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11-5-2015

Synthesis, Characterization and FerroelectricProperties of LN-Type ZnSnO3 NanostructuresCorisa KonsUniversity of South Florida, corisa@mail.usf.edu

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Scholar Commons CitationKons, Corisa, "Synthesis, Characterization and Ferroelectric Properties of LN-Type ZnSnO3 Nanostructures" (2015). Graduate Thesesand Dissertations.http://scholarcommons.usf.edu/etd/5976

Synthesis, Characterization and Ferroelectric Properties of LN-Type ZnSnO3

Nanostructures

by

Corisa Kons

A thesis submitted in partial fulfillment

of the requirements for the degree of

Master of Science

Department of Physics

College of Arts and Science

University of South Florida

Co- Major Professor: Anuja Datta, Ph.D.

Co- Major Professor: Pritish Mukherjee, Ph.D.

Manh-Huong Phan, Ph.D.

Date of Approval:

August 6, 2015

Keywords: nanowire, nanoparticle, nanoflake, hysteresis, solvothermal, pulsed-laser-deposition

Copyright © 2015, Corisa Kons

i

Table of Contents

List of Figures ................................................................................................................................ iii

Abstract ............................................................................................................................................v

Chapter 1: What are Ferroelectric Materials and Their Applications? ............................................1

1.1 Theory of Ferroelectrics .................................................................................................1

1.2 Pb-Free Nanostructured Ferroelectrics ..........................................................................5

1.2.1 LN-Type ZnSnO3 ............................................................................................6

1.3 Applications of Ferroelectric Materials .........................................................................8

1.4 References ......................................................................................................................9

Chapter 2: Experimental Methods and Characterization of LN-type ZnSnO3 Nanostructures .....12

2.1 Synthesis Equipment and Facilities .............................................................................12

2.1.1 Step I: Pulsed Laser Deposition (PLD) System for Thin Film Growth ........12

2.1.2 Step II: Chemical Synthesis of Nanostructures ............................................12

2.2 Characterization Equipment and Facilities ..................................................................13

2.2.1 Physics Materials Diagnostics Facility .........................................................13

2.2.2 Facility for Optical Characterization of Materials ........................................14

2.2.3 Other Resources and User Facilities Used In This Work at University of

South Florida ....................................................................................................15

2.3 General Synthesis and Characterization Overview ......................................................16

2.3.1 Deposition of Al-doped ZnO Template-Layer by PLD ................................16

2.3.2 Solvothermal Synthesis of LN-type ZnSnO3 Nanostructures .......................16

2.3.3 Fabrication of ZnSnO3 Nanostructure arrayed Ferroelectric Devices for

Polarization Measurements ..............................................................................17

2.4 Specific Experimental Parameters ...............................................................................18

2.4.1 Structural Growth of ZnSnO3 Nanowire Arrays by Physical/Chemical

Methods............................................................................................................18

2.4.2 Structural Growth of Hybrid ZnSnO3 Nanowire-Nanoparticle Arrays ........18

2.4.3 Structural Growth of Other ZnSnO3 Nanostructures ....................................19

Chapter 3: Ferroelectricity in LN-type ZnSnO3 Nanowire Arrays ................................................20

3.1 References ....................................................................................................................28

Chapter 4: Ferroelectricity in LN-type ZnSnO3 Nanowire-Nanoparticle Arrays ..........................30

4.1 References ....................................................................................................................40

ii

Chapter 5: Ferroelectricity in Pb-free LN-type ZnSnO3 Other Nanostructure Arrayed Thick

Films ........................................................................................................................................42

5.1 References ....................................................................................................................46

Chapter 6: Summary and Future Work ..........................................................................................47

6.1 References ....................................................................................................................48

Appendices .....................................................................................................................................49

Appendix 1: Publication List .............................................................................................49

1.1 Peer-Reviewed Journal Articles .......................................................................49

1.2 Peer-Reviewed Conference Proceedings .........................................................49

1.3 Conference Presentations .................................................................................49

Appendix 2: Copyright Permissions ..................................................................................51

iii

List of Figures

Figure 1.1 A broad overview of the 32 crystal classes, or point groups, showing a break-

down of each category leading to possible ferroelectric materials ............................................2

Figure 1.2 A typical electric polarization-electric field hysteresis plot of ferroelectricity ..............3

Figure 1.3 A graph depicting the relationship between spontaneous polarization and

temperature ................................................................................................................................4

Figure 1.4 Projection of ZnSnO3 crystal structure on (a) (110), (b) (101), and (c) (011)

planes .........................................................................................................................................7

Figure 1.5 Schematic diagram of the various applications of ferroelectric materials .....................8

Figure 2.1 Equipment used for substrate preparation and synthesis ..............................................13

Figure 2.2 Equipment used for characterization of ZnSnO3 arrayed films....................................14

Figure 2.3 A picture of the ferroelectric probe for detailed analysis of the ferroelectric

behavior of the ZnSnO3 nanostructured samples .....................................................................15

Figure 3.1 Overview of structural growth and fabricated device ..................................................21

Figure 3.2 SEM images taken at various stages of synthesis ........................................................22

Figure 3.3 Structural characterization of ZnSnO3 nanowire arrayed film .....................................23

Figure 3.4 (a) A top-down SEM image of the dense, non-porous ZnSnO3 nanowire fused

over-layer covering the substrate and hiding the nanowire arrays beneath .............................24

Figure 3.5 Results of ferroelectric behavior in the nanowire arrayed film ....................................26

Figure 4.1 (a) XRD of the as-prepared LN-type ZnSnO3 nanowire-nanoparticle arrayed |

film confirming phase purity as compared to Inaguma et al ...................................................31

Figure 4.2 (a and b) SEM images showing the densely packed, vertically aligned ZnSnO3

nanowires with attached nanoparticles ....................................................................................32

Figure 4.3 Fabricated sample alongside the corresponding hysteresis loop ..................................34

iv

Figure 4.4 Structural characterization and various hysteresis loops ..............................................35

Figure 4.5 Interaction of nanoparticle dipoles with nanowires and the resulting hysteresis .........37

Figure 4.6 Results of sample with improved nanoparticle concentration ......................................39

Figure 5.1 Structural results of various ZnSnO3 nanostructures....................................................43

Figure 5.2 Ferroelectric hysteresis loops of various ZnSnO3 nanostructures ................................45

v

Abstract

With increasing focus on the ill health and environmental effects of lead there is a greater

push to develop Pb-free devices and materials. To this extent, ecofriendly and earth abundant

LiNbO3-type ZnSnO3, a derivative of the ABO3 perovskite structure, has a high theoretically

predicted polarization making it an excellent choice as a suitable alternative to lead based

material such as PZT. In this work we present a novel synthesis procedure for the growth of

various ZnSnO3 nanostructures by combined physical/chemical processes. Various ZnSnO3

nanostructures of different dimensions were grown from a ZnO:Al template layer on a Si (100)

substrate deposited by pulsed laser deposition followed by a strategic solvothermal process. The

ferroelectric properties of each sample were explored and a remanent polarization as high as

nearly 30 µC/cm2 was found in aligned nanowire arrayed films. An in-depth understanding of the

structure-property relationship is key to the future development of this material and is the subject

of future investigations.

1

Chapter 1:

What are Ferroelectric Materials and Their Applications?

1.1 Theory of Ferroelectrics

Ferroelectricity is a property unique to materials that exhibit a reversible spontaneous

electric polarization in the presence of an applied external electric field, it is a cooperative

phenomenon that arises from the lattice of the polar material. In ABO3 perovskite oxides and

derivatives of such structures the spontaneous polarization is due a small displacement between

the cation and oxygen ion. The ability for ferroelectricity is limited to select crystal groups; of

the thirty-two point groups only twenty-one are non-centrosymmetric (NCS), or without a center

of symmetry, a trait necessary for ferroelectricity. The lack of an inversion point in NCS crystals

is essential for charge separation leading to polarization, an occurrence necessary for the

piezoelectric (PE) effect experienced by twenty of the NCS point groups. In the cubic point

group 432 symmetry arguments restrict the ability of a crystal to induce a dipole moment and so

cannot be PE.[1] Half the PE point groups feature a unique polar axis leading to spontaneous

polarization, for all materials of this nature a pyroelectric effect is also observed due to the

temperature dependence of the intrinsic polarization.[2,3]

All ferroelectric (FE) materials are a subset of the pyroelectric point groups, as shown in

Figure 1.1, but while all FEs are pyroelectrics the converse is not true. For a pyroelectric material

to be considered FE it must satisfy the Anderson and Blount conditions for ferroelectricity; there

must be a continuous phase transition from a centrosymmetric structure to one without inversion

symmetry and have a unique polar axis.[4] The change from a paraelectric centrosymmetric

2

structure to a polarized NCS one is necessary for spontaneous polarization since it is a result of

symmetry breaking due to the structural transition.[5]

Figure 1.1 A broad overview of the 32 crystal classes, or point groups, showing a breakdown of each

category leading to possible ferroelectric materials.

In all FE materials there is an accompanying non-linear behavior relating polarization and

electric field, a typical P-E hysteresis curve is shown in Figure 1.2 Electric polarization is a

measure of the average dipole moment volume density; it is an indicator of how strong and well-

aligned the dipoles are in a material. When an electric field is applied to a FE material the

polarization rapidly increases before becoming linear, it is at this point that extrapolation to the

zero electric field determines the value of spontaneous polarization (PS).[6] The electric field that

returns the polarization to zero from its maximum value is known as the coercive field (EC).

When the electric field is reduced to zero the remanent polarization (Pr) is what remains of the

spontaneous polarization.[7] The maximum value Pr can attain is that of the spontaneous

polarization (PS), it is a marker of the permanent dipole moment. Squareness of a hysteresis loop

is defined as the ratio of remanent polarization to spontaneous polarization (Pr/PS); a higher

3

squareness value is sought for FE materials as it means the remanent polarization is approaching

the theoretical maximum. Anomalous FE behavior can lead to asymmetry or constriction in P-E

hysteresis loops as a result of defect dipoles or other charge effects that cause deviation from

typical hysteresis loop behavior.

Figure 1.2 A typical electric polarization-electric field hysteresis plot of ferroelectricity. The spontaneous

polarization (PS) is determined by the line extrapolated at high electric field to the zero field. The electric

field that brings polarization to zero is known as the coercive field (EC). Remanent polarization (Pr) is the

remaining polarization present in the material in the absence of an applied electric field.

The pyroelectric effect is seen in FE materials in the form of temperature dependence of

spontaneous polarization. An increase in temperature is accompanied by a reduction in

spontaneous polarization due to the rising thermal energy that disorients the dipoles. The height

and width of the hysteresis curve shrink until the temperature surpasses the transition

temperature, or FE Curie temperature (TC), at which a phase transition occurs and the material

becomes paraelectric, meaning no spontaneous polarization; for this reason, spontaneous

polarization is greatest at temperature well below TC, as shown in Figure 1.3 below.[8] Above

4

the phase transition temperature there are no symmetry breaking distortions so the material can

only be centrosymmetric, meaning charge separation cannot occur.

Figure 1.3 A graph depicting the relationship between spontaneous polarization and temperature. Above

Tc the material becomes paraelectric and has no spontaneous polarization.

Since polarization is a vector, there is a direction associated with its magnitude, groups of

similarly oriented dipoles are known as a domain.[8] Domain walls are the boundaries between

different dipoles, groups of dipoles aligned in varying directions; much like a fence separates

yards.[6] The application of an external electric field will change the direction of the polarization

within a domain; the dipoles will want to align with the applied electric field, leading to a

process known as domain switching.[7] In some cases the presence of an internal bias field due

to defect dipoles can affect the field necessary for domain switching resulting in anomalous

behavior of the hysteresis loop. The alignment of dipoles at changing electric field will vary

spontaneous polarization, meaning the hysteresis curve is a measure of the work required to

move domain walls and the energy barrier that separates the different polarization directions.[9]

5

1.2 Pb-Free Nanostructured Ferroelectrics

Nanotechnology is already in use for next-generation computing, communications, and

other electronic applications to provide faster, smaller, energy-efficient systems that can manage,

and store information (memory).[10-15] In addition to enhancing the performance of functional

nanoscale-devices, a major concern for scientists is also to develop preparative processes and

materials towards cleaner environment and affordable technology. [16-21] The family of lead-

based perovskites such as Pb(Zr,Ti)O3 (PZT) has been extensively pursued as viable ferro- and

piezo-materials owing to its’ high-polarization (coercive field of about 1 kV/mm and a Pr of

about 35 µC/cm2 in bulk). [10-13],[22,23] Despite the efficacy of PZT, the widespread concern

on using toxic Pb in memory devices have resulted in efforts to find high-efficiency

environment-friendly materials.[16-21]

In this direction, an increased interest has developed around the polar NCS oxide

materials, which possess multi-functionality with high dielectric and PE coefficients, switchable

FE polarization, non-linear optical and electrical properties, and high mechanical stability.

[24,25] These properties identify them as next-generation ‘smart materials’, which are of high

academic and commercial interest. NCS oxides containing main-group cations with the

electronic configuration of (n-1)d10ns0, such as ZnO and have been widely studied to possess

enhanced symmetry dependent properties.[11,16,17] LiNbO3 (LN)-type compounds, which can

be described as a derivative of the perovskite-type structure (ABO3) as shown in Figure 1.4a,[24]

because both LN-type and perovskite-type compounds possess three-dimensionally corner-

sharing BO6 octahedra. The cooperative cation shift along the c-axis direction against close-

packed anions, results in spontaneous polarization in these types of oxide compounds. In such

compounds, ferroelectricity may be the result of B-site distortions, or more rarely, A-site.

6

1.2.1 LN-Type ZnSnO3: The growing concern of the health and environmental impact of

lead (Pb) in electronics and FE materials [26,27] has spurred much research in developing Pb-

free FE materials.[28-30] To this end, perovskite oxides of ABO3 phase offer a plethora of

possibilities due to the versatility of the structure in which the A and B sites can accommodate

nearly any element resulting in an impressive range of properties that can be adjusted based on

suitable A and B element choices.[31] Composed of earth abundant elements LN-type ZnSnO3 of

space group R3c (No. 161) has been reported to have a theoretical maximum polarization of ≈ 59

μC/cm2, [32] while experimental reports of epitaxial (111) ZnSnO3 thin films feature a high

remanent polarization of ~ 47 μC/cm2 at a coercive field of ~ 130 kV/cm and a maximum

polarization of 58μC/cm2.[33] Lattice parameters have also been theoretically calculated for LN-

type ZnSnO3; a = 5. 3441 Å and c = 14.2206 Å.[34]

In this work, ZnSnO3 adopts the LiNbO3 hexagonal structure with rhombohedral

symmetry, as seen in Figure 1.4. Containing no ferroelectrically-active cations (d0) or lone-pair

cations, and a tolerance factor less than 1 it is surprising that zinc stannate exhibits

ferroelectricity at all but unlike most perovskite oxides the reason for its intrinsic polarization is

tied to the displacement of the A site cation.[24] In the hexagonal structure the layers follow a

Zn-Sn-Vacancy-Zn-Sn-Vacancy-… pattern, as can be seen in Figure 1.4b-c, and the Zn ion is

distorted along the c-axis toward the vacant site possibly due to an electrostatic field within the

lattice tied to the strong ionic character of the cation.[35] This distortion leads to different two

different Zn-O bond lengths so Zn bonds differently to three O compared to the other three O

and this variation allows for spontaneous polarization in the c-direction.[32] The smaller radius

of the Zn cation (1.31 Å) compared to the Sn atom (1.41 Å) allows for more displacement of the

Zn in the vacancies.[33]

7

Figure 1.4 Projection of ZnSnO3 crystal structure on (a) (110), (b) (101), and (c) (011) planes. Reprinted

from [34] with permission from Elsevier.

Inaguma et al. reported that ZnSnO3 with a hexagonal LiNbO3(LN)-type structures

synthesized under high pressure, possess high-polarization (exhibiting long range alignment of

electric dipoles).[32,36] Reports on functional LN-type oxides relative to perovskite-type oxides

have been limited due to the difficulty in synthesizing these metastable phases by traditional

solid state synthesis processes under ambient conditions. In case of ZnSnO3, this may be due to

the ease of disproportionation of Zn2+ to Zn4+ and Zn metal to form hexagonal symmetry lattice.

An enhanced covalence qualifies the electronic structure as it can be inferred from analyzing the

chemical bonding. This lead us to search for a novel, and less conventional, non-equilibrium

bottom-up synthesis scheme such as combination of pulsed laser deposition (PLD) and

solvothermal in synthesizing these phases. As an advanced tool for growing high-quality

complex oxide thin films, PLD has made a significant impact in terms of growth of single- and

multi-layered heterostructures of multi-component materials.[32,36,37] When combined with

8

inexpensive, scalable chemical synthesis techniques, PLD can qualify to be an easy and rapid

way to design nanostructures of many important FE materials of high quality and high efficiency

and is the main focus of this thesis. We are the first to synthesize LN-type ZnSnO3 nanowire

arrays on silicon (Si) substrates using a novel physical-chemical approach encompassing pulsed

laser deposition (PLD) deposited ZnO:Al seed-layer and conventional hydrothermal processes

(to be discussed in later chapters).[37,38]

1.3 Applications of Ferroelectric Materials

Figure 1.5: Schematic diagram of the various applications of ferroelectric materials. Reproduced from

[15] with permission of The Royal Society of Chemistry.

The polarization of FE materials can be changed by using three forms of energy, i.e.

electric, thermal and mechanical or vice versa. These three inter-dependent properties of FEs

have been widely exploited for many functional applications.[15,39] The various applications of

FEs are schematically shown in Figure 1.5. The reversible polarization of a FE material can be

9

used in nonvolatile memory applications, as the direction of polarization switching represents the

binary ‘1’ and ‘0’ in data storage. [15,39] The FE thin films have already shown potential as

FeRAMs in memory storage applications. [15,39] The piezo effect in FEs can be used to convert

mechanical energy to electrical energy, or vice versa, and find wide-spread use as actuators,

transducers and micro-electromechanical (MEMS) devices. [15,39] As all FE materials are

pyroelectric, they can also generate an electrical current in response to any change in ambient

temperature; this principle, finds use in infrared thermal imaging.[15]

Significant efforts have been made recently to achieve high efficiency FE memory

devices, through the assistance of nanostructuring.[15,39,40] Different types of nanodevice

architectures with improved features have been realized, based on the reversible polarization of

the FEs. By scaling down the size of individual memory cells, the storage efficiency of FeRAMs

can be greatly improved. PE nanostructures, especially nanowires, have been studied extensively

for their potential energy harvesting capabilities from collective mechanical movements.

[15,39,40] Electrical energy collected from PE nanostructures has already been used effectively

to power nanoelectronics devices and sensors. One dimensional nanostructures, especially

nanowires and rods, have mostly been used for piezotronics applications due to their large

mechanical strain tolerance, especially the vertically or horizontally aligned arrays of PE

nanowires.

1.4 References

[1] M.M. Julian, Foundations of crystallography with computer applications, CRC Press,

Boca Raton, 2008, p. 135.

[2] W.-D. Cheng, C.-S. Lin, L. Geng, Z.-Z. Luo, W.-L. Zhang, H. Zhang, Syntheses and

Properties of Some Bi-Containing Compounds with Noncentrosymmetric Structure in H.

Li, Z.M. Wang (Eds.), Bismuth-containing compounds. Springer, New York, 2013, p.

323

[3] B. Jiménez, J.A. Gonzalo, Ferroelectricity, Wiley-VCH Verlag GmbH, 2007, p. 1.

[4] P.W. Anderson, E.I. Blount, Phys Rev Lett 14 (1965) 217.

10

[5] V. Keppens, Nat Mater 12 (2013) 952.

[6] S.L. Kakani, A. Kakani, Material science, New Age International (P) Ltd., Publishers,

New Delhi, 2004.

[7] G. Ravichandran, in: W.N. Sharpe, Jr. (Ed.), Springer Handbook of Experimental Solid

Mechanics, Springer US, 2008, p. 159.

[8] P. Maheshwari, Electronic components and processes. [electronic resource], New Delhi :

New Age International, c2006., 2006.

[9] S.C. Abrahams, K. Nassau, in: R.J. Brook (Ed.), Concise Encyclopedia of Advanced

Ceramic Materials, Pergamon, Oxford, 1991, p. 152.

[10] J.F. Scott, Ferroelectric memories, Springer, Berlin ; New York, 2000.

[11] A.J. Moulson, J.M. Herbert, Electroceramics: Materials, Properties, Applications, Wiley,

2003.

[12] S. Xu, B.J. Hansen, Z.L. Wang, Nat Commun 1 (2010) 93.

[13] K. Lubitz, C. Schuh, T. Steinkopff, A. Wolf, in: N. Setter (Ed.), Piezoelectric Materials

and Devices, Lausanne, 2002.

[14] H. Han, Y. Kim, M. Alexe, D. Hesse, W. Lee, Advanced Materials 23 (2011) 4599.

[15] J. Varghese, R.W. Whatmore, J.D. Holmes, J Mater Chem C 1 (2013) 2618.

[16] P. Hiralal, H.E. Unalan, G.A.J. Amaratunga, Nanotechnology 23 (2012).

[17] Z.L. Wang, J.H. Song, Science 312 (2006) 242.

[18] A. Navrotsky, Chemistry of Materials 10 (1998) 2787.

[19] J.M. Wu, C. Xu, Y. Zhang, Y. Yang, Y.S. Zhou, Z.L. Wang, Advanced Materials 24

(2012) 6094.

[20] J.M. Wu, C.Y. Chen, Y. Zhang, K.H. Chen, Y. Yang, Y.F. Hu, J.H. He, Z.L. Wang, Acs

Nano 6 (2012) 4369.

[21] J.M. Wu, C. Xu, Y. Zhang, Z.L. Wang, Acs Nano 6 (2012) 4335.

[22] N. Izyumskaya, Y. Alivov, S.J. Cho, H. Morkoc, H. Lee, Y.S. Kang, Crit Rev Solid State

32 (2007) 111.

[23] J. Kim, S.A. Yang, Y.C. Choi, J.K. Han, K.O. Jeong, Y.J. Yun, D.J. Kim, S.M. Yang, D.

Yoon, H. Cheong, K.-S. Chang, T.W. Noh, S.D. Bu, Nano Lett 8 (2008) 1813.

[24] N.A. Benedek, C.J. Fennie, J Phys Chem C 117 (2013) 13339.

[25] P.S. Halasyamani, K.R. Poeppelmeier, Chemistry of Materials 10 (1998) 2753.

[26] H. Black, Environmental Health Perspectives 113 (2005) A682.

[27] J. Pareja-Carrera, R. Mateo, J. Rodríguez-Estival, Ecotoxicology and Environmental

Safety 108 (2014) 210.

[28] I.P. Raevski, S.A. Prosandeev, Journal of Physics and Chemistry of Solids 63 (2002)

1939.

[29] N.K. Noel, S.D. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A.-A. Haghighirad, A.

Sadhanala, G.E. Eperon, S.K. Pathak, M.B. Johnston, A. Petrozza, L.M. Herz, H.J.

Snaith, Energy & Environmental Science 7 (2014) 3061.

[30] J. Rödel, K.G. Webber, R. Dittmer, W. Jo, M. Kimura, D. Damjanovic, Journal of the

European Ceramic Society 35 (2015) 1659.

[31] D.G. Schlom, L.-Q. Chen, X. Pan, A. Schmehl, M.A. Zurbuchen, Journal of the

American Ceramic Society 91 (2008) 2429.

[32] Y. Inaguma, M. Yoshida, T. Katsumata, J Am Chem Soc 130 (2008) 6704.

[33] J.Y. Son, G. Lee, M.H. Jo, H. Kim, H.M. Jang, Y.H. Shin, J Am Chem Soc 131 (2009)

8386.

11

[34] H. Wang, H. Huang, B. Wang, Solid State Communications 149 (2009) 1849.

[35] M. Nakayama, M. Nogami, M. Yoshida, T. Katsumata, Y. Inaguma, Advanced Materials

22 (2010) 2579.

[36] Y. Inaguma, D. Sakurai, A. Aimi, M. Yoshida, T. Katsumata, D. Mori, J. Yeon, P.S.

Halasyamani, Journal of Solid State Chemistry 195 (2012) 115.

[37] A. Datta, D. Mukherjee, C. Kons, S. Witanachchi, P. Mukherjee, Small 10 (2014) 4093.

[38] D. Mukherjee, A. Datta, C. Kons, M. Hordagoda, S. Witanachchi, P. Mukherjee, Applied

Physics Letters 105 (2014).

[39] J. Rodel, W. Jo, K.T.P. Seifert, E.M. Anton, T. Granzow, D. Damjanovic, Journal of the

American Ceramic Society 92 (2009) 1153.

[40] S. Hong, Nanoscale phenomena in ferroelectric thin films, Kluwer Academic Publishers,

Boston, 2004.

12

Chapter 2:

Experimental Methods and Characterization Techniques of LN-type ZnSnO3 Nanostructures

2.1 Synthesis Equipment and Facilities

2.1.1 Step I: Pulsed Laser Deposition (PLD) System for Thin Film Growth: The state-of-

the-art PLD systems located in the Laboratory for Advanced Materials Science and Technology

(LAMSAT), Interdisciplinary Sciences (ISA) building, Department of Physics, USF, was used

for the deposition of seed-layers on hard conducting substrates. The various custom-built PLD

systems available consist of vacuum deposition chambers with pulsed lasers such as 248 nm KrF

excimer, 1064 nm (532 nm) Nd:YAG, and 10.6 µm CO2 lasers. The vacuum systems are capable

of sub-microtorr background pressures which can be back filled with inert or reactive gasses, as

required. A multi-target changer is available allowing in-situ deposition of multi-layers with

clean interfaces. An intensified charge-coupled detector (ICCD) imaging system aligned

perpendicular to the plume propagation enables two-dimensional imaging and spectroscopic

analysis of the laser-induced plume, allowing in-situ monitoring of the process. An illustration of

the PLD setup can be seen in Figure 2.1a.

2.1.2 Step II: Chemical Synthesis of Nanostructures: The established chemical synthesis

facilities used in this work include: fumehood for handling chemicals, Fisherprice Magnetic

heater and stirrers for preparing homogenized precursor solutions, SRC Ultrasonic cleaners for

cleaning substrates, Yamato gravity convection oven with controlled PID controller operational

up-to 300 ˚C for solvothermal reaction of the precursors in presence of the seed-layers,

Panasonic Commercial microwave oven for easy and rapid reaction of the seed-layers thin-films,

13

digestion reactors for solvothermal synthesis both for convection oven (parr instruments),

Champion variable speed centrifuge for cleaning reactants, vacuum drying oven for drying

samples (custom-made). Pictures of the autoclave and reaction chamber can be seen in Figure

2.1b.

Figure 2.1 (a) A schematic of the PLD setup used for deposition of ZnO:Al on Si (100) substrate. (b)

Equipment used for solvothermal synthesis.

2.2 Characterization Facilities and Equipment

2.2.1 Physics Materials Diagnostics Facility: User Facilities available at the Department

of Physics, University of South Florida, were extensively used to obtain results discussed in this

thesis. The Physics Materials Diagnostics Facility equipment used include an X-ray

diffractometer, Scanning Electron Microscope, and Energy Dispersive X-ray Spectroscopy. X-

Ray Diffractometer (Figure 2.2a) for characterization including crystal orientation, phase

identification, quantitative phase analysis, crystallite size, and lattice dimensions. The X-ray

Diffractometer (XRD) is a Bruker-AXS D8 Focus powder diffractometer with a Cu anode (Kα1

λ=1.5406Å), Bragg-Brentano geometry configuration, fixed divergence slits (0.1 to 6mm), high-

speed detector, and intuitive operation. The XRD is used for non-destructive characterization of

(a) (b)

14

materials in the form of powders, thin-films, single crystals, and bulk. The Scanning Electron

Microscope (SEM), shown in Figure 2.2b, is a JEOL JSM-6390LV with tungsten filament, low-

vacuum mode option, and intuitive operation. The SEM scans the sample with a focused beam of

electrons to characterize the surface morphology of the samples at a magnification range of 5x to

300,000x, producing high-resolution photo-quality images. Intricate morphological details are

obtained through this technique. The Energy Dispersive X-ray Spectroscopy (EDS) system is an

Oxford Instruments; INCAx-sight 7582M, liquid-nitrogen cooled EDS. The EDS system is

integrated with the JEOL JSM-6390LV SEM. The EDS (shown in Figure 2.2b) is an analytical

technique used for qualitative or quantitative elemental analysis or chemical characterization of

materials capable of detecting concentrations as low as 1% with ±0.1% accuracy. The high-

energy beam of electrons, provided by the SEM, stimulates the emission of characteristic X-rays

that allows the composition of the sample to be determined.

Figure 2.2 (a) The X-ray diffractometer used for structural characterization alongside (b) the Scanning

electron Microscope and Energy-dispersive X-ray spectroscope used for structural characterization as

well. (c) The Raman equipment used for optical studies.

2.2.2 Facility for Optical Characterization of Materials: The Raman spectroscopy system

(shown in Figure 2.2c) is a Horiba Jobin Yvon T64000 Advanced Research Raman System with

a Synapse charge-coupled device (CCD) detection system, and a confocal LabRAM Raman

microprobe. It has an integrated triple spectrometer design for unprecedented optical stability.

(c) (b) (a)

15

This spectroscopic system is designed to provide a versatile platform for Raman spectroscopy

analysis to observe vibrational, rotational, and other low-frequency modes providing a

fingerprint by which materials can be identified.

2.2.3 Other Resources and User Facilities Used In This Work at University of South

Florida: Low resolution Transmission Electron Microscope (FEI Morgagni TEM) for

microstructural imaging of samples. A cross-sectional TEM sample was prepared by milling a

100 nm thick 5 μm x 10 μm rectangular strip from the film surface using a focused ion beam

(FIB) (JOEL 4500 FIB/SEM) and Pt-welding it to a Cu TEM grid. High resolution Transmission

Electron Microscope (TEM-HRTEM, FEI Tecnai F20 S-Twin TEM) for imaging of atomic

structure and cross-sectional sample preparation through Focused ion-beam milling (JOEL 4500

IB/SEM). Atomic Force Microscope (AFM, Digital Instruments DI-III) for surface

characterization of samples.

Figure 2.3 A picture of the ferroelectric probe for detailed analysis of the ferroelectric behavior of the

ZnSnO3 nanostructured samples.

The Ferroelectric (FE) Tester with Microprobe Station, seen in Figure 2.3: This

instrument available at Laboratory for Advanced Materials Science and Technology (LAMSAT),

Interdisciplinary Sciences (ISA) building, Department of Physics, USF, was used for measuring

16

the FE properties of nanostructured thin-films. The set-up consists of a Precision LC Materials

Analyzer (FE tester), an optical microscope with video monitor, a probe station, and control PC,

the Zeiss microscope is equipped with a video camera for monitoring microprobe position, probe

station includes sample stage holder, x-y-z micro-manipulators and pole pieces, the micro-

manipulators on the probe station are KRN-01A 'DC' positioners from J-Micro Technology. In

addition, this system is also capable of I-V, C-V, leakage current, and resistivity measurements.

2.3 General Synthesis and Characterization Overview

2.3.1 Deposition of Al-doped ZnO Template-Layer by PLD: A carefully crafted bottom-

up synthesis procedure including a combination of physical/chemical techniques was developed

for the selective growth of all LN-type ZnSnO3 nanostructure arrays on ZnO:Al/Si. The substrate

was prepared using pulsed laser deposition (PLD) to deposit a layer of ZnO doped with 2 at. %

Al on Si (100). A Zn0.98Al0.02 target was ablated with a KrF excimer laser (Lambda Physik;

wavelength, λ = 248 nm) of laser fluence (energy density) of 2 J/cm2 at a laser repetition rate of

10 Hz in a background atmosphere of 100 mTorr oxygen. The substrate was kept 4 cm from the

target in an atmosphere at 400 ˚C. A portion of the finished substrate was wrapped securely in

Teflon tape for preservation to later serve as the bottom electrode in the assembled FE device. A

van der Pauw configuration at 300 K was used to measure resistivity (ρ) of the ZnO:Al layer (ρ =

1.2 x 10-4 Ωcm), which was much less than that of an undoped ZnO thin-film (ρ = 2.1 x 10-2

Ωcm) grown under the same conditions by PLD.

2.3.2 Solvothermal Synthesis of LN-type ZnSnO3 Nanostructures: The prepared

ZnO:Al/Si substrates were partially wrapped with teflon tape to preserve a portion of the

conductive ZnO:Al layer that would act as a bottom electrode during for FE measurements. The

substrate was reacted by a template-free, low-temperature solvothermal process in the basic pH

precursor solution composed of 0.01 mol urea (CH4N2O, 98 %), 0.38 mmol sodium stannate

17

trihydrate (H6Na2O6Sn, 95 % ), and 0.38 mmol zinc nitrate (Zn(NO3)2·6H2O, 98 %) in 36 mL of

a water/anhydrous ethanol or water/Polyethylene-glycol (PEG) or water/ethylene-glycol (EG)

solution at a 60/40 ratio. The precursor solution was vigorously stirred and additional heat added

as needed to ensure uniform dissolution of the chemical constituents while the pH of the solution

was adjusted to 10 by the addition of urea as necessary. The mixed precursor solution and

prepared PLD deposited ZnO:Al/Si substrates were placed inside the teflon chamber and

transferred to a teflon-lined solvothermal autoclave. The solvothermal reactions were performed

at 150- 180 °C for 6 - 12 hours. Specific reaction conditions and any additional reaction time will

be discussed in the individual nanostructure synthesis sections. After allowing sufficient time to

cool to room temperature the thick, white deposition on the substrate, indicating the growth of

ZnSnO3, was cleaned repeatedly with absolute ethanol and distilled water and allowed to air dry.

The combined physical/chemical synthesis strategy allowed for the controlled synthesis

of various nanostructures by assistance of the deposited ZnO layer. Due to similar crystal

symmetry and lattice parameters the ZnO acted as a seed layer for the initiation of ZnSnO3

allowing for the dense formation of various one-dimensional nanostructures. The doped ZnO

layer was also chosen due to its good conductive properties (ρ=1.2 x 10-4Ωcm) compared to

undoped Zno (ρ=2.1 x 10-2Ωcm). Through this strategic synthesis process we were able to

control the morphology, crystalline, and packing density of the grown nanostructures that aided

in FE measurements and electrode placement using a simple sandwich configuration. After

synthesis the samples were structurally characterized by various means.

2.3.3 Fabrication of ZnSnO3 Nanostructure arrayed FE Devices for Polarization

Measurements: A FE device was fabricated from the ZnSnO3 nanostructure arrayed film by

using the portion of the conducting ZnO:Al template-layer preserved by Teflon tape during

18

synthesis as the bottom electrode while top Pt electrodes were sputtered onto the nanostructure

arrayed film (CRC-100 Sputtering system, Plasma Sciences Inc.) by means of a shadow mask

that yielded 100 μm diameter dots for a thickness of 100 nm. The device was then annealed at

400 ˚C for 20 min in air after top electrode placement. The FE measurements were performed

using Precision LC Materials Analyser (Radiant Technologies, Inc.) equipped with a micro-

probe station. A constant standard bipolar input profile at 1 kHz with applied voltages between

100 V to 300 V was used during hysteresis measurements. The leakage current density versus

voltage was measured at 1 kHz using a 20 V DC bias voltage and a maximum applied voltage of

240 V.

2.4 Specific Experimental Parameters

2.4.1 Structural Growth of ZnSnO3 Nanowire Arrays by Physical/Chemical Methods:

Tunability of reaction conditions such as temperature and synthesis time duration, and thickness

of the ZnO:Al seed layers results in variation in nanostructures and their corresponding

dimensions. The ZnSnO3 nanowire arrays to be discussed in Chapter 3 were synthesized in

stages from 4-12 hours at a reaction temperature of 180 °C following the synthesis procedures

outlined above. A profilometer determined the thickness of the doped ZnO template layer as 400

± 10 nm. No post-annealing was required on this sample.

2.4.2 Structural Growth of Hybrid ZnSnO3 Nanowire-Nanoparticle Arrays: The LN-type

ZnSnO3 nanoparticle-nanowire hybrid structures were synthesized in a nearly idential process as

the nanowires but with minor adjustments. The same physical/chemical synthesis process was

employed but the ZnO:Al seed layer deposited by PLD on Si (100) was increased to a thickness

of ≈ 600 ± 10 nm, and reaction temperature was decreased to 150 ˚C for 12 hours and increased

to 180 ˚C for another 4 hours. The first 12 hour reaction time frame allowed formation of the

nanowires while the subsequent 4 hours initiated the growth of the nanoparticles on the nanowire

19

surfaces. The sample require post-annealing at 12 hours at 600 ºC and FE measurement were

retaken on the post-annealed sample. To increase nanoparticle density a new sample was

synthesized using the aforementioned synthesis procedure and increasing the ramping

temperature to 200 ºC for 6 hours. The results are discussed in Chapter 4.

2.4.3 Structural Growth of Other ZnSnO3 Nanostructures: A similar bottom-up synthesis

approach was utilized for the synthesis of various ZnSnO3 nanostructures described in Chapter 5

(nanorods, other nanowires, and nanoflakes) as described previously. The ZnO:Al seed layer was

deposited atop the Si (100) substrate at thicknesses ranging from 400 - 600 nm by PLD and the

finished substrate was reacted in the aforementioned precursor solution for 12 hours at 180 °C

while also wrapping a portion of the ZnO:Al/Si layer to preserve for later use as a bottom

electrode in the assembled FE device. After reaction the thick, white ZnSnO3 deposition top

layer was carefully rinsed with distilled water and ethanol followed by structural characterization

to confirm the LN-type ZnSnO3 phase. The FE device was fashioned in the same manner

previously described; 100 µm Pt-top electrodes were sputtered onto the top of the sample while

the preserved ZnO:Al layer served as the bottom electrode. The nanorod and nanoflake arrays

were post-annealed at 200 °C for 6 hours to improve inter-structural density and reduce porosity.

20

Chapter 3:

Ferroelectricity in LN-type ZnSnO3 Nanowire Arrays1

In this work, ferroelectric (FE) LN-type ZnSnO3 nanowire arrays were successfully

fabricated by implementing a bottom-up synthesis approach through a combination of physical

and chemical processes. The ZnSnO3 nanostructures were grown via a solvothermal process

atop a ZnO:Al conducting layer deposited by pulse-laser deposition on a Si substrate. Previous

reports of LN-type ZnSnO3 nanowires have not been synthesized by a template-free, low-

temperature hydrothermal process on economic Si substrates, a significant achievement to this

study. The doped ZnO layer was crucial to the growth of the ZnSnO3 nanowires since the similar

crystal symmetry between the two materials acted as a template of sorts during the growth stage

leading to a dense array of well aligned and self-supported nanowires with a packing density

greater than 0.8. Ferroelectricity was also observed in these nanowire arrays displaying a

remanent polarization (Pr) value of nearly 30 µC/cm2. Various structural analyses showed an

intriguing pattern of fusing along the length of the wires and at the tip, the formation of this

impenetrable canopy layer and subsequent decrease in porosity aided in the direct measurement

of the FE polarization of these structures.

A pictorial representation of the process by which the nanowires grow and self-assemble

on the ZnO:Al template layer of Si substrate is shown in Figure 3.1. The importance of the ZnO

layer goes beyond acting as the seed and template layer for the growth of the ZnSnO3 nanowires

it also serves as the bottom electrode for the FE device made from the synthesized nanowire

1 Reproduced from [15] with permission from John Wiley and Sons.

21

arrays on the Si substrate. The Al-doped ZnO was selected over undoped ZnO since it

demonstrates a lower resistivity (ρ=1.2 x 10-4Ωcm compared to ρ=2.1 x 10-2Ωcm) while

maintaining similar crystallinity.[1,2]

The first step in the synthesis process is preparation of the Si substrate by depositing a

layer of ZnO:Al. The well-ordered, c-axis oriented ZnO layer is composed of ~100 nm sized

hexagonally structured crystals deposited onto Si by pulse-laser deposition with an average

thickness of 400 nm. After 2 hours of the solvothermal reaction in the precursor solution

nucleation of the LN-type ZnSnO3 grains ~50-80 nm diameter in size begin from the ZnO:Al

template layer (Figure 3.2a). Growth of ZnSnO3 nanowires initiates after 4 hours of reaction time

in the precursor solution guided by the zinc stannate grains acting as a seed layer, as shown in

Figure 3.2b. Cross sectional SEM image (Figure 3.2c) shows that after the solvothermal

synthesis has carried on for 6 hours the nanowires have achieved vertical growth with an average

diameter of ~25 nm and lengths ranging from 10-12 µm. Further SEM analysis of Figure 3.2d

taken from a top-down perspective show a well dispersed, uniform growth of the LN-type

ZnSnO3 nanowires on the substrate; the nanowires appear to group together and form clusters,

(c)

(a)

(b)

Figure 3.1 A generalized schematic of the synthesis and growth process (a-c) of the self-supported LN-

type ZnSnO3 nanowire arrays on the PLD deposited ZnO:Al coated Si substrate and the fabricated device

for ferroelectric measurements. (a) An illustration of the deposition of the ZnO:Al layer onto the Si

substrate by PLD. After 2 hours of reaction time the initial stages of ZnSnO3 nanowire growth, shown in

(b), begin with nucleation in the ZnO crystals that later guide the growth of the upright ZnSnO3 nanowires

(c). The ferroelectric device is fabricated by using a preserved portion of the ZnO:Al layer as the bottom

electrode and sputtering Pt electrodes atop the ZnSnO3 nanowire over-layer, as shown in (d). The inset to

(d) shows an SEM image of the deposited top electrodes on the ZnSnO3 nanowire over-layer.

(d)

22

possibly to reduce high surface stress due to the large aspect ratio, while the ends fuse together

creating an even layer across the top of the nanowires. This grouping effect leads to holes in the

top layer, making difficult the task of FE measurements without use of dielectric fillers in the

porous openings to prevent shortage of electrodes in the device.

The XRD spectrum of the sample taken after 6 hours of reaction time compared to the

standard pattern of hexagonal LN-type ZnSnO3 taken from Inaguma et al. and calculated in

PowderCell, all shown in Figure 3.3a, reveals highly oriented growth with a tendency for (110)

plane for the ZnSnO3 nanowire arrays.[3] The evolution of the growth of ZnSnO3 nanowires is

(d) 12h

(b) 4h (a) 2h

(c) 6h

Figure 3.2 SEM images taken at various stages of synthesis. (a) After 2 hours, nucleation of ZnSnO3

begins. (b) ZnSnO3 nanowires begin to grow from the nucleated sites near the 4 hour mark. (c) Six

hours into the reaction the nanowires begin to bundle and fuse. (d) Increasing reaction time to 12

hours results in self-supported nanowires beneath a compact over-layer.

23

evident in the XRD pattern II of the sample after the reaction has carried on for 12 hours and still

shows a preference for (110) plane. No secondary phases were observed in the XRD patterns of

the various stages of growth, confirming the phase purity of the sample as LN-type ZnSnO3. The

* is an artifact of the PSD detector and, hence, is present for all spectra. The overlap of the ZnO

(0002) and ZnSnO3 (110) XRD peaks indicate heteroepitaxial growth in the sample while energy

dispersive spectroscopy (EDS) over various regions of the substrate confirm the presence of Zn

and Sn by average atomic percent in stoichiometric ratio of 1:1.04 as seen Figure 3.3b.

Increasing reaction time to 12 hours produces a sample with a highly dense, uniform

ZnSnO3 nanowires layer with an average thickness of ~20 µm featuring an impenetrable over-

layer created by the fusing of the nanowire tips, as shown in Figure 3.2d. XRD spectrum of the

sample shown in 3.3a verifies phase purity of the sample by revealing the formation of single-

(a)

I. ZnSnO36 h reaction time

keV

Co

un

ts

Zn:Sn (at. %) 6.45:6.47

(b)

Figure 3.3 Structural characterization of ZnSnO3 nanowire arrayed film. (a) The evolution of the

growth of LN-type ZnSnO3nanowires from the c-axis oriented ZnO:Al seed layer (Labeled Template-

layer ZnO:Al/Si) can be seen in the XRD spectra at the 6 hour mark (Labeled I) and after 12 hours of

reaction time (Labeled II.). Comparison to the XRD pattern taken from Inaguma et al. and calculated

by PowderCell is shown at the bottom. The * is an artifact of the PSD detector and the peak detected

from the Si substrate has also been noted. (b) EDS of the sample taken after 6 hours of reaction time

confirming the stoichiometric atomic % of Zn and Sn in different areas of the sample.

II. ZnSnO312 h reaction time

time

24

phase LN-type ZnSnO3 along with peaks from the Si substrate.[3] Due to the compact and

uniformly dispersed over-layer, cursory SEM imaging revealed only a nanoparticulate surface

layer that effectively hid the nanowire arrays underneath (see Figure 3.4a). Further SEM imaging

show cracks that developed along the edges of the sample revealing the self-supported nanowire

arrays underneath and the fused tips that create the over-layer (see Figure 3.4b). SEM also

corroborates a minimum packing density of 0.8 while TEM images provide greater insight into

the fusing of the nanowire arrays.

(a)

(b)

(c)

(d)

Figure 3.4 (a) A top-down SEM image of the dense, non-porous ZnSnO3 nanowire fused over-layer

covering the substrate and hiding the nanowire arrays beneath. (b) A cross-sectional SEM image of a

chipped portion shows all layers of the grown sample including the densely packed nanowires and

uniform thickness of the layers (~20 µm). (c) TEM image of broken bundles of fused nanowire arrays

and a more detailed inset picture showing the welding along the axial lengths of individual nanowires.

(d) Clear indication that structural fusing is the result of nanowires crossing paths as seen by the

partial fusing.

25

The overlap of adjacent nanowires goes beyond being a simple contact point, the

individual nanowires are structurally fused as can be clearly seen in 3.4d, however there is no

exact orientation relationship and the selection process for determining which nanowires will

fuse is random or opportunistic at best; only if two nanowires pass or cross along their axial sides

can welding occur. Figure 3.4c displays a low resolution TEM image of broken clusters of the

fused nanowire collected by drop-casting the scrapped off nanowires from the prepared sample

while the inset shows more clearly that the nanowires bind along their axial lengths. There are

varying degrees to which adjacent nanowires will weld; nearly the entirety of the lengths may

join (see Figure 3.4c inset) or partial binding may occur at the ends of the wires. Results from

SEM and TEM show the welding of the nanowire ends creates an impenetrable canopy layer on

the order of 1-2 µm thick while profilometer and cross-sectional SEM image calculations

approximate the thickness of the entirety of the nanowire film as between 20-25 µm.

As described previously the top-down and bottom-up synthesis approaches for

nanostructured materials have been the subject of much research and well explored, however,

fabrication of FE devices using these synthesized structure has not been easy. [4,5] To this end,

joining and integration of nanostructure arrays into devices is anticipated to be an efficient way

of optimizing device performance.[6] The fused top layer of the dense, self-supported ZnSnO3

nanowire arrays is advantageous for device fabrication due to reduced porosity, thereby

increasing electrode contact area and preventing failure of the device due to “shorting”

Overcoming this caveat poises a major challenge to measuring polarization of nanostructured

thin-film capacitors based on the Sawyer-Tower circuit.[7] The reduced porosity also eliminates

the need for a dielectric or polymeric fillings that introduce significant dielectric losses and

reduce polarization in the overall device, it can also prevent shortage due settling of the top

26

electrodes within the pores.[8] The prepared FE device was designed by depositing Pt electrodes

100 µm in diameter atop the ZnSnO3 nanowire thin-film while a preserved portion of the

conductive ZnO:Al layer served as the bottom electrode; a schematic of the finished device is

shown in 3.1d. The inset of 3.1d shows why the dense, canopy layer is so beneficial to the

measurement process; the top-down SEM image features the Pt electrodes deposited on top the

ZnSnO3 nanowire arrays without any settling.

Figure 3.5 (a) A ferroelectric hysteresis loop from the Pt/ZnSnO3 nanowire layer/ZnO:Al capacitor at

room temperature with an applied voltage of 220 V at 1 kHz; this yielded a Pr ≈30 µC/cm2 and a Ec ≈ 25

kV/cm. (b) A leakage current density (J) versus voltage (V) plot, the J-V graph is asymmetric with a high

leakage current. (c) A multitude of hysteresis plots from the Pt/ZnSnO3 nanowire layer/ZnO:Al capacitor

at applied voltages ranging from 130 – 220 V.

The culmination of the results of the fabrication of the FE device are present in Figure

3.5; shown is a polarization-electric field hysteresis loop on the Pt/ZnSnO3 layer/ZnO:Al sample

at an applied voltage of 220 V, a plot of leakage current density versus voltage, and another

hysteresis plot at various applied voltages. A well saturated hysteresis plot is depicted in Figure

3.5a of the Pt/ZnSnO3 layer/ZnO:Al capacitor; at an applied voltage of 220 V, the 20 µm thick

ZnSnO3 layer exhibited a Pr of nearly 30 µC/cm2 and a coercive field (Ec) around 25 kV/cm.

Extrapolation to zero field yields a spontaneous polarization (PS) ≈ 38 µC/cm2 with a hysteresis

squareness of ≈ 79% as determined by the ratio of Pr to PS. Polarization was measured across

27

several top Pt electrodes while maintaining the same bottom ZnO:Al electrode and consistent

values were found, reinforcing the findings of uniformly distributed of ZnSnO3 nanowires across

the sample. Previously reported Pr of 47 µC/cm2 for hetero-epitaxially grown ZnSnO3 film [9] is

larger than the Pr of 30 µC/cm2 exhibited by our ZnSnO3 nanowire arrays and is the subject of

future work. The coercive field (Ec = 25 kV/cm) of the fabricated device was significantly lower

than that of the reported ZnSnO3 film (Ec = 130 kV/cm), largely believed to be related to the

smaller domain sizes due to the nanostructuring achieved during synthesis.

Figure 3.5b depicts the leakage current density versus voltage for the Pt/ZnSnO3

nanowire array/ZnO:Al capacitor. The asymmetry in the curves can be partially attributed to

differences in the work function of the top and bottom electrodes that establish different potential

barrier heights at the top and bottom interfaces between the ZnSnO3 capacitor. [10] The work

function of ZnO:Al and Pt are 4.978 eV [11] and 5.3 eV, [12] respectively, the higher leakage

current density in the negative voltage region is tied to a lower potential barrier due to the greater

work function of the Pt/ZnSnO3 interface. Our J-V graphs for ZnSnO3 films depict a larger

leakage current when compared to other reports of PZT thin films, [10] but this effect has been

reported in other ZnSnO3 nanowires as well. [13] Whereas polycrystalline films suffer from a

clear correlation between grain boundaries and increased leakage currents, and charge trapping

defects [14] by contrast, our monocrystalline ZnSnO3 nanowires may feature improved carrier

mobility. From Figure 3.5c it can be seen that the high leakage current density does not impede

the FE properties displayed by the ZnSnO3 nanowire arrays. From the hysteresis graph a clear

voltage dependence is revealed; it can be seen that the Pr rises from 26 µC/cm2 at 130 V to 30

µC/cm2 with 220 V applied. With this we have undoubtedly probed the FE nature of our ZnSnO3

nanowire arrayed capacitors.

28

In conclusion, the FE properties of LN-type ZnSnO3 nanowire arrays grown by a

selective synthesis process were well explored. The selective approach involved PLD of ZnO:Al

on a Si substrate and solvothermal synthesis to produce the densely packed, self-supported,

highly crystalline nanowire arrays. The fusing of individual nanowires along their axial length

led to a compact arrayed film with a packing density no less than 0.8 with an impenetrable over-

layer that made possible FE measurements without the need of a dielectric filler. This optimized

approach to fabricating a ZnSnO3 nanowire arrayed film culminated in a high Pr of 30 µC/cm2

with a low Ec of 25 kV/cm never before reported in other nanostructure ZnSnO3 work. Any

practical applications of the device are limited by the high leakage current and large applied

voltages (130-220 V) required for operation.

Future work would seek to reduce the operating voltage by growing similarly compact

nanowires of smaller length without sacrificing the impenetrable over-layer that increases

contact with the electrodes to prevent shorting.[15] We should also like to definitively explore

the effect of grain size of the bottom ZnO:Al seed layer on the spatial distribution of the ZnSnO3

nanowires and how this effects leakage current and, in turn, polarization. An in-depth analysis of

the structure-property relationship of ZnSnO3 nanostructures will also be explored in future work

to further enhance the FE properties of this material.

3.1 References

[1] D. Mukherjee, T. Dhakal, H. Srikanth, P. Mukherjee, S. Witanachchi, Phys Rev B 81

(2010).

[2] A.V. Singh, R.M. Mehra, N. Buthrath, A. Wakahara, A. Yoshida, J Appl Phys 90 (2001)

5661.

[3] Y. Inaguma, M. Yoshida, T. Katsumata, J Am Chem Soc 130 (2008) 6704.

[4] J. Varghese, R.W. Whatmore, J.D. Holmes, J Mater Chem C 1 (2013) 2618.

[5] X.Q. Liu, Y.L. Liu, W. Chen, J.C. Li, L. Liao, Nanoscale Res Lett 7 (2012).

[6] Z. Gu, H. Ye, D. Gracias, D. Gracias, Jom 57 (2005) 60.

[7] I. Vrejoiu, M. Alexe, D. Hesse, U. Gosele, J Vac Sci Technol B 27 (2009) 498.

29

[8] J. Kim, S.A. Yang, Y.C. Choi, J.K. Han, K.O. Jeong, Y.J. Yun, D.J. Kim, S.M. Yang, D.

Yoon, H. Cheong, K.S. Chang, T.W. Noh, S.D. Bu, Nano Lett 8 (2008) 1813.

[9] J.Y. Son, G. Lee, M.H. Jo, H. Kim, H.M. Jang, Y.H. Shin, J Am Chem Soc 131 (2009)

8386.

[10] Masruroh, M. Toda, Appl Mech Mater 110-116 (2012) 294.

[11] M. Wei, C.-F. Li, X.-R. Deng, H. Deng, Energy Procedia 16, Part A (2012) 76.

[12] J. Scott, in: H. Ishiwara, M. Okuyama, Y. Arimoto (Eds.), Ferroelectric Random Access

Memories, Springer Berlin Heidelberg, 2004, p. 3.

[13] X.Y. Xue, Y.J. Chen, Q.H. Li, C. Wang, Y.G. Wang, T.H. Wang, Applied Physics

Letters 88 (2006) 182102.

[14] K. McKenna, A. Shluger, V. Iglesias, M. Porti, M. Nafría, M. Lanza, G. Bersuker,

Microelectronic Engineering 88 (2011) 1272.

[15] A. Datta, D. Mukherjee, C. Kons, S. Witanachchi, P. Mukherjee, Small 10 (2014) 4093.

30

Chapter 4:

Ferroelectricity in LN-type ZnSnO3 Nanowire-Nanoparticle Arrays2

A hysteresis plot is a unique identifier of the ferroelectric (FE) material to which it

belongs, it is a culmination of the effects of such intrinsic properties as nanostructures, grain

boundaries, doping, phase purity; structural defects; and extrinsic conditions such as applied

voltage, frequency, or temperature.[1] It is not uncommon for hysteresis loops of FE thin-films

or nanostructures to deviate from an idealized square one and experience pinching, asymmetry or

other anomalous behavior as a result of space charge effects such as defect dipoles, oxygen

vacancies or other charge defects fashioned during the growth states of the FE structures.[2-4]

These space charge effects can lead to a pinning effect that inhibits the motion of domain walls

resulting in a pinched or constricted hysteresis loop, [5] but distinguishing the trapped charge

effects from the intrinsic FE behavior can be difficult since ferroelectricity is an accumulation of

the effects of the spatial ordering of dipoles.

Polarization and long- and short- range dipole order are strongly dependent on the

interaction between many factors such as charge defects, domain wall movement,

crystallographic structure, strain effects, etc. so it is of little wonder that they have garnered

much research interest.[6-8] The aforementioned factors range from a several hundred to several

thousand angstroms so to fully understand they vital role they play in polarization a study of such

phenomenon must be at comparable scales.[9] To this end, careful nanostructuring of the FE

2 Reprinted with permission from [13]. Copyright 2014, AIP Publishing LLC.

31

materials by a selective physical/chemical synthesis process has met with great success in

achieving a hysteresis loop with high degree of squareness and large remanent polarization.[10]

In this thesis we will explore the FE properties of LN-type ZnSnO3 nanowire-

nanoparticle arrays. Reports on materials of different composites of nanostructures have shown

enhanced physical properties making them an attractive choice for device improvement.[11-13]

The FE measurements of the LN-type ZnSnO3 hybrid nanowire-nanoparticle thin film reveal a

pinched hysteresis loop with a remanent polarization (Pr) of ≈ 26 μC/cm2. The physical

mechanism responsible for the pinched hysteresis was deduced using the Preisach model; it is

believed that the interaction of the dipoles from the nanoparticles with their environment (the

hybrid nanostructured arrays) is responsible for the constriction of the hysteresis loop.

Figure 4.1 (a) XRD of the as-prepared LN-type ZnSnO3 nanowire-nanoparticle arrayed film confirming

phase purity as compared to Inaguma et al. [14] (b) Cross-sectional SEM of the nanostructured arrays

showing the vertical growth of the nanowires along with the nanoparticles.

Structural characterization began with comparing the XRD pattern (shown in Figure 4.1a)

of the as-synthesiszed sample with the standard LN-type ZnSnO3 taken from Inaguma et al. and

calculated from PowderCell.[14] It confirms the phase purity of the polycrystalline sample with

(012)

(122)

(110)

(202)

(006)

(113)

(116)

(018)

(104)

Si

20 30 40 50 60 70 80

Inte

nsi

ty (

arb

. u

nit

s)

2 (in degrees)

LiNbO3type

(b) (a)

32

favored growth in the (012) plane. The experimental lattice parameters were also calculated and

deteremined to be a = 0.52776 nm and c = 1.39676 nm, matching well with previously reported

LN-type ZnSnO3 experimental data as well as theoretical values. [14-16] Figure 4.1b is a SEM

image of a cross sectional view of the nanostructured array; a dense, compact layer of vertically

aligned ZnSnO3 nanowire-nanoparticles of uniform thickness (~ 10 µm) can be seen. Also

visible is the ZnO:Al layer atop the Si substrate.

Figure 4.2 (a and b) SEM images showing the densely packed, vertically aligned ZnSnO3 nanowires with

attached nanoparticles. (c) Low resolution TEM showing the various attachment arrangement of

nanoparticles to the nanowires. They may uniformly attach along the length of the nanowire and increase

its width (shown by a double arrow) or in clusters at randomly selected locations (indicated by single

arrows). (d) High resolution TEM of a single nanowire uniformly encrusted in nanoparticles confirming

the single crystalline nature of each structure.

The compact nature of the nanostructured ZnSnO3 film can be attributed to the even

dispersion of the nanoparticles within the nanowire arrays, as can be seen in Figure 4.2a and

Figure 4.2b. TEM image seen in Figure 4.2c show nanowires of diameter between 20-30 nm and

nanoparticles on the order of 5-15 nm that account for about 30-40 % of the volume of the

(a) (b)

(c) (d)

33

arrayed film. The nanoparticles attach to the surface of nanowires with no discernable pattern;

they can uniformly attach and increase the overall thickness of a nanowire (indicated by a double

arrow) or nanoparticles may stick to a nanowire in clusters rather than dispersing (shown by

single arrows). High resolution TEM shown in Figure 4.2d shows the nanowires and

nanoparticles are single crystalline with preferred growth along the (012) plane and random

orientations for the nanostructures, respectively. The dense top layer is once again useful for FE

measurements as it negates the need for any dielectric or polymeric filling to prevent shorting of

the device.

The FE device was fabricated by sputtering 100 μm Pt top electrode pads and using the a

portion of the preserved ZnO:Al layer to serve as the bottom electrode, as illustrated by Figure

4.3a. Measured at room temperature and an operating voltage of 100 V at 1 kHz the resulting

hysteresis loop (shown in Figure 4.3b) of the ZnSnO3 arrayed film shows a well saturated graph

with a high Pr ≈ 26 µC/cm2) and a coercive field (Ec) of ≈ 15 kV/cm based on a 10 µm thick LN-

type ZnSnO3 nanowire-nanoparticle arrayed layer. The Pr of the hybrid nanowire-nanoparticle

arrayed film is less than that reported in the previous chapter for the ZnSnO3 nanowires (Pr ≈ 30

µC/cm2) but the spontaneous polarization is similar to that exhibited by the singly crystalline

ZnSnO3 nanowire arrayed device (refer to Figure 4.5a). It is believed that existence of the

nanoparticles within the structured array create a pinning effect that inhibits domain wall motion

and thereby, lowering Pr compared to the nanowire only structure. The shift in the negative

direction of the FE loop and asymmetry of the leakage current versus voltage (inset of Figure

4.3b ) are indicative of a built-in internal bias field related to space charge effects.[17] The low

leakage current can be attributed again to the existence of the nanoparticles which enlarge grain

boundaries, leading to carrier entrapment which results in limiting carrier mobility.[18]

34

Figure 4.3 (a) An illustrated setup of the assembled ferroelectric device showing the top Pt electrode, the

preserved ZnO:Al bottom electrode and the ZnSnO3 nanowire-nanoparticle layer sandwiched between the

top and bottom electrodes. (b) The pinched hysteresis from the nanostructured array revealing a remanent

polarization of ≈ 26 µC/cm2 at 100 V applied at 1 kHz; the inset shows the asymmetric leakage current

versus voltage characteristics.

The pinched behavior of the hysteresis loop can clearly be seen in Figure 4.3b as a result

of the interaction of polarization with charge carriers, [19] if this abnormal behavior can be

attributed to space charge effects, such as oxygen vacancies, or grain boundary effects that serve

as entrapment regions for charges then post-annealing to de-age the material can combat these

effects.[20-22] To determine the origination of the pinched hysteresis loop the sample was post-

annealed for 12 hours at 600 ºC and FE measurements retaken, the results of which are

summarized in Figure 4.4. SEM reveals even denser film was obtained after annealing while

maintaining the nanowire-nanoparticle structure of uniform thickness with increased

agglomeration (Figure 4.4a-b) while XRD showed no change in phase or crystallinity (Figure

4.4c) compared to the initial sample.

(a) (b)

Pt top electrodes

Microprobe connectors

to Ferrotester

LN-type ZnSnO3

NP-NW Array

@ 300 K

Constricted hysteresis

-100 -50 0 50 100

-50

-25

0

25

50

-100 -50 0 50 10010

-6

10-5

10-4

10-3

Electric Field (kV/cm)

Po

lari

zati

on

(

C/c

m2)

Leakag

e C

urr

en

t (A

/cm

2)

Voltage (V)

35

Figure 4.4 (a and b) SEM images of cross-sectional views of the ZnSnO3 nanowire-nanoparticle array

after post-annealing at 600 ºC for 12 hours highlighting the uniformity in thickness and density and

preservation of the nanoparticle and nanowires.(c) XRD of the post-annealed sample confirming phase

purity and continued crystallinity compared to the as-prepared sample. (d) P-E curves comparing the as-

prepared sample to the post-annealed one. (e) Exploring the time dependence of the sample at varying

frequency yielded slight increasing in pinching at high frequencies. (f) Hysteresis loop of the original

sample poled at room temperature as compared to being hot poled at 100 ºC.

Despite post-annealing to negate the effects of the internal bias and ease pinching, Figure

4.4d clearly shows the FE hysteresis remained unchanged compared to the as-prepared sample.

The FE measurements were retaken at frequencies in the range of 0.1 kHz to 2 kHz to explore

the time dependent relationship of the space charge effects (Figure 4.4e) but the pinched loops

still remained with increased constriction at higher frequencies. It was determined the random

orientation of the dipoles in the nanoparticles has a significant impact on the overall polarization

and domain wall motion in the hybrid nanowire-nanoparticle structured array.[23,24] The

-100 -50 0 50 100

-50

-25

0

25

50

As-prepared

Post-annealed

Po

lari

zati

on

(

C/c

m2)

Electric Field (kV/cm)

(d)

(b) (a)

NP

NW

-100 -50 0 50 100

-50

-25

0

25

50 As-prepared

Hot-poled

Po

lari

za

tio

n (

C/c

m2)

Electric Field (kV/cm)

(e)

(c)

20 30 40 50 60 70 80

Si

Post-annealed

2 (in degrees)

As-prepared

Inte

nsi

ty (

arb

. un

it)

-100 -50 0 50 100

-50

-25

0

25

50

1 kHz

0.1 kHz

Electric Field (kV/cm)

Po

lari

zati

on

(C

/cm

2)

1.5 kHz

2 kHz

(f)

36

continued existence of the pinched behavior exhibited by the sample despite repeated poling and

annealing points to the intrinsic nature of this feature. Poling measurements were repeated at 100

ºC on the annealed ZnSnO3 nanowire-nanoparticle array and constriction of the loop relaxed

slightly in the hot-poled sample compared to the as-prepared sample, as shown in Figure 4.4f.

The relaxation may be attributed to the reorientation of the dipoles in the nanoparticles along the

direction of the applied field due to improved motion of domain walls at higher

temperatures.[25] This confirms the suspicions that the pinched behavior of the FE hysteresis

loops are due to the interaction of the orientation of dipoles in the nanoparticles with the

polarization exhibited by the nanowires in the LN-type ZnSnO3 structured array.

Models that attempt to understand such anomalous FE responses as pinched hysteresis

loops often employ strict boundary condition, such as thin films or superlattices, that hinder their

application to the hybrid nanostructured arrays. [26-28] While previously only applied to

ferromagnetic hysteresis loops, the Preisach model has been employed as a tool for analysis of

FE materials. [29,30] This model is assembled on the premise that the hysteretic system contains

switchable bistable units (domain wall motion, defect dipoles, etc.) where each unit can be

described by an internal bias field and a coercive fields.[31] To fit the nanowire-nanoparticle

hybrid arrays to the Preisach model it was assumed the nanoparticles attached to the nanowires

act as single domain units with switchable dipoles aligned in a sole direction. The exchanges of

the nanoparticle dipoles may be subject to internal electric fields (Ei) when in the presence of an

external field (see Figure 4.5a); the sign of such an internal field will shift the hysteresis loop

depending on the alignment of the nanoparticle dipoles with the overall polarization orientation

of the nanowires. This kind of asymmetry can be seen in any of the above hysteresis loops of the

LN-type ZnSnO3 nanowire-nanoparticle arrays. The dispersion of the nanoparticles throughout

37

the arrays creates regions of localization due to the varied internal fields that change the coercive

field required for dipole switching in that area, the summation of the different Ec’s leads to a

distribution of such values. The relation of the magnitude of Ec and Ei may inhibit the motion of

domain walls where such an effect might culminate as a pinched, or constricted, hysteresis

loop.[29]

Figure 4.5 (a) A generalized sketch of the exchanges between the nanoparticle dipoles with the

surrounding polarization of the nanowires under an applied field. It is these interactions that lead to

internal fields with the nanowire-nanoparticle arrayed sample. (b) The P-E loops taken under the

conditions of a sinusoidal voltage of 100 V as compared to the generated Preisach model curve.

The measured FE behavior of the LN-type ZnSnO3 nanowire-nanoparticle film was fitted

with the Preisach expression for pinched hysteresis loops as derived by Robert et al. based on a

sinusoidal input voltage and a symmetric distribution of the bistable units (nanoparticle dipoles).

[29] The FE measurements were retaken using a 100 V sinusoidal operating voltage at 1 kHz to

be consistent with the constraints of the theoretical model; the experimental and Preisach

modeling results are shown in Figure 4.5b as is the sinusoidal voltage input. A left shifted

pinched hysteresis loop is shown with a remanent polarization of ≈ 26 μC/cm2 and asymmetric

Interacting dipoles from LN-type ZnSnO3 FE NPs

External Electric Field

Internal Field

(a) (b)

-100 -50 0 50 100

-50

-25

0

25

50

0.0 0.5 1.0

-100

-50

0

50

100

Experiment

Preisach model

Po

lari

zati

on

(

C/c

m2)

Electric Field (kV/cm)

Vo

ltag

e (

V)

Time (ms)

38

coercive fields of Ec- (- 20 kV/cm) and Ec

+ (+ 10 kV/cm) were recorded with results similar to

those previously shown. The pinched electric hysteresis loop was obtained from the following

equation based on the Preisach model:

𝑃 = (𝑑 + 𝐸𝑚

3

12𝑘 −

𝐸𝑚5

80) 𝐸 + (

𝐸𝑚

12𝑘 −

𝐸𝑚3

24) 𝐸3 −

𝐸𝑚

80𝐸5

+ 𝑆𝑖𝑔𝑛 (𝑑𝐸

𝑑𝑡) [−

𝐸𝑚4

48𝑘 +

𝐸𝑚6

480+ (−

𝐸𝑚2

8𝑘 +

𝐸𝑚4

32) 𝐸2 + (

7𝑘

48+

𝐸𝑚2

32) 𝐸4

− 31

480 𝐸6] (1)

where 𝐸 = 𝐸𝑚 sin 𝜔𝑡 and d and k were chosen to be 5 and 25, respectively, in suit with Robert et

al.[29] while the value of for the applied field Em= 100 kV/cm was determined by experimental

conditions. The generated polarization values taken from Equation (1) were normalized to fit the

experimental values since the equation parameters were arbitrarily chosen. From Figure 4.5b it

can be seen that the experimental FE results match well with the P-E hysteresis figure generated

from the Preisach model with the same sinusoidal voltage used for poling. This confirms the

pinched FE behavior of the LN-type ZnSnO3 nanowire-nanoparticle arrays as an intrinsic

property and testifies to the strong contact between the nanoparticle dipoles and the highly

orientated dipole moments in the nanowires. The difference in the experimental and theoretical

values at low applied fields can be attributed to the assumption that the nanoparticle dipoles were

symmetrically distributed throughout the sample while a random distribution was more likely in

the sample. Regardless, the modified Preisach model for FE behavior fits well with the

experimental results and can effectively explain the pinched behavior exhibited by the LN-type

ZnSnO3 nanostructured array.

39

Figure 4.6 (a) SEM of a LN-type ZnSnO3 nanowire-nanoparticle arrayed sample synthesized at a 200 ºC

ramping temperature for 6 hours featuring increased nanoparticle concentration (50 -55 %). (b) Hysteresis

loop of the sample with increased nanoparticle density as compared to the original as-prepared sample

showing a clear cut correlation between concentration of nanoparticle and degree of pinching.

While there is a clear relation between the dipoles of nanoparticles and pinching of the

hysteresis loop the effect of nanoparticle concentration on FE behavior was also of interest.

Nanoparticle volume was increased to 50 – 55 % by using a higher ramping temperature (200°C)

for a longer period (6 hours) compared to the original synthesis parameters, SEM of the newly

prepared ZnSnO3 nanowire-nanoparticle arrayed film shown in Figure 4.6a reveals a greater

density of nanoparticles within the film while still maintaining the size of the nanowires and

nanoparticles compared to the original synthesis. Increased pinching in the FE behavior can be

seen in Figure 4.6b from the newly prepared sample with the increased nanoparticle

concentration. The clear correlation between nanoparticle volume % and hysteresis constriction

reaffirms the relationship between FE behavior and the interaction of the LN-type ZnSnO3

nanoparticles with the nanowires.

(a) (b)

-100 -50 0 50 100

-50

-25

0

25

50

-100 -50 0 50 100

30-40 vol. % NP

Po

lari

zati

on

(C

/cm

2)

Electric Field (kV/cm)

50-55 vol. % NP

40

To summarize, prepared LN-type ZnSnO3 nanowire-nanoparticle arrayed film exhibited

high Pr of ≈ 26 μC/cm2 with anomalous intrinsic pinched FE behavior. Post-annealing and

repeated poling at room temperature and high temperatures reaffirms the theory that the pinched

behavior is an intrinsic property of the hybrid nanostructured film. The exchanges between

nanoparticle dipoles with the overall polarization of the nanowires was reaffirmed by increasing

volume occupancy of the nanoparticles in the prepared sample and noting the resulting elevated

degree of pinching in hysteresis loops. Overall, the experimental results were well fitted to the

modified Preisach model for ferroelectricity which revealed a unique mechanism for charge

ordering in the prepared ZnSnO3 nanostructured films due to the arrangement of nanoparticle

dipoles in the nanowire environment.

4.1 References

[1] L. Jin, F. Li, S. Zhang, Journal of the American Ceramic Society 97 (2014) 1.

[2] X. Wei, Y. Feng, L. Hang, S. Xia, L. Jin, X. Yao, Materials Science and Engineering: B

120 (2005) 64.

[3] Masruroh, M. Toda, Appl Mech Mater 110-116 (2012) 294.

[4] P. Zubko, D.J. Jung, J.F. Scott, J Appl Phys 100 (2006) 114112.

[5] L. BaoShan, L. GuoRong, Y. QingRui, Z. ZhiGang, D. AiLi, C. WenWu, Journal of

Physics D: Applied Physics 38 (2005) 1107.

[6] H. Han, Y. Kim, M. Alexe, D. Hesse, W. Lee, Advanced Materials 23 (2011) 4599.

[7] J. Varghese, R.W. Whatmore, J.D. Holmes, J Mater Chem C 1 (2013) 2618.

[8] P. Suryanarayana, K. Bhattacharya, J Appl Phys 111 (2012) 034109.

[9] V. Nagarajan, C. Ganpule, R. Ramesh, in: H. Ishiwara, M. Okuyama, Y. Arimoto (Eds.),

Ferroelectric Random Access Memories, Springer Berlin Heidelberg, 2004, p. 47.

[10] A. Datta, D. Mukherjee, C. Kons, S. Witanachchi, P. Mukherjee, Small 10 (2014) 4093.

[11] D. Bhadra, S.C. Sarkar, B.K. Chaudhuri, RSC Advances 5 (2015) 36924.

[12] Y. Song, Y. Shen, H. Liu, Y. Lin, M. Li, C.-W. Nan, Journal of Materials Chemistry 22

(2012) 8063.

[13] D. Mukherjee, A. Datta, C. Kons, M. Hordagoda, S. Witanachchi, P. Mukherjee, Applied

Physics Letters 105 (2014).

[14] Y. Inaguma, M. Yoshida, T. Katsumata, J Am Chem Soc 130 (2008) 6704.

[15] H. Gou, F. Gao, J. Zhang, Computational Materials Science 49 (2010) 552.

[16] H. Wang, H. Huang, B. Wang, Solid State Communications 149 (2009) 1849.

[17] D. Fu, K. Suzuki, K. Kato, H. Suzuki, Appl. Phys. A 80 (2005) 1067.

[18] L. Pintilie, Charge Transport in Ferroelectric Thin Films, 2011.

[19] S. Sun, Y. Wang, P.A. Fuierer, B.A. Tuttle, Integrated Ferroelectrics 23 (1999) 25.

41

[20] Z. Ye, M.-h. Tang, C.-p. Cheng, Y.-c. Zhou, X.-j. Zheng, Z.-s. Hu, Transactions of

Nonferrous Metals Society of China 16, Supplement 1 (2006) s71.

[21] S.K. Upadhyay, V.R. Reddy, K. Sharma, A. Gome, A. Gupta, Ferroelectrics 437 (2012)

171.

[22] S.K. Pandey, O.P. Thakur, A. Kumar, C. Prakash, R. Chatterjee, T.C. Goel, J Appl Phys

100 (2006) 014104.

[23] R.F. Klie, Y. Ito, S. Stemmer, N.D. Browning, Ultramicroscopy 86 (2001) 289.

[24] P. Han, K.-J. Jin, H.-B. Lü, J.-F. Jia, J. Qiu, C.-L. Hu, G.-Z. Yang, Chinese Physics

Letters 26 (2009) 027301.

[25] A.B. Kounga, T. Granzow, E. Aulbach, M. Hinterstein, J. Rödel, J Appl Phys 104 (2008)

024116.

[26] L. Baudry, J Appl Phys 86 (1999) 1096.

[27] C.K. Wong, F.G. Shin, J Appl Phys 96 (2004) 6648.

[28] P. Wu, X. Ma, Y. Li, V. Gopalan, L.-Q. Chen, Applied Physics Letters 100 (2012)

092905.

[29] G. Robert, D. Damjanovic, N. Setter, Applied Physics Letters 77 (2000).

[30] A.T. Bartic, D.J. Wouters, H.E. Maes, J.T. Rickes, R.M. Waser, J Appl Phys 89 (2001)

3420.

[31] G. Robert, D. Damjanovic, N. Setter, A.V. Turik, J Appl Phys 89 (2001) 5067.

42

Chapter 5:

Ferroelectricity in Pb-free LN-type ZnSnO3 Other Nanostructure Arrayed Thick Films3

Here we discuss the results of structural characterization and the FE properties of

partially aligned nanowires, nanorods, and nanoflakes. The XRD results of each of the three

nanostructures (nanowires, nanorods, and nanoflakes) are shown in Figure 5.1a stacked against

the spectra of the ZnO:Al layer and LN-type ZnSnO3 database. [1] The different morphologies

are results of the various solvents (water/ethanol, EG, or PEG) used with different chemical

reactivities used during the solvothermal reaction that helped to stabilize some complexes as

intermediate steps and guided the growth of different crystal planes within the ZnSnO3, leading

to such varied structures as one dimensional nanowire and nanorods to the two dimensional

nanoflakes. The growth of hexagonal ZnSnO3 with space group R3c was guided by the ZnO seed

layer due to similar lattice constants (a = 5.26 Å and a = 3.29 Å for ZnO and ZnSnO3,

respectively) that stabilized this phase on the Si substrate. Good phase purity is shown by the

XRD spectra in Figure 5.1a as well as the polycrystalline nature of ZnSnO3 nanostructures.

Figure 5.1 b-d show SEM images of the partially aligned nanowires, nanorods, and

nanoflakes synthesized in a solution of water/ethanol, water/PEG/, and water/EG, respectively.

Unlike the nanowires discussed in Chapter 3 these nanowires are only partially aligned, Figure

5.1b displays the fusing along the axial lengths and tips to create an impenetrable over layer

3 Corisa Kons and Anuja Datta, “Facile Growth of Functional Perovskite Oxide Nanowire Arrays by

Hybrid Physical-Chemical Techniques”, MRS Proceedings, Volume 1751, 2015, reproduced with

permission.

43

(inset to Figure 5.1b) suitable for electrode placement. After 12 hours of synthesis they achieved

a length of ≈ 20 µm and an average diameter of 20 nm, slightly smaller than the previously

discussed nanowires but this may be a result of a different ZnO seed layer thicknesses. The thick

canopy layer is a nanoparticulate composite that, once again, deceitfully hides the structure

nanowire arrays beneath. The density and reduced porosity of the surface eliminated the need for

a dielectric or polymeric filling used to prevent the electrodes from settling between the

nanostructures and ‘shorting’ the device during FE measurements.

Figure 5.1 (a) XRD spectra of ZnSnO3 nanowire arrays, nanorod arrays, nanoflake congregations and

ZnO:Al seed-layer on Si substrates compared to the standard XRD pattern of LN-type ZnSnO3. (b) Cross

– sectional SEM image of densely packed nanorods with the dense over-layer of fused nanowires shown

in the inset. (c) Top-down SEM images of ZnSnO3 nanorods with the post-annealed sample shown in the

inset with improved density. (d) SEM image of nanoflake groupings showing the interconnectivity of the

structured array; the inset shows the reduced porosity post-annealing.

20 30 40 50 60 70 80

200 300 400 500 600

Si

297

E(L

O)

433

E(L

O)

E(L

O)

226

Inte

nsi

ty (

a.

u.)

Raman shift (cm-1

)

NF film

NW arrays

NR arrays

(012)

(312)

(300)

(116)

(113)

(024)(1

10)

(0002)

(0004)

(104)

ZnO

ZnOZnOSi

Si

Rel

ati

ve

Inte

nsi

ty (

arb

. u

nit

s)

ZnO:Al/Si seed layer

LN-type ZnSnO3

2 (degrees)

B

(a) (b)

(c) (c) (d)

44

The ZnSnO3 nanorod arrays, shown in Figure 5.1c were synthesized in a water/PEG

mixture and are randomly orientated with columns that narrow towards the top, the base has an

average diameter of 30 nm while the structure tapers to 5 nm at the tip with an overall structural

thickness of ~ 6 µm. The as-prepared sample was too porous to allow sputtering of the Pt top

electrodes which inhibits the ability for FE measurements; to increase the density of the nanorods

the sample was annealed for 6 hours at 200 °C. The inset to Figure 5.1c shows cross-sectional

SEM of the ZnSnO3 nanorod assembly with improved compactness and preserved nanorod

structures after annealing successfully allowing for the deposition of top electrodes and FE

measurements.

When the precursor solution is composed of 60/40 vol. % water/EG the resulting

structural growth is of nanoflakes, as shown in Figure 5.1d. The SEM image shows the

nanoflakes are 5 -10 nm thick and interconnected to each other. As with the nanorods, the flakes

were annealed to improve inter-structural density for 6 hours at 200 °C, seen in the inset to

Figure 5.1d; the film averaged 2 µm thick and porosity was decreased to < 10 vol. % post-

annealing making it suitable for FE measurements.

Improving the structural porosity enabled the design of thick film capacitors for FE

measurements from the ZnSnO3 nanostructured arrays without the need for dielectric fillers that

ultimately introduce dielectric losses during polarization testing by aiding in the deposition of the

top Pt electrodes. As previously, the doped ZnO layer served as the bottom electrode. The results

of FE measurements at room temperature with an applied voltage of 220 V are shown in Figure

5.2 of the LN-type ZnSnO3 partially aligned nanowire, nanorod, and nanoflakes arrays,

respectively. Of the three nanostructured arrays the nanowires had the highest remanent

polarization (Pr) (Figure 5.2a) of ≈ 28 µC/cm2 with an estimated coercive field (Ec) of ≈ 25

45

kV/cm calculated from the sample thickness of 20 μm determined by SEM (refer to Figure 5.1b).

The extrapolated spontaneous polarization at 220 V is ≈ 38 µC/cm2 giving the partially aligned

nanowire arrayed thick film hysteresis loop a squareness of ≈ 79 %. The FE measurements taken

from various top Pt electrodes and the same bottom ZnO:Al bottom electrode show similar

polarization values as a result of the uniform distribution of nanowires throughout the surface of

the substrate.

Figure 5.2 Ferroelectric hysteresis loops measured at room temperature from the Pt/ZnSnO3

nanostructure arrayed films/ZnO:Al capacitors using an applied voltage of 220 V of the (a) nanowires, (b)

nanorods, and (c) nanoflake groups.

The ZnSnO3 partially aligned nanowire arrays and nanoflake assemblies exhibited much

lower Pr values of ~ 11 µC/cm2 and 4.7 µC/cm2, respectively. The coercive fields were estimated

to be are ≈ 40 kV/cm for the nanorods and ≈ 38 kV/cm for the nanoflakes based on the

respective film thicknesses from SEM. The Pr values are significantly lower than the values

reported for epitaxial (111) ZnSnO3 nanostructured films (Pr = 47 µC/cm2) and is the subject of

future investigations while the coercive field values, especially for the nanowire arrayed film, are

much lower than those previously reported.[1] It is clear though that the partially aligned

nanowire arrays display far superior FE properties to those of the other nanostructures and holds

much potential as a suitable alternative to future Pb-free FE devices.

-100 -50 0 50 100

-40-30-20-10

010203040 NW arrays

Po

lari

zati

on

(

C/c

m2)

Electric Field (kV/cm)

-100 -50 0 50 100-30

-20

-10

0

10

20

30NR Arrays

Po

lari

zati

on

(

C/c

m2)

Electric Field (kV/cm)-100 -50 0 50 100

-15

-10

-5

0

5

10

15NF Assemblage

Po

lari

zati

on

(

C/c

m2)

Electric Field (kV/cm)

(a) (b) (c)

46

To summarize, by a strategic synthesis approach involving PLD and a solvothermal

process, LN-type ZnSnO3 partially aligned nanowire, nanorods, and nanoflakes were grown on

Si substrates due to the doped ZnO seed layer. This resulted in a densely packed, highly

crystalline film as a result of the similar crystal symmetry between the ZnO layer and LN-type

ZnSnO3. The fusing of the nanowire and the densely packed structures allowed for direct

measurement of FE properties while the nanorods and nanoflakes samples required low

temperature annealing to reduce porosity before electrodes could be deposited for testing. The

best Pr of 28 µC/cm2 at a low coercive field of 25 kV/cm was exhibited even by the partially

aligned nanowire arrays. Accounts of ferroelectricity in LN-type ZnSnO3 nanostructures have

not been well reported to date so this report could provide value insight into the design of future

FE devices centered around Pb-free materials.

5.1 References

[1] Y. Inaguma, M. Yoshida, T. Katsumata, J Am Chem Soc 130 (2008) 6704.

47

Chapter 6:

Summary and Future Work

Ferroelectric (FE) perovskites with switchable polarization properties are identified as

next generation ‘smart materials’, which can convert changes in mechanical energies into

electrical signals (piezoelectricity) and potentially provide faster, smaller, energy-efficient

systems that can manage, sense and store information efficiently (ferroelectricity). Despite

adverse environmental impact, lead-zirconium-titanate (PZT) has been the dominant component

in such applications. The aim of the work discussed in this thesis is to develop a novel synthesis

process to grow nanostructured films of lead-free FEs that functionally challenge the widely used

PZT and to acquire fundamental understanding of the new growth mechanism for extending the

process to a large-area growth capability in future. The high piezoelectric (PE) and FE properties

predicted and recently observed in environmentally benign, zinc-based perovskite-type oxide

ZnSnO3 has the potential to replace the current industry standard of lead-based ceramics.

Our research is aimed to advance the current state of research that aims to replace PZT in

a multitude of devices. The large-scale synthesis and integration of these materials with ordered

low-dimensional structures and controlled crystal orientations is a challenge due to the narrow

window of parameters for which the phases are stable. Our material synthesis scheme

encompasses a unique physical/chemical technique that include pulsed laser deposition (PLD)

and solvothermal processes to fabricate oriented nanostructure arrays on both hard and flexible

substrates. Initially nanograined seed layers of oriented zinc oxide thin-films were deposited on

the substrates by the PLD process, which were then chemically reacted in controlled ionic

48

precursor environment to promote the stabilization and growth of the desired nanostructured

phases. We are the first to demonstrate a generalized scale-up synthesis scheme for Pb-free and

earth-abundant elements based ZnSnO3, as different dimensional nanostructure arrays on

industrially viable hard substrates. We investigated their structure-dependent FE properties and

developed a fundamental understanding of the structure, stability and growth of these hitherto

unexplored perovskite material in nanostructured forms to help in designing next-generation

high-performance ferro- and piezo-devices.

Zinc stannate is also a direct band gap material (2.42 eV) that may have light-harvesting

applications if the band gap can be reduced to a level where it can absorb the full solar

spectrum.[1] Future work would focus on band gap tuning by introducing dopants into the

crystal structure as in this material the band gap is a strong function of the lattice constants. The

FE photovoltaics are unique in that it is the inherent polarization that drives photocurrent

throughout the device and improves charge separation efficiency.[2]

6.1 References

[1] H. Gou, F. Gao, J. Zhang, Computational Materials Science 49 (2010) 552.

[2] Y. Yuan, Z. Xiao, B. Yang, J. Huang, Journal of Materials Chemistry A 2 (2014) 6027.

49

Appendices

Appendix 1: Publication List

The work presented in this dissertation is based in part on the following journal articles:

1.1 Peer-Reviewed Journal Articles

1. Devajyoti Mukherjee, Anuja Datta, Corisa Kons, Mahesh Hordagoda, Sarath Witanachchi and

Pritish Mukherjee, “Intrinsic anomalous ferroelectricity in vertically-aligned LiNbO3-type

ZnSnO3 hybrid nanoparticle-nanowire arrays” Applied Physics Letters 2014 (105) 212903

2. Anuja Datta, Corisa Kons, Devajyoti Mukherjee, Sarath Witanachchi and Pritish Mukherjee,

“Evidence of superior ferroelectricity in aligned ZnSnO3 nanowire arrays” Small 2014 (10) 4093

1.2 Peer-Reviewed Conference Proceedings

1. “Measurements of Polarization Switching in LiNbO3-type ZnSnO3/ZnO Nanocomposite Thin

Films”D Mukherjee, M Hordagoda, C Kons, A Datta, S Witanachchi, P. Mukherjee, Fall MRS

Proceedings 1729, mrsf14-1729-m11-07 (2015).

2. “Ferroelectricity in Strategically Synthesized Pb-Free LiNbO3-type ZnSnO3 Nanostructure

Arrayed Thick Films”. Anuja Datta, Devajyoti Mukherjee, Corisa Kons, Sarath Witanachchi and

Pritish Mukherjee, Fall MRS Proceedings 1729, mrsf14-1729-m11-02 (2015).

3. “Facile Growth of Functional Perovskite Oxide Nanowire Arrays by Hybrid Physical-Chemical

Techniques”. Corisa Kons, and Anuja Datta, MRS Proceedings 1751, mrsf14-1751-ll13-07

(2015).

1.3 Conference Presentations

1. Corisa Kons, Anuja Datta, Devajyoti Mukherjee , Pritish Mukherjee “Band Gap Tuning in ZnSnO3

Nanorods by Cation Doping and Core-Shell Method for Solar Cell Applications” Florida Annual Meeting

and Exposition (FAME) by the Florida Section of the American Chemical Society, Innisbrook,

Florida, May 7 – 9, 2015.

2. Corisa Kons, Anuja Datta, Pritish Mukherjee, “Band-Gap Tuning in Perovskite-type Ferroelectric

ZnSnO3 by Doping and Core-Shell Approach for Solar Cell Applications” American Physical

Society, Session L34: Thin Film Photovoltaics (Perovskites, etc), March 4, 2015.

3. Anuja Datta, Devajyoti Mukherjee, Corisa Kons, Sarath Witanachchii, and Pritish Mukherjee,

“Ferroelectricity in Strategically Synthesized Pb-free LiNbO3-type ZnSnO3 Nanostructure

50

Arrayed Thick Films” 2014 MRS Fall Meeting & Exhibit, Boston, MA, November 30 -

December 5, 2014.

4. Corisa Kons and Anuja Datta, “Facile Growth of Functional Perovskite Oxide Nanowire Arrays

by Hybrid Physical-Chemical Techniques” 2014 MRS Fall Meeting & Exhibit, Boston, MA,

November 30 - December 5, 2014.

5. Mahesh Hordagoda, Corisa Kons, Devajyoti Mukherjee, Anuja Datta, Sarath Witanachchi1, and

Pritish Mukherjee, “Evidence of Polarization Switching in LiNbO3-Type ZnSnO3/ZnO

Nanocomposite Thin Films” 2014 MRS Fall Meeting & Exhibit, Boston, MA, November 30 -

December 5, 2014.

6. Corisa Kons, Anuja Datta, Devajyoti Mukherjee, and Pritish Mukherjee “Band Engineering in

ZnSnO3 Nanorods by Doping and Core-Shell Approach for Solar Cell Applications” 2014 MRS

Fall Meeting & Exhibit, Boston, MA, November 30 - December 5, 2014.

7. Corisa Kons, Anuja Datta, Devajyoti Mukherjee, and Pritish Mukherjee, “Band Gap

Modification in ZnSnO3 by Cation Doping and Core-Shell Approach for Solar Cell Applications”

NanoFlorida 2014 - The 7th Annual Nanoscience Technology Symposium Sept. 25-26, Miami,

Fl, USA, 2014.

8. Boeing Smith, Corisa Kons, and Anuja Datta, “Band Energy Modification of Ferroelectric

ZnSnO3 for Photovoltaic Applications” REU-RET Poster Symposium, University of South

Florida, July 31, Tampa, Fl, USA, 2014.

9. Corisa Kons and Anuja Datta “Seed-Assisted Synthesis and Characterization of Ferroelectric

ZnSnO3 Nanostructures” Florida Annual Meeting and Exposition (FAME) by the Florida Section

of the American Chemical Society, Innisbrook, Florida, May 8 – 10, 2014.

10. Corisa Kons and Anuja Datta, “Growth and Characterization of Eco-friendly Lead-Free

Ferroelectric ZnSnO3 Nanostructures” 38th International Conference & Exposition on Advanced

Ceramics & Composites, Daytona Beach, Florida, January 26-31, 2014

51

Appendix 2: Copyright Permissions

Excerpts from the license agreements for use of material from previously published articles are

reproduced below.

JOHN WILEY AND SONS LICENSE

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Jul 29, 2015

This Agreement between Corisa Kons ("You") and John Wiley and Sons ("John Wiley and

Sons") consists of your license details and the terms and conditions provided by John Wiley

and Sons and Copyright Clearance Center.

License Number 3678320890919

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Licensed Content Publisher John Wiley and Sons

Licensed Content

Publication Small

Licensed Content Title Evidence of Superior Ferroelectricity in Structurally Welded

ZnSnO3 Nanowire Arrays

Licensed Content Author Anuja Datta,Devajyoti Mukherjee,Corisa Kons,Sarath

Witanachchi,Pritish Mukherjee

Licensed Content Date Jun 23, 2014

Pages 7

Type of Use Dissertation/Thesis

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Format Print and electronic

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Number of Pages 12

Will you be translating? No

Title of your thesis /

dissertation

Synthesis, Characterization and Ferroelectric Properties of LN-

Type ZnSnO3 Nanostructures

Expected completion date Aug 2015

Expected size (number of

pages) 51

AIP PUBLISHING LLC LICENSE

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Jul 29, 2015

52

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Publisher AIP Publishing LLC

Publication Applied Physics Letters

Article Title Intrinsic anomalous ferroelectricity in vertically aligned LiNbO3-

type ZnSnO3 hybrid nanoparticle-nanowire arrays

Author Devajyoti Mukherjee,Anuja Datta,Corisa Kons, et al.

Online Publication Date Nov 25, 2014

Volume number 105

Issue number 21

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Portion Figure/Table

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AIP Publishing LLC ("AIPP"") hereby grants to you the non-exclusive right and license to

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Jul 29, 2015

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("Cambridge University Press") provided by Copyright Clearance Center ("CCC"). The

license consists of your order details, the terms and conditions provided by Cambridge

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Facile Growth of Functional Perovskite

Oxide Nanowire Arrays by Hybrid

Physical-Chemical Techniques

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Ferroelectric Properties of LN-Type

ZnSnO3 Nanostructures

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applications

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Licensed content date Feb 11, 2013

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Properties of LN-Type ZnSnO3 Nanostructures