DSpace at VNU: Ferroelectric-based nanocomposites: fabrication and characteristic properties

Ferroelectric-based nanocomposites: fabrication and characteristic properties Pham Duc Thang* Laboratory for Micro and Nanotechnology, Faculty of Engineering Physics and Nanotechnology

Ferroelectric-based nanocomposites: fabrication and characteristic properties

Pham Duc Thang*

Laboratory for Micro and Nanotechnology, Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology,

Vietnam National University, Hanoi, Building E3, 144 Xuan Thuy, Cau Giay, Hanoi, Vietnam Email: pdthang@vnu.edu.vn

*Corresponding author

Mai Thi Ngoc Pham

Faculty of Chemistry, University of Natural Science, Vietnam National University, Hanoi,19 Le Thanh Tong, Hoan Kiem, Hanoi, Vietnam Email: m.t.n.pham@gmail.com

Abstract: Ferroelectric ceramics are widely used in a variety of applications,

ranging from high dielectric capacitors, mechanical actuators, micro-transducers to memory devices Here we present chemical and physical processes to obtain nanocomposites based on ferroelectric perovskites, from metal-oxide to dual-phase oxide materials The changes in the structural, dielectric and magnetic properties are studied Interesting observations such as the percolation effect and anomaly in temperature dependence of magnetisation are also mentioned The results are discussed in view of effects of the spatial distribution of each phase, the stress-induced structural phase transition and the ferroelectric-magnetic coupling Additionally, recent results on epitaxial growth of PZT film on Si are presented

Keywords:PZT ferroelectrics; nanocomposites; high-k dielectric; ferroelectric-magnetic coupling

Reference to this paper should be made as follows: Thang, P.D and Pham,

M.T.N (2013) ‘Ferroelectric-based nanocomposites: fabrication and characteristic properties’,Int J Nanotechnology,Vol 10, Nos 3/4,pp.363–371

Biographical notes: Pham Duc Thang obtained the PhD degree in

Experimental Physics from the University of Amsterdam in 2003 From 2003

to 2006 he worked as a Postdoctoral Researcher at the University of Twente In

2006 he joined the University of Engineering and Technology, VNU as a research staff at the Faculty of Engineering Physics and Nanotechnology

He became an Associate Professor of the university in 2011 His current research interests are focused on nanostructured magnetic materials, functional ferroelectrics, piezoelectrics and multiferroics, micro-nano fabrication and devices

Mai Thi Ngoc Pham obtained the PhD degree in Materials Chemistry from the University of Twente in 2005 In 2006 she joined the University of Natural Science, VNU as a Research Staff at the Faculty of Chemistry Her current research interests are analytical chemistry, nanostructured functional materials and their applications

This paper is a revised and expanded version of a paper entitled ‘Ferroelectric-based nanocomposites: fabrication, characterization and properties’ presented

at the ‘3rd International Workshop on Nanotechnology and Application (IWNA’2011)’, Vung Tau, Vietnam, 10–12 November 2011

1 Introduction

Ferroelectric materials were discovered in the early 1940s when the phenomenon of ferroelectricity was found to be the source of the unusually high dielectric constant in BaTiO3 capacitors Since that time, they became important for many industrial applications, ranging from high-dielectric constant capacitors to later developments in mechanical actuators, micro-transducers and memory devices [1–3] It is known that the properties of ferroelectric materials can be tailored to meet the requirements for a specific application, e.g by varying the synthesis conditions, doping with a foreign element

or adding a second phase Enhancement in the fracture toughness was observed for dual-phase metal-ferroelectric composites PZT-Ag [4], PZT-Pd-Ag [5] and PZT-Pt composites [6] Additionally, the electrical properties are modified by the presence of the metallic phase In the presence of small amounts of Ag the ferroelectric-paraelectric transition of PZT-Ag [7], PZT-Pd-Ag [5] shifts to higher temperatures Recently ferroelectric composites have been engineered to develop high performance in advanced technology Ceramic-polymer composites, for example PVDF-PZT, combining a large piezoelectric effect of the ferroelectric ceramic with flexibility of the polymer, are widely used as acoustic devices or hydrophones [8] Ferroelectric-ferromagnetic composites like PZT-CoFe2O4 or BaTiO3-CoFe2O4 [9–10] exhibit magneto-electricity by coupling electrostrictivity of the ferroelectric phase and magnetostrictivity of the magnetic phase

In this work, we present our study on the synthesis of PZT-Pt bulk nanocomposite by sol-precipitation method and of PZT-CoFe2O4 film nanocomposite by pulsed laser deposition (PLD) The structural, dielectric and magnetic properties are analysed The results are discussed in view of effects of the shape and spatial distribution of each phase, the stress-induced structural phase transition and the ferroelectric-magnetic coupling For practical application, films deposited on Si wafer are desired However, PZT film directly grown on Si often show degradation of ferroelectric properties because at the interface Pb and Si easily diffuse into each other and react to produce various silicate compounds In order to overcome these problems, insulating buffer layers preventing inter-diffusion of

Si and Pb are necessary for PZT film deposition on Si substrates For example ferroelectric thin films have been grown on the oxide electrode, such as RuO2 and SrRuO3 (SRO) Such films, however, often grow along (111) or (110) direction that leads

to decreasing in their properties [11, 12]

In the last part of this work, buffer layers of YSZ and perovskite (La,Sr)MnO3

(LSMO) and SRO have been prepared prior to the deposition of PZT films

Using suitable combinations between these materials, one can obtain PZT films with (001)-preferred orientation The role of buffer layers in the growth is also discussed

2 Fabrication process and characterisation

2.1 High-k dielectric PZT-Pt nanocomposites

Sol-precipitation method was used to prepare PZT-Pt composites The starting materials are PZT powder with a Zr/Ti ratio of 53/47 in % and Pt with the concentration in the composites varied between 0 and 30 vol.% In this method an aqueous solution of

H2PtCl6 was reduced to form nano-sized Pt particles by using hydrazine, sodium boronhydrate, methanol or sodium citrate as a reducing agent PVP was added as a protective agent against agglomeration Subsequently, PZT powder was added to the solution, which was stirred thoroughly to ensure good mixing The composite powder was obtained by filtering the suspension through a 0.1 μm filter The composite pellet was formed after pressing at 4000 MPa and sintering at 1150°C

In the TEM image of composite PZT-Pt powder, presented in Figure 1, Pt particles appear as grey particles covering the entire surface of a PZT grain The XRD patterns of sintered PZT and the PZT-Pt composite with 10 vol.% Pt are shows in Figure 2 All reflections are in agreement with XRD database of PZT and Pt

Figure 1 TEM picture of as-prepared PZT-Pt mixture: Pt particles are adsorbed on PZT

Figure 2 XRD patterns of PZT and PZT-Pt

* Pt peaks

*

*

* PZT-Pt10

PZT

SEM image of sintered PZT-Pt sample with 10 vol.% Pt is presented in Figure 3 As is apparent from this, the round-shaped Pt-particles are spread over the PZT surface with a preferred location at the grain junctions The average size of PZT grains is estimated to

be 2–3 μm, while that of Pt particles is between 100 and 500 nm When the Pt content increases, the shape of Pt particles changes from spherical to irregular aggregates The average particle size increases to 5 μm for 30 vol.% Pt but the grain size of PZT decreases down to 1–2 μm in PZT-Pt sample with 30 vol.% Pt (see Figure 4) In this sample Pt already forms conductive paths as was confirmed by electrical measurements

This can be attributed to a high mobility of Pt, which favours the agglomeration of

Pt particles

Figure 3 SEM image of PZT-Pt with 10 vol.% Pt

Figure 4 SEM image of PZT-Pt with 30 vol.% Pt

In Figure 5, we present the Pt content dependence of the dielectric constant of PZT-Pt samples It is clear that at low Pt content, the dielectric constant is slightly increased but around 25–30 vol.% Pt, it sharply increases, reaching a six times enhancement at

28 vol.% of Pt relative to that observed for bulk ceramic PZT

Pt

Figure 5 Dielectric constant of PZT-Pt composites as a function of Pt content

0 2000 4000 6000 8000

Pt content (vol %)

The sol-precipitation route provides both the proper phases and a homogeneous phase distribution The increase in the dielectric constant is attributed to the formation of multiple capacitors inside the composite At metal concentrations just below the percolation threshold, the transition associated with formation of a continuous path for electrical transport, the capacity reaches extremely large values due to creation of infinitely large surface area for the capacitor electrodes and a small spacing between them This is called the percolation threshold, depending on the shape and spatial distribution as well as the size of the conducting particles relative to that of the insulating particles

Multilayered ferroelectric-ferromagnetic composite of PZT-CoFe2O4 were grown on single-crystal (001) SrTiO3 (STO) substrates by using pulsed laser deposition technique

The deposition chamber was vacuumed to a base pressure of 5 × 10–6 mbar The PZT and CoFe2O4 (CFO) layers were grown at 600°C in oxygen pressure (pO2) of 0.1 mbar and 0.05 mbar, respectively The energy density of laser (E) was 3.5 J/cm2 and 2.5 J/cm2 for PZT and CFO, respectively The thickness of ferroelectric layer is 1 µm and the thickness

of ferromagnetic layer is 280 nm

The typical XRD pattern of CFO/PZT/CFO film is presented in Figure 6 The

presence of two sets of (00l) peaks contributed from CFO and PZT layers reveals that

PZT/CFO film is epitaxially grown on the substrate From AFM images, the first CFO

layer show a very smooth surface (rms = 0.5 nm) and follow the terrace of the substrate

(see Figure 7a) As long as the PZT layer is deposited, its particles formed resulted in a

rough surface with rms = 2.5 nm (see Figure 7b)

The capacitance-voltage characteristic of PZT/CFO film shows a well-defined butterfly and the dielectric constant, derived from the capacitance value at zero applied voltage, is around 800 The magnetic hysteresis loops of PZT/CFO film measured in-plane and perpendicular to the film display an in-in-plane magnetic anisotropy with a coercivity of 200 kA/m (not presented here) This magnetic anisotropy can be explained

in terms of the stress in the film, originated from the lattice mismatch between CFO layer

and the substrate and between CFO layers and PZT layer Since the lattice parameter of

cubic CFO is 8.39 Å, layers grown on STO substrate (a = b = c = 3.91 Å) and PZT layer (a = b = 4.03 Å, c = 4.14 Å) are under compression in the film plane Due to its negative

magnetostriction, a strong in-plane stress anisotropy will be induced

Figure 6 XRD pattern of PZT/CFO film

(004)PZT (003)PZT

(001)PZT

(002)PZT

(008)CFO

(004)CFO

Figure 7 AFM image of CFO (a) and PZT/CFO (b) films (see online version for colours)

The temperature dependence of the in-plane magnetisation of PZT/CFO film is represented in Figure 8 The curve shows a magnetic ordering temperature of about 530°C In addition, an anomaly is observed at 369°C close to the PZT Curie temperature

(TC(E) = 360–390°C) This observation can be understood as the magnetoelectric coupling between the magnetostrictive and piezoelectric parts in the two-phase nanostructure

At temperatures higher than TC(E), CFO is compressed in plane due to the lattice

mismatch with cubic PZT For temperatures below TC(E), the tetragonal distortion in PZT lattice further increases this deformation in CFO layers and results in a decrease in

magnetisation as temperature undergoes TC(E)

Figure 8 Temperature dependence of in-plane magnetisation of PZT/CFO film

T = 369 °C

T (°C)

2.3 Epitaxial PZT film on Si substrate

Ferroelectric PZT films with two configurations of YSZ/SRO/PZT and YSZ/

LSMO/SRO/PZT were grown on single-crystal (001) Si substrates by means of pulsed laser deposition Si substrates were chemically etched in HF solution for 2’ to remove native oxide layer before loading into the deposition chamber The deposition chamber was vacuumed to a base pressure of 2 × 10–6 mbar The 100 nm YSZ buffer layer were deposited at the substrate temperature of 800°C with an energy density of 2.1 J/cm2 To minimise the effect of the native amorphous SiOx layer on the Si surface, this process firstly was carried out in 0.1 mbar Ar for 24’ followed by 30’ in 0.02 mbar O2 prior

to the deposition Then the substrate was cooled down to 600°C for the 200 nm SRO bottom electrode deposition In this process, the experimental parameters used were

E = 2.5 J/cm2, pO2 = 0.13 mbar For the second film configuration, a thin LSMO layer was fabricated on YSZ layer at 600°C, E = 2.1 J/cm2, pO2 = 0.35 mbar Finally the

500 nm PZT layer was grown at 600°C in an oxygen environment with the ambient pressure of 0.1 mbar at 2.9 J/cm2

In Figure 9, a typical XRD θ-2θ pattern of YSZ/SRO/PZT film is illustrated The

presence of only (00l) peaks of YSZ indicate that YSZ epitaxially grown on Si (001)

substrate The same orientation of YSZ and Si substrate can be well understood since

they have a very small lattice mismatch (a = 5.43 Å for Si and 5.14 Å for YSZ)

However, with the addition of only one buffer layer of SRO as a bottom electrode and followed by growing of the PZT film, the orientation of these layers already changed

to (110)-direction This is due to the large difference in lattice parameter of SRO (a = 3.94 Å) compared to YSZ, which results to an alignment of the SRO <111> unit-cell body diagonal with the <110> face diagonal of YSZ

XRD pattern of YSZ/LMSO/SRO/PZT configuration, also presented in Figure 1, however shows that all the layers, including PZT film have only one preferred (00l) orientation This means that the addition of another perovskite layer of LSMO between SRO and YSZ promote the epitaxial growth of the film LSMO with its lattice parameter almost match SRO lattice parameter would be a suitable buffer layer for SRO and PZT

growth It can be assumed that the unit cells of LSMO and SRO rotated 45° on YSZ, resulted in the epitaxial growth of the PZT film

Figure 9 XRD patterns of YSZ/SRO/PZT and YSZ/LSMO/SRO/PZT films

3 Conclusion

PZT-Pt nanocomposites have been successfully prepared by the sol-precipitation route, in which PZT powder is wet mixed with a sol containing Pt nano-particles Their dielectric enhancements are explained by the role of space charge at the PZT/Pt interface in the composites and the diluting effect of non-ferroelectric Pt phase Their property enhancement can be used for specific applications, such as in super-capacitors and dynamic RAM

Epitaxial multilayered PZT-CFO multiferroics have been successfully grown by laser deposition These ferroic phase nanostructures have a large in-plane magnetic anisotropy, reasonable ferroelectric properties and especially, a good magnetostrictive-piezoelectric coupling The obtained result in multilayered nanostructure facilitates the interconversion

of energy stored in electric and magnetic fields

Moreover epitaxial PZT films grown on Si using different buffer layers have been obtained The results provide a great potential for practical applications of these materials

in MEMS and NEMS

Acknowledgements

This research was supported by project 103.02.87.09 of the National Foundation for Science and Technology Development (NAFOSTED) of Vietnam

References

1 Haertling, G.H (1999) ‘Ferroelectric ceramics: history and technology’, Journal of American Ceramic Society, Vol 82, p.797

2 Scott, J and Araujo, C.A (1989) ‘ Ferroelectric memories’, Science, Vol 246, p.1400

3 Auciello, O., Scottand, J.F and Ramesh, R (1998) ‘The physics of ferroelectric memories’,

Physics Today, p.22

4 Hwang, H.J., Yasuoka, M and Sando, M (1999) ‘Fabrication, sinterability, and mechanical

properties of Lead Zirconate Tinatate/Silver composites’, Journal of American Ceramic Society, Vol 82, p.2417

5 Sato, Y., Kanai, H and Yamashita, Y (1996) ‘Effect of Silver and Palladium doping on the dielectric properties of 0.9Pb(Mg 1/3 Nb 2/3 )O 3 -0.1PbTiO 3 ceramic’, Journal of American Ceramic Society, Vol 79, p.261

6 Hwang, H.J., Tajima K and Sando, M (1999) ‘Fatigue-free PZT-based nanocomposites’, Key Engineering Materials, Vols 161–163, p.431

7 Hwang, H.J., Tajima K and Ohji, T (1998) ‘Curie temperature anomaly in Lead Zirconate

Titanate/Silver composite’, Journal of American Ceramic Society, Vol 81, p.709

8 Aggarwal, M.D et al (2007) Polymer-Ceramic Composite Materials for Pyroelectric Infrared Detectors: An Overview, National Aeronautics and Space Administration Center for

AeroSpace Information, 7115 Standard Drive, Hanover, MD 21076-1320

9 Gao, X et al (2010) ‘Microstructure and properties of well-ordered multiferroic Pb(Zr,Ti)O 3 /CoFe 2 O 4 nanocomposites’, ACS Nano, Vol 4, p.1099

10 Bai, F., Zhang, H., Li,J and Viehland, D (2010) ‘Magnetic and magnetoelectric properties of as-deposited and annealed BaTiO 3 -CoFe 2 O 4 nanocomposite thin films’, Journal of Physics D:

Applied Physics, Vol 43, p.285002

11 Chrisey, D and Hubler, G.K (Eds) (1994) Pulsed Laser Deposition of Thin Films,

John Wiley & Sons, New York

12 Brodoceanu, D et al (2003) Pulsed Laser Deposition of Oxide Thin Films, PPLA World

Scientific Publishing Co Pte Ltd, Singapore