PhD Oral Examination - Shuhong Liu (Wednesday, May 14, 2008, 1:30 pm)

Shuhong Liu shhliu at
Wed Apr 30 13:33:23 PDT 2008

Controlled Deposition of Organic Semiconductor Single Crystals and its  
Application in Field-Effect Transistors

Shuhong Liu
Department of Materials Science and Engineering
Advisor: Professor Zhenan Bao

Wednesday, May 14, 2008, 1:30 pm, Packard building, Room 101
(Refershment to be served at 1:15 pm)


The search for low-cost, large area, flexible devices has led to a  
remarkable increase in the research and development of organic  
semiconductors. Single-crystal organic field-effect transistors are  
ideal device structures for studying fundamental science associated  
with charge transport in organic materials and have demonstrated high  
mobility and outstanding electrical characteristics. For example, an  
exceptionally high carrier mobility of 20 cm2/Vs has been demonstrated  
for rubrene single crystal field effect transistors. 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 by  
handpicking a single crystal and placing it onto the device substrate.  
This makes it impossible to mass-produce at high density with  
reasonable throughput. Therefore, there is a great need for a  
high-throughput method for depositing large arrays of organic  
semiconductor single crystals directly onto device structures.

In this talk, I?ll present several approaches towards realizing this  
goal. In the first part, I?ll introduce a solution-processing  
technique that relies on solvent wetting and de-wetting on substrates  
with patterned wettability to selectively direct the deposition or  
removal of organic crystals. The assembly of different organic  
crystals over centimeter-squared areas on Au, SiO2 and flexible  
plastic substrates is demonstrated. By designing line features on the  
substrate, alignment of needle-like crystals is also achieved. As a  
demonstration of the potential application of this approach, arrays of  
organic single crystal FETs are fabricated by patterning organic  
single crystals directly onto and between transistor source and drain  
electrodes. Besides organic single crystals, our self-assembly  
strategy is also be applicable for patterning other objects such as  
metallic nanowires.

In the second part, I?ll present a vapor-processing technique that  
patterns organic single crystals using carbon nanotube (CNT) bundles  
as templates. Several organic semiconductor materials are successfully  
patterned, including p-type pentacene, tetracene, sexiphenylene, and  
sexithiophene, as well as n-type tetracyanoquinodimethane (TCNQ). This  
study suggests that the selective growth of crystals onto patterned  
carbon nanotubes is most likely due to the coarse topography of the  
CNT bundles. Moreover, we observe that the crystals nucleate from CNT  
bundles and grow onto CNT bundles in a conformal fashion. The crystal  
growth can be directly applied onto transistor source-drain electrodes  
and arrays of organic single-crystal field effect transistors are  
demonstrated. To investigate the impact of CNTs on device performance,  
CNT bundles are incorporated into thin-film FETs and a mobility  
enhancement of organic semiconductors is observed.

In the third part, I?ll present a method that offers the control of  
the size and shape of organic single crystals using patterned Au films  
as templates. It is observed that ?-sexithiophene (?-6T) crystals  
nucleate from the edge or the top surface of Au films and then grow  
two dimensionally on SiO2 surface. The sizes and shapes of ?-6T  
crystals are precisely determined by that of the Au patterns. After  
removing Au templates, large arrays of ?-6T crystals with controlled  
sizes and various shapes such as stripes, squares, hexagons, etc. are  
achieved. Top-contact FETs made of ?-6T ribbons are demonstrated.  
Besides organic single crystals, Au templates can also act as  
templates to pattern vapor- and solution-deposited organic  
semiconductor thin films. Patterned organic thin-film FETs exhibit  
superior performance compared to unpatterned devices.

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