or Yb-Doped Y2O3 Transparent Ceramics - American Chemical Society

Article pubs.acs.org/JPCA

Cathodoluminescence and Raman Spectroscopic Analyses of Nd- or Yb-Doped Y2O3 Transparent Ceramics Wenliang Zhu,†,‡ Yiquan Wu,*,§ Andrea Leto,∥ Jing Du,⊥ and Giuseppe Pezzotti*,† †

Ceramic Physics Laboratory, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, 606-8585 Kyoto, Japan Department of Orthopedic Surgery, Osaka University Medical School, 2-2 Yamadaoka, Suita 565-0871, Japan § Kazuo Inamori School of Engineering, New York State College of Ceramics, Alfred University, 2 Pine Street, Alfred, New York 14802-1296, United States ∥ Piezotech Japan Ltd., Mukaihata-cho 4, Ichijoji, Sakyo-ku, 606-8326 Kyoto, Japan ⊥ Department of Mechanical Engineering, University of Rochester, Rochester, New York 14623, United States ‡

ABSTRACT: In this article, we discuss a study of the influence of Nd3+ and Yb3+ dopants on the spectroscopic behavior of Y2O3 ceramic polycrystals, by using Raman and cathodoluminescence (CL) spectroscopy. Doping with Yb at 1 at % results in a blue shift of the Raman band but has no pronounced influence on the intrinsic emission of Y2O3, while doping with Nd shows a red shift of the Raman band and markedly enhances the oxygen vacancy related 380 nm CL band. Lattice distortion induced by the alien ion incorporation and variations of effective absorption coefficients by adding different dopants were assessed using Raman spectroscopy. Moreover, both CL and Raman spectroscopies were applied to examine the homogeneity and distribution of the dopants throughout the ceramic microstructure. Visualization of sample homogeneity was made available by hyperspectral imaging of the local intensity of CL bands, while spectral deconvolution was performed to retrieve local structural variations at grain boundaries. We also confirmed that the combination of Raman and CL spectroscopies leads to a reliable and useful methodology for the examination of dopants in yttria ceramic materials. When compared with Nd3+-doped Y2O3 ceramics, Yb3+-doped Y2O3 ceramics are more attractive for wavelength-tunable or ultrashort pulse lasers because of its broad emission bandwidths. Because of the weak absorption cross-section of Yb3+ in the region where strong pulsed-pump sources exist, codoping of the laser material with Nd3+ is able to absorb efficiently from pump sources in various ranges and to transfer the excitation to the Yb3+ ions, making possible exploitation of the good storage capacity for generation of very high energy Qswitched laser emission.9,10 In practical applications, the use of a laser material requires the availability of crystals of exceptionally high optical quality since low optical quality of the active laser medium might directly result in a poor laser performance or even no laser oscillation at all. Loss of crystals can be caused by inclusions like gas bubbles or precipitates, strain induced birefringence, or lattice defects including grain boundaries. Another important point to consider is the homogeneous distribution of the dopant ions since inhomogeneities can cause irregular heating of the crystal resulting in inhomogeneous thermal lensing up to

1. INTRODUCTION In recent years, ceramic laser materials have been extensively investigated due to the functional and practical advantages they possess in terms of good optical properties (i.e., comparable to those of single-crystal materials), low cost, easy fabrication of large-size samples, and the possibility to incorporate high doping concentrations within the material.1,2 Among the studied materials, (cubic) Y2O3 ceramics in its polycrystalline state stood out the most as being an attractive material for use as laser host materials because of its excellent optical, thermal, chemical, and mechanical properties. Its phase stability and high thermal conductivity can reduce the sensitivity of the system toward heating up during service, and a large splitting of the material’s energy levels helps reduce the thermal population of the lower laser level. In this latter context, the sesquioxide of yttrium doped with lanthanide ions has been deeply investigated, with its main focus on the possibility of fabricating ceramic lasers doped and/or codoped with different rare earth (RE) ions, e.g., Nd3+, Yb3+, and Er3+.3−10 Neodymium ion is one of the earliest trivalent RE ions used in solid-state lasers. The potential of laser operation for 1.5 at % Nd3+-doped Y2O3 ceramics has been clearly demonstrated over more than a decade ago, which included the ceramic material exhibiting a significantly high laser power with a slope efficiency of 32%.3 © 2013 American Chemical Society

Received: January 17, 2013 Revised: April 2, 2013 Published: April 2, 2013 3599

dx.doi.org/10.1021/jp400552b | J. Phys. Chem. A 2013, 117, 3599−3607

The Journal of Physical Chemistry A

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the destruction of the crystal due to local thermal expansion.11 Concurrent with processing efforts, by which defect concentration can be considerably reduced (i.e., through a careful choice of growth methods and growth parameters), a quantification of the extent to which such defect removal is achieved should require the development of sensitive and quick experimental techniques for high spatial/spectral resolution assessments of the light emitted from the ceramic sample. Among the available techniques, spectrally and spatially resolved cathodoluminescence (CL) spectroscopy, as performed in a field-emission-gun scanning electron microscope (FEG-SEM), represents a viable method for fulfilling such requirements.12,13 CL spectral mapping with a nanometric spatial resolution has routinely been achieved using the beamscanning facility available in conventional SEM devices. The resulting three-dimensional CL data sets, or hyperspectral images of luminescence intensity, can be analyzed using either spectral or panchromatic imaging. Another possibility is represented by confocal Raman spectroscopy, as an analytical tool for quantitative analysis of defects and microstructural changes in ceramics, owing to its high spectral sensitivity to molecular vibrations and nonuniform distortions of the crystal lattice.14−16 Confocal Raman devices can screen relatively large sampling areas (in the millimeter scale) with a micrometric spatial resolution (namely, a resolution several orders of magnitude lower than CL), which includes the possibility of sampling along the depth of transparent ceramics without damaging the sample. In this article, we attempt to study the influence of Nd3+ and Yb3+ RE dopants on the spectroscopic behavior of Y2O3 ceramic polycrystals, with an emphasis placed on the local lattice distortion induced by their incorporation in the crystallographic lattice. Lattice distortion could be assessed by Raman spectroscopy, which also can provide precious information on the variations of an effective absorption coefficient obtained with adding different dopants. Moreover, we also examined the homogeneity and distribution of the dopants throughout the ceramic microstructure using both CL and Raman spectroscopy. Hyperspectral imaging of the local intensity of CL bands was used for visualizing sample homogeneity, while spectral deconvolution was performed to retrieve local structural variations at grain boundaries. Aside from assessing the effect of different dopants on the Y2O3-based laser ceramics, we also confirmed that the combination of Raman and CL spectroscopies leads to a reliable and useful methodology for the examination of dopants in ceramic laser materials.

grains), while the Yb−Y2O3 sample homogeneously experienced a typically submicrometer-sized microstructure. Raman spectroscopic experiments were carried out in a backscattering configuration using a triple monochromator (T64000, Horiba/Jobin-Yvon, Kyoto, Japan) equipped with a liquid nitrogen-cooled charged coupled device (CCD). The excitation source in the present experiments was the 488 nm line of an Ar-ion laser (Stabilite 2017, Spectra-Physics, Mountain View, CA). A confocal configuration of the Raman probe was adopted throughout the experiments, which involved the use of a 100× objective lens. The collected Raman spectra were analyzed using a commercially available software package (Labspec 4.02, Horiba/Jobin-Yvon, Kyoto, Japan), while spectral fitting was performed with employing Gaussian− Lorentzian functions. CL spectra were collected in a field-emission gun scanning electron microscope (FEG-SEM, SE-4300, Hitachi Co., Tokyo, Japan). The nominal spatial resolution of the electron beam at the sample surface was 1.5 nm. The microscope was also equipped with a CL device consisting of an ellipsoidal mirror, and a bundle of optical fibers, used to collect and focus, respectively, the electron-stimulated luminescence emitted by the sample into a high, spectrally resolved light monochromator (Triax 320, Jobin-Yvon/Horiba Group, Tokyo, Japan). The collected light was analyzed with a 600 gr/mm grating device and a liquid-nitrogen-cooled 1024 × 256 pixel CCD was used to display the CL emission from the samples with a spectral resolution of 0.03 nm. Spectral lines were analyzed with the same software package mentioned above for Raman spectroscopy.

2. EXPERIMENTAL PROCEDURE The investigated Y2O3 ceramics were prepared utilizing a conventional powder metallurgy method, starting with high purity raw materials Y2O3, Nd2O3, and Yb2O3 powders. The raw powders were blended by selecting a weight percentage 1% of Nd2O3 or Yb2O3 and then ball-milled with agate balls. After drying, the powders were pressed into disk samples with a dimension 15 mm in diameter and 1−1.5 mm in thickness. The disk samples were then compacted by cold isostatically pressing (CIP). Transparent Y2O3 ceramics were obtained by sintering the CIPed pellets under vacuum conditions around 1750 °C. Prior to CL/Raman spectroscopic characterizations, the samples were mirror polished using a fine diamond paste. The average grain size in both the Nd−Y2O3 and the pure Y2O3 samples was >2 μm (with some larger abnormally grown

Figure 1. CL spectra of Nd−Y2O3, Yb−Y2O3, and pure Y2O3. Related spectral deconvolutions are given in the insets.

3. RESULTS AND DISCUSSION 3.1. Influence of Nd or Yb Doping on CL and Raman Spectra of Y2O3 Ceramics. Figure 1 shows typical CL spectra

of the three ceramic samples, Nd−Y2O3, Yb−Y2O3, and pure Y2O3, in the wavelength range between 250 and 1000 nm. Under excitation by electrons, only broad and overlapping luminescence bands could be observed for pure Y2O3 in the range from 250−800 nm. According to the procedure reported in ref 17, spectral deconvolution was attempted, as displayed in the left inset to Figure 1. The CL spectrum of Y2O3 consisted of seven sub-bands located at 340, 380, 448, 521, 608, 683, and 752 nm, respectively. The luminescence band at 380 nm is directly related to the anion sublattice, a decrease in oxygen vacancy resulting in an increase in the intensity of this band. The other bands can be interpreted as a result of radiative 3600



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fitted values for the selected band are also shown), so as to take advantage of spectral averaging in order to remove the influence of grain orientation and dopant inhomogeneity on the measured band intensities. As shown in Table 1, the influence of Yb doping and Nd doping is different: a red shift in band position (i.e., a shift toward lower wavenumber (Raman) or to higher wavelength (CL)) together with an increase in Raman intensity is observed for Nd doping, while Yb doping involves a blue shift of the Raman band and a decrease in its intensity. Moreover, the bandwidth of yttria is broadened by Yb doping, but sharpened by Nd doping. From the observation of surface morphology through the use of scanning electron microscopy, we confirmed that roughness changes on the sample surface could be, in first approximation, neglected. Consequently, the observed changes in CL/Raman bands should be mainly dependent on sample property variations. Accordingly, the 380 nm CL band is considered to be only influenced by oxygen deficiency, while the Raman scattering intensity will be mainly determined by the amount of polarizability change. With the introduction of Yb3+ or Nd3+ into the Y2O3 cubic structure, the dopant is expected to substitute for Y3+. However, in the case of a coordination number of 6 in Y2O3, the ionic radius of Yb3+, RYb3+ = 0.868 Å, is slightly smaller than that of Y3+, RY3+=0.90 Å, while RNd3+ = 0.983 Å is larger than that of RY3+ by a comparable amount.19 Therefore, a lattice distortion of difference will be caused by the incorporation of Yb3+ and Nd3+ ions in the lattice structure. As already clarified for the C-type bixybite structure of Y2O3, the cations occupy two crystallographically inequivalent sixcoordinate sites: one site, with C2 symmetry, has the cation at the center of a distorted cube with two O2− ion vacancies on one face diagonal, while the other site, with S6 symmetry, involves the cation being at the center of a distorted cube with two O2− ion vacancies on one body diagonal (cf. ref 20). Accordingly, the lattice distortion induced by alien ions, especially the larger Nd3+ ions, can reduce the presence of O2− ion vacancies, which we also confirmed by observing an increase of the CL band at 380 nm. However, only the radiative recombination in the excited Y3+−O2− pairs with the nearest yttrium−oxygen distance (i.e., the 340 nm band) is clearly affected by lattice distortion, while with an increase in the Y−O distance, the morphological influence on the CL sub-bands significantly decreases. Similarly, the lattice distortion also causes a change in the polarizability of the Y−O vibration in the Y−O octahedra; the greater the change in polarizability of the functional group, the larger the intensity of the Raman scattering effect. Accordingly, the blue/red shift of Yb/Nd doping is assumed to arise from an increase/decrease in binding strength, caused by shortening/ increasing of the distance between Yb3+/Nd3+ and its coordinated oxygen ions. In fact, taking advantage of previously reported data (band positions located at 349 and 330 cm−1, for 17.45 at % Nd−Y2O321 and pure Nd2O3,22 respectively), a nearly linear relationship between the peak position of the 377

recombination in the excited donor−acceptor Y3+−O2− pairs, with each band corresponding to a certain yttrium−oxygen distance. Doping with Yb was found to have no pronounced influence on the intrinsic emission from the Y2O3 lattice, while doping with Nd seemed to markedly enhance the 380 nm band. In the spectral range between 850 and 1000 nm, no luminescence band was found for pure Y2O3. However, for Nd−Y2O3, several emission bands located around 890 nm were found, corresponding to the 4F3/2 → 4I9/2 transition. Only a single band originating from the 2F5/2 → 2F7/2 transition could be observed for Yb−Y2O3. It is apparent that the intensity of such transition bands, originating from the incorporation of Nd or Yb ions in the lattice, should be sensitive to the concentration of the dopant. Therefore, they could be used for examining dopant distribution in the materials when a highresolution spectral line scan is performed within a local region of the sample, as it will be shown in the following section. Figure 2 shows typical Raman spectra of undoped and REdoped Y2O3 samples. At ambient temperature and pressure,

Figure 2. Raman spectra collected from pure Y2O3, Yb−Y2O3, and Nd−Y2O3 samples.

Y2O3 has a cubic structure, belonging to space group Ia3 with 16 formula units in the elemental cell. According to group theory, cubic Y2O3 is predicted to have 22 first-order Raman active modes.18 However, not all such bands could be observed here. From an experimental viewpoint, the peaks labeled in Figure 2 are located at around 158, 191, 316, 328, 376, 430, 469, and 595 cm−1, which can be assigned to: Fg + Ag, Fg + Eg, Fg, Fg + Eg, Fg + Ag, Fg + Eg, Fg + Ag, and Fg + Ag modes, respectively.18 Because of the polycrystalline nature of the investigated yttria samples, the measured CL/Raman spectra, especially the band intensities, may differ from one another at different measuring locations, owing to variations of grain orientation and dopant distribution homogeneity. Therefore, in order to obtain statistical reliability, a representative trend of variation of the 380 nm CL band and of the 376 cm−1 Raman band for the Y2O3 samples is given in Table 1, as averaged over 250 CL/ Raman spectra collected at different locations on each sample under the same measurement conditions (scattering of the

Table 1. Fitting Parameters of the 380 nm CL Band and the 376 cm−1 Raman Band for Nd−Y2O3 CL band pure Y2O3 Nd−Y2O3 Yb−Y2O3

Raman band

position (nm)

intensity (a.u.)

fwhm (nm)

position (cm−1)

intensity (a.u.)

fwhm (cm−1)

380.45 ± 8.0 387.40 ± 7.5 375.60 ± 9.3

3.3 ± 1.2 11.6 ± 4.1 3.2 ± 1.4

80.6 ± 8.9 70.6 ± 5.2 86.0 ± 13.35

375.99 ± 0.25 375.71 ± 0.2 376.15 ± 0.3

196 ± 25 590 ± 86 44 ± 6

11.1 ± 0.56 7.65 ± 0.34 13 ± 0.77

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cm−1 band, ωNd−Y2O3, and dopant concentration, cNd, could be found, showing ωNd−Y2O3 = ωY2O3 − 0.46 × cNd, which is exhibited in Figure 3. Despite the lack of experimental data for

transparency range (0.2−8 mm), with an absorption-edge at about 230 nm. Doping with RE3+ may result in strong absorption at specific wavelengths due to the presence of the introduced dopant,10 and the absorption coefficients of the absorption peaks increase with the doping concentration (e.g., a nearly linear increase of the bands located at 594, 748, and 820 nm in the case of Nd3+ doping23). The inset to Figure 4 shows

Figure 3. Plot of the peak position of the 377 cm−1 band as a function of content of Nd3+ or Yb3+ in Nd- or Yb-doped Y2O3.

pure Yb2O3 to make this assessment over a wide concentration interval, a linear relationship could also be obtained for Yb− Y2O3, showing ωYb−Y2O3 = ωY2O3 + 0.16 × cYb. Such kinds of linear relationships can be useful for determining local dopant concentrations for inhomogeneous distributions of dopants in a local region, provided that a spatially resolved spectroscopy is used. With respect to pure Y2O3, the experimental scattering of the Raman intensity values for the Yb3+ and Nd3+-doped Y2O3 showed no clear difference, which might suggest homogeneity in the distribution of dopant on the micrometer scale (i.e., on the scale of the Raman probe). For polycrystalline samples, the presence of grain boundaries is the main contributor to band broadening. The smaller fwhm for the Nd−Y2O3 can be attributed to a much larger grain size of Nd−Y2O3 with respect to the other two samples (cf. section 3.3) and thus a smaller contribution from grain boundaries within the unit probe volume. The reason why Nd−Y2O3 showed the largest grain size lies in the fact that the lattice distortion, as induced by the larger size of Nd3+ as compared to Y3+, will enhance the speed of the mass transport at grain boundaries, as well as grain growth during sintering. Hou et al. have reported that grain size increases with increasing Nd content in Nd−Y2O3, showing values of 18.91, 22.79, 27.20, and 30.44 μm for the mean grain size in the case of the Nd3+ ion concentration being 0.1, 0.7, 3.0, and 7.0 at %, respectively.23 3.2. Influence of Nd or Yb Doping on CL and Raman Probes in Polycrystalline Y2O3. It has been long known that both CL and Raman spectra collected at any given location of the sample contain information arising from different portions of material comprised within the finite probe volume, while the local intensity contributed by different portions obeys a threedimensional distribution.24,25 As we have already shown in previous papers,25−27 in the case of Raman scattering measurements, the in-plane size of the laser spot on the focal surface of the sample (about 1 μm in the present case) is mainly determined by the objective lens, while the in-depth probe size is mainly determined by the transparency of the material and can be obtained by calibrating the depthdependent Raman intensity in a defocusing measurement scan (i.e., defocusing the laser along the in-depth direction and measuring the intensity variation). However, the optical properties of yttria, such as transmittance, can be altered upon doping the sample with RE ions. Pure Y2O3 has a wide

Figure 4. Defocusing behaviors of pure Y2O3, Yb−Y2O3, and Nd− Y2O3 samples. The inset shows the photographs of the three yttria samples.

the photographs of the investigated pure Y2O3, Yb−Y2O3, and Nd−Y2O3 samples. Both pellets of pure Y2O3 and Yb−Y2O3 exhibit high transparency, while the Nd-doped sample presents slightly translucent due to the light absorption. Therefore, in order to analyze the population of dopant in the ceramic sample, the probe size should first be calibrated. Accordingly, defocusing experiments were performed on the three investigated polycrystalline yttria samples. The focal position (z0) of the incident laser was gradually shifted from above the sample surface (z0 < 0) toward inside locations (z0 > 0). The defocusing behaviors of the 377 and 470 cm−1 Raman modes for each sample are shown in Figure 4. With respect to the pure yttria sample, no dramatic change in the profile could be found in the Yb-doped sample, while a pronounced difference was observed in the Nd-doped sample. A series of locations were investigated, but no significant changes in the defocusing behavior could be found at different locations of the same sample. According to a theoretical analysis of the laser probe described in previous papers, when the laser beam is focused on an arbitrary location (x0,y0,z0) in the sample, the observed spectrum is obtained by combining all the spectral contributions originating from different points within the analyzed region, as follows:25 ∞

Iobs(v , x0 , y0 , z 0) ∝

∞

∞

∫0 ∫−∞ ∫−∞ I(v)

p2

e−2αeff z p2 + (z − z 0)2 ⎡ (x − x )2 + (y − y )2 ⎤ 0 0 ⎥ × exp⎢ −2 dx dy dz ⎢⎣ ⎥⎦ B2

(1)

where I(v) is the local spectrum rising from a location z, B is the laser beam radius in the focal plane, p is the beam response parameter, and αeff is the effective absorption coefficient that takes into account the optical absorption of both incident and 3602



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within the probe volume. An analytical expression for the PRF in the plane perpendicular to the sample surface was introduced by Donolato as follows:29

scattered light, as well as the influence of the collection crosssection for inhomogeneous scattering. We set up a computational program for an iterative computation using commercially available software (MATHEMATICA, Wolfram Research, Inc.), in order to solve eq 1 at different depths, z0, and to determine the appropriate values of the probe response parameters p and αeff by best fitting the experimentally observed intensity variations. The fitting parameters retrieved for different samples are listed in Table 2. As can be seen, Yb

G(x , y , z ; x0 , y0 , z 0 = 0; V )

∝

Table 2. Values of Probe Response Parameters and Probe Sizes for Raman and CL Probes

pure Y2O3 Yb−Y2O3 Nd− Y2O3

p (μm)

α (μm−1)

zd (μm)

Rel (V = 5 kV) (μm)

RCL (V = 5 kV) (μm)

6.0

0.0002

35.6

0.125

0.081

5.8 6.4

0.0002 0.0054

34.5 23.5

0.104 0.104

0.067 0.067

p

∞

∫0 e−2αeff z ×

∫ ∫

p 2

2

p + (z − z 0)

+∞

+∞

2π[C 2(V ) + 0.1z 2] ⎧ (x − x0)2 + (y − y0 )2 ⎫ ⎪ ⎪ ⎬ × exp⎨ − 2 2 ⎪ ⎪ 2[C (V ) + 0.1z ] ⎭ ⎩

2.76 × 10−2 × A 1.67 V ρZ 0.889

(5)

R CL

⎡ ⎛ z ⎞2 ⎤ 1.14 × exp⎢−7.5⎜ − 0.3⎟ ⎥ ⎝ R el ⎠ ⎥⎦ ⎢⎣ 2π[C 2(V ) + 0.1z 2]

⎧ (x − x0)2 + (y − y0 )2 ⎫ ⎬dz × exp⎨− ⎩ 2[C 2(V ) + 0.1z 2] ⎭

∞

⎡ ⎛ z ⎞2 ⎤ 1.14 × exp⎢−7.5⎜ − 0.3⎟ ⎥ ⎝ R el ⎠ ⎥⎦ ⎢⎣ 2π[C 2(V ) + 0.1z 2]

= 0.9 ⎧ (x − x0) + (y − y0 ) ⎫ ⎬dz × exp⎨− ⎩ 2[C 2(V ) + 0.1z 2] ⎭ 2

dz

(6)

In the case of low acceleration voltage, generally C ≈ Rel > z; so eq 6 can be simplified to become

(2)

∫ ∫

0

R el

⎡ exp⎢ −7.5 ⎣

z R el

⎡ exp⎢ −7.5 ⎣

z R el

(

(

2⎤ − 0.3 ⎥dz ⎦

)

2⎤

)

− 0.3 ⎥dz ⎦

= 0.9 (7)

Under such experimental conditions, the CL probe size has been shown to be comparable in magnitude with the primary electron probe range, Rel. According to eqs 5 and 7, the dimension of the luminescence probe at an acceleration voltage of 5 kV for the three yttria samples were calculated and tabulated in Table 2. Both doping with Nd and Yb slightly reduced the CL probe size, with respect to the pure yttria sample. As can be seen, the CL probe size is on the order of tens of nanometers and thus much smaller than the grain size. Therefore, local information can be retrieved from the CL spectrum collected within each individual grain. 3.3. Distribution of Dopants around the Grains in Y2O3. As mentioned above, in the case of doping with RE ions in Y2O3 ceramics, the distribution of dopants in the material is an important factor in practical applications. In this section, we

I (x , y , z )G

(x , y , z ; x0 , y0 , z 0 ; V )dx dydz

R CL

0

+∞

∫0 ∫−∞ ∫−∞

2

0

= 0.9

The calculated data is also listed in Table 2. In the current Raman measurements, the laser probe size for Nd−Y2O3 was found to be the smallest, but even in this sample it was still larger than the grain size. As far as CL experiments are concerned, luminescence intensity at different wavelengths is generated as a consequence of the excitation of the incident electrons, and thus it is strongly dependent on electron diffusion paths. In other words, it is mainly determined by the size of the electron probe, if selftransition of photons can be neglected. When an incident beam impinging the sample with acceleration voltage, V, is focused at an arbitrary point, (x0,y0,z0), the energy distribution inside the probe can be described by the so-called probe response function (PRF), G(x,y,z;x0,y0,z0;V), and the observable CL spectrum can be expressed according to the following triple integral:28 Iobs(λ) =

)

0

dz

2

2⎤ − 0.3 ⎥ ⎦

where ρ is the crystal density, A its atomic mass number, and Z its atomic number. Note that the primary electron probe size, Rel, is generally regarded as corresponding to the region in which the electron beam has dissipated 99% of its energy, while neglecting any other influence like temperature variation. Similarly, the CL probe size or probe depth, RCL, can be calculated according to the following equation (using 90% of the intensity as a threshold value):

2

p2 + (z − z 0)2

z R el

where C is a probe-related parameter, and the primary electron probe range, Rel, is a function of acceleration voltage that can be calculated according to the Kanaya−Okayama equation:30

doping seemed to have a negligible influence on both fitting parameters, while Nd doping increased the effective absorption coefficient at around 500 nm (as confirmed by the absorption spectra given in ref 23), although the probe response parameter was barely altered. It should be noted that, because of the large grain size of the Nd−Y2O3 ceramic, the opacity might also partially result from incoherent light scattering at grain boundaries. However, when the size of the grain boundary is well below the size of the wavelength of the light being scattered (i.e., visible light), light scattering no longer occurs to any significant extent. The respective probe sizes, as expressed in terms of penetration depth (or probe depth), zd, can be calculated according to the following equation (using 90% of the emitted intensity as a threshold value): z

(

(4)

2R el =

∫0 d e−2αeff z ×

⎡ 1.14 × exp⎢ −7.5 ⎣

(3)

where Iobs(λ) is the observed CL spectrum and I(x,y,z) is the amount of CL emission intrinsically generated at each location 3603



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Y2O3 samples, showing areas with grain sizes of about 4 and >10 μm, respectively, which is consistent with the observed presence of big grains in Nd−Y2O3 by Hou et al.23 However, no grain boundaries could be visualized in the Yb−Y2O3 sample, due to its smaller grain size and/or because of the homogeneity of the dopant. Accordingly, spectral line scans were performed at the same path on the surface of each sample by using Raman and CL spectroscopy. Because of the incapability of grain visualization owing to its small grain-size nature, the scanning path on Yb− Y2O3 was randomly selected on the surface (cf. line in Figure 5e). In the case of Nd−Y2O3, the scanning path involved three different grains and crossed two different grain boundaries (see line in Figure 5f). Figure 6a,b shows the variation of band position, width, and intensity of the 377 cm−1 Raman mode and the 892 nm CL band, respectively, for the Nd−Y2O3 sample. Variations of the line-shape parameters of the 377 cm−1 mode and the 970 nm CL band in the line scan are shown in Figure 6c,d, respectively, for the Yb−Y2O3 sample. Presumably because of the small grain size of the Yb−Y2O3 sample, no pronounced variation of both CL and Raman spectra could be observed, which indicated the homogeneity of Yb3+ distribution in the sample down to a submicrometer resolution. However, for Nd−Y2O3, with the line scan proceeding from one grain to another grain, a marked decrease in the intensity of the CL band could be observed at grain boundaries, in addition to a red shift of the band position toward a higher wavelength. Since the CL probe size is smaller than the size of the grain boundary zone, the decrease in intensity can be taken to arise from an increase in dislocation density, due to grain misorientation at grain boundaries and possibly from an inhomogeneous distribution of the dopant in this region. However, the latter assumption of a decrease of

attempt to analyze the local distribution of dopants by using both CL and Raman spectroscopy. Figure 5a−c shows SEM images of pure Y2O3, Yb−Y2O3, and Nd−Y2O3, respectively. No residual porosity was found in this

Figure 5. SEM (a−c) and CL (d−f) images of (a,d) pure Y2O3, (b,e) Yb−Y2O3, and (c,f) Nd−Y2O3 samples, respectively.

SEM observation. No relevant instructive information could be obtained from these images, except for the presence of some scratches on the sample surface due to polishing treatments. In previous studies,14,31 we have shown the possibility of visualizing domain textures on the nanometer scale through CL spectroscopy assessments. In this study, CL hyperspectral images were collected for pure Y2O3, Yb−Y2O3, and Nd−Y2O3, using bands located at 380, 970, and 900 nm, respectively, as exhibited in Figure 5d−f. As shown, the grain morphology can be observed from these CL maps for both pure Y2O3 and Nd−

Figure 6. Variation of band position, width, and intensity of (a) the 377 cm−1 Raman mode and (b) the 892 nm CL band for Nd−Y2O3, respectively, and (c) the 377 cm−1 mode and (d) the 970 nm CL band, respectively, for Yb−Y2O3 in the line scans. Dashed lines represent the locations of the two boundaries observed from the CL image shown in Figure 5f. 3604



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dopant content at grain boundaries is contrary to the general knowledge that segregation of dopants happens at grain boundaries. The Raman measurements also showed a decrease of band intensity at the grain boundaries, but a gradual shift of the band position to a lower wavenumber. As we have shown above, an increase in dopant concentration results in a shift to lower wavenumber for the 377 cm−1 Raman mode and an increase in the band intensity. Consequently, from the viewpoint of dopant contribution, the observation of intensity decrease and band shift to lower wavenumber conflict one another. Moreover, no clear band broadening could be found at the grain boundaries for this sample. As a result, we can reach the conclusion that the assumed inhomogeneous distribution of the dopant at grain boundaries should be negligible. As the Raman probe size is much larger than the size of the grain boundary zone, the density loss due to the presence of dislocations can be neglected. This kind of decrease in the Raman intensity is presumed to be due to the light scattering at the grainboundary grooves associated with light transmittance losses: the light transmittance decreases with increasing grain boundary surface area.32 The observed variation in band intensity at different grains is thus the result of different grain orientations under the incidence of a polarized laser source. Since the observed Raman/CL band shifts do not originate from chemical shifts owing to an inhomogeneous distribution of dopants in the material, it is speculated that they might be caused by grain interactions during fabrication in the form of residual stresses stored inside the material. Yttria exhibits a phase transition from the hexagonal high temperature phase to the cubic room temperature phase slightly below its melting point. This phase transition causes strong stresses in the grown crystals and therefore lowers the crystal quality if the stress cannot be released. A study of pressure dependence of the yttria phase has shown that an increase of pressure (compression) results in a shift of the Raman modes to higher wavenumber (e.g., a linear relationship with a slope of −3.3 cm−1/GPa was found for the 377 cm−1 Raman band when the pressure is lower than 15 GPa).33 In the meanwhile, the pressure dependence of luminescence bands from RE ions have shown that the emission band originating from the RE ions exhibits a redshift, i.e., shifts to lower wavelength as the pressure increases.34,35 Therefore, the variation of band shifts observed in the line scans by CL and Raman spectroscopy are consistent with each other, if one assumes that such shifts are both contributed to by residual stress. As a rough estimation, the trace of the hydrostatic component of the residual stress tensor, σ, can be determined from the spectral shift, Δω, with respect to a stress free band position, according to the piezo-spectroscopic relationship for polycrystalline materials expressed as25,36 Δω =

Πii σ 3

Figure 7. Variation of calculated stress on the surface of Nd−Y2O3.

For further confirmation on the role of chemistry, energy dispersive X-ray spectroscopy (EDX) measurements were also performed in the same region as that shown in Figure 5c on the Nd−Y2O3 ceramic to analyze the distribution of the elements. Figure 8 shows the EDX maps of the elements of Y, O, and Nd,

Figure 8. EDX maps of individual elements of (a) Y, (b) O, and (c) Nd, respectively.

respectively, at an acceleration voltage of 15 kV. According to eq 5, the electron probe in the EDX measurement could be calculated, showing Rel = 0.85 μm, which is still much smaller than the grain size. No distinguishable inhomogeneity in the distribution of Nd ions could be observed in the Nd−Y2O3 ceramic, which supported the conclusion made above from the Raman and CL studies about the homogeneity of dopant distribution in the material. Finally, it should be noted that, in the case of doping in the sesquioxide of yttrium with RE ions, an optimal content of dopant may exist because of the quenching effect due to crossrelaxation interactions between RE ions. For instance, for Nd doping, a content of around 1−2 at % Nd was found to show the highest emission intensity.23,37 Because of its high-energy migration rates, trivalent ytterbium, especially at high concentrations, is also extremely sensitive to luminescence quenching. However, doping with the RE ions are not only limited to applications for laser materials since heavier doping and various growth methods could be involved in the fabrication. Such different fabrication methods include growth of RE-crystalline yttrium oxide films on Si or sapphire substrates by molecular beam epitaxy in planar waveguide lasers, deposition of Yb−Y2O3 thin films on sapphire for solar cell photon harvesting, and fabrication of nanosized RE-Y2O3 polycrystalline solids for advanced phosphor applications.21,37,38 The combined protocols of CL/Raman spectroscopy can be applied as a reliable and useful methodology for the examination of dopants in all such kinds of systems.

(8)

where Πii is the trace of the piezo-spectroscopic tensor or the sum of the phonon deformation potentials (here, Πii = −3.3 cm−1/GPa for the 377 cm−1 band). Therefore, with respect to the reference averaged peak position (ω0 = 375.71 cm−1), the variation of band position in the line scan can be translated into stress, as done in the plot shown in Figure 7. As can be found, a nonuniform local stress field exists in the material, showing some grains in compression with some others in tension. 3605



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Quantum Defect-Limited Efficiency. Opt. Express 2011, 19, A1082− A1087. (9) Petersen, G. E.; Bridenbaugh, P. M. Application of Resonance Cooperation of Rare Earth Ions Nd 3+ and Yb 3+ to Lasers Na0.5RE0.5WO4. Appl. Phys. Lett. 1964, 4, 201−202. (10) Lupei, A.; Lupei, V.; Ikesue, A.; Gheorghe, C. Spectroscopic and Energy Transfer Investigation of Nd/Yb in Y2O3 Transparent Ceramics. J. Opt. Soc. Am. B 2010, 27, 1002−1010. (11) Artemyev, V. K.; Folomeev, V. I.; Ginkin, V. P.; Kartavykh, A. V.; Mil’vidskii, M. G.; Rakov, V. V. The Mechanism of Marangoni Convection Influence on Dopant Distribution in Ge Space-Grown Single Crystals. J. Cryst. Growth 2001, 223, 29−37. (12) Zhu, W. L.; Porporati, A. A.; Matsutani, A.; Lama, N.; Pezzotti, G. Spatially Resolved Crack-Tip Stress Analysis in Semiconductor by Cathodoluminescence Piezo-Spectroscopy. J. Appl. Phys. 2007, 101, 103531. (13) Pezzotti, G.; Leto, A. Contribution of Spatially and Spectrally Resolved Cathodoluminescence to Study Crack-Tip Phenomena in Silica Glass. Phys. Rev. Lett 2009, 103, 175501−1−4. (14) Matsutani, A.; Zhu, W. L.; Pezzotti, G. Spectroscopic Assessments of Domain Texture in Barium Titanate: II. Cathodoluminescence Analysis. J. Am. Ceram. Soc. 2010, 93, 265−271. (15) Zhu, W. L.; Zhu, J. L.; Meng, Y.; Wang, M. S.; Zhu, B.; Zhu, X. H.; Zhu, J. G.; Xiao, D. Q.; Pezzotti, G. Structural Characteristics of Mg-Doped (1 − x)(K0.5Na0.5)NbO3−xLiSbO3 Lead-Free Ceramics As Revealed by Raman Spectroscopy. J. Phys. D: Appl. Phys. 2011, 44, 505303−1−8. (16) Deluca, M.; Pezzotti, G. First-Order Transverse Phonon Deformation Potentials of Tetragonal Perovskites. J. Phys. Chem. A 2008, 112, 11165−11172. (17) Bordun, O. M. Influence of Oxygen Vacancies on the Luminescence Spectra of Y2O3 Thin Films. J. Appl. Spectrosc. 2002, 69, 430−433. (18) Repelin, Y.; Proust, C.; Husson, E. Beny, Vibrational Spectroscopy of the C-Form of Yttrium Sesquioxide. J. Solid State Chem. 1995, 118, 163−169. (19) Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr. 1976, A32, 751−767. (20) Silver, J.; Withnall, R. Probes of Structural and Electronic Environments of Phosphor Activators: Mossbauer and Raman Spectroscopy. Chem. Rev. 2004, 104, 2833−2855. (21) Allieri, B.; Depero, L. E.; Marino, A.; Sangaletti, L.; Caporaso, L.; Speghini, A.; Bettinelli, M. Growth and Microstructural Analysis of Nanosized Y2O3 Doped with Rare-Earths. Mater. Chem. Phys. 2000, 66, 164−171. (22) Ubaldini, A.; Carnasciali, M. M. Raman Characterisation of Powder of Cubic RE2O3 (RE = Nd, Gd, Dy, Tm, and Lu), Sc2O3 and Y2O3. J. Alloys Compd. 2008, 454, 374−378. (23) Hou, X.; Zhou, S.; Jia, T.; Lin, H.; Teng, H. Effect of Nd Concentration on Structural and Optical Properties of Nd:Y2O3 Transparent Ceramic. J. Lumin. 2011, 131, 1953−1958. (24) Atkinson, A.; Jain, S. C. Spatially Resolved Stress Analysis Using Raman Spectroscopy. J. Raman Spectrosc. 1999, 30, 885−891. (25) Zhu, W. L.; Pezzotti, G. Spatially Resolved Stress Analysis in Al2O3/3Y-TZP Multilayered Composites Using Confocal Fluorescence Spectroscopy. Appl. Spectrosc. 2005, 59, 1042−1048. (26) Wan, K. S.; Zhu, W. L.; Pezzotti, G. Methods of PiezoSpectroscopic Calibration of Thin Film Materials: I, Ball-on-Ring Biaxial Flexure. Meas. Sci. Technol. 2006, 17, 181−190. (27) Zhu, W. L.; Zhu, J. L.; Ge, W. Y.; Pezzotti, G. A PhotoStimulated Spectroscopic Method for Spatially Resolved Stress Analysis in Hetero-Epitaxial Films. J. Phys. D: Appl. Phys. 2009, 42, 015505. (28) Munisso, M. C.; Zhu, W. L.; Leto, A.; Pezzotti, G. Stress Dependence of Sapphire Cathodoluminescence from Optically Active Oxygen Defects as a Function of Crystallographic Orientation. J. Phys. Chem. A 2007, 111, 3526−3533.

4. CONCLUSIONS We have shown a study of the effect of RE dopants on the spectroscopic behavior of yttria ceramics by means of a combined protocol of Raman and CL spectroscopies. Doping with Yb at 1 at % had no pronounced influence on the intrinsic emission of Y2O3, while doping with Nd markedly enhances the 380 nm band, reducing the amount of intrinsic oxygen vacancies. Owing to the lattice distortion induced by alien ions, a red shift of the position of Raman bands, as well as the 380 nm CL band, together with an intensity increase, was found for Nd doping, while Yb doping showed a blue shift of the Raman band and a decrease in Raman intensity. Moreover, the Raman probe size was greatly reduced by Nd doping because of an increase in the effective absorption coefficient, while Yb doping had a negligible influence on the probe response parameters. Both doping with Nd and with Yb slightly reduced the CL probe size, with respect to the pure yttria. Grain profiles could be clearly visualized by hyperspectral imaging of the local CL band and spectral deconvolution of Raman and CL bands, performed to retrieve local spectral variations at grain boundaries. This procedure showed a homogeneous distribution of the dopants in the ceramics as far as the resolution of the CL probe could allow us to assess. Finally, we confirmed that a combination of Raman and CL spectroscopies could be a reliable and useful method for the examination of dopants in ceramic laser materials.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-75-724-7568. E-mail: [email protected] (G.P.). Tel: 607-871-2662. E-mail: [email protected] (Y.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the US Air Force Office of Scientific Research (AFOSR) through the YIP program (contract FA9550-10-1-0067) for partially funding and supporting this research.

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REFERENCES

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