Lithium adsorption on MgO(100) and its defects: Charge transfer, structure, and energetics

The adsorption energetics and growth of lithium vapor on MgO(100) at 300 K was studied using microcalorimetry, in combination with low-energy electron diffraction (LEED), low-energy ion scattering (ISS), Auger electron spectroscopy (AES), and work-function measurements. The MgO(100) samples were films of $\sim 4\text{nm}$ thickness grown on a Mo(100) single crystal. The initial sticking probability of lithium was $\sim 0.97$, reaching unity by 0.5 monolayer (ML). The AES and ISS signals vary with Li coverage up to 3 ML as expected if the Li atoms stay within the layer where they initially hit (i.e., with no interlayer transport). Initially, lithium adsorbs strongly at the intrinsic surface defects and as two-dimensional (2D) lithium clusters, with a heat of adsorption of 260 kJ/mol. The heat approaches the heat of sublimation of bulk Li (159 kJ/mol) by 0.4 ML, due to the growth of 2D and then three-dimensional (3D) Li islands. Argon-ion sputtering of the surface increases the defect density and the probability for adsorbing Li to find a defect, and thus the heat of adsorption at low coverages. When defects only are being populated, Li exhibits a heat of adsorption of 410 kJ/mol. Comparing heats with recent density functional theory (DFT) calculations suggests that the defect sites are under-coordinated O atoms at steps or kinks, or related structures at dislocations. The work function decreases by $\sim 1.8\text{eV}$ within the first 0.5 ML and then increases to near the value of bulk Li(solid), $\sim 2.6\text{eV}$, by 3 ML. These results support recent DFT calculations predicting stronger electron transfer from Li to the MgO when at steps and kinks than at terraces, and decreasing charge transfer as 2D Li clusters grow. The work function starts to increase when the growth mode becomes dominated by growth of 3D Li(solid). In spite of a large amount of electron transfer from Li to MgO, Li adatoms have attractive interactions that lead to 2D clustering. For 1-nm-thick MgO films, the heat of adsorption was higher by 60–20 kJ/mol than for 4 nm films in the entire range from 0 to 0.7 ML, where adsorption in the first layer dominates.

Figure 1

The sticking probability of Li onto MgO(100) at room temperature as a function of the total amount of Li dosed.

Figure 2

The integrated Li (○) and Mg (●) AES intensities normalized to bulk Li and the clean MgO(100) surface, respectively, versus coverage for the room temperature growth of Li on a thin MgO(100) film grown on Mo(100). The solid curves correspond to what is expected if the Li film grows in a layer-by-layer manner. Dashed curves correspond to what is expected in the “No down-step/up-step model,” wherein diffusion between layers is kinetically forbidden (see text). The inelastic mean-free path used for calculating the curves was 0.43 nm for Mg and 0.47 nm for Li, obtained from Ref. 67, and the detection angle was $45°$. The definition of one ML $\left(1.12\times {10}^{15}\text{atoms}/{\text{cm}}^2\right)$ and the bulk density of Li(solid) give the ML thickness of 0.25 nm used here.

Figure 3

Evolution of the normalized Mg (○) ISS signal as a function of Li coverage on a thin MgO(100) film grown on Mo(100) at room temperature. The solid curve shows the ISS signal for a layer-by-layer growth. The dashed curve shows a fit to the data using the no down-step/up-step model, whereby Li atoms stick in the layer in which they hit, with no diffusion between layers.

Figure 4

The change in the work function versus Li coverage on MgO(100). The line is a guide to the eye. The inset shows the low coverage region.

Figure 5

The heat of adsorption versus Li coverage at room temperature on (●) a 4-nm-thick MgO(100) film and on (○) a 1–nm-thick MgO(100) film. Each data point is due to a pulse of 0.006–0.01 ML of Li (pulsed at 1/2 Hertz), and is the average of four experimental runs. 1 ML is defined as $1.12\times {10}^{15}\text{atoms}/{\text{cm}}^2$, which is the MgO(100) unit-cell density.

Figure 6

The heat of adsorption versus Li coverage at room temperature; on (●) pristine 4-nm-thick MgO(100), (○) a film irradiated with a small ${\text{Ar}}^{+}$ ion dose, and $(\oplus )$ a film irradiated with a larger ${\text{Ar}}^{+}$ ion dose.