Xingzhan Wei

The surface plasmon modes of self-assembled gold nanocrystals

The three-dimensional (3D) self-assembly of nanocrystals constitutes one of the most important challenges in materials science. A key milestone is the synthesis of simple, regular structures, such as platonic solids, composed of nanocrystal building blocks. Such objects are predicted to have unique optical and electronic properties such as polarization-independent light-scattering and intense local fields. Here we present a two-stage process for fabricating well-defined and highly symmetric, 3D gold nanocrystal structures, including tetrahedra, 3D pentamers and 3D hexamers. Polarized scattering spectra are used to elucidate the plasmon modes present in each structure, and these are compared with computational models. We conclude that self-assembly of highly symmetric, polarization-independent structures with interparticle spacings of order 0.5 nm can now be fabricated. Drastically, enhanced local fields, 1000 times higher than the incident field strength, are produced within the interstices. Fano resonances are generated if the symmetry is broken.

Spring constant calibration of atomic force microscope cantilevers of arbitrary shape

The spring constant of an atomic force microscope cantilever is often needed for quantitative measurements. The calibration method of Sader et al. [Rev. Sci. Instrum. 70, 3967 (1999)] for a rectangular cantilever requires measurement of the resonant frequency and quality factor in fluid (typically air), and knowledge of its plan view dimensions. This intrinsically uses the hydrodynamic function for a cantilever of rectangular plan view geometry. Here, we present hydrodynamic functions for a series of irregular and non-rectangular atomic force microscope cantilevers that are commonly used in practice. Cantilever geometries of arrow shape, small aspect ratio rectangular, quasi-rectangular, irregular rectangular, non-ideal trapezoidal cross sections, and V-shape are all studied. This enables the spring constants of all these cantilevers to be accurately and routinely determined through measurement of their resonant frequency and quality factor in fluid (such as air). An approximate formulation of the hydrodynamic function for microcantilevers of arbitrary geometry is also proposed. Implementation of the method and its performance in the presence of uncertainties and non-idealities is discussed, together with conversion factors for the static and dynamic spring constants of these cantilevers. These results are expected to be of particular value to the design and application of micro- and nanomechanical systems in general.

Surface plasmon coupling in end-to-end linked gold nanorod dimers and trimers

Colloidal gold nanorods were aligned end-to-end via dithiol coupling. The scattering properties of the resultant nanostructures were investigated at the single particle level by combining dark-field microscopy and high resolution scanning electron microscopy. The longitudinal surface plasmon resonance of end-to-end coupled Au nanorods exhibited a red-shift as the number of rods in the chain increased. The nanostructures exhibited polarization-dependent optical properties, due to selective excitation of collective bonding and anti-bonding modes. The surface plasmon peak energy was not strongly dependent on the angle of rod-sphere-rod trimers. The experimental scattering spectra were compared with the results obtained from theoretical calculations using the Finite Element Method (FEM) and found to be in good agreement.

2D/3D perovskite hybrids as moisture-tolerant and efficient light absorbers for solar cells

The lifetime and power conversion efficiency are the key issues for the commercialization of perovskite solar cells (PSCs). In this paper, the development of 2D/3D perovskite hybrids (CA2PbI4/MAPbIxCl3-x) was firstly demonstrated to be a reliable method to combine their advantages, and provided a new concept for achieving both stable and efficient PSCs through the hybridization of perovskites. 2D/3D perovskite hybrids afforded significantly-improved moisture stability of films and devices without encapsulation in a high humidity of 63 ± 5%, as compared with the 3D perovskite (MAPbIxCl3-x). The 2D/3D perovskite-hybrid film did not undergo any degradation after 40 days, while the 3D perovskite decomposed completely under the same conditions after 8 days. The 2D/3D perovskite-hybrid device maintained 54% of the original efficiency after 220 hours, whereas the 3D perovskite device lost all the efficiency within only 50 hours. Moreover, the 2D/3D perovskite hybrid achieved comparable device performances (PCE: 13.86%) to the 3D perovskite (PCE: 13.12%) after the optimization of device fabrication conditions.

A high refractive index metamaterial at visible frequencies formed by stacked cut-wire plasmonic structures

A type of metamaterial composing of stacked cut-wire plasmonic structures has been designed, fabricated, and characterized. The excitation of electric resonance caused by the surface plasmons effect dominates the electric field states, so that the effective refractive index can behave as a dramatic increase at visible frequencies, which is intimately associated with the resonance strength and can be modulated by reasonably changing the structure geometries. The phenomenon has been demonstrated with the measured transmittances being in agreement with simulation results. And the effective parameters judged by Kramers–Kronig relations have been uniquely retrieved from the simulated transmission and reflection data.

Localized surface plasmon nanolithography with ultrahigh resolution

A localized surface plasmon nanolithography (LSPN) technique is proposed and demonstrated to produce patterns with a sub-20nm line width. High transmission efficiency is realized by adjusting the period of grating. The well-regulated grating structures in metallic mask are employed to excite surface plasmon polaritons (SPPs) on the illuminated side. The SPP waves propagate toward the tip along the taper surfaces which cause most of energy accumulation at the tip and give rise to high local field enhancements in a near-field region around the tip. The amplitude of local electric field intensity is quite large and the line width can be confined within sub-20nm, at the same time, the contrast and spatial resolution are greatly enhanced, which can facilitate nanolithography efficiently with simple ultraviolet light sources.

Spectroelectrochemistry of Silver Deposition on Single Gold Nanocrystals.

We report the electrodeposition of metallic silver onto gold nanostars adsorbed to ITO electrodes. The electrochemical process was studied at the single particle level by correlated in situ dark field spectroscopy and scanning electron microscopy (SEM). Underpotential deposition avoids bulk silver formation on the ITO substrates. SEM proves that deposition occurs on all surfaces of the gold nanostars when polyvinylpyrrolidone (PVP) is stabilizing the nanostars or preferentially at the nanostar tips when the ligand is removed. The surface plasmon resonance blue-shifts by more than 100 nm following the formation of a 5 nm Ag film on PVP stabilized gold nanostars, moving the scattered color from the near-infrared to red or orange. The spectral shifts can be accurately modeled using finite element simulations. These results demonstrate that the morphology and composition of individual bimetallic nanocrystals can be engineered electrochemically.

Nanolithography method by using localized surface plasmon mask generated with polydimethylsiloxane soft mold on thin metal film

We propose a photolithographic method to fabricate nanostructures by employing a localized surface plasmon (LSP) mask generated by a soft mold on a thin metal film. The soft mold can be formed by transparent materials, such as polydimethylsiloxane, contacting firmly to the metal film. The pattern edges of the mold, serving as the fine tapers, can be used to excite LSPs and accumulate a large amount of localized energy from the incident light field, providing a modulated optical field in the resist with nanometer feature size. Nanolithographic results with a minimum feature size of 30 nm are demonstrated.

Superlens nano-patterning technology based on the distributed Polystyrene spheres

Based on surface plasmon resonant enhancement, a method to realize photolithography beyond diffraction limit by using polystyrene spheres (PSs) self-assembled on silver slab was proposed in this paper. The optimum parameters for PS with different diameters were presented. In order to verify this method, numerical simulation on a typical configuration with 1.5 microm diameter of PS was carried out by using the finite-difference time-domain (FDTD) method, and the minimum feature size of 88 nm beyond diffraction limit at 365 nm working wavelength was obtained.

Nanofabrication with controllable localization energy based on the interference modulation of surface plasmons

A nanolithography technique based on the interference of surface plasmons (SPs) is proposed and demonstrated to modulate the localized exposure energy. The SP waves participating in interference are excited by two distinct structures, namely, the grating and the nanotaper. Constructive or destructive interference, which ultimately causes an enhanced or reduced modulation to the localized energy, can be obtained merely by adjusting the distance of the grating and the taper. Detailedly speaking, the localized energy can be modulated consecutively with a constant periodicity, and the modulation range of energy is extremely wide, for instance, the maximum energy is nearly 3 orders of magnitude larger than the minimum by our FDTD simulation results. Moreover, since the localized electric field at the taper tip, which leads to the exposure of the photoresist, is extremely sensitive to interference, it suggests a potential way to produce patterns with different depths and critical widths in one chip via beforehand programming and reasonably controlling the corresponding interference of SPs.