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Kang, Kyungnam,Yang, Daejong,Park, Jaeho,Kim, Sanghyeok,Cho, Incheol,Yang, Hyun-Ho,Cho, Minkyu,Mousavi, Saeb,Choi, Kyung Hyun,Park, Inkyu Elsevier 2017 Sensors and actuators. B Chemical Vol.250 No.-
<P><B>Abstract</B></P> <P>Integration of heterogeneous sensing materials in microelectronic devices is essential to accomplish compact and highly integrated environmental sensors. For this purpose, a micro-patterning method of electrospun metal oxide nanofibers based on electrohydrodynamic (EHD) printing process was developed in this work. Several types of metal oxide (SnO<SUB>2</SUB>, In<SUB>2</SUB>O<SUB>3</SUB>, WO<SUB>3</SUB> and NiO) nanofibers that were produced by electrospinning, fragmented into smaller pieces by ultrasonication, and dissolved in organic solvents were utilized as inks for the printing. Constant or pulsed wave bias consisting of base and jetting voltages were applied between a nozzle and a substrate to generate a jetting of nanofiber solutions. Several parameters for EHD printing such as pulse width, inner diameter of the nozzle, distance from the nozzle to the substrate, and stage speed, were optimized for accurate micro-patterning of electrospun nanofibers. By using optimized printing parameters, microscale patterns of electrospun nanofibers with a minimum diameter less than 50μm could be realized. Gas sensors were fabricated by EHD printing on the microelectrodes and then used for the detection of toxic gases such as NO<SUB>2</SUB>, CO and H<SUB>2</SUB>S. Four kinds of metal oxides could detect down to 0.1ppm of NO<SUB>2</SUB>, 1ppm of H<SUB>2</SUB>S and 20ppm of CO gases. Also, heterogeneous nanofiber gas sensor array was fabricated by the same printing method and could detect NO<SUB>2</SUB> using the sensor array platform with microheaters. Furthermore, microscale patterns of nanofibers by EHD printing could be applied to the suspended MEMS platform without any structural damage and this sensor array could detect NO<SUB>2</SUB> and H<SUB>2</SUB>S gases with 20mW power consumption.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A new method for the microscale patterning of 1D metal oxide for integrated and low-power gas sensor array is proposed. </LI> <LI> Electrohydrodynamic (EHD) printing enables finer patterning of 1D metal oxide nanomaterials than other methods. </LI> <LI> Highly integrated and low-power MEMS gas sensor array has been realized by EHD printing of heterogeneous nanomaterials. </LI> </UL> </P>
Kang, Kyungnam,Lee, Sanghwa,Kim, Jungho,Kim, Sungchul,Han, Younho,Baek, Seungin IEEE 2016 IEEE photonics journal Vol.8 No.5
<P>We propose a simple numerical modeling method to consider the effect of the incoherent thick substrate on the absorption characteristics of thin-film solar cells. In the proposed 'equispaced thickness method' (ETM), the incoherent optical characteristics of the thick substrate are modeled by adding an additional thickness that gives an equispaced phase shift to the incoherent substrate and averaging the coherent simulation results over several equispaced thicknesses. The proposed ETM can be used to consider the effect of the incoherent glass substrate without complicated mathematical and computational procedures and is applicable to not only planar but also surface-textured thin-film solar cells. By applying the proposed method to the numerical modeling based on the finite element method (FEM), we calculate the reflectance spectra in planar and surface-textured thin-film solar cells, respectively. The simulation condition of the FEM, such as mesh size, is determined to match the numerical results based on the ETM with the analytical results obtained by the generalized transfer matrix method. For comparison, the reflectance spectra in the same structures are calculated by taking the average over coherent calculation results for a large number of random thicknesses of the incoherent layer. According to the comparison of the calculated statistical deviations from the exact solution between the ETM and the random thickness method, the ETM reduces the number of simulations by at least a factor of 50 with the same accuracy.</P>