Interface Distances Research Articles

Spatial Resolution of a Type II Heterojunction in a Single Bipolar Molecule

Bipolar molecules incorporating donor and acceptor components within a single molecule create exciting device opportunities due to their possible use as nanoscale p-n heterojunctions. Here we report a direct characterization of the internal electronic structure of a single bipolar molecular heterojunction, including subnanometer features of the intramolecular donor-acceptor interface. Angstrom-resolved scanning tunneling spectroscopy was used to map the energy levels and spatial extent of molecular orbitals across the surface of an individual bipolar molecule, bithiophene naphthalene diimide (BND). We find that individual BND molecules exhibit type II heterojunction behavior with orbital energy shifts occurring over subnanometer intramolecular interface distances. Comparison of this behavior with first-principles theoretical modeling provides new insights into the optimization of these molecular systems.

Computer-simulated diamond/graphite structure based on the diffraction pattern

Theoretical diffraction patterns have been computed with the use of a known spatial structure of a spherical two-phase nanocrystal particle consisting of a diamond core and an outer graphite shell. This approach not only allowed direct determination of the structure of nanosized crystals but also revealed more exactly its minor details, namely, the effect of the interface (distances between outer diamond core atoms and inner graphite shell atoms) on the diffraction of two-phase nanoparticles.

Process during laser implantation and ablation of Coumarin 6 in poly (butyl methacrylate) films

Coumarin 6 (C6) molecules were implanted into poly butyl methacrylate (PBMA) polymer films following excitation using a pulsed dye laser coupled with a microscope. The mechanism of this microscopic laser induced molecular implantation method (LIMIT) is discussed on the basis of experimental parameters affecting the implanted area's size. The C6 molecules were ejected from a C6 source film facing a PBMA target film with various interface distances between them. The area size variation was observed in terms of the density distribution of the ejected C6 molecules in the plume and the plume shape was estimated at different laser fluences. The size and the shape of the ejected C6 molecules are discussed in terms of the processes taking place in the plume. Based on the obtained results, the implantation method was optimized so that the formation of extended fine-art quality green-color fluorescent patterns consisting of C6 molecular dots could be demonstrated.

First-Principles Study of Molecule/Al Interfaces

The structural and electronic properties of C n H 2n+2 /Al(110) interfaces (n = 5) have been studied by first-principles calculations using a plane-wave pseudopotential method coupled with an efficient electronic minimization scheme for large systems. We have examined the stability of vertical and parallel adsorption of a C 5 H 12 molecule on an Al(110) surface for various adsorption sites and initial interface distances. It has been found that image interactions between the C-H polar bonds and the metal surface with physorption characters dominate the interfacial interactions for both the vertical and parallel cases of the C 5 H 12 /Al interfaces. However, for the C 5 H 11 molecule with a dehydrogenated terminal C atom, we have observed the formation of a strong interfacial C-Al bond with both covalent and ionic characters.

Comparison of exact and asymptotic results for the focusing of electromagnetic waves through a plane interface

Recently, exact Kirchhoff solutions and the corresponding asymptotic solutions for the focusing of electromagnetic waves through a plane interface between two different dielectrics were reported. But the computation of exact results takes a long time because it requires the quadruple integration of a rapidly oscillating integrand. By using asymptotic techniques to perform two of the integrations, one can reduce the computing time dramatically. Therefore it is important to establish the accuracy and the range of validity of the asymptotic technique. To that end, we compare the exact and the asymptotic results for high-aperture, near-field focusing systems with a total distance from the aperture to the focal point of a few wavelengths and with a distance from the aperture to the interface as small as a fraction of a wavelength. The systems examined have f-numbers in the range from 0.6 to 0.9 and Fresnel numbers in the range from 0.4 to 3.5. Our results show that the accuracy of the asymptotic method increases with the aperture-interface distance when the aperture-focus distance is kept fixed and that it increases with the aperture-focus distance when the aperture-interface distance is kept fixed. To an accuracy of 7.8%, the asymptotic techniques are valid for aperture-interface distances as small as 0.5lambda as long as the total distance from the aperture to the focal point exceeds 8lambda. It is also shown that an accuracy of better than 1% can be obtained for the same aperture-interface distance of 0.5lambda and for interface-observation-point distances as small as 0.1lambda as long as the total distance from the aperture to the focal point exceeds 12lambda. By use of the asymptotic technique the computing time is reduced by a factor of 10(3).

Thermal-wave imaging of epoxy/aluminum interfaces

Laser-excited thermal-wave image magnitude and phase were acquired by photoacoustic detection in the 10–1000-Hz modulation frequency range on epoxy/aluminum interface systems with surface (black) and bulk (white) laser-absorbing epoxy, different planar interface distances, surface and subsurface flaws, and different surface angle boundaries. The following conclusions were reached from the thermal-wave results: (1) thermally thin epoxy on aluminum layer thicknesses can be estimated from variable modulation frequency data; (2) planar and subsurface defect interface structures of epoxy/aluminum systems are best studied by examining both magnitude and phase images; (3) thermal-wave images are more straightforward to interpret for the systems that absorb light at the irradiated surface rather than in the bulk. These conclusions are consistent with thermal diffusion length and thermal-wave reflection effects predicted by theory for surface light-absorption conditions.

Stress behaviour in the vicinity of a crack approaching a bimaterial interface

The integral approach of Cook and Erdogan was used to obtain the asymptotic behaviour of a crack close to a bimaterial interface. It was found that, in addition to the square-root singularity which dominates the stresses in the vicinity of a conventional crack, there are two additional contributions to stress which should not be neglected. One contribution is due to an “image singularity” and the second is due to an O(1) term which can become very large for small tip to interface distances. It is shown how these features of the stress distribution can be used to advantage in the numerical calculation of stress intensity factors.

The effect of some composite structures on the thermal resistance of substrates and integrated circuit chips

An exact three-dimensional solution to the differential equation for steady-state heat transfer in composite media has been derived. Detailed results for 0.050-in-square chips mounted on 1- and 2-in-square substrates show that the thermal resistance of alumina substrates may be expected to increase by as much as 80 percent upon application of a glaze to the surface. The thermal resistances of beam lead chips varying in size from 0.050-to 0.150-in square are determined for chip-substrate interface distances from 0.0005 to 0.002 in, both with and without a thermal conducting compound as the interface media.

The identification of the crystal systems by the method of `constant parameters'

A new general analytical method is suggested for identifying a crystal system from a powder X-ray pattern with a computer. The formulae relating interferential indices and interplanar spacing of a line with the definite ratio of parameters of the Bravais cell for each crystal system are given. The identification of the system is carried out by selection of three definite lines, indices and interface distances, all of which satisfy the corresponding formulae.