Summary of science materials doctoral thesis: Abrication and investigation of characteristics of photonic microcavity 1D for optical sensors

The objectives of the thesis: Research and fabricate the one-dimensional (1D) – nanoporous silicon microcavity (1D-NPSMC) structures by using electrochemical etching method with the selectivity of wavelength in visible range from 200 nm to 800 nm. The 1D-NPSMC structures has high reflectivity, narrow linewidth of the pass-band and homogeneous pores.

MINISTRY OF EDUCATION VIETNAM ACADEMY AND TRAINING OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY  NGUYEN THUY VAN FABRICATION AND INVESTIGATION OF CHARACTERISTICS OF PHOTONIC MICROCAVITY 1D FOR OPTICAL SENSORS Chuyên ngành: Materials for Optics Optoelectronics and Photonics Code: 62.44.01.27 SUMMARY OF SCIENCE MATERIALS DOCTORAL THESIS Hanoi - 2018 The thesis was completed at Key Laboratory for Electronic Materials and Devices, Institute of Materials Science, Vietnam Academy of Science and Technology Supervisors: Assocc Prof Dr Pham Van Hoi Assocc Prof Dr Bui Huy Reviewer 1: Reviewer 2: Reviewer 3: The dissertation will be defended at Graduate University of Science and Technology, 18 Hoang Quoc Viet street, Hanoi Time: , , 2018 The thesis could be found at: - National Library of Vietnam - Library of Graduate University of Science and Technology - Library of Institute of Science Materials LIST OF PUBLICATIONS LIST OF PUBLICATIONS USED FOR THE THESIS Huy Bui, Van Hoi Pham, Van Dai Pham, Thanh Binh Pham, Thi Hong Cam Hoang, Thuy Chi Do and Thuy Van Nguyen, Development of nano-porous silicon photonic sensors for pesticide monitoring, Digest Journal of Nanomaterials and Biostructures, volume 13, No.1, January – March 2018 H Bui, V H Pham, V D Pham, T H C Hoang, T B Pham, T C Do, Q M Ngo, and T Van Nguyen, “Determination of low solvent concentration by nano-porous silicon photonic sensors using volatile organic compound method,” Environ Technol., pp 1–9, May 2018 Van Hoi Pham, Huy Bui, Thuy Van Nguyen, The Anh Nguyen, Thanh Son Pham, Van Dai Pham, Thi Cham Tran, Thu Trang Hoang and Quang Minh Ngo, “Progress in the research and development of photonic structure devices”, Adv Nat Sci.: Nanosci Nanotechnol 7, 015003, 17pp, 2016 Van Hoi Pham, Thuy Van Nguyen, The Anh Nguyen, Van Dai Pham and Bui Huy, “Nano porous silicon microcavity sensor for determination organic solvents and pesticide in water”, Adv Nat Sci.: Nanosci Nanotechnol 5, 045003, 9pp, 2014 Bui Huy, Thuy Van Nguyen, The Anh Nguyen, Thanh Binh Pham, Quoc Trung Dang, Thuy Chi Do, Quang Minh Ngo, Roberto Coisson, and Pham Van Hoi, “A Vapor Sensor Based on a Porous Silicon Microcavity for the Determination of Solvent Solution”, Jounal of the Optical Society of Korea, Vol 18, No 4, pp 301-306, 2014 Van Hoi Pham, Huy Bui, Le Ha Hoang, Thuy Van Nguyen, The Anh Nguyen, Thanh Son Pham, and Quang Minh Ngo, “Nanoporous Silicon Microcavity Sensors for Determination of Organic Fuel Mixtures”, Jounal of the Optical Society of Korea, Vol 17, No 5, pp 423-427, 2013 Nguyen Thuy Van, Pham Van Dai, Pham Thanh Binh, Tran Thi Cham, Do Thuy Chi, Pham Van Hoi and Bui Huy, “A microphotonic sensor based on resonant porous silicon structures for liquid enviroment monitoring”, Proc of Advances in optics Photonics Spectroscopy & application, Ninh Binh city, Vietnam November - 10, 2016, ISBN 978-604-913-578-1, pp 471-475, 2017 Phạm Văn Hội, Bùi Huy, Nguyễn Thúy Vân, Nguyễn Thế Anh, “Thiết bị cảm biến quang tử phương pháp để đo nồng độ dung môi hữu chất bảo vệ thực vật môi trường nước” sáng chế số: 16527, cấp theo định số: 5424/QĐ-SHTT, ngày 24.01.2017 LIST OF PUBLICATIONS RELATED TO THE THESIS Pham Van Dai, Nguyen Thuy Van, Pham Thanh Binh, Bui Ngoc Lien, Phung Thi Ha, Do Thuy Chi, Pham Van Hoi and Bui Huy, “Vapor sensor based on porous silicon microcavity for determination of methanol content in alcohol”, Proc of Advances in optics Photonics Spectroscopy & application, Ninh Binh city, Vietnam November - 10, 2016, ISBN 978-604-913-578-1, pp 404-408, 2017 Nguyen Thuy Van, Nguyen The Anh, Pham Van Hai, Nguyen Hai Binh, Tran Dai Lam, Bui Huy and Pham Van Hoi, “Optical sensors for pesticides determaination in water using nano scale porous silicon microcavity ”, Proc of Advances in Optics, Photonics, Spectrscopy & Applications VIII, ISSN 1859-4271, pp.603-608,2015 Thuy Van Nguyen, Huy Bui, The Anh Nguyen, Hai Binh Nguyen, Dai Lam Tran, Roberto Coisson and Van Hoi Pham, “An improved nano porous silicon microcavity sensor for monitoring atrazine in water”, Proc of The 7th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN2014)- November 02-06, 2014- Ha Long City, Vietnam, ISBN: 978-604-913-301-5, pp.173-179, 2015 INTRODUCTION The urgency of the thesis In recent years, photonic sensors have generated an increasing interest because of their already well-known advantages, as immunity to electromagnetic interferences, high sensitivity, no impact noise and working in harsh environment Photonic sensors are generally classified according to the physical principle including endogenous sensors and exogenous sensors Exogenous sensors often use the physical principle that light is altered in intensity of spread; reflex; scattering; refraction; or wavelength conversion due to interaction with the external environment These sensors are relative easiness of fabrication, but the processing of light signals varies due to the complexity of the external environment requiring high sensitivity The endogenous photonic sensor uses the physical principle that the optical properties of sensor structure is changed when interacting with the environment Therefore, they have very high sensitivity, easiness of signal processing and compact device size However, the disadvantage of endogenous photonic sensor is the ability to reuses and selectivity Endogenous photonic sensors are being promoted in research because of their extremely high sensitivity which can be combined with many specializations in chemistry and biology At present, the sensitivity and selectivity of endogenous photonic sensors can be enhanced and have had some very good results In general, scientists and technologists have proposed the standard approach of quantitative analysis of components with extremely small concentrations by using gas chromatography or liquid chromatography (GC / MS, LC / MS or HPLC / MS-MS) [1]-[4], liquid chromatography combined with UV-Vis [5] These methods have played a key role in the analysis of residues of low organic dissolved organic substances in the process of controlling or controlling the environment However, these methods suffer some drawbacks since thay require professional laboratories with specialized personel and expensive equipment In the field of electrochemical sensors [6-7], the enzyme-linked immunosorbent assay (ELISA) has been developed for determination of residues of organic matter based on the principle of antigen antibody The ELISA technique has high sensitivity, easiness of manipulation and rapid analysis time, so there are many models of sensor devices using the ELISA principle The disadvantage of the ELISA approach is the low accuracy in harsh environment, inflexible due to the dependence on the chemicals of the manufacturer Thus, finding new analytical methods is more convenient than the goal of many sensing laboratories in the world Endothelial photonic sensors based on the principle of changing the refractive index of the sensor environment due to the interaction with environment are being extensively studied for the development of sensors in the world Principles of transmission, interference, scattering and refraction of light is studied and applied radically in the photonic sensor based on changing the refractive index of the environment The most recent publish reported that the optical fiber Bragg grating is capable of detecting the refractive index change to as low as 7.2x10-6 in liquid environment [8] which allows the determination of solution at low concentration Endothelial photonic sensors based on the 1D – nanoporous silicon microcavity (1DNPSMC) have high sensitivity, low cost and ability to analyse substances quickly and easily [9] In recent years, scientists have promoted research on endogenous photonic sensors for determining concentrations of solvents, biological antibodies [10], cadavers petroleum contamination norms and petroleum products [11], determination of pesticide residues in water and sludge (recorded pesticide concentration at ppm) [12], determination of concentration DNA level (0.1 mol / mm2 DNA concentration) [13], chemical sensor [14] Current trends in the development of endogenous photonic sensors in the world are enhancing the sensitivity of the sensor (down to ppm), the selectivity of closeoptical properties and portable sensor devices In addition, the nano porous silicon with different porosity have different refractive indexes, so that the multilayer porous silicon can easily form an optical resonance cavity with cost low, durable in the environment for application in photonic sensor technology The research results show that photonic sensors based on resonant cavity have the ability to measure the concentration of solvents and pesticides in the aqueous medium at extremely low concentrations So that PSMC devices show promise for a simple and portable instruments for measuring the level of water pollution caused by organic solvents from industrial production or agricultural protective substances Based on the large surface area of the porous silicon, the porous silicon material has become the ideal material for liquid and vapor phase sensors The principle of pSi-sensors is a determination of the optical spectral shift caused by refractive index change of the porous silicon layers in the device due to the interaction with liquid and/or gas The advantages of photonic sensors are highly sensitivity, so that they are suitable for determination of organic solvents or pesticides at low concentrations Therefore, “Fabrication and investigation of characteristics of photonic microcavity 1D for optical sensors” has been selected as a research topic of the thesis The objectives of the thesis i) Research and fabricate the one-dimensional (1D) – nanoporous silicon microcavity (1D-NPSMC) structures by using electrochemical etching method with the selectivity of wavelength in visible range from 200 nm to 800 nm The 1D-NPSMC structures has high reflectivity, narrow linewidth of the pass-band and homogeneous pores ii) Design the photonic sensor device based on 1D-NPSMC structure which is capable of measuring in two modes: liquid phase (used for determination pesticides) and vapor phase (used for determination organic solvents) iii) Determinate the low concentrations of pesticides and organic solvents in aqueous medium The main contents of the thesis i) Research and fabricate 1D-NPSMC structures based on porous silicon ii) Calculate and simulate optical characteristics of 1DNPSMC structures by using Transfer Matrix Method (TMM) iii) Design the photonic sensor device based on 1D-NPSMC structure which is capable of measuring in two modes: liquid phase (used for determination pesticides) and vapor phase (used for determination organic solvents) iv) Determinate the low concentrations of pesticides and organic solvents in aqueous medium Thesis structure: This thesis consists of 148 pages: introduction, five chapters in content, conclusion The main results were published on 06 articles was published on international journal, 01 presentation at an international workshop and 01 patent Chapter 1: OVERVIEW ABOUT PHOTONIC MICROCAVITY 1D AND POROUS SILICON: In this chapter, we introduce photonic crystals from the concept to the structure of all 1D, 2D and 3D photonic crystals Particularly, this chapter details the structure of the 1D photonic resonator and the formation of silicon by electrochemical etching method The advantages of silicon and its application in the field of sensing are detailed in this chapter Chapter 2: DESIGN AND SIMULATUION OF THE 1D MICROCAVITY STRUCTURES BASED ON POROUS SILICON This chapter describes the basic physics theory of one dimensional photonic crystals and the transmission of optical waves in layered media The Kronig-Penny model is reviewed as a rigorous model for one dimensional periodically layered dielectric media Next, the Transfer Matrix Method (TMM) is developed and its uses in calculating the band gaps of the non-defect PhCs and the reflection properties of defects introduced PhC structures are presented This simulation work explored the effect of the refractive indices variation, the thickness of each layer and the number of layers on the formation of band gaps and on resonant transmissions in 1-D PhC microcavities The obtained band gap was compared with the simulation result based on the Kronig-Penny model, and the structure parameters defined from the simulated reflection spectra laid the foundation for the following fabrication work Parameters affecting the sensitivity of the optical sensor based on the 1D microcavity structure on the silicon wafer are also detailed Chapter 3: FABRICATION OF THE 1D – MICROCAVITY BASED ON POROUS SILICON 3.1 Principle, process of fabrication of the 1D microcavity based on porous silicon 3.1.1 Fabricating principles This part introduces the principle of fabricating 1D microcavity based on porous silicon by using electrochemical etching method 3.1.2 Process of fabricating 1D microcavity structure This section details the steps of fabrication of 1D microcavity structure 3.2 Design and fabrication of 1D microcavity structure The microcavity structure consists of two parallel reflectors separated by a spacer layer Usually the reflectors used are λ/4 DBR with optical thickness of the layers λ/4 The optical thickness of the spacer layer can be either λ or λ/2 Porous silicon microcavities are formed Figure 3.5 (a) Schematic illustration of microcavity structure represented by a half-wave optical thickness defect layer between two Bragg mirrors The Bragg mirrors consist of alternating layers of high and low refractive index quarter-wave optical thickness layers (b) Reflectance spectrum of microcavity The defect layer introduces a narrow resonance in the middle of the high reflectance stopband 10 high and low refractive index layers, respectively, 4.5 and 5.0 mean four and half and five pairs of HL, because this gives a good reflectivity spectrum, possibly controlling the porosity of layers, and easily repeatable electrochemical etching method Figure 3.23 presents images of microcavity structure samples in the visible range 3.5 Design of photonic sensor device based on 1D-porous silicon microcavity Figure 3.34 is a block diagram of a photonic sensor device used in a thesis including a liquid method (application of non-volatile analytical substance) and vapor organic compounds (application for volatile compounds) Figure 3.26 The schematic of photonic sensor device Hình 3.23 Phổ phản xạ cấu trúc vi cộng hưởng quang tử 1D sau chia cho cường độ phản xạ mẫu Figure 3.27 Schematic of the pesticide concentration measurement by liquid-drop method using the porous silicon microcavity sensor Figure 3.28 Schematic of the concentration measurement for VOC using a sensor based on the porous silicon microcavity 11 Figure 3.33 Overall drawing of equipment and sensor systems Figure 3.29 The image of photonic sensor device Chapter 4: DETERMINATION OF PESTICIDE RESIDUES IN AQUATIC ENVIRONMENT BASED ON POROUS SILICON MICROCAVITIES 4.1 Principle of optical sensing Principle of interferometric transduction is used, in which the molecular recognition events are converted into optical signals via the change of Figure 4.1 Schematic Diagram of Sensor Principle the refractive index As shown in the schematic diagram (Fig 4.1), light reflected from the top interface (air-PS) and bottom interface (PS-Si substrate) interfere with Figure 4.2 Wavelength shift in the reflectance spectra of sensor device before and after analyte substance absorption 12 each other and form the typical Fabry-Perot fringes in the reflectance spectrum For bare 1D-NPSMC structure (without any analyte), the refractive index of the structure is n When the pores are filled with an analyte (e.g., chemicals or bio-chemicals), the effective refractive index of the structure increases from n to n+Δn with shift in wavelength from λ+Δλ in the reflectance spectra due to increased optical thickness of the structure Hence, by analyzing the wavelength shift in the reflectance spectra, capture and detection of the analyte is done Figure 4.2 shows principle of optical sensing based on porous silicon microcavity 4.2 The optical sensing applications based on the 1Dnanoporous silicon microcavity For converting the surface of the silicon nano-crystals from hydrophobic to hydrophilic, the as-prepared sample was oxidized in an ozone atmosphere for 45 by using the ozone generator (H01 BK Ozone with a capacity of 500 mg/h) Futhermore, the controlled process of pSi oxidation Figure 4.3 The reflection spectra of the microcavity before (curve 1) and after oxidization (curve 2) improved the durability of skeletal structure and for long life time of ageing pSi Figure 4.3 shows the reflection spectra of the microcavity before and after oxidization The reflection spectra were carried out on a spectrometer (USB-4000, Ocean Optics) and a halogen light source (HL-2000 Ocean Optics) The blue shift of the resonant wavelength after oxidization is due to a 13 decrease in the effective index of the porous layers in the microcavity 4.3 Determination of solvent solutions using 1D-nano porous silicon microcavity 4.3.1 Characteristics of liquid-phase photonic sensors Table 4.1 Various organic solvents with known refractive index and resonant wavelength of sensors based on porous silicon microcavity dipped in corresponding solvent Refractive Resonant index wavelength (nm) Air 1.0003 504.75 Methanol (99.5%) 1.3280 572.05 Ethanol (99.7%) 1.3614 579.00 Isopropanol (99.7%) 1.3776 583.17 Methylene chloride (99.5%) 1.4242 592.85 Organic solvent Sensitivity (Δλ/Δn) is one of the most important parameters to evaluate the performance of the sensors Using the experimental data in Table 1, we calculate the sensor sensitivity of about 200 nm/RIU The Spectrophotometer Varian Cary 5000 is able to detect a wavelength shift of 0.1nm, corresponding to the minimum detectable refractive index change in the porous silicon layer of less than 10 -3 Experiment shows that after complete evaporation of organic solvent, the reflectance spectra of the sensors return to their original waveform positions (as in the air) In our case the evaporation of organic solvents in open air at room temperature was carried out for 40-50 minutes, but this process can occur in 20 seconds when the samples are in a vacuum chamber with 10 -1 torr That means, the 14 change of sensor reflectance spectra are temporary, and it is useful for reversible optical sensing 4.3.2 Determination of concentrations of organic solvents in the gasoline The based been microcavitysensors applied determination have to of different solutions of ethanol and methanol in the commercial gasoline A92 Figure shows the measured results of the resonant wavelength shift of the microcavity sensor Figure 4.8 Response characteristics of the sensor wavelength shift for mixture of methanol and ethanol in different concentrations and commercial gasoline A92 immersed into gasoline A92 with different concentrations of ethanol and methanol In the case of a mixture of ethanol/A92, a resonant wavelength shift is 3.6 nm, when ethanol concentration changed in the range from 5% to 15% in the gasoline With the sensitivity of the sensor as described above, the minimum determination of ethanol concentration change in the gasoline is about 0.4% In the case of methanol/A92, wavelength shifts are 7.2 nm between the 5% and 15% methanol mixtures, From these experimental data, we suppose that the enhanced sensor can distinguish change of about 0.2% in concentration of methanol in the gasoline 15 4.4 Determination of pesticides residues in the aquatic environment Figure 4.9 demonstrates the reflection spectra of pSi-microcavity sensor in the air and in pure water The wavelength shift measured by spectrometer USB-4000 is of 39.8 nm in water and this value is kept for referent data of used sensor for liquiddrop Figure 4.9 The reflection spectra of pSi-microcavity sensor in air (curve 1) and in pure water (curve 2) Inset: Image of pSi-sensor with surface area of about 0.8 cm2 measurement method Fig.4.11 shows a linear relation between the different concentrations of atrazine and the wavelength shift Each experimental point was the average on five measurements, the representing the independent error Figure 4.11 Peak shift of PSMC as a function of Atrazine concentration in both aqueous and humic solutions bar standard deviation The calibration plot of obtained sensor device indicates a good and linear response to atrazine within the concentration range from to 22 pgmL-1 We could calculate the sensitivity of the Figure 4.12 Wavelength shift of pSi-sensor as a function of atrazine concentration in water and humic acid solutions at different times 16 sensor device as the slope of the linear curve interpolating the experimental points Thus, we obtained the value of 0.3 and 0.6 nm/pgmL-1 for Atrazine queous and humic solution, respectively From these numbers, we also estimated the limit of detection (LOD), as the ratio between the instrument resolution and sensitivity LOD numerical value is 1.4 and 0.8 pgmL-1 for Atrazine queous and humic solution, respectively Also, it was observed that the higher wavelength shift was observed in the case of atrazine in HA because atrazine with HA contains dissolved organic matter as component which have higher refractive index compare to water Fig 4.13 presents the results of detection of αand β- endosulfan concentration in water The endosulfan of α- and β- isomers can be specified by the different slope of the dependence between wavelength shift endosulfan and concentrations The LOD of pSi-sensor is obtained of Figure 4.13 Wavelength shift of pSi-sensors as a function of α- and β-endosulfan concentrations in water 0.32 µg.mL-1 and of 0.21 µg.mL-1 for α- and β-endosulfan, respectively This LOD is still low level of detection in comparison with gas chromatography method [25] (about 0.12 - 0.15 ng.mL-1), but the pSi-sensor method has advantage in the low cost, simple sample preparation process and that is suitable for detection of endosulfan in the out-door field work 17 Chapter DETERMINATION OF SOLVENT CONCENTRATION BY USING 1D NANO-POROUS SILICON PHOTONIC SENSORS 5.1 Experimental setup for VOC method 5.1.1 Theoretical basis The capillary deposition of vapour in the pores which caused an increase in the effective refractive index of the porous layer and a shift of the reflection spectra of the sensor is represented by the following Kelvin equation: rK   2 M  RTSe ln( (5.1) P ) P0 where γ, M, and ρ are the surface tension, molecular weight and density of vapour molecules, respectively, P is the observed vapour pressure, and P0 is saturation vapour pressure of analyte, R is ideal gas contant, TSe is the temperature of the sensor chamber, rK is the Kelvin radius that characterizes the process of capillary deposition 5.2 Determination of solvent concentration by VOC method Table 5.1 Some relevant physical-chemical properties of organic solvents used in sensing experiment Substances n ρ (g/cm3) VP (kPa) BP (0C) Ethanol 1,3614 0,785 5,9 78,5 Methanol 1,3284 0,791 12,8 64,6 Acetone 1,3586 0,791 24 56,2 Water 1,3330 0,998 1,75 100 18 5.2.1 The dependence of the sensor response on temperature of the solution and velocity of the air stream flowing through the solution Figure 5.4 The dependence of the Figure 5.3 The dependence of the wavelength wavelength shift on the airflow velocity in shift on ethanol concentration when the velocity the range 0-2.5ml.s-1 of air flow (V) and temperature of solution (T) work as parameters in the measurements The Figure 5.3 shows the curves 1-3 received from measurements with pairs of these parameters such as V= 0.84ml.s-1 dependence of the resonant and T=30℃, V=0.84 ml.s-1, T=45℃, wavelength shift Δλ(C) on V=1.68ml.s-1 and T=30℃, respectively ethanol concentration, when velocity of the airflow (V) and temperature of the solution (T) work as parameters in the measurements It can be seen in Figure that the curve described by Δλ(C) is linear and its slope, i.e sensitivity of the measurement, increases as V and T increase Figure 5.4 shows the dependence of Δλ on V, Δλ (V), at a temperature of 300C when concentration of ethanol and acetone work as the parameters It can be seen in Fig that curves describing Δλ (V) are separate straight lines with different concentrations of acetone and ethanol This shows that empirical function ϑ (V) is a linear function of V 19 5.2.2 Comparative sensitivity of the three methods (volatile organinc compound (VOC), liquid drop and saturated vapour pressure) Parameters TSo and V had been changed from 30oC to 100oC and from 1.68 mL.s-1 to 2.22 mL.s-1, respectively Figure 5.4 The dependence of the resonant wavelength shift (a), Δλ, and the sensitivity (b), S, on the volume concentration of ethanol in water, C, from the liquid, saturated vapour pressure and VOC measurements when solution temperature, T, and velocity of the air flow, V, work as parameters Fig 5.4 shows the dependence of the resonant wavelength shift, Δλ, and the sensitivity, S, on the volume concentration of ethanol in water, C, from the liquid drop, saturated vapour pressure and the VOC methods when solution temperature, TSo, and velocity of the air flow, V, work as parameters Among those, the VOC method provides the highest sensitivity at low solvent volume concentrations because it can create a high vapour pressure of the analyte on the sensor surface owing to the capillary deposition of organic solvent into the silicon pores 5.3 Determination of methanol content in alcohol based on 1DNPSMC 20 5.3.1 Determination of methanol concentrations in alcohol The prepared solution samples to simulate contaminated beverages (for example, vodka) with total alcohol content of 30% v/v (CE = 30%) 45% v/v and of (CE = 45%) Contamination was simulated by adding methanol to the samples in proportions ranging from to 5% v/v Fig 5.8 shows the dependence of the wavelength shift, Δλ, on methanol concentration, Cm, Figure 5.8 Dependence of wavelength shift on methanol concentration in the drinking alcohol with ethanol concentration of 30% and 45%v/v at the temperature of sensor chamber TSe= 22oC and the temperature of solution changed in 45% and 30% alcohol at sensor temperature of 22 oC when the solution temperature, Tso, works as a parameter Note that the dependence of the wavelength shift on methanol concentration can be described as a linear curve whose slope increases with the concentration of ethanol in solution and the solution temperature Normally, with the increase of pressure, the curve describing the dependence of the wavelength shift on vapor pressure shows alternately the low slope in the mechanism of physisorption, the high slope in the capillary deposition and then the significantly reduced slope in the wetting regime [13] In our case, the relatively large wavelength shift at methanol concentration of 0% shows that the capillary deposition has occurred at the solution temperature from 45o to 55oC for both ethanol solutions Working in the capillary deposition, response of sensor is linear in the narrow 21 range of concentrations It is evident that the sensitivity calculated as slope of the curve interpolating the experimental points is proportional to the solution temperature Fig 5.9 depicts the dependence of the wavelength shift, Δλ, on methanol concentration, Cm, in the ethanol-water at the solution temperature of 550C when the sensor temperature TSe decreased from 28 to 14 C Obviously, curves from to describing the dependence of Δλ on Cm are linear and their slope increases with the increase of the concentration of ethanol in solution Figure 5.9 The dependence of the wavelength shift on methanol concentration in 45% and 30% alcohol at the solution temperature of 550C when the sensor temperature, TSe, works as a parameter and with the decrease of the sensor temperature In curve 8, the response of sensor is linear for methanol concentrations lower than 3% and then the shift increases slowly with concentration until saturation at about 5% At this concentration, as mentioned above, the sensor works in the wetting regime with a significant reduction in sensitivity 5.3.2 Determination of methanol and ethanol in industrial alcohol At present, there are many types of counterfeit wine mixed with industrial alcohol with high methanol content These counterfeit drinks cause poisoning to drinkers that can lead to death Therefore, the purpose of this section is to determine the content of methanol contained in vodka prepared from industrial alcohol As in the 22 previous section, I have examined the methanol content in alcohol with the known ethanol concentration Therefore, in this section I conduct the experimental steps as follows: first I determined the ethanol content in industrial alcohol, then I mixed this industrial alcohol into two types of alcohol with ethanol concentration corresponding to the concentration ethanol surveyed the previous section, followed by the vaporization of organic compounds to determine the methanol content of the two types of alcohol Thus, it can be concluded that the method of vaporizing organic compounds can be fully applied to determine the methanol content found in alcoholic beverages based on industrial alcohol With this varnish, I determined a methanol concentration of 7.3% with an alcohol content of 91% v / v This is also an important result, opening up a way to allow us to control the spirits that are circulating in the market using optical sensors based on spongy silicon Vaporization methods of these organic compounds have very high sensitivity and are capable of detecting solvents at very low concentrations Figure 5.11 The dependence of the wavelength shift on ethanol concentration in industrial alcohol Figure 5.13 The dependence of the wavelength shift on methanol concentration in 45% and 30% alcohol and methanol concentration in 45% and 30% industrial alcohol 23 CONCLUSION The thesis has focused on the study and fabrication of 1D - NPSMC for optical sensor From the results obtained, the thesis can conclude with the following main points: One dimensional (1D) – nanoporous silicon microcavity (1DNPSMC) structures were successfully fabricated by electrochemical etching of p-type Si wafer (, 0.01-0.03 Ωcm, 625 µm The 1DNPSMC structures has the selectivity of wavelength in visible range with the sharp photonic resonance dip centred at 650 nm The photonic sensor device based on 1D-NPSMC structure was successfully designed for determination of organic solvents and pesticides in liquid environment The photonic sensor device is capable of measuring in two modes: liquid phase (used for determination pesticides) and vapor phase (used for determination organic solvents) In case of liquid phase, the photonic sensor device is used for determination of low concentration of atrazine and alpha (α); beta (β) isomer endosulfan concentration in aquatic environment The limit of detection (LOD) of sensors is of 1.4 pg.mL-1, 0.32 µg.mL-1 and 0.21 µg.mL-1 for atrazine and α- and β-endosulfan, respectively Experiment shows that atrazine’s half-life in the aqueous phase ranges from 60 to 150 days in the humic acid and/or water, respectively These pSi-sensors can contribute to the detection of persistent pesticides in the aqueous medium with short measuring time and high sensitivity for improving agricultural and food productions with sustainable environmental protection In case of vapor phase, the photonic sensor device is used for determination of organic solvents in liquid environment by using volatile organic compound (VOC) method The VOC method provides the highest sensitivity at low solvent volume concentrations because it can create a high vapour pressure of the analyte on the sensor surface owing to the capillary deposition of organic solvent into the silicon pores This VOC method consists of three steps: heating the solution with its particular boiling temperature, controlling the flowing gas through liquid and cooling sensor It 24 delivers the highest sensitivity of 6.9 nm/% at concentration of 5% and the limit of detection (LOD) of pSi-sensor is 0.014% in case of ethanol in water when using an optical system with a resolution of 0.1 nm Especially, the VOC method is capable of detecting low volume concentration of methanol in two tested ethanol solutions of 30% (v/v) and 45% (v/v) with the LOD of pSi-sensor up to 0.01% and 0.04%, respectively This result will help pave a way to control the quality of contaminated liquor beverages In particular, the volatile organic compound method is capable of detecting low concentrations with high sensitivity of organic solvents (from 0-5% v/v methanol in alcohol) when cooling the sensor chamber Based on this method, it is possible to determine the content of methanol contained in wines and counterfeit liquors manufactured from industrial alcohol This result opens a new way to control the quality of alcoholic beverages using nanofiber silicon photon sensors ... for determination of organic solvents or pesticides at low concentrations Therefore, “Fabrication and investigation of characteristics of photonic microcavity 1D for optical sensors has been... Key Laboratory for Electronic Materials and Devices, Institute of Materials Science, Vietnam Academy of Science and Technology Supervisors: Assocc Prof Dr Pham Van Hoi Assocc Prof Dr Bui Huy Reviewer... workshop and 01 patent Chapter 1: OVERVIEW ABOUT PHOTONIC MICROCAVITY 1D AND POROUS SILICON: In this chapter, we introduce photonic crystals from the concept to the structure of all 1D, 2D and 3D photonic