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Patent Issued for Methods and Apparatus for Detecting Neutral Chemical Units via Nanostructures

June 4, 2014

By a News Reporter-Staff News Editor at Journal of Engineering -- From Alexandria, Virginia, VerticalNews journalists report that a patent by the inventors Hau, Lene V. (Cambridge, MA); Golovchenko, Jene A. (Lexington, MA); Goodsell, Anne (Arlington, MA), filed on March 24, 2011, was published online on May 20, 2014.

The patent's assignee for patent number 8729495 is President and Fellows of Harvard College (Cambridge, MA).

News editors obtained the following quote from the background information supplied by the inventors: "The ability to create high electric fields near charged carbon nanotubes has stimulated the development of nanotube-based electron field emission sources for a variety of applications, including low-power video displays, atom sensing, and atom interferometry. For example, nanotube-based electron field emission sources could be used in low power video displays and in sensitive gas sensors whose ionization currents depend on the gas pressure and species present in the gas. Typical nanotube-based devices include an array, or forest, of nanotubes or nanowires that extend longitudinally from a substrate. The quantitative behavior of such devices depends on the fact that the large fields used in operation can be obtained at the tips of nanotubes or nanowires in an inhomogeneous forest of nanotubes or nanowires.

"Due to shielding effects, active nanotubes in a forest are generally separated from each other by many inactive nanotubes. The effective active area of a forest-based device is thus often a small fraction of the area occupied by the forest. For instance, a forest of nanowires extending over a 1 cm.times.2 cm area typically includes 10.sup.10 nanowires, of which only about 1000 are active in tip ionization. The resulting efficiency of the forest is about 10.sup.-7. Furthermore, interpretation of the behavior of forest-based devices is complicated by variability of nanotube lengths, density, and tip geometry, including tip geometry that changes with time, among the nanowires in the forest. As a result, devices based on nanowire forests are often unsuitable for reliable, sensitive atom detection or atom interferometry."

As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventors' summary information for this patent: "Embodiments of the inventive subject matter include charged, substantially one-dimensional nanostructures that can be used to convert hard-to-detect neutral chemical units (e.g., atoms, molecules, and condensates) into charged particles (e.g., cations and anions) that are relatively easy to detect. Exemplary methods and apparatus disclosed herein facilitate ionization of one or more single atoms with high efficiency, making it possible to detect appreciably small concentrations of neutral chemical units in gases and other fluids with significantly high sensitivities. Exemplary nanostructures also can be used for detecting fringe patterns generated by interfering matter waves with sufficient spatial resolution to implement highly sensitive atom interferometers, gyroscopes, graviometers, and gravity gradiometers.

"Inventive nanotube-based apparatus can be used to practice methods of ionizing neutral chemical units as follows. Applying a charging voltage, which may be about 200 V, about 300 V, or more, from a voltage source between a substantially one-dimensional nanostructure and a reference potential creates an electric field in a vicinity of the nanostructure. When ionizing rubidium atoms, the electric field may be about 3 V/nm or larger; for non-alkali atoms, the field may be larger (e.g., 20 V/nm). This electric field captures a neutral chemical unit and attracts the neutral chemical unit towards a position along a length of the nanostructure. Eventually, the neutral chemical unit gets close enough to the position along the length of nanostructure to ionize; ionization transforms the neutral chemical unit into a charged chemical unit whose polarity measures the polarity of the electric field. Like-like repulsion ejects the charged chemical unit from the vicinity of the nanostructure.

"The charged chemical unit can be detected with an ion detector, such as a channel-electron multiplier (CEM), a position-sensitive microchannel plate, a mass spectrometer, an energy spectrometer, and ion time-stamping equipment. In some cases, ions optics or another suitable device may 'magnify' ion-based image of the nanostructure sensed by the detector. Alternatively, the nanostructure can be curved away from the source of the neutral chemical unit such that charged chemical units ejected from the vicinity of the nanostructure travel along diverging, rather than parallel, propagation vectors away from the nanostructure. This also has the effect of 'magnifying' any detected image, although the detected image may be affected by pincushion-type distortion, which can be compensated based on the nanostructure's radius of curvature.

"Some embodiments include position-sensitive detectors (with or without ion magnifiers) capable of determining the position along the length of the nanostructure at which the charged chemical unit was generated. These embodiments may determine the position with a resolution of about 1 nm or less. They may also detect a circumferential position of the nanostructure at which the charged chemical unit was generated, e.g., with a resolution of about 1 nm or less.

"In some cases, the voltage source varies the charging voltage applied to the nanostructure as a function of time or to account for variations in angular momentum (e.g., due to changes in temperature) of incident neutral chemical units. For instance, the charging voltage may be increased in a linear or stepwise fashion (e.g., along rungs of a quantum ladder) to selectively ionize incident neutral chemical units as a function of their ionization energies, masses, and polarizabilities. Such variation can also be used to produce indications of the presence (or absence) of particular types of neutral chemical units as by scanning the charging voltage across a range corresponding to different ionization thresholds.

"Alternatively, nanostructures can be arranged in an array to perform parallel detection of different types of neutral chemical units. Such detection can be performed with an array of identical nanostructures charged to different voltages, an array of different nanostructures charged to the same voltages, or combinations of different nanostructures charged to different voltages, depending on the application. Nanostructure arrays are particularly useful for discriminating among neutral chemical units in a fluid flowing within the capture range of the charged array.

"Exemplary substantially one-dimensional nanostructures can have be at least about ten times longer than they are wide (i.e., their lengths are at least ten times greater than their diameters). They can include or be formed of any of a single-walled carbon nanotube, a multi-walled carbon nanotube, a nanowire, a nanorod, a nanocylinder, a strip of a sheet of graphene, and an edge of a sheet of graphene. Example nanostructures can be fabricated using any suitable technique, including those found in U.S. application Ser. No. 12/449,141 entitled 'Methods, Systems, and Apparatus for Storage, Transfer and/or Control of Information via Matter Wave Dynamics,' filed on Sep. 11, 2009, by Lene V. Hau and in International Publication No. WO 2005/000739 A1, published Jan. 6, 2009, entitled 'Carbon Nanotube Device Fabrication,' each of which is incorporated herein in by reference in its entirety.

"In some embodiments, at least a portion of the nanostructure can be suspended over a gap in a substrate, with a first electrode disposed on a substrate with a first end of the nanostructure. The width of the first electrode can be less than about two times the length of the nanostructure. Such embodiments may also include a supplemental voltage source configured to apply a supplemental voltage between the first end of the nanostructure and a second end of the nanostructure.

"Yet further embodiments include a source of neutral chemical units, such as a magneto-optical trap, a cold atom source, a thermal vapor, or a thermal beam. Neutral chemical units can be directed from the source toward the nanostructure with an appropriately manipulated potential or flow. For example, the neutral chemical units may be directed toward the nanostructure after release from a magneto-optical trap or by flowing a gas including the neutral chemical units past the nanostructure. The source may also release a plurality of neutral chemical units (either at once or sequentially), and a detector may detect a number of ions leaving the nanostructure as a function of the charging voltage applied to the nanostructure. An optional probe laser may illuminate neutral chemical units released from the source before ionization.

"Exemplary apparatus may also include a laser or electron beam source that excites the neutral chemical unit to an excited state before the neutral chemical unit is captured by the electric field and/or ionization.

"Additional inventive embodiments include atom interferometers and corresponding methods of converting a matter-wave interference patterns to ionized fringe patterns. Neutral chemical units propagate about and interfere at the end of the a beam path to generate a matter-wave interference pattern in the vicinity of a substantially one-dimensional nanostructure. A voltage source generates an electric field about the nanostructure by applying a charging voltage between the nanostructure and a reference potential so as to capture the matter-wave interference pattern in the electric field. The captured matter-wave interference pattern ionizes along a length of the substantially one-dimensional nanostructure to generate the ionized fringe pattern along the length of the nanostructure.

"Detecting the ionized fringe pattern with a position-sensitive ion detector yields a measurement of the ionized fringe pattern's period and/or fringe position(s). (The ionized fringe pattern may also be magnified before detection with ion optics or by using a curved nanostructure.) The measured period and/or shift can be used to determine an angular orientation of the nanostructure, a velocity (linear, angular, or both) of the nanostructure, a force applied to the nanostructure, and a potential applied to the nanostructure. Information about the fringe pattern can also be used to determine changes in orientation, velocity, force, and potential.

"The capture and ionization processes can be carried out for single atoms or molecules interacting with a single, charged nanotube, nanowire, or other one-dimensional nanostructure.

"High cross sections for sidewall ionization by a single nanotube or nanowire can be obtained, even for relatively modest voltages applied to the nanotube. For instance, a cross section for sidewall ionization of cold atoms can be achieved that is five times higher than the cross section for tip ionization of cold atoms. Similarly, the cross section for sidewall ionization of thermal atoms is three times higher than the cross section for tip ionization of thermal atoms. In general, using sidewall ionization, a single nanotube can achieve a capture cross section comparable to that achieved by a macroscopically sized forest of nanotubes or nanowires with tip ionization.

"The large critical impact parameter for atom capture perpendicular to the length of the nanotube can be used to capture atoms or molecules even at large distances from the nanotube. This wide capture range gives rise to a high signal-to-noise ratio and high sensitivity.

"Sidewall ionization can be applied to form a compact, chip-integrated neutral atom or molecule detector with single atom or molecule sensitivity even for ground state atoms or molecules. Time resolution in the nanosecond regime can be achieved; the spatial resolution is determined by the capture cross-section of the nanotube in the transverse direction and can be at the nanometer level with use of time discrimination. The threshold voltage for ionization is a sensitive probe of the species being ionized. With the appropriate sample geometry and the use of ion optics, the detector's efficiency for atom or molecule counting can approach 100%. Such a detector has applications in sensitive gas detection; the development of compact, cold-atom based interferometers, interferometers based on thermal (non-cooled) atoms, and atom counting and/or quantum correlation measurements in cold and in thermal atomic gases.

"With a position-sensitive microchannel plate and ion optics, spatial resolution at the single nanometer or sub-nanometer level can be obtained along the length of the nanotube. Combined with the large atom capture range perpendicular to the nanotube, fringes of interfering matter waves can be measured directly and with high sensitivity, for example in a compact chip-based atom interferometer.

"Additionally, quantum degenerate gases can be manipulated into extreme conditions.

"It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the inventive subject matter disclosed herein. It should also be appreciated that terminology explicitly employed herein that also may appear in any disclosure incorporated by reference."

For additional information on this patent, see: Hau, Lene V.; Golovchenko, Jene A.; Goodsell, Anne. Methods and Apparatus for Detecting Neutral Chemical Units via Nanostructures. U.S. Patent Number 8729495, filed March 24, 2011, and published online on May 20, 2014. Patent URL:

Keywords for this news article include: Nanowire, Fullerenes, Nanostructural, Nanostructures, Nanotechnology, Carbon Nanotubes, Emerging Technologies, President and Fellows of Harvard College.

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Source: Journal of Engineering

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