as an Anisotropic Layered Semiconducting Material

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Letter 15

Rediscovering MP Family (M = Li, Na, and K) as an Anisotropic Layered Semiconducting Material Yanhan Yang, Nan Tian, Yongzhe Zhang, Danmin Liu, Dong Zhang, Kai Chang, and Hui Yan J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02817 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 25, 2017

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The Journal of Physical Chemistry Letters

Rediscovering MP15 Family (M = Li, Na, and K) as an Anisotropic Layered Semiconducting Material Yanhan Yang1, 2‖, Nan Tian1,3‖, Yongzhe Zhang1, 2*, Danmin Liu1, 3, Dong Zhang4, Kai Chang4, Hui Yan1, 2 1

Beijing Key Laboratory of Microstructure and Properties of Advanced Material, Beijing University of Technology, Beijing 100124, China 2

College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China

3

Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, China

4

State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, 100083, China

‖: These authors contributed equally to this work. *E-mail: [email protected]

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Abstract: Binary alkaline-metal phosphides family MP15 (M = Li, Na, K) exhibiting layered structure nature and in-plane anisotropy is discussed through first-principles. Their thickness dependent bandstructures were reported for the first time. Furthermore, the transport studies demonstrate that single-layer MP15 exhibits a large anisotropic ratio for carrier mobility (both electron and hole) (~101-102 magnitude) between two special crystal directions, which is the record high value among the reported two-dimensional anisotropic materials. Additionally, the chemical stability under ambient conditions and the binding energy which related to experimental exfoliation were also investigated. The high anisotropy of the layered semiconducting MP15 family could open up considerable promise for anisotropic optics, electronics, optoelectronics device, as well as energy storage application.

Table of Contents Image:


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Recent decade has witnessed the dramatic explosion of the investigation of two-dimensional (2D) materials, including graphene itself,1-3 transition metal dichalcogenides (TMDCs),4-7 hexagonal boron nitride (hBN),8 and black phosphorene,9-11 as candidate materials for future applications, since the experimental realization of exfoliated few-layered graphene.12 Among them, graphene and most of the well-studied two-dimensional high-lattice-symmetry TMDCs exhibit isotropic in-plane behavior, while a few low-symmetry 2D materials, including the puckered honeycomb structured black phosphorene,13 group-IV monochalcogenides semiconductors (SnSe, GaSe, GaTe and SnTe),14-18 and 1T structure TMDCs (such as ReS2, ReSe2, and WTe2),19-23 offer in-plane anisotropic optical, electrical, mechanical and thermal properties. We focus on a new kind of low-symmetry layered materials, the binary alkaline-metal phosphides, with the general composition MP15 (M = Li,24 Na,25 K26-28), layered structure nature, and anisotropic in-plane configuration, which were featured tubular [P-15] units that are analogical to the framework of the strands found in the violet and the fibrous red phosphorus allotropes.29-31 The pioneering work in this field was performed by von Schnering et al. , synthesizing KP15, in 1967.26 In the past 50 years since their discovery, several fundamental properties of the MP15 have been characterized including the typical Raman spectrum, the decomposition and thermal properties,28 and the vibrational and photoluminescence properties.25 Up to now, all the works about MP15 family are about their bulk properties, ignoring the layered structure nature of them, let alone their anisotropic properties. In this paper, we present the theoretical discovery of this kind of layered semiconductors, MP15 (M = Li, Na, K). MP15 is a new anisotropic phosphorus-rich alkaline phosphide family consisting of antiparallel-arranged pentagonal phosphorus tubes connected by alkaline atoms. We demonstrate that few-layered MP15, from monolayer up to bulk, are chemically stable with a bandgap decreases with



the thickness increasing. We predict the carrier mobility at room temperature and observe a large anisotropic ratio of carrier mobility between two special crystalline orientations. The results not only first demonstrate the layer dependent bandgap and bandstructure information of MP15 family, but also reveal their anisotropic nature in the transport properties, which is of great significance for the selective design of anisotropic layered MP15-based electronic and optoelectronic devices. Density functional theory (DFT) simulations were performed within the Vienna ab initio simulation package (VASP)32-33 in the framework of the projector augmented wave (PAW) method,34 using the gradient-corrected Perdew-Burke-Ernzeerhof (GGA-PBE) functionals.35 The energy cutoff for the plane-wave basis was set to 500 eV for all calculations. Two Monkhorst-Pack k-meshes36 of 8×6×4 and 8×6×1 were adopted to sample the first Brillouin-Zone of the conventional unit cell of bulk and few-layer MP15 with a Gaussian smearing of 0.05 eV near Fermi level (EF). Atomic sites and lattice constants were relaxed until the residual forces were less than 0.001 eV/Å in each bulk species. All the few-layered configurations were simulated employing vacuum surrounding the sheet of 30 Å. In optimizing the system geometry, van der Waals interactions were considered by the Tkatchenko and Scheffler (TS) approach.37 The most results were obtained based on the optimized structure through PBE+TS approach unless otherwise stated. We also performed basic PBE and optB88-vdW calculations and the related results could find in the Supporting Information. For more details about carrier mobility calculations, see the Supporting Information S3. Figure 1 shows the schematic drawing of MP15 crystal structure. The unit cell of MP15 contains 32 atoms and it exhibits low crystalline symmetry, which could be defined as the P1 space group. One layer of MP15 consists two lines of parallel P fibers and alkaline atoms between them (as shown in the bottom right of figure 1). Between layers there were van der Waals interactions. The detailed


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crystal characterization was described in the Supporting Information S1 as well as the optimized crystal parameters (listed in the Table S1). Compared with the experimental results, our optimized structure parameters show excellent agreement (within 1% error).24,38 Considering the optimized structure from PBE+TS is in excellent agreement with the experimental value (the error is below 1%) , we use it to determine the electronic structure and transport properties of MP15 family in the following, which has been employed in the violet phosphorene (having the similar P-tubes of pentagonal cross section with MP15).39 Results obtained using the basic PBE, and optB88-vdW approaches could also find in Table S1.

Figure 1. Crystal structure of MP15 (M = Li, Na, K). Yellow spheres represent phosphorus atoms, while the dark gray spheres represent alkaline atoms M (M = Li, Na, K). The semitransparent gray tubes are added as guides to the eye only to emphasize the P fibrous nature ([P-15] units) of MP15.

To evaluate the possibility of mechanical and liquid exfoliation, we consider the energetics of few-layered MP15 (M = Li, Na, K). The calculated relative energies of n-layers of MP15 (relative to their bulk configurations) are shown in figure 2(a). The binding energy of all the species of MP15 increases with the decreasing layer number monotonically, attaining about 0.045 eV/atom at n = 1.



The relative energies for 1-4 layers black phosphorus and for 1-3 layers violet phosphorus were shown in the same figure, which reaches a larger value of 0.095 eV/atom and a comparable value of 0.044 eV/atom when n = 1, respectively, employing the similar van der Waals amendment.39 For monolayer (n = 1), the relative energy becomes to the binding energy per layer per area, which is helpful to evaluates the difficulty to exfoliate the bulk form into monolayers. For MP15 family, we obtain the value about 0.38 J/m2 (0.39 J/m2 for LiP15, 0.39 J/m2 for NaP15, and 0.35 J/m2 for KP15), similar to those of graphite (0.37 J/m2),40 MoS2 (0.42 J/m2), the black (0.40 J/m2) and violet phosphorene (0.35 J/m2), suggesting that exfoliate MP15 sheets from its bulk form should be possible.

Figure 2. (a) Relative energy of MP15 with n layers with respect to their bulk forms. Results for n layers the violet-P and


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black-P are also shown to compare.39 (b) Bandgap of MP15 as a function of thickness. (c) Sketch of the density of states (DOS) for the monolayer MP15, and the energy diagram of the DOS of aqueous oxygen acceptor with solvent reorganization. The blue arrows point to the proposed light-induced oxidative reaction.

For the first time, we report the bandgap and band structure of MP15 family for their bulk and 1-3 layers forms. As shown in figure 2(b), monolayer LiP15 has the smallest bandgap (1.32 eV) while its value for NaP15 is the largest (1.52 eV). Similar to other 2D materials, their bandgap decreases with the increasing thickness. The bandgap of monolayer MP15 is not observably larger than the value of its corresponding bulk form: the largest difference between the bandgap for monolayer and bulk forms appears on KP15 (0.23 eV), while LiP15 has the smallest bandgap variation (0.16 eV). When the layer number increases to 3 or more, the bandgap of NaP15 and KP15 decreases to their bulk value. This property eliminates the difficulty for applications that researchers would not having to select specific numbers of layers accurately for particular wavelength responded device. Table S2 in the Supporting Information lists the bandgap and type of bandgap of the monolayer MP15 (M = Li, Na, K) compared with those of the violet phosphorene. The difference in bandgap values employing PBE and PBE+TS approach should arise from that the latter could take into account variations caused by the van der Waals interactions of atoms derives from their local chemical environment when performing structure optimizing.



Figure 3. Band structure and density of states of (a) bulk LiP15, (b) monolayer LiP15, (c) bulk NaP15, (d) monolayer NaP15, (e) bulk KP15, and (f) monolayer KP15. (g) The schematic of the reciprocal lattice with the high symmetry points indicated for bulk (left) and monolayer (right) MP15.

The black phosphorene would degrade in the wet atmosphere since the energy levels of O2(aq) and O 2(aq)

fluctuated over a range of ±1 eV with the result that they are possibly overlap the density of


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states (DOS) for monolayer black phosphorus, which makes electrons transfer from the black phosphorene to the solvated oxygen acceptor state, leading to the light-induced oxidative reaction.41-42 For this reason, we check the chemical stability of MP15. The band edges of the monolayer MP15 relative to a vacuum are well below that of the aqueous O2 acceptor state (-3.1 eV), which prevents the carrier transfer between them and makes monolayer MP15 remains stable under ambient conditions, as shown in figure 2(c) (the LUMOs are -3.96 eV, -3.94 eV, and -3.95 eV, while the HOMOs are -5.28 eV, -5.46 eV, and -5.44 eV for monolayer- LiP15, NaP15, and KP15). The calculated band structure of MP15 (M = Li, Na, K) (bulk and monolayer forms) and the corresponding density of states (DOS) are shown in Figures 3 (a)-(f). MP15 family are indirect-gap semiconductors, no matter for their bulk or monolayer forms. For the monolayer forms (figure 3(b), (d), and (f)), the fundamental bandgap located between the valence band maximum (VBM) located at Γ point and the conduction band minimum (CBM) situated at C point (for LiP15 and NaP15, the CBM located between the C and Γ points, closed to C point). For their bulk forms, the locations of VBM located between the C and D points, while the CBM sited between the Z and F points in the Brillouin-Zone. There are two isolate bands located at the VBM of their monolayer forms, which becomes obvious from LiP15, NaP15, to KP15, as shown in figures 3 (b), (d), and (f). Comparing to the P element, the contribution of electronic states to the band edge from alkali metal is really less. Therefore, we perform the project density of states of P element in monolayer MP15 as shown in the Supporting Information S2. These bands are mostly contributed by the p-orbit of P element (as shown in figure S2). Additionally, uniaxial strain in the xy plane of monolayer MP15 has a significantly smaller (< 0.1 eV under ± 2% strain) effect on the bandgap (figure S1 a-c) than the case in the black phosphorus (~0.26 eV under ± 2% strain).43



In generally, reducing the symmetry of the 2D lattice system could lead to high structural anisotropy and further cause a strong anisotropy on their electronic structures and optical properties. For instance, the band dispersion is very flag along the C → Γ for the CBM of monolayer KP15, which would have a larger electron effective mass compared to C → N direction, confining the transport of carriers along this direction and leading to anisotropic carrier transport in MP15. Therefore, we next consider the carrier mobility of monolayer MP15 by applying a standard 2D model as discussed in the Supporting Information S3. We define the direction along the fiber-like [P-15] unit as direction x, while the direction perpendicular to [P-15] unit (in-plane) was treated as direction y. Three properties, namely the carrier effective mass, the deformation potential Eiα, and the elastic modulus Cα in the transport direction, are considered as the most relevant factors determining the mobility. The calculated carrier mobility µα, elastic modulus Cα, deformation potential Eiα, and effective mass m*α for MP15 (M = Li, Na, K) employing PBE+TS are summarized in Table1. The electron (as well as hole) has a larger effective mass along y-direction compared with x-direction for all MP15 and the effective mass of electron along x-direction shows the smallest value in each member. Different from the case of violet phosphorus which has an almost isotropic elastic moduli (50 J/m2) for both special directions,39 all MP15 members have highly anisotropic elastic moduli. For all MP15 members, x-direction has larger value of elastic moduli than y-direction and the ratio Cx/Cy is above 7. This anisotropic elastic moduli ratio is larger than that in the black phosphorene case, which is 3.52.44 It's more complicated in the situation of the deformation potential Eiα . In each member of MP15, the deformation potential in x-direction appears higher anisotropy than that in the y-direction and the E hole 1x

being the smallest in magnitude. These three factors contribute together and result in a very high


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anisotropic carrier mobility of monolayer MP15: the carriers (both electron and hole) transport more easily along the fiber-like [P-15] unit (x-direction) for all MP15. At this direction, the charge prefers to transport through the hole rather electron. The ratio of hole and electron mobility along x-direction is about 12 for LiP15, 3 for NaP15, and 30 for KP15. Attributing to the square dependence of Eiα, the integrated effect generates a large hole mobility along x-direction for LiP15, which reaches 19450 cm2 V−1 s−1. In all MP15 members, the hole mobility along the [P-15] unit direction is the largest respectively. Considering that the optB88-vdW approach could get similar optimized structure with PBE+TS, we list the results employing the optB88-vdW approach in the Table S3. From the results we could suggest that through the specific values are different, the basic characters and change law are similar to which obtained by PBE+TS approach. Detailed results and discussion could be found in the Supporting Information S3.

Table 1. Carrier mobility µα, Elastic modulus Cα, Deformation Potential Eiα, and effective mass m*α for MP15 (M = Li, Na, K) employing PBE+TS. LiP15

NaP15

KP15

electron

hole

electron

hole

electron

hole

m*x /m0

0.43±0.005

0.70±0.006

0.51±0.006

1.47±0.014

0.44±0.003

1.31±0.022

m*y /m0

1.91±0.016

1.89±0.003

2.29±0.023

3.46±0.036

10.89±0.13

1.82±0.009

E1x[eV]

1.72±0.14

0.35±0.03

1.70±0.15

0.39±0.03

1.54±0.02

0.60±0.11

E1y[eV]

1.21±0.12

1.00±0.01

1.09±0.05

1.06±0.06

0.57±0.08

0.59±0.09

2

90.70±0.96

90.70±0.96

89.15±0.60

89.15±0.60

83.41±0.53

83.41±0.53

2

9.99±0.12

9.99±0.12

12.36±0.06

12.36±0.06

10.98±0.29

10.98±0.29

-1 -1

µx[10 cm V s ]

1.65±0.31

19.54±3.51

1.18±0.23

3.73±0.51

0.08±0.03

2.46±0.95

µy[103 cm2 V-1 s-1]

0.08±0.02

0.10±0.004

0.09±0.01

0.03±0.003

0.03±0.009

0.24±0.09

Cx[J/m ] Cy[J/m ] 3

2

In conclusion, we investigated the electronic and transport properties of the binary alkaline-metal phosphides family MP15 (M = Li, Na, K). We found them having the indirect bandgap around 1.2 eV in their bulk form, decreasing with the increasing thickness, reaching about 1.5 eV for monolayer



form. More importantly, the carrier transport property of MP15 shows high anisotropy: all members have higher hole mobility along the [P-15] unit direction, which enlarged the species of 2D anisotropic materials. Unlike the black phosphorene, the lowest unoccupied molecular orbitals (LUMO) of MP15 are well below that of the aqueous O2 acceptor state, which makes monolayer MP15 remains stable under ambient conditions. Additionally, the binding energy is in comparison to those of the black and violet phosphorus, suggesting the possibility of the experimental exfoliation. The MP15 family combines the advantages found for good chemical stability and appropriate bandgap for visible and near-infrared light range, with the advantage of having a high and highly anisotropic hole mobility, thereby making this family an excellent candidate for designing novel electronic and optoelectronic devices such as polarizers, polarization sensors, and 2D logic circuits device, which cannot be realized in isotropic 2D materials easily. We should also note that though MP15 shows promising application potential based on theoretical prediction, there are also still a lot of works to be done in the process to applications in the future. For example, the present synthetic method of MP15 is chemical vapor transport (CVT) which needs to seal the reactants in the quartz tube and heat them for more than 12 hours. Researchers should find out other rapid, large scale, and low-cost growth and doping technics, for example the chemical vapor deposition (CVD), which has been employed in other two-dimensional materials.45-47 In device preparation, referring to their relative high work function (about 5.0-5.2 eV), we suggest connecting MP15 with high-work-function metal, like Au or Pd, to lower the contact potential. Further research is also necessary for the issues about its compatibility with the traditional semiconductor technology.

Supporting Information Available


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Geometric properties and additional electric properties discussion of MP15, computational methods for carrier mobility supplied as Supporting Information.

Acknowledgments Financial support by the National Natural Science Foundation of China (NSFC) under Grant Nos. 61575010 and 51671006, the Beijing Municipal Natural Science Foundation (BNSF) under Grant No. 4162016, the Beijing Municipal Science and Technology Commission (BSTC) under Grant Nos. Z141109001814053 and Z151100003315018. The work was carried out at National Supercomputer Center in Tianjin, and the calculations were performed on TianHe-1 (A).

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as an Anisotropic Layered Semiconducting Material

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