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Auger decay of multiple excitons represents a significant obstacle to photonic applications of semiconductor quantum dots (QDs). This nonradiative process is particularly detrimental to the performance of QD-based electroluminescent and lasing devices. Here, we demonstrate that semiconductor quantum shells with an "inverted" QD geometry inhibit Auger recombination, allowing substantial improvements to their multiexciton characteristics. By promoting a spatial separation between multiple excitons, the quantum shell geometry leads to ultralong biexciton lifetimes (10 ns) and a large biexciton quantum yield. Furthermore, the architecture of quantum shells induces an exciton-exciton repulsion, which splits exciton and biexciton optical transitions, giving rise to an Auger-inactive single-exciton gain mode. In this regime, quantum shells exhibit the longest optical gain lifetime reported for colloidal QDs to date (6 ns), which makes this geometry an attractive candidate for the development of optically and electrically pumped gain media.

Electrically coupled quantum dots (QDs) can support unique optoelectronic properties arising from the superposition of single-particle excited states. Experimental methods for integrating colloidal QDs within the same nano-object, however, have remained elusive to the rational design. Here, we demonstrate a chemical strategy that allows for the assembling of colloidal QDs into coupled composites, where proximal interactions give rise to unique optoelectronic behavior. The assembly method employing "adhesive" surfactants was used to fabricate both homogeneous (e.g., CdS-CdS, PbS-PbS, CdSe-CdSe) and heterogeneous (e.g., PbS-CdS, CdS-CdSe) nanoparticle assemblies, exhibiting quasi-one-dimensional exciton fine structure. In addition, tunable mixing of single-particle exciton states was achieved for dimer-like assemblies of CdSe/CdS core-shell nanocrystals. The nanoparticle assembly mechanism was explained within the viscoelastic interaction theory adapted for molten-surface colloids. We expect that the present work will provide the synthetic and theoretical foundation needed for building assemblies of many inorganic nanocrystals.

Using the cation-exchange method, colloidal PbS nanoribbons are converted completely into CdS nanoribbons. This process expands the emission spectrum of the nanoribbons from infrared to visible. The morphology of nanoribbons remains the same after cation exchange, but the crystal structure changes from rock salt to zincblende. CdS nanoribbons exhibit blue band-edge photoluminescence under ultraviolet-light excitation. Cathodoluminescence spectroscopy of the CdS nanoribbons shows multicolor (blue, green, and red) emissions. Further time-resolved photoluminescence spectroscopy studies show that the lifetime of the midgap states is more than 2 orders of magnitude longer than that of the band-edge states.

Two-dimensional (2D) semiconductor nanocrystals processed from solution have become an attractive material platform for the development of optoelectronic devices. The synergy of the 2D charge carri...

With continuing progress in the chemical synthesis of colloidal semiconductor nanocrystals (NC), one property that remains elusive to the rational design is the ensemble photoluminescence (PL) line...

Low-dimensional colloidal semiconductors are inexpensive to process and offer a broad spectrum of attractive quantum-mechanical properties. The notorious problem of these nanostructures lies in their limited performance under high energetic loads, when more than one exciton per particle is created. Multiple excitons undergo fast Auger recombination, causing efficiently roll-off in energy-intensive applications, including high-brightness LEDs, X-ray scintillators, and solar cells. This presentation will discuss an emerging type of low-dimensional semiconductors, known as colloidal quantum shells that allow avoiding such multi-exciton (MX) energy losses. The geometry of quantum shells benefits from the spatial separation of multiple excitons, which leads to extraordinary improvements to MX lifetimes and MX quantum yield. We will compare quantum shells with other nanoscale geometries in three categories: Auger suppression, multi-exciton level structure, and solution-processed film conductivity, highlighting the potential benefits quantum shells can contribute to various applications. Theoretical analysis discussing the role of the quantum shell geometry in suppressing Auger recombination will also be presented.

Photonic applications of semiconductor quantum dots (QDs) had significant drawbacks due to their ultrafast Auger decay of multiple excitons. These nonradiative processes have hindered the performance of electroluminescent and lasing devices. The emergence of the colloidal quantum well (QW) architecture where semiconductor quantum shell with an inverted QD geometry has led to suppressed Auger recombination. Ultra long biexciton lifetime (>10 ns) and large biexciton quantum yields has been achieved through spatial separation promoted by the QW geometry. Moreover, architecture induced exciton-exciton repulsions have led to the splitting of exciton and biexciton optical transitions which resulted in the single exciton gain mode along with the biexciton gain mode. High biexciton lifetime and the longest reported single exciton gain lifetime (>6 ns) has rendered this geometry a worthy candidate for the development of optically and electrically pumped gain media

Photocatalytic systems comprising a hydrogenase-type catalyst and CdX (X = S, Se, Te) chalcogenide quantum dot (QD) photosensitizers show extraordinary hydrogen production rates under visible light excitation. What remains unknown is the mechanism of energy conversion in these systems. Here, we have explored this question by comparing the performance of two QD sensitizers, CdSe and CdTe, in photocatalytic systems featuring aqueous suspensions of a [Fe2 (μ-1,2-benzenedithiolate) CO6] catalyst and an ascorbic acid sacrificial agent. Overall, the hydrogen production yield for CdSe-sensitized reactions QDs was found to be 13 times greater than that of CdTe counterparts. According to emission quenching experiments, an enhanced performance of CdSe sensitizers reflected a greater rate of electron transfer from the ascorbic acid (kAsc). The observed difference in the QD-ascorbic acid charge transfer rates between the two QD materials was consistent with respective driving forces for these systems. Financial support from projects RTI2018-099407-B-I00, RTI2018-093310-B-I00, RTC2019-006820-5 (MCIU/AEI/FEDER, UE) and ‘HYLANTIC’-EAPA_204/2016 (Interreg Atlantic/FEDER UE) is gratefully acknowledged. Juan Corredor is grateful to FPI contract grant (BES-2016-079201). MZ and DH were supported by the Award DE-SC0016872 (MZ) funded by the U.S. Department of Energy, Office of Science.

Artificial solids of CsPbX3 perovskite nanocrystals (NC) are well known for their promising charge transport characteristics. The long-range diffusion of photoinduced charges in these materials is ...

Precise tuning of optoelectronic properties of solid-state materials assembled from colloidal semiconductor nanocrystals (quantum dots, QDs) is of utmost importance for future optoelectronic techno...

Multiple-exciton (MX) generation is beneficial to many applications of semiconductors, including photoinduced energy conversion, stimulated emission, and carrier multiplication. The utility of MX p...

We demonstrate that solution-phase semiconductor nanocrystals (NCs) undergo photoinduced rotation in an external electric field. Present measurements backed by theoretical calculations show that the rotation of colloidal NCs is driven by the excited-state dipole moment, which is counterbalanced by the solvent viscosity drag. Corresponding angular velocities range from 0.5°/ns for cubic CsPbBr3 NCs to 3°/ns for nanoparticles with a large photoinduced charge separation (CdSe/CdS core-shell and dot-in-a-rod NCs). Because of photoinduced rotation, solution-phase semiconductor NCs exhibited an order-of-magnitude increase in the spectral changes caused by the quantum confined Stark effect (QCSE), compared to solid NC assemblies. The enhanced QCSE of colloidal NCs reflected their global alignment in solution, which could be retained in a solid environment by slow crystallization. Overall, we expect that the demonstrated phenomenon of the colloidal nanocrystal rotation in an electric field will open up new avenues for developing electro-optical and voltage-sensitive applications.

Colloidal semiconductor nanocrystals (NCs) represent a promising class of nanomaterials for lasing applications. Currently, one of the key challenges facing the development of high-performance NC optical gain media lies in enhancing the lifetime of biexciton populations. This usually requires the employment of charge-delocalizing particle architectures, such as core/shell NCs, nanorods, and nanoplatelets. Here, we report on a two-dimensional nanoshell quantum dot (QD) morphology that enables a strong delocalization of photoinduced charges, leading to enhanced biexciton lifetimes and low lasing thresholds. A unique combination of a large exciton volume and a smoothed potential gradient across interfaces of the reported CdSbulk/CdSe/CdSshell (core/shell/shell) nanoshell QDs results in strong suppression of Auger processes, which was manifested in this work though the observation of stable amplified stimulated emission (ASE) at low pump fluences. An extensive charge delocalization in nanoshell QDs was confirmed by transient absorption measurements, showing that the presence of a bulk-size core in CdSbulk/CdSe/CdSshell QDs reduces exciton-exciton interactions. Overall, present findings demonstrate unique advantages of the nanoshell QD architecture as a promising optical gain medium in solid-state lighting and lasing applications.

Plasmonics is a rapidly developing field at the boundary of fundamental sciences and device engineering, which exploits the ability of metal nanostructures to concentrate electromagnetic radiation. The principal challenge lies in achieving an efficient conversion of the plasmon-concentrated field into some form of useful energy. To date, a substantial progress has been made within the scientific community in identifying the major pathways of the plasmon energy conversion. Strategies based on the hot electron injection and the near-field energy transfer have already shown promise in a number of proof-of-principle plasmonic architectures. Nevertheless, there are several fundamental questions that need to be addressed in the future to facilitate the transition of plasmonics to a variety of applications in both light amplification and optical detection. Of particular interest is a plasmon-induced resonance energy transfer (PIRET) process that couples the plasmon evanescent field to a semiconductor absorber via dipole-dipole interaction. This relatively unexplored mechanism has emerged as a promising light conversion strategy in the areas of photovoltaics and photocatalysis and represents the main focus of the present minireview. Along these lines, we highlight the key advances in this area and review some of the challenges associated with applications of the PIRET mechanism in nanostructured systems.

Controlling the morphology of colloidal semiconductor nanocrystals (NCs) remains to be a challenging task. Traditional growth strategies employ a high concentration of monomers to promote particle nucleation, which tends to oversaturate the solution with reactive species. This leads to secondary nucleation events and other dispersion-broadening processes. Here, we explore monomer-deprived synthetic conditions as a bilateral strategy for tuning both the shape and the surface-ligand chemistry of semiconductor colloids. Rather than controlling the nucleation phase, the present method employs a postsynthetic treatment based on low-temperature digestive ripening, where small particles grow at the expense of larger ones. The feasibility of the present approach was demonstrated by observing a 4-fold reduction in the CdSe nanoparticle size dispersion during the digestive ripening reaction, which was induced by high concentrations of L-type (amines) or X-type (oleic acid) ion-solubilizing ligands. In the latter ca...

Sensing strategies utilizing Förster resonance energy transfer (FRET) are widely used for probing biological phenomena. FRET sensitivity to the donor-acceptor distance makes it ideal for measuring the concentration of a known analyte or determining the spatial separation between fluorescent labels in a macromolecular assembly. The difficulty lies in extracting the FRET efficiency from the acceptor-induced quenching of the donor emission, which may contain a significant non-FRET contribution. Here, we demonstrate a general spectroscopic approach for differentiating between charge transfer and energy transfer (ET) processes in donor-acceptor assemblies and apply the developed method for unravelling the FRET/non-FRET contributions in cyanine dye-semiconductor quantum dot (QD) constructs. The present method relies on correlating the amplitude of the acceptor emission to specific changes in the donor excitation profile in order to extract ET-only transfer efficiencies. Quenching of the donor emission is then utilized to determine the non-ET component, tentatively attributed to the charge transfer. We observe that the latter accounts for 50-99% of donor emission quenching in QD-Cy5 and QD-Cy7 systems, stressing the importance of determining the non-FRET efficiency in a spectroscopic ruler and other FRET-based sensing applications.

Digestive ripening (DR) represents a powerful strategy for improving the size homogeneity of colloidal nanostructures. It relies on the ligand-mediated dissolution of larger nanoparticles in favor of smaller ones and is often considered to be the opposite of Ostwald ripening. Despite its successful application to size-focusing of metal colloids, digestive ripening of semiconductor nanocrystals has received little attention to date. Here, we explore this synthetic niche and demonstrate that ligand-induced ripening of semiconductor nanocrystals exhibits an unusual reaction path. The unique aspect of the DR process in semiconductors lies in the thermally activated particle coalescence, which leads to a significant increase in the nanocrystal size for temperatures above the threshold value (Tth = 200–220 °C). Below this temperature, nanoparticle sizes focus to an ensemble average diameter just like in the case of metal colloids. The existence of the thermal threshold for coalescence offers an expedient strate...

The generation and transfer of triplet excitons across semiconductor nanomaterial-molecular interfaces will play an important role in emerging photonic and optoelectronic technologies, and understanding the rules that govern such phenomena is essential. The ability to cooperatively merge the photophysical properties of semiconductor quantum dots with those of well-understood and inexpensive molecular chromophores is therefore paramount. Here we show that 1-pyrenecarboxylic acid-functionalized CdSe quantum dots undergo thermally activated delayed photoluminescence. This phenomenon results from a near quantitative triplet-triplet energy transfer from the nanocrystals to 1-pyrenecarboxylic acid, producing a molecular triplet-state 'reservoir' that thermally repopulates the photoluminescent state of CdSe through endothermic reverse triplet-triplet energy transfer. The photoluminescence properties are systematically and predictably tuned through variation of the quantum dot-molecule energy gap, temperature and the triplet-excited-state lifetime of the molecular adsorbate. The concepts developed are likely to be applicable to semiconductor nanocrystals interfaced with molecular chromophores, enabling potential applications of their combined excited states.

A general strategy for measuring the energy-transfer efficiencies in multifluorophore assemblies is demonstrated. The present method is based on spectral shaping of the excitation light with molecular solutions representing donor and acceptor fluorophores, which causes a suppressed excitation of the respective donor and acceptor molecules in the sample. The changes in the acceptor emission resulting from spectral shaping of the excitation light are then used to determine the energy-transfer efficiencies (Ex) associated with all participating donor–acceptor pairs. Here, the technique is demonstrated through energy-transfer (ET) measurements in a 4-fluorophore construct featuring a DNA supported assembly of three donor/donor-relay (Cy3, Cy3.5, and Cy5) and one acceptor (Cy5.5) molecules. The resulting Ex were validated using the standard photoluminescence (PL) quenching approach as well as measurements of partial 2- and 3-dye assemblies. The present work highlights general benefits of the spectrally shaped ...

Ratiometric sensing strategy relies on the ratio of the two photoluminescence (PL) signals originating from the same nano-object for detecting the changes in the surrounding media. Recently, such dual-color emission has been demonstrated in semiconductor colloids, where the PL signal from a quantum-confined domain was complemented with the secondary emission from transition metal ions or a bulk-like structure. Here, we report on the development of dual-color nanocrystal colloids featuring a combination of two quantum-confined emitters within the same nano-object. The reported morphology relies on double-well core/barrier/shell arrangement, where zero-dimensional excitons of the core component (PbS) can coexist with two-dimensional excitons of the shell domain (CdSe). As a result, the core and shell emission bands can be independently tuned across 880–1500 nm and 600–650 nm spectral windows, respectively. A CdS potential barrier at the PbS/CdSe interface was designed to suppress the energy and charge diffu...

The superior optical extinction characteristics of noble metal nanoparticles have long been considered for enhancing the solar energy absorption in light-harvesting devices. The energy captured through a plasmon resonance mechanism can potentially be transferred to a surrounding semiconductor matrix in the form of excitons or charge carriers, offering a promising light-sensitization strategy. Of particular interest is the plasmon near-field energy conversion, which is predicted to yield substantial gains in the photocarrier generation. Such a short-range interaction, however, is often inhibited by processes of backward electron and energy transfer, which obscure its net benefit. Here, we employ sample-transmitted excitation photoluminescence spectroscopy to determine the quantum efficiency for the plasmon-induced energy transfer (ET) in assemblies of Au nanoparticles and CdSe nanocrystals. The present technique distinguishes the Au-to-CdSe ET contribution from metal-induced quenching processes, thus enabl...

The emerging generation of quantum dot optoelectronic devices offers an appealing prospect of a size-tunable band gap. The confinement-enabled control over electronic properties, however, requires nanoparticles to be sufficiently small, which leads to a large area of interparticle boundaries in a film. Such interfaces lead to a high density of surface traps which ultimately increase the electrical resistance of a solid. To address this issue, we have developed an inverse energy-gradient core/shell architecture supporting the quantum confinement in nanoparticles larger than the exciton Bohr radius. The assembly of such nanostructures exhibits a relatively low surface-to-volume ratio, which was manifested in this work through the enhanced conductance of solution-processed films. The reported core/shell geometry was realized by growing a narrow gap semiconductor layer (CdSe) on the surface of a wide-gap core material (CdS) promoting the localization of excitons in the shell domain, as was confirmed by ultrafast transient absorption and emission lifetime measurements. The band gap emission of fabricated nanoshells, ranging from 15 to 30 nm in diameter, has revealed a characteristic size-dependent behavior tunable via the shell thickness with associated quantum yields in the 4.4-16.0% range.

Assemblies of metal nanostructures and fluorescent molecules represent a promising platform for the development of biosensing and near-field imaging applications. Typically, the interaction of molecular fluorophores with surface plasmons in metals results in either quenching or enhancement of the dye excitation energy. Here, we demonstrate that fluorescent molecules can also engage in a reversible energy transfer (ET) with proximal metal surfaces, during which quenching of the dye emission via the energy transfer to localized surface plasmons can trigger delayed ET from metal back to the fluorescent molecule. The resulting two-step process leads to the sustained delayed photoluminescence (PL) in metal-conjugated fluorophores, as was demonstrated here through the observation of increased PL lifetime in assemblies of Au nanoparticles and organic dyes (Alexa 488, Cy3.5, and Cy5). The observed enhancement of the PL lifetime in metal-conjugated fluorophores was corroborated by theoretical calculations based on the reverse ET model, suggesting that these processes could be ubiquitous in many other dye-metal assemblies.

Excitonic energy transfer is a versatile mechanism by which colloidal semiconductor nanocrystals can interact with a variety of nanoscale species. While this process is analogous to dipole–dipole coupling in molecular systems, the corresponding energy transfer dynamics can deviate from that of molecular assemblies due to manifestations of bulk-like features in semiconductor colloids. In particular, weak exciton binding, small singlet–triplet exciton splitting, and the energy disorder across nanocrystal ensembles can all play distinctive roles in the ensuing energy conversion processes. To characterize the variety of energy transfer schemes involving nanocrystals, this feature article will discuss the latest research by both our group and other groups on the key scenarios under which nanocrystals can engage in energy transfer with other nanoparticles, organic fluorophores, and plasmonic nanostructures, highlighting potential technological benefits to be gained from such processes. We will also shed light on experimental strategies for probing the energy transfer in nanocrystal-based assemblies, with a particular emphasis on novel characterization techniques.

Understanding the energy flow in quantum dot solids represents an important step toward designing artificial systems with configurable optoelectronic properties. The growing complexity of nanoparticle assemblies and deposition techniques calls for advanced methods of characterization and control of the underlying exciton diffusion, which is pervasive in these materials. Along these lines, the Perspective will review recent strategies for measuring the energy transfer processes in assemblies of semiconductor nanocrystals with particular emphasis on emerging avant-garde characterization techniques. We will also shed light on novel experimental methods for controlling the energy diffusion in quantum dots solids, highlighting the role of assembly architecture in ensuing processes of exciton diffusion and dissociation. Novel energy transfer mechanisms recently observed in perovskite quantum dots and triplet-sensitizer nanocrystals will also be discussed.

Catalyst-free, one-pot synthesis of colloidal PbSe nanorods is demonstrated. The co-solvent chloroalkanes play a critical role in driving a one-dimensional growth of PbSe nanorods. The formation of the nanorod is likely governed by the anisotropic growth of the crystal due to the different reactivity of the facets. The photoluminescence quantum yield of the nanorods is above 25%, indicating well-passivated surfaces. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Quantum dot (QD) solids represent an important class of functional materials that holds strong promise for future applications in technology. Their optoelectronic properties are determined by energy diffusion processes, which character can often be inferred from the temporal and spectral analysis of the film’s photoluminescence (PL). Here, optical techniques based on PL lifetime, bulk quenching, and temperature-dependent PL will be discussed. These techniques complement the electrical conductivity measurements by mapping the flow of optically induced excitons through undepleted, contact-free films with an unprecedented temporal and spatial resolution. By correlating the QD solid morphology with the ensuing photoluminescence (PL) dynamics, these methods allow estimating important transport characteristics, including exciton and charge carrier diffusion lengths, the rate of interparticle energy transfer, carrier mobility, and the exciton diffusivity. The review will cover most popular PL-based strategies and summarize the key experimental findings resulting from these works.

We demonstrate a general strategy for the synthesis of colloidal semiconductor nanocrystals (NCs) exhibiting size dispersion below 5%. The present approach relies on the sequential deposition of fully saturated cationic and anionic monolayers onto small-diameter clusters, which leads to focusing of nanocrystal sizes with the increasing particle diameter. Each ionic layer is grown through a room-temperature colloidal atomic layer deposition process that employs a two-solvent mixture to separate the precursor and nanocrystal phases. As a result, unreacted precursors can be removed after each deposition cycle, preventing the secondary nucleation. By using CdS NCs as a model system, we demonstrate that a narrow size dispersion can be achieved through a sequential growth of Cd2+ and S2– layers onto starting CdS cluster “seeds”. Besides shape uniformity, the demonstrated methodology offers an excellent batch-to-batch reproducibility and an improved control over the nanocrystal surface composition. The present s...

Triplet excitons are pervasive in both organic and inorganic semiconductors but generally remain confined to the material in which they originate. We demonstrated by transient absorption spectroscopy that cadmium selenide semiconductor nanoparticles, selectively excited by green light, engage in interfacial Dexter-like triplet-triplet energy transfer with surface-anchored polyaromatic carboxylic acid acceptors, extending the excited-state lifetime by six orders of magnitude. Net triplet energy transfer also occurs from surface acceptors to freely diffusing molecular solutes, further extending the lifetime while sensitizing singlet oxygen in an aerated solution. The successful translation of triplet excitons from semiconductor nanoparticles to the bulk solution implies that such materials are generally effective surrogates for molecular triplets. The nanoparticles could thereby potentially sensitize a range of chemical transformations that are relevant for fields as diverse as optoelectronics, solar energy conversion, and photobiology.

Nanoshell quantum dots (QDs) represent a novel class of colloidal semiconductor nanocrystals (NCs), which supports tunable optoelectronic properties over the extended range of particle sizes. Traditionally, the ability to control the bandgap of colloidal semiconductor NCs is limited to small-size nanostructures, where photoinduced charges are confined by Coulomb interactions. A notorious drawback of such a restricted size range concerns the fact that assemblies of smaller nanoparticles tend to exhibit a greater density of interfacial and surface defects. This presents a potential problem for device applications of semiconductor NCs where the charge transport across nanoparticle films is important, as in the case of solar cells, field-effect transistors, and photoelectrochemical devices. The morphology of nanoshell QDs addresses this issue by enabling the quantum-confinement in the shell layer, where two-dimensional excitons can exist, regardless of the total particle size. Such a geometry exhibits one of the lowest surface-to-volume ratios among existing QD architectures and, therefore, could potentially lead to improved charge-transport and multi-exciton characteristics. The expected benefits of the nanoshell architecture were recently demonstrated by a number of reports on the CdSbulk/CdSe nanoshell model system, showing an improved photoconductivity of solids and increased lifetime of multi-exciton populations. Along these lines, this perspective will summarize the recent work on CdSbulk/CdSe nanoshell colloids and discuss the possibility of employing other nanoshell semiconductor combinations in light-harvesting and lasing applications.

The visualization of distinct molecular species represents an important challenge of bio-imaging research. In past decades, the development of multicolor fluorescent (FL) labels has greatly improved our ability to track biological analytes, paving the way for important advances in understanding the cell dynamics. It remains challenging, however, to visualize a large number of different fluorephores simultaneously. Owing to a spectrally broad absorption of fluorescent dyes, only up to five color categories can be resolved at once. Here, we demonstrate a general strategy for distinguishing between multiple fluorescent targets in acquired microscopy images with improved accuracy. The present strategy is enabled through spectral shaping of the excitation light with an optical filter that uniquely attenuates the light absorption of each fluorophore in the investigated sample. The resulting emission changes, induced by such excitation modulation, are therefore target-specific and can be used for identifying various fluorescent species. The technique is demonstrated through an accurate identification of 8 different CdSe dyes with absorption maxima spanning the 520-620 spectral range. It is subsequently applied for accurate measurements of the pH balance in buffers emulating a metabolism of tumor cells.

The shape of colloidal nanoparticles grown via hot-injection routes is largely determined by the reaction-limited rate of monomer nucleation. This offers an important synthetic benefit of tuning the morphology of colloidal nanocrystals simply by controlling the rate of monomer release during the thermal conversion of precursors. Unfortunately, the monomer concentration in colloidal reactions is difficult to track in situ, which obscures the actual effect of the temperature, monomer solubility, and ligand density on the probability of nanoparticle nucleation. Here, we develop an experimental strategy for monitoring the time-dependent monomer concentration during the hot-injection synthesis of Ag nanocrystals. This approach employs Au nanoparticles as chemical probes of the Ag monomer build-up in the reaction flask. The precipitation of Ag on the surface of Au nanoparticles is diffusion-limited and results in a blue-shift of the plasmon resonance that is used to gauge the Ag monomer concentration, [Ag0]. By...

The plasmon resonance of small-diameter metal nanoparticles has a unique decay character, as it produces almost no far field scattering. Instead, most of the absorbed light is emitted in the near field (NF), a phenomenon also known as strong field localization. Such nonradiative emission is short-lived and decays primarily via the production of heat, which prevents efficient harvesting of the NF energy. Here, we demonstrate a general strategy for coupling the NF radiation of surface plasmons to long-lived optical excitations in semiconductor nanocrystals (NCs). This concept was manifested through the observation of an enhanced exciton generation in CdSe NCs coupled to 5 nm Au nanoparticles. To distinguish the plasmon antenna effect from photoinduced charge transfer processes, both Au and CdSe nanoparticles were encapsulated into insulating CdS or ZnS matrices. A unique signature of the plasmon to exciton energy transfer was observed in photoexcitation measurements that unambiguously correlate the increase...

We describe surface enhanced Raman spectroscopy (SERS) experiments in which molecular coverage is systematically varied from 3.8 × 105 to 3.8 × 102 to 0.38 molecules per μm2 using electrospray deposition of ethanolic 4,4′-dimercaptostilbene (DMS) solutions. The plasmonic SERS substrate used herein consists of a well-characterized 2-dimensional (2D) array of silver nanospheres (see El-Khoury et al., J. Chem. Phys., 2014, 141, 214308), previously shown to feature uniform topography and plasmonic response, as well as intense SERS activity. When compared to their ensemble averaged analogues, the spatially and temporally averaged spectra of a single molecule exhibit several unique features including: (i) distinct relative intensities of the observable Raman-active vibrational states, (ii) more pronounced SERS backgrounds, and (iii) broader Raman lines indicative of faster vibrational dephasing. The first observation may be understood on the basis of an intuitive physical picture in which the removal of averaging over multiple molecules exposes the tensorial nature of Raman scattering. When an oriented single molecule gives rise to the recorded SERS spectra, the relative orientation of the molecule with respect to vector components of the local electric field determines the relative intensities of the observable vibrational states. Using a single molecule SERS framework, described herein, we derive a unique molecular orientation in which a single DMS molecule is isolated at a nanojunction formed between two silver nanospheres in the 2D array. The DMS molecule is found lying nearly flat with respect to the metal. The derived orientation of a single molecule at a plasmonic nanojunction is consistent with observations (ii) and (iii). In particular, a careful inspection of the temporal spectral variations along the recorded single molecule SERS time sequences reveals that the time-averaged SERS backgrounds arise from individual molecular events, marked by broadened SERS signatures. We assign the broadened spectra along the SERS time sequence – which sum up to a SERS background in the averaged spectra – to instances in which the π-framework of the DMS molecule is parallel to the metal at a classical plasmonic nanojunction. This also accounts for Raman line broadening as a result of fast vibrational dephasing, and driven by molecular reorientation at a plasmonic nanojunction. Furthermore, we report on the molecular orientation dependence of single molecule SERS enhancement factors. We find that in the case of a single DMS molecule isolated at a plasmonic nanojunction, molecular orientation may affect the derived single molecule SERS enhancement factor by up to 5 orders of magnitude. Taking both chemical effects as well as molecular orientation into account, we were able to estimate a single molecule enhancement factor of ∼1010 in our measurements.

Multiple exciton generation, in which two electron–hole pairs are generated from the absorption of one high-energy photon, has been demonstrated to improve efficiency in quantum-dot-based solar cells. Now, a photoelectrochemical system using PbS quantum dots is shown to drive hydrogen evolution with external quantum efficiency over 100%.

Tracking the energy flow in nanoscale materials is an important yet challenging goal. Experimental methods for probing the intermolecular energy transfer (ET) are often burdened by the spectral crosstalk between donor and acceptor species, which complicates unraveling their individual contributions. This issue is particularly prominent in inorganic nanoparticles and biological macromolecules featuring broad absorbing profiles. Here, we demonstrate a general spectroscopic strategy for measuring the ET efficiency between nanostructured or molecular dyes exhibiting a significant donor-acceptor spectral overlap. The reported approach is enabled through spectral shaping of the broadband excitation light with solutions of donor molecules, which inhibits the excitation of respective donor species in the sample. The resulting changes in the acceptor emission induced by the spectral modulation of the excitation beam are then used to determine the quantum efficiency and the rate of ET processes between arbitrary fluorophores (molecules, nanoparticles, polymers) with high accuracy. The feasibility of the reported method was demonstrated using a control donor-acceptor system utilizing a protein-bridged Cy3-Cy5 dye pair and subsequently applied for studying the energy flow in a CdSe

The visible extinction of noble metal nanoparticles surpasses those of semiconductor and molecular dyes by several orders of magnitude. Such superior light-harvesting characteristics are highly attractive for photocatalytic applications. Of a particular interest is the process of the plasmon near-field energy conversion, which is predicted to yield substantial gains in the photocarrier generation. Here, we employ the sample-transmitted excitation photoluminescence (STEP) spectroscopy to determine the quantum efficiency for the plasmon induced resonant energy transfer (ET) in assemblies of Au nanoparticles and semiconductor quantum dots (CdSe, PbS, etc.). The present technique distinguishes the Au-to-QD ET contribution from metal-induced quenching processes thus enabling accurate estimates of the photon-to-exciton conversion efficiency. We show that in the case of 9.1-nm Au nanoparticles, only 1-2% of the Au absorbed radiation is converted to excitons in the surrounding CdSe nanocrystal matrix. For larger, 21.0-nm Au, the photon-to-exciton conversion efficiency increases to 29.5%. The results of present measurements were used to develop an empirical model for estimating the maximum gain in the plasmon-induced carrier generation versus the mass-fraction of Au.

One of the key challenges facing the realization of functional nanocrystal devices concerns the development of techniques for depositing colloidal nanocrystals into electrically coupled nanoparticle solids. This work compares several alternative strategies for the assembly of such films using an all-optical approach to the characterization of electron transport phenomena. By measuring excited carrier lifetimes in either ligand-linked or matrix-encapsulated PbS nanocrystal films containing a tunable fraction of insulating ZnS domains, we uniquely distinguish the dynamics of charge scattering on defects from other processes of exciton dissociation. The measured times are subsequently used to estimate the diffusion length and the carrier mobility for each film type within the hopping transport regime. It is demonstrated that nanocrystal films encapsulated into semiconductor matrices exhibit a lower probability of charge scattering than that of nanocrystal solids cross-linked with either 3-mercaptopropionic acid or 1,2-ethanedithiol molecular linkers. The suppression of carrier scattering in matrix-encapsulated nanocrystal films is attributed to a relatively low density of surface defects at nanocrystal/matrix interfaces.

Photochemistry of copper(II) monochlorocomplexes in methanol and acetonitrile solutions is studied by UV-pump/broadband deep-UV-to-near-IR probe femtosecond transient absorption spectroscopy. Upon 255 and 266 nm excitation, the complexes in acetonitrile and methanol, respectively, are promoted to the excited ligand-to-metal charge transfer (LMCT) state, which has a short (sub-250 fs) lifetime. From the LMCT state, the complexes decay via internal conversion to lower-lying ligand field (LF) d-d excited states or the vibrationally hot ground electronic state. A minor fraction of the excited complexes relaxes to the LF electronic excited states, which are relatively long-lived with lifetimes1 ns. Also, in methanol solutions, about 3% of the LMCT-excited copper(II) monochlorocomplexes dissociate forming copper(I) solvatocomplexes and chlorine atoms, which then further react forming long-lived photoproducts. In acetonitrile, about 50% of the LMCT-excited copper(II) monochlorocomplexes dissociate forming radical and ionic products in a ratio of 3:2. Another minor process observed following excitation only in methanol solutions is the re-equilibration between several forms of the copper(II) ground-state complexes present in solutions. This re-equilibration occurs on a time scale from sub-nanoseconds to nanoseconds.

The present study investigates the photocatalytic activity of ZnSe/CdS core/shell semiconductor nanocrystals. These nanoparticles exhibit a spatial separation of photoinduced charges between the core and the shell domains, which makes them potentially viable for photocatalytic applications. Unfortunately, one of the excited charges remains inside the core semiconductor and thus cannot efficiently react with the external environment. Here, we explore this issue by investigating the mechanisms of hole extraction from the ZnSe core to the surface of the CdS shell. In particular, the effect of shell thickness in ZnSe/CdS core/shell nanocrystals on the ability of core-localized charges to perform oxidative reactions was determined. By using a combination of time-resolved spectroscopy and electrochemical techniques, we demonstrate that the use of hole-scavenging surfactants facilitates an efficient transfer of core-localized holes to the surface even in the case of shells exceeding 7 nm in thickness. These meas...

A general strategy for the assembly of all-inorganic light-emitting nanocrystal films with emission quantum yield in the 30–52% range is reported. The present methodology relies on solution-processing of CdSe nanocrystals into a crystalline matrix of a wide band gap semiconductor (CdS, ZnS), which replaces the original molecular ligands on nanocrystal surfaces with an inorganic medium. Such matrices efficiently protect nanoparticles from the surrounding environment and preserve the quantum confinement of electrical charges in embedded CdSe NCs. In addition to strong emission, fabricated films show excellent thermal and chemical stability, and a large refractive index, which avails their integration into emerging solid-state nanocrystal devices, including light-emitting diodes, solar concentrators, and quantum dot lasers.

Degussa P-25 TiO2 bearing surface-anchored Pt(dcbpy)Cl2 [dcbpy = 4,4′-dicarboxylic acid-2,2′-bipyridine] prepared with systematically varied surface coverage produced Pt0 nanoparticles under bandgap illumination in the presence of methanol hole scavengers. Energy-dispersive X-ray spectroscopy confirmed the presence of elemental platinum in the newly formed nanoparticles during scanning transmission electron microscopy (STEM) experiments. According to the statistical analysis of numerous STEM images, the Pt0 nanoclusters were distributed in a segregated manner throughout the titania surface, ranging in size from 1 to 3 nm in diameter. The final achieved nanoparticle size and net hydrogen production were determined as a function of the Pt(dcbpy)Cl2 surface coverage as well as other systematically varied experimental parameters. The hybrid Pt/TiO2 nanomaterials obtained upon complete decomposition of the Pt(dcbpy)Cl2 precursor displayed higher photocatalytic activity (300 μmol/h) for hydrogen evolution in aq...

A general strategy for low-temperature processing of colloidal nanocrystals into all-inorganic films is reported. The present methodology goes beyond the traditional ligand-interlinking scheme and relies on encapsulation of morphologically defined nanocrystal arrays into a matrix of a wide-band gap semiconductor, which preserves optoelectronic properties of individual nanoparticles while rendering the nanocrystal film photoconductive. Fabricated solids exhibit excellent thermal stability, which is attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells.

The effect of hole localization on photocatalytic activity of Pt-tipped semiconductor nanocrystals is investigated. By tuning the energy balance at the semiconductor-ligand interface, we demonstrate that hydrogen production on Pt sites is efficient only when electron-donating molecules are used for stabilizing semiconductor surfaces. These surfactants play an important role in enabling an efficient and stable reduction of water by heterostructured nanocrystals as they fill vacancies in the valence band of the semiconductor domain, preventing its degradation. In particular, we show that the energy of oxidizing holes can be efficiently transferred to a ligand moiety, leaving the semiconductor domain intact. This allows reusing the inorganic portion of the "degraded" nanocrystal-ligand system simply by recharging these nanoparticles with fresh ligands.