e-Journal of Surface Science and Nanotechnology

14 March 2015

Conference - ISSS-7 -e-J. Surf. Sci. Nanotech. Vol. 13 (2015) 93-98

Electronic Structure of Ni2 P(0001) Studied by Resonant Photoelectron Spectroscopy∗
Shuhei Ishida, Yuichi Sugizaki, Takuya Nakamura, and Kazuyuki Edamoto†Department of Chemistry, Rikkyo University, Toshima-ku, Tokyo 171-8501, Japan

Masaharu Matsunami
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan

Tetsuya Hajiri
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan

Shinichi Kimura
Graduate School of Frontier Biosciences, Osaka University, Suita, 565-0871, Japan(Received 24 December 2014; Accepted 20 February 2015; Published 14 March 2015)
The electronic structure of Ni2 P(0001) has been investigated by photoelectron spectroscopy (PES) utilizing
synchrotron radiation. A Ni 3d–P 3p hybrid band was observed at 1-5 eV, and the band had peaks at 1.5 eV and 3.4
eV together with a shoulder around 2.4 eV. The resonant PES study showed that the Ni 3d component was includedin the whole binding energy region of the band. The change in the surface electronic structure of Ni2 P(0001) inducedby the segregation of P atoms was investigated by annealing temperature-dependent measurements of PES, andit was found that the change proceeded via following two steps: the segregation of P atoms proceeded at about200◦ C, and the bonding between surface Ni atoms and segregated P atoms was facilitated at > 300◦ C. The bonding
between Ni and P atoms induced substantial stabilization of Ni 3d levels, which is contrary to the case of Fe2 P(0001)
where the segregation of P atoms induced little change in DOS of Fe 3d levels. [DOI: 10.1380/ejssnt.2015.93]
Keywords: Synchrotron radiation photoelectron spectroscopy; Surface electronic phenomena; Surface segregation; Phosphides; Low index single crystal surfaces

I.

INTRODUCTION

The surface properties of transition metal phosphides(TMPs) have attracted much attention because these havebeen found to have high catalytic activities for hydroprocessing reactions such as hydrodesulfurization (HDS) andhydrodenitrogenation (HDN) for petroleum fuels [1,2]. Of
all TMPs studied thus far, it has been found that Ni2 P
has the highest catalytic performance for these reactions[1,2]; the HDS conversion for dibenzothiophene is nearly
100% [1]. The conversion rate is substantially higher thanthat of currently used sulﬁde-based catalysts, and Ni2 P isa promising material as the catalyst of HDS in the next
generation. On the other hand, the catalytic activity of
Fe2 P presents striking contrast to that of Ni2 P; the HDS
conversion for dibenzothiophene is only several % [1]. It
is a wonder that the catalytic activities of Ni2 P and Fe2 P
are much diﬀerent from each other, because the electronic
structures of these materials are thought to be similar
with each other; these are typical metal-rich phosphides
and both crystals are expected to have metallic electronic
structures [1]. In addition, Fe2 P and Ni2 P crystals belong
to the same space group (P62m) and both crystals consist of two types of atomic layers with the stoichiometryof M3 P and M3 P2 (M = Ni, Fe) which are stacking alternatively along the [0001] direction [3]. One of the possible
reasons for the diﬀerence in catalytic activity is that thesurface electronic structures of these materials, especiallythose of active metal sites, are diﬀerent with each other.

∗ This paper was presented at the 7th International Symposiumon Surface Science, Shimane Prefectural Convention Center (Kunibiki Messe), Matsue, Japan, November 2-6, 2014.
† Corresponding author: edamoto@rikkyo.ac.jp

Therefore, in order to elucidate the origin of the high catalytic performance of TMPs, it is useful to explore thediﬀerence in the surface electronic structures of Fe2 P and
Ni2 P.
The characterization of a Ni2 P single-crystal surface
has been mainly attempted for a (0001) surface [3-12].
An ideal (0001) surface should be terminated with either a Ni3 P- or a Ni3 P2 -layer, and theoretical studies by
Liu et al. using density functional theory (DFT) calculations have predicted that the Ni3 P2 -terminated surface
is more stable than the Ni3 P-terminated surface by 2.75
eV/unit cell [4,5]. Li and Hu made ﬁrst-principles calculations of the electronic structure of Ni2 P(0001), andalso concluded that the Ni3 P2 -terminated surface is morestable [6]. However, previous structural analyses using
low-energy electron diﬀraction (LEED) and scanning tunneling microscopy (STM) have revealed that, in addition
to a (1×1) termination,
several reconstructed phases such√√as 3/2 × 3/2 and 3 × 3 R30◦ are also realized on thesurface [7-9]. Furthermore, Hernandez et al. made a dynamical LEED study for the (0001) surface, and proposedthat even the (1 × 1) surface is not an ideal surface andshould be a Ni3 P2 -terminated surface on which P atoms
sit above threefold sites of three Ni atoms (P-terminatedNi3 P2 -surface) [10]. As for the electronic structure, ourgroup performed an angle-resolved photoemission spectroscopy (ARPES) study on Ni2 P(0001) and found thatthe normal-emission spectra consist of a Ni 3d–P 3p hybrid band (main band) and a satellite at 0-4 eV and 8eV, respectively [11,12]. As for Fe2 P surfaces, our group
recently performed an angle-integrated PES study for
Fe2 P(0001), and found that a Fe 3d–P 3p hybrid band
was formed at 0-3 eV, and the band consisted of a peak
at 0.2 eV and a shoulder at 2.0 eV [13]. The band seemed
to be formed in the lower binding energy side with respectc 2015 The Surface Science Society of Japan (http://www.sssj.org/ejssnt)
ISSN 1348-0391 ⃝93Ishida, et al.

Volume 13 (2015)

II.

EXPERIMENTAL

A Ni2 P single-crystal was grown by Dr. S. Otani of
the National Institute for Materials Science using a ﬂoating zone method. The crystal was cut at an orientation of(0001) by spark erosion into a disk of 1 mm thickness, andthe surface was polished mechanically to a mirror ﬁnish.
The surface was cleaned in a vacuum chamber by several
cycles of Ar+ ion sputtering (1-3 kV, 30 min) and annealing at 600◦ C for 30 min. The clean surface prepared bythis procedure gave a hexagonal (1 × 1) LEED pattern,
and no impurity was observed within the detection limitof Auger electron spectroscopy (AES). The PES measurements were performed at BL-5U of the UVSOR Facility,
Institute for Molecular Science, using an electron energy
analyzer of MBS-Toyama A-1. The acceptance angle of
the analyzer was ±18◦ . The total experimental resolution was estimated from the Fermi edge in the spectra ofNi2 P(0001) measured at room temperature, and was estimated to be 110 meV at hν = 60 eV, which includes
thermal broadening contribution to the width at 300 K.
The spectra presented below were normalized by the photocurrent of a gold mesh which was inserted at the entrance of the analysis chamber. The incidence angle ofthe light was 45◦ from the surface normal direction, andthe analyzer axis was parallel to the surface normal direction. The base pressure in the vacuum system was
1.8 × 10−8 Pa.

III.

RESULTS AND DISCUSSIONSThe PES spectra of Ni2 P(0001) were measured at various photon energies of hν = 46 − 80 eV, and the spectra
measured at hν = 62 − 74 eV are shown in Fig. 1. An intense band is observed at 1-5 eV, and the band has peaks
at 1.5 eV and 3.4 eV together with a shoulder around 2.4
eV independent of photon energy. The peaks are called
peak A and peak B, respectively, and the shoulder is called
peak S, hereafter. The peak B has not been clearly observed in previous ARPES studies [11, 12], and the reason
will be discussed later. The band observed at 1-5 eV isascribed to a Ni 3d–P 3p hybrid band (main band) [4-6].
Though a clear cut-oﬀ is observed at the Fermi level (EF )independent of hν, the density of states (DOS) around EF
is suppressed due to the hybridization of Ni 3d levels with
P 3p levels forming a pseudo gap. A band is also observed
at 8 eV, and previous resonant PES studies showed thatthe peak is ascribed to a satellite which is associated with
94

A

Ni2P(0001)

B S
hν
(eV)
74
73

Intensity (Arb. Units)

to the Ni 3d–P 3p band in Ni2 P [13], suggesting that thestabilization of metal sites owing to the bonding with P
atoms is ineﬀective in Fe2 P. The stabilization of metalsites is thought to be important to prevent deactivationof the surface induced by accumulation of S atoms on active metal sites during HDS reactions. In this paper, we
report the results of an angle integrated PES study on
Ni2 P(0001) using the same experimental set-up as thatin our previous study of Fe2 P(0001) [13]. The possible
origin of the diﬀerence in the catalytic activities of Ni2 P
and Fe2 P will be discussed.

72
71
70
69
68
67
66
65
64
63
62
15

10

5

0

Binding Energy (eV)FIG. 1. PES spectra of Ni2 P(0001) measured at various photon energies of 62-74 eV.

the Ni 3d photoemission leading to a two-hole bound ﬁnal state [11,12]. An additional broad band, which shifts
to the higher binding energy side with increasing hν, isobserved at hν > 69 eV, and the band is ascribed to a Ni
M23 VV Auger peak [11,12].
The PES spectra of Ni2 P(0001) after Ar+ ion sputtering (3 kV, 10 min) and of the surface subsequently annealed at elevating temperatures are shown in Fig. 2. The
spectra are measured at hν = 60 eV. The spectrum of thesurface after annealing at 600◦ C (the topmost spectrum)is the same as that before sputtering. When the surface issputtered, the peak A slightly shifts to the lower binding
energy side by 0.2 eV and the peak B is almost suppressed,
and the overall band intensity is enhanced as comparedwith those in the spectra of the surface before sputtering.
As the Ni2 P(0001) surface is subjected to Ar+ ion sputtering, it has been proved that P atoms in the surface region
are selectively removed [3]. The photoionization cross section of Ni 3d (9 Mb) is much higher than that of P 3p (0.5Mb) at hν = 60 eV [14], and therefore the enhancementof the band intensity is explained to be due to the increaseof the component of Ni 3d electrons in the valence banddue to the removal of P atoms in the surface region, because the formation of the Ni 3d–P 3p bond should resultin the partial donation of Ni 3d electrons to the P 3p levels [11,12]. As the surface is annealed at 150-300◦ C, theband is decreased in intensity though the spectral proﬁleis nearly unchanged. A previous XPS study by Kanama et
al. showed that the segregation of bulk P atoms toward
the Ni2 P(0001) surface proceeds by annealing the sputtered surface at around 200◦ C [3], and thus the decrease of
the band intensity by annealing at 150-300◦ C is ascribedto the increase of the component of P 3p electrons in thevalence band due to the surface segregation of P atoms.
The spectrum is substantially changed by annealing at

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Ni2P(0001)

annealing
temperature

Intensity (Arb. Units)600◦ C; the peak A shifts to the higher binding energy
side and the peak B grows, and overall band intensity isfurther decreased. Essentially the same result has beenobtained by Kanama et al. using He II UPS [3], though
the peak B is more clearly observed in this study. Though
the XPS Ni/P intensity ratio is nearly constant by annealing at higher than 200◦ C [3], the substantial change
in the surface electronic structure proceeds by annealingat > 300◦ C. Therefore, we propose that the change inthe surface electronic structure of Ni2 P(0001) induced by
P segregation proceeds via following two steps; the segregation of bulk P atoms toward the surface proceeds byannealing at around 200◦ C, and the bonding between Ni
and segregated P atoms in the surface region is facilitatedby annealing at > 300◦ C. The state of the segregated P
atoms in the ﬁrst step is unknown at present. The spectral
shape of the valence band is not changed by the P segregation, and thus one of the possible situations is that the
segregated P atoms occupy some interstitial sites aroundthe surface, which is thought to give little eﬀect on the
spectral shape due to the low photoionization cross section of P states. However, since the overall valence band
intensity is decreased also in the ﬁrst step, partial changetransfer from Ni atoms to segregated P atoms may occur
in this stage. A normal-emission spectrum of Ni2 P(0001)
at hν = 60 eV measured in our previous ARPES studies [11,12] is shown as a dotted line in Fig. 2. Though
the spectrum was measured in an angle-resolved mode,the spectral proﬁle was nearly independent of hν and detection angles (θd ) [11,12], unlike the case of the ARPES
study of Ni2 P(1010) in which the θd -dependent dispersion
was observed [15]. The origin of the lack of the dispersion of ARPES from Ni2 P(0001) is unknown, and one ofpossible reasons proposed in Refs. [11] and [12] was thatthe spectra reﬂect three dimensional DOS. However, thespectral proﬁle is not the same as that obtained in thepresent study; the valence band seems to consist of peaks
A and S, while the peak B is not observed, and the peak
A locates in the lower binding energy side with respect tothat observed in the present work. This is thought to be
due to the diﬀerence in the annealing condition; the surface is annealed at 600◦ C for 10 min in our previous works[11,12] while the surface was annealed at 600◦ C for prolonged time (30 min) in the present work. The formationof Ni–P bond should be a slow process, and the peak B isthought to arise from the hybridization of the 3d levels of
surface Ni atoms with the 3p levels of segregated P atoms.
The segregation behavior of P atoms was also studied for
Ni2 P(1010), and it was proposed that the P atoms in thesurface region were selectively removed by Ar+ ion sputtering, and that the annealing of the sputtered surface
induced P segregation which proceeded at 260-330◦ C [1618]. The segregation behavior is qualitatively the sameas that of Ni2 P(0001), though the segregation occurs at
relatively low temperature region for Ni2 P(0001).
The integrated intensities of the main band in the spectra of Ni2 P(0001) are plotted as a function of hν inFig. 3(a) (open circles). The background drawn by the
Shirley procedure was subtracted from each raw data before integration. In Fig. 3(a), the integrated intensities ofthe satellite are also plotted as ﬁlled circles. The background approximated to be a polynominal function was
subtracted from each raw data to estimate the satellite

Volume 13 (2015)

600Υ

300Υ
250Υ
200Υ
150Υ

as sputtered
5

2.5

0

Binding Energy (eV)FIG. 2. PES spectra of Ni2 P(0001) after Ar+ ion sputtering (3 kV, 10 min), and of the surface subsequently annealed
at elevating temperatures (hν = 60 eV). A normal-emission
spectrum of Ni2 P(0001) measured in a previous study [11](hν = 60 eV) is shown as a dotted line.

intensity. The satellite is overlapping with a Ni M23 VV
Auger peak around hν = 68 eV, and the sum of the intensities of the satellite and the Auger peak is plottedat hν ≧ 68 eV. The hν-dependence of the satellite intensity is identical to that reported in previous resonant
PES studies; the intensity is maximized around the Ni 3p
photoexcitation threshold (∼ 70 eV) [11,12]. The intensity maximum is interpreted as caused through the resonance arising from the interference between a direct Ni
3d photoemission process and a process induced by theNi 3p → 3d photoexcitation followed by a Ni 3d electron
emission through an Auger process [11,12]. The latterprocess leads to a ﬁnal state with two correlated d holes,
and thus the satellite is associated with the Ni 3d photoemission leading to the two-hole bound ﬁnal state. On
the other hand, the hν-dependence of the main band intensity is characterized by the existence of a minimum
around the Ni 3p photoexcitation threshold (hν = 67 eV).
The existence of the minimum of the main band intensity around the Ni 3p photoexcitation threshold was also
observed in the previous resonant PES studies [11,12],though the dip structure is more prominently observed in
the present study. The intensity minimum was explained
to be caused through the same resonance process as thatfor satellite; the super-Coster-Kronig decay of the 3p →3d excited state leads to the two-hole ﬁnal state, and thesatellite and the main band are resonated when the ﬁnal
state is localized and delocalized, respectively [19,20]. In
this case, the hν-dependence of the main band intensity issimulated to be a Fano line-shape with a minimum aroundthe 3p photoexcitation threshold [19,20]. On the other

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hand, Hüfner gave a more simple explanation that the intensity minimum is originated from the fact that manyof the photons impinging on the Ni atoms are used forthe 3p → 3d excitation rather than the direct 3d photoemission around the Ni 3p photoexcitation threshold [21].
In either case, the appearance of the dip around 67 eV ischaracteristic of the Ni 3d photoemission. The heights ofpeaks A, B and S in each spectrum after subtracting the
Shirley background are plotted as a function of hν in theinset of Fig. 3(a). The intensities of all three peaks shownearly the same hν-dependence and are minimized around67 eV, indicating that the Ni 3d components are includedin the whole valence band region. Though the peak B
should arise from the hybridization between the 3d levelsof surface Ni atoms and the 3p levels of the segregationof P atoms, the resonant PES study shows that the stateincludes substantial contribution of Ni 3d components asin the case of peaks A and S.
The valence band spectra of Fe2 P(0001) also consistof a main band and satellites [13], and the PES spectra
measured at hν = 49 and 53 eV are shown in the inset
of Fig. 3(b). A Fe 3d–P 3p hybrid band (main band)
is observed at 0-3 eV, and satellites are observed at 4.5
and 7 eV. The origin of the two-peak structure of thesatellite is unknown at present. In Fig. 3(b), the integrated intensities of the main band (open circles) andthose of the sum of satellites (ﬁlled circles) in the PES
spectra of Fe2 P(0001) are plotted as a function of hν[13]. At hν > 55 eV, a Fe MVV Auger emission peakis overlapping with the satellites, and the intensity athν > 55 eV (ﬁlled circles) is the sum of the satellites andAuger peak intensities. As in the case of Ni2 P(0001), thesatellite intensity shows a maximum and the main band
intensity shows a minimum around the Fe 3p photoexcitation threshold (54 eV). The resonant behavior of thephotoemission from Fe2 P(0001) is similar to that from
Ni2 P(0001). However, the comparison of the plots in
Figs. 3(a) and 3(b) shows that the resonant enhancementof the satellite intensity is more intense for Fe2 P(0001)
than that for Ni2 P(0001); the intensity of the satellite atresonance maximum relative to that in the oﬀ-resonanceregion is 21 for Fe2 P(0001) while that is 10 for Ni2 P(0001).
It has been revealed that the intensity of the resonancedepends on the number of 3d holes [22,23], which qualitatively explains the diﬀerence in the resonance intensities of
Fe2 P(0001) and Ni2 P(0001). Chandesris et al. estimatedthe intensity of the resonance of PES of several 3d metals
by the use of a parameter deﬁned as the satellite intensity
relative to the main band intensity (Isat /Imain ) at resonance, and reported that the resonance intensity of Fe isabout 2.8 times of that of Ni [22]. However, the resonanceintensity (Isat /Imain ) is about 0.2 for Ni2 P(0001) whilethat is 9.5 for Fe2 P(0001), and thus the resonance intensity of Fe2 P(0001) is about 48 times of that of Ni2 P(0001),
indicating that the ratio of the resonance intensity of Fe
3d relative to that of Ni 3d is enhanced by the formation of phosphides. We think that this is mainly due tothe diﬀerence in the valence electronic structures formed
through the metal–P bonding. For Fe2 P, a Fe 3p3/2 peak
was observed at 53 eV [13], and the resonance maximum
is observed at hν = 54 eV. Therefore, the resonance iscaused through the excitation of Fe 3p electrons to the unoccupied states just above EF . The valence band spectra
96

FIG. 3. (a) Integrated intensities of the main band (open circles) and the satellite band (ﬁlled circles) in PES spectra ofNi2 P(0001) as a function of hν. The latter plots correspond to
the sum of the intensities of the satellite peak and the intensities of an Auger peak at hν ≧ 68 eV. The heights of peaks
A, B and S are plotted as a function of hν in the inset. (b)Integrated intensities of the main band (open circles) and thesatellite band (ﬁlled circles) in PES spectra of Fe2 P(0001) as afunction of hν. The latter plots correspond to the sum of theintensities of the satellite peak and the intensities of an Auger
peak at hν ≧ 55 eV. PES spectra of Fe2 P(0001) measured athν = 49 and 53 eV are shown in the inset.

of Fe2 P(0001) is characterized by the existence of a sharp
peak just below EF as shown in the inset of Fig. 3(b) and
Fig. 4, and thus the band with high DOS is formed around
EF , which implies that the Fe 3p → 3d excitation occurs
with high photoabsorption intensity. For Ni2 P, a Ni 3p3/2
peak was observed at 67 eV [11,12], and the resonancemaximum is observed at ∼ 70 eV (Fig. 4(a)), suggestingthat the ﬁnal state of the Ni 3p → 3d excitation locates atslightly above EF . In the case of Ni2 P, DOS around EF is
suppressed due to the hybridization of Ni 3d levels with P
3p levels forming a pseudo gap (Fig. 1). Therefore, the Ni
3p → 3d photoexcitation probability becomes relatively
low as compared with the case of Fe2 P. The diﬀerence inthe electronic structures of Fe2 P(0001) and Ni2 P(0001)
will be also discussed in the next paragraph. The otherpossible origin of the diﬀerence in resonance intensities isthe diﬀerence in the charged states of metal sites. Previous core-level PES studies showed that the 2p3/2 level

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IV.

Ni2P(0001)
Intensity (Arb. Units)of bulk P atoms locate at 129.3 eV for Fe2 P(0001) [13]and at 129.6 eV for Ni2 P(0001) [11,12], while the 2p3/2
level of simple substance of P has been observed at 130.2130.9 eV [24]. Therefore, both of the P atoms in Fe2 P and
those in Ni2 P are negatively charged, and core-level PES
studies show that the P atoms in Fe2 P is more negative
than those in Ni2 P. Therefore, the increase of 3d holes
due to the formation of phosphide is more prominent in
Fe2 P, which should be one of the reasons of the higher
resonance intensity for the photoemission from Fe2 P.
In order to compare the electronic structures of
Ni2 P(0001) and Fe2 P(0001), the valence band spectraof these surfaces measured in the oﬀ-resonance region(hν = 60 and 49 eV, respectively) are shown in Fig. 4(solid lines). The photoionization cross sections of Ni 3d(9 Mb) and Fe 3d (10 Mb) are much higher than that of
P 3p (0.5 Mb) at these photon energies [14], and thus the
spectra mostly reﬂect DOS of metal 3d components. Bothof Ni and Fe metals are known to have large DOS at EF ,because the Fermi level intersects the 3d bands [22,23].
However, the valence band of Ni2 P(0001) is observed at
1-5 eV and DOS around EF is relatively small, indicating that the Ni 3d levels are eﬀectively stabilized throughthe hybridization with P 3p levels. On the other hand,the spectrum of Fe2 P(0001) has large emission aroundEF and the spectral shape is similar to that of a Fe metal[22], and thus the Fe 3d levels are not eﬀectively stabilized
in Fe2 P. The electronic states around EF should play a
crucial role in surface reactions, and Fe atoms in Fe2 P
should remain reactive in spite of bonding with P atoms.
In Fig. 4, the spectra of these surfaces after Ar+ ion sputtering are also shown as dotted lines. The sputtering of
Ni2 P(0001) (3 kV, 10 min) induces a substantial spectralchange; the band is totally shifted to the lower binding
energy side and the intensity is enhanced, and the spectrum reversibly returns back to original one by annealing
the surface. These results mean that the 3d levels of surface Ni atoms are stabilized through the bonding withsegregated P atoms after annealing. On the other hand,the sputtering of Fe2 P(0001) induces little change in the
PES spectrum (Fig. 4). Though the sputtering condition
is mild (1 kV, 5 min) relative to that for Ni2 P(0001), it
has been conﬁrmed by P 2p core-level measurements that
P atoms in the surface region are selectively removed bythis sputtering condition [13]. Therefore, it is concluded
that the stabilization of the surface Fe atoms due to thebonding with the segregated P atoms is not eﬀective on
Fe2 P(0001). The stabilization of the 3d levels of active
metal sites is denoted as a “ligand eﬀect” [4], and is predicted to be important to maintain the activity of TMP
catalysts in HDS reactions through the prevention of theaccumulation of S atoms on the active metal sites. The
present study suggests that the ligand eﬀect is eﬀectiveon Ni2 P(0001), while that is not eﬀective on Fe2 P(0001),
which should be one of the reasons for the diﬀerence inthe catalytic activities of Ni2 P and Fe2 P for HDS.

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Fe2P(0001)

5

2.5

0

Binding Energy (eV)FIG. 4. PES spectra of Ni2 P(0001) and Fe2 P(0001) clean surfaces (solid lines) and of the surfaces after slight Ar+ ion sputtering (3 kV for 10 min and 1 kV for 5min, respectively) (dotted lines). The spectra are measured at hν = 60 eV and 49 eV,
respectively.

band) and the satellite were observed at 1-5 eV and at 8
eV, respectively. The main band consisted of peaks at 1.5
eV and 3.4 eV together with a shoulder around 2.4 eV.
The peak at 3.4 eV was formed through the bonding between surface Ni atoms and segregated P atoms, while theresonant PES study showed that the intensities of all threepeaks were minimized around the Ni 3p photoexcitationthreshold, indicating that substantial contributions of Ni
3d levels are included in all three peaks. The change inthe surface electronic structure of Ni2 P(0001) induced by
the segregation of P atoms was investigated by annealing
temperature-dependent measurements of PES, and it was
found that the substantial change of the surface electronic
structure was induced by annealing at > 300◦ C, while
the P segregation occurred at about 200◦ C. These results
suggest that the change proceeds via following two steps:
the segregation of P atoms proceeds at about 200◦ C, andthe bonding between surface Ni atoms and segregated P
atoms is facilitated at > 300◦ C. The bonding between
Ni and P atoms induced substantial stabilization of Ni 3dlevels, which is contrary to the case of Fe2 P(0001) where
the segregation of P atoms induced little change in DOS
of Fe 3d levels.

CONCLUSIONACKNOWLEDGMENTS

The valence electronic structure of Ni2 P(0001) was investigated by photoelectron spectroscopy (PES) utilizing
synchrotron radiation. The Ni 3d–P 3p hybrid band (mainWe are pleased to thank Dr. S. Otani of the National
Institute for Materials Science for providing us Ni2 P and

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Fe2 P single-crystal samples. This work has been performed under the UVSOR Proposal (Proposal No. 24-

522, 25-807). This work has been supported by MEXTSupported Program for the Strategic Research Foundation at Private Universities, 2013-2017.

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