ARTICLE pubs.acs.org/JPCC
Optimization of Sol Gel-Formed ZnO:Al Processing Parameters by Observation of Dopant Ion Location Using Solid-State 27Al NMR Spectrometry Tim Kemmitt,*,†,‡ Bridget Ingham,†,‡ and Rachael Linklater†,§ †
Industrial Research Ltd., PO Box 31-310, Lower Hutt 5040, New Zealand The MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, Wellington, New Zealand § Department of Chemistry, University of Waikato, Hamilton, New Zealand ‡
bS Supporting Information ABSTRACT: We report the discrimination of Al doping sites in sol gel-formed ZnO powders by solid-state 27Al nuclear magnetic resonance (NMR) spectrometry. A degree of control of dopant placement is demonstrated by modifying sol precursors and processing parameters. ZnO powders containing 1 8 at. % aluminum ions were prepared from aqueous citrate-amino-alcohol-based gels calcined at 500 °C. The powders were characterized using 27Al NMR spectrometry, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Solid-state 27Al NMR spectrometry clearly distinguished between different Al environments and was effective in determining the relative amounts of incorporation of Al dopant ions into the Zn lattice sites in the zincite structure. This allowed a synthesis protocol to be developed to optimize the doping effectiveness. Relatively minor variations in processing conditions could influence the degree and mode of Al incorporation. Optimal conditions were found to include a 90 °C drying step, followed by placement in a preheated (500 °C) furnace for 1 h. An ethanolamine-containing precursor was shown to perform much better than precursors containing alternative amino alcohols.
’ INTRODUCTION Studies on the properties of zinc oxide (ZnO) have been well-reviewed,1 3 with many applications proposed such as thin-film solar cells1 and optoelectronic materials.3 The performance of doped ZnO as a transparent conducting oxide (TCO) prepared using techniques such as magnetron sputtering, chemical vapor deposition, and pulsed laser deposition has provided a solid body of knowledge.1 Zn is cheap and abundant, making doped ZnO a favored candidate to replace TCOs based on indium tin oxide (ITO) for thin-film photovoltaic applications. Typical dopants used to prepare ZnO-based TCOs include F, B, Al, Ga, In, and Sn. The sol gel route has been proposed as a cost-effective alternative to vacuumassisted deposition of films for large-area TCOs, and a number of studies have examined the influence of processing details (heating profile, temperature, solvents, etc.) on film properties and performance.4 20 However, despite the identification of optimum processing conditions, there is limited understanding as to how and why the processing variables influence electrical properties such as resistivity. Previous studies have shown, for example, that the texturing and resistivity of solutionprocessed films are sensitive to both the concentration5 and heating profiles used to calcine the films.6,7 Rapid thermal annealing appears to result in highly (002)-oriented films with low resistivities,7,8 agreeing with other observations that higher r 2011 American Chemical Society
conductivities and transmittances occur in preferred (002)oriented films.10 However, other studies appear to suggest that more random orientations are preferred.11 There is general agreement that ZnO:Al TCOs with low resistivities appear to require around 1 at. % added Al dopant and postsynthesis annealing at ca. 400 500 °C.8 14 Estimates of lattice deformation made from X-ray diffraction (XRD) data using Bragg’s equation to attempt to quantify the effective dopant concentration suggest that the effective Al concentration is much lower than the added Al concentration.12 A dopant ion introduced to modify the electronic properties of a material needs to be incorporated into the crystal structure of the host material (either in lattice sites or interstitially). In the case of a ZnO:Al TCO, the Al3+ ion is required to occupy a Zn2+ lattice site in order to provide a free electron (charge carrier) and enhance the conductivity of the ZnO. A simplistic representation is shown, 1, in which an Al3+ ion occupies the site of a Zn2+ ion, producing a charged defect.14 A quantum chemical approach has been applied to calculate the structural, electrical, and electronic properties of ZnO due to the Al doping and explains the increase in the n-type electrical conductivity.15 To Received: May 8, 2011 Revised: June 28, 2011 Published: June 29, 2011 15031
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The Journal of Physical Chemistry C attain optimum performance, the material would be expected to incorporate a uniform and homogeneous distribution of dopant ions in the correct locations, up to a limiting concentration. A concentration that is too high could lead to dopant ions occupying adjacent or proximal lattice sites as in 2 causing ion vacancies, resulting in a neutral defect that would not contribute to conductivity.14
In the hexagonal close-packed lattice of the ZnO wurtzite structure, half of the tetrahedral sites and all of the octahedral sites are empty, providing further possible dopant sites. The Al3+ ion is smaller than the Zn2+ ion and could easily be accommodated in either site; however, taking into account geometry preference rules, one would expect a preference of Al for the octahedral site.21 Occupation of these interstitial sites requires that the Al ion behave as an acceptor, which would decrease conductivity.22 Unincorporated dopant could also reside in a nonconductive intergrain layer, resulting in electrical isolation of the individual ZnO crystallites. High-temperature (>800 °C) processing of Al-doped ZnO has previously been demonstrated to result in the ready formation of intergrain impurity phases.23 The importance of the dopant ion location for the electrical and electronic properties of ZnO led us to attempt to more accurately track the fate of the Al dopant ions and to determine whether the proportion of Al ions that are actually incorporated into the appropriate sites of the crystalline ZnO can be more accurately quantified. Our interest is in developing a specific understanding of the critical differences in materials prepared using different thermal processing and solution chemistries and to examine the ability to influence properties in a controlled way by varying processing conditions. Herein, we report a systematic study of the formation of Al-doped ZnO powders using solidstate 27Al NMR spectrometry in conjunction with XRD and scanning electron microscopy (SEM) to design a synthesis protocol aimed at maximizing Al occupation of the zincite tetrahedral sites.
’ EXPERIMENTAL SECTION Synthesis of ZnO Powders. All reagents were used as received. Al-doped zinc oxide powders were produced by a sol gel method utilizing zinc citrate prepared from the reaction of aqueous zinc acetate (Scharlau) with 1 mol equiv of citric acid (Scharlau). The liberated acetic acid was removed by repeated rotary evaporation of the samples to dryness at 80 °C, followed by redissolution or resuspension of the residue in distilled water. Then, 2 mol equiv of an aminoethanol chosen from methyl diethanolamine (MDEA), diethanolamine (DEA), aminoethoxyethanol (AEE), propanolamine (PA), and ethanolamine (EA) (Acros) was added to the citrate to solubilize the zinc citrate salt in water. In a related study on thin-film formation, it was reported18 that using EA or DEA as the amino alcohol in the precursor solution had an impact on the crystallographic orientation of the film. An aluminum citrate solution was prepared from aluminum sec-butoxide
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(Sigma) and anhydrous citric acid (Applichem) (1: 1 ratio) in isopropanol. Removal of the solvents under reduced pressure was carried out by rotary evaporation, and the solid residue was redissolved in distilled water to prepare a standard solution. Aliquots were added to the Zn solutions to form mixtures containing Al in concentrations of 0, 1, 2, 4, and 8 mol % with respect to Zn. The Zn concentration of the solutions was standardized at 0.5 mol L 1. Aliquots of the solutions were placed in shallow crucibles and thermally processed by gelation and calcination to form ZnO:Al powders. A preliminary drying step carried out at between 70 and 160 °C in an oven was followed by heating in a furnace to 500 °C, either by ramping the temperature at a heating rate of 5 °C min 1 and then holding for 1 h or by placing the sample in a preheated oven at 500 °C and holding for 1 h. Characterization. Laboratory XRD data were collected on a Philips PW3700 series diffractometer using Co KR radiation (λ = 1.7889 Å). Synchrotron XRD data were collected at the Powder Diffraction beamline of the Australian Synchrotron. Finely ground powder samples were loaded into 0.3-mm borosilicate glass capillaries. The X-ray beam energy was 16 keV (λ = 0.77442 Å, refined using LaB6 660b NIST standard powder), and the beam size was 3 0.5 mm (h v). Data were collected using a Mythen detector spanning 80° of arc. The crystallite sizes and unit cell dimensions of the zincite formed were determined using TOPAS XRD software (Bruker). Solid-state 27Al MAS NMR data were collected at 11.7 T [Bruker Avance 500 spectrometer with 4-mm Doty magic-angle-spinning (MAS) probe spun at 10 12 kHz]. Spectra were acquired at 130.245 MHz using a 15° pulse of 1 μs and a recycle time of 1 s and are referenced to Al(H2O)63+. Freshly prepared samples were treated identically and were stored in sealed vials, and they were run within 2 days of preparation. Hydrating the samples to reduce the quadrupolar line-shape effects was not considered to be suitable because of the formation of Zn Al OH phases (see the Results and Discussion). Thermal analysis was carried out on samples predried at 90 °C. Samples were loaded directly from the drying oven onto the microbalance under flowing dry air to prevent reabsorption of moisture. Sample sizes were limited to below 10 mg to ensure good temperature control during combustion. Data were collected using an Alphatech SDT Q600 thermoanalyzer under flowing air (50 mL min 1) at a heating rate of 10 °C min 1. SEM imaging was performed on a JEOL JSM-6500F field-emission scanning electron microscope. Samples were deposited onto a Si wafer and coated with 10 nm of Pt in a JEOL JFC-1500 ion sputtering device. Standard scattered-electron digital images were obtained at an accelerating voltage of 10 15 kV and viewed at a working distance of 9 11 mm.
’ RESULTS AND DISCUSSION Analysis Method. Figure S1 (Supporting Information) shows a typical 27Al NMR signal observed for a sample prepared using 2% Al. Octahedrally coordinated Al is generally observed at 0 ( 15 ppm, whereas tetrahedral Al is seen at 65 ( 15 ppm.24 The signals observed in the present samples include a narrow signal at ca. 81.2 ppm (tetrahedral) and much broader signals centered at 75 ppm (tetrahedral) and 8.2 ppm (octahedral). The narrow signal observed at ca. 81.2 ppm does not appear to exhibit a quadrupolar line shape. It is indicative of Al in a highly symmetrical tetrahedral environment and is likely to correspond to Al3+ 15032
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Figure 1. 27Al NMR spectra of 2 at. % Al-doped ZnO samples prepared using methyl diethanolamine (MDEA), diethanolamine (DEA), aminoethoxyethanol (AEE), propanolamine (PA), and ethanolamine (EA).
located in isolated Zn2+ sites of crystalline ZnO, as proposed in 1. We cannot totally rule out the possibility that this signal is due to Al3+ occupying the tetrahedral holes of the zincite structure. This would be undesirable as it would not contribute to improved conductivity in the material. Having observed that conducting materials can indeed be produced by this method, we can infer that tetrahedral hole occupancy by Al3+ is unlikely. The signal is at slightly lower field than is normally observed for aluminum oxides,24 which could reflect the local charge density or field strength. The signals were deconvoluted by constructing Gaussian envelopes in Origin software for each tetrahedral signal, for the purpose of measuring the areas contributed by the different peaks. This method was chosen over simple integration of the peak areas because of the overlap of the crystalline and amorphous tetrahedral peaks. A representative example is shown in Figure S1 (Supporting Information). The octahedral signal is asymmetric because of quadrupolar dispersions. However, the area was adequately determined by using two Gaussian signals, namely, one under the maximum intensity and a broader one under the tail. The combined area was taken as a representative indication of the relative occupancy of Al in an octahedral environment. The resulting relative proportions of each signal type were then plotted to more easily discern the differences between spectra. It is important to note that the integration of solid-state 27Al NMR signals does not provide reliable quantitative data on the relative abundance of each signal type. However, the change in relative intensities between the different signals can be used to provide a comparative, semiquantitative estimate of the relative Al site occupations of the samples. The quadrupole-induced shift in 27Al NMR spectra is influenced by the moisture content of the sample. Exposing a material to a humid atmosphere for 12 24 h can be used to minimize the quadrupolar effects. However, it has been observed that ZnO nanocrystals are not stable under these conditions,25,26 so that we could not carry out this experiment with any degree of confidence. The 27Al NMR spectrum of a freshly prepared sample stored and manipulated in an atmosphere-controlled glovebox was compared with a spectrum run a week after preparation and exhibited no significant change. Figure S2 (Supporting Information) shows the 27Al MAS NMR spectral comparison
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between a freshly prepared sample, the same sample rerun 1 year later, and a fresh sample stored in a 100% humidity chamber for 72 h. The spectra show that there is a continuing evolution of the sample, beyond the initial 72 h of exposure. A powder XRD pattern of the year-old sample shows the unambiguous presence of a mixed zinc aluminum hydroxide carbonate hydrate salt (Figure S3, Supporting Information), confirming the instability of the sample to moist air. For this reason, we adopted a semiquantitative approach, using a standard sample treatment procedure so that the spectra would be directly comparable. The analysis does not give an exact indication of the proportions of each Al site occupancy but does allow relative proportions of lattice-site aluminum between samples to be identified. Aluminum-containing materials can sometimes contain “NMR-invisible Al”, which occurs because of very large quadrupolar coupling constants that broaden the 27Al NMR signal beyond detection. In some cases, full quantitative spectra can be obtained using 27Al multiple-quantum magic-angle-spinning (MQMAS) NMR spectrometry24 or TRAPDOR (transfer of populations in double resonance)27 experiments. These NMR-invisible Al phases arise from Al in highly locally disordered sites, and thus, we would expect that these phases to be proportional to the observable amorphous signal. Therefore, the results comparing the ratio of crystalline to amorphous Al signals are qualitatively valid; a sample for which the crystalline-to-amorphous ratio is high will also have a high crystalline-to-invisible ratio. Precursor Formulation. The composition of a sol gel precursor influences many of its properties, such as rheology, wetting, and other surface coating characteristics. We have employed a citrate route in preference to the more commonly used acetate route because citrates have been demonstrated to provide thicker films when used in related systems.28 The acetate route generally necessitates the application of multiple coatings to form films of sufficient thickness to provide the required conductivity. This is likely to be a significant barrier to commercial adoption, as it would add extra cost and complexity to the manufacturing process. A complicating factor with Zn is that its carboxylates have low solubility in water unless the pH is raised. This can be achieved using ammonia or an amine. Zinc monocitrate requires 2 mol equiv of amine to enable full dissolution in water, as opposed to a single molar equivalent required for the diacetate. Simple amines or ammonia are not preferred, as they can lead to crystallization of the precursor complex in the drying film. This not only destroys the continuity of the film, but also reduces the doping homogeneity. An amino alcohol provides the possibility for cross-linking with the citrate carboxylate groups, enhancing the film formation characteristics. Acetate-based solutions have largely used ethanolamine or diethanolamine. We examine here a selection of amino alcohols to determine whether they exert any observable influence on the doping characteristics of the ZnO. The current study focused on the formation of ZnO bulk gels and powders to provide sufficient material to carry out NMR studies. The 27Al NMR spectra of ZnO samples containing 2 at. % Al synthesized using the amino alcohols EA, PA, AEE, DEA, and MDEA are shown in Figure 1, and the relative areas of the three primary features are shown in Figure 2. These samples were dried at 90 °C prior to calcination in a 500 °C preheated furnace. The relative proportions of the different Al types vary significantly between different amino alcohol samples. It is difficult to draw a direct relationship between the amino alcohol ligand architecture and the resulting NMR response, as there are too many variables 15033
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Figure 2. Proportional peak areas deconvoluted from the 27Al NMR spectra shown in Figure 1. Abbreviations: MDEA, methyl diethanolamine; DEA, diethanolamine; AEE, aminoethoxy ethanol; PA, propanolamine; and EA, ethanolamine.
Figure 3. Thermogravimetry (thick solid line) and differential thermal analysis (thin solid line) of a Zn precursor sol dried overnight at 90 °C.
between each ligand type. Solvolysis and condensation rates between the zinc and aluminum oxides are likely to vary with the solution pH and the co-ordinating ability of the different amino alcohols. Differential rates could result in Al-rich or Al-depleted regions in the gel, which would carry through to the resulting material upon calcination. The sample prepared using EA exhibited the largest proportion of the 81.2 ppm signal, making this the most promising formulation to examine for further studies. These included changing the heating profile and varying the maximum calcination temperature and the time held at that temperature. Initial variations were carried out using a 2 at. % Al dopant level to provide sufficient Al for observation in the Al NMR spectra. However, we also examined a series of Al dopant levels to examine the change in dopant site occupancy at different concentrations. Thermal Evolution of ZnO Precursor. To develop some understanding of processes occurring during pyrolysis and crystallization, we further examined the thermal evolution of the ZnO precursor solution. The differential scanning calorimetric (DSC) and thermogravimetric (TG) curves of a dried
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Figure 4. XRD patterns of 2 at. % Al-doped samples heated for 1 h at various temperatures (as indicated). The Miller indices for the ZnO peaks are labeled.
Figure 5. 27Al NMR signals in 2 at. % Al-doped samples heated for 1 h at various temperatures (as indicated).
Zn(citrate) 3 2EA solution are shown in Figure 3. Oven drying at 90 °C overnight removed all of the water and 1 mol equiv of the EA, leaving a zinc citrate EA gel. Thermal decomposition of the residue occurred in three main events. First, at 210 °C, an endothermic weight loss consistent with the dehydration and transformation of the citrate to itaconate29 occurred with loss of H2O and CO2. The exothermic decomposition and combustion of the complex occurred in two steps, with an initial onset at 350 °C and a second event beginning at 425 °C. The combustion was fully completed by around 500 °C, leaving crystalline ZnO. The coincident exothermic combustion of the organic ligands masks any possible observation of the ZnO crystallization enthalpy. The total of the weight loss from 25 to 600 °C was 73.9%, which corresponds to a theoretical weight loss of 74.3% in the case of complete conversion of Zn(citrate) 3 EA to ZnO. To further examine the differences in materials prepared at different heating rates, we prepared a sequence of calcined samples quenched at intermediate temperatures. XRD indicated the onset of ZnO crystallization above 325 °C (Figure 4). This parallels the NMR data, which showed the first appearance of tetrahedral Al at the same temperature (Figure 5). Comparison 15034
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Figure 6. Proportional NMR peak areas of 2 at. % Al-doped samples heated to 500 °C for 1 h, in a preheated oven (left) and at a heating rate of 2 °C min 1 (right).
of these data with the TG data shows that some ZnO began to crystallize as soon as the combustion step started. These observations are broadly in line with previous XRD studies of zinc acetate amino alcohol studies.22 Effect of Heating Profile. To achieve fully reproducible results, we observed that it was necessary to follow heating profiles very precisely. The samples used in the following study were all prepared using EA and contained a standard 2 at. % Al dopant level. The final intensity of the 81.2 ppm NMR signal was surprisingly sensitive to the gel drying temperature. Samples were dried for up to 16 h at intermediate temperatures (70 250 °C) prior to full calcination at 500 °C, and the most intense 81.2 ppm signal was observed using a 90 °C drying step. (Figure S5, Supporting Information). Differential solvolysis and condensation rates of Zn or Al complexes in the materials heated at 160 °C, for example, was found to influence the local dopant ion concentration, thus interfering with the homogeneity of the dopant in the final materials. To test this hypothesis, we repeated syntheses with sols containing 40% (v/v) glycerol heated from room temperature at 5 °C min 1, with a 1-h dwell time at 160 °C. 27 Al NMR spectrometry of these samples indicated almost exclusively octahedral aluminum (Figure S6, Supporting Information). This is an important result, as an intermediate heating step is often used in the sol gel formation of ZnO films.7 11,16,30 In the case of our citrate-based sols at least, this heating profile now looks to be detrimental to the doping efficiency. Following the drying step, it was discovered that a rapid calcination in a preheated furnace (500 °C) resulted in the highest proportion of the lattice site Al, compared to use of a lower furnace ramping rate. For example, a 2 °C min 1 ramp rate resulted in a lower proportion of the tetrahedral lattice site Al, predominantly in favor of octahedral Al; see Figures 6 and S4 (Supporting Information). It has been demonstrated that low heating rates produce more densely packed larger crystallites, which would be expected to improve conductivity through better crystal connectivity and fewer grain boundaries.20 The observation that these conditions are detrimental to efficient dopant placement could explain why rapid thermal annealing has been demonstrated to result in low-resistivity films.7,8 Influence of Calcination Temperature. Heating the samples at higher temperatures broadened the sharp 27Al NMR signal at 81.2 ppm and changed the relative proportions of the
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Figure 7. Proportional NMR peak areas of 2 at. % Al ZnO samples heated for 1 h at 90 °C and then for 1 h at various temperatures up to 800 °C.
different Al types (Figure S7, Supporting Information). Deconvoluting the signals in these spectra provided the relative proportions of Al in the different coordination environments as the precursor gel was heated above 300 °C (Figure 7). Prior to pyrolysis, all of the Al existed in an octahedral geometry in the amorphous citrate precursor. Crystallization of the ZnO commenced between 325 and 350 °C, whereupon the sharp signal at 81.2 ppm appeared (presumed to be due to Al ions in zincite lattice sites) and the intensity increased to a maximum of around 15% of the total integrated intensity at 500 °C. The proportion of amorphous tetrahedral coordination sites observed at 75 80 ppm continued to increase up to 600 °C, reaching a total of 70% of the total integrated intensity. A rapid decrease in tetrahedral Al occurred above 600 °C, marking the onset of the crystallization of a minor (ZnAl2O4) phase (gahnite, JCPDS library file 05-0669). If the temperature were increased above 800 °C, we would expect the proportion of octahedral Al to increase because of the preferential formation of gahnite. Reflections due to gahnite in the XRD patterns of the doped ZnO were too weak to be observed at the lower Al dopant levels; however, they were clearly observed in a sample calcined at 800 °C containing 8 at. % dopant level (Figure S8, Supporting Information). To confirm the behavior observed here, we prepared a precursor containing a Zn/Al ratio of 1:2, consistent with the ZnAl2O4 gahnite phase. Calcination at 500 °C gave an 27Al NMR spectrum consisting of three broad peaks, centered at 7, 37.4, and 70 ppm (Figure S9, Supporting Information), roughly in proportion to the intensity of the broader peaks seen in the 8% Al-doped samples (see Figure S10, Supporting Information). This sample was amorphous according to XRD; however, the gahnite phase commenced crystallization upon calcination at 600 °C (Figure S11, Supporting Information). Although no other phase was observed at this temperature, the peaks were relatively broad and weak. Further heating at higher temperature was necessary to produce a well-crystallized material. Some tetrahedral (75 ppm) Al remained at 600 °C, but above 700 °C, the 27Al NMR spectrum shows a single sharp signal exhibiting a quadrupolar line shape centered at ca. 10 ppm (Figure S9, Supporting Information). Thus, crystallization of the spinel structure of gahnite results in the Al ions exclusively adopting octahedral 15035
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Figure 8. Proportional NMR peak areas scaled to Al levels in different Al coordination sites obtained from 27Al NMR spectra of ZnO samples containing 1 8 at. % Al.
Figure 10. XRD patterns for ZnO samples containing a range of Al dopant levels. The TOPAS fits and residual intensities are also shown.
Figure 9. Al dopant levels incorporated into the zincite lattice sites from 27 Al NMR spectra.
lattice sites. The observation of this phase was reported previously14,17 and shown to occur at the ZnO grain boundaries.17,23 Variation of Dopant Level. To explore the effects of Al concentration on the incorporation of Al into the various lattice sites and other properties such as crystallite size, a combined NMR spectrometry, XRD, and SEM study was undertaken. Samples containing 1, 2, 4, and 8 at. % Al were prepared to observe trends in Al coordination. These samples were all prepared using EA in the precursor solution, which was dried at 90 °C prior to calcination in a 500 °C preheated furnace for 1 h. The individual NMR signals (Figure S10, Supporting Information) were deconvoluted, and relative proportions of the different Al types are plotted in Figure 8. An additional broad feature centered at ca. 45 ppm was also visible in the 8 at. % doped sample. This could be attributed either to pentacoordinate Al31 or distorted tetrahedral Al. The data were scaled to take into account the different levels of added dopant. This shows a relatively straightforward growth of both the amorphous tetrahedral and octahedral Al with added dopant level. Notably, the level of tetrahedral Al in the zincite lattice sites plateaud when the added dopant level reached around 4 at. %. However, a plot (Figure 9) showing the relative proportions in the lattice sites indicates that the highest proportion of added
Figure 11. Variation of ZnO crystallite size with Al dopant concentration determined using TOPAS from fitting the synchrotron XRD data (Figure 10). Uncertainties are contained within the points.
dopant was incorporated at the lowest level of added dopant examined (1 at. %). Assuming optimal distribution of the Al throughout the crystal, the 4% doping level should result in the highest intragrain conductivities, although the higher levels of extraneous insulating intergrain material would reduce the intergrain conductivity. The higher intergrain connectivity 15036
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achievable at lower dopant levels should produce overall higher conductivities at the macroscale. It is worthwhile re-emphasizing at this point that the scaled peak area ratios determined here can be used only in a comparative sense; no actual quantitative values are implied in any way by using this method. For reasons explained in the Experimental Section, the actual levels of amorphous intergrain material are likely to be much higher than discussed here. If so, this would have a much larger net reduction in conductivity with increase in dopant added, because of a higher rate of accumulation at the grain boundaries. The net influences of the increase in conductivity of the lattice Al versus the increased resistance due to the excess dopant will be examined in a later publication. Studies have shown that solid-state 27Al NMR spectrometry is a powerful tool for examining the fate and efficiency of the dopant in the ZnO:Al system. 27Al NMR spectrometry has previously been used to examine the limits of solid solution of Al in ZnO,14,17 which showed a very low (
