Article pubs.acs.org/crystal
Bulk Crystal Growth and Optical and Thermal Properties of the Nonlinear Optical Crystal L-Histidinium-4-nitrophenolate 4Nitrophenol (LHPP) Tianliang Chen,† Zhihua Sun,† Cheng Song,‡ Yan Ge,† Junhua Luo,*,† Wenxiong Lin,† and Maochun Hong† †
Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ‡ Department of Chemistry, Jiangxi Normal University, Nanchang 330022, China ABSTRACT: Good-quality single crystals of the nonlinear optical (NLO) crystal L-histidinium-4-nitrophenolate 4-nitrophenol (LHPP) with a large size of 40 × 25 × 10 mm3 have been grown by the slow cooling method. Its morphology has been indexed to reveal that the crystal is a rhombohedra with major forms of (−100) and (−101). The optical transmittance range has been identified by UV−vis NIR studies. Secondharmonic generation (SHG) on powder samples has been measured using the Kurtz and Perry technique, and the results display that LHPP is a phase-matchable NLO material with its SHG efficiency being 3.55 times as large as that of KDP. Its specific heat and the principal thermal expansion along the principal crystal axes have been measured at different temperatures. The average principal thermal expansion coefficients have been calculated on the basis of experimental measurements between 298 and 410.5 K, and the relationship between the structure and the thermal properties has been discussed. Surface morphologies of the laser-induced damaged crystal were observed with an optical microscope, and the nature origin of the damage was analyzed. species.11,12 Therefore, many 4-nitrophenol derivative NLO materials had been synthesized and investigated.13,14 However, little research has been reported on the combination of the above two building units to obtain new NLO materials.15 Recently, Dhanalakshmi and co-workers introduced 4-nitrophenol into L-histidinium to obtain a material with a relatively large second-order nonlinear susceptibility (χ(2)).16 However, only small sized crystals of L-histidinium-4-nitrophenolate 4nitrophenol (LHPP) were successfully grown, which were unsuitable for optical and other physical investigations. Herein, we report the bulk growth of LHPP single crystal in detail and the growth morphology, thermal expansion, and laser damage threshold characterization of the grown crystals.
1. INTRODUCTION Organic NLO materials have attracted consistent attention because of the advantages offered by organic systems including high electronic susceptibility (χ(2)), fast response time, facile modification, and relative ease of device processing.1−3 Among organic NLO materials, α-amino acid salts are one of the directions for searching for new NLO materials due to some specific features,4 such as molecular chirality, weak van der Waals and hydrogen bonds, and the absence of strongly conjugated bonds, wide transparency ranges in the visible and UV spectral regions, and zwitterionic nature of the molecule which favors crystal hardness.5 Besides, L-histidine as the only α-amino acid with an imidazole side chain with a pKa near neutrality became an interesting alternative for optical secondharmonic generation after Marcy et al.6 reported L-histidine tetrafluoroborate with higher NLO properties than L-arginine phosphate (LAP). Since then, many compounds of L-histidine have been reported to be second-harmonic generation (SHG) active, for example, L-histidine hydrochloride monohydrate,7 Lhistidine hydrofluoride dehydrate,7 L-histidinium perchlorate,8 9 10 L-histidine acetate, L-histidinium dinitrate, etc. Meanwhile, 4nitrophenolate (NP) is a classic dipolar NLO-phore and a typical one-dimensional (1D) donor−acceptor π system, and the presence of proton transfer of the phenolic OH of 4nitrophenolate with various organic and inorganic bases results in an enhancement of the hyperpolarizability of both © 2012 American Chemical Society
2. EXPERIMENTAL SECTION Bulk Crystal Growth. All of the starting materials were analytical-grade reagents. The initial materials of LHPP were prepared by dissolving L-histidine and 4-nitrophenol with a molar ratio of 1:2 in deionized water, as shown in Scheme 1. The solution was stirred and subsequently kept for evaporation at 50 °C for several days. The synthesized salt was purified by successive recrystallization and utilized for growth of LHPP Received: February 22, 2012 Revised: March 27, 2012 Published: April 4, 2012 2673
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Scheme 1. Synthesis Reaction of LHPP Crystal
crystal. In order to optimize the growth conditions, the solubility curve for LHPP crystal was determined in the range between 25 and 45 °C by the gravimetrical method, as shown in Figure 1. The LHPP exhibits a positive solubility Figure 3. Morphology of LHPP crystal deduced from BFDH theory.
Figure 2. Photograph of (a) LHPP crystal with dimensions of 40 × 25 × 10 mm3 and (b) some polished crystal samples.
Rigaku MM007 CCD diffractometer at room temperature, equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The structural model was refined using SHELXL97. Using a polished crystal sample with a thickness of 1.1 mm, the optical transmission spectra of the LHHP single crystal along (−100) and (−101) from UV to NIR in the wavelength range from 450 to 1500 nm were recorded on a Lambda-900 UV/vis/NIR spectrophotometer. The NLO property of LHPP crystal was tested by the Kurtz-powder SHG test using a Nd:YAG laser (1064 nm) with an input pulse of 350 mV. Samples were ground into a powder and sieved using a series of mesh sizes in the range of 25−210 μm. The SHG of 990 mV was obtained for standard KDP with a size of 170−200 μm. Thermal Properties Characterization. Specific heat was measured under a nitrogen atmosphere in the range from 298 to 410.5 K on a NETZSCH DSC F3 instrument. Thermal expansion of the LHPP crystal was measured with a temperature up to 70 °C using a thermal dilatometer (Diamond TMA) made by the Perkin-Elmer Co. Four cuboid bars with a size of 5 × 5 × 4.8 mm3 were cut from this crystal for measuring its thermal expansion. (−100) and (001) faces were determined by X-ray diffraction, and the other two crystal faces were cut and polished based on the crystallographic relation to the two X-ray diffraction-determined faces. Laserinduced damage threshold measurement was carried out using a xenon laser with a frequency of 1 Hz and pulse width of 8.3 ns at a wavelength of 1064 nm. The laser beam was focused on the (−100) plane by a lens.
and 2b presents the as-grown LHPP crystal and some polished samples, respectively. Its morphology is deduced from BFDH theory, which assumes that the growth rate of a given face is proportional to 1/d hkl taking account into symmetry elements,17,18 as shown in Figure 3. The perfect crystal morphology of LHPP is a polyhedron with six basal and six top faces. Among all the crystal faces, (−100) and (−101) are the most well-developed faces. The dominant faces have also been experimentally confirmed by X-ray diffraction. For the grown crystal, the apposed largest face is indexed as (−100) and the adjacent face with (−100) is indexed as (−101). As crystal growth progressed, some faces such as (−1−11), (−111), and (001) often disappear, which may be attributable to the growth environment, such as pH value, temperature field gradient, type of solvent, impurities, etc. Crystal Structure and Optical Properties Characterization. Single-crystal X-ray diffraction data were collected on a
3. RESULTS AND DISCUSSION Crystal Structure. To confirm the crystal structure, singlecrystal X-ray diffraction data were collected. It is observed that the crystal belongs to the monoclinic system with a space group of P21, and the unit cell parameters are a = 8.858(4) Å, b = 8.827(3) Å, c = 12.493(5) Å, and β = 102.580(6)°. These values agreed well with the reported values.16 Figure 4a and 4b illustrates the packing of the molecules in the solid-state structure. The histidinium cations along (100) are connected through H bonds to form zigzag sheets. The 4-nitrophenol molecules bond to these sheets in an almost perpendicular arrangement by C−H···O hydrogen bonds. All O−H and N−H groups are involved in the extensive hydrogen-bonding network, giving rise to a relative density of 1.517 g/cm3. Further, one of the nitrogen atoms in the 4-nitrophenolate ion forms a bifurcated H bond with the 4-nitrophenol molecule. In short, the presence of the amino, imidazolium, nitro, and phenol/phenolate groups in the molecules leads to formation
Figure 1. Solubility curve of LHPP crystal in water.
temperature gradient in water, and the solubility almost increases linearly with temperature. Before starting the crystal growth process, the solution temperature was kept 2−3 °C above the saturated point for 3−4 h. Then, good optical-quality seed crystals obtained from spontaneous nucleation were used for bulk crystal growth. The temperature was reduced initially at a rate of 0.2 °C/day and subsequently 0.4 °C/day as growth progressed. An orange-colored single crystal of size 45 mm × 25 mm × 10 mm was obtained in a period of 30 days. Figure 2a
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Figure 4. (a) Crystal structure packing viewed along the a axis, and (b) crystal structure packing viewed along the b axis.
of a three-dimensional extensive hydrogen-bonding network, which could induce the large charge-transfer enhancement to result in the high SHG efficiency of LHPP. Optical Properties. The transparence spectra of the LHPP crystal along different orientations (−101) and (−100) in the wavelength range from 450 to 1500 nm are shown in Figure 5,
Figure 6. Second-harmonic intensity for LHPP powders as a function of particle size. (Insert) Oscilloscope traces of SHG signals of KDP and LHPP.
Figure 5. Transmission spectrum of LHPP crystal along the (−100) and (−101) planes.
which displays that the UV transparency cutoff wavelength is 480 nm for the grown crystals. LHPP crystal exbibits good transparency from 510 to 1500 nm with the transmittance more than 50% on average and 86% and 87% for 1064 nm with respect to the crystal plane (−101) and (−100), respectively. The absence of absorption in the region below 1200 nm is an advantage as it is the key requirement for materials having NLO properties.19 The second-order NLO property of LHPP crystal was studied following the Kurtz and Perry powder technique.20 The SHG efficiency of the grown crystal was obtained relative to KDP crystal, that is, 3.55 times as large as that of KDP. The relatively high SHG efficiency can be attributed to the zigzag sheet structure and the charge-transfer enhancement by H bonds. The intensity of the SHG output as a function of particle size was measured and is plotted in Figure 6, which indicates LHPP crystal is a phase-matchable NLO material. Specific Heat. For NLO crystals, the damage threshold and possible applications can be greatly influenced by the specific heat. Figure 7 shows the dependence of the specific heat of the LHPP crystal on temperature. It can be seen from Figure 7 that the specific heat of LHPP crystal increases almost linearly from 1.117 to 1.409 J·g−1·K−1 with temperature from 298 to 380 K. It is a slightly lower value than the specific heat of NPNa dihydrate,21 which possesses a large damage threshold. Therefore, LHPP will have a small temperature gradient
Figure 7. Dependence of the specific heat of LHPP crystal with temperature.
when a laser beam passes through the crystal, and it could be used in high-power laser systems. Thermal Expansion. The thermal expansion coefficient is not only an important parameter for crystal growth and processing but also for practical application of a NLO crystal. Under intense laser beam irradiation, optical absorption of the crystal causes thermal gradients that disturb its phase-matching properties and even gives rise to mechanical stress. If the thermal stress is too large, it may be cracked. For the monoclinic crystal, the thermal expansion coefficients should be indicated with a second-rank tensor as below 2675
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Crystal Growth & Design ⎛ α11 0 α13 ⎞ ⎟ ⎜ ⎜ 0 α22 0 ⎟ ⎟ ⎜ ⎝ α31 0 α33 ⎠
Article
⎡ α11 0 α13 ⎤ ⎢ ⎥ αi = (sin ξi 0 cos ξi)⎢ 0 α22 0 ⎥ ⎢ ⎥ ⎣ α13 0 α33 ⎦
(1)
There are four independent principal thermal components, α11, α31 (= α13), α22, and α33. In order to determine αij, we cut four samples from the crystal along four different orientations, as shown in Figure 8. In the (010) plane, the expansion coffecients
= (sin ξiα11 + (cos ξiα31 0 sin ξiα31) + cos ξiα33) ⎛ sin ξi ⎞ ⎜ ⎟ ⎜ 0 ⎟ ⎜ ⎟ ⎝ cos ξi ⎠ = sin 2 ξiα11 + sin 2ξiα31 + cos2 ξiα33
where i = a*, c, and c* and ξi is the angle with respect to the crystallographic c axis. Therefore, α11, α31, and α33 can be calculated to be 6.52 × 10−5, −0.508 × 10−5, and 3.96 × 10−5 K−1. The radius of Mohr’s circle rm2 = (1/4)(α33 − α11)2 + α312 = 1.377 × 10−5. The principal expansion coefficients α1 = (1/ 2)(α11 + α33) − rm and α3 = (1/2)(α11 + α33) + rm are, respectively, 3.863 × 10−5 and 6.617 × 10−5 K−1. The principal expansion coefficient α2 is 2.08 × 10−5 K−1, which coincides with the crystallographic b axis. The orientation of the principal axes φ is calculated to be 21.65°, where u is the counterclockwise angle from the α3-axis to the crystallographic c-axis. The relationship between the principal thermal expansion orientation and the crystallographic orientation is shown in Figure 10. The thermal expansion can be interpreted according
Figure 8. Orientation schematic diagram of the four processed samples.
corresponding to the three orientations ξ = 90°, 0°, and 120°, with respect to the crystallographic c axis, are along a*, c, and c*. Figure 9 shows the relative thermal expansion curves along
Figure 10. Thermal expansion ellipsoid of LHPP crystal.
to its crystal structure. As we can see from Figure 4, the b axis is vertical to the C−C chain of L-histidine or in an angle to the benzene ring of 4-nitrophenol, which is a little sensitive to temperature change. Thus, the thermal expansion coefficient α2 is the smallest. The a axis is parallel to the C−C chain of Lhistidine, and the c-axis is parallel to the benzene ring of 4nitrophenol. When th temperature is fluctuating, the stretching vibration will result in a larger thermal expansion. Moreover, it can be seen that the volume of LHPP crystal is mainly attributed to the 4-nitrophenol group. Thus, the thermal expansion coefficient α3 is larger than that of α1 when heating and the thermal expansion coefficient along the c axis is the largest one. Table 1 shows the thermal expansion coefficients of a few NLO crystals. As we can see, the thermal expansion coefficients of LHPP crystal are comparatively larger than that of L-arginine phosphate monohydrate (LAP),24 deuterate L-arginine phosphate (DLAP),24 and L-arginine trifluoroacetate (LATF).22 More interesting, it possesses weaker anisotropy of the thermal expansion than LAP, DLAP, LATF, etc., which enhances crystal growth, processing, and applications of LHPP. Laser Damage Threshold. Laser-induced damage is one of most important considerations in the choice of a material for NLO application. The maximum output power can be obtained
Figure 9. Relative thermal expansion curves along α*, b, c, and c* orientations.
the four different orientations. It can be seen that the relative thermal expansion is almost linear over the entire measured temperature. The average expansion coefficients along the four different orientations αi (i = a*, b, c, and c*) can be obtained by means of the equation αi(T0 → T ) =
ΔL 1 L0 ΔT
(2)
where αi(T0 → T) is the average linear thermal expansion along the measured orientation over the temperature range from T0 to T, L0 is the sample length at T0, ΔL is the length change with the change of the temperature from T0 to T, and ΔT is the temperature change from T0 to T. According eq 2, the four thermal coeffiencts αa*, αc, αc*, and αb were calculated as 3.96 × 10−5, 6.52 × 10−5, 6.32 × 10−5, and 2.08 × 10−5 K−1. The respective values of three variables α11, α31, and α33 can be obtained by means of the equation22,23 2676
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Table 1. Thermal Expansion Coefficients for LHPP and Some Other Known NLO Crystals thermal expansion coefficients (×10−5 K−1) compound L-arginine
phosphate monohydrate (LAP)24 deuterate L-arginine phosphate (DLAP)24 L-arginine trifluoroacetate (LATF)22 L-arginine bis(trifluoroacetate) (LABTF)22 urea-(d) tartaric acid (UDT)25 sodium P-nitrophenolate dihydrate (NPNa dihydrate)26 manganese(II) mercury(II) thiocyanate-bis(dimethyl sulfoxide) (MMTD)27 L-histidinium-4nitrophenolate 4nitrophenol (LHPP)
crystallographic system
α1
α2
α3
monoclinic
−1.75
0.96
1.64
monoclinic
5.74
0.96
1.83
monoclinic
5.12
0.75
1.64
monoclinic
9.87
−0.86
7.04
orthorhombic
5.23
3.86
3.57
orthorhombic
7.48
0.43
1.99
Figure 11. Micrograph of the laser-induced damage pattern.
orthorhombic
5.33
4.50
3.10
monoclinic
3.86
2.08
6.62
can be attributed to the inhomogeneous single-crystal quality. The crack results from anisotropic thermal expansion of the crystal.
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CONCLUSION Bulk optical-quality crystals of LHPP were grown using the controlled temperature cooling solution growth technique. The thermal properties of the LHPP crystal were studied by measuring the thermal expansion and specific heat. The specific heat of the crystal is 1.117 J g−1 K−1 at 298 K. The calculated average principal expansion coefficients α1, α2, and α3 are 3.863 × 10−5, 2.08 × 10−5, and 6.617 × 10−5 K−1, respectively. The relationship between the structure and the thermal expansion properties has been discussed. The crystal LHPP exhibits weak anisotropy thermal expansion, which is good for crystal growth and processing. The large laser-induced damage threshold benefits application of LHPP crystal as a NLO material. Thus, LHPP seems to be a promising NLO material.
by increasing the power density of the fundamental beam which is proportional to the harmonic conversion efficiency. However, a high power intensity beam can often cause damage of the crystal. Hence, the damage studies become important for newly discovered materials. The laser damage threshold of the grown crystals can be evaluated by the following equation22
I=
E τA
where I is the energy required to cause damage, τ is the pulse width, and A is the area of the laser spot. Laser-induced surface damage threshold measurements were performed using a highpower xenon laser with an energy of 333 mJ and the diameter of 1.3 mm. Thus, the calculated result is 3.02 GW/cm2. A direct comparison of the laser-induced damage threshold results is impossible as the testing conditions, such as wavelength and pulse widths, are different. However, it can easily be seen that longer pulses prevent any thermal relaxation, thereby reducing the damage resistance. Therefore, the measured value for LHPP can be roughly compared with a few known inorganic NLO crystals [BBO (2.6 GW/cm2 at 1064 nm and 10 ns), KTP (1.5−2.2 GW/cm2 at 1064 nm and 11 ns), LI (3.2 GW/cm2 at 1064 nm and 12 ns)] and organic NLO crystals [MHBA (2 GW/cm2 at 1064 nm and 10 ns), LAP (10−13 GW/cm2 at 1064 nm and 1 ns)].28,29 It has been well established that in the long-pulse regime (τ > 100 ps) the damage process is controlled by the rate of thermal conduction through the atomic lattice, whereas in the shortpulse width (τ < 10 ps) the optical breakdown is dominated by various nonlinear ionization mechanisms such as the multiphoton process and avalanche multiplication. The single-shot magnified damage profile of LHPP crystal is presented in Figure 11, which might reveal the nature of damage and possible origin. The damage pattern of LHPP shows some blobs and a crack surrounding the core of the damage. The damage morphologies of the material surface is uneven and disordered, which can be attributed to melting and resolidification.30 Because the more perfect crystalline material can lead to a higher melting point,31 the rough damaged surface
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Nature Science Foundation of China (51102231 and 21171166), the 973 key programs of the MOST (2010CB933501 and 2011CB935904), 863 program of MOST (2011AA030208) and the One Hundred Talent Program of the Chinese Academy of Sciences.
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