Devesh C. Deshpande

Investigation of femtosecond laser assisted nano and microscale modifications in lithium niobate

A study of the physicochemical modifications at micro and nano scales as a result of femtosecond laser processing is essential to explore the viability of this process to write surface and subsurface structures in transparent media. To this end, scanning probe and transmission electron microscopy and spectroscopy techniques were used to study these modifications in lithium niobate. A variable power Ti:Sapphire system (800nm,300fs) was used to determine the ablation threshold of (110) lithium niobate, and to write these structures in the substrate for subsequent analysis. Higher processing energies were used to amplify the laser-induced effects for a clear understanding. Evidences of a number of simultaneously occurring mechanisms such as melting, ablation, and shockwave propagation are observed in the scanning electron microscope (SEM) micrographs. X-ray diffraction (XRD), Auger and electron dispersive spectroscopy (EDS) studies indicate loss of lithium and oxygen from the immediate surface of the process...

Aligned nanowire growth using lithography-assisted bonding of a polycarbonate template for neural probe electrodes

This research presents a fabrication method of vertically aligned nanowires on substrates using lithography-assisted template bonding (LATB) towards developing highly efficient electrodes for biomedical applications at low cost. A polycarbonate template containing cylindrical nanopores is attached to a substrate and the nanopores are selectively opened with a modified lithography process. Vertically aligned nanowires are grown by electrochemical deposition through these open pores on polyimide film and silicon substrates. The process of opening the nanopores is optimized to yield uniform growth of nanowires. The morphological, crystalline, and electrochemical properties of the resulting vertically aligned nanowires are discussed using scanning electron microscopy (SEM), x-ray diffraction (XRD), and electrochemical analysis tools. The potential application of this simple and inexpensive fabrication technology is discussed in the development of neural probe electrodes.

Investigation of Nanoscale Electro Machining (nano-EM) in Dielectric Oil

There is a great need to produce nanoscale features on diverse and sometimes difficult-to-machine materials for electronic and bio-medical applications. This paper describes an electro machining (EM) process at nanoscale, implemented using a scanning probe platinum-iridium (Pt-lr) electrode tool (15-20 nm radius) in dielectric oil. The experiments were conducted by maintaining a gap of 2 nm between the tool and the work-piece. These experiments have successfully demonstrated that controlled and consistent machined features, as small as 10 nm in diameter, on atomically flat gold can be obtained. It is suggested that resonant tunnelling through dielectric oil molecules causes the removal of material.

Nanoscale surface and subsurface defects induced in lithium niobate by a femtosecond laser

In this letter, electron and ion microscopy techniques have been used to characterize the changes that result when single crystals of lithium niobate are processed using a focused femtosecond laser. The prevailing observation is that of competing processes—ablation and partial redeposition, thermal shock, and extreme quenching, as well as effects associated with shock wave propagation, resulting in both amorphization and heavily defective regions at the focal point of the laser pulse. The observed microstructural defects have a direct implication in optical memory or waveguide writing, where the goal is to realize consistent structural features with uniform optical properties.

Investigation of Femtosecond Laser-assisted Micromachining of Lithium Niobate

Lithium Niobate has a potential for applications in electronics and communication industries due to its unique electro-optical, piezoelectric and nonlinear properties. Femtosecond laser machining offers the best alternative to machine the mechanically fragile and optically delicate lithium niobate crystal. This paper reports a study of the effect of femtosecond laser machining on the surface integrity of lithium niobate. The transmission electron microscopy reveals a 100nm thin amorphous region and a void. The chemical analysis shows a loss of lithium and oxygen from the surface and sub-surface. Optical illumination facilitates the selective readout of the written spots of 2 microns size.

Development of a nanoscale heterostructured glucose sensor using modified microfabrication processes

We discuss the development of a novel amperometric sensor to detect glucose concentrations in solution. Inorganic, vertically aligned nanowire arrays were employed as the sensing electrode in place of planar electrodes to utilize the unique properties of nanostructures, resulting in enhanced sensing signal from a smaller area. Heterostructured gold/platinum nanowires were used so that the dual functions of the nanoelectrodes for covalent immobilization of glucose oxidase and enhanced oxidation of hydrogen peroxide can be achieved using modified microfabrication methods. Two different enzyme immobilization methods—using self-assembled monolayers of alkanethiols and a porous conducting polypyrrole matrix—were investigated as methods for functionalizing the electrodes. Glucose sensing results were compared for planar and nanowire electrodes and the heterostructured nanowire electrodes. The results indicate that the unique structure of the sensing electrode delivers superior sensing performance from a smaller geometric area of electrodes, thus enabling further miniaturization of the sensor.

Enzyme electrodes immobilized on hetero-structured metallic nanowire array for glucose sensing

The fabrication of hetero-structured vertically aligned nanowire arrays and enzyme immobilization on their surface is presented for a glucose sensor with high sensitivity. Hetero-structured nanowires of gold and platinum are fabricated by hybrid polycarbonate membrane assembly and electrochemical deposition processes and glucose oxidase are attached on their surface by covalent immobilization. Platinum and gold hetero-structured nanoelectrodes with enzyme are evaluated to detect hydrogen peroxide produced in the enzyme reaction without the need for the artificial redox mediator, which is not viable on a homogenous gold electrode. Chronoamperometric current behavior is demonstrated with various concentrations from 0.5 mM to 28 mM. In this research, the combination of enzyme immobilization and sensing surfaces on nanowire arrays has shown superior performance with regards to the sensitivity and response time.

Nanodevices for biosensing applications

This research discusses the development of biosensors with vertically aligned nanowires, and the evaluation of their physical properties, electrochemical performance and biocompatibility. The developments include neurotransmitter (dopamine) sensors, glucose sensors for continuous monitoring, potassium ion sensors and integration of those sensors. A hemi-cylindrical nanocavity structure has been developed for dopamine sensing using redox cycling with radial diffusion within the cavities. By immobilization of enzymes in a conducting polymer matrix on vertically aligned nanowires, glucose sensing electrodes have been obtained with high sensitivity and selectivity. In addition, potassium sensing, potentially useful for monitoring changes of extracellular potassium concentration during myocardial ischemia, has been demonstrated using ion selective membranes (ISM) on nanowires. Sensor developments and measurement results are included in the presentation along with descriptions of top-down and bottom-up nano-/micro-fabrication technologies such as lithography and thin film deposition.

Development of amperometric glucose sensors with heterostructured nanowire arrays for continuous subcutaneous monitoring

This research discusses the development of a novel amperometric sensor to detect glucose concentrations in solution without the need for an artificial mediator. Since the intended goal of this research is to develop a glucose sensor to subcutaneously monitor glucose levels in the body, it is important that the sensor does not require a mediator, since such chemicals would prove harmful to the body. Nanowire arrays were used as the sensing electrode in place of planar electrodes to utilize the unique properties of nanostructures. Heterostructured Au/Pt nanowires were used so that the dual roles of covalent immobilization of glucose oxidase and oxidation of hydrogen peroxide could be carried out by the sensing electrode. Glucose oxidase was immobilized on these nanowires using self- assembled monolayers of alkanethiols and using a conducting polypyrrole matrix. Results indicate that the unique structure of the sensing electrode delivers superior performance with regards to sensitivity and response time in the absence of an artificial mediator. The development of such a sensor would assist the treatment of patients in an effective and timely manner. Ongoing efforts will help understand the process fabrication and analysis in detail.