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We report effective charges and diffusion constants of several different single-stranded DNA oligonucleotides trapped in an MspA nanopore. Nucleotide identity is found to have a substantial influence on effective charges and diffusion constants. These quantities are determined from escape time experiments for a DNA molecule attached to a NeutrAvidin molecule that, unlike the DNA, does not pass through the pore. Correlations are reported between oligonucleotide effective charges and current blockages, and between their diffusion constants and DNA-induced current blockage fluctuations. We also report an unanticipated source of current fluctuations that reflects a discrete blockage current level structure. We posit that this is associated with interactions between the NeutrAvidin molecule and the MspA nanopore.

We probe the molecular dynamics and states of an avidin protein as it is captured and trapped in a voltage-biased cytolysin A nanopore using time-resolved single-molecule electrical conductance signals. The data for very large numbers of single-molecule events are analyzed and presented by a new method that provides clear visual insight into the molecular scale processes. Avidin in cytolysin A has surprisingly rich conductance spectra that reveal transient and more permanently trapped protein configurations in the pore and how they evolve into one another. We identify a long-lasting, stable, and low-noise configuration of avidin in the nanopore into which avidin can be reliably trapped and released. This may prove useful for single-molecule studies of other proteins that can be biotinylated and then transported by avidin to the pore via their coupling to avidin with biotin-avidin linking. We demonstrate the sensitivity of this system with detection of biotin attached to avidin captured by the pore.

We probe the molecular dynamics of a protein, avidin, as it is captured and trapped in a nanopore, ClyA, with time-resolved single-molecule electrical conductance measurements, and we present a method for visualizing this process from the data. The case of avidin in ClyA has rich time-dependent conductance spectra of discrete levels that correlate with different configurations of the protein in the pore. One is very long-lasting, stable and noise-free, and portends the use of this system as a platform for more general studies of proteins and other molecules, where avidin acts as a shuttle that ferries analytes into the pore for probing. We demonstrate this by the sensitive detection of a biotin molecule attached to avidin captured by the pore. We also present an approach to determining the nanopore size based on a 3D printed model of the pore.

As population growth continues to outpace development of water infrastructure in many countries, desalination (the removal of salts from seawater) at high energy efficiency will likely become a vital source of fresh water. Due to its atomic thinness combined with its mechanical strength, porous graphene may be particularly well-suited for electrodialysis desalination, in which ions are removed under an electric field via ion-selective pores. Here, we show that single graphene nanopores preferentially permit the passage of K+ cations over Cl− anions with selectivity ratios of over 100 and conduct monovalent cations up to 5 times more rapidly than divalent cations. Surprisingly, the observed K+/Cl− selectivity persists in pores even as large as about 20 nm in diameter, suggesting that high throughput, highly selective graphene electrodialysis membranes can be fabricated without the need for subnanometer control over pore size., Sub-nanometer graphene nanopores are usually required to create graphene-based reverse osmosis membranes. Here, Rollings et al. show that membranes with larger pores are highly ion selective and a hundred times more permeable to potassium ions than to chloride ions, making them useful for electrodialysis.

Graphene is an ideal thin membrane substrate for creating molecule-scale devices. Here we demonstrate a scalable method for creating extremely small structures in graphene with atomic precision. It consists of inducing defect nucleation centers with energetic ions, followed by edge-selective electron recoil sputtering. As a first application, we create graphene nanopores with radii as small as 3 Å, which corresponds to 10 atoms removed. We observe carbon atom removal from the nanopore edge in situ using an aberration-corrected electron microscope, measure the cross-section for the process, and obtain a mean edge atom displacement energy of 14.1 ± 0.1 eV. This approach does not require focused beams and allows scalable production of single nanopores and arrays of monodisperse nanopores for atomic-scale selectively permeable membranes.

Isolated, atomically thin conducting membranes of graphite, called graphene, have recently been the subject of intense research with the hope that practical applications in fields ranging from electronics to energy science will emerge. The atomic thinness, stability and electrical sensitivity of graphene motivated us to investigate the potential use of graphene membranes and graphene nanopores to characterize single molecules of DNA in ionic solution. Here we show that when immersed in an ionic solution, a layer of graphene becomes a new electrochemical structure that we call a trans-electrode. The trans-electrode’s unique properties are the consequence of the atomic-scale proximity of its two opposing liquid–solid interfaces together with graphene’s well known in-plane conductivity. We show that several trans-electrode properties are revealed by ionic conductance measurements on a graphene membrane that separates two aqueous ionic solutions. Although our membranes are only one to two atomic layers thick, we find they are remarkable ionic insulators with a very small stable conductance that depends on the ion species in solution. Electrical measurements on graphene membranes in which a single nanopore has been drilled show that the membrane’s effective insulating thickness is less than one nanometre. This small effective thickness makes graphene an ideal substrate for very high resolution, high throughput nanopore-based single-molecule detectors. The sensitivity of graphene’s in-plane electronic conductivity to its immediate surface environment and trans-membrane solution potentials will offer new insights into atomic surface processes and sensor development opportunities.

We present a mathematical model for Joule heating of an electrolytic solution in a nanopore. The model couples the electrical and thermal dynamics responsible for rapid and extreme superheating of the electrolyte within the nanopore. The model is implemented numerically with a finite element calculation, yielding a time and spatially resolved temperature distribution in the nanopore region. Temperatures near the thermodynamic limit of superheat are predicted to be attained just before the explosive nucleation of a vapor bubble is observed experimentally. Knowledge of this temperature distribution enables the evaluation of related phenomena including bubble nucleation kinetics, relaxation oscillation, and bubble dynamics.

The development of solid state nanopores1–7, inspired by their biological counterparts8–15, shows great potential for the study of single macromolecules16–21. Applications such as DNA sequencing6,22,23 and exploration of protein folding6 will require understanding and control of the dynamics of a molecule’s interaction with the pore, but DNA capture by a solid state nanopore is not well understood24–26. By recapturing individual molecules soon after they pass through a nanopore, we reveal the mechanism by which double stranded DNA enters the pore. Observed recapture rates and times agree with solutions of a drift-diffusion model. Electric forces draw DNA to the pore over micron distances, and, upon arrival at the pore, molecules begin translocation almost immediately. Repeated translocation of the same molecule improves measurement accuracy, offers a way to probe chemical transformations and internal dynamics of macromolecules on sub-millisecond time and sub-micron length scales, and demonstrates the ability to trap, study, and manipulate individual macromolecules in solution.

We measured the UV optical absorption of single-stranded DNA bound to single-walled carbon nanotubes (DNA/SWNT). The nucleotide absorbance from DNA/SWNT provides the first experimental confirmation that DNA binds to nanotubes through pi-stacking. Because the hypochromic absorbance typical of pi-stacked structures are expected to occur primarily for DNA dipole transitions that lie along the axis of the optically anisotropic SWNTs, the absorbance changes following binding of DNA to nanotubes reveal the preferred orientation assumed by each of the four bound nucleotides with respect to the nanotube's long axis.

The single-molecule accuracy of nanopore sequencing has been an area of rapid academic and commercial advancement, but remains insufficient for the de novo analysis of genomes. We introduce here a novel algorithm for the error correction of nanopore data, utilizing statistical models of the physical system in order to obtain high accuracy de novo sequences at a range of coverage depths. We demonstrate the technique by sequencing M13 bacteriophage DNA to 99% accuracy at moderate coverage as well as its use in an assembly pipeline by sequencing λ DNA at a range of coverages. We also show the algorithm’s ability to accurately classify sequence variants at far lower coverage than existing methods.

We report on an experiment and calculations that determine the thermal motion of a voltage-clamped single-stranded DNA-NeutrAvidin complex in a Mycobacterium smegmatis porin A nanopore. The electric force and diffusion constant of DNA inside a Mycobacterium smegmatis porin A pore were determined to evaluate the thermal position fluctuations of DNA. We show that an out-of-equilibrium state returns to equilibrium so quickly that experiments usually measure a weighted average over the equilibrium position distribution. Averaging over the equilibrium position distribution is consistent with results of state-of-the-art nanopore sequencing experiments. It is shown how a reduction in thermal position fluctuations can be achieved by increasing the electrophoretic force used in nanopore sequencing devices.

Nanopores in graphene membranes can potentially offer unprecedented spatial resolution for single molecule sensing, but their fabrication has thus far been difficult, poorly scalable, and prone to contamination. We demonstrate an in-situ fabrication method that nucleates and controllably enlarges nanopores in electrolyte solution by applying ultra-short, high-voltage pulses across the graphene membrane. This method can be used to rapidly produce graphene nanopores with subnanometer size accuracy in an apparatus free of nanoscale beams or tips.

The accuracy of sequencing single DNA molecules with nanopores is continually improving, but de novo genome sequencing and assembly using only nanopore data remain challenging. Here we describe PoreSeq, an algorithm that identifies and corrects errors in nanopore sequencing data and improves the accuracy of de novo genome assembly with increasing coverage depth. The approach relies on modeling the possible sources of uncertainty that occur as DNA transits through the nanopore and finds the sequence that best explains multiple reads of the same region. PoreSeq increases nanopore sequencing read accuracy of M13 bacteriophage DNA from 85% to 99% at 100× coverage. We also use the algorithm to assemble Escherichia coli with 30× coverage and the λ genome at a range of coverages from 3× to 50×. Additionally, we classify sequence variants at an order of magnitude lower coverage than is possible with existing methods.

Voltage biased solid-state nanopores are used to detect and characterize individual single stranded DNA molecules of fixed micrometer length by operating a nanopore detector at pH values greater than approximately 11.6. The distribution of observed molecular event durations and blockade currents shows that a significant fraction of the events obey a rule of constant event charge deficit (ecd) indicating that they correspond to molecules translocating through the nanopore in a distribution of folded and unfolded configurations. A surprisingly large component is unfolded. The result is an important milestone in developing solid-state nanopores for single molecule sequencing applications.

Nanostructures can be patterned with focused electron or ion beams in thin, stable, conformal films of water ice grown on silicon. We use these patterns to reliably fabricate sub-20 nm wide metal lines and exceptionally well-defined, sub-10 nanometer beam-induced chemical surface transformations. We argue more generally that solid phase condensed gasses of low sublimation energy are ideal materials for nanoscale patterning, and water, quite remarkably, may be amongst the most useful.

We report the design of an “ion sculpting” instrument that enables the controlled fabrication of nanometer-sized structures in solid-state materials. The instrument employs a beam of kilo-electron-volt argon ions that impinge on a solid-state membrane containing prefabricated structures such as holes, slits, or cavities whose properties are to be modified. By controlling both the ion beam parameters and sample temperature, the instrument can be adjusted to either deliver or remove material from these articulations, for example opening or closing holes of various shapes. The instrument is unique in its use of feedback control for the crafting of structures that define a hole through which a component of the incident ion beam is permitted to pass and be monitored. Electrostatic ion optics refocus ions transmitted unimpeded through the hole, onto a detector capable of registering single ions. The transmission rate is a direct, real-time measure of the transmitting area that is used as a feedback signal to tr...

Single nanometer-sized pores in suspended graphene membranes have been used to detect and control DNA molecules, and could be the basis for next-generation single-molecule DNA sequencing devices. The extremely thin graphene membrane can offer superior longitudinal resolution to protein nanopores, leading to lower error rates for sequencing. We demonstrate a novel method of creating graphene pores by applying short electrical pulses across graphene membranes already wet in solution, allowing rapid fabrication of pores with subnanometer precision. These pores readily exhibit translocation of double and single stranded DNA, and are a robust and scalable platform for sequencing devices.

Atomic layer deposition of alumina enhanced the molecule sensing characteristics of fabricated nanopores by fine-tuning their surface properties, reducing 1/f noise, neutralizing surface charge to favor capture of DNA and other negative polyelectrolytes, and controlling the diameter and aspect ratio of the pores with near single Angstrom precision. The control over the chemical and physical nature of the pore surface provided by atomic layer deposition produced a higher yield of functional nanopore detectors.

We report the formation of a tunable single DNA molecule trap near a solid-state nanopore in an electrolyte solution under conditions where an electric force and a pressure-induced viscous flow force on the molecule are nearly balanced. Trapped molecules can enter the pore multiple times before escaping the trap by passing through the pore or by diffusing away. Statistical analysis of many individually trapped molecules yields a detailed picture of the fluctuation phenomena involved, which are successfully modeled by a one-dimensional first passage approach.

Manipulating matter at the nanometre scale is important for many electronic, chemical and biological advances, but present solid-state fabrication methods do not reproducibly achieve dimensional control at the nanometre scale. Here we report a means of fashioning matter at these dimensions that uses low-energy ion beams and reveals surprising atomic transport phenomena that occur in a variety of materials and geometries. The method is implemented in a feedback-controlled sputtering system that provides fine control over ion beam exposure and sample temperature. We call the method "ion-beam sculpting", and apply it to the problem of fabricating a molecular-scale hole, or nanopore, in a thin insulating solid-state membrane. Such pores can serve to localize molecular-scale electrical junctions and switches and function as masks to create other small-scale structures. Nanopores also function as membrane channels in all living systems, where they serve as extremely sensitive electro-mechanical devices that regulate electric potential, ionic flow, and molecular transport across cellular membranes. We show that ion-beam sculpting can be used to fashion an analogous solid-state device: a robust electronic detector consisting of a single nanopore in a Si3N4 membrane, capable of registering single DNA molecules in aqueous solution.

A new monoenergetic MeV positron beam source was designed and constructed at the Lawrence Livermore National Laboratory (LLNL) positron facility. The positron source provides a low emittance beam with high energy resolution for channeling, scattering, and in-flight annihilation experiments. New beam optics for extracting slow positrons from a moderator mounted in a Pelletron electrostatic accelerator is presented. A beam line was designed and constructed to energy analyze the output of the accelerator and to provide adjustable size and angular divergence beams to a target station. A post target large angle acceptance magnet separates positrons transmitted through the target from forward gamma ray emission.

As a step towards developing fast positrons as a probe to measure electron density and spin magnetic density in crystals, an experiment has been performed to measure the multiple scattering distribution of MeV positrons penetrating through thin polycrystalline films in both random and channeling directions. Image plate detectors were used to determine the angular distribution of emergent positrons. Image plate sensitivity was measured for high-energy positrons and for the background radiation encountered in our experiment. The image plates showed good sensitivity and good linearity for MeV positrons. The multiple scattering results are compared with the theory for random direction and show disagreement due to crystal effects and quantum mechanical considerations.

Energy-resolved two-quantum annihilation in flight of positrons with energies ranging from 10 to 71.6 keV was observed. An energy-dispersive two-detector coincidence system was used to observe the sum and difference energies of the {gamma} rays from annihilating positron-electron pairs. For positrons penetrating carbon foils the c/v dependence of the annihilation cross section is confirmed. Spectra obtained from gold-coated carbon foils show evidence of in-flight annihilation with gold M-shell electrons. (c) 1999 The American Physical Society.

Energetic, positively charged particles travelling along a low-index crystal direction undergo many highly correlated, small-angle scattering events; the effect of these interactions is to guide or ‘channel’ (refs 1,2,3,4,5,6,7,8) the particles through the lattice. Channelling effectively focuses positive particles into the interstitial regions of the crystal: nuclear collisional processes such as Rutherford backscattering are suppressed, while the number of interactions with valence electrons increases. The interaction of channelled positrons with electrons produces annihilation radiation that can in principle9,10,11,12 serve as a quantitative, spatially selective probe of electronic charge and spin densities within the crystal: in the interstitial regions, two-photon annihilation is enhanced relative to single-photon annihilation, because the latter process requires a nuclear recoil to conserve momentum. Here we report observations of single- and two-photon annihilation from channelled positrons, using a monoenergetic beam flux of 105 particles per second. Comparison of these two annihilation modes demonstrates the ability of channelled positrons to selectively sample valence electrons in a crystal. Useful practical implementation of the technique will require the development of more intense positron beams with fluxes approaching 107 particles per second.

We report on the first direct energy resolved measurements of two quantum annihilation in flight (TQAF) using monoenergetic positrons below 100 keV. A monoenergetic positron beam at Washington State University with variable kinetic energy up to 70 keV was guided onto thin aluminum targets. To clearly identify in flight positron annihilation a two dimensional Doppler broadening technique was employed utilizing two HPGe detectors. The method uniquely identifies TQAF by providing the total energy of the annihilating electron-positron pair. A basic understanding of the fundamental TQAF process has been obtained using this high resolution technique. With this fundamental understanding we believe several new avenues of experimental exploration are possible.

X-ray bound states trapped on a curved surface are realized by coupling subangstrom photons at grazing angles into a bent silicon wafer with adjustable radius. At small radii of curvature the usual ray approximation breaks down and new phenomena associated with wave optical behavior of x-ray surface eigenmodes are revealed. For such samples, tunneling is shown to extinguish all but the ground state surface eigenmode, whose presence may be detected after extremely long distances of propagation along the surface.

We have measured the ion-channeling minimum yield ({chi}{sub min}) and angular width ({Psi}{sub 1/2}) for Y, Ba, Cu, and O in 2000 {Angstrom} (001)-oriented films of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} on MgO. The measurements mapped out a 30 K region around the critical temperature (T{sub c}) in 1{endash}2 K steps, and T{sub c} was determined {ital in situ}. A {sup 16}O({alpha},{alpha}){sup 16}O resonance was used to study the O motions. {chi}{sub min} increases and {Psi}{sub 1/2} decreases with increasing temperature for all elements, as expected for a smooth increase in vibration amplitude. We see no anomalous jumps in either parameter, which differs from previous reports on YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} bulk single crystals. {copyright} {ital 1997} {ital The American Physical Society}

We report experimental escape time distributions of double-stranded DNA (dsDNA) molecules initially threaded halfway through a thin solid-state nanopore. We find a universal behavior of the escape time distributions consistent with a one-dimensional first passage formulation notwithstanding the geometry of the experiment and the potential role of complex molecule-liquid-pore interactions. Diffusion constants that depend on the molecule length and pore size are determined. Also discussed are the practical implications of long time diffusive molecule trapping in the nanopore.

Voltage-biased solid-state nanopores are well established in their ability to detect and characterize single polymers, such as DNA, in electrolytes. The addition of a pressure gradient across the nanopore yields a second molecular driving force that provides new freedom for studying molecules in nanopores. In this work, we show that opposing pressure and voltage bias enables nanopores to detect and resolve very short DNA molecules, as well as to detect near-neutral polymers.

The subtleties of the interplay between classical and quantum pictures are revealed particularly clearly in connection with the effect of channeling. The role of this interplay is described and related to a new probe for valence electron and spin magnetic densities in thin films, which is based upon two-photon annihilation in flight of channeled MeV positrons. The channeling effect is used to steer positrons into the interstices of a crystal, where the valence electron density is high. Transmission measurements have been performed to test our quantitative understanding of the spatial and momentum distribution of channeled positrons. We calculate annihilation rates in a gold crystal and show that the probe is highly sensitive to valence electrons. Channeling states in iron are shown to be sensitive to different regions of the spin magnetic density. With a polarized beam of MeV positrons and the possibility for selective state population, we show that the spatial distribution of the magnetization density can be probed.

Uniform coaxial coating of suspended single-walled carbon nanotubes with high-quality dielectric silicon nitride has been obtained by low-pressure chemical vapor deposition. A three-terminal device has been demonstrated by coating a suspended metallic nanotube grown directly on contacting metal electrodes with subsequent patterning of a top gate electrode. Large charging energies have been observed in the suspended nanotubes and the conversion factor from gate voltage to the electrostatic potential in the nanotube approaches unity, which can be attributed to the device geometry.

A fabrication method for positioning and embedding a single-walled carbon nanotube (SWNT) across the diameter of a solid state nanopore is presented. Chemical vapor deposition (CVD) is used to grow SWNTs over arrays of focused ion beam (FIB) milled pores in a thin silicon nitride membrane. This typically yields at least one pore whose diameter is centrally crossed by a SWNT. The final diameter of the FIB pore is adjusted to create a nanopore of any desired diameter by atomic layer deposition (ALD), simultaneously embedding and insulating the SWNT everywhere but in the region that crosses the diameter of the final nanopore, where it remains pristine and bare. This nanotube-articulated nanopore is an important step towards the realization of a new type of detector for biomolecule sensing and electronic characterization, including DNA sequencing.

X-ray standing-wave measurements, along with first-principles local-density molecular-cluster calculations, have determined the Bi-dimer orientation, location, and bond length for the Si(001)-(1{times}2):Bi surface. The results for Bi directly scale with the covalent radii and with adsorption characteristics of other group-V elements (As and Sb) on Si(001).

X-ray standing-wave (111) and (11[bar 1]) measurements have been performed on the room-temperature and high-temperature phases of a Pb layer on Ge(111). At room temperature, our results support a four-atom--unit-cell surface structure. On passing into the high-temperature phase, we find that there is an increase in the Fourier coefficient along the surface [11[bar 2]] direction for the Pb density distribution. Our model, which requires a 0.6-A in-plane thermal vibration amplitude, is similar to one previously proposed by Hwang and Golovchenko and disagrees with a strictly two-dimensional-liquid interpretation.

We have studied the high coverage phases of Pb on a Si(111) surface using a scanning tunneling microscope (STM). For annealed samples, 1 ML Pb forms an incommensurate (IC) phase composed of alternating domains of two types of trimer regions and a quasi-1 × 1 region. The detailed morphology of the domains depends sensitively on the stress fields resulting from imperfections on the surface. We observe that the stress fields originating from nearby reconstructions transform the IC phase into a 1 × 1 structure. The transformation involves only small displacements of Pb atoms. We observe that the 1/3 ML √3 phase suppresses the formation of trimers in the 1 × 1 phase, but Pb islands and Pb clusters enhance their formation. The same structural transformation from IC to 1 × 1 can occur at high temperatures. The local transformation temperature depends strongly on surface imperfections nearby. The high coverage phase for room temperature (RT) deposition, Pb/Si-7 × 7, is also resolved. Pb deposition on this phase causes interesting changes in its morphology. The 1 × 1 adatoms in Pb/Si-7 × 7 appear to relax normal to the surface relative to the surrounding unreconstructed regions. The formation of the IC phase for Pb/Si(111) offers a novel system for understanding incommensurate surface reconstructions in a chemisorption system.

Single-walled carbon nanotubes have been grown on a variety of substrates by chemical vapor deposition using low-coverage vacuum-deposited iron as a catalyst. Ordered arrays of suspended nanotubes ranging from submicron to several micron lengths have been obtained on Si, SiO 2 , Al 2 O 3 , and Si 3 N 4 substrates that were patterned on hundred nanometer length scales with a focused ion beam machine. Electric fields applied during nanotube growth allow the control of growth direction. Nanotube circuits have been constructed directly on contacting metal electrodes of Pt/Cr patterned with catalysts. Patterning with solid iron catalyst is compatible with modern semiconductor fabrication strategies and may contribute to the integration of nanotubes in complex device architectures.

We demonstrate that voltage-biased solid-state nanopores can transiently localize DNA in an electrolyte solution. A double-stranded DNA (dsDNA) molecule is trapped when the electric field near the nanopore attracts and immobilizes a nonend segment of the molecule across the nanopore orifice without inducing a folded molecule translocation. In this demonstration of the phenomenon, the ionic current through the nanopore decreases when the dsDNA molecule is trapped by the nanopore. By contrast, a translocating dsDNA molecule under the same conditions causes an ionic current increase. We also present finite-element modeling results that predict this behavior for the conditions of the experiment.

Electron beam (e-beam) lithography using polymer resists is an important technology that provides the spatial resolution needed for nanodevice fabrication. But it is often desirable to pattern nonplanar structures on which polymeric resists cannot be reliably applied. Furthermore, fragile substrates, such as free-standing nanotubes or thin films, cannot tolerate the vigorous mechanical scrubbing procedures required to remove all residual traces of the polymer resist. Here we demonstrate several examples where e-beam lithography using an amorphous ice resist eliminates both of these difficulties and enables the fabrication of unique nanoscale device structures in a process we call ice lithography. (1, 2) We demonstrate the fabrication of micro- and nanostructures on the tip of atomic force microscope probes, microcantilevers, transmission electron microscopy grids, and suspended single-walled carbon nanotubes. Our results show that by using amorphous water ice as an e-beam resist, a new generation of nanodevice structures can be fabricated on nonplanar or fragile substrates.

We study a reversible phase transition of 1 monolayer (ML) of Pb on a Ge(111) surface using a scanning tunneling microscope (STM) over a temperature range from below to above the transition (\ensuremath{\sim}175 \ifmmode^\circ\else\textdegree\fi{}C). Below the transition temperature, a periodic array of Pb trimers centered on ${\mathrm{H}}_{3}$ sites is resolved. The controversial high-temperature phase is actually a simple Pb-terminated 1\ifmmode\times\else\texttimes\fi{}1 structure. Just above the transition temperature, we observe a strongly fluctuating structure. Between 1/3 and 1 ML of Pb coverage, the transition is found to start in the vicinity of the phase boundary with a 1/3-ML \ensuremath{\surd}3 phase. At 1 ML we also observe the transition beginning at the step edges on the upper terraces. A sharp increase in the transition temperature occurs near 1 ML. The transition temperature is also found to depend on the domain size. The role of surface-strain fields on the phase transformation is discussed.

The design of a novel‐type of atomic beam source which provides for long term, stable operation at high emission rates is reported. The heart of the design is the ‘‘candlestick’’ where liquid source material is transported by capillary action to a localized hot emission region. A surrounding cavity kept at the melting point for the source material shields the vacuum chamber walls from this region. The atomic beam escaping from the source is collimated, and uncollimated atoms are transported back to the liquid reservoir at the bottom of the ‘‘candlestick’’ by capillary action. This design has advantages over traditional oven designs: localized heating provides for large emission rates under high vacuum conditions, collimation is combined with recycling and conservation of source material, and the use of capillarity allows any orientation of the beam source. The source has been tested with sodium, and we believe that the design is useful for a broad range of applications including thin‐film evaporation, mol...

X-ray standing-wave measurements and tunneling microscopy have been combined to solve the atomic geometry of the [radical]3[times][radical]3[ital R]30[degree] honeycomb phase of Bi on Si(111). The standing-wave measurements utilize three different diffracting planes to triangulate the surface position of Bi atoms. The unoccupied surface sites required to completely determine the structure can be deduced from Rutherford-backscattering coverage and low-energy electron-diffraction symmetry arguments. These arguments are completely confirmed by a tunneling-microscope study, which is free of the ambiguities of previous studies. The final result is a 2/3-ML [radical]3[times][radical]3[ital R]30[degree] structure with Bi atoms in the [ital T][sub 1] sites directly above first-layer Si atoms.

Using the x-ray standing-wave method, we have measured directly the thermal vibration amplitudes [l angle][ital u][sup 2][r angle] of symmetric As dimers on a Si(001)-(2[times]1) surface. For sample temperatures in the range 300 K [le][ital T][le]650 K, the results are Debye-like. Above 650 K [l angle][ital u][sup 2][r angle] varies more rapidly, indicating the onset of defect-mediated processes. We also found that at room temperature the bond length of the As dimers is 2.58[plus minus]0.04 A and that, independent of temperature, they sit 1.40[plus minus]0.01 A above the top bulk-extrapolated silicon (004) plane. These results provide a critical test for theoretical structure calculations.

We report the successful application of a new approach, ice lithography (IL), to fabricate nanoscale devices. The entire IL process takes place inside a modified scanning electron microscope (SEM), where a vapor-deposited film of water ice serves as a resist for e-beam lithography, greatly simplifying and streamlining device fabrication. We show that labile nanostructures such as carbon nanotubes can be safely imaged in an SEM when coated in ice. The ice film is patterned at high e-beam intensity and serves as a mask for lift-off without the device degradation and contamination associated with e-beam imaging and polymer resist residues. We demonstrate the IL preparation of carbon nanotubes field effect transistors (FETs) with high quality trans-conductance properties.

We study a reversible phase transition of one monolayer of Pb on a Ge(111) surface with a tunneling microscope over a temperature range from below to above the transition (∼175 o C). The controversial high temperature phase is actually a simple Pb-terminated 1×1 structure. Below the transition temperature, Pb atoms are displaced from their T 1 positions and form trimers with √3×√3 periodicity. A fluctuating structure with patches of trimers can be resolved slightly above the transition temperature. The important effects of boundaries and domain size on the phase transformation are discussed

In molecular-beam epitaxy a monolayer of Pb on the Si(111) surface induces single-crystal growth at temperatures well below those required for similar growth on a bare surface. We demonstrate that the suppression of dopant segregation at the lower temperatures attainable by Pb-mediated growth allows the incorporation of As donors at concentrations reaching a few atomic percent. When Pb and Si are deposited on an As-terminated Si(111) substrate at 350 °C, the Pb segregates to the surface without doping the Si film while the As is buried within nanometers of the substrate–film interface. The resulting concentration of electrically active As, 1.8×1021 cm−3, represents the highest concentration of As donors achieved by any delta-doping or thin-film deposition method.

Laser-cooled rubidium atoms are captured by the inhomogeneous electric field surrounding a single charged suspended carbon nanotube. The atom experiences an induced dipole attractive potential with a singular inverse square scaling. The ultimate fate of the captured atom is field ionization near the wall of the nanotube. Field ionization detection is fast, efficient, and position sensitive, and could become the detection method for a new generation of atom-nanotube traps.