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Protic ruthenium complexes using the dihydroxybipyridine (dhbp) ligand combined with a spectator ligand (N,N = bpy, phen, dop, Bphen) have been studied for their potential activity vs. cancer cells and their photophysical luminescent properties. These complexes vary in the extent of π expansion and the use of proximal (6,6′-dhbp) or distal (4,4′-dhbp) hydroxy groups. Eight complexes are studied herein as the acidic (OH bearing) form, [(N,N)2Ru(n,n′-dhbp)]Cl2, or as the doubly deprotonated (O− bearing) form. Thus, the presence of these two protonation states gives 16 complexes that have been isolated and studied. Complex 7A, [(dop)2Ru(4,4′-dhbp)]Cl2, has been recently synthesized and characterized spectroscopically and by X-ray crystallography. The deprotonated forms of three complexes are also reported herein for the first time. The other complexes studied have been synthesized previously. Three complexes are light-activated and exhibit photocytotoxicity. The log(Do/w) values of the complexes are used herein to correlate photocytotoxicity with improved cellular uptake. For Ru complexes 1–4 bearing the 6,6′-dhbp ligand, photoluminescence studies (all in deaerated acetonitrile) have revealed that steric strain leads to photodissociation which tends to reduce photoluminescent lifetimes and quantum yields in both protonation states. For Ru complexes 5–8 bearing the 4,4′-dhbp ligand, the deprotonated Ru complexes (5B–8B) have low photoluminescent lifetimes and quantum yields due to quenching that is proposed to involve the 3LLCT excited state and charge transfer from the [O2-bpy]2− ligand to the N,N spectator ligand. The protonated OH bearing 4,4′-dhbp Ru complexes (5A–8A) have long luminescence lifetimes which increase with increasing π expansion on the N,N spectator ligand. The Bphen complex, 8A, has the longest lifetime of the series at 3.45 μs and a photoluminescence quantum yield of 18.7%. This Ru complex also exhibits the best photocytotoxicity of the series. A long luminescence lifetime is correlated with greater singlet oxygen quantum yields because the triplet excited state is presumably long-lived enough to interact with 3O2 to yield 1O2.

Colloidal CdSe quantum dots (QDs) designed with a high degree of asymmetric internal strain have recently been shown to host a number of desirable optical properties including subthermal room-temperature line widths, suppressed spectral diffusion, and high photoluminescence (PL) quantum yields. It remains an open question, however, whether they are well-suited for applications requiring emission of identical single photons. Here we measure the low-temperature PL dynamics and the polarization-resolved fluorescence line narrowing spectra from ensembles of these strained QDs. Our spectroscopy reveals the radiative recombination rates of bright and dark excitons, the relaxation rate between the two, and the energy spectra of the quantized acoustic phonons in the QDs that can contribute to relaxation processes. In comparison to conventional colloidal CdSe/ZnS core/shell QDs, we find that in asymmetrically strained CdSe QDs over six times more light is emitted directly by the bright exciton. These results are therefore encouraging for the prospects of chemically synthesized colloidal QDs as emitters of single indistinguishable photons.

Colloidal semiconductor quantum dots (QDs) are promising materials for realizing high-performance liquid-junction photovoltaic cells. Solar cells based on Zn:CuInSe2 QDs show high efficiency despite a large abundance of native defects typical of ternary I–III–VI2 semiconductors. To elucidate the reasons underlying the remarkable defect tolerance of these devices, we conduct side-by-side photovoltaic and spectroscopic studies of as-prepared and surface-modified Zn:CuInSe2 QDs. Using surface ligands with different lengths and binding affinities to the TiO2 surface, we tune the rates of both defect-related relaxation and QD-to-TiO2 electrode electron transfer. Despite their profound influence on photoluminescence dynamics, surface modifications have surprisingly little effect on photovoltaic performance suggesting that intragap defects do not impede but actually assist the photoconversion process in Zn:CuInSe2 QDs. These intragap states, identified as shallow surface-located electron traps and native Cu1+ hole-trapping defects, mediate QD interactions with the TiO2 electrode and the electrolyte, respectively, and help achieve consistent photovoltaic performance with ~85% photon-to-electron conversion efficiencies and highly reproducible power conversion efficiencies of 9–10%. Defects are believed to detrimentally affect the efficiency of quantum-dot-sensitized solar cells. Now, we show that charge-trapping defects actually assist the photoconversion process, while the quantum dot density in the mesoporous electrode is a primary limiting factor in device performance.

Colloidal quantum dots (QDs) are attractive materials for the realization of solution-processable laser diodes. Primary challenges towards this objective are fast optical-gain relaxation due to nonradiative Auger recombination and poor stability of colloidal QD solids under high current densities required to obtain optical gain. Here we resolve these challenges and achieve broad-band optical gain spanning the band-edge (1S) and the higher-energy (1P) transitions. This demonstration is enabled by continuously graded QDs with strongly suppressed Auger recombination and a current-focusing device design, combined with short-pulse pumping. Using this approach, we achieve ultra-high current densities (~1000 A cm

We present time-resolved photoluminescence (PL) spectroscopy of a series of colloidal CdSe/CdS core/shell nanoplatelets with different core and shell thicknesses. Exciton numbers are determined fro...

On-demand generation of quantum photons is one of the four major targets of Quantum Information Science (QIS), identified by the NSF as one of the five “industries of the future.” Colloidal semiconductor quantum dots (QDs) are viable candidates for quantum materials, owing to their tunable, bright, and stable emission. Identifying a suitable semiconductor material and engineering long exciton coherence times is key to producing single coherent photons on demand. We show that colloidal compositionally-graded CdSe/ZnS QDs designed with a high degree of asymmetric internal strain demonstrate superior quantum emission properties compared to conventional sharp-interface CdSe/ZnS QDs. In comparison to conventional colloidal CdSe/ZnS core/shell QDs, we find that, in asymmetrically strained CdSe QDs, over six times more light is emitted directly by the bright exciton. Temperature-dependent PL dynamics and fluorescence line narrowing (FLN) spectroscopy reveals the radiative recombination rates of bright and dark excitons, the relaxation rate between the two, and the energy spectra of the quantized acoustic phonons in the QDs that can contribute to relaxation processes. We attribute the six-fold improvement to the four-fold deceleration of the bright-dark relaxation and the doubled radiative recombination rate in the strained QDs. The next step in this research is to bring these improvement from the visible to the near-IR emission, where most communication bandwidths lie. This area has been traditionally dominated by III-V materials, which are chemically challenging in the colloidal state. We design, synthesize, and characterize alternative strong near-IR materials and assess them as potential quantum emitters. Last but not least, we demonstrate applications of electrochemistry in programmable hot syntheses of colloidal QDs.

Quasi-two-dimensional nanoplatelets (NPLs) possess fundamentally different excitonic properties from zero-dimensional quantum dots. We study lateral size-dependent photon emission statistics and carrier dynamics of individual NPLs using second-order photon correlation (g(2)(τ)) spectroscopy and photoluminescence (PL) intensity-dependent lifetime analysis. Room-temperature radiative lifetimes of NPLs can be derived from maximum PL intensity periods in PL time traces. It first decreases with NPL lateral size and then stays constant, deviating from the electric dipole approximation. Analysis of the PL time traces further reveals that the single exciton quantum yield in NPLs decreases with NPL lateral size and increases with protecting shell thickness, indicating the importance of surface passivation on NPL emission quality. Second-order photon correlation (g(2)(τ)) studies of single NPLs show that the biexciton quantum yield is strongly dependent on the lateral size and single exciton quantum yield of the NP...

Patterning without polymers Nanoscale patterning usually requires multiple steps to mask, pattern, and develop sequential layers. One overriding concern is to obtain compatibility between all the materials, as well as the patterning techniques used, to ensure accurate and clean processing. Wang et al. use light-responsive ligands to change the solubility of nanocrystals in specific solvents, so that development can be done by simple redispersion of nanocrystals in dark regions (see the Perspective by Striccoli). The process can fully utilize the advantages of conventional semiconductor processing, but without the need for photoresists, because the nanocrystals are only deposited where they are exposed to light. Science , this issue p. 385 ; see also p. 353

In contrast to molecular systems, which are defined with atomic precision, nanomaterials generally show some heterogeneity in size, shape, and composition. The sample inhomogeneity translates into a distribution of energy levels, band gaps, work functions, and other characteristics, which detrimentally affect practically every property of functional nanomaterials. We discuss a novel synthetic strategy, colloidal atomic layer deposition (c-ALD) with stationary reactant phases, which largely circumvents the limitations of traditional colloidal syntheses of nano-heterostructures with atomic precision. This approach allows for significant reduction of inhomogeneity in nanomaterials in complex nanostructures without compromising their structural perfection and enables the synthesis of epitaxial nano-heterostructures of unprecedented complexity. The improved synthetic control ultimately enables bandgap and strain engineering in colloidal nanomaterials with close to atomic accuracy. To demonstrate the power of the new c-ALD method, we synthesize a library of complex II-VI semiconductor nanoplatelet heterostructures. By combining spectroscopic and computational studies, we elucidate the subtle interplay between quantum confinement and strain effects on the optical properties of semiconductor nanostructures.

Switching on quantum dot lasers The optical properties of quantum dots can be tailored with alterations to their size and composition. Developing quantum dots as low-threshold laser sources requires overcoming problems associated with carrier recombination and stability. Kozlov et al. report success in this direction by demonstrating that a mix of processing and postsynthesis charging can overcome these problems. Synthesis of compositionally graded core-shell quantum dots followed by a carrier charging step provides stable, low-threshold lasing. Science , this issue p. 672

Semiconductor nanorods can emit linear-polarized light at efficiencies over 80%. Polarization of light in these systems, confirmed through single-rod spectroscopy, can be explained on the basis of the anisotropy of the transition dipole moment and dielectric confinement effects. Here we report emission polarization in macroscopic semiconductor-polymer composite films containing CdSe/CdS nanorods and colloidal CdSe nanoplatelets. Anisotropic nanocrystals dispersed in polymer films of poly butyl-co-isobutyl methacrylate (PBiBMA) can be stretched mechanically in order to obtain unidirectionally aligned arrays. A high degree of alignment, corresponding to an orientation factor of 0.87, was achieved and large areas demonstrated polarized emission, with the contrast ratio I∥/I⊥ = 5.6, making these films viable candidates for use in liquid crystal display (LCD) devices. To some surprise, we observed significant optical anisotropy and emission polarization for 2D CdSe nanoplatelets with the electronic structure of quantum wells. The aligned nanorod arrays serve as optical funnels, absorbing unpolarized light and re-emitting light from deep-green to red with quantum efficiencies over 90% and high degree of linear polarization. Our results conclusively demonstrate the benefits of anisotropic nanostructures for LCD backlighting. The polymer films with aligned CdSe/CdS dot-in-rod and rod-in-rod nanostructures show more than 2-fold enhancement of brightness compared to the emitter layers with randomly oriented nanostructures. This effect can be explained as the combination of linearly polarized luminescence and directional emission from individual nanostructures.

There have been multiple demonstrations of amplified spontaneous emission (ASE) and lasing using colloidal semiconductor nanocrystals. However, it has been proven difficult to achieve low thresholds suitable for practical use of nanocrystals as gain media. Low-threshold blue ASE and lasing from nanocrystals is an even more challenging task. Here, we show that colloidal nanoplatelets (NPLs) with electronic structure of quantum wells can produce ASE in the red, yellow, green, and blue regions of the visible spectrum with low thresholds and high gains. In particular, for blue-emitting NPLs, the ASE threshold is 50 μJ/cm(2), lower than any reported value for nanocrystals. We then demonstrate red, yellow, green, and blue lasing using NPLs with different thicknesses. We find that the lateral size of NPLs does not show any strong effect on the Auger recombination rates and, correspondingly, on the ASE threshold or gain saturation. This observation highlights the qualitative difference of multiexciton dynamics in CdSe NPLs and other quantum-confined CdSe materials, such as quantum dots and rods. Our measurements of the gain bandwidth and gain lifetime further support the prospects of colloidal NPLs as solution-processed optical gain materials.

Using time-resolved photoluminescence spectroscopy, we show that two-exciton Auger recombination dominates carrier recombination and cooling dynamics in CdSe nanoplatelets, or colloidal quantum wells. The electron-hole recombination rate depends only on the number of electron-hole pairs present in each nanoplatelet, and is consistent with a two-exciton recombination process over a wide range of exciton densities. The carrier relaxation rate within the conduction and valence bands also depends only on the number of electron-hole pairs present, apart from an initial rapid decay, and is consistent with the cooling rate being limited by reheating due to Auger recombination processes. These Auger-limited recombination and relaxation dynamics are qualitatively different from the carrier dynamics in either colloidal quantum dots or epitaxial quantum wells.

Nonradiative Auger recombination limits the efficiency with which colloidal semiconductor nanocrystals can emit light when they are subjected to strong excitation, with important implications for the application of the nanocrystals in light-emitting diodes and lasers. This has motivated attempts to engineer the structure of the nanocrystals to minimize Auger rates. Here, we study Auger recombination rates in CdSe/CdS core/shell nanoplatelets, or colloidal quantum wells. Using time-resolved photoluminescence measurements, we show that the rate of biexcitonic Auger recombination has a nonmonotonic dependence on the shell thickness, initially decreasing, reaching a minimum for shells with thickness of 2–4 monolayers, and then increasing with further increases in the shell thickness. This nonmonotonic behavior has not been observed previously for biexcitonic recombination in quantum dots, most likely due to inhomogeneous broadening that is not present for the nanoplatelets.

Colloidal nanomaterials represent an important branch of modern chemistry. However, we have very little understanding of molecular processes that occur at the nanocrystal (NC) surface during synthesis and post-synthetic modifications. Here we show that potentiometry can be used to study the surface of colloidal NCs under realistic reaction conditions. Potentiometric titrations of CdSe and InP nanostructures provide information on the active surface area, the affinity of ligands to the NC surface, and the surface reaction kinetics. These studies can be carried out at different temperatures in polar and nonpolar media for NCs of different sizes and shapes. In situ potentiometry can provide real-time feedback during synthesis of core-shell nanostructures.

Colloidal semiconductor nanocrystals, also known as “quantum dots” (QDs), represent an example of a disruptive technology for display and lighting applications. Their high luminescence efficiency and tunable, narrow emission are nearly ideal for achieving saturated colors and enriching the display or TV color gamut. Our contribution will discuss the next generation of inorganic nanostructures with electronic structure optimized for achieving emission characteristics beyond traditional near-spherical QDs. For example, nano-heterostructures with spherical CdSe QDs epitaxially integrated into CdS quantum rods combine high luminescence efficiency with giant extinction coefficients, large Stokes shifts, and linearly polarized emission. Such a set of characteristics can be ideal for LCD backlighting. The other class of emitters includes colloidal quantum wells (QWs) whose ensemble luminescence is significantly narrower than emission spectra of the best QD samples. Moreover, we show that colloidal QWs produce amplified spontaneous emission (ASE) with pump-fluence thresholds as low as 6 μJ/cm2 and gain as high as 600 cm−1, on par with the best values for any solution-processed material.

We report measurements of electron transfer rates for four isoenergetic donor–acceptor pairs comprising a molecular electron acceptor, methylviologen (MV), and morphology-controlled colloidal semiconductor nanoparticles of CdSe. The four nanoparticles include a spherical quantum dot (QD) and three differing lateral areas of 4-monolayer-thick nanoplatelets (NPLs), each with a 2.42 eV energy gap. As such, the measurements, performed via ultrafast photoluminescence, relate the dependence of charge transfer rate on the spatial extent of the initial electron–hole pair wave function explicitly, which we show for the first time to be related to surface area in this regime that is intermediate between homogeneous and heterogeneous charge transfer as well as 2D to 0D electron transfer. The observed nonlinear dependence of rate with surface area is attributed to exciton delocalization within each structure, which we show via temperature-dependent absorption measurements remains constant.

Semiconductor quantum rings are of great fundamental interest because their non-trivial topology creates novel physical properties. At the same time, toroidal topology is difficult to achieve for colloidal nanocrystals and epitaxially grown semiconductor nanostructures. In this work, we introduce the synthesis of luminescent colloidal CdSe nanorings and nanostructures with double and triple toroidal topology. The nanorings form during controlled etching and rearrangement of two-dimensional nanoplatelets. We discuss a possible mechanism of the transformation of nanoplatelets into nanorings and potential utility of colloidal nanorings for magneto-optical (e.g., Aharonov–Bohm effect) and other applications.

Direct experimental measurements of the thermo-optic for fixed temperature intervals (20–200 °C, 200–500 °C, 500–660 °C, 660–780 °C) in fused silica fiber containing fiber Bragg gratings (FBGs) were conducted. The diffraction efficiency of a FBG fluctuated with temperature between 2.01 × 10 − 4 and 0.17 × 10 − 4 while the temperature shift of the Bragg's peak was monitored between 1300 and 1311 nm with sub-Angstrom precision. Numerical simulations were focused on FBG's diffraction efficiency calculations accounting for the temperature drift of the gratings, and found to be in excellent agreement with obtained experimental data. It was found that the first-order thermo-optic coefficient changes between 1.29 and 1.85 × 10 − 5 K − 1 for the linear fit and at T = 0 °C its value was found to be close to 2.37 × 10 − 5 K − 1 for the polynomial fit of experimental data. The average thermo-optic coefficient undergoes a minimum in the vicinity of 440 °C. Additional observation indicates a negative sign of the second-order thermo-optic coefficient. The value of thermal expansion coefficient was much less (0.5 × 10 − 6 K − 1 ) than that for the average thermo-optic coefficient. Based on the energy dispersive spectroscopy it was determined that thermal erasing of the FBGs at a temperature around 780 °C corresponds to germanium monoxide diffusion out of core in silica-based fibers.

Quantum wells (QWs) are thin semiconductor layers than confine electrons and holes in one dimension. They are widely used for optoelectronic devices, particularly semiconductor lasers, but have so far been produced using expensive epitaxial crystal-growth techniques. This has motivated research into the use of colloidal semiconductor nanocrystals, which can be synthesized chemically at low cost, and can be processed in the solution phase. However, initial demonstrations of optical gain from colloidal nanocrystals involved high thresholds. Recently, colloidal synthesis methods have been developed for the production of thin, atomically flat semiconductor nanocrystals, known as nanoplatelets (NPLs). We investigated relaxation of high-energy carriers in colloidal CdSe NPLs, and found that the relaxation is characteristic of a QW system. Carrier cooling and relaxation on time scales from picoseconds to hundreds of picoseconds are dominated by Auger-type exciton-exciton interactions. The picosecond-scale cooling of hot carriers is much faster than the exciton recombination rate, as required for use of these NPLs as optical gain and lasing materials. We therefore investigated amplified spontaneous emission using close-packed films of NPLs. We observed thresholds that were more than 4 times lower than the best reported value for colloidal nanocrystals. Moreover, gain in these films is 4 times higher than gain reported for other colloidal nanocrystals, and saturates at pump fluences more than two orders of magnitude above the ASE threshold. We attribute this exceptional performance to large optical cross-sections, relatively slow Auger recombination rates, and narrow ensemble emission linewidths.

Yugang Sun, Leonardo Scarabelli, Nicholas Kotov, Moritz Tebbe, Xiao-Min Lin, Ward Brullot, Lucio Isa, Peter Schurtenberger, Helmuth Moehwald, Igor Fedin, Orlin Velev, Damien Faivre, Christopher Sorensen, Regine Perzynski, Munish Chanana, Zhihai Li, Fernando Bresme, Petr Kral, Emre Firlar, David Schiffrin, Joao Batista Souza Junior, Andreas Fery, Elena Shevchenko, Ozgur Tarhan, Armand Paul Alivisatos, Sabrina Disch, Rafal Klajn and Suvojit Ghosh

Fluorescence resonance energy transfer (FRET) enables photosynthetic light harvesting, wavelength downconversion in light-emitting diodes (LEDs), and optical biosensing schemes. The rate and efficiency of this donor to acceptor transfer of excitation between chromophores dictates the utility of FRET and can unlock new device operation motifs including quantum-funnel solar cells, non-contact chromophore pumping from a proximal LED, and markedly reduced gain thresholds. However, the fastest reported FRET time constants involving spherical quantum dots (0.12-1 ns; refs 7-9) do not outpace biexciton Auger recombination (0.01-0.1 ns; ref. 10), which impedes multiexciton-driven applications including electrically pumped lasers and carrier-multiplication-enhanced photovoltaics. Few-monolayer-thick semiconductor nanoplatelets (NPLs) with tens-of-nanometre lateral dimensions exhibit intense optical transitions and hundreds-of-picosecond Auger recombination, but heretofore lack FRET characterizations. We examine binary CdSe NPL solids and show that interplate FRET (∼6-23 ps, presumably for co-facial arrangements) can occur 15-50 times faster than Auger recombination and demonstrate multiexcitonic FRET, making such materials ideal candidates for advanced technologies.

Semiconductor nanocrystals can be synthesized using inexpensive, scalable, solution-based techniques, and their utility as tunable light emitters has been demonstrated in various applications, including biolabeling and light-emitting devices. By contrast, the use of colloidal nanocrystals for optical amplification and lasing has been limited by the high input power densities that have been required. In this work, we show that colloidal nanoplatelets (NPLs) produce amplified spontaneous emission (ASE) with pump-fluence thresholds as low 6 uJ/cm2 and gain as high as 600 cm-1, both a 4-fold improvement over the best reported values for colloidal nanocrystals; in addition, gain saturation occurs at pump fluences two orders of magnitude higher than the ASE threshold. We attribute this exceptional performance to large optical cross-sections, slow Auger recombination rates, and the narrow emission linewidth of the NPL ensemble. The NPLs bring the advantages of quantum wells as an optical gain medium to a colloidal system, opening up the possibility of producing high-efficiency, solution-processed lasers.

Fiber Bragg gratings (FBGs) embedded in conventional fibers may serve as temperature sensors over a wide temperature range and withstand temperatures around 1200 K. A variety of linearly polarized (LP) modes for the wavelengths between 400 and 700 nm may be sustained in fibers with and without FBGs. The composition of the LP modes and their competition is instrumental for understanding physics of thermo-optics and thermal expansion effects in silica-based fibers. The first objective of this work was to model mathematically the competition between LP modes and modal distribution using the solutions of Bessel equations for the fibers with and without the gratings. Computer generated modes were constructed and the cut-off V-numbers (and Eigen values W and U ) were determined. Theoretical results then were compared with experimental observations of LP modes for two separate ranges of temperatures: 77– 300 K and 300-1200 K. To study the formation of LP modes over the first temperature range, liquid nitrogen was used to cool down the fiber and a thermocouple was used to monitor the temperature of the fiber. Real time recording of the modal structure was performed using digital imaging and data acquisition instrumentation. To study LP modes between 300– 1200 K, the fibers were inserted into a tube furnace with temperature control. The wavelength of the infrared radiation was reflected by a FBG and detected by an optical spectrum analyzer. Radiation at the visible wavelength propagated through the fibers, and transmitted visible light was collected, analyzed and recorded with a CCD camera to monitor distribution of the LP modes in the samples with and without the FBGs.