Reaction kinetics of ultrathin NaCl films on Ag(001) upon electron irradiation

We report on an electron-induced modification of alkali halides in the ultrathin film regime. The reaction kinetics and products of the modifications are investigated in the case of NaCl films grown on Ag(001). Their structural and chemical modification upon irradiation with electrons of energy 52–60 eV and 3 keV is studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), respectively. The irradiation effects on the film geometry and thickness (ranging from between two and five atomic layers) are examined using scanning tunneling microscopy (STM). We observe that Cl depletion follows different reaction kinetics, as compared to previous studies on NaCl thick films and bulk crystals. Na atoms produced from NaCl dissociation diffuse to bare areas of the Ag(001) surface, where they form Na-Ag superstructures that are known for the Na/Ag(001) system. The modification of the film is shown to proceed through two processes, which are interpreted as a fast disordering of the film with removal of NaCl from the island edges and a slow decrease of the structural order in the NaCl with formation of holes due to Cl depletion. The kinetics of the Na-Ag superstructure growth is explained by the limited diffusion on the irradiated surface, due to aggregation of disordered NaCl molecules at the substrate step edges.

Figure 1

STM topography of the NaCl films before electron irradiation. (a) $680\times 680{\mathrm{nm}}^2$ image and (b) $272\times 272{\mathrm{nm}}^2$ image ($U_b=4$ V, $I_t=0.7$ nA) of an ultrathin NaCl film grown on Ag(001) at 413 K (10 min deposition). (c) $435\times 435{\mathrm{nm}}^2$ image ($U_b=1$ V, $I_t=0.8$ nA) and (d) $435\times 435{\mathrm{nm}}^2$ image ($U_b=4$ V, $I_t=0.12$ nA) of two ultrathin NaCl films grown at 500 K, differing in nominal thickness by a factor of 4 to 5 (according to AES measurements), the latter is the thickest. All STM images are recorded at low temperature (78 K). The NaCl film thickness in atomic monolayers (ML), as determined from the STM height variations, and the orientation of the $\left[1\overline{1}0\right]$ and $\left[110\right]$ axes of Ag(001) is indicated in the images.

Figure 2

NaCl dissociation dynamics upon irradiation with the 3-keV electron beam of an AES. (a) Temporal evolution of the intensity ratio between the Cl and Ag AES peaks taken at Auger electron energies of 181 and 358 eV, respectively, vs irradiation time. The sample current is $0.5\pm 0.2\mu \mathrm{A}$ with a beam spot area of about 1 ${\mathrm{mm}}^2$. The experimental data are fitted using a monoexponential decay model. (b) Statistics from the model parameters yield a mean decay rate coefficient $〈\gamma 〉=0.011\pm 0.001{\mathrm{s}}^{-1}$ with no significant dependence of $\gamma$ on the decay amplitude $\beta$. The data shown in this figure were obtained for NaCl films grown on Ag(001) at 500 K.

Figure 3

Background-subtracted LEED images, obtained from ultrathin NaCl films grown on Ag(001) at 500 K. (a) NaCl on Ag(001) before dissociation. (b) Appearance of a $p(2\times 1)$ structure in the early steps of NaCl dissociation, attributed to missing-row reconstruction of the substrate surface. (c) Longer exposure to low-energy electrons [images (b) and (c) are separated in time by 218 s] yields almost complete disappearance of the NaCl spots and appearance of an incommensurate structure (weak elongated spots), attributed to 1D chains of Na atoms adsorbed on the reconstructed substrate. (d) Starting from a thicker NaCl film yields a $p(3\times 3)$ structure, which is associated to the formation of an ordered Ag-Na surface alloy. All LEED images measured at electron energy 52 eV, except (d) recorded at 49 eV.

Figure 4

LEED spot intensity vs time, measured on ultrathin NaCl films grown on Ag(001) at 500 K. [(a),(c),(d)] Temporal variations of the diffraction spots upon irradiation with the electron beam of the LEED. The intensity of the $(\overline{1},0)$ spots is monitored for the Ag(001) surface and its $p(2\times 1)$ missing-row reconstruction and for the (001)-terminated NaCl film, whereas intensity is averaged over several spots for the linear incommensurate chains of Na atoms. In (a), the time axis features a 600 s break during which the electron beam is blocked, before irradiation restarts at $t\approx 1180$ s. (b) Background-subtracted LEED image recorded at electron energy 60 eV during the same experiment as in (a) at $t\approx 1250$ s and revealing a $p(4\times 2)$ superstructure. In (c), the temporal variation of the NaCl spot intensity [same data as in (a)] is fitted with a biexponential decay function with optimized short (${\tau }_1$) and long (${\tau }_2$) time constants. In order to improve visibility, some of the data are multiplied by 2 or 4 [when indicated in (a) and (d)]. As a relative thickness indication, AES measurements give Cl/$\mathrm{Ag}(t=0$) at $0.5$ for the NaCl film considered in (a) to (c) and $0.12$ for that used in (d). LEED measurements and irradiation were conducted at electron energy of 60 eV for the thicker film and 53 eV for the thinner one.

Figure 5

LEED spot intensity vs time, measured on ultrathin NaCl films grown on Ag(001) at 413 K. Temporal variations of the diffraction spots upon irradiation with the electron beam of the LEED. The intensity of the $(\overline{1},0)$ spots is monitored for the Ag(001) surface and its $p(2\times 1)$ missing-row reconstruction and for the (001)-terminated NaCl film, whereas intensity is averaged over several spots for the $p(3\times 3)$ Na-Ag surface alloy. In (b), the NaCl film is thicker than in (a), since the deposition time is 30 min, as compared to 10 min at the same cell temperature in (a).

Figure 6

STM topography of NaCl films after electron irradiation. [(a) and (b)] $680\times 680{\mathrm{nm}}^2$ images ($U_b=4$ V, $I_t=0.7$ nA) of an ultrathin NaCl film grown on Ag(001) at 413 K (10 min deposition) after 600 s electron irradiation using the electron beam of the LEED (electron energy 52 eV) at sample current of $0.40$ mA. All STM images are recorded at low temperature (78 K). The orientation of the $\left[1\overline{1}0\right]$ and $\left[110\right]$ axes of Ag(001) is indicated in the images. (I) NaCl island contour; (II) substrate step edge; and (III) branch-shaped holes.

Figure 7

STM topography of NaCl films after irradiation at different electron doses. [(a)–(c)] $680\times 680{\mathrm{nm}}^2$ images ($U_b=4$ V, $I_t=0.7$ nA) of an ultrathin NaCl film grown on Ag(001) at 413 K (30 min deposition) after 60 s electron irradiation using the electron beam of the LEED (electron energy 52 eV) at sample current of $0.40$ mA. The three images are obtained on different areas of the sample, at the center and the periphery of the irradiated zone, which have thus received different electron doses (the electron beam is about 1 mm in diameter at the sample and the top side of the sample is 6 mm in diameter). The effective electron dose increases from (a) to (c) in this figure. (d) $197\times 197{\mathrm{nm}}^2$ image ($U_b=1$ V, $I_t=1$ nA) of the same irradiated NaCl film, revealing the geometry of the disordered areas. All STM images are recorded at low temperature (78 K). The orientation of the $\left[1\overline{1}0\right]$ and $\left[110\right]$ axes of Ag(001) is indicated in the image on the left. (I) dots at the silver step edges and (II) serrated island edges.

Figure 8

Differential conductance (dI/dV) spectra of ultrathin NaCl films grown on Ag(001) at 413 K (a) before and [(b)–(d)] after electron irradiation. (a) dI/dV spectra obtained on the as-grown NaCl domains shown in Fig. 1, whose thickness ranges from two to five NaCl atomic layers (monolayer, ML), and on the bare Ag(001) surface. [(b) and (c)] dI/dV spectra measured on the irradiated NaCl film shown in Fig. 6. (d) dI/dV spectra measured on the same NaCl film as shown in Fig. 7. All the dI/dV spectra are measured in opened-loop conditions (the initial STM parameters are $U_b=4$ V and $I_t=0.7$ nA) by varying the bias voltage from 0 to 4 V by steps of 100 mV. The dI/dV spectra are measured at the position indicated by the apex of the arrows in the STM images ($U_b=4$ V, $I_t=0.7$ nA) shown in inset. All STM images are recorded at low temperature (78 K).