3. Results and Discussion
A TEM observation result of C
59N@SWNTs is shown in
Figure 1(a), which indicates that C
59N molecules with spherical symmetry are filled into both individual and bundled SWNTs. In comparison with an empty pristine SWNT, C
59N molecules with spherical symmetry are clearly observed in the individual SWNTs, forming a one-dimensional chain-like structure inside the SWNTs, as illustrated by arrows in the TEM image. Therefore, the TEM observation provides strong evidence that C
59N molecules are encapsulated inside SWNTs. The energy dispersive X-ray analysis is also used to distinguish the elements in the C
59N@SWNTs during TEM measurements, as indicated in
Figure 1(b). However, only C and Cu originated from the TEM copper grid are detected, and there is no signal of N found in the spectrum due to very small amount of N from C
59N. A schematic diagram of FET device with a C
59N@SWNT as current channel is illustrated in
Figure 2(a), and its corresponding AFM image of the FET device is shown in
Figure 2(b), in which a C
59N@SWNT contacting two Au electrodes is clearly observed, which well confirms that an individual C
59N@SWNT indeed plays the role of the current channel with gap width of 500 nm.
The transport properties of pristine semiconducting SWNTs are well known to exhibit the
p-type behavior, as shown in
Figure 3(a) [
11], where a characteristic curve of source-drain current
IDS versus gate voltage
VG is described for source-drain voltage
VDS = 1 V.
Figure 3(b) presents the transport property of C
59N@SWNTs where the
IDS-
VG curve is measured at
VDS = 1 V. In contrast, the transport property of C
59N@SWNTs drastically changes to an
n-type semiconductor. This
n-type characteristic is attributed to the charge transfer between C
59N and local parts of SWNTs [
11–
13]. It is found that such azafullerene-induced characteristics have been observed in many independent SWNTs devices and they have good reproducibility under measurements performed with different source-drain voltages, which has been confirmed in our previous work [
11].
Under light illumination, it is noticed that the photoresponse of transfer characteristics for C
59N@SWNT-FET is strikingly different at different temperatures.
Figure 4 shows the transfer curves of a C
59N peapod FET device, in which the
IDS-
VG curves are recorded for the C
59N@SWNT-FET in both dark and upon 400 nm light illumination for the two temperatures at 300 K and 10 K, respectively. Obviously, the prominent response of the device at room temperature to light is the decrease of transconductance, and the light irradiation results in ∼95% decrease in conductance (
Figure 4(a)), which has been mentioned in our previous report [
12]. However, at low temperatures such as 10 K, a different photoresponse phenomenon is observed in the transport property of C
59N peapod FET device under light illumination, as shown in
Figure 4(b), and the source-drain current displays a several times increase under the same light illumination, which is the exactly opposite phenomenon to that observed at 300 K. It is necessary to mention that such a phenomenon has never been observed in pristine SWNT FET devices. This finding indicates that the response of transport properties of C
59N peapod FET device significantly depends on the variations of the temperature. On the other hand, the above interesting phenomenon implies a clear photoinduced electron transfer process. To further investigate the photoswitching characteristics at low temperatures, we have further exposed the device to the light pulse (1 s) during sweeping the
IDS-
VG curves at 10 K, as seen in
Figure 5. As the gate voltage is continually swept (with sweeping speed ∼1.4 V/s), the current at
VG = 21 V shows a sudden increase and the current value is about 6 times larger than its original one. After scanning to the high positive gate voltage
VG = 40 V, the measured
IDS is two times larger compared with the case of no light illumination. Again, when the 400 nm light pulse exposure is given at
VG = 26 V during
IDS-
VG sweeping, the similar sharp increase of current is observed, suggesting that such an effect of current increase is fully reproducible by exposure to the light pulse and disappears without light illumination, demonstrating the complete restoration of photoswitching effect. The results confirm well that the C
59N@SWNT FET device also exhibits an ultra fast response (on the level of millisecond) to the pulsed light, and the measured current is drastically enhanced under instantaneous UV illumination (400 nm, 1 s), which is entirely consistent with the result observed in
Figure 4(b). Moreover, the
IDS measured (
VG = 20,
VDS = 0.5 V) as a function of time at 10 K under exposure of a light pulse for a C
59N@SWNT-FET device is shown in
Figure 6, suggesting strong evidence for the great increase in current under the light pulse. This finding is well consistent with the measured results in
Figure 5, which is different from that observed at room temperature.
In order to understand the effect of temperatures, we have further measured photoinduced characteristics of current
vs. light pulse at different temperatures, as indicated in
Figure 7. Interestingly, the current increase is found to depend inversely on the temperature, and becomes gradually negligible when the temperature is increased from 10 to 90 K. As the temperature is further increased to 140 K, a clear negative photocurrent,
i.e., a decrease in current is observed, as seen in
Figure 7(c), upon pulsed light illumination. Up to 300 K, a significant decrease of current upon the light pulse is observed, just in agreement with the result in
Figure 4(a). Therefore, the above results suggest that it is possible to read the temperature by monitoring the optoelectronics signal of C
59N@SWNT-FET.
Figure 8 presents the ratio of the changed current (Δ
IDS) caused by instantaneous light illumination to the original current (Δ
IDS/
IDS) as a function of temperature in the range of 10–300 K. A variation of photoinduced current
vs. temperature indicates that when the temperature is decreased and increased from 90 K in the range of 10–300 K, the positive and negative photocurrents rise, respectively. In other words, the photocurrent is found to depend inversely on the temperature, and it becomes gradually negligible when the temperature is increased from 10 to 90 K. As the temperature is further increased from 90 K to 300 K, a negative photocurrent is observed upon pulsed light illumination. This finding reveals that it is possible to read the temperature by monitoring the optoelectronics signal of C
59N@SWNTs. In particular, sensing low temperatures would become more convenient and easy by giving a simple light pulse. In order to understand the photoswitching mechanism, we have further measured the transport properties of C
60 fullerenes encapsulated SWNT (C
60@SWNT) under the same experimental conditions of light illumination. There is no big change in the conductance under light illumination when the sample was measured at room and low temperatures, implying that the azafullerene is responsible for the decrease of conduction. According to our previous work [
11–
14], the
n-type transport behavior of C
59N@SWNT is considered to be due to the charge transfer from monomer C
59N to SWNT by the weak C-C bonding since the azafullerenes C
59N can easily lose or gain electrons through regioselective reactions. According to theoretical calculations [
15], such bonding can easily undergo homolysis under photolysis or thermolysis conditions, resulting in the formation of azafullerenenyl radical C
59N
•. When the light energy is higher than the bonding energy, the thermolysis of the bond will lead to the stop of charge transfer, leading to a decrease in current at room temperature. To confirm this, we have also measured the transport behavior of C
59N@SWNT at high temperatures (supporting information), and find that the current becomes unstable when the temperature reaches 400 K which indicates that the bond between C
59N and SWNT will break due to the high system temperature or light absorption. However, the thermal effect is reduced in the low temperature environment, as a result, the weak bonding between C
59N and SWNT may remain linked under light illumination. On the other hand, it was reported that the C
59N
+ exhibits distinguishing absorption spectral features in absorption spectra in the range of 1–3 eV [
16]. The light energy exerted on the C
59N@SWNT-FET device is in the range of 1.24–3.1 eV. Therefore, the multiplication effect cannot be ruled out since the quantum efficiency will be greatly enhanced at low temperatures. The above phenomenon might lead to the increase of photoinduced current at low temperatures although the exact mechanism is still unclear at this stage.