S. J. Stranick

Atomic-Scale Dynamics of a Two-Dimensional Gas-Solid Interface

The interface between a two-dimensional (2D) molecular gas and a 2D molecular solid has been imaged with a low-temperature, ultrahigh-vacuum scanning tunneling microscope. The solid consists of benzene molecules strongly bound to step edges on a Cu{111} surface. Benzene molecules on the Cu{111} terraces move freely as a 2D gas at 77 kelvin. Benzene molecules transiently occupy well-defined adsorption sites at the 1D edge of the 2D solid. Diffusion of molecules between these sites and exchange between the two phases at the interface are observed. On raised terraces of the copper surface, the 2D gas is held in a cage of the solid as in a 2D nanometer-scale gas bulb.

Nanometer-scale phase separation in mixed composition self-assembled monolayers

Mixed composition monolayers of similar n-alkanethiols on are formed by self-assembly. While the average surface composition of these films accurately reflects the composition of the deposition solution, scanning tunneling microscopy and secondary ion mass spectroscopy measurements show that the films phase separate on the nanometer scale. Scanning tunneling microscopy has been used to follow molecular motions within these films. We discuss our observations in terms of the formation and stability of the phase-segregated domains, and their potential importance in nanoscale applications.

Interactions and dynamics of benzene on Cu{111} at low temperature

We have used ultrahigh vacuum scanning tunneling microscopy to study the structure and dynamics of benzene on Cu{111} at low coverage and low temperature. We find that at 77 K adsorbed benzene molecules preferentially form two-dimensional (2D) solid structures along Cu substrate step edges while benzene molecules on terraces move freely across the surface as a 2D molecular gas. At the interface between the 2D solid and 2D gas phases we observe lateral diffusion, 2D adsorption, and 2D desorption. The motion of the benzene is detected by observing the partial and transient occupation of preferred adsorption sites at the interface. The outward growth of the 2D benzene solid is assisted by the modulations in the local density of states due to Cu{111} surface state electrons scattered from step edges and from the tightly bound benzene molecules. These modulations in the local density of states strongly influence the binding and dynamics of the benzene molecules and are also imaged using the scanning tunneling microscope. The relative strengths of the interactions at various step and terrace sites due to nanometer-scale variations in the surface electronic structure are discussed and the resulting benzene overlayer structures described.

A versatile microwave‐frequency‐compatible scanning tunneling microscope

Challenged by the need to transmit a microwave frequency signal to and from the tunnel junction of a scanning tunneling microscope (STM), an STM has been designed and built that provides minimal microwave signal loss. The STM has been operated over a wide frequency range, dc to 20 GHz, without the use of a microwave resonance cavity. This design also provides a flexible feedback scheme allowing selection of the required piezoelectric gain and stability. A reliable, easy to fabricate sample approach is also described that adds to the stability of this design. The operation of this microscope by imaging a lead silicate glass surface with a bias voltage modulation frequency and detection frequency of 7.73 GHz is demonstrated.

A TUNABLE MICROWAVE FREQUENCY ALTERNATING CURRENT SCANNING TUNNELING MICROSCOPE

By modulating the scanning tunneling microscope junction bias voltage at microwave frequencies, imaging and spectroscopy of insulating surfaces have become possible. In order to explore the spectroscopic capabilities of this instrument, we have developed a tunable microwave frequency alternating current scanning tunneling microscope. We combine the reliable beetle‐style sample approach with coaxial sample and tip contacts. This provides us with a stable microwave‐frequency‐compatible scanning tunneling microscope. This alternating current scanning tunneling microscope design is compatible with ultrahigh vacuum and low‐temperature operation.

A low temperature, ultrahigh vacuum, microwave‐frequency‐compatible scanning tunneling microscope

To expand the capabilities of the microwave frequency alternating current scanning tunneling microscope to include the ability to study isolated adsorbates and highly reactive surfaces, we have developed a low temperature, ultrahigh vacuum alternating current scanning tunneling microscope. In this alternating current scanning tunneling microscope, we employ the reliable beetle‐style sample approach mechanism with a number of other components unique to a low temperature scanning tunneling microscope. These include the sample transfer, delivery, retrieval, storage, sputtering, and heating systems. This alternating current scanning tunneling microscope operates at 77 and 4 K.

Scanning tunneling microscopy studies of self‐assembled monolayers of alkanethiols on gold

Scanning tunneling microscopy was used to probe the molecular scale structures of pure and mixed composition self‐assembled monolayers of CH3(CH2)15SH and CH3O2C(CH2)15SH on gold. A predominant defect structure is found for both films, and is assigned as a void defect in the thiolate overlayer. The mixed composition films are observed to phase segregate into domains of the pure component thiolates. Time‐lapse series of scanning tunneling microscope images reveal kink‐driven motion at step edges and exchange of thiolate molecules on terraces.

Alternating current scanning tunneling spectroscopy of self‐assembled monolayers on gold

We demonstrate the spectroscopic capabilities of an ac scanning tunneling microscope. This ac scanning tunneling microscope has a bias voltage modulation frequency tunable over the range dc–20 GHz and is thus capable of recording images and local spectra of insulating as well as conducting substrates. In this article we detail the spectral sensitivity and the tip‐sample separation dependence of the spectra recorded by simultaneously tuning the modulation and detection frequencies. Spectra of an evaporated gold film and of self‐assembled monolayers of CH3(CH2)15SH and of CH3O2C(CH2)15SH on gold are presented.

Nucleation, formation, and stability of benzene islands on

Abstract : We have imaged stable islands of benzene molecules on the surface of Cu (111) at 77K using a low temperature ultrahigh vacuum scanning tunneling microscope. The islands consist of a small number of benzene molecules and nucleate into straight lines on the Cu (111) surface. The ordering and stability of these clusters are discussed in terms of the molecular interactions with the substrate surface state electrons and the intermolecular interactions.

Atomic-scale view of motion on surfaces

Scanning tunneling microscopy has been used to obtain an atomic-scale view of the motion of adsorbates on surfaces. For sufficiently slow diffusion, we have followed this motion in real time. For faster adsorbate motion we were able to observe the partial and transient occupancy of surface sites on the time scale of our measurements. For lateral motion induced by surface processes such as adsorption or chemical reaction, we were able to analyze the final positions of the adsorbates or reaction products in order to measure the distances covered and to elucidate the means by which energy is accommodated to the surface.