The assembly of colloidal nanocrystals (NCs) into superstructures with long-range translational and orientational order is sensitive to the molecular interactions between ligands bound to the NC surface. We illustrate how ligand coverage on colloidal PbS NCs can be exploited as a tunable parameter to direct the self-assembly of superlattices with predefined symmetry. We show that PbS NCs with dense ligand coverage assemble into face-centered cubic (fcc) superlattices whereas NCs with sparse ligand coverage assemble into body-centered cubic (bcc) superlattices which also exhibit orientational ordering of NCs in their lattice sites. Surface chemistry characterization combined with density functional theory calculations suggest that the loss of ligands occurs preferentially on {100} than on reconstructed {111} NC facets. The resulting anisotropic ligand distribution amplifies the role of NC shape in the assembly and leads to the formation of superlattices with translational and orientational order.

The reactions of tetrakis(dimethylamido)titanium, Ti[N(CH(3))(2)](4), with alkyltrichlorosilane self-assembled monolayers (SAMs) terminated by -OH, -NH(2), and -CH(3) groups have been investigated with X-ray photoelectron spectroscopy (XPS). For comparison, a chemically oxidized Si surface, which serves as the starting point for formation of the SAMs, has also been investigated. In this work, we examined the kinetics of adsorption, the spatial extent, and stoichiometry of the reaction. Chemically oxidized Si has been found to be the most reactive surface examined here, followed by the -OH, -NH(2), and -CH(3) terminated SAMs, in that order. On all surfaces, the reaction of Ti[N(CH(3))(2)](4) was relatively facile, as evidenced by a rather weak dependence of the initial reaction probability on substrate temperature (T(s) = -50 to 110 degrees C), and adsorption could be described by first-order Langmuirian kinetics. The use of angle-resolved XPS demonstrated clearly that the anomalous reactivity of the -CH(3) terminated SAM could be attributed to reaction of Ti[N(CH(3))(2)](4) at the SAM/SiO(2) interface. Reaction on the -NH(2) terminated SAM proved to be the "cleanest", where essentially all of the reactivity could be associated with the terminal amine group. In this case, we found that approximately one Ti[N(CH(3))(2)](4) adsorbed per two SAM molecules. On all surfaces, there was significant loss of the N(CH(3))(2) ligand, particularly at high substrate temperatures, T(s) = 110 degrees C. These results show for the first time that it is possible to attach a transition metal coordination complex from the vapor phase to a surface with an appropriately functionalized self-assembled monolayer.

The chemisorption of hydrogen on both the Ir(111) and Pt(110)-(1×2) surfaces has been examined under ultrahigh vacuum conditions with thermal desorption mass spectrometry, LEED, and contact potential difference measurements. No ordered adsorbate superstructures were observed on either surface at any fractional coverage and at surface temperatures from 100 to 700 K, and the (1×2) reconstruction of the Pt(110) surface was stable in all cases. Hydrogen adsorbs dissociatively on the Ir(111) surface, the adsorption reaction described by second-order Langmuir kinetics with an initial probability of adsorption of 7×10−3. The rate parameters describing the second-order desorption reaction of hydrogen from the Ir(111) surface are weakly dependent on coverage between fractional coverages of 0.1 and 0.3, and are given by Ed ≂12.6 kcal mol−1 and k(2)d ≂2×10−6 cm2 s−1. Beyond a fractional coverage of 0.3, however, both rate parameters decrease with increasing coverage. Hydrogen adsorbs dissociatively on the Pt(110)-(1×2) surface into two distinct β2 and β1 adstates, and the ratio of the saturation densities of these two states, β2:β1, is 1:2. Adsorption into the higher binding energy β2 adstate is described by first-order Langmuir kinetics with an initial probability of adsorption of 0.46, whereas adsorption into the β1 adstate is described by second-order Langmuir kinetics with an ‘‘initial’’ probability of adsorption of 0.022. The rate parameters describing the desorption reaction of hydrogen from the Pt(110)-(1×2) surface are strongly dependent on the coverage. In the coverage regime characteristic of the β2 adstate (θ≤0.32) the rate parameters are approximately symmetric about one-half of saturation of this state. Specifically, from the values for the zero-coverage limit of Ed ≂18 kcal mol−1 and k(2)d ≂10−4 cm2 s−1, the parameters first increase to maximum values of Ed ≂26.5 kcal mol−1 and k(2)d ≂0.3 cm2 s−1 at θ=0.15, and subsequently decrease approximately to the values for the zero-coverage limit at θ=0.32 In the coverage regime characteristic of the β1 adstate (θ>0.32), the activation energy decreases continuously with increasing coverage from a value of Ed ≂17 kcal mol−1 at θ=0.35, whereas the preexponential factor remains essentially constant with a value of 3×10−4 cm2 s−1. The contact potential difference for hydrogen on Pt(110)-(1×2) increases continuously with coverage to a value of 0.17 eV at θ=0.30. As the coverage increases further, however, it decreases continuously approaching a value of −0.50 eV at saturation. Probable binding states for the β2 and β1 adstates on the Pt(110)-(1×2) surface are inferred from both the adsorption and desorption kinetics and the contact potential difference measurements. Comparisons of the results obtained on the (111) and (110)-(1×2) surfaces of both iridium and platinum suggest strongly that local surface structure (e.g., ‘‘step’’ sites vs terrace sites) has a profound influence on the kinetics of adsorption of hydrogen on these surfaces. Surface structure apparently also has a profound influence on the desorption kinetics of hydrogen via the mediation of adatom–adatom interactions. Whereas both attractive and repulsive interactions are clearly manifest within the β2 adstates on the (110)-(1×2) surfaces, only repulsive interactions are apparent on the (111) surfaces and for the β1 adstates on the (110)-(1×2) surfaces.

Synchrotron-based x-ray reflectivity is increasingly employed as an in situ probe of surface morphology during thin film growth, but complete interpretation of the results requires modeling the growth process. Many models have been developed and employed for this purpose, yet no detailed, comparative studies of their scope and accuracy exists in the literature. Using experimental data obtained from hyperthermal deposition of pentane and diindenoperylene (DIP) on SiO2, we compare and contrast three such models, both with each other and with detailed characterization of the surface morphology using ex-situ atomic force microscopy (AFM). These two systems each exhibit particular phenomena of broader interest: pentacene/SiO2 exhibits a rapid transition from rough to smooth growth. DIP/SiO2, under the conditions employed here, exhibits growth rate acceleration due to a different sticking probability between the substrate and film. In general, independent of which model is used, we find good agreement between the surface morphology obtained from fits to the in situx-ray data with the actual morphology at early times. This agreement deteriorates at later time, once the root-mean squared (rms) film roughness exceeds about 1 ML. A second observation is that, because layer coverages are under-determined by the evolution of a single point on the reflectivity curve, we find that the best fits to reflectivity data -corresponding to the lowest values of χ 2 ν -do not necessarily yield the best agreement between simulated and measured surface morphologies. Instead, it appears critical that the model reproduce all local extrema in the data. In addition to showing that layer morphologies can be extracted from a minimal set of data, the methodology established here provides a basis for improving models of multilayer growth by comparison to real systems.

We demonstrate that small-molecule organic thin films of pentacene deposited from thermal and supersonic molecular beam sources can undergo significant reorganization under vacuum or in N 2 atmosphere, beginning immediately after deposition of thin films onto SiO 2 gate dielectric treated with hexamethyldisilazane (HMDS) and fluorinated octyltrichlorosilane (FOTS). Films deposited on bare SiO 2 remain unchanged even after extended aging in vacuum. The changes observed on low-energy surfaces include the depletion of molecules in the interfacial monolayer resulting in the population of upper layers via upward interlayer transport of molecules, indicating a dewetting-like behavior. The morphology of pristine, as-deposited thin films was determined during growth by in situ real-time synchrotron X-ray reflectivity and was measured again, ex situ, by atomic force microscopy (AFM) following aging at room temperature in vacuum, in N 2 atmosphere, and in ambient air. Important morphological changes are observed in ultra-thin films (coverage < 5 ML) kept in vacuum or in N 2 atmosphere, but not in ambient air. AFM measurements conducted for a series of time intervals reveal that the rate of dewetting increases with decreasing surface energy of the gate dielectric. Films thicker than $5 ML remain stable under all conditions; this is attributed to the fact that the interfacial layer is buried completely for films thicker than $5 ML. This work highlights the propensity of small-molecule thin films to undergo significant molecular-scale reorganization at room temperature when kept in vacuum or in N 2 atmosphere after the end of deposition; it should serve as a cautionary note to anyone investigating the behavior of organic electronic devices and its relationship with the initial growth of ultra-thin molecular films on low-energy surfaces.Small-molecule thin films of pentacene exhibit among the highest field effect mobilities reported to date, in large partly because of the propensity of pentacene to form highly ordered thin films with efficient p-stacking along the channel of the organic thin film transistor (OTFT). 1-3 The charge carrier mobility in pentacene thin films is presumed to be severely affected by defects and trap sites at grain boundaries located near the semiconductordielectric interface. 4-7 The first 2 or 3 monolayers (ML) of pentacene are especially important to OTFT applications, as they account for the majority of charge transport during transistor operation. 8 Attempts have been made to improve charge transport by chemically modifying the surface of the dielectric (typically SiO 2 ) with self-assembled monolayers (SAMs) and interfacial organic layers, 9,10 or by employing hydrophobic polymers as gate dielectrics. 11,12 The use of these hydrophobic coatings, such as hexamethyldisilazane (HMDS), octadecyltrichlorosilane (ODTS) and others, has improved device performance significantly, resulting in systematically higher values of field effect mobility for pentacene films deposited from both thermal and superson...