Methods of fabricating complex two-dimensional conductive silicides

The embodiments disclosed herein relate to the fabrication of complex two-dimensional conductive silicide nanostructures, and methods of fabricating the nanostructures. In an embodiment, a conductive silicide includes a plurality of connected and spaced-apart nanobeams linked together at an about 90-degree angle, the plurality of nanobeams forming a two-dimensional nanostructure having a mesh-like appearance. In an embodiment, a method of fabricating a two-dimensional conductive silicide includes performing chemical vapor deposition, wherein one or more gas or liquid precursor materials carried by a carrier gas stream react to form a nanostructure having a mesh-like appearance and including a plurality of connected and spaced-apart nanobeams linked together at an about 90-degree angle.

FIELD

The embodiments disclosed herein relate to the fabrication of complex two-dimensional conductive nanostructures, and more particularly to the fabrication of complex two-dimensional conductive silicide nanostructures by a chemical vapor deposition method.

BACKGROUND

Simple nanostructures (e.g. nanowires) form complex nanomaterials when connected by single crystalline junctions, offering better mechanical strength and superior charge transport while preserving unique properties associated with the small dimensions. Great research interest has been attracted to study this new class of materials, especially in the field of electronics and energy applications. Synthesis of these materials is challenging, necessitated by the combined features of low dimensionality and high complexity; the former requires growth suppressions whereas the latter demands growth enhancement. To this end, two-dimensional complex nanostructures are exceedingly difficult to grow chemically.

SUMMARY

Complex two-dimensional conductive silicide nanostructures and methods of fabricating the nanostructures are disclosed.

According to aspects illustrated herein, there is provided a conductive silicide that includes a plurality of connected and spaced-apart nanobeams linked together at an about 90-degree angle, the plurality of nanobeams forming a two-dimensional nanostructure having a mesh-like appearance. In an embodiment, the plurality of nanobeams are flexible. In an embodiment, the silicide has an electrical resistivity of approximately 10 μΩ·cm. In an embodiment, the conductive silicide can be used in a nanoelectronics device. In an embodiment, the conductive silicide can be used in an energy-related device. In an embodiment, the conductive silicide can be used in a planar electronic device. In an embodiment, the conductive silicide can be used in an optoelectronics device. In an embodiment, the conductive silicide can be used in a nanophotonics device.

According to aspects illustrated herein, there is provided a conductive silicide nanostructure comprising a plurality of two-dimensional nanonet sheets, wherein each of the nanonet sheets include connected and spaced-apart nanobeams linked together at an about 90-degree angle. In an embodiment, the plurality of nanonet sheets are stacked approximately horizontally. In an embodiment, the plurality of nanonet sheets have an electrical resistivity of approximately 10 μΩ·cm.

According to aspects illustrated herein, there is provided a method of fabricating a two-dimensional conductive silicide that includes performing chemical vapor deposition, wherein one or more gas or liquid precursor materials carried by a carrier gas stream react to form a nanostructure having a mesh-like appearance and including a plurality of connected and spaced-apart nanobeams linked together at an about 90-degree angle.

DETAILED DESCRIPTION

Silicides are highly conductive materials formed by alloying silicon with selected metals. They are commonly used in Si integrated circuits to form ohmic contacts. The most frequently used silicides in advanced integrated circuits are silicides of titanium (TiSi2), cobalt (CoSi2), and nickel (NiSi). Titanium silicide (TiSi2) is an excellent electronic material and is one of the most conductive silicides (resistivity of about 10 micro-ohm-centimeters (μΩ·cm)). TiSi2has recently been demonstrated to behave as a good photocatalyst to split water by absorbing visible lights, a promising approach toward solar H2as clean energy carriers. Better charge transport offered by complex structures of nanometer-scaled TiSi2is desirable for nanoelectronics and solar energy harvesting. Capabilities to chemically synthesize TiSi2are therefore appealing. Synthetic conditions required by the two key features of complex nanostructures, low dimensionality and complexity, however, seem to contradict each other. Growth of one-dimensional (1D) features involves promoting additions of atoms or molecules in one direction while constraining those in all other directions, which is often achieved either by surface passivation to increase energies of sidewall deposition (such as solution phase synthesis) or introduction of impurity to lower energies of deposition for the selected directions (most notably the vapor-liquid-solid mechanism). Complex crystal structures, on the other hand, require controlled growth in more than one direction. The challenge in making two-dimensional (2D) complex nanostructures is even greater as it demands more stringent controls over the complexity to limit the overall structure within two dimensions. The successful chemical syntheses of complex nanostructures have been mainly limited to three-dimensional (3D) ones. In principle, 2D complex nanostructures are less likely to grow for crystals with high symmetries, e.g. cubic, since various equivalent directions tend to yield a 3D complex structure; or that with low symmetries, e.g. triclinic, monoclinic or trigonal, each crystal plane of which is so different that simultaneous growths for complexity are prohibitively difficult.

Methods of fabricating 2D conductive silicide nanostructures are disclosed herein. In an embodiment, the 2D conductive silicide nanostructures are free-standing nanostructures. In an embodiment, the nanostructures are single crystalline complex 2D networks composed of a plurality of nanonet (NN) sheets, formed by optimizing various processing parameters during fabrication. In an embodiment, the nanostructures include a plurality of nanonet sheets that are stacked on top of each other. In an embodiment, the nanonet sheets are stacked approximately horizontally to each other. In an embodiment, each nanonet sheet is a complex structure made up of nanobeams that are linked together by single crystalline junctions with 90-degree angles. In an embodiment, each nanobeam is approximately 15 nm thick, 20-30 nm wide, and at least about 1 μm long. Crystals with hexagonal, tetragonal, and orthorhombic lattices are good choices for 2D complex nanostructures of the present disclosure.

The following definitions are used to describe the various aspects and characteristics of the presently disclosed embodiments.

As used herein, the term “CVD” refers to chemical vapor deposition. In CVD, gaseous mixtures of chemicals are dissociated at high temperature (for example, CO2into C and O2). This is the “CV” part of CVD. Some of the liberated molecules can then be deposited on a nearby substrate (the “D” in CVD), with the rest pumped away. Examples of CVD methods include but are not limited to, “plasma enhanced chemical vapor deposition” (PECVD), “hot filament chemical vapor deposition” (HFCVD), and “synchrotron radiation chemical vapor deposition” (SRCVD).

As used herein, the term “electrical resistivity” refers to a measure of how strongly a nanostructure of the presently disclosed embodiments opposes the flow of electric current.

As used herein, the term “mesh-like appearance” or “nanonet appearance” refers to a complex 2D nanostructure of the presently disclosed embodiments fabricated to form a plurality of connected nanobeams of conductive silicide. The nanobeams making up the nanostructure can exist either parallel or perpendicular to another nanobeam(s). The nanobeam(s) that are perpendicular to other nanobeam(s) are at an about 90-degree angle to one another. Spaces exist between nanobeams, forming the mesh-like appearance. The entire nanostructure is single crystalline.

Structural stability improvements achieved by the methods of the presently disclosed embodiments results in a significant increase in conductivity as compared to bulk C49 TiSi2. The 2D conductive silicide nanostructures of the presently disclosed embodiments show remarkable mechanical integrity and good electrical conductivity. In an embodiment, the 2D conductive silicide nanostructures of the present disclosure can be used in the field of nanoelectronics, where the nanostructures represent dimensions and complexities far beyond that can be reached by lithography methods. This will lead to significant progress of electronics miniaturizations. In an embodiment, the 2D conductive silicide nanostructures of the present disclosure can be used for developing energy-related devices such as solar cells and batteries, benefited from the new structures and outstanding electrical conductivities achieved. Planar electronic devices made using the 2D conductive silicide nanostructures of the presently disclosed embodiments can be employed as ultra-sensitive sensors, which will be useful in chemical detection and medical diagnosis. In an embodiment, the 2D conductive silicide nanostructures of the present disclosure act as nano-antennas, and can be used for optoelectronics and nanophotonics applications. In an embodiment, the 2D conductive silicide nanostructures of the present disclosure find use as a fractal antenna.

The methods disclosed herein generate novel complex 2D conductive silicide nanostructures by optimizing various process parameters during fabrication. In an embodiment, careful control of the feeding of the synthesis precursors is necessary for obtaining the nanostructures disclosed herein. Inbalanced feeding of either the precursors or the overall concentration in the reaction chamber, can lead to failed growth of the nanostructures. In an embodiment, careful control of the carrier gas is necessary for obtaining the nanostructures disclosed herein, as the carrier gas reacts with both precursors, as well as acts as a protecting gas by providing a reductive environment.

An important distinguishing characteristic of the methods disclosed herein is that the nanostructres are spontaneously formed, without the need for supplying growth seeds. This eliminates the limitations that many other nanostructure synthesis methods require, and thus extend the nanostructures applications in fields where impurities (from hetergeneous growth seeds) are detrimental. The substrates that the disclosed nanostructures can be grown on are versatile, so long as the substrate sustains the temperatures required for the synthesis. In an embodiment, the nanostructures are grown on a transparent substrate. The nanostructures fabricated according to the methods of the presently disclosed embodiments can provide superior conductivity, excellent thermal and mechanical stability, and high surface area.

In an embodiment, the 2D conductive silicide nanostructures are titanium silicide nanostructures. The following detailed description will focus on the fabrication of 2D titanium silicide (TiSi2) nanostructures. However, it should be noted that other 2D conductive silicide nanostructures can be fabricated using the methods of the presently disclosed embodiments, including, but not limited to, nickel silicide, iron silicide, platinum silicide, chromium silicide, cobalt silicide, molybdenum silicide and tantalum silicide.

FIG. 1shows a CVD system100used for an embodiment of a method of fabricating 2D conductive nanostructures of the present disclosure. The CVD system100has automatic flow and pressure controls. Flow of a precursor gas and a carrier gas are controlled by mass flow controllers101and102respectively, and fed to a growth (reaction) chamber107at precise flow rates. The flow rate for the precursor gas is between about 20 standard cubic centimeters per minute (sccm) and about 100 sccm. In an embodiment, the flow rate for the precursor gas is about 50 sccm. In an embodiment, the precursor gas is present at a concentration ranging from about 1.3×10−6mole/L to about 4.2×10−6mole/L. In an embodiment, the precursor gas is present at a concentration of about 2.8±1×10−6mole/L. The flow rate for the carrier gas is between about 80 standard cubic centimeters per minute (sccm) and about 130 sccm. In an embodiment, the flow rate for the carrier gas is about 100 sccm. A precursor liquid is stored in a cylinder104and released to the carrier gas mass flow controller102through a metered needle control valve103. The flow rate for the precursor liquid is between about 1.2 sccm and 5 sccm. In an embodiment, the flow rate for the precursor liquid is about 2.5 sccm. In an embodiment, the precursor liquid is present at a concentration ranging from about 6.8×10−7mole/L to about 3.2×10−6mole/L. In an embodiment, the flow rate for the precursor liquid is present at a concentration of about 1.1±0.2×10−6mole/L. All precursors are mixed in a pre-mixing chamber105prior to entering the reaction chamber107. The pressure in the reaction chamber107is automatically controlled and maintained approximately constant by the combination of a pressure transducer106and a throttle valve108. In an embodiment, the system100is kept at a constant pressure of about 5 Torr during growth. The variation of the pressure during a typical growth is within 1% of a set point. All precursors are kept at room temperature before being introduced into the reaction chamber107. A typical reaction lasts from about five minutes up to about twenty minutes. The reaction chamber107is heated by a horizontal tubular furnace to temperature ranging from about 650° C. to about 685° C. In an embodiment, the reaction chamber107is heated to a temperature of about 675° C.

In an embodiment, the precursor liquid is a titanium containing chemical. Examples of titanium containing chemicals include, but are not limited to, titanium beams from high temperature (or electromagnetically excited) metal targets, titanium tetrachloride (TiCl4), and titanium-containing organomettalic compounds. In an embodiment, the precursor gas is a silicon containing chemical. Examples of silicon containing chemicals include, but are not limited to, silane (SiH4), silicon tetrachloride (SiCl4), disilane (Si2H6), other silanes, and silicon beams by evaporation. In an embodiment, the carrier gas is selected from the group consisting of hydrogen (H), hydrochloric acid (HCl), hydrogen fluoride (HF), chlorine (Cl2), fluorine (F2), and an inert gas.

The 2D conductive silicide nanostructures disclosed herein are spontaneously fabricated in the chemical vapor deposition system100when the precursors react and/or decompose on a substrate in the growth chamber107. This spontaneous fabrication occurs via a seedless growth, i.e., no growth seeds are necessary for the growth of the 2D conductive silicide nanostructures. Therefore, impurities are not introduced into the resulting nanostructures. The fabrication method is simple, no complicated pre-treatments are necessary for the receiving substrates. The growth is not sensitive to surfaces (i.e., not substrate dependent). The substrates that the disclosed nanostructures can be grown on are versatile, so long as the substrate sustains the temperatures required for the synthesis. In an embodiment, the 2D conductive silicide nanostructures are grown on a transparent substrate. No inert chemical carriers are involved (the carrier gas also participates the reactions). It is believed that due to the nature of the synthesis of the 2D conductive silicide nanostructures disclosed herein, a continuous synthesis process may be developed to allow for roll-to-roll production.

Fabrication of Complex 2D Conductive TiSi2Nanostructures

A chemical vapor deposition system, as described above and shown inFIG. 1, was used for fabricating a complex 2D conductive TiSi2nanostructure of the presently disclosed embodiments. Briefly, SiH4was selected as the precursor gas, H2was selected as the carrier gas, and TiCl4was selected as the precursor liquid. Fifty (50) standard cubic centimeter per minute (sccm) of SiH4(10% diluted in He) and TiCl4vapor with an equivalent flow of two-and-a-half (2.5) sccm is transported by one hundred (100) sccm H2flow. All precursors were kept at room temperature before being introduced into the reaction chamber that was heated to about 675° C. with temperatures with ±1° C. accuracy. The system was kept at a constant pressure of about 5 Torr during growth, and the reaction lasted approximately fifteen (15) minutes.

FIG. 2shows electron micrographs of the complex 2D conductive TiSi2nanostructure200fabricated as described above.FIG. 2Ais a scanning electron micrograph showing the complex nanostructure200. The nanostructure200is composed of a plurality of nanonet (NN) sheets201. At relatively low magnifications, the nanostructure200packs to resemble tree leaves, except that each NN sheet201is composed of nanobeams202, as revealed by the close-up inset. (Scale bars: 5 μm in main frame, and 100 nm in the inset). The nanostructure200is better seen under transmission electron microscope,FIG. 2B. Within each of the NN sheets201are approximately 25 nm wide and approximately 15 nm thick nanobeams202, all linked together by single crystalline junctions with 90° angles. One of the frames is twisted at the bottom of the picture (arrow), demonstrating belt-like characteristics.

A series of tilted transmission electron micrographs confirm the 2D characteristics of each of the NN sheets201, as shown inFIG. 2C-2E. The inset electron diffraction pattern inFIG. 2Cwas on the NN sheets201in the vertical orientation, revealing the single crystalline nature of the NN sheets201, and that the plane of the NN sheets201is perpendicular to <010> directions (presence of strong diffraction spot of (060)). Similar series of tilted images using the scanning electron microscope, seeFIG. 3A-C, shows similar results. As best seen inFIG. 9B, 2D TiSi2NN sheets901bend and roll up when pushed by a S™ tip910during TEM characterization, further verifying the 2D nature and suggesting that the nanostructures are highly flexible as a result of the thinness.

High resolution transmission electron microscopy images and electron diffraction (ED) patterns of different regions of the nanobeam202fromFIG. 2B, reveal that the entire nanobeam202structure is single crystalline, including the 90° joints (FIG. 4A), the middle (FIG. 4B) and the ends (FIG. 4C). The ends of the nanobeams202within any NN sheet201, are free of impurities,FIG. 4C. Scale bars forFIG. 4Ais 5 nm,FIG. 4Bis 5 nm,FIG. 4Cis 5 nm, andFIG. 4Dis 2 nm. The frames are nanobelts based on two main observations: loose ends often bend on TEM supporting films, showing characteristics of nanobeams (see arrow inFIG. 2B), and the thickness of a NN sheet (approximately 15 nm) is thinner than the width of a NN sheet (approximately 25 nm), as evidenced in the tilted TEM image (FIG. 2C,FIG. 3A, andFIG. 4F).

Further analyses of HRTEM images and associated selected-area electron diffraction (SAED) patterns show that the NN sheets201are C49 structure with the b axis perpendicular to the plane (seeFIG. 2C, andFIG. 3A). That is, the NN sheets201primarily grow along a and c directions. Using a NN sheet having dimensions of 2 μm wide and long and 15 nm thick as an example, the growth selectivity of different crystal directions (a/b or c/b, i.e. width/thickness)>100, a remarkable ratio considering that no growth seeds are involved. Without being bound by any particular theory, this can be explained by the orthorhombic symmetry of C49 TiSi2and corresponding atomic arrangements. In a conventional C49 TiSi2unit cell (a=3.62 Å, c=3.61 Å and b=13.76 Å), there exist atomic layers entirely composed of Si along b direction, which are less susceptible to depositions of TiSi2required for continuous crystal growth (see,FIG. 6). The Si layer is further passivated by —Cl terminations to protect the {010} planes from additional growth, as confirmed by X-ray photoelectron spectroscopy (XPS), seeFIG. 4E. XPS spectra from the TiSi2NN sheets were taken with an Al K-alpha irradiation source (1486.69 eV) using a Kratos AXIS Ultra Imaging X-ray Photoelectron Spectrometer with 0.1 eV resolution. An internal C 1 s standard was utilized to calibrate the binding energies. Composition analysis by XPS shows that Si:Ti ratio on the surface is much greater than 2. This confirms that Si contents are richer on the surface, suggesting Si terminations. In contrast, other planes such as {100} and {001} are always composed of both Ti and Si atoms, favoring additions of both chemical species and leading to highly anisotropic growth. As a result, 2D structures are created by promoted growth of {100} and {001} planes and inhibited depositions on the {010} planes.

The sidewalls of the nanobeams are likely passivated by Cl and H as well, although less stable than those of the {010} planes. When the passivation is destabilized by continuous Ti and Si deposition on the side of a frame, branching occurs. Since TiSi2preferably grows along <100> and <001>, angles between connecting branches are always 90°, yielding the unique 2D network nanostructure disclosed herein. When two growing frames meet, one of the frames either changes growth direction to form a 90° kink or melts into the second frame to form a single crystalline connection (FIG. 8). NN sheets composed of wider, but not thicker, nanobeams are obtainable for extended periods of growth (e.g., 1 hr), implying the {100} and {001} sides are indeed susceptible to further growth. Noticeably, multiple kinks can be formed as seen inFIG. 8A. Scale bars 100 nm, 5 nm and 5 nm, from left to right. Arrows inFIG. 8BandFIG. 8Cindicate the growth direction.

When growth conditions are changed, for example using different pressures, temperatures and precursor gas ratios, different structures are obtained. For example, as shown inFIG. 7, high quality nanowires (NWs) are also obtainable by using the methods of the presently disclosed embodiments and manipulating the growth parameters. The general trend is that lower pressure, lower SiH4:TiCl4ratios, and higher temperature favor NWs growth, while the opposite produces more NN sheets. Careful studies of the microstructures, however, revealed that although belonging to the same symmetry group (orthorhombic), NWs obtained by the methods of the presently disclosed embodiments are C54 structure (a=8.236 Å, b=4.773 Å and c=8.523 Å) and grow along b direction. The structural difference can be confirmed by Raman spectrum (seeFIG. 7C), as well as TEM characterizations (FIG. 7B). Relatively higher Si concentrations (afforded by higher SiH4ratios, higher pressures, and lower temperatures) help passivate {010} planes of the C49 structure and therefore lead to NN sheet growth. The degree of supersaturation of TiSi2in the gas phase can also play a role. The microstructures are evidenced by the high resolution imaging, ED patterns, as well as micro-Raman measurements, seeFIG. 7C. Raman spectra were taken on a home-built Raman spectrometer at a laser excitation wavelength of 647 nm, with a power level of 1 mW and 100× object lens. Scale bars: 5 μm inFIG. 7Aand 5 nm inFIG. 7B. TiSi2nanowires are favored for growth conditions with relatively lower Si concentration, e.g. lower pressure and higher temperature.

For bulk TiSi2, C49 phase is reported to form first during solid-state reactions and then is converted to C54 at high temperatures (e.g. 700° C.). C49 TiSi2has been regarded as the metastable phase that has higher resistivity, due to stacking faults along the b direction. It has been shown that the 2D TiSi2nanostructures of the presently disclosed embodiments are extremely stable the nanostructure is preserved after up to about 90° C. annealing in H2for over 30 minutes. The 2D TiSi2nanostructures of the presently disclosed embodiments are also highly conductive. The remarkable stability may result from the small dimensions; 15 nm film thickness means approximately 10-12 unit cells along <010> direction, within which stacking faults are unlikely events.

The complex 2D conductive silicide nanostructures of the presently disclosed embodiments link low dimensional nanomaterials by high quality single crystalline junctions, providing better charge transport between individual components and stronger mechanical support. Thus, the complex 2D conductive silicide nanostructures of the presently disclosed embodiments are of significant interest for nanoelectronics and emerging solar energy harvesting.

FIG. 9shows electrical measurements of a TiSi2NN sheet of the presently disclosed embodiments. The electrical transport measurements on the TiSi2 NN sheet were conducted using a commercial STM-TEM holder (Nanofactory® Instruments AB, ST1000). The NN sheet was adhered to a sharp and fresh gold needle by gently dragging the needle on the surface of the as prepared sample. Another sharp gold probe was piezo-driven to approach the nanonets protruding the gold needle inside the TEM (JOEL 2010F). Electron beams were blocked during the measurements to avoid interferences. Care was also taken to minimize air exposure time during sample preparation, thus to limit surface oxide growth. When pushed by the STM tip, the NN sheet rolled up, seeFIG. 9B. The structural change is reversible, demonstrating a remarkable flexibility (the structure survives repeatable bending of curves with radii as small as less than 500 nm). Scale bar: 500 nm. Current-voltage (I-V) curves were obtained by applying biases in the two-terminal configuration, seeFIG. 9C. All measurements were conducted under vacuum conditions (<10−5Pa). The gold probes and needles were obtained by etching gold wires (0.010 and 0.013 inches in diameter, respectively) in a 37 weight percent HCl aqueous solution with initial etching currents of 2.00 and 2.25 mA, with a bias of approximately 1 Volt.FIG. 9Dshows how annealing was found necessary to form Ohmic contacts between the STM tip and TiSi2NN sheet of the presently disclosed embodiments. Constant current (50 μA) at large bias (3V) helps from Ohmic contacts.

Electrical resistivity is the resistance of a material in slowing down the electrical current when the material is subject to a potential difference. Electrical resistivity is calculated as:
ρ=VA/(I×l), where:V is the potential difference across the material,A is the cross-section area,I is the electrical current flowing through it, andl is the length of the material.

Lower resistance leads to lower power consumption and faster responses to electrical signals. Lower resistance also allows for higher current as a result of the lower power consumption (hence reduced Joule heating.) Electronics built on low-resistivity materials run faster under the same power consumption or consumes less power while running at the same speed, compared to those made of conventional materials. In energy-related applications such as solar cells, lower resistivity yields better efficiencies by reducing energy lost in transporting light-induced electricity. As shown in the current-voltage curves, the 2D TiSi2NN sheets of the presently disclosed embodiments are excellent conductors, with low-resistivity. Assuming the thickness of 15 nm and width of 30 nm for a single beam within the NN, and regarding the charge transport path as shortest distance between contacting electrodes, e.g., about 1 μm, the electrical resistivity of the NN sheets are approximately 10 μΩ·cm, in good agreement with that from bulk C54 and significantly better than bulk C49 TiSi2. Without being bound by any particular theory, the absence of defects in the nanostructures of the presently disclosed embodiments, which have been determined to be detrimental in electrical conductance in bulk C49 TiSi2, may play a role in the nanostructures high current ability.

Methods of fabricating two-dimensional conductive silicides include performing chemical vapor deposition, wherein one or more gas or liquid precursor materials carried by a carrier gas stream react to form a nanostructure having a mesh-like appearance and including a plurality of connected and spaced-apart nanobeams linked together at an about 90-degree angle.

The method of the presently disclosed embodiments can be used to synthesize a new 2D nanonet structure. The products are high quality single crystalline complex structures composed of perpendicular nanobeams. The morphology results from the orthorhombic crystal symmetry, and is sensitive to growth conditions; lower Si concentration in the precursor mixture favors NW growth. The high quality single crystalline NN sheets disclosed herein represent one of the most conductive silicides, and opens new doors to new exciting electronic and energy-related applications.

All patents, patent applications, and published references cited herein are hereby incorporated by reference in their entirety. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.