Lecture 3 More on Adsorption and Thin Films Monolayer adsorption Several adsorption sites

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:


The flux of gas molecules to a surface (molecules/cm2-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 1022P/(MT)1/2


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/Nmax = Θ << 1, we have S ~ S0

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 = S0(1-Θ) ~ 0, so eventually, the surface coverage does not increase.

Single Adsorption Site: Monolayer adsorption:


This is certainly the case for O2 on Fe(110) at 90 K (Smentkowski and Yates)


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 1st 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


This is the case H2 desorption following Si2H6 adsorption on Si(100)(2x1) for adsorption at exposures of up to 2 L at 300 K.

A single H2 peak is observed at ~ 800 K. Why is this first order?

Adapted from Nix and Wu, Surf. Sci. 306 (1994) 59


Lecture 3: More on adsorption 8

Substrate : fcc(100) surface with unit cell lattice vectors a1, a2

Adsorbate overlayer( 1 type of adsorption site)

b1 = 2a1

b2 = 2a2

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 E1

XSite 2, 4-fold hallow site, desorption energy E2

Now, assume that E1 > E2 Therefore , the desorption temperature for E1

(TD1) is higher than for E2L TD1>> TD29Lecture 3: More on adsorption

X

XX

X X

X

X

X

Which site gets filled first?

Suppose we “dose” the surface (expose it to) H2 at 300 K, then do TPD. Will both sites get filled at the same time?

No!

Atoms (including dissociated H2) are generally mobile on surfaces

The sites with highest EB (highest TD) get filled first…


H H

HHHH HHH

H

First, the sites with the highest EB are filled….

Then, the sites with the next highest EB are filled…11Lecture 3: More on adsorption

HHH

H2

At low exposures, only Type 1 sites occupied, desorption spectrum will have only one peak

(2nd order reaction shown)

PH2

T

At increasing doses, 2nd order peak moves to lower desorption temperatures, finally, all the type 1 sites are saturated…


HHHHH

HHH

H2

T2

H2

T1


Desorption of H2 from W(100), Madey and Yates, Surf. Sci. 49 (1975) 465


This picture holds even for more complex adsorbates

e.g., Si2H6 on Si(100)(2x1)


Wu and Nix, Surf. Sci. 306 (1994) 59

Consider H2 desorption from Si2H6/Si(100)(2x1)…

1st or 2nd order desorption?


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.



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 1st, 2nd, 3rd monolayer, etc.

Examples:

Deposition of a metal film on an unreactive substrate.

One example is the MBE of C(111) (graphene) on Co3O4(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 = FtS0A (where A = sample area). 19Lecture 3: More on adsorption


Lecture 3: More on adsorption 21

Summary:

Mulitple absorption sites Multiple Peaks

Sites with highest EB fill up first

Sites with smallest EB 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.

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Lecture 3 More on Adsorption and Thin Films Monolayer adsorption Several adsorption sites