WO2008095081A2

WO2008095081A2 - Markers comprising light-emitting semiconductor nanocrystals and their use

Info

Publication number: WO2008095081A2
Application number: PCT/US2008/052620
Authority: WO
Inventors: San Ming Yang; Luis A. Sanchez; James C.M. Hayes
Original assignee: Evident Technologies
Priority date: 2007-02-01
Filing date: 2008-01-31
Publication date: 2008-08-07
Also published as: WO2008095081A3

Abstract

The invention includes markers comprising light-emitting semiconductor nanocrystals, as well as methods for their use in an optical tracking system. The use of semiconductor nanocrystals having different peak emission wavelengths permits discrimination of individual markers among a plurality of markers. In some embodiments, the semiconductor nanocrystals are patterned onto a substrate.

Description

MARKERS COMPRISING LIGHT-EMITTING SEMICONDUCTOR NANOCRYSTALS

AND THEIR USE

CROSS-REFERENCE TO RELATED APPLICATIONS The present application claims the benefit of co-pending US Provisional Patent Application No. 60/898,681 , filed 01 February 2007, which is hereby incorporated herein.

TECHNICAL FIELD

The present invention relates generally to markers for motion tracking and to a system which is capable of optically monitoring and recording full or partial object movements and, more particularly, to markers comprising semiconductor nanocrystal complexes. The present invention also relates to methods of formulating liquid inks containing semiconductor nanocrystals for the preparation of such markers.

BACKGROUND OF THE INVENTION

Semiconductor nanocrystals are crystals typically made of M-Vl, Ml-V, IV-VI, or ll-alloyed I-IM-VI materials (e.g., CdSe, CdS, CdTe, CnS, ZnSe, PbS, PbSe, PbTe, CuInGaS, CuInGaSe, ZnCuInGaS, ZnCuInGaSe, InP, or InGaP) that have a diameter between about 1 nanometer (nm) and about 20 nm. In the strong confinement limit, the physical diameter of the nanocrystals is smaller than the bulk excitation Bohr radius, causing quantum confinement effects to predominate. In this regime, the nanocrystal is a 0-dimensional system that has both quantized density and energy of electronic states where the actual energy and energy differences between electronic states are a function of both the nanocrystal composition and physical size. Larger nanocrystals have more closely spaced energy states and smaller nanocrystals have the reverse. Because interaction of light and matter is determined by the density and energy of electronic states, many of the optical and electric properties of nanocrystals can be tuned or altered simply by changing the nanocrystal geometry (i.e., physical size).

Nanocrystals or populations of nanocrystals exhibit unique optical properties that are size tunable. Both the onset of absorption and the photoluminescent wavelength are a function of nanocrystal size and composition. The nanocrystals will absorb all wavelengths shorter than the absorption onset; however, photoluminescence will always occur at the absorption onset. The bandwidth of the photoluminescent spectra is due to both homogeneous and inhomogeneous broadening mechanisms. Homogeneous mechanisms include temperature- dependent Doppler broadening and broadening due to the Heisenberg uncertainty principle, while inhomogeneous broadening is due to the size distribution of the nanocrystals. The narrower the size distribution of the nanocrystals (i.e., a more monodisperse population of nanocrystals), the narrower the full-width half max (FWHM) of the resultant photoluminescent spectra. In 1991 , Brus wrote a paper reviewing the theoretical and experimental research conducted on colloidally grown semiconductor nanocrystals, such as cadmium selenide (CdSe) in particular; Brus L., "Quantum Crystallites and Nonlinear Optics," Applied Physics A, 53 (1991 ). That research, precipitated in the early 1980s by the likes of Efros, Ekimov, and Brus himself, was greatly accelerated by the end of the 1980s, as demonstrated by the increase in the number of papers concerning colloidally-grown semiconductor nanocrystals. Triangulation is described in numerous systems for measuring object surface or point locations. A typical system sends a beam of collimated light onto an object and forms images of that light through a sensor. The parallax displacement along the axis between the projector and the sensor is used to compute a range to the illuminated point. This type of system is exemplified in US Patent Nos. 5,198,877 to Schulz, RE35,816 to Schulz, 5,828,770 to Leis et al., 5,622,170 to Shulz, and 6,801 ,637 to Voronka.

Known target locating systems are used to track specific body points for medical purposes or to provide the means to capture an object's surface points for the purpose of three-dimensional digitization of object geometry. In all such systems, targets are either projected from scanned, collimated light sources or are active light-emitting markers affixed to the object that is being tracked. Some known systems utilize linear CCD sensors that capture light through cylindrical lens systems. Some utilize more than one active emitter, where multiple emitters are distinguished from each other through geometrical marker identification (not time multiplexing). Some known systems describe a tag or marker controller that is synchronized with the imaging sensor systems.

Unlike the present invention, no known system describes the use of passive tags or markers containing emissive semiconductor nanocyrstals. These nanoparticle-loaded tags or markers emit spectroscopically-specific and narrow emission bandwidth signals upon a broad illumination of short wavelength (300-450 nm). The different luminescent colors of the tags can be computer-processed with modern video editing software. SUMMARY OF THE INVENTION

The invention relates to markers comprising emissive semiconductor nanocrystals, where the markers are used in an optical system capable of tracking the motion of an object, including the human body or portions thereof. A system according to the invention provides a simultaneous measurement of multiple photoluminescent markers, preferably attached to an object or person to be tracked, the markers having the same or different emission wavelength bands.

Markers according to the invention are illuminated with a short wavelength light source and emit light in specific wavelength bands within the spectrum where the peak emission wavelength is a function of composition and diameters of the nanocrystals in the marker. Nonlimiting examples of short wavelength light sources include, for example, ultraviolet lamps (mercury vapor, xenon flashlamps, deuterium lamps), light emitting diodes (blue, violet, UV), and excimer lasers. Other light sources having a peak emission wavelength less than the peak emission wavelength of the nanocrystals of the marker may also be employed.

An optical system according to the invention may include one or more imaging devices having an optical filtering apparatus for discriminating between individual markers. Such a system may be used to track the spatio-temporal location of each marker by triangulation and data processing.

In one embodiment of the present invention, each marker (identifying an object or a portion of an object) has a different emission wavelength(s). Alternatively, each object may be decorated with a plurality of markers, each of the same wavelength, with different objects being decorated with a plurality of markers of different wavelengths. Because each marker has a unique emission wavelength, multiple objects can be tracked simultaneously via an imaging system having optical filters capable of distinguishing the different wavelengths emitted by individual markers or sets of markers. Unlike known tracking systems, a system according to the invention does not require temporal synchronization of the emitters and detectors.

In another embodiment of the invention, nanocrystals are prepared as an ink. A marker is constructed by printing the nanocrystal ink onto a substrate in a specific spatial pattern. A tracking system comprising one or more imaging devices can discriminate between markers or sets of markers by identifying both unique emission wavelengths as well as unique spatial patterns.

A first aspect of the invention provides a system for tracking an object, the system comprising: at least one marker capable of fixation to an object to be tracked, the at least one marker including semiconductor nanocrystals having a peak emission wavelength; an illuminating system for exciting the semiconductor nanocrystals of the at least one marker, the illuminating system being capable of transmitting a wavelength shorter than the peak emission wavelength of the semiconductor nanocrystals; and an imaging system for imaging a wavelength emitted by the semiconductor nanocrystals.

A second aspect of the invention provides a marker comprising: a substrate; and semiconductor nanocrystals on a surface of the substrate, wherein the semiconductor nanocrystals emit light at a peak emission wavelength in response to excitation by a wavelength shorter than the peak emission wavelength.

A third aspect of the invention provides a method of tracking an object, the method comprising: affixing to the object at least one marker, the at least one marker including semiconductor nanocrystals capable of emitting light at a peak emission wavelength in response to excitation by a wavelength shorter than the peak emission wavelength; exciting the semiconductor nanocrystals with a wavelength shorter than the peak emission wavelength; imaging the wavelength emitted by the semiconductor nanocrystals using an imaging device; and tracking the object based on a change in spatiotemporal position of the wavelength emitted by the semiconductor nanocrystals.

The illustrative aspects of the present invention are designed to solve the problems herein described and other problems not discussed, which are discoverable by a skilled artisan.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the invention, in which:

FIG. 1 shows an illustrative tracking system according to an embodiment of the invention;

FIG. 2 shows absorption and emission (525 nm) spectra of illustrative semiconductor nanocrystals according to the invention;

FIG. 3 shows illustrative spatial patterns suitable for use with ink-based semiconductor nanocrystals;

FIG. 4 shows an absorption spectrum of a color filter manufactured by Gamcolor Inc., suitable for use in a camera for resolution enhancement.

It is noted that the drawings of the invention are not to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a general schematic of a semiconductor nanocrystal tracking system according to an embodiment of the invention. The present invention provides for a motion tracking system 10. An illuminating source 20 emits light with a wavelength(s) less than the peak emission wavelength of the nanocrystals 25 of the marker 30. The illuminating source may be a short wavelength light source, such as a filtered or unfiltered UV lamp (mercury vapor, xenon, deuterium), blue, violet, or UV LED or LED array, excimer laser, solid state laser, or similar device. Semiconductor nanocrystals 25 are applied to a surface of the marker 30, which is attached to a movable object 40. The movable object 40 is illuminated by illuminating source 20. Light emitted from the marker 30 is recorded by one or more imaging system 50 with one or more optical filters 60. Imaging data are sent to a computer 70 and the spatiotemporal position of the marker 30 is calculated and displayed using digital video software.

Light from the illuminating source 20 may be long UV (about 350 nm to about 385 nm), violet (about 405 nm to about 410 nm), or blue (about 440 nm to about 470 nm). It should be noted that, unlike traditional phosphors, nanocrystals have broadband absorption spectra and can therefore be excited by any light having a wavelength shorter than the peak emission wavelength of the nanocrystals. Typically, semiconductor nanocrystals may be excited by wavelengths that are less than the emission wavelengths of the semiconductor nanocrystal, as illustrated in FIG. 2. Different markers 30 may emit distinct colors, the emitted color being dependent on the size of the semiconductor nanocrystal. Emission from the marker 30 is then recorded by a plurality of imaging devices of the imaging system 50. Nonlimiting examples of imaging devices include digital cameras (e.g., CCD or CMOS focal plane array cameras) and imaging devices using film.

In one embodiment of the invention, markers having semiconductor nanocrystals with different emission wavelengths are used to discriminate among multiple objects or between multiple parts of one or more objects. Tracking the spatiotemporal location of each marker permits the tracking of each object or each part of the object(s).

In another embodiment of the invention, semiconductor nanocrystals are formulated in the form of inkjet ink and used to print a spatial pattern on the marker, such as the patterns shown in FIG. 3. In other embodiments of the invention, semiconductor nanocrystals may be formulated in the form of flexographic ink, screen printing ink, or a thermal transfer ribbon. Other forms for printing spatial patterns are also possible and within the scope of the invention.

By multiplexing both wavelength and spatial patterns, many more markers may be discriminated. Hence, many more objects and/or parts of objects may be simultaneously tracked by an optical imaging system. The object or objects to be tracked may be, for example, a person or an appendage of a person or persons. Such persons may be located in an immersive environment, such as a movie studio, where there are ambient lighting conditions. The tracked person(s) is preferably outfitted with one or more markers oriented such that they may be illuminated with a short wavelength light source and that the light emitted from the markers may be observable to an optical imaging system. Optical filters 60 (FIG. 1 ) may be used to reduce background light and to discriminate among the emissions of multiple markers, thereby reducing the complexity of the image processing algorithms and improving overall system performance. Such a system works well in indoor conditions where diffuse incandescent or fluorescent light is present.

The markers 30 may be patches affixed to the objects or parts of objects to be tracked. Fixation may be by any known or later-developed technique, such as sewing, hooks and loops, and adhesives (e.g., pressure-sensitive tape, double-sided tape). Where a marker 30 is to be affixed to a non-planar surface, a movable surface, an outfit designed for wearing on a human body, or any similar surface, it is preferable that the marker 30 be of a soft, flexible material.

Once the motion of an object, multiple objects, or parts of an object or objects are recorded by an imaging device of the imaging system 50, the data are analyzed using digital movie software or similar software. The motion of an individual marker (and therefore of an individual object or part of an object) can then be extracted from the data by selecting a specific emission color of the marker of interest. Where a patterned marker is employed, the motion of the marker of interest may be based on both the emission color and the specific pattern of the marker.

Example 1 - InkJet ink

CdSe nanocrystals (1 ml_, 40 mg/mL in toluene, emission maximum 530 nm) were evaporated under a stream of nitrogen. The solid residue was redissolved in chloroform (10 ml_). The solution was then mixed with a chloroform solution of poly(maleic anhydride -alt-octadecene) (1 g in 10 ml_). The solution was then evaporated until dry. The solid residue was then added to a 4% dimethylamine solution (5 g) and heated to 80 degrees Celsius to form a yellow solution. To this solution was added glycerin (1.2 g), 2-pyrrolidinone (0.8 g), and 10% sodium dodecylsulfonate (0.5 g), and the final solution was brought up to 10 g with deionized water. The viscosity of the ink is about 4-5 cps, with a surface tension of about 40-50 dyne/cm. The ink was then loaded into an empty Epson cartridge (T060120) and printed from an Epson C88+ printer.

Example 2 - Flexo ink

CdSe nanocrystals (1 ml_, 40 mg/mL in toluene, emission maximum 530 nm) were evaporated until dry under a nitrogen stream. The solid was dissolved into 10 ml_ dichloromethane and mixed with 10 g of Integrity 1100D (from Hexion). The mixture was then ultrasonicated in a Branson Ultrasonic disperser for 2 minutes to form a milky emulsion. The emulsion was then heated to 60 degrees Celsius to remove the dichloromethane, and resulted in a clear yellow solution. The viscosity is about 20 cps in a Zahn Cup #3. The ink was then ready for flexographic printing.

Example 3 - Screen ink

CdSe nanocrystals (1 ml_, 20 mg/mL in toluene, emission maximum 530 nm) were added to 1.5 g polyvinyl acetate) (XX210 from AirProducts), with 0.5 g of water. The mixture was then ultrasonicated in a Branson Ultrasonic disperser for 2 minutes to form a milky emulsion. The ink was then ready for screen printing.

Example 4 - Thermal transfer ink

CdSe nanocrystals (1 ml_, 40 mg/mL in toluene, emission maximum 530 nm) were evaporated until dry under a nitrogen stream. The solid was dissolved into 10 ml_ dichloromethane and mixed with an SP330 resin solution in dicholoromethane (15%). The total resin content was increased to about 30-35% by evaporating off the dichloromethane. The solution was then coated onto a PET film to form a thermal transfer ribbon.

The foregoing description of various aspects of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of the invention as defined by the accompanying claims.

Claims

CLAIMS What is claimed is:

1. A system for tracking an object, the system comprising: at least one marker capable of fixation to an object to be tracked, the at least one marker including semiconductor nanocrystals having a peak emission wavelength; an illuminating system for exciting the semiconductor nanocrystals of the at least one marker, the illuminating system being capable of transmitting a wavelength shorter than the peak emission wavelength of the semiconductor nanocrystals; and an imaging system for imaging a wavelength emitted by the semiconductor nanocrystals.

2. The system of claim 1 , comprising a plurality of markers, wherein each marker is adapted to have a different peak emission wavelength.

3. The system of claim 2, wherein the imaging system further comprises at least one filter for differentiating an individual marker from the plurality of markers.

4. The system of claim 1 , wherein the at least one marker comprises a substrate to which the semiconductor nanocrystals are applied.

5. The system of claim 4, wherein the semiconductor nanocrystals are applied in a pattern.

6. The system of claim 1 , wherein the semiconductor nanocrystals comprise a core semiconductor having a diameter less than about 20 nm.

7. The system of claim 6, wherein the semiconductor nanocrystals comprise a core semiconductor having a diameter less than about 10 nm.

8. The system of claim 1 , wherein the semiconductor nanocrystals are selected from a group consisting of: M-Vl semiconductors, Ml-V semiconductors, IV-VI semiconductors, and ll-alloyed I-III-VI semiconductors.

9. The system of claim 8, wherein the semiconductors are selected from a group consisting of: CdSe, CdS, CdTe, CnS, ZnSe, PbS, PbSe, PbTe, CuInGaS, CuInGaSe, ZnCuInGaS, ZnCuInGaSe, InP, and InGaP.

10. A marker comprising: a substrate; and semiconductor nanocrystals on a surface of the substrate, wherein the semiconductor nanocrystals emit light at a peak emission wavelength in response to excitation by a wavelength shorter than the peak emission wavelength.

11. The marker of claim 10, wherein the semiconductor nanocrystals form a pattern on a surface of the substrate.

12. The marker of claim 10, wherein the semiconductor nanocrystals comprise a core semiconductor having a diameter less than about 20 nm.

13. The marker of claim 12, wherein the semiconductor nanocrystals comprise a core semiconductor having a diameter less than about 10 nm.

14. The marker of claim 10, wherein the semiconductor nanocrystals are selected from a group consisting of: M-Vl semiconductors, Ml-V semiconductors, IV-VI semiconductors, and ll-alloyed I-III-VI semiconductors.

15. The marker of claim 14, wherein the semiconductors are selected from a group consisting of: CdSe, CdS, CdTe, CnS, ZnSe, PbS, PbSe, PbTe, CuInGaS, CuInGaSe, ZnCuInGaS, ZnCuInGaSe, InP, and InGaP.

16. A method of tracking an object, the method comprising: affixing to the object at least one marker, the at least one marker including semiconductor nanocrystals capable of emitting light at a peak emission wavelength in response to excitation by a wavelength shorter than the peak emission wavelength; exciting the semiconductor nanocrystals with a wavelength shorter than the peak emission wavelength; imaging the wavelength emitted by the semiconductor nanocrystals using an imaging device; and tracking the object based on a change in spatiotemporal position of the wavelength emitted by the semiconductor nanocrystals.

17. The method of claim 16, further comprising: filtering wavelengths received by the imaging device to discriminate between the emissions of a plurality of markers having different peak emission wavelengths.

18. The method of claim 16, wherein tracking is further based on a pattern of the semiconductor nanocrystals.

19. The method of claim 16, wherein the semiconductor nanocrystals are selected from a group consisting of: M-Vl semiconductors, Ml-V semiconductors, IV-VI semiconductors, and ll-alloyed I-III-VI semiconductors.

20. The method of claim 19, wherein the semiconductors are selected from a group consisting of: CdSe, CdS, CdTe, CnS, ZnSe, PbS, PbSe, PbTe, CuInGaS, CuInGaSe, ZnCuInGaS, ZnCuInGaSe, InP, and InGaP.