High performance surface-enhanced Raman scattering substrates of Si-based Au film developed by focused ion beam nanofabrication
© Gao et al.; licensee BioMed Central Ltd. 2012
Received: 17 April 2012
Accepted: 14 June 2012
Published: 17 July 2012
A novel method with high flexibility and efficiency for developing SERS substrates is proposed by patterning nanostructures on Si substrates using focused ion beam direct writing (FIBDW) technology following with precise thermal evaporation of gold film on the substrate. The effect of SERS on the substrate was systematically investigated by optimizing the processing parameters and the gold film thickness. The results proved that small dwell time could improve the machining accuracy and obtain smaller nanogap. The Raman-enhanced performance of the substrate was investigated with 10−6mol/L Rhodamine 6 G solution. It was indicated that the elliptic nanostructures with 15-nm spacing on Si substrates, coated with approximately 15-nm thick gold film, have exhibited a high-enhanced performance, but dramatic performance degradation was found as the gold film thickness further increased, which most probably resulted from changes of the nanostructures’ morphology such as elliptical tip and spacing. To avoid the morphological changes effectively after depositing gold film, optimization design of the nanostructures for FIBDW on Si substrates was proposed. Besides, a similar phenomenon was found when the gold film was less than 15nm because there was little gold remaining on the substrate. The method proposed in this paper shows a great potential for the higher performance SERS substrates development, which can further reduce the spacing between hot spots.
KeywordsSERS Si-based Au nanostructures Focus ion beam nanofabrication Thermal evaporation
Surface-enhanced Raman scattering (SERS) is one of the most powerful tools for trace detections and biochemical applications because of its ultrasensitivity, low-cost, and real-time characteristics[1–6]. In 1928, Raman and Krishnan first observed a special phenomenon that monochromatic light incident on molecules resulted in normal Rayleigh scattering as well as modified scattered radiation of different frequencies. This ‘feeble’ phenomenon is known as Raman scattering, which is attributable to the excitation (or relaxation) of vibration modes of a molecule. Therefore, the Raman spectrum could be used to identify the target molecules up to single molecule in chemical and biological systems because every molecule has its unique Raman spectrum, and different functional groups have different characteristic vibration energies. But in a long time after the Raman scattering was discovered, the applications in biosensing had been limited by its inherent much weak signal until 1977, when Jeanmaire and Van Duyne indicated that the magnitude of Raman scattering signal can be largely enhanced by roughened noble metal surface[7, 8]. Later, this phenomenon was defined as SERS. The main underlying enhancement mechanism was attributed to the localized surface plasmon resonance (LSPR) that electronic collective oscillation contributes to the electromagnetic enhancement, occurring when the nanostructure is much smaller than the excitation wavelength. The chemical mechanism would also play a role in SERS enhancement with less contribution than LSPR.
The most critical aspect of SERS is the research of efficient SERS-active substrates, such as nanostructured surface or nanoparticles of noble metals with suitable physical parameters such as their material, size, shape, and spacing[12, 13]. Generally, Ag and Au nanoparticles are regarded as one of the best candidates for SERS substrate studies. During its development, many methods were put forward to fabricate SERS-active substrates, such as roughened electrodes, noble metal colloidal nanoparticles, silver island films, metal films over nanostructured surfaces, acid-etched metal foils, and lithographically produced nanoparticle arrays[15–17]. Nevertheless, fabrication of SERS substrates with both high sensitivity and high stableness remains difficult, and it is costly for routine SERS detection.
Materials and instruments
In this work, Si wafers were cut into squares, cleaned in an ultrasonic bath with methanol for 20min, and dried in the air. Rhodamine 6 G was diluted to 10−6mol/L with deionized water. The adsorption peak of R6G molecule in deionized water was 557nm, and fluorescence wavelength was 610nm. Therefore, in the experiment, a laser wavelength of 785nm was chosen so as to avoid fluorescence.
FIB system (Nova 200, NanoLab, MA, USA) was used to fabricate patterning nanostructures. It combines ultra-high resolution field emission SEM and precise focused ion beam etch, and could be used for nanoscale prototyping, machining, and on-line high resolution SEM measurements. The thickness of gold film was measured by white light interferometry (NT 9300, Veeco Instruments Inc., Shanghai, China). Raman spectra were obtained with an inVia Raman microscope (Renishaw plc, Gloucestershire, UK) with a CCD detector and a × 50 objective measuring the probe molecules. The nanostructure SERS substrate, which was placed into the R6G solution for 2h and dried in the air, was irradiated by 785nm wavelength with a laser power of approximately 70mW and integration time of 10s. The Raman shift ranges from 550 to 2,000cm−1. All tests were carried out on three different places from the substrate, and the final spectra were averaged by those measurements.
Nanostructures developed on Si substrates by FIB
Description of the dwell time, etching time, and spacing
Based FIB Si
FIB machining parameters
Dwell time (μs)
Etching time (min)
30 kV voltage and 10 pa accelerate current
15 ± 1
18 ± 1
22 ± 2
Preparation of SERS-active substrates via thermal evaporation
Results and discussion
Effect of FIB fabrication conditions on SERS
By optimizing the FIBDW parameters, the nanostructure’s dimension and form accuracy on Si substrates can be well controlled, and the nanostructure’s spacing can be reduced to 15nm, as shown in Table 1 and Figure 2. However, bigger dwell time and longer FIB etching time result in apparent changes of the nanostructure’s profile. For example, compared to A_1, the nanostructure’s spacings of A_2 and A_3 increased by 20% and 46.7%, respectively, and the curvature of the elliptical tip becomes bigger. It can be clearly seen in Figure 2. This phenomenon could be attributed to the ion beam etching. Longer FIB fabrication time can induce large-depth structures, but it would also degrade the nanostructures accuracy accordingly.
Therefore, in order to obtain high performance and high electromagnetic field enhancement Si-based gold film SERS substrates, FIB fabrication parameters should be optimized.
Effect of Au film thickness on SERS
Optimization of the nanostructures
This paper presents a high enhancement SERS substrate development method with FIB nanofabrication in advance and subsequently with gold film thermal evaporation. It is found that acceleration current, dwell time, and etching time play a major role in precisely controlling nanostructures during the FIB nanofabrication on Si substrates. SERS spectra study revealed the relativities between Raman enhancement and thickness of gold film. As the gold film increased from 15 to 70nm, the Raman signal decreased and disappeared finally because thermal evaporation has covered many hot spots. It is concluded that the spacing and curvature of structures are the key factors for the electromagnetic field enhancement and SERS performance. The method integrated the advantages of high sensitivity and repeatability, and would significantly facilitate practical SERS substrate preparation.
FF is a professor working in Tianjin University from 2005. His main research interests are micro/nano manufacturing and freeform optics. He is the president of the International Society for Nanomanufacturing (ISNM). He is also a Fellow of CIRP - International Academy for Production Engineering, and an Editor-in-Chief of the International Journal of Nanomanufacturing (IJNM).
The authors appreciate the supports of the National Basic Research Program of China (973 Program, grant no.: 2011CB706703) and the National Natural Science Foundation of China (grant nos.: 90923038 and 50905126), Tianjin Natural Science Foundation (grant no.: 10JCYBJC06400). The authors would like to thank Mr. B. Tang for the assistance in coating the Au thin films and valuable discussions with Prof. Yongqi Fu from University of Electronic Science and Technology of China.
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