Water-mediated height artifacts in dynamic atomic force microscopy.

Amplitude modulation atomic force microscopy is one of the most broadly used techniques for the nanoscale characterization of a large variety of surfaces because it can routinely provide topography images with nanometer and subnanometer resolution in air, i.e. under ambient conditions, using available commercial instruments. The topographic map results from the convolution of the different interactions (van der Waals, capillary, adhesion, etc.) sensed by the probe and the presence of nanometer-thick water films on both the surface and the tip of the probe, as is usually the case under ambient conditions, can lead to apparent heights markedly different from the real heights due to formation and rupture of water menisci, particularly when the surfaces exhibit regions with different affinity to water (hydrophilic and hydrophobic). In order to systematically explore such a well-known but usually ignored phenomenon, we have performed a combined experimental and theoretical study using (hydrophobic) self-assembled monolayers of stearic acid grown on (hydrophilic) freshly cleaved mica surfaces and a simplified point mass on a spring model to simulate the tip dynamics. We show that, depending on the operation parameters (free oscillation amplitude and setpoint), the apparent heights can vary in magnitude and sign (contrast inversion) and, most important, that the true height cannot be measured in the presence of water layers when surface affinity to water is not homogeneous even if menisci are not formed. We suggest to revise, within the perspective of the present investigation, those published works where the determination of heights is critical.

Dipolar origin of water etching of amino acid surfaces.

The etching induced by water on hydrophobic (001) surfaces of enantiomeric L-, D- and racemic DL-valine crystals has been characterized by means of atomic force microscopy (AFM) at ambient conditions. Well-defined chiral parallelepipedic shallow patterns, one bilayer deep, are observed for the enantiomeric crystals with sides (steps) oriented along low index crystallographic directions. Hence, chirality can be readily identified by visual inspection of an AFM image after etching. The formation of such regular patterns can be rationalized using basic concepts of electrical dipolar interactions. The key factor that determines the relative etching rate for each step and thus defines the shape of the etching patterns is the orientation of the molecular dipoles with respect to the step edge. The simplicity of the approach allows the prediction of the effect of water etching on other amino acid crystals as well as the effect of the interaction of water with amino acid molecules forming part of more complex structures.

Strong water-mediated friction asymmetry and surface dynamics of zwitterionic solids at ambient conditions: L-alanine as a case study.

Water molecules strongly interact with freshly cleaved (011) surfaces of L-alanine single crystals at low relative humidity (below 10%) promoting diffusion of L-alanine molecules. Species mobility is enhanced above ~40% leading to the formation of two-dimensional islands with long-range order through Ostwald ripening. Scanning force microscopy experiments reveal that both, islands and terraces, are identical in nature (composition and crystallographic structure) but a relevant friction asymmetry appearing upon water-surface interaction evidences that orientation dependent properties exist between them at the molecular level. We interpret this observation as due to water incorporation in the topmost surface crystal structure. Eventually, for high humidity values, surface dissolution and roughening occur.

Orbital specific chirality and homochiral self-assembly of achiral molecules induced by charge transfer and spontaneous symmetry breaking.

We study the electronic mechanisms underlying the induction and propagation of chirality in achiral molecules deposited on surfaces. Combined scanning tunneling microscopy and ab initio electronic structure calculations of Cu-phthalocyanines adsorbed on Ag(100) reveal the formation of chiral molecular orbitals in structurally undistorted molecules. This effect shows that chirality can be manifest exclusively at the electronic level due to asymmetric charge transfer between molecules and substrate. Single molecule chirality correlates with attractive van der Waals interactions, leading to the propagation of chirality at the supramolecular level. Ostwald ripening provides an efficient pathway for complete symmetry breaking and self-assembly of homochiral supramolecular layers.

Two-dimensional wetting: the role of atomic steps on the nucleation of thin water films on BaF2(111) at ambient conditions.

The interaction of water with freshly cleaved BaF(2)(111) surfaces at ambient conditions (room temperature and under controlled humidity) has been studied using scanning force microscopy in different operation modes. The images strongly suggest a high surface diffusion of water molecules on the surface indicated by the accumulation of water at step edges forming two-dimensional bilayered structures. Steps running along the 110 crystallographic directions show a high degree of hydrophilicity, as evidenced by small step-film contact angles, while steps running along other directions exhibiting a higher degree of kinks surprisingly behave in a quite opposite way. Our results prove that morphological defects such as steps can be crucial in improving two-dimensional monolayer wetting and stabilization of multilayer grown on surfaces that show good lattice mismatch with hexagonal ice.

Amphiphillic organic crystals.

The amphiphillic character, that is, the capacity to simultaneously attract and repel water, has been traditionally reserved to organic molecules such as phospholipids and surfactants, containing both hydrophilic and hydrophobic groups within the same molecule. However, this general concept can be extended to artificial structures such as micrometer-sized particles, the so-called Janus particles, and patterned surfaces. Here we provide an example of an amphiphillic crystalline solid, l-alanine, by combining atomic force microscopy measurements performed on two different cleavage surfaces showing contrasting behaviors when exposed to water vapor, with computer simulations that allow us to clarify the dipolar origin of this behavior. Although we take l-alanine as an example, our results should apply quite generally to dipolar molecular crystals.

Influence of the macroscopic shape of the tip on the contrast in scanning polarization force microscopy images.

We demonstrate that a quantitative analysis of the contrast obtained in electrostatic force microscopy images that probe the dielectric response of the sample (scanning polarization force microscopy (SPFM)) requires numerical simulations that take into account both the macroscopic shape of the tip and the nanoscopic tip apex. To simulate the SPFM contrast, we have used the generalized image charge method (GICM), which is able to accurately deal with distances between a few nanometers and several microns, thus involving more than three orders of magnitude. Our numerical simulations show that the macroscopic shape of the tip accounts for most of the SPFM contrast. Moreover, we find a quasi-linear relation between the working tip-sample distance and the contrast for tip radii between 50 and 200 nm. Our calculations are compared with experimental measurements of the contrast between a thermally grown silicon oxide sample and a few-layer graphene film transferred onto it.

Thin water films grown at ambient conditions on BaF(2)(111) studied by scanning polarization force microscopy.

The interaction of water with freshly cleaved BaF(2)(111) surfaces has been studied using scanning force microscopy operated in different modes at room temperature and under controlled humidity. The Kelvin probe microscopy (KPM) mode has been used to study the evolution of the surface potential differences (SPDs). In the 20%-50% relative humidity (RH) range, adsorbed water forms two-dimensional solidlike bilayers (islands). The SPD between water islands and the bare substrate surface exhibits a sign crossover from negative ( approximately -30 mV) at low RHs to positive ( approximately +50 mV) at higher RHs, evidencing a cooperative and irreversible flipping of the preferential orientation of water dipoles, from pointing toward the surface evolving into the opposite direction. The KPM results suggest that the classical hexagonal (I(h)) bilayer configuration is not the most favorable structure.

In situ nanocalorimetry of thin glassy organic films.

In this work, we describe the design and first experimental results of a new setup that combines evaporation of liquids in ultrahigh vacuum conditions with in situ high sensitivity thermal characterization of thin films. Organic compounds are deposited from the vapor directly onto a liquid nitrogen cooled substrate, permitting the preparation and characterization of glassy films. The substrate consists of a microfabricated, membrane-based nanocalorimeter that permits in situ measurements of heat capacity under ultrafast heating rates (up to 10(5) K/s) in the temperature range of 100-300 K. Three glass forming liquids-toluene, methanol, and acetic acid-are characterized. The spikes in heat capacity related to the glass-transition temperature, the fictive temperature and, in some cases, the onset temperature of crystallization are determined for several heating rates.

Full-wafer fabrication by nanostencil lithography of micro/nanomechanical mass sensors monolithically integrated with CMOS.

Wafer-scale nanostencil lithography (nSL) is used to define several types of silicon mechanical resonators, whose dimensions range from 20 µm down to 200 nm, monolithically integrated with CMOS circuits. We demonstrate the simultaneous patterning by nSL of ∼2000 nanodevices per wafer by post-processing standard CMOS substrates using one single metal evaporation, pattern transfer to silicon and subsequent etch of the sacrificial layer. Resonance frequencies in the MHz range were measured in air and vacuum. As proof-of-concept towards an application as high performance sensors, CMOS integrated nano/micromechanical resonators are successfully implemented as ultra-sensitive areal mass sensors. These devices demonstrate the ability to monitor the deposition of gold layers whose average thickness is smaller than a monolayer. Their areal mass sensitivity is in the range of 10(-11) g cm(-2) Hz(-1), and their thickness resolution corresponds to approximately a thousandth of a monolayer.

Long-range repulsive interaction between molecules on a metal surface induced by charge transfer.

The adsorption of a molecular electron donor on Au(111) is characterized by the spontaneous formation of a superlattice of monomers spaced several nanometers apart. The coverage-dependent molecular pair distributions obtained from scanning tunneling microscopy data reveal an intermolecular long-range repulsive potential, which decreases as the inverse of the molecular separation. Density functional theory calculations show a charge accumulation in the molecules due to electron donation into the metal. Our results suggest that electrostatic repulsion between molecules persists on the surface of a metal.

Electronic structure of Ag2Cu2O4. Evidence of oxidized silver and copper and internal charge delocalization.

The electronic structure of the recently isolated silver copper oxide Ag(2)Cu(2)O(4) is analyzed along with its precursor Ag(2)Cu(2)O(3) and similar binary oxides, Ag(2)O, AgO, CuO, and NaCuO(2), using X-ray photoemission (XPS) and X-ray absorption (XAS) measurements. The results for Ag(2)Cu(2)O(4) reveal an electronic distribution in which silver and copper share a delocalized valence scheme with both metals in formal oxidation states larger than the usual Ag(I) and Cu(II). Only one type of crystallographic silver or copper is found, but disorder-strain parameters are considerable and the possibilities of thermal disorder, atomic motion, oxygen contribution, mixed valence, and internal charge delocalization are considered. Classical coordination descriptions for oxidized silver are revisited in terms of this new internal charge delocalization framework found for the electronic structure.

Nanoindentation: Toward the sensing of atomic interactions.

The mechanical properties of surfaces of layered materials (highly oriented pyrolytic graphite, InSe, and GaSe) and single-crystal ionic materials (NaCl, KBr, and KCl) have been investigated at the nanometer scale by using nanoindentations produced with an atomic force microscope with ultrasharp tips. Special attention has been devoted to the elastic response of the materials before the onset of plastic yield. A new model based on an equivalent spring constant that takes into account the changes in in-plane interactions on nanoindentation is proposed. The results of this model are well correlated with those obtained by using the Debye model of solid vibrations.