Session X28: Focus Session: Charge Transport in Nanostructures III

Controlling Electronic States and Transport Properties at the Level of Single Molecules

Since molecular electronics has been rapidly growing as a promising alternative of conventional electronics towards the ultimate miniaturization of electronic devices through the bottom-up strategy, it has become a long-term desire to understand and control the transport properties at the level of single molecules. In this presentation we show that one may modify the electronic states of single molecules, and thus control their transport properties through designing and fabrication of functional molecules or manipulating molecules with scanning tunneling microscopy. We demonstrated that the rectifying effect of single molecules can be realized by designing donor-barrier-acceptor architecture of Pyridine-$\sigma $-C$_{60}$ molecules to achieve the Aviram-Ratner rectifier through azafullerene C$_{59}$N molecules The effect of the negative differential resistances can be realized by appropriately matching the molecular oribital symmetries between a cobalt phthalocyanine (CoPc) molecule and a Ni electrode. The electronic states and transport properties of single molecules, such as CoPc and melamine molecules, can be altered through manipulation or modifying molecular structures, leading to functionalized molecular devices.   

Revealing the role of anchoring groups in the electrical conduction through single-molecule junctions

Using the mechanically controllable breakjunction technique we have performed transport experiments through single ethyne molecules attached to gold electrodes via thiol, nitro, and cyano anchoring groups. We have measured current-voltage characteristics inside a liquid cell. By fitting the experimental curves to a single-level resonant tunneling model we are able to extract both the position of the molecular orbital closest to the Fermi energy and the strength of the metal-molecule coupling. We compare the results to ab initio calculations which give further insight into the transport properties. The dependence of the I-V characteristics on the various anchoring groups shows clearly that these groups affect the coupling strength between metal and the molecules as well as the position of the molecular energy levels.   

Single Molecule Conductance Measurements at Low Temperatures in Ultra High Vacuum

We image and measure the conductance of 2,3,5,6 Tetramethyl-p-phenylenediamine on a single crystal Au(111) surface in an ultra high vacuum (UHV) chamber of a scanning tunneling microscope (STM) with a solid gold tip. Images reveal an ordered layer of molecules deposited on the surface. Single molecule conductance is measured by smashing the tip into the molecule-covered substrate until metal-to-metal contact is established$_{ }$and then pulling the tip out until tunneling conditions are reestablished. Current traces recorded while the junction is stretched reveal plateaus below 1G$_{0}$ due to the formation of single-molecule junctions between the sample and tip. Conductance histograms made from thousands of traces reveal a well-defined peak that agrees well with previous measurements in ambient conditions and allows an unambiguous measurement of single molecule conductance in UHV at temperatures from 50 to 300K. Scanning tunneling spectroscopy measurements are also presented and compared with IV measurements of single molecule junctions.   

Trends in Conductance through Single Molecule Junctions Formed with Double-Layered Molecules

We compare experimentally measured single-molecule conductances with Density-Functional Theory (DFT) based calculations for a series of ``double-decker'' molecules in which two parallel backbones are held between a common contact group on each end, consisting of S(CH$_{2})_{2}$ units. In the simplest example, the two backbones are both phenyl groups, but an extensive series has been studied in which one or both parallel backbones are varied to include either short alkane segments, phenyl groups with substituents or fluorenes. Single molecule transport measurements using the STM-break-junction technique reveal that these molecules have multiple conducting states, suggesting that the single molecule junction geometry plays an important role in the outcome of these experiments. Density Functional Theory based calculations are used to explore the role of the link geometry on the conductance and to understand the relationship between the double-decker structure and the conductance.   

The first-principle study of single molecular switch based on Hydrogen Tautomerization sandwiched between atomic chain electrode

Using DFT based NEGF technique, we investigated electrical transport properties of the single-molecular switch, phthalocyanine (H2Pc), based on hydrogen tautomerization. The molecule is coupled to 1-D electrodes in the form of semi- infinite metallic chains of gold and carbon. Hydrogen tautomerization affects the electronic state of H2Pc by switching the alignment of the molecular orbital energies between HOMO and HOMO-1, and causes a substantial change in the tunnelling current. As a consequence switching can be achieved already in the low-bias regime for both electrode models. For gold chain electrode, molecule-metal interaction at the junction leads to modification of the electronic structure of H2Pc, and hence multi-peak NDR is obtained. It is revealed that the even-numbered Au atoms provide stronger NDR than odd- numbered Au atom. Owing to interesting switching characteristics of H2Pc based on hydrogen tautomerization, it could potentially function as a molecular switch in nano electronic circuits.   

Theory of charge transport in nanostructures: graphene and molecular junctions

We discuss some recent progress on the theory and computation of charge transport in nanostructures. Selected results on two classes of systems of current interest -- single molecule junctions and graphene nanostructures -- are presented. Electronic level alignment at the metal-molecule contact (central to the conductance) is shown to be strongly renormalized by many-electron interaction (self-energy) effects. By including the environment-dependent quasiparticle self-energy correction to the molecular resonance levels, we obtain conductance values for amine-gold linked single-molecule junctions in good agreement with experiment, illustrating the importance of these many-electron effects in off-resonant conduction. The calculations also provide insights into the nature of the conducting molecular states and the influence of contact geometries. In graphene, the low-energy electronic states behave like two-dimensional massless Dirac fermions with pseudospin character. We show that this unique electronic structure leads to a number of novel properties in graphene and graphene-based nanostructures. Graphene nanoribbons are semiconductors with unusual electronic, magnetic and optical properties. The carrier dynamics in graphene exhibits anomalous anisotropy when subjected to an external periodic potential of nanometer dimensions (called graphene superlattices). Under appropriate conditions, these graphene superlattices are predicted to be electron supercollimators and new generations of massless Dirac fermions may be created. Charge transport across grain boundaries in graphene is also found to be highly unusual.   

Molecule heating in resonant molecular tunnel junctions

Charge transfer in metal/molecule/metal junctions is accompanied by a release of power \textit{IV} which is partly spent to excite molecular vibrations. Resonant tunneling is the major mechanism of molecular conductivity which occurs via electronic charged states of the molecule. Therefore, the major physical quantity, governing the vibrational excitations is the reorganization energy $E_{r}$: the amount of energy $E_{r}$ is released for every electron (hole) tunneling through the molecule. The mechanisms of energy dissipation of molecular vibrations determining molecular temperature are considered and possibilities of their experimental observation are discussed.   

Measurements of Bond Breaking Forces in Single Atom Contacts and Single Molecule Junctions

We measure the force required to break a single-atom Au-point contact formed between the tip of a gold-coated micro-cantilever and a gold substrate using a modified conducting atomic force microscope (AFM) while simultaneously measuring the conductance of the contact. Repeated measurements are performed to study the distributions and average forces required to break a single gold-gold bond as a function of the speed at which the surface is separated from the cantilever. Experimental results are compared to existing theoretical models, which predict non-Gaussian asymmetric distributions of breaking forces, to obtain microscopic parameters of the adhesive potential. These techniques are then applied to analyze the force data acquired while breaking single metal-molecule-metal junctions. In addition, an alternative approach involving application of constant pulling force on the junction through feedback control loop is explored.