Microelectromechanical (MEMS) manipulators for control of nanoparticle coupling interactions

A nanopositioning system for producing a coupling interaction between a first nanoparticle and a second nanoparticle. A first MEMS positioning assembly includes an electro-static comb drive actuator configured to selectively displace a first nanoparticle in a first dimension and an electrode configured to selectively displace the first nanoparticle in a second dimensions. Accordingly, the first nanoparticle may be selectively positioned in two dimensions to modulate the distance between the first nanoparticle and a second nanoparticle that may be coupled to a second MEMS positioning assembly. Modulating the distance between the first and second nanoparticles obtains a coupling interaction between the nanoparticles that alters at least one material property of the nanoparticles applicable to a variety of sensing and control applications.

FIELD OF THE INVENTION

The present invention relates generally to the field of controlled nanoparticle interaction. More particularly, the present invention relates to microelectromechanical (MEMS) manipulators and methods of using a MEMS manipulator to control nanoparticle location to produce coupling interactions that modulate one or more properties of the nanoparticles.

BACKGROUND OF THE INVENTION

A common approach to making closely spaced metal nanoparticles is through advanced lithographic methods. Electron beam lithography is frequently used to make nanostructures on the scale of a few 10 s of nanometers in size and with similar spacing between nanostructures. However, prior efforts in this regard suffer from the inability to reliably produce features on this length scale with high precision. Defects in shape, size, and/or spacing generally arise when producing nanostructures on the scale of 10 nm in length using lithographic techniques. Currently, lithography techniques cannot reliably fabricate nanostructures with a spacing of 10 nm or less. Furthermore, lithography yields static structures and does not allow for optimization of optical and electronic coupling between the nanoparticles, as real-time control over the distance between nanoparticles is not achievable.

As a result of these limitations, attempts have been made to use rough mechanical means to essentially push two particles together. [J. Merlein et al., Nature Photonics 2, 230 (2008)] Electron beam lithography has been used to create two nanoparticles with an initial spacing of about 85 nm. Subsequently, an atomic force microscope tip is used to push one of the particles closer to the other. This is a crude method to move nanoparticles closer, and suffers from deficiencies such as damaging the nanoparticles, the substantial time needed to accomplish movement, and a complete lack of reversibility in the degree of coupling between the nanoparticles. Attempts at applying MEMS to move two “swords” of silicon coated Au structures to near contact have also been made [IEEE Transducers 2009 conference (Jun. 21-25, 2009)]. Here, the silicon swords had micron lengths and included coarse position control.

Achieving nanoparticle separation on the scale of less than 10 nm is generally beyond the reliable capability of modem nanofabrication tools. In addition to the nanoparticle spacing issue, the ability to modulate this distance in a reversible manner would be of value for a wide range of next generation nanoscale devices. For example, controlled coupling of nanoparticles could be used as a switching framework to control optical and electrical energy flow in nanoscale devices. Another example that could benefit from this technology would be a sensor where strongly interacting fields of the involved nanoparticles produce shifts in the absorption and scattering resonances of the particles. Another benefit of close nanoparticle coupling would be the gain in the sensitivity of spectroscopies, including surface enhanced Raman spectroscopy (SERS), that result from large field confinement and enhancement effects in the gaps between nanoparticles. For example, by positioning molecules within this small region, molecular spectroscopy of a single molecule can be achievable. Precise control of the distance between nanoparticles is also needed to manage the efficiency of bow-tie nanoantennas. Nanoantenna structures are best known for providing a mechanism for focusing light into the nanoscale gap between conductors. It is expected that these structures could ultimately provide optical readout for on chip nanophotonic logic or light routing devices.

SUMMARY

It thus would be beneficial to obtain closely spaced nanoparticles that produce coupling interactions that modulate properties of the nanoparticles, for example, electronic and optical properties. This is a challenging goal, because the degree of interaction between nanoparticles generally increases with decreasing distance between the nanoparticles. The degree of interaction is generally nonlinearly dependant on the distance. In other words, a small distance between the nanoparticles is desired, and a distance of less than 10 nm is preferred.

MEMS technology may be applied to dynamically control the distance between nanoparticles and, as a consequence, control their interaction. Several MEMS devices and different types of actuation mechanisms (electrostatic, thermal, piezoelectric, optical and magnetic) can be used for this purpose. Electrostatic actuation can provide fast and very precise position control. The advanced MEMS positioning device of one embodiment of the present invention comprises an array of electrostatically actuated comb-drive MEMS actuators coupled to a positioning element such as a micron-size needle. The comb-drive actuators include a releasable comb portion and an anchored comb portion. The MEMS positioning device may further include a restoring force element coupled to the comb-drive actuator and a portion of the micro-needle. The nanoparticle and/or nanostructure may be disposed on the micro-needle, and in one preferred embodiment, is located at the distal tip of the micro-needle.

The configuration of the present system enables real-time adjustment of nanoparticle position to modulate the gap between nanoparticles within a precision of less than about 1 nm. In operation, when a voltage is applied to the array of comb-drive actuators, the releasable comb portion moves toward the anchored comb portion. This motion displaces the coupled micro-needle along a first axis and induces a mechanical deformation of the coupled restoring force element. The mechanical deformation of the restoring force element provides a restoring force useful to control the displacement of the coupled micro-needle. The MEMS positioning device may further include a lateral electrode to induce lateral motion of the micro-needle along a second axis. The second axis may be substantially orthogonal with respect to the first axis. As such, the array of comb-drives provide motion control in a first dimension and the lateral electrodes allow perpendicular displacement of the micro-needle in a second dimension. In this way, the relative position of the nanoparticles may be independently modulated in two-degrees of freedom. Modulation in additional degrees of freedom may further be achieved by including additional electrodes and/or positioning elements associated with the positioning device.

In an embodiment, a first MEMS positioning device and a second MEMS position device are arranged with respect to each other such that the nanoparticles disposed on the respective micro-needles may be brought into close proximity, thereby decreasing the distance between the nanoparticles with respect to an initial position. In a preferred embodiment, the nanoparticles may be brought within less than about 10 nm of each other. In another preferred embodiment, the nanoparticles may be brought within less than about 1 nm of each other.

Accordingly, an object of the present system is to achieve nanoparticle-nanoparticle spacing of less than about 10 nm. According to another object of the present system, electrically controllable and tunable spacing between nanoparticles in two-degrees of freedom is provided. Yet another object of the present system is to obtain positioning capabilities of a plurality of nanoparticles with precision better than about 1 nm. Still another object, is to provide a method for achieving these capabilities.

In one set of embodiments, a nanopositioning system for producing a coupling interaction between a first nanostructure and a second nanostructure comprises a first MEMS positioning assembly. The first MEMS positioning assembly includes a first static comb drive actuator and a first dynamic comb drive actuator. The first static comb drive actuator is configured to selectively displace the first dynamic comb drive actuator along a first axis in relation to the first static comb drive actuator upon application of a voltage to the first static comb-drive actuator. The first MEMS positioning assembly further includes a first restoring force element having a first end and a second end. The first end of the restoring force element is operatively attached to the first dynamic comb drive actuator, and the second end is attached to the first static comb drive actuator. The first MEMS positioning assembly also includes a first positioning electrode configured to selectively displace the first dynamic comb drive actuator along a second axis in relation to the first static comb drive actuator upon application of a voltage to the first positioning electrode. A first nanostructure is operatively connected to the first dynamic comb drive actuator. Accordingly, the first nanostructure is selectively positionable along the first axis and along the second axis, modulating a distance between the first nanostructure and the second nanostructure to obtain a coupling interaction that alters at least one property of the first nanostructure and the second nanostructure.

In another set of embodiments, a method for selectively modifying a property of a plurality of nanoparticles through a coupling interaction between the plurality of nanoparticles comprises providing a first nanoparticle, providing a first MEMS positioning assembly having a first positioning portion. The first MEMS positioning assembly is configured to selectively displace the first positioning portion along a first axis in response to a first voltage and to selectively displace the first positioning portion along a second axis substantially normal to the first axis in response to a second voltage. The method further comprises coupling the first nanoparticle to the first positioning portion, providing a second nanoparticle spaced a distance from the first nanoparticle, and modulating the distance between the first nanoparticle and the second nanoparticle by displacement of the first positioning portion along at least one of the first axis and the second axis to obtain a coupling interaction between the first nanoparticle and the second nanoparticle. The coupling interaction alters at least one property of the first nanoparticle and the second nanoparticle.

In a further set of embodiments, an electrostatic nanopositioning system for modulating a distance between a first nanostructure and a second nanostructure to control an interaction between the first nanostructure and the second nanostructure comprises a first nanopositioner configured to receive a first nanoparticle, a second nanopositioner configured to receive a second nanoparticle spaced a distance from the first nanoparticle so as to cause a coupling interaction between the first nanoparticle and the second nanoparticle. Each of the first nanopositioner and the second nanopositioner include a positioning portion selectively moveable along a respective first path by an array of electrostatic comb drive actuators and selectively moveable along a respective second path by an electrode. Each of the first nanopositioner and the second nanopositioner further include a restoring force element coupled to the positioning portion. The restoring force element is deformable in response to movement of the positioning portion along the respective first path. The coupling interaction between the first nanostructure and the second nanostructure is controllable by modulating the distance between the first nanostructure and the second nanostructure by movement of at least one of the first nanostructure and the second nanostructure along at least one of the respective first paths and the respective second paths.

DETAILED DESCRIPTION OF EMBODIMENTS

An exemplary nanoparticle MEMS positioning system10shown inFIG. 1comprises one or more voltage controlled actuators detailed below. In other embodiments, a thermal, a piezoelectric, an optical and/or a magnetic MEMS positioning system may be used. The voltage controlled actuator may be configured to work in concert with a restoring force element to produce a nanopositioning device with better than 1 nm positioning resolution in a plurality of dimensions.FIGS. 1-7illustrate a nanoparticle MEMS positioning system10constructed in accordance with an embodiment of the present invention. The MEMS positioning system10comprises an array of electrostatically actuated comb-drive actuators20coupled to at least one positioning element such, as a micron-size micro-needle40. The array of comb-drive actuators20include a releasable comb portion21and an anchored comb portion22. The MEMS positioning system10may further include a first restoring force element50operatively coupled to the releasable comb portion21and operatively coupled to the anchored comb portion22. A nanoparticle and/or nanostructure60may be disposed on a portion of the micro-needle40, and in an embodiment, is located proximate a distal tip42of the micro-needle40. At least a portion of the MEMS positioning system10may be located on a substrate70.

With reference toFIGS. 1 and 2, the array of comb-drive actuators20includes a plurality of actuator elements23. Each of the plurality of actuator elements23comprises a static comb portion24and a dynamic comb portion25associated with the static comb portion24. Each of the static comb portion24and the dynamic comb portion25is characterized by a plurality of comb teeth26coupled to a comb base27and a plurality of comb openings28disposed between each of the plurality of comb teeth26. Each of the plurality of comb teeth26extend substantially perpendicular from the comb base27in a plane proximate the substrate70. Each comb base27associated with the static comb portion24is coupled to a static link member29that connects each of the static comb portions24. As depicted inFIG. 1, the static link member29may be substantially parallel to the plurality of comb teeth26associated with the anchored comb portion22. The static link member29further electrically couples each of the static comb portions24to a first electrical contact90. The array of the comb-drive actuators20may be fabricated from, for example, silicon.

The dynamic comb portion25is disposed in an actuator opening31in a base portion30of the MEMS positioning system10. Each of the comb bases27associated with a dynamic comb portion25is coupled to a dynamic link32. The dynamic link32and each of the comb bases27that are associated with the dynamic comb potion25may include a plurality of openings37to minimize the mass of the releasable comb portion21. The dynamic link32may include a boom33that extends away from the base portion30. As depicted inFIG. 1, the boom33extends in a plane substantially parallel to the substrate70and substantially parallel to the plurality of comb teeth26. However, in other embodiments, the boom33may be configured to achieve any desired position and/or displacement profile.

With reference toFIGS. 2 and 3A, a first restoring force element50may operatively couple the boom33and the static comb portion24. As depicted, the first restoring force element50comprises an elongated “U” spring element with a static comb end51and a boom end52. However, in various embodiments, the first restoring force element50may comprise other configurations. As shown inFIG. 2, the static comb end51is coupled to a leading edge34of the static comb portion24. The boom end52is coupled to the boom33. As explained in detail below, the first restoring force element50moderates the displacement of the boom33and the attached micro-needle40.FIG. 2depicts the first restoring force element50in an extended configuration with a first lateral distance d1between the static comb end51and the boom end52.FIG. 3Ashows the first restoring force element50in a retracted configuration with a second distance d2, less than d1, between the static comb end51and the boom end52. The first restoring force element50may be configured such that the retracted configuration is the natural position of the first restoring force element50.FIG. 6shows another embodiment of the MEMS positioning system10having a centrally located boom33disposed between a first static comb portion24aand first dynamic comb portion25aand a second static comb portion24band second dynamic comb portion25b. In this embodiment, a pair of first restoring force elements50aand50bare provided and associated with opposite lateral sides of the boom33.

By applying a voltage to the MEMS positioning system10the electrostatically actuated comb-drive actuators20move the nanopositioners closer together or further apart along a substantially linear first axis. In particular, applying a voltage to the first electrical contact90the dynamic comb portion25moves generally along the X axis as shown inFIGS. 1 and 2relative to the static comb portion24.FIG. 2shows the MEMS positioning system10in a fully extended orientation where the comb teeth26of the dynamic comb portion25are partially received in the comb openings28of the static comb portion24. In this orientation, the first restoring force element50is extended and the boom33and micro-needle40are extended away from the static comb portion24.FIG. 1shows the MEMS positioning system10in a fully retracted orientation where the comb teeth26of the dynamic comb portion25lie outside the comb openings28of the static comb portion24. In this orientation, the first restoring force element50is retracted and the boom33and micro-needle40translated toward the static comb portion24.

The MEMS positioning system10is reversible. That is, the MEMS positioning system10is selectively moveable between the orientations ofFIGS. 1 and 2. The MEMS positioning system10may be further be positioned at any intermediate orientation with respect to the orientations depicted inFIGS. 1 and 2. Accordingly, a nanostructure coupled to the MEMS positioning system10may be selectively positioned with a high degree of precision in a first dimension substantially defined by the X axis. In an embodiment, the MEMS positioning system10is configured to provide adjustable position control of an attached nanoparticle to within about 1 nm or better resolution. Precise and adjustable positioning permits customized control of coupling interactions between two or more nanoparticles60for a variety of applications.

As shown inFIG. 1, the MEMS positioning system10may further include a second restoring force element55. The second restoring force element55, as depicted, couples the dynamic comb portion25to the base portion30. The second restoring force element55may be configured substantially similar to first restoring force element50. For example, inFIG. 1, the second restoring force element55comprises an elongated “U” spring element with a dynamic comb end56and a base end57. However, the second restoring force element55may comprise other configurations. As shown inFIG. 1, the dynamic comb end56is coupled to a trailing edge35of the dynamic comb portion25. The base end57is coupled to the base portion30. The second restoring force element55works in conjunction with the first restoring force element50to moderate the displacement of the boom33and the attached micro-needle40.

The MEMS positioning system10may also include a lateral electrode80. As shown inFIG. 1, the lateral electrode80is located proximate at least a portion of the dynamic link32and may be substantially parallel to the dynamic link32. Alternatively, the lateral electrode80may be located near the boom33or another location where it may act on the dynamic comb portion25. The lateral electrode80is electrically coupled to a second electrical contact91. The lateral electrode80is capable of producing lateral motion of the dynamic comb portion25, including the boom33and the micro-needle40. In other words, application of a voltage to the lateral electrode80causes a displacement substantially in the plane of and substantially normal to the axial displacement of the dynamic link32. In general, the MEMS positioning system10is configured such that the range of lateral displacement is less than the range of the axial displacement. However, fine control of the position of MEMS positioning system10, and any coupled nanostructure, simultaneously in two dimensions (along the X and Y axes) is achieved through electrical control of the electrostatically actuated comb-drive actuators20and the lateral electrode80. Position control in additional dimensions (e.g., along the Z axis and/or rotations about one or more axes) may be achieved by associating additional electrodes or other positioning elements with the dynamic comb portion25and/or the MEMS positioning system10generally.

With reference toFIGS. 3A and 3B, the micro-needle40may be coupled to a distal end36of the boom33. The micro-needle40may comprise various configurations as necessitated by the application of the MEMS positioning system10. However, as depicted, the micro-needle40is a narrow member that extends substantially coaxially from a distal end36of the boom33. The micro-needle40and the boom33may be constructed from a single member or the micro-needle40may be coupled to the boom33using fabrication and/or attachment techniques known in the art. In various embodiments, the micro-needle40may be eliminated entirely.

A nanostructure60as shown inFIGS. 3B and 3Cmay be disposed on one or more surfaces of the micro-needle40. For example, a nanodot, nanosphere, nanorod, nanoplate, nanotriangle, nanopyramids, and other structures may be operatively and/or directly coupled to the micro-needle40. Additionally, the various nanostructures may be disposed on a portion of the boom33. As described below, the configuration of the nanostructure may be selected in view of the particular application or other parameter or material property. As shown inFIGS. 3B and 3C, a nanodot D (or D′) may be located on an upper surface41of the micro-needle40.

FIGS. 4A and 4Bshow a nanopyramid P located on an upper surface41of the micro-needle40.FIGS. 5A and 5Bdepict a nanosphere S (or S′) located on an upper surface41of the micro-needle40.

Various nanostructures may be associated with the MEMS positioning system10using standard deposition tools. For example, electron beam lithography is one technique that may be used to form the desired nanostructure that may be associated with the MEMS positioning system10. Alternatively, a focused ion beam technique may be used to directly deposit the nanostructure on the MEMS positioning system10. As seen inFIGS. 3C,4B and5B, the various nanostructures60can be placed with a high degree of precision on the micro-needle40, including proximate a distal edge42. However, one or more nanostructures60may located anywhere on the micro-needle40or on another portion of the MEMS positioning system10. The nanostructures can encompass a wide range of materials for various applications in sensing, nanooptics, and nanoelectronics. The various nanostructures that may be associated with the MEMS positioning system10may be constructed of any appropriate material or materials capable of forming a nanostructure. For example, nanostructures of various noble metals (e.g., Au, Ag, Pt, etc.), carbon and other materials may be constructed and associated with the MEMS positioning system10. For instance,FIGS. 3A and 3Bdepict a pair of plasmonic Pt nanodots D/D′.FIGS. 4A and 4Bshow a pair Au nanopyramids P/P′. In various embodiments, nanostructures of different materials may be positioned relative to each other using one or more of the MEMS positioning systems10.

A nanostructure MEMS positioning system may comprise two or more MEMS positioning systems10configured for interaction such that two or more nanostructures60may be selectively positioned with respect to each other. As depicted inFIGS. 3A-3C, a pair of MEMS positioning systems10are (seeFIGS. 3A and 3B) opposingly arranged to locate a pair of nanodots D/D′ (seeFIG. 3C) in relation to each other. The boom33of the first MEMS positioning system10is substantially coaxial with the boom33′ of the second MEMS positioning system10′. As such, axial displacement of the micro-needle40by the first MEMS positioning system10(substantially along the X axis) modulates the distance, i.e. increases or decreases the distance, between the nanodots D/D′. Alternatively, and/or in conjunction with the displacement of the first micro-needle40by the first MEMS positioning system10, the second MEMS positioning system10′ may be actuated to cause an axial displacement of the respective micro-needle40′ substantially along the X axis.FIG. 3Cshows the nanodots D/D′ positioned to have a separation distance of about 2 μm.FIGS. 4A and 4Bshow the nanopyramids P/P′ selectively positioned in proximity to each other such that the respective apexes of the nanopyramids P/P′ are aligned. Accordingly, a tunable bow-tie nanoantenna is formed where operation of the nanoantenna may be selectively controlled by modulating the distance between the nanopyramids P/P′.

Additionally, the MEMS positioning systems10may be configured to modulate the position of the attached nanostructures in a second dimension, e.g. along the Y axis. For example, displacement in the second dimension, substantially normal to axial displacement along the X axis, is achieved via the lateral electrode80(seeFIG. 1). As such, precise and reliable coupling of the nanoparticles may be achieved by approximating the MEMS positioning systems10/10′ relative to each other along the X and Y axes. Multi-axis positioning of the nanoparticles can produce a range of applications as described below, based on the control of the optical and electronic properties induced by closely spaced, interacting nanoparticles. Further, positioning control may be achieved by providing additional electrodes and or positioning elements as needed.

In other embodiments, other nanostructure60placement configurations may be used, depending on, for example, the application and/or structure. For example,FIG. 7shows an arrangement of a pair of MEMS positioning systems10and10′ (not fully shown) where the respective micro-needles40/40′ are configured to at least partially overlap. In the embodiment ofFIG. 7, the booms33/33′ are substantially parallel and disposed in a plane but are laterally offset by a distance approximately equal to the width of one of the micro-needles40/40′. In various embodiments the booms33/33′ may be coaxial and the lateral offset may be achieved by positioning one or more of the micro-needles40/40′ in the lateral (Y-axis) direction by imparting a displacement via the lateral electrode80. In still other embodiments, two or more MEMS positioning systems10may be arranged in space as necessary to provide positioning control of a plurality of nanostructures.

The described embodiments may provide opportunities to develop new sensor technologies and switchable control of energy flow in various forms, e.g., optical, electronic, heat, etc. In particular, the present embodiments may be applied to create coupled plasmonic nanoparticles, a key element under development for opto-electronic applications, including sensors, communications, interconnects and wavefront control. The foregoing systems and methods have numerous applications, including, but not limited to:

An optical or plasmonic switch Plasmons are of significant interest for next generation sub-wavelength waveguides, but researchers currently do not have a reliable way to externally control the flow of plasmons between nanostructures. An optical or plasmonic switch may be constructed by, for example, bringing together a first plasmonic nanoparticle and a second plasmonic nanoparticle using the devices and methods already described. By modulating the distance between the first plasmonic nanoparticle and the second plasmonic nanoparticle, a shift in the resonance of the particles may be used as a nanoscale optical switch by varying the absorption at different wavelengths. According to the embodiments described herein, an electrically addressable plasmonic switch might be formed by modulating the distance between the nanostructures, i.e., decreasing the distance between the nanostructures to induce plasmon flow and then separating the nanostructures to turn off plasmon flow.

A spatial plasmon modulator A critical limitation in the area of optical interconnects is the absence of a technology to modulate optical signals that are smaller than the wavelength of the light used in the signal. Plasmons are electromagnetic waves localized in very small regions; but there is not a reliable method to manipulate plasmons in a controlled manner. Integration of metallic nanoparticles with MEMS or NEMS devices as in the described embodiments may allow for control of the relative amplitude or phase of various plasmons in arbitrary spatial locations. In doing so, an ideal device for applications like inter-chip optical interconnects may be constructed. Devices constructed according the various embodiments described herein may have the potential to reliably and controllably manipulate plasmons for such applications.

A sensor, and in a particular embodiment a surface enhanced Raman scattering (SERS) sensor Tuning the distance between nanoparticles according to embodiments described herein can optimize field confinement and enhancement effects that other SERS nanostructures cannot currently approach. By positioning molecules in this small region, molecular spectroscopy of a single molecule can be achievable through SERS techniques. The gains in field strength are not limited to SERS, as other material properties such as absorption and emission can be modified through controllable field enhancement by modulation of nanoparticle position.

Tunable nanoscale antennae Plasmonic antennae that can operate at the nanoscale are key to next generation nanophotonic structures. Antennae act to collect and confine electromagnetic waves for use in photonic devices. A “bow-tie” configuration, such as depicted inFIGS. 4A and 4B, is one such antenna structure and comprises two triangular structures with a gap between the apex of the two triangles. However, miniaturizing antennae structure for use in nanophotonics requires gaps to be formed that are generally too small to be constructed with current lithography technology. The embodiments described herein enable both a sufficiently small gap and selective tunability in the gap distance—for optimization of the antennae collection performance—to be achieved. It is expected that these structures may ultimately provide optical readout for on chip nanophotonic logic or routing light devices.

Nanoscale control of energy and charge transport In addition to plasmon coupling, new techniques to control energy and charge transport within nanostructures are needed. By placing energy or charge donors and acceptors on opposing nanopositioners, tunable control of energy and charge flow could be achieved. This applies to both organic and inorganic materials that are integrated with the nanopositioners.

Advanced heterodyne methods of detection Embodiments described herein could also enable the positioning of nanoparticles to be modulated through the use of an AC field and the ability to use lock-in detection of the modulated properties for extraordinarily enhanced signal to noise in sensing and spectroscopy applications.

Multiplexing Embodiments of the MEMS positioning devices described herein could permit multiplexing of a signal across multiple nanopositioners for parallel signal processing and achieve corresponding gains in efficiency for sensing and spectroscopy.