APPLIED PHYSICS LETTERS 87, 131909 共2005兲
Infrared-to-visible upconversion in thin films of LaEr„MoO4…3 D. M. Bubba兲 Department of Physics, Rutgers University-Camden, Camden, New Jersey 08102 and Naval Research Laboratory, Code 6365, Washington, DC 20375
D. Cohen Department of Physics, Seton Hall University, South Orange, New Jersey 07079
S. B. Qadri Naval Research Laboratory, Code 6372, Washington, DC 20375
共Received 25 April 2005; accepted 3 August 2005; published online 22 September 2005兲 LaEr共MoO4兲3 thin films have been grown by pulsed laser deposition. The films were characterized by x-ray diffraction, Rutherford backscattering, and fluorescence measurements. The results show that the deposited films were epitaxial with their c axis oriented along the surface normal. Films illuminated with 980 nm laser light show visible emission spectra. This visible emission arises as a result of the Er 4f − 4f transitions and their lifetimes. Such so-called “upconverting phosphors” are important to the development of new chemical and biological sensing applications. © 2005 American Institute of Physics. 关DOI: 10.1063/1.2067712兴 Upconverting phosphors have received recent attention as promising materials for biomedical and other applications.1,2 In general, materials that contain erbium are candidates for upconversion as they can have dipole forbidden 4f − 4f transition become allowed due to asymmetry in the crystal field. This level-splitting results in a large number of possible transitions from the near infrared to the ultraviolet 共1500–380 nm兲. LaEr共MoO4兲3 is a ferroelectric material with interesting optical properties.3 In this letter, we describe the deposition and characterization of LaEr共MoO4兲3 thin films that, when exposed to 980 nm laser light, emit green and red light. The 980 nm absorption line 共 4I15/2 → 4I11/2兲 has a sufficiently long lifetime that a “1 + 1” scheme can be envisioned where the absorption of two infrared photons leads to the emission of visible photons. As noted in Ref. 3, it is extremely difficult to grow single crystals of LaEr共MoO4兲3 of sufficient quality for optical applications. The crystals grown in that study were cloudy and had visible imperfections. Therefore, we attempted to stabilize LaEr共MoO4兲3 by vapor phase epitaxy during pulsed laser deposition. This physical vapor deposition technique has been a successful tool in the past for depositing metastable materials such as cubic4 NbN and rhombohedral5 Ba1−xSrxTiO3. The thin films were grown utilizing a KrF excimer laser 共248 nm兲 that was focused to reach a fluence of ⬃2 J / cm2 and incident upon a rotating target of LaEr共MoO4兲3. The substrates were 1 cm2 pieces of 共100兲 oriented yttriastabilized zirconate wafers. Films were deposited in a partial pressure of 350 mT O2 gas and the substrate temperature was 660 °C. Films that were deposited for 20 000 laser pulses were estimated to be about 6400-Å-thick by Rutherford backscattering and step-edge profilometry. Correspondingly, the deposition rate was determined to be about 0.3 Å per shot, which is typical for an oxide material when using a dense target.
Films were characterized for structure and composition using x-ray diffraction and Rutherford backscattering. For the x-ray diffraction measurements, a Huber four circle diffractometer using Cu K␣1 radiation in triple axis geometry was used. The monochromator and analyzer are very high quality Ge共1,1,1兲 crystals. In Fig. 1, the diffractograms are shown. To the right of every substrate peak, we find a corresponding film peak. The combination of in plane and out of plane measurements has led us to conclude that the film is registering epitaxially with the substrate and is highly oriented along the c axis. No extra peaks were seen in the x-ray diffraction scans indicating that the film was only in the single phase with orthorhombic symmetry. Based on the observed reflections the space group is consistent with Pba2. The calculated lattice parameters for this phase are a = 10.494, b = 10.541, and c = 11.636 Å. The in-plane lattice parameters are very close to bulk 共⬃10.5 Å兲, but the c axis lattice parameter is significantly larger 共bulk ⬃10.8 Å兲, probably due to a combination of strain induced by the mis-
a兲
FIG. 1. X-ray diffraction results for a LaEr共MoO4兲3 thin film deposited by pulsed laser deposition.
Author to whom correspondence should be addressed; electronic mail:
[email protected]
0003-6951/2005/87共13兲/131909/3/$22.50 87, 131909-1 © 2005 American Institute of Physics Downloaded 19 Oct 2005 to 165.230.100.195. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
131909-2
Appl. Phys. Lett. 87, 131909 共2005兲
Bubb, Cohen, and Qadri
FIG. 2. Excitation and emission scans for a LaEr共MoO4兲3 thin film.
match in lattice parameters between the thin film and yttriastabilized zirconate along with oxygen deficiency, which is a typical occurrence in thin films of oxides. The Rutherford backscattering results suggested a slight erbium excess in the deposited film in comparison with lanthanum 共⬃1.05/ 1兲, but the La/ Mo and Er/ Mo ratios were very near 3. When coupled with the x-ray diffraction results, this suggests that the film is single phase with a composition that is close to the target material. In addition to yttriastabilized zirconate, a number of other substrates were attempted, including MgO共100兲, Gd3Ga5O12 , SrTiO3, Si共100兲 and 共111兲, and others. We were unsuccessful in growing thin films on top of these other materials thus suggesting that it is necessary for the LaEr共MoO4兲3 films to be stabilized by epitaxy or it will not be deposited at all. Fluorescence spectra were recorded for laser-deposited thin films in a Jobin Yvon Spex FL1T1-TAU 2 spectrometer which allowed for independent control of the excitation and emission monochromator. The emission and excitation scans are very similar to the results in Ref. 1 and are shown in Fig. 2. There is a peak in the excitation scan at 379 nm which coincides with the 4I15/2 to 4G11/2 transition.6 The main peaks in the emission spectrum can be assigned to the 2H11/2 and 4 S3/2 to 4I15/2 transitions. Additionally, a weak peak is seen at around 650 nm that corresponds to the 4F9/2 to 4I15/2 transition.1 As expected, the fluorescence spectrum is completely dominated by the 4f − 4f transitions. In Fig. 3, we display an upconversion fluorescence spectrum recorded during excitation at 980 nm 共optical parametric oscillator −7 pulse width兲 of the thin films of LaEr共MnO4兲3 using an Ocean Optics fiber-coupled spectrometer in the range 200–800 nm. The excitation laser was operated at a repetition rate of 15 Hz with a pulse energy of about 2.3 mJ so that the average power was approximately equal to 35 mW. The excitation beam was weakly focused on the surface of the sample and an infrared filter was used to avoid contamination of the spectrum. A similar spectrum was obtained with a 200 mW 980 nm cw laser diode as well. In the inset of Fig. 3, a schematic of the relevant energy levels is shown. After two successive absorptions from the ground state, the 4F7/2 level is reached. There are both radiative and nonradiative channels for relaxation to the three levels which ultimately produce the visible fluorescence 共2H11/2, 4S3/2, and
FIG. 3. Upconversion spectrum for the same film when excited by a 980 nm laser. The inset has a schematic which shows the relevant energy levels. A similar diagram appears in Ref. 1.
F9/2兲. Erbium also fluoresces in the midinfrared7 and there are nonradiative decay channels as well6 that could be at work here. Finally, the wavelength dependence of the upconversion intensity at 548 nm is measured for the range 965–995 nm for the excitation beam in 1 nm increments. The profile has a full width at half maximum of about 15 nm and coincides with the 4I15/2 → 4I13/2 transition as one might expect. There is a strong enhancement at 980 nm where the absorption coefficient is at a maximum in this region. This result, displayed in Fig. 4, demonstrates the resonant nature of the upconversion. Finally, in the inset of Fig. 4, the intensity of emitted green light 共548 nm兲 is plotted against the square of the exciting laser power at 980 nm. The linear dependence of emission intensity on the square of the excitation laser power confirms that a two photon process is responsible for the fluorescence. Thin films of LaEr共MoO4兲3 have been deposited by pulsed laser deposition. The films are single phase and epitaxial. They fluoresce strongly and the excitation and emission scans are very similar to that observed for related ma4
FIG. 4. Dependence of the upconversion intensity on the excitation wavelength. The inset shows the dependence of the green emission intensity upon the infrared laser power. Downloaded 19 Oct 2005 to 165.230.100.195. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
131909-3
Appl. Phys. Lett. 87, 131909 共2005兲
Bubb, Cohen, and Qadri
terials. When excited by a 980 nm laser, the films upconvert the infrared radiation to green and red emission in a 1 + 1 scheme. These results are especially pertinent to biological and chemical sensing applications. D.M.B. would like to thank J. M. Joseph, W. R. Murphy, S. P. Kelty, and J. Hanson for helpful discussions and use of equipment. This work has been supported by the National Science Foundation Award No. DMI-0323621 and by a Cottrell College award from the Research Corporation. The authors also thank David Knies for performing the Rutherford backscattering measurements.
1
G. Yi, B. Sun, F. Yang, D. Chen, Y. Zhou, and J. Cheng, Chem. Mater. 14, 2910 共2002兲. 2 D. A. Zarling, M. J. Rossi, N. A. Peppers, J. Kane, G. W. Faris, M. J. Dyer, S. Y. Ng, and L. V. Schneider, U.S. Patent No. 6,537,829 共25 March 2003兲. 3 G. A. Thomas, D. M. Bubb, T.-Y. Koo, D. J. Werder, S.-W. Cheong, and J. F. Federici, J. Appl. Phys. 90, 2678 共2001兲. 4 R. E. Treece, M. S. Osofsky, E. F. Skelton, S. B. Qadri, J. S. Horwitz, and D. B. Chrisey, Phys. Rev. B 51, 9356 共1995兲. 5 D. M. Bubb, S. B. Qadri, J. S. Horwitz, D. Knies, and D. M. Potrepka, Appl. Surf. Sci. 223, 275 共2004兲. 6 D. M. Gill, L McCaughan, and J. C. Wright, Phys. Rev. B 53, 2334 共1996兲. 7 S. M. Kirkpatrick, L. B. Shaw, S. R. Bowman, S. Searles, B. J. Feldman, and J. Ganem, Opt. Express 1, 78 共1997兲.
Downloaded 19 Oct 2005 to 165.230.100.195. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
