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One-Dimensional Nanostructures of Ferroelectric Perovskites Per Martin Rørvik, Tor Grande, and Mari-Ann Einarsrud* of reviews of 1D nanostructures have been published,[1–3] providing an overview of research directions in synthesis and applications of 1D nanostructures. The review by Xia et al.[2] presents an excellent introduction to 1D nanostructures of a variety of materials. The amount of literature on 1D nanostructures is increasing rapidly, showing the widespread and growing research field that these structures represent. While much of the literature so far has focused on the synthesis and basic properties of 1D nanostructures, demonstrations of more advanced applications and controlled assembly into larger nanostructures are emerging. 1D nanostructures have been fabricated in several types of morphologies. While hollow 1D nanostructures are all called nanotubes, a variety of names are used for dense 1D nanostructures: nanorod, nanowire, nanobelt, nanofiber, nanowhisker, and nanofinger. A nanorod and a nanowire can be differentiated by the aspect ratio (length/diameter). For instance, Murphy and Jana[4] defined nanorods as nanostructures with a width of ≈1–100 nm and aspect ratios greater than 1 but less than 20 and nanowires as nanostructures that have aspect ratios greater than 20. However, as no standard is given, these two terms are widely mixed. Nanobelts are 1D nanostructures with a rectangular cross-section, with a width markedly different from the height. 1D nanostructures made by electrospinning are usually called nanofibers, a name also often used to describe clearly polycrystalline 1D nanostructures. Nanowhisker and nanofinger are seldom used. Often 1D structures with diameters of a few hundred nanometers are also named “nano”, although “submicrometer” would be a more precise prefix. The synthesis of 1D nanostructures has mainly been directed towards metals, semiconductors, binary oxides, and carbonaceous materials,[1–3] but syntheses of complex ternary oxide materials have been emerging rapidly over the last 5 years.[3c,k,n,o] Among the ternary crystal structures, the perovskite structure (ABX3) is the most multifunctional, as the functional properties can easily be tailored by chemical substitution. Perovskite oxides (ABO3) are by far the most studied, with unique electronic, magnetic, and optical properties, including ferroelectricity, high-temperature superconductivity, and colossal magnetoresistance.[5] The ideal (cubic) perovskite structure consists of corner-sharing oxygen-octahedra with the B cation in the center, and with the A cation in the 12coordinated position between 8 octahedra. The three main classes of perovskites are AIBVO3, AIIBIVO3, and AIIIBIIIO3, with
Nanorods, nanowires, and nanotubes of ferroelectric perovskites have recently been studied with increasing intensity due to their potential use in non-volatile ferroelectric random access memory, nano-electromechanical systems, energy-harvesting devices, advanced sensors, and in photocatalysis. This Review summarizes the current status of these 1D nanostructures and gives a critical overview of synthesis routes with emphasis on chemical methods. The ferroelectric and piezoelectric properties of the 1D nanostructures are discussed and possible applications are highlighted. Finally, prospects for future research within this field are outlined.
1. Introduction The synthesis and deposition of nanoscale structures have attracted extensive attention in the past two decades as a result of their novel size-dependent properties. Intense experimental efforts have been spent to prepare nanoparticles, ultrathin films, nanowires, nanotubes, and 3D arrays of nanostructures. Among these, 1D structures such as nanowires and nanotubes are the smallest dimensional structures that can be used for efficient transport of electrons and optical excitations and are thus expected to be critical to the function and integration of components at the nanoscale.[1] The application of 1D nanostructures in electronic devices is currently still in the initial phase, but they are expected to play an important role as both interconnects and functional units in the fabrication of electronic, optoelectronic, electrochemical, and electromechanical devices with nanoscale dimensions.[2] The novel properties of 1D nanostructures open up for new avenues of applications in several fields such as electronics, photonics, sensors, catalysts, energy harvesting, information storage, and mechanical strength enhancement. Applications of 1D nanostructures are in most cases dependent on the inherent properties of a certain material (electrical, optical, magnetic, or thermoelectric properties), so general applications for such nanostructures are few. The advantage of the large surface area is generally attractive for catalysis and sensors, and the anisometric shape can be used in composites for improving the mechanical strength. A number
Dr. P. M. Rørvik, Prof. T. Grande, Prof. M.-A. Einarsrud Department of Materials Science and Engineering Norwegian University of Science and Technology 7491 Trondheim, Norway E-mail:
[email protected]
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the Roman numerals depicting the valence state of the cations. Solid solutions between these three are also possible giving mixed valence state on both A and B site. Several of the perovskites are ferroelectric, which means that they have a spontaneous electric polarization that can be switched by an external electric field. Ferroelectricity is closely related to piezoelectricity and pyroelectricity; all ferroelectrics are also piezoelectric and pyroelectric, but not all piezoelectrics are pyroelectric, and not all pyroelectrics are ferroelectric.[6] The prototype ferroelectric perovskite is BaTiO3. The ferroelectricity in BaTiO3 arises from a displacement of the titanium ion in the [001] direction of the tetragonal perovskite structure, and BaTiO3 is therefore labeled a displacive ferroelectric. At high temperature, BaTiO3 has a paraelectric cubic perovskite structure (Figure 1a). At 120 °C, it transforms from the cubic phase to a ferroelectric tetragonal phase (Figure 1b,c).[6,7] Among the ferroelectrics there are a few important material classes that closely resemble the perovskites and which therefore will be included here: LiNbO3 and layered oxide ferroelectrics. LiNbO3 has a rhombohedral unit cell with a structure composed of oxygen octahedra containing the B atom and surrounded by the A atoms. Compared to the perovskite structure, the oxygen octahedra have been rotated around [111], such that the A atoms only have 6 oxygen first neighbors, rather than 12 as in the cubic perovskite structure.[7] The layered oxide ferroelectrics belong to the family of Aurivillius compounds with a general formula (Bi2O2)2+(Am−1BmO3m+1)2−, consisting of m perovskite-like blocks sandwiched between bismuth oxide layers where the A ion is Na, Sr, Ca, Ba, Pb, or Bi, the B ion is Ti, Ta, or Nb, and m can be an integer or half-integer.[7,8] Examples include SrBi2Ta2O9 (m = 2) and Bi4Ti3O12 (m = 3). In addition, multiferroic composites composed of nanoparticles of a ferroelectric oxide and nanoparticles of a ferromagnetic oxide will be briefly addressed. Advances in science and technology of ferroelectrics the last decade have resulted in the development of nanoscale ferroelectric structures and devices ( molten salt and hydrothermal syntheses, alkali cations such as 14.0. These observations are consistent with other reports; Na+ and K+ are usually present in significant amounts. Na+ and Zhu et al.[79a] adjusted the pH to 13.0 and obtained PX-phase K+ may substitute the Ba2+ or Pb2+ ions in the lattice, creating [ 72 ] nanowires, and Wang et al. filtered the precursor suspension oxygen vacancies to maintain charge neutrality.[117a] In addition,
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www.advmat.de www.MaterialsViews.com Table 3. A summary of synthesis studies of 1D nanostructures of ferroelectric III-III perovskites. Material BiFeO3
Synthesis method
Reference(s)
Sol-gel deposition in AAO template + calcination
[127]
Electrospinning + calcination at 550 °C
[128]
Ultrasonic irradiation of solution + annealing at 400 °C
[129]
Annealing of thin film at 600 °C (vapor-solid mechanism)
[130]
Hydrothermal
[131]
Bi0.9La0.1FeO3
related ferroelectric III-III perovskites. Table 3 gives a summary of the available literature. A challenge during synthesis of BiFeO3 is the thermodynamic instability in the temperature region of about 600–800 °C.[132] In this temperature interval BiFeO3 spontaneously transforms to Bi25FeO39 and Bi2Fe4O9, so annealing within this temperature region should be avoided. In addition BiFeO3 reacts easily with Al2O3 at temperatures of 800 °C and above, forming Bi2Fe4–xAlxO9, as Al2O3 is more soluble in Bi2Fe4O9 than in BiFeO3.[133] Thus, calcination of BiFeO3 in AAO templates should ideally be performed below 600 °C, possibly at long calcination times. The III-III perovskites includes several non-ferroelectric oxides and several of these have been fabricated as 1D nanostructures: La1−xCaxMnO3,[123c,134] La1−xSrxMnO3,[123d,135] La0.5Ba0.5MnO3,[136] La0.5(Ba,Sr)0.5MnO3,[137] LaFeO3,[138] La1−xSrxCoO3–δ,[139] LaNiO3,[140] and Pr0.5Ca0.5MnO3.[141] These compounds are highly relevant in various applications, such as catalysis, magnetic sensors, and solid oxide fuel cells, and in the study of colossal magnetoresistance. 3.4. Layered Oxide Ferroelectrics A summary of the available literature on 1D nanostructures of layered oxide ferroelectrics, made by chemical methods, is given in Table 4. The products are mainly polycrystalline due to Table 4. A summary of synthesis studies of 1D nanostructures of layered oxide ferroelectrics. Material
Synthesis method
Reference(s)
Bi2VO5.5
Sol-gel deposition in AAO template + calcination at 650 °C
[142]
Bi3.15Nd0.85Ti3O12
Sol-gel deposition in AAO template + calcination
[143]
Bi3.25La0.75Ti3O12
3.3. III-III Perovskites The most studied ferroelectric III-III perovskite today is BiFeO3.[126] BiFeO3 is ferroelectric below 820–830 °C and antiferromagnetic below 370 °C and is therefore termed a multiferroic material because it combines two of the ferroic properties. Compared to the ferroelectric I−V and II−IV perovskites, there are relatively few studies on 1D nanostructures of BiFeO3 and Adv. Mater. 2011, 23, 4007–4034
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OH− may substitute O2−, creating cation vacancies.[117b] The protons can be removed from the structure by annealing. Heat treatment up to 600 °C may be necessary to remove all the protons from the lattice; however, a significant number of protons are removed at lower temperatures.[118] Although the hydrothermal synthesis route is a low-temperature synthesis route, subsequent high-temperature annealing to annihilate defects in the structure may be necessary in certain cases. Several of the syntheses in Table 2 include advanced chemistry and non-trival procedures. The synthesis procedures are often delicate and small variations in the procedure may yield a different product. Often, the necessary information to precisely duplicate the experimental procedure is not described in detail and therefore it may be problematic to reproduce the synthesis. At minimum, the amount of chemicals used should be included, as the concentration often is a vital parameter. For instance, although the early paper by Urban et al.[14] reporting on the synthesis of BaTiO3 and SrTiO3 nanowires is highly cited, no follow-up studies using the same method by other research groups have been published. In general, most of the non-template methods have been used only by one research group. It must also be mentioned that some of the literature are of questionable quality, especially regarding the determination of the phase composition of the 1D nanostructures by selected area electron diffraction (SAED) and X-ray diffraction. Careful and critical interpretation of the diffraction data is necessary, as the 1D nanostructures may be minority phases in the reaction product. In addition to the literature cited in Table 2, a few studies of non-ferroelectric AIIBIVO3 materials as 1D nanostructures have been reported. BaMnO3 and BaTi0.5Mn0.5O3 nanorods have been made by a low-temperature molten salt method using NaOHKOH eutectic as the reaction medium.[119] CuGeO3 nanowires with a pyroxene-related structure with GeO4 tetrahedra in line have been made by hydrothermal synthesis from GeO2 and Cu substrate.[120] Compared to the number of studies using bottom-up chemical methods, the production of 1D nanostructures of perovskites using top-down physical methods have scarcely been reported. One notable exception is the studies by Schilling et al. of focused ion beam milling of BaTiO3 single-crystals into nanowires.[121] PZT nanowires were recently made by reactive ion etching of a PZT thin film, using Ni nanowires made by lithographic techniques as a mask.[122] In addition, physical deposition techniques such as magnetron sputtering and pulsed laser deposition have been used to deposit BaTiO3 and PZT onto positive templates[123] and PbZrO3 in AAO templates.[124] PZT stripes have been formed on SrTiO3 substrates containing growth-controlling gold particles by thermal evaporation.[125]
Hydrothermal
[144]
Electrospinning + calcination
[145]
Sol-gel deposition in AAO template + calcination
[146]
Electrospinning + calcination at 700 °C
[147]
Sol-gel deposition in AAO template + calcination at 700 °C
[148]
Sol-gel electrophoretic deposition in AAO template + calcination at 700 °C
[149]
Liquid source misted chemical deposition in porous silicon templates + calcination
[50c,150]
SrBi2Ta2O9
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www.MaterialsViews.com Table 5. A summary of synthesis studies of 1D nanostructures of multiferroic composites and core-shell structures based on ferroelectric perovskites. Material CoFe2O4-BaTiO3 core/shell CoFe2O4-PZT composite PZT-CoFe2O4 core/shell NiFe2O4-PZT composite NiFe2O4-PZT core/shell
Synthesis method
Reference(s)
Pulsed laser deposition onto carbon nanotubes
[123b]
Electrospinning
[152]
Sol-gel deposition in AAO template + calcination (for both materials)
[88f ]
Electrospinning
[153]
Sol-gel deposition of PZT in AAO template + calcination; electrodeposition of Ni/Fe + annealing at 800 °C
[88d]
the synthesis methods used, but Hu et al.[144] describe synthesis of single-crystalline Bi3.15Nd0.85Ti3O12 using a hydrothermal method. PVA was used as a structure-directing agent and the nanorods grew in the [104] direction. 3.5. Multiferroic Composite Materials Composites or core-shell structures consisting of a ferroelectric material and a ferromagnetic material represent an alternative to true multiferroic materials.[151] A summary of the available literature on 1D nanostructures of such structures based on ferroelectric perovskites is given in Table 5. The composites were made by first preparing a sol for each compound. Then a mixture of the two sols was electrospinned followed by drying and calcination.[152,153] The composite nanofibers consisted of nanocrystals of ferroelectric PZT and ferromagnetic spinel (CoFe2O4 or NiFe2O4). The core/shell structures naturally necessitate a sequential processing and for these structures templates have been used, either carbon nanotubes[123b] or AAO templates.[88d,88f ]
spectroscopy studies. When used in spectroscopy mode to do switching measurements at a certain location of the sample surface, the technique is sometimes called piezoresponse force spectroscopy (PFS).[155b] In conventional PFM of 1D nanostructures, a conductive atomic force microscopy (AFM) probe tip is brought into contact with the sample top surface in contact mode. An ac bias is applied between the probe and the conductive sample back surface (Figure 9a), which induces local oscillatory structural deformation due to the converse piezoelectric effect. This deformation is measured using a lock-in amplifier and the piezoresponse (P) is obtained by P = A × cosθ from the amplitude (A) and phase (θ) signals. PFM can be operated in two modes, vertical and lateral. In the vertical PFM mode, the induced local vertical deformation is measured through the vertical deflection of the AFM cantilever and the out-of-plane polarization is studied, while in the lateral PFM mode, the local shear deformation of the sample is measured through the torsional twisting of the AFM cantilever and the in-plane polarization is studied. There exist certain limitations in the conventional PFM setup for in-plane polarization study. Because of the concern of electric breakdown and the localized nature of the electric field, whenever a high bias is needed for in-plane polarization switching or a sample segment needs to be uniformly polarized, the conventional lateral PFM setup is incapable. Wang et al.[156] have reported a setup where the dc bias is applied along the length direction of a nanowire (Figure 9b). The ac bias is still applied through the AFM tip, but the dc bias used
4. Ferroelectric and Piezoelectric Properties 4.1. Characterization Methods at the Nanoscale Until about a decade ago, high-resolution studies of ferroelectrics were limited to electron microscopy. Scanning probe microscopy (SPM) techniques have emerged as a powerful tool for high-resolution characterization of ferroelectrics, providing an opportunity for non-destructive visualization of ferroelectric domain structures.[154] SPM has made it possible to map the surface potential at the nanoscale, to evaluate local electromechanical properties, and to measure non-linear optical and dielectric constants. Among the SPM techniques for nanoscale characterization of ferroelectrics, piezoresponse force microscopy (PFM) is by far the most popular.[9b,155] The high spatial resolution, easy implementation, effective manipulation and control of nanoscale domains, and local spectroscopy capabilities make PFM a well-suited tool for nanoscale ferroelectric studies. PFM is used both for imaging and non-imaging
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Figure 9. Schematic illustration of a) the biasing condition in conventional PFM of nanowires and b) the axial biasing setup in the study of Wang et al.[156] Reproduced with permission.[156] Copyright 2006, American Institute of Physics.
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4.2. Finite Size Effects Ferroelectricity is traditionally expected to vanish at a small scale due to decreased long-range ordering of dipoles, but during the last decade it has been shown that ferroelectricity can be maintained in nanostructures with dimensions down to a few unit cells. Theoretical investigations have demonstrated the possibility for dipoles to orient in unconventional toroidic structures. Ferroelectricity at the nanoscale is limited by the relevant electrical and mechanical boundary conditions, and also by the practical issues involved in producing high-quality samples on the extreme nanoscale.[161] Size effects can be of intrinsic nature (related to the changes in atomic polarization at small scales) or extrinsic nature. Extrinsic effects are caused by the patterning and processing of materials or more complicated effects, which include the influence of inhomogeneous strain, incomplete polarization screening at the surface, and defect microstructure. In the early studies of size effects the extrinsic ones were dominant. The information is therefore rather contradictory and scattered even for the same materials prepared with different processing routes.[9b] The development of thinfilm deposition techniques and characterization methods have made it possible to deposit and characterize epitaxial thin films of complex oxides with thicknesses down to a single unit cell.[9a,162] For instance, Fong et al.[162] made epitaxial PbTiO3 films on SrTiO3 substrates and demonstrated that the ferroelectric phase is stable for thicknesses down to 3 unit cells (1.2 nm). Their results thereby imply that there is no thickness limit for practical devices by the intrinsic ferroelectric size effect. The advance in synthesis and structuring has also made it easier to fabricate 0D nanoparticles and islands with fewer defects and which are suitable for investigations of the ferroelectricity. Recent research on BaTiO3 nanoparticles suggests that there
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exists no particular critical size at which ferroelectricity in perovskites is completely lost.[163] Rather, ferroelectricity decays gradually with decreasing size through a disordering process in which the atomic ordering becomes cubic-like on average while maintaining local tetragonal distortion. Compared to nanoparticles and thin films, the finite size effects on ferroelectricity in 1D nanostructures have been much less studied experimentally. This is largely because of the difficulty in producing ferroelectric 1D nanostructures with such small dimensions. In one of the earlier synthesis studies Urban et al.[14] managed to produce BaTiO3 nanowires with diameters of 5–60 nm. Spanier et al.[164] have studied the polarization perpendicular to the nanowire axis by electrostatic force microscopy (EFM). Their measurements show that the ferroelectric phase transition temperature (Curie temperature, TC) is depressed as the nanowire diameter (dnw) decreases, following a 1/dnw scaling (Figure 10). The diameter at which TC falls below room temperature was determined to be ≈3 nm, and extrapolation of the data indicated that nanowires with dnw as small as 0.8 nm can support ferroelectricity at lower temperatures.[164] They also presented density functional theory (DFT) calculations of bare and molecule-covered BaTiO3 surfaces, which indicate that ferroelectricity is stabilized by molecular adsorbates such as OH and carboxylates in these nanowires. The adsorption screens a significant amount of the polarization charge on the surface and thereby reduces the depolarization field relative to bare BaTiO3.[164] Various theoretical calculations on ferroelectric nanowires and nanotubes have shed light on the finite size effects in these structures. From these studies, ferroelectricity is generally conserved down to nanowire diameters of only a few unit cells.[165] For instance, Geneste et al. used first-principles calculations to show that the ferroelectric distortion along the wire axis in
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to switch the in-plane polarization of the nanowire is applied directly between the two ends of the nanowire. This provides in-plane polarization control while simultaneously allowing the study of ferroelectric hysteresis and shear piezoresponse of a nanowire with PFM. We note that hysteresis loops should be evaluated carefully to secure that the hysteresis actually originate from a ferroelectric origin. As humorously demonstrated by Scott[157] a closed loop of switched charge (Q) versus applied voltage (V) does not necessarily imply a ferroelectric material; hysteresis loops should be saturated and have a region in Q versus V that is concave, instead of being cigar-shaped. In particular for multiferroics such as BiFeO3, the polarization-electric field loops are often dominated by leakage currents. Although SPM and especially PFM techniques are developing rapidly, electron microscopy remains an important tool for characterization of nanoscale ferroelectrics. Schilling et al. have, for instance, used scanning transmission electron microscopy (STEM) to image 90° domains in BaTiO3 lamellaes,[158] nanocolumns,[121] and nanodots[159] that were cut from BaTiO3 single crystals by a focused ion beam. More importantly, the new generation of aberration-corrected TEMs offers such high spatial resolution to allow direct imaging of atomic displacements associated with the electrical dipoles in ferroelectrics.[160]
Figure 10. Ferroelectric phase transition temperature (TC) as a function of BaTiO3 nanowire diameter (dnw). The solid circles are the experimentally determined TC and the error bars are uncertainties. The solid line is the result of the fit to the data using the 1/dnw scaling relation. The inset plots TC as a function of 1/dnw and illustrates the inverse diameter dependence. Reproduced with permission.[164] Copyright 2006, American Chemical Society.
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BaTiO3 nanowires disappears below a critical diameter of about 1.2 nm.[165b] Shimada et al. found that the surface termination had a clear influence of the axial polarization in PbTiO3 nanowires; PbO-terminated nanowires with a cross-section of only 1 × 1 unit cell can show ferroelectricity, while ferroelectricity disappears in TiO2-terminated nanowires with a crosssection smaller than 4 × 4 unit cells.[165g] As the size of nanowires decreases, the surface tension becomes more important. The surface tension on 1D nanostructures induces an internal pressure in the radial direction that will influence the ferroelectric properties. The effect of the pressure depends on the polarization direction. If the polarization is oriented along the axis the polarization and TC are expected to increase with decreasing diameter (up to the point at which long-range interactions favoring ferroelectricity become weakened), while for polarization perpendicular to the axis TC decreases with decreasing diameter.[166] By taking into account this surface tension effect, the results of Spanier et al.,[164] showing decrease in TC with decreasing nanowire diameter (Figure 10), have been verified theoretically using Landau– Ginzburg–Devonshire theory.[165c,166] Similarly, the effect of surface tension on ferroelectric nanowires with polarization along the axis has been studied.[165d,h,i,167] The polarization and TC are typically maximal at 2–5 nm nanowire radius. Figure 11 shows the calculated increase in TC and polarization to above bulk values in PbTiO3 nanowires as a function of the radius.[165h] The situation without surface tension is also shown for comparison; it is evident that the surface tension has a significant influence at nanoscale. The increase in TC can be of special importance in multiferroics; when the ferroelectric transition temperature approaches the magnetic one, the magnetoelectric coupling can be enhanced by several orders of magnitude.[126,168] The phase diagrams of multiferroic nanorods are therefore dramatically changed with the reduction in size. Calculations have also been performed for BaTiO3 nanotubes with the polarization along the nanotube axis and similar size effects as for nanowires were observed.[169] Experimentally, Yang et al.[73d] observed a shift in TC of PbTiO3 nanotubes to 620 °C (TC/TC,bulk = 1.17) by capacitance versus temperature measurements, supported by high-temperature Raman spectroscopy and X-ray diffraction. The increased TC was explained by compressive stress in the nanotubes. In a noteworthy study, Nonnenmann et al.[170] made Ag/ PZT core/shell nanowires and showed by PFM that the piezoelectric response in the radial direction is much higher than corresponding PZT thin films with the same thickness. They supported their observations with modified Landau–Ginzburg model calculations (Figure 12), which indicate increased piezoelectricity and increased TC in thin curved PZT shells. In contrast to the expected scaling of a depression of TC with inverse thickness, the calculations indicate that geometric curvature-driven polarization gradients in ultrathin films result in increased TC. Zhang et al. have used a molecular dynamics method to study the hysteresis behavior of thin BaTiO3 nanowires with crosssection lengths (d) of 0.8, 1.2, and 2.0 nm.[171] For d = 2.0 nm, corresponding to 5 × 5 unit cells, a stepwise hysteresis loop was obtained, which was attributed to a core/shell domain configuration with different composition proportions at different electrical loading levels resulting in a sequential switching of
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the three layers. In comparison, for the 1.2 nm nanowire the hysteresis loop has only one step due to the 3 × 3 unit cells, and no loop is obtained for the 0.8 nm nanowire (2 × 2). In the bulk material, the hysteresis loop may be regarded as the superposition of infinite steps, such that the step height is too small to be distinguished and the loop becomes smooth. 4.3. Domain Configuration and Polarization Direction In ferroelectrics, domains form to minimize the energy associated with the depolarization field. Small domains are favored to minimize the depolarization energy, but the creation of domains also creates domain walls, which cost energy and favor larger domains. The depolarization energy can be compensated by free charges, which will favor a single domain. The exact domain configuration in a crystal depends on many factors including the crystal symmetry, the electrical conductivity, the defect structure, the magnitudes of the spontaneous polarization and elastic and dielectric compliances, the geometry of the
Figure 11. Increase in a) the Curie temperature (TC) and b) spontaneous polarization (P) along the nanowire axis of PbTiO3 nanowires as a function of nanowire radius. The solid and dashed lines represent the results with surface tension and without surface tension, respectively. Reproduced with permission.[165h] Copyright 2009, American Institute of Physics.
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Wang et al.[91d,92] observed contrast lines 45° to the axis in their TEM study of PZT nanowires that were made by hydrothermal synthesis and later annealed. These contrast lines were attributed to domain walls, resulting in a domain configuration similar to the one found by Schilling et al. We have studied hydrothermally synthesized PbTiO3 nanorods by TEM and PFM and observed that the polarization direction can be changed from parallel to perpendicular to the nanorod axis by heat treatment above TC.[172c] The PbTiO3 nanorods were made at 180 °C, far below the TC of PbTiO3 (≈490 °C). Thus, they grow in the ferroelectric tetragonal state Figure 12. Left) Calculated (300 K) radial polarization profile Pr as a function of radial position ρ for selected values of shell inner and outer radii a and b, respectively (solid lines) and for and the observed polarization direction in the planar, stress-free thin films of corresponding thicknesses (dashed lines). Right) Calculated as-grown nanorods was along the nanorod volume-averaged Pavg as a function of temperature for the nanoshells of selected diameters axis. As the nanorods were heated to the and thicknesses (solid lines) and for planar, stress-free thin films of corresponding thicknesses cubic paraelectric phase and cooled through (dashed lines) plotted in the left figure; the inset illustrates the model geometry. The bulk T a new domain configuration developed, C polarization value is denoted by horizontal dashed black lines and in the right figure the bulk with the polarization oriented perpendicular TC value is denoted by a vertical dashed line. Reproduced with permission.[170] Copyright 2010, to the axis and with domain walls oriented American Chemical Society. on {101} planes along the axis. At the same time a surface rearrangement occurred, from mainly {110} surfaces to mainly {100}/{001} surfaces. The comcrystal, the thermal history of the crystal, and the preparation bination of imaging the nanorods perpendicular and along the method.[6] The allowed angles for domain walls depend on axis (cross-section samples) allows a 3D image of the nanorod the orientations of the polarization allowed by symmetry. In morphology and domain configuration and is therefore very tetragonal perovskites such as BaTiO3 and PbTiO3, the domains useful for determining the domain configuration, especially align with 180° and 90° domain walls. In orthorhombic perwhen combined with PFM. ovskites the polarization orients parallel to [011] and the angles SPM techniques have been used to study the domain conbetween two adjacent polarization vectors can be 60°, 90°, 120°, figuration and polarization direction in BaTiO3 nanorods made or 180°, while in rhombohedrally distorted perovskites, there by molten salt synthesis.[172b,173] Wang et al.[173a] compared the are no 90° domain walls, but instead 71° and 109° walls. amplitude and phase loops obtained by lateral and vertical PFM The ferroelectric domain configuration in 1D nanostructures and observed that the polarization of BaTiO3 nanowires was of perovskites has not been studied in much detail. Most studies oriented along the axis of the nanowires, as hysteresis loops of ferroelectric tetragonal perovskites report that the polarization is oriented along the nanorod axis, but several studies show results that indicate that the polarization can at least partially be perpendicular to the axis.[58e,62,91d,92,121,164,172] When TEM techniques are used for domain imaging, 90° domains are in general easier to distinguish than 180° domains due to the difference in crystallographic directions. As mentioned previously, Schilling et al. used STEM to image 90° domains in BaTiO3 nanocolumns with square cross-sections that were cut from BaTiO3 single crystals using a focused ion beam.[121] They attributed the observed patterns to domain patterns with the polarization sequentially along and perpendicular to the axis and with domain walls oriented 45° to the axis. When the dimensions of the nanocolumns were intentionally varied, they observed changes in the domain pat- Figure 13. STEM images of BaTiO3 nanocolumns and schematic illustrations of the domain configurations. The nanocolumns have the same thickness in the x-direction (200–250 nm). tern that indicate that the nonaxial polariza- When the column width in the y-direction was changed from above 250 nm (left) to below tion component oriented perpendicular to the 200 nm (right) the change in the observed pattern indicates that the non-axial component of shortest dimension in the column to decrease the polarization orients perpendicular to the shortest dimension in the column. Reproduced the depolarization energy (Figure 13).[121b] with permission.[121b] Copyright 2007, American Chemical Society.
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were only obtained by lateral PFM (Figure 14). In a follow-up study, by using the modified setup shown in Figure 9b, they were able to apply the poling bias directly through the length of the BaTiO3 nanowire and observed that the polarization switching was non-remanent.[173b] Their results suggest the existence of a preferred polarization direction in the nanowire. This was explained by a core/shell polarization structure with an unswitchable shell of ≈10 nm, which made the switched core polarizations spontaneously switch back to conform to the surface polarizations. In another study, Berweger et al.[172b] studied similar BaTiO3 nanowires by tip-enhanced Raman spectroscopy and identified domains through selective probing of different transverse optical phonon modes in the system. Their results
indicate spontaneous domain formation with polarization both along and perpendicular to the nanowire axis. An axial setup with control of polarization along the axis has also been used to study the properties of individual KNbO3 nanowires.[174] A multidomain configuration was observed and it was found that only 120° and 180° domains could fulfill the orientation requirements to explain the observed domain walls. Multiple domains have also been observed in KNb0.8Ta0.2O3 nanorods.[42b] It is interesting to note that these I−V nanorods were grown by hydrothermal synthesis at 130–200 °C and consist of multiple domains, while II−IV nanorods grown by hydrothermal synthesis typically are single-domain with polarization along the axis in the as-grown state. Bulk single crystals
Figure 14. a) 3D AFM representation of a BaTiO3 nanowire lying on Au/Pd-coated substrate surface. b,c) Lateral PFM in-field hysteresis phase loop (b) and amplitude loop (c). d,e) Vertical PFM phase loop (d) and amplitude loop (e). All loops were obtained from the same spot on the nanowire. Reproduced with permission.[173a] Copyright 2006, American Institute of Physics.
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size and amount of spontaneous deformation. At the end of the nanorod, the domain configuration is different from the rest of the rod in order to screen the electrostatic stray field.
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of KNb0.8Ta0.2O3 undergo a tetragonal to orthorhombic transition at ≈140 °C;[175] thus it is reasonable that the nanorods grow in the tetragonal phase and go through a phase transformation during cooling that may induce a change in the domain configuration. However, in pure KNbO3 this phase transition occurs at 225 °C and can not directly explain the multidomain configuration in the KNbO3 nanowires. In bulk, both KNbO3 and BaTiO3 undergo phase transitions from the high-temperature cubic phase to tetragonal phase, and further to orthorhombic and finally rhombohedral, with decreasing temperature. Recently, Louis et al.[176] found evidence of an unreported ferroelectric ground state of monoclinic Cm symmetry for nanowires instead of the rhombohedral ground state that is stable in bulk. This monoclinic phase is of interest due to high piezoelectric responses. They also found numerically that the size of the cross-section of nanowires strongly influences the polarization direction in the lowtemperature monoclinic phase, making it possible to optimize the properties of such nanowires. In very small ferroelectric nanoparticles, various calculations have predicted the existence of vortex domains with a toroidal ordering of dipoles.[177] Such a vortex configuration leads to zero net polarization. If one of the dimensions of the nanoparticle is elongated to make a nanorod and further a nanowire, the configuration goes from a single-vortex type to a multivortex type and finally the dipoles orient in one direction.[178] Ponomareva et al. studied the effect of strain and screening of the depolarization field on the polarization distribution in PZT nanowires and found that a multivortex configuration was stable for low screening values and with compressive strain in the direction along the axis and one of the directions perpendicular to the axis.[179] Pilania et al. found evidence of a vortex configuration with the toroidal moment along the axis in BaTiO3 nanowires with TiO2-terminated side walls and 1.6 × 1.6 nm2 square crosssection.[165f ] For PbTiO3 nanowires of similar size, Pilania et al. found that TiO2 termination and compression along the axis favored a vortex polarization state, while PbO termination and tension along the axis favored polarization along the axis.[165j] Ferroelectric nanotubes have also been investigated and simulations show that the domain structures are highly dependent on the compensation of polarization-induced surface charges; a vortex configuration is formed under open-circuit boundary conditions to reduce any net polarization.[180] Besides a possible toroidal ordering of dipoles, the surface charge compensation and side wall terminations also affect the directed ordering of dipoles. Ponomareva et al. demonstrated that for sufficient screening of the polarization field, polarization perpendicular to the axis is the stable configuration in PZT nanowires with a square cross-section of 4.8 × 4.8 nm2.[179] Otherwise, the polarization is oriented along the axis. In thin BaTiO3 nanowires, the polarization field can orient in complex configurations in the nonaxial directions; the manner in which the polarization field orients depends on whether the side walls are TiO2- or BaO-terminated.[165f,181] Slutsker et al. studied the domain configuration in ferroelectric nanorods of different sizes and shapes embedded into a non-ferroelectric film clamped by a substrate.[182] They found that a circuit of electrostatically compatible 180° and 90° domains is formed, but the exact configuration depends on the
4.4. Piezoelectric Properties In very thin nanowires, the piezoelectric coefficient is lower than the bulk value. Zhang et al. observed that from a zero value at 0.4 nm diameter, the piezoelectric coefficient increases gradually with increasing diameter and approaches its counterpart for bulk material when the diameter is larger than 2.4 nm.[183] Thus, as it is very challenging to make such small nanowires, there is no diameter limit for the use of the piezoelectric properties of ferroelectric nanowires in practical devices. To use the direct piezoelectric effect of piezoelectric nanowires in devices, the response to the external mechanical force should be highly sensitive. Simulations of the behavior of a uniaxially pulse-loaded PbTiO3 nanowire have shown that the nanowire can produce an alternating current voltage under a mechanical load of suitable magnitude and frequency.[165d] The response has also been studied experimentally. Wang et al. demonstrated production of a periodic voltage generation from a BaTiO3 nanowire when a periodically varying tensile mechanical strain applied with a precision mechanical stage (Figure 15).[184] The measured voltage generation was found to be proportional to the applied strain rate. Power generation from piezoelectric PZT nanotubes has been demonstrated by Xu and Shi, who recorded the output voltage when a steel nugget was dropped onto an electrode on top of PZT nanotubes embedded in an AAO structure.[88i] The output voltage was shown to increase up to 469 mV when the drop height increased due to the increased impact energy. In another setup, direct piezoelectric potential measurement of a single PZT nanofiber with an effective length of 4 μm was measured under bending using a nanomanipulator inside a scanning electron microscope chamber.[93g] A potential of ≈0.4 mV was generated when a bending moment was applied to the PZT nanofiber by the nanomanipulator tungsten tip.
5. Potential Uses for 1D Nanostructures of Ferroelectric Perovskites Ferroelectrics find three main technological applications based on three related physical characteristics.[185] First, because of their spontaneous electric polarization, they can be used to store binary data by letting the opposite directions of polarization represent the 1 or 0 data bits. Second, ferroelectrics can convert mechanical energy into electrical energy and vice versa via the piezoelectric effect. This makes ferroelectrics useful in various transducer applications such as piezoelectric actuators and sonar detectors. Finally, they have very large dielectric permittivity leading to applications in capacitors. Ferroelectric nanostructures will be important within all these applications. Here, some of the ideas that have been proposed for the use of ferroelectric 1D nanostructures are presented. The roadmap for ferroelectric random access memory (FeRAM) development[186] calls for fully 3D devices by 2017.
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Figure 15. a) Schematic showing the experimental setup for piezoelectric charge detection from an individual BaTiO3 nanowire. b) SEM image of a suspended nanowire under test. c) Acquired output signal from the charge amplifier for a BaTiO3 nanowire under a periodic tensile load. A square-wave voltage signal was applied onto a piezostack in the flexure mechanical stage to deform the nanowire. Reproduced with permission.[184] Copyright 2007, American Chemical Society.
The drive for implementation of more complex 3D structures into FeRAM, instead of the conventional 2D plate capacitor arrangements, is caused by the requirement that FeRAM capacitors must have large enough electrode surfaces to generate sufficient switched charge for the sense amplifiers to reliably discriminate between the 1 and 0 data bits.[121]b] The down-sizing of the current 2D arrangements will result in electrode surfaces too small to accomplish the necessary charge. Schemes for 3D development include both free-standing nanotubes and nanowires[50c,88g] and, perhaps more importantly, ferroelectric nanotubes made by coating electronically conducting positive templates (such as carbon nanotubes[94a,94b] or noblemetal-covered Si or ZnO nanowires[94c,123a]) with a ferroelectric material, as this results in a large electrode surface. Such metal–ferroelectric–metal composite nanotubes are good candidates for 3D capacitors for non-volatile FeRAMs approaching Tb/in2 storage density.
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Most other possible applications of 1D nanostructures of ferroelectric perovskites do not make use of the ferroelectricity itself, but of the related piezoelectric, pyroelectric, and dielectric properties. Pyroelectrics are useful in a variety of imaging and detection applications, and pyroelectric 1D nanostructures could be well suited for sensors. Morozovska et al. have studied the pyroelectric response of arrays of ferroelectric nanowires fixed in a flat capacitor and demonstrated that such devices are candidates for harvesting of electric energy from different heat sources.[187] Piezoelectrics are typically used in electromechanical devices. Piezoelectric nanotubes have, for instance, been presented as candidates for microfluidic delivery systems, making ultrasmall ink-jet printers and nanosyringes possible.[50c] These applications take use of the reverse piezoelectric effect–mechanical deformation of the nanostructures because of an applied electric field. For such devices the attachment of the electrodes for controlling the piezoelectric displacement is a serious challenge.[9b] Applications using the direct piezoelectric effect (production of an electric potential when stress is applied) have been studied more frequently, particularly using piezoelectric 1D nanostructures in energy-harvesting devices that can generate electrical energy. Wang et al.[188] have shown that ZnO nanowire arrays can generate electrical energy when the nanowires are deflected, thus enabling a self-powering nanosystem that harvests its operating energy from the mechanical and vibrational energy of the environment. Considering the relatively low piezoelectric coefficient (d33) of ZnO, it is desirable to use nanowires made of piezoelectric perovskites with higher piezoelectric coefficients, such as BaTiO3 and PZT.[184,189] Several demonstrations of the direct piezoelectric effect in a single nanowire show the possibility of using such piezoelectric perovskite nanowires for energy-harvesting applications.[93g,165d,184] Chen et al.[189] have made an energy-harvesting device based on PZT nanofibers by electrospinning nanofibers onto Pt electrodes on a Si substrate and then depositing a polymer matrix on top (Figure 16). An applied pressure to the surface was transferred to the PZT nanofibers through the polymer matrix and resulted in charge generation because of the combined tensile and bending stresses in the PZT nanofibers. The peak output voltage from this nanogenerator was 1.63 V, and the output power was 0.03 μW with a load resistance of 6 MΩ. The polymer matrix helped to prevent the nanofibers from being damaged, thus extending the life of the generator. In a different setup, Xu et al.[91f ] made a nanogenerator from vertically aligned PZT nanowires on a doped SrTiO3 substrate, with a Pt-coated Si wafer as the electrical contact on top of the nanowire array. With a single nanowire array the nanogenerator produced a peak output voltage of 0.7 V, current density of 4 μA cm−2, and an average power density of 2.8 mW cm−2. Ferroelectric perovskites are photovoltaic and can produce a steady voltage or direct current when illuminated by highenergy light. Zheng et al.[190] have studied the piezophotovoltaic properties of Pb1−xLaxZr1−yTiyO3 nanowires sandwiched between short-circuited metal electrodes and found that the photoelectric current is highly dependent on the applied stress near the para/ferroelectric phase transition. There exist an ultrasensitive stress regime in which a few percent change in the applied stress can be magnified into hundreds of times in
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photomechanical sensors and other nanophotovoltaic devices.[190] An impedance-type humidity sensor has been fabricated based on BaTiO3 nanofibers.[66g] To fabricate the sensor, electrospun nanofibers were formed into a mat on top of a ceramic substrate with electrodes. The sensor exhibited fast response and recovery and was stable over several weeks. The humidity sensing mechanism is based on adsorption of water molecules on the BaTiO3 surface and the high surface–volume ratio is therefore important for the sensing properties. The optical properties of 1D perovskite nanostructures are useful in applications such as photonic crystals and subwavelength optics. The important fundamental property of photonic crystals lies in the photonic bandgaps that emerge in periodic dielectric structures with refractive index contrast. The photonic bandgaps result from the diffraction of electromagnetic waves generated in the periodic dielectric structures, creating standing wave conditions. Periodic dielectric structures with a large refractive index contrast produce photonic bandgaps at limited frequencies, where light of the same frequencies is forbidden to exist inside the perfect photonic crystals.[191] This enables control of photons in a wide range of possible applications in wave-guiding devices. Photonic crystals of BaTiO3 with regular hexagonal arrays of BaTiO3 nanorods have been made by templating and have been shown to exhibit photonic bandgaps in good agreement with calculations.[53] These photonic crystals require perfect periodicity of the dielectric structures, and the template methods are therefore the most likely candidates for the synthesis of such structures. Another novel optical application has been described by Nakayama et al.[192] They used an optically trapped single KNbO3 nanowire as a tunable nonlinear optical probe in a scanning light microscopy setup. The KNbO3 nanowire exhibit efficient second harmonic generation, Figure 16. Concept and power generation mechanism of the PZT nanofiber-based nanogendoubling the frequency of the trapping light erator. a) Schematic view of the PZT nanofiber generator. b) SEM image of the PZT nanofiber mat across the interdigitated electrodes. c) Cross-sectional SEM image of the PZT nanofibers and then waveguiding this locally generated in the polydimethylsiloxane (PDMS) matrix. d) Schematic cross-sectional view of the generator. light to its ends. The large second-order suse) Schematic view explaining the power output mechanism of the PZT nanofibers working in ceptibility χ(2) of KNbO3 nanowires facilitates the longitudinal mode. The color presents the stress level in PDMS due to the application the generation of tunable, coherent visible of pressure on the top surface. Reproduced with permission.[189] Copyright 2010, American radiation that is sufficient for in situ scanChemical Society. ning and fluorescence microscopy. A strong second harmonic generation response has the change in the photocurrent. It is also interesting that by also been observed in LiNbO3 nanowires.[35] Grange et al. also varying the dimensions of the 1D nanostructure this stress rotated a LiNbO3 nanowire using dielectrophoretic forces while regime can be shifted. The piezophotovoltaic effect observed simultaneously monitoring the second harmonic signal and by Zheng et al. is of interest for applications in high-sensitivity observed a signal attenuation corresponding to a rotation of the
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signal. This effect can be used to detect the signal of objects below the diffraction limit of optical systems.[35] In the last decade there has been a significant interest in using terahertz (THz) technology for noninvasive imaging and spectroscopy of materials, for instance for detecting explosives and drugs in security controls.[193] Interestingly, Scott et al.[194] have observed intense THz emission from PZT nanotubes and, as such emission is absent in bulk PZT and PZT films, the effect was attributed to the nanoscale geometry of the nanotubes. The effect is probably related to higher carrier mobility at the surface than in bulk; since the surface–volume ratio is high for nanostructures the effect is only observed in the nanotubes and not in bulk. The THz radiation was emitted within 0.2 ps and the spectrum exhibited a broad peak of 2–8 THz. This emission range supplements the conventional semiconductor THz devices; THz emitters and devices bases on PZT nanostructures via the optical rectification effect can therefore be useful in future devices. Because of the high surface–volume ratio of 1D nanostructures, they should be attractive for catalytic devices. For instance, good photocatalytic activity has been shown for 1D nanostructures of KNbO3,[42e] NaNbO3,[36,41] NaTaO3,[46] SrTiO3,[86] and BiFeO3.[127d] Hong et al.[195] observed direct water splitting when sonication was turned on in water containing 1D micro- and nanostructures of ZnO and BaTiO3. A piezoelectrochemical effect was proposed for the water splitting; when the piezoelectric 1D structures were bent because of vibration of ultrasonic waves a strain-induced electric charge developed on the surface, which triggered the redox reaction of water to produce H2 and O2. Nanostructures of certain perovskites can also be useful as bioactive materials. SrTiO3 nanotube arrays have been demonstrated as a possible strontium delivery platform on Ti-based osteoporotic bone implants.[84] Such arrays can release Sr at slow rate for a long time. In vitro experiments have demonstrated that the SrTiO3 nanotubes can induce precipitation of hydroxyapatite from simulated body fluids, and related cell culture experiments indicated good biocompatibility of the nanotubes. Other applications of 1D nanostructures of perovskites include inclusion in composites and as templates for textured ceramics.[196] For instance, to obtain lead-free materials with piezoelectric and ferroelectric properties corresponding to PZT, templating can be an important tool.[31] As is evident from this overview, the applications and practical use of 1D perovskite nanostructures lie behind the synthesis studies of these structures; however, the coming years will certainly see a flourishing of the application of 1D nanostructures of ferroelectrics.
6. Outlook From the body of work presented in this review, it is clear that research on 1D nanostructures of ferroelectric perovskites has advanced significantly during the past decade. While the focus of research so far has mainly been on synthesis, the focus in the next decade will be more on the piezoelectric and ferroelectric properties of the nanostructures and on applications. Further
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studies of the growth mechanisms are also desired in order to be able to tailor the growth of nanostructures. At the beginning of this review we mainly presented chemical routes to prepare 1D nanostructures, simply because there are relatively few studies using physical and top-down methods. In general, the synthesis of 1D nanostructures of ternary oxides by chemical means is challenging, but compared to physical methods, chemical methods are cost-effective (at least on the laboratory scale) and offer the potential for highvolume production of a variety of materials. The most used chemical methods are deposition in AAO templates, electrospinning, hydrothermal, and molten salt (see Table 1–5). The most common morphology offered by these methods is not the same: the AAO template-based methods generally produce polycrystalline nanotubes, electrospinning produces larger polycrystalline nanowires or nanofibers, hydrothermal methods produce single-crystalline nanowires, and synthesis in molten salt can produce single-crystalline nanorods if a rodlike precursor is used. The simplest method to obtain 1D nanostructures of perovskites is the template-assisted method using a template with channels, such as porous alumina. For the fabrication of polycrystalline nanotubes this method should be preferred for most materials. Advantages are that arrays of nanotubes can be obtained, that up-scaling is relatively easy, and that the dimensions of the 1D nanostructures are controlled by the pore size. By using more advanced template methods, it is also possible to fabricate segmented nanowires.[116] It is also interesting to combine porous templates with hydrothermal processing to obtain single-crystalline nanostructures, as demonstrated for LiFePO4 nanorod arrays,[197] avoiding the calcination step. For fabrication of polycrystalline nanowires or nanofibers, electrospinning is probably the most versatile method. Single-crystalline 1D nanostructures are obtained by thermolysis, the molten salt method or the hydrothermal method. A 1D-shaped precursor can be used to direct the morphology; however, the obtained 1D nanostructures are typically polycrystalline if a polycrystalline precursor is used, such as nanotube arrays. If a single-crystalline precursor with 1D morphology is used, single-crystalline perovskite nanorods and nanowires can be prepared. The most interesting chemical synthesis method from a research perspective appears to be the hydrothermal method. This method typically yields single-crystalline nanostructures and can be used to obtain hierarchical structures and arrays of nanorods on substrates. The chemistry involved to obtain 1D growth can be quite complex and further development calls for further studies of the growth mechanisms. Moreover, growth of 1D nanostructures into more complex structures is important for practical use. Large-scale production is challenging by the hydrothermal method, but promise has been shown by the continuous processes developed for fabrication of 0D nanostructures. The guidelines outlined above are based on the current knowledge on synthesis. As discussed by Beier et al.[198] and Tao et al.[199] for 0D BaTiO3 nanocrystals, in an ideal situation one would be able to synthesize 1D nanostructures of perovskites under ultrabenign and facile conditions on the bench top, i.e., at room temperature, ambient pressure, and near-neutral pH,
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Most of the syntheses routes offer only 1D nanostructures with diameters much larger than the size range where ferroelectric size effects are expected to occur. Thus, the experimental verification of novel ferroelectric phenomena may still occur some years into the future. Understanding the relationship between nanostructure morphology and ferroelectric properties is important for future applications. For instance, Schilling et al. deliberately changed the morphology of BaTiO3 nanowires and observed a change in the domain structure.[121b] It is also important to test the long-term properties of the 1D nanostructures: the mechanical stability, the domain stability, stability in different atmospheres, possible toxicological effects, and so on. To conclude, several steps are still to be climbed before 1D nanostructures of ferroelectric perovskites can be applied in future electronic and electromechanical devices. First, one must be able to control the morphology of the structures over large areas with reproducible synthesis conditions. Second, a detailed characterization of the properties and the long-term stability of these nanostructures must be carried out. Third, the use of the structures in applications must be tested. The combination of fundamental research on new nanostructures and promising applications makes this research field interesting for several years to come.
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using environmentally friendly solvents and precursors. Such a “green chemistry” approach would also be energy-efficient and potentially low-cost. Several studies have shown the possibility of using methods that mimic biomineralization to produce nanostructures. Biofaciliated room-temperature syntheses using biomolecules such as peptides have, for instance, been used to produce BaTiO3 nanocrystals with diameters as small as to 4 nm.[198–200] Low-temperature synthesis methods are in general interesting for ferroelectric perovskites, as the nanostructures then grow in the tetragonal ferroelectric state instead of the cubic paraelectric state. To facilitate the application of chemically synthesized 1D nanostructures it will be necessary to scale up the production volume and to fabricate larger assemblies of 1D nanostructures, either by oriented growth on substrates or by templates or by assembling individual 1D nanostructures into larger assemblies. The challenge of upscaling chemical syntheses is that several of the routes are sensitive to small variations in the synthesis conditions, and modifications of the procedure are often necessary. Oriented growth of perovskite nanorods and nanotubes on substrates has been demonstrated but the achievements lie far behind the development, for instance, of semiconducting nanowires made by the VLS method on various substrates. Direct growth of ferroelectric nanowires on conducting substrates is interesting for applications such as energy-harvesting devices or FeRAM. For the epitaxial growth of nanostructures on substrates, the crystal lattice match or mismatch between the material and the substrate is of course vital, so the substrate material has to be chosen carefully. Directed assembly of individual nanowires is another challenge, which can be obtained by using various forces: molecular forces, electrostatic interactions, shear forces, magnetic fields, and electric fields.[201] For ferroelectric nanowires electric fields are suitable; Grange et al.[35] controlled the alignment of a single LiNbO3 nanowire with an electric field, due to dielectrophoretic response. Beside the synthesis methods based on a solution or a dispersion, there are also possibilities of fabricating 1D nanostructures of perovskites by vapor deposition techniques such as sputtering, molecular beam epitaxy, pulsed laser deposition, and metal-organic chemical vapor deposition. Such methods have rarely been used for the growth of ferroelectric 1D nanostructures, but they are interesting for integration into current semiconductor processes.[76a] Methods that are used for fabricating 0D nanostructures should in many cases also be possible to use for fabrication of 1D nanostructures. Dip-pen lithography has, for instance, been used to produce well-ordered arrays of PbTiO3 nanodots with lateral dimension down to 37 nm.[202] Lithography should in principle allow for the construction of other advanced nanostructures, however with limited height. The importance of studying the ferroelectric and piezoelectric properties of 1D perovskite nanostructures has been addressed. The promising predictions from theory need to be confirmed by experiments and the progress in computational science and theory is important for guidance in experimental science. Inspite of the progress in the science of materials synthesis, it is still very challenging to fabricate extremely small 1D nanostructures (diameter < 10 nm) with a narrow size distribution.
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