External-Strain-Induced Raman Scattering

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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(ๆŽๅปทไผš)1** , LI Hai-Tao(ๆŽๆตทๆถ›)2 , PAN Jiang-Hong(ๆฝ˜ๆฑŸๆดช)3 ,
GUO Jun-Hong(้ƒญไฟŠๅฎ)4 , HU Fang-Ren(่ƒก่Šณไป)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 ๐›ฝ-C3 N4 , defect zinc-blende
cubic-C3 N4 , ๐›ผ-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 ๐ต
were predicted to be 427, 425, 425, and 496 GPa,
respectively.[14] The graphitic-C3 N4 phase with very
low bulk modulus (๐ต = 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 ๐‘”C3 N4 nanostructures. However, our understanding of
strain-dependent Raman scattering properties of ๐‘”C3 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
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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 ๐‘šC /๐‘šN = 0.86,
๐‘ŸC1−N1 /๐‘ŸC2−N2 = 0.91, respectively. The optimized
cell eigenfrequency can be simply written as[21]
๐œ”02 = 1/๐‘šC(N)
(b)
๐‘›
∑๏ธ
๐‘˜C1N1(C2N2) ,
(2)
๐‘–=1
where ๐‘šC(N) is the C(N) atom mass and ๐‘˜C1N1(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 ๐‘ฅ or ๐‘ฆ direction, as
shown in Fig. 1(d).
The optimized primitive (1 × 1) ๐‘”-C3 N4 with ๐‘Ž =
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)
๏œฑ๏œฑ
๏œตโŠฒ
๏œธ๏œถ
C1
A
A
y
A1(1)
C2
๏œฑ๏œฑ
๏œทโŠฒ
๏œน๏œณ
๏œท
๏œฑโŠฒ๏œด
๏œณ
๏œฑโŠฒ๏œณ
N1
N2
x
(e)
(f)
S(1)
Raman intensity (arb. units)
(a)
S(2)
Fig. 1. (Color online) Schematic Raman modes of the ๐‘”C3 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 ๐‘”C3 N4 are ๐‘Ž = 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œ“ ′ ๐‘’−๐‘–๐‘ž·๐‘Ÿ๐›ผ ,
(1)
๐ท๐›ผ,๐›ผ
′ = √
๐‘€๐‘˜ ๐‘€๐‘˜′ ๐›ผ ๐›ผ,๐›ผ
′
๐‘˜,๐‘˜
where ๐œ“๐›ผ,๐›ผ
′ 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‘ž = 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 ๐‘ฅ-direction.
To more clearly display the Raman feature, the
Raman spectra of ๐‘”-C3 N4 with asymmetric deformation along the ๐‘ฅ 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
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CHIN. PHYS. LETT. Vol. 32, No. 10 (2015) 106201
(a)
Raman intenstiy (arb. units)
(b)
deformation along the ๐‘ฅ-direction, while the peak positions of S1 and S2 (as shown in Figs. 5 and 6) are
different. For comparison, the Raman spectra of ๐‘”C3 N4 with symmetric deformation induced by biaxial
strain are also calculated, as shown in Fig. 4. We can
see that the symmetric deformation (๐‘ฅ๐‘ฆ = 0.02) also
leads to A1(1) mode splitting, and the S1 and S2 positions can be blue-shifted (๐‘ฅ๐‘ฆ = −0.02) and red-shifted
(๐‘ฅ๐‘ฆ = 0.04) by the biaxial strain, which are consistent
with those of the asymmetric deformation induced by
uniaxial strain. However, further stretched deformation (๐‘ฅ๐‘ฆ = 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 (๐‘ฅ๐‘ฆ = 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 ๐‘ฅ-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 ๐‘ฅ-direction increases (๐‘ฅ = 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 (๐‘ฅ = −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 ๐‘ฅ-direction, the lattice constants will be changed
into ๐‘Ž = 4.711 Å, ๐‘ = 4.801 Å form ๐‘Ž = 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 ๐‘ฅ๐‘ฆ-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 ๐‘ฆ-direction.
To further disclose the asymmetric deformation effect, the Raman spectra of ๐‘”-C3 N4 with asymmetric deformation along the ๐‘ฆ-direction are also calculated, as shown in Fig. 3. The Raman spectrum feature of ๐‘”-C3 N4 with asymmetric deformation along
the ๐‘ฆ-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 ๐‘ฅ, ๐‘ฆ, ๐‘ฅ๐‘ฆ) is calculated, as
shown in Figs. 5 and 6, respectively. When the ๐‘”C3 N4 structure is compressed by the external strain
(๐‘ฅ = −0.03, ๐‘ฆ = −0.03, and ๐‘ฅ๐‘ฆ = −0.03), the
Raman mode of S1 is blue-shifted form 1032.4 cm−1
(the ๐‘ฅ-direction) to 1050.6 cm−1 (the ๐‘ฆ-direction)
and 1100.8 cm−1 (the ๐‘ฅ๐‘ฆ-direction). The asymmetric stretched deformation (along the ๐‘ฅ- or ๐‘ฆ-direction)
makes the S1 mode decrease monotonously, and the
difference of the S1 peak position between ๐‘ฅ-strain
and ๐‘ฆ-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
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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
(๐‘ฅ, ๐‘ฆ and ๐‘ฅ๐‘ฆ) 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
(๐‘ฅ, ๐‘ฆ and ๐‘ฅ๐‘ฆ) as a function of the applied strain, respectively.
The splitting energy (โˆ†๐‘† = ๐‘†2 − ๐‘†1) 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 ๐‘ฅ- and ๐‘ฆ-directions. The โˆ†๐‘† values of ๐‘”-C3 N4 with asymmetric deformation along
the ๐‘ฅ-direction (marked by ๐‘†๐‘ฅ ) is larger than that
of deformation along the ๐‘ฆ-direction (marked by ๐‘†๐‘ฆ ),
which can be attributed to the structural symme-
(cm
1100
1000
900
800
700
600
try. With increasing the symmetric deformation
(marked by ๐‘†๐‘ฅ๐‘ฆ ), the splitting energy increases firstly
to 443.6 cm−1 (๐‘ฅ๐‘ฆ = 0.06) and then decreases to
320.9 cm−1 (coincidence with value of ๐‘†๐‘ฅ ). 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 โˆ†๐‘† differences in ๐‘†๐‘ฅ , ๐‘†๐‘ฆ , and ๐‘†๐‘ฅ๐‘ฆ strains are
also related to its structure characteristic. Symmetric
deformation (๐‘†๐‘ฅ๐‘ฆ ) leads to larger structural changes,
therefore the โˆ†๐‘† values in ๐‘†๐‘ฅ๐‘ฆ 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 (โˆ†๐‘†) to the external strain is also different. Thus there are obvious โˆ†๐‘†
differences in ๐‘†๐‘ฅ and ๐‘†๐‘ฆ 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
)
๐‘ฅ๐‘ฆ = 0.06, and then increases slightly to 599.7 cm−1 at
๐‘ฅ๐‘ฆ = 0.08. Those results indicate that the S1 Raman
peak is more sensitive to symmetric deformation (the
๐‘ฅ๐‘ฆ-direction), and the difference between two asymmetric deformations (the ๐‘ฅ- and the ๐‘ฆ-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 (๐‘ฅ, ๐‘ฆ, ๐‘ฅ๐‘ฆ < 0), while values are obviously
larger than those of the S1 mode. In addition, the S2
value difference induced by asymmetric deformation
along the ๐‘ฅ- and ๐‘ฆ-directions is effectively enlarged
with increasing the strain. However, the S2 peak position of ๐‘”-C3 N4 with increasing symmetric deformation
(the ๐‘ฅ๐‘ฆ-direction) is more approaching the values of
asymmetric deformation along the ๐‘ฆ-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‘† = ๐‘†2 − ๐‘†1) between S2 peak and S1 peak of ๐‘”-C3 N4
with different deformations (๐‘ฅ, ๐‘ฆ and ๐‘ฅ๐‘ฆ) as a function of
the applied strain, respectively.
In summary, the deformation induced by the external strain can destroy the perfect crystal symmetry,
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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. With
increasing the uniaxial strain, the calculated Raman
spectra acquired from ๐‘”-C3 N4 with asymmetric deformation reveal that their Raman modes are split and
shifted, which are strongly dependent on their microstructural changes. Their Raman peak decreases
and increases caused by the strain changes can be used
as a fingerprint to identify the inner lattice and symmetric changes. The Raman spectra feature of ๐‘”-C3 N4
with symmetric deformation becomes more complex,
their changes cannot be described by a simple linear
relation. The complex microstructural changes induced by asymmetric or symmetric deformation can
be clearly displayed by the Raman spectra.
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