Jene Andrew Golovchenko

Ion-beam Sculpting at Nanometre Length Scales

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.

Graphene as a subnanometre trans-electrode membrane

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.

Optical Matter: Crystallization and Binding in Intense Optical Fields

Properly fashioned electromagnetic fields coupled to microscopic dielectric objects can be used to create arrays of extended crystalline and noncrystalline structures. Organization can be achieved in two ways: In the first, dielectric matter is transported in direct response to the externally applied standing wave optical fields. In the second, the external optical fields induce interactions between dielectric objects that can also result in the creation of complex structures. In either case, these new ordered structures, whose existence depends on the presence of both light and polarizable matter, are referred to as optical matter.

Time‐resolved reflectivity of ion‐implanted silicon during laser annealing

The time‐resolved reflectivity at 0.63 μm from arsenic‐implanted silicon crystals has been measured during annealing by a 1.06‐μm laser pulse of 50‐ns duration. The reflectivity was observed to change abruptly to the value consistent with liquid silicon and to remain at that value for a period of time which ranged from a few tens of nanoseconds to several hundreds of nanoseconds, depending on the annealing pulse intensity. Concurrently, the transmission of the primary annealing beam dropped abruptly. These observations confirm the formation of a metallic liquid phase at the crystal surface during the annealing process.

Recapturing and Trapping Single Molecules with a Solid State Nanopore

The development of solid-state nanopores, inspired by their biological counterparts, shows great potential for the study of single macromolecules. Applications such as DNA sequencing and the exploration of protein folding require control of the dynamics of the molecules interaction with the pore, but DNA capture by a solid-state nanopore is not well understood. By recapturing individual molecules soon after they pass through a nanopore, we reveal the mechanism by which double-stranded DNA enters the pore. The observed recapture rates and times agree with solutions of a drift-diffusion model. Electric forces draw DNA to the pore over micrometer-scale 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 the chemical transformations and internal dynamics of macromolecules on sub-millisecond time and sub-micrometre length scales, and demonstrates the ability to trap, study and manipulate individual macromolecules in solution.

Atomic-scale surface modifications using a tunnelling microscope

The desire to modify materials on the smallest possible scale is motivated by goals ranging from high-density information storage to the purposeful transformation of genetic material. Here we report an atomic-scale modification of the surface of a nearly perfect germanium crystal, effected by the tungsten tip of a tunnelling microscope. We believe this to be the smallest spatially controlled, purposeful transformation yet impressed on matter and we argue that the limit set by the discreteness of atomic structure has now essentially been reached.

Atom-by-atom nucleation and growth of graphene nanopores

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.

Molecule-hugging graphene nanopores

It has recently been recognized that solid-state nanopores in single-atomic-layer graphene membranes can be used to electronically detect and characterize single long charged polymer molecules. We have now fabricated nanopores in single-layer graphene that are closely matched to the diameter of a double-stranded DNA molecule. Ionic current signals during electrophoretically driven translocation of DNA through these nanopores were experimentally explored and theoretically modeled. Our experiments show that these nanopores have unusually high sensitivity (0.65 nA/Å) to extremely small changes in the translocating molecule’s outer diameter. Such atomically short graphene nanopores can also resolve nanoscale-spaced molecular structures along the length of a polymer, but do so with greatest sensitivity only when the pore and molecule diameters are closely matched. Modeling confirms that our most closely matched pores have an inherent resolution of ≤0.6 nm along the length of the molecule.

The Tunneling Microscope: A New Look at the Atomic World

A new instrument called the tunneling microscope has recently been developed that is capable of generating real-space images of surfaces showing atomic structure. These images offer a new view of matter on an atomic scale. The current capabilities and limitations and the physics involved in the technique are discussed along with specific results from a study of silicon crystal surfaces.

Growth and morphology of Pb on Si(111)

Abstract Tunneling microscopy, thermal desorption, Rutherford backscattering, and low-energy electron diffraction are used to study the structures and coverages of the phases of Pb on the Si(111)7 × 7 surface. For room-temperature deposition at low coverage on the 7 × 7 surface, Pb atoms occupy sites above the rest atoms and between the Si adatoms (with a preference for the faulted half of the unit cell). At 0.6 ML, the Pb forms an ordered overlayer based on the 7 × 7 unit cell. The Pb grows epitaxially up to 3 ML at which point Pb crystals start to form. On annealed samples at low coverage, Pb atoms occupy Si(111)7 × 7 adatom sites. At 1 6 ML, a new √3 × √3 mosaic phase consisting of alternating chains of Pb and Si adatoms with a high melting point is produced. At 1 3 ML the standard √3 × √3 phase is observed. Between 1 3 and 1 ML, a 1 × 1 Pb overlayer is found on annealed samples, while above 1 ML, a rotated incommensurate phase is observed. As more Pb is added above 1 ML, the 2D Pb density is increased, and the incommensuration is reduced, until island formation begins. We link our discussion of the atomic structure of the interface to the variations in Schottky barrier heights observed for Pb/Si(111) diodes.