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We introduce the controllable doping from hydrogen silsesquioxane (HSQ) to graphene by changing its electron-beam exposure dose. Using HSQ as the dopant, a fine-resolution electron-beam resist allows us to selectively dope graphene with an extremely high spatial resolution of a few nanometers. Therefore, we can design and demonstrate the single quantum dot (QD)-like transport in the graphene nanoribbon (GNR) with the opening of the energy gap. Moreover, we suggest a rough geometric design rule in which a relatively short and wide GNR is required for observing the single QD-like transport. We envisage that this method can be utilized for other materials and for other applications, such as p–n junctions and tunnel field-effect transistors.

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.

Multiple sub-band transport in bilayer graphene nanoribbons (GNRs) with localized charge states has not been experimentally investigated owing to the difficulty of accessing into the upper sub-band. In this letter, we utilize current annealing to heavily p-dope graphene from a capping hydrogen-silsesquioxane layer. As a result, Fermi energy can be tuned into the upper sub-valence band with reasonably small gate voltage. Owing to the localized charge states, quantum dot-like characteristics are measured in bilayer GNRs, as a result of the tunnelling transport of holes in the upper sub-band through these charge puddles. In contrast to monolayer GNRs, this phenomenon appears in a considerable conductive regime since the carrier transport in the lower sub-band is still diffusive. Moreover, by electrically tuning the Fermi energy, the localized charge puddles in the upper sub-band can be resized and isolated from each other.

We report the experimental and theoretical investigation of single carrier transport property on graphene single-constriction devices. As the Fermi wavelength is comparable with the width of the constriction and shorter than the length of the constriction in our device, the single quantum dot-dominated characteristic is observed by back-gate modulation. The first-principles calculation shows that discrete states are induced in the narrower constriction due to the mode mixing in the narrower constriction and wider leads. It is found that the geometry of constriction-wider lead junctions plays a crucial role in single-dot formation in graphene constriction structure.