Pressure-Induced Switching between Amorphization and

Jun 27, 2013 - Combining in situ high-pressure X-ray scattering with transmission electron microscopy, we investigated the pressure-induced structural...
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Pressure-Induced Switching between Amorphization and Crystallization in PbTe Nanoparticles Zewei Quan,†,§ Zhiping Luo,∥,⊥ Yuxuan Wang,‡ Hongwu Xu,§ Chenyu Wang,† Zhongwu Wang,# and Jiye Fang*,†,‡ †

Department of Chemistry and ‡Materials Science and Engineering Program, State University of New York at Binghamton, Binghamton, New York 13902, United States § Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ∥ Microscopy and Imaging Center, Texas A&M University, College Station, Texas 77843, United States ⊥ Department of Chemistry and Physics, Fayetteville State University, Fayetteville, North Carolina 28301, United States # Cornell High Energy Synchrotron Source, Wilson Laboratory, Cornell University, Ithaca, New York 14853, United States ABSTRACT: Combining in situ high-pressure X-ray scattering with transmission electron microscopy, we investigated the pressure-induced structural switches between the rock salt and amorphous phases as well as the associated mechanisms of their crystallization and growth in 6 nm PbTe nanocrystal. It was observed that rock salt PbTe nanocrystal started to become amorphous above 10 GPa and then underwent a low-to-high density amorphous phase transformation at pressures over 15 GPa. The low-density amorphous phase exhibited a structural memory of the rock salt phase, as manifested by a backward transformation to the rock salt phase via single nucleation inside each nanoparticle upon the release of pressure. In contrast, the high-density amorphous phase remained stable and could be preserved at ambient conditions. In addition, electron beam-induced heating could drive a recrystallization of the rock salt phase on the recovered amorphous nanoparticles. These studies provide significant insights into structural mechanisms for pressure-induced switching between amorphous and crystalline phases as well as their associated growth processes. KEYWORDS: Amorphization, crystallization, high pressure, PbTe nanoparticle, phase transformation, synchrotron XRD

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contains about 100−10 000 atoms, are an ideal class of materials for such studies, since they are not only smaller than the fragment domains in a bulk single crystal (without being further fractured under compression)4 but can also be prepared to be defect-free. As demonstrated in the study of CdSe NCs by Alivisatos and Tolbert,2 the single nucleation domain controls the crystallization and growth of high pressure metastable phase in NCs. Therefore, use of NCs may facilitate our investigations of the underlying mechanisms of pressureinduced phase transformations. Another advantage of NCs is their unique surface structures that could be utilized to stabilize high-pressure metastable phases. A feasible harvest of the metastable structural polymorphs at ambient pressure may enable newly manifested properties for a broad range of applications.5 Unlike a diamond that nucleates from graphite at high pressure and stabilizes under ambient conditions, most pressure-induced metastable phases are unstable and frequently reverse back to their original

tructural transformations are important to many research topics, including studies of solids at high pressures and of engineering materials with enhanced properties, as they are often accompanied by appreciable and exploitable changes in physical and chemical properties.1,2 Despite the importance for developing advanced materials, experimental investigations of the microscopic mechanisms of solid−solid phase transformations remain a daunting task due to several reasons, of which highly inhomogeneous transformation kinetics in an extended solid is a major factor.3 In bulk solids, metastable high-pressure phases nucleate in multiple domains, and thus various structural features may play roles in modification of the kinetics and mechanisms of associated phase transformations. Even in a bulk single crystal, crystalline domains of nanometer or micrometer scale exist, which are associated with various stacking faults, twins, fractures, and other defects.3 Such microstructural complexities in bulk materials make it challenging to elucidate microscopic mechanisms of their phase transformations. To circumvent these difficulties, synthesis and utilization of “defect-free” single crystals for high-pressure phase transformation studies have been implemented. Nanocrystals (NCs), each of which © XXXX American Chemical Society

Received: May 7, 2013 Revised: June 14, 2013

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Figure 1. TEM images of the starting PbTe NCs. (a) Low-magnification TEM image; (b) HRTEM image; (c) SAED pattern. The inset in (a) is a constructed model for quasi-spherical PbTe NCs.

structures upon release of pressure.3 Due to the increased ratio of surface-to-volume in NCs as compared with that of the extended solids, the contribution of surface energies could overwhelm the bulk free energies. As a result, the high-pressure behavior of nanomaterials is dramatically different from that of their bulk counterparts. When NCs are smaller than a certain critical size, the increased surface energies could even cause the reversal of size-dependent relations of physical properties, such as mechanical strength and phase stability.4,6 Therefore, one may fine-tune either or both of the size and morphology of NCs, allowing for not only harvesting the pressure-induced metastable phases at ambient pressure, but also producing novel metastable substances even without pressurization.7 Inspired by recent successes, tremendous interest has been incurred to study pressure-induced phase transformations in a variety of NC materials.4,6,8−13 PbTe NCs not only display relatively low transition pressures but can be also synthesized readily with a delicately controlled size, shape, and crystallinity.14−17 Due to these characteristics, PbTe NCs become one of the ideal systems for exploiting microscopic mechanisms and kinetics of pressure-induced structural transformations. Recently, we have observed a series of size-dependent pressure-induced structural changes in PbTe NCs.6 When the particle size is smaller than 9 nm, rock salt PbTe NCs can directly transform to an amorphous phase at high pressure.6 They do not behave like large NCs or bulk PbTe, which transform to the orthorhombic structure at high pressure. Meanwhile, several end members in the chalcogenide group as the Ge−Sb−Te system display a laser-driven “reversible crystalline ↔ amorphous phase transformation” that has been widely used in fabrication of rewritable DVDs for data storage.18,19 However, many aspects associated with these

intriguing structurally switching processes remain largely unclear. In particular, the microscopic mechanisms and detailed nucleation/growth kinetics of the “crystalline ↔ amorphous” transformation are unknown. This study takes advantage of the pressure-induced reversible phase transformations between crystalline and amorphous structures in PbTe nanoparticles (NPs) and electron beaminduced postheating recrystallization processes. In this work, we chose 6 nm PbTe NCs as a model system and investigated their pressure-induced structural transformations using synchrotron X-ray diffraction coupled with the diamond anvil cell (DAC) technique. Our results show that rock salt PbTe NCs first transformed to an amorphous phase and then underwent a lowto-high density amorphous phase transformation. Control of the pressure applied onto the PbTe NCs allowed for either switching between a low-density amorphous (LDA) phase and the initial rock salt structure or retaining a high-density amorphous (HDA) phase at ambient conditions. Transmission electron microscopy (TEM) imaging further revealed nucleation process from the LDA and HDA phases. These results provide important insights into pressure-induced transformations between crystalline and amorphous phases under various conditions. Experiments and Characterization. Chemicals. Oleic acid (OA, 90%), 1-octadecene (ODE, 90%), trioctylphosphine (TOP, 90%), and Te shots (99.999%) were purchased from Sigma-Aldrich. Lead oxide (PbO, 99.99%) was received from Alfa Aesar. A stock solution of 0.5 M trioctylphosphine telluride (TOP-Te) was prepared in a glovebox by mixing 5 mmol of Te shots and 10 mL of TOP, followed by stirring and heating at 150 °C overnight. B

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Figure 2. WAXS patterns of 6 nm PbTe NCs under (a) compression and (b) decompression with the maximum pressure of 15.6 GPa; (c) HRTEM image; and (d) SAED pattern of the recovered PbTe NCs. The black and gray triangles in (a) and (b) indicate visible diffraction peaks.

Figure 3. Several typical WAXS patterns of 6 nm PbTe NCs under (a) compression and (b) decompression with a peak pressure of 19.7 GPa; (c) HRTEM image of the recovered PbTe NPs; and (d) SAED pattern of the PbTe NCs after the HRTEM imaging. The peaks marked with star symbols symbols in (a) and (b) are from the stainless steel gasket.

Synthesis. PbTe NCs with an average diameter of 6 nm were prepared using a solution-based high-temperature hot-injection method described elsewhere.14−17 In brief, 0.45 g of PbO (2 mmol), 4.2 mL of OA, and 12.0 mL of ODE were loaded in a three-neck round-bottom flask under airless condition, heated to 170 °C for 30 min, and subsequently stabilized at 150 °C for 5 min under agitation and argon protection. The heating source was then immediately removed to allow the temperature of the system to drop down slowly. Once the solution temperature approach ∼110 °C, 2 mL of Te-TOP (0.5 M) was rapidly injected into this transparent Pb-OA complex solution with vigorous stirring to initiate the formation of PbTe NCs. At the seventh min after the hot-injection, the flask was placed in a cold water bath to completely cease the particle growth. PbTe NCs were precipitated by adding methyl alcohol and acetone in sequence followed by centrifugation and redispersion in hexane and then repeated with ethanol and hexane for two cycles. The isolated PbTe NCs were finally dispersed in hexane, forming black transparent colloidal solutions. High-Pressure WAXS Determination. In situ high-pressure wide-angle X-ray scattering (WAXS) measurements were conducted at B2 station of the Cornell High Energy Synchrotron Source (CHESS).20 Details of the DAC used in this study were described previously.6 The samples were loaded by drop-casting a suspension of PbTe NCs into the gasket hole seated on the tip of one diamond anvil. This procedure was repeated 2−3 times to ensure a sufficient amount of PbTe NCs in the sample chamber. Several pieces of small ruby chips were placed together with PbTe NCs, and a standard laser-excited ruby fluorescence technique was used to monitor the pressure

Figure 4. A plot of the (200) d-spacing of PbTe NCs as a function of applied pressure. Three sections of rock salt, LDA, and HDA phases are categorized based on the variations of d-spacing and curve slope.

applied on the sample.21 Two separate high pressure runs were performed to reach the peak pressures of 15.6 and 19.7 GPa, respectively, for exploring possible difference of phase transformation behaviors of the PbTe NCs. Monochromatic X-rays optimized at a wavelength of 0.485946 Å was collimated to 100 μm in diameter for simultaneous collection of WAXS images from the sample at each pressurized step. A large MAR345 area detector was employed to record the X-ray signals scattered C

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Figure 5. Several typical 2D WAXS images showing the two types of processes and their correlations.

from the sample. Using the Fit2D program, two-dimensional (2D) images were integrated into patterns with plots of intensity as a function of 2θ (degree) for further structural analyses. Upon release of pressure to ambient conditions, the sample was carefully transferred from the gasket hole to a copper grid for successive TEM investigation. TEM Characterization. An FEI Tecnai G2 F20 ST and a JEOL 2010 microscope, operated at 200 keV, were employed to obtain TEM results of the starting and high-pressured PbTe NPs, including TEM images, high-resolution TEM (HRTEM) images, and selected area electron diffraction (SAED) patterns. Results and Discussion. As revealed in Figure 1, the asprepared PbTe NCs possess a narrow size distribution and high crystallinity. They are quasi-spherical with an average diameter of ∼6 nm (Figure 1a). As illustrated in Figure 1b, the lattice planes can be clearly seen across the entire domain of each NC. The SAED pattern (Figure 1c) further shows the highly crystalline nature of each distinct NC. As a series of slightly faceted nanopolyhedra, these quasi-spherical NCs can be reconstructed on surfaces through the Wulff crystallographic theory by the three low-index facets of (200), (220), and (222) (inset of Figure 1a) that normally have relatively lower surface energies. Previous studies suggest that PbTe NCs with a size of larger than 9 nm display a first-order phase transformation from the rock salt structure (face-centered-cubic structure) to an orthorhombic structure, and their size-dependent transformation pressures are higher than that for the bulk PbTe, 6 GPa.6 More interestingly, when their sizes are smaller than 9 nm, PbTe NCs undergo a pressure-induced crystalline-to-amorphous phase transformation. Furthermore, the amorphization pressure decreases with a further reduction of the particle size. To further understand the amorphization mechanism of PbTe NCs, this study chooses 6 nm PbTe NCs that not only hold good crystallinity but also have a lower transition pressure to amorphous phase. Figure 2a presents several representative WAXS patterns of 6 nm PbTe NCs during compression up to 15.6 GPa. At ambient condition, the rock salt PbTe NCs displayed an apparent size-

induced broadening feature of their X-ray diffraction peaks. Upon a pressure increase, PbTe NCs remained stable in the rock salt structure, but all of the X-ray peaks shifted to higher angles, indicating a pressure-induced lattice contraction. At pressures above 10.4 GPa, the two peaks of (220) and (222) (gray triangles in Figure 2a) began to weaken. Above 12.9 GPa, both the peaks became indistinguishable and hidden in the strong diffuse background. Meanwhile, the (200) peak (indicated by black triangle) was left as the only noticeable one, and the peak width became much broader. Such overall features suggest the appearance of a pressure-induced amorphous phase. Upon a pressure release from the maximum pressure of 15.6 GPa (Figure 2b), the amorphous phase remained stable to a pressure as low as 2.3 GPa and then transformed back to the rock salt structure at ambient pressure. HRTEM image (Figure 2c) and SAED pattern (Figure 2d) of the PbTe NCs recovered from this experiment showed wellordered lattice fringes and sharp diffraction rings, confirming the crystalline nature of the recovered PbTe NCs. It can be thus concluded that the structural transformations between the crystalline and amorphous phase are reversible when the loading pressure is below 15.6 GPa. A further comparison of diffraction peaks between the starting and recovered PbTe NCs does not reveal any significant variation in terms of the peak widths. It appears that the full cycle of compression did not break PbTe NCs down to smaller pieces. In other words, the NC size of 6 nm is probably smaller than the critical fragment domain size of PbTe. Therefore, the observed structural switches between the rock salt and amorphous phases were initialized mainly by a single nucleation event within each NC. TEM results (Figure 1) reveal that the quasi-spherical PbTe NCs are terminated at surfaces by a series of polyhedral facetsmainly three lattice planes, (200), (220), and (222). The increased surface areas dominated by these low surface energy lattice facets are most likely responsible for the enhancement of structural stability and different types of phase transformations. The rock salt-to-orthorhombic phase transformation observed in bulk and large PbTe NCs is displacive in nature, in D

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which the rock salt structure distorts without breaking its chemical bonds to form the orthorhombic structure.22 As for smaller NCs in which single nucleation event controls the phase transformation, the nucleated phase grows larger and quickly extends across the entire NC region. As a common characteristic of displacive transformations, the coherence and homogeneity of crystal growth in each NC may reshape the NC morphology.9 Accordingly, the NC surfaces were reconstructed by a series of high-index facets (with higher surface energy) of the newly formed metastable phase.3 The magnitude of energy barrier that prevents from the occurrence of phase transformation increases and scales with the reduction of particle size, so the size-tuned increase of energy barrier dramatically alters the nature of the solid−solid phase transformation. This is apparently true in the case of PbTe NCs, in which the eventual state was an amorphous phase, rather than the orthorhombic phase that was observed in bulk and large NCs. As for the amorphous phase, if the applied pressure is equivalent or slightly greater than the starting amorphization pressure, the atoms become frustrated in a disordered way but could still retain certain structural connection to long-range ordered features. If the original structure features were memorized, we expect that release of pressure could be able to switch the amorphous phase back to the rock salt phase. However, continuous elevation of pressure to a critical point could be most likely breaking such a hidden structural connection between the low-pressure crystalline and highpressure amorphous phases. To test this hypothesis, we conducted additional WAXS runs at a higher pressure. Figure 3a presents WAXS patterns of PbTe NCs collected at pressures up to 19.7 GPa, and the corresponding variation of the (200) d-spacings with pressure is shown in Figure 4. Once the pressure reached above 10 GPa, the d-spacing of (200) peak suddenly shifted to a smaller value in a nonlinear fashion and the pressure-dependent d-spacing curve displayed an apparent discontinuity. This implies the first-order phase transition of PbTe NCs from rock salt to LDA occurs at a pressure over 10 GPa, which matches well with the WAXS patterns in Figure 2. This LDA phase possesses comparable compressibility with the rock salt phase, as revealed by their similar curve slope from 10 to 15 GPa. Apparently, continuous pressurization above ∼15 GPa (marked with yellow square brackets in Figure 3a) resulted in a significant broadening of the only remnant (200) peak and noticeable loss of its intensity. The incompressibility became relatively greater as compared with that at pressures below 15 GPa (Figure 4). These typical features suggested that a HDA phase formed above 15 GPa. This amorphous phase is significantly different from the LDA phase that formed at lower pressure with a structure memory effect of the rock salt phase. A pressure release from the peak value of 19.7 GPa to the ambient pressure did not transform the HDA phase back to the rock salt structure (Figure 3b). This result indicates that a compression of PbTe NCs at a pressure above the critical value (15 GPa) completely broke the structural connection between the low-pressure crystalline and amorphous phases. Therefore, the collapse of entire crystalline structure and loss of structure memory resulted in the formation of a new HDA state and its excellent preservation at ambient conditions. Based on a TEM imaging taken using very weak electron beams (Figure 3c), we verified that the recovered PbTe NCs from 19.7 GPa were almost in an amorphous state. This is consistent with the broad diffuse feature of the amorphous phase as shown in its SAED pattern (Figure 3d).

Figure 6. TEM results of PbTe NPs after the high-pressure treatment with a maximum loading of 19.7 GPa. (a−f) In situ SAED patterns of PbTe NPs along with the increase of electron dose level (the unit is e/ nm2), and (g) their corresponding intensity profiles of SAED patterns shown in a−f; (h) HRTEM image of PbTe NPs directly obtained after the electron irradiation dose of 27.2 × 106 e/nm2. The selected sample region is similar to that used in Figure 3c.

To illustrate the experimental results obtained from the two high-pressure X-ray runs, several typical 2D WAXS images with azimuthal plots are presented in Figure 5. PbTe NCs crystallized in a rock salt structure as a starting phase under ambient conditions. Upon compression, PbTe NCs gradually lost their long-range order. At a pressure above 10.4 GPa, it became amorphous. This amorphous phase represented a LDA phase and remained stable up to the peak pressure of 15.6 GPa. Upon a release of the pressure, the LDA phase transformed back to the rock salt structure. On further compression, however, this LDA phase transformed into a HDA phase. The HDA phase appeared to be more incompressible than LDA and could not revert back to the original rock salt structure. Instead, this HDA phase was preserved at ambient conditions. Apparently, control of the magnitude of the loading pressure was capable of harvesting either an ordered or an amorphous E

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Figure 7. Schematic phase transitions associated with Gibbs free energy observed in PbTe NPs, as well as the HRTEM images of the samples recovered from 15.6 GPa (left inset) and from 19.7 GPa at the initial stage of electron beam irradiation (right inset). The scale bars in both TEM images represent 5 nm.

HRTEM image (Figure 6h), which shows well-developed lattice fringes. One additional SAED was taken for the sample exposed at a higher dose of electron beam (Figure 6f). However, no significant difference was discerned in the patterns of crystalline PbTe NCs. To compare the intensity change, all of the SAEDs were processed with the Crisp ELD program with a background subtraction using Materials Studio Reflex module.24 The results are shown in Figure 6g, indicating that the crystallization of the rock salt phase developed gradually as a function of the electron beam dose. Based on the above results, we propose a model for the transformation mechanisms as shown in Figure 7, illustrating relative changes in the Gibbs free energies of observed phases (i.e., the initial rock salt phase, LDA, and HDA). The 6 nm PbTe NCs underwent a pressure-induced crystalline-toamorphous phase transformation, rather than the conventional rock salt-to-orthorhombic transformation observed in bulk and in larger PbTe NCs. This disparity is likely due to the formation of high-index facets at the small NC surfaces and the absence of structural defects in the “perfect” single NCs. These defects, which are energetically unfavorable, would serve as nucleation sites for the high-pressure orthorhombic phase, and their absence results in the increase in the energy barrier, thereby suppressing the formation of the high-pressure crystalline phase. In other words, to promote the conventional rock saltto-orthorhombic transition, a greater pressure (equivalent to more energy) must be applied on NCs to overcome the increased energy barrier. However, before the formation of orthorhombic phase, the rock salt structure can’t sustain higher pressures and accordingly collapses into disordered amorphous

phase in PbTe NPs. Interestingly, we also noticed that, with controlled beam irradiation onto these recovered amorphous structures, renucleation and growth of the rock salt phase could occur in PbTe NPs. To further understand how the rock salt phase nucleates and grows from the HDA phase, the quenched amorphous sample was loaded into a TEM chamber for a series of in situ TEM observations upon a gradual increase of the irradiation dose of electron beam. The area in Figure 3c that was originally amorphous was selected for the controlled irradiation experiment. It is known that part of the energy dissipated from interactions between NPs and electron beam (e.g., inelastic scattering of the incident electrons) was converted into heat.23 Upon irradiation of the area using a very weak electron beam (we suppose that the electron dose of this pattern is close to 0 e/nm2), a SAED pattern was immediately taken after the specimen was moved to the recording area (Figure 6a). The pattern showed only a broad diffuse scattering ring, indicating an amorphous structure. Upon irradiation of an electron beam with increased dose, a series of SAEDs were sequentially taken (Figure 6b−f). At the 5.9 × 106 e/nm2 level (Figure 6b), PbTe NPs started to crystallize but still showed weak diffraction rings. These electron diffraction rings became gradually stronger (Figure 6c−f). When a dose level of 8.8 × 106 e/nm2 was achieved, the crystallization was complete. Note that the sample was very sensitive to electron irradiation. To monitor the process of crystal nucleation and growth, HRTEM images could not be taken as they required high dose levels of electron beams. Only in the final crystallization stage of the sample exposed at 27.2 × 106 e/nm2, we were able to obtain an F

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laboratory-directed research and development (LDRD) program of Los Alamos National Laboratory, which is operated by Los Alamos National Security LLC under DOE Contract No. DE-AC52-06NA25396. Z.L. thanks Dr. Masahiro Kawasaki from JEOL USA Inc., for assistance in the electron dose calculations. CHESS is supported by the NSF award DMR0936384.

configuration. Certainly, at local scales, the structure could still retain some features of the rock salt structure, such as atomic coordination numbers, thus leading to certain memory of the path to the original rock salt structure. Therefore, below a critical pressure value, if the pressure was released, the memory effect would revert the amorphous phase back to the rock salt structure. When the loaded pressure exceeded 15 GPa, the LDA phase transformed to a HDA phase. It is obvious that the HDA phase completely loses the structure features of the rock salt phase. Thermodynamically, this metastable phase may behave like diamond that nucleates from compression of graphite and has a large energy barrier that prevents from the return of rock salt structure. Nevertheless, the energy barrier between the recovered amorphous and the rock salt structure of PbTe NCs could be overcome by additional thermal energy through electron irradiation induced heating, thus resulting in recrystallization of the rock salt phase from the HDA phase. Finally, the relative crystallization region in a NP domain of PbTe recovered from 15.6 GPa (Figure 2c or the left inset of Figure 7) seems different from that from 19.7 GPa (Figure 3c or the right inset of Figure 7), indicating that the pressureinduced crystallization of the PbTe rock salt structure recovered from the LDA phase and the heat-driven crystallization of the PbTe rock salt structure generated from the HDA phase may take different mechanisms of development. Conclusions. High-pressure X-ray diffraction studies of 6 nm PbTe NCs were carried out toward understanding the microscopic mechanisms of the reversible transformations and structural switches between crystalline and amorphous phases. Upon a loading pressure of above 10 GPa, instead of undertaking the conventional rock salt-to-orthorhombic phase transformation, PbTe NCs transform to a LDA phase. Continuous compression to a pressure value of over 15 GPa drives this LDA phase to transform to a HDA phase. Upon the release of pressure, the LDA phase switches back to the rock salt structure, but the HDA phase remains stable to ambient conditions. Crystallization of the rock salt phase from the HDA phase was also monitored using an in situ TEM SAED technique. The structural switching mechanisms from the LDA phase and from the HAD phase to the PbTe rock salt phase seems different. Such characteristics of PbTe NCs render them an ideal model system for investigation of the solid−solid phase transformations between different crystalline and amorphous phases. Systematic studies of this system not only provide insights into how pressure effectively controls multiple structure switches between crystalline and amorphous phases but also help us understand at which sites the crystallization and growth of different metastable phases are initiated. These results have important implications for design and fabrication of advanced materials with metastable structures and new properties for a variety of technological applications.

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REFERENCES

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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 partially supported by S3IP at Binghamton University and DOE STTR program. Z.Q. acknowledges the J. Robert Oppenheimer (JRO) fellowship supported by the G

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