MRS Communications (2015), 5,27–44 © Materials Research Society, 2015 doi:10.1557/mrc.2015.8 Prospective Articles Synthesis, physics, and applications of ferroelectric nanomaterials Mark J. Polking, Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138 A. Paul Alivisatos, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720; Department of Chemistry, University of California, Berkeley, California 94720 Ramamoorthy Ramesh, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720; Department of Materials Science and Engineering, University of California, Berkeley, California 94720 Address all correspondence to A. Paul Alivisatos, Ramamoorthy Ramesh at [email protected]; [email protected] (Received 23 November 2014; accepted 11 February 2015) Abstract Improvement of both solution and vapor-phase synthetic techniques for nanoscale ferroelectrics has fueled progress in fundamental under- standing of the polar phase at reduced dimensions, and this physical insight has pushed the boundaries of ferroelectric phase stability and polarization switching to sub-10 nm dimensions. The development and characterization of new ferroelectric nanomaterials has opened new avenues toward future nonvolatile memories, devices for energy storage and conversion, biosensors, and many other applications. This pro- spective will highlight recent progress on the synthesis, fundamental understanding, and applications of zero- and one-dimensional ferroelec- tric nanomaterials and propose new directions for future study in all three areas. Introduction basic properties of ferroelectrics for the unfamiliar reader. As Since the discovery of ferroelectricity in Rochelle salt nearly a the dielectric analog of more familiar ferromagnetic materials, century ago, ferroelectrics have transitioned from an academic ferroelectrics possess many of the basic characteristics of the curiosity to serious contenders in the fields of nonvolatile mem- latter class of materials, including an order parameter with ory devices, actuators for microelectromechanical systems multiple stable states, hysteresis during cycling between (MEMS) devices, mechanical energy conversion, and many these states, and the formation of domain structures (Fig. 1). other applications at the forefront of modern science.[1–3] In analogy with ferromagnets, which exhibit a permanent Although research on ferroelectrics remains primarily confined magnetic moment that may be reoriented with a sufficiently to the realm of epitaxial thin films, new methods for the synthe- strong magnetic field, the order parameter in ferroelectrics is sis, processing, and integration of ferroelectric nanomaterials in a spontaneous electrical polarization that can be similarly zero and one-dimensional forms is poised to reinvigorate re- reoriented under a sufficiently strong electric field.[6–8] search on both the applications and fundamental science of fer- Different spatial orientations of the spontaneous polarization, roelectric materials. The last decade has witnessed the rapid as with ferromagnets, give rise to domain structures. To under- development of new synthetic pathways to low-dimensional fer- stand the origin of this spontaneous polarization, we turn to the roelectric nanomaterials, and these synthetic developments have archetypal inorganic ferroelectric, BaTiO3. As the material is both transformed our understanding of the fundamental physics cooled below a critical temperature, known as the Curie tem- of nanoscale ferroelectricity and spawned tantalizing new appli- perature in analogy with ferromagnets, the originally cubic cations in solar energy conversion,[4] biosensing,[5] and many crystal undergoes a spontaneous symmetry-breaking distor- other areas. In this review, we summarize these recent achieve- tion to a tetragonal phase through the relative displacement ments related to the synthesis, physics, and applications of these of the Ti ionic sublattice with respect to the negatively charged nanomaterials. In addition, we highlight areas in which the con- oxygen sublattice, leading to the separation of the centers of current development of powerful new characterization tools, negative and positive charge in the unit cell and an electric new synthetic pathways offering improved material control, dipole moment (Fig. 1). The alignment of many of these and an improved base of fundamental understanding offer new dipole moments across an entire crystal, like the coherent opportunities to address longstanding questions within the field. alignment of electronic spins in a ferromagnet, leads to a net, macroscopic polarization. In some cases, the orientation of this spontaneous dipole moment can be reversed by appli- An introduction to ferroelectrics cation of an electric field without inducing dielectric break- Prior to reviewing recent developments in the field of ferroelec- down. We term these crystals ferroelectric, and we label tric nanomaterials, we shall begin with a brief overview of the those crystals with a spontaneous polarization that cannot be MRS COMMUNICATIONS • VOLUME 5 • ISSUE 1 • www.mrs.org/mrc ▪ 27 Figure 1. An introduction to the basics of ferroelectricity. (a) Unit cell of the prototypical perovskite ferroelectric BaTiO3 above (left) and below (right) the Curie temperature (Tc). The tetragonal distortion and displacement of the Ti cations (red arrow) below Tc result in a separation of the centers of negative and positive charge, leading to a spontaneous electric dipole moment. Alignment of these dipole moments across an entire crystal leads to a net macroscopic spontaneous polarization. (b) Schematic illustration of the domain structure of a ferroelectric material. Domains with the same polarization orientation (red arrows) form to minimize electrostatic energy, similar to the formation of ferromagnetic domains. (c) Polarization switching in a ferroelectric. Cycling of the electric field results in switching of the direction of the spontaneous polarization and a characteristic hysteresis loop resembling the magnetization-magnetic field hysteresis loops of ferromagnetic materials. reoriented as pyroelectric. These two classes of materials, phase.[7] Many other classes of ferroelectrics exist, however, in- which we refer to as polar materials, are the focus of this re- cluding semiconductors such as GeTe and SbSI and hydrogen- view. The utility of these materials arises not only from the ex- bonded materials such as potassium dihydrogen phosphate istence of multiple stable polarization states, but also from the (KDP).[8] In addition, in materials with a magnetic sublattice fact that all pyroelectric crystals are necessarily piezoelectric such as BiFeO3 and YMnO3, ferroelectric ordering may be ac- by symmetry, allowing for interconversion of mechanical companied by ferromagnetic ordering, in many cases leading to and electrical impulses. magnetoelectric coupling between the order parameters.[7] The The vast majority of common ferroelectrics are complex discovery of new ferroelectrics remains an active area of re- oxide materials, and most of these are based on the perovskite search, and these new semiconducting and multiferroic materi- structure, defined by the general formula ABO3, and, in many als continue to push the frontiers of ferroelectricity in the cases, a small symmetry-breaking distortion from a cubic present day. 28 ▪ MRS COMMUNICATIONS • VOLUME 5 • ISSUE 1 • www.mrs.org/mrc Prospective Articles Synthesis of ferroelectric serious challenges. Nanostructures of BaTiO3, PbTiO3, and Pb nanomaterials (ZrxTi1−x)O3 (PZT) have been prepared by hydro/solvothermal The last three decades have featured an explosion in the devel- means, often through reaction of Ti isopropoxide with Ba metal [16,17] opment of both zero and one-dimensional nanostructures.[9,10] or acetate salts. Wong and co-workers also prepared both The vapor–liquid–solid (VLS) growth scheme, colloidal chem- BaTiO3 and SrTiO3 using oxalate chemistry at high pressure [18] istry, and hydro/solvothermal techniques have exhibited partic- (Fig. 3). This general approach involving high-temperature, ular versatility. Despite the considerable successes of these high-pressure reaction of metal salts has also been employed methods for a broad range of materials, quality syntheses of fer- for synthesis of other perovskites, such as the colossal magne- [19] roelectric nanomaterials have been noticeably lacking in the lit- toresistive material La1−xBaxMnO3. In addition, Caruntu erature, and most of the successes have been confined to a and co-workers have recently reported the growth of reasonably single material, BaTiO3. This dearth of literature is curious well-dispersed nanocrystals of the ferroelectric (and leading given the rapidly increasing interest in ferroelectrics and related nonlinear optical material) LiNbO3 via decomposition of a [20] polar materials such as multiferroics, and is rendered still more single-source precursor under high pressure. The hydrother- curious by the success of both vapor and solution-based tech- mal approach has recently shown promise for the synthesis of niques in the synthesis of complex oxides with other function- the multiferroic perovskite BiFeO3 through reaction of Bi and alities, including the magnetic spinel ferrites.[11] In this section, Fe salts in the presence of tartaric acid or ethylene glycol fol- [21,22] we shall examine progress in the synthesis of ferroelectric lowed by calcination