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Vanishing Schottky Barriers in Blue Phosphorene/MXene Heterojunctions Hefei Wang, Chen Si, Jian Zhou, and Zhimei Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07642 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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The Journal of Physical Chemistry

Vanishing Schottky Barriers in Blue Phosphorene/MXene Heterojunctions

Hefei Wang1, Chen Si1,2, Jian Zhou1,2, Zhimei Sun1,2* 1

School of Materials Science and Engineering, Beihang University, Beijing 100191, China

2

Center for Integrated Computational Materials Engineering, International Research Institute for

Multidisciplinary Science, Beihang University, Beijing 100191, China

*Correspondence and requests for materials should be addressed to Z. M. Sun: E-mail: [email protected]

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Abstract An appropriate electrode material is crucial for two-dimensional (2D) semiconductors, where a vanishing Schottky barrier is ideal but is a great challenge. Blue Phosphorene (BlueP) is a promising 2D semiconductor for electronic and optoelectronic applications. Here, we report that Zr-, Hf- and Nb-based 2D transition metal carbides (MXenes) are ideal electrode materials for BlueP based on extensive investigations of the electronic properties and interfacial Schottky barrier characteristics of BlueP/MXene heterojunctions by first-principles calculations. Our results show that the strong interaction between BlueP and bare MXenes destroys the semiconducting character of BlueP, and thus bare MXenes are not ideal contact electrodes. While with the surface functionalization of MXene, the intrinsic electronic feature of BlueP is well preserved in the BlueP/surface-engineered MXene heterojunctions. Furthermore, the interfacial Schottky barriers of the heterojunctions are affected by the terminal surface groups on MXenes and vanishing Schottky barriers are achieved in some MXenes with the formula Zrn+1CnF2, Hfn+1CnF2, Zrn+1Cn(OH)2, Hfn+1Cn(OH)2 and Nbn+1Cn(OH)2. Finally, we demonstrate that the work functions of MXenes and the interface dipole induced by charge rearrangement are two underlying factors to determine the magnitude of Schottky barriers. This work provides fundamentals for selecting ideal electrode material for BlueP and is also beneficial for optimizing electrodes for other 2D semiconductors.

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1. Introduction Semiconductor devices based on two-dimensional (2D) materials have drawn increasing attention due to their fascinating properties.1-4 Among these 2D semiconductors, phosphorenes, including black phosphorene (BlackP) and blue phosphorene (BlueP), have been recently investigated as potential applications in electronic and optoelectronic devices. For example, BlackP displays excellent performance in the field of field-effect transistors (FET) due to its higher charge carrier mobility than MoS2 and the more suitable gap than graphene.5-6 BlueP, an allotrope of BlackP, is found to possess the same high stability as BlackP.7 Unlike the narrow band gap of BlackP,8-9 BlueP shows a wide band gap which is suitable for electronic applications.10-14 Furthermore, the band gap of BlueP can be tuned by controlling the electric field or strain,15 and the carrier mobility of BlueP is predicted to be higher than that of MoS2,7 indicating its usefulness in band gap engineering and semiconductor devices. To date, BlueP has been successfully fabricated through molecular beam epitaxial growth using black phosphorus as the precursor in experiment,16 and its promising application as the electronic devices is anticipated. Yet, in electronic devices, 2D semiconductors always form a high resistance with the contact electrode, and hence generate Schottky barriers at the interface of semiconductor-metal heterojunctions, which is a big challenge because the Schottky barriers will impede the carrier transport and degrade the device performance.17-20 Therefore, it is of great importance to select an appropriate metallic material as the contact electrode to construct the heterojunction with BlueP, where an ideal case is that the Schottky barrier is approaching zero. The basic requirements for the contact electrodes of BlueP are that the electrodes should be metallic 3



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and are good electrical conductors. In this case, a newly discovered 2D family of transition metal carbides offers promising opportunities due to their metallic properties and good electrical conductivity.21-25 Transition metal carbides, referred to as MXenes, are fabricated by extracting the aluminum layers from their parent ternary transition metal carbides (MAX phases).26-29 As electrode materials, MXenes exhibit some advantages superior to the conventional metals, for instance, various transition elemental compositional and surface functionalized possibilities of MXenes offer a wide range to search suitable electrode candidates for BlueP. In addition, the good flexibility of MXenes contribute to flexible electronic applications.30 According to previous reports,31 the surface-engineered MXenes have been predicted to form Schottky-barrier-free contacts with transition metal dichalcogenides (TMDs). Yet, when forming the heterojunctions, their interface characteristics including the binding mechanism, interaction intensity and the electron transfer induced band alignment, are still unclear. In addition, each atom of BlueP has a single lone pair electrons on the surface, which exhibit higher chemical activity than bonding electrons,32 indicating a complex interaction with the electrode material. Thus, it is highly desirable to have comprehensive understanding on the electronic properties and interface characteristics of BlueP/MXene heterojunctions and find the appropriate systems to achieve the low-resistance contact. In this work, we have systematically studied the electronic structures and interface properties of heterojunctions composed of BlueP and MXenes using first-principles calculations. By comparing the electronic properties of BlueP/bare MXene and BlueP/surface-engineered MXene heterojunctions, we find that surface-engineered MXenes are potential electrode candidates for BlueP. In particular, the 4


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Schottky barriers in BlueP/surface-engineered MXene can be controlled by the surface groups, and vanishing Schottky barriers are realized in all OH-terminated and some of the F-terminated systems. Finally, we further reveal the underlying mechanism which determines the magnitude of Schottky barriers at interfaces.

2. Methods and computational details First-principle calculations based on Density Functional Theory (DFT) are performed using the Vienna ab initio simulation package (VASP) within the projector-augmented wave (PAW) method.33-34 The generalized gradient approximation35 (GGA) of Perdew-Burke-Ernzerhof (PBE) is applied for the exchange-correction functional. Because of the effect of vdW interaction, the vdW-corrected density functional of optB8836 is considered in all calculations. The plane wave cutoff energy of 500 eV is used and the vacuum region perpendicular to the surface is set larger than 20 Å in order to avoid the interaction between adjacent units. A dipole correction is adopted to avert the energy and force errors as a result of the asymmetric arrangement in the periodic boundary condition (PBC).37-38 The electronic optimization stops as energy convergence criterion of 10-5 eV is reached and all structures are fully relaxed until the residual force on each atom is below 10-2 eV/Å. Considering the GGA-PBE functional always underestimates the band gap of the semiconductor in heterojunctions,

we

have

also

calculated

the

electronic

band

structures

using

the

Heyd–Scuseria–Ernzerhof (HSE06)39 hybrid functional for some typical systems to testify the reliability and the robustness of our conclusions, because the HSE06 functional has been proven to provide more accurate band gaps and band edges.40 Meanwhile, we have checked the possible magnetism in all 5



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heterojunctions and calculated the spin-polarized projected band structures for the spin-polarized systems (see Table S1 and Figure S1). Finally, in order to check whether the spin-orbit coupling (SOC) effect in MXenes influences the electronic structures and Schottky barriers in BlueP/MXenes heterojunctions, we have also calculated the band structures of some representative heterojunctions with the effect of SOC for comparison.

3. Results and discussion As illustrated in Figure 1a, the crystal structure of BlueP is hexagonal and the optimized lattice constant aBlueP is calculated to be 3.296 Å, with the P-P bond length and the buckling (∆) of the 2D phosphorus plane being 2.273 Å and 1.244 Å, respectively. Freestanding BlueP is a typical semiconductor with an indirect band gap of approximately 2.0 eV based on GGA calculations (see Figure 1b), in agreement with previous reports.7, 10-11 While using the HSE06 hybrid functional which is found to give much more accurate band gaps for semiconductors,40 the band gap of BlueP is calculated to be 2.7 eV (Figure 1b), in good agreement with the value reported by Barun Ghosh.15 To construct a heterojunction by stacking BlueP and a specific 2D material together where the latter can be used as a good electrode material for the former, this 2D material is expected to satisfy the following two conditions at the first step: (i) it is metallic with a high conductivity; (ii) its Bravais lattice is also hexagonal, comparable with that of BlueP, and the lattice mismatch between them should be small. In this respect, 2D transition metal carbides (Mn+1Cn, also referred to as MXenes) attract our attention. MXenes whose geometric structures depicted in Figure 1c offer a wide range to search suitable electrode material candidates due to their hexagonal lattices, various transition elemental compositional and 6


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surface functionalized possibilities and flexible thickness controllability (n = 1, 2 or 3). After a global screening of all the bare MXenes Mn+1Cn with M focused on all related transition metal elements (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo) and the surface-engineered MXenes Mn+1XnT2 (T stands for the surface terminal functionalized groups, T = F, O or OH), we find that Zrn+1Cn, Hfn+1Cn and Nbn+1Cn as well as their derivatives Zrn+1CnT2, Hfn+1CnT2 and Nbn+1CnT2 (except Zr2CO2 and Hf2CO2 which are found to be semiconductors) are potential candidates that satisfy the above two conditions. Figure 1d displays the diagrammatic sketches of BlueP/MXene heterojunctions constructed by positioning BlueP on the top of bare or surface-engineered MXenes. We have also calculated the magnetism in freestanding MXenes and the heterojunctions. It is found that bare MXenes, including Zr2C, Zr3C2, Zr4C3, Hf3C2 and Hf4C3 exhibit significantly spin-polarized properties, while all surface-engineered MXenes exhibit non-magnetic properties. When forming the heterojunctions, only BlueP/Zr3C2 and BlueP/Zr4C3 show obvious magnetic properties (see Table S1). Due to the similar lattice constants between BlueP and the chosen MXenes, we use the unit cells of BlueP and MXenes to construct the heterojunctions. As the properties of BlueP are sensitive to its lattice parameter,7 we fix the lattice constant of BlueP to its optimized value and adapt the lattice constants of MXenes accordingly.

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Figure 1. (a) The optimized lattice structure of BlueP. (b) The band structure of BlueP with GGA-PBE functional (blue solid line) and with HSE06 functional (red dashed line). The Fermi level is set at zero. (c), (d) The optimized lattice structures of MXenes, BlueP/bare MXene heterojunction and BlueP/surface-engineered MXene heterojunction, respectively.

Among these heterojunctions, the lattice mismatches between BlueP and MXenes, defined as |ε| = |aMXene aBlueP /aBlueP × 100%|, are smaller than 2%, 3% and 5% for Zr-, Hf- and Nb-based MXenes, respectively, as shown in Figure 2a. These small lattice mismatches, in combination with the good flexibilities of MXenes and BlueP, suggest that BlueP/MXene heterojunctions can be formed by epitaxial stacking BlueP and MXenes, where the lattice mismatches can be compensated by small strain. Note that the extremely small strain caused by the lattice mismatch will not damage the electronic 8


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structural stability, which is checked later in the article. Therefore, next we will focus on the BlueP/MXene heterojunctions composed of single-layer BlueP and Zr-, Hf- or Nb-based MXenes. Given that besides BlueP, the Zr-, Hf- and Nb-based MXenes, including Zr3C2, Hf2C, Hf3C2, Nb2C and Nb4C3, have also been successfully produced in experiments recently,41-45 the syntheses of the corresponding BlueP/MXene heterojunctions are anticipated.

Figure 2. (a) Lattice mismatch between BlueP and Zr-, Hf- or Nb-based MXenes. (b) Binding energy (Eb) of BlueP/MXene heterojunctions under the most stable stacking pattern.

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For each type of heterojunctions, we consider six possible stacking patterns for BlueP on Mn+1Cn or MnCn+1T2 MXenes (see Figure S2), and determine the most stable stacking pattern. The binding energy Eb is defined as the following equation, Eb =

EBlueP/MXene EBlueP EMXene N

,

where EBlueP/MXene , EBlueP and EMXene represent the total energies of the bilayer BlueP/MXene heterojunction, single-layer BlueP and MXene, respectively, and N is the number of P atoms in the unit cell of the heterojunction. Figure 2b summarizes Eb for each type of heterojunctions under the most stable stacking pattern. It is clearly seen that the binding energies of BlueP/bare MXene heterojunctions are about -0.8 eV, whereas the binding energies of BlueP/surface-engineered MXene heterojunctions range from -0.1 eV to -0.3 eV. The negative binding energies indicate that these heterojunctions are stable, and the differences for MXenes with and without surface groups imply that BlueP strongly interacts with bare MXenes, whereas it is relatively weakly adsorbed on the surface-engineered MXenes. Next, we take BlueP/Hf3C2 and BlueP/Hf3C2T2 heterojunctions as examples to elucidate the interface interaction and electronic properties of BlueP/MXenes. Figure 3a illustrates the band structure of the BlueP/Hf2C3 heterojunction where the red circles represent the contribution from BlueP. We observe that the bands of BlueP are significantly perturbed by strong interaction with bare MXene, and the states of BlueP at the band edge spread into the original gap region, resulting in a metallic character for BlueP. Similar phenomenon is observed in the band structure based on the HSE06 calculations (see Figure S3a). This can be further clearly evidenced by the partial density states (PDOS) of P atoms where 10


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the intrinsic band gap for the isolated BlueP completely disappears, as shown in Figure 3b.

Figure 3. (a) The projected band structure for BlueP/Hf3C2 heterojunction. The contribution of electronic orbitals from BlueP is represented by red circles in the band structure. The Fermi level is set at zero. (b) The partial density of states (PDOS) for BlueP/Hf3C2. (c) The electron localization functions (ELF) plots projected on the (110) plane for BlueP/Hf3C2.

Generally, the interface interaction comes from the electronic states hybridized between the contacting components in the heterojunctions. The effect of hybridization has to fulfill two conditions simultaneously: energetic and spatial overlapping of orbitals. For BlueP/Hf3C2, the electronic states 11



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around the Fermi level are mainly contributed by the d orbitals of Hf and the p orbitals of P (see Figure 3b). The similarity between Hf-d orbitals and P-p orbitals indicates their energetic overlapping around the Fermi level. Besides, the interlayer vertical distance between the bottom P atoms in BlueP and the surface Hf atoms in Hf3C2 is only 2.01 Å, which can maximize the spatial overlapping at the interface region. Furthermore, the lone pair electrons on P atoms also contribute to the strong binding of BlueP with Hf3C2. In BlueP, each P atom is bonded with its three nearest neighbors. Based on the valence shell electron pair repulsion theory, we can conclude that each atom of BlueP also has a single lone pair electrons. The lone pair electrons which have relatively high chemical reactivity interact more easily with the transition metal atoms on the surface of bare MXenes, resulting in strong binding between BlueP and bare MXenes. This strong interaction could be also visualized through the electron localization functions (ELF) analysis. As illustrated in Figure 3c, the ELF plots are projected on the (110) plane for BlueP/Hf3C2. On the upper surface of BlueP, the deep color indicates the densely localized electrons from the lone pair p electrons in BlueP. While the electronic localization for bottom P atoms is relatively weak and the covalent bonds for BlueP and Hf3C2 are generated at the interface of the heterojunction. The destructive disturbance of the electronic structure in BlueP owing to the strong interaction with bare MXenes suggests that bare MXenes are not favorable contact electrodes for BlueP. Thus we need to explore methods to modify the interfacial characteristics of BlueP/MXene heterojunctions. Depending different synthesis process, the surfaces of these MXenes can be terminated by O, F or OH groups. This brings a chance to tune the properties of MXenes by appropriate surface functionalization. Interestingly, 12


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when BlueP is on the surface-engineered MXenes, its intrinsic electronic structure is well preserved. Electronic structures of BlueP/Hf3C2O2, BlueP/Hf3C2F2 and BlueP/Hf3C2(OH)2 heterojunctions are displayed in Figure 4a-c. It is visualized that after contacting with Hf3C2T2, the bands of BlueP are unperturbed, with a gap of about 2.0 eV, similar to the case in isolated BlueP. Obviously, the unaltered band structure of BlueP on surface-engineered MXene is closely related with the weak interface binding. Take BlueP/Hf3C2O2 as an instance, Figure 4d illustrates the ELF plots projected on the (110) plane for this heterojunction. Compared with the electron distribution in BlueP/Hf3C2 mentioned above, the lone pair electrons of P atoms appear on both upper and bottom surfaces of BlueP, and there are no covalent bonds for BlueP and Hf3C2O2 in BlueP/Hf3C2O2, indicating an extremely weak interaction between BlueP and the surface-engineered MXene. In fact, the terminal functionalized groups T on the surface of MXene can be considered as an intercalation into the BlueP/MXene interface, breaking the BlueP-MXene interaction. Meanwhile, the T layer is chemically relatively inert in essence: these functionalized groups are saturated by getting electrons from MXene. In other words, the intercalated T layer will only weakly interact with the top BlueP layer. The combining effects of the above two factors result in the weak binding between BlueP and surface-engineered MXene, realizing the unperturbed characteristics for the 2D semiconductor of BlueP. Compared with the GGA method, the band structures of BlueP/Hf3C2O2, BlueP/Hf3C2F2 and BlueP/Hf3C2(OH)2 heterojunctions are also calculated by the HSE06 method (see Figure S3). Unlike the semimetal properties of Hf3C2O2 in BlueP/Hf3C2O2 with GGA method shown in Figure 4a, Hf3C2O2 is calculated to be a semiconductor with a narrow band gap of about 0.2 eV by the HSE06 functional. For BlueP/Hf3C2F2 and BlueP/Hf3C2(OH)2, it is seen that the 13



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band gap of BlueP increases to be 2.7eV and 2.8 eV using the HSE06 method, respectively, while the shapes and characteristics of the band structures remain virtually unchanged.

Figure 4. Projected band structures of (a) BlueP/Hf3C2O2, (b) BlueP/Hf3C2F2 and (c) BlueP/Hf3C2(OH)2 heterojunctions. The contribution from BlueP electronic orbitals is represented by red circles in the band structure. The Fermi level is set at zero. (d) The ELF plots projected on the (110) plane for BlueP/Hf3C2O2.

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One of the most crucial characters for semiconductor-metal contacts is the Schottky barrier height (SBH).46 The Schottky barrier height for electrons (ΦBn) is measured by the energy difference between the heterojunction’s Fermi level and the semiconductor’s conduction band edge (see the inset in Figure 5). Difficulties for electrons transferring from the metal to the semiconductor are considerably dominated by the ΦBn value and if ΦBn vanishes, or in other words, if the value of ΦBn approaches zero or negative, an Ohmic contact will be generated at the interface of the heterojunction. Figure 5 presents the calculated ΦBn in all related heterojunctions of BlueP with surface-engineered MXenes in the study. A sizable or vanishing ΦBn is signified by the upward or downward arrow, respectively, and a vanishing ΦBn implies the spontaneous injection of electrons from the MXene substrate to BlueP. It is clearly seen that all Zrn+1Cn(OH)2, Hfn+1Cn(OH)2 and Nbn+1Cn(OH)2 have a zero or negative ΦBn with BlueP. For BlueP/F-terminated MXenes, the Schottky barriers in Zr- and Hf-based systems disappear while the values in Nb-based systems are sizable. As for heterojunctions composed of BlueP and O-terminated MXenes, sizable Schottky barriers are generated in all these systems. To sum up, vanishing Schottky barriers are obtained in all the OH-terminated and some of the F-terminated heterojunctions. In order to check whether the GGA functional influences the accuracy of the Schottky barriers, the electronic structures of some representative BlueP/surface-engineered MXene heterojunctions are recalculated based on the HSE06 hybrid functional (see Figure S3). It is clearly seen that the Schottky barrier heights still remain zero or negative in BlueP/Hf3C2F2 and BlueP/Hf3C2(OH)2 with the HSE06 method (-0.01 eV for BlueP/Hf3C2F2 and -0.17 eV for BlueP/Hf3C2(OH)2), which is comparable with the heights obtained with the GGA method in Figure 5 (-0.08 eV for BlueP/Hf3C2F2 and -0.27 eV for 15



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BlueP/Hf3C2(OH)2). Based on the similarity in electronic properties between these two Hf-based systems and other heterojunctions with the calculated vanishing barriers in Figure5, we can conclude that vanishing Schottky barriers are maintained in these systems with both GGA and HSE06 methods.

Figure 5. The Schottky barrier heights of electrons (ΦBn) for the semiconductor-metal contact in heterojunctions composed of BlueP and surface-engineered MXenes. For convenience, the horizontal axis just gives corresponding MXenes in heterojunctions. The up and down arrows signify a sizable and vanishing ΦBn, respectively. The inset shows the schematic illustration for the measurement of ΦBn.

We have also checked the stability of the electronic structures and interfacial properties in the BlueP/surface-engineered MXene heterojunctions in terms of the small strain and the spin-orbital coupling (SOC) effect. In the above calculations, we adapt the lattice constants of MXenes to that of BlueP when constructing the heterojunctions. Now we choose Nb3C2F2 and Nb3C2(OH)2 which have the 16


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largest lattice mismatches (4.8% for Nb3C2F2 and 3.7% for Nb3C2(OH)2) with BlueP to build the heterojunctions by fixing the lattice constants of MXenes and adjusting the lattice of BlueP. The comparison of the band structures for the heterojunctions with fixed BlueP lattice or fixed MXene lattice is plotted in Figure S4. It is observed that the shape of bands from MXenes and the value of Schottky barriers slightly change, while the shape of bands from BlueP and the sign of barrier values are almost unaltered, indicating the stableness of the interfacial properties in the heterojunctions. Owing to the existence of the heavy transition metal atoms in MXenes, we have calculated the band structures for some representative heterojunctions with the effect of SOC (see Figure S5). The result suggests that SOC only slightly splits off some orbits from MXenes but without varying the shapes and properties of the band structures, consequently, the SOC effect does not affect the conclusions of our work. For the semiconductor-metal contact, it is difficult to form the vanishing Schottky barriers in most cases as the result of the strong binding and Fermi level pinning at the interface of the semiconductor and its metallic electrode.47-48 In previous literatures,18,49 the problem is mainly treated by inserting an insulating monolayer or absorbing passivating atoms at the interface to weaken the strong interaction and reduce the interface states. Yet it can be expected that such methods involving at least three types of materials produces a great experimental complexity. By comparison, the functionalized groups are more easily intercalated into the BlueP/MXene interfaces because when synthesizing MXene, it is a self-assembly process for the chemical groups spontaneously covering on the surface. This suggests the experimental feasibility for the synthesis of BlueP/MXene heterojunctions with vanishing Schottky barriers. 17



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After calculating the Schottky barriers of the systems, we will further demonstrate the underlying mechanisms by which the Schottky barriers are determined at the interfaces of BlueP/MXene heterojunctions. Firstly, the work functions of isolated MXenes as well as their relative energetic positions to the band edges of BlueP are shown in Figure 6a. It is viewed that the MXenes with the formula of Zrn+1Cn(OH)2, Hfn+1Cn(OH)2, Nbn+1Cn(OH)2, Zrn+1CnF2 and Hfn+1CnF2 have relatively low work functions, leading to their Fermi levels located above the conduction band minimum (CBM) of BlueP. Interestingly, these MXenes are exactly the satisfying candidates to form vanishing Schottky barriers with BlueP as mentioned above, indicating the close relationship between the work functions of electrode materials and the Schottky barriers of heterojunctions. This correlation can be interpreted by the Schottky–Mott Rule written as ΦBn = WM – χBlueP , where WM is the work function of MXene and χBlueP is the electron affinity of BlueP. Upon more detailed analysis, we find that the differences between work functions of OH-terminated MXenes and the CBM of BlueP are about 2.0 eV, while for Zrn+1CnF2 and Hfn+1CnF2, the values are 0.2-1.0 eV. However, the observed Schottky barrier heights are extremely close to zero or slightly below zero, deviating from the Schottky–Mott Rule, so there must be other factors to modulate the band alignment of heterojunctions. One of the reasons for the electronic level change in heterojunctions is related to charge redistribution at the interface.47 Accordingly, we investigate the electron transfer and the interface dipole on the contact between BlueP and MXene when building the heterojunction. The plane-averaged electron difference density calculation is performed along the vertical orientation to the interface of 18


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BlueP and MXene. As an illustrative example, the electron difference density of BlueP/Hf3C2F2 heterojunction is plotted in Figure 6b. The charge redistribution at the interface is visualized on the basis of the following equation, ∆ρ = ρBlueP/MXene ρBlueP ρMXene , where ρBlueP/MXene , ρBlueP and ρMXene represent the plane-averaged charge densities of the heterojunction, the single-layer BlueP and MXene, respectively. By integrating ∆ρ(z) in the regions belonging to BlueP or MXene, we can confirm the direction of electron transfer. It is visualized that electrons accumulate at the interface region belonging to BlueP and deplete at the region belonging to Hf3C2F2, indicating that the electrons transfer from Hf3C2F2 to BlueP when constructing the heterojunction. The identical electron transfer direction could be obtained at the interfaces between BlueP and other MXenes, hence the behaviors of charge rearrangement at interfaces in all BlueP/MXene heterojunctions are similar. As depicted in Figure 6c, an interface dipole is formed at the interface due to the charge redistribution, decreasing the Fermi level and leading to a closer position of the Fermi level with respect to the CBM of BlueP. These results correspond with the concrete values of SBH derived from BlueP/MXene heterojunctions in Figure 5. Therefore, beside the work functions of MXenes, the band alignment of these heterojunctions is also impacted by the interface dipole induced by the charge redistribution at interfaces.

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Figure 6. (a) Work functions of MXenes with different terminations. The dashed lines indicate relative positions of the conduction band minimum (CBM) and valence band maximum (VBM) in BlueP. (b) Plane-averaged electron difference density ∆ z of BlueP/Hf3C2F2 heterojunction. Atomic positions normal to the interface are indicated by solid circles. (c) Schematic illustration of band alignment for BlueP/Hf3C2F2 after the charge redistribution. An interface dipole is formed at the interface. 20


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4. Conclusions In summary, we have investigated the electronic properties and interfacial Schottky barrier characteristics of BlueP/MXene heterojunctions using first-principles calculations. We show that the lattice matches between BlueP and the Zr-, Hf- and Nb-based MXenes as well as their binding energies ensure the structural stability. For heterojunctions composed of BlueP and bare MXenes, the strong interaction and electronic states hybridization of contacting components destroy the semiconducting nature of BlueP, indicating bare MXenes are not ideal electrode materials for BlueP. While a semiconductor-metal junction is formed when BlueP contacts with surface-engineered MXenes. The Schottky barriers in BlueP/surface-engineered MXene heterojunctions could be tuned by the functionalized groups absorbed on MXenes. In particular, vanishing Schottky barriers are achieved in the systems of BlueP and MXenes with the formula Zrn+1CnF2, Hfn+1CnF2, Zrn+1Cn(OH)2, Hfn+1Cn(OH)2 and Nbn+1Cn(OH)2. We further point out that the energetic position of MXene’s work function with respect to the band edges of BlueP and the electron transfer induced interface dipole codetermine the values of Schottky barriers in the heterojunctions. Our work proposes the potential electrode materials for BlueP and benefits its future applications in electronic devices.

Supporting information Calculations for possible magnetism in freestanding MXenes and BlueP/MXene heterojunctions (Table S1 and Figure S1). Different stacking patterns for BlueP/MXene heterojunctions (FigureS2). Electronic structure calculations for BlueP/MXene heterojunctions with HSE06 hybrid functional 21



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(FigureS3). The examination of the effect of small strains on the electronic structures for BlueP/MXene heterojunctions (FigureS4). The examination of the spin-orbit coupling (SOC) effect on the electronic structures for BlueP/MXene heterojunctions (FigureS5).

Acknowledgements This work is financially supported by the National Key Research and Development Program of China (Grant No. 2017YFB0701700), the National Natural Science Foundation for Distinguished Young Scientists of China (Grant No. 51225205) and National Natural Science Foundation of China (No. 61274005).

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