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CHIN. PHYS. LETT. Vol. 32, No. 10 (2015) 106201 External-Strain-Induced Raman Scattering Modification in ๐-C3 N4 Structures * LI Ting-Hui(ๆๅปทไผ)1** , LI Hai-Tao(ๆๆตทๆถ)2 , PAN Jiang-Hong(ๆฝๆฑๆดช)3 , GUO Jun-Hong(้ญไฟๅฎ)4 , HU Fang-Ren(่ก่ณไป)4** 1 2 College of Electronic Engineering, Guangxi Normal University, Guilin 541004 Department of Physics, Chengde Teachers College for Nationalities, Chengde 067000 3 College of Physical and Technology, Guangxi Normal University, Guilin 541004 4 School of Optoelectronic Engineering and Grüenberg Research Centre, Nanjing University of Posts and Telecommunications, Nanjing 210023 (Received 7 May 2015) Regulation of optical properties and electronic structure of graphitic carbon nitride (๐-C3 N4 ) via external strain has attracted much attention due to its potential in photocatalyst and electronic devices. However, the identification of ๐-C3 N4 structure transformation induced by strain is greatly lacking. In this work, the Raman spectra of ๐-C3 N4 with external strain are determined theoretically based on the density function theory. Deformation induced by external strain not only regulates the Raman mode positions but also leads to a Raman mode splitting, which can be ascribed to crystal symmetry destruction by strain engineering. Our results suggest the use of Raman scattering in structural identification in deformed ๐-C3 N4 structure. PACS: 62.23.Eg, 67.80.dm, 68.55.−a DOI: 10.1088/0256-307X/32/10/106201 Graphitic carbon nitride (๐-C3 N4 ) nanostructures have attracted much attention due to their interesting applications such as degradation of organic contaminants, water-splitting to prepare hydrogen and oxygen, photocatalyst, and optoelectronic devices.[1−8] Therefore, many nanostructures with hypothetical crystalline phases such as ๐ฝ-C3 N4 , defect zinc-blende cubic-C3 N4 , ๐ผ-C3 N4 , and cubic-C3 N4 are fabricated to explore their physical properties.[9−13] Many reports disclose that their optical, electronic, phonon characteristics are strongly dependent on the intrinsic structural transformation. Their bulk moduli ๐ต were predicted to be 427, 425, 425, and 496 GPa, respectively.[14] The graphitic-C3 N4 phase with very low bulk modulus (๐ต = 51 GPa) was found to be the most energetically favorable structure, and becomes a research focus. In experiments, different nanostructures can be obtained, which exhibit obvious, different physical characteristics. Comprehensive experimental and theoretical analysis disclose that its excellent physical characteristics is strongly related with intrinsic microstructures transformations. Thus the identification of ๐-C3 N4 structure becomes an important physical problem. Raman scattering has proved to be a useful tool to identify the microstructure changes,[15−17] which is sensitive to slight variation in lattice symmetry. The morphology transformation and intrinsic defect will result in a local deformation, which is equated with an applied strain. This physical mechanism may be related with its novel phonon behavior. For this purpose, one needs to understand the responses of ๐-C3 N4 structural deformation to different strain fields. However, the understanding of strain-dependent Raman scattering in ๐-C3 N4 is quite limited so far. External strain is unavoidable especially in the fabrication of the nanostructures and thin films. The responses of a nanostructure to the external strain are determined by its mechanical properties, which are strongly influenced by introducing a specific substrate in the fabrication of the ๐-C3 N4 structure. For instance, it has recently been proposed that the strain induced by absorption of atoms on graphene nanoribbons can be strong so that it may fold the nanoribbons forming single-walled nanotubes.[18] We have known that the structural deformation can affect the ๐-C3 N4 structure phonon behavior; if the strain is applied on this system, many new physical phenomena will be presented. This may lead to more complicated behavior in their Raman spectra in strain-related ๐C3 N4 nanostructures. However, our understanding of strain-dependent Raman scattering properties of ๐C3 N4 is very limited so far. In this work, ๐-C3 N4 is chosen as the model system to study the relationship between external strain and Raman scattering. The theoretical derivation discloses that asymmetric deformation induced by uniaxial strain not only regulates the Raman mode positions but also leads to the Raman mode at ∼1032.5 cm−1 split into two Raman peaks. However, this transition is obviously different from symmetric deformation induced by biaxial strain and some new vibration modes * Supported by the National Natural Science Foundation of China under Grant Nos 61264008 and 61274121, and the Natural Science Foundation of Jiangsu Province under Grant No BK2012829. ** Corresponding author. Email: [email protected]; [email protected] © 2015 Chinese Physical Society and IOP Publishing Ltd 106201-1 CHIN. PHYS. LETT. Vol. 32, No. 10 (2015) 106201 are activated to display in their Raman spectra, and their behavior is also strongly dependent on the applied strain, which can be used as a fingerprint to identify the deformed ๐-C3 N4 microstructure. (a) and bond length of C and N atoms are ๐C /๐N = 0.86, ๐C1−N1 /๐C2−N2 = 0.91, respectively. The optimized cell eigenfrequency can be simply written as[21] ๐02 = 1/๐C(N) (b) ๐ ∑๏ธ ๐C1N1(C2N2) , (2) ๐=1 where ๐C(N) is the C(N) atom mass and ๐C1N1(C2N2) is the corresponding parameters determined by C1N1 and C2-N2 bond lengths and force constants. Those formulas indicate that the Raman mode behavior strongly depends on their symmetrical structure change. The deformed ๐-C3 N4 structure is characterized by two different types along ๐ฅ or ๐ฆ direction, as shown in Fig. 1(d). The optimized primitive (1 × 1) ๐-C3 N4 with ๐ = ๐ = 4.808 Å lattice constants is shown in Fig. 1(d). The bond lengths and angles of C1–N1 and C2–N2 are different obviously, and the optimized structure leads to three typical Raman modes E(3), A1(4) and A1(1), as shown in Figs 1(a)–1(c), respectively. We can infer that those Raman modes with different vibration directions can exhibit different Raman features, which also strongly depend on the structural deformation. A1(4) E(3) (c) (d) ๏ฑ๏ฑ ๏ตโฒ ๏ธ๏ถ C1 A A y A1(1) C2 ๏ฑ๏ฑ ๏ทโฒ ๏น๏ณ ๏ท ๏ฑโฒ๏ด ๏ณ ๏ฑโฒ๏ณ N1 N2 x (e) (f) S(1) Raman intensity (arb. units) (a) S(2) Fig. 1. (Color online) Schematic Raman modes of the ๐C3 N4 structure. Grey and blue spheres stand for C and N atoms, respectively. Different C and N atoms are also marked. Our calculations were performed under the framework of density functional theory as implemented in the CASTEP package. Electron–ion interactions were described by the projector augmented plane wave method, and the wave function was expanded in a plane wave basis set with an energy cutoff of 500 eV. The ๐ points in the Brillouin zone were sampled on a 5 × 5 × 1 mesh. The norm-conserving pseudopotential method is chosen together with the gradient correction and the Perdew–Burke–Ernzerhof potential function.[19,20] The calculated lattice constants of ๐C3 N4 are ๐ = 4.80766 Å and ๐ = 4.80766 Å, respectively. Finally, the optimized geometrical structure is employed to construct and to diagonalize the Hessian matrix ∑๏ธ ๐,๐′ 1 ๐,๐′ ๐ ′ ๐−๐๐·๐๐ผ , (1) ๐ท๐ผ,๐ผ ′ = √ ๐๐ ๐๐′ ๐ผ ๐ผ,๐ผ ′ ๐,๐ where ๐๐ผ,๐ผ ′ are the matrix force constants related with the bond length and the bond angle. The vibrational frequencies are obtained as the square roots of the phonon wavevector ๐ = 0. The ratio of atom mass (b) =-0.02 E(3) (c) =0.00 A1(1) S1 A1(4) =0.02 S2 (d) =0.04 (e) =0.06 (f) 600 =0.08 800 1000 1200 Raman shift (cm 1400 -1 ) Fig. 2. (Color online) Calculated Raman spectra of the ๐-C3 N4 structure with asymmetric deformation induced by the strain along the ๐ฅ-direction. To more clearly display the Raman feature, the Raman spectra of ๐-C3 N4 with asymmetric deformation along the ๐ฅ direction are calculated, as shown in Fig. 2. Without the external strain, three typical Raman modes are located at 1032.9 cm−1 (A1(1) mode), 1471.5 cm−1 (A1(4) mode), and 682.7 cm−1 (E(3) mode), which are in agreement with the previous report.[14] To verify the correctness of our calculated results, comparisons between calculated results and experiments are also carried out. The measured Raman spectra display an obvious broad Raman peak at 1480 cm−1 (A1(4) mode),[1] which is consistent with 106201-2 CHIN. PHYS. LETT. Vol. 32, No. 10 (2015) 106201 (a) Raman intenstiy (arb. units) (b) deformation along the ๐ฅ-direction, while the peak positions of S1 and S2 (as shown in Figs. 5 and 6) are different. For comparison, the Raman spectra of ๐C3 N4 with symmetric deformation induced by biaxial strain are also calculated, as shown in Fig. 4. We can see that the symmetric deformation (๐ฅ๐ฆ = 0.02) also leads to A1(1) mode splitting, and the S1 and S2 positions can be blue-shifted (๐ฅ๐ฆ = −0.02) and red-shifted (๐ฅ๐ฆ = 0.04) by the biaxial strain, which are consistent with those of the asymmetric deformation induced by uniaxial strain. However, further stretched deformation (๐ฅ๐ฆ = 0.06) will render the S2 mode further splitting into three small peaks, which can be ascribed to degenerated degradation of the Raman mode. The splitting energy between those peaks (marked by S2) is further increased (๐ฅ๐ฆ = 0.08), and the S1 mode also begins to split, meanwhile the average position is blue-shifted instead. All results imply that the larger symmetric deformation can lead to many new Raman modes, which has a more complicated Raman scattering behavior. (a) =-0.02 E(3) A1(1) S1 (c) =0.00 A1(4) S2 =-0.02 (b) Raman intensity (arb. units) our calculated conclusion. The slightly different peak position can be ascribed to the lower crystallization of the experimental sample. When a uniaxial strain along the ๐ฅ-direction is applied, the degenerate Raman mode (A1(1) mode) is split into two Raman peaks (marked by S1 and S2). As asymmetric deformation induced by uniaxial strain along the ๐ฅ-direction increases (๐ฅ = 0.02–0.08), the separation between S1 and S2 peaks is strengthened obviously, the S1 modes remain constant at around A1(1) mode position, while the S2 modes are red-shifted sharply. On the contrary, the E(3) and A1(4) modes with asymmetric deformation cannot be shifted significantly. The compressed deformation (๐ฅ = −0.02) not only causes A1(1) mode splitting but also leads to S1 and S2 peak blue-shifts obviously, which can be attributed to its force constant changes. When an asymmetric strain is applied along the ๐ฅ-direction, the lattice constants will be changed into ๐ = 4.711 Å, ๐ = 4.801 Å form ๐ = ๐ = 4.808 Å (they are not equivalent). The asymmetric deformation will make the bond length and the bond angle change (for example, the C2–N2 bond is compressed into 1.44 Å from 1.47 Å), the structural transformation finally leads to the splitting of S1 and S2. The atom vibration model illustrations of S1 and S2 are displayed in Figs. 1(e) and 1(f), respectively. We can see that the symmetrical destruction makes the new Raman mode exhibit in Raman spectra, which can be used to identify the ๐-C3 N4 microstructure transformation. =0.02 E(3) (c) A1(1) =0.00 S2 S1 (f) 450 =0.04 S1 S1 675 S2 =0.06 S2 900 =0.08 1125 Raman shift (cm (d) =0.04 (e) =0.06 (f) 600 1000 1200 Raman shift (cm 1350 1575 -1 ) Fig. 4. Calculated Raman spectra of the ๐-C3 N4 structure with symmetric deformation induced by the strain along the ๐ฅ๐ฆ-direction. =0.08 800 =0.02 (d) (e) A1(4) 1400 -1 ) Fig. 3. Calculated Raman spectra of the ๐-C3 N4 structure with asymmetric deformation induced by the strain along the ๐ฆ-direction. To further disclose the asymmetric deformation effect, the Raman spectra of ๐-C3 N4 with asymmetric deformation along the ๐ฆ-direction are also calculated, as shown in Fig. 3. The Raman spectrum feature of ๐-C3 N4 with asymmetric deformation along the ๐ฆ-direction is similar to that of the asymmetric To compare the change of peak position, the S1 and S2 peak positions as a function of the external strain (deformation along ๐ฅ, ๐ฆ, ๐ฅ๐ฆ) is calculated, as shown in Figs. 5 and 6, respectively. When the ๐C3 N4 structure is compressed by the external strain (๐ฅ = −0.03, ๐ฆ = −0.03, and ๐ฅ๐ฆ = −0.03), the Raman mode of S1 is blue-shifted form 1032.4 cm−1 (the ๐ฅ-direction) to 1050.6 cm−1 (the ๐ฆ-direction) and 1100.8 cm−1 (the ๐ฅ๐ฆ-direction). The asymmetric stretched deformation (along the ๐ฅ- or ๐ฆ-direction) makes the S1 mode decrease monotonously, and the difference of the S1 peak position between ๐ฅ-strain and ๐ฆ-strain is very weak. However, the S1 peak position of ๐-C3 N4 with increasing symmetric deformation red-shifts reaches a minimum value (567.8 cm−1 ) at 106201-3 CHIN. PHYS. LETT. Vol. 32, No. 10 (2015) 106201 ) -1 Peak (S2) position (cm 1200 1150 1100 1050 1000 950 900 S2 S2 S2 -0.02 0.00 0.02 0.04 0.06 0.08 Strain Fig. 6. (Color online) The calculated S2 mode peak position of the ๐-C3 N4 structure with different deformations (๐ฅ, ๐ฆ and ๐ฅ๐ฆ) as a function of the applied strain, respectively. S1 S1 S1 Strain 0.04 0.06 360 0.08 ) 0.02 -1 -0.02 0.00 450 Fig. 5. (Color online) The calculated S1 mode peak position of the ๐-C3 N4 structure with different deformations (๐ฅ, ๐ฆ and ๐ฅ๐ฆ) as a function of the applied strain, respectively. The splitting energy (โ๐ = ๐2 − ๐1) between S2 peak and S1 peak of ๐-C3 N4 with different strains is calculated, as shown in Fig. 7. We can see that the splitting energy is increased with asymmetric deformation along the ๐ฅ- and ๐ฆ-directions. The โ๐ values of ๐-C3 N4 with asymmetric deformation along the ๐ฅ-direction (marked by ๐๐ฅ ) is larger than that of deformation along the ๐ฆ-direction (marked by ๐๐ฆ ), which can be attributed to the structural symme- (cm 1100 1000 900 800 700 600 try. With increasing the symmetric deformation (marked by ๐๐ฅ๐ฆ ), the splitting energy increases firstly to 443.6 cm−1 (๐ฅ๐ฆ = 0.06) and then decreases to 320.9 cm−1 (coincidence with value of ๐๐ฅ ). This can be explained by the Raman spectra behavior of symmetric deformation being more complicated than that of asymmetric deformation (as shown in Figs. 5 and 6). The โ๐ differences in ๐๐ฅ , ๐๐ฆ , and ๐๐ฅ๐ฆ strains are also related to its structure characteristic. Symmetric deformation (๐๐ฅ๐ฆ ) leads to larger structural changes, therefore the โ๐ values in ๐๐ฅ๐ฆ strain are more obvious. The Raman mode (S1 and S2) is originated from a different vibration mode (see Figs. 1(e) and 1(f)), the response of Raman mode splitting (โ๐) to the external strain is also different. Thus there are obvious โ๐ differences in ๐๐ฅ and ๐๐ฆ strains. Those results indicate that their Raman behavior is strongly dependent on deformation induced by the external strain, and the strain can effectively affect their Raman spectra feature, as a main factor should be considered in the experimental analysis. D Peak (S1) position (cm -1 ) ๐ฅ๐ฆ = 0.06, and then increases slightly to 599.7 cm−1 at ๐ฅ๐ฆ = 0.08. Those results indicate that the S1 Raman peak is more sensitive to symmetric deformation (the ๐ฅ๐ฆ-direction), and the difference between two asymmetric deformations (the ๐ฅ- and the ๐ฆ-direction) is not obvious. Exploring the S2 Raman mode behavior in Fig. 6, we find that the position of S2 Raman mode can also be linearly blue-shifted by compressed deformation (๐ฅ, ๐ฆ, ๐ฅ๐ฆ < 0), while values are obviously larger than those of the S1 mode. In addition, the S2 value difference induced by asymmetric deformation along the ๐ฅ- and ๐ฆ-directions is effectively enlarged with increasing the strain. However, the S2 peak position of ๐-C3 N4 with increasing symmetric deformation (the ๐ฅ๐ฆ-direction) is more approaching the values of asymmetric deformation along the ๐ฆ-direction, while the behavior becomes more complicated. As a general feature, the Raman mode position can be blue-shifted and red-shifted by compressed and stretched deformation, while the Raman mode values may be different. S1 and S2 are originated from different Raman modes, as shown in Figs. 5(e), 5(f), 6(e) and 6(f), which are related with the intrinsic structure characteristic. The S2 mode are mainly related with the C2-N2 bond vibration, which is more sensitive to the external strain. The S1 mode not only relates with the C2–N2 band vibration, but also depends on the C1–N2 band vibration (it is insensitive to external strain). Therefore, the slope of peak position in the S1 mode is much larger. 270 S S S 180 90 0 -0.02 0.00 0.02 Strain 0.04 0.06 0.08 Fig. 7. (Color online) The calculated splitting energy (Δ๐ = ๐2 − ๐1) between S2 peak and S1 peak of ๐-C3 N4 with different deformations (๐ฅ, ๐ฆ and ๐ฅ๐ฆ) as a function of the applied strain, respectively. In summary, the deformation induced by the external strain can destroy the perfect crystal symmetry, 106201-4 CHIN. PHYS. LETT. Vol. 32, No. 10 (2015) 106201 cause A1(1) mode splitting into two strong Raman modes and display into their Raman spectra. 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