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FEATURE ARTICLE
www.rsc.org/materials | Journal of Materials Chemistry
Exfoliated oxide nanosheets: new solution to nanoelectronics† Minoru Osada*ab and Takayoshi Sasaki*ab
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Received 12th November 2008, Accepted 19th February 2009 First published as an Advance Article on the web 18th March 2009 DOI: 10.1039/b820160a Two-dimensional (2D) nanosheets obtained via exfoliation of layered compounds have attracted intense research in recent years. In particular, the development of exotic 2D systems such as stable graphene and transition-metal oxide nanosheets has sparked new discoveries in condensed matter physics and nanoelectronics. Here, we review the progress made in the synthesis, characterization and properties of oxide nanosheets, highlighting emerging functionalities in electronic and spin-electronic applications. We also present a perspective on the advantages offered by this class of materials for future nanotechnology.
a International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Tsukuba, Ibaraki, 305-0044, Japan. E-mail:
[email protected];
[email protected] b CREST, Japan Science and Technology Agency, Kawaguchi, Saitama, 332-0012, Japan † This paper is part of a Journal of Materials Chemistry theme issue on Layered Materials. Guest editors: Leonardo Marchese and Heloise O. Pastore.
double hydroxides,19 producing positively charged nanosheets in contrast to the majority of polyanionic nanosheets derived from cation-exchangeable layered materials. These 2D nanosheets, which possess nanoscale dimensions only in thickness and have infinite length in the plane, are emerging as important new materials due to their unique properties. Research into such exotic 2D systems recently intensified as a result of emerging progress in graphene (carbon nanosheet)20,21 and novel functionalities in oxide nanosheets.2–4,8–18,22–31 In particular, oxide nanosheets are exceptionally rich in both structural diversity and electronic properties, with potential application in areas ranging from catalysis to electronics. Now, by using the exfoliation approach, it is possible to investigate dozens of different 2D oxide nanosheets in search of new phenomena and applications. One of the most important and attractive aspects of the exfoliated nanosheets is that various nanostructures can be fabricated using them as 2D building blocks.32–38 It is even possible to tailor superlattice-like assemblies, incorporating into the nanosheet galleries a wide range of materials39–45 such as organic molecules, polymers, and inorganic and metal nanoparticles. Sophisticated functionalities or nanodevices may be designed through the selection of nanosheets and combining materials, and precise control over their arrangement at the molecular scale. In this context, many projected applications in
Minoru Osada received his PhD degree in Materials Science from Tokyo Institute of Technology in 1998. He is now senior researcher in International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Tsukuba, Japan. His current research interests focus on physical properties of oxide nanosheets.
Takayoshi Sasaki received his PhD degree in Chemistry from the University of Tokyo in 1985. He is now principle investigator in International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, Tsukuba, Japan, and Professor at the University of Tsukuba. His group is one of the pioneers in oxide nanosheet research.
1. Introduction The delamination of layered compounds is attracting an increasing amount of attention, opening up new fields in the science and technology of two-dimensional (2D) nanomaterials.1,2 The resulting individual layers can be regarded as a new class of nanoscale materials, referred to as ‘‘nanosheets’’ due to their 2D morphology.3,4 Delamination is reported for various layered compounds having cation-exchange properties: smectite clay minerals,5 chalcogenides,6 metal phosphates7 and oxides.8 In particular, the successful delamination of layered perovskites,8–12 titanates3,4,13,14 and manganese oxides15–18 has triggered keen interest in the nanosheets because of their attractive functionalities. These layered materials are exfoliated into colloidal polyanionic nanosheets, typically by intercalation of bulky guests such as quaternary ammonium ions. More recently, attention has focused on the delamination of layered
Minoru Osada
This journal is ª The Royal Society of Chemistry 2009
Takayoshi Sasaki
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current nanodevices would involve the use of 2D structures by forming quantum wells, multilayers, superlattices or heterostructures, and much of the impetus for research has been the enormous diversity afforded by 2D electron gas and/or spinpolarized electrons, allowing unique electrical and magnetic effects.46,47 In particular, due to their true 2D nature with semiconducting characteristics, oxide nanosheets have become a pivotal architectural element as a new channel material in fieldeffect transistors and molecular electronics. An alternative route to nanosheet-based electronics is to consider the nanosheet as a dielectric layer. The idea is to exploit the fact that, unlike other high-k materials, dielectric nanosheets are stable down to true nanometer sizes.48 These features may open a possibility to fabricate flexible transparent optoelectronic devices, in which everything including conducting channels, gate dielectrics and conductive sheets can be made from oxide nanosheets. Apart from these practical viewpoints, a fundamental understanding of electronic structures in oxide nanosheets is also important for physics in low-dimensional systems, because the 2D quantum confinement may result in intriguing properties that differ from those of bulk systems. Indeed, novel physical and chemical properties, such as quantum confinement or surface effects, were revealed in these nanosheets, and are associated with the unusual 2D structural feature. Table 1 Reported materials of oxide nanosheets Ti oxide3,4,13,14
Mn oxide15–18 Ti-Nb oxide22 Ti-Ta oxide22 Nb oxide23,24 Ta oxide25 Layered perovskite8–12,26–29
Ru oxide30 W oxide31
Ti0.91O2, Ti0.87O2, Ti3O7, Ti4O9, Ti5O11 Ti1xCoxO2 (x # 0.2), Ti1xFexO2 (x # 0.4), Ti1xMnxO2 (x # 0.4), Ti0.8x/4Fex/2 Co0.2x/4O2 (x ¼ 0.2, 0.4, 0.6) MnO2, Mn3O7, Mn1xCoxO2 (x # 0.4), Mn1xFexO2 (x # 0.2) TiNbO5, Ti2NbO7 TiTaO5 Nb3O8, Nb6O17 TaO3 LaNb2O7, La0.90Eu0.05Nb2O7, Eu0.56Ta2O7, SrTa2O7, Bi2SrTa2O9, Ca2Nb3O10, Sr2Nb3O10, NaCaTa3O10, CaLaNb2TiO10, La2Ti2NbO10, Ba5Ta4O15, W2O7 RuO2.1 Cs4W11O36
In this feature article, we review the current status of research on oxide nanosheets. Particular focus is placed on recent progress that has been made in the synthesis, characterization and properties of oxide nanosheets, highlighting emerging functionalities in electronic and spin-electronic applications.
2. Synthesis and structure of oxide nanosheets Various nanosheets based on transition-metal oxides have been synthesized by delaminating the precursor crystals of layered oxides into their elemental layers (Table 1, Fig. 1).2–4,8–18,22–31 The most well-established method of synthesizing oxide nanosheets is the intercalation reaction with bulky guest species such as tetrabutylammonium (TBA) ions.3,4,8–10,17 In this approach, layered transition-metal oxides such as Cs0.7Ti1.825,0.175O4 (,: vacancy),3,4 K0.45MnO217 and KCa2Nb3O108–10 can be used as the starting material for the nanosheet. A common feature of these host compounds is cation-exchange properties involving interlayer alkali metal ions, which are a key to facilitating exfoliation.3,4,8–10,17 As the first step to delamination, these layered materials are acid-exchanged into protonated forms such as H0.7Ti1.825,0.175O4$H2O, H0.13MnO2$0.7H2O and HCa2Nb3O10$ 1.5H2O, in which the interlayer alkali metal ions can be completely removed under suitable conditions while maintaining the layered structure.3,4,8–10,17 The resulting protonic oxides are subsequently delaminated through reaction with a solution containing TBA ions, producing turbid colloidal suspensions of Ti0.91O2, MnO2 and Ca2Nb3O10 nanosheets.3,4,8–10,17 Such an exfoliation process is quite general: exfoliation of the other layered host compounds proceeds in a similar fashion.3,4,8–18,22–31 These materials have prompted many efforts to elucidate their structural properties.4,12,17 The formation of unilamellar nanosheets was confirmed by direct observation with atomic force microscopy (AFM) and transmission electron microscopy (TEM). Fig. 2 depicts AFM images for Ti0.91O2, MnO2 and Ca2Nb3O10 nanosheets. The AFM data clearly reveal a sheet-like morphology, which is inherent to the host layer in the parent compounds. The average thickness and the standard deviation were 0.93 0.06 nm for Ti0.91O2, 0.74 0.10 nm for MnO2 and 1.84 0.10 nm for Ca2Nb3O10. The values obtained are nearly
Fig. 1 Representative structures of selected oxide nanosheets. (a) MnO2, (b) Ti1dO2 (d ¼ Ti deficiencies), (c) TiNbO5, (d) Ca2Nb3O10, (e) Cs4W11O36.
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Fig. 2 AFM images of Ti0.91O2, MnO2 and Ca2Nb3O10 nanosheets. A tapping-mode AFM (SII nanotech E-Sweep) in vacuum conditions was used to evaluate the morphology of the nanosheets on Si substrates. Height profiles are shown in the bottom panels.
comparable to the crystallographic thickness of the host layer in the corresponding parent compounds, supporting the formation of unilamellar nanosheets.4,12,17 On the other hand, the lateral size depends on the choice of starting materials. For nanosheets derived from polycrystalline powder samples, the lateral size ranges from submicrometers to several tens of micrometers.3,4,9,10,17 After tuning the exfoliation conditions by using fluxgrown single crystals, the technique provides high-quality nanosheet crystallites up to 100 mm in size,49 which are suitable for electronic applications. The structural features have been characterized by means of TEM electron diffraction,50–52 in-plane X-ray diffraction53,54 and X-ray absorption fine-structure spectroscopy.55 In the case of Ti0.91O2, for example, all techniques yield the picture of a 2D crystal that is basically a double layer of octahedrally coordinated Ti cations periodically arranged along the sheet plane [Fig. 1(b)], with a rectangular unit cell of 0.38 0.30 nm. This arrangement is very similar to the structure of individual lamellae in the stacked parent titanate, whose orthorhombic 3D unit cell is of the lepidocrocite type. First-principles calculations confirm the experimentally derived structure.56
this approach. Furthermore, control of particulate shape as thin flakes and hollow spheres has been achieved through freeze- or spray-drying techniques.58,59 One of the highlights is the fabrication of nanocomposite films of organic polymer/nanosheet materials that exhibit useful properties. Several groups have demonstrated that the electrostatic LbL self-assembly via sequential adsorption32–38 and Langmuir–Blodgett (LB) procedure60 are effective for this purpose. Sequential LbL assembly, often called ‘‘molecular beaker epitaxy’’, is one of the most powerful methods of fabricating nanostructured multilayer films with precisely controlled
3. Materials synthesis using oxide nanosheets Oxide nanosheets are an important and promising component for creating new materials. Oxide nanosheets have an extremely high 2D anisotropy of the crystallites: thickness is 1 nm while lateral size ranges from submicrometers to 100 mm. In addition, these nanosheets are obtained as negatively charged crystallites that are dispersed in a colloidal suspension. These aspects make the nanosheets suitable building blocks for designing nanostructured films. In practice, colloidal nanosheets can be organized into various nanostructures or combined with a range of foreign materials at the nanometer scale by applying wet-process synthetic techniques involving flocculation and layer-by-layer (LbL) self-assembly. Through these processes, oxide nanosheets can be combined with a wide range of polyions39–45,57 such as organic polyelectrolytes, metal complexes, clusters and even oppositely charged nanosheets, which is a major advantage of This journal is ª The Royal Society of Chemistry 2009
Fig. 3 UV-visible absorption spectra in the multilayer buildup processes for (PDDA/Ti0.91O2)10 (a) and (PDDA/Ti0.91O2/PDDA/MnO2)10 (b). The insets indicate the designed stacked structures of the nanosheets.
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composition, thickness and architecture on a nanometer scale. In this LbL process, a multilayer assembly can be built up by alternately dipping the substrate in a colloidal suspension of nanosheets and an aqueous solution of suitable polyelectrolytes. Polycations such as poly(diallyldimethylammonium chloride) (PDDA) and poly(ethyleneimine) (PEI) are usually used as a counterpart of the oxide nanosheets.32–38 Fig. 3(a) depicts an example of the multilayer film of (PDDA/Ti0.91O2)10 on a quartz glass substrate, showing UV-visible absorption spectra in the fabrication process. The absorption peak at 265 nm, attributable to the Ti0.91O2 nanosheets, was progressively enhanced as the number of deposition cycles increased, clearly indicating the repeated adsorption of nearly equal amounts of nanosheets.36,37 XRD data provide important evidence for the formation of multilayer films by the evolution of Bragg peaks and their progressive enhancement.36,37 The diffraction peaks are attributable to a repeating nanostructure of inorganic nanosheet and organic polymer. Other characterizations by ellipsometry, FT-IR and AFM all support the growth of multilayer nanocomposite films.37 Multilayer films of other nanosheets were fabricated by similar procedures. Such LbL assembly of various nanosheets also allows us to tailor superlattices or heterostructures by tuning the number of nanosheets and their stacking sequences. Fig. 3(b) shows UVvisible absorption spectra for the superlattice assembly composed of MnO2 and Ti0.91O2 nanosheets.61 The observed spectral changes clearly indicate that the films grew as designed. The superlattice approach makes it possible to design complex functions that cannot be achieved using a single material. Multilayer films of similar quality can be synthesized via the LB procedure.60 Although this technique has been used for decades, its application for nanoparticles and nanorods is often frustrated by defects ranging from pinholes to larger reorganization of the layers. In the case of nanosheets, the LB technique provides nearly perfect mono- and multilayer films with atomically flat surfaces. Recent studies on high-k dielectric Ti0.87O2 nanosheets have also demonstrated that the LB-based LbL approach with the use of an atomically flat substrate is effective for fabricating atomically uniform and highly dense nanofilms.62 Fig. 4 shows a cross-sectional high-resolution TEM image of
Fig. 4 Cross-sectional high-resolution TEM image of a 10-layer (9.4 nm thick) Ti0.87O2 film on a SrRuO3 substrate. LB-based LbL assembly of Ti0.87O2 nanosheets is effective for room-temperature fabrication of oxide nanofilms with a well-ordered lamellar structure. Note that the film/substrate interface is atomically flat without an interfacial layer between (Ti0.87O2)n and SrRuO3 substrate. The nanofilms of this quality show excellent dielectric properties as will be discussed in section 4.1.
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a 10-layer (9.4 nm thick) Ti0.87O2 film on a SrRuO3 substrate. The image clearly reveals a stacking structure corresponding to the LbL assembly of nanosheets. Such LB-deposited nanofilms are very suitable for a number of applications in electronic devices. A clear benefit of these LbL approaches is the interface engineering, which appears to be a key step in the design of film properties. Physical methods such as vapor deposition and laser ablation are currently the main methods of fabricating oxide films. These techniques, however, usually require a complex and difficult deposition process involving high-temperature postannealing (>600 C), which can cause degradation in the filmsubstrate interface arising from both nonstoichiometry and thermal stress.63 The bottom-up fabrication using oxide nanosheets provides new opportunities for room-temperature fabrication of oxide nanoelectronics, while eliminating integration problems encountered in current film-growth techniques.
4. Functionalities in electronic and spin-electronic applications The development of a wide range of nanosheets with various properties is very important in the design of nanodevices with sophisticated functionalities. Currently, extensive effort is being made to develop oxide nanosheets with new physical and chemical properties. The range of applications of nanoassemblies could therefore be widened significantly (Table 2). Here, we describe the current status of research on oxide nanosheets, highlighting emerging functionalities in electronic and spinelectronic applications. 4.1 Electronic properties In nanosheets, 2D structures created by lateral confinement can potentially lead to not only the modification of electronic Table 2 Classification of physical properties in oxide nanosheets Materials
Physical properties
Ref.
MnO2, RuO2.1
Metallic [or semimetallic] Semiconducting [or insulating (dielectric)]
15–18,30
Ferromagnetic
83–86
Electrochromic Photochromic Photoluminescent
15–18 31 26–28
Ti0.91O2, Ti0.87O2, Ti3O7, Ti4O9, Ti5O11 TiNbO5, Ti2NbO7, TiTaO5, Nb3O8, Nb6O17, TaO3, LaNb2O7, Ca2Nb3O10, Sr2Nb3O10, Ba5Ta4O15, W2O7 Ti1xCoxO2 (x # 0.2), Ti1xFexO2 (x # 0.4), Ti1xMnxO2 (x # 0.4), Ti0.8x/4Fex/2Co0.2x/4O2 (x ¼ 0.2, 0.4, 0.6) MnO2 Cs4W11O36 La0.90Eu0.05Nb2O7, Eu0.56Ta2O7, SrTa2O7, Bi2SrTa2O9
3,4,13,14, 22–29,48
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structures but also the modulation of electron-transport phenomena that arise from the quantum confinement effect. Research into such exotic 2D systems recently intensified as a result of emerging progress in graphene20,21 and its novel functionalities. In graphene, a number of unique conducting phenomena have already been found, such as anomalous quantum Hall effect, bipolar supercurrent, etc.20,21 Despite the similar 2D structural nature, oxide nanosheets are quite different electronically (Table 2). Most oxide nanosheets synthesized to date are d0 transition metal oxides (with Ti4+, Nb5+, Ta5+, W6+), where the empty d orbitals of the metal mix with the filled p orbitals of the ligands. Such d0 oxide nanosheets are not electronically interesting, but are useful as semiconducting or insulating materials. A few exceptions include MnO2 and RuO2.1 nanosheets which are either redox-active or semimetallic.15,30 Current research on oxide nanosheets has thus focused on their use as a semiconducting host or a dielectric nanoblock. As concerns semiconducting properties, titania nanosheets have attracted prime attention because of the marked similarities between nanosheets and bulk TiO2 systems and possibilities to create various photonic functionalities such as photocatalysis,64,65 photoconductivity66 and photoluminescence.26–28 Ti0.91O2 nanosheets possess semiconducting properties similar to those of bulk TiO2, such as rutile and anatase, except for some modifications due to size quantization.66 Ti0.91O2 nanosheets generate anodic photocurrent upon ultraviolet irradiation with wavelengths shorter than 320 nm, corresponding to a wider band gap energy of 3.8 eV.67 Similar semiconducting properties have also been reported in other d0 oxide nanosheets with Ti4+, Nb5+, Ta5+, W6+ (see Table 2).13,14,22–29,31 Another enticing possibility is the use of titania nanosheets in high-k dielectrics. Most importantly, titania nanosheets consist only of highly polarizable TiO6 octahedra (Fig. 1b), the key building blocks of Ti-based dielectrics, which makes an ideal base for high-k dielectrics with a critical thickness. Such a materials design in nanodielectrics is a challenge, but recent development of high-k Ti0.87O2 nanosheets is an encouraging step towards new high-k dielectrics using nanosheets.48 Fig. 5 summarizes the maximum values of 3r for various high-k oxides. Ti0.87O2 nanosheets exhibit both low leakage current density (
