Structure and Properties of Egyptian Blue Monolayer Family

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Letter

Structure and Properties of Egyptian Blue Monolayer Family: XCuSiO (X = Ca, Sr and Ba) 4

10

Yu Chen, Min Kan, Qiang Sun, and Puru Jena J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02770 • Publication Date (Web): 14 Jan 2016 Downloaded from http://pubs.acs.org on January 14, 2016

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The Journal of Physical Chemistry Letters

Structure and Properties of Egyptian Blue Monolayer Family: XCuSi4O10 (X = Ca, Sr and Ba) AUTHOR NAMES Yu Chen a, Min Kan a, b, Qiang Sun a, c, d,*, and Puru Jena d AUTHOR ADDRESS

a

Department of Materials Science and Engineering, Peking University, Beijing

100871, China b

Kuang-Chi Institute of Advanced Technology, Shenzhen 518057, China

c

Center for Applied Physics and Technology, Peking University, Beijing 100871,

China d

Department of Physics, Virginia Commonwealth University, Richmond, VA 23284,

USA AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

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ABSTRACT

Motivated by the recent experimental advances in exfoliating Egyptian Blue monolayers, we have carried out extensive calculations using density functional theory to understand their geometry, stability, mechanical properties, electronic structures and magnetism. Upon exfoliation from their bulks, XCuSi4O10 (X = Ca, Sr and Ba) monolayers are found to change symmetry from tetragonal to orthorhombic. They all satisfy Born criteria and are mechanically stable. Each Cu site carries a magnetic moment of 1.0 μB but with degenerate ferromagnetic and anti-ferromagnetic coupling states. From Ca to Sr and Ba, as the atomic number increases, the thickness, elastic constants, Young’s moduli and Poisson’s ratios of the monolayers increase, while the band gaps decrease. Applying strain can tune the magnitude of energy band gaps but the direct gap feature remains. Complementing the widely studied graphene, Mxenes, black phosphorus, and dichalcogenide sheets, the Egyptian Blue monolayers add additional features to the family of two-dimensional materials. TOC GRAPHICS


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Egyptian Blue (CaCuSi4O10) and Han Blue (BaCuSi4O10) are two of the valuable blue pigments with tremendous historical and cultural significance. Egyptian Blue, synthesized about 3600 BC in ancient Egypt, is the earliest artificial pigment found so far. It was widely used as decorations and then disseminated to the Mediterranean basin in Mesopotamia, Greece and areas of the ancient Roman Empire1-5. Han Blue has been used at least since 800 BC. Named by Fitzhugh in honor of the extraordinary achievements in the Han Dynasty (202 BC-220 AD), Han Blue was found in Chinese pottery, wall paintings and famous Chinese cultural heritage Terracotta Army of Qin Shihuang in Qing Dynasty1-2, 6-7. XCuSi4O10 (X = Ca, Sr and Ba), belonging to the larger gillespite-type group series of XMSi4O10 (X = Ca, Sr and Ba, M = Fe, Cu and Cr)8-9 were also found in natural minerals such as cuprorivaite, effenbergerite and wesselsite10-12, respectively. It is interesting to note that the XCuSi4O10 family has layered structures with the eight fold-coordinated alkaline-earth metal ions occupying the remaining half of the hole sites and acting as the counterions positioned between the two silicate layers. The [SiO4]4- tetrahedra are linked by the square-planar copper ions forming the framework of the layered structure and the chromophore copper ions are tightly bound in the silicate matrix, showing good in-layer stability.

Recently, effort has been made to exfoliate monolayers from Egyptian Blue and Han Blue materials. For example, Johnson-McDaniel et al.

13-14

discovered that

CaCuSi4O10 in Egyptian Blue can be exfoliated into monolayer nanosheets by simply stirring in hot water (~80℃), while BaCuSi4O10 in Han Blue can be exfoliated by



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ultrasonication in organic solvents. These studies13-15 reported exfoliation via basal plane cleavage along the {001} faces of the bulk crystal, the resulting monolayer structures have been characterized by transmission electron microscopy (TEM), powder X-ray diffraction (PXRD) and atomic force microscope (AFM). The strong near infrared (NIR) luminescence properties13-17 of both the bulk and monolayer forms of Egyptian blue and Han Blue family have wide applications in NIR-based biomedical imaging, silica-based optical amplifiers, security ink, laser technology and high-performance optical sensors. Although experimental advances have been made for synthesizing monolayers from Egyptian Blue and Han Blue, comprehensive theoretical understanding of these monolayers is still lacking, which motivated the present study. Here for the first time we carry out extensive first-principles calculations based on density functional theory (DFT) for shedding light on the stability, mechanical, magnetic and electronic properties of XCuSi4O10 (X = Ca, Sr, Ba) monolayers.

Our first-principles calculations are based on density functional theory (DFT) as implemented

in

Vienna

ab

initio

Simulation

Package

(VASP)18.

Perdew-Burke-Ernzerhof (PBE) function19 for generalized gradient approximation (GGA) is used for the electronic exchange-correlation potential. The interaction between electrons and nuclei is treated using the projected augmented wave (PAW) method20 with a plane-wave basis set. Due to the strong correlation in d subshells of Cu atoms, GGA + U method21 is used to calculate the electronic property of


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XCuSi4O10 monolayers, where Ueff = 2.5 eV is used22. For the calculations in 2D system, vacuum space of ～20 Å along the z direction is applied to avoid interactions between two neighboring images. All the structures have been fully relaxed through conjugated gradient scheme without any symmetric constraints. We apply (2 × 2) supercells to simulate the magnetic coupling between magnetic atoms. The reciprocal space is represented by the Monkhorst-Pack special k point method23 with (9 × 9 × 1) and (7 × 7 × 1) grid meshes for the (1 × 1) unit cell and the (2 × 2) supercell, respectively. The energy cutoff is set to 520 eV and the criteria of total energy and Hellman-Feymann force are 1 × 10−4 eV and 0.01 eV/Å, respectively. The accuracy of our computation is tested by calculating the lattice constants of CaCuSi4O10 bulk; the computed results (a = b = 7.32 Å, c= 15.12 Å) are in good agreement with the experimental values24 (a = b = 7.30 Å, c= 15.13 Å).

We first optimized the geometry of XCuSi4O10 monolayers as shown in Figure 1. One can see that monolayer can still keep the main features of the copper silicate framework. The geometric parameters are summarized in Table 1 and compared with their corresponding bulk phases24. In contrast to the tetragonal symmetry (space group P4/ncc) of their bulk phases, the crystallographic symmetry of monolayers reduces to orthorhombic with the space group of P21212 due to the lattice deformation caused by exfoliation. We find that [CuO4]6- square-planar ligands slightly distort in monolayer structures due to the O(1)-Cu-O(2) bonds and O(3)-Cu-O(4) bonds warping upward and downward along the diagonal directions, respectively, resulting in the O-Cu-O



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angle no longer being equal to 90ºand the Cu-O bond length of ~1.96 Å being larger than 1.90 Å in the bulk phase24. It is worth noting that the thicknesses of XCuSi4O10 monolayers and the perpendicular distances from X2+ to [CuO4]6- planar ligand (∆z and ∆d in Table 1) decrease significantly from their bulk values (here we define ∆z in XCuSi4O10 bulk as the half length of parameter c of XCuSi4O10 plus the diameter of corresponding alkali earth metal ions). From the data we find that ∆z in XCuSi4O10 bulk is approximately equal to ∆z in monolayers plus twice the difference of ∆d between bulk and monolayer, namely,  z  b u lk    z  m o n o la yer   2  [  d  b u lk    z  m o n o la yer  ].

This is consistent with the structural deformations caused by exfoliation. Hence, we conclude that the reduced thickness of XCuSi4O10 monolayers is mainly due to the reduction of the coordination numbers of alkaline-earth metal ions in monolayer structures resulting in alkaline-earth metal ions being closer to the copper silicate network. We note that the calculated thickness of CaCuSi4O10 monolayer is smaller than the experimental value.13 We believe that the experimental liquid exfoliation methods and surface hydration may keep the alkaline-earth metal ions from closing to the silicate network.

Table 1. Structural properties of the XCuSi4O10 monolayers. Shown are the lattice constants, a and b and the thicknesses, ∆z, the perpendicular distances from X2+ to [CuO4]6- planar ligand, ∆d, the bond lengths and bond angles in the monolayers compared to XCuSi4O10 bulks. 6 ACS Paragon Plus Environment

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Bulk (experiment24)

Monolayer CaCuSi4O10

SrCuSi4O10

BaCuSi4O10

CaCuSi4O10

SrCuSi4O10

BaCuSi4O10

a /Å

7.29

7.32

7.36

7.30

7.37

7.44

b /Å

7.79

7.87

7.87

7.30

7.37

7.44

∆z /Å

7.74

8.49

9.45

9.57

10.16

10.76

∆d /Å

0.32

0.54

0.87

1.24

1.38

1.52

Cu-O(1) /Å

1.96

1.96

1.96

1.92

1.92

1.92

Cu-O(3) /Å

1.96

1.97

1.97

1.92

1.92

1.92

O(1)-Cu-O(3) /º

83.3

84.6

86.2

90.0

90.0

90.0

O(2)-Cu-O(3) /º

97.6

95.3

93.6

90.0

90.0

90.0

Figure 1. (a) Top view and (b) side view of XCuSi4O10 monolayers with green, pink, gold, red balls representing X (X= Ca, Sr and Ba), Cu, Si and O, respectively. The unit cell is marked with the dashed box.

The mechanical properties of XCuSi4O10 monolayers are studied by calculating the total energy as a function of strain ε near the equilibrium position25. The resulting 7 ACS Paragon Plus Environment


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elastic constants are given in Table 2. According to the previous studies26-28, the elastic energy U(ε) of the orthorhombic 2D crystal can be expressed as U 

1 2

C 1 1 x x  2

1 2

C 22

2 yy

 C 12 xx 

 2 C 44 xy , 2

yy

where the x (y) axis refers to the x (y) direction in the unit cell (Figure 1), εij’s are the infinitesimal strain tensors and the Cij’s are the corresponding linear elastic constants to take care of Voigt index. For the orthorhombic system, there are nine independent components of the elastic constants, namely C11, C12, C13, C22, C23, C33, C44, C55, and C66, According to the Born criteria29, the mechanically stable conditions for orthorhombic crystals require satisfying the following relationships: (C 11  C 22  2 C 12 )  0 , (C 11  C 33  2 C 13 )  0 , (C 22  C 33  2 C 23 )  0 , C 11  0 , C 22  0 , C 33  0 , C 44  0 , C 55  0 , C 66  0 , (C 11  C 22  C 33  2 C 12  2 C 13  2 C 23 )  0 .

Due to the symmetry of 2D XCuSi4O10 monolayers, only four elastic constants (C11, C12, C22 and C44) given in the following relations need to be considered, (C 11  C 22  2 C 12 )  0 , C 11  0 , C 22  0 , C 44  0 , (C 11  C 22  2 C 12 )  0 .

From Table 2 we see that the calculated elastic constants satisfy the above criteria, indicating that XCuSi4O10 monolayers are mechanically stable. We plot the Young’s modulus E(θ) and Poisson’s ratio ν(θ) in the polar diagrams in Figure 2 by expressing the Young’s modulus and Poisson’s ratio along an arbitrary direction θ (θ being the angle relative to the positive x axis in the unit cell) as25


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C 1 1C 2 2  C 1 2 2

E ( ) 

C 1 1C 2 2  C 1 2 2

C 11 s  C 22 c  ( 4

4

C 44

C 1 1C 2 2  C 1 2

 2 C 12 )c s 2

2

2

(C 11  C 22 

2

)c s

2

C 44

 ( ) 

C 1 1C 2 2  C 1 2

 C 12 (c

4

 s ) 4

2

C 11 s

4

 C 22 c

4

 (

C 44

 2 C 12 )c s 2

2

where c = cos θ and s = sin θ.

Figure 2. Polar diagrams for the (a) E(θ) and (b) ν(θ) of XCuSi4O10 monolayers. Blue, magenta and orange lines are for monolayers of CaCuSi4O10, BaCuSi4O10 and SrCuSi4O10, respectively.

Table 2. Effective independent elastic constants (Cij, N∙m-1), Young’s modulus (E, N∙m-1), the average Young’s moduli (Eav, N∙m-1) and Poisson’s ratio (ν) of XCuSi4O10 monolayers. 9 ACS Paragon Plus Environment


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C11

C22

C12

C44

E

Eav

ν

CaCuSi4O10

84.9

104.8

7.1

33.7

79.6-104.2

87

0.07-0.20

SrCuSi4O10

88.5

115.6

10.2

34.2

82.6-114.4

92

0.09-0.24

BaCuSi4O10

92.4

125.9

15.1

36.1

87.6-123.4

98

0.12-0.26

We find that the elastic constants gradually increase from CaCuSi4O10 to SrCuSi4O10 and BaCuSi4O10. As shown in Figure 2a, E(θ) and ν(θ) exhibit large anisotropic linear elastic features. The Young’s modulus along the x direction is unequal to that along the y direction due to the orthorhombic symmetry. The largest Young’s modulus appears in the y direction and Emax values for CaCuSi4O10, SrCuSi4O10 and BaCuSi4O10 monolayers are 104.2, 114.4 and 123.4 N∙m-1, respectively. While the minimum values of the Young’s modulus for CaCuSi 4O10, SrCuSi4O10 and BaCuSi4O10 monolayers occur near the diagonal directions with the Emin of 79.6, 82.6 and 87.6 N∙m-1, respectively, the average Young’s moduli (Eav) of XCuSi4O10 monolayers are 87, 92 and 98 N∙m-1, respectively. These are comparable to MoS230 monolayer but only about one-third of that in graphene25, indicating relatively good mechanical property and more stretchable nature of XCuSi4O10 monolayers. The Poisson’s ratio of XCuSi4O10 monolayers shows petal-shaped feature in Figure 2b, which is consistent with the symmetrical characteristic of monolayer structure. The maximum (νmax) and minimum (νmin) values occur in the diagonal direction and x direction listed in Table 2, respectively. From calculations, we find that the mechanical properties are enhanced as the atomic number increases and relatively good mechanical properties of XCuSi4O10 monolayers indicate that these 10 ACS Paragon Plus Environment

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2D materials may have potential for nanometer device applications.

Figure 3. (a) Isosurface (0.05 e/Å3) and (b) 2D slice of spin density (ρ↑- ρ↓) of CaCuSi4O10 monolayer. (c) Energy level diagram of Cu2+ ion in tetragonally distorted octahedral crystal field environment.

The spin-polarized calculations are carried out to study the magnetic properties of XCuSi4O10 monolayers, we found that the spin magnetic moment per unit cell is 2 μB. Detailed analysis indicates that each Cu2+ ion carries a magnetic moment of 0.62 μB and polarizes its neighboring O(1) [O(2)] and O(3) [O(4)] atoms which contribute only a small magnetic moment of about 0.09 μB and 0.07 μB, respectively. Analyzing the contributions to the magnetic moments, we find that they are mainly originate from the unpaired dx2-y2 orbital of copper ion and partially spin polarized O atoms linked with Cu2+ ion. This can be visualized from the isosurface and 2D slice of spin charge density distribution (ρ↑ - ρ↓) in Figure 3a and 3b. The octahedral crystal field of Cu2+ ion changes to square-planar crystal field as a result of lattice constraints and the Jahn-Teller effect10. As shown in Figure 3c, the d orbitals of Cu2+ ion in square-planar ligand are divided into two components, namely T2g orbitals (dxy, dxz 11 ACS Paragon Plus Environment


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and dyz ) and Eg orbitals (dx2-y2 and dz2 ). The dx2-y2 orbital in A1g state possesses the highest energy due to the ligand atoms directly affecting the central Cu2+ ion along the axial direction. We note that the original degenerate dxz and dyz orbitals in Eg state further split thus lifting the degeneracy due to the distortions in the [CuO4]6square-planar crystal field. To further understand the magnetic and corresponding electronic properties of the system, we plot the band structures and partial density of states (PDOS) of XCuSi4O10 monolayers in Figure 4. Examination of PDOS near the Fermi level shows that the Cu 3d-orbitals hybridize with O 2p-orbitals, and the magnetic property is mainly contributed by the Cu 3d-electrons and O 2p-electrons. The valence and conduction bands are quite flat leading to large effective masses of electrons and holes. Band structures show that XCuSi4O10 monolayers are direct band-gap semiconductors with the valence band maximum (VBM) and the conduction band minimum (CBM) locating at the S point in the spin-up as well as in spin-down channel. The band gaps are found to be slightly reduced from 1.77 to 1.75 and 1.73 eV, when going from CaCuSi4O10 to SrCuSi4O10 and BaCuSi4O10 monolayers. For CaCuSi4O10 monolayer (Figure 4a), we find that the band gap is larger than its bulk value of 1.6 eV22. In the spin-up channel, VBM and CBM respectively locate at the S and Γ points resulting in an indirect band gap of 3.60 eV. In the spin-down channel, on the other hand, the valence bands locate below the Fermi level and both VBM and CBM are at the S point with a direct band gap of 2.30 eV. For SrCuSi4O10 and BaCuSi4O10 monolayers (Figure 4b and 4c), we observe that the electronic structures are similar to those of 12 ACS Paragon Plus Environment

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CaCuSi4O10 monolayer due to the isomorphs nature, but the band gaps decrease to 3.55 and 3.39 eV in the spin-up channel, and to 2.24 and 2.18 eV in the spin-down channel.

Figure 4. Valence bands (VB) and conduction bands (CB) near the Fermi level, band structures and the corresponding PDOS for monolayers of (a) CaCuSi4O10, (b) SrCuSi4O10 and (c) BaCuSi4O10. The high symmetric k points are Γ (0, 0, 0), X (0, 1/2, 0), S (1/2, 1/2, 0) and Y (0, 1/2, 0) in reciprocal space.

Since a tensile strain can be applied experimentally to a 2D material31-34, we study 13 ACS Paragon Plus Environment


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if biaxial tensile strain can be used to tune the electronic properties in these materials (Figure 5a). We define the biaxial tensile strain as (L - L0)/L0*100%, where L0 (namely a and b in Table 1) and L are the lattice constants of XCuSi4O10 monolayers in equilibrium and in strained states, respectively. Table 3 summarizes the functional relations of energy band gaps of XCuSi4O10 monolayers with respect to the strain. Interestingly, as shown in Figure 5b, the energy band gaps decrease linearly with the magnitude of strain, but these structures still retain a direct band gap. With 9% tensile strain, we find that the total energy band gaps of XCuSi4O10 monolayers may be tuned in a range of about 0.6 eV. In the spin-up channel, the band gaps quadratically change with the strain, resulting in the maximum band gaps around ~3% strain for CaCuSi4O10 and SrCuSi4O10 monolayers and around ~4% strain for BaCuSi4O10 monolayer. In the spin-down channel, the band gaps linearly decrease with strain. When we apply 9% strain, the VBM in the spin-up and spin-down channels nearly converge, thus, tuning the total energy band gaps to the same value as those in the spin-down channel. Importantly, the direct band gaps exhibited in the monolayers can be effectively tuned to 1.5 eV with less than 5% strain. Thus, the Egyptian Blue monolayers with direct band gaps of this magnitude can be applied for solar energy harvesting as well as solar related optoelectronic devices.


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Figure 5. (a) Illustration of biaxial tensile strain of XCuSi4O10 monolayers. (b) Band gaps as a function of the biaxial tensile strain. Blue, magenta and orange fitting curves are for CaCuSi4O10, SrCuSi4O10 and BaCuSi4O10 monolayers, respectively.

Table 3. Functional relations of energy band gaps (Eg/eV) of XCuSi4O10 monolayers with respect to the biaxial tensile strain (xs/%). CaCuSi4O10

SrCuSi4O10

BaCuSi4O10

Band gap

Eg = 1.772 - 0.058*xs

Eg = 1.754 - 0.065*xs

Eg = 1.731- 0.070*xs

Spin-down

Eg = 2.302 - 0.119*xs

Eg = 2.244 - 0.124*xs

Eg = 2.182 - 0.126*xs

Spin-up

Eg = 3.604 - 0.008*xs2

Eg = 3.537 - 0.009*xs2

Eg = 3.390 - 0.010*xs2

+ 0.046*xs

+ 0.059*xs

+ 0.086*xs

To further study the preferred magnetic coupling of XCuSi4O10 monolayers, we calculate the total energy based on (2 × 2) supercells for different magnetic coupling configurations (Figure 6): (a) ferromagnetic (FM) coupling state and (b - d) three possible anti-ferromagnetic (AFM 1-3) coupling states (We also calculated other magnetic coupling configurations but are not discussed here due to their higher energies). For CaCuSi4O10 monolayer, calculations show that the AFM-2 and AFM-3 15 ACS Paragon Plus Environment


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coupling states are nearly degenerate in energy and FM coupling state has the lowest energy, which lies 0.13 meV and 0.97 meV lower than the AFM-1 and AFM-2 (AFM-3) coupling states, respectively. Same situations are found for SrCuSi 4O10 and BaCuSi4O10 monolayers, where the AFM-2 and AFM-3 states are also degenerate, and the FM coupling state is found to be the most stable state. For SrCuSi4O10 monolayer, the total energies of FM states lie only 0.25 meV and 0.89 meV lower in energy than those of AFM-1 and AFM-2 (AFM-3) coupling states, and 0.22 meV and 0.69 meV for BaCuSi4O10 monolayer. Thus, the magnetic exchange energy Ex (Ex = EFM - EAFM) between FM and AFM states is significantly smaller than thermal fluctuation kT (~26 meV) at room temperature. Hence, we conclude that the ferromagnetic and antiferromagnetic states are degenerate and XCuSi4O10 monolayers are paramagnetic at room temperature. The paramagnetic state of XCuSi4O10 monolayers can be understood by analysing the geometric and electronic structures. In fact, the magnetic coupling between two copper ions at long distance requires effective conduction of p-electrons in the silicate framework, though the PDOS shows O 2p-electrons hybridized with Cu 3d-eletrons. The semiconducting properties of the system make the magnetic mediation less effective, thus resulting in very small magnetic exchange energies and paramagnetic behavior. In fact, a recent report35 showed that Han Blue (BaCuSi4O10) bulk material is paramagnetic over a wide temperature range, which is in good agreement with our results due to the isolated copper ions in the structure. In contrast, the Chinese purple BaCuSi2O6 shows spin gapped antiferromagnetic property due to the presence of Cu-Cu dimers and dark blue BaCu2Si2O7 exhibits 16 ACS Paragon Plus Environment

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quasi-one-dimensional antiferromagnetic property36. Thus, Han Blue displays different magnetic properties as compared to Chinese purple and dark blue materials.

Figure 6. Different magnetic coupling configurations and the corresponding magnetic moments of XCuSi4O10 monolayers with different spin configurations: (a) ferromagnetic (FM), (b), (c) and (d) anti-ferromagnetic (AFM-1, AFM-2 and AFM-3).

Due to the similarity between XCuSi4O10 monolayers, we consider CaCuSi4O10 as an example to discuss the electronic properties of different magnetic coupling states. The electronic structure of FM state is similar to our previous discussion in Figure 4a. As shown in Figure 7a - 7c, there is no spin splitting between spin-up and spin-down states in the band structures, which are significantly different from the FM states. We find that the AFM coupling states are still direct band-gap semiconductors with similar band gaps of 1.77 and 1.78 eV for AFM-1 state and AFM-2 (AFM-3) state, respectively. For AFM-2 and AFM-3 states, the VBM and CBM are also located 17 ACS Paragon Plus Environment


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at the S point, while for AFM-1 state the VBM and CBM are both located at the Г point. The PDOS of AFM states show similar distribution of Cu d-electrons and O p-electrons near the Fermi level. From the isosurface of spin density of AFM states (Figure 7d - 7f), one can find that all O atoms are ferromagnetically spin-polarized by the neighboring Cu2+ ion and the distribution of spin charges is consistent with the corresponding antiferromagnetic coupling configurations.

Figure 7. (a - c) Electronic structures and (d - f) corresponding spin density isosurface (0.05 e/Å3) for AFM-1, AFM-2 and AFM-3 coupling states of CaCuSi4O10 monolayer, 18 ACS Paragon Plus Environment

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red and blue shading are for spin-up and spin-down charges, respectively.

In summary, motivated by the recent experimental advances in the synthesis of Egyptian Blue monolayer family, we carried out extensive calculations using spin-polarized density functional theory to study the geometry, stability, mechanical properties, electronic structures and magnetism of XCuSi4O10 (X = Ca, Sr and Ba) monolayers. We found that when exfoliated from their bulks, these materials change symmetry from tetragonal to orthorhombic. They are found to be mechanically stable and show mechanical anisotropy. When going from Ca to Sr and Ba, as the atomic number increases, the thickness, elastic constants, Young’s modulus and Poisson’s ratios of the monolayers gradually increase while the direct band gaps decrease. Applying biaxial tensile strain can linearly tune the magnitude of energy band gaps but the direct gap feature remains. Each Cu site carries a magnetic moment of 1.0 μB but the ferromagnetic and anti-ferromagnetic coupling states are degenerate due to the small magnetic exchange energy. These findings not only provide a better understanding of the ancient pigments in two-dimensional (2D) configurations but also expand the family of 2D materials with many potential applications. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

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The authors declare no competing financial interests. ACKNOWLEDGMENT This work is partially supported by grants from the National Natural Science Foundation of China (NSFC-11274023 and 21573008), and from the National Grand Fundamental Research 973 Program of China (2012CB921404). PJ acknowledge the grants from the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award # DE-FG02-96ER45579 and # DE-FG02-11ER46827. We acknowledge Professor Young Hee Lee at Sungkyunkwan University for his stimulating discussions. REFERENCES (1) Berke, H. Chemistry in Ancient Times: The Development of Blue and Purple Pigments. Angew. Chem. Int. Ed. 2002, 41, 2483-2487. (2) Berke, H. The Invention of Blue and Purple Pigments in Ancient Times. Chem. Soc. Rev. 2007, 36, 15-30. (3) Bredal-Jorgensen, J.; Sanyova, J.; Rask, V.; Sargent, M. L.; Therkildsen, R. H. Striking Presence of Egyptian Blue Identified in a Painting by Giovanni Battista Benvenuto from 1524. Anal. Bioanal. Chem. 2011, 401, 1433-1439. (4) Pages-Camagna, S.; Colinart, S.; Coupry, C. Fabrication Processes of Archaeological Egyptian Blue and Green Pigments Enlightened by Raman Microscopy and Scanning Electron Microscopy. J. Raman Spectrosc. 1999, 30, 313-317. (5) Jaksch, H.; Seipel, W.; Weiner, K. L.; Goresy, A. E. Egyptian Blue — Cuprorivaite a Window to Ancient Egyptian Technology. Naturwissenschaften 1983, 70, 525-535. (6) Zycherman, E. W. F. a. L. A. An Early Man-Made Blue Pigment from China: Barium Copper Silicate. Stud. Conserv. 1983, 28, 15-23. 20 ACS Paragon Plus Environment

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(7) FitzHugh, E. W.; Zycherman, L. A. A Purple Barium Copper Silicate Pigment from Early China. Stud. Conserv. 1992, 37, 145-154. (8) Hazen, R. M.; Burnham, C. W. The Crystal Structures of Gillespite I and II: A Structure Determination at High Pressure. Am. Mineral. 1974, 59, 1166-1176. (9) Miletich, R.; Allan, D. R.; Angel, R. J. The Synthetic Cr2+ Silicates BaCrSi4O10 and SrCrSi4O10: The Missing Links in the Gillespite-Type ABSi(4)O(10) Series. Am. Mineral. 1997, 82, 697-707. (10) Pozza, G.; Ajò, D.; Chiari, G.; De Zuane, F.; Favaro, M. Photoluminescence of the Inorganic Pigments Egyptian Blue, Han Blue and Han Purple. J. Cult. Herit. 2000, 1, 393-398. (11) Giester, G.; Rieck, B. Effenbergerite, BaCuSi4O10, a New Mineral from the Kalahari Manganese Field, South-Africa - Description and Crystal-Structure. Mineral. Mag. 1994, 58, 663-670. (12) Giester, G.; Rieck, B. Wesselsite, SrCuSi4O10, a Further New Gillespite-Group Mineral from the Kalahari Manganese Field, South Africa. Mineral. Mag. 1996, 60, 795-798. (13) Johnson-McDaniel, D.; Barrett, C. A.; Sharafi, A.; Salguero, T. T. Nanoscience of an Ancient Pigment. J. Am. Chem. Soc. 2013, 135, 1677-1679. (14) Johnson-McDaniel, D.; Salguero, T. T. Exfoliation of Egyptian Blue and Han Blue, Two Alkali Earth Copper Silicate-Based Pigments. J. Visualized Exp. 2014, 86, 51686-51695. (15) Chen, Y.; Shang, M.; Wu, X.; Feng, S. Hydrothermal Synthesis, Hierarchical Structures and Properties of Blue Pigments SrCuSi4O10 and BaCuSi4O10. CrystEngComm 2014, 16, 5418-5423. (16) Chen, W.; Shi, Y.; Chen, Z.; Sang, X.; Zheng, S.; Liu, X.; Qiu, J. Near-Infrared Emission and Photon Energy Upconversion of Two-Dimensional Copper Silicates. J. Phys. Chem. C 2015, 119, 20571-20577. (17) Borisov, S. M.; Wurth, C.; Resch-Genger, U.; Klimant, I. New Life of 21 ACS Paragon Plus Environment


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Ancient Pigments: Application in High-Performance Optical Sensing Materials. Anal. Chem. 2013, 85, 9371-9377. (18) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. (19) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (20) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758-1775. (21) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Electron-Energy-Loss Spectra and the Structural Stability of Nickel Oxide: An Lsda+U Study. Phys. Rev. B 1998, 57, 1505-1509. (22) Li, Y. J.; Ye, S.; Wang, C. H.; Wang, X. M.; Zhang, Q. Y. Temperature-Dependent near-Infrared Emission of Highly Concentrated Cu2+ in CaCuSi4O10 Phosphor. J. Mater. Chem. C 2014, 2, 10395-10402. (23) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188-5192. (24) Kendrick, E.; Kirk, C. J.; Dann, S. E. Structure and Colour Properties in the Egyptian Blue Family, M1−xM′xCuSi4O10, as a Function of M, M′ Where M, M′=Ca, Sr and Ba. Dyes Pigm. 2007, 73, 13-18. (25) Cadelano, E.; Palla, P. L.; Giordano, S.; Colombo, L. Elastic Properties of Hydrogenated Graphene. Phys. Rev. B 2010, 82, 235414-235421. (26) Singh, R.; Bester, G. Hydrofluorinated Graphene: Two-Dimensional Analog of Polyvinylidene Fluoride. Phys. Rev. B 2011, 84, 155427-155433. (27) Yu, J.; Sun, Q.; Kawazoe, Y.; Jena, P. Stability and Properties of 2d Porous Nanosheets Based on Tetraoxa[8]Circulene Analogues. Nanoscale 2014, 6, 14962-14970. (28) Zhang, W. B.; Song, Z. B.; Dou, L. M. The Tunable Electronic Structure and Mechanical Properties of Halogenated Silicene: A First-Principles Study. J. 22 ACS Paragon Plus Environment

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TOC graphic 49x49mm (300 x 300 DPI)

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Figure 1. (a) Top view and (b) side view of XCuSi4O10 monolayers with green, pink, gold, red balls representing X (X= Ca, Sr and Ba), Cu, Si and O, respectively. The unit cell is marked with the dashed box. 80x81mm (300 x 300 DPI)



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Figure 2. Polar diagrams for the (a) E(θ) and (b) ν(θ) of XCuSi4O10 monolayers. Blue, magenta and orange lines are for monolayers of CaCuSi4O10, BaCuSi4O10 and SrCuSi4O10, respectively. 98x129mm (300 x 300 DPI)


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Figure 3. (a) Isosurface (0.05 e/Å3) and (b) 2D slice of spin density (ρ↑- ρ↓) of CaCuSi4O10 monolayer. (c) Energy level diagram of Cu2+ ion in tetragonally distorted octahedral crystal field environment. 41x11mm (300 x 300 DPI)



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Figure 4. Valence bands (VB) and conduction bands (CB) near the Fermi level, band structures and the corresponding PDOS for monolayers of (a) CaCuSi4O10, (b) SrCuSi4O10 and (c) BaCuSi4O10. The high symmetric k points are Γ (0, 0, 0), X (0, 1/2, 0), S (1/2, 1/2, 0) and Y (0, 1/2, 0) in reciprocal space. 129x224mm (300 x 300 DPI)


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Figure 5. (a) Illustration of biaxial tensile strain of XCuSi4O10 monolayers. (b) Band gaps as a function of the biaxial tensile strain. Blue, magenta and orange fitting curves are for CaCuSi4O10, SrCuSi4O10 and BaCuSi4O10 monolayers, respectively. 52x19mm (300 x 300 DPI)



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Figure 6. Different magnetic coupling configurations and the corresponding magnetic moments of XCuSi4O10 monolayers with different spin configurations: (a) ferromagnetic (FM), (b), (c) and (d) anti-ferromagnetic (AFM-1, AFM-2 and AFM-3). 85x97mm (300 x 300 DPI)


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Figure 7. (a - c) Electronic structures and (d - f) corresponding spin density isosurface (0.05 e/Å3) for AFM1, AFM-2 and AFM-3 coupling states of CaCuSi4O10 monolayer, red and blue shading are for spin-up and spin-down charges, respectively. 145x141mm (300 x 300 DPI)


Structure and Properties of Egyptian Blue Monolayer Family

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