Abstract
In this study, porous silicon (PSi) was used to manufacture gas sensors for acetone and ethanol. Samples of PSi were successfully prepared by photoelectrochemical etching and applied as an acetone and ethanol gas sensor at room temperature at various current densities J= 12, 24 and 30 mA/cm2 with an etching time of 10 min and hydrofluoric acid concentration of 40%. Well-ordered n-type PSi (100) was carefully studied for its chemical composition, surface structure and bond configuration of the surface via X-ray diffraction, atomic force microscopy, Fourier transform infrared spectroscopy and photoluminescence tests. Results showed that the best sensitivity of PSi was to acetone gas than to ethanol under the same conditions at an etching current density of 30 mA/cm2, reaching about 2.413 at a concentration of 500 parts per million. The PSi layers served as low-cost and high-quality acetone gas sensors. Thus, PSi can be used to replace expensive materials used in gas sensors that function at low temperatures, including room temperature. The material has an exceptionally high surface-to-volume ratio (increasing surface area) and demonstrates ease of fabrication and compatibility with manufacturing processes of silicon microelectronics.
1 Introduction
Over the last few decades, the production of nanoparticles has garnered a great deal of interest. These materials possess physical characteristics that crystalline semiconductors do not [1]. Given its unique physical characteristics, research on porous silicon (PSi) as a novel morphological form of monocrystalline silicon is on the rise [2]. PSi is a network of nanoscale silicon wires and voids that is formed when crystalline silicon wafers are electrochemically etched in a hydrofluoric acid-based electrolyte solution under constant anodization conditions, such as etching time, current density, HF concentration and Si orientation. PSi structures have good mechanical robustness, chemical stability and compatibility with existing silicon technology, so they have a broad range of potential applications such as waveguides, 1D photonic crystals, chemical sensors, biological sensors and photovoltaic devices [3]. PSi is an optimum choice for sensing applications because these structures cannot be formed in other porous materials (zeolites and aluminium oxides). PSi might also be easily functionalised using surface chemistry techniques or infiltrating it with other materials to provide specificity to target molecules [4]. Photoelectrochemical PSi is a simple process that uses a gold cathode and a silicon wafer anode submerged in a hydrofluoric acid (HF) electrolyte. It has a steady current source, which is commonly used to maintain a consistent HF tip concentration, leading to a layer with homogeneous porosity [5]. PSi has intriguing properties such as a low refraction index, a highly textured surface that can improve light trapping and minimise reflection losses in a solar cell and tenability of the band gap of PSi that can be utilised to maximise sunlight absorption [6]. Thus, PS is a promising platform for developing biosensors, gas sensors and optical devices [7,8,9].
The main objective of this paper is to manufacture a gas sensor using PSi layers prepared with different current densities. The PSi sensor's sensitivity was based on the nanowire structure of the surface and the nano size of the pore. Results demonstrated the good sensitivity of the PS surface for acetone and ethanol, and performance varied with the different current densities of PS fabrication.
2 Materials and methods
An n-type (100) silicon wafer with 580±0.25 μm thickness of 1.5–4.cm resistance was used to make the PSi layer. To prepare porous silicon the needed etching solution of 40% HF and electrochemical cell made of Teflon, 48% HF (Germany) was diluted and utilized with ethanol of high purity (99.9%). In addition, Si-wafers were cut into small pieces measuring approximately 2.5 cm × 2.5 cm and ultrasonically cleaned for 5 min in ethanol to remove any contamination on the surface. Furthermore, the photo-electrochemical etching (PECE) approach was used to create a homogeneous PS layer on the front surface related to the n-Si wafer.
PECE consists of a halogen lamp, power supply for the current source, two electrodes, ammeter for measuring current, gold grid serving as the cathode and Si wafer serving as the anode. Such samples were etched into electrolytes consisting of HF: ethanol solution with various current densities J=12, 24, and 30 mA/cm2. The schematic related to the PECE setup is shown in Figure 1. Atomic force microscopy (AFM) was conducted to examine the PSi surface roughness and pore diameter with nano range. An X-ray diffraction (XRD) spectrometer (Shimadzu) was utilized to collect the XRD patterns of bulk silicon and PSi surface layers, as well as the Fourier transform infrared spectroscopy (FT-IR) results from chemical bonds for PSi. A Varian Cary Eclipse fluorescence spectrometer was used for steady-state measurements.
A schematic diagram of PECE set up for porous silicon at different etching current densities, etching time 10min, and HFC 40%.
The sensor was measured by a homemade device, and the measurements were carried out in a 2.5 L glass desiccator chamber. Hence, the measurement stage was fixed within the chamber. A heater heated the substrate, and its temperature was adjusted by a variable transformer (Variac) linked across a heater. The resistance change was measured using the Mable application, which was linked to a precision millimeter (Keithley 616) through a personal computer. During the experiment, a rotating pump was used to refresh the chamber. All sensor measurements were conducted at room temperature.
3 Results and discussion
The XRD patterns of PSi and c-Si samples at etching current densities of 12 and 30 mA/cm2 are shown in Figure 2. XRD is a technique for identifying the atomic and molecular structures of a crystal in which the crystalline atoms cause an X-ray beam to diffract in different directions. A crystallographer may create a 3D image of the density of electrons within the crystal by measuring the angles and intensities of these diffracted beams. From this electron density, the mean positions of the atoms in the crystal can be determined, as well as their chemical bonds, disorder and other information. In addition, the layers of PSi remain crystalline in plane (100) with diffraction angles of roughly (2Θ) = 69.727°, 69.15° and 69.461° for bulk-silicon and PSi at J=12 and J=30 mA/cm2, respectively. The PSi layer remained crystalline but shifted slightly to a small diffraction angle. These results were due to the strain's effect, which caused a slight rise in the lattice parameter and the resulting displacement of the PS peak to a minor diffraction angle. The diffraction peak broadened as the crystal size was reduced to nanometer scale, and the breadth of the peak was directly proportional to the nanocrystalline domain's size [10].
The XRD diffraction patterns of the a) PSi and bulk silicon at various etching current densities of b) 12 c) 30 mA / cm2, HFC of 40%, and etching time 10 min.
We determined the crystal size of PSi by measuring the peak's full width half-maximum (FWHM) via the Scherrer–Debye, which is shown in Eq. (1):
Where the X-ray beam wavelength is 0.154606 nm, D is the crystalline size (nm), β is the FWHM and k is a constant (k=0.89).
We can determine the strain by using the W-H plot, which is expressed as Eq. (2).
Figure 3 shows the SEM images of PSi with different etching current densities of 12 and 30 mA/cm2. From these images, the formation of the high density of pores was observed, and they were distributed randomly on the surface layer of silicon samples. To increase the visibility of the pores created on the silicon surface, the SEM image showed two magnifications (4000X and 14000X) for all samples. The average pore diameters were 2.22 and 1.68 μm for drilling current densities of 30 and 12 mA/cm2, respectively.
SEM images of PS at different etching current densities (a) 12 and (b) 30mA/cm2, etching time 10min. and HFc 40%.
The image (a) shows that the pore diameters at J= 12mA/cm2 were small (see image (b)), but they increased when the current density increased. At J= 30 mA/cm2, the highest amount of pore diameter was obtained, with the thinnest walls separating the neighbour pores. This phenomenon was attributed to the effective dissolution of Si at the wall of pores, resulting in an increase in pore size and minimised distance of inter-pore ‘wall thicknesses. This result was due to the rapid dissolution on the silicon surface by HF solution, which attacked directly via oxidation [10].
Figure 4 shows an AFM image with nanoscale in which 3D and 2D images are displayed when scanning at about ~ 500 nm × 500 nm for PS layers. The average pore diameters were 23.05 and 26.23 nm at etching current densities J= 12 and 30 mA/cm2, respectively. These images illustrated that PS exhibited a sponge-like structure with high homogeneity and densely branching pores. This figure clearly showed that the rate of bore diameter and surface roughness increased when the etching current density increased. Table 2 illustrates the increase in the pore diameter and roughness with etching current density. At J= 12 mA/cm2, a small uniform porous layer was produced, with diameter D= 23.05 nm and roughness R= 2.44 nm. By contrast, as the etching current density increased to 30 mA/cm2, the average pore diameter increased but the pore wall thickness and pore diameter decreased to D=26.23 nm and roughness R=8.63 nm. These changes were due to the increased dissolution of Si inside the porous layer and condensation of hydrogen bubbles by the preparation method [11]. Any change in the composition state and current density led to an abrupt change in the microstructure of the film [12].
Illustrated the 2-D and 3-D AFM images (a) at J= 12 and (b) J= 30 mA/cm2, HFc 40%, and etching time 10 mints.
Figure 5 shows the photoluminescence (PL) of a PSi sample in the air at room temperature with an etching current density of J= 12 mA/cm2. The greatest PL intensity was at 580 nm in the visible range, with a PL intensity of 0.75, compared with 1100 nm in C-Si bulk. This feature was related to the quantum confinement effect of nano-sized PSi [13]. The PL spectra presented emission peaks centred at approximately 515–680 nm. Such green-yellow light-emitting optical characteristics were believed to be responsible for the quantum confinement effect in the nanostructure of the produced PS film. We tested one sample to determine the PL range of PSi samples and calculated the energy gap (2.110315 eV). PL is a process in which a molecule absorbs a photon in the visible region, exciting one of its electrons to a higher electronic excited state, and then radiates a photon as the electron returns to a lower energy state.
Photoluminescence spectra (PL) of a PSs layer created with etching current densities of J= 12 mA/cm2, etching duration of 10 minutes, and HFc 40%.
PL of PSi yields information of a cross-sectional view of the silicon nanocrystalline, which remains amongst the pores, because of the large optical penetration depth. PL is directly related to the electronic structure and transitions [14], so it depends on the surface roughness.
The energy gap is calculated by
The FT-IR spectra related to PSi at J= 12 mA/cm2 etching current density are shown in Figure 6 with an etching time of 10 min and HF concentration of 40%. The transmittance peak at 611 cm−1 [15] indicated Si-Si stretching, that at 458 cm−1[16] was attributed to Si-O, the peak at 1083 cm−1 [17] was due to Si-O-Si bending, that at 1439 cm−1 [18] was a result of Si, that at 1510.11 cm−1 [15] was due to CH3 symmetric stretching, the peak at 1720 cm−1 [19] was attributed to C=O, that at 2357 cm−1 [16] indicated Si-C stretching and the peak at 3690 cm−1[20] was the result of Si-OH stretching. These chemical bonds play an active role in the deposition or adhesion of gas atoms on the surface of PSi, so the amount of resistance changes in the presence of these gases. The sensitivity of the gases on the surface of PSi can be calculated.
The Fourier Transform Infrared Spectroscopy (FTIR) of PS was produced with an etching current density of 12 mA / cm2, an etching period of 10 minutes, and HFC content of 40%.
The sensing characteristics of PSi samples for acetone vapour and ethanol are shown in Figure 7. Sensor sensitivity (Sg) is defined as Ra/Rg [21], where Ra is the sensor's electrical resistance in air, and Rg is its resistance in mixed gas and air. Figures 7a and 7b show the sensitivity of samples that were prepared with etching current density J= 12, 24 and 30 mA/cm2, 40% HFC etching solution and etching time of 10 min in the presence of 500 ppm methanol and ethanol, respectively. Sensitivity was determined by calculating the sample's resistance with a constant voltage. When the sample was exposed to acetone or ethanol, the sensitivity improved. The best sensitivity to acetone was 2.413 at J= 30 mA/cm2, whereas the best sensitivity of ethanol was 1.809 at J= 12 mA /cm2. The long reaction time was due to the large surface area and the presence of nanopores, which required a long period to achieve equilibrium adsorption. This result was consistent with the AFM results presented in Table 1, which showed that the sample made with J= 30 mA/cm2 presented the most observable porous structure [22, 23].
Typical gas sensor responses (Ra / Rg) to a PS with different etching current densities: 12, 24, 30 mA/cm2 sensor for (a) acetone and (b)ethanol gases at an operating room temperature.
Crystal size and strain for PS with different current densities at 30, 12 mA/cm2, 10 min. & HFc 40%.
| Samples | 2Θ | FWHM (β) | Crystal size (D(nm)) | ɛ*10−3 (nm−2) |
|---|---|---|---|---|
| PS(J=30mA/cm2) | 69.36059 | 2.86064 | 3.3757584 | 18.03940059 |
| PS(J=12mA/cm2) | 69.47454 | 0.11181 | 86.427719 | 0.70358557 |
| 69.67084 | 0.11745 | 82.3753899 | 0.736379216 |
The AFM results of PS samples at J=12, 30 mA/cm2.
| J (mA/cm2) | Average diameter (nm) | Average roughness (nm) |
|---|---|---|
| 12 | 23.05 | 2.44 |
| 30 | 26.23 | 8.63 |
High porosity means a large surface area and a large reactive surface for the acetone gas sensor. Figure 7 shows that the gas sensitivity increased after injection of acetone gas into the reaction chamber and reached the saturation level; it then decreased when the sample was exposed to air. The gas response was found to be Ra/Rg= 2.413, 1.6376 and 1.6845 at etching current densities of J= 12, 24 and 30 mA/cm2, respectively, where Rg is the resistance when the sample is exposed to acetone. The higher response was at J= 30 mA/cm2 compared with the other samples at 12 and 24 mA/cm2. The high sensitivity of the PSi film prepared with PECE technology may be attributed to the optimal number of uniform pores on the surface, large surface area, high surface roughness and high acetone oxidation rate. These results were consistent with the findings of AFM. The effect of changing the etching current densities in making PSi was evident on the pore diameter and surface roughness, consistent with the results of AFM examination, where the pore roughness was equal to 2.44 nm and the pore diameter was equal to 23.05 nm at J= 12 mA/cm2. At J= 30 mA/cm2, the surface roughness was 8.63 nm, and the pore diameter was 26.23 nm. These findings were also consistent with the results of SEM examination, where the pore diameter increased with the etching current density, leading to an increase in the surface area exposed to the gas and an increase in the energy gap on the surface [8, 15].
The lowest acetone gas response was also obtained for Ra/Rg=1.63 at an etching current density of 12 mA/cm2. For ethanol gas, the highest sensitivity was 1.8090 at J= 30 mA/cm2, followed by Ra/Rg= 1.4209 at J= 24 mA/cm2; the lowest was Ra/Rg= 1.3132 at J= 12 mA/cm2.
Figure 8 illustrates the variation in response time as a function of the sensitivity of different etching current densities. Response time represents the time it takes for the sample to interact with the gas, and the recovery time represents the time it takes for the sample to return to its normal state (i.e. the state of the sample before pumping the gas). The response time increased with time, had a high value of 31s at a current density of 30 mA/cm2, and then decreased again. The large response time and recovery times may be due to the high gas absorption rate and gas absorption agreement [24]. The variation in response and recovery times for the two gases used was due to the fact that the melting and boiling temperatures of acetone were relatively higher than those of ethane. Moreover, acetone can form strong bonds such as hydrogen bonds, whereas ethane cannot. Table 3 shows the recovery and response times of the PSi samples [25]. Tetrapodal ZnO networks were functionalized with carbon nanotubes (CNTs) to form a highly efficient hybrid sensing material (ZnO-T-CNT) for ultrasensitive, selective, and rapid detection of ammonia (NH3) at room temperature. Additionally, the response and recovery times were improved (by decreasing them from 58 and 61 s to 18 and 35 s, respectively). Devices can be used in automotive, environmental monitoring, chemical industry, and medical diagnostics. Results confirm the efficient use of CNTs doped ZnO nanopartacial materials in the gas sensor.
Response and recovery time versus start time for a PS with different current densities for (a) acetone and (b) ethanol at 500 ppm.
The modified samples’ recovery and response times.
| J(mA/cm2) | GAS | Resistance air (KOHM) | Resistance gas (KOHM) | Sensitivity | Response time (sec) | Recover time (sec) |
|---|---|---|---|---|---|---|
| 12 | Acetone | 55.93207 | 34.1539 | 1.63764802 | 10 | 19 |
| 24 | 66.20434 | 39.302 | 1.684503193 | 11 | 20 | |
| 30 | 259.6996667 | 107.587 | 2.413857313 | 31 | 22 | |
| 12 | Ethanol | 4.23 | 3.211 | 1.3132 | 10 | 23 |
| 24 | 7.266 | 5.1135 | 1.4209 | 17 | 29 | |
| 30 | 1.2446 | 0.688 | 1.8090 | 23 | 45 |
4 Conclusions
In summary, the sequential preparation method with different etching current densities was successfully used as an acetone and ethanol gas sensor. This research revealed that PSi surface sensitivity and response and recovery times of acetone gas were significant and changed with the etching current density used in the preparation of PSi. This phenomenon was due to the apparent change in the structure and shape of the surface with the variation in the etching current density. This alteration led to a change in the pore diameter and surface roughness. Subsequently, the surface area of the sample exposed to the gas varied. These factors played an important role in changing the amount of gas absorbed and reduced. The highest response was at the current density of 30 mA/cm2; the optimum sensitivity of the PSi-based gas sensor was 2.413 at an acetone concentration of 500 ppm at room temperature. This impressive result also showed a response time of 31 s and recovery time of 22 s. By contrast, in ethanol, the best result was observed at current density J= 30 mA/cm2, and the gas sensor was about 1.8090.
In general, the beast sensing results were observed with acetone gas and etching current density J= 30 mA/cm2.
Acknowledgement
We would like to thank the University of Technology and the Applied Science Department in Baghdad, Iraq, for their assistance in conducting this study.
Funding information:
The authors state no funding involved.
Author contributions:
All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest:
The authors state no conflict of interest.
References
[1] Abdulkhaleq NA, Hasan AK, Nayef UM. Enhancement of photodetectors devices for silicon nanostructure from study effect of etching time by photoelectrochemical etching technique. Optik. 2020;206:164325.10.1016/j.ijleo.2020.164325Search in Google Scholar
[2] Timokhov DF, Timokhov FP. Influence of injection level of charge carriers in nanostructured porous silicon on electroluminescence quantum efficiency. Microelectron Eng. 2005;81(2–4):288–92.10.1016/j.mee.2005.03.021Search in Google Scholar
[3] Kadhim RG, Ismail RA, Abdulridha WM. Structural and Optical Properties of Porous Silicon Prepared by Electrochemical Etching. J Korean Ceram Soc. 2002;39(2):109–12.10.4191/KCERS.2002.39.2.109Search in Google Scholar
[4] Alothman ZA. A review: Fundamental aspects of silicate meso-porous materials. Materials (Basel). 2012;5(12):2874–902.10.3390/ma5122874Search in Google Scholar
[5] S. Dubey R. Electrochemical Fabrication of Porous Silicon Structures for Solar Cells. Nanosci Nanoeng. 2013;1(1):36–40.10.13189/nn.2013.010105Search in Google Scholar
[6] Alfeel F, Awad F, Alghoraibi I, Qamar F. Using AFM to Determine the Porosity in Porous Silicon. J Mater Sci Eng A. 2012;2(9):579–83.Search in Google Scholar
[7] Abdul Hussien AM, Nayef UM, Hussein HT. Preparation of chemical vapor sensor by reduction graphene oxide doped with nanoparticles of gold on porous silicon using photoluminescence. Optik. 2020;206:163576.10.1016/j.ijleo.2019.163576Search in Google Scholar
[8] Hussein HT, Nayef UM, Hussien AMA. Synthesis of graphene on porous silicon for vapor organic sensor by using photoluminescence. Optik. 2019;180:61–70.10.1016/j.ijleo.2018.10.222Search in Google Scholar
[9] Zamel RS, Mohammed BK, Yaseen AS, Hussein HT, Nayef UM. Preparation and characterization of Ag nanoparticles on porous silicon for photo conversion. J Res Lepid. 2019;50(3):82–95.10.36872/LEPI/V50I3/201027Search in Google Scholar
[10] Sulaiman AA. Effect of γ - irradiation on the n- porous silicon structures prepared by electrochemical etching. Raf J Sci. 2018;27(3):173–80.Search in Google Scholar
[11] Bisi O, Ossicini S, Pavesi L. Porous silicon: A quantum sponge structure for silicon based optoelectronics. Surf Sci Rep. 2000;38(1):1–126.10.1016/S0167-5729(99)00012-6Search in Google Scholar
[12] Mohammed IM, Shnieshil AH. Characteristics study of porous silicon produced by electrochemical etching technique. Int J Innov Technol Manag. 2013;2(9):77–80.Search in Google Scholar
[13] Shokri B, Firouzjah MA, Hosseini SI. FTIR analysis of silicon dioxide thin film deposited by metal organic-based PECVD. Proc 19th Int Plasma Chem Soc. 2009;1–4.Search in Google Scholar
[14] Kumar P. Effect of Silicon Crystal Size on Photoluminescence Appearance in Porous Silicon. ISRN Nanotechnol. 2011;2011:1–6.10.5402/2011/163168Search in Google Scholar
[15] Nayef UM, Hussein HT, Abdul Hussien AM. Study of photoluminescence quenching in porous silicon layers that using for chemical solvents vapor sensor. Optik. 2018;172(July):1134–9.10.1016/j.ijleo.2018.07.112Search in Google Scholar
[16] Launer PJ, Arkles B. Infrared Analysis of Orgaonsilicon Compounds. Silicon Compounds: Silanes & Silicones. 3rd Ed. Morrisville: Galest. 2013;175–8.Search in Google Scholar
[17] Dian J, Macek A, Nižňanský D, Němec I, Vrkoslav V, Chvojka T, et al. SEM and HRTEM study of porous silicon - Relationship between fabrication, morphology and optical properties. Appl Surf Sci. 2004;238:169–74.10.1016/j.apsusc.2004.05.218Search in Google Scholar
[18] Bondy AL, Kirpes RM, Merzel RL, Pratt KA, Banaszak Holl MM, Ault AP. Atomic Force Microscopy-Infrared Spectroscopy of Individual Atmospheric Aerosol Particles: Subdiffraction Limit Vibrational Spectroscopy and Morphological Analysis. Anal Chem. 2017;89(17):8594–8.10.1021/acs.analchem.7b02381Search in Google Scholar PubMed
[19] Munajad A, Subroto C, Suwarno. Fourier transform infrared (FTIR) spectroscopy analysis of transformer paper in mineral oil-paper composite insulation under accelerated thermal aging. Energies. 2018;11(2).10.3390/en11020364Search in Google Scholar
[20] H. H. Hadi. Study of the characteristics of porous silicon by electrochemical etching. J Southwest Jiaotong Univ. 2013;31(1):34–8.Search in Google Scholar
[21] Patil PT, Anwane RS, Kondawar SB. Development of Electrospun Polyaniline/ZnO Composite Nanofibers for LPG Sensing. Procedia Mater Sci. 2015;10:195–204.10.1016/j.mspro.2015.06.041Search in Google Scholar
[22] Zhao YF, Sun YP, Yin X, Yin GC, Wang XM, Jia FC, et al. Effect of Surfactants on the Microstructures of Hierarchical SnO2 Blooming Nanoflowers and their Gas-Sensing Properties. Nanoscale Res Lett. 2018;13.10.1186/s11671-018-2656-5Search in Google Scholar PubMed PubMed Central
[23] Saxena N, Kumar P, Gupta V. CdS nanodroplets over silica microballs for efficient room-temperature LPG detection. Nanoscale Adv. 2019;1(6):2382–91.10.1039/C9NA00053DSearch in Google Scholar
[24] Alwan AM. Study of the Influence of Incorporation of Gold Nanoparticles on the Modified Porous Silicon Sensor for Petroleum Gas Detection. 2017;35(8):811–5.10.30684/etj.35.8A.4Search in Google Scholar
[25] Schütt F, Postica V, Adelung R, Lupan O. Single and Networked ZnO-CNT Hybrid Tetrapods for Selective Room-Temperature High-Performance Ammonia Sensors. ACS Appl Mater Interfaces. 2017;9(27):23107–18.10.1021/acsami.7b03702Search in Google Scholar PubMed
© 2021 Muna H. Kareem et al., published by De Gruyter
This work is licensed under the Creative Commons Attribution 4.0 International License.
