Haizhou Ren

Surface Enhanced Raman Scattering Sensing with Nanostructures Fabricated by Soft Nanolithography

The nanospike structures formed with femtosecond laser irradiations have been successfully replicated on the surface of a polyurethane (PU) polymer using a low cost soft nanolithography method. The surface enhanced Raman scattering (SERS) of rhodamine 6G (Rh6G) and dinitrotoluene (DNT) molecules have been measured with silver coated PU nanospike surfaces by a simple portable Raman spectrometer. Compared to a flat silver coated surface, where no Raman Scattering of the molecules can be detected by the simple portable Raman spectrometer, the Raman spectra are enhanced by more than 4 orders of magnitudes. This indicates that the high area/volume ratio and small size of the PU nanospikes can be used for SERS sensing.

The electric field effect on the sensitivity of tin oxide gas sensors on nanostructured substrates at low temperature

A novel low-temperature SnO2 gas sensor was prepared and studied on silicon nanostructures formed by femtosecond laser irradiation. By applying a bias voltage on the silicon substrate to alter the charge distribution on the surface of the SnO2, carbon monoxide (CO), and ammonia (NH3) gas can be distinguished by the same sensor at room temperature. The experimental results are explained with a mechanism that the sensor works at low temperature because of adsorption of gas molecules that trap electrons to the surface of the SnO2.

Highly sensitive gas sensors on low-cost nanostructured polymer substrates

SnO2 thin-film gas sensors have been successfully fabricated on nanospiked polyurethane polymer surfaces, which are replicated by a low-cost soft nanolithography method from silicon nanospike structures formed with femtosecond laser irradiations. Measurements revealed significant response to carbon monoxide (CO) gas at room temperature, which is considerably different from the sensors of SnO2 thin films coated on smooth surfaces that show no response to CO gas at room temperature. The high area/volume ratio and sharp structures of the nanospikes enhance the sensitivity of SnO2 at room temperature. This will greatly decrease the electrical power consumption of the gas sensor and the cost for calibrations, and has great potential for application in other sensing systems.

Using metal nanostructures to form hydrocarbons from carbon dioxide, water and sunlight

Based on experimental results, we propose a mechanism that allows the use of metal nanostructures to synthesize hydrocarbons and carbohydrates from carbon dioxide, water and sunlight. When sunlight impinges on cobalt nanostructures in a glass chamber, its intensity is greatly enhanced around the tips of the nanostructures through surface plasmon excitations focusing effect, and it then photodissociates the water and carbon dioxide molecules through enhanced photon absorptions of ions around the tips of the nanostructures. The photodissociated molecules in excited states remain on the cobalt nanostructure surfaces and various hydrocarbons and carbohydrates then will be formed around the surfaces at temperatures much lower than 100 oC.

Low-cost self-cleaning room temperature SnO2 thin film gas sensor on polymer nanostructures

We have successfully fabricated SnO2 thin film CO gas sensors on nanospiked polyurethane (PU) polymer surfaces that are replicated with a low-cost soft nanolithography method from nanospiked silicon surfaces formed with femtosecond laser irradiations. The sensors show sensitive responses to the CO gas at room temperature because of the sharp structures of the nanospikes. This is much different from the sensors of SnO2 thin film coated on smooth surfaces that show no response to the CO gas at room temperature. To make the nanostructure sensor surface behave self-cleaning like lotus leaves, we deposited a silane monolayer on the surface of the sensors with the 1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFOTS) which has low surface energy. The contact angle measurement conducted on the PFOTS monolayer-coated SnO2 gas sensors indicates that a super-hydrophobic surface formed on the nanospike sensor. The CO gas response sensitivity of the PFOTS-coated SnO2 sensors is almost the same to that of the as-fabricated SnO2 sensors without the PFOTS coating. Such a super-hydrophobic surface can protect the sensors exposed to moisture and heavy particulates, and can perform cleaning-in-place operations to prolong the lifetime of the sensors. These results show a great potential to fabricate thousands of identical gas sensors at low cost.

Femtosecond laser irradiation enhanced room temperature tin oxide nanostructure gas sensor

Tin oxide (SnO2) thin film gas sensors that function at room temperature have been fabricated on nanostructured substrates. After femtosecond laser irradiation of the surface of the SnO2, the sensitivity to gases, for example, carbon monoxide, increased noticeably. The dependence of the sensitivity on the number of laser pulses has been investigated. It is believed that the femtosecond laser pulses generate defects in a thin layer on the SnO2 sensor surface. These defects may result in a potential energy well creating surface bound states for electrons to move on the surface, which increases the sensitivity to gases.

Effects of surface electric field on SnO2 room temperature gas sensors fabricated on nanospike substrates

SnO2 thin film room-temperature gas sensors have been fabricated on silicon nanospike surfaces prepared by femtosecond pulsed laser irradiation. The surface morphologies of the as-fabricated silicon nanospikes and SnO2 thin film gas sensors indicate that the surface roughness increased significantly after the SnO2 layer was deposited. The surface morphology and electric field distribution of the silicon nanospikes were studied with atomic force microscopy (AFM) and the electric force microscopy (EFM), respectively. The comparison between AFM morphology and EFM images shows that the aspect ratio of the nanostructures in the EFM image was larger than that in the AFM image, which indicates that the nanospikes on the silicon surface can induce an enhanced electric field around their sharp features. The electric field around the tips is further enhanced when there is electric current flowing through the SnO2 layer. The enhanced electric field and increased surface area on the nanospike structures are the main contributors to the high sensitivity of these room temperature gas sensors.

Failure study of SnO 2 room temperature gas sensors fabricated on nanospike substrates

SnO2 gas sensors were fabricated on polyurethane (PU) polymer surfaces with nanospike structures. These nanospikes are replicated with a low-cost soft nanolithography method from silicon nanospike surfaces formed by femtosecond pulsed laser irradiation. The hydrophobicity of the sensing surface was enhanced by a monolayer coating of silane (1H,1H,2H,2H-perfluorooctyltrichlorosilane, PFOTS). The resulting self-cleaning behavior enabled sensing in environments with high moisture and heavy particulate content, while performing cleaning-in-place operations to prolong the lifetime of the sensors. Failure studies were performed to quantify the effects on the sensitivity of water washing. Contact angle measurements showed that the hydrophobicity was weakened after many cycles of droplet washing due to wear of the PFOTS film and/or damage of the nanoscale spike structure. It was also found that the baseline signal increased with droplet washing, while the sensitivity changed randomly within about 7.5%, so that the sensitivity of the gas sensor remained at a constant level after several thousand cycles of water washing.

Enhancing sensitivity of semiconductor-based gas sensors on nanostructured surfaces

The metal oxide semiconductor thin film gas sensors have been successfully fabricated on a nanospiked silicon surface formed with femtosecond laser irradiations. The sensors show significant response to CO gas at room temperature. It is well-known that the C-O is polarized with positive charges on oxygen atom and negative charges on carbon atom. When the currents pass through the semiconductor sensitive layer, some electrons may accumulate on the tips of the nanospikes to maintain the same electric potential on the surface, which results in strong local electrical fields near the tips of the nanospikes. Then more CO molecules will be pulled onto the tips of the nanospikes and this will enhance the sensitivity of the sensor. A gate bias enhancement has been studied on silicon/oxide layer/semiconductor architecture with the underlying silicon substrate as the back gate. The bias voltage applied on the gate can further enhance the sensitivities of the gas sensors by alternating the electron (or hole) concentration on the surfaces of the metal oxide semiconductor thin film.