ARTICLE pubs.acs.org/JACS Collectively Induced Quantum-Confined Stark Effect in Monolayers of Molecules Consisting of Polar Repeating Units Ferdinand Rissner,†,|| David A. Egger,†,|| Amir Natan,‡,z Thomas K€orzd€orfer,§ Stephan K€ummel, Leeor Kronik,‡ and Egbert Zojer*,† § †Institute of Solid State Physics, Graz University of Technology, 8010 Graz, Austria ‡Department of Materials and Interfaces, Weizmann Institute of Science, 76100 Rehovoth, Israel §Theoretical Physics IV, University of Bayreuth, 95440 Bayreuth, Germany bS Supporting Information ABSTRACT: The electronic structure of terpyrimidinethiols is inves- tigated by means of density-functional theory calculations for isolated molecules and monolayers. In the transition from molecule to self- assembled monolayer (SAM), we observe that the band gap is substantially reduced, frontier states increasingly localize on opposite sides of the SAM, and this polarization in several instances is in the direction opposite to the polarization of the overall charge density. This behavior can be analyzed by analogy to inorganic semiconductor quantum-wells, which, as the SAMs studied here, can be regarded as semiperiodic systems. There, similar observations are made under the inﬂuence of a, typically external, electric ﬁeld and are known as the quantum-conﬁned Stark eﬀect. Without any external perturbation, in oligopyrimidine SAMs one encounters an energy gradient that is generated by the dipole moments of the pyrimidine repeat units. It is particularly strong, reaching values of about 1.6 eV/nm, which corresponds to a substantial electric ﬁeld of 1.6 107 V/cm. Close-lying σ- and π-states turn out to be a particular complication for a reliable description of the present systems, as their order is inﬂuenced not only by the docking groups and bonding to the metal, but also by the chosen computational approach. In the latter context we demonstrate that deliberately picking a hybrid functional allows avoiding pitfalls due to the infamous self-interaction error. Our results show that when aiming to build a monolayer with a speciﬁc electronic structure one can not only resort to the traditional technique of modifying the molecular structure of the constituents, but also try to exploit collective electronic eﬀects. Snoble metals have raised enormous interest over the past elf-assembled monolayers (SAMs) of organic molecules on decades1 6 and have become important for a number of applica- tions.7 10 SAMs, and even individual molecules, are subject to research also as the active elements in molecular electronic devices.8,11 24 There, especially, the alignment of the states in the SAM relative to the metal Fermi-level as well the degree of “delocalization” of the transport channels between the electrodes are of uttermost importance.25 29 When functional molecules are assembled into SAMs as the active region of electronic devices, control of the electronic structure can be achieved by tuning the properties of the isolated molecule via targeted chemical substitution. Relating molecular and SAM properties, is, however, far from trivial, and the peculiarities of SAM electrostatics have been pointed out repeatedly.30 35,6,36 40 They are essentially related to the fact that well-ordered SAMs of polar molecules correspond to quasi-inﬁnite dipolar layers split- ting space into two regions with diﬀerent vacuum energies with the magnitude of this diﬀerence depending on the molecular dipole moments.38 The latter can be modiﬁed by attaching polar donor or acceptor substituents (so-called tail groups) to otherwise apolar molecules, which in the individual molecules signiﬁcantly modiﬁes the positions of the eigenstates such as the highest occupied molecular orbital (HOMO) relative to the common vacuum level. Surprisingly, however, the level alignment between the SAM states and the Fermi-level of the substrate is typically tail-group independent.33,41 This ﬁnding has been rationalized by an electrostatic decoupling of the attached polar groups from the molecular backbones in combination with depolarization eﬀects within the SAM, thus underlining the impact of collective interaction of molecules on the properties of monolayers.42,43 Another approach to adjust molecular dipole moments is to use polar building blocks such as pyrimidines to form molecular backbones, which then can be bonded to metal surfaces by suitable docking groups, e.g., thiolates.44 By attaching this dock- ing group to one or the other “end” of the oligopyrimidine molecule, two variants with opposite dipole orientation can be realized (cf., “3Nup” and “3Ndown” in Figure 1a).44 Monolayers formed from these molecules display a number of intriguing and Received: April 19, 2011 Published: September 29, 2011 r 2011 American Chemical Society 18634 dx.doi.org/10.1021/ja203579c | J. Am. Chem. Soc. 2011, 133, 18634–18645