Resolving Band-Structure Evolution and Defect-Induced States of Single Conjugated Oligomers by Scanning Tunneling Microscopy and Tight-Binding Calculations

Abstract

We study single conjugated polyphenylene oligomers consisting of 3n (2 n 12) phenyl units by means of cryogenic scanning tunneling microscopy and spectroscopy. The spatially resolved local densities of states reveal a progressive development of a continuous conduction band out of discrete molecular orbitals as the length of the oligomers increases. The experimental results are satisfactorily described by tight-binding calculations which gave a conduction band bandwidth of 4:5 AE 0:2 eV and a band gap of 3:1 AE 0:2 eV for an infinitely long polymer. We observed two types of defects, known as conformational torsional angle misfit and metasite kink. Tight-binding as well as density-functional theory model calculations confirm that both types of defects effectively destroy the delocalization. DOI: 10.1103/PhysRevLett.106.206803 PACS numbers: 73.22.Àf, 73.61.Ph, 68.37.Ef, 82.35.Cd One-dimensional (1D) conjugated polymers (CP) are both interesting scientifically and important for technological applications [1]. Theoretical descriptions of CP follow one of two routes: a delocalized band theory of solid-state physics or a localized molecular orbital approach of quantum chemistry [2][3][4]. In spite of their different perspectives, both approaches predict that the molecular orbitals of short oligomers develop progressively into a broad band in the long-length limit [5][6][7]. The bandlike features have been detected experimentally for short oligomers only [8,9]. The majority of attempts follow the so-called oligomer approach, in which the characteristics of short oligomers of increasing chain length are extrapolated to infinite polymers [10,11]. There are still significant discrepancies among different theories, as well as between theories and experimental results, and many fundamental properties of CP including bandwidth, band gap, and the influence of various structural defects, etc., are under debate [7].In particular, precise experimental determination of band structure, i.e., the energy-momentum dispersion, of CP or long oligomers has had limited successes largely due to the difficulty of growing high-purity single crystalline samples [12]. Above all, the existence of defects presents a severe challenge since defects in a 1D system can strongly alter its electronic structure [13]. Thus to be able to measure the electronic structure of a single CP whose molecular structure, including the details of defects, can be resolved simultaneously is highly desirable. Recently, Repp et al. demonstrated that scanning tunneling microscopy (STM) can provide an unprecedented view of the electronic structures of individual conjugated oligomers [14]. In this Letter we report the conductance band structures of single poly(para-phenylene) (PPP) oligomers determined by cryogenic (5 K) STM. We examine the energy-momentum dispersion of the PPP oligomers of up to 16 nm in length (36 phenyl units) and derive the conduction band bandwidth, energy gap, and the effective mass of charge carriers. Furthermore, we demonstrate and discuss how...

Two-Orbital Kondo Screening in a Self-Assembled Metal–Organic Complex

“…The higher intensity observed on one side of the molecules is ascribed to the tilt of the phenyl rings with respect to the surface and to each other. 25,28,29 Notice that very similar results have been obtained for Fe atoms buried by NC-Ph 5 -CN molecules (see Supporting Information).…”

Quasi one-dimensional band dispersion and surface metallization in long-range ordered polymeric wires

“…We have also carried out DFT calculations in the gas phase using B3LYP/6–31g( d ) that yield a bandgap of 3.05 eV ( Supplementary Note 8 ). In another study, STS measurements were performed on isolated PPP chains grown on Cu(111) using a different precursor 32 . The bottom of the conduction band was found to be 0.9 eV above the Fermi level for long chains, in the unoccupied electronic states, that is, shifted by 1.3 eV compared with our measurements.…”

“…Evidence of π -conjugation was shown by combining X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure 26 27 28 29 (NEXAFS). Bandgaps can be deduced by scanning tunnelling spectroscopy (STS) and/or angle-resolved photoelectron spectroscopy (ARPES) and supported by theoretical calculations 30 31 32 33 34 35 . However, a full-band dispersion in polymeric chains has not been reported to date, due to the difficulty in obtaining ordered phases at sufficiently long range.…”

Influence of Molecular Conformations and Microstructure on the Optoelectronic Properties of Conjugated Polymers

“…In summary, different types of defects that can occur in organic semiconductors, including conformational torsional angle misfit or various chain kinks lead to localization of π-electrons, as has been convincingly shown on paraphenylene oligomers [106]. Thereby, localization of π-electrons over a multitude of distances due to defects is translated into a multitude of absorption and PL responses [107].…”