Nanofabrication by Miniaturization
A new technique has been proposed and demonstrated for nano-fabrication using hydrogels. The technique can be used both for 3D objects as well as 2D patterning. The essential idea is to repeatedly shrink the hydrogel molded replicas of an original micro-object to sub-micron size by controlled drying.
Soft Lithographies
Three new soft lithography techniques based on self-organization of polymer thin films have been developed. These are based on the control of self-organization of surface features in thin soft polymer films as they come in adhesive contact with another surface (stamp). In the first method (Elastic Contact Lithography), reproduction occurs by the contact instability of a soft surface, morphology of which is controlled by the stamp movement. In the second method (Adhesive Force Assisted Lithography), flexible thin foils of metal, PVA and PDMS have been used for imprinting of curved surfaces by adhesive conformal contacts. Both the techniques have large application potential in patterning of large areas on sub-micron levels. An Indian patent and PCT has been filed for the first technique.
The third technique is based on controlled dewetting of thin (< 100 nm) polymer films to produce large area sub-micron patterns.
In a related development, we (I collaboration with JNC) have shown that application of external electric fields can control the surface instability of soft polymers. Further work has been pursued to better control this technique for a novel soft lithography.
Superhydrophobic Surfaces
Controlled growth of silica sol-gel films at oil-water interfaces have produced highly asymmetric.
Super-hydrophobic (face in contact with aqueous medium) and hydrophilic (face in contact with oil) films that are further being probed for their preparation and applications in textile treatments.
Nano-scale Forces
We have measured a new type of electrostatic double layer ATTRACTION between two negatively charged surfaces I the presence of trace amounts of a cationic surfactant. This result has far reaching implications in surface and bio-surface adhesion and colloidal and nanoparticle dispersion stability.
Micro/Nano Hierarchal Carbon Webs for Catalytic Adsorption and Carbon MEMS
We have demonstrated (1) preparation of activated and metal nano-particle functionalized carbon micro/nano webs for catalytic adsorption of Sox, NOx and VOCs.
In a related development, we have developed novel techniques for fabrication of carbon MEMS, which are being tested for micro-battery and bio-platform applications.
Coexistence of superconductivity and magnetism in 2-dimensional nanosystems consisting of doped transition metal oxides
Oriented LSMO-YBCO-LSMO films have been successfully synthesized on (110) STO substrates. To the best of our knowledge, ours is the first successful effort in growing this difficult structure. Since the CuO2 planes in these structures are perpendicular to the manganite layer, we will be able to inject spin polarized carriers directly along the fully gapped directions of the Fermi surface. Such experiments are potentially very exciting. We have measured the interlayer exchange coupling in these films as a function of temperature in the superconducting as well as the normal state of the YBCO. These data are being analyzed currently. We are also installing a broad beam ion gun for patterning of these films.
Field–induced superconductivity has been studied in nanoengineered structures of CoPt and NbN created by pulsed laser ablation followed by focused ion beam milling. These structures show a novel reentrant superconducting behavior.
Synthesis of nanoparticles of doped oxides and multicomponent metal alloys using the technique of liquid phase pulsed laser ablation
Several chemical methods have been developed in recent years to synthesize monodispersed colloids of ferromagnetic nanoparticles. While these techniques work well for simple elemental and oxides based ferromagnets, there are difficulties in producing alloy nanoparticles through this route as the process involves toxic and often inflammable metalloganics combined with difficulties in stoichiometry control. We have developed a pulsed laser ablation in aqueous medium (PLAAM) technique to synthesize nanoparticles of a variety of materials. Nanoparticles of silver and CoPt have been synthesized and investigated in detail. This technique has also been used to successfully make nanoparticles of BiFeO3 and YBa2Cu3O7.
In the case of silver, the colloidal solution has a bright yellow colour due to surface plasmon modes of silver nanoparticles. Transmission electron microscopy measurements on Ag and CoPt solutions reveal a particle size of <5 nm.
Magnetic relaxation and superparamagnetism of non-interacting, disordered CoPt nanoparticles
A plate of Co0.5Pt0.5 alloy synthesized by arc melting of high purity (4N) cobalt and platinum chunks followed by homogenization at high temperature was suspended horizontally in a bath of triply distilled milliq water. A focussed beam of pulsed Nd-YAG laser (Quantel Big Sky, pulse width ~6 nanoseconds) irradiated the plate from the top. The laser was operated at 20 Hz with a combined energy density of 14 J/cm2 at 1064 nm and 532 nm wavelengths. A light brown but clear solution, with no evidence of any sedimentation, was obtained after 25 minutes of ablation. In order to avoid agglomeration of particles, sodium dodecylsulfate (SDS) was added to the solution immediately after the ablation process and homogenized by ultrasonification for 10 minutes. The colloidal solution was concentrated through the process of vacuum evaporation of excess water. The larger particles and agglomerates have been separated subsequently through centrifugation at ~3000 g of the resultant solution for 15 minutes.
As evident from the micrographs, the nanoparticles have a droplet type of morphology and their size varies from ~1.5 nm to ~6 nm. A careful examination of the morphology of larger droplets indicates agglomeration of much smaller particles.
Cobalt platinum nanoparticles dispersed in surfactant polymer matrix were synthesized successfully using a simple and versatile technique based on pulsed laser ablation of a composite alloy target in an aqueous medium. The technique is particularly suitable for preparation of binary and multi elemental alloy nanoparticles, whose size can be controlled to some extent by a judicious choice of laser ablation parameters such as laser fluence, pulse repetition rate and laser wavelength. A choice of liquid medium may also allow in-situ coating of the nanoparticles to form a core shell structure. For CoPt nanoparticles prepared using this technique the zero-field-cooled and field-cooled-magnetization measurements at low-fields reveal two distinct temperatures (260 K and 4 K) where the ZFC and FC curves bifurcate from each other, corresponding to the characteristic blocking of an aggregate of superparamagnetic particles. Magnetization vs. field measurements at different temperatures shows strong evidence of superparamagnetic nature of the particles. For temperatures below 4 K, the magnetization curves are hysteretic. The low coercive field (~400 Gauss) of this assembly at 2K is essentially a characteristic of low temperature fcc phase of Co0.5Pt0.5 nanoparticles. Magnetization measurements suggest a distribution of two different particle sizes centered at ~ 1.3 nm and ~ 5.9 nm, which is consistent with electron microscopy observations. The magnetic anisotropy energy of these particles is 0.9 X 10-6 J/m3 and 0.1 X 10-6 J/m3 respectively. A two-particle distribution model agrees with the behavior of isothermal magnetization and magnetic relaxation data.
We note that with the anisotropy energy of 0.1 X 10-6 J/m3 a ~16 nm diameter single domain CoPt nanoparticle would have blocking temperature of ~300 K. The anisotropy energy and hence the blocking temperature of CoPt can be enhanced through stabilization of the ordered fct phase on annealing at high (? 600 0C) temperatures. Future research in this area would involve in-situ growth of a ceramic shell around CoPt nanoparticles so that high temperature annealing can be realized without agglomeration.
BiFeO3 nanoparticles:
The magnetic properties of multiferroic perovskite nanoparticles have been studied at low temperatures, using prototypical compound bismuth ferrite BiFeO3. Bismuth ferrite compound was synthesized using solid state sintering route and characterized by powder X-ray diffraction. Pulsed laser ablation technique was used to make its nanoparticles. Estimate of the size of nanoparticles was obtained using Transmission Electron Microscopy and EDAX measurement was done to characterize the nanoparticles. Magnetic properties were studied in Superconducting Quantum Interference Device at low temperatures.
YBa2Cu3O7 nanoparticles:
Bulk sample of YBCO superconductor is prepared by mixing, grinding and heating three compounds Y2O3, BaCO3 and CuO. Small pallets of this bulk sample is prepared with diameter 1.5 cm. Bulk YBCO is tested and is found to be superconducting at 93 K. Pulsed Laser Ablation is used to produce nanoparticles suspended in hexane-isopropanol solution. TEM of this solution reveals presence of particles of ~30 nm. EDAX and SQUID magnetometer are used to determine structural and magnetic properties of the prepared sample.ZnGa2O4 bulk and nano phosphors (M.Sc. Project of Bhubanjyoti Bhattacharya):
The photoluminescence properties of the phosphor ZnGa2O4 doped with Mn2+ were studied. The origin of emission from the pellets of the phosphor material, under ultra-violet excitation was traced to inner transitions of the 3d electrons in the Mn2+ ions located on three different sites of the host crystal. Low temperature photoluminescence measurements show that competing mechanisms are responsible for the emission from the phosphor. An approach has been made towards searching for the origin of light emission from the phosphor when doped with the transition metal Mn. Nanoparticles of ZnGa2O4 have been synthesized using the liquid phase ablation technique with some success. Attempts will be made to enhance the photoluminescence characteristics of these particles.
Studies of spin polarized percolation in 2-dimensions: Studies of spin polarized transport in mesoscopic systems is an emerging area of research. We have successfully created 2D fractal and percolating networks of Iron on MgO substrate using the technique of pulsed laser ablation. The ac and dc electrical transport in these systems is being investigated in the presence of a magnetic field.
New and Significant Observations
Adhesion at soft interfaces can be enhanced by upto two orders of magnitude by imbedding micro/nano structures in an elastomeric film such as air pockets and liquid carrying channels
Nano-fabrication using soft hydrogels by replica molding and gel shrinkage is an effective method of fabrication for 3d objects as well as 2D patterning.
Electrostatic double layer attraction between like charge surfaces tunable by pH, salt and surfactants.
Growth of highly asymmetric nano-porous silica films at aqueous-oil interfaces.
Morphology and length scale of elastic contact induced instability can be tuned by film thickness and movement of adhering surfaces.
Application of electric fields can deform surfaces of soft polymers like PDMS in a periodic way with a well defined wavelength. The process can be further tuned by spatio-temporal variation of the electric field. This can have implications in a new soft lithography.
A Larkin-Ovchinnikov-Fulde-Ferrel type of inhomogeneous superconducting state has been observed for the first time in ultra thin heterostructures of doped oxides.
Reentrant superconductivity has been seen for the first time in a designer nanostructure consisting of CoPt and NbN.
Percolating and fractal 2-dimensional networks of Iron has been grown for the first time using pulsed laser ablation. These reveal fascinating aspects of spin polarized electron transport.
Innovations
A novel micro/nano fluidic adhesive has been prepared and shown to be vastly more effective.
A novel nano fabrication method based on gel miniaturization has been proposed.
Three novel methods for self-organized sub-micron patterning of polymers on large areas have been developed and currently being further fine-tuned for applications.
A novel highly asymmetric nano-porous silica surface has been synthesized. One face of this surface is super-hydrophobic and the other is hydrophilic. Potential applications are in textile and barrier coatings.
Application of electric fields to pattern the surfaces of soft visco-ealstic materails on sub-micron lengthscales by spatio-temporal variation of the e-field.
A novel technique for synthesis of colloidal solution of nanoparticles has been developed. This pulsed laser based method allows synthesis of multicomponent oxide and metal alloy nanoparticles, which is rather difficult to achieve with the know methods.
Stress tuning has been used to create novel 2-dimensional fractal networks of iron.
Application Potential
Pressure sensitive super adhesives that are reusable.
Soft micro/nano fabrication for complex objects.
Elastic Contact Lithography and Adhesive Force Assisted Lithography can be useful for large area, inexpensive sub-micron patterning of polymers for opto-electronics, MEMS and bulk nano-applications including control of wettability, adhesion and optical properties.
The colloidal solution of metal nanoparticles can be used as inks in inkjet printing of circuits and magnetic structures.
Asymmetric silica films can make textile coatings that allow an easy passage of sweat but are water resistant from outside. Inner feel is like cotton, but outside is like very non-wetable polyster.
Process conditions for the growth of 2-dimensional Ferromagnet-Superconductor-Ferromagnet have been optimized. Rich dividends are expected from this research in terms of understanding and applications of oxides based nanostructures.
