Chirality Induced Spin Selectivity in Functionalized Carbon Nanotubes
Creators: Rahman, Md Wazedur
Publisher: name: University of Alberta Library
Year: 2022

<p>In recent years it has been established that electrons become strongly spin polarized when transmitted through chiral compounds and their polarization is dependent on the chirality of the medium. This effect is known as chirality induced spin selectivity or CISS. CISS could be used to realize efficient spin injectors and detectors, which are expected to significantly improve the performance of spintronic devices. There are a number of organic molecules that can exhibit the CISS effect, including nucleic acids and peptides. The organic molecules found in nature are typically insulators that have extremely low electrical conductivities, making them unsuitable for use in practical electronic devices. In this study, we employ carbon nanotubes (CNTs) as transport channels to enhance conductivity, while chiral compounds are used as functionalization entities to induce spin polarization via CISS.<br>
CNTs helically functionalized with single-stranded DNA (ssDNA), have recently emerged as a spin filter material. Inversion asymmetric helical potential of DNA creates a spin filtering effect, which polarizes carrier spins in the nanotube. Tube lengths in the submicron range can achieve ~70% spin polarization, although, according to theoretical predictions spin polarization is expected to increase with tube length. In an effort to study length-dependent CISS effect, we described a fabrication process that can yield DNA-wrapped nanotubes of length ∼1−4 μm. Results from this study indicate presence of highly spin-polarized carriers in these systems.<br>
Tuning of DNA-CNT coupling is expected to affect the degree of carrier spin polarization in nanotubes. Also, in CISS, carrier spins are locked with their momentum directions, and in one-dimensional (1D) systems such as nanotubes, momentum flip must be accompanied by a simultaneous spin flip. This constraint can have a profound impact on charge transport in nanotubes. We have explored DNA-CNT spin filters in which CNTs have been functionalized with two different classes of sequences, exhibiting different degrees of coupling with CNT. They induce different degrees of spin polarization in the channel, with significant impact on temperature-dependent charge transport and associated phenomena. Observed phenomena are consistent with spin-momentum locking expected in CISS systems. The observed negative background magnetoresistance, which results from interference effects between the forward and backward hopping routes, was examined further. CISS-induced spin polarization has been estimated to increase the carrier localization length by an order of magnitude in the low temperature range and it affects the magnetoresistance effect in a non-trivial way that is not observed in conventional systems. <br>
Going beyond the helical geometry of DNA, by using the versatility of peptide chemistry, we demonstrate how spin polarization depends on molecular structural features such as chirality (in non-helical systems) as well as molecule-nanotube interactions. Our findings show that spin polarization can indeed be induced in two-dimensional (2D) carbon nanotube networks by “certain” molecules and the spin signal routinely survives length scales significantly exceeding 1𝜇m. In addition to the more common chirality-dependent effect, a novel form of chirality-independent effect was discovered, and the total spin signal was found to be a mix of both. Finally, the magnetic field dependency of the spin signals was investigated, and the &quot;chirality dependent&quot; signal was shown to exist only at specific field angles. <br>
The influence of multiple and mixed chiral functionalizations on 2D carbon nanotube networks is being investigated in the follow-up experiments. The initial results show that when two or more chiral systems are present, both chirality-dependent and chirality-independent processes contribute to the overall spin polarization. This finding suggests that the CISS effect can be fine-tuned further by manipulating the chiral centers in the medium. </p>