One Carbon Matters: The Origin and Reversal of Odd–Even Effects in

Article pubs.acs.org/JPCC

One Carbon Matters: The Origin and Reversal of Odd−Even Effects in Molecular Diodes with Self-Assembled Monolayers of FerrocenylAlkanethiolates Li Yuan,† Damien Thompson,‡,§ Liang Cao,† Nisachol Nerngchangnong,† and Christian A. Nijhuis*,†,∥ †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore Department of Physics and Energy, University of Limerick, Limerick, Ireland § Materials and Surface Science Institute, University of Limerick, Limerick, Ireland ∥ Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore ‡

S Supporting Information *

ABSTRACT: We investigated the origin of odd−even effects in molecular diodes based on self-assembled monolayers (SAMs) of ferrocenyl-terminated n-alkanethiolates S(CH2)nFc with n = 6−15 on Ag or Au surfaces contacted with EGaIn top electrodes. These SAMs have different M−S−C bond angles of 180° when M = Ag and 104° when M = Au causing a multitude of odd−even effects in the performance of the diodes. By changing the M−S−C bond angles and using several characterization techniques, we were able to systematically identify and rationalize odd−even effects in the electronic structure of the device. Changing n from 6 to 15 resulted in an odd−even effect in the tilt angle of the Fc units (α), which, in turn, caused odd−even effects in the surface dipole, work function, and HOMO onset (HOMO = highest occupied molecular orbital). These odd−even effects caused an odd−even modulation of the tunneling current across the diode in the on state (the current that flows across the junctions when the diode allows the current to pass through). The current that flows across the diodes in their off state (the leakage current) also followed an odd−even effect that was related to an odd−even effect in the packing energy: SAMs with small tilt angles of the ferrocenyl units α (with the Fc units standing up) pack better than SAMs with large α values (with the Fc units in a parallel orientation with the plane of the electrode). All these odd−even effects were completely and consistently reversed when the Ag electrodes were replaced with Au electrodes proving they are induced by the M−S−C bond angle.

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INTRODUCTION Molecular and organic electronic devices (or junctions) are complicated physical−organic systems in which it is challenging to unravel how each of the components (electrodes, interfaces, and the organic part) contributes to the electrical properties of the devices. The performance of these devices depends not only on the chemical structure of the molecules inside them but also on how the molecules are organized at the supramolecular level and how they couple to the electrodes. In thin film devices, it is well-known that molecular packing affects the device performance.1−8 However, the supramolecular structures of molecular tunnel junctions are difficult to characterize and control without changing other parts of the junctions (such as the molecule− electrode coupling, surface dipoles, or work functions), and therefore it is often not clear how supramolecular structure affects the junction properties. This paper describes the dependence of the charge transport properties on the subtle changes in the supramolecular and electronic structure of molecular diodes based on self© 2015 American Chemical Society

assembled monolayers (SAMs) of ferrocenyl (Fc) terminated n-alkanethiolates (S(CH2)nFc, henceforth SCnFc, with n = 6− 15) immobilized on bottom electrodes of ultraflat templatestripped (TS) Ag and Au in contact with Ga2O3/EGaIn top electrodes (the native layer of Ga2O3 is highly conductive and ∼0.7 nm thick9−12). We controlled the supramolecular structure of the SAM while keeping potential changes to the SAM−electrode interactions to a minimum and the fabrication methods of the electrodes unchanged, by exploiting so-called odd−even effects by simply changing the number of CH2 units (i.e., the value of n). We found that incremental changes of 1 in n changed the orientation of the Fc units by 5° which in turn resulted in an odd−even effect in the SAM packing energies.6 These subtle changes in the supramolecular structure of the SAM impacted the performance of molecular diodes in terms of Received: May 20, 2015 Revised: July 8, 2015 Published: July 9, 2015 17910

DOI: 10.1021/acs.jpcc.5b04797 J. Phys. Chem. C 2015, 119, 17910−17919

Article

The Journal of Physical Chemistry C

odd−even effects. We isolated each of these effects and studied how they affect the electrical characteristics of the molecular diodes. The electronic properties of molecular junctions are difficult to predict. The tunneling decay coefficient (β in nC−1) and the injection current (J0 in A/cm2) vary, respectively, from 0.4 to 1.3 nC−1 and over 9 orders of magnitude across testbeds for junctions with SAMs of SCn.10,13−15 Whitesides et al. showed in a series of papers how varying a large number of functionalities in the SAM structure does not significantly affect the tunneling rates in EGaIn-based junctions.16−18 In contrast, numerous studies indicated that the molecular component can in some cases affect the electronic properties of these junctions.6,19−21 These uncertainties also complicate experimental studies of molecular diodes, and promising molecular diodeswith onpaper ideal chemical structuresoften result in junctions with disappointingly low rectification ratios12,22−25 (apart from a few exceptions26−29). Most of these studies have mainly focused on the chemical structure of the molecular component and/or on the energy level alignment of the molecular frontier orbitals (the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) with the Fermi levels of the electrodes.30−32 In general, the values of R are low ( 9 but increases to ∼4.3 eV when n decreases from 9 to 6. The reason could be that the surface coverage decreases from 5 × 10−10 to 3 × 10−10 mol/cm2 with decreasing n for n < 9 (see Supporting Information, Figure

least absolute deviation (LAD)

Gaussian surfaces odd

SAM on Ag SAMeven on Ag SAModd on Au SAMeven on Au

log|J0| (A/cm2)

β (Å−1)

log|J0| (A/cm2)

β (Å−1)

0.36 ± 0.14

0.90 ± 0.08

0.34 ± 0.03

0.88 ± 0.01

0.15 ± 0.07

0.75 ± 0.09

0.16 ± 0.03

0.76 ± 0.01

0.92 ± 0.07

1.04 ± 0.03

0.93 ± 003

1.04 ± 0.01

0.35 ± 0.08

0.95 ± 0.05

0.36 ± 0.02

0.96 ± 0.01

These values were obtained from fits to eqs 2 and 3 to the average log|J| at −1.0 V using two analysis methods (Gaussian and LAD) as explained in the text. bThe error bars represent 95% confidence levels.

a

(p values determined by Z-tests are 0.7−25 × 10−3 for Gaussian fits and