Conference Programme

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Overview
Session
P-04: Theory & Modelling I
Time:
Tuesday, 20/Jun/2017:
1:30pm - 3:30pm

Session Chair: Jonatan Dubi, Ben-Gurion University
Session Chair: Sarah Guerin, University of Limerick
Location: Rm 302

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Presentations
1:30pm - 2:00pm
Invited

Operational Usefulness of the Single Level Model in Approaching Charge Transport through Molecular Junctions

Ioan BALDEA

Heidelberg University, Germany

In spite of impressive efforts, the microscopic theory of charge transport through molecular junctions continues to be confronted with important difficulties. Attempts to account for the chemical specificity of the active molecule as well as the nature of the contacts to electrodes mostly emphasize differences between properties of various molecular junctions. At the other extreme, the approach – to be examined in this talk - based on a compact single-level model (ideally with prameters backed by quantum chemical calculations [1]) is able: (i) to emphasize common features and even predict a universal behavior (“law of corresponding states” [2]) in out-of-equilibrium situations, which has been validated against experimental I-V data measured for completely different classes of molecules and electrodes mostly utilized to fabricate nanojnctions [2,3], (ii) to quantitatively analyze fluctuation effects in molecular junctions and to demonstrate that these effects can be extremely weak [4], and (iii) to demonstrate [5] that strongly temperature dependent currents measured experimentally [6] can be fully compatible with the transport mechanism by tunneling. A number of possible pitfalls [7] related to the application of the single level model to interpret I-V measurements in molecular junctions will be also presented.

References:

[1] Z. Xie, I. Baldea, C. Smith, Y. Wu, and C. D. Frisbie, ACS Nano 9 (2015) 8022.

[2] I. Baldea, Z. Xie and C. D. Frisbie, Nanoscale 7 (2015) 10465.

[3] Z. Xie, I. Baldea, S. Oram, C. Smith, and C. D. Frisbie, ACS Nano9 (2016) ASAP.

[4] Z. Xie, I. Baldea, T. Demisssie, C. Smith, Y. Wu, G. Haugstad, and C. D. Frisbie, in preparation.

[5] I. Baldea, submitted.

[6] C. Smith, Z. Xie, I. Baldea, and C. D. Frisbie, submitted.

[7] I. Baldea, PCCP 17 (2015) 20217; 31260.


2:00pm - 2:30pm
Invited

Generic, Empirical Quantification of the Voltage Effect on Tunneling-Dominated Charge Transport

Ayelet VILAN

Weizmann Institute of Science, Israel

Any technological prospect for molecular electronics requires generic description of the current-voltage relations. While tunneling is accepted as the transport mechanism, there is a large ambiguity on the details of the tunneling process and its derived mathematical model. This leads to a puzzling situation where a given experimental current-voltage trace can be fitted equally well to various mathematical relations, yielding vastly different physical parameters, such as the energy barrier for tunneling. Use of Normalized Differential Conductance (NDC) offers an elegant alternative to curve-fitting. Qualitatively, the NDC provides a current-normalized scale that is graphically similar to the transmission probability across the junction as a function of the applied bias. It magnifies small differences and, in certain cases, it reveals clear deviation from the widely accepted Lorentzian transmission function. Quantitatively, the bias where NDC=2 is identical to the so-called “transition bias”. This arbitrary threshold voltage reflects the sensitivity of the transport to the applied bias, or a “scaling bias”. Reporting experimental observations in terms of scaling bias bypasses the need to agree on a specific transport mechanism. In this sense the scaling bias is the bias-equivalent of the tunneling length decay and temperature activation energy, all empirical parameters that are non-specific to a given tunneling model. My presentation will compare the pros and cons of fundamental and empirical models, and will demonstrate the use of NDC and scaling bias on variety of molecular junctions.


2:30pm - 2:45pm
Oral

First Principles Design of Organic Piezoelectric Devices

Sarah GUERIN, Tofail SYED, Damien THOMPSON

University of Limerick, Ireland

Piezoelectric materials exhibit the unique characteristic of becoming electrically charged when strained and conversely, becoming deformed in the presence of an electric field. Inorganic piezoelectric materials have been exploited for decades as nanogenerators, biosensors, resonators, acoustics, and in scanning probe microscopy (SPM). Here we present organic crystals as a basis for such applications, based on experimentally validated quantum mechanical models. Our models can quantify the piezoelectric response of small biomolecular crystals, and uncover significant electromechanical coupling along hidden crystallographic planes. In this project we present both our theoretical and experimental data on amino acid crystals, highlighting their low permittivity and high flexibility. Amino acids are the building blocks of proteins and other biological structures and in their uncrystallised form regulate a number of our bodies’ functions. Here we discuss the future that crystallised biomolecules could have in energy harvesting and sensing technologies, as well as the implications of discovering piezoelectricity at the foundations of our biology.


2:45pm - 3:00pm
Oral

Control of the Energy Gap in Junction for Making a Molecular Rectifier

Colin Jean-Louis VAN DYCK, Mark RATNER

Northwestern University, United States

Single Molecule junctions are the constitutive components of Molecular Electronics circuits. For any potential application, the energy gap in the junction, i.e., the accumulated energy difference between the electrode Fermi level and the two frontier energy levels of the molecule, is a key property. In this presentation, we show that the gap of the molecule inserted between electrodes can differ largely from the gap of the same molecule, at the isolated level. It can be widely compressed by the alignment mechanism at each metal/molecule interface. [1]

This behavior is important to consider for characterizing and designing molecular junctions. We show that this is particularly true for a new mechanism of rectification that we recently introduced. [2] This latter opposes resonant to non-resonant tunneling and is based on the control of the energy gap in junction. We show that both the structure of the molecule and the anchoring group drive the value of this gap, at equilibrium and under bias. In the end, this will highlight the crucial importance, and benefit, of the contact for Molecular Electronics.

References:

[1] Van Dyck and Ratner, J. Phys. Chem. C, 2017, 10.1021/acs.jpcc.6b07855

[2] Van Dyck and Ratner, Nano Letters, 2015, 10.1021/nl504091v


3:00pm - 3:30pm
Invited

Playing with Molecular Junctions – Two Tales from the South

Jonatan DUBI

Ben-Gurion University, Israel

The ultimate goal of molecular electronics is to create technologies that will complement—and eventually supersede—Si-based microelectronics technologies. To reach this goal, the field of single-molecule electronics is aiming at recognizing and characterizing single-molecule devices that mimic at least some of the behaviors of today's semiconductor components. I will review two such single-molecule devices, based on an ongoing collaboration with the experimental group of Prof. B.-Q. Xu at the University of Georgia. The first device is a DNA-based single-molecule rectifier, which is to date the world’s smallest diode. The second is an electro-optical (photo-conductance) switch composed of a single molecule. I will describe the experiments and theory behind these devices, the puzzles they presented and their resolution.



 
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