Silver related emitting defects and aging ZnO nanocrystals

Accepted Manuscript Silver related emitting defects and aging ZnO nanocrystals T.V. Torchynska, B. El Filali, Ch. Ballardo Rodriguez, G. Polupan, L. Shcherbyna PII:

S0254-0584(18)30183-4

DOI:

10.1016/j.matchemphys.2018.03.021

Reference:

MAC 20424

To appear in:

Materials Chemistry and Physics

Received Date: 24 September 2017 Revised Date:

12 December 2017

Accepted Date: 7 March 2018

Please cite this article as: T.V. Torchynska, B. El Filali, C. Ballardo Rodriguez, G. Polupan, L. Shcherbyna, Silver related emitting defects and aging ZnO nanocrystals, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.03.021. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Silver related emitting defects and aging ZnO Nanocrystals T.V. Torchynska1*, B. El Filali 2, Ch. Ballardo Rodriguez2,

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Instituto Politécnico Nacional, ESFM, U.P.A.L.M. México City, 07738, México 2

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Instituto Politécnico Nacional, UPIITA, Av. IPN, México City, 07320, México

Instituto Politécnico Nacional, ESIME, U.P.A.L.M. México City, 07738, México

V. Lashkaryov Institute of Semiconductor Physics at NASU, Kyiv, 03028, Ukraine Abstract

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G. Polupan3 and L. Shcherbyna4

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The emission, morphology, structure and chemical composition have been studied in Ag-doped ZnO nanocrystals (NCs) in as-grown state and after aging in ambient air by means of SEM, X-ray diffraction (XRD), photoluminescence (PL) and X-ray photo electronic spectroscopy (XPS) methods. PL spectra of as-grown Ag-doped ZnO NCs include a set of elementary PL bands: near band edge (NBE) emission, green and orange

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PL bands, as well as Ag related PL bands with the peaks at 2.68 and 2.89 eV. The PL intensity of Ag- related PL bands falls down at aging. Simultaneously, the PL intensity of orange PL band increases. Joint XRD analysis and PL study permit identifying the optical

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transition connected with the substitutional AgZn defects. At XPS research the variation of

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AgZn state and the formation of Ag oxide at aging has been revealed. The radiative defect transformations at aging have been analyzed and discussed.

Key words: Photoluminescence; Aging; Ag-doped ZnO nanocrystals; AgZn defect modification

*Corresponding author e mail: [email protected]

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1. Introduction ZnO nanocrystals (NCs) gotten the great interest in the last decade due to potential applications in short-wavelength and high-temperature optoelectronic devices, such as:

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light emitting diodes, UV photodetectors, ultraviolet lasers [1-5], field emission cathodes [6, 7], as well as photocatalytical [8] and antibacterial [9] materials. To fabricate electronic devices n- and p-type ZnO NC layers need to be prepared. However, the growth of p-type

in ZnO: zinc interstitials or oxygen vacancies [10].

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ZnO NC films is difficult to realize owing to the high concentration of native sallow donors

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To obtain the p-type ZnO NC films the different impurities were studied that includes the group IA (Li or Na [11, 12]), group V (N, P, As [13-17]), or group IB (Cu, Ag, Au [1824]) elements. Note that IA group metals, due to the strong self-compensation, cannot be efficient shallow acceptors in the p-type ZnO films [11]. In contrary, Ag is a promising

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acceptor for p-type ZnO due to the formation a shallow acceptor level in comparison with other 1B elements (Cu or Au) [20-24].

The analysis of published papers related to Ag-doping ZnO NCs has shown that the

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position of Ag-acceptor levels in ZnO is still unclear. The deep level (0.23eV) below the conduction band was attributed to Ag defects in [25]. Other authors assigned to Ag the

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acceptor level 0.20 eV above the valence band of ZnO [23, 26]. However, the theoretically calculated energies ε (0/−) for Cu, Ag, and Au in substitutional sites were estimated as 0.7, 0.4, and 0.5 eV, respectively, above the valence band [20]. Thus, the energy level of substitutional AgZn defects in ZnO still is not definite and the types of optical transitions for Ag related defects are not established yet. The aim of present paper deals with the study the structure and emission of ZnO NCs with the different Ag contents (0-4at%), as well as to investigate the emission stability of ZnO NCs at aging in ambient air. 2

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Experimental details ZnO NC films were deposited by an ultrasound spray pyrolysis method on the soda-

lime glass substrates at the temperature equal to Ts=400 °C. ZnO films were deposited

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from a 0.4 M solution of zinc (II) acetate [Zn(O2CCH3)2] (from Alfa, 98 %), dissolved in a mixture of deionized water, acetic acid [CH3CO2H] (from Baker, 98%), and methanol [CH3OH] (from Baker, 98%). The volume ration of mentioned components was

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100:100:800. Separately, a solution of silver nitrate (Ag(NO3) from Baker, 98%) dissolved in a mixture of deionized water and acetic acid in the 1:1 volume proportion was prepared

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to obtain ZnO NCs with the various [Ag]/[Zn] rations of 1, 2, 3 and 4 at.% Ag. The deposition system was presented elsewhere [27]. To stimulate the crystallization the Ag doped ZnO films were annealed at 450 °C in a nitrogen flow for 4 hrs. Film aging was realized at keeping the ZnO:Ag NC films in ambient air at 300K for 2 years.

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The morphology of ZnO:Ag NC films were verified by a scanning electronic microscope (SEM), model Quanta 3D FEG-FEI. X-ray diffraction (XRD) experiments were done using the equipment of Model XPERT MRD with the Pixel detector, three axis

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goniometry and parallel collimator, with the resolution of 0.0001 degree and X ray beam from the Cu source, Ka1 line λ=1.5406 Ǻ. PL spectrum measurements were carried out

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using the equipment on the base of a spectrometer SPEX 500 with a Hamamatsu photomultiplier described early in [28, 29]. A closed-cycle He cryostat (CCS-450) (Janis Research Inc.) was used when the temperature is varied in the range of 10–310K. PL spectra were excited by the 325 nm line of a He–Cd laser (KIMMON Model: IK3102R-G) with an emission power 76 mW. To detect the elements and the chemical composition of inclusions presented in ZnO NC films, X-ray photoelectron spectroscopy (XPS) has been used that was realized in 3

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Thermo Scientific™ K-Alpha™ XPS spectrometer. X-rays were obtained from the Al anode (K-alpha radiation 1486.7eV) operated at 15kV (90W) at a pressure of 1.33x10-7Pa during the data collection. The 400 µm spot of X-ray beam was set in two pass energy

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modes of 160 and 40eV. To analyze the XPS spectra the Thermo Avantage V5.938 software was applied. 3. Experimental results and discussion

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SEM images of as-grown ZnO NC films with Ag contents of 1 and 4at.% are shown in figure 1a,b. ZnO NCs have the nanorod shape with the hexagonal cross section. The size of

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ZnO grains decreases versus Ag contents from the 150-300 nm at 1at%Ag (Fig.1a) down to 50-150nm for 4at.%Ag (Fig.1b). The surface SEM images of as-grown and aged ZnO NCs with 2at%Ag are presented in figures 1c,d. The comparison shows that aging in ambient air for 2 years did not change the shape of ZnO NCs, but stimulates the amorphous phase

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formation on the ZnO NC surface (Fig.1d).

XRD data of ZnO NCs with different Ag doping (1, 2, 3 and 4at.%) exhibit a set of peaks corresponding to the diffraction from the (100), (002), (101), (102), (110), (103) and

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(112) crystal planes in the wurtzite ZnO crystal structure (JCPDS file no. 36-1451). The examples of XRD patterns for Ag doped ZnO NCs in as-grown and aged states are shown

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in figure 2. The 2theta (2Θ) positions of most intense XRD peaks for all studied films are summarized in Table 1. XRD peaks shift to higher 2theta (2Θ) values at increasing Ag concentrations from 1 to 3at.% (Table 1). These changes of XRD parameters can be interpreted as decreasing the inter-planar distances in the ZnO crystal lattice at Ag-doping (1-3at.%). In contrary, at the higher Ag content (4at.%) in ZnO NCs all XRD peaks shift to lower 2Θ values (Table 1). Thus at Ag doping equal to 4at.% the increase of inter-planar distances in ZnO NCs has been detected. In addition, at high Ag concentrations (3-4at.%) 4

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the new diffraction peaks related to the face-centered-cubic crystal phase of metallic Ag inclusions in ZnO NCs (JCPDS file no. 04-0783) were detected (Table 1). The half widths of all XRD peaks (Fig.2) increase after aging that testify on ZnO NC size decreasing. Thus

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the ZnO NC oxidation continues at 300K in ambient air. The last process is accompanied by amorphous phase appearing on the NC surface (Fig.1d) and decreasing NC sizes (Fig.2). We have shown early [27] that PL bands related to Ag doping with the peaks at 2.68

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and 2.89 eV can be detected in PL spectra of ZnO:Ag NCs only at low temperatures from the range of 10-180K (Fig.3a). When the temperature increases (T≥ 100K) the Ag related

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PL bands decay very fast and only the near band edge (NBE) emission, as well the green and orange PL bands are seen in PL spectra (Fig.3a). Thus, to study the Ag doping impact on emission PL spectra need to be investigated and compared at low temperatures. Normalized PL spectra measured at 10K for as-grown ZnO NC films with Ag contents

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1, 2, 3 and 4 at.% are shown in figure 3b. PL spectra can be represented as a superposition of five PL bands in visible and UV spectral ranges (Fig.4a). The PL spectrum of as-grown ZnO:Ag NCs includes the near band edge emission (3.18eV), well known defect-related

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green (2.40eV) and orang (1.90eV) PL bands [30], as well as Ag doping related PL bands peaked at 2.68 and 2.89 eV (Fig.3 and 4a). The incorporation of 1-3at%Ag in ZnO NCs

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leads to raising the intensity of 2.68 and 2.89 eV PL bands (Fig.3b). In contrary, the PL intensities of 2.68 and 2.89 eV PL bands decrease in PL spectra of ZnO NCs with the 4at% Ag content (Fig.3b). At the same time, PL intensities of green and orange PL bands fall down at low Ag doping (1-3at.%) ZnO NCs, but at the higher Ag content (4at%) the PL intensity of orange PL band increases (Fig.3b). The PL spectrum transformation with aging has been seen from the comparison of PL spectra presented in figures 4a,b. PL intensities of 2.68 and 2.89 eV PL bands decrease 5

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significantly at aging and the PL intensity of orange PL band at 10K increases (Fig.4a,b). The orange band peak shifts with aging to higher energy up to 2.16 eV (Fig.4b). The orange PL band was studied early and attributed to emission via oxygen

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interstitials, Oi (2.02eV) [31], or hydroxyl groups (2.10 eV) [32]. The green PL band in undoped ZnO was assigned to emission via zinc vacancies [33] or surface defects [34, 35]. Aging ZnO NCs in ambient air corresponds to the treatment at O- and N- rich conditions.

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The acceptor type defects favored for these conditions are: oxygen interstitial, Oi, and zinc vacancy, VZn [33]. Thus, the photo curries recombination via Oi and VZn defects is

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accompanied by emission of the orange and green PL bands, respectively (Fig.4b). The shift of orange PL peak to the higher energy (2.16eV) at aging can be explained by the concentration growth of oxygen interstitials in ZnO NCs and, probably, by the donoracceptor nature of orange emitting centers.

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In ZnO NCs the silver ions can occupy the substitutional AgZn and interstitial AgI sites, but the occupation of substitutional sites (zinc positions) is energetically more favorable [20, 36]. Thus the Ag ions occupy the substitutional positions in ZnO at a low Ag contents

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(1-3at %). The size of Ag+ ions (1.26Ǻ) is bigger than the size of Zn2+ ions (0.74Ǻ) [37]. The last fact leads to increasing a compressive strain in ZnO NCs at the Ag dissolution by

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occupying the zinc sites. Actually, the increase of compressive strains in ZnO NCs with 13at% Ag is confirmed by the shift of all XRD peaks to higher 2Θ values (Table 1) owing to decreasing the inter-planar distances in ZnO [38]. Thus we can suppose that the 2.89eV PL band is connected with emission via the substitutional AgZn acceptor defects. The theoretically calculated energy ε (0/−) for Ag levels in the substitutional sites was estimated as 0.4eV above the valence band [20]. The intensity variations of 2.68eV PL band versus Ag doping and aging correlate with the behavior of 2.89eV PL band. The last fact permits 6

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to assign the 2.68eV PL band to emission via some defects that accompany Ag doping. Actually, the 2.95eV PL band was detected early at 3.6K in PL spectra of un-doped ZnO NCs and attributed to the carbon contamination [39]. The spectral position of this PL band

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is close to the 2.89eV PL band that is discussed in the present paper. But 2.89eVPL band disappeared in aged ZnO NCs, when the carbon concentration (detected by XPS) increased. Thus it is not any reason to attribute the 2.89eV PL band to carbon contamination in the

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studied ZnO NCs.

The shift of XRD peaks to lower 2Θ values (Table 1) at the high Ag content (4at%)

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testifies on increasing the inter-planar distances in ZnO Ag NCs. For this Ag concentration the silver ions, apparently, occupy both the substitutional AgZn and interstitial AgI sites in ZnO NCs. Simultaneously, the formation of metallic Ag inclusions has been detected by XRD (Table 1). The formation of Ag-rich second phases at high Ag doping was supposed

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early in [23]. In the case of second phase formation the change of crystal lattice constants in ZnO NCs was expected as well [23].

XPS spectra were studied for as-grown and aged Ag doped ZnO NCs with the aim to

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investigate the reasons of PL intensity decreasing the Ag related PL bands (2.68 and 2.89 eV) at aging. XPS peaks of four elements Zn, C, O and Ag have been seen clearly in the

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XPS spectrum of as-grown ZnO NCs (Fig.5a). High resolution XPS spectra permit to detect the Zn doublet: Zn 2p1/2 and Zn2p3/2 at 1045 and 1022 eV, respectively (Fig.5b). Zn doublet positions with the energy difference of 23 eV between peaks are typical for Zn2+ ions in the ZnO crystal lattice [40, 41]. The Zn 2p related XPS peaks did not change at aging ZnO:Ag NCs.. The peak of 530.4 eV in the high resolution XPS spectrum of as-grown ZnO NCs (Fig.5c) is connected with the lattice oxygen, O2-, in ZnO [40,41]. The second peak at 532 7

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eV (Fig.5c) is attributed to chemisorbed oxygen inside of the surface hydroxyl group or in a single bound to carbon (C-O) [40-42]. Oxygen related XPD peaks do not change their positions at aging as well. In XPS spectra of aged ZnO:Ag NCs the high intensity C1s peak

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has appeared at 284.1 eV. This peak is attributed to the carbon contamination of the ZnO NC surface [40, 41].

The significant modification at aging is detected in high resolution XPS spectra of the

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Ag related doublet (Fig.5d). Two peaks Ag3d5/2 and Ag3d3/2 at 368.8 (1) and 374.8 eV (2),

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respectively, with the energy difference of 6 eV between them, have been detected in XPS spectra of as-grown ZnO:Ag NCs. Both Ag doublet peaks have shifted to lower energy in the XPS spectrum of aged ZnO:Ag NC films (Fig.5d). Actually the XPS spectrum of aged ZnO:Ag NCs includes the low intensity Ag3d5/2 and Ag3d3/2 doublet (1, 2), with the same peak positions as in as-grown ZnO:Ag NCs, and the new high intensity Ag3d5/2 and

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Ag3d3/2 doublet with the positions 367.3 (3) and 373.3 eV (4), respectively (Fig.5d). Note that the Ag3d3/2 peak in XPS spectra (Ag0 state) of metallic silver was reported to

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be in the range from 367.9eV to 368.8 eV [32]. It is known that the Ag3d3/2 peak position in XPS spectra of silver oxide shifts to the ranges: 367.6 - 368.5eV for Ag2O and 367.3 -

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368.1eV for AgO [43, 44]. Thus the peak Ag3d3/2 shifts to lower energy in silver oxide in comparison with its position in XPS spectra of metallic silver. In studied ZnO:Ag NCs the Ag related peaks shift at aging from their positions 368.8 (1) and 374.8 eV (2), typical for metallic silver (or Ag0), to lower energy positions 367.3 (3) and 373.3 (4) eV, keeping the difference between them 6eV. New positions of Ag doublet peaks testify on the formation of AgO inclusions in ZnO:Ag NCs at aging. The last process is accompanied by

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concentration decreasing the silver related radiative defects (AgZn) together with falling down the PL intensities of 2.68 and 2.89 eV PL bands.

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4. Conclusions The impacts of Ag doping ZnO NCs and aging in ambient air have been investigated. It is revealed that Ag doping ZnO NCs for 1-3at% leads to the emission enhancement of 2.68 and 2.89eV PL bands. The last PL band was attributed to emission via

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the substitutional AgZn defects. It is shown that aging in ambient air is connected with the

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diffusion of oxygen interstitials into ZnO NCs. This process leads to the amorphous phase formation on the ZnO:Ag NC surface and the PL intensity enhancement of the orange PL band in aged ZnO:Ag NCs. At the same time oxygen interstitials participate in the formation of AgO inclusions in ZnO NCs that is accompanied by decreasing the PL intensities of 2.89 and 2.68eV PL bands.

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Acknowledgements The authors thank the Secretary of Investigation and Postgraduate Study at National Polytechnic Institute (projects 20170821), and National Council of

support.

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Science and Technology (CONACYT) of Mexico (project 258224) for the financial

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Figure captions. Fig.1 SEM images of as-grown ZnO NCs doped with 1at%Ag (a) and 4at%Ag (b). The comparison of SEM images for as-grown (c) and aged (d) ZnO:2at%Ag NCs.

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Fig.2. XRD diagram for as-grown (a) and aged (b) ZnO:3at%AgNCs.

Fig.3 PL spectra of ZnO Ag NCs measured in the range 10-310K (a). Normalized PL spectra measured at 10K (b) for as-grown ZnO:NCs with the Ag contents: 1at%, 2at%,

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3at% and 4at% .

Fig.4 PL spectra measured at 10K for as-grown (a) and aged (b) ZnO:3at%Ag NCs. Dashed

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curves present the results of PL spectrum deconvolution on the five PL bands. Fig.5. XPS spectra of as-grown ZnO:Ag NCs (a) and the high resolution XPS spectra for Zn and O elements (b, c). The comparison (d) of Ag3d related XPS doublets obtained in high resolution regime for as-grown and aged ZnO:Ag NCs.

ZnO (002)

ZnO (101)

Ag (111)

Ag (200)

ZnO (102)

ZnO (110)

ZnO (103)

ZnO (112)

31.8371

34.4631

36.2471

-

-

47.6151

56.6801

62.9521

68.2441

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ZnO (100)

31.8391

34.4831

36.2661

-

-

47.7621

56.7781

63.0011

68.3421

31.8671

34.6301

36.2961

38.1581

44.6261

47.8111

56.8271

63.0501

68.3951

31.7391

34.5321

36.2471

38.2071

44.6261

47.6641

56.6311

63.0011

67.9991

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Asgrown samples ZnO:Ag 1at% ZnO:Ag 2at% ZnO:Ag 3at% ZnO:Ag 4at%

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Table 1. XRD parameters of ZnO:Ag NCs versus Ag contents

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b. As-grown

ZnO:4%Ag NCs

ZnO:2%Ag NCs

Fig.1

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c. As-grown

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a. As-grown ZnO:1%Ag NCs

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d. Aged ZnO:2%Ag NCs

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b

1600 (100)

(101) (102)

(103) (112) (110)

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800

ZnO:3%Ag NCs aged

(002)

0 2400

ZnO:3%Ag NCs as-grown

a

1600

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XRD Intensity (arb. un.)

2400

800

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0 20 25 30 35 40 45 50 55 60 65 70 75 80

2θ (degree)

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Fig.2

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Fig.3a

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4

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1

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ZnO:Ag 3at%

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ZnO:Ag 4at%

ZnO:Ag 2at%

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Normalized PL intensity (arb.un.)

10 K

ZnO:Ag 1at%

2,1 2,4 2,7 3,0 Emission energy (eV)

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1,8

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Fig.3b

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ZnO:3%Ag NCs as-grown

10K a

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3

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4

1

ZnO:3%Ag NCs aged

10K

SC

PL intensity (arb. un.)

2

1,8

M AN U

b

2,1

2,4

2,7

3,0

TE D

Emission energy (eV)

AC C

EP

Fig.4

18

3,3

ACCEPTED MANUSCRIPT

O1s 400000

Ag3d

200000

C1s

0

1200 1000 800

600

400

200

1050

0

1030

1020

1010

SC

a.

M AN U

b.

ZnO:3%AgNCs as grown

ZnO:3%Ag NCs aged

TE D

2

c

AC C

ZnO:3%Ag NCs as-grown Ag3d3/2

Ag3d5/2

526

EP

534 532 530 528 Binding Energy (eV)

Counts /s

O1s

1

536

1040

Binding Energy (eV)

Binding Energy (eV)

538

Zn2p1/2

Counts /s

600000

Counts /s

Counts /s

800000

Zn2p3/2

ZnO:3%Ag NCs as-grown

ZnO-3%Ag NCs as-grown

RI PT

Zn2p

1000000

380

375

370

Binding Energy (eV)

d Fig.5

19

365

ACCEPTED MANUSCRIPT

Highlight

RI PT

1. Emission and crystal structure of ZnO NCs with different Ag contents are studied. 2. New AgZn related PL bands peaked at 2.68 and 2.89 eV have been detected.

3. The variation of PL spectra at aging ZnO:Ag NCs in ambient air has been revealed.

AC C

EP

TE D

M AN U

SC

4. The formation of Ag oxide inclusions in ZnO:Ag NCs at aging has been detected.