Effect of dielectric coating on the positron work function of a metal

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by  V. V. Pogosov

Effect of dielectric coating on the positron work function of a metal
V. V. Pogosov, A. V. Babich
Department of Micro- and Nanoelectronics, Zaporozhye National
Technical University, Zhukovsky Str. 64, Zaporozhye 69063, Ukraine
(Dated: January 28, 2011)
We show that the dielectric coating of the metal surface leads to the change in the sign of the
positron work function.
1
PACSnumbers: 78.70.Bj,71.60.+z
1
0
2
Positron work function is of importance for a proper keeping the same metal (or the crystal face).

n description of both bulk and surface states in a metal. The effect of sign for Al Cu is illustrated in Fig. 1.
a Electron work function We− for metals is always posi- For polycrystalline Zn / Zn (0001), the effect of ε does
J tive (metal can be consideredas a potential box for e−). notresultinanydramaticchanges: Wp =+0.61/−2.59
7 For some metals, positronwork function We+ has a neg- and +2.01/− 1.23 eV for ε = 1 and 80, respectively.
2
ative sign [1, 2] (these metals are equivalent to the po- Similar conclusions can be drawn for Al (110) / (100)

tentialbarrierforemittede+). Amongthemetalswitha [W = −4.86/−2.23; −3.39/−0.55 eV] and Cu (110)
] p
l negative positron work function are cooper, aluminium, / (100) [W = −4.52/− 2.64; −3.02/ − 0.95 eV ] at
l p
a iron, molybdenum, nickel, chromium, and titanium. For
h
these metals, the value of W < 0 was obtained from
- e+
s reemitted positron energy distribution relatively to the
e vacuum level (see Refs. [1–3] and references therein).
m
Electronandpositronworkfunctionsarecontrolledby
. their bulk contributionsas wellasby the surfaceelectro-
t
a static dipole barrier D, which enables one to distinguish
m
between different crystal faces:
-
nd We− =Web−ulk+eD, We+ =Web+ulk−eD, (1)
o where e is the unit positive charge and the term Wbulk
c e+
[ includes the positron band shift energy and positron-
electron correlation contribution. In the present pa-
1 per, for Al, Cu, and Zn, we use the following values of FIG. 1: Thecalculated dependenceWe+(ε).
v Wbulk : 3.97, 2.82 and 3.80 eV, respectively, these val-
5 e+
ues being extracted from the literature (see Ref. [2] and
8
2 references therein). ε=1; 80, respectively.
5 By assuming that the value of the electrostatic poten- Note that the positronium work function can be cal-
. tialfar beyondthe metal is equalto zeroit is possible to culated from the Born cycle,
1
find the dipole barrier D =−φ¯, where φ¯<0 is the bulk
0
1 valueoftheelectrostaticpotential[4]. Owingtoinequal- WPs =We− +We+ −Ry/2.
1 ities Wbulk, D > 0, the strong competition between the
e+
v: terms in Eq. (1) leads to the negative sign of We+. WPs does not depend on D(ε) but it is determined en-
i InRef. [4],wereportedonthemethodtocalculateen- tirely by the bulk properties.
X
ergycharacteristicsofmetallicsurface(includingD)cov- To conclude, we have demonstrated that the sign of
r eredbyaninsulatorcoatingasfunctions ofthe dielectric the positron work function is sensitive to the dielectric
a
constant ε. The calculations are performed by using the coating of the metal surface. The reported results on
Kohn-Shammethodandstabilizedjelliummodel. Strong the work function sign change can be of importance for
dependenceofDonεallowsonetoestablishthatthesign applicationsinpositrondiagnosticsofadsorbatesoroxide
of W can be changed by changing ε of coating, while at surface and possibly in nanotechnologies.
e+
[1] P. J. Schultz, K.G. Lynn,Rev.Mod. Phys.60, 3 (1988). C. Lei, Phys.Rev. B44, 12166 (1991).
[2] M. J. Puska, R. M. Nieminen, Rev. Mod. Phys. 66, 841 [4] A.V.Babich,V.V.Pogosov, Surf.Sci.603, 2393(2009).
(1994).
[3] M.Jibaly,A.Weiss,A.R.Koymen,D.Mehl,L.Stiborek,