Photoluminescence Brightening of Isolated Single-Walled Carbon Nanotubes

The addition of dithiothreitol (DTT) to a suspension consisting of either DNA- or sodium-dodecyl-sulfate (SDS)-wrapped single-walled carbon nanotubes (SWCNTs) caused significant photoluminescence (PL) brightening from the SWCNTs, whereas PL quenching to different extents was observed for other surfactant–SWCNT suspensions. PL lifetime studies with high temporal resolution show that the addition of DTT mitigates nonradiative decay processes but also surprisingly increases the radiative decay rate for DNA– and SDS–SWCNTs. Completely opposite effects on the decay rates are found for the other surfactant–SWCNTs that show PL quenching. We propose that the PL brightening results from a surfactant reorganization upon DTT addition

Uncovering Hot Hole Dynamics in CdSe Nanocrystals

Single and multiple exciton relaxation dynamics of CdSe/CdZnS nanocrystal quantum dots (QDs) monitored at the two lowest optical transitions, 1S_e–1S_3/2 and 1S_e–2S_3/2, have been examined using ultrafast transient absorption (TA) spectroscopy. For the CdSe/CdZnS QDs studied, the 1S_e–1S_3/2 and 1S_e–2S_3/2 transitions are widely separated (∼180 meV) compared to bare CdSe QDs (∼50–100 meV), allowing for clearly distinguishable TA signals attributable to hot hole relaxation. Holes depopulate from the 2S_3/2 state with a lifetime of 7 ± 2 ps, which is consistent with the predictions for hole relaxation via a phonon coupling pathway to lower-energy hole states, with possible contributions from hole trapping as well. These results suggest that tuning the surface chemistry of semiconductor QDs is a viable route to measure and possibly control their hot hole relaxation dynamics

Spectroscopic Investigation of Electrochemically Charged Individual (6,5) Single-Walled Carbon Nanotubes

Individual single-walled carbon nanotubes (SWNTs) of (6,5) chirality were investigated by means of optical spectroscopy while their charge state was controlled electrochemically. The photoluminescence of the SWNTs was found to be quenched at positive and negative potentials, where the onset and offset varied for each individual SWNT. We propose that differences in the local environment of the individual SWNT lead to a shift of the Fermi energy, resulting in a distribution of the oxidation and reduction potentials. The exciton emission energy was found to correlate with the oxidation and reduction potential. Further proof of a correlation was found by deliberately doping individual SWNTs and monitoring their photoluminescence spectral shift

Tuning and Enhancing Quantum Coherence Time Scales in Molecules via Light-Matter Hybridization

Protecting quantum coherences in matter from the detrimental effects introduced by its environment is essential to employ molecules and materials in quantum technologies and develop enhanced spectroscopies. Here, we show how dressing molecular chromophores with quantum light in the context of optical cavities can be used to generate quantum superposition states with tunable coherence time scales that are longer than those of the bare molecule, even at room temperature and for molecules immersed in solvent. For this, we develop a theory of decoherence rates for molecular polaritonic states and demonstrate that quantum superpositions that involve such hybrid light-matter states can survive for times that are orders of magnitude longer than those of the bare molecule while remaining optically controllable. Further, by studying these tunable coherence enhancements in the presence of lossy cavities, we demonstrate that they can be enacted using present-day optical cavities. The analysis offers a viable strategy to engineer and increase quantum coherence lifetimes in molecules

Aqueous Photogeneration of H₂ with CdSe Nanocrystals and Nickel Catalysts: Electron Transfer Dynamics

CdSe quantum dots (QDs) and simple aqueous Ni²⁺ salts in the presence of a sacrificial electron donor form a highly efficient, active, and robust system for photochemical reduction of protons to molecular hydrogen in water. Using ultrafast transient absorption (TA) spectroscopy, the electron transfer (ET) processes from the QDs to the Ni catalysts have been characterized. CdSe QDs transfer photoexcited electrons to a Ni–dihydrolipoic acid (Ni–DHLA) catalyst complex extremely fast and with high efficiency: the amplitude-weighted average ET lifetime is 69 ± 2 ps, and ∼90% of the ultrafast TA signal is assigned to ET processes. The impacts of Auger recombination, QD size and shelling on ET are also reported. These results help clarify the reasons for the exceptional photocatalytic H₂ activity of the CdSe QD/Ni–DHLA system and suggest direction for further improvements of the system

Recovery of Active and Efficient Photocatalytic H₂ Production for CdSe Quantum Dots

Recently, colloidal semiconductor quantum dots (QDs) have shown great promise as photocatalysts for the production of chemical fuels by sunlight. Here, the efficiency of photocatalytic hydrogen (H₂) production for integrated systems of large diameter (4.4 nm) CdSe QDs as light harvesting nanoparticles with varying concentrations of nickel–dihydrolipoic acid (Ni–DHLA) small molecule catalysts is measured. While exhibiting excellent robustness and longevity, the efficiency of H₂ production for equimolar catalyst and QDs is relatively poor. However, the efficiency is found to increase substantially with increasing Ni–DHLA/QD molar ratios. Surprisingly, this high activity is only observed with the use of 3-mercaptopropionic acid (MPA) ligands, while CdSe QDs capped with dihydrolipoic acid (DHLA) exhibit poor performance in comparison, indicating that the QD capping ligand has a substantial impact on the catalytic performance. Ultrafast transient absorption spectroscopic measurements of the electron transfer (ET) dynamics show fast ET to the catalyst. Importantly, an increase in ET efficiency is observed as the catalyst concentration is increased. Together, these results suggest that for these large QDs, tailoring the QD surface environment for facile ET and increasing catalyst concentrations increases the probability of ET from QDs to Ni–DHLA, overcoming the relatively small driving force for ET and decreased surface electron density for large diameter QDs

Defects Enable Dark Exciton Photoluminescence in Single-Walled Carbon Nanotubes

Variable temperature photoluminescence excitation spectroscopy of three (<i>n</i>,<i>m</i>) species of single-walled carbon nanotubes revealed that at resonant S₂₂ excitation, in addition to allowed excitonic optical transitions, several sidebands that should be forbidden based on selection rules were observed and appeared to have a strong temperature dependence. In particular, we found that a sideband located approximately 130 meV away from the bright S₁₁ exciton peak relating to the K-momentum dark exciton state, called X₁, decreased in intensity 5-fold as the samples were cooled. Direct optical excitation of this dark state is nominally forbidden, thus calling into question how the state is populated and why it is so prominent in the photoluminescence spectrum. Interestingly, the ratio of the integrated photoluminescence intensities of X₁ to S₁₁ scales with a Boltzmann factor unrelated to the phonon that is thought to be responsible for depopulating the K-momentum dark exciton state: an in-plane transverse optical phonon, A₁′. Furthermore, photoluminescence spectra from individual (7,5) nanotubes show that only a small fraction exhibit the X₁ feature, with varying oscillator strength, thus suggesting that intrinsic processes such as phonon scattering are not responsible for populating the dark state. Alternatively, we suggest that populating the K-momentum dark exciton state requires scattering from defects, which is consistent with the increased magnitude of the X₁ feature for samples with increased sample purification and processing. Thus, the presence of an X₁ peak in photoluminescence is an extremely sensitive spectroscopic indicator of defects on single-walled carbon nanotubes

Fabrication of Tapered Microtube Arrays and Their Application as a Microalgal Injection Platform

A template-synthesis method that enables fabrication of tapered microtube arrays is reported. Track-etched poly­(ethylene terephthalate) membranes are used as the template, with closed-tipped conical pores having length and base diameter of 6.27 ± 0.28 and 1.21 ± 0.05 μm, respectively. A conductive layer of Pt is deposited by atomic layer deposition (ALD) to enable the successive electrodeposition of Ni. By decreasing the Pt precursor pulse duration from 10 to 1 s during the ALD step, the heights of the microtubes are controlled from the maximal full length (∼6 μm) to only a fraction (1–2 μm) of the template pore. Using a pulsed-current electrodeposition (PCD) method, a smooth and uniform Ni deposit is achieved with a thickness that can be controlled as a function of the PCD cycle. The microtubes’ lumen is confirmed to stay open even after 2000 cycles of Ni PCD. A potential application of the prepared array as a microinjection platform is demonstrated via successful injection of 10 nm sized CdZnS/ZnS core/shell quantum dots into Chlamydomonas reinhardtii microalgae cells with intact cell walls. The direct delivery method demonstrated in this paper offers novel opportunities for extending the growing interest in array-based microinjection platform to microalgal systems

Photocatalytic Hydrogen Generation by CdSe/CdS Nanoparticles

The photocatalytic hydrogen (H₂) production activity of various CdSe semiconductor nanoparticles was compared including CdSe and CdSe/CdS quantum dots (QDs), CdSe quantum rods (QRs), and CdSe/CdS dot-in-rods (DIRs). With equivalent photons absorbed, the H₂ generation activity orders as CdSe QDs ≫ CdSe QRs > CdSe/CdS QDs > CdSe/CdS DIRs, which is surprisingly the opposite of the electron–hole separation efficiency. Calculations of photoexcited surface charge densities are positively correlated with the H₂ production rate and suggest the size of the nanoparticle plays a critical role in determining the relative efficiency of H₂ production

Single-Molecule Analysis of Cytochrome <i>c</i> Folding by Monitoring the Lifetime of an Attached Fluorescent Probe

Conformational dynamics of proteins are important for function. However, obtaining information about specific conformations is difficult for samples displaying heterogeneity. Here, time-resolved fluorescence resonance energy transfer is used to characterize the folding of single cytochrome <i>c</i> molecules. In particular, measurements of the fluorescence lifetimes of individual cytochrome <i>c</i> molecules labeled with a single dye that is quenched by energy transfer to the heme were used to monitor conformational transitions of the protein under partially denaturing conditions. These studies indicate significantly more conformational heterogeneity than has been described previously. Importantly, the use of a purified singly labeled sample made a direct comparison to ensemble data possible. The distribution of lifetimes of single proteins was compared to the distribution of lifetimes determined from analysis of ensemble lifetime fluorescence data. The results show broad agreement between single-molecule and ensemble data, with a similar range of lifetimes. However, the single-molecule data reveal greater conformational heterogeneity