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Tabatabaeipour G.,Electronic Research Center | Hajghassem H.,University of Tehran | Mohtashamifar M.,University of Tehran
EPJ Applied Physics | Year: 2015

Digital microfluidics is an emerging technology that is able to manipulate droplets individually. To develop this technology it is needed to use cheaper and more accessible materials for its fabrication. At present, materials commonly used for the hydrophobic layer in these devices are expensive materials that require a legal agreement. In this paper, polydimethylsiloxane (PDMS), which is a common and cheap material in the field of microfluidic, is used as a hydrophobic material for both top and bottom plates of digital microfluidic devices. The droplet cannot be actuated using DC voltage in a surrounding air environment. The reason for the droplet pinning is the high contact angle hysteresis of PDMS surface and considerable thickness of hydrophobic layer on the top plate. In order to overcome this problem, we have exploited a suitable AC voltage (230 Vrms and 8 kHz) as well as changing the surrounding environment to the olive oil. Therefore this paper demonstrates the feasibility of using PDMS as a hydrophobic layer in two-plate digital microfluidics. © EDP Sciences, 2015.


Akhavan O.,Sharif University of Technology | Abdolahad M.,University of Tehran | Esfandiar A.,Sharif University of Technology | Mohatashamifar M.,Electronic Research Center
Journal of Physical Chemistry C | Year: 2010

TiO2 nanoparticles were physically attached to chemically synthesized single-layer graphene oxide nanosheets deposited between Au electrodes in order to investigate the electrical, chemical, and structural properties of the TiO2/graphene oxide composition exposed to UV irradiation. X-ray photoelectron spectroscopy showed that after effective photocatalytic reduction of the graphene oxide sheets by the TiO2 nanoparticles in ethanol, the carbon content of the reduced graphene oxides gradually decreased by increasing the irradiation time, while no considerable variation was detected in the reduction level of the reduced sheets. Raman spectroscopy indicated that, at first, the photocatalytic reduction resulted in a significant increase in the graphitized sp2 structure over the disorders in the graphene oxides. After that, as the carbon content decreased by UV irradiation, further disorders appeared in the reduced graphene oxide sheets, confirming degradation of the reduced sheets after the photocatalytic reduction. Based on the current-voltage characteristic, the optimum time for the photocatalytic reduction resulted in a sharp decrease in the electrical resistivity of the reduced graphene oxide. However, longer photocatalytic processes caused a high increase in the resistivity, due to dominating the photodegradation process over the nearly completed photocatalytic reduction. © 2010 American Chemical Society.


Hajghassem H.,Shahid Beheshti University | Erfanian A.,K. N. Toosi University of Technology | Mohtashamifar M.,Electronic Research Center | Aliahmadi M.,Electronic Research Center | And 2 more authors.
Journal of Porous Media | Year: 2010

An efficient and reliable method for patterning a two-dimensional porous Si (PS) is presented in this work. The Cr/Au (50/200 nm) layer is used as a mask for electrochemical prosification of Si to form a two-dimensional pattern of porous Si. Quality of the formed 8 × 8 pads of porous Si is analyzed using statistical image analysis, an atomic force microscope, a scanning electron microscope, and also images from an optical microscope. It is shown that a combination of photoresist patterning and Cr/Au deposition can be used to generate arrays of silicon nanoporous in selected surface regions. Our results indicate that using the Cr/Au layer as a mask during prosification of Si results in sharper patterns with higher resolution in comparison to conventionally used photoresist masks such as Shipley and SU 8 photoresists. The sharpness of the formed patterns is also investigated by calculating the correlation coefficient using image processing of the formed pattern with the ideal pattern. The correlation coefficient of the patterns made by the Cr/Au mask is 0.99 and the maximum deviation of the formed pattern from the original Cr/Au pattern is 3.44 μm, which shows an almost perfect correlation of the generated array with the ideal pattern. © 2010 by Begell House, Inc.


Banihashemian S.M.,Islamic Azad University at Qom | Hajghassem H.,Shahid Beheshti University | Erfanian A.,K. N. Toosi University of Technology | Aliahmadi M.,Electronic Research Center | And 2 more authors.
Sensors | Year: 2010

Nanosize porous Si is made by two step controlled etching of Si. The first etching step is carried on the Si surface and the second is performed after deposition of 75 Å of platinum on the formed surface. A platinum silicide structure with a size of less than 25 nm is formed on the porous Si surface, as measured with an Atomic Forced Microscope (AFM). Differential resistance curve as a function of voltage in 77 K and 100 K shows a negative differential resistance and indicates the effect of quantum tunneling. In general form, the ratio of maximum to minimum tunneling current (PVR) and the number of peaks in I-V curves reduces by increasing the temperature. However, due to accumulation of carriers behind the potential barrier and superposition of several peaks, it is observed that the PVR increases at 100 K and the maximum PVR at 100 K is 189.6. © 2010 by the authors.

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