Article pubs.acs.org/JPCC
Morphology, Work Function, and Silver Ad-Structures of HighTemperature Grown Ultrathin MgO Films on Ag(001) Fayal Mechehoud† and Clemens Barth*,† Aix-Marseille University, CNRS, CINaM UMR 7325, 13288 Marseille, France S Supporting Information *
ABSTRACT: We characterized the high-temperature growth of ultrathin MgO films on Ag(001) at 783 K through analyzing the film’s morphology and work function by noncontact atomic force microscopy (nc-AFM), scanning tunneling microscopy (STM), Kelvin probe force microscopy (KPFM) and assisting low-energy electron diffraction (LEED). At submonolayer coverage, one-atomic-plane-high MgO islands are formed, which are preferentially embedded into the silver surface and which are larger and more regular than the islands of those films normally grown between 500 and 600 K. KPFM shows that embedded MgO islands decrease the silver work function (WF) by ΔϕMgO−Ag = −50 ± 20 meV, whereas a small amount of supported islands increase the WF by ΔϕMgO−Ag = +50 ± 20 meV. When the MgO film starts to cover entirely the silver surface, ad-islands, nanoparticles (NPs), and much larger particles of silver are formed, which is due to the high mobility of silver and the embedding of the MgO islands into the silver surface. Our work opens the general perspective to prepare a thin oxide film and at the same time metal NPs from the metal surface via a high-temperature growth.
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INTRODUCTION Magnesium oxide (MgO) has become the focus of intensive research because of its important role as a magnetic tunnel junction or high-k dielectric interface in electronic devices1 and particularly as a standard substrate in heterogeneous model catalysis.2−5 One of the most fascinating characteristic of the ultrathin film is that when grown in particular on Ag(001) the reduction of the silver work function (WF) by MgO6 may lead to an immediate intrinsic charging of supported metal atoms and nanoparticles (NPs) as exemplified with gold.7,8 A large amount of work has been done to characterize the surface morphology of ultrathin MgO films by scanning tunneling microscopy (STM),9 noncontact atomic force microscopy (nc-AFM), and Kelvin probe force microscopy (KPFM);10−12 however, despite improvements in obtaining large crystalline MgO films,12 a relatively low film quality is always found characterized by a high density of small islands with mean sizes ranging from 10 to 20 nm and a high density of defects at the island borders.12 Recently, Pal et al. suggested that MgO ultrathin films on Ag(001) can be prepared in a high quality if they are grown in a high-temperature growth mode at a temperature of 773 K,13 which is much higher than the one between 500 and 600 K regularly used for the MgO thin-film preparation (lowtemperature growth). In surface regions not larger than 30 × 30 nm2 the authors could observe that the high-temperature grown film is composed of wide, one monolayer (ML) thin MgO(001) islands extending over a few tens of nanometers.13 Unlike in the case of MgO films grown at low temperatures, © XXXX American Chemical Society
Density functional theory (DFT) predicts that at high temperatures oxygen atoms are incorporated into the first layer of silver, reducing the stress between metal and oxide and hence being responsible for the formation of extended MgO layers.14 The incorporation of oxygen into the silver layer is responsible for smaller WF changes between MgO/Ag(001) and Ag(001), which stands in contrast with ultrathin MgO(001) grown at low temperatures: A WF reduction of >1 eV was calculated by DFT7,15 and measured by KPFM.11 The temperature of 773 K is certainly sufficiently high for an increased mobility of silver atoms, as indirectly observed after a low-temperature growth at ∼673 K.12 A recent work states that after a film growth at 573 K some nanometer-sized silver adislands are formed, which are responsible for an enhanced catalytic activity of the MgO/Ag(001) surface in the oxidation of CO.16 A requisite for understanding the high-temperature growth of ultrathin MgO films therefore is to study the surface morphology and at the same time the surface WF on the nanometer scale, in particular, in view of the important role of mobile silver during the film growth, which has not been considered so far. In this work, we present STM, nc-AFM, KPFM, and assisting low-energy electron diffraction (LEED) measurements conducted in ultrahigh vacuum (UHV), which helped to characterize the high-temperature growth of ultrathin Received: August 2, 2015 Revised: September 28, 2015
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DOI: 10.1021/acs.jpcc.5b07465 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. Ultrathin MgO film grown at 783 K in the sub ML range (0.5 ML nominal thickness, effective MgO flux: 0.033 ML/min). Shown are the topography image (a) obtained by nc-AFM and corresponding Kelvin image (c) obtained by KPFM. A line fitting procedure was applied onto the Kelvin image; see the Supporting Information for more details. The profiles (b,d) show typical contrast features visible in the topography (a) and Kelvin image (c), respectively. Image sizes: 100 × 100 nm2, scanning speed: 0.5 Hz, Δf = −12.2 Hz.
MgO films at 783 K. We show that a closed crystalline MgO film with a relatively high quality is obtained and characterized by coalesced, up to 50 nm wide and 1 ML thin MgO islands, which only slightly modify the WF of silver in the tens of millivolts range. Ad-islands and particles with a nanometer and much larger size are formed during the growth, which is due to the mobility of silver and the embedding of the MgO islands.
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sputtering (E = 1.5 keV) and UHV annealing at 823 K. The MgO ultrathin films were grown by evaporation of Mg from a homemade Knudsen cell (alumina crucible, evaporation temperature: 473 ± 10 K) in the UHV chamber backfilled with molecular oxygen (99,995% purity, Linde MINICAN, Munich, Germany) onto the Ag substrate held at 783 K. The effective flux of MgO, which was determined from the amount of MgO that remained on the silver surface at 783 K, was between 0.033 and 0.17 ML/min, depending on the type of experiment. We have chosen a partial oxygen pressure of pO2 = 1.5 × 10−5 mbar and a slow cooling speed of
