Optical sensing nanostructures for porous silicon rugate filters
© Li et al; licensee Springer. 2012
Received: 9 October 2011
Accepted: 17 January 2012
Published: 17 January 2012
Porous silicon rugate filters [PSRFs] and combination PSRFs [C-PSRFs] are emerging as interesting sensing materials due to their specific nanostructures and superior optical properties. In this work, we present a systematic study of the PSRF fabrication and its nanostructure/optical characterization. Various PSRF chips were produced with resonance peaks that are adjustable from visible region to near-infrared region by simply increasing the periods of sine currents in a programmed electrochemical etching method. A regression analysis revealed a perfect linear correlation between the resonant peak wavelength and the period of etching current. By coupling the sine currents with several different periods, C-PSRFs were produced with defined multiple resonance peaks located at desired positions. A scanning electron microscope and a microfiber spectrophotometer were employed to analyze their physical structure and feature spectra, respectively. The sensing properties of C-PSRFs were investigated in an ethanol vapor, where the red shifts of the C-PSRF peaks had a good linear relationship with a certain concentration of ethanol vapor. As the concentration increased, the slope of the regression line also increased. The C-PSRF sensors indicated the high sensitivity, quick response, perfect durability, reproducibility, and versatility in other organic gas sensing.
Porous silicon [PSi], a material with unique structural and optical properties, can be prepared by anodic etching of silicon in ethanolic hydrofluoric acid solution [1–3]. By changing the current density, the porosity of PSi that decides the refractive index can be tailored in a wide range, and thus, it is able to obtain many types of PSi optical structures such as porous silicon microcavities and porous silicon rugate filters [PSRFs]. Among them, PSRFs are a class of multilayered photonic crystal with a sinusoidal refractive index distribution that is normal to the surface. Light incident on the surface of a rugate filter will be reflected in a narrow spectral range, and spectral position is dependent on the refractive index of the material . PSRFs were introduced and further improved afterwards by different groups [5–9]. For example, by placing the electrode in a different way, a porous silicon band filter gradient that displayed rainbow colors on its surface was prepared , and by employing an amplitude-modulated sinusoidal refractive index apodized with a Gaussian function, the optical property was greatly improved as a stop band at a wavelength of 850 nm yet with the full-width at half maximum [FWHM] being only 5 nm [7, 8]. Notably, the location of its peak was found to be determined by the period of the waveform used in the preparation .
Recently, combination PSRFs [C-PSRFs] have been developed. One kind of C-PSRF was designed with two peaks in their reflectance spectra and combined with two multilayered mirrors, among which the hydrophobic one was at the top and the hydrophilic one, at the bottom [10, 11]. These C-PSRFs have been used to manipulate the movement of liquid droplets  and local heating . Another kind of C-PSRF was constituted by combined multilayered mirrors and generated by coupling n sine waves with different frequencies used for PSRF preparation . By contrast, these C-PSRFs were applied for biomolecular screening  or encoded microcarriers . However, the nanostructure and optical characterization have not been studied systematically.
Herein, we reported a systematic study on the fabrication and characterization of PSRFs and C-PSRFs, especially on the relationships of position, FWHM, and intensity of their optical resonance peaks with the period of sinusoidal current density applied in the synthesis. Also, a scanning electron microscope and a microfiber spectrophotometer were employed to analyze their physical nanostructure and feature spectra, respectively. The novel C-PSRF with three synchronous peaks was used to detect the response to ethanol vapor, and the red shifts of the C-PSRF peaks had a good linear relationship with a certain concentration of ethanol vapor. As the concentration increased, the slope of the regression line also increased. The C-PSRF sensors indicated high sensitivity, quick response, perfect durability, and reproducibility.
Thickness and configuration measurements of the resulting PSRFs were carried out with a field-emission scanning electron microscope [FESEM] (S4700, Hitachi High-Tech, Minato-ku, Tokyo, Japan). The reflectance spectra (200 to 1,100 nm) of the PSRFs were collected by a fiber optic spectrometer (AvaLight-DHS, Avantes BV, Apeldoorn, The Netherlands) with a halogen lamp as light source. The resolution of the spectrometer is 0.8 nm.
The highly doped p-type Si wafers (boron-doped, 0.002 to 0.004 Ω cm resistivity) were obtained from Silicon Valley Microelectronics, Inc. (Santa Clara, CA, USA). Hydrofluoric acid [HF] was obtained from Chemical Reagent Company, Ltd. of Dongguan City, Guangdong, China. All other chemicals used in this study were of analytical reagent grade and used without further purification. A JL-RO100 Milli-pore-Q Plus water (Millipore Co., Billerica, MA, USA) purifier supplied deionized water with a resistivity of 18.25 MΩ cm.
Three sine waves with 30 periods and different frequencies (e.g., PSRF1, PSRF2, and PSRF3; Figure 1) were coupled to generate a combined waveform (Figure 1) that was then converted into a current-time waveform by the computer-controlled current source (2400 SourceMeter, Kiethley Instruments Inc., Cleveland, OH, USA). The converted waveform was then applied to etch a porosity-depth profile in the Si wafer, yielding the C-PSRFs . Other conditions were kept the same as described in the PSRF preparation.
Prior to anodization, all the silicon wafers were dipped in 5 wt.% HF solution to remove the native oxides. After anodization, all the samples were rinsed with ethanol and then dried under a gentle stream of nitrogen gas.
For ethanol monitoring experiments, the PSRF wafers were placed in a sealed steel chamber with a window that was covered with a quartz glass, in which the detecting vapors were transported at room temperature. The dry ethanol vapors were produced by the bubbling of nitrogen with a flow rate of 100 sccm into an ethanol aqueous solution with different concentrations and dried by passing through a pipe filled with anhydrous Na2CO3.
Results and discussion
It was obvious that the PSRFs with peaks ranging from short to long wavelengths could be obtained by increasing the thickness of each layer, which was controlled by the duration of the etch cycle .
where λ was the wavelength of peak in the spectrum, h was Planck's constant, c was the speed of light, and energy (E, joules) and work (W, electron volt) were photon energies. As can be seen, the wavelength (λ) red shifts resulted in the lower photon energy (W).
When the ethanol vapors were desorbed through exposure of N2, the peaks would quickly return to their exact initial positions (see a3, b3, and c3 in Figure 6). This process was completely reversible and repeatable even after several cycles of exposure. The same value for each measurement within the experiment error has been obtained from the process that has been repeated three times. The response time to ethanol vapors and recovery time from N2 were as rapid as within 5 s.
While these values reflect the sensitivity to a given volume (or mass) of the analyte, it is important to point out that the slope coefficient for a given analyte is profoundly influenced by the adsorption and microcapillary condensation processes operative in these PSRF sensors. The high surface area and the existence of a large volume fraction of micropores both contribute in concentrating the analyte vapors in the sensor. Consequently, the slope coefficient is expected to be substantially reduced, in particular, for molecules that have high sticking probabilities and low vapor pressures .
In summary, we have presented a systematic study on the preparation of PSRFs with resonant peaks varying from the visible region to the near-infrared region. Those PSRFs were electrochemically produced in a program-controlled current etching by systematically tuning the periods of the sine currents. The PSRF resonant peak wavelength was observed as a perfect linear correlation with the period of etching current. Moreover, C-PSRFs with several peaks in the feature spectra were produced using a combination sine current that was coupled with several single sine waves with different frequencies. The relationships between the wavelength of the resonant peak, FWHM, peak intensity, and the periods were revealed. Based on the C-PSRFs' unique nanostructure and multiplex peak detection, the sensing properties of C-PSRFs were explored in the ethanol vapor. The sensing process of C-PSRFs exhibited high sensitivity, quick response, and reproducible abilities, implying their promising applications in gas sensor and biosensing fields.
We gratefully acknowledge the financial support of the National Natural Science Foundation of China (grant nos. 20875062 and 81071249), Shenzhen Science and Technology Projects (SY200806300225A), the 'Hundred Talents Program' of Chinese Academy of Sciences, and the Science and Technology Projects of Guangdong Province (grant no. 2009A030301010).
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