Composite semiconductor crystallites involving CdSe grown on an ZnS seed,and vice versa, have been synthesized and "capped" with organic ligands in inverse micelle solutions. These composite particles, as well as capped seed crystallites of CdSe and ZnS, are isolated, purified, and characterized for relative atomic composition, structure, and electronic properties. The Debye X-ray scattering equation, when solved for these layered particles, shows that powder X-ray scattering is insensitive to a small foreign inclusion. A simple theoretical model for the LUMO and HOMO of layered crystallites shows that a small (<15-A diameter) interior foreign seed causes only small shifts of the lowest excited state, to either higher or lower energies. The capped CdSe seed and the capped CdSe portion of the layered particle grown on a ZnSe seed undergo low-temperature (169 "C) annealing to give near-single-crystal X-ray scattering. However, CdSe annealing is blocked by a surface ZnS layer which is ca. 4 A thick. While growth to make composite particles does occur, neither particle shows evidence for epitaxial growth.Semiconductor quantum crystallites (20-60-A diameter) have incomplete band structure development, even though the bulk unit cell is present. Their optical and electronic properties are size dependent, and within the past 5 years synthetic procedures have been developed to prepare and stabilize uniform crystallites.I~* Inverse micelle solutions provide a medium for synthesis of stable, size-selected semiconductor colloid^.^ CdSe crystallites in inverse micelles do not fuse with one another because of the surfactant, yet they can grow larger if either ionic or organometallic sources of Cd and Se atoms are added.4 This surface reactivity was exploited in an organic "capping" reaction:(1) Steigerwald, M. L.; Brus, L. E.
Newly commercialized PEDOT:PSS products CLEVIOS PH1000 and FE-T, among the most conducting of polymers, show unexpectedly higher Seebeck coefficients than older CLEVIOS P products that were studied by other groups in the past, leading to promising thermoelectric (TE) power factors around 47 μW/m K(2) and 30 μW/m K(2) respectively. By incorporating both n and p type Bi(2)Te(3) ball milled powders into these PEDOT:PSS products, power factor enhancements for both p and n polymer composite materials are achieved. The contact resistance between Bi(2)Te(3) and PEDOT is identified as the limiting factor for further TE property improvement. These composites can be used for all-solution-processed TE devices on flexible substrates as a new fabrication option.
The interaction of vapor-deposited Al atoms with self-assembled monolayers (SAMs) of HS(CH2)15CH3 and HS(CH2)15CO2CH3 chemisorbed at Au{111} surfaces was studied using X-ray photoelectron spectroscopy, infrared spectroscopy, time-of-flight secondary ion mass spectrometry, and spectroscopic ellipsometry. For the CH3-terminated SAM, no reaction with C−H or C−C bonds was observed. For total Al doses up to ∼12 atoms/nm2, penetration to the Au−S interface occurs with no disruption of the average chain conformation and tilt, indicating formation of a highly uniform ∼1:1 Al adlayer on the Au. Subsequently, penetration ceases and a metallic overlayer begins to form at the SAM−vacuum interface. These results are explained in terms of an initial dynamic hopping of the −S headgroups on the Au lattice, which opens transient diffusion channels to the Au−S interface, and the closing of these channels upon completion of the adlayer. In contrast, Al atom interactions with the CO2CH3-terminated SAM are restricted to the vacuum interface, where in the initial stages discrete organometallic products form via reaction with the CO2CH3 group. First, a 1:1 complex forms with a reduced CO bond and an intact CH3 moiety. Further exposure leads to the additional reaction of about four Al atoms per ester, after which a metallic overlayer nucleates in the form of clusters. After the growth progresses to ∼30 Å, the clusters coalesce into a uniform metallic film. These results illustrate the extraordinary degree of control that organic substrates can exert during the course of metal film formation.
The interaction of vapor-deposited Al atoms with self-assembled monolayers (SAMs) of HS-(CH(2))(16)-X (X = -OH and -OCH(3)) chemisorbed at polycrystalline Au[111] surfaces was studied using time-of-flight secondary-ion mass spectrometry, X-ray photoelectron spectroscopy, and infrared reflectance spectroscopy. Whereas quantum chemical theory calculations show that Al insertion into the C-C, C-H, C-O, and O-H bonds is favorable energetically, it is observed that deposited Al inserts only with the OH SAM to form an -O-Al-H product. This reaction appears to cease prior to complete -OH consumption, and is followed by formation of a few overlayers of a nonmetallic type of phase and finally deposition of a metallic film. In contrast, for the OCH(3) SAM, the deposited Al atoms partition along two parallel paths: nucleation and growth of an overlayer metal film, and penetration through the OCH(3) SAM to the monolayer/Au interface region. By considering a previous observation that a CH(3) terminal group favors penetration as the dominant initial process, and using theory calculations of Al-molecule interaction energies, we suggest that the competition between the penetration and overlayer film nucleation channels is regulated by small differences in the Al-SAM terminal group interaction energies. These results demonstrate the highly subtle effects of surface structure and composition on the nucleation and growth of metal films on organic surfaces and point to a new perspective on organometallic and metal-solvent interactions.
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