3.3 Adsorption or Segregation
61
one with segregation, but they are often different from the viewpoint of thermodynamic equilibrium. Usually, the surface composition realized by segregation is at
thermodynamic equilibrium, whereas a surface with adsorption is not in many cases.
Because a surface with adsorption is realized when an adsorbate is supplied from
the outside, it cannot be reproduced when the adsorbate is removed for some reason.
On the other hand, a surface with segregation can, in many cases, be reproduced by
heating because an adsorbate can be supplied from inside to achieve thermodynamic
equilibrium.
When the work function is modified by adsorption or segregation, the position
of the adsorbate on the surface is strongly dependent on the method of modification. Figure 3.27a, b demonstrate an example of Al segregation in Cu-9 at% Al(111)
obtained by first-principles calculations [29]. The work functions are different when
Al is on the surface (adatom), inside the top layer (replace), and in the bulk. The
position of the adsorbate affects the surface electron distribution decisively and thus
the surface term of the work function. In practice, only one state is thermodynamically stable and realized under equilibrium. Furthermore, the distance between an
adsorbate and the top surface of the substrate also affects the work function. The
effect of the distance on the work function of N-adsorbed W determined by firstprinciples calculations is shown in Fig. 3.28 [30]. It can be seen that the direction
of the change in the work function, either an increase or decrease, upon nitrogen
adsorption varies with the distance. In reality, the distance is determined by nature,
and the work function is accordingly determined by the stable distance.
Note that the work function does not necessarily increase when an element with a
larger work function (than the substrate) is adsorbed. Not only the electron transfer
between the adsorbate and the substrate but also the surface roughness at the atomic
level affects the work function. Figure 3.29 shows experimentally obtained changes
in the work function upon the adsorption of various elements on W(110) as a function
of coverage [31]. At the first stage of adsorption up to a coverage of one monolayer,
the work function decreases upon adsorption for most elements.
There is one more type of surface segregation that is not under equilibrium, as
schematically shown in Fig. 3.30. When a thin film is deposited and annealed, rapid
diffusion and surface segregation of an underlayer element are often observed [32–
34]. The mechanism of this type of segregation [35] and the method of generally
predicting whether this type of segregation occurs for a specific combination of an
underlayer element and overlayer film element [36, 37] have been reported. Underlayer elements reach the surface through grain boundary diffusion, and segregation
is achieved under a quasi-stable state. Although it is not under thermodynamic equilibrium, a surface with this type of segregation can also be reproduced by heating,
as in the case of equilibrium surface segregation. Therefore, a change in the work
function occurs with this type of segregation. Some examples are given in Fig. 3.31
[38, 39].
Adsorption or segregation has been utilized to modify the work function in practical applications. So-called thoria-coated tungsten or iridium (tungsten or iridium
coated with thorium dioxide, ThO 2 ) shows a stable low work function and has been
used as an electron emitter for vacuum gauges and other electron sources. A zirconia
61
one with segregation, but they are often different from the viewpoint of thermodynamic equilibrium. Usually, the surface composition realized by segregation is at
thermodynamic equilibrium, whereas a surface with adsorption is not in many cases.
Because a surface with adsorption is realized when an adsorbate is supplied from
the outside, it cannot be reproduced when the adsorbate is removed for some reason.
On the other hand, a surface with segregation can, in many cases, be reproduced by
heating because an adsorbate can be supplied from inside to achieve thermodynamic
equilibrium.
When the work function is modified by adsorption or segregation, the position
of the adsorbate on the surface is strongly dependent on the method of modification. Figure 3.27a, b demonstrate an example of Al segregation in Cu-9 at% Al(111)
obtained by first-principles calculations [29]. The work functions are different when
Al is on the surface (adatom), inside the top layer (replace), and in the bulk. The
position of the adsorbate affects the surface electron distribution decisively and thus
the surface term of the work function. In practice, only one state is thermodynamically stable and realized under equilibrium. Furthermore, the distance between an
adsorbate and the top surface of the substrate also affects the work function. The
effect of the distance on the work function of N-adsorbed W determined by firstprinciples calculations is shown in Fig. 3.28 [30]. It can be seen that the direction
of the change in the work function, either an increase or decrease, upon nitrogen
adsorption varies with the distance. In reality, the distance is determined by nature,
and the work function is accordingly determined by the stable distance.
Note that the work function does not necessarily increase when an element with a
larger work function (than the substrate) is adsorbed. Not only the electron transfer
between the adsorbate and the substrate but also the surface roughness at the atomic
level affects the work function. Figure 3.29 shows experimentally obtained changes
in the work function upon the adsorption of various elements on W(110) as a function
of coverage [31]. At the first stage of adsorption up to a coverage of one monolayer,
the work function decreases upon adsorption for most elements.
There is one more type of surface segregation that is not under equilibrium, as
schematically shown in Fig. 3.30. When a thin film is deposited and annealed, rapid
diffusion and surface segregation of an underlayer element are often observed [32–
34]. The mechanism of this type of segregation [35] and the method of generally
predicting whether this type of segregation occurs for a specific combination of an
underlayer element and overlayer film element [36, 37] have been reported. Underlayer elements reach the surface through grain boundary diffusion, and segregation
is achieved under a quasi-stable state. Although it is not under thermodynamic equilibrium, a surface with this type of segregation can also be reproduced by heating,
as in the case of equilibrium surface segregation. Therefore, a change in the work
function occurs with this type of segregation. Some examples are given in Fig. 3.31
[38, 39].
Adsorption or segregation has been utilized to modify the work function in practical applications. So-called thoria-coated tungsten or iridium (tungsten or iridium
coated with thorium dioxide, ThO 2 ) shows a stable low work function and has been
used as an electron emitter for vacuum gauges and other electron sources. A zirconia