Supplementary material for the paper

Details about the electrical transportation

Figure s5 shows the I–V curves relative to the connections described in Section II.

Figure s5. (a) and (b) show the selected positively and negatively biased I–V curves of the CNT–fullerene–CNT contact, respectively. The labels correspond to the different contacting geometries in Figure s2. (c) and (d) show the log–log plot for the I–V curves, where the positive and negative parts are shown simultaneously.

1. 
   Fluctuation observed at a higher bias voltage
   

In some of the I–V curves, small fluctuations and asymmetries were observed. The fluctuations of the curves were more evident under a high bias of 1.5 V to 2 V, due to the change in the contact geometry during the I–V measurement caused by the larger Faraday force or/and the high-temperature-induced interface bond breakage. In some cases, detachment of the contact was observed, accompanied by the current jumps to zero, such as in the I–V curve labeled as “IIa –” in Figure 3 (see also “b,” “e,” and “h” in Figure s5). All the breaking points in the curves were found to occur at a relatively lower current, and their I–V curves are relatively lower than the others (smaller current under the same bias). Considering that their corresponding TEM figures show almost the same contact area, the failure of the contact could be more probably due to the weaker covalent carbon bonds at the CNT–fullerene contact.

2. 
   SCLC fitting
   

The I–V relationship based on different electrical transportation models was analyzed to understand the conducting mechanism better. Fitting with different kinds of transportation models failed to give an accurate description of the I–V curves. However, an acceptable fitting using the space-charge-limited current (SCLC) model gives a linear relationship of . [r1–r3]. The SCLC relationship shows a linear relationship in the log–log plot of the I–V relationship, as shown in the lower panel of Figure 3 (see also Figure s5). Curve fitting gives a slope of α = 1.0–3.5, but it does not show a clear dependence on the contact geometry. The observation of the SCLC process indicates that the Ohmic contact was formed between the CNT and the fullerene [r4]. Considering that the two CNTs have relatively large diameters and more than three walls, they should be treated as metallic-like. For the fullerene with a size the similar to that of C₇₂₀, the previous calculation gives the band gap of around 0.5 eV for a single-shelled fullerene [37, 38]. For a double-layered fullerene, the band gap was assumed to be smaller but still semiconductor-like because of the unavailability of reference data [r5, r6]. Thus, the fullerene hinge forms a symmetrical metal–semiconductor–metal structure. The SCLC could be attributed to the semiconductor nature of the fullerene.

Note that although SCLC transportation was observed in the system, the bulk and contact effects are difficult to distinguish because of the small dimension of the fullerene compared with the size of the contact. Different values of α = 1.0–3.5 were observed in the “fullerene hinge” with changing contact geometry, and the difference in the interface structures could be the most probable reason for the diversity in the α values. According to the SCLC model, different α values indicate different levels of trap occupation in the band gap or different relationships between the concentration of the injected carrier and the carrier in the semiconductor. α = 1 refers to the Ohmic behavior, indicating that there are enough free carriers for current transportation. α = 2 corresponds to the SCLC region, where all trap levels have been occupied. α = 1–2 is the transition region from Ohmic to SCLC conduction, and α > 2 refers to the SCLC in the trap-filling region [r1–r3]. In the current study, the SCLC transportation process is tentatively attributed to the formation of different interface bond structures between the CNT and the fullerene, because the interface bond structure, such as the type (single/double/triple) and number of bonds, significantly affects electron transportation [34–37]. The change in the interface structure has a similar effect with the different levels of trap-filling processes. The formation of the interface bond increases the electron density of state in the band gap of the fullerene, which serves as the channel for electron transportation [34–37]. The changes in the bond type and number will cause the change in the conducting channels. This change has a similar significance to that of the trap-filling process. More conducting channels tend to induce Ohmic conductivity, that is, smaller α values near 1 and lesser conducting channels tend to induce SCLC conduction, that is, larger α values [r7, r8]. Although the type and number of bonds under the current stage are difficult to determine, the movement of the fullerene at the contact and the change in the contact geometry are believed to be the causes of the change in the interface bond structure.

3.