In situ real-time investigation of Organic Ultra-Thin-Film transistors: growth, electrical properties and biosensing applications

Organic electronics has grown enormously during the last decades driven by the encouraging results and the potentiality of these materials for allowing innovative applications, such as flexible-large-area displays, low-cost printable circuits, plastic solar cells and lab-on-a-chip devices. Moreover, their possible field of applications reaches from medicine, biotechnology, process control and environmental monitoring to defense and security requirements. However, a large number of questions regarding the mechanism of device operation remain unanswered. Along the most significant is the charge carrier transport in organic semiconductors, which is not yet well understood. Other example is the correlation between the morphology and the electrical response. Even if it is recognized that growth mode plays a crucial role into the performance of devices, it has not been exhaustively investigated. The main goal of this thesis was the finding of a correlation between growth modes, electrical properties and morphology in organic thin-film transistors (OTFTs). In order to study the thickness dependence of electrical performance in organic ultra-thin-film transistors, we have designed and developed a home-built experimental setup for performing real-time electrical monitoring and post-growth in situ electrical characterization techniques. We have grown pentacene TFTs under high vacuum conditions, varying systematically the deposition rate at a fixed room temperature. The drain source current IDS and the gate source current IGS were monitored in real-time; while a complete post-growth in situ electrical characterization was carried out. At the end, an ex situ morphological investigation was performed by using the atomic force microscope (AFM). In this work, we present the correlation for pentacene TFTs between growth conditions, Debye length and morphology (through the correlation length parameter). We have demonstrated that there is a layered charge carriers distribution, which is strongly dependent of the growth mode (i.e. rate deposition for a fixed temperature), leading to a variation of the conduction channel from 2 to 7 monolayers (MLs). We conciliate earlier reported results that were apparently contradictory. Our results made evident the necessity of reconsidering the concept of Debye length in a layered low-dimensional device. Additionally, we introduce by the first time a breakthrough technique. This technique makes evident the percolation of the first MLs on pentacene TFTs by monitoring the IGS in real-time, correlating morphological phenomena with the device electrical response. The present thesis is organized in the following five chapters. Chapter 1 makes an introduction to the organic electronics, illustrating the operation principle of TFTs. Chapter 2 presents the organic growth from theoretical and experimental points of view. The second part of this chapter presents the electrical characterization of OTFTs and the typical performance of pentacene devices is shown. In addition, we introduce a correcting technique for the reconstruction of measurements hampered by leakage current. In chapter 3, we describe in details the design and operation of our innovative home-built experimental setup for performing real-time and in situ electrical measurements. Some preliminary results and the breakthrough technique for correlating morphological and electrical changes are presented. Chapter 4 meets the most important results obtained in real-time and in situ conditions, which correlate growth conditions, electrical properties and morphology of pentacene TFTs. In chapter 5 we describe applicative experiments where the electrical performance of pentacene TFTs has been investigated in ambient conditions, in contact to water or aqueous solutions and, finally, in the detection of DNA concentration as label-free sensor, within the biosensing framework.

Experimenting with the role of performance of hygiene and motivators on customer satisfaction: studying the temporal effect

Marketers continuously attempt to identify important attributes and innovate in order to understand how attribute performance could lead to customer satisfaction in the short term and in the long term. Understanding the impact of customer satisfaction may offer a competitive edge to companies. Researchers are discussing the importance of performance attributes in leading to satisfaction; however, there is no clear understanding of whether an attribute that leads to satisfaction at one time (e.g., short run) can cause it also in the long run, without excluding the possibility that it could lead to dissatisfaction and no satisfaction. The present research tries to understand anomalies related to asymmetric attribute performance and satisfaction over time with the help of Herzberg's (1967) Two-Factor Theory (TFT) and construal level theory (CLT). More precisely, there are main purposes of this dissertation. First, the present research tries to understand whether positive or negative hygiene attribute performance and motivator attribute factors exert different weights on overall customer satisfaction depending on the time elapsed from the service experience. Second, to test if positive or negative hygiene/motivator attribute performance affect to revisit intention and to word of mouth by considering mediating role of satisfaction. The results reveal that in the near past (NP) experience, the positive performance of hygiene concrete attributes creates a differential effect on overall satisfaction higher than the negative performance of hygiene concrete attributes. Results also confirmed mediating role of satisfaction in the relationship between attribute performance and revisit intention for near past condition but not for distant past. Likewise significant relationship was found for the mediating role of satisfaction in the relationship between attribute performance and word of mouth (WOM) for near past condition but not for distant past.