Concept of Embedded Dipoles as a Versatile Tool for Surface Engineering

Abstract

Controlling the physical and chemical properties of surfaces and interfaces is of fundamental relevance in various areas of physical chemistry and a key issue of modern nanotechnology. A highly promising strategy for achieving that control is the use of self-assembled monolayers (SAMs), which are ordered arrays of rodlike molecules bound to the substrate by a suitable anchoring group and carrying a functional tail group at the other end of the molecular backbone. Besides various other applications, SAMs are frequently used in organic electronics for the electrostatic engineering of interfaces by controlling the interfacial level alignment. This is usually achieved by introducing a dipolar tail group at the SAM-semiconductor interface. Such an approach, however, also changes the chemical character of that interface, for example, affecting the growth of subsequent layers. A strategy for avoiding this complication is to embed polar groups into the backbones of the SAM-forming molecules. This allows disentangling electronic interface engineering and the nucleation of further layers, such that both can be optimized independently. This novel concept was successfully demonstrated for both aliphatic and aromatic SAMs on different application-relevant substrates, such as gold, silver, and indium tin oxide. Embedding, for example, ester and pyrimidine groups in different orientations into the backbones of the SAM-forming molecules results in significant work-function changes. These can then be fine-tuned over a wide energy range by growing mixed monolayers consisting of molecules with oppositely oriented polar groups. In such systems, the variation of the work function is accompanied by pronounced shifts of the peaks in X-ray photoelectron spectra, which demonstrates that electrostatically triggered core-level shifts can be as important as the well-established chemical shifts. This illustrates the potential of X-ray photoelectron spectroscopy (XPS) as a tool for probing the local electrostatic energy within monolayers and, in systems like the ones studied here, makes XPS a powerful tool for studying the composition and morphology of binary SAMs. All these experimental observations can be rationalized through simulations, which show that the assemblies of embedded dipolar groups introduce a potential discontinuity within the monolayer, shifting the energy levels above and below the dipoles relative to each other. In molecular and monolayer electronics, embedded-dipole SAMs can be used to control transition voltages and current rectification. In devices based on organic and 2D semiconductors, such as MoS₂, they can reduce contact resistances by several orders of magnitude without adversely affecting film growth even on flexible substrates. By varying the orientation of the embedded dipolar moieties, it is also possible to build p- and n-type organic transistors using the same electrode materials (Au). The extensions of the embedded-dipole concept from hybrid interfaces to systems such as metal-organic frameworks is currently underway, which further underlines the high potential of this approach.

Figure 1

(a and b) Three- and two-ring SAM-forming molecules with embedded dipolar pyrimidine groups along with their names used for the remainder of the manuscript (P = phenyl, Pm = pyrimidine, 1 = single methylene spacer, up/down = direction of dipole moment (red arrows) relative to the anchoring group). (c and d) WFs of the SAMs grown on Au(111) substrates for the three-ring (c) and two-ring (d) systems. (e) X-ray photoelectron (XP) spectra of the PPP1-based SAMs; the peaks associated with the top and bottom rings are marked in blue and red. Adapted with permission from refs ( and 4). Copyright 2015 Wiley-VCH (ref (1)). Copyright 2018 The Authors (ref (4)). Published by Wiley under a Creative Commons Attribution 4.0 International (CC BY 4.0) License. https://creativecommons.org/licenses/by/4.0/.

Figure 2

(a and b) Representative mid-ester substituted molecules and reference C16 system; the direction of the dipole moment relative to the molecular backbone is schematically shown. (b) SAM-induced WF changes and (c) C 1s XP spectra of the three systems shown in panel a. Individual peaks in the spectra are color-coded, accompanied also by the schematic drawings of the embedded groups. Adapted with permission from ref (28). Copyright 2017 American Chemical Society.

Figure 3

(a) Schematics of the electronic structure of the interface between a metal substrate and an embedded-dipole SAM comprising two dipole layers due to the bond dipoles (BDs) and the embedded dipoles (EDs). The SAM-induced change in the WF, ΔΦ (determined by the change in the offset between the Fermi level (E_F) and the vacuum level (E_vac) right above the sample), and the change in the core-level energies and the resulting kinetic energies of the photoelectrons (E_kin¹ and E_kin²) are illustrated for the up and down orientations of the embedded dipoles. (b) DFT-calculated electrostatic energy averaged over a plane parallel to the interface for alkanethiolates containing an embedded ester group in two different orientations and for a C16 SAM as reference. Adapted with permission from refs ( and 28). Copyright 2016 and 2017 American Chemical Society.

Figure 4

(a) DFT-calculated C 1s core-level energies in an alkanethiolate SAM containing an embedded ester group (C10EC5-up, left panel) with the resulting simulated XP spectrum compared to the actual experimental data (right panel). For further details see ref (2). (b) Comparison of the calculated electrostatic energy for a densely packed C10EC5-up SAM and for an isolated, upright-standing C10EC5-up molecule on a Au substrate. Adapted with permission from ref (2). Copyright 2016 American Chemical Society.

Figure 5

Dependence of the WF of the single-component and mixed PPmP1-up/down (a and b) and C10EC10-up/down (c and d) SAMs on Au(111) (a and c) and Ag(111) (b and d) on the concentration of PPmP1-up (a and b) or C10EC10-up (c and d) molecules in the solutions from which the monolayers were grown. The red lines serve as guides to the eye highlighting the character of the dependencies. Adapted with permission from refs (3), (36), and (37). Copyright 2016, 2017, and 2018 American Chemical Society.

Figure 6

Electrostatic energy plotted for the models of phase separated (a) and homogeneous (b) SAMs comprising a 50:50 mixture of PPmP1-up and PPmP1-down molecules. (c) Weighted superposition of the C 1s XP spectra of the single-component PPmP1-up and -down SAMs, mimicking large scale phase separation in the mixed films. (d) Experimental C 1s XP spectra of the mixed PPmP1-up/down SAMs. The dominant peak corresponds to the core-level excitations for C atoms in the top ring; its position is shifted electrostatically by the embedded dipoles. Weights and compositions in parts c and d are color-coded. Adapted with permission from ref (3). Copyright 2016 American Chemical Society.

Figure 7

(a) Schematic structure of the Au/SAM//Ga₂O₃/EGaIn junctions featuring the PPP1-based SAMs; EGaIn is a eutectic GaIn alloy covered by a thin (∼0.7 nm) oxide layer. (b) I–V curves and (c) current asymmetry (log of the ratio of the currents for forward and reverse bias as a function of the absolute value of the bias) for junctions comprising PPmP1-up and PPmP1-down SAMs. Panel d illustrates transition voltage (V_T) vs WF change for these junctions. V_T was measured for both positive (V_T⁺) and negative (V_T^–) bias. (e) I–V curves for junctions featuring the PP-based SAMs consisting of molecules with only two aromatic rings and no methylene spacer (see Figure 1b). Adapted with permission from refs (39), (40), and (26). Copyright 2016 Authors (refs (39) and (40)). Published by Royal Society of Chemistry under a Creative Commons Attribution 3.0 International (CC BY 3.0) License. https://creativecommons.org/licenses/by/3.0/. 2018 American Chemical Society (ref (26)).

Figure 8

(a and b) Schematic of the energy level alignment between the Au electrode and p- and n-type organic semiconductor before and after introducing suitable up/down embedded-dipole SAMs. (c and d) Typical output characteristics of p-type pentacene OTFTs with the Au electrodes modified by the PPm-down (c) and PmP-up (d) SAMs. (e and f) analogous characteristics of n-type C₆₀ OTFTs with the Au electrodes modified by the PPm-down (e) and PmP-up (f) SAMs. Adapted with permission from ref (4). Copyright 2018 The Authors. Published by Wiley under a Creative Commons Attribution 4.0 International (CC BY 4.0) License. https://creativecommons.org/licenses/by/4.0/

Figure 9

(a) Schematic structure of an OTFT with electrodes modified by distributed-dipole SAMs. (b) Contact resistances of the source electrodes in pentacene-based transistors as a function of the orientation of the embedded dipoles and the length of the backbones. (c) Devices with SAM-modified electrodes on a flexible substrate and (d) a 5-stage ring oscillator with a buffer stage fabricated on a flexible substrate. Adapted with permission from ref (4). Copyright 2018 The Authors. Published by Wiley under a Creative Commons Attribution 4.0 International (CC BY 4.0) License. https://creativecommons.org/licenses/by/4.0/.

Figure 10

(a) Chemical structures of the distributed dipole SAMs studied in ref (48) and (b) WF modifications they induce when adsorbed onto a Au(111) surface. (c) Density of states (as a function of position and energy) of a monolayer consisting of more complex embedded-dipole molecules containing oppositely oriented polar groups consisting of bipyrimidine units. The alignment of the dipoles creates a quantum well for electrons in the central section of the monolayer. (d) Evolution of the DFT-calculated electrostatic energy for a MOF thin film consisting of 1,4-benzenedicarboxylate-linked Zn-paddlewheel sheets connected by seven layers of apical, polar (partly fluorinated) bipyridine linkers (see top-right inset). Adapted with permission from refs (−50). Copyright 2020 American Chemical Society (ref (48)). Copyright 2015 (ref (49)) and 2020 (ref (50)) The Authors. Published by Wiley and MDPI under a Creative Commons Attribution 4.0 International (CC BY 4.0) License. https://creativecommons.org/licenses/by/4.0/.