Part I: Synthesis and Study of Nonacene Derivatives; Part II: Optoelectronic Properties of Metal-Semiconductor Nanocomposites in Strongly Coupled Regime

Acenes are polycyclic aromatic hydrocarbons (PAHs) consisting of linearly fused benzene rings. In the recent past, acenes have been of interest from fundamental and applied perspectives. Smaller acenes such as benzene, naphthalene, and anthracene are among the most studied organic compounds and their properties are well explored. Pentacene has received considerable attention as the most promising active semiconductor for use in organic thin film transistors (TFT) because of its high charge-carrier mobility; however, poor environmental stability is one of the problems limiting its practical application. As the number of rings increases, the members of the acene family become increasingly reactive. The successful synthesis of heptacene developed by Mondal et al used the Strating-Zwanenberg photodecarbonylation reaction. The lesser stability of the tetracene moieties in the nonacene photoprecursor compared to the anthracene moieties of the heptacene process make its synthesis more challenging. The latter scheme requires 2,3-dibromoanthracene as one of the starting materials. Besides the poor solubility of 2,3-dibromoanthracene, failure was also due to insufficient formation of anthracyne upon treatment of 2,3-dibromoanthracene with n-BuLi. Although the initial idea didn\u27t work we used the same scheme replacing 2,3-dibromoanthracene with 7,8-dibromo-1,4-dihydroanthracene. The reaction of the latter with 5,6,7,8-tetramethylenebicyclo[2.2.2]oct-2-ene gave 1,4,7,8,9,12,15,18,19,20-octadecahydro-8,19-diethenononacene albeit in low yield. Multiple attempts to dehydrogenate the non-aromatic rings using DDQ and other reagents under various conditions failed to produce the desired compound. Recently Miller reported the synthesis of relatively stable heptacene derivatives having a combination of arylthio and o-dialkylphenyl substituents. Miller\u27s scheme used 1,2,4,5-tetrakis(bromomethyl)-3,6-bis(4\u27-t-butylthiophenyl)benzene as the core precursor. Another synthetic approach has been undertaken that employs Miller\u27s 1,2,4,5-tetrakis(bromomethyl)-3,6-bis(4\u27-t-butylthiophenyl)benzene in its core. First attempts to react the latter with 1,4-anthraquinone to produce nine linearly fused ring system were unsuccessful. Interestingly in both approaches we used, a dienophile benzyne-type and quinone-like with more than one fused ring were unreactive in subsequent Diels-Alder reactions. So similarly to the prior scheme, a dienophile with terminal nonaromatic ring (6,7,8,9-tetrahydro-1,4-anthraquinone) was used along with 1,2,4,5-tetrakis(bromomethyl)-3,6-bis(4\u27-t-butylthiophenyl)benzene to yield a nine-ring backbone structure which was treated with mesityl magnesium bromide followed by reduction to yield 1,2,3,4,12,13,14,15-Octahydro-8,19-bis(4\u27-t-butylphenylthio)nonacene. Unfortunately this compound wasn\u27t isolated or properly characterized. Combining metal and semiconductor domains in a single nanocrystal offer a unique opportunity for the development of hybrid nanoscale composites with functionalities that extend beyond those of isolated materials. The presence of powerful carrier confinement in these nanoparticles joint with tunable geometry of the semiconductor-metal interface gives rise to novel optoelectronic properties that can potentially add up to a wide range of applications. Recently, Au/CdS and Au/CdSe heterostructures containing gold domains grown onto cadmium chalcogenide semiconductor nanorods (NRs) have come forward as a model system for studying such hybrid nanomaterials. In this work we have developed several chemical routes to CdSe/CdS core-shell nanocrystals (NCs) with each of them leading to different shape of nanocrystals. Also a simple chemical method for growing Au domains onto CdS nanorods and CdSe/CdS NCs in oleylamine was developed. The size of Au NCs can be precisely tuned by adjusting the temperatu..

Part I: Synthesis and Study of Nonacene Derivatives; Part II: Optoelectronic Properties of Metal-Semiconductor Nanocomposites in Strongly Coupled Regime

Acenes are polycyclic aromatic hydrocarbons (PAHs) consisting of linearly fused benzene rings. In the recent past, acenes have been of interest from fundamental and applied perspectives. Smaller acenes such as benzene, naphthalene, and anthracene are among the most studied organic compounds and their properties are well explored. Pentacene has received considerable attention as the most promising active semiconductor for use in organic thin film transistors (TFT) because of its high charge-carrier mobility; however, poor environmental stability is one of the problems limiting its practical application. As the number of rings increases, the members of the acene family become increasingly reactive. The successful synthesis of heptacene developed by Mondal et al used the Strating-Zwanenberg photodecarbonylation reaction. The lesser stability of the tetracene moieties in the nonacene photoprecursor compared to the anthracene moieties of the heptacene process make its synthesis more challenging. The latter scheme requires 2,3-dibromoanthracene as one of the starting materials. Besides the poor solubility of 2,3-dibromoanthracene, failure was also due to insufficient formation of anthracyne upon treatment of 2,3-dibromoanthracene with n-BuLi. Although the initial idea didn\u27t work we used the same scheme replacing 2,3-dibromoanthracene with 7,8-dibromo-1,4-dihydroanthracene. The reaction of the latter with 5,6,7,8-tetramethylenebicyclo[2.2.2]oct-2-ene gave 1,4,7,8,9,12,15,18,19,20-octadecahydro-8,19-diethenononacene albeit in low yield. Multiple attempts to dehydrogenate the non-aromatic rings using DDQ and other reagents under various conditions failed to produce the desired compound. Recently Miller reported the synthesis of relatively stable heptacene derivatives having a combination of arylthio and o-dialkylphenyl substituents. Miller\u27s scheme used 1,2,4,5-tetrakis(bromomethyl)-3,6-bis(4\u27-t-butylthiophenyl)benzene as the core precursor. Another synthetic approach has been undertaken that employs Miller\u27s 1,2,4,5-tetrakis(bromomethyl)-3,6-bis(4\u27-t-butylthiophenyl)benzene in its core. First attempts to react the latter with 1,4-anthraquinone to produce nine linearly fused ring system were unsuccessful. Interestingly in both approaches we used, a dienophile benzyne-type and quinone-like with more than one fused ring were unreactive in subsequent Diels-Alder reactions. So similarly to the prior scheme, a dienophile with terminal nonaromatic ring (6,7,8,9-tetrahydro-1,4-anthraquinone) was used along with 1,2,4,5-tetrakis(bromomethyl)-3,6-bis(4\u27-t-butylthiophenyl)benzene to yield a nine-ring backbone structure which was treated with mesityl magnesium bromide followed by reduction to yield 1,2,3,4,12,13,14,15-Octahydro-8,19-bis(4\u27-t-butylphenylthio)nonacene. Unfortunately this compound wasn\u27t isolated or properly characterized. Combining metal and semiconductor domains in a single nanocrystal offer a unique opportunity for the development of hybrid nanoscale composites with functionalities that extend beyond those of isolated materials. The presence of powerful carrier confinement in these nanoparticles joint with tunable geometry of the semiconductor-metal interface gives rise to novel optoelectronic properties that can potentially add up to a wide range of applications. Recently, Au/CdS and Au/CdSe heterostructures containing gold domains grown onto cadmium chalcogenide semiconductor nanorods (NRs) have come forward as a model system for studying such hybrid nanomaterials. In this work we have developed several chemical routes to CdSe/CdS core-shell nanocrystals (NCs) with each of them leading to different shape of nanocrystals. Also a simple chemical method for growing Au domains onto CdS nanorods and CdSe/CdS NCs in oleylamine was developed. The size of Au NCs can be precisely tuned by adjusting the temperatu..

Measuring the Time-Dependent Monomer Concentration during the Hot-Injection Synthesis of Colloidal Nanocrystals

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 <i>in situ</i>, 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 measuring [Ag⁰] immediately before the nucleation burst, we were able to elucidate the effect of several reaction parameters on the nucleation dynamics and the ultimate morphology of Ag nanocrystals. In particular, we show that the nucleation rate is independent of the reaction temperature but is highly sensitive to the concentration of free ligands in solution

Mapping the Exciton Diffusion in Semiconductor Nanocrystal Solids

Colloidal nanocrystal solids represent an emerging class of functional materials that hold strong promise for device applications. The macroscopic properties of these disordered assemblies are determined by complex trajectories of exciton diffusion processes, which are still poorly understood. Owing to the lack of theoretical insight, experimental strategies for probing the exciton dynamics in quantum dot solids are in great demand. Here, we develop an experimental technique for mapping the motion of excitons in semiconductor nanocrystal films with a subdiffraction spatial sensitivity and a picosecond temporal resolution. This was accomplished by doping PbS nanocrystal solids with metal nanoparticles that force the exciton dissociation at known distances from their birth. The optical signature of the exciton motion was then inferred from the changes in the emission lifetime, which was mapped to the location of exciton quenching sites. By correlating the metal–metal interparticle distance in the film with corresponding changes in the emission lifetime, we could obtain important transport characteristics, including the exciton diffusion length, the number of predissociation hops, the rate of interparticle energy transfer, and the exciton diffusivity. The benefits of this approach to device applications were demonstrated through the use of two representative film morphologies featuring weak and strong interparticle coupling

Colloidal Synthesis of Monodisperse Semiconductor Nanocrystals through Saturated Ionic Layer Adsorption

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 S^2– 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 synthesis is amenable to other types of semiconductor nanocrystals and can potentially offer a viable alternative to traditional hot-injection strategies of the nanoparticle growth

One-Dimensional Carrier Confinement in “Giant” CdS/CdSe Excitonic Nanoshells

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

Plasmon-Induced Energy Transfer: When the Game Is Worth the Candle

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 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 the present measurements were used to develop an empirical model for estimating the maximum gain in plasmon-induced carriers versus the mass fraction of Au in a film

Lifting the Spectral Crosstalk in Multifluorophore Assemblies

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 (<i>E</i>_<i>x</i>) 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 <i>E</i>_<i>x</i> 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 excitation approach to measuring donor–acceptor energetics, including the ability to resolve the spectral cross talk between multiple fluorophores and to exclude charge transfer contributions into donor PL quenching