The present disclosure is directed to the field of nanoscale devices. The present disclosure is also directed to the field of enhancing emissions in nanoscale devices.
The manipulation of radiative properties of light emitters coupled with surface plasmons is important for engineering new nanoscale optoelectronic devices, including lasers, detectors and single photon emitters. However, so far the radiative rates of excited states in semiconductors and molecular systems have been enhanced only moderately, typically by a factor of 10-50, producing emission mostly from thermalized excitons.
In direct-bandgap semiconductors, optical excitation with energies greater than the exciton ground state is followed by rapid relaxation of the excess energy of the exciton by emission of longitudinal optical (LO) phonons and eventually by interactions with acoustic phonons to thermalize to k˜0 momentum states. The relaxation timescales of LO and acoustic phonons are typically sub-picosecond and ˜100 ps respectively and are much faster than the exciton recombination timescale (>ns). As the relaxation processes for LO phonons are much faster than the exciton radiative decay, hot-exciton emission has typically been observed only in high-quality crystals or specially designed quantum-well structures that decrease the exciton lifetime. However, even when hot-exciton emission has been observed it is only with very low emission yields. Achieving accurate control over the emissive properties of the exciton states is critical to design nanoscale photonic devices and also to study their fine structure, which may not be possible when recombination occurs from thermalized excitons.
Although some have reported on modifying the optical properties of emitters placed near metallic nanostructures, they have focused on modifying normal luminescence. Hot electroluminescence has been observed in molecules placed between a metallic nanotip and a substrate, but the emission intensities were very low and the underlying physics remains to be clarified.
Semiconducting nanowires placed in the vicinity of flat metal substrates have shown a decrease in their radiative lifetime to ˜100 ps; however, this is not enough to significantly modify their emission properties, as this timescale is comparable to the interactions of acoustic phonons, which mostly thermalize the excitons.
Resonant plasmonic nanocavities have the potential to significantly modify the excited-state relaxation processes by means of near-field coupling of strongly enhanced electromagnetic fields with excitons to provide enhanced light emitters. Furthermore, plasmonic nanocavities integrated with semiconducting nanowires in a core-shell architecture can be tuned in and out of resonance with lower-order plasmonic whispering gallery modes by using the nanowire diameter to manipulate their optical response in order to further enhance light emitters.
One object of the present invention is a nanowire with increased emissions.
Another object of the present invention is a nanowire employing plasmonic nanocavities.
Still yet another object of the present invention is a plasmonic device that include an indirect bandgap material core.
Still yet another object of the present invention is a plasmonic device wherein a diameter of the core corresponds to a resonant mode of the plasmonic cavity.
An aspect of the present invention is a plasmonic device comprising: a core composed of a direct bandgap material; an interlayer surrounding the nanowire; and a metallic shell surrounding the interlayer.
Another aspect of the present invention is a plasmonic nanoscale device comprising: a core composed of a bandgap material having a diameter; an interlayer surrounding the nanowire; a metallic shell surrounding the interlayer; and wherein a plasmonic nanocavity is formed and a diameter of the core or a thickness of the interlayer corresponds to a plasmonic resonant cavity mode.
This disclosure also provides plasmonic nanowires, comprising: a core that comprises a direct bandgap material; an interlayer that at least partially surmounts the core; and a shell that at least partially surmounts the interlayer.
Also disclosed are plasmonic nanowires, comprising a core having a diameter and comprising a bandgap material; an interlayer having a thickness and at least partially surmounting the core; and a shell at least partially surmounting the interlayer, wherein a plasmonic nanocavity is formed and a diameter of the core or a thickness of the interlayer corresponds to a plasmonic resonant cavity mode.
Additionally provided are plasmonic nanowires, comprising a core having a diameter and comprising an indirect bandgap material; an interlayer having a thickness and at least partially surmounting the core; and a shell that at least partially surmounts the interlayer.
Further provided are emitters, comprising: a excitation source; a plasmonic nanowire comprising a core having a diameter and comprising a bandgap material; an interlayer having a thickness and at least partially surmounting the core; a metallic shell that at least partially surmounts the interlayer, wherein a diameter of the core or a thickness of the interlayer corresponds to the plasmonic resonant cavity mode of the plasmonic nanowire; and a detector capable of detecting an emission from the plasmonic nanowire.
Still yet another aspect of the present invention may be a plasmonic nanowire comprising: a core composed of an indirect bandgap material; an interlayer surrounding the nanowire; and a metallic shell surrounding the interlayer.
Another aspect of the present invention may be an emitter comprising: An emitter comprising: a source; a plasmonic nanowire comprising: a core composed of a bandgap material; an interlayer surrounding the nanowire; a metallic shell surrounding the interlayer; wherein a diameter of the core or a thickness of the interlayer corresponds to the plasmonic resonant cavity mode; and a detector (e.g, a PMT, a CCD, and the like).
These and various other advantages and features of novelty that characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.
The summary, as well as the following detailed description, is further understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings exemplary embodiments of the invention; however, the invention is not limited to the specific methods, compositions, and devices disclosed. In addition, the drawings are not necessarily drawn to scale or proportion. In the drawings:
a-3d show hot-exciton emission from plasmonic and photonic nanowires;
a-6d show temperature and polarization dependent properties of the hot exciton emission;
a-8e show the size-dependent properties of the whispering-gallery plasmon nanocavity;
a and 11b are diagrams of a nanowire using indirect bandgap material for the core;
The present invention may be understood more readily by reference to the following detailed description taken in connection with the accompanying figures and examples, which form a part of this disclosure. It is to be understood that this invention is not limited to the specific devices, methods, applications, conditions or parameters described and/or shown herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting of the claimed invention. Also, as used in the specification including the appended claims, the singular forms “a,” “an,” and “the” include the plural, and reference to a particular numerical value includes at least that particular value, unless the context clearly dictates otherwise. The term “plurality” as used herein, means more than one. When a range of values is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “approximately,” “essentially,” or “about,” it will be understood that the particular value forms another embodiment. All ranges are inclusive and combinable, and any documents cited herein are incorporated by reference in their entireties for any and all purposes.
It is to be appreciated that certain features of the invention which are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination. Further, references to values stated in ranges include each and every value within that range.
It should be understood that the term “diameter” is used in this disclosure to refer to a cross-sectional dimension of a nanowire, a nanowire core, or other feature. The term “diameter” is—in some places—used for the sake of convenience, as the disclosed devices need not be circular in cross-section. Accordingly, the term “diameter” should be understood to refer to a cross-sectional dimension of the component to which “diameter” refers and should not be understood as necessarily requiring a circular cross-section.
Plasmonic nanowires are a type of nanowire with increased emissions. With plasmonic nanowires the highly concentrated fields of plasmonic nanocavities are integrated with a core to completely alter the excited-state optical processes. In the embodiments discussed herein, either a direct or indirect bandgap material may be used in the core. By integrating the plasmonic nanocavities, emission from highly non-thermalized excitons are observed, indicating very high radiative rate enhancement.
It should be understood that the term “diameter” is used in this disclosure to refer to a cross-sectional dimension of a nanowire, a nanowire core, or other feature. The term “diameter” is—in some places—used for the sake of convenience, as the disclosed devices need not be circular in cross-section. Accordingly, the term “diameter” should be understood to refer to a cross-sectional dimension of the component to which “diameter” refers and should not be understood as necessarily requiring a circular cross-section.
Shown herein is the generation of dominant hot-exciton emission, which is to say, luminescence from non-thermalized excitons that are enhanced by the highly concentrated electromagnetic fields supported by the resonant whispering-gallery plasmonic nanocavities of nanowire devices. By tuning the plasmonic cavity size to match the whispering-gallery resonances, an almost complete transition from thermalized exciton to hot-exciton emission can be achieved, which reflects exceptionally high radiative rate enhancement of >103 and subpicosecond lifetimes. Core-shell plasmonic nanowires are an ideal test bed for studying and controlling strong plasmon-exciton interaction at the nanoscale and opens new avenues for applications in ultrafast nanophotonic devices.
To modify the exciton relaxation and emission properties, a semiconductor nanowiredielectric metal core-shell plasmonic nanocavity structure was manufactured. An exemplary plasmonic nanowire 100 is shown in
The core 10 shown in
The core 10 (e.g., made of CdS) may operate as a source of semiconductor excitons. The core 10 has a cross-sectional dimension (e.g., diameter D) that may be between 20-200 nm, preferably between 50-150 nm, and even between about 10 nm to about 1 micrometer. The dimensions of the interlayer and metal shell may vary depending on the core's dimensions. For example, if the core is 10 nm, then the inter-layer may be 2-5 nm thick and the metal shell is suitably thicker than the diameter of the thickness of the core and interlayer.
The thickness of interlayer may be anywhere between about 5% to about 95% of a cross-sectional dimension of the core. Likewise, the thickness of the metal shell may be equal to the combined cross-sectional dimension of the core and the interlayer. The thickness of the metal shell may be equal to (or greater than) the thickness of the interlayer plus a cross-sectional dimension of the core. The thickness of the metal shell may also be equal to or greater than the total cross-sectional dimension of the core and interlayer.
The core (and associated layers) may have virtually any length that the user desires. Lengths of about 1, 5, 10, 50, 100, or even about 500 nm are all suitable. The core may also have a length of a micrometer, tens of micrometers, hundreds of micrometers, or even a length of millimeters, depending on the user's needs. Lengths of centimeters or greater are also considered suitable for the disclosed devices.
The diameter of core 10 may be larger or smaller than the foregoing ranges. The thickness of interlayer 20 may be selected so as to correspond to a whispering gallery resonant mode of the formed plasmonic nanocavity. Without being bound to any particular theory, the role of the interlayer is to prevent exciton quenching by the metal layer. The energy (or frequency) at which the whispering gallery mode (this is the surface plasmon mode) occurs is sensitive to the device geometry, which includes the diameter of the core, interlayer, and metal shell, but again. Controlling the energy of the whispering gallery mode is not necessarily the primary purpose of the interlayer. The diameter D of the core 10 may, in some embodiments, be referred to as the thickness of the nanowire.
Suitably surrounding the core 10 is an interlayer 20, which in the embodiment shown in
The second role of the interlayer 20 is to retard or even prevent rapid quenching of excitons because of the metallic shell 30 (which may be made of Ag or other metal). A preferred thickness for an interlayer 20 in the embodiment shown is 5 nm. The thickness chosen is a compromise to prevent exciton quenching while ensuring strong interaction of plasmonic modes with the CdS core 10. The thickness T1 of the interlayer 20 may be in the range of from about 1 nm to about 10 nm or even 20 nm or even 50 nm. The thickness is preferably between 3-8 nm, and most preferably between 4-6 nm. The thickness of the interlayer 20 may be selected to correspond to a whispering gallery resonant mode of the formed plasmonic nanocavity.
The metallic shell 30 of the plasmonic nanowire 100 may be, for example, an Ag nanoshell, in the embodiment shown in
Structural and chemical analysis by energy-dispersive X-ray spectroscopy was used to confirm the conformal coating of SiO2 and Ag. In evaluating the effect of the plasmonic cavity on the emission properties, 5 nm SiO2-coated CdS nanowires without a Ag metallic shell were prepared.
Figures representing the layers would show transmission of an electron microscope image showing the SiO2 layer on a CdS nanowire produced by atomic layer deposition, lines indicate the thickness of the SiO2 interlayer 20; a transmission electron microscope micrograph of the fabricated CdS—SiO2—Ag core-shell structure, with lines indicating the 15 nm Ag shell thickness and an elemental mapping image showing the highly conformal coating of SiO2 and Ag on the CdS nanowire measured by energy-dispersive X-ray spectroscopy.
In fabricating the plasmonic nanowire 100, single crystalline CdS nanowires were grown on Si(100) substrates by an Au/Pd nanoparticle-catalyzed vapor-liquid-solid method using a quartz tube vacuum furnace. At a temperature of 760° C., pure CdS powder (99.999% Sigma Aldrich) was evaporated for 3 hours in an argon flow of 100 SCCM. The growth substrate was subsequently moved to an atomic layer deposition system (Cambridge Nanotech) to passivate the nanowires with a 5 nm SiO2 layer by alternating O3, 3-Aminopropyltriethoxysilane (APTES), and H2O pulses at a temperature of 150 deg. C. A 15 nm Ag shell was conformally coated using an e-beam evaporator at a low deposition rate of 0.2 A/s.
Nanowires on growth substrates were dry-transferred onto 400 nm-thick SiO2 coated Si substrates to carry out optical experiments using a home-built microscope equipped with a 60×, 0.7NA objective (Nikon) with a spatial resolution of 500 nm. A continuous wave argon-ion laser (Coherent), tuned at a wavelength of 457.9 nm, was focused to a beam spot size of 800 nm to pump individual nanowires on the sample at low excitation powers (<100 kW/cm2), thus avoiding heating of the samples. Photoluminescence spectra were collected using a spectrometer (Acton) and a cooled CCD (charge-coupled device) (Pixis 2K, Princeton Instruments) with a spectral resolution of 0.1 nm. Low-temperature and temperature-dependent measurements were conducted using a liquid nitrogen cooled cryostat (Janis Research). To avoid oxidation of the Ag shell, the samples were stored in vacuum.
The optical properties of the plasmonic nanowire 100 and of the photonic nanowire (i.e. the structure without the metallic shell 30) were measured by means of microphotoluminescence carried out at 77 K for individual nanowires with the focused laser excitation energy of the 2.708 eV (457.9 nm) line of an Ar+ ion laser. The photoluminescence spectra of the photonic nanowires show emission from the recombination of free A- and B excitons of CdS, with peaks at 2.544 eV and 2.559 eV, respectively, this is shown in
In
New narrow emission lines appear for the plasmonic nanowire 100 at an equidistant progression below the laser excitation energy, with separations corresponding to the energy of the CdS LO phonon of ˜38 meV, reflecting the modified spontaneous exciton emission (
Another feature of the spectra of the plasmonic nanowire 100 is that the main peak now corresponds to the 4LO hot exciton, with significant line narrowing superimposed on the Bexciton position, indicating dominant emission from hot excitons.
To further ensure that hot-exciton emission has been observed by LO-phonon coupling, plasmonic nanowire spectra were measured with a different laser excitation energy (2.662 eV). The resulting spectra confirmed that the spectral position of the LO progression is sensitive to the excitation energy.
No hot-exciton peaks were observed for SiOrcoated CdS nanowires on planar Ag films. Without being bound to any single theory, this suggests that a plasmonic nanocavity supports the large field enhancements needed to observe emission from non-thermalized excitons. This is shown in
In
The differences may arise—again without being bound to any single theory—to whether the process requires the participation of real exciton states (hot-exciton emission) or not (resonance Raman scattering).
Further studies of the temperature and polarization dependent properties of the hotexciton peaks were conducted. At higher temperatures, a pronounced quenching of the emission intensity is expected for the hot exciton because of the increased nonradiative decay rate, whereas the resonance Raman scattering intensity is not very sensitive to the temperature. As shown in
Furthermore, with increasing temperature, the enhanced emission intensity of the 4LO hot exciton, which is in resonance with the B-exciton level at 77 K, is drastically decreased because the B-exciton energy shifts whereas the 4LO peak does not, as shown in
The hot-exciton emission feature can also be inferred from the polarization properties of plasmonic nanowires. The polarization of hot-exciton peaks should be similar to CdS excitons (A and/or B), whereas that of Raman scattering should follow the pump laser polarization. For CdS crystals, the A-exciton is linearly polarized perpendicular to the c axis and the B-exciton is isotropic, which is consistent with the measurements for the photonic nanowires shown in
To study the effect of the plasmon nanocavity size on the hot-exciton emission, a series of photoluminescence measurements were performed on 44 plasmonic nanowires ranging from 48 to 157 nm in diameter. As the B-exciton peak is strongly modified by the resonance with the 4LO hot exciton in plasmonic nanowires, shown in
The 60 nm wire shows very intense 3LO and 4LO peaks, which overwhelm the normal A-exciton emission, indicating an almost complete transition from the thermalized exciton to the hot-exciton emission. Taking into account the fast, sub-picosecond relaxation time of the LO phonons, it is evident that the modified radiative decay time must become comparable to the relaxation time of the LO phonons. Using simple kinetic modeling based on exciton kinetic energy relaxation, the modified radiative decay time due to the plasmonic cavity was estimated to be sub-picosecond for 60 nm wires, a marked reduction by more than three orders of magnitude from >1 ns for photonic nanowires. Furthermore, size-dependent enhancements with a series of peaks suggest a strong role played by the whispering-gallery plasmon cavity modes in the emission process.
To investigate the origin and effect of field enhancement in the plasmonic cavities due to the excitation of the surface-plasmon modes on exciting the CdS core with the laser, numerical simulations were performed to calculate the field confinement for photonic and plasmonic nanowires at the 4LO hot-exciton energy. From the calculations, it was observed that, for sub-150 nm cores 10 of CdS, the plasmonic nanowires 100 supported whispering-gallery modes, shown in
Numerical calculations reveal that the enhancement in the electric field intensity for the 60 nm plasmonic nanowire 100, which supports the m=2 resonance mode with a cavity quality factor of 55, is as large as 40,000 in comparison with the same sized photonic nanowire as shown in 8d, for m=3,
Taking into account the effects of coupling the light in and out through the Ag shell 30, by full 3D numerical calculations, a direct comparison of the spectra (photon counts) between a 60 nm plasmonic and photonic nanowire, shown in
This, combined with the shorter excited state lifetime for the plasmonic system suggests large radiative rate enhancements. As the excited-state lifetime of the plasmonic nanowires with an average size of 140±50 nm is measured to be 7 ps, it is estimated that the 60 nm plasmonic wire with a strong whispering-gallery resonance mode should have a subpicosecond lifetime, as a result of the ˜10 times enhancement observed for this nanowire size in comparison with the 140 nm sized wires shown in
The estimated sub-picosecond lifetime of the 60 nm plasmonic nanowire is also consistent with the Purcell factor calculations predicting an enhancement of 3.8×103. The calculated size dependence of the field density inside the plasmonic cavity, shown in
While the above discussion involved the plasmonic nanowire 100 with a direct bandgap material used in the core 10. Another embodiment of the present invention involves using an indirect band-gap material in the core in order to generate efficient light emission in the nanowire by using plasmon nanocavity structures.
Indirect bandgap material include, for example, Si, Ge, Si—Ge, SiC, GaP, and the like, as some non-exhaustive examples. Other light-emitting, or potentially light-emitting materials are also suitable core materials. The indirect bandgap plasmonic nanowire 200 is a light-emitting device that comprises a semiconductor nanowire-dielectric-metal core-shell plasmonic nanocavity structure. The structure comprises a semiconductor nanowire of any indirect bandgap material used as the core 210 of the plasmonic nanowire 200. The core 210 is a source of electron-hole pairs (excitons). The plasmonic nanowire 200 further comprises an interlayer 220 made of any dielectric/insulating material. For example, the interlayer 220 may be SiO2. The plasmonic nanowire 200 may further comprise a metallic shell 230. The metallic shell 230 supports the plasmon cavity modes.
Similar to the interlayer 20 discussed above, the insulating interlayer 220 plays two roles; it electronically passivates the surface defects of the semiconductor nanowire core 210 to remove nonradiative recombination sites on the nanowire surface. The second role of the insulating interlayer 220 is the prevention of rapid quenching of electron-hole pairs due to the metallic shell 230. An optimal thickness of the insulating interlayer 220 may be a few nanometers (2-5 nm) which are a good compromise for preventing electron-hole pair quenching but ensuring strong interaction of plasmonic modes with the semiconductor core.
In indirect bandgap semiconductors such as silicon, the radiative lifetime of electronhole pair is typically in the millisecond range, which is much longer than that of direct bandgap materials because the momentum mismatch between electrons and holes in the conduction and the valence bands extrema is extremely large. As shown in
The plasmonic nanowire 200 of the present invention may generate very intense electromagnetic fields due to the plasmon nanocavity (by surface plasmons). The plasmonic nanowire 200 can produce radiative recombination from hot carriers (high-energy electron-hole pairs) in the indirect bandgap semiconductor nanowire core 210, which results in efficient light emission from excited states before the relaxation of hot carriers by emitting optical phonons. The strongly enhanced electromagnetic field inside the plasmon nanocavity dramatically shortens the radiative lifetime to a sub-picosecond, which is comparable with the relaxation time so that the sub-picosecond lifetime gives rise to UV-VIS light emission from indirect bandgap bulk silicons, shown in
Even at room temperature, the plasmonic nanowire 200 emitted an efficient bluegreen light emission. The experiments demonstrate that the plasmonic nanowire 200 can obtain light emission from Si and other indirect bandgap materials.
As the current silicon-based electronics has faced limitations on the speed for data processing due to the inherent issues of RC time delay and Joule heating, silicon optoelectronics, which can provide ultrafast optical data processing on a silicon microchip, has been intensively studied by many researchers both in academia and industry.
One challenge has been to obtain light emission from Si, which is not possible due to the low emission efficiency in indirect bandgap materials due to momentum mismatch. Also, integration of direct bandgap materials which emit light efficiently on Si substrates has been challenging due to the mismatch in their crystal structures. Light emission from nanocrystalline and nanoporous silicon has been reported and researched extensively; however, the efficiency is low and these structures are not amenable for the fabrication of electrical injection devices. Therefore, it is desirable to develop device technologies for efficient emission of light from silicon and other indirect bandgap materials which are also compatible with electrical injection devices.
One may use nanoscale silicon structures (not necessarily quantum confined length scales, i.e., <5 nm). These can be made in efficient electrical injection devices and hence can be used as a monolithically integrated light source on optically interconnected silicon microchips for ultrafast data processing.
As another application, the silicon light emitting device can be exploited as an optical probe for optical bio-sensors integrated on a silicon chip, of which the advantages are the compatibility with well-developed silicon technologies and the low manufacturing cost.
For the last two decades, it has been known that a light emission from silicon at a moderate quantum yield can only be possible when the silicon has a form of quantum confined structures such as porous silicon and silicon quantum dot. Since the exciton Bohr radius in silicon is about 5 nm, a tiny structure smaller than 5 nm would be required to obtain the light emission from indirect bandgap materials such as silicon and germanium, but it is still very difficult to fabricate the quantum confined structures with control over their size and properties, and then to be able to electrically connect them.
The indirect-gap light emitting device of this invention does not require a quantum confined nanostructure, but a semiconductor nanowire with the dimension of a few ten to a few hundred nanometers which can be easily fabricated at high precision using current top-down or bottom-up fabrication technologies. In addition, the size-scale of the present invention is compatible with efficient electrical injection devices, unlike quantum dots or nanoporous silicon.
The plasmonic nanowires disclosed herein (which may feature whispering gallery, Fabry-Perot modes or both, depending on configuration) can alter the radiative decay rates of excitons as a result of resonances with highly concentrated optical fields. The observation of the complete transition to non-thermalized exciton emission demonstrates that the spontaneous emission properties of semiconductors can be tuned over a very wide range with ultra-small mode volume plasmon nanocavities in spite of their low quality factors. The tunability of the excited state relaxation processes may open up novel ways to develop new device concepts for high performance deep-subwavelength optoelectronic devices that can be modulated at very high frequencies owing to the significantly shortened excited-state lifetime.
Additional Discussion
In bulk silicon, emission from hot-carriers (non-thermalized carrier recombination) has been observed by injecting carriers at large applied bias using a scanning tunneling microscope10, but the measured quantum efficiency is extremely low because of much faster hot-carrier relaxation time (intra-band; 0.1-1 ps) in comparison to the long radiative lifetime11-13. Since the reported radiative lifetime for hot luminescence in bulk silicon is ˜10 ns at the Γ point,14 and the hot-carrier relaxation time is <1 ps,12, 13 the efficiency for hot luminescence across the direct band-gap is expected to be ˜10−4 at the Γ point. Furthermore, as hot-carriers relax, the radiative lifetime would increase due to the involvement of phonons in emission processes, resulting in lower quantum efficiencies. However, visible light emission from hot-carriers in “bulk” silicon can be efficient if the radiative lifetime becomes comparable with the hot-carrier relaxation time. In addition, enhanced emission from hot-carriers in silicon can enable studies of photophysics of indirect bandgap materials, which is otherwise challenging due to low emission quantum yields. Although silicon photonic crystal nanocavities have recently demonstrated enhancements up to 100, the emission was mostly generated from thermalized carriers in the near-infrared wavelength range.15,16.
Provided here is light emission with a high quantum yield (>1%) at room temperature from “bulk-sized” (no quantum confinement; >30 nm) silicon (as one example) integrated with a plasmonic nanocavity via Purcell enhancement effect17,18. Highly concentrated electromagnetic fields inside plasmon nanocavities induce phonon-assisted light emission from hot-carriers before their thermalization to the lowest energy state (near X-point) in the conduction band (
To generate light emission from hot-carriers, one may fabricate the plasmonic nanocavity on single silicon nanowires (30-80 nm diameter range) by depositing a 5 nm SiO2 interlayer (to prevent recombination of carriers at the metal surface while maintaining strong nanocavity plasmon fields in silicon), followed by a 100 nm-thick silver Ω-shaped cavity (see description elsewhere herein) to support surface plasmon polariton modes (
Room temperature micro-photoluminescence measurements were carried out on individual nanowire devices with an Ar+ laser excitation source (2.708 eV). Bright visible light emission was observed from single-plasmonic silicon nanowires (
To further study the effect of size-tunable plasmonic nanocavity resonances on hot luminescence, various-sized silicon nanowires (d, 30 to 80 nm) coupled with an Ω-shaped cavity were examined. The intensity of hot luminescence band (integrated counts) reached a maximum at a resonant size (d=70 nm), with a clear peak structure reflecting phonon-assisted hot luminescence processes (
Polarization-dependent photoluminescence measurements were made. Because surface plasmon polaritons are transverse-magnetic waves at the metal-dielectric interface22, the electric-field of plasmon cavity modes should be polarized perpendicular to the nanowire long axis. Indeed, the resonant-sized plasmon cavity (d=70 nm) showed perpendicularly-polarized hot luminescence bands with the emission polarization ratio, ρ, of 0.56 where ρ=(I⊥−I//)/(I⊥+I//) (
Phonon-assisted hot luminescence is enhanced when the hot-carriers assisted by phonons with the highest density of states are resonantly coupled with the cavity plasmons. The simulated size dependence of plasmon cavity modes (
Light emission is possible when phonons scatter the hot-carriers to the almost vertical light-line (k˜0). This phonon assisted scattering process should satisfy momentum conservation, k′=ke±q≈0, where k′ and ke are the momentum values of hot-carriers at the light line and at the initial electronic state respectively, and q is the phonon momentum. Thus, the hot-carrier population would depend on the modes with the highest phonon density of states where phonon dispersion has almost zero slope23, and those specific phonons can scatter the hot-carrier efficiently to the light line. The phonon dispersion of silicon along the <110> direction shows that the density of states is relatively high for momentum values of ˜2π/a(0.6, 0.6, 0) for transverse optical (TO) and transverse acoustic (TA) and ˜2π/a(0.7, 0.7, 0) for longitudinal optical (LO) and TA phonons, situated between Γ and K points24 (
The absorption process upon laser excitation involves interaction with a phonon, followed by intra-band relaxation of hot-carriers by phonon emission (lower k values); this can occur by either a 1-phonon process involving an optical phonon near the Brillouin zone center (Γ-point) with low k values, or a 2-phonon (acoustic) process with k values with opposite signs near the Brillouin zone boundaries26. Because the density of states of transverse acoustic (TA) phonons near the zone boundary is much higher than that of longitudinal acoustic (LA) phonons in crystalline silicon as seen in their dispersion24, intra-band relaxation process would be dominated by 2-TA phonons. Based on the known phonon dispersion in silicon27, the phonon-assisted hot luminescence processes has been explained elsewhere herein. The broadening of the peaks with increasing energy separation from the laser excitation suggests that many other phonon relaxation pathways also start to contribute.
This work demonstrates the unique interplay of three (quasi) particle systems; carriers, phonons and cavity plasmons, which provide an interesting test bed to study such complex processes that can also lead to new properties in engineered materials not found otherwise. The ability to obtain visible light emission from silicon devices which are compatible with lengthscales in current electronics (>20 nm) opens up new ways to integrate active Si-based photonics with other conventional functionalities. This method obtains light emission from any indirect bandgap semiconductor and is useful for the fabrication of monolithic devices utilizing optics for ultrafast data processing.
Thus, in one embodiment the present disclosure provides plasmonic nanowire devices. These devices suitably include a core that comprises a direct bandgap material; an interlayer that at least partially surmounts the core; and a shell that at least partially surmounts the interlayer.
In some embodiments, a plasmonic nanocavity is defined by the nanowire. and a diameter of the core corresponds to a plasmonic resonant cavity mode. A plasmonic cavity may be formed by the nanowire and a thickness of the interlayer corresponds to a plasmonic resonant cavity mode.
A cross-sectional dimension (e.g, diameter, radius, and the like) of the core may be in the range of from about 10 nm to about 1 micrometer, or from about 100 nm to about 500 nm. As explained elsewhere herein, it should be understood that the core may be circular in cross-section, but may also be triangular, hexagonal, or otherwise polygonal or even irreglular or non-symmetric in cross-section. A core may include CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, GaN, GaAs, InP, InAs, InN, CuO, PbS, PbSe, PbTe, and the like and combinations thereof. An interlayer may include, e.g., SiO2, Si3N4, MaF2, TiO2, Al2O3, HfO2, MgO, and the like or any combination thereof. A shell may include a metal, e.g., Au, Al, Pt, Cu, Pd, graphene, and the like or any combination thereof. Ag is considered an especially suitable shell material.
Also disclosed are plasmonic nanowire devices, the devices including a core having a diameter and comprising a bandgap material; an interlayer having a thickness and at least partially surmounting the core; and a shell at least partially surmounting the interlayer, wherein a plasmonic nanocavity is formed and a diameter of the core or a thickness of the interlayer corresponds to a plasmonic resonant cavity mode.
The bandgap material may be an indirect bandgap material or a direct bandgap material, depending on the user's needs. The core may have a cross-sectional dimension in the range of from about 10 nm to about 1 micrometer, or even from about 100 nm to about 500 nm. The core may include, e.g., CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, GaN, GaAs, InP, InAs, InN, CuO, PbS, PbSe, PbTe, and the like or any combination thereof. Alternatively, the core may include Si, Ge, Si—Ge, SiC, GaP, and the like or any combination thereof.
An interlayer of the disclosed devices may include SiO2, Si3N4, MaF2, TiO2, Al2O3, HfO2, MgO, and the like or any combination thereof. Suitable shell materials are described elsewhere herein, and may include Ag, Au, Al, Pt, Cu, Pd, graphene and the like or any combination thereof.
Further provided are plasmonic nanowire devices comprising a core having a cross-sectional dimension and comprising an indirect bandgap material; an interlayer having a thickness and at least partially surmounting the core; and a shell that at least partially surmounts the interlayer.
A cross-sectional dimension of the core or a thickness of the interlayer may corresponds to a plasmonic resonant cavity mode. Suitable core materials are described elsewhere herein, as are suitable interlayer and shell materials.
The present disclosure also provides emitters. An emitter may include a excitation source; suitable excitation sources may be either optical (such as a laser, or white-light source) or electrical.
The emitter also suitably includes a plasmonic nanowire device. Such a device suitably includes a core having a cross-sectional dimension and comprising a bandgap material; an interlayer having a thickness and at least partially surmounting the core; a metallic shell that at least partially surmounts the interlayer, wherein a diameter of the core or a thickness of the interlayer corresponds to the plasmonic resonant cavity mode of the plasmonic nanowire; and a detector capable of detecting an emission from the plasmonic nanowire. A detector that can count photons (exiting the nanowire) is considered especially suitable.
The disclosed devices may be applied in a number of fields, e.g., silicon photonic-circuit components, silicon-based light emitting diodes (LEDs), ultrasensitive biodetectors. deep sub-wavelength optoelectronic devices, which may be modulated at very high frequencies due to the very short excited state lifetime.
Methods
Device Fabrication
Single-crystalline undoped silicon nanowires (Sigma-Aldrich) were dispersed in ethanol and transferred onto a 150 μm-thick glass substrate. A 5 nm SiO2 interlayer was deposited by atomic layer deposition (ALD) (Cambridge Nanotech) by alternating O3, 3-Aminopropyltriethoxysilane (APTES), and H2O pulses at a temperature of 150° C. A 100 nm Ag thin film was coated to form an Ω-shaped plasmonic cavity by using an e-beam evaporator at a low deposition rate of 0.2 Å/s for the first 50 nm and 0.5 Å/s for the remaining film. Ag bowtie structures were fabricated on Si substrates after etching the native oxide. Silicon substrate was covered with 5 nm SiO2 layer grown via ALD. After the ALD process, bowties were patterned by electron-beam lithography followed by the deposition of 30 nm-thick Ag by electron-beam evaporation.
Optical Measurements at Room Temperature
Silicon nanowires coupled with Ω-shaped plasmonic nanocavities were optically excited through the 150 μm-thick glass substrate using a home-built microscope equipped with a 60×, 0.7 NA objective (Nikon), having a spatial detection resolution of 500 nm. Planar Si with the bowtie structures was excited from the top side. A continuous wave argon-ion laser (Coherent) tuned at a wavelength of 457.9 nm was focused to pump the individual nanowires with the beam spot size of 2 μm on the sample with an excitation power of ˜250 kW/cm2. For UV excitation, a frequency doubled femtosecond pulsed Ti:Sapphire laser (Chameleon) was tuned at a central wavelength of 355.7 nm. Photoluminescence spectra were collected using a spectrometer (Acton-SP 500i) and a cooled charge coupled device (CCD) (Pixis 2K, Princeton Instruments) with a spectral resolution of 0.5 nm.
Numerical Calculation
Simulations were performed for the silicon nanowire coupled with the Ω-shaped plasmonic nanocavity structures with a pulsed point dipole source inside the cavity using a commercial finite-difference-time-domain (FDTD) software package (Lumerical). The eigenmodes, quality factors (Q), and field intensity profiles were analyzed by Fourier transforming the calculated time-domain data to the frequency-domain. By performing three-dimensional FDTD simulations, the Purcell factor, Γ/Γ0=3Q(λ/2n)3/2πVeff, was obtained by calculating the effective mode volume (Veff) where λ and n is the free space wavelength and refractive index respectively18. The effective mode volume of the plasmonic cavity modes can be expressed by28
where ∈(r) is the material dielectric constant. Edip was taken in the silicon medium at the position where a dipole emitter would experience the calculated Purcell factor and the electric field intensity is integrated over the entire mode structure. The frequency-dependent real and imaginary parts of the dielectric function of Ag were obtained via an analytical fit to experimental data29 and the real and imaginary parts of refractive indices of Si and SiO2 were taken from Palik, Handbook of Optical Constants of Solids (1998).
Quantum Efficiency Estimation
Optical power collection efficiency was estimated in the optical fiber coupled spectrometer with CCD, which was used for the photoluminescence measurements. By measuring a known laser power from the integrated counts with taking into account the quantum yield and the sensitivity of the CCD, the collection efficiency was estimated to be 1%. From the integrated photon counts of the photoluminescence spectrum from the sample, the photoluminescence power through the objective was measured to be 0.8 nW. Three-dimensional FDTD calculations were carried out for Si nanowire coupled with Ω-shaped cavity (d=70 nm, l=8 μm) to obtain an overall far-field out-coupling efficiency of 0.059% through the entire emission spectral range after considering the numerical aperture of objective, resulting in the actual power of 1.4 μW emitted from Si. In addition, the absorption efficiency of 1% for the same plasmonic silicon with Ω-shaped cavity was calculated at the laser wavelength of 457.9 nm with a 2 μm beam size (FWHM) using the FDTD technique to fully take into account possible absorption enhancement due to the antenna effect (
Discussion of Phonon-assisted Hot Luminescence Process (
The broad spectrum of hot luminescence originates from a large number of discrete phonon assisted events with many pathways interacting with various types of phonons on each crystallographic direction of the electronic dispersion of silicon. However, the existence of specific phonon modes with high density of states (zero slopes in dispersion) provides highly emissive pathways for the phonon assisted hot luminescence process, which due to the energy and momentum conservation, results in the observed three strong emission bands and additional fine peak structure.
These data, which compare resonant (˜70 nm Si diameter;
It should be noted that light emission is only possible when a phonon scatters the hot carriers to the almost vertical light line (k˜0). This phonon assisted scattering process should follow energy and momentum conservation, k′=ke±q≈0, where k′ and ke are the momentum values of hot carriers at the light line and at the initial electronic state respectively, and q is the phonon momentum. Thus, the emissive hot carrier population would depend on the modes with the highest phonon density of states where phonon dispersion has almost zero slope23, and those specific phonons can scatter the hot carrier efficiently to the light line. The phonon dispersion of silicon along the <110> direction shows that the density of states is relatively high for momentum values of ˜2π/a(0.6, 0.6, 0) for transverse optical (TO) and transverse acoustic (TA) and ˜2π/a(0.7, 0.7, 0) for longitudinal optical (LO) and TA phonons, situated between Γ and K points (
As revealed in the polarization-dependent measurements along with the simulations, the hot luminescence can be strongly enhanced when the specific emission channels with high density of states are resonant with the cavity plasmons. It is reasonable to believe that the sharp peaks originate from phonons with the highest density of states. From these spectral peak positions (labeled 1-5), the relaxation and emission processes involving carrier-phonon interactions can be inferred. The absorption process upon laser excitation involves interaction with a phonon, followed by the intra-band relaxation of hot carriers by emitting phonons. The intra-band energy relaxation process involves carrier scattering with phonons with small k; this can occur by either a 1-phonon process involving an optical phonon near the Brillouin zone center (Γ-point) with small k values, or a 2-phonon (acoustic) process with k values with opposite signs near the Brillouin zone boundaries. Because the density of states of transverse acoustic (TA) phonons near the zone boundary is much higher than that of longitudinal acoustic (LA) phonons in crystalline silicon as seen in their dispersion, intra-band relaxation process is expected to be dominated by 2-TA phonons.
In the first strong emission band (at ˜2.51 eV), only the two peaks (peaks 1 and 2) are predominant and no further strong peak at ˜32 meV lower than peak 2 is observed in the measurements. This can be explained by taking into account the narrow energy window for the phonon modes with the highest density of states, which correspond to the first strong emission band (
Although numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in details within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.
This application claims priority to U.S. application 61/562,685, “Emission in Nanowires Via Nanocavity Plasmons,” filed Nov. 22, 2011, the entirety of which is incorporated herein for any and all purposes.
This invention was made with government support under Grant No. W911NF0910477 awarded by the Army Research Office and under Grant No. DP2 OD007251-01 awarded by the National Institutes of Health. The government has certain rights in the invention.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/US2012/066184 | 11/21/2012 | WO | 00 | 5/21/2014 |
Number | Date | Country | |
---|---|---|---|
61562685 | Nov 2011 | US |