Session B16: Transport in Nanostructures -- Single Molecules and Molecular Devices

Light Emission in Single-Molecule Molecular Junctions

The basic mechanism for light emission in Scanning Tunneling Microscopes (STM) is described by inelastic tunneling electrons which couple to localized surface plasmons. There is also growing interest in charge transfer plasmons which appear when electrically coupling metallic nanoparticles. Here, we use single molecules to form conductive bridges between an STM tip and substrate and study the complex relationship between tunneling electrons, charge transfer plasmons, and luminescence in molecular junctions. We employ the STM break junction technique to form thousands of Au point contacts and molecular junctions at ambient conditions and room temperature and measure the light emitted from these junctions using a Si photomultiplier. Since the current, junction bias, and emission signals are collected simultaneously, fluctuations in light emission can be correlated with changes in current and junction bias. Because the break junction procedure continuously modifies the local geometry, limits on the emission efficiency are experimentally determined. Finally, we explore the effect of different molecules on light emission.   

Investigating Molecule-Electrode Binding in Single Molecule Junctions Using Linker Group Chemistry and Machine Learning

Designing circuits based on individual molecules has the potential to produce smaller and more powerful electronic devices. However, a continued challenge to studying the properties of such single molecule circuits is understanding the behavior of the linker group used to connect the molecule to electrodes. This linker plays a critical role because it controls both the electronic coupling and the available binding configurations for the molecule-electrode connections. We have studied a series of single molecule circuits using a mechanically controlled break junction set-up with custom high-speed amplification electronics. By changing both the identity and chemical environment of the molecular linker groups we can gain insight into how and why these linkers affect the value of and variation in molecular conductance. We also develop and use novel data analysis tools including advanced clustering methods to help disentangle the different types of junction behavior in an unbiased manner.   

Two-stage Kondo effect in single Manganese phthalocyanine molecule transistor

Charge and spin manipulation over single molecule is important for understanding various fundamental physical processes down to molecular level. In single molecule device, when the coupling between molecule and electrodes is enhanced, localize electron spin strongly interacts with electrons in the electrodes, forming Kondo effect. In this talk, I will discuss our efforts on manipulation of Kondo effect in single Manganese Phthalocyanine (MnPc) device. We use electromigration method fabricating single MnPc molecule field effect transistor successfully. Two stage Kondo and S=1/2 Kondo resonance were observed in different charge states. Two-stage Kondo effect was examed against temperature and electric field. Evolution of Kondo temperatures of the two stages in the field will be discussed. We also observed magnet field induced quantum phase transition between spin singlet and spin triplet in this system. These results illustrate spin control in multi electrons system at single molecule scale.   

Many-body states description for transport and optical properties of molecular junctions with intra-molecule Coulomb interaction

Recent progress of nano-fabrication and laser techniques at nanometer scale makes it possible to perform optical measurements in current-carrying molecular junctions. These advances give rise to the new branch of research coined molecular optoelectronics. With electrons involved in both quantum transport and optical scattering, theoretical challenge is description of these processes on the same footing. Here, we present theoretical analysis of quantum transport and optical response in molecular junctions with strong intra-molecule Coulomb interaction. The study employs molecular many-body states as a basis of consideration, which allows to take intra-molecular interactions exactly. To account for molecule-contacts coupling we use diagrammatic technique for the Hubbard nonequilibrium Green’s functions (NEGF). We verify the methodology by comparing with other available techniques at the model level, and apply it to analyze experimental measurements of electroluminescence and photocurrent generation in molecular junctions.   

Polaronic transport on conductive metal-organic frameworks from first principles

Hybrid organic-inorganic materials are an emerging class of functional materials with important technological applications in solar energy conversion, catalysis and carbon capture. Among these, metal–organic frameworks (MOFs) correspond to three-dimensional porous materials with potential applications in batteries and fuel cells. Recent studies have shown that polarons, localized quasi-particles formed by excess electronic charge and its self-induced local lattice distortion, play an important role in the transport and optoelectronic properties of electrically conductive MOFs; and therefore, in their overall performance in real devices. In this work, we revisit recent experimental results [1,2] and use theoretical and computational first principles methods to investigate the structural stability and electronic properties of electron polarons, as well as their transport properties. We describe our theoretical results for two specific hybrid organic-inorganic materials: the metallic doped system K_xFe₂(BDP)₃ (0 < x < 2; BDP^2- = 1,4-benzenedipyrazolate) [1], and the two-dimensional ferrimagnetic and conductive system CrCl₂(pyz)₂ (pyz = pyrazine) [2]. References: [1] Aubrey et al., Nature Materials 17, 625 (2018); [2] Pedersen et al., Nature Chemistry 10, 1056 (2018).   

Extracting Quantitative Information of electronic structures from Tunneling Molecular Junction I-V Characteristics using a Compact Analytical Model

One of the central challenges of molecular electronics is to establish clear connections between molecular structure, the ensuing electronic structure, and the current-voltage (I-V) characteristics of molecular junctions. In particular, the offset ε_h of the Fermi level relative to the appropriate frontier molecular orbital (HOMO in this case) and the electrode-molecule coupling strength Γ are recognized as two main factors that determine the electrical properties of a typical molecular junction. We show that a compact analytical model derived from the Landauer formalism provides a quantitative fit to the I-V data and yields values of ε_h and Γ that vary systematically with molecular structure and choice of electrode materials. We will present transport data and theoretical analysis of tunnel junctions based on oligophenylene monothiols and dithiols – with systematically varying lengths – and electrodes fabricated from Ag, Au and Pt metals. Furthermore, Ultraviolet photoelectron spectroscopy (UPS) was employed to determine the ε_h to be compared with the values predicted by the model from transport data. We will emphasize that the compact analytical analysis facilitates structure-property correlations and a powerful physical organic chemistry approach to molecular electronics.   

Angstrom-distance rulers using single molecule conductance measurements

Electron transport through single metal-molecule-metal junctions are exquisitely sensitive to the atomic scale geometry of the junction. Here, we use conductance signatures of single molecules on metal electrodes measured using the Scanning Tunneling Microscope-based Break Junction (STM-BJ) technique to map the Angstrom-scale arrangement of the atoms in the junctions. Using our “pull-push” technique, where we repeatedly pull apart, hold and then push the electrodes together in the presence of diamine molecules, we determine the conductance and geometry signatures of a series of these molecules. We find that the molecular conductance can be used to determine the distance between electrodes with Angstrom-scale precision. We discover that the odd or even number of carbon atoms in an alkanediamine molecule affects the mean elongation that the single molecule junction can sustain. Furthemore, by analyzing the statistical probability of forming the junctions with molecules of different lengths, we can we infer the average shape of our electrodes.   

Single Molecule Conductance of Ferrocene at Cryogenic and Room Temperature

Ferrocene and other metallocenes are organometallic species with a metal atom sandwiched between two aromatic rings whose overall spin, spectrum and other properties can be tuned synthetically by choice of metal atom. There is interest in these molecules as candidates for applications in single molecule electronics and spintronics. However, few electron transport measurements of single metallocene molecules on metal have been performed to date. Previous conductance measurements of ferrocene using single molecule break junctions employed anchoring groups to attach the ferrocene to the electrodes, thereby changing the molecular orbitals energy levels and attenuating electron transport. Here, we use Scanning Tunneling Microscope-based break junction technique to measure conductance of ferrocene on gold at room temperature and at 4 K in UHV. We are able to effectively trap the molecule between the tip and substrate while pushing the junction together and find that the molecule binds to gold directly through carbon atoms in the cyclopentadiene organic ligands sandwiching the iron, and has an average conductance of 0.012 G₀. Using this technique, we investigate the effect of the metal atom on transport properties by measuring conductance through other metallocene molecules.   

Signatures of Conformational Dynamics and Electrode-Molecule Interactions in the Conductance Profile During Pulling of Single-Molecule Junctions

We demonstrate that conductance can act as a sensitive probe of conformational dynamics and electrode-molecule interactions during the equilibrium and non-equilibrium pulling of molecular junctions. To do so, we use a combination of classical molecular dynamics simulations and Landauer electron transport computations to investigate the conductance of a family of Au-alkanedithiol-Au junctions as they are mechanically elongated. The simulations show an overall decay of the conductance during pulling that is due to a decrease in the through-space electrode-molecule interactions, and that sensitivity depends on the electrode geometry. In addition, characteristic kinks induced by level alignment shifts (and to a lesser extent by quantum destructive interference) were also observed superimposed to the overall decay during pulling simulations. The latter effect depends on the variation of the molecular dihedral angles during pulling and therefore offers an efficient solution to experimentally monitor conformational dynamics at the single-molecule limit.   

charge transport in molecular transistor incorporating a pair of Mn-phthalocynine molecule

charge transport phenomena of single molecule transistor are of great interests not only to understand quantum transport in confined structure, but also to study molecular properties at individual molecule level. Single molecule transistors have been realized successfully in the past years and findings including Kondo resonance, quantum phase transition among others were discovered. In this talk, we will discuss our new experimentals finding when two molecules were incorporated in a molecular devices. When two Mn phthalocynine molecules are close to each other, they will coupled to each other though electric static field and in some case exchange electrons in between, leading to unique and different physics picture depending on coupling strength. molecular gating and exchange coupling will be adressed.   

Origin of high thermal conductivity in complex molecular crystals: an ab initio study of polythiophene

Thermally conductive molecular crystals are of fundamental and practical interest in part because they are unlike typical complex crystals, which conduct heat poorly owing to their large phonon scattering phase space. While molecular crystals with high thermal conductivity in the range of tens of Wm^-1K^-1 have been known experimentally for decades, the microscopic origin of this property has remained unclear. Ab-initio methods that have been successfully applied to simple crystals have proved difficult to adapt to molecular crystals due to quantum nuclear motion and their complex primitive cells. Here, we report the thermal transport properties of crystalline polythiophene with 28 atoms per primitive cell using an ab-initio approach that rigorously includes finite temperature anharmonicity and quantum nuclear effects. The calculated room temperature thermal conductivity is 176 Wm^-1K^-1, a high value that arises from exceptional phonon focusing along the chain for certain branches and despite short lifetimes in the picosecond range. Our finding suggests that many complex molecular crystals with stiff intra-chain bonds are intrinsically good thermal conductors as phonon focusing occurs in any crystal with anisotropic bonds.   

Modeling structure and conductivity of atomic-scale break junctions

A large number of single-molecule conductance measurements rely on the break junction approach in which a metal wire (typically Au) is stretched to the breaking point in the presence of the molecule of interest. As the wire breaks, a single molecule can bridge the gap between the electrodes and its conductance can be measured. This is a complex process during which many atomic rearrangements occur, resulting in complicated current versus distance traces that are difficult to interpret. We employ density functional theory (DFT) structure relaxations to investigate the possible geometries during a break junction experiment with and without the presence of molecules. Our focus is on Au electrodes, but we also consider other elements such as Ag and Pt. Next, we the relaxed structures and study their conductance properties with the non-equilibrium Green’s function technique coupled with DFT (NEGF-DFT). Our results reveal an intriguing relationship about the evolution of conductance as the junction is stretched, which can be attributed to atomic rearrangements and orbital alignment. These computational analyses can provide guidance in interpreting experimental data.   

Modeling Nonreactive Molecule−Surface Systems on Experimentally Relevant Time and Length Scales: Dynamics and Conductance of Polyfluorene on Au(111)

We propose a computationally efficient strategy to accurately model nonreactive molecule–surface interactions that adapts density functional theory calculations with the Tkatchenko–Scheffler scheme for van der Waals interactions into a simple classical force field. The resulting force field requires just two adjustable parameters per atom type that are needed to capture short-range and polarization interactions. The developed strategy allows for classical molecular dynamics simulation of molecules on surfaces with the accuracy of high-level electronic structure methods but for system sizes (10³ to 10⁷ atoms) and timescales (picoseconds to microseconds) that go well beyond what can be achieved with first-principles methods. Parameters for H, sp² C, and O on Au(111) are developed and employed to atomistically model experiments that measure the conductance of a single polyfluorene on Au(111) as a continuous function of its length. The simulations qualitatively capture both the gross and fine features of the observed conductance decay during initial junction elongation and lead to a revised atomistic understanding of the experiment.

Z. Li, A. Tkatchenko and I. Franco, J. Phys. Chem. Lett., 9, 1140 (2018)   

Ab initio study of the single molecule conductance of TCNQ and F4TCNQ using NEGF-DFT

By investigating the conductance of single molecules we gain insight into the fundamental physics of electron transport. Such an understanding can lead to molecular-based electronic components and sensors. We study the transport properties of tetracyanoquinodimethane (TCNQ) and tetrafluoro-TCNQ (F₄TCNQ) at the level of density functional theory within the non-equilibrium Green's function formalism (NEGF-DFT). Experiments show at least three distinct conductance values for a single molecule of TCNQ and F₄TCNQ. We construct a model system which consists of two gold electrodes connected via a single molecule arranged in different orientations. Four orientations are defined: bidentate-bidentate (bi-bi), mono-bi, mono-mono, and flat. We find that conductance depends on the molecule's orientation between the electrodes which ranges over an order of magnitude. In both molecules, the bi-bi and mono-bi produce lower conductance (0.02 - 0.1 G₀) while the flat orientation gives a slightly higher conductance of 0.2 G₀. Surprisingly, the mono-mono shows the largest conductance at 0.4 and 0.6 G₀ for F₄TCNQ and TCNQ respectively. The results are in qualitative agreement with scanning tunneling microscopy break junction (STM-BJ) experiments, which find two low and one high conductance value.   

Auxiliary quantum master equation for nonequilibrium dual-fermion approach

We introduce auxiliary quantum master equation - dual fermion approach (AQME-DF) and argue that it presents a convenient way to describe steady states of non-equilibrium correlated impurity systems. The combined scheme yields an expansion around a reference system much closer to the true system than in the previous considerations. This scheme also avoids long time propagation and hence is numerically cheaper. Results for Anderson impurity model (AIM) are presented as a benchmark.