Research Article www.acsami.org
Novel Green Luminescent and Phosphorescent Material: Semiconductive Nanoporous ZnMnO with Photon Confinement Sejoon Lee,*,†,‡ Youngmin Lee,‡ and Gennady N. Panin‡,§,# †
Department of Semiconductor Science, Dongguk UniversitySeoul, Seoul 04623, Korea Quantum-Functional Semiconductor Research Center, Dongguk UniversitySeoul, Seoul 04623, Korea § Nano Information Technology Academy, Dongguk UniversitySeoul, Seoul 04623, Korea # Institute of Microelectronics Technology and High-Purity Materials Russian Academy of Sciences, 142432 Chernogolovka, Russia ‡
S Supporting Information *
ABSTRACT: A novel green luminescent and phosphorescent material of semiconductive nanoporous ZnMnO was synthesized by the thermal nucleation of nanopores in the 20-period Zn0.93Mn0.07O/Zn0.65Mn0.35O multilayer structure. Nanoporous ZnMnO showed an n-type semiconducting property and exhibited an extremely strong green light emission in its luminescence and phosphorescence characteristics. This arises from the formation of the localized energy level (i.e., green emission band) within the energy band gap and the confinement of photons. The results suggest nanoporous ZnMnO to have a great potential for the new type of semiconducting green phosphors and semiconductor lightemitting diodes with lower thresholds, producing an efficient light emission. In-depth analyses on the structural, electrical, and optical properties are thoroughly examined, and the formation mechanism of nanoporous ZnMnO and the origin of the strong green light emission are discussed. KEYWORDS: nanoporous ZnMnO, multilayers, strong green emission, green luminescence, green phosphorescence, photon confinement, functional semiconducting-oxide light,12−14 to adjust the color of light,15−18 and to improve the speed of data transmission.9,10 Very recently, additionally, the voltage-tunable multicolor emission was demonstrated on an organic−inorganic hybrid LED, which had been composed of highly luminescent polymer and semiconductive phosphorescent materials.19 These give us a hint to realize the highperformance optoelectronic devices for future SSL and VLC thorough utilizing the novel-functional semiconductive lightemitting or light-converting materials. Motivated by all the above virtues, we have fabricated a new type of the high-quality semiconductive luminescent and phosphorescent material−nanoporous ZnMnO. In this Research Article, we report on the fabrication and the characterization of nanoporous ZnMnO as a semiconducting green luminescent and phosphorescent material with the confinement of carriers and photons. The structural, electrical, and optical characteristics of nanoporous ZnMnO are thoroughly examined, and the formation mechanism and the origin of the strong green luminescence for nanoporous ZnMnO are discussed in details.
1. INTRODUCTION The development of light-emitting and light-converting materials remains a major challenge for solid-state lighting (SSL) devices1−3 and visible light communication (VLC) systems,4−10 where the light-emitting diode (LED) or the laser diode (LD) can be utilized as an active light source. In comparison with conventional light-emitting devices and lower frequency communication systems (e.g., Wi-Fi, Bluetooth, etc.), the SSL devices and the VLC modules have many advantages, such as a high energy efficiency and an environmental friendliness.8 Typically, white-light sources in SSL lamps and VLC systems are made of light emitting material-based blueLEDs or blue-LDs with light-converting phosphors, which partially convert the blue light into the green, yellow, and red lights.11 Among various light-converting materials, the green phosphor is currently of great interest because of possible redgreen-blue (RGB) schemes2,3 for more efficient white-light emissions, compared to other schemes (e.g., blue-yellow mixing). In addition, the semiconductor LDs with RGB phosphors come to provide better efficiencies for highperformance laser projectors that can display or transmit the information data. Furthermore, the usage of semiconductive RGB light-emitters and light-converters with a shorter carrier lifetime enables us to electrically switch the wavelength of © 2017 American Chemical Society
Received: February 1, 2017 Accepted: June 1, 2017 Published: June 1, 2017 20630
DOI: 10.1021/acsami.7b01557 ACS Appl. Mater. Interfaces 2017, 9, 20630−20636
Research Article
ACS Applied Materials & Interfaces
2. RESULTS AND DISCUSSION Figure 1 shows the surface textures of the as-grown and annealed ZnMnO multilayers (i.e., 20-period of Zn0.93Mn0.07O
Figure 2. XRD patterns of the Zn0.93Mn0.07O (∼700 nm) and the Zn0.65Mn0.35O (∼700 nm) films. (Inset) Diffraction patterns near the Bragg angle at (0002) for the Zn0.93Mn0.07O and the Zn0.65Mn0.35O films measured by HR-XRD.
interstitials and/or a formation of MnOx precipitates although no diffraction peaks from secondary phases are observable in the sample. According to Fujimura et al.,20 the (000l) plane has the lowest surface energy to form a c-axis-preferential hexagonal array of wurtzite film textures. Hence, we can conjecture that Zn0.93Mn0.07O has a much stronger c-axis preference with a lower surface energy, compared to Zn0.65Mn0.35O. Additionally, it should be noted that the Bragg angle (i.e., peak position of the (0002) phase) is shifted toward the lower angle region when the film contains a larger concentration of manganese (see the inset of Figure 2). We attribute this feature to an increased lattice expansion because, in Zn0.65Mn0.35O, the larger amount of bigger Mn2+ ions (0.89 Å)21 are substituted to smaller Zn2+ sites (0.74 Å).22 In other words, the lattice spacing of Zn0.65Mn0.35O is bigger than that of Zn0.93Mn0.07O. As illustrated in Figure 3, therefore, one may expect the Zn0.65Mn0.35O layer and the Zn0.93Mn0.07O layer to receive the compressive stress and the tensile stress, respectively. And also, the layers might have different surface energies in each other, as discussed earlier. These lead to the grooving and agglomeration in the whole stack of the multilayers during thermal annealing at Ta above the recrystallization temper-
Figure 1. SEM images of the ZnMnO multilayers (i.e., 20 periods of Zn0.93Mn0.07O (∼8 nm)/Zn0.65Mn0.35O (∼8 nm)) before and after thermal annealing: (a) as-grown, (b) annealed at 900 °C, (c) annealed at 950 °C, (d) annealed at 1000 °C, (e) annealed at 1050 °C, and (f) annealed at 1100 °C.
(∼8 nm)/Zn0.65Mn0.35O (∼8 nm)), where the compositions of Zn and Mn were measured through X-ray photoelectron spectroscopy (XPS). For the as-grown sample, the scanning electron microcopy (SEM) image of the top layer displays a smooth surface with no visible pinholes or hillocks (Figure 1a). This depicts that the ZnMnO multilayers were well grown layer-by-layer on the Al2O3 substrate. After annealing at 900 °C, the surface becomes wrinkle and exhibits a lot of nanocraters (Figure 1b). The craters begin to change into the nanopores when the annealing temperature (Ta) is increased to 950 °C (Figure 1c). As Ta increases up to 1050 °C, the pores become clearer and deeper (Figures 1d and 1e). When Ta is further increased to 1100 °C, the sample clearly reveals plenty of hexagonal pores with the diameter of 300−500 nm on average (Figure 1f). The ratio of Zn to Mn concentrations in porous ZnMnO was confirmed by XPS to be approximately 8:2 (i.e., Zn0.8Mn0.2O). The formation of porous ZnMnO is thought to result from the thermal nucleation of the ZnMnO multilayers because each layer will receive the biaxial stress, leading to grooving and agglomeration during thermal annealing, as discussed in detail later. To clarify this hypothesis, we examined the structural properties of the Zn0.93Mn0.07O (∼700 nm) and Zn0.65Mn0.35O (∼700 nm) films through X-ray diffraction (XRD) measurements. As shown in Figure 2, both the Zn0.93Mn0.07O and the Zn 0.65 Mn 0.35 O films exhibit (0002) and (0004) peaks corresponding to the wurtzite lattice phases. The peak intensity of the (0002) phase is 12-times greater for Zn0.93Mn0.07O than that for Zn0.65Mn0.35O. The degradation of the (0002) intensity in Zn0.65Mn0.35O might arise from an existence of Mn
Figure 3. Schematic configuration of the ZnMnO multilayers, where the Zn0.93Mn0.07O and the Zn0.65Mn0.35O layers receive biaxial stresses, resulting in grooving and agglomeration during the thermal nucleation process. 20631
DOI: 10.1021/acsami.7b01557 ACS Appl. Mater. Interfaces 2017, 9, 20630−20636
Research Article
ACS Applied Materials & Interfaces ature.23,24 As a result, the nanopores will be formed in the entire region of the ZnMnO multilayers. In Mn-doped ZnO, a verification of valence states and local structures of Mn is of significant importance because those are crucial for determining a key feature of the material characteristics in the entire material system. Thus, we first characterized chemical bonding states of Mn in nanoporous ZnMnO through XPS measurements. As can be seen from the inset of Figure 4, the sample clearly shows the XPS peaks from
Figure 5. EPR spectrum of nanoporous ZnMnO. (Inset) First derivate of the EPR data.
native donors (e.g., oxygen vacancies (VO), zinc interstitials (Zni), etc.)30−33 or acceptors (e.g., zinc vacancies (VZn), oxygen interstitials (Oi), etc.).30−33 We, accordingly, examined the electrical properties of the ZnMnO multilayers through Halleffect measurements. As listed in Table 1, for the as-grown Table 1. Electrical Properties of the ZnMnO Multilayers Examined by Hall-Effect Measurements
Figure 4. Valence states of Mn in nanoporous ZnMnO. (Inset) XPS spectrum of the Mn 2p core levels in nanoporous ZnMnO.
samples
Mn 2p3/2 and Mn 2p1/2 core levels, which represents an effective incorporation of Mn in nanoporous ZnMnO. To analyze the valence states of Mn, we deconvoluted the overlapped band of Mn 2p3/2 into three distinct peaks by Gaussian fittings (Figure 4). The peaks at ∼640.0, ∼642.6, and ∼645.0 eV are attributed to Mn2+, Mn3+, and Mn4+ valence states, respectively.25,26 Although small portions of Mn3+ and Mn4+ states are included, the most relevant valence-state of Mn is Mn2+ in our nanoporous ZnMnO. Since the number of unpaired electrons is different in each valence state, fine transitions can be easily distinguished in electron paramagnetic resonance (EPR) because they have different g-values.27 For example, five, four, and three fine transitions can occur in Mn2+ (S = 5/2), Mn3+ (S = 4/2), and Mn4+ (S = 3/2), respectively. In addition, three types of Mnrelated EPR spectra can be distinguished; that is, Mn2+ substituted to the cation site of the crystal lattice, Mn2+ near the crystal surface, and Mn2+ located at clusters/precipitates.28,29 We, therefore, carried out EPR measurements to complement the bonding state with the local structure of Mn in nanoporous ZnMnO. The sample clearly show magnetic resonance at around 3200 G (Figure 5), which is consistent with the EPR signal from Mn2+ substituted on tetrahedral sites in ZnO.29 The values of hyperfine splitting (i.e., deviation of the peak-to-peak magnetic field, ΔHpp) are regular for our case; however, both ΔHpp of ∼76 G (from bulk sites)29 and ΔHpp of ∼90 G (from surface sites)29 coexist in the first derivate of the EPR signal (see inset of Figure 5). This verifies that Mn2+ ions are located at both the bulk-like ZnMnO region and the surface/edge of ZnMnO pores. The microstructural changes would alter the electrical properties of the solid-state system because the structural defects at the surface and the interface create the metastable bonding states in nanostructured materials. In the host material ZnO, particularly, most of all intrinsic point-defects act as
as-grown annealed at 900 °C annealed at 950 °C annealed at 1000 °C annealed at 1050 °C annealed at 1100 °C
carrier type
carrier concentration (cm−3)
carrier mobility (cm2 V−1 s−1)
resistivity (Ω-cm)
N/A N/A
N/A N/A
N/A N/A
N/A N/A
n
5.1 × 1017
39.2
8.4 × 10−2
n
7.9 × 1017
38.8
7.6 × 10−2
n
4.7 × 1018
34.6
2.8 × 10−2
n
8.9 × 1018
31.3
2.1 × 10−2
sample, the carrier concentration and the carrier mobility were unmeasurable because of high sheet-resistance (>10 MΩ/sq.). After thermal annealing at Ta > 950 °C, however, the samples became highly conductive. Furthermore, the carrier concentration was significantly increased with increasing Ta. We ascribe the observed n-type conductivity to the increased oxygen-related donor-like defects (e.g., VO+, VO2+, etc.),34−36 because the great number of green luminescence centers might be created in nanoporous ZnMnO, as discussed in details below. In semiconductor materials, native point-defects play a key role for optical radiative-transitions. For ZnO, particularly, every donor- and acceptor-like defects are closely associated with the luminescence centers. To elucidate the origin of free carrier sources in nanoporous ZnMnO, thus, we analyzed the optical properties of the 1100 °C-annealed sample, which showed the highest carrier concentration among our samples. As can be seen from the photoluminescence (PL) spectrum (Figure 6), the sample clearly reveals a strong green luminescence peak with small peaks from ultraviolet (UV), blue, and red emissions. The strong green luminescence is attributed to both the VO-related radiative transition in ZnMnO (i.e., VO → EV)34−37 and the impact ionization-induced 20632
DOI: 10.1021/acsami.7b01557 ACS Appl. Mater. Interfaces 2017, 9, 20630−20636
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has also a green phosphorescence characteristic, suggesting a potential application in field-emission device. Different from PL, however, the blue and red emissions are diminished probably because of the extremely high intensity of the greenemission from a large volume of the nanoporous material due to deeper penetration depth of the e-beam (∼3−5 μm) compared with the photons (∼0.3 μm). According to the defect chemistry model of ZnO, the formation energies of VO are much smaller than those of VZn and Zni,30,31 particularly for oxygen-deficient conditions. In ZnO-based materials, therefore, VO should be abundant whereas VZn and Zni would be rare. Furthermore, we previously observed that the green emission is relevant to oxygen vacancies in ZnMnO.44 Therefore, we believe our nanoporous ZnMnO to involve the large number of VO. On the basis of all the above, we conclude that both VO and Mn2+ provide the green luminescence and phosphorescence centers in nanoporous ZnMnO; and also, the donor-like defect (i.e., VO) supplies free carriers in the material system. To monitor the micropositional sites of VO and Mn2+, we measured and compared the CL properties at different positions. As shown in the inset of Figure 7, the green emission is much stronger at the edge area of the ZnMnO nanopores (e.g., positions B and C) than that at the bulk-like regions (e.g., positions A and D). This implies that VO defects and Mn2+ ions are enriched at the edge area of the pores. For more clarity, we carried out the monochromatic CL measurements at λCL = 525 nm. Compared to bulk-like regions, the green emission is much stronger at the edge area of the pores (see the insets of Figure 8). Furthermore, the monochromatic CL intensity (i.e., ICL at λCL = 525 nm) is increased in the edge regions of the pores when scanning ICL along the lines of X1 → X2 and Y1 → Y2 (Figure 8). This confirms that the concentration of green emission centers increases in nano-
Figure 6. PL spectrum of nanoporous ZnMnO at 300 K. (Inset) Green luminescence-related optical transitions in ZnMnO represented in the energy-band scheme.
intraband transition in Mn2+ core−shells (i.e., 4T1 → 6A1)38,39 (see the inset of Figure 6). The UV peak originates from the near-band-edge emission in the host material,40,41 the blue emission arises from the Zni-related radiative recombination,42,43 and the red emission comes from optical transitions relating to excess oxygens, possibly involving VZn complexes.40,41 From PL features, one can expect the VO- and Mn2+-related emissions to be most prominent in nanoporous ZnMnO. For further insight into light-emission sources in nanoporous ZnMnO, we examined the cathodoluminescence (CL) characteristics of the material with a spatial resolution. For collecting CL signals from nanoporous ZnMnO, first, we arranged and focused on the ZnMnO nanopores (i.e., scanned area = 5 μm × 6 μm) by using the SEM setup equipped with the CL system (see the inset of Figure 7). When irradiating an e-beam with the acceleration voltage (Vacc) of 10 kV, a strong green emission is observed at λCL = 515−560 nm (i.e., VO- and Mn2+-related emissions). This verifies that nanoporous ZnMnO
Figure 8. Dependence of the green light emission intensity (ICL at λCL = 525 nm) on the position of ZnMnO nanopores. (Left-hand-side inset) Top-view SEM image of nanoporous ZnMnO used for CL measurements. The dashed lines along X1 → X2 and Y1 → Y2 indicate the positions, from which green light emission intensities of were collected. (Right-hand-side inset) Monochromatic CL image measured at λCL = 525 nm.
Figure 7. CL spectrum of nanoporous ZnMnO at 300 K. (Left-handside inset) Top-view SEM image of nanoporous ZnMnO used for CL measurements at different positions of A−D. (Right-hand-side inset) CL spectra of nanoporous ZnMnO obtained from different positions. 20633
DOI: 10.1021/acsami.7b01557 ACS Appl. Mater. Interfaces 2017, 9, 20630−20636
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ACS Applied Materials & Interfaces
and the emission can be enhanced. Confined systems can lead to the reduction in size and power requirements for optical devices.46 Photonic bandgap materials provide strong photon confinement within volumes on the order of (λ/2n)3, where λ is the emission wavelength and n is the refractive index.47,48 Thus, nanoporous ZnMnO may confine green photons (λ ≈ 525 nm) within 150−200 nm ZnMnO volumes (n = 1.8) to enhance the luminescence. In addition, the photon confinement in the nanoporous cavity49 could lead to a high efficiency of the green luminescence because of the decreased nonradiative chargecarrier recombination. The formation of Eloc can be explained by the effects of band bending and carrier confining near the ZnMnO nanopores. In other words, since the donor-like defects (i.e., VO) at nanopores’ edges provide negative charge carriers, the electrochemical potential will decrease at the edge region of the ZnMnO pore (see left-hand-side inset of Figure 9). This gives rise to an increase in the free carrier confinement and results in a formation of Eloc for nanoporous ZnMnO. Therefore, it can be concluded that the extremely strong green luminescence and phosphorescence properties in nanoporous ZnMnO arise from the localized energy level, originating from donor-like defects of VO in semiconductive nanoporous ZnMnO. Since ZnO-based materials are well-known as green phosphors with a short decay time-constant (e.g., τdecay ≈ 0.7 ns for ZnO:Ga ceramics50 and τdecay < 2 ms for Mn2+-incorporated ZnO-based phosphors,51 etc.), the application avenue of nanoporous ZnMnO can be further extended forward various optical devices with a scintillant operation.
porous ZnMnO, leading to a photon confinement in the present type of the material system. Finally, we characterized the optical transmittance property of nanoporous ZnMnO by using ultraviolet−visible (UV−vis) spectrophotometry measurements. It should be noted that two features of the spectrum appear at different wavelengths (i.e., one at the UV region, and the other at the green region.) (Figure 9). The transmittance value at the UV region (i.e., λ
520 nm) is 87% on average. This indicates that nanoporous ZnMnO absorbs the UV light and transmits the light with longer wavelengths (>520 nm). Because the photon or the electron energies absorbed from UV or e-beam can stimulate both VO- and Mn2+-related green emissions, as confirmed in PL and CL, one may expect nanoporous ZnMnO to have a great potential for various SSL device applications. For example, the semiconducting properties of green-luminescent nanoporous ZnMnO could allow us to demonstrate a light-emitting p-n junction device,19 and the green-phosphorescent characteristics would enable us to realize a warm-white light emitters45 or a high performance field-emission devices (see also Supporting Information). We can take into account of the light filtering effect through the green-emission band, which is energetically localized in the energy gap of ZnMnO. To determine the optical band gap energy (Eg) and the localized energy level (Eloc), we evaluated the absorption coefficient (α) of the sample as a function of the photon energy. As shown in the right-hand-side inset of Figure 9, the curve of α2 displays two different slopes. One is steep at the higher photon-energy region, and the other is gentle at the lower photon-energy region. Both slopes are straight and are clearly distinguishable, as indicated by dashed lines. This means that nanoporous ZnMnO involves a definite localized-energylevel (green emission band) within the band gap. Using (αhν)2 = (hν − Eg(opt)), we estimated Eg and Eloc to be ∼3.3 and ∼2.4 eV, respectively. The magnitude of Eloc corresponds to the photon energy for the green emission in PL and CL (see also the inset of Figure 6). Furthermore, the confinement of light to small volumes of the porous cavity material can take place and has important consequences on the properties of optical emission. The density of electromagnetic states may be significantly modified
3. CONCLUSION Nanoporous ZnMnO was formed by the thermal nucleation of the nanopores in 20-period Zn 0.93 Mn 0.07 O (∼8 nm)/ Zn0.65Mn0.35O (∼8 nm) multilayers. Because of both the surface energy differences and the biaxial stresses between the Zn0.93Mn0.07O and the Zn0.65Mn0.35O layers, ZnMnO nanopores were successfully created via thermal grooving and agglomeration during high-temperature annealing at 1100 °C. Nanoporous ZnMnO exhibited an extremely strong green emission in both the luminescence and the phosphorescence characteristics. We found that the strong green emission is associated with the optical emissions from VO and Mn2+ concentrated near the nanopores. In the optical transmittance characteristics, the sample showed to have a localized energy level (i.e., green band) within the energy band gap. In addition, the sample revealed an n-type semiconducting behavior (n > 1018 cm−3) because the donor-like defects (i.e., VO) exist in nanoporous ZnMnO. Since the photon confinement allows a week nonradiative charge carrier recombination, nanoporous ZnMnO cavities can provide a high efficiency of green light emission. These suggest that nanoporous ZnMnO holds promise for various applications as a novel semiconducting green luminescent and phosphorescent material. 4. METHODS 4.1. Growth of ZnMnO Multilayers. The ZnMnO multilayers (i.e., 20 periods of Zn0.93Mn0.07O (∼8 nm)/Zn0.65Mn0.35O (∼8 nm)) were sequentially deposited on (0001) Al2O3 substrates by radio frequency (RF) magnetron sputtering using ZnMnO (Mn, 5 at. %) and ZnMnO (Mn, 30 at. %) targets. During the sputter-growth of ZnMnO multilayers, the mixture gases of Ar and O2 (Ar, 15 sccm; O2, 15 sccm) were supplied, and the working pressure was kept at 10 mTorr. The RF power for performing plasma discharge was 120 W, 20634
DOI: 10.1021/acsami.7b01557 ACS Appl. Mater. Interfaces 2017, 9, 20630−20636
Research Article
ACS Applied Materials & Interfaces and the substrate temperature was maintained at 600 °C for whole growth processes. The compositions of Zn, Mn, and O in ZnMnO layers (and nanoporous ZnMnO) were measured through X-ray photoelectron spectroscopy (XPS). 4.2. Formation of Nanoporous ZnMnO. Nanoporous ZnMnO was formed by the thermal nucleation of pores in the sputter-grown ZnMnO multilayers. To perform the thermal nucleation, we carried out high-temperature annealing at 900−1100 °C in N2. 4.3. Characterization of Microstructural and Structural Properties. The microstructural properties of the nanoporous ZnMnO multilayers were monitored through scanning electron microscopy (SEM) measurements by using an FE SEM XL-30 system. The structural properties were characterized through X-ray diffraction (XRD) measurements by using a Bede D3 system equipped with a Cu Kα source. 4.4. Characterization of Valence States and Local Structures of Mn. The chemical bonding states of Mn in nanoporous ZnMnO were studied by XPS measurements using a SIGMA PROBE ESCA system (ThermoVG). The local structures of Mn were characterized by electron paramagnetic resonance (EPR) measurements at X band of approximately
