Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-30

Investigation of surface alloy formed with coadsorption of Pb and Bi/Sb/Sn onCu(001) by low energy electron diffraction
1
Rezwan Ahmed , Mohammad Kabiruzzaman 1, Takeshi Nakagawa 1, Seigi Mizuno 1
1.

Department of MMS, Kyushu University, Fukuoka 816-8580, Japan

Abstract
Surface alloy obtained by the coadsorption of Pb and Bi/Sb/Sn on Cu(001) substrate has been
determined by tensor low energy electron diffraction (LEED). The best-fit structure of all ofcombinations can be described as a ternary alloy having 1D chain arrangement of Pb andBi/Sb/Sn atoms in the [110] direction between the Cu rows in the same direction. This type of
1D arrangement is previously obtained by sole Pb adsorption on the same substrate with c(4×4)
periodicity. In all of the coadsorption cases, creation of a vacancy causes a large displacementof the Pb atoms from the fourfold-hollow site and the atoms having smaller atomic radius, Bior Sb or Sn, located in the exact fourfold-hollow site. The structure of the Pb-Bi, Pb-Sb and PbSn coadsorption on Cu(001) that we have obtained in our previous works are compared anddescribed here.
1. Introduction
Coadsorption of more than one element on
the substrate is a familiar method to produce
surface new materials. The surface phases
produced by coadsorption and the determinationof this phases may be used to realize theproperties of the newly formed materials. [1] It
would be a good study of coadsorption of twomaterials of which one is Pb, and other is one ofthe Bi/Sb/Sn materials on the substrate Cu(001).
In this study, we investigated the coadsorption ofthese materials in the combination of (Pb, Bi),(Pb, Sb), and (Pb, Sn) on Cu(001). We
considered Pb, because it is individually used assurfactant in the thin film growth. [1] On the other
hand, Bi/Sb/Sn are materials having large
magnetic moment and promising material for themagnetic applications. [2] Interestingly, thesethree combinations have also bulk alloy.
Individually, Pb, Bi, Sn on Cu(001) makes
different alloy and overlayer structures at thedifferent coverages except the Sb atoms. [3-4]Only p(2x2) alloy structure is possible for Sb onCu(001). [5]After coadsorption of these three systems,
we got a c(4x4) phase for (Pb, Bi) [6] and p(4×4)
phase for (Pb, Sb/Sn) systems all of which wereobtained at the coverage of Pb of ~0.25 andBi/Sb/Sn of ~0.125. For all the three systems, wegot 1D chain arrangement Pb and Bi/Sb/Sn
atoms between Cu rows in the [110] direction.

2. EXPERIMENTAL

The experiments were done in an ultrahighvacuum (UHV) chamber equipped with four grid
low energy electron diffraction (LEED) system.
The base pressure of the chamber was always
maintained better than 3×10-8 Pa during thewhole experiment. The Cu(001) substrate wascleaned by several cycles of Ar+ ion sputtering(1 kV, 6.5 μA, 15 min) and subsequent annealing
at 800 K until a sharp (1×1) LEED pattern wasobtained. The best-fit model was obtained
through best agreement between theexperimental and theoretical I(E) curves, which
was decided by minimizing Pendry’s reliability
factor (Rp). [7] The error bars on the structuralparameters were calculated by variance of Rp,
ΔR = Rmin(8|Voi|/ΔE)1/2, where Rmin is theminimum Rp value and ΔE is the total energy
range of the experimental I(E) curves. [8]3. Results and Discussion
3.1 c(4x4)-Pb,Bi coadsorption
The room temperature coadsorption of Pb
and Bi was carried out by starting the depositionof Pb atoms on Cu(001) at a certain coverage.
After that, Bi atoms were deposited on the Pb
pre-adsorbed surface. The coverage of Bi atoms
was increased gradually and we checked theLEED patterns. When we reversed thedeposition order, the same LEED patterns wereobtained. In this way, the c(4×4) structure wasobtained at the coverage of Pb at ~0.25 and Bi at83

Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-30~0.125. According to the coverage calculation ofthe c(4×4) unit cell, 0.375 corresponds to thethree adsorbate atoms. Thus, there are two Pbatoms and one Bi atom in the unit cell, and thecoverages of Pb and Bi are 2/8 and 1/8,respectively. Considering this 3 adsorbates, thebest-fit structure of this phase is given in thefollowing figure 1.

structure having 1D chain arrangement of –SnPb-Pb-Sn- in the [110] direction. The best-fit
structure of this phase is shown in figure 2(b).
We confirmed this structure by the Rp 0.21
Cu 2nd layer
Cu top layer
SbCu 2nd layer

(a)

(b)

SnFigure 2: The best-fit structure of the (a) p(4x4)(Pb, Sb) coadsorption system and (b) the p(4x4)(Pb, Sn) coadsorption system. Upper one is topview and the lower one is the side view.

Cu top layer
PbBiFigure 1: The best-fit structure of the c(4x4)-(Pb,Bi) coadsorption system. Upper one is top viewand the lower one is the side view.

4. CONCLUSION
In summary, it is possible to obtain thesurface alloy of heavy and larger atomic radius
metals on the lighter and smaller atomic radiusof Cu(001). Although the different phases are
obtained for these three combinations but thestructure are similar. The structures are definedas the 1D arrangement of –Bi/Sb/Sn-Pb-PbBi/Sb/Sn- between the Cu rows in the directionof [110]. Also, it can be predicted that Pb may
have significant effect to make such type ofchain arrangement as individually Pb makes thistype of structure and the theoretical investigationis needed for this purpose.

We confirmed this structure by the lower
Rp factor of 0.21. This structure can be definedas 1D chain arrangement of –Bi-Pb-Pb-Bi- in
the [110] direction. 6
3.2 p(4x4)-Pb Sb coadsorption
In case of Pb and Sb coadsorption, we gotp(4x4) phase at the coverage of Pb at ~0.25 andSb at ~0.125. This coverage combination is thevery similar to the above Pb and Bi coadsorption
system. Moreover, the LEED pattern of thep(4×4) phase was very clear up to ~220 eV ofthe beam energy. Above this energy, only c(4×4)
points were visible and other points of p(4×4)
points are totally invisible. According to thecoverage calculation of the p(4×4) unit cell,
0.375 corresponds to the six adsorbate atoms.
Thus, there are four Pb atoms and two Sb atoms
in the unit cell. Considering this 6 adsorbates, thebest-fit structure of this phase is given in thefigure 2(a). We confirmed this structure by thelower Rp factor of 0.23. This structure also canbe defined as 1D chain arrangement of –Sb-PbPb-Sb- in the [110] direction.

ACKNOWLEDGEMENTS
The financial support for this work was obtained
from JSPS KAKENHI Grant Numbers
JP15H03677 and JP15K13504.
REFERENCES[1] D.P. Woodruff, Surface Alloys and Alloy
Surfaces, (1st ed., Elsevier, Netherlands, 2002).
[2] Pierluigi Gargiani et.al. The j. of chemicalphysics 132 (2010) 174706.
[3] Y. Gauthier, W. Moritz, W. Hosler, Surf. Sci., 345(1996) 53-63.
[4] Hisashi Yoshida et. Al. Surface Science 604(2010) 535–540.
[5] E. AlShamaileh, et. Al. Surf. Sci., 566–568(2004) 52–57.
[6] M. Kabiruzzaman, R. Ahmed, T. Nakagawa, S.
Mizuno, Surf. Sci., 664 (2017) 70–75.
[7] S. Mizuno, H. Tochihara, A. Barbieri and M.A.
Van Hove, Phys. Rev. B, 52 (1995) R11658.
[8] J.B Pendry, J. Phys. C, 13 (1980) 937.

3.3 p(4x4)-Pb Sn coadsorption
In case of Pb and Sn coadsorption, we also
got p(4x4) phase at the coverage of Pb at ~0.25
and Sb at ~0.125. And the p(4x4) fraction points
becoming weaker in this case also. Considering
these similarities with the Pb and Sbcoadosorption system, we got similar best-fit84