This PDF file contains the front matter associated with SPIE Proceedings Volume 9549, including the Title Page, Copyright information, Table of Contents, Invited Panel Discussion, and Conference Committee listing.

The phenomenon of electric field-induced emission quenching is important in organic light-emitting diodes because operating conditions involve large electric fields. Past experimental work on light-emitting polymers and oligomers showed that field-induced quenching (FIQ) efficiencies are higher in non-rigid molecules such as poly(pphenylene vinylene) or PPV, than in similar, more planar molecules. Based on this relationship, we previously proposed that the applied field enhances internal conversion decay channels. Our further studies built on this idea by examining FIQ in PPV oligomers of varying length using computational methods. Calculations performed at the INDO/S-CI level showed the presence of free electron-hole pair (FEHP) states which are stabilized by the uniform external electric field. These FEHP states undergo an avoided crossing with the fluorescent 1B_u bound exciton state at sufficiently high field magnitudes. The magnitude of the electronic coupling between the FEHP and 1B_u state, determined from these avoided crossings, is found to be a function of the field at which these states cross. This function is universal in that it applies to all FEHP states in PPV. This includes FEHP states on different length oligomers and the multiple FEHP states on a given length oligomer. Combining this universal function with simple models for the surrounding dielectric medium and effects of disorder allowed Marcus theory to be used to develop a model of FIQ. The resulting model yields reasonable quantitative agreement with FIQ magnitudes, dependence on oligomer length, and threshold field strengths at which quenching is observed. Here, the universal curve, relating electronic coupling to the field at which the FEHP and 1B_u states cross, is examined for planar, ordered oligomers of polyfluorene and ladder-type poly(p-phenylene). The dependence is similar to PPV, suggesting that this curve is universal not just across states, but also across this family of conjugated polymers. Given that the electronic couplings are similar, the observed differences in FIQ may be attributed to other factors, including especially the reduced degree of structural disorder present for these more rigid systems.

The diffusion of singlet and triplet excitons along single polyfluorene chains in solution has been studied by monitoring their transport to end traps. Time-resolved transient absorption and steady state fluorescence were used to determine fractions of excitons that reach the end caps. In order to accurately determine the singlet diffusion coefficient, the fraction of polymer ends that have end traps was determined through a combination of NMR and triplet quenching experiments. The distributions of polymer lengths were also taken into account and the resulting analysis points to a surprisingly long singlet diffusion length of 34 nm. Experiments on triplet transport also suggest that the entire 100nm+ chain is accessible to the triplet during its lifetime suggesting a lack of hindrance by defects or traps on this timescale. Time Resolved Microwave Conductivity measurements were also performed on a series of different length oligo- and polyfluorenes in solution allowing a global fit to be performed to extract an accurate intrachain mobility of 1.1 cm²/Vs.

With increasing knowledge of the role of the different phases in the bulk heterojunction organic solar cell, the primary site for charge generation is now considered to be the mixed phase, and not the clean interface between neat polymer and neat fullerene. To gain a better understanding of the primary charge generating and recombination steps in this region of the system, we focus our studies on the role of the solid-state microstructure of neat polymers and light-doping of these polymers with a variety of electron-accepting dopants at low concentration. This presentation will describe some recent work on the doping of polythiophene and polyfluorene derivatives with fullerenes, phthalocyanines and perylenes, which provide a range of reduction potentials that serve to control the driving force for electron transfer processes. Results from flash photolysis, time-resolved microwave conductivity (fp-TRMC), femtosecond transient absorption spectroscopy (fTA) and photoluminescence spectroscopy will be presented.

We have used a femtosecond-resolved spectroscopic technique based on the Stark effect (electromodulated differential absorption) in order to investigate free charge generation and charge drift in solar cell devices of neat conjugated polymer pBTTT and in its 1:1 (by weight) blend with PCBM. In the latter, the fullerene molecules intercalate between the polymer side-chains, yielding a co-crystal phase. Our results show that free charge generation in both materials is ultrafast and strongly dependent on the applied reverse bias. Charge drift to the electrodes (under strong reverse bias) occurs with comparable dynamics on the 1.2 ns time scale for neat pBTTT and the blend, and is probably dominated by hole transport within/between polymer chains.

Exciton dissociation at organic semiconductor donor-acceptor (D-A) heterojunctions is critical for the performance of organic photovoltaic (OPV) structures. Interfacial charge separation and recombination processes control device efficiency. We have investigated these fundamental interfacial issues using time-resolved two-photon photoemission (TR-2PPE), coupled with the formation of well-controlled D-A structures by organic molecular beam epitaxy. The interfacial electronic and molecular structure of these model interfaces was well-characterized using scanning tunneling microscopy and ultraviolet photoemission. Exciton dissociation dynamics were investigated by using a sub-picosecond pump pulse to create Pc π→π* transitions, producing a population of singlet (S1) Pc excitons. The subsequent decay dynamics of this population was monitored via photoemission with a time-delayed UV pulse. For CuPcC60 interfaces, S1 exciton population decay in the interfacial CuPc layer was much faster than decay in the bulk due to interfacial charge separation. The rate constant for exciton dissociation was found to be ≈ 7 x 10 12 sec-1 (≈ 140 fs). Excitons that lose energy via intersystem crossing (ISC) to triplet levels dissociate approximately 500 to 1000 times slower. The dependence of exciton dissociation on separation was also studied. Exciton dissociation falls of rapidly with distance from the interface. Dissociation from the 2nd, and subsequent, layers of H2Pc is reduced by at least a factor of 10 from that in the interfacial layer. Finally, investigations of the relative efficiency for interfacial exciton dissociation by alternative acceptors based on perylene cores, (perylene tetracarboxylic dianhydride, or PTCDA) compared to fullerene-based acceptors such as C60 will also be discussed.

The quinoxaline-based polymer TQ1 (poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5- diyl]) is a promising candidate as electron donor in organic solar cells. In combination with the electron acceptor [6,6]- phenyl-C₇₁- butyric acid methyl ester (PC₇₀BM), TQ1 has resulted in solar cells with power conversion efficiencies of 7 %.

We have studied TQ1 films, with and without PC₇₀BM, spin-casted from different solvents, by fluorescence spectroscopy and UV/VIS absorption spectroscopy. We used chloroform (CF), chlorobenzene (CB), and odichlorobenzene (o-DCB) as solvents for the coating solutions and 1-chloronaphthalene (CN) as solvent additive. CN addition has been shown to enhance photo-conversion efficiency of these solar cells. Phase-separation causes lateral domain formation in the films and the domain size depends on the solvent . These morphological differences coincide with changes in the spectroscopic patterns of the films.

From a spectroscopic point of view, TQ1 acts as fluorescent probe and PC₇₀BM as quencher. The degree of fluorescence quenching is coupled to the morphology through the distance between TQ1 and PC₇₀BM. Furthermore, if using a bad solvent for PC₇₀BM, morphological regions rich in the fullerene yield emission characteristic for aggregated PC₇₀BM. Clear differences were found, comparing the TQ1:PC₇₀BM blend films casted from different solvents and at different ratios between the donor and acceptor. The morphology also influences the UV/VIS absorption spectra, yielding further information on the composition.

The results show that fluorescence and UV/VIS absorption spectroscopy can be used to detect aggregation in blended films and that these methods extend the morphological information beyond the scale accessible with microscopy.

Although the interfaces between two isotropic media are of primary importance in many areas of science and technology, their properties are only partially understood. Our strategy to obtain an insight into these properties is to investigate the ultrafast excited-state dynamics of environment-sensitive molecular probes at liquid interfaces using time-resolved surface second harmonic generation, and to compare it with the dynamics of the same molecules in bulk solutions. Additionally, this approach gives rich information on how the chemical reactivity may change when going from the bulk phase to the interface. This is illustrated by an investigation performed with a series of fluorescent DNA probes at the dodecane/water interface without and with the presence of DNA in the aqueous phase. Substantial differences in the conformation of these cyanine dyes (aggregated or not) and in the excited-state dynamics are observed when going from bulk solutions to the interface. Moreover, the presence of double-stranded DNA in the aqueous phase induces some chirality at the interface.

Broadband pump-probe spectroscopy over the entire time range (200 fs to 500 ns) relevant to monitor the polaron generation and recombination dynamics were performed on the bulk heterojunction composites of poly (3- hexylthiophene) (P3HT) and poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%) with [6,6]- phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor. The modeling of the polaron dynamics with the Debye- Smoluchowski diffusion model provides the charge separation length at the polymer:fullerene interface. Furthermore, the computed polaron yield using the polaron cross-section over the entire time range reveals the amount of photo-generated charges in P3HT:PCBM and P3HTT-DPP-10%:PCBM bulk-heterojunction thin films.

We examined photoinduced charge-generation dynamics of the poly(3-hexylthiophene) (P3HT)/titanyl phthalocyanine (TiOPc) bilayer and the P3HT/TiOPc/C₆₀ trilayer using the combination of flash-photolysis time-resolved microwave conductivity experiments (fp-TRMC) and classic pump-probe transient absorption (TA) spectroscopy following dominant excitation of the P3HT layer. The superlinear increase of φΣμ for the P3HT/TiOPc bilayer, compared to the φΣμ sum of each P3HT and TiOPc layer suggest photoinduced carrier-generation. Furthermore, the superlinear increase of φΣμ of the P3HT/TiOPc/C₆₀ trilayer with respect to the each P3HT/TiOPc and TiOPc/C₆₀ bilayers evinces charge migration from one interface to the other interface. In addition, with selective photoexcitation on the P3HT layer, both amorphous and H-aggregated P3HT domains participate in electron transfer ([P3HT*/TiOPc]→[P3HT^•+/TiOPc^•-]), contrasting to the previous observation where with selective excitation of the TiOPc layer, only the H-aggregated P3HT domain involves in hole transfer ([P3HT/TiOPc→[P3HT^•+/TiOPc^•-]) to produce P3HT^•+/TiOPc^•- in J. Phys. Chem. B 119(24), 7729—7739 (2015). These results under different excitation conditions are consistent with calculated energetic driving force (ΔE_CS) for charge generation which is -0.58 eV and -0.73 eV for amorphous and H-aggregated P3HT domains under the P3HT layer excitation, while 0.04 eV and -0.11 eV for amorphous and H-aggregated P3HT domains under the TiOPc layer excitation.

A reversible color change and large absorption band shift have been observed for the disperse red-13 (DR-13) attached on the surface of the monodisperse silica spheres. Two step synthetic processes including urethane bond formation and hydrolysis-condensation reactions were used to attach the DR-13 on the surface of the silica spheres. After the reaction, the characteristic absorption peak at 2270 cm^-1 representing the –N=C=O asymmetric stretching vibration disappeared, and the a new absorption peak at 1700 cm^-1 corresponding the C=O stretching vibration appeared. A visual and reversible color change was observed before and after wetting in alcohol. Although the absorption peak of DR-13 in alcohol is at 510 nm, the absorption peak shifts to 788 nm when it is dried. The absorption peak shifts to 718 nm when it is wetted in alcohol. This result can be explained by the formation of intramolecular charge transfer band.

Nano-optical spectroscopic imaging of MoS2: probing 2D materials at the length scales that matter (Presentation Recording)

Ultrafast two-dimensional infrared spectroscopy (2D IR) spectroscopy is performed in attenuated total reflectance (ATR) geometry with the Kretschmann configuration in order to measure femtosecond to picosecond dynamics of self-assembled monolayers on gold-coated solid-liquid interfaces. In the monolayers low-absorbing (<200 M^-1 cm^-1) nitrile functional groups are used as local vibrational probes to monitor vibrational relaxation and spectral diffusion in dependence of different environments of the nitrile group. By comparing spectral diffusion dynamics of the vibrational probe in bulk solution and in the monolayer we find that the dynamics are slowed down by more than a factor of 20 upon immobilization of the sample. Moreover, spectral diffusion dynamics are affected by the local environment within the monolayers as evidenced by 2D ATR IR experiments on mixed monolayers with different aliphatic and aromatic co-adsorbates. The results are interpreted in terms of absent excitation energy-transfer as well as solvation dynamics around the nitrile vibrational probe. Our results demonstrate that 2D ATR IR spectroscopy offers the possibility to obtain ultrafast dynamics from sub-monolayer coverages of even low-absorbing vibrational probes such as nitrile functional groups.

In solar cells that incorporate semiconductor polymers as electron donors and fullerene derivatives as acceptors, a number of reports based on ultrafast optical probes reveal that charges can be generated on timescales significantly faster than ~100 fs in certain solid-state microstructures. Techniques that have been applied in these studies include variants of visible transient absorption and photoluminescence spectroscopy, terahertz spectroscopy, time-resolved infrared spectroscopy, and femtosecond stimulated Raman spectroscopy. These probes allow measurement of population dynamics of relevant photoexcitations (excitons, polarons) but do not reveal directly how these interact to produce photocarriers. Here, we present a non-linear coherent spectroscopy, photocurrent-detected two-dimensional spectroscopy (2DPC), which is an ultrafast optical thechnique belonging to a family of 2D Fourier- domain spectroscopies that allows measurement of correlations between optical transitions induced by short optical pulses. In our implementation, spectral correlations are detected via the time-integrated photocurrent produced in a photovoltaic diode. Four collinear ultrashort laser pulses (10 fs, centered at 600 nm in our experimental setup) excite the semiconductor polymer in the solar cell, with a variable delay that is independently controlled between each pulse in the sequence. Each pulse separately excites a quantum wavepacket with spectral phase and amplitude imparted by that pulse, while the effect of the pulse sequence is to collectively excite multiple quantum coherences. Interferences between the various combinations of the wavepackets determine linear and non-linear contributions to the material optical response. The fourth-order signal terms of the detected photocurrent are read using phase-sensitive detection schemes with reference waveforms corresponding to a modulation of specific phase combinations of the four femtosecond excitation pulses. By scanning the time delay between the pulses 1 and 2, as well as that between pulses 3 and 4 (coherence times), at a fixed delay between pulses 2 and 3 (population waiting time), one measures a two-dimensional coherence decay function that is Fourier transformed to produce a 2D photocurrent correlation excitation spectrum. Measurement of such spectra at different population waiting times provides insight into the role of spectral correlations and state coherence in photocurrent generation in such complex functional materials. We focus on solar cells produced by blends of a common carbazole-thiophene-benzothiadiazole polymer, PCDTBT (the donor polymer), and PCBM (the fullerene acceptor), in which we analyse the dynamics of total photocurrent generation via the time evolution of diagonal and off-diagonal spectral correlations. We address the role of vibronic coherence as well as resonant tunneling in charge separation pathways on ultrashort timescales.

Using an ultrafast electronic-vibrational pulse-sequence, we show that the outcome of light-induced ET can be radically altered by mode-specific infrared (IR) excitation of vibrations which are coupled to the ET-pathway. IR-control is particularly challenging in condensed phase systems due to the ultrafast timescales involved, in particular rapid intramolecular vibrational redistribution. We demonstrate how an IR-pulse following UV-excitation perturbs nuclear-electronic (vibronic) interactions within a donor-bridge-acceptor system similar in design to those utilized in (bio)chemical light-harvesting, and alters charge-transport pathways and product state yields.

We study exciton coupling and interconversion between neutral and charged states of different spin in pi-stacked conjugated polymer aggregates. Rigorous self-assembly approaches are used to prepare aggregate nanofibers that permit reliable control of polymer chain conformational and packing (intra- and interchain) order within these structures. Exciton coupling can be tuned between the H- and J-aggregate limits, which has important implications for determining the fates of excitons and polarons. Single molecule intensity modulation spectroscopy was performed on individual nanofibers and large quenching depths of emissive singlet excitons by triplets are found in J-aggregate type structures. We propose that high intrachain order leads to exciton delocalization that effectively lowers singlet-triplet energy splittings thus increasing triplet yields. Exciton-polaron and polaron-polaron interactions are next investigated in both H- and J-type nanofibers where polarons are injected by charge transfer doping. We find that the enhanced intrachain order of J-aggregates enables efficient intrachain polaron transport and leads to significantly larger doping efficiencies than less ordered H-aggregates. As polaron densities increase, signatures of spin-spin interactions between polarons on adjacent chains become appreciable leading to the formation of a spinless bipolaron. Overall, these studies demonstrate the potential for controlling and directing exciton and polaron interactions via tuning of subtle intra- and interchain ordering characteristics of aggregates, which could benefit various polymeric optoelectronic applications.

Nanometer scale iron particles have a variety of technological applications. They are vastly utilized in optical and microwave devices. Thin films with varying compositions of iron (III) nitrate and ethylene glycol were deposited on glass substrate using a spin coating technique. The thicknesses of the films were controlled by the spin rate. Precursor films on the substrate were then annealed to different temperatures ranging from 200°C to 600°C for 1-3 hours in air. The microstructures of iron particles in films prepared under different conditions were investigated using X-ray Absorption spectroscopy and Mossbauer spectroscopy. The main absorption edge peak position and pre-edge energy position were identical in samples with different numbers of layers, but prepared under similar conditions. This indicates that there was no change in the charge state of the iron regardless of the number of layers. However the intensity of the pre-edge feature decreases as the number of layers increases, which shows a decrease of Fe-O compounds as the number of layers increases. Mossbauer spectrum of these iron particles contains only quadrupole doublets. The absence of six-linespectrum confirms the nano-size nature of the particles.

The structural photomodification of the multicomponent cyanine films was studied. The spectrum of the film shows different bands of components, formed by dye molecules. Optical properties of the films were determined by the relative concentrations of the molecular forms and their orientation angles. Laser-induced changes in the absorption spectra of film associated with the variation of the relative concentrations of the forms and their orientation angles. The dependence of angle on the total photoexcitation energy had the form of saturating functions. Maximum limiting angles depend on the initial orientation angles of the component.

Second Harmonic Generation (SHG) a nonlinear optical process sensitive to medium structure deviation from centrosymmetry has been used to investigate the bulk and the surface of an aqueous phase. Using the combination of incoherent SHG, also named Hyper Rayleigh Scattering (HRS), and interface coherent SHG, we have investigated the neat air-water interface. In this paper, we report an analysis where the experimental conditions have been investigated to have the best contrast between the surface and the volume contributions. Our data are described within a simple model allowing us to normalize the surface contribution to the volume one.

Photothermal microscopy has achieved single molecule sensitivity. However, the analytes are usually restricted to be natural absorbers in the visible light region. Mid-infrared (MIR) imaging, on the other hand, provides a wealth of information, but encounters difficulties of diffraction-limited spatial resolution and scarcity of ideal detectors. Here we present Mid-IR photothermal heterodyne imaging (MIR-PHI) microscopy as a high sensitivity, super-resolution mid-IR imaging technique. In MIR-PHI, a tunable Mid- IR pulsed laser at 150 kHz is used to excite a micron sized particle. Energy relaxation creates a temperature gradient around the particle, changing the refractive index of the surrounding solvent and creating a thermal lens. A collinear, counter propagating probe beam (a 532 nm CW laser) is modified by the thermal lens and generates a super-resolution photothermal image. We studied 1.1 μm polystyrene beads at the single particle level using this technique. Various solvents with different heat capacities and refractive indices are tested for the best image contrast. The wide applicability and potentially high sensitivity of this technique make it promising for biological imaging and identification.

Transient absorption microscopy is an experimental technique that allows nanomaterials to be studied with ultrafast time resolution and diffraction limited spatial resolution. This paper describes recent results from using transient absorption microscopy to investigate energy relaxation processes in single metal and semiconductor nanowires. The processes that have been examined include charge carrier trapping in semiconductor nanostructures, the motion of surface plasmon polaritons in metal nanowires, and the damping of the acoustic breathing modes of metal nanowires by high viscosity solvents.

Interfacial adsorption and transport are the chemical and physical processes that underlie separations. Although separations technology accounts for hundreds of billions of dollars in the global economy, the process is not well-understood at the mechanistic level and instead is almost always optimized empirically. One of the reasons is that access to the underlying molecular phenomena has only been available recently via single-molecule methods. There are still interesting challenges because adsorption, desorption, and transport are all dynamic processes, whereas much of the advances in super-resolution imaging have focused on imaging static materials. Our lab has focused in recent years on developing and optimizing data analysis methods for quantifying the dynamics of adsorption and transport in porous materials at nanometer-resolution spatial scales. Our methods include maximizing information content in dynamic single-molecule data and developing methods to detect change-points in binned data. My talk will outline these methods, and will address how and when they can be applied to extract dynamic details in heterogeneous materials such as porous membranes.

As a phenothiazine derivative, Promethazine may undergo structural modifications when it is exposed to light. This process consists in the degradation of the initial compound and in the generation of new photoproducts with possible anti-infectious qualities. Stability studies are necessary in order to establish the proper use of drug solutions in different applications. At the same time, these investigations are important in the context of the generation of side-products induced by environmental conditions that bring new benefits to the compound.

This study reports the stability of Promethazine aqueous solutions, based on their absorption spectra acquired before and after Nd:YAG laser irradiation sessions or under different temperature and illuminating storage conditions. Samples of Promethazine solutions in ultrapure water, at a concentration range between 10^-6 M – 10^-2 M, were kept in dark at 22°C, and 4°C as well as at 22°C in ambient light up to a time interval of three months. Absorption spectra were recorded periodically in order to determine any changes of the optical properties. Also, solutions of 20 mg/mL were exposed for different time intervals to laser radiation emitted at 266 nm by the Nd:YAG laser. The stability of the optical properties of irradiated Promethazine solutions for 4 h was investigated up to two months.

The laser irradiated samples show similar but more rapid and intense changes compared to solutions exposed to ambient light, suggesting molecular modifications that could be due to the production of more polar phenothiazine derivatives.

To improve the efficiency of optoelectronic devices, it is critical to understand the carrier dynamics of photoactive materials and the mechanisms involved, including those effects caused by different surface stoichiometry and/or interfacial interactions. A good example is CdTe, which exhibits cost-effective high performance in thin-film photovoltaic cells; it is also known to show surface oxidation, which may affect device efficiency and hence limit the production methods used. In this contribution, we present ultrafast carrier dynamics of crystalline CdTe specimens with different surface conditions using transient reflectivity measurements, following a femtosecond above-gap excitation. The distinct differences observed in the dynamics and the time constants for oxidized and stoichiometrically restored specimens indicate the major role of surface tellurium oxide on the relaxation of photoinduced carriers. The much slower recovery observed on oxidized surfaces is attributed to a transfer (and trapping) of electrons to the tellurium atoms with a high oxidation state, which signifies a charge separation near the surface. To distinguish the effect caused by oxygen adsorption, we also examined the carrier dynamics of CdTe surfaces covered by a thin layer of water molecules for comparison. These results, which show clear interfacial effects, may have broader implications for the understanding of carrier dynamics in nanostructured and polycrystalline specimens under different chemical environments, as such materials exhibit a high surface-to-volume ratio.

In this paper, the Second Harmonic light intensity scattered off a liquid solution upon illumination by an incident fundamental frequency beam is written within a general framework in order to describe its coherent and incoherent contributions. It is shown that this formulation requires the introduction of a correlation function in time, position and orientation. We discuss this framework in light of recent experiments where the interface and the bulk of liquid solutions can be investigated simultaneously. We apply here this analysis to a neat water solution to compare the bulk volume and the interface correlation functions.

Organic electronics promise to provide flexible, large-area circuitry such as photovoltaics, displays, and light emitting diodes that can be fabricated inexpensively from solutions. A major obstacle to this vision is that most conjugated organic materials are miscible, making solution-based fabrication of multilayer or micro- to nanoscale patterned films problematic. Here we demonstrate that the solubility of prototypical conductive polymer poly(3-hexylthiophene) (P3HT) can be reversibly “switched off” using high electron affinity molecular dopants, then later recovered with light or a suitable dedoping solution. Using this technique, we are able to stack mutually soluble materials and laterally pattern polymer films using evaporation of dopants through a shadow mask or with light, achieving sub-micrometer, optically limited feature sizes. After forming these structures, the films can be dedoped without disrupting the patterned features; dedoped films have identical optical characteristics, charge carrier mobilities, and NMR spectra as as-cast P3HT films. This method greatly simplifies solution-based device fabrication, is easily adaptable to current manufacturing workflows, and is potentially generalizable to other classes of materials.

Gold nanoclusters hold many potential applications such as biosensing and optics due to their emission characteristics, small size, and non-toxicity. However, their low quantum yields remain problematic for further applications, and their fluorescence mechanism is still unclear. To increase the low quantum yields, various methods have been performed: doping, tuning structures, and changing number of gold atoms. In the past, most characterizations have been performed on spherical shaped nanoclusters; in this paper, several characterizations of various rod-shaped Au nanoclusters specifically on Au₂₅ are shown. It has been determined that the central gold atom in Au₂₅ nano-rod is crucial in fluorescence. Furthermore, single molecule analysis of silver doped Au₂₅ nano-rod revealed that it has more photo-stability than conjugated polymers and quantum dots.

Here, we show how the surface free energy of the electron-collecting oxide contact has a very pronounced effect on the nucleation free energy of solution-processed organolead halide perovskite thin films, which influences the crystal size/orientation, band-edge energies, conductivity and, ultimately, the performance of solar cell devices. While a great deal of the research community’s attention has been focused on the perovskite deposition methodology (e.g., starting precursors, annealing conditions, etc.), we demonstrate how the surface free energy of the oxide contact itself can be modified to control morphology and optoelectronic properties of the resulting hybrid perovskite thin films. The surface free energy of high-quality oxide contacts deposited by chemical vapor deposition (CVD) and atomic layer deposition (ALD) is modified by functionalization with a variety of self-assembled monolayers. We explore a number of deposition methodologies (e.g., a variety of single step and sequential step approaches) and their effect on the morphological and electronic properties of the resulting perovskite thin films deposited on these modified oxide contacts. Standard atomic force microscopy (AFM) and its conductive analog (cAFM) show how the oxide surface free energy ultimately affects the nanoscale morphology and charge transport characteristics of these semiconductor films. Photoelectron spectroscopy is used to elucidate the chemical composition (e.g., X-ray photoelectron spectroscopy - XPS), band edge energies (e.g., ultraviolet photoelectron spectroscopy - UPS), and the presence of gap states above the valence band (high sensitivity UPS measurements near the Fermi energy) of the hybrid perovskite materials as a function of the oxide surface free energy.

The interaction of light with molecular conduction junction is attracting growing interest as a challenging experimental and theoretical problem on one hand, and because of its potential application as a characterization and control tool on the other. From both its scientific aspect and technological potential it stands at the interface of two important fields: molecular electronics and molecular plasmonics. I shall review the present state of the art of this field and our work on optical response, Raman scattering, temperature measurements, light generation and photovoltaics in such systems.

Models in which each aromatic ring of a conjugated polymer is treated as a single site provide useful coarse grained models of electronic properties. The ability of such models to describe the lowest-energy, exciton, state is explored by parameterizing various forms of site models to data from time-dependent density functional theory (TDDFT). TDDFT data is generated for polythiophene oligomers with between 2 and 4 rings, and with randomly chosen tosional angles between adjacent rings. The best site models can reproduce the TDDFT data with a mean absolute error of about 0.13 eV. The coupling between adjacent rings is found to have a cosine dependence on the interplanar angle between these rings.

Hamaker-Lifshitz constants are material specific constants that are used to calculate van der Waals interaction forces between small particles in solution. Typically, these constants are size-independent and material specific. According to the Lifshitz theory, the Hamaker-Lifshitz constants can be calculated by taking integrals that include the dielectric permittivity, as a function of frequency, of the interacting particles and the medium around particles. The dielectric permittivity of interacting metal nanoparticles can be calculated using the Drude model, which is based on the assumption of motion of free conducting electrons. For bulk metals, the Drude model does not predict any sizedependence of the dielectric permittivity. However, the conducting electrons in small noble metal nanoparticles (R ~ 10nm) exhibit surface scattering, which changes the complex permittivity function. In this work, we show theoretically that scattering of the free conducting electrons inside silver and gold nanoparticles with the size of 1 – 50 nm leads to size-dependent dielectric permittivity and Hamaker-Lifshitz constants. We calculate numerically the Hamaker-Lifshitz constants for silver and gold nanoparticles with different diameters. The results of the study might be of interests for understanding colloidal stability of metal nanoparticles.

Ternary lipid bilayer systems assembled from mixtures of dipalmitoylphosphatidylcholine (DPPC), dioleoylphosphatidylcholine (DOPC), and cholesterol have been studied using coarse-grained molecular dynamics at biologically relevant temperatures (280 K to 310 K), which are between the chain melting temperatures of the pure lipid component. Free lipid bilayers were simulated using the MARTINI model (Stage I) and a variant with water-water interactions reduced to 76% (Stage II). The latter was subsequently used for preparing supported lipid bilayer simulations (Stage III). Clustering of like lipids was observed, but the simulation timescale did not yield larger phaseseparated domains.

Organic light-emitting diodes (OLEDs) have received a significant attention over the past decade due to their energy-saving potential. We have recently synthesized two novel carbazole-based donor-acceptor compounds and analyzed their optical properties to determine their suitability for use as blue emitters in OLEDs. These compounds show remarkable photo-stability and high quantum yields in the blue region of the spectrum. In addition, they have highly solvatochromic emission. In non-polar solvents, bright, blue-shifted (λmax ≈ 398 nm), and highly structured emission is seen. With increasing solvent dielectric constant, the emission becomes weaker, red-shifted (λmax ≈ 507 nm), and broad. We aim to determine the underlying cause of these changes. Electronic structure calculations indicate the presence of multiple excited states with comparable oscillator strength. These states are of interest because there are several with charge-transfer (CT) character, and others centered on the donor moiety. We theorize that CT states play a role in the observed changes in emission lineshape and may promote charge mobility for electrofluorescence in OLEDs. In the future, we plan to use Stark spectroscopy to analyze the polarity of excited states and transient absorption spectroscopy to observe the dynamics in the excited state.