External control electrochemical approach

External controls to drive the nucleation in confined areas. The external control can be any external driving force promoting the growth of MOFs (e.g., electrochemical approach) or controlling/depositing the amount of MOF precursors in precise locations (e.g., microcontact printing and evaporation-induced growth). 

The difference between the electrolytic ceU potential and the potential (voltage) when the current passes in the external circuit is due to ohmic losses. The main sources of ohmic losses are the resistance of the electrolyte, contact resistances of the leads, and the film formed on the electrode-electrolyte interface. The circuit ohmic resistance decreases the equilibrium potential by an amount equal to iR, where is the current passing between the working and counter electrode and R is the net resistance in the circuit. Current passes through the cell only when the voltage applied to the system consists of thermodynamically controlled equilibrium potential and the potential drop that compensates for the ohmic losses. The potential drop is not thermodynamically controlled and depends on the current density and the resistance in the circuit. It approaches zero when the current is shut off, and increases immediately when the current is switched on [8,9]. The iR drop in volts is equal to i°l/k, where i° is the current density in A/cm, is the thickness of the electrolyte in cm, and k is the specific conductivity of the electrolyte 1/Qcm. Various techniques are employed to measure the ohmic losses in an electrochemical cell. These measurement techniques include current interruption and four probe methods, among others that are discussed later in the book [8-10],... 

The basic difference between conventional processing and nanofabrication is the dimension of the structures to be fabricated. There are basically two possible approaches top-down and bottom-up approaches. In the top-down approach, micro and nanostructures are achieved by controlled removal of extra amount of material by applying an external source of energy such as mechanical, thermal, chemical, and electrochemical energy. The top-down approach of micro and nanofabrication is schematically shown in Fig. 1.2. This approach is difficult to apply at nanoscale however at microscale, it has been utilized successfully by various means. In the bottom-up approach, positions of atoms or molecules are manipulated to build up the nanodevices or nanostmctures, as illustrated in Fig. 1.3. Various techniques of this approach are under development at the laboratory level and need further improvements. 

SECM instruments suitable for imaging require a PC equipped with an interface board to synchronize acquisition of the electrochemical data with the movement of the tip. Building an SECM for kinetic experiments at fixed tip position or approach curve measurements is relatively easy, but fairly sophisticated software and some electronic work is necessary to construct a computer-controlled apparatus for imaging applications. Details on the construction of SECM instruments can be found elsewhere [6, 13-18, 53, 55]. An SECM is now available commercially from CH Instruments, Inc. (Austin, TX, USA). The instrument employs piezoelectric actuators, a three-axis stage, and a bipotentiostat controlled by an external PC under a 32-bit Windows environment. Various standard electrochemical techniques are incorporated along with SECM imaging, approach curves, and the modes described in Sect. 3.3.I.I. 

In Chapter 1 we explored the fundamental relationship between the electrode potential and a redox couple in solution. It was also pointed out that if the potential of an electrode is controlled externally, the solution can be made to adjust by electron transfer to approach equilibrium with the electrode potential. In many electrochemical experiments, the solution initially has only one form of a redox couple present, and the electrode is initially set at a potential such that this form does not undergo electron transfer. This ensures that the experiment begins at zero faradaic current. The electrode potential is then changed to a position that favors electron transfer. The manner in which the potential is changed gives rise to a profusion of electrochemical controlled potential techniques. 

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