Surface Modification of ZnO(0001)–Zn with Phosphonate-Based Self-Assembled Monolayers: Binding Modes, Orientation, and Work Function

Abstract

We used partially fluorinated alkyl and aromatic\ud phosphonates as model systems with similar molecular dipole\ud moments to form self-assembled monolayers (SAMs) on the\ud Zn-terminated ZnO(0001) surface. The introduced surface\ud dipole moment allows tailoring the ZnO work function to tune\ud the energy levels at the inorganic−organic interface to organic\ud semiconductors, which should improve the efficiency of charge\ud injection/extraction or exciton dissociation in hybrid electronic\ud devices. By employing a wide range of surface characterization\ud techniques supported by theoretical calculations, we present a\ud detailed picture of the phosphonates’ binding to ZnO, the\ud molecular orientation in the SAM, their packing density, as\ud well as the concomitant work function changes. We show that\ud for the aromatic SAM the interaction between neighboring molecules is strong enough to drive the formation of a more densely packed monolayer with a higher fraction of bidentate binding to ZnO, whereas for the alkyl SAM a lower packing density was found with a higher fraction of tridentate binding

Energy-Level Engineering at ZnO/Oligophenylene Interfaces with Phosphonate-Based Self-Assembled Monolayers

“…Furthermore, the O 1s core level analysis of the pCF 3 PhPAmodified ZnO surface in Figure 2d confirms our previous findings. 13 The fraction of bidentate to tridentate binding is similar in all three cases. Assuming the same number of binding sites theoretically available on the ZnO surface, 13 a similar fraction of bidentate to tridentate binding in the present study points to a similar surface coverage on ZnO for the three aromatic phosphonates (i.e., on the order of 4 molecules/nm 2 , as discussed in ref 13).…”

“…24 These findings are in good agreement with our previously obtained results for the unmodified ZnO surface, which was annealed under similar conditions. 13 After surface modification (Figure 2b−d), the two initial components (bulk oxygen and surface−OH) are markedly decreased. Furthermore, the O 1s spectrum of each SAMmodified ZnO surface can be fitted with three additional components, which were previously assigned on the basis of their binding energy shifts (with respect to the bulk oxygen) to two different PA binding modes, 13,25 i.e., bidentate and tridentate binding (red and blue components in Figure 2b−d).…”

“…In contrast to that, no appreciable change in the valence structure is observed for the same nominal thickness (i.e., 12 Å) L4P-sp3 deposited onto the pCF 3 PhPA-modified ZnO surface (see Figure 5b). Since pCF 3 PhPA molecules adopt an approximately upright-standing orientation on ZnO(0001), 13 this difference can be explained by a lower sticking of the L4P-sp3 molecules on the partially fluorinated pCF 3 PhPA-modified ZnO surface (i.e., "Teflon-like" behavior), as compared to the (nonpolar) PhPAand PyPA-modified ZnO surfaces. Therefore, for the same nominal L4P-sp3 thickness, it is expected that the pCF 3 PhPAmodified ZnO surface is only partially covered by L4P-sp3 islands (i.e., valence spectrum mainly resembles that of pCF 3 PhPA), whereas both the PyPA-and the PhPA-modified ZnO surface are completely covered by L4P-sp3 (i.e., valence spectrum resembles that of L4P-sp3).…”

Etching of Crystalline ZnO Surfaces upon Phosphonic Acid Adsorption: Guidelines for the Realization of Well-Engineered Functional Self-Assembled Monolayers

“…25 In another study, an alternative preparation protocol was employed that was based on three preparation cycles with shorter immersion times in the PAs solution to avoid strong etching and the formation of precipitates. 26 Therefore, particular attention was paid to use carefully dried ethanolic solutions for our film preparation. Though after 1 day of immersion no deposits are visible by the naked eye, thorough AFM measurements reveal the presence of small precipitates that steadily grow with increasing immersion time.…”

Electronic and Optical Excitations at the Pyridine/ZnO(101¯0) Hybrid Interface

“…[1][2][3] These properties together with a type-I level alignment at the hybrid interface would be ideally suited for light-emitting applications. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] The adsorption of a molecular layer on inorganic surfaces has also been exploited to tune the work function of the inorganic component. [6][7][8][9][10] Prerequisite for that is the lightinduced creation of hybrid or charge-transfer excitons, which exhibit the hole to a large extent on one side of the interface while the excited electron would reside on the other side.…”