Programmable SERS active substrate for chemical and biological sensing applications using amorphous/crystalline hybrid silicon nanomaterials

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We propose a unique nanostructured amorphous/crystalline hybrid silicon material that exhibits surface-enhanced Raman scattering (SERS) activity. This nanomaterial is an interconnected network of amorphous/crystalline nanospheres, forming a nanonet structure. As far as we know, this material has not been observed before and has not been used as a SERS sensing material. The material is formed using femtosecond synthesis technology, which is conducive to the formation mechanism of laser plume ion condensation. By fine-tuning the laser plume temperature and ion interaction mechanism in the laser plume, we can accurately program the relative ratio of crystalline silicon to amorphous silicon content in the nanospheres, as well as the size distribution of each nanosphere and the size of the Raman hot spot nanogap. By using rhodamine 6G (R6G) and crystal violet (CV) chemical dyes, compared with bulk silicon wafer substrates, we were able to observe that the maximum enhancement factors of hybrid nanomaterials were 5.38×106 and 3.72×106, respectively. By creating a silicon-based nanomaterial that can detect analytes through SERS, this work proves that the role of nanostructured Si in nano-Raman sensing applications has been redefined from inactivity to SERS activity.
The field of nano-Raman sensing is growing year by year, and with the further development of enhanced and mature sensing materials1,2. Raman scattering is an important technology in the field of chemical and biosensing because it provides the ability to detect these analytes at a single molecule concentration3. Nano Raman technology also has the ability to detect multiple analytes. Nano Raman equipment can not only detect various chemical substances, including pollutants in water supply4, explosive chemicals5, but also can be used for pharmaceutical chemical identification6. Nano Raman technology can also be used for many biomolecule sensing technologies . Nano Raman sensor also has the ability to detect various cancer cells 7, 8, bacteria 9, RNA and DNA 10, 11 and viruses 12.
The main problem of nano-Raman scattering observation is the intensity of the scattered signal. Only 10-12 incident photons 13 will undergo inelastic scattering, making Raman characterization an impractical tool that will not significantly enhance the signal. Due to the extremely high enhancement factor and the extraordinary sensitivity that SERS activation can achieve, SERS enhancement has become the main focus of current nano-Raman sensing research. It has been determined that the main source of SERS enhancement of metallic nanomaterials comes from a phenomenon called surface plasmon resonance (SPR). This is the collective oscillation of electrons around the nucleus caused by incident electromagnetic radiation. The increased sensitivity to the analyte results from the enhancement of the electric field caused by SPR, which is transferred to the analyte molecule, resulting in a larger cross section of the Raman scattered photon14. Precious metals (gold, silver, etc.) have been widely used as SERS materials because they have the recognized characteristics of Raman lasers with surface plasmon resonance (SPR) in the visible and near infrared (NIR) spectra (most wavelength ranges)15. Although SPR is the main source of Raman enhancement, SPR alone cannot provide the huge observed enhancement factor of these nanostructured materials. The physical form also plays an important role and works in conjunction with SPR. Many different nanostructures have been created and exhibit enhanced Raman scattering, including nanostars 16, nanomembranes arranged on nanowire arrays 17, hollow or solid nanocubes 18 or nanoshells 19. The common reason for the enhancement of most of these nanostructures is the generation of concentrated localized areas of electromagnetic fields, which are called Raman or plasma hot spots. Hot spots are usually formed in the nano gap 20 between two nanostructures, the simplest being the gap 15 between two nanoparticles, but hot spots 21 can also be formed between many types of nanostructures. However, this means that the generation of nanogap is highly dependent on the size of the nanostructure, the wavelength of the Raman laser, and the size of the analyte molecule13, but single-molecule detection can be performed by precise control3.
However, there are a lot of research focuses on the development and utilization of semiconductor-based nanomaterials, especially silicon, as materials for SERS enhancement substrates. This is due to the fact that the nano-structured silicon material is relatively in a dormant state in terms of direct SERS activation when using conventional synthesis techniques. For example, Wells et al. When zinc phthalocyanine has been detected on silicon nanopillar arrays using nanolithography, the EF value is 510. Other researchers also reported EF values ​​of 8–2823, 7.1–7024, and 10325. Although significant achievements have been made in Si-based SERS activation, these EF values ​​are several orders of magnitude lower than those reported for precious metal nanostructures. For example, Garcia-Leis et al. [26] reported that the EF value of silver nanostars was 1.72×105, Wang et al. [27] reported that the maximum EF value of gold nanoparticle arrays was 107, and Tao et al. [28] reported EF. value. For Ag nanowire single layer film, it is 2×109. Therefore, in the currently studied SERS nanomaterials, nanostructured silicon is mainly used as the scaffold or substrate 29, 30, 31 of the noble metal Raman active nanostructure; and nanostructured silicon has no major role in Raman enhancement. The energetic motivation is to fill the gap in SERS active silicon nanomaterials, because silicon is the basic material of all existing electronic devices, and the ability to obtain Raman active nanostructured silicon materials will not require the development of Raman sensing devices. New integration technology. . A lot of research has been conducted to study the Raman activity of semiconductor nanomaterials, and has produced promising results. SERS enhancement was observed in ZnO32, ZnS33, CdS34 and CuO35 in the colloidal suspension, respectively. Competitive EF values ​​of about 106 were also observed in the three-dimensional TiO2 nanostructures 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, and 46. Due to the wide controllability of the material properties of semiconductors (including band gap, dopants, physical form, stoichiometry, phase crystallinity, nanostructure size distribution, etc.), these semiconductor materials and a wider range of silicon are more suitable as SERS materials. Attractive. More than metal. Therefore, Si nanomaterials with high SERS activity provide new opportunities for SERS sensing substrates, instead of being too costly and difficult to manufacture precious metal SERS nanostructures that lack depth of control over material properties and physical forms. The use of silicon-based SERS nanomaterials may lead to the proliferation of economical SERS detection equipment in many applications.
Through our current research, we have observed a new phenomenon of using nanostructured Si to activate SERS, which we can precisely control through the formation mechanism that allows us to create SERS active materials from dormant Raman Si wafer materials. In this research, we created a brand-new nanomaterial, which is currently not able to be produced by other methods. We are able to create mixed crystalline/amorphous silicon nanospheres in the laser ionization plume, which are fused and deposited on a silicon substrate as a network of interconnected nanofibers. Through ultra-fast femtosecond laser synthesis, we can create nanospheres instead of individual crystalline and amorphous nanoparticles, and the entire nanosphere has crystalline and amorphous silicon states. By changing the ionization mechanism and temperature in the laser plume, we can precisely control the structural arrangement of silicon ions to form this hybrid silicon nanomaterial. We can not only program the crystalline/amorphous content of a single nanosphere, but also manipulate the morphology of the nanospheres and how they are arranged when deposited on the substrate surface. Due to this unique particle formation mechanism, these formed nanospheres are unique in structural features. Due to the higher grain boundary concentration in the nanospheres, these nanospheres have greater SERS activity than single crystal silicon. This combination of amorphous/crystalline grains in the individual nanospheres of the interconnected nanosphere network is a new concept of nano-Raman activation. Figure 1 is the overall schematic diagram and SERS enhancement diagram of molecular dyes on Si nanofiber network structure.
The purpose of this research is to prove that we can synthesize a new type of silicon nanomaterial with SERS enhancement properties, which does not exist in bulk silicon materials or other nanostructured silicon materials, and we can precisely control this The production of unique nanomaterials, and this is the formation mechanism of other nanomaterials that cannot be achieved. In addition, we demonstrated the feasibility of Si Raman active nanomaterials as chemical sensing substrates without the need for traditional precious metal SERS activation.
A pulsed Y-doped fiber amplified femtosecond laser is used to fabricate silicon nanostructures. In order to control the manufacture of nanofiber webs to the greatest extent, certain parameters of the laser need to be fixed. In this experiment, the laser wavelength (1030 nm), polarization (circular) and laser power (16 W) are fixed. The changed parameters are repetition rate, dwell time and pulse width.
The nanostructures were created on a 5×5 mm dot array with a dot pitch of 50μm. Use EzCAD software to draw the array, and use piezoelectric driven raster scanner for control. The silicon substrate used is a crystalline silicon wafer with an orientation of <110>.
Before and after applying the dye to the ablation using the B&W Tek, IncNanoRam® handheld Raman system, inspect each ablation area. The Raman excitation laser used has a wavelength of 785 nm at a power of 350 mW. A Raman laser was used to analyze the exposed nanofiber ablation area to determine how the nanostructure changes the Raman spectrum compared to the unablated substrate, depending on the Raman intensity and any structural/composition changes caused by ablation. The dyes used to test the SERS enhancement factor of silicon nanostructures are Rhodamine 6 G and Crystal Violet. Because of their large Raman cross section, they are popular dyes for SERS analysis. In order to determine the sensitivity range of the Si nanofiber network structure, each dye was applied to a separate ablation area at two concentrations of 8×10-3 M and 8×10-6 M. Apply one drop of each dye concentration to the two dyes and move them to a separate ablation zone before Raman analysis. Each obtained Raman spectrum was acquired at a collection time of 3 s, repeated three times, and then averaged.
To confirm the existence of the silicon nanomesh network and characterize its structure, a high-resolution scanning electron microscope (HRSEM) was used. SEM is used to determine the size distribution of nanospheres and nanogap in each ablation zone. Using the HRSEM image of the nanofiber web substrate and ImageJ image processing software, the average size of the nanospheres was calculated. Use ImageJ to manually calculate the particle size; first adjust the scale according to the magnification of the image, and then measure the sphere size according to the outer diameter of each sphere. The same technique is used to measure the nanogap size distribution. In order to obtain a cross-sectional image, the substrate is cut through the ablation zone from the middle, and then the nanofibers are imaged at a certain angle. The gold sputtering of the nanofiber web is necessary because the fibers are made of silicon, so they are too charged.
High-resolution transmission electron microscopy (HRTEM) is also used to image and analyze the shape and size of nanowebs and nanospheres. HRTEM involves wiping the sample with a carbon grid to attach nanospherical clusters to the grid, and then scanning the grid. Using Fast Fourier Transform (FFT) analysis, we can determine the crystal orientation within each nanosphere.
X-ray diffraction is used to analyze the composition of nanostructures and the relative proportion of crystalline and amorphous content of nanostructures. A Bruker AXS D8 Advance micro-diffraction system equipped with a Cu-K source and a graphite monochromator to eliminate unwanted Cu-K-beta lines was used to collect XRD data. In order to obtain the relative ratio of crystalline content to amorphous content, Reitveld analysis was performed on XRD data.
The unique nanomaterial that we have been able to manufacture with ultrafast femtosecond lasers is a material that, as far as we know, cannot be formed using any other manufacturing technology. Lasers with longer pulse widths (nanoseconds, picoseconds, etc.) cannot produce this material, because only lasers with femtosecond pulse widths can cause such high temperatures that when the pulse hits the silicon surface, Si atoms are immediately destroyed. Ionize and form an ion plume above the silicon surface without losing energy to heat the substrate 41. FIG. 2 is a schematic diagram showing the formation mechanism of the laser ion plume with the SEM image of the formed nanofiber network structure. The nanomaterials we have observed are an interconnected network of mixed amorphous/crystalline nanospheres to form a nanofiber network structure. Figure 3 shows the nanoweb structure of the nanoweb material and the properties of the interconnected nanospheres.
We have inferred that this mixing of crystalline and amorphous phases in a single nanosphere is due to rapid fluctuations in plume temperature caused by laser pulses. The temperature of the plume changes rapidly, resulting in a higher temperature and lower temperature torque 42. At higher temperatures, Si ions are more likely to arrange themselves into an amorphous structure when interacting, because they will have higher thermal energy, so they behave in a poorly organized way 43, leading to more random assembly . However, between pulses, the plume temperature will be reduced by 42 because the laser will not provide additional thermal energy. Therefore, the energy of the Si ions will be reduced and they will assemble themselves into a more orderly and lower energy arrangement, thus forming Crystal structure. Over time, these Si atoms that have formed an amorphous or crystalline arrangement will collide and bond together to form a random arrangement of amorphous and crystalline nanoparticles. These will then coalesce and deposit themselves on the surface of the substrate in the form of nanospheres that have amorphous silicon and crystalline silicon within the same sphere. Figure 4 is a schematic diagram of this process.
Schematic diagram of the formation mechanism of theoretical hybrid amorphous/crystalline nanospheres.
The XRD results (Figure 5) show that in our nanofiber mesh samples, we have observed the presence of crystalline and amorphous silicon and amorphous SiO2. These spectra also showed three sharp peaks, indicating the presence of multiple orientations of crystalline silicon. For each sample, there is a sharp peak from the {111} plane to the single crystal Si, and a sharp peak on the {220} and {311} planes. The existence of these multiple orientations of crystalline silicon supports our hypothesis that we are creating nanospheres with randomly oriented silicon grains. These peaks appear in each XRD spectrum in Figure 5. Therefore, no matter what the plume conditions are, we can produce nanospheres with randomly oriented Si grains.
Another main observation result of XRD spectroscopy is that each nanofiber mesh layer has a considerable amount of amorphous content, from which we can infer that in addition to the multiple crystalline orientations of Si present in each particle, During the formation process, the amorphization of silicon will also occur to form an oxidation process and an oxidation process of amorphous SiO243. This is consistent with the proposed mechanism, in which each nanosphere is composed of silicon grains and amorphous silicon grains with different crystallographic orientations.
Using Reitveld analysis, we determined the ratio of crystalline to amorphous material (Figure 6). Due to the equivalent sample composition between 214 fs and 1428 fs at 4 MHz, the Reitveld analysis spectrum of 214 fs at 4 MHz is not included. For the same reason, the 1478 fs sample spectrum at 26 MHz is not included. Reitveld analysis showed that at 214 fs, the sample was 76% amorphous and 24% crystalline, and at 1428 fs, the sample was 40% amorphous and 60% crystalline.
The schematic diagram shows the hybrid nature of the nanospheres. HRSEM and HRTEM images show the nanofiber network structure of the nanofiber network formed at high peak power (left) and lower peak power (right) and the amorphous/ Grain and Rietveld spectra.
Through our level of control over the conditions in the Si ion plume, we can influence the particle formation mechanism to build the selected physical form of the deposited nanofiber network layer. The two aspects of physical morphology emphasized in this study are the diameter of the deposited hybrid nanospheres and the thickness of the nanomesh layer on the substrate surface. In order to design the morphology of the nanofiber mesh layer, we modified the way the laser interacts with the surface of the substrate. We can control the time interval of each laser pulse (called pulse width), the time between two laser pulses (called repetition rate), and the time it takes to ablate each ablation point (called dwell time).
Like the formation of hybrid structures, controlling the morphology of a single nanosphere, especially the diameter of the nanosphere, depends on the temperature within the ion plume. At a higher plume temperature, enough energy is provided to the nanoparticles so that they can continue to grow and combine with other nanoparticles 44, thereby increasing their size. However, when the temperature of the plume drops below a certain point, the growth rate of crystal grains is inhibited, effectively inhibiting the growth of crystal grains 41, and the formation stage of nanospheres is initiated and the particle size is determined. Since we can use laser properties to define plume temperature, we can directly control the diameter of the nanosphere.
This change in particle size is attributed to the effect of repetition rate and pulse width on the temperature in the ion plume in which the particles are forged. By changing the repetition rate, we changed the number of pulses that hit the sample surface per second. By changing the pulse width, we can change the time between the laser pulse hitting the substrate. Both of these laser parameters will affect the peak power of the laser pulse and thus the energy of the pulse. The peak power and pulse energy are calculated using the following two formulas:
Where Ppeak is the peak power, Pavg is the average power, tp is the pulse width, fp is the repetition rate, and Ep is the pulse energy
At higher energies, since more energy can be transferred to the substrate, allowing greater phonon excitation, it is expected that the plume will have a higher total temperature. When the average plume temperature is high, the growth rate of nanoparticles is high, resulting in larger particles at higher plume temperatures. This is because the particle growth rate is related to temperature. The grain growth is represented by the following formula:
Where rgrowth is the growth rate, A is an independent coefficient, Q is the activation energy required to initiate particle growth, k is Boltzmann’s constant, and T is the temperature 47. As the temperature of the system increases, the growth rate will increase exponentially, so by increasing the peak power, we will transfer more energy to the substrate and laser plume, thereby increasing the plume temperature and increasing the growth rate And size. The particles formed. As a result, both the repetition rate and pulse width of the laser pulse will have a significant impact on the particle size.
These results show that we can precisely control the material structure of each nanosphere and the morphology of the nanosphere and nanofiber network layer by controlling the interaction between the ultrafast laser pulse and the substrate material. Therefore, we have direct control over the formation mechanism of spheroids within the ion plume. The Raman properties of the formed nanofiber network layer depend on several different sources, and we can control each source by controlling the formation mechanism of nanospheres.
Another key feature of our proposed formation mechanism is the presence of polycrystalline grains and amorphous grains in a single nanosphere. In order to confirm the crystalline/amorphous grains in the nanospheres, we took TEM images of the dispersed nanospheres to determine the crystal orientation and amorphous regions.
The enhancement effect of our unique nanomaterials on SERS and Raman is dual; the hybrid structure of a single nanosphere and the physical morphology of the nanosphere on the substrate surface help to detect the analyte. The sum of these contributions results in the observed Raman signal enhancement of the R6 G dye analyte.
The enhancement effect of our unique nanomaterials on SERS and Raman is dual; the hybrid structure of a single nanosphere and the physical shape of the nanosphere on the substrate surface facilitate the detection of analytes. The sum of these contributions results in an increase in the observed Raman signal of silicon or any deposited analyte. To quantify this enhancement, the following equation 48 is used.
The ratio is to quantify the ratio of Raman enhancement between the matrix and the nanostructure by comparing the peak intensity at the characteristic Raman shift of the analyte on the surface of the matrix and the nanostructure. The proportionality needs to take into account the interaction volume of the Raman laser and the number of dye molecules that contribute to Raman enhancement in this volume. These factors are nanostructure and substrate respectively. Considering the effective surface area of ​​the nanostructure and the absorption of the nanonet structure by the dye in the Raman interaction volume. Some assumptions were made when calculating the sum. Assuming that the density of the nanostructures is the same as that of crystalline silicon, the absorbance and surface area of ​​the dye are similar to those established by Maznichenko et al. [46]. This is because the same ion plume formation mechanism is used to produce TiO2 and hybrid silicon nanostructures.
The schematic diagram of this part of the experiment, the comparison of the Raman spectrum of the nano-network structure with the spectrum of the bulk silicon wafer substrate, the Raman spectrum of each nano-network structure and the scaled spectrum of the 520 cm-1 Si peak, and each The enhancement factor of the substrate is shown in FIG. 7.
Raman enhancement schematic diagram of bare Si nanofiber mesh, Raman spectrum silicon peak on Si nanofiber mesh and Si wafer substrate, Si Raman spectrum 520 cm-1 on Si nanofiber mesh and Si nanofiber mesh The EF value of Si is the peak function power at 520 cm-1.
The schematic diagram of this part of the experiment, in which molecular dyes are used, the Raman spectra of the dye-coated substrate, the Raman spectra of each nanofiber network structure, and the 1360 cm-1 R6 G peak and 1621 cm-1 CV zoom spectra are shown respectively The maximum peaks of R6 G (@ 1360 cm-1) and CV (@ 1621 cm-1) at 10-3 M and 10-6 M concentrations and the enhancement factor of each concentration on the nanofiber substrate are compared with each The relationship between concentration and peak power), as shown in Figure 8.
Schematic diagram of Raman enhanced sensing with molecular dyes on Si nanonets, Raman spectra of R6 G and CV dyes on Si nanonets and Si wafer substrates, and R6 G and CV on Si nanonets under each laser ion plume condition Raman spectroscopy.
From the Raman spectra and EF values ​​(Figures 8 and 9), we observe that compared with the dye on the Si substrate, the intensity of the characteristic Raman peaks of the two dyes on the nanofiber mesh substrate has been significantly improved . When coated with R6 G dye or CV dye, we have observed that the overall Si substrate spectrum does not respond to the presence of the dye. However, when the dye is coated on the nanofiber mesh layer substrate, the characteristic peaks of the relevant dye are clear and clear. We observed that at a concentration of 10-3 M, the maximum enhancement of R6 G and CV dyes was 5.21×106 and 3.72×106, respectively, and the peak power was 18.7 MW. For R6 G and CV concentrations of 10-6 M, although the EF value is several orders of magnitude lower than the EF value at higher dye concentrations, the Si nanofiber network structure can still observe the enhancement of each dye spectrum. This provides a basis for the theory that our new material has high Raman activity and is easier to detect analytes than bulk silicon. Although it has been demonstrated that rough precious metal substrates show SERS enhancement 49 compared with precious metal smooth substrates, these results show that our Si nanofiber network structure alone can enhance molecular dye detection without the use of precious metal SERS enhancement .
The EF values ​​of R6 G and CV dyes on the Si nanofiber network structure at concentrations of 10-3 M and 10-6 M are a function of peak power.
Due to current theoretical advances, the SERS enhancements we have observed can be attributed to a series of chain resonances 32, 33, 34, 35 that are only possible with semiconductor nanomaterials. The resonances that realize the EF cooperative work we have observed are surface plasmon resonance, molecular resonance, charge transfer resonance and exciton resonance.
The amorphous/crystalline hybrid structure of each nanosphere provides a unique source of Raman enhancement due to its unconventional structure. The hybrid nanoparticle structure of nanospheres results in a high concentration of grain boundaries between crystalline silicon grains ([111], [220], [311]) with different orientations, and the grain boundaries between crystalline silicon grains The gain of amorphous silicon within each lattice is high nanospheres. Through TEM imaging, FFT analysis and XRD spectroscopy, we have observed that there are multiple orientation grains of crystalline silicon and amorphous silicon in each nanosphere. The concentration of the grain boundary affects the scattering mode of the Raman laser light in the nano-spherical structure. Veprek et al. have demonstrated that the grain boundaries in nanocrystalline silicon films exhibit enhanced Raman scattering intensity, but only in the process involving the coupling of electromagnetic fields to the microcrystalline body through fluctuations in the charge density in the grain boundaries. Due to bond stretching and compression (bond expansion) occurring at the grain boundary, a local electric dipole moment is formed, thereby enhancing the coupling with the electromagnetic field. Therefore, as the concentration of grain boundaries increases, the coupling with the electromagnetic field becomes stronger, and therefore the Raman enhancement increases. At higher peak powers, we observe more amorphous content, from which we can infer a higher grain boundary concentration (Figure 10). Among all the nanofiber web structures formed, the Raman enhancement observed for the nanofiber web base material produced at a higher peak power is the largest.
HRTEM images of mixed amorphous/crystalline nano-solids and single-crystal nano-solids, only amorphous nano-spheroids, polycrystalline nano-solids and amorphous/crystalline nano-hybrid solids. Schematic diagram of Mann scattering.
The material structure of nanospheres is not the only source of Raman enhancement of our materials. The size of nanospheres and the assembly of nanospheres on the surface of the substrate also provide enhancement. By changing the laser parameters to control the peak power, we can adjust the sphere size distribution of the silicon nanospheres in the nanomesh. Figure 11 shows the size distribution of nanospheres with different nanofiber network structures.
The relationship between the size distribution and enhancement factor of the nanospheres and the peak power at 0.431 MW, 2.88 MW and 18.7 MW.
These figures show that as the peak power decreases, the distribution of nanosphere sizes widens and the median shifts to larger sphere sizes. This result is consistent with our understanding of the effect of femtosecond lasers on plume temperature and nanostructure condensation and grain growth. The Raman spectrum enhancement factor of R6 G dye and CV dye on the silicon nanonet (Figure 11) shows that as we increase the size distribution of nanospheres and effectively form larger-volume large nanospheres, the enhancement of the dye peak decreases. Therefore, at high peak power, we will produce larger nanospheres, which will cause greater enhancement. However, this correlation needs further study, because as the peak power increases, not only will the nanospheres decrease, but the size of the nanogap will also decrease (Figure 11).
It has been proved and fully demonstrated that the size of the nano-gap between Raman active nanostructures has a direct and vital influence on the Raman enhancement factor of the material [44,50].
The unique structure of the nanofiber mesh layer adds an additional reinforcement effect due to the formation of nano-gaps between each nanosphere. These nano gaps are formed when silicon nanospheres fuse together on the surface of the substrate to form a fibrous structure. These nanogaps can concentrate electromagnetic fields, which can enhance SERS enhancement51. It has been shown that when the nanogap size is significantly reduced, an order of magnitude increase of 20, 43, 52 is observed. We have measured the nano gaps of three different nanofiber mesh layers, each nano layer is created with a different peak power, and the nano gap size distribution is shown in Figure 12.
The size distribution and enhancement factor of the nanogap are a function of the peak power of the nanofiber webs generated at 0.431 MW, 2.88 MW and 18.7 MW.
These distributions clearly indicate that as the peak power decreases, the size of the nanogap between the nanospheres increases and becomes non-uniform. The increase in size correlates with the decrease in Raman enhancement in FIG. 12. The maximum EF was obtained at a peak power of 18.7 MW, and the median nanogap size observed with this particular nanofiber web was about 5.15 nm. Although this value is different from the optimal nanogap size 53, 51, 52, 53, 54, 55, 56, 57, and 58 of other SERS active nanomaterials, this hybrid nanofiber mesh material is different in material properties , And the number of single nanospheres that contribute to EF due to grain boundary scattering, which is different from the currently determined optimal nano-gap size value of precious metal nanostructures for our truly silicon-based Raman active materials. The role of nanogap distribution also plays an important role in SERS enhancement59,60,61; the nanogap distribution observed in Figure 12 clearly shows evidence of the wider nanogap size as the peak power increases. Although the size of the nanogap we observed cannot be maintained as the standard value of other nanomaterials, the principle of controlling the narrow distribution of nanogap size Raman enhancement can still be applied to our nanostructures. As observed from FIG. 12, as the width of the nanogap distribution increases, the enhancement factor decreases. This is because the Raman scattering efficiency of the nano-slit 48 is insufficient, and the concentration of the nano-slit is small in the optimal range of our hybrid Si nanomaterial.
Lombardi and Birke40 established a comprehensive theory to describe and predict SERS scattering in semiconductor materials. Their theory suggests that the SERS enhancement observed with semiconductor nanomaterials is not mainly due to SPR like metal nanomaterials, but also due to the chain resonance between molecular resonance, charge transfer resonance and exciton resonance. Their theory also shows that not only semiconductors can achieve EF values ​​equivalent to precious metal nanostructures, but semiconductor nanomaterials also have the ability to achieve single-molecule sensitivity in chemical sensing applications.
When the valence band and conduction band of the semiconductor nanomaterial are similar in energy level to the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), the contribution of molecular and charge transfer resonance to the enhancement of SERS occurs. . When this energy level similarity occurs, charge transfer may occur between the valence band edge and the LUMO or the conduction band edge and the HOMO, resulting in enhanced SERS. The most intense phenomenon occurs when the transition ends at the band edge. In addition, exciton resonance occurs when excitons (electron-hole pairs) are generated in the semiconductor nanomaterial by light absorption. The exciton energy levels formed by these excitons go from below the conduction band to above the valence band, forming a sequence in the absorption spectrum. When the size of the nanoparticle is reduced below the Bohr radius of the semiconductor material, the exciton energy level diverges due to quantum confinement, resulting in the dependence of the SERS enhanced spectrum on the size of the semiconductor nanoparticle. With the further development of these hybrid silicon nanostructures, each contribution of semiconductor SERS enhancement will be explored as a research topic.
In this article, we demonstrated the unique ability of making Raman active nanomaterials from non-active Raman bulk materials. Using this silicon-based Raman active material, we are able to uniquely increase the cross section of Raman scattered photons without using mature SERS active nanomaterials. We can not only activate the Raman sensitivity with our nanofiber mesh material, but also precisely adjust the Raman activity of this nanomaterial by controlling the ion plume formation mechanism and the laser/substrate interaction parameters. The material we formed is a nanofiber mesh material, which consists of an interconnected network of mixed amorphous/crystalline nanospheres. The hybrid nature of single nanospheres, the concentration of nano-interstitials of the nano-network structure and the resonance phenomenon of the links in the SERS active Si nano-network semiconductor material are the main reasons for the increase in Raman activity. We also proved that this nanomaterial has the potential to be used as a chemical sensing device. We observed that the maximum enhancement factor of the chemical dye R6 G is 5.38×106, while the maximum enhancement factor of the CV dye is 3.72×106. With these results, we have opened up a new way to use silicon as a Raman active material for chemical sensing devices.
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Department of Mechanical and Industrial Engineering, Ultrashort Laser Nanofabrication Research Facility, Ryerson University, 350 Victoria Street, Toronto, Ontario, Canada, M5B 2K3
350 Victoria Street, Toronto, Ontario, Nano Imaging Laboratory, Department of Aerospace Engineering, Ryerson University, M5B 2K3, Canada
JAP, Dr. KV and Dr. BT co-designed the project. JAP completed the experiment and wrote the manuscript. Dr. KV and BT assisted in the results, discussion and editing of the manuscript.
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Powell, J., Venkatakrishnan, K. & Tan, B. Programmable SERS active substrate using amorphous/crystalline hybrid silicon nanomaterials in chemical and biosensing applications. Scientific Representative 6, 19663 (2016). https://doi.org/10.1038/srep19663
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Post time: Nov-04-2020