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On the effect of order-disorder phase transition in adsorbed films on work function
Surface Science 0 North-Holland
93 (1980) L98 -LlOO Publishing Company
SURFACE SCIENCE LETTERS ON THE EFFECT OF ORDER-DISORDER PHASE TRANSITION ADSORBED FILMS ON WORK FUNCTION
IN
A.G. FEDORUS and A.G. NAUMOVETS Institute of Physics ofthe Academy 252650 Kiev 28, USSR
Received
17 August
of Sciences of the Ukrainian SSR, Prospekt Nauki, 144,
1979
Recently Mora’n-Mpez and Ten Bosch 111 have studied theoretically the influence of the order-disorder phase transition in adsorbed films on the work function of covered surfaces. They suggest their model the most suitable for the description of the films of alkaline elements adsorbed on noble and transition metals. The theory [ 11 predicts a strong effect of the order-disorder phase transition on the value of the work function change A& induced by an adsorbed film: A@, can be changed by several times and the dependence of A$, versus the coverage degree can acquire a qualitatively different shape. Here WC want to draw attention to the fact that these conclusions, at least for the films of alkaline and alkaline-earth elements on refractory metals, are completely inconsistent with experimental data. Ten years ago we established [2] that the change of the work function S@<,d accompanying the order-disorder phase transition in sodium films on tungsten (011) plane does not exceed 0.05 eV. whereas A@, amounts in this case to several eV. Later these measurements were repeated with a higher accuracy and have revealed that for the systems Na-W(O1 1) [3], Cs-W(O11) [4], and Li--W(112) [S] the Qo~ values are not larger than O.Ol& 0.02 eV. Similar results were obtained for the alkaline-earth films ~ Ba on W(Ol1) and Mo(O11) [6] and Sr on W(O11) [7]. Thus, there exists considerable evidence that disturbance of the long-range order in the adfilms of the mentioned type does not significantly change the work function. We explain this experimental fact assuming that the work function is determined mainly by the short-range order which remains after the disturbance of the long-range order in the adsorbed layer. The retaining of the short-range order is evidenced by the presence of diffuse rings on the LEED patterns [4,8] and the results of Monte Carlo simulation of the order-disorder transition in adsorbed films [9,10]. In the presence of the short-range order, the nearest neighbours around each adatom are located essentially in the same way as with the long-range order. The influence of the far neighbours on the adatom depends only insignificantly on their particular positions on the substrate. It should also be taken into account that L98
A.G. Fedorus, A.G. Naumovets /Effect
of order--disorder
phase transitions
L99
the correlation function characterizing the adatom arrangement in the film with the short-range order is in general markedly affected by the surface potential relief. The above arguments are supported by the observations revealing that. the work function actually undergoes noticeable changes when the short-range order in the layer is changed with the temperature variation. This can be the case in the following situations: (1) If the adsorbed film consists of two different phases coexistent in equilibrium, the temperature increase results in a redistribution of the adsorbate between the phases (the amount of the adsorbate on the surface remains constant). Evidently this redistribution must be accompanied by some changes in the short-range order: The experiments with the systems Ba-Mo(O1 1) [6] and. Sr-W(O11) [7] have shown that in the regions of phase diagrams corresponding to two-dimensional condensation in the adfilms the temperature increase leads to the work function changes -0.1-0.2 eV. For these systems A@, is -3 eV. (2) If a film deposited on a cooled substrate is amorphous, its ordering in the course of annealing causes generally noticeable changes in the work function. It should be noted, however, that the alkaline and alkaline-earth adatoms have rather high mobility and the films of these elements on close-packed surfaces can not been obtained in the completely amorphous state even at liquid helium temperature [7]. For this reason the effect is most pronounced for low mobility adsorbates, such as scandium [ 111 or yttrium [ 121 on W(O1l). Annealing of these films is accompanied by a very strong change of the work function (-1 eV). It should be emphasized that in this case we are concerned with the transition of the film from a metastable state to an equilibrium ordered state (with corresponding changes in the short-range as well as long-range order) rather than with the reversible order-disorder phase transition. In our opinion, the results of these experiments indicate that a theory describing the dependence of the work function on the order-disorder phase transition in an adsorbed layer must take into account the short-range order in the adatom arrangement. The neglect of the short-range order in the theory [l] seems to be the main reason of its disagreement with the experimental data.
References [l] J.L. Morin-Lbpez and A. ten Bosch, Phys. Rev. B18 (1978) 2555. [2] A.G. Naumovets and A.G. Fedorus, Pis’ma v ZhETF 10 (1969) 11; Soviet Phys.-JETP Letters 10 (1969) 6. [3] V.K. Medvedev, A.G. Naumovets and A.G. Fedorus, Fiz. Tverd. Tela 12 (1970) 375; Soviet Phys.-Solid State 12 (1970) 301. (41 A.G. Fedorus and A.G. Naumovets, Surface Sci. 21 (1970) 426. [5] V.K. Medvedev, A.G. Naumovets and T.P. Smereka, Surface Sci. 34 (1973) 366. [6] A.G. Fedorus, A.G. Naumovets and Yu.S. Vedula, Phys. Status Solidi (a) 13 (1972) 445. [7] O.V. Kanash, A.G. Naumovets and A.G. Fedorus, Zh. Eksperim. i Teor. Fiz. 67 (1974) 1818; Soviet Phys.-JETP 40 (1975) 903.
LIOO [B] [9] [lo] [ 1 l]
A. G. Fedonrs, A.G. Naurnovers / Ejfect of order--disorder
phase transitions
R.L. Gerlach and T.N. Rhodin, Surfacc Sci. 17 (1969) 32. G. Doyen, G. Ertl and M. Plancher. J. Chem. Phys. 62 (1975) 2957. V.K. Medvcdev and I.N. Yakovkin, I%.. Tverd. Teln 19 (1977) 1587. V.B. Voronin and A.C. Naumovcts, Ukr. 1%. Zh. 13 (1968) 1389; Ukr. IXz. Zh. (English Transl.) 13 (1968) 991. [ 121 V.B. Voronin and A.G. Naurnovets, Izv. Akad. Nauk SSSR, Ser. Fiz. 35 (1971) 355.
93 (1980) L98 -LlOO Publishing Company
SURFACE SCIENCE LETTERS ON THE EFFECT OF ORDER-DISORDER PHASE TRANSITION ADSORBED FILMS ON WORK FUNCTION
IN
A.G. FEDORUS and A.G. NAUMOVETS Institute of Physics ofthe Academy 252650 Kiev 28, USSR
Received
17 August
of Sciences of the Ukrainian SSR, Prospekt Nauki, 144,
1979
Recently Mora’n-Mpez and Ten Bosch 111 have studied theoretically the influence of the order-disorder phase transition in adsorbed films on the work function of covered surfaces. They suggest their model the most suitable for the description of the films of alkaline elements adsorbed on noble and transition metals. The theory [ 11 predicts a strong effect of the order-disorder phase transition on the value of the work function change A& induced by an adsorbed film: A@, can be changed by several times and the dependence of A$, versus the coverage degree can acquire a qualitatively different shape. Here WC want to draw attention to the fact that these conclusions, at least for the films of alkaline and alkaline-earth elements on refractory metals, are completely inconsistent with experimental data. Ten years ago we established [2] that the change of the work function S@<,d accompanying the order-disorder phase transition in sodium films on tungsten (011) plane does not exceed 0.05 eV. whereas A@, amounts in this case to several eV. Later these measurements were repeated with a higher accuracy and have revealed that for the systems Na-W(O1 1) [3], Cs-W(O11) [4], and Li--W(112) [S] the Qo~ values are not larger than O.Ol& 0.02 eV. Similar results were obtained for the alkaline-earth films ~ Ba on W(Ol1) and Mo(O11) [6] and Sr on W(O11) [7]. Thus, there exists considerable evidence that disturbance of the long-range order in the adfilms of the mentioned type does not significantly change the work function. We explain this experimental fact assuming that the work function is determined mainly by the short-range order which remains after the disturbance of the long-range order in the adsorbed layer. The retaining of the short-range order is evidenced by the presence of diffuse rings on the LEED patterns [4,8] and the results of Monte Carlo simulation of the order-disorder transition in adsorbed films [9,10]. In the presence of the short-range order, the nearest neighbours around each adatom are located essentially in the same way as with the long-range order. The influence of the far neighbours on the adatom depends only insignificantly on their particular positions on the substrate. It should also be taken into account that L98
A.G. Fedorus, A.G. Naumovets /Effect
of order--disorder
phase transitions
L99
the correlation function characterizing the adatom arrangement in the film with the short-range order is in general markedly affected by the surface potential relief. The above arguments are supported by the observations revealing that. the work function actually undergoes noticeable changes when the short-range order in the layer is changed with the temperature variation. This can be the case in the following situations: (1) If the adsorbed film consists of two different phases coexistent in equilibrium, the temperature increase results in a redistribution of the adsorbate between the phases (the amount of the adsorbate on the surface remains constant). Evidently this redistribution must be accompanied by some changes in the short-range order: The experiments with the systems Ba-Mo(O1 1) [6] and. Sr-W(O11) [7] have shown that in the regions of phase diagrams corresponding to two-dimensional condensation in the adfilms the temperature increase leads to the work function changes -0.1-0.2 eV. For these systems A@, is -3 eV. (2) If a film deposited on a cooled substrate is amorphous, its ordering in the course of annealing causes generally noticeable changes in the work function. It should be noted, however, that the alkaline and alkaline-earth adatoms have rather high mobility and the films of these elements on close-packed surfaces can not been obtained in the completely amorphous state even at liquid helium temperature [7]. For this reason the effect is most pronounced for low mobility adsorbates, such as scandium [ 111 or yttrium [ 121 on W(O1l). Annealing of these films is accompanied by a very strong change of the work function (-1 eV). It should be emphasized that in this case we are concerned with the transition of the film from a metastable state to an equilibrium ordered state (with corresponding changes in the short-range as well as long-range order) rather than with the reversible order-disorder phase transition. In our opinion, the results of these experiments indicate that a theory describing the dependence of the work function on the order-disorder phase transition in an adsorbed layer must take into account the short-range order in the adatom arrangement. The neglect of the short-range order in the theory [l] seems to be the main reason of its disagreement with the experimental data.
References [l] J.L. Morin-Lbpez and A. ten Bosch, Phys. Rev. B18 (1978) 2555. [2] A.G. Naumovets and A.G. Fedorus, Pis’ma v ZhETF 10 (1969) 11; Soviet Phys.-JETP Letters 10 (1969) 6. [3] V.K. Medvedev, A.G. Naumovets and A.G. Fedorus, Fiz. Tverd. Tela 12 (1970) 375; Soviet Phys.-Solid State 12 (1970) 301. (41 A.G. Fedorus and A.G. Naumovets, Surface Sci. 21 (1970) 426. [5] V.K. Medvedev, A.G. Naumovets and T.P. Smereka, Surface Sci. 34 (1973) 366. [6] A.G. Fedorus, A.G. Naumovets and Yu.S. Vedula, Phys. Status Solidi (a) 13 (1972) 445. [7] O.V. Kanash, A.G. Naumovets and A.G. Fedorus, Zh. Eksperim. i Teor. Fiz. 67 (1974) 1818; Soviet Phys.-JETP 40 (1975) 903.
LIOO [B] [9] [lo] [ 1 l]
A. G. Fedonrs, A.G. Naurnovers / Ejfect of order--disorder
phase transitions
R.L. Gerlach and T.N. Rhodin, Surfacc Sci. 17 (1969) 32. G. Doyen, G. Ertl and M. Plancher. J. Chem. Phys. 62 (1975) 2957. V.K. Medvcdev and I.N. Yakovkin, I%.. Tverd. Teln 19 (1977) 1587. V.B. Voronin and A.C. Naumovcts, Ukr. 1%. Zh. 13 (1968) 1389; Ukr. IXz. Zh. (English Transl.) 13 (1968) 991. [ 121 V.B. Voronin and A.G. Naurnovets, Izv. Akad. Nauk SSSR, Ser. Fiz. 35 (1971) 355.