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Photodeposition is a chemical deposition technique in which a material is irradiated with light, decomposes, and then the decomposed particles deposit onto the surface of a substrate. The first step of this process is known as photodecomposition or photodissociation. Many different materials can be decomposed this way, the most common being polymers and refractory metals. Visible light as well as all the other forms of electromagnetic radiation act as though they were packets of energy, known as photons. If the material to be decomposed is a metal, the photons, usually in the range of ultraviolet radiation, will strike the metal, transfer energy, and either oxidize or reduce the metal. The newly modified metal will then decompose, or break off from the rest of the metal, opening up more of the metal to be struck with photons [1]. If the material is a polymer, the photon will strike the bonds holding the polymers together, and add enough energy for these bonds will break. The newly cleaved polymer bonds will connect, and the polymers will begin to crosslink, resulting in small polymer fragments [1]. The decomposed metal or polymer fragments will then deposit onto a substrate. Controlling the wavelength, power, and duration of the incident UV radiation can be used to control the rate of growth of deposited material onto the substrate [2]. One irradiation event can be used to cause decomposition and nucleation of the material, and then a second controlled irradiation can control the growth of metal nanoparticles on the substrate [2]. There is an almost linear relationship between the amounts of deposited metal as a function of illumination time [3].

History of photodeposition: Deutsch et al. were the first to use photodeposition lasers to generate metal films on SiO2 surfaces in 1979. They used ultraviolet laser photons with wavelengths of 193 nm and 257 nm to break the organometallic bonds in trimethylaluminum, dimethyl cadmium, and tetramethyl tin to produce free metal atoms of aluminum, cadmium, and tin which then condensed on a substrate. Iodine was also photodeposited on surfaces using the precursor CF3I. (5) For the next two decades, their method of photodeposition was used to deposit a variety of metals and organics on surfaces for over 5000 various applications.

Process: Methods to photodeposit material involve essentially two steps, (1) photolysis of targeted precursor molecules affording charged or excited species and (2) deposition of these excited species onto nucleation sites such as metals. The various manners of chemical excitations are found to provide optimal conditions for specific material classes (e.g. changing the power, dose, or pulsation parameters provide varying properties in pore size and spacial localization). The main component in this technique is the employment of a light source instead of other electrochemical means such as heat (used in molecular beam epitaxy-MBE) or external electric fields (used in electrospray deposition) involved with other chemical deposition processes.

Generally, a directed irradiation source (UV, visible, or laser) is grated towards a sample (solid, colloidal liquid, liquid, or gas) affording excitation of incident precursor molecules. Depending on the source and the ability to penetrate substrate composition, interactions at the interface of photoactive media and substrates or bulk volume of the photoreactor may be observed. The excited chemical target is transferred to the substrate through various electron transfer processes. In turn, a thin film polymer, metal, metal oxide, or mixed matrix is deposited on the substrate over time for advantageous material properties. This new material contains mixed properties of both precursor materials. The largest variance in this technique is found with the preparation of the sample to be photodeposited on the predetermined surface. The most common of which include:

Liquid Phase Photodeposition (LPPD)

This technique involves the photolysis of precursors either dissolved in solution or colloidal in solution. The benefits of this type of preparation include longer shelf life, lower temperatures, ease of preparation, and the decreasing need for vacuum environments. Both film thickness deposition rate and latteral writing velocity are favored in comparison to gas phase by a factor of 1000. The activation of the colloidal particles is potentially more difficult than other types.

Gas Phase Photodeposition (GPPD)

The energy of the irradiation source is sufficient to vaporize the precursor molecules. These excited molecules are then deposited on activated centers. See Vapor Deposition for more detail.

Typical deposition layers afforded through gas phase photodeposition (GPPD). It is shown that the deposition can be controlled to form ordered films (picture C) instating that both directional and conformal deposition can be achieved. (1)

Types of photodeposition:

The two primary types of photodeposition that are observed are heterogeneous and homogeneous deposition. They are differentiated by the uniformity of deposition onto the substrate.

Homogeneous Photodeposition

Homogeneous deposition occurs when the material to be deposited onto the substrate is first decomposed by photons and the decomposed pieces are dissolved into a solution containing both the substrate and the rest of the material to be decomposed [1]. The now dissolved material is adsorbed onto the surface of the substrate. This method is preferred because it allows for uniform coating of material onto the substrate.

Heterogeneous Photodeposition

Heterogeneous deposition is less preferred, as it does not create a uniform coating. Heterogeneous deposition results from a direct photocatalytic decomposition [1]. This mechanism…[Requires more research]

Analytical Methods to Determine Mechanism

It is impossible to visibly distinguish between these two deposition routes. Advanced analytical techniques such as scanning electron microscopy, X-ray photoelectron spectroscopy and atomic force microscopy are required to determine the morphology, size and distribution of the deposited particles. One method for determining which mechanism has occurred starts off by creating an ordered linear array of substrate. An ordered array of substrate can be created by depositing the substrate onto a highly oriented pyrolytic graphite surface. X-ray photoelectron spectroscopy can be used to observe the difference in deposition between selective depositions onto a part of the preferred substrate vs. the total deposition over both the substrate and the graphite surface [1]. A homogeneous mechanism results in deposition over both the substrate and graphite, while a heterogeneous mechanism will result in only deposition on the surface of some of the substrate particles.

Kinetics & Uniformity of Different Photodeposition Mechanisms

Homogeneous photodeposition is the kinetically preferred mechanism, and it usually occurs more often. The heterogeneous mechanism also occurs, but at a much smaller frequency [1]. The identity of both the substrate and deposited material can change which type of mechanism will happen and the kinetic rates of each [1]. When copper and silver are both deposited onto a titanium oxide substrate, the copper deposited on with relatively high uniformity in particle size, with particles ranging from 50 to 120 nanometers in size. These particles were also deposited onto the substrate with a much more even distribution on the substrate. The size of the silver particles that deposited has a huge range of sizes, from 100 to over 500 nanometers [2]. Silver has a much larger molecular weight than copper, which makes it more difficult to deposit with uniformity. Silver also has a much higher electrode potential, which means that the decomposed and ionic silver atoms in solution will be reduced back to the uncharged, solid metal more rapidly, and so will deposit faster and therefore more uncontrolled [2]. This results in a larger range of particle sizes as well.

Figure 1: The left image shows the large range of silver particles deposited onto TiO2, and the right image shows the much smaller range of copper particles [2]

Multiple processes involving photodeposition have been patented for specific uses. Blum et al used ultraviolet wavelengths of less than 200 nm to deposit molybdenum, tungsten, and chromium hexacarbonyl refractory metals onto a GaAs substrate to form a gate electrode containing a continuous layer of metal. Other substrates using this process also include quartz, glass, semiconductors such as Si, polymers, and metals. This method using wavelengths of less than 200 nm is advantageous because it gives superior metal deposition. Additionally, focused ultraviolet beams are not required and thus heat buildup at the substrate can be avoided. (8) Even now, new methods of photodeposition are being developed for specific uses. Should this paragraph be moved into applications? It ties in with the Deutsch paper though...

References will need reordering once information is reordered.

Typical sequence of depositing films using photodeposition. The colloidal solution can be exchanged for other means of sample preparation including gas and solid phases. (16)(10)

Applications of photodeposition: A wide variety of applications for photodeposition exist, primarily suited for microelectronics and optoelectronics. Photodeposition is important for these applications because this technique can be used to generate thin films on the order of one micron. Photodeposition-generated polymer films from materials such as polydiacetylene have optical quality superior to that of films grown by standard crystal growth techniques. (4)

Materials photodeposition is used on: Photodeposition is used on a variety of materials. It is most common to deposit neutral metal particles, such as those of Silver, Nickel, and Copper, onto a titania or platinum surface in an effort to improve the oxidation-reduction capabilities of the substrate. Additionally, it is possible to deposit more complex precursors, such as metal sulfide quantum dots, onto similar surfaces. (11,12) It is also possible to deposit a neutral metal coating onto a more complicated substrate, such as the deposition of Ag onto ZnO nanorods. (13) This allows for the same uniformity of deposition afforded by the self-assembled monolayer (SAM) and successive ionic layer adsorption and reaction (SILAR) techniques, while negating the need for multiple rinses of the surface and large quantities of laboratory chemical waste.

Properties of materials Photodeposition may be used to achieve a number of goals. Deposition of neutral metal particles onto the surface of a nanorod or nanoparticle may help to increase the stability of the high energy particle. The deposition of a metal “cover” may provide a continuous surface around the particle, preventing decomposition. Additionally, photodeposition may be used to couple a reactive particle to a secondary catalytic substrate, such as the deposition of narrow gap semiconductor quantum dots into a titanium (II) dioxide surface in an effort to increase photocatalytic reactivity. (12) Increasingly often, the deposition of a metal onto a Titania surface is utilized in an attempt to increase the photocatalytic activity toward Carbon monoxide, alcohols, and hydrogen. (14) These compounds have potential applications in solar cells. Additionally, compounds generated via a similar method have seen applications in the removal of potential toxins from waste water. (15)

Laser photodeposition, using ultraviolet radiation, can vaporize and deposit micron-sized metal ions such as copper and silver on a surface. Electrodes uniformly deposited with Ag-Cu metallic nanoparticles using photodeposition have been coupled with TiO2 and used in photovoltaic devices and photocatalysts. The deposition of the metal is critical because the Ag-Cu metallic nanoparticles deposited on the surface of TiO2 shift the valence band and the conduction band to lower energy, which improves the light-harvesting capacity. Additionally, the deposition of metal nanoparticles also decreases the charge transfer resistance and enhances the electron injection efficiency. Move to Properties of Materials section.(2) Laser photodeposition has useful applications in several areas of microelectronics including metallization, etching, and growth of semiconductor films. Laser photodeposition allows the direct generation of metal patterns without the intermediate photolithographic steps required in conventional microelectronic fabrication. In addition to metals, compounds can be deposited by applying laser photodecomposition to two-component gas mixtures. GaAs and ZnSe thin films have been synthesized this way. (5) Photodeposition can also be used to fabricate three-dimensional patterned polymer microstructures, providing micrometer-scale photopatterning for the fabricated structures. (6) Photodeposition is also frequently used in the synthesis of catalysts and can even be used for such diverse applications as treating waste water containing metal and organic contaminants by photoreducing the metal contaminants onto the photocatalyst, and then removing the metal contaminants by photoreductive deposition so the catalyst can be recycled. (7)

Hilton et al used ferritin, an iron storage protein, to target precise locations and deposit metal nanoparticles on a surface. Initially, ferritin is deposited on precise locations on the target surface using ink jet deposition for precise patterns. Then through a photochemical reaction gold and other metal ions are reduced to their elemental form and deposited on the surface where the ferritin is located. This method is useful in any application where precise nucleation of metal nanoparticles is desired, such as positioning electrical leads on a printed circuit board. This method is advantageous because the photochemical byproduct oxalate is very clean, and the solution chemistry doesn’t require the expensive equipment used in chemical vapor deposition reactions. (9)

References: 1) Taing, J.; Cheng M.; Hemminger J. Photodeposition of Ag or Pt onto TiO2 Nanoparticles Decorated on Step Edges of HOPG. ACS Nano, 2011, 5, 6325-6333.

2) Chen, F.; Liu, J.; Plasmon Enchanced Photoelectrochemical Activity of Au-Cu Nanoparticles on TiO2 / Ti Substrates. Int. J. Electrochem. Sci, 2012, 7, 9560-9572.

3) Yoneyama, H.; Nishimura, N.; Tamura, H. Photodeposition of Palladium and Platinum onto Titanium Dioxide Single Crystals. J. Phys. Chem. 1981, 85, 268-272. 4) Paley, M.S.; Frazier, D.O.; Abdeleyem, H.; Armstrong, S.; McManus, S.P. Photodepositon of Amorphous Polydiacetylene Films from Monomer Solutions onto Transparent Substrates. J. Am. Chem. Soc. 1995, 117, 4775-4780. 5) Deutsch, T.F.; Ehrlich, D. J.; Osgood, R. M. Laser Photodeposition of Metal Films with Microscopic Features. Applied Physics Letters, 1979, 2, 175-177. (6) http://www.google.com/patents/US6200737 (7) http://www.google.com/patents/US5332508 (8) http://www.freepatentsonline.com/4451503.html (9) http://techtransfer.byu.edu/?products=photo-deposition-of-metal-nanoparticles-on-surfaces (10) Liquid phase photodeposition from Colloidal Solutions. Photo-excited processes, Diagnostics and Applications. 2004 pp251-280 (11) a) Korzhak, A.V.; Ermokhina, N.I.; Stroyuk, A.L.; Bukhtiyarov, V.K.; Raevskaya, A.E.; Litvin, V.I.; Kuchmiy, S.; Ilyin, V.G.; Manorik, P.A. Photocatalytic hydrogen evolution over mesoporous TiO2/metal nanocomposites; J. Photochem. and Photobiol. A: Chem, 2008, 198, 126 b) Pacholski, C.; Kornowski, A.; Weller, H. Site-Specific Photodeposition of Silver on ZnO Nanorods; Angew. Chem. Int. Ed., 2004, 43, 4774 (12) Tada, H.; Fujishima, M.; Kobayashi, H. Photodeposition of metal sulfide quantum dots on titanium(IV) dioxide and the applications to solar energy conversion Chem. Soc. Rev., 2011, 40, 4232 (13) Pacholski, C.; Kornowski, A.; Weller, H.; Site-Specific Photodeposition of Silver on ZnO Nanorods, Angew. Chem. Int. Ed., 2004, 43, 4774 (14) a) Horiuchi, Y.; Toyao, T.; Saito, M.; Mochizuki, K.; Iwata, M.; Higashimura, H.; Anpo, M.; Matsuoka, M.; Visible-Light-Promoted Photocatalytic Hydrogen Production by Using an Amino-Functionalized Ti(IV) Metal−Organic Framework J. of Phys. Chem. C, 2012, 116, 20848. b) Bamwenda, G.; Tsubota, S.; Nakamura, T.; Haruta, M.; The influence of the preparation methods on the catalytic activity of platinum and gold supported on TiO2 for CO oxidation Cat. Lett., 1997, 83. (15) a) Férnandez-Rodríguez, C.; Doña-Rodríguez, J.M.; González-Díaz, O.; Seck, I.; Zerbani, D.; Portillo, D.; Perez-Peña, J.; Synthesis of highly photoactive TiO2 and Pt/TiO2 nanocatalysts for substrate-specific photocatalytic applications Appl. Cat. B: Env., 2012, 125, 383. b) Han, Y.; Zhou, J.; Wang, W.; Wan, H.; Xu, Z.; Zheng, S.; Zhu, D.; Enhanced selective hydrodechlorination of 1,2-dichloroethane to ethylene on Pt–Ag/TiO2 catalysts prepared by sequential photodeposition Appl. Cat. B: Env., 2012, 125, 172 (16) Sha Jin,Fumihide Shiraishi; Photocatalytic activities enhanced for decompositions of organic compounds over metal-photodepositing titanium dioxide. Chem. Enginerring Journal, 2004, 97 (2-3), pp 203-211