The XRD patterns of the Si NWs for the n-type and p-type samples are shown in Figure 1. The XRD peak for the n₄₀ sample is observed at 2θ = 69.16°, assigned (004) plane, and for the p₄₀ sample is observed at 2θ = 68.8°, lattice plane (004). The sharp and narrow peak observed in the n₄₀-type indicates improvements in crystallinity (Westra et al., 2010; Ghahramanifard et al., 2018).

The average crystalline size of Si NWs was calculated by the Scherrer equation (Minin and Minin, 1994): D = 0.9 λ β   cos ⁡   θ (1) where D is the crystalline size (average), θ is the diffraction angle, λ is the wavelength (0.154 nm) of radiation used for the diffraction, and β is the full-width at half maximum (FWHM) of the observed peaks (∼0.002 for both samples). The estimated crystalline size of the Si NWs is 89 nm and 73 nm for the n₄₀ and p₄₀ samples, respectively.

Scanning electron micrographs of Si NWs are depicted in Figure 2. Figures 2A–D depict magnified 45^o-tilt and cross-sectional FESEM images of Si NWs on n (Figures A,B) and p (Figures C,D) Si (100) wafers (1–10 Ω cm), keeping other parameters constant, such as chemical concentration and etching time (i.e., 40 min). The length of the Si NWs is calculated using ImageJ software; it is approximately 10 μm and 11 µm for n₄₀ and p₄₀ Si NWs, respectively, as shown in Figure 3.

Figures 2A–D reveal that, as we deal with different doping (n₄₀ and p₄₀), the length of deposited Si NWs increases due to sufficient oxidizing species present in the solution to oxidize the Si (Peng et al., 2011). The p₄₀ sample has a denser structure and has a greater length compared to the n₄₀ sample, as shown in Figure 3.

The average size (diameter) estimated using ImageJ software, of the Si NWs in n₄₀ and p₄₀ are 97 nm and 80 nm, as shown in Figures 4A,B, respectively. According to previous studies the variation in the shape of Si NWs is due to the occurrence of inhomogeneous nucleation of Ag on the Si (100) wafer (Hutagalung et al., 2017; Naffeti et al., 2020; Rouis et al., 2021). We conclude that the lower size of the p₄₀-type Si NW compared to the n₄₀-type Si NW is attributable to the energy band gap because as the size reduces the band gap increases. As discussed earlier, the p-type has a band gap of 1.89 eV, while the n-type has a band gap of 1.64 eV (Rouis et al., 2021). Figure 4 depicts the average diameter of n-type and p-type Si NWs, which we calculated using ImageJ software by selecting the Si NWs from the FESEM images. By selecting the diameter of all images of the Si NWs, we take the average length, which comes out as 80 nm and 97 nm for n-type and p-type Si NWs, respectively.

The total reflectance of Si NWs onto n₄₀ Si (100) and p₄₀ Si (100) in the wavelength range of 250 nm–850 nm is shown in Figures 5A,B. It is observed that the p-type Si NW has better light-trapping than the n-type Si NW. In the n₄₀ sample, the observed reflectance was ∼9% in the UV region (<300 nm), ∼6% in the visible region (300–700 nm), and about 3.5% in the IR region (>700 nm), which is less than the percentage of reflectance given by crystalline Si (c-Si) (reflectance = 66%). In the p₄₀ sample, the observed reflectance was ∼3% in the UV region (<300 nm), ∼4% in the visible region (300 nm–700 nm), and about 3% in the IR region (>700 nm). Moreover, the total reflectance depends upon on the length of the Si NWs; this corresponds to the reflection, which becomes lower with longer lengths of Si NWs. Total reflectance decreased for the p₄₀ sample because the charge carrier gradually increases on the textured surface (Kashyap et al., 2022).

The Kubelka–Munk function was used to transform reflection into absorption spectra (Chiu and Li, 2009; Yogi et al., 2017; Kashyap et al., 2021b): F ( R ) = ( 1 − R ) 2 2 R = K S   (4) where s denotes the scattering coefficient. If s → 0 (no scattering occurs), all the light will be either transmitted or absorbed, while if k → 0 (no absorption or transmission occur), all the light will be reflected.

The optical band gap can be estimated by (Shukla and Dixit, 2016; Venkatesan et al., 2019) F ( R ) h   υ = A ( h   υ − E g ) n (5) where F(R) is the Kubelka–Munk function, which is equivalent to the optical absorption coefficient, E_g is the optical band gap of the Si NW, A is a constant, and n is the power factor which is impacted by several electronic transitions. The plot between (F(R) hν)^1/2 vs. photon energy (E_g) for the n₄₀ type- and p₄₀-type Si NWs is depicted in the inset of Figures 5A,B, given by (i) and (ii), respectively. The estimated values are 1.64 and 1.89 eV for the n₄₀- and p₄₀-typetype samples, respectively. It is observed that the n₄₀ sample has a lesser band gap in comparison to the p₄₀ sample because the conductivity of the n₄₀ sample is higher.

This might be explained by a growth in crystallite size and a reduction in imperfections. The plot between (F(R) hν)^1/2 vs. photon energy (E_g) for n₄₀-type and p₄₀-types in the inset of Figure 5 [i.e., (i) and (ii)] illustrates that the Si NWs are linear throughout a large range of photon energy. This suggests that the synthesized Si NWs have a direct optical band gap. The optical band gap of these materials can be calculated by extrapolating the linear portion of the curve to the energy axis. It is noticed that the resulting band gap falls from 1.89 eV (for the p₄₀-type) to 1.64 eV (for the n₄₀-type). According to the usual relationship between band gap and crystallite size, the band gap rises as the crystallite size decreases (summarized in Table 1). Due to the size confinement in the Si NWs, the observed values of E_g are greater than the value of the bulk Si (1.12 eV) (Gao et al., 2007). The variation of different parameters with crystalline size is also confirmed in Figure 6.

The Raman spectra for the n₄₀ and p₄₀ samples are given in Figure 7, along with a comparison with bare Si. A Raman peak at 520 cm⁻¹ with a FWHM of 3.5 cm⁻¹ of c-Si was found due to the Raman active zone center symmetry point Γ = 0 of the Brillouin zone. The Raman spectra for the n-type and p-type bare Si were found to be identical, with no asymmetry, as shown in Figure 7. Also, the various parameters calculated from the Raman analysis has been given in Table 2

Figure 7 shows that in comparison to their bulk equivalent, the Raman spectra for Si NWs are red-shifted and asymmetrically widened. An asymmetric broadening of the Raman line-shape is caused by the combination of the Fano effect and phonon confinement (Kumar, 2013; Yogi et al., 2016; Saxena et al., 2017; Bhujel et al., 2018). Analysis of the Raman line-shape characteristics such as peak position, peak asymmetry, and FWHM which also reveals the presence of phonon confinement in both samples. The wavenumber was observed at 517.4 ± 0.26, and 518.4 ± 0.18 cm⁻¹ for the n₄₀ and p₄₀ samples respectively, with varying asymmetry ratios due to the quantum confinement (QC) effect. This range of wavenumbers of the line is generally defined with a long-range Raman scattering line.

The asymmetry arises due to the presence of nano-scale size confinement (Li et al., 2005; Kumar, 2013). Phonon confinement in Si NWs is thought to be the cause of the red-shifted, asymmetrically widened Raman line-shapes, as shown in Figure 7. The Raman line-shapes are wider on the lower energy side of the peak for n₄₀ semiconductors (asymmetry ratio 1.3), but wider on the higher energy side of the peak for p₄₀ semiconductors (asymmetry ratio 1.5), as there is more electron–phonon confinement in the n₄₀ sample.

In vertically aligned Si NW arrays, metal electrodes must be linked to the ends of the Si NWs. Copper wire was used for the connecting wires and silver epoxy was used to join the wires after they had been tested and chosen for ohmic contact formation. The Si nanowire tips must be in complete contact with a metal electrode; otherwise, a short circuit would result. Figure 8 is a representation of the ohmic contact made with the fabricated samples of Si NWs.

The change of electrons in the presence of the electric field is responsible for the change in the capacitance of the Si NWs. The dielectric properties of the Si NWs were investigated in the form of a thin film of dimensions 2 mm × 2  mm with 0.00265 mm thickness. The electrodes were deposited on thin film using silver metal to make ohmic contact. The dielectric study of Si NWs was performed in air atmosphere. The dielectric permittivity was calculated by the following formula (Pawar et al., 2021): ε ′ = ∁ t ε 0 A   (6) where ε ′ is the dielectric permittivity of material, ε 0 is the dielectric permittivity of the free space, ∁ is the capacitance, t is the thickness of the thin film, and A is the area of the thin film. The dielectric permittivity of Si NWs as a function of frequency with varying temperature is shown in Figures 9A,B. The maximum dielectric permittivity of both types of Si NWs was found at low frequency, decreasing rapidly with increasing frequency then becoming almost constant after the 1–8 MHz frequency range. The Si NWs had a more conductive nature; hence, the dielectric permittivity of the samples was higher at low frequency because of the Debye orientational polarization (Rasool et al., 2012). As the temperature decreased from 110 ℃ to 30 ℃ , the dielectric permittivity decreased and became lowest at 30 ℃ , nearly 965 and 1,128 for n-type and p-type Si NWs, respectively, because the dipoles tend to rearrange and follow the direction of the applied electric field. The optimum value of dielectric permittivity at room temperature is a strong indication for the sensing performance of the fabricated device.

The dielectric loss showed a very low loss of energy in operation with varying frequency and time for both devices, which indicates that the devices will show enhanced sensing phenomenon for the detection of NO₂ gas. The dielectric loss as a function of frequency with variation of temperature is depicted in Figures 9C,D. The dielectric loss decreased rapidly as the frequency initially increased until 1 MHz and 0.5 MHz for n-type and p-type Si NWs, respectively, and became constant but as the temperature increased from 30 ℃ to higher temperatures the dielectric loss increased with an increase in frequency because of the ion exchange anisotropy phenomenon (Khan et al., 2020), where at high temperature the ions tend to move from the barrier and produce polarization at a large scale.

The AC conductivity as a function of frequency with varying temperature is depicted in Figure 10. It is clearly observed from Figures 9A,B that as the frequency increased from 100 Hz to 8 MHz and the AC conductivity also increased in linear order and became highest at 8 MHz for both types of sensing materials. The AC conductivity of the n₄₀ Si NW increased with increasing temperature from 30°C to 110°C and became highest at 110°C, while this was found to be lower for the p₄₀ Si NW. The change in conductivity with increasing temperature is observed due to the hopping conduction mechanism (Saidi et al., 2017).

The instrument used for the gas-sensing analysis is shown in Figure 11, which comprised a LCR meter furnace followed by a gas-sensing arrangement.

The sensing mechanism behind the response toward oxidizing gas (NO₂) is based on the change in resistance or conductance of the semiconductor devices in the presence of air and an oxidizing gas environment. The change in resistance as a function of frequency at 30 ℃ temperature for n₄₀-type and p₄₀-type Si NWs devices is depicted in Figures 12A,C. It is observed from Figures 12A,C that as the frequency increased the resistance decreased rapidly until 1 MHz and then became almost constant at higher frequencies. Furthermore, the resistance of both the devices decreased as the temperature increased from 30 ℃ to 110 ℃ , which is depicted in Figures 12B,D. The reason behind the decrement of sensor resistance in an air atmosphere at higher temperatures is due to adsorbed oxygen molecules losing electrons at the surface of the sensor, which further move through the sensor and increase the flow of current; hence, the resistance of the sensor decreases simultaneously. The maximum resistance of both sensor devices was observed at 30 ℃ in air; hence, the gas-sensing experiments were performed at 30 ℃ with varying concentrations of oxidizing gas from lower to higher concentrations (10 ppm–250 ppm).

The ratio of change in resistance matter for the identification of gas response was calculated using Eqs 7, 8. The objective of the present study is to investigate the change of resistance at 30°C with varying concentrations of NO₂ gas for the fabrication of room-temperature detection devices.

Figures 11B,D show the change in resistance as a function of frequency with varying temperatures of the sensor devices in the presence of oxidizing gas. The change in resistance or conductance also depends on the n-type or p-type behavior of the devices. Usually, the resistance of n₄₀ type Si NWs in the presence of oxidizing gas such as NO₂ increases with increasing the concentration of the target gas (10 ppm–250 ppm) while decreasing for p₄₀-type Si NWs owing to the dual modulation of space charge constituency between the adsorbed oxygen with the sensing element and NO₂ gas atmospheres. This occurs because of the physically powerful electron-withdrawing capabilities of NO₂; the accumulation of a hole in p₄₀-type Si NW based sensors occurs due to the withdrawing of electrons, which increases the conductivity of the sensor (Cao et al., 2013) with increasing concentrations, while decreasing for n₄₀-type Si NW based sensors.

The gas-sensing behavior of the sensors was investigated by observing the change in resistance in air and oxidizing gas atmosphere using the following relationship (Pawar et al., 2020): R   ( % ) = ( ∆ R R g ) × 100 = ( R a − R g R g ) × 100 i   R g > R a (7) R   ( % ) = ( ∆ R R a ) × 100 = ( R a − R g R a ) × 100 i f   R a > R g (8) where R (%) is the sensitivity of the target gas, ∆ R is the change in resistance, and R a   a n d R g are the resistance of the air and the target gas, respectively. n₄₀-type and p₄₀-type Si NW based sensors were used for the detection of oxidizing gas such as NO₂.

The gas-sensing response as the function of frequency with varying gas concentrations from 10 ppm to 250 ppm is shown in Figures 13A,B. The sensing response of the n₄₀-type Si NW based sensor showed that the response toward NO₂ gas initially decreased with increasing frequency, reaching a minimum at a certain frequency and then increasing again at higher frequencies. The concentration of the oxidizing gas introduced in the gas-sensing unit, varying from 10 ppm to 250 ppm, exhibited an excellent gas response at lower concentrations, which is at 50 ppm, and a minimum response at higher concentrations at all frequency ranges because the fabricated sensor had a small dimension suitable to detect gas precisely at lower concentrations, but at higher concentrations, the adsorbed layer of the O⁻² and gas-interacting surface was saturated; hence, the free electrons were not available in the sensor element, which can propagate current through the device, and as a result the resistance increased rapidly, which reduced the gas response of the sensor, while decreasing in the case of the p₄₀-type Si NW sensor. In the presence of NO₂ gas, as the frequency increased, the resistance of the sensing device decreased due to saturation polarization which improved the gas sensitivity. A chemiresistive sensor’s sensitivity is a resistance-dependent mechanism as the maximum change in resistance was observed in the presence of NO₂ gas, while in the air atmosphere, the gas response increased.

The gas response R (%) of n₄₀-type and p₄₀-type Si NW sensors as a function of concentrations at specific frequencies of 1, 4, and 8 MHz is depicted in Figures 13C,D.

Figure 13C shows the gas response of the n₄₀-type Si NW based gas sensor, with the maximum sensing performance at 50 ppm found to be 35%, 63%, and 68% at 1, 4, and 8 MHz, respectively, while this was 75%, 68%, and 62% for the p₄₀-type Si NW based sensor, as illustrated in Figure 10D. The aforementioned results revealed that the p₄₀-type Si NW based sensor is the best for the detection of NO₂ gas at room temperature and at lower frequencies, while the n₄₀-type Si NW-based sensor is the best for the detection of NO₂ gas at room temperature and higher frequencies.