## 3 Results and Discussion  
Figure 2a shows the transfer curves of Device #1, whose optical image is shown in Figure 2b. The device initially behaves as an ambipolar transistor with a channel height of 6 nm (Figure 2c) and the smallest current, which corresponds to a charge neutrality condition of approximately \(-\)28 V. The device was first maintained under ambient conditions for two days to investigate the effect of air exposure without any treatment. It was then heated on a hot plate at 100 \({}^{\circ}\)C to induce oxygen adsorption on the surface, which led to a shift in V\({}_{\text{th}}\). As the air-heating time increased, the threshold voltages of the MoTe\({}_{2}\) FETs, which were originally located in the negative gate bias regime, were consistently shifted in the positive direction. After 3 h of air annealing, the current on/off ratio improved from 4.89 \(\times\) 10\({}^{4}\) to 1.03 \(\times\) 10\({}^{5}\) and the charge neutrality point was nearest to zero. However,  
Figure 1: (**a**) Fabrication flow for FET devices and (**b**) schematic illustration of MoTe\({}_{2}\) FET. the SS also increased, which may be attributed to undesirable residual adsorption on the channel surface under ambient conditions. To investigate the effect of air heating alone precisely, each step of air heating was performed without rinsing in between. This can affect the pristine surface, which can lead to the deterioration of carrier mobility and an increase in SS. Therefore, the overall electrical performance appears to be degraded after a certain period of air heating owing to the long duration of exposure to ambient conditions (Supplementary Figure S1). However, a consistent hole-doping effect was observed in all the MoTe\({}_{2}\) FETs, regardless of the channel profile. The shifting of the charge neutrality point shown in Figure 2 (Device #1) indicates that hole doping resulted from O\({}_{2}\) molecules on the channel surface [6]. The oxygen/water redox couple adsorbed on the MoTe\({}_{2}\) channel is known to cause electron transfer from MoTe\({}_{2}\) to the redox couple, resulting in a p-doping effect [6; 12]. The effect of air annealing can be compared to the plain air-exposure result. As shown in Figure 2a, after two days of air exposure, the transfer (I\({}_{\text{ds}}\) vs. V\({}_{\text{gs}}\)) characteristics of the MoTe\({}_{2}\) FET remained almost the same, with a slightly smaller current in the positive gate bias region and a small shift in the charge neutrality point to the p-side. As the air heating started, the transfer curve showed a smaller off current, resulting in a larger current on/off ratio, which was not observed in the air-exposed sample. An improvement in the on/off ratio continued to be observed, as shown in Figure 2d. After 2 h of air heating at 100 \({}^{\circ}\)C, the smallest current was observed at a gate voltage of \(-\)5 V. The output characteristics under a drain bias of 0 V without gate biasing showed that the device was turned on in the pristine state with a current flow of less than a few nanoamperes (Figure 2e). As the air heating proceeded, the current flow at zero gate bias gradually decreased and reached a few femtoamperes after 3 h of air heating. Ambipolar transistors have the limitation of intrinsically large off currents compared with unipolar transistors [28]. Our technique not only modifies the threshold regime but also rectifies the large off current issue in ambipolar MoTe\({}_{2}\) devices. While the device exhibited a significant positive shift in V\({}_{\text{th}}\) by 23 V, the N\({}_{\text{it}}\) value was not significantly affected, as it only changed from 1.91 \(\times\) 10\({}^{13}\) cm\({}^{-2}\) eV\({}^{-1}\) to 3.90 \(\times\) 10\({}^{13}\) cm\({}^{-2}\) eV\({}^{-1}\).