Surface Science 552 (2004) 8–16
www.elsevier.com/locate/susc
Photoemission and LEED characterization of Ni2P(0 0 0 1)
Daisuke Kanama a, S. Ted Oyama a, Shigeki Otani b, David F. Cox
a
a,*
Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
b
National Institute for Materials Science, 1-1, Namiki, Tsukuba, Ibaraki 305, Japan
Received 5 November 2003; accepted for publication 13 January 2004
Abstract
Ion bombardment of Ni2 P(0 0 0 1) produces a disordered, Ni-rich surface. For annealing temperatures above 450 K,
long range ordering occurs, and the surface Ni/P ratio decreases. Annealing above 700 K produces an ordered (1 · 1)
hexagonal surface with a composition near that expected for a simple stoichiometric termination of the bulk structure.
Variations in photoemission spectra are observed along with the changes in composition associated with the long range
ordering of the surface. UPS spectra change from Ni metal like to Ni2 P like, with a corresponding decrease in the
density of states at the Fermi level. The splitting between the Ni 2p3=2 main and satellite peaks increases as the Ni/P ratio
increases, likely due to poorer screening of the photoemission final state as a result of the decreased density of states at
the Fermi level.
2004 Elsevier B.V. All rights reserved.
Keywords: Phosphorus; X-ray photoelectron spectroscopy; Visible and ultraviolet photoelectron spectroscopy; Surface electronic
phenomena (work function, surface potential, surface states, etc.)
1. Introduction
Recent legislation in the US, Europe, and Japan
has mandated steep reductions in the levels of
sulfur allowed in transportation fuels [1,2], and
this has stimulated much research in the development of new catalysts for hydrodesulfurization
(HDS) and hydrodenitrogenation (HDN). Catalysts currently employed in industry are sulfides,
consisting of Mo or W promoted by Co or Ni, and
are used with or without supports [3]. Recently a
new class of catalysts for hydrodesulfurization has
been reported, the transition metal phosphides.
These materials are very interesting because on a
*
Corresponding author. Tel.: +1-540-231-6829; fax: +1-540231-5022.
E-mail address: [email protected] (D.F. Cox).
site basis, as titrated by chemisorption, they are
more active than the sulfides [4]. Recent work has
shown that MoP [5–9], WP [10,11], Ni2 P, and
Co2 P [12–14] are highly active for HDS and HDN
of petroleum feedstocks. Of all phosphides, nickel
phosphide, Ni2 P, is the most active [4]. As a prelude to future investigations of the chemistry of
model Ni2 P surfaces, we have undertaken a characterization study of the surface properties of the
(0 0 0 1) face of single crystal Ni2 P.
Ni2 P [15] adopts the hexagonal Fe2 P structure
(space group: P62m , D23h , Struckturbericht notation:
revised C22) with lattice parameters, a ¼ b ¼
0:5859 nm, c ¼ 0:3382 nm (Fig. 1). The structure
of the metal-rich phosphides is based on trigonal
prisms, which can well accommodate the relatively
large phosphorus atoms. The prisms are similar to
those in sulfides, but phosphides do not take on
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2004.01.038
D. Kanama et al. / Surface Science 552 (2004) 8–16
9
along [0 0 0 1] gives the full Ni2 P stoichiometry of
the bulk. An ideal, simple (0 0 0 1) termination of
the bulk structure allows for a surface plane
of either composition. Interestingly, for either
simple (0 0 0 1) termination the phosphorous surface atoms take on a six-fold coordination compared to a bulk coordination number of 9.
3. Experimental
Fig. 1. Hexagonal bulk crystal structure of Ni2 P.
layered structures and are metallic conductors, not
insulators or semiconductors [16,17]. Lack of layers leads to a more isotropic crystal morphology
and potentially better exposure of surface metal
atoms to fluid phase reactants.
2. Ni2 P(0 0 0 1)
Along the [0 0 0 1] direction in bulk Ni2 P, there
is an alternation between two compositionally inequivalent atomic layers with stoichiometry of
Ni3 P and Ni3 P2 (Fig. 2). A two-plane repeat unit
Experiments were conducted in a turbopumped, dual-chamber ultrahigh vacuum system.
X-ray photoelectron spectroscopy (XPS) experiments were conducted in an analysis chamber
equipped with a Leybold EA-11 hemispherical
analyzer and a Mg anode X-ray source. Low-energy electron diffraction (LEED) experiments were
conducted in a preparation chamber equipped with
a set of Vacuum Generators three-grid reverse view
LEED optics. The base operating pressure for this
study was 1.3 · 1010 Torr in both chambers.
All reported XPS experiments were run at
normal emission, with a pass energy of 60 eV
which gives a Ag 3d5=2 line width of 1.06 eV. The
Fig. 2. Illustration of the two different (0 0 0 1) atomic layers in Ni2 P. Bulk Ni2 P is comprised of alternating layers with compositions
Ni3 P1 and Ni3 P2 . A two plane repeat unit along the [0 0 0 1] direction gives the correct Ni2 P stoichiometry. The top half of the figure
shows the compositions of the two separate planes, while the bottom of the figure shows the opposite sides of a two plane unit
representing the two possible ideal stoichiometric surfaces formed from a simple termination of the bulk.
10
D. Kanama et al. / Surface Science 552 (2004) 8–16
binding energy scale is referenced to the sample
Fermi level. All XPS spectra shown for Ni and P
have been modified by X-ray satellite subtraction
and Shirley background subtraction [18,19]. Shirley background subtractions were accomplished
using the CONTUR program of Contini and
Turchini [20]. Reported XPS Ni/P atomic ratios
were obtained by integrating the areas under the
Ni 2p and P 2p peaks (following X-ray satellite and
Shirley background subtraction) and correcting
with Leybold atomic sensitivity factors. All UPS
data were run with a pass energy of 10 eV which
gives an analyzer resolution (DE) of 0.15 eV.
The sample was oriented to within 1 of the
(0 0 0 1) surface by Laue backreflection. The oriented sample was mounted on a tantalum holder
that was fastened to LN2 -cooled copper electrical
conductors in a sample rod manipulator. The
holder was heated resistively, and the temperature
was monitored with a type-K thermocouple attached to the back of the sample through a hole in
the holder using Aremco No. 569 ceramic cement.
4. Results
4.1. Simulated XPS Ni/P ratios for stoichiometric
Ni2 P(0 0 0 1)
Examination of the chemical composition of
compound single crystal surfaces using atomic
sensitivity factors for electron spectroscopic techniques such as AES and XPS can sometimes yield
atomic ratios (or apparent surface stoichiometries)
that deviate from the known bulk composition of
the crystal, even in situations where the actual
surface composition is similar to that of the bulk.
In our experience, these variations tend to be the
largest when the compositions of the planes parallel
to the surface vary, and when the difference in kinetic energies (and hence inelastic mean free paths)
of the collected electrons for the different elements
is large. Both of these conditions are met for
Ni2 P(0 0 0 1) where the alternating layers along the
[0 0 0 1] direction in bulk Ni2 P vary in composition,
and where the XPS binding energies for Ni 2p and
P 2p are 860 and 130 eV, respectively. For this
reason, we undertook a simple simulation of the
expected XPS Ni/P ratio for stoichiometric Ni2 P(0 0 0 1) surfaces. Such estimated values can be
useful for comparison to experimental XPS composition measurements (reported below) if the
mean free paths used in the estimates are consistent
with the empirical sensitivity factors [21] applied to
the experimental data.
The calculation of the expected Ni/P ratios was
accomplished using conventional layer-by-layer
summations assuming: (1) an exponential decay of
signal intensity with distance for normal emission;
(2) no diffraction effects; 1 and (3) inelastic mean
for Ni 2p (Ekin ¼ 392 eV) and
free paths of 9.45 A
for P 2p (Ekin ¼ 1123 eV) photoelectrons.
19.7 A
The inelastic mean free paths were estimated with
the NIST Electron Inelastic-Mean-Free-Path Database [23–25]. The (0 0 0 1) layer compositions and
interlayer distances were assumed to be unchanged
from the bulk structure [15]. The estimated XPS Ni/
P ratios for the two simple terminations of the bulk
crystal with Ni3 P1 and Ni3 P2 layers are 1.02 and
0.99, respectively. 2 These estimates indicate that
1
The arrangement of atoms in adjacent layers (Fig. 2) does
not lend itself to any strong forward scattering effects from
nearest neighbors along the surface normal that might skew the
intensity of the photoemission features for one element relative
to another [22]. Diffraction effects should, therefore, be negligible provided the photoemission signal intensities are smoothly
varying with take-off angle, and show no anomalous maxima
or minima near the normal emission angle chosen for the
measurements. Note also that some angular integration about
the normal emission angle is expected in our XPS apparatus
because the transfer lens of the analyzer images a 1 mm · 8 mm
sample area and is not optimized for true angle-resolved
photoemission with a narrow angular acceptance.
2
The mean free paths used in the estimate are consistent
with the Leybold sensitivity factors used to generate the
experimental Ni/P ratios. The ratio of the Leybold sensitivity
factors for P and Ni, SP =SNi is 0.046. The mean free path
dependence in the ratio of sensitivity factors can be checked
(using the form for empirical atomic sensitivity factors [21]) as
P
SP =SNi rrNiPkkNiP =KE
=KENi , where ri is the photoionization cross
section of the core level for element i, ki is the mean free path
of the photoemitted electron from element i, and KEi is the
kinetic energy of the emitted photoelectron which accounts for
the 1=E dependence of the transmission function of the
analyzer. Using published cross sections [29] and the estimated
mean free paths gives a ratio of sensitivity factors, SP =SNi , of
0.041. The similarity in the ratio of Leybold and estimated
sensitivity factors indicates a consistent mean free path dependence in the experimental and estimated XPS Ni/P ratios.
D. Kanama et al. / Surface Science 552 (2004) 8–16
11
stoichiometric Ni2 P(0 0 0 1) surfaces should give
XPS Ni/P ratios near 1.0 rather than 2.0 as might be
expected based on the bulk composition of the
material.
4.2. Experimental observations
Ion bombardment and annealing experiments
were conducted to track thermally driven changes
in the surface composition and long-range order.
The sample was bombarded with a 10 lA beam
of 3 keV Ar ions for 30 min, then heated to successively higher temperatures. Following a given
annealing treatment, the sample was cooled to
room temperature and XPS spectra were collected
and the LEED periodicity was examined before
heating again to the next temperature in sequence.
The annealing treatments were performed by
heating for 5 min at temperatures between 400 and
1000 K in 100 K increments, except over the range
between 450 and 480 K in which the sample was
annealed in 5 K increments.
4.2.1. LEED
For annealing temperatures up to 450 K, no
LEED pattern is observed at any beam energy
from an initially ion-bombarded surface; only
diffuse background is observed. For annealing to
455 and 460 K, surface ordering begins and broad
diffuse spots appear in the LEED pattern. For
annealing temperatures over 465 K, the spots
sharpen and the diffuse background decreases. Fig.
3 shows a typical LEED pattern taken with a 66
eV beam energy following annealing to 700 K. The
(1 · 1) hexagonal periodicity remains unchanged
for annealing temperatures up to 1000 K.
4.2.2. XPS
Fig. 4 shows XPS spectra of Ni 2p and P 2p
regions as a function of annealing temperature.
XPS spectra of Ni in Fig. 4 include the main peak
for Ni 2p1=2 , Ni 2p3=2 and also the broadened satellite features associated with each. The main
peaks and satellite features for each Ni core level
are associated with two separate final states in the
photoemission process [26]. All Ni spectra in Fig. 4
have been modified by X-ray satellite and Shirley
background subtraction [18–20]. Annealing the
Fig. 3. (1 · 1) hexagonal LEED pattern observed at a 66 eV
beam energy following annealing to 700 K.
disordered (ion bombarded) surface to successively
higher temperatures causes an increase in the
binding energy of all the features in the Ni 2p
spectrum. The main 2p3=2 and 2p1=2 features shift
up in binding energy by 0.6 eV, while the position
of the Ni 2p3=2 satellite peak moves up by 1.4 eV.
Fig. 4 also shows the variation in the unresolved
P 2p photoemission features with annealing. These
spectra were obtained after each annealing treatment in conjunction with the Ni 2p data. As seen
with the Ni photoemission features, the P 2p peak
also shifts to higher binding with annealing, but
only by about 0.2 eV.
Fig. 5 compares the variation in the XPS Ni/P
ratio and the Ni 2p3=2 binding energy with annealing temperature. The value of the XPS Ni/P
ratio is near 2.0 just after the ion bombardment,
and decreases to 1.8 with annealing temperatures
up to about 450 K. Annealing treatments between
450 and 470 K result in a rapid decrease in the Ni/
P ratio to 1.2. The onset of the observed composition change corresponds with the first observation of surface ordering with LEED. Annealing
to higher temperatures, up to 700 K, causes a
further decrease in the ratio to about 1.0, similar to
the expected XPS Ni/P ratio for a stoichiometric
surface termination based on the layer-by-layer
summations described above (Section 4.1). For
12
D. Kanama et al. / Surface Science 552 (2004) 8–16
Ni 2p
P 2p
2p3/2
2p1/2
500 K
470 K
N(E)
N(E)
500 K
460 K
460 K
450 K
450 K
300 K
300 K
(ion bombarded)
(ion bombarded)
880 875 870 865 860 855 850 845
(a)
470 K
Binding Energy (eV)
138 136 134 132 130 128 126 124
(b)
Binding Energy (eV)
Fig. 4. Selected Ni 2p and P 2p XPS data for an ion bombardment and annealing series. The 300 K spectra correspond to the ion
bombarded and disordered surface.
LEED pattern
first appears
XPS Ni/P ratio
2.0
853.4
1.8
1.6
1.4
853.2
ion bombarded
surface
XPS Ni/P ratio
Ni 2P3/2 Binding Energy
853.6
853.8
1.2
1.0
854.0
0.8
200 300 400 500 600 700 800 900 1000 1100
Ni 2p3/2Binding Energy (eV)
2.2
Temperature (K)
Fig. 5. Variation in the XPS Ni/P ratio and Ni 2p3=2 binding
energy with annealing temperature.
further heating to higher temperatures up to
1000 K the Ni/P ratio stays near 1.0. Examination
of the individual Ni and P peak areas (not shown)
indicates that the decrease in the XPS Ni/P ratio
from near 2.0 to near 1.0 with annealing is associated with a 20% decrease in the Ni peak area,
and a 60% increase in the P peak areas. These
variations suggest that the composition change is
primarily driven by an out-diffusion of P from the
bulk.
The variation in binding energy of the Ni 2p3=2
feature with annealing temperature is also shown
in Fig. 5. The Ni 2p3=2 binding energy for the ionbombarded surface (300 K) is 853.2 eV. The
binding energy increases with increasing annealing
temperature, for a final Ni 2p3=2 binding energy of
854.0 eV following annealing at 1000 K. Note that
the binding energy scale on the right hand side of
Fig. 5 runs in the opposite direction as the surface
composition scale on the left hand side. With
increasing temperature, the Ni/P ratio decreases,
while the Ni 2p3=2 binding energy increases. The
similarity in the two data sets as shown in Fig. 5
indicates that the decrease in the XPS Ni/P ratio
and the increase in the Ni 2p3=2 binding energy are
coincident with annealing temperature, and associated with the ordering of the surface observed
with LEED. These results show that the well-ordered (1 · 1) hexagonal periodicity resulting from
high temperature annealing is associated with an
XPS Ni/P ratio near 1.0 and a Ni 2p3=2 binding
energy of 854.0 eV.
4.2.3. UPS
Fig. 6 shows He II UPS data for variations in
the valence band density of states with ion bombardment and annealing. Following ion bombardment, the UPS spectrum is very similar to that
of a metallic Ni surface (see for example Refs.
[27,28]). While the character of the valence band
features have not yet been fully attributed to specific atomic orbitals, the most intense feature near
D. Kanama et al. / Surface Science 552 (2004) 8–16
13
5. Discussion
He II
Ni 3d
N(E)
5.1. XPS ratio of Ni and P
ion
bombarded
300 K
450 K
475 K
1000 K
10
5
0
Binding Energy (eV)
Fig. 6. Selected He II UPS spectra for an ion bombardment
and annealing series. The 300 K spectra correspond to the ion
bombarded and disordered surface.
1 eV is clearly associated with Ni 3d states which
have a photoionization cross section (8.36 Mb)
more than an order of magnitude greater than
Ni 4s (0.13 Mb) or P 3s (0.45 Mb) and 3p (0.51
Mb) electronic states [29]. Following annealing at
450 K where ordering is observed with LEED and
the surface Ni/P ratio decreases, a shoulder appears on the low binding energy edge of the
spectrum near the top of the valence bands, and
the main peak shifts slightly to higher binding
energy leading to a decrease in the density of states
at the Fermi level. Increased annealing temperatures cause further definition of the shoulder near
the Fermi level, and following annealing at 700 K
and above, the main Ni 3d feature shifts to near
1.5 eV leaving a distinct shoulder and a lower
density of states near the Fermi level. In addition
to the changes in shape, the overall intensity of the
valence band features decreases. This decreased
intensity can be attributed to the increased fraction
of P in the surface layer with annealing, and the
lower sensitivity of He II photoemission to P valence electronic states.
The observed experimental XPS Ni/P ratios
(near 2.0 for ion-bombarded surfaces and 1.0 for
annealed and ordered surfaces) might at first seem
to suggest that ion bombardment produces surfaces near the bulk stoichiometric composition,
and annealing produces a P-rich (or Ni-deficient)
surface. This observation is contrary to expectations that a Ni-rich surface should be produced by
ion bombardment since preferential removal of
lighter atoms is often observed for ion bombardment. However, when compared to the expected
(simulated) XPS Ni/P ratio determined by the
simple layer-by-layer summation, the value near
1.0 expected for a simple stoichiometric termination of the bulk structure is seen to compare
favorably with the experimental value near 1.0 for
the annealed and ordered surface. Hence, in contrast to the initial expectation that the changes in
experimental Ni/P ratio indicate a stoichiometric
Ni2 P composition following ion bombardment
and a P-rich surface following annealing, our
estimated ratios indicate that the ion-bombarded
surface is actually Ni rich, while the annealed and
ordered surface has a more stoichiometric Ni2 P
composition. This discrepancy in the measured
XPS Ni/P ratio and the actual surface composition
provides a clear example of the difficulties that can
be encountered in extracting meaningful surface
composition information from single crystal compound surfaces using simple atomic sensitivity
factors when the photoelectron lines of the atomic
species have widely different kinetic energies.
Ideally, one would like to extract information
concerning the nature of the terminating layer for
the annealed and ordered surface from the XPS
composition measurements. While the measured
XPS Ni/P ratio near 1.0 is seen to correspond to a
stoichiometric Ni2 P composition by comparison
to the estimated ratios, unfortunately the 0.03
variation in the expected Ni/P ratio for the two
simple terminations (either Ni3 P1 or Ni3 P2 ) is
smaller than the error associated with the experimental Ni/P determinations. Hence, while the
XPS composition measurements suggest that an
14
D. Kanama et al. / Surface Science 552 (2004) 8–16
ordered surface of stoichiometric composition is
formed by annealing in vacuum, no insight into
the nature of the terminating layer is gained. Also,
it appears that the XPS Ni/P ratio of 2.0 corresponds to a Ni-rich surface, as might be expected
following ion bombardment.
5.2. Ni XPS: main and satellite peaks
The Ni core level photoemission features of
Ni2 P consist of a main peak and an associated
satellite peak at a 6.7–7.5 eV higher binding energy
than the main peak (Fig. 4). The main peaks and
satellite features of Ni are attributable to two
different electronic final states in the photoemission process [26,30–33]. In a recent examination of
the photoemission behavior of conductive Ni
minerals (Ni metal, NiS, NiAs), Nesbitt et al. [26]
concluded that the main Ni 2p3=2 peak is not sensitive to the formal Ni oxidation state or the nature
of the ligand (anion) since the variation in the
main peak binding energy is small (DBEmain 6 0:5
eV). Given this small variation in the binding energy, Nesbitt et al. [26] suggested that the two final
state electronic configurations must involve predominantly metal (Ni) character, with electronic
states of ligand character contributing little to the
main peak binding energies. The main peak and
satellite peak in these conductive Ni compounds
were attributed to c1 3d10 4s1 and c1 3d9 4s2 final
states (where c1 represents a core hole), respectively, similar to Ni metal [30]. Compared to the
3d9 4s1 ground electronic state of Ni metal [30], the
additional valence electron in the final state is due
to an increase in the Coulombic potential at the Ni
center resulting from the core hole formed by
photoionization. The increase in the potential due
to the core hole lowers the energy of initially
unoccupied conduction band states below the
Fermi level, allowing the localization of a conduction electron on a Ni center. Since Ni2 P is also
a conductive compound like NiS and NiAs, a
similar argument applies. Table 1 compares the
main peak and satellite peak binding energies for
Ni2 P and the conductive Ni compounds described
by Nesbitt and co-workers [26,34,35].
While the variation in binding energy of the
main Ni 2p3=2 peak is small (DBEmain 6 0:5 eV), the
variation in binding energy of the satellite features
is significantly greater (DBEsatel 6 2:2 eV) for the
conductive compounds (Ni metal, NiS, and NiAs).
These differences in the variation in binding energy
of the two features are most readily seen in terms
of the difference in splitting between the Ni 2p3=2
main peak and satellite peak of the different
compounds as shown in Table 1. While Nesbitt
et al. have suggested that the final states for the
two main photoemission features in conductive
minerals primarily involves Ni-derived electronic
states, they have also suggested that the binding
energy of the satellite peaks are sensitive, indirectly, to the nature of the ligand (anion) in the
mineral [26]. From a comparison of the results for
NiS and NiAs, they proposed that a decrease in
the number of Ni 3d (nonbonding) electrons due to
electron donation to the Ni–L bond (L ¼ ligand)
Table 1
XPS peak positions for conducting Ni compounds
Compound
Peak
Ni2 P
Ni 2p3=2
NiAs
P 2p
Ni 2p3=2
NiS
Ni 2p3=2
Ni metal
Ni 2p3=2
Binding energy (eV)
Main
Satellite
Main
Satellite
Main
Satellite
Main
Satellite
Ref.
After
sputtering
Annealing to 500 K
DBE satel ) main
853.2
859.9
130.3
853.8
861.2
130.5
6.7
853.0
860.5
853.1
859.7
852.6
858.3
7.4
This work
n/a
7.5
[26,34,35]
6.6
[26,34,35]
5.7
[26,34,35]
D. Kanama et al. / Surface Science 552 (2004) 8–16
decreases the valence band screening of the core
hole, with a resulting decrease in the photoelectron
kinetic energy and increase in the measured binding energy. The higher binding energy for NiAs
relative to NiS has been attributed to the 4s2 4p3
electronic configuration of As atoms which are
expected to bind and additional electron in comparison to S atoms with 3s2 3p4 electronic configuration [26].
Our results provide an alternative test of this
idea. Given the actual variation in composition of
the surface region from a Ni-rich regime following
ion bombardment to a stoichiometric Ni2 P regime
following annealing, variations in the splitting
between the main peak and the satellite can be
observed with changes in ligand (anion) concentration rather than changes in the nature of the
coordinating ligand. The 3s2 3p3 electronic configuration of the group 5A element P is similar to the
4s2 4p3 electronic configuration of As. The 0.7 eV
increase in the binding energy of the main Ni 2p3=2
peak is similar to the variation observed between
Ni metal and NiAs, and the splitting between the
main and satellite features varies from 6.7 eV, near
the reported value for NiS, to 7.5 eV, near the
reported value for NiAs. The increase in splitting
in XPS corresponds qualitatively to the increase in
binding energy of the Ni 3d states and the decrease
in density of states at the Fermi level in UPS as the
spectrum changes from Ni metal like to Ni2 P. The
increase in splitting with the decrease in the density
of states at the Fermi level is consistent with the
idea of Nesbitt et al. [26] that reduced screening by
conduction electrons from the Fermi sea can account for variations in the splitting between the
main peak and satellite in Ni core level photoemission.
6. Summary and conclusions
For Ni2 P(0 0 0 1), ion bombardment and
annealing in vacuum above 700 K produces an
ordered (1 · 1) hexagonal surface with a composition near that expected for a simple stoichiometric
termination of the bulk structure. Ion bombardment preferentially removes phosphorous, giving a
disordered and Ni-rich surface. Heating near 450
15
K in vacuum causes sufficient diffusion to initiate
surface reordering and replenish the phosphorous
deficiency caused by ion bombardment.
Two photoemission features are seen for each
Ni core level in this conductive material, and are
attributed to different electronic final states following Nesbitt et al. [26]: c1 3d10 4s1 for the main
peak and c1 3d9 4s2 for the satellite peak. The
splitting between the main and satellite features in
XPS increases as the surface concentration of P
increases and the surface orders due to thermal
annealing. These changes occur as the surface
electronic structure observed with UPS changes
from Ni metal like, to Ni2 P like. The increased
splitting in XPS appears to be related to poorer
core hole screening as density of states at the
Fermi level decreases.
Acknowledgements
D.K. acknowledges Yokohama National University, Virginia Tech, and the International Student Exchange Program for financial support.
D.F.C. and S.T.O. gratefully acknowledge the
Chemical Sciences, Geosciences and Biosciences
Division of the US DOE, Office of Basic Energy
Sciences for partial support of this work through
grants #DE-FG02-97ER14751 and #DE-FG02963414669.
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