The performance of many nanomaterials and polymers is dictated by the interfacial architecture, which in turn is controlled by interactions ranging from the atomic to submicron scale. Moreover, the ensuing properties fail to follow simple additivity rules, and in reality are complex functions of a huge parameter space. CombiSci concepts therefore provide a highly effective pathway to interrogate these materials, and ask critical questions such as:

1) what is the impact of interfacial properties on bulk material behavior; and

2) how do structures at various length scales influence function? By integrating library synthesis, high throughput screening, and informatics, we examine how interfacial interactions affect the chemistry, physics, and engineering of these systems.

Combinatorial Strategies for Design and Testing of Adhesives (Narasimhan, Porter, Salapaka, Zou): Polymer adhesion is a complex function that encompasses surface chemistry, miscibility, molecular weight, crystallinity, and viscoelasticity. Screening to identify optimal candidates for specific applications is, however, time-consuming, cost-prohibitive, and often strongly empirical. To overcome these challenges, we have developed CombiSci techniques to understand adhesion mechanisms. In particular, we have addressed the challenge of poor adhesion at low surface tension materials like polypropylene (PP) by using thin film libraries of acrylic-based adhesives. PP is the world's #2 commodity polymer, and improvement in its adhesive strength is of major industrial significance. We have created libraries with continuous and discrete orthogonal gradients in adhesive composition (phi) and temperature (T) by modifying existent methods. High-throughput characterization of adhesion between the library and PP is carried out using PP-modified AFM tips in conjunction with high precision nanopositioning via force-volume mapping. By scanning across a film library, adhesion strength as a function of f and T is rapidly determined. The results serve as a primary screen of f-T space to identify optimal regimes and answer key questions governing adhesion mechanisms.

Organic Light Emitting Device (OLED) Arrays (Shinar, Narasimhan, Woo): There is growing realization that white OLEDs may become the ultimate general-purpose light source in the emerging market for OLED-based displays. In addition, a new platform for chemical and biological sensors, based on the structural integration of an OLED (as the light source) and sensing element, has enormous potential in meeting a host of needs related to health, homeland security, and the environment. We recently demonstrated the power of CombiSci in discovering new regimes in the complex structure-property map in OLEDs by fabricating and optimizing Combinatorial arrays of the shortest wavelength OLEDs reported to date. These UV/violet OLED arrays, with an indium tin oxide (ITO)/[Cu phthalocyanine (CuPc)]/[carbazolylbiphenyl (CBP)]/[butylphenyl-oxadiazole (Bu-PBD)]/CsF/Al structure, were fabricated using a sliding shutter technique. This enables a systematic variation of layer thickness, composition and bias voltage. Thus, in addition to improving the efficiency and lifetime of OLEDs, we have extended emissivity into the UV region. We will explore the development of white OLEDs with optimized efficiency, lifetime, and color-rendering index. We have also fabricated and screened libraries of structurally integrated OLED-based luminescent chemical and biological sensors. High throughput screening methods automate the movement of the array in these libraries from pixel to pixel in a test circuit, and synchronize current-voltage, electroluminescence-voltage, or sensor response measurements at each pixel. This integrated approach not only lets us probe more deeply into issues related to the understanding of high-efficiency, high-performance, deeper-UV, whiter-white OLEDs, but has real potential as a foundation for a new generation of sensitive long-lived chemical and biological sensors. Parallel plans are in place for investigations of polymer LEDs, another important class of materials for these devices that can be fabricated by ink-jet library design methodologies.

Molecularly tailored self-assembled monolayer (SAM) libraries as adaptive lubricants (Sundararajan, Porter): In the molecular design of alkane-based SAM systems for adaptive tribological performance, the ideal molecule arrangement should exhibit low friction and good wear resistance. In addition, novel monolayer lubricant systems that incorporate mixed SAMs and functionalized MoS2 and/or graphitic carbon nanoparticles exhibit enhanced interfacial friction and durability under varying conditions. In these systems, the design parameters are monolayer chain length, presence/absence of an aromatic group, cross-linking of adjacent molecules, and nanoparticle concentration.

Thus, CombiSci methods are invaluable in understanding the interplay between the chemistry/structure and nanotribological properties. Using micro-contact printing and conventional lithography, micron-sized addresses of various SAMs are deposited on a single substrate with systematic variation of the design parameters. By photopattering, a large library of cross-linked SAMs can be formed on a single substrate, engendering reliable comparative studies. Microtribometry and AFM at loads ranging from nano- to micro-newton levels are used to study the tribological behavior of SAMs. The friction and wear response of several SAMs can be obtained in a single pass of the microtribometer probe tip. Similarly, nanoscale friction, adhesion, and stiffness characteristics can be obtained by repeated positioning of the AFM stage. This research will result in the design of new adaptive lubricants and provide a molecular-level understanding of tribological phenomena in SAMs.