Yuqing Cheng

Sub-10 nm low current resistive switching behavior in hafnium oxide stack

In this letter, a tip-induced cell relying on the conductive atomic force microscope is proposed. It is verified as a referable replica of an integrated resistive random access memory (RRAM) device. On the basis of this cell, the functionality of sub-10 nm resistive switching is confirmed in hafnium oxide stack. Moreover, the low current switching behavior in the sub-10 nm dimension is found to be more pronounced than that of a 50 × 50 nm2 device. It shows better ON/OFF ratio and low leakage current. The enhanced memory performance is ascribed to a change in the shape of the conductive filament as the device dimensions are reduced to sub-10 nm. Therefore, device downscaling provides a promising approach for the resistance optimization that benefits the RRAM array design.

Strongly enhanced Raman scattering of graphene by a single gold nanorod

Individual gold nanorods (AuNRs) and monolayer graphene hybrid system is investigated experimentally. Surface enhanced Raman scattering (SERS) signal of the graphene is observed due to a single AuNR with enhancement factor up to ∼1000-fold. The SERS intensity is strongly polarization dependent and the enhancement effect varies with the detuning between the excitation laser and the AuNR resonance. The SERS effect is highest when the resonant wavelength of the AuNRs matches well with the excitation light. By correlating the scattering and photoluminescence, it is demonstrated that the conventional background in SERS ascribes to the photon emission of metallic nanostructures.

Multimode resistive switching in nanoscale hafnium oxide stack as studied by atomic force microscopy

The nanoscale resistive switching in hafnium oxide stack is investigated by the conductive atomic force microscopy (C-AFM). The initial oxide stack is insulating and electrical stress from the C-AFM tip induces nanometric conductive filaments. Multimode resistive switching can be observed in consecutive operation cycles at one spot. The different modes are interpreted in the framework of a low defect quantum point contact theory. The model implies that the optimization of the conductive filament active region is crucial for the future application of nanoscale resistive switching devices.

A hybrid nanoantenna for highly enhanced directional spontaneous emission

Spontaneous emission modulated by a hybrid plasmonic nanoantenna has been investigated by employing finite-difference time-domain method. The hybrid nanoantenna configurations constituted by a gap hot-spot and of a plasmonic corrugated grating and a metal reflector sandwiching a SiO2 thin layer which appears promising for high spontaneous emission enhancement devices. Simulation assays show that the coupling between the gap-antenna and plasmonic corrugations reaches an ultra-high near-field enhancement factor in the excitation process. Moreover, concerning the emission process, the corrugations concentrate the far-field radiated power within a tiny angular volume, offering unprecedented collection efficiency. In the past decades, many kinds of optical antennas have been proposed and optimized to enhance single molecule detection. However, the excitation enhancement effect for single individual or dimmer plasmonic nanostructure is limited due to intrinsic nonradiative decay of the nanoparticle plasmon and ...

Surface enhanced fluorescence by metallic nano-apertures associated with stair-gratings

Metallic nano-apertures associated with stair-gratings are proposed for surface enhanced fluorescence with high excitation enhancement and narrow emission beaming effect. Fluorescence correlation spectroscopy method was utilized to analyze the fluorescence trace and fluorescence enhancement, and the angular patterns of fluorescent emission were measured with the back focal plane imaging method. The stair-grating presents a strong optical response which covering well both the excitation and the emission bands of the photoluminescence process. Such high enhancement and narrow directionality by the stair-gratings would enable the detection of single molecules with low numerical aperture objective effectively.

Nano-gap between a gold tip and nanorod for polarization dependent surface enhanced Raman scattering

We demonstrate experimentally that a nano-gap could be constructed by using a scanning probe microscope to allow a gold tip to approach a gold nanorod immobilized on a glass coverslip. The nano-gap can enhance Raman scattering of graphene sandwiched between the tip and the nanorod. The Raman intensity was strongly dependent on the incident light polarization. Here, linear, radial, azimuthal, and intermediate states between radial and azimuthal polarization were investigated and compared in detail. The maximum surface-enhanced Raman scattering effect of the nano-gap occurred for the intermediate states between the radial and azimuthal polarized light.

Plasmonic nano-resonator enhanced one-photon luminescence from single gold nanorods

Strong Stokes and anti-Stokes one-photon luminescence from single gold nanorods is measured in experiments. It is found that the intensity and polarization of the Stokes and anti-Stokes emissions are in strong correlation. Our experimental observation discovered a coherent process in light emission from single gold nanorods. We present a theoretical mode, based on the concept of cavity resonance, for consistently understanding both Stokes and anti-Stokes photoluminescence. Our theory is in good agreement of all our measurements.

Light Emission from Plasmonic Nanostructures Enhanced with Fluorescent Nanodiamonds

In the surface-enhanced fluorescence (SEF) process, it is well known that the plasmonic nanostructure can enhance the light emission of fluorescent emitters. With the help of atomic force microscopy, a hybrid system consisting of a fluorescent nanodiamond and a gold nanoparticle was assembled step-by-step for in situ optical measurements. We demonstrate that fluorescent emitters can also enhance the light emission from gold nanoparticles which is judged through the intrinsic anti-Stokes emission owing to the nanostructures. The light emission intensity, spectral shape, and lifetime of the hybrid system were dependent on the coupling configuration. The interaction between gold nanoparticles and fluorescent emitter was modelled based on the concept of a quantised optical cavity by considering the nanodiamond and the nanoparticle as a two-level energy system and a nanoresonator, respectively. The theoretical calculations reveal that the dielectric antenna effect can enhance the local field felt by the nanoparticle, which contributes more to the light emission enhancement of the nanoparticles rather than the plasmonic coupling effect. The findings reveal that the SEF is a mutually enhancing process. This suggests the hybrid system should be considered as an entity to analyse and optimise surface-enhanced spectroscopy.

Understanding photoluminescence of metal nanostructures based on an oscillator model

Scattering and absorption properties of metal nanostructures have been well understood based on the classic oscillator theory. Here, we demonstrate that photoluminescence of metal nanostructures can also be explained based on a classic model. The model shows that inelastic radiation of an oscillator resembles its resonance band after external excitation, and is related to the photoluminescence from metallic nanostructures. The understanding based on the classic oscillator model is in agreement with that predicted by a quantum electromagnetic cavity model. Moreover, by correlating a two-temperature model and the electron distributions, we demonstrate that both one-photon and two-photon luminescence of the metal nanostructures undergo the same mechanism. Furthermore, the model explains most of the emission characteristics of the metallic nanostructures, such as quantum yield, spectral shape, excitation polarization and power dependence. The model based on an oscillator provides an intuitive description of the photoluminescence process and may enable rapid optimization and exploration of the plasmonic properties.

Luminescence quantum yields of gold nanoparticles varying with excitation wavelength

Luminescence quantum yields (QYs) of gold nanoparticles including nanorods, nanobipyramids and nanospheres are measured elaborately at single nanoparticle level with different excitation wavelengths. It is found that the QYs of the nanostructures are essentially dependent on the excitation wavelength. The QY is higher when the excitation wavelength is blue-detuned and close to the nanoparticles’ surface plasmon resonant peak. A phenomenological model based on plasmonic resonator concept is proposed to understand the experimental findings. The excitation wavelength dependent of QY is attributed to the wavelength dependent coupling efficiency between the free electrons oscillation and the intrinsic plasmon resonant radiative mode. These studies should contribute to the understanding of one-photon luminescence from metallic nanostructures and plasmonic surface enhanced spectroscopy.