The Shankar Research Group - Excitonics and Nanostructures Laboratory



40. Ma AM, Gupta M, Chowdhury FR, Shen M, Bothe K, Shankar K, Tsui YY and Barlage DW, ZnO Thin Film Transistors with Schottky Source Barriers, Solid-State Electronics 74 104-108 2012.

Abstract: Schottky barrier field effect transistors (SB FETs) with Schottky source injection barrier contacts are fabricated using zinc oxide (ZnO) thin films deposited by pulsed laser deposition at room temperature. In these devices, we utilize a gold Schottky barrier for the source and an aluminum ohmic metal for the drain contacts. The transistors exhibit field effect mobilities as high as 0.1 cm2 V−1 s−1, a current on/off ratio of 105, and a low saturation voltage of 6 V. When using ohmic source and drain contacts, transistor characteristics are not observed. Furthermore, the devices’ transconductance- and capacitance–voltage characteristics show a transition in the dominant carrier injection mechanism at the source barrier from thermionic emission to tunneling at a gate bias of approximately 8 V. These results demonstrate the promise of the Schottky source barrier FET architecture for building ZnO-based transistors.

39. Adl AH, Ma A, Gupta M, Benlamri M, Tsui YY, Barlage DW and Shankar K, Schottky barrier thin film transistors using solution-processed ZnO, ACS Applied Materials & Interfaces 4(3) 1423-1428 2012.

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Abstract: Solution-processed ZnO thin films are attractive as active materials in thin film transistors (TFTs) for low-cost electronic device applications. However, the lack of true enhancement mode operation, low mobility, and unreliability in transistor characteristics due to the high density of traps and other defects present challenges in using such TFTs in circuits. We demonstrate in this report that the electrical characteristics of such TFTs can be improved by source injection barriers. Asymmetrical Schottky source metal–oxide–semiconductor field-effect transistors (MOSFETs) have been fabricated by utilizing heavily doped solution-processed ZnO as the active layer. n+-ZnO was obtained by using triethylamine as the stabilizer in the solution process instead of the more commonly used monoethanolamine. Au was chosen for source metallization to create a Schottky contact to the ZnO and an Al ohmic contact was chosen as the drain. Voltage applied to the gate induced field emission through the Schottky barrier and allowed modulation of the drain current by varying the width of the barrier. By operating the asymmetrical MOSFET when the Schottky contact is reverse biased, effective control over the transistor characteristics was obtained.

38. Ren J, Ding X, Greer JJ and Shankar K, Increased detection of human cardiac troponin I by a decrease of nonspecific adsorption in diluted self-assembled monolayers, Applied Surface Science, 2012 258(13) 5230-5237.

Abstract: In this paper, we tested the hypothesis that there is an increased sensitivity for detecting and measuring disease biomarkers (such as human cardiac troponin I, cTnI) by a decrease of nonspecific adsorption in diluted self-assembled monolayers (SAMs) on planar sputtered gold films. Combining grazing angle Fourier-transform infrared spectroscopy (FT-IR) and antibody–antigen–antibody (sandwich) fluorescence-based immunoassay, we examined the relationship of sensitivity, specificity of detection of cTnI and the level of nonspecific protein adsorption in the following SAMs: pure MHA (16-mercaptohexadecanoic acid, 1 mM, with head COO−, x = 1.0), a mixed SAM comprising MHA (0.1 mM) and UDT (1-undecane thiol, 0.9 mM, with hydrophobic head CH3, x = 0.1UDT), and a mixed SAM comprising MHA (0.1 mM) and MUD (11-mercapto-1-undecanol, 0.9 mM, with hydrophilic head OH, x = 0.1MUD). Our data revealed that nonspecific binding to SAMs is favored in the following order: CH3 > COO− > OH, consistent with previous studies. Compared with pure SAMs, diluting MHA SAMs with MUD increases the sensitivity of cTnI, whereas diluted MHA SAMs with UDT has the same sensitivity of detection of cTnI, suggesting it is the nature of the second diluting thiol that plays an important role on the amount of adsorbed protein on the surface. We obtained a 10-fold increase in the limit of detection of cTnI to 10 ng/ml using x = 0.1MUDdue to a decrease of nonspecific binding. Further, specific binding between the antigen cTnI and its antibody is unaltered on pure and diluted SAMs.

37. Kar P, Pandey A, Greer JJ and Shankar K, Ultrahigh sensitivity assays for human cardiac troponin I using TiO2 nanotube arrays Lab on a Chip, 2012 12 821-828.

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Abstract: Rapid, highly sensitive troponin assays for the analysis of serum at the point-of-care are particularly desirable for the effective treatment of myocardial infarction (MYI). TiO2 nanotube arrays constitute a low cost, high surface area, semiconducting architecture with great promise for biosensing applications due to their compatibility with multiple detection techniques. Using TiO2nanotube arrays functionalized with highly robust and ordered carboxyalkylphosphonic acid self-assembled monolayers, we have developed a simple and highly sensitive fluorescence immunoassay which can detect concentrations of human cardiac troponin I as low as 0.1 pg ml−1without the use of enzymatic amplification. Varying the morphological parameters of the nanotube arrays allows tuning the detection range over 6 orders of magnitude of the troponin concentration from 0.1 pg ml−1–100 ng ml−1.