Chemical Nanomanipulation of Two-Dimensional Nanosheets and Its Applications

Two-dimensional (2D) nanosheets obtained via exfoliation of layered compounds have attracted intensive research in recent years, opening up new fields in the science and technology of 2D nanomaterials.1-6 These 2D nanosheets, which possess atomic or molecular thickness and infinite planar dimensions, are emerging as important new materials due to their unique properties. Research in such exotic 2D systems recently intensified as a result of emerging progress in graphene (carbon nanosheet)1, 2 and novel functionalities in oxide nanosheets.3-5 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 important and attractive aspects of the exfoliated nanosheets is that various nanostructures can be fabricated using them as 2D building blocks.7-18 It is even possible to tailor superlattice-like assemblies, incorporating into the nanosheet galleries a wide range of materials 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 chapter, we review the current research on oxide nanosheets. Our particular focus is placed on recent progress that has been made in the synthesis and properties of oxide nanosheets, highlighting emerging functionalities in electronic applications.


Introduction
2][3][4][5][6] These 2D nanosheets, which possess atomic or molecular thickness and infinite planar dimensions, are emerging as important new materials due to their unique properties.4][5] 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.][9][10][11][12][13][14][15][16][17][18] It is even possible to tailor superlattice-like assemblies, incorporating into the nanosheet galleries a wide range of materials 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 chapter, we review the current research on oxide nanosheets.Our particular focus is placed on recent progress that has been made in the synthesis and properties of oxide nanosheets, highlighting emerging functionalities in electronic applications.

Synthesis of oxide nanosheets
Various nanosheets based on transition-metal oxides have been synthesized by delaminating the precursor crystals of layered oxide into their elemental layers (Table 1).Chemical exfoliation is the most facile route for making isolation of single layers (oxide nanosheets) separately from thicker layered compounds (Fig. 1).These procedures have attracted much attention as an efficient method for preparing single layers with lateral sizes of up to several micrometers.Pioneering works in this line appeared in the 1990s by Sasaki  19,20 reporting the successful delamination of layered titanates into single titanate nanosheets.Prompted by the findings related to functional oxide nanosheets, several strategies on functional oxides can be found in the literature.
Fig. 1.Schematic illustration for the exfoliation of a layer compound into nanosheets.
In the case of metal oxides, protonation usually resulted in electrostatic repulsions that facilitated exfoliation.By this procedure, single layers of Ti oxides, [19][20][21][22] Mn oxides, 23 Nb/Ta oxides, [24][25][26] Mo oxides, 27 Ru oxides, 28 and W oxides, 29 as well as sheets of several perovskites 7,9,[30][31][32][33][34][35] have been separated from bulk samples (Fig. 2).In these cases, a chemical intercalator that assists the separation of layers and hampers the reassembly of the bulk lamellar material is always required.Tetrabutylammonium (TBA) is the most commonly used intercalator, but also tetrametylammonium and ethylammonium have been used successfully for these purposes.Layered transition-metal oxides such as Cs 0.7 Ti 1.825  0.175 O 4 (: vacancy), K 0.45 MnO 2 , and KCa 2 Nb 3 O 10 can be used as the starting material for the nanosheet. 19,20,23,31 A ommon feature of these host compounds is cationexchange properties involving interlayer alkali metal ions, which are a key to facilitating exfoliation.As the first step to delamination, these layered materials are acid-exchanged into protonated forms such as H 0.7 Ti  These materials have prompted many efforts to elucidate their structural properties.The formation of unilamellar nanosheets was confirmed by direct observation with atomic force microscopy (AFM), x-ray diffraction (XRD), and transmission electron microscopy (TEM). 36- 40Fig. 3 depicts an AFM image for Ti 0.87 O 2 nanosheets.The AFM data clearly reveals a sheetlike morphology, which is inherent to the host layer in the parent compounds.The average thickness was 0.93 ± 0.1 nm.This value is nearly comparable to the crystallographic thickness of the host layer in the corresponding parent compound, supporting the formation of unilamellar nanosheets.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.After tuning the exfoliation conditions by using flux-grown single crystals, the technique provides high-quality nanosheet crystallites up to ~100 µm in size, which is suitable for electronic applications. 21g. 3. AFM image of Ti 087 O 2 nanosheets dispersed on a Si substrate. www.intechopen.com

Chemical nanomanipulation of oxide nanosheets
Oxide nanosheets are an important and promising component for creating new materials.
Oxide nanosheets have a high 2D anisotropy of the crystallites: thickness is ~1 nm wheras lateral size ranges from submicrometers to ~100 µm.In addition, these nanosheets are obtained as negatively charged crystallites that are dispersed in a colloidal suspension.These aspects make the nanosheets a suitable building block 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 polyions such as organic polyelectrolytes, metal complexes, clusters and even oppositely charged nanosheets, which is a major advantage of this approach.Furthermore, control of particulate shape as thin flakes and hollow spheres has been achieved through freeze-or spray-drying techniques.
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 adsorption and Langmuir-Blodgett (LB) procedure are effective for this purpose (Fig. 4).The superlattice approach makes it possible to design complex functions that cannot be achieved using a single material.LB deposition has been proved much simple and effective as another approach for organizing 2D nanosheets [Fig.3][44][45] LB film deposition, the formation of a floating monolayer on water surface in a Langmuir trough followed by an appropriate level of compression, is preferable for achieving dense packing or neat tiling.Through verticaldipping/lifting, the monolayer is deposited onto a flat substrate in LbL fashion.Pioneering work 42 has demonstrated that exfoliated nanosheets could float by adhering to amphiphilic ammonium cations at the air/water interface through electrostatic interaction, and thus the ordinary LB procedure is applicable for fabricating nanosheet films.Although LB 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.The LB-based LbL approach with the use of an atomically flat substrate is effective for fabricating atomically uniform and highly dense nanofilms of oxide nanosheets.Fig. 6 shows a cross-sectional high-resolution TEM image of a 5-layer (7.5 nm thick) Ca 2 Nb 3 O 10 film on a SrRuO 3 substrate. 46The 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 engineering of the clean interface, which appears to be a key step in the design of film properties.Currently, physical depositions such as vapor deposition and laser ablation are 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 film-substrate interface arising from both nonstoichiometry and thermal stress.In that scence, the solution-based 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.

Electronic applications
The development of a wide range of nanosheets with various properties is very important in the design of nanodevices with sophisticated functionality.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.Here, we describe the current status of researches on oxide nanosheets, highlighting emerging functionalities in electronic applications.

Electronic devices
In nanosheets, 2D structures created by lateral confinement can potentially lead to not only the modification of electronic structures but also the modulation of electron-transport phenomena that arise from the quantum confinement effect.Research in such exotic 2D systems recently intensified as a result of emerging progress in graphene and its novel www.intechopen.com Chemical Nanomanipulation of Two-Dimensional Nanosheets and Its Applications 159 functionalities. 1,2,47 I graphene, a number of unique conducting phenomena have already been found, such as anomalous quantum Hall effect, bipolar supercurrent, etc.Despite the similar 2D structural nature, oxide nanosheets are quite different electronically (Table 1).Most oxide nanosheets synthesized to date are d 0 transition metal oxides (with Ti 4+ , Nb 5+ , Ta 5+ , W 6+ ), where the empty d orbitals of metal mix with the filled p orbitals of the ligands. 48Such d 0 materials are not electronically interesting, but semiconducting or insulating materials.In current research on oxide nanosheets, experimental efforts have thus focused on their use as a semiconducting host or dielectric.Ti 0.91 O 2 nanosheets possess semiconducting properties similar to those of bulk TiO 2 , such as rutile and anatase except for some modifications due to size quantization. 49Ti 0.91 O 2 nanosheets generate anodic photocurrent by band gap excitation under light irradiation with wavelengths shorter than 320 nm, corresponding to wider band gap energy of 3.8 eV. 50n contrast, MnO 2 nanosheets have a broad absorption peak centered at 372 nm, which results from d-d transitions in the MnO 2 nanosheets. 13Various interesting and useful properties have also been developed by organizing or assembling these oxide nanosheets into composite materials or multilayer films.Ti 0.91 O 2 nanosheets flocculated with lanthanide cations emitted intense photoluminescence at room temperature through effective energy transfer from the semiconducting nanosheet host. 15,51 ighly stable photoinduced charge separation was attained in a composite film of restacked Ti 0.91 O 2 nanosheets and mesoporous silica or clay minerals, in which electron donors and acceptors are spatially separated at a distance of micrometers.Another enticing possibility is the use of oxide nanosheets in high-κ dielectrics, a key material for future semiconducting technology.[54][55] Fig. 7 summarizes the ε r values for oxide nanosheets and various high-κ oxides.In the ultrathin region (< 20 nm), the ε r values of Ti 0.87 O 2 and Ca 2 Nb 3 O 10 nanosheets are larger than the values reported for any other high-κ materials.It should be noted that the high ε r values of Ti 0.87 O 2 and Ca 2 Nb 3 O 10 nanosheets persist even in the < 10 nm region, which is in sharp contrast to a size-induced dielectric collapse in (Ba 1-x Sr x )TiO 3 . 56,57 Tese results suggest that Ti 0.87 O 2 and Ca 2 Nb 3 O 10 are a very promising candidate for highκ applications such as high-density capacitors and gate dielectrics.
Oxide nanosheets are reported to be an excellent material for electric batteries.In particular, owing to their unique 2D morphology, it is expected that laterally confined 2D nanosheet crystals can significantly enhance the host capabilities of active electrode materials.RuO 2.1 nanosheets showed high performance as electrochemical supercapacitors. 28][60] Multilayer films of MnO 2 nanosheets prepared on ITO substrate also exhibited electrochromic behavior associated with the electrochemical redox process between Mn 3+ and Mn 4+ . 61This electrochromic efficiency is estimated to be 64.2 cm 2 C -1 at 385 nm, which is a relatively high value among manganese oxides.3][64] The idea is to exploit the advantage of oxide nanosheets having high thermal stability even in ultrathin form.Such a technique is expected to have great potential for advances in thin-film technology.

Spin-electronic devices
Recent interest in room-temperature (RT) ferromagnetic semiconductors and lowdimensional magnetic nanostructures (motivated by possible application in spin-electronic devices) has stimulated research in the synthesis and characterization of TiO 2 nanosheet based materials.Titania nanosheets substituted with magnetic elements (Co, Fe, Mn) are ferromagnetic at room temperature. 65The magnetization of Ti 0.8 Co 0.2 O 2 nanosheets is anisotropic due to the 2D nature, and a maximum magnetic moment of 1.4 μ B /Co for H // film is obtained, which is greater than the spin moment of 1μ B /Co theoretically expected for low-spin Co 2+ as well as that in Co-doped anatase with semiconducting (0.3 μ B /Co) and insulating (1.1 μ B /Co) grounds. 65,66 imilar ferromagnetic properties have also been reported in a series of substituted and co-substituted titania nanosheets, including Ti 0.8 Co 0.2 O 2 , Ti 0.6 Fe 0.4 O 2 , 67 Ti 0.8-x/4 Fe x/2 Co 0.2-x/4 O 2 (0  x  0.8) 68 , and Ti (5.2-2x)/6 Mn x/2 O 2 (0  x  0.4) 69 , and Co 1/3 Al 2/3 (OH) 2 .Spin-glass behavior was recently observed in the dried aggregate of tetramethylammonium (TMA)/MnO 2 nanosheets, in which the geometrical frustration was caused by the triangular arrangement of the mixed-valence Mn 4+ /Mn 3+ ions in the MnO 2 layer. 70oncerning applications, ferromagnetic nanosheets have become a pivotal architectural element in magneto-optical (MO) and magneto-electronic devices, because low-dimensional nanostructures make use of the advantage offered by spin-polarized electrons and realize the integration of ferromagnetic materials into nanoelectronics.Indeed, the 2D nature of the electronic state of ferromagnetic nanosheets leads to a gigantic MO response, superior to that of bulk systems (Fig. 8).Multilayer films of Ti 0.8 Co 0.2 O 2 and Ti 0.6 Fe 0.4 O 4 nanosheets exhibited a robust MO effect (~10 4 deg cm -1 ) near the absorption edge at 280 nm, the shortest operating wavelength attained so far. 67More interestingly, alternating stacking (Ti 0.8 Co 0.2 O 2 /Ti 0.6 Fe 0.4 O 2 ) 5 caused a strong enhancement in MO response (~3  10 5 deg cm -1 ) at 400-550 nm, which stems from the interlayer transitions (Co 2+ -Fe 3+ ) between adjacent nanosheets. 67A similar MO response (~210 5 deg cm -1 ) at 400-750 nm was also observed in (Co/Fe)-cosubstituted titania nanosheets, Ti 0.8-x/4 Fe x/2 Co 0.2-x/4 O 2 (x = 0.2, 0.4, 0.6). 68These www.intechopen.com MO materials are also important from a practical viewpoint as a key component for optical isolators in optical communication and data storage devices.In particular, such a large MO response including the blue light region offers potential for short-wavelength MO applications.These ferromagnetic nanosheets are also a model experimental system for future spintronics studies, and their assembly has great potential for the rational design and construction of complex nanodevices, even combined with transparent electronics and molecular devices.
Although we focus here only on MO devices, the assembled structure is naturally viewed as a tunnel junction, which could obviously be used in novel devices such as spin-tunneling switches, spin valves and optical interconnectors.

Conclusion
The current status of research on oxide nanosheets was reviewed.A variety of physical and chemical properties of oxide nanosheets have been developed to functionalize nanosheets for electronic and spin-electronic applications, and further studies will yield new information on their physics.2D nanosheets also teach us how to handle and process 2D nanomaterials and develop nanotechnology in general.Although we have focused here only on high- properties in oxide nanosheets, 2D nanosheets exist in a whole class of functional materials, including metals, semiconductors, ferromagnetic, redox-active, photoluminescence, photochromic etc. 2D nanosheets with having regulated 2D would create the unconventional interactions of electrons as well as the confinements of electrons and ions inside the 2D nanospace or quantum well.Through new chemical design of 2D nanosheets, we can expect new or unprecedented functionalities in the 2D confined system.Furthermore, we can utilize nanosheet-based LbL technology as a new tool to develop advanced fusion functions by promoting the cooperative interaction between organized components, which are difficult to attain with the current synthetic techniques and thin-film technologies.
Oxide nanosheets provide an ideal model to study phenomena in 2D systems.Previously restricted to theoretical study, 2D nanomaterials with their exotic properties are now open to experimentation using the individual 2D system.Graphene has already been found to exhibit a number of unique phenomena such as anomalous quantum Hall effect, bipolar supercurrent, half-metallic, etc.Although current experimental and theoretical efforts mainly focus on graphene, similar properties may be available with oxide nanosheets.We hope that all aspects described here demonstrate the great potential of oxide nanosheets, introducing more exciting physics and wide-ranging applications.

Fig. 4 .Fig. 5 .
Fig. 4. Schematic illustration for chemical nanomanipulation of oxide nanosheets.(a) Electrostatic sequential deposition and (b) Langmuir-Blodgett deposition.Electrostatic sequential deposition is one of the most powerful methods of fabricating nanostructured multilayer films with precisely controlled composition, thickness and architecture on a nanometer scale [Fig.4(a)].This technique, often called "molecular beaker epitaxy", has been first developed by Decher41 and applied to various charged materials.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 polyelectroytes.Polycations such as poly (diallyldimethylammonium chloride) (PDDA) and poly (ethylenimine) are usually used as a counterpart of the oxide nanosheets.Fig.5(a) depicts an

Fig. 6 .
Fig. 6.Cross-sectional high-resolution TEM image of a 5-layer (7.5 nm thick) Ca 2 Nb 3 O 10 film on a SrRuO 3 substrate.Note that the film/substrate interface is atomically flat without an interfacial layer between Ca 2 Nb 3 O 10 and SrRuO 3 substrate.The nanofilms of this quality show an excellent dielectric property as will be discussed in section 4.1.

Fig. 8 .
Fig. 8. (a) Magneto-optical spectra for multilayer assemblies of (Ti 0.8 Co 0.2 O 2 ) 10 and (Ti 0.6 Fe 0.4 O 2 ) 10 .(b) Magneto-optical spectra for (Ti Co 0.2 O 2 /Ti 0.6 Fe 0.4 O 2 ) 5 superlattice and (Ti 0.75 Fe 0.1 Co 0.15 O 2 ) 10 .We used magnetic circular dichroism (MCD) spectroscopy for the characterization of nanosheets.The MCD spectra were measured at RT on the basis of the difference in the absorption of right and left circularly polarized light.1° of MCD corresponds to a 7% difference of optical absorption.