Forrest L. Carter

The molecular device computer: Point of departure for large scale cellular automata

Abstract Switching is possible at the molecular size level because of the conformational changes that occur. Three of the most promising switching mechanisms include electron tunnelling in short periodic arrays, soliton switching and soliton valving. Assuming a 3-d architecture and molecular dimensions, memory and switching elements with densities of 10 15 to 10 18 elements per cc are possible. The active elements are connected together conceptionally with “molecular wires” like polysulfur nitride (SN) x and polyacetylene (CH) x . Simple cellular automata involving soliton propagation in conjugated systems would include soliton valves and cyclic configurations of valves. In the latter, soliton propagations becomes isomorphous with group operations giving rise to possible non-binary finite-state machines. The development of a molecular electron device (MED) synthetic capability in combination with the above devices would suggest that large 3-d arrays of parallel processors will be possible with automata, biological, and crystallographic implications.

Vacancy and charge ordering in the Th3P4 related structures

Abstract A tetragonal structural model for cationic vacancy ordering in the lanthanide sesquichalcogenides having the Th3P4 structure is developed from considerations involving the metal site stereohedra or Wigner-Seitz cells and tested by Madelung constant calculations. In addition the anion stereohedra are discussed as well as the Voronoi polyhedra for both metal and nonmetal sites. The electrostatic results include crystal potential calculations as a function of charge. Madelung constants and potentials were also calculated for charge ordering in semiconducting materials like Eu3S4 as a function of atomic position and c a ratio. In these materials electrostatic stability is associated with increasing vacancy and charge ordering.

Preparation of the anhydrous rare earth trichlorides, tribromides, and triiodides

Abstract A new method is presented for preparing an anhydrous rare earth trihalide using the reaction of excess mercuric halide with the rare earth metal. The trihalide products may be purified by vacuum distillation.

Gas phase and solid state X-ray photoelectron spectra of the substituted acetylenes (SF5)n C2 (CF3)2−n (n = 0, 1, 2): A comparison of the pentafluorosulfur and trifluoromethyl groups

Abstract The core level X-ray photoelectron spectra (XPS) of CF 3 CCCF 3 , CF 3 CCSF 5 and SF 5 CCSF 5 have been measured in the solid state. Gas phase spectra of CF 3 CCCF 3 and CF 3 CCSF 5 have also been obtained. The XPS data, interpreted with the point charge potential model and semiempirical MNDO (minimum neglect of differential overlap) molecular orbital calculations, indicate that the electron withdrawing effect of the −CF 3 group is greater than that of the −SF 5 group. Results further suggest that sulfur 3 d orbitals do not play a detectable role in the bonding or charge distribution in these molecules. Carbon 1 s linewidths of −CF 3 carbon atoms are found to be much narrower than those arising from the acetylenic carbon atoms. The narrower lines correlate with the much higher binding energy of the −CF 3 carbon atoms. Large shifts (nearly 1 eV) in heteroatom core level binding energy differences (for example, F 1 s — C 1 s ) between the gas phase and solid state data are observed. These shifts are attributed to solid state effects (Madelung potential, intermolecular bonding interactions, and/or extramolecular relaxation contributions). From these comparisons it is clear that solid state effects are not uniform in their influence on the photoionized sites in these molecules.

Atomic volume contraction in intermetallic hydride formers: A valuable new clue

Abstract Recent calculations of individual atomic volumes for rare earth intermetallics suitable for hydrogen storage provide valuable new clues in the search for new hydrogen storage materials and give new insights into factors which are important in hydride formation. Polyhedral atomic volumes (PAVs) calculated for good rare earth hydride formers involving a small transition element (nickel) show that the rare earth (RE) volume undergoes a large contraction (35–40%) as the partial coordination number coefficient for RE-Ni bonds approaches a value of unity (where only RE-Ni bonds exist for the RE). This volume contraction is primarily due to two effects: (1) an RE bonding electron rehybridization and (2) a size difference effect in which bonding electrons are attracted to the bond axis. This rare earth PAV contraction in LaNi 5 and its alloys is substantially reversed as they are fully hydrided. Lanthanum regains 95% of its elemental volume. Similar effects are discussed in detail for TiFe and TiCu hydrides. These considerations then suggest that intermetallic hydride formers are searched for among transition metal compounds which demonstrate a large volume contraction during compound formation.

Chemistry and microstructures: Fabrication at the molecular size level

Abstract The case for the interdisciplinary fabrication of small structures is made using molecular and atomic forces in a building-up approach as a complimentary alternative to the semiconductor carving-out mode. Not only does this approach permit the modular chemical fabrication of molecular conductors but also gives rise to new forms of “lithography” at the molecular size level. Several chemical sensors responding to femto quantities or less of reagents are proposed, as are new active molecular moieties for non-linear optical nano-composite structures fabricated via Langmuir-Blodgett techniques.

The application of gas phase and solid state X-ray photoelectron spectroscopy to the investigation of derivatives containing the repeating SN unit☆

Abstract Solid state X-ray photoelectron spectra of S 2 N 2 , S 4 N 4 , (SNBr 0.04 ) x , and (SNBr 0.25 ) x have been obtained and the gas phase spectrum of S 2 N 2 is also reported. Both the solid state and gas phase core level spectra, as well as MNDO and CHELEQ calculations, show that there is greater S → N charge transfer in S 4 N 4 than in S 2 N 2 . The solid state data indicate that the charge distributions in S 2 N 2 and (SN) x are the same. All of the data can be satisfactorily explained without recourse to N p π → S d π bonding. Bromination of an (SN) x film and single crystals results in complicated, broad N 1s and S 2p envelopes. Changes in the relative core level intensities on bromination suggest that the bromine resides largely between and/or on the (SN) x fibrils, rather than penetrating into the fibrils.

Bonding and polyhedral atomic volumes for the transition metal borides

Abstract The results of Paulings metallic radii and PAV calculations provide an estimate of effective charges, valences, atomic volumes, and non-integral coordination numbers for a wide selection of rare earth and transition metal binary borides. This uniform treatment suggests that effective charges for the transition metals are generally less than one, and that their valence is lower than the usual Pauling maximum of six. In addition, it was noted that the boron PAV volume was related to the non-isonomicity of the cell and that the use of ƒ orbitals in the BOA technique provides a useful discussion of bonding in some of the rare earth diborides not readily treated by s , p , and d hybridization alone.

Molecular level fabrication techniques and molecular electronics devices

Abstract In anticipation of the continued size reduction of switching elements to the molecular level, new approaches to materials, memory, and switching elements have been developed. In particular, switching moieties or groups of molecular size whose structures are conductive to switching phenomena, are emphasized. Techniques of building up from the molecular level using molecular forces are also suggested as an alternative to current semiconductor practice. New techniques for accomplishing lithography at the molecular scale must also be devised. Three approaches possible involving biological and Langmuir-Blodgett materials will be described.

Atomic Volume Contraction in Rare Earth Nickel Intermetallics as a Function of Partial Coordination Number Coefficient

Molecular volume contraction upon formation of rare earth inter-metallic compounds from the elements is a very well-known phenomenon. While a method for calculating intermetallic atomic volumes was introduced by the author in 1971 as a partial argument for predicting the unstability of SmCo5 (1) and again in 1974 in the discussion of the Pu2C3 structure (2), systematic studies have only recently included rare earth semimetals and intermetallics (3, 4). As might be expected from their large size it is the rare earth atoms that are primarily responsible for the volume contraction. The rare earth atom volume is found herein to decrease approximately linearly with respect to its coefficient of partial coordination number with nickel. This latter concept is a measure of the relative importance and number of rare earth-nickel bonds and arises from the generalization of the concept of coordination number (3, 5). The amount of the rare earth volume contraction is surprisingly large, i.e., greater than 30%. Nickel on the other hand shows a smaller volume expansion upon alloying. These volume changes are thought to be due primarily to size difference effects upon bonding density distribution.