Beatriz H. Cardelino

Growth of bulk single crystals of organic materials for nonlinear optical devices - An overview

Abstract Highly perfect single crystals of nonlinear optical organic materials are required for use in optical devices. An overview of the bulk crystal growth of these materials by melt, vapor, and solution processes is presented. Additionally, methods that may be used to purify starting materials, detect impurities at low levels, screen materials for crystal growth, and process grown crystals are discussed.

Experimental and theoretical study of the structure of N, N-dimethyl-4-nitroaniline derivatives as model compounds for non-linear optical organic materials

Abstract Molecular and crystal structure of a series of derivatives of N , N -dimethyl-4-nitroaniline has been studied by both X-ray diffraction method and high-level ab initio calculations. According to these data, the dimethylamino groups were found to have a trigonal-pyramidal configuration and are considerably turned with respect to the ring plane in all molecules having a substituent in the ortho -position; on the contrary, this group is planar in the meta -substituted molecules. Topological analysis of the electron density function for all molecules studied within the framework of Baders ‘atoms in molecules’ (AIM) theory revealed that introduction of a substituent into the ortho - or meta -position of the ring results in increasing of the contribution of the resonance forms different from the quinoid one. Contribution of the latter form is predominant for the structure of N , N -dimethyl-4-nitroaniline ( 1 ). Topological analysis of the electron density distribution was used to explain a decreasing of the molecular hyperpolarisabilites of the ortho - and meta -substituted compounds as compared with those for 1 .

Positive chemical ionization triple-quadrupole mass spectrometry and ab initio computational studies of the multi-pathway fragmentation of phthalates

We report the first positive chemical ionization (PCI) fragmentation mechanisms of phthalates using triple-quadrupole mass spectrometry and ab initio computational studies using density functional theories (DFT). Methane PCI spectra showed abundant [M + H](+), together with [M + C(2)H(5)](+) and [M + C(3)H(5)](+). Fragmentation of [M + H](+), [M + C(2)H(5)](+) and [M + C(3)H(5)](+) involved characteristic ions at m/z 149, 177 and 189, assigned as protonated phthalic anhydride and an adduct of phthalic anhydride with C(2)H(5)(+) and C(3)H(5)(+), respectively. Fragmentation of these ions provided more structural information from the PCI spectra. A multi-pathway fragmentation was proposed for these ions leading to the protonated phthalic anhydride. DFT methods were used to calculate relative free energies and to determine structures of intermediate ions for these pathways. The first step of the fragmentation of [M + C(2)H(5)](+) and [M + C(3)H(5)](+) is the elimination of [R-H] from an ester group. The second ester group undergoes either a McLafferty rearrangement route or a neutral loss elimination of ROH. DFT calculations (B3LYP, B3PW91 and BPW91) using 6-311G(d,p) basis sets showed that McLafferty rearrangement of dibutyl, di(-n-octyl) and di(2-ethyl-n-hexyl) phthalates is an energetically more favorable pathway than loss of an alcohol moiety. Prominent ions in these pathways were confirmed with deuterium labeled phthalates.

Hydrogen rearrangement and ring cleavage reactions study of progesterone by triple quadrupole mass spectrometry and density functional theory.

The fragmentation mechanisms of progesterone have been studied by triple quadrupole tandem mass spectrometry (MSMS) and density functional theory (DFT). Mechanisms leading to major product ions are proposed. The data suggest that progesterone fragments preferentially via hydrogen and other rearrangements lead to neutral losses. These fragmentations are quite complex and are preceded by σ-bond cleavages in most cases. Four major pathways for progesterone fragmentation are proposed involving: (1) cleavage of ring B at C9-C10, (2) cleavage of C6-C7 bond in ring B through m/z 191, (3) two types of cleavages of ring D, and (4) ketene elimination in ring A. Pathways (1)-(3) proceed via charge-remote fragmentations while pathway (4) proceeds via charge-site initiated mechanism. The geometry of product ions in these pathways were optimized using DFT at the B3LYP/6-311G(d,p) level of theory from which the free energies of the pathways were calculated. The effect that the choice of basis sets and density functionals has on the results was tested by performing additional calculations using B3LYP/6-31G(d) and B3PW91/6-311G(d,p).

MOLECULAR STATIC THIRD-ORDER POLARIZABILITIES OF CARBON-CAGE FULLERENES AND THEIR CORRELATION WITH THREE GEOMETRIC PROPERTIES : SYMMETRY, AROMATICITY, AND SIZE

Abstract The static third-order polarizabilities ( γ ) of C 60 , C 70 , five isomers of C 78 and two isomers of C 84 were analyzed in terms of three properties, from a geometric point of view: symmetry; aromaticity; and size. The polarizability values were based on the finite field approximation using a semiempirical Hamiltonian (AM1) and applied to molecular structures obtained from density functional theory calculations. Symmetry was characterized by the molecular group order. The selection of six-member rings as aromatic was determined from an analysis of bond lengths. Maximum interatomic distance and surface area were the parameters considered with respect to size. Based on triple linear regression analysis, it was found that the static linear polarizability ( α ) and γ in these molecules respond differently to geometrical properties: α depends almost exclusively on surface area while γ is affected by a combination of number of aromatic rings, length and group order, in decreasing importance. In the case of α , valence electron contributions provide the same information as all-electron estimates. For γ , the best correlation coefficients are obtained when all-electron estimates are used and when the dependent parameter is ln( γ ) instead of γ .

Advanced computational modeling for growing III-V materials in a high-pressure chemical vapor-deposition reactor

A numerical model was developed to simulate vapor deposition in high-pressure chemical vapor-deposition reactors, under different conditions of pressure, temperature, and flow rates. The model solved for steady-state gas-phase and heterogeneous chemical kinetic equations coupled with fluid dynamic equations within a three-dimensional grid simulating the actual reactor. The study was applied to indium nitride (InN) epitaxial growth. The steady-state model showed that at 1050-1290 K average substrate temperatures and 10 atm of total pressure, atomic indium (In) and monomethylindium [In(CH3)] were the main group III gaseous species, and undissociated ammonia (NH3) and amidogen (NH2) the main group V gaseous species. The results from numerical models with an inlet mixture of 0.73:0.04:0.23 mass fraction ratios for nitrogen gas (N2), NH3 and trimethylindium [In(CH3)3], respectively, and an initial flow rate of 0.17 m s-1, were compared with experimental values. Using a simple four-path surface reaction scheme, the numerical models yielded a growth rate of InN film of 0.027 μm per hour when the average substrate temperature was 1050 K and 0.094 μm per hour when the average substrate temperature was 1290 K. The experimental growth rate under similar flow ratios and reactor pressure, with a reactor temperature between 800 and 1150 K yielded an average growth rate of 0.081 μm per hour, comparing very well with the computed values.

Development of an advanced computational model for OMCVD of indium nitride

An advanced computational model is being developed to predict the formation of indium nitride (InN) film from the reaction of trimethylindium (In(CH 3 ) 3 ) with ammonia (NH 3 ). The components are introduced into the reactor in the gas phase within a background of molecular nitrogen (N 2 ). Organometallic chemical vapor deposition occurs on a heated sapphire surface. The model simulates heat and mass transport with gas and surface chemistry under steady state and pulsed conditions. The development and validation of an accurate model for the interactions between the diffusion of gas phase species and surface kinetics is essential to enable the regulation of the process in order to produce a low defect material. The validation of the model will be performed in concert with a NASA-North Carolina State University project.

Second-order nonlinear optical crystal susceptibility: computational approach

A computer program has been developed to calculate crystal susceptibility tensor components. In addition to previous considerations where molecular polarization tensors have been treated as 1D or 2D, the present program allows the use of the 3D case that appears necessary for non-planar molecules. Calculation of crystal susceptibility is based on the approximation of relatively weak intermolecular forces in relation to intramolecular ones. Local field corrections have been estimated using simple Lorentz form. To provide molecular second-order polarizability, a semiempirical quantum chemical calculation has been carried out using the finite field method incorporated in the MOPAC program.

Aromaticity and conjugation effects on the nonlinear optical properties of multidimensional molecules

This investigation explores the effect that aromatic subgroups have on the nonlinear optical properties of highly conjugated multi-dimensional molecules. In particular, carbon-cage fullerenes, porphyrins and phthalocyanines have been studied The optimized geometries were determined from all-electron ab-initio calculations. The nonlinear properties were obtained using the finite field approximation. Data of polarization versus static electric field was obtained from valence-electron semi-empirical calculations using the AMI Hamiltonian. The static electric fields were created using a variety of conditions. Polynomial fits were performed with 14 to 400 data points. The nonlinear properties were extracted from expansions of order four to sixteen. These last three conditions allowed estimation and minimization of the uncertainty in the results. Aromaticity was evaluated by analyzing the molecular geometry.

The growth of InN and related alloys by high-pressure CVD

The growth of high-quality InN and indium rich group III-nitride alloys are of crucial importance for the development of high-efficient energy conversion systems, THz emitters and detectors structures, as well as for high-speed linear/nonlinear optoelectronic elements. However, the fabrication of such device structures requires the development of growth systems with overlapping processing windows in order to construct high-quality monolithic integrated device structures. While gallium and aluminum rich group III-nitrides are being successfully grown by organometallic chemical vapor deposition (OMCVD), the growth of indium rich group III-nitrides presents a challenge due to the high volatility of atomic nitrogen compared to indium. In order to suppress the thermal decomposition at optimum processing temperatures, a new, unique high-pressure chemical vapor deposition (HPCVD) system has been developed, allowing the growth of InN at temperatures close to those used for gallium/aluminum-nitride alloys. The properties of InN layers grown in the laminar flow regime with reactor pressures up to 15 bar, are reported. Real-time optical characterization techniques have been applied to analyze gas phase species and are highly sensitive the InN nucleation and steady state growth, allowing the characterization of surface chemistry at a sub-monolayer level. The ex-situ analysis of the InN layers shows that the absorption edge in the InN shifts below 0.7 eV as the ammonia to TMI precursor flow ratio is lowered below 200. The results indicate that the absorption edge shift in InN is closely related to the In:N stoichiometry.