A systematic scanning tunneling microscopy (STM) study of alkanethiol self-assembled monolayers (SAMs) is presented as a function of the bias voltage, tunneling current, and tip-termini separation. Stable and etch-pit free SAMs of close-packed undecanethiol/Au(111) were obtained after annealing in ultrahigh vacuum. STM revealed two distinct c(4x2) structures with four nonequivalent molecules per unit cell. For both structures, reversible contrast variations occur upon systematically tuning the bias voltage, the current, and the tip-termini distance. These contrast transitions originate from probing the corresponding local density of states (LDOS) of each molecule and not from the reorientation of the alkanethiol chains. The STM contrast is particularly sensitive to the tip-termini separation in the range of 0.5-2.5 A, reflecting the distance-dependence of LDOS. At a fixed tip elevation, the STM contrast is less sensitive to changes in bias within 0.1-1.2 V. For the first time, we demonstrate that LDOS may override the physical height variations in the STM topographic contrast for alkanethiol SAM systems.

The structure of aldehyde-terminated alkanethiol self-assembled monolayers (SAMs) on Au(111) is investigated using scanning tunneling microscopy (STM), atomic force microscopy (AFM), and density functional theory (DFT) calculations. For the first time, the structures of aldehyde-terminated SAMs are revealed with molecular resolution. SAMs of 11-mercapto-1-undecanal exhibit the basic (radical3xradical3)R30 degrees periodicity and form various c(4x2) superstructures upon annealing. In conjunction with DFT studies, the models of the (radical3xradical3)R30 degrees and the c(4x2) superstructures are constructed. In comparison with alkanethiol SAMs, the introduction of aldehyde-termini results in smaller domain size, lower degree of long-range order, large coverage of disordered areas, and higher density of missing molecules and other point defects within domains of closely packed molecules. The origin of these structural differences is mainly attributed to the strong dipole-dipole interactions among the aldehyde termini.

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The atomic structure of In 0.81 Ga 0.19 As/InP alloy layers was examined using in situ scanning tunneling microscopy. The ͑2ϫ3͒ reconstruction observed during growth by reflection high-energy electron diffraction represents a combination of surface structures, including a ␤2͑2ϫ4͒ commonly observed on GaAs͑001͒ and InAs͑001͒ surfaces, and a disordered ͑4ϫ3͒ that is unique to alloy systems. The proposed ͑4ϫ3͒ structure is comprised of both anion and cation dimers. Empty and filled states images show that the features reverse contrast with sample bias, in agreement with the model.

The resonant response of the complex x-ray scattering factor has been used in conjunction with the coherent Bragg rod analysis phase-retrieval algorithm to determine the composition and strain profiles of ultrathin layers of GaAs grown on InGaAs buffers. The buffer layers are nominally latticed matched with the InP substrate and the subsequent GaAs growth is compared at two different temperatures: 480 and 520°C. We show that electron density maps extracted from Bragg rod scans measured close to the Ga and As K-edges can be used to deconvolute roughness and intermixing. It is found that indium incorporation and roughening lead to a significant reduction of the strain in this system.