Growth and characterization of an efficient new NLO single crystal L-phenylalanine D-methionine for frequency conversion and optoelectronic applications

Article abstract of DOI:10.1016/j.physb.2017.08.037

Optically active single crystals of L-phenylalanine D-methionine (LPDM) were grown by slow evaporation technique by co-crystallization of amino acids L-phenylalanine and D-methionine in water. The unit cell dimensions have been identified from single crystal X-ray diffraction technique. The existences of various hydrocarbyls were examined by FTIR and FT-Raman spectroscopy. The carbon and hydrogen environment of the grown crystals were analyzed by FT NMR spectrum. The optical absorption studies show that the crystal is transparent in the visible region with a lower cut-off wavelength of 259 nm and there by optical band gap energy E_g is calculated to be 5.35 eV. The Urbach energy, extinction coefficient, reflectance were calculated from UV-absorption data. Further, the thermal stability and accurate melting point has been investigated by TG/DSC techniques. The Kurtz powder SHG was confirmed using Nd:YAG laser with fundamental wavelength of 1064 nm. The dielectric behavior of the specimen has been determined for various temperatures (313 K, 333 K, 353 K, 373 K) at different frequencies. Fluorescence study and the time resolved decay calculation was also performed for the LPDM crystal. Optical nonlinear susceptibility was measured in LPDM and the real and imaginary part of χ³ was evaluated by Z-scan technique using open and closed apertures.

Full text of DOI:10.1016/j.physb.2017.08.037

Author’s Accepted Manuscript
Growth and characterization of an efficient new
NLO single crystal L-phenylalanine D-methionine
for frequency conversion and optoelectronic
applications
P. Sangeetha, P. Jayaprakash, M. Nageshwari, C.
Rathika Thaya Kumari, S. Sudha, M. Prakash, G.
Vinitha, M. Lydia Caroline
www.elsevier.com/locate/physb
PII:
S0921-4526(17)30522-7
http://dx.doi.org/10.1016/j.physb.2017.08.037
PHYSB310177
DOI:
Reference:
To appear in:
Physica B: Physics of Condensed Matter
Received date: 8 July 2017
Revised date: 15 August 2017
Accepted date: 16 August 2017
Cite this article as: P. Sangeetha, P. Jayaprakash, M. Nageshwari, C. Rathika
Thaya Kumari, S. Sudha, M. Prakash, G. Vinitha and M. Lydia Caroline,
Growth and characterization of an efficient new NLO single crystal L-
phenylalanine D-methionine for frequency conversion and optoelectronic
applications, Physica
B:
Physics
of
Condensed
Matter,
http://dx.doi.org/10.1016/j.physb.2017.08.037
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Growth and characterization of an efficient new NLO single crystal
L-phenylalanine D-methionine for frequency conversion and optoelectronic applications
P. Sangeetha1, P. Jayaprakash1, M. Nageshwari1, C.Rathika Thaya Kumari1, S. Sudha1, M.Prakash3,
G. Vinitha4, M. Lydia Caroline1, 2*
1
PG & Research Department of Physics, Arignar Anna Govt. Arts College, Cheyyar - 604 407,
Tamil Nadu, India
2
Department of Physics, Dr Ambedkar Govt. Arts College, Vysarpadi, Chennai – 600 039,
Tamil Nadu, India
3 PG & Research Department of physics, Urumu Dhanalakshmi College, Thiruchirappalli - 19,
Tamil Nadu, India
4Department of Physics, School of Advanced Sciences, VIT, Chennai – 600127, Tamil Nadu, India
Abstract:
Optically active single crystals of L-phenylalanine D-methionine (LPDM) were grown by
slow evaporation technique by co-crystallization of amino acids L-phenylalanine and
D-methionine in water. The unit cell dimensions have been identified from single crystal X-ray
diffraction technique. The existences of various hydrocarbyls were examined by FTIR and
FT-Raman spectroscopy. The carbon and hydrogen environment of the grown crystals were
analysed by FT-NMR spectrum. The optical absorption studies show that the crystal is
transparent in the visible region with a lower cut-off wavelength of 259 nm and there by optical
band gap energy Eg is calculated to be 5.35 eV. The Urbach energy, extinction coefficient,
reflectance were calculated from UV-absorption data. Further, the thermal stability and accurate
melting point has been investigated by TG/DSC techniques. The Kurtz powder SHG was
confirmed using Nd:YAG laser with fundamental wavelength of 1064 nm. The dielectric
behavior of the specimen has been determined for various temperatures (313 K, 333 K, 353 K,
373 K) at different frequencies. Fluorescence study and the time resolved decay calculation was
1

also performed for the LPDM crystal. Optical nonlinear susceptibility was measured in LPDM
and the real and imaginary part of χ3 was evaluated by Z-scan technique using open and closed
apertures.
Keywords: Nonlinear optical material; X-ray diffraction; optical properties; dielectric;
fluorescence; third harmonic generation.
Corresponding author Email address and mobile no:
lydiacaroline2006@yahoo.co.in(M.Lydia Caroline);
Tel: 044-25520151: Fax: 044 - 2552 1852
+91 9841720216
1. Introduction
From the past few decades it was observed that the amino acid based compounds have
potential nonlinear optical (NLO) behavior and disclose good mechanical strength [1]. Nonlinear
optical materials are proved to be a suitable candidate for photonics and optoelectronic industries
[1]. These materials are efficiently utilized for frequency conversion, optical switching, device
fabrication, and large capacity communications because, further application of these may be
provided in all optical systems. Among organic and inorganic materials, the organic one shows
extremely outstanding characteristics of NLO behavior due to weak Vander Waals forces and
hydrogen bonds with conjugate electrons [1]. Amino acids based materials catches the
attention in the field of NLO applications as they contain a proton donor carboxylic acid
(COOH) group and the proton accepter amino (NH2) group in them [2]. Nowadays
crystallization is one of the most important separation and purification processes in the
pharmaceutical and chemical industries. One method for race mate separation is “resolution by
entrainment”, which is usually referred to as preferential crystallization also adopted for growing
2

nonlinear optical crystals [3]. Anisotropic L-phenylalanine has many different positive effects
anxiety relief with increased focus and concentration, and improved motivation [4]. Methionine
is one of the sulfur containing amino acid which it is structurally different from other amino
acids. Optically active methionine is required in the human diet for normal metabolism and
growth, since it is synthesized in humans. D-methionine is often used as an additive in animal
feedstuffs, and it is also used in production of medicines and active pharmaceutical ingredients,
and as a precursor to other amino acids [3]. The growth and characterization of complexes of
L-phenylalanine single crystals were reported in the literature earlier [5-7]. L-phenylalanine
D-methionine (LPDM) is the simplest acentric member of the family crystallized by Yamamoto
et al [8]. Our present work was carried out to harvest optically clear crystals from L-
phenylalanine and D-methionine taken in the equimolar ratio (1:1) and to extent our work to
linear and nonlinear optical studies to prove its suitability for photonic applications. The peptide
(LPDM) includes hydrogen bond donors and acceptors in the peptide bonds in addition to the C-
+
terminal-COO- carboxylate group and the N-terminal –NH3 amino group [9]. Already the
stoichiometric crystallization of the title compound was studied by Gorbitz et al [10]. Also much
interest has been concentrated on organic materials as third-order nonlinear optical materials
among which some organic materials with conjugated -electrons show remarkably large
nonlinearities [11,12]. Since several third order nonlinear optical materials show intensity
dependent refractive indices since they evince fast response time and low absorption [13]. Here
by we report various characterizations for the grown specimen including SXRD, UV-absorption
studies, FTIR, FT-Raman, FT-NMR, Fluorescence and Life time, dielectric studies, second
(SHG) and third harmonic generation (THG) studies suitable for opto electronics and NLO
applications.
3

2. Materials and methods
The commercially available raw materials L-phenylalanine (LOBA, 99%) and D-methionine
(LOBA, 99%) in the purest form were taken in an equimolar (1:1) ratio and dissolved in the
double distilled water. After continuous stirring for 8 h at room temperature homogenous
saturated solution was obtained which was then filtered in the vessel using whatman filter paper
at room temperature. After a time span of 30 days optically good quality crystals were harvested
from the mother solution by solution evaporation method. The chemical reaction is depicted in
Fig.1. The crystallization of L-phenylalanine with D-methionine produced good quality crystals
finds suitable for enhanced nonlinear optical efficiency in the field of nonlinear optical
applications. The photograph of grown crystals of LPDM is shown in Fig.2.
Fig.1. Reaction scheme for LPDM compound.
4

Fig.2.Photograph of single crystals of LPDM grown slow evaporation method
2.1. Characterization studies
The grown L-Phenylalanine D-methionine crystal was subjected to various
characterization techniques like single crystal X-ray diffraction, Fourier Transform Infra-Red
(FTIR), Fourier Transform Raman (FT-Raman) vibrational, UV-visible spectral, Nuclear
Magnetic Resonance (NMR), TG/DSC, Nonlinear Optical (NLO) study, Fluorescence lifetime
and dielectric studies. BRUKER KAPPA APEX II CCD diffractometer with MoKα
(λ=0.71073Å) radiation was utilized to measure the cell dimensions. FTIR spectrum was
recorded using PERKIN ELMER spectrometer in the range 4000-400cm-1 by KBr pellet
technique. Powder FT-Raman spectrum was recorded on a BRUKER: RFS 27 spectrometer in
the range 4000-450cm-1. A BRUKER AMX 400 spectrometer was utilized to study the carbon-
hydrogen frame work in the molecule using DMSO as solvent. UV-visible absorption spectrum
was recorded in the region 200-700 nm using PERKIN ELMER LAMADA 35
spectrophotometer. The TG/DSC of LPDM has been recorded by using NETZSCH STA 449F3
5

thermal analyzer at a heating rate of 20˚C min-1 in the temperature range of 20-1000˚C. With the
performance of PERKIN ELMER LS-45 spectrofluorophotometer using a continuous 450 W
xenon lamp excitation source, fluorescence spectrum was obtained in the range of 240-800 nm.
The dielectric loss and dielectric constant as a function of frequency for the temperatures 313K,
333K, 353K and 373K were studied using a HIOKI 3532-50 LCR HiTESTER meter. The
surface exploration of LPDM was inspected accomplishing FE1 quanta FEG 200 High resolution
scanning electron microscope with acceleration voltage of 20KV. To analyze the nonlinear
optical property of LPDM powder a pulsed Nd:YAG laser was employed using Kurtz Perry
powder method. The third order nonlinearity for LPDM was analyzed by the method of Z-scan
using 632.8 nm diode pumped CW Nd-YAG laser.
3. Results and discussion
3.1. Single crystal X-ray diffraction
From single crystal X-ray diffraction analysis, it was confirmed that the crystal belongs to
monoclinic system with noncentrosymmetric space group P21. The attained lattice parameters are
presented in Table1.The unit cell parameters are found to be consistent with the reported
crystallographic structure data [10].
6

Table 1
Single crystal XRD data of LPDM.
Cell parameters
Present work
Reported work [10]
Monoclinic
P21
Monoclinic
P21
Space group
a (Å)
10.16
4.75
10.11
b (Å)
4.70
c (Å)
16.83
106.74
778
16.68
(˚)
Volume (Å3)
106.66
760.6
3.2. Fourier transform infra-red and Fourier transform Raman vibrational studies
FTIR and FT-Raman spectrum was qualitatively analyzed to know the vibrational interactions
of distinct functional groups and the corresponding spectra recorded for LPDM are shown in
Fig.3 (a) and Fig.3 (b) respectively. The description on IR absorption reports of L-phenylalanine
is well understood in the literature [14-16]. The FTIR wave number observed at 3438 cm-1 is
assigned to O-H stretching carboxylic acid vibration of COOH group [14]. The FT-Raman peaks
+
observed at 3055 cm-1 is attributed to NH3 asymmetric stretching [15]. The CH2 stretching is
found to appear at 2928 cm-1 and 2914 cm-1 respectively in FTIR and FT-Raman spectra. The
wave numbers 2084 cm-1 and 2090 cm-1 gives CH stretching vibration in IR and Raman spectra
[17]. The FTIR and FT-Raman counterpart shows C=O stretching vibrations at 1605 cm-1 and
1606 cm-1 respectively. The peaks at 1441 cm-1 in IR and FT-Raman in 1442 cm-1 are assigned to
COO‾ symmetric stretching in Raman. The wave numbers 1336 cm-1 and 1338 cm-1 indicate the
presence of aromatic vibrations both in IR and Raman spectrum [16]. The C-N stretching
vibrations observed depicted the presence of phenylalanine ions at 1207 cm-1 and 1249 cm-1 in
7

IR and Raman spectra counterpart [16]. The peaks seen at around 1070 cm-1, 1153 cm-1,
1065 cm-1 and 1156 cm-1 illustrate the C-C aromatic stretching respectively in both spectra. The
bands at 867 cm-1 and 855 cm-1 are due to deformation of benzene ring. The intense sharp band
at 766 and 743 establishes the presence of CH2 rocking vibration in both spectra [14]. The bands
at 484 cm-1 and 483 cm-1 are attributed due to C-C deformation in both IR and FT-Raman
spectra. Thus, the presence of all the functional groups present in LPDM was confirmed.
Fig. 3. (a) FTIR spectrum and (b) FT-Raman Spectrum of LPDM.
8

3.3. FT-NMR spectral analysis
The analysis of carbon hydrogen network is performed by suitably applying 1H and 13C NMR
analytical technique. These techniques are used to identify and confirm the structure of organic
1
compounds. Herewith, the H NMR spectrum of LPDM crystal recorded is presented in
Fig.4.The sharp resonance peak at δ=2.5 ppm could be attributed due to CH proton of
D-methionine. The multiplets that appears in the range 7.20-7.34 ppm shows with less intensity
confirms the presence of phenolic protons in the L-phenylalanine (aromatic) aminoacid [18]. The
chemical shift seen at δ= 3.6 ppm could be assigned to proton environment in the amino group
+
(NH3 ) in both L-phenylalanine and D-methionine residues. The 13C NMR spectrum of LPDM is
13
depicted in Fig. 5. The C NMR spectrum shows signals in the region 39.5-40.5 ppm which is
an indicative of different carbon environment in amino acid D-methionine (aliphatic) [19]. The
presence of carbon atom in the phenolic ring of L-phenylalanine is confirmed due to the
appearance of resonance peaks at 128.8 ppm and 129.8 ppm respectively.
9

Fig. 4. 1H NMR profile of LPDM
Fig. 5. 13C NMR profile of LPDM
10

3.4. Optical studies
The Ultra Violet visible absorbance study of LPDM crystal exemplifies the linear optical
properties. The optical absorption spectrum recorded in the region 200-700 nm is shown in
Fig. 6(a). Ultra Violet visible spectral analysis gives information about electronic transitions of
the compound [16]. Also it would help to understand the electronic structure and the optical band
gap of the crystal. The lower optical cut-off wavelength of the LPDM crystal was found to be
259 nm, which is in the UV- region confirms the π-π* electronic transition occurrence at this
region [19]. Hence this optical absorption spectrum reveals the usefulness of LPDM crystal
suitability in optoelectronic applications.
The optical electronic band gap (Eg) of the material is very closely related to the atomic
and electronic band structures [16]. The optical absorption coefficient (α) was computed using
the formula,
2.3026log(1/T)
 
Eq. (A.1)
t
where T,t denotes the transmittance, thickness of the crystal. The absorption coefficient (α) was
computed using the formula,
hυα = A(hυ - Eg)1/2
Eq. (A.2)
In this equation A is a constant, Eg is electronic band gap energy, h is plank’s constant, υ
is frequency of the incident photon. The electronic band gap energy of L-Phenylalanine
D-methionine (LPDM) single crystal was evaluated from linear part of the tauc’s plot by plotting
(hυα)2 versus hυ shown in Fig. 6(b). The figure clearly shows the realization of direct allowed
transition for LPDM crystal over the range 5.0-5.5 eV. The extrapolation of the straight line
11

down to (αhυ)2 = 0 gives the value of direct electronic band gap energy Eg = 5.35eV. Hence, this
grown LPDM crystal having such wide optical band gap can be a suitable candidate for UV
tunable laser and NLO device applications effectively.
Fig. 6. (a) UV-vis profile and (b) Tauc’s plot of LPDM.
3.4. 1. Determination of optical constants
The performance of a material deeply relies on the optical behaviour which forms a
platform to evaluate its usage in optoelectronic devices [16]. The influence of optical absorption
coefficient (α) with the high photon energy (hυ) helps to study the band structure and the type of
transition of the electron. The different optical constants have been calculated using the
following theoretical formulae [19]. Thereby absorption coefficient (α) correlated to the
extinction coefficient (K) is shown as,

4
K 
Eq. (B.1)
12

The reflectance (R) in terms of optical absorption coefficient (α) and refractive index (nx) are
calculated [20] and depicted in Fig. 7(a) which shows that R and K completely depend on the
high photon energy [19].
1 exp(t)  exp(t)
R 1
Eq. (B.2)
Eq. (B.3)
1 exp(t)
2



(R 1)  3R 10R 3


n  

x
2(R 1)




Fig. 7 (b) illustrates the twain reflectance (R) and extinction coefficient (K) dependence on
absorption coefficient. The refractive index (nx) states that how much light is bent, or refracted,
when enters the material. Since the internal efficiency of the device also controlled by the
incident photon energy, by carefully tailoring the incident photon energy (hυ) one can aspire to
have the material meant to fabricate the optoelectronic devices. Fig. 7(c) evinces the variation of
refractive index (nx) with photon energy (hυ). From the figure it is clear that refractive index (nx)
decreases with increase photon energy (hυ). This lowering of refractive index emphasize that the
LPDM crystal exhibit the regular dispersion behaviour. The evaluated value of linear refractive
index (nx) of 1.6373 for the LPDM crystal at 380 nm, with the high optical transparency decides
LPDM as a prominent material for antireflection coating in solar thermal devices and NLO
applications [19].
The photonic response due to optical conductivity (σop) of the material when irradiated
with light is admissible to the refractive index (nx) and the speed of light (c) is shown below,
nxc
4
op

Eq. (B.4)
13

and also the electrical conductivity associated with optical conductivity of the LPDM crystal is
as follows,
2 op
ele

Eq. (B.5)

Fig.7 (e) concludes that the optical conductivity (σop) increases with photon energy (hυ), having
high magnitude (108 (Ω m)-1) influences the presence of high photo tunable nature of the
material [18]. From Fig. 7 (e) and Fig. 7 (b) it is concluded that the low extinction value (10-5)
and electrical conductivity (1.0 x 1013 (Ω m)-1) reinforce semiconducting nature of the material.
Hence this material could be suitable for device applications in computing ultrafast optical data
[19].
From the optical constants, the electric susceptibility (χc) could be evaluated from the following
relation [19],
2
r 0  4c  nx  K2
Eq. (B.6)
2
nx  K 2 0
c 
Eq. (B.7)
4
where ε0 is the dielectric constant in absence of any contribution from free carriers. The
calculated value of electric susceptibility χc is 0.2261 at λ= 380 nm (Fig. 7(d)).
The real part (εr) and imaginary part (εi) of the dielectric constants can be rated from the
following relations [19],
2
εr = nx - K2 and εi = 2nxK
Eq. (B.8)
14

The value of real part (εr) and imaginary part (εi) of the dielectric constants at λ= 380 nm are
2.6806 x 10-6 and 17.191 x 10-6 respectively. Hence the optical constants of a material such as
electronic optical band gap and extinction coefficient are indeed quite necessary to judge the
material’s potential in optoelectronic applications [18].
15

Fig.7. (a) Plot of reflectance (R) and extinction coefficient (K) vs. photon energy (b) Variation of
reflectance (R) and extinction coefficient (K) with absorption coefficient (α) (c) Plot of refractive
16

index (n0) vs. photon energy (d) electric susceptibility (χc) vs. photon energy (e) Variation of
electrical conductivity and optical conductivity vs. photon energy.
3.4. 2. Urbach energy
The Urbach relationship which is an exponential edge region [21] depicts the importance of
absorption coefficient (α),





h
Eu


 0 exp
Eq. (C.1)

where α0 is a constant and Eu is Urbach energy, which adhere to an indication of depth-of tail
levels stretching into the forbidden electronic band gap below the absorption edge, where h is
plank’s constant and υ is frequency of radiation [22]. The observed slope 7.284 of the linear
portion of the plot was calculated from logarithm of the absorption coefficient (α) as a function
of high photon energy (hυ) demonstrate that the crystal in highly crystalline in nature. Urbach
energy (Eu) was determined by taking the reciprocal of the slope of linear portion of the plot
drawn between ln(α) versus hυ as illustrated in Fig. 8. The computed Urbach energy value of
0.1372 eV being low, point out a decrease in the structural defect in as-grown LPDM crystal
[23].
17

Fig. 8 . Plot ln(α) vs. hυ for the LPDM
3.5. Thermal studies
The specimen was subjected to thermal analysis technique to estimate the weight loss (TG)
and melting point (DSC) of the sample with respect to the temperature. Thermogravimetric
(TGA) and differential scanning calorimetry (DSC) was conducted in an atmosphere of nitrogen
at a heating rate of 20˚C min-1 in the temperature range of 20-1000˚C are shown in Fig.9. The
TGA thermogram indicates no weight loss up to 250˚C. But in the temperature region
250˚C-990˚C weight loss occurs in three steps, one starting from 250˚C and ends at 275˚C and
second weight loss occur between 275˚C-375˚C shows a decrease from 55% to 20%
respectively, could be due to the decomposition process. In the third stage at a temperature of
about 400˚C to 990˚C, almost the compound is completely decomposed (weight loss 10%)
leaving only the residue. The DSC trace of the crystal exhibits sharp exothermic peak at 268˚C is
18

attributed to the melting of the material, which coincides well with the TG-trace. This
temperature of melting is the maximum temperature applicable for NLO applications.
Fig. 9. TG-DSC profile of LPDM
3.6. Dielectric studies
The electrical response, molecular anisotropy, structural changes, polarization
mechanism and transport phenomena within crystalline materials were analysed by dielectric
measurements [24]. The dielectric constant and dielectric loss of the grown LPDM crystal was
carried out for various temperatures 313 K, 333K, 353 K and 373 K in the frequency range 50
Hz to 5MHz. The dielectric constant (ε') and dielectric loss (ε'') of LPDM were calculated using
the following equation:
Ct

 
Eq. (C.1)
A0
19

and
εʹʹ = εʹ tan δ
Eq. (C.2)
respectively. Where C is the capacitance in µF, t is the thickness of the sample in mm, ε0 is the
vacuum dielectric constant (ε0 = 8.854 x 10-12 Fm-1), A- Area of the sample in mm2 and tan δ is
the dissipation factor. From Fig. 10 (a) and 10 (b) it is spotted that increase of frequency there is
a decrease of dielectric constant and dielectric loss. The large values of (ε') at low frequencies is
due to the outcome of the contribution of all the four polarizations namely electronic, space
charge, orientation and ionic polarization. Also the low value of (ε') at high frequency region is
triggered by inability of the dipoles in consent with the external field [18]. Thus, the attributed
very low value of dielectric constant and dielectric loss at higher frequencies divulge the
enhanced optical quality with defect less LPDM crystal, in order to meet the requirements in
opto-electronic applications and photonic applications [24].
3. 6. 1. A.C conductivity
The a.c conductivity σac (Ω m)-1 has been calculated from the following formula:
ac  2f0'tan
Eq. (D.1)
where f is the applied frequency of ac field (Hz). The variation of log ω with ac conductivity at
various temperatures has been plotted and shown in Fig. 12(c).
3. 6. 2. Activation energy
The activation energy (Ea) is evaluated using the following relation:


Ea



 0 exp 
Eq. (E.1)


kBT

20

Where σac is the conductivity at temperature T, Ea - activation energy, kB is the Boltzmann
constant (kB = 1.38 x 1023 J/K). Fig. 12(d) shows the variation of log σac versus 1000/T. This is
almost linear in behavior and the slope of this graph is used to estimate the activation energy (Ea)
using the relation,
Ea = -slope x 1000 x kB
Eq. (F.2)
The value of the activation energy comes out to be 0.00270, 0.00732, 0.00858 and 0.01997 eV at
frequencies 4 KHz, 100 KHz, 2 MHz and 5 MHz respectively evince that LPDM possesses less
defect owing to less value of activation energy. Flawless crystals become more useful materials
for device fabrications [19].
21

Fig.10. Plot of (a) Log f vs. dielectric constant (b) Log f vs. dielectric loss (c) Log σac vs. log
ω and (d) Log σac vs. 1000/T.
3. 7. Fluorescence and life time measurement
The fluorescence profile (Fig. 11(a)) of LPDM crystal at excitation wavelength 259 nm
was studied to outlook over the emission spectrum in regard with that particular excited state of
the system and decaying components [24]. The emission peaks with maximum intensity is found
occurring at 284 nm (4.37 eV), which is just near to the visible region in accordance with the
electromagnetic spectrum. The existence of 2-day (prompt and delayed) component of
fluorescence materials paves the way to neutron-gamma discrimination. A distinctive feature of
22

the organic scintillator as a detector application is to manifest various lifetime at the same
emission wavelength. The lifetime quantification was performed with the help of
FLUOROCUBE spectrofluorometer with the wavelength of 284 nm. The lifetime profile was
evaluated by adopting time-correlated single photon counting method (TCSPC) [24]. Fig. 11(b)
represents the three exponential decay time profiles for the LPDM single crystal. The analysis of
decay time measurement was fitted with a three exponential decay function is of the form,
F(t)  Aet /  A2et /  A et /
1
2
3
Eq. (G.1)
1
3
Where A1, A2 and A3 denotes amplitudes and τ1, τ2 and τ3 are lifetimes of prompt and delayed
emissions. The residual fit presented in Fig.11(c) specifies the extent of best fitting done to the
actual decay curve. The quality of the curve fit was accomplished by the inspection of the
residuals, and the value of the reduced χ2 ratio. For the organic molecular crystals the fast
scintillation decay time lies in between 2 and 30 ns [24]. The present study exhibited the shortest
decay component which is detailed in Table 2.
Table 2
Fluorescence lifetime analysis data of LPDM crystal.
Single crystal
Analysis
Lifetime (ns)
Amplitude
χ2
τ1
τ2
τ3
A1
A2
A3
LPDM Three exponential
2
7
9
43.85 32.52 23.62
1.06
23

Fig.11. (a) Emission spectrum (b) Fluorescence lifetime spectrum (c) Residual fit of LPDM.
3.8. SEM analysis
The surface investigation was performed using FE1 quanta FEG 200 High resolution
scanning electron microscope with acceleration voltage of 30KV and the sample is kept at high
vacuum. Fig.13(a,b,c,) represents the recorded SEM micrograph of LPDM crystal with
resolutions 30000 X, 20000 X, 16000 X respectively reveal the layers of atoms of different sizes
and also portrays the surface of the microcrystal as sharp fine morphology free from cracks. The
overlapping of crystals occurred due to agglomeration effected by the thermal effect during
synthesis process.
24

a
c
b
Fig. 12(a,b,c). SEM Micrograph of LPDM crystal at a resolution of 30000 X,20000 X,16000
X with (a)1 µm (b)2 µm (c) and 3 µm
3.9. NLO studies
The second harmonic generation (SHG) conversion efficiency was individualized by the
Kurtz-Perry powder method [25], and the efficiency of the sample was correlated with the
microcrystalline powder of KH2PO4 (KDP). In this method the sample made into a very fine
powder with average particle size of 100-115μ was tightly packed in a microcapillary tube, and
illuminated using spectra physics quanta Nd:YAG laser emitting a first harmonic output of
1064 nm with the pulse width of 10 ns. The input laser beam energy 0.69 J was passed through
the IR reflector and then aimed on the microcrystalline powder sample of KDP. The frequency-
doubled output pulse values of 8.91 mJ and 5.72 mJ were obtained for KDP and LPDM
respectively. The KDP was used as the reference material and the second harmonic generation
conversion efficiency of the LPDM was found to be 0.64 times that of KDP. Some of the
reported organic noncentrosymmetric chromospheres with space group and NLO efficiency are
listed in Table 3 for comparison. Hence the wavelength conversion efficiency in LPDM
25

concludes it as a suitable candidate to develop NLO devices such as optical communication
system.
Table 3
Comparison of NLO efficiency of non-centrosymmetric crystals.
Crystal
spacegroup
NLO efficiency
Reference
L-phenylalanine nitric acid
L-alanine
P21
P21
C2
0.26
0.33
0.35
[14]
[6]
L-phenylalanine dl-mandelic acid
L-phenylalanine D-methionine
[16]
P21
0.64
[Present work]
3.10. Z -Scan studies
The third-order optical nonlinearities have been evaluated for the organic LPDM material
by the THG measurement using single beam Z-scan technique equipped with He-Ne laser of
intensity 5 mW (λ = 632.8 nm), focused by a lens of 3.5 cm focal length. It is a simple and
highly sensitive but very accurate method to determine nonlinear index of refraction (n0),
nonlinear absorption coefficient (β) and third-order nonlinear optical susceptibility (χ(3)) by
closed and open aperture mode respectively [24]. Measurement of the closed aperture, open
aperture and ratio of the closed-to-open normalized transmittance as a function of sample
position Z have been done for LPDM crystal and illustrated in Fig. 13 (a), (b) and (c). In closed
aperture pattern (Fig. 13(a)), we could see that the peak followed by a valley intensity, signifies
that the sign of the refraction nonlinearity is negative, i.e., the occurrence of self-defocusing
effect [23]. The open aperture Z-scan sign (Fig. 13(b)) suggests that the sample exhibits saturable
26

)
absorption. The measurable quantity (TPV can be defined as the difference between the
0
transmittance change of peak and valley TP - TV. The on-axis phase shift at the focus
given by,
is
TPV  0.406(1 S)0.25 0
Eq. (H.1)




2 


 2ra
a
Eq. (H.2)
S 1 exp
2

where S is the linear transmittance aperture, ra denoting the radius of the aperture and ωa is the
0
beam radius at the aperture. The on-axis phase shift
refractive index of the crystal by,
is related to the third-order nonlinear
0
n2 
Eq. (H.3)
KI0Leff
where I0 is the intensity of the laser beam at focus (Z=0) and Leff is the effective thickness of the
L [1 exp(L)]/
sample which can be evaluated by using the relation
, where α and L is
eff
the linear absorption coefficient and the thickness of the sample respectively. From the open
aperture curve, the nonlinear absorption coefficient (β) can be obtained using the relation,
2 2T
I0Leff
 
Eq. (H.4)
where ∆T is the one valley value at the open aperture Z-scan trace. The value of β is negative for
saturated absorption and positive for two-photon absorption processes [23]. The experimental
determination values of n2 and β were used to estimate the real and imaginary parts of the third-
order nonlinear optical susceptibility:
27

2
4
2

2 
10 (0C no n2 ) cm
Re (3) (esu) 
Im (3) (esu) 




Eq. (H.5)
Eq. (H.6)

W


2
2

2 
10 (0C n0 ) cm




4 2
W


where ε0 is the permittivity of free space (8.8518 x 10-12Fm-1), C is the velocity of the light in a
vacuum and n0 is the linear refractive index of the crystal. The absolute value of third-order
nonlinear optical susceptibility χ(3) was calculated according to the following relation,
(3)  (Re((3) ))2  (Im((3) ))2
Eq. (H.7)
Table 3 personate the experimental details and the results of the Z-scan technique for
LPDM crystal. Thus, the LPDM could be a suitable candidate for the protection of night vision
optical sensors, optical limiting applications [24].
Table 4
Obtained nonlinear optical parameters from Z-scan measurements for LPDM.
Parameters
Measured values for LPDM
crystal
Laser beam wavelength (λ)
532 nm
Linear absorption coefficient (α)
237.94
Nonlinear refractive index (n2)
5.77 x 10-8 cm2W-1
0.050 x 10-4 cmW-1
3.08 x 10-6 esu
0.34 x 10-6 esu
3.10 x 10-6 esu
Nonlinear absorption coefficient (β)
Real part of the third-order susceptibility [Re (χ3)]
Imaginary part of the third-order susceptibility [Im (χ3)]
Third-order nonlinear optical susceptibility (χ3)
28

Fig. 13. (a) Closed aperture (b) Open aperture and (c) Ratio of the closed-to-open Z-scan traces
of LPDM.
Conclusion
Organic chromophore LPDM was synthesized and good quality crystals were grown by
the slow solvent evaporation solution growth technique. The cell parameters and crystal structure
of LPDM were examined by single crystal XRD. The presence of functional groups was
identified by FTIR and FT-Raman spectral analyses. The Nuclear Magnetic Resonance (NMR)
analysis identifies the functional groups that contribute the grown crystal. The UV-vis spectra of
the LPDM crystals showed that they are highly transparent in nature in the visible region, with
the cut-off wavelength at 259 nm. The calculated optical direct band-gap value was evaluated to
be 5.13 eV using Tauc’s plot. Optical parameters such as absorption coefficient (α), reflectance
29