- Email: [email protected]
Zn nanoparticle formation in FIB irradiated single crystal ZnO
Accepted Manuscript Title: Zn nanoparticle formation in FIB irradiated single crystal ZnO Authors: M. Pea, G. Barucca, A. Notargiacomo, L. Di Gaspare, V. Mussi PII: DOI: Reference:
S0169-4332(17)33074-X https://doi.org/10.1016/j.apsusc.2017.10.128 APSUSC 37471
To appear in:
APSUSC
Received date: Revised date: Accepted date:
1-8-2017 3-10-2017 16-10-2017
Please cite this article as: M.Pea, G.Barucca, A.Notargiacomo, L.Di Gaspare, V.Mussi, Zn nanoparticle formation in FIB irradiated single crystal ZnO, Applied Surface Science https://doi.org/10.1016/j.apsusc.2017.10.128 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.
Title page Zn nanoparticle formation in FIB irradiated single crystal ZnO M. Pea1, G. Barucca2, A. Notargiacomo1, L. Di Gaspare3 and V. Mussi4* 1
Institute for Photonics and Nanotechnologies - CNR, via Cineto Romano 42, 00156 Rome, Italy Department SIMAU, Università Politecnica delle Marche, Via Brecce Bianche 12, 60131 Ancona, Italy 3 Department of Science, Roma TRE University, Via della Vasca Navale 79, 00146 Roma, Italy 4 Institute of Complex Systems - CNR, via del Fosso del Cavaliere 100, 00137 Rome, Italy 2
*
Corresponding author: [email protected]
Graphical Abstract
Research highlights ► The formation of Zn NPs is induced by Ga+ focused ion beam on ZnO single crystal ►NPs size, concentration and localization depend on the used ion dose ►Zn NPs have the same hexagonal lattice and orientation of the ZnO matrx ►Raman spectroscopy can be used to monitor Zn clustering in a non-destructive way
1
Abstract We report on the formation of Zn nanoparticles induced by Ga+ focused ion beam on single crystal ZnO. The irradiated material have been studied as a function of the ion dose by means of atomic force microscopy, scanning electron microscopy, Raman spectroscopy and transmission electron microscopy, evidencing the presence of Zn nanoparticles with size of the order of 5-30 nm. The nanoparticles are found to be embedded in a shallow amorphous ZnO matrix few tens of nanometers thick. Results reveal that ion beam induced Zn clustering occurs producing crystalline particles with the same hexagonal lattice and orientation of the substrate, and could explain the alteration of optical and electrical properties found for FIB fabricated and processed ZnO based devices.
Keywords: Zinc nanoparticles; ZnO; Focused Ion Beam; Raman spectroscopy; Transmission electron microscopy
1 Introduction Zinc oxide has been the subject of intense research in the last years, focusing on its exceptional electrical, optical and physical properties [1] that allow its application in a huge number of different fields, from optoelectronics to sensing and actuators [2]. It has been used in flexible and transparent electronics, for next generation solar cells, gas sensing, mechanical energy harvesting, and as an efficient light emitting material [3]. Most of these applications require the realization of suitable ZnO micro/nanostructures for achieving high efficiency devices and systems, so that different approaches have been developed for fine tailoring of size and geometry of the fabricated features. An emerging tool for top-down prototyping on ZnO is the focused ion beam (FIB), which is a maskless manufacturing technique exploited to obtain patterning resolution down to the nanoscale [4]. FIB fabrication can be used directly on ZnO as active materials for achieving surface structures as well as complex 3D patterns not obtainable with conventional surface machining. Moreover, its 2
ability to mill almost every known material with high efficiency and optimal control allows to process easily both bulk and coated (or multilayered) samples. Inevitably, interaction with ions produce defects that can alter in some extent the properties of the receiving material. Nonetheless, such defectivity can be somewhat limited by a suitable choice of the sample materials and structure, as we recently reported in the case of ZnO coated with properly tailored ohmic metal stacks [5]. It is worth pointing out that this “side-effect” FIB induced modification of materials is not always just a drawback, but can serve to bring new capabilities and features, such as in the case of the local modification of electrical properties of single crystal ZnO [6]. Here we demonstrate that, under appropriate conditions, it is possible to use ion beam based machining of ZnO crystals to induce the clustering of Zn atoms and the consequent formation of Zn nanoparticles (NPs), which have promising applications for optical and photonic devices. Indeed, Zn NPs have peculiar optical and plasmonic properties [7-9], thus their presence can induce in the hosting material new functionalities exploitable in many fields, such as photovoltaics (enhanced absorption and efficiency), filtering, non-linear optics (large third order susceptibility, fast response, modified and tunable refractive index and luminescence properties with respect to ZnO surrounding matrix) [10-12]. Usually, embedded metal NPs are produced by ion implantation and clustering of the metallic species to modify the properties of target materials [13-16]. Differently, we show that the same use of FIB structuring allows to generate Zn colloids on the ZnO crystal surface. The NPs formation is studied as a function of the ion dose by means of atomic force microscopy (AFM), Raman spectroscopy and transmission electron microscopy (TEM), to demonstrate that the process is not only efficient but also controllable, and can be used to tailor the properties of the starting material. Moreover, several papers in the literature show an increase in conductivity due to focused Ga ion treatment of ZnO, which is usually explained by the introduction of new energy levels in the band structure due to the formation of lattice defects or oxygen vacancies. In light of what was observed for the first time, to our knowledge, in the present work, percolation pathways
3
among Zn nanoparticles should be considered as one of the possible mechanisms explaining the increased conductivity of the material that has undergone FIB treatment.
2 Materials and methods Single crystal zinc oxide substrates (O face polished top surface) with c-axis (001) orientation and resistivity in the 500÷1000 Ω x cm range were irradiated by Ga+ ions. A dual-beam FIB (FEIHelios Nanolab 600) with a Ga+ liquid metal ion source was used at 30keV acceleration energy and 54 pA ion current. A field-emission scanning electron microscope (SEM) in the same dual beam system vacuum chamber was employed for imaging. SEM images were collected using a beam current of 680pA and an acceleration voltage of 5 kV. Areas with different size up to 20 μm x 20 μm were treated using a pattern generator which allowed to set both the size/shape and the ion dose assigned to each pattern: different doses were obtained by using a fixed dwell time of 1 µs and varying the number of exposure passes. Fig. 1 shows the SEM picture (tilted view) of a single-crystal ZnO substrate after FIB treating areas at ion doses in the range 9.6 1015 - 1.5 1017 ions/cm. The doses are indicated on the right inset in the figure. Material removal due to FIB induced sputtering is apparent as well as changes in the roughness of the treated areas. Topography measurements and surface roughness evaluation were made by AFM using VeecoDigital Instruments D3100 and Bruker Dimension ICON atomic force microscopes, employed in Tapping Mode with silicon probes having nominal tip radius of 5-10 nm and spring constant ~40 N/m. Raman measurements were realized with a DXR Thermo Fisher Scientific Raman Microscope, with a 532 nm excitation, 8 mW power and a 50× objective. The analysed spectral range was 50-650 cm1
, and each spectrum resulted from 1s acquisition time and 100 accumulations. Spectra were
collected on the ZnO single crystal before the treatment (“substrate”), and after FIB irradiation at the various ion doses. To better compare the obtained data, all the spectra have been corrected for 4
the baseline and normalized to the peak located at about 440 cm−1, which corresponds to the E2high vibration mode of ZnO and can be considered as a common underlying reference for all the studied samples. The inner structure of the FIB treated samples was investigated by TEM using a Philips CM200 electron microscope equipped with a LaB6 filament and operating at 200 kV. The same FIB equipment used for ion treatment was employed to prepare TEM lamellae by both depositing a protective C:Pt layer at the place of cutting and thinning the lamellae.
3 Results and discussion As already reported in ref. [6] the overall effect of FIB irradiation of the ZnO surface is the formation of a shallow defected layer which partially turns to amorphous by increasing the ion dose. Most of the impinging Ga+ atoms are confined in this defective and amorphous layer. Here in the following we report a more detailed study of the ZnO sample treated in a selected dose range. A SEM investigation of the samples is reported in fig. 2, showing images acquired on the areas treated by FIB at increasing ion dose. The image referring to untreated ZnO substrate does not report any significant feature (not shown). At the lowest dose used (D1, fig 2a) an initial surface structuring is visible, which becomes more evident at dose D2 (fig 2b). By increasing further the ion dose (D3, fig. 2c) bright spots start to appear, and from dose D4 (fig. 2d) rounded and elongated features develop, becoming more apparent at doses D5-D6 (fig. 2e-f). Figure 3 shows the 2D AFM views of the ZnO substrate before any treatment (panel a), and upon increasing the ion dose (panels b to g). The corresponding root-mean-square roughness (Rq) values calculated from AFM data are reported in fig. 3h. The untreated single crystal ZnO substrate (fig 3a) displays an irregular surface due to the polishing process, which is characterized by an Rq value slightly below 1 nm. At increasing ion dose up to D3, the surface tends to become more homogenous, keeping Rq values between 0.5 and 1.0 nm. At higher doses (fig. 3e-g, D4-D6), the sample develop apparent protruding features superimposed uniformly on a hill-and-valley 5
structured surface. The corresponding Rq values are increased by more than 50%, and are pinned at around 1.5 nm. Raman spectra collected on the untreated crystalline substrate and on five regions exposed to the FIB at different ion doses are imaged in fig. 4. The spectrum obtained on the ZnO bulk crystal, shown in Fig. 4a, is characterized by four main spectral contributions at about 440 cm−1 (E2 high vibration mode), 410 cm−1 (appearing as a lower shoulder of the E2 peak and ascribed to TO component of E1 phonon), 99 cm−1 (a low frequency mode dominated by the vibrations of the Zn sub-lattice), and about 336 cm−1 (due to the combination E2 high – E2 low) [17]. As shown in a previous paper [6], ion irradiation at the lowest dose does not modify substantially the crystal spectrum, but some additional contributions appear in the range 500–600 cm−1, that are associated to superimposed second order bands, disorder activated “silent modes” [18], and to the two close modes A1(LO), at 574 cm−1, and E1(LO) at 583 cm−1. These vibrational contributions, that have low Raman cross section in crystalline ZnO, are usually treated as a single band named LO band, and their activation after ion irradiation can be attributed to the disordering of the material [19, 6]. The intensity of the added contributions has a typical dependence on the used dose, characterized by an initial increase, associated to the structure perturbation, followed by a significant reduction for the highest doses, due to the balance between further sample damaging and surface milling [6]. Here we have analysed the low wavenumber region, where peculiar changes are clearly visible. In particular, fig. 4b presents the spectra collected on the crystalline substrate before the treatment (black line) and on the regions irradiated with the FIB at different ion doses (namely D1, D2, D3, D4 and D6). An overall signal increase is apparent, with a dose behaviour similar to that of the damage induced added spectral contributions mentioned above. A sharp peak at about 78 cm-1 is visible, emerging at dose D2, increasing in intensity up to dose D4, and decreasing back at the highest used dose, D6 (see inset of Fig. 4b reporting the band intensity obtained by a fitting procedure). As the ion dose increases, this peak undergoes a relevant shift towards low wavenumbers, reaching a stable position at about 74 cm-1 at doses D4-D6. This newly appearing 6
spectral feature is compatible with the occurrence of Zn clustering, i.e. formation of Zn nanoparticles induced by the Ga+ ion irradiation. This assignment is supported by the comparison of our Raman data with the reported properties of long-wavelength optical phonons in zinc single crystal, even if the relevant peak is in this case located at about 70 cm-1 [20]. Furthermore, an identical band has been revealed (but not precisely assigned) in ZnO nanofilms obtained by chemical bath deposition technique activated by microwaves [21], in which a relevant concentration of interstitial Zn has been found. The presence of metallic Zn aggregates in our samples is confirmed by transmission electron microscopy observations. Figure 5 shows TEM cross-sectional analyses performed on the sample subjected to a Ga+ dose of 3.8 x 1016 ions/cm2 (D3). In figure 5a, a 20-30 nm thick layer with a bright contrast is clearly distinguishable on the top of the sample. Increasing the magnification (inset of fig. 5a) this layer appears composed of a kind of columnar nanocrystals immersed in an amorphous or poor crystalline matrix. The amorphous-like nature of the matrix was confirmed by high-resolution TEM observations. The lateral size of these grains ranges between 5 and 10 nm, while their height is between 5 and 30 nm. The nature of the nanocrystals is revealed by selected area electron diffraction (SAED) measurements. In particular, fig. 5b shows the SAED pattern corresponding to the region imaged in fig. 5a. Two kind of reflections are visible: large spots regularly distributed in a rectangular symmetry and small spots distributed on short circle arcs. Analysing the spots geometry and the associated lattice distances, it is possible to attribute the large spots to the ZnO sample in [100] zone axis orientation, while the small spots are due to metallic Zn nanocrystals, fig. 5c. Considering that Zn reflections are manly distributed on short circle arcs, it is possible to deduce that nanocrystals grow preferentially oriented with respect to the ZnO substrate. In detail, they have the same hexagonal lattice of the ZnO substrate and tend to grow with approximately the same orientation: [001]Zn // [001]ZnO ; (100)Zn // (100)ZnO. For completeness, it must be stressed that TEM observations performed on the sample treated with the D1 dose did not reveal the presence of Zn nanocrystals, in perfect agreement with Raman data. 7
In light of recent papers [22, 23] the possibility of an electron beam effect in the formation of the Zn nanoparticles during TEM observations has been considered. However, the accelerating potential (200kV) and the current densities at the sample (lower than 100 A/cm2) used during observations are insufficient to induce significant knock-on or radiolysis effects on ZnO. Furthermore, TEM analysis performed in the same operational conditions (electrons energy, density current, observation time) on sample regions irradiated with lower Ga-ions doses or not implanted did not show the presence of Zn nanocrystal, definitely excluding, in our case, an electron beam Zn nanocrystals formation. The above reported analysis suggests that a preferential ion sputtering of oxygen atoms induced by normal incidence Ga+ FIB irradiation at 30keV causes the formation of small Zn clusters preserving the hexagonal crystal structure of the ZnO matrix. In this interpretative framework, the shift of the new peak appearing in our Raman spectra, with respect to the Zn single crystal one reported in [20], is likely associated to the nanometric size of the aggregates formed in the sample surface layer after FIB processing. A similar red shift has been in fact reported for confined longitudinal optical phonons in spherical ZnO quantum dots (QD) with wurtzite crystal structure [24]. It was also observed that the entity of the red shift decreases by decreasing the size of the quantum dots, i.e. the phonon mode frequency in the smaller nanoaggregate slightly approaches the bulk one [24] (in fact, the calculated ZnO bulk mode z,LO is located at 579 cm-1, but it is found at 588 cm-1 for 8 nm QDs, and at 584 cm-1 for 4 nm QDs). This behaviour with the QD size is imputed to the contribution from the excited exciton states and to the stronger deviation from the perfect spherical shape of smaller nanoaggregates. A similar effect is likely found in our case for the Zn nanoaggregates formed after FIB irradiation, suggesting a variation of the nanoparticle size with the ion dose. In particular, at dose D2, the concentration and size of the metallic clusters becomes sufficiently high that the corresponding signal can be detected by means of Raman spectroscopy, giving rise to the new band at around 78 cm-1, red-shifted with respect to the bulk Zn mode, while the AFM and SEM techniques do not evidence any nanometric feature or apparent change in the sample morphology. This is probably due to the development of Zn clusters inside the 8
shallow amorphous ZnO layer formed by FIB at low doses. By increasing the dose, the proceeding of surface milling removes the amorphous material and induces a reduction of the size of the formed nanoaggregates, causing the shift of the Raman band towards 74 cm-1. At this point, the nanometric clusters start to be visible also by means of AFM and SEM. At dose D4, a sort of equilibrium is reached between damaging and milling mainly associated, as said, with an intensity reduction of the “disorder activated Raman bands” located at 500-600 cm-1 [6]. Finally, between the last two doses, D4 and D6, the size of the Zn nanoparticles remains stable, so that the position of the new peak at 74 cm-1 does not change, but their concentration decreases, therefore even the intensity of this Raman peak starts reducing.
5 Conclusion We reported, for the first time to our knowledge, on the formation of Zn nanoparticles embedded in, or partially covered by, a thin ZnO amorphous layer induced by normal incidence Ga+ FIB irradiation at 30keV. The combined morphological and structural characterization using AFM, SEM, TEM and Raman spectroscopy showed that Zn clustering occurs forming nanoparticles with the same hexagonal lattice and orientation of the ZnO substrate. Moreover, we showed that Raman spectroscopy is a highly sensitive technique capable of characterizing the particle formation in a non-destructive way and allowing to tailor and control the shallow formation of Zn NPs by checking both the intensity and position of the related Raman peak. In this context, the combined use of high resolution pattern-based ion irradiation processes and micro-Raman spectroscopy can be envisaged as powerful tool for processing and diagnostic monitoring of spatially controlled Zn clustering on single crystal ZnO. In light of our findings, we can hypothesize that current percolation pathways among Zn nanoparticles can be one of the mechanisms likely explaining the increased conductivity of ZnO material that has undergone FIB treatment, and further studies are planned to investigate this specific issue. Furthermore, additional
9
experiments will be performed by varying the energy of the Ga+ ions to investigate in more detail the physical mechanism originating the transformation from ZnO to Zn.
Acknowledgment This work has been supported by “Progetto FIRB Futuro in Ricerca RBFR10VB42 (MIUR ITALY)”. We acknowledge LIME LAB of Università degli Studi Roma TRE, and Thermo Fisher Scientific for technical support.
References [1] A. Janotti, C.G.V. de Walle, 2009 Fundamentals of zinc oxide as a semiconductor, Rep. Prog. Phys. 72 (2009) 126501-30. [2] U. Özgür, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, H. Morkoç, A comprehensive review of ZnO materials and devices, J. Appl. Phys. 98 (2005) 041301103. [3] C. Klingshirn, J. Fallert, H. Zhou, J. Sartor, C. Thiele, F. Maier-Flaig, D. Schneider, H. Kalt, 65 years of ZnO research - old and very recent results, Phys. Status Solidi B. 247 (2010) 1424–1447. [4] A.A. Tseng, Recent developments in nanofabrication using focused ion beams Small (Weinh. Bergstr. Ger.) 1 (2005) 924–939. [5] M. Pea, L. Maiolo, E. Giovine, A. Rinaldi, R. Araneo, A. Notargiacomo, “Electrical characterization of FIB processed metal layers for reliable conductive-AFM on ZnO microstructures”, Appl. Surf. Sci. 371, 83-90 (2016) [6] M. Pea, V. Mussi, G. Barucca, E. Giovine, A. Rinaldi, R. Araneo, A. Notargiacomo, Focused ion beam surface treatments of single crystal zinc oxide for device fabrication, Materials & Design 112 (2016) 530-538. [7] J.-H. Lin, Y.-J. Huang, Y.-P. Su, C.-A. Liu, R.S. Devan, C.-H. Ho, Y.-P. Wang, H.-W. Lee, C.M. Chang, Y. Liou Y and Y.-R. Ma, Room-temperature wide-range photoluminescence and 10
semiconducting characteristics of two-dimensional pure metallic Zn nanoplates, RSC Advances 2 (2012) 2123–2127. [8] S.-S. Chang, S.O. Yoon, H.J. Park, A. Sakai, Luminescence properties of Zn nanowires prepared by electrochemical etching Mat. Lett. 53 (2002) 432–436. [9] S.-S. Sung-Sik Chang, S. O. Yoon, H. J. Park, A. Sakai, Luminescence properties of anodically etched porous Zn, Appl. Surf. Sci. 158 (2000) 330–334. [10] J.-H. Lin, R. A. Patil, R. S. Devan, Z.-A. Liu, Y.-P. Wang, C.-H. Ho, Y. Liou, and Y.-R. Ma, Photoluminescence mechanisms of metallic Zn nanospheres, semiconducting ZnO nanoballoons, and metal-semiconductor Zn/ZnO nanospheres, Scien. Rep. 4 (2014) 6967. [11] H. Zeng, W. Cai, Y. Li, J. Hu, and P. Liu, Composition/Structural Evolution and Optical Properties of ZnO/Zn Nanoparticles by Laser Ablation in Liquid Media, J. Phys. Chem. B 109 (2005) 18260-18266 [12] I. G. Morozov, O. V. Belousova, D. Ortega, M.-K. Mafina, M. V. Kuznetcov, Structural, optical, XPS and magnetic properties of Zn particles capped by ZnO nanoparticles, J. All. Comp. 633 (2015) 237–245. [13] P. Umapada, and O. Peña Rodríguez, Ion Implantation for the Fabrication of Plasmonic Nanocomposites: A Brief Review In Ion Implantation (2012) edited by Goorsky M.; InTech, pp.327-360. [14] J. Wang, G. Jia, B. Zhang, H. Liu, and C. Liu, Formation and optical absorption property of nanometer metallic colloids in Zn and Ag dually implanted silica: Synthesis of the modified Ag nanoparticles, J. Appl. Phys. 113 (2013) 034304-8. [15] A. Meldrum, L. A. Boatner, C. W. White, Nanocomposite formed by ion implantation: Recent developments and future opportunities, Nucl. Instrum. Methods Phys. Res. B 178 (2001) 7-16. [16] M. M. Mikhailov and V. V. Neshchimenko, Characteristic Features of Colloid Centers in the Absorption Spectra of Preliminarily Heated Zinc Oxide Powders Irradiated by Protons, Journal of Surface Investigation. X ray, Synchrotron and Neutron Techniques 9 (2015) 144–152. 11
[17] T. C. Damen, S. P. S. Porto, B. Tell, Raman effect in zinc oxide, Phys. Rev. 142 (1966) 570– 574. [18] Manjón F J, Marí B, Serrano J and Romero A H 2005 Silent Raman modes in zinc oxide and related nitrides. J. App. Phys. 97 053516-1 [19] F. J. Manjón, B. Marí, J. Serrano, and A. H. Romero, A. Multi-wavelength Raman scattering of nanostructured Al-doped zinc oxide, J. Appl. Phys. 115 (2014) 073508-1. [20] G. A. Bolotin, Y. I. Kuz’min, Y. V. Knyazev, Y. S. Ponosov, C. Thomsen, Analysis of dispersion of long-wavelength optical phonons in zinc with the help of light scattering, Phys. Sol. Sta. 43 (2001) 1801-1806. [21] J. D. Reyes, J. M. Juárez, D. Hernández de la Luz, Characterization of ZnO nanofilms deposited by CBD-AW, Revista Colombiana de Materiales 5 (2014) 103-110. [22] S. B. Lee, Rocksalt ZnO nanocrystal formation by beam irradiation of wurtzite ZnO in a transmission electron microscope, Physica E: Low-dimensional Systems and Nanostructures 84 (2016) 310-315. [23] S. B. Lee, J. Park and P. A. van Aken, Formation of Pt–Zn Alloy Nanoparticles by ElectronBeam Irradiation of Wurtzite ZnO in the TEM, Nanoscale Research Letters (2016) 11:339. [24] V. A. Fonoberov A. A. and Balandin, Interface and confined polar optical phonons in spherical ZnO quantum dots with wurtzite crystal structure, Phys. Stat. Sol. (c) 1 (2004) 2650–2653.
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Fig 1: Electron microscopy image (tilt angle: 52°) showing rectangular areas (2 μm x 5 μm) treated using FIB at different ion dose. The ion dose labels are reported below each treated area and corresponding values are listed in the table in the figure panel. The scale bar indicates 2 μm.
Fig 2: SEM images of FIB treated areas at different ion doses: (a) D1, (b) D2, (c) D3, (d) D4, (e) D5, and (f) D6, respectively. Ion dose increases from left to right and from top to bottom.
13
Fig 3: AFM 2D images of FIB treated areas at different ion dose: (a) ZnO substrate, and sample treated at doses (b) D1, (c) D2 (d) D3, (e) D4, (f) D5 and (g) D6. (h) RMS roughness values as a function of the ion dose.
Fig 4: (a) Raman spectra of the bare single crystal ZnO substrate. (b) Low wavenumber region of the Raman spectra of the single crystal ZnO sample before and after treatments at increasing ion dose. The inset shows the amplitude of the peak around 76 cm-1.
14
Fig 5: TEM investigation on ZnO sample treated at D3 ion dose. a) TEM bright field images showing the presence of columnar nanocrystals (arrows) on the treated surface at different magnifications; b) corresponding diffraction pattern; c) attribution of the diffraction spots to ZnO and Zn.
15
S0169-4332(17)33074-X https://doi.org/10.1016/j.apsusc.2017.10.128 APSUSC 37471
To appear in:
APSUSC
Received date: Revised date: Accepted date:
1-8-2017 3-10-2017 16-10-2017
Please cite this article as: M.Pea, G.Barucca, A.Notargiacomo, L.Di Gaspare, V.Mussi, Zn nanoparticle formation in FIB irradiated single crystal ZnO, Applied Surface Science https://doi.org/10.1016/j.apsusc.2017.10.128 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.
Title page Zn nanoparticle formation in FIB irradiated single crystal ZnO M. Pea1, G. Barucca2, A. Notargiacomo1, L. Di Gaspare3 and V. Mussi4* 1
Institute for Photonics and Nanotechnologies - CNR, via Cineto Romano 42, 00156 Rome, Italy Department SIMAU, Università Politecnica delle Marche, Via Brecce Bianche 12, 60131 Ancona, Italy 3 Department of Science, Roma TRE University, Via della Vasca Navale 79, 00146 Roma, Italy 4 Institute of Complex Systems - CNR, via del Fosso del Cavaliere 100, 00137 Rome, Italy 2
*
Corresponding author: [email protected]
Graphical Abstract
Research highlights ► The formation of Zn NPs is induced by Ga+ focused ion beam on ZnO single crystal ►NPs size, concentration and localization depend on the used ion dose ►Zn NPs have the same hexagonal lattice and orientation of the ZnO matrx ►Raman spectroscopy can be used to monitor Zn clustering in a non-destructive way
1
Abstract We report on the formation of Zn nanoparticles induced by Ga+ focused ion beam on single crystal ZnO. The irradiated material have been studied as a function of the ion dose by means of atomic force microscopy, scanning electron microscopy, Raman spectroscopy and transmission electron microscopy, evidencing the presence of Zn nanoparticles with size of the order of 5-30 nm. The nanoparticles are found to be embedded in a shallow amorphous ZnO matrix few tens of nanometers thick. Results reveal that ion beam induced Zn clustering occurs producing crystalline particles with the same hexagonal lattice and orientation of the substrate, and could explain the alteration of optical and electrical properties found for FIB fabricated and processed ZnO based devices.
Keywords: Zinc nanoparticles; ZnO; Focused Ion Beam; Raman spectroscopy; Transmission electron microscopy
1 Introduction Zinc oxide has been the subject of intense research in the last years, focusing on its exceptional electrical, optical and physical properties [1] that allow its application in a huge number of different fields, from optoelectronics to sensing and actuators [2]. It has been used in flexible and transparent electronics, for next generation solar cells, gas sensing, mechanical energy harvesting, and as an efficient light emitting material [3]. Most of these applications require the realization of suitable ZnO micro/nanostructures for achieving high efficiency devices and systems, so that different approaches have been developed for fine tailoring of size and geometry of the fabricated features. An emerging tool for top-down prototyping on ZnO is the focused ion beam (FIB), which is a maskless manufacturing technique exploited to obtain patterning resolution down to the nanoscale [4]. FIB fabrication can be used directly on ZnO as active materials for achieving surface structures as well as complex 3D patterns not obtainable with conventional surface machining. Moreover, its 2
ability to mill almost every known material with high efficiency and optimal control allows to process easily both bulk and coated (or multilayered) samples. Inevitably, interaction with ions produce defects that can alter in some extent the properties of the receiving material. Nonetheless, such defectivity can be somewhat limited by a suitable choice of the sample materials and structure, as we recently reported in the case of ZnO coated with properly tailored ohmic metal stacks [5]. It is worth pointing out that this “side-effect” FIB induced modification of materials is not always just a drawback, but can serve to bring new capabilities and features, such as in the case of the local modification of electrical properties of single crystal ZnO [6]. Here we demonstrate that, under appropriate conditions, it is possible to use ion beam based machining of ZnO crystals to induce the clustering of Zn atoms and the consequent formation of Zn nanoparticles (NPs), which have promising applications for optical and photonic devices. Indeed, Zn NPs have peculiar optical and plasmonic properties [7-9], thus their presence can induce in the hosting material new functionalities exploitable in many fields, such as photovoltaics (enhanced absorption and efficiency), filtering, non-linear optics (large third order susceptibility, fast response, modified and tunable refractive index and luminescence properties with respect to ZnO surrounding matrix) [10-12]. Usually, embedded metal NPs are produced by ion implantation and clustering of the metallic species to modify the properties of target materials [13-16]. Differently, we show that the same use of FIB structuring allows to generate Zn colloids on the ZnO crystal surface. The NPs formation is studied as a function of the ion dose by means of atomic force microscopy (AFM), Raman spectroscopy and transmission electron microscopy (TEM), to demonstrate that the process is not only efficient but also controllable, and can be used to tailor the properties of the starting material. Moreover, several papers in the literature show an increase in conductivity due to focused Ga ion treatment of ZnO, which is usually explained by the introduction of new energy levels in the band structure due to the formation of lattice defects or oxygen vacancies. In light of what was observed for the first time, to our knowledge, in the present work, percolation pathways
3
among Zn nanoparticles should be considered as one of the possible mechanisms explaining the increased conductivity of the material that has undergone FIB treatment.
2 Materials and methods Single crystal zinc oxide substrates (O face polished top surface) with c-axis (001) orientation and resistivity in the 500÷1000 Ω x cm range were irradiated by Ga+ ions. A dual-beam FIB (FEIHelios Nanolab 600) with a Ga+ liquid metal ion source was used at 30keV acceleration energy and 54 pA ion current. A field-emission scanning electron microscope (SEM) in the same dual beam system vacuum chamber was employed for imaging. SEM images were collected using a beam current of 680pA and an acceleration voltage of 5 kV. Areas with different size up to 20 μm x 20 μm were treated using a pattern generator which allowed to set both the size/shape and the ion dose assigned to each pattern: different doses were obtained by using a fixed dwell time of 1 µs and varying the number of exposure passes. Fig. 1 shows the SEM picture (tilted view) of a single-crystal ZnO substrate after FIB treating areas at ion doses in the range 9.6 1015 - 1.5 1017 ions/cm. The doses are indicated on the right inset in the figure. Material removal due to FIB induced sputtering is apparent as well as changes in the roughness of the treated areas. Topography measurements and surface roughness evaluation were made by AFM using VeecoDigital Instruments D3100 and Bruker Dimension ICON atomic force microscopes, employed in Tapping Mode with silicon probes having nominal tip radius of 5-10 nm and spring constant ~40 N/m. Raman measurements were realized with a DXR Thermo Fisher Scientific Raman Microscope, with a 532 nm excitation, 8 mW power and a 50× objective. The analysed spectral range was 50-650 cm1
, and each spectrum resulted from 1s acquisition time and 100 accumulations. Spectra were
collected on the ZnO single crystal before the treatment (“substrate”), and after FIB irradiation at the various ion doses. To better compare the obtained data, all the spectra have been corrected for 4
the baseline and normalized to the peak located at about 440 cm−1, which corresponds to the E2high vibration mode of ZnO and can be considered as a common underlying reference for all the studied samples. The inner structure of the FIB treated samples was investigated by TEM using a Philips CM200 electron microscope equipped with a LaB6 filament and operating at 200 kV. The same FIB equipment used for ion treatment was employed to prepare TEM lamellae by both depositing a protective C:Pt layer at the place of cutting and thinning the lamellae.
3 Results and discussion As already reported in ref. [6] the overall effect of FIB irradiation of the ZnO surface is the formation of a shallow defected layer which partially turns to amorphous by increasing the ion dose. Most of the impinging Ga+ atoms are confined in this defective and amorphous layer. Here in the following we report a more detailed study of the ZnO sample treated in a selected dose range. A SEM investigation of the samples is reported in fig. 2, showing images acquired on the areas treated by FIB at increasing ion dose. The image referring to untreated ZnO substrate does not report any significant feature (not shown). At the lowest dose used (D1, fig 2a) an initial surface structuring is visible, which becomes more evident at dose D2 (fig 2b). By increasing further the ion dose (D3, fig. 2c) bright spots start to appear, and from dose D4 (fig. 2d) rounded and elongated features develop, becoming more apparent at doses D5-D6 (fig. 2e-f). Figure 3 shows the 2D AFM views of the ZnO substrate before any treatment (panel a), and upon increasing the ion dose (panels b to g). The corresponding root-mean-square roughness (Rq) values calculated from AFM data are reported in fig. 3h. The untreated single crystal ZnO substrate (fig 3a) displays an irregular surface due to the polishing process, which is characterized by an Rq value slightly below 1 nm. At increasing ion dose up to D3, the surface tends to become more homogenous, keeping Rq values between 0.5 and 1.0 nm. At higher doses (fig. 3e-g, D4-D6), the sample develop apparent protruding features superimposed uniformly on a hill-and-valley 5
structured surface. The corresponding Rq values are increased by more than 50%, and are pinned at around 1.5 nm. Raman spectra collected on the untreated crystalline substrate and on five regions exposed to the FIB at different ion doses are imaged in fig. 4. The spectrum obtained on the ZnO bulk crystal, shown in Fig. 4a, is characterized by four main spectral contributions at about 440 cm−1 (E2 high vibration mode), 410 cm−1 (appearing as a lower shoulder of the E2 peak and ascribed to TO component of E1 phonon), 99 cm−1 (a low frequency mode dominated by the vibrations of the Zn sub-lattice), and about 336 cm−1 (due to the combination E2 high – E2 low) [17]. As shown in a previous paper [6], ion irradiation at the lowest dose does not modify substantially the crystal spectrum, but some additional contributions appear in the range 500–600 cm−1, that are associated to superimposed second order bands, disorder activated “silent modes” [18], and to the two close modes A1(LO), at 574 cm−1, and E1(LO) at 583 cm−1. These vibrational contributions, that have low Raman cross section in crystalline ZnO, are usually treated as a single band named LO band, and their activation after ion irradiation can be attributed to the disordering of the material [19, 6]. The intensity of the added contributions has a typical dependence on the used dose, characterized by an initial increase, associated to the structure perturbation, followed by a significant reduction for the highest doses, due to the balance between further sample damaging and surface milling [6]. Here we have analysed the low wavenumber region, where peculiar changes are clearly visible. In particular, fig. 4b presents the spectra collected on the crystalline substrate before the treatment (black line) and on the regions irradiated with the FIB at different ion doses (namely D1, D2, D3, D4 and D6). An overall signal increase is apparent, with a dose behaviour similar to that of the damage induced added spectral contributions mentioned above. A sharp peak at about 78 cm-1 is visible, emerging at dose D2, increasing in intensity up to dose D4, and decreasing back at the highest used dose, D6 (see inset of Fig. 4b reporting the band intensity obtained by a fitting procedure). As the ion dose increases, this peak undergoes a relevant shift towards low wavenumbers, reaching a stable position at about 74 cm-1 at doses D4-D6. This newly appearing 6
spectral feature is compatible with the occurrence of Zn clustering, i.e. formation of Zn nanoparticles induced by the Ga+ ion irradiation. This assignment is supported by the comparison of our Raman data with the reported properties of long-wavelength optical phonons in zinc single crystal, even if the relevant peak is in this case located at about 70 cm-1 [20]. Furthermore, an identical band has been revealed (but not precisely assigned) in ZnO nanofilms obtained by chemical bath deposition technique activated by microwaves [21], in which a relevant concentration of interstitial Zn has been found. The presence of metallic Zn aggregates in our samples is confirmed by transmission electron microscopy observations. Figure 5 shows TEM cross-sectional analyses performed on the sample subjected to a Ga+ dose of 3.8 x 1016 ions/cm2 (D3). In figure 5a, a 20-30 nm thick layer with a bright contrast is clearly distinguishable on the top of the sample. Increasing the magnification (inset of fig. 5a) this layer appears composed of a kind of columnar nanocrystals immersed in an amorphous or poor crystalline matrix. The amorphous-like nature of the matrix was confirmed by high-resolution TEM observations. The lateral size of these grains ranges between 5 and 10 nm, while their height is between 5 and 30 nm. The nature of the nanocrystals is revealed by selected area electron diffraction (SAED) measurements. In particular, fig. 5b shows the SAED pattern corresponding to the region imaged in fig. 5a. Two kind of reflections are visible: large spots regularly distributed in a rectangular symmetry and small spots distributed on short circle arcs. Analysing the spots geometry and the associated lattice distances, it is possible to attribute the large spots to the ZnO sample in [100] zone axis orientation, while the small spots are due to metallic Zn nanocrystals, fig. 5c. Considering that Zn reflections are manly distributed on short circle arcs, it is possible to deduce that nanocrystals grow preferentially oriented with respect to the ZnO substrate. In detail, they have the same hexagonal lattice of the ZnO substrate and tend to grow with approximately the same orientation: [001]Zn // [001]ZnO ; (100)Zn // (100)ZnO. For completeness, it must be stressed that TEM observations performed on the sample treated with the D1 dose did not reveal the presence of Zn nanocrystals, in perfect agreement with Raman data. 7
In light of recent papers [22, 23] the possibility of an electron beam effect in the formation of the Zn nanoparticles during TEM observations has been considered. However, the accelerating potential (200kV) and the current densities at the sample (lower than 100 A/cm2) used during observations are insufficient to induce significant knock-on or radiolysis effects on ZnO. Furthermore, TEM analysis performed in the same operational conditions (electrons energy, density current, observation time) on sample regions irradiated with lower Ga-ions doses or not implanted did not show the presence of Zn nanocrystal, definitely excluding, in our case, an electron beam Zn nanocrystals formation. The above reported analysis suggests that a preferential ion sputtering of oxygen atoms induced by normal incidence Ga+ FIB irradiation at 30keV causes the formation of small Zn clusters preserving the hexagonal crystal structure of the ZnO matrix. In this interpretative framework, the shift of the new peak appearing in our Raman spectra, with respect to the Zn single crystal one reported in [20], is likely associated to the nanometric size of the aggregates formed in the sample surface layer after FIB processing. A similar red shift has been in fact reported for confined longitudinal optical phonons in spherical ZnO quantum dots (QD) with wurtzite crystal structure [24]. It was also observed that the entity of the red shift decreases by decreasing the size of the quantum dots, i.e. the phonon mode frequency in the smaller nanoaggregate slightly approaches the bulk one [24] (in fact, the calculated ZnO bulk mode z,LO is located at 579 cm-1, but it is found at 588 cm-1 for 8 nm QDs, and at 584 cm-1 for 4 nm QDs). This behaviour with the QD size is imputed to the contribution from the excited exciton states and to the stronger deviation from the perfect spherical shape of smaller nanoaggregates. A similar effect is likely found in our case for the Zn nanoaggregates formed after FIB irradiation, suggesting a variation of the nanoparticle size with the ion dose. In particular, at dose D2, the concentration and size of the metallic clusters becomes sufficiently high that the corresponding signal can be detected by means of Raman spectroscopy, giving rise to the new band at around 78 cm-1, red-shifted with respect to the bulk Zn mode, while the AFM and SEM techniques do not evidence any nanometric feature or apparent change in the sample morphology. This is probably due to the development of Zn clusters inside the 8
shallow amorphous ZnO layer formed by FIB at low doses. By increasing the dose, the proceeding of surface milling removes the amorphous material and induces a reduction of the size of the formed nanoaggregates, causing the shift of the Raman band towards 74 cm-1. At this point, the nanometric clusters start to be visible also by means of AFM and SEM. At dose D4, a sort of equilibrium is reached between damaging and milling mainly associated, as said, with an intensity reduction of the “disorder activated Raman bands” located at 500-600 cm-1 [6]. Finally, between the last two doses, D4 and D6, the size of the Zn nanoparticles remains stable, so that the position of the new peak at 74 cm-1 does not change, but their concentration decreases, therefore even the intensity of this Raman peak starts reducing.
5 Conclusion We reported, for the first time to our knowledge, on the formation of Zn nanoparticles embedded in, or partially covered by, a thin ZnO amorphous layer induced by normal incidence Ga+ FIB irradiation at 30keV. The combined morphological and structural characterization using AFM, SEM, TEM and Raman spectroscopy showed that Zn clustering occurs forming nanoparticles with the same hexagonal lattice and orientation of the ZnO substrate. Moreover, we showed that Raman spectroscopy is a highly sensitive technique capable of characterizing the particle formation in a non-destructive way and allowing to tailor and control the shallow formation of Zn NPs by checking both the intensity and position of the related Raman peak. In this context, the combined use of high resolution pattern-based ion irradiation processes and micro-Raman spectroscopy can be envisaged as powerful tool for processing and diagnostic monitoring of spatially controlled Zn clustering on single crystal ZnO. In light of our findings, we can hypothesize that current percolation pathways among Zn nanoparticles can be one of the mechanisms likely explaining the increased conductivity of ZnO material that has undergone FIB treatment, and further studies are planned to investigate this specific issue. Furthermore, additional
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experiments will be performed by varying the energy of the Ga+ ions to investigate in more detail the physical mechanism originating the transformation from ZnO to Zn.
Acknowledgment This work has been supported by “Progetto FIRB Futuro in Ricerca RBFR10VB42 (MIUR ITALY)”. We acknowledge LIME LAB of Università degli Studi Roma TRE, and Thermo Fisher Scientific for technical support.
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Fig 1: Electron microscopy image (tilt angle: 52°) showing rectangular areas (2 μm x 5 μm) treated using FIB at different ion dose. The ion dose labels are reported below each treated area and corresponding values are listed in the table in the figure panel. The scale bar indicates 2 μm.
Fig 2: SEM images of FIB treated areas at different ion doses: (a) D1, (b) D2, (c) D3, (d) D4, (e) D5, and (f) D6, respectively. Ion dose increases from left to right and from top to bottom.
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Fig 3: AFM 2D images of FIB treated areas at different ion dose: (a) ZnO substrate, and sample treated at doses (b) D1, (c) D2 (d) D3, (e) D4, (f) D5 and (g) D6. (h) RMS roughness values as a function of the ion dose.
Fig 4: (a) Raman spectra of the bare single crystal ZnO substrate. (b) Low wavenumber region of the Raman spectra of the single crystal ZnO sample before and after treatments at increasing ion dose. The inset shows the amplitude of the peak around 76 cm-1.
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Fig 5: TEM investigation on ZnO sample treated at D3 ion dose. a) TEM bright field images showing the presence of columnar nanocrystals (arrows) on the treated surface at different magnifications; b) corresponding diffraction pattern; c) attribution of the diffraction spots to ZnO and Zn.
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