Session X34: Focus Session: Nano VI: Junctions and Transport

Electronic transport through a light-driven azobenzene molecule switch: A revisit by density functional theory study

Azobenzene, a molecule that changes conformation between \emph{trans} and \emph{cis} configurations, is a candidate light-driven molecule switch. Recent experiments showed that the ``on'' state with larger measured conductance is associated with the \emph{cis} isomer, which is in contrast with our previous theoretical prediction. Here we reconsider the issue of the molecule-electrode and electrode-electrode coupling by performing a first-principles study of the electronic structures and transport properties of Au-azobenzene-Au molecule junctions. Specifically, we investigate the dependence of the conductance and the current-voltage characteristics in two types of Au electrode, 2-D Au(111) surface and 1-D Au STM tip. We find that, not only the \emph{trans} to \emph{cis} transformation of the molecule, but also the electrode-electrode coupling plays a critical role in determining the conductance near the Fermi level.   

Reliable anchoring groups for single-molecule junctions

In the field of molecular electronics, thiols have been extensively used as the most common anchoring groups to bind molecules to gold electrodes. However, other anchoring groups as amines can provide interesting advantages. Recently, C$\-{60}$ has been also proposed as a possible very efficient binding group. In this talk, I will present our studies on molecular junctions formed by thiol-, amine-, and C$\-{60}$-terminated molecules. We use a STM (scanning tunneling microscope) break-junction technique to create and characterized single-molecule junctions both in ambient and liquid environment. We compare thiols and amines on the alkane family and an oligo(phenylene ethynylene). Our study of the molecular-junction stretching length allows us to conclude that thiols affect atomic rearrangement at the electrodes significantly more than amines. Using C$\-{60}$-terminated molecules, we have recently introduced a new technique for controllably wiring one molecule at a time. We first get STM images to located isolated molecules on a gold substrate, which are then specifically targeted and contacted using a STM gold tip. This technique offers a significant improvement over other techniques, as it guaranties that one and only one molecule is contacted at a time between the electrodes.   

Novel quantum interference effects in transport through molecular radicals

In molecules with an unpaired electron (radicals), we predict a correlation-induced `Mott-node' in the transmission spectrum arising from destructive interference between transport contributions from different charge states of the molecule. This class of quantum interference effect has no single-particle analog and cannot be described by effective single-particle theories. Large errors in the thermoelectric properties and nonlinear current-voltage response of molecular radical junctions are introduced when the complementary wave and particle aspects of the electron are not properly treated. A method to accurately calculate the low-energy transport through a radical-based junction using an Anderson model is given.   

Electron Transport through Porphyrin in Nanoscale Junctions

As electronic devices become exceedingly small, incorporation of molecules as circuit elements is an attractive option due to their small size and the new functions that molecules can bring to the existing microelectronics. To realize such devices we have fabricated nanogaps of size 2-3 nm on gold wires and positioned porphyrin molecules in the gap. I-V characteristics with and without molecules in the junction is performed and signatures of molecular transport has been identified. We measure inelastic electron tunneling spectra (IETS) from molecular junctions at 4K in ultra-high vacuum to study its vibrational modes. These measurements will be compared to Raman spectra. We discuss how IETS can provide valuable insight to the metal-molecule coupling and the role ligated atoms can play in electron transport properties.   

Modeling Surface and Stress-Anisotropy Effects on Transformations in Lead Sulfide Nanocrystals Under Pressure

The semiconductor PbS, which displays a small band gap and large excitonic Bohr radius, presents an ideal candidate material for such devices as infrared photon detectors and nanocrystal solar cells.\footnote{J. Choi, et al., Nano Lett., {\bf9}, 3749 (2009)} This has motivated a number of studies into the effect of pressure on bulk PbS and PbS nanocrystals (NCs), which organize into highly periodic superlattices with interesting mechanical properties.\footnote{P.~Podsiadlo, \emph{et al.}, Nano Lett.,{\bf11}, 579 (2010)} The ambient-pressure NaCl-type structure of PbS undergoes a transformation to an orthorhombic structure close to 2.5 GPa, which itself transforms to the CsCl-type structure at 21.5 GPa. We have identified competitive minimum energy paths between the different modifications of PbS using density-functional calculations, and have calculated the associated enthalpy barriers over a range of pressures. In empirical molecular dynamics simulations of the PbS NC transformation under pressure, the effect on the transformation of anisotropic stresses, applied perpendicular to the $\{100\}$- and $\{111\}$-type facets of the NC, has been investigated. The effect of the NC surface on the stability of metastable modifications in PbS NCs is also considered.   

Compositional Distribution and Electronic Structure of Ternary Compound Semiconductor Nanocrystals

We present a first-principles-based theoretical study of compositional distribution and the resulting electronic structure of ternary quantum dots (TQDs) of compound semiconductor nanocrystals. The analysis is based on first-principles Density Functional Theory (DFT) calculations of atomic and electronic structure and on Monte Carlo (MC) simulations of compositional distribution according to DFT-parameterized valence force field models. We report results for ZnSe$_{1-x}$S$_{x}$ (type-I), ZnSe$_{1-x}$Te$_{x}$ (type-II), and In$_{x}$Ga$_{1-x}$As (Reverse type-I) TQDs with nm-scale diffusion lengths and large surface-to-volume ratios. The equilibrium compositional distribution is predicted as a function of overall composition (x) and TQD diameter and its impact on the electron density distribution, electronic density of states, and band gap of the TQDs is analyzed. We find that thermodynamically stable atomic distributions allow for optimal band-gap tenability and wave function confinement in TQDs. Our findings explain the possibility for compositional redistribution that may cause, over time, favorable or adverse changes of the TQD electronic properties with far reaching implications for the synthesis and applications of such nanostructures in devices.   

The Role of morphology and interface in photoluminescent properties in CdSe/CdS Heterostructure Nanocrystals

Semiconductor core/shell colloidal nanocrystals (NCs) are promising materials for many applications such as luminescent solar concentrators and lasing media, due to their high PL quantum yield (PLQY) and their charge separation effect. The role of interface~in PLQY and in band alignment type is studied in this nanoscale heterojunction. Rod-shaped CdSe/CdS NCs have been synthesized by colloidal chemical approach via epitaxial process, which gives us a fine-tuning of both the spherical core size and rod-like shell length. The modification of the relative morphology changes the effective core/shell band alignment, which impacts the electron and hole delocalization into the nanorods. The evolution of the PLQY and PL lifetimes has been studied as a function of the relative core/shell sizes. High PLQY of up to 80{\%} and PL lifetime of 36 ns have been obtained for a shell-excitation wavelength of 450 nm with a large quasi-Stokes shift ($\sim $100nm). Radiative decay rates have been correlated with the rod volume and the band alignment type has been identified as being core size dependent. Finally, UltraSTEM and EXAFS analyses have been used to characterize the crystalline configuration at the core/shell interface and the lattice strain.   

Electrostatic gating and single-molecule Raman spectroscopy

SERS(surface enhanced raman spectroscopy) is a useful tool for single molecule spectroscopic investigations. We fabricated nanoscale Au bowtie structures to function as SERS substrates. Following electromigration, these metal nanostructures possess nanometer-scale interelectrode gaps that support highly localized surface plasmon resonances, resulting in SERS electromagnetic enhancements sufficient for single-molecule studies. These structures have also proven suitable for single-molecule electronic transport experiments, in which the underlying substrate can function as a gate electrode to shift molecular electronic levels relative to the metal source and drain. We will present preliminary results of the effect of gate modulation on the SERS and electrical properties of molecules in such junctions.   

Thermoelectricity and transmission eigenchannels in buckyball junctions

Transmission through nanoscale junctions consisting of a single Buckminsterfullerene molecule between two Pt or Au electrodes is investigated in the Coulomb blockade regime using the nonequilibrium Green's function approach. The Green's function of the buckyball is calculated in the isolated-resonance approximation, including the degenerate HOMO and LUMO orbitals. Electron-electron interactions were included in a constant-interaction model derived from $\pi$-electron effective field theory. For junctions with Pt electrodes, we find two transmission channels (despite the 5-fold degenerate HOMO and 3-fold degenerate LUMO resonances) and a positive thermopower. For Au electrodes, the thermopower is strongly affected by quantum interference, and we find just one transmission channel.   

Study of Raman Stark Effect in Self-Aligned Nanojunctions

Plasmonically-active nanojunctions have been used to study the electrical and optical properties of single molecules by using surface-enhanced Raman spectroscopy (SERS). A new, ``self-aligned'' fabrication technique has been developed to mass-produce more robust nanojunctions. Similar to nanogaps made by electromigration, these self-aligned nanojunctions have been shown to exhibit strong SERS signal. In addition to having the capabilities of fabricating SERS substrates on a massive scale, the self-aligned technique also produces devices with a longer shelf-life than those fabricated by electromigration. Preliminary studies of the electomigrated devices have demonstrated Raman Stark shifts under DC bias. This work aims to study the Stark effect more in-depth and with the self-aligned nanogaps geometry by integrating the self-aligned structures into electrical circuits. Initial findings and current progress of these simultaneous optical and electrical measurements of a single molecule's vibrational modes will be discussed.   

Hopping And Trapping of F4TCNQ on h-BN Nanomesh

A single layer of hexagonal boron nitride (h-BN) on Rh(111) (nanomesh) [1] is an excellent template for trapping and self-assembly of molecules. This hexagonal structure has a periodicity of 3.22 nm and an appearance with strongly bound h-BN regions called ``pores'' of about 2 nm in diameter surrounded by h-BN regions called ``wires.'' The trapping mechanism traces back to a corrugated electrostatic potential at the surface [2]. We have investigated the adsorption behavior of an electron acceptor molecule, tetrafluoro-tetracyano-quinodimethane (F4TCNQ) on nanomesh by combining XPS, UPS, STM and DFT calculation. The work function increase upon F4TCNQ adsorption indicates electron transfer to the molecules, and is in good agreement with the DFT result. At room temperature, F4TCNQ adsorbs on the ``wires'' and in the ``pores'' because of a high mobility. STM measurements allow us to detect the hopping rate of molecules. Upon cooling, molecules are trapped inside of pores, which gives insight into the behavior of a negatively charged molecule, compared to neutral species, in the trapping potential of h-BN nanomesh.   

Quantum-Enhanced Thermoelectric Effects in Polycyclic Molecular Junctions

We calculate the thermoelectric response of a polycyclic molecular junction including electron-electron interactions. To do this, the molecular Green's function is determined via a Lanczos-based technique and $\pi$-electron effective field theory is used to model the degrees of freedom most relevant to transport. In these junctions we find that the presence of multiple rings leads to higher order quantum interference features giving rise to dramatic enhancements of molecular thermoelectric effects, consistent with previous predictions based on Hueckel theory, which neglected electron correlations.   

Decoherence Assisted Single Electron Trapping at Room Temperature

In this work, we theoretically investigate electron transport in heterostructure semiconductor nanowire (NW). We develop a new mechanism to trap an electron in a quantum dot (QD) by means of decoherence. There are six QDs in the NW. Bias voltage (Vb) is applied across the NW and gate voltage (Vg) is applied to the auxiliary QD to control single charge tunneling. The single electron dynamics along the NW is calculated by means of the generalized master equation based on the tight binding model taking into account electron LO phonon interaction (ELOPI) and thermal broadening inside the QDs. It is shown that the decoherence, which is in the pico-second (ps) regime, speeds up the trapping of the electron in the central QD with probability of 70{\%} in less than 2 ps. Our results can be used for the implementation of high temperature single photon source (SPS) or single electron transistor (SET). We acknowledge support from NSF (Grant No. ECCS-0725514), DARPA/MTO (Grant No. HR0011-08-1-0059), NSF (Grant No. ECCS-0901784), AFOSR (Grant No. FA9550-09-1-0450), and NSF (Grant No. ECCS-1128597).   

First-Principles Studies on Photoinduced Charge Transfer in Functionalized Carbon Nanotubes

We have studied the binding energy, electronic structure, optical excitation, and relaxation of dinitromethane molecules (CH$_{2}$N$_{2}$O$_{4})$ on semiconducting carbon nanotubes (CNTs) of chiral index (n, 0) (n=7,10,16,19). The electronic structures calculated from density functional theory (DFT) show that the dinitromethane introduces a localized state inside the band gap of CNT systems of n=10,16 and19, which indicates that the state can trap an electron when the CNT is photoexcited. The dynamics of intra-band relaxations in such systems has been investigated using reduced density matrix formalism combined with DFT. For pristine CNTs, we have found that the calculated charge relaxation time constants agree well with the experimental time scales. Upon adsorption, these constants are modified and there is not a clear trend for the direction and magnitude of the change. However, our calculations predict that electron relaxation in the conduction band is faster than hole relaxation in the valence band, for CNTs with and without molecular adsorbates.   

Understanding Crosstalk Between Parallel Molecular Wires

Cooperative effects between molecular wires affect conduction through the wires, and studies have yet to clarify the conditions under which these effects enhance (diminish) conduction. Using a simple but general model, we attribute this crosstalk to the duality of energetic splitting and phase interference between the wires' conduction channels. In most cases, crosstalk increases (decreases) conductance when the Fermi level is far from (close to) an isolated wire's resonance. Finally, we discuss strategies for controlling crosstalk between parallel molecular wires. \\[4pt] [1] M. G. Reuter et al. J. Phys. Chem. Lett. 2, 1667-1671 (2011).\\[0pt] [2] M. G. Reuter et al. Nano Lett. 11, 4693-4696 (2011).