First-principles density functional theory and non-equilibrium Green function calculations have been conducted to explore the electronic properties of the graphene-like 2D materials. It is found that zigzag boron phosphide nanoribbons (zBPNRs) exhibit non-magnetic direct bandgap semiconducting property and bandgap is about 1 eV. The heterostructure zSiC-BP-SiC nanoribbons are found to display an obvious negative differential resistance (NDR), which are tunable by changing the length of BPNRs. It is also found that for the armchair MoS2/WS2NRs heterostructures, with the number of WS2NR unit cell increasing, the NDR effect can be modulated. Especially for the case of M(edge) with W atoms doping on the edges, it not only exhibits a significant NDR effect but also owns a fast current transport. Therefore, these graphene-like 2D materials may possess potential for the application in logic transistor.
- density functional theory
- non-equilibrium Green function
- BP nanoribbons
- MoS2 nanoribbons
Due to the unique electronic properties and peculiar structure of graphene, such as the zero bandgap and its massless charge carriers, graphene-based nanoscale electronics and photonic devices have been attracting increasing attention [1–7]. Many researchers focused on graphene field effect transistors (FETs) as a result that FETs are the fundamental unit of the integrated circuits. Most studies have presented that graphene shows an exceptionally high carrier mobility, which enables it to easily fabricate into fast-speed FETs [8–12]. However, pristine graphene cannot been applied in the logic transistors as it does not possess a bandgap, not enabling it to gain a large on/off ratio [10, 13, 14]. Therefore, for graphene, opening a sizable bandgap becomes an urgent issue, which should to be tackled by both theoretical and experimental studies. Although considerable methods have been used to solve this problem, such as chemical decorations [15–18], applying a high electric field to bilayer graphene [19–22], and cutting graphene into graphene nanoribbons (GNRs) [23–25], these methods add complexity and depress mobility. To tackle this issue, seeking other graphene-like semiconducting materials would be a better choice.
Nowadays, graphene-like materials have gained considerable research interest because they own significant advantages in low-dimensional scientific research and in nanodevice applications. In 2012, in Alejandro work, they predicted the existence of boron phosphide (BP) nanoflakes, which exhibit extraordinary electronic properties . Interestingly, zigzag SiC nanoribbons (zSiCNRs) share a very similar lattice constant with zigzag BP nanoribbons (zBPNRs), but their electronic properties are different from each other. More recently, 2D transition metal dichalcogenides (TMDs), which are composed of atomic layers coupled by van der Waals forces, have gained considerable interest because of the various electronic properties that they can exhibit, for example, metallic, semiconductor, superconductor, and charge density wave [27–34]. Single-layer MoS2, which is a semiconductor in a member of TMD family, is not like graphene but have a large bandgap. Hence, in the application of field effect transistor (FET), the MoS2 nanoflake is a most promising candidate [35, 36]. MoS2 monolayer consists of a Mo atom layer, which is enclosed within two S layers with hexagonal structure and these layers are held together by weakly coupled van der Waals interactions . Due to such sandwich layer Mo-S-Mo structure, MoS2 single layer could be probably fabricated repeatedly one by one from bulk materials by micromechanical cleavage  or exfoliation . Recently, MoS2 nanoribbons and other TMD materials with a width down to several nanometers have been synthesized [40–43]. Importantly, among these TMDs, WS2 is expected to have similar properties with MoS2 [44–47], and the first experimental demonstrations of monolayer WS2 have just been reported . Armchair MoS2 and WS2 nanoribbons have the same crystal structure and share the very similar lattice constants. Therefore, these nanoribbons might be utilized to construct heterostructures with stable coherent interfaces and multifunctionality.
2. Nanoelectronics constructed using boron phosphide and silicon carbide nanoribbons
2.1. Simulation method
All calculations were performed by the first-principles density functional theory (DFT) and the non-equilibrium Green function method with the Atomistix ToolKit software package [49, 50]. Norm-conserving pseudopotentials were used together with the double-zeta single polarized basis sets. Perdew-Burke-Ernzerhof generalized gradient approximations were employed to describe the exchange-correlation energy. The mesh cutoff was 150 Re. Monkhorst-Pack k-point was set to 1 × 1 × 50 to calculate the Brillouin zone integration. The electron temperature of 300 K was used. The supercells of all structures were built large that vacuum layers between neighboring cells were set to at least 15 Å in order to avoid interactions between periodic images. The geometries of the devices and of the zBPNR crystal structures were optimized until the forces of the atoms were less than 0.05 and 0.01 eV Å−1, respectively. The tolerance for energy convergence was 10− eV. The device current
2.2. Results and discussion
2.2.1. The electronics properties of zBPNRs
Figure 1a presents the structure of the hexagonal BP sheet. After optimization, B-P bond length we calculated was 1.858 Å, which is very close to the previously calculated Si-C bond length (1.787 Å) of the hexagonal SiC sheet . The difference between them is less than 5%, so constructing hybrid BP-SiC structures with coherent interfaces was possible. As shown in Figure 1c, hexagonal BP sheet is a direct bandgap semiconductor with its highest occupied valence band maximum (HOVBM) and lowest unoccupied conductance band minimum (LUCBM) both at the k-point in the Brillouin zone. P and B atoms
Figure 1b presents the structure of zigzag BP nanoribbons with the edges passivated by hydrogen. These nanoribbons are denoted as
To understand whether the width of the ribbon has effect on the bandgaps (
2.2.2. Hybrid zSiC-BP-SiC nanoribbon devices
Previous studies have shown that the ferrimagnetic state of the 6zSiCNR is a semiconductor with a very small bandgap, while the non-magnetic state of the 6zSiCNR is also a semiconductor with a negligible bandgap and the energy difference between the states of them is very small . Meanwhile, the spin-polarized state would become unstable when external fields are applied . Therefore, we employ a spin-unpolarized calculation to investigate the electron transport properties of the in-plane zigzag SiC and BP(zSiC-BP-SiC) nanoribbon heterostructures. Figure 3a shows a structure of such heterostructures, which consists of the scattering region, a hybrid zSiC-BP-SiC structure, and source and drain electrodes, denoted by dashed boxes. We study the current-voltage characteristics of this heterostructure, as shown in Figure 3b. An obvious non-linear feature was found, due to the discrete energy levels of the nanomaterial. Furthermore, from the
To understand the physical mechanism appearing at the electron transport properties of this heterostructure, transmission spectrum and local density of states (LDOS)  at different voltage should be calculated, as shown in Figure 4. The LDOS was averaged in the
2.2.3. Diode-like structure constructed using zBP-SiC nanoribbons
We also studied the electron transport properties of a diode-like structure consisting of the zigzag BP and SiC nanoribbons, denoted as zBP-SiC. The current-voltage curve of a 5zBP-SiCdiode-like device was shown in Figure 5. Due to the semiconducting property of the zigzag BP nanoribbons, we can see there was no current appearing until the bias of ±0.5 V in the current-voltage curve. In addition, when the bias of ±1 V was applied, 5zBP-SiC diode-like device exhibited a slight NDR effect. We can see from the transmission spectrum and the LDOS under different bias that such NDR effect also originated from the change in coupling between the zigzag BP and SiC nanoribbons, as shown in Figure 6. As a result, in Figure 6e, as the applied bias is −0.9 V, electrons of the lowest unoccupied molecular orbital of the zBPNR contributed to the increase of the current and the electrons of SiC flowed through both the highest occupied molecular orbital and the lowest unoccupied molecular orbital. However, when further decreasing the bias to −1.0 V, we can see from Figure 6c and e that the contribution of the highest occupied molecular orbital of the zSiCNR moving away from the bias window and the current was significantly depressed, leading to the decrease of the current in Figure 5a. In Figure 5b, we also study the current-voltage curve of this diode-like device with a wider width, which showed that NDR effect weakened and disappeared as the width of the ribbon increased.
To investigate whether the zBPNRs played an important role in the heterostructures mentioned above, we investigate the electron transport properties of folded 4zSiCNRs, as shown in Figure 7, with the overlapping area decreasing from device a to d. The transmission spectrum of the four cases indicates that there was no transmission energy gaps near the Fermi energy, because the non-magnetic 4zSiCNR is a narrow-gap semiconductor. As mentioned above, it is concluded that only the zSiCNR structure is difficult to construct into nanoelectronics.
3. In-plane heterostructures constructed using MoS2 and WS2 nanoribbons
3.1. Simulation method
All calculations were performed by the first-principles density functional theory (DFT) and the non-equilibrium Green function method with the Atomistix ToolKit software package [49, 50]. Norm-conserving pseudopotentials were used together with the double-zeta single polarized basis sets. Perdew-Burke-Ernzerhof generalized gradient approximations were employed to describe the exchange-correlation energy. The mesh cutoff was 150 Re. Monkhorst-Pack k-point was set to 1 × 1 × 100 to calculate the Brillouin zone integration. The electron temperature was employed by 300 K. The supercells of all structures were built large that vacuum layers between neighboring cells were set to at least 15 Å in order to avoid interactions between periodic images. The geometries of the devices and of the crystal structures were optimized until the forces of the atoms were less than 0.05 and 0.01 eV Å−1, respectively. The tolerance for energy convergence was 10−5 eV. The device current
3.2. Results and discussion
Figure 8 shows the devices consisting of an armchair MoS2NRs/WS2NRs heterostructure. The devices are placed along the
3.2.1. Heterostructure of WS2/MoS2 with zigzag direction interface
In Gong et al.  work, they found that in-plane heterostructure of zigzag direction interface between WS2 and MoS2could be mostly appeared in the MoS2/WS2 hybrid structures. Therefore, we first construct M(
We now investigate the physical origin of the NDR; the transmission spectrum is shown in Figure 10(a) for M(3a) at several typical bias voltages. When a small bias of 0.6 V is applied, a little transmission peak occurs in the transmission energy window and spontaneously the current begins to appear initially. Further increasing the bias to 0.9 V, the transmission peak under the bias window broadens, resulting in the increase of current and up to the maximum current. Interestingly, as applied the bias to 1.0 V, the height of transmission peak drops largely, which play the most dominant contribution to the current. Although there are additional two small peaks appearing in the bias window, the negative contribution to the current can be ignored. Therefore, the NDR effect can be found in the Figure 8. And when the bias continuously increases to 1.4 V, the transmission peaks under the expanding bias window increase and broaden, which results in a steady increase in current. In Figure 10(b) and (c), to explain whether the NDR effect of these three heterostructures is different from each other, transmission spectrum at bias of 0.9 and 1.0 V is calculated. In the case of the applied bias of 0.9 V, all the cases have the single peak under the bias window. However, when the length of the WS2NR enlarges, the height of transmission peak also increases but slowly, such that the current peak of these cases has a small increase, while for the cases of 1.0 V bias, with
To identify the nature states of the electron transport properties, equilibrium density of states (DOS) of these devices and their corresponding band structure of crystal structures are calculated and shown in Figure 11. In Figure 11(a), the curve of DOS indicates that zero electron states occur at
3.2.2. Heterostructure of WS2/MoS2 with armchair direction interface
In addition, as Gong et al.  work mentioned, besides the preferred zigzag interface, in-plane heterostructures of WS2/MoS2with the armchair interface were also occasionally observed. Therefore, we also study the electron transport properties of such armchair MoS2/WS2NR heterostructures with the interface along the armchair direction with the increasing length of WS2NR. In Figure 12, the current-voltage curves of M(
To understand the observed NDR effect, it is useful to analyze the evolution of the transmission spectrum as the bias potential is ramped up for M(1z) and some typical biases are shown in Figure 13(a). When a small bias of 0.6 V is applied, transmission peak begins to appear in the bias window, which results in an increase in the current. Under the bias of 0.9 V, only one transmission peak appeared in the bias window and peak height gradually enhances as the bias is applied. Simultaneously, the weight of the transmission spectrum in the bias window is enlarged, leading to the increase of current. When the bias is 1.0 V, an additional transmission peak appears in the expanding bias window, but the only peak in the bias window of 0.9 eV drops significantly. Compared to each other, the height of peak reduction outweighs another small peak appearing in the bias window, resulting in the decrease of current. As a result, current-voltage curves exhibit the NDR effect (Figure 12). When further increasing the bias to 1.4 V, the transmission peaks in the bias window broaden and there are some small peaks moving into the extending bias window, leading to a steady increase in the current. For the case of M(4z), with a wider WS2NR, transmission spectrum at some several typical biases is presented in Figure 13(b). It is more obvious that the initial transmission peak appears in the bias window and with the bias increasing, the height of the peak enhances. When the bias exceeds 0.9 V, the contribution of transmission spectrum both at the bias of 1.0 and 1.4 V under the bias window all decrease slightly, leading to twice NDR effect with the ignored PVR. In Figure 13(c) and (d), we study the distinction of these four heterostructures, and transmission spectra at bias of 0.9 and 1.0 V are calculated. When the width of WS2NR increases from 1 to 4, the transmission peaks both at two biases broaden and therefore both the current peak and valley increase. After investigating the variation of PVR, we find that as the width of WS2NR broadens, the increasing extent for bias of 1.0 V is more obvious than the bias of 0.9 V. From the above discussion, it is explained the reason why the NDR effect is depressed.
To investigate the mechanism of the electron transport properties, we calculate the equilibrium TDOS and PDOS on the edges of M(1z), as shown in Figure 14(a). It can be seen that a significant electron state gap appear around
3.2.3. MoS2/WS2 heterostructure with W atoms doping on the edges
Lastly, we study the another heterostructure denoted by M(edge) that Gong et al.  did not discuss, in which the Mo atoms on the edges are replaced by W atoms. Due to the same amount of W doping, M(1z) makes a comparison with this heterostructure to investigate whether edge states influence electronic transport properties. In Figure 15, we calculate current-voltage curves for M(edge). Compared to M(1z), a significant NDR effect also appears in M(edge), but the NDR window enlarges to 0.3 V. When the bias exceeds 0.6 V, an initial current appears and increases more rapidly than the case of M(1z), and then the current continues to enhance up to its maximum at 0.9 V. Although the current peak is almost similar to the case of M(4z), there is a lowest current valley among these heterostructures, which leads to the best NDR effect and largest PVR is 18.4462. Interestingly, M(edge) and M(1z) are doped the same amount of W atoms. However, the M(edge) performs more excellently than M(1z) and exhibits a better NDR effect and the faster current transport. In fact, this phenomenon can be explained that the W atoms on the edge possess the higher energy, resulting in the electrons of W atoms easily transporting through the edges than from the inside. From the previous discussion, M(edge) exhibits not only a significant NDR effect but also a fast current transport, which would make it the best candidate in the application of logic transistor.
To understand the observed NDR effect, transmission spectra at four typical biases are calculated, as shown in Figure 16(a). When applied a small bias of 0.6 V, the transmission energy window occurs a little transmission peak, resulting in an initial current. When the bias is 0.9 V, the single transmission peak in the bias window broadens up to its maximum. Further increasing the bias, this only transmission peak reduces obviously into two small peaks. Therefore, it displays a significant NDR effect. To clearly investigate the distinction between the M(1z) and M(edge), we calculate the transmission spectrum at the bias of 0.9, 1.0, and 1.2 V, respectively, as shown in Figure 16(b-d). For the bias of 0.9 and 1.0 V, the transmission spectrum of M(edge) contributes larger than M(1z) in the bias window and results in a higher current. However, in the case of 1.2 V bias, for M(edge), the height of two peaks under the bias window are both lower, resulting in a small current.
First-principles DFT and non-equilibrium Green function calculations have been used to study the electronic properties of the graphene-like 2D materials. It was found that zBPNRs exhibited a non-magnetic direct bandgap semiconducting property and bandgap was about 1 eV. We also found that when the width of zBPNRs increases, the bandgap decreases below the level of BP monolayer, resulting from electrons transport from the P edge to the B edge of the zBPNRs. Moreover, a heterostructure, which consists of a zBPNR and two zSiCNRs, was constructed and the electron transport property was studied. For these zSiC-BP-SiC two-probe devices, a significant NDR effect was observed, arising from the change in the SiC-BP coupling under various biases, and when changing the length of zBPNR, the NDR effect of these heterostructures can be modulated. In addition, we also studied the electron transport properties of a diode-like structure consisting of the zigzag BP and SiC nanoribbon. For these zBP-SiC two-probe devices, the NDR effect weaken with the increasing width of ribbon, and zBPNRs played an important role in these heterostructures.
For all the armchair MoS2/WS2NRs heterostructures, they are found to be a direct bandgap semiconductor. When enlarging the width of WS2NR, bandgap of these heterostructures narrows slightly, which leads to the current increasing fast and owning a higher current peak. Moreover, when the width of the WS2NR increases, for the case of M(na) with the interfaces along the zigzag directions, the NDR effect becomes a little better, while for M(nz) with the interfaces along the armchair directions, the NDR effect becomes inferior but with a higher current peak. Interestingly, for M(edge) with W atoms doping on the edges, it not only exhibits a significant NDR effect but also a fast current transport. Therefore, M(edge) may possess the great potential for the application in logic transistor.
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