Haiyang Mao

Microfluidic Surface-Enhanced Raman Scattering Sensors Based on Nanopillar Forests Realized by an Oxygen-Plasma-Stripping-of-Photoresist Technique

A novel surface-enhanced Raman scattering (SERS) sensor is developed for real-time and highly repeatable detection of trace chemical and biological indicators. The sensor consists of a polydimethylsiloxane (PDMS) microchannel cap and a nanopillar forest-based open SERS-active substrate. The nanopillar forests are fabricated based on a new oxygen-plasma-stripping-of-photoresist technique. The enhancement factor (EF) of the SERS-active substrate reaches 6.06 × 10(6) , and the EF of the SERS sensor is about 4 times lower due to the influence of the PDMS cap. However, the sensor shows much higher measurement repeatability than the open substrate, and it reduces the sample preparation time from several hours to a few minutes, which makes the device more reliable and facile for trace chemical and biological analysis.

Highly‐Ordered, 3D Petal‐Like Array for Surface‐Enhanced Raman Scattering

Despite the great potential of the application of surface-enhanced Raman scattering (SERS), the difficulty in fabricating suitable SERS substrates is still a problem. Based on the self-assembly of silica nanoparticles, a simple method is here proposed to fabricate a highly-ordered, 3D, petal-like arrayed structure (3D PLAS) that serves as a promising SERS substrate for both its high reproducibility and enormous SERS enhancement. Such a novel structure is easily achieved by anisotropically etching a self-assembly bilayer of silica nanoparticles, followed by metal deposition. The SERS performance of the 3D PLAS and its relationship with the main parameters, including the etching time, the diameter of silica nanoparticles, and the deposited metal film, are characterized using 632.8 nm incident light. With Rhodamine 6G as a probe molecule, the spatially averaged SERS enhancement factor is on the order of 5 × 10(7) and the local enhancement factor is much higher, both of which can be improved further by optimizing the parameters.

The fabrication of diversiform nanostructure forests based on residue nanomasks synthesized by oxygen plasma removal of photoresist

A simple lithography-free approach for fabricating diversiform nanostructure forests is presented. The key technique of the approach is that randomly distributed nanoscale residues can be synthesized on substrates simply by removing photoresist with oxygen plasma bombardment. These nanoresidues can function as masks in the subsequent etching process for nanopillars. By further spacer and then deep etching processes, a variety of forests composed of regular, tulip-like or hollow-head nanopillars as well as nanoneedles are successfully achieved in different etching conditions. The pillars have diameters of 30-200 nm and heights of 400 nm-3 microm. The needles reach several microns in height, with their tips less than 10 nm in diameter. Moreover, microstructures containing these nanostructure forests, such as surface microchannels, have also been fabricated. This approach is compatible with conventional micro/nano-electromechanical system (MEMS/NEMS) fabrication.

Fabrication of nanopillar forests with high infrared absorptance based on rough poly-Si and spacer technology

Nanopillar forests with high infrared (IR) absorptance are fabricated based on a highly flexible, controllable and micro-fabrication compatible parallel approach. The key technique of the approach is using rough surfaces of poly-Si films as support structures in spacer technology. In a wavelength region of 1.5–5 μm, IR absorptance of the large-area nanopillar forests reaches a minimum of 95%, which is much higher than that of poly-Si films and Si3N4-based IR absorbers. As the approach is compatible with conventional micro-fabrication process, the nanopillar forests can be photo-patterned and generated on microstructures or devices. It is expected that the nanopillar forests can be employed as absorbers in micro-electro-mechanical system IR sensors to improve performance. (Some figures may appear in colour only in the online journal)

Fabrication of polyimide sacrificial layers with inclined sidewalls based on reactive ion etching

Polyimide is used as a sacrificial material because of its low stress, its removable ability and its compatibility with standard micromachining processes. In this work, polyimide structures with inclined sidewalls are fabricated with a reactive ion etching process, where SiO2 is used as the hard-mask material. The structures can be further used as sacrificial layers in micro-electro-mechanical systems infrared (IR) sensors to support IR absorbers, to realize the thermal connections between the absorbers and the thermopiles, and to scale down the size of the sensors. As a result, IR sensors with low-residual-stress absorption, high structural stability, low heat loss and small dimensions can be achieved.

Microfluidic surface-enhanced raman scattering sensors for online monitoring trace chemical mixing and reaction

We report a microfluidic surface-enhanced Raman scattering (SERS) sensor for online monitoring the mixing and reaction between chemicals of low concentration. The SERS sensor is made up of a SERS-active substrate and a polydimethylsiloxane cap with microchannels for delivering solutions. The SERS substrate consists of noble-metal covered silicon nanopillar-forest patterns, which are fabricated based on an oxygen-plasma-treating-on-photoresist technique. Enhancement factor of the device is self-calibrated to be on the order of 5.2×105, by utilizing the flat-metal areas beside the nanopillar-forest patterns as references. The mixing process of Crystal Violet and Rhodamine 6G trace solutions is characterized in the SERS sensor online and in real-time. Other kinetic features, including flow and diffusion, are also revealed.

Silicon nanopillar-forest based microfluidic surface-enhanced Raman scattering devices

We report a novel microfluidic surface-enhanced Raman scattering (SERS) device, which is achieved by bonding a polydimethylsiloxane cap with a microchannel structure onto an SERS-active substrate composed of noble-metal covered silicon nanopillar forests. The silicon nanopillar forests are fabricated by using nanomaterial dots, which are introduced in oxygen-plasma bombardment of photoresist, as etching masks. Analytes are uniformly distributed in the nanopillar forests by flowing through the microchannels, and thus higher measurement repeatability is obtained compared with regular open SERS-active substrates. Moreover, the enhancement factor (EF) of the devices can be self-calibrated by utilizing the flat metal areas beside the forests as references. Accordingly, such SERS devices have an EF on the order of 5.2×105. Meanwhile, the devices can reduce the measurement time from several hours to about 10 minutes.

Fabrication of Nanopillars based on Silicon Oxide Nanopatterns Synthesized in Oxygen Plasma Removal of Photoresist

We report for the first time a facile lithography-free approach for fabricating nanopillars over large areas or in patterns. The key technique of this approach is that randomly-distributed nanoscale SiO2 patterns can be synthesized on substrates simply by removing photoresist with oxygen plasma bombardment. Those SiO2 nanopatterns may further function as masks in the following etching process for nanopillars. Based on this approach, a variety of microstructures containing nanopillars with diameters of 30~200 nm, which include surface micro channels, micro-cantilever probes and nanofences, have been fabricated. This approach can be applied both to silicon and metal substrates compatible with conventional micro-electromechanical systems (MEMS) fabrication.

Multi-resonant wideband energy harvester based on a folded asymmetric M-shaped cantilever

This article reports a compact wideband piezoelectric vibration energy harvester consisting of three proof masses and an asymmetric M-shaped cantilever. The M-shaped beam comprises a main beam and two folded and dimension varied auxiliary beams interconnected through the proof mass at the end of the main cantilever. Such an arrangement constitutes a three degree-of-freedom vibrating body, which can tune the resonant frequencies of its first three orders close enough to obtain a utility wide bandwidth. The finite element simulation results and the experimental results are well matched. The operation bandwidth comprises three adjacent voltage peaks on account of the frequency interval shortening mechanism. The result shows that the proposed piezoelectric energy harvester could be efficient and adaptive in practical vibration circumstance based on multiple resonant modes.

Self-assembly nanoparticle based tripetaloid structure arrays as surface-enhanced Raman scattering substrates

This paper reports a novel highly ordered tripetaloid structure array (TPSA) which performs very well as an active surface-enhanced Raman scattering (SERS) substrate. The TPSA is easily fabricated by anisotropic etching of a self-assembly silica-nanoparticle bilayer and a subsequent metal deposition step, with notable uniformity and reproducibility. Electromagnetic simulation indicates that the narrow inter-gaps and edge protrusions in the TPSA act as hot spots. In addition, the peak electromagnetic field intensity in the inter-gaps changes slightly and periodically as the polarization of the incident light varies from 0° to 360°. SERS experiments show that the SERS enhancement factor (EF) of a Au-film-covered TPSA is 12 times higher than that of regular Au-film-over-nanoparticles, and not sensitive to the polarization of the incident light. The spatially averaged EF of the TPSA is as high as 5.7 × 10(6), and the local EF of its hot spots is much higher.