Surface-Enhanced Raman Spectroscopy of 2D Organic Semiconductor Crystals

Abstract

Recently developed ultrathin two-dimensional (2D) organic semiconductor crystals are a promising platform for advanced organic electronic devices. Remarkable quality of such crystals results in charge-carrier mobilities comparable to those of bulk crystals, but their structure and orientation are hard to study because of their extremely small thickness. Here, we applied surface-enhanced Raman spectroscopy (SERS) to investigate the structure of the thinnest 2D single crystalsmonolayers, which are based on thiophene-phenylene co-oligomers: 1,4-bis­(5′-decyl-2,2′-bithiene-5-yl)­benzene and 1,4-bis­(5′-hexyl-2,2′-bithiene-5-yl)­benzene. Their Raman spectra were calculated as a function of the molecule orientation and SERS microscopy maps were acquired. High sensitivity of SERS allowed us to study monolayer single-crystal domains with the optical spatial resolution. Raman anisotropy was used to probe the orientations of single-crystal domains and the molecule orientation within them. Notably, the SERS microscopy detected the presence of a submonolayeramorphous material between the crystalline domains, which is practically inaccessible to optical or conventional atomic force microscopies (AFMs). The submonolayer was also studied by lateral-force AFM, which showed notably higher friction and adhesion. We found that the measured Raman anisotropy significantly reduced by the metal-covered substrate still allowing us to distinguish orientations of molecules in the 2D crystals and in the submonolayer. Anisotropy-sensitive SERS was shown to be promising for studying 2D organic semiconductor crystals.

Controlled Growth and Self‐Assembly of Multiscale Organic Semiconductor

“…[74] Usually, the thickness of 2D OSs does not reach its saturated mobility distance. [75,76] The structure of 2D OSs usually has a π-conjugated molecular plane. [77] While considering the properties, 2D OSs can effectively eliminate interlayer shielding, thus providing an ideal platform for studying the effects of disorder and interface effects on electron transport.…”

“…Nevertheless, it is still a powerful tool to investigate the structure of OS assemblies. [75,126,131] For example, OSs with crystalline structures present an extra complexity of various terminations decorated with different functional groups. Thus, it is a significant challenge to clearly identify crystal surfaces.…”

“…[177] Organic crystal films with ultrathin thicknesses have attracted considerable attention because of their outstanding photoelectric properties. [25,75] The formation of a supramolecular crystalline structure in aqueous solution is an effective way to prepare organic crystal thin films. Currently, it is still a significant challenge to grow organic crystals with a single-molecule thickness over a large area.…”

Polarization Raman Imaging of Organic Monolayer Islands for Crystal Orientation Analysis

“…Raman signals from organic molecule monolayers are usually very weak, so several techniques are sometimes utilized for signal enhancement, such as the resonant Raman effect 34 and plasmonic enhancement. 35 , 36 However, we realized that these techniques are not very suitable for our purpose. The resonant excitation resulted in strong photoluminescence from DPh-DNTT.…”

Bulk and Surface‐Mediated Polymorphs of Bio‐Inspired Dyes Organic Semiconductors: The Role of Lattice Phonons in their Investigation

“…In the last decade, low-wavenumber Raman microscopy has proven to be a versatile, reliable and fast way for the identification and the spatial mapping of crystalline phases in molecular material-based devices. [18,19] Different structures can in fact be identified on the basis of their spectrum in the THz region, which represents the fingerprint of the crystal lattice dynamics. [20] The modes observed here hold the character of lattice phonons, or vibrations in which molecules oscillate as a whole around the lattice equilibrium positions, subjected to the intermolecular force field.…”