Ferroelectric nanomaterials for electronic technique M. D. Glinchuk Institute for Problems of Materials Science National Academy of Sciences of Ukraine Kiev, Ukraine E-mail: glin@ipms.kiev.ua

I. Main fields of ferroelectric materials application in electronic technique capacitors piezoelectric devices (ultrasound sources, resonators, filters, phones, microphones, piezotransformators etc.) radiolocation and communication systems (the devices on surface acoustic waves, waveguides, coders and decoders etc.) devices for night vision, high harmonic generation systems for recording and reading of information piezoelectric devices, heat-visioners for medicine and biology posistors for the systems of thermal regime control

II. Anomalous properties near the phase transitions as the basis for applications Giant dielectric permittivity of relaxor ferroelectric (Pb,La)TiO3 [B.-G. Kim et al., Phys. Rev. Lett. 86, 3404 (2001)] Temperature dependence of electrical resistance of (Ba,Sr)TiO3:Y3+:Nb5+ ceramics [Z. He et al., J. Phys.:Cond. Matter 16, 6961 (2004)] Shortcomings of bulk ferroelectrics: strong dependence of the properties on temperature and scattering of phase transition temperatures while for application we mostly need room temperature; the necessity for integration with semiconductors and metals; the demands for the devices miniaturization; the materials, energy and money savings. All these shortcomings can be overcome in the nanomaterials.

III. Size effects of ferroelectric nanomaterials properties Size dependence of the transition temperature from ferroelectric (FE) to paraelectric (PE) phase Dependence of dielectric permittivity on nanoparticles mean sizes Glinchuk M.D., Eliseev E.A., Stephanovich V.A., J. Appl. Phys., 93(2), 1150 (2003); Physica B, 332, 356 (2002) Glinchuk M.D., Morozovska A.N., Phys. Stat. Sol., 238, 81 (2003) Powder PbTiO3 Powder PbTiO3 E. Erdem et al., J. Phys.: Condens. Matter 18, 3861 (2006)

III. Size effects of ferroelectric nanomaterials properties The physical background of the properties size effects, including size-driven ferroelectric–paraelectric phase transition is the influence of surface which can not be neglected at nanosizes. The main peculiarities is the appearance of the properties gradients and so their inhomogeneity, the influence of mechanical conditions on the surface (confinement conditions) and the essential contribution of depolarization field. As the result the new properties absent in bulk materials appear in ferroelectric nanos. In what follows I will demonstrate some of the new properties.

IV. New phases originated from mechanical conditions on the surface of ferroelectric thin films PbTiO3 film on SrTiO3 substrate PbTiO3 film on SrTiO3: Nb substrate The deformation of the film due to mismatch between the parameters of the film and substrate Um = (b – a)/b (a, b are their lattice constants) and the surface piezoelectric effect induce built-in electric field. This field lead to electret state in the thinnest film. With the thickness increase the self-polarized ferroelectric phase appears. The phenomenon of self-polarization is very useful for the films application e.g. in pyroelectric devices because it allows to omit the costly process of polarization by external field. Ferroelectric phase can be conserved in the thinnest film (up to monolayer) by special choice of the pair film–substrate. Glinchuk M.D., Morozovska A.N., Eliseev E.A., J. Appl. Phys. 99, 114102 (2006) Glinchuk M.D., Morozovska A.N., J. Phys.: Cond. Matter 16, 3517 (2004) Eliseev E.A., Glinchuk M.D., Phys. Stat. Sol. (b) 241, R52 (2004) Glinchuk M.D., Morozovska A.N., Eliseev E.A., Integrated Ferroelectrics, 64, 17 (2005)

V. Conservation and enhancement of ferroelectric properties in nanotubes and nanowires Effective piezoresponse of PbZr0.52Ti0.48O3 nanotube (outer diameter 700 nm, thickness of wall 90 nm, the length about 30 mm) in dependence on applied voltage. Symbols – experimental data; solid line – theory. Scheme of perovskite structure deformation under external pressure Pressure on lateral surface of nanowire due to surface tension enhances the polar properties of nanos A.N.Morozovska, E.A.Eliseev and M.D. Glinchuk, Phys. Rev. B. 73, 214106 (2006)

Nanorods of Rochel salt (RS) V. Conservation and enhancement of ferroelectric properties in nanotubes and nanowires Nanorods of Rochel salt (RS) r = 30 nm, l = 500 nm Remanent polarization PSV dependence on temperature and hysteresis loop for RS nanorods with radius 15 nm. Symbols – experimental data; solid lines – theory. D. Yadlovker and S. Berger, Phys. Rev. B 71, 184112 (2005) A.N.Morozovska, E.A.Eliseev and M.D. Glinchuk, Phys. Rev. B. 76, 014102 (2007)

V. Conservation and enhancement of ferroelectric properties in nanotubes and nanowires Nanorods Nanotubes with walls of different thickness Effective surface tension and decrease of depolarizaton field on cylindrical surface are the reasons of ferroelectric properties enhancement in nanorods and nanotubes of perovskite (Q12 < 0) ferroelectrics. This effect is very useful for development of new nanomaterials with high polar properties (better than in bulk). A.N. Morozovska, E.A Eliseev and M.D. Glinchuk, Physica B, 387, 358 (2007); Phase Transitions 80, 71 (2007)

VI. Phase states and giant magnetoelectric effect induced by surface tension in ferroic nanoparticles Ferroelectric phase has to appear at room temperature in nanorods and nanowires with r  50 nm of incipient ferroelectrics, which conserve paraelectric phase up to 0 K in bulk. The existence of ferroelectric phase at room temperature will be useful for development of new generation of the devices on the basis of nanostructures. The ferromagnetic phase observed in spherical nanoparticles with radius 7–30 nm at room temperature of nonmagnetic in bulk materials CeO2, Al2O3, ZnO, SnO2 etc. (Phys. Rev. B 74, 161306(R) (2006)) can be induced by surface tension that increases inversely proportional to the particle radius. The giant magnetoelectric coupling which increase dielectric tunability about two orders of magnitude was predicted in multiferroic nanorods due to surface tension influence. The phenomenon is very important for application because it will allow to write information by electric field and read it by magnetic field. A.N.Morozovska, E.A.Eliseev and M.D. Glinchuk, Phys. Rev. B 76, 014102 (2007) M.D. Glinchuk, E.A.Eliseev, A.N.Morozovska and R.Blinc, Phys. Rev. B 77, 024106 (2008)

VII. Electronic technique devices for nowadays or nearest future applications 1. Pyroelectric infrared sensors on the basis of thin (from tens to hundreds nm) ferroelectric films PbZr0.5Ti0.5O3 on the Si–SiO2 substrates, which are generating electric current at temperature changing (e.g. due to infrared irradiation) are widely used as the detectors sensing the living organisms presence (frequently nonwanted), sensing the temperature enhancement under the fire beginning and producing the alarm signals; they are useful also for medical examination of humans health. Nowadays there is multimillion production of pyrosensors and transducers in Simens, Marconni, Muratta etc. The important advantage of thin ferroelectric films is their self-polarization, its nature being found out by us.

VII. Electronic technique devices for nowadays or nearest future applications 2. The ferroelectric devices for an information recording. The concept of recording is the polarization PS № 0 and PS = 0 corresponds respectively to 1 and 0 (binary system), i.e. it is related to the switching of polarization. The coercive field in the bulk ferroelectrics is about several kV/cm, while semiconductor devices in the integrated scheme are working at a few V/cm. Because of this it is necessary to use thin films, where the coercive field is small enough. In comparison with magnetic films ferroelectric ones have the advantage of larger memory, of energy saving, of higher rate of information exchange e.g. in mobile phones, photocameras, videocameras etc. In these important consumer equipment many ferroelectric capacitors are use to provide the work of the devices on necessary frequencies. The main problem of ferroelectric memory is the process of reading related to depolarization current, that lead to the film deterioration with time. Contrary in the magnetic films the information recording process can destruct the film. Because of this the scientists and engineers are discussing now the possibility to use multiferroics, i.e. the materials with coexistence of ferroelectric and ferromagnetic order with strong magnetoelectric coupling, so that an information can be recorded with the help of electric field and can be read with the help of magnetic field. It was mentioned earlier that such possibility can occur in nanoparticles as our calculations had shown. Many firms in all the world (Toshiba, Samsung, Integrated Semiconductors, Matsushita etc.) are working on the problems related to the production of improved ferroelectric films for memory devices.

VII. Electronic technique devices for nowadays or nearest future applications 3. Other applications Nanoparticles and nanocomposites (sphere, tubes, rods, wires) Nanofilms and their multilayers (epitaxial, textured, polycrystalline) Profiled nanostructures (nanodomains, superlattices) Applications Micro- and nano-electromechanical systems. Energy transducers and accumulators. Long-term memory elements on the basis of thin films. Multielemental piezo- and pyrosensors. Optical devices Optical generators of higher harmonics, transducers, frequency converters, parametric amplifiers. Memory systems of new generation. Ferroelectric micro- and nanolithography Pyroelectric detectors. Three-dimention architecture of ferroelectric memory. Photon devices. Electrooptical and nonlinear optical devices. New generation of printers Advantages Space-saving, integrability with modern semiconductor devices, the decrease of manipulated voltage in 10–103 times. High density of information recording, minor time of access. Development of ferroelectric nanos with new properties in comparison with bulk materials Enlargement of spectral region, controlling of angular dependence. Increase of recording density, enhancement of stability. High decoupling ability Enlargement of sensitivity. High density of information recording. Improvement of quality factors, cost decrease

Conclusions In the system nanofilm–substrate the choice of substrate can essentially influence the film phase diagram and properties, namely: For compressed films self-polarized ferroelectric state can be conserved up to several lattice constants thickness. Self-polarized films are very useful for the development of high-quality pyrosensors and modern memory systems. For stretched films built-in electric field originated from the mechanical stress in the system nanofilm–substrate induces elektret-like polar state. Its properties can be useful in construction of pyroelectric detectors and actuators without hysteresis. The possibility to obtain giant dielectric permittivity (from 105 to 106) is shown in multilayers of thin films relaxor ferroelectric–paraelectric, which is important for development of space-saving capacitors of new generation. The enhancement of ferroelectric properties in the cylindrical nanoparticles (rods, wires, tubes) of ferroelectrics with perovskite structure arises due to surface tension on the lateral surface and to the decrease of depolarization field for cylindrical geometry. Array of cylindrical nanoparticles can be used for the creation of modern memory systems, pyroelectric detectors. It was shown for the first time that surface tension which increase at nanoparticle radius decrease can lead to the appearance of ferroelectric or/and magnetic state with giant magnetoelectric effect in nanoparticles with radius smaller than 50 nm. This effect will be useful to control the work of the devices both by electric or magnetic field.

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