Session B14: Focus Session: Molecular-Scale Electronics and Sensors II

Transport Through A Molecule Or A Dot: Capacitance, Hysteresis, DNA And Switching

Although simple tunneling transport through molecular wires has been extensively investigated, small modifications in the coupling conditions can lead to transport regimes that are (so far) only poorly understood. We will utilize a combination of non-equilibrium Green’s functions and polaron theory to discuss hysteresis and switching in molecular current transport junctions, and simple capacitance models to discuss transport through gold quantum dots held in place with DNA. The effectiveness of relatively simple models in understanding (at least on a semi-quantitative basis) complex transport structures will be emphasized, and some conclusions will be drawn concerning the appropriate limits in which simple pictures can be recovered.   

Scanning Tunneling Microscope Observation of Current Rectification by Self-Assembled Monolayers of Conjugated Organic Molecules

The current ($I)$ voltage ($V)$ characteristics of HS-C$_{6}$H$_{4}$-CH=N-C$_{6}$H$_{4}$-N=CH-C$_{6}$H$_{4}$-SH$_{,}$ 4,4'-[1,4-phenylenebis(methylidynenitrilo)]bisbenzenethiol (PMNBT), molecules adsorbed on Au (111)/Mica substrate has been determined by scanning tunneling microscope (STM) measurements. The self-assembled monolayers (SAMs) of PMNBT molecules exhibit rectification of the tunneling current. In contrast, SAMs of $\sigma $-bonded dodecanemonothiolate molecules do not exhibit rectification. The observed rectification in the case of PMNBT SAMs is attributed to a combination of (i) charge transfer from Au to molecule at Au-S interface and (ii) a highly delocalized $\pi $-electron charge in the donor level (highest occupied molecular orbital) of the molecule.   

Scanned Probe Imaging of Nanoscale Conducting Channels in Pt/alkanoic acid monolayer/Ti Devices

The mechanisms responsible for switching in metal/molecule/metal systems are subjects of intense research. We report here a scanned probe technique that maps the conductance of a planar molecular junction (Pt/stearic acid monolayer/Ti) under mechanical perturbation by using an atomic force microscope (AFM) to apply a localized force to a molecular junction while measuring the junction conductance. Such mechano-conductance maps reveal that transport through the molecular device is dominated by nanoscale conducting channels, which emerged or disappeared when the junction is switched into higher or lower conductance states. The experimental conductance data across a wide range of device conductance are consistent with the formation of nanoscale quantum point contacts, and can be effectively described by a quantitative model that combines quantum tunneling with the growth of nano-asperities.   

Modeling the Inelastic Electron Tunneling Spectra of Molecular Wire Junctions

A method to predict inelastic electron tunneling (IET) spectra through molecular junctions is proposed. The quality, reproducibility and richness of information that these measurements provide suggest that this technique could be soon become a standard way to characterize molecular junctions. However, the information contained in a IET spectrum can be useful only in the presence of a predictive model that allows its univocal assignment. Standard quantum chemical techniques are adapted to compute the Green's function derivatives with respect to the normal vibrational coordinates, and these quantities are used to calculate the intensities of the IET peak for each vibration. The agreement between the computed spectra and the experimental measurements for three different molecules recently presented by Kushmerick \textit{et al.} [Nanoletters 4, 639, 2004] is excellent. The possibility of assigning easily an IET spectrum increases the usefulness of the technique, which, in principle can be used for \textit{all} molecular junctions.   

First-principles study of the spin-polarized electron tunneling in a self-assembled molecular BDT monolayer

Understanding of the ``controlled transport of spin-polarized electrons'' through a molecular spacer has been an ultimate goal and has attracted much attention in recent years for its potential applications in spin-based molecular electronic devices. In this talk, we will describe the results of the periodic density functional calculations on a self-assembled benzene-1,4-dithiolate (BDT) molecular monolayer. Specifically, we will investigate the effect of spin configurations of the magnetic substrate and atomic magnetic tip on the spin-polarized electron tunneling of the molecular device.   

Non-equilibrium quantum transport properties of organic molecules on silicon

Electronic and quantum transport properties of organic molecules on Si surfaces are studied within density functional theory. This system is synthetically accessible and has potential applications in chemical sensors, resonant tunneling devices and molecular logic. Since the bonding of organic molecules on Si is well defined and often well characterized, it constitutes an ideal model system for systematic comparisons with experiment. The 1,4-diethynylbenzene molecule on Si(111) was chosen as a first example. A non-equilibrium Green's function approach with an optimized localized orbital basis is employed to investigate transport properties under different biases. Due to variational optimization, only a small number of basis functions per atom are needed, enabling studies of systems containing up to 1000 atoms. For each system, the interface structure is optimized by massively parallel total energy calculations. The I-V curves show a number interesting features, including a plateau and a negative differential resistance. The origins of these features and applications to other systems will be discussed.   

Nanoparticle-Organic Composite Electronic Materials for Chemical Sensors

Molecular electronic based chemical vapor sensors were assembled using noble metal nanoparticles and short conjugated phenylene ethynylene (PE) based molecules. Sacrificial capping ligands on the nanoparticles were replaced by tighter binding PE ligands. The films were assembled between pairs of electrodes by iteratively exposing the substrates to solutions of the nanoparticles and PE crosslinking bridging ligands. Some of the conjugated bridging molecules contained an electron deficient phenol to provide a simple platform for developing sensor applications. The phenol is calculated to have a significant change in its HOMO/LUMO gap in the presence of specific analytes. Judicious combination of nanoparticle size and ligand structure provides a film in which the organic bridging ligands dramatically affect film conductance. Specifically, $\pi $-conjugated ligands lower resistance more in films with smaller particles. Thus the sensing mechanism of these films is not based on the typical swelling mechanism but rather on the modulation of the molecular electronic structure of the conducting PE bridging ligands.   

Electrostatic force microscopy of DNA molecules

The electrical properties of DNA molecules are investigated by electrostatic force microscopy (EFM) experiments. The phase shift of the oscillation of the cantilever can be related to the conductivity of the sample, allowing us to study the electrical properties of these samples without attaching leads. By stretching DNA and zinc-doped DNA on silicon oxide or polystyrene film, we found that the DNA bundles are slightly positively charged and insulating in micrometer range. We found that the phase shift signal depends strongly on the diameter of the DNA bundle: single strands and small bundles show a qualitatively different behavior than large bundles. The results will be discussed in terms of charge transport in DNA molecules.   

Single Protein Structural Analysis with a Solid-state Nanopore Sensor

We report on the use of solid-state nanopore sensors to detect single polypeptides. These solid-state nanopores are fabricated in thin membranes of silicon nitride by ion beam sculpting...[1]. When an electrically biased nanopore is exposed to denatured proteins in ionic solution, discrete transient electronic signals: current blockages are observed. We demonstrate examples of such transient electronic signals for Bovine Serum Albumin (BSA) and human placental laminin M proteins in Guanidine hydrochloride solution, which suggest that these polypeptides are individually translocating through the nanopore during the detecting process. The amplitude of the current blockages is proportional to the bias voltage. No transient current blockages are observed when proteins are not present in the solution. To probe protein-folding state, pH and temperature dependence experiments are performed. The results demonstrate a solid-state nanopore sensor can be used to detect and analyze single polypeptide chains. Similarities and differences with signals obtained from double stranded DNA in a solid-state nanopore and single stranded DNA in a biological nanopore are discussed. [.1] Li, J., D. Stein, C. McMullan, D. Branton, M.J. Aziz, and J.A. Golovchenko, \textit{Ion-beam sculpting at nanometre length scales.} Nature, 2001. \textbf{412}(12 July): p. 166-169.   

PH Dependence of Single DNA Molecules Translocation through a Nanopore Device

We report here the pH dependence of DNA translocation through a nanopore made in a silicon nitride membrane. We demonstrate that silicon nitride nanopores can tolerate extreme pH conditions of surrounding ionic solution. When an electrically biased nanopore is exposed to DNA in ionic solution, discrete transient electronic signals: current blockages are observed. In our experiment, linear DNA molecules, about 3 and 10 kbp, have been electrophoretically driven through nanopores ranged from 3 to 12 nm in diameter. The translocation experiments are performed from pH=3 to 13. By measuring the amplitude change and dwell time of each current blockage that represents a single molecule translocation event, different behaviors of the DNA molecules are observed following pH changes. An alkaline pH (12.7, for example) denatures the molecules, resulting single stranded DNA. A folded DNA state is favored at low pH (3.7), indicating a reduced charge of the phosphate groups.   

Electronic Coupling of Organics to Semiconductors Through Quantum Resonance

We have simulated attaching various organic species to semiconductor surfaces for the formation of molecular electronics and sensors using DFT. Although strong attachment and limited decomposition of the molecular species to Si and Ge surfaces is often achieved, the electronic structure of the molecular-semiconductor interface is not suitable for many devices. We have found that certain organics are stabilized on semiconductor surfaces by quantum mechanical resonance. In some cases, this enhances the growth of organic nanowires and other structures. Furthermore, this resonance leads to stronger electronic coupling between the attached organic and the semiconductor substrate which might be useful in improving electrode-molecule charge injection for molecular transistors and sensors. We will present several examples to illustrate the effect as well as to provide general guidelines for determining how to design molecules to exhibit this property for attachment to Si and Ge.   

Spin Polarized Electron Transport in a Magnetically Coupled Molecular Wire

Nanostructured materials present new opportunities for spintronics and for coherent applications such as quantum information processing. In order to make progress in these areas, new materials that are optimized for spin transfer are required. We will present theoretical studies of electronic structure and transport in magnetically coupled bridging molecules that reveal how coherent spin transport between nanostructures may be enhanced by suitable molecular design. We will present a study on a bridge consisting of two molecules coupled by a vanadium atom and sandwiched by two magnetic contacts (Ni) at both ends. Our first-principles density functional calculations suggest that by controlling the spin of the vanadium atom, one can control the spin polarized transport through the magnetically coupled molecules. The ground state of the magnetically coupled molecular bridge when sandwiched between similar magnetic cluster contacts at both ends (e.g. magnetic clusters such as Ni), prefers anti-parallel to parallel spin configurations. Control of the vanadium spin could be implemented by magnetic STM tips. The large difference in resistance between the two spin configuration states of such a system could be used to make GMR like devices.   

Detection of NO2 down to ppb levels using individual and multiple In2O3 nanowire devices

We demonstrate detection of NO$_{2}$ down to ppb levels using transistors based on both single and multiple In$_{2}$O$_{3}$ nanowires operating at room temperature. This represents orders-of-magnitude improvement over previously reported metal oxide film or nanowire/nanobelt sensors. A comparison between the single and multiple nanowire sensors reveals that the latter have numerous advantages in terms of great reliability, high sensitivity and simplicity in fabrication. Furthermore, selective detection of NO$_{2}$ can be readily achieved with multiple-nanowire sensors even with other common chemicals such as NH$_{3}$, O$_{2}$, CO and H$_{2}$ around.