RESEARCH ON BENZENE VAPOR DETECTION USING POROUS SILICON

Size: px

Start display at page:

Download "RESEARCH ON BENZENE VAPOR DETECTION USING POROUS SILICON"

Reynard Sullivan
5 years ago
Views:

1 Section Micro and Nano Technologies RESEARCH ON BENZENE VAPOR DETECTION USING POROUS SILICON Assoc. Prof. Ersin Kayahan 1,2,3 1 Kocaeli University, Electro-optic and Sys. Eng. Umuttepe, 41380, Kocaeli-Turkey 2 Kocaeli University-LATARUM Laboratory, 41275, Yenikoy-Kocaeli-Turkey 3 Kocaeli University, Hereke MYO, 41800, Hereke-Kocaeli-Turkey ABSTRACT Porous silicon (PS) has been an attractive material for enhancing the optical properties of silicon. Its large surface area for sensor applications and compatibility with siliconbased technologies has been the driving force for this technology development. In this study, benzene vapor detection properties of porous silicon have been investigated at room temperature. Electrical (DC) and photoluminescence (PL) spectra measurements in a controlled atmosphere (Nitrogen gas and the benzene vapor mix) were performed to test the sensor response towards the benzene vapor. It was find that PS surface very sensitive against to the vapor and electrical/optical properties changes with exposure the vapor. The experimental results suggested that PS surface is candidate a promising material for sensing the benzene vapor. Keywords: Porous silicon, benzene vapor sensing, luminescence, DC measurement INTRODUCTION Benzene is a carcinogen with a maximum permitted exposure limit in the atmosphere of 5 ppb. There is a need for an inexpensive instrument for measuring benzene concentrations, particularly in urban areas [1]. Benzene was widely used in industry as cleaning agent and solvent. Benzene vapor is carcinogen and it is hazard in urban areas as it occurs in the exhaust fumes from motor vehicle [1]. Therefore, it must be detected continuously in our living environment. The traditional methods as chromatography detecting of organic gas have costly equipment and need professional laboratory with specialized researcher [2]. it is also expensive and has long time. Therefore, we need new modern sensors cheap and capable quickly sensing of benzene vapor or other harmful gases. Semiconductor gas sensors are most attractive because they are compact, sensitive, low cost, and have low power consumption [3]. The semiconductor must have a large specific area (surface area to volume ratio) to produce a higher charge exchange rate for a high sensitivity gas detection. This can be realized in practice by using porous materials due to their large specific area [4]. High surface to volume ratio can be practically obtained by chemically etching of porous silicon in HF based solution. Therefore, PS has very high surface to volume ratio, it is a promising candidate for gas sensing. As a matter of fact, many authors have reported the modification of PL and electrical transport properties during exposure to volatile compounds [5,6]. 159

2 14 th SGEM GeoConference on Nano, Bio and Green Technologies for a Sustainable Future The objective of this study is to investigate benzene vapor sensing using porous silicon. Therefore, it was measured the DC current response and photoluminescence spectra response of PS in sensing the benzene vapor. The structure of PS layer was characterized by Scanning Electron Microscopy (SEM). EXPERIMENTAL The porous silicon samples were prepared from p-type Boron-doped crystal-line (c-si) silicon wafer with resistivity of 2.5 Ωcm. Before electrochemical etching, a solution of 1:24 volume ratio of 50 % HF acid and de-ionized (DI) water were used for cleaning the surface of the c-si substrate in 5 minutes. We used an aqueous HF-ethanol solution with a 25% concentration (1:3 volume ratios of 50% Hydrofluoric acid (HF) and 99.9% absolute ethanol (ETH)) for etching solution. Anodisation current density is 8.85 ma/cm 2. After electrochemical etching, the porous silicon samples were rinsed with ethanol and dried with nitrogen gas. An experimental setup was used for benzene vapor sensing as seen in figure 1. The PL quenching measurements of PS were performed in optical module and DC electrical measurements were performed electrical module. Where, nitrogen gas was used as carried of the benzene vapor. Two flow-meters from Sierra which have maximum flow 200 scc/m and 4 scc/m were used as Nitrogen gas controller. Nitrogen gas separated two lines. A line connected to a bubble bottle to obtain organic vapor-gas ( ) mix. 254 nm wave length of light from a UV lamp (Konrad Benda) was used for PL excitation. PS surface was coated with a metal interdigital-electrode (IDE) (Aluminum, 0,2g, 99,999%) used with thermal evaporation system for electrical measurements. Etching process and sensor measurements were carried out room temperature. UV lamp (254nm) Spectrometer Flow-meter (200 scc/m) PS Inlet Optic module Flow-meter (4 scc/m) Benzene vapor + Benzene Bubble bottle +Benz. Electrical module PS Outlet Source-meter (Keithley-1601) Figure 1. Schematic diagram of electrical and optical sensor test chambers. 160

Section Micro and Nano Technologies RESULTS and DISCUSSION Surface morphology of the porous silicon was studied by scanning electron microscope as shown in Fig. 2.

3 Section Micro and Nano Technologies RESULTS and DISCUSSION Surface morphology of the porous silicon was studied by scanning electron microscope as shown in Fig. 2. From the figure it was clear that there is a continuous distribution of pore sizes ranging between 1 and 3 µm. Figure 2. SEM micrographs of the PS surface. Figure 3 shows electrical response of PS sensors during exposure to various concentrate benzene vapor. It is shown from the figure that the electrical signal increases during exposure to benzene vapor and then decreases during exposure to nitrogen gas (carrier gas). This phenomenon can be described with capillary condensation of the benzene vapors in pores of the PS [7]. During exposure to benzene vapor, air in the pores of the PS is replaced with benzene vapor. Therefore, we can see electrical signal increase due to the phenomenon of the capillary condensation. Therefore, surface conductivity changes are mainly due to changes in the free electron concentration due to charge exchange between adsorbed species from the gas and the PS surface as mentioned in ref [8]. 90% Benzene Current (A) 10% Benzene 30% 50% 70% Benzene Benzene Benzene Time (s) Figure 3. Electrical response of PS sensors during exposure to various concentrate benzene vapor. 161

14 th SGEM GeoConference on Nano, Bio and Green Technologies for a Sustainable

Photoluminescence spectra of porous silicon during exposure various concentrate

PL intensity change (au) 7000 6000 5000 4000 3000 Benzene + (200 scc/m) 2000 1000

4 14 th SGEM GeoConference on Nano, Bio and Green Technologies for a Sustainable Future Figure 4. Photoluminescence spectra of porous silicon during exposure various concentrate benzene vapor. The spectra were taken 5 minute intervals. PL intensity change (au) Benzene + (200 scc/m) ,1 scc/m 0,5 scc/m 1 scc/m 2 scc/m 3 scc/m 4 scc/m Benzene vapor quantity (scc/m) Figure 5. The changes of photoluminescence intensity with various concentrate benzene vapor. 162

5 Section Micro and Nano Technologies Before sensor measurements, gas was filled optic module and then taken continuous PL spectra for seeing PL stability. After the PL spectra were stabile about in one minute, benzene nitrogen mixture was filled the module and then periodically (5 min) changed this process with various benzene concentrations. Similar process was taken for electrical module. Figure 4 shows photoluminescence spectra of PS response to exposure various concentrate the benzene vapor and PL intensity changes versus benzene concentration are also shown in figure 5. It is shown from the figures change of PL intensity decrease during exposure to decreasing the benzene vapor. From the figures, it can be conclude that PL intensity depends on the concentrate of the benzene vapor and PS surface is more sensitive to benzene vapor. CONCLISIONS In this study, we explored the possibility of an electrical and optical sensor based on porous silicon. We fabricated porous silicon on silicon substrate and investigate its photo-luminescence spectra and electrical properties during exposure to various concentrated the benzene vapor. We observed that PL intensity decrease, whereas, electrical signal (current) increase during exposure to increasing benzene vapor level because of the capillary condensation of the organic vapors into the pores. The measurement results indicated that PS surface showed good vapor sensing performances to the benzene vapor. It can be conclude that PS is an ideal candidate material for fabricating high performance, selective electrical/optical vapor sensor in the future. However, we need to new scientific studies for using porous silicon as benzene sensor. Acknowledgements This research is financially supported by the Scientific and Technological Research Council of Turkey (Project no. 111T357). REFERENCES [1] M. Mabrook and P. Hawkins, Benzene sensing using thin films of titanium dioxide operating at room temperature, Sensors, 2(2002) [2] N.K. Ali, M.R. Hashim, A. Abdul-Aziz, Effects of surface passivation in porous silicon as H2 gas sensor, Solid-State Electronics 52 (2008) [3] Leigh Canham, Properties of Porous Silicon, INSPEC, The Institution of Electrical Engineers, London, United Kingdom, (1997). [4] L. Seals, J.L. Gole, L.A. Tse, P.J. Hesketh, Rapid, reversible, sensitive porous silicon gas sensor, J Appl. Phys. 91(2002) [5] I. Schechter, M. Ben Chorin, A. Kux, Gas sensing properties of porous silicon, Anal. Chem. 67(1995) [6] H-J Kim, Y-Y Kim and K-W Lee, Sensing characteristics of the organic vapors according to the reflectance spectrum in the porous silicon multilayer structure, Sensor Actuat A, 165(2011)

6 14 th SGEM GeoConference on Nano, Bio and Green Technologies for a Sustainable Future [7] S. Dhanekar, S.S. Islam, T. Islam, A.K. Shukla Harsh, Organic vapour sensing by porous silicon: Influence of molecular kinetics in selectivity studies, Physica E, 42(2010) [8] V. Polishchuk, E. Souteyrand, J.R. Marin, V.I. Strikha, V.A. Skryshevsky, A study of hydrogen detection with palladium modified porous silicon, Anal Chim Acta, 375(1998)

PHOTOACOUSTIC TECHNIQUE FOR MEASURING BAND-GAP ENERGY OF POROUS SILICON LAYER ON n-si SUBSTRATE. Department of Physics,

PHOTOACOUSTIC TECHNIQUE FOR MEASURING BAND-GAP ENERGY OF POROUS SILICON LAYER ON n-si SUBSTRATE. Chan Kok Sheng 1, W. Mahmood Mat Yunus 1, Wan Md. Zin Wan Yunus 2, Zainal Abidin Talib 1 and Anuar Kassim