Search

From studies of single-layer graphene, the authors find that atomic deuteration indeed does lead to reversible chemisorption. However, they find that atomic deuterium treatment of many-layer epitaxially grown graphene on C-face 4H-SiC only affects the surface graphene layer and the buried graphene/SiC interface. Raman and x-ray diffraction experiments reveal that only a small portion of the graphene is affected, showing no interlayer incorporation of deuterium. However, x-ray reflectivity and cross-sectional transmission electron microscopy demonstrate a change of the buried graphene/SiC interface, which resembles a delamination of graphene from the substrate. In some cases, multiple atomic treatments lead to complete delamination of the graphene film.From studies of single-layer graphene, the authors find that atomic deuteration indeed does lead to reversible chemisorption. However, they find that atomic deuterium treatment of many-layer epitaxially grown graphene on C-face 4H-SiC only affects the surface graphene layer and the buried graphene/SiC interface. Raman and x-ray diffraction experiments reveal that only a small portion of the graphene is affected, showing no interlayer incorporation of deuterium. However, x-ray reflectivity and cross-sectional transmission electron microscopy demonstrate a change of the buried graphene/SiC interface, which resembles a delamination of graphene from the substrate. In some cases, multiple atomic treatments lead to complete delamination of the graphene film.

The performance of organic field-effect transistors (OFETs) largely depends on the nature of interfaces of dissimilar materials. Metal–semiconductor interfaces, in particular, play a critical role in the charge injection process. This work demonstrates the unique potential of Raman and surface-enhanced Raman scattering (SERS) for the investigation of physical phenomena at the nanoscale in pentacene–metal interfaces in OFETs. A large enhancement in the Raman intensity (SERS) is observed from pentacene films under thermally evaporated Au films. Comparing experiments with density functional theoretical calculations of the Raman spectrum of pentacene indicates the presence of disordered sp2 carbons. Changes in the Raman spectra are further tracked after biasing the devices. Raman maps across the pentacene–Au interface provide a powerful visualization tool for correlating the device performance with structural changes of the molecule.

Two donor-acceptor diketopyrrolopyrrole (DPP)-based copolymers (PDPP-BBT and TDPP-BBT) have been synthesized for their application in organic devices such as metal-insulator semiconductor (MIS) diodes and field-effect transistors (FETs). The semiconductor-dielectric interface was characterized by capacitance-voltage and conductance-voltage methods. These measurements yield an interface trap density of 4.2 × 10(12) eV⁻¹ cm⁻² in TDPP-BBT and 3.5 × 10¹² eV⁻¹ cm⁻² in PDPP-BBT at the flat-band voltage. The FETs based on these spincoated DPP copolymers display p-channel behavior with hole mobilities of the order 10⁻³ cm²/(Vs). Light scattering studies from PDPP-BBT FETs show almost no change in the Raman spectrum after the devices are allowed to operate at a gate voltage, indicating that the FETs suffer minimal damage due to the metal-polymer contact or the application of an electric field. As a comparison Raman intensity profile from the channel-Au contact layer in pentacene FETs are presented, which show a distinct change before and after biasing.

Polyfluorenes reveal a complex interplay between emissive properties and intra- and intermolecular structure. Vibrational frequencies and intensities determined by Raman spectroscopy are strongly influenced by variations in the backbone as well as side chain conformations. Changes in the structural and electronic properties of two side group substi- tuted polyfluorenes (PF): ethyl-hexyl substituted PF (PF2/6) and dioctyl substituted PF (PF8) as a function of solvent, thermal cycling, and hydrostatic pressure via Raman scattering are presented. The vibrational frequencies and intensities of fluorene oligomers with various alkyl side chain conformations are calculated using hybrid density-functional theory. A comparison of the computed vibrational spectra of single chain fluorene oligomers with our experimental data shows that the conformational isomers in PF8 are a direct consequence of the side chain conformation.

Through an electron beam evaporation process, silver nanoparticles (Ag-nps) were introduced into dielectric thin-films. Silver incorporation into the oxide films resulted in a distribution of insulating Ag-nps at a density of ~10^12 particles/cm^2. For these samples, an enhancement of about two fold in dielectric constant was observed in both Al2O3 and HfO2. We attribute the dielectric constant enhancement to the polarizability of metallic nanoparticles. The dipole moment of the Ag-nps increased the stack dielectric constant in a frequency dependent manner. Because of the inherent relaxation frequency of the Ag-np dipoles and space-charge polarization, the dielectric constant was more strongly enhanced at lower frequencies (down to 100 Hz) relative to higher frequencies (up to 1 MHz). Gate leakage currents remained comparable between the control and nanocomposite samples. The Ag-nps also produced a memory effect. Potential applications of these Ag-np based films in CMOS devices are discussed.

Endohedral fullerenes belong to a new class of compounds which are technologically and scientifically important owing to their unique structures and optoelectronic properties. This review focuses on theoretical calculations and spectroscopic (electronic, vibrational, and nuclear magnetic resonance (NMR)) studies of endohedral fullerenes thus far published. A theoretical background, with various computational methods used for determining energy-optimized electronic structure and calculation of vibrational spectra, is presented. Further, theoretical and spectroscopic investigations of individual endohedral fullerenes are discussed. Such studies provide structural information about the carbon cage, position of the encapsulated species, and the degree of charge transfer. In particular, 13C NMR spectroscopy is indispensable for the determination of the cage symmetry. In some cases, NMR signals from 45Sc encapsulated species yield information about dynamic behavior inside the cage. Vis–NIR absorption spectra determine the HOMO–LUMO band-gap energy. IR and Raman spectroscopy play an important role in elucidating the nature of interaction between the cage and encapsulated species. Novel vibrations resulting from these interactions appear in the low-frequency region, and the corresponding force constants serve as a measure of the strength of their interaction.

The effects of hydrostatic pressure on conjugated polymers, in particular polyphenyls (see Figure for monomer unit) have been studied using photoluminescence, absorption, photo-induced absorption, and Raman spectroscopy. The effect of pressure on the singlet and triplet excitons and polarons allows an understanding of localized and delocalized electronic states. Changes in the intensity ratios of Raman bands that correspond to vibrations of a perpendicular and a coplanar array of phenyl rings in the chain, and comparison with calculated intensities, demonstrate the influence of pressure on the polymer's conformation.

We present a study of the Raman phonons in YBa{sub 2}(Cu{sub 0.96}Ni{sub 0.04}){sub 4}O{sub 8} under hydrostatic pressure in the superconducting phase. A comparison with our earlier work on the undoped YBa{sub 2}Cu{sub 4}O{sub 8} shows that the pressure coefficients of two vibrational modes involving oxygen atoms differ significantly from those of the corresponding modes in the undoped material. These are the O(1) chain mode which shifts 33{percent} faster and the O(2)-O(3) in-phase mode which shifts 23{percent} slower than the undoped counterparts. The other Raman modes in the Ni-doped sample shift in a manner similar to the undoped material. The observed behavior of the O(1) chain and O(2)-O(3) in-phase modes in YBa{sub 2}(Cu{sub 0.96}Ni{sub 0.04}){sub 4}O{sub 8} under pressure and the softening of the Cu(1) A{sub g} mode frequency with increasing Ni doping suggest that the Ni atoms substitute for the Cu atoms in the chain, which in turn decreases the compressibility in the vicinity of the Cu(Ni) chain atom. {copyright} {ital 1999} {ital The American Physical Society}

Low-operating voltage, high mobility, and stable organic field-effect transistors (OFETs) using polymeric dielectrics such as pristine poly(4-vinyl phenol) (PVP) and poly(methyl methacrylate) (PMMA), dissolved in solvents of high dipole moment, have been achieved. High dipole moment solvents such as propylene carbonate and dimethyl sulfoxide used for dissolving the polymer dielectric enhance the charge carrier mobilities by three orders of magnitude in pentacene OFETs compared with low dipole moment solvents. Fast switching circuits with patterned gate PVP-based pentacene OFETs demonstrated a switching frequency of 75 kHz at input voltages of |5 V|. The frequency response of the OFETs is attributed to a high degree of dipolar-order in dielectric films obtained from high-polarity solvents and the resulting energetically ordered landscape for transport. Remarkably, these pentacene-based OFETs exhibited high stability under bias stress and in air with negligible shifts in the threshold voltage. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1533–1542

The exciton diffusion length in organic semiconductors strongly impacts the efficiency of energy harvesting organic optoelectronics. We demonstrate an experimental technique to estimate the diffusion length of triplet excitons in a ladder-type conjugated organic semiconductor from quenching of the triplet-triplet absorption in the presence of acceptor molecules. The diffusion lengths are consistent with a phenomenological random-walk model. Experiments on two different molecular weights of the polymer sample show that the diffusion length of triplet excitons is in the few micrometers range and depends upon the size of the $\ensuremath{\pi}$-conjugated system.

The blue emission of ethyl-hexyl substituted polyfluorene (PF2/6) films is accompanied by a low energy green emission peak around 500 nm in inert atmosphere. The intensity of this 500 nm peak is large in electroluminescence (EL) compared to photoluminescence (PL)measurements. Furthermore, the green emission intensity reduces dramatically in the presence of molecular oxygen. To understand this, we have modeled various nonradiative processes by time dependent quantum many body methods. These are (i) intersystem crossing to study conversion of excited singlets to triplets leading to a phosphorescence emission, (ii) electron-hole recombination (e-hR) process in the presence of a paramagnetic impurity to follow the yield of triplets in a polyene system doped with paramagnetic metal atom, and (iii) quenching of excited triplet states in the presence of oxygen molecules to understand the low intensity of EL emission in ambient atmosphere, when compared with that in nitrogen atmosphere. We have employed the Pariser-Parr-Pople Hamiltonian to model the molecules and have invoked electron-electron repulsions beyond zero differential approximation while treating interactions between the organic molecule and the rest of the system. Our time evolution methods show that there is a large cross section for triplet formation in the e-hR process in the presence of paramagnetic impurity with degenerate orbitals. The triplet yield through e-hR process far exceeds that in the intersystem crossing pathway, clearly pointing to the large intensity of the 500 nm peak in EL compared to PL measurements. We have also modeled the triplet quenching process by a paramagnetic oxygen molecule which shows a sizable quenching cross section especially for systems with large sizes. These studies show that the most probable origin of the experimentally observed low energy EL emission is the triplets.

Dioctyl-substituted polyfluorene (PF) is especially well known for the presence of an unusual spectral feature identified as the beta phase, known to originate in regions of enhanced chain planarity. Although this phase appears as a minority constituent it dominates the emission, resulting in a red shift of the luminescence. We present Raman scattering studies of poly[9,9'-(di n, octyl) fluorene] as a function of thermal cycling that establish a connection between the conformational isomers and chain morphology. Density-functional theory calculations of the vibrational spectra of single chain oligomers in conjunction with our experimental results demonstrate the incompatibility of the beta phase with the overall alpha crystalline phase. Further, electroluminescence and photoluminescence measurements from PF-based light-emitting diodes are presented and discussed in terms of the crystalline phases and chain morphologies in the PFs.

Diketopyrrolopyrrole (DPP) containing copolymers have gained a lot of interest in organic optoelectronics with great potential in organic photovoltaics. In this work, DPP based statistical copolymers, with slightly different bandgap energies and a varying fraction of donor-acceptor ratio are investigated using monochromatic photocurrent spectroscopy and Fourier-transform photocurrent spectroscopy (FTPS). The statistical copolymer with a lower DPP fraction, when blended with a fullerene derivative, shows the signature of an inter charge transfer complex state in photocurrent spectroscopy. Furthermore, the absorption spectrum of the blended sample with a lower DPP fraction is seen to change as a function of an external bias, qualitatively similar to the quantum confined Stark effect, from where we estimate the exciton binding energy. The statistical copolymer with a higher DPP fraction shows no signal of the inter charge transfer states and yields a higher external quantum efficiency in a photovoltaic structure. In order to gain insight into the origin of the observed charge transfer transitions, we present theoretical studies using density-functional theory and time-dependent density-functional theory for the two pristine DPP based statistical monomers.

Highly luminescent semiconductor quantum dots have been synthesized in porous materials with ultraviolet and x-ray lithography. For this, the pore-filling solvent of silica hydrogels is exchanged with an aqueous solution of a group II metal ion together with a chalcogenide precursor such as 2-mercaptoethanol, thioacetamide or selenourea. The chalcogenide precursor is photodissociated in the exposed regions, yielding metal chalcogenide nanoparticles. Patterns are obtained by using masks appropriate to the type of radiation employed. The mean size of the quantum dots is controlled by adding capping agents such as citrate or thioglycerol to the precursor solution, and the quantum yield of the composites can be increased to up to about 30% by photoactivation. Our technique is water-based, uses readily available reagents, and highly luminescent patterned composites are obtained in a few simple processing steps. Polydispersity, however, is high (around 50%), preventing large-scale usage of the technique for the time being. Future developments that aim at a reduction of the polydispersity are presented.

Crystallization of amorphous silicon (a-Si) thin film occurred by the self-propagation of copper oxide/aluminum thermite nanocomposites. Amorphous Si films were prepared on glass at a temperature of 250°C by plasma enhanced chemical vapor deposition. The platinum heater was patterned on the edge of the substrate and the CuO∕Al nanoengineered thermite was spin coated on the substrate that connects the heater and the a-Si film. A voltage source was used to ignite the thermites followed by a piranha solution (4:1 of H2SO4:H2O2) etch for the removal of residual products of thermite reaction. Raman spectroscopy was used to confirm the crystallization of a-Si.