Nanostructured copper/porous silicon hybrid systems as efficient sound-emitting devices
© Recio-Sánchez et al.; licensee Springer. 2014
Received: 22 May 2014
Accepted: 22 August 2014
Published: 11 September 2014
In the present work, the photo-acoustic emission from nanostructured copper/porous silicon hybrid systems was studied. Copper nanoparticles were grown by photo-assisted electroless deposition on crystalline silicon and nanostructured porous silicon (nanoPS). Both the optical and photo-acoustic responses from these systems were determined. The experimental results show a remarkable increase in the photo-acoustic intensity when copper nanoparticles are incorporated to the porous structure. The results thus suggest that the Cu/nanoPS hybrid systems are suitable candidates for several applications in the field of thermoplasmonics, including the development of sound-emitting devices of great efficiency.
Nowadays, in a wide range of research fields including photonics, optoelectronics, engineering, and biomedicine, there is an increasing demand for multifunctional and affordable materials. In this regard, nanostructured porous silicon (nanoPS) has been demonstrated to be a good candidate for the development of multifunctional materials [1–3]. In addition to its tunable morphological and physicochemical properties, which greatly depend on the fabrication parameters, the relative simplicity and low cost of nanoPS processing make nanoPS a promising material for its use in many fields, photonics among them. Moreover, the compatibility of Si with standard electrochemical processes allows the fabrication of integrated metal/semiconductor hybrid systems .
In the particular case of sound-emitting devices, nanoPS has been demonstrated as a versatile material to fabricate efficient devices . Since its optical and thermal properties can be easily tuned by changing its porosity, nanoPS offers great versatility aiming at adjusting its photo-acoustic response . In addition, devices based on this material have been proposed as sound transmitters  or even to obtain three-dimensional images . Furthermore, the photo-acoustic emission from noble metal nanoparticles is a property which can be exploited for different applications in several fields such as biomedicine , energy , microfluids , sound emitters , etc., since they are able to act as localized heat sources. This effect is based on the plasmon resonance of noble metal nanoparticles and can be optimized by adjusting their size and shape or by combining them with dielectric materials .
In the present work, the incorporation of Cu nanoparticles into nanoPS structures by photo-assisted electroless deposition is studied in detail. This experimental method represents an efficient and low-cost technique for the fabrication of metal/semiconductor hybrid systems. The photo-acoustic response of the resulting hybrid systems is analyzed, and the results show that these systems can be good candidates for the development of efficient sound-emitting devices.
Porous silicon layers were fabricated by electrochemical etching of boron-doped p+-type silicon wafers (resistivity 0.01 to 0.05 Ω · cm; orientation <100>). The composition of the solution was 1:2 hydrofluoric (HF) (48 wt.%):ethanol (98 wt.%). The wafers were galvanostatically etched under illumination from a 100-W halogen lamp. The applied etching density current was 80 mA · cm-2 for 20 s to grow layers around 75% porosity and 750 nm of thickness.
The deposition of Cu nanoparticles over p+-type crystalline Si and nanoPS layers was carried out by immersing the substrates in CuSO4 (50 mM) and H2SO4 (1 mM) aqueous solutions at room temperature during different times. H2SO4 is used to stabilize the pH. The deposition was activated by turning on the 100-W halogen lamp during the appropriate time. Once the deposition is finished, the samples are washed and rinsed twice in water and twice in ethanol and subsequently dried with N2.
Field emission scanning electron microscopy (FESEM) images were acquired using a XL 30S FEG SEM (Philips, Amsterdam, The Netherlands). Fourier transform infrared spectroscopy (FTIR) was carried out using a Bruker IF-S66v spectrophotometer (Bruker AXS, Inc., Madison, WI, USA). Optical characterization was carried out using a Jasco V-560 UV-vis spectrophotometer (Halifax, Canada) equipped with a Hamamatsu R928 photomultiplier (Hamamatsu Photonics, Iwata, Japan). All measurements were taken in the wavelength range between 350 and 850 nm with a 1-nm interval and with 1-s integration time.
The photo-acoustic measurements were performed using a photo-acoustic cell (10 mm in diameter and 3.5 mm in height, MTEC Model 300, MTEC Photoacoustics, Inc., Ames, IA, USA). The samples were irradiated with a 785-nm wavelength laser whose intensity was modulated sinusoidally between 2 and 40 mW. The photo-acoustic signal was measured by means of a lock-in amplifier and recorded as a function of the laser modulation frequency over the range 1 to 100 kHz.
Results and discussion
On the other hand, the photo-acoustic response of the nanoPS layer is about two times higher than that of c-Si. This increase is mainly due to the lower thermal conductivity and higher light absorption of the nanoPS layers. When Cu nanoparticles are deposited into nanoPS, the photo-acoustic response significantly increases, although the absorption at 785 nm is quite similar. This increase can be associated to the Cu nanoparticles acting as localized heat sources near the surface where the heat exchange between the Cu nanoparticles and air is more efficient, increasing the total temperature of the devices and therefore the photo-acoustic intensity.
On the other hand, the thermal conductivity of the nanoPS layer under Cu NPs (approximately 0.6 [W m-1 K-1])  is in the same order of magnitude as that of the porous column layer (approximately 0.2 [W m-1 K-1])  and expected to be a thermal insulation layer as the porous column layer. In fact, the calculated Ppsi/Psio2, where Ppsi represents the photo-acoustic amplitude of nanoPS + Cu, agrees well with the experimental results, suggesting that the strong photo-acoustic emission from nanoPS + Cu can be well understood by the low thermal conductivity of the PSi layer. Consequently, we can conclude that hybrid systems composed of nanoPS and noble metal nanoparticles can be used for thermoplasmonic applications, having much simpler structures than the local plasmon resonator and can be fabricated without a vacuum system.
In this report, the photo-assisted electroless deposition of copper on crystalline silicon and nanostructured porous silicon has been studied. This method has been demonstrated as an efficient technique to deposit copper nanoparticles not only on the surface but also inside the porous structures. The photo-acoustic response of the resulted devices has been measured showing a notable increase in the intensity when copper nanoparticles are deposited into nanostructured porous silicon. Accordingly, nanostructured copper/porous silicon hybrid systems have been shown to be very promising candidates for several applications in the field of thermoplasmonics, including the development of efficient sound-emitting devices.
The authors thank L. García-Pelayo for the technical support in the realization of this work and gratefully acknowledge the financial support from Comunidad de Madrid (Spain) under Project Microseres.
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