Monoatomic Layer Electronics Constructed by Graphene and Boron

Jul 24, 2012 - The electron transport properties of devices based on such GNR-BNNR-GNR heterostructures are systematically investigated. The effects o...
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Monoatomic Layer Electronics Constructed by Graphene and Boron Nitride Nanoribbons J. C. Dong and H. Li* Key Laboratory for Liquid−Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, People’s Republic of China S Supporting Information *

ABSTRACT: A new and simple kind of heterostructure nanoelectronics, which are free of metal electrodes, is constructed by a boron nitride nanoribbon (BNNR) seamlessly connected between two pieces of graphene nanoribbons (GNRs). The electron transport properties of devices based on such GNR-BNNR-GNR heterostructures are systematically investigated. The effects of vacancy, chirality, width, and boundary of nanoribbons on the electron transport properties of these devices are discussed. Energy gaps over 1 eV are observed in the electron transmission spectra of devices composed of these heterojunctions, indicating their pronounced field effect transistor (FET) characters. Removing hydrogen atoms at the boron edge of zigzag BNNR can result in 100% electron spin polarization in the GNR-BNNR-GNR FET. This study has implications for developing high-performance monatomic layer nanoelectronics with simple heterojunctions.

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INTRODUCTION In recent years, much attention is paid to developing graphene electronics due to the peculiar electronic properties of graphene, such as the zero band gap semiconducting character and its massless Dirac fermions.1−6 Among these electronics, graphene field effect transistors (FETs) occupy a very special position because FETs are the fundamental components of integrated circuits. Many researchers have reported the fabrication of graphene FETs and their extremely high carrier mobilities as expected.7−10 However, the exceedingly small on− off ratios of these FETs induced by the nearly zero band gap of large area graphene seriously restrict their potential applications in logics. Therefore, opening a sizable band gap for graphene becomes an urgent issue that needs to be solved. Theoretical and experimental studies show that cutting graphene into GNRs is capable of opening band gaps inversely proportional to the ribbon width.11−14 A band gap can also be created by applying an electric field perpendicular to bilayer graphene.15−17 Other methods to generate a band gap of graphene are chemical decorations of graphene surfaces and edges.18−20 Although the above methods can enhance the switching performance of graphene FETs to some extent, all these graphene FETs use metal electrodes as the source and drain, which can not only cause unstable performance due to the © 2012 American Chemical Society

interfaces between graphene and electrodes but also make graphene FETs rather difficult to be fabricated and integrated into circuits. Therefore, developing a new device structure free of these limitations is of great significance. Analogous to graphene, hexagonal BN sheets have honeycomb lattice structures, in which B and N atoms are alternatively arranged in the form of sp2 hybridization.21 However, BN sheets are semiconductors with a band gap of about 4.5 eV. The electronic properties of BNNRs are also found to be rather different from those of GNRs. Both armchair and zigzag BNNRs with hydrogen passivation of edge atoms have band gaps larger than 3 eV that are not very susceptible to the ribbon width.22 Moreover, hydrogen passivation of different edges can result in half-metallic, antiferromagnetic, and ferromagnetic properties for zigzag BNNRs.23 Therefore, it would be of great interest to develop nanoelectronics with heterojunctions composed of GNRs, and BNNRs. On the one hand, excellent experimental work has been done to successfully synthesize monolayers of hybridized graphene and BN sheet with band gaps over 1 eV.24 On the other hand, Received: May 1, 2012 Revised: July 23, 2012 Published: July 24, 2012 17259

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Figure 1. Schematics of GNR-z(a)BNNR-GNR two-probe devices and three-terminal FETs. (a) Structure of GNR-5zBNNR6-GNR two-probe device. The source and drain electrodes are marked by yellow. The dashed box denotes the unit cell of the BNNR. The whole ribbon is situated in a supercell represented by the outmost solid rectangle. (b) Structure of GNR-3aBNNR5-GNR two-probe device. (c) Structure of GNR-5zBNNR6GNR three-terminal FET. The gate electrode is denoted by the gray box with a thickness of 0.5 Å. The purple box represents the dielectric layer with a dielectric constant of 4 and thickness of 3 Å. (d) Structure of GNR-3aBNNR5-GNR three-terminal FET.

theoretical studies have been carried out on tuning the band gap of the graphene sheet using BN flakes or regulating the electronic structure of the BN sheet with graphene flakes.25−27 Recent calculations demonstrated that the electronic and magnetic properties of BNNRs can be effectively tuned by carbon doping.28 In addition, the electronic properties of nanoribbons composed of GNRs and BNNRs, which are arranged in parallel, have also been studied.29−32 However, little has been done to investigate the electron transport properties of nanodevices with heterojunctions consisting of a BNNR seamlessly connected between two pieces of GNRs. How do the GRN-BNNR boundaries, ribbon chirality, and defects affect the electron transport properties of these devices? Can large energy gaps occur in the electron transmission spectra of devices with these heterojunctions? All these questions are essential for monatomic layer electronics and need to be thoroughly studied. In this article, we propose a very simple kind of heterostructure for nanoelectronics composed of a BNNR seamlessly connected between two pieces of GNRs. Systematic first-principles calculations are performed to study the electron transport properties of both two-probe devices and three terminal FETs with these heterojunctions.

The quantum electron transport properties of these devices are calculated using the Nonequilibrium Green Function combined with the DFT by the softpackage Atomistix ToolKit.33,34 The electronic structures of source and drain electrodes are obtained with periodic boundary conditions in all directions. Open boundary conditions in the z direction are applied to calculating the electronic structure of the channel region. For two-probe device calculations, the boundary conditions in x and y directions of the channel region are periodic, while for three terminal FETs, the boundary conditions are determined by the negative gradient of the Hartree potential. Double-zeta single polarized basis sets are used as the local atomic numerical orbitals. The generalized gradient approximations (GGA) are adopted for the exchange correlation functional, and norm-conserving pseudo potentials are employed. The mesh cutoff for the electrostatic potentials is 75 Ha. A 1 × 1 × 50 Monkhorst sampling in the Brillouin zone is utilized, where 50 is only for the calculation of electrodes. The temperature in the Fermi function is set as 300 K. These heterojunctions are modeled within a supercell with over 10 Å of vacuum space between neighboring cells to avoid interactions between periodic images. Before calculating the electron transport properties of GNR-BNNR-GNR devices, all atoms in the channel region are relaxed with a force tolerance of 0.05 eV/Å. The structure of GNR-5zBNNR6-GNR changes very little after relaxation. The current Id is manifested by the Landauer−Büttiker equation:

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COMPUTATIONAL METHODS The devices composed of GNR-BNNR-GNR heterojunctions are shown in Figure 1. Heterojunctions are placed along the z axis, and its surface vector is parallel with the x axis. The twoprobe device is divided into two parts, namely, the electrodes and the channel. The source and drain electrodes are made of GNRs marked by yellow, between which is the electron transfer channel (heterojunctions). Three-terminal FETs are constructed by adding a gate electrode under the channel of twoprobe devices. One dielectric layer is used to insulate the channel from the gate. These devices are denoted using GNRmz(a)BNNRn-GNR, where m is the number of BNNR unit cells in the channel region, z(a) accounts for the zigzag or armchair shaped ribbon edge, and n indicates the number of boron−nitride dimers characterizing the ribbon width.

I=

2e h

∞

∫−∞ dE(T(E , V )(f1 (E) − f2 (E)))

where T(E,V) is the quantum mechanical transmission probability of electrons, f1,2(E) are the Fermi functions of source and drain electrodes, e is the electron charge, and h is Planck's constant.

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RESULTS AND DISCUSSION GNR-mzBNNRn-GNR Two-Probe Device Calculation. To investigate the effect of BNNR length on the electron transport properties of GNR-zBNNR-GNR heterojunctions, the equilibrium nonspin-polarized transmission spectra of 17260

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Figure 2. Equilibrium transmission spectra and total and projected DOS of GNR-3zBNNR6-GNR (a), GNR-4zBNNR6-GNR (b), GNR-5zBNNR6GNR (c), and GNR-6zBNNR6-GNR (d) two-probe devices. The primary transmission eigenstates at Ef are shown in the corresponding insets with an isovalue of 1.

GNR-mzBNNR6-GNR two-probe devices, where m ranges from 3 to 6, are calculated and illustrated in Figure 2. The transmission at each energy point is calculated by summing up the transmission eigenvalues of every electron transport channel, which are obtained by diagonalizing the transmission matrix.33,34 Figure 2 shows that the transmission in some energy regions exceeds 1 because more than one electron transport channel exists in these energy regions. Interestingly, energy gaps larger than 2 eV occur in these transmission spectra and become wider and wider with the increase of the BNNR length. This is the exact character that is being urgently pursued by many researchers who desire to develop graphene nanoelectronics with pronounced switching performance. However, it is worth noting that prominent transmission peaks at Fermi level (Ef) are observed for the GNR-3zBNNR6GNR and GNR-4zBNNR6-GNR two-probe devices, which can result in large leakage current at the off state for FETs. Compared to the above, the transmission values at Ef are considerably small for the GNR-5zBNNR6-GNR and GNR6zBNNR6-GNR two-probe devices. To elaborate the transmission at Ef, the dominant eigenstates (from the left electrode) of the transmission matrix at Ef are calculated and shown in the insets of Figure 2. Obviously, all the four transmission eigenstates distribute at the nanoribbon edges with π characteristics, indicating that it is the px electrons that are mainly responsible for the electron transport. Notably, the transmission eigenstate in GNR-3zBNNR6-GNR is delocalized

throughout the nanoribbon, resulting in the pronounced transmission peak at Ef. Moreover, this transmission eigenstate at the BNNR region is mainly comprised of the px orbitals of the nitrogen atoms near the nitrogen edge. When the BNNR length increases, the transmission eigenstate gradually changes from delocalization throughout the nanoribbon to localization at the two ending GNRs, which reflects the transition of the resonance tunneling electron transport in GNR-3zBNNR6GNR to the tunneling electron transport in GNR-6zBNNR6GNR at Ef. For a deep insight into the electron transport properties of these devices, their equilibrium density of states (DOS) are calculated. Figure 2 shows that there are sharp peaks at Ef in all the total density of states (TDOS). The projected density of states on the carbon atoms (PDOS-C) and the boron and nitrogen atoms (PDOS-BN) indicate that the TDOS at Ef mainly derives from the PDOS-C. Moreover, the PDOS-BN at Ef changes little with the increase of the BNNR length, which implies that the PDOS-BN at Ef mainly originates from the boron and nitrogen atoms near the junction boundaries between BNNR and GNRs. When the length of the BNNR is short enough, these states can extend through the BNNR and thus open up electron transport channels as shown in Figure 2a. We can see that no energy gaps occur in the PDOS-C curves. In contrast, there are significant energy gaps in the PDOS-BN curves, which correspond well to those in the transmission 17261

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Figure 3. Equilibrium transmission spectra and total and projected DOS of GNR-1aBNNR5-GNR (a), GNR-2aBNNR5-GNR (b), GNR3aBNNR5-GNR (c), and GNR-4aBNNR5-GNR (d) two-probe devices. Their LDOS at Ef − 1 eV are shown in the insets with an isovalue of 0.1.

semiconducting properties of aGNRs and BNNRs, which results in the zero electron transmission at Ef. The TDOS and PDOS-C of GNR-1aBNNR5-GNR almost coincide with each other in the energy range from Ef − 2 to Ef + 2 eV, and the PDOS-BN is considerably small in this energy range. It seems that the electron transmission in this energy region should be very small due to the small PDOS-BN. However, the energy gap of the transmission curve only spans from Ef − 0.7 to Ef + 0.7 eV for GNR-1aBNNR5-GNR. To illuminate this discrepancy, local density of states (LDOS) at Ef − 1 eV is calculated and given in the insets of Figure 3. As clearly shown in the insets, the LDOS at GNR is much larger than that at BNNR. Importantly, the electron states at Ef − 1 eV in the BNNR mainly localize at the nitrogen atoms close to the boundaries between GNR and BNNR, indicating that the electron states at the two boundaries can expand through the BNNR only if the length of BNNR is short enough. Thus, as for GNR-1aBNNR5-GNR, despite that the PDOS-BN is very small, the transmission at Ef − 1 eV is still very large due to the short BNNR length. With the increase of BNNR length, the energy gaps in the TDOS and PDOS-C of these devices changes little, and the PDOS-BN in the energy range from Ef − 2 to Ef + 2 eV increase slightly, further proving that it is the boundary electron states and the length of BNNR that determine the energy gaps of transmission curves. The equilibrium transmission curves and DOS of GNRmaBNNR6-GNR and GNR-maBNNR4-GNR two-probe devices, where m ranges from 1 to 4, are shown in the Supporting Information. Similar to GNR-mzBNNRn-GNR two-probe devices, the electron transport properties of GNRmaBNNRn-GNR two-probe devices show little dependence on ribbon width. In addition, to find out whether the

spectra, indicating that the semiconducting character of BNNR plays a critical role in the electron transport of these devices. As a comparison, the electron transport properties of GNRmzBNNR8-GNR and GNR-mzBNNR4-GNR two-probe devices, where m ranges from 3 to 6, are calculated to investigate the effect of the ribbon width. Similar electron transport behaviors are observed in these devices except for a very slight change of the energy gaps in the transmission spectra (shown in the Supporting Information), suggesting that the performance of these devices depends little on ribbon width. GNR-maBNNRn-GNR Two-Probe Device Calculation. Although both zigzag GNRs and armchair GNRs are expected to be semiconductors, their band gaps depend sensitively on the chirality.11 BNNRs also show a significant semiconducting character dependent on chirality.22 Hence, investigating the effect of chirality on the electron transport properties of GNRBNNR-GNR heterojunctions is of great importance. In Figure 3, the transmission spectra of GNR-maBNNR5-GNR twoprobe devices, where m ranges from 1 to 4, are shown. Apparently, energy gaps over 1 eV appear in all these transmission spectra, and they broaden with the increase of the BNNR length. Different from GNR-mzBNNRn-GNR twoprobe devices, no transmission resonance peaks at Ef are observed in these devices. Moreover, when m ranges from 2 to 4, sharp slopes are found at the edges of energy gap in the transmission spectra. The above results indicate that GNRmaBNNRn-GNR heterojunctions possess better prospects in FET applications than GNR-mzBNNRn-GNR. To clarify the distinct difference between the two kinds of heterojunctions, equilibrium TDOS and PDOS of GNR-maBNNR5-GNR twoprobe devices are calculated. These DOS curves show that there are no electron states at Ef due to the intrinsic 17262

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Figure 4. Equilibrium electron transmission spectra of GNR-BNNR-GNR two-probe devices with the same BNNR length but different composition ratios. The inset shows the structure of these devices.

Figure 5. Equilibrium transmission spectra of GNR-5zBNNR6-GNR two-probe devices with nitrogen vacancy at the boundary site (a) and central region (b) of BNNR (noted as GNR-5zBNNR6-GNRVn1 and GNR-5zBNNR6-GNRVn2) and with boron vacancy at the boundary site (c) and central region (d) of BNNR (noted as GNR-5zBNNR6-GNRVb1 and GNR-5zBNNR6-GNRVb2).

composition ratio of the BNNR in these heterojunctions can affect the magnitude of electron transmission energy gaps, we calculate the electron transmission spectra of two groups of two-probe devices. Each group of devices has the same BNNR length but different composition ratios. As shown in Figure 4, interestingly, the transmission curves of devices with the same BNNR length are almost the same, indicating that the electron transmission energy gap is related to the absolute length rather than the composition ratio of the BNNR. Effect of Nitrogen and Boron Vacancy. In practice, it is difficult to fabricate GNR-BNNR-GNR devices without defects. Therefore, we take nitrogen and boron vacancies as examples to investigate the effect of defects. In this section, both GGA and spin-polarized GGA (SGGA) for the exchange correlations are used for comparison. Figure 5 shows the equilibrium transmission spectra of GNR-5zBNNR6-GNR two-probe

devices with nitrogen and boron vacancies located at different positions. Compared to the equilibrium transmission spectra of the perfect device (Figure 2c), transmission peaks appear within the transmission energy gap for these four defected devices. Moreover, it can be seen that both the position and the species of vacancies have a profound effect on the position and magnitude of these transmission peaks. For GNR-5zBNNR6GNR two-probe devices with a nitrogen vacancy (Figure 5a,b), distinct transmission peaks around Ef in the transmission spectra occur. Interestingly, the magnitude of these transmission peaks of the device with a nitrogen vacancy situated at the center of BNNR is much larger than those of the device with a nitrogen vacancy located near the BNNR-GNR boundary. Boron vacancy, however, can result in transmission peaks within the transmission energy gap below the Ef (Figure 5c). Moreover, moving the boron vacancy from the boundary 17263

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Figure 6. Equilibrium LDOS (up panel) and primary transmission eigenstates (lower panel) of GNR-5zBNNR6-GNRVn1 at Ef − 0.075 eV with isovalues of 0.3 and 1.0, respectively (a), GNR-5zBNNR6-GNRVn2 at Ef + 0.25 eV with isovalues of 0.4 and 1.0, respectively (b), GNR-5zBNNR6GNRVb1 at Ef − 0.925 eV with isovalues of 0.1 and 1.0, respectively (c), and GNR-5zBNNR6-GNRVb2 at Ef − 0.85 eV with isovalues of 0.4 and 2.0, respectively (d).

Figure 7. Equilibrium transmission spectra of GNR-3aBNNR5-GNR two-probe devices with nitrogen and boron vacancies situated at different sites of BNNR.

vacancies can lead to spin polarizations. Most of these spin

position to the center of BNNR can improve the magnitude of transmission peaks (Figure 5d). From the point of view of logic applications, GNR-5zBNNR6-GNR two-probe devices with a boron vacancy seem to be superior to devices with a nitrogen vacancy because the former ones would be much easier to be turned off than the latter ones. It is worth noticing that the transmission spectra obtained from the SGGA indicate that

polarizations are so weak that they are not suitable for spintronics. However, the GNR-5zBNNR6-GNRVn2 two-probe device is an exception (Figure 5b). The transmission peaks around Ef are mainly caused by spin-down electron states, suggesting that this device can be used as a spin filter. 17264

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Figure 8. FET characteristics of GNR-5zBNNR6-GNR. (a) Structure of GNR-5zBNNR6-GNR (top panel) and side view of the FET (bottom panel); (b) transfer curve of the FET at Vd = 2 V; (c) transmission spectra of the FET at Vd = 2 V and different Vg; (d) comparison between equilibrium spin polarized transmission curves and equilibrium nonspin polarized transmission curve; (e) side view of the optimized FET structure at Vg = Vd = 2 V; (f) transmission spectrum of panel e.

To have an insight into the above changes on the electron transport properties induced by vacancies, LDOS and primary transmission eigenstates corresponding to transmission peaks within the transmission energy gap are calculated based on GGA. As clearly shown in Figure 6, the transmission peaks within the transmission energy gap can be at least partially attributed to the electron states induced by vacancy. For GNR5zBNNR6-GNRVn1 (Figure 6a), the electron states caused by nitrogen vacancy around the boundary site of BNNR penetrate into the center of BNNR, improving the probability for electrons to transmit through the BNNR and thus leading to a small transmission peak at Ef − 0.075 eV. When the nitrogen vacancy is situated at the center of BNNR (Figure 6b), the electron states resulted from this vacancy spread throughout the BNNR and open up electron transmission channels. It is almost the same case for GNR-5zBNNR6-GNR two-probe devices with a boron vacancy (Figure 6c,d). The effect of vacancies on the electron transport properties of the GNR-3aBNNR5-GNR two-probe device is also studied. Figure 7 shows the equilibrium transmission spectra of GNR-

3aBNNR5-GNR two-probe devices with nitrogen and boron vacancies situated at different regions of BNNR. Surprisingly, there are no transmission peaks within the transmission energy gaps of the first three devices for both GGA and SGGA calculations (Figure 7a−c). In addition, only a small peak appears around Ef − 1.0 eV within the transmission energy gap of the last device for the GGA calculation, and for the accurate SGGA calculation, this small peak abates significantly (Figure 7d). These results indicate that the performance of the GNR3aBNNR5-GNR two-probe device is much less sensitive to vacancies than that of the GNR-5zBNNR6-GNR two-probe device. Thereby, the GNR-3aBNNR5-GNR is more suitable for logic applications than the GNR-5zBNNR6-GNR. FET and Spin Filter Characteristics of GNR-BNNR-GNR. Since GNR-mz(a)BNNRn-GNR heterojunctions show prominent applications in FETs, it is necessary to demonstrate their FET current−voltage characteristics. To this end, the transfer curve of the GNR-5zBNNR6-GNR FET is calculated when the source-drain voltage (Vd) is fixed at 2 V. Figure 8a shows the structure of this FET. As clearly illustrated in Figure 8b, this 17265

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Figure 9. Spin polarized electron transport properties of GNR-5zBNNR6-GNR FET of which only the boron edge is passivated. (a) Equilibrium spin-down (upper panel) and spin-up (lower panel) transmission eigenstates at Ef of this FET. (b) Equilibrium spin polarized transmission spectra of this FET.

8c), indicating the decrease of Id at Vd = Vg = 2 V caused by the ribbon bending. What is more important is that the transmission energy gap of the optimized FET changes little compared to that of the unoptimized one, and thus, the performance of GNR-BNNR-GNR FETs can be considerably stable in practical applications. When only the boron edge is passivated by hydrogen, zBNNRs exhibit half-metallic properties. To investigate their possible applications in spintronics, the electron transport properties of the GNR-5zBNNR6-GNR FET, in which the hydrogen atoms at the nitrogen edge of the BNNR are removed, are calculated. Figure 9a shows the structure of the heterojunction. As clearly demonstrated in Figure 9b, the equilibrium spin-polarized transmission spectra indicate a giant spin splitting. The electrons flowing through the heterojunction around the Ef are 100% spin polarized. The half-metal gap, defined as the difference between the Ef and topmost spin-up valent band, is approximated at 0.35 eV. This strong spin polarization can be well understood from the primary spindown and spin-up transmission eigenstates at Ef shown in Figure 9a. The spin-down transmission eigenstate (upper panel) is delocalized throughout the heterojunction, acting as an electron transfer channel for spin down electrons and resulting in the large transmission probability. On the contrary, the spin-up transmission eigenstate (lower panel) is localized at the GNR region of the heterojunction, preventing spin up electrons from transferring through this device. Moreover, the spin-down transmission eigenstate at the BNNR region distributes at the nitrogen edge with distinct π characters, indicating that the dangling bond states and the px states of the nitrogen atoms play a critical role in the spin-down transmission channel.

FET exhibits a good n-type transistor behavior with an on−off ratio of 150. The subthreshold swing S, defined as S = (d log Id/ dVg)−1, is estimated at 0.7 V/decade. Figure 8c shows the transmission spectra of this FET at Vd = 2.0 V and different gate voltages (Vg). As expected, large energy gaps are observed in these curves. Positive gate voltages can shift the transmission curves to higher energies due to the enhancement of nanoribbon energy levels. In contrast, negative gate voltages move the transmission curves toward lower energies. The transmission curve in the energy range from μ1 to μ2, where μ1,2 are electrochemical potentials of source and drain electrodes, determines the current (Id) of the FET. As a result, a proper regulation of Vd and Vg can effectively switch this FET from the off to on state and significantly promote its switching performance. Moreover, from Figure 8b,c, we think that this FET is capable of showing ambipolar behaviors because large negative gate voltages can also draw transmission states to the μ1−μ2 energy window and result in the on state. It is well-known that the electronic structures of zGNRs depend sensitively on the electron spin polarization. When there is no spin polarization, zGNRs exhibit metallic properties,35 which can be displayed from the PDOSC shown in Figure 2. Band gaps open when the spin polarization is considered for zGNRs.36 Thus, it is very necessary to investigate the effect of electron spin polarization on the electron transport properties of GNR-zBNNR-GNR heterojunctions. As shown in Figure 8d, the spin polarized transmission spectra of the GNR-5zBNNR6-GNR FET at Vg = Vd = 0 V show large energy gaps comparable to that of nonspin polarized transmission curve. Moreover, the spin-up and spin-down transmission spectra almost coincide with each other, indicating that spin polarization has little effect on the electron transport properties of GNR-zBNNR-GNR FETs. In addition, the coincidence of spin-up and spin-down transmission spectra reflects the fact that zBNNRs with two edges passivated by hydrogen are nonmagnetic.22 When Vd and Vg are applied, the nanoribbon of the GNR5zBNNR6-GNR FET may bend due to the electric field force. Figure 8e gives a side view of the optimized FET at Vd = Vg = 2 V. Apparently, the nanoribbon bends down at the boundaries between GNR and BNNR, especially at the left side boundary. To find out whether this ribbon bending can result in destructive effects on the electron transport properties, the electron transmission spectrum at Vd = Vg = 2 V of the optimized FET is calculated. As disclosed in Figure 8f, the electron transmission probability within the μ1−μ2 energy window is smaller than that of the unoptimized FET (Figure

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CONCLUSIONS A new and simple heterojunction model consisting of a BNNR seamlessly connected between two pieces of GNRs is put forward in this study. Devices with these heterojunctions are free of metal electrodes that would complicate fabrication processes and cause unstable performance. Theoretical calculations using the Nonequilibrium Green Function combined with the DFT reveal that this kind of heterojunction is an ideal candidate for nanoelectronics. Energy gaps over 1 eV appear in the electron transmission spectra of both GNRaBNNR-GNR and GNR-zBNNR-GNR devices. In addition, a significant transmission peak would appear at Ef within these energy gaps if the length of zBNNR is short enough. We attribute this transmission peak to the metallicity of zGNRs 17266

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The Journal of Physical Chemistry C

Article

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under nonspin-polarized conditions. The electron transport properties of these devices show a great dependence on the electron states around the boundaries between GNRs and BNNRs. Shortening BNNRs can improve the electron transmission probability and thus decrease the energy gaps in the electron transmission spectra. Interestingly, boron and nitrogen vacancies can result in electron transmissions within the transmission energy gaps of GNR-zBNNR-GNR two-probe devices. For devices with armchair heterojunctions, boron and nitrogen vacancies have little effect on their electron transport properties. The pronounced n-type transistor character of the GNR-5zBNNR6-GNR FET reveals the possibility to fabricate monatomic layer FETs with large on−off ratios using proper GNR-BNNR-GNR heterojunctions. Removal of the hydrogen atoms at nitrogen edge of BNNR can induce 100% electron spin polarization in the GNR-5zBNNR6-GNR FET, suggesting their possible applications in spintronics. These simple GNRBNNR-GNR devices may lead to a new prototype for monatomic layer nanoelectronics.

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ASSOCIATED CONTENT

S Supporting Information *

Electron transport spectra of two-probe devices based on the GNR-mzBNNR8-GNR, GNR-mzBNNR4-GNR, GNRmaBNNR6-GNR, and GNR-maBNNR4-GNR. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge the support from the National Basic Research Program of China (Grant No.2012CB825702). This work is also supported by the National Natural Science Foundation of China (Grant No. 50971081).

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REFERENCES

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dx.doi.org/10.1021/jp304189w | J. Phys. Chem. C 2012, 116, 17259−17267