This paper demonstrates that the electrical properties of suspended graphene nanomesh (GNM) can be tuned by systematically changing the porosity with helium ion beam milling (HIBM). The porosity of the GNM is well-controlled by defining the pitch of the periodic nanopores. The defective region surrounding the individual nanopores after HIBM, which limits the minimum pitch achievable between nanopores for a certain dose, is investigated and reported. The exponential relationship between the thermal activation energy (E A ) and the porosity is found in the GNM devices. Good E A tuneability observed from the GNMs provides a new approach to the transport gap engineering beyond the conventional nanoribbon method.Micromachines 2020, 11, 387 2 of 13 lattice structure of the graphene [25]. It has a larger driving current and higher on-off ratio than the narrow GNR [26,27]. Most of all, GNM has been reported to observe a transport gap opening both in theoretical simulations [28][29][30] and experiments [31,32]. These results pave the way to use the GNM in electrical logic devices. In addition, GNMs have also been proposed to be a promising candidate for applications including gas sensing [33,34], phonon engineering [35,36], battery electrodes [37], and quantum technology [38]. Although these remarkable properties of GNM were previously reported, the electrical properties of the suspended GNM with systematically controlled porosity has not been clearly investigated experimentally, especially in the sub-10 nm nanopores regime. Although the GNMs can be patterned by the electron-beam irradiation, the patterning speed is relatively low, about a few seconds for single nanopore [39]. Thus, patterning the large area of GNMs via electron-beam irradiation is impractical. The focused ion beam milling overcomes the limitation of the speed and also provides a reasonable resolution to observe the nanopore array [32].In this work, the suspended GNM devices were fabricated and patterned by focused helium ion beam milling (HIBM) (Figure 1). By optimizing the pattern location, stable suspended GNMs were obtained with different pitches of nanopores, which avoided cracking at edges during the HIBM. The electrical properties of the suspended GNM devices were measured at different temperatures. An exponential relationship was found between the porosity and the thermal activation energy of the GNMs. The results demonstrate that the GNM transport gap could be tuned by controlling the porosity.