A forrest of crystals

Organic electronics promise many technological advances, such as transistors, solar cells, electronic skin, and a rainbow of new materials to explore. One key to ensuring performance comparable with traditional inorganic materials is controlling charge transport through organic crystals by manipulating the molecular orientation and packing between adjacent molecules [1]. One dimensional (1D) organic single-crystalline nanostructures have attracted particular attention, both as model systems for understanding the transport mechanisms in organic semiconducting materials, and more importantly, as potential high performance ingredients in photophysical molecular electronic devices [2].

The molecular orientation can significantly affect the material characteristics such as light absorption, charge transport and energy level in the films and crystals. For planar π-conjugated molecules, the charge transport is preferred along the stacking direction due to the π–π intermolecular interactions between neighboring molecules, while the transport perpendicular to the stacking axis is less favorable and exhibits weak electronic coupling. For example in organic solar cells, the desired charge transport direction is normal to the substrate surface, therefore a vertical π–π stacking of molecules is highly desirable. On the other hand, in the case of organic field-effect transistors, charge transport is preferred along the substrate plane. In addition, molecular orientation at organic-organic and organic-electrode interfaces are also of paramount importance, because it determines the pathway and energy barrier for the exciton dissociation and migration at these interfaces [3]. To maximize excitonic dissociation and minimize the polaron-pair recombination, both order and crystallinity within the bulk and appropriate disorder to decrease the electronic coupling interaction at the donor-acceptor interface are required and recently have been demonstrated by theoretical and experimental studies [4].

There is a general interest in the effect of defects (e.g., chemical impurities and molecular packing disorder) on the properties of organic single crystals. In order to investigate defects in discrete 1-D nanostructures, a dense array of well-separated, vertically oriented crystals is required. In this context, organic nanostructures have been obtained from growth via physical vapor transport. By combining a vacuum-based vapor transport technique, and using graphene as substrate, a large array of isolated nano- and micro-pillars of the light-absorbing small molecule, tetraazaterrylene, was obtained [5]. Such pillars, of extremely high crystal quality can be grown at an areal density appropriate for optical measurements, serve as a test bed for single microcrystal photophysics. Because we can also control the nucleation density, these “organic micro forrests” will open new opportunities in areas such as energy harvesting, batteries, supercapacitors, and sensors.

This green colorized image is acquired via a Magellan 400 scanning electron microscope (SEM) at 5 keV electron energy. The dimensions of the micro-pillars are about 0.5–1 μm in diameter and 5 μm in length, and the tilt angle of each pillar in respect to the graphene substrate is nearly identical to the tilt angle of molecules within the micro-crystal pillar due to the inherent “slipped” π–π stacking in the unit cell. There is no photographic blurring to the image, rather, the depth of field itself give this illusion to the vertical crystals at the rear of the image.

Further reading

[1] A.L. Briseno, et al. Mater. Today, 11 (4) (2008), p. 38.
[2] Y. Zhang, et al. J. Am. Chem. Soc., 132 (33) (2010), p. 11580.
[3] J.G. Xue, et al. Adv. Mater., 17 (1) (2005), p. 66.
[4] J.D. Zimmerman, et al. Nano Lett., 12 (8) (2012), p. 4366.
[5] J. Fan, et al. Org. Lett., 14 (4) (2012), p. 1024.

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DOI: 10.1016/j.mattod.2014.10.004