Difference between revisions of "Organic Field Effect Transistor Using Pentacene Single Crystals Grown by a Liquid-Phase Crystallization Process"

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==Soft matter keywords==
 
==Soft matter keywords==
[[Liquid-Phase Crystallization]], Pentazene, Organic Field Effect Transistor (OFET), Pentacene, Tricholorbenzene.
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Liquid-Phase Crystallization, Pentazene, [[Organic Field Effect Transistor]], Pentacene, Tricholorbenzene.
  
 
==Summary==
 
==Summary==
The purpose of this study was to investigate a novel self assembly on a mica surfaceWhen the researchers allowed (dodecyldimethylammonioacetoxy)diethyltriamine trichloride (DTAD) to assemble on a mica surface nanoscaled parallel stripes formedThis interesting formation is quite unique compared to islands that form on normal surfaces.  A suggested mechanism for this ordering was also proposed that draws on many different forces including hydrophbicity and lattice matching.
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The purpose of this study was to investigate a different fabrication technique for creating single crystals of pentacene to be used for organic field effect transistorsThe liquid-phase crystallization process creates single crystals by dissolving pentacene in solution, then precipitating it as a single crystalThe pentacene crystals formed by this method then had their semiconducting properties measured and these properties were compared to pentacene crystals grown using different methods.
  
 
==Soft matter discussion==
 
==Soft matter discussion==
[[Image:Pentacene1.jpg|500px|thumb|left|The top image is an atomic force microscopy image of pentacene single crsytal.  The bottom image is the height along the white arrow from the top image.]]
 
  
[[Image:Stripes.jpg|500px|thumb|right| The top photo shows the parallel stripes with a few islands corresponding to a DTAD concentration of 10mM.  The next photo shows more stripes that are longer and thicker corresponding to a DTAD concentration of 20mM.]]
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[[Image:Pentacene2.jpg|500px|thumb|right|The structure of pentacene.]]
  
  
Future devices may have a need for nanoscale structures, so research in this area is booming even without direct applications in mindTo create these structures, many scientists have already noticed that surfactant molecules can self assemble in unique ways; much research has been done on single tailed, or two tailed surfactants.  The people in this study however, have used a trimeric structure, DTAD, pictured left.
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[[Image:Pentacene1.jpg|500px|thumb|left|The top image is an atomic force microscopy image of pentacene single crsytalThe bottom image is the height along the white arrow from the top image.]]
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Organic field effect transistors (OFETs) have many favorable properties over current field effect transistors (FETs).  The most popular OFET, due to its optimal electronic properties derived from its delocalized pi bond, is pentacene.  These properties however, are degraded by the existence of grain boundaries in polycrystalline pentacene as a result of the reduced mobility in charge carriers.  Therefore, single crystals of pentacene are primarily used.  The most popular way of synthesizing the single crystalline pentacene is currently by physical vapor deposition.
  
The researchers used the critical concentration to create micelles from DTAD, 0.29mM, as a basis for their studyThey added concentrations of 0.08, 2, 10, and 20 mM, corresponding to 0.2, 5, 25, and 50 times its cmc, respectively to a mica surfaceAt the lowest concentration typical islands were observed, then as the CMC was passed, parallel stripes began formingAs the concentration was then increased the islands gave way to longer and thicker parallel stripesThese can be seen in the figure on the right.  The dimensions of these structures is about 1um in length, 40-80 nm in width, and 2-4nm in height.
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An alternative method of creating pentacene single crystals is by liquid-phase crystallizationIn this process pentacene dissolved in trichlorobenzene is purified via various methodsThen the pentacene solution is heated and very slowly cooled so that precipitation occurs as a single crystalIn fact, in this experiment the pentacene was heated to 200C, then cooled at 0.1C per hourThe phase change from solute to solid single crystal is clearly applicable to the soft matter discussionThe image on the left shows the single crystal as seen from AFM.  Each step shown is a single molecule step.  The entire image has an area of 4umx4um.
  
I will refer you to the paper to look into the proposed theory for why this structure arisesIt has to do with many different factors, including hydrophobic interactions, lattice matching, and electrostatic bindingA proposed structure is shown in the image to the right.
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The crystalline pentacene was then fabricated onto a thin film of heavily doped silicon oxide to create the OFETThe OFET electrical properties were measured by standard methods.  The most important of these properties was the field effect mobility which was measured to be <math>0.4 \frac{cm^2}{Vs}</math>This number is similar to PVD single crystal pentacene which shows that the fabrication technique was a success.  Liquid-phase crystallization is a practical method in producing pentacene single crystals for OFETs.
  
[[Image:Mica.jpg|500px|thumb|left|Proposed structure for DTAD on micaThe orange hexagonal structure is the mica lattice, while the blue and pink represent the DTAD head groupThe yellow bonding in image B is the DTAD hydrocarbon chains.]]
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There are some differences between this method and PVD. For example, there seems to be a large anisotropy in the method used hereThat means that the orientation of the crystal, and the direction by which you measure your voltage across makes a differenceThis created mobilities that were both higher and lower than expected.  Perhaps this can be tuned to create an even better OFET.

Latest revision as of 03:17, 28 October 2009

Original entry: William Bonificio, AP 225, Fall 2009

Information

Organic Field Effect Transistor Using Pentacene Single Crystals Grown by a Liquid-Phase Crystallization Process. Yasuo Kimura, Michio Niwano, Naohiko Ikuma, Kenichi Goushi, Kingo Itaya. Langmuir 2009 25 (9), 4861-4863

Soft matter keywords

Liquid-Phase Crystallization, Pentazene, Organic Field Effect Transistor, Pentacene, Tricholorbenzene.

Summary

The purpose of this study was to investigate a different fabrication technique for creating single crystals of pentacene to be used for organic field effect transistors. The liquid-phase crystallization process creates single crystals by dissolving pentacene in solution, then precipitating it as a single crystal. The pentacene crystals formed by this method then had their semiconducting properties measured and these properties were compared to pentacene crystals grown using different methods.

Soft matter discussion

The structure of pentacene.


The top image is an atomic force microscopy image of pentacene single crsytal. The bottom image is the height along the white arrow from the top image.

Organic field effect transistors (OFETs) have many favorable properties over current field effect transistors (FETs). The most popular OFET, due to its optimal electronic properties derived from its delocalized pi bond, is pentacene. These properties however, are degraded by the existence of grain boundaries in polycrystalline pentacene as a result of the reduced mobility in charge carriers. Therefore, single crystals of pentacene are primarily used. The most popular way of synthesizing the single crystalline pentacene is currently by physical vapor deposition.

An alternative method of creating pentacene single crystals is by liquid-phase crystallization. In this process pentacene dissolved in trichlorobenzene is purified via various methods. Then the pentacene solution is heated and very slowly cooled so that precipitation occurs as a single crystal. In fact, in this experiment the pentacene was heated to 200C, then cooled at 0.1C per hour. The phase change from solute to solid single crystal is clearly applicable to the soft matter discussion. The image on the left shows the single crystal as seen from AFM. Each step shown is a single molecule step. The entire image has an area of 4umx4um.

The crystalline pentacene was then fabricated onto a thin film of heavily doped silicon oxide to create the OFET. The OFET electrical properties were measured by standard methods. The most important of these properties was the field effect mobility which was measured to be <math>0.4 \frac{cm^2}{Vs}</math>. This number is similar to PVD single crystal pentacene which shows that the fabrication technique was a success. Liquid-phase crystallization is a practical method in producing pentacene single crystals for OFETs.

There are some differences between this method and PVD. For example, there seems to be a large anisotropy in the method used here. That means that the orientation of the crystal, and the direction by which you measure your voltage across makes a difference. This created mobilities that were both higher and lower than expected. Perhaps this can be tuned to create an even better OFET.