Abstract:
A fuel cell is disclosed which is formed on a semiconductor wafer by etching channel in the wafer and forming electronics on the substrate electronically coupled to the fuel cell that controls generation of power by the fuel cell through electrical communication with the fuel cell. A hydrogen fuel is admitted into one of the divided channels and an oxidant into the other. The hydrogen reacts with a catalyst formed on an anode electrode at the hydrogen side of the channel to release hydrogen ions (protons) which are absorbed into the PEM. The protons migrate through the PEM and recombine with return hydrogen electrons on a cathode electrode on the oxygen side of the PEM and the oxygen to form water.

Description:
RELATED APPLICATIONS 
     This application is a Continuation of U.S. application Ser. No. 12/220,787, filed Jul. 28, 2008, which is a Continuation of U.S. application Ser. No. 11/521,593, filed on Sep. 14, 2006, which is a Continuation of U.S. application Ser. No. 11/322,760, filed Dec. 29, 2005, which claims priority to and is a continuation application of U.S. application Ser. No. 10/953,038 filed on Sep. 29, 2004, now U.S. Pat. No. 6,991,866, and of U.S. application Ser. No. 10/985,736 filed on Nov. 9, 2004, now U.S. Pat. No. 7,029,779, which are a divisional application and a continuation application, respectively, of U.S. application Ser. No. 09/949,301 filed Sep. 7, 2001, now U.S. Pat. No. 6,815,110, which is a continuation of U.S. application Ser. No. 09/449,377, filed Nov. 24, 1999, now U.S. Pat. No. 6,312,846. 
     The entire teachings of the above applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The electro-chemical fuel cell is not new. Invented in 1839 by William Robert Grove, it has recently been the subject of extensive development. NASA used its principals in their 1960&#39;s space program, but the latest push into this technology is being driven largely by the automotive industry. Daimler-Chrysler and Ford Motor Co. together have invested about $750 million in a partnership to develop fuel cell systems. As environmental concerns mount and legislation toughens, development of “green” energy sources becomes more justified as a course of action, if not required. 
     The information age has ushered in the necessity for new ways to examine, process, manage, access and control the information. As the basic technologies and equipment evolve to handle these new requirements, there is a growing need for a smaller, lighter and faster (to refuel/recharge) electrical energy source. Portable computing and communications, in particular, would benefit greatly from a miniature fuel cell based power source. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a method and apparatus is provided which uses a combination of SAMs (self-assembled monolayers), MEMS (micro electrical mechanical systems), “chemistry-on-a-chip” and semiconductor fabrication techniques to create a scalable array of power cells directly on a substrate, preferably a semiconductor wafer. These wafers may be “stacked” (i.e., electrically connected in series or parallel), as well as individually programmed to achieve various power (V*I) characteristics and application driven configurations. 
     One preferred embodiment of the invention is formed by fabricating a plurality of individual fuel cells on a planar semiconductor wafer into which flow channels are formed by etching or other well-known semiconductor processes. Oxygen is admitted into one side of a channel and hydrogen into the other side; with the two gases being separated by a membrane. Electrodes are formed on opposite sides of the membrane and a catalyst is provided in electrical communication with the electrode and membrane on both sides. Lastly, a gas impermeable cover or lid is attached to the cell. 
     Preferably, the membrane is a PEM (Proton Exchange Membrane) formed by depositing or otherwise layering a column of polymers into etched channels in the substrate to create a gas tight barrier between the oxygen and hydrogen, which is capable of conveying hydrogen ions formed by the catalyst across the barrier to produce electricity across the contacts and water when the H-ions combine with the oxygen in the other channel. 
     In addition, a number of fuel cells can be electrically interconnected and coupled to gas sources on a portion of the same wafer to form a “power chip”. Traditional electrical circuitry can be integrated on the wafer along with the chips to provide process monitoring and control functions for the individual cells. Wafers containing multiple chips (power discs) or multiple cells can then be vertically stacked upon one another. 
     In one embodiment, the present invention is method of forming a fuel cell. The steps include forming a fuel cell on a substrate, and forming electronics on the substrate electronically coupled to the fuel cell that controls generation of power by the fuel cell through electrical communication with the fuel cell. 
     A further understanding of the nature and advantages of the invention herein may be realized with respect to the detailed description which follows and the drawings described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is a schematic plan view of a semiconductor fuel cell array in accordance with the invention. 
         FIG. 2  is a simplified schematic cross-sectional view taken along the lines II-II of a fuel cell  12  of the invention. 
         FIGS. 3( a )-( h )  is a schematic sectional process view of the major steps in fabricating a PEM barrier structure  30  of the invention. 
         FIG. 4  is a cross-sectional schematic view illustrating an alternate cast PEM barrier invention. 
         FIG. 5  is a sectional view of a PEM structure embodiment. 
         FIG. 6  is a sectional view of an alternate of the PEM structure. 
         FIG. 7  is a sectional view of another alternate PEM structure. 
         FIG. 8  is a block diagram of circuitry which may be integrated onto a fuel cell chip. 
         FIG. 9  is a schematic of the wiring for an integrated control system for the operation of individual cells or groups of cells. 
         FIG. 10  is a schematic side view of a manifold system for a power cell. 
         FIG. 11  is a schematic plan view of a plurality of cells arranged side-by-side on a wafer to form a power chip and stocked on top of each other to form a power disc. 
         FIG. 12  is a fragmented side-view of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of preferred embodiments of the invention follows. 
     A description of preferred embodiments of the invention follows. 
     Referring now to  FIG. 1 , there is shown in plan view a conventional semiconductor wafer  10  upon which a plurality of semiconductor fuel cells  12  have been fabricated. A plurality of cells may be electrically interconnected on a wafer and provided with gases to form a power chip  15 . For simplicity, fuel cells  12  and chips  15  are not shown to scale in as much as it is contemplated that at least 80 million cells may be formed on a 4″ wafer. One such cell is shown in fragmented cross-section in  FIG. 2 . In its simplest form, each cell  12  consists of a substrate  14 , contacts  16 A and B, and a conductive polymer base  18  formed on both sides of a first layer  20 ( a ) of non-conductive layered polymer support structure  20  and in intimate contact with the metal electrical contacts. 
     A conductive polymer  22  with embedded catalyst particles  28  on both sides of the central structure  20  forms a PEM barrier separating the hydrogen gas on the left side from the oxygen gas on the right side. Etched channels  50 B and  50 A respectively for admittance of the O 2  and H 2  gas and a heatsink lid  40  over the cell  12  is also shown in  FIG. 2 . 
       FIGS. 3 a -3 h    are a series of schematic sectional views showing the relevant fabrication details of the PEM barrier  30  in several steps.  FIG. 3 a    shows the bottom of a power cell channel which has been etched into the semiconductor substrate  14 . It also shows the metal contacts  16  which are responsible for conveying the electrons out of the power cell  12  to the rest of the circuitry. These metal contact are deposited by well-known photolithographic processes in the metalization phase of the semiconductor fabrication process. 
       FIG. 3 b    shows the conductive polymer base  18  as it has been applied to the structure. Base  18  is in physical/electrical contact with the metal contacts  16  and has been adapted to attract the conductive polymer  22  of the step shown in  FIG. 3 a   - 3   h.    
       FIG. 3 c    shows the nonconductive polymer base  20 (a) as it has been applied to the structure. It is positioned between the two conductive polymer base sites  18  and is adapted to attract the nonconductive polymer  20 . 
       FIG. 3 d    shows a polymer resist  21  as applied to the structure. Resist  21  is responsible for repelling the polymers and preventing their growth in unwanted areas. 
       FIG. 3 e    shows the first layer  20 B of nonconductive polymer as it has been grown on its base  20 A. This is the center material of the PEM barrier. It helps support the thinner outer sides  22  when they are constructed. 
       FIG. 3 f    shows the subsequent layers of nonconductive polymer  20  which are laid down, in a layer by layer fashion to form a vertical barrier. This vertical orientation allows for area amplification. 
       FIG. 3 g    shows the first layer  22   a  of conductive polymer grown on its base  18 . This is the outside wall material with catalyst of the PEM barrier. 
       FIG. 3 h    shows the subsequent layers of conductive polymer  22  laid down, in a layer by layer fashion on to the structure.  FIG. 2  shows the completed structure after removal of the polymer resist layer  21  and the addition of lid  40  and the pre-existing sidewalls  52  left out of  FIG. 3 a -3 h    for simplicity. This resist removal may not be necessary if layer  21  was originally the passivation layer of the final step in the semiconductor fabrication process. 
     Referring now to  FIG. 2  again further details of the elements forming the fuel cell  12  will be explained. The protein exchange membrane shown generally at  30  forms a barrier between the fuel H 2  and the oxidant O 2 . 
     The PEM barrier  30  is made up of three parts of two materials. There is the first outside wall  22 B, then the center  20 , and finally the second outside wall  22 C. It is constructed with a center piece  20  of the first material in contact with the two outside walls which are both made of the second material. 
     The material  20  forming the center piece, is preferably an ionic polymer capable of passing the hydrogen ions (protons) through from the hydrogen side to the oxygen side. It is electrically nonconductive so that it does not, effectively, short out the power cell across the two contacts  16 A and  16 B. It may be made of Nafion® or of a material of similar characteristics. An external load  5  as shown in dotted lines may be coupled across the contacts to extract power. 
     The second material  22 , forming the two outside walls, is also a similar ionic polymer capable of passing the hydrogen ions. In addition, it is doped with nano catalyst particles  28  (shown by the dots), such as, platinum/alloy catalyst and is also electrically conductive. 
     By embedding the catalyst particles  28  into the polymer  22 , maximum intimate contact is achieved with the PEM  30 . This intimate contact provides a readily available path which allows the ions to migrate freely towards the cathode electrode  16 B. Catalysis is a surface effect. By suspending the catalytic particles  28  in the polymer  22 , effective use of the entire surface area is obtained. This will dramatically increase the system efficiency. 
     By making the second material  22  electrically conductive, an electrode is produced. The proximity of the electrode to the catalytic reaction affects how well it collects electrons. This method allows the catalytic reaction to occur effectively within the electrode itself. This intimate contact provides a readily available path which allows the electrons to migrate freely towards the anode  16 A. This will allow for the successful collection of most of the free electrons. Again, this will dramatically increase the system efficiency. 
     In addition to the electrical and chemical/functional characteristics of the PEM  30  described above, there are some important physical ones, that are described below: 
     This self assembly process allows for the construction of a more optimum PEM barrier. By design it will be more efficient. 
     First, there is the matter of forming the separate hydrogen and oxygen path ways. This requires that the PEM structure to be grown/formed so that it dissects the etched channel  50  fully into two separate channels  50 A,  50 B. This means that it may be patterned to grow in the center of the channel and firmly up against the walls of the ends of the power cell. It may also be grown to the height of the channel to allow it to come into contact with an adhesive  42  on the bottom of lid  40 . 
     Second, there is the matter of forming a gas tight seal. This requires that the PEM structure  30  be bonded thoroughly to the base structures  18  and  20 A, the substrate  14  and the end walls (not shown) of the power cell and to an adhesive  42  which coats the lid  40 . By proper choice of the polymers, a chemical bond is formed between the materials they contact in the channel. In addition to this chemical bond, there is the physical sealing effect by applying the lid  40  down on top of the PEM barrier. If the height of the PEM  30  is controlled correctly, the pressure of the applied lid forms a mechanical “0 ring” type of self seal. Growing the PEM  30  on the substrate  14  eliminates any fine registration issues when combining it with the lid  40 . There are no fine details on the lid that require targeting. 
     Turning now to  FIG. 4 , there is shown in simplified perspective an alternate embodiment of a PEM barrier involving a casting/injecting process and structure. 
     Using MEMS machining methods three channels  60 A,  60 B and  60 C are etched into a semiconductor substrate  140 . The outside two channels  60 A and  60 C are separated from the middle channel  60 B by thin walls  70 A,  70 B. These walls have a plurality of thin slits S 1 -S n  etched into them. The resultant tines T 1 -T n+1  have a catalyst  280  deposited on them in the area of the slits. At the bottom of these thin walls  70 A,  70 B, on the side which makes up a wall of an outside channel  60 A,  60 C, a metal electrode  160 A,  160 B is deposited. A catalyst  280  is deposited on the tines after the electrodes  160  are in place. This allows the catalyst to be deposited so as to come into electrical contact and to cover to some degree, the respective electrodes  160  at their base. In addition, metal conductors  90  are deposited to connect to each electrode  160 , which then run up and out of the outside channels. 
     A lid  400  is provided with an adhesive layer  420  which is used to bond the lid to the substrate  140 . In this way, three separate channels are formed in the substrate; a hydrogen channel  60 A, a reaction channel  60 B, and an oxygen channel  60 C. In addition, the lid  400  has various strategically placed electrolyte injection ports or holes  500 . These holes  500  provide feed pathways that lead to an electrolyte membrane of polymer material (not shown) in the reaction channel  60 B only. 
     The structure of  FIG. 4  is assembled as follows: 
     First, the semiconductor fabrication process is formed including substrate machining and deposition of all electrical circuits. 
     Next, the lid  400  is machined and prepared with adhesive  420 . The lid  400  is bonded to the substrate  140 . Then, the electrolyte (not shown) is injected into the structure. 
     The thin walls  70 A,  70 B of the reaction channel  60 B serve to retain the electrolyte during its casting. The slits S 1 -S N  allow the hydrogen and oxygen in the respective channels  60 A,  60 B access to the catalyst  280  and PEM  300 . Coating the tines T 1 -T 1+n  with a catalyst  280  in the area of the slits provides a point of reaction when the H 2  gas enters the slits. When the electrolyte is poured/injected into the reaction channel  60 B, it fills it up completely. The surface tension of the liquid electrolyte keeps it from pushing through the slits and into the gas channels, which would otherwise fill up as well. Because there is some amount of pressure behind the application of the electrolyte, there will be a ballooning effect of the electrolyte&#39;s surface as the pressure pushes it into the slits. This will cause the electrolyte to be in contact with the catalyst  280  which coats the sides of the slits S 1 -S N . Once this contact is formed and the membrane (electrolyte) is hydrated, it will expand even further, ensuring good contact with the catalyst. The H 2 /O 2  gases are capable of diffusing into the (very thin, i.e. 5 microns) membrane, in the area of the catalyst. Because it can be so thin it will produce a more efficient i.e. less resistance (I2R) losses are low. This then puts the three components of the reaction in contact with each other. The electrodes  160 A and  160 B in electrical contact with the catalyst  280  is the fourth component and provides a path for the free electrons, through an external load (not shown), while the hydrogen ions pass through the electrolyte membrane to complete the reaction on the other side. 
     Referring now to the cross-sectional views of  FIGS. 5-7 , various alternative configurations of the PEM structure  30  of the invention will be described in detail. In  FIG. 5 , the central PEM structure  20  is formed as a continuous nonconductive vertical element, and the electrode/catalyst  16 / 28  is a non-continuous element to which lead wires  90  are attached.  FIG. 6  is a view of an alternative PEM structure in which the catalyst  28  is embedded in the non-conductive core  20  and the electrodes  16  are formed laterally adjacent the catalyst. Lastly, in  FIG. 7 , the PEM structure is similar to  FIG. 5  but the center core  201  is discontinuous. 
       FIG. 8  is a schematic block diagram showing some of the possible circuits that may be integrated along with a microcontroller onto the semiconductor wafer  10  to monitor and control multiple cells performance Several sensor circuits  80 ,  82 ,  84  and  86  are provided to perform certain functions. 
     Temperature circuit  80  provides the input to allow the micro processor  88  to define a thermal profile of the fuel cell  12 . Voltage circuit  82  monitors the voltage at various levels of the configuration hierarchy or group of cells. This provides information regarding changes in the load. With this information, the processor  88  can adjust the system configuration to achieve/maintain the required performance. Current circuit  84  performs a function similar to the voltage monitoring circuit  82  noted above. 
     Pressure circuit  86  monitors the pressure in the internal gas passages  50 A,  50 B. Since the system&#39;s performance is affected by this pressure, the microprocessor  88  can make adjustments to keep the system running at optimum performance based on these readings. An undefined circuit  81  is made available to provide a few spare inputs for the microprocessor  88  in anticipation of future functions. 
     In addition, configuration circuit  94  can be used to control at least the V*I switches to be described in connection with  FIG. 9 . The output voltage and current capability is defined by the configuration of these switches. Local circuitry  92  is provided as necessary to be dynamically programmed, such as the parameters of the monitoring circuits. These outputs can be used to effect that change. Local subsystems  94  are used by the microprocessor  98  to control gas flow rate, defect isolation, and product removal. A local power circuit  96  is used to tap off some part of the electricity generated by the fuel cell  12  to power the onboard electronics. This power supply circuit  96  will have its own regulation and conditioning circuits. A two-wire communications I/F device  98  may be integrated onto the chip to provide the electrical interface between communicating devices and a power bus (not shown) that connects them. 
     The microcontroller  8  is the heart of the integrated electronics subsystem. It is responsible for monitoring and controlling all designated system functions. In addition, it handles the communications protocol of any external communications. It is capable of “in circuit programming” so that its executive control program can be updated as required. It is capable of data storage and processing and is also capable of self/system diagnostics and security features. 
     Referring now to  FIG. 9 , further details of the invention are shown. In this embodiment, the individual power cells  12   1 ,  12   2 - 12   n  are formed on a wafer and wired in parallel across power buses  99 A and  99 B using transistor switches  97  which can be controlled from the microprocessor  88  of  FIG. 8 . Switches  97 B and  97 A are negative and positive bus switches, respectively, whereas switch  97 C is a series switch and switches  97 D and  97 E are respective positive and negative parallel switches, respectively. 
     This allows the individual cells or groups of cells (power chip  15 ) to be wired in various configurations, i.e., parallel or series. Various voltages are created by wiring the cells in series. The current capacity can also be increased by wiring the cells in parallel. In general, the power profile of the power chip can be dynamically controlled to achieve or maintain a “programmed” specification. Conversely, the chip can be configured at the time of fabrication to some static profile and, thus, eliminate the need for the power switches. By turning the switches on and off and by changing the polarity of wiring, one can produce both AC and DC power output. 
     To implement a power management subsystem, feedback from the power generation process is required. Circuitry can be formed directly on the chip to constantly measure the efficiencies of the processes. This feedback can be used to modify the control of the system in a closed loop fashion. This permits a maximum level of system efficiency to be dynamically maintained. Some of these circuits are discussed next. 
     The quality of the power generation process will vary as the demands on the system change over time. A knowledge of the realtime status of several operational parameters can help make decisions which will enable the system to self-adjust, in order to sustain optimum performance. The boundaries of these parameters are defined by the program. 
     For example, it is possible to measure both the voltage and the current of an individual power cell or group of power cells. The power output can be monitored and, if a cell or group is not performing, it can be removed if necessary. This can be accomplished by the power switches  97  previously described. 
     An average power level can also be maintained while moving the active “loaded” area around on the chip. This should give a better overall performance level due to no one area being on 100% of the time. This duty cycle approach is especially applicable to surge demands. The concept here is to split the power into pieces for better cell utilization characteristics. 
     It is expected that the thermal characteristics of the power chip will vary due to electrical loading and that this heat might have an adverse effect on power generation at the power cell level. Adequate temperature sensing and an appropriate response to power cell utilization will minimize the damaging effects of a thermal build up. 
     The lid  40  is the second piece of a two-piece “power chip” assembly. It is preferably made of metal to provide a mechanically rigid backing for the fragile semiconductor substrate  14 . This allows for easy handling and provides a stable foundation upon which to build “power stacks”, i.e., a plurality of power chips  15  that are literally stacked on top of each other. The purpose being to build a physical unit with more power. 
       FIG. 10  illustrates how the fuel and oxidant/product channels  50 A (and  50 B not shown) may be etched into the surface of the substrate  14 . These troughs are three sided and may be closed and sealed on the top side. The lid  40  and adhesive  42  provides this function of forming a hermetic seal when bonded to the substrate  14  and completes the channels. A matrix of fuel supply and oxidant and product water removal channels is thereby formed at the surface of the substrate. 
     The lid  40  provides a mechanically stable interface on which the input/output ports can be made. These are the gas supply and water removal ports. The design may encompass the size transition from the large outside world to the micrometer sized features on the substrate. This is accomplished by running the micrometer sized channels to a relatively much larger hole H. This larger hole will allow for less registration requirements between the lid and substrate. The large holes in the lid line up with the large holes in the substrate which have micrometer sized channels also machined into the substrate leading from the larger hole to the power cells. 
     Each wafer may have its own manifolds. This would require external connections for the fuel supply, oxidant and product removal. The external plumbing may require an automated docking system. 
       FIGS. 11 and 12  illustrates one of many ways in which several cells  12  (in this example three cells side-by-side can be formed on a wafer  14  to form a power chip  15 . Power disks can be stacked vertically upon each other to form a vertical column with inlet ports,  50 HI,  50 OI coupled to sources of hydrogen and oxygen, respectively. The vertical column of wafers with power chips formed therein comprise a power stack ( 93 ). 
       FIG. 12  illustrates how stacking of a number of power discs  15  may be used to form power stacks ( 93 ) with appreciable power. The use of the word “stacking” is reasonable for it suggests the close proximity of the wafers, allowing for short electrical interconnects and minimal plumbing. In reality, the stacking actually refers to combining the electrical power of the wafers to form a more powerful unit. They need only to be electrically stacked to effect this combination. However, it is desirable to produce the most amount of power in the smallest space and with the highest efficiencies. When considering the shortest electrical interconnect (power bussing) alternatives, one should also consider the possibility of using two of the main manifolds as electrical power busses. This can be done by electrically isolating these manifold/electrical power buss segments and using them to convey the power from each wafer to the next. This reduces the big power wiring requirements and permits this function to be done in an automated fashion with the concomitant increased accuracy and reliability. 
     A desirable manifold design would allow for power disc stacking. In this design the actual manifold  95  would be constructed in segments, each segment being an integral part of the lid  40 . As the discs are stacked a manifold (tube) is formed. This type of design would greatly reduce the external plumbing requirements. Special end caps would complete the manifold at the ends of the power stack. 
     In summary, one of the primary objects of this invention is to be able to mass produce a power chip  15  comprised of a wafer  10  containing multiple power cells  12  on each chip  15  utilizing quasi standard semiconductor processing methods. This process inherently supports very small features. These features (power cells), in turn, are expected to create very small amounts of power per cell. Each cell will be designed to have the maximum power the material can support. To achieve any real substantially power, many millions will be fabricated on a single power chip  15  and many power chips fabricated on a “power disc” (semiconductor wafer  10 ). This is why reasonable power output can be obtained from a single wafer. A 10 uM×10 uM power cell would enable one million power cells per square centimeter. The final power cell topology will be determined by the physical properties of the constituent materials and their characteristics. 
     The basic electro-chemical reaction of the solid polymer hydrogen fuel cell is most efficient at an operating temperature somewhere between 80 to 100° C. This is within the operating range of a common semiconductor substrate like silicon. However, if the wafers are stacked additional heatsinking may be required. Since a cover is needed anyway, making the lid  40  into a heatsink for added margin makes sense. 
     The fuel and oxidant/product channels are etched into the surface of the semiconductor substrate. These troughs are three-sided and may be closed and sealed on the top side. The lid  40  provides this function. It is coated with an adhesive to form a hermetic seal when bonded to the semiconductor substrate and completes the channels. This forms a matrix of fuel supply and oxidant and product water removal channels at the surface of the semiconductor substrate. The power cells two primary channels are themselves separated by the PEM which is bonded to this same adhesive. Thus, removing any fine grain critical alignment requirements. 
     Equivalents 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. 
     For example, while silicon because of its well-defined electrical and mechanical properties is the material of choice for the substrate  14 , other semiconductor materials may be substituted, therefore, such as Gd, Ge, or III-V compounds such as GaAs. Alternatively, the substrate for the cell may be formed of an amorphous material such as glass or plastic, or phenolic; in which case, the controls for the cells can be formed on a separate semiconductor die and electrically coupled to the cells to form a hybrid structure. The interface between the PEM&#39;s structure is preferably an assembled monolayer (SAM) interface formed of gold, however, other metals such as silver or platinum, may be used in place thereof. Likewise, although the PEM is formed of many molecular chains, it preferably has a base with an affinity for gold so that it will bond to the gold SAM feature. Again, other pure metals such as platinum and silver may be substituted therefore.