Observation of the triplet exciton in EuS-coated single-walled nanotubes

Chirality Dependence of Triplet Excitons in (6,5) and (7,5) Single-Wall Carbon Nanotubes Revealed by Optically Detected Magnetic Resonance

The excitonic structure of single-wall carbon nanotubes (SWCNTs) is chirality dependent and consists of multiple singlet and triplet excitons (TEs) of which only one singlet exciton (SE) is optically bright. In particular, the dark TEs have a large impact on the integration of SWCNTs in optoelectronic devices, where excitons are created electrically, such as in infrared light-emitting diodes, thereby strongly limiting their quantum efficiency. Here, we report the characterization of TEs in chirality-purified samples of (6,5) and (7,5) SWCNTs, either randomly oriented in a frozen solution or with in-plane preferential orientation in a film, by means of optically detected magnetic resonance (ODMR) spectroscopy. In both chiral structures, the nanotubes are shown to sustain three types of TEs. One TE exhibits axial symmetry with zero-field splitting (ZFS) parameters depending on SWCNT diameter, in good agreement with the tighter confinement expected in narrower-diameter nanotubes. The ZFS of this TE also depends on nanotube environment, pointing to slightly weaker confinement for surfactant-coated than for polymer-wrapped SWCNTs. A second TE type, with much smaller ZFS, does not show the same systematic trends with diameter and environment and has a less well-defined axial symmetry. This most likely corresponds to TEs trapped at defect sites at low temperature, as exemplified by comparing SWCNT samples from different origins and after different treatments. A third triplet has unresolved ZFS, implying it originates from weakly interacting spin pairs. Aside from the diameter dependence, ODMR thus provides insights in both the symmetry, confinement, and nature of TEs on semiconducting SWCNTs.

Delayed Fluorescence from Carbon Nanotubes through Singlet Oxygen-Sensitized Triplet Excitons

Single-wall carbon nanotubes (SWCNTs) in liquid suspension have been observed to emit delayed, microsecond-scale fluorescence arising from upconverted triplet excitons that are directly created through energy transfer from singlet oxygen molecules (¹O₂). The singlet oxygen is produced through quenching of an optically excited organic sensitizer. The mechanism of this delayed fluorescence has been deduced from measurements of time-resolved emission kinetics, delayed emission spectra, and polarization-resolved excitation-emission spectra. The observed strong dependence of ¹O₂ sensitization efficiency on SWCNT structure suggests that (7,6) triplet excitons have an energy near 970 meV. The yields for E₁₁^T → E₁₁^S upconversion are found to be in the range of several percent. These yields increase with increasing temperature and decrease with increasing excitation intensities, reflecting thermal activation and triplet-triplet exciton annihilation processes.

Hidden Fine Structure of Quantum Defects Revealed by Single Carbon Nanotube Magneto-Photoluminescence

Organic color-center quantum defects in semiconducting carbon nanotube hosts are rapidly emerging as promising candidates for solid-state quantum information technologies. However, it is unclear whether these defect color-centers could support the spin or pseudospin-dependent excitonic fine structure required for spin manipulation and readout. Here we conducted magneto-photoluminescence spectroscopy on individual organic color-centers and observed the emergence of fine structure states under an 8.5 T magnetic field applied parallel to the nanotube axis. One to five fine structure states emerge depending on the chirality of the nanotube host, nature of chemical functional group, and chemical binding configuration, presenting an exciting opportunity toward developing chemical control of magnetic brightening. We attribute these hidden excitonic fine structure states to field-induced mixing of singlet excitons trapped at sp³ defects and delocalized band-edge triplet excitons. These findings provide opportunities for using organic color-centers for spintronics, spin-based quantum computing, and quantum sensing.

Critical Role of the Sorting Polymer in Carbon Nanotube-Based Minority Carrier Devices

A prerequisite for carbon nanotube-based optoelectronic devices is the ability to sort them into a pure semiconductor phase. One of the most common sorting routes is enabled through using specific wrapping polymers. Here we show that subtle changes in the polymer structure can have a dramatic influence on the figures of merit of a carbon nanotube-based photovoltaic device. By comparing two commonly used polyfluorenes (PFO and PFO-BPy) for wrapping (7,5) and (6,5) chirality SWCNTs, we demonstrate that they have contrasting effects on the device efficiency. We attribute this to the differences in their ability to efficiently transfer charge. Although PFO may act as an efficient interfacial layer at the anode, PFO-BPy, having the additional pyridine side groups, forms a high resistance layer degrading the device efficiency. By comparing PFO|C₆₀ and C₆₀-only devices, we found that presence of a PFO layer at low optical densities resulted in the increase of all three solar cell parameters, giving nearly an order of magnitude higher efficiency over that of C₆₀-only devices. In addition, with a relatively higher contribution to photocurrent from the PFO-C₆₀ interface, an open circuit voltage of 0.55 V was obtained for PFO-(7,5)-C₆₀ devices. On the other hand, PFO-BPy does not affect the open circuit voltage but drastically reduces the short circuit current density. These results indicate that the charge transport properties and energy levels of the sorting polymers have to be taken into account to fully understand their effect on carbon nanotube-based solar cells.

Solid state carbon nanotube device for controllable trion electroluminescence emission

Semiconducting carbon nanotubes (CNTs) have a direct chirality-dependent bandgap and reduced dimensionality-related quantum confinement effects, which are closely related to the performance of optoelectronic devices. Here, taking advantage of the large energy separations between neutral singlet excitons and charged excitons, i.e. trions in CNTs, we have achieved for the first time all trion electroluminescence (EL) emission from chirality-sorted (8,3) and (8,4) CNT-based solid state devices. We showed that strong trion emission can be obtained as a result of localized impact excitation and electrically injected holes, with an estimated efficiency of ∼5 × 10(-4) photons per injected hole. The importance of contact-controlled carrier injection (including symmetric and asymmetric contact configurations) and EL spectral stability for gradually increasing bias were also investigated. The realization of electrically induced pure trion emission opens up a new opportunity for CNT film-based optoelectronic devices, providing a new degree of freedom in controlling the devices to extend potential applications in spin or magnetic optoelectronics fields.

Spatially resolved photoexcited charge-carrier dynamics in phase-engineered monolayer MoS2

A fundamental understanding of the intrinsic optoelectronic properties of atomically thin transition-metal dichalcogenides (TMDs) is crucial for its integration into high performance semiconductor devices. Here, we investigate the transport properties of chemical vapor deposition (CVD) grown monolayer molybdenum disulfide (MoS2) under photoexcitation using correlated scanning photocurrent microscopy and photoluminescence imaging. We examined the effect of local phase transformation underneath the metal electrodes on the generation of photocurrent across the channel length with diffraction-limited spatial resolution. While maximum photocurrent generation occurs at the Schottky contacts of semiconducting (2H-phase) MoS2, after the metallic phase transformation (1T-phase), the photocurrent peak is observed toward the center of the device channel, suggesting a strong reduction of native Schottky barriers. Analysis using the bias and position dependence of the photocurrent indicates that the Schottky barrier heights are a few millielectron volts for 1T- and ∼ 200 meV for 2H-contacted devices. We also demonstrate that a reduction of native Schottky barriers in a 1T device enhances the photoresponsivity by more than 1 order of magnitude, a crucial parameter in achieving high-performance optoelectronic devices. The obtained results pave a way for the fundamental understanding of intrinsic optoelectronic properties of atomically thin TMDs where ohmic contacts are necessary for achieving high-efficiency devices with low power consumption.

Spin manipulation with magnetic semiconductor barriers

Magnetic semiconductors with unique spin-filtering property and the ability to create excessive internal magnetic fields can open myriads of new phenomena.

Visible-wavelength polarized-light emission with small-diameter InN nanowires

Group III nitrides are widely used in commercial visible-wavelength optoelectronic devices, but materials issues such as dislocations, composition fluctuations, and strain negatively impact their efficiency. Nitride nanostructures are a promising solution to overcome these issues and to improve device performance. We used first-principles calculations based on many-body perturbation theory to study the electronic and optical properties of small-diameter InN nanowires. We show that quantum confinement in 1 nm wide InN nanowires shifts optical emission to the visible range at green/cyan wavelengths and inverts the order of the top valence bands, leading to linearly polarized visible-light emission. Quantum confinement on this scale also leads to large exciton binding energies of 1.4 eV and electronic band gaps in excess of 3.7 eV. Our results indicate that strong quantum confinement in InN nanostructures is a promising approach to developing efficient visible-wavelength light emitters.

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.

Dynamics and transient absorption spectral signatures of the single-wall carbon nanotube electronically excited triplet state

We utilize femtosecond-to-microsecond time domain pump-probe transient absorption spectroscopy to interrogate for the first time the electronically excited triplet state of individualized single-wall carbon nanotubes (SWNTs). These studies exploit (6,5) chirality-enriched SWNT samples and poly[2,6-{1,5-bis(3-propoxysulfonic acid sodium salt)}naphthylene]ethynylene (PNES), which helically wraps the nanotube surface with periodic and constant morphology (pitch length = 10 ± 2 nm), providing a self-assembled superstructure that maintains structural homogeneity in multiple solvents. Spectroscopic interrogation of such PNES-SWNT samples in aqueous and DMSO solvents using E(22) excitation and a white-light continuum probe enables E(11) and E(22) spectral evolution to be monitored concomitantly. Such experiments not only reveal classic SWNT singlet exciton relaxation dynamics and transient absorption signatures but also demonstrate spectral evolution consistent with formation of a triplet exciton state. Transient dynamical studies evince that (6,5) SWNTs exhibit rapid S(1)→T(1) intersystem crossing (ISC) (τ(ISC) ~20 ps), a sharp T(1)→T(n) transient absorption signal (λ(max)(T(1)→T(n)) = 1150 nm; full width at half-maximum ≈ 350 cm(-1)), and a substantial T(1) excited-state lifetime (τ(es) ≈ 15 μs). Consistent with expectations for a triplet exciton state, T(1)-state spectral signatures and T(1)-state formation and decay dynamics for PNES-SWNTs in aqueous and DMSO solvents, as well as those determined for benchmark sodium cholate suspensions of (6,5) SWNTs, are similar; likewise, studies that probe the (3)[(6,5) SWNT]* state in air-saturated solutions demonstrate (3)O(2) quenching dynamics reminiscent of those determined for conjugated aromatic hydrocarbon excited triplet states.

Quasi-molecular fluorescence from graphene oxide

Aqueous dispersions of graphene oxide (GO) have been found to emit a structured, strongly pH-dependent visible fluorescence. Based on experimental results and model computations, this is proposed to arise from quasi-molecular fluorophores, similar to polycyclic aromatic compounds, formed by the electronic coupling of carboxylic acid groups with nearby carbon atoms of graphene. Sharp and structured emission and excitation features resembling the spectra of molecular fluorophores are present near 500 nm in basic conditions. The GO emission reversibly broadens and red-shifts to ca. 680 nm in acidic conditions, while the excitation spectra remain very similar in shape and position, consistent with excited state protonation of the emitting species in acidic media. The sharp and structured emission and excitation features suggest that the effective fluorophore size in the GO samples is remarkably well defined.

Time-resolved red luminescence from europium-catalyzed single walled carbon nanotubes

The photo-physical properties of Eu-SWCNTs indicate that intrinsic excitonic properties of SWCNTs sensitize the lanthanoid element europium (Eu) to emit time-resolved red luminescence.

Brightening of triplet dark excitons by atomic hydrogen adsorption in single-walled carbon nanotubes observed by photoluminescence spectroscopy

Adsorption and desorption of atomic hydrogen on single-walled carbon nanotubes were observed by photoluminescence spectroscopy. A satellite peak appeared at the lower energy side of the E11 photoluminescence emission peak after exposure to atomic hydrogen and then disappeared after annealing at 300 °C in vacuum. The energy difference between the satellite peak and E11 peak was 40-80 meV, depending on the tube diameter. The satellite peak was attributed to the triplet dark exciton that became optically active because of the effectively enhanced spin-orbit interaction induced by adsorbed hydrogen atoms.