Eelectrical Transport Properties of C 59 N Azafullerene Encapsulated Double-Walled Carbon Nanotube

Electrical transport properties of double-walled carbon nanotubes (DWNTs) are modulated by encapsulating the azafullerene C59N which is synthesized via a plasma ion-irradiation method. The encapsulation of C59N molecules inside DWNTs has been confirmed by both transmission electron microscopy and Raman spectroscopy. The pristine DWNTs with outer diameter 4 5 nm are found to exhibit an ambipolar semiconducting behavior due to their small band gap. It is found that C60 fullerene encapsulated DWNTs exhibit a unipolar p-type semiconducting behavior. By comparison, C59N encapsulated DWNTs display an n-type semiconducting behavior. Our findings demonstrate that C59N operates as an electron donor compared with the acceptor behavior of C60, which is further clarified by photoelectron emission spectroscopy.


Introduction
Recently, double-walled carbon nanotubes (DWNTs) serving as nanoelectrical materials has received extensive attentions owing to their great potential applications [1,2].DWNTs represent a good candidate since they possess more stable mechanical properties and thermal stability than single-walled carbon nanotubes (SWNTs) because of their intrinsic coaxial structure.In particular, the large inner diameter of DWNTs makes them especially advantageous as an effective atom/molecule container.Therefore, DWNTs are interesting as material in engineering various kinds of nanoelectronic devices.However, most of previous experiments to date focus on the empty DWNTs which initially show an ambipolar or a p-type behavior when fabricated as the channels of fieldeffect transistor (FET) devices [3][4][5].The extensive research using different kinds of nanotubes with controllable electronic properties to construct nanoelectronic devices is extremely important for the progress in this field.Unfortunately, the number of reports on the transport properties of DWNTs is still limited.Moreover, there are few systematic studies concerning the electronic properties of DWNTs modified with electron dopants [6,7].
Here, we have investigated the transport properties of FET devices fabricated based on DWNTs which are modulated with the C 59 N azafullerene for the first time.
The encapsulation of C 59 N in DWNTs is proven by transmission electron microscopy and Raman spectroscopy.Pristine DWNTs are found to show either metallic or ambipolar semiconducting behavior owing to their narrow bandgap.However, after the C 59 N encapsulation, DWNTs can exhibit a unipolar n-type semiconducting behavior in contrast to the p-type behavior of C 60 encapsulated DWNTs, indicating that the electronic structure of DWNTs is strongly modified upon the insertion of C 59 N azafullerene in contrast to the case of pristine DWNTs and C 60 encapsulated DWNTs.

Experimental
The azafullerene C 59 N was synthesized by a nitrogen plasma irradiation method [8].A plasma was produced by applying an RF power with a frequency of 13.56 MHz, and nitrogen ions in the plasma were generated and accelerated toward a substrate by a sheath electric field in front of the deposited C 60 fullerene.Detailed experimental conditions are given as follows: plasma density n p ~ 10 9 cm -3 , electron temperature T e ~ 0.5 eV, and nitrogen-ion irradiation energy E i = 10 -40 eV.The fullerene C 60 after the plasma irradiation was dissolved in toluene and its mixture was separated into a residue and a solution.The mass spectroscopy analysis of the formed C 59 N azafullerene was performed using a laser-desorption time-of-flight mass spectrometer (LD-TOF-MS, Shimadzu AXIMA-CFR+).
The DWNTs used in this work were fabricated by an arc discharge method with Fe as catalyst.C 59 N azafullerene or C 60 fullerene molecules encapsulated DWNTs were synthesized by a vapor diffusion method.The purified DWNTs together with azafullerene or fullerene powders were first sealed in a glass tube under the vacuum condition ~10 -5 Torr.After that, the sealed glass tube was heated at 500˚C for 48 h to encapsulate the C 59 N azafullerene or C 60 fullerene in DWNTs.The encapsulated samples were obtained after the above process, and examined in detail by field emission transmission electron microscopy (FE-TEM, Hitachi HF-2000) operated at 200 kV and Raman Spectroscopy with a laser wavelength of 633 nm.
The electronic transport properties of various DWNTs are investigated by fabricating them as the channel of FET devices.During the fabrication process, DWNT samples are firstly dispersed by sonication in N, N-dimethylformamide (DMF) solvent and then spincoated on a substrate, which consists of Au electrodes on a SiO 2 insulator layer.A heavily doped Si substrate is used as a backgate, and the back-gate electrode is prepared by Al evaporation.The fabrication process for nanotube FET devices has been described in detail in our previous studies [9,10].The transport property measurements are carried out at room temperature in a vacuum using a semiconductor parameter analyzer (Agilent 4155C).Copyright © 2011 SciRes.OJM are observed.One strong peak at 1476 cm -1 corresponds to the intermolecular Raman active frequency (tangential mode) Ag (2) of C 60 molecules, and the other weak peak at 1437 cm -1 near the D-band can be attributed to the Hg (7) mode of C 60 molecules.A very weak peak for the Ag (2) mode observed in C 59 N encapsulated DWNTs compared with that observed for C 60 encapsulated DWNTs may possibly be explained in terms of their different electronic structure.

Transport Properties of C 59 N Encapsulated DWNTs
The electrical transports properties of DWNTs are measured based on an FET configuration, as schematically illustrated in the inset of  evidently that there is the strong electron transfer from C 59 N to the encapsulated DWNT; as a result, the electron density of conduction band of DWNT is strongly modified.In contrast, for the C 60 -encapsulation, the transport characteristic is completely opposite to that observed for the C 59 N encapsulated DWNTs, and the unipolar p-type semiconducting DWNTs are obtained, as given in  can be understood by the charge transfer from C 59 N to DWNTs, which shifts the Fermi level towards the conduction band.In other words, the Fermi level of DWNTs is strongly modified by the interaction between DWNTs and C 59 N azafullerene.

Summary
TEM observations and Raman spectra have confirmed that the C 59 N azafullerene has successfully been filled inside DWNTs.Electrical transport measurements indicate that pristine DWNTs can exhibit the amibipolar semiconducting behavior.On the other hand, unipolar n-type semiconducting DWNTs are significantly observed after the C 59 N encapsulation, proving the electronic structure of DWNTs is strongly modified.Compared with C 60 with the electron accepting behavior, the C 59 N azafullerene shows the interesting electron-donating behavior, which is confirmed by photoelectron emission spectra.The DWNTs are a promising candidate for creating FET devices showing various properties including p-type, n-type and ambipolar behaviors.

Figure 1 ( 2 .Figure 1 .
Figure 1(a) shows the mass spectrum of synthesized C 59 N, in which the peak at 722 is the most distinct, corresponding to the C 59 N azafullerene.While the peak at 720 is well known for the C 60 fullerene, its peak density is much lower than that of C 59 N, suggesting that C 59 N is the dominant material in the sample.Such C 59 N molecules are encapsulated into DWNTs by a vapour diffusion method.Figures 1(b) and (c) give TEM images of individual pristine DWNT and DWNT filled with the C 59 N molecules.In Figure 1(b), a pristine DWNT with inner diameter 4 nm and outer diameter 4.8 nm is clearly observed.In contrast, Figure 1(c) shows the TEM image of an individual DWNT filled with the C 59 N molecules.Our results indicate that they have been filled in DWNTs in the amorphous-phase state (indicated by arrows), which is similar to the case of C 60 encapsulated DWNTs[11], but is different from the chain-like C 59 N observed in SWNTs[12] because of large diameter of DWNTs.

Figure 3 .Figure 3 .
Our measurements demonstrate that the transport properties of pristine semiconducting DWNTs show an ambipolar behavior, as shown in The characteristics of source-drain current versus gate voltage (I DS -V G ) curves indicate that the device conducts either electrons or holes depending on the gate bias when different source-drain voltages (V DS ) from 0 to 1 V are applied.The region on the left-hand for V G  -20 V corresponds to the p-type conduction and the n-type conductance is observed in the right-hand region for V G  -20 V.The current-voltage characteristics of the device indicate that the source-drain current increases strongly with increasing the negative gate voltage in the p-channel and increasing the positive gate voltage in the n-channel, respectively.Particularly, the observed saturated conductance in the p-channel typically appears to be two or three times larger than that observed in the n-channel for pristine DWNT-FETs.In contrast, unipolar n-type DWNT-FETs can be obtained by the C 59 N-encapsulation, as shown in Figure4(a), where the characteristics of I DS -V G measured at different V DS ranging from 0 to 0.1 V in steps of 0.02 V indicate clearly that the FET device exhibits an excellent n-type semiconducting behavior, and no amibipolar behavior is found due to the strong electron-donating property of C 59 N. The threshold voltage (V th ) necessary to completely deplete the nanotubes is about -20 V at V DS = 0.1 V, which is similar to the value of V th for the n-type region in the pristine ambipolar DWNTs.To further estimate the performance of the n-type FET device, the I DS -V DS curves are measured with V DS ranging from -0.1 to 0.1 V by applying different gate voltages from -30 V to 20 V, as shown in Figure4(b).The conductance of device is significantly suppressed by decreasing the gate voltages from 20 V until the gate voltage reaches about -40 V, which also exhibits a reproducible characteristic for the n-type nanotube FETs, being consistent with the result in Figure4(a).The above result demonstrates

Figure 3 .
Figure 3. (a) Drain-source current versus gate voltage (I DS -V G ) characteristics for an amibipolar semiconducting DWNT-FET measured with bias voltage (V DS ) ranging from 0 to 1 V in steps of 0.2 V.The inset shows the FET configuration.

Fig- ures 4 Figure 4 .
Figure 4. (a) I DS -V G characteristics for an n-type C 59 N encapsulated DWNT with V DS ranging from 0 to 0.1 V in steps of 0.02 V. (b) I DS -V DS curves for an n-type C 59 N encapsulated DWNT with V G ranging from 20 to -30 V. (c) I DS -V G characteristics for a p-type C 60 encapsulated DWNT with V DS ranging from -1 to 1 V in steps of 0.2 V. (c) I DS -V DS output characteristics for a p-type C 60 encapsulated DWNT measured with V G ranging from -40 to 40 V.

Figure 5 .
Figure 5. Photoelectron emission spectra of C 59 N and C 60 .