The Division of Chemistry supports Timothy Anglin of the University of Minnesota as an American Competitiveness in Chemistry Fellow. Dr. Anglin will work with Prof. Aaron Massari to measure the molecular ordering of semiconductor polymers at the polymer dielectric interface in model organic field effect transistor (OFET) systems. The measurements will be carried out using vibrational sum-frequency generation (VSFG) a technique that is exquisitely sensitive to material interfaces and insensitive to the bulk. In addition, Dr. Anglin will collaborate with scientists at Argonne National Laboratory to probe longer-scale structure in the same materials using x-ray scattering techniques. For his plan for broadening participation, the PI will organize a career education fair and develop informational materials for local high school/community college students or in the initial stages of their undergraduate careers to illustrate careers that are possible with degrees in chemistry. The fair will be carried out in partnership with groups on the U. Minnesota Campus (Lando, REU and MSROP summer programs) as well as local community colleges.
Research like that of Dr. Anglin is aimed ultimately at developing improved materials for electronic devices. Robust, easily processible polymer electronics could yield substantial savings in the costs of fabrication of a myriad of technologically important devices. The efforts at broadening participation being pursued by Dr. Anglin are aimed at encouraging young people, especially those from groups underrepresented in the sciences, to pursue higher education in chemistry, by providing a roadmap to an attractive career as a productive member of the American technological workforce.
Understanding molecular structure at buried interfaces: Building electronic devices from organic polymers presents a potential low cost alternative to conventional inorganic semiconductors. However, the electrical performance of most organic electronic devices falls well short of what is needed to compete with existing technologies. One major limitation to the operation of such devices is the disorder that is present in organic materials due to the fact that neighboring molecules are held together only weakly. As charges must move from one molecular fragment to another to pass through a device, poor packing or alignment of neighboring molecules can act as a barrier to charge transport. This work has sought to uncover the organization of polymer semiconductors in organic field-effect transistors (OFETs). In an OFET, the semiconducting polymer is coated over electrical contacts and is separated from a gate electrode by a thin insulating dielectric. Application of a potential between the gate and top electrodes causes the device to switch on and charges to move through the polymer layer. In such an OFET, the charge transport occurs only in a molecularly thin (~1nm) region of the polymer film residing at the dielectric surface. This makes characterization of the structure especially difficult, as one requires a technique sensitive to this polymer-dielectric boundary but that is insensitive to the remaining thickness of the polymer film (as the structure of this inactive region plays no significant role in the electrical behavior of the device). To accomplish this we have used Vibrational Sum Frequency Generation (VSFG), an interface-specific vibrational spectroscopy that is capable of assessing the molecular orientation and degree of order for the polymer residing at the dielectric interface. Additionally, we used grazing incidence x-ray scattering techniques to understand the organization of the polymer throughout the entire thickness of the film in order to understand how the interfacial layer differs from the overlying material. Our findings reveal both that the interfacial structure can significantly differ from the overlying material with respect to the orientation of polymer segments and their relative order and that the interfacial orientation and ordering of the polymer depends strongly on the chemical makeup of the dielectric surface. A final question that we sought to address was how these materials behave upon heat treatment. Heat treatment of OFETs is a common way to improve molecular packing and is therefore widely used in an effort to affect an improvement in charge transport. Our findings reveal that the chemistry of the dielectric layer becomes critically important to the temperature dependent response of the polymer that contacts it. Fluorocarbon coated surfaces show reversible structural change with little change in the degree of packing and orientation of polymer molecules at the dielectric interface even when heated for long times at high temps (>220 ºC). However, when bare silica glass surfaces are used the heat treatment process can lead to significant reorientation of the interfacial polymer layer and a loss of net order within this interfacial region. Importantly, this differs from the response of those polymer molecules residing in the bulk region above the surface. For those molecules not in immediate contact with the surface, packing and ordering of neighboring polymer species appears to improve at all temperatures. These results highlight the importance of choosing an appropriate technique for surface characterization in the case of OFETs, where only a small fraction of the polymer film is responsible for the electrical behavior of the device. Moreover, these new findings emphasize the critical role that dielectric layer surface chemistry plays in these devices and open new avenues for chemical modification of organic electronic materials and electronic material design.
