A computer simulation study of silver-gold cluster formation on AgBr tabular microcrystals with AgIBr cores

Computer simulation is used to study how an iodobromide core in an AgBr tabular microcrystal affects the efficiency of photoinduced metal cluster formation. Iodide ions can trap photogenerated holes and act as recombination centers. But interstitial silver ion formation from lattice silver ions adjacent to the trapped holes can reduce the charge on the site, reducing the recombination cross section and increasing the overall efficiency of metal cluster formation. This process is modeled by varying the recombination radius of the iodide-trapped hole over an 8-fold range. Several arrangements of electron traps provided by a chemical treatment of the microcrystal surface are studied. The simplest arrangement is that of a uniform placement. In this case, the iodobromide core can improve efficiency but only when the recombination radius is smaller than that due to intrinsic hole traps. When the electron traps are positioned at the corners of the microcrystal and given a large trapping radius and trap depth, the iodobromide core is unable to improve the efficiency, even when its recombination radius is 25% of that for an intrinsically trapped hole. Locating some of the electron traps at the thin edges of the tabular microcrystal benefited from the iodobromide core only when the traps had the same trapping radius as those on the face of the microcrystal. Similar results were found when the electron traps were confined to a small region of the edge. Comparison with limited experimental data suggests that the interstitial-silver-ion formation process at the site of an iodide-trapped hole appears to be competitive with recombination at that site. This situation leads to an efficiency improvement in those cases where there is excessive recombination, even after optimum chemical treatment of the microcrystal surface.