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Geometrical Separation of Defect States in the ZnO Nanorod and Their Morphology-Dependent Correlation between Photoluminescence and Photoconductivity Cheolmin Park, Jihye Lee, and Won Seok Chang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04033 • Publication Date (Web): 29 Jun 2015 Downloaded from http://pubs.acs.org on July 2, 2015

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The Journal of Physical Chemistry

Geometrical Separation of Defect States in the ZnO Nanorod and Their Morphology-Dependent Correlation between Photoluminescence and Photoconductivity

Cheolmin Park,a,b Jihye Lee,a,b Won Seok Chang a,b,* a

Department of Nano-mechatronics, Korea University of Science and Technology (UST), 217

Gajeong-ro Yuseong-gu, Daejeon 305-333, Korea b

Nano-convergence Mechanical System Research Division, Korea Institute of Machinery and

Materials (KIMM), 156 Gajungbukno, Yuseong-gu, Daejeon 305-343, Korea *To whom correspondence should be addressed. E-mail: [email protected]

ACS Paragon Plus Environment

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Abstract An understanding of the morphology-dependent correlation between photoluminescence and photoconductivity in the nanostructured ZnO is important for elucidating the carrier dynamics and expanding its use in optoelectrical applications. In this study, we investigated this relationship using distinctly different ZnO nanorods with diameters greater than 100 nm, which were produced by a hydrothermal method. Further, in order to study the effects of its defect states on the correlation, we thoroughly characterised the defect states of the ZnO nanorods in terms of the light-penetration depth during photoluminescence. The photoconductivities of the nanorods were measured using light sources with wavelengths of 355, 405, and 532 nm to confirm the influences of the visible-emission-generating defects on carrier transport. We found that the intensity of the near-band-edge emission was almost comparable to the amount of photocurrent generated under ultraviolet (UV) light; this could be attributed to the crystallinity of the inside of ZnO nanorods. However, the concentration of the surface defects resulting from the size and morphology probably had an effect leading to the observed differences in the photocurrent sensitivity under low-intensity UV light, the dark current level, the amount of photocurrent under a specific wavelength of light within the visible range, and the persistent photoconductivity. The results of this study could aid research on carrier dynamics in nanostructured ZnO and further its use in optoelectronics.

Keywords Zinc oxide, nanostructure, defect states, photoconductivity, photoluminescence


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Introduction Various types of ZnO nanostructures can be synthesised readily by solution-phase and vapourphase processes such as molecular beam epitaxy,1 metal-organic chemical vapour deposition,2 pulsed laser deposition,3 hydrothermal growth,4 electrochemical deposition,5 and template deposition,6 to name a few. This is owing to the polarity characterisitics of ZnO depending on the lattice plane of its surface in the wurtzite crystal structure.7 With the ability to be fabricated to diverse nanostructures, a direct wide band gap (3.37 eV) with large exciton binding energy (60 meV) at room temperature,8 the piezoelectric and pyroelectric properties,9,10 suggest that ZnO has great potential for use in various applications. The use of nanostructures with specific morphologies can improve the efficiency, activity, and sensitivity of ZnO devices such as ultraviolet (UV) nanolasers,11 solar cells,12 photonic cyrstals,13 chemical sensors,14 and UV photodetectors,15 etc. However, it is well-known that the properties of nanostructured ZnO are determined mainly by its size, morphology and growth conditions; these factors, in turn, are related to the presence of native defect states.8 Since these defect states affect the carrier dynamics and thus determine the optoelectric properties, significant efforts have been made to investigate the relationship between specific ZnO nanostructures and their characteristics. One method of analyzing crystal quality of ZnO nanostructure with shallow and deep energy levels is to measure the photoluminescence (PL) arising from carrier recombination. However, PL studies have a limitation in that they are not suited for investigating the conductivity originating from carrier transport because of the different mechanisms of carrier dynamics. Since the transport mechanism is critical with respect to nanostructured ZnO-based optoelectronics, the effect of nanostructured ZnO morphology on its conductivity should be examined. To investigate the conductivity of nanostructured ZnO involving its defect states, the photoconductivity for various wavelengths of light should be determined. Thus, a comparative analysis of the PL and


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photoconductivity could result in a clear and detailed understanding of the carrier dynamics for specific nanostructures. In the present study, we determined the morphology-dependent correlation between the PL and photoconductivity using ZnO nanorods with two distinct morphologies grown through a hydrothermal method. First, the defects on the surfaces and in the bulk of the samples were identified with precision. This was done by comparing the spectra of macro-PL, micro-PL, and cathodoluminescence (CL), because the defect states could be nonuniform and varied with the depth. Further, the morphology dependency of the defect density was characterized visually on the basis of spatially resolved micro-PL (i.e., by mapping the micro-PL spectra). In addition, the carrier dynamics regarding to the defect states was determined using time-resolved PL (TRPL) by comparing both samples. Then, to investigate the effects of the defect states on the photoconductivity, we measured photocurrents under laser irradiation with wavelengths of 355, 405, and 532 nm; this was done using a metal-semiconductor-metal (MSM)-type device. Owing to the use of radiation with different wavelengths, comparative study was performed between the defect-related photoconductivities and the PL spectra.

Experimental Section Sample synthesis To hydrothermally grow the ZnO nanostructures on 2-inch SiO2/Si wafers, we deposited a nanoline seed layer by UV-nanoimprint lithography (UV-NIL). The deposition process has been described in detail previously.16 The ZnO precursor for UV-NIL, which contained resin, was prepared as follows: (1) the molar equivalent of monoethanolamine (NH2CH2CH2OH, Aldrich, 99.5%), 0.5 mol zinc acetate dehydrate (Zn(CH3COO)2∙2H2O, Aldrich, 99.5%), and 2-nitrobenzaldehyde (UV-linker, Aldrich) were dissolved in 2methoxyethanol (CH3OCH2CH2OH, Aldrich, 99.5%); (2) the resultant solution was stirred at


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24°C for 3 h and at 75°C for 24 h to produce a homogeneous solution. Hydrothermal growth was achieved by immersing the substrates into this homogeneous solution at 70°C for 25 min (for the thick nanorods) and at 90°C for 25 min (for the thin nanorods). Then, the substrates were rinsed with deionized water and dried by blowing N2 gas. The solution for the hydrothermal growth consisted of 0.834 mM polyethylenimine (Aldrich, molecular weight of 1300 g/mol) and 25 mM zinc nitrate hexahydrate (Zn(NO3)2 ∙ 6H2O, Aldrich, 98%) in aqueous water, along with 25 mM hexamethylenetetramine (C6H12N4, Aldrich, 99.5%). To measure the photoconductivity of the samples, an MSM-type device was fabricated using photolithography and the lift-off process. Electrodes of Ti/Au (10 nm/150 nm) were deposited using the thermal evaporation method (KVE-T2000, Koreava). The active area of the device was 5 × 5 µm2. Characteristics The morphologies of the synthesised ZnO nanostructures were characterised using cold-type scanning electron microscopy (SEM) (S-4800, Hitachi), which was performed at an acceleration voltage of 1 kV and emission current of 8.6 pA. The crystallinities and orientations of the two types of ZnO nanorods were determined using X-ray diffraction (XRD) analysis (D8 Advance, Bruker) performed with Cu Kα radiation (wavelength, λ = 1.5418 Å). The 2θ range was 31–38° and the scan speed was 0.2°/min. The macro-PL spectra for determining the surface-layer emissions were obtained using a spectrofluorometer (FluoroLog®-3, Horiba) for wavelengths of 300–700 nm; the resolution was 0.35 nm. The samples were positioned at 45° between the excitation light (which had a peak at 265 nm; 1.12 mW/cm2) and the detector, which had a measured area of 102.8 mm2. The micro-PL spectra for studying the bulk emissions were measured using a Plan Fluor 100× objective lens (numerical aperture of 0.9) and a 355 nm excitation laser (4.36 MW/cm2); the spot size was about 0.18 µm2. To determine the relative carrier densities of the samples, which were associated with the defect states, their spatially resolved micro-PL spectra were observed using a Peltier-cooled charge-coupled device detector


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(Newton, Andor). The CL measurements were performed for wavelengths of 280–900 nm in situ using an SEM instrument integrated with a Gatan MonoCL system. An acceleration voltage of 10 keV was used, and the measured area was 9 µm2. To study the carrier dynamics of the ZnO defect states generating visible emissions, the TRPL were obtained using a 405-nm laser diode as the excitation source; it provided an optical pulse with duration of less than 200 ps and a frequency of 1 MHz. The emission detector was a photomultiplier tube. The dark currents and the photocurrents produced under the light sources with wavelengths of 532, 405, and 355 nm were measured with a high-speed source/monitor unit (E5262A, Agilent Technologies) in a dark room. The intensities of the light sources were identical at 2 mW/cm2. All the measurements were performed at room temperature.

Results and Discussion The morphologies of the ZnO nanorods grown hydrothermally are shown in Figures 1a–c. The seed layer was deposited on the 2-inch wafers by means of UV-NIL, to measure the macro-PL spectra. The ZnO nanoparticles on the substrate acted as nucleation sites after being subjected to wet etching with a 0.25% HNO3 solution, as shown in Figure S1 (Supporting Information); this led to the synthesis of randomly oriented nanorods (hereafter called rods) during the hydrothermal growth processes. The morphologies of the thick rods (Figure 1a) and the thin rods (Figure 1b) had aspects ratios of 1 and 3.3, respectively. The thick rods exhibited a distinct and large Zn-terminated (0001) polar plane on the hexagonal top surface and a (1010) rectangular side plane as the nonpolar surface. However, the thin rods exhibited only nonpolar planes owing to the pyramidal (1011) plane on the top surface.17 The crystallinities of the two samples were determined from their XRD patterns as shown in Figure 1d. The XRD peaks at 31.7°, 34.4°, and 36.1° corresponded to the (1010), (0002), and (1011) planes of the wurtzite crystal structure.


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Although the both samples grew preferentially along the c-axis, their grain sizes, obtained from the full width at half maximum (FWHM) of the peak at 34.4° using the Scherrer equation, were 79 nm (thick rods) and 89 nm (thin rods), respectively. The smaller grain size of the thick rods can be attributed to their relatively lower growth temperature, because hydrothermal growth at low temperatures (70°C) results in poor crystal quality.8,18

Figure 1. Top-view SEM images (a) the thick rods and (b) the thin rods. (c) Cross-sectional SEM images (upper image is of the thick rods and the lower one is of the thin rods). In the top view, the thick rods have hexagonal top surface, while the thin rods are pyramidal. (d) XRD patterns of the thick and thin rods. The scale of the y-axis in the stacked graphs is identical. Both


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types of rods grew preferentially along the c-axis of the wurtzite hexagonal structure, as shown in the inset of (d). The diameters/lengths of the thick and thin rods were approximately 200/200 nm and 150/500 nm, respectively.

As mentioned previously, PL studies have been employed to analyze the crystallinity of ZnO. However, with regard to nanostructured ZnO, Djurišić et al.8 and Fabbri et al.17 have suggested that determining the crystallinity on the basis of the ratio of the near-band-edge (NBE) emission and the deep-level emission (DLE) can lead to an incorrect result. In addition, a few previous reports have shown the observed discrepancy regarding the origin of the DLE. For instance, through studies about the size and morphology dependency on the properties of ZnO nanostructures, Shalish et al.19 reported that the DLE arises mainly from surface layer, whereas Chang et al.20 have suggested that it originates from the bulk. We assume that this disagreement could be associated with the penetration depth of the excitation light for the specific ZnO nanostructures, in which the defect states vary with the depth. Therefore, in this study, to determine the defect states in nanostructured ZnO on the basis of its photoemission, we measured the macro- and micro-PL spectra for excitation lights with different wavelength and intensities, in order to obtain PL spectra from dissimilar penetration depth. (Figure S2a). Figures 2a and 2b show the schematics of the set ups for measuring the macro-PL and the micro-PL spectra. The thickness of the surface layer in the nanostructured ZnO was expected as approximately 30 nm;19 this would correspond to an absorption of 48.7% of the excitation intensity on the macro-PL as shown in the Figure S2a because the penetration depth of the light was shallow due to its short wavelength of 265 nm and low intensity (1.12 mW/cm2). Therefore, it cannot induce strong PL emission from the deep depth of ZnO rods. However, with a comparatively long wavelength of 355 nm and high intensity (4.36 MW/cm2) of the excitation


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light, which were achieved by focusing beam using a high NA 100× objective lens, the microPL can have emission from the inside bulk of the both rods almost completely. Furthermore, by the largely different measurement areas (macro-PL: 102.8 mm2, micro-PL: 0.1817 µm2), the macro-PL has more surface emission than the micro-PL. That is why we fabricated both samples in the large area by UV-NIL with hydrothermal growth. Consequently, the macro-PL spectra are mainly generated from the surface layer, while the micro-PL spectra represent mostly the photoemissions from the bulk.

Figure 2. Schematics showing the set up used for measuring the (a) macro- and (b) micro-PL spectra. In case of the macro-PL spectra, the wavelength of the excitation light was 265 nm (1.12 mW/cm2) and the spot area was 102.8 mm2, whereas for the micro-PL spectra, the excitation wavelength was 355 nm (4.36 MW/cm2) and the spot area was 0.1817 µm2. In addition, the excitation light of the micro-PL was focused into the bulk; this is represented by the pink spot in


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the (b). (c) and (d) show the macro- and micro-PL spectra, respectively. The violet rectangle in (c) presents the violet-blue emission range. The inset in (d) shows the corresponding normalized micro-PL spectra.

As can be seen from the macro-PL spectra in Figure 2c, the thick rods exhibited several broad peaks of spectral emissions, such as a NBE (a peak at 392 nm), violet-blue (a peak at 466 nm) and green-yellow emissions (a peak at 562 nm). Normally, the peak of NBE emission is around 380 nm at room temperature. However, the reason of the largely red shifted NBE emission peak (392 nm) could be due to the competition of emissions in the various energy levels, which has been well-known. Furthermore, especially, since the macro-PL contains most of emission from the surface layer, the influence could be increased due to the surface defects in the ZnO nanostructure. In the case of the thin rods featuring only non-polar planes, violet-blue emissions (peak at 439 nm) were mostly observed. If we assume that the emission depends on the nonpolar planes based on this observation, it can be deduced that both NBE and green-yellow emissions in the thick rods are relatively generated from the polar plane on the top surface. The origins of all emission in the both rods are well documented as native defect states. The peaks at 439 and 508 nm in the thin rods were attributable to the recombination of the interstitial zinc (Zni) with zinc vacancies (Vzn)21 and the combination of singly charged oxygen vacancies (Vo+) with photo-excited holes in the valence band, respectively.22 The peaks at 466 and 562 nm in the thick rods were respectively caused by the various extended Zni,23 and the doubly charged oxygen vacancies (Vo++)24 in the grain-boundary-induced depletion regions. It should be noted that, as was evident from their macro-PL spectrum, the thin rods did not exhibit NBE emissions. This indicates that the thin rods had a higher density of surface defects. In their micro-PL spectrum, which was clearly different from their macro-PL spectrum, as can be seen from Figure


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2d, the thin rods exhibited higher-intensity NBE emissions and a lower-intensity DLE than did the thick rods. This phenomenon indicates that the thin rods had better crystallinity, which might be attributable to their highly c-axis-oriented crystal structure and larger grain size (see XRD data). However, although the thin rods had better crystallinity, the inset of Figure 2d shows that their NBE emission was red shifted compared to that of the thick rods. We believe this red shift is caused by the high density of surface defects. These could be associated with the surface-tovolume ratio of the nanorods, which depends on their size and morphology.25,26 Therefore, the thin rods,

Figure 3. (a) CL curves of the two types of rods; the spectral range was 280–900 nm. The measured area was 3 × 3 µm2, and the penetration depth of the electron beam was 458 nm at the 10 keV acceleration voltage (Figure S2b). The strong visible emission in the thick rods was


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deconvoluted into six Gaussian functions. (b) The normalized micro-PL and CL spectra in the visible spectral range; the overall shapes of the curves were mostly similar, and they primarily consisted of green-yellow-orange emissions. (c) Normalized visible spectra, which show a comparison of the micro-PL and CL curves. The measured area of CL (9 µm2) is larger than the focused spot area of the micro-PL (0.1817 µm2). The vertical blue dashed line indicates the inflection point of the pink areas, which exhibited a significant change of the area. The acid green rectangle indicates the spectral range for the green-yellow emissions.

which had a high aspect ratio and pyramidal shape on the top surface, exhibited a narrower band gap due to high density of defect states, which caused the red shift. Further, this red shift is the same phenomenon with the nonexistence of the NBE emission in the macro-PL. To visually confirm the difference of defect densities between the two samples, micro-PL mapping was performed (Figure S3). The results showed that the thick rods exhibited only medium-intensity NBE emissions, while the thin rods exhibited mainly low-intensity emissions with locally highintensity emissions, suggesting that the density of surface defects in the thick rods was lower than that in the thin rods. The locally high intensity of emissions in the thin rods (Figure S3b) could be attributed to the good crystallinity of their bulk and high aspect ratio. In case of the visible emissions in the micro-PL spectra, both rod types exhibited predominantly green-yellow emissions (a peak at 593 nm); when comparing the macro-PL spectra, this could indicate that there are nonuniform defect states between on the surface and in the bulk. To verify the defect states depending on the depth in a broader spectral range, we performed CL measurements based on SEM (see the description of Figure 3 for measuring condition of CL). Figure 3a shows the CL spectra of the two samples. The intensity ratios of the NBE emissions in the both samples were similar to that for the micro-PL spectra. In the visible range, the CL curves


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of both samples had similar shapes although their intensities were dissimilar. Further, the shape of the curves was analogous to the micro-PL spectra, as can be seen from Figure 3b. Thus, the micro-PL and CL spectra confirmed that the defects in the bulks of the two samples mainly generated broad green-yellow-orange emissions with a peak at approximately 585 nm. Especially, the thick rods exhibited DLE with a remarkably higher intensity. Considering the surface-to-volume ratios, this result confirmed that the DLE was also associated with the internal bulk of the hydrothermal grown ZnO nanorods, which had a diameter around 150 nm. This observation is in keeping with the results reported by Chang et al.20. The assumption for the defect states classified into surface and bulk was confirmed through a comparison of the macroand micro-PL spectra. Further, by comparing the CL and micro-PL spectra, this can be verified as shown in the Figure 3c, owing to the nonidentical measurement conditions such as the largely measured area of CL and the focused excitation beam of the micro-PL. In the figure, the cyan areas are the highly affected zones by the high-intensity of the micro-PL emissions from the bulk, while the pink areas are due to the surface emissions by CL. The pink areas and the points of vertical dashed arrows agreed well respectively with the intensity curve of the macro-PL spectra.


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Figure 4. Energy levels in the (a) thick rods and (b) thin rods, showing the surface defects (lightblue) and bulk defects (royalblue). As can be seen from the results of the emission study, the violet-blue emissions mostly arise from the surface, while the green-yellow-orange emissions are mainly generated from the bulk. The thick lines of recombination present the high intensity of emissions.

The energy levels related to the defects identified on the surface and in the bulk from the measurement data of macro-, micro-PL and CL are shown in Figure 4. The energy level diagrams depict a probability of existing high densities of defect states. Therefore, the surface layer of the thin rods also could have the yellow-green emission due to Vo++. Likewise, the surface layer of the thick rods could have Vo+ energy levels. In the diagrams, owing to the violetblue emissions from the surface layer, the native defects were mainly considered to be Zni, Vzn, and Vo+ in the surface layer. Similar bulk defects were mostly observed in both samples as the green-yellow-orange emissions; these were related to Zni, Oi, and Vo++. Especially, it has been known that the origin of orange emissions arises from the bulk, caused by the recombination of Zni with the oxygen interstitials (Oi),27 which was also confirmed by our experiments.


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Figure 5. TRPL data for the (a) thick rods and the (b) thin rods. Using excitation light with a wavelength of 405 nm, visible-spectrum PL decay curves were obtained as 500, 550, and 600 nm. This suggests the effect of the defect states on the carrier dynamics. The maximum intensities of PL decay curves at the detection wavelengths of 500, 550, and 600 nm were dissimilar with the micro-PL and CL. This can be ascribed to the low energy (405 nm) of the excitation light.

Table 1. Magnitudes and decay time constants of exponential fitting functions at 500, 550, and 600 nm in the PL decay curves (unit of 𝐴: counts, unit of 𝜏: nsec) Thick

𝑨𝟏

Thin

Thick

𝝉𝟏

Thin

Thick

𝑨𝟐

Thin

Thick

𝝉𝟐

Thin

Thick

𝑨𝟑

Thin

Thick

𝝉𝟑

Thin

500 nm

14629

2072

0.79

0.90

704

468

3.7

3.9

45

29

43.9

Geometrical Separation of Defect States in ZnO ... - ACS Publications

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