James E. Whitten

Encapsulation of Zinc Oxide Nanorods and Nanoparticles

A simple method for encapsulating zinc oxide nanoparticles within an organic matrix is described that consists of dispersing them in an ethanolic solution, adding an organothiol, and stirring while heating. Electron microscopy, photoemission, Raman spectroscopy, and thermal gravimetric analyses demonstrate that partial dissolution of the oxide occurs, accompanied by encapsulation within a matrix consisting of a 1:2 zinc/thiol complex. Using this methodology, it is possible to surround ZnO within diverse matrices, including fluorescent ones. The process is demonstrated for 1-dodecanethiol (DDT) and fluorescent 2-naphthalenethiol (NPT). For DDT, ZnO nanorods become surrounded by a layer of the zinc-thiol complex that is greater than 100 A thick. In the case of NPT, significantly greater dissolution of the ZnO occurs, with the encapsulated rods taking on a spherical geometry, as evidenced by electron microscopy.

Prevention of the cathode induced electrochemical degradation of [Ru(bpy)3](PF6)2 light emitting devices

Light emitting electrochemical cells based on the tris(2,2′ bipyridyl) ruthenium(II) complex show improved performance if electrochemically stable materials such as Ag are used as the cathode material. In contrast, if Al is used as the cathode such devices undergo degradation when stored in the off-state in inert atmosphere. In this work, the mechanism of the aluminum-induced degradation is investigated. X-ray photoelectron spectroscopy shows that some of the Ru(II) complexes are reduced in the presence of the Al cathode to Ru(I). In addition, secondary ion mass spectrometry depth profiles indicate degradation of the indium tin oxide in devices with Al cathodes. Because of the mixed-valent Ru(II)/(I) states, devices with Al cathodes exhibit unipolar charge injection at voltages below the turn-on voltages. The unipolar charge injection can be described by a theory of charge hopping in mixed-valent redox systems. In addition, impedance analysis data at 0 V bias of devices with Al or Ag cathodes can be fit u...

Anomalous vapor sensor response of a fluorinated alkylthiol-protected gold nanoparticle film.

Monolayer-protected gold nanoparticle films generally swell and increase their electrical resistance when exposed to organic vapors. Films of gold nanoparticles protected by 1H,1H,2H,2H-perfluorodecanethiol (PFDT) exhibit an anomalous response in which the resistance decreases for all vapors investigated. Electron microscopy illustrates that the PFDT-functionalized gold nanoparticles are hexagonally ordered with an interparticle separation of 3 nm. Quartz crystal microbalance measurements confirm substantial mass uptake, but the relatively large interparticle separation and insulating properties of the gold particles lead to a porous film whose electrical resistance is strongly influenced by changes in the relative permittivity and reversible, vapor-induced changes in film morphology.

The interaction of aluminum with a urethane-substituted polythiophene with electroluminescence applications

Abstract The interaction of aluminum with spin-cast films of poly[2(3-thienyl)ethanol n-butoxycarbonylmethylurethane], a polythiophene having a hydrogen-bond-forming urethane side chain, has been studied by X-ray photoelectron spectroscopy (XPS). The aluminum was evaporated in situ under ultra-high vacuum conditions, and changes in the Al(2p), C(1s), O(1s), N(1s) and S(2p) electronic levels were monitored as a function of aluminum coverage. O(1s) and N(1s) shifts to lower binding energies indicate that initially deposited aluminum preferentially bonds with oxygen atoms of the urethane side-chain, transferring electrons to it. Interaction with the side-chain is so strong that it prevents electron donation into the thiophene ring, as evidenced by a lack of aluminum-induced S(2p) features. This is in contrast to reactive metal deposition on aliphatic-substituted polythiophenes, such as poly(3-hexylthiophene). Al(2p) spectra show that the metal in contact with the polymer electronically resembles that of aluminum oxide, but deposition beyond about 4×1015 Al/cm2 leads to metallic aluminum. Comparison of aluminum intensities on the polymer with aluminum on gold suggest limited diffusion of metal into the bulk.

ELECTRON-STIMULATED DESORPTION OF NEUTRALS FROM METHANOL-DOSED AL(111) : VELOCITY DISTRIBUTIONS AND ADSORBATE DECOMPOSITION DETERMINED BY NONRESONANT LASER IONIZATION

Abstract Electron-stimulated desorption (ESD) of neutrals from methanol-dosed Al(111) is studied using laser ionization at 193 nm coupled with time-of-flight (TOF) mass spectrometry. At room temperature and at very low laser intensity, mass spectrometry of the neutral ESD species indicates the presence of desorbing CH 3 O, the methoxy radical. At higher laser intensity, this species is efficiently photolyzed to C + and HCO + fragments. The velocity distributions of these photofragments, indicative of the velocity distribution of the methoxy parent, are measured for methanol dosed onto both clean and pre-oxidized single crystal surfaces. Both of the surfaces yield similar non-Boltzmann distributions with peak velocities of ∼ 900 m/s, corresponding to a peak kinetic energy of ∼ 0.1 eV for the methoxy parent. The similar results may find explanation in terms of oxidation of the Al(111) surface by the initial methanol exposure. The major ionic desorbate observed from this methanol-dosed Al(111) is H + , and its kinetic energy distribution peaks at ∼ 4 eV, a value which is typical of that observed in other ESD studies of ionic desorbates. The order of magnitude difference in kinetic energies between the desorbed ions and neutrals is discussed in terms of possible desorption mechanisms. Neutral ESD, combined with X-ray photoelectron spectroscopy (XPS) is also used as a probe of changes in surface adsorbate composition as a function of temperature and of electron beam dose for methanol/Al(111). The surface concentration of the methoxy species, as monitored via the HCO + photofragment, is found to decrease linearly with increasing temperature. An increase in C + signal at ∼ 470 K is attributed to the formation of a thermal decomposition product with either a higher desorption cross section or a higher laser ionization/fragmentation cross section than the methoxy species. Electron beam damage studies of the methoxy/aluminum system at an electron beam energy of 3 keV give a cross section of 3 ± 1 × 10 −17 cm 2 for loss of methoxy from the surface at this energy.

Photocatalytic Activity and Fluorescence of Gold/Zinc Oxide Nanoparticles Formed by Dithiol Linking.

Monolayer-protected gold nanoparticles (AuNPs) with average diameters of 2-4 nm have been covalently attached to zinc oxide nanorods using dithiol ligands. Electron microscopy and Raman spectroscopy show that ozone treatment or annealing at 300 or 450 °C increases the average diameter of the AuNPs to 6, 8, and 14 (±1) nm, respectively, and decomposes the organic layers to various degrees. These treatments locate the AuNPs closer to the nanorods. Heating and subsequent ozone exposure changes the color of the as-prepared nanocomposite powder from blue to purple due to oxidation of the outer layer of the AuNPs, and heating to 300 °C changes it to pink due to oxygen desorption. ZnO nanorods have a bimodal photoluminescence spectrum that consists of an ultraviolet excitonic peak and a visible, surface defect-related peak. Ozone treatment and annealing of the nanocomposite decreases the intensities of both peaks due to quenching by the AuNPs, but the visible peak is affected less. The photocatalytic efficiency of the nanocomposites toward oxidative degradation of rhodamine B has been measured and follows the order 300 °C > 450 °C > ozone treated ≈ as-prepared ≈ bare ZnO. The greater efficiency of the annealed samples likely arises from decreased electron-hole pair recombination rates.

Thiol Adsorption on and Reduction of Copper Oxide Particles and Surfaces

The adsorption of 1-dodecanethiol at room temperature and at 75 °C on submicron cuprous and cupric oxide particles suspended in ethanol has been investigated by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and transmission electron microscopy. Thiol adsorption occurs in all cases via Cu-S bond formation, with partial dissolution of CuO at 75 °C and formation of a copper-thiolate complex replacement layer. Regardless of temperature, the surface of the CuO particles is essentially completely reduced to either Cu2O or metallic copper, as evidenced by loss of the characteristic Cu(2+) XPS features of dried powder samples. Companion ultrahigh-vacuum studies have been performed by dosing clean, oxygen-dosed, and ozone-treated single crystal Cu(111) with methanethiol (MT) gas at room temperature. In the latter case, the surface corresponds to CuO/Cu(111). XPS confirms MT adsorption in all cases, with an S 2p peak binding energy of 162.9 ± 0.1 eV, consistent with methanethiolate adsorption. Heating of MT-covered Cu(111) and oxygen-dosed Cu(111) leads to decomposition/desorption of the MT by 100 °C and formation of copper sulfide with an S 2p binding energy of 161.8 eV. Dosing CuO/Cu(111) with 50-200 L of MT leads to only partial reduction/removal of the CuO surface layers prior to methanethiolate adsorption. This is confirmed by ultraviolet photoelectron spectroscopy (UPS), which measures the occupied states near the Fermi level. For both the colloidal CuO and single crystal CuO/Cu(111) studies, the reduction of the Cu(2+) surface is believed to occur by formation and desorption of the corresponding dithiol prior to thiolate adsorption.

Electron impact on benzene layers on W(110)

Electron impact on benzene chemisorbed on W(110) and on benzene multilayers physisorbed on top of chemisorbed benzene was carried out. The only observable desorption products were H+ and traces of H2. There was no loss of C from chemisorbed benzene and less than 10% from benzene multilayers, as determined by Auger and XPS measurements. The decay cross sections thus correspond to C-H bond breaking only and are 3 × 10−17 cm2 for chemisorbed benzene and 2 × 10−17 cm2 for low coverage of physisorbed benzene. For thicker deposits, 5 langmuir or more, a second decay regime with a cross section of 2 × 10−18 cm2 is also seen. On heating, the H+ yield from chemisorbed benzene (measured at 90 K) increases up to 800 K, and then decreases. H+ can still be detected at 1400 K, indicating that C-H bonds still exist at this temperature. A loss at 6 eV is seen on heating multilayers after electron impact to T ≥ 650 K and is attributed to a graphite plasmon. Work function measurements give a value of 4.4 eV after electron impact at 90 K; this increases to 4.5 eV after heating to 1000 K. These values are close to that reported for bulk graphite. A spot, 1.8 mm in diameter was prepared by irradiating a multilayer deposit with a 2 keV electron beam from an Auger gun. After heating to 200 K to remove unreacted physisorbed benzene the carbon spot was found to be stable to > 1100 K; two adjacent spots were found to be unchanged after exposure to air, and bakeout at 200°C. Retardation measurements indicate that carbon layers formed by electron impact on benzene multilayers do not charge up at current densities up to 25 microampere/cm2. The present results indicate that it should be possible to make carbon nanostructures by electron beam writing on benzene layers, physisorbed at 77–120 K, with unreacted benzene subsequently removed by heating to 200 K.

Interaction of oxygen and Ni on W(110)

Abstract The adsorption of Ni on O-covered W(110) and of oxygen on Ni-covered W(110) and subsequent behavior on heating have been studied via thermal desorption, XPS, Auger, work function, and LEED measurements. Adsorption of Ni on O/W(110) does not lead to NiO formation, but on heating to T > 400 K to dewetting and at even higher temperatures to segregation which seems complete near 1000 K. After segregation Ni forms (111)-oriented islands, while O is forced into a p(1 × 8) structure, which corresponds to p(1 × 1) with regularly spaced defect lines every eighth (non-primitive) unit cell, running along 〈100〉 directions. There is evidence for a dead zone between Ni and O islands, as also observed in other systems. Ni desorption temperature is reduced appreciably, relative to Ni on clean W(110). Adsorption of oxygen on Ni W (110) occurs on a saturated Ni monolayer (corresponding to Ni W = 1.29 , i.e. virtually to the density of a Ni(111) plane) with constant sticking coefficient s = 0.74 at 90 K up to O W = 0.9 . Maximum O uptake corresponds to O Ni ≈ 1 or O W ≈ 1.3 . For lower Ni coverage, Ni W = 0.94 ( Ni 0.73 W(110) relative to saturation Ni coverage), s is initially higher but decreases more rapidly than on Ni 1 W (110) . In both cases O adsorption leads to disorder of the composite surface. For Ni W (110) chemisorption occurs up to O W = 0.3 at 90 K, accompanied by a linear increase in work function. Above this coverage oxidation sets in, as indicated by Ni core level shifts; φ remains constant to nearly saturation coverage, where a steep increase, attributable to O2 species occurs. On heating these desorb at 150–200 K. NiO decomposes between 400 and 600 K with O becoming chemisorbed on the W surface. Additional heating leads to desorption of WO2 between 1000 and 1250 K, with remaining O coverage O/W = 0.7. Ni-O segregation occurs above 600 K. Oxygen adsorption also occurs on Ni1O0.6/W(110) to the extent of O W = 0.7. Similarities and differences between these results and those obtained for O-Cu, O-Pd and O-Ag interactions on W(110) are discussed.

Synthesis and characterization of transition metal complexes of (2-(2-methoxyethoxy)ethyl)diphenylphosphine and (2-(2-methoxyethoxy)ethyl)dimethylarsine

Abstract Two new, potentially tridentate ligands, Ph 2 P(CH 2 CH 2 O) 2 Me (POO) and Me 2 As(CH 2 CH 2 O) 2 Me(AsOO), have been synthesized and characterized by multinuclear NMR spectroscopy. Mononuclear complexes of these ligands, cis -(CO) 4 Mo(EOO) 2 (E = P, As), cis,cis,trans - and cis,trans,cis -(CO) 2 Cl 2 Ru(POO) 2 , [(1,5-cod)Rh(POO) 2 ][ClO 4 ] and Cl 2 M(POO) 2 , (M = Pd and Pt), have also been prepared and characterized by multinuclear NMR and IR spectroscopy. In all cases, these ligands are coordinated only through the group 15 donor atom. Different procedures have been developed to give either the cis,cis,trans or cis,trans,cis isomers of Cl 2 (CO) 2 Ru(POO) 2 complex in high yields. The latter isomer is unusual and has not previously been reported with ligands of this type. The reactions of the cis -(CO) 4 Mo(EOO) 2 complexes with methyllithium have been examined. The carbonyl ligands in these complexes do not react with methyllithium at room temperature. These results are in direct contrast to those of Powell and coworkers who reported facile reactions between methyllithium and the carbonyl ligands in similar cis -(CO) 4 Mo(Ph 2 PO(CH 2 CH 2 O) 3 PPh 2 complexes. These results confirm Powells conclusions that the number and type of the hard donor atoms in these complexes greatly affect the reactivity of methyllithium towards the carbonyl ligands.