WO2010071137A1

WO2010071137A1 - Optical reading method

Info

Publication number: WO2010071137A1
Application number: PCT/JP2009/070920
Authority: WO
Inventors: 幹育中西
Original assignee: メトロ電気株式会社
Priority date: 2008-12-16
Filing date: 2009-12-15
Publication date: 2010-06-24
Also published as: JP5339381B2; JPWO2010071137A1; CN102257508B; CN102257508A; HK1162722A1

Abstract

Disclosed is an optical reading method with which can be realized a system that compensates for handling of an information pattern not recognizable to the naked eye, wherein a nonvisible information pattern such as a bar code or a QR code is printed on an information recording medium or an object to be managed, and that provides high security or guaranteed product quality. When an information recording medium (11) having an information pattern (12) is placed in or transported to a prescribed position, the information pattern (12) is irradiated with excitation light (H1), which is emitted from a light-emitting diode (13) and has higher energy than the band gap of a semiconductor quantum dot, and the semiconductor quantum dot is thermally expanded. Then, light is emitted from the thermally-expanded quantum dot with a wavelength which transitions to the long-wavelength side compared to the wavelength emitted from the quantum dot prior to thermal expansion, this light beam is received by an optical sensor (14), which has a sensitivity to the longer wavelength portion of light which is emitted and transitions to the long-wavelength side, and the information pattern (12) is read by an information judgment unit (15).

Description

Optical reading method

The present invention relates to an optical reading method for reading an information pattern such as a bar code or a QR code printed on an information recording medium or an object to be managed.

In recent years, management and authentication checks have been performed by printing information patterns such as barcodes or QR codes for management of various articles, various prepaid cards or pass cards, and reading them using an optical reader. Various security systems have been proposed in which these information patterns are subjected to anti-counterfeiting means and read by an optical reading system to determine whether or not the information pattern is forged.

For example, a non-visible information pattern such as a barcode is printed with non-visible ink containing a phosphor, the semiconductor pattern is irradiated with a semiconductor laser to excite the phosphor, and the fluorescence emitted from the phosphor is received and non-printed. A security system for reading information from a visible information pattern with an optical reader has been implemented. In this case, the information pattern is invisible and not noticed except at the time of reading. At the time of reading, the phosphor of the information pattern shifts the wavelength of the emission spectrum to the longer wavelength side with respect to the wavelength of the emission spectrum of the semiconductor laser diode. Since the emission spectrum of the semiconductor laser diode has a narrow wavelength width, the wavelength peaks of the emission spectrum of the semiconductor laser diode and the emission spectrum of the phosphor are separated, and only the emission spectrum of the phosphor can be read. For this reason, when a special optical filter is used in front of the light receiving element, it is possible to receive only the fluorescence with the light receiving element and read the information of the phosphor mark while irradiating the phosphor mark (information pattern) with a laser. In this security system, the phosphor mark is difficult to be counterfeited and the technology for reading the phosphor mark is difficult to counterfeit, so that security can be provided.

However, since the above-described security system uses a semiconductor laser diode as the light source, the light source driving circuit of the optical reader is complicated and large, and has a disadvantage of high cost.

Therefore, when a small and low-priced optical reader is required, it is considered to use a normal light-emitting diode as a light source, which is less expensive than a semiconductor laser diode and has a small drive circuit scale.

However, when irradiating the phosphor mark (information pattern) with the spectrum of excitation light emitted from the light emitting diode instead of the laser and receiving the fluorescence by the light receiving element and reading the information of the phosphor mark, the light is emitted unlike the laser. Since the spectrum of the excitation light emitted from the diode is wide, it partially overlaps the emission spectrum of the phosphor. For this reason, it is difficult to separate both lights by the optical filter, and there is a possibility that an erroneous determination occurs in the optical reader.

In order to avoid such overlapping of emission spectra, it is necessary to select the type of phosphor and light emitting diode to be used. However, this has the disadvantage that the selection range of the light source is limited.

Therefore, an optical reading system that can be reduced in size and is inexpensive has been proposed (Patent Document 1). This optical reading system optically reads a printed layer printed with invisible ink, and the phosphor is made of an inorganic oxide to which neodymium is added as an activation element. This eliminates the overlap between the emission center wavelength of the light emitting element that excites the phosphor and the wavelength region where the light receiving element that receives fluorescence from the print layer can receive light, thereby eliminating the possibility of erroneous determination.

In Patent Document 2, in an optical data carrier containing metal particles, an increase in the volume of the data carrier resulting from thermal expansion as a result of absorption of radiation causes a shift of the absorbed light to a longer wavelength. It is disclosed.

Japanese Patent Laid-Open No. 6-274677 JP 2008-512807 A

However, when the inventor optically reads and tests the information of the printed layer (information pattern) using the phosphor invisible ink of Patent Document 1, it often occurs that reading is impossible. This is thought to be because, in the technique of Patent Document 1, when reading information using invisible ink, the wavelength range of light emission is separated from the excitation light, and strong light emission cannot be obtained.

Therefore, the present inventor can adopt light from a light emitting diode as excitation light as in Patent Document 1, can be miniaturized, and can be an inexpensive optical reading system, and is a method different from Patent Document 1. The inventors have eagerly searched for a printing layer and an optical reading method that can receive light emitted from a printing layer (information pattern) excited by excitation light and can avoid receiving excitation light.

Furthermore, the present inventor pays attention to the shift of the wavelength to the longer wavelength side due to thermal expansion in Patent Document 2, and uses it for avoiding the reception of excitation light at the time of reading. Stacked.

The present invention has been devised in view of the above circumstances. The purpose of the present invention is to print an invisible information pattern such as a barcode or QR code on an information recording medium or an object to be managed, and visually check the information pattern. It is extremely difficult to maliciously develop an optical reader that can compensate for unrecognized handling and can read information pattern information even if it is noticed, and can realize a high security system or provide high quality assurance. An object of the present invention is to provide an optical reading method capable of realizing visible traceability.

In order to solve the above-mentioned problems, the optical reading method of the present invention prints an invisible information pattern with an invisible ink containing core / shell type semiconductor quantum dots on an information recording medium or an object to be managed. The semiconductor quantum dot is irradiated with an excitation beam having energy higher than the band gap of the semiconductor quantum dot, and the semiconductor quantum dot is thermally expanded or contracted, and the semiconductor quantum dot that has been thermally expanded or contracted is shifted to a longer wavelength side. To emit light having a wavelength that shifts to the short wavelength side or to the short wavelength side, and receives the light by a photosensor having sensitivity to the long wavelength side portion or the short wavelength side portion of the shifted wavelength and reads the information pattern It is characterized by this.

According to this configuration, an invisible information pattern is printed on an information recording medium (or an object to be managed), and when read, the core / shell type semiconductor quantum dots are excited by thermal expansion or contraction, and light is emitted at that time. The light having a wavelength that shifts to the long wavelength side or the short wavelength side is received and information is decoded. For this reason, the presence of an invisible information pattern is not noticed except during reading. At the time of reading, the core / shell type semiconductor quantum dot emits strong light, the wavelength of the light emission changes due to thermal expansion or contraction, and the information is decoded by using an optical reader capable of decoding the wavelength. .

If necessary, it is effective to thermally expand or contract the semiconductor quantum dots by applying heat rays, high heat, or cold to the semiconductor quantum dots.

Whether the core / shell type semiconductor quantum dot is (1) CdSe / ZnS, or (2) a core obtained by adding Te to ZnSe nanoparticles, and a shell having ZnS nanoparticles (3 ) Whether ZnS nanoparticles containing Mn ions are used as the core, (4) ZnS nanoparticles are used as the core, or (5) Zn-In-Ag-S based semiconductor nanoparticles are used as the core. Or (6) those having silicon nanoparticles as the core.

According to the above configuration, an invisible information pattern can be printed on an information recording medium or an object to be managed, and when reading, the emission wavelength region can be changed and strong light emission can be realized. The information reading of the information pattern can be reliably decoded by an optical reading device that can compensate for handling that cannot be recognized by the naked eye and can decode the shifted wavelength. In addition to being able to detect with high sensitivity using ordinary light receiving elements such as silicon photodiodes, it is difficult to imitate the reader, so it is possible to realize an optical reading method that can realize a system that gives high security or quality assurance, credit card, Prevents counterfeiting, alteration, and falsification of cash cards, telephone cards, ID cards, student ID cards, stamp cards, point cards, etc., and enables system miniaturization, space saving, or invisible traceability. it can.

FIG. 1 is a conceptual diagram for explaining the optical reading method of the first embodiment. FIG. 2 is a graph showing the relationship between the wavelength and intensity of light emitted by exciting semiconductor quantum dots at room temperature. FIG. 3 is a graph showing the relationship between the wavelength and intensity of light emitted by exciting a semiconductor quantum dot by thermal expansion. FIG. 4 is a graph showing the relationship between the wavelength and intensity of light emitted by exciting a semiconductor quantum dot by thermal contraction, according to the optical reading method of the second embodiment.

DESCRIPTION OF SYMBOLS 10 Information recording card 11 Information recording medium 12 Information pattern 13 Light emitting diode 14 Optical sensor 15 Information determination part

Hereinafter, embodiments of the optical reading method of the present invention will be described with reference to the drawings.

[First Embodiment]
First, a configuration for carrying out the optical reading method will be described. FIG. 1 is a conceptual diagram for explaining the optical reading method of this embodiment. In FIG. 1, an information recording card 10 includes an information recording medium (or may be a management target) 11 and an information pattern 12 such as an invisible bar code or QR code printed on the information recording medium 11. .

The information recording medium 11 is, for example, a card body, and is composed of a vinyl chloride sheet or the like in which a white pigment such as titanium oxide is dispersed and held, and has a property of reflecting infrared rays, visible rays, and ultraviolet rays. As the information recording medium 11, for example, a cash card, various prepaid cards, a telephone card, a traffic card, a health insurance card, or the like is applied.

The information pattern 12 is a non-visible information pattern printed with an invisible ink containing core / shell type semiconductor quantum dots on an information recording medium or a management object.

”Here, invisible means that it is almost impossible to see with eyes, and includes invisible things that cannot be seen with eyes. The invisible ink and the information pattern 12 are substantially transparent, difficult to reflect visible light, and the ground color of the part forming the information pattern 12, for example, substantially the same color as the information recording medium 11 and so on.

The core / shell type refers to a type in which a core made of an inner material is covered with a shell made of an outer material, and a quantum dot is a nanometer-sized fine particle such as a semiconductor. In this embodiment, a CdSe / ZnSe core / shell type semiconductor quantum dot is used. When a core / shell type semiconductor quantum dot of CdSe / ZnS is used, it can be dispersed well with toluene or the like and aggregation does not occur, it can be used as an invisible ink, and an information pattern can be printed well.

The information pattern 12 is printed as a non-visible information pattern 12 with a non-visible ink containing a transparent binder that scatters and holds the semiconductor quantum dots, and is hidden by a masking film (not shown). Is done.

The optical reading method of this embodiment includes a light emitting diode 13 that is an inexpensive electronic component that emits excitation light H1, an optical sensor 14 that is an inexpensive electronic component that receives light H2 emitted by excitation, and an information determination unit 15. And an optical reader (no symbol). The light emitting diode 13 is provided so as to irradiate the security information with the excitation light H1, and the optical sensor 14 receives the light beam H2 that is emitted when the core / shell type semiconductor quantum dots included in the information pattern 12 are excited. Is provided. In order to read information from the information pattern 12, the information recording card 10 having the information pattern 12 is relative to the light emitting diode 13 and the optical sensor 14 while the positional relationship between the light emitting diode 13 and the optical sensor 14 is fixed. The scanning movement needs to be performed linearly, and this scanning movement may be either by a mechanical mechanism provided in the optical reader or by manual operation.

The light emitting diode 13 emits excitation light H1 having energy higher than the band gap of the core / shell type semiconductor quantum dots included in the information pattern 12. The excitation light H1 excites the core / shell type semiconductor quantum dots included in the information pattern 12 by irradiation, and heats the information pattern 12 to cause thermal expansion of the semiconductor quantum dots. For this reason, it is configured such that the excitation light H1 of the light emitting diode 13 is condensed through a condensing lens as necessary and irradiated to the information pattern 12. As the light emitting diode 13, a high output one is selected, or a plurality of light emitting diodes are used for irradiation, or a light emitting diode that emits infrared light and a light emitting diode that emits ultraviolet light are used in combination.

As the optical sensor 14, for example, a photodiode can be used. This optical sensor 14 needs to be sensitive to the wavelength of the long wavelength side portion of the light emitted by shifting to the long wavelength side in a state where the information pattern 12 including the core / shell type semiconductor quantum dots is thermally expanded. There is.
Specifically, as shown in FIG. 2, when the CdSe / ZnSe core / shell type semiconductor quantum dots are excited without being thermally expanded, they emit a plurality of visible rays having different wavelengths for different particle sizes. Emits light. On the other hand, as shown in FIG. 3, when excited in a thermally expanded state, a plurality of visible rays having different wavelengths are emitted every time the particle diameter greatly shifted to the long wavelength side is different. Therefore, in FIG. 3, the optical sensor 14 having sensitivity in the wavelength region indicated by the range Y deviating from the wavelength region shown in FIG. 2 to the longer wavelength side is used. The same relationship is assumed when the core particles of the core / shell type semiconductor quantum dots have a single particle diameter.
Here, the temperature before the information pattern 12 is heated and the temperature after the heating can be measured by a radiation temperature sensor directed to the information pattern 12, and the temperature before the information pattern 12 is heated (for example, 15 ° C. to 30 ° C.). The time until the temperature after heating (for example, 60 ° C. to 85 ° C.) can be measured, and the optical sensor 14 is the length of light emitted by the optical sensor 14 when the time required for the temperature rise has elapsed. It is assumed that the wavelength of the wavelength side portion can be received.

The information determination unit 15 is configured to amplify the electrical signal (rectangular signal) received by the optical sensor 14 and optically read the code information of the information pattern 12.

In the optical reading method of this embodiment, when the information recording medium 11 having the information pattern 12 is placed or sent in a predetermined position, the light-emitting diode 13 emits light, and the band gap of the semiconductor quantum dots with respect to the information pattern 12 Excitation light H1 having higher energy than that is irradiated. The excitation light H1 excites the semiconductor quantum dots and heats the information pattern 12 to thermally expand the semiconductor quantum dots. The thermally expanded semiconductor quantum dots emit light having a wavelength that shifts to the longer wavelength side shown in FIG. 3 as compared to the wavelength emitted from the semiconductor quantum dots when not thermally expanded (FIG. 2). CdSe / ZnSe core / shell type semiconductor quantum dots have particularly strong light emission. This light emission is received by the optical sensor 14 having sensitivity to the long wavelength side portion Y. The information determination unit 15 patterns the received signal to read the information pattern 12, and further determines whether the information pattern 12 matches the data recorded in the database. Judge authenticity.

According to this embodiment, the information pattern 12 can be excited by light emission of a normal inexpensive light emitting diode 13 and strong light emission can be realized from the semiconductor quantum dots (quantum size effect). It can be detected with high sensitivity by a light receiving element such as an inexpensive silicon photodiode.

A non-visible information pattern such as a barcode or QR code is printed on an information recording medium or management target to compensate for handling that the information pattern is not recognized by the naked eye. Even if the organization notices the presence of an information pattern due to the minute rise of the printed film on the surface of the information recording medium, it is difficult to imitate the ink, and the light source of the information pattern is a core / shell type semiconductor quantum dot. Even if an optical reader is developed that can analyze information by receiving light emitted by normal excitation of the semiconductor quantum dots based on that analysis, the information pattern can be decoded depending on the optical reader. Absent.

Since the optical reader is configured to decode information using an optical sensor that is sensitive to light emission that has shifted to the long wavelength region, it is difficult to imitate the optical reader and an optical reading method with high security can be realized.

Light that is sensitive to the long wavelength side portion of the light that excites the core / shell type semiconductor quantum dots, which are the light source of the information pattern, with thermal expansion and transitions to the long wavelength side at that time. Since the sensor receives the emitted light and decodes the information, the information can be decoded for the first time by using a special optical reader, and a system that can provide higher security or quality assurance can be realized.

As a modification of this embodiment, a heating means for thermally expanding the semiconductor quantum dots may be provided. For example, an electric heater for applying high heat to the information recording card and thermally expanding the semiconductor quantum dots of the information pattern 12 is provided as a heating means on the lower side of the information recording card 10 and on the back side on which the information pattern 12 is printed. Also good. Moreover, a means for irradiating the information pattern 12 with heat rays (infrared rays or the like) may be provided as a heating means.

[Second Embodiment]
In this embodiment, the semiconductor quantum dots are thermally contracted, and the semiconductor quantum dots subjected to the thermal contraction are allowed to emit light having a wavelength that transitions to the short wavelength side, and the short wavelength side portion in the transitioned wavelengths In this configuration, the light emission is received by an optical sensor having sensitivity and the information pattern is read. In addition, about the part which overlaps with 1st Embodiment, the same code | symbol is attached | subjected and description is abbreviate | omitted.

In this embodiment, a cooling means (not shown) is provided that thermally cools the semiconductor quantum dots by applying cold heat to the semiconductor quantum dots. Here, the cooling means blows air of 0 ° C. to −10 ° C. on the information pattern 12 to cool the information pattern 12 to around 0 ° C., and heat shrinks the semiconductor quantum dots of the information pattern 12. . Note that means for spraying cooler air or cooler gas may be provided as long as the information pattern 12 and the information recording card 11 are not damaged. As the optical sensor, one having sensitivity in the wavelength region indicated by Ya shown in FIG. 4, which is a range deviating from the wavelength region shown in FIG. 2 to the short wavelength side, is used.

In this embodiment, when the information recording medium 10 having the information pattern 12 is placed or sent in a predetermined position, the cooling means cools the information pattern 12 and heat shrinks the semiconductor quantum dots. Then, the light emitting diode 13 emits light, and the information pattern 12 is irradiated with excitation light H1 having energy higher than the band gap of the semiconductor quantum dots. The heat-shrinkable semiconductor quantum dots emit light having a wavelength that shifts to the short wavelength side shown in FIG. 4 compared to the wavelength emitted from the semiconductor quantum dots when the heat-shrinkable semiconductor quantum dots do not shrink (FIG. 2). This light emission is received by an optical sensor having sensitivity to the long wavelength side portion Ya. The information determination unit 15 patterns the received signal to read the information pattern 12, and further determines whether the information pattern 12 matches the data recorded in the database. Judge authenticity.

According to this embodiment, the core / shell type semiconductor quantum dot that is the light emission source of the information pattern 12 is excited so as to be accompanied by thermal contraction, and at that time, the short wavelength in the light emitted by transitioning to the short wavelength side is emitted. Since the optical sensor with sensitivity on the wavelength side receives the emitted light and decodes the information, the information can be decoded for the first time by using a special optical reader, and the security or quality assurance is much higher. Can be realized.

[Other Embodiments]
The present invention is not limited to the above-described embodiment, but includes variously modified forms within the technical scope without departing from the gist of the invention described in the claims. In the above embodiment, the CdSe / ZnSe core / shell type semiconductor quantum dots are used. However, security information may be printed by the following invisible ink using the core / shell type semiconductor quantum dots. .

(1) A core / shell type semiconductor quantum dot having a core obtained by adding Te to ZnSe nanoparticles and using a ZnS nanoparticle as a shell may be used.
(2) The core / shell type semiconductor quantum dots may be core / shell type semiconductor quantum dots having ZnS nanoparticles containing Mn ions as a core.
(3) A core which is a ZnS nanoparticle (Zn _(1-2x) In _x Ag _x S) doped with In ³⁺ and Ag ⁺ by thermally decomposing a thiol complex containing Zn ²⁺ , In ³⁺ and Ag ⁺ / Shell type semiconductor quantum dots may be used.
(4) A core / shell type semiconductor quantum dot whose core is a ZnS nanoparticle containing Mn ions may be used.
(5) A core / shell type semiconductor quantum dot having Zn—In—Ag—S based semiconductor nanoparticles as a core may also be used.

(6) The coefficient of thermal expansion of erbium is 7.6 × 10 ⁻⁶ / ° C. at 20 ° C., which is 2.5 × 10 ⁻⁶ / ° C. at 20 ° C. It is about 3 times. For this reason, it is most preferable to use erbium in order to realize a wavelength shift due to expansion. In this case, a core / shell type semiconductor quantum dot in combination with silicon nanoparticles is preferable.
This includes two types: silicon nanoparticles as the core and erbium nanoparticles as the shell, and erbium as the core and silicon nanoparticles as the shell.

When silicon nanoparticles are used as the core and erbium nanoparticles are used as the shell, the silicon nanoparticles are preferably 1.9 nm to 4.3 nm, and the erbium nanoparticles are preferably 1.9 nm to 4.3 nm.

For semiconductor quantum dots having silicon nanoparticles as the core and erbium nanoparticles as the shell, the semiconductor quantum dots are dispersed in water and the water temperature is increased in the range of 22 ° C. to 90 ° C., and then excitation light having a wavelength of 325 nm is used. It has been confirmed that light emission with a wavelength of 720 to 740 nm can be obtained from semiconductor quantum dots.

When erbium nanoparticles are used as the core and silicon nanoparticles are used as the shell, the erbium nanoparticles should be 1.9 nm to 4.3 nm, and the silicon nanoparticles should be 1.9 nm to 4.3 nm. preferable.

For semiconductor quantum dots having erbium nanoparticles as the core and silicon nanoparticles as the shell, the semiconductor quantum dots are dispersed in water and the water temperature is increased in the range of 22 ° C. to 90 ° C., and then excitation at a wavelength of 325 nm is performed. It has been confirmed that strong light emission with a wavelength of 720 to 740 nm can be obtained from semiconductor quantum dots when irradiated with light.

Further, when silicon nanoparticles having oxygen doped on the surface, or silicon oxide nanoparticles whose inside is oxidized are used as a core or shell, and a core / shell type semiconductor quantum dot using erbium as a shell or core, It is preferable because it emits stronger light.

Furthermore, in the core / shell type semiconductor quantum dot having silicon nanoparticles as the core, a plurality of hydrocarbon groups are bonded to respective Si atoms in the silicon nanoparticles, and the surface of the Si atoms is covered with the hydrocarbon groups. A thing may be used. This semiconductor quantum dot is prevented from lowering the emission wavelength and emission efficiency, and can emit visible light by ultraviolet excitation. Further preferably it is possible to adjust the particle size by the reaction conditions of the _Mg 2 Si and SiCl ₄ (silicon tetrachloride).

The present invention can be used not only for authenticating various cards but also for reading invisible information of traceability for tracking product items. Note that the present invention does not exclude using a semiconductor laser diode as a light source of excitation light. In addition, a visible information pattern such as a barcode or QR code is printed with normal visible ink on top of the invisible information pattern or arranged in the vicinity of one side, and the optical reading device optically converts the invisible information pattern. The present invention includes not only reading but also optical reading of a visible information pattern.

The present invention can prevent counterfeiting, alteration, and falsification of cards and certificates, and can reduce the size and space of the system, or can realize invisible traceability, and can be used in fields that require information retention. It can contribute widely.

Claims

An invisible information pattern is printed on an information recording medium or an object to be managed by an invisible ink containing core / shell type semiconductor quantum dots, and excitation light having energy higher than the band gap of the semiconductor quantum dots is printed on the information pattern. The semiconductor quantum dots are thermally expanded or contracted, and the semiconductor quantum dots that have been thermally expanded or contracted emit light having a wavelength that transitions to the long wavelength side or a wavelength that transitions to the short wavelength side. An optical sensor having sensitivity to a long wavelength side portion or a short wavelength side portion of the shifted wavelength, and reading the information pattern by reading the information pattern.
2. The optical reading method according to claim 1, wherein the semiconductor quantum dots are thermally expanded or contracted by applying heat rays, high heat or cold to the semiconductor quantum dots.
3. The optical reading method according to claim 1, wherein the core / shell type semiconductor quantum dots are CdSe / ZnS.
3. The optical reading method according to claim 1, wherein the core / shell type semiconductor quantum dots are obtained by adding Te to ZnSe nanoparticles as a core, and using ZnS nanoparticles as a shell.
3. The optical reading method according to claim 1, wherein the core / shell type semiconductor quantum dots have ZnS nanoparticles containing Mn ions as a core.
3. The optical reading method according to claim 1, wherein the core / shell type semiconductor quantum dots have ZnS nanoparticles as a core.
3. The optical reading method according to claim 1, wherein the core / shell type semiconductor quantum dots have Zn—In—Ag—S based semiconductor nanoparticles as a core.
3. The optical reading method according to claim 1 or 2, wherein the core / shell type semiconductor quantum dots have silicon nanoparticles as a core.
The optical reading method according to claim 8, wherein the core / shell type semiconductor quantum dots have erbium as a shell.
3. The optical reading method according to claim 1, wherein the core / shell type semiconductor quantum dots have erbium as a core and silicon nanoparticles as a shell.