Study of adsorption of titanium on tungsten and rhenium by field electron emission

Abstract

The deposition and layer growth of titanium, evaporated on to tungsten and rhenium field emitter tips has been studied, and field emission average work-functions measured at varying titanium layer thicknesses from less than a monolayer to several monolayers. Titanium evaporated on to a clean tungsten tip at 293 K in the submonolayer region tended to collect preferentially on atomically rough planes. With further deposition, a “pseudo-clean” emission pattern was obtained at about a monolayer coverage when the titanium atoms were concluded to be in epitaxial registry with the BCC substrate; at about this coverage the work-function for the system passed through a minimum. Further titanium deposition in excess of three monolayers resulted in a progressive roughening of the surface due to microcrystallinity: brief heating (~ 1 min, 1300 K) of such a surface resulted in a well ordered layer identified as ß-titanium (BCC) presumably epitaxed to the tungsten. This epitaxed layer remained indefinitely stable at 293 K. In the deposition of titanium on to a clean rhenium emitter tip at 293 K, titanium again appeared to collect on atomically rough planes in the submonolayer region. Further deposition in excess of two monolayers resulted in the formation of a well ordered layer identified as α-titanium (HCP) probably epitaxed to the rhenium. The field emission average work-functions of α-titanium and ß-titanium were determined as 4.00 ± 0.05 and 3.65 ± 0.05 eV respectively.