Surface Plasmons are coherent electron oscillation that propagates along the metal-dielectric interface together with an electromagnetic wave. These are the pillar stones of applications as varied as ultrasensitive optical biosensing, photonic metamaterials, light harvesting, optical nanoantennas and quantum information processing. Here, I propose to use graphene plasmons for optical antennae which provide a suitable alternative to noble-metal plasmons because they exhibit much tighter confinement, lower losses and relatively long propagation distances, with the advantage of being highly tunable via electrostatic gating. Plasmon resonances in graphene can be tuned over broad Terahertz frequency range by changing width of graphene ribbon width and in-situ electrostatic doping and so can be used as novel materials for optical antennas. Also, graphene plasmons are confined to volumes of the order of ~10⁶ times smaller than the diffraction limit, thus facilitating for strong light matter interactions.

INTRODUCTION

Surface plasmons (SPs) are being fascinated by wide spectrum of scientists, ranging from physicists, chemists and materials scientists to biologists. Renewed interest in SPs comes from recent advances that allow metals to be structured and characterized on the nanometre scale. SPs are waves that propagate along the surface of a conductor, usually a metal, where, light waves that are trapped on the surface because of their interaction with the free electrons of the conductor and the free electrons respond collectively by oscillating in resonance with the light wave[1]. The resonant interaction between the surface charge oscillation and the electromagnetic field of the light constitutes the SP and gives rise to its unique properties. For researchers in the field of optics, one of the most attractive aspects of SPs is the way in which they help us to concentrate and channel light using subwavelength structures[3]. The interaction between the surface charges and the electromagnetic field that constitutes the SP has two consequences. First, the interaction between the surface charge density and the electromagnetic field results in the momentum of the SP mode, ℏk_SP, being greater than that of a free-space photon of the same frequency, ℏk₀  (k₀ = ω/c is the free-space wavevector). Solving Maxwell’s equations under the appropriate boundary conditions yields the SP dispersion relation, that is, the frequency-dependent SP wave-vector k_SP [1],

k_SP =Ko √(εd.εm/(εd+εm))

The frequency-dependent permittivity of the metal, εm, and the dielectric material,εd , must have opposite signs if SPs are to be possible at such an interface. This condition is satisfied for metals because εm is both negative and complex (the latter corresponding to absorption in the metal). The second consequence of the interaction between the surface charges and the electromagnetic field is that, in contrast to the propagating nature of SPs along the surface, the field perpendicular to the surface decays exponentially with distance from the surface, so called evanescent or near field in nature. Once light has been converted into an SP mode on a flat metal surface it will propagate but will gradually attenuate owing to losses arising from absorption in the metal [1]. The difficulty in controlling and varying permittivity functions of noble metals and the existence of material losses-esp. at visible wavelengths-degrade the quality of the plasmon resonance and limit the relative propagation length of surface plasmon waves along the interface between metals and dielectric materials [8].

Optical antenna can be defined as a device designed to efficiently convert free-propagating optical radiation to localized energy, and vice versa. In the context of nanoscopy, an optical antenna effectively replaces a conventional focusing lens or objective, concentrating external laser radiation to dimensions smaller than the diffraction limit. Optical antennas are strongly analogous to their RF and microwave counterparts, but there are crucial differences in their physical properties and scaling behavior. At optical frequencies, metals are not perfect conductors, but are instead strongly correlated to plasmas described as free electron gas. Optical antennas can take various unusual forms (tips, nanoparticles, etc) and their properties may be strongly shape and material dependent owing to surface plasmon resonances. A receiver or transmitter which is ideally an elemental quantum absorber or emitter, such as an atom, ion, molecule, quantum dot, or defect center in a solid, interacts with free optical radiation via an optical antenna. Also, antenna properties depend on those of the receiver-transmitter, and it becomes evident that two must be regarded as a coupled system.

WHY GRAPHENE PLASMONICS?

Graphene is one atom thick planar sheet of  sp²-bonded carbon atoms densely packed in a honeycomb crystal lattice, which has grabbed noticeable attention to be used as a next generation electronic material, due to its outstanding properties including high current density, ballistic transport, chemical inertness, high thermal conductivity, optical transmittance and super hydrophobicity at nanometer scale[4]. Graphene can be doped to high values of electron or hole concentrations by applying voltage externally, much like field effect transistors (FET). This electrical gating leads to a dramatic change in optical properties of graphene because of its impact on the strong interband transitions. Jablan et al. investigated plasmons in doped graphene and demonstrated that they simultaneously enable low losses and significant wave localization for frequencies of the light smaller than the optical phonon frequency ℏω_Oph ≈ 0.2 eV [5]. Interband losses via emission of electron-hole pairs can be blocked by sufficiently increasing doping level, which pushes the interband threshold frequency ω_inter toward higher values. Experimentally doping levels had already been achieved that can push it even up to near infrared frequencies.

Fig. 1: a)Schematic of graphene system and TM plasmon modes. The profile of the fields looks the same as the fields of  Surface Plasmons. b) Electronic band structure of graphene. c) Intraband (green arrow) and interband (red arrow) single particle excitations that can lead to large losseswhich can be avoided by implementing a sufficiently high doping[5].

Recently, Long et al. reported the first study of tunable plasmon excitations and light plasmon coupling at terahertz frequencies in graphene microribbon arrays, the simplest form of sub-wavelength infrared metamaterials. In such arrays, plasmon excitations correspond to collective oscillation of electrons across width(w) of the micro-ribbons. It was found that we can control the plasmon excitations by varying micro-ribbon width (w) where plasmon frequency scales as w^-1/2, which is the characteristic of two-dimensional electron gas (2DEGs)[6].

Fig. 2: Plasmon resonance in gated graphene micro-ribbon arrays. a) Top-view illustration of graphene micro-ribbon array. The array was fabricated on transferred large-area CVD using optical lithography and plasma etching. b) Side view of a typical device incorporating the graphene micro-ribbon array on a Si/SiO₂ substrate. The carrier concentration in graphene is controlled using the ion-gel top gate[6].

Further, we can tune the plasmon resonance using electrostatic gating where plasmon frequency varies with carrier concentration (n) as n^1/4, characteristics of massless Dirac electrons. The observed light-plasmon coupling from massless electrons in graphene was remarkably strong. Hence, Graphene plasmon excitations showed prominent terahertz absorption peaks at room temperature, however, low temperatures (4.2K) were required to measure plasmon absorption in conventional 2DEGs.This strong and tunable plasmon-light interaction together with the outstanding electrical properties of graphene and it’s compatibility with micro/nano fabrication holds great promise for optical antennae[6].

PLASMON DISPERSION RELATION IN GRAPHENE

For sufficiently high doping (E_F > ℏω) graphene can sustain p-polarized SPs propagating along the sheet with wave vector k_SP ≈ i(ε+1)ω/4πσ , where σ is conductivity of graphene[7]. The remarkable degree of confinement provided by the graphene is clear from the ratio of SP to free space light wavelengths

λsp/λo ≈ (4α/(ε+1))(E_F/ℏω)

In addition, out of plane wave vector ~ik_SP indicates an equally tight confinement to dimensions ~λsp/2π  in the transverse direction z. Interestingly, the in-plane propagation distance (1/e decay in amplitude), given by 1/Im(Ksp), reaches value well above 100λsp and drops rapidly at high energies when the plasmon has sufficient energy to generate e-h pairs and the dispersion relation enters the interband region. Fig. 3 clearly shows that for increasing E_F the plasmons become narrower because the damping rate 1/τ (τ is relaxation time) decreases relative to photon frequency [7].

Fig 3: Plasmon dispersion relation in Graphene[7].

GRAPHENE PLASMONICS Vs GOLD PLASMONICS

Comparing 1 monolayer of gold(thickness L=0.24 nm) to graphene (L=0.33nm), it was found that propagation length is much larger in graphene which indicate low losses in graphene and also confinement of plasmon was very low for graphene as compared to gold monolayer. Calculations are shown below:

Graphene,

CONCLUSIONS

Hence, graphene plasmonics are better than noble metal plasmonics to use it as optical antennae because of higher field confinement, higher plasmon localization, longer plasmon propagation (with lower losses) and we can engineer plasmon resonances easily. Further with nanostructuring, we can achieve extremely high field confinement and plasmon localization. The most important advantage of graphene over thin metal layers and metal interfaces is the capability to dynamically tune the conductivity of graphene by means of chemical doping or gate voltage [8].

References:

1. William et al. Nature 424, 2003, 824-830
2. Bharadwaj et al. Optical antennae, 2009, 440-472
3. Hu et al. Applications: Nanophotonics and Plasmonics
4. Geim et al.  Scientific American , 2008,298: 90
5. Jablan et al. Phys. Rev. B 80,245435 (2009)
6. Long et al. Nature Nanotech. 6, 2011, 630
7. Koppens et al., NanoLett. 11, 3370 (2011)
8. Ashkan Vakil et al. Science, 332, 2011, 1291-1294

Use the propagation matrix approach to calculate the transmission coefficient T for an array of N rectangular potential barriers, each having width L = 1 nm, height V₀ = 0.3 eV and a barrier-barrier separation a = 1 nm. Plot T as a function of energy for an increasing number of barriers. Do you see any oscillations in the transmission? Any regions where the transmission is heavily reduced? What is the underlying physical explanation? Play with the parameters, and compare your results with those of theory (e.g. the Kronig-Penney potential model). In what limit is the theoretical model valid? Can you predict where the regions of high transmission (bands) will appear? Tip: Instead of just plotting the transmission T (E), also try to plot the damping (defined as −ln(T (E)).

Problem C2.2(Transmission through a parabolic potential)

Use (a modified version of) your program and calculate the transmission through the potential step defined by

V (x) =(x²/L² )   if |x| <= L,

                            0 elsewhere.

Use L = 5 nm. Look at the bound state energies and compare your results with those of the harmonic oscillator. How does the quality of your result depend on the number of barriers used (the potential ”roughness”)? Tip: If needed, plot your transmission using a logarithmic scale.

Answer is attached below with pdf file along with MATLAB Program.

Fund of Nanoscience – Project_II_Ravi Sharma Dulal

Use the propagation matrix approach to calculate the transmission coefficient T of a rectangular potential step with width L=1 nm and a height Vo=0.3 eV. Plot T as a function of energy. Do you see any oscillations in the propability? Why? Change the barrier width and height. What happens? Compare your results with theory. Do they agree?

Problem C1.2(Bound states)

Modify the program from problem 1 to treat bound states in a rectangular potential well. Extract the bound state energies from the transmission coefficient and compared with theory (e.g. the bound states of a particle in a box). When is this comparison relevant?

Answer is attached with pdf file below along with MATLAB Program

Fund of Nanoscience – Project(I)_Ravi Sharma Dulal

Byeong-Su Kim, Seung Woo Lee, Hyeonseok Yoon, Michael S. Strano, Yang Shao- Horn, and Paula T. Hammond

INTRODUCTION

This paper presents a simple and versatile method for the generation of all MWNT (Multi wall carbon nanotube) multilayers over ploy(dimethylsiloxane) PDMS with controlled nanoscale thickness and then pattern transferred onto various substrates. Though CNTs have unique physical, chemical and mechanical properties and have potential application in electronic devices, biosensors, mechanical thin film applications, it is very critical to develop efficient method to maneuver the geometry structure and positioning of CNT arrays at the micrometer and nanometer scale. There has been some progress in patterning CNT thin films but with lots of challenges and complicated procedures for patterning over large areas. However, in this technique of solution based processing for CNTs fabrications, it includes Layer by Layer (LbL) assembly combined with soft lithography which can create highly tunable, conformal thin films with nanoscale control over film composition and structure with precisely controlled position (figure 1). Moreover, patterned MWNT networks can be generated without any prior surface templating without incorporation of any surface template avoiding the issues of selective deposition, masking or etching, making the process faster and more convenient. Finally, this paper also demonstrates the application of this approach to create the biosensor for detecting glucose selectively and quantitatively.

EXPERIMENT PROCEDURE AND OBSERVATIONS

Chemically modified MWNTs in stable aqueous form were prepared by oxidation of MWNT in strong acid forming negatively charged MWNTs with carboxylic acid groups (MWNT-COOH) and further reaction with excess ethylenediamine forming positively charged MWNTs with free amine group (MWNT-NH₂). The presence of reactive functional group on the surface of MWNTs was successfully confirmed by X-ray photoelectron spectroscopy (XPS) and zeta potential measurement shows +43 mV for MWNT-NH₂ at pH 2.5 and -35 mV for MWNT-COOH at pH 3.5. These stable suspensions of MWNTs are used to construct MWNTs films on a micrometer scaled patterned PDMS stamp via LbL assembly. The main driving force for this LbL assembly is electrostatic interactions between MWNT-NH₃⁺ and MWNT-COO^- .

To achieve final transfer onto the desired substrate, proper tailoring is required for the adhesion of MWNT multilayer film between PDMS surface(where it is assembled) and the desired substrate (where it is transferred). To ensure the successful pattern transfer upon contact to desired substrate, main prerequisite is to have proper control over the weak hydrophobic interactions of the PDMS interface with the multi layer as compared to the strong attractive electrostatic interactions between the MWNT top layer and the substrate. Here, affinity of MWNT-NH₃⁺ to hydrophobic PDMS (low surface energy 19.8 mJ/m²) was sufficient to create the initial deposition of MWNTs on PDMS substrate as a priming layer. Then the sequential deposition of MWNTs suspensions with opposite charges was followed as PDMS/ MWNT-NH₃⁺ /(MWNT-COO^- /MWNT-NH₃⁺ )_n, typically n = 5, 10, 20 and 30 bilayers with a positively charged MWNT-NH₃⁺ as a final top layer. Then, the substrate was pre-treated with O2 plasma to create negatively charged surface before patterning it with MWNTs multilayer. By the addition of drop of water on the MWNT film surface prior to stamping, it enhanced the flexibility of multilayer film and contact between film and substrate and hence improve the quality of pattern transfer. With the help of pair of paper clips, slight pressure was applied for 3 hours which was essential for conformal contact during the pattern transfer. Successful pattern transfer was achieved irrespective of the pattern size or shape. Also, 5-bilayer MWNT multilayer was successfully transferred onto a flexible ITO-coated PET substrate, proving the generality of this method to any substrate. Hence, without need of any additional materials that might hinder electrochemical and/or electronic performance, MWNTs porous thin films were directly transferred to a number of electrode surfaces. This paper also demonstrates direct stamping of single MWNT layer (suspension ink of MWNT-NH₃⁺) onto a neutral surface (silicon wafer) and considering that as template, additional MWNTs layer were grown via LbL assembly selectively. The thickness of transferred films and directly patterned films (non-transferred) were found to be similar. Another, interesting observations was the thicker film (30 bilayer) was considerably show more roughness as compared to thinner films (5 and 10 bilayer films) because of increase in surface roughness with increased film thickness.

APPLICATION AS BIO-SENSOR

This paper also explained for making the MWNT film as biosensor for glucose detection. The glucose oxidase (GOx) enzyme was covalently immobilized to the surface end with carboxyl groups of MWNTs via N-ethyl-N’-(3-dimethyl aminopropyl) carbodiimide methiodide (EDC) mediated reaction after transferring onto a interdigitated microelectrode array (IDA). Then the patterned MWNT film on IDA showed ohmic contact without significant loss in conductivity, which was observed from current-voltage characteristics (figure 2c. This proves that pattern that we can make reliable electrical contact to the electrodes using MWNT multilayer. When glucose interact with GOx, hydrogen peroxide is formed (figure 2a) which affect the charge transport property of CNTs. There are two gold electrode bands of IDA serving as source(S) and drain (D) and the MWNT film on IDA was immersed in a phosphate buffered solution(10 mM, pH 7.0) with gate potential applied between Ag/AgCl reference electrode and the source electrode through the buffer solution(figure 2b). Upon addition of glucose analyte, current I_SD increased gradually and reached saturated value. No such remarkable response was observed for MWNT without GOx attached for same process above (figure 2c).

MOTIVATION FOR THIS TOPIC

As the transistor gate length has been reduced to nanometer level, lots of complications scientists are facing to tackle the short channel effects and they are forced to search some other future alternatives for silicon technology. Carbon nanotubes (CNTs) offers very good promising technology in electronic applications because of it’s unique physical, chemical and mechanical properties. By increasing the thickness of CNT thin films, the transition from semiconducting  to metallic form can occur. Also, films with thickness in the range  of 10-100 nm show high optical transparency and electrical conductivity and can be used as a replacement for Indium-Tin-Oxide (ITO) electrodes. Micrometer-thick CNT films are nanoporous and used as electrodes for supercapacitors, fuel cells, and battery applications. Also by functionalizing the surface of thin films of MWNTs multilayer, we can used it as nano sensor. Though there are so many potential applications in electronic fields, it is very critical to pattern MWNTs multilayer with well defined nano-structures and precise position. This paper describes the solution based techniques which combine Layer by Layer (LbL) assembly with soft Lithography which is very simple, efficient, convenient and reliable method. By this method, we can patterned MWNT onto any substrates. We can easily control the thickness of MWNTs multilayer which helps to obtain semiconducting or metallic one according to our requirements. Also, just by making the surface of thin films MWNTs multilayer functionalized, we can use it as nano-sensor.

As we are dealing with the nanometer control of thickness of thin films MWNTs multilayer, where MWNTs were chemically modified in stable aqueous form to obtain negatively charged MWNTs and positively charged MWNTs and form layers one by one with opposing charged MWNTs on one another, we can obviously say this paper deals with “Chemistry of Nanometer Scale”. The main driving force for this assembly is LbL where we can form the priming layer of  MWNT-NH₃⁺  onto PDMS surface and then we will follow the architecture PDMS/ MWNT-NH₃⁺ /(MWNT-COO^- /MWNT-NH₃⁺ )_n, typically n = 5, 10, 20 and 30 bilayers with a positively charged MWNT-NH₃⁺ as a final top layer, that occurs due to electrostatic interactions between oppositely charged layers. In order to transfer that MWNT multilayers onto any substrates, we need to treat the surface of substrates to make it negatively charged and then we can transferred pattern, which is having positive charge on its surface, onto that substrate.

This is an important development because it offers a unique potential platform for integrating active nanomaterials for advanced electronic, energy and sensor applications. We can use these MWNTs layer as transparent and conducting electrode for many optoelectronic devices applications, including solar cells, flat panel displays, touch panels, organic light emitting diodes, electroluminescent lighting, and many others. Currently, the most commonly used transparent conducting films (TCF) is Indium-Tin-Oxide (ITO) because of its good electrical properties and ease of fabrication. However, these thin films are usually fragile and have problems such as lattice mismatch and stress-strain constraints lead to restrictions in possible uses for TCFs. ITO has been shown to degrade with time when subject to mechanical stresses. Recent increases in cost are also forcing many to look to carbon nanotube films as a potential alternative.

Also, we are not using the surface template that avoids the issues of selective deposition, masking or etching and prevents from affecting physical, chemical and mechanical properties of MWNTs and also makes the process simple, more convenient and faster. This provides the precise control of the geometry structure of MWNTs layer and we can control the thickness of multilayers that also determines the conducting property of MWNTs multilayers.

The reason behind selecting this paper is due to my interests of doing researches towards Carbon Nanotube based Field effect Transistor (CNT based FET). This is one of the best approach for patterning the MWNT multilayers with more convenient, less risk for degrading the electronic and mechanical properties of device by eliminating masking, etching, etc. processes and also cheaper process which is very feasible from industrial approaches. In the past, the main difficulty for CNT based FET to realize practically was the deposition of CNT as channel in between source and drain. Also we can use this application for CNT-interconnect by growing above critical thickness where we can sure that it will act as metallic interconnect. Increasing the thickness of MWNTs, its bandgap decreases and ultimately transition occurs from semiconducting nanotubes to metallic nanotubes. Also, using CNTs as interconnect, it provides very good thermal managements as it is having very good thermal conductivity. This CNT based FET has so many applications which can be used as an alternative to current CMOS transistors, which is suffering from so many challenges to realize it below 10 nm, and it can also be used as bio sensors which has already been explained above for glucose detection.

One of the major remaining roadblocks to commercializing device applications is to find a scalable and reliable method to separate metallic and semiconducting CNTs. The separation of CNTs will benefit two major applications. Semiconducting CNTs will benefit thin film transistors, and metallic CNTs will benefit highly conductive electrodes for transparent conductor applications. These two immediate applications areas of CNT thin films will be the dominant applications and are of much industrial interests in commercializing.

NANOTOXICITY ISSUES

There are very potential risks of nanoparticles due to their capacity to penetrate cells and potentially translocate to other cells, tissues and organs remote from the portal of the entry to the body. There is superficial resemblance between CNTs and asbestos fibres which causes the potential health risks of CNT exposure for which main target is lungs. Also, cytotoxicity and inflammatory responses were analyzed following the exposure of CNTs. Using micro imaging, slow damaging effects of CNT to epidermis and dermis of rat skin was successfully observed.

By measuring the Nitric Oxide (NO) production, we can identify the level of inflammation. Upon exposure of CNTs to epithelial cells, NO productions were dramatically increased as the concentrations of CNTs increased. At higher concentrations of CNTs, parts of cell layers were detached, indicating cytotoxic response of cells. Also metabolic activity of cells was found to be decreased as concentrations of CNTs increased, especially for epithelial cells.

Recent study of CNTs introduced into the abdominal cavity of mice was done to analyze the nanotoxicity issues of CNTs. Four samples of commercial MWNTs were taken, two with high aspect ratio and other two with low aspect ratio. Samples were prepared for in vivo use by ultrasonication using a sterile 0.5% BSA/Saline solution. Then, 50 grams of each dose were injected into the peritoneal cavity of mice and after 7 days, they found the inflammation of cell layer that cover the chest peritoneal cavities and also the formation of scar like structures called granulomas, which can be assessed by increased level of polymorphonuclearwhite blood cells (PMN WBC). MWNTs with high aspect ratio caused much more PMN exudation and granulomas on  the peritoneal side of the diaphragm.

The mechanisms of CNTs’ toxicity may be closely related to their structure, functional group and surface charge on the molecule. Functionalized CNTs may undergo different process to affect inflammatory and cytotoxic responses. Due to their structure with high aspect ratio, they have structural unusual toxicity.

REFERENCES:

1. Srinivasan et al. Current Science Vol 95, 3, 2008
2. Sharma et al. NSTI-Nanotech Vol 2, 2009
3. http://www.wikipedia.org

I am trying to operate the Nano-resonator in non-linear hysteric regime and trying to tune to the state where I can have access to bistable state with equal probability. Then I am sending 2 input signals after electrically adding and modulate it to the AC driving signal, that drives the resonator. The role of noise here I can describe using quantum phenomena, where 2 states (high and low) that we can also called as threshold levels when described as twin well potential(one well is low state and another is high stste), the modulating signals asymmetrize  this twin well potential by lowering barrier potential of corresponding state and hence noise assists in  helping to go for corresponding state by hopping.

The main problem here is we can perform this logical operation within certain noise window only. However, we have no control over noise power so what we can do is we can play with non-linearity parameter of resonator which helps in tuning internal noise floor so that noise can fully cooperate with the modulating signal with periodicity.

1. Off-axis illumination : In off-axis illumination, the light passes through mask at an oblique angle rather than perpendicularly i.e. light,in this case, is not parallel to the axis of the optical system. This causes all the diffraction orders of light to be tilted and more higher orders are able to pass through the projection lens. As, we know higher order of diffraction contains fine details of images that’s why by this technique, we can get better quality images on the substrate.

2. Optical proximity correction: This is also photo lithography enhancement technique which is used to compensate the errors in the images due to diffraction or process effects. Lens normally acts as low pass filter in which limited amount of higher order spatial frequencies passes for image formation. So, imaging close to the resolution limit of the optical system, we face different errors in the images due to loss of finer details of image which can be provided by higher order spatial frequencies. Errors are actually due to inability of lens to maintain edge placement integrity of the original design causing rounding at the corners, shortening of line at the end, line width roughness, etc. In dense line, lens capture only zero and first order diffraction (not higher order diffraction) so that it causes imbalance in image formation. From optical proximity enhancement techniques, we can solve this problem. If the line on the wafer is thin, we make it thicker on the mask, if lines are too dense on the wafer, we do reverse on the mask, if line is too short, we make it long on the mask. For maintaining edge at the corners, we can place serifs ot hats at the corners on mask.

3. Phase shifted mask: Phase shifted mask are designed to sharpen the intensity profile or thin resist profile, which allows to sharpen the image to be printed. In alternating phase shifted mask, each alternate windows are 180 degree phase shifted in comparison to theirs neighbors so that the effect of adjacent light diffraction will have minimum effect to get sharpen image.

4. Immersion Lithography: In this case, immersion fluid passes between wafer and lens continuously so that lens substantially changes the light path, which enables higher angles of the incident light i.e image can have more finer details. This causes numerical aperture of lens to be larger than 1 thereby improving the resolution.

Casimir Force

]]> http://nanoravi.wordpress.com/2011/04/19/casimirs-force-or-van-der-waals-force-interaction-ambiguity-in-nems-logic-design/feed/ 0 nanoravi Picture2 Picture1 http://nanoravi.wordpress.com/2011/04/12/biomaterials-for-regenerative-medicine/ http://nanoravi.wordpress.com/2011/04/12/biomaterials-for-regenerative-medicine/#comments Tue, 12 Apr 2011 09:58:12 +0000 nanoravi http://nanoravi.wordpress.com/?p=214 ]]> One of the most interesting applications of Nanotechnology is related to regenerative medicine. The vision put by Nano-Scientists for Medical applications are really marvelous and too much challenging. The vision of regenerative medicine is to regenerate the soft and the hard tissues, organs and nerves responsible for major human disability. Main focus are being done on CNS (Central Nervous System), the disability is mainly due to spinal cord injury, neuro-degenerative diseases, vision degeneration and stroke. Also the main target is given to Heart where the pacemaker transplantation is nowadays available in most parts of the World.The repairing of other Organs such as the pancreas would restore quality of life to humans with juvenile diabetes e.g. transplantation of B-Cell or pumping instrument that regularly inject insulin according to demand of our body.

Here, I am describing about how Supramolecular design can help to treat the paralyse problem that occur due to accident. First, we require strong amphiphillic structures which can be designed to form nanocylinders, not tubes or twisted fibrils.Second, it should contain peptide segment that has specific amino acid sequences. Around the fibre, there should be high densities of functional units perpendicular to long axis of fibre, which favours for cell signalling. As these structures are of high concentration, so the main issue will be how to inject this highly viscous substances inside body. We must make them non-viscous outside body, after injecting due to electrolytes presence inside body, it turns into viscous substance inside body which is our requirements. We can change the structures from solution to gel form either by changing pH (making it lower) or by adding electrolytes (due to screening effect of electrolytes, gel formation occurs).

By the recruitment of specific cells from he biological environment into the artificial matrix space and possibly the accumulation at the site of specific proteins, we can trigger regeneration. The nanostructures would be designed to attract specific proteins such as growth factors. A designed nanofiber network is to encapsulate neural progenitor cells and control their differentiation. These progenitor cells have potential to replace the lost CNS cells after degeneration or trauma. Isoleucine-Lysine-Valanine-Alanine-Valine(IKLVAV) are known to promote the neurite sprouting and direct neurite growth. The formation of a bubble gel in which nanofibres surrounds the cells without compromising viability, helps to induce rapid/selective differentiation of progenitor cells into neurons and also suppressed development of astrocytes( cell that impedes the repairing of neurons after spinal cord injury and therefore, block regeneration).

Hence, by this we can stimulate our body for neuron cells regeneration to prevent people from paralysis/disabilities. Upon successful implementation of this kind of regenerative medicine will definitely be a boon in  the field of medical science. Because of the nanotechnology, medical science is progressing a lot and so many complicated issues have been turned to minorities. Still, we have far to go and I hope within 2020, we will be able to change this world into the new era of Nanotechnology.

NEMS – Nano Electro Mechanical System logic gates are the NEMS devices that can perform Boolean algebra like the logic gates that are composed of solid state transistors (FET, BJT, etc). The proposed NEMS logic gates are expected to have more applications than NEMS switches and also can be used to construct a full-Mechanical computer. The advantages of this mechanical computer over present days computer are that it can work under severe temperatures and strong ion-radiation environments. Under such conditions, conventional solid state transistors that are used in present days computers fails.

The propose of my project is to design a NEMS logic gate that can be fabricated by Surface micro-machining, which uses same mechanical structure to perform either NAND or NOR gate function depending on the electrical interconnects (just by reversing the voltage polarity, NAND gate switch to NOR gate or vice versa). As we know, it is possible to realize entire digital circuitry by NAND and NOR gate functions, so solely from the proposed NEMS logic gate it has potential application to realize mechanical computer.

But, here I am describing about MEMS logic and later I will describe about NEMS logic design after doing researches at IMEC in the upcoming posts.

Translational 3- Layer Design

It is shown in figure 1., where the device has one shuttle electrode in the middle and two fixed electrode on the top and bottom. When the fixed electrodes are biased at the voltages of Vcc+ and Vcc-, the shuttle electrode moves either up or down, depending on the voltages applied to it.The output terminal is connected to shuttle electrode which can connect to top or bottom electrode and ultimately gives brings the output. Hence, the logic function in this structure is carried by the motion of shuttle electrode. In this logic gate, there are two gaps d1 and d2 which are different in size, d1>d2 whereas two effective areas Aa and Ab are same. Depending on application of bias at Va and Vb, motion of shuttle occurs and thus following four situations occur.

1. Va = Vcc+ and Vb= Vcc- : Here voltage difference between upper half and lower half is same but  d1>d2, upper electrostatic force is smaller than down one. The shuttle electrode moves downward and connects the bottom electrode making the output voltage Vcc+.
2. Va = Vcc- and Vb= Vcc+ : This is the same situation as above so output voltage is Vcc+.
3. Va = Vcc+ and Vb= Vcc+ : Lower half electrostatic force is zero as there is no voltage drop between bottom electrode and shuttle electrode. So, shuttle electrode moves upward connecting the top electrode making output volatge Vcc-.
4. Va = Vcc- and Vb= Vcc- : Upper half electrostatic force is zero as there is no voltage drop between top electrode and shuttle electrode. So, shuttle electrode moves downward connecting the bottom electrode making output volatge Vcc+

Hence this gate performs NAND logic (Vcc+ =logic 1 and Vcc- = logic 0) similar to the logic gate composed of solid state transistors. We can switch this NAND logic gate to NOR logic gate by reversing the bias between top and bottom electrode hence can used same mechanical structure for both NAND and NOR logic gate. This structure has no leakage current which is major advantages over FET logic gate. Also output terminal is always connected to either top or bottom electrode so this design has no undefined state which is an issue in some logic devices composed of solid state transistors. However, due to three layered structure, this design is difficult to fabricate by using Surface micromachining. However, this design can be rotated by 90 degree and fabricated by bulk micro machining.

Torsional 2- Layer design

Two layered structure, shown in figure 2, is suitable to fabricate by surface micro-machining which is possible to integrate with existing solid state transistors. Here the shuttle electrode does a see-saw motion which connects the output terminal on each end to export the corresponding output voltage. In three layer design, logic function is controlled by different voltages and different gap whereas in two layer design that is done by different voltages and different dimensions of actuation pad,s Al and Ar.

The actuation torque is always positive for two input signals both at Vcc+ while it is always negative for both input signals at Vcc-. Here micro flexure is designed such that resilient torque remain in between the actuation torque of above both cases. Hence, net torque at (1,1) is always positive and the plate can be actuated from any angle to angle where it connects the output terminal on the right. Similarly, in case (0,0), the plate can be actuated from any angle to angle where it connects the output terminal on the left.

When the input terminals are the combination of either (Vcc+, Vcc-) or (Vcc-, Vcc+) , the plate should be rotated clockwise at any initial angle and then connects the output terminals on the right. Hence, it performs as NOR gate and reversing the voltage polarity at the shuttle electrode it can be switched to NAND gate.

Silicon on Insulator transistor has buried oxide layer as soon in figure which plays the main role for providing several advantages. By reducing parasitic drain/source junction capacitances, SOI devices yield improved switching speed and reduced power consumption. By isolating channel from substrate bias effect, it also improves the operating speed. It also provides perfect lateral and vertical isolation from substrate which makes the device free from latch up and inter-device leakage problems.

SOI transistors are of 2 types – PDSOI (partial depleted SOI) and FDSOI ( fully depleted SOI). In PDSOI, silicon film on the BOX layer is thicker than the depletion layer formed beneath the gate oxide, has problems like floating body effects, low threshold voltage variation with temperature as it possess self heating process. This problem has been removed in FDSOI, where silicon film thickness is thin enough or lowly dopes to be fully depleted. However, FDSOI are more sensitive to process variation. One important question is why we are not adopting this properties in present day devices though it has so many several advantages (this ques. was asked by my prof. to me during seminar ), the reason behind this is the use of FINFET devices which INTEL and TSMC are using in present day devices, which accompanies property like SOI.

In double- gate structure, electric field lines from S/D underneath the device terminate on the bottom of the electrode, and therefore cannot reach the channel region. we can see in the figure that how subthreshold swing has been decreased by the use of double gate structure relative to planar bulk transistor. Double gate structure improves the electrostatic control of gate over channel and no. of equivalent gate is 2 for double gate structure which will provides almost twice drain current than normal planar bulk transistor.

The main advantage of using Multi gate devices is that it supress the drain field much more efficiently. There is a parameter called “Natural length” which  represents length of the region of the channel  that is controlled by drain. Effective gate length should be 5-10 times larger than natural length in order to free from short channel effects. Current drive in Multi Gate FET is equal to current in single gate device times the equivalent no. of gates. So our main purpose is to decrease the natural length parameter. We can see the formula how the natural length parameter depends on dielectric constant of gate oxide and silicon, thickness of silicon and gate oxide. Bu using the no. of equivalent gate, obviously it will decrease the natural length by improving the electrostatic control over channel (i.e. by increasing gate coupling). Also, it proves that how high K material improve the device performance that is obvious by reducing the natural length parameter. In triple gate like pi-Gate and Omega-Gate, equivalent no. of gate is more than 3 (between 3 and 4), hence improve the performance by increasing drain current.

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