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Quantum confinement and surface effects (SEs) dramatically modify most solid state phenomena as one approaches the nanometer scale, and superconductivity is no exception. Though we may expect significant modifications from bulk superconducting properties when the system dimensions become smaller than the characteristic length scales for bulk superconductors-such as the coherence length or the penetration depth-it is now established that there is a third length scale which ultimately determines the critical size at which Cooper pairing is destroyed. In quasi-zero-dimensional (0D) superconductors (e.g. nanocrystalline materials, isolated or embedded nanoparticles), one may define a critical particle diameter below which the mean energy level spacing arising from quantum confinement becomes equal to the bulk superconducting energy gap. The so-called Anderson criterion provides a remarkably accurate estimate of the limiting size for the destabilization of superconductivity in nanosystems. This review of size effects in quasi-0D superconductors is organized as follows. A general summary of size effects in nanostructured superconductors (section 1) is followed by a brief overview of their synthesis (section 2) and characterization using a variety of techniques (section 3). Section 4 reviews the size-evolution of important superconducting parameters-the transition temperature, critical fields and critical current-as the Anderson limit is approached from above. We then discuss the effect of thermodynamic fluctuations (section 5), which become significant in confined systems. Improvements in fabrication methods and the increasing feasibility of addressing individual nanoparticles using scanning probe techniques have lately opened up new directions in the study of nanoscale superconductivity. Section 6 reviews both experimental and theoretical aspects of the recently discovered phenomena of 'parity effect' and 'shell effect' that lead to a strong, non-monotonic size dependence of the superconducting energy gap and associated properties. Finally, we discuss in section 7 the properties of ordered heterostructures (bilayers and multilayers of alternating superconducting and normal phases) and disordered heterostructures (nanocomposites consisting of superconducting and normal phases), which are primarily controlled by the proximity effect.

The phase transition from the hexagonal 4H polytype of silver to the commonly known 3C (fcc) phase was studied in detail using x-ray diffraction, electron microscopy, differential scanning calorimetry and Raman spectroscopy. The phase transition is irreversible and accompanied by extensive microstructural changes and grain growth. Detailed scanning and isothermal calorimetric analysis suggests that it is an autocatalytic transformation. Though the calorimetric data suggest an exothermic first-order phase transition with an onset at 155.6 °C (for a heating rate of 2 K min(-1)) and a latent heat of 312.9 J g(-1), the microstructure and the electrical resistance appear to change gradually from much lower temperatures. The 4H phase shows a Raman active mode at 64.3 cm(-1) (at 4 K) that undergoes mode softening as the 4H → 3C transformation temperature is approached. A first-principles density functional theory calculation shows that the stacking fault energy of 4H-Ag increases monotonically with temperature. That 4H-Ag has a higher density of stacking faults than 3C-Ag, implies the metastability of the former at higher temperatures. Energetically, the 4H phase is intermediate between the hexagonal 2H phase and the 3C ground state, as indicated by the spontaneous transformation of the 2H to the 4H phase at -4 °C. Our data appear to indicate that the 4H-Ag phase is stabilized at reduced dimensions and thermally induced grain growth is probably responsible for triggering the irreversible transformation to cubic Ag.

We report the optical, electronic, vibrational and mechanical properties of a stable, anisotropic, hexagonal (4H) form of silver. First principles calculations based on density functional theory were used to simulate the phonon dispersion curves and electronic band structure of 4H-Ag. The phonon dispersion data at 0 K do not contain unstable phonon modes, thereby confirming that it is a locally stable structure. The Fermi surface of the 4H phase differs in a subtle way from that of the cubic phase. Experimental measurements indicate that, when compared to the commonly known face-centered cubic (3C) form of silver, the 4H-Ag form shows a 130-fold higher, strongly anisotropic, in-plane resistivity and a much lower optical reflectance with a pronounced surface plasmon contribution that imparts a distinctive golden hue to the material. Unlike common silver, the lower symmetry of the 4H-Ag structure allows it to be Raman active. Mechanically, 4H-Ag is harder, more brittle and less malleable. Overall, this novel, poorly metallic, anisotropic, darker and harder crystallographic modification of silver bears little resemblance to its conventional counterpart.

The quasi-one-dimensional, chiral crystal structure of Selenium has fascinating implications: we report simultaneous magnetic and ferroelectric order in single crystalline Se microtubes below ≈40 K. This is accompanied by a structural transition involving a partial fragmentation of the infinite chains without losing overall crystalline order. Raman spectral data indicate a coupling of magnons with phonons and electric field, while the dielectric constant shows a strong dependence on magnetic field. Our first-principles theoretical analysis reveals that this unexpected multiferroic behavior originates from Selenium being a weak topological insulator. It thus exhibits stable electronic states at its surface, and magnetism emerges from their spin polarization. Consequently, the broken two-fold rotational symmetry permits switchable polarization along its helical axis. We explain the observed magnetoelectric couplings using a Landau theory based on the coupling of phonons with spin and electric field. Our work opens up a new class of topological surface-multiferroics with chiral bulk structure.

We show that controlled clustering in electrochemically grown silver nanorods results in up to 50% enhancement in their field emission performance. Larger cluster size and nanorod length lead to a lower turn-on electric field, a higher current density and a larger enhancement factor. However, beyond a critical length (≈30 µm), the nanorods begin to form disordered ridges instead of well separated conical clusters and the field emission performance proceeds to deteriorate. With the help of finite element modelling, we show that a larger cluster size indeed leads to an enhanced electric field at the cluster edges, effectively reducing screening effects and thereby enhancing the field emission performance. Though our present work pertains to silver nanorods, a similar clustering of nanorods is observed in many types of electrochemically grown nanorods, and even in carbon nanotubes and Si nanowires grown by other techniques. Hence, we expect this study to have general applicability in the design of better nanorod-based field emitters.

We report a simple, versatile, low cost fabrication technique for synthesizing nanorod arrays whose architecture is suited for many applications spanning the nanometer to micrometer range. Specifically, we have covered the range of nanorod diameter from 50 to 1200 nm. From a detailed study of the growth parameters involved in the synthesis of the ZnO nanorod arrays from an aqueous solution, we report, in particular, the effects of varying the capping agent, substrate and substrate-seeding. We find that seeding the substrate and selecting the appropriate capping agent play the most crucial roles in the alignment of nanorod arrays. Our study on the use of different precursor materials and varied substrates for the growth of ZnO nanorod arrays should lead to an enhanced understanding of the controllable growth of ZnO crystals and nanostructures.

We show that hydrogen titanate (H(2)Ti(3)O(7)) nanotubes form strongly associated reversible nano-bio-conjugates with the vital respiratory protein, cytochrome c. Resonance Raman spectroscopy along with direct electrochemical studies indicate that in this nano-bio-conjugate, cytochrome c exists in an equilibrium of two conformational states with distinctly different formal redox potentials and coordination geometries of the heme center. The nanotube-conjugated cytochrome c also showed enhanced peroxidase activity similar to the membrane-bound protein that is believed to be an apoptosis initiator. This suggests that such a nanotube-cytochrome c conjugate may be a good candidate for cancer therapy applications.

We report a study of the relative effects of dimensional and kinetic constraints on the stabilization of metastable, polytypic forms of metallic silver. We show that the hexagonal 4H polytype (hitherto observed only in size-constrained systems) can be produced in the form of bulk thin films by suitably slowing down the growth kinetics. Further, using extremely slow growth conditions, we have been successful in depositing a novel, two-dimensional, metastable polytype (2H) of silver, which is highly reactive (easily oxidized) and has a density 23% lower than normal silver. First-principles calculations based on density functional theory confirm that the 4H structure is relatively stable. However, local stability analysis via a determination of the phonon dispersion of the 2H structure reveals that it is only marginally stable with an energy surface that is rather flat or weakly varying with respect to many of the modes. This makes a large contribution to the configurational entropy and is probably the reason for the metastability of the observed 2H polytype with an unusually large lattice constant along the c-direction.

Measurements of the local susceptibility and 3d spin relaxation rate for single Fe impurities embedded in a nanocrystalline Nb host indicates the emergence of a local moment on Fe at and below a critical size of 11 nm. Our ab initio electronic structure calculations show that the moment formation occurs due to Stoner enhancement arising from a size dependent lattice expansion and a consequent shift in the Fermi level. We also show that a size-induced positive host spin polarization of the Nb-4d band electrons strongly influences the fluctuation rate of the Fe moment.

Zinc oxide nanorods were synthesized by a direct electrodeposition technique on indium tin oxide plates. The effect of a systematic variation of the deposition potential and the inter-electrode distance on the morphology of ZnO nanorods was investigated. X-ray diffraction studies indicated that the nanorods are highly c-axis oriented. The diameter of the nanorods depends mainly on the deposition potential and the electrode separation. Our detailed study of the synthesis-morphology correlations provide the optimal electrochemical conditions required for oriented ZnO nanorods with minimum diameter.

The formation of amorphous phases in immiscible alloys with a large positive enthalpy of mixing is thermodynamically unfavorable. Co-sputter deposited Cu-Nb films exhibit a nanoscale phase separation into Cu-rich and Nb-rich amorphous regions. They show relatively high room temperature resistivity, a negative temperature coefficient of resistance (TCR), and an incomplete superconducting transition with onset at 3.7 K. Annealing the nanophase-separated amorphous films at 200 °C results in the nucleation of fcc Cu-rich nanocrystals within an Nb-rich amorphous matrix. This film exhibits multiple resistance steps, eventually showing a sharp drop with (T(C))(onset) = 3.7 K. Annealing at 350 °C leads to complete devitrification via the formation of large bcc Nb-rich grains encapsulating the existing fcc Cu nanocrystals. These films show low room temperature resistivity, positive TCR, and a sharp superconducting transition with onset at 5.2 K. The electrical transport and superconducting behavior appear to be consistent with a two-stage crystallization process.

The superconducting transition temperature (T(C)) in nanostructured Pb decreases from 7.24 to 6.4 K as the particle size is reduced from 65 to 7 nm, below which superconductivity is lost rather abruptly. In contrast, there is a large enhancement in the upper critical field (H(C2)) in the same size regime. We explore the origin of the unusual robustness of T(C) over such a large particle size range in nanostructured Pb by measuring the temperature dependence of the superconducting energy gap in planar tunnel junctions of Al/Al(2)O(3)/nano-Pb. We show that below 22 nm, the electron-phonon coupling strength increases monotonically with decreasing particle size, and almost exactly compensates for the quantum size effect, which is expected to suppress T(C).

The application of a dc voltage to an array of copper nanorods causes field evaporation of atoms from the tips, resulting in their progressive sharpening and a further increase in the local field. The process is self-limited by the build-up of space charge on the nanorod tips. From an analysis of the conductance noise recorded across the nanorod array, we show that the conduction mechanism bears a strong analogy with the stick-slip problem in sliding friction. The in situ sharpening results in an unprecedented lowering of the breakdown voltage of air by over 90%, as compared to plane parallel electrodes.

A parallel array of isolated metal nanowires is expected to be hydrophilic. We show, however, that a clustering of such nanowires brought about by vacuum drying produces a 'dual-scale roughness' and confers a strongly hydrophobic property to the surface. The mean size of the nanowire clusters as well as the contact angle are both found to be related to the wire length, and the critical wire length above which the surface becomes hydrophobic is ≈10 µm. Surface roughness is generally known to enhance water-repellent properties, but this is the first report of roughness-induced hydrophobicity on a bare (uncoated) metallic surface.

Silver nanoparticles were sputter deposited through self organized hexagonally ordered porous anodic alumina templates that were fabricated using a two-step anodization process. The average pore diameter of the template was 90 nm and the interpore spacing was 120 nm. Atomic force microscope studies of the sputter-deposited silver nanoparticle array on a Si substrate indicate an approximate replication of the porous anodic alumina mask. The nature of the deposition depends strongly on the process parameters such as sputtering voltage, ambient pressure and substrate temperature. We report a detailed study of the sputtering conditions that lead to an optimal deposition through the template.

A direct measurement of the superconducting energy gap by point contact spectroscopy in nanostructured Nb films shows that the gap decreases with a reduction in the average particle size. The superconducting T(c), obtained from transport and magnetic measurements, also decreases with size and scales with the energy gap. The size dependence of the superconducting properties in this intermediate coupling type II superconductor is therefore governed by changes in the electronic density of states rather than by phonon softening. Consistent with the Anderson criterion, no T(c) was observed for sizes below 8 nm.

The application of semiconductor quantum dots in important new areas such as random lasing and quantum-information processing requires knowledge of the coherence of the optical emission from such systems. We report the first direct experimental estimation of the coherence in the light emitted by a nanoparticle ensemble. The photoluminescence from a two-phase nanocomposite CdS-ZnO thin film (with a characteristic grain size of 2-3 nm for both the chemical phases) possesses an appreciable degree of spatial and temporal coherence at room temperature. The degree of spatial coherence was estimated from the classical Young's double slit experiment. We also discuss a simple technique for estimating the degree of spectral coherence of the photoluminescence from thin films.