Optical absorption and photoluminescence studies of gold nanoparticles deposited on porous silicon
© Amran et al.; licensee Springer. 2013
Received: 4 September 2012
Accepted: 13 January 2013
Published: 18 January 2013
We present an investigation on a coupled system consists of gold nanoparticles and silicon nanocrystals. Gold nanoparticles (AuNPs) embedded into porous silicon (PSi) were prepared using the electrochemical deposition method. Scanning electron microscope images and energy-dispersive X-ray results indicated that the growth of AuNPs on PSi varies with current density. X-ray diffraction analysis showed the presence of cubic gold phases with crystallite sizes around 40 to 58 nm. Size dependence on the plasmon absorption was studied from nanoparticles with various sizes. Comparison with the reference sample, PSi without AuNP deposition, showed a significant blueshift with decreasing AuNP size which was explained in terms of optical coupling between PSi and AuNPs within the pores featuring localized plasmon resonances.
Coupling system involving semiconductor nanocrystals (NCs) and metal nanoparticles (NPs) has been a subject of great interest for the scientific community . Due to the plasmon resonance in metal NPs, the interplay between NCs and NPs can modify the spectral features of NCs to improve emission efficiency as it involves the charge transfer across the semiconductor/metal interfaces . Gold nanoparticles (AuNPs) are the subject of increasing interests due to their essential properties and localized surface plasmon resonance in the visible spectrum wavelength . The interplay effect in combining the gold and silicon is widely used in electronic devices in controlling their lifetime and resistivity [4, 5].
The AuNPs are mostly fabricated using a combination of chemical e-beam lithography and self-assembly techniques [6, 7] or by electron beam evaporation . However, the challenge is to control the size and position of the nanoparticles because these techniques tend to show a slightly broader size distribution. Mafuné et al.  have developed the laser ablation and laser-induced method to control the size of AuNPs without contamination. Nevertheless, this technique is very costly to implement. As an alternative, electrodeposition technique can offer a solution to the problems as it is known for its simplicity and low processing cost .
Instead of using silicon as the substrate for the AuNP deposition, Fukami et al.  discovered the use of porous Si to control the shape and alignment of metal nanostructures. In this paper, we demonstrate that AuNPs supported on zinc oxide (ZnO) that was synthesized via the deposition-precipitation method can be deposited into porous silicon (PSi) using electrochemical deposition (ECD) technique. The deposition-precipitation method has been proven to produce gold particles of size less than 5 nm . The growth parameters such as pore size distribution of PSi, metal solution concentration, and exposure time may have major influence on the AuNP growth.
Preparation of porous silicon using pulsed technique
An n-type <100 > −oriented silicon wafer with a resistivity of 1 to 10 Ω cm was used to fabricate the PSi substrate. The substrate was cleaned in a wet chemical etching process, using RCA cleaning method. After cleaning, the samples were prepared using pulsed anodic etching method . Output signal from the pulse current generator was used to feed the current at a constant peak of 10 mA/cm2 by adjusting the pause time (Toff) at 4 ms with cycle time Tall (14 ms). The electrolyte solution used was a mixture of hydrofluoric acid and ethanol, 1:4 by volume. Anodization process was carried out for 30 min for all samples under illumination of a 100-W incandescent white light, 15 cm away from the samples. After the etching, the samples were rinsed in deionized water and dried in ambient air.
Preparation of gold nanoparticle supported on zinc oxide
The AuNPs were prepared using the procedure basically similar to that described in our previous work  using deposition-precipitation method. The solution of 100 mL of HAuCl4 solution was heated to 80°C where the pH was adjusted by dropwise mixing with 0.5 M NaOH. Relatively, 1.00 g of zinc oxide support was immersed into the solution. In order to maintain the pH after the support was inserted, dropwise addition of 1.5 M HCl was prepared. The suspension was thermostated at 80°C and underwent vigorous stirring for 2 h. After that, the precipitates were washed with distilled water to remove residual sodium, chloride ions, and unreacted Au species. This process was repeated until there were no AgCl precipitates detected when a filter was added to the AgNO3. The resulting precipitate was gathered by centrifugation and dried at 100°C overnight. The calcination procedure was brought out at 450°C under ambient air for 4 h and a temperature gradient of 50°C min−1. About 0.6 g of black powder was finally obtained. The mean diameter of AuNPs less than 5 nm at pH 7 was obtained.
Fabrication of AuNPs using electrochemical deposition method
The as-prepared Au nanoparticle powder was dispersed in aqua regia  and diluted with deionized water, forming yellow solutions with a mass concentration of approximately 2.8 mg/mL. The aqua regia was prepared by mixing one part of concentrated HNO3 with four parts of concentrated HCL to dissolve the gold. It was stirred using the hot plate magnetic stirrer at 20°C for 15 min. The solvent then was used in electrochemical deposition process using direct current at different current densities of 1.5, 2.5, 3.5, and 4.5 mA/cm2 for 30 min. Gold wire (99.999% purity) was an anode, and PSi was a cathode. The distance between the two electrodes was approximately about 0.5 cm. After that, the samples were dried under nitrogen flow and followed by annealing at 350°C for 15 min. The deposited samples were characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and photoluminescence spectroscopy (PL).
Results and discussion
Transmission electron microscopy
Scanning electron microscopy
This resulted in a removal of positive gold ions (Au3+) from the solution and allowed further oxidation of gold to take place, and so, the gold was dissolved. In addition, Cl− (from hydrochloric acid) removed Au3+ from the solution, encouraging NO3− to dissolve a bit more gold. The longer the process goes on, the larger the Au particles become in size. We believe that there is an appropriate current density in which the formation of a high percentage of Au particle could be accelerated. Accordingly, from the SEM and EDX analyses, when the current density increases from 1.5 to 2.5 mA/cm2, the deposition rate of AuNPs increases. Consequently, at 3.5 mA/cm2, the deposition rate produces Au colloidal crystal with smaller sizes that widely distributed on the substrate. Beyond 3.5 mA/cm2, the deposition rate increases, and it enhances the fabrication of Au grain size. Thus, larger sizes of AuNPs were produced.
The current density, angle (2 θ ), FHWM and Au crystallite size for all the samples
Current density (mA/cm2)
Crystallite size (nm)
AuNPs exhibit the local electric field caused by the localized surface plasmon resonance behavior. When incident laser hits onto the surface, it caused the surface plasmon excitation and locally enhanced the electromagnetic field near the AuNPs and gave rise to PL emission. It has been known that the intensity of the of the plasmon peak is greatly dependent on the size and shape of AuNPs . Therefore, as arranging them in crystallite size, we can see a general trend of increased emitted energy with decreasing crystallite size. Generally, as the gold crystallite size becomes smaller, the PL intensity becomes higher and stronger. Sample d shows the highest PL intensity by a factor of 4 compared to the other samples. It also noted that the plasmon peak exhibits blueshift with decreasing particle size. The observed blueshift in the peak position of plasmon absorption can be attributed to the quantum size effects from the AuNPs . The optical properties of this composite structure, PSi containing AuNPs, reveal dielectric changes when the electromagnetic fields from AuNPs couple with PSi emission that leads to a clear blueshift (relative to the PSi sample) . It is therefore noted that a current density of 4.5 mA/cm2 for electrodeposition produced the highest PL intensity.
In conclusion, we have succeeded growing AuNPs on the surface of PSi using a cost-effective and simple method of electrochemical deposition. We showed that current density of the deposition process defined the size of the formed AuNPs. The surface morphology showed a gold colloidal crystal network with varying sizes. XRD spectra confirmed the element inside the Au/PSi whereby the peaks of Au (111) emerged as the preferred orientation. The samples contain cubic gold phase with crystallite size in the range of 40 to 58 nm. The blueshifted spectrum was caused by the interface interactions between AuNPs and the PSi, which are interpreted in terms of localized surface plasmon resonance of the AuNPs due to the change of the size condition which leads to a spectral shift of the spectrum in the PL.
TSTA is a MSc student of the University Sains Malaysia (USM) together with HY. MRH is professor at the USM. NKA is the senior lecturer at the University Teknologi Malaysia (UTM). RA is an associate professor at USM.
The authors would like to thank the Material Innovations and Nanoelectronics research group (MINE), University Teknologi Malaysia, for their help and support. The authors are grateful for the financial support in part from the Ministry of Science, Technology and Innovation (MOSTI). Support grant from the Research University Grant USM-RU-PGRS grant: 1001/PFIZIK/833030 and Universiti Teknologi Malaysia GUP grants are gratefully acknowledged.
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