5.3 Compatibility Issues Between Molecules and TILs
6.1.2 Structure-coverage relationship
Figure 6.1 shows the various adsorption sites on a fcc(111) surface. In this figure and throughout the paper, adatoms in fcc, hcp, and bridge sites are coloured blue, pink, and green, respectively. As Cl atoms are added to the Cu(111) surface they form chains, where the adjacent atoms alternate between adsorption in fcc and hcp sites, or small (√3×√3) R30° islands with the Cl atoms in fcc sites.460 As
the coverage increases, a porous structure is formed, with voids enclosed by the aforementioned Cl chains or (√3×√3) R30° ribbons. The voids close as more Cl atoms are added to the surface, until the coverage reaches 1/3 ML, where a uniform (√3×√3)R30°adlayer is formed.456
Above 1/3 ML, Cl atoms form crowdion interstitials where some of the Cl atoms surrounding the interstitial are displaced from fcc sites to hcp or bridge sites.470 These crowdion interstitials condense into domain walls as the number
density increases.459A section of a domain wall is shown in Fig. 6.1, where the Cl
atoms transition from a(√3×√3)R30°domain in which the Cl atoms occupy fcc sites, to one in which they occupy hcp sites, by passing through a line of atoms in bridge sites. The size of these domains shrinks with increasing Cl coverage until, at 0.4 ML, the fcc and hcp domains are each one atomic row wide.470 Between
0.4 ML and 5/12 ML, the mesh is further compressed such that some of the Cl atomic rows are in sites intermediate between bridge and hollow. Above 5/12 ML, further uniaxial compression of the surface mesh is unfavourable because it
would result in Cl-Cl nearest-neighbour distances smaller than the Van der Waals diameter of chlorine.471
Figures 6.2(a) and 6.2(b) show the (19×√3) structure, corresponding to a coverage of 7/19 ML. Here domains, alternating between two and three atomic rows wide, are separated by rows of Cl atoms adsorbed in bridge sites. The atoms in bridge sites can be distinguished from those in hollow sites by analysing the topography. Since the inter-row distance is shorter between a row of Cl atoms in hollow sites and a row of Cl atoms in bridge sites than between two rows of atoms in hollow sites, the contrast between the atoms is reduced. Therefore, where there is greater contrast between two atomic rows, indicated by the cyan arrow in Fig. 6.2(a), atoms in these rows can be assigned to hollow sites. Where there is reduced contrast between an atomic row and the row on either side of it, indicated by yellow arrows, the atoms in this row can be assigned to bridge sites. Alternatively, the bridge sites can be identified by noticing that atomic rows in adjacent domains do not align exactly. The atomic row (along the uncompressed direction) where there is an offset between adjacent rows in both of the other two (compressed) directions, as indicated by the black lines in Fig. 6.2(b), can be identified as the bridge site.
Due to the symmetry of the structure, the fcc domains cannot be differen- tiated from the hcp domains by analysis of the topography alone. To identify the adsorption site, this adlayer was modified by locally applying a large current and desorbing some of the Cl atoms to form large domains [see left side of Fig. 6.2(c)]. Since it was previously established that Cl atoms occupy fcc sites in the (√3×√3)R30°adlayer,463 due to the slightly larger adsorption energy relative
to the hcp sites,472 the large domain is assumed to consist of Cl atoms occupying
fcc sites.
The relationship between Cl coverage and structure in the uniaxial compres- sion regime is described in Fig. 6.3. At 1/3 ML the chlorine atoms occupy the Cu(111) fcc sites in the isotropic(√3×√3) R30°structure,456,458which is marked
by the lower dashed line in Fig. 6.3(a) and shown schematically in Fig. 6.3(b). As the coverage increases crowdion interstitials appear on the surface, which then condense into domain walls resulting in a non-uniform uniaxial compression of the (√3×√3) R30° structure.470 The domains get progressively smaller as the
coverage increases. The (8×√3) structure shown in Fig. 6.2(a) is indicated in Fig. 6.3(a), and shown schematically in Fig. 6.3(c). The (19 ×√3) struc- ture, shown schematically in Fig. 6.3(d), was also observed after electrospraying chloroform on clean Cu(111). The (5×√3) structure, indicated in Fig. 6.3(a),
(c)
fcc hcp bridge
(a) (b)
Figure 6.2: Topographic images of uniaxially compressed Cl adlayers on Cu(111). The compression direction is indicated with grey arrows. Part (a) shows atomic resolution of the Cl adlayer, imaged at 1.0 V, 50 pA. The striped pattern allows us to differentiate between Cl atoms adsorbed in hollow sites and those in bridge sites. Part (b) shows the enlarged area indicated by the red square in (a). Here, atoms in bridge sites can be distinguished by following the atomic rows along the compressed axes. The blue, pink, and green circles represent Cl atoms occupying fcc, hcp, and bridge sites, respectively. These were assigned by analysing the image in panel (c), where high current was applied to desorb some of the Cl atoms, forming a large (√3× √3)R30° domain. Since in the uncompressed (√3×√3)R30° structure, the Cl atoms all occupy fcc sites, we assign the Cl atoms in the largest domain to fcc sites.
corresponding to alternating fcc and hcp domains one atomic row in width, has been previously reported for Cl on Ag(111).470
Between 1/3 and 2/5 ML, where Cl atoms occupy fcc, hcp, and bridge sites only, the relationship between coverage in monolayers Θ, the number of chlorine atoms per unit cell NA, and the length of the unit cell L are given by:
Θ = NA
2L, (6.1)
where NA is related to the number of atomic rows in the fcc domain Rfcc and
the number of atomic rows in the hcp domain Rhcp according to the following
equation:
NA= (b+ 1)(Rfcc+Rhcp+ 2), (6.2)
and the unit-cell length is:
L= b+ 1
(a) (5×√3) (8×√3) - (c) (19×√3)- (d) 0 10 20 30 40 50 0.33 0.36 0.39 0.42 Θ (ML) Rfcc+Rhcp (b) (b) (c) (d)
Figure 6.3: Coverage vs structure relationship in the uniaxial compression regime. Rfcc and Rhcp represent the number of atomic rows in the fcc and hcp domains,
respectively. The orange, light brown, and dark brown circles represent the cop- per atoms in the first, second, and third layer from the surface, respectively. Blue, pink, and green circles represent Cl atoms occupying fcc, hcp, and bridge sites, respectively. Part (a) shows the coverage decaying exponentially to 1/3 ML (indicated by the lower dashed line), corresponding to the(√3×√3)R30°struc- ture [presented in (b)], as the domain widths increase. The upper dashed line in (a) corresponds to the maximum coverage structure in the uniaxial compression regime. Part (c) shows the (8×√3) structure where domains, two atomic rows wide, of Cl atoms occupying fcc sites are separated from hcp domains by a row of Cl atoms in bridge sites. Part (d) shows a model of the (19×√3) structure, shown in Fig. 6.2(a), where the fcc and hcp domains are both three and two atomic rows wide, respectively.
where b= (Rfcc+Rhcp) mod 2.
At coverages between 0.4 ML and 5/12 ML, single row fcc and hcp domains are separated by domain walls several rows wide, where the chlorine atoms occupy adsorption sites that are less symmetric than bridge sites. The most compressed structure possible under the uniaxial compression regime is the(12×√3)struc- ture,459,471corresponding to a coverage of 5/12 ML [indicated by the upper dashed
line in Fig. 6.3(a)].
6.1.3 Electronic properties of uniaxially compressed chlorine adlayers