1. Lecture 3 More on Adsorption and Thin Films 1.Monolayer adsorption 2.Several adsorption sites 3.Thin Films (S ~ constant, multilayer adsorption) 1Lecture 3: More on adsorption

2. Consider a surface in equilibrium with a gas for a given length of time. We will assume that the rate of desorption at the adsorption temperature is negligibe: 2Lecture 3: More on adsorption

3. The flux of gas molecules to a surface (molecules/cm 2 -sec is given by: F = P[2πkTM] -1/2 So, the flux is proportional to the pressure. Putting P in Torr, M in grams, and T in Kelvin we have (see Somorjai) F = 3.51 x 10 22 P/(MT) 1/2 3Lecture 3: More on adsorption

4. The number of molecules or atoms adsorbed on a surface, is then given as N = FtS Where t = time, and S = sticking coefficient, which typically varies with coverage: S = S(N) However, when N/N max = Θ << 1, we have S ~ S 0 Therefore, we can expect that adsorbate coverage will vary linearly with exposure in the early stages. However, for Θ~ 1 (larger exposures) we then have S = S 0 (1-Θ) ~ 0, so eventually, the surface coverage does not increase. Single Adsorption Site: Monolayer adsorption: 4Lecture 3: More on adsorption

5. This is certainly the case for O 2 on Fe(110) at 90 K (Smentkowski and Yates) 5Lecture 3: More on adsorption

6. If there is only one type of adsorption binding site, with one characteristic adsorbate binding energy, then the desorption spectra will look fairly straightforward: If desorption is 1 st order, then the desorption spectrum is fairly simple: The peak is always at the same temperature, and the intensity is proportional to the number of adsorbed molecules: Increasing initial exposure P TT 6Lecture 3: More on adsorption

7. This is the case H 2 desorption following Si 2 H 6 adsorption on Si(100)(2x1) for adsorption at exposures of up to 2 L at 300 K. A single H 2 peak is observed at ~ 800 K. Why is this first order? Adapted from Nix and Wu, Surf. Sci. 306 (1994) 59 7Lecture 3: More on adsorption

8. 8 Substrate : fcc(100) surface with unit cell lattice vectors a 1, a 2 Adsorbate overlayer( 1 type of adsorption site) b 1 = 2a 1 b 2 = 2a 2

9. Top view, simple Cubic lattice Let’s assume that a surface as two adsorption sites for atomic H Site 1, a-top site, desorption energy E 1 X Site 2, 4-fold hallow site, desorption energy E 2 Now, assume that E 1 > E 2 Therefore, the desorption temperature for E 1 (TD 1 ) is higher than for E 2 L TD 1 >> TD 2 9Lecture 3: More on adsorption X XX XX X X X

10. Which site gets filled first? Suppose we “dose” the surface (expose it to) H 2 at 300 K, then do TPD. Will both sites get filled at the same time? No! Atoms (including dissociated H 2 ) are generally mobile on surfaces The sites with highest E B (highest TD) get filled first… 10Lecture 3: More on adsorption

11. H H HH H H H H H H First, the sites with the highest E B are filled…. Then, the sites with the next highest E B are filled… 11Lecture 3: More on adsorption

12. H H H H2H2 At low exposures, only Type 1 sites occupied, desorption spectrum will have only one peak (2 nd order reaction shown) P H2 T  At increasing doses, 2 nd order peak moves to lower desorption temperatures, finally, all the type 1 sites are saturated… 12Lecture 3: More on adsorption

13. H HH H H HH H H2H2 T2T2 H2H2 T1T1 13Lecture 3: More on adsorption

14. Desorption of H 2 from W(100), Madey and Yates, Surf. Sci. 49 (1975) 465 14Lecture 3: More on adsorption

15. This picture holds even for more complex adsorbates e.g., Si 2 H 6 on Si(100)(2x1) 15Lecture 3: More on adsorption

16. Wu and Nix, Surf. Sci. 306 (1994) 59 Consider H 2 desorption from Si 2 H 6 / Si(100)(2x1)… 1 st or 2 nd order desorption? 16Lecture 3: More on adsorption

17. If there are several occupied adsorption sites, you can desorbe from the lower energy site at the lower desorption temperature, without desorbing from the higher temperature. 17Lecture 3: More on adsorption

18. 18Lecture 3: More on adsorption

19. Multilayer adsorption: In this case, the sticking coefficient does not go to zero at full monolayer coverage. Instead, the sticking coefficient remains roughly constant with formation of 1 st, 2 nd, 3 rd monolayer, etc. Examples: Deposition of a metal film on an unreactive substrate. One example is the MBE of C(111) (graphene) on Co 3 O 4 (111) (M. Zhou, et al., JPCM 24 (2012) 072201) In this case, the total number of atoms on the surface is simply given by N = FtS 0 A (where A = sample area). 19Lecture 3: More on adsorption

20. 20Lecture 3: More on adsorption

21. 21 Summary:  Mulitple absorption sites  Multiple Peaks  Sites with highest E B fill up first  Sites with smallest E B desorb at the lowest temperature  Desorption from different sites can obey different desorption kinetics  Deposition of, e.g., thin films does not obey monolayer adsorption, sticking coefficient does not go to zero.