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Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results
  6. 4 Discussion
  7. 5 Conclusions
  8. Acknowledgments
  9. References

Congruent crystallization of antimony sulphoiodide (SbSI) glass of stoichiometric composition, which is prepared successfully for the first time using rapid melt-quenching, has been investigated using differential scanning calorimetry (DSC). The results for glass powder show a glass transition at 127°C and two separate exothermal peaks with maxima around 140°C and 190°C. The ratio of the intensities of the exothermal peak at ~190°C to the peak at ~140°C increases as the particle size and heating rate are increased, but their total enthalpy remains constant at 62 ± 2 J/g for all DSC runs. Surface heating of the glass induced by a 520 nm CW laser shows two contracted regions: needle-like crystalline formations at low temperature and bulk crystallization at high temperature. The observed phenomena and DSC results suggest two different kinds of crystallization of the SbSI phase: one-dimensional crystallization at low temperature which starts from the sample surface and three-dimensional bulk crystallization that continues the transformation to crystalline state at higher temperatures. The origin of the two different crystallizations can be traced to the strong anisotropy of the SbSI crystal structure due to the weak van der Waals interaction between covalent-ionic chains (Sb2S2I2)n.


1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results
  6. 4 Discussion
  7. 5 Conclusions
  8. Acknowledgments
  9. References

Crystallization by laser irradiation is a novel method to produce 3-D crystal architectures within preselected regions of a glass[1-10] and to construct active optical elements. Thus far, this technique has been applied primarily to oxide glasses with compositions typically used for glass-ceramics applications. Usually, glass-ceramics are fabricated using well-controlled heat treatments in an electric furnace and the desired crystals are formed at the surface or in the bulk of the glass. The method of laser-induced crystallization differs from conventional formation of glass-ceramics in a furnace in that it induces oriented crystal growth in spatially selected regions (bulk or surface) and there is a much faster temperature increase in the laser-irradiated area. In order to fabricate crystals of the desired phase by laser irradiation, the search for, and development of, suitable precursor glasses is one of the most important steps.

In order to form specific single crystal patterns in glass, the first important consideration is the choice of a composition which both forms glass easily and crystallizes only into the desired phase. If the chemical composition of the desired crystal is located in the glass-forming region, then the basic conditions of laser crystallization are satisfied a priori, and patterned crystals may be formed without significant difficulty.[1-10] However, in general, this condition may not be satisfied. For example, antimony sulphoiodide (SbSI) is a chalcogenide compound which exhibits high values of the dielectric constant, spontaneous polarization, and pyroelectric and pyro-optic coefficients. These characteristics set it apart from other ferroelectrics and make it suitable for use in uncooled pyro-optic and pyroelectric infrared detectors.[11-13] Unfortunately, the stoichiometric composition of the ferroelectric SbSI compound lies outside of the conventional glass-forming region of the Sb–S–I system, which introduces interesting challenges for its laser crystallization. In this study, we explore these challenges for SbSI, which can serve as a model system for other systems, as well. Here, we describe crystallization processes in the SbSI glass under conventional conditions using differential scanning calorimetry (DSC) as a thermal probe and compare the results with continuous wave (CW) laser crystallization.

2 Experimental Procedure

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results
  6. 4 Discussion
  7. 5 Conclusions
  8. Acknowledgments
  9. References

2.1 Fabrication of Glass and Crystalline Samples

As mentioned above, SbSI composition does not form glass readily, but the addition of GeS2 facilitates glass formation.[14, 15] Nevertheless, we attempted to form pure SbSI glass by emulating the conditions previously established for the SbSI–GeS2 system.[16] An elemental powder mixture (about 5 g) of stoichiometric SbSI composition was prepared directly from Sb, S, and I elemental powders. The mixture was sealed in an evacuated 11 mm inner diameter and 10 cm long silica (quartz) tube under 10−2 torr pressure. To prevent explosion, the ampoule was slowly heated in a rocking furnace at 1 K/min to 128°C, 250°C, 450°C, and 650°C, and kept for an hour at each temperature. It was then heated to the final step at 730°C and maintained there for 12 h. After slowly cooling down to 650°C, the ampoules were quenched in cold water to form glass.

To eliminate stresses arising from nonuniform cooling during quenching, the as-cast samples were annealed in a convection oven preheated to 70°C. After 2 h, the ampoule was slowly cooled to room temperature and then cut to obtain solid castings. We found that even though stoichiometric SbSI composition had been considered to be outside of the conventional glass-forming region of Sb–S–I system, when the batch size was small and the quench rate was sufficiently fast (~100K/s–200K/s or faster), we could obtain glassy samples. Visual inspection and further X-ray powder diffraction (XRPD) analysis showed that the regions of sample next to ampoule walls formed the amorphous phase, whereas the interior was partly or fully crystalline (see Fig. 1). Decreasing the ampoule temperature from 650°C to 500°C before quenching in cold water increased the size of the glassy region but did not eliminate completely the crystalline region. It was determined that the low thermal conductivity of the ampoule walls and the SbSI compound limited the cooling rates within the SbSI melt. To achieve faster heat transfer, 6 mm ID ampoules were used to increase the surface area relative to the volume. The as-cast samples [see Fig. 2(a)] obtained in this manner were completely glassy as shown by X-Ray Powder Diffraction (XRPD).

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Figure 1. SbSI sample (a) and X-ray diffraction patterns from different regions (b).

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Figure 2. SbSI glass (a) and crystals (b).

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To make SbSI crystal as a reference, we prepared the melt as before, except that the ampoules were cooled slowly from 730°C to room temperature at the last step of preparation. XRPD analysis shows that the druse of needles, as seen in Fig. 2(b), is simply the SbSI crystalline phase; there are no signs of other likely phases such as Sb2S3 and SbI3.

For energy-dispersive X-ray (EDS) analysis a small “needle” shaped crystal with length 5 mm and thickness 0.3–0.4 mm was selected and was mounted in epoxy resin. Samples were polished successively using 600-, 1000- and 1200-grit SiC abrasive papers with water as a polishing medium, followed by polishing with the suspension of Al2O3 powder of 3, 1, 0.3, 0.1, and 0.05 μm particle sizes.

The powdered sample was divided into fractions of different particle sizes by sieving. We selected six sieves with different mesh sizes (66, 178, 251, 354, 500, and 710 μm) and placed them on the base of the shaker in increasing order. Glass sample was crushed in an agate mortar and transferred to the upper sieve with 710 μm mesh size. After shaking the sieve's stack for 5 min, the particles from the upper sieve were crushed again and the shaking operation was repeated.

A CW diode laser operating at wavelength λ = 520 nm was used for writing crystal lines. The laser beam was first focused on the polished surface of the glass sample by a microscope objective (numerical aperture 0.75) to a spot of 5 μm diameter. We had previously observed selective evaporation of SbI3 from SbSI surface under irradiation from an Argon ion laser operating at 488 nm whose intensity was modulated by a combination of a wave plate and polarizer.[17, 18] In an attempt to avoid this undesirable change of composition and gain a more precise control over the laser intensity, the 520 nm diode laser was employed. This laser provided the ability to precisely control the intensity through the application of an analog voltage, and simultaneously eliminated a significant amount of the noise present in the Argon laser. Thus, we were able to create the spots by slowly (5–10 s) ramping the power density from 0 to 0.05 mW/μm2. This procedure minimized surface evaporation of SbI3 and induced crystallization under the surface of glass sample. In the second stage, the lines were fabricated by translating the sample at a speed of 0.1 μm/s while maintaining the final power density of 0.05 mW/μm2.

2.2 Methods of Characterization

XRPD and DSC were used for identifying crystalline phases and phase transformations, respectively. The XRPD analyses were performed on a Rigaku “MiniFlex II” diffractometer (Tokyo, Japan). The diffraction data were recorded between θ = 10° and 50°, with 0.02° scan step and 0.5 s step time. The glass transition (Tg) and maximum crystallization (Tc) temperatures were determined with a DSC system (model Q2000; TA Instruments, New Castle, DE). The measurements were conducted with a heating rate from 3 to 20 K/min on powders with five different-sized particles: 66–178, 178–251, 251–354, 500–710 μm and >2 mm from room temperature to 250°C.

The chemical composition of glass and crystal samples was determined by an EDS spectroscopy device attached to a scanning electron microscope (SEM) Hitachi 4300 SE (Dallas, TX) in low vacuum environment to eliminate the charging effects usually observed on insulating samples. For EDS analysis, an acceleration voltage of 20 kV and water vapor pressure of 30 Pa were chosen. The spectra were collected and analyzed using EDAX-Genesis software package (EDAX Inc., Mahwah, NJ). The parameters for data acquisition (time, full scale for intensity, and pulse processing time) were kept the same for all the samples. The EDS spectra for crystal and glass samples are shown in Fig. 3, which compare Sb, S, and I in crystal and glass samples. The chemical composition, as calculated following the ZAF procedure [corrections for atomic number effects (Z), absorption (A) and fluorescence (F)] for the three major elements, is summarized in Table 1.

Table 1. Energy-Dispersive X-Ray Data from Different Regions in Crystal and Glass
SampleSb (at.%)S (at.%)I (at.%)
Crystal34.9 ± 2.032.0 ± 2.033.1 ± 2.0
34.8 ± 2.031.8 ± 2.033.4 ± 2.0
35.2 ± 2.031.4 ± 2.033.4 ± 2.0
Glass35.7 ± 2.033.4 ± 2.030.9 ± 2.0
35.4 ± 2.033.6 ± 2.031.0 ± 2.0
35.7 ± 2.032.9 ± 2.031.4 ± 2.0
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Figure 3. Energy-dispersive X-ray spectra of SbSI glass (1) and crystal (2).

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The laser-induced lines were observed with scanning electron (Hitachi 4300 SE) and optical microscopies. The optical images were received using a PAX-IT camera attached to an Olympus BH-2 light optical microscope with PAX-IT digital imaging software (Center Valley, PA). Sample topography was determined by atomic force microscopy (Solver Next AFM/STM, NT-MDT, Zelenograd, Russia) with a conductive Pt-coated NSG10 tip in semicontact mode. The amorphous or crystalline nature of the lines was checked by electron back-scatter diffraction analysis.

3 Results

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results
  6. 4 Discussion
  7. 5 Conclusions
  8. Acknowledgments
  9. References

3.1 Calorimetric Measurements of Crystallization Process

Figure 4 shows a nonisothermal DSC plot for a bulk (>2 mm) fragment of SbSI glass recorded using the heating rate of 15 K/min. It shows a glass transition with inflection point of 127°C and two different exothermal peaks with maximum at 152°C and 192°C. The corresponding endothermic step and exothermal peaks were absent on curves obtained during second heating of the same sample. Despite the two different exothermal peaks present in the first of the two DSC scans (up to 250°C), subsequent XRPD measurements showed the presence of the SbSI phase only. No additional peaks were detected in this pattern (Fig. 5).

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Figure 4. Differential scanning calorimetry data for one big piece (~20 mg) of SbSI glass.

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Figure 5. X-ray powder diffraction data for glass samples heated to 140°C and 250°C.

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To determine the origin of the two different exothermal peaks in the DSC trace, the effect of particle size on the peak profile was investigated by heating glass particles of varying sizes at different rates. The DSC plots for four different sizes and heating rates are shown in Figs. 6 and 7. In contrast to the DSC trace for the large particles (Fig. 4), the trace for particles with average size 66–178 μm shows only one strong exothermal peak at a temperature close to Tg [Fig. 6(a)]. The peak maximum shifts to higher temperatures with increasing heating rate. The enthalpy of the observed exothermal effect is given by the area under the peak, which is found to be practically the same within experimental error (62 ± 2 J/g) for all the four heating rates. The XRPD patterns for these small samples, which were heated to 140°C and held for 15 min, show only the SbSI phase (Fig. 5).

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Figure 6. Differential scanning calorimetry data for 66–178 μm (a) and 500–710 μm (b) size particles of the SbSI glass with different heating rates: 3 K/min, 10 K/min, 15 K/min, and 20 K/min.

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Figure 7. Differential scanning calorimetry data for different sized particles of SbSI glass with heating rate 15 K/min (a) and 20 K/min (b).

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Similar to the case of bulk glass fragment, two DSC peaks (approximately at ~140°C–150°C and ~190°C) are observed for powder samples with 500–710 μm size particles at different heating rates [Fig. 6(b)]. At 3 K/min, only the first peak is present. When the heating rate is increased from 10 to 20 K/min, the intensity of the peak at ~190°C also increases but that of the first peak at ~140°C decreases in comparison with the case of slow heating. At 20 K/min, the second peak is stronger than the low temperature peak [Fig. 6(b)]. Note that the total enthalpy of the two observed exothermal effects (one for 3 K/min) obtained from DSC measurements with four different heating rates is 66 ± 4 J/g. This value is the same as for the particles with size 66–178 μm, which showed only one exothermal peak. Aside from SbSI, no other crystalline phase was detected from XRPD patterns for these samples after DSC measurements.

It is noteworthy that within the DSC data obtained on powders at sufficiently slow heating rate, the endothermic change representing glass transition is not visible. As the exothermic crystallization peak moves to lower temperatures with decreasing heating rate, the glass transition is masked by its presence and cannot be discerned.

The thermogravimetric analysis did not show any evidence of decomposition of the samples of any size when heated up to 250°C. Thus, both of the observed exothermal peaks are caused by the same process, viz. congruent crystallization of glass to the SbSI phase.

In principle, the surface to volume ratio for a fixed weight of the glass powder should decrease with increasing particle size. A similar behavior for the ratio of intensities of exothermal peaks at ~140°C and ~190°C is observed when varying the particle size of the glass powders (Fig. 7). For the samples with size less than 251 μm, the peak at ~190°C is absent. Such particles are fully crystallized in a lower temperature range – around the glass transition temperature (127°C). As the size of particles increases, the intensity of the peak at ~190°C also increases in contrast to the intensity of the first peak at the lower temperature, which decreases (Fig. 7). Such a dependence of the exothermic peaks on particle size can be explained readily by two different types of SbSI crystallization – one dominating at the surface and the other deep within the bulk. At temperatures just above Tg, crystallization of the SbSI phase starts from the sample surface. If the heating rate is low (3 K/min) and the size of particles is smaller than 710 μm, samples are devitrified and fully crystallized in this low temperature range via surface crystallization (Fig. 6). Evidently, the rate of growth from the surface is limited, and at higher heating rates, as well as for bigger particle sizes (>500 μm), bulk crystallization begins to dominate in the transformation of SbSI glass to its crystalline phase peaking at ~190°C (Figs. 4, 6(b), and 7).

From Fig. 7 we can see shifts in both the DSC crystallization peaks to lower temperatures when the size of the particles decreases. This is especially evident for the first low temperature peak, which shifts, for example, from 150°C for powder samples with 500–710 μm sized particles to 135°C for particles with 66–178 μm size [Fig. 7(a)]. This is probably due to a significantly larger amount and greater variety of surface nuclei formed during the grinding procedure for making the powder.[19]

3.2 Analysis of Calorimetric Data

With the assumption that nucleation and growth rates are independent of time, the temperature dependence of the crystallization rate constant k(T) can be expressed in an Arrhenian form. The rate of crystallization dα/dt can then be written as[20]:

  • display math(1)

where the function f(α) depends on the reaction model and is independent of temperature, E is the overall activation energy, R is the gas constant, and T is the absolute temperature. On the other hand, the rate of crystallization dα/dt is proportional to the measured specific heat flow ϕ[21]:

  • display math(2)

where ΔHc corresponds to the total enthalpy of exothermal processes associated with crystallization. By combining Eqs. (1) and (2), the following logarithmic expression is obtained[20]:

  • display math(3)

The first term on the right-hand side of Eq. (3) is constant for a given value of crystallized fraction α. Thus, the overall activation energy E can be evaluated from the slope of the plot ln(ϕ)α versus 1/T for a constant value of α.[19, 20]

We have used this approach to analyze the present nonisothermal DSC data obtained from the experiments conducted at different heating rates. Figure 8 shows plots for activation energy of the low temperature (surface) crystallization for two (66–178 and 500–710 μm) different sized particles. The activation energy for particles with 500–710 μm size was calculated for α in the range of 0.2–0.5 only, because outside this range these particles crystallized in the high temperature range [see Fig. 6(b)].

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Figure 8. The activation energy of SbSI low-temperature crystallization for particles with 66–178 μm (1) and 500–710 μm (2) size.

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According to Kissinger's model, the effective crystallization activation energy can be determined from the variation in the peak crystallization temperature Tp with heating rate β. For the evaluation of this activation energy, the following relation was used[22]:

  • display math(4)

The slope of the plots of ln(β/Tp2) vs. 1/Tp gives the value of the activation energy of crystallization Ec. The straight lines are obtained for the low temperature peak, and their slopes give 101 ± 5 kJ/mol for the smallest particles (66–178 μm) and 121 ± 14 kJ/mol for 500–710 μm particles (Table 2). The calculated values of Ec are shown as horizontal lines on Fig. 8.

Table 2. Parameters of SbSI Low-Temperature Crystallization
Sample size (μm)66–178500–710
Kissinger's activation energy (kJ/mol)101 ± 5121 ± 14
DSC rate (K/min)Avrami parameter value
31.9 ± 0.41.7 ± 0.4
101.9 ± 0.41.7 ± 0.4
151.9 ± 0.41.7 ± 0.4
202.0 ± 0.41.8 ± 0.4

For the identification of crystallization processes, the crystal growth dimensionality, n (also known as the Avrami parameter) was calculated according to the Augis–Bennett equation[23]:

  • display math(5)

where Δω is the full width at half maximum of the crystallization exothermic. Equation (5) indicates that a broad peak signifies a lower dimension (surface, one-dimensional growth) reaction, whereas a sharp peak implies a higher dimension reaction (bulk, three-dimensional growth).[23] Prior knowledge of the activation energy Ec is required to determine n. To calculate the crystal growth dimensionality for low temperature crystallization (see Table 2), we used the values of the activation energy calculated using Kissinger's model [Eq. (4)]. Then Eq. (5) gives similar values of ≤ 2 within the experimental error for the different sized particles (500–710 μm), which indicates low dimensionality of this crystallization process.

The above described approach using Eqs. (3)-(5) cannot be applied to the analysis of the high-temperature SbSI crystallization because this process is secondary and depends on the low-temperature crystallization process. Nevertheless, for one large piece of SbSI glass, where secondary crystallization is dominant, the width of the corresponding high-temperature exothermal peak (Δω) is a few times smaller than the width of the DSC low-temperature peak (Fig. 4). This indicates a higher dimension of the high temperature crystallization – probably bulk, three-dimensional crystal growth.

3.3 Laser-Induced Crystallization

Two different types of transformation (crystallization) were observed in laser-treated SbSI glass. An analysis of the optical microscopy images collected with unpolarized and polarized light showed the formation of different crystalline regions upon laser irradiation (see Fig. 9). At the beginning of line formation, we observed a crater (marked as region C on Figs. 9 and 10) with a depth of ~1 μm and diameter of 5–6 μm, which correspond to the spot diameter of the laser beam. Deviation of the chemical composition from that of the starting stoichiometric SbSI glass was observed only at the spot crater and a narrow region D in the central part of line. Thus this crater appeared due to SbI3 evaporation at the beginning of laser irradiation (Figs. 10(a) and (b)].

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Figure 9. Optical microscopy images of laser-induced line (laser power 0.95 mW; ramp time 5 s; time spot exposure 5 s; speed 0.1 μm/s) received in unpolarized light (a) and crossed polarizations (b) and corresponding schematic of the observed regions (c). See text for explanation of marked regions. Scale bar corresponds to 5 μm.

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image

Figure 10. Scanning electron microscope (a) and AFM (b) images of laser-induced line (laser power 1 mW; ramp time 5 s; time spot exposure 5 s; speed 0.1 μm/s). Marked regions correspond to those shown in Fig. 9. Profiles of surface (c) and (d) correspond to the lines marked on AFM image (b) by numbers 1 and 2, respectively. Scale bars correspond to 5 μm for X- and Y-directions and 0.2 μm for AFM Z-direction.

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Around the starting spot crater, we find a shrinkage region (marked as A) with 0.3 μm depth, which is approximately two times wider than the laser beam diameter [Fig. 10(c)]. This region appears during the first stage (spot creation) and can propagate along the line (L region on Figs. 9 and 10) during the scanning stage. Evidently, due to different thermal expansion (contraction) coefficients for crystalline and glass phases, this region is bounded by the surrounding areas by a sharp boundary. Sometimes this boundary develops into a crack.

Aside from the shrinkage in region A, another region (marked as B) can be observed on optical microscopy images (Fig. 9). Region B, which surrounds region A as well as extends around the line of the laser scan, shows smaller contraction (0.15 μm) than region A [Fig. 10(d)]. In contrast to region A, this region is bounded by glass areas with a fuzzy boundary. At some places in region B, in the middle part of the line we observed many long and thin “needles,” which were oriented at angles less than 30°–40° with respect to the direction of the laser beam scanning. We hypothesize that these two shrinkage crystallization regions appear in laser-treated SbSI glass due to the two different crystallization processes which were detected by DSC in the ranges of 140°C and 190°C (Figs. 6 and 7). Our attempts to confirm the crystallinity of both regions by electron-backscattered diffraction were unsuccessful, most likely due to the surface roughness in region A and/or because sometimes laser crystallization occurs just beneath the surface where temperature is lower due to exposure to air.[10]

4 Discussion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results
  6. 4 Discussion
  7. 5 Conclusions
  8. Acknowledgments
  9. References

Raman spectroscopy of glasses in the Sb–S–I system shows that the glass matrix is built through a weak interaction between SbS3 and SbI3 structural groups.[24-26] The quasi-eutectic structure of the Sb–S–I system glasses is amenable to decomposition and evaporation of SbI3 molecules from the surface, especially in the liquid state.[18]

From another perspective, the crystalline SbSI has a strongly anisotropic double chain structure.[27-29] This structure belongs to the orthorhombic system where double chains (Sb2S2I2)n are oriented in the c-direction of the cell. It contains strong covalent-ionic bonds within the double chains and weak van der Waals interaction between the double chains. This extremely large anisotropy in bonding forces predetermines strong anisotropy of crystal growth as well as a number of physical properties. For example, the growth rate along the c-axis is 2 orders of magnitude larger than along the a- or b-axis. As a result, only thin needle-like crystals or polycrystalline forms could be obtained in spite of various attempts.[30, 31]

Taking into account the strong anisotropy of the SbSI crystal structure and correspondingly high crystal growth rate along (Sb2S2I2)n double chains, we can expect that the crystallization process detected by DSC around the glass transition corresponds to the formation of SbSI mainly along the c-direction. For such crystallization to occur, some bonds of SbS3 and SbI3 units must switch or establish long-range ordering in line with the already existing SbSI chains in the glass. The stoichiometric composition of the SbSI compound lies outside of the glass-forming region of the Sb–S–I system and a glassy state was obtained only upon very fast quenching with cooling rates ~200°C/s. So we can expect that numerous crystalline nuclei would form during quenching. This explains the observation that crystallization starts in the temperature range of glass transition. Another likely source of a large amount of existing nuclei is the grinding procedure, which induces crystalline embryos on the surface of glass particles. The crystallization of SbSI chains would then begin from nuclei located at the surface of the particle, as supported by the observed shift in the DSC peak to low temperatures with decreasing particle sizes (see Fig. 7).

For the growth of SbSI crystal in a direction normal to chains, a more complicated shifting, switching, and diffusion of SbS3 and SbI3 units, or reorientation of existing (Sb2S2I2)n double chains is needed. Such a complex process of crystallization would require higher activation energy and hence begin at higher temperatures. In this case, SbSI crystalline grains would grow in volume (three-dimensional growth – 3D) in contrast to low-temperature crystallization when they increase basically only in length (1D). Note that the crystalline “needles,” which appear at lower temperatures and grow into the bulk of the sample, would serve as crystal seeds for the high-temperature bulk crystallization of SbSI.

Following the above model comprising of growths of two different dimensionalities, we can evaluate the average rate of one-dimensional SbSI crystal growth which starts at the sample surface. According to DSC data presented in Fig. 7(a), the particles with maximal size of 350 μm were almost fully crystallized at a heating rate of 15 K/min in the temperature range from 120°C to 165°C. It means that needle-like crystals radially penetrate from the surface to the center at a depth of 175 μm within 4.33 min. At a heating rate of 20 K/min and corresponding time of crystallization 2.5 min, SbSI needles penetrate to a depth of 125 μm [Fig. 7(b)]. In both cases, simple calculation gives approximately the same value −0.6 to 0.7 μm/s, which is comparable to the value 0.2 μm/s of growth rate for SbSI crystals in the c-direction from vapor phase.[31]

The proposed model consisting of two different kinds of SbSI crystallizations explains the observed two different crystallization regions under laser-induced writing (Figs. 9 and 10). During laser-induced fabrication of lines we keep the laser power density in the range less than the decomposition threshold but higher than heat removal at room temperature.[18] The temperature of the spot surface then reaches a steady level after some time of irradiation and the laser heat deposition becomes equal to the loss of heat. The maximum value of temperature is established at the center of the illuminated region with the thermal gradient extending radially outward. According to DSC data, 3D bulk SbSI crystallization occurs in the temperature range above 190°C. So, at first (spot formation – Fig. 9) and at the beginning of the second stage (line formation – Fig. 10), the temperature in region A reaches this range and viscous supercooled liquid devitrifies growing crystals in all directions (3D bulk crystallization). Due to the different densities of the crystalline and glassy states, this region shows strong shrinkage. In region B, surrounding region A, the temperature is lower and only one-dimensional crystallization is possible, where we observe “needles.” Certainly, needle-like SbSI crystals are incorporated in the glass matrix and the magnitude of contraction is correspondingly less than in region A, where bulk crystallization is possible.

5 Conclusions

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results
  6. 4 Discussion
  7. 5 Conclusions
  8. Acknowledgments
  9. References

Glass with a stoichiometric composition of SbSI that lies outside of the glass-forming region of the Sb–S–I system has been prepared. Maximum crystallization of SbSI was observed at two different temperature ranges around 140°C and 190°C. The DSC scans and XRPD measurements of glass particles of different sizes allow the identification of one-dimensional surface, and three-dimensional bulk crystallizations of the SbSI phase, which are predetermined by the strong anisotropy due to the crystal structure consisting of chains. The laser-crystallized lines consisting of two different regions of SbSI crystals were patterned at the glass surface by scanning the focused beam from a 520 nm CW laser. Crystallization induced by the laser also shows two crystallization regions: needle-like crystal formation at low temperature and bulk crystallization at high temperature.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results
  6. 4 Discussion
  7. 5 Conclusions
  8. Acknowledgments
  9. References

This work was supported by the Basic Energy Sciences Division, Department of Energy (project DE-SC0005010). The National Science Foundation supported MS as a summer REU student through IMI grant DMR 0844014. BK, who contributed to the laser irradiation part of the work, is supported by NSF (DMR-0906763). We thank Karel Palka for help with AFM measurements.

References

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental Procedure
  5. 3 Results
  6. 4 Discussion
  7. 5 Conclusions
  8. Acknowledgments
  9. References
  • 1
    T. Komatsu, R. Ihara, T. Honma, Y. Benino, R. Sato, H. G. Kim, and T. Fujiwara, “Patterning of non-Linear Optical Crystals in Glass by Laser-Induced Crystallization,” J. Am. Ceram. Soc., 90, 699705 (2007).
  • 2
    T. Fujiwara, T. Honma, S. Mizuno, N. Iwafuchi, Y. Benino, and T. Komatsu, “Order/Disorder Hybrid Structures in Photonic Glass Materials,” Adv. Mater. Res., 11–12, 536 (2006).
  • 3
    T. Honma, “Laser-Induced Crystal Growth of Nonlinear Optical Crystal on Glass Surface,” J. Ceram. Soc. Jpn., 118, 716 (2010).
  • 4
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