Liquid solutions or suspensions using water or other solvents (organic, ionic liquids, molten salts) represent an alternative method to PVD for the growth of materials either in bulk form or as thin films. Liquid phase deposition (LPD), as the name implies, in- volves the formation of solid deposits on a substrate from a liquid solution. The material precursors are either dissolved or suspended in a liquid medium and the deposition may occur by either chemical or physical mechanisms. Over time, LPD has developed into various important methods for material processing and synthesis. LPD techniques have certain advantages over vapour phase methods including the lower processing tempera- tures involved, high deposition rates and the lower cost of the precursors. Furthermore, LPD methods have recently found applications for the preparation of nanoscale struc- tures by deposition within mesoporous templates.
Chapter 3. Materials Deposition inside Microstructured Optical Fibres 39
3.3.1
Electrochemical Deposition
Electrochemical deposition refers to the growth of a material or alloys from a conductive solution (electrolyte) containing ions of the material to be deposited. The deposition occurs on the cathode’s surface, by the flow of an electric current between two elec- trodes immersed in the solution. Many metals have been deposited within the holes of porous alumina, polycarbonate and polymer templates by the electrochemical reduc- tion of metals ions [131, 132, 133]. Figure 3.6 shows examples of gold wires formed by electrodeposition within alumina templates. The membrane employed has cylindrical pore geometry, with monodisperse diameters that extend through the entire thickness of the membrane. The deposition is done by simply coating one end of the template with a metal layer and using this film as a cathode. The deposition is initiated on the cathode and the material grows following the geometry of the pores. It usually results in well-defined, high aspect ratio, nanowires or nanotubes. If desired, after the deposi- tion the template can be dissolved away leaving behind the resulting metal structures. Nanowires and nanotubes of metals like gold, silver, platinum, copper, nickel have been prepared by electrochemistry. This technique has been prefered for the deposition of metals however many researchers have applied it for the deposition of semiconductor films on planar [134] and porous [135, 136] substrates.
Although electrochemistry has shown to be an excellent bottom-up template processing technique, the deposition conditions required in this method make it difficult to use for the impregnation of materials within MOFs templates. The main problem is that electrodeposition needs a conductive layer onto which the materials are grown. Because of this, materials could only be deposited at one end of a very long fibre but not in the central part of it. Another limitation of electrochemical deposition is the need for liquid precursors solutions, which can not easily diffuse inside centimetre long optical fibres and would thus prohibit the sufficient mass transport of precursors.
3.3.2
Electroless Deposition
Electroless deposition involves catalytic reactions in an aqueous solution which, in con- trast to electrodeposition, occur without an external electric current. This technique has been used to fabricate metal nanotubes within nanoporous templates [137, 138, 139]. Figure 3.7 shows a very interesting experiment by Monk and Walt [38], in which mil- limetre long gold tubes are grown within microscale capillaries of an optical fibre. The electroless deposition process can be briefly described as follows, a catalyst is first applied to all surfaces of the porous membrane, which is afterwards immersed into a solution containing metal ions and a chemical reduction agent (usually hydrogen). The
Chapter 3. Materials Deposition inside Microstructured Optical Fibres 40
Figure 3.6: Synthesis of gold nanowires in porous alumina template by elec-
trochemical deposition. Left: TEM image of a microtomed1 section of alumina membrane. Right: TEM of Au nanotubes that are 70 nm in diameter within
the pores of a membrane, from reference [131].
reduction of the metal solution to solid metal only occurs on the surface (where the cat- alyst was previously deposited) and metal films are formed on the walls. This technique has resulted in the preparation of gold nanotubes ensembles whose wall thickness can be controlled from a few nanometres to some micrometres. Nanotubes with inside diam- eters of less than 1 nm, made by electroless deposition of gold, have shown interesting applications for molecular filtering [137].
Figure 3.7: Synthesis of gold micro-tubes in a 5µm silica capillary by electro-
less deposition (end and side views). Silica capillaries (A) are modified with a monolayer of bifunctional silane for gold adhesion (B), deposition of a monolayer of colloidal gold (C). Deposition of bulk gold from solution increases the size of the gold layer, eventually forming a uniform layer (D). As deposition progresses, the gold thickness approaches the radius of the capillary (E), microscope images
of the resulting gold tubes (F G H). Taken from reference [38]
Both LPD methods, electrochemical and electroless deposition, require the diffusion of the precursor solutions through the template pores. Thus, these techniques are usually
1A microtome is a mechanical instrument used to cut samples into thin segments for micro-
Chapter 3. Materials Deposition inside Microstructured Optical Fibres 41 limited to thin substrates (≃ 100µm), or else deposition must be conducted over long periods of time (days) or cannot be achieved due to insufficient mass transport of the precursor far into the capillaries [38]. More than likely, this would be an inherently time consuming process for high aspect ratio features like MOFs capillaries. Furthermore,the removal of the liquid precursor solution and reaction by-products from the fibre holes would be a difficult task after the deposition inside long MOFs.