Defects, Lattice Correspondence, and Optical Properties of Spinel-like

Department of Materials and Optoelectronic Science, Institute of Materials Science and Engineering, Center for Nanoscience and Nanotechnology, Nationa...
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J. Phys. Chem. C 2009, 113, 16356–16363

Defects, Lattice Correspondence, and Optical Properties of Spinel-like Cr3O4 Condensates by Pulsed Laser Ablation in Water C. H. Lin,† S. Y. Chen,‡ and P. Shen*,† Department of Materials and Optoelectronic Science, Institute of Materials Science and Engineering, Center for Nanoscience and Nanotechnology, National Sun Yat-sen UniVersity, Kaohsiung, Taiwan, R.O.C., and Department of Mechanical and Automation Engineering, I-Shou UniVersity, Kaohsiung, Taiwan, R.O.C. ReceiVed: May 8, 2009; ReVised Manuscript ReceiVed: July 29, 2009

Analytical electron microscopic observations indicated that the chromium oxide nanocondensates fabricated by pulsed laser ablation in water are predominantly dodecahedral Cr3O4 with varied extent of tetragonal (t) distortion from the spinel (sp) type following the Bain relationship [211j]sp//[011j]t; (011)sp//(100)t. The t-Cr3O4 nanocondensates have {101} twinning due to tetragonal distortion and/or a coalescence event. The additional (3/2)×(200) and 2×(211) commensurate superstructures can be attributed to the periodic presence of Cr2+, Cr3+, and/or H+ in the 4- and/or 6-coordinated lattice sites with an internal compressive stress up to ∼5 GPa according to X-ray photoelectron and vibrational spectroscopic evidence. The presence of internal stress and Cr2+ ion caused red shift of the UV absorbance, thus shedding light on potential optoelectronic applications of the Cr3O4 nanocondensates. I. Introduction The motivation of this research is to study the effect of a water environment on chromium oxide condensation by the pulsed laser ablation (PLA) technique. We focused on the morphology, defect clustering, phase transformation mechanism, and optical properties of the predominant spinel-like nanocondensates under the influence of periodic bond chains (PBC, i.e., strong bonding directions), atom attachment, OH- signature, and the precondensation effect on the crystallization process.1-3 Crystalline and/or amorphous chromium oxide film is commonly produced by dc magnetron reactive sputtering,4 chemical vapor deposition (CVD),5-10 and pulsed laser deposition11,12 for potential applications in optoelectronics such as components for flat panel display devices. PLA was employed in air to fabricate Si4+-doped chromium oxide nanocondensates which have a considerable internal compressive stress due to rapid heating/cooling and hence pressure effect of the process.13 The predominant corundumtype Si4+:R-Cr2O3 nanocondensates thus formed were found to be hexagonal in shape with a significant internal compressive stress, whereas the minor spinel-like Si4+:Cr3O4 nanocondensates were found to be octahedral in shape with a considerable tetragonal distortion similar to the case of furnace-cooled Cr3O4.14 This PLA result13 indicated that condensation would be as effective as a solidification process to form Cr3O4 during laser-heated pedestal growth (LHPG).15 (Nonepitaxial Cr3O4 crystallites with spinel-like structure were found to segregate on the {112} side surfaces of the YAG fibers grown by LHPG process at a relatively low speed.15 It was suggested that such Cr3O4 crystallites were formed via an intermediate surface melt during LHPG.) In all cases, the crystallographic relationship between the spinel-type and its tetragonal (t) derivative of Cr3O4, and the underlying transformation mechanism, was not clarified. * To whom correspondence should be addressed. Fax: +886-7-5254099. E-mail: [email protected]. † National Sun Yat-sen University. ‡ I-Shou University.

PLA was also conducted in a vacuum16 to fabricate and quench amorphous Cr2O3 with corrugated lamellar layers which are similar to the specific lattice planes of the stable R-type structure, i.e., (1j104) and (112j0) having the Cr-filled octahedral sites assembled as 0 and 1 PBC, respectively. Such amorphous nanocondensates were observed in situ to become more polymerized by forming (011j2)-like layers (with 2 PBCs) and then fully crystallized as R-Cr2O3 for further (011j2)-specific coalescence when irradiated by electron beam. The partially crystallized lamellae showed a higher frequency for the strongest Raman band due to an internal compressive stress up to ca. 4 GPa.16 In other words, there is a rather tight 6-coordination of Cr3+ in the rapidly quenched amorphous phase. PLA in liquid (PLAL) was known to have an even higher heating-cooling rate and hence a more pronounced pressure effect to form smaller and denser nanocondensates than the PLA process.17,18 The PLAL route has been used to fabricate highpressure phases such as diamond.18 Here we followed this route to fabricate dense chromium oxide nanocondensates with a predominant spinel-like structure, focusing on the effect of water environment on the shape, defects, lattice correspondence, mechanism of phase transformation, and optical properties of the spinel-type derived Cr3O4. II. Experimental Procedure Cr (Solar Applied Materials Technology, 99.9% pure) plate 1 mm in thickness was immersed in deionized (DI) water in a glass beaker and then subjected to energetic Nd:YAG laser (Lotis, 1064 nm wavelength, beam mode TEM00) pulse irradiation at a specified laser parameter, i.e., 1100 mJ/pulse with a pulse time duration of 240 µs at 10 Hz on a focused area of 0.03 mm2. Under such laser conditions, the average power density and peak power density are 1.5 × 108 and 1.5 × 107 W/cm2, respectively, with the latter being related to the shock wave induced pressure according to an analytical model by Fabbro et al.,19 as discussed later. The upper surface of the Cr target was 5 mm below the water level in a beaker 6 cm in

10.1021/jp904288n CCC: $40.75  2009 American Chemical Society Published on Web 08/20/2009

Spinel-like Cr3O4 Condensates by PLA in Water diameter full of DI water ca. 15 cm3 in volume during such an ablation process. An optimal synthesis time of 10 min, i.e., a total of 6000 pulses given 10 Hz, was adopted for a satisfactory yield of nanocondensates, yet to circumvent severe plasma breakdown effects, which typically occur at a much higher power density of 10 GW/cm2.20 (Generation of a high amplitude shock wave by laser plasma in a water confinement regime has been investigated for an incident 25-30 ns/40 J/λ ) 1064 µm pulsed laser beam.20 Above a 10 GW/cm2 laser intensity threshold, saturation of the peak pressure was shown to occur while the pressure pulse duration was reduced by parasitic plasma occurring in the confining water.20) The nanoparticles in the upper portion of the colloidal solution (ca. 1 wt % in concentration) then settled and were collected on Cu grids or glass substrate without centrifugation for transmission electron microscopy and optical property studies. The samples thus prepared have negligible cible pieces according to optical and electronic microscopic observations. The composition and crystal structures of the condensates were characterized by field emission transmission electron microscopy (TEM, FEI Tecnai G2 F20 at 200 kV) coupled with selected area electron diffraction (SAED), and point-count energy dispersive X-ray (EDX) analysis at a beam size of 10 nm. Bright field images (BFI) taken by TEM were used to study the morphology and agglomeration of the nanocrystals. Lattice images coupled with two-dimensional (2-D) Fourier transform and inverse transform were used to characterize the planar defects of the as-formed nanocondensates. The condensates collected on a silica glass substrate were used for UV-visible absorption (U-3900H, Hitachi, with a resolution of 0.1 nm in the range 200-900 nm) and Raman spectroscopic study. Raman spectra were made using semiconductor laser excitation (532 nm) having a resolution of 2 cm-1 (Jobin-Yvon Triax 320 Micro-Raman microprobe) calibrated with that of a reagent-grade R-Cr2O3 powder (Cerac, 5 µm in diameter, 99.8% pure). The condensates collected on silica glass were also used for X-ray photoelectron spectroscopy (XPS, JEOL JPS-9010MX photoelectron spectrometer with Mg KR X-ray source) calibrated with a standard of C 1s at 284 eV to analyze the position of Cr 2p3/2 peak for the possible presence of Cr2+ in the condensates. The condensates mixed with KBr were studied by Fourier transform infrared spectroscopy (FTIR, Bruker 66v/S. 64 scans with 4 cm-1 resolution) for the extent of the OH- signature.

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Figure 1. TEM (a) BFI and (b) SAED pattern of minor R-Cr2O3 and predominant spinel-type Cr3O4 nanocondensates which are in random orientation as indicated by ring diffractions labeled as hkil and hkl, respectively. (c) Point-count EDX spectrum of the condensates showing Cr and O counts with extra Cu counts from the sample supporting copper grid. Sample produced by laser ablation on pure Cr target at 1100 mJ/ pulse in DI water and then collected by a carbon-coated collodion film.

III. Results TEM. TEM BFI and corresponding SAED pattern indicated that the chromium oxide condensates produced by PLAL are equiaxed nanocrystallites 10-30 nm in diameter and in random orientation (Figure 1a). Despite the superimposition of the diffraction rings in Figure 1b, the mixed phases of the nanocondensates can be reasonably estimated as predominant (90%) Cr3O4 of the spinel-derived type and minor (10%) R-Cr2O3 of the corundum-type structure based on the lattice image analysis of the individual particles. Point-count EDX spectrum of the nanocondensates (Figure 1c) shows counts of Cr and O whose ratio varied from place to place depending on the size/shape of the condensates and the exposition of the underlying carbon-coated collodion film. The minor R-Cr2O3 nanoparticles were (11j02), (0001), and j (1104) faceted and commonly twinned over (0001) to form a coherent twin boundary as revealed by lattice image coupled with 2-D forward and inverse Fourier transforms in the [112j0] zone axis (Figure 2). The lattice plane corrugation observed in the local area may be due to defect clusters as discussed later.

Figure 2. TEM (a) lattice image, (b) 2-D forward Fourier transform, and (c) inverse Fourier transform of the square region in (a) showing minor R-Cr2O3 nanoparticles have (11j02), (0001), and (1j104) facets and a coherent (0001) twin boundary viewed edge on in [112j0] zone axis. The slightly corrugated lattice planes at the upper-right-hand corner of the bicrystals may be due to defect clusters (cf. text). This is the same specimen as in Figure 1.

The predominant t-Cr3O4 nanoparticles tended to form faceted polyhedra with varied extent of distortion from the spinel-like

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Figure 3. TEM (a) lattice image of a t-Cr3O4 nanoparticle partially transformed from a parental spinel-type structure showing welldeveloped (011)t surface viewed edge on in [011j] zone axis. The 2-D forward (b, d) and inverse (c, e) Fourier transforms from the square regions I and II, respectively, in (a) show varied extent of distortion of (011)t lattice planes. This t-nanoparticle was partly derived from the spinel type (diffractions underlined) following [211j]sp//[011j]t; (011)sp// (100)t according to the simulation indexing of the t-type (c/a ) 1.229, JCPDS No. 12-0559) and spinel type in (f) and (g), respectively, with interfacial angles specified (cf. text). Note vague superlattice diffraction at 1/2(113) of the spinel, or 1/2(211) of the t-phase in (b). This is the same specimen as in Figure 1.

parent phase. Figure 3a shows the lattice image of such a t-Cr3O4 nanoparticle with a well-developed (011) surface edge on and other inclined surfaces with traces parallel to {211} when viewed in the [011j] zone axis. The 2-D forward and inverse Fourier transforms from local region I (Figure 3b,c) and II (Figure 3d,e) of the image showed different distortions of the (011) lattice planes, presumably due to inhomogeneous distribution of defect clusters and phase transformation induced strain. This t-Cr3O4 nanoparticle is in fact partially converted from the spinel (sp) type in view of the simulation indexing of the

Lin et al. tetragonal type (c/a ) 1.229, JCPDS No. 12-0559) and spinel type in Figure 3f and 3g, respectively. (The interfacial angle 32.8 ( 0.8° defined by the considerably broadened diffractions in Figure 3b fits reasonably well with the ideal 32.8° between (200) and (21j1j) for the t-type (c/a ratio ) 1.142 in this case) and 31.5° between (022) and (1j31) for the spinel type.) The phase transformation follows [211j]sp//[011j]t; (011)sp//(100)t (cf. Figure 10 in the Appendix for the stereographic projection of this relationship), which is analogous to the Bain relationship reported for the fcc-bct transformation of steel.21 There are vague superlattice diffractions at 1/2(113) of the spinel or 1/2(211) of the t-phase in Figure 3b, indicating a periodic presence of chromium vacancies and charge/volume compensating defects as addressed later. Cr3O4 nanoparticles fully transformed to tetragonal symmetry were also observed. Figure 4 shows a lattice image coupled with 2-D forward and inverse Fourier transforms of such a typical t-Cr3O4 nanoparticle with well-developed {1j10} and {101j} surfaces viewed edge on in the [111] zone axis. This t-Cr3O4 nanoparticle was likely transformed from a cuboctahedral spinel type in the [011j] zone axis, having the interfacial angle changed from 70.5° (between (1j11) and (111) of the spinel type) to 66.9 ( 0.8° (between (101j) and (011j) of the t-type with an observed c/a ratio of 1.253), which is in good agreement with the simulated angle 66.5° of t-Cr3O4 using c/a ) 1.229 (JCPDS No. 12-0559). Figure 5a shows an almost completely transformed t-Cr3O4 nanoparticle which has planar defects besides the inclined facets parallel to the traces of (012j) and {312j} when viewed in the [021] zone axis. The 2-D forward Fourier transform (Figure 5b,d) and inverse Fourier transform (Figure 5c,e) from the square regions I and II, respectively, of this particle indicated that only the latter region showed commensurate superstructure diffraction (2/3)(200). In this case, the measured interfacial angle 56.0 ( 0.8° between (112j) and (1j12j) with the c/a ratio 1.254 (Figure 5b) fits well with the simulated angle 55.2° of t-Cr3O4 using c/a ) 1.229 (JCPDS No. 12-0559) (Figure 5f), but does not fit the simulated angle 60.0° among {022} of the spinel type (Figure 5g). The twinned t-Cr3O4 nanoparticles were also commonly found. Figure 6a shows the lattice image of such twinned bicrystals. The 2-D forward Fourier transform (Figure 6b) and inverse Fourier transform (Figure 6c) indicate that the twinned bicrystals have well-developed {101j} surfaces and coherent (101j) twinning plane as viewed edge on in the [111] zone axis. As discussed later, the twin could be due to (101j)-specific coalescence and/or deformation as a result of spinel-type to t-type transformation. Raman shift and FTIR. The Raman spectrum (Figure 7a) of the chromium oxide condensates deposited on silica glass shows a strong Raman band at 549 cm-1 and much weaker bands at 301 and 682 cm-1 for the Cr ion in octahedral site. The most intense band can be assigned as A1g symmetry,22 although at a higher wavenumber (549 cm-1) than 533 cm-1 of the standard powder under ambient condition (not shown). This indicates an internal compressive stress up to ∼5 GPa for the constituting CrO6 polyhedra of the nanocondensates based on the Raman band dependence of the applied pressure.22 The shoulder (∼575 cm-1) of the most intense band can be attributed to Cr2+ and/or Cr3+ in the tetrahedral sites of the t- and spineltype Cr3O4, which coexist with the R-Cr2O3 according to XPS. The FTIR spectrum of the same specimen (Figure 7b) shows a rather sharp signal near 3452 cm-1 which can be assigned as OH stretching vibration23 analogous to the broad band (3400-2500 cm-1) for the R-Cr2O3 prepared by a thermal

Spinel-like Cr3O4 Condensates by PLA in Water

Figure 4. TEM (a) lattice image coupled with 2-D forward Fourier transform (b) and inverse Fourier transform (c) from the square region in (a) showing a t-Cr3O4 nanoparticle completely transformed from the spinel-type structure with well-developed {1j10}t and {101j}t surfaces edge on in [111] zone axis. The measured interfacial angle in (b) fits the t-Cr3O4 simulation in (d) using c/a ) 1.229 (JCPDS No. 12-0559), but not the spinel simulation in (e) (cf. text). This is the same specimen as in Figure 1.

decomposition process.24 The extra bands can be attributed mainly to octahedral-coordinated Cr ions in association with the hydroxyl group analogous to those assigned for R-Cr2O3. (The R-Cr2O3 via a thermal decomposition process24 showed such bands at ∼1700, ∼1680, and 1400 cm-1 besides 719 cm-1 for the A2u mode and ∼590 cm-1 for the vibration of a symmetric CrO6 octahedra in R-Cr2O3.) Again, different wavenumbers of the bands can be attributed to tramp Cr2+ and/or Cr3+ in the tetrahedral sites of the additional t- and spinel-type Cr3O4 phases. XPS and UV-Visible Absorption. The XPS spectra of the chromium oxide condensates collected on a cover glass substrate

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Figure 5. TEM (a) lattice image of a defective t-Cr3O4 nanoparticle almost completely transformed from the spinel-type structure showing inclined surfaces with their traces parallel to (012j)t and {312j}t viewed edge on in [021] zone axis. The 2-D forward and inverse Fourier transforms from the square region I and II in (b, c) and (d, e), respectively, showing rather strong commensurate superlattice diffraction at 2/3(200)t and weak double diffraction at 1/3(200)t from region II. The superstructure is also labeled in the real space in (e). This is the same specimen as in Figure 1. The measured interfacial angle in (b) fits the t-Cr3O4 simulation in (f) using c/a ) 1.229 (JCPDS No. 12-0559), but not the spinel simulation in (g) (cf. text).

show O 1s (Figure 8a) and Cr 2p3/2 peaks (Figure 8b). The curve fitting of the 2p3/2 peak indicates an average Cr2+/Cr3+ atomic ratio of 0.66. It is an open question whether a higher ratio of Cr2+/Cr3+ for the present nanocondensates than the ideal value 0.5 for Cr3O4 is due to nonstoichiometry and/or protonation, i.e., H+ signature with charge-compensating defects via a dynamic PLAL process as discussed later. Figure 9 shows the UV-visible absorption spectrum of the chromium oxide condensates. There is a significant red shift of the absorbance in the UV region for the present condensates

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Figure 6. TEM (a) lattice image of twinned t-Cr3O4 bicrystals with well-developed {101j}t surfaces and a coherent (101j)t twinning plane edge on in [111] zone axis as indicated by 2-D forward Fourier transform (b) and inverse Fourier transform (c) of the square region in (a). This is the same specimen as in Figure 1.

Figure 7. Vibrational spectra of the chromium oxide condensates prepared by PLAL and then collected on silica glass: (a) Raman; (b) FTIR (cf. text for band assignment).

fabricated by PLAL than those by PLA in air (Figure 11in the Appendix).25 This can be attributed to charge and phase differences of the samples as discussed later.

and Z is the reduced shock impedance between target and the confining water defined by the relation

IV. Discussion Thermodynamic and Kinetic Factors via PLAL. The thermodynamic and kinetic factors of laser ablation of solids in liquids can greatly influence the phase behavior, including the formation of dense phases such as diamond by Ogale and co-workers,26 based on the understanding of the evolution of laserinduced plasma.19 In general, the liquid confinement, which implies the accretion process, induces a shock wave (and so on a high pressure) and a high temperature. These conditions may promote crystallographic phases which are not accessible by the PLA method, for example, diamond phase in the case of carbon.26 The maximum shock pressure generated by the laser plasma in water confined regime is given by an analytical model:19

 R +R 3 √Z (g cm

P (GPa) ) 0.01

s )√I0 (GW cm-2)

-2 -1

(1)

where R is the fraction of internal energy devoted to thermal energy (typically R ∼ 0.25), I0 is the incident power intensity,

2 1 1 + ) Z Zwater Ztarget

(2)

where Zwater and Ztarget are the shock impedances of the water and the target, respectively. The shock wave induced pressure can then be calculated as 0.2 GPa given a peak power density of 1.5 × 107 W/cm2 as adopted in this study. As for the sizeinduced pressure ∆P of the nanoparticles 10-30 nm in size, it is estimated to be 1.46-0.49 GPa based on the Laplace-Young equation ∆P ) 2γ/r, where r is the radius and γ is the surface energy 7.307 J/m2 after ref 27. A higher internal compressive stress, up to ∼5 GPa according to X-ray photoelectron and vibrational spectroscopic evidence, than that contributed from the shock pressure and capillarity force can be attributed to the additional effect of extremely rapid heating and cooling, which is up to108 K/s according to the gray-body radiation calculation for the Cr2O3 condensates 10 nm in size.13,25 A very rapid heating-cooling rate under similar PLA conditions was also known to be able to quench such sized TiO2 nanocondensates with high-pressure R-PbO2- and fluoritetype derived structures.28,29

Spinel-like Cr3O4 Condensates by PLA in Water

J. Phys. Chem. C, Vol. 113, No. 37, 2009 16361 R-Cr2O3 nanocondensates. (According to the ideal unrelaxed atom disposition,16 the {1j 104} and {011j 2} of R-Cr2O3 are K and energetically favorable F faces having the cation-filled octahedral sites assembled as 0 and 2 PBCs, respectively.1) The precondensation effect is, however, obscured for the predominant t-Cr3O4 nanocondensates with a tetragonal bodycentered lattice (I41/amd, JCPDS No. 12-0559).14 These nanoparticles turned out to form {101}t surfaces, i.e., the most closepacked and energetically favorable for such a lattice. Such {101}t surfaces were likely derived from {111} of the spinel type following the Bain relationship (Figure 10). Defect Chemistry and Defect Clusters of Cr3+ and H+ Codoped Cr3O4. Cation deficiency is common for natural chromite due to charge-compensating impurities.31 The present nanocondensates have a rather high Cr2+/Cr3+ atomic ratio and OH- signature according to XPS and FTIR results, respectively. In other words, the Cr3+, Cr2+, and H+ ion species were co-incorporated in the Cr3O4 lattice of the spinel type and its tetragonal derivative when fabricated by PLAL. Under such a circumstance, charge-compensating defect clusters [VCr′′ + CrCr•] in association with the interstitial proton would occur through the following equation in Kro¨ger-Vink notation:32 H2O + Cr2O3

Cr3O4 98 3Hi• + 3[VCr′′ + CrCr•] + 4OOx Figure 8. XPS spectra of the chromium oxide condensates fabricated by PLAL: (a) O 1s and (b) Cr 2p3/2 with curve fitting of the Cr2+ and Cr3+ species (cf. text). This is the same specimen as in Figure 7a.

(3) Here Hi• signifies single positively charged hydrogen in the interstitial octahedral and/or tetrahedral sites; CrCr• is the dominating single positively charged chromium at chromium sites, and VCr′′ is double negatively charged chromium vacancies in the crystal lattice. In such a defect chemistry scheme, the Cr3+ (effective ionic radius 0.0615 nm) would replace a larger Cr2+ (0.073 and 0.080 nm for low and high spin, respectively) in CN 6,33 for the volume compensation of interstitial protons. It is also possible that the volumecompensating effect, due to the undersized Cr3+ dopant in the Cr2+ site, forced CrCr• to enter the interstitial tetrahedral site, i.e., Cri•••, and hence more charge-compensating cation vacancies through the following equation:

3CrCr• + 3[VCr′′ + CrCr•] f CrCr• + [Cri••• + 4VCr′′ + 4CrCr•] (4) Figure 9. UV-visible absorption spectrum of the chromium oxide condensates fabricated by PLAL. This is the same sample as Figure 7a.

Effect of H+/OH- on the Size, Shape, and Phase Identity of the Condensates. Precondensation such as solidification or crystallization from melt/solutions may favor surprising faces not following the PBC predictions.2 For example, the S and K faces of barite crystal were suggested to be favored by dehydration and impurities.3 In addition, diamonds tend to form dendrite with unusual {100} faces rather than {111} under the combined effects of H2O and a large driving force.30 By analogy, the precondensation effect in PLAL process accounts for the attachment of ions/atoms on (11j 02) (F face with two PBCs as mentioned16) and hence the formation of an otherwise unfavorable K face, i.e., {1j 104} for the minor

The defect clustering of Cr3+ and H+ codoped Cr3O4 through eqs 3 and 4 may end up with lattice plane corrugation and the (3/2)×(200) and 2×(211) commensurate superstructures. The possible presence of Cr4+ with an effective radius of 0.041 nm in the tetrahedral interstitial site33 can be excluded by the XPS result. It is not clear, however, if Cr2+ became enriched in water environment at high temperature to form nonstoichiometric Cr3O4-δ having CrCr′ and charge-compensating Hi• assembled via an alternative defect chemistry scheme. Martensitic-like Transformation of Cr3O4. As mentioned, the t- and spinel-type structures follow the Bain relationship in the individual Cr3O4 nanocondensates and the completely transformed t-Cr3O4 has well-developed {101}t surfaces and {101}t twinning plane. These observations suggest that the t-type with a body-centered tetragonal structure was transformed in a martensitic manner from the parental spinel type with an fcc lattice. This transformation scheme is basically the same as that

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of γ-iron to martensite transformation by rapid cooling.21 Being nanosized and hence site saturated, a single variant rather than multiple twin variants were formed via the Bain distortion. The single {101} twinning plane can be alternatively attributed to a coalescence event. Cr Ion Dependent Optical Absorbance in UV Region. There is a significant red shift of UV-visible absorbance for the nanocondensates fabricated via PLAL (Figure 9) in comparison with those by PLA in air (Figure 11). The red shift can be attributed to a much higher Cr2+ content in the dominating Cr3O4 by PLAL than the predominant R-Cr2O3 by PLA in air.25 Besides the common effect of internal stress in both cases, the H+/OH- signature in the PLAL case may affect the trigonal field and hence red shift of absorbance for the nanocondensates. The absorbance peaks at 263 and 368 nm (Figure 9) correspond to minimum band gaps of 3.0 and 2.8 eV, based on their intersections with the baseline at 410 and 440 nm, respectively. The minimum band gap of 2.8 eV could be attributed to Cr2+ in the spinel-like Cr3O4 with varied extent of tetragonality. It is not clear, however, if the tetragonality of the spinel-type Cr3O4 has anything to do with the spin change of Cr ions upon rapid quenching of the condensates. The minimum band gap of the Cr3+-containing substances was reported to be ∼4.7-5.0 eV for poorly crystallized chromium oxide formed by dc magnetron reactive sputtering,4 3.4 eV for Cr3+ in hydrated KCr(SO4)2 · 12H2O,34 and 3.09-2.98 eV for R-Cr2O3 by sublimating a precursor at high temperatures.35 The present Cr3O4 nanocondensates showed the combined effects of internal compressive stress and the presence of Cr2+ ion on the red shift of the UV absorbance and hence a band gap as low as 2.8 eV. This sheds light on potential optoelectronic applications of the Cr3O4 nanocondensates via a PLAL route.

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Figure 10. Stereogram showing the specified crystallographic relationship [211j]sp//[011j]t; (011)sp//(100)t between the parental spinel- and t-type Cr3O4 (using c/a ) 1.229, JCPDS No. 12-0559) with their plane normals denoted as black and red circles, respectively. Note that {111} of the spinel type and {101} of the t-type are nearly superimposed.

V. Conclusions 1. The chromium oxide nanocondensates fabricated by PLAL are predominantly dodecahedral Cr3O4 with a varied extent of tetragonal (t) distortion from spinel (sp)-type structure following the Bain relationship [211j]sp//[011j]t; (011)sp//(100)t. 2. The t-Cr3O4 nanocondensates have {101} twinning due to tetragonal distortion and/or a coalescence event, and (3/2)×(200) and 2×(211) commensurate superstructures due to periodic defect clusters as a result of incorporation of Cr2+, Cr3+, and H+ in the octahedral and/or tetrahedral sites with a significant internal compressive stress up to 5 GPa. 3. Despite the coexistence of a minor R-Cr2O3 type, the presence of internal compressive stress and Cr2+ in the Cr3O4 nanocondensates accounts for the red shift of UV absorbance and sheds light on potential optoelectronic applications. Acknowledgment. We thank Dr. C. N. Huang for PLAL runs with us and Miss S. Y. Shih for help with XPS analysis. This work was supported by the Center for Nanoscience and Nanotechnology at NSYSU and partly by the National Science Council, Taiwan, ROC, under Contract No. NSC98-2221-E110-040-MY3. Appendix Figure 10 is a stereogram showing the specified crystallographic relationship [211j]sp//[011j]t; (011)sp//(100)t between the parental spinel- and t-type Cr3O4. Figure 11shows the UV-visible absorption spectrum of the chromium oxide condensates fab-

Figure 11. UV-visible absorption spectrum of the chromium oxide condensates fabricated by PLA in air under 1.5 × 108 W/cm2 power density and an oxygen flow rate of 50 L/min.

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