Lamellar Chromium

J. Phys. Chem. C 2008, 112, 17559–17566

17559

Condensation and Crystallization of Amorphous/Lamellar Chromium Sesquioxide Chun-Hung Lin,† Pouyan Shen,† Shuei-Yuan Chen,‡,* and Yuyuan Zheng† †

Institute of Materials Science and Engineering, Department of Materials and Optoelectronic Science, National Sun Yat-sen UniVersity, Kaohsiung, Taiwan, ROC and ‡ Department of Mechanical and Automation Engineering, I-Shou UniVersity, Kaohsiung, Taiwan, ROC ReceiVed: August 2, 2008; ReVised Manuscript ReceiVed: September 10, 2008

Amorphous chromium oxide nanocondensates were fabricated by energetic pulsed laser ablation on a Cr target in vacuum for a fine particle size and a pronounced quenching effect. Analytical electron microscopy indicated the amorphous phase thus quenched has corrugated lamellar layers with bimodal interspacings of 0.259-0.266 and 0.355-0.371 nm, which are close to that of the specific lattice planes of the stable R-type structure, that is, (1j104) and (112j0), having the Cr-filled octahedral sites assembled as 0 and 1 periodic bond chains, respectively. Such amorphous nanocondensates were observed in situ to become more polymerized by forming (011j2)-like layers and then fully crystallized as R-Cr2O3 for further (011j2)-specific coalescence when irradiated by electron beam. The partially crystallized lamellae showed Raman shifts similar to that of the ambient R-Cr2O3, yet at higher frequencies due to a residual compressive stress up to ∼4 GPa. This implies a rather tight 6-coordination of Cr3+ in the corundum-like structure for the rapidly quenched amorphous phase. I. Introduction Crystalline and amorphous chromium oxide film has been commonly produced by DC magnetron reactive sputtering,1 chemical vapor deposition (CVD),2-7 and pulsed laser deposition (PLD)8,9 for potential applications in optoelectronics, such as components for flat panel display devices. (In general, the Cr2O3 thin films on a Si substrate have better crystallinity at a higher deposition temperature based on x-ray diffraction study.9) Structure transformations and their relation to optoelectronic properties of amorphous chromium oxide thin film prepared by PLD9 and atmospheric pressure CVD are of concern for such applications.10 The X-ray diffraction peaks with very low intensity within the broad band of the as-deposited amorphous phase was matched to the strongest peaks of R-Cr2O3 (104) and (110) indexed according to the (hkl) scheme.10 However, the extent of polymerization of the CrO6 polyhedra and possible density of the amorphous Cr2O3 remain to be studied. Pulsed laser ablation (PLA) of metal targets in air has been used to fabricate dense sesquioxides with closely packed structures, that is, γ-Al2O3 of spinel-like structure11 and Crdoped γ- and R-Al2O3,12 as well as Si-doped R-Cr2O313 of corundum isostructure. In general, there is a significant residual compressive stress for such sesquioxide nanocondensates due to rapid heating/quenching and, hence, a pressure effect in the dynamic fabrication process. For example, a residual nonhydrostatic stress up to ∼2 GPa was estimated for the Cr-doped R-Al2O3 particulates according to Cr3+ photoluminescence (PL) R1 and R2 line (2E-4A2 transition) measurements.12 By contrast, Al2O3 nanoparticles of extremely fine size (down to 10 nm) were condensed as an amorphous phase, which became crystallized as γ-Al2O3 upon electron irradiation, implying that 4- and 6-coordination of Al3+ in the spinel-like structure may also occur in the amorphous phase.14 * To whom correspondence should be addressed. Fax: 886-7-6578853. E-mail: [email protected].

Here, we report that amorphous Cr2O3 nanocondensates with a considerable residual stress of the constituting CrO6 octahedra can be condensed by energetic PLA on a Cr target in vacuum, rather than air,13 for a finer particle size and a more pronounced quenching effect. We focused on the corrugated lamellar layers of the rapidly quenched amorphous phase with corundum-like d-spacings and their transformation into faceted R-Cr2O3 domains for further coalescence upon electron dosage. This knowledge is of concern for the phase behavior of analogous sesquioxides via a dynamic PLA process and optoelectronic activation for potential applications. II. Experimental The Cr target with negligible impurities (99.9% pure) was subjected to energetic Nd-YAG-laser (Lotis, 1064 nm in wavelength, beam mode: TEM00) pulse irradiation in a chamber pumped to a moderate vacuum of 3 × 10-5 torr having oxygen gas (99.999% purity) introduced at a flow rate of 20 sccm to oxidize and cool the condensates. A specified laser power density of 1.5 × 108 W/cm2, that is, 1100 mJ/pulse with a pulse time duration of 240 µs at 10 Hz on a focused area of 0.03 mm2, was employed for PLA. The condensates formed during such a PLA process for a total of 20 s were collected by copper grids overlaid with a carbon-coated collodion film and fixed at a specified distance of 5 cm from the target for transmission electron microscopic (TEM) study using the instrument FEI Tecnai G2 F20 operating at 200 kV. Bright field images (BFI), selected area electron diffraction (SAED), and point-count energy dispersive X-ray (EDX) analyses were used to characterize the phase and morphology of the condensed nanoparticles. Lattice imaging coupled with two-dimensional Fourier transform and inverse transform was used to characterize the crystal structure of the individual nanoparticles and their phase transformation upon electron irradiation under 200 kV at a specified beam diameter of 100 nm, a beam current of 5 nA; that is, a beam current density of 2 ×10-4 nA nm-2. The d-spacings measured from

10.1021/jp806896k CCC: $40.75  2008 American Chemical Society Published on Web 10/22/2008

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SAED patterns were used for least-squares refinement of the lattice parameters. The condensates deposited on silica glass by the same PLA process as for the TEM samples yet with a longer time of 10 min for a thickness of several micrometers were studied by optical microscopy under plane polarized light and scanning microscopy (SEM, JSM6700 at 10 kV) under the secondary electron image (SEI) mode for microstructure characterization. The phase identity of the deposit was also characterized by X-ray diffraction (XRD, SIEMENS D1, Cu KR at 40 kV, 50 mA, and 3s for each 0.05° increment from 15° to 80° of 2θ angle) and Raman spectroscopy (JOBIN-YVON TRIAX 320 MicroRaman microprobe coupled with an Olympus microscope objective for the spatial resolution of 0.37 m. The back-scattering (180°) spectra were obtained using a 532 nm line of semiconductor laser for 10 s of accumulation. The present first-principle density functional calculations on the relaxed (1j104), (112j0), and (011j2) atomic planes of R-Cr2O3 were based on the plane-wave pseudopotential and PerdewBurke-Ernzerhof generalized gradient approximation.15 Electroncore interactions have been described by ultrasoft pseudopotentials.16 (The valence states are 2s and 2p shells for O with 6 valence electrons, and 3s, 3p, 3d, and 4s states for Cr with 14 valence electrons.) The smooth part of the wave functions was expanded in plane wave with a kinetic-energy cutoff, 300 eV. A 5 × 5 × 2 mesh of k points was used to sample the Brillouin zone for R-Cr2O3. Structural optimizations were carried out by the Broyden-Flecher-Goldfarb-Shanno minimization algorithm. The convergence of displacement was set to less than 0.002 Å, and the energy difference was set to less than 0.2 × 10-5 eV/atom. All calculations were performed by the CASTEP module of the software material studio.17 The refined hexagonal (space group R3jc) cell parameters are a ) 0.4923 nm, c ) 1.3495 nm given the internal fractional coordinate u ) 0.1523 for the position of Cr ions at (0, 0, ( u), (0, 0, 1/2 ( u), (2/3, 1/3, 1/3 ( u), (2/3, 1/3, 5/6 ( u), (1/3, 2/3, 2/3 ( u) and (1/3, 2/3, 7/6 ( u). Such cell parameters are considerably different from the ambient values a ) 0.4959 nm, c ) 1.3594 nm (JCPDS 38-1479). III. Results TEM Observations of Individual Condensates. The nanocondensates collected by a carbon-coated collodion film during PLA in vacuum suffered considerable phase and microstructural changes during TEM observations. The as-condensed sample consists of a predominant amorphous phase and minor R-Cr2O3, as indicated by the BFI (Figure 1a) and SAED pattern (Figure 1b) taken within 0.5 min of electron dosage. The diffraction intensity profile and Gauss fits of Figure 1a (see Figure 10) showed a broad diffraction of the predominant amorphous lamellar phase, which is nearly superimposed with (101j4) and (112j0); that is, the most and second-most intense peaks, respectively, of the minor R-Cr2O3, similar to previous XRD results of the chromium oxide film produced by CVD.10 There is no low angle scattering below 2 cm-1 for the amorphous phase. The amorphous Cr2O3 condensates survived further electron irradiation for up to 2 min (Figure 1b) and 6 min (Figure 1c), although another diffraction close to the (011j2) of R-Cr2O3 emerged. Point-count EDX analysis (Figure 1d) indicated the condensates have nearly stoichiometric Cr2O3 composition with negligible impurities. Lattice image coupled with 2-D forward and inverse Fourier transform further revealed the corrugated lamellar layers of the amorphous condensates, which turned into R-Cr2O3 when

Figure 1. TEM BFI (left) and corresponding SAED pattern (right) of amorphous lamellar Cr2O3 upon electron irradiation for (a) 0.5, (b) 2, and (c) 6 min, showing progressive crystallization of R-Cr2O3 as indexed. (d) Point-count EDX spectrum of the condensate showing Cr and O counts with negligible impurities, regardless of electron irradiation. The Cu counts are from the sample-supporting copper grid. The sample was produced by laser ablation on the Cr target in a vacuum under an oxygen flow rate of 20 sccm and collected by a carbon-coated collodion film.

subjected to a short dwelling time (1.5 min) of electron irradiation (Figure 2). The lamellar edge showed a bimodal interspacing of 0.259-0.266 nm (Figure 2b and c) and 0.355-0.371 nm (Figure 2d), which are close to the (101j4) and (011j2) d-spacings of R-Cr2O3, respectively, as indicated by 2-D forward and inverse Fourier transform of the specified areas in Figure 2a. The latter interspacing appears to be predominant according to additional observations in other areas of the specimen (Figure 11). Careful scrutiny of the TEM sample with double tilting indicated that the lamellar fringes have nothing to do with the R-Cr2O3 lattice imaged under two beam conditions. Both interspacings of the lamella can be rationalized

Chromium Sesquioxide

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Figure 2. TEM (a) lattice image of the amorphous lamellar Cr2O3 nanocondensates with modulated layer, which survived a short time duration (1.5 min) of electron irradiation; (b-d) 2-D forward (left) and inverse (right) Fourier transform of the square regions in (a) showing lamellar interspacing in the range of 0.259-0.266 nm (b, c) and 0.355-0.371 nm (d), close to the (101j4) and (011j2) d-spacings of R-Cr2O3.

Figure 3. TEM (a) lattice image of the Cr2O3 nanocondensates which were originally amorphous lamellae but became R-Cr2O3 with better developed (011j2), (1j012) facets than (112j0), for impingement upon electron irradiation for 5 min; (b, c) 2-D forward and inverse Fourier transform, respectively of the square regions in (a) showing corrugated {011j2} interface with misfit dislocations denoted by T, which is a 70° tilt boundary about the [22j01] zone axis.

Figure 4. TEM (a) lattice image of the R-Cr2O3 nanoparticles in Figure 3 subjected to further electron irradiation for up to 7 min to form an additional twist boundary (22j05) with Moire´ fringes in the top view along the [22j01] zone axis; (b, c) 2-D forward and inverse Fourier transform, respectively, of the square regions in (a) showing Moire´ fringes due to overlapping of the impinged nanoparticles, still with 73° misorientation and misfit dislocations denoted by T, despite the rotation of the particles and migration of the interface (see text).

by the specific polymerization scheme of the CrO6 octahedra, as discussed later. The R-Cr2O3 domains with well-developed (112j0), (1j012), and (011j2) lattice fringes and facets in the [22j01] zone axis

were emerged and impinged upon electron irradiation for 5 min (Figure 3a). The 2-D Fourier transform (Figure 3b) and inverse transform (Figure 3c) showed corrugated {011j2} interface with misfit dislocations, which is, in fact, a 70° tilt boundary about


Figure 5. TEM (a) lattice image of the R-Cr2O3 nanoparticles in Figures 3 and 4 after further electron irradiation for up to 10 min; (b) 2-D Fourier transform in the [22j01] zone axis; and (c) inverse Fourier transform of the square region in (a) showing the nanoparticle at lower right hand corner has further developed (1j104), (112j0), and {011j2} faces during its unification with the upper left hand corner one. Note Moire´ fringes due to overlapping of an approaching particle on the lower left corner.

the [22j01] zone axis. Further electron irradiation for up to 7 min caused slight orientation change of the impinged bicrystals (Figure 4a) having 73° misorientation and corrugated {011j2} interface with misfit dislocations as indicated by 2-D Fourier transform (Figure 4b) and inverse transform (Figure 4c), respectively, in the [22j01] zone axis. There was an additional twist boundary (22j05) that emerged to form Moire´ fringes at this moment. Finally, the impinged R-Cr2O3 bicrystals were unified to have further developed {011j2}, (112j0), and (1j104) faces within 10 min of electron irradiation, as indicated by the lattice image (Figure 5a) coupled with 2-D Fourier transform (Figure 5b) and inverse transform (Figure 5c), respectively, in the [22j01] zone axis. The amorphous Cr2O3 with wormlike atom clusters appeared to develop into a planar structure near the (011j2) face in such a case. Optical and Structural Observations of the Deposit on Silica Glass. XRD indicated that the deposit on silica glass produced by the same laser ablation as that for the TEM sample contains amorphous chromium oxide and metallic R-Cr of bcc structure (Figure 6a). The low-angle scattering in the 2θ range of 15-25 full width at half height is due to the glass substrate. The deposit on the glass is brownish to the naked eye. Optical microscopic observation under an open polarizer further showed

Lin et al.

Figure 6. (a) XRD trace (Cu KR) of the deposit on silica glass by laser ablation on the Cr target in vacuum under an oxygen flow rate of 20 sccm showing preferred (110) diffraction of R-Cr with bcc structure and broad diffraction from amorphous Cr2O3, yet with negligible diffractions from R-Cr2O3. (b, c) Corresponding optical micrographs under open polarizer and crossed polarizer, respectively, with accessory λ retardation plate showing brownish and partially devitrified Cr2O3 matrix and the opaque R-Cr particulates partially oxidized as birefringent R-Cr2O3.

that the fine matrix of the deposit consisting of the partially devitrified amorphous phase is brownish, in contrast to opaque R-Cr particulates of much larger size (Figure 6b). The spherical R-Cr particulates were partially oxidized at the surface to show convergent birefringence under a crossed polarizer (Figure 6c), analogous to the case of partially oxidized Al particulates.11 SEM observations of the deposit further showed uniformly distributed chromium oxide nanocondensates and much largersized R-Cr particulates, presumably due to rapid solidification of melt (Figure 7a). Abundant microcracks of the film due to shock loading of the particulates were also observed (Figure 7b and c). The fine matrix of the deposit consisting of partially devitrified amorphous phase was further found by micro-Raman spectroscopy to have active Eg and A1g modes similar to that of ambient R-Cr2O3, though at different frequencies. The R-Cr2O3 powder (Cerac, 99.8% pure, 5 µm in size), dry-pressed as an ambient standard, has five characteristic Raman shifts at 296, 328, 484, 533, and 590 cm-1 under ambient conditions (Figure 8a), the strongest band at 533 cm-1. By contrast, the present lamellar amorphous Cr2O3 nanocondensates have Raman bands at significantly higher frequencies; that is, 339, 492, 546, and 608 cm-1, respectively, except the one at 297 cm-1 (Figure 8b), which deviates within experimental error from the ambient values. The Raman shifts for the powdered R-Cr2O3 scale measured at high pressure and room temperature18 also have



Figure 8. Raman spectrum of Cr2O3 (a) ambient reagent-grade R-Cr2O3 powders with five characteristic Raman modes at about 296, 328, 484, 533, and 590 cm-1 as a reference; (b) as-condensed amorphous lamellae showing corresponding Raman shifts at about 297, 339, 492, 546, 608 cm-1. The same specimen as in Figure 6.

Figure 7. (a) SEM SEI of the laser ablation deposit on silica glass showing R-Cr particulates rapidly solidified from melt and much finersized Cr2O3 condensates accumulated in the matrix. (b, c) Further magnified images showing shock loading of the particulates has caused cracks of the fine deposit. The same specimen as in Figure 6.

higher frequencies (307, 350, 524, 551, and 610 cm-1, respectively, having the most intense band at 551 cm-1) than the ambient values. The most intense band can be assigned as the active vibration of A1g symmetry (one degenerate mode), whereas others possess Eg symmetry (double degenerate mode).18 IV. Discussion Laser Parameter Dependence of Metastable Cr2O3. The present PLA in vacuum caused metastable amorphous Cr2O3 nanocondensates, which are in drastic contrast to the hexagonal disklike R-Cr2O3 condensates prepared by PLA in air for a much larger particle size.13 The formation and retention of such a metastable sesquioxide with a significant residual compressive stress depends on rapid heating and cooling for a pressure effect, as in the case of the formation of dense R-Al2O3 nanocondensates.11 The cooling rate, u, of the individual Cr2O3 condensate

depends on its size, temperature, and heat capacity as well as radiant emissivity of specific phase. The cooling rate for gray body radiation19 can be expressed as u ) (6/DFCp)εσ(T4 - T04), where D is the particulate diameter, F is the material density, Cp is the heat capacity of the material, ε is the radiant emissivity, σ is the Stefan-Boltzmann constant, T is the temperature, and T0 is the ambient temperature. Assuming the physical properties of the molten state are similar to those of the solid state, namely, F ) 5.22 g/cm3, as for R-Cr2O3; Cp ) 28.53 + 2.20 × 10-3 T - 3.74 × 105 T-2 cal/mol K at 1800 K (i.e., 0.892 J g-1 K-1 given a molecular weight 152), typical T ∼ Tm ) 2708 K, ε ) 0.60,20 and σ ) 5.67 × 10-8 W m-2 K-4, the cooling rate u was estimated to be 108 K/s for the Cr2O3 condensates with D ) 10 nm. Such a cooling rate is on the same order of magnitude as that for the effective quench for 10-nm-sized metastable/ nonstable analogue oxides (e.g., amorphous Al2O3 nanocondensates (108 K/s)14) and an order of magnitude lower than that for such sized TiO2 nanocondensates with a high-pressure R-PbO2-type structure (109 K/s)21 in the same dynamic laser ablation process. Pressure-induced amorphization (PIA)22,23 could also happen for the present Cr2O3 condensates, because they have been subjected to a shock front in the laser ablation process. Thermodynamically, PIA can be explained by the interception along the compression or decompression path of the metastable extension of the melting curves, as for the silica analogue.23 Miniature size may also facilitate PIA of the Cr2O3 and


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Figure 9. Schematic drawing of the specific atomic planes of R-Cr2O3: (a) top view of the (22j05) plane (i.e., the [22j01] zone axis with diffraction pattern inset) showing the unrelaxed O (red) and Cr (gray) atom positions of the (1j104), (112j0), and (011j2) planes edge-on along dotted (yellow), dashed (green), and solid (white) traces, respectively. (b, c, d) Top view of (1j104), (112j0), and (011j2) planes showing 0, 1, and 2 periodic bond chains, respectively, in terms of edge-shared CrO6 monomers and Cr2O10 dimers. The lattice parameters of ambient R-Cr2O3 used for the drawing refer to JCPDS file no. 38-1479.

Figure 10. Diffraction intensity profile and Gauss fits of Figure 1a showing a broad diffraction of the predominant amorphous lamellar phase (denoted as L) nearly superimposed with (101j4) and (112j0) (i.e. the most and second intense peaks), respectively, of the minor R-Cr2O3 (JCPDS file no. 38-1479) in the curve fitting to the furthest left. Higher angle diffractions of R-Cr2O3 are also noted by the curve fittings to the right.

previously studied Al2O313 condensates in view of the sizedependent PIA of nanoscale TiO2, as recently proved experimentally by XRD and Raman shift coupled with the diamond anvil cell high-pressure technique.24 Residual Stress of Partially Devitrified Cr2O3 Condensate. Our previous study13 showed that the hexagonally shaped Si4+: R-Cr2O3 condensates prepared by PLA in air have a rather large residual stress of ∼18 ( 3 GPa,13 based on the least-squares refinement of the SAED lattice parameters (a ) 0.4837 ( 0.0012nmandc)1.3343(0.0049nm)andtheBirch-Murnagham

equation of state, assuming the isothermal bulk modulus, B ) 240 GPa,25 and B′ (i.e., the pressure derivative of B ) 3.525 for R-Cr2O3) are valid for the Si4+:R-Cr2O3 condensates. In the present case of partially devitrified Cr2O3 condensates, the d-spacings are too scarce and broad for a reliable estimation of residual stress. The observed Raman shift, however, sheds light on the residual stress of the constituting CrO6 polyhedra in the condensates as follows. The static compression study of R-Cr2O3 scale at room temperature in a diamond anvil cell18 showed a pressuredependent Raman shift. With an increase in pressure, as of concern in the present nanocondensates, the bands except the one close to 300 cm-1 (cf. Figures 4 and 5 of ref 18) were found to shift linearly toward a higher frequency. The change of the most intense Raman peak (i.e., that assigned as A1g symmetry) from 533 cm-1 for ambient R-Cr2O3 to 546 cm-1 for the present partially devitrified Cr2O3 condensates thus implies a residual compressive stress of ∼4 GPa. Such a stress level is significantly lower than that based on the cell parameters for the hexagonally shaped Si4+:R-Cr2O3 condensates fabricated via a similar dynamic laser ablation process.13 Such a difference can be attributed to the absence of size-mismatched dopant (i.e. Si4+), and the presence of a relatively relaxed lamellar amorphous phase in the present condensates. It should be noted, however, that the additional lamellar amorphous phase in partial epitaxial relationship with R-Cr2O3 would cause deviatory mismatch stress to affect the Raman shift. In any case, the stress level is not high enough to cause splitting of two Eg (near 296 and 590 cm-1 in the present case) corresponding to a phase transition to the monoclinic V2O3-type (I2/a) structure that typically occurs at 15-30 GPa under cold compression,26



Figure 11. TEM (a) lattice image of amorphous lamellar Cr2O3 nanocondensates with modulated layer, which survived short electron irradiation for 1.5 min, (b, c) 2-D forward (inset) and inverse Fourier transform of the square regions in part a, showing the lamellar has a predominant interspacing of 0.355-0.371 nm.

although a reddish brown color relevant to Cr-O bond-length change at lower pressures26 appeared to occur (Figure 6b). The residual stress would be affected little by oxygen deficiency via the present ablation process in a vacuum because the Raman bands would otherwise show a frequency decrease rather than increase.27 In this regard, the annealing experiments on nanophase Cr2O3 in air or vacuum to form R-Cr2O3 10-100 nm in size showed that the red shift and broadening of the Raman shift near 530-550 cm-1 is due to oxygen deficiency.27 It is by no means clear whether the residual stress of the CrO6 polyhedra in the amorphous lamellar phase was inherent from the molten state analogous to the case of dense amorphous germination with significant distortion and even coordination number change from 4 to 6 when subjected to high-pressure conditions.28 Comparative Relaxation/Crystallization Path of Sesquioxide Condensates. The amorphous Al2O3, either liquid, supercooled liquid, or glass,29 has an Al ion with a coordination number (CN) of 4 for the nucleation site of γ-Al2O3 having spinel-like structure with cations with a CN of 4 and 6.30 The 4-coordinated Al3+ ions in liquid aluminum oxide thus act as nucleation sites for the crystallization of metastable γ-Al2O3, rather than the stable R-Al2O3.14 By contrast, the amorphous Cr2O3 has a Cr ion with a CN of 631 and, hence, favors the crystallization of R-Cr2O3, also with cations with a CN of 6. It should be noted that the amorphous Al2O3 condensate above a critical size of 20 nm via the PLA process was found to crystallize as a γ-Al2O3 core from the center rather than the surface upon electron irradiation,14 indicating that surface energy is of less importance than elastic strain energy on such a nucleation event. It was suggested that the amorphous yet rigid condensate could have the highest stress at the center upon cooling, analogous to the effect of hydrostatic pressure or carbon onion pressure cell.32 Thus, the condensate center could be the preferred nucleation site. As for a much more rapid devitrification process of the present Cr2O3 nanocondensates, a hydrostatic pressure state was hardly built up for the nucleation of R-Cr2O3 at particle center. Instead, the (1j104)-, (112j0)-, and (011j2)-like motifs in the order of increasing polymerization of the CrO6 monomers and Cr2O10 dimers in the amorphous lamellar Cr2O3 could act as nucleation sites or transformation intermediates of the stable R-Cr2O3, which has fully polymerized CrO6 octahedra; that is, all edges shared for the (0001) plane. Having no apparent charge-

compensating defect clusters, the present condensates were almost stoichiometric and dioctahedral; that is, one-third of the octahedral sites are vacant. According to theoretical simulation and experimental results, the (101j2) plane has vacancy octahedra alternately tilted and, hence, tends to form a slightly corrugated (101j2) surface and cleavage.33 Still, the (101j2) surface is nonpolar and has the lowest energy among the low-index surfaces, such as {0001}, {112j0}, {101j0}, {101j1}, {101j2}, and {112j6} of R-Cr2O3.34 This accounts for the further developed (1j104), (112j0), and {011j2} faces in the order of increasing polymerization for the R-Cr2O3 when derived from monomer or dimmer clusters in amorphous lamellae upon electron irradiation (Figure 5). The (1j104), (112j0), and {011j2} of R-Cr2O3 are, in fact, K, S, and F faces; that is, with 0, 1, and 2 periodic bond chains (PBCs; i.e., strong bonding directions), respectively,35 in terms of edge-shared CrO6 monomers and Cr2O10 dimers (Figure 9). We suggest that the (1j104)-, (112j0)-, and (011j2)-like lamellar motifs solve the structure problem of accommodating edgeshared Cr2O10 dimers with increasing polymerization for the amorphous Cr2O3. It is likely that the bimodal interspacing of the nearly superimposed (1j104)- and (112j0)-like and later (011j2)-like lamellar motifs are due to varied distribution of octahedral vacant sites, varied extent of polymerization within the lamellar layers, or both. It is by no means clear whether the varied lamellar interspacing has anything to do with amorphous phase separation under the combined thermal and electron irradiation effects. The growth of nucleated R-Cr2O3 domains via {11j02}specific coalescence is analogous to the lateral impingement over well-developed {11j02} surfaces of the hexagonal disklike condensates.13 Thermally activated Brownian rotation of the imperfectly coalesced condensates would proceed during a dynamic PLA process or electron irradiation until a parallel epitaxial relationship of beneficial low interfacial energy was reached. In such a unification process, the migration of the stepwise {11j02} structural ledges/steps along the corrugated interface, such as a special Σ tilt boundary, analogous to the assembly of the ZrO2 nanocrystals over a single crystal NaCl substrate36 and twist boundary, such as (22j05) in Figure 4, would occur.

17566 J. Phys. Chem. C, Vol. 112, No. 45, 2008 V. Concluding Remarks The present experimental results indicate that the Cr2O3 nanocondensates synthesized by laser ablation in vacuum consist of dense amorphous lamellar layers with an interspacing close to that of specific lattice planes of the R-type structure having the Cr-filled octahedral sites assembled as PBCs. Such amorphous nanocondensates became more polymerized by forming (011j2)-like layers and then fully crystallized as R-Cr2O3 for further (011j2)-specific coalescence when irradiated by electron beam. Raman spectroscopy indicated a significant residual compressive stress up to ∼4 GPa for the partially crystallized nanocondensates. The Cr2O3 nanocondensates, being in the form of dense amorphous lamellar clusters with a large surface area and corundum-like structure units sensitive to optoelectronic activation for transformation into the stable R-form, may have potential catalytic37 and silicon etch-stop applications, as the case of a deposited film.38 Acknowledgment. We thank Dr. C. N. Huang for the help on laser ablation process, Dr. D. Gan and N.J. Ho for helpful discussions and an anonymous referee for constructive comments. Supported by the Center for Nanoscience and Nanotechnology of NSYSU and National Science Council, Taiwan, under contract NSC 95-2221-E-110-032-MY3. References and Notes (1) Hong, S.; Kim, E.; Kim, D. W.; Sung, T. H.; No, K. J. Non-Cryst. Solids 1997, 221, 245. (2) Suzuki, K.; Tedraw, P. M. Phys. ReV. B 1998, 58, 11597. (3) Suzuki, K.; Tedraw, P. M. Appl. Phys. Lett. 1999, 74, 428. (4) Lip, X. W.; Gupta, A.; McGuire, T. R.; Cuncombe, P. R.; Xiao, G. J. Appl. Phys. 1999a, 85, 5585. (5) Li, X. W.; Gupta, A.; Xiao, G. Appl. Phys. Lett. 1999b, 75, 713. (6) Gupta, A.; Li, X. W.; Guha, S.; Xiao, G. Appl. Phys. Lett. 1999, 75, 2996. (7) Rabe, M.; Pommer, J.; Samm, K.; Ozyilmaz, B.; Ko¨nig, C.; Fraune, M.; Ru¨diger, U.; Gu¨ntherodt, G. S.; Senz Hesse, D. J. Phys.: Condens. Matter 2002, 14, 7. (8) Shima, M.; Tepper, T.; Ross, C. A. J. Appl. Phys. 2002, 91, 7920. (9) Tabbal, M.; Kahwaji, S.; Christidis, T. C.; Nsouli, B.; Zahraman, K. Thin Solid Films 2006, 515, 1976. (10) Ivanova, T.; Gesheva, K.; Cziraki, A.; Szekeres, A.; Vlaikova, E. J. Phys. Conf. Series 2008, 113, 012030.

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