Kathiravan Vaiyapuri | ThangaveI Subramani | Ashok Kumar Rajamani | Muthu Lakshmi ThangaveI | Satheesh Kumar Ganesan | SeIvarajan PaIanisamy | Kumaresavanji MaIaiveIusamy

Abstract—Single crystals of L-alanine cadmium iodide (LACI) were grown by the slow evaporation technique at room temperature.A single-crystal X-ray diffraction (SXRD) model was used to evaluate the crystal structure of the as-grown LACI crystal.The energy dispersive X-ray (EDX) analysis and ultraviolet-visible-near infrared (UV-vis-NIR)transmittance studies were carried out,and the results reveal the presence of elements in the title compound.From the transmittance data,the optical bandgap as a function of photon energy was estimated,and the different optical constants were calculated.A fluorescence study was performed for the LACI crystal.Thermogravimetric and differential thermal analyses have also been studied to investigate the thermal property of the LACI crystal.The efficiency of the second harmonic generation (SHG) of the title crystal was investigated.The magnetic and electrical properties were estimated by the vibrating sample magnetometer (VSM) analysis and impedance study,respectively.

1.Introduction

Semi-organic non-linear optical (NLO) crystals are used for various applications,such as frequency conversion,frequency doubling,frequency tripling,and optical switching.Amino acid-based semi-organic crystals have good exposure as conceivable possible second-order NLO materials.Photonic crystals are used in image processing techniques.These materials show NLO impacts,and thus there is a tremendous requirement for high-quality single crystals[1]-[3].Much work has gone into combining amino acids with interesting inorganic materials to create better materials that can compete with conventional inorganic materials,like niobates,borates,and potassium dihydrogen phosphate (KDP)[4],[5].The advancement of science in a few areas of the modern world has been cultivated through the production of single crystals.Because of the higher NLO coefficient,which favors mechanical and thermal stability and a high degree of chemical inertness,inorganic materials in combination with amino acids are widely used in various applications.In recent years,researchers are focusing their efforts on discovering new artificial NLO materials in a single-crystal form,which have high optical transparency,physico-chemical solid properties,high laser damage thresholds,and high efficiency of the second harmonic generation (SHG)[6],[7].Since they include both the proton-donor carboxyl acid (-COO) and the proton-acceptor amino (-NH2) groups,amino acid-based organic materials are fascinating for NLO applications[8].The existence of dipole gives amino acids some unusual characteristics,such as sub-atomic chirality,which ensures acentric structures,and the non-appearance of unequivocally shaped bonds,which results in wide visible (vis) and ultraviolet (UV)ranges.The zwitterionic nature of the molecule makes crystals easier to work with.

L-alanine is an excellent organic NLO substance belonging to the amino acid category,with a melting point of 297 °C and belonging to the orthorhombic crystal structure with the space group of P212121.With a molecular weight of 89.09 g/mol,L-alanine is one of the smallest chiral naturally-occurring amino acids[9],[10].Many researchers have carried out studies on L-alanine complex crystals[11]-[18].By using the slow evaporation process,L-alanine is combined with cadmium iodide to produce L-alanine cadmium iodide (LACI) crystals.This paper is focused on the growth and characterization of the LACI crystal.

2.ExperimentaI Procedure: CrystaI Growth

LACI was made by combining L-alanine and cadmium iodide in a 3:1 molar ratio in double-distilled water.A magnetic stirrer was used to continuously stir the prepared solution for 6 h at room temperature (30oC).The solution was then filtered using the Whatman filter paper,and it was kept in the growth vessel covered with a perforated sheet.Due to slow evaporation,spontaneous nucleation resulted in tiny seed crystals with excellent transparency.In the solution,a defectfree seed crystal was suspended and allowed to evaporate at room temperature.Following completion of the nucleation and growth processes,monomers from the mother solution were collected at the seed-crystal locations,resulting in large single crystals.After a 21-day growing period,LACI crystals were harvested through slow solvent evaporation.Fig.1 illustrates the picture of the as-grown LACI crystal.

Fig.1.As-grown LACI crystal.

3.ResuIts and Discussion

3.1.SingIe-CrystaI X-Ray Diffraction (SXRD) AnaIysis

With the support of a Bruker Kappa Apex-II diffractometer,the SXRD test was performed on the as-grown LACI crystal.It belongs to the orthorhombic structure with the space group of P212121,according to the findings of the SXRD investigation.The values of lattice parameters are given in Table 1.

3.2.Energy Dispersive X-Ray (EDX or EDAX)AnaIysis

The EDX analysis was carried out by using the Vega 3 Tescan scanning electron microscope.The recorded EDX spectrum of the LACI crystal is depicted in Fig.2,which confirms the existence of the title compound.The presence of the elements including carbon (C),oxygen (O),nitrogen (N),cadmium (Cd),and iodide (I) in different proportions is indicated by the respective peaks.The weight and atomic percentages of these elements in the LACI crystal are given in Table 2.It is mentioned here that the atomic percent is based on the number of atoms in a sample,and the weight percent is based on the mass or atomic weight of the elements in the sample.It is possible that the atomic percent can be converted into the weight percent and vice versa.

Table 1: Values of lattice parameters of the LACI crystal

Fig.2.EDX spectrum of the LACI crystal.

Table 2: Weight and atomic percentages of different elements in the LACI crystal

3.3.Linear OpticaI Studies and ReIevant Constants

3.3.1.UV-vis-Near Infrared (UV-vis-NIR) Transmission Spectrum Studies

The linear optical properties of the LACI crystal were analyzed for studying the UV-vis-NIR optical transmission.Fig.3 (a) shows the measured transmittance spectrum of the LACI crystal in the wavelength range of 190 nm to 1100 nm using the Perkin Elmer Lambda 35 UV-visible spectrometer.The title compound has maximum transmittance of 94% in the vis and infrared (IR) regions,with a lower cut-off wavelength of 240 nm.

Fig.3.Investigation of optical parameters: (a) UV-vis-NIR spectrum and (b) Tauc’s plot of the LACI crystal.

The optical absorption coefficient (α) can be determined by

whereTdenotes the transmittance andddenotes the crystal thickness.

Using the values ofαand the Tauc’s relation in (2),the optical bandgap energy (Eg) can be calculated:

wherehis the Planck’s constant,νis the frequency,andBis a constant[19].Heren=1/2 or 3/2 for a natural transformation,depending on whether the transition is permitted or prohibited in a quantum mechanical context.Similarly,for indirect permitted and prohibited transitions,n=2 or 3.In direct transitions,there will be a single linear region;in indirect transitions,there will be two linear regions.Fig.3 (b) shows Tauc’s plot of the LACI crystal and a single linear region is observed here.Hence,the LACI crystal can be considered as the direct bandgap material[19].The bandgap energy of the LACI crystal was calculated from the linear part of Tauc’s plot by plotting (αhν)2versus photon energy (hν).Extrapolating the linear portion of the plot to intercept at the photon energy (hν) axis yields the value ofEgas 5.97 eV.The high transmittance in the vis region and defectless concentration in the as-grown crystal are verified by the large bandgap of the LACI crystal[20].As a result,the LACI crystal with a large optical bandgap may be a good candidate for actual applications,such as UV tunable lasers and NLO devices.

3.3.2.Determination of Optical Constants

For the applications of NLO crystals,measuring the refractive index (n) is important for frequency doubling experiments and calculating optical parameters.The following theoretical formulae were used to measure the other miscellaneous optical constants.

The extinction coefficient (k) can be calculated by[21]

whereλis the wavelength of radiation.The reflectance (R) and refractive index (n) of the as-grown LACI crystal are obtained from the following relations[21]:

From Fig.4,it is clear that the extinction coefficient (k) and reflectance (R) strongly depend on photon energy in the higher-value range.The internal energy of the system is determined by the absorption coefficient,and the amount of light that reaches the materials is determined by the refractive index (n).

The refractive index (n) of a substance specifies how much light is bent or refracted as it enters.Since the internal performance of the system is determined by incident photon energy (hν),one may achieve the desired material for fabricating optoelectronic devices by carefully tailoring the value and tuningEgof the material.The change inndenotes that the LACI crystal could cause a normal dispersion behavior in the material,as shown in Fig.4 (c).Due to its high optical clarity and low refractive index in the UV-vis-NIR area,LACI is an excellent material for antireflection coatings in solar thermal devices and NLO applications.

Fig.4.Determination of optical constants: (a) extinction coefficient (k),(b) reflectance (R),(c) refractive index (n),and(d) optical conductivity (σop) versus incident photon energy (hν) for the LACI crystal.

When a material is irradiated with light,its photoresponse of optical conductivity (σop) is related to the refractive index (n) and the speed of light (c) in the following way[22]:

For the LACI crystal,Fig.4 (d) depicts the variation of optical conductivity (σop) as a function of photon energy.As indicated by (6),the optical conductivity of a material depends onα,which is dependent on the wavelength.Hence,the optical conductivity relates to photon energy.

The optical conductivity of the LACI crystal remains constant up to 5.97 eV,and a steep increase of the optical conductivity is noticed.The high magnitude of optical conductivity (1012s-1) of the LACI crystal confirms the high photoresponse of the material.This property enhances the material’s suitability for information processing and computer device applications.

The relationship for the electrical conductivity(σele) with the optical conductivity (σop) and absorption coefficient (α) can be expressed as[23]

As shown in Fig.5,the electrical conductivity of the LACI crystal varies with photon energy.The electrical conductivity decreases as photon energy increases.Equation (7) indicates that the electrical conductivity depends on the optical conductivity and the wavelength of radiation.The low electrical conductivity value demonstrates the dielectric nature of the material.

Fig.5.Electrical conductivity as a function of photon energy for the LACI crystal.

Fig.6.Fluorescence spectrum of the LACI crystal.

3.4.FIuorescence Studies

The fluorescence emission spectrum of the LACI crystal was recorded using a Perkin Elmer LS 45 fluorescence spectrophotometer,and the result ranging from 490 nm to 560 nm is shown in Fig.6.A peak is observed at 530 nm in the emission spectrum,indicating the emission of green fluorescence.Hence,the as-grown crystal is suitable for optoelectronic devices[24].

3.5.NLO Studies

The Kurtz and Perry method was employed to study the SHG behavior of the crystal[25].In this process,an Nd:YAG laser (λ=1064 nm) with a pulse length of 6 ns was passed through the as-grown sample.The output laser with green emission at the wavelength of 532 nm was achieved,confirming the SHG behavior of the crystal.Therefore,it has the ability to be used in frequency conversion.For the input energy of 0.64 J,a 19-mV SHG signal for the LACI crystal was obtained.For the same input energy,the standard KDP crystal generated an SHG signal of 35 mV.As a result,the SHG efficiency of the as-grown LACI crystal is 0.54 times higher than that of the standard KDP crystal.

3.6.ThermaI Properties

The thermal analysis is an effective method for determining the thermal stability of the crystal[26].The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) techniques were used in this investigation.The experiment was carried out in the temperature range of 27 °C to 600 °C at a moderate heating rate of 10 K/min in the nitrogen atmosphere using the NETZCHSTA449F3 simultaneous TGA/DTA analyzer.For the measurement,a sample weighing 3.635 mg was taken.The thermogram of the LACI crystal is illustrated in Fig.7.

It can be seen from the TGA curve that the loss begins at 235 °C.Dehydration induces the decomposition stage.However,no weight loss is observed in the temperature range of 0 to 235 °C.The lack of the weight loss in the LACI crystal up to 100 °C indicates the absence of a water molecule during the crystallization process.Hence,the title compound is stable up to 235 °C.In the DTA curve,the endothermic peak at 281 °C is observed.The sharpness of the endothermic peaks points out that the LACI crystal has a good degree of crystallinity[27].According to the finding,the LACI crystal is suitable for the fabrication of optoelectronic devices up to the temperature of 235 °C.

Fig.7.TGA/DTA thermogram of the LACI crystal.

3.7.Impedance AnaIysis

The impedance was measured at room temperature using an impedance analyzer (Model: Versa STAT MC).In terms of the complex impedance (Z＊),the frequency-dependent electrical property is expressed asZ′-jZ′,where j is the imaginary element;Z′andZ′are the real and imaginary components of the impedance,respectively,which are shown in Fig.8.

Fig.8.Impedance analysis: (a) real (Z′) and (b) imaginary (Z′) parts of impedance versus log ν for the LACI crystal at room temperature.

The DC conductivity (σDC) of the crystal can be calculated by

whereArepresents the crystal surface area andRbis the bulk resistance.Fig.9 shows the Nyquist plot of the LACI crystal.The DC conductivity of the LACI crystal was found to be 5.9631×10-6(Ω·m)-1at room temperature.A single semicircular arc is obtained from Fig.9,and this semicircle demonstrates that the electrical properties of the material are primarily determined by bulk effects[28],which yieldsRb=8447 Ω.

Fig.9.Nyquist plot for the LACI crystal.

3.8.Vibrating SampIe Magnetometer (VSM)AnaIysis

The applied field dependence of magnetization is measured for the LACI crystal using VSM at room temperature (Lakeshore model 7404),and the result is shown in Fig.10.When the applied field is increased,the magnetization increases sharply,and at the higher field,the magnetization values decrease.These results exhibit that the sample is ferromagnetic at the lower applied field and diamagnetic at a higher field.Such a hysteresis behavior indicates that the diamagnetic property is reduced in the LACI crystal and induces a shortrange ferromagnetic order.Magnetism in transition metal halides,such as cadmium iodide,is caused by the angular momentum of partially filled d-orbitals.The interaction with the coordinating anions splits the five d-orbitals into a variety of energy levels in an octahedral coordination.Hund’s rule indicates that the d-orbital electrons initially fill the levels singly,with their spins parallel.Along with the spin,electrons in levels have orbital angular momentum.As a result,the LACI crystal exhibits magnetism and hysteresis behaviors[29].

Fig.10.Magnetization versus magnetic field for the LACI crystal.

We have estimated the saturation magnetization(Ms),retentivity (Mr),coercivity (Hc),and squareness ratio (Mr/Ms) from the hysteresis curve,and the obtained values are displayed in Table 3.The value of the squareness ratio near 0.5 confirms a shortrange ferromagnetic order in the LACI crystal with a larger grain size.

Table 3: Magnetic parameters of the LACI crystal

4.ConcIusion

The slow evaporation technique was used to grow an optically excellent LACI crystal at room temperature.The lattice parameter values were determined using the SXRD study.The various elements presented in the LACI crystal were identified using the EDAX analysis.From the UV-vis-NIR transmittance study,the bandgap energy was found to be 5.97 eV.The optical constants,like the extinction coefficient,reflectance,refractive index,optical conductivity,and electrical conductivity,as a function of photon energy were also calculated from the transmittance data,confirming its suitability for optical device fabrication.The fluorescence spectrum ensures that the crystal has the emission of green fluorescence.The improved Kurtz-Perry powder technique was used to observe the NLO property of the LACI crystal using green radiation with the Nd:YAG laser as a source.The thermal analysis reveals that the sample is thermally stable and has a high degree of crystallinity.The DC conductivity of the as-grown LACI crystal was calculated from the Nyquist plot.The coercivity and retentivity of the LACI crystal were measured from the hysteresis curve as 154.667 Oe and 2.3056×10-7emu/g,respectively.Based on our findings,the LACI crystal appears to be a good candidate for NLO systems due to their excellent optical quality,moderate thermal stability,and increased SHG performance.

AcknowIedgment

The authors are grateful for the support from the research centers including the Sophisticated Analytical Instruments Facility (SAIF),Indian Institute of Technology (IIT),Madras;the Archbishop Casimir Instrumentation Centre (ACIC),St.Joseph’s College,Tiruchirappalli;the National College Instrumentation Facility (NCIF),National College,Tiruchirappalli;Alagappa University,Karaikudi.The authors would like to acknowledge the support extended to this research by Abraham Panampara Research Centre (APRC),Sacred Heart College,Tirupattur;the Central Instrumentation Facility (CIF),Pondicherry University,Pondicherry;Indian Institute of Science (IISC),Bangalore for their full-fledged help in carrying out the characterization measurement.

DiscIosures

The authors declare no conflicts of interest.