Polarization-Sensitive Photoluminescence from Aligned Carbon Chains Terminated by Gold Clusters

We synthesize a thin film composed of long carbyne chains terminated by gold clusters and study its optical properties. The presence of gold particles stabilizes longer chains and leads to their alignment. We show that the gold clusters also act as a source of electron doping, thus, changing the intensity of photoluminescence from quadratic dependence on the pumping intensity without gold to linear with gold. We also observe that the excitation of the film at the gold plasmon frequency causes the blue shift of photoluminescence and estimate, on the basis of this effect, the minimum length of the carbyne chains. The high degree of alignment of the gold-terminated carbyne chains results in strongly anisotropic light absorption characterized by a distinctive cosine dependence on the angle between the carbyne molecule and polarization plane of the excitation. This paves the way for a new class of ultimately thin polarization sensitive emitters to be used in future integrated quantum photonics devices.

Photochemical spin-state control of binding configuration for tailoring organic color center emission in carbon nanotubes

Incorporating fluorescent quantum defects in the sidewalls of semiconducting single-wall carbon nanotubes (SWCNTs) through chemical reaction is an emerging route to predictably modify nanotube electronic structures and develop advanced photonic functionality. Applications such as room-temperature single-photon emission and high-contrast bio-imaging have been advanced through aryl-functionalized SWCNTs, in which the binding configurations of the aryl group define the energies of the emitting states. However, the chemistry of binding with atomic precision at the single-bond level and tunable control over the binding configurations are yet to be achieved. Here, we explore recently reported photosynthetic protocol and find that it can control chemical binding configurations of quantum defects, which are often referred to as organic color centers, through the spin multiplicity of photoexcited intermediates. Specifically, photoexcited aromatics react with SWCNT sidewalls to undergo a singlet-state pathway in the presence of dissolved oxygen, leading to ortho binding configurations of the aryl group on the nanotube. In contrast, the oxygen-free photoreaction activates previously inaccessible para configurations through a triplet-state mechanism. These experimental results are corroborated by first principles simulations. Such spin-selective photochemistry diversifies SWCNT emission tunability by controlling the morphology of the emitting sites.

Chemical functionalization of the sidewalls of single-wall carbon nanotubes (SWCNTs) is an emerging route to introduce fluorescent quantum defects and tailor the emission properties. Here, authors demonstrate that spin-selective photochemistry diversifies SWCNT emission tunability by controlling the morphology of the emitting sites.

Coupling between Emissive Defects on Carbon Nanotubes: Modeling Insights

Covalent functionalization of single-walled carbon nanotubes (SWCNTs) with organic molecules results in red-shifted emissive states associated with sp³-defects in the tube lattice, which facilitate their improved optical functionality, including single-photon emission. The energy of the defect-based electronic excitations (excitons) depends on the molecular adducts, the configuration of the defect, and concentration of defects. Here we model the interactions between two sp³-defects placed at various distances in the (6,5) SWCNT using time-dependent density functional theory. Calculations reveal that these interactions conform to the effective model of J-aggregates for well-spaced defects (>2 nm), leading to a red-shifted and optically allowed (bright) lowest energy exciton. H-aggregate behavior is not observed for any defect orientations, which is beneficial for emission. The splitting between the lowest energy bright and optically forbidden (dark) excitons and the pristine excitonic band are controlled by the single-defect configurations and their axial separation. These findings enable a synthetic design strategy for SWCNTs with tunable near-infrared emission.

Experimental and theoretical study of energy transfer in a chromophore triad: What makes modeling dynamics successful?

Simulation of electronic dynamics in realistically large molecular systems is a demanding task that has not yet achieved the same level of quantitative prediction already realized for its static counterpart. This is particularly true for processes occurring beyond the Born-Oppenheimer regime. Non-adiabatic molecular dynamics (NAMD) simulations suffer from two convoluted sources of error: numerical algorithms for dynamics and electronic structure calculations. While the former has gained increasing attention, particularly addressing the validity of ad hoc methodologies, the effect of the latter remains relatively unexplored. Indeed, the required accuracy for electronic structure calculations to reach quantitative agreement with experiment in dynamics may be even more strict than that required for static simulations. Here, we address this issue by modeling the electronic energy transfer in a donor-acceptor-donor (D-A-D) molecular light harvesting system using fewest switches surface hopping NAMD simulations. In the studied system, time-resolved experimental measurements deliver complete information on spectra and energy transfer rates. Subsequent modeling shows that the calculated electronic transition energies are "sufficiently good" to reproduce experimental spectra but produce over an order of magnitude error in simulated dynamical rates. We further perform simulations using artificially shifted energy gaps to investigate the complex relationship between transition energies and modeled dynamics to understand factors affecting non-radiative relaxation and energy transfer rates.

Controlling Defect-State Photophysics in Covalently Functionalized Single-Walled Carbon Nanotubes

ConspectusSingle-walled carbon nanotubes (SWCNTs) show promise as light sources for modern fiber optical communications due to their emission wavelengths tunable via chirality and diameter dependency. However, the emission quantum yields are relatively low owing to the existence of low-lying dark electronic states and fast excitonic diffusion leading to carrier quenching at defects. Covalent functionalization of SWCNTs addresses this problem by brightening their infrared emission. Namely, introduction of sp³-hybridized defects makes the lowest energy transitions optically active for some defect geometries and enables further control of their optical properties. Such functionalized SWCNTs are currently the only material exhibiting room-temperature single photon emission at telecom relevant infrared wavelengths. While this fluorescence is strong and has the right wavelength, functionalization introduces a variety of emission peaks resulting in spectrally broad inhomogeneous photoluminescence that prohibits the use of SWCNTs in practical applications. Consequently, there is a strong need to control the emission diversity in order to render these materials useful for applications. Recent experimental and computational work has attributed the emissive diversity to the presence of multiple localized defect geometries each resulting in distinct emission energy. This Account outlines methods by which the morphology of the defect in functionalized SWCNTs can be controlled to reduce emissive diversity and to tune the fluorescence wavelengths. The chirality-dependent trends of emission energies with respect to individual defect morphologies are explored. It is demonstrated that defect geometries originating from functionalization of SWCNT carbon atoms along bonds with strong π-orbital mismatch are favorable. Furthermore, the effect of controlling the defect itself through use of different chemical groups is also discussed. Such tunability is enabled due to the formation of specific defect geometries in close proximity to other existing defects. This takes advantage of the changes in π-orbital mismatch enforced by existing defects and the resulting changes in reactivities toward formation of specific defect morphologies. Furthermore, the trends in emissive energies are highly dependent on the value of mod(n-m,3) for an (n,m) tube chirality. These powerful concepts allow for a targeted formation of defects that emit at desired energies based on SWCNT single chirality enriched samples. Finally, the impact of functionalization with specific types of defects that enforce certain defect geometries due to steric constraints in bond lengths and angles to the SWCNT are discussed. We further relate to a similar effect that is present in systems where high density of surface defects is formed due to high reactant concentration. The outlined strategies suggested by simulations are instrumental in guiding experimental efforts toward the generation of functionalized SWCNTs with tunable emission energies.

Computational design of p-(dimethylamino)benzylidene-derived push-pull polyenes with high first-hyperpolarizabilities

Multiple theoretical investigations on three new series of donor-bridge-acceptor substituted compounds are employed to aid in the design of NLO-phores with high first-hyperpolarizability β. The effect of varying the acceptor (rhodanine, thiohydantoin and thiobarbituric acid derivative-based) and bridge parts of these D-π-A systems was analyzed in terms of geometric and optoelectronic parameters such as bond length alternation, ground state dipole moments, HOMO and LUMO energies, UV-vis absorption spectra, transition dipole moments, and electronic absorption energies. Various functionals with the AUG-cc-pVDZ basis set including B3LYP, PBE38, and ωB97XD, and the Hartree-Fock method were employed to calculate β values, and the solvent effect was also considered by employing the SMD model. The variation of first-hyperpolarizabilities has been explained satisfactorily in terms of the PBE38/AUG-cc-pVDZ level calculated spectroscopic properties in the light of the sum-over-states method and the two-level model. The comprehensive study indicates that the most worthwhile targets for development as NLO-phores are compounds that include a longer π-bridge.

Electron–phonon scattering and excitonic effects in T-carbon

Through first-principles calculations combining many-body perturbation theory, we investigate electron–phonon scattering and optical properties including the excitonic effects of T-carbon. Our results reveal that optical and acoustic phonons dominate the scattering around the valence and the conduction band edges, respectively. In addition, the relaxation lifetimes of holes (0.5 ps) are longer than those of electrons (0.24 ps) around the band edges due to the weaker scattering. We also predict that mean free paths of hot holes are as high as 80 nm while only 15 nm for hot electrons, resulting in different hot carrier extraction ranges in T-carbon. Moreover, we demonstrate that there exist lowest energy dark excitons in T-carbon with radiative lifetime of about 3.4 s, which is revealed to be much longer than that of bright excitons and would lower the photoluminescence quantum yield of T-carbon.

Through first-principles calculations combining many-body perturbation theory, we investigate electron–phonon scattering and optical properties including the excitonic effects of T-carbon.

Optical properties of semiconducting zigzag carbon nanotubes with and without defects

The excited states of a series of semiconducting zigzag (n, 0) tubes are studied using the GW method and the Bethe-Salpeter equation within the ab initio many-body perturbation theory. The optical variation rule of the excitation energy with the tube diameter exhibits a family pattern, which arises from the electronic structure of the pristine tube and depends on the value of n mod 3. The introduction of single vacancy and Stone-Wales defects with different orientations affords an effective route for modulating the band structures and optical spectra, resulting in the variation of the selection rules of the excitons and turning dipole-forbidden excitons into dipole-allowed ones. The new localized impurity states in defected tubes will provide additional optically allowed transitions and give rise to pronounced satellite red-shifted peaks. These findings provide inspiration for the tune of optical properties of carbon nanotubes in the future for applications in optoelectronics.

Solvent- and Wavelength-Dependent Photoluminescence Relaxation Dynamics of Carbon Nanotube sp3 Defect States

Photoluminescent sp³ defect states introduced to single wall carbon nanotubes (SWCNTs) through low-level covalent functionalization create new photophysical behaviors and functionality as a result of defect sites acting as exciton traps. Evaluation of relaxation dynamics in varying dielectric environments can aid in advancing a more complete description of defect-state relaxation pathways and electronic structure. Here, we exploit helical wrapping polymers as a route to suspending (6,5) SWCNTs covalently functionalized with 4-methoxybenzene in solvent systems including H₂O, D₂O, methanol, dimethylformamide, tetrahydrofuran, and toluene, spanning a range of dielectric constants from 80 to 3. Defect-state photoluminescence decays were measured as a function of emission wavelength and solvent environment. Emission decays are biexponential, with short lifetime components on the order of 65 ps and long components ranging from around 100 to 350 ps. Both short and long decay components increase as emission wavelength increases, while only the long lifetime component shows a solvent dependence. We demonstrate that the wavelength dependence is a consequence of thermal detrapping of defect-state excitons to produce mobile E₁₁ excitons, providing an important mechanism for loss of defect-state population. Deeper trap states (i.e., those emitting at longer wavelengths) result in a decreased rate for thermal loss. The solvent-independent behavior of the short lifetime component is consistent with its assignment as the characteristic time for redistribution of exciton population between bright and dark defect states. The solvent dependence of the long lifetime component is shown to be consistent with relaxation via an electronic to vibrational energy transfer mechanism, in which energy is resonantly lost to solvent vibrations in a complementary mechanism to multiphonon decay processes.

Correction Scheme for Comparison of Computed and Experimental Optical Transition Energies in Functionalized Single-Walled Carbon Nanotubes

Covalent functionalization of single-walled carbon nanotubes (SWCNTs) introduces red-shifted emission features in the near-infrared spectral range due to exciton localization around the defect site. Such chemical modifications increase their potential use as near-infrared emitters and single-photon sources in telecommunications applications. Density functional theory (DFT) studies using finite-length tube models have been used to calculate their optical transition energies. Predicted energies are typically blue-shifted compared to experiment due to methodology errors including imprecise self-interaction corrections in the density functional and finite-size basis sets. Furthermore, artificial quantum confinement in finite models cannot be corrected by a constant-energy shift since they depend on the degree of exciton localization. Herein, we present a method that corrects the emission energies predicted by time-dependent DFT. Confinement and methodology errors are separately estimated using experimental data for unmodified tubes. Corrected emission energies are in remarkable agreement with experiment, suggesting the value of this straightforward method toward predicting and interpreting the optical features of functionalized SWCNTs.

Effect of Aspect Ratio on Multiparticle Auger Recombination in Single-Walled Carbon Nanotubes: Time Domain Atomistic Simulation

Many-particle Auger-type processes are common in nanoscale materials due to a combination of high densities of states that can support multiple excitations and substantial Coulomb coupling between charges enhanced by quantum confinement. Auger decay dynamics in (10,5) semiconductor carbon nanotubes (CNT) with different aspect ratios and particle densities are simulated in time domain using global flux surface hopping, recently developed and implemented within Kohn-Sham tight-binding density functional theory. Despite an increasing density of states, the multiparticle Auger recombination rate decreases in longer CNTs. The atomistic simulation shows that the effect is directly related to the coupling between electronic states, which decreases as the aspect ratio becomes larger. The dependence on tube length is stronger for three-exciton than two-exciton recombination and the calculated time scale ratio approaches the experimental value measured for long CNTs. Phonon-assisted transitions play a particularly important role during Auger recombination. Electron-phonon relaxation is faster than the recombination, and Auger transitions are assisted by phonons over a range of frequencies up to the G-mode. The involvement of phonons strongly enhances the probability of transitions involving asymmetric electron-hole pairs. The time-domain atomistic simulation mimics directly time-resolved optical experiments and provides a detailed, systematic analysis of the phonon-assisted Auger dynamics.

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 via use of larger diameter SWCNTs.

Computational study on thermally activated delayed fluorescence of donor–linker–acceptor network molecules

Using density functional theory we have investigated the structure–property relationships of organic molecules with a donor–linker–acceptor (DLA) framework, which can be used as precursors of OLED materials.

Hyperspectral imaging of exciton photoluminescence in individual carbon nanotubes controlled by high magnetic fields

Semiconducting carbon nanotubes (CNTs) provide an exceptional platform for studying one-dimensional excitons (bound electron-hole pairs), but the role of defects and quenching centers in controlling emission remains controversial. Here we show that, by wrapping the CNT in a polymer sheath and cooling to 4.2 K, ultranarrow photoluminescence (PL) emission line widths below 80 μeV can be seen from individual solution processed CNTs. Hyperspectral imaging of the tubes identifies local emission sites and shows that some previously dark quenching segments can be brightened by the application of high magnetic fields, and their effect on exciton transport and dynamics can be studied. Using focused high intensity laser irradiation, we introduce a single defect into an individual nanotube which reduces its quantum efficiency by the creation of a shallow bound exciton state with enhanced electron-hole exchange interaction. The emission intensity of the nanotube is then reactivated by the application of the high magnetic field.

Asymmetry in the electron and hole transfer at a polymer-carbon nanotube heterojunction

To achieve a high photon-to-charge conversion efficiency, the electron-hole pair generated by photon absorption in organic photovoltaic systems must overcome the Coulomb attraction, which often results in voltage loss. Bearing this in mind, we performed ab initio time-domain simulations of the charge separation and energy relaxation across an interface formed by poly(3-hexylthiophene) (P3HT) and a single-walled carbon nanotube (CNT). The dynamics of the positive and negative charges showed strong asymmetry. Photoexcitation of the polymer leads to a 100 fs electron transfer, in agreement with the experiment, followed by a loss of 0.6 eV of energy within 0.5 ps. Photoexcitation of the CNT leads to hole transfer, which requires nearly 2 ps, but loses only 0.3 eV of energy. The strong disparity arises due to the differences in the localization of the photoexcited donor states, the number densities of the acceptor states, and the phonon modes involved. Used as a chromophore, P3HT produces faster charge separation but leads to larger energy losses and cannot harvest light in the red region of the solar spectrum. In contrast, CNT absorbs a broader range of photons and reduces energy losses but gives a less efficient charge separation. The complementary properties of the two chromophores can be utilized to improve the performance of solar cells by optimizing simultaneously light harvesting, charge separation, and energy relaxation, which affect the photovoltaic yield, current, and voltage.

Brightening of the lowest exciton in carbon nanotubes via chemical functionalization

Using time-dependent density functional theory, we found that chemical functionalization at low concentrations of single-walled carbon nanotubes (SWNTs) locally alters the π-conjugated network of the nanotube surface and leads to a spatial confinement of the electronically excited wave functions. Depending on the adsorbant positions, the chemisorption significantly modifies the optical selection rules. Our modeling suggests that photoluminescent efficiency of semiconducting SWNT materials can be controlled by selective chemical functionalization.

Optoelectronic Properties of Carbon Nanorings: Excitonic Effects from Time-Dependent Density Functional Theory

The electronic structure and size-scaling of optoelectronic properties in cycloparaphenylene carbon nanorings are investigated using time-dependent density functional theory (TDDFT). The TDDFT calculations on these molecular nanostructures indicate that the lowest excitation energy surprisingly becomes larger as the carbon nanoring size is increased, in contradiction with typical quantum confinement effects. In order to understand their unusual electronic properties, I performed an extensive investigation of excitonic effects by analyzing electron-hole transition density matrices and exciton binding energies as a function of size in these nanoring systems. The transition density matrices allow a global view of electronic coherence during an electronic excitation, and the exciton binding energies give a quantitative measure of electron-hole interaction energies in the nanorings. Based on overall trends in exciton binding energies and their spatial delocalization, I find that excitonic effects play a vital role in understanding the unique photoinduced dynamics in these carbon nanoring systems.