral measurements. The micro-resonators fabricated via rolled-up nanotechnology and GeSn/Ge NMs in this work may have great potential in photonic micro- and nanodevices.

Asymmetrically Curved Hyperbolic Metamaterial Structure with Gradient Thicknesses for Enhanced Directional Spontaneous Emission

We demonstrate hyperbolic metamaterials (HMMs) on a curved surface for an efficient outcoupling of nonradiative modes, which lead to an enhanced spontaneous emission. Those high-wavevector plasmonic modes can propagate along the curved structure and emit into the far field, realizing a directional light emission with maximal fluorescent intensity. Detailed simulations disclose a high Purcell factor and a spatial power distribution in the curved HMM, which agrees with the experimental result. Our work presents remarkable enhancing capability in both the Purcell factor and emission intensity, which could suggest a unique structure design in metamaterials for potential application in, e.g., high-speed optical sensing and communications.