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A study of the self-assembly of 1,4-benzenedimethanethiol (BDMT; HS−CH2−(C6H4)−CH2−SH) monolayers on gold is presented. Self-assembled monolayers (SAMs) are characterized by reflection−absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE) measurements. The ensemble of measurements consistently shows that well-organized BDMT SAMs, with “standing-up” molecules, can be obtained on high quality gold films with incubation in n-hexane provided that N2-degassed solutions are used and all preparation steps are performed at 60 °C in the absence of ambient light. SE data indicate that the optical interface properties of the BDMT−Au system are different from those of simple alkanethiol SAMs. A possible mechanism for the formation of the “standing-up” phase from the lying-down phase via a hydrogen exchange reaction involving chemisorbed lying-down and free dithiol molecules is discussed. [ABSTRACT FROM AUTHOR]

PbS colloidal quantum dots (QDs) are a promising material for the realization of low-cost, high-responsivity near-infrared photodetectors. Previously reported attempts showed high responsivity but a fast performance decay in air-exposed devices, demanding encapsulation of the photodetectors. Conversely, devices with very high air stability have been demonstrated but the low trap-state density hinders photoconductive gain and reduces overall responsivity. In this paper, photoconductive devices prepared with partially tetrabutylammonium iodide exchanged PbS QDs are presented with enhanced air stability and high responsivity at low voltage, low optical power. [ABSTRACT FROM AUTHOR]

Westudied the structural and compositional transformations ofcolloidal covellite (CuS) nanocrystals (and of djurleite (Cu1.94S) nanocrystals as a control) when exposed to divalent cations, asCd2+and Hg2+, at room temperature in organicsolvents. All the experiments were run in the absence of phosphines,which are a necessary ingredient for cation exchange reactions involvingcopper chalcogenides, as they strongly bind to the expelled Cu+ions. Under these experimental conditions, no remarkablereactivity was indeed seen for both CuS and Cu1.94S nanocrystals.On the other hand, in the covellite structure 2/3 of sulfur atomsform covalent S–S bonds. This peculiarity suggests that thecombined presence of electron donors and of foreign metal cationscan trigger the entry of both electrons and cations in the covellitelattice, causing reorganization of the anion framework due to therupture of the S–S bonds. In Cu1.94S, which lacksS–S bonds, this mechanism should not be accessible. This hypothesiswas proven by the experimental evidence that adding ascorbic acidincreased the fraction of metal ions incorporated in the covellitenanocrystals, while it had no noticeable effect on the Cu1.94S ones. Once inside the covellite particles, Cd2+andHg2+cations engaged in exchange reactions, pushing theexpelled Cu+ions toward the not-yet exchanged regionsin the same particles, or out to the solution, from where they couldbe recaptured by other covellite nanoparticles/domains. Because nogood solvating agent for Cu ions was present in solution, they essentiallyremained in the nanocrystals. [ABSTRACT FROM AUTHOR]

Colloidal nanocrystal films of different materials (semiconductors,metals) and shapes (spheres and rods) were dipped in solutions ofpropyltrichlorosilane (PTCS) in acetonitrile. This process removedmost of the surfactants covering the surface of the tested nanocrystals,leaving their surface either unpassivated or passivated with chlorineatoms, depending on their composition. PTCS was reactive toward mostof the surfactants used in nanocrystal synthesis and therefore sucha procedure could be applied to a large variety of materials. Allsamples were characterized with FTIR, XRD, and XPS measurements. Innanocrystal films, the reduction of the separation between the nanocrystalsresulting from the removal of surfactants led to an enhancement inboth dark and photocurrent. The surface of Au nanocrystals is leftunpassivated by the reaction with PTCS, which makes the process potentiallyuseful for applications in catalysis and plasmonics. [ABSTRACT FROM AUTHOR]