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A model scanning probe microscope, designed using similar principles of operation to research instruments, is described. Proximity sensing is done using a capacitance probe, and a mechanical linkage is used to scan this probe across surfaces. The signal is transduced as an audio tone using a heterodyne detection circuit analogous to that used in the theremin (one of the first electronic musical instruments, invented in the early 20th century). The instrument is useful for demonstrations and hands-on activities that introduce fundamentals of scanning probe microscopy and, by extension, nanoscience and nanotechnology. The details of instrument construction are provided, along with instructions for assembly and troubleshooting.

The process of molecular self-assembly on solid surfaces is essentially one of crystallization in two dimensions, and the structures that result depend on the interplay between intermolecular forces and the interaction between adsorbates and the underlying substrate (1). Because a single hydrogen bond typically has an energy between 15 and 35 kilojoules per mole, hydrogen bonding can be a strong driver of molecular assembly; this is apparent from the dominant role of hydrogen bonding in nucleic-acid base pairing, as well as in the secondary structure of proteins. Carboxylic acid functional groups, which provide two hydrogen bonds, are particularly promising and reliable in creating and maintaining surface order, and self-assembled monolayers of benzoic acids produce structure that depends on the number and relative placement of carboxylic acid groups (2-6). Here we use scanning tunnelling microscopy to study self-assembled monolayers of ferrocenecarboxylic acid (FcCOOH), and find that, rather than producing dimeric or linear structures typical of carboxylic acids, FcCOOH forms highly unusual cyclic hydrogen-bonded pentamers, which combine with simultaneously formed FcCOOH dimers to form two-dimensional quasicrystallites that exhibit local five-fold symmetry and maintain translational and rotational order (without periodicity) for distances of more than 400 angstroms., Quasicrystals--first observed experimentally in 1982--exhibit long-range, non-periodic order, and may contain extended regions with 'forbidden' (5, 8, 10 or 12-fold) rotational symmetry (7,8). The majority of known quasicrystals are binary [...]

The use of a pulse-injection technique is reported to deposit the quantum-dot cellular automata (QCA) candidate molecule trans-[Cl(dppe)2Ru[C(triple bond)C]6Ru(dppe)2Cl] on a Au(111) surface. The isolated molecules are clearly observed with a submolecular 'dumbbell'-like structure in high-resolution scanning tunneling microscopy (STM) images.

The structure and dynamics of partial monolayers of C(sub 60) on the Au(111) surface were observed by scanning tunneling microscopy (STM) under ambient conditions. The motion of surface-adsorbed C(sub 60) is predominantly the result of thermal diffusion and the extent of pertubation by the STM tip is determined to be minimal, using a quantitative analysis of a time series of STM images.

A study was conducted to analyze the tile reaction by monitoring the ethyl radical product using resonance-enhanced multiphoton ionization proceeding at 242 nm through highly dissociative Rydberg states. Core-extracted ion arrival profiles were determined with the photolysis laser polarization method. The measured C2H5 spatial anisotropy was then utilized to estimate the product center-of-mass velocity and change the speed distribution tot he differential cross area.

Low-temperature scanning tunneling microscopy is used to observe self-assembled structures of ferrocenedicarboxylic acid (Fc(COOH)2) on the Au(111) surface. The surface is prepared by pulse-deposition of Fc(COOH)2 dissolved in methanol, and the solvent is evaporated before imaging. While the rows of hydrogen-bonded dimers that are common for carboxylic acid species are observed, the majority of adsorbed Fc(COOH)2 is instead found in six-molecule clusters with a well-defined and chiral geometry. The coverage and distribution of these clusters are consistent with a random sequential adsorption model, showing that solution-phase species are determinative of adsorbate distribution for this system under these reaction conditions. [ABSTRACT FROM AUTHOR]

Scanning tunneling microscopy was used to investigate the reaction of octanethiolate self-assembled monolayers (SAMs) with atomic chlorine. We have found that exposing a SAM to low fluxes of radical Cl results primarily in the formation of new defects in areas with close-packed alkanethiolates, but has little to no effect on the domain boundaries of the SAM. Dosing high quantities of atomic chlorine results in the near-complete loss of surface order at room temperature, but not the complete removal of the thiolate monolayer. These observations are in stark contrast to the results of previous measurements of the reaction of atomic hydrogen with alkanethiolate SAMs. [ABSTRACT FROM AUTHOR]

Ultrahigh-vacuum scanning tunneling microscopy (STM) was used to study trans-[Cl(dppe)2Ru(C=C)6Ru(dppe)2Cl] [abbreviated as Ru2, diphenylphosphinoethane (dppe)] on Au(111). This large organometallic molecule was pulse deposited onto the Au(111) surface under ultrahigh-vacuum (UHV) conditions. UHV STM studies on the prepared sample were carried out at room temperature and 77 K in order to probe molecular adsorption and to characterize the surface produced by the pulse deposition process. Isolated Ru2 molecules were successfully imaged by STM at room temperature; however, STM images were degraded by mobile toluene solvent molecules that remain on the surface after the deposition. Cooling the sample to 77 K allows the solvent molecules to be observed directly using STM, and under these conditions, toluene forms organized striped domains with regular domain boundaries and a lattice characterized by 5.3 and 2.7 Å intermolecular distances. When methylene chloride is used as the solvent, it forms analogous domains on the surface at 77 K. Mild annealing under vacuum causes most toluene molecules to desorb from the surface; however, this annealing process may lead to thermal degradation of Ru2 molecules. Although pulse deposition is an effective way to deposit molecules on surfaces, the presence of solvent on the surface after pulse deposition is unavoidable without thermal annealing, and this annealing may cause undesired chemical changes in the adsorbates under study. Preparation of samples using pulse deposition must take into account the characteristics of sample molecules, solvent, and surfaces. [ABSTRACT FROM AUTHOR]

Collisions with high-energy xenon atoms (1.3 eV) induce structural changes in octanethiol self-assembled monolayers on Au(111). These changes are characterized at the molecular scale using an in situ scanning tunneling microscope. Gas-surface collisions induce three types of structural transformations: domain boundary annealing, vacancy island diffusion, and phase changes. Collision-induced changes that occur tend to increase order and create more stable structures on the surface. We propose a mechanism where monolayer transformations are driven by large amounts of vibrational energy localized in the alkanethiol molecules. Because we monitor incremental changes over small regions of the surface, we can obtain structural information about octanethiol monolayers that cannot be observed directly in scanning tunneling microscopy images. [ABSTRACT FROM AUTHOR]

In situ scanning tunneling microscopy is used to measure the effect of hyperthermal rare-gas bombardment on octanethiol self-assembled monolayers. Close-packed monolayers remain largely unchanged, even after repeated collisions with 0.4 eV argon and 1.3 eV xenon atoms. In contrast, gas-surface collisions do induce structural changes in the octanethiol film near defects, domain boundaries, and disordered regions, with relatively larger changes observed for xenon-atom bombardment. [ABSTRACT FROM AUTHOR]

Scanning tunneling microscopy (STM) is used to image and study the [{Cp*(dppe)Fe(C[triple bond]C-)}.sub.2](1,3-[C.sub.6][H.sub.4]) both as a neutral molecule, Fe(II)-Fe(II), and as a mixed-valence complex, Fe(II)-Fe(III). The observation of charge localization in mixed-valence complexes outside of the solution environment offer new routes for the control and patterning of charge on surfaces which could find potential applications in smart materials and molecular electronic devices.

The scanning tunneling microscopy technique is employed to demonstrate the incorporation of gold atoms in the alkanethiol/Au(111) self-assembled monolayers. The impact of addition of the hydrogen atoms to the octanethiol monolayers is also discussed.

Studies the complete chlorine (Cl) atom molecular frame photofragment angular momentum distributions from the photodissociation of Cl and ICl using time-of-light (TOF) mass spectroscopy with laser detection. Techniques for obtaining TOF profiles from which photofragment polarization parameters are extracted; Methods to calculate detection sensitivities.

Examines the reaction of atomic chlorine with methane using resonance-enhanced multiphoton ionization. Indication of possible reaction of spin-orbit excited chlorine; Vibrations created by large enhancements in reactivity; Importance of vibrationally excited reagents to thermal kinetics of the reaction of chlorine with methane.

Scanning tunneling microscopy is used to investigate adsorbate interactions and reactions on metal surfaces. The manners in which adsorbates perturb the surface electronic structure in their vicinity are discussed.

Photolysis of Cl2 initiates the title reaction at a sharply defined collision energy of 0.24±0.03 eV. Nascent product rotational state distributions for HCl (v=0) are determined using resonance enhanced multiphoton ionization (REMPI), center-of-mass scattering distributions are measured by the core-extraction technique, and the average internal energy of the C2H5 product is deduced from the dependence of the core-extracted signal on the photolysis polarization. The HCl product has little rotational excitation, but the scattering distribution is nearly isotropic. Although seemingly contradictory, both of these features can be accounted for by using the simple line-of-centers model presented to explain earlier results for the Cl+CH4 reaction. In contrast to the Cl+CH4 reaction, the data suggest that the Cl+C2H6 reaction proceeds through a loosely constrained transition-state geometry. The reactions of atomic chlorine with ethane, C2H6, and perdeuteroethane, C2D6, yield virtually identical results. These findings, along with the low energy deposited by the reaction into the ethyl product (200±120 cm-1), demonstrate that the alkyl fragment acts largely as a spectator in this hydrogen abstraction reaction. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Discusses the use of the photoloc technique that permits the measurement of state-to-state differential cross section and the complete product polarization dependence for all moments of orientation and alignment. Location of the D-atom transfer as the cause of large differences in the Arrhenius pre-exponential factors of the Cl + C[sub 2]D[sub 6] reaction.

Describes procedures for the measurement of the differential-cross-section moments of the velocity distribution of the state-selected products of photoinitiated bimolecular reactions. Use of resonance-enhanced multiphoton ionization detection; Effects of product polarization on experimental signals.

The mechanism for the reaction of atomic chlorine with vibrationally excited methane is investigated by measurement of correlated state and scattering distributions using the method of core extraction (see preceding paper). Laser photolysis of molecular chlorine creates monoenergetic chlorine atoms (>=98% Cl 2P3/2) that react with vibrationally excited methane molecules prepared by linearly polarized infrared laser excitation. The resulting HCl product population distributions are determined by (2+1) resonance-enhanced multiphoton ionization (REMPI), and the differential cross section for each product rovibrational state is measured by core extraction. Approximately 30% of the product is formed in HCl(υ=1,J) with a cold rotational distribution; the remaining population is formed in HCl(υ=0,J) and is more rotationally excited. We observe a rich variation of the scattered flux that is dependent on the internal-energy state of the product. The HCl(υ=1) product is sharply forward scattered for low J and becomes nearly equally forward–backward scattered for high J; the HCl(υ=0,J) product is back and side scattered. The reactions of Cl with C–H stretch-excited methane (CH4) and C–H stretch-excited CHD3 are found to have similar angular and internal-state distributions. Observation of the spatial anisotropy of the HCl(υ=0, J=3) product shows that significant vibrational excitation of the methyl fragment does not occur.The measured spatial anisotropy is most consistent with a model in which backscattered HCl(υ=0, J=3) is formed in coincidence with slight methyl vibrational excitation and the forward-scattered HCl(υ=0, J=3) is formed in coincidence with no methyl excitation. The approach of the attacking chlorine atom with respect to the C–H stretch direction can be varied by rotating the plane of polarization of the infrared excitation. A marked steric effect is observed in which Cl atoms approaching... [ABSTRACT FROM AUTHOR]

We describe a method we call core extraction for measuring the speed distributions of products from photoinitiated bimolecular reactions for the purpose of determining state-to-state differential cross sections. Core extraction is demonstrated by determination of the state-to-state differential cross section for the reaction Cl+CH4(υ3=1)→HCl(υ=1, J=1)+CH3. The method of core extraction measures three-dimensional projections of the velocity distribution using a time-of-flight mass spectrometer equipped with a mask to reject off-axis scattered products. This three-dimensional projection is then converted to a state-to-state differential cross section via simple transformations. Competition between instrumental resolution and signal in core extraction is discussed, and the behavior of our system is checked with simple velocity distributions that result from photodissociation of Cl2. Core extraction is compared with other methods for the measurement of state-resolved differential cross sections. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]