Abstract:
A luminescent system includes a layer of donor material, an acceptor material and a barrier layer therebetween. The energy transfer between the donor and acceptor is biased to the acceptor layer, by an asymmetric energy transfer efficiency created by the barrier layer. Energy from the donor material is converted into photo-luminescence in the acceptor layer by discouraging photo-luminescence quenching caused by energy backflow.

Description:
TECHNICAL FIELD 
     The present invention generally relates to luminescent light sources. 
     More particularly, the present invention relates to luminescent cables, films, as well as nanostructures such as luminescent nano-wires, nano-ribbons, nano-particles and so on. 
     BACKGROUND INFORMATION 
     Presently, erbium-doped silicon or silicon-related materials are used for their luminescence at the wavelength of about 1.5 μm. This luminescence is due to  4 I 13/2 -to- 4 I 15/2 intra-4-f-electron shell transition of erbium ion Er 3+  that can be excited both optically and electrically. 
     Accordingly, erbium-doped silicon is one of the most effective materials with which silicon-based light sources may be made. Such materials may be used as films, in optic fibres or nanowires. In particular, nanowires are useful in electronic and optical devices including integrated circuit, transistors, photodetectors, biochemical sensors etc. (Generally, a nanostructure has one or two aspects of its dimensions being in the order of hundreds of nanometers. Therefore, the radius of the luminescent nano-cable, nanowire and nano-particle is typically equal to or less than 500 nms, and the thickness of nano-ribbon and layered nano-film is equal to or less than 500 nm.) 
     However, the efficiency of erbium-doped silicon photonics is hampered by inherent problems causing the luminescence of erbium-doped silicon to be easily quenched at between 77 K to room temperature, e.g. about 300 K. The quenching mechanisms are mainly Auger and phonon-assisted de-excitations (via Dexter mechanisms). 
     Auger de-excitation is the process in which one electron of the excited species gets into a higher electronic state using energy from another electron from the same species, thereby releasing the excitation energy without giving off a photon, so-called a non-radiative de-excitation. Phonon-assisted de-excitation is also a non-radiative de-excitation process, in which the energy of the excited species is released via phonons, without giving off a photon. At a concentration in excess of 10 22  cm −3  in silicon or silica, erbium atoms tend to aggregate in clusters. This close proximity results in dipolar-dipolar interaction between the erbium ions, leading to a process in which an excited erbium ion de-excites non-radiatively by transferring energy to a neighboring excited erbium ion, promoting the neighboring erbium ion to an even higher excited state. This process is generally known as up-conversion, also a non-radiative de-excitation. 
     Quenching of erbium luminescence in erbium-doped silicon is primarily associated with energy backflow from excited erbium ions to free carriers in silicon via the three processes described above. The free carriers are, in turn, made more active by a higher temperature. As a result, erbium ion luminescence in an erbium-doped silicon is extremely weak at room temperature. 
     These mechanisms limit erbium excitation and are primarily responsible for thermal quenching of 1.5 μm luminescence of Er 3+  ions embedded in crystalline silicon. 
     Furthermore, it is not easy to dope silicon or silicon-related material with erbium due to solubility limitation. This may only be achieved to an insufficiently low erbium concentration. 
     Therefore, it is desirable to propose a luminescent silicon structure in which these problems are reduced, eliminated or minimized. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the invention comprises a luminescent system comprising a barrier between a donor material and an optically-active compound, wherein the barrier allows energy from the donor material to transfer to the layer of optically-active compound to excite the optically active compound into photo-luminescent emission, and the barrier restricts energy transfer from the optically-active compound to the donor material, such that quenching of the photo-luminescent emissions from the optically-active compound is reduced. 
     Preferably, the barrier layer is made of silica or a dielectric or insulating layer, in which the carriers are few, bound and not free, and unlike erbium in silicon, the erbium ions are less subjected to room- or high-temperature related de-excitation due to presence of the barrier layer. 
     In a second aspect, the invention comprises a method of preparing a luminescent system comprising the steps of heating silicon oxide (SiO) powder to provide silicon vapors, precipitating the silicon vapors to form silicon structures having a surface layer of silica, heating erbium metal to provide erbium vapors and precipitating the erbium vapors to form an erbium oxide layer on the silica surface of the silicon structures, the layer of silica being a barrier between the silicon structure and the erbium oxide layer, the barrier allowing energy from the silicon structure to transfer to the erbium oxide layer to excite the erbium oxide into photo-luminescent emission, and the barrier restricting energy transfer from the erbium oxide layer to the silicon structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the invention will now be described in a non-limitative manner with references to the following figures, in which like numerals refer to like parts, in which 
         FIG. 1   a  illustrates a first example of the invention in the form of a silicon nanowire; 
         FIG. 1   b  is a model of asymmetric energy transfer in the example of  FIG. 1   a;    
         FIGS. 2   a  and  2   b  illustrate one example of how silicon nanowires of the example of  FIG. 1   a  are made; 
         FIG. 3   a  is a Scanning Electron Microscope (SEM) image of the example of  FIG. 1   a;    
         FIG. 3   b  is a High-Resolution Transmission Electron Microscopic (HRTEM) image of the example of  FIG. 1   a;    
         FIG. 4   a  is another SEM image of the example of  FIG. 1   a;    
         FIG. 4   b  is a HRTEM image of an edge of the example of  FIG. 1   a;    
         FIG. 4   c  shows erbium mapping in the sample of  FIG. 4   b;    
         FIG. 4   d  shows silicon mapping in the sample of  FIG. 4   b;    
         FIG. 4   e  shows oxygen mapping in the sample of  FIG. 4   b;    
         FIG. 4   f  is another illustration of the example of  FIG. 1   a;    
         FIG. 5  shows the photo-luminescence spectrum of the example of  FIG. 1   a  at room temperature and at 1000 mW excitation power; 
         FIG. 6  shows photo-luminescence intensity as a function of excitation power at 11.3 0 K and 291 0 K, of the example of  FIG. 1   a;    
         FIG. 7(   a ) shows the photo-luminescence spectrum of the first example of  FIG. 1   a;    
         FIG. 7(   b ) shows the photo-luminescence spectrum of a second example of the invention; 
         FIG. 7(   c ) shows the photo-luminescence spectrum of a third example of the invention; 
         FIG. 8(   a ) shows another TEM image of the example of  FIG. 1   a ; and 
         FIG. 8(   b ) is a higher magnification of the TEM image of  FIG. 8(   a ). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates an embodiment which is a silicon nanowire comprising a silicon (Si) donor or host layer  101 , a silica (SiO 2 ) barrier layer  103  surrounding the silicon donor layer  101 , and an acceptor layer  105  of crystalline erbium oxide (Er 2 O 3 ) surrounding the silica barrier layer  103 . 
     The donor layer  101  is capable of absorbing light and transferring energy to an acceptor to cause the acceptor to emit photo-luminescence. 
     The acceptor layer  105  contains an optically active compound (an acceptor), which is strongly fluorescent and can be activated via energy from the donor layer  101  to photo-luminescence. The acceptor layer  105  is sometimes referred to as the ‘dopant’ layer since it is typical for the layer to be ‘doped’ with the optically active compound. However, in this embodiment, the acceptor layer is made of crystalline erbium oxide (Er 2 O 3 ). 
     The materials, and therefore the energy levels, of the donor layer  101  and the acceptor layer  105  are chosen such that excitation energy tends to be forward-transferred from the donor layer  101  to the acceptor layer  105  by the Förster mechanism. Förster mechanism is a system-dependent energy transfer mechanism which has an energy transfer range of about 10 nm between Si/SiO 2 . 
     Thus, when energy is transferred from the silicon to the erbium oxide  105 , the resulting Er 3+  ion excitation produces luminescence. 
     Furthermore, the donor layer  101  and the acceptor layer  105  are separated by a barrier layer  103  which is sufficiently thick to prevent energy backward transferred from the acceptor layer  105  to the donor layer  101  by the Dexter mechanism. In comparison to the Förster mechanism, the Dexter mechanism is a short-range mechanism having an energy transfer range of only 2 nm between silicon and silica. 
     In this embodiment, the silica barrier layer  103  has a thickness of 2-5 nm, which is thin enough to allow efficient Förster energy transfer from the silicon donor layer  101  to the erbium oxide acceptor layer  105 , but thick enough to prevent or suppress energy backflow or de-excitation from the erbium oxide acceptor layer  105  to the silicon donor layer  101  by Dexter mechanism. Thus, the silica barrier layer  103  between the silicon donor layer  101  and the erbium oxide acceptor layer  105  improves the photo-luminescence efficiency of the erbium oxide acceptor layer  105  by creating an asymmetry in energy transfer between the donor layer  101  and the acceptor layers  105  and biasing the energy transfer from the donor layer  101  to the acceptor layer  105 . This ensures that more of the excitation energy transferred to the erbium oxide is efficiently transformed into photo-luminescent emissions than quenched. 
       FIG. 1   b  illustrates this principle. The energy in the silicon donor layer  101  is transferred to the erbium ions in the erbium oxide layer  105  despite the barrier layer of silica  103  by Foster mechanism, as indicated by the curved arrow  111 . Typically, some of the energy in the consequently excited erbium ions may be quenched by flow of the energy back to the silicon donor layer  101 , as indicated by arrow  113 . However, the mechanism by which energy is transferred from the erbium ions to the silicon donor layer  101  is the Dexter mechanism, which is blocked by the silica barrier layer (hence the arrow  113  indicating this is dotted). Therefore, the silica barrier layer  103  favors energy transfer from the silicon donor layer  101  to the erbium oxide acceptor layer  105 , and restricts back-transfer of excitation energy from the erbium oxide acceptor layer  105  to the silicon donor layer  101 . 
     The photo-luminescence of the embodiment is further enhanced by the fact that Auger energy from excited erbium ions does not backflow as efficiently to bound carriers in the silica barrier layer  103  or in silicon layer  101 . Thus, the silica barrier layer  103  proximate the erbium ions is able to suppress short-range direct interaction of the excited erbium ions with the free carriers and thermal phonons in the underlying silicon donor layer  101 . In the absence of such a barrier layer  103 , strong thermal quenching or backward energy transfer readily occurs due to thermally-excited phonon- and Auger-assisted energy transfer, causing drastic photo-luminescence decrease at elevated temperatures. 
     It is preferable that the concentration of the acceptor atoms is high enough for maximum photo-luminescence but not too dense for aggregation to cause up-conversion losses. Crystalline erbium oxide  105  provides a favorably higher concentration of erbium ions than that obtainable by doping silicon with erbium. Furthermore, by using crystalline erbium oxide, the requirement and difficulty of having to dope silicon with erbium is removed. Experiments show that erbium oxide-clad silicon maintains strong erbium emission at 1.54 μm (˜0.8 eV) at room-temperature, indicating that there are high concentrations of erbium activated for luminescence. However, the oxygen coordination number of the body-centre-cubic (bcc) and fcc phases of crystalline erbium oxide are 8 and 12, respectively. For this reason, the oxygen is capable of screening neighboring erbium atoms from energy backflow, i.e. absorption of energy by neighboring ions leading to quenching of luminescence. This helps to reduce the probability of up-conversion. Furthermore, the 4f electrons of erbium are shielded by the outer 5s 2 5p 6  electrons from the full effects of the donor crystal field, thus the intra-4-f shell optical transitions are nearly independent of the donor materials. This means that the photo-luminescence or optical property of erbium is localized and independent of its surrounding, and thus stays almost constant in any chemical environment or compound. 
     Furthermore, it is preferable that there is a one-to-one or optimal acceptor-to-donor ratio at close enough proximity to ensure optimal donor-to-acceptor Förster energy transfer. Thus, the layer of crystalline erbium oxide near the silicon core provides an Er:Si ratio of about 1:1 at sufficiently close proximity. This leads to efficient excitation energy transfer from the silicon to the erbium oxide. Thus, photo-luminescence of crystalline erbium oxide is superior to the photo-luminescence of erbium-doped silicon. 
     Accordingly, an embodiment has been described which provides a luminescent system comprising a barrier (the silica layer  103 ) between a donor material (in the donor layer  101 ) and an optically-active compound (in the acceptor layer  103 ), wherein the barrier allows energy from the donor material to transfer to the layer of optically-active compound to excite the optically active compound into photo-luminescent emission, but the barrier restricts energy transfer from the optically-active compound to the donor material. In this way, such that quenching of the photo-luminescent emissions from the optically-active dopant is reduced. 
     The following is an example of the steps by which the above embodiment may be made using the oxide-assisted growth (OAG) method, as shown in  FIGS. 2   a  and  2   b.  
         1. Silicon oxide (SiO) powder  207  is placed on an alumina plate  201  (3 cm in length and 2 cm in width) and positioned at the centre of a horizontal alumina tube  203  mounted inside a high-temperature tube furnace (not shown).   2. The tube atmosphere is then evacuated and a carrier gas consisting of 95% Ar and 5% H 2  is introduced and maintained at a pressure of 500 Torr and a constant flow rate of 50 sccm.   3. The temperature at the alumina boat is kept at 1300° C. for 5 hours. Consequently, silicon nanowires  205  are formed at the tube wall or on the aluminum plate  201  downstream of the Ar/H 2  flow, where the temperature is lower at about 930° C., i.e. a temperature gradient exists across the alumina tube.       

     The silicon nanowires  205  obtained this way typically consist of a crystalline silicon core cladded by a silica layer, with atomically smooth sidewalls. These nanowires  205  have a diameter within the range of tens of nanometers and a length of up to millimeters. 
       FIGS. 3(   a ) and ( b ) show the silicon nanowires having an average diameter of about 20 nm, including a silica thickness of about 2 nm.
         1.  FIG. 2   b  shows that, subsequently, erbium metal  217  is placed on an alumina plate  211  of 2 cm in length and 1 cm wide, into the same furnace and positioned at the centre of the alumina tube  203 .   2. The atmosphere in the system is then evacuated again and the pressure is reduced to pressure of 10 −4  Torr. Argon flow is then provided through the alumina tube at 50 sccm (standard cubic centimeters per minute),   3. The erbium  217  source is then heated to 1150° C. and kept at this temperature for 2 hours.   4. The argon flow carries generated erbium vapors to be deposited and accumulated on the silicon nanowires  205  at the tube wall or on the alumina plate  201 .   5. The nanowires  205  are then annealed in the erbium vapors at 1100° C. for 5 minutes to form a layer of erbium oxide on the nanowires  215 .   6. The erbium coated nanowires  215  are then collected after cooling to room temperature.       
     The erbium oxide-coated silicon nanowires  215  obtained have an average diameter of about 40 nm, as shown in  FIGS. 4   a  to  4   f.    
       FIG. 4   a  shows the SEM image of Er 2 O 3 -coated OAG-grown Si nano-wires. 
       FIG. 4   b  is a High-Resolution Transmission Electron Microscopic (HRTEM) image of the edge of a nanowire obtained using the method, showing a continuous crystalline erbium oxide acceptor layer. The lattice spacing is about 0.258 nm, which is in good agreement with the spacing of the 200 plane (0.258 nm) of face-centre-cubic (fcc) Er 2 O 3 . The agreement in the spacing shows the Er 2 O 3  is in the crystalline form of face-centre-cubic (fcc) structure. The figure inset in  FIG. 4   b  is the corresponding low-magnification TEM image of a silicon nanowire, showing a diameter of about 30 nm and a light contrast shell of about 4 nm. 
       FIGS. 4   c ,  4   d , and  4   e  show respectively the element mapping of erbium, silicon, and oxygen in the nanowire, each corresponding to the respective parts in the HRTEM image of  FIG. 4   b . The element mapping indicates that almost no silicon is present in the erbium oxide layer, while erbium and oxygen are uniformly distributed over the entire length of the wire. Erbium is not detected inside the silicon nanowires despite the erbium-coating process, which is due to the slow diffusion rate of erbium in silicon and silica. 
       FIG. 4   f  illustrates the structure of silicon nanowires after erbium coating and modelled into a coaxial cable consisting of a crystalline silicon core and a crystalline erbium oxide shell or cladding, separated by a thin silica interlayer of a few nanometers. 
     Experiments to measure the photo-luminescence of the erbium oxide-clad silicon nanowires were performed using an argon laser, at the wavelength of 514.5 nm, for excitation with a spot size of 1 mm 2  on a sample. 
     The photo-luminescence spectra were recorded at both room temperature (291 0 K) and at a low temperature (11.3 0 K) and are shown in  FIG. 5 . A distinct broad emission band can be clearly observed in the spectral region of 1458-1500 nm, which can be primarily attributed to the formation of Er—O complexes. The photo-luminescence spectrum is dominated by the intense peak at 1537 nm with weak peaks at 1554 and 1599 nm; they are associated with the radiative  4 I 13/2 -to- ∝ I 15/2  transition arising from Er-related centres of cubic symmetry. 
     The power dependence of photo-luminescence intensity of the erbium oxide-clad silicon nanowires is illustrated in  FIG. 6 , showing that the photo-luminescence intensity increases almost linearly with increasing excitation power, and does not saturate even up to an excitation power of 2000 mW at both room-temperature (291 0 K) and low-temperature (11.3 0 K). 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Incident Power (mW) 
                 100 
                 200 
                 1000 
               
               
                   
                 PL Intensity Ratio (I RT /I LT ) 
                 0.73 
                 0.57 
                 0.47 
               
               
                   
                   
               
             
          
         
       
     
     Table 1 shows the ratio I 291 K /I 11.3 K  of the integrated photo-luminescent intensity at 291 0 K and 11.3 0 K, versus excitation power intensity. The values of I 291 K /I 11.3 K  are in the range of 0.41-0.73, showing only a slight decrease despite significant increase in excitation power. This indicates strong suppression of Auger and phonon-assisted de-excitations of the luminescence of erbium oxide-clad silicon nanowires, as the intensity of luminescence could not be proportional to the excitation power if there is significant de-excitation of the luminescence at different temperatures. 
     Furthermore, the temperature dependency of the integrated photo-luminescence intensity of the embodiment is over 10 times smaller or the I 291 K /I 11.3 K  ratio is over 10 times larger than that (ratio&lt;1%) reported previously for erbium-doped silicon materials. This result also indicates that temperature quenching of photo-luminescence is strongly suppressed. Consequently, the room-temperature photo-luminescence intensity of the embodiment remains nearly as strong as that of low-temperature photo-luminescence e.g. 11.3 0 K. 
     Advantageously, the described structure of donor layer  101 , barrier layer  103  and acceptor layer  105  facilitates independent tuning of each layer to meet specific requirements for an optimum photo-luminescence system. For example, the materials used in the three-layered nanostructures can be tailored to emit in any desirable wavelength. 
     Furthermore, the acceptor layer  105  can be advantageously designed to contain the most suitable or most optically active “acceptor” in optimum concentration. For example, the concentration of erbium in crystalline erbium oxide in the first embodiment exceeds the concentration of erbium limited by erbium solubility in silicon. 
     The thickness of the barrier layer  103  may also be advantageously pre-determined accordingly by the material used for the donor and acceptor layers  101  and  105 , respectively, to ensure asymmetric energy transfer and efficient donor-acceptor optical coupling. 
     Advantageously, embodiments may include luminescent nanostructures useable in photonics, optoelectronics and electronic devices, such as LEDs, laser diodes, waveguides or detectors. Furthermore, embodiments of other nanostructures are possible, for example, nano-ribbons (NRs), nano-particles (NPs), and also non-nano, normal size structures such as cables or films. 
     Instead of the described Si/SiO 2 /Er 2 O 3  configuration, embodiments using other materials are also possible, such as configurations of ZnS/SiO 2 /MnS (such as for nano-ribbons) and Y 2 O 3 /SiO 2 /Eu 2 O 3  (such as for nano-particles). 
       FIG. 7  shows the room-temperature photo-luminescence spectra of three nanostructure photo-luminescent systems, i.e. (a) Si/SiO 2 /Er 2 O 3  nano-wires, (b) ZnS/SiO 2 /MnS nano-ribbon; and (c) Y 2 O 3 /SiO 2 /Eu 2 O 3  nano-particles, yielding photo-luminescence in three distinctive wavelengths: 1.5 μm, 590 nm, and 611 nm respectively. 
     More specifically,  FIG. 7(   a ) shows the photo-luminescence spectrum of Si/SiO 2 /Er 2 O 3  nanowires, demonstrating a lasing peak at 1565 nm and a super linear relationship between photo-luminescence output and input power (inset), having a lasing threshold at 20 to 30 mW. 
       FIG. 7(   b ) illustrates the photo-luminescence spectrum of a second embodiment, which is nano-ribbons comprising layers of ZnS/SiO 2 /MnS  701  (Zinc Sulphide, Silicon dioxide and Manganese Sulphide). As illustrated, the nano-ribbons exhibit photo-luminescence fine structures (lasing modes) of nano-ribbon cavity originating mainly from two transitions, one at 579 nm and the other at 589 nm. The sharp and narrow (&lt;0.8 nm) lasing modes with a spacing of several nm occur along the ribbon axis. The photo-luminescence spectrum of nano-ribbons made of Mn-doped silicon  702  (second lowest line in the graph) and the photo-luminescence spectrum of nano-ribbons made of pure ZnS  703  (the bottom-most line) are superimposed for comparison. The graphs show that the photo-luminescence of both Mn-doped silicon nano-ribbons and pure ZnS nano-ribbons are weak and featureless when compared to that 701 of the ZnS/SiO2/MnS structure under the same measurement conditions. 
       FIG. 7(   c ) illustrates the photo-luminescence spectrum of a third embodiment, which is nano-particles comprising layers of Yttrium (III) Oxide, silica and Europium Oxide, i.e. Y 2 O 3 /SiO 2 /Eu 2 O 3 . The spectrum of  FIG. 7(   c ) shows nano-particles having Eu emissions due to  5 D 0 -to- 7 F j (j=0, 1, 2, 3) intra-4-f-electron shell transitions of Eu 3+ . The intense peak at 611 nm ( 5 D 0 -to- 7 F 2 ) is much stronger than that of the conventionally used yttrium-based photo-luminescence configuration (not provided), i.e. Y 2 O 3  layered with Eu-phosphor-doped silicon. The intense peak is attributed to both the optimal photo-luminescence configuration and random resonance effect, i.e. coherent recurrent light scattering among nano-particles in a random medium which further enhances photo-luminescence.  FIG. 8(   a ) is a corresponding TEM image of Y 2 O 3 /SiO 2 /Eu 2 O 3  nano-particles.  FIG. 8(   b ) is a higher magnification of the TEM image, showing the Y 2 O 3 /SiO 2 /Eu 2 O 3  nano-particles to have an average diameter of about 50 nm. 
     In the case of each of the above embodiments, the excitation was performed in a wavelength non-resonant with the acceptor, so that the energy was absorbed by the donor atoms/ions in the donor layer  101 , and transferred to the acceptor atoms/ions in the acceptor layer  105 . All spectra were obtained at room temperature proving that thermal quenching due to phonon-assisted and Auger-decay backflow was small, i.e. the barrier layer  103  was sufficiently thick for asymmetric energy transfer. 
     While several aspects of the present invention have been described and depicted herein, alternative aspects may be effected by those skilled in the art to accomplish the same objectives. Accordingly, it is intended by the appended claims to cover all such alternative aspects as fall within the true spirit and scope of the invention. 
     For example, although the described embodiments use SiO 2  as the barrier layer material, it is possible to use other material capable of providing the energy transfer asymmetry between the donor layer and the acceptor layer, such as Si 3 N 4 , Al 2 O 3 , SiO x N y  and those materials having no or little free carriers. Moreover, although the embodiments use a crystalline form of a dopant material, e.g. Er 2 O 3 , as the acceptor layer  105 , it is nevertheless possible to have an embodiment that uses the conventional Er-doped silicon as the acceptor layer  105  but the performance of which is improved by the asymmetric energy transfer provided by the silica barrier layer  103 . Furthermore, alternative production methods other than the afore-described oxide-assisted growth (OAG), such as sol-gel processing or a combination of such processes, is useable to coat a silicon/silica core.