Article pubs.acs.org/JPCC
FePc/Metal Interfaces Driven by the Electronic States of Different Low-Dimensional Ag Structures Formed on Si(111) Emilia Annese,*,†,‡ Agnese Rosi,† Jun Fujii,§ and Kazuyuki Sakamoto∥ †
Department of Physics, Università degli Studi Modena e Reggio Emilia, via Campi 213/A, I-41100 Modena, Italy Elettra Synchrotron S.C.p.A Trieste, SS 14, km 163.5, I-34149 Trieste, Italy § TASC Laboratory, IOM-CNR, SS 14, km 163.5, I-34149 Trieste, Italy ∥ Department of Nanomaterials Science, Chiba University, Chiba 263-8522, Japan ‡
ABSTRACT: The characteristic electronic structure of low-dimensional Ag structures on semiconductor, i.e., Si(111)-√3 × √3-Ag superstructure and Ag quantum well state (QWS), were used to explore the interaction between two-dimensional electron gas and a metal−organic molecule, Fe−phthalocyanine (FePc), using angle-resolved photoelectron and X-ray absorption (XAS) spectroscopies. The characteristic surface states of the Ag low-dimensional (LD) system close to the Fermi level (EF), produced either by an excess of Ag atoms on the Si(111)-√3 × √3-Ag superstructure or by confinement near the surface as in Ag-QWS, are actively involved in the charge reorganization at the FePc/Ag(LD) interfaces. In the first case, the surface state S1 undergoes a change in its binding energy (BE) and an increase in its electron population; in the latter, the diameter of the Fermi map of the surface state (SS) decreases by 50% suggesting the reduction of its electron population. The two FePc/Ag(LD) systems manifest interface states located at dissimilar BE and with different FePc orbital character as established by photon- and polarization-dependent photoemission measurements.
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depending on the preparation conditions and film thickness (t): (i) t ≤ Ag monolayer (ML), Ag shows characteristic superstructure Si(111)-√3 × √3-Ag (among others) and the electrons are confined in the surface forming two-dimensional electron gas (2DEG). The Si(111)-√3 × √3-Ag can be converted from the semiconducting surface to a metal in a controlled way22,23 by adding extra Ag atoms (hereafter we refer to this surpestructure as Si(111)-√3 × √3-Ag + Ag); (ii) t ≥ Ag 5 ML, the confinement of the electrons by Ag boundaries brings the formation of quantum well states.24−26 It has been demonstrated that QSE can favor molecular selfassembling and reaction at the Pb film quantum well state (QWS).18,19 In this work we would like to understand how and if QSE can affect not only the molecular organization but also its electronic properties. We exploited the characteristic structure of low-dimensional Ag structures on semiconductor (Si(111)-√3 × √3-Ag + Ag and Ag QWS) to explore the interaction between 2DEG and Fe-phthalocyanine (FePc) by using angle-resolved photoemission and X-ray absorption spectroscopy (XAS). In both cases the surface states close to the Fermi level changed their electron population as a consequence of the molecule−substrate interaction. At the same time, molecular interface states appear with different FePc orbital character as demonstrated by photon- and polarizationdependent photoemission and XAS spectra. The interaction between delocalized (low-dimensional Ag film) and localized
INTRODUCTION In the fast expanding field of organic molecular electronics and spintronics the interaction between the molecules and the different substrates1 still remains the crucial issue for potential future applications2 inspiring the recent research. In this regard, metal−organic molecules have been used to construct nanoarchitectures with different functionalities (i.e., varying metal central atoms or radical) owing to their unprecedented ability of self-assembly on various surfaces. 3d transition metal atoms hosted within an organic molecule, for example M-phthalocyanine (M-Pc) and M-porphyrin (M-P), add magnetic property to the molecules making them promising in organic spintronics. FePc electronic configuration3−8 can be modified when the molecule is adsorbed on the metal surface, and the absence of biunivocal correspondence between pure molecular and interface states indicates the formation of mixed FePc−Ag states. In some cases the interaction with the substrate leads to the formation of new states as recently observed for CoPc and FePc on Au.9−15 Different strategies have been proposed over the years ranging from vicinal metal surface16,17 to metallic film exhibiting quantum size effect (QSE)18,19 to favor the molecular self-assembling and to control their properties. In the latter approach, the quantum phenomena are the consequence of confinement of the electrons into nanometer scale (electron wavelength, λe, comparable to the atomic distance) and appear in thin metal films on semiconductor, artificial nanostructures,17 self-assembled 1D chains,20 and quantum dots.21 Ag film on Si(111) is a prototype system where QSE manifests in two forms of 2D electron confinement © 2015 American Chemical Society
Received: December 20, 2014 Revised: August 5, 2015 Published: August 5, 2015 20065
DOI: 10.1021/jp512720p J. Phys. Chem. C 2015, 119, 20065−20073
Article
The Journal of Physical Chemistry C
All measurements were performed in situ at the APE beamline, Elettra (Italian Synchrotron Radiation facility). For more details about the beamline setup, sample preparation, and spectroscopic measurements we refer to ref 29.
electrons (molecular orbital) does not go beyond the change in the population of 2DEG states close to the FE.
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EXPERIMENTAL SECTION Si(111)-√3 × √3-Ag + Ag. Ag was deposited on the Si(111)_(7 × 7) clean surface, previously annealed at ∼1500 K, from a Knudsen cell with the sample kept at room temperature. To obtain the Si(111)-√3 × √3-Ag structure, the sample was heated at 750 K. The Si(111)- √3 × √3 -Ag can be turned into a metallic surface by electron doping with an extra Ag atom on the √3 × √3 matrix, and by varying the annealing conditions of the sample23,27 we obtained Si(111)-√3 × √3Ag + Ag. Low energy electron diffraction (LEED) was used to check the quality of the surface as well as angle-resolved photoemission spectroscopy (ARPES) measurements. Ag-QWS. Ag was deposited on the Si(111)_(7 × 7) clean surface from a Knudsen cell with the sample kept at around 80 (20) K. Ag film was then annealed to around room temperature and cooled back to 80 K, leading to Ag(111) films of desired coverage and a bulk-like lattice constant.28 We prepared two Ag-QWS surfaces with Ag thickness (t) of 5 and 18 ML. FePc. FePc from Sigma-Aldrich powders was well outgassed in situ in UHV, with several annealing cycles, and deposited on the metal surface in a base pressure
