Xupeng Zhu

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.

Vapor-phase preparation of gold nanocrystals by chloroauric acid pyrolysis

We report that gold nanocrystals can be prepared from vapor phase using chloroauric acid (HAuCl4) as the precursor. By tuning the vapor-phase deposition parameters, the size and space distribution of the gold nanocrystals can be well controlled on substrates. Systematic control experiments demonstrate that intermediate AuCl and AuCl3 products pyrolyzed from HAuCl4 play an essential role in this vapor-phase deposition process. Compared to conventional wet-chemical synthesis process, vapor-phase process enables direct deposition of gold nanoparticles on solid substrates with better coverage and uniformity, which may find applications in surface-enhanced Raman scattering and plasmon-enhanced photocatalysis.

Surface enhanced Raman scattering of gold nanoparticles supported on copper foil with graphene as a nanometer gap

Gaps with single-nanometer dimensions (<10 nm) between metallic nanostructures enable giant local field enhancements for surface enhanced Raman scattering (SERS). Monolayer graphene is an ideal candidate to obtain a sub-nanometer gap between plasmonic nanostructures. In this work, we demonstrate a simple method to achieve a sub-nanometer gap by dewetting a gold film supported on monolayer graphene grown on copper foil. The Cu foil can serve as a low-loss plasmonically active metallic film that supports the imaginary charge oscillations, while the graphene can not only create a stable sub-nanometer gap for massive plasmonic field enhancements but also serve as a chemical enhancer. We obtained higher SERS enhancements in this graphene-gapped configuration compared to those in Au nanoparticles on Cu film or on graphene-SiO2-Si. Also, the Raman signals measured maintained their fine features and intensities over a long time period, indicating the stability of this Au-graphene-Cu hybrid configuration as an SERS substrate.

Vapor-phase preparation of single-crystalline thin gold microplates using HAuCl4 as the precursor for plasmonic applications

We report a facile vapor-phase method to synthesize single-crystalline, atomically smooth gold microplates with thickness less than 100 nm. Compared to the existing methods to synthesize gold plates, the current method uses only chloroauric acid as the precursor and thus allows us to obtain clean gold microplates without any ligands and carbon contamination. By carrying out control experiments, it is hypothesized that Cl− ions play a crucial role in the anisotropic growth of microplates through their selective adsorption on the (111) surface of gold. With their clean, single crystalline and ultrasmooth features, we demonstrate that the synthesized gold microplates may be used to define high-quality plasmonic nanostructures using focused ion beam lithography.

Plasmon Modes and Substrate-Induced Fano Dip in Gold Nano-Octahedra

In this work, we have carried out a systematic study on the optical properties of gold nano-octahedra via numerical simulations. We obtained broadening scattering spectra dominated by a hybridized bonding mode originated from the interaction between intrinsic plasmon modes when increasing the size of gold nano-octahedra in vacuum. Once placing the nano-octahedra on a high refractive index dielectric substrate, a Fano dip can be induced in the scattering spectra due to the mutual interference between a dipolar mode and a quadrupolar mode. Moreover, we found that the interference become stronger by increasing the refractive index in substrate, and the broadening scattering spectra split into two hybridized modes in a single gold nano-octahedron: the anti-bonding mode and the bonding mode. Our results for the first time give a clear description of the complex plasmonic properties of a gold nano-octahedron and provide insights in designing appropriate nano-octahedral structures for applications.

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.

Transmissive structural color filters using vertically coupled aluminum nanohole/nanodisk array with a triangular-lattice

We demonstrate a configuration to generate transmissive structural colors through triangular-lattice square nanohole arrays in aluminum (Al) film with Al nanodisks on the bottom of the nanoholes. By using a simple nanofabrication process, colors covering the entire visible light with different brightness and saturation are achieved by tuning both the period of arrays and the size of nanoholes. The optical behaviors of the structures are systematically investigated by both experimental and theoretical methods. The results indicate that the localized surface plasmon resonance of nanohole arrays plays the key role in the extraordinary transmission and meanwhile the coupling of disks and holes is also of importance for the enhanced transmission. With the wide color gamut, these kinds of vertically coupled Al nanohole/nanodisk arrays show the capabilities for high-resolution full-color printing. Compared to existing transmissive plasmonic color filters, the configuration in this work has the advantages of a simple fabrication process and using cheap aluminum materials.