Ultraviolet and blue cathodoluminescence from cubic Y₂O₃ and Y₂O₃:Eu³⁺ generated in a transmission electron microscope

Abstract

Herein we describe the investigation of cubic spherical submicron particles of non-doped Y₂O₃ and Y₂O₃ doped with Eu³⁺ in a transmission electron microscope (TEM) equipped with a spectrometer to detect cathodoluminescence from individual particles. Each submicron particle was made up of nanometre sized crystals. We found that these crystals showed a broad emission band at 353 nm upon bombardment with 200 keV or 80 keV electrons. Upon increasing the Eu³⁺ concentration from 0 to 2 mol% this UV/blue emission was gradually quenched: at Eu³⁺ concentrations >2 mol% no UV/blue emission was detected, only the well-known cathodoluminescence (CL) spectrum of Y₂O₃:Eu³⁺ could be recorded. This UV/blue emission has been attributed to the intrinsic luminescence of Y₂O₃ caused by self-trapped excitons. We found that the UV/blue luminescence was strongly temperature dependent and that the trap depth of the self-trapped excitons was 0.14 eV. The ratios of the spectral radiances of ⁵D₁ → ⁷F_J and ⁵D₀ → ⁷F_J (J = 0, 1…6) Eu³⁺ transitions in the CL-TEM spectra of Y₂O₃:Eu³⁺ at low Eu³⁺ concentrations was about a factor of 10 larger than those recorded at 15 keV. This phenomenon has been explained by absorption of the intrinsic luminescence of Y₂O₃ by Eu³⁺.

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1. Introduction

Yttrium oxide (Y₂O₃:Eu³⁺) is a well-known phosphor from its application as the red emitting phosphor in fluorescent lamps and projection cathode ray tubes (CRTs).^1–3 Due to the industrial applications and the attractive properties of Y₂O₃ as host lattice, Y₂O₃ doped with Eu³⁺ and other rare earth ions has been well studied and documented. The expected application of Y₂O₃:Eu³⁺ in low-voltage field emission displays has stimulated intensive research in the synthesis and characterisation of nanosized phosphors, recently reviewed by Li and Lin.⁴ In our laboratory we have studied the enhancement of cathodoluminescence by applying double layers of ZnO:Zn and Y₂O₃:Eu³⁺.⁵ Non-doped Y₂O₃ also yields photoluminescence when excited with UV radiation. Although this has been known for more than 50 years,⁶ the luminescence of non-doped Y₂O₃ has not been studied in much detail until recently. The reason for this recent interest is the possible application of non-doped Y₂O₃ and Y₂O₃ doped with Eu³⁺, Tb³⁺, Nd³⁺ or Tm³⁺ as a scintillation material.^7–15 When non-doped Y₂O₃ is excited by α-particles,⁷ UV-radiation (207 nm)⁸ or X-rays,^9–13 a broad luminescence band is observed between 340 nm and 500 nm. Table 1 summarizes the wavelength at maximum (λ_max) of this broad luminescence band recorded in the literature. The values for λ_max listed in Table 1 vary substantially. This erratic behaviour of the luminescence of Y₂O₃ elicited the following statement from Fukabori et al.⁷ “Light yields of Y₂O₃ ceramics are different from specimen to specimen. Nature of this phenomenon is not clear yet”. Konrad et al.⁸ found that the size of nanoparticles plays an important role and might explain a variation of about 30 nm in λ_max; however, this does not explain the variation shown in Table 1. Fukabori et al.⁷ found a relation between the scintillation light output and crystal size in their Y₂O₃ samples: the larger the crystallites, the higher the light output.

Table 1 λ _max of luminescence band of non-doped cubic Y₂O₃

λ max (nm)	Type of excitation	Ref.
410	UV: 248 nm	6
340	UV: 207 nm	8
344	UV: 207 nm	12
350	α-particles	7
410	X-ray	9
364	X-ray	10
500	X-ray	11
385	X-ray	13

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Wood and Hayes¹⁰ reported that the 364 nm emission band is particularly strong upon X-ray excitation at 1.6 K; Tanner et al.¹² also found that the 344 nm band in Y₂O₃:Eu³⁺ with either 0.1 mol% or 1 mol% Eu³⁺ is strongly temperature dependent.

Besides the position of λ_max there is another interesting characteristic of the intrinsic luminescence of Y₂O₃: the quenching of this luminescence upon doping with rare earth ions. This was first noticed by Wickersheim and Lefever⁶ and later by Jacobsohn et al.⁹ and Tanner et al.¹² Jacobson et al. observed a strong reduction of the intrinsic Y₂O₃ luminescence upon doping with 20 ppm Tb³⁺, while the intrinsic Y₂O₃ luminescence virtually disappeared at doping with 0.08 at% Tb³⁺. Hayes et al. mention the weak intrinsic luminescence of Y₂O₃ upon doping with Eu³⁺.¹⁴ This phenomenon has been observed by other workers as well in studies of Y₂O₃ doped with other rare earth ions, although without comment.^15,16 Fukabori et al.¹⁵ measured a weak intrinsic band at 350 nm upon doping Y₂O₃ with 1 mol% Nd, while Fujimoto et al.¹⁶ measured a weak emission band at 360 nm upon doping with 0.15 mol% Tm³⁺. These latter authors assigned this band to the emission of Tm³⁺; however, we believe that the measured emission band is too broad for an emission line of Tm³⁺. Upon exciting Y₂O₃:Eu³⁺ (5 mol% Eu³⁺) and Y₂O₃:Tb³⁺ (0.5 mol% Tb³⁺) with α-particles, Cress et al.¹⁷ could not detect any broad emission band in the near UV-blue. This indicates that at these rare earth concentrations the intrinsic Y₂O₃ emission has been quenched completely. Comparing this result with the observation of Hayes et al.,¹⁴ it may be concluded that Tb³⁺ is more effectively quenching the intrinsic Y₂O₃ luminescence than Eu³⁺. Ato et al.¹⁸ reported a strong thermo-luminescence peak of Y₂O₃:Eu³⁺ upon irradiation with γ-rays from a Co⁶⁰ source at low and high dose rates. They did not suggest at that time that Eu³⁺ was being reduced by γ-rays: this mechanism was suggested nine years later by Ozawa,¹⁹ a co-author of Ato.¹⁸

The intrinsic luminescence of Y₂O₃ has been explained by three different mechanisms: (1) oxygen vacancies,⁹ (2) self-trapped excitons (STE)^{7,8,10,12,14,20,21} and (3) ligand-to-metal charge-transfer involving the empty 3d orbitals of the Y³⁺ ion.¹³ The latter two mechanisms are more plausible, because luminescence due to STEs¹⁴ or charge-transfer generates broad bands, while Hayes et al.¹⁴ also concluded that the STE-mechanism is likely from their measurements of optical detection of magnetic resonance (ODMR). No explanation is apparent in the literature for the quenching of the intrinsic luminescence of Y₂O₃ upon doping with rare earth ions.

The current density in a projection CRT at normal operating conditions has a maximum at about 0.25 A m⁻². There have been no reports of finding blue CL from Y₂O₃:Eu³⁺ in such CRTs. In a scanning electron microscope the beam current at the specimen is normally between 0.01 nA and 1 nA, dependent on the settings of the condenser lenses,²² and the spot size is often of the order of 1.5 nm, leading to current densities between 6 × 10⁶ A cm⁻² and 6 × 10⁸ A cm⁻², which are 7 to 9 orders of magnitude larger than in a CRT. If the electron transmission at 200 keV is about 99.99% in 200 nm crystals of Y₂O₃, then the effective current density is still 3 to 5 orders of magnitude higher. This difference in effective current density could explain why blue emission was never observed before or previously reported. Another reason is the quenching of this emission at Eu³⁺ concentrations greater than 2 mol% (which is typical for CRTs and fluorescent lamps red phosphors). In our laboratories we are currently involved in cathodoluminescence (CL) and photoluminescence (PL) studies of nanosized and submicron Y₂O₃ doped with Eu³⁺,Tb³⁺ and other rare earth ions.^23–26 Yttrium oxide is a stable compound and when doped with Eu³⁺, nobody (to our knowledge) has observed broad emission bands between 350 and 500 nm as mentioned above. It was therefore a surprise when investigating Y₂O₃:Eu³⁺ samples in our transmission electron microscope (TEM) equipped with an optical spectrometer that we observed a broad emission band at 350 nm. Preliminary results of this observation have been presented at the 22nd International Display Workshops in Japan.²⁷ Herein we describe further measurements and present an analysis of the intrinsic emission of non-doped Y₂O₃ and Y₂O₃:Eu³⁺ in the electron beam of a TEM.

2. Materials and methods

Materials and synthesis

Yttrium oxide (99.99%, Ampere Industrie, France) and europium(III) oxide (99.99%, Neo Performance Materials, UK) were used to prepare the europium-doped yttrium nitrate stock solutions. Urea, nitric acid, oxalic acid and isopropanol (IPA) were supplied by (Fisher Scientific, UK). All chemicals were used as received.

The synthesis of sub-micrometre spherical Y₂O₃:Eu³⁺ precursor particles via a homogeneous precipitation route utilising the hydrothermal decomposition of urea method, followed by annealing the precursor particles at 980 °C resulting in cubic Y₂O₃:Eu³⁺ has been described extensively in our earlier work.^23–26 The concentration of Eu³⁺ in Y₂O₃ was adjusted to 0.1, 0.5, 1, 1.5, 4, 20, 60 and 100 mol%. For comparison reasons three samples of micrometre sized Y₂O₃:Eu³⁺ precursor particles were prepared via an oxalate precipitation route.^1,3 The first sample was made by co-precipitation of Eu³⁺-oxalate (2%) and Y³⁺-oxalate. The second and third samples were made by separately precipitating Y³⁺-oxalate and Eu³⁺-oxalate (6% and 2%) and then slurry-mixing (SM) these precipitates before annealing. These samples were then annealed for four hours at 980 °C in air.

Transmission electron microscope

The submicron spherical Y₂O₃ and Y₂O₃:Eu³⁺ samples were investigated with a TEM (2100F, JEOL, Japan) equipped with a Schottky-type field emission gun. When operated in scanning mode (STEM), the spot size of the e-beam at the specimen was adjusted to 0.2 nm or 1.5 nm. Initial work demonstrated the need to reduce the X-rays in the column generated from the condenser lens aperture, which were found to significantly contribute to disperse excitation of phosphor samples. These X-rays excited the phosphor and caused the emission of visible light when the electron beam was not on the sample, leading to unwanted interference and a loss of resolution. To reduce this X-ray excitation of the sample, a hard X-ray aperture was inserted into the column, which reduced the background noise in CL imaging and spectroscopy modes considerably. The TEM was equipped with a Vulcan™ CL detector, Gatan, USA, for imaging and spectroscopic purposes. This system used a Czerny–Turner spectrometer with back-illuminated CCD and a grating with 1200 lines per mm (blazed at 500 nm) for collection of CL emission spectra. Light was collected from the sample using a mirror above and below the sample, which enabled a solid angle of about 5 sr, which is almost half of a sphere. This high solid angle made light collection highly efficient and enabled the collection of CL at low intensity. Unfortunately, the cooled detector of this spectrometer did not allow the recording of spectra at λ < 380 nm. In the subsequent sections spectra recorded with this spectrometer will be represented at λ > 400 nm. Spectra between 200 and 400 nm were recorded with the Black Comet spectrometer of StellarNet Inc. (USA) for undoped Y₂O₃. This spectrometer had an uncooled detector and the spectra were therefore much noisier.

By collecting the visible light with the Vulcan system simultaneously with JEOL's high-angle annular dark-field (HAADF) detector, it was possible to observe the visible light that was emitted from the particles. A small cryostat connected to the sample holder enabled cooling of the samples in the TEM down to 102 K (−171 °C); adjustment of the sample temperature anywhere between 102 K and 303 K could be made. A Gatan electron energy loss spectrometer (EELS) was used to map the position of europium ions at the surface of the nanocrystals.

X-ray powder diffraction

The crystalline phases of the products were determined by X-ray powder diffraction (XRPD) using a Bruker D8 Advance X-ray powder diffractometer fitted with a nickel-filtered copper source and a LynxEye™ silicon strip detector. Data were recorded from 5 to 100 2θ degrees at 25 °C. The diffractometer was previously calibrated using an aluminium oxide line position standard from Bruker and a LaB₆ NIST SRM 660a line profile standard. Diffractograms were collected using the annealed powders in a conventional holder. The emission of the nickel filtered Cu source and hence the instrumental line broadening was determined by fitting the NIST standard using Bruker Topas version 3. Phases in the combusted products were identified from the XRD patterns by peak search matching using the ICCD PDF-2 data files.

3. Results

Transmission electron microscope analysis

Fig. 1a is a TEM image of the urea-precipitated cubic spherical submicron Y₂O₃:Eu³⁺ phosphor particles with diameters between 200 nm to 300 nm. Fig. 1b presents a TEM image of a single particle, which is composed of a number of tessellated nanocrystals from 40 nm to 80 nm.

Fig. 1 TEM images, (a) and (b), of Y₂O₃:Eu³⁺ particles: (a) urea-precipitated route, (b) higher magnification image of a single particle from (a). (c) HAADF image oxalate-precipitated particles.

In Fig. 1c the oxalate-precipitated cubic micrometre sized Y₂O₃:Eu³⁺ phosphor particles are presented. These have an irregular morphology with a size range between 1 and 3 micrometres.

X-ray diffraction

X-ray diffraction was only used for the analysis of the oxalate-precipitated samples of Y₂O₃:Eu³⁺, because urea-precipitated samples of Y₂O₃ and Y₂O₃:Eu³⁺ after annealing at 980 °C for 4 hours are known to consist of the cubic phase for 100%.^23–26Fig. 2 shows the diffractogram for oxalate-precipitated Y₂O₃:Eu³⁺. This diagram proved that this material after annealing for 4 hours at 980 °C also consisted for >99% of the cubic phase.

Fig. 2 Powder X-ray diffractogram of Y₂O₃:Eu³⁺ (2%) oxalate method co-precipitated. Fired at 980 °C for 4 hours.

UV/blue emission band

Fig. 3 shows a spectrum of undoped Y₂O₃ excited with an electron beam of 200 keV at −120 °C. The as-recorded spectrum is the noisy curve (1), which is represented by a Gaussian profile (3) corrected for the background (2). The Gaussian profile has been fitted to the spectrum with a least squares algorithm using Microsoft's Excel solver.

Fig. 3 Spectrum of undoped Y₂O₃ excited with an electron beam of 200 keV and spot size 1.5 nm at −120 °C (1). Recorded with the StellarNet spectrometer with an integration of 16 s. (2): Background. (3): Gaussian profile fitted to the data points and corrected for background.

By averaging the values for λ_max and the full width at half maximum (FWHM) from spectra recorded at various temperatures, we determined that λ_max = 353 nm (28 300 cm⁻¹) and FWHM = 5735 cm⁻¹. This value of λ_max is identical to the value published by Fukabori et al.⁷ and close to the values published by Konrad et al.,⁸ Wood and Hayes¹⁰ and Tanner et al.¹² Fukabori et al. found that in some of their samples the emission extended much further in the visible region. This has not been confirmed in our measurements, neither in those of the other workers.^8–10,12,13 Nevertheless, the broad UV emission band represented in Fig. 4 extends substantially into the visible part of electromagnetic radiation. At 405 nm the spectral radiance is almost 5 times smaller than at 353 nm; however, for comparing the intrinsic Y₂O₃ luminescence with the Eu³⁺ emission lines for the doped samples, we used the Gatan spectrometer with the cutoff at about 390 nm, basically because of the detector limitation of the StellarNet spectrometer.

Fig. 4 CL spectra of Y₂O₃:Eu³⁺ recorded at −171 °C, 200 keV beam voltage and spot size of 1.5 nm. (A) non-doped Y₂O₃, inset: spectrum at larger scale between 600 nm and 900 nm; (B) 0.1 mol% Eu³⁺; (C) 0.5 mol% Eu³⁺; (D) 1 mol% Eu³⁺. The sharp lines in the spectrum are Y₂O₃:Eu³⁺ transitions; the strongest is the ⁵D₀ → ⁷F₂ Eu³⁺ transition at 611 nm.

Fig. 4 shows how we were able to turn the red-emitting phosphor Y₂O₃:Eu³⁺ into a UV/blue-emitting phosphor in the TEM: the radiance of the UV/blue luminescence is dwarfing the (area) radiance of the well-known emission lines of Eu³⁺ in Y₂O₃ at 0.1 mol% Eu³⁺. Important conditions for observing the broad UV luminescence are: low temperature of the sample: −171 °C, large spot size of the e-beam of 1.5 nm, high beam current by maximising the condenser lens aperture and a specimen thickness of at least 100 nm. The energy of the electron beam does not seem paramount: we observed the UV/blue luminescence both at 200 keV at 80 keV. In Fig. 4A the sharp emission lines of Eu³⁺ are not present: whereas the strongest transition in Fig. 4B–D is that due to the ⁵D₀ → ⁷F₂ Eu³⁺ transition at 611 nm. The vertical scales of the graphs in Fig. 4 cannot be compared due to the different integration times during spectra recording and different specimen thicknesses. It can be seen that the spectral radiance (normalised to the spectral radiance at 611 nm) of the blue emission at 400 nm decreases strongly when the concentration of Eu³⁺ is increased from 0.1 to 1 mol%. At Eu³⁺ concentrations ≥2 mol% in urea-precipitated Y₂O₃:Eu³⁺ we could not detect any UV/blue emission at 400 nm. Beside the strong UV/blue emission at 400 nm, two very weak long wavelength bands can be observed (inset of Fig. 4A), one at about 675 nm and the other at about 770 nm. Electron bombardment of the carbon-coated Cu-grid without Y₂O₃:Eu³⁺ particles did not show any CL; hence, interference from the Cu-grid holder material can be excluded.

The luminescence spectrum between 400 nm and 500 nm in Fig. 4A (and the weak long wavelength bands) did not noticeably change its shape upon reducing the energy of the electron beam from 200 keV to 80 keV. This therefore excludes Cherenkov radiation being the origin of the observed emission bands at 353 nm, 675 nm and 770 nm.²⁸ At 400 nm the refractive index of Y₂O₃ is 1.98: to calculate the refractive index of Y₂O₃ as a function of λ, use was made of the dispersion formula given by Nigara.²⁹ The condition for Cherenkov radiation in a medium with a refractive index of 1.98 is satisfied when the electron velocity >1.52 × 10⁸ m s⁻¹, which starts at an electron energy of 80 keV. So, at 80 keV there cannot be Cherenkov radiation in Y₂O₃ at wavelengths >∼405 nm. From this straightforward calculation it can be concluded that any light at wavelengths >405 nm that is emitted upon bombarding non-doped, transparent Y₂O₃ crystals with electrons at 80 keV cannot be ascribed to Cherenkov radiation. Since the position and shape of the blue emission did not change upon increasing the electron energy to 200 keV, we can reasonably exclude Cherenkov radiation for 200 keV electrons as well. Additional evidence for this conclusion is the strong temperature dependence of the UV/blue and red emission, as we shall discuss in the following paragraphs. Cherenkov radiation is virtually not affected by temperature as long as the refractive index and the density of the specimen do not change substantially. In ref. 27 it was supposed that the blue emission of Y₂O₃:Eu³⁺ at 400 nm was caused by reduction of Eu³⁺ to Eu²⁺. From Fig. 4A it must be concluded that this is not the case and that the blue emission in all graphs of Fig. 4 is intrinsic luminescence of Y₂O₃. In other words, it is concluded that the UV/blue luminescence observed in the TEM is identical to the blue/UV emission from undoped Y₂O₃ and Y₂O₃ doped with small amounts of rare earth ions upon excitation by other sources of ionizing radiation.^7–16

In order to quantify the quenching of the intrinsic luminescence of Y₂O₃ upon increasing the Eu³⁺ concentration we define a quenching factor η:

where SR₄₀₅ is the spectral radiation of the UV/blue emission at 405 nm and SR₆₁₁ is the spectral radiance of the ⁵D₀ → ⁷F₂ Eu³⁺ transition at 611 nm. For non-doped Y₂O₃η = 1 and for Eu³⁺ concentrations >2 mol%, η = 0.

Fig. 4B–D refer to Y₂O₃ doped with 0.1, 0.5 and 1 mol% Eu³⁺. Apart from the diminishing intrinsic luminescence of the host material there is another interesting phenomenon, viz. the changing ratios between the spectral radiance of the strongest Eu³⁺ transition at 611 nm and the spectral radiances of Eu³⁺ transitions at λ < 611 nm. Furthermore, these latter transitions are about two orders of magnitude stronger than the corresponding transitions in the photoluminescence (PL) spectrum of Y₂O₃:Eu³⁺ as represented in Ozawa's book on page 165.³ We shall discuss these phenomena together with the intrinsic luminescence of Y₂O₃ in the next section.

Fig. 5 is a HAADF image of urea-precipitated Y₂O₃:Eu³⁺ (0.1 mol% Eu³⁺) at −168 °C and 200 keV. The image illustrates the effect of positioning of the e-beam on η. The square denoted by “Spatial Drift” indicates the image that was used to compensate the thermal drift of the sample holder during cooling and warming up. This feature enabled a stable position of the electron beam on the nanocrystal during recording of the spectra, which required more than 2 minutes in some cases. The sites SI.1 to SI.5 in Fig. 5 refer to spots with different particle thickness: SI.1 is a very small particle, whereas SI.2 and SI.3 refer to spots with two particles on top of each other.

Fig. 5 Quenching factor η at various sites for urea-precipitated Y₂O₃:Eu³⁺ (0.1 mol% Eu³⁺) at −168 °C and 200 keV.

This is reflected in the rather high value for the Eu³⁺ emission. In the other urea-precipitated samples of Y₂O₃:Eu³⁺ containing larger Eu³⁺ concentrations, the variation of the quenching factor η was less than a factor of two. Since some Y₂O₃:Eu³⁺ particles are partially hollow, it is impossible to determine a relation between R₄₀₅ and R₆₁₁ and specimen thickness from Fig. 5. It should be kept in mind that the radiance R_b of the Y₂O₃ emission band, defined as

where b− and b+ are the integration limits, is outstripping the radiance of the ⁵D₀ → ⁷F₂ Eu³⁺ transition at 611 nm at low Eu³⁺ concentrations, because the UV emission band is much broader than the 611 nm peak.

Fig. 6 shows the effect of temperature on the spectral radiance of the intrinsic emission of undoped Y₂O₃: the lower the temperature, the stronger the UV/blue luminescence. The drift corrector facility, indicated in Fig. 5, guaranteed that the spectra shown in Fig. 6 were recorded at the same spot. Since the time for recording the spectra (integration time) was also kept constant, the spectra for different temperatures can be compared directly: i.e. there is no effect of thickness. The intrinsic blue luminescence in Y₂O₃:Eu³⁺ also increased by more than a factor of 10 upon decreasing the temperature from 31 °C to −171 °C.

Fig. 6 Spectra of undoped Y₂O₃ recorded at 80 keV and various temperatures. The inset shows the spectra between 600 and 900 nm at a different vertical scale.

The two long wavelength bands shown in the inset of Fig. 6 are much weaker than the UV/blue emission and are also stronger at −172 °C than at 30 °C. It can be seen that the temperature behaviour of the band at 675 nm deviates from that of the other band at 770 nm and the UV/blue band. Upon excitation of Y₂O₃ or Y₂O₃:Eu³⁺ by X-rays or α-particles no emission was observed from these bands by other workers.^{7,9–11,13–16} These bands also disappeared when the Eu³⁺ concentration was increased. The origin of these two bands is unknown.

Fig. 7 is an Arrhenius plot of the spectral radiance (SR) measured at 353 nm with the StellarNet spectrometer (data points 1) and SRs measured at 405 nm with the Gatan spectrometer (data points 2 and 3). The data points of (1) refer to undoped Y₂O₃ and 200 keV beam energy, the data points of (2) refer also to undoped Y₂O₃ but at 80 keV and the data points of (3) refer to Y₂O₃:Eu³⁺ with 0.1 mol% Eu³⁺ and 200 keV. The drift corrector facility enabled us to stay at the same point during recording of the spectra and after changing the temperature. In some cases a tiny shift was applied to deal with beam degradation. However, the measured effect of beam degradation on spectral radiance was <2%, which was smaller than the noise level in the spectra, especially in the StellarNet spectra, as shown in Fig. 3. The curves in Fig. 7 have been fitted to the data points with eqn (4) that describes the temperature-dependent effect of STEs. This will be discussed in the next section. The trap depth that describes the experimental data for non-doped Y₂O₃ is 0.13 eV and it is 0.16 eV for Y₂O₃:Eu³⁺ with 0.1 mol% Eu³⁺. For Y₂O₃:Eu³⁺ with other Eu³⁺ concentrations the Arrhenius plots are similar to that of 0.1 mol% Eu³⁺ as indicated in Fig. 7. In view of the spread in the data we consider that the trap depth of the STEs in undoped and doped Y₂O₃ is identical and has a value of 0.14 eV.

Fig. 7 Arrhenius plot of spectral radiance versus 1000 K. (1): SR₃₅₃ of undoped Y₂O₃ recorded at 200 keV; (2): SR₄₀₅ of undoped Y₂O₃ recorded at 80 keV; (3): SR₄₀₅ of Y₂O₃:Eu³⁺ with 0.1 mol% Eu³⁺ recorded at 200 keV. The dashed curves have been fitted to the experimental data with eqn (4).

In Fig. 8 we have summarized the quenching factor η (for the blue band) as a function of Eu³⁺ concentration for all samples investigated in the TEM. There are three types of samples collected in Fig. 8, viz. urea-precipitated (without a number), oxalate co-precipitated (no. 1) and SM-oxalate precipitated (no. 2). As mentioned above, blue luminescence could not be observed for the urea-precipitated samples at Eu³⁺ concentrations >1.5 mol%, whereas the SM-oxalate samples showed very large blue luminescence for 2 and 6 mol% Eu³⁺. The spread in η was about a factor of 3 in the SM-oxalate samples; in the urea-precipitated samples it was less.

Fig. 8 Quenching factor η as a function of Eu³⁺ concentration in Y₂O₃. (1): oxalate-co-precipitated Y₂O₃:Eu³⁺, (2) SM-oxalate-precipitated Y₂O₃:Eu³⁺. Other points refer to urea-precipitated samples. The abscissa is quasi-logarithmic, because the point with 0 mol% Eu³⁺ dope has been indicated.

Fig. 8 illustrates the different behaviour of the SM-oxalate-precipitated and urea-precipitated samples, whereas the oxalate co-precipitated sample (point 1 in Fig. 8) with molecular mixing of the Eu³⁺ and Y³⁺ ions indicates that this sample behaves as the urea-precipitated samples. The deviating behaviour of the SM-oxalate-precipitated sample is explained by its inhomogeneity, in which Eu³⁺-rich areas in the crystals are alternating with areas with very low Eu³⁺ concentration. The penetration depth of 200 keV electrons in crystalline Y₂O₃ is about 75 μm,^5,30 which is much larger than the size of the oxalate-precipitated crystals. The Eu³⁺ rich areas in the crystal hit by the e-beam do not contribute to the build-up of the blue band because of concentration quenching, whereas the areas with very low Eu³⁺ concentration take care of the strong blue band signal. This hypothesis was confirmed by the very low Eu³⁺ signal in the EELS (not shown) at various spots. From this consideration it can be concluded that the SM-oxalate precipitated sample should be inserted at a much lower concentration in Fig. 8. Since the effective Eu³⁺ concentration is unknown, the best we could do is plotting the result at the as-made Eu³⁺ concentration.

Finally we would like to mention that we also observed blue luminescence in the TEM when bombarding undoped monoclinic Y₂O₃ crystals with 200 keV electrons at 1.5 nm spot size. The monoclinic material was unstable under these electron bombardment conditions, decomposing partly into the more stable cubic form, and therefore we do not reproduce any results of these measurements in this article.

Discussion of blue luminescence in Y₂O₃ and transition ratios

When an electron beam of 200 keV hits an Y₂O₃ crystal it produces besides defects, backscattered and secondary electrons, X-rays and holes by inelastic scattering processes. Electrons and holes can combine to form free excitons and STEs. Herein we adopt the model that STEs are responsible for the blue luminescence in undoped Y₂O₃ and Y₂O₃:Eu³⁺ as indicated in the literature.^{7,8,10,12,14,20,21} According to Mikhailik and Kraus³¹ and Blasse³² the radiance B of a scintillator upon excitation can be written as:

B = αN_e–hSQ (3)

where α is a proportionality constant, N_e–h is the number of electron–hole pairs that is generated by the electron beam inside the crystal, S is the probability of transferring energy from an e–h pair to a luminescent centre, e.g. a Eu³⁺ ion, and Q is the luminescence quantum efficiency. Undoped Y₂O₃ is a so-called self-activated or intrinsic scintillator,³¹ in which S is 1 by definition. The intrinsic luminescence in non-doped Y₂O₃ is considered to come from O-2p levels and Y-3d levels after an STE has combined with a luminescent centre in Y₂O₃.¹¹ The quantum efficiency Q in eqn (3) can be described by Mott's equation on thermal quenching:³¹where P and C are constants to be fitted, E_A is the activation or trap energy, k is Boltzmann's constant and T is the absolute temperature. In Fig. 8 we have fitted Q according to eqn (4) to the experimental points for undoped Y₂O₃ and Y₂O₃:Eu³⁺ with 0.1 mol% Eu³⁺. It can be seen that the experiments are satisfactorily represented with E_A = 0.14 eV both for non-doped Y₂O₃ and 0.1 mol% Y₂O₃:Eu³⁺. We interpret this activation energy as the trap depth of the self-trapped exciton.

It should be kept in mind that eqn (3) for self-activated Y₂O₃ refers to the radiance of the blue emission band. When Y₂O₃ is doped with Eu³⁺, energy will be transferred from the host lattice to the Eu³⁺ luminescent centres: hence, the CL spectrum of Y₂O₃:Eu³⁺ can be detected. The CL of Y₂O₃:Eu³⁺ can also be described with an energy transfer probability S in eqn (3), which is 0 for undoped Y₂O₃ and has a maximum value, albeit < 1, for a dopant concentration of 2 mol% and larger.

As mentioned above, Fig. 4 shows that the spectral radiance of Eu³⁺ transitions at λ < 580 nm decreases in the sequence Fig. 4B–D. We have plotted this behaviour for some transitions in Fig. 9.

Fig. 9 Ratio of spectral radiance between ⁵D₁ → ⁷F₁ (533 nm) and ⁵D₀ → ⁷F₂ (611 nm): R₁ and ratio of spectral radiance between ⁵D₀ → ⁷F₄ (713 nm) and ⁵D₀ → ⁷F₂ (611 nm): R₂. For R₁ the measurements at −170 °C (cold) and room temperature (RT) gave different results.

Fig. 9 represents the ratios R₁ and R₂, which are the ratios of the spectral radiances ⁵D₁ → ⁷F₁ (533 nm) and ⁵D₀ → ⁷F₂ (611 nm) and that of ⁵D₀ → ⁷F₄ (713 nm) and ⁵D₀ → ⁷F₂ (611 nm) respectively. It can be seen that R₁ decreases almost by a factor of 10 upon increasing the dopant concentration from 0.1 mol% to 4%, whereas R₂ is constant, both at cryogenic and room temperature. The latter ratio is constant, because it refers to ⁵D₀ transitions. The transitions of the ⁵D₂ and ⁵D₃ states to the ⁷F_J manifold show a similar behaviour as the ⁵D₁ → ⁷F₁ transition; however, due to the lower spectral radiances the error bar in the graphs is larger and therefore these will not be reproduced here. The partial concentration quenching of the ⁵D₁ transition of the Eu³⁺ ion represented in Fig. 9 is well known and it has been described by Blasse and Grabmaier² and Klaassen et al.³³ Blasse and Grabmaier explained the behaviour of R₁ in terms of the following cross relaxation in Eu³⁺:

⁵D₁ + ⁷F₀ → ⁵D₀ + ⁷F₃. (5)

This cross relaxation is facilitated by the ten times faster decay rate from ⁵D₁ levels as that from ⁵D₀ levels as indicated by Klaassen et al.³³ These latter authors described the partial quenching of ⁵D_J (for J > 0) transitions in terms of radiative decay time and the rate of non-radiative transfer processes. However, neither cross relaxation nor rate constants explain why R₁ is so large in Fig. 9 at low Eu³⁺ concentrations.

For a qualitative explanation we shall make use of the energy diagram depicted in Fig. 10. Most levels indicated in Fig. 10 have been reported by Wen et al.,³⁴ only the lowest energy level of the manifolds is indicated. The energy level of the intrinsic Y₂O₃ emission band has been centred at 28 300 cm⁻¹ above the ground level ⁷F₀ of Eu³⁺, based on the spectrum depicted in Fig. 4. The level of the STE Y₂O₃ emission band overlaps with several Eu³⁺ levels; hence, it seems obvious that energy can radiationless be transferred from an STE in Y₂O₃ to the ⁵D₄ level of Eu³⁺, which is only slightly lower. This process, which takes place at Eu³⁺ concentrations >2 mol%, is indicated by arrow 4. From the ⁵D₄ level energy will radiationless trickle down to the ⁵D₃, ⁵D₂, ⁵D₁ and ⁵D₀ levels, indicated by arrows 7. As mentioned above, the decay times of transitions from ⁵D_J levels with J > 0 are about 10 times shorter than these from ⁵D₀,³³ which enhances efficient energy transfer and hence rather fast quenching of the Y₂O₃ emission upon increasing the Eu³⁺ concentration.^2,35 In terms of the probability S in eqn (3) it means that energy from an e–h pair is transferred to a Eu³⁺ ion.

Fig. 10 Energy levels of Eu³⁺ in Y₂O₃. The broad intrinsic luminescence of Y₂O₃ has been represented at 28 300 cm⁻¹ by arrow (3). The arrows (0), (1) and (2) refer to the ⁵D₀ → ⁷F₂, ⁵D₁ → ⁷F₁ and ⁵D₀ → ⁷F₄ Eu³⁺ transitions respectively, while (4) indicates the radiationless energy transfer from Y₂O₃ to Eu³⁺. For arrows (5), (6) and (7) we refer to text.

At low Eu³⁺ concentrations <2 mol% the strong intrinsic emission at 28 300 cm⁻¹ may be absorbed by Eu³⁺ ions in the lattice: this process is indicated by arrows (5) and (6) in Fig. 10. It has been indicated in this figure that the Y₂O₃ UV radiation is absorbed from the ground level ⁷F₀; hence, the ⁵D₄ Eu³⁺ level will be populated more densely. As mentioned above, from ⁵D₄ energy will trickle down to the lower ⁵D_J levels. The process indicated by arrows (5) and (6) will stop when the intrinsic luminescence of Y₂O₃ has quenched at large Eu³⁺ concentration. The latter process explains the high value of R₁ in Fig. 9 and similarly the rather strong ⁵D₁ → ⁷F_J and ⁵D₂ → ⁷F_J transitions in Fig. 4B–D. The foregoing consideration also explains why the ratio R₁ in Fig. 9 at low temperature is higher than at room temperature: the process indicated by the arrows (5) and (6) is more dominant at low temperature because of the much stronger UV luminescence of Y₂O₃.

4. Conclusions

In the preceding sections we have described the cathodoluminescence of undoped Y₂O₃ and Y₂O₃:Eu³⁺ crystals in the TEM by high-energy electron bombardment. At low temperatures we observed a broad emission band at about 353 nm, which has been ascribed to the migration of excitons to luminescence centres in Y₂O₃. We found that the UV/blue luminescence is strongly temperature dependent and that at concentrations >2 mol% Eu³⁺ no blue light could be detected.

The temperature behaviour of the intrinsic luminescence of Y₂O₃ has been explained with a model for the self-trapped excitons. The depth of these traps was found to be 0.14 eV. The concentration dependence of the UV/blue luminescence has been explained by the good overlap between the level of the blue Y₂O₃ band and the ⁵D₄ level of the Eu³⁺ ion. The strong radiance of ⁵D₁ → ⁷F_J and ⁵D₂ → ⁷F_J Eu³⁺ transitions in the spectra excited at 200 keV in Y₂O₃:Eu³⁺ with low Eu³⁺ concentration has been explained by absorption of the intrinsic Y₂O₃ radiation by Eu³⁺ ions. Although we have presented a detailed explanation of the concentration and temperature behaviour of the UV/blue emission, we have also generated new questions. The most important is about the nature of the red emission bands at 675 nm and 770 nm.

Based on the results described in this article we would like to make a suggestion to other scientists working with electron microscopes in the field of biomedical imaging with phosphors. This fast growing technology has recently been reviewed by Gai et al.³⁶ Since undoped Y₂O₃ or Y₂O₃:Eu³⁺ with low Eu³⁺ concentration will manifest the strong intrinsic luminescence of Y₂O₃ in the e-beam of a TEM (and likely also in a SEM) at low temperatures, we think that this work opens new perspectives for labelling (biological) samples with nanocrystals of Y₂O₃:Eu³⁺.

Acknowledgements

We are grateful to the EPSRC and the Technology Strategy Board (TSB) for funding the PURPOSE (TP11/MFE/6/I/AA129F; EPSRC TS/G000271/1), CONVERTED (JeS no. TS/1003053/1) and PRISM (EP/N508974/1) programs. We are also grateful to the TSB for funding the CONVERT program.

References

1. Phosphor Handbook, ed. W. Yen, S. Shionoya and H. Yamamoto, CRC Press, Boca Raton, 2nd edn, 2007.
2. G. Blasse and B. C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, 1994.
3. L. Ozawa, Cathodoluminescence, Theory and Applications, Kodansha & VCH, Verlag, Tokyo, 1990.
4. G. Li and J. Lin, Chem. Soc. Rev., 2014, 43, 7099.
5. D. den Engelsen, P. G. Harris, T. G. Ireland and J. Silver, ECS J. Solid State Sci. Technol., 2014, 3, R53.
6. K. A. Wickersheim and R. A. Lefever, J. Electrochem. Soc., 1964, 111, 47.
7. A. Fukabori, L. An, A. Ito, V. Chani, K. Kamada, A. Yoshikawa, T. Ikegami and T. Goto, Ceram. Int., 2012, 38, 2119.
8. A. Konrad, U. Herr, R. Tidecks, F. Kummer and K. Samwer, J. Appl. Phys., 2001, 90, 3516.
9. L. G. Jacobsohn, B. L. Bennett, R. E. Muenchausen, J. F. Smith and D. W. Cooke, Proc. SPIE, 2006, 6321, 63210J.
10. R. L. Wood and W. Hayes, J. Phys. C: Solid State Phys., 1982, 15, 7209.
11. T. C. de Oliveira, M. Souza da Silva, L. Menezes de Jesus, D. Vieira Sampaio, J. C. Alves dos Santos, N. R. da Silva Souza and R. Santos da Silva, Ceram. Int., 2014, 40, 16209.
12. P. A. Tanner, L. Fu and B. M. Cheng, J. Phys. Chem. C, 2009, 113, 10773.
13. G. Blasse and L. H. Brixner, Eur. J. Solid State Inorg. Chem., 1991, 28, 767.
14. W. Hayes, M. J. Kane, O. Salminen and A. I. Kuznetsov, J. Phys. C: Solid State Phys., 1984, 17, L383.
15. A. Fukabori, V. Chani, J. Pejchal, K. Kamada, A. Yoshikawa and T. Ikega, Opt. Mater., 2011, 34, 452.
16. Y. Fujimoto, T. Yanagida, Y. Yokota, A. Ikesue and A. Yoshikawa, Opt. Mater., 2011, 34, 448.
17. C. D. Cress, C. S. Redino, B. J. Landi and R. P. Raffaelle, J. Solid State Chem., 2008, 181, 2041.
18. Y. Ato, R. Huzimura and L. Ozawa, Jpn. J. Appl. Phys., 1968, 7, 1497.
19. L. Ozawa, Appl. Phys. Lett., 1977, 31, 694.
20. W. Hayes, J. Lumin., 1984, 31, 99.
21. A. Lushchik, M. Kirm, Ch. Lushchik, I. Martinson and G. Zimmerer, J. Lumin., 2000, 87–89, 232.
22. D. B. Williams and C. B. Carter, Transmission Electron Microscopy, A Textbook for Materials Science, Springer, New York, 2009.
23. D. den Engelsen, P. G. Harris, T. G. Ireland and J. Silver, ECS J. Solid State Sci. Technol., 2015, 4, R1.
24. D. den Engelsen, P. G. Harris, T. G. Ireland, R. Withnall and J. Silver, ECS J. Solid State Sci. Technol., 2013, 2, R201.
25. X. Jing, T. Ireland, C. Gibbons, D. J. Barber, J. Silver, A. Vecht, G. Fern, P. Trogwa and D. C. Morton, J. Electrochem. Soc., 1999, 146, 4654.
26. J. Silver, T. G. Ireland and R. Withnall, J. Electrochem. Soc., 2004, 151, H66.
27. G. R. Fern, A. Lipman, J. Silver, A. Howkins, T. G. Ireland, P. Marsh, D. den Engelsen, Proceedings of 22nd International Display Workshops, December 9–11, Otsu, Japan, 2015.
28. J. V. Jelley, Čerenkov Radiation and its applications, Pergamon Press, London, 1958.
29. Y. Nigara, Jpn. J. Appl. Phys., 1968, 7, 404.
30. D. den Engelsen, P. G. Harris, T. G. Ireland and J. Silver, ECS J. Solid State Sci. Technol., 2015, 4, R1.
31. V. B. Mikhailik and H. Kraus, Phys. Status Solidi B, 2010, 247, 1583.
32. G. Blasse, J. Lumin., 1994, 60–61, 930.
33. D. B. M. Klaasen, R. A. M. van Ham and T. G. M. van Rijn, J. Lumin., 1989, 43, 261.
34. J. Wen, L. Hu, M. Yin and S. Xia, Current Appl. Phys., 2012, 12, 732.
35. D. L. Dexter, J. Chem. Phys., 1953, 21, 836.
36. S. Gai, C. Li, P. Yang and J. Lin, Chem. Rev., 2014, 114, 2342.

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