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

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

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 P/(MT) 1/2 3Lecture 3: More on adsorption

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

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

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

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 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

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

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

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

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

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

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

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

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

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

18Lecture 3: More on adsorption

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) ) 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

20Lecture 3: More on adsorption

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.