Direction and strain controlled anisotropic transport behaviors of 2D GeSe-phosphorene vdW heterojunctions

Inspired by the discovery of two-dimensional (2D) atomically layered graphene, a number of nano-materials, including hexagonal boron nitride (h-BN) [1], phosphorene [2, 3], transition metal dichalcogenides (TMDCs) [4], and group IV–VI metal dichalcogenide [5] are quite accessible using synthetic methods in experiment. However, the single 2D material with restricted and monotonous physical behaviors limit its utilization and development in nanoscale electronic devices. In recent years, some researchers proposed a new way to constitute a vdW heterostructure to break through the limitation of single material. VdW heterojunctions are formed by stacking different various 2D atomic crystals and have a series of attractive physical properties, such as high electrical conductivity and mobility, ultrafast charge transfer, novel quantum Hall effect and spin/valley polarization [6–9]. Recently, the vertical heterostructure based on vdW interaction have attracted massive attentions, which is constructed by stacking different 2D materials with one on top of the other exhibiting excellent electronic properties, and the combined vdW heterostructures exhibited enhanced mobility and reduced carrier inhomogeneity far beyond single one [10, 11]. To date, high-quality lateral 2D heterostructures have been synthesized in the laboratory, and vertical stacking of graphene on h-BN is the most successful example. They led to the opening of a band gap in graphene without weakening its electronic mobility [12]. Meanwhile, the vdW p–n heterojucntion composing by p-type BP and n-type monolayer MoS₂ had been synthesized successfully in 2014, which exhibit a strong gate-tunable current-rectifying I–V characteristics [13]. Recent report displayed that the SnSe/MoS₂ vdW heterojunction presents excellent electrical transport behaviors with a distinct rectification effect and a high current on/off ratio [14]. Thus, the vdW heterojucntion expand the applications of 2D material and offered an ideal platform in versatile electronic applications.

Group IV–VI metal dichalcogenides, such as GeSe, GeS, SnSe and SnS₂, form another family of 2D semiconducting materials. Analogues to black phosphorus, the large bonding anisotropy of their layered crystals can be considered as a distorted NaCl structure, which lead to a strong anisotropy in electronic band structures. Among the group-IV monochalcogenides, only GeSe monolayer has a direct band gap (1.16 eV), which has been extensively studied owing to its high stability and unique electronic properties, and the Earth abundant and environmental friendship make GeSe particularly attractive in applications of semiconductors [15]. Moreover, a great enhancement in the electrical conductivity of GeSe monolayer was achieved as its structure could be changed under high pressure and high temperature [16]. Monolayer GeSe is an isoelectronic analogue of phosphorene, which can be viewed as consisting of six-membered rings in chair conformation. Recently, the combination of GeSe with other 2D layered materials has already been used to construct some heterostructures [17–20]. The strong covalent bonds within the layer but weak vdW interactions between the layers lead to the elimination of dangling bonds and surface states, which provides chemically inert surfaces and considerably high chemical and environmental stabilities of GeSe nanosheets. Recently, Yu et al reported the an intrinsic type-II band alignment and indirect band gap of GeSe–BP vdW p–n heterostructure, and found that an intriguing indirect–direct and insulator–metal transition can be induced by strain [21]. Xu et al found that by adjusting the interlayer distance between GeSe and graphene layer, the height of Schottky barrier can be tuned effectively and a gap opened in GeSe-graphene heterostructure [19]. He et al reported that GeSe/BP heterostructure exhibits a semiconductor to metal transition after incorporating K atoms, indicating that the GeSe–BP heterostructure has great potential for application in advanced electrode materials in K-ion batteries [20]. However, the electronic transport properties of the vdW heterostructure composed with monolayer GeSe and phosphorene have not been carefully studied, which somewhat hindered its applications in nano-electronic devices.

Here, we report on the electronic structure and transport properties of GeSe–BP based on a 2D vdW heterojunction consisting of orthorhombic-structured GeSe and phosphorene layers. Monolayer GeSe is a direct band gap semiconductor with tunable gap and small carrier effective mass, and phosphorene, an emerging 2D material, which has a high carrier mobility and a high on/off ratio for field effect transistors. Thus it is interesting to explore the 2D vdW device composed with monolayer GeSe and phosphorene, and some issues, like how is the electronic anisotropy in 2D GeSe–BP nano-devices or how can the current be tuned under strains, should be raised and solved. To answer these questions, we systematically studied the electronic transport properties of GeSe–BP based 2D devices by using density functional theory and nonequilibrium Green's function approaches. The I–V curves revealed that 2D GeSe–BP devices own an anisotropic I–V behaviors as the lead setting along the two orthogonal directions, in which the current along y-direction is much larger than that along x-direction, and this anisotropic characteristics independent of the GeSe–BP stacking styles. Interestingly, the electronic structures are significantly modified by the considered strains. The results revealed that big transmission gap remains in device when x-direction is applied. It is just reversed for the small gap become smaller when changed the external strain from compressed to stretched strains (from +10% to −10%) along the y-direction. Our work may be a promising candidate for nano-switching materials with a stable structure and high on/off ratio.

The four typical AA- AB-, AC- and AD-stacking of GeSe–BP heterostructures, which has 8 atom unit cell, are plotted in figure 1(a). For AA-stacking, the top monolayer GeSe directly and completely located above the bottom layer phosphorene. Although the similar analogous structure between GeSe and phosphorene layers, a mismatch of 4.36% and 1.36% still remains as the optimal lattice parameters of monolayer GeSe (a = 4.39 Å and b = 3.83 Å) and BP (a = 4.59 Å and b = 3.31 Å) along the a and b lattice direction, respectively. The mismatch was defined by –(L^GeSe–L^BP)–/L_max, where L^GeSe is the lattice constant of GeSe domain, L^BP is the lattice constant of BP domain, and L_max is the maximum value of L^GeSe and L^BP. The mismatches reported here do not matter as the good flexibility and puckered structure of GeSe and phosphorene monolayers [22]. The AB-stacking can be viewed as shifting the bottom BP layer of the AA-stacking by half of the cell along b direction. For AC-stacking, the top layer GeSe and bottom layer BP are mirror images of each other, and the AD-stacking can be obtained by shifting the BP layer of the AC-stacking by half unit along b direction. After structural relax, we found that the layer distance between GeSe and BP is 4.44 Å in AA and AB marked d₁ and d₂, and 4.47 Å for AC and AD marked d₃ and d₄, respectively. We calculate the binding energy to reveal the stable configuration in four stacking patterns of the GeSe–BP nanojunctions. The binding energy can be defined as E_b = E_GeSe–BP–E_GeSe–E_BP, where, E_GeSe–BP, E_GeSe and E_BP denote the total energy of the whole GeSe–BP bilayer, the isolated single-layer GeSe, and the free-standing BP monolayer, respectively. Based on this definition, a smaller negative value of E_b implies a more stable configuration. The lower E_b forces the heterostructures to approach the lower-energy state and therefore a more stable configuration, suggesting that the heterostructures can be synthesized in experiments more easily [21, 23, 24]. Here, the binding energy is calculated to be about −0.85 eV for AA and AB, and about −0.84 eV for AC and AD, respectively, indicating the structures of these considered GeSe–BP stacking nanojunctions should be stable. What's more, we found that the band structures of these four stacking are similar to each other. Therefore, we mainly consider the electronic structure and transport properties of AA- and AD-stacking. Here, the calculated lattice parameters of GeSe–BP heterostructure are a = 4.55 Å, b = 3.58 Å in GeSe–BP heterostructures, which is in good agreement with some theoretical published results [20, 21]. The sample structures of two-probe systems basing on GeSe–BP heterostructures are shown in figures 2(a) and (b), including the side views. The blue and pink shadow regions indicate the electrodes and their extensions. As the electronic properties GeSe–BP heterostructures regardless of their stacking styles, we mainly explore the transport properties of AA- and AD-stacking systems along the two orthogonal directions for instance.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

All the structures were optimized before calculation. The electronic structures and the transport properties were employed in Atomistix Toolkit(ATK) package [25] by using density functional theory and the nonequilibrium Green's function method (i.e. NEGF-DFT) [26, 27]. The exchange-correlation functional was Perdew–Burke–Ernzerhof within the generalized gradient approximation among electrons [28]. For all Ge, Se and P atoms, their core electrons are described by the optimized Norm-Conserving Vanderbilt pseudopotentials and the wave functions of valence states are expanded as linear combinations of atomic orbitals [29, 30]. The atomic structure is relaxed until all residual force on each atom is smaller than 0.01 eV Å⁻¹ [31, 32]. The total energy tolerance is below 10⁻⁶ eV, and the maximum stress is 0.02 eV Å⁻¹. We also use a 150 × 150 × 1 Monkhorst–Pack-points grid to sample the 2D Brillouin zone for the structural optimization and band calculations. The transmission is computed using the double-ζ polarized basis set, the density mesh cutoff of 75 Hartree and the Monkhorst–Pack κ-points grid 1 × 13 × 150, using 13 κ-points in the periodic direction and 150 κ-mesh in the transport direction to achieve a balance between the accuracy and cost.

Figure 1(b) show the top and side views of AA-stacking and AD-stacking GeSe–BP heterostructures. The middle panel shows the band structure of 2D AA-stacking and AD-stacking GeSe–BP heterostructures along the y-/x-direction in figure 1(c), which is located on the κ-lines from Γ to Y (Γ–Y) and from Γ to X (Γ–X), respectively. The band structures along two κ-lines are completely different, which shows a large anisotropic behaviors of GeSe–BP heterostructure [33]. One can see that the indirect small gap (∼0.10 eV) along the Γ–Y line in AA-stacking, while it exhibits a large direct band gap (∼0.53 eV) along the Γ–X line. Moreover, the band structure of AD-stacking is also plotted with a much larger band gap along Γ–X than the Γ–Y line. Block function of valence band maximum (VBM) and conduction band minimum (CBM) in figure 1(d) show that the BP mainly contributes to the CBMs (marked 1 and 3 in band structure of AA-stacking) and the GeSe contributes to the VBMs (marked 4). More interestingly, both GeSe and BP contribute in VBM of state 2, leading to a relative much more conductive behavior along the Γ–Y line. The different separation of Block function of VBMs (marked 2 and 4) indicating an anisotropy of AA-stacking GeSe–BP heterostructure. Thus the anisotropy of the electronic structure of different stacking GeSe–BP heterostructures could lead to the intrinsic different transport behaviors of 2D GeSe–BP based nano-devices along the x and y directions.

Moreover, the electronic transport properties depend on the band structures behaviors. As the electron transmission contains contributions from intra- and inter-band transitions around the Fermi level (E_F), we present the results for the electronic transport properties of AA- and AD-stacking of GeSe–BP based nano-devices in this work. Figure 2 depicts the configurations and transmission spectra of the AA and AD two-probe systems of the 2D GeSe–BP nano-devices. The electronic transmission function is calculated by using semi-infinite left and right electrodes, and the device region as the central part. The left panel in figure 2 shows the models with the top and side view of the two nano-devices, and the right panel shows the transmission spectra at the zero bias with transport direction along the x-/y-direction for both AA- and AD-stacking(AA-/AD-X and AA-/AD-Y) nano-devices. One can see that the transmission spectra in figures 2(c) and (d) are almost the same with each other, and all of them exhibit a quantization step characteristics, which is similar to that of graphene and borophene [33–35]. Although a transmission gap happens in 2D GeSe–BP nano-devices regardless of the electronic transport direction, the gap in AA-Y (∼0.09 eV) is much smaller than in AA-X (∼0.59 eV) systems. Moreover, it is interesting to see that the transmission platform in AA-Y is almost 2 ∼ 3 times larger than AA-X in the energy range below the E_F, and the transmission spectra in AA-Y is much larger than that in AA-X above the E_F. Analysis of the local density of states (LDOS) can reveal these anisotropic transport behaviors, as shown in figure S1; available online at stacks.iop.org/NANO/30/445703/mmedia. From figures S1(a) and (b), one can see that electrons can easily cross the scattering region in up and down layers, which provides good conductive channels for the transmission of AA-Y in figures S1(c) and (d). Moreover, the LDOS of AA-X are wholly located at the bottom layer of phosphorene, which directly lead the big transmission gap in AA-X. The separation of LDOS indicate a ignorable interaction between the GeSe and BP. As interlayer interaction between GeSe and BP nanosheet is quite weak, thus the intralayer interactions dominated the transport behaviors of GeSe–BP nanojunctions. Moreover, we further tested how the distance between GeSe and BP affect the transport behaviors of 2D GeSe–BP nano-devices. The transmission spectra with layer distance changing from 4.44 to 3.14 Å in AA-stacking systems as shown in figure S2. A transmission gap always appear in AA-X systems and it slightly decreases as decreasing the layer distance. Furthermore, a negligible transmission gap stands at the E_F in the transmission spectra of AA-Y systems regardless of the layer distance. Therefore, the anisotropic transport behaviors always happen in 2D GeSe–BP heterojunctions.

Such remarkable difference will bring about distinct I–V behaviors when the bias is applied to the two semi-finite electrodes. When applied a voltage, the electrochemical potentials of right and left electrodes are shifted accordingly. By using the Landauer–Büttiker formula [36], the current through the 2D GeSe–BP nano-devices can be defined as,

I(V_b) = $\tfrac{2e}{{\hslash }}$ ${\int }_{-\infty }^{\infty }$ T(E,V_b)[f_L(E–μ_L) − f_R(E–μ_R)] where f_L(R)(E, μ) denotes the Fermi–Dirac distribution functions for the left/right electrode regions, and f_L(R) = E_F ± eV/2 presents the electrochemical potential of the Fermi energy under external bias. T(E) is the transmission function at given bias voltage V and at given energy which can be defined as functions of energy E as T(E) = Tγ[${{\rm{\Gamma }}}_{L}{G}^{R}{{\rm{\Gamma }}}_{R}{G}^{A}$], Where G^R(A) denotes the retarded/advanced Green's function of the scattering region and presents the coupling matrix of the left/right electrode.

Figure 3 shows the I–V characteristics of AA- and AD-stacking GeSe–BP based nano-devices along the x- and y-direction. Some interesting phenomena can be found, like (1) the variation tendency of currents in AA and AD systems are similar to each other at the same magnitude of bias along the same direction. (2) The currents along the y-direction are much larger than that along the x-direction when the bias below 0.8 V in two systems, but it reverses when the bias from 0.9 to 1.5 V. The currents transport along the x-direction become stronger, which are larger than the currents along the y-direction. (3) Based on such an outstanding electrical anisotropy property of I–V behaviors in GeSe–BP based nano-devices, we propose that these GeSe–BP heterojunction devices could be used as nano-switches, in addition to its electronic anisotropy property [21]. More explicitly, the I–V cures of AA and AD systems are calculated and plotted in figures 3(a) and (b). One can see that the currents along the x-direction are relatively small, and they rapidly increase with the rise of voltage in figure 3(a), while the electrons transport along the y-direction are much more stable, the I–V curves keep as a horizontal line within the bias range. Based on such a different I–V behaviors systems along the x- and y-direction, we propose that GeSe–BP heterojunctions could be used as a switch device.

Zoom In Zoom Out Reset image size

We further calculated the on/off ratio (I_Y/I_X), as shown in figure 3(c). One can see that the ratio decrease when increasing the bias, it always keeps over 10⁵ as the bias below 0.6 V, and it further decreases. The decreases of ratio can be explained by the separation transmission spectra in figure 2. There is a big transmission gap in AA-/AD-X within the energy range of [−0.3, 0.3 eV], when further increasing the bias above 0.6 V, the first transmission platform comes to the bias range, leading to the relative large current appears in AA-X/AD-X systems, and the ratio turns very small at the bias of 0.7 or 0.8 V, as shown in figure 3(c). Things reverse as the bias more than 0.9 V, the currents in AA-/AD-X systems become slightly larger than the AA-/AD-Y systems in figure 3(b). Thus we redefined and calculated the on/off ratio as I_X/I_Y in figure 3(d). One can see that the ratio reaches to about 10 at the bias of 1.5 V. This anisotropic ratio in these GeSe–BP based nano-devices are much larger than the conductive borophene sheet reported earlier [35, 37, 38]. The anisotropy of I–V characteristics of GeSe–BP systems can be understood by presenting the LDOS of AA-X and AA-Y at the bias of 0.6 and 1.2 V in figure 4. From figures 4(a) and (c), we can see that electrons are completely blocked at the left side of BP layer in AA-X under 0.6 V, while electrons can easily cross the central region from right to left for GeSe and BP layers provided good conductive channels for AA-Y under 0.6 V, leading to high on/off ratio of I_Y/I_X. Furthermore, the LDOS of AA-X and AA-Y are wholly discrete under 1.2 V, the good conductive behaviors appear indicating a slight small ratio between I_X and I_Y in figure 3(d). This indicates that GeSe–BP based nano-devices have an outstanding electrical anisotropy property, which can be exploited for practical device purposes.

Zoom In Zoom Out Reset image size

As the anisotropy of 2D material can be tuned by using external mechanical strain [21, 39–41], it is significant to emphasize the effect of external strain to tune the electronic and transport properties of GeSe–BP nano-devices. Here, we applied strain separately in the electron-transport direction along two mutually perpendicular directions along the x- and y-direction, respectively. Here, the applied strain can be defined as ε = (a − a₀)/a₀ or ε = (b − b₀)/b₀, where a(b) and a₀(b₀) are the lattice constants for the strained and original structures, respectively. We use the '−' to represent compressed strain, and '+' represents stretched strain. Figure 5 shows the transmission function with ±4% along the x- and y-direction for AA and AD systems at the bias of zero. The left panel shows the transmissions in the x-direction, and the right panel shows the transmissions in the y-direction under the respective strain. We found that electronic transport behaviors in AA and AD systems under the same strain are just similar to each other. The transport behaviors of GeSe–BP nano-devices are independent of their stacking styles under the same strain, only the electronic behaviors of AA-stacking style have been displayed. Further studies show that the transmission gap become larger (0.92 eV) under the compressed strain of −4%, while it cut down to 0.43 eV when the strain changed to +4% along the x-direction in figure 5(a). Figure 5(b) reveals the transmission spectra of AA-Y under the strain of ±4% along the y-direction. On the contrary, the transmission gap becomes larger under the +4% strain (0.26 eV) and smaller under the −4% strain (0.06 eV) to compare with zero strain situation (0.09 eV), giving rise to a decrease in stretched strain and increase in compressed strain uniformly. Therefore, the transmission gap can be further narrow when we applied the stretched strain along the x-direction and the compressed strain along the y-direction together, agreeing well with previous investigations [21]. Figures 5(c)–(d) show the I–V characteristics of AA-stacking GeSe–BP system to observe how the strain affects the anisotropy and currents of GeSe–BP systems. The results show that the I–V behaviors along the two directions changed slightly upon both stretched and compressed strain. The currents remain very small at low bias along the x-direction, while it increases as increasing the value of applied bias. The currents under the strain of +4% are much larger than that under −4% situation in the whole bias range, and they increase from 1.33 to 22.5 μA at the bias from 0.9 to 1.5 V in +4% case along the x-direction. On the other hand, in figure 5(d), strain in the y-direction gives significant changes. Currents under the stain −4% is always 10² larger than that +4% case. Meanwhile, the current under the strain of −4% increases rapidly in the range of (0, 20 μA), when increased the bias along the y-direction. The current increases slowly from 4.2 × 10⁻² to 3.66 × 10⁻¹ μA as the bias increasing in +4% case. Therefore, the electronic transport behaviors of GeSe–BP system can be efficiently modulated by in-plane strain, and the anisotropic the I–V characteristics are mainly attributed to the intrinsic difference between the band structures along the Γ–X and the Γ–Y lines. Based on the I–V behaviors of GeSe–BP system under the in-plane strain of ±4% along the x and y directions, we found the currents in AA-Y are much larger than that in AA-X under the strain of +4% or −4% cases, corresponding to the high on/off ratio in original AA systems without strain.

Zoom In Zoom Out Reset image size

Considering how the strain affect the transport properties of GeSe–BP heterojunctions, we redefined the on/off ratio as I_+4%/I_−4% or I_−4%/I_+4% with the applied stretched and compressed strain along the same direction in AA-/AD-X or AA-/AD-Y systems. As the currents under +4% are larger than that under −4% in AA-/AD-X systems, we plotted the ratio of I_+4%/I_−4% in figure 6(a). One can see that the ratio reaches up to 10⁸ at the bias of 0.3 V, and it still very high when the bias is below 0.9 V. But as the bias over 0.9 V, both of the I_+4% and I_−4% are relative large and stable, so the on/off ratios become smaller and smaller as the bias increasing. However, the ratio of I_−4%/I_+4% in AA-/AD-Y systems are quite different from that in AA-/AD-X. As only a relative small transmission gap induced in AA-/AD-Y under strain of ±4%, it is easy for the transmission peaks get into the bias range when applied an extra voltage, especially for AA-Y under the strain of −4% with a small transmission gap. The ratio of I_−4%/I_+4% is calculated and plotted in figure 6(b). One can see that the tunable ratio is more than 10² in a wide bias range of (0.4, 1.4 V) for AA-Y. Thus, the anisotropy is robust in terms of electronic transport with the applied strain.

Zoom In Zoom Out Reset image size

We also tested other situations with the strain changing from ±2% to ±10%, the anisotropy is preserved under the in-plane strain in both directions. The direction and strain controlled anisotropic transport behaviors of AA vdW heterojunction along the x-/y-direction are calculated and plotted in figure 7. One can see from figures 7(a) and (b), the transmission gap is deeply dependent on the direction and magnitude of strains. We conclude the results as follow: (1) The transmission gap in AA-X systems are relative large indicating a semi-conductive behaviors of AA-X systems with stretched and compressed strain along the x-direction. The gap increases at first under the compressed strain from 0% to −4%, while it decreases when increased the compressed strain from −6% to −10%, and the biggest transmission gap can reach to 0.92 eV under the compressed strain of −4% along the x-direction. (2) The situation changed under stretched strains. The gap monotonously decreases with the value changing from 0.6 to 0.28 eV when increased the magnitude of the stretched strain, as shown in figure 7(c). (3) Different from AA-X systems, the gap decreases when increasing the compressed strain along the y-direction, and the gap disappears when the compressed strain reaches over −6%. The AA-Y systems become metallic under the stretched strain of −6%, −8%, −10%. (4) It reverses in the stretched strain situations, the transmission gap becomes larger and larger when increase the magnitude of the compressed strains from +2% to +10%. The gap can change from 0 to 0.48 eV by modulating the magnitude of strains in the AA-Y systems. The anisotropic transport behaviors of AA vdW heterojunction can be illustrated by the band structures under respective stress in figure S3. As the applied strain being set separately in the electron-transport direction along two mutually perpendicular directions, and we only plotted the band structures along the direction of applied strain. One can see that the variation trend of band gap matches well with the zero bias transmission spectra for AA-X and AA-Y systems with the compressed and stretched strain in figure 7. Band gap increases from the bottom of −10% and reached it is maximum under −4%. Then it decreases when the strain changed from −2% to +10% in AA-X. However, a metal-semiconductor transmission can be induced by the strain changed from −10% to 10% in AA-Y. Therefore, one can easily gain a switch device only by controlling the direction and magnitude of the strain along the y-direction in GeSe–BP heterojunctions.

Zoom In Zoom Out Reset image size

In summary, we used DFT in combination with the NEGF approach to investigate the electronic and transport properties of GeSe–BP heterojunctions along the x- and y-direction. At first, we tested the anisotropic band structure of the original AA- and AD-stacking heterostructures, and found that electrons are more easily transport along the y-direction than that along the x-direction. The anisotropic band structures reveal the anisotropic transport properties of 2D GeSe–BP nano-devices. A wide transmission gap appears in AA-/AD-X systems in company with a small gap in AA-/AD-Y, which is supported by the underlying electronic structures along Γ–X and Γ–Y lines. We proved that the anisotropic transmission behaviors of 2D GeSe–BP heterojunctions are independent of the considered layer distances. The transmission spectra indicate a high ratio between the I_Y and I_X in GeSe–BP systems. The currents in conductive AA-/AD-Y systems are much larger than AA-/AD-X ones leading to the ratio over 1 × 10⁵ in the bias range of (0.1, 0.5 V), while it is reversed when the bias over 0.9 V with currents in AA-/AD-X slightly larger than AA-/AD-Y systems, and the highest ratio of I_X/I_Y is over 10 at 0.5 V. The distribution of on/off ratio can be well explained by the LDOS of GeSe–BP based systems. Moreover, the electronic transport properties of GeSe–BP heterojunctions under external in-plane strain are tested. The conductivity of GeSe–BP nano-devices can be efficiently improved by applying the stretched strain along the x-direction or the compressed strain along the y-direction. A tunable anisotropy can also be found in the I–V characteristics of GeSe–BP heterojunctions. The maximum ratio of I_+4%/I_−4% is more than 1 × 10⁸ in AA-/AD-X systems, and the ratio of I_−4%/I_+4% can be tuned from 13 to 235 in AA-/AD-Y. Moreover, a semiconductor-metal changes in AA-Y systems when the strain changed from the compressed strain of −10% to the stretched strain of +10%. The giant electrical anisotropy makes the GeSe–BP based heterojunctions a promising candidate for intriguing nano-switches with a high on/off ratio, which might hold great application in electronic devices.

We thank Professor Hong Guo and Dr Xiaodong Xu, Dr Qing Shi from McGill University for kind and helpful discussion, and we acknowledge the supports from Natural Science Foundation of China (Grant No. 11747004, 11764018), the China Scholarship Council (Grant No. 201808360072), the Natural Science Foundation of Jiangxi Province (20192BAB212001), the Scientific Research Fund of Jiangxi Provincial Education Department (Grant No. GJJ160661), and the Program of Qingjiang Excellent Young Talents, Jiangxi University of Science and Technology(Grant No. JXUSTQJYX201805).