Ilia N. Ivanov

A Compilation of Physical, Spectroscopic and Photophysical Properties of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAH),t also known as polynuclear aromatic hydrocarbons or polyarenes, constitute a large class of organic compounds. They are formed and released into the environment through natural and man-made sources. Natural sources include volcanoes and forest fire, while the man-made sources come from wood burning, automobile exhaust, industrial power generators, incinerators, production of coal tar, coke, asphalt and petroleum, incomplete combustion of coal, oil, gas, garbage, tobacco and charbroiled meat. Polycyclic aromatic heterocyclic analogs containing one or more nitrogen, oxygen or sulfur atoms are also present in substantial quantities in coal tar and petroleum residues produced during the refining process. In the atmosphere, they are principally generated from the combustion of fossil fuels, wood or forest burning, refuse burning and coal tar. Although anthropogenic sources account for the major portion of atmospheric PAH pollution (l), vehicle emissions are believed to be responsible for 35% of the total PAH emission in highly populated and industrialized urban areas of the United States (2). Deposition of PAH in surface and ground waters can take place from a variety of sources such as airborne PAH, municipal wastewater discharge, effluents from wood treatment plants and other industries, oil spills and petroleum pressing. Accumulation of PAH in soils is believed to result from atmospheric deposition after long-range transport. The concentration of PAH found in soil around urban and industrialized areas are sometimes up to two orders of magnitude higher than those in less-developed areas (derived from forest fires and airborne pollution). Once released into the environment, PAH can partition between air, water, soil or sediments. An extensive body of work on this subject has appeared in the literature in recent years. Representative examples include partitioning of PAH between the gas and suspended particle phase (3,4), aidwater

Real Space Mapping of Li-Ion Transport in Amorphous Si Anodes with Nanometer Resolution

The electrical bias driven Li-ion motion in silicon anode materials in thin film battery heterostructures is investigated using electrochemical strain microscopy (ESM), which is a newly developed scanning probe microscopy based characterization method. ESM utilizes the intrinsic link between bias-controlled Li-ion concentration and molar volume of electrode materials, providing the capability for studies on the sub-20 nm scale, and allows the relationship between Li-ion flow and microstructure to be established. The evolution of Li-ion transport during the battery charging is directly observed.

Perovskite Solar Cells with Near 100% Internal Quantum Efficiency Based on Large Single Crystalline Grains and Vertical Bulk Heterojunctions

Imperfections in organometal halide perovskite films such as grain boundaries (GBs), defects, and traps detrimentally cause significant nonradiative recombination energy loss and decreased power conversion efficiency (PCE) in solar cells. Here, a simple layer-by-layer fabrication process based on air exposure followed by thermal annealing is reported to grow perovskite films with large, single-crystal grains and vertically oriented GBs. The hole-transport medium Spiro-OMeTAD is then infiltrated into the GBs to form vertically aligned bulk heterojunctions. Due to the space-charge regions in the vicinity of GBs, the nonradiative recombination in GBs is significantly suppressed. The GBs become active carrier collection channels. Thus, the internal quantum efficiencies of the devices approach 100% in the visible spectrum range. The optimized cells yield an average PCE of 16.3 ± 0.9%, comparable to the best solution-processed perovskite devices, establishing them as important alternatives to growing ideal single crystal thin films in the pursuit toward theoretical maximum PCE with industrially realistic processing techniques.

Fast and highly anisotropic thermal transport through vertically aligned carbon nanotube arrays

This letter reports on fast and highly anisotropic thermal transport through millimeter-tall, vertically aligned carbon nanotube arrays (VANTAs) synthesized by chemical vapor deposition on Si substrates. Thermal diffusivity measurements were performed for both longitudinal and transverse to the nanotube alignment direction, with longitudinal values as large as 2.1±0.2cm2∕s and anisotropy ratios as large as 72. Longitudinal thermal conductivities of 15.3±1.8W∕(mK) for porous 8±1vol% VANTAs in air and 5.5±0.7W∕(mK) for epoxy-infiltrated VANTAs already exceed those of phase-changing thermal interface materials used in microelectronics. Data suggest that further improvements are possible through optimization of density and defects in the arrays.

White light-emitting diodes based on ultrasmall CdSe nanocrystal electroluminescence.

We report white light-emitting diodes fabricated with ultrasmall CdSe nanocrystals, which demonstrate electroluminescence from a size of nanocrystals (<2 nm) previously thought to be unattainable. These LEDs have excellent color characteristics, defined by their pure white CIE color coordinates (0.333, 0.333), correlated color temperatures of 5461-6007 K, and color rendering indexes as high as 96.6. The effect of high voltage on the trap states responsible for the white emission is also described.

Synthesis of Millimeter-Size Hexagon-Shaped Graphene Single Crystals on Resolidified Copper

We present a facile method to grow millimeter-size, hexagon-shaped, monolayer, single-crystal graphene domains on commercial metal foils. After a brief in situ treatment, namely, melting and subsequent resolidification of copper at atmospheric pressure, a smooth surface is obtained, resulting in the low nucleation density necessary for the growth of large-size single-crystal graphene domains. Comparison with other pretreatment methods reveals the importance of copper surface morphology and the critical role of the melting-resolidification pretreatment. The effect of important growth process parameters is also studied to determine their roles in achieving low nucleation density. Insight into the growth mechanism has thus been gained. Raman spectroscopy and selected area electron diffraction confirm that the synthesized millimeter-size graphene domains are high-quality monolayer single crystals with zigzag edge terminations.

PS‐b‐P3HT Copolymers as P3HT/PCBM Interfacial Compatibilizers for High Efficiency Photovoltaics

A conducting diblock copolymer of PS-b-P3HT was added to serve as a compatibilizer in a P3HT/PCBM blend, which improved the power-conversion efficiency from 3.3% to 4.1% due to the enhanced crystallinity, morphology, interface interaction, and depth profile of PCBM.

In situ growth rate measurements and length control during chemical vapor deposition of vertically aligned multiwall carbon nanotubes

Time-resolved reflectivity is employed as an in situ diagnostic in thermal chemical vapor deposition of vertically aligned arrays of multiwall carbon nanotubes (VAA–MWNT). Fabry–Ṕerot interference fringes and attenuation of a reflected HeNe laser beam are used to measure the length of VAA–MWNT throughout the first 3–8 μm of growth yielding in situ measurements of growth rates and kinetics and the capability to observe the onset and termination of growth. VAA–MWNT growth is characterized between 565 and 750 °C on Si substrates with evaporated Al/Fe/Mo multilayer catalysts and acetylene feedstock. Nanotube lengths were controlled by rapid evacuation of the chamber at predetermined reflectivities, and it was demonstrated that growth can be restarted at later times. The extinction coefficients of the VAA–MWNT were studied and correlated with nanotube wall structure. Growth rates for VAA–MWNT are found to vary depending on the catalyst preparation, temperature, and time. Both the highest growth rates (0.3 μm/s...

Electrical and thermal conductivity of low temperature CVD graphene: the effect of disorder

In this paper we present a study of graphene produced by chemical vapor deposition (CVD) under different conditions with the main emphasis on correlating the thermal and electrical properties with the degree of disorder. Graphene grown by CVD on Cu and Ni catalysts demonstrates the increasing extent of disorder at low deposition temperatures as revealed by the Raman peak ratio, IG/ID. We relate this ratio to the characteristic domain size, La, and investigate the electrical and thermal conductivity of graphene as a function of La. The electrical resistivity, ρ, measured on graphene samples transferred onto SiO2/Si substrates shows linear correlation with La(-1). The thermal conductivity, K, measured on the same graphene samples suspended on silicon pillars, on the other hand, appears to have a much weaker dependence on La, close to K∼La1/3. It results in an apparent ρ∼K3 correlation between them. Despite the progressively increasing structural disorder in graphene grown at lower temperatures, it shows remarkably high thermal conductivity (10(2)-10(3) W K(-1) m(-1)) and low electrical (10(3)-3×10(5) Ω) resistivities suitable for various applications.

Symmetry relationship and strain-induced transitions between insulating M1 and M2 and metallic R phases of vanadium dioxide.

The ability to synthesize VO2 in the form of single-crystalline nanobeams and nano- and microcrystals uncovered a number of previously unknown aspects of the metal-insulator transition (MIT) in this oxide. In particular, several reports demonstrated that the MIT can proceed through competition between two monoclinic (insulating) phases M1 and M2 and the tetragonal (metallic) R phase under influence of strain. The nature of such phase behavior has been not identified. Here we show that the competition between M1 and M2 phases is purely lattice-symmetry-driven. Within the framework of the Ginzburg-Landau formalism, both M phases correspond to different directions of the same four-component structural order parameter, and as a consequence, the M2 phase can appear under a small perturbation of the M1 structure such as doping or stress. We analyze the strain-controlled phase diagram of VO2 in the vicinity of the R-M2-M1 triple point using the Ginzburg-Landau formalism and identify and experimentally verify the pathways for strain-control of the transition. These insights open the door toward more systematic approaches to synthesis of VO2 nanostructures in desired phase states and to use of external fields in the control of the VO2 phase states. Additionally, we report observation of the triclinic T phase at the heterophase domain boundaries in strained quasi-two-dimensional VO2 nanoplatelets, and theoretically predict phases that have not been previously observed.