K. W. Hipps

Scanning Tunneling Microscopy of Metal Phthalocyanines: d7 and d9 Cases

Scanning tunneling microscopy (STM) images of cobalt(II) phthalocyanine (CoPc), copper(II) phthalocyanine (CuPc), and mixtures of the two adsorbed on the Au(111) face are reported. Based upon the stability and ease of obtaining molecular images, CoPc appears to adsorb more strongly than CuPc on Au(111), but both species provide images showing submolecular structure. The mixed CoPc and CuPc films also provide high-quality images showing details of the internal structure of the metal phthalocyanine. A particularly exciting aspect of this work is the strong influence of the metal ion valence configuration on the observed tunneling images. Unlike CuPc, wherein the central metal appears as a hole in the molecular image, the cobalt atom in CoPc is the highest point (about 0.3 nm) in the molecular image. These data are interpreted as indicating that the Co(II) d7 system has significant d-orbital character near the Fermi energy while the Cu(II) d9 system does not. This interpretation is consistent with theoretica...

Differing HOMO and LUMO mediated conduction in a porphyrin nanorod.

In this communication we provide the first UHV-STM images and STM-based current-voltage (I-V) and orbital mediated tunneling spectroscopy (OMTS) data on a self-assembled porphyrin nanostructure at the single structure level. We will show that transverse conductivity over distances less than 10 nm can occur by barrier type tunneling but that long distance conduction solely occurs through the LUMO band. These nanorods are very highly rectifying.

Single Molecule Imaging of Oxygenation of Cobalt Octaethylporphyrin at the Solution/Solid Interface: Thermodynamics from Microscopy

For the first time, the pressure and temperature dependence of a chemical reaction at the solid/solution interface is studied by scanning tunneling microscopy (STM), and thermodynamic data are derived. In particular, the STM is used to study the reversible binding of O(2) with cobalt(II) octaethylporphyrin (CoOEP) supported on highly oriented pyrolytic graphite (HOPG) at the phenyloctane/CoOEP/HOPG interface. The adsorption is shown to follow the Langmuir isotherm with P(1/2)(298K) = 3200 Torr. Over the temperature range of 10-40 °C, it was found that ΔH(P) = -68 ± 10 kJ/mol and ΔS(P) = -297 ± 30 J/(mol K). The enthalpy and entropy changes are slightly larger than expected based on solution-phase reactions, and possible origins of these differences are discussed. The big surprise here is the presence of any O(2) binding at room temperature, since CoOEP is not expected to bind O(2) in fluid solution. The stability of the bound oxygen is attributed to charge donation from the graphite substrate to the cobalt, thereby stabilizing the polarized Co-O(2) bonding. We report the surface unit cell for CoOEP on HOPG in phenyloctane at 25 °C to be A = (1.46 ± 0.1)n nm, B = (1.36 ± 0.1)m nm, and α = 54 ± 3°, where n and m are unknown nonzero non-negative integers.

Interaction of wide band gap single crystals with 248 nm excimer laser radiation. IV. Positive ion emission from MgO and NaNO3

We report quadrupole mass‐selected, time‐of‐flight measurements of Mg+ from polished, single crystal MgO and Na+ from cleaved, single crystal NaNO3 exposed to 248 nm (5 eV) laser radiation. A large fraction of the ions emitted from these materials have energies well above the energy of the incident photon. As the fluence is raised from low values, the ion intensities show thresholdlike behavior with a high‐order fluence dependence (roughly sixth order). At still higher fluences, the fluence dependence of Mg+ from MgO decreases to roughly second order. We attribute these emissions to weakly bound ions adsorbed atop surface electron traps; when the underlying vacancy is photoionized, the adsorbed ion is electrostatistically ejected at high energy. We argue that several photons are required to ionize a surface electron trap beneath an adsorbed ion, accounting for the high‐order fluence dependence and satisfying conservation of energy. (Several 5 eV photons are required to produce a 10 eV ion.) We show that a...

Low adhesion, non-wetting phosphonate self-assembled monolayer films formed on copper oxide surfaces

Self-assembled monolayer (SAM) films have been formed on oxidized copper (Cu) substrates by reaction with 1H,1H,2H,2H-perfluorodecylphosphonic acid (PFDP), octadecylphosphonic acid (ODP), decylphosphonic acid (DP), and octylphosphonic acid (OP) and then investigated by X-ray photoelectron spectroscopy (XPS), contact angle measurement (CAM), and atomic force microscopy (AFM). The presence of alkyl phosphonate molecules, PFDP, ODP, DP, and OP, on Cu were confirmed by CAM and XPS analysis. No alkyl phosphonate molecules were seen by XPS on unmodified Cu as a control. The PFDP/Cu and ODP/Cu SAMs were found to be very hydrophobic having water sessile drop static contact angles of more than 140 degrees , while DP/Cu and OP/Cu have contact angles of 119 degrees and 76 degrees , respectively. PFDP/Cu, ODP/Cu, DP/Cu, and OP/Cu SAMs were studied by friction force microscopy, a derivative of AFM, to better understand their micro/nanotribological properties. PFDP/Cu, ODP/Cu, and DP/Cu had comparable adhesive force, which is much lower than that for unmodified Cu. ODP/Cu had the lowest friction coefficient followed by PFDP/Cu, DP/Cu, and OP/Cu while unmodified Cu had the highest. XPS data gives some indication that a bidentate bond forms between the alkyl phosphonate molecules and the oxidized Cu surface. Hydrophobic phosphonate SAMs could be useful as corrosion inhibitors in micro/nanoelectronic devices and/or as promoters for anti-wetting, low adhesion surfaces.

Morphology and orientation of nanocrystalline AlN thin films

Abstract This paper describes the surface morphology, composition, and structure of aluminum nitride (AlN) thin films formed by single ion-beam sputtering of Al. Either pure N 2 or a N 2 (75%) + H 2 (25%) gas mixture was used as the feed gas for the ion gun. The films formed from the hydrogen containing feed gas were deposited on substrates either at room temperature or at 200°C. The film surface was extremely smooth for films grown under all the deposition conditions: scanning force microscopy indicated ⩽ 1 nm average roughness over a 500 × 500 nm 2 area. Transmission electron microscopy examination of the films showed that they were highly textured: the c axis of the AlN grains is preferentially oriented perpendicular to the film/substrate interface. The films grown with pure N 2 consisted of grains in the order of 100 nm in diameter whereas films formed with a hydrogen containing feed gas had at least an order of magnitude smaller grain size.

A Single Molecule Level Study of the Temperature-Dependent Kinetics for the Formation of Metal Porphyrin Monolayers on Au(111) from Solution

Scanning tunneling microscopy was used to make the first molecular scale measurements of the temperature dependence of composition of an adlayer at the solution-solid interface. We conclusively demonstrate that metal porphyrins adsorb very strongly on Au(111) at the solution solid interface such that the monolayer composition is entirely kinetically controlled below about 100 °C. The barrier for desorption is so great in fact that a temperature of 135 °C is required to induce desorption over a period of hours. Moreover, cobalt(II) octaethylporphyrin (CoOEP) and NiOEP desorb at different rates from different sites on the surface. We have measured the rate constant for desorption of CoOEP into phenyloctane to be 6.7 × 10(-5)/s at 135 °C. On the basis of these measurements, an upper bound can be set for the desorption rate of NiOEP into phenyloctane as 6.7 × 10(-4)/s at 135 °C. For solutions of the order of 100 μM in NiOEP or CoOEP, a dense monolayer is formed within seconds, and the adsorption rate constants fall within 40% of each other. The structures of NiOEP and CoOEP monolayers are essentially identical, and the molecular spacing for both can be described by A = 1.42 ± 0.02 nm, B = 1.32 ± 0.02 nm, and α = 57° ± 2°. The solubility of CoOEP and NiOEP in phenyloctane at room temperature was measured to be 0.228 and 0.319 g/L, respectively.

Fabrication of graphene with CuO islands by chemical vapor deposition.

Graphene prepared on Cu foil by chemical vapor deposition was studied as a function of post growth cooling conditions. CuO islands embedded in the graphene film were discovered and studied by scanning electron microscopy, atomic force microscopy, and X-ray photoemission spectroscopy. It is shown that nanostructured holes can be formed within a graphene film by reduction using hydrogen cooling immediately after film growth. We also observe the formation of symmetrical oxide islands in these holes. This study provides an easy way to fabricate a graphene + CuO composite, and the method may be extended to other graphene based structures.

Investigation of the temperature dependence of ruthenocene photoluminescence

Abstract Computer analysis of the luminescence decay times of ruthenocene in the range of 4.2 to 77 K has yielded energies and decay parameters for the lowest excited states. The derived level pattern has been rationalized on the basis of a 3E1 (dd) term, split by ≈ 500 cm-1 by spin-orbit coupling into A2 + A1 + E1 + E2 components in order of increasing energy. From a preliminary Franck-Condon analysis of the well-structured luminescence observed at 4.2 K, the molecule is shown to be expanded in the excited A2 state, primarily along the principal symmetry axis.

Scanning tunneling microscopy of 1, 2, and 3 layers of electroactive compounds.

Bilayer and trilayer organic films grown on Au(111) were studied by scanning tunneling microscopy (STM). Studies were carried out under UHV conditions with the sample cooled to either 80 or 100K. Cobalt(II)phthalocyanine [CoPc] was deposited from vapor onto Au(111), followed by vanadyl phthalocyanine, VOPc. CoPc coverages studied were 0.5 and 1 monolayer, while VOPc coverages were about 0.5 monolayer. Constant current images were acquired at high tunneling gap resistance, of the order of 30GOmega. Two different physical structures were observed for VOPc on CoPc, and each had a characteristic I(V) curve indicating significantly different unoccupied state density. It is also demonstrated that the transmission factor for two layers of VOPc is not simply the product of the transmission factors for each layer.