Single-crystal organic field-effect transistors (OFETs) are ideal device structures for studying fundamental science associated with charge transport in organic materials and have demonstrated outstanding electrical characteristics. However, it remains a technical challenge to integrate single-crystal devices into practical electronic applications. A key difficulty is that organic single-crystal devices are usually fabricated one device at a time through manual selection and placing individual crystals. To overcome this difficulty, Bao et al. successfully developed two high-throughput approaches to pattern organic single crystal arrays. In the first method, organic crystals are patterned on electrode regions through solution (de)wetting on heterogeneously wettability-patterned substrates. This solution processing technique potentially has very low-cost. In the second approach, organic semiconductors are vapor-deposited on substrates using carbon nanotubes as templates. In both techniques, large arrays of single crystals OFETs with superior performance are successfully fabricated.

aStanford University MRSEC 0213618 S. Liu1, A. Briseno,2 S.C.B. Mannsfeld,1 J. Locklin,1 W. You, H. Lee, Y. Xia,2 Z. Bao,1 A. Sharei, S. Liu,1 M.E. Roberts1 1Stanford University and 2University of Washington, Berkeley 10 m1Fig.1 Schematic of the solution-based patterning process (left) and scanning electron micrographs (SEM) of patterned single crystal OFETs (right).Patterning Organic Semiconductor Single Crystal Field-Effect Transistors Fig.2 Schematic of the vapor patterning process (left) and SEM images of patterned organic single crystals.

Figure 1 is a schematic of the cross slot device. Fluid flows in the two horizontal channels in the directions indicated, and exits the two outflow arms oriented at ninety degrees to the inflow axis. A stagnation point is created at the center of the cross, at which a single DNA molecule can be trapped and stretched. The steady state extension of the molecule is determined by the flow strength (the flow rate into the inflow channels). Flow focusing can be used to direct the DNA and sequence-specific probes or other reagents to the stagnation point.

Figure 2 is an SEM of the microfluidic four roll mill. As in the cross slot, a central stagnation point exists at the center of this device; DNA molecules and other microscale objects can be trapped and studied at this point. By varying the ratio of flow rates in the various inlet and outlet channels of this device, the flow type near the stagnation point can be varied from pure extension to shear to pure rotation. This device has been fabricated in silicon, and used to study the tumbling dynamics of DNA in mixed flows. These tumbling dynamics are in quantitative agreement with Brownian dynamics simulations.

Figure 3 is an image of a lambda bacteriophage DNA molecule to which a sequence specific probe has been bound ex situ. The DNA-probe complex was then introduced into the cross slot, trapped, stretched, and imaged. The intensity trace at right (taken along the DNA molecule at left) shows that the probe has bound at the correct binding location.