Sensing devices and methods for forming the same

A sensing device includes an optical cavity having two substantially opposed reflective surfaces. At least one nanowire is operatively disposed in the optical cavity. A plurality of metal nanoparticles is established on the at least one nanowire.

BACKGROUND

The present disclosure relates generally to sensing devices and to methods for forming the same.

Since the inception of semiconductor technology, a consistent trend has been toward the development of smaller device dimensions and higher device densities. As a result, nanotechnology has seen explosive growth and generated considerable interest. Nanotechnology is centered on the fabrication and application of nano-scale structures, or structures having dimensions that are often 5 to 100 times smaller than conventional semiconductor structures. Nanowires are included in the category of nano-scale structures.

Nanowires are wire-like structures having at least one linear dimension ranging from about 3 nm to about 200 nm. Nanowires are suitable for use in a variety of applications, including functioning as conventional wires for interconnection applications or as semiconductor devices. Nanowires are also the building blocks of many potential nano-scale devices, such as nano-scale field effect transistors (FETs), p-n diodes, light emitting diodes (LEDs) and nanowire-based sensors, to name a few. A primary drawback of conventional sensing techniques is a small area of the sensing surface, and, thus, limited ability to interact with a sample.

DETAILED DESCRIPTION

Embodiments of the sensing device and system disclosed herein advantageously have enhanced optical intensity and enhanced sensitivity. Without being bound to any theory, it is believed that this may be due, at least in part, to the addition of nanowires having metal nanoparticles thereon to an optical cavity having two opposed reflective surfaces. The optical cavity of the sensing device may advantageously enhance the light and/or signal that is/are produced during spectroscopic analysis. Moreover, it has been found that conventional sensing techniques may be augmented with more effective techniques that are especially advantageous when nanowires are used. For example, surface-sensitive techniques, such as surface-enhanced Raman spectroscopy (SERS) may be dramatically enhanced by providing a very large area of nano-structured surface to interact with the sample. Furthermore, the optical cavity may be designed to have a desirable quality (“Q”) factor, and may be designed for a wavelength of interest.

Referring now toFIG. 1, a general embodiment of a method for forming a sensing device is depicted. The method includes forming an optical cavity having at least two substantially opposed reflective surfaces; and operatively disposing at least one nanowire (on which metal nanoparticles are subsequently established) in the optical cavity. It is to be understood that embodiments of the method shown inFIG. 1are described in further detail in reference to the other figures hereinbelow.

Methods for forming nanowires having metal nanoparticles subsequently established thereon (also known as “decorated” nanowires) are described in the parent pending U.S. Patent Application referenced above, filed on Sep. 29, 2006; which application generally describes these decorated nanowires established on a surface.

FIGS. 2A,2B and2C together depict an embodiment of the method for forming an embodiment of the sensing device10. Very generally,FIG. 2Adepicts a substrate12having a spacer layer14thereon, and a second layer16established on spacer layer14.FIG. 2Bdepicts the removal of a portion of the spacer layer14to form the optical cavity18in an area between the substrate12and second layer16. As shown inFIG. 2B, the optical cavity18has two opposed reflective surfaces20,22.FIG. 2Cdepicts the formation of nanowires24, with nanoparticles26subsequently established thereon. It is to be understood that nanoparticles26may be formed from any suitable materials, including but not limited to gold, silver, copper, platinum, palladium, alloys thereof, and combinations thereof. It is to be further understood that nanoparticles26may be bi-material nanoparticles. Some examples of bi-material nanoparticles26include core-shell type structures. In one embodiment, the core and shell of such a structure may be formed from different metallic elements or alloys, some non-limiting examples of which are listed immediately above. In another embodiment, the core of a core-shell structure may be formed from a metallic element or alloy, and the shell may be a dielectric, including, for example, silicon oxide, silicon nitride, metal oxides, glass and/or the like.

Referring more specifically toFIG. 2A, the substrate12and/or second layer16may be formed of a reflective material or of a stack of materials (the individual layers of which are not shown) having at least one reflective material therein to form reflective surfaces20,22, or having layers with varying refractive indices. In any of these embodiments, the reflective surfaces20,22(shown inFIG. 2B) are formed integrally with the respective substrate12and second layer16.

In an embodiment, second layer16generally acts as a partially transparent mirror. Second layer16may be configured with a reflective top surface17, and/or a reflective underside surface22, or a distributed Bragg reflector50(seeFIG. 2E) formed from any desired number of layers, e.g., layers40,42,44,46, built within the layer16. As defined herein, “reflective” for use within optical cavity18(or within optical cavity18′, described further below) generally means R˜0.9 (cavity Q˜10, low) to R˜0.999999 (Q˜106, very high), or in some cases, even higher.

When a reflective material is selected as the substrate12and/or second layer16, it is to be understood that any reflective material may be suitable that is capable of serving as a substrate or layer on which nanowires24can be grown or attach. Non-limiting examples of such materials include silicon, germanium, gallium arsenide, silicon germanium, indium phosphide, gallium nitride, aluminum arsenide, or the like, or combinations thereof. These materials are available in the form of wafers or other like substrate forms, and/or may be established as layers on some other substrate.

When a stack of materials is selected as the substrate12, it is to be understood that the stack is generally formed prior to establishing the spacer layer14thereon. Alternately or additionally, the stack of materials may be deposited, e.g., via conformal deposition, after etching cavity18so that the stack is formed on second layer16, as well as on substrate12. Such deposition may be desirable if the stack of materials is very thin (as in the case of a Bragg reflector, which generally may be about 2-10 wavelengths or more thick), and thus would benefit from mechanical support provided by, e.g., a substrate. When the stack of materials is placed on the second layer16before etching the cavity18, it is to be understood that the stack and second layer16may be formed on the spacer layer14, or may be formed separately and then established as a single unit on the spacer layer14. The stack(s) may include any number of materials, as long as one of the materials has a refractive index different from the other materials. The materials are chosen to optimize the reflectivity of the total stack of materials. In an embodiment, the stack is a Bragg reflector, where the constructive interference from all the layers with varying (generally alternating) refractive indices yields high mirror reflectivity. Suitable techniques for forming the stack(s) include, but are not limited to electron-beam evaporation, sputtering, electro-deposition, electroless deposition, and atomic-layer deposition (ALD).

Generally, the materials within the stack(s) and the thickness of each layer in the stack may be tuned or selected to achieve maximum reflectivity at a desirable wavelength. Furthermore, if nanowires24are to be grown from the stack of materials forming the substrate12and/or second layer16, it is to be understood that the stack and/or the nanowire growth process may be designed so that none of the layers in the stack subtantially interfere with nanowire growth.

FIGS. 2A and 2Btogether depict the presence and partial removal of the spacer layer14. It is desirable that the substrates12,16remain substantially unaltered after removal of the portion of the spacer layer14. As such, the spacer layer14may be any material that exhibits different etch characteristics than those of the selected substrate12and second layer16. For example, spacer layer14may have a different etch rate than the etch rate of the selected substrate12and second layer16, and/or spacer layer14may be etched by a material(s) to which selected substrates12,16are resistant. As a non-limiting example, if silicon is used as substrate12and layer16, silicon dioxide may be incorporated as the spacer layer14. As another non-limiting example, if gallium arsenide is used as the substrate12and layer16materials, aluminum gallium arsenide may be used as the spacer layer14. Removal of the spacer layer14may be accomplished by wet chemical etching, isotropic plasma etching, or combinations thereof. Generally, reactive ion etching is not suitable for removal of the spacer layer14, in part, because a lateral undercut is generally desirable.

The removal of the portion of the spacer layer14exposes a portion of each of the substrate12and layer16, and forms a cavity defined by the substrate12, layer16and the remaining portion of the spacer layer14. In an embodiment in which the exposed substrate12and layer16materials are also desirable reflective surfaces20,22, the removal process completes the formation of the optical cavity18.

In another embodiment, in which one or both of the exposed substrate12and layer16materials do not form desirable reflective surfaces20,22(e.g., a higher or different reflectivity is desirable), another layer of reflective material may be established on one or both of the exposed substrate12and layer16to form surface(s)20,22. As such, it is to be understood that it may be desirable to establish the additional layer on one or both of substrate12and layer16to form reflective surfaces20,22, respectively. In this embodiment, the addition of the reflective material completes the formation of the optical cavity18. Further, it is to be understood that the additional layer of reflective material is selected so as not to inhibit the growth or attachment of nanowires24to or from the substrate12or layer16. Alternatively, the additional layer(s) may be added after nanowire24growth. Any suitable process for establishing the additional reflective layer(s) may be used, including but not limited to growing the nanowires, followed by selective deposition of reflective material (such as a single layer of metal) (e.g., via atomic-layer deposition (ALD), electrochem/electroless plating, and/or the like) onto the surfaces20,22. The additional layer(s) may also be established via non-selective deposition processes (i.e., may be established on a portion(s) of the nanowires24).

Examples of suitable reflective materials for establishing as a layer(s) on surface20and/or surface22include, but are not limited to Si, Ge, silicon oxides (including SiO, SiOx, SiO2), TiO2, GaAs, AlGaAs (e.g., on a GaAs surface20,22), InGaAs (e.g., on a GaAs surface20,22), SiGe (on a Si or Ge surface20,22), Al, Au, Rh, Ag, Pt, Ni, Cu, or the like, or combinations thereof.

Referring now to the embodiment shown inFIG. 2C, the nanowires24may be grown from the substrate12to the second layer16, from the second layer16to the substrate12, and/or from both of substrate12and layer16toward each other to form a single nanowire(s)24. In each of these non-limiting examples of growth scenarios, the formed nanowires24contact each of the reflective surfaces20,22.

FIGS. 2D through 2Fshow alternate non-limiting examples of suitable devices10according to embodiments of the present disclosure, where nanowires24contact a single surface (not necessarily one of the reflective surfaces20,22, as demonstrated inFIG. 2E).

FIG. 2Ddepicts decorated nanowires24extending from a respective reflective surface20,22. Although nanowires24are shown extending from each of surfaces20,22, it is to be understood that nanowires24could extend solely from surface20or solely from surface22.

FIG. 2Edepicts decorated nanowires24extending from a surface21of spacer layer14, as well as a Bragg reflector50forming layer16(it is to be understood that a Bragg reflector50may be used as suitable and/or as desired in any of the embodiments described throughout this disclosure). Bragg reflector50includes two or more layers having varying refractive indices, for example, layers40,42,44,46.

FIG. 2Fdepicts branched decorated nanowires24′ extending from each of the reflective surfaces20,22, as well as a branched nanowire24′ contacting each of reflective surfaces20,22. The branched nanowires24′ shown at the right and left sides of the figure further include branches extending from branches, and thus may be referred to as “hyper-branched” nanowires.

In an embodiment in which the substrate12and layer16define the reflective surface(s)20,22, “contact” means that the nanowire24,24′ is grown from or attaches to the reflective surface(s)20,22. In an embodiment in which one or more additional layers of reflective material define the reflective surface(s)20,22, “contact” means that a portion of the nanowire24,24′ extends through the reflective surface(s)20,22, or that nanowire24,24′ is attached to the one or more additional layers.

As previously stated, it is to be understood that the reflecting surfaces20,22(whether formed integrally with the substrate12or layer16or as an additional layer) are selected so that nanowire growth from the substrate12and/or layer16and/or additional reflective layer(s) is not substantially inhibited.

The substrate12and/or layer16from which the nanowires24,24′ are grown may be formed with the surface plane being a (111) crystal lattice plane. Such a substrate12or layer16is referred to as a (111) oriented Si substrate or layer. In this embodiment, the (111) plane is considered to be horizontally oriented with respect to the Cartesian coordinate system.

In this embodiment, the term “horizontal” generally refers to a direction or a plane that is parallel with plane P shown inFIG. 2C, while the term “vertical” generally refers to a direction or plane that is substantially or approximately perpendicular to the plane P shown inFIG. 2C.

The spacer layer14(shown inFIG. 2E) from which the nanowires24are grown may be formed with its surface21being a (111) crystal lattice plane. In this embodiment, the (111) plane is considered to be vertically oriented with respect to the Cartesian coordinate system.

It is to be understood that the specific use of the terms “horizontal” and “vertical” throughout this disclosure to describe relative characteristics is to facilitate discussion, and is not intended to limit embodiments of the present disclosure.

The (111) surface orientation of enables growth of the nanowires24,24′; as such, any suitable method may be used to grow the nanowires24,24′. In an embodiment, nanowire24,24′ growth is accomplished by establishing a catalyst on one or both of the reflective surface(s)20,22, or the surface21, and exposing the catalyst to a precursor gas that initiates growth of the nanowire24,24′. The formation of nanowires24,24′ is further described in U.S. patent application Ser. No. 10/982,051, filed on Nov. 5, 2004 (U.S. Publication No. 2006/0097389, published May 11, 2006), incorporated by reference herein in its entirety.

Non-limiting examples of the types of nanowires24,24′ that may be formed include silicon nanowires, germanium nanowires, compound semiconductor nanowires (including lattice mis-matched nanowires (e.g., indium phosphide nanowires grown on silicon with a lattice mis-match of about 8%)), or the like, or combinations thereof.

As depicted inFIG. 2C, the nanowires24include metal nanoparticles26established thereon. As mentioned above, the formation of metal nanoparticles on nanowires is described in further detail in the parent U.S. patent application filed on Sep. 29, 2006. Such formation of metal nanoparticles on nanowires is also described in “Growth and use of metal nanocrystal assemblies on high-density silicon nanowires formed by chemical vapor deposition” by Yasseri et al., published online Dec. 1, 2005, and published in 2006 in volume 82 ofApplied Physics Aat pages 659-664.

In any of the embodiments disclosed herein, while establishing the nanoparticles26on nanowires24,24′ (and36,36′ shown inFIG. 3Aet seq.), one should ensure that the chemicals used in solution and processing do not substantially and/or undesirably degrade the optical properties of reflective surfaces20,22.

Referring now toFIGS. 3A,3B and3C together, another embodiment of the method for forming another embodiment of the sensing device10′ is depicted. Very generally,FIG. 3Adepicts a substrate28, andFIG. 3Bdepicts the removal of a portion of the substrate28to define the optical cavity18having at least two opposed cavity sidewalls30,32(which as described further hereinbelow may be, or may have established thereon, reflective surfaces20,22) and a cavity bottom34.FIG. 3Cdepicts the formation of nanowires36in the optical cavity18. As shown, the nanowires36have nanoparticles26established thereon.

Specifically referring toFIGS. 3A and 3Btogether, the substrate28may be a single material (e.g., silicon), a mixture of materials, or layers of different materials (e.g., a silicon-on-insulator (SOI) wafer (not shown)). Etching of the substrate28may be accomplished via anisotropic wet etching (e.g., with KOH), with directional dry etching (i.e., reactive ion etching), ion milling, and/or other like etching processes. When etching a single material, the depth of the cavity18depends, at least in part, on the amount of time during which etching takes place. When etching a silicon-on-insulator wafer, the insulator of the wafer acts as an etch stop. As such, the depth of the cavity18depends on the thickness of the silicon layer on the insulator. Other semiconductor-on-insulator structures may also be used. In some instances, a GaAs wafer may be used with appropriate measures taken to smooth sidewalls30,32and grow epitaxial layers of different refractive indices to form the reflecting stack.

As previously described, etching results in the formation of cavity sidewalls30,32and a cavity bottom34. The cavity sidewalls30,32either form the reflective surfaces20,22of the cavity18, or they support an additional layer (not shown) formed of reflective material that functions as the reflective surfaces20,22. In an embodiment in which the substrate28is formed of a desirable reflective material, the sidewalls30,32form the reflective surfaces20,22.

In an embodiment in which the substrate28is formed of an undesirable material for the reflective surface(s)20,22, a layer having a desirable reflectivity may be established on the sidewall(s)30,32. Such a layer may be epitaxially grown on the cavity sidewall(s)30,32, or it may be deposited by evaporation or other techniques.

Referring now to the embodiment shown inFIG. 3C, the nanowires36may be grown laterally or horizontally from one sidewall30to another sidewall32, from one sidewall32to another sidewall30, and/or from both of the sidewalls30,32toward each other to form nanowires36. In each of these non-limiting examples of growth scenarios, the formed nanowires36contact each of the reflective surfaces20,22.

FIGS. 3D and 3Eshow alternate non-limiting examples of suitable devices10′ according to embodiments of the present disclosure, where nanowires36,36′ contact a single surface (not necessarily one of the reflective surfaces20,22, as demonstrated inFIG. 3D).

FIG. 3Edepicts decorated nanowires36and branched nanowires36′ extending from a respective reflective surface20,22. Although nanowires36,36′ are shown extending from each of surfaces20,22, it is to be understood that nanowires36,36′ could extend solely from surface20or solely from surface22.

In an embodiment in which the sides30,32of the etched cavity18define the reflective surface(s)20,22, “contact” means that the nanowire36,36′ is grown from or attaches to the reflective surface(s)20,22. In an embodiment in which one or more additional layers of reflective material define the reflective surface(s)20,22, “contact” means that a portion of the nanowire36,36′ extends through the reflective surface(s)20,22, or that nanowire36,36′ is attached to the one or more additional layers.

As previously stated, it is to be understood that the reflecting surfaces20,22(whether formed integrally with the sides30,32or as an additional layer) are selected so that nanowire growth from the substrate28is not substantially inhibited.

In an embodiment, the substrate28(or top layer of an SOI substrate) from which the nanowires36,36′ are grown may be cut or polished with the surface plane being a (110) crystal lattice plane. Such a substrate28(or top layer) is referred to as a (110) oriented Si substrate. As used herein, the (110) plane is considered to be horizontally oriented with respect to the Cartesian coordinate system. The (110) oriented substrate28(or top layer) further has (111) planes of the Si crystal lattice, at least some of which are approximately perpendicular to, and intersect with, the horizontally oriented (110) surface of the substrate28(or top layer). These intersecting (111) planes are referred to herein as vertically oriented (111) planes or surfaces, noting that the (111) planes are approximately vertically oriented relative to the horizontal (110) surface of the substrate28(or top layer).

In another embodiment, the cavity bottom34may be configured to have the (111) crystal lattice planes, so that nanowires36,36′ may be grown therefrom. In this embodiment, the (111) plane is considered to be horizontally oriented with respect to the Cartesian coordinate system.

In these embodiments, the term “horizontal” generally refers to a direction or a plane that is parallel with plane P shown inFIG. 3C, while the term “vertical” generally refers to a direction or plane that is substantially or approximately perpendicular to the plane P shown inFIG. 3C.

The (111) surface orientation enables growth of the nanowires36,36′; as such, any suitable method may be used to grow the nanowires36,36′. In an embodiment, nanowire36,36′ growth is accomplished by establishing a catalyst on the reflective surface(s)20,22, and exposing the catalyst to a precursor gas that initiates growth of the nanowire36,36′. A non-limiting example of the formation of nanowires36,36′ is described in U.S. patent application Ser. No. 10/738,176 filed on Dec. 17, 2003 (U.S. Publication No. 2005/0133476, published on Jun. 23, 2005), incorporated by reference herein in its entirety.

Non-limiting examples of the types of nanowires36,36′ that may be formed include silicon nanowires, germanium nanowires, compound semiconductor nanowires (including lattice mis-matched nanowires (e.g., indium phosphide nanowires grown on silicon with a lattice mis-match of about 8%)), or the like, or combinations thereof.

As depicted inFIG. 3C, the nanowires36,36′ include metal nanoparticles26established thereon, as discussed above.

In any of the embodiments of nanowires24,24′,36,36′ discussed herein, it is to be understood that there may be segments of different material composition along the length of an individual nanowire24,24′,36,36′. The different segments may advantageously be used to position the nanoparticles26along the length of a respective nanowire(s)24,24′,36,36′, which position may be utilized to maximize signal intensity.

Embodiments of the sensing device10,10′ have a cavity surface that is substantially parallel to the nanowires24,24′,36,36′. For example, the nanowires24,24′ may be substantially parallel to the remaining portion of the spacer layer14(which defines one side of the optical cavity18, though it is to be understood that, with the nanowires24,24′, the recessed surface of the etched layer14may not necessarily be vertical), and the nanowires36,36′ may be substantially parallel to the cavity bottom34. It is to be understood that generally it is desirable for these portions of the cavity18to remain relatively non-reflective when compared to the reflective surfaces20,22. As such, a substantially non-reflective spacer layer14is selected for the embodiment of the sensing device10shown, for example, inFIG. 2C. For the nanowires36,36′ of the embodiment of the sensing device10′ shown, for example, inFIG. 3C, a layer of suitable index of refraction and thickness may form, or be placed on the bottom surface34of the cavity18(in an example, this layer would be the insulator of an SOI substrate).

An alternate embodiment of optical cavity18′ of sensing device10″ is shown inFIG. 4. In this embodiment, cavity18′ is not an enclosed cavity. Rather, cavity18′ is formed by opposed, substantially parallel surfaces of an external mirror16′ and reflective substrate12′, and may be a macro-scale cavity18′, e.g., from about 1 mm to about 1 m. It is to be understood that mirror16′ may be formed from any of the materials and by any of the processes described above regarding layer16, and substrate12′ may be formed from any of the materials and by any of the processes described above regarding substrate12. It is to be further understood that materials and processes that may not be compatible with the materials and processes used to form the monolithic structures shown in the series ofFIGS. 2 and 3may be used to form the external mirror16′.

In the embodiments ofFIGS. 2A-2FandFIGS. 3A-3E, the optical cavity18is micro-scale, e.g., from about 0.1 microns to about 100 microns, and monolithic. As defined herein, “monolithic” means that the cavity is formed within or upon a single substrate.

In any of the cavities18,18′ described herein, the definition of the cavity spacing may be, but is not necessarily equal to the distance between surfaces20,22. An example of when the cavity spacing is not equal to the inter-surface20,22distance is when, for example, the ‘mirror’12,12′,16,16′,28has a physical thickness, as in the case of a distributed Bragg reflector50or two coupled, partially transparent surfaces. In this case, the actual cavity spacing or length boundary will be located within the physical thickness of the mirror(s) (12′,16′, this cavity spacing18′ being depicted schematically inFIG. 4).

In any of the embodiments herein, the nanowires24,24′,36,36′ may be conducting, semiconducting, or insulating. Some non-limiting examples of suitable nanowire materials are elements or compounds from Group IV, Group III-V, or Group II-VI. In an embodiment, the nanowire materials are selected from Si, Ge, SiGe, GaAs, InP, GaN, and combinations thereof.

In the embodiments shown in the various figures, the nanowires24,24′,36,36′ are depicted as being substantially vertical or substantially horizontal with respect to one or more surfaces of the cavity18,18′. However, it is to be understood that the nanowires24,24′,36,36′ may be grown at any suitable angle with respect to one or more of the cavity surfaces and/or with respect to other nanowire(s)24,24′,36,36′ within a respective cavity18,18′.

Referring now toFIG. 5, there is depicted an embodiment of a system1000for using an embodiment of the sensing device10,10′. A sample containing one or more species (e.g., present as atoms, molecules, ions, chemical and/or biological complexes, etc.) is introduced into the optical cavity18. Without being bound to any theory, it is believed that the metal nanoparticles26present on the nanowires24,24′,36,36′ provide locally enhanced electromagnetic fields and thus enhance electromagnetic scattering by any species in proximity to the metal nanoparticles26. Spectroscopic analysis (e.g. Raman spectroscopy, including surface-enhanced Raman spectroscopy), may be performed by shining a light of a single wavelength (e.g., a laser) from a light source100(for example, normal to the cavity18, as shown) into the cavity18. In this embodiment, the length of the optical cavity18is designed equal to (n/2)(λillumination source), where n is an integer. The species located at or near the metal nanoparticles26scatters the light and shifts the wavelength. The shifts in wavelength may be measured by any suitable wavelength selective detector110, which is generally not normal to the cavity18, though it could be normal thereto in some instances, if appropriate and/or desired. The spectrum of wavelength shifts provides a “fingerprint” that is indicative of one or more of the species in the sample that was proximate to the nanowire24,24′,36,36′.

It is to be understood that the light source100may be integrated with the device10,10′,10″, or it may be separate therefrom, but operatively positioned adjacent thereto. Similarly, the wavelength selective detector110may be integrated with the device10,10′,10″, or it may be separate therefrom, but operatively positioned adjacent thereto. Yet further, light source100may be integrated with device10,10′,10″ while detector110is separate therefrom; or vice versa.

It is believed that the combination of the metal nanoparticle coated nanowires24,26and the optical cavity18advantageously enhances the scattered signal from the one or more species. The optical cavity18provides for an enhanced electromagnetic field within the cavity18when illuminated by a single wavelength source (e.g. a laser) that is resonant with the cavity18. It is believed that the electromagnetic field is enhanced on a local, nanometer length scale in proximity to the metal nanoparticles26. The cavity enhancement and the separate nanoparticle enhancement are expected to combine in a multiplicative or even power-law manner to yield a scattered signal significantly enhanced from either the cavity18or the nanoparticles26alone. The dimensions and materials selected for the optical cavity18tune the optical cavity for a particular wavelength. In an embodiment, the spacing of the reflective surfaces20,22forming the optical cavity18is a one half integer multiple of the wavelength of light that is used during spectroscopic analysis (e.g., Surface Enhanced Raman Spectroscopy (SERS)).

Furthermore, the quality factor (i.e., “Q”) of the optical cavity18may be selected to enhance the overall sensitivity of the sensing device10,10′. Generally, the sensitivity of the device10,10′ may be enhanced by the quality factor or by a square of the quality factor. Altering the dimensions and materials of the cavity18alters the quality factor. As such, the quality factor of the optical cavity18is tunable so that optimal sensitivity may be achieved. Optimal sensitivity may include detecting as little as a few atoms, molecules, ions, etc. of the species present in the sample. Generally, the optical intensity inside the cavity18is enhanced by the quality factor, so a higher Q is generally better. Scattered light will be at a different wavelength than the incident light, and at least in principle, the (high Q) mirrors20,22,50may be designed to reflect the laser light but pass the scattered light.

Embodiments of the sensing device10,10′,10″ include, but are not limited to the following advantages. Without being bound to any theory, it is believed that the optical cavity18of the sensing device10,10′,10″ and the metal nanoparticle26coated nanowires24,24′,36,36′ each advantageously enhances the light and the signal(s) that is/are produced during spectroscopic analysis. The optical cavity18may be designed to have a desirable quality (“Q”) factor so as to greatly enhance the intensity of the light incident on the species being measured, and therefore the sensitivity of the device10,10′,10″. The nanoparticles26enhance the response to the light incident on the species. Therefore, the combination of the optical cavity18,18′ and the nanoparticles26greatly enhances the sensitivity of the device10,10′,10″.