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Application of nanogaps for electron sources is fascinating in surface-conduction electron-emitter display. In contrast to rather complicated fabrication processes of the focused ion beam technique for the extremely narrow fissure, nanogaps fabricated by hydrogen embrittlement (HE) have thus been proposed as novel surface-conduction electron emitters due to their low turn-on voltage, high emission current, high focus capability, and high emission efficiency. In this paper, we theoretically investigate effects of the separation width and the tilted angle of the nanogaps fabricated by HE method on the field emission efficiency using a 3-D finite-difference time-domain particle-in-cell simulation technique. The structure with a large tilted angle may result in a high emitted current, but the collected current on the anode is suppressed due to the strong local field around the tip. A small structure prevents the emitted electrons from spreading out, and thus, no current could be collected by the anode. Also, the structure with a wide (or a narrow) separation of gap weakens (or enhances) the field around the tip and reduces the collected electrons. For better emission efficiency and focus capability, the separation width and the tilted angle of the examined structure could vary from 57 to 117 nm and 30deg to 60deg, respectively.