Person:

Branton, Daniel

We describe the design of an instrument that can fully implement a new nanopatterning method called ice lithography, where ice is used as the resist. Water vapor is introduced into a scanning electron microscope (SEM) vacuum chamber above a sample cooled down to 110 K. The vapor condenses, covering the sample with an amorphous layer of ice. To form a lift-off mask, ice is removed by the SEM electron beam (e-beam) guided by an e-beam lithography system. Without breaking vacuum, the sample with the ice mask is then transferred into a metal deposition chamber where metals are deposited by sputtering. The cold sample is then unloaded from the vacuum system and immersed in isopropanol at room temperature. As the ice melts, metal deposited on the ice disperses while the metals deposited on the sample where the ice had been removed by the e-beam remains. The instrument combines a high beam-current thermal field emission SEM fitted with an e-beam lithography system, cryogenic systems, and a high vacuum metal deposition system in a design that optimizes ice lithography for high throughput nanodevice fabrication. The nanoscale capability of the instrument is demonstrated with the fabrication of nanoscale metal lines.

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. 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.

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, 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.

We quantify the base dependent interactions between single stranded DNA and single walled carbon nanotubes (SWNTs) in solution. DNA/SWNT hybrids hold the promise of applications ranging from nanoscale electronics and assembly of nanotube based materials, to drug delivery and DNA sequencing. These applications require control over the hybrid assembly and disassembly. Our analytical assay reveals the order of nucleobase binding strengths with SWNTs as G>C>A>T. Furthermore, time dependent fixed temperature experiments that probe the kinetics of the dissociation process provide values for the equilibrium constants and dissociation enthalpies that underlie the microscopic interactions. Quantifying the base dependency of hybrid stability shows how insight into the energetics of the component interactions facilitates control over hybrid assembly and disassembly.

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.

A variety of different DNA polymers were electrophoretically driven through the nanopore of an α-hemolysin channel in a lipid bilayer. Single-channel recording of the translocation duration and current flow during traversal of individual polynucleotides yielded a unique pattern of events for each of the several polymers tested. Statistical data derived from this pattern of events demonstrate that in several cases a nanopore can distinguish between polynucleotides of similar length and composition that differ only in sequence. Studies of temperature effects on the translocation process show that translocation duration scales as ∼T−2. A strong correlation exists between the temperature dependence of the event characteristics and the tendency of some polymers to form secondary structure. Because nanopores can rapidly discriminate and characterize unlabeled DNA molecules at low copy number, refinements of the experimental approach demonstrated here could eventually provide a low-cost high-throughput method of analyzing DNA polynucleotides.

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.

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 measure current blockade and time distributions for single-stranded DNA polymers during voltage-driven translocations through a single α-hemolysin pore. We use these data to determine the velocity of the polymers in the pore. Our measurements imply that, while polymers longer than the pore are translocated at a constant speed, the velocity of shorter polymers increases with decreasing length. This velocity is nonlinear with the applied field. Based on this data, we estimate the effective diffusion coefficient and the energy penalty for extending a molecule into the pore.

Single-stranded DNA and RNA molecules in solution can be driven through a nanoscopic pore by an applied electric field. As each molecule occupies the pore, a characteristic blockade of ionic current is produced. Information about length, composition, structure, and dynamic motion of the molecule can be deduced from modulations of the current blockade.