The remaining of the Ag nanoparticles would sink into holes and longitudinal and lateral dissolution of silicon triggering the formation of SiNWs arrays [ 59 ] Figure 3. Generally, fabrication of SiNWs via a top-down approach which employed the application of advanced nanolithography tools on silicon-on insulator SOI is mostly compatible with conversional complementary metal oxide-semiconductor CMOS technology that typically consist of deposition, etching and patterning steps.
Basically, the SiNWs fabrication started from the bulk material and scaled down into a single SiNW or SiNW array that can be formed with the help of nanolithography techniques such as electron beam lithography EBL [ 61 ], lithography patterned nanowires electrodeposition, nanoimprint lithography [ 62 ], and photolithography.
For example, Park et al. Pham et al. Tong et al. This technique is suitable for any conventional microtechnology clean room facility. This novel wafer-scale technology process uses a combination of angled thin-film deposition and etching of a metal layer in a precisely defined cavity with a single micrometer-scale photolithography step.
The key factor to provide an improved dimensional control compared to other methods is a precisely defined cavity that permits controlled removal of the metal layer with an angled wafer level ion beam that resembles a nanostencil structure patterned directly on the wafer surface, which minimizes lateral spread of the deposited metal. Chen et al. In the study reported by He et al. Lateral bridging growth was first demonstrated for GaAs nanowires [ 69 ] and recently for Si nanowires [ 70 ].
However, well controlled growth and device operation were not achieved. He et al. Table 1 shows a brief description on the SiNW synthesis as reported above. In this section, we demonstrate the latest applications of SiNW based sensor using different detection methods including surface-enhanced Raman scattering SERS , fluorescence, electrochemical methods, and field-effect transistors FET that have been fabricated.
Surface-enhanced Raman scattering spectroscopy based on a metal nanostructure has gained attention due to the enhancement of Raman signal that reached 10 12 —10 15 compared to normal Raman signals. The authors further explored the application of SiNW arrays coated with Ag nanoparticle as SERS substrate for protein and immunoglobulin detection [ 72 ]. This may be due to the fact that the immune reaction between migG and gamIgG changed the conformation structure in terms of amino acid residue, functional group, and orientation bonds thus displaying different Raman signals.
Zhang et al. Study of Shao et al. Furthermore, the group of Jiang et al. Han et al. The authors also studied the detection of E. Su et al. Since the position of carboxyfluoresceine is close proximity with AuNPs-SiNWs in terms of stem loop conformation structure, leading feeble intensity of fluorescence. When DNA hybridization happened, the stem loop of MBs underwent conformation changes resulting in spatial separation of the carboxyfluorescein and AuNPs-SiNWs, thus enhancing the fluorescence intensity.
There is another research by Maxwell et al. It was found that the optical sensor has high selectivity as it has the lower fluorescence signal with no complementary DNA due to the absence of Cy3 labeled target DNA, which is more than 30 lower than complementary DNA.
Another application of SiNWs has been reported by Han et al. Their finding demonstrated that fluorescence intensity as the result of the binding of both anti-Ig G and anti-Ig M was greatly enhanced using SiNWs compared with planar substrates Figure 8.
New type of optical sensor based on SiNWs for Cu II detection, an important element for hematopoiesis, metabolism, growth, and immune system, was constructed by the group of Mu et al. The presence of other metal ions such as mercury, zinc, cadmium, ferrum, cobalt, and plumbum in this study did not have significant interference effect on the selectivity of an optical sensor based on QIEOT-SiNWs.
Miao et al. It was found that the modified SiNWs fluorescence sensor MsiNWs showed a rapid fluorescence response towards NO in a few seconds and was stable for days at room temperature. Interestingly, the fluorescence images of single MSiNW before and after reacting with NO showed a fine spatial resolution when it was combined with microscopy techniques.
In the study of Zhuo et al. Meanwhile, unsupported metal nanoparticle without SiNWs was easily aggregated due to the high surface energy of the small nanoparticle and the large particles were expected to meet stronger steric hindrances in the coupling. The basic principle of electrochemical detection is based on redox reaction as a result of chemical reaction between immobilized biomolecule or chemical species on working electrode and target analyte which finally produces measurable electrical current [ 83 ].
The novel nonenzymatic method for detection of hydrogen peroxide H 2 O 2 with high sensitivity and selectivity based on electrochemical method using nanostructure of Ni OH 2 -SiNWs was reported by Yan et al.
In their study, the SiNW array was prepared using a chemical etching process followed by deposition of nickel film through electroless technique.
Based on previous studies, there is a great interest in the application of SiNWs functionalized with metal nanoparticle due to enhancement of electron transfer of enzyme activity and electrical conductivity.
The authors demonstrated that SiNWs-AuNPs modified carbon electrode exhibits high sensitivity compared with the unmodified carbon electrode Figure 9.
It was clearly shown that SiNWs enable to increase the electrical conductivity of modified electrode and facilitate electron transfer of acetylcholinesterase AChE for organophosphate pesticide detection.
The authors found that the SiNWs modified electrode showed rapid response in the detection of acetylcholine in the range of 1. According to Su et al. The author found that SiNWs electrode produced a weak peak current. Kwon et al. SiNWs-FET sensor consists of three electrodes, which are source, drain, and gate electrode, and its work is based on conductive change of the carrier on the surface of SiNWs either accumulation or depletion charge.
When negative charged molecules bind on n-type SiNW surface it results in accumulation of the negative carriers thus increasing the resistance reading and vice versa if using p-type SiNWs [ 92 ]. Gao et al. In this work, they managed to improve the sensitivity of SiNWs-FET sensor by optimization of probe concentration, buffer ionic strength, and the gate voltage. Since DNA probe possesses a negative charge due to the phosphate group that binds on SiNW surfaces via SAM layer of amine group and carboxyl group as described before, leading to an increase of resistance and same observation obtained when hybridization occurred.
The authors found that E-DNA probe helps to enhance sensitivity of hybridization signal in terms of resistance change, which was This can be explained such that E-DNA used in their work does not have an anionic backbone of the phosphate group. The results showed a smaller conductance change of 8. The authors utilized sol-gel approach to immobilize anti-CRP and anti-PSA on SiNW arrays instead of using chemical modification to avoid loss of protein activity and maintain conformation of antibody.
It was found that integration of sol-gel method exhibited high sensitivity with a low amount of serum for simultaneous detection of CRP and PSA in the range of 0. Moreover, Zhang et al. Svendsen et al. For example, Bi et al. Table 2 summarized the applications of SiNW in different techniques as described above.
We noticed that the hybrid of SiNWs with metal nanoparticles such as gold nanoparticles AuNPs and silver nanoparticles AgNPs presents a new generation of sensing material electrodes with excellent catalytic activity and high conductivity that can greatly enhance the performance of sensors in terms of sensitivity and selectivity.
We believe that the integration of SiNWs as sensing nanomaterials has great interesting in future for fabrication of of miniaturized sensor devices due to their unique properties. In our opinion, the electrochemical and electrical detection showed a great promise in realizing a miniaturized sensor based on SiNWs due to its advantages including high detection, portability, and simplicity of the procedure.
However, a few challenges must be overcome. Firstly, the fabrication technique of SiNWs either bottom-up approach or top-down approach must be strongly developed to ensure the reliable electrochemical and electrical SiNW sensor.
Highly controlled SiNW fabrication in terms of surface, diameter, length, alignment, and so forth should become the main barrier in the bottom-up technique and therefore the parameter manipulation of SiNW synthesis has to be established as the initial step for development of reproducible sensor based SiNWs. Secondly, since most of the bottom-up techniques produce SiNW suspension followed by dispersing method for the SiNW integration in sensor system, it is quite hard to control the distribution align and identical desired direction.
Therefore, there is a need for the development technique of casting or alignment of SiNWs in order to control their distribution and quantity. In contrast, top-down approach can provide high control of SiNW synthesis and alignment; however, the high cost of fabrication of SiNW sensors became the main barrier to develop a low cost portable sensor involving advanced lithography tools.
For the top-down approach, there are great efforts to find another low costing and effective method for fabrication of reliable sensors. In summary, SiNW is the promising nanomaterial sensing in the future.
This is an open access article distributed under the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Article of the Year Award: Outstanding research contributions of , as selected by our Chief Editors. Read the winning articles. Journal overview. Special Issues. Received 04 Sep Accepted 03 Nov Published 28 Dec Abstract The application of silicon nanowire SiNW as a sensing nanomaterial for detection of biological and chemical species has gained attention due to its unique properties.
Introduction In the last decades, biological and chemical sensor technologies have received a tremendous interest among research areas in various applications due to their efficiency in monitoring and regulating many areas such as toxicology testing [ 1 , 2 ], food industry [ 3 , 4 ], medical diagnostics [ 5 — 7 ], environmental monitoring [ 8 , 9 ], and drug industries [ 10 , 11 ].
Vapour Liquid Solid VLS Wagner and Ellis have reported for the first time silicon wire synthesis in vapor phase condition using silicon substrate coated with liquid Au droplet [ 39 ]. Figure 1. Figure 2. Reprinted with permission from [ 50 ]. Figure 3. Reprinted with permission from [ 59 ]. Figure 4. Single mask silicon nanowires DEA fabrication process. Reprinted with permission from [ 63 ]. Table 1. Figure 5. Reprinted with permission from [ 73 ]. Figure 6. Reprinted with permission from [ 74 ].
Figure 7. Background and noncomplemetary sequence are presented as control. Reprinted with permission from [ 75 ]. Figure 8. Immunoassays with micropatterned SiNWs. Reprinted with permission from [ 78 ].
Figure 9. Reprinted with permission from [ 47 ]. Figure Scan rate was 0. Reprinted with permission from [ 85 ]. Hybridization was demonstrated by 0. The error marks the point when the solution was changed. Reprinted with permission from [ 93 ]. The silicon surface exhibited excellent stability in solution water, chloroform, acid, or basic solution even if a small oxidation was observed when exposed to air under ambient conditions. The authors suggested that alkyl monolayer formation was based on a radical mechanism initiated by diacyl peroxide.
This latter undergoes homolytic cleavage to form two acyloxy radicals which decompose to carbon dioxide and an alkyl radical. This latter, in turn, can abstract hydrogen atoms at an adjacent Si-H site to give a new silicon radical and thus promote a new alkene addition site Figure 3 b [ 45 ].
However, further investigations by Chidsey et al. Mechanism for radical-based hydrosilylation, a Silicon radical formation process, b Reaction of alkene molecules with silicon radical. This leads to a silicon radical which later reacts with olefine to form alkyl monolayers, stable up to K under vacuum. Bateman et al. Ferrocenyl-derivatized porous silicon was also prepared in the same way and characterized by cyclic voltammetry [ 47 ].
Feng and co-workers also found that thermally-driven chemical reactions occurred between C60 monolayer molecules and H-terminated Si substrate surfaces. In , Sieval et al. Monolayer formation was characterized by water contact angle measurements, attenuated total reflection ATR infrared spectroscopy, and X-ray reflectivity.
From the water contact angle data, the authors deduced ordered monolayers on the Si surface comparable to the values reported for thiols on gold [ 49 ]. In addition, the infrared spectroscopy spectra showed bands in the C-H stretching region [ 50 ]. The main limitation of the thermal hydrosilylation approach is the large excess of unsaturated molecules required.
Henriksson et al. Pioneered by Terry et al. The process is generally easy to set up, and favours both chemical stability and good surface coverage. The argument of a homolytic dissociation of surface Si-H and Si-Si not being possible to sustain under white light illumination, the proposed mechanism would be light-induced radical formation from impurities in solution and a hole-related mechanism favouring the reaction on illuminated substrates [ 53 , 54 ].
Streifer et al. The reaction starts by the covalent attachment of primary amines, protected by t-BOC, to SiNWs using ultraviolet light at a wavelength of nm. After the deprotection step, these amine groups react with an intermediate cross-linker sulfo-succinimidyl 4- N-maleimidomethyl cyclohexane carboxylate, which will subsequently react with the DNA. Surface characterization performed by XPS has proved that the photochemical attachment of the t-BOC-protected amine proceeds without any significant oxidation of the sample.
This strategy was also followed by Zhang et al. From TEM characterization, the authors concluded that the surface was uniform and oxide free. Electrochemistry was used by Allongue et al. The latter can then abstract a surface hydride to form silicon radicals Scheme 2 b , which can react with another aryl radical to form the silicon—carbon bond Scheme 2 c. The proposed mechanism of electrochemical reduction of diazonium salts on hydride silicon is summarized as follows [ 56 ]:.
This approach has the advantage of limiting silicon oxidation in the final surface because the cathodic process makes the surface rich in electrons during the reaction, and thus less susceptible to nucleophilic attack by water. Some examples of organic compounds modified by this method are shown in Figure 4. Terminal alkynes can also form silicon-bonded monolayers by electrografting under a negative bias [ 58 ]. Note that the reaction is more specific for acetylenes.
The reaction scheme is described as follows:. Apart from the various hydrosilylation techniques describe above, Lewis acid e. This allows a high selectivity and specificity of the corresponding solution-phase reaction. H atoms are the most commonly used passivating agent for surface preparation, but some approaches based on halogen-terminated surfaces have been reported [ 61 ]. Haick et al.
The chlorine terminated SiNWs were alkylated by the Grignard route [ 62 ]. The silanization reaction is described as the binding of organofunctional groups to the oxide shell covering the silicon surface to form siloxane bonds Si-O-Si [ 63 ].
Firstly, we have the Head group -SiX 3 which forms the chemical bond with the substrate. A high density of these leads to a good surface coverage. Thereafter, we have an alkyl chain - CH 2 n- functioning as spacer groups, and finally the surface group -R , which can be substituted with different functional groups according to the desired applications Figure 6.
Schematic diagram showing the different surfactant molecules linking to the Si-substrate. Silane compounds are commonly used on oxide surfaces, where they can covalently bind to the surface by the transfer of a proton from the hydroxylated surface to a silane leaving group, eliminating an alcohol in the case of methoxy or ethoxysilanes or HCl in case of chlorosilanes [ 64 , 65 , 66 , 67 ].
However, the layer formed was physisorbed, and therefore weaker [ 68 ]. The oxide layer formed on the silicon surface, at room temperature, contains a high density of traps, which are unfavourable for electrical purposes. For this reason, it is recommended to eliminate this layer and to thermally grow a new one. In all cases, deep cleaning of the substrate is a prerequisite to obtain a clean oxide layer with high density of silanol groups Si-OH on the surface, acting as anchor sites for silanization reactions.
In general, the functionalization is carried out by the simple immersion of the freshly hydroxylated surface into an organic solution containing a silane derivative [ 69 , 70 ]. Toluene is commonly used as a reaction medium. The SAM formation begins with the physisorption of silane molecules on the hydrated silicon surface. It should also be noted that silanol groups attached to the silicon surface are susceptible to condensing with these neighbours [ 71 , 72 ]. Silanization mechanism. Reprinted from The Lancet, , D.
Aswal et al. As a selective sensor can generally be configured from silicon nanowire devices by linking recognition receptor groups to the surface of the nanowire [ 73 ], silanization remains the most frequently-applied method to functionalize the native silicon oxide coating on silicon nanowires. Patolsky et al. With a view to preventing nonspecific adsorption of the target species, the authors passivated unreacted aldehyde groups on the silicon surface using an ethanolamine solution.
To demonstrate the surface modification procedures, electrical measurements were carried out [ 74 ] Figure 8. Silicon nanowire modification scheme for DNA recognition [ 74 ].
Salhi et al. The alkyl-modified silicon surface was produced by soaking the hydroxyl surface in hexane solution with M octadecyltrimethoxysilane OTS for 2 h at room temperature under a nitrogen atmosphere.
Surface functionalization of the SiNWs by sulfonate groups was performed in two steps. The first was to shape the thiol layer on the silicon surface by submerging it in 3-mercaptopropyltrimethoxysilane dissolved in toluene. The success of the oxidation reaction was confirmed by XPS analyses. We should point out that a long hydrocarbon chain tends to reduce thermal stability and change the mechanical properties. Likewise, the alkyl surface concentration decreases as the n-alkyl chain length increases [ 84 ].
Let us focus for a moment on 3-aminopropyltriethoxysilane APTES , an alkoxysilane which is a commonly used to modify silicon nanowires and is the main coupling agent to synthesize aminated silane films.
Indeed, APTES favours the adhesions of polymer films on glass [ 85 ], promotes protein adhesion [ 86 ] for biological implants, and is used to attach metal nanoparticles to silica substrates [ 87 ]. However, APTES films are subject to forming disordered monolayers or multilayers because of the favourable head-and-tail group interactions [ 88 , 89 ].
Even though the problem of disordered monolayers can be overcome by using an intense electrical field [ 90 ], it is necessary to establish a set of conditions favouring good film quality.
The authors highlighted the fact that the morphology and groth kinetics of APTES films were affected by reaction time, solution concentration, and temperature. They showed that silanization enabled a higher density of organic compounds around SiNWs and thus maximized the intensity of spectra recorded by spectroscopic techniques. There exists an alternative approach to modifying the silicon oxide layer by electrostatic interaction.
Known as layer-by-layer LbL deposition [ 92 ], the process takes place in solution and involves the successive adsorption of oppositely-charged species on the substrate surface, as schematically outlined in Figure 9. Film deposition can be carried out by different techniques, including dip-coating, spin-coating, spray-coating.
Adsorption times per layer range from seconds to minutes [ 76 , 92 ]. Film deposition is generally achieved using an adsorbate concentrations of several milligrams per millilitre to avoid impoverishment of the solution during the fabrication of the multilayer stack.
Note that the film thickness increases linearly with the number of deposition cycles. Polyelectrolytes polyanion or polycation are very often preferred to small molecules because of their strong electrostatic attraction, favouring the good adhesion of the layer to the underlying substrate. However, the properties of the final materials are closer to those of the polymer than those of underlying substrate. Vu et al. This sequence was repeated several times to build a stack of six bilayers [ 93 ].
Steps 1 and 3 correspond respectively to the adsorption of the polyanion and to the polycation while steps 2 and 4 are washing steps; b simplified image of the film formed on the substrate during steps 1 and 3; c chemical structures of the polyions used: sodium salt of poly styrene sulfonate violet and poly allylamine hydrochloride [ 92 ].
Liu et al. The sample was washed after each soaking with ethanol to remove excess reactant from the surface. The authors indicated that the thickness of the MOF shell was about 80— nm after 40 cycles, implying an average thickness of 2—3 nm for each deposition cycle. Claus et al.
As before, several steps are involved in the fabrication process. The substrates were first modified by Naminoethylaminopropyl trimethoxysilane via silanization. A positive charge was applied to the amine monolayer by adjusting the pH and immersion in an aqueous solution of polyimide precursor at pH 8.
By the repetition of these two processes in a cyclic fashion, a multilayer film was obtained, in principle without any limitation on the thickness of the final materials Figure The major advantages of layer-by-layer adsorption from solution are that many different materials can be incorporated into individual multilayer films, and the film architectures are completely determined by the deposition sequence.
Illustration of multilayer formation by electrostatic molecular self-assembly. Reprinted from Liu, Wang, and Claus, Appl. The Table 1 summarize the main characteristics of each functionalization approach. For nanosized materials, surface properties are become paramount due to their large surface-to-buck ratio. This makes them particularly attracting in the applications where such properties are exploited.
On the grounds that silicon surface can be facilely functionalized, leading in this way to different surface properties high surface-to-volume ratio favourable for molecule adsorption, efficient conduction, mechanical and optical properties , the chemical sensitivity on silicon nanowires has been exploited to design chemical biosensors and sensors [ 16 , 20 , 95 , 96 ]. The principle of sensors defined as devices that incorporated with sensing materials detection is based on interaction between a molecular probe and its target, thus the change of physical properties of the sensing material is translated into a quantifiable signal via a transducer.
Indeed, the radio frequency identification RFID is one of the major technologies in the field of identification and has grown considerably, since its principle was introduced 60 years ago [ 97 ]. This is a technique for the automatic capture of information contained in a label, by radio waves with a remote reading facility. Even if the information exchanged is mainly for identification purposes, more and more tags are used as wireless sensors [ 98 ].
However, the RFID solution is complex; it requires the use of a chip and a communication protocol which induces costly tags. It is why new identification and sensor systems are expected. Among them, a solution without any chip is very promising [ 98 ].
This is why the development of chipless radar tags in RF has grown significantly in recent years [ 98 , 99 ]. The principle of information encoding is based on the generation of a specific electromagnetic signature, on the basis of a radar principle a wave is sent to the tag, and the significant information is contained in the tag backscattered signal.
The shape of the conductive pattern forming the tag is imposed in order to have a specific and perfectly recognizable signature. Apart from that, the appearance of the tag is similar to that of a barcode. Thus, the information is no longer stored with an electronic chip, as can be done in traditional RFID tags, but directly "written" on the label. Therefore, a key point is the relationship between the geometry of the conductor pattern and the RF signature expected. Most often these shapes define resonators whose resonance frequencies are in the UWB band between 3.
A significant number of papers have now been published that explore the use of silicon nanowires in sensor devices. Many of them are based on the reading principle used in chipless RFID. An example, RFID humidity sensors were realized by deposing few drops of SiNWs dissolved in ethanol in the center of the scatterer where the electric field is maximum [ 95 ].
A random distribution of nanowires was observed after alcohol evaporation. Briefly, RF measurement consists in irradiating a target with an electromagnetic field then measuring the backscattered field. The variation of electromagnetic response is related to the use of some sensitive materials which induce variable conductivity and permittivity. The change in conductivity will in turn lead to a variation in the tag response level, whereas the permittivity will impact the resonant frequency or the phase of the scatterer.
The Figure 11 shows the experimental set-up made of two antennas in an anechoic chamber, a plastic box partially filled with water in front of antennas and a chipless tag modified with SiNWs placed inside the box. A probe immersed in the water and connected to a hygrometer is used to monitor changes in temperature and relative humidity RH.
Radio frequency measurements set up for hygrometry variation [ 95 ]. The observed change was attributed to the strong sensitivity of the SiNWs to the humidity. The reproducibility of measurement was studied by comparing the variation in resonance frequency of four different measurements carried out on the same tag. They observed maximum error of 9. These results reveal that the sensors do not provide an absolute value but a threshold value.
In order to verify the influence of silicon nanowires, the authors performed the same measurements on prototype without nanowires which shown no significant variation in the same humidity range Figure 12 b. This example provides proof of an extremely simple use of SiNWs for the realization of a wireless sensor. Therefore, using a radar approach with a label consisting solely of a conductive pattern and a localized deposit of SiNWs, it is possible to trace the humidity value in the air at a distance of a few tens of centimeters.
A silicon nanowire array was employed as a substrate to covalently graft various fluoroionophores for the detection of heavy metals ions [ ] since it permits high carrier mobility, and so high sensitivity to analytes adsorbed on their surfaces. Fluorescence method was used to investigate the sensing effect due to its high sensitivity and simplicity for environmental analysis. First of all, suspensions of SiNWs-based fluoroionophores was dispersed in a mixture of ethanol and 2-[4- 2-hydroxyethyl piperazinyl] ethanesulfonic acid HEPES buffer at pH 7.
However, it is well known that rhodamine molecules have non-fluorescence in the spirocyclic form and fluorescence in its open form.
The need for gas detection devices to control air pollution in real time continues to grow due to environmental and health concerns. Semiconductor materials such as metals oxides have been intensively studied as conductometric sensors due to their good chemical stability and functional properties [ , ]. The latter leads to a decrease in conductance of the structure.
However, the negatively charged oxygen species can be removed by exposing the semiconductor to a reducing gas and therefore improve its conductance.
In short, oxidizing gases reduces the electrical conductance in n-type semiconductor while reducing gases leads to an increase in the conductance.
The same trends are observed for p-type gas sensors but in the opposite direction. Note that the target gases can be adsorbed on the surface of a semiconductor either by: physisorption weak interaction and chemisorption strong interaction of gas molecules with solids. Jump to site search. You do not have JavaScript enabled. Please enable JavaScript to access the full features of the site or access our non-JavaScript page.
Issue 44, From the journal: Journal of Materials Chemistry. Silicon nanowires: the key building block for future electronic devices. Lih J. Chen a. Secondly, the radius of the second semicircle greatly increases indicating larger thickness of SiO x layer. The choice of the frequency was dictated by the necessity to attain depletion of the charge carriers while avoiding diffusion limitations. Resulting plots for different pH of etching solutions and derived flat band potential values are summarized in Figure 4 and Table 1.
This effect corresponds well to smaller band bending and smaller capacitance of the interface layers. Total reflectance spectra of SiNW layers are presented in Figure 5. Also in this case, reflection peaks appear at and nm, which are associated with the c-Si direct band gap.
Low total reflection of SiNW layers can be explained by the strong scattering and absorption of light in the visible region of the spectrum, which can lead to a partial localization of light in nanowires Gonchar et al. Figure 5. The inset in Figure 6A shows a close view of the Raman scattering peaks.
SiNW's diameter is about 50— nm and far from the quantum confinement regime. That's why peaks and shapes of the interband PL and Raman scattering for all samples are similar to c-Si. At the same time the intensities of interband PL and Raman scattering for SiNWs increase strongly as opposed to corresponding value for c-Si. This effect can be explained by the light localization in such inhomogeneous optical medium as SiNW layers Gonchar et al. Figure 6. The signal intensity of the samples here was normalized to the signal intensity of c-Si substrate dash line.
Thus, the intensity of Raman scattering and interband PL increases by 3—5 times and 3 times, respectively, for all SiNWs layers in comparison with c-Si. Let's remember, that the shape and length of SiNWs is changed with the increasing of pH value of H 2 O 2 :NH 4 F: the length is decrease and the shape is changing from vertical cilinders to pyramidal like structures see Figure 1.
Based on this, we can conclude that the intensity of Raman scattering and interband PL depends not only on the length of SiNW, but also on their shape. The structural and optical properties of SiNWs, prepared by the metal assisted chemical etching method, where the commonly used hydrofluoric acid HF has been successfully replaced with ammonium fluoride NH 4 F , and their dependence from the pH of the etching H 2 O 2 :NH 4 F solutions were studied in detail for the first time.
It is shown that as the pH of H 2 O 2 :NH 4 F decrease, the shape of the nanowires changes from pyramidal to vertical. The length of SiNW arrays demonstrated non-linearly pH dependence. By impedance and Mott-Schottky measurements it was shown that the SiO x layer thickness and electrolyte potential are strongly affected by pH. With increasing pH of electrolyte OCP of the cell decreases reducing silicon oxidation rate.
Also the intensities of interband PL and Raman scattering for SiNWs increase strongly as opposed to corresponding value for c-Si, but depends both from the length and the shape of SiNWs: they were larger for long pyramidal nanowires. This effect can be explained by the light localization in such inhomogeneous optical medium as SiNW layers Thus, SiNW, manufactured using weakly toxic NH 4 F, have great potential for applications in the field of photovoltaics, photonics, and sensorics.
GZ performed the SEM measurements. AE performed impedance and Mott-Schottky measurements. KG and LO performed the general data analysis and discussion of the obtained data. All authors read and approved the final manuscript. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Bai, F. Bertolini, J. Hydrofluoric acid: a review of toxicity.
Emergency Med. Optical properties of individual silicon nanowires for photonic devices. ACS Nano 4, — Bruggeman, D.
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