Nanoparticle, method for producing a nanoparticle, and light-emitting device

Abstract

A nanoparticle (1) is specified. The nanoparticle (1) comprises a core (2) comprising a first semiconductor material, an emission shell (3) comprising a second semiconductor material, and an outer shell (4) comprising a third semiconductor material, wherein the emission shell (3) is arranged between the core (2) and the outer shell (4), and a lattice mismatch between the first semiconductor material and the second semiconductor material is at least 3%. Furthermore, a method for producing a nanoparticle and a light-emitting device, in particular comprising a micro-LED, are specified.

Description

[0001] Description

[0002] NANOPARTICLE, METHOD FOR PRODUCING A NANOPARTICLE, AND LIGHTEMITTING DEVICE

[0003] A nanoparticle, a method for producing a nanoparticle, and a light-emitting device are specified.

[0004] It is an object to provide a nanoparticle having improved properties. Furthermore, an efficient method for producing such a nanoparticle shall be specified. Additionally, a light-emitting device with improved properties shall be provided .

[0005] A nanoparticle is specified. In particular, the nanoparticle is a discrete structure. For example, the nanoparticle is a solid. Here and in the following, the nanoparticle is a particle having a size in the nanometer range, for example between and including 1 nanometer and 100 nanometers. In particular, the nanoparticle converts electromagnetic radiation of a first wavelength range into electromagnetic radiation of a second wavelength range. In other words, the nanoparticle has wavelength-converting properties. The nanoparticle can absorb the electromagnetic radiation of the first wavelength range and emit the electromagnetic radiation of the second wavelength range. The nanoparticle is, for instance, a quantum dot.

[0006] According to at least one embodiment, the nanoparticle comprises a core. In particular, the core comprises or consists of a first semiconductor material. For instance, the core does not emit the electromagnetic radiation of the second wavelength range. However, it is possible that the core absorbs the electromagnetic radiation of the first wavelength range.

[0007] According to at least one embodiment, the nanoparticle comprises an emission shell. In particular, the emission shell comprises a second semiconductor material. The second semiconductor material is, for example, different from the first semiconductor material. For instance, the emission shell at least partially, in particular completely, surrounds the core. The emission shell and the core can be in direct mechanical contact. In particular, the emission shell is directly grown onto the core. The emission shell can emit the electromagnetic radiation of the second wavelength range.

[0008] According to at least one embodiment, the nanoparticle comprises an outer shell. In particular, the outer shell comprises a third semiconductor material. The third semiconductor material is, for example, different from the second semiconductor material. The third semiconductor material can be different from the first semiconductor material. However, it is also possible that the first semiconductor material and the third semiconductor material are the same. In other words, the core and the outer shell can comprise the same semiconductor material. The outer shell is, for example, a passivation shell for the emission shell. In particular, the outer shell increases a stability of the nanoparticle. For example, the outer shell prevents a degradation of the emission shell by oxidation.

[0009] According to at least one embodiment, the emission shell is arranged between the core and the outer shell. For instance, the outer shell at least partially, for example completely, surrounds the emission shell. The outer shell and the emission shell can be in direct mechanical contact . In particular, the outer shell is directly grown onto the emission shell . However, it is also possible that a further semiconductor layer is arranged between the emission shell and the outer shell . The emission shell and the outer shell are in particular layers surrounding the core .

[0010] In particular, the nanoparticle comprises a spherical quantum well . The spherical quantum well is , for example , formed by the core , the emission shell , and the outer shell . The spherical quantum well in particular comprises discrete energy values . Thus , the nanoparticle can emit electromagnetic radiation of defined wavelength ranges .

[0011] According to at least one embodiment of the nanoparticle , a lattice mismatch between the first semiconductor material and the second semiconductor material is at least 3% or at least 5% , in particular at least 7 % , for example at least 10% . In particular, the lattice mismatch arises from di f ferences in the crystal structure of the first semiconductor material and the second semiconductor material . For example , the lattice mismatch refers to a di f ference in lattice parameters between the first semiconductor material and the second semiconductor material .

[0012] According to at least one embodiment , the nanoparticle comprises the core comprising the first semiconductor material , the emission shell comprising the second semiconductor material , and the outer shell comprising the third semiconductor material , wherein the emission shell is arranged between the core and the outer shell , and the lattice mismatch between the first semiconductor material and the second semiconductor material is at least 3% , in particular at least 7 % .

[0013] Due to the lattice mismatch of at least 3% , in particular at least 7 % , a thickness of the emission shell is limited . In other words , the nanoparticle can have a thin emission shell . Due to the thin emission shell , the second wavelength range can be shi fted towards lower wavelengths compared to other nanoparticles . Furthermore , the emission shell has a larger volume compared to a nanoparticle with an emissive core .

[0014] Thus , Auger scattering can be reduced with the emission shell compared to the nanoparticle with an emissive core . This leads to an increased ef ficiency of the nanoparticle described herein even at high light fluxes .

[0015] Furthermore , the emission shell can have a uni form shape due to the lattice mismatch . Due to the uni form shape , the emission of the nanoparticle is narrower compared to other nanoparticles .

[0016] According to at least one embodiment of the nanoparticle , a thickness of the emission shell is between and including 1 . 5 monolayers and 5 monolayers . In particular, this thickness arises from the lattice mismatch between the first semiconductor material and the second semiconductor material .

[0017] According to at least one embodiment of the nanoparticle , a thickness of the emission shell is at most 5 nanometers , in particular at most 2 nanometers . Such a thickness of the emission shell can arise from the lattice mismatch between the first semiconductor material and the second semiconductor material . According to at least one embodiment of the nanoparticle, a thickness of the outer shell is between and including 1 nanometer and 20 nanometers.

[0018] According to at least one embodiment of the nanoparticle, the emission shell has a magic size. In particular, the magic size arises from the lattice mismatch between the first semiconductor material and the second semiconductor material and / or a size of the core. Here and in the following magic size means a size, in particular a thickness, of the emission shell which is especially thermodynamically stable. Adding another layer of material to the emission shell having the magic size is thermodynamically unfavored. Each magic size of the emission shell, for example, corresponds to a local free energy minimum. In particular, the magic size is quantized. This means that nanoparticles with emission shells having intermediate sizes between two magic sizes of the emission shell cannot be isolated as they are unstable. Thus, advantageously size focusing is possible.

[0019] According to at least one embodiment of the nanoparticle, the first semiconductor material has a wider bandgap than the second semiconductor material. In other words, the nanoparticle is an inverse type I quantum dot. The smaller bandgap in the second semiconductor material advantageously leads to a delocalization of charge carriers in the shell. Additionally or alternatively, the wider bandgap of the first semiconductor material leads to a superior charge carrier confinement .

[0020] According to at least one embodiment of the nanoparticle, the third semiconductor material has a wider bandgap than the second semiconductor material. Due to the wider bandgap of the third semiconductor material, a quantum well can be generated in the emission region. Advantageously, wider bandgap materials have stronger bonds. Thus, the wider bandgap materials have an increased mechanical stability and a higher resistance against degradation and / or corrosion.

[0021] According to at least one embodiment of the nanoparticle, the core has one of the following shapes: spherical shape, cubic shape, tetrahedral shape, tetrapod shape, rod shape, hexagonal prism shape, star shape.

[0022] According to at least one embodiment of the nanoparticle, the emission shell has one of the following shapes: spherical shape, cubic shape, tetrahedral shape, rod shape, tetrapod shape, star shape.

[0023] According to at least one embodiment of the nanoparticle, the outer shell has one of the following shapes: spherical shape, cubic shape, tetrahedral shape, rod shape, tetrapod shape, star shape.

[0024] According to at least one embodiment, the nanoparticle has one of the following shapes: spherical shape, cubic shape, tetrahedral shape, rod shape, tetrapod shape, star shape.

[0025] Here and in the following, the shape of an element such as the nanoparticle, the core, the emission shell, and the outer shell is the outline or contour of the respective element.

[0026] According to at least one embodiment of the nanoparticle, the shape of the core and the shape of the emission shell are different. In other words, the shape of the emission shell can be independent of the shape of the core. However, it is also possible that the shape of the core and the shape of the emission shell are the same. In other words, the emission shell recreates the shape of the core.

[0027] According to at least one embodiment of the nanoparticle, the shape of the emission shell and the shape of the outer shell are different. In particular, a shape of the outer shell defines the shape of the nanoparticle.

[0028] The cubic shape of the nanoparticle has the advantage that a packing density of the nanoparticle can be increased compared to spherically shaped nanoparticles. Thus, an on-chip load of the nanoparticles can advantageously be increased. Furthermore, due to an increased possible contact area between two cubically shaped nanoparticles, transport properties and / or conductivity properties of the nanoparticles are, in particular, improved.

[0029] In particular, it is possible that one, two, or all three of the core, the emission shell, and the outer shell has / have a tetrahedral shape. For example, the first semiconductor material, the second semiconductor material, and / or the third semiconductor material crystalize ( s ) in the zinc blende structure. It is possible that, thus, a tetrahedral shape for the core, the emission shell, the outer shell, and / or the whole nanoparticle arises.

[0030] In particular, a particle, such as the core or the nanoparticle, or an element, such as the emission shell or the outer shell, having a tetrapod shape comprises a center and arms. The center can be a tetrahedron. The arms of the tetrapod extend from each facet of the tetrahedron. The arms can be rod-shaped. For example, the emission shell forms arms of the tetrapod . However, it is also possible that the emission shell is a shell surrounding a tetrapod-shaped core .

[0031] In particular, i f the core has a star shape , the emission shell also has a star shape . However, it is also possible that the emission shell surrounds a star-shaped core in such a way that the tips of the star-shaped core remain free of the emission shell .

[0032] In particular, the nanoparticle can have a dot-in-a-rod structure . In particular, in the dot-in-a-rod structure the core has a spherical shape . The emission shell and / or the outer shell , however, has / have a rod shape . It is also possible that the emission shell in the dot-in-a-rod structure has a spherical shape .

[0033] According to at least one embodiment of the nanoparticle , the first semiconductor material and the third semiconductor material are I I-VI compound semiconductor materials , and the second semiconductor material is a I I I-V compound semiconductor material . It is also possible that the first semiconductor material and the second semiconductor material are I I I-V compound semiconductor materials and the third semiconductor material is a I I-VI compound semiconductor material .

[0034] I I I-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 . 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.

[0035] According to at least one embodiment of the nanoparticle, the first semiconductor material is ZnS, GaP, ZnSe, CdZnS, ZnSeS, InGaP or InGaZnP. Advantageously, these semiconductor materials have a wide bandgap.

[0036] According to at least one embodiment of the nanoparticle, the second semiconductor material is InP, GaP, or CdSe. In particular, these semiconductor materials have a narrow bandgap. Thus, an emission shell comprising InP, GaP, or CdSe is advantageous.

[0037] According to at least one embodiment of the nanoparticle, the third semiconductor material comprises or consists of ZnS. In particular, ZnS has a wide bandgap compared to other semiconductor materials. Thus, it is particularly effective as the outer shell. Furthermore, ZnS is chemically stable even under the influence of water and / or oxygen.

[0038] According to at least one embodiment of the nanoparticle, the first semiconductor material comprises or consists of ZnS or ZnSe. For example, a lattice mismatch between ZnS and InP is about 7.7%. Thus, the lattice mismatch is particularly high with this combination of materials.

[0039] According to at least one embodiment, the nanoparticle further comprises an intermediate shell. In particular, the intermediate shell comprises a fourth semiconductor material. For instance, the fourth semiconductor material is a III-V compound semiconductor material or a II-VI compound semiconductor material. For example, the intermediate shell is arranged between the emission shell and the outer shell or between the core and the emission shell. Advantageously, the intermediate shell can be used to adjust a lattice mismatch between the emission shell and the core or between the emission shell and the outer shell. The intermediate shell may also be used to tune the emission properties of the nanoparticle .

[0040] According to at least one embodiment of the nanoparticle, the fourth semiconductor material has a wider bandgap than the second semiconductor material. In particular, the bandgap of the fourth semiconductor material is between the bandgap of the second semiconductor material and the bandgap of the third semiconductor material.

[0041] According to at least one embodiment of the nanoparticle, a thickness of the intermediate shell is between and including 0.3 nanometers and 10 nanometers.

[0042] According to at least one embodiment, the nanoparticle comprises one of the following material combinations: ZnSe (S) / GaP / InP / ZnS (Se) , ZnSe (S) / InP / GaP / ZnS (Se) , GaP / InP / ZnS (Se) , GaP / InP / ZnSe / ZnS, GaP / InP / ZnS / Au (Pt, Pd, Ag) , ZnS / InP / ZnS, ZnSe / InP / ZnS, ZnS / InP / ZnSe / ZnS, ZnSe / InP / ZnSe / ZnS , wherein the succession of materials corresponds to a succession of core, emission shell, outer shell and, if present, intermediate shell. The chemical elements given in parentheses are alternatives or additions to the chemical element mentioned directly before. Thus, ZnSe(S) means, for example, ZnSe, ZnS and ZnSeS.

[0043] Furthermore, a method for producing a nanoparticle is specified. In particular, the method is used for producing the nanoparticle described herein . Thus , embodiments , features , and advantages disclosed in combination with the nanoparticle also apply to the method and vice versa .

[0044] According to at least one embodiment , the method comprises the following steps :

[0045] - providing a core comprising a first semiconductor material ,

[0046] - adding the core to a first precursor of a second semiconductor material ,

[0047] - adding a second precursor of the second semiconductor material to form a first shell comprising the second semiconductor material , wherein a lattice mismatch between the first semiconductor material and the second semiconductor material is at least 3% ,

[0048] - forming an outer shell comprising a third semiconductor material . In particular, the first semiconductor material , the second semiconductor material , and / or the third semiconductor material is / are epitaxially grown .

[0049] In particular, the steps are carried out in the order given . For example , a plurality of nanoparticles is produced with the method described . To achieve this , for example , a plurality of cores is provided . The second precursor is , in particular, added all at once or incrementally, for example using a syringe pump or an addition funnel .

[0050] Because of the lattice mismatch between a material of the core and a material of the emission shell , a thickness of the emission shell can be determined . Thus , advantageously, there is less need to precisely know the number of cores added to the first precursor when a plurality of nanoparticles are produced . Furthermore , the formation of the emission shell is not a kinetically controlled process that must be quickly stopped when the emission shell reaches a desired thickness as a thickness can be controlled by the lattice mismatch between the materials of the core and the shell .

[0051] According to at least one embodiment of the method, adding the second precursor of the second semiconductor material is performed under air- free conditions . In particular, adding the second precursor is performed under an oxygen- free or inert atmosphere . Thus , an oxidation during the reaction of the first precursor and the second precursor can be prevented . Accordingly, the emission shell can be provided free of oxides .

[0052] According to at least one embodiment of the method, the first precursor comprises a cation of the second semiconductor material , and the second precursor comprises an anion of the second semiconductor material . The cation of a I I I-V compound semiconductor material is the element of group 13 of the periodic table , whereas the anion of a I I I-V compound semiconductor material is the element of group 15 of the periodic table . The cation of a I I-VI compound semiconductor material is the element of group 2 or 12 of the periodic table , whereas the anion of a I I-VI compound semiconductor material is the element of group 16 of the periodic table .

[0053] Furthermore , a light-emitting device is speci fied . In particular, the light-emitting device comprises the nanoparticle described herein . Thus , embodiments , features and advantages described in combination with the nanoparticle and the method for producing the nanoparticle also apply to the light-emitting device and vice versa . 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 the first wavelength range . For example , the first wavelength range is in the ultraviolet to blue spectral range .

[0054] According to at least one embodiment , the light-emitting device comprises a conversion element . In particular, the conversion element comprises a nanoparticle described herein . For example , the conversion element comprises a plurality of the nanoparticles . The conversion element converts the electromagnetic radiation of the first wavelength range into electromagnetic radiation of the second wavelength range . In particular, the second wavelength range is at least partially di f ferent from the first wavelength range . The second wavelength range comprises wavelengths having a lower energy compared to wavelengths in the first wavelength range . The wavelength conversion properties of the conversion element arise from the nanoparticles therein as the nanoparticles convert the electromagnetic radiation of the first wavelength range into electromagnetic radiation of the second wavelength range .

[0055] According to at least one embodiment , the light-emitting device comprises the light-emitting semiconductor chip and the conversion element , wherein the conversion comprises the nanoparticle described herein .

[0056] Due to the improved properties of the nanoparticle , the light-emitting device can be operated at higher fluxes without damaging the nanoparticles in the conversion element . Furthermore , because of the narrow emission of the nanoparticle , the light-emitting device can be used in display applications . In particular, the light-emitting device can be used for illumination, automotive displays , and backlighting units .

[0057] According to at least one embodiment of the light-emitting device , the light-emitting semiconductor chip comprises or is a micro-LED . Here and in the following, LED is the abbreviation for light-emitting diode .

[0058] As a broad definition, a micro-LED could be seen as any light-emitting diode ( LED) with a particularly small si ze . Micro-LEDs 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 of 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 , 1 . 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 .

[0059] 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 .

[0060] Advantageous embodiments and developments of the nanoparticle , the method for producing a nanoparticle , and the light-emitting device will become apparent from the exemplary embodiments described below in conj unction with the figures .

[0061] In the figures :

[0062] Figures 1 , 2 , 3A, 3B, 3C, 4A, 4B, 5A, 5B, 6A, 6B, 7A, 7B, 8A, and 8B show schematic cross-sections of a nanoparticle according to exemplary embodiments .

[0063] Figure 9 schematically shows steps of a method for producing a nanoparticle according to an exemplary embodiment .

[0064] Figure 10 shows a schematic cross-section of a light-emitting device according to an exemplary embodiment .

[0065] 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 .

[0066] The exemplary embodiment of a nanoparticle 1 shown in figure 1 comprises a core 2 which is spherically shaped . The core 2 is surrounded by an emission shell 3 and an outer shell 4 . The emission shell 3 and the outer shell 4 presently completely surround the core 2 . The emission shell 3 is arranged between the core 2 and the outer shell 4 . The emission shell 3 and the outer shell 4 are spherically shaped . The core 2 comprises a first semiconductor material , which is presently ZnSe or ZnS . The emission shell 3 comprises a second semiconductor material , which is presently InP . The outer shell 4 comprises a third semiconductor material , which is presently ZnSe or ZnS . The nanoparticle 1 converts electromagnetic radiation of a first wavelength range into electromagnetic radiation of a second wavelength range . The electromagnetic radiation of the second wavelength range is emitted via the emission shell 3 .

[0067] The emission shell 3 has a magic si ze . In other words , a thickness of the emission shell 3 is such that adding another layer of the second semiconductor material would lead to a less stable emission shell 3 . A lattice mismatch between the first semiconductor material and the second semiconductor material is at least 3% . For example , the lattice mismatch between ZnSe and InP is about 3 . 3% . The lattice mismatch and a si ze of the core 2 determine the thickness of the emission shell 3 . In particular, the thickness of the emission shell 3 is limited by the strain between the first semiconductor material and the second semiconductor material and lattice constants of these semiconductor materials . A bulk-like si ze of the core 2 creates an extra degree of incompatibility between the first semiconductor material and the second semiconductor material . This ef fect enables a precise control of the emission wavelength of the nanoparticle 1 .

[0068] The exemplary embodiment of a nanoparticle 1 shown in figure 2 has the same structure as the nanoparticle 1 of figure 1 . However, the shape of the nanoparticle 1 , the core 2 , the emission shell 3 , and the outer shell 4 is presently not spherical , but the shape is cubic instead .

[0069] Figures 3A, 3B, and 3C show exemplary embodiments of nanoparticles 1 each having a tetrahedron shape . The nanoparticles 1 comprise a core 2 which is surrounded by an emission shell 3 and an outer shell 4 . The core 2 is in direct mechanical contact with the emission shell 3 . The emission shell 3 is in direct mechanical contact with the outer shell 4 . The emission shell 3 is arranged between the core 2 and the outer shell 4 . Materials for the core 2 , the emission shell 3 , and the outer shell 4 can be the same as discussed in connection with figure 1 . A thickness of the emission shell 3 is , for example , between and including 1 . 5 monolayers and 5 monolayers of the second semiconductor material .

[0070] In the exemplary embodiment of the nanoparticle 1 of figure 3A, the core 2 , the emission shell 3 , and the outer shell 4 have a tetrahedron shape . In the exemplary embodiment of the nanoparticle 1 of figure 3B, the core 2 is spherically shaped, whereas the emission shell 3 and the outer shell 4 have a tetrahedron shape . The nanoparticle 1 of figure 3C has a core 2 and an outer shell which have a tetrahedral shape . However, the emission shell 3 is spherically shaped . The tetrahedral shape of the nanoparticle 1 , the core 2 , the emission shell 3 , and / or the outer shell 4 can be traced back to semiconductor materials which crystalli ze in the zinc blende crystal structure . In particular, the shape of the nanoparticle 1 , the core 2 , the emission shell 3 , and / or the outer shell 4 is determined by the synthetic conditions used during production of the nanoparticle 1 . The exemplary embodiment of a nanoparticle 1 shown in figure 4A has a core 2 having a tetrahedral shape . An emission shell 3 is arranged around the core 2 . Presently, the emission shell 3 has the shape of arms which extend from facets of the core 2 . In this way, the core 2 and the emission shell 3 have the shape of a tetrapod . In particular, the arms have the shape of a rod . The lengths of the rods are magic-si zed . Thus , the emission shell 3 has a magic si ze .

[0071] The emission shell 3 is formed from a di f ferent semiconductor material than the core 2 . A lattice mismatch between the two semiconductor materials is at least 5% . The core 2 and the emission shell 3 are surrounded by an outer shell 4 . The outer shell 4 comprises a third semiconductor material . Presently, the outer shell 4 completely surrounds the core 2 and the emission shell 3 . The first semiconductor material of the core 2 is ZnS , for example . The second semiconductor material of the emission shell 3 is CdSe , for example . The outer shell 4 comprises or consists of ZnS .

[0072] The nanoparticle 1 shown in figure 4B also has a tetrapod shape . However, in contrast to the nanoparticle 1 of figure 4B, the core 2 and the emission shell 3 are tetrapod-shaped, as is the outer shell . The core 2 comprises a tetrahedrally shaped center comprising a first semiconductor material , which is ZnS . The tetrahedrally shaped center is surrounded by rod-shaped arms also comprising the first semiconductor material , such that the core 2 has the tetrapod shape .

[0073] The core 2 is completely surrounded by the emission shell 3 , which replicates the shape of the core 2 . The emission shell 3 comprises a second semiconductor material which has a lattice mismatch to the first semiconductor material of at least 5% or at least 3% , in particular at least 7 % , for example at least 10% . A thickness of the emission shell 3 is at most 5 nanometers . The second semiconductor material is CdSe . The second semiconductor material is epitaxially grown onto the core 2 , for example .

[0074] The nanoparticle 1 further comprises a tetrapod-shaped outer shell 4 , which is in direct mechanical contact with the emission shell 3 . The outer shell 4 comprises a third semiconductor material , which is presently ZnS .

[0075] Figures 5A and 5B show exemplary embodiments of rod-shaped nanoparticles 1 . The nanoparticles 1 each comprise a core 2 , an emission shell 3 surrounding the core 2 , and an outer shell 4 surrounding the emission shell 3 . A second semiconductor material of the emission shell 3 is di f ferent from a first semiconductor material of the core 2 and a third semiconductor material of the outer shell 4 . The second semiconductor material has a smaller bandgap than the first semiconductor material and the third semiconductor material . In other words , the nanoparticle has an inverse type I structure . A lattice mismatch between the first semiconductor material and the second semiconductor material is at least 7 % , at least 5% or at least 10% . A thickness of the emission shell 3 can be between and including 1 . 5 monolayers and 5 monolayers of the second semiconductor material .

[0076] The nanoparticle 1 of figure 5A has a core 2 having a rod shape or a hexagonal prism shape . The emission shell 3 conformally surrounds the core 2 . In contrast to this , the nanoparticle 1 of figure 5B has an emission shell 3 which is thicker in the long dimension of the rod of the core 2 . In other words , a thickness of the emission shell 3 in a direction parallel to a main extension direction of the core 2 is greater than a thickness of the emission shell 3 in a direction perpendicular to the main extension direction of the core 2 .

[0077] Figures 6A and 6B show nanoparticles 1 according to the exemplary embodiments which have a dot-in-a-rod structure . For both nanoparticles 1 the core 2 comprising a first semiconductor material has a spherical shape . The core 2 is surrounded by an emission shell 3 comprising a second semiconductor material . The core 2 can be arranged of f-center or at the center of the nanoparticle 1 . The first semiconductor material has a wider bandgap than the second semiconductor material . A lattice mismatch between the first semiconductor material and the second semiconductor material is at least 5% or at least 7 % . The emission shell 3 is surrounded by an outer shell 4 , which comprises a third semiconductor material . The third semiconductor material has a wider bandgap than the second semiconductor material . The outer shell 4 acts as a passivation for the emission shell 3 .

[0078] The emission shell 3 of the nanoparticle 1 shown in figure 6A has a rod shape . A thickness of the rod-shaped emission shell 3 is magic-si zed . In other words , adding another layer of the second semiconductor material to the emission shell 3 leads to a less stable emission shell 3 .

[0079] The emission shell 3 of the nanoparticle 1 shown in figure 6B replicates the shape of the core 2 . Thus , the emission shell 3 has a spherical shape . Figure 7A shows a nanoparticle 1 according to an exemplary embodiment where the core 2 has the shape of a star . The core

[0080] 2 comprises a first semiconductor material . An emission shell

[0081] 3 comprising a second semiconductor material is arranged around the core 2 such that it recreates the star shape of the core 2 . Accordingly, the emission shell 3 also has a star shape . The emission shell 3 comprises a second semiconductor material which has a smaller bandgap than the first semiconductor material . A lattice mismatch between the first semiconductor material and the second semiconductor material is at least 7 % . The emission shell 3 has a thickness of 1 . 5 monolayers to 5 monolayers of the second semiconductor material . The emission shell 3 presently completely covers the core 2 . The nanoparticle 1 further comprises an outer shell 4 , which is arranged such that the emission shell 3 is arranged between the core 2 and the outer shell 4 . The outer shell 4 comprises a third semiconductor material which has a wider bandgap than the second semiconductor material . Presently, the outer shell 4 has a spherical shape .

[0082] In contrast to the exemplary embodiment of the nanoparticle 1 shown in figure 7A, the nanoparticle 1 of the exemplary embodiment shown in figure 7B has an emission shell 3 which does not completely cover the star-shaped core 2 . Otherwise , the structures of the nanoparticles 1 of figure 7A and 7B are the same . The emission shell 3 is arranged such that tips of the star-shaped core 2 are free of the emission shell 3 .

[0083] It is possible that metal tips are deposited on each tip of the star-shaped core 2 . Thus , the nanoparticle 1 may be used in catalysis , for example for water splitting or hydrogen evolution . A magic-si zed emission shell 3 could aid wave function separation and improve an electron transport to the metal tip .

[0084] Figures 8A and 8B show further exemplary embodiments of nanoparticles 1 having a spherical shape . The nanoparticles 1 have a structure as discussed in combination with figure 1 . However, presently an additional , intermediate layer 5 is present . The intermediate shell 5 comprises a fourth semiconductor material . The fourth semiconductor material has a bandgap between the second semiconductor material and the first semiconductor material and / or the third semiconductor material . As can be seen from figure 8A, the intermediate shell 5 can be arranged between the emission shell 3 and the outer shell 4 . However, it is also possible , as shown in figure 8B, that the intermediate shell is arranged between the core 2 and the emission shell 3 . The intermediate shell 5 can be used to adj ust a lattice mismatch between the first semiconductor material and the second semiconductor material .

[0085] Figure 9 schematically shows steps of a method for producing a nanoparticle 1 . In a first step S I , a core 2 comprising a first semiconductor material is provided . The first semiconductor material is , for example , ZnS , GaP, ZnSe , CdZnS , ZnSeS , InGaP, or InGaZnP .

[0086] The core 2 is inj ected to a solution containing a first precursor for a second semiconductor material during the second method step S2 . The first precursor is the precursor for the cation of the second semiconductor material , for example Cd2+or In3+.

[0087] Then, a second precursor of the second semiconductor material is added . The second precursor is the precursor for the anion of the second semiconductor material . The second precursor is added all at once or introduced slowly using a syringe pump or an addition funnel . The second precursor is added under oxygen- free conditions to avoid the formation of undesired oxides . The second precursor and the first precursor then react to form the second semiconductor material , which forms an emission shell 3 around the core 2 . The reaction conditions are tuned such that the emission shell 3 is grown but nucleation of the second semiconductor material is prevented . In particular, the concentration of the first precursor and the second precursor are kept low . In contrast , the concentration of the core 2 should be high . Furthermore , the reaction temperature should be low, for example between and including 120 ° C and 240 ° C .

[0088] A lattice mismatch between the first semiconductor material and the second semiconductor material is at least 3% , at least 5% , or at least 7 % . Thus , there is less need to precisely know the concentration of the cores 2 inj ected to the solution containing the first precursor .

[0089] In a third step S3 , an outer shell 4 is formed . The outer shell 4 surrounds the emission shell 3 . In particular, the outer shell 4 is directly formed on the emission shell 3 . With this method it is possible to obtain the structures of nanoparticles described in combination with figures 1 to 8 .

[0090] An exemplary embodiment of a light-emitting device 10 is shown as a cross-section in figure 10 . The light-emitting device 10 comprises a light-emitting semiconductor chip 11 , which is presently a micro-LED . The light-emitting semiconductor chip 11 comprises an epitaxial semiconductor layer sequence 111 with an active layer 112 . The light- emitting semiconductor chip 11 is configured to generate electromagnetic radiation of a first wavelength range in the active layer 112 . The electromagnetic radiation of the first wavelength range can be emitted via a radiation exit surface of the light-emitting semiconductor chip 11 .

[0091] A conversion element 12 is arranged on the radiation exit surface of the light-emitting semiconductor chip 11 . The conversion element 12 comprises a plurality of nanoparticles

[0092] I . The nanoparticles can be configured like the exemplary embodiments of figures 1 to 8 . Presently, the conversion element 12 is arranged directly on the light-emitting semiconductor chip 11 , that is without an adhesive layer arranged in between . The nanoparticles 1 convert the electromagnetic radiation of the first wavelength range into electromagnetic radiation of a second wavelength range which is at least partially di f ferent from the first wavelength range .

[0093] The conversion element 12 presently further comprises a matrix material 121 . The nanoparticles 1 are embedded in the matrix material 121 . The matrix material 121 can be organic or inorganic . For example , the matrix material 121 is a polysiloxane or silica . Presently, the conversion element 12 is a layer arranged on the light-emitting semiconductor chip

[0094] I I . However, it is also possible that the conversion element 12 is a casting at least partially surrounding the lightemitting semiconductor chip .

[0095] Alternatively, the conversion element 12 is free of the matrix material 121 . In other words , the conversion element 12 comprises or consists of close-packed nanoparticles 1 . 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 .

[0096] This patent application claims the priority of US provisional patent application 63 / 655 , 110 , the disclosure content of which is hereby incorporated by reference .

[0097] 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 .

[0098] References

[0099] 1 nanoparticle

[0100] 2 core 3 emission shell

[0101] 4 outer shell

[0102] 5 intermediate shell

[0103] 10 light-emitting device 11 light-emitting semiconductor chip

[0104] 111 semiconductor layer sequence

[0105] 112 active layer

[0106] 12 conversion element

[0107] 121 matrix material

Claims

Claims1. Nanoparticle (1) comprising- a core (2) comprising a first semiconductor material,- an emission shell (3) comprising a second semiconductor material, and- an outer shell (4) comprising a third semiconductor material, wherein- the emission shell (3) is arranged between the core (2) and the outer shell (4) , and- a lattice mismatch between the first semiconductor material and the second semiconductor material is at least 3%.

2. Nanoparticle (1) according to the previous claim, wherein a thickness of the emission shell (3) is between and including 1.5 monolayers and 5 monolayers.

3. Nanoparticle (1) according to any of the previous claims, wherein a thickness of the emission shell (3) is at most 5 nanometers .

4. Nanoparticle (1) according to any of the previous claims, wherein the first semiconductor material has a wider bandgap than the second semiconductor material.

5. Nanoparticle (1) according to any of the previous claims, wherein the third semiconductor material has a wider bandgap than the second semiconductor material.

6. Nanoparticle (1) according to any of the previous claims, wherein the core (2) has one of the following shapes: spherical shape, cubic shape, tetrahedral shape, tetrapod shape, rod shape, hexagonal prism shape, star shape.

7. Nanoparticle (1) according to any of the previous claims, wherein the emission shell (3) has one of the following shapes: spherical shape, cubic shape, tetrahedral shape, rod shape, tetrapod shape, star shape.

8. Nanoparticle (1) according to any of the previous claims, wherein the shape of the core (2) and the shape of the emission shell (3) are different.

9. Nanoparticle (1) according to any of the previous claims, wherein- the first semiconductor material and the third semiconductor material are II-VI compound semiconductor materials, and- the second semiconductor material is a III-V compound semiconductor material.

10. Nanoparticle (1) according to any of the previous claims, wherein the second semiconductor material is InP, GaP, or CdSe.

11. Nanoparticle (1) according to any of the previous claims, wherein the third semiconductor material comprises or consists ofZnS .

12. Nanoparticle (1) according to any of the previous claims, comprising an intermediate shell comprising a fourth semiconductor material.

13. Nanoparticle (1) according to the previous claim, wherein the fourth semiconductor material has a wider bandgap than the second semiconductor material.

14. Method for producing a nanoparticle (1) , comprising the steps of:- providing a core (2) comprising a first semiconductor material ,- adding the core (2) to a first precursor of a second semiconductor material,- adding a second precursor of the second semiconductor material to form a first shell (3) comprising the second semiconductor material, wherein a lattice mismatch between the first semiconductor material and the second semiconductor material is at least 3%,- forming an outer shell (4) comprising a third semiconductor material .

15. Method according to claim 14, wherein adding the second precursor of the second semiconductor material is performed under air-free conditions.

16. Method according to any of claims 14 and 15, wherein- the first precursor comprises a cation of the second semiconductor material, and- the second precursor comprises an anion of the second semiconductor material.

17. Light-emitting device (10) comprising- a light-emitting semiconductor chip (11) , and- a conversion element (12) , wherein- the conversion element (12) comprises a nanoparticle according to any of claims 1 to 13.

18. Light-emitting device according to claim 17, wherein the light-emitting semiconductor chip (11) comprises a microLED.

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