1. Energy-related Materials
(go back to top)

Research projects conducted by graduate students and post doctoral researches at LAMSAT encompass multiple disciplines that are critical for future technologies. Some of the currently ongoing projects are described below.

• Materials for next-generation solar cells:
One of the limiting factors of the current pn-junction solar devices is the generation of one electron-hole pair per absorbed photon. Quantum dots of semiconductors such as PbSe and PbS can generate multiple electron-hole pairs (excitons) from a single high energy photon. Incorporation of these QDs into organic and inorganic solar devices has the potential to enhance device current due to multi-exciton generation-dissociation. The quantum dots of diameters in the range of 2-8 nm are grown by a solvothermal process. A Laser Assisted Spray (LAS) technique has been developed at LAMSAT to deposit the QDs on a substrate without any organic surfactants. QDs are deposited on nanorods of TiO₂ or ZnO to enhance current transport. The goal of this project is to develop an efficient flexible solar device.

• Thermoelectric materials:
Most mechanical and electrical devices that consume energy dispel part of the energy as heat.  For example, in an automobile, less than 30% of the energy produced by burning fuel is used for locomotion, while the rest is wasted as heat.  Thermoelectric power conversion devices offer the potential to convert most of this waste energy into electrical energy and help to reduce the energy cost.  

In order to convert thermal power into electrical power (IV) the following properties are desirable for the material; (a) low thermal conductivity (b) relatively high electrical conductivity (c) a high voltage drop across the material per unit temperature, which is the Seebeck coefficient.  Two of the material system that are under investigation at LAMSAT are the type I clathrate (Ba₈Ga₁₆Ge₃₀) films by the dual-laser ablation process and naoparticle coatings of the layered cobalt (Co) oxide Ca₃Co₄O₉ by a microwave plasma assisted spray process.      

• Nanomaterials for Solid State Lighting:
The need to replace the highly inefficient incandescent light bulb with a solid state Light Emitting Diode (LED), which is an order of magnitude more efficient, is at the forefront of the energy revolution. LEDs produce light in a narrow spectral bandwidth around a specific wavelength. Combinations of blue, green and red LEDs are being used to produce white light. However the color rendering (reproduction of ambient light) is poor. One of the approaches to convert narrowband emission from LEDs to broadband spectra is the use of phosphor materials. Better yet, if the phosphors can be excited by an external voltage source to generate electroluminescence efficiencies can be very high. New approaches are needed to generate sufficient intensities to produce a light source.
At LAMSAT the microwave assisted spray technique is being used to fabricate nanophosphors which will be embedded in a layered structure fabricated by the laser ablation technique. The goal of this project is to develop a nanophosphore based electroluminescence device that can produce high white light intensities with high efficiency.

2. Materials for Sensors and Communication
(go back to top)

• Multiferroics:
Ferroelectric (FE) and ferromagnetic (FM) materials and their composites fall in the group of functional materials which have applications in smart sensors, magnetic memory devices and communication devices.  Epitaxially grown FE and FM layered structures experience interfacial strain.  If the FE and FM order parameters can be coupled via the interfacial strain, the resulting magnetoelectric effect produces a multiferroic structure where the magnetic field can be altered by an applied electric field, and vice versa. Memory devices fabricated with multiferroics have the potential to quadruple the capacity.
At LAMSAT the dual laser ablation technique is being used to fabricate and characterize FE/FM heterostructures of variety of materials that include, CoFe₂O₄/PZT, ZnO:Mn/ZnO:V, BaTiO₃, and transition metal doped PZT. 

• PZT nanorods:
The piezoelectric material PZT exhibits multifunctionality and thin films and nanostructures of the material find applications in a variety of devices including smart sensors and energy harvesting. Application of a strain gives rise to charge separation in the material. This effect changes the transport properties of PZT/conductor layered structures and thus can be used to measure strain.

At LAMSAT an oblique incident dual-laser ablation technique is used to form vertically aligned nanorods of PZT that can be used to fabricate a nanoforce sensor that will have applications in MEMS devices.

3. Biomaterials
(go back to top)

Blood vessel damage elicits a natural response from the body to stop the bleeding. The cessation of bleeding, or hemostasis, is achieved by the coordinated activation of three mechanisms: (i) vessel spasm, which constricts blood flow (vasoconstriction), (ii) interaction of platelets in blood with exposed collagen at site of damage, producing adenosine triphosphate (ATP), adenosine diphosphate (ADP) and a growth factor that causes proliferation of vascular endothelial cells, smooth muscle cells and fibroblasts, and (iii) formation of a fibrous mesh from interacting activated platelets and chemicals released by damaged tissue. ADP release attracts additional platelets and promotes their adhesion to form a platelet plug. The fibrous mesh causes red blood cells to form a clot, stopping further bleeding. If the injury is severe, however, profuse bleeding may limit the ability of platelets of agglomerate and form a blood clot that is large enough to plug the opening.
A recent and innovative approach to controlling internal bleeding is to use artificial platelets made of biocompatible polymers. Nanoparticles made of the block copolymer poly(lactic-co-glycolic acid)-poly-L-lysine (PLGA-PLL) were used as the synthetic platelet.

At LAMSAT the LAS process for making nanoparticles will be used to develop a method of encapsulating within a PEG shell both the artificial platelet and ADP, and to investigate the effectiveness of these capsules as blood clotting agents. The capsules that are functionalized with relevant proteins will help to guide them towards the wound.

4. Dynamic Optical Plasma Studies
(go back to top)

Laser-generated plasmas have wide applicability in areas as diverse as laser-driven fusion, x-ray and ultra-uv generation for lithography and materials growth. In particular, the unique advantages of laser ablation for the growth of thin films and heterostructures are well-established. These include in-situ epitaxial growth of films due to high species energies and subsequent adatom mobility on the substrate enabled by laser ablation, the possibility of reactive deposition in a gaseous ambient, as well as the capability of depositing stoichiometric multi-component materials. A critical aspect in realizing the potential of materials growth using laser ablation is the determination of the dynamic evolution of the composition and energetics of the laser-ablated plume constituents prior to deposition.

At LAMSAT we have expertise in two-dimensional plume imaging applicable to complex systems using laser ablation, as well as microwave and laser-assisted spray pyrolysis-generated plasmas. Species-resolved 2-D plume dynamics using optical gated imaging has been demonstrated in multi-component systems. Past studies have included detailed optical analyses of the effect of temporal synchronization in dual-laser ablation. An example of the resultant change in plume expansion and dynamics is shown in the figure. Optical studies permit accurate manipulation of plume expansion and dynamics. This results in precise control of the plasma temperature in dual-laser ablation by nanosecond synchronization of the coupling of the CO₂ laser into the excimer laser-generated plasma of the ablated materials. Plasma temperatures above 30,000K with resultant plume ionization enhancements from 8% to above 90% now permit manipulation of the highly ionized plasma via bias voltages to achieve high species velocities. This dramatically alters plume reactivity with ambient species and the quality of the deposited film.

In addition to standard diagnostic species-resolved, two-dimensional, line-of-sight, time-gated optical imaging of high-energy plasmas with nanosecond precision, current and future experiments are directed at the development of novel optical imaging techniques combining the advantages of ultrafast spectroscopy and computed tomography. The objective is to generate real-time, three-dimensional, species-resolved images of multi-component laser-ablated plumes, thereby permitting previously unattained insight into the hydrodynamic flow, high temperature energy exchange and stoichiometric dynamics of a single plume as well as chemical reactivity of multiple high-energy colliding plumes. This imaging system will further the potential use of laser-generated plumes in the growth of thin films and nanostructures using laser-based techniques, and establish a paradigm for real-time three-dimensional visualization extendable to other plasma systems for scientific and industrial purposes.