Facile Synthesis and Photoluminescence Mechanism of ZnO

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Facile synthesis and photoluminescence mechanism of ZnO nanowires decorated with Cu nanoparticles grown by atomic layer deposition Qing-Hua Ren, Yan Zhang, Tao Wang, Wen-Jie Yu, Xin Ou, and Hong-Liang Lu ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.9b00338 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

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ACS Applied Electronic Materials

Facile synthesis and photoluminescence mechanism of ZnO nanowires decorated with Cu nanoparticles grown by atomic layer deposition Qing-Hua Ren1, 2, Yan Zhang1, Tao Wang1, Wen-Jie Yu2, Xin Ou2, Hong-Liang Lu1, * 1State

Key Laboratory of ASIC and System, Shanghai Institute of Intelligent

Electronics & Systems, School of Microelectronics, Fudan University, Shanghai 200433, China 2State

Key Laboratory of Functional Materials for Informatics, Shanghai Institute of

Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China

Email: [email protected]

ABSTRACT Vertically aligned ZnO nanowires (NWs) decorated with Cu nanoparticles (NPs) were successfully prepared on Si (100) by hydrothermal method together with atomic layer deposition (ALD) technique. By varying the number of ALD cycles, size and concentration of Cu NPs on the ZnO NWs can be accurately controlled. Scanning electron microscopy and transmission electron microscopy were applied to demonstrate the formation of Cu NPs on ZnO NWs. Study of the photoluminescence (PL) characteristics of ZnO NWs decorated with different density of Cu NPs show gradual change in the near band edge emission and deep level emission with increasing ALD cycles. The enhancement of PL with decreasing ALD cycles was due 1

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to decreasing the size of Cu NPs. The results are discussed considering two major physical phenomena taking place at the interface between Cu NPs and ZnO NWs, namely passivation of surface dangling bonds of ZnO NWs by Cu NPs and formation of Schottky contact at the boundaries between them. This work not only indicates that ALD is an excellent technique to grow conformal Cu NPs with highly controllable size, but also demonstrates that the surface passivation and electron transfer between Cu and ZnO can efficiently manipulate the PL intensities of ZnO NWs.

KEYWORDS: photoluminescence, ZnO nanowires, Cu nanoparticles, atomic layer deposition, surface passivation, Schottky contact

2


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1. Introduction One-dimensional ZnO nanostructures such as nanowires (NWs), nanorods and nanobelts have been extensively explored for future devices in the past few years. ZnO has many superior properties, for instance, direct wide bandgap (3.37 eV), large exciton binding energy (~60 meV) and high melting point (1975 ºC).1-3 They have great potential applications in development of short-wavelength light sources such as high-efficiency light emitting diode, high density ultraviolet laser diode devices, photosensors,4-8 photodetectors and gas sensors.9-11 Generally, the preparation method and morphology of ZnO nanostructures have great influence on its photoluminescence (PL) features. It is widely known that the PL emissions of typical ZnO contains two peaks, one is the near band edge (NBE) emission located in the ultraviolet (UV) region and the other is the deep level emission (DLE) in the visible region. The former comes from the recombination of excitons and donors, the latter from the radiative recombination of photogenerated holes with electrons occupying the singly ionized oxygen vacancies.12,13 Owing to the large specific surface areas of ZnO nanostructures, they usually have a high density of defects or impurities on the surface that will trap excitons and lead to degradation of PL efficiency.14,15 Several efforts have been made to enhance the luminescence efficiency of ZnO. Post treatment of ZnO with Ar plasma have strong influence on NBE emission intensity of ZnO NW by hydrogen incorporation. It was corroborated by strong hydrogen-donor-bound exicton line after plasma treatment.16-19 Coating of ZnO NWs with polymers has also been reported towards 3



stable enhancement of PL efficiency.20,21 It was observed that when ZnO NWs were covered with Polymethylmethacrylate (PMMA), the NBE emission linearly increases about three times compared to that of the bare ZnO NW. In another report, strong enhancement of ultraviolet PL band from ZnO-MgO nanocomposites (similar to heterojunction like structure) was observed compared to pure ZnO NW.22-24 It was due to depletion region formed between Zn-MgO nanocomposites, which modifies the electron states for visible emission related defects and result in strong enhancement of NBE emission. ZnO/Al2O3 core-shell nanostructure has also demonstrated strong enhancement of surface exciton band and reduction of green defect emission with respect to NBE emission.25 In similar report, when SnO2 NWs were coated with ZnO nanocrystals, strong near-band ultraviolet emission was observed compared to pure ZnO NWs.26-29 It has also been reported that when noble metals such as Au, Ag, and Pt nanoparticles (NPs) has been deposited on ZnO NWs, strong enhancement of NBE emissions were observed.19 Low work function metal NPs such as Zn,30 Ti,30 and Pd31 are frequently used to adjust the PL of ZnO due to metal-semiconductor contact. When metal and semiconductor are brought together, either Ohmic or Schottky contact are formed based on the work-function of the metal and semiconductor. When metal and semiconductor as difference in the work-function, then under thermal equilibrium energy band of the semiconductor bends at the interface. Due to band bending at the interface, variation in the direction of exciton transfer results into significant influence on the PL properties. Other metals such as Au,32-35 Ag,36-41, Pt,42,43 Al,30,44 and Cu45,46 have also attracted great attention towards regulation of the 4


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emissions of ZnO nanostructures due to localized surface plasmonic resonance effect at the interface between metal and semiconductor. In particular, Cu is a luminescence activator and a deep acceptor in ZnO. It can passivate the surface defects and trap electrons, enhancing the PL emission and improving the photodetector performance by reducing the dark current. Unlike other noble metal, Cu is low cost. Moreover, the size, the density of Cu coated on ZnO NWs can be controlled precisely by ALD method. This will facilitate to study the mechanism of PL change. Several fabrication techniques for preparation of metal NPs have been reported such

as

chemistry

methods,47,48

electrochemical

deposition,49

sputtering,50

photo-deposition,51 and lithography.52 Among these fabrication methods, chemical reaction need accurately control the surfactant dosages, reaction rate, temperature and the subsequent separation and purification process is complicated. Also, they show large particle size distribution and the particle dispersion cannot control precisely. Electrochemical deposition method is a simple, rapid and low-cost technique but also cannot accurately control the size distribution and the uniformity of particles. The other methods including lithography, sputtering or photo-deposition are used restrictedly considering the cost and output, and usually suitable to pattern only small areas. The atomic layer deposition (ALD) technique is a promising method to grow uniform, conformal thin films by sequentially exposing the substrate to each gas-phase precursor separately to achieve monolayer by monolayer growth. Due to the self-limiting surface reactions of ALD, film thickness can be accurately controlled 5



in the atomic-scale, and obtain conformal coating on substrates with complex geometries, and even porous structure.53-57 ALD has been developed in recent years to grow metal oxides,58 nitrides,59,60 sulfides,61 and even pure metal.62 Physical vapor deposition (PVD) process like direct current magnetron sputtering has already been applied for the decoration of ZnO NWs with pure metallic NPs like Zn, Ag, Ni, Au, Al, and Cu.30-46 However, PVD process is limited due to non-conformal coating of the material. It is not suitable on 3D surfaces like NWs and NPs. Therefore, the PL property of ZnO NWs decorated with different density of Cu NPs grown by ALD has been studied in detail in this paper. In this paper, we fabricate vertically aligned ZnO NWs decorated with different density of Cu NPs by hydrothermal method combined with ALD technique, as shown schematically in Figure 1. ALD was employed to deposit a ZnO seed layer on p-Si wafers, followed by hydrothermal method growth of vertically aligned ZnO NWs. Then, Cu NPs were deposited along the whole surface of ZnO NWs by ALD. By varying the ALD cycles, we have succeeded in obtaining the ZnO NWs decorated with different density of Cu NPs. Finally, we performed a comparative study of their PL properties for different hybrid samples decorated with different ALD cycles of Cu and the internal mechanism was systematically analyzed based upon the localized surface plasmon and the energy band structure.

2. Experimental Section Reagents 6


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The p-Si wafers (thickness about 550 µm) were served as substrates. All the chemicals employed here were analytical pure grade. The purity of gases was UHP grade (Airgas). The water come from a Millipore Q purification system (resistivity >18 MΩ⋅cm). ALD of Cu nanoparticles on ZnO Nanowires Vertically aligned ZnO NWs were grown on cleaned p-Si substrates by ALD technique combined with hydrothermal method. More details about preparation of ZnO NWs have been described in our previous works.63-65 Then Cu NPs were deposited on surface of the ZnO NWs by thermal ALD at 200 ºC. Copper (II)-hexafluoroacetylacetonate [Cu(hfac)2] and diethylzinc [DEZ, (C2H5)2Zn] were used as precursors. In order to get sufficient precursor saturated vapor pressure, Cu(hfac)2 was held in stainless bubblers at 75 °C and DEZ was maintained at room temperature. The precursors were separately introduced to the reactor chamber by using high purity N2 as the carrier gas and purging gas. The ALD cycle for prepare Cu consisted of 0.5 s Cu(hfac)2 pulse, 4 s N2 purge, 1 s DEZ pulse and 4 s N2 purge. Repeating the ALD cycles for 0, 5, 10, 20, 50, 100, respectively and we obtained ZnO NWs decorated with different density of Cu NPs samples labeled as bare ZnO NWs, ZnO/Cu-5, ZnO/Cu-10, ZnO/Cu-20, ZnO/Cu-50, ZnO/Cu-100. The preparation process is illustrated in Figure 1. Characterization and measurements Spectroscopic ellipsometry (GES-5E) system was used to measure the thicknesses of ZnO seed layer deposited on Si substrates. The crystallinity of the ZnO 7



NWs and ZnO/Cu samples with various ALD cycles of Cu were characterized on a Bruker-D8 X-ray diffractometer. The morphologies were characterized in a field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F) and high-resolution transmission electron microscopy (HRTEM, TECNAI G2 F20 S-TWIN) operating at 200 kV. PL properties of the ZnO/Cu composites with different ALD cycles of Cu were carried out by PL measurements at room temperature on a luminescence spectrometer (KR1801C) with the wavelength of 325 nm from an He-Cd laser and the laser power of 30 mW. Each sample was tested at least three points to ensure the reliability of the results.

3. Results and Discussion Figure 2 shows the XRD results of ZnO/Cu composite samples with different ALD cycles of Cu. There are two sets of diffraction peaks: peaks marked by the black solid round corresponds to ZnO and the ones marked by black solid snowflake corresponds to Cu. It can be seen from the Figure 2 that the diffraction peaks located at (2θ) 31.8◦, 34.6◦ and 36.2◦ are corresponding to the (100), (002) and (101) crystal faces of hexagonal wurtzite structure of ZnO (JCPDS No. 36-1451). The peaks at (2θ) 43.3◦ and 50.4◦ correspond to the (111) and (200) planes of cubic Cu (JCPDS No. 04-0836). In addition, the characteristic peaks of Cu can be clearly seen in sample ZnO/Cu 200, as displayed in Figure S1 in the supporting information. The peaks of Si substrate and its native oxide layer SiO2 are detected. Absence of any other peak in the ZnO/Cu composite samples indicates high purity of the samples. There appears to be no any shift on the peak position as well as the full width at half maximum 8


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(FWHM) value of Cu and ZnO, suggesting that Cu NPs are adhered but not doped to the ZnO NWs. Figure 3 shows the surface and cross-section SEM images of different samples. Figure 3a displays the as-grown ZnO NWs on a 20 nm ZnO seeded p-Si substrate after hydrothermal method. It can be seen that ZnO NWs grown vertically along the surface of Si substrate with uniform hexagonal end planes (as shown in the inset image), which is in good conformity with the XRD results. The average diameter of ZnO NWs is about 50 nm. Figure 3b gives the cross-sectional images of the ZnO NWs. The average length is around 1.3 µm. After a 20 ALD cycle of Cu NPs deposited on ZnO NWs, the morphologies, diameter and length of the ZnO NWs are almost the same as the bare ZnO NWs (Figure 3c-d). It can be clearly seen that the ZnO NWs are covered uniformly with small size of Cu NPs. Increasing the number of ALD cycles to 100, the Cu NPs are clearly seen on the tips and sidewalls of the ZnO NWs. Moreover, the diameters of Cu-coated ZnO NWs (~ 60 nm) are larger than the bare ZnO NWs. In addition, the hexagonal tips of ZnO NWs gradually become almost circular due to the thick Cu coating, as shown in Figure 3e-f. For further demonstrate the crystal structures of ZnO NWs and ZnO/Cu composite samples, typical TEM images are performed. Figure 4a shows the low-magnification TEM images of a ZnO NW. The diameter is 45 nm, which is in consistent with the data detected by SEM in Figure 3a. Figure 4b is a HRTEM image of the position marked by the red dotted boxed rectangle in Figure 4a. There are two types of lattice spaces measured about 0.261 nm and 0.281 nm, respectively, 9



corresponding to (002) and (100) crystal planes of wurtzite ZnO. In addition, the ZnO NWs grow in preferred orientation along the c-axis. The inset image of Figure 4b displays the selected area electron diffraction (SAED) pattern, which further affirms that the ZnO NWs have an excellent single-crystalline structure. Figure 4c gives the energy dispersive X-ray spectrometry (EDS) spectrum detected in the position marked by the pink dash circle in Figure 4a. It can be clearly seen that the Zn, O elements are ascribed to ZnO NWs. The peak of C and Cu come from the surface contamination and copper foil, respectively. The atomic ratio of O to Zn was about 1.1, which further verifies the ZnO NWs have hexagonal wurtzite structure. Figure 4d shows a low-magnification TEM image of the ZnO/Cu -50. The diameter of ZnO NW is ~ 47 nm, which is in alignment with the data obtained in Figure 4a. The outer Cu shell is composed of a number of nanoparticles. The thickness of Cu is ~ 5 nm, which is consistent with the estimation of applied ALD cycles as the ALD growth rate of Cu NPs is 0.1 nm per cycle. This is also indicating the excellent thickness controllability of ALD technique. In addition, there appears some twins resulting from the coalescence of Cu grains, which demonstrating that the ALD growth of Cu here is island-growth mode, as shown in Figure S2 in the supporting information. Furthermore, the Cu NPs are well-distributed, which is almost covering the whole surface of the ZnO NWs. Figure 4e displays a HRTEM image of an individual ZnO/Cu composite corresponding to the position marked by the green dotted boxed rectangle in Figure 4d, from which we can clearly see that the Cu NPs are coated on the surface other than doping into the lattice of ZnO. There are two types of fringe 10


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patterns coming from ZnO NW and polycrystalline structure of Cu, respectively. The lattice spacing is 0.209 nm and 0.180 nm, corresponding to the (111) and (200) planes of the cubic Cu, which are distinguished from those of ZnO. The lattice spacing of ZnO NWs and outer Cu are further tested and clearly shown in Figure S3 in the supporting information. The corresponding SAED pattern is shown in inset of Figure 4e, which includes the strong reflection spots of the (002) and (100) reflections of ZnO and a diffraction ring corresponding to polycrystalline structure of Cu. Furthermore, Figure 4f is the EDS spectrum detected in the position marked by the orange dotted boxed circle in Figure 4d of ALD deposited Cu. It can be clearly seen that there are include Cu, Zn, C, and O elements. As above-mentioned, the peak of C comes from the surface contamination, the intensities of Zn and O from ZnO/Cu sample are decreased greatly compared with the EDS result of pure ZnO, as the tested region of EDS marked by the orange dotted boxed circle in Figure 4d focus on the outer Cu shell layer and very few ZnO core are detected. The atomic ratio of O to Zn was about 1.3, which was very close to the stoichiometry of the hexagonal wurtzite ZnO. The atomic percent of Cu increased because the ALD deposited Cu shell layer. And there are no oxide states of copper in the shell layer. Further evidence for the composition and chemical states of the prepared samples is obtained by x-ray photoelectron spectra (XPS) measurement. In order to eliminate the effect of surface contamination, sample ZnO/Cu 20 are tested as-grown or with surface treatment by Ar plasma. The spectra are illustrated in S4 in supporting information, from which one can see that it is necessary to treat the surface of sample 11



before test. Therefore, XPS data of samples in this paper is treated by Ar plasma for 10 minutes before test. Figure 5a presents the typical survey XPS spectra of ZnO/Cu-5 (Figure 5a, the blue line) and ZnO/Cu-20 (Figure 5a, the red line), which demonstrates the existence of Cu, Zn, O, C elements in both two samples. The carbon may come from the inevitable contamination during measurement. However, the Cu peaks in sample ZnO/Cu-5 were not obvious because the lower concentration of Cu deposited by ALD technique. To investigate chemical state of each element, high-resolution spectra of Cu-2p, Zn-2p, O-1s, and C-1s core level regions are analyzed. All the binding energies are charge corrected based on the peak of C1s at 284.6 eV. Figure 5b and Figure 5c show the Cu 2p spectrum of sample ZnO/Cu-5 and ZnO/Cu-20, respectively. The Cu peaks of ZnO/Cu-5 sample were very weak while the peak intensity detected from ZnO/Cu-20 sample was high enough. The peaks of Cu 2p1/2 and Cu 2p3/2 appeared at 951.9 eV and 932.2 eV can be assigned to metallic Cu.66 The Zn 2p spectra in Figure 5d shows two symmetric peaks located at 1044.3 and 1021.2 eV corresponding to Zn 2p1/2 and Zn 2p3/2. The spin orbit splitting energy is calculated to be 23 eV, which matches the value with the Zn2+ in ZnO.67 Figure 5e gives the O 1s spectrum, in which the peak can be deconvoluted into two peaks one is at 529.8 and the other is at 531.2 eV, corresponding to O2- on wurtzite ZnO and O2− ions in oxygen-deficient regions within the ZnO matrix, respectively.68,69 The detailed XPS spectrum of C1s was shown in Figure 5f, it can be divided into three peaks with peak value locating at 284.6 eV, 285.8 eV, and 288.4 eV correspond to the C-C, C-OH, and C=O bonds, respectively.70,71 12


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Figure 6 shows the typical PL spectra of ZnO/Cu composites with different ALD cycles of Cu under the excitation wavelength of 325 nm. All the spectra consist of two typical emission peaks including a strong NBE peak and a weak DL emission peak of ZnO, the peak center of the former one is located at 377.8 nm and the latter one is ranging from 425 to 700 nm.72 The variation of both emission peak intensities with increasing the cycles of ALD Cu is summarized in insert, respectively. It can be observed clearly that the variation tendency of the two peak intensity is consistent with each other, when deposited 5 cycles of Cu NPs (~ 0.5 nm) on ZnO NWs, the intensity of both peaks increased slightly. Increasing the ALD cycles of depositing Cu NPs to 10, 20, 50 and 100, the intensity of both emissions decreased dramatically, indicating a quenching of PL emissions. The surface plasmon dispersion relation curves are shown in Figure S5 in the supporting information, from which the surface plasmon resonance energies can be calculated from the dispersion relation. It is found that the coupling between ZnO excitons and Cu surface plasmons is weak in ZnO/Cu composites. Therefore, the intensity of ZnO emission peak can be enhanced by coating few ALD cycles of Cu. The extraordinary change can be explained as follows: Figure 7a gives oxygen molecules adsorbed on ZnO surface (up) and the energy band diagram of bare ZnO NWs (below). The work function of ZnO is 4.45 eV. Electrons in ZnO NWs, excited by incident light, will move to the surface and gather there. These electrons are supposed to recombine with holes in the valance band or in the intrinsic defect level of ZnO NW arising from the slight oxygen vacancy, producing the strong UV emission and the weak defect emission (Figure 6). However, 13



when 5 cycles of Cu nanoparticles (~ 0.5 nm) were deposited on ZnO NWs, the intensity of both the NBE and DLE emissions increased slightly, this can be interpreted as the elimination of the surface defect related capture process, increase the recombination of excited electrons with holes thus increasing the PL emissions. Considering the large surface to volume ratio of ZnO NWs, there exist some surface defects and absorbed O2, which will trap the excited electrons on the conduction band of ZnO, result in decreasing the recombination of excited electrons with holes on the valence band of ZnO. Few ALD cycles of Cu are thick enough to passivate most of the surface defects, which can block the O2 adsorption process and prevent the capture of the excited electrons, as shown in Figure 7b. Under the UV illumination, more excited electrons escaped from the capture state and can be recombined with holes in the valance band or in the intrinsic defect level. Therefore, the NBE and DLE emission can be increased. Increasing the ALD cycles of depositing Cu NPs, the intensity of both emissions decreased gradually and down to the lowest when the cycles of ALD Cu are 100 (the thickness of Cu is ~10 nm). This reduced trend was attributed to the excited electrons transfer in the interface of Cu and ZnO when they are in direct contact, which will induce the band bending of the ZnO at the interface.30 In order to better understand our explanation, energy band bending diagram of ZnO/Cu composite (thick Cu) is schematically plotted in Figure 7c. The work function of Cu is 4.65 eV, which is higher than the electron affinity of ZnO (4.35 eV). In ZnO/Cu composite, there will form a Schottky contact leading to the energy band of ZnO bending upward near the 14


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contact region.30 The electrons in ZnO, excited by the incident light, will easily move from ZnO to the Fermi level of Cu. This will reduce the density of excited electrons, decline the recombination greatly and lead to the decrement of both the NBE and DLE emissions. The intensity of both emissions reduced gradually with the increase of the cycles of ALD Cu NPs until 100 cycles (~10 nm). In order to further testify the effect of surface passivation and Schottky contact, typical I-V curves of samples were measured in the dark and under a 365 nm UV illumination, respectively. The dark I-V characteristics of p-Si/ZnO NWs with various ALD cycles of Cu are shown in Figure 8a. The inset shows the schematic diagram of the structure for measurement. The rectified current of p-Si/n-ZnO NWs appears at the sleep voltage from -5 to +5 V in the dark indicates the typical photodetector property of p-n junction with obvious rectification. It can be clearly seen that the dark current in positive voltage range was remarkably decreased in the ZnO/Cu-5 and then increased gradually with further increasing the ALD cycles of Cu to 10. For ZnO/Cu-20 sample, the rectification behavior disappeared. Figure 8b-d show the I-V curves of p-Si/n-ZnO/Cu in dark and under UV illumination with different ALD cycles of Cu, respectively. The photoresponse was enhanced in ZnO/Cu-5 sample then decreased sharply when the ALD cycles of Cu increase. This is because the combined effects of both surface passivation and the Schottky barrier formed between Cu and ZnO. In dark condition, a p-n junction is formed between p-Si substrate and n-ZnO NWs, as well as a thickness of depletion region. Due to the large surface to volume ratio, oxygen molecules adsorbed on ZnO NWs will trap the free electrons 15



from conduction band of ZnO NWs [O2 (g) + 2e- → O2- (ad)], leading the Fermi level of ZnO NWs shift down to a lower level and the depletion thickness of p-n junction decrease. When coated the ZnO NWs with 5 cycles Cu, the surface to volume ratio decreased and the surface defects related trapping effect weakened, free electrons in conduction band of ZnO NWs increased, therefore the Fermi level of ZnO NWs shift upward, as shown in Figure 7b (down). The depletion thickness of p-n junction increased, which leads to the reduction of dark current (red line in Figure 8a). Increasing the ALD cycles of Cu to 10, there formed a Schottky contact between Cu and ZnO, the electrons will easily transfer from ZnO to the Fermi level of Cu, which will reduce the Fermi level of ZnO, as shown in Figure 7c (down). Therefore, the thickness of the depletion layer between p-Si and n-ZnO decreased, leading to increasing the dark current (blue line and pink line in Figure 8a). Under illumination by 365 nm UV light, more electron-hole pairs were generated [hν → e- + h+], the photogenerated holes in valence band of ZnO will be trapped by O2- to become O2 then escape out from the surface [O2- (ad) + h+ → O2 (g)], so the photocurrent increases because of the increasing free carrier concentration. After coating ZnO with Cu, the transfer process of photoelectrons become more easier due to the lower barrier height, thus increasing the photocurrent. The photodetector result is consistent with the PL result. The performance of common metal decorated ZnO nanostructure photodetector before and after UV illumination are reported and some of them are summarized in table 1. 16


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Metal/ ZnO

Idark

IUV

IUV/Idark

Refs.

Ag/ZnO NRs

1.3×10-5 A

2.16×10-5 A

1.67

[73]

Au/ZnO NRs

1.3×10-7 A

5.83×10-6 A

45

[74]

Pt/ZnO NRs

1.3×10-4 A/cm2

5×10-4 A/cm2

3.8

[42]

Cu/ZnO QD

1.1×10-3 A

3.58×10-3 A

3.2

[75]

Pd/ZnO NPs

~2.8×10-3 A

~9×10-3 A

3.24

[76]

Cu/ZnO NWs

~1×10-6 A

1×10-4 A

~100

This work

photodetector

4. Conclusions In summary, we have successfully synthesized ZnO NWs decorated with different density of Cu NPs via a simple hydrothermal method with ALD technique. Because of the excellent ALD method, we can precisely control the size, particle spacing and the thickness of Cu NPs covered on the surface of ZnO NWs by varying the ALD cycles. The morphologies of different samples have been characterized by SEM and TEM, the PL and photodetector properties of these samples have been systematically investigated. Remarkably, as increasing the cycles of ALD Cu NPs, the NBE and DLE emissions show a consistent change with each other. 5 ALD cycles of Cu NPs will passivate the surface dangling bonds and greatly reduce the surface defects and reduce the electron trapping effect by the surface adsorption of O2 molecules thus leading to a slightly increase of the intensity of both the NBE and DLE emission. Because of the different work function of Cu and ZnO, when the ALD cycles of Cu increases to 10, the excited electrons will transfer from ZnO to Cu, which will reduce the excited electrons, decline the recombination greatly and lead to 17



the decrement of both the NBE and DLE emissions. These results can not only help one to understand the mechanism of PL but exploit new designs for modulating the emission properties of nanostructured semiconductors with metal NPs and related area.

Supporting Information Supporting Information is available from online or from the author.

Author information Corresponding Author: Hong-Liang Lu *E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgements This work is supported in part by the National Key R&D Program of China ((No.2016YFE0110700), National Natural Science Foundation of China (No. U1632121, 11804055 and 51861135105), and Natural Science Foundation of Shanghai (No. 18ZR1405000).

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Figure captions TOC Figure 1 Schematic illustration of the preparation process of ZnO NWs decorated with Cu NPs Figure 2 X-ray diffraction patterns of different samples (gray solid round corresponds to ZnO and gray solid rectangles correspond to peaks of Cu.) Figure 3 Surface morphologies of a bare ZnO NWs, c ZnO/Cu-20, and e ZnO/Cu-100, measured by SEM at different resolutions. Cross-section SEM of b bare ZnO NWs, d ZnO/Cu-20, and f ZnO/Cu-100, corresponding to a, c, e, respectively Figure 4 a low-magnification and b high-resolution TEM image of bare ZnO NWs, c EDS spectrum detected in the area marked in the pink dotted circle in a, d low-magnification and e HRTEM image of ZnO/Cu-50, f EDS spectrum detected in the area marked in the orange dotted circle in d. The insets in b and e show the SAED pattern corresponding to b and e Figure 5 a Typical survey XPS spectra of ZnO/Cu-5 and ZnO/Cu-20, b XPS spectra of Cu 2p of ZnO/Cu-5, c - f represent the XPS spectra of Cu 2p, Zn 2p, O 1s and C 1s of ZnO/Cu-20, respectively. Figure 6 Room-temperature PL spectra of bare ZnO NWs and ZnO/Cu composites with different ALD cycle of Cu Figure 7 Schematic diagram for the mechanism of the emissions in a bare ZnO NWs , b Cu-ZnO composites with thin Cu (5 ALD cys), and c Cu-ZnO composites with thick Cu (> 5 ALD cys). Figure 8 I-V characteristic curves of p-Si/n-ZnO/Cu with different ALD cycles of Cu a in dark condition. The inset shows a schematic diagram of the structure for measurement. In the dark and under UV illumination with different ALD cycles of Cu, b 0, c 5, d 10 27



Figure 1 157x91mm (300 x 300 DPI)


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Figure 2 297x209mm (150 x 150 DPI)



Figure 3 118x135mm (300 x 300 DPI)


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Figure 4 225x117mm (300 x 300 DPI)



Figure 5 220x265mm (300 x 300 DPI)


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Figure 6 297x209mm (150 x 150 DPI)



Figure 7 474x279mm (300 x 300 DPI)


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Figure 8 217x160mm (300 x 300 DPI)


Facile Synthesis and Photoluminescence Mechanism of ZnO

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