Patent Publication Number: US-2012036919-A1

Title: Nanowire sensor having a nanowire and electrically conductive film

Description:
BACKGROUND 
     Sensors for detecting species in fluids (gases or liquids) have become increasingly important in recent years for process control and personnel safety. Of particular importance are sensors that employ materials at nanoscale dimensions because they offer relatively large surface areas while occupying relatively small sizes, they exhibit relatively uniform properties, and they often have better performance than other types of sensors. For instance, the nanoscale sensors are more sensitive to chemical reactions with many target fluids than are other types of sensors to the same target fluids. 
     Nanoscale sensors are often configured to detect the species based upon sensing a property, such as, a change in electrical resistance. As such, the volume of the sensing element in the nanoscale sensors is often minimized to increase the surface to volume ratio, which for instance, increases the fraction of the volume that is affected by surface changes on the sensing element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments are illustrated by way of example and not limited in the following figure(s), in which like numerals indicate like elements, in which: 
         FIG. 1  illustrates a cross-sectional side view of a nanowire sensor, according to an embodiment of the invention; 
         FIGS. 2A and 2B  illustrate respective cross-sectional axial views of sensing elements, according to embodiments of the invention; 
         FIG. 3  illustrates a perspective view of an array, according to an embodiment of the invention; 
         FIG. 4A  illustrates a diagrammatic view of an array, according to another embodiment of the invention; 
         FIG. 4B  illustrates logic functions performed by the array shown in  FIG. 4A , according to an embodiment; and 
         FIG. 5  illustrates a flow diagram of a method of forming a nanowire sensor. 
     
    
    
     DETAILED DESCRIPTION 
     For simplicity and illustrative purposes, the principles of the embodiments are described by referring mainly to examples thereof. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments. It will be apparent however, to one of ordinary skill in the art, that the embodiments may be practiced without limitation to these specific details. In other instances, well known methods and structures are not described in detail so as not to unnecessarily obscure the description of the embodiments. 
     Disclosed herein is a nanowire sensor that includes a first electrode, a second electrode, and a sensing element connecting the first electrode and the second electrode. The sensing element is composed of at least one nanowire and an electrically conductive film covering the at least one nanowire and extending between and contacting the first electrode and the second electrode. According to an embodiment, the at least one nanowire may have a width in the range of about 5 nm to 500 nm. In another embodiment, the at least one nanowire may have a width in the range of about 10 nm to 200 nm. In addition, the at least one nanowire may have a length in the range of about 500 nm to 50 micrometers, and another embodiment, about 2 micrometers to 20 micrometers. 
     The conductance through the electrically conductive film alone or through the combination of the electrically conductive film and the nanowire is configured to change when at least one species interacts with the electrically conductive film. Consequently, the nanowire sensor disclosed herein may be implemented to determine a presence and, in certain instances, a quantity, of at least one species contained in a fluid (gas or liquid). 
     The at least one species may be any organic or inorganic material. In addition, the at least one species may be a charged or an uncharged species. In instances where the at least one species comprises charged species, the species that become attached to the electrically conductive film may cause a field effect to occur in the sensing element. The at least one species may also diffuse into the electrically conductive film and act as a dopant, changing the conductance (or conversely, the resistance) of the electrically conductive film. The change may occur throughout the film or, in some embodiments, at grain boundaries. In instances where the at least one species comprises uncharged species, the species that become attached or diffuse into the electrically conductive film may change the grain boundaries of the electrically conductive film, thus making it easier or more difficult for charge carriers to cross the grain boundaries of the electrically conductive film, which effectively changes the conductance (or conversely, the resistance) of the electrically conductive film. Interaction of the uncharged species with the conductive layer may also create a charged species which then interacts with the sensing element, as described above for a charged species. 
     In various embodiments, the at least one species attached to or embedded in the electrically conductive film may be removed. The at least one species may be removed through application of thermodynamic and/or kinetic effects on the electrically conductive film. For instance, the at least one species may be removed through application of a cleansing fluid over the nanowire sensor. However, where the at least one species has a relatively tight binding on the surface of the electrically conductive film, it may be necessary to add some additional energy to remove the at least one species. The additional energy may be imparted through, for instance, heating of the nanowire and the electrically conductive film by passing current through the nanowire and the electrically conductive film. 
     Alternatively, the bonds attaching the at least one species to the surface of the electrically conductive film may be broken by application of light waves when the bonds are sufficiently weak to enable such separation. As a further example, the pressure surrounding the electrically conductive film may be reduced to facilitate desorption of the at least one species from the electrically conductive film. 
     Thus, through application of various processes, the change in conductance through the electrically conductive film caused by the at least one species may be at least partially reversed by removing the at least one species. In some cases, the conductance of the electrically conductive film and, in certain instances, the underlying nanowire, may revert to the conductance prior to introduction of the at least one species. 
     With reference first to  FIG. 1 , there is shown a cross-sectional frontal view of a nanowire sensor  100 , according to an embodiment. It should be understood that the nanowire sensor  100  depicted in  FIG. 1  may include additional components and that some of the components described herein may be removed and/or modified without departing from a scope of the nanowire sensor  100 . 
     As depicted in  FIG. 1 , the nanowire sensor  100  includes a first electrode  102 , a second electrode  104 , and a sensing element  106 . The sensing element  106  is depicted as being composed of a nanowire  108  covered by an electrically conductive film  110 . In addition, the sensing element  106  is depicted as contacting and extending between the first electrode  102  and the second electrode  104 . The sensing element  106  is further depicted as being suspended away from a surface of a substrate  116  on which the first electrode  102  and the second electrode  104  are located. In one regard, suspending the sensing element  106  away from a surface of the substrate  116  creates a larger surface area on the electrically conductive film  110  upon which species may become attached. Suspending the sensing element  106  also places the sensing element  106  further into the flow region so that the sensing element  106  is less affected by boundary-layer effects than if it were on the surface of the substrate  116 . In other cases, the sensing element  106  is positioned on the surface of the substrate  116 . 
     The nanowire sensor  100  is further depicted as including a measurement device  112  and a voltage source  114 . According to an example, the measurement device  112  comprises a hardware device, such as, an ammeter. According to another example, the measurement device  112  is configured to perform additional processing operations and thus comprises a combination of hardware and software modules, the software comprising code stored, for instance, in a volatile or non-volatile memory, such as DRAM, EEPROM, MRAM, flash memory, floppy disk, a CD-ROM, a DVD-ROM, or other optical or magnetic media, and the like. By way of example, the measurement device  112  may be configured to analyze the information pertaining to detected currents through the electrically conductive film  110  alone or in combination with the nanowire  108 . 
     In operation, the nanowire sensor  100  is configured to detect a species that is present in a fluid when the electrical conductance (or conversely, the electrical resistance) through the electrically conductive film  110  alone or in combination with the nanowire  108  changes. More particularly, the measurement device  112  may measure the electrical conductance (or resistance) of the sensing element  106  prior to introduction of a fluid containing the at least one species and may measure the electrical conductance after the fluid has been introduced. In this example, the electrically conductive film  110  may be configured to interact with particular types of species that may be contained in the fluid. If the electrical conductance of the electrically conductive film  110  changes, it is likely that the particular type of species is present in the fluid due to the interaction between the species and the electrically conductive film  110 . 
     The effect the at least one species has on the conductance of the electrically conductive film  110  depends on the specific interaction of the at least one species with the electrically conductive film  110 . More particularly, for instance, the electrical conductance of the electrically conductive film  110  may be altered by a charge on the surface of the electrically conductive film  110 , by a charge diffusing into the electrically conductive film  110 , by the uniformity with which the charge has diffused into the electrically conductive film  110  through the grains or along grain boundaries, etc. In other words, various types of species may interact differently with the same type of electrically conductive film  110 . For instance, a first type of species may become attached to the surface of the electrically conductive film  110 , whereas, a second type of species may diffuse into the electrically conductive film  110 . Because the manner in which the electrical conductance of the electrically conductive film  110  becomes altered affects in different ways the electrical conduction in the electrically conductive film  110  and, in certain instances, the charge induced in the nanowire  108 , detection of the electrical conductance may be used to differentiate between different species. 
     With reference now to  FIGS. 2A and 2B , there are shown respective cross-sectional axial views  200  and  220  of the sensing element  106  depicted in  FIG. 1 ,  106 ′, according to an embodiment. It should be understood that the nanowire  108  and the electrically conductive film  110  depicted in  FIGS. 2A and 2B  may include additional components and that some of the components described herein may be removed and/or modified without departing from a scope of the nanowire sensor  100  disclosed herein. For instance, a plurality of nanowires  108  and/or a plurality of layers of electrically conductive film  110  may be provided in place of the nanowire  108  and the electrically conductive film  110 . In another example, although the nanowire  108  in  FIGS. 2A and 2B  have been shown to have a circular cross section, it should be understood that the nanowire  108  may have other cross sectional shapes, such as a hexagonal shape, a square shape, a trapezoidal shape, a faceted surface, an irregular shape, etc. 
     As shown in  FIG. 2A , the nanowire  108  and the electrically conductive film  110  are coaxial electrical conductive regions. The conductance of the nanowire  108  is based upon factors including the geometry, the resistivity, and the thickness of the materials that comprise the nanowire  108 . The nanowire  108  may comprise any suitable material for forming nanowires, such as, silicon, boron, germanium, GaAs, InP, other III-V and II-VI compounds, etc. Similarly, the conductance of the electrically conductive film  110  is based upon factors including the geometry, the resistivity, and the thickness of the materials that comprise the electrically conductive film  110 . The electrically conductive film  110  may comprise any suitable material, such as, tin oxide, zinc oxide, other metal oxides, etc. By varying the factors that affect the conductance of the nanowire  108  and the conductance of the electrically conductive film  110 , the ratio of conductance between the nanowire  108  and the electrically conductive film  110  may be altered. 
     The ratio of conductance between the nanowire  108  and the electrically conductive film  110  may be also be altered by controlling the number of dopant atoms that are added during growth of the nanowire  108  as described in greater detail herein below. In one regard, the electrically conductive film  110  may be configured to have a relatively higher conductance level as compared with the nanowire  108  to thus enable changes in the electrical conductance through the electrically conductive film  110  to be more readily identified. 
     As shown in  FIG. 2B , in addition to the nanowire  108  and the electrically conductive file  110 , the sensing element  106 ′ includes an electrically insulating layer  230 . The electrically insulating layer  230  may comprise any suitable material for at least partially reducing electrical conduction between the nanowire  108  and the electrically conductive film  110 . In addition, the insulating layer  230  may extend at least partially between the first electrode  102  and the second electrode  104 . 
     In either or both of the sensing elements  106 ,  106 ′, the electrically conductive film  110  may also be functionalized, by which molecules or other substances are attached to a surface of the electrically conductive film  110 . The electrically conductive film  110  may be functionalized by, for instance, the addition of materials that enable the electrically conductive film  110  to interact with a particular type of species. According to an example, the electrically conductive film  110  may be functionalized to at least one of prevent attachment of species other than at least one particular species and vary sensitivity to different species. 
     With reference now to  FIG. 3 , there is shown a perspective view of a nanowire sensor array  300  employing a plurality of nanowire sensors  310 , according to an embodiment. It should be understood that the nanowire sensor array  300  depicted in  FIG. 3  may include additional components and that some of the components described herein may be removed and/or modified without departing from a scope of the nanowire sensor array  300 . 
     As depicted in  FIG. 3 , the array  300  includes a plurality of nanowire sensors  310  positioned on a substrate  306 . Although two nanowire sensors  310  have been depicted in  FIG. 3 , the nanowire sensor array  300  may include additional nanowire sensors  310 . Each of the nanowire sensors  310  is depicted as including a first electrode  302 , a second electrode  304  and multiple sensing elements  308  connecting the first electrode  302  and the second electrode  304 . Although not particularly shown, each of the multiple sensing elements  308  is formed of either or both of the sensing elements  106 ,  106 ′ depicted in  FIGS. 2A and 2B . 
     Although the first electrodes  302  and the second electrodes  304  of the nanowire sensors  310  are depicted in  FIG. 3  as being in the same horizontal plane, it should be understood that the first electrodes  302  and the second electrodes  304  may also be in different planes. For example, the second electrode  304  may be vertically aligned above the first electrode  302  with the sensing elements  308  extending vertically to connect the two electrodes  302  and  304 . In other examples, the first and second electrodes  302  and  304  may be offset with respect to each other to make electrical connection easier or to be compatible with nanowire  108  growth directions. 
     According to a first example, the electrically conductive films  110  of each of the plurality of sensing elements  308  are composed of the same or similar material with respect to each other. In addition, the nanowires  108  of each of the plurality of sensing elements  308  are composed of the same or similar material with respect to each other. In addition, or alternatively, the electrically conductive films  110  of the sensing elements  308  may be doped and/or functionalized in similar manners with respect to each other to enable the electrically conductive films  110  to interact in one or more manners with the same or similar types of species. As a further alternative, the electrically conductive films  110  of the sensing elements  308  may be doped and/or functionalized in different manners with respect to each other to enable electrically conductive films  110  formed of different materials to interact with the same or similar types of species. 
     According to another example, at least one of the plurality of nanowire sensors  310  is differently configured from at least another one of the plurality of nanowire sensors  310 . In this example, at least one of the nanowire sensors  310  includes sensing elements  308  that differ from the sensing elements  308  of another one of the nanowire sensors  310 . The sensing elements  308  may differ through being composed of nanowires  108  formed of differing materials, electrically conductive films  110  formed of differing materials, differing functionalization and/or doping, etc. 
     The array  300  may have differing nanowire sensor arrays  310  to detect multiple types of species. For instance, each differently configured nanowire sensor  310  may be configured to detect a different type of species. According to an example, the differently configured nanowire sensors  310  may be employed to identify false positives by verifying and/or removing certain positive identifications detected by the sensing elements  308 . In addition, further differentiation between detected species may be obtained by observing the response to the selective desorption of particular species by photons of selected wavelengths. 
     Electronic circuitry may be integrated on the substrate  306 . These electronics may be used, for instance, to amplify the small signals from the multiple nanowire sensors  310 , to compare the signals to a reference structure not exposed to the at least one species, to convert analog signals to digital signals, etc. In addition, the electronic circuitry may be used to determine the concentration of each species in test gases from the different responses of each of the nanowire sensors  310  of the array  300 . 
       FIG. 4A  illustrates a diagrammatic view of a nanowire sensor array  400 , according to an embodiment. As shown, the nanowire sensor array  400  includes a plurality of nanowire sensors  310 . The array  400  has differently configured nanowire sensors  310  as indicated by the labels “A, B, C, and D”. While the array  400  is depicted with four nanowire sensors  310 , a person having ordinary skill in the art will appreciate that the array  400  may have any number of differently configured nanowire sensors  310  arranged in any configuration. Moreover, the array  400  may include other components not illustrated in FIG.  4 A, such as the measurement device  112  described above, with respect to FIG. 
     The differently configured nanowire sensors  310  may be configured to interact with species  1 ,  2 ,  3 , and  4 , respectively. For example, an analyte  410  may be exposed to the array  400 , such that the analyte  410  flows and contacts each of the differently configured nanowire sensors  310  (A, B, C, and D), as indicated by the arrows shown in  FIG. 4A . The array  400  may detect four different species because the array  400  may contain four differently configured nanowire sensors  310 , each of which being capable of interacting with a different species. 
     Logic may be built into the array  400 , such that the array  400  performs an operation equivalent to a logic gate as a result of an exposure of the array  400  to an analyte. The operation may result in an indication that certain species may be present in the analyte  410 . For example, the array  400  may be exposed to an analyte  410  containing some or all of species  1 ,  2 , and  3 . These species may interact with the differently configured nanowire sensors A, B, or C and may change the differently configured nanowire sensors A, B, or C from a non-conducting state to a conducting state, or otherwise vary the conductance through the nanowire sensors  310 . 
       FIG. 4B  shows an example of the logic functions  450  performed by the nanowire sensor array  400 . For example, pairs of the differently configured nanowire sensors  310  A, B, C, and D in the nanowire sensor array  400  perform an AND function, as illustrated by AND gates  430  and  431 , respectively. For example, if differently configured nanowire sensors  310  A and B interact with species  1  and  2 , respectively, species  1  and  2  have been detected. If differently configured nanowire sensors  310  C and D interact with species  3  and  4 , respectively, species  3  and  4  have been detected. Because differently configured nanowire sensors  310  A and B are connected in parallel with differently configured nanowire sensors  310  C and D, which is represented by the OR gate  420  in  FIG. 4B , if either differently configured nanowire sensors  310  A, and B and differently configured nanowire sensors  310  C, and D are conducting, species ( 1  and  2 ) or species ( 3  and  4 ) are detected. 
     Conversely, in other embodiments, the differently configured nanowire sensors  310  A, B, C, and D may be “turned off” as a result of an interaction between a species and a differently configured nanowire sensors  310 . That is, the differently configured nanowire sensors  310  A, B, C, and D are designed so that current flows through the nanowire sensors  310  before the nanowire sensor array  400  is exposed to an analyte  410 . However, the differently configured nanowire sensors  310  A, B, C, and D may be configured such that the interaction of a species and a nanowire sensor  310  substantially reduces the current flowing through the sensing element  308 . In this embodiment, each pair of the differently configured nanowire sensors  310  A, B, C, and D functions as an OR gate and the connection in parallel performs the function of an AND gate. Different connections of nanowire sensors  310  to perform other logical functions are possible, as will be evident to a person of ordinary skill in the art. 
     A smaller amount of higher quality information is obtained from the nanowire sensor array  400 , as compared to the combination of conventional sensors needed to accomplish the same detection. The number of electronic signals that need to be transported and the amount of required computation external to the nanowire sensor array  400  is reduced by performing some of the computation within the array  400  itself. For example as illustrated in  FIG. 4B , if the conductance of either differently configured nanowire sensors  310  A and B or differently configured nanowire sensors  310  C and D are above a threshold, then the array  400  may indicate simply that either species  1  and  2  or species  3  and  4  caused the signal, and, thus, that either species  1  and  2  or species  3  and  4  are present in the analyte  410 . Further calculations performed external to the array  400  are thus not required to obtain an accurate result. 
     Moreover, the array  400  is extremely sensitive allowing for the detection of a small quantity of a species in a fluid, because the nanowire sensors  310  A, B, C, and D provide a large surface area relative to the volume of the nanowire, where the fluid may interact with the sensing elements  308 . Detection often relies on sensing a property such as a change in conductance, so the volume of the nanowire sensors  310  A, B, C, and D may be reduced as much as feasible to increase the surface to volume ratio and, therefore, the fraction of the volume that is affected by surface charges. 
     Turning now to  FIG. 5 , there is shown a flow diagram of a method  500  of fabricating nanowire sensors  100  and  310  according to an embodiment. It should be understood that the method  500  of fabricating the nanowire sensors  100  and  310  depicted in  FIG. 5  may include additional steps and that some of the steps described herein may be removed and/or modified without departing from a scope of the method  500 . Thus, for instance, the nanowire  108  may be formed prior to the formation of the first electrode  102  and the second electrode  104 . 
     At step  502 , a first electrode  102  is formed through any suitable formation process, such as one or more of, growing, chemical vapor deposition, sputtering, etching, lithography, etc. 
     At step  504 , a second electrode  104  is formed through any suitable formation process, such as one or more of, growing, chemical vapor deposition, sputtering, etching, lithography, etc. According to an example, steps  502  and  504  are performed concurrently. 
     At step  506 , a nanowire  108  connecting the first electrode  102  and the second electrode  104  is formed through any suitable formation process, such as one or more of, metal catalyzed nanowire growth, chemical vapor deposition, sputtering, etching, lithography, etc. According to an example, at least one other process that adds material for forming the nanowire  108  may be performed at step  506 . The nanowire  108  may be suspended between the first electrode  102  and the second electrode  104  as depicted in  FIG. 1  either during growth of the nanowire  108  or after the nanowire  108  has been formed. 
     During formation of the nanowire  108 , a dopant may be added in order to vary the conductance of the nanowire  108 . The dopant may be a p-type dopant or an n-type dopant and may be added in the gas phase. For instance, where the nanowire  108  comprises boron, diborane (B 2 H 6 ) may be added as p-type dopant. Alternatively, the dopant may be added after the nanowire  108  is formed. 
     At step  508 , the electrically conductive film  110  is formed on the nanowire  108  through any suitable process or combination of processes, such as chemical vapor deposition, physical vapor deposition, chemical reaction, diffusion, masking, etc. 
     During formation of the electrically conductive film  110  or after its formation, a dopant may be added in order to vary the conductance of the electrically conductive film  110 . The dopant may be a p-type dopant or an n-type dopant and may be added in the gas phase during deposition or it may be added after deposition. The conductance of the electrically conductive film  110  may also be altered where the formation process alters the microstructure of the electrically conductive film  110 . After formation of the electrically conductive film  110 , the electrically conductive film  110  may be functionalized so that it interacts with one or a selected group of species in the analyte. 
     An array  300  of differently configured nanowire sensors  310  may be formed where each nanowire sensor  310  or set of nanowire sensors  310  is selectively coated with a different electrically conductive film  110 , with the electrically conductive film  110  limited to the selected set of nanowire sensors  310  by any of a number of different methods. For instance, the electrically conductive film  110  may be applied to one set of nanowire sensors  310  by a technique analogous to ink-jet printing at the selected set of nanowire sensors  310 . Alternately, the other sets of nanowire sensors  310  may be masked by a physical shadow mask or covered with a protective layer so that the electrically conductive film  110  is only deposited on the desired set of nanowire sensors  310 . A protective coating may be applied to the entire array  300  of nanowire sensors  310  and then removed selectively from one set by ultraviolet photon desorption focused on the selected set of nanowire sensors  310  or by current passing through the nanowires  108  of the selected set of nanowire sensors  310 . 
     What has been described and illustrated herein is an embodiment along with some of its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations are possible within the spirit and scope of the subject matter, which is intended to be defined by the following claims—and their equivalents—in which all terms are meant in their broadest reasonable sense unless otherwise indicated.