36

37

Optical Properties of Nanoscale Transition Metal Oxides

5.1 Physical, Chemical, and Size-Shape Tunability in Transition Metal Oxides

The interplay between charge, structure, and magnetism is the origin of rich physics in complex oxides. Because these interactions are so strong, oxides are “on the knife’s edge”, straddling several competing regions of phase space. [1-8] These competing interactions give rise to very rich H-T-P phase diagrams, often with exotic electronic and magnetic ground states that are the result of delicately balanced charge-spin-lattice coupling. Signatures of this coupling include, for instance, inhomogeneous spin and charge texture and coupled excitations like electromagnons, magnon sidebands, and electron-phonon interactions. [9-14] An important consequence of the aforementioned phase proximity is the physical property tunability. Small external perturbations can change important energy and length scales, driving the system into a new state with very different properties. Here, the Goodenough-Kanamori-Anderson rules are often used to understand the influence of charge, structure, and orbital overlap on magnetism. [15] Occasionally, one discovers the opportunity to drive completely new functionality by tuning across phase boundaries. For instance, temperature can induce metal-insulator, chargeand magneticordering, or superconducting transitions. [16–24] Likewise, high magnetic fields, pressure, and electric fields can be used to drive superconducting, density wave, ferromagnetic, and other exotic magnetic transitions. [25–38]

Chemical substitution is another powerful method for tuning the properties of a material. The copper oxide superconductors are a classic example. [21, 23] In these doped antiferromagnets, oxygen stoichiometry controls the carrier concentration. [39] Chemical substitution also gives rise to exotic phase diagrams in the manganites. [3, 9] Here, rare earth doping drives the system through a series of ground states whose properties depend upon the charge (and spin) of the Mn centers and coupling to the lattice. Chemical substitution can also be used to manipulate the properties of ferroelectric perovskites such as Pb(Zr1-xTix)O3. [40-42] According to recent theoretical predictions, a magnetic ion placed on the perovskite “A site” may activate ferroelectrically-induced ferromagnetism. [43, 44] The ability of chemical substitution to manipulate properties is, of course, not limited to oxides or extended solids. For instance, molecular hydrogen-bonded systems [45–53] and molecular magnets [54–62] employ subtle chemical substitutions to control architecture, bonding, and reactivity, often with the use of electron donating or withdrawing substituents as structure directing agents. Photomagnetic metal-cyanide compounds [63–72] also display rich chemical tunability.

The discovery that complex solids can form nanoscale objects provides an exciting opportunity to investigate bulk versus nanoscale chemistry using molecular-level strain as the tuning parameter. [73–78] At very small length scales, surface states and strain, defect states, local structure, local composition, and finite size effects dominate the properties. Some of the beautiful, flexible, and functional nanomorphologies include tubes, wires, octahedra, particles, tetrahedra, and spheres. [79–114] These systems demonstrate the promise of molecular-level control of size and shape, and hold out the potential for control of confinement and crossover related effects. Flagship materials include organics such as carbon nanotubes and graphene, and inorganics such as gold, CdSe, ZnO, PtFe, and MoS2. [77, 79, 82, 90, 93, 108, 109, 115–117] Here, quantum confinement influences transport, magnetic, optical, and mechanical properties. [79, 105, 106, 118–123] Interestingly, inorganic nanomaterials have many of the same scientific challenges as their bulk counterparts with regard to superconductivity and magnetism, mixing of spin-lattice-charge-orbital degrees of freedom, unusual phase transitions, microand nanoscopic “texture”, and strain.[124–130] As a consequence of intrinsic physical and chemical interactions, competition between different states in nanoscale solids remains an area with challenging scientific opportunities. [131]

The goal of this review is to illustrate how optical spectroscopy can reveal fundamental aspects of nanoscale transition metal oxides. Just as physical and chemical tuning are at the heart of our ability to design, understand, and control advanced materials in their bulk form, size-shape tuning is of emerging importance in the field of functional oxide nanomaterials. This is because it (i) expands the usable structure-property phase space and (ii) it can drive emergent phenomena. And in the same way that optical spectroscopy reveals the dynamics of bulk materials, it is poised to do so in nanoscale compounds. I selected several physical systems that illustrate these opportunities, placing an emphasis on chemically simple materials where both bulk and nanoscale properties have been investigated. In making these selections, I carefully considered the definition of a nanomaterial and elected to take a broad view encompassing nanoscale texture in bulk materials, thin films oxides, more traditional nanoscale objects such as particles and tubes, as well as chemical aspects of binding in porous materials. In addition to advancing the fundamental understanding of novel materials, basic research on the optical properties of nanomaterials can lead to practical applications [66, 132–156] particularly in energy-related technologies.

5.2 Optical Spectroscopy as a Probe of Complex Oxides

Optical spectroscopy is a sensitive microscopic probe of various physical phenomena in complex solids. [157, 158] A common target of an infrared investigation is the dipole-allowed vibrational modes, typically characterized as intramolecular and intermolecular features for a molecular material, or collective phonons and lattice modes for a more extended solid. [159–161] Vibrational spectroscopy is an important complement to neutron scattering for the investigation of complex materials. Molecular group theory, correlation group, and double group methods are employed to assess symmetry properties of a material. [162, 163] There is a natural connection between vibrational spectra and molecular dynamics theory because such calculations provide resonance frequencies, displacement patterns, and symmetry. [107, 164–167] From a combined analysis of vibrational mode trends and displacement patterns, it is possible to assess local lattice distortions, the local chemical environment, and charge ordering patterns. [161, 168–171] Rattling, soft, and collective modes are other types of low-energy excitations and can contribute to ionic conductivity, negative thermal expansion, thermoelectric response, density wave and ferroelectric transitions, and superconductivity. [138, 172–174]

Electronic effects can also be investigated by optical techniques. These include the mobile carrier response, localization, confinement effects, charge ordering/disproportionation, dimensional behavior (such as van Hove singularities and Luttinger liquid behavior), charge transfer, color properties and crystal field splitting, effective mass, oscillator strength, and the nature of the optical gap, just to name a few. [107, 164, 171, 175–196] Many of these effects occur within 200 meV (∼1600 cm-1) of the Fermi surface, making them sensitive to physical and chemical modifications. For certain features, electronic structure calculations (and partial densities of states information) offer insight into the microscopic origins of observed electronic excitations, gaps, and impurity levels; calculated geometry and magnetic properties are also of interest. [101, 102, 104, 107, 197–210] A variety of interaction mechanisms lift traditional spectroscopic selection rules. For instance, vibronic coupling connects many well-known degrees of freedom with appropriate symmetry in molecular solids, [18, 211] activates crystal field excitations, [196] and gives nanoscale texture in superconducting cuprates. [212] Microscopic strain, local structural modifications, intermolecular interactions and bonding, and the charge environment each affect the vibrational response via local symmetry considerations. [26, 121, 162, 213, 214] Low-energy magnetic excitations can also be spectroscopically probed due to the lifting of traditional selection rules. [215–218] For instance, spin-orbit coupling can activate the singlet-triplet gap, [217, 219–221] and spin-phonon coupling has been of contemporary interest in molecular magnets222, 223 and frustrated systems. [224]

Clearly, optical spectroscopy is a flexible and microscopic probe of the dispersive and lossy response of a material. [157, 158] Considering the developing evidence for nanoscale analogs of most functional oxides, optical spectroscopy of these systems is a rich and wide-open area of research, and it uniquely provides microscopic information that complements various bulk properties studies where different length and time scales are important. Combining optical spectroscopy with physical and chemical tuning techniques makes it even more powerful. [225] Characteristic energy scales for temperature, magnetic field, pressure, and electric field effects are summarized in Table 5.1. It is clearly of great interest to evaluate how nanoscale strain compares (and competes) with characteristic temperature, magnetic field, pressure, and electric field ranges.

Table 5.1 Characteristic energy scales probed by optical spectroscopy compared with typical temperature, magnetic field, pressure, and electric field scales

Low Energy

Medium Energy

High Energy

30 cm-1 (1 THz)

100 3000 cm-1 (12 meV-0.4 eV)

0.5-20 eV

10 K

100-1000 K

104-105 K

1 T

50 T

1000 T

1 kbar

25 kbar

1 Mbar

1 mV/cm

100 mV/cm-1 V/cm

1000 kV/cm

Magnetic excitations, lattice modes, free carrier processes

Vibrational modes, low energy charge transfer excitations

Localized electronic, charge transfer excitations

5.3 Quantitative Models

As alluded to above, optical spectroscopy is a technique that is blessed with an excellent fundamental connection to theory. This is because the matrix element of an allowed excitation is, at its heart, simply a measurement of the joint density of states with a particular moment operator. As a result, traditional electronic structure theory, lattice dynamics calculations, and group theory are excellent tools with which to understand the electronic excitations, vibrational properties, symmetry, and selection rules. These approaches work well in e.g. polar oxide thin films and carbon nanotubes where periodic boundary conditions, crystal symmetries, and helical symmetries can be invoked. That said, one is often presented with issues (e.g. a size-dependent band gap, the need to account for inhomogeneities, or the need to compare the wavelength of light with the length scale of a nanostructure) that do not regularly arise with a single crystalline sample. Cases where local structure and symmetry are important (e.g. at an interface or at the uncompensated edge of ananoparticle) are also less amenable to standard approaches, although they still represent a reasonable place to start an analysis. Sometimes, quantitative methods of bulk structures fail completely to describe the relevant science. In this case, other models must be sought to quantify finite length scale effects. Several prominent approaches are discussed below.

5.3.1 Confinement Models

One example of a framework that was developed to capture quantum size effects is the confinement model. These models take on a variety of forms depending upon their intended application, as described in Yoffe’s excellent review article. [226] All involve foundational assumptions regarding particle size and the characteristic size of the exciton. Kayanuma [227] defines three different regimes that depend upon the R/a*B ratio (Fig. 5.1). Here, R is the radius of the quantum well (or particle) and a*B is the excitation Bohr radius. This is a sliding scale that depends essentially upon the chemical nature of the material (which determines the exciton Bohr radius) and the characteristic size of the nano-scale object (which depends upon synthetic techniques). From the point of view of dramatically different or emergent properties, the strong confinement regime is the most interesting.

Fig. 1 Sliding scale of confinement for a spherical electron-hole system that depends upon the radius of the quantum well (R) and the exciton Bohr radius (a∗B ).[227] When R/a∗B >> 1, confinement is weak and the character of the exciton remains quasi-particle-like. The exciton looses its quasi-particle character in the strong confinement regime where R/a∗B << 1

Brus presented a useful analytical expression for the first excited electronic state of a nanoparticle [228] that is relevant to the strong confinement regime in Fig. 1. This model is based on the effective mass approximation and was extended by Kayanuma [227] and others. [229–232] Here,

EMBED Equation.DSMT4

222

*

,,

2**

111.8

()0.248

2R

gapnanogapbulkRyd

eh

he

EEE

Rmm

p

=++--

ò

.(5.1)

The second particle-in-a-box-like term in this expression describes quantum localization effects and acts to increase the energy of the first excited state. It has the practical effect of blue-shifting electronic excitations from their position in the bulk material. The third term describes Coulomb interactions and acts to shift Egap,nano to lower energy as R−1 . Because of the competition between these terms, the apparent band gap will always increase for small R but show variations at larger R depending on the importance of various terms. The fourth term describes binding energy effects and acts to decrease the apparent gap when E*Ryd is large. Egap,nano is predicted to asymptotically approach Egap,bulk with increasing particle size (Fig. 2), [228] an effect that has been observed in a wide variety of materials.[226] TiO2 and ZnO nanomaterials provide a classic illustration of spectral blue shifts due to quantum confinement and are discussed in later sections of this review. [233, 234] CdO and Bi2O3 nanoparticles display similar trends. [235,236] Although these and other materials display spectral blue shifts due to confinement, it is worth noting that there are several emerging cases where opposite effects are observed. Another interesting prediction of these models is a confinement-induced redistribution of oscillator strength. [227, 232] This modification of the optical properties derives from the breakdown of translational invariance. In the exciton confinement regime (large R/a*B), oscillator strength grows as R3, whereas oscillator strength asymptotically approaches an enhanced value in the individual particle confinement regime. [227]

Fig. 2 Calculated energy of the cluster lowest excited electronic state in relation to the bulk band gap. [228]

Confinement effects also manifest themselves in phase transition behavior. This is because phase transitions are intrinsically long-range, cooperative phenomena that are governed by an appropriate correlation length. [237] Interesting things happen when sample size is tuned through the length scale of the important correlation. For instance, in triglycine sulphate, the ferroelectric polarization and Curie temperature are reduced with decreasing film thickness as ΔT = Tc(∞)−Tc(D)~1/D where D is the film thickness. [238, 239] The superconducting transition temperature depends on thickness in α-MoGe thin films as well. [240] In model ferroelectrics like PbTiO3 and BaTiO3 , x-ray diffraction and Raman scattering were combined to correlate the reduced ferroelectric transition temperature vs. nanoparticle size with soft mode trends and local structure distortions. [241–243] In PbTiO3, the E(1TO) soft mode frequency decreases with particle size as ∼1/(D - Dcrit ), where D is particle diameter and Dcrit is the critical particle diameter. [241, 243] Even in the absence of microscopic models for these trends, there are clearly many opportunities to understand phonon confinement and its relation to phase transitions in functional oxide nanomaterials, particularly in systems where the particle size distribution is under good control.

5.3.2 Descriptions of Inhomogeneous Media

The spectroscopic response of inhomogeneous media is an area that benefits from the well-developed Maxwell-Garnett and Bruggeman Effective Medium Approximation models. [244–246] These models were traditionally applied to mixtures and compounds, as described by Carr et. al in their excellent 1985 review article. [247] They are now finding application in nanoscale materials. The Maxwell-Garnett and Bruggeman Effective Medium Approximation models are both based upon the assumption that particle (or domain) size is much larger than the wavelength of light used in the spectroscopic analysis. This approach allows us to consider inhomogeneous media to be homogeneous on the length scale of the light and to possess a well-defined effective dielectric response function. The models differ mainly in the way in which the medium surrounding the grain under consideration is treated whether it is surrounded by one of the mixture constituents or by a medium characterized by averaged properties. [247] The Maxwell-Garnett model is most appropriate for dilute systems, where the gains of the minority constituent are well separated. [244, 245] We can write the effective dielectric response, typically labeled εMGT as

3(

(1)(3

ab

MGTbb

abb

f

f

-)

=+

--)+

òò

òòò

òòò

,

(5.2)

where ϵa is the dielectric function of the grain in question, b is the dielectric function of the surrounding medium, and f is the filling factor. Two issues that immediately arise are (i) the symmetric nature of this expression and (ii) the continuous variation of the effective dielectric response of the mixture, ϵMGT. The first issue can be finessed with a focus on well-separated minority grains in a surrounding matrix. The second issue is more physical and relates to the lack of a predicted percolation threshold, a limitation that can be traced back to the assumption that embedded grains are not in contact. That said, the Maxwell-Garnett model has been widely employed to understand stained glass, metallic particles like Al, Au, and Ag, granular superconductors like Sn and Pb, and carbon nanotubes (with an appropriate depolarization factor). [244, 245, 248–251] The Effective Medium Approximation provides an alternate framework for the description of inhomogeneous media. [246] In a two-component system with spherical grains, ϵa is the dielectric response of the first component (present with volume fraction f), and ϵb is the dielectric response of the second component (present with volume fraction (1-f)). In this model, each individual grain is considered to be surrounded by a homogeneous host that possesses the average dielectric properties of the medium. Within the Effective Medium theory, the effective response of a two-component system with spherical grains is given as

(1)

22

aEMAbEMA

aEMAbEMA

ff

--

+-

++

òòòò

òòòò

.

(5.3)

Equation 5.3 can be extended to induced a depolarization factor that depends on the shape of a grain or component. This approach differs from the Maxwell-Garnett theory in that (i) it is not restricted to a particular range of concentration and (ii) it predicts a metal-insulator transition at a critical volume fraction of ⅓. Overall, the Effective Medium Approximation gives a superior description of the optical properties for most systems. It has been used to understand the optical properties of small Ag grains, α-MoGe thin film superconductors in magnetic field, and the temperature-driven insulator-metal transition in VO2. [124, 252–254] In the α-MoGe superconductor, the effective medium approximation provides a good fit to changes in transmittance ratios with magnetic field, from which the authors find that the gap shrinks due to field-assisted pair breaking. [253] The superconducting fraction also decreases linearly with magnetic field.

The optical properties of nanocomposites can be complicated by many other factors including particle-particle interactions and scattering effects. [255–257] These effects are important in a variety of dense media such as coatings, paints, and device applications. For larger particles (particle size ≥ λ), the effective medium approximation breaks down and both absorption and scattering must be taken into account. [258] Both dependent and independent scattering models can be used to describe these effects, depending upon particle size, loading, and agglomeration effects.

5.3.3 Inhomogeneous Media and Surface Plasmons

A surface plasmon is a natural, collective oscillation of the electron gas inside of a metallic nanomaterial that propagates parallel to the surface. [259] It is different than the bulk plasma frequency for a material, ωp. [157] The basic problem is solved by considering a metallic sphere with an incident electromagnetic wave with electric field E0e−iωt. This incident wave activates the collective excitation. Writing the internal electric field as a function of the Drude dielectric response of the metallic sphere, the host dielectric, and the applied electric field, one extracts a surface plasmon resonance frequency of

0

2

p

w

w

=

+

ò

.

(5.4)

Within the effective medium approximation, the surface plasmon peak develops naturally above the percolation threshold. This electric dipole-allowed excitation displays a characteristic peak in the absorption spectrum that often determines the color properties of a material. It dominates the optical properties of a traditional system like Ag nanoparticles embedded in an insulating matrix. [260] It is easy to understand why the plasmon resonance frequency is sensitive to quantum size effects. As indicated in Eqn. 5.3, the resonance frequency depends on the dielectric function of the nanomaterial (which determines ωp) and the dielectric properties of the host. The former is very sensitive to size effects, making it an incisive probe of confinement in nanomaterials. To the extent that shape affects the complex dielectric properties of a material, it will also influence the plasmon resonance frequency. Studies of shape and aspect-ratio effects in silver nanoparticles indicate important changes to both absorption profiles and scattering cross-sections that emanate from the way electric field (and charge) are concentrated on areas of high curvature or corners (Fig. 5.3). [257, 261–264] Nanoshell structures offer electric field enhancements (and “hot volume” effects) for similar reasons. [265]

Fig. 3 E-field enhancement contours external to the Ag trigonal prism, for a plane that is perpendicular to the trigonal axis and that passes midway through the prism. The light is chosen to have k along the trigonal axis and E along the abscissa. Left: 770 nm. Right: 460 nm. Side length = 100 nm, and thickness = 16 nm. [261]

5.3.4 Charge and Bonding Models

Quantitative methods to evaluate charge and bonding in nanomaterials have also attracted attention. [162,196] Here, Born’s original formalism was extended in a powerful way to include powdered samples - an approach that is very useful for nanomaterials. Born effective charge (Z*B) can be calculated mode by mode from a knowledge of oscillator strength (Sj) and the transverse optic phonon frequency (ωTO,j) as

*2

2

222

,

0

()

4

Bk

TOjj

jk

k

Z

Nc

cS

Vm

pw

=

åå

ò

.

(5.5)

With appropriate density and orientation corrections, this rendering is well-suited to the analysis of nanomaterials and was recently employed to evaluate the Born effective charge for both 2H-MoS2 and analogous nanoparticles (Fig. 4). [162] In MoS2, the intralayer Born effective charge of the nanoparticles is decreased significantly compared to the layered bulk. Of course, Born effective charge describes the static and dynamic polarizations. [196] Local charge (Z*) and polarizability (α) provide a way to separate these effects. They can be extracted as

*

*

1

B

Z

Z

N

n

V

a

=

-

(5.6)

and

()1

1

N

V

N

n

V

a

a

¥=+

-

ò

.

(5.7)

In the MoS2 nanoparticles, the significant decrease in the intralayer Born effective charge was attributed to structural strain and the resulting change in polarizability in the nanoparticles. [162] Thus far, this method has been applied only to bulk and nanoscale MoS2, but it can be extended to oxides, where it is anticipated that confinement will also modify ionicity.

5.4 Charge-Structure-Function Relationships in Model Nanoscale Materials

The remainder of this article focuses on charge-structure-function relations in nanomaterials using a set of case studies selected for chemical simplicity as well as variety. Simple model systems are most amenable to quantitative approaches, although there is still a lot to learn from the theoretical perspective about confinement, local strain, charge and bonding, excitons, the role of defects, and how the spectroscopic signatures of these features vary with size.

Fig. 4 (a) Crystal structure [266] and displacement patterns of infrared active E1u and A2u vibrational modes [162,267] in 2H-MoS2. These modes are sensitive to charge and bonding in the intralayer and interlayer directions, respectively. (b) Close-up view of the 300 K reflectance spectra of bulk and nanoscale MoS2 along with oscillator fits. [162] The inset shows a high-resolution TEM image of the nanoparticles. (c) Born effective charge, local charge, and polarizability of 2H- and IF-MoS2 in the two principle directions and a schematic view of the intralayer electron clouds where spheres indicate a generalized orbital. [162]

Another advantage of model systems is that they generally form the chemical basis for a much wider class of materials. As a result, studies of finite length scale effects in chemically simple model compounds have wide applicability. Several physical systems are of interest including (i) the Mott-Hubbard compound VO2 , (ii) La1/2Sr3/2MnO4 in high magnetic field, (iii) pristine and chemically-substituted vanadium oxide nanoscrolls, (iv) quantum size effects in ZnO and TiO2 , (v) polar oxide thin films and nanoparticles based upon BiFeO3, and (vi) H2 binding in metal-organic frameworks. Discussion will focus on the consequences of nanometer sized physical length scales in these materials on functionality.

5.4.1 Mott Transition in VO2 Revealed by Infrared Spectroscopy

Vanadium dioxide (VO2) is a chemically simple transition metal oxide that is at the heart of the correlated electron vs. band structure/Peierls instability debate. [268, 269] At issue is whether electron correlations or electron-phonon interactions dominate the physics, particularly very near a insulator-metal transition. The general facts have been clear for some time. VO2 is a monoclinic insulator at low temperature. It displays an insulator-to-metal transition at ~342.5 K, above which VO2 is tetragonal and metallic. The infrared properties of VO2 thin films in the low and high temperature phases are consistent with this picture, with an 0.5 eV gap in the insulating range and a broad Drude-like feature in the metallic regime. [124,254,270]

Fig. 5 (a) Optical conductivity of VO2 as a function of frequency for various temperatures. (b) Images of near-field scanning amplitude over a 4 × μm area showing the insulating and metallic domain structure and how it develops through the transition regime. (c) Optical conductivity of the metallic domains extracted from a modified Effective Medium analysis. (d and e) The relaxation rate and effective mass vs. temperature. [124]

The optical response through the insulator-metal transition is of special interest. Here, the gap fills gradually (borrowing strength from various high energy electronic excitations [270]), and there is an isosbestic point in the optical conductivity (Fig. 5(a)) that is seen in other Mott systems, which in chemical kinetics, is indicative of a simple, straightforward mechanism. Combining these far-field infrared results with near-field infrared spectroscopy offers a way forward. [124,254] The near-field results, obtained with scanning near-field infrared microscopy in tapping mode, provide detailed information on scattering amplitudes of the insulating and metallic components and their relative spatial positioning. This visualization (and the subsequent analysis) is made possible because of the excellent dielectric contrast between the insulating and metallic phases at the probe frequency (at 930 cm−1) and the 10 - 20 nm linear system resolution. Images of the near field scattering amplitude clearly show the development of metallic domain structure with increasing temperature in the transition regime (Fig. 5(b)). [124,254] At the onset of the transition, these metallic islands are embedded in an insulating matrix. They increase in size and begin to connect with increasing temperature. The observed coexistence of metallic and insulating regimes in the transition regime demonstrates the percolative nature of the insulator-to-metal transition. As the metallic islands grow, their optical properties evolve into those of the high temperature rutile phase of VO2. The transition is complete by 360 K, as evidenced by the lack of contrast in the scanning images. The metallic puddles in the transition regime are small (≤1 μm2) below the percolation threshold, and they have several unusual properties, as described below. Combining the far- and near-field infrared data with a modified Effective Medium approach, the authors extracted the optical conductivity of the metallic puddles. [124,254] The latter shows a very narrow Drude-like feature along with a pinned electronic band centered at 0.25 eV, quite different than the spectrum of the fully metallic high temperature state. The metallic puddles also display a divergent effective carrier mass and a pseudo-gap-like structure in the relaxation rate (Fig. 1.5(c)). Both signal the importance of electron correlation. The diverging effective mass in particular is an unambiguous signature of the Mott transition. [271] This indicates that while lattice effects may play a role in the temperature-driven insulator-to-metal transition, VO2 is primarily a correlated electron material. This experimental approach will be very useful in addressing questions regarding charge dynamics in other inhomogeneous correlated oxides like cuprates and manganites.

5.4.2 Visualizing Charge and Orbitally Ordered Domains in La1/2Sr3/2MnO4 at High Magnetic Fields

Interest in micro- and nanoscale texture in high magnetic fields is also driving new instrumentation. An important example is a recently developed polarizing microscope equipped with a variable temperature cryostat, pulsed magnet (to 35 T), and high speed camera. [272] This system captures domain structure images and their changes with magnetic field, essentially representing a fusion of magneto-optical techniques and spatially resolved imaging. This setup was recently used to visualize and elucidate domain structure changes through the field-induced collapse of the charge and orbitally ordered state in the mixed valent manganite La1/2Sr3/2MnO4 (Fig. 6). [272] In these images, the charge ordered state is birefringent and appears as a bright area under crossed polarizers. Local intensity is thus proportional to the order parameter of the transition from the charge and orbitally ordered state to the quenched state, although the functional form is not yet clear. The dark image in Fig. 6(d) at full field demonstrates that the charge and orbital ordering is disordered, at least over the length scales probed in these measurements (4.8 micrometer/pixel). Clearly, development and widespread use of this instrumentation will enable major contributions to our understanding of magneto-optical effects in complex transition metal oxides, especially as the image resolution improves. The magneto-structural transitions recently reviewed in Ref. 273 may provide especially interesting opportunities.

Fig. 6 Polarizing microscope images in the ab-plane of La1/2Sr3/2MnO4 at 180 K in magnetic fields of (a) 0 T, (b) 20 T, (c) 26 T, and (d) 27 T. (e) Averaged intensity of a part of the polarizing microscope images [marked by red squares in (a)-(d)] as a function of applied field. The data at each temperature are normalized so as to reproduce the intensity at each temperature in zero field [272]

5.4.3 Discovery of Bound Carrier Excitation in Metal Exchanged Vanadium Oxide Nanoscrolls and Size Dependence of the Equatorial Stretching Modes

The discovery that low-dimensional inorganic solids can curve or fold into nanoscale objects provides an exciting opportunity to investigate bulk versus nanoscale chemistry using molecular-level strain as the tuning parameter. [73,76,77] Among the transition metal oxides, vanadates show particularly rich chemistry due to the tunable oxidation state and flexible coordination environment, which ranges from octahedral to square pyramidal to tetrahedral with increasing vanadium oxidation state. [274] They also display open framework structures, making them prospective materials for ion intercalation, exchange, and storage. [111,112,275,276] One nanoscale system of interest is mixed-valent (amine)yVOx with x ∼ 2.4. These compounds are actually nanoscrolls, consisting of vanadium oxide layers between which organic molecules are intercalated. [111,112,277–279] The latter controls sheet distance –essentially the scroll winding. The optical response of mixed-valent vanadate nanoscrolls and their metal exchanged derivatives was investigated in order to understand the charge dynamics in these compounds (Fig. 7). Unexpectedly, a bound carrier excitation was observed in the metal exchanged nanoscrolls, rather than the predicted free carrier response. [140] This excitation is localized near 400 cm−1 (0.05 eV) at all temperatures and yields an extrapolated dc conductivity of ∼1 Ω−1 cm−1, suggesting that the Mn2+ exchanged nanoscrolls are weakly metallic in their bulk form. [280] Detailed analysis suggests that this excitation is localized due to a combination of inhomogeneous charge disproportionation, electron-electron correlation, and chemical disorder, motivating us to propose an alternate band filling picture that accounts for these effects (Fig. 7(c)). [280] Resistivity and thermopower measurements find a similar energy required to propagate charge by polaronic mechanisms (0.09 eV). [281] The higher energy electronic structure of the (amine)yVOx materials consists of an 0.56 eV charge gap at 300 K that does not depend on scroll size, superimposed V4+ d → d and V4+ → V5+ transitions centered at ~1 eV, and a series of charge transfer excitations above 3.2 eV. [280, 282] The feature near 5 eV depends on scroll size, suggesting that the excitation is polarized preferentially in the equatorial direction.

Fig. 7 (a) Structural information on (C12H25NH3)-VOx scrolls. (b) 300 K optical conductivity of pristine VOx scrolls and the Mn exchanged compound in the far-infrared regime. The dotted line guides the eye, highlighting the additional bound carrier contribution in the substituted scrolls. (c) Schematic representation of possible electronic structure changes emanating from the ion exchange process in the scrolled vanadates, showing the effects of additional carriers, electron-electron interactions, and chemical disorder. (d) Expanded view of the 300 K optical conductivity of pristine VOx scrolls and the Mn exchanged compound [280]

The vibrational properties of the (amine)yVOx scrolls also display a sheet distance dependence. In the pristine (amine)yVOx materials, the equatorial V-O-V stretching modes are particularly sensitive to sheet curvature, blueshifting and broadening with increasing sheet distance. [282] The blue shift is a typical quantum size effect, and the broadening and peak asymmetry at small scroll sizes emanate from k ≠ 0 mode contributions to the response. Axially-directed modes are much less sensitive to the microscopic manifestations of strain. The 113 cm−1 mode, assigned as screw-like motion of the VOx tubes in analogy to the radial breathing mode in carbon nanotubes, is a unique feature of these nanoscrolls. [282] Analysis of vibrational structures in the metal substituted scrolls indicated that ion exchange modifies local charge and symmetry. The 575 cm−1 V-O-V equatorial stretching mode is especially sensitive to ion substitution. [280]

5.4.4 Classic Test Cases: Quantum Size Effects in ZnO and TiO2

The photophysical response of the II-VI compound semiconductor ZnO provides a classic example of confinement effects. [232,283,284] Since the Bohr radius of ZnO is ≈2 nm, small nanoscale objects are the most interesting (Fig. 1). [232] Here, the direct gap is determined by the exciton. [283,284] It blue shifts from a room temperature value of ~400 nm in the bulk to 340 and 325 nm in 9.3 and 6.1 nm particles, respectively – a clear signature of quantum size effects. [283] The exciton binding energy is ~60 meV, making ZnO useful for short wavelength optical device applications. Light emission in nanoscale ZnO has been extensively investigated. [233,285–290] Figure 8 displays a comparison of the photoluminescence for ZnO in thin film and nanoparticle form. [233] Importantly, light emission is observed at significantly higher energies in the nanoparticles. This blue shift is attributed to the quantum confinement effect. Kim et. al. estimate the band gap enhancement in nanoscale ZnO using the second term of Eqn. 5.1 along with appropriate electron and hole masses. [233] Other authors include the Coulomb contribution [286] (third term in Eqn. 5.1) and the exciton binding contribution [285] (the fourth term in Eqn. 5.1). Even these modified expressions predict exciton shifts that are somewhat larger than observed in experiment, [233,285,286] a discrepancy that may be related to the actual barrier potential in ZnO as well as assumptions about the importance of Coulomb interactions in the model. The additional width of the nanoparticle spectrum in Fig. 8 is attributed to size distribution effects. [233,285,286] In the future, ZnO nanostructures with novel shapes, engineered defects, and different chemical doping [291–297] may allow quantitative testing of models related to size dependence of shallow and deep trapped states. [228]

Fig. 8 Photoluminescence spectra of the ZnO thin film and quantum dots grown on the SiO2/Si (111) substrate at 550 ◦C measured at 10 K. (a) ZnO thin film grown for 60 min and (b) ZnO quantum dots grown for 90 s. The free exciton peak at 3.377 eV is labeled “EX”. It corresponds to the 10 K bulk bandgap. [233]

Nanoscale TiO2 also provides a classic illustration of quantum size effects. [234,298–300] Rutile and anatase forms of TiO2 have indirect band gaps at 3.0 and 3.2 eV, respectively, followed by direct gaps at 3.3 and 3.4 eV, respectively. [301–303] The recent synthetic breakthrough by Satoh et. al. involving the use of dendrimers to control nanoparticle size and distribution enabled very important studies of confinement and growth in both the rutile and anatase forms. [234] Particle sizes below 2 nm are again most interesting for confinement studies, and loss of translational invariance at small sizes seems to convert an indirect transition to a direct one. The spectral blueshift with decreasing size is clearly observed for both crystalline forms of TiO2 (Fig. 9), and the authors fit their data with a modified version of the Brus equation [228,229] (Eqn. 5.1) that better accounts for finite well depth effects. [234] Clearly, fundamental understanding of band gap trends will impact photocatalytic and solar cell applications. [304,305] Analysis of small cluster trends combined with modeling studies is revealing distinctions in the early stage growth mechanism of rutile vs. anatase morphologies as well. [234]

Fig. 1.9 Bandgap measurements for 6TiO2 (green), 14TiO2 (red) and 30TiO2 (blue). Optical wave guide spectra for hydrolysed (rutile) TiO2 (a) and thermolysed (anatase) TiO2. (b) The insets shows Tauc plots for the hydrolysed and thermolysed TiO2, respectively. From the relation (αhν)0.5 ~ (hν Eg), we can obtain Eg from the Tauc plots for the indirect bandgap. Taking into account the lowest direct bandgap, we obtained the regression lines in the regions 4.00 - 4.26, 3.60 - 3.94 and 3.47 - 3.78 eV for the hydrolysed 6TiO2, 14TiO2 and 30TiO2 and 3.88 - 4.10, 3.67 - 3.92 and 3.44 - 3.77 eV for the thermolysed 6TiO2, 14TiO2 and 30TiO2, respectively (estimated error in energies ≤0.02 eV, coefficients of determination ≥95%). [234]

5.4.5 Optical Properties of Polar Oxide Thin Films and Nanoparticles

Iron-based compounds and mixed metal oxides are promising materials with which to harvest solar energy. [306] One system that has attracted attention in this regard is BiFeO3, currently the only single phase room temperature multiferroic. Important predictions from first principles electronic structure calculations include strong transition metal-oxygen hybridization, a stereochemically-active Bi lone pair that mixes Bi and O states, and insulating behavior in calculations that employ strong exchange correlation. [307–310] The optical properties of BiFeO3 and related systems are of great current interest for understanding the nature and size of the band gap as well as chemical bonding and hybridization. [196,311] The preparation of high-quality thin films with precisely controlled oxygen stoichiometry, chemical substitution, and epitaxial strain (to stabilize non-equilibrium phases) allows us to compare predictions from first principles electronic structure calculations with optical property measurements. Recent reports of photoconductivity, [311] a photovoltaic effect, [312] and a 0.8-0.9 V open circuit voltage in a working ferroelectric solar device [313] illustrate the potential of polar oxides as active photovoltaic materials. [311–313] The light harvesting efficiency in BiFeO3 thin films is, however, still limited by a variety of factors including carrier lifetimes, recombination rates, and matching the optical band gap an overall electronic structure with the solar spectrum. Optical spectroscopy can support the development of new thin film polar oxides for solar devices by tackling the band gap problem. It is, of course, worth remembering that oxides are attractive because they are cheap, abundant, and stable, and their properties can be tuned by chemical substitution. Among the oxides, ferroelectrics present an alternative pathway to charge separation of photoexcited carriers potentially eliminating the need for a p-n junction. The latest BiFeO3 - based all-oxide heterostructures are characterized by open-circuit voltages greater than 0.8 - 0.9 eV and quantum efficiencies up to 10% when illuminated with light of energies above the bandgap. [313] This efficiency is at least an order of magnitude larger than the maximum efficiency under sunlight (AM 1.5) thus far reported for ferroelectric thin film devices. [313] To extend the impact of this discovery and possibly provide a new approach to solar energy conversion, it is desirable to tune the bandgap and electronic excitations to coincide with the 550 - 700 nm peak in the solar spectrum and to take advantage of the long near infrared tail. This approach to light harvesting is complementary to many others such as catalytically-driven H2O-splitting reactions, dyesensitized cells, and molecular electronics-based assemblies. [314] The discovery that BiFeO3 also forms nanoparticles [315–317] provides additional avenues for exploration.

The optical properties of rhombohedral BiFeO3 (Fig. 10(a)) are well documented. [196,311,322,323] The 300 K direct charge gap is at 2.67 eV. [324] It is preceded by a small shoulder centered at ~2.5 eV, which yields an absorption onset near 2.2 eV. Peaks at 3.2 and 4.5 eV are dipole-allowed charge transfer excitations. [196,311,322,323] Our efforts to tune the band gap have focused on (i) tetragonal BiFeO3 stabilized via a compressive strain from the substrate (Fig. 10(b)), (ii) a series of BiFeO3 nanoparticles (Fig. 1.10(c)), and (iii) a series of alloys with Mn substitution, Bi(Fe,Mn)O3 , including the end member BiMnO3 (Fig. 10(d)). [318–320] Taken together, these systems offer an excellent test of band gap tuning methodologies, previewing how chemical substitution and nanoscale strain can impact the match with the solar spectrum, and by corollary, energy conversion efficiencies of a device. The optical properties of tetragonal and nanoscale BiFeO3 allow us to evaluate the potential of strain as a tuning parameter. The absorption spectrum of the tetragonal film is overall blue-shifted compared with that of the rhombohedral material. [318] It shows an absorption onset near 2.25 eV, a direct 3.1 eV bandgap, and charge transfer excitations that are ~0.4 eV higher than those of the rhombohedral counterpart (Fig. 10(b)). In the nanoparticles, the band gap decreases from 2.7 eV to ~2.3 eV, and the well-known 3.2 and 4.5 eV charge transfer excitations split into multiplets (Fig. 10(c)). [319] Again, the band gap is defined by the charge transfer edge. The spectral redshift with decreasing particle size is different from normal confinement-induced blue shifts [228] (Eqn. 5.1) but in line with recent results on BiFeO3 micro-cubes, [325] indicative of important Coulomb interactions in BiFeO3. These results can be understood in terms of structure, strain, and symmetry breaking. Focusing on the onset to optical absorption and the direct gap analysis via an (αE)2 vs. E plot in Fig. 10(d), we find that Mn substitution yields an overall redshift of the oscillator strength and a 1.1 eV charge gap in the BiMnO3 end member. [320,321] This gap emanates from strongly hybridized O 2p + Mn 3d → Mn 3d charge transfer excitations. Taken together, this data demonstrates the potential of band gap tuning to improve polar oxide-based ferroelectric solar cell efficiency. Given the recent report of a piezoelectric response in strained BiFeO3, [326] control of the strain-driven morphotropic phase boundary may also give rise to electro-optical effects.

Fig. 10 (a) 300 K optical absorption of a rhombohedral BiFeO3 thin film compared with the solar spectrum. [196, 311] (b) Response of a tetragonal phase BiFeO3 film compared with that of the rhombohedral material. [318] Here, strain blue-shifts the gap and the electronic excitations. (c) Optical properties of 31 nm nanoparticles showing a red-shifted gap compared with the rhombohedral film. [319] (d) 300 K optical absorption spectrum of BiMnO3, the end member in a series of alloy films, compared with BiFeO3. [320,321] The Mn-substituted material displays a lower band gap

5.4.6 Spectroscopic Determination of H2 Binding Sites and Energies in Metal-Organic Framework Materials

Local charge and structure govern the properties of many complex materials including superconductors, thermoelectrics, metamaterials, and multiferroics. [280,327–331] Charge and structure are also intimately connected to functionality in porous materials like metal-organic framework (MOF) compounds that have nanomater-sized pores, channels, and active sites. [78,332] Here, absorption, binding, and chemical reactivity are the functionalities of interest, and they have direct application to hydrogen storage, carbon/nitrogen sequestration, and catalysis. [333–340] Infrared vibrational spectroscopy is well-suited to analyze the chemical and physical aspects of small molecules absorbed on high surface area materials like metal-organic frameworks because, as a microscopic technique, it can provide information on the relative energies of different binding sites in these materials, especially when combined with first principles calculations.

Pore size, shape, and local charge seem to determine much of the hydrogen binding chemistry in metal-organic framework materials. [341] Corners in the framework structure or exposed metal coordination sites within the pore structure concentrate charge, change the local acid/base character, and present convenient attachment sites. Monte Carlo calculations predict optimum pore sizes to be ~7 and 10 Å at 300 and 77 K, respectively. [342] From the bulk properties point of view, H2 has weak surface interactions at 300 K, so it is hard to get good high temperature performance in metal-organic frameworks. This is because thermal energy exceeds binding energy. Reasonable bulk hydrogen uptake data can, however, be obtained at 77 K. [343,344] In order to advance hydrogen storage applications, it clearly is desirable to measure, tune, and control the H2 binding energy.

Fig. 11 Dependence of the equilibrium constant for absorption, on 1/T for H2 adsorbed on MOF-5 sites labeled 1, 2, and 3 in the inset. Adsorption enthalpies (ΔH) calculated from slope lines are 7.4 ± 0.2 kJ/mol for sites 1 and 2 and 3.5 ± 0.1 kJ/mol for site 3. Inset: Infrared spectra of H2 adsorbed on MOF-5 as a function of decreasing temperature (upper curve 30 K and 0.019 bar equilibrium pressure; lower curve 110 K and 0.035 bar equilibrium pressure). [345]

MOF-5 is one system for which the uptake and binding site problem is reasonably well understood. [343,345] The structure of MOF-5 consists of a cubic lattice of Zn4O tetrahedra connected by 1,4-benzenedicarboxylate groups. [346] The zinc centers are fully coordinated and unexposed. In their work, Bordiga et. al exploit the fact that interaction of the surface with a hydrogen molecule breaks local symmetry, perturbs the charge density, and activates the symmetric stretching mode of H2. Interaction with the surface also induces a frequency shift that can be related to the local charge, structure, and accessibility of the binding site. Because the frequency shift increases with increasing interaction energy, different absorbing sites yield distinct infrared features. [345] Figure 11 displays a close up view of the infrared absorption response of MOF-5 as a function of H2 uptake. [345] With increasing uptake, sharp new bands appear in the 4110 - 4150 cm−1 range, direct evidence for at least three distinct binding sites. [345] Interaction energies can be determined from an equilibrium analysis with the van’t Hoff equation. [345] Here,

2

()

[1

ads

H

T

K

P

q

q

=

-(T)]

,

(5.8)

where θ(T) is the fraction of sites covered by H2 (determined from the relative infrared intensity), PH2 is the loading pressure, and Kads is the equilibrium constant for the absorption process. The slope of lnKabs vs. 1/T gives the absorption energy. These dependencies are plotted in Fig. 11 for the active sites in MOF-5. [345] The strongest H2 binding (3.5 ± 0.1 kJ/mol) occurs at the corners of the pores, at the carboxylate O2− centers behind which lie the Zn2+ ions. [345]

Metal-organic framework materials with exposed metal sites are providing a way forward with improved bulk H2 uptake results and significantly larger H2 ... metal binding energies. [344,346,347] For instance, a site-specific absorption energy was recently determined for CPO-27-Ni, a metal-organic framework system with accessible Ni2+ sites. Vibrational spectroscopy reveals an interaction energy of 13.5 kJ/mol. [346] Clearly, exposed metal sites are poised to play a very important role in the design of new metal-organic framework materials. Charge variation on the unsaturated metal site may provide both control of the H2 binding energy and mechanistic insight. At the same time, structures that contain a higher density of absorption sites may offer additional opportunities to boost H2 uptake. [346]

5.5 Summary and Outlook

Finite length scale effects are a major new theme for transition metal oxides. They present compelling opportunities for discovery-class science and new areas of phase space within which to search for exciting physical properties. Optical spectroscopy is well-positioned to capitalize on these opportunities. This is because it is a versatile and sensitive microscopic probe that can easily be combined with various physical tuning techniques. Certainly, one goal of this review is to advance the idea that size-shape effects are on par with temperature, magnetic field, pressure, and electric field in their potential to tune complex materials. At the same time, optical spectroscopy connects easily with existing models and can be used to build upon and test new theoretical approaches. This review takes the case study approach, highlighting recent findings in several prototypical transition metal oxides including (i) the MottHubbard compound VO2, (ii) La1/2Sr3/2MnO4 in high magnetic field, (iii) pristine and chemically-substituted vanadium oxide nanoscrolls, (iv) quantum size effects in ZnO and TiO2, (v) polar oxide thin films and nanoparticles based upon BiFeO3, and (vi) H2 binding in metal-organic frameworks. In these examples, efforts were made to bring simple chemical systems together with appropriate models of insulator-to-metal transitions, confinement, local strain, charge and bonding, excitons, and the role of defects for high level physical understanding. Discussion focused on the consequences of quantum size effects on properties and functionality. There are clearly many opportunities for spectroscopists to advance the field of nanoscience. Investigations that emphasize systematic structure-property correlations or combine optical spectroscopy with physical tuning techniques and mechanistic models will be particularly valuable.

Acknowledgements

JLM thanks the Materials Science Division, Basic Energy Sciences, U.S. Department of Energy and the Joint Directed Research and Development Program at the University of Tennessee for support of this work.

References

1. Rao, C.N.R., Raveau, B.: Transition metal oxides, VCH Publishers, New York, (1995)

2. Hwu, S.-J.: Structurally confined transition-metal oxide layers, chains and oligomers in molecular and extended magnetic solids. Chem. Mater. 10(10), 2846–2859 (1998)

3. Tokura, Y. (ed.): Colossal magnetoresistive oxides, Gordon and Breach, New York (2000)

4. Julien, C., Pereira-Ramos, J.P., Momchilov, A. (eds.): New trends in intercalation compounds for energy storage, NATO Science Series II: Mathematics, Physics, and Chemistry, Vol. 61. Kluwer Academic Publishers, (2002)

5. Grüner, G.: The dynamics of charge density waves, Rev. Mod. Phys. 60, 1129–1181 (1988)

6. Greenblatt, M.: Monophosphate tungsten bronzes: a new family of low-dimensional, charge-density-wave oxides. Acc. Chem. Res. 29, 219–228 (1996)

7. Kahn, O: Molecular magnetism. VCH Publishers, New York, (1993)

8. Canadell, E.: Dimensionality and Fermi surface of low-dimensional metals. Chem. Mater. 10, 2770–2786 (1998)

9. Dagotto, E.: Nanoscale phase separation and colossal magneto-resistance, Springer Series on Solid State Sciences. Springer, Berlin, (2003)

10. Cuk, T., Lu, H.H., Zhou, X.J., Shen, Z.-X., Devereaux, T.P., Nagosa, N.: A review of electron-phonon coupling seen in the high-Tc superconductors by angle-resolved photoemission studies. Phys. Status Solidi (b) 242, 11–29 (2005)

11. Pimenov, A., Rudolf, R., Mayr, F., Loidl, A., Mukhin, A.A., Balbashov, A.M.: Coupling of phonons and electromagnons in GdMnO3. Phys. Rev. B 74, 100403(R) (2006)

12. Sushkov, A.B., Valdés-Aguilar, R., Park, S., Cheong, S-W., Drew, H.D.: Electromagnons in multiferroic YMn2O5 and TbMn2O5. Phys. Rev. Lett. 98, 027202 (2007)

13. White, R.M., Yen, W.M.: On the discovery of magnon sidebands in insulating antiferromagnets. Low. Temp. Phys. 31, 777–779 (2005)

14. Born, M., Huang, K.: Dynamical theory of crystal lattices. Oxford University Press, London, (1954)

15. Goodenough, J.: Magnetism and the chemical bond. Wiley, New York, (1963)

16. Egami, T., Billinge, S.J.L.: Underneath the Bragg peaks: structural analysis of complex materials, Pergamon Materials Series Vol. 7, Pergamon Press. Elsevier Ltd., Oxford, (2003)

17. Kamihara, Y., Watanabe, T., Hirano, M., Hosono, H.: Iron-based layered superconductor La[O1−xFx]FeAs (x = 0.05–0.12) with Tc = 26 K. J. Am. Chem. Soc. 130, 3296–3297 (2008)

18. Dressel, M., Drichko, N.: Optical properties of two-dimensional organic conductors: signatures of charge ordering and correlation effects. Chem. Rev. 104, 5689–5716 (2004)

19. Zhou, H.D., Lumata, L.L., Kuhns, P.L., Reyes, A.P., Choi, E.S., Dalal, N.S., Lu, J., Jo, Y.J., Balicas, L., Brooks, J.S., Wiebe, C.R.: Ba3NbFe3Si2O14: A new multiferroic with a 2D triangular Fe3+ motif. Chem. Mater. 21(1), 156–159 (2009)

20. Sasmal, K., Lev, B., Lorenz, B., Guloy, A.M., Chen, F., Xue, Y.-Y., Chu, C.-W.: Superconducting Fe-based compounds (A1−xSrx)Fe2As2 with A=K and Cs with transition temperatures up to 37 K. Phys. Rev. Lett. 101, 107007 (2008)

21. Orenstein J., Millis, A.J.: Advances in the physics of high-temperature superconductivity. Sci. 288, 468–474 (2000)

22. Yoo, C.S., Maddox, B., Klepeis, J.-H.P., Iota, V., Evans, W., McMahan, A., Hu, M.Y., Chow, P., Somayazulu, M., Husermann, D., Scalettar, R.T., Pickett, W.E.: First-order isostructural Mott transition in highly compressed MnO. Phys. Rev. Lett. 94, 115502 (2005)

23. Basov D.N., Timusk, T.: Electrodynamics of high-Tc superconductors. Rev. Mod. Phys. 77, 721 (2005)

24. Hur, N., Park, S., Sharma, P.A., Ahn, J.S., Guha, S., Cheong, S.-W.: Electronic polarization reversal and memory in a multiferroic materials induced by magnetic fields. Nat. 429, 392–395 (2004)

25. Brooks, J.S.: Magnetic field dependent and induced ground states in organic conductors. Rep. Prog. Phys. 71, 126501 (2008)

26. Snow, C.S., Karpus, J.F., Cooper, S.L., Kidd, T.E., Chiang, T.-C.: Quantum melting of the charge density wave state in 1T-TiSe2. Phys. Rev. Lett. 91, 136402 (2003)

27. Goddard, P.A., Singleton, J., Sengupta, P., McDonald, R.D., Lancaster, T., Blundell, S.J., Pratt, F.L., Cox, S., Harrison, N., Mason, J.L., Southerland, H.I., Schlueter, J.A.: Experimentally determining the exchange parameters of quasi-two-dimensional Heisenberg magnets. New J. Phys. 10, 083025 (2008)

28. dela Cruz, C.R., Lorenz, B., Sun, Y.Y., Wang, Y., Park, S., Cheong, S-W., Gospodinov, M.M., Chu, C.W.: Pressure-induced enhancement of ferroelectricity in multiferroic RMn2O5 (R=Tb,Dy,Ho). Phys. Rev. B 76, 174106 (2007)

29. Graf, D., Choi, E.S., Brooks, J.S., Almeida, M.: Pressure-induced quantum limit in a Q1D system in high magnetic fields. J. Low Temp. Phys. 142, 179–184 (2006)

30. Uji, S., Terashima, T., Nishimura, M., Takahide, Y., Konoike, T., Enomoto, K., Cui, H., Kobayashi, H., Kobayashi, A., Tanaka, H., Tokumoto, M., Choi, E.-S., Tokumoto, T., Graf, D., Brooks, J.S.: Vortex dynamics and the Fulde-Ferrell-Larkin-Ovchinnikov state in a magnetic-field-induced organic superconductor. Phys. Rev. Lett. 97, 157001 (2006)

31. Gupta, R., Kim, M., Barath, H., Cooper, S.L., Cao, G.: Field- and pressure-induced phases in Sr4Ru3O10: A spectroscopic investigation. Phys. Rev. Lett. 96, 067004 (2006)

32. Kim, K.H., Harrison, N., Jaime, M., Boebinger, G.S., Mydosh, J.A.: Magnetic field-induced quantum critical point and competing order parameters in URu2Si2. Phys. Rev. Lett. 91(25), 256401 (2003)

33. Zapf, V.S., Zocco, D., Hansen, B.D., Jaime, M., Harrison, N., Batista, C.D., Kenzelmann, M., Niedermayer, C., Lacerda, A., Paduan-Filho, A.: Bose-Einstein condensation of S = 1 nickel spin degrees of freedom in NiCl2-4SC(NH2)2. Phys. Rev. Lett. 96, 077204 (2006)

34. dela Cruz, C.R., Lorenz, B., Sun, Y.Y., Chu, C.W., Park, S., Cheong, S.-W.: Magnetoelastic effects and the magnetic phase diagram of multiferroic DyMn2O5, Phys. Rev. B. 74, 180402 (2006)

35. Sologubenko, A.V., Berggold, K., Lorenz, T., Rosch, A., Shimshoni, E., Phillips, M.D., Turnbull, M.M.: Magnetothermal transport in the spin-1/2 chains of copper pyrazine dinitrate. Phys. Rev. Lett. 98, 107201 (2007)

36. Kunes, J., Lukoyanov, A.V., Anisimov, V.I., Scalettar, R.T., Pickett, W.E.: Collapse of magnetic moment drives the Mott transition in MnO. Nat. Mater. 7, 198–202 (2008)

37. Itkis, M.E., Brill, J.W.: Electromodulated infrared transmission in blue bronze. Phys. Rev. Lett. 72, 2049 (1994)

38. Ahn, C.H., Triscone, J.-M., Mannhart, J.: Electric field effect in correlated oxide systems. Nat. 424, 1015–1018 (2003)

39. In addition to controlling carrier concentration, oxygen vacancies are also related to domain wall pinning, leakage currents, degradation, resistances switching behavior in thin film samples.

40. Jaffe, B., Cook, W.R., Jaffe, H.: Piezoelectric ceramics. Academic Press, London, (1971)

41. Cooper, V.R., Grinberg, I., Martin, N.R., Rappe, A.M.: Local structure of PZT. In: Cohen, R.E. (ed.) Fundamental Physics of Ferroelectrics, pp. 26-35. AIP (2002)

42. Egami, T.: Local structure and dynamics of ferroelectric solids, in Structure and Bonding, Vol. 124, Ferro-and Antiferroelectricity. Dalal, N.S., Bussmann-Holder, A. (eds.), Springer-Verlag, Berlin, (2007)

43. Ederer, C., Fennie, C.J.: Electric-field switchable magnetization via the Dzyaloshinskii-Moriya interaction: FeTiO3 versus BiFeO3. J. Phys.: Condens. Matter 20, 434219 (2008).

44. Fennie, C.J.: Ferroelectrically induced weak ferromagnetism by design. Phys. Rev. Lett. 100, 167203 (2008)

45. Holman, K.T., Pivovar, A.M., Ward, M.D.: Engineering crystal symmetry and polar order in molecular host frameworks. Sci. 294, 1907–1911 (2001)

46. Ferrer, J.R., Lahti, P.M., George, C., Oliete, P., Julier, M., Palacio, F.: Role of hydrogen bonds in banzimidazole-based organic magnetic materials: crystal scaffolding or exchange linkers. Chem. Mater. 13, 2447–2454 (2001)

47. Murata, H., Aboaku, S., Lahti, P.M.: Molecular recognition in a heteromolecular radical pair system with complementary multipoint hydrogen-bonding. Chem. Comm. 29, 3441–3443 (2008)

48. Hayward, M.A., Cussen, E.J., Claridge, J.B., Bieringer, M., Rosseinsky, M.J., Kiely, C.J., Blundell, S.J., Marshall, I.M., Pratt, F.L.: The hydride anion in an extended transition metal oxide array: LaSrCoO3H0.7. Sci. 295, 1882–1884 (2002)

49. Ward, M.D.: Directing assembly of molecular crystals. MRS Bull. 30, 705–712 (2005)

50. Orendacova, A., Kajnakova, M., Cernak, J., Park, J.-H., Cizmar, E., Orendac, M., Vlcek, A., Kravchyna, O.V., Anders, A.G., Feher, A., Meisel, M.W.: Hydrogen bond mediated magnetism in [CuII(en)2(H2O)][CuII(en)2Ni2CuI2(CN)10]·2H2O. Chem. Phys. 309, 115–125 (2005)

51. Baddeley, C., Yan, Z., King, G., Woodward, P.M., Badjic, J.D.: Structure-function studies of modular aromatics that form molecular organogels. J. Org. Chem. 72(19), 7270–7278 (2007)

52. Manson, J.L., Conner, M.M. Schlueter, J.A., McConnell, A.C., Southerland, H.I., Malfant, I., Lancaster, T., Blundell, S.J., Brooks, M.L., Pratt, F.L., Singleton, J., McDonald, R.D., Lee, C., Whangbo, M.-H.: Experimental and theoretical characterization of the magnetic properties of CuF2(H2O)2 (pyz) (pyz = pyrazine): A two-dimensional quantum magnet arising from supersuperexchange interactions through hydrogen bonded paths. Chem. Mater. 20(24), 7408–7416 (2008)

53. Murata, H., Miyazaki, Y., Inaba, A., Paduan-Filho, A., Bindilatti, V., Fernandes Oliveira Jr., N., Delen, Z., Lahti, P.M.: 2-(4,5,6,7-Tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl, A hydrogen-bonded organic quasi-1D ferromagnet. J. Am. Chem. Soc. 130, 186–194 (2008)

54. Manriquez, J.M., Yee, G.T., McLean, R.S., Epstein, A.J., Miller, J.S.: A room temperature molecular/organic-based magnet. Sci. 252, 1415–1417 (1991)

55. Kaul, B.B., Durfee, W.S., Yee, G.T.: Dialkyldicyanofumarate diesters: tunable building blocks for molecular-based ferromagnets. J. Am. Chem. Soc. 121, 6862–6866 (1999)

56. Berlinguette, C.P., Baughn, D., Canada-Vilalta, C., Gala´n-Mascaros, J.R., Dunbar, K.R.: A tironal-bipyramidal cyanide cluster with single molecular behavior: synthesis, structure, magnetic properties of [MnII(tmpen)2]3[MnIII(CN)6]2. Angew, Chem., Int. Ed. 42, 1523–1526 (2003)

57. Miller, J.S., Epstein, A.J.: Designer magnets. Chem. Eng. News 73(40), 30 (1995)

58. Hill, S., Anderson, N., Wilson, A., Takahashi, S., Petukhov, K., Chakov, N.E., Murugesu, M., North, J.M. del Barco, E., Kent, A.D., Dalal, N.S., Christou, G.: A comparison between high-symmetry Mn12 single-molecule magnets in different ligand/solvent environments. Polyhedron 24, 2284–2292 (2005)

59. Miller, J.S.: Organometallic and organic based magnets: New chemistry and new materials for the new millennium. Inorg. Chem. 39, 4392–4408 (2000)

60. Pokhodnya, K.I., Epstein, A.J., Miller, J.S.: Thin film V(TCNE)x magnets. Adv. Mater. 12, 410–413 (2000)

61. Pejaković, D.A., Manson, J.L., Miller, J.S., Epstein, A.J.: Photoinduced magnetism, dynamics, cluster glass behavior of a molecule-based magnet. Phys. Rev. Lett. 85, 1994–1997 (2000)

62. Gîrtu, M.A., Wynn, C.M., Zhang, J., Miller, J.S., Epstein, A.J.: Magnetic properties and critical behavior of Fe(tetracyanoethylene)2 · x(CH2Cl2): a high-Tc molecule-based magnet. Phys. Rev. B 61, 492–500 (2000)

63. Sato, O., Iyoda, T., Fujishima, A.: Photoinduced magnetization of a cobalt-iron cyanide. Sci. 272, 704–705 (1996)

64. Miller, J.S.: Three-dimensional network-structured cyanide-based magnets. MRS Bull. 25, 60–64 (2000)

65. Ohta, H., Nomura, K., Hiramatsu, H., Ueda, K., Kamiya, T., Hirano, M., Hosono, H.: Frontier of transparent oxide semiconductors. Solid-State Electron. 47, 2261–2267 (2003)

66. Kaye, S.S., Long, J.R.: Hydrogen storage in the dehydrated pressian blue analogues Mn3[Co(CN)6]2(M = Mn, Fe, Co, Ni, Cu, Zn). J. Am. Chem. Soc. 127, 6506–6507 (2005)

67. Culp, J.T., Park, J.H., Frye, F., Huh, Y.D., Meisel, M.W., Talham, D.R.: Magnetism of metal cyanide networks assembled at interfaces. Coord. Chem. Rev. 249, 2642–2648 (2005)

68. Berlinguette, C.P., Dragulescu-Andrasi, A., Sieber, A., Galan-Mascaros, J.R., Güdel, H., Achim, C., Dunbar, K.R.: A charge-transfer-induced spin transition in the descrete cyanide-bridged complex [Co(tmphen)2]3[Fe(CN)6]2. J. Am. Chem. Soc. 126, 6222–6223 (2004)

69. Holmes, S.M., Girolami, G.S.: Sol-gel synthesis of KVII[CrIII(CN)6] · 2H2O: a crystalline molecule-based magnet with a magnetic ordering temperature above 100̊ C. J. Am. Chem. Soc. 121, 5593–5594 (1999)

70. Escax, V., Bleuzen, A., Itie, J.P., Munsch, P., Varret, F., Verdaguer, M.: Nature of the long-range structural changes induced by the molecular photoexcitation and by the relaxation in the Prussian blue analogs Rb1.8Co4[Fe(CN)6]3.3 · 13H2O:a synchrotron diffraction study. J. Phys. Chem. B. 107, 4763–4767 (2003)

71. Li, D., Clérac, R., Roubeau, O., Harté, E., Mathoniére, C., Le Bris, R., Holmes, S.M.: Magnetic and optical bistability driven by thermallyand photo-induced intramolecular electron transfer in a molecular cobalt-iron Prussian blue analogue. J. Am. Chem. Soc. 130, 242–259 (2007)

72. Berlinguette, C.P., Draguleschu-Andrasi, A., Sieber, A., Güdel, H.-U., Achim, C., Dunbar, K.R.: A charge-transfer-induced spin transition in a discrete complex: the role of extrinsic factors in stabilizing three electronic isometic forms of a cyanide-bridged Co/Fe cluster. J. Am. Chem. Soc. 127, 6766–6779 (2005

73. Rao, C.N.R. Nath, M.: Inorganic nanotubes. Dalton Trans. 1, 1–24 (2003)

74. Bonnell, D.A.: Materials in nanotechnology: new structures, new properties, new complexity. J. Vac. Sci. Technol. A 21, S194–S206 (2003)

75. Tenne, R., Rao, C.N.R.: Inorganic nanotubes. Philos. Trans. R. Soc. A 362, 2099–2125 (2004)

76. Halford, B.: Inorganic menagerie. Chem. Eng. News 83, 30–33 (2005)

77. Tenne, R.: Inorganic nanotubes and fullerene-like nanoparticles. Nat. Nanotechnol 1, 103–111 (2006)

78. Long, J.W., Rolison, D.R.: Architectural design, interior decoration, three-dimensional plumbing en route to multifunctional nanoarchitectures. Acc. Chem. Res. 40, 854–862 (2007)

79. Nobile, C., Kudera, S., Fiore, A., Carbone, L., Chilla, G., Kipp, T., Heitmann, D., Cingolani, R., Manna, L., Krahne, R.: Confinement effects on the optical phonons in spherical, rod-, tetrapod-shaped nanocrystals detected by Raman spectroscopy. Phys. Stat. Sol. (a) 204, 483 (2007)

80. Moses, M.J., Fettinger, J.C., Wichhorn, B.W.: Interpenetrating As20 fullerene and Ni12 iscosahedra in the onion-skin [As@Ni12@As20]. Sci. 300, 778–780 (2003)

81. Wu, Y., Messer, B., Yang, P.: Superconducting MgB2 nanowires. Adv. Mater. 13, 1487–1489 (2001)

82. Wang, Z.L., Kong, X.Y., Ding, Y., Gao, P., Hughes, W.L., Yang, R., Zhang, Y.: Semiconducting and piezoelectric oxide nanostructures induced by polar surfaces. Adv. Funct. Mater. 14, 943–956 (2004)

83. Liu, C., Rondinone, A.J., Zhang, Z.J.: Sol-gel synthesis of free-standing ferroelectric lead zirconate titanate nanoparticles. J. Am. Chem. Soc. 123, 4344–4345 (2001)

84. Kovtyukhova, N.I., Mallouk, T.E., Mayer, T.S.: Templated surface sol-gel synthesis of SiO2 nanotubes and SiO2-insulated metal nanowires. Adv. Mater. 15, 780–785 (2003)

85. Tian, M., Wang, J., Han, T., Kumar, N., Kobayashi, Y., Liu, Y., Mallouk, T.E., Chan, M.W.H.: Superconductivity in granular Bi nanowires fabricated by electrochemical deposition at ambient pressure. Nano. Lett. 6, 2773–2780 (2006)

86. Rusakova, I., Ould-Ely, T., Hofmann, C., Prieto-Centurión, D., Levin, C.S., Halas, N.J., Lüttge, A., Whitmire, K.H.: Nanoparticle shape conservation in the conversion of MnO nanocrosses into Mn3O4. Chem. Mater. 19, 1369–1375 (2007)

87. Ould-Ely, T., Prieto-Centurion, D., Kumar, A., Guo, W., Knowles, W.V., Asokan, S., Wong, M.S., Rusakova, I., Lüttge, A., Whitmire, K.H.: Manganese(II) oxide nanohexapods: Insight into controlling the form of nanocrystals. Chem. Mater. 18, 1821–1829 (2006)

88. Tian, M.L., Wang, J.G., Kurtz, J.S., Liu, Y., Chan, M.H.W., Mayer, T.S., Mallouk, T.E.: Dissipation in quasi-one-dimensional superconducting single crystal Sn nanowires. Phys. Rev. B. 71, 104521 (2005)

89. O’Dwyer, C., Navas, D., Lavayen, V., Benavente, E., Santa Ana, M.A., Gonzales, G., Newcomb, S.B., Sotomayor Torres, C.M.: Nanourchin: the formation and structure of high-density spherical clusters of vanadium oxide nanotubes. Chem. Mater. 18, 3016–3022 (2006)

90. Danier, M.-C., Astruc, D.: Gold nanoparticles: Assembly, supramolecular chemistry, quantum size related properties, applications toward biology, catalysis, and nanotechnology. Chem. Rev. 104, 293–346 (2004)

91. Mai, L.W., Lao, C.S., Hu, B., Qi, Y.Y., Chen, W., Gu, E.D., Wang, Z.L.: Structure and electrical transport for single-crystal NH4 V3 O8 nanobelts. J. Phys. Chem. B 110, 18138–18141 (2006)

92. Szlufarska, I., Nakano, A., Vashishta, P.: A crossover in the mechanical response of nanocrystalline ceramics. Sci. 309, 911–914 (2005)

93. Wu, B., Heidelberg, A., Boland, J.J.: Mechanical properties of ultrahigh-strength gold nanowires. Nat. Mater. 4, 525–529 (2003)

94. Tenne, R., Margulis, L., Genut, M., Hodes, G.: Polyhedral and cylindrical structures of tungsten disulphide. Nat. 360, 444–446 (1992)

95. Margulis, L., Salitra, G., Tenne, R.: Nested fullerene-like structures. Nat. 365, 113–114 (1993)

96. Parilla, P.A., Dillon, A.C. K.M. Jones, G. Riker, D.L. Schulz, D.S. Ginley, Heben, M.J.: The first true inorganic fullerenes? Nat. 397, 114–114 (1999)

97. Yao, B.D., Chan, Y.F., Zhang, X.Y., Zhang, W.F., Yang, Z.T., Wang, N.: Formation mechanism of TiO2 nantoubes. Appl. Phys. Lett. 82, 281–283 (2003)

98. Remskar, M., Mrzel, A., Skraba, Z., Jesih, A., Ceh, M., Demsar, J., Stadelmann, P., Lévy, F., Mihailovic, D.: Self-assembly of subnanometer-diameter single-wall MoS2 nantobues. Sci. 292, 479–481 (2001)

99. Mickelson, W., Aloni, S., Han, W.-Q., Cumings, J., Zettl, A.: Packing C60 in boron nitride nanotubes. Sci. 300, 467–469 (2003)

100. Levy, P., Leyva, A.G., Troiani, H.E., Sánchez, R.D.: Nanotubes of rare earth manganese oxide. Appl. Phys. Lett. 83, 5247–5249 (2003)

101. Cabria, I., Mintmire, J.W.: Stability and electronic structure of phosphorus nanotubes. Euro. Phys. Lett. 65, 82–88 (2004)

102. Seifert, G., Enyashin, A.N.: Structure, stability, electronic properties of TiO2 nanostructures. Phys. Stat. Sol. 242, 1361–1370 (2005)

103. Enyanshin, A.N., Seifert, G., Ivanovskii, A.L.: Calculation of the electronic and thermal properties of C/BN nanotubular heterostructures. Inorg. Mater. 41, 595–603 (2005)

104. Saha-Dasgupta, T., Valenti, R., Capraro, F., Gros, C.: Na2V3O7, a frustrated nanotubular system with spin-1/2 diamond rings. Phys. Rev. Lett. 95, 107201 (2005)

105. Cao, J., Choi, J., Musfeldt, J.L., Lutta, S., Whittingham, M.S.: Effect of sheet distance on the optical properties of vanadate nanotubes. Chem. Mater. 16, 731–736 (2004)

106. Cao, J., Choi, J., Musfeldt, J.L., Lutta, S., Whittingham, M.S.: Vibrational response of vanadium oxide nanotubes: Exploring sheet distance effects and variable temperature properties, in Nano-Scale Materials: From Science to Technology. Sahu, S.N., Choudhury, R.K., Jena, P. (eds.), Nova Science Publishers (2006)

107. Baruah, T., Zope, R.R., Richardson, S.L., Pederson, M.R.: Electronic structure and rebonding in the onionlike As@Ni12@As20. Phys. Rev. B. 68, 241404 (2003)

108. Chen, S.H., Wang, Z.L., Ballato, J., Foulger, S.H., Carroll, D.L.: Monopod, bipod, tripod and tetrapod gold nanocrystals. J. Am. Chem. Soc. 125, 6186–16187 (2003)

109. Kong, X.Y., Wang, Z.L.: Spontaneous polarization-induced nanohelixes, nanosprings, nanorings of piezoelectric nanobelts. Nano. Lett. 3, 1625–1631(2003)

110. Dames, C., Poudel, B., Wang, W.Z., Huang, J.Y., Ren, Z.F., Sun, Y., Oh, J.I., Opeil, C., Naughton, M.J., Chen, G.: Low-dimensional phonon specific heat of titanium dioxide nanotubes. Appl. Phys. Lett. 87, 031901 (2005)

111. Muhr, J.-J., Krumeich, K., Schonholzer, U.P, Bieri, F., Neiderberger, M., Gauckler, L.J., Nesper, R.: Vanadium oxide nanotubes: a new flexible vanadate nanophase. Adv. Mater. 12, 231–234 (2000)

112. Worle, M., Krumeich, F., Bieri, F., Muhr, H.-J., Nesper, R.: Flexible V7O16 layers as the common structural element of canadium oxide nanotubes and a new crystalline vanadate. Z. Anorg. Allg. Chem. 628, 2778–2784 (2002)

113. Friedman, J.R., Sarachik, M.P., Tejada, J., Ziolo, R.: Macroscopic measurement of resonant magnetization tunneling in high-spin molecules. Phys. Rev. Lett. 76, 3830–3833 (1996)

114. Tasiopoulos, A.J., Vinslava, A., Wernsdorfer, W., Abboud K.A., Christou, G.: Giant single-molecule magnets: A Mn84 torus and its supramolecular nanotubes, Angew. Chem. Int. Ed. 43, 2117–2121 (2004)

115. Iijima, S.: Helical microtubules of graphitic carbon. Nat. 354, 56–58 (1991)

116. Bethune, D.S.: Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls. Nat. 363, 605–607 (1993)

117. Geim, A.K., Novoselov, K.S: The rise of grapheme. Nat. Mater. 6 183–191 (2007)

118. Hone, J., Batlogg, B., Benes, Z., Johnson, A.T., Fischer, J.E.: Quantized Phonon Spectrum of Single-Wall Carbon Nanotubes. Sci. 289, 1730–1733 (2000)

119. Sun, C.Q., Pan, L.K., Li, C.M., Li, S.: Size-induced acoustic hardening and optic softening of phonons in InP, CeO2, SnO2, CdS, Ag, Si nanostructures. Phys. Rev. B 72, 134301 (2005)

120. Bersani, D., Lottici, P.P., Ding, X.-Z.: Phonon confinement effects in the Raman scattering by TiO2 nanocrystals. Appl. Phys. Lett. 72, 73–75 (1998)

121. Luttrell, R.D., Brown, S., Cao, J., Musfeldt, J.L., Rostenveig, R., Tenne, R., Understanding the dynamics of bulk vs. nanoscale WS2: Local strain and charging effects. Phys. Rev. B. 73, 035410 (2006)

122. Brown, S., Musfeldt, J.L., Mihut, I., Betts, J.B., Migliori, A., Zak, A., Tenne, R.: Bulk vs. nanoscale WS2: Finite size effects and solid state lubrication. Nano. Lett. 7, 2365–2369 (2007)

123. Smith, M.B., Page, K., Siegrist, T., Redmond, P.L., Walter, E.C., Seshadri, R., Brus, L.E., Steigerwald, M.L.: Crystal structure and the paraelectric-to-ferroelectric phase transition of nanoscale BaTiO3. J. Am. Chem. Soc. 130, 6955–6963 (2008)

124. Qazilbash, M.M.. Brehm, M., Chae, B.-G., Ho, P.-C., Andreev, G.O., Kim, B.-J., Yun, S.J., Balatsky, A.V., Maple, M.B., Keilmann, F., Kim, H.-T., Basov, D.N., Mott transition in VO2 revealed by infrared spectroscopy and nano-imaging. Sci. 318, 1750–1753 (2007)

125. Filippetti, A., Hill, N.A.: Magnetic stress as a driving force of structural distortions: the case of CrN. Phys. Rev. Lett. 85, 5166–5169 (2000)

126. Sun, Z., Chuang, Y.-D., Fedorov, A.V., Douglas, J.F., Reznik, D., Weber, F., Aliouane, N., Argyriou, D.N., Zheng, H., Mitchell, J.F., Kimura, T., Tokura, Y., Revcolevschi, A., Dessau, D.S.: Quasiparticle-like peaks, kinks, electron-phonon couping at the (π,0) regions in the CMR oxide La2−2xSr1+2xMn2O7. Phys Rev Lett. 97, 056401 (2006)

127. Mazin, I.I., Johannes, M.D.: A critical assessment of the superconducting pairing symmetry in NaxCoO2 · yH2O. Nat. Phys. 1, 91–93 (2005)

128. Monteverde, M., Nunez-Regueiro, M., Rodado, N., Regan, K.A., Hayward, M.A., He, T., Lourerir, S.M., Cava, R.J.: Pressure dependence of the superconducting transition temperature of magnesium diboride. Sci. 292, 75–77 (2001)

129. Homes, C.C., Vogt, T., Shapiro, S.M., Wakimoto, S., Ramirez, A.P: Optical response of high dielectric constant perovskite-related oxide. Sci. 293, 673–676 (2001)

130. Lake, B., Tennant, D.A., Frost, C.D., Nagler, S.E.: Quantum criticality and universal scaling of a quantum antiferromagnets. Nat. Mater. 4, 329–334 (2005)

131. Feynman, R.P.: There’s Plenty of Room at the Bottom. http://www.zyvex.com/nanotech/feynman.html (1959).

132. Aviram, A.: Molecules for memory, logic, amplification. J. Am. Chem. Soc. 110, 5687–5692 (1988)

133. Wang, Y.Z., Gebler, D.D., Blatchford, J.W., Jessen, S.W., Wang, H.L., Epstein, A.J.: AC light emitting device based on conjugated polymers. Appl. Phys. Lett. 68, 894–896 (1996)

134. Lui, C., Pan, H. Fox, M.A., Bard, A.J.: High-density nanosecond charge trapping in thin films of the photoconductor ZnODEP. Sci. 261, 897–899 (1993)

135. Balasubramanian, K., Burghard, M., Kern, K., Scolari M., Mews, M.: Photocurrent imaging of charge transport barriers in carbon nanotube devices. Nano Lett. 5, 507–510 (2005)

136. Leuenberger, M.N., Loss, D.: Quantum computing in molecular magnets. Nat. 410, 789–793 (2001)

137. Kagan, C.R., Mitzi, D.B., Dimitrakopoulos, C.D.: Organic-inorganic hybrid materials as semiconducting channels in thin-film field-effect transitors. Sci. 286, 945–947 (1999)

138. Sales, B.C., Mandrus, D., Williams, R.K.: Filled skutterudites antimonides: a new class of thermoelectric materials. Sci. 272, 1325–1328 (1996)

139. Rueckes, T., Kim, K., Joselvich, E., Tseng, G.Y., Cheung, C.L., Lieber, C.M.: Carbon nanotube-based non-volatile random access memory for molecular computing. Sci. 289, 94–97 (2000)

140. Krusin-Elbaum, L., Newns, D.M., Zeng, H., Derycke, V., Sun, J.Z., Sandstrom, R.: Room-temperature ferromagnetic nanotubes controlled by electron or hole doping. Nat. 431, 672–676 (2004)

141. Rothschild, A., Cohen, S.R., Tenne, R.: WS2 nanotubes as tips in scanning probe microscopy. Appl. Phys. Lett. 75, 4025–4027 (1999)

142. Maeda, K., Eguchi, M., Youngblood, W.J., Mallouk, T.E.: Niobium oxide nanoscrolls as building blocks for dye-sensitized hydrogen production from water under visible light irradiation. Chem. Mater. 20, 6770–6778 (2008)

143. Rapoport, L., Fleischer, N., Tenne, R.: Fullerene-like WS2 Nanoparticles: superior lubricants for harsh conditions. Adv. Mater. 15, 651–655 (2003)

144. Nemanic, V., Zumer, M., Zajec, B., Pahor, J., Remskar, M., Mrzel, A., Panjan, P., Mihailovic, D.: Field-emission properties of molybdenum disulfide nanotubes, Appl. Phys. Lett. 82, 4573–4575 (2003)

145. Hill, S., Edwards, R.S., Aliaga-Alcalde, N., Christou, G.: Quantum coherence in an exchange-coupled dimer of single molecule magnets. Sci. 302, 1015–1018 (2003)

146. Wang, J., Stucky, G.D.: Mesostructured composite materials for multibit-per-site optical data storage. Adv. Funct. Mater. 14, 409–415 (2004)

147. Li, S., Yu, Z., Yen, S.-F., Tang, W.C., Burke, P.J.: Carbon nanotube transistor operation at 2.6 GHz. Nano Lett. 4, 753–756 (2004)

148. Jacoby, M.: Single-nanotube photodetector. Chem. Engin. News 81(27), 5 (2003)

149. Moore, J.G., Lochner, E.J., Ramsey, C., Dalal, N.S., Stiegman, A.E.: Transparent, superparamagnetic KCo[Fe(CN)6]-silica nanocomposites with tunable photomagnetism. Angew. Chem. Int. Ed. 42, 2741–2743 (2003)

150. Novoselov, K.S., Jiang, Z., Zhang, Y., Morozov, S.V., Stormer, H.L., Zeitler, U., Maan, J.C., Boebinger, G.S., Kim, P., Geim, A.K.: Room-temperature quantum Hall effect in grapheme. Sci. 315, 1379 (2007)

151. Zŭtić, I., Fabian, J., Das Sarma, S.: Spintronics: fundamentals and applications. Rev. Mod. Phys. 76, 323–410 (2004)

152. Cage, B., Russek, S.E., Shoemaker, R., Barker, A.J., Stoldt, C., Ramachandaran, V., Dalal, N.S.: The utility of the single-molecule magnet Fe8 as a magnetic resonance imaging contrast agent over a broad range of concentration. Polyhedron 26(12), 2413–2419 (2007)

153. Cao, G., Durairaj, V., Chikara, S., DeLong, L.E., Schlottmann, P.: Observation of strong spin valve effect in bulk Ca3(Ru1-xCrx)2O7. Phys. Rev. Lett. 100, 016604 (2008); Phys. Rev. Lett. 100, 159902 (2008)

154. Poudel, B., Hao, Q., Ma, Y., Lan, Y., Minnich, A., Yu, B., Yan, X., Wang, D., Muto, A., Vashaee, D., Chen, X., Liu, J., Dresselhaus, M.S., Chen, G., Ren, Z.: High thermoelectric performance of nanostructured bismuth antimoney telluride bulk alloys. Sci. 320 634–638 (2008)

155. Bibes, M., Barthélémy, A.: Multiferroics: Towards a magnetoelectric memory. Nat. Mater. 7, 425–426 (2008)

156. Chapline, M.G., Wang, S.X.: Room-temperature spin filtering in a CoFe2O4/MgAl2O4/Fe3O4 magnetic tunnel barrier. Phys. Rev. B 74, 014418 (2006)

157. Wooten, F.: Optical properties of solids. Academic Press, New York (1972)

158. Pankove, J.I.: Optical processes in semiconductors, Dover, New York, (1971)

159. Love, S.P., Worl, L.A., Donohoe, R.J., Huckett, S.C., Swanson, B.I.: Origin of the fine structure in the vibrational spectrum of [Pt(C2H8N2)2] [Pt(C2H8N2)2] (ClO4)4: vibrational localization in a quasi-1D system. Phys. Rev. B. 46, 813–816 (1992)

160. Jones, B.R., Varughese, P.A., Pigos, J.M., Landee, C.P., Turnbull, M.M., Olejniczak, I., Carr, G.L.: Lattice dynamics of the 1D S=1/2 Heisenberg antiferromagnet CuPzN. Chem. Mater. 13, 2127–2134 (2001)

161. Brown, S., Cao, J., Musfeldt, J.L., Conner, M.M., McConnell, A.C., Southerland, H.I., Manson, J.L., Schlueter, J.A., Phillips, M.D., Turnbull, M.M., Landee, C.P.: Hydrogen bonding and multiphonon structure in copper pyrazine coordination polymers. Inorg. Chem. 46, 8577–8583 (2007)

162. Sun, Q.-C., Xu, X.S., Vergara, L.I., Rosentsveig, R., Musfeldt, J.L.: Dynamical charge and structural strain in inorganic fullerne-like MoS2 nanoparticles. Phys. Rev. B 79, 205205 (2009)

163. Xu, X.S., Brinzari, T.V., McGill, S., Zhou, H.D., Weibe, C.R., Musfeldt, J.L.: Absense of spin liquid behavior: Magneto-optical study of Nd3Ga5SiO14. Phys. Rev. Lett. 103, 267402 (2009)

164. Forró, L., Mihály, L.: Electronic properties of doped fullerenes. Rep. Prog. Phys. 64, 649-699 (2001)

165. Kortus, J., Pederson, M.R.: Magnetic and vibrational properties of the uniaxial Fe13O8 cluster. Phys. Rev. B. 62, 5755–5759 (2000)

166. Zhu, Z.-T., Musfeldt, J.L., Kamarás, K., Adams, G.B., Page, J.B., Kashevarova, L.S., Rakhmanina, A.V., Davydov, V.A.: Far-infrared vibrational properties of linear C60 polymers: A comparison between neutral and charged materials. Phys. Rev. B 67, 045409 (2003)

167. Fateley, W.G., Dollish, F.R., McDevitt, N.T., Bentley, F.F.: Infrared and Raman selection rules for molecular and lattice vibrations: the correlation method. Wiley-Intersience, New York, (1972)

168. Barath, H., Kim, M., Cooper, S.L., Abbamonte, P., Fradkin, E., Mahns, I., Rübhausen, M., Aliouane, N., Argyriou, D.N.: Domain fluctuations near the field-induced incommensurate-commensurate phase transition of TbMnO3. Phys. Rev. B. 78, 134407 (2008)

169. Gasparov, L.V., Brown, K.G., Wint, A.C., Tanner, D.B., Berger, H., Margaritondo, G., Gaal, R., Forro, L.: Phonon anomaly at the charge ordering transition in 1T-TaS2. Phys. Rev. B. 66, 094301 (2002)

170. Musfeldt, J.L., Brown, S., Mazuumdar, S., Clay, R.T., Mas-Torent, M., Rovira, C., Dias, J.C., Henriques, R.T., Almeida, M.: Infrared investigation of the charge ordering pattern in the organic spin ladder candidate (DTTTF)2Cu(mnt)2. Solid State Sci. 10, 1740–1744 (2008)

171. Choi, J., Musfeldt, J.L., He, J., Jin, R., Thompson, J.R., Mandrus, D., Lin, X.N. Bondarenko, V.A., Brill, J.W.: Probing localization effects in Li0.9Mo6O17: An optical properties investigation. Phys. Rev. B. 69, 085120 (2004)

172. Barath, H., Kim, M., Karpus, J.F., Cooper, S.L., Abbamonte, P., Fradkin, E., Morosan, E., Cava, R.J.: Quantum and classical mode softening near the charge density wave-superconductor transition in CuxTiSe2. Phys. Rev. Lett. 100, 106402 (2008)

173. Choi, J., Musfeldt, J.L., Whangbo, M.H., Galy, J., Millet, P.: Optical investigation of Na2V3O7 nanotubes. Chem. Mater. 14, 924–930 (2002)

174. Ernst, G., Broholm, C., Kowach, G.R., Ramirez, A.P.: Phonon density of states and negative thermal expansion in ZrW2O8. Nat. 396, 147–149 (1998)

175. Frey, G.L., Elani, S., Homyonfer, M., Feldman, Y., Tenne, R.: Optical absorption spectra of inorganic fullerene-like MS2 (M = Mo, W). Phys. Rev. B. 57, 6666–6671 (1998)

176. Bommeli, F., Degiogi, L., Wachter, R., Legez, Ö., Janossy, A., Oszlanyi, G., Chauvet, O., Forro, L.: Metallic conductivity and metal-insulator transition in (AC60 )n (A =K, Rb, Cs) linear polymer fullerides. Phys. Rev. B. 51, 14794–14797 (1995)

177. Hicks, L.D., Dresselhaus, M.S.: Effect of quantum-well structures on the thermodynamic figure of merit. Phys. Rev. B. 47, 12727–12731 (1993)

178. Hasegawa, T., Akutagawa, T., Nakamura, T., Mochida, T., Kondo, R., Kagoshima, S., Iwasa, Y.: Neutral-ionic phase transition of (BEDT-TTF)(ClMeTCNQ) under pressure. Phys. Rev. B. 64, 085106 (2001)

179. Schrama, J.M., Rzepniewski, E., Edwards, R.S., Singleton, J., Ardavan, A., Kurmoo, M., Day, P.: Millimeter-wave magneto-optical determination of the anisotropy of the superconducting order parameter in the molecular superconductor κ-(BEDTTTF)2Cu(NCS)2. Phys. Rev. Lett. 83, 3041–3044 (1999)

180. Nuttall, C.J., Hayashi, Y., Yamazaki, K., Mitani, T., Iwasa, Y.: Dipole dynamics in the endohedral metallofullerene La@C82. Adv. Mater. 14, 293–296 (2002)

181. Islam, M.F., Milkie, D.E., Kane, C.L., Yodh, A.G., Kikkawa, J.M.: Direct measurement of the polarized optical absorption cross section of single-wall carbon nanotubes. Phys. Rev. Lett. 93, 037404 (2004)

182. Tsvetkov, A.A., Mena, F.P., van Loosdrecht, P.H.M., van der Marel, D., Ren, Y., Nugroho, A.A., Menovsky, A.A., Elfimov, I.S., Sawatzky, G.A.: Structural, electronic, and magneto-optical properties of YVO3, Phys. Rev. B 69, 075110 (2004)

183. Turner, G.M., Beard, M.C., Schmuttenmaer, C.A.: Carrier localization a