Growth of L-asparagine monohydrate and its structural, optical, mechanical, thermal and electrical studies for nonlinear optical applications

In the recent past, single crystals of organic materials exhibiting nonlinear optical behaviour have garnered much attention in the field of optical data storage and laser technology. In the present article, amino acid crystals of L-asparagine monohydrate (LAM) have been grown successfully by conventional slow evaporation solution growth technique from aqueous solution at room temperature (RT), with double distilled water used as a solvent. Single crystal x-ray diffraction was used to confirm the orthorhombic crystal structure (with P212121 space group) and lattice parameters of the grown crystal were determined. The crystallinity, purity, and phase of L-asparagine monohydrate were also determined from the powder x-ray diffraction data. The optical behaviour of LAM crystals was investigated by UV-Visible-NIR spectroscopy, and no absorption was found in the entire visible region. From the Tauc plot, the optical energy band gap was evaluated to be 5.09 eV. Valence band (EVB) and conduction band (ECB) positions of LAM were calculated from the knowledge of electron affinity (Ea), ionisation energy (Ei) of the elements and optical energy gap. Thermal parameters of the crystal were measured by photoacoustic spectroscopy. In addition, the mechanical stability of L-asparagine monohydrate single crystal was evaluated via shock damage threshold measurement. The dielectric studies as a function of frequency and temperature were carried out to understand the electrical behavior of the grown crystal. From the Z-scan analysis, nonlinear refractive index (n2) and nonlinear absorption coefficient (β) of the LAM crystal were also determined.


Introduction
In the past few decades, the development of standardized methods of single crystal growth has tremendous impact in various fields like photonics, optoelectronics, semiconductor technology etc [1,2]. Inorganic single crystals such as KDP (abbreviation of potassium dihydrogen orthophosphate, KH 2 PO 4 ) and LiNbO 3 (lithium niobate) have been the foundation on which optical communications have been developed, but recently, organic crystals with highly anisotropic optical response have become increasingly important [3]. The degree of charge separation (polarization) which is induced by energy and intensity of the incident light is responsible for the optical properties of these NLO materials. The NLO activity is enhanced in organic material as compared to inorganic material due to the presence of the conjugated π-electron system, containing donor and acceptor groups [4]. NLO activity in inorganic materials such as LiNbO 3 , LiTaO 3 and KDP originates from their ionic bonding. In organic materials, the molecules are connected with weak van der Waals and hydrogen bonds, resulting in greater structural diversity and ease of design and fabrication [5][6][7]. Among these, the amino acids crystals exhibit fascinating NLO properties. Usually, amino acid-based compounds crystallize in a non-centrosymmetric (NCS) crystal system, which is a key requirement for demonstrating second harmonic generation (SHG).
L-asparagine, with 20 amino acids, is one such promising organic NLO material, with space group P2 1 2 1 2 1 [8,9] and possessing 4 molecules in the unit cell. Some other names of this compound are 2-Amino-3carbamoyl propanoic acid, (S)-(+)-2-Amino succinamic acid, monohydrated (S)-2-Aminosuccinic acid 4-amide, and L-Aspartic acid 4-amide. Its second harmonic generation (SHG) response has been found to be 0.32 times that of KDP [10]. Amino acids contain two groups: carboxyl group (-COOH) and amino group (-NH 2 ) and in the solid state, the carboxylic group behaves as a proton donor and amino group acts as a proton acceptor, and forms (COO − ) and ( + NH 3 ), within the molecule to establish the dipole nature of the compound [11][12][13]. Because of this dipole nature of amino acids, we come across some fascinating physical and chemical properties with their NLO behaviour making them appropriate candidates for applications. Hence, it is the need of the hour to produce good quality defect free single crystals to fulfill this aim. There are few reports of detailed determination of the optical behaviour, shock damage threshold (SDT), photoacoustic measurements (PAM) etc for LAM, which are essential parameters for fabricating devices.
In the present study, we achieve the growth of L-asparagine monohydrate (LAM) single crystal by using a conventional slow evaporation solution growth technique (SEST) from aqueous solution at room temperature (RT). The LAM crystals are analysed by using single crystal x-ray diffraction (SCXRD), powder x-ray diffraction (PXRD), UV-Visible-NIR spectroscopy, shock damage threshold, photoacoustic measurement, photoconductivity, dielectric studies (DS) and Z-scan studies.

Crystal growth
Commercially available L-asparagine monohydrate salt (LOBA Chemie, 99%) is dissolved in double distilled water. For making a homogeneous solution, the solution was continuously stirred for the period of 26 h at 40°C by using a magnetic stirrer. Then the prepared homogeneous solution of L-asparagine monohydrate was filtered by using Grade 1 Whatman filter paper (pore size 11 μm and thickness 180 μm), to extract the impurities from the solution. A beaker containing filtered homogeneous solution was covered with a perforated thin plastic sheet for avoiding dust particles, with a few pinholes made for evaporation, and kept at RT. After a period of 45 days, single crystals of LAM were obtained from the solution. To get a flat surface, grown crystals of LAM were well polished with double distilled water (figure 1) and then subjected to characterisations to examine its structural, optical, mechanical, thermal, electrical, and nonlinear optical properties.

Characterization techniques
Single crystal x-ray diffraction (SCXRD) of the grown crystal was performed using a Bruker AXS Kappa single crystal (D8 QUEST) diffractometer equipped with CMOS detector, with graphite monochromatic MoKα radiation source (λ=0.710 73 Å ) at RT. Powder XRD was analysed using the Rigaku smart lab x-ray diffractometer with CuK α radiation (λ=1.540 59 Å) at RT, with an electron current of 30 mA and voltage 40 kV. The sample was scanned in the continuous scan mode with 2θ ranging from 5°-90°at a scan speed of 3°/ min, with 0.02°step size. For the optical study, UV-Visible-NIR spectral analysis was performed using the spectrophotometer of Perkin Elmer Lambda 35 over the wavelength range from 200 nm to 1100 nm at room temperature. A semiautomatic shock tube which can generate shock waves from 1.2 to 4.5 Mach number was used for mechanical characteristics. An in-house-developed photoacoustic spectrophotometer was used for measuring the thermal parameters of the grown LAM crystals. A 250 W halogen lamp was employed as a light source. At RT, both photo current and dark current were measured using the Keithley pico-ammeter (MODELM 6487) for applied voltages of 0-50 volts (V) in step of 1 volt. For electrical study, PSM 1735 LCR instrument was used for dielectric measurement at different temperature range as a function of frequency range from 2 kHz to 1 MHz. A continuous wave (CW) neodymium-doped yttrium aluminium garnet (Nd:YAG) laser radiation of wavelength 532 nm with input power 100 mW was used to execute the Z-scan experiment for the grown LAM crystal in the present study. The laser beam was focused using a 10.1 cm focal length (L) lens.

Powder x-ray diffraction (PXRD) study
PXRD analysis is the most powerful method for determining the crystallinity, phase and purity of the grown compound. For measurement, initially, a grown crystal is crushed with the help of agate mortar and pestle, and made into a fine powder. The obtained diffraction pattern is presented in figure 2. In figure 2, we observe that the peaks are sharp with no broadening, indicating the high crystallinity that is obtained. As per the crystallographic notation, the diffraction peaks corresponding to (h k l) plane such as

UV-Visible-NIR spectroscopy studies
UV-Visible-NIR spectrophotometry is an efficient technique used to investigate the optical properties of materials such as optical transition type, electronic band structure, absorption coefficient etc [15,16]. For UV- Visible measurement, well-polished 2 mm thick optically transparent LAM wafer is used to record the transmittance spectrum. The obtained transmittance spectrum of LAM single crystal is displayed in figure 3(a) which indicates that the LAM crystals have a 'lower cut-off wavelength' at 240 nm and the crystal indicates ∼80% transmittance without any significant absorption in the entire UV-Visible-NIR range [250-1100 nm]. Thus, the crystal has good optical behaviour along with lower cutoff wavelength which is a measure of the potential of this material to be used in optoelectronic and NLO applications [17,18].

Optical absorption coefficient
For materials, the absorption coefficient (α) varies with the incident photon energies and for different wavelength α it can be estimated by the transmittance data using the following relation [19,20]: where t the sample thickness (2 mm) and T is the transmittance. The optical energy bandgap of grown LAM single crystal is calculated using the Tauc's plot ((αhν) 2 on the y-axis versus photon energy (hν) on the x-axis), as illustrated in figure 3(b). The dependence of the optical absorption strength depends on the difference between the photon energy and band gap as [21][22][23][24][25][26]: where h is Planck's constant (6.626×10 −34 joule-seconds), ν is the incident photon frequency, A is constant, E g is the optical energy bandgap, and n is an index. The value of n signifies the type of allowed electronic transitions, with value n=1/2 for direct allowed transition, n=2 for an indirect allowed transition, n=3 for forbidden indirect transition, and n=3/2 for forbidden direct transition [23]. The tangent line from the linear region in the graph is used to determine the band gap value as its intercept at the x-axis. For the LAM crystal of this study, (αhν) 2 is used (with n=1/2) which shows the direct allowed transition [24]. From the Tauc plot, the value of optical energy bandgap (E g ) for LAM crystal is obtained to be 5.09 eV in figure 3(b), which is comparable with values from the literature of 5.1 eV and 5.08 eV [24]. Theoretically, the optical band gap energy value can be calculated by using the equation: where c is the light speed in vacuum and λ max is cutoff wavelength (240 nm for LAM crystal). From the equation (3), the obtained energy band gap value is measured 5.17 eV which is close to our experimental value.

Determination of valence band position and conduction band position
where, M=C, H, N, and O, respectively and E CB is the energy of conduction band potential, E g is the optical energy bandgap of LAM crystal (5.09 eV), E e is the dissociation energy of the hydrogen molecules which is equal to 4.47 eV [31], and X is the energy parameter. By putting these constant values of electron affinity (E a ) and ground state ionization energy (E i ) in terms of eV of C, H, N, and O, the energy parameter (X) can be calculated from the equation (

Shock damage threshold measurement
To determine the mechanical characteristics of the crystals for their performance in device applications, shock damage threshold measurement (SDT) is carried out, which is a unique technique to understand the behavior of the crystals at extreme conditions. Formation of shock waves leads to energy release in the form of sharp increase in temperature and pressure, like an explosion [32,33]. Generally, the nature of the shock wave is destructive [34]. A transparent and good quality LAM crystal (with size 5×5×2 mm 3 ) is located in the sample holder that is fixed at 1 cm away from the open end of the driven section and the shock pulses are loaded on the crystal by varying the Mach number from 1.2 to 2.2. After each shock, the crystal surface is carefully checked using an optical microscope (WESWOX, BXLTR, 2010) to confirm the damage. Figure 5 When the number of shock pulses increased one by one, the concentration of the defect points also increased (figures 5(c)-(g)). After the 15th shock, the crystal has cracked, as illustrated in figures 5(h), (i)). Since this crystal could withstand up to 2.0 Mach number, it is determined that this crystal can be used in vibrating and fastmoving devices.

Photoacoustic measurement
Photoacoustic spectroscopy (PAS) is a nondestructive tool to measure thermophysical properties of material like solid, liquid and gases. During this, the light energy absorbed from the sample is converted to heat energy, causing pressure fluctuation in the surrounding air [35,36]. At modulation frequency, a microphone takes the pressure fluctuation as an acoustic signal. The crystal is positioned in a photoacoustic (PA) cell and a sensitive microphone is used as a detector. A chopper is fixed in front of the sample for capturing the modulated signal. The spectrophotometer is calibrated with the standard material like KDP, quartz and BK7 glass with a dimension of up to (10×10×1 mm 3 ). A light beam produced from the lamp is collimated and modulated, and it enters into the sample, generating a PA signal, and these signals are detected by the microphone. With the help of a computer using sound recording software, the resultant PA signal is documented. In our study, a crystal of LAM with dimensions of 4×4.1×1.3 mm 3 is placed for measurement. The PA signal recorded with different-2 chopping frequency from 10 to 120 Hertz (Hz). The graph of normalized amplitude of photoacoustic (PA) signal versus square root of chopping frequency (f 1/2 ) is plotted and is displayed in figure 6. In this graph, we observed that as chopping frequency is increased, the normalized amplitude of photoacoustic (PA) signal is decreased considerably. The thermal diffusivity (α) of the sample is estimated from the curve fitting method [37][38][39][40][41]. The thermal effusivity (e) and conductivity (k) of the specimen are calculated with the help of thermal diffusivity value (α) using the following relations as: where k denotes thermal conductivity, α denotes thermal diffusivity, ρ denotes the density, e is the thermal effusivity and C p is the specific heat capacity of the crystal, respectively. The measured thermal parameters i.e. k, α, and e are cited in table 2. The higher values of thermal parameters of the crystal indicate that the crystal may be used for high power laser action [35].

Photoconductivity measurement
For measurement, a good quality polished LAM crystal is chosen and two electrodes are made on the surface of crystal at a finite distance (1 mm) by using the high grade silver paste. After that, the crystal is fixed inside a cryostat which is evacuated to 10 −6 mbar vacuum. For the measurement of photocurrent (I p ), the sample is exposed to 50 W halogen lamp which acts as a source of radiation and dark current (I d ) is reported when the lamp is in the off condition. From figure 7, we observed that the photocurrent (I p ) is less than the dark current (I d ) for different applied fields. This indicates that the LAM single crystal shows the negative photoconductivity. Usually, the loss of water molecules in the crystal may be responsible for showing the negative photoconductivity [42]. However, under illumination the decrease in the number of charge carriers (CCs) or their lifetime give rise to negative photoconductivity. According to this relation the lifetime,    speed of charge carriers, s denotes the capture cross-section of the recombination centers, and N denotes the carrier concentration). Under illumination, the decrease in the lifetime occurs with an increase in applied voltage, due to a rise in carrier velocity and possibility increases the trapping [43]. By the Stockman model, the phenomena of negative photoconductivity could be explained in detail with the two level scheme [44]. According to this scheme, there exist two energy states within the forbidden bandgap, one lies between the Fermi level and conduction band edge (E CB ), which is denoted as the upper energy level, while the other is in proximity to the valence band edge (E VB ) or in between Fermi level and valence band edge, and is called lower energy level. The second level has a higher capture cross section for charge carriers (i.e. holes and electrons). Since it captures electrons from the conduction band (E CB ) and holes left behind by the electrons from the valence band (E VB ). This prevents the immediate recombination of charge carriers under illumination while it reduces the number of mobile charge carriers which leads to the negative photoconductivity.
The I p and I d values of grown LAM crystal at 50 V are 1.2269×10 −10 and 1.9178×10 −10 A, respectively. The photosensitivity (S) of grown LAM crystal can be measured from the relation: where I ph is equal to the difference of (I p -I d ), I p and I d is photocurrent and dark current, respectively. From the relation (9), the photosensitivity (S) of LAM crystal is obtained 0.36 at 50 V. The negative photoconductivity anomaly observed for some materials like L-Prolinium Trichloroacetate, L-Prolinium Picrate, L-Proline Succinate, L-Alaninium Oxalate etc [12,[45][46][47]. So, it can be concluded that the material which shows the negative photoconductivity, can be used in numerous applications purposes like photo sensors and switches etc.

Dielectric and conductivity study
To know the electrical behavior of solids, dielectric measurement is one of the elemental characterisations. Particularly for nonconducting material, the dielectric behaviour is linked with the electro-optic properties of the crystal. For measurement, a well-polished LAM crystal is selected and then coated on both sides by using high-grade silver paste, which acts as a parallel plate capacitor. Figures 8(a) and (b) show the variation of dielectric constant and dielectric loss of LAM single crystal at a various temperature range (308 to 358 K) and range of frequency from (2 kHz to 1 MHz). From figure 8(a), we observed that the value of dielectric constant is high at low frequency region and decreases rapidly with frequency range and remains constant at higher frequency range. The higher value of dielectric constant at low frequencies is due to the presence of four types of polarizations (i.e. space-charge, orientational, ionic, and electronic polarizations) [48][49][50]. As we increase frequency, the value of dielectric constant gets decreased, as the induced dipoles are not able to chase the variation of the applied electric field. Therefore, space-charge polarization and orientational polarization do not contribute at a higher frequency, and only ionic and electronic polarization contributes to the low value of dielectric constant. In figure 8(a), it is also observed the dielectric constant increased with temperature which happens due to the hopping of the charge carriers in the lattice sites which get thermally activated [51]. From figure 8(b), it can be observed that the dielectric loss value (tan δ) is decreasing with increased frequency range. The large value of dielectric loss at lower frequencies is due to the polarisation of trapped charge carriers. However, at higher frequency region, the dielectric loss decreases due to the reduction of the contribution of various types of polarisation. At low frequencies, dielectric response peak in dielectric loss, is attributed to response time (τ=1/ω max ) [52]. The low value of dielectric loss suggests that the LAM crystal has fewer defects/impurities, and it reveals that the grown crystal has good optical quality from the transmittance analysis which is an important property for the NLO applications [53,54]. Due to the presence of the π-electron conjugated system, these organic crystals behave like an insulator in the high purity state and its conductivity is lesser than ca 10 −14 ohm. cm −1 at RT or below. However, defects induced by the addition of impurities in these crystals can make them behave like a semiconductor at a moderate temperature [53]. The AC conductivity (s ac ) is calculated by dielectric data at different temperature using the equation where w is the angular frequency (is equal to 2πf, f is frequency of the applied field), e 0 is the permittivity of free space charge carriers (8.8541×10 −12 Farad/meter), e r is the relative permittivity and d tan is a dielectric loss. As shown in figure 9, the σ ac is frequency dependent and increases linearly with frequency and temperature. The increase in σ ac with frequency is correlated with the polaron hopping. As the conduction is possible by the mechanism of the hopping of charge carriers, the mobility determines the hopping rate. The drift mobility of the charge carriers and the carrier density are affected due to the defects. The defects play a significant role in the electrical conduction mechanism of such a system, as thermally ionized charged carriers get released from these defects, resulting in increase of conductivity. The topology of an organic molecule also affects its conductivity. At  low temperature, there is low electrical conduction of LAM crystal, caused by the trapping of charge carriers at defect sites which is illustrated in figure 9. With the increase in temperature, more carriers are available due to thermal activation, and there is an increase in conductivity [56].

Z-Scan study
An effective tool for measuring nonlinear refraction (NLR), nonlinear absorption coefficient (NLA), and thirdorder nonlinear optical susceptibility (χ (3) ) is the Z-scan method. The nonlinear refractive index (n 2 ) and nonlinear absorptive properties can be revealed through closed and open aperture Z-scan experiments. Generally, nonlinear optical absorption (NLOA) is classified into two types: (i) saturable absorption (SA), which includes multiphoton absorption in which, with increases in optical intensity, transmittance of the sample increases, and (ii) reverse saturable absorption (RSA), which includes two-photon absorption (2PA) in which, with increase in optical intensity, transmittance decreases.
The sample is moved along the direction of the propagation of the laser beam from −Z to +Z position with Z=0 is the focal point. At the open aperture (OA) and closed aperture (CA) conditions, the transmitted intensity was measured. The open aperture, closed aperture and ratio of closed to open (C/O) normalized Zscan curve for LAM crystal are shown in figure 10. The open aperture (OA) Z-scan curve of grown LAM crystal is depicted in figure 10(a), the experimental results illustrate the sample shows the RSA with positive absorption coefficient (β) which affirms the occurrence of the two photon absorption (2PA) process in grown LAM crystal [57]. To obtain required parameters for optical limiting applications (OLA), this is required documentation. Materials which exhibit saturable absorption (SA) have maximum transmittance in focus at valley while RSA exhibiting materials has minimum transmittance (T) in the focus at the valley. The peak followed by a valley transmittance (T) in the closed aperture (CA) Z-scan curve as displayed in figure 10(b) demonstrates the signature of negative nonlinearity cause the self-defocusing effect (SDE) [58]. The cause of self-defocusing effect (SDE) is the local variation of refractive index (n 2 ) with temperature. The nonlinear optical coefficients are measured using standard equations [59] and cited in table 3.

Conclusions
In summary, single crystals of LAM have been successfully grown by a slow evaporation solution growth technique. The confirmation of the crystal structure system is affirmed from the SCXRD analysis that the grown LAM crystal belongs to orthorhombic structure with non-centro symmetry space group P2 1 2 1 2 1 . From PXRD study, good crystallinity is confirmed of the grown LAM crystal. Optical constants are calculated from the UV-Visible-NIR spectral analysis and high optical transparency is observed in the entire visible region and wide band gap (5.09 eV) of LAM crystal. From theoretical calculation, the positions of the valence band and conduction band for LAM crystal have also been investigated. The shock damage threshold of LAM single crystal is evaluated, and bubble-like projection were found to appear after applying the shock waves with a Mach number of 2.2. The thermal parameters are estimated from photoacoustic spectroscopy. The negative photoconductivity is also found for LAM single crystal. The variation in dielectric constant and dielectric loss is considered with varying frequencies and at different temperature ranges. The nonlinear parameters of LAM crystal have been evaluated from the Z-scan study, which affirmed that the nonlinearity is in the form of self-defocusing and two photon absorption (2PA) with reverse saturable absorption. Table 3. Optimized parameters while performing Z-Scan measurement.