The role of intermolecular and molecule-substrate interactions in the stability of alkanethiol nonsaturated phases on Au(111)

The structure and stability of alkanethiols self-assembled on Au(111) have been studied as a function of the molecular chain length by means of atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXD). Below saturation, phases consisting of molecules with different tilt angles and periodicities are formed. Differences in the mechanical stability of these phases are revealed by AFM experiments and discussed in terms of the competition between intermolecular and molecule-substrate interactions as a function of chain length. For long molecules, intermolecular interactions play a dominant role which stabilizes the formation of closed packed 30 degrees tilted ( radical 3x radical 3)R30 degrees structures. For short molecules, the van der Waals interaction with the gold substrate favors the formation of a 50 degrees tilted phase in which the molecules are arranged in a rectangular configuration.     

Sensing dipole fields at atomic steps with combined scanning tunneling and force microscopy

The electric field of dipoles localized at the atomic steps of metal surfaces due to the Smoluchowski effect were measured from the electrostatic force exerted on the biased tip of a scanning tunneling microscope. By varying the tip-sample bias the contribution of the step dipole was separated from changes in the force due to van der Waals and polarization forces. Combined with electrostatic calculations, the method was used to determine the local dipole moment in steps of different heights on Au(111) and on the twofold surface of an Al-Ni-Co decagonal quasicrystal.     

The low-mass Kπ system in K−p → KπN reactions at 14.3 GeV/c

We present experimental results and a partial-wave analysis of the low-mass $\text{(}\text{K}\text{π)}{}^{\mathrm{<mtext>0</mtext>}}$ systems produced in the reactions $\text{K}{}^{\mathrm{?}}\text{p}\text{→}\text{K}\text{π}\text{N}$ at 14.3 GeV/c. The main results concern the production mechanisms of the $\text{K}{}^1\text{(890)}\text{and}\text{K}{}^1\text{(1420)}$. We also extract the s-wave component of the $\text{K}\text{π}$ system as a function of mass.     

Some three-body final states in K−p reactions at 14.3 GeV/c

We present experimental results on a number of K^?p reactions at 14.3 GeV/c that have three bodies in the final state. The final states are $\text{K}{}^{\mathrm{?}}\text{ω}\text{p}\text{,}\text{K}{}^{\mathrm{?}}\text{π}\text{p}\text{, Λπ}{}^{\mathrm{+}}\text{π}{}^{\mathrm{?}}\text{, Λ}\text{K}{}^{\mathrm{+}}\text{K}{}^{\mathrm{?}}\text{, Λp}\text{p}\text{,}\text{K}{}^1{}^{\mathrm{?}}\text{ω}\text{p}\text{, Λ(1520)}\text{K}{}^{\mathrm{+}}\text{K}{}^{\mathrm{?}}\text{and}\text{Λ(1520)}\text{p}\text{p}$. Whenever, with one exception explained by the Zweig rule, there is a K^? or a proton in the final state, there is a diffractive-like threshold enhancement in the mass spectrum of the two recoiling particles. These enhancements account for a large fraction of the events in all but the Λπ⁺π^? final state, where they cannot occur, and which is dominated by resonance production. We find evidence for the Q₁(1300) decaying into K^?ω.     

Wetting and molecular orientation of 8CB on silicon substrates

The wetting properties of 8CB ( 4(')-n-octyl-4-cyanobiphenyl) on silicon wafers have been studied with scanning polarization force microscopy (SPFM). Layer-by-layer spreading of 8CB droplets is observed. With the help of the surface potential mapping capability of SPFM, we found that the molecular dipole of the first monolayer of 8CB is parallel to the surface. A layer of nearly vertical molecular dimers on top of the monolayer has an associated surface potential of 40 mV, which is attributed to a distortion of the dimer. The dimer distortion propagates to the subsequent smectic bilayers, producing an additional 7 mV potential increase in the second layer, 2 mV on the third, and approximately 1 mV on the fourth.     

Growth and structure of water on SiO2 films on Si Investigated by Kelvin probe microscopy and in Situ X-ray spectroscopies

The growth of water on thin SiO2 films on Si wafers at vapor pressures between 1.5 and 4 Torr and temperatures between -10 and 21 degrees C has been studied in situ using Kelvin probe microscopy and X-ray photoemission and absorption spectroscopies. From 0 to 75% relative humidity (RH), water adsorbs forming a uniform film 4-5 layers thick. The surface potential increases in that RH range by about 400 mV and remains constant upon further increase of the RH. Above 75% RH, the water film grows rapidly, reaching 6-7 monolayers at around 90% RH and forming a macroscopic drop near 100%. The O K-edge near-edge X-ray absorption spectrum around 75% RH is similar to that of liquid water (imperfect H-bonding coordination) at temperatures above 0 degrees C and is ice-like below 0 degrees C.     

Noncontact to contact tunneling microscopy in self-assembled monolayers of alkylthiols on gold

The mechanical interaction between a scanning tunneling microscopy (STM) probe and hexadecane (C16) alkylthiol molecules in a self-assembled monolayer was investigated by sensing the force during constant current mode STM imaging. The force regime changed from attractive to repulsive over the insulating molecule islands under feedback control of the current. The repulsive force on the molecule was strongly dependent on the setpoint value of the current during STM operation. In our experiments, the threshold for contact was found at a tunneling current of 1 pA when the sample bias is 2 V. At higher current, the apparent height of molecular islands changed logarithmically with current. In addition, the current as a function of applied load revealed a stepwise increase, indicative of discrete molecular tilting events. A tunneling decay constant beta of =0.53+/-0.02 A(-1) was obtained based on the measurement of the height of molecules and the tunneling current.     

Induced water condensation and bridge formation by electric fields in atomic force microscopy

We present an analytical model that explains how, in humid environments, the electric field near a sharp tip enhances the formation of water menisci and bridges between the tip and a sample. The predictions of the model are compared with experimental measurements of the critical distance where the field strength causes bridge formation.