Effect of Acidic Polymers on the Morphology of Laser-Induced Nucleation of Cesium Chloride.

An approach to controlling morphology and size is presented through the combination of laser-induced nucleation and polymer additives. Here, we apply the technique of non-photochemical laser-induced nucleation to irradiate a supersaturated solution (S = 1.15) of cesium chloride (CsCl). The solution immediately responds to laser exposure, and spherical crystallites are produced along the laser pathway. The crystals gradually grow into snowflake-like crystals with different sizes. In this report, two types of acidic polymers including polyepoxysuccinic acid (PESA) and polyaspartic acid (PASA) were individually added in supersaturated CsCl solution to shape its crystalline morphology; we found that a particular property of this control from PESA is uniformity in modification of crystal sizes. Additionally, we observed that both PESA and PASA were able to decrease crystal growth velocity and the quantity of crystals after laser irradiation. With the effect of more than 0.2 wt % PESA in solution, spherical crystallites were initially induced by laser; after that, crystal growth velocities and sizes became slower and smaller with increase in mass fraction of PESA, which led to identical crystal sizes. With the effect of more than 5 wt % PESA, the resulting crystalline morphology obtained by laser was flower-like crystals, whilst cuboid-shaped crystals could be obtained by spontaneous nucleation. Classical nucleation theory, crystal growth rate, and additives as large-size impurities were discussed to analyze the underlying mechanism of the change in morphology.

Introduction

Crystalline materials are essential to our daily life, from scientific devices and electronic products to pharmaceuticals. Obtaining a crystal with a desirable shape is a great request for scientific and social demand; for example, crystals with suitable shapes and sizes could lead crystalline devices to achieve a greater testing performance and research efficiency and speed up scientific development.[1−4] Crystal habits also can be influenced by polymorphs, which would result in different thermal and physical properties.[5] The polymorphic outcome of crystalline materials could be selected by morphology control;[5] for example, several polymorphic forms of sulfathiazole can be dominated through morphology control using a range of alcohols. The ability of controlling crystal morphology could also enhance rational drug design; this is due to the fact that most pharmaceuticals are made from crystals, and drugs with different crystalline morphology and size could give rise to a different performance, including solubility, friability, bulk density, melting point, and dissolution rate;[2−4] thus, modifying the shape of insoluble drugs could highly increase solubility and their efficacy.[6] With the certain morphology, crystals could be informed the specific time to dissolve, which could aid drug design to reach the optimal medical therapy. As crystal morphology can determine many physical aspects of crystalline products, such as packaging, tableting, and bioavailability properties,[6−8] morphology control is becoming more demanding as the efficiency and applicability of pharmaceuticals can be obtained by a suitable morphology design.

Based on the fact that a particular morphology of a growing crystal is determined by the different growth rates of the crystal planes,[9] an approach through controlling the crystal growth rate could provide an effective process for morphological design; this process can be achieved by some acidic polymers, such as poly(acrylic acid), poly(methacrylic acid), poly(epoxysuccinic acid) (PESA), and poly(aspartic acid) (PASA). In general, those soluble macromolecular organic additives have been widely used as crystal growth modifiers for the shape control of inorganic crystals including calcium carbonate (CaCO3),[10−13] barium sulfate (BaSO4),[14,15] calcium oxalate (CaC2O4),[16−18] calcium sulfate (CaSO4),[19] potassium sulfate (K2SO4),[20] and calcium hydrogen phosphate (CaHPO4).[21] As crystallization is a process involving two steps, that of nucleation followed by crystal growth, crystallization control should contain the control from the nucleation to crystal growth. The addition of acidic polymers could offer the control on crystal growth; for nucleation control, previous methods to control nucleation include usual cooling,[22] evaporate crystallization,[23] ultrasound,[24,25] and the application of electric field and magnetic field.[26−29] One particular nucleation method called laser-induced nucleation (LIN) is the most documented technique, which could highly control nucleation sites by precisely adjusting the pulsed laser energy; morphological control could be obtained by LIN through adjusting the laser power density to selectively overcome the energy barriers of different surfaces of crystals. These benefits are observed in the work of ZnO morphology control by adjusting the pulsed laser energy to differentiate the growth rate on a Si(100) surface, as reported by S. Liu and C. R. Liu.[30] With respect to the continuous-wave laser light, the polymorph of a glycine crystal could be selectively controlled by tuning laser polarization and power, according the account of Sugiyama and their co-workers.[31] More recently, Cheng et al. applied optical trapping with a focused continuous-wave near-infrared laser to demonstrate the morphology evolution of KCl crystals.[32] Another type of LIN was first observed in the urea work by Garetz et al.;[33] they found urea needle-shaped crystals appeared shortly after laser irradiation, while the laser wavelength is in the nonabsorbing zone of urea molecules; this crystallization cannot be related to photochemistry. Thus, this unique technique is called non-photochemical laser-induced nucleation (NPLIN). Since NPLIN could offer various control over nucleation, a series of studies to discover its advantages by Garetz and co-workers.[33−36] In the earlier work of Garetz et al., they demonstrated polymorph-controlled crystallization of glycine[34−36] and l-hisdine[37] through NPLIN. A similar research was then launched by Tasnim et al., where two different morphologies of glycine could be produced by NPLIN from supersaturated glycine solution in agarose gels,[38] implying that laser irradiation could have a potential usage in morphological modification by addition of agarose gels. This addition had been first exhibited in NPLIN of KCl crystals by Duffus et al.;[39] KCl crystals in an agarose gel were regularly arranged into a word “LASER” through laser exposure, which demonstrated that laser irradiation could offer spatiotemporal control over crystal nucleation.

In this report, crystallization of cesium chloride (CsCl) was studied by NPLIN. CsCl can be widely used in different areas, such as molecular genetic applications, beer brewing industry, fluorescent screens, and ultracentrifuge separations. In general, the normal purification process of CsCl is fairly complicated, and the technique of NPLIN could be used to purify CsCl in a simple way. On the basis of the experiment launched by Liu et al.,[40] two different acidic polymers, PESA and PASA, were used to shape crystal morphology, in combination with NPLIN. As NPLIN could offer several controls over nucleation and organic macromolecules could exert control on crystal growth, this method exhibited control on the entire crystallization process. CsCl crystals in supersaturated cesium chloride solution were induced by a nanosecond laser pulse, and studies have investigated the effect of the limited quantity of PESA and PASA on CsCl crystalline morphology after laser exposure. As laser irradiation and acidic polymers could tune the crystal shape, attempts to explore a comprehensive approach, that NPLIN incorporate the addition of an acidic polymer to modify the morphology of CsCl crystals, were made in this research. Several related mechanisms of NPLIN are discussed in order to analyze the underlying influence of acidic polymers on CsCl crystals. Even though inorganic crystals of CsCl were studied in this work, the way of controlling crystal morphology could be transferred and applied to other crystalline systems; here, we present a comprehensive way of modifying the morphology of CsCl crystals through crystallization control from nucleation to crystal growth.

Results

Crystal Growth after NPLIN

The process of crystal growth after laser irradiation of supersaturated solutions can be classified into three steps, as shown in Figure : (a) spherical crystallites occur along the laser pathway; (b) the aggregation of CsCl solute molecules around spherical crystallites will form simple branches of snowflake-like crystals; and (c) the growth of branches will gradually become snowflake-like crystals. These snowflake-like crystals are fluffy and soft, and they could be easily dispersed into tiny spherical crystals by slightly shaking the sample vials.

Figure 1

SEM images of crystal growth of CsCl crystals after laser exposure. (a) Spherical crystallites obtained by NPLIN; (b) crystallites form branches of snowflake-like crystals; and (c) formation of snowflake-like crystals. The times of images from (a–c) is around 15 s.

Crystal Size Dependence of Acidic Polymers

The results of crystals caused by NPLIN with and without acidic polymers are summarized in Figure . Two experiment videos are given in the Supporting Information (Videos S1 and S2). As illustrated, the number of crystals and crystal sizes were largely decreased by the addition of 0.2 wt % PASA and PESA, compared with that of the sample without additives. Specifically, the sizes of crystals at 5 s after laser irradiation were ranging from 0.52 to 1.47 mm for the original samples, their mean size was 0.97 mm, and crystal sizes were heterogeneous and scattered in a large distribution, as shown in Figure d. With the effect of 0.2 wt % PESA, the mean crystal size largely decreased to 0.28 mm, and crystal size distribution became narrow, ranging from 0.12 to 0.45 mm, implying that sizes of entire crystals could be controlled by PESA. As expected, an increase of the mass fraction of PESA could extensively control the crystal size and crystal growth velocities. Specifically, the crystal size distribution of 0.3 wt % PESA was ranging from 0.15 to 0.19 mm, and the mean size substantially decreased to 0.17 mm, which decreased by 39% compared with that of 0.2 wt % PESA; with respect to 0.5 wt % PESA, the crystal size became homogeneous, leading to a marginal distribution as all crystal sizes decreased to 0.11 mm. when the mass fraction of PESA increased to 1.0 wt % in solution, the crystal size had no change and remained at 0.11 mm. Compared to the same mass fraction of PESA, PASA has the similar effect to decease the crystal size, and the data of PASA are given in Table S2 of the Supporting Information. When the solution contained 5–8 wt % PESA/PASA, all crystal sizes in a spherical shape decreased to 0.06 mm at 5 s after the laser pulse, crystal growth became distinctly slow, and crystal morphology changed to rigid flower-like crystals with four planar petals from softer snowflake-like crystals, whilst for the spontaneous nucleation event, the resulting morphology at 5–8 wt % PESA/PASA became rigid cuboid-shaped crystals (see Figure ). In Figure d, three different morphologies were analyzed by X-ray powder diffraction (pXRD), showing that all of them were CsCl crystals. Crystal data were collected, showing that the crystal system for diffraction patterns in this study should be cubic with a space group of Pm3̅m (221) and Z = 1.

Figure 2

Crystal growth (a–c) and crystal size distribution (d) for NPLIN of CsCl crystals in supersaturated solutions with and without 0.2 wt % additives. (a) Growth of CsCl crystals 2, 5, 15, and 60 s after laser irradiation without additives; (b) growth of CsCl crystals in 0.2 wt % PASA at 2, 5, 15, and 60s after laser irradiation; (c) growth of CsCl crystals in 0.2 wt % PESA at 2, 5, 15, and 60s after laser irradiation; and (d) CsCl crystal sizes in a2, b2, and c2 versus frequency of crystal particles.

Figure 3

Three morphologies of CsCl crystals and their partial XRD patterns. (a) Dendritic crystal; (b) flower-like crystal with four planar petals; and (c) cuboid-shaped crystal. All scale bars in red lines represent 2.9 (a), 5.5 (b), and 7.5 mm (c), respectively. (d) XRD patterns are specifically for cubic (blue), flower-like (red), and dendritic (black) crystals. The hkl indices of several peaks are labeled in green rectangles with dashed lines, corresponding to the crystallographic planes (100), (110), (111), (200), (210), and (211).

Discussion

The results show that flower-like crystals and snowflake-like crystals can be induced by laser with PESA and without PESA, respectively; the original cubic crystals were spontaneously nucleated in PESA without laser irradiation. Even though the peak intensities for each crystallographic plane of the three different morphologies in Figure were slightly different, the change in peak intensity is not sufficient enough to give reliable deduction on the morphology modification of the crystals as the change of peak intensity can be determined by many factors. Here, we applied the classical nucleation theory to explain why PESA could decrease the number of crystals and why cubic crystals can be easily obtained from spontaneous nucleation, rather than flower-like crystals. Also, the effect of PESA on the crystal growth rate was analyzed to deduce the underlying mechanism on morphology modification of CsCl crystals. According to the current research from Alexander and Camp,[42] the mechanism of NPLIN was not only due to the effect of electric field but also based on photothermal and photomechanical factors, such as bubble or cavitation generation by optical absorption of impurity nanoparticles. Since PESA and PASA used in our work can be large-size impurities, their effect on crystalline morphology has been discussed.

Classical Nucleation Theory

As the cuboid CsCl crystal has no preferential axis of alignment due to its cubic crystalline structure, crystallization of CsCl through NPLIN was explained by the mechanism proposed by Alexander et al.[39,41−44] Pervious works on crystallization of inorganic molecules with isotropic polarizability through NPLIN assumed that the interaction of the electronic polarization of subcritical clusters with the electric field of laser can activate some of the subcritical clusters to become nuclei. Therefore, based on the classical nucleation model of NPLIN modified by Alexander et al.,[41−43] the free energy of forming a spherical cluster of radius r in the external electric field is given bywhere r is the radius of a subcritical cluster, γ is the solution–crystal interfacial tension, S is the supersaturation (S = 1.15), A = ρsRT/M, where ρs and M is the density and the molar mass of the solute (which for CsCl have values of 3.99 g mL–1 and 168.36 g mol–1, respectively), E is the electric field caused by the laser, and the parameter a is decided by dielectric constants of the particle (εS) and solvent (εL), and their relationship can be written byWhere jpeak is the peak power density of the laser (16.3 MW cm–2) and c is the speed of light in vacuum (3 × 109 m s–1). According to the Maxwell relation, dielectric constant ε ≈ n2, where n is refractive indices at 532 nm, and here, εS for CsCl is 2.707,[45] εL1 for water is 1.783, and εL2 for the solvent containing 8 wt % PESA obtained from the dielectric constant tester (ITACA, DKV1) is 1.695, as detailed in the Supporting Information. From eqs and 2, the critical radius of a cluster becoming a nucleus can be written

In the equation, the value of aE2 has little influence on the critical radius due to the fact that the value of c in eq is higher than other parameters. Hence, the value r can be achieved as

In the equation, the value of A and S is fixed, the ratio of rc0 of the solution without additives to rc8 of the solution containing 8% PESA would be γ0/γ8, and the value of γ can be achieved by the Mersmann calculation, which is based on cubic-shaped clusters. The Mersmann calculation applies to binary systems, whereas the solution in our work is a ternary system. Interfacial tension between the solvent and crystal should be considered in a binary system, whilst in a ternary system, interfacial tension between the additive and crystal would occur due to the addition of acidic polymers, and the proportions of the three components would also influence the value of γ; this would highly increase the complexity of the calculation of γ. Thus, we use the Mersmann equation for an approximate calculation. As our initial crystallite induced by laser is spherical, as shown in Figure a, the corresponding correction to assume a spherical shape instead of cuboid is modified as[44]where kB is Boltzmann’s constant, T is temperature, NA is the Avogadro constant, the value of ρS is given in eq , w is the mass fraction of the solute in the solution (w0 for the solution without additives is 68.3%, and w8 for the solution containing 8% PESA is 63.5%), and ρL is the density of the solution (ρL0 for the solution without additives is 1.75 g mL–1, and ρL8 for the solution containing 8 wt % PESA is 1.84 g mL–1). In this equation, the parameters w and ρL are changed by the mass fraction of PESA, and the ratio of γ0 to γ8 equals 0.98, which is the value of rc0/rc8. According to the ratio of γ0 to γ8, the addition of PESA could effectively increase the interfacial tension between the solution and crystal, which in turn increase the free energy ΔG (r, E) with respect to eq and disfavors the growth of clusters. In the solution with 8% PESA, when the cluster sizes are bigger than rc8, the formation of a crystal nucleus becomes favorable. However, rc8 is bigger than rc0, indicating that the solution with 8 wt % PESA could contain a fewer quantity of clusters (≥rc8) than that of clusters (≥rc0) in the original solution and result in fewer nucleation sites, as clusters (PESA. This nucleation model is in good agreement with our experimental result: solutions without additives could experimentally produce more than 50 crystals caused by laser, whilst fewer than 20 crystals could be obtained in solutions with PESA (≥0.3 wt %) after laser irradiation, as shown in Figure . Thus, acidic polymers could highly decrease nucleation sites, leading to a fewer number of crystals induced by laser.

Figure 4

Crystals in the bottom view of vials 30 s after laser irradiation. (a) 13 crystals induced by laser in 0.3 wt % PESA; (b) 6 crystals induced by laser in 0.4 wt % PESA.

In the classical nucleation theory,[46,47] the energy barriers of homogenous nucleation could be written as

In this equation, γ is the solution–nuclei interfacial amd ΔGv is the free energy density for the formation of the new phase; the free energy change for heterogeneous nucleation is lower than that for homogeneous nucleation. The relationship is ΔGhet = f (θ) ΔGhom, where f(θ) is the structure factor of the contact angle θ between nuclei and the foreign solid surface (0 ≤ θ ≤ 180°), as shown in Figure . The function of f(θ) is given by

Figure 5

Schematic diagram of heterogeneous nucleation with contact angle θ = 90° for a cubic nucleus and θ = 180° for a spherical nucleus.

In this equation, it is clearly seen that ΔGhet is less than ΔGhom. If CsCl heterogeneously nucleated to form a cubic structure, and the nucleus was assumed to be a cubic shape, the structure factor would be f(90°) = 0.5, ΔGhet is half of ΔGhom; if it is formed into flower-like crystals, and the nucleus was assumed to be a spherical shape as initial crystallites are spherical, the contact angle is 180°, and the energy barrier nearly equals 0.9 ΔGhom. In this case, the energy barrier for the formation of a cuboid crystal is less than that of flower-like crystals, indicating that CsCl is easily spontaneously nucleated to cubic-shaped crystals according to the classical nucleation theory; this is consistent with the experimental result.

The absolute viscosity η0 of supersaturated CsCl solution is 1.504 mPa·s,[41] and the addition of PESA could increase the viscosity due to its macromolecular structure; η1 for the solution with 1–8 wt % PESA obtained by an Ubbelohde viscometer (MW6500) is higher than 5 mPa·s (see details in the Supporting Information). In general, low viscosity tends to result in the aggregation of solute molecules on the edge and point of the crystal to form a dendrite crystal through a diffusion process of solute molecules. However, as laser irradiation could simultaneously induce several nucleation sites, the ratio of supersaturation to the number of nucleation sites decreases, and the number of solute molecules for each nucleation site to form a crystal is fewer. Thus, the number of solute molecules for the growth of a complete crystal will be not sufficient as solute molecules initially assemble at the edge and point, and this would cause the formation of a dendritic crystal.

Effect of PESA on Crystal Growth Rate

The mechanism based on eq could explain our experiment results. However, eq is not capable to account for all observations of NPLIN. Thus, the absorption by impurity particles accounts for the observations, as discussed in the following content. According to Figure , crystal sizes in PESA at 5 s after laser irradiation are much smaller than that without PESA. Additionally, the sizes of initial crystallites induced by laser in PESA are fairly bigger than that without PESA, which has been explained by eqs and 5 in Section . Thus, the addition of PESA could highly decrease the crystal growth rate. Without PESA, the crystal growth rate is relatively higher, and more nucleation sites are induced by laser; this could be more likely to induce the formation of dendritic crystals. With PESA, the relatively lower crystal growth rate and fewer nucleation sites would conduce to a large and strong crystal, as a higher mass fraction of PESA leads to fewer nucleation sites induced by laser, and as illustrated in Figure , the final crystal would be large and strong with a flower-like structure. If crystals are spontaneously nucleated in PESA without laser irradiation, fewer nucleation sites and a lower crystal growth rate would attribute to form original cubic crystals.

Figure 6

Plots of mean particle size against mass fraction of PESA at 20 °C. Solid squares represent the mean size of particles, exponential function fits to the data are shown as solid lines, and the function is given in the Supporting Information; error bars representing standard deviations at each point are also detailed in the Supporting Information.

Interaction between Acidic Polymers and Crystals

The structure of acidic polymers is likely to affect crystal growth; the fundamental reason is due to the fact that both PESA and PASA are carboxylic acids. Based on the work of polyacrylate-modified sodium oxalate crystallization,[48] polymer anions could incorporate into the crystalline matrix, and the functional group could capture a small portion of Cs+ ions, which explains the lower crystal growth rate at a higher polymer concentration. The adsorption of the functional groups from acidic polymers on the surface of the nucleus or crystallites could be the key factor for those performances, which could enhance interfacial energy and lead to larger radii for critical nuclei, fundamentally forming fewer nucleation sites. As can be seen in Figure , PESA shows a stronger effect on the decrease of the crystal size than PASA when PESA/PASA have the same proportion in the solution; this could be on account of their functional group. According to the non-crystallographic branching model,[49] the inclusion of impurities could cause defects during crystal growth and, therefore, result in branching. However, there is no evidence to prove this branching model on whether polymer is being included or not in the solid, which would require some experimental analysis. With respect to this model, PESA and PASA could be large-size impurities, and the anisotropy of crystal growth could be modified by inducing internal stress when impurity anions incorporate into the growing crystals, thereby resulting in a different crystalline morphology.

Conclusions

In summary, crystals of CsCl can be easily induced by the technique of NPLIN, and crystal shapes after laser irradiation were similar, whilst crystal sizes became heterogeneous. With the addition of acidic polymers, the crystal size and morphology achieved a uniform control; the crystal size and the quantity of crystals can be modified by an adjustable mass fraction of additives in solution. According to the modified classical nucleation theory, the free energy of forming a subcritical nucleus would be increased by the addition of PESA due to the increase of interfacial tension between the solution and nucleus, which disfavors the formation of nucleus and decreases the number of nucleation sites. With respect to spontaneous nucleation, CsCl crystals with a cubic shape are more likely produced from heterogenous nucleation instead of forming a flower shape. Additionally, the presence of PESA could result in high viscosity and a decreased crystal growth rate. In the event of spontaneous nucleation, fewer nucleation sites lead to a sufficient number of solutes for the formation of crystals in each nucleation site; this would lead to a large and strong cubic crystal as the energy barrier for the formation of a cuboid crystal is less than that of flower-like crystals. In the application of NPLIN, high-level PESA could lead to the presence of crystals with a flower-like due to a relatively lower crystal growth rate and more nucleation sites induced by the laser. With respect to the interaction between acidic polymers and crystals, the adsorption of the functional groups from acidic polymers on the surface of growing crystals could also be a factor to decrease the crystal growth rate. According to the non-crystallographic branching model, the anisotropy of crystal growth could be changed through incorporating impurity anions into the growing crystals; this could cause different crystalline morphologies to occur.

Experimental Section

Sample Preparation

Cesium chloride powder (99.99% metals basis, Macklin), ultrapure water (18.2 MΩ cm), PESA solution (40 wt % aqueous solution), and PASA solution (40 wt % aqueous solution) as organic additives are made for samples. Supersaturated CsCl solutions with different mass fractions of PESA from 0.12 to 1.0 wt % were prepared. If the quantity of ultrapure water was known and fixed, higher mass fractions of PESA in solution would be made by adding more quantity of PESA solution, which would in turn increase the entire mass of the solvent. Thus, the quantity of CsCl powder added should also increase to ensure the consistency of the concentration of CsCl solutions. Both 60 wt % water in added PESA solution and the quantity of ultrapure water were considered to be the entire solvent for CsCl solute; for example, the solution with supersaturation of 1.15 and 1.0 wt % PESA should be made by 22.58 g CsCl powder and 0.84 g PESA solution if 10 g ultrapure water is added. The plot of the mass fractions of PESA versus the mass of CsCl solute is given in the Supporting Information (Figures S1 and S2). Solutions with concentration of 2.15 g g–1 at 20 °C were prepared,[45] corresponding to a supersaturation of 1.15; the addition of 0.12–1.0 wt % PESA was based on Figure S2. As water in PESA solution reduces supersaturation, the addition of each component in solutions with the same supersaturation of 1.15 is listed on Table S1. As solubility of CsCl could be modified by the addition of acidic polymers, the value of solubility at 20 °C was measured by adding CsCl powder at a fixed mass of ultrapure water and acidic polymer; we observed that the solubility slightly increased, by 1.6%, when mass fraction of acidic polymers increased to 8% in solution. Thus, the change in supersaturation can be negligible with the effect of acidic polymers. Solutions were placed inside an oven (60 °C) until fully dissolved, and then, samples were made by transferring those heated solutions into to 5 mL vials (Pyrex 1.7 cm diameter, plastic screw-on caps with rubber inserts), after which all vials were then reheated 2 h inside an oven to dissolve any spontaneous nucleation events, after which they were placed to a temperature-controlled incubator at 20 °C. Vials were left untouched and allowed to fully cool to 20 °C (3 h) before laser work.

Laser-Induced Nucleation

Samples were exposed with the laser by placing the cooled vials in the laser pathway and shooting each vial with a single laser pulse with 532 nm wavelength, which was generated by a 1064 nm Q-switched Nd3+/YAG laser (Quantel Q-smart 450) with a harmonic generator (2w) for frequency doubling; see Figure . The linearly polarized laser light was passed through a beam attenuator module (BAM) to control the power of the beam. The diameter of the unfocused beam was 2.5 mm, modified by a variable beam reducer (Zolix, GCO-2501); the input beam was delivered through the center of the vial. The mean power of the unfocused beam was recorded by a power meter (Ophir, NovaII). As the beam could be focused by the cylindrical lens of vials, laser power was converted to peak power density by taking into account a 6 ns pulse duration and the area of the beam at the exit of the vial. With an input area of 0.049 cm2, the area of the beam at the exit of the vial was 0.021 cm2. The experiment was conducted at a peak power density of 16 MW cm–2 with the power of 0.095 J cm–2. Crystals immediately occurred along the laser pathway after a single laser pulse. Videos of NPLIN were recorded using a camera (Sony, DSC-HX30) to obtain a good contrast between the crystal growth in different groups. Crystal sizes were measured by a Nano Measurer 1.2 which is particle size analysis software, and the actual crystal sizes would be slightly bigger than the sizes measured from images due to a lensing effect from the cylindrical shape of the vial. Nevertheless, this research mainly focused on the change of crystal sizes, and the value of crystal sizes measured from videos was considered as the main assessment for crystal size changes. The resulting crystals were ground into powder in order to perform pXRD analysis, and CsCl crystals produced by laser or spontaneous nucleation were kept at 200.02 K during data collection from 15 to 90° in steps of 6° min–1 through a “Bruker D8 Advance” diffractometer. The X-ray source is Cu Kα radiation with a λ of 1.5406 Å, a current of 40 mA, and a voltage of 40 kV.

Figure 7

Schematic layout of the optical setup for crystal size control in NPLIN of supersaturated CsCl sample. The output wavelength of Nd3+/YAG laser is 1064 nm, frequency doubling to 532 nm through a second-harmonic generation module (2w); laser power is controlled by inserting a beam attenuator module (BAM) between them. Coated laser mirror (M) is used to adjust the beam pathway. The diameter of the input beam is reduced to 2.5 mm using a viable beam reducer (BR) from an original diameter of 6.5 mm, and the center of the sample vial (V) is in the laser pathway, as illustrated.