Ursula Mazur

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.

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.

Structure and composition studies for silicon nitride thin films deposited by single ion bean sputter deposition

Abstract Silicon nitride films were deposited on Al/glass, NaCl, KBr, and silicon substrates by single ion beam sputter deposition. Films were prepared at various N2+ ion beam voltages over the range from 500 to 1200 V. The resulting films were characterized by using secondary ion mass spectrometry, Fourier transform infrared spectroscopy, transmission electron microscopy, and transmission electron diffraction. The stoichiometry of these silicon nitride films was controlled by adjusting the ion beam voltage. Films deposited under conditions of ion beam voltage below about 700 V were nitrogen rich while films made with beam voltages in excess of 900 V were silicon rich. The film structure changed with the substrate temperature from amorphous at room temperature to nanocrystalline/amorphous at 300 °C. The lowest occupied (LO) phonon band for stoichiometric Si3N4 was approximately 1124 cm−1 and the stoichiometry of thin silicon nitride films could be assessed with the LO phonon position.

A tunneling spectroscopy study of the adsorption of ferrocyanide from water solution by Al2O3

Abstract Tunneling spectra of Fe( 12 C 15 N) 4− 6 and Fe( 12 C 14 N) 4− 6 adsorbed on Al 2 O 3 from dilute H 2 16 O and 50% H 2 18 O solutions are compared to infrared and Raman spectra of the K 4 Fe(CN) 6 salt. C–N stretching frequencies and isotopic shifts indicate that the iron remains in the divalent state after adsorption. IETS provides more information than IR, and somewhat different information than Raman spectroscopy.

An iets study of alumina and magnesia supported —sihx and their use as substrates for inorganic vibrational spectroscopy

Abstract Tunneling spectra of Al 2 O 3 /—SiH x , MgO/—SiH x , Al 2 O 3 /—SiD x , and Al 2 O 3 /—SiH x + NCS − are reported. Analysis of the vibrational spectra observed from isotopic substitution of the barriers obtained by deposition of a thin film of SiO onto alumina and magnesia indicate that the supported species is —SiH. The Al 2 O 3 /—SiH barrier can be used as a support for studying inorganic ions by IETS.

Charge transfer induced chemical reaction of tetracyano-p-quinodimethane adsorbed on graphene

Raman spectroscopy was used to study the interaction of 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) deposited on graphene films grown by Chemical Vapor Deposition (CVD). Different thickness layers (1 ML to 7 ML) of TCNQ deposited on single layer graphene were investigated. A distinct blue shift of the G band of graphene was observed depending on the TCNQ coverage, which indicated charge transfer from graphene to TCNQ. No charge transfer phenomenon was observed when TCNQ was adsorbed on graphite, reflecting the intrinsic difference in the electronic structure of graphene and graphite. The vibrational modes of TCNQ were identified. Moreover, new Raman bands not associated with TCNQ were discovered on the TCNQ/graphene sample and assigned to vibrational modes of α,α-dicyano-p-toluoylcyanide (DCTC−1). Our observations indicate that charge transfer occurs between graphene and TCNQ followed by chemical reaction with the atmosphere to form DCTC−1. We estimate the transfer of ≤0.03 electrons per C atom to the TCNQ adlayer. This study clearly demonstrates the potential of TCNQ to produce p-type graphene. It also demonstrates that the charged species at the graphene/organic layer may be readily susceptible to chemical reaction and that the resulting surface species may be more complex than a simple negative (or positive) ionization of the parent adsorbate. The final charge state of the graphene may depend upon the free energy change associated with the surface chemical reaction. Any detailed understanding of the graphene–adsorbate charged interface must include a thorough chemical study of the potential redox reactions that can occur following deposition of the primary organic species.

Highly ordered thin films prepared with octabutoxy copper phthalocyanine complexes

Langmuir-Blodgett (LB) films of copper (II) 1,4,8,11,15,18,22,25-octabutoxyphthalocyanine, nCuPc(OBu)(8), (non-peripheral substitution) and copper (II) 2,3,9,10,16,17,23,24-octabutoxyphthalocyanine, pCuPc(OBu)(8), (peripheral substitution), were fabricated and characterized by optical spectroscopy and scanning probe microscopy. The LB films were transferred onto hydrophilic substrates by vertical dipping. Although they posses relatively short polar substituents both compounds form smooth, uniform, dense, and highly stable LB monolayers composed of linear arrays of cofacial oligomers. The long range discotic assemblies of LB and spun cast films of pCuPc(OBu)(8) and nCuPc(OBu)(8) posses physical and chemical properties favorable for molecular electronic device application.

Atomic force microscopy observation of the morphology of tetracyanoquinodimethane (TCNQ) deposited from solution onto the atomically smooth native oxide surface of Al(111) films

Abstract The morphology of tetracyanoquinodimethane (TCNQ) and of benzoic acid deposited from solutions onto atomically smooth native oxide surfaces of Al(111) films was observed by atomic force microscopy (AFM). A surface height analysis (bearing analysis) of the AFM images of TCNQ-deposited atomically smooth oxidized Al films gave a bimodal height distribution of the adsorbed TCNQ on the surfaces. The height distribution showed that TCNQ is adsorbed on the oxide surface as both a uniform film on a nanometer scale and micrometer-sized particles with the height ranging from 10 to 100 nm above the surface. The large particles are easily identified by AFM as microcrystallites of neutral TCNQ and they have many morphologies. The uniform film with the nanometer scale height distribution results predominantly from corrosion of the oxide surface by the TCNQ anion formation reaction. The volumes of the large particles and of the thin uniform film were obtained separately from the analysis of the bearing histogram and bearing area curve. Generally, the volume (approx. 2 μm 3 for a 30×30-μm 2 area) of the thin uniform film is 4–5 times larger than the volume of one monolayer of TCNQ or TCNQ salt. Benzoic acid treated surfaces, on the other hand, did not show a bimodal distribution nor large particles, and surface roughness is consistent with monolayer formation of aluminum benzoate. Discrepancies in inelastic electron tunneling spectroscopy and X-ray photoelectron spectroscopy results for TCNQ adsorption on alumina were discussed.