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Sculpting nanoscale functional channels in complex oxides using energetic ions and electrons Ritesh Sachan, Eva Zarkadoula, Xin Ou, Christina Trautmann, Yanwen Zhang, Matthew F. Chisholm, and William J. Weber ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02326 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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ACS Applied Materials & Interfaces

Sculpting nanoscale functional channels in complex oxides using energetic ions and electrons Ritesh Sachan1*†, Eva Zarkadoula1, Xin Ou3, Christina Trautmann4,5, Yanwen Zhang1,2, Matthew F. Chisholm1, and William J. Weber2,1ǂ 1

Material Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA 2 Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN, 37996, USA 3 State Key Laboratory of Functional Material for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, 200050, China 4 GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstrasse, 1, D-64291, Darmstadt, Germany 5 Materialwissenschaft, Technische Universität Darmstadt, Darmstadt, 64287, Germany *

email: [email protected] Present address: Materials Science Division, Army Research Office, Research Triangle Park, Durham, North Carolina 27695, USA ǂ email: [email protected] †

Keywords: Ion irradiation, electron irradiation, in-situ scanning transmission electron microscopy, phase transformation, molecular dynamics

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ABSTRACT The formation of metastable phases has attracted significant attention due to their unique properties and potential functionalities. In the present study, we demonstrate the phase conversion of energetic ion induced amorphous nanochannels/tracks into metastable defectfluorite in A2B2O7 structured complex oxides by electron irradiation. Through in situ electron irradiation experiments in a scanning transmission electron microscope, we observe electroninduced epitaxial crystallization of the amorphous nanochannels in Yb2Ti2O7 into the defectfluorite. This energetic electron induced phase transformation is attributed to a coupled effect of ionization-induced electronic excitations and local heating, along with subthreshold elastic energy transfers. We also show the role of ionic radii of A-site cations (A = Yb, Gd and Sm) and B-site cations (Ti and Zr) on facilitating electron beam induced crystallization of the amorphous phase to the defect fluorite structure. The formation of the defect-fluorite structure is eased by the decrease in the difference between ionic radii of A- and B-site cations in the lattice. Molecular dynamics simulations of thermal annealing of the amorphous phase nanochannels in A2B2O7 draw parallels to the electron irradiation induced crystallization and confirm the role of ionic radii in lowering the barrier for crystallization. These results suggest that employing guided electron irradiation with atomic precision is a useful technique for selected area phase formation in nanoscale printed devices.


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INTRODUCTION The atomic rearrangement in complex oxides by non-equilibrium methods has enthralled the scientific community. These non-equilibrium processes enable the formation of various metastable atomically disordered phases that, due to the lack of distinct atomic sites for cations, exhibit unique physical properties and potential novel functionalities. For instance, atomically disordered defect-fluorite phase in A2B2O7 structured oxides are reported to have high radiation tolerance,1,2 as well as the fast ionic conductivity3–6. The spinel phased ferrites (AFe2O4) exhibit unusual magnetic properties through cation antiferromagnetic coupling.7 The heavily doped defective structures show high-temperature superconductivity characteristics due to lattice atom substitution and the formation of Cooper pairs.8,9 Among various routes to achieve such phase transformation, energetic electron-irradiation (e-irradiation) is seen as an effective technique, especially for nanoscale devices with locally preferential phase transformations. Guided eirradiation, such as in scanning transmission electron microscope (STEM), enables specific area phase transformations and patterning at a nanoscale level.10–12 It is well-known that, in addition to atomic resolution imaging, electron-atom interactions in STEM give rise to nuclear and electronic energy-loss that has been used in atomic-level sculpting and phase transitions in SrTiO3.10,13–15 Various studies have reported phase changes or more specifically crystallization of amorphous phase under e-beam irradiation in STEM, yet there are few application-driven studies that project future implementation technology.16,17 From the viewpoint of applications, electron beam (e-beam) induced patterning by e-beam lithography in a scanning electron microscope is widely used to fabricate structures at the nanoscale.18 Irradiation using the scanning e-beam in a STEM is a useful extrapolation of the 3 ACS Paragon Plus Environment


same technique, which in principle achieves subnanometer level nanofabrication and advances interface engineering science by creating epitaxial complex nanostructures.16,19,20 The present work follows the same direction and focusses on the spatially controlled phase transformation of amorphous phase nanochannels embedded in pyrochlore structured A2B2O7 matrix. The choice of A2B2O7 structured complex oxides is based on its unique atomic structure providing potential applications, such as fast ionic conductors, topological insulators, matrix for immobilization of nuclear waste or thermal barrier coatings.1,4–6,21,22 These materials show remarkable capability of disordering by atomic rearrangement under high energy ion irradiation and preferentially transform to disordered crystalline defect-fluorite phase or amorphous phase under specific conditions .23–25 In the present study, we take advantage of highly energetic ion (> 50 MeV) to create wellseparated amorphous nanochannels/tracks along the irradiation direction in A2B2O7 (A = Yb, Gd, Sm and B = Ti, Zr) materials. Since A2B2O7 structured titanates and zirconates exhibit a tendency to form a disordered metastable defect-fluorite phase under specific extreme conditions, we envision demonstrating a selective nanopatterning of defect-fluorite nanochannels for possible fast oxygen transport applications. We demonstrate the effect of the kinetic energy of incident electrons on the phase transformation behavior of the amorphous nanochannels. This study provides insights on the role of ionic radii of A3+-site and B4+-site cations on e-beam induced crystallization of amorphous nanochannels. We subsequently use molecular dynamics (MD) simulations to further understand the crystallization process. With this in situ experimentation, we demonstrate focused e-beam irradiation in a STEM as a tool to nanopattern metastable phases with atomic precision and attempt to forge a connection between material design and envisioned properties for nanoscale devices.


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RESULTS AND DISCUSSION To demonstrate the nanopatterning in Yb2Ti2O7, a series of the high-angle annular dark field (HAADF) images are acquired simultaneously with the e-irradiation in an in situ electron irradiation experiment as a function of scanning rates and accelerating voltages. The progression of crystallization of the disordered defect-fluorite phase from the amorphous phase under 200 keV e-irradiation is shown in Figure 1 for a nanochannel in Yb2Ti2O7. The nanochannels were created by 55 MeV I ion irradiation at a fluence of 5x1010 ion/cm2 to ensure the well-separated nanochannels. Figure 1b and 1c are the HAADF images of the originally amorphous nanochannel (Figure 1a), taken after 25 and 43 seconds of total e-beam exposure, respectively, while keeping the 5 µs/pixel e-beam scanning rate for 512x512 pixels. The fast Fourier transforms (FFTs), which correspond to regions that initially existed as ordered pyrochlore (Figure 1d, g, j), disordered defect-fluorite plus amorphous (Figure 1e, h, k) and amorphous (Figure 1f, i, l), further display the transition of the amorphous phase into the defect-fluorite structure under e-irradiation. It is evident from the images that the amorphous phase in the nanochannel transforms to the defect-fluorite phase epitaxially in the radially inward direction. This transformation begins from a pre-existing epitaxial (2-3 monolayer thick) concentric defectfluorite shell, acting as a template, around the amorphous phase. Unlike the phenomenon of ionbeam-induced epitaxial crystallization (IBIEC),26–28 which requires elevated temperatures and sufficient elastic energy transfers to cause significant displacement of atoms, this electron-beam induced crystallization processes occurs at room temperature and at energies insufficient to displace atoms. The progression of the defect-fluorite phase transformation has been captured systematically through a series of HAADF images and is provided as a movie S1 in the Supporting Information. During electron-irradiation, energetic electrons interact with the solid



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and contribute to the phase transformation via inelastic (electronic energy loss) and elastic (nuclear energy loss) and energy transfer. While the maximum elastic energy transfer required for atomic displacements of Yb, Ti and O is 3.0, 10.9 and 32.7 eV, respectively, the cross-section of such scattering events is generally very small as compared to the ionization cross-section associated with inelastic energy transfer.15,29,30 The ionization processes locally produce a high concentration of hot electrons that thermalize, transferring their energy to atoms via electronphonon coupling and forming some localized electronic excitations. The energy transferred to atoms causes ultrafast local heating, or thermal spike, while the local electronic excitations affect bonding and atomic mobility. Both of these processes may contribute to enhanced local atomic hopping and restructuring that lead to both defect recovery and crystallization. This crystallization

under

e-beam

irradiation

is

found

to

be

more

favorable

at

the

amorphous/crystalline (a/c) interface as the critical activation energy for epitaxial and heterogeneous crystallization at the interface is lower than the homogeneous nucleation of defect-fluorite phase from the amorphous phase.31 The effects of e-irradiation on nanochannels in Yb2Ti2O7 were also investigated as a function of scanning rate. Figure 2a shows the decrease in the amorphous fraction during formation of the defect-fluorite phase for an ion track of 4.5 nm diameter as a function of e-beam exposure time for scanning rates of 5 and 32 µs/pixel. The results indicate that the rate of defect-fluorite phase conversion is independent of the electron scanning rate. In all the experiments, the scanned region contained 512x512 pixels, and the ebeam current was ~28 pA. The effect of the incident electron energy on defect-fluorite formation has also been studied by irradiating nanochannels of different diameters with 100 and 200 keV ebeams, as shown in Figure 2b. Interestingly, the crystallization rate at 200 keV is observed to be about a factor of 2 faster than that at 100 keV. While this observation is counter-intuitive due to


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the slightly higher electronic energy-loss at 100 keV (0.057 eV/nm),32as compared to 200 keV (0.040 eV/nm), it can be attributed to the a combination of localized excitations that break bonds, reduce ionic radii or reduce barriers to atomic mobility and to subthreshold nuclear energy transfers (a few eV), which is slightly (~10%) higher at 200 keV. The crystallization of amorphous channels to the defect-fluorite phase under e-beam has also been investigated as a function of ionic radii of the constituent cations. Figure 3 shows the effect of A3+ ionic radii on the crystallization of the amorphous phase in A2Ti2O7 by substituting the A site cation with Yb, Gd and Sm (ionic radii Yb3+>Gd3+>Sm3+). The HAADF images of same ion track were acquired after 43s of electron irradiation at 200 keV. It is clearly evident from the figure that only Yb2Ti2O7 undergoes a phase transformation to the defect-fluorite phase (Figure 3a, d), while the amorphous tracks in Gd2Ti2O7 (Figure 3b, e) and Sm2Ti2O7 (Figure 3c, f) remain unchanged under electron irradiation. As the difference between the ionic radii of A-site cations (rYb3+= 0.985 Ao, rGd3+ = 1.053 Ao and rSm3+ = 1.08 Ao) and B-site cations (rTi4+ = 0.603 Ao) decreases from Sm to Yb, the feasibility of Yb and Ti atoms randomly occupying A and B cation sites increases, lowering the energy barrier for formation of the defect-fluorite phase.1,33–35 Thus, under electron-irradiation, A2Ti2O7 amorphous structures with large A-site ionic radii (i.e. Gd and Sm) relative to Ti do not easily undergo crystallization to the defect-fluorite phase, while Yb2Ti2O7 with a smaller Yb ionic radius does crystallize. Similarly, the role of B-site ionic radii on e-beam induced crystallization has been studied. In this study, the amorphous nanochannels were created in Gd2Ti2O7 and Gd2TiZrO7 by 2.2 GeV Au and 2.3 GeV Pb ions, respectively, as shown in Figure 4a and b. Since it is well-known that the Zr atom at the B-site tends to enhance the formation of the defect-fluorite phase under high energy ion irradiation.24 The present study is conducted on the Gd2TiZrO7, where half of the Ti atoms 7 ACS Paragon Plus Environment


are replaced with Zr, and the 2.3 GeV Pb ions create an amorphous ion track surrounded by a thicker concentric shell of the defect-fluorite phase. Figure 4 shows the direct comparison of the crystallization process after electron irradiation for 60 s, where the amorphous phase completely converts into defect-fluorite phase in Gd2TiZrO7, while Gd2Ti2O7 remains unchanged. This difference in electron-beam response is due to the decreased difference between A-site ionic radius (rGd3+ = 1.053 Ao) and B-site ionic radii (rTi4+ = 0.603 Ao, rZr4+ = 0.718 Ao), which decreases the energy barrier for formation of the defect-fluorite phase formation in Gd2TiZrO7. It is worth noting that the crystallization rate due to B-site substitution is relatively slower and required larger incident electron density than for the A-site substitution. To further understand the phase transformation of the amorphous phase into the defect-fluorite phase under the electron irradiation, atomistic MD simulations were performed, as illustrated in Figure 5. An amorphous nanochannel is first created in the pyrochlore structured Yb2Ti2O7 by introducing a thermal spike profile that is predicted from the electronic energy loss for 55 MeV I ions. This thermal spike leads to local to local melting, quenching and restructuring over tens to hundreds of picoseconds. While the phase transformation induced by electron irradiation is due to the local energy transfer from the incident electrons to electrons and atomic nuclei, the MD simulations only considered thermal annealing over short times at extreme temperatures. The original amorphous nanochannel is shown in Figure 5a. Images of the thermally annealed nanochannel structures after 85 ps at 3000 and 5000 K are shown in Figures 5b and c, respectively, exhibiting partially and fully crystallized stages of the phase transformed nanochannel. Assuming first order kinetics, these results suggest that thermal epitaxial crystallization of the amorphous nanochannel would occur at about 700 K over several hours, and not on the time scale observed under in situ electron beam irradiation.


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Similar MD simulations were performed at to create and anneal amorphous nanochannels in Gd2Ti2O7 and Gd2TiZrO7, as shown in Figure 6. The created amorphous nanochannels (Figure 6a and 6b) were subsequently annealed at 4000 K for 100 ps. While the nanochannel remains amorphous in Gd2Ti2O7 (Figure 6c), the amorphous nanochannel in Gd2TiZrO7 transforms to the defect-fluorite phase. Overall, the consistency between the MD simulations and experimental results suggests that the amorphous phase in Yb2Ti2O7 readily transforms to the defect-fluorite phase due to closer ionic radii of Yb and Ti atoms as compared to the Gd and Ti atoms. Similarly, by replacing half the Ti atoms with larger Zr atoms, the phase transformation process is facilitated in Gd2TiZrO7 due to lowering of the energy barrier for defect-fluorite phase formation because of the increase in average ionic radii of the B4+ atoms. To further discuss the electron irradiation and its analogical thermal annealing effects, we draw a conclusion that the thermalization of hot electrons and subsequent localized heating dominate the phase transformation process.36 We emphasize that the electron-irradiation induced ionization, which is primarily responsible for the crystallization at a/c interface, is significantly different from the solid-phase epitaxial equilibrium crystallization by thermal annealing. The crystallization rate via these processes at the a/c interface can be described as: ܸ(ܶ) = ܸ௜௥௥ exp ቀ−

ா೔ೝೝ ௞்

ቁ + ܸ௧௛ exp ቀ−

ா೟೓ ௞்

ቁ

(1)

where, k is the Boltzmann constant, Eirr and Eth are activation energies and Virr and Vth are the prefactors for irradiation enhanced and thermal epitaxial processes, respectively. While thermal annealing requires large increase in the temperature (>700 K), as suggested by the MD simulations, the overall temperature rise via electron irradiation could be an order of magnitude lower, between few K to 50 K. This can be calculated using the formula:37



ூ

௱ா

ଶ௕

߂ܶ = గ఑௘ ቀ ௗ ቁ ݈݊ ௗ

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(2)

బ

where, I is the beam current, κ is the thermal conductivity, b is the distance of radial conduction, d0 is the diameter of incident beam, e is the electron charge, and ∆E is the total average energy loss per electron (~0.04 eV/nm for 200 keV electrons) in a sample of thickness d.38 Assuming a thermal conductivity of the amorphous phase of ~10-3 Wm-1K-1,36,37,39 specimen thickness of 50 nm, probe current of 28 nA, probe size of 0.1 nm, it is calculated that the temperature rise in the amorphous region is ~50 K which is an order of magnitude lower than that required for thermal annealing. Thus, the contribution from thermal annealing can be conveniently neglected in Equation 1 for the crystallization at a/c interface. The interaction of the electron beam with material gives rise to phenomena on multiple energy scales, ranging from inelastic energy transfer to electrons and high-energy electron-atom elastic collisions at primary electron beam energies to ultimately thermalization of the hot electrons and generation of local heating. The heat generation occurs due to inelastic collisions of incident electrons with the electrons in the material. The amorphous phase exhibits larger electronphonon coupling and orders of magnitude lower thermal conductivity that assist in generating a considerably larger ionization-induced thermal spike in the disordered/amorphous phase, as compared to its ordered crystalline variant;29,40–43 this could increase the local heating above the 50 K value estimated above. An additional contribution from subthreshold energy transfers due to minor nuclear scattering events can also be considered based on the results obtained from the irradiations at 100 and 200 keV, which could assist the ionization induced crystallization process.14 Similar observations were reported earlier in the studies on atomic-level sculpting of crystalline phase from amorphous SrTiO3

and LaAlO3 systems by guided electron beam

irradiation.10,31 An additional variable is the local excitations due to trapped electrons and holes, 10 ACS Paragon Plus Environment

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that affect bonding and atomic mobility. Recent ab initio MD calculations have shown that the crystalline to amorphous transition temperature in A2Ti2O7 pyrochlores is significantly reduced with increasing concentration of electronic excitation,44–46 and one might expect that the amorphous to crystalline transition temperature is likewise reduced. CONCLUSIONS With this study, we present e-irradiation in a STEM as a tool to do selective phase transformation for nanopatterning with atomic precision, as demonstrated in Figure 7. The figure shows a schematic of e-beam irradiation of nanochannels and a HAADF image containing two nanochannels, one being completely converted to defect-fluorite phase and other in the amorphous state. Overall, the present study clearly shows the effect of electron irradiation on the crystallization of amorphous phase A2B2O7 structured complex oxides through in situ electron microscopy. In this study, we also showed that amorphous nanochannels in both Yb2Ti2O7 and Gd2TiZrO7 readily transformed to the defect-fluorite crystalline phase under e-beam irradiation, while amorphous nanochannels in Gd2Ti2O7 and Sm2Ti2O7 remained unaffected by e-irradiation. It is attributed to the difference between ionic radii of A-site cations (Sm, Gd and Yb) and B-site cation (Ti) that decreases from Sm to Yb, facilitating the atomic rearrangement to form defectfluorite phase. Similar results were observed in Gd2TiZrO7, while partially replacing Ti with larger Zr atoms. It is concluded that decreasing the difference in ionic radii of cations lowers the energy barrier for formation of the defect-fluorite phase. This study also proposes the coupling of energetic ion irradiation and electron irradiation for the formation of disordered defect-fluorite nanochannels in an ordered crystalline matrix. Such disordered channels can be nanopatterned with atomic precision as well as functionalized for fast ion-conduction in energy conversion and storage applications. 11 ACS Paragon Plus Environment


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EXPERIMENTAL SECTION Ion

irradiation

All

the

samples

were

prepared

by

the

sol-gel

method,

followed by a sintering process at 1875 K for 50 hrs in air. Amorphous nanochannels were formed in various samples by high energy ion irradiation. The samples of Yb2Ti2O7, Gd2Ti2O7 and Sm2Ti2O7 (used in Figure 1-3) were irradiated by 55 MeV I at the Helmholtz-Zentrum Dresden-Rossendorf ion accelerator facility in Dresden, Germany. The other set of Gd2Ti2O7 and Gd2TiZrO7 samples (used in Figure 4) were irradiated by 2.2 GeV Au and 2.3 GeV Pb respectively at the UNILAC linear accelerator facility in the GSI Helmoltz Center for Heavy Ion Research in Darmstadt, Germany.

For creating isolated nanochannels, the fluence in each

irradiation was kept at 5x1010 ions/cm2. STEM Characterization HAADF imaging was performed on various samples in a 5th order aberration corrected scanning transmission electron microscopes, operating at 200 keV (Nion UltraSTEM200) and 100 keV (UltraSTEM100), respectively. HAADF imaging was done using an inner semi-angle of 65 mrad. The electron probe current used in the experiment was 28±2 pA. The samples for STEM analysis were prepared by conventional mechanical thinning, precision polishing and ion-milling in the liquid N2 environment as described in detail elsewhere.47 Molecular Dynamics simulations For the MD simulations, the DL_POLY MD code was used.48 The systems consist of about 160000 and 176000 atoms, and the MD boxes have sizes about 20 nn × 20 nm × 50 nm. We have used the Minervini and Grimes empirical potentials,33 joined with the short range ZBL potentials.49 The ZBL potentials were used for all atomic pairs. For the irradiation simulations, the energy deposition corresponding to 10.5 keV/nm energy-loss in Yb2Ti2O7 and 18 keV/nm energy-loss in Gd2Ti2O7 and Gd2TiZrO7 were used, described by a Gaussian profile with a 2 nm standard deviation.50 Equilibration of the systems under the NPT (constant pressure and temperature) ensemble at 300 K preceded the irradiation which was 12 ACS Paragon Plus Environment

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performed along the z direction under the NVE (constant-volume, constant-energy) ensemble at 300 K. A Langevin thermostat was connected to the atoms contained in a layer of 10 Å along the x and y boundaries of the MD box to emulate the effect of energy dissipation into the sample. For the thermal annealing, the systems were heated in the NPT ensemble, and cooled down for 85-100 ps. Conflict of Interest The authors declare no competing financial interest. Acknowledgements This research was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. A portion of this work was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. The computational research used resources of the National Energy Research Scientific Computing Center, supported by the Office of Science, US Department of Energy, under Contract No. DEAC02-05CH11231. References (1)

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Figure captions Figure 1 HAADF images of an amorphous nanochannel after (a) 0 s, (b) 25 s and (c) 45 s of ebeam irradiation in Yb2Ti2O7. The FFTs and magnified HAADF images corresponding to the regions initially containing the ordered matrix (d, g and j), interface (e, h and k) and amorphous phase (f, I and l) are shown in the figure. This clearly shows the subsequent transformation of the amorphous phase to the defect-fluorite phase in radially inward direction. The amorphous nanochannels were formed by 55 MeV I ion irradiation. Figure 2 (a) The crystallization of amorphous phase in Yb2Ti2O7 as a function of irradiation time at 5 and 32 µs/pixel scanning rates at 200 keV. (b) The crystallization of amorphous nanochannel as a function of irradiation time for nanochannels of different diameter at 100 and 200 keV. It qualitatively shows that the crystallization occurs relatively faster at 200 keV as compared to 100 keV due to coupled effect of nuclear and electronic energy-loss during electron-atom interaction. Figure 3 HAADF images of amorphous nanochannels before (a, b and c) and after (d, e and f) electron beam irradiation for 45s in (a) Yb2Ti2O7, Gd2Ti2O7 and (c) Sm2Ti2O7. The nanochannels in each material was formed by 55 MeV I ion irradiation. Figure 4 HAADF images of nanochannels in (a, c) Gd2Ti2O7 and (b, d) Gd2TiZrO7, showing the crystallization of amorphous phase (b) into defect-fluorite phase (d) in Gd2TiZrO7 after e-beam irradiation for 60 s. The amorphous phase in Gd2TiZrO7 remains unchanged in similar experiment. The nanochannels in Gd2Ti2O7 was formed by 2.2 GeV Au and in Gd2TiZrO7 by 2.3 GeV Pb irradiation. Figure 5 Molecular dynamics simulations of the annealing process of a nanochannel in Yb2Ti2O7. Figure shows crystallization of amorphous phase (a) to the defect-fluorite phase in a nanochannel ion track while annealing it at (b) 3000 K and (c) 5000 K for 85 ps. Figure 6 MD simulations of amorphous nanochannel in (a) Gd2Ti2O7 (b) Gd2TiZrO7. (c) No crystallization is observed in the amorphous nanochannel in Gd2Ti2O7 after thermal annealing. Simulation time is 100 ps. (d) Recrystallization to the defect-fluorite structure of the track in Gd2TiZrO7 due to annealing at 4000 K. Frame shown is at 100 ps. Figure 7 (a) A schematic of ebeam irradiation on the nanochannels (b) a HAADF image containing two nanochannels in Yb2Ti2O7, where one nanochannel is completely phase transformed to the defect-fluorite and other remained in the amorphous phase.


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Figures

Figure 1 HAADF images of an amorphous nanochannel after (a) 0 s, (b) 25 s and (c) 45 s of ebeam irradiation in Yb2Ti2O7. The FFTs and magnified HAADF images corresponding to the regions initially containing the ordered matrix (d, g and j), interface (e, h and k) and amorphous phase (f, I and l) are shown in the figure. This clearly shows the subsequent transformation of the amorphous phase to the defect-fluorite phase in radially inward direction. The amorphous nanochannels were formed by 55 MeV I ion irradiation.



Figure 2 (a) The crystallization of amorphous phase in Yb2Ti2O7 as a function of irradiation time at 5 and 32 µs/pixel scanning rates at 200 keV. (b) The crystallization of amorphous nanochannel as a function of irradiation time for nanochannels of different diameter at 100 and 200 keV. It qualitatively shows that the crystallization occurs relatively faster at 200 keV as compared to 100 keV due to coupled effect of nuclear and electronic energy-loss during electron-atom interaction.


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Figure 3 HAADF images of amorphous nanochannels before (a, b and c) and after (d, e and f) electron beam irradiation for 45s in (a) Yb2Ti2O7, Gd2Ti2O7 and (c) Sm2Ti2O7. The nanochannels in each material was formed by 55 MeV I ion irradiation.



Figure 4 HAADF images of nanochannels in (a, c) Gd2Ti2O7 and (b, d) Gd2TiZrO7, showing the crystallization of amorphous phase (b) into defect-fluorite phase (d) in Gd2TiZrO7 after ebeam irradiation for 60 s. The amorphous phase in Gd2TiZrO7 remains unchanged in similar experiment. The nanochannels in Gd2Ti2O7 was formed by 2.2 GeV Au and in Gd2TiZrO7 by 2.3 GeV Pb irradiation.


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Figure 5 Molecular dynamics simulations of the annealing process of a nanochannel in Yb2Ti2O7. Figure shows crystallization of amorphous phase (a) to the defect-fluorite phase in a nanochannel ion track while annealing it at (b) 3000 K and (c) 5000 K for 85 ps.



Figure 6 MD simulations of amorphous nanochannel in (a) Gd2Ti2O7 (b) Gd2TiZrO7. (c) No crystallization is observed in the amorphous nanochannel in Gd2Ti2O7 after thermal annealing. Simulation time is 100 ps. (d) Recrystallization to the defect-fluorite structure of the track in Gd2TiZrO7 due to annealing at 4000 K. Frame shown is at 100 ps.


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Figure 7 (a) A schematic of ebeam irradiation on the nanochannels (b) a HAADF image containing two nanochannels in Yb2Ti2O7, where one nanochannel is completely phase transformed to the defect-fluorite and other remained in the amorphous phase.



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