Crystal growth, structural and optical characterization of a semi-organicsingle crystal for frequency conversion applications

P. Anandan a,b,*, G. Parthipan b, T. Saravanan b, R. Mohan Kumar c, G. Bhagavannarayana d, R. ]ayavel a

a Crystal Growth Centre. Anna University. Chennai 600 025. Indiab Department of Physics, Thiruvalluvar College of Engineering and Technology. Vandhavasi 604 505. IndiaC Department of Physics. Presidency College. Chennai 600 005, Indiad Materials Characterization Division. National Physical Laboratory. Dr. K.S. Krishnan Road. New Delhi I/O 012. India

ABSTRACT

Single crystals of semi-organic L-histidine hydrobromide have been grown by slow evaporationtechnique from a mixture of L-histidine and hydrobromic acid in aqueous solution at ambienttemperature. From high-resolution X-ray diffraction analysis, the crystalline perfection of the growncrystal has been studied. Single crystal X-ray diffraction analyses, Nuclear Magnetic Resonance spectralanalysis, Thermo-Gravimetry (TG), Differential Thermal Analysis (DTA) and hardness test have beenemployed to characterize the as-grown crystals. The UV cutoff wavelength of the grown crystal is below300 nm and has a wide transparency window, which is suitable for second harmonic generation of laserin the blue region. Nonlinear optical characteristics have been studied using Q switched Nd:YAG laser(), = 1064 nm). The second harmonic generation conversion efficiency of the grown crystals confirmstheir suitability for frequency conversion applications.

1. Introduction

In recent decades, growth of nonlinear optical crystals hasbeen given much importance, because of their potential applica­tions like wave guides, frequency conversion devices, parametricoscillators, etc. Discovery of semi-organic nonlinear properties inL-arginine phosphate (LAP) has played a major role in bothattracting the attention to crystalline salts of amino acids andworking out the concept of semi-organic crystals [1]. Petrosyanet al. [2] derived some semi-organic nonlinear optical crystals ofL-histidine salts and reported the interaction with various acids.Recently our group has reported the effect of solvent normality onthe growth of L-tyrosine bromide single crystals [3]. Mukerjiand Kar [4] have grown and investigated the structural, thermaland spectroscopic properties of L-arginine hydrobromide singlecrystals. Only a few papers have reported the growth andproperties of L-histidine hydrobromide (LHHB) single crystals[5-7]. Ben Ahmed et al. [8] studied the structural and theoreticalstudies of the same.

In the present work we succeeded in growing L-histidinehydrobromide (LHHB) single crystals of comparatively larger size

than the previous ones by slow evaporation solution growthtechnique (SEST). Furthermore, the grown LHHB single crystal hasbeen observed to have large second harmonic generationconversion efficiency and it is better than that of KDP. Thecrystalline perfection of the grown crystal was studied by high­resolution X-ray diffraction (HRXRD) analysis.

2. Experimental

2.1. Synthesis and growth of L-histidine hydrobromide (LHHB)

The analytical grade L-histidine (99% pure) and Merck productof hydrobromic acid (HBr) were taken as starting materials tosynthesize L-histidine hydrobromide (LHHB). The calculatedamount of hydrobromic acid was dissolved in deionized waterof resistivity 18.2 MQ cm. The measured amount of L-histidinewas added to the solution slowly with stirring. The preparedsolution was kept in a constant temperature bath controlledto an accuracy of ± 0.01 O( and allowed to evaporate at roomtemperature. The seed obtained by slow evaporation methodwas used for the bulk growth by slow evaporation technique.The microbes were not observed during the growth of LHHB,probably due to the presence of hydrobromic acid (HBr), whichacts destructively on the growth of microbes. Bulk crystalsof dimension 25 x 20 x 11 mm3 were harvested after a typical

growth period of 30 days. The grown crystal was stable atambient temperature and was non-hygroscopic. The as-growncrystal is shown in Fig. 1.

2.2. Characterization techniques

The crystalline perfection of the grown single crystal was studiedby HRXRD by employing a multi-crystal X-ray diffractometerdeveloped at NPl (9). The well-collimated and monochromatedMOKc,l beam obtained from the three monochromator Si crystals setin dispersive (+, -, - ) configuration was used as the exploring X-raybeam. The specimen crystal was aligned in the (+, -, -,+ )configuration. Due to the dispersive configuration, though latticeconstants of the monochromator crystal(s) and the specimen aredifferent, the unwanted dispersion broadening in the diffractioncurve (DC) of the specimen crystal is insignificant. The specimen canbe rotated about the vertical axis, which is perpendicular to theplane of diffraction, with minimum angular interval of 0.4 arcsec.Rocking or diffraction curves were recorded by changing theglancing angle (angle between the incident X-ray beam and thesurface of the specimen) around the Bragg diffraction peak positionBB (taken as zero for the sake of convenience) starting from asuitable arbitrary glancing angle and ending at a glancing angle afterthe peak so that all the meaningful scattered intensities on bothsides of the peak can be included in the diffraction curve. The DCwas recorded by the so-called w scan, wherein the detector was keptat the same angular position 2BB with a wide opening for its slit. Thisarrangement is very appropriate to record the short range orderscattering caused by the defects or the scattering from local Braggdiffractions by agglomerated point defects or low and very low anglestructural grain boundaries [10]. Before recording the diffractioncurve, the specimen was first lapped and chemically etched in anon-preferential etchant of water and acetone mixture in 1:2volume ratio to remove the non-crystallized solute atoms remainingon the surface of the crystal and the possible layers, which maysometimes form on the surfaces of crystals grown by solutionmethods (11), and also to ensure the surface planarity.

Single crystal X-ray diffraction analysis was carried out usingan ENRAF CAD-4 diffractometer with MoKe< U=0.7107 A) radia­tion to identify the crystal system and to estimate the latticeparameter values. The ITIR spectrum of lHHB crystal wasrecorded in the range 400-4000 cm -1 using a Perkin-Elmerspectrometer by KBr pellet method in order to study the presenceof various functional groups. The IT-Raman spectrum wasrecorded using a Bruker IFS66V IT-Raman spectrometer. The l3C

NMR spectrum of lHHB crystal was recorded using a JEOl GSX400 instrument at 30 dc. Linear optical properties of the crystalswere studied using a UV-visible spectrophotometer. To confirmthe nonlinear optical property, the grown crystals were subjectedto Kurtz powder SHG test [12]. Hardness ofthe grown crystal wasstudied by Vickers's micro-hardness test. The refractive index ofthe grown crystal was measured using a Metricon 2010 prismcoupler instrument. Thermo-Gravimetry (TG), Derivative Thermo­Gravimetric (DTG) and Differential Thermal Analysis (DTA) forlHHB crystals were carried out in nitrogen atmosphere by aNETZSCH Thermal Analyzer to study the thermal properties of theas-grown crystal.

3. Results and discussion

3.1. High-resolution X-ray diffraction study

Fig. 2 shows the high-resolution diffraction curve (DC) recordedfor a typical SEST grown lHHB single crystal using (023)diffracting planes in symmetrical Bragg geometry by employingthe multi-crystal X-ray diffractometer with MoI<ct1 radiation. Asseen in the figure, the DC contains a single peak, which indicatesthat the specimen is free from structural grain boundaries.The full width at half maximum (FWHM) of the curve is18 arcsec, which is only slightly more than that expected for anideally perfect crystal according to the plane wave theory ofdynamical X-ray diffraction [13), but close to that expected for anearly perfect real crystal.

It is important to note the asymmetry of the DC. For aparticular angular deviation (1'1B) of glancing angle with respect tothe peak position, the scattered intensity is much more in thepositive direction as compared to that in the negative direction.This feature clearly indicates that the interstitial type of defectsare predominant in the crystal than the vacancy defects. This canbe well understood by the fact that due to interstitial defects (self­interstitials or impurities at interstitial sites), which may be dueto fast growth and/or impurities present in the raw material, thelattice around these defects undergoes compressive stress [14]and the lattice parameter d (inter-planar spacing) decreasesand leads to more scattered (also known as diffuse X-rayscattering) intensity at slightly higher Bragg angles (BB) as dand sin BB are inversely proportional to each other in the Bragg

150100

Interstitial

defect

-50 0 50

Glancing angle [arc s]

-100

LHHB

(023) Planes

MoKa 1

(+,-,-,+)

o'--~_L.....~--'-----~--"-----~--'--~--'----~-------'

-150

>.~ 200c::.2.~

>.~

X"0

~ 100

~o

Fig. 2. High-resolution X-ray diffraction curve recorded for a typical LHHB singlecrystal specimen using (023) diffracting planes.Fig. 1. Photograph of as-grown crystal of LHHB.

equation (2d sin BB= n.?c, nand .?c being the order of reflection andwavelength, respectively, which are fixed). However, the singlediffraction curve with reasonably low FWHM indicates that the

Table 1Crystal data of LHHB.

Identification code

Empirical formulaTemperatureWavelength

Crystal systemSpace groupUnit cell dimension

Volume (A3)

Density, p (gjcm3)

Single crystal data

C6HlON30 2Br293 K

0.7107 AOrthorhombicP2,2,2,

Q=7.046A

b=9.061 Ac=15.271 A

974.959

1.73

crystalline perfection is excellent. The density of such interstitialdefects is however very low and in almost all real crystals,including nature gifted crystals, these defects are commonlyobserved and are many times unavoidable due to thermodynamicconditions and hardly affect the device performance. More detailsmay be obtained from the study of high-resolution diffuse X-rayscattering measurements [9], which are however not the mainfocus of the present investigation. It is worth mentioning herethat the observed scattering due to interstitial defects is of shortorder nature as the strain due to such minute defects is limited tothe very defective core, long order could not be expected andhence the change in the lattice parameter of the crystal is notpossible. It may be mentioned here that the minute informationlike the asymmetry in the DC is possible in the present sampleonly because of the high resolution of the multi-crystal X-raydiffractometer used in the present investigation.

3.2. Single crystal X-ray diffraction study

Single crystal X-ray diffraction analysis has been carried out todetermine the lattice parameters. From the single crystal XRDanalysis, it was observed that the grown crystals possessorthorhombic structure with P2 12121 space group and the latticeparameter values are a= 7.046 A. b=9.061 Aand c= 15.271 A. Thesingle crystal data are in good agreement with reported values [5].and are shown in Table 1.

In order to analyze the presence of various functional groups inLHHB qualitatively, Fourier transform infrared (FTIR) andFT-Raman spectra have been recorded and are shown in Figs. 3and 4, respectively. The absorption peaks from 2000 to 3500 cm- 1

include overlapping of absorption peaks due to O-H stretch of-COOH and N-H stretch of NH;. The CH2 vibrations, whichgenerally lie just below 3000 cm -1, are not clearly resolved in theFTIR spectrum. The broadening of absorption peak is due tohydrogen bonding. These absorption peaks are clearly resolvedin the FT-Raman spectrum. The CH2 bending mode appears at

3.3. Fourier transform infrared and FT-Raman studies

400ICXD

O'--__-'-__--'-__---l...__----l.__----' -'-__--"-'

4000

1000--------------------------,

Fig. 3. ITIR spectrum of LHHB.

20

eo

0013 r:=--------------------------------r--,N

!

0011

0009..'c:>c:0E 00070

~~..

uc:0-e 0005.8<l

0<l03

0001

I~

Wavenumber lcm- I)

Fig. 4. IT-Raman spectrum of LHHB.

1335 cm- 1 in the ITIR spectrum and it is at 1344 cm- 1 in theRaman spectrum. The C=O vibration of -COOH lies at 1636 cm- 1

in the ITIR spectrum, whereas in the Raman spectrum it lies at1610 cm -1. The torsional oscillation of NH; occurs nearly at527 cm -1 in both the spectra. The wave number and theassignments for ITIR and IT-Raman spectra are presented inTable 2.

v..........

3.4. l3C nuclear magnetic resonance spectral study o9v....

NMR spectrum has been used to identify the carbon atompresent in various chemical environments and to confirm thefunctional groups in the grown LHHB crystal. Fig. 5 shows therecorded 13C NMR spectrum. The carboxyl carbon gives its signalat 172.5 ppm. The aliphatic carbon carrying NH; and imidazolering gives its signal at 53.54 and 25.75 ppm, respectively. Theimidazole ring carbon produces their characteristic signalbetween 110 and 140 ppm. The ring carbon carrying the alkylposition gives its signal at 127.39 ppm. The ortho- and meta­carbon of the ring with respective alkyl substituent give theirsignal at 134 and 117.72 ppm, respectively. As no other peaks areobserved, the compound has been successfully synthesized and itis LHHB as a pure material.

~N...

160

..

...."-N

120 80 40

(o)ppm

Fig. 5. 13C NMR spectrum of LHHB.

o

-664981 1~8

Steps

1803

OL- -'- ...L L- --' --'

2626

Since absorption, if any, in the NLO material near thefundamental or second harmonic leads to loss of conversionefficiency, it is necessary to have the crystal with good opticaltransparency in the UV region. Optical transmission spectrum hasbeen recorded in the range between 300 and 1200 nm for theLHHB crystals. The crystals possess UV cutoff below 300 nm andthere is no absorption in the visible range of the spectrum. Hence,LHHB is useful for optoelectronic applications and a secondharmonic generation (frequency conversion) from the Nd:YAG

3.5. Linear and nonlinear optical property study

Fig. 6. Plot of angle of incidence vs. intensity.

Table 2Wave numbers (cm -I) of ITIR absorption peaks and IT-Raman lines in the spectraof LHHB Crystal.

Wave number (cm- 1)

ITIR

3443310230002955260116361574149214311412133513041160

959914865822804696628527490

IT-Raman

315030102984

1610

14791420

1344129511861149984915884834814706

647,614528498392271

Assignment

N-H asym. stretchingN-H sym. stretchingC- H stretchingCH2 asym. stretchingNHi asym. and sym. stretchingC= 0 stretchingNHi stretchingNHi bendingC- N stretchingN-H bendingCH2 deformationNHi in plane bendingC-H deformationC-H in plane bendingN-H deformationC-H out of plane bendingC-N deformationRing deformationC- N deformationc=o deformationRing deformation or C-N deformationTorsion oscillation of NHiO-H in plane bendingC-C twistingC-N deformation

Laser. The SHG efficiency of LHHB was studied by Kurtz andPerry's [12] powder technique. The powder sample prepared fromthe grown crystal has been subjected to the SHG test and theefficiency of the energy (frequency) conversion is confirmed bythe emission of green light from the powder sample. A KDPsample was used as the reference material and output powerintensity of LHHB has been found to be 1.6 times that of KDP.

3.6. Refractive index study

To measure the refractive index, the mirror polished crystal ofsize 10 x 10 x 2 mm3 was used. This sample was clamped againstthe prism and the index was determined by measuring the criticalangle ee for the sample-prism interface. Nitrogen gas pressurewas used to make a contact between the crystal and the prism.The bulk crystals usually show a clear critical angle knee, fromwhich the refractive index have been calculated automatically.The knee observed is shown in Fig, 6 and the calculated refractiveindex is 1.6393.

3.7. Vickers' micro-hardness study

Hardness of the grown crystal was estimated by Vickers'micro-hardness test. The indentations were made at roomtemperature on the prominent plane with a constant dwelltime of 5 s. The distance between two indentation points was

maintained to more than three times the diagonal length so as toavoid the mutual interference of indentations. The indentationmarks were made on the surface by varying the load from 5 to45 g. The diagonal length of the indentation impression wasmeasured using a Leitz Metallax II microscope. To get accuratemeasurements for each applied load three indentations weremade on the sample and the average diagonal length (d) of theindenter impressions has been measured. Vickers' micro-hardnessnumber Hv of the crystal was calculated using the equationHv = 1.8544P/d2 kg/mm2

, where P is the applied load and d theaverage diagonal length of the indenter impression. It has beenobserved that the hardness value increases (Hv = 132.2 kg/mm2

)

with applied load (P=45 g) due to work hardening of the surfacelayer. The hardness value of LHHB is much higher than those ofL-arginine maleate dihydrate (LAMD) and zinc thiourea chloride(ZTC) crystals [15,16]. Above 45 g load cracks were formed due tothe release of internal stress generated locally by indentation andhence the hardness value decreased. The variation in hardnessnumber with load is shown in Fig. 7.

3.8. Thermal study

stability of the material for fabrication, where a considerableamount of heat is generated during the cutting process. Thermalanalysis was performed on the grown crystals to study thethermal stability and melting point. The Thermo-GravimetricAnalysis (TGA) of LHHB was done between room temperature(28°C) and 1400 °C at a heating rate of 10 °C/min. The experimentwas performed in nitrogen atmosphere and the TG, DTG andDTA plots are shown in Fig. 8. The TGA trace appears nearlystraight up to 162°C and a careful examination of DifferentialThermal Analysis (DTA) and Derivative Thermo-Gram (DTG)reveals an endothermic peak at 162.5 0c, which could be themelting point and first decomposition temperature, since from162°C onwards, decrease in weight was observed in various stepsup to 950 0c, which signifies the decomposition of the sample. Attemperatures above 500°C, the final stage of decompositionoccurred, giving a total loss equal to 39.42%. Hence, from thisstudy, it can be inferred that the crystal can retain texture up to162 0c. However in the literature [5,61 it is reported that themelting point is nearly 140°C. This deviation in melting pointmay due to the increased bond energy due to the perfectarrangement of molecules and also due to the purity of thematerial synthesized.

TG and DTA analyses are of immense importance as far asfabrication technology is concerned, as they provide thermal

4. Conclusions

Fig. 7. Plot of load vs, hardness number.

140

~ 120E

~ 100Qi

.Q

E 80~

z(/)

60(/)Q)c:1:C\l 40I

20

0 10 20 30Load P (gm)

40 50

Optical quality single crystals of LHHB have been grown byslow evaporation solution growth technique. HRXRD studyconfirms the crystalline perfection with very low order interstitialdefect. Optical transmission studies reveal that LHHB is trans­parent in the entire visible region and there is no absorption oflight to any appreciable extent in the visible range of theelectromagnetic spectrum. The refractive index value shows thatthe grown crystal is a suitable NLO candidate. ITIR, IT-Raman and13CNMR spectral studies reveal the structural and optical proper­ties of the LHHB crystals. Hardness value of the crystal wasmeasured to be higher than those of some of the known semi­organic crystal. Thermal analysis reveals that the sample isthermally stable up to 162°C. The NLO property studiesconfirmed that the second harmonic conversion efficiency of thecrystal is comparable to those of many NLO crystals and betterthan that of KDP. Thus, the optical, NLO and thermal properties ofthe LHHB confirm their suitability for frequency conversionphotonic device applications.

_____-'-CIl'-A'-li..-"-399-,-O'-"-.,,-71-·"-----i iO

o

o

eo

20

60~C>....

2

-6

-8

14001200I()(X)

/-._'-'--'-'-'-'~\0

./

BOO600400200

texo

\ f

\ i\ i\ i'v'

UlOTG.IUO·C

4

go 3:>:>

:t....o 2

Temperofurel'tl

Fig. 8. TG, DTG and DTA curves of LHHB crystal.

Acknowledgement

One of the authors (P.A) is grateful to Mrs. BalamaniArunachalam, Chairperson, Thiruvalluvar College of Engineeringand Technology, Vandhavasi 604505, India, for her constantencouragement and support. The authors are thankful toProfessor D. Rajan Babu, VIT University, Tamil Nadu, for his helpin the measurement of refractive index.

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