Electron-bombarded ⟨110⟩-oriented tungsten tips for stable tunneling electron emission.

Area of Science:

• Materials Science
• Surface Science
• Physics

Background:

• Stable electron emission requires a clean tungsten (W) tip apex with a robust atomic plane.
• Aqueous chemical etching creates impurity layers on W tips, necessitating ultra-high vacuum heating.
• Overheating during treatment can melt the tip apex, leading to unstable electron emissions.

Purpose of the Study:

• To quantitatively investigate W tip apex structures.
• To develop a method for preparing W tips with stable tunneling electron emissions.
• To determine the optimal electron-bombardment heating power for tip preparation.

Main Methods:

• Field emission current-voltage (I-V) curves.
• Scanning electron microscopy (SEM).
• X-ray diffraction (XRD) including transmitted Debye-Scherrer and Laue methods with micro-parabola capillary.
• Field ion microscopy (FIM).
• Field emission microscopy (FEM).

Main Results:

• Electron-bombardment heating of 10-40 W for 10 seconds effectively removes oxide layers and yields stable electron emission.
• A heating power of approximately 60 W for 10 seconds was identified as the threshold for tip radius increase (+10 ± 5 nm), indicating the onset of melting.
• A grain size of approximately 1000 nm in polycrystalline W wires is crucial for achieving a conical tip apex shape.

Conclusions:

• Controlling electron-bombardment heating power is a viable method for preparing W tips with stable tunneling electron emissions.
• Optimal heating parameters prevent tip melting while ensuring surface cleanliness.
• The grain size of the W wire significantly influences the resulting tip apex morphology and emission stability.