Silicon (Si) nanoparticles (NPs)
and self-organized high spatial
frequency laser (HSFL) induced periodic surface structures were fabricated
by means of femtosecond ablation of bulk Si target in acetone. The
ablation was performed with ∼40 fs (fwhm) pulses and different
input energies of ∼500, ∼200, ∼150, ∼100,
∼50, and ∼10 μJ. Fabricated NPs and nanostructures
(NSs) were characterized by UV–visible absorption spectroscopy,
photoluminescence (PL) spectroscopy, Raman spectroscopy, transmission
electron microscopy, and field emission scanning electron microscopy.
The average sizes of the NPs were estimated to be in the 4–135
nm range. From the PL studies of Si NPs of different sizes, we have
observed a size-dependent shift toward blue spectral region. We could
tune the observed PL peak in the spectral range of 440–515
nm. The crystalline and amorphous nature of the Si nanoparticles and
nanostructures was investigated using selected area electron diffraction
and Raman spectra. Complex refractive index, conduction band electron
density of the Si NPs, estimated by measuring the effective spot size
corresponding to each input energies, were observed to play a crucial
role in determining the periodicity of HSFL induced periodic surface
structures. Experimentally measured periodicity of gratings was in
good agreement with the theory.
Three-dimensional silver nanoparticles decorated vertically aligned Si nanowires (Si NWs) are effective surface-enhanced Raman spectroscopy (SERS) substrates for molecular detection at low concentration levels. The length of Si NWs prepared by silver assisted electroless etching is increased with an increase in etching time, which resulted in the reduced optical reflection in the visible region. These substrates were tested and optimized by measuring the Raman spectrum of standard dye Rhodamine 6G (R6G) of 10 nM concentration. Further, effective SERS enhancements of $10 5 and $10 4 were observed for the cytosine protein (concentration of 50 lM) and ammonium perchlorate (oxidizer used in explosives composition with a concentration of 10 lM), respectively. It is established that these three-dimensional SERS substrates yielded considerably higher enhancement factors for the detection of R6G when compared to previous reports. The sensitivity can further be increased and optimized since the Raman enhancement was found to increase with an increase in the density of silver nanoparticles decorated on the walls of Si NWs.
This
review presents a comprehensive synopsis of the recent developments
and achievements in the research of nanosensors composed of plasmonic
nanoparticles (NPs) and silicon nanostructures (NSs) for effective
trace-level molecular detection. This review focuses intensively on
the methodologies for the preparation and enforcement of a variety
of SiNSs including (a) metal nanoparticles decorated silicon nanowires
(NWs), (b) metal nanodendrites (NDs) on Si substrate, (c) plasmonic
NPs decorated nanocrystalline porous silicon (pSi), and (d) silicon
composed hybrid nanostructures with favorable parameters of importance
in sensing. Furthermore, their potency in wide molecular sensing applications,
especially chemical, biological, and explosive molecules based on
surface enhanced Raman scattering (SERS) phenomenon is discussed in
detail. Various demonstrations and categorizations are provided on
the topic of Si-based NSs for a clear understanding to diverse readers.
A roadmap is also provided at the end for achieving superior sensing
materials or devices in the future.
We report results from our extensive studies on the fabrication of ultra-thin, flexible, and cost-effective Ag nanoparticle (NP) coated free-standing porous silicon (FS-pSi) for superior molecular sensing. The FS-pSi has been prepared by adopting a simple wet-etching method. The deposition time of AgNO3 has been increased to improve the number of hot-spot regions, thereby the sensing abilities are improved efficiently. FESEM images illustrated the morphology of uniformly distributed AgNPs on the pSi surface. Initially, a dye molecule [methylene blue (MB)] was used as a probe to evaluate the sensing capabilities of the substrate using the surface-enhanced Raman scattering (SERS) technique. The detection was later extended towards the sensing of two important explosive molecules [ammonium nitrate (AN), picric acid (PA)], and a pesticide molecule (thiram) clearly demonstrating the versatility of the investigated substrates. The sensitivity was confirmed by estimating the analytical enhancement factor (AEF), which was ∼107 for MB and ∼104 for explosives and pesticides. We have also evaluated the limit of detection (LOD) values in each case, which were found to be 50 nM, 1 µM, 2 µM, and 1 µM, respectively, for MB, PA, AN, and thiram. Undeniably, our detailed SERS results established excellent reproducibility with a low RSD (relative standard deviation). Furthermore, we also demonstrate the reasonable stability of AgNPs decorated pSi by inspecting and studying their SERS performance over a period of 90 days. The overall cost of these substrates is attractive for practical applications on account of the above-mentioned superior qualities.
We report the fabrication and performance evaluation of cost-effective, reproducible silver nanodendrite (AgND) substrates, possessing high-density trunks and branches, achieved by a simple electroless etching and used for the trace detection of RDX and Ammonium Nitrate.
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