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Abstract
Zinc oxide (ZnO) is a material that, depending on its properties, can be found in applications ranging from the most simple (food additives) to the most sophisticated ones (UV lasers). Because of such versatility, it is natural to explore alternative forms of ZnO that, ultimately, can be used to develop new ZnO-related properties or devices. Stimulated by these facts, this work was concerned with the light emission characteristics of ZnO in the form of micrometer-sized flake-like structures (μfk-ZnO). The samples were produced by thermal oxidation of a Zn film deposited by sputtering, and their main properties were investigated by morphological-structural-optical techniques. Along with a concise description involving the μfk-ZnO formation and its luminescent characteristics, the work also suggests the suitability of μfk-ZnO as a broad-band (white) light source.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
For decades, zinc oxide (ZnO) has been fascinating the scientific community because of its outstanding physical-chemical properties [1]. ZnO is a II-VI semiconductor whose thermo-dynamically most stable phase is (hexagonal) wurtzite. Moreover, ZnO presents optical properties (direct bandgap Egap ~3.37 eV and exciton binding energy Eexc ~60 meV) that, allied to the possibility of controllable n- and p-type doping, highlight its potential use in several applications. Indeed, ZnO is one of those materials in which the number of functional devices seems to surpass its various morphologies [2,3], and typical examples include: laser sources [4], light-emitting diodes [5,6], gas sensors [7], solar cells [8,9], photocatalysis media [10], infrared metamaterials [11], etc.
The ZnO properties and morphology are closely related and highly susceptible to the synthesis method and conditions − those associated with temperature being the most important ones. Hence, future advances in the development of ZnO-based devices rely on the precise (and reproducible) manufacture of different ZnO morphologies. The systematic study of these structures is also crucial and forms the basis of the present contribution. More precisely, this work reports on the production of ZnO micrometer-sized flake-like structures (μfk-ZnO) and their investigation by means of different experimental techniques. Special attention was given to the optical properties of these so-called μfk-ZnO aiming at the development of new and/or more convenient light emission materials or devices [12].
2. Experimental details
The μfk-ZnO structures were produced by thermal annealing, at increasing temperatures, a film of metallic Zn. The film was deposited onto mirror polished (100)-oriented silicon substrates by sputtering a Zn target (99.99% pure) in a plasma of argon gas (99.999%). Typical experimental conditions comprised: a deposition pressure of 1.5x10−4 Torr (preceded by a base pressure of 2x10−6 Torr), (150 ± 15) °C substrate temperature, 0.8 W cm−2 radio frequency (13.56 MHz) power density, and 7 min deposition time (rendering a uniform 500 nm thick film). After deposition, the film + substrate system was cut into various pieces of ~1x1 cm2 and annealed under a flow of oxygen gas (99.99%). The treatments were cumulative, 30 min long, and took place at 300, 450, 600, 750, and 900 °C. Annealing details also involved a fast temperature raise (< 30 sec) and relaxation at room conditions.
Both as-deposited (AD) and thermally-annealed (ZnO_300 to ZnO_900) samples were investigated according to their morphology (FEG-SEM − field-emission gun scanning electron microscopy), composition (EDX − energy-dispersive X-ray analysis), atomic structure (Raman scattering), and optical properties (UV-VIS DRS − ultraviolet-visible diffuse reflectance spectroscopy, CL and PL − cathodo- and photoluminescence). All measurements were carried out at room-temperature.
3. Results and discussion
Based on the FEG-SEM results (some of the images shown in Fig. 1), the surfaces of both AD and ZnO_300 are constituted by Zn particles with sizes in the 500-1000 nm range. Thermal annealing at 450 °C transforms the Zn particles into flake-like structures − most of them in the micrometer range, porous, and covered with nanometer-sized whiskers. Treatments at increasing temperatures do not alter the typical size of the observed flakes, but the whiskers disappear and the structures become covered by droplets (ZnO_600 and ZnO_750) and, finally, by exhibiting smooth-rounded borders (ZnO_900).
Fig. 1 Scanning electron micrographs (5 keV accelerating voltage) of Zn films deposited onto crystalline Si substrates: as-deposited [(a)] and after thermal annealing at increasing temperatures [(b) to (d)]. Notice the development of flake-like microstructures after treatment at ~450 °C.
Along with the transformation of Zn particles into flakes, EDX measurements indicate changes in the oxygen concentration of the samples from ~5-10 at.% (AD and ZnO_300) to ~36 at.% (ZnO_450 to ZnO_900). The chemical composition of a commercial ZnO powder (99.995% pure) was also determined, following exactly the same experimental procedure, and showed that [O] = (45 ± 1) at.%.
Most of the above morphological-compositional characteristics derive from the starting material and/or thermal treatment details in the sense that: (a) because of the deposition method and conditions [13], the formerly Zn particles present no specific shape nor definite size; (b) considering that Zn metal melts at ~420 °C, treatments at 300 °C are not enough to induce appreciable changes in the samples morphology, nor in their oxygen content; (c) treatments at 450 °C, on the contrary, soften the Zn particles that react with oxygen species in what seems to be (under the adopted experimental conditions) the most stable form to ZnO, i.e., micrometer-sized flake-like structures; and (d) whereas further increase of the annealing temperature does not improve the oxygen content of the samples (now, best described by ZnO-covered microstructures), it makes the microstructures smoother and presenting a denser distribution.
This phenomenological description is consistent with both the structural (Raman) and optical (DRS) characterizations (Fig. 2). In the former case, the excess of Zn metal in ZnO_AD and ZnO_300 inhibited the Raman effect [14] − in contrast with the remaining samples, in which the Raman spectra [Fig. 2(a)] clearly indicated their polycrystalline nature [15]. In the latter [Fig. 2(b)]: (a) the optical reflection of ZnO_AD and ZnO_300 (not shown) is similar to that obtained from rough metallic surfaces, and (b) samples with the highest oxygen content (~36 at.%) produced reflection spectra that resemble a wide bandgap material. In fact, analysis of the UV-VIS diffuse reflectance spectra indicates that Egap ~(3.23 ± 0.05) eV is the average optical bandgap of samples ZnO_450 to ZnO_900, and Egap = (3.32 ± 0.05) eV for the ZnO powder. The bandgap analysis took into consideration the Kubelka-Munk correction of the reflectance spectra [16], the Tauc's method [17], and the fact that ZnO is a direct bandgap semiconductor [1].
Fig. 2 (a) Raman scattering spectra (632.8 nm excitation) of ZnO samples as obtained by thermal annealing a Zn metal film at 450, 600, and 900 °C. The Raman spectrum of a commercial ZnO powder (99.995% pure) sample, along with their main phonon modes in the ~100-800 cm−1 range, is also shown. The spectra were normalized and vertically shifted for comparison reasons. (b) Diffuse reflectance spectra of some ZnO samples and of the ZnO powder. The spectrum of a diffuse reflectance standard (Spectralon®) is also shown.
According to the literature, whereas it is clear the correspondence between the ZnO-related UV-VIS light emission and their morphological-compositional aspects, their origin is still matter of debate [1]. In the UV, luminescence takes place in the ~365-385 nm range and is attributed to exciton emission from the conduction to the valence bands, or to shallow donor-acceptor-pair recombination [18]. In the VIS, at least three main contributions can be recognized − all of them being associated to defects [1]: in the ~400-500 nm (blue) range due to Zn interstitials (Zni) or vacancies (VZn); in the ~500-600 nm (green) range due to oxygen vacancies (VO) or antisities (OZn); and in the ~600-750 nm (red) range due to oxygen vacancies (VO) or interstitials (Oi).
Most of these features are present in the room-temperature CL and PL spectra of the μfk-ZnO samples, as can be seen from Fig. 3 that, additionally, displays the results from ZnO powder and a white light-emitting diode. Because of their zinc-rich composition, no luminescence was detected from ZnO_AD and ZnO_300. For the remaining samples, however, thermal annealing at increasing temperatures improved the total luminescence intensity of both CL (by a factor of ~8) and PL (~4), produced almost no change in the UV emission wavelength [CL peak at ~(380 ± 2) nm] and PL peak at ~(384 ± 1) nm], and favored the green ZnO-related emission in detriment of the red one. Small differences in the shapes of the CL and PL spectra are also evident from Fig. 3, and are associated with the physical processes involving the samples excitation by electrons or photons. Roughly, these processes can be divided according to their nature (and corresponding effect): (CL1) energy in the range of keV (rendering hundreds-thousands of electron-hole pairs and exciting almost all the luminescence mechanisms present in the material); (CL2) sample probed depths that depend on the energy of the incident electron (c.a. 500 nm for the present μfk-ZnO samples when excited with 7.5 keV electrons [19]); (PL1) energy around 1-4 eV (typically, producing one electron-hole pair per incident photon and involving specific excitation processes); and (PL2) shallow photon penetration depths (< 100 nm for the present μfk-ZnO samples when excited with 3.5 eV, corresponding to an absorption coefficient of ~105 cm−1 [20]).
Fig. 3 (a) CL cathodoluminescence (7.5 keV electron energy) spectra of μfk-ZnO samples after annealing at different temperatures. (b) PL photoluminescence (350.7 nm photon excitation) spectra of the same μfk-ZnO samples shown in (a). PL at ~760 nm is an artifact (i.e., second-order of the UV peak at ~385 nm). All CL and PL spectra were normalized (see the multiplying factors) and vertically shifted for comparison purposes. The figures also include the CL and PL spectra of the ZnO powder sample as well as the light emission pattern of a commercial (Ce-doped YAG) white LED.
The above CL and PL specificities are also behind the different UV-to-VIS intensity ratio (IUV/IVIS) values that, usually, are associated to the optical quality of ZnO [1]. According to the spectra of Fig. 3, the IUV/IVIS CL ratios stayed in the ~0.3-2.8 range, whereas the PL ones varied between ~0.4 and 1.8. Overall, in spite of the great morphological-compositional changes induced by the thermal treatments, both CL and PL spectra (including the results due to ZnO_powder) clearly indicate that μfk-ZnO contains a lot of structural defects.
The most remarkable characteristic of the μfk-ZnO samples, however, is the color perception that they provide. Such effect can be noticed by plotting the x,y color coordinates of all μfk-ZnO samples, as obtained from their CL and PL spectra, in a CIE chromaticity diagram [21]. The results of this analysis, along with the data achieved from the ZnO powder and the commercial white LED, are shown in Fig. 4. As can be seen from the diagrams, whereas the light emission patterns of ZnO powder and white LED assume a bluish character, all the μfk-ZnO samples stay toward the green-yellow region. The only exceptions occur with sample ZnO_450, in which its CL spectrum behaves like an illuminant E [Fig. 4(a)], and its PL spectrum tends to the yellow-orange region [Fig. 4(b)]. By definition, an illuminant E is a standard light source of even color, in which equal wavelengths applies [21]. Its color coordinates are (x,y) = (0.33,0.33) − very close to (0.34,0.36), as achieved for the CL of ZnO_450. Altogether, it is clear the role played by the sample preparation method (and conditions) onto the morphological-compositional-optical properties of the μfk-ZnO samples, as is evident that μfk-ZnO samples give rise to efficient whitish light emission following either electron or photon excitation.
Fig. 4 CIE Commission Internationale de l'Eclairage 1931 color diagrams, as obtained from the CL (a) and PL (b) spectra of μfk-ZnO samples. The x,y color coordinates corresponding to the results due to commercially available ZnO powder and white LED are also shown.
At this point, it is worth noticing that the production and characterization of μfk-ZnO structures is not a new subject [22–24]. However, in most of the cases, the studies involved either a complex sample preparation procedure or did not present a comprehensive experimental characterization. Within this context, the present work adds valuable information to the field of ZnO-related materials and properties. More specifically: the work is based on a very simple method (thermal oxidation of a Zn film), and it presents a consistent description of the main morphological-compositional-optical properties of the μfk-ZnO structures. Moreover, the work contains a brief discussion relating the color characteristics of the μfk-ZnO samples.
4. Concluding remarks
Motivated by the many fascinating properties of ZnO, as well as by the continuing interest for new light-emitting materials, this work reports on the systematic investigation of micrometer-sized flake-like ZnO (μfk-ZnO) structures. The samples were produced by the thermal oxidation of a Zn film, as deposited from sputtering onto a crystalline silicon substrate. The thermal treatments were conducted in the 300-900 °C temperature range, under a continuous flow of oxygen gas. Following this approach, the initial Zn particles (samples ZnO_AD and ZnO_300) developed into the ZnO structures (ZnO_450 to ZnO_900) with oxygen concentrations around 36 at.%. Further experimental characterization indicates that these figures are perfectly consistent with the optical bandgap values (Egap ~3.2 eV) and luminescence features presented by the μfk-ZnO structures. Indeed, both CL and PL results suggest the suitability of the present ZnO-based materials in generating broad (whitish) light sources for future technological application. Finally, in addition to a succinct (but comprehensible) experimental investigation, this work proposes an alternative (low-temperature + catalyst-free) method to produce μfk-ZnO structures whose main properties can be useful in the development of devices like UV optical detectors, LEDs, etc.
Funding
FAPESP (Grant 2012/10127-5) and CNPq (Grant 304314/2013-7).
Acknowledgments
The author is indebted to Professor M. Siu Li (IFSC) for the UV-excited PL measurements.
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