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Persistent Spectral Hole-Burning in Diffuse Reflection: Application to Nanocrystalline LiGa5O8:Co2þ Hans Riesen* and Baran Yildirim School of Physical, Environmental and Mathematical Sciences, The University of New South Wales, ADFA, Northcott Drive Canberra, ACT 2600 Australia
ABSTRACT Persistent spectral hole-burning of an electronic transition measured in diffuse reflection is reported for the first time. The technique is applied to the electronic origin of the 4A2(F)f4T1(P) transition of a 400 μm thick film of nanocrystalline LiGa5O8:Co(II) (average particle size ≈ 50 nm). For this particular material, observed signal-to-noise ratios are about 16 times better in diffuse reflection compared to hole-burning spectra in luminescence excitation measurements. The possibility of conducting hole-burning experiments by diffuse reflectance spectroscopy enables many possible high-resolution measurements of electronic transitions in opaque nanocrystalline materials and for adsorbed molecules on surfaces. Importantly, it facilitates hole-burning experiments of low- or nonluminescent materials such as nanocrystalline metal-oxide catalysts. SECTION Kinetics, Spectroscopy
O
ptical properties of nanocrystalline materials can be significantly different in comparison to macroscopic single crystals of the same chemical composition, and this has generated a great deal of interest over recent years with potential for a range of applications.1-3 Nanocrystalline materials are often prepared as precipitates, and films or pressed pellets thereof are, in general, opaque. As a consequence, absorption spectra, for example, have to be measured in diffuse reflection unless the powder is suspended in a solvent or a solid matrix with a similar refractive index.4 It is well established that electronic transitions in the solid state suffer from the effect of inhomogeneous broadening; point defects, dislocations, unintentional impurities, isotope distributions, and deviations in bond lengths and bond angles lead to variations of the local field acting on individual optical centers, and this results in a distribution of transition frequencies.5-8 Nanomaterials are prone to lattice defects, as they are often prepared by rapid synthetic methods, e.g., coprecipitation, combustion reactions, and so forth, and their surface-to-volume ratio is very high, i.e., leading to a high number of surface defects per unit volume. Thus, it is not surprising that the inhomogeneous broadening is often significantly larger than in macroscopic crystals, as the latter are carefully grown over long periods of time, resulting in a much lower defect density. Severe inhomogeneous broadening obscures valuable spectroscopic information, and hence laser techniques that can overcome this broadening, such as fluorescence line narrowing or spectral hole-burning,6-9 become even more important for nanomaterials in comparison with well-defined macroscopic crystals. Spectral hole-burning can be based on three mechanisms: photochemical, nonphotochemical, and transient hole-burning.6-8 All three mechanisms are basically frequency selective bleaching, leading to a
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ground-state depletion of optical centers in resonance with the laser light. Spectral holes are usually measured in transmission (absorption) or luminescence (fluorescence, phosphorescence)-excitation mode. For opaque materials, only the latter mode is practical, although diffuse transmission may also be possible for thin films. It can be difficult to conduct spectral hole-burning in luminescence excitation if the luminescence quantum yield is low or if the monitored luminescence is close to or resonant with the laser wavelength. Moreover, many (nano)materials of interest are not luminescent at all and hence do not allow the luminescence excitation readout mode. Diffuse reflectance spectroscopy has been applied to conventional electronic spectroscopy for many years.10 Thus it is somewhat surprising that there is no report of spectral holeburning measured in diffuse reflection of an electronic transition. We note here that some room-temperature CO2 laserbased work has been conducted on silicates and some 150 GHz (5 cm-1) and 1500 GHz (50 cm-1) wide “holes” (spectral changes) were observed in infrared reflection spectra.11 The present paper reports on our observation of holes in the electronic origin of the 4A2(F)f4T1(P) transition in opaque films of nanocrystalline LiGa5O8:Co(II) (0.6% Co(II) concentration) in diffuse reflection spectra. LiGa5O8 exists in two polymorphs that are based on close-packed lattices of oxygen ions with tetrahedral and octahedral interstices.12-14 At temperatures higher than 1138 C, the structure is disordered in the octahedral sites and forms an inverse-spinel system with space group Fd3m. The room-temperature structure displays Received Date: June 13, 2010 Accepted Date: July 13, 2010 Published on Web Date: July 15, 2010
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DOI: 10.1021/jz100806d |J. Phys. Chem. Lett. 2010, 1, 2380–2384
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Figure 1. Representative TEM micrograph of nanocrystalline LiGa5O8:Co2þ prepared by a combustion reaction at 600 C with subsequent annealing at 900 C for 14 h.
1:3 ordering of the lithium and trivalent ions along the [110] direction, and its cubic primitive space group is P4132. We have measured X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra (not graphically illustrated here); from these measurements it follows that a significant percentage of Co ions enter the lattice, indeed, also in the octahedral sites, and that both the þ2 and þ3 oxidation states are present. The visible spectrum of LiGa5O8:Co(II) and the associated deep blue color are caused by the Co2þ ions in tetrahedral geometry as d-d transitions are enhanced by the large odd-parity field, resulting in relatively large oscillator strengths of about 10-3. The lowest-energy ligand field transition in the visible region is the 4A2(F)f4T1(P) interconfigurational transition with an electronic origin at around 660 nm.15-19 Figure 1 shows a representative TEM micrograph of the investigated nanocrystalline LiGa5O8:Co2þ sample. From this micrograph it follows that the average crystallite size is about 50 nm, in accord with an analysis of powder X-ray diffraction patterns.19 Luminescence and diffuse reflection spectra of a 400 μm film of nanocrystalline LiGa5O8:Co2þ at 3 K and room temperature are illustrated in Figure 2. The minor wavelength shift (0.5 nm) of the inhomogeneously broadened electronic origin of the 4A2f4T1 transition at 660 nm, between the diffuse reflection and luminescence spectra, is due to the zero field splittings of the 4A2 and 4T1 multiplets, which are on the order of several wavenumbers.16,17 The inhomogenous width of the electronic origin is about 1200 GHz; this is about 40% larger than the width observed in macroscopic single crystals.17 In accord with high-temperature luminescence spectra,19 the electronic origin is not discernible in room-temperature diffuse reflectance spectroscopy (inset ii) in Figure 2). The electronic origin carries about 10% of the total intensity, which indicates a relatively small displacement of the excited state potential, i.e., relatively low electron-phonon coupling. We note here that there are slight variations in the vibrational sideband compared to bulk crystals. Spectral hole-burning measurements by diffuse reflectance spectroscopy were conducted as is schematically illustrated in Figure 3 and described in the Experimental Methods below. Figure 4 illustrates results for hole-burning in the 4A2f4T1
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Figure 2. Luminescence (red trace) and diffuse reflection (blue trace) spectra of nanocrystalline LiGa5O8:Co2þ (0.6%) in the region of the 4A2f4T1 electronic origin at 3 K. Inset “i” shows the entire luminescence spectrum of the 4T1f4A2 transition at 3 K. The asterisk indicates a transition (R-line) due to a chromium(III) impurity. Inset “ii” shows the diffuse reflection spectrum (blue trace) and its K-M transform (green trace) at room temperature (298 K).
Figure 3. Schematic diagram of experimental setup for spectral hole-burning by diffuse reflectance spectroscopy. The laser is at normal incidence on the sample (90) to minimize specular reflection from reaching the collimating optics and the photo detector.
electronic origin at 661.06 nm of nanocrystalline LiGa5O8: Co2þ. (0.6%). Panels a and d show spectral holes in the electronic origin observed in diffuse reflection employing laser power densities of 6 mW/cm2 and 0.8 mW/cm2, respectively. In diffuse reflection, a spectral hole in an optical transition leads to higher reflectance. For comparison, the same experiment was undertaken in fluorescence-excitation mode as is illustrated in panels c and f of Figure 4. We note here that we have previously reported spectral hole-burning experiments in fluorescence-excitation mode in ref 19, but the present letter is the first report of electronic hole-burning spectroscopy in diffuse reflection. In the case of luminescence excitation, a spectral hole leads to a reduction of luminescence intensity at the frequency of the spectral hole. From a comparison of panel a
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DOI: 10.1021/jz100806d |J. Phys. Chem. Lett. 2010, 1, 2380–2384
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Figure 4. Spectral hole-burning experiments at 3 K and 661.06 nm in the 4A2(F)f4T1(P) electronic origin in diffuse reflection (a,d) and in luminescence excitation (c,f). Panels b and e show the data of panels a and d after applying the K-M function. The spectra in panels a and c were measured in repetitive burn-read mode (800 and 400 μs burn and readout periods) over 120 s with a laser power density of 6 mW/ cm2. The spectra in panels d and f were measured by burning a hole for 50 s with a laser power density of 6 mW/cm2 with subsequent readout in 30 s with a reduced power density of 0.8 mW/cm2. Lorentzian line shape fits are shown as dashed traces.
Thus, in the case of LiGa5O8:Co2þ, the diffuse reflectance spectroscopy renders SNRs that are about 16 times better than for excitation spectra. Panels b and e of Figure 4 show the data of a and d, respectively, after applying the KubelkaMunk (K-M) function. The K-M function is given in eq 3, where R¥ is the reflectance of an infinitely thick layer of the material, and A and S are absorption and scattering factors. The absorption factor is proportional to concentration, whereas the scattering factor is determined by particle size, shape, and packing density, and is, in general, hard to control.
with c and panel d with f, it clearly follows that the signal-tonoise ratio (SNR) is significantly better in the diffuse reflection spectra. If we define the SNR by eq 1, where SP and SB are the signal at the peak and the baseline, respectively, and we calculate the root mean square (RMS) noise by the standard eq 2, we calculate SNRs of 214, 14, 70, and 4 for the spectra shown in panels a, c, d, and f, respectively. SNR ¼
fRMS
SP - SB fRMS
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z T2 1 ¼ ½f ðtÞ2 dt T2 - T1 T1
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ð1Þ
ð2Þ
f ðR¥ Þ ¼
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ð1 - R¥ Þ2 ¼ A=S 2R¥
ð3Þ
DOI: 10.1021/jz100806d |J. Phys. Chem. Lett. 2010, 1, 2380–2384
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Although the K-M function assumes an infinitely thick absorbing layer, the 400 μm film of nanocrystalline LiGa5O9:Co2þ seems to be deep enough to yield consistent results. The apparent hole-depth observed in diffuse reflection spectra seems to be about 3 times as large as the corresponding one in excitation mode. This is most likely due to the fact that the two spectroscopic modes are probing different volumes of the film. However, it may also be due to some residual pickup of laser light in the luminescence excitation measurement. The former explanation is somewhat supported by the observation of slightly larger hole-widths in diffuse reflectance (1.7 GHz) compared with the excitation measurement (1.4 GHz). We note here that Macfarlane and Vial observed a hole-width of about 1.3 GHz for a 10% deep hole in a single crystal of LiGa5O8:Co2þ. The spectral holes read out by diffuse reflectance spectroscopy appear to be slightly asymmetric. This could be based on a minor but systematic baseline correction problem; however, it may also be due to some pickup of specular reflection. More work is needed to rule the latter out. An apparent minor spectral feature at 6-7 GHz (panel a) is definitley due to baseline problems since sideholes have to occur symmetrically on both sides of the central hole for samples with such a large inhomogeneous width. The hole-burning mechanism is based on a two-photon ionization process with Co3þ in the neighborhood acting as electron traps.17,19 Spontaneous hole-filling appears to be faster in the nanocrystalline sample (shorter recombination length) compared to results for macroscopic single crystals.19 This points to the fact that spectral diffusion is also faster in the nanomaterial. We note here that a relatively high luminescence quantum efficiency (QE) of about 10% has been reported for bulk LiGa5O8:Co2þ crystals.17 We have measured the excited state lifetime of nanocrystalline LiGa5O8:Co2þ and found τ = 90 ( 20 ns, compared to 200 ns for macroscopic crystals. This reduction is possibly caused by surface modes that accelerate nonradiative relaxation. Hence, the QE of the nanocrystalline material is still on the order of 5%. Despite this relatively high QE, the holeburning spectra in diffuse reflection are significantly better than in luminescence excitation mode; diffuse reflection holeburning will be even more important for low QE materials. Importantly, the diffuse reflection mode may enable holeburning experiments of nonluminescent materials that cannot be measured in luminescence excitation mode. For example, metal-ion catalysts on roughened oxide surfaces could be investigated in diffuse reflection.
refrigerator Janis/Sumitomo SHI-4.5. Luminescence spectra were measured by employing a Spex 1404 monochromator equipped with a 1200 gr/mm 750 nm blazed grating and a Hamamatsu R928 photomultiplier. A 10 mW Nd:YAG laser, attenuated to
