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Researchers at the University of Erlangen-Nuremberg’s Department of Chemistry, Pharmacy and Interdisciplinary Center for Molecular Materials in collaboration with Facultad de Químíca Universidad Complutense recently developed stable hybrids of single walled carbon nanotubes (SWCNTs) and nanographene (NG) with a dye, heptamethine cyanine1.
When light was used to stimulate the hybrid systems, the heptamethine cyanine molecule transferred an electron to the carbon nanostructures, which returned to the ground state after a few nanoseconds. This group of Researchers were able to selectively influence the hybrid structures’ properties for the first time demonstrating the possibility of customizing the carbon nanostructures for specific purposes1.
Graphene is a two dimensional, atomic scale thick, allotrope of carbon that has a hexagonal honeycomb-like lattice with sp2 hybridized (C) atoms arranged at the vertices of each hexagon. Graphene can form nanostructures such as NG or carbon nanotubes (CNTs), which are cylindrical structures made from the same hexagonal lattice of the carbon allotrope – graphene.
CNTs occur as two types, single walled carbon nanotubes (SWCNTs) where the hexagonal lattice forms one hollow cylinder and multi walled carbon nanotubes (MWCNTs) where more than one cylinders made of the hexagonal lattice are enclosed within each other.
Due to their exceptional and unique physicochemical properties, these nanostructures of C are of great interest to a variety of fields such as nanotechnology, electronics, optics and material science.
The SWCNTs have unique electrical properties compared to MWCNTs. While the MWCNTs behave as zero band gap metals, the band gap in SWCNTs could vary from 0 to 2 eV thereby exhibiting both metallic and semi conducting behavior2.
Fine-tuning the electrical properties of SWCNTs could have potential applications to be additives in polymers, catalysts, electron field emitters for cathode ray lighting elements, displays, electromagnetic wave absorption and shielding, sensors, supercapacitors and many more2.
Alexandra Roth’s team have combined the nanostructures of C, NG and SWCNT with an anionic dye, heptamethine cyanine to create two hybrid nanostructures that have the potential to utilize energy from both visible light and near Infra-red (near-IR) regions1.
This group of Researchers demonstrated that this process of doping the carbon nanostructures utilizing functionalization schemes by incorporating dyes would allow for controlling the nanomaterial properties without affecting the conjugated sp2 hybridized C framework.
Furthermore, the excited state dynamics of the hybrids were also studied in the current research to investigate for the presence of charge transfer process1.
The visualization of the electronic interactions in the ground state was facilitated by the near IR absorption spectrum of heptamethine cyanine in the current study. Changes in the photovoltaic properties of the hybrid structures as compared to the original carbon nanostructures confirmed the formation of hybrid structures1.
Statistical Raman assays further concluded that the electronic interactions resulted in stable n-doping of both the nanostructures, NG and SWCNT as evidenced by the downshifted 2D and G-modes for SWCNTs and the upshifted 2D and downshifted G-modes for the NG1. For the first time, this group of Researchers determined that a complete but metastable transfer of charges as accompanied by a radical-ion-pair state that occurs for several nanoseconds.
This process of shift in charge density is attributed to the electron donating character of heptamethine cyanine1.
By creating hybrid structures of carbon nanostructures and heptamethine cyanine, the Researchers were able to manipulate the electronic properties of these nanostructures suggesting that the properties of nanostructures could be fine-tuned to employ them in a variety of applications1. For example, these new hybrid systems could be used in next generation solar power generators wherein these hybrid structures would be able to absorb light in near IR region along with the visible region, whereas the conventional solar power generators can only utilize light in the visible region.
Similarly, these hybrid systems could be used in a variety of applications including sensor technology, electrodes for touch screens, field effect transistors and other energy conversion applications1,2.
References and Further
- “Low-Dimensional Carbon Allotropes: Found- and Excited- State Charge Transfer with NIR-Absorbing Heptamethine Cyanine” A. Roth, C. Schieri, et al. Chem. (2017). DOI: 10.1016/j.chempr.2017.05.003.
- “Single-walled Carbon Nanotubes (SWNTs, SWCNTs)” – U.S. Research Nanomaterials, Inc.
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