Computational Studies of Nucleotide Selectivity in DNA–Carbon Nanotube Hybrids

Using classical molecular mechanical techniques, we have investigated the morphological properties and interaction mechanisms of single-walled carbon nanotubes (SWNT) functionalized by DNA. Our simulations evidence sequence preference trends for the homogeneous single-strand DNA on the (6,5) vs the (9,1) SWNTs. The DNA–SWNT interaction strength is ordered as G ≳ A > C > T for (9,1) and A > C > G > T for (6,5) hybrids. This preference in a DNA sequence versus tube chiralities is accompanied by variations in wrapping geometries, demonstrating large wrapping angles (40–60°) for the (6,5) and two ranges of wrapping angles (10–30° and 45–60°) for the (9,1) systems. In addition, we found correlations between the most stable wrapping angles of SWNT–DNA hybrids and the most preferential DNA orientations on the graphene with respect to its lattice vectors, which could elucidate the sensitivity of SWNT–DNA structures to the DNA sequence

Morphology and Optical Response of Carbon Nanotubes Functionalized by Conjugated Polymers

Noncovalent functionalization of single wall carbon nanotubes (SWNTs) by biological and conjugated polymers promises significant improvements in their properties important for future nanotube-based optoelectronic and photovoltaic devices. Using a combination of molecular mechanics and quantum chemistry methods, we investigate how the deposition of poly-phenylene vinylene (PPV), a conjugated polymer, on the surface of selected SWNTs affects their morphology, as well as their electronic and optical properties. We found that the interaction between PPV and the nanotube is relatively weak (0.1–0.3 eV per repeat unit), and the most stable structures exhibit small coiling angles (≤20°) of PPV chains around the nanotube. PPV functionalization weakly affects optical excitations of the SWNT, resulting in slight red-shifts of the first and second optical bands of the nanotube. In contrast, the absorption spectra of PPV are strongly affected by specific conformational structures of the wrapped polymer. Our analysis identifies and explains a significant blue-shift of the excited energy and much broader line-width of the coiled PPV compared to that of the respective isolated polymer structures. These trends convey that signatures of polymer wrapping around SWNTs can be detected in experimental optical spectra of hybrid composites

Thickness-Controlled Quasi-Two-Dimensional Colloidal PbSe Nanoplatelets

We demonstrate controlled synthesis of discrete two-dimensional (2D) PbSe nanoplatelets (NPLs), with measurable photoluminescence, via oriented attachment directed by quantum dot (QD) surface chemistry. Halide passivation is critical to the growth of these (100) face-dominated NPLs, as corroborated by density functional theory studies. PbCl₂ moieties attached to the (111) and (110) of small nanocrystals form interparticle bridges, aligning the QDs and leading to attachment. We find that a 2D bridging network is energetically favored over a 3D network, driving the formation of NPLs. Although PbI₂ does not support bridging, its presence destabilizes the large (100) faces of NPLs, providing means for tuning NPL thickness. Spectroscopic analysis confirms the predicted role of thickness-dependent quantum confinement on the NPL band gap

Low-Temperature Single Carbon Nanotube Spectroscopy of sp³ Quantum Defects

Aiming to unravel the relationship between chemical configuration and electronic structure of sp³ defects of aryl-functionalized (6,5) single-walled carbon nanotubes (SWCNTs), we perform low-temperature single nanotube photoluminescence (PL) spectroscopy studies and correlate our observations with quantum chemistry simulations. We observe sharp emission peaks from individual defect sites that are spread over an extremely broad, 1000–1350 nm, spectral range. Our simulations allow us to attribute this spectral diversity to the occurrence of six chemically and energetically distinct defect states resulting from topological variation in the chemical binding configuration of the monovalent aryl groups. Both PL emission efficiency and spectral line width of the defect states are strongly influenced by the local dielectric environment. Wrapping the SWCNT with a polyfluorene polymer provides the best isolation from the environment and yields the brightest emission with near-resolution limited spectral line width of 270 μeV, as well as spectrally resolved emission wings associated with localized acoustic phonons. Pump-dependent studies further revealed that the defect states are capable of emitting single, sharp, isolated PL peaks over 3 orders of magnitude increase in pump power, a key characteristic of two-level systems and an important prerequisite for single-photon emission with high purity. These findings point to the tremendous potential of sp³ defects in development of room temperature quantum light sources capable of operating at telecommunication wavelengths as the emission of the defect states can readily be extended to this range <i>via</i> use of larger diameter SWCNTs