Near infrared Er3+ photoluminescence from erbium metallofullerenes

27 February 1998

Chemical Physics Letters 284 Ž1998. 171–176

Near infrared Er 3q photoluminescence from erbium metallofullerenes K.R. Hoffman a , B.J. Norris a , R.B. Merle a , M. Alford a b

b

Department of Physics, Whitman College, Walla Walla, WA 99362, USA TDA Research Inc., 12345 West 52nd AÕe., Wheat Ridge, CO 80033, USA Received 5 December 1996; in final form 15 August 1997

Abstract Near infrared photoluminescence spectra of Er 3q have been measured in samples containing erbium metallofullerenes. The structured emission lines in the region of 1.52 microns are typical of Er 3q emission. Mass spectrometry measurements on the samples reveal the presence of erbium metallofullerenes in the samples. Possible sources of the observed signal are discussed in terms of specific erbium metallofullerenes identified in the mass spectrum. In addition, differences between the emission spectra of samples generated from raw soot and toluene extract are presented and discussed. q 1998 Elsevier Science B.V.

1. Introduction Lanthano-metallofullerenes consist of a rare earth atom encapsulated in a fullerene cage w1,2x. The most commonly studied lanthano-metallofullerenes have Sc, La and Y as the dopant atoms, although endohedral forms of most rare earth atoms have been generated w3–5x. Progress in understanding these materials has been impeded by the difficulty in isolating metallofullerenes from empty fullerenes. Multistage high performance liquid chromatography Žhplc. w6x has produced small quantities of high purity samples, however, this technique is limited to soluble metallofullerenes which are only a subset of all metallofullerenes present in the soot w7x. Determining the structure of metallofullerenes has been a major focus of studies on these molecules. In fact, only recently has the endohedral nature of metallofullerenes been confirmed directly using X-ray diffraction w8x. ESR measurements of metallofullerenes indicated that

ionization of the trapped atom occurs with the electrons transferring to the cage w9–12x. Theoretical considerations of La@C 82 supported this conclusion and placed the ion off to one side of the cage w13–15x. Similarly, multiple endohedral ions, such as La 2 @C 80 and Sc 3 @C 82 , donate up to six valence electrons to the cage w16–19x. The resulting picture is that endohedral lanthanide ions obtain their preferred ionization by donating valence electrons to the fullerene cage. UV–VIS absorption measurements have been made on the few metallofullerenes isolated by hplc. Incorporation of a single, trivalent ion in C 82 induces a broad absorption band peaking at 1400 nm and extending down to 2300 nm w11,12,20,21x. This absorption feature is attributed to the unpaired electron in a half filled p molecular orbital. When the trapped ions donate 6 electrons to the cage, as in Sc 3 @C 82 , absorption does not occur below 1100 nm w22x. With six electrons on C 82 the HOMO is filled w18,19x

0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 7 . 0 1 4 1 9 - X

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resulting in a larger energy separation between the LUMO and HOMO. All metallofullerenes exhibit moderate to strong absorption of visible light. Optical transitions of trivalent rare earth ions are well characterized and have long been used as optical probes w23x. Because the 4f orbitals are shielded by the 5s and 5p electrons, only small changes in the optical spectra of rare earth ions occur as a result of crystal or ligand fields. Consequently, the presence of trivalent rare earth ions inside metallofullerenes can be determined optically. Furthermore, changes in the obtained spectra may enable different metallofullerenes to be distinguished by their characteristic emission. We previously reported photoluminescence ŽPL. spectra which we attributed to Er 3q in erbium doped metallofullerenes w24x. We present here a more complete study in which the presence of endohedral erbium fullerenes has been directly measured using mass spectrometry. Based on the metallofullerenes identified in the mass spectrometric data and absorption measurements, we suggest a possible source of the observed signal. In addition, we compare the emission spectra from a variety of sample types. We discuss variations in the emission spectra from films grown by sublimation and evaporation.

2. Experimental details The fullerenes used in these experiments were generated using the carbon arc method w25x. The rods consumed in the arc were packed with a mixture of graphite and Er2 O 3 . A significant improvement in yield was achieved by including a binder to prevent the mixed powders from being blown out of the reaction region before ionization. To remove contaminants, the rods were heated in an evacuated chamber Ž1 = 10y6 T. for one hour prior to placing them in the arc chamber. Typical parameters that led to successful metallofullerene production include: 150 T ambient He pressure in the reaction chamber, a 22 V DC arc at 100 A, and an Er to C ratio of 1 to 60. Fullerenes and metallofullerenes were extracted by washing the soot with toluene through filter paper. The samples used for optical measurements were prepared using two different procedures. Evaporated samples were produced by washing the soot through

a filter with toluene and subsequently drying the toluene extract on a glass slide w21x. The resulting aggregate was dark brown and of poor optical quality; these samples are essentially a powder attached to a glass slide. Uniform thin films of higher quality were produced by sublimation. We utilized both dried toluene extract and raw soot as starting material for the sublimated films. Sublimation was achieved in an evacuated chamber Ž1 = 10y6 T. containing an electrically heated molybdenum boat ŽRD Mathis., a shutter and a substrate. Initially the boat temperature was set to 4008C for at least one hour to bake contaminants out of the soot. The films were grown by opening the shutter and slowly raising the temperature through some range of values. A typical range for our films was 450–8008C. The substrate temperature was not controlled but some heating occurred due to its proximity to the boat. PL from the samples was excited by the 488 nm line from a chopped cw argon ion laser ŽLexel.. The emission spectrum was measured by a 0.5 m monochrometer ŽChromex. and detected by a cooled Ge detector ŽNorth Coast Scientific.. A lock-in amplifier ŽStanford Research. was utilized to extract the signal and the data was stored on a personal computer. The samples were cooled in a He flow cryostat ŽCryo industries. permitting measurements down to 20 K. Time-of-flight mass spectrometry ŽTOFMS. measurements utilized a two laser desorptionrionization method. Laser desorption was accomplished by using low power Ž- 100 mJ, 1 mm spot. pulses of 532 nm light from a Nd : YAG laser. Ionization of the neutral species desorbed from the sample was accomplished using 193 nmŽ; 1 mJrcm2 . light from an ArF excimer laser. The ions were extracted from the ionization zone using a delayed voltage pulse and mass analyzed with a reflection type spectrometer.

3. Results and discussion TOFMS measurements on toluene extracted fullerenes produced in our reaction chamber are presented in Fig. 1. The labels identify the predominant fullerenes and metallofullerenes in the toluene extract. The relative peak areas suggest that Er2 @C 82

K.R. Hoffman et al.r Chemical Physics Letters 284 (1998) 171–176

Fig. 1. LD-TOF measurements of toluene extract from soot produced by arcing erbium packed graphite rods. Erbium metallofullerenes are identified in the spectrum.

is the most abundant metallofullerene in the sample. This effect appears to be only weakly dependent of the metalrcarbon ratio and has also been observed for Ho metallofullerenes w26x. Focusing on erbium metallofullerenes only, the relative peak areas indicate that the Er2 @C 82 concentration exceeds that of the next most abundant metallofullerene by a factor of 20. Interpretation of the absolute concentrations of the empty and metallofullerenes shown in Fig. 1 is difficult owing to the large difference in ionization potentials between the two species. Empty fullerenes, with an IP of roughly 7.6 eV for C 60 , require two 193 nm Ž6.42 eV. photons for ionization. The IP of La@C n Ž n s 60, 74 and 82. has been determined by gas phase charge exchange reactions w27x to be between 6.2 and 6.4 eV and requires only one 193 nm photon for ionization. The Er metallofullerenes should have IP’s similar to La containing fullerenes. Assuming similar adsorption cross sections, the difference between the linear one photon and quadratic two photon ionization processes results in a strong enhancement of the metallofullerene signal. Experiments designed to correct for the enhancement suggest that the Er2 @C 82 concentration shown in Fig. 1 is comparable to the higher empty fullerenes such as C 84 w28x. Low temperature PL spectra from evaporated and sublimated films are shown in Fig. 2. Note: both samples were made from the same toluene extract.

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The measured spectrum from the evaporated sample possesses sharp lines superimposed on a broad shoulder extending from 1500 nm to 1650 nm. The strongest line appears at 1520 nm with additional peaks at 1526, 1545 and 1600 nm. Erbium emission from the sublimated film is very weak owing to the small amount of material available to form the film. Nevertheless, a single peak appears at 1520 nm, coincident with the strongest emission line from the evaporated sample. The linewidth of the emission from the sublimated film is noticeably smaller than that observed in the evaporated film: 6 cmy1 compared to 25 cmy1 . The difference suggests sample quality can effect the inhomogeneous broadening of the erbium centers. The broad nature of the emission from the evaporated sample obscures details of the emission spectrum. Repeated measurements on several samples suggest a grouping of certain lines in the spectrum. Using the lines observed in Ref. w24x as a starting point, fits to the overall lineshape can be made. We obtain narrow linewidths, ; 25 cmy1 , for the features appearing at 1520, 1526 and 1545 nm. The rest of the lines appear to be broader by a factor of two or more. We attribute the three narrow lines to transitions terminating on three stark components of the Er 3q ground state. The broader features at lower energy are likely a superposition of electronic and vibrational transitions because we see more than eight lines which is the maximum number of nondegenerate states for the 4 I 15r2 ground state.

Fig. 2. PL spectra, measured at 30 K, from sublimated and evaporated films using the same toluene extract as the starting material. Similar pump laser intensities were utilized in both measurements.

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Fig. 3. A diagram of the energy dynamics possibly occurring in Er2 @C 82 and Er@C 82 . ŽA. shows the case where the HOMOLUMO gap of the host molecule is larger than the ground excited state splitting of the Er 3q ion. ŽB. shows the likely dynamics when the HOMO-LUMO gap is smaller than the ground-excited state splitting.

Lacking the facilities to isolate metallofullerene species we turn to other considerations for indirect clues to the source of the observed signal. The observation of emission from trapped erbium requires the LUMO–HOMO splitting of the host fullerene be larger than the ground-excited electronic state splitting of Er 3q Ž; 6500 cmy1 .. If the LUMO–HOMO splitting is smaller, then an excited erbium ion can relax by transferring energy back to the fullerene cage without emitting a photon Žsee Fig. 3B.. This nonradiative decay process would likely quench any erbium emission. Furthermore, the host fullerene likely acts as a sensitizer for the enclosed erbium ions. If a cage electron is excited it will rapidly decay nonradiatively to the LUMO state. If this lies below the excited state of the erbium ions then energy transfer to the trapped ions will not occur at low temperatures. Measurement of the LUMO–HOMO splitting in metallofullerene hosts will permit elimination of systems that exhibit absorption at energies below ; 6500 cmy1 . Assuming Er@C 82 behaves like other M@C 82 systems described in the introduction ŽM-rare earth., y1 . Ž the LUMO energy of the Cy3 cage lies 82 5000 cm well below the first excited state energy of Er 3q. As 3y a result, no energy transfer from the C 82 cage to the trapped erbium ion will occur at low temperature. In addition, if energy transfer to the trapped ion oc-

curred from a higher molecular orbital, nonradiative relaxation back to the cage would quench the luminescence. These processes are depicted in Fig. 3B. 6y Conversely, the LUMO energy of C 82 , as measured y1 in Sc 3 @C 82 Ž9000 cm ., lies well above the first excited state of Er 3q. Therefore energy transfer from 6y the excited C 82 cage to one of the enclosed erbium ions in Er2 @C 82 can occur. Possible energy transfer processes for this system are shown in Fig. 3A. The large splitting also decreases the likelihood of backtransfer from the erbium ion to the fullerene. Higher erbium metallofullerenes Ž2 n ) 82. also possess filled p molecular orbitals making it possible for emission from the trapped ions to be observed. We are not aware of any absorption measurements of these molecules so LUMO–HOMO gap is unknown. However, for empty fullerenes the LUMO–HOMO splitting decreases as the cage size increases. Assuming this trend applies to metallofullerenes as well suggests that the LUMO–HOMO splitting would decrease as 2 n increases. As the LUMO–HOMO splitting decreases, nonradiative decay processes due to back-transfer of energy from the excited erbium ions to the host should become more effective, thereby decreasing the emission efficiency. Based on these arguments erbium ions should emit more efficiently in Er2 @C 82 than in the higher metallofullerenes. We attribute the observed emission to transitions between the 4 I 13r2 and 4 I 15r2 stark levels of Er 3q ions in Er2 @C 82 molecules. The arguments presented above favor this molecule as the source. While some higher metallofullerenes could also contribute to the emission signal, we expect them to be less efficient emission sources compared to Er2 @C 82 . Also, the relative concentration of the metallofullerenes suggests that Er2 @C 82 emission will dominate other possible emitters. Recently, luminescence measurements from isolated Er2 @C 82 metallofullerenes have been observed in powder samples w29x. The observed spectra exhibit a strong peak at 1520 nm and possess an overall line shape similar to our data. During the revision of this manuscript another group published results demonstrating emission from Er2 @C 82 w30x. The position and lineshape of these results are similar to those described here. Both results provide further support for our assignment of the observed emission.

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emission and are independent of the fullerenes. It is possible that a smaller, insoluble mono- or di-metallofullerene is responsible for the observed spectrum. If true, the technique would permit direct studies of metallofullerenes previously inaccessible for characterization. Further studies are required to elucidate this question.

4. Conclusions

Fig. 4. PL spectrum, measured at 30 K, from a film produced by sublimating raw soot directly. The inset spectra compare higher resolution scans of two different films showing some variation between samples.

To increase the number of metallofullerenes in our sublimated film, we used raw soot as the starting material. The partial selectivity of the extraction process is lost in this procedure, however, a much larger amount of starting material was available. Fig. 4 shows an emission spectrum from one of these films. The better quality of the sublimated film is reflected in the narrow emission lines, ; 5 cmy1 FWHM. More interesting is the wavelength shift of the prominent features in the spectrum. The strong line appears at 1511 nm compared to 1520 for the toluene extracted samples. While the overall shape of the emission spectrum is qualitatively similar, details of the spectrum are different. There appear to be fewer lines in the case of the sublimated film. In addition, the separation between some of the observed lines is altered. Evidently, the erbium centers in the film occupy slightly different sites than those ions emitting light from the powder. Interpretation of the lines obseved in these samples is hindered by not knowing what is present on the film. Because metallofullerenes as well as other complexes are present in the soot, the source of the emission from the film is unclear. Measurements made by Macfarlane et al. w30x suggest that the local environment does not effect the emission lines of Er2 @C 82 . Carrying this argument to our films suggests that Er2 @C 82 is not the source. We have ruled out Er2 O 3 as the source, however, other erbium complexes could also form in the reactor that lead to

We measured luminescence from samples containing erbium metallofullerenes which correspond to transitions between the 4 I 13r2 and 4 I 15r2 stark levels of Er 3q. Samples obtained by extracting metallofullerenes with toluene exhibited relatively broad emission lines with the major feature appearing at 1520 nm. From films sublimated directly from soot the emission lines are shifted to higher energy. In both cases we attribute the emission signal to Er 3q ions in Er2 @C 82 molecules. We attribute the differences between the emission spectra to the presence of toluene adducts on samples made from toluene extract. These results demonstrate the utility of optical measurements for characterizing metallofullerenes. We can directly determine if the rare earth attains the 3 q valence. Furthermore, the well defined energy levels of the erbium ions permit different centers in our samples to be distinguished. Our results suggest that emission spectra from derivitized metallofullerenes are distinguishable from the isolated molecule. Therefore, properties such as reaction rates and stability of metallofullerene derivatives can be studied.

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