Darayas Patel

STRONG VISIBLE UPCONVERSION IN RARE EARTH ION-DOPED NaYF4 CRYSTALS

NaYF4:Er3+, Yb3+ crystals were prepared by simple synthetic method. Under 980 nm laser excitation, 408 nm, 539 nm and 655 nm upconversion emissions were recorded. Laser power and signal intensities of the upconverted emissions were obtained to understand the upconversion mechanisms.

Investigation of transmembrane protein fused in lipid bilayer membranes supported on porous silicon

This article investigates a device made from a porous silicon structure supporting a lipid bilayer membrane (LBM)fused with Epithelial Sodium Channel protein. The electrochemically-fabricated porous silicon template had pore diameters in the range 0.2~2 µm. Membranes were composed of two synthetic phospholipids: 1,2-diphytanoyl-sn-glycero-3-phosphoserine and 1,2-diphytanoyl-sn-glycero-3-phosphoethanolamine. The LBMwas formed by means of the Langmuir-Blodgett and Langmuir-Schaefer techniques, at a monolayer surface tension of 26 m Nm−1 in room temperature and on a deionized water subphase, which resulted in an average molecular area of 0.68–0.73 nm2. Fusion of transmembrane protein was investigated using Atomic Force Microscopy. Initial atomic force microscopy results demonstrate the ability to support lipid bilayers fused with transmembrane proteins across a porous silicon substrate. However, more control of the membrane’s surface tension using traditional membrane fusion techniques is required to optimize protein incorporation.

Electrophysiology of Epithelial Sodium Channel (ENaC) Embedded in Supported Lipid Bilayer Using a Single Nanopore Chip

Nanopore-based technologies are highly adaptable supports for developing label-free sensor chips to characterize lipid bilayers, membrane proteins, and nucleotides. We utilized a single nanopore chip to study the electrophysiology of the epithelial Na+ channel (ENaC) incorporated in supported lipid membrane (SLM). An isolated nanopore was developed inside the silicon cavity followed by fusing large unilamellar vesicles (LUVs) of DPPS (1,2-dipalmitoyl-sn-glycero-3-phosphoserine) and DPPE (1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine) to produce a solvent-free SLM with giga-ohm (GΩ) sealed impedance. The presence and thickness of SLM on the nanopore chip were confirmed using atomic force spectroscopy. The functionality of SLM with and without ENaC was verified in terms of electrical impedance and capacitance by sweeping the frequency from 0.01 Hz to 100 kHz using electrochemical impedance spectroscopy. The nanopore chip exhibits long-term stability for the lipid bilayer before (144 h) and after (16 h) incorporation of ENaC. Amiloride, an inhibitor of ENaC, was utilized at different concentrations to test the integrity of fused ENaC in the lipid bilayer supported on a single nanopore chip. The developed model presents excellent electrical properties and improved mechanical stability of SLM, making this technology a reliable platform to study ion channel electrophysiology.

Lipid Bilayer Membrane in a Silicon Based Micron Sized Cavity Accessed by Atomic Force Microscopy and Electrochemical Impedance Spectroscopy

Supported lipid bilayers (SLBs) are widely used in biophysical research to probe the functionality of biological membranes and to provide diagnoses in high throughput drug screening. Formation of SLBs at below phase transition temperature (Tm) has applications in nano-medicine research where low temperature profiles are required. Herein, we report the successful production of SLBs at above—as well as below—the Tm of the lipids in an anisotropically etched, silicon-based micro-cavity. The Si-based cavity walls exhibit controlled temperature which assist in the quick and stable formation of lipid bilayer membranes. Fusion of large unilamellar vesicles was monitored in real time in an aqueous environment inside the Si cavity using atomic force microscopy (AFM), and the lateral organization of the lipid molecules was characterized until the formation of the SLBs. The stability of SLBs produced was also characterized by recording the electrical resistance and the capacitance using electrochemical impedance spectroscopy (EIS). Analysis was done in the frequency regime of 10−2–105 Hz at a signal voltage of 100 mV and giga-ohm sealed impedance was obtained continuously over four days. Finally, the cantilever tip in AFM was utilized to estimate the bilayer thickness and to calculate the rupture force at the interface of the tip and the SLB. We anticipate that a silicon-based, micron-sized cavity has the potential to produce highly-stable SLBs below their Tm. The membranes inside the Si cavity could last for several days and allow robust characterization using AFM or EIS. This could be an excellent platform for nanomedicine experiments that require low operating temperatures.

Polymer-inorganic nanocomposite thin film emitters, optoelectronic chemical sensors, and energy harvesters produced by multiple-beam pulsed laser deposition

Large class of new photonic devices, including light emitters, chemical sensors, and energy harvesters, can be made of the polymer-inorganic nanocomposite thin films produced by the new multiple-beam pulsed laser deposition process (MB-PLD). We describe the PLD system and the film deposition process itself, particularly the multiple-beam matrix assisted pulsed laser evaporation (MB-MAPLE) version with laser beam scanning and plume direction control. We also report on the results of the investigation of optical and performance characteristics of three types of the fabricated nanocomposite thin film devices: upconversion light emitters, chemical (ammonia) sensors, and thermoelectric energy harvesters. The emitters were made of poly(methyl methacrylate) (PMMA) film impregnated with the nanoparticles of rare-earth (RE) fluorides such as NaYF4: Yb3+, Er3+ and NaYF4: Yb3+, Ho3+. They demonstrated bright upconversion emission in visible region being pumped with a 980-nm infra-red laser. The same films, but doped with an indicator dye, were tested as ammonia sensors. They demonstrated the drop of upconversion emission (registered by a photodetector) due to the rise of the optical absorption of the indicator dye affected by ammonia. The capability of detecting fractions of one percent (molar) of ammonia was established. The thermoelectric energy harvesters were made of nanocomposite films of aluminum-doped zinc oxide (AZO) impregnated with polymer nanoparticles. The role of the nanoparticles was to reduce the thermoconductivity and increase electroconductivity thus contributing to the improvement of the thermoelectric figure-of-merit ZT.

Multi-beam pulsed laser deposition: new method of making nanocomposite coatings

Huge number of new photonic devices, including light emitters, chemical sensors, and energy harvesters, etc. can be made of the nanocomposite coatings produced by the new multi-beam pulsed laser deposition (MB-PLD) process. We provide a short review of the conventional single-beam PLD method and explain why it is poorly suitable for making nanocomposite coatings. Then we describe the new MB-PLD process and system, particularly the multiple-beam matrix assisted pulsed laser evaporation (MB-MAPLE) version with laser beam scanning and plume direction control. The latter one is particularly designed to make organic (polymer) - inorganic functionalized nanocomposite coatings. Polymer film serves as a host for inorganic nanoparticles that add a specific functionality to the film. We analyze the properties of such coatings using the examples of poly(methyl methacrylate) (PMMA) films impregnated with the nanoparticles of rare-earth (RE) upconversion phosphors. They demonstrated the preservation of microcrystalline structure and bright upconversion emission in visible region of the phosphor nanoparticles after they were transferred in the polymer matrix during the MB-MAPLE process. The proposed technology has thus proven to serve its purpose: to make functionalized polymer nanocomposite coatings for a various potential applications.

Ammonia sensor based on polymer-inorganic nano-composite thin film upconversion light emitter prepared by double-beam pulsed laser deposition

One of the unique properties of the sp2-carbon allotropes, such as fullerenes, carbon nanotubes and graphenes, is that their electronic structures differ significantly among them according to characteristic electron confinement based on their dimensionality and geometric structures, which can be influenced not only by charge injection and chemical bonding but also structural modification. In this talk, I will discuss the electronic structures of various sp2-carbon allotropes on metal substrates investigated by scanning tunneling microscopy and spectroscopy. In particular, it is focused on the one dimensional (1D) electronic structure in a graphene nano wrinkle (GNW) of an epitaxially grown graphene (EG) sheet on Ni(111), the width of which was small enough (less than 5 nm) to cause 1D electron confinement. Use of spatially resolved, scanning tunneling spectroscopy revealed band-gap opening and a 1D van Hove singularity in the GNW, as well as the chemical potential distribution across the GNW. Our demonstration of 1D electron confinement in an EG is the novel possibility of controlling its electronic properties not by chemical modification but by mechanical structuring in a controlled manner. Graphene-based carbon materials such as fullerenes, carbon nanotubes, and graphenes have distinct and unique electronic properties that depend on their dimensionality and geometric structures. Graphene wrinkles with pseudo one-dimensional structures have been observed in a graphene sheet. However, their one-dimensional electronic properties have never been observed because of their large widths. Here we report the unique electronic structure of graphene nanowrinkles in a graphene sheet grown on Ni, the width of which was small enough to cause one-dimensional electron confinement. Use of spatially resolved, scanning tunnelling spectroscopy revealed bandgap opening and a one-dimensional van Hove singularity in the graphene nanowrinkles, as well as the chemical potential distribution across the graphene nanowrinkles. This observation allows us to realize a metallic-semiconducting-metallic junction in a single graphene sheet. Our demonstration of one-dimensional electron confinement in graphene provides the novel possibility of controlling its electronic properties not by chemical modification but by ‘mechanical structuring’. Graphene wrinkles, which are one-dimensional (1D) folded graphene structures, have generally been observed in graphene produced by chemical vapour deposition. These structures have been thought to be the result of the difference in the thermal expansion coefficient between graphene and its substrate. A graphene wrinkle is chemically bonded with surrounding planar epitaxial graphene. Therefore, its unique geometric structure is distinct from those of carbon nanotubes and graphene nanoribbons which are indisputably 1D structures. Hence, we define a graphene wrinkle as a ‘pseudo 1D structure’ to indicate that it has a 1D shape, but is still a part of a two-dimensional structure. In the following, we demonstrate the 1D electron confinement in graphene nanowrinkle (GNW) by scanning tunnelling microscopy/spectroscopy (STM/STS), whose width is <5 nm. Moreover, spatially resolved electronic structures have been investigated, and the manipulation of graphene geometry by STM tip has been demonstrated. Our results imply that a semiconducting property can be realized by the mechanical deformation of the graphene geometry not by chemical modification, which is analogous to the case of a strain-induced pseudo magnetic field that was discovered in deformed ‘graphene nanobubbles’. The lack of surface functionalization in our approach can prevent the mobility decline due to chemical defects. Moreover, the covalent bonding at the metallic pEG-semiconducting GNW junction can reduce the contact resistance. Our results demonstrate that the interfacial interaction between graphene and the metal substrate provides a novel way to realize a metallic-semiconducting-metallic junction within a single graphene sheet. Results: Structural characterization of GNWs Epitaxial graphene with GNWs was synthesized by dissociating acetylene on a clean Ni(111) surface. A rapid cooling process is necessary, which is the most critical step to synthesize GNWs. Most of the GNWs were observed in the region where the terrace width of the underlying Ni surface was as small as several tens of nanometres. These GNWs have been recoloured with orange, and a line profile along the white arrow, which shows that the GNWs on the terrace have larger widths and lower heights than the GNWs at the step edges. We should note that all GNWs were formed at the step edges (red triangles or propagated from kinks at the step edges of the Ni surface, the implication being that the geometrical structure of the underlying Ni must play a crucial role in the formation of GNWs To analyse the structure of the GNWs in detail, we obtained atomically resolved STM images from an isolated GNW on the terrace under different scanning conditions. The top and bottom regions of the GNW in were scanned at a sample bias (Vs) of 1 V and a feedback current (If) of 1 nA, whereas the centre region was scanned with a smaller tip–sample distance.The objective of the paper was to demonstrate feasibility of an ammonia sensor using polymer –inorganic nano-composite thin film upconversion light emitters made by the new double-beam pulsed laser deposition method. The existing pulsed laser deposition vacuum chamber was modified to accommodate two laser beams of different wavelengths for the in-situ ablation of two targets: a polymer host poly(methyl methacrylate) mixed with indicator dye Phenol Red and the brilliant rare earth doped upconversion phosphor NaYF4:Yb3+, Er3+. Nano-composite films were deposited on silicon substrates by the proposed method with near-infra-red laser radiation (1064-nm wavelength) ablating the polymer target dissolved in Gamma-butyrolactone together with the indicator dye, and frozen in circulating liquid nitrogen (matrix assisted pulsed laser evaporation – MAPLE), and visible radiation (532 nm) ablating the inorganic target. The deposited nano-composite films retained bright green upconversion fluorescence with a spectral peak at 540 nm attributed to the inorganic phosphor nano-particles pumped with the 980-nm infrared laser diode. The spectrum of the green emission matched the absorption band of the indicator dye exposed to ammonia. When the films were exposed to ammonia, they demonstrated an optical response in the form of the drop of the intensity of green radiation monitored with a silicon photodiode. The sensitivity of the opto-electronic sensor of ammonia based on the nano-composite films was measured to be close 0.4% ammonia in air, and the response time was 5 minutes.Superconductors possess unique properties such as zero electrical resistance and expulsion of magnetic fields below a critical temperature Tc. They can carry electric current without any energy loss and have many applications. However, understanding superconductivity is a great challenge. Especially, anomalously small isotope effect in some high and low Tc superconductors such as YBa2Cu3O7 (YBCO), Nb3Sn, Zr, created a great challenge for understanding. To solve the puzzle, a new methodology is implemented by integrating first-principles calculations of electronic structures of the materials into the theory of many-body physics for superconductivity. The aim is to seek a unified methodology to study the electronic and superconducting properties of the materials. It is demonstrated from first-principles that the extended saddle point singularities in the electronic structures of the materials such as YBCO, Nb3Sn, Zr, strongly correlate with the anomalous isotope effect in these superconductors. Some guidance for finding new high Tc superconductors will also be discussed.

Double pulse laser deposition of polymer nanocomposite: NaYF4:Tm3+,Yb3+ films for optical sensors and light emitting applications

The main objective of this paper is to demonstrate the feasibility of producing operationally Nanocomposite polymeric thin films for sensor and light emitting applications using the innovative modified double pulsed laser deposition (DPLD) technique. The existing PLD vacuum chamber was modified to accommodate multiple wavelength laser beams for in-situ-double-ablation/Deposition (DPLD) of multiple targets of host and dopants. Special design was made for cooling of the target to the threshold of the polymer ablation without interrupting the continuity of the ablation process. Multilayered of nanocomposites of acrylic polymers and nanoparticles of NaYF4:Tm3+ ,Yb3+ are fabricated using ultra-violet (UV) radiation (355 nm) ablating polymer targets and near-infra-red (near-IR) radiation (1064 nm) ablating inorganic targets. The films were characterized using the reflected high energy electron diffraction (RHEED), XRD , XRF, AFM, and FTIR absorption spectroscopy and tested as possible chemical sensors and light emitters.

Synthesis, spectroscopic studies, and AFM analysis of Ho3+ doped zirconium oxide nanoparticles

Nanometer sized crystals has attracted a lot of attention in the recent years because of the size dependent optical and electrical properties. Rare-earth ions in insulating hosts find use in a variety of applications, including phosphors for fluorescence lighting displays, monitors, lasers, fiber optic communication, biolabeling, bioimaging and solar-energy converters. Nanocrystals of trivalent holmium ion doped zirconium oxide crystals have been synthesized by using simple synthetic method. Spectroscopic studies of these nanocrystals have been conducted using visible and infrared laser excitation. Size distribution of the nanoparticles will also be discussed using atomic force microscope.

Optical gain in capillary light guides filled with NaYF 4 : Yb 3+ , Er 3+ nanocolloids.

A capillary light guide optical amplifier using nanocolloids of Yb3+-Er3+ co-doped NaYF4 as a filler was successfully demonstrated. A 7-cm-long and 150-micron-inner-diameter capillary light guide was capable to amplify a pulsed optical signal at 1550 nm with a gain coefficient of 0.15 cm-1 at a pump power of 4 mW (980-nm wavelength). The nanocolloid gain medium was prepared by pulverizing the phosphor powder with a high-speed planetary ball mill. Ball milling of the powder in water produced nanoparticles with a size of approximately 130 nm that after drying were transferred to a liquid with high refractive index (1.551 at 1550 nm) required to maintain light confinement within the fused silica capillary light guide. The results show that RE-doped colloids of nanocrystals can be potentially used as liquid gain media fillers in capillary light guide lasers and amplifiers with high photostability and low toxicity.