Zinc-substituted cytochrome c (Zn-cyt c) is noncovalently bound to single-walled carbon nanotubes (SWNTs), causing the Zn-cyt c fluorescence to be quenched by up to 95%, primarily due to photoinduced charge transfer. Deposition of Zn-cyt c/SWNT films onto conductive oxides allows for harvesting of photoexcited electrons with an internal quantum efficiency of over 5%.

Here we discuss the photon-driven transport of protons and electrons over hundreds of microns through a membrane based on vertically aligned single-walled carbon nanotubes (SWNTs). Electrons are photogenerated in colloidal CdSe quantum dots that have been noncovalently attached to the carbon nanotube membrane and can be delivered at potentials capable of reducing earth-abundant molecular catalysts that perform proton reduction. Proton transport is driven by the electron photocurrent and is shown to be faster through the SWNT-based membrane than through the commercial polymer Nafion. The potential utility of SWNT membranes for solar water splitting applications is demonstrated through their excellent proton and electron transport properties as well as their ability to interact with other components of water splitting systems, such as small-molecule electron acceptors.

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.Keywords: hydrogen generation; Nanoparticles; photocatalysis; water splitting;

Correlated measurements of fluorescence and topography were performed for individual single-walled carbon nanotubes (SWNTs) on quartz using epifluorescence confocal microscopy and atomic force microscopy (AFM). Surprisingly, only ∼11% of all SWNTs in DNA-wrapped samples were found to be highly emissive on quartz, suggesting that the ensemble fluorescence quantum yield is low because only a small population of SWNTs fluoresces strongly. Qualitatively similar conclusions were obtained from control studies using a sodium cholate surfactant system. To accommodate AFM measurements, excess surfactant was removed from the substrate. Though individual SWNTs on nonrinsed and rinsed surfaces displayed differences in fluorescence intensities and line widths, arising from the influence of the local environment on individual SWNT optical measurements, photoluminescence data from both samples displayed consistent trends.

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.

We report the fabrication of membranes hundreds of micrometers thick that demonstrate efficient electron conduction and proton transport through vertically aligned arrays of multiwalled carbon nanotubes (NTs) impregnated by epoxy. Electrical transport was Ohmic with a conductivity of 495 mS cm–1. Protons traversed the membrane through the NT bore with a current of 5.84 × 10–6 A. Good electron and proton transport, chemical robustness, and simple fabrication suggest NT membranes have potential in artificial photosynthesis applications.

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.

The impact of pulsed versus continuous wave (cw) laser excitation on the photophysical properties of single quantum dots (QDs) has been investigated in an experiment in which all macroscopic variables are identical except the nature of laser excitation. Pulsed excitation exaggerates the effects of photobleaching, results in a lower probability of long ON fluorescence blinking events, and leads to shorter fluorescence lifetimes with respect to cw excitation at the same wavelength and average intensity. Spectral wandering, biexciton quantum yields, and power law exponents that describe fluorescence blinking are largely insensitive to the nature of laser excitation. These results explicitly illustrate important similarities and differences in fluorescence dynamics between pulsed and cw excitation, enabling more meaningful comparisons between literature reports and aiding in the design of new experiments to mitigate possible influences of high photon flux on QDs.

Single and multiple exciton relaxation dynamics of CdSe/CdZnS nanocrystal quantum dots (QDs) monitored at the two lowest optical transitions, 1Se–1S3/2 and 1Se–2S3/2, have been examined using ultrafast transient absorption (TA) spectroscopy. For the CdSe/CdZnS QDs studied, the 1Se–1S3/2 and 1Se–2S3/2 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 2S3/2 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.Keywords: Auger process; biexciton; hot hole relaxation; Poisson population; transient absorption;

Individual single-walled carbon nanotubes (SWNTs) were suspended in water using amphipathic β-sheet peptides of the general sequence Ac-(XKXE)2-NH2. By substituting natural and nonnatural amino acids of varying aromatic and hydrophobic character in the X position, the interactions between the peptide and the nanotube sidewall could be systematically varied. Surprisingly, enhancing the degree of favorable π–π and hydrophobic interactions, which strongly influence the self-assembly properties of these peptides, did not correlate with an improvement in nanotube dispersion efficiency. We found that substituents in the X-position of the peptides play a significant role in SWNT interaction and contributes to (n,m) structure specificity.

Semiconductor nanocrystal quantum dots (QDs) have been the subject of much interest for fundamental and applied studies. The synthesis of QDs has developed over the past 30 years such that production of monodisperse, photostable QDs with a near-exact size and shape are readily achievable. However, an understanding of the chemical reaction mechanism behind the synthesis of QDs has lagged the ability to synthesize high-quality nanoparticles. This review will discuss recent studies of QD synthetic mechanisms that have been proposed for metal-chalcogenide (ME) semiconductor QDs, particularly CdE and PbE. Although the focus here will be on the initial metal–chalcogenide bond formation, we will also discuss growth models for QDs as well as attempt to provide a future outlook for how understanding reaction mechanism can be leveraged to make improved QDs with easily tailored properties.Keywords: mechanism; nanocrystals; semiconductor quantum dots; synthesis;

Colloidal CdS quantum dots (QDs) were synthesized with tunable surface composition. Surface stoichiometry was controlled by applying reactive secondary phosphine sulfide precursors in a layer-by-layer approach. The surface composition was observed to greatly affect photoluminescence properties. Band edge emission was quenched in sulfur terminated CdS QDs and fully recovered when QDs were cadmium terminated. Calculations suggest that electronic states inside the band gap arising from surface sulfur atoms could trap charges, thus inhibiting radiative recombination and facilitating nonradiative relaxation.

Applications of colloidal semiconductor quantum dots (QDs) have recently begun to move from the laboratory into the commercial sector. This article provides a brief description of QDs and their associated optical properties, highlighting the concept that QD size is now a parameter used to tune photophysical properties. Additionally, three major applications of QDs are discussed: biological imaging, photovoltaic devices, and light-emitting devices. Progress in each area is highlighted, as well as potential advantages over existing technologies when QD products are realized. Finally, some of the challenges to the further development of QDs for each respective application and in the field overall are addressed.

Single-walled carbon nanotubes (SWNTs) have unique photophysical properties but low fluorescence efficiency. We have found significant increases in the fluorescence efficiency of individual DNA-wrapped SWNTs upon addition of reducing agents, including dithiothreitol, Trolox, and β-mercaptoethanol. Brightening was reversible upon removal of the reducing molecules, suggesting that a transient reduction of defect sites on the SWNT sidewall causes the effect. These results imply that SWNTs are intrinsically bright emitters and that their poor emission arises from defective nanotubes.

Upon absorption of single photons, multiple excitons were generated and detected in semiconducting single-walled carbon nanotubes (SWNTs) using transient absorption spectroscopy. For (6,5) SWNTs, absorption of single photons with energies corresponding to three times the SWNT energy gap results in an exciton generation efficiency of 130% per photon. Our results suggest that the multiple exciton generation threshold in SWNTs can be close to the limit defined by energy conservation.

We demonstrate that Zn(II) porphyrin in Zn(II)cytochrome c (Zn cyt c) is a fluorescence resonance energy transfer (FRET) donor to an Alexa660 dye acceptor. The energy transfer efficiency is dependent on the distance between the two fluorophores as shown through protein denaturation studies of five Zn cyt c variants labeled with Alexa660 in different positions. The relative quantum yield, excitation and emission energies, and labeling efficiencies of this donor−acceptor pair allow for a method of analysis based on sensitized emission of the acceptor. These studies show that Zn(II) porphyrin is an effective energy donor for measurement of molecular-scale distances by FRET.

We have investigated the reaction mechanism responsible for QD nucleation using optical absorption and nuclear magnetic resonance spectroscopies. For typical II−VI and IV−VI quantum dot (QD) syntheses, pure tertiary phosphine selenide sources (e.g., trioctylphosphine selenide (TOPSe)) were surprisingly found to be unreactive with metal carboxylates and incapable of yielding QDs. Rather, small quantities of secondary phosphines, which are impurities in tertiary phosphines, are entirely responsible for the nucleation of QDs; their low concentrations account for poor synthetic conversion yields. QD yields increase to nearly quantitative levels when replacing TOPSe with a stoiciometric amount of a secondary phosphine chalcogenide such as diphenylphosphine selenide. Based on our observations, we have proposed potential monomer identities, reaction pathways, and transition states and believe this mechanism to be universal to all II−VI and IV−VI QDs synthesized using phosphine based methods.

The usefulness of semiconductor nanocrystals is severely limited by the fact that they 'blink': they turn on and off intermittently under continuous excitation. Here, ternary core/shell CdZnSe/ZnSe nanocrystals are realized, in which the transition between CdZnSe and ZnSe seems to be radially graded rather than abrupt, and which show completely non-blinking behaviour and strong photoluminescence.

Single-walled carbon nanotubes (SWNTs) are cylindrical graphitic molecules that have remained at the forefront of nanomaterials research since 1991, largely due to their exceptional and unusual mechanical, electrical, and optical properties. The motivation for understanding how nanotubes interact with light (i.e., SWNT photophysics) is both fundamental and applied. Individual nanotubes may someday be used as superior near-infrared fluorophores, biological tags and sensors, and components for ultrahigh-speed optical communications systems. Establishing an understanding of basic nanotube photophysics is intrinsically significant and should enable the rapid development of such innovations. Unlike conventional molecules, carbon nanotubes are synthesized as heterogeneous samples, composed of molecules with different diameters, chiralities, and lengths. Because a nanotube can be either metallic or semiconducting depending on its particular molecular structure, SWNT samples are also mixtures of conductors and semiconductors. Early progress in understanding the optical characteristics of SWNTs was limited because nanotubes aggregate when synthesized, causing a mixing of the energy states of different nanotube structures. Recently, significant improvements in sample preparation have made it possible to isolate individual nanotubes, enabling many advances in characterizing their optical properties. In this Account, single-molecule confocal microscopy and spectroscopy were implemented to study the fluorescence from individual nanotubes. Single-molecule measurements naturally circumvent the difficulties associated with SWNT sample inhomogeneities. Intrinsic SWNT photoluminescence has a simple narrow Lorentzian line shape and a polarization dependence, as expected for a one-dimensional system. Although the local environment heavily influences the optical transition wavelength and intensity, single nanotubes are exceptionally photostable. In fact, they have the unique characteristic that their single molecule fluorescence intensity remains constant over time; SWNTs do not “blink” or photobleach under ambient conditions. In addition, transient absorption spectroscopy was used to examine the relaxation dynamics of photoexcited nanotubes and to elucidate the nature of the SWNT excited state. For metallic SWNTs, very fast initial recovery times (300–500 fs) corresponded to excited-state relaxation. For semiconducting SWNTs, an additional slower decay component was observed (50–100 ps) that corresponded to electron–hole recombination. As the excitation intensity was increased, multiple electron–hole pairs were generated in the SWNT; however, these e−h pairs annihilated each other completely in under 3 ps. Studying the dynamics of this annihilation process revealed the lifetimes for one, two, and three e−h pairs, which further confirmed that the photoexcitation of SWNTs produces not free electrons but rather one-dimensional bound electron–hole pairs (i.e., excitons). In summary, nanotube photophysics is a rapidly developing area of nanomaterials research. Individual SWNTs exhibit robust and unexpectedly unwavering single-molecule fluorescence in the near-infrared, show fast relaxation dynamics, and generate excitons as their optical excited states. These fundamental discoveries should enable the development of novel devices based on the impressive photophysical properties of carbon nanotubes, especially in areas like biological imaging. Many facets of nanotube photophysics still need to be better understood, but SWNTs have already proven to be an excellent starting material for future nanophotonics applications.

Fluorescence spectroscopy is utilized to investigate photodarkening and photobrightening behaviors in PbS quantum dots (QDs) subjected to various environmental conditions. We are able to separate contributions from charge trapping to a long-lived optically dark state (single particle fluorescence blinking) and irreversible photooxidation to the overall photodarkening behavior. Both processes produce effects that are potentially detrimental for emission-based technological applications. Charge trapping is the dominant mechanism on short time scales (<3 s), exhibits no particle size- or environmental-dependence, is reversible, and is an order of magnitude faster compared to CdSe QDs. Photooxidation is the dominant mechanism on long time scales (50–100 s), is strongly dependent on particle size and environmental atmosphere, and results in irreversible decreases in emission intensity, large blue shifts of emission maximum, and increases in particle size distribution.

The impact of pulsed versus continuous wave (cw) laser excitation on the photophysical properties of single quantum dots (QDs) has been investigated in an experiment in which all macroscopic variables are identical except the nature of laser excitation. Pulsed excitation exaggerates the effects of photobleaching, results in a lower probability of long ON fluorescence blinking events, and leads to shorter fluorescence lifetimes with respect to cw excitation at the same wavelength and average intensity. Spectral wandering, biexciton quantum yields, and power law exponents that describe fluorescence blinking are largely insensitive to the nature of laser excitation. These results explicitly illustrate important similarities and differences in fluorescence dynamics between pulsed and cw excitation, enabling more meaningful comparisons between literature reports and aiding in the design of new experiments to mitigate possible influences of high photon flux on QDs.