Abstract
Low-frequency Raman scattering from self-assembled bioinspired diphenylalanine (FF) nanotubes/microtubes (NTs/MTs) has been observed for the first time. Four double peaks are identified as the three-dimensional localized collective (acoustic phonon) vibrations of FF molecules in the subnanometer crystalline structure (biological building block) forming the FF NTs/MTs. The increased energy separations between two subpeaks caused by the loss of water in the nanochannel cores are due to the enhancement of vibrational couplings between the FF molecules as a result of the reduction of the influence from water on the coupling. The results provide experimental evidence of localized but still weakly coupled vibrations in organic crystalline nanostructures in the low-frequency region.
Highlights
Low-frequency (LF) Raman scattering in inorganic nanocrystals (NCs) has been an attractive research subject for the past two decades because of fundamental issues related to the size, shape, surface/interface structure, and confined property of NCs [1,2,3,4,5,6]
It was shown that quantum confinement effect that had been observed only from inorganic semiconductor NCs could occur in such selfassembled from self-assembled bioinspired diphenylalanine (FF) NTs/MTs made of biological building blocks in the form of helices with six FF molecules per turn and side chains emanating from the channel core filled with water molecules [circle, Fig. 1(a)] [14,15,16,17,18,19]
Since the LF modes stem from the collective vibrations of some large molecular groups in the FF molecule, water loss weakens the vibration coupling between these groups with water, leading to strengthening vibration coupling between the large groups themselves, thereby creating significant energy separation between the two subpeaks
Summary
Low-frequency (LF) Raman scattering in inorganic nanocrystals (NCs) has been an attractive research subject for the past two decades because of fundamental issues related to the size, shape, surface/interface structure, and confined property of NCs [1,2,3,4,5,6]. It was shown that quantum confinement effect that had been observed only from inorganic semiconductor NCs could occur in such selfassembled FF NTs/MTs made of biological building blocks (subnanometer crystalline structures, SCSs) in the form of helices with six FF molecules per turn and side chains emanating from the channel core filled with water molecules [circle, Fig. 1(a)] [14,15,16,17,18,19] These SCSs are arranged regularly in the tube walls and have almost the same subnanometer scales, for example, three side lengths of a SCS unit cell a = b = 2.407 nm and c = 0.545 nm as reported by Gorbitz [18,19]. The new results provide experimental evidence of localized but still weakly coupled vibrations in organic crystalline nanostructures in the lowfrequency region
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