Danny Porath

Long-range charge transport in single G-quadruplex DNA molecules

DNA and DNA-based polymers are of interest in molecular electronics because of their versatile and programmable structures. However, transport measurements have produced a range of seemingly contradictory results due to differences in the measured molecules and experimental set-ups, and transporting significant current through individual DNA-based molecules remains a considerable challenge. Here, we report reproducible charge transport in guanine-quadruplex (G4) DNA molecules adsorbed on a mica substrate. Currents ranging from tens of picoamperes to more than 100u2005pA were measured in the G4-DNA over distances ranging from tens of nanometres to more than 100u2005nm. Our experimental results, combined with theoretical modelling, suggest that transport occurs via a thermally activated long-range hopping between multi-tetrad segments of DNA. These results could re-ignite interest in DNA-based wires and devices, and in the use of such systems in the development of programmable circuits.

SP1 protein-based nanostructures and arrays.

Controlled formation of complex nanostructures is one of the main goals of nanoscience and nanotechnology. Stable Protein 1 (SP1) is a boiling-stable ring protein complex, 11 nm in diameter, which self-assembles from 12 identical monomers. SP1 can be utilized to form large ordered arrays; it can be easily modified by genetic engineering to produce various mutants; it is also capable of binding gold nanoparticles (GNPs) and thus forming protein-GNP chains made of alternating SP1s and GNPs. We report the formation and the protocols leading to the formation of those nanostructures and their characterization by transmission electron microscopy, atomic force microscopy, and electrostatic force microscopy. Further control over the GNP interdistances within the protein-GNP chains may lead to the formation of nanowires and structures that may be useful for nanoelectronics.

Energy level tunneling spectroscopy and single electron charging in individual CdSe quantum dots

We directly show the evolution of the electronic structure of semiconductor quantum dots (QDs) with QD size in the strong confinement regime by employing low-temperature tunneling current–voltage spectroscopy to individual electrodeposited CdSe QDs. From the spectra we measure the values of Eg, map discrete energy levels in both valence and conduction “bands,” and show the occurrence of single-electron tunneling effects for isolated QDs of different sizes (2, 3, and 4.5 nm diameter). Since tunneling is not limited by optical selection rules, we are able to directly measure energy level spacings not necessarily accessible by optical spectroscopy. Our spectra demonstrate the interplay between charging and energy level spacing, resulting in a rich and controllable structure that forms a basis for QD nanoelectronic devices.

Electrical characterization of self-assembled single- and double-stranded DNA monolayers using conductive AFM

We recently reported electrical transport measurements through double-stranded (ds)DNA molecules that are embedded in a self-assembled monolayer of single-stranded (ss)DNA and attached to a metal substrate and to a gold nanoparticle (GNP) on opposite ends. The measured current flowing through the dsDNA amounts to 220 nA at 2 V. In the present report we compare electrical transport through an ssDNA monolayer and dsDNA monolayers with and without upper thiol end-groups. The measurements are done with a conductive atomic force microscope (AFM) using various techniques. We find that the ssDNA monolayer is unable to transport current. The dsDNA monolayer without thiols in the upper end can transport low current on rare occasions and the dsDNA monolayer with thiols on both ends can transport significant current but with a much lower reliability and reproducibility than the GNP-connected dsDNA. These results reconfirm the ability of dsDNA to transport electrical current under the appropriate conditions, demonstrate the efficiency of an ssDNA monolayer as an insulating layer, and emphasize the crucial role of an efficient charge injection through covalent bonding for electrical transport in single dsDNA molecules.

Scanning tunneling microscopy studies and computer simulations of annealing of gold films

The effect of thermal annealing on the surface morphology of thin gold films is studied using a scanning tunneling microscope (STM) and computer simulations. The gold films were thermally evaporated onto glass substrates, and were then measured with the STM at room temperature before and after annealing. The annealing treatments were done at temperatures between 200 and 500u2009°C and for periods ranging from 1 to 200 h. We present data showing the evolution of the average surface‐grain size and root‐mean‐square roughness amplitude of the gold films as a function of annealing temperature and duration. Our data suggest that surface diffusion is the main process active at low annealing temperatures of 300u2009°C and below. At higher annealing temperatures grain coarsening, which can be explained by recovery and recrystallization (secondary grain growth), is the dominant process contributing to large scale morphology changes. Computer simulations based on these processes account well for the experimental results, wi...

Logic implementations using a single nanoparticle–protein hybrid

A Set-Reset machine is the simplest logic circuit with a built-in memory. Its output is a (nonlinear) function of the input and of the state stored in the machines memory. Here, we report a nanoscale Set-Reset machine operating at room temperature that is based on a 5-nm silicon nanoparticle attached to the inner pore of a stable circular protein. The nanoparticle-protein hybrid can also function as a balanced ternary multiplier. Conductive atomic force microscopy is used to implement the logic input and output operations, and the processing of the logic Set and Reset operations relies on the finite capacitance of the nanoparticle provided by the good electrical isolation given by the protein, thus enabling stability of the logic device states. We show that the machine can be cycled, such that in every successive cycle, the previous state in the memory is retained as the present state. The energy cost of one cycle of computation is minimized to the cost of charging this state.

Annealing study of gold films using scanning tunneling microscopy

We have studied thermal annealing effects on the surface morphology of 800 A thick gold films, using scanning tunneling microscopy. The gold films were thermally evaporated onto glass substrates and were then measured with the scanning tunneling microscope at room temperature before and after annealing. The annealing treatments were done at temperatures between 200 and 500u2009°C and for periods ranging from 1 to 60 h. We present data showing the evolution of the average surface‐grain size and rms roughness amplitude of the gold films as a function of annealing temperature and duration. Our data suggest that surface diffusion is the main process active at low annealing temperatures of 300u2009°C and below. The activation energy for surface self‐diffusion of gold in our samples was around 1.1 eV. Other mechanisms seem to be dominant at higher temperatures.

Scanning tunneling microscopy studies of annealing of gold films

Abstract We have studied thermal annealing effects on the surface morphology of 800 A thick gold films, using scanning tunneling microscopy. The gold films were thermally evaporated onto glass substrates, and were then measured with the scanning tunneling microscope at room temperature before and after annealing. The annealing treatments were done at temperatures between 200 and 500°C and for times from 3 to 60 h. The topographic images were analyzed using various statistical methods and image-processing techniques. We present data showing the evolution of the average surface-grain size and rms roughness amplitude of the gold films as a function of annealing temperature and duration. The typical grain size was found to increase with time for all annealing temperatures with a rate that increases with temperature. The rms roughness amplitude, on the other hand, shows a more complex dependence on the annealing temperature. Our data suggest that surface diffusion is the main process active at low annealing temperatures, but other mechanisms are dominant in the high-temperature range.

Low temperature scanning tunneling microscopy studies of granular metal films

Cryogenic scanning tunneling microscopy is used to study local electrical transport properties of thin granular Au/Al2O3 films in the vicinity of the percolation threshold. The current–voltage characteristics are found to vary dramatically from one tip position to another over distances of the order of a few nanometers. The characteristics often exhibit single electron tunneling effects such as the Coulomb blockade and the Coulomb staircase. This behavior is similar to that observed for tunneling into a single isolated nanometer size metallic particle which was explained in terms of a double‐barrier tunnel junction model. Some of the characteristics show, however, novel Coulomb‐staircase structures having unusual variations in step widths and heights due to complex tunneling paths. A triple‐barrier tunnel junction model, where the electron tunnels through two metallic particles along its path, accounts quantitatively for the experimental results.

Direct measurements of electrical transport through DNA molecules

We present direct measurements of electrical charge transport through single DNA molecules. The molecules are electrostatically trapped between two metal nanoelectrodes separated by 8 nm and current flow through the DNA is measured upon application of a voltage between these electrodes. The measured current is negligible up to a threshold voltage followed by a sharp rise of the current. The conductance curves show a peak structure as a function of voltage, which suggests that the charge transport is mediated by the energy bands of the measured DNA. The existence of DNA between the electrodes is verified using DNase I enzyme control experiments.