Epitaxial growth of ultrathin, crystalline Au/Ag and Al/Ag metal heterostructures on silicon for plasmonic applications

The plasmonic response of ultrathin (less than 5 nm), nanostructured Ag could be powerful means of confining and manipulating light, but such films are unstable in ambient conditions without a passivating layer, which may also impact the overall electronic and optical properties. Here, we demonstrate the epitaxial growth of Au/Ag and Al/Ag heterostructures on (7x7)-Si(111) exhibiting well-defined mid-infrared plasmons after exposure to air and nanopatterning by electron beam lithography. Angle-resolved photoemission spectroscopy, X-ray photoemission spectroscopy, scanning tunneling microscopy, and low-energy electron diffraction are used to monitor the heterostructure growth. The high quality of the metal films is further monitored by resolving the electronic band structure and quantum well states via photoemission as a function of Ag thickness on the Si substrate and subsequent coverage of the Ag by Au or Al. The results suggest that a wide variety of ultrathin, crystalline superlattices could be obtained for electronic, spintronic, or photonics applications by using Ag films on Si(111) as a platform.