Nanoparticle and light-emitting device

Abstract

A nanoparticle (1) is specified. The nanoparticle (1) comprises a core (2) comprising a first, non-absorbing semiconductor material, a first shell (3) comprising a second, absorbing and emitting semiconductor material, and a second shell (4) comprising a third, non-absorbing semiconductor material. Furthermore, a light-emitting device (10), in particular a micro-LED, is specified.

Description

[0001] Description

[0002] NANOPARTICLE AND LIGHT-EMITTING DEVICE

[0003] A nanoparticle and a light-emitting device are specified.

[0004] It is an object to provide a nanoparticle having an improved efficiency. Furthermore, a light-emitting device with an improved efficiency shall be provided.

[0005] According to at least one embodiment, a nanoparticle is specified. Here and in the following, a nanoparticle is a particle having a size in the nanometer range. In particular, the nanoparticle has a diameter of between and including 1 nanometer and 100 nanometers, in particular between and including 1 nanometer and 50 nanometers. The nanoparticle is, for example, a discrete particle. For instance, the nanoparticle is crystalline. In other words, the nanoparticle can be composed of crystalline materials. The nanoparticle can be a quantum dot.

[0006] According to at least one embodiment, the nanoparticle converts electromagnetic radiation of a first wavelength range into electromagnetic radiation of a second wavelength range. In other words, the nanoparticle has wavelengthconverting properties. For example, the electromagnetic radiation of a first wavelength range comprises wavelengths corresponding to a higher energy than the wavelengths of the electromagnetic radiation of the second wavelength range. For example, the first wavelength range comprises wavelengths in the ultraviolet to blue region of the electromagnetic spectrum. The second wavelength range can comprise wavelengths in the green to red region of the electromagnetic spectrum .

[0007] According to at least one embodiment , the nanoparticle comprises a core . The core may also be referred to as an endoshell . In particular, the core comprises or consists of a first , non-absorbing semiconductor material . Here and in the following, "non-absorbing" means that no or almost no electromagnetic radiation of the first wavelength range is absorbed by the first semiconductor material . The first , nonabsorbing semiconductor material can be crystalline . For instance , the core has a diameter of between and including 0 . 2 nanometers and 25 nanometers , in particular between and including 2 nanometers and 12 nanometers , for example between and including 8 nanometer and 12 nanometers .

[0008] According to at least one embodiment , the nanoparticle comprises a first shell . For example , the first shell is arranged around the core . The first shell can be in direct mechanical contact with the core . In particular, the first shell at least partially, for example completely surrounds the core .

[0009] In particular, the first shell comprises a second, absorbing and emitting semiconductor material . For example , the second semiconductor material is di f ferent from the first semiconductor material . The second semiconductor material can be a narrow-bandgap material .

[0010] Here and in the following, "absorbing" means that a material is configured to absorb the electromagnetic radiation of the first wavelength range . In other words , the absorbing material can take up the energy of the electromagnetic radiation of the first wavelength range . Here and in the following, "emitting" means that a material is configured to emit the electromagnetic radiation of the second wavelength range . In other words , the emitting material releases the electromagnetic radiation of the second wavelength range .

[0011] According to at least one embodiment , the nanoparticle comprises a second shell . In particular, the second shell is arranged around the first shell . In other words , the first shell can be arranged between the core and the second shell . The second shell is , for example , arranged in direct mechanical contact with the first shell . In particular, the second shell at least partially, for example completely surrounds the first shell and / or the core .

[0012] In particular, the second shell comprises a third, nonabsorbing semiconductor material . For example , the second semiconductor material is di f ferent from the third semiconductor material . A thickness of the second shell is , for instance , between and including 0 . 2 nanometers and 15 nanometers .

[0013] According to at least one embodiment , the nanoparticle comprises the core comprising the first , non-absorbing semiconductor material , the first shell comprising the second, absorbing and emitting semiconductor material , and the second shell comprising the third, non-absorbing semiconductor material .

[0014] Advantageously, the nanoparticle with the first shell comprising the second, absorbing and emitting semiconductor material has a large a volume of emitting material compared to other nanoparticles . Furthermore , due to the first shell comprising the second, absorbing and emitting semiconductor material , a level of absorption is much higher compared to a nanoparticle comprising the absorbing material in the core . Thus , the presently described nanoparticle has an increased ef ficiency compared to other nanoparticles . At the same time is not obvious that putting non-absorbing material into the center of a nanoparticle will allow for a more absorbing nanoparticle .

[0015] In particular, the nanoparticle or a plurality of the nanoparticles can be used as a conversion material in a conversion element of a light-emitting device . For example , a conversion element with a low thickness and high absorption properties can be obtained with the nanoparticles described herein .

[0016] According to at least one embodiment of the nanoparticle , a thickness of the second shell is greater than a thickness of the first shell . Such a thickness ratio advantageously ensures that the first shell is ef ficiently protected from environmental influences .

[0017] According to at least one embodiment of the nanoparticle , the first semiconductor material and the third semiconductor material have a wider bandgap than the second semiconductor material . In other words , the nanoparticle is an inverse type I quantum dot .

[0018] In particular, the second semiconductor material is surrounded by semiconductor materials with a wider bandgap . Such a configuration of the semiconductor material in the nanoparticle advantageously ensures that absorption of the electromagnetic radiation of the first wavelength range and emission of the electromagnetic radiation of the second wavelength range mainly or exclusively occurs in the first shell . Furthermore , advantageously, due to this structure , more absorbing material can be packed into a nanoparticle material having a predetermined thickness . This allows , for example , to provide more ef ficient conversion elements , as more absorbing material can be packed into a given film thickness .

[0019] According to at least one embodiment of the nanoparticle , the nanoparticle is a spherical nanoparticle . In other words , the nanoparticle has a spherical shape . In particular, the core has a spherical shape . Thus , the first shell and the second shell may also have a spherical shape . For instance , the shape of the nanoparticle is determined by the second shell . Advantageously, due to the spherical shape , the nanoparticles can be ef ficiently packed . In particular, with spherical nanoparticles an occupation of 74 % of a total volume can be obtained .

[0020] According to at least one embodiment , the nanoparticle comprises or consists of a quantum well structure . In particular, the nanoparticle comprises or consists of a spherical quantum well structure , for example a quantum well structure which has quantum confining ef fects . For instance , the quantum well structure is formed by arranging the first shell between the core and the second shell . For example , the confining ef fects of the quantum well occur in the first shell . Advantageously, the spherical quantum well structure allows for a much higher absorption compared to a nanoparticle comprising an absorbing and emitting core when the focus is on minimi zing the absolute layer thickness of a conversion element . According to at least one embodiment of the nanoparticle , a thickness of the second shell is greater than 0 . 5 nanometers , in particular greater than 1 . 5 nanometers , for example at least 3 nanometers . It is also possible that the second shell comprises a thickness greater than 5 nanometers or greater than 10 nanometers . Advantageously, such a thick second shell allows for a more stable and reliable nanoparticle .

[0021] According to at least one embodiment of the nanoparticle , a thickness of the second shell is between and including 6 nanometers and 15 nanometers , in particular between and including 10 nanometers and 15 nanometers .

[0022] According to at least one embodiment of the nanoparticle , the second shell comprises at least 20 monolayers , in particular at least 30 monolayers , for example at least 36 monolayers , of the third semiconductor material .

[0023] According to at least one embodiment of the nanoparticle , the first semiconductor material and / or the third semiconductor material is a I I-VI compound semiconductor material . I I-VI compound semiconductor materials comprise at least one element from group 2 or 12 of the periodic table , for example Mg, Ca, Sr, Ba, Zn, Cd, and at least one element from group 16 of the periodic table , for example 0, S , Se . In particular, the first semiconductor material and / or the third semiconductor material comprises or consists of a semiconductor material of the group consisting of ZnSe , ZnS , and combinations thereof .

[0024] According to at least one embodiment of the nanoparticle , the second semiconductor material is a I I I-V compound semiconductor material. III-V compound semiconductor materials comprise at least one element from group 13 of the periodic table, for example B, Al, Ga, In, and at least one element from group 15 of the periodic table, for example N, P, As, Sb. In particular, the second semiconductor material comprises or consists of InP.

[0025] According to at least one embodiment of the nanoparticle, the second semiconductor material is a II-VI compound semiconductor material. In particular, the second semiconductor material comprises or consists of CdSe. With the second semiconductor material being a II-VI compound semiconductor material, it is possible to provide a nanoparticle which solely comprises II-VI compound semiconductor materials as semiconductor materials.

[0026] According to at least one embodiment of the nanoparticle, the core comprises or consists of a material selected from the group consisting of ZnSe, ZnS, ZnSeS, and combinations thereof. In other words, the first semiconductor material is a material selected from the group consisting of ZnSe, ZnS, ZnSeS, and combinations thereof. The core comprising or consisting of ZnSeS may comprise ZnSeS as a homogenous alloy. Alternatively, it is possible that a proportion of S in ZnSeS varies in a radial direction from a center of the core.

[0027] According to at least one embodiment of the nanoparticle, the first shell comprises or consists of InP or CdSe. In other words, the second semiconductor material is InP or CdSe.

[0028] According to at least one embodiment of the nanoparticle, the second shell comprises or consists of ZnSe and / or ZnS. In other words , the third semiconductor material is ZnSe and / or ZnS .

[0029] According to at least one embodiment , a photoluminescence quantum yield ( PLQY) of the nanoparticle is at least 60% of an initial photoluminescence quantum yield after irradiation with light having an intensity of 60 mW / cm2in air for 200 min . Advantageously, such a nanoparticle is particularly suitable for being used in a light-emitting device .

[0030] Furthermore , a light-emitting device is speci fied . In particular, the light-emitting device comprises the nanoparticle described herein . Thus , embodiments , features , and advantages of the nanoparticle also apply to the lightemitting device and vice versa .

[0031] According to at least one embodiment , the light-emitting device comprises a light-emitting semiconductor chip . In particular, the light-emitting semiconductor chip is configured to emit electromagnetic radiation of a first wavelength range . For example , the first wavelength range is in the ultraviolet to blue range of the electromagnetic spectrum .

[0032] In particular, the light-emitting semiconductor chip comprises an epitaxially grown semiconductor layer sequence . The epitaxially grown semiconductor layer sequence can comprise an active layer . In particular, the electromagnetic radiation of the first wavelength range is generated in the active layer . The electromagnetic radiation of the first wavelength range is , for example , emitted via a radiation exit surface of the light-emitting semiconductor chip . According to at least one embodiment , the light-emitting device comprises a conversion element . In particular, the conversion element comprises a plurality of the nanoparticles described herein . For example , the conversion element is arranged on or above the radiation exit surface of the lightemitting semiconductor chip . The conversion element is , for instance , a layer comprising the plurality of nanoparticles .

[0033] According to at least one embodiment of the light-emitting device , the conversion element converts the electromagnetic radiation of the first wavelength range into electromagnetic radiation of a second wavelength range . In other words , the conversion element has radiation-converting properties . In particular, the conversion element has its radiationconverting properties due to the presence of the nanoparticles . The electromagnetic radiation of the second wavelength range can comprise wavelengths having a lower energy than the wavelengths of the first wavelength range .

[0034] According to at least one embodiment , the light-emitting device comprises the light-emitting semiconductor chip configured to emit the electromagnetic radiation of the first wavelength range and the conversion element comprising the plurality of nanoparticles described herein . The conversion element converts the electromagnetic radiation of the first wavelength range into electromagnetic radiation of the second wavelength range .

[0035] Advantageously, the light-emitting device described herein can be used in display applications , for example for mobiles and / or in the automotive area . The light-emitting device can be used for applications including virtual reality, augmented reality, near-to-eye displays , and other displays . Furthermore , due to using the nanoparticles in the conversion element , a particularly thin conversion element having a high level of absorption of the electromagnetic radiation of the first wavelength range is advantageously provided . This is because the nanoparticles described herein can be highly emissive and particularly stable in a light-emitting device .

[0036] According to at least one embodiment , the light-emitting device is a micro-LED . Here and in the following, LED is the abbreviation for light-emitting diode .

[0037] As a broad definition, a micro-LED could be seen as any light-emitting diode with a particularly small si ze . MicroLEDs may comprise a width, a length, a thickness and / or a diameter smaller than or equal to 100 micrometers , in particular smaller than or equal to 70 micrometers , for example smaller than or equal to 50 micrometers . In particular, micro-LEDs , for example rectangular micro-LEDs , have an edge length, in particular in plan view of layers of the semiconductor layer sequence , of a radiation exit surface smaller than or equal to 70 micrometers , for example smaller than or equal to 50 micrometers . For example , a micro-LED is a light-emitting diode with a growth substrate removed, such that a thickness of the micro-LED is in the range between and including, for example , 0 . 5 micrometers and 10 micrometers . For example , the micro-LED is provided on a wafer having releasable retaining structures . The micro-LED can be detached from the wafer in a non-destructive manner .

[0038] In particular, micro-LEDs are mainly used in displays . The micro-LEDs form pixels or subpixels and emit light of a defined color . Small pixel si zes and a high density with close distances make micro-LEDs suitable , among others , for small monolithic displays for augmented reality applications , especially data glasses . In addition, other applications are being developed, in particular regarding their use in data communication or pixelated lighting applications .

[0039] Advantageously, even though the light-emitting device is configured as a micro-LED, still a highly ef ficient red light-emitting device can be provided with the structure comprising the conversion element . In contrast , micro-LEDs having a light-emitting semiconductor chip emitting red light from its active layer have a low ef ficiency .

[0040] According to at least one embodiment of the light-emitting device , the conversion element is configured for full conversion . Here and in the following, " full conversion" means that at most 5% , at least 1 % , or at least 0 . 5% of the electromagnetic radiation of the first wavelength can pass through the conversion element without being absorbed . Thus , the electromagnetic radiation emitted by the light-emitting device mainly comprises the electromagnetic radiation of the second wavelength range .

[0041] In particular, the light-emitting device shows a reduced blue bleed-through due to a more complete absorption of the electromagnetic radiation of the first wavelength by the nanoparticles because of their higher absorption per unit volume .

[0042] Typically, absorption coef ficients are reported in per mol or per gram . In the case of a conversion element for a micro- LED, it is more appropriate to consider the absorption per unit volume because of the limited volume available above the LED for the conversion of the electromagnetic radiation of the first wavelength range into the electromagnetic radiation of the second wavelength range to occur .

[0043] A film of the conversion element above the light-emitting semiconductor chip in particular absorbs most of the high- energy light from the light-emitting semiconductor chip, that is the electromagnetic radiation of the first wavelength range , and converts it to lower-energy light , that is the electromagnetic radiation of the second wavelength range , but a practical limit of the film thickness is approximately the width of the chip ; beyond that thickness or height , the conversion element becomes an unstable "tower" that may be mechanically unstable . The conversion element may also have signi ficant crosstalk with neighboring pixels .

[0044] I f the conversion element comprises or consists of particles such as nanoparticles , the particles have a maximum packing ef ficiency that is si ze-independent ; for example , cubic particles have a maximum packing ef ficiency of 100% whereas spherical particles can be packed with a maximum ef ficiency of 74 % . To determine an absorbance of a conversion element , a volumetric absorption coef ficient of the absorber materials and the volume fraction of the particle that they occupy have to be considered . In the case of a conversion element comprising InP / ZnS nanoparticles , the most important factor for determining the absorption per unit volume is the InP volume fraction of the nanoparticle .

[0045] The nanoparticles described herein exhibit an increase in absorption per unit volume compared to nanoparticles having an absorbing and emitting core because the nanoparticles described herein comprise the first shell comprising the first , absorbing and emitting semiconductor material .

[0046] According to at least one embodiment of the light-emitting device , the conversion element has an optical density of at least 2 . 0 for the electromagnetic radiation of the first wavelength range . In particular, an optical density of 2 . 0 corresponds to an absorption of 99% .

[0047] Advantageous embodiments and developments of the nanoparticle and the light-emitting device will become apparent from the exemplary embodiments described below in conj unction with the figures .

[0048] In the figures :

[0049] Figure 1 shows a schematic cross section of a nanoparticle according to an exemplary embodiment .

[0050] Figure 2 shows a schematic cross section of a light-emitting device according to an exemplary embodiment .

[0051] Figures 3 and 4 show an InP content of nanoparticles according to exemplary embodiments and comparative examples as a function of a ZnS shell thickness .

[0052] Figures 5 and 6 show a percent di f ference between the InP content of nanoparticles according to exemplary embodiments and nanoparticles according to comparative examples as a function of a ZnS shell thickness .

[0053] Figure 7 shows a PLQY retention of nanoparticles according to exemplary embodiments after irradiation . In the exemplary embodiments and figures , similar or similarly acting constituent parts are provided with the same reference signs . The elements illustrated in the figures and their si ze relationships among one another should not be regarded as true to scale . Rather, individual elements may be represented with an exaggerated si ze for the sake of better representability and / or for the sake of better understanding .

[0054] The exemplary embodiment of a nanoparticle 1 shown in figure 1 comprises a core 2 . The core 2 comprises or consists of a first , non-absorbing semiconductor material . In the present case , the first semiconductor material is ZnS or ZnSe . The core 2 is surrounded by a first shell 3 . The first shell 3 completely surrounds the core 2 . The first shell 3 comprises a second, absorbing and emitting semiconductor material . The second semiconductor material is di f ferent from the first semiconductor material , and the first semiconductor material comprises a wider bandgap than the second semiconductor material . In the present case , the second semiconductor material is CdSe or InP .

[0055] The first shell 3 is further surrounded by a second shell 4 . In other words , the first shell 3 is arranged between the core 2 and the second shell 4 . The first shell 3 is in direct mechanical contact with the core 2 and the second shell 4 . The second shell 4 comprises or consists of a third, nonabsorbing semiconductor material . The third semiconductor material comprises a wider bandgap than the second semiconductor material . The third semiconductor material is in the present case ZnS or ZnSe . The third semiconductor material can be the same as , or di f ferent from the first semiconductor material . As can be seen from figure 1 , the nanoparticle 1 has a spherical shape . Thus , the nanoparticle 1 is a spherical nanoparticle . Advantageously, with such a nanoparticle a high packing density can be obtained in a conversion element 12 of a light-emitting device 10 .

[0056] In the present case , the nanoparticle 1 comprises a quantum well structure . Furthermore , as the nanoparticle 1 is spherical , the nanoparticle may comprise a spherical quantum well . The nanoparticle 1 can absorb electromagnetic radiation of a first wavelength range and emit electromagnetic radiation of a second wavelength range , for example both in the first shell 3 . The nanoparticle 1 is thus able to convert the electromagnetic radiation of the first wavelength range into electromagnetic radiation of the second wavelength range . The first wavelength range comprises wavelengths of the ultraviolet to blue region of the electromagnetic spectrum, whereas the second wavelength range comprises wavelengths of the green to red region of the electromagnetic spectrum .

[0057] For example , the core 2 of the nanoparticle has a diameter of about 2 . 1 nanometers . The first shell 3 may have a thickness of 0 . 6 nanometers and the second shell 4 may have a thickness of 10 nanometers . With such a configuration 4 % of the volume of the nanoparticle 1 are formed by an absorbing material .

[0058] In comparison, for a nanoparticle 1 comprising a core 2 formed from InP and a shell formed from ZnS , wherein the core 2 has a diameter of 1 . 9 nanometers and the shell has a thickness of 10 nanometers , only 1 % of the volume is formed by an absorbing material . Accordingly, the nanoparticles 1 described herein have a higher amount of absorbing material and thus have a higher ef ficiency than other nanoparticles 1 . Here and in the following, a nanoparticle 1 comprising a core 2 formed with an absorbing material and a shell formed with a non-absorbing material is also referred to as a core / shell nanoparticle .

[0059] Calculations of spherical InP / ZnS core / shell nanoparticles with a 1 . 89 nanometer diameter InP core and varied ZnS shell thicknesses show that the volume percent of InP in the core / shell nanoparticle drops from 43% for a single monolayer of ZnS to 1 . 5% for 10 monolayers of ZnS . I f thicker ZnS shells impart improved stability, there is a clear trade-of f between stability and film thickness for full conversion in a conversion element 12 of a light-emitting device 10 .

[0060] A similar wavelength-emitting nanoparticle 1 can be made using a 2 . 1 nanometer ZnSe core 2 , two monolayers of InP as a first shell 3 , and a second shell 4 comprising ZnS . This structure forms a spherical quantum well ( SQW) . The SQW comprises a volume percentage of 47 % InP with a single monolayer ZnS shell , which is a modest improvement compared to the core / shell nanoparticle . However, a volume percentage of 3 . 5% InP is obtained with a second shell 4 comprising 10 monolayers of ZnS . This is 2 . 8 times the InP content of the core / shell nanoparticle . The result of this increased InP or other absorbing material content is improved optical absorption in a conversion element 12 for SQWs compared to core / shell nanoparticles .

[0061] Figure 2 shows an exemplary embodiment of a light-emitting device 10 . The light-emitting device 10 is a micro-LED, for example . The light-emitting device 10 comprises a light- emitting semiconductor chip 11 having a semiconductor layer sequence 111 . The light-emitting semiconductor chip 11 emits electromagnetic radiation of a first wavelength range , which is for example in the blue region of the electromagnetic spectrum . The electromagnetic radiation of the first wavelength range is generated in an active layer 112 of the semiconductor layer sequence 111 .

[0062] The light-emitting device 10 further comprises a conversion element 12 which is arranged on a radiation exit surface of the light-emitting semiconductor chip . The conversion element 12 comprises a plurality of nanoparticles 1 , for example as described in combination with figure 1 . The nanoparticles 1 can be embedded in a matrix material . However, it is also possible that the nanoparticles 1 form the conversion element 12 . In this way, the nanoparticles 1 can be particularly closely packed, which leads to a conversion element 12 having a reduced thickness . In this way, the light-emitting device 10 can be further miniaturi zed .

[0063] Due to the nanoparticles 1 , the conversion element 12 shows a more complete absorption of the electromagnetic radiation of the first wavelength range . This reduces bleed-through of the electromagnetic radiation of the first wavelength range . Thus , a blue color filter may be omitted .

[0064] The nanoparticles 1 in the conversion element 12 convert the electromagnetic radiation of the first wavelength range into electromagnetic radiation of a second wavelength range . For example , the electromagnetic radiation of the second wavelength range comprises wavelengths in the green or red region of the electromagnetic spectrum . Figures 3 and 4 show a volume percentage of InP c(InP) as a function of a thickness of a ZnS shell t(ZnS) in nanometers. The nanoparticles 1 resulting in the curves of figure 3 emit electromagnetic radiation of a second wavelength range in the green region of the electromagnetic spectrum, whereas the nanoparticles 1 resulting in the curves of figure 4 emit electromagnetic radiation of a second wavelength range in the red region of the electromagnetic spectrum.

[0065] The nanoparticles 1 resulting in curve VI comprise an absorbing and emitting core 2 having a diameter of 1.9 nanometers and a shell. The core 2 is formed from InP. The shell is formed from ZnS. the nanoparticles 1 resulting in curve VI are core / shell nanoparticles.

[0066] The nanoparticles 1 resulting in curve El comprise a core 2, a first shell 3 which comprises a second, absorbing and emitting semiconductor material, and a second shell 4. The core 2 has a diameter of 2.1 nanometers and is formed from ZnSe. The first shell 3 has a thickness of 0.6 nanometers and is formed from InP. The second shell 4 is formed from ZnS.

[0067] The nanoparticles 1 resulting in curve El comprise a spherical quantum well (SQW) .

[0068] The nanoparticles 1 resulting in curve V2 comprise an absorbing and emitting core 2 having a diameter of 2.8 nanometers and a shell. The core 2 is formed from InP. The shell is formed from ZnS. the nanoparticles 1 resulting in curve V2 are core / shell nanoparticles.

[0069] The nanoparticles 1 resulting in curve E2 comprise a core 2, a first shell 3 which comprises a second, absorbing and emitting semiconductor material, and a second shell 4. The core 2 has a diameter of 2.1 nanometers and is formed from ZnSe. The first shell 3 has a thickness of 1.2 nanometers and is formed from InP. The second shell 4 is formed from ZnS.

[0070] The nanoparticles 1 resulting in curve E2 comprise a spherical quantum well.

[0071] The nanoparticles 1 resulting in curves VI and V2 are comparative examples, whereas the nanoparticles 1 resulting in curves El and E2 are exemplary embodiments.

[0072] As can be seen from figures 3 and 4, the volume percentage of the absorbing and emitting material InP in the nanoparticles 1 is higher for the nanoparticles 1 according to the exemplary embodiments comprising the spherical quantum well compared to the nanoparticles 1 of the comparative examples. The difference in volume percentage of the absorbing and emitting material InP in the nanoparticles 1 according to the exemplary embodiments and in the nanoparticles 1 according to the comparative examples is further shown in figures 5 and 6.

[0073] Figures 5 and 6 show a percent difference %D between the InP content of nanoparticles 1 according to the exemplary embodiments and nanoparticles 1 according to the comparative examples as a function of a ZnS shell thickness t(ZnS) in nanometers. The nanoparticles 1 have the same structure as described in combination with figures 3 and 4. Figure 5 shows the percent differences of the InP content of the green emitting nanoparticles 1. Figure 6 shows the percent difference of the InP content of the red emitting nanoparticles 1. The percent difference is calculated as follows : %D = (c(InP) of nanoparticle comprising the SQW) / (c(InP) core / shell nanoparticle) . It can be seen from figures 5 and 6 that the advantage of the nanoparticles 1 having the spherical quantum well structure regarding the InP content increases with an increasing thickness of the ZnS shell .

[0074] Figure 7 shows a retention of the photoluminescence quantum yield %R of nanoparticles 1 comprising a core 2 , a first shell 3 , and a second shell 4 . The core 2 comprises ZnSe . The first shell comprises InP . The second shell comprises ZnS . A thickness of the second shell varies between and including 3 monolayers (ML ) of ZnS and 36 monolayers of ZnS . The photoluminescence quantum yield ( PLQY) is measured after irradiation of the nanoparticles 1 with blue light having an intensity of 60 mW / cm2in air .

[0075] It can be seen from figure 7 that with an increased thickness of the second shell 4 also the retention of the PLQY increases . For example , a nanoparticle 1 comprising a second shell 4 having a thickness of at least 19 monolayers of ZnS shows a PLQY retention of over 60% after irradiation for 200 min . Accordingly, such nanoparticles 1 are particularly suitable for being used in a conversion element 12 of a light-emitting device 10 .

[0076] The features and exemplary embodiments described in connection with the figures can be combined with each other according to further exemplary embodiments , even i f not all combinations are explicitly described . Furthermore , the exemplary embodiments described in connection with the figures may have alternative or additional features as described in the general part . This patent application claims the priority of US provisional patent application 63 / 677 , 412 , the disclosure content of which is hereby incorporated by reference . The invention is not restricted to the exemplary embodiments by the description on the basis of said exemplary embodiments . Rather, the invention encompasses any new feature and also any combination of features , which in particular comprises any combination of features in the patent claims and any combination of features in the exemplary embodiments , even i f this feature or this combination itsel f is not explicitly speci fied in the patent claims or exemplary embodiments .

[0077] References

[0078] 1 nanoparticle

[0079] 2 core

[0080] 3 first shell

[0081] 4 second shell

[0082] 10 light-emitting device

[0083] 11 light-emitting semiconductor chip

[0084] 111 semiconductor layer sequence

[0085] 112 active layer

[0086] 12 conversion element

[0087] VI , V2 curve

[0088] El , E2 curve c volume percentage t thickness

[0089] %D percent di f ference

[0090] %R PLQY retention

[0091] T time

Claims

Claims1. A nanoparticle (1) comprising- a core (2) comprising a first, non-absorbing semiconductor material ,- a first shell (3) comprising a second, absorbing and emitting semiconductor material, and- a second shell (4) comprising a third, non-absorbing semiconductor material.

2. The nanoparticle (1) according to claim 1, wherein a thickness of the second shell (4) is greater than a thickness of the first shell (3) .

3. The nanoparticle (1) according to any of the previous claims, wherein the first semiconductor material and the third semiconductor material have a wider bandgap than the second semiconductor material .

4. The nanoparticle (1) according to any of the previous claims, wherein the nanoparticle (1) is a spherical nanoparticle.

5. The nanoparticle (1) according to any of the previous claims, wherein the nanoparticle (1) comprises a quantum well structure.

6. The nanoparticle (1) according to any of the previous claims, wherein a thickness of the second shell (4) is greater than 0.5 nanometers .

7. The nanoparticle (1) according to any of the previous claims, wherein a thickness of the second shell (4) is between and including 6 nanometers and 15 nanometers.

8. The nanoparticle (1) according to any of the previous claims, wherein the second shell (4) comprises at least 20 monolayers of the third semiconductor material.

9. The nanoparticle (1) according to any of the previous claims, wherein the first semiconductor material and / or the third semiconductor material is a II-VI compound semiconductor material .

10. The nanoparticle (1) according to any of the previous claims, wherein the second semiconductor material is a III-V compound semiconductor material.

11. The nanoparticle (1) according to any of the previous claims, wherein the second semiconductor material is a II-VI compound semiconductor material.

12. The nanoparticle (1) according to any of the previous claims, wherein- the core (2) comprises a material selected from the group consisting of ZnSe, ZnS, ZnSeS, and combinations thereof,- the first shell (3) comprises InP or CdSe, and- the second shell (4) comprises ZnSe and / or ZnS.

13. The nanoparticle (1) according to any of the previous claims, wherein a photoluminescence quantum yield of the nanoparticle (1) is at least 60% of an initial photoluminescence quantum yield after irradiation with light having an intensity of 60mW / cm2in air for 200 min.

14. A light-emitting device (10) comprising- a light-emitting semiconductor chip (11) configured to emit electromagnetic radiation of a first wavelength range, and- a conversion element (12) comprising a plurality of nanoparticles (1) according to any of claims 1 to 12, wherein the conversion element (12) converts the electromagnetic radiation of the first wavelength range into electromagnetic radiation of a second wavelength range.

15. The light-emitting device (10) according to claim 14, wherein the light-emitting semiconductor device (10) is a micro-LED.

16. The light-emitting device (10) according to claim 14 or15, wherein the conversion element (12) is configured for full conversion .

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