Free-Standing Atomically Thin ZnO Layers via Oxidation of Zinc

Mar 30, 2017 - Monolayer ZnO represents a class of new two-dimensional (2D) materials that are expected to exhibit unique optoelectronic properties an...
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Free-Standing Atomically Thin ZnO Layers via Oxidation of Zinc Chalcogenide Nanosheets Zheng Wang, Lu Gan, Haiping He,* and Zhizhen Ye* State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China S Supporting Information *

ABSTRACT: Monolayer ZnO represents a class of new two-dimensional (2D) materials that are expected to exhibit unique optoelectronic properties and applications. Here we report a novel strategy to synthesize free-standing atomically thin ZnO layers via the oxidation of hydrothermally grown ultrathin zinc chalcogenide nanosheets. With micrometer-scaled lateral size, the obtained ultrathin ZnO layer has a thickness of ∼2 nm, and the layered structure still maintained well after high temperature oxidation. The thermal treatment strongly improves the crystal quality as well without inducing cracks or pinholes in the ultrathin layers. The atomically thin ZnO layers are highly luminescent with dominant green emission. High quality white light is obtained from the mixed phosphors containing the ZnO layers, exhibiting their potential as compelling ultraviolet-excited phosphors. KEYWORDS: ZnO, two-dimensional materials, free-standing, light emission

1. INTRODUCTION The rapid development1,2 and various applications3−6 of graphene have set off a new wave of research on other twodimensional (2D) materials, such as hexagonal boron nitride (h-BN)7 and transition-metal dichalcoges (TMDs).8,9 Nowadays, these atomically thin 2D materials have attracted enormous attention in the field of optoelectronic devices, especially transistors10 and photodetectors.11 However, these inorganic graphene analogues (IGAs) are still limited to layered compounds,12 in other words, the van der Waals solids.13 Recently, there were growing interests in synthesizing nonlayered materials into 2D structures, and the researchers have made a big breakthrough in this field.14−17 Among those nonlayered materials, II−VI binary chalcogenide compounds such as ZnO, ZnS, and ZnSe are of great research interest due to their remarkable physical and chemical properties18 and potential applications in light-emitting diodes,19 photoconductors,20 chemical sensors,21 transistors,22 nanogenerators,23 and photovoltaic devices.24 Although numerous reports on relatively thick nanosheets of these materials have been reported,25,26 the work of creating ultrathin and free-standing 2D morphology from II−VI binary chalcogenide compounds is still challenging and needs further exploration.27 A class of layered II−VI semiconductors with unique structures that interconnected or separated by organic insulators have been successfully developed,13 including ZnS, ZnSe, CdS, and CdSe. The thickness of these semiconductors is at nanometer or subnanometer scale. On this basis, flexible and free-standing ZnSe monolayers have been successfully fabricated.12 However, there are few reports on monolayer II oxides such as ZnO. It has been predicted28 by first-principles © XXXX American Chemical Society

calculations that at least ten kinds of II−VI semiconductors, including ZnO, BeO, MgO, CaO, CdO, CaS, SrS, SrSe, BaTe, and HgTe, exhibit promising dynamic stability of hexagonal monolayer. As for ZnO, besides the theoretical prediction,28 2− 5 monolayer thick ZnO films have been deposited on Ag(111) surface.29 Other studies have also achieved graphene-like ZnO on metal substrates including Pd,30 Pt,31 and Au.32 In these cases, however, the substrates are mandatory for the formation of graphene-like ZnO, and the synthesis of free-standing graphene-like ZnO is still challenging. The growth of freestanding graphene-like ZnO was first demonstrated with the assistance of electron-beam-produced pores in graphene.33 Nevertheless, the lateral size of ZnO membranes in this case was too small. Very recently, Wang et al.18 prepared nanometer thick ZnO nanosheets with sizes up to tens of micrometers, which were directed by surfactant monolayer. Here we report the synthesis of free-standing atomically thin ZnO layers simply by oxidizing the hydrothermally grown fewlayer zinc chalcogenides. We find that the layer morphology can be well maintained after high temperature oxidation. The resultant ZnO ultrathin layers exhibit strong deep-level emission, which enables them as possible candidate for UVexcited phosphors or single photonic sources.

2. EXPERIMENTAL SECTION 2.1. Synthesis. Zn(NO3)2·6H2O (98%), S powders (98%), and npropylamine (pa, 99%) of analytical grade were purchased from Alfa Received: February 18, 2017 Accepted: March 30, 2017 Published: March 30, 2017 A

DOI: 10.1021/acsami.7b02425 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic diagram of the fabrication process of atomically thin ZnO layers. (b) XRD pattern of the ZnO thin layers. All the diffraction peaks can be indexed to wurtzite ZnO. (c) TEM image of the ZnO thin layers. Scale bar: 1 μm. Inset shows the crystalline interplanar spacing and the direction of c-axis. Scale bar: 2 nm. (d) Representative AFM image of the ZnO thin layers. The bottom panel shows the line scan along the white line in the image, revealing lateral size of several micrometers and thickness of ∼2 nm. Aesar. Ca1.6Sr0.4AlN3:Eu red phosphor was purchased from Shandong Yingguang Advanced Materials Co. Ltd. CdS(Se)/ZnS core−shell quantum dots were purchased from Najing Tech Co. Ltd. All chemicals were used as received without further purification. The atomically thin ZnO layers were formed by oxidation of atomically thin zinc chalcogenides. We followed the hydrothermal method reported in ref 13 to synthesize the atomically thin zinc chalcogenides (ZnS or ZnSe). Briefly, the reactants (Zn(NO3)2·6H2O, 2.97 g; S, 0.16 g; pa, 20 mL) were put into the reactors (50 mL) by the sequence of solid and solvents. The reactors were then sealed and heated in oven at 110 °C for 5 days. While the reactions were complete, the reactors were naturally cooled down to room temperature. The products were washed by 30% ethanol and 80% ethanol in centrifuge tubes (50 mL), followed by the procedure of ultrasonic exfoliation (power 240 W, 30−60 min) and drying at 45 °C in the vacuum oven for 12 h. These intermediate materials were transferred into tube furnace and underwent thermal treatment in oxygen ambience at different temperatures for 2 h with a gas flow of 100 sccm. 2.2. Characterization. For structural characterization, X-ray diffraction (XRD) measurements were conducted on an X’pert PRO diffractometer (PANalytical) operated at 40 keV and 40 mA with Cu Kα radiation (λ = 0.154 06 nm). For morphological characterization, atomic force microscopy (AFM) measurements were carried out on multimode Nanoscope 3D (Veeco) with a rtesp probe (Bruker) in the tapping mode. For AFM measurements, the samples were dispersed in ethanol and then drop-casted on SiO2/Si substrates. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) analysis, and energy-dispersive X-ray spectroscopy (EDS) were recorded using a Tecnai G2 F20 S-Twin (FEI, Hillsboro, USA) operated at 200 keV to investigate the microstructure of the products. For TEM and HRTEM analysis, the samples were dispersed in ethanol and dropped onto the carbon film of copper grids. X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate the composition and chemical states of the samples on an Escalab 250 system (Thermo Scientific), using Al Kα radiation of 1486.6 eV with a pass energy of 30 eV and step of 0.1 eV. Fourier transform infrared (FTIR) spectra were used to investigate the ligands on the surface of nanosheets, thus to monitor the transformation of ZnS into ZnO. The spectra were obtained by a Bruker Vector 22 spectrophotometer. Photoluminescence (PL)

measurements were used to evaluate the light-emitting properties and defects in the samples. The spectra were recorded on a FLS920 fluorescence spectrometer (Edinburg Instruments) using the 325 nm line of a He−Cd laser as the excitation source (excitation power density: 0.8 W/cm2). The micro-PL measurements were carried out on an MS350Li system (SOL, Belarus) using the 355 nm laser as an excitation source. 2.3. Fabrication and Evaluation of Mixed Phosphors. 100 mg of layered ZnO sample was mixed homogeneously with 30 mg of Ca1.6Sr0.4AlN3:Eu red phosphor (PL wavelength 630 nm). 0.2 mL of solution (100 μg/mL) of CdS(Se)/ZnS quantum dot (core−shell structure with average size of 5 nm, PL wavelength 455 nm) was then added to the mixture to ensure that the quantum dots were mixed homogeneously with the powders. The prepared mixed phosphors were then mixed with epoxy resin homogeneously and coated on the quartz plate. A UV-violet LED chip with 390 nm wavelength was used as the light source to excite the mixed phosphors.

3. RESULTS AND DISCUSSION The strategy for atomically thin ZnO layers is based on the transformation of atomically thin zinc chalcogenides, such as ZnS and ZnSe,12,13 by means of thermal oxidation. The fabrication procedures are schematically described in Figure 1a. First, hybrid ZnS (pa) intermediate were synthesized by a hydrothermal process following Li’s method.13 After ultrasonic exfoliation, free-standing lamellar hybrid ZnS (pa) could be obtained. Subsequently, the lamellar hybrid ZnS (pa) were oxidized to ZnO (pa), and the pa molecules were removed during the thermal treatment of 600−950 °C. Figure 1b shows the XRD pattern of the sample thermally oxidized at 950 °C. The pattern exhibits a series of sharp diffraction peaks, which can be indexed to wurtzite ZnO, indicating that the synthesized sample is crystalline ZnO with the crystal structure unchanged from bulk ZnO. Free-standing folded ZnO layers, with lateral size of several micrometers, can be clearly seen in the TEM images (Figure 1c and Figure S1). Revealed from the contrast of Figure 1c, the thickness of ZnO B

DOI: 10.1021/acsami.7b02425 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. XPS core level spectra of (a) Zn 2p and (b) S 2p of the samples before and after thermal treatment in O2 atmosphere, respectively. The spectra are vertically separated for clarity. (c) XPS core level spectrum of O 1s of the sample after thermal treatment in O2 atmosphere. The spectrum can be well fitted with three Gaussian components labeled as OI, OII, and OIII. (d) FTR spectra of the samples before and after thermal treatment in O2 atmosphere.

The results indicate that our samples are few-layer-thick ZnO.18 Throughout the process, pa acts as the spacer that preventing interactions among the neighboring inorganic fragments, thus leading to strong quantum confinement and contributing to the formation of layered structure. We note that the strategy described above also works when replacing the ZnS precursor with ZnSe. As shown in Figure S4, the ZnO thin layers produced from ZnSe present very similar morphology and thickness to that from ZnS. The transformation from ZnS into ZnO is also evidenced by XPS spectra. Figure 2 shows the XPS core level spectra of Zn 2p and S 2p of the samples before and after thermal treatment in O2 atmosphere at 950 °C, respectively. The Zn 2p peak does not show clear shift after thermal treatment, which is expected because the chemical shift of the Zn 2p peaks is normally very small. While for the S 2p spectra, one can find disappearance of the 162.3 eV peak after thermal treatment. The tiny peak around 169 eV in Figure 2b may stem from sulfate radical

layer can be estimated to be approximately 2−5 nm since the thickness of underlying carbon film is ∼8 nm. The layered structure of ZnO is further supported by the observation of multilayer nanosheets under TEM as well as its corresponding EDS analysis (Figure S2). HRTEM (inset of Figure 1c) was performed to reveal the microstructure of the ZnO thin layers. The crystalline interplanar spacing is 0.282 nm, which corresponds to (100) plane of wurtzite ZnO, indicating that the surface plane of the ZnO layers is perpendicular to the caxis. The representative AFM image of ZnO thin layers is displayed in Figure 1d. A ZnO nanosheet with holes is shown. The lateral size is several micrometers, and the thickness is determined to be ∼2 nm. Through a large number of observations (Figures S3 and S4) we find that the majority of ZnO layers are 2−4 nm in thickness and 3−10 μm in lateral size. The layer thickness is also supported by the statistical thickness distribution of a representative sample (Figure S5). C

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works that getting ZnO films by thermal oxidation of ZnS films.37−39 It should be noted that the structure of ZnO is wurtzite and does not change for all temperatures. However, the diffraction peaks exhibit slight shift compared with the standard value (34.42°) of bulk ZnO (Figure 3b). This is likely due to strain arising from the cell shrinkage from ZnS to ZnO. Figures 3c and 3d show typical AFM images for the ZnO layers oxidized at 600 and 900 °C, respectively. Significantly, we find that the thin-layer morphology can be maintained completely when the samples underwent treatment in the entire temperature range of 600−950 °C. The cell shrinkage does not deal much damage such as cracks or pinholes to the ultrathin layers. We suggest that there are two causes for the structural integrity of the sheet upon oxidation. One is the very small thickness and the free-standing nature of the nanosheets. It is well-known in epitaxial growth that thin films can tolerate large structural strain when the film thickness is below the critical thickness (typically from several nanometers to tens of nanometers). The free-standing nature of the nanosheets further facilitates the strain release. The other is the formation of defects to release the strain. The defected lattice can be partly repaired by the thermal treatment that also acts as annealing. The full width at half maximum of the diffraction peaks decreases with increasing temperature in the range of 400−950 °C, indicating improved crystal quality. In addition, it is found that the (002) peak shifted to higher degree at 800 °C, which can be interpreted by the formation of zinc vacancies, thus inducing lattice shrinkage.40 This phenomenon is consistent with the XPS result and the red-shift of deep-level emission, which will be discussed later. As the temperature further increases from 800 to 1000 °C, the (002) peak shifts to lower degree toward the standard position, which may represent the gradual release of defects-induced lattice strain at higher temperature. The undamaged phase translation, as well as the unique optical properties discussed below, may both originate from the ultrathin layer structure. To evaluate the effects of the process parameters on the properties of ZnO layers, a series of different parameters were tried to synthesize the samples with reaction temperature varying from 80 to 140 °C while reaction time varying from 3 to 7 days. Thermal treatment at 950 °C was conducted after hydrothermal growth to transform all the samples to ZnO, as confirmed by XRD results (Figures S6h and S7f). AFM measurements were used to find out the optimal parameters (Figures S6 and S7). We find that when the reaction temperature is lower than 110 °C, the layers of ZnO samples are relatively thick. When the reaction temperature is higher than 110 °C, it is very difficult to find the layered structures, while the temperature approaches 140 °C, only nanoparticles can be found. On the other hand, when the reaction time is shorter than 5 days, the layered structure is still in the process of formation. When the reaction time is longer than 5 days, the layered structure starts to stack. These results indicate that reaction temperature of 110 °C and reaction time of 5 days are the optimal parameters for the growth of atomically thin ZnO layers. The PL spectra of ZnO ultrathin layers fabricated at various temperatures show dominant deep-level emission, as illustrated in Figure 4a for ensemble layers and Figure S8 for an individual layer. The chemical nature of green emission is controversial and complicated. So far, various defects including oxygen vacancy and zinc vacancy have been invoked to interpret its origin.41−43 In ZnO nanostructures, however, it is generally

during the oxidation. A similar strategy has been demonstrated to achieve ZnO films and nanowires from ZnS.34 The core level spectrum of O 1s of the sample after thermal treatment was illustrated in Figure 2c. The spectrum can be decomposed to three peaks at 529.8, 531.0, and 532.0 eV, which corresponds to the fully coordinated O2− in ZnO lattice (OI), O2− in oxygendeficiency region (OII), and chemisorbed oxygen (OIII), respectively.35,36 The Zn/O atomic ratio when excluding the chemisorbed oxygen is slightly lower than 1 (0.98), suggesting the formation of zinc vacancies after 950 °C oxidation. The organic groups were confirmed to be removed after thermal oxidation from the results of FTIR measurement (Figure 2d). Since the ZnO thin layers stem from ZnS and ZnSe through oxidation, one may concern whether the thin layer morphology can be completely maintained because ZnO and ZnS/ZnSe have different crystal structures (ZnO is wurtzite while ZnS/ ZnSe is zinc-blende) and cell volumes. Therefore, it is necessary to study the role of thermal treatment in the formation of ZnO thin layers. Figure 3a exhibits the XRD patterns of the ZnO samples treated from 400 to 1000 °C. When oxidized at 400 °C, the sample reveals coexistence of sharp ZnO peaks with broad ZnS peak at 28.6° (inset of Figure 3a). When the oxidation temperature is increased to 600 °C, the diffraction peaks of ZnS disappear, indicating the complete transformation. The transition temperature is in accordance with previous

Figure 3. (a) XRD pattern of the ZnO thin layers underwent thermal treatment at 400−1000 °C. Inset shows the magnified pattern of 400 °C sample, revealing the ZnS peak at 28.6°. The red arrow indicates the (002) peak shown in (b). (b) The magnified patterns of (002) diffraction peak of ZnO thin layers. The black dashed line shows the standard position of (002) peak of bulk ZnO. (c) AFM image of the ZnO thin layers oxidized at 600 °C. (d) AFM image of the ZnO thin layers oxidized at 900 °C. Bottom panels show that the thin-layer morphology can be maintained completely when the samples underwent thermal treatment. D

DOI: 10.1021/acsami.7b02425 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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around 2.3 eV, while oxygen vacancies are responsible for the emission around 2.5 eV.43 The result is consistent with the shift of XRD peak at 800 °C shown in Figure 3b because the formation of vacancies will induce lattice shrinkage.38 To obtain more information on the green emission in such ultrathin ZnO layers, we carried out temperature-dependent PL measurements, as shown in Figure 4b,c. It is found that the green emission is dominant within the entire temperature range, with its integrated intensity ∼5 times higher than that of the near band edge emission. The dominance of green emission is consistent with the large surface-to-volume ratio of the layered structure. Interestingly, the integrated PL intensity shows a negative thermal quenching behavior in the temperature range of 160−250 K, indicating a detrapping/delocalization of photogenerated carriers.45 Such effect results in high internal quantum efficiency46 of ∼30% at room temperature. Here, the internal quantum efficiency is obtained from the relative integrated PL intensity by assuming that the internal quantum efficiency at low temperature is very close to unit. Such assumption is reasonable because the nonradiative recombination channels are thermally activated; that is to say, at very low temperature the recombination is approximately all radiative. Such method has been widely used to estimate internal quantum efficiency in various semiconductors.46,47 It suggests that the free-standing atomically thin ZnO layers might be applied as single quantum emitters48,49 or green phosphors,41,50 taking advantage of their strong defect-related emission. It has been reported that the spectral distribution of the green emission in ZnO closely matches the dark-adopted human eye response,51 making them compelling UV-excited phosphors. As a preliminary attempt, we demonstrate below the mixing of our ZnO sample with commercial Ca1.6Sr0.4AlN3:Eu red phosphor and CdSSe/ZnS quantum dots to form white-light-emitting phosphors. Under the excitation of 390 nm UV-LED, the prepared phosphor shows white light emission. The characteristic emissions at 1.98, 2.45, and 2.68 eV from the phosphor are illustrated in Figure 5a. By adjusting the weight ratio of the three components, white light emission with CIE coordinates of (0.325, 0.336), as indicated by the blue circle in Figure 5b, correlated color temperature (CCT) of 5840 K, and color rendering index (CRI) of 70, can be achieved. The CIE coordinates are close to the pure white light (0.33, 0.33), indicating the high quality of our white light. For comparison, the mixture of Ca1.6Sr0.4AlN3:Eu red phosphor and CdSSe/ZnS quantum dots without ZnO shows slightly shifted PL peaks at 1.96 and 2.71 eV. The spectrum corresponds to CIE coordinates of (0.416, 0.200), as illustrated by the cyan circle in Figure 5b, which is far from the pure white light. The results indicate that our ZnO sample plays a significant role in the white light emission. The mixture phosphors can be easily coated on the quartz plate and placed in the front of UV-violet LED chips, as shown in Figure 5c. The result indicates that our ZnO ultrathin layers may find application in light-converted white-light LEDs.

Figure 4. (a) Room temperature PL spectra of the atomically thin ZnO layers oxidized at 400−950 °C. The intensities are normalized to the deep-level emission. (b) Temperature-dependent PL spectra of the atomically thin ZnO layers underwent thermal treatment at 950 °C. The spectra were recorded at 15, 25, 40, 60, 80, 120, 160, 200, 250, and 300 K. (c) Arrhenius plots of the integrated PL intensities for the green and near band edge emissions. The intensities are normalized to the green emission at 15 K.

accepted that the deep-level emission is attributed to surface intrinsic defects which are strongly manifested when the surface-to-volume ratio is high.44 Thus, it is reasonable for our ultrathin ZnO layers to show dominant and strong green emission due to the high surface-to-volume ratio. The result is consistent with the large fraction of O2− in the oxygendeficiency region revealed in Figure 2c. As the oxidation temperature increases, the near band edge emission increases slightly, consistent with the improved crystallinity as revealed in Figure 3a. Meanwhile, the green emission shows a slight redshift, which may results from the formation of zinc vacancy upon annealing in oxygen-rich ambience because zinc vacancies are reported to be responsible for the deep-level emission

4. CONCLUSION In summary, free-standing atomically thin ZnO layers were successfully synthesized via oxidizing hydrothermally grown ultrathin zinc chalcogenides layers. The lateral size of ZnO ultrathin layers is several micrometers, and the thickness is determined to be as thin as ∼2 nm. The ultrathin-layer morphology of ZnO can be maintained completely throughout various oxidation temperatures. The cell volume shrinkage does E

DOI: 10.1021/acsami.7b02425 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Haiping He: 0000-0001-8246-0286 Author Contributions

The manuscript was written through contributions of all authors. Z.W. carried out the experiments, measurements, and data analysis and drafted the manuscript. H.H. conceived the research, directed the experiments, analyzed the results, and revised the manuscript. L.G. provided advice on experiments as well as the revise of the manuscript. Z.Y. supervised the project. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Nos. 91333203 and 51372223) and Program for Innovative Research Team in University of Ministry of Education of China (No. IRT13037).

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Figure 5. (a) Room temperature PL spectra of the mixed phosphors with and without ZnO ultrathin layers. The white-light phosphors (black line) were prepared by mixing 100 mg of ZnO ultrathin layers with 30 mg of commercial Ca1.6Sr0.4AlN3:Eu red phosphor and 20 μg of blue-emitting CdSSe/ZnS quantum dots solution. (b) CIE of the white light from the mixed phosphors with (blue circle) and without (cyan circle) ZnO ultrathin layers under 390 nm excitation. (c) Photograph of the white light emission under excitation of 390 nm LED chips.

not ruin the ultrathin layer structure, possibly due to the release of strains by forming defects. Furthermore, the thermal treatment improves the crystal quality. In the entire temperature range of 15−300 K, the atomically thin ZnO layers shows strong deep-level emission, which enables them as possible candidates for UV-excited phosphors or single photonic sources.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02425. Additional TEM, AFM, and micro-PL of ZnO thin layers (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.H.). *E-mail: [email protected] (Z.Y.). F

DOI: 10.1021/acsami.7b02425 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.7b02425 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX