Edge-Sensitive Semiconducting Behaviour in Low-Defect Narrow Graphene Nanoribbons

Low-defect graphene nanoribbons (GNRs) derived from the unzipping of carbon nanotubes have exhibited large energy band gaps (transport gaps), despite having widths in the order of ~100 nm. Here, we report on the unique semiconducting behaviour of very narrow, low-defect GNRs, with widths of less than 20 nm. Narrow GNRs are highly resistive, and additional annealing is required to reduce their resistivity. The GNRs display ambipolar rather than evident semiconducting behaviour (p- and n-types), exhibiting normalized Ion/Ioff as great as ~106 (close to those in a few nm-order-width GNRs) and which are very sensitive to the atmosphere and the termination of the GNRs' edges by foreign atoms (hydrogen for n-type and oxygen for p-type). It is also revealed that the activation energy (Ea ~35 meV) estimated from the temperature dependence of the minimum conductance is smaller than those in ~100 nm width GNRs. The observed sharp conductance peak on back-gate voltage (Vbg) dependence and its strong correlation with the Ea value suggest the presence of possible resonant tunnelling through shallow impurity levels with the small Ea introduced by the edge terminations by foreign atoms, which provides the observed unique behaviour, including the high Ion/Ioff. An energy band gap as large as ~215 meV is also confirmed from the Ioff voltage region on Vbg. These narrow GNRs must open the door to large-scale graphene integration circuits based on CMOS-like behaviour.


Introduction
Graphene has attracted considerable attention due to its unique electronic properties [1,2]. In particular, it is important to realize well-controlled semiconducting behaviours in graphene to enable its future use in largescale integration (LSI) circuits because pristine graphene is a semimetal (a gapless semiconductor). At least two effective approaches have been used for this: (1) the formation of bilayer graphenes [3][4][5][6][7] and (2) the formation of graphene nanoribbon (GNR; pseudo one-dimensional (1D) strip-lines of graphenes) structures with armchairtype edges .
For GNRs, the quasi-1D confinement of carriers and the presence of armchair edges induce an intrinsic energy gap in the single-particle spectrum [22][23][24]. Given this, we have reported the fabrication of fieldeffect transistors (FETs) using low-defect GNRs derived from a non-lithographic method (i.e., the unzipping of carbon nanotubes (CNTs) with the airblow method and three-step annealing [15]). In the low-defect GNRs, even with a width as wide as ~75 nm, we confirmed an intrinsic energy band gap as large as ~50 meV, which was seven-times greater than that in lithographically-defined high-density-defect GNRs, as well as some semiconducting behaviours.
On the other hand, other groups have reported on the semiconducting behaviours of narrower GNRs (width <40 nm) [18][19][20]; e.g., p-type semiconducting behaviour with on/off ratios of Isd upon Vbg change (Ion/Ioff) up to 10 6 in ∼2 nm-wide GNRs, changing from p-type to ntype behaviours with Ion/Ioff ∼ 10 5 in similar ∼5 nm-wide GNRs with electron doping in an ammonia atmosphere, and the evident single-electron charging effect (the socalled artificial Kondo effect) in CNT-derived smoothedge GNRs with sub-10 nm widths [18]. Extremely large magnetoresistance arising from the unique symmetry of band gaps is also predicted in GNRs for the generation of highly spin-polarized current [26]. Even fast and reliable DNA-sequencing devices are predicted, utilizing the distinct conductance behaviours of GNRs [27].
The appearance of such semiconducting behaviour has been understood based on the shifting of the Fermi level (EF) -which is sensitive to carrier doping via the GNR edges [29][30][31][32][33][34] -through the 1D electronic density of states (EDOS) with van Hove singularities (VHSs). However, the correlation of semiconducting behaviour with the presence of impurity levels introduced via edge termination by foreign atoms has not thus far been experimentally reported, although there are many theoretical reports (particularly as regards calculating resonant tunnelling via impurity levels originating from edge termination by foreign atoms) [35][36][37][38]. In the present study, we observe this correlation in low-defect narrow GNRs (<∼20 nm width). Narrow GNRs are highly resistive, and additional annealing is required to reduce their resistivity. The GNRs show not ambipolar (i.e., charge-neutral points) but rather evident semiconducting behaviours (like p-and ntypes) exhibiting normalized Ion/Ioff as great as ∼10 6 , which are highly sensitive to atmospheres and the termination of the GNRs' edges by foreign atoms [29][30][31][32][33][34]. We noticed the presence of a conductance peak at the low voltage region and discussed its correlation with resonant tunnelling via impurity levels, which can realize the observed semiconducting behaviours. images before the three-step annealing. d, e, Scanning electron microscope (SEM) images of FET electrode patterns with Ti/Au (15 nm/100 nm) for short (d, 600 nm) and long (e, 3600 nm) GNRs. The inter-electrode distance is 400 nm for (d), and 400 nm between electrodes 1 and 2 and 3,000 nm between electrodes 1 and 3 for (e). f, Typical drain current (Isd) vs. drain voltage (Vsd) relationship of the sample shown in Figs. 1b and 1d at 1.5 K before performing the fourth annealing.

Sample fabrication and characterization
The GNRs fabricated in the present study were prepared by oxidization and the longitudinal unzipping of multiwalled CNTs, basically following our previous methods [15][16][17]. As a significant difference, in order to separate the entangled as-grown narrow GNRs and obtain suitable rectangle-high aspect-ratio structures of individual GNRs on the SiO2 substrate, an air gun or conventional heat dryer was used. Air flow from either of these was applied to the H2O droplet containing the as-grown GNRs, and the air flow direction was changed every 60 s a total of ~80 times. The strength of the air flow was tuned by controlling the distance between the droplet and the air gun. The droplet was moved around on the substrate by this action and the entangled as-grown GNRs were separated. (Figs. 1a and 1b). Next, the substrate surface was dried at 400 °C in high vacuum (10 -6 Torr).
Subsequently, three-step annealing was carried out following our previously-described protocol [15]. As the fourth step, annealing was performed under high vacuum (10 -6 Torr) at 700 °C immediately after the formation of the metal electrodes. This temperature is critical to ensuring that no damage occurs to the Au/Ti metal electrode.
The low-defect concentration of the fabricated GNRs was confirmed by the observation of a high-intensity G band and a low-intensity D band, along with a low-intensity 2D band in the Raman spectrum obtained after the annealing as well as the case in [15]. The D-band peak becomes slightly larger than those in previous, wider GNRs [15], due to the induced influence of the residual edge disorder in narrow GNRs. The absence of defects was also inferred from the observation of a clear hexagonal graphene lattice with small-amount defects in high-resolution transmission electron microscope (HRTEM) images ( Figure 1c). Although some defects still remain -because it was observed before annealing -it shows a much better HRTEM image compared with lithographically fabricated GNRs. The reduction of the D peak height after annealing suggests the disappearance of these defects.
Using these low-defect narrow GNRs, we fabricated FETs with short-length GNRs (600 nm length: Figure 1d) and long GNRs (3,600 nm length: Figure 1e). Each electrodeincluding the back gate -is composed of Ti/Au and the electrode pads were placed on SiO2. Figure 1f shows the typical drain current (Isd) values as a function of the drain voltage (Vsd) of an FET fabricated using a short-length GNR (Figure 1d) as the current channel at a temperature (T) of 1.5 K. The current amplitude is small, with the presence of a strong zero-bias conductance (G0) anomaly (a voltage width of ΔVSD ∼±1 V even at a back-gate voltage (Vbg) of ±20 V). We observed that most of the fabricated narrow GNRs (width ∼20 nm) were highly resistive (i.e., a small magnitude of Isd) with strong G0 anomalies even after the three-step annealing [15] when compared with our previously-fabricated wide GNRs (width >75 nm).

Basic property and 4th annealing
Although the width of the narrower GNRs is less than ∼1/3 of the width of the wider GNRs, the resistivity is at least 10-times larger. This result cannot be understood by the conventional relationship of resistivity vs. the width of the GNRs, which is an inversely proportional relationship. Moreover, the G0 anomaly cannot be attributed to the single-electron charging effect [15] (see Author's note). However, after we carried out the fourth annealing process under high vacuum at 700 °C immediately after the formation of the metal electrodes (methods), the amplitude of Isd exhibited a significant recovery. This implies that the previously-observed high resistivity ( Figure 1f) was due to poor interface contact between the metal electrode and the narrow GNRs. After the fourth annealing, further deoxidization from the GNR can at least occur. The GNRs present were fabricated by the unzipping of CNTs by the oxidation of defects. Thus, after the formation of the GNRs, oxygen (O)-atoms remained. Here, even if the number of the remaining O atoms is very small and mostly the same between the wide and narrow GNRs after the three-step annealing, the resistivity at the interface of the electrodes/GNR is significantly affected only in the narrow GNRs because the ratio of the O atoms/GNR width is larger in the narrow GNRs. This can result in larger interface resistivity. After the fourth annealing, O atoms beneath the electrodes move and can be excluded further, resulting in low interface resistivity. Figure 2a shows the typical Isd vs. Vbg curve for the sample with a 600 nm length at T = 300 K, after the fourth annealing. The Isd value is significantly enhanced by the fourth annealing step. In the Vbg range between ±20 V, the device does not exhibit ambipolar behaviour but instead evident n-type semiconductor behaviour, where Isd drastically increases only when the applied Vbg is positive. This behaviour is qualitatively consistent with the behaviour reported in previously fabricated carrierdoped GNRs with sub-10 nm-widths and graphene nanomeshes with an inter-pore distance (i.e., GNR width) of ∼20 nm [18][19][20]. However, an Ion/Ioff ratio as high as ∼10 4 is observed at Vsd = 0.2 V, even with the leakage Isd at Vsd = 0 V ( Figure 2b with the y-axis representing the logarithmic-scale values of the Isd value in Figure 2a). This Ion/Ioff ratio is 10 3 times greater than those observed in previously fabricated ∼20 nm-width GNRs [19,30]. Surprisingly, this value is equal to ~10 6 as reported in 2 nm-wide GNRs, when the Ion is normalized for the GNR width (i.e., normalized Ion~ 200 μA/μm = Ion ~ 4μA/20 nm (Figure 2a) × 1μm/20 nm) [20].

Semiconducting behaviours on edge termination
As mentioned above, the observed semiconducting behaviour that does not exhibit transport gaps (or charge neutrality points, band gaps) in sub-10 nm GNRs has been understood only by the shift in EF through the VHSs of 1D EDSs caused by edge carrier doping. This shift can also produce large Ion/Ioff values due to a significant increase in EDOS caused by the EF shifting along a VHS. However, in the presence 20 nm-GNRs, no evident 1D EDOS exists because the GNR width is greater than 10 nm and, hence, the observed semiconducting behaviour with the extremely large normalized Ion/Ioff values of ∼10 6 cannot be fully understood by this model alone. We discuss our model (considering EF pinning at impurity levels) of the origin of this semiconducting behaviour in a later section. Figure 2c shows the typical Isd-Vbg relationship obtained for the sample corresponding to Figure 2a when placed in atmospheric air for two weeks at room temperature. Because our fabricated GNRs are not covered with passivation films, the surfaces and edges of the GNRs are directly exposed to air and the edges are automatically oxidized through this process. We found that the initially observed n-type behaviour (Figure 2a) drastically changes to p-type behaviour (Figure 2c). The normalized Ion/Ioff ratio (as large as ∼10 6 ) observed in Figure 2b is also approximately observed in Figure 2d (with a logarithmic y-axis scale as in Figure 2b), although it is reduced by the larger leakage Isd. Because the wider GNRs (∼100 nm) showed no such change, this edge-sensitive characteristic is unique to the narrow (<20 nm) GNRs present. We confirmed qualitatively similar changes in five samples. This is qualitatively consistent with the evident semiconducting behaviour observed in Figure 2a, and this result strongly suggests that the narrow GNRs' electronic structures are extremely sensitive to the termination of the dangling edge bonds by foreign atoms. It is known that the H-termination of GNR edges leads to electron doping and n-type behaviour [40], while Otermination by this process causes hole doping and ptype behaviour [41]. Evident changes between the n-and p-type behaviours in Figs. 2a and 2c may correspond to these edge terminations. Indeed, in the wider GNRs fabricated following the same CNT-derived method, ptype behaviour in an as-grown one and its transformation into ambipolar behaviour were reported after airpumping in high vacuum for a long time [39]. This supports our results and the argument given above, although our narrow GNRs are much more sensitive to the atmosphere because the GNRs were only placed in air, without performing any intentional procedures. This result indicates that some portions of edge C-H bonds are chemically unstable and tend to be replaced by C=O bonds when the edges are exposed in air for a long time in the absence of covering passivation films. In the narrow and short GNRs (e.g., <20 nm width and <400 nm length), such a partial replacement can drastically change the electronic behaviours of all of the GNRs. The result also indicates that such GNRs are suitable for the fabrication of p-and n-type semiconducting devices via carrier doping from the edges and, hence, for CMOS-like circuits. Figure 2e shows the typical Isd-Vbg characteristics of ∼3,600 nm-long GNRs (measured between electrodes 1 and 3 in Figure 1e) after O-termination of the edges by exposing the sample to atmospheric air. The curves indicate p-type semiconducting behaviour, as in the case of Figure 2c. However, the onset value of Vbg, at which point Isd starts to increase, is different in this case. The onset Vbg shifts to Vbg ∼-4 V in Figure 2e. In contrast, the Isd-Vbg relationship between electrodes 1 and 2 with a 400 nm distance on the same GNR (Figure 1e) is similar to that observed in Figure  2c, with the onset being Vbg = 0 V. These differences in the onset Vbg at the different portions of the long GNR can also be understood from the perspective of the poor uniformity of O-termination for the long GNR edges, as explained in a later section. This suggests that the electronic transport in narrow-width GNRs is highly sensitive not only to different types of foreign atoms but also to their quantities involved in edge termination. This result also implies the possibility of fabricating GNR FETs with different threshold voltages by using differentlength GNRs; this can prove useful in manufacturing LSIcircuit components using GNRs. In the LSI circuits, the tuning of the threshold voltages (current-onset voltage) is the most important. In the present experiments, the short GNRs with high homogeneous edge termination by the impurity atoms showed the lowest onset Vbg, while the long GNRs with inhomogeneous edge termination showed the highest onset Vbg. Thus, combining and hybridizing such different-length GNRs with different onsets of Vbg make the realization of LSI possible. Of course, the onset Vbg is not controllable in the present work, because only the unintentional termination of the edges was employed. If one could control it in the future, it would lead to the formation of an effective LSI circuit.

Small activation energy for impurity level
As mentioned, we have previously reported Ea values as large as ∼50 meV in our fabricated wide GNRs, with minimum widths of ∼75 nm. In order to compare the Ea value of the present narrow GNRs with the wide GNRs, we measured the relationship for minimum conductivity (Gmin) vs. temperature for Vbg = 0 V in a few of the Hterminated n-type samples ( Figure  2a). The corresponding Arrhenius plots are shown in Figure 3. The relationship is mostly linear when T >20 K, following a thermal-activation relationship as indicated by the dotted line, while the relationship is not clearly defined below T = 20 K, and the Gmin value becomes extremely low while exhibiting a constant value when T <10 K (i.e., insulating behaviour). The linear part for T >20 K (the dotted line for1/T <0.05) yields Ea ∼35 meV from fitting with Gmin ∝ exp(−Ea/2kBT). Despite the narrow width of the GNRs (20 nm), this Ea value is less than the maximum Ea value of ∼50 meV obtained for our previously-fabricated wider GNRs (minimum width ∼75 nm), for which only the linear part was observed in the Arrhenius plots for the estimation of Ea. In general, the Ea value is inversely proportional to the GNR width W (i.e., Ea = AW -1 , where A denotes the coefficient). Although the A values were small in the previously-fabricated wide GNRs because of the large width region (W >75 nm), these low-defect GNRs exhibited large Ea values because of strong electron confinement in the clean 1D space of the low-defect GNRs [15]. This type of confinement should ideally be induced in the present narrower (W <20 nm) GNRs and, thus, the A value should increase. Moreover, the influence of armchair edges should become also significant in sub-10 nm-width GNRs. In our present near-sub-10 nm GNRs, such an influence should become non-trivial, thereby resulting in the enhancement of Ea. Nevertheless, the Ea observed in the present case is relatively smaller. This suggests that the anomalously small Ea value of ∼35 meV is not characteristic of intrinsic energy band gaps. Because no power-law behaviour (i.e., Gmin ∝ T α for hopping conductance) is observed even in the temperature regions where the linearity is smeared out (i.e., 10 K <T<20 K), the small value of Ea cannot be attributed to defect-originated energy levels, as in the case of lithographically fabricated GNRs [8][9][10].
On the other hand, it is known that the termination of GNR edges and carrier doping theoretically lead to the introduction of impurity levels [35 -39]. Indeed, this Ea value of ∼35 meV estimated from Figure 3 is close to the impurity levels reported in [35]. The observed Ea value, thus, could be the activation energy of the electrons emitted from the impurity level to the bottom of the conduction band (Figure 4d) because of the measured Vbg = 0V.

Conductance peak at low Vbg fields
Here, in certain such small-Ea GNRs, we observe a sharp and small conductance peak in low Vbg regions (Vbg = ±3.5 V) of the Isd-Vbg curves at T = 1.5 K (Figure 4a). Five of eight samples which showed the small Ea exhibited the conductance peaks. This suggests the strong correlation of the conductance peak with the impurity level with small Ea values. The conductance peak appears to be mostly independent of Vsd ( Figure 4c). As the temperature increases, the conductance peak decreases and it disappears entirely at T ∼30 K (Figure 4b). It can be, however, potentially the main cause for the abovementioned small Ea -which was observed at Vbg ∼0over a wide temperature range in Figure 3(a).
When EF is pinned around this impurity level, it can cause resonant tunnelling [35][36][37] and, thus, produce this sharp conductance peak. Here, in Figure 4(a), the conductance peaks exist around Vbg = ±3.5 V. This Vbg is the voltage applied from the back side of the silicon substrate and, thus, the applied VBG modulates the electronic states of the GNRs via the silicon substrate, so electronic charging in the substrate (QBG = CBGΔVBG) gives rise to the true energy values [15].
Considering this, the energy in the single particle energy spectrum is given by Δ = (h/2π) vF√(2πCBGΔVBG/|e|), where vF ∼10 6 m/s is the Fermi velocity of graphene and CBG is the capacitive coupling of the GNR to the back-gate electrode. When CBG is taken to be 230 aF μm -2 , which is 1/3 times smaller than that for our previous wide GNR [15], Δ for Vbg = 3.5 V in the present narrow GNRs is estimated to be ∼70 meV. This means that the resonant tunnelling for the conductance peak occurs when EF shifts in ∼70 meV from the Dirac point at Vbg = 0V and is pinned by the impurity level.
We also discuss this issue in the next section, considering also other phenomena mentioned above.
Alternatively, carrier doping via edge termination by foreign atoms without considering the presence of impurity levels has been considered as the origin of the appearance of n-or p-type semiconductive behaviour, particularly in sub-10 nm-GNRs. However, in the present GNRs with ∼20 nm widths, no 1D EDOS evidently exists because the width is >10 nm. Thus, the impurity levels with the small Ea ∼35 meV introduced by edge termination can consistently explain the observed phenomena, as elucidated below (see Figure 4d).

Discussion
Theoretically, resonant tunnelling through the impurity levels theoretically produces conductance peaks [36,37]. Semiconducting ultra-narrow-armchair GNRs with impurities exhibited sharp peaks in DOS between VHSs as well as conductance due to resonant tunnelling, depending on the impurity concentration [36]. As the impurity concentration increased, the resonant-peak height increased. Moreover, the two connected GNRs with impurities also showed resonant tunnelling and conductance peaks, depending on the width of the GNRs and the hybridization energy of the impurity atoms in two GNRs [37].
As mentioned above, there is a strong correlation between the observed conductance peak and the impurity level with the small Ea value in the present, narrow GNRs. This suggests the robust possibility that the conductance peak originates from resonant tunnelling through the impurity level with an Ea value of ∼35 meV, when EF shifts ∼70 meV from the Dirac point at Vbg = 0V and when EF is pinned by the impurity level. Assuming this, all of the other three phenomena can be consistently understood qualitatively, as follows.
(1) The absence of transport gaps (Figure 2) can be understood by the model whereby EF is strongly pinned around the high-intensity impurity level, which exists close to the bottom of the conduction band (Ea value of ∼35 meV; Figure 4d) for H-terminated edges and close to the top of the valence band for O-terminated edges (Figure 2c; hole doping). The application of Vbg, thus, cannot shift EF effectively through all the energy−regions present within the band gaps, resulting in the absence of transport gaps. (2) The high sensitivity upon the appearance of these n-and p-type behaviours to different atmospheres can also be understood by the closest position of EF to the impurity level and the EF pinning, especially for the present short GNRs (e.g., <500 nm length). The replacement of C-H bonds by edge C=O bonds can easily change the n-type behaviours to p-type behaviours due to the formation of high-intensity impurity levels in such short and narrow GNRs. (3) The large Ion/Ioff value can also be explained by the model whereby drastic electron (hole) emissions can be caused due to the high-intensity of the impurity levels present, even upon slightly varying the applied Vbg and shifting the EF. Indeed, Isd rapidly increases right above Vbg = 0 V, both in Figs. 2b and 2d, resulting in the large Ion/Ioff value. This is strong evidence for the EF pinning model.
In the present narrow GNRs of width ∼20 nm and length <∼500 nm, the densities of the impurity levels will not be uniform through all the regions along the longitudinal direction because the termination by impurity atoms (H and O) is not uniform. It is speculated that some very small portions have the highest densities for tunnel barriers. When electrons run through such small tunnel barriers and Vbg is pinned by the impurity level, the electron must engage in resonant tunnelling. Moreover, the combination of two portions including the impurity levels in a GNR may correspond to the hybridization of the impurity level, like [37]. In this case, the emergence of a sharp conductance peak like that in Figure 5(a) can actually be predicted, depending on the hybridization energy. The models that are relevant to the present case should be clarified by further experiments.
Here, the difference in the onset Vbg value for different GNRs' lengths (Figs. 2c and 2e) can also be interpreted by poorer uniformity in the density of the impurity levels, caused by the occasional O-termination of the long GNR edges (i.e., the presence of extremely low-quantity C=O bonds at some edge sections). This is because the GNRs were unintentionally exposed to an air atmosphere.
The onset Vbg is determined by the energy difference between the initial position of EF at Vbg = 0 V and the impurity levels. In the short GNRs, the edge-termination by H and O is relatively homogeneous with a high density. Thus, the impurity level density is high and homogeneous and hence the initial position of EF locates close to the impurity level. Indeed, we estimated the initial position of EF to be only 70 meV from the impurity level, as mentioned above. In contrast, in the unintentionally-oxidized long GNRs, some portions should have much lower O-termination and lower impurity levels as a matter of convention. In such portions, the initial EF at Vbg = 0 V locates far from the impurity level according to the theory of semiconductors and, hence, the larger Vbg is needed in order to cause the onset to Isd. The onset Vbg in the overall region of a long GNR is determined by such a portion, which has the largest resistivity. Therefore, the large onset Vbg appears in the long GNRs.
In Figure 2, Isd cannot easily flow even under the applied maximum Vbg (i.e., Vbg = -20 V for Figure 2a and Vbg = +20 V for Figure 2c), except for the leaked Isd. When we assume that EF locates close to the impurity level at Vbg = 0 V and shifts directly following Vbg without undergoing pinning by the impurity level, we can estimate the energy difference Δ between the impurity level and the other band edges (i.e., the top of the valence band for the sample corresponding to Figure 2a and the bottom of the conduction band for that corresponding to Fig 2c). This energy difference can be also given by Δ = (h/2π) vF√(2πCBGΔVBG/|e|) ∼180 meV for the present GNR. This suggests that the energy band gaps Δgap of the present ∼20 nm-wide GNRs are at least greater than [(Δ = 180) + (Ea = 35) meV = 215 meV]. This result is in qualitatively good agreement with the observed trend of GNR width vs. Δgap [15].

Conclusion
We reported on the unique semiconductive behaviours of narrow, low-defect GNRs, with widths of less than 20 nm. We found that the narrow GNRs were highly resistive and that additional annealing was required to reduce their resistivity. The GNRs did not show ambipolar but instead evident semiconductive behaviours (p-and ntypes), exhibiting normalized Ion/Ioff as great as ∼10 6 (close to those in a few nm-order-width GNRs), which were very sensitive to the atmosphere and the termination of the GNR edges by foreign atoms (i.e., H atoms for n-type and O atoms for p-type). It was also anomalously revealed that the Ea (∼35 meV) estimated from the temperature dependence of the minimum conductance was smaller than those in ∼100 nm-width GNRs. The sharp conductance peak observed at low Vbg in the Isd-Vbg curves exhibited a strong correlation with the presence of shallow impurity levels with the small Ea, which were introduced by the edge terminations by H atoms. This possibly originated from resonant tunnelling via the pinning of EF by the impurity level, which provided the observed, unique semiconducting behaviour. An energy band gap as large as ∼215 meV was also confirmed from the Isd off-voltage region on Vbg.
The narrow GNRs fabricated in this study are suggestive of the utility of high-efficiency semiconducting graphenes bearing edges terminated by a variety of hetero-atoms. The FETs prepared with such GNRs can lead to the fabrication of efficient graphene LSI circuits based on CMOS-like operations, because they easily provide for n-and p-type semiconducting behaviours as well as different onset voltages. As other unique applications of graphenes, nanoresonators (e.g., utilizing the enhanced mass-sensitivity due to the effective strain in the non-linear oscillation [42] as well as the unique lattice dynamics' properties in twisting bilayer graphene [43]) have recently been proposed. Moreover, it was predicted that ZnO nanowire without polar surfaces can exhibit high thermal conductivity [44]. The present, very thin GNRs, may also be good candidates for such applications. Moreover, the introduction of spin-orbit interaction [45] and the utilization of edge-polarized spins [29][30][31] in the present thin GNRs must open the door to novel spintronic devices.