Physical Vapor Deposition and Inmobilized metal na Essay

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The application of metal nanoparticles has grown greatly over the past years.Immobilized metallic particles are much easier to fabricate via normal wet chemistry, giving various choices with regard to shape and sizeCITATION Mar05 l 1033[1]. Furthermore, SERS hotspots can be cheaply realizedthrough the aggregation of immobilizedmetallic particles from their suspensionsthrough the use of salts or any analyte of interest.Nevertheless, the use of aggregated and dispersedimmobilized metallic particles as SERS substrates within real analyticalcomplications is limited as a result of the poor modification factor reproducibility.The issue of reproducibility could be solved through advanced metallic nanoparticles immobilization together with some solid supportCITATION LeR09 l 1033[2].

The easiest SERS experiments are achieved with metallic nanoparticles under the presence of particular analyte concentration[6]. However, suspension of metallic nanoparticles should be mixed with the SERS analyte solution, a sampling demand that may be hampering some applications. Regardless, of the reproducibility andpossible sampling shortcomings, metallic nanoparticles are widely used as SERS substrate because of their good stability, high SERS performance and easy fabrication[9].Furthermore,theypromote the formation of more stable metallic particles.
Another methodology comprises of generating some SERS substrates through immobilizing the metallic nanoparticles under a planar foundation[10]. The metallic nanoparticles adhesion to solid supports is occasionally so poor and particular immobilization methodologies have to be devised to retain the performance and integrity of SERS substrate over time[5].For example the chemical attachment of metallic nanoparticles to solid substrates where bi-functional molecules are used for its immobilizationCITATION Fre95 l 1033[3]. The ideology behind this is to anchor the moleculeto the surface through the use of one of its functional sets, hence leaving the other functional set free to bind the metallic nanoparticle.Glass slides surfaces are functionalized with thiol or amine groups with the aid of a surface polymerization procedure coupled withDeepingthe functionalized glass to the metallic nanoparticles suspension for some time periodCITATION DMM10 l 1033[4].Benefits of using Glass in this case includelarge enlargement factor, low cost, electrochemical addressability, flexibility with regard to glass surface geometry, better reproducibility as opposed to metallic nanoparticles in suspension and the fact that glass has a less di-electric constant that affects the Rama/SERS signal compared to other substances such as PDMS[7].

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Apart from the above approach, some other efficient surface chemical modification avenues exists which have been used to immobilize metallic nanoparticles. For instance, the introduction of amino functionalityto a silicon surfacethrough the application of Silane chemistry[2]. The amino group was successively clapped using a carboxyl alkanethiol. The thiol group then reacted with the metallic nanoparticles.SERS substrates could also be achieved through fabricated biochips by soft lithographyCITATION Fre95 l 1033[3]. A set of nanofabrication technique is established to build nano-pillars frameworks within a silicon wafer as a parent molding copy, then the other nano-wells frameworks on polydimethylsiloxane.PDMS are established through soft lithography[2]. The selection of metallic deposition on the nanowells is used to establish SERS active sites prior to the integration with glass microfluidic that works as a sample delivery device as well as an optical transparent window for imaging of the SERS spectroscopic.

PDMS is an off the-shelve available chemically and physically stable silicone rubber. It contains some unique flexibility that cannot be compared to glass with shear elastic modulus as a result of one of the lowest glass temperature transition of any polymer. In addition, PDMS are a bit some low change within the shear elastic modulus as opposed to temperature typically no change in elastic modulus versus high compressibility and frequency. Due to its clean processability, the high flexibility and low temperature, thechances of change to any of its functional components as well as property drift over temperature and time, as opposed to glass, PDMS is suitable for chemical and mechanical sensors as it has many desirable features than can be found in glass when producing SERS signals and further helps in making the SERS morestronger.Furthermore, the di-electric properties of PDMS is an advantage to Surface Plasmon generated on metal nanoparticles which is much greater that the di-electric properties of glass. The great di-electric properties help in the creation of force sensors which respond to several forces much easily.

Part 2
Physical vapor deposition is a term commonly used to elucidate a set of coating procedures.In this case I used thermal evaporation using a tungsten wire coilCITATION DMM10 l 1033[4]. This art of vaporization is obviously very high as opposed to other vaporization techniques. The substrates are mounted within some distance from the source of evaporation so as to minimize substrate radiant heating through the vaporization source.All these procedures come through vacuum under working pressure and often integrate substrate bombardment to be coated using positively charged energetic ions within the coating procedure to foster high density.Furthermore, reactive gases emitted like nitrogen, oxygen or acetylene might be brought into the vacuum chamber in the metal deposition process to establish severalcompound coating compositions. The outcome is a formidable bond between the toolingand coatingsubstrateand tailored physical as well as tri-biological and structural properties of the film.
Immobilized substrates can be directly obtained through vapor depositing small metal layers on solid supports. In the event that the thickness of the metal is small, the evaporated metal seems to clusterin particles or islands rather than a flat film. Such process types have been widely applied to fabricateimmobilized substrates on various substances, like nano-structured optical fiber cablesCITATION Sto04 l 1033[3].Metal nanorods could also be made via a procedure referred to as oblique angle deposition. The technique involves setting tilted substrates relative to incoming metal vapor which gives long nanorods with an aspect ratio of approximately 5. There is also the use of vapor depositionin the nanosphere process where a layer of nanosphere is self-assembled within a solid support, plus by metal vapor deposition directly on the nanosphere layers.
Thermal evaporation vs. Lithography
Item
Thermal evaporation
Lithography
Scaling-up
Difficult
Good
Number of depositions
One deposition per change
Many depositions per target
Changes in source material
Easy
Expensive
Substrate heating
Very low
Heating is substantial
Rate
Thousands of atomic layers per second
One atomic layer per second
Purity
Better
Possibility of incorporating impurities
Uniformity
Difficult
Easy over large areas
Surface damage
Very low x-ray damage possible
Ionic bombardment damage
Choice of material
Limited
Unlimited
Physicalvapor deposition coatings are often used to improve immobile substrates wear resistance, hardness as well as their resistance to oxidation[8]. Hence such coatings can then be used for automotive, aerospace, fire arms, medical equipment and cutting tools.The best advantage of applying physical vapor decomposition as opposed to other deposition techniques is the temperature requirements.Other deposition techniques such as chemical vapor deposition processes operate at a much higher temperature requirements more than processes of physical vapor deposition.

Normally, heat is provided by some furnace or laser but in most cases it heats the substrate.Substrates which cannot live up to this temperature should have films deposited through the physical nature of vapor deposition.Physical vapor deposition processes molecular beam epitaxy have some unique benefit of atomic control level of chemical composition, film thickness, chemical composition and transition sharpness.Furthermore, physical vapor deposition processes do not need the application of expert precursor materials as it is done in other deposition methodsCITATION Mar05 l 1033[1].
The other physical vapor deposition benefit over other deposition methods is the aspect of material safety that are applied in other deposition methods.It is understood that a few precursors and some of its by-products are so toxic, corrosive or pyrophoric[4]. This could lead to major issues with regard to material storage and handling.Also, materials could deposit materials with advanced properties as opposed to the material substrates. Almost all types of inorganic materials could be applied together with other organic materials. Lastly, physical vapor deposition is friendlier to the environment as opposed to other processes.

Thermal evaporation vs. sputtering
Physical vapor deposition (Thermal Evaporation)
Sputtering
Advantages
Disadvantages
Advantages
Disadvantages
No radiation
Contamination
Better step coverage
Plasma damage
Highest purity due to low pressures
Poor step coverage forming alloys can be hard
Less contamination
Hood for ohmics
High throughput
Lower throughput due to lower vacuum
Easier to deposit alloys
Expensive
Cheaper
Poor coverage
References
BIBLIOGRAPHY
[1]
M. Martin, “Surface-enhanced Raman spectroscopy: a brief retrospective,”Journal of Raman Spectroscopy,vol. 36, no. 6-7, p. 485-496, 2005.
[2]
L. ,. a. R. P. V. D. Christy, “Nanosphere Lithography: A Versatile Nanofabrication Tool for Studies of Size-Dependent Nanoparticle Optics,”The Journal of physical Chemistry,p. 5599-5611, 2001.
[3]
D. L. C. Z. &. V.-D. T. Stokes, Surface-enhanced-Raman-scattering-inducing nanoprobe for spectrochemical analysis. Applied spectroscopy,, 2004.
[4]
M. M. D, Handbook of physical vapor deposition (PVD) processing, Norwich, N.Y.: William Andrew ; Oxford, 2010, pp. 2-4.

[5]
C. C. L.-C. P. Z. J. A. M. Carlos, “Thiol-immobilized silver nanoparticle aggregate films for surface enhanced Raman scattering,”Journal of Raman Spectroscopy,vol. 39, no. 9, p. 1162-1169, 2008.
[6]
R. G. K. A. K. B. R. D. J. G. A. H. M. J. M. S. P. W. D. N. M. Freeman, Self-Assembled Metal Colloid Monolayers: An Approach to SERS Substrates., US National Library of Medicine , 1995.
[7]
S. T. V. Singamaneni, Nanostructured surfaces and assemblies as SERS media., US National Library of Medicine , 2008.
[8]
J. Thornton, “Physical vapor deposition.,”Noyes Data Corporation, Noyes Publications, Semiconductor Materials and Process Technology Handbook for Very Large Scale Integration(VLSI) and Ultra Large Scale Integration(ULSI),,pp. 329-454., 1988.
[9]
H. H. Y. &. Z. Y. Chu, Silver nanorod arrays as a surface-enhanced Raman scattering substrate for foodborne pathogenic bacteria detection. Applied spectroscopy, 2008.
[10]
E. C. a. P. G. E. Le. Ru, Principles of surface enhanced Raman scattering and related plasmonic effects, 2009.

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