Thickness-Insensitive Properties of α-MoO3 Nanosheets by Weak Interlayer Coupling.

van der Waals (vdW) materials have shown unique electrical and optical properties depending on the thickness due to strong interlayer interaction and symmetry breaking at the monolayer level. In contrast, the study of electrical and tribological properties and their thickness-insensitivity of van der Waals oxides are lacking due to difficulties in the fabrication of high quality two-dimensional oxides and the investigation of nanoscale properties. Here we investigated various tribological and electrical properties, such as, friction, adhesion, work function, tunnel current, and dielectric constant, of the single-crystal α-MoO3 nanosheets epitaxially grown on graphite by using atomic force microscopy. The friction of atomically smooth MoO3 is rapidly saturated within a few layers. The thickness insensitivity of friction is due to very weak mechanical interlayer interaction. Similarly, work function (4.73 eV for 2 layers (hereafter denoted as L)) and dielectric constant (6 for 2L and 10.5-11 for >3L) of MoO3 in MoO3 showed thickness insensitivity due to weak interlayer coupling. Tunnel current measurements by conductive atomic force microscopy showed that even 2L MoO3 of 1.4 nm is resistant to tunneling with a high dielectric strength of 14 MV/cm. The thickness-indifferent electrical properties of high dielectric constant and tunnel resistance by weak interlayer coupling and high crystallinity show a promise in the use of MoO3 nanosheets for nanodevice applications.

Atomic scale study of black phosphorus degradation.

Black phosphorus (BP) is a promising two-dimensional (2D) material for future electronic devices due to its unique properties of high carrier mobility and large band gap tunability. However, thinner crystalline BP is more readily degraded under ambient conditions. For BP-based electronic devices, degradation of the exfoliated BP is a key issue. However, the nanometer scale study of BP degradation is rare so far. Herein, we report an atomically resolved degradation process of the BP surface using atomic force microscopy under temperature- and humidity-controlled environments. The atomically resolved crystal surface of BP deteriorated due to surface etching after cleavage, and showed monolayer etching. The etching process is accelerated by applying a bias voltage to BP via a conductive tip. After the voltage-assisted BP etching, the BP etching product shows crystalline BP confirmed by Raman spectroscopy and atomic force microscopy. Our atomic scale study of BP will be useful for the future 2D-based electronic devices to overcome conventional silicon-based electronic devices.

A tip-attached tuning fork sensor for the control of DNA translocation through a nanopore.

In this work, we demonstrate that a tuning fork can be used as a force detecting sensor for manipulating DNA molecules and for controlling the DNA translocation rate through a nanopore. One prong of a tuning fork is glued with a probe tip which DNA molecules can be attached to. To control the motion and position of the tip, the tuning fork is fixed to a nanopositioning system which has sub-nanometer position control. A fluidic chamber is designed to fulfill many requirements for the experiment: for the access of a DNA-attached tip approaching to a nanopore, for housing a nanopore chip, and for measuring ionic current through a solid-state nanopore with a pair of electrodes. The location of a nanopore is first observed by transmission electron microscopy, and then is determined inside the liquid chambers with an optical microscope combined with local scanning the probe tip on the nanopore surface. When a DNA-immobilized tip approaches a membrane surface near a nanopore, free ends of the immobilized DNA strings can be pulled and trapped into the pore by an applied voltage across the nanopore chip, resulting in an ionic current reduction through the nanopore. The trapped DNA molecules can be lifted up from the nanopore at a user controlled speed. This integrated apparatus allows manipulation of biomolecules (DNA, RNA, and proteins) attached to a probe tip with sub-nanometer precision, and simultaneously allows measurement of the biomolecules by a nanopore device.

Graphene edges and beyond: temperature-driven structures and electromagnetic properties.

The atomic configuration of graphene edges significantly influences the various properties of graphene nanostructures, and realistic device fabrication requires precise engineering of graphene edges. However, the imaging and analysis of the intrinsic nature of graphene edges can be illusive due to contamination problems and measurement-induced structural changes to graphene edges. In this issue of ACS Nano, He et al. report an in situ heating experiment in aberration-corrected transmission electron microscopy to elucidate the temperature dependence of graphene edge termination at the atomic scale. They revealed that graphene edges predominantly have zigzag terminations below 400 °C, while above 600 °C, the edges are dominated by armchair and reconstructed zigzag edges. This report brings us one step closer to the true nature of graphene edges. In this Perspective, we outline the present understanding, issues, and future challenges faced in the field of graphene-edge-based nanodevices.

Dielectrophoretic stretching of DNA tethered to a fiber tip.

In this work, we studied the stretching of λ phage DNA molecules immobilized on an optical fiber tip attached to a force sensitive tuning fork under ac electric fields. We designed a two electrodes stretching system in a small chamber: one is a gold-coated optical fiber tip electrode, and the other is a gold-coated flat electrode. By applying a dielectrophoretic (DEP) force, the immobilized λ DNA molecules on the tip are stretched and the stretching process is monitored by a fluorescent microscope. The DNA stretching in three-dimensional space is optimized by varying electrode shape, electrode gap distance, ac frequency, and solution conductivity. By observing the vibrational amplitude change of a quartz tuning fork, we measured the effects due to Joule heating and the DEP force on the tethered DNA molecules in solution. This work demonstrates a method to manipulate and characterize immobilized λ DNA molecules on a probe tip for further study of single DNA molecules.

DNA motion induced by electrokinetic flow near an Au coated nanopore surface as voltage controlled gate.

We used fluorescence microscopy to investigate the diffusion and drift motion of λ DNA molecules on an Au-coated membrane surface near nanopores, prior to their translocation through solid-state nanopores. With the capability of controlling electric potential at the Au surface as a gate voltage, Vgate, the motions of DNA molecules, which are presumably generated by electrokinetic flow, vary dramatically near the nanopores in our observations. We carefully investigate these DNA motions with different values of Vgate in order to alter the densities and polarities of the counterions, which are expected to change the flow speed or direction, respectively. Depending on Vgate, our observations have revealed the critical distance from a nanopore for DNA molecules to be attracted or repelled-DNA's anisotropic and unsteady drifting motions and accumulations of DNA molecules near the nanopore entrance. Further finite element method (FEM) numerical simulations indicate that the electrokinetic flow could qualitatively explain these unusual DNA motions near metal-collated gated nanopores. Finally, we demonstrate the possibility of controlling the speed and direction of DNA motion near or through a nanopore, as in the case of recapturing a single DNA molecule multiple times with alternating current voltages on the Vgate.

Threading immobilized DNA molecules through a solid-state nanopore at >100 µs per base rate.

In pursuit of developing solid-state nanopore-based DNA sequencing technology, we have designed and constructed an apparatus that can place a DNA-tethered probe tip near a solid-state nanopore, control the DNA moving speed, and measure the ionic current change when a DNA molecule is captured and released from a nanopore. The probe tip's position is sensed and controlled by a tuning fork based feedback force sensor and a nanopositioning system. Using this newly constructed apparatus, a DNA strand moving rate of >100 µs/base or <1 nm/ms in silicon nitride nanopores has been accomplished. This rate is 10 times slower than by manipulating DNA-tethered beads using optical tweezers and 1000 times slower than free DNA translocation through solid-state nanopores reported previously, which provides enough temporal resolution to read each base on a tethered DNA molecule using available single-channel recording electronics on the market today. This apparatus can measure three signals simultaneously: ionic current through a nanopore, tip position, and tip vibrational amplitude during the process of a DNA molecule's capture and release by a nanopore. We show results of this apparatus for measuring λ DNA's capture and release distances and for current blockage signals of λ DNA molecules biotinylated with one end and with both ends tethered to a tip.

Directly observing the motion of DNA molecules near solid-state nanopores.

We investigate the diffusion and the drift motion of λ DNA molecules near solid-state nanopores prior to their translocation through the nanopores using fluorescence microscopy. The radial dependence of the electric field near a nanopore generated by an applied voltage in ionic solution can be estimated quantitatively in 3D by analyzing the motion of negatively charged DNA molecules. We find that the electric field is approximately spherically symmetric around the nanopore under the conditions investigated. In addition, DNA clogging at the nanopore was directly observed. Surprisingly, the probability of the clogging event increases with increasing external bias voltage. We also find that DNA molecules clogging the nanopore reduce the electric field amplitude at the nanopore membrane surface. To better understand these experimental results, analytical method with Ohm's law and computer simulation with Poisson and Nernst-Planck (PNP) equations are used to calculate the electric field near the nanopore. These results are of great interest in both experimental and theoretical considerations of the motion of DNA molecules near voltage-biased nanopores. These findings will also contribute to the development of solid-state nanopore-based DNA sensing devices.

Probing Access Resistance of Solid-state Nanopores with a Scanning Probe Microscope Tip.

An apparatus that integrates solid-state nanopore ionic current measurement with a Scanning Probe Microscope has been developed. When a micrometer-scale scanning probe tip is near a voltage biased nanometer-scale pore (10-100 nm), the tip partially blocks the flow of ions to the pore and increases the pore access resistance. The apparatus records the current blockage caused by the probe tip and the location of the tip simultaneously. By measuring the current blockage map near a nanopore as a function of the tip position in 3D space in salt solution, we estimate the relative pore resistance increase due to the tip, ΔR/R(0), as a function of the tip location, nanopore geometry, and salt concentration. The amplitude of ΔR/R(0) also depends on the ratio of the pore length to its radius as Ohm's law predicts. When the tip is very close to the pore surface, ~10 nm, our experiments show that ΔR/R(0) depends on salt concentration as predicted by the Poisson and Nernst-Planck equations. Furthermore, our measurements show that ΔR/R(0) goes to zero when the tip is about five times the pore diameter away from the center of the pore entrance. The results in this work not only demonstrate a way to probe the access resistance of nanopores experimentally, they also provide a way to locate the nanopore in salt solution, and open the door to future nanopore experiments for detecting single biomolecules attached to a probe tip.

Probing access resistance of solid-state nanopores with a scanning-probe microscope tip.

An apparatus that integrates solid-state nanopore ionic current measurement with a scanning-probe microscope is developed. When a micrometer-scale scanning-probe tip is near a voltage-biased nanometer-scale pore (10­100 nm), the tip partially blocks the flow of ions to the pore and increases the pore access resistance. The apparatus records the current blockage caused by the probe tip and the location of the tip simultaneously. By measuring the current blockage map near a nanopore as a function of the tip position in 3D space in salt solution, the relative pore resistance increases due to the tip and ΔR/R0 is estimated as a function of the tip location, nanopore geometry, and salt concentration. The amplitude of ΔR/R0 also depends on the ratio of the pore length to its radius as Ohm's law predicts. When the tip is very close to the pore surface, ≈10 nm, experiments show that ΔR/R0 depends on salt concentration as predicted by the Poisson and Nernst­Planck equations. Furthermore, the measurements show that ΔR/R0 goes to zero when the tip is about five times the pore diameter away from the center of the pore entrance. The results in this work not only demonstrate a way to probe the access resistance of nanopores experimentally; they also provide a way to locate the nanopore in salt solution, and open the door to future nanopore experiments for detecting single biomolecules attached to a probe tip.

Printed magnetic FePt nanocrystal films.

Patterned monolayers and multilayers of FePt nanocrystals were printed onto substrates by first assembling nanocrystals on a Langmuir-Blodgett (LB) trough and then lifting them onto prepatterned polydimethylsiloxane (PDMS) stamps, followed by transfer printing onto the substrate. Patterned features, including micrometer-size circles, lines, and squares, could be printed using this approach. The magnetic properties of the printed nanocrystal films were also measured using magnetic force microscopy (MFM). Room-temperature MFM could detect a remanent (permanent) magnetization from multilayer (>3 nanocrystals thick) films of chemically ordered L1(0) FePt nanocrystals.