Synthesis and Structure-Dependent Optical Properties of ZnO

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Synthesis and Structure-Dependent Optical Properties of ZnO Nanocomb and ZnO Nanoflag Yuting Nie, Zhiqiang Wang, Jian Wang, Feng Bao, Jinping Zhang, Yanyun Ma, Tsun-Kong Sham, and Xuhui Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08016 • Publication Date (Web): 20 Oct 2017 Downloaded from http://pubs.acs.org on October 22, 2017

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The Journal of Physical Chemistry

Synthesis and Structure-Dependent Optical Properties of ZnO Nanocomb and ZnO Nanoflag Yuting Niea, Zhiqiang Wangb, Jian Wangc, Feng Baoa, Jinping Zhangd, Yanyun Maa *, TsunKong Shama,b *, Xuhui Suna * a

Soochow University-Western University Centre for Synchrotron Radiation, Institute of

Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, China. b

Department of Chemistry, University of Western Ontario, London, ON, N6A 5B7, Canada

c

Canadian Light Source, Saskatoon, Saskatchewan, S7N 0X4, Canada

d

Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese academy of sciences,

Suzhou 215123, China. *Corresponding author: [email protected] (X. Sun), Phone: +86-512-65880943, Fax: +86512-65880280; [email protected] (Y. Ma), Phone: +86-512-65884530; [email protected] (T. Sham), Phone: +01-519-661-2111 ext. 86341

ACS Paragon Plus Environment

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ABSTRACT. The structure-dependent optical properties of ZnO nanostructures have attracted considerable attention due to their fascinating optoelectronic properties and the great structural diversity. Novel ZnO nanocomb and ZnO nanoflag have been successfully synthesized by CVD method using Au nanoparticles (NPs) as the catalyst at the deposition temperature of 900 °C and 950 °C, respectively. X-ray diffraction and high-resolution transmission electron microscopy results show that the ZnO nanocomb handle and its teeth grow in [0 111] and [0001] orientation, respectively, while the ZnO nanoflag sheet and its pole grow along [0001] and [2 1 10] orientation, respect0ively. Au NPs as well as deposition temperature played an important role in the growth of the nanocomb handle and nanoflag pole. Synchrotron transmission X-ray microscopy (STXM) reveals the thickness distribution and the crystallinity of ZnO nanocomb and ZnO nanoflag. For the near surface emission, photoluminescence and cathode luminescence spectra of these two ZnO nanostructures show band gap emission from both nanocomb and nanoflag but green emission from only ZnO nanocomb. Synchrotron-based two-dimensional Xray absorption near edge structure - X-ray excited optical luminescence (2D XANES-XEOL) further reveal that the green (defect) emissions come from both surface and bulk of nanostructures. In the ZnO nanocomb, the O excitation channel contributes more favorably to the band gap emission compared to the defect emission, while the Zn excitation channel contributes less favorably to the band gap emission than the defect emission. Meanwhile, ZnO nanoflag displays an excellent crystallinity with weak defect emission, the Zn and O excitation channel both contribute predominantly to the band gap emission.

INTRODUCTION


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With the deep development of semiconductor industry, zinc oxide (ZnO) nanostructures with a wide direct band gap (3.37 eV) and a large exciton binding energy (60 meV), have attracted considerable attention due to their fascinating optoelectronic properties and the great structural diversity.1-5 ZnO nanostructures have exhibited extensive application in various industrial fields such as field effect transistors,6-8 lasers,9-10 photodetectors,11-13 solar cell and batteries,14 and chemical and biological sensors.15-16 During the past decades, ZnO nanostructures with plentiful morphologies, such as nanowires and needles,17-19 nanosheets and nanodiscs,20 tetrapods nanorod,21 nanomembranes,22 helix structures and nanobelts,23 have been successfully synthesized by various methods including chemical vapor deposition (CVD), solution synthesis and hydrothermal method.17, 24-25 In particular, CVD method is the most popular technique to prepare high quality crystalline structures. Among these CVD syntheses, most ZnO nanostructures have been grown through vapor-liquid-solid (VLS) mechanism with gold catalysts.26-27 Numerous studies have demonstrated that the optical luminescence property of ZnO nanostructures depends uniquely on their corresponding structure and size.17 Investigation of the structure-function relationship could help understand the mechanism of optical emission from ZnO nanomaterials. Generally, ZnO nanostructures exhibit widespread luminescence range, containing UV excitonic emission and visible emission at different emission wavelengths due to intrinsic or extrinsic defects.6 The structure-dependent luminescence revealed the effects of crystallinity, defect concentration and surface roughness.28 Although previous works have explored the emission origin, especially for the green emission, the detailed mechanism has been still controversial. So far, various hypotheses have been proposed to explain the nature of green emission.29-31 Mostly, it has been demonstrated that the green emission is typically associated


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with oxygen deficiency. For example, decades ago, the common assumption for the green emission was the transition between a singly charged oxygen vacancy and a photo excited hole.29 After that, it was reported that the green luminescence originated from surface defects.31 In addition, it was assumed that the visible emission is ascribed to the transition between an electron close to the conduction band edge and a deeply trapped hole in the bulk of ZnO particle.30 Among the reported works, the optical properties of ZnO nanostructures are mostly studied by photoluminescence (PL) and cathode luminescence (CL) with a limited detectiondepth, which mainly collected the photoelectron information near the surface. Thus, it still lacks a comprehensive understanding about the optical emissions both from bulk and surface. The identification of the exact origin of the optical emission therefore re-mains a challenge. In order to further investigate the optical photon emission in the bulk of nanostructures, X-ray excited optical luminescence (XEOL), a powerful synchrotron-based spectroscopy technique, has been developed to explore the nature of the optical luminescence.32 XEOL is a kind of deexcitation spectroscopy which measures the optical response of the system upon excitation using X-ray photons with selected energy, often across an absorption edge of a given element, providing elemental and site specificity.32-33 XEOL has a strong function of the excitation energy, and hence penetration depth, as well as the turn-on of any shallow core levels, and can be used together with the X-ray absorption near-edge structure (XANES) to track the chemical environment and the energy transfer to the optical channels (band gap and defects). XANES is an element and chemical specific technique, it records the modulation of the absorption coefficient across an absorption edge of an element of interest, when the element is placed in a chemical environment. Thus, XANES can be recorded using the photoluminescence yield (PLY). XANES in the soft X-ray region with advanced detectors of total electron yield (TEY) and partial


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fluorescence yield (PFY) can yield excellent signal to noise ratio without distortion in thin samples, which truly reflects the electronic structure at surface and bulk of the specimen, respectively.32 Both XANES and XEOL can be used to reveal the element or the site that is responsible for the luminescence. For example, recently, XANES-XEOL measurements reveal that the XEOL from indium-doped GaN-ZnO solid-solution nanostructures is primarily due to oxygen defects in the indium oxide surface layer and effective energy transfer from the GaNZnO solid solution.34 In this study, the novel ZnO nanostructures of nanocomb and nanoflag with uniform sizes have been synthesized with gold catalysts by CVD method via changing the deposition temperature. Especially, ZnO nanoflag, as a new type of 2D nanostructure, is reported for the first time. The morphology, crystal structure and composition of ZnO nanocombs and nanoflags were characterized by SEM, TEM, XRD and synchrotron transmission X-ray microscopy (STXM), the corresponding optical properties were systematically studied by PL, CL and 2D XANESXEOL. Moreover, the monochromic cathode luminescence (mono-CL) images of the single nanocomb and nanoflag were applied to distinguish between the emission distribution of UV and visible region. The compared analyses on structure-dependent luminescence of ZnO nanocombs and nanoflags reveal that the emissions of nanocomb are associated with both surface defects and bulk crystallinity, while nanoflag shows a main luminescence concentration originated from the bulk. In this work, various optical detecting techniques were combined together to demonstrate a more comprehensive and accurate analysis approach for the optical property study of the luminescent nanostructures. MATERIALS AND METHODS


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1. ZnO Nanostructures Synthesis. Prior to the growth process, a precleaned Si-(001) wafer as the deposition substrate was covered with 2 nm Au by sputtering as the catalyst and then put in the down-stream region of a horizontal quartz tube in a tube furnace. The pure ZnO powder (99.9%, Alfa Aesar) was placed upstream in the center of tube at the heating temperature of 1080 °C. ZnO nanocombs and ZnO nanoflags can be obtained on the silicon substrate at 900 °C and 950 °C maintained for 90 min, respectively. During the growth processes, pure Ar (99.99%) was used as the carrier gas for the first 30 min at the flow rate of 80 sccm (standard cubic centimeters per minute), exposing one end of the tube in the air thereafter. As-prepared ZnO nanocomb and ZnO nanoflag were directly used to be characterized without any treatment. 2. Structural and Chemical Characterization. The morphology and size of ZnO nanocomb and nanoflag were investigated by scanning electron microscopy (SEM, FEI Quanta 200 FEG), transmission electron microscopy (TEM, FEI Tecnai G2 F20) equipped with energy dispersive X-ray spectroscopy (EDX) for the chemical composition analysis and synchrotron based scanning transmission X-ray microscope (STXM) using the spectromicroscopy (SM) beamline at the Canadian Light Source (CLS). In STXM, the monochromatic X-ray beam is focused by a Fresnel zone plate to a spot (~30 nm) on the sample, which is raster-scanned with synchronized detection of transmitted X-rays to generate image sequences (stacks) over a range of photon energies often across the absorption edge of the element of interest. The crystal structures of the ZnO nanocombs and nanoflags were studied by high-resolution TEM (HRTEM, equipped at the acceleration voltage of 200kV), selected-area electron diffraction (SAED) and X-ray diffraction (XRD, Broker Empyrean, Cu K radiation, λ=1.504 nm). 3. Optical Characterization. The optical properties were characterized by PL (LaRAM HR800 Confocal laser Raman spectrometer, the wave-length of the laser is at 325nm (equals to


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~3.8 eV), performed at room temperature), mono-CL (equipped in FEI Quanta 400 FEG -SEM at the accelerated energy of 5kV) and 2D XANES-XEOL (using the Spherical Grating Monochromator (SGM) beamline at CLS). For the collection of the 2D XANES-XEOL data (x axis: emission wavelength, y axis: excitation photon energy; z axis: luminescence intensity color coded in a 2D map), the sample was mounted on a carbon tape with an angle of incidence of 45 degrees, and XEOL (UV-NIR, ~200-1000 nm) spectra were collected using a dispersive optical spectrometer (Ocean Optics, QE6500) at every excitation photon energy across the O K-edges and the Zn L3, 2-edges for ZnO nanocomb and ZnO nanoflag. From the 2D XANES-XEOL, via a horizontal cut yields an excitation energy-selective XEOL spectrum. While the wave-lengthselected PLY spectrum can be achieved by vertical cuts, setting a desired wavelength window from the 2D XANES-XEOL spectra, and then the wavelength-selected collected PLY XANES spectra can be obtained as the integrated yield from a specific wavelength window, such as 360400 nm (band gap), 450-750 nm (defect) and 200-1000 nm (zero order) chosen in our study. Moreover, total electron yield (TEY) and partial fluorescence yield (PFY using Si drift solid state detector) detection modes were used to collect absorption spectra in the SGM beamline. All spectra were normalized to the incident photon flux. RESULTS AND DISCUSSION The SEM images of the sample prepared at the temperature of 900 °C are shown in Figure 1a. It is clear that the product mainly exhibits a comb-like shape with comb handle and comb teeth. These nanocombs are uniform in shape with the length in a range of 3~5 µm. An Au nanoparticle was found on the tip of the comb handle. Interestingly, the lengths of comb teeth exhibit gradient changes with the intervals between each other being almost constant (around 250 nm), as shown in the inset of Figure 1a. In addition, closer examination reveals a serious of obvious stripes


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pattern on the surface of the nanocomb handle. The other sample grown at the temperature of 950 °C exhibits a uniform flag-like 2D nanostructure (Figure 1b), dramatically different from the comb structure. The magnified SEM image of an individual nanoflag (the inset of Figure 1b) shows that the flagpole is belt-like with an Au nanoparticle at the tip and the flag sheet is nearly square with a smooth surface of several micrometers. To confirm the elemental composition of these samples, EDX-mapping was performed under scanning transmission electron microscopy (STEM) mode. Figure S1 and S2 (Electronic Supplement Information, ESI) show the TEM images, high-angle annular dark field (HAADF) STEM images and the corresponding elemental mapping (Au, Zn and O) of nanocomb and nanoflag, respectively, in which both samples show the uniform element distribution of Zn and O with the gold mainly exists on the tip of the comb handle and the flagpole. The EDX quantitation of Au, Zn and O can be extracted from the EDXmapping data. Only Au can be found in the nanoparticle on the tip of the nanocomb and there is no any Au found in the area below Au NP. However, in the ZnO nanoflag image the Au NPs was found to be partly immersed in the tip of the nanoflag pole. The EDX data of the part of Au NP in the nanoflag pole tip shows all Au, Zn and O elements with the different contents (e.g. average atomic ratio of Au: Zn: O=25: 42: 33 found in Figure S2). However, only Au can be found in the nanoparticle outside the tip with no any Zn and O signal. The difference in the shape and structure of Au NPs on the tip of ZnO nanocomb and nanoflag is probably attributed to the different growth temperature for the two structures. The crystal structures of the nanocombs and nanoflags were further confirmed by XRD (Figure 1c). Both XRD patterns can be indexed to the hexagonal wurtzite ZnO (JCPDS 79-0206). Compared to the pattern of ZnO nanocomb and the standard card, the pattern of ZnO nanoflag shows a significant preferential orientation on (10 10) face and (1120) face whose intensities are


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even higher than those of (10 11) face and (0002) face, respectively, probably resulting from the novel 2D structure of nanoflags. In addition, all the feature diffraction peaks of ZnO nanocomb show obvious splitting peaks, indicating the variation in crystallinity and/or the deformation of crystal structures. This is not unexpected as STXM shows below that the teeth are considerably thinner than the comb handle hence a slightly relaxed crystal spacing in the teeth is a good possibility.

Figure 1. SEM images of ZnO nanostructures deposited on the silicon wafers: (a) nanocombs, (b) nanoflags. (c) XRD patterns of nanocombs and nanoflags. HRTEM and SAED were employed to further investigate the crystal structures and the growth orientation of ZnO nanocomb and ZnO nanoflag. Figure 2a and 2b are the typical TEM image and the corresponding SAED pattern of ZnO nanocomb, respectively. Figure 2c and 2d are the HRTEM images corresponding to the square regions marked as 1 and 2 (in red) in Figure 2a, respectively. In Figure 2c, the lattice fringes could be indexed to (10 10) face and (0002) face


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which agree well with the results in Figure 2b. Combining those information from SAED (Figure 2b) and TEM (Figure 2a and 2c), it can be inferred that the nanocomb handle grew in [0 111] orientation induced by Au NP catalyst. Similarly, the (10 10) face and (0002) face are also found in Figure 2d, and the growth orientation of nanocomb teeth is along [0001] which has been suggested to be the fastest growth direction of hexagonal ZnO.4-5 This is consistent with the resultant teeth pattern. Crystal lattice dislocations near the surface of nanocomb teeth are observed in Figure 2d. In addition, the interplanar crystal spacing of 0.267nm in small regions are also observed in the new grown tooth of the nanocomb as shown in Figure S3b (HRTEM image marked as 3 with red square in the Figure S3a) due to the distortion of (0002) faces (d=0.260 nm) probably by the stretching stress. This result is consistent with the XRD peak splitting in the (0002) face of nanocomb.

Figure 2. TEM characterization of ZnO nanocomb (a-d) and ZnO nanoflag (e-h): (a, e) TEM images; (b, f) SAED patterns; (c, d) HRTEM images of the marked 1 and 2 area in red, respectively; (g, h) HRTEM images of the marked 1and 2 areas in green, respectively.


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In Figure S4, eight ZnO nanocombs were chosen to measure the length of the comb teeth (colored in red) and the intervals between the comb teeth (colored in blue) from SEM images. It is found that the intervals between the comb teeth keep almost the same spacing around 250 nm and the length of the comb teeth shows a linear change with the growth of comb handle. In the inset of Figure S4, the angle between comb handle ([0 111] growth orientation) and comb tooth ([0001] growth orientation) is 61.4°, which matches the induced angle of (0 111) face (equivalent to (10 11) face) and (0002) face. Moreover, the angle between the connecting line of the tip of comb teeth (following [0 11 1] crystal orientation) and comb handle ([01 11] growth orientation) is 57.2°. The growth orientation of ZnO nanoflag was investigated as well. The TEM image (Figure 2e) and the corresponding SAED (Figure 2f) clearly show that the nanoflag sheet and the nanoflag pole grows along [0001] and [2 1 10] orientations, respectively. Figure 2g and 2f are the HRTEM images of the marked 1 and 2 areas (in green) in Figure 2e, respectively, showing the lattice fringes of (1120) and (0002) faces, displaying the same growth orientation of nanoflag as mentioned above. In Figure S3d (HRTEM image marked as 3 with green square in the Figure S3c), the corner of the nanoflag sheet and the pole also shows the growing orientations along [0001] and [2 1 10] orientations, respectively. The dislocations are also observed in the HRTEM image of the corner, as shown in the white dotted box marked in Figure S3d. According to the previous studies on the growth mechanism of ZnO nanostructures via CVD method, and grow following the well-known vapor-liquid-solid (VLS) mechanism. The detailed process and growth mechanism could be summarized in a brief description. Firstly, the Au film on the Si substrate as the catalyst precursor was melt and coalesced to the gold nanoparticles


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(NPs) at the elaborated temperature. When ZnO powder was evaporated, and carried by Ar gas to the Si substrate, Au-ZnO liquid alloy was formed and reached to a saturation state by the continue feeding of ZnO, and then ZnO was precipitated out to form ZnO nanostructures, lifting Au NP to the top. The gold catalyst induced the ZnO to grow in the specific crystal faces (e.g. (10 10) face in the nanocomb handle and (1120) face in the nanoflag pole). With the continuous feeding of ZnO in the following reaction time frame, the crystal growth orientation [0001] (in the nanocomb teeth and nanoflag sheet) with polar charge growth to minimize the energy had the high nucleation rate5, leading to spontaneous formation of teeth (for nanocomb) and sheet (for nanoflag) at the growth temperature of 900 °C and 950 °C, respectively. The growth temperature is a critical factor for the crystal growth to form the different morphology and structure. In addition, the Au film on the Si substrate can be form the near liquid gold nanoparticles with different sizes at different tempareture.27 Thus, we suggest that the Au film on the silicon substrate heating at 900 °C and ~950 °C could lead to the different gold liquid sizes and influences the absorption of evaporated ZnO, and finally it may result in the ZnO growth at the [0 111] growth orientation to form the nanocomb handle and [2 1 10] orientations to form the nanoflag pole. For the ZnO nanoflag (see in the Figure S2), the shape of the Au nanoparticle on the tip of the nanoflag was more likely tend to be irregular structure that is different from the shape of the ZnO nanocomb (see in the Figure S1) which was more likely tend to be hexagonalspherical structure. The detail growth mechanism of ZnO nanocomb and nanoflag need to be further investigated. As a powerful tool for the electronic structure study, STXM, combined with XANES spectroscopy and microscopy, can provide both the electronic and local structural information with good spatial resolution.35 With a nanoscale beam size, STXM is very suitable to probe the


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electronic structure of an individual nanomaterial, especially the complex nanomaterial with various morphologies, different elemental distribution, or the existence of defects and impurities.36 The as-prepared novel ZnO nanostructures of nanocomb and nanoflag were measured by STXM at SM beamline in CLS with ~30 nm beam size, as shown in Figure 3. From the color composite images of thickness maps (Figure S5, the insets of Figure 3a and 3b), it is clear that ZnO nanostructures are of significant thickness variation. This will have some bearings in the luminescence properties of the nanostructures. The nanocomb handle shows the largest thickness (~160 nm to ~220 nm) which is almost two to three times that of the nanocomb teeth (~50 nm to ~110 nm), while the nanoflag are relative thinner, the pole of the nanoflag (~70 nm to ~120nm) is 1.3 times of that of the nanoflag sheet (~50 nm to ~70nm), the top of the nanoflag pole is nearly twice times of the sheet (~115 nm to ~150 nm). XANES O K-edge and Zn L3, 2edge spectra of ZnO nanocomb at different regions show similar absorption features (Figure 3a), suggesting high purity and uniformity of ZnO. Since STXM measures the absorption in transmission, the spectra edge jump reflects the thicknesses at different regions, the higher the intensity, the thicker the sample. Similar results can be observed in ZnO nanoflag (Figure 3b). Furthermore, the normalized XANES spectra (relative to edge jump) of the O K-edge from the middle regions of nanocomb and nanoflag were compared in Figure 3c. There are three features at around 534.5 eV (feature A), 538 eV (feature B) and 540 eV (feature C). According to the dipole-transition selection rule, features A, B and C are attributed to the electron excitations from O 1s-derived states to 2p-derived (along the bilayer), O 2pσ -derived (along the bilayer) and O 2pπ -derived (along the c axis) states, respectively, which are approximately proportional to the density of the unoccupied O 2p-derived states.37-39 Compared to the nanoflag, the obvious enhancement of the feature A in nanocomb represents the enhanced local densities of states


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(DOSs) arisen from the defects or dangling bonds in the edge and teeth of ZnO nanocomb.40 This result well agrees with the corresponding HRTEM image of nanocomb teeth (Figure 2d). Figure 3d shows the normalized XANES spectra of Zn L3, 2-edge from the middle regions in nanocomb and nanoflag. Two features at around 1026.6 eV (feature A) and 1028.7 eV (feature B) present the unoccupied Zn s- and d-derived states, respectively, which are consistent with that of ZnO nanostructures in the previous reports.39, 41 The feature A and feature B are related to the electron excitations from 2p3/2 electrons to unoccupied 4s and 4d states, respectively. Comparing to the nanoflag, the intensity of the feature B increased in the nanocomb, which indicates that the number of unoccupied Zn 4d states near the conduction band minimum is increased in the nanocomb, suggesting that more Zn-interstitial on the surface of the nanocomb compared to the nanoflag.40


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Figure 3. STXM XAS spectra and maps of ZnO nanocomb (a) and ZnO nanoflag (b); Normalized XANES spectra of O K-edge (c) and Zn L-edge (d) from the middle regions in (a) and (b), comparing the nanocomb and nanoflag. It has been well known that ZnO nanostructures present the diversity of optical property, which is primarily dependent on the shape, size and structure. Figure 4 shows the PL spectra of ZnO nanocomb and nanoflag, in which the emission peaks at 377 nm and 530 nm of nanocomb attribute to the band gap emission (BG) of ZnO and the defect emission (DE) resulting from O or


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Zn deficiency and/or interstitial,6 respectively. The intensity ratio of these two emissions (IBG/IDE) is 0.53 in the nanocomb. It has been demonstrated that the green emission in PL is not only originated from O vacancies, but also from Zn vacancies.42 However, the PL spectrum of ZnO nanoflag shows only one emission peak at 382 nm without the green emission at around 530 nm, indicating an excellent crystallinity compared to the nanocomb due to the higher growth temperature for the nanoflag. The band gap emission (at 378 nm) of nanocomb exhibits a little blue-shift compared with that of nanoflag (at 382 nm) due to the widening of the band gap (quantum confinement in the teeth). For ZnO nanostructures, the band gap increase with the increase of surface-to-volume ratio or the decrease of the diameter of the exciton.43

Figure 4. Photoluminescence spectra of ZnO nanocomb and nanoflag. The excitation energy of laser is ~3.8 eV. The intensity ratio of IBG/IDE is 0.53 in the nanocomb. IBG: the intensity of band gap emission; IDE: the intensity of defect emission. To further investigate the optical property of these novel ZnO nanostructures, the monochromatic cathode luminescence (mono-CL) measurement under the SEM observation was performed to obtain mono-CL images of the single nanostructure at the selected wavelength,


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probing the specific position of the band edge emission and the defect emission. The CL area scan spectrum of a single ZnO nanocomb (Figure 5a) displays two main emission features, including the band gap emission around 378 nm and defect emission around 520 nm. The intensity ratio of IBG/IDE is 5.55 for the single ZnO nanocomb, which is much higher than that of 0.53 in average PL obtained from multiple ZnO nanocombs, indicating that energy transfer to the defect channel is more efficient in PL than CL with the later sampling much deeper into the bulk. CL can detect band edge emission into the bulk due to its higher energy (5 keV) of the electron beam. In addition, the mono-CL images excited by monochrome electron energy with optical yield at 378 nm (the inset III) and 517 nm (the inset IV) are shown in the insets of Figure 5a. It is interesting to note that more intense green emission comes from the nanocomb handle than that from the nanocomb teeth. Compared to ZnO nanocomb, ZnO nanoflag shows only band gap emission around 382 nm in its CL area scan spectrum (Figure 5b), and the mono-CL image of band edge emission excited by monochrome electron energy with optical yield at 382 nm (the inset II of Figure 5b) exhibits an obvious emission on both the nanoflag pole and the nanoflag sheet. The deficiency of green emission mainly attributes to the excellent crystallinity of ZnO nanoflag with less defects due to the higher growth temperature. Meanwhile, CL spot scan spectra were obtained to compare the optical properties in different positions of a single nanocomb as show in Figure 6.


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Figure 5. CL spectra, mono-CL images and SEM images of ZnO nanocomb (a) and ZnO nanoflag (b). (a) CL area scan spectra of a single nanocomb and the corresponding SEM image (I), color overlapped CL image (II), CL image of band edge emission (III) excited by monochrome electron energy at 378 nm, CL image of defect emission (IV) excited by monochrome electron energy at 517 nm. (b) CL area scan spectra of a single nanoflag and the corresponding SEM image (I), CL image of band edge emission (II) excited by mono-chrome electron energy at 382 nm.


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It should be noted that energy transfer to the optical channel in CL is mainly based on the thermalization of the exciting electrons undergoing inelastic scattering. The thermalization path can be understood in terms of inelastic mean free path (IMFP) or the escape depth of electrons. Thus, the 5keV excitation electron can be completely thermalized after three “absorption (attenuation) lengths” or ~ 15 nm in the solid. Therefore, in the CL spot scan, given a uniform sample, the thicker the sample, the more intense the luminescence. It is apparent from Figure 6 that both of the tip (marked as a) and the middle (marked as h) positions of the comb handle display the strong band gap emission (around 380 nm) and the weak defect emission (around 520 nm) (Figure 6a). While the other positions (marked as b-f) in the middle of different comb teeth show relatively weak band gap emission (Figure 6b, the spectra were normalized by divided the highest intensity of the defect emission in each spectrum). It is clear that the emission intensity of the comb handle is much stronger than that of the comb teeth simply because of its larger thickness as revealed from the STXM study described above. Based on the enlarged CL spot scan spectra from 360 nm to 405 nm, detailed information of band gap emission from a single nanocomb were obtained: i) the emission peak of position a (378 nm) shows a slight blue-shift compared with position h (381 nm), probably resulting from more bulk UV emission in position h; ii) position g (the edge of comb handle) shows the same emission peak position but lower peak intensity compared with position h; iii) the emission peaks of position b-f are gradually blue-shifted and the emission intensities decrease step-by-step from position b to position f; thus the blue shift indicates the longer comb teeth gradually become thinner exhibiting quantum confinement and the decrease in intensity indicates increasing defect.


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Figure 6. (a) CL spot scan spectra of different positions of a single ZnO nanocomb marked a-h (b-f: the middle positions of comb teeth) in the inset of SEM image. (b) The enlarged CL spot scan spectra from 360 nm to 405 nm in (a), in order to comparing the band gap, the spectra were normalized by divided the highest intensity of the defect emission in each spectrum. Although PL and CL have provided much information about the optical properties via detecting the defects of nanocrystals near the surface or the sub-surface, there is still a great limitation for them to measure the fine structure in the interior of bulk crystals due to the low excitation energy. Recently, a powerful synchrotron-based spectroscopy technique, XEOL combined with XAS has been developed to systematically explore the nature of the optical luminescence. For the ZnO nanocomb and ZnO nanoflag, the 2D XANES-XEOL spectra were obtained to compare their optical features. Figure 7 (a, b, c and d) display the 2D XANES-XEOL maps of the as-prepared ZnO nanocomb and ZnO nanoflag, in which the colors represent the intensities of the XEOL excited across the O K-edge and the Zn L3, 2-edge. Representative wavelength-selective PLY (vertical cuts taken in the 2D XANES-XEOL map) are shown in Figure 7 (e, f, g and h). For comparison, TEY, and PFY are also shown. All PLY spectra show similar features as those of TEY and PFY spectra at O K-edge and Zn L3, 2-edge in both nanocomb and nanoflag samples, suggesting that O and Zn excitation transfer (mainly via the


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thermalization of Auger electrons) their energy to both optical channel more or less proportionally. The wavelength-selected PLY XANES spectra recorded from the band gap emission and defect emission regions were studied in details. In the PLY spectra of ZnO nanocomb shown in Figure 7e and 7f, the PLYDE spectrum is almost the same as the zero order PLY spectrum while PLYBG is very weak at both O K-edge and Zn L3, 2-edge, but with similar appearance at the edge jump. This is not unexpected since defect emission dominates, indicating that the PLY of ZnO nanocomb is proportional to the absorption coefficient albeit there is noticeable but not too dramatic change in the branching ratio of the BG and the DE. In Figure 7g, the PLYBG spectrum of the nanoflag shows nearly the same shape as that of PLYDE at O K-edge. Similarly, at the Zn L3, 2-edge from ZnO nanoflag (Figure 7h), these results confirm better crystallinity of the nanoflag than that in the nanocomb and reveals some bulk defects in the nanoflag not observable in PL and CL. It has been reported that green defect emission can be assigned to oxygen vacancies in the near surface region of ZnO nanostructures, while band gap emission yield represents the bulk properties.44 Thus, the relative intensity of the band gap emission (IBG) and defect emission (IDE) has bearings on the validity of this model. It has been shown that the IBG/IDE ratio is a measure of the crystallinity of ZnO and in a nearly perfect ZnO nanostructure, band gap emission is very intense and defect emission is negligible. It should be noted that three factors influence this ratio: (i) kinetic energy of the primary electrons involved in the thermalization (ii) the penetration depth of the excitation photon and (iii) distribution of the defects in the sample. (i) and (ii) are associated with the probing photon energy and the elements and edges of interest and (iii) is the property of the sample. It must be noted that (i) and (ii) can change dramatically across an absorption edge, especially with soft X-rays since at excitation above an absorption edge, new Auger channels are turned on (O KVV Auger at the O K-edge


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and Zn LVV at the Zn L3, 2-edge for example), the thermalization path will be different than those excited with energy below the edge (in the case of O, below the O K-edge, only valence electrons are excited, no core hole is created; in the case of Zn L3, 2-edge, O KVV Auger are excited below the edge and both O KVV and Zn LVV Auger are excited above the edge and the latter becomes a dominant process when it is turned on). Thus, the thermalization process across the absorption edge will show qualitative changes despite the complexity of the process in which Auger plays a dominant role in converting the X-ray energy into optical photons.

Figure 7. 2D XANES-XEOL map of ZnO nanocomb with excitations at O K-edge (a) and Zn L3, 2-edge

(b); 2D XANES-XEOL map of ZnO nanoflag with excitations at O K-edge (c) and Zn L3,

2-edge

(d). The x-axis is the emission wavelength in nm and the y-axis is the excitation energy in

electron voltage (eV). The color represents the intensity of XEOL excited across the absorption edge. Wavelength selective integral PLY (vertical cuts taken in the 2D XANES-XEOL map at a selected wavelength) of ZnO nanocomb (e) O K-edge and (f) Zn L3, 2-edge; and ZnO nanoflag (g) O K-edge and (h) Zn L3, 2-edge. The weak PLY signals from a minor emission band are


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amplified compared to PLY from a major emission band and the zero-ordered PLY. All PLY, TEY, and FLY intensity are normalized to the incident photon flux. For a given ZnO nanostructure with defects, the IBG/IDE will remain nearly the same if the defects are evenly distributed in the sample but will change if the distribution of defects are different near the surface and in the bulk. In Figure 8a, the XEOL spectra of ZnO nanocomb excited across the O K-edge show the UV emission around 378 nm and the visible emission in the range of 450~675 nm, attributable to the band gap emission and defect emission, respectively. The similar optical feature can also be observed from the XEOL spectra excited across the Zn L3, 2-edge (Figure 8b). From the inset of Figure 8a, the IBG/IDE shows an increase from 0.08 to 0.20 when the exciting energy (Eex) changes across the threshold of the O K-edge, after that, IBG/IDE is kept around 0.19. The XEOL spectra excited across the Zn L3, 2-edge show the stable feature with the ratio of the IBG/IDE focused on 0.18, as shown in the inset of Figure 8b. Comparing the value of IBG/IDE in the different element excitation of O K-edge and Zn L3, 2-edge in the ZnO nanocomb, the energy transfer to the band gap emission channel is almost the same effective with O K-edge than Zn L3, 2-edge excitation, except a lower effect in the pre-edge of O K-edge excitation. In contrast to ZnO nanocomb, both of the XEOL spectra of ZnO nanoflag excited across the O K-edge (Figure 8c) and the Zn L3, 2-edge (Figure 8d) show intense UV emission around 378 nm and weak visible emission in the range of 450~700 nm. From the inset of Figure 8c, the value of IBG/IDE excited across the O K-edge show the range of 5.2 to 7.3 (Eex changes from 520 eV to 575 eV). However, the IBG/IDE excited across the Zn L3, 2-edge (the inset of Figure S7d) shows a fluctuant change from 3.5 to 6 (from Eex =1015 eV to Eex =1070 eV) and both of the O K-edge and Zn L3, 2-edge excited IBG/IDE variation tendency match TEY spectral features. Comparing the value of IBG/IDE in the different element excitation of O K-edge and Zn


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L3, 2-edge in the ZnO nanoflag, the contributions of the band gap emission favors O excitation. Obviously, the ratios of IBG/IDE for ZnO nanoflag excited across both of O K-edge and Zn L3, 2edge are much higher than those of ZnO nanocomb, revealing a much better crystallinity of nanoflag albeit also revealing some bulk defects not detected in the PL and CL defects which are unable to be detected by PL or CL.32, 45-46 Moreover, both Zn and O are contributed to the UV emission of ZnO nanocomb and nanoflag, and the UV emission originated from oxygen excitation channels shows a little bit more than that of zinc excitation channels.

Figure 8. XEOL spectra of the O K-edge (a) and Zn L3, 2-edge (b) of ZnO nanocomb (horizontal cuts taken in the 2D XANES-XEOL map of Figure 7a and 7b, respectively); XEOL spectra of the O K-edge (c) and Zn L3, 2-edge (d) of ZnO nanoflag (horizontal cuts taken in the 2D XANES-XEOL map of Figure 7c and 7d, respectively. The insets of (a-d) show the intensity ratios of IBG/IDE (height ratio) versus excitation energy. CONCLUSION


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ZnO nanocomb and ZnO nanoflag were successfully synthesized for the first time by CVD method with Au NPs as the catalyst. STXM characterization confirms the thickness variation between ZnO nanocomb and ZnO nanoflag and reveals more defect sites in ZnO nanocomb compared to ZnO nanoflag. PL measurements with a weak excitation energy of 3.8 eV show that there are both band gap emission and green emission in ZnO nanocomb, but only band gap emission in ZnO nanoflag. Meanwhile, CL spectra of ZnO nanocomb and ZnO nanoflag exhibit a similar result except a much higher IBG/IDE in CL because CL could get more bulk information due to the deeper detection depth via a higher electron accelerating voltage (5 keV) under SEM measurement. Spot CL clearly reveals that the teeth in the nanocomb are more crystalline than the comb hand. 2D XANES-XEOL were applied to systematically investigate the optical properties by distinguishing the XEOL spectra of the O K-edge and Zn L3, 2-edge. In the ZnO nanocomb, the O K-edge excitation channel contributed more effective to the band gap emission than the Zn L3, 2-edge excitation channel. ZnO nanoflag displays an excellent crystallinity with weak defect emission revealed with XEOL, not observed in PL and CL. This work has presented a comprehensive and systematic study on the morphology, growth orientation, structural features and the luminescence mechanism of ZnO nanostructures using a combination of powerful techniques, especially the synchrotron-based STXM and 2D XANES-XEOL, which can be used for the study of a wide variety of luminescent nanostructures. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. STEM mapping images, HRTEM images, Profiles of the lengths of comb teeth and the intervals


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between the comb teeth and SEM image of nanocomb, STXM images and thickness tables of the uniform ZnO nanocomb and nanoflag (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (X. Sun), Phone: +86-512-65880943, Fax: +86-512-65880280; [email protected]

(Y. Ma), Phone: +86-512-65884530; [email protected] (T. Sham),

Phone: +01-519-661-2111 ext. 86341 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (NSFC) (U1432249), the Priority Academic Program Development of Jiangsu Higher Education Institutions. This is also a project supported by Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices and Collaborative Innovation Centre of Suzhou Nano Science & Technology. Synchrotron experiments were performed at the Canadian Light Source, which is supported by NSERC, NRC, CIHR and the University of Saskatchewan. We thank Xionghui Zeng and Shunan Zhen of SINANO for their technical assistance for the CL and PL measurements, respectively. Research at the University of Western Ontario is supported by NSERC, CRC, CFI, and IDI. We thank Tom Regier of the CLS for his technical assistance for the XEOL measurements. REFERENCES


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Synthesis and Structure-Dependent Optical Properties of ZnO

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