Materials chemistry and design from first principles: A study of hybrid organic/semiconductor structures
|Location: Dahlem Center Seminar Room
Time: Wednesday, January 12, 2011, 14 h c.t.
The chemistry of hybrid structures composed of organic molecules and semiconductor surfaces is opening up exciting new areas of development in molecular electronics, nanoscale sensing devices, and surface lithography. Covalent attachment of organic molecules to a semiconducting surface can yield active devices, such as molecular switches and sensors or passivating insulating layers. Moreover, it is assumed that the actions can be controlled by “engineering” speciﬁc modiﬁcations to organic molecules, suggesting possible new lithographic techniques. One of the goals of controlling the surface chemistry is the creation of ordered nanostructures on semiconducting surfaces, and there has previously been some success in obtaining locally ordered structures on the hydrogen terminated Si(100) surface. These methods require a dangling Si bond without a hydrogen to initialize the self-replicating reaction. Another popular approach eliminates the initialization step by exploiting the reactivity between surface dimers on certain reconstructed surfaces with the π bonds in many organic molecules. The challenge with this approach lies in designing the surface and/or the molecule so as to eliminate all but one desired reaction channel. Charge asymmetries, such as occur on the Si(100)-2×1 surface, lead to a violation of the usual Woodward-Hoﬀman selection rules, which govern many purely organic reactions, allowing a variety of possible[4+2] and[2+2] surface adducts. Multiple reactive sites, such as occur on some of the SiC surfaces, also allow for a variety of possible adducts. The exploration of hybrid organic-semiconductor materials and the reactions associated with them is an area in which theoretical and computational tools can play an important role. Indeed, modern theoretical methods combined with high-performance computing, have advanced to a level such that the thermodynamics and reaction mechanisms can be routinely studied. These studies can aid in the interpretation of experimental results and can leverage theoretical mechanisms to predict the outcomes of new experiments. This talk will focus on a description of one set of such techniques, namely, those based on density functional theory and ﬁrst-principles or ab initio molecular dynamics. As these methods employ an explicit representation of the electronic structure, electron localization techniques can be used to follow local electronic rearrangements during a reaction and, therefore, generate a clear picture of the reaction mechanism. In addition, statistical mechanical tools can be employed to obtain thermodynamic properties of the reaction products, including relative free energies and populations of the various products. The basic methodology will be brieﬂy described, and then we will present a series of applications to conjugated dienes reacting with diﬀerent semiconductor surfaces. We will explore the role of surface thermal motions on the reaction mechanisms, and we will demonstrate how the predicted mechanisms can be used to rationalize product distributions. We will investigate how the surface structure inﬂuences the thermodynamics of the reaction products and how these thermodynamic properties can be used to “reverse engineer” the molecule and/or the surface in order to control the product distribution and associated free energies. Finally, we will describe the problem of computing theoretical scanning tunneling microscopy (STM) images and the challenges inherent in simple perturbative schemes.
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