Field emission properties of single crystalline W5O14 and W18O49 nanowires

Single crystalline tungsten oxides in a form of W5O14 and W18O49 nanowires were synthesized by iodine transport method. The morphology, work functions and field emission properties of these nanowires were investigated. Work functions of the W5O14 (4.20-4.34 eV) and W18O49 (4.55-4.57 eV) nanowires (NWs) have been measured by Kelvin probe force microscopy (KPFM) in ultra-high vacuum. Field emission (FE) measurements of individual nanowires were performed in ultra-high vacuum at microscopic and macroscopic distances between the emitter and electron collector. The obtained FE curves at microscopic distances were analyzed in the framework of the Fowler-Nordheim (F-N) theory. Field enhancement factors of W5O14 at the emitter-collector distance of 2, 4 and 5 mu m were calculated to be 110 +/- 10, 180 +/- 25 and 210 +/- 30, respectively, and 125 +/- 15 for W18O49 at 2 mu m. At macroscopic distances, the F-N theory revealed unrealistic high field enhancement factors: for W5O14 at 1mm it was 17,000 +/- 500, and for W18O49, the field enhancement factors were 5050 +/- 30 and 6450 +/- 30 at 600 mu m and 800 mu m emitter-collector distance, respectively. Therefore, more realistic model was discussed. The lower work function and typically smaller diameter of the W5O14 nanowires in comparison with the W18O49 wires, range the W5O14 nanowires to the promising sources of electrons in field emission devices.