X-Ray photoelectron spectroscopy for chemical and electrical characterization of devices extended to liquid/solid interfaces

Süzer, Şefik
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Bilkent University
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Understanding of electrical and electrochemical devices in operating conditions is vital for development of new technologies. Many important characteristics that determine the performance of such devices lie on their surfaces and interfaces which significantly deviate from the bulk properties. However, particularly for the liquid based devices, carrying out surface analysis is challenging and requires highly sophisticated instrumentation. In this PhD. thesis, we aim to unravel the potential development on liquids, dielectrics as well as the liquid/solid interfaces during AC and DC excitation in a chemically resolved fashion using the UHV compatible non-aqueous liquids in a basic electrowetting on dielectrics configuration within X-Ray Photoelectron Spectroscopy (XPS) chamber. Low molecular weight Polyethylene glycol (PEG) and a particular ionic liquid Diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide [DEME][TFSI] are used to represent two extreme cases as being non-ionic and fully-ionic liquids. Application of external electrical bias to these devices either from the top or the bottom electrode during data acquisition enabled us to investigate the electrowetting phenomenon, in a chemically addressed fashion In the first part of the thesis, geometrical changes that the drop undergoes during electrowetting have been monitored both by steady state areal maps and by dynamic XPS point analysis where the potential was altered periodically. In the second part, we have focused only on the DC electrowetting of liquids. We probed the potential developments in the dielectric layer and on the liquid by monitoring the changes in the binding energy of the representative XPS peaks with respect to the applied potential. We showed that the conductivity of the liquid has no influence on the potential and the entire potential drop occurs at the liquid/dielectric interface. Dielectric breakdown and its effect on the potential developments were also investigated in this part. In the third part, we have tried to understand the frequency dependent potential developments of the ionic liquid and the polyethylene glycol based EWOD devices by AC electrowetting. Our time dependent XPS measurements under AC excitation with sweeping frequency have demonstrated that EWOD devices exhibit two different behaviors separated by a critical frequency, which is dependent on the AC resistance (impedance) or ionic content of the liquid and also the electrical characteristics of the dielectric layer. Below the critical frequency, XPS spectra are mainly affected by the capacitive component of the dielectric, hence the liquid completely screens the applied electrical field. However, for frequencies above the critical, the resistive component of the liquid dominates and the drop behaves like a resistive strip, resulting in the formation of equipotential surface contours which are shown experimentally for the first time in this study. In the last part of the thesis, an equivalent circuit model was developed to electrically describe the electrowetting behavior of PEG on dielectric and also to generate a solid-state mimicking device to produce the same XPS spectral observations.

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