Mengjie Zheng

Fabrication of single-crystal silicon nanotubes with sub-10 nm walls using cryogenic inductively coupled plasma reactive ion etching.

Single-crystal silicon nanostructures have attracted much attention in recent years due in part to their unique optical properties. In this work, we demonstrate direct fabrication of single-crystal silicon nanotubes with sub-10 nm walls which show low reflectivity. The fabrication was based on a cryogenic inductively coupled plasma reactive ion etching process using high-resolution hydrogen silsesquioxane nanostructures as the hard mask. Two main etching parameters including substrate low-frequency power and SF6/O2 flow rate ratio were investigated to determine the etching mechanism in the process. With optimized etching parameters, high-aspect-ratio silicon nanotubes with smooth and vertical sub-10 nm walls were fabricated. Compared to commonly-used antireflection silicon nanopillars with the same feature size, the densely packed silicon nanotubes possessed a lower reflectivity, implying possible potential applications of silicon nanotubes in photovoltaics.

Optimization of the particle density to maximize the SERS enhancement factor of periodic plasmonic nanostructure array

Low-cost surface-enhanced Raman scattering (SERS) substrate with the largest possible enhancement factor is highly desirable for SERS-based sensing applications. In this work, we systematically investigated how the density of plasmonic nanostructures affects the intensity of SERS signal. By directly depositing of metallic layer on electron-beam-lithography defined dielectric nanoposts, plasmonic structures array with different densities were reliably fabricated for SERS measurements. Two main experimental phenomena were obtained: (1) the SERS intensity did not increase monotonically when increasing the density of plasmonic structures, and (2) these ultra-dense plasmonic structures resulted in the maximal SERS intensity. These results could be well explained based on finite-difference time domain (FDTD) simulations and provide robust experimental evidences to guide the design of the best possible SERS substrate.

Sensitive SERS detection at the single-particle level based on nanometer-separated mushroom-shaped plasmonic dimers

Elevated metallic nanostructures with nanogaps (<10 nm) possess advantages for surface enhanced Raman scattering (SERS) via the synergic effects of nanogaps and efficient decoupling from the substrate through an elevated three-dimensional (3D) design. In this work, we demonstrate a pattern-transfer-free process to reliably define elevated nanometer-separated mushroom-shaped dimers directly from 3D resist patterns based on the gap-narrowing effect during the metallic film deposition. By controlling the initial size of nanogaps in resist structures and the following deposited film thickness, metallic nanogaps could be tuned at the sub-10 nm scale with single-digit nanometer precision. Both experimental and simulated results revealed that gold dimer on mushroom-shaped pillars have the capability to achieve higher SERS enhancement factor comparing to those plasmonic dimers on cylindrical pillars or on a common SiO2/Si substrate, implying that the nanometer-gapped elevated dimer is an ideal platform to achieve the highest possible field enhancement for various plasmonic applications.

Low-voltage-exposure-enabled hydrogen silsesquioxane bilayer-like process for three-dimensional nanofabrication.

We report a bilayer-like electron-beam lithographic process to obtain three-dimensional (3D) nanostructures by using only a single hydrogen silsesquioxane (HSQ) resist layer. The process utilizes the short penetration depth of low-energy (1.5 keV) electron irradiation to first obtain a partially cross-linked HSQ top layer and then uses a high-voltage electron beam (30 keV) to obtain self-aligned undercut (e.g. mushroom-shaped) and freestanding HSQ nanostructures. Based on the well-defined 3D resist patterns, 3D metallic nanostructures were directly fabricated with high fidelity by just depositing a metallic layer. As an example, Ag-coated mushroom-shaped nanostructures were fabricated, which showed lower plasmon resonance damping compared to their planar counterparts. In addition, the undercut 3D nanostructures also enable more reliable lift-off in comparison with the planar nanostructures, with which high-quality silver nanohole arrays were fabricated which show distinct and extraordinary optical transmission in the visible range.

Sensitive Surface-Enhanced Raman Scattering Detection Using On-Demand Postassembled Particle-on-Film Structure

Highly sensitive and low-cost surface-enhanced Raman scattering (SERS) substrates are essential for practical applications of SERS. In this work, we report an extremely simple but effective approach to achieve sensitive SERS detection of molecules (down to 10-10 M) by using a particle/molecule/film sandwich configuration. Compared to conventional SERS substrates which are preprepared to absorb analyte molecules for detection, the proposed sandwich configuration is achieved by postassembling a flexible transparent gel tape embedded with plasmonic nanoparticles onto an Au film decorated with to-be-detected analyte molecules. In such a configuration, the individual plasmonic gel tape and Au film have low or no SERS activity but the final assembled sandwich structure shows strong SERS signal due to the formation of numerous hot spots at the particle-film interface, where the analyte molecules themselves serve as both spacer and signal probes. Because of its simple configuration, we demonstrate that the proposed SERS substrate can be obtained over a large area with extremely low cost. Particularly, because of the on-demand nature and the flexibility, such a postassembly strategy provides an ideal solution to detect the pesticide residue on fruit surfaces with significantly enhanced sensitivity.

Stepwise-Nanocavity-Assisted Transmissive Color Filter Array Microprints

Visible-light color filters using patterned nanostructures have attracted much interest due to their various advantages such as compactness, enhanced stability, and environmental friendliness compared with traditional pigment or dye-based optical filters. While most existing studies are based on planar nanostructures with lateral variation in size, shape, and arrangement, the vertical dimension of structures is a long-ignored degree of freedom for the structural colors. Herein, we demonstrate a synthetic platform for transmissive color filter array by coordinated manipulations between height-varying nanocavities and their lateral filling fractions. The thickness variation of those nanocavities has been fully deployed as an alternative degree of freedom, yielding vivid colors with wide gamut and excellent saturation. Experimental results show that the color-rendering capability of the pixelated nanocavities can be still retained as pixels are miniaturized to 500 nm. Crosstalk between closely spaced pixels of a Bayer color filter arrangement was calculated, showing minimal crosstalk for 1 µm2 square subpixels. Our work provides an approach to designing and fabricating ultracompact color filter arrays for various potential applications including stained-glass microprints, microspectrometers, and high-resolution image sensing systems.

Polarization-dependent scattering properties of single-crystalline silicon nanocylindroids

Silicon nanostructures have been attracting increasing attention as nanoscale Mie scatters for various applications due to the subwavelength light concentration capability endowed by its high refractive index and the fabrication compatibility with the chip manufacturing processes. In this work, we investigate the polarization-dependent scattering properties of lithographic single-crystalline silicon nanocylindroids at the visible range. Both simulated and experimental studies were carried out to reveal the electric and magnetic resonance modes that occur in the silicon nanocylindroids. Systematic control experiments were conducted to demonstrate the polarization and size dependence of the resonance-induced scattering peaks. The unique anisotropic optical property of lithographically fabricated Si nanostructures at the single particle resolution provides an extra freedom to design silicon-based optical elements at the visible range for enhanced light-matter interactions.