Work‐Function Modification of Au and Ag Surfaces upon Deposition of Self‐Assembled Monolayers: Influence of the Choice of the Theoretical Approach and the Thiol Decomposition Scheme

Rodríguez‐González

Osella

et al. 2018

Advcd Theory and Sims

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We present a detailed theoretical characterization of the energetic alignment between the HOMO level of a series of thiolated oligophenylenes of increasing chain size, and the Fermi level of gold electrodes, using density functional theory (DFT) calculations for molecular self-assembled monolayers (SAMs) chemisorbed on an Au (111) surface, and the nonequilibrium Green's function (NEGF) formalism coupled to DFT for single molecule junctions. The additional role of the dynamic electronic polarization effects neglected in standard DFT calculations is also discussed. Interestingly, whereas the HOMO energy varies significantly among the unsubstituted oligomers in the gas phase, their alignment with respect to the Fermi level of the electrode is almost insensitive to chain size upon chemisorption, thus pointing to a strong pinning effect. The energy at which the HOMO is pinned strongly depends on the degree of interfacial hybridization, and hence on the contact geometry, as well as on the degree of surface coverage although a different mechanism enters into play.anchoring unit. Over the years, a large body of knowledge has been accumulated on the morphologies of such SAMs and on structure-property relationships for the work function shifts [3,4] ; in molecular electronics, many theoretical and experimental studies have focused on the changes in the transmission spectra and I/V characteristics when elongating the size of conjugated oligomers. [5][6][7] In contrast, less attention has been paid to structure-property relationships driving the alignment of the frontier electronic levels with respect to the Fermi level of the metallic electrodes; this energy offset can be deduced by ultraviolet photoelectron spectroscopy (UPS) measurements, [8,9] estimated from I/V characteristics of molecular junctions by using simple analytical models, [8,[10][11][12] or measured photocurrent spectra. [13] The alignment of the HOMO is clearly a key quantity in SAMs, providing electronic levels that favor charge injection in organic layers or in molecular junctions.Theory can prove useful in this context to shed light on this issue, even though it remains extremely challenging to predict the Fermi level alignment quantitatively via first-principles calculations, typically based on density functional theory (DFT). [14][15][16][17][18] This is due to the fact that the ionization potential/electron affinity (IP/EA) of SAM-forming molecules attached to metallic electrodes is governed by several effects not present for the isolated molecules: (i) the electronic polarization of the metal (i.e., image effects) and of the neighboring molecules in presence of a net charge, that both lower the IP and increase the EA of the molecules (i.e., stabilize the formation of a positive charge or negative charge on the molecules); these effects are neglected when performing standard DFT calculations on neutral systems; (ii) the hybridization of the orbitals of the anchoring group with the orbitals of the metallic atoms and the resulting charge reorganization ...

Section: Methodssupporting

confidence: 84%

Section: Resultsmentioning

confidence: 98%

Energy Level Alignment at Interfaces Between Au (111) and Thiolated Oligophenylenes of Increasing Chain Size: Theoretical Evidence of Pinning Effects

Rodríguez‐González

Osella

et al. 2018

Advcd Theory and Sims

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“…Figure summarizes the charge density distribution Δρ upon SAM formation along the z ‐axis and the resulting BD potential for PA‐DAE on ZnO(0001) and ZnO(000‐1). Δρ is calculated by the following equation

Δ ρ = ρ_{SAM} + ρ_{normalH} - ρ_{ZnO} - ρ_{mol}

…”

Section: Resultsmentioning

confidence: 99%

Dynamic Photoswitching of Electron Energy Levels at Hybrid ZnO/Organic Photochromic Molecule Junctions

Wang

Ligorio

et al. 2018

Adv Funct Materials

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The functionality of interfaces in hybrid inorganic/organic (opto)electronic devices is determined by the alignment of the respective frontier energy levels at both sides of the heterojunctions. Controlling the interface electronic landscape is a key element for achieving favourable level alignment for energy and charge transfer processes. Here, it is shown that the electronic properties of polar ZnO surfaces can be reversibly modified using organic photochromic switches. By employing a range of surface characterization techniques combined with density functional theory calculations, it is demonstrated that self-assembled monolayers (SAMs) of photochromic phosphonic acid diarylethenes (PA-DAEs) can be employed to reversibly change the electronic properties of polar ZnO/ SAM structures by light stimuli. The highest occupied molecular orbital level of PA-DAE is raised by 0.7 eV and the lowest unoccupied one lowered by 0.9 eV, respectively, upon illumination by ultraviolet light and the levels shift back to their original position upon illumination by green light. The results thus provide a pathway to tailor hybrid interface electronic properties in a dynamic manner upon simple light illumination, which can be exploited to reversibly tune the electrical properties of photoswitchable (opto)electronic devices.

“…Poisson's equation is used and requires the evaluation of the charge density difference Dρ(z). The latter is calculated in the so-called "molecular scenario" as: 33,34 (…”

Section: Theoretical Understanding Of the Surface Potential Changementioning

confidence: 99%

Dynamically Switching the Electronic and Electrostatic Properties of Indium–Tin Oxide Electrodes with Photochromic Monolayers: Toward Photoswitchable Optoelectronic Devices

Wang

Dell’Elce

et al. 2019

ACS Appl. Nano Mater.

Self Cite

The chemical modification of electrodes with organic materials is a common approach to tune the electronic and electrostatic landscape between interlayers in optoelectronic devices, thus facilitating charge injection at the electrode/semiconductor interfaces and improving their performance. The use of photochromic molecules for the surface modification allows dynamic control of the electronic and electrostatic properties of the electrode and thereby enables additional functionalities in such devices. Here, we show that the electronic properties of a transparent indium tin oxide (ITO) electrode are reversibly and dynamically modified by depositing organic photochromic switches (diarylethenes) in the form of self-assembled monolayers (SAMs). By combining a range of surface characterization and density functional theory calculations, we present a detailed picture of the SAM binding onto ITO, the packing density of molecules, their orientation, as well as the work function modification of the ITO surface due to the SAM deposition.Upon illumination with ultraviolet and green light, we observe a reversible shift of the frontier occupied levels by 0.7 eV, and concomitantly a reversible work function change of ca. 60 meV. Our results prove the viability of dynamic switching of the electronic properties of the electrode with external light stimuli, which could be used to fabricate ITO-based photo-switchable optoelectronic devices.