SYMPOSIUM ON SURFACE SCIENCE 2011 Baqueira Beret, Lleida ...

Electronic properties of NiO thin films: A combined STM,
STS, and 2PPE study
Mario Kiel, Stephan Großer, Anke Höfer, Klaus Duncker, and Wolf Widdra
Institute of Physics, Martin-Luther Universität Halle, 06120 Halle, Germany
(Corresponding author: W. Widdra, e-mail: wolf.widdra@physik.uni-halle.de)
By combining scanning tunneling microscopy and spectroscopy (STM, STS) with two-photon
photoemission (2PPE) the electronic structure of ultrathin NiO(001) films has been
investigated in the region of unoccupied states.
For the 2PPE experiments a fiber-based laser system (IMPULSE, Clark-MXR) which drives
two non-collinear optical parametric amplifiers at a repetition rate of 1.5 MHz is used. It
delivers independently tunable laser beams in the
range of 500 – 670 nm and 700 – 950 nm with pulse
widths of 20 - 30 fs after compression. The photoelectron
spectra have been recorded with a
hemispherical electron energy analyzer (Phoibos
150, Specs) which is equipped with a 2D
channelplate detector. The STM and STS
experiments have been performed in a different
UHV chamber with a homebuilt instruments which
is operated at 80 K.
Based on STM for the NiO monolayer which has
been grown by reactive metal deposition in an O2
atmosphere on a Ag(001) substrate, an uniaxially
compressed (2×1) structure is found as shown in
Fig.1 (a). The NiO monolayer is characterized by
two well-developed peaks in the corresponding STS
spectra at 2.25 and 3.8 eV (Fig.1 (b)). For NiO films
from 2 ML to 9 ML, STM and LEED show an
Fig. 1: (a) STM image of 1.2 ML NiO deposited on
Ag(001) at RT. (b) STS spectra in the region of
unoccupied states for local NiO thickness of 1 and 2
ML recorded at 80k. (c) Time-resolved two-color
2PPE spectra for 2 ML NiO as function of pumpprobe
delay.
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