Photoluminescence Properties of Ordered Mesoporous ZnO

Jan 5, 2011 - Michael Tiemann,*. ,§ and Sangam Chatterjee*. ,†. †. Faculty of Physics and Material Sciences Center, Philipps University, Renthof ...
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Photoluminescence Properties of Ordered Mesoporous ZnO Alexej Chernikov,† Swantje Horst,† Thomas Waitz,‡ Michael Tiemann,*,§ and Sangam Chatterjee*,† †

Faculty of Physics and Material Sciences Center, Philipps University, Renthof 5, D-35032 Marburg, Germany Institute of Chemical Didactics, Tammannstrasse 4, D-37077 G€ottingen, Germany § Faculty of Science, Department of Chemistry, University of Paderborn, Warburger Strasse 100, D-33098 Paderborn, Germany ‡

ABSTRACT: Nanoporous ZnO powders with high surface-to-mass ratios (SMR) between 15 and 70 m2 g-1 are synthesized, structurally characterized, and studied by time-resolved photoluminescence (PL). A strong dependence of the recombination dynamics and spectral width on SMR is observed at T = 10 K, and pronounced disorder-induced effects are found in the temperature dependence. Both the thermally induced shift of the PL maximum and the spectrally integrated PL intensity are interpreted by appropriate theoretical models. This consistent quantitative analysis of the experimental data yields a characteristic energy of 15 meV for the disorder scale in the nanoporous ZnO sample with an intermediate SMR.

’ INTRODUCTION Research on zinc oxide (ZnO) has rapidly developed over the past decade motivated by the search for (i) ferromagnetism in ZnO doped with transition metal elements, (ii) p-doping for realization of ZnO-based optoelectronic devices, e.g., LEDs, and (iii) fascinating new properties of ZnO nanostructures, available due to advanced growth techniques. The luminescence properties of nanostructures such as quantum dots, wires, and whiskers aiming for new optoelectronic devices have been studied by steady-state and time-resolved photoluminescence experiments.1-10 Different radiative recombination channels are attributed to free and impurity-bound excitons, surface excitons, and transitions involving deep defects.1-4,7-9 The influence of different surface-to-mass ratios (SMRs) on the optical emission properties is believed to be small for the different nanostructures studied so far.3 Fallert et al. compared nanocrystals with different particle sizes from commercial suppliers, but neither the preparation methods and conditions nor their influence on the nanocrystal properties were reported.4 Metal oxide nanostructures with well-defined SMRs are nowadays available by chemical synthesis using the structure replication (nanocasting) technique;11-14 this includes nanoporous ZnO.15-17 Here we present optical properties and carrier dynamics obtained from time-resolved photoluminescence experiments on a series of crystalline, nanoporous ZnO materials with variable SMRs between 15 and 70 m2 g-1. The samples exhibit uniform tubular mesopores and consist of crystallites in the size range of approximately 7 nm. The optical properties are correlated with the SMR. r 2011 American Chemical Society

’ EXPERIMENTAL SECTION Synthesis and Structural Characterization of Nanoporous ZnO. Mesoporous CMK-3 carbon was prepared according to a

literature procedure.18 For a typical synthesis of mesoporous ZnO, 0.5 g of CMK-3 carbon was immersed in 20 mL of a solution of Zn(NO3)2 in THF (1.5 mol L-1), followed by stirring at room temperature for 6 h. After filtration, the impregnated carbon was dried at ambient temperature, heated under an air atmosphere to 573 K at a constant rate of 2.5 K min-1, and kept at that temperature for 2 h to convert zinc nitrate to zinc oxide. This procedure was repeated twice. Powder X-ray diffraction (XRD) was carried out on a Bruker AXS D8 Advance diffractometer equipped with a secondary monochromator and automatic divergence slits (filtered Cu KR radiation, 40 kV, 40 mA; counting time 4 s in steps of 2θ = 0.01° for low-angle measurements and 1 s in steps of 2θ = 0.02° for wide-angle measurements). N2 physisorption was conducted at 77 K on a Quantachrome Autosorb 6; samples were degassed at 393 K for 24 h prior to measurement. Pore size evaluation was carried out by NLDFT-based analysis using the cylindrical pore/equilibrium model data kernel. Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) were performed on a Philips CM30-ST microscope. Photoluminescence Experiments. For the time-resolved photoluminescence (TRPL) measurements, the powdered ZnO samples were pressed into cylindrical pellets inside a copper Received: May 11, 2010 Revised: December 9, 2010 Published: January 5, 2011 1375

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Figure 1. Transmission electron microscopic (TEM) image and selectedarea electron diffraction (SAED, inset) pattern of ordered, mesoporous ZnO.

sample holder. A ZnO epitaxial layer of about 300 nm thickness was used as a bulk reference. All samples were excited by a frequency-tripled Ti:sapphire laser emitting 100 fs pulses at a repetition rate of 80 MHz and a center wavelength of 290 nm, corresponding to a photon energy of 4.3 eV. The photon flux in front of the sample was set to 1.5  10-11 cm-2 s-1. The PL was spectrally dispersed using an imaging spectrometer and temporally resolved in a streak camera with a resolution of 5 meV and 2 ps full widths at half-maximum (fwhm), respectively.

Figure 2. X-ray diffraction (XRD) pattern of mesoporous ZnO.

’ RESULTS AND DISCUSSION Structural Characterization. A series of nanoporous ZnO samples with distinct surface-to-mass ratios (SMR) from 15 to 70 m2 g-1 were prepared by nanocasting, using mesoporous CMK-3 carbon as a structure matrix and by varying the amounts of Zn(NO3)3 precursor for the synthesis. This procedure generally leads to a periodically ordered two-dimensional mesopore structure, as shown in the transmission electron microscopic (TEM) image of an example material with a specific SMR (BET surface area) of 50 m2 g-1 (Figure 1). In terms of structural properties, this sample is representative to the other materials used in this study. The selected-area electron diffraction pattern (SAED, inset) consists of distinct spots arranged in circles and superimposed on diffuse rings, suggesting that the nanostructure consists of small ZnO crystallites (see below). However, in the case of a ZnO synthesis, the utilization of mesoporous carbon matrices often fails to produce a 100% true structural replica. Instead, non-negligible fractions of the products may consist of granular material without defined mesoporosity. This can be explained by the fact that substantial amounts of heat are released during the thermal combustion of the carbon matrix which may lead to undesired sintering of ZnO crystallites and, hence, to partial deterioration of its mesostructure. Fractions of such granular material were found by TEM (not shown) in most of the samples presented in this study. However, since the granules are only a few nanometers in size (see below), the overall surfaceto-mass ratio is still high, and the voids between adjacent granules are in the regime of large mesopores/small macropores.

Figure 3. Nitrogen physisorption isotherms and pore size distribution (inset) of mesoporous ZnO.

These findings are confirmed by powder X-ray diffraction (XRD). Figure 2 shows the diffraction pattern of the same sample as in Figure 1. All reflections in the wide-angle region can be indexed according to the hexagonal wurtzite-analogous structure of ZnO. From the width of the 110 reflection (2θ = 56.6°, fwhm = 1.2°), the crystallite size is estimated by the Scherrer method as ca. 7 nm, but it should be noted that the Scherrer method tends to underestimate the crystallite size quite a bit. The low-angle region of the XRD pattern (Figure 2, inset) shows a strong increase in intensity toward low diffraction angles attributable to 1376

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Figure 4. TRPL intensity in false colors as a function of energy (x-axis) and time (y-axis) of the samples with SMRs of 30 (a) and 50 m2 g-1 (b). The temperature is set to 10 K. The corresponding spectrally and temporally resolved data are shown at the bottom and to the right of the TRPL image, respectively.

the experimental geometry (artifact). However, a broad reflection can be identified in this region, attributable to the nanoscale periodicity of the mesopore system. Since the ordered, mesoporous fractions of the sample may be assumed to possess the same 2D hexagonal symmetry as its parent CMK-3 carbon structure matrix (space group p6mm), this reflection can be indexed as 100. The reflection angle can only be roughly estimated as 2θ = 0.9°, corresponding to a d-value of approximately d100 = 9.8 nm. From this value, the cell constant √ of the hexagonal nanostructure can be calculated as a = 2d100/ 3 = 11.3 nm. Subtracting the average pore diameter D = 6.5 nm of the ordered mesopores (see below), a pore wall thickness of a - D = 4.8 nm is obtained. This value is below the above-obtained (Scherrer method-based) crystallite size of ca. 7.2 nm, which further confirms the presence of larger granules without ordered mesoporosity. Figure 3 shows the nitrogen physisorption isotherm at 77 K of the same sample as in Figure 1. The isotherm shape reflects a combination of type-II (macroporous) and type-IV characteristics with H1-type hysteresis according to the IUPAC classification.19 The latter corresponds to the uniform, nearcylindrical mesopores in those parts of the sample which possess the replica structure of the CMK-3 carbon matrix. The resulting pore size distribution (inset), calculated by the NLDFT-based method,20 has its maximum at 6.5 nm. The strong increase in adsorbed volume at a relative pressure close to p/p0 = 1 is attributable to the above-mentioned presence of larger mesopores/ small macropores, i.e., to the voids between submicrometer-sized granules without intrinsic, ordered mesoporosity.

Surface-to-Mass Ratio Dependence of PL Spectra. The samples are investigated by photoluminescence (PL) spectroscopy as a function of surface-to-mass ratio (SMR) at T = 10 K. Exemplary, Figure 4 shows a low-temperature TRPL set of data of nanoporous ZnO samples with SMRs of 30 and 50 m2 g-1 in (a) and (b), respectively. The PL intensity is plotted in false colors as a function of energy (x-axis) and time (y-axis). The emission spectra exhibiting the common signatures for ZnO are below the TRPL images. At low temperatures, the PL is dominated by localized, impurity-bound excitons (BX), emitting slightly below the optical gap of 3.38 eV.21 Additionally, equally spaced peaks are observed and attributed to the phonon-replica of the BX. Due to the strong electron-phonon coupling in ZnO, the radiative recombination of excitons is often accompanied by inelastic phonon scattering.22 The simultaneous creation of n longitudinal optical phonons leads to the distinct emission features on the low-energy side of the BX luminescence (þnLO). All spectral features are broadened for the sample with higher SMR. The temporal evolution of the emission is shown on the right side of Figure 4. The PL signal decays multiexponentially on a short time scale of several 10 ps, with faster decay for the 50 m2 g-1 sample. Next, the effect of the SMR on the PL decay is discussed. An overview of the 1/e - lifetimes as a function of the SMR for all nanoporous samples is given in Figure 5. The PL decay correlates strongly with the SMR, decreasing from 37 to 5 ps as the SMR increases. Such ultrafast recombination is naturally explained by the presence of surface-related impurity states, generating 1377

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Figure 5. 1/e - lifetime of the PL for various SMRs at T = 10 K. The solid line is a guide to the eye. The inset shows the fwhm of the PL spectra for comparison.

Figure 6. PL emission energies for the ZnO bulk reference and the nanoporous sample with the SMR of 30 m2 g-1. The absolute PL maximum and the FX emission are separately plotted for the bulk sample. The solid lines are guides to the eye.

additional nonradiative decay channels. The experimental findings are thus comparable to the material systems with high SMRs, like nanoparticels or nanowires.23 Also, the distribution of the impurity states broadens for larger surfaces, as it is clearly observed in the dependence of the spectral fwhm on the SMR, plotted in the inset of the Figure 5. Temperature Dependence of PL Spectra. The influence of the temperature on the PL properties of the nanoporous ZnO is investigated. The PL peak energy is plotted as a function of temperature in Figure 6, exemplary for the sample with the SMR of 30 m2 g-1 and for the ZnO bulk reference. The PL of the free propagating excitons (FX) and the absolute PL maximum are spectrally distinguishable in the bulk sample. The FX line simply follows the temperature-induced band gap narrowing. The absolute PL maximum corresponds to the BX emission at

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Figure 7. Spectrally and temporally integrated PL emission of the nanoporous ZnO sample with the SMR of 30 m2 g-1 and the bulk reference as a function of 1/T on a semilogarithmic scale. The solid lines are fits with the Bose-model for luminescence quenching.

low temperatures and exhibits an instantaneous shift at T = 130 K due to the ionization of bound excitons. As the lattice temperature is further increased, the FX related signal dominates the spectrum. Altogether, the reference data show the typical signatures of a high-quality ZnO crystal.24 In stark contrast to that, the PL maximum of the nanoporous sample exhibits anomalous temperature dependence. At T = 10 K, the emission energy corresponds to the BX signature of the reference sample. However, as the temperature increases, the PL clearly exhibits a so-called S-shape, commonly observed in disordered materials.25,26 The disorder in the nanoporous sample is most likely introduced by the energy distribution of the surface-related BX states. The broadening of the BX signature with increasing SMR corroborates this interpretation. Generally, the S-shape appears due to the interplay of relaxation, recombination, and ionization of carriers trapped in spatially localized states. The details of the underlying processes and a proper theoretical description are found elsewhere.25,27 It is possible to extract the characteristic energy scale of the density-of-states (DOS) distribution in a disordered system using the temperature at the lowest PL energy within the S-shape. Its position in energy is indicated by the dotted arrow in Figure 6 and corresponds to the energy of εmin=5 meV. According to the theoretical analysis,27 the characteristic energy ε0 is calculated by ε0 = εmin/m, with m in the range of 0.2-0.3 for a Gaussian and 0.6-0.8 for an exponential distribution, respectively. Hence, we estimate the disorder scale in the nanoporous sample between 7 and 20 meV, depending on the form of the energy distribution. The carriers are trapped in the disordered states at low temperatures. When the temperature is increased, they become delocalized as their kinetic energy exceeds the mean disorder potential. For the nanoporous sample, the critical temperature is in the range of 130-150 K. In this regime, the PL maximum follows the temperature dependence of the bulk emission, i.e., of the band gap. However, the absolute luminescence energy of the nanoporous sample is still about 50 meV lower than the bulk FX. Such behavior is often observed in ZnO samples with fast nonradiative surface-related recombination for the FX line,28 leaving the first phonon replica as the main emission signature at room temperature. Finally, the temperature dependence of the spectrally and temporally integrated PL is studied to support the above findings. Figure 7 shows the luminescence intensities of the nanoporous 1378

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The Journal of Physical Chemistry C ZnO and the bulk emission as a function of 1/T in a semilogarithmic plot. Again, the PL behavior is similar for both samples at elevated temperatures. In the low-temperature regime, however, an additional emission-quenching channel appears for the nanoporous samples. Therefore, we have to expand the standard Bose-model29 commonly used to describe thermal-activated decrease of the luminescence efficiency, for the proper description of the experimental findings X Ij ð1þAj eEa, j =kB T Þ - 1 ð1Þ IPL µ j ¼ 1, 2 Here, two carrier fractions (1,2) are introduced with the corresponding activation energies Ea,(1;2) of the PL quenching processes. Fits of the experimental data with the theoretical model are shown by solid lines in Figure 7. Excellent agreement is achieved for both samples. The standard Bose-model, including only one carrier fraction, i.e., I2 = 0, is sufficient to reproduce the bulk data with a typical Ea of about 35 ((5) meV. Similar activation energy is found for the high-temperature quenching channel in the nanoporous ZnO. The low-temperature behavior in this sample yields energy of 15 ((3) meV for the thermally activated PL loss, perfectly matching the value of the previously estimated disorder scale. Therefore, the low-temperature dependence of the PL intensity is also governed by carriers, localized within the disorder potential of the surface-related impurity states.

’ CONCLUSIONS In summary, optical properties of the nanoporous ZnO powders are studied by TRPL as a function of the surface-tomass ratio and temperature. The strong correlation of the PL decay, spectral line-width, and the SMR at low temperature is attributed to the appearance of the surface-related impurity states in the nanoporous samples, typical for ZnO nanostructures.20 Clear evidence of disorder is observed in the temperature dependence of the luminescence. A quantitative analysis of the observed S-shape in the PL emission yields an estimate of 7-20 meV for the characteristic energy scale. The disorder is most likely introduced by the broad distribution of the surface-related states, already identified in the low-temperature data. The integrated PL intensity of the nanoporous ZnO reveals an additional thermally activated PL quenching channel. A quantitative description using an extended Bose model yields activation energy of 15 ((3) meV, strongly supporting the above interpretation by the excellent agreement of the two independent approaches.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected].

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’ ACKNOWLEDGMENT Funding by the German Science Foundation and the Optodynamic Research Center is gratefully acknowledged. ’ REFERENCES (1) Fonoberov, V. A.; Alim, K. A.; Balandin, A. A.; Xiu, F.; Liu, J. Phys. Rev. B 2006, 73, 165317. 1379

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