Publications

We present our research results on membrane pores. The study was divided into two primary sections. The first involved the formation of protein pores in free-standing lipid bilayer membranes. The second involved the fabrication via surface micromachining techniques and subsequent testing of solid-state nanopores using the same characterization apparatus and procedures as that used for the protein pores. We were successful in our ability to form leak-free lipid bilayers, to detect the formation of single protein pores, and to monitor the translocation dynamics of individual homogeneous 100 base strands of DNA. Differences in translocation dynamics were observed when the base was switched from adenine to cytosine. The solid state pores (2-5 nm estimated) were fabricated in thin silicon nitride membranes. Testing of the solid sate pores indicated comparable currents for the same size protein pore with excellent noise and sensitivity. However, there were no conditions under which DNA translocation was observed. After considerable effort, we reached the unproven conclusion that multiple (<1 nm) pores were formed in the nitride membrane, thus explaining both the current sensitivity and the lack of DNA translocation blockages.

Density functional calculations show that the electric field effect on Si ad-dimer diffusion on Si(0 0 1) is largely a reflection of the position dependence of the ad-dimer's dipole moment. Surface diffusion barriers' dependence on perpendicular electric fields can be used to discriminate between diffusion mechanisms. Since the previously accepted mechanism for ad-dimer diffusion on Si(0 0 1) has the opposite field dependence to what is observed, it cannot be the one that dominates mass-transport. We identify an alternate process, with a similar barrier at zero electric field and field dependence in agreement with measurements. For rotation, calculations to date show linear field dependence, in contrast to experiments. © 2003 Elsevier Science B.V. All rights reserved.

Abstract not provided.

We change the diffusion mechanism of adsorbed Ge-Si dimers on Si(001) using the electric field of a scanning tunneling microscope tip. By comparing the measured field dependence with first-principles calculations we conclude that, in negative field, i.e., when electrons are attracted towards the vacuum, the dimer diffuses as a unit, rotating as it translates, whereas, in positive field the dimer bond is substantially stretched at the transition state as it slides along the substrate. Furthermore, the active mechanism in positive fields facilitates intermixing of Ge in the Si lattice, whereas intermixing is suppressed in negative fields. © 2003 The American Physical Society.

Abstract not provided.

Abstract not provided.

We use atom-tracking scanning tunneling microscopy to study the diffusion of Pd in the Pd/Cu(001) surface alloy. By following the motion of individual Pd atoms incorporated in the surface, we show that Pd diffuses by a vacancy-exchange, mechanism. We measure an effective activation energy for the diffusion of incorporated Pd atoms of 0.88 eV, which is consistent with an ab initio calculated barrier of 0.94 eV.

The authors carry out a comparative study of the energetic and dynamics of Si-Si, Ge-Ge, and Ge-Si ad-dimers on top of a dimer row in the Si(001) surface, using first-principles calculations. The dynamic appearance of a Ge-Si dimer is distinctively different from that of a Si-Si or Ge-Ge dimer, providing a unique way for its identification by scanning tunneling microscopy (STM). Its rocking motion, observed in STM, actually reflects a 180{degree} rotation of the dimer, involving a piecewise-rotation mechanism. The calculated energy barrier of 0.74 eV is in good agreement with the experimental value of 0.82 eV.

Quantitative measurements of the diffusion of adsorbed mixed Ge-Si dimers on the Si(100) surface have been made as a function of temperature using atom-tracking scanning tunneling microscopy. These mixed dimers are distinguishable from pure Si-Si dimers by their characteristic kinetics--a 180-degree rotation between two highly buckled configurations. At temperatures at which the mixed dimers diffuse, atomic-exchange events occur, in which the Ge atom in the adsorbed dimer exchanges with a substrate Si atom. Re-exchange can also occur when the diffusing Si-Si dimer revisits the original site of exchange.

The positions of Ge atoms intermixed in the Si(100) surface at very low concentration are identified using empty-state imaging in scanning tunneling microscopy. A measurable degree of place exchange occurs at temperatures as low as 330 K. Contrary to earlier conclusions, good differentiation between Si atoms and Ge atoms can be achieved by proper imaging conditions.

The authors have used low-energy electron microscopy to investigate the dynamics of the Si(111) 7 x 7 {r_arrow} 1 x 1 phase transition. Because the densities of the two phases differ, the phase transformation is analogous to precipitation in bulk systems: additional material must diffuse to the phase boundaries in order for the transformation to occur. By measuring the size evolution of an ensemble of domains, and comparing the results to simulations, they have identified a new mechanism of precipitate growth. The source of material necessary for the transformation is the random creation of atom/vacancy pairs at the surface. This mechanism contrasts sharply with classical theories of precipitation, in which mass transport kinetics determine the rate of transformation.

This work is comprised of two major sections. In the first section the authors develop multivariate image classification techniques to distinguish and identify surface electronic species directly from multiple-bias scanning tunneling microscope (STM) images. Multiple measurements at each site are used to distinguish and categorize inequivalent electronic or atomic species on the surface via a computerized classification algorithm. Then, comparison with theory or other suitably chosen experimental data enables the identification of each class. They demonstrate the technique by analyzing dual-polarity constant-current topographs of the Ge(111) surface. Just two measurements, negative- and positive-bias topography height, permit pixels to be separated into seven different classes. Labeling four of the classes as adatoms, first-layer atoms, and two inequivalent rest-atom sites, they find excellent agreement with the c(2 x 8) structure. The remaining classes are associated with structural defects and contaminants. This work represents a first step toward developing a general electronic/chemical classification and identification tool for multivariate scanning probe microscopy imagery. In the second section they report measurements of the diffusion of Si dimers on the Si(001) surface at temperatures between room temperature and 128 C using a novel atom-tracking technique that can resolve every diffusion event. The atom tracker employs lateral-positioning feedback to lock the STM probe tip into position above selected atoms with sub-Angstrom precision. Once locked the STM tracks the position of the atoms as they migrate over the crystal surface. By tracking individual atoms directly, the ability of the instrument to measure dynamic events is increased by a factor of {approximately} 1,000 over conventional STM imaging techniques.

We create mobile surface vacancies on vicinal Si(001) by bombarding the surface with 300 eV Xe ions at a substrate temperature of 465{degrees}C. The vacancies preferentially annihilate at the rough steps retracting them with respect to their smooth neighbors. This process leads to a bimodal terrace width distribution. The retraction of the rough steps due to the vacancy annihilation is in competition with the healing process by which the surface tries to maintain its equilibrium configuration of equally spaced steps. As the two competing processes balance, the surface reaches steady state and subsequent removal of surface atoms is manifest as simple step flow.