Field Emission Properties of Carbon Nanotubes with Boron Doping and H 2 O Adsorption

Gas adsorption and atom doping usually present when preparing carbon nanotubes (CNTs) and can affect the field emission properties of carbon nanotubes. H 2 O molecule and boron atom are the most important adsorbates, respectively. Using ab-initio calculations, we have investigated the electron field emission performance of CNTs simultaneously adsorbed with one H 2 O molecule and doped with one boron atom (BCNT+H 2 O) in this paper. The results indicate that the electrons localize at the top of BCNT+H 2 Oand the electronic density of states (DOS) around the Fermi level is enhanced obviously. It is expected that BCNT+H 2 O will be more propitious to the field emission of electrons based on the calculations of DOS, HOMO/LUMO, and Mulliken charge population.


Introduction
Carbon nanotubes (CNTs) have attracted much attention due to their high aspect ratio and remarkable mechanical, electronic, and chemical properties [1][2][3].The quasi-one-dimensional structures and unique electric properties make CNTs have a lot of potential applications in different fields [4][5][6].Especially, because of the excellent field emission properties, CNTs have been promising candidates for flat-panel displays [7,8].Up to now, many experimental [9] and theoretical [10][11][12] studies have shown that the gas adsorption is of great advantage to the electron emission of the CNTs.For example, Kim et al. calculated the DOS and field emission properties of closed CNTs under electric field [13], Maiti et al. explained the reason why H 2 O adsorption on the top of CNTs can improve the electron emission properties [14], and Zhang et al. pointed out that CNTs doped with boron and nitrogen are easy to emit electrons [15].
Naturally, some kinds of gas can markedly change the energy band structure of carbon nanotubes, whereas the work of simultaneous doping and adsorption is rarely reported until now.Therefore, it is very important to study the emission properties of CNTs doped with other atoms and adsorbed with gases under electric field.In this paper we adopted the density functional theory (DFT) to investigate the performances of BCNT+H 2 O.

Models and Calculation Methods
A single-walled armchair (5,5) CNT with a cap (Figure 1) is adopted.The cap is a hemisphere (30 carbon atoms) of C 60 at one end of the CNT and the dangling bonds at the other end are saturated by hydrogen atoms.The position of boron dope is selected at the top layer of the CNT and the distance of H 2 O adsorption from the top pentagon is about 0.3 nm.Therefore the whole system contains 93 atoms.
All the calculations were carried out by using first-principle DFT provided by DMOL 3 code [16].The geometrical optimization was based on the local density approximation (LDA) during self-consistent field iteration with the Perdew-Wang parameterization of the local exchange correlation energy.The generalized gradient approximation (GGA) was adopted to calculate the total energy and various electronic properties with the Perdew-Wang-91 (PW91) function [17].The all-electron Kohn-Sham wave functions were expanded in the local atomic orbital basis set with orbital cutoff of 4.5 Å. Self-consistent field procedure was done with a convergence criterion of 10 −5 a.u. on the energy and electron density.

Results and Discussion
In order to study the electronic field emission properties of BCNT+H 2 O, a uniform external electric field with the direction from the top to the end of the CNT is applied, whose intensities are  = 0 and  = 10 eV/nm, respectively.In real field emission conditions, a short tip of the structures is considered.The electric field gradient along CNTs axis is ignored and the whole structure is supposed to be in a uniform external electric field [17].
The adsorption distance and adsorption energy are shown in Table 1.The adsorption energy ( ad ) under the different electric field is defined in (1) and (2), respectively.Consider the following: ad(=10 eV/nm) =  CNTs(=10 eV/nm)  1, we can see that the adsorption distance is about 0.3 nm and the adsorption energy is very small for BCNT+H 2 O and CNT+H 2 O systems in the absence of electric field.When the electric field increases to 10 eV/nm, their adsorption energy is remarkably enhanced, while their adsorption distance decreases slightly.Moreover, Table 1 shows that the adsorption energy of BCNT+H 2 O is much higher than that of CNT+H 2 O under the same strength of electric field, which indicates that the structure of BCNT+H 2 O is more stable [18] than that in a electron transmitter.
Figure 2 presents the DOS of the two systems under the different electric fields and the local density of states (LDOS) of the boron atom and the nearest three carbon atoms on the top of BCNT+H 2 O under  = 10 eV/nm.Obviously, the DOS plots of the two systems shift towards the low energy direction and their corresponding peak positions of antibond states are more close to   when the electric fields increase from 0 to 10 eV/nm.This indicates that the probability of electron occurrence at antibond states increase, which is well consistent with Kim's conclusion [13].
To determine the pseudogap, an advanced peak separation technique [19] is adopted.Furthermore, the DOS at   is also calculated, as listed in Table 2.When the electric fields increase from 0 to 10 eV/nm, the DOS of the two systems at   enhances by a big margin.Especially the DOS of BCNT+H 2 O is about 179% that of CNT+H 2 O at  = 10 eV/nm.The reasons can be found from Figure 2  To further investigate the field emission property of BCNT+H 2 O, we have performed the electronic orbital analysis.As shown in Figure 3, the HOMOs of CNT+H 2 O and BCNT+H 2 O are all localized at their body and the distribution is very similar no matter whether the electric field is applied or not.However, the LUMO of later is mainly localized at the tip of CNTs at  = 10 eV/nm.This means that there are more electrons congregating on the top region of BCNT+H 2 O. Once the stronger electric field is applied these electrons immediately emit to the surrounding space.It can be seen that the conclusions of orbital analysis are well consistent with that of the discussion above.
When a boron atom substitutes a carbon atom in the top layer, the charges distribution and the electronic structures change accordingly.Figure 4 exhibits the results of Mulliken population analysis for the CNT+H 2 O and BCNT+H 2 O in the presence of  = 10eV/nm.We can see that the boron atom carries a little negative charge and each of the nearest three carbon atoms carries much more negative charges than other atoms located in the first and second layer [20].The analysis results show that the boron doping can cause more electrons aggregation at the top.Essentially, the electron congregation becomes more obvious with the increase of applied electric field and extra electrons on the tip of system will congregate in the conduction band.Consequently the Fermi level shifts toward the vacuum level.This will lower the potential barrier of BCNT+H 2 O tip and make the field emission more easily.All of these discussions indicate that BCNT+H 2 O can enhance the electron field emission.

Conclusions
In summary, we investigate the electronic structure and physical mechanism of the fieldemission of the capped SWNT which are simultaneously adsorbed with one H 2 O molecule and doped by a boron atom in the top layer.The calculations and analysis confirm that BCNT+H 2 O has a higher DOS at   than CNT+H 2 O.Although the HOMO of BCNT+H 2 O changes hardly with and without the applied electric field, the LUMO is highly localized at the top.Furthermore, the Mulliken population analysis shows that a lot of electrons congregate at the cap of BCNT+H 2 O. Naturally, present results of the LDOS, the pseudogap, the HOMO/LUMO, and Mulliken population analysis congruously indicate that

Figure 1 :
Figure 1: The structures of BCNT+H 2 O and CNT+H 2 O. (a) and (b) are the top views; (c) and (d) are the side views of BCNT+H 2 O and CNT+H 2 O, respectively.

Figure 2 :
Figure 2: DOS plots of CNT+H 2 O and BCNT+H 2 O under the different electric field.(a) and (c) are DOS of CNT+H 2 O and BCNT+H 2 O without electric field, (b) and (d) are DOS of CNT+H 2 O and BCNT+H 2 O under  = 10 eV/nm, and (e) is LDOS of a boron and 3 carbon atoms of BCNT+H 2 O under  = 10 eV/nm, respectively.

Table 1 :
The adsorption distance (nm) and adsorption energy (eV) of CNT + H 2 O and BCNT+H 2 O systems with and without electric field (eV/nm).
where the  CNTs(=0 ) ,  H 2 O(=0) , and  CNT+H 2 O(=0) are the total energy of the pure CNT, the adsorbate (a H 2 O molecule), and the adsorbed CNT system, respectively, under the strength of electric field of 0 eV/nm, and where  CNTs(=10 eV/nm) and  CNT+H 2 O(=10 eV/nm) have the same meaning under the strength of electric field of 10 eV/nm.From Table