Optical and spectroscopic studies of potassium p-nitrophenolate dihydrate crystal for frequency doubling applications

Article abstract of DOI:10.1016/j.saa.2011.11.002

Non centrosymmetric potassium p-nitrophenolate dihydrate single crystals have been grown by employing the technique of slow solvent evaporation from aqueous solution by slightly adjusting the pH and growth temperature. The grown crystals have been identif

Full text of DOI:10.1016/j.saa.2011.11.002

Spectrochimica Acta Part A 86 (2012) 495–499
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Optical and spectroscopic studies of potassium p-nitrophenolate dihydrate
crystal for frequency doubling applications
M. Josea, R. Uthrakumarc, A. Jeya Rajendranb, S. Jerome Dasc,∗
a Department of Physics, Vel Tech Multi Tech Dr. Rangarajan Dr. Sakunthala Engineering College, Avadi, Chennai-62, India
b Department of Chemistry, Loyola College, Chennai 600034, India
c Department of Physics, Loyola College, Chennai 600034, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 July 2011
Received in revised form 24 October 2011
Accepted 5 November 2011
Non centrosymmetric potassium p-nitrophenolate dihydrate single crystals have been grown by employ-
ing the technique of slow solvent evaporation from aqueous solution by slightly adjusting the pH and
growth temperature. The grown crystals have been identified from single crystal XRD analysis, FTIR and
FT Raman spectroscopic techniques. The high resolution X-ray diffraction experiments substantiate good
quality of the title material. Between 510 and 2000 nm, the material is observed to be nearly transpar-
ent allowing it to be explored for potential use in device fabrication. In addition, the photoluminescence
spectrum of the grown crystal at room temperature shows a stable broad violet-blue emission around the
383–550 nm wavelengths with the maximum centered at 436 nm. Owing to its excellent non linear figure
of merit and strong PL emission, the title crystal can have technological applications in opto-electronic
devices.
Keywords:
Optical materials
Crystal growth
Fourier transform infrared spectroscopy
(FTIR)
Optical properties
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
thermal properties which otherwise lack in organic materials. In
recent years, the NLO properties of semi-organic complex prod-
Nonlinear optics is at the forefront of current research because
of its importance in providing the key functions of frequency shift-
ing, optical modulation, optical switching, optical logic, and optical
memory for the promising technologies in areas such as telecom-
munications, signal processing, and optical interconnections [1–3].
Devices with enhanced figures of merit have developed over the
last three decades as a major force to help drive nonlinear optics
from the laboratory to real applications. The fast growing develop-
ment of optical fiber communication systems has stimulated the
search for better nonlinear materials capable of fast and efficient
processing of optical signals. It has been demonstrated that organic
crystals have large nonlinear susceptibilities compared with inor-
ganic crystals, but their use in device fabrication is obstructed
by low optical transparency, reduced mechanical properties, low
laser damage threshold, and the inability to produce and pro-
cess large single crystals [4,5]. Purely inorganic materials typically
have excellent mechanical and thermal properties with relatively
modest optical nonlinearities because of the lack of extended ␲-
electron delocalization. In semi-organic NLO crystals, however,
polarizable organic molecules are stoichiometrically bonded with
an organic host making them exhibit superior mechanical and
ucts of p-nitrophenol have attracted great interest because these
metal-organic complexes combine the high optical nonlinearity
and chemical flexibility of organics with the physical ruggedness
of inorganics [6–11]. The contribution from the delocalized ␲ elec-
trons belonging to the organic ligand results in high nonlinear optic
and electro optic coefficients in this kind of materials. Potassium
p-nitrophenolate dihydrate (NPK·2H2O) is one such semiorganic
non linear material wherein the nitro group in the crystal lattice
is bonded with the metal ion potassium through the conjugated ␲
electron bond system of donor and acceptor ions, which provides
high SHG efficiency of about 1.5 times than that of lithium nioboate
[12].
Furthermore, solid state laser sources often use the association
of a luminescent crystal and a SHG crystal. Recently there has been a
great interest for self-doubling materials which exhibit both lumi-
nescence and nonlinear optical properties simultaneously. With
the use of such crystals laser sources can be more compact [13–15].
In this paper, we present the growth of stable, optically transpar-
ent, NLO active NPK·2H2O single crystal by judiciously adjusting the
pH and growth temperature employing slow solvent evaporation
technique from aqueous solution. The FTIR and FT Raman spectra of
NPK·2H2O are described on the basis of characteristic frequencies of
the molecular groups – NO2 and phenyl ring; they were compared
to those of the similar aromatic nitro compounds. Consequently, for
the assignment of peaks, we refer on one hand to our own exper-
imental results and on the other hand to works treating similar
∗
Corresponding author. Tel.: +91 44 2817 5662; fax: +91 44 2817 5566.
E-mail addresses: sjeromedas2004@yahoo.com, jerome@loyolacollege.edu (S.
Jerome Das).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.11.002

496
M. Jose et al. / Spectrochimica Acta Part A 86 (2012) 495–499
Table 1
The vibrational modes of NPK·2H2O single crystal and their tentative assignments.
IR (cm−1
)
Raman (cm−1
)
Assignment
–
618
–
700
758
–
846
–
993
1110
–
539
–
635
–
–
833
–
862
–
–
1113
–
1171
1321
–
NO2 bending vibration (rocking)
Ring bending or stretching vibration
Ring bending or stretching vibration
Ring torsion
C–H out-of-plane symmetric deformation (wagging)
C–H out-of-plane bending
C–H out-of-plane bending
C–H out-of-plane bending
C–H in-plane bending
C–H in-plane bending
C–H in-plane bending
C–H in-plane bending
C–H in-plane bending
NO2 symmetric stretching
NO2 symmetric stretching
NO2 asymmetric stretching
NO2 asymmetric stretching
NO2 asymmetric stretching
Ring bending or stretching vibration
Ring bending or stretching + C–NO2 vibration
C–O stretching vibration
C–H stretching vibration
C–H stretching vibration
C–H stretching vibration
OH stretching vibration
1170
–
–
1332
1458
–
–
1570
–
1599
2852
2923
–
3192
3356
3465
–
1508
1531
–
1597
–
–
–
3057
–
Fig. 1. As grown single crystal of NPK·2H2O.
belongs to the non-centrosymmetric monoclinic crystal system and
˚
˚
˚
the lattice parameters are a = 22.195 A, b = 3.870 A, c = 21.291 A, and
˛ = ꢀ = 90◦, ˇ = 121.499◦, which are in good agreement with that of
reported values in the literature [12].
–
–
OH stretching vibration
OH stretching vibration
3.2. Morphology studies
compounds [16–18]. On the basis of such a comparison, in Table 1,
we give a tentative assignment of the various modes observed in
NPK·2H2O. HRXRD measurements substantiate reasonable quality
of the grown crystal. Moreover, we demonstrate in the ensuing
discussion that the title material exhibited strong violet-blue pho-
toluminescence emission. On account of its outstanding non linear
figure of merit and spectacularly good PL emission, the title crystal
can have potential applications in opto-electronic devices.
Due to difference in the relative growth rate of crystal faces, crys-
tals often present various morphological habits. For equilibrium of a
steadily growing crystal, the growth rates of faces are evidently pro-
portional to the distances from the crystal center to the respective
h k l faces. In this case, the growth morphology may be predicted
according to the relative growth rates of individual crystal faces.
Morphology was measured using ENRAF NONIUS CAD4 diffrac-
tometer using ω − 2Â scan mode. The grown crystal presented a
hexagonal morphology wherein it grows faster along [0 1 0] direc-
2. Experimental
¯
tion. The habit mainly consists of (0 1 0), (0 0 2), (2 0 0) and (2 0 2)
faces with (2 0 0) face well developed and more dominating. As a
result the crystals have a shape extended in the [0 1 0] direction as
shown in Fig. 2.
2.1. Material synthesis
The commercial reagent p-nitrophenol and potassium hydrox-
ide pellets are dissolved thoroughly in
a container in 1:1
3.3. FTIR and FT Raman analyses
stoichiometric ratio with millipore water as solvent. The solution
was stirred well for about 48 h using a magnetic stirrer to ensure
homogeneous concentration over entire volume of the solution.
Efforts made to grow the crystals under supersaturation condi-
tions (pH = 8.54) at room temperature turned futile for not only
it resulted in the growth of needle like crystals but also lost
their transparency immediately after they were removed from the
mother solution. After several test runs, we were able to grow trans-
parent and stable single crystals of the title material after the pH
of the solution and growth temperature were adjusted to be 7.54
and 323 K, respectively, by employing the technique of slow sol-
vent evaporation. The grown crystal was then subjected to various
characterization studies to elucidate its structural and optical prop-
erties. The photograph of the as grown single crystal is shown in
Fig. 1.
The Fourier Transform Infra Red (FTIR) spectrum of the title
material (Fig. 3) was recorded using KBr pellet technique between
450 and 4000 cm−1 by Brukker IFS 66v FTIR spectrometer. FT Raman
spectrum (Fig. 4) was recorded between 20 and 4000 cm−1 at room
temperature using a Bruker RFS 100/S FT Raman spectrometer. NO2
symmetric and asymmetric stretching vibrations of aromatic nitro
group are observed at 1332 cm−1 (IR) and 1458 cm−1 (IR), respec-
tively, in good agreement with the values given in the literature
[12]. The symmetric C–H vibrations of the out-of-plane deforma-
tion (wagging) are identified in the IR spectrum at 758 cm−1. The
Raman band at 539 cm−1 can be assigned to the NO2 bending vibra-
tion (rocking). In accordance with the various data published in the
literatures [16,17], the bands at 618 cm−1 and 700 cm−1 in the IR
spectrum are assigned to the bending or stretching vibrations of
the ring and to its torsion vibrations, respectively. However, the
band at 646 cm−1 in the IR spectrum assigned to the C–NO2 and
C–O stretching and ring bending or stretching vibrations in NPNa
[18] is not well resolved in NPK·2H2O. The IR band at 846 cm−1
(862 cm−1 in Raman) is attributed to the C–H out-of-plane bending.
The vibrational modes corresponding to the C–H in-plane bend-
ing were observed in the IR spectrum at 993, 1110 (1113 cm−1
in Raman) and at 1170 cm−1(1171 cm−1 in Raman), respectively.
3. Results and discussion
3.1. Single crystal XRD analysis
A carefully selected single crystal of NPK·2H2O was subjected to
single crystal XRD analysis using an ENRAF NONIUS CAD-4 X-ray
diffractometer. The X-ray diffraction study reveals that the crystal

M. Jose et al. / Spectrochimica Acta Part A 86 (2012) 495–499
497
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0
500
1000
1500
2000
2500
3000
3500
4000
Wave number (cm-1
)
Fig. 4. FT Raman spectrum of NPK·2H2O single crystal.
However, the very weak Raman bands at 129, 187 and 262 cm−1
are assigned to the hydrogen bonds in accordance with the values
available in the literatures [16].
3.4. High-resolution X-ray diffraction analysis
A multicrystal crystal X-ray diffractometer designed and devel-
oped at National Physical Laboratory [19] has been used to study
the crystalline perfection of the grown single crystal. Fig. 5 shows
the high-resolution diffraction curve (DC) recorded for the grown
NPK·2H2O single crystal along (2 0 0) planes using symmetrical
Bragg geometry by employing the multicrystal X-ray diffractome-
ter with MoK␣1 radiation. The solid line (convoluted curve) is well
fitted with the experimental points represented by the filled cir-
cles. On deconvolution of the diffraction curve, it is clear that the
curve contains an additional peak, which is 51 arc s away from the
main peak. This additional peak depicts an internal structural very
low angle boundary. As seen in Fig. 5, two regions of the crystal are
misoriented by a finite angle ˛ also known as tilt angle. The two
regions may be perfect. If the value of ˛ is ≤1 arc min, we may call
it as very low angle boundary. If ˛ > 1 arc min but less than a degree,
we call it as low angle boundary. For more details of such structural
grain boundaries including their affect on physical properties, ref-
erence is made available elsewhere [20,21]. The angular separation
Fig. 2. Morphology of NPK·2H2O.
The bands at 1570 cm−1 in IR or at 1597 cm−1 in Raman and at
1599 cm−1 in IR are assigned to ring bending or stretching vibra-
tion and ring bending or stretching plus C–NO2 and C–O stretching
vibrations, respectively. In the infrared, the C–H stretching vibra-
tion bands of the aromatic ring appear at 2852 cm−1 and 2923 cm−1
while it produce a peak at 3057 cm−1 in the Raman spectrum. There
is a broad envelope carrying peaks due to OH stretch at 3465,
3356 and 3192 cm−1, respectively, due to hydrogen-bonded lattice
water. The 1,4-di substitution is revealed by the peak at 843 cm−1
.
The presence of hydrates of potassium metal ion is confirmed by
the peak at 485 cm−1 in IR [12] and at 496 cm−1 in Raman spectrum.
Moreover, there are no well defined peaks in the Raman spectrum
between 1672 and 3000 cm−1 and between 3100 and 4000 cm−1
.
900
51"
120
100
80
60
40
20
0
600
300
20"
82"
0
-100
0
100
200
0
500
1000 1500 2000 2500 3000 3500 4000
Wavenumber (cm-1)
Glancing angle [arc s]
Fig. 3. Fourier Transform Infra Red Spectrum.
Fig. 5. Diffraction curve recorded for NPK·2H2O single crystal.

498
M. Jose et al. / Spectrochimica Acta Part A 86 (2012) 495–499
4000
3500
3000
2500
2000
1500
1000
500
4.5
4
3.5
3
2.5
2
1.5
0
0
500
1000
1500
2000
2500
0
1
2
3
4
5
6
7
Wavelength (nm)
hν (eV)
Fig. 6. UV–vis–NIR absorption spectrum.
Fig. 7. Variation of photon energy (hv) with (˛hv)2.
between the two peaks gives the tilt angle ˛ which is 40 arc s for
the specimen as seen in the figure. The FWHM (full width at half
maximum) of the main peak and the very low angle boundary are
20 and 82 arc s respectively. These low values reveal the fact that
both the regions of the crystal are nearly perfect as one can expect
such low values only for crystals with reasonable quality. Though
the specimen contains a very low angle boundary, the relatively low
angular spread of around 200 arc s of the diffraction curve and the
low FWHM values show that the crystalline perfection is reason-
ably good. Thermal fluctuations or mechanical disturbances during
the growth process could be responsible for the observed very low
angle boundary. It may be mentioned here that such very low angle
boundaries (which may hardly deteriorate the properties) could
be detected with well resolved peaks in the diffraction curve only
because of the high-resolution of the diffractometer, characterized
by very low values of wavelength spread i.e. ꢁꢂ/ꢂ and horizontal
divergence for the exploring or incident beam, which are, respec-
tively, around 10−5 and much less than 3 arc s of the multicrystal
X-ray diffractometer used in the present studies.
where A is the absorbance and t the thickness, neglecting the
reflection coefficient, which is negligible and insignificant near the
absorption edge. The relation between ˛ and Eg for a direct transi-
tion is given by
(˛hꢃ)2 = A(Eg − hꢃ)
(2)
where Eg is the optical band gap of the crystal and A is a con-
stant. The variation of (˛hv)2 with hv in the fundamental absorption
region is plotted in Fig. 7. The band gap of the crystal can be evalu-
ated by extrapolation of the linear part which is found to be 5.1 eV.
The wide band gap of the title material confirms the large trans-
mittance in the visible region.
3.7. Photoluminescence studies
Fluorescence may be expected generally in molecules that are
aromatic or contain multiple conjugated double bonds with a
high degree of resonance stability. Hence the emission spectrum
of NPK·2H2O was recorded using JOBIN YVON FLUROLOG-3-11
spectroflurometer at room temperature and is shown in Fig. 8. It
exhibited distinct violet-blue photoluminescence with the emis-
sion peaks at 410, 436 and 461 nm on excitation at 354 nm. It
is found that the emission peak intensity decreases quickly after
461 nm. Moreover, the PL emission from NPK·2H2O is found to be
quite strong compared to sodium p-nitrophenolate dihydrate sin-
gle crystal reported elsewhere [24]. Such a strong PL signal is an
indicator of high quality surface [25]. The strong PL emission of
the title material may find potential applications in optoelectronic
devices [26].
3.5. UV–vis–NIR absorption studies
Absorption spectrum is very important for any NLO material
because a nonlinear optical material can be of practical use only if
it has wide transparency window. Hence, to assess its suitability for
NLO applications, the grown single crystal with a thickness of about
2 mm was subjected to UV–vis–NIR studies at room temperature
in the wavelength range from 200 to 2500 nm using a Varian Cary
5E UV–vis–NIR spectrometer. As seen in Fig. 6, between 510 and
2000 nm, the material is observed to have low absorption and better
transparency. Although this crystal has a lower optical cut off at
510 nm, it is still better compared to those of 3-methyl-4-methoxy-
40-nitrostilbene (MMONS) [22] and m-nitroaniline (m-NA) [23],
with lower optical cut offs at 520 nm and 515 nm, respectively. The
low absorption in the entire region from 510 nm to 2000 nm enables
it to be a good candidate for electro-optic and NLO applications. Just
below 510 nm, the absorbance raises more than four units. This is
due to the electronic transitions in the aromatic ring of the title
material.
15400000
12900000
10400000
7900000
5400000
2900000
400000
3.6. Determination of optical band gap
The variation of optical absorption coefficient (˛) with photon
energy (hv) was found to obey this relation:
382
432
482
532
582
632
682
Wavelength (nm)
2.303A
˛ =
(1)
t
Fig. 8. Photoluminescence spectrum of NPK·2H2O single crystal.

M. Jose et al. / Spectrochimica Acta Part A 86 (2012) 495–499
499
4. Conclusions
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Good optical quality single crystals of NPK·2H2O have been
grown by adjusting the growth parameters employing the tech-
nique of slow solvent evaporation from its aqueous solution. Single
crystal XRD analysis, FTIR and FT Raman spectroscopic studies
confirm the identity of the title material. High resolution XRD mea-
surements substantiate reasonably good quality of the grown single
crystal. Between 510 and 2000 nm, the material is observed to be
nearly transparent making the material suitable for non linear opti-
cal applications. The band gap of the material was found to be 5.1 eV
from UV absorption data. Although the investigation of the prop-
erties of NPK·2H2O is still in progress, NPK·2H2O crystal can be
expected to have potential applications in opto-electronic devices
in view of its excellent non linear figure of merit and strong PL
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Acknowledgements
One of the authors (M.J.) acknowledges the Government of Tamil
Nadu for partial financial assistance. The support rendered by Dr.
G. Bhagavannarayana, NPL, New Delhi towards HRXRD studies is
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