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Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising materials for solar energy conversion and photodetectors. Fullerenes and their derivatives, being widely employed as electron ...

The synthesis of graphene nanoribbons on complementary metal–oxide–semiconductor-compatible substrates is a significant challenge hindering their integration into commercial semiconductor electroni...

Carbon-nanotube (CNT) aggregates are promising light-absorbing materials for photovoltaics. The hopping rate of excitons between CNTs directly affects the efficiency of these devices. We theoretically investigate phonon-assisted exciton hopping, where excitons scatter with phonons into a same-tube transition state, followed by intertube Coulomb scattering into the final state. Second-order hopping between bright excitonic states is as fast as the first-order process (∼1 ps). For perpendicular CNTs, the high rate stems from the high density of phononic states; for parallel CNTs, the reason lies in relaxed selection rules. Moreover, second-order exciton transfer between dark and bright states, facilitated by phonons with large angular momentum, has rates comparable to bright-to-bright transfer, so dark excitons provide an additional pathway for energy transfer in CNT composites. As dark excitons are difficult to probe in experiment, predictive theory is critical for understanding exciton dynamics in CNT com...

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have attracted significant attention as a photoactive component in thin film photovoltaic solar cells and photodetectors due to their strong optical absorptivity and high charge transport mobility. However, the external quantum efficiency (QE) of s-SWCNT/acceptor heterojunction solar cells has been limited by poor exciton harvesting efficiency. Exciton trapping and quenching at defects are a suspected source of loss. Here, we study the influence of defects on bilayer s-SWCNT/C60 planar heterojunction photovoltaic devices via both experiment and modeling. First, diazonium chemistry is used to introduce covalent sp3 sidewall defects to s-SWCNTs at various densities that are estimated using Raman and transient absorption spectroscopy. s-SWCNT/C60 heterojunction photovoltaic cells are then fabricated that show a significant decrease in peak external QE (e.g., from 40% to 8%) with increasing defect density. Second, a diffusion-limited contact quenching M...

We compute the exciton transfer (ET) rate between semiconducting single-wall carbon nanotubes (SWNTs). We show that the main reasons for the wide range of measured ET rates reported in the literature are 1) exciton confinement in local quantum wells stemming from disorder in the environment and 2) exciton thermalization between dark and bright states due to intratube scattering. The SWNT excitonic states are calculated by solving the Bethe-Salpeter equation using tight-binding basis functions. The ET rates due to intertube Coulomb interaction are computed via Fermi's golden rule. In pristine samples, the ET rate between parallel (bundled) SWNTs of similar chirality is very high ($\sim 10^{14}\;\text{s}^{-1}$) while the ET rate for dissimilar or nonparallel tubes is considerably lower ($\sim 10^{12}\;\text{s}^{-1}$). Exciton confinement reduces the ET rate between same-chirality parallel SWNTs by two orders of magnitude, but has little effect otherwise. Consequently, the ET rate in most measurements will be on the order of $ 10^{12}\;\text{s}^{-1}$, regardless of the tube relative orientation or chirality. Exciton thermalization between bright and dark states further reduces the ET rate to about $10^{11}\;\text{s}^{-1}$. The ET rate also increases with increasing temperature and decreases with increasing dielectric constant of the surrounding medium.

Polarized two-dimensional white-light (2D-WL) spectra are reported for thin films of semiconducting carbon nanotubes. The orientational responses for 4-point correlation functions are derived for samples that are isotropic in two dimensions. Spectra measured using ⟨−45°,+45°,0°,90°⟩ polarizations eliminate the diagonal peaks in the spectra arising from S1 transitions to uncover cross peaks to a weaker transition that is assigned to radial breathing modes. In nanotubes purified by unwrapping PFO-BPY polymer using metal chelation, an absorption at 1160 nm is observed that is assigned to hole doping that forms trions. The trion peak may have a transition dipole nonparallel to the S1 transitions, and so its cross peak is prominent in polarized 2D-WL spectra. Energy transfer of photoexcitons to the trion peak occurs within 1 ps. Identifying and understanding the effects of purification on the electronic structure of thin films of semiconducting carbon nanotubes is important for learning how the inherent photop...

Graphene-enhanced Raman scattering (GERS) is a promising characterization technique which uses a single layer of graphene. As the electronic coupling of adsorbates with graphene leads to enhancement in the Raman signal, it is of immense interest to explore the factors that affect the coupling of the adsorbates with graphene. To probe this effect we have designed and synthesized a series of dipolar molecules with the general structure, N-ethyl-N-(2-ethyl(1-pyrenebutyrate)-4-(4-R-phenylazo)aniline) where the R-groups are varied from methoxy (−OCH3), methyl (−CH3), hydrogen (−H), nitrile (−CN), nitro (−NO2) to tricyanofuran (TCF) groups. This systematically changes the dipole moments and electronic/optical band gap of the molecules. By noncovalently interfacing these molecules on graphene, the Raman signal is enhanced by a factor of 40–90 at the excitation wavelength of 532 nm. Measurements of the Raman enhancement factor and Raman cross section are complemented with DFT calculations to correlate the dipole ...

Previous work has demonstrated that the growth temperature for the chemical vapor deposition of graphene can be decreased by tailoring the precursor composition. Here, we fix the precursor as CH4 and instead explore the effects of the catalyst facet, synthesis conditions, and growth rate on the quality of graphene grown at reduced temperature on Cu. We find that in order to obtain graphene films with low defect density, it is critical to maintain a slow growth rate, which is achieved using a low CH4 partial pressure. Furthermore, growth on Cu(111) is more efficient than on other low index Cu facets. Using optimized growth conditions, we achieve high-quality continuous monolayer graphene films, with a low nucleation density of 4 × 10–2 μm–2 and a negligible Raman D:G ratio, at 750 °C, which is 125 °C cooler than previous reports achieving full graphene coverage on Cu. The field-effect mobility is 2600 cm2 V–1 s–1 at a carrier concentration of 1012 cm–2, and low-energy electron microscopy indicates that the...

Highly transparent and conductive monolayer graphene was used as a template to tune the crystal orientation of pentacene from generic standing-up (001) to lying-down (022) in neat films. Spatially resolved Kelvin probe force microscopy (KPFM) was used to reveal the energy levels of pentacene thin films grown on substrates with and without the template graphene layer, as well as the energy level alignment in various pentacene-containing organic–organic heterojunctions. A correlation between crystal domain orientation and the work function was directly observed using KPFM. Up to 0.36 eV shifts in work function were observed in neat pentacene films over large areas (>0.5 in.2) upon orientation transition, likely due to the transition from Fermi level pinning (standing-up pentacene on ITO) to vacuum level alignment (lying-down pentacene on graphene–ITO). Morphology-induced energy level shifts versus interfacial electronic equilibration effects were disentangled using atomic force microscopy, KPFM, X-ray diffr...

We show a large enhancement in the Raman signal from a highly polarizable molecule attached to single layer graphene. Through spatial mapping of the Raman signal and wavelength-dependent Raman measurements from a dipolar chromophore latched to a graphene/SiO2 substrate and to a bare SiO2 substrate, we show that strong electronic coupling in the hybrid structure contributes to the enhancement. The dipolar molecule is a pyrene tethered Disperse Red 1 (DR1P) that noncovalently binds to graphene. Upon comparison of the Raman signal of DR1P on single layer graphene with that on a bare SiO2/Si substrate, we found that the enhancement factor is in the range 29–69 at 532 nm excitation. As the surface coverage of DR1P on graphene increases, Raman intensity also increases and saturates at a certain concentration. The saturation of the Raman signal intensity at higher DR1P concentrations were accompanied by shifts in the G band and the 2D band of graphene due to p-doping. We further show that the Raman enhancement t...

We report on photovoltaic diodes based on bilayer heterojunctions between nearly monochiral, polymer wrapped (7,5) semiconducting carbon nanotube photoabsorbing films and C60. The internal quantum efficiencies (IQEs) for exciton dissociation and subsequent charge collection at the nanotubes’ visible E22 and near-infrared E11 and E11 + X resonances are 84% ± 7%, 85% ± 5%, and 84% ± 14%, respectively. The high IQE at each transition shows that recombination losses during relaxation and/or direct dissociation of “hot” E11 + X and E22 excitons are negligible. A peak external quantum efficiency (EQE) of 34% is achieved at the E11 transition. Zero-bias photoresponsivity is invariant up to short-circuit current densities of at least 23 mA cm–2, indicating negligible losses via trion, charge-exciton, and charge–charge recombination relaxation pathways. An open circuit voltage of 0.49 V and power conversion efficiency of 7.1% are achieved in response to monochromatic excitation of the diodes at the E11 transition....

We investigate exciton dynamics in isopycnically enriched (6,5) nanotube-DNA suspensions using femtosecond time-resolved pump−probe spectroscopy. The ground state recovery is characterized by a t-0.45 ± 0.03 power law behavior, indicative of a one-dimensional diffusion-limited reaction that is tentatively attributed to subdiffusive trapping of dark excitons. Spectral transients of bright singlet excitons within the E11 and E22 manifolds exhibit a photobleach (PB) and a photoabsorption (PA) signal of similar strength. The PA is blue-shifted with respect to the PB-signal by 7.5 meV and is attributed to a transition from the dark singlet exciton to a + state within the two exciton E11 manifold.