In-Situ Regrowth and Purification by Zone Melting of Organic Single-Crystal Thin Films Yielding Significantly Enhanced Optoelectronic Properties

Liu, Chong Yang; Bard, Allen J.

doi:10.1021/cm000353x

Cited by 76 publications

(60 citation statements)

References 17 publications

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“…Many pathways are being explored to achieve self-assembled highly connected networks of π-π interacting molecules, including both chemical modifications [2][3][4] and processing techniques. [5][6][7][8][9][10][11][12] Discotics, i.e., planar, disc-shaped aromatic molecules, have been intensely studied during the past two decades. To make them soluble and to control the thermal properties, they are substituted with side chains at the periphery.…”

Section: Introductionmentioning

confidence: 99%

In Situ Studies of Phase Transitions in Thin Discotic Films

et al. 2005

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The crystalline to liquid crystalline (Cr-LC) phase transition in thin films of zone-cast hexa-perihexabenzocoronene sixfold substituted with dodecyl side chains (HBC-C 12 H 25 ) has been studied in detail using grazing incidence X-ray diffraction (GID), electron diffraction (ED), and variable angle spectroscopic ellipsometry (VASE), When heating the material, a first minor transition is observed around 42°C. This change is attributed to alterations of the crystalline alkyl chain packing, which only slightly changes the electronic properties of the material. At higher temperatures of about 90°C, but still significantly below the previously reported transition temperature in bulk, the Cr-LC transition is observed. An accompanying large increase in optical anisotropy is compatible with the X-ray data, showing a transition from the as-cast herringbone-like crystalline state to a highly ordered discotic hexagonal columnar LC phase. The structural transition has the macroscopic effect of increasing the film thickness. The high structural order of the as-cast low-temperature phase is only partly recovered after cooling, and the phase transition exhibits a large hysteresis. From the ellipsometry data, the dielectric tensor of HBC-C 12 H 25 was refined to unprecedented detail. IntroductionOrganic electronics have promising prospects, with the main motivation being to make cheap plastic sensors and circuits. 1 The importance of well-defined molecular order to exploit the highly anisotropic properties of the specifically designed molecules is well-recognized. A requirement for good mobility in films of organic conjugated molecules is a good overlap of the π-electrons of neighboring molecules. Many pathways are being explored to achieve self-assembled highly connected networks of π-π interacting molecules, including both chemical modifications 2-4 and processing techniques. [5][6][7][8][9][10][11][12] Discotics, i.e., planar, disc-shaped aromatic molecules, have been intensely studied during the past two decades. To make them soluble and to control the thermal properties, they are substituted with side chains at the periphery. The molecules tend to segregate into stacked columns of the aromatic cores, separated by less-ordered side chains. These stacks can be considered one-dimensional molecular wires, whereas the side chains, typically alkyl, are electronically inactive. A relatively high mobility of typically 1.0 cm 2 /V s is obtained in these materials. 13 The molecules studied here, hexa-peri-hexabenzocoronenes (HBC; see Figure 1) are graphenes, having delocalized electrons on both sides of the molecular plane. Underivated HBC is highly crystalline and poorly soluble. Because of electrostatic quadrupolar interactions, the molecules tend to pack in the so-called herringbone structure.

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Section: Introductionmentioning

confidence: 99%

In Situ Studies of Phase Transitions in Thin Discotic Films

et al. 2005

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“…On the other hand, the 'face-on' arrangement, with DLC columnar directors perpendicular to the substrate, is generally best for photovoltaic devices and light emitting diodes. [10] Recently, various alignment techniques utilizing temperature gradient, [11] concentration gradient, [12] pre-aligned layer, [13] electric field [14] and magnetic field [15] have been reported. However, more facile approaches to achieve uniaxially oriented long-range ordering of DLC columnar superstructures with clear identification of the ordered structures are still being actively explored.…”

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confidence: 99%

Uniaxially Oriented, Highly Ordered, Large Area Columnar Superstructures of Discotic Supramolecules using Magnetic Field and Surface Interactions

et al. 2008

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The unique anisotropic physical properties of columnar discotic liquid crystals (DLCs) have attracted considerable interest for their potential applications as electronic devices, such as field effect transistors (FETs), [1,2] photovoltaics, [3] and light emitting diodes (LEDs). [4,5] The typical DLC molecule consists of a relatively rigid aromatic core with several flexible aliphatic side chains. Depending on their molecular structure and thermodynamic state, DLCs exist in various phases such as nematic, hexagonal columnar and rectangular columnar. [6] In the columnar phase, the intermolecular p-p orbital overlap between neighboring aromatic cores is significant, providing extremely anisotropic and order-dependent, quasi-one-dimensional, charge carrier mobility along the stacked cores. [7,8] For many practical applications, however, it is crucial to obtain a thin film of uniaxially oriented and highly ordered columnar superstructures of DLC molecules covering the macroscopic area of a substrate. [9] For example, the unidirectional 'edge-on' alignment, with the DLC columnar directors parallel to the substrate, is required for field effect transistors. On the other hand, the 'face-on' arrangement, with DLC columnar directors perpendicular to the substrate, is generally best for photovoltaic devices and light emitting diodes. [10] Recently, various alignment techniques utilizing temperature gradient, [11] concentration gradient, [12] pre-aligned layer, [13] electric field [14] and magnetic field [15] have been reported. However, more facile approaches to achieve uniaxially oriented long-range ordering of DLC columnar superstructures with clear identification of the ordered structures are still being actively explored.Here, we report a simple method to obtain a uniaxially oriented and highly ordered columnar superstructure of a DLC, using the alkyl-substituted metalloporphyrinoid, cobalt octa(n-decylthio)porphyrazine (CoS10), on substrates over a centimeter length scale. This method simultaneously utilizes the interactions of DLCs with both an easily accessible external magnetic field (ca. 1.0 T) and a functionalized surface of substrate. The substrate was functionalized with octadecyltrichlorosilane (OTS) to increase the affinity of the surface with the alkyl side chains of the discotics, resulting in an 'edge-on' arrangement of DLC columns. The detailed structures of the ordered and oriented columnar DLCs on substrates were investigated using the grazing incidence small angle X-ray scattering (GISAXS) technique, which is a very powerful tool to investigate the internal nanoscale structures in thin films. [16,17] The GISAXS measurements show that when a spin-coated CoS10 thin film on an OTS-functionalized silicon wafer is cooled from its isotropic phase to the liquid crystal phase in the presence of an external magnetic field of 1.07 T, it forms highly oriented 'edge-on' columnar superstructures with the columnar directors perpendicular to the applied magnetic field. The oriented superstructure shows a long range o...

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“…The technique has also been adopted by Forrest's group in the fabrication of organic crystalline microcavities of anthracene grown by the melted compound (Kena-Cohen et al 2008, Kena-Cohen et al 2010. Another example has been reported (Liu & Bard 2000) on the growth and purification by zonemelting of organic single-crystals. Our group has similarly employed the melt-growth technique to grow crystals of the compounds TPB and TPP.…”

Section: Melt Growthmentioning

confidence: 99%