Proceedings Volume 8459

Physical Chemistry of Interfaces and Nanomaterials XI

Jenny Clark, Carlos Silva
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Proceedings Volume 8459

Physical Chemistry of Interfaces and Nanomaterials XI

Jenny Clark, Carlos Silva
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Volume Details

Date Published: 11 October 2012
Contents: 7 Sessions, 14 Papers, 0 Presentations
Conference: SPIE NanoScience + Engineering 2012
Volume Number: 8459

Table of Contents

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Table of Contents

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  • Front Matter: Volume 8459
  • Excitons in Organic Semiconductors I
  • Excitons in Organic Semiconductors II
  • Electronic Processes at Nanostructured Interfaces II
  • Electronic and Photonic Processes at Model Interfaces
  • Interfaces in Quantum-Confined Nanostructures II
  • Poster Session
Front Matter: Volume 8459
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Front Matter: Volume 8459
This PDF file contains the front matter associated with SPIE Proceedings Volume 8459, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.
Excitons in Organic Semiconductors I
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Optical signatures of the interplay between intermolecular and intramolecular coupling in plastic semiconductors
Francis Paquin, Maciej Sakowicz, Natalie Stingelin, et al.
Polymeric semiconductors such has regioregular poly(3-hexylthiophene) have electronic proprieties that can be tuned by proper control of the solid-state microstructure. We process thin films of P3HT of different molecular weight ranging from 2 kg/mol to 341 kg/mol. The polymer undergo a transition from a paraffinic, non-entangled microstructure to a two-phase microstructure defined by entangled chains embedded in amorphous regions at around 50 kg/mol. We observe an abrupt decrease in the intermolecular coupling from an average of ~20 meV for molecular weight below 50 kg/mol to ~5 meV above 50 kg/mol. We assign this decrease in the interchain coupling and associated free-exciton bandwidth at higher molecular weight to a transition from a one-phase morphology to a two-phase morphology defined above. In steady-state photoluminescence, we associate the lower Huang-Rhys factors at higher molecular weight to more planar backbone.
Excitons in Organic Semiconductors II
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Singlet fission in carotenoid aggregates: insights from transient absorption spectroscopy
Chen Wang, Maria Angelella, Chun-Hung Kuo, et al.
The excited-state dynamics of three types of zeaxanthin aggregates are probed with transient absorption spectroscopy on the femtosecond-to-microsecond timescale. Triplet excited states form via singlet fission in all three aggregates within several hundred femtoseconds. The transient absorption spectra are consistent with an S2, but not S1, parent state for singlet fission. The quantum yield of triplet states in one of the weakly-coupled aggregates is at least 60-80% immediately following photoexcitation. The same aggregate has a 10-30% yield of S1 excited states, which have a dominant decay time of ~8 ps. For the strongly-coupled H-aggregate, a new transient absorption band with maximum 400−420 nm is found. The band is assigned to a triplet state with T1→Tn transition that is strongly exciton-coupled to either the S0→S2 transition of surrounding ground-state chromophores, or a T1→Tn transition of a nearby triplet excited state. The yield of triplet states could be 180% or more in the strongly coupled aggregate, as inferred from the absence of S1 signal. Fast annihilation depletes most of the triplet population in the aggregates on the picosecond timescale, however a measurable fraction persists beyond 1 μs.
Electronic Processes at Nanostructured Interfaces II
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Electronic energy transport in nanomaterials: influence of host structure
D. L. Andrews, J. S. Ford
The transport of electronic energy within molecular nanomaterials generally entails a multi-step migration of excitation between chromophores possessing readily distinguished and characterized absorption and fluorescence spectra, such that each step of the migration is well described by a standard Förster model. When the associated chromophores are sited within a superstructure of significantly different composition, the simplest picture of the host influence is commonly given in terms of a dependence on local refractive index. Such a representation is deployed for structures ranging from photosynthetic systems to a wide variety of multi-chromophore materials including light-harvesting dendrimers, but the oversimplification fails to register the electronic effect of material specifically in the vicinity of the energy transfer. In photosynthetic systems, for example, successive stages of energy transport can occur in very different portions of a protein superstructure. In this initial analysis the methods of quantum electrodynamical analysis are brought to bear on these general issues. Exploiting a state-sequence methodology, the development of theory extends earlier studies by several research groups. It leads to new results that allow the identification of specific optical and electronic attributes that can locally expedite or inhibit energy transport. One newly discovered feature is a significant interplay of influence between the local architecture, as determined by the disposition and relative orientations of the donor and acceptor chromophores, with the structural symmetry of the host material within which they reside.
Electronic and Photonic Processes at Model Interfaces
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Single-molecule studies of proton transfer in guest-host materials
Philip J. Reid, Erin A. Riley, Chelsea Hess, et al.
Although the promise of organic materials for a variety of photonic applications has been recognized for some time, the wide-spread use of these materials is limited by photochemical processes that result in irreversible material decomposition. In this talk, the use of single-molecule microscopy to investigate the photochemistry of organic molecules in guest-host systems is described. A new method for analyzing the photoluminescence intermittency (PI) or “blinking” exhibited by single emitters is presented. This method allows for a statistically robust method for analyzing PI data, and for determining if an external perturbation results in a significant modification of the blinking statistcs. This method is applied to the blinking exhibited by CdSe/CdS quantum dots (QDs) in poly(methyl methacrylate) and the rhodamine derivative violamine R (VR) isolated in single crystals of potassium acid phthalate (KAP). For the QDs our analysis demonstrates that the blinking statistics are not power-law distributed. For VR in KAP we find that deuterium substitution significantly alters the PI exhibited by VR consistent with proton-transfer contributing to PI.
Mechanism of two-photon fluorescence increment via cross-linked bovine serum albumin
C.-Y. Lin, C.-H. Lien, C.-Y. Chang, et al.
The two-photon excited fluorescence (TPEF) increments of two dyes via bovine serum albumin (BSA) microstructures fabricated by the two-photon crosslinking technique were investigated. One is Rose Bengal (RB) with a high nonradiative decay rate, while the other is Eosin Y with a low non-radiative decay rate. Experimental results demonstrate that the quantum yield and lifetime of RB are both augmented via crosslinked BSA microstructures. Compared with theoretical analysis, this result indicates that the non-radiative decay rate of RB is decreased; hence, the quenched effect induced by BSA solution is suppressed. However, the fluorescence lifetime of Eosin Y is acutely abated despite the augmented quantum yield for the two-photon crosslinking processing from BSA solution. This result deduces that the radiative decay rate increased. Furthermore, the increased TPEF intensity and lifetime of RB correlated with the concentration of fabricated crosslinked BSA microstructures through pulse selection of the employed femtosecond laser is demonstrated and capable of developing a zone-plate-like BSA microstructure.
Interfaces in Quantum-Confined Nanostructures II
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Wavefunction engineering in core-shell semiconductor nanocrystals: from fine-tuned exciton dynamics and suppressed Auger recombination to dual color electroluminescence
Sergio Brovelli, Florencio García-Santamaría, Ranjani Viswanatha, et al.
Using semiconductor nanocrystals (NCs) one can produce extremely strong spatial confinement of electronic wave functions not accessible with other types of nanostructures. As a result, NCs exhibit important physical properties which, in combination with the chemical stability and solution processability, make this class of functional materials particularly appealing for several technological fields, such as solid-state lighting, lasers, photovoltaics, and electronics. Generally, the tunability of their physical properties is achieved through particle-size control of the quantum confinement effect. Wavefunction engineering adds a degree of freedom for manipulating the physical properties of NCs by selectively confining the carriers in specific domains of the material, thereby controlling the spatial overlap between the electron and hole wavefunctions. This design has been applied to several material systems in different geometries and has been shown to successfully control the emission energy and recombination dynamics as well as to reduce nonradiative Auger recombination, a process in which, as a consequence of strong spatial confinement, the energy of one electron-hole pair is nonradiatively transferred to a third charge carrier. The focus of this presentation is on nanocrystal heterostructures that comprise a small CdSe core overcoated with a thick shell of wider-gap CdS. These quasi-type II structures show greatly suppressed Auger recombination, which allows us to realize broadband optical gain (extends over 500 meV)1, and are a remarkable class of model compounds for investigating the influence of nanoengineered electron-hole overlap on the exciton fine structure.2 We indeed recently showed that this quasi-type II motif can be used to tune the energy splitting between optically active (“bright”) and optically passive (“dark”) excitons due to strong electron-hole exchange interaction, which is typical of quantum-confined semiconductor nanocrystals. This design provides a new tool for controlling excitonic dynamics including absolute recombination time scales and temperature and magnetic field dependences separately from the confinement energy.

As a result of reduced Auger recombination, in combination with essentially complete suppression of energy-transfer in thick-shell NCs films, we recently fabricated bright, monochrome LEDs based on these nanostructures. Our results indicate that the luminance and efficiency can be improved dramatically by increasing the shell thickness without detrimental effects of increased turn-on voltage.3 Detailed structural and spectroscopic studies reveal a crucial role of interfaces on the Auger recombination process ion these heterostructures. Specifically, we observe a sharp transition to Auger-recombination-free behavior for shell thickness ~1.8-2.5 nm, accompanied by the development of an intense phonon mode characteristic of a CdSeS alloy.4 These results suggest that the likely reason for suppressed Auger recombination in these nanostructures is the “smoothing out” of the otherwise sharp confinement potential due to formation of a graded interfacial CdSeS layer between the CdSe core and the CdS shell, as was recently proposed by theoretical calculations by Cragg and Efros.5
Poster Session
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Positive magnetoresistance in hydrogenated amorphous alloys silicon nickel a- Si1-yNiy:H at very low temperature with magnetic field
Abdelfattah Narjis, Abdelhamid El Kaaouachi, Abdelghani Sybous, et al.
We present results of an experimental study of magnetoresistance phenomenon in an amorphous siliconnickel alloys a-Si1-yNiy:H (where y=0.23) on the insulating side of the metal-insulator transition (MIT) in presence of magnetic field up to 4,5T and at very low temperature. The electrical resistivity is found to follow the Efros-Shklovskii Variable Range Hopping regime (ES VRH) with T -1/2. This behaviour indicates the existence of the Coulomb gap (CG) near the Fermi level.
Variable range hopping in hydrogenated amorphous silicon-nickel alloys
Abdelfattah Narjis, Abdelhamid El Kaaouachi, Abdelghani Sybous, et al.
On the insulating side of the metal-insulator transition (MIT), the study of the effect of low Temperatures T on the electrical transport in amorphous silicon-nickel alloys a-Si1-yNiy:H exhibits that the electrical conductivity follows, at the beginning, the Efros-Shklovskii Variable Range Hopping regime (ES VRH) with T-1/2. This behaviour showed that long range electron-electron interaction reduces the Density Of State of carriers (DOS) at the Fermi level and creates the Coulomb gap (CG). For T higher than a critical value of temperature TC, we obtained the Mott Variable Range Hopping regime with T-1/4, indicating that the DOS becomes almost constant in the vicinity of the Fermi level. The critical temperature TC decreases with nickel content in the alloys.
Study of electrical conductivity and scale theory in metallic n-type GeSb
Abdelghani Sybous, Abdelhamid El Kaaouachi, Abdelfattah Narjis, et al.
We present measurements of the electrical conductivity of barely metallic n-type GeSb that are driven to the metal-insulator transition (MIT) by impurity concentration. The experiments were carried out at low temperature in the range (4.2 -0.066 K) and with impurity concentrations up 6.41017 cm−3 . On the metallic side of the MIT, the electrical conductivity is found to behave like σ =σ0 + mT1/2 down to 66 mK. Physical explanation to the temperature dependence of the conductivity is given in metallic side of the MIT using a competition between two effects involved in the mechanisms of conduction, like electron-electron interaction effect, and weak localization effect.
Negative magnetoresistance behaviour and variable range hopping conduction in insulating NbSi amorphous alloys at very low temperature with magnetic field
Abdelghani Sybous, Abdelhamid El Kaaouachi, Abdelfattah Narjis, et al.
We present results of an experimental study of magnetoresistance (MR) in insulating NbSi amorphous alloys sample showing Variable Range Hopping (VRH) conductivity ; The MR is found to be negative in a wide range of low temperature (4.2-20 K) and in the range of moderate magnetic fields (0-4 T). We made tentative analysis using three theoretical models which are the model of quantum interference, the model of Zeeman effect and the model of localized magnetic moments.
Variable range hopping conduction in insulating n-type InSb semiconductor
Abdelghani Sybous, Abdelhamid El Kaaouachi, Said Dlimi, et al.
Longitudinal and positive magnetoresistance behaviour was used to determine what of the Variable Range Hopping (VRH) conduction regime is found in insulating InSb sample, Mott VRH regime or Efros- Shklovskii (ES) VRH regime. Experimental results are reported on field longitudinal magnetoresistance in insulating n-type InSb sample in which range hopping occurs at low temperatures. Positive magnetoresistance associated with VRH conduction has been observed. Experimental data are tentatively compared with available theoretical models in the insulating regime.
Spectroscopic studies on ultra-thin films of indium tin oxide under electro-chemical modulation
Indium tin oxide (ITO) is one of the most used conductive transparent oxide (CTO). Its electrical and optical properties under different environments are crucial to several applications. In this study, single-mode, broadband, integrated optical waveguide (IOW) technique was used to investigate the spectroscopic properties of ultra-thin (~ 13 nm) ITO films under electrical potential modulation. Optical absorbance changes under cyclic voltammetry at various probing wavelengths were measured. Electric potential for minimum absorbance against wavelength was analyzed, and we observed a clear linear correlation between them. Understanding the mechanisms behind this unusual spectroscopic change under potential modulation may have an important impact on several technological applications of ITO.
Vanadium and rare metals extraction via nano-porous intercalation of porphyrine-contained asphaltenes
Vladimir F. Sapega, Michael K. Rafailov
Here we report a phenomenon of metal-organics nano-porous trapping that occurs via intercalation of high-molecular components of oil disperse system. Metal-organics, particularly porphyrines that are part of asphaltenes and resins may replace water molecules in nano-porous while intercalating in formation or substrate material - whatever it is clay, minerals or high porosity substrate. Such processes are naturally occurs in oil sands, specifically in so called poor grade oil sand ore that has relatively low mass concentration of bitumen. Mass concentration of trapped metal-organics strongly depends on surface characteristics, porosity and volume of nano-porous material. It may happen that practically all metal-organics such as vanadyl porphyrine contained asphaltenes are trapped in nano-porous. Therefore nano-porous metal trapping provides three benefits at once: first, it makes crude oil or bitumen practically vanadium and other metals free. Second, crude oil or bitumen after such asphaltene intercalation may contain much more of lighter fractions of carbohydrates and much less asphaltenes and resins then initial crude or bitumen. Third, nano-porous materials were metal-organics have been trapped is highly enriched ore for mining of V, Re, Mo, Ni etc metals.