Gold substrates were prepared by thermal evaporation of approx. 1 nm Cr and 45 nm Au (99.999 pure, Alfa Aesar) onto BK7 glass microscope slides. Silicon wafers (Silicon Quest) and glass slides (VWR) were cut into 2x3 cm rectangles. The substrates were sonicated 5 minutes in acetone, rinsed with acetone, sonicated 5 minutes in isopropyl alcohol, rinsed with isopropyl alcohol, sonicated 1 minute in water, and rinsed with water. The substrates were dried in a stream of nitrogen gas and baked in a 150 • C oven for 1 hour. The substrates were then treated for 15 minutes in a UV-ozone cleaner.