Polysilane-Wrapped Carbon and Boron-Nitride Nanotubes: Effects of

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Polysilane Wrapped Carbon and Boron-Nitride Nanotubes: Effects of B or P doping on Electron Transport Xiu Yan Liang, Guiling Zhang, Yan Shang, Zhao-di Yang, and Xiao Cheng Zeng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11979 • Publication Date (Web): 22 Feb 2016 Downloaded from http://pubs.acs.org on March 3, 2016

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The Journal of Physical Chemistry

Polysilane Wrapped Carbon and Boron-Nitride Nanotubes: Effects of B or P doping on Electron Transport Xiu Yan Liang,1 Guiling Zhang,*,1 Yan Shang,1 Zhao-Di Yang,1 and Xiao Cheng Zeng*,2 1

College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150080, China. 2

Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588, USA

ABSTRACT: We perform a comprehensive study of effects of wrapping either undoped or doped polysilane (PSi) around the outer surface of a carbon nanotube (CNT) or a boron-nitride nanotube (BNNT) using density functional theory and nonequilibrium Green’s function calculations. For CNT, because the wrapping of either undoped PSi or B-doped PSi has little effect on the electronic band structure near the Fermi surface Ef, the conductivity of the wrapped CNT is still dominated by the CNT π state. This behavior is also confirmed by using the two-probe device model system with a unit cell of undoped or B-doped PSi-wrapped CNT sandwiched between two Au electrodes. For P-doped PSi/CNT, the P dopant can introduce electron donor state in the valence band. However, such a P-dopant effect is still suppressed and the conductivity is still controlled by the CNT π state based on the two-probe device computation. Contrary to CNT, the PSi-wrapped BNNT can markedly influence the band structure of the BNNT. The wrapping of either undoped or doped PSi can significantly increase the conductivity. For undoped PSi/BNNT, the valence band stems from the BNNT π state while the conduction band stems from the PSi σ state. For B-doped PSi/BNNT, B atoms introduce an electron-acceptor band just above the Ef while in the P-doped PSi/BNNT, P atoms introduce an electron-donor band just below the Ef. For the B-doped PSi/BNNT two-probe system, the B-dopant state can participate in electron transport and exhibit a notable negative differential resistance (NDR) feature. However, for the P-doped PSi/BNNT two-probe system, the P-dopant contribution is suppressed, akin to the P-doped PSi/CNT system.

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Carbon

nanotubes

and

boron

nitride

nanotubes

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are

prototype

quasi-one-dimensional nanostructures with unique electronic, optical, mechanical properties. CNTs can be either metallic or semiconducting while BNNTs are wide-gap semiconductor.1-4 Recently, noncovalent modification of CNTs and BNNTs has attracted particular interest as it is an effective approach to tailor CNTs’ (or BNNTs’) properties while preserving many intrinsic properties of the host.5 Previous experimental studies have shown that various π-conjugated conducting polymers such as

poly(p-phenylenevinylene)

(PPV),6-8

poly(vinylpyrrolidone)

(PVP),9,10

polythiophene (PT),11-16 polypyrrole (PPy),17,18 poly(phenylethynylenes) (PPE),19-25 and Poly(phenylacetylene)s (PPA)26-30 can wrap around the outer surface of either CNTs or BNNTs to form polymer/NT nanocomposites. Experiments reveal that the conductivity of polymer/CNT nanocomposite can be increased by several orders of magnitude compare with that of neat polymer.31-34 The band gaps of polymer/BNNT nanocomposites can be also tuned through noncovalent functionalization of organic molecules.35-38 Many theoretical efforts have been devoted to understanding the wrapping mechanisms by using molecular dynamics (MD) simulations and density functional theory (DFT) computation.39-54 Thus far, however, theoretical study of the electronic and transport properties of the polymer/NT nanocomposites is scarce. Very recently, Naito, M. et al. reported a novel composite nanostructure by wrapping polysilanes (PSi) around outer wall of CNT (PSi/CNT) in a helical fashion, using a mechanochemical high-speed vibration milling technique.55,56 PSi have been utilized in device applications such as chemical or biological sensors,57,58 field-effect transistors,59 solar cells,60 as well as lithium battery anodes.61 Contrary to the π-conjugated carbon-based polymers, charge carrier transport in PSi through delocalized σ-conjugated bonds. Such a unique electronic structure would inevitably 2


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bring about intriguing electron transport properties. For example, room temperature negative differential resistance (NDR) has been observed in PSi.62-64 On the other hand, no NDR phenomenon has been seen in any CNT-based nanomaterials.65-69 Note that the use of semiconducting PSi in electronic devices is strongly conditioned by the presence of heteroatom dopant which is able to move the Fermi level (Ef) close to the band edges.70-73 Boron and phosphorus are well known to be the most common seemliness heteroatom dopants for PSi to form p-type and n-type semiconductors, respectively, because they induce little damage of the silicon backbones as shown from both experimental74,75 and theoretical76 studies. Our previous studies have shown that the B- or P-doping upon oligosilanes can significantly affect the electronic and transport properties, e.g., resulting in multiple NDR behavior or rectifying effect.77-79 It is expected that new properties could be introduced by wrapping undoped or doped PSi on CNT or BNNT surfaces. In this work, we investigate the electronic and transport properties of undoped and doped PSi/CNT and PSi/BNNT nanocomposites using DFT and nonequilibrium Green’s function (NEGF) methods. For computing electronic structures, the infinitely long PSi/CNT and PSi/BNNT are modeled by using the periodic condition in the axial direction (z direction). Here, metallic CNT (4, 4) and insulating BNNT (4, 4) are selected as the host NT to be wrapped by PSi. Specifically, a 28-backbone-atom long polymer is wrapped helically onto the surface of a 272-atom long NT, forming a unit cell (Figure 1). Here the pitch length, p, is just the same as a lattice parameter of the unit cell, L, in the axial direction. The cell dimensions in the x and y directions are set to be 32 Å, large enough to avoid direct PSi/NT-PSi/NT interactions. The periodic systems are fully optimized until the maximum absolute force is less than 0.02 eV/Å. The computations 3



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are carried out based on combination of DFT and the NEGF methods implemented in the Atomistix ToolKit (ATK) software package.80-83 The generalized gradient approximation (GGA) within the Perdew-Burke-Ernzerhof (PBE) formalism is employed to describe the exchange correlations between electrons. A double-ζ basis functional with polarization (DZP) is used for all atoms. The (1×1×100) k-point in string Brillouin zone (x, y, z directions, respectively) is used. A cutoff energy of 150 Ry is applied to describe the periodic wave function. Only the Γ point is adopted for the k-point sampling. Figure 1 shows optimized structures of the unit cell of twelve PSi/NT nanocomposites, including the neat undoped PSi/CNT (a), one-B-doped (per unit cell) PSi/CNT (B-a), two-B-doped PSi/CNT (BB-a), one-P doped PSi/CNT (P-a), two-P doped PSi/CNT (PP-a), and B- and P-codoped PSi/CNT (BP-a); the neat undoped PSi/BNNT (b), one-B-doped PSi/BNNT (B-b), two-B-doped PSi/BNNT (BB-b), one-P doped PSi/BNNT (P-b), two-P doped PSi/BNNT (PP-b), and B- and P-codoped PSi/BNNT (BP-b). For BB-a, BB-b, PP-a, PP-b, BP-a, and BP-b systems, the two heteroatoms are separated apart by 14 Si atoms in the polymer backbone, while the lattice parameter of unit cell, L (i.e., the helix pitch length, p), is ~40.6 Å. The helical PSis’ outer diameters R range from 6.14 to 6.35 Å (where R is defined by the Si backbone) while the inner NT cores’ diameters r range from 2.73 to 2.91 Å. So the radial separation between PSi and NT is within the range of 3.23-3.62 Å, suggesting a van der Waals interaction between the PSi sheath and the NT core. Wrapping PSi onto NT surface induces a slight shrink of the NT diameter as reflected from the smaller r compared to bare NT. The Si-Si bond lengths are about 2.39 Å, slightly longer than that in pristine linear PSi (2.37 Å)84 due to the curvature effect of 4


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the inner NT. The computed Si-B and Si-P bond lengths are about 2.02 and 2.37 Å, respectively, slightly shorter than those in linear PSi (Si-B: 2.07 Å; Si-P: 2.43 Å).76 Thermodynamic stability of the PSi-wrapped NT is evaluated by computing the interaction energy per unit cell in the wrapping state [NT + PSi → nanocomposite ∆Er]. Here, the computed values of ∆Er are negative for all nanocomposites considered, indicating exothermic energies (Table 1). Hence, wrapping undoped and doped PSi around either CNT or BNNT is thermodynamically favorable. Figure 2 displays computed band structures of bare CNT, the undoped and doped PSi/CNT nanocomposites a, B-a, BB-a, P-a, PP-a, and BP-a, respectively, as well as the corresponding Kohn-Sham orbitals near the Ef. For bare CNT, the valence band and the conduction band cross at the Ef with large dispersion, a typical metallic character (Figure 2(a)). The wrapping of undoped PSi on CNT has little effect on the overall band structure since both the valence and the conduction bands of a are still largely dictated by the CNT π states (Figure 2(b)). Figure 3(a) plots the average projected density of states (PDOS) of carbon atoms of CNT and nanocomposite a. In a, each C atom of CNT contributes a constant in PDOS near the Ef, nearly the same as that for pure CNT, while the Si atom contributes little. By comparison the band structures of a (Figure 2(b)), B-a (Figure 2(c)), and BB-a (Figure 2(d)), it can be seen that B-doping has almost no influence on the band structures. The Kohn-Sham orbitals and the PDOS (Figure 3(a)) also suggest that the B state contributes little to the valence and conduction bands in both B-a and BB-a. In contrast, in P-a and PP-a, the P atom contributes evident electron donor states to their valence band (Figure 2(e,f)). Figure 3(a) shows PDOS peaks of P state just below the Ef in P-a and PP-a. Therefore, for infinitely long P-a and PP-a, the heteroatom P may participate in the electron transport through electron hoping to CNT. However, in P-a and PP-a, the 5



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dispersion of the CNT valence band shrinks, compared to that of bare CNT and a. Hence, the P-doping can have two opposing effects on the electron transport: one is to enhance the conductivity via electron hoping from P atom to CNT, whereas another is unfavorable to the conductivity due to narrowed CNT transport channel. As expected, the band structure of BP-a is similar to that of P-a (Figure 2(g) vs. 2(e)), correlating well with the different effect of B- and P-doping. The PDOS of BP-a in Figure 3(a) is also comparable to that of P-a. To obtain more quantitative transport properties of PSi/CNT nanocomposites and analyze the effect of the undoped and doped PSi on the transport properties, we construct a model system such that a unit cell of a, B-a, BB-a, P-a, PP-a, or BP-a is sandwiched between two Au electrodes to form the two-probe devices (Figure 4). The longitudinal axis of the nanotube is denoted as z direction. The unit-cell lengths of nanocomposites (~40.6 Å) are long enough to neglect the electrode-electrode interaction. The semi-infinite electrodes are modeled by two Au(111)-(6×7) surfaces, and five layers are used for both the left and right side. Such an Au surface extends a dimension of 20.19 Å × 24.47 Å, large enough to neglect interaction among periodic images of nanocomposites in x and y directions. The nearest atomic distance between the NT and the Au is set to be 1.93 Å, very close to their covalent bond length. Transport current is computed by changing the applied bias voltage in the step of 0.2 V over the range of -1.0 to 1.0 V. The same computational parameters were chosen for the two-probe devices as for the infinitely long systems. The computed I-V curves of pure CNT and PSi/CNT nanocomposites, based on the two-probe devices, are shown in Figure 5(a). Both undoped and doped PSi/CNT nanocomposites exhibit the metallic feature with proximity currents within the bias voltage range of -1.0 to 1.0 V, indicating that the conductivity is mainly controlled by 6


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the CNT π states, consistent with the computed electronic structures. With the attached PSi (undoped or doped), the magnitude of current is slightly lower than the pure CNT. This may be attributed to the scatter effect of the PSi chain. To further investigate the PSi effect on the conductivity, the energy spectra (ES) and molecular projected self-consistent Hamiltonian (MPSH) states at 0.0 and 1.0 V bias voltage were also calculated and the results are shown in Figures 6 and 7. Since the calculated results for PSi/CNT at -1.0 V are very similar to that at 1.0 V, and thus the ES and MPSH states at -1.0 V are not given here. Regardless of the bias voltage, both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PSi/CNT nanocomposites originate from the CNT π states with a strong delocalizing character, indicating that CNT offers a major electron transport path. However, the HOMO-LUMO gap of pure CNT is slightly narrower than that of PSi/CNT nanocomposites, and hence pure CNT entails a high current. In B-a, BB-a, and BP-a, no evident B MPSH state is found in the range of -0.6 to 0.6 eV at either 0.0 (Figure 6(c, d, g)) or 1.0 V (Figure 7(c, d, g)) bias voltage, indicating that the negligible effect of the B-doping, in line with the computed electronic structures of their infinite long systems. In P-a, PP-a, and BP-a, the MPSH state of P appears at deep levels of HOMO-3 and HOMO-4 at 0.0 V bias voltage (Figure 6(e, f, g)). This differs from their infinitely-long systems, where the P state is located at shallow levels of valence bands (Figure 2(e, f, g)). This feature is mainly attributed to the electrode effect (see below). Under the 1.0 V bias voltage, the P contribution is further suppressed, no significant P state is seen in the energy region of -0.6 to 0.6 eV (Figure 7(e, f, g)). Therefore, attaching the undoped and doped PSi onto the CNT still retains the intrinsic transporting properties of CNT. Figure 8(a) shows the transmission spectra (TS) for all systems considered at 1.0 7



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V bias voltage. The current can be determined from TS in the bias windows indirectly,85 where the bias window refers to [-V/2, V/2]. In general, the larger the integral area S of the resonant peaks is within the bias window, the higher the current. Clearly, the value of S for pure CNT is slightly larger than that of PSi/CNT nanocomposites so that pure CNT exhibits a slightly higher current. The data of S for all the PSi/CNT nanocomposites are within a small range of 0.53 - 0.59, again reflecting negligible effect of B- or P-doping. In addition, we calculate electric potential distribution (see Figure 9). Clearly, the electric potential is well dispersed along the PSi/CNT nanocomposites. Thus, PSi/CNT nanocomposites are good conductors. Figure 10 shows the computed band structures of pure BNNT and PSi/BNNT nanocomposites b, B-b, BB-b, P-b, PP-b, and BP-b, together with the corresponding Kohn-Sham orbitals near Ef. Contrary to CNT, adding undoped or doped PSi onto BNNT has significant influence to the band structures. Pure BNNT possesses a band gap of Eg = 4.29 eV (Figure 10(a)). With wrapping the undoped PSi, the valence band still stems from the BNNT π state, (Figure 10(b)) while the conduction band stems from the delocalized PSi σ state. Electron transport in b may be through carrier hopping from BNNT to PSi by across an Eg = 3.83 eV band gap (Figure 10(b)). With doping B (or P) atoms to PSi, the heteroatom bands (i.e., impurity band) arises in the band gap region of PSi/BNNT, leading to a split of the band gap Eg into two small subgaps Eg1 and Eg2. As a result, the conductivity can be enhanced. B-b introduces an electron acceptor band (Figure 10(c)) and BB-b introduces two degenerate electron acceptor bands (Figure 10(d)) above the Ef, rendering both systems with p-type character. In contrast, P-b introduces an electron donor band (Figure 10(e)) and PP-b introduces two degenerate electron donor bands (Figure 10(f)) below Ef, rendering 8


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both systems n-type behavior. The doping concentration however cannot significantly affect the magnitude of subgaps Eg1 and Eg2. In B-b, BB-b, P-b, and PP-b, electron transport may correlate with an indirect intermolecular hopping pathway. For B-b and BB-b, BNNT π electron first hops to the B acceptor state by crossing the subgap Eg1 (2.51 eV in B-b and 2.47 eV in BB-b), and then transports along PSi by crossing the subgap Eg2 (1.33 eV in B-b and 1.40 eV in BB-b) (Figure 10(c, d)). For P-b and PP-b, P atom provides electron that tunnels along PSi by overcoming the subgap Eg2 (3.00 eV in P-b and 2.94 eV in PP-b), and BNNT π electron moves to the P state to compensate the P electron lost by striding the subgap Eg1 (0.84 eV in P-b and 0.85 eV in PP-b) (Figure 10(e, f)). As for BP-b, both B acceptor and P donor states appear in the band gap region of PSi/BNNT (Figure 10(g)), and consequently more transport channels may coexist in BP-b including electron hopping from BNNT to B, from P to BNNT, and from P to B. Figure 3(b) shows computed average PDOS of each atom for pure BNNT and PSi/BNNT nanocomposites. In b, the BNNT π orbitals still dominate the valence band, while the Si σ orbitals contribute a peak above Ef, as a part of conduction band. For B-b, BB-b, and BP-b, sharp PDOS peaks from B are located just above Ef, while for P-b, PP-b, and BP-b, sharp PDOS peaks from P are located just below Ef. Transport properties of a unit cell of b, B-b, BB-b, P-b, PP-b, and BP-b are also computed by using the two-probe device as for CNT nanocomposites (Figure 4). The computed I−V curves are shown in Figure 5(b). For undoped and doped PSi/BNNT nanocomposites, the currents are less than 0.6 µA, far less than those of PSi/CNT nanocomposites (~104 µA). No evident TS peaks are found within the bias window in Figure 8(b). Wrapping undoped or doped PSi can increase the conductivity. Figures 9



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11 and 12 show computed ES, as well as the MPSH states of pure BNNT and PSi/BNNT nanocomposites at 0.0 and 1.0 V bias voltage, respectively. Notably, when BNNT and PSi/BNNT nanocomposites are sandwiched between two Au electrodes, the BNNT valence states are localized strongly onto one end (Figures 11 and 12), implying a blocking effect of BNNT in electron transport. For all the PSi/BNNT nanocomposites considered (undoped or doped) at 0.0 V bias voltage, the BNNT π state serves as the valence state and lies just below Ef. At 1.0 V bias voltage, the electric potential cannot be effectively dispersed along BNNT due to the blocking character. As a result, the BNNT valence states located at the right side are pushed upwards above Ef (Figure 11 vs. 12). For b at 1.0 V bias voltage, electron is injected from the right electrode to the BNNT valence state, then transfers to the PSi σ state and transports along the PSi chain to reach the left electrode (Figure 12(b)). This behavior is consistent with the band structures of the corresponding infinite system. The calculated electric potential distribution (Figure 13) also suggests that the BNNT entails a block effect on electron transport while the PSi helix can offer a transport path. For one-B-atom doped B-b at 0.0 V bias voltage, B contributes an electron acceptor state above Ef and below the PSi state (Figure 11(c)). Similarly, for two-B-atom doped BB-b, two degenerate electron acceptor states arise (Figure 11(d)), in line with the band structure of corresponding infinitely long system. However, for B-b at 1.0 V bias voltage, the BNNT valence states are upshifted and the B acceptor state is pushed downward below Ef (Figure 12(c)). For BB-b, the two degenerate B 10


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states split at 1.0 V bias voltage: the right B state is still located between the BNNT valence and the PSi conduction state, while the left B state shifts below the Ef (Figure 12(d)). We also calculate the ES and MPSH states for B-b and BB-b at -1.0 V bias voltage (see Supporting Information Figure S1). Contrary to the 1.0 bias voltage for B-b, the B acceptor state still lies between the BNNT valence and the PSi conduction states. For BB-b the left B state is still located between the BNNT valence and the PSi conduction state, while the right B state shifts below Ef. Thus, for B-b and BB-b, the B state close to the negative electrode may participate in electron transport; this B state may first accept electrons from the BNNT valence state and then the electrons transport along the PSi. A very interesting feature is the NDR behavior for B-b and BB-b. Figure 5(b) shows a clear NDR feature when the applied bias voltages are in the range of 0.6 - 1.0 V and ±0.2 - ±0.6 V for B-b and BB-b, respectively. The NDR behavior can be explained based on the ES (Figure S2). B-b has a smaller subgap for electron transport from BNNT valence to PSi conduction state at 0.8 V bias voltage, compared to those at 0.6 and 1.0 V bias voltage. Similarly, for BB-b at -0.4 V bias voltage, electron needs to overcome smaller subgaps to transfer from BNNT valence to the left B acceptor state and then transports along PSi, compared to those at -0.2 and -0.6 V bias voltages. Hence, B-b and BB-b can display the NDR behavior within the bias voltage range of -1.0 to 1.0 V. It is worthy of noting that b, P-b, and PP-b show similar ES and MPSH states regardless of the bias voltage, suggesting that the P-doping has little effect on electron 11



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transport. Indeed, at 0.0 V bias voltage, the P level is deeply below Ef as HOMO-9 (Figure 11(e)). This is in stark contrast to the band structures of the corresponding infinitely long systems (Figure 10(e)) for which the P state contributes to the shallow levels of HOMO, HOMO-1, and HOMO-2. This deference mainly arises from the influence of Au electrodes. To investigate the electrode effect, we also construct two two-probe device models of P-b by setting the P-b-Au distances to be 3.5 and 5.0 Å, respectively. In the former model, the MPSH state of P appears at HOMO-6, while in the latter, the MPSH state of P shifts upward to become HOMO-4. Thus, the Au electrode can suppress the P contribution. This may be a reason why P-doping plays little role in electron transport, and P-b and PP-b show comparable conductivity as undoped b (Figure 5(b)). Likewise, BP-b shows similar conductivity as B-b (Figure 5(b)) due to the different effect of B- and P- doping. Unlike in the case of the linear B- and P-codoped oligosilanes,77-79 BP-b cannot exhibit rectifying effect.

CONCLUSIONS Effects of wrapping undoped and doped PSi on the outer wall of CNT and BNNT are investigated by using density functional theory and nonequilibrium Green’s function calculations. It is found that the attachment of either undoped PSi or B-doped PSi upon CNT has little effect on the band structure near Ef. Both valence and conduction bands are dominated by the CNT π state. However, when the PSi is doped by P atom, the P atom can contribute electron donor state hybridizing with the valence CNT band. In the PSi/CNT two-probe devices, the conductivity is also mainly 12


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controlled by the CNT π states. The P contribution is suppressed at the valence state in the two-probe device due to the influence of the Au electrodes. Contrary to CNT, the wrapping of PSi onto BNNT can notably influence the band structure. The wrapping of either undoped or doped PSi can increase the conductivity. With the undoped PSi, the valence band stems from the BNNT π state, while the conduction band stems from the PSi σ state. Upon doping with B or P atoms to PSi, the heteroatom bands arise in the band gap region of PSi/BNNT, resulting in a split of the band gap into two subgaps. The B-doping introduces an electron acceptor band just above Ef, while P-doping introduces an electron donor band just below Ef. In the two-probe devices, the BNNT valence state is raised upwards above Ef at 1.0 eV from below Ef at 0.0 V. This is mainly because at 1.0 V bias voltage, the electric potential on the BNNT valence state cannot be effectively dispersed along BNNT due to a BNNT blocking behavior. In contrast, the electric potential on the CNT valence state at 1.0 V bias voltage can be well dispersed due to the metallic character of CNT. Lastly, for the B-doped PSi/BNNT two-probe systems, the B state can participate in electron transport and exhibit a clear NDR feature. But in the P-doped PSi/BNNT two-probe systems, the P contribution is suppressed, as the P-doped PSi/CNT system.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Computed energy spectra of B-a and BB-a two-probe devices at -1.0 V bias voltage and the MPSH states corresponding to the energy levels near Ef; Computed band structure of B-b at 0.6, 0.8, and 1.0 V bias voltages and of BB-b at -0.2, -0.4, 13



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and -0.6 V bias voltages. This information is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Author Contributions Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT GLZ is supported by a grant from the NSFC (grant No. 51473042). XCZ is supported by a grant from the National Science Foundation (NSF) through the Nebraska Materials Research Science and Engineering Center (MRSEC) (grant No. DMR-1420645). SY is supported by the SF for youth reserve talent of Harbin of China (grant No. 2014RFQXJ075), the NSF of Heilongjiang Province of China (grant No. E201236), and the foundation for the department of education of Heilongjiang Province of China (grant No. 12521074). ZDY is supported by a scientific start-up fund of National Ministry of Education for returned overseas (grant No. [2014]1685) and the NSF of Heilongjiang Province of China (grant No. LC2015005).

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Table 1 Computed total energies ETol and interaction energies ∆Er per unit cell of nanocomposites Species

ETol/eV

∆Er/eV

a B-a BB-a P-a PP-a BP-a

-53336.0658 -53224.5130 -53109.2376 -53407.8600 -53478.3578 -53293.0836

-6.98 -7.30 -7.82 -7.06 -7.16 -7.39

b B-b BB-b P-b PP-b BP-b

-53691.3739 -53588.3878 -53461.4565 -53760.9504 -53830.2735 -53646.0097

-3.37 -3.30 -3.61 -3.64 -3.57 -3.84

20


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Figure 1. Optimized structures of unit cells of (a) CNT and PSi/CNT nanocomposites a, B-a, BB-a, P-a, PP-a, BP-a and of (b) BNNT and PSi/BNNT nanocomposites b, B-b, BB-b, P-b, PP-b, BP-b.

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Figure 2. Computed band structures of CNT, a, B-a, BB-a, P-a, PP-a, and BP-a and the Kohn-Sham orbitals corresponding to the energy levels (highlighted in color lines) near Ef at the Γ point. The iso-surface value is 0.005 (e/Å3).

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Figure 3. Computed projected density of states (PDOS) of (a) CNT and PSi/CNT nanocomposites a, B-a, BB-a, P-a, PP-a, BP-a and of (b) BNNT and PSi/BNNT nanocomposites b, B-b, BB-b, P-b, PP-b, BP-b. 23



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Figure 4. Two-probe devices exemplified by one unit cell of a or b sandwiched between two Au(111)-(6×7) electrodes, five layers were used for the left and right electrodes.

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Figure 5. Computed I−V curves of (a) CNT, a, B-a, BB-a, P-a, PP-a, and BP-a two-probe devices and of (b) BNNT, b, B-b, BB-b, P-b, PP-b, and BP-b two-probe devices.

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Figure 6. Computed energy spectra of CNT, a, B-a, BB-a, P-a, PP-a, and BP-a two-probe devices at 0.0 V bias voltage and the MPSH states corresponding to the energy levels (highlighted in color lines) near Ef. The iso-surface value is 0.005 (e/Å3). 26


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Figure 7. Computed energy spectra of CNT, a, B-a, BB-a, P-a, PP-a, and BP-a two-probe devices at 1.0 V bias voltage and the MPSH states corresponding to the energy levels (highlighted in color lines) near Ef. The iso-surface value is 0.005 (e/Å3).

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2

V=1.0 V

S=0.71

CNT 0

a

S=0.55

V=1.0 V

BB-a

S=0.53

V=1.0 V

PP-a

S=0.58

V=1.0 V

2 S=0.54

B-a

Transmission

2

0

2 V=1.0 V

0

0

2

2

S=0.59

P-a

V=1.0 V

0 2

0 -1

BP-a

0

0 Energy/eV

1

Energy/eV

V=1.0 V

S=0.59

0 -1

1

(a) 2 BNNT

2

V=1.0 V

0

B-b

2

V=1.0 V

0

V=1.0 V

BB-b

V=1.0 V

PP-b

V=1.0 V

0

2

P-b

2

V=1.0 V

0 2

0 -4

BP-b

0 -4

b

0

2

Transmission

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-2

V=1.0 V

0 Energy/eV

2

-2

0 Energy/eV

2

4

4

(b) Figure 8. Transmission spectra (TS) of (a) CNT, a, B-a, BB-a, P-a, PP-a, and BP-a two-probe devices and of (b) BNNT, b, B-b, BB-b, P-b, PP-b, and BP-b two-probe devices.

28


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Figure 9. Computed contour plot of potential distribution for CNT, a, B-a, BB-a, P-a, PP-a, and BP-a two-probe devices at 0.0 and 1.0 V.

29



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Figure 10. Computed band structures of pure BNNT, b, B-b, BB-b, P-b, PP-b, and BP-b and the Kohn-Sham orbitals corresponding to the energy levels (highlighted in color lines) near Ef at the Γ point. The iso-surface value is 0.005 (e/Å3). 30


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Figure 11. Computed energy spectra of BNNT, b, B-b, BB-b, P-b, PP-b, and BP-b two-probe devices at 0.0 V bias voltage and the MPSH states corresponding to the energy levels (highlighted in color lines) near Ef. The iso-surface value is 0.005 (e/Å3). 31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 12. Computed energy spectra of BNNT, b, B-b, BB-b, P-b, PP-b, and BP-b two-probe devices at 1.0 V bias voltage and the MPSH states corresponding to the energy levels (highlighted in color lines) near Ef. The iso-surface value is 0.005 (e/Å3). 32


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Figure 13. Computed contour plot of potential distribution for BNNT, b, B-b, BB-b, P-b, PP-b, and BP-b two-probe devices at 0.0 and 1.0 V.

33



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TOC Graphic

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