OPTOELECTRONIC-GRADE ANISOTROPIC INDIUM PHOSPHIDE NANOCRYSTALS

TECHNOLOGICAL FIELD

The invention generally contemplates optoelectronic grade InP nanocrystals and processes for their preparation.

BACKGROUND OF THE INVENTION

Colloidal semiconductor nanocrystals stand out in the tunability of their optoelectronic properties enabled by control over their composition, size, shape, and crystal structure. Accompanied by the flexible surface chemistry, they emerge as outstanding building blocks for diverse applications ranging from biological tagging to light emitters in information displays. Nanocrystals of ionic semiconductors of the II- VI and IV-VI families, in particular cadmium chalcogenides, have reached an exquisite level of control, over their size and shape, allowing to achieve quantum dots, nanorods and nanoplatelets; and also over the lattice structure, allowing to achieve high degree of light polarization and to study the symmetry of the different optical transitions.

However, due to regulatory concerns, alternative materials are necessary. InP nanocrystals of the III-V semiconductor family with cubic (zincblende) isotropic crystal structure are already utilized in large scale in information display applications due to their lower toxicity and tunable band gap. As the covalent nature of the bonds compared to II- VI semiconducting nanocrystals (SCNCs), yields also deeper surface trap states due to dangling bonds, significant efforts addressed the achievement of efficient band gap emission. This included mild etching with HF along with ZnSe and ZnS multishell growth bringing this material to high photoluminescence intensity.

Nonetheless, achieving narrow linewidth is essential for meeting future color standards in displays, and remains a challenge. Moreover, anisotropic crystal structure control and shape control in colloidal InP nanocrystals are still only at a very rudimentary level. This challenging synthesis is attributed to the covalent nature of III- V semiconductors. An elegant alternative path to the direct synthesis route for shape control and for size control that is hindered in this case by these limitations, is the approach of cation exchange. Numerous studies have utilized cation exchange reactions to expand the selection of nanocrystals shape and crystal phase for different materials, starting from a well-controlled nanocrystal sample of a convenient composition synthesized directly. Copper based systems play a major role in this approach, either as the beginning nanocrystal sample, or as an intermediate, due to their high density of vacancies. Following this idea, wurtzite phase InP nanocrystals were achieved by cation exchange from copper phosphide nanocrystals. However, for these systems as well as for prior experiments on semiconductor nanocrystals produced by cation exchange, high optical quality in terms of resolved transitions and strong emission was not achieved.

Additionally, the known cubic phase isotropic colloidal InP nanocrystals are with spherical-like or tetrahedral pyramid shapes. Shape control for other morphologies is highly limited. For example, rods were made only via the metal seed mediated growth approach, which suffer from lower quality in terms of length control and also the photoluminescence quantum yields are low due to the presence the metal seed that can quench the fluorescence. Plate-like morphology of InP nanocrystals is also unknown, unlike the anisotropic Cd-chalcogenide nanocrystals.

BACKGROUND PUBLICATIONS

1. De Trizio, L. et al. Cu 3- x P Nanocrystals as a Material Platform for NearInfrared Plasmonics and Cation Exchange Reactions. Chem. Mater. 27, 1120-1128 (2015).

2. Shan, X., Li, B. & Ji, B. Synthesis of Wurtzite In and Ga Phosphide Quantum Dots Through Cation Exchange Reactions. Chem. Mater. 33, 5223-5232 (2021).

3. Liu, J. et al. Triphenyl Phosphite as the Phosphorus Source for the Scalable and Cost-Effective Production of Transition Metal Phosphides. Chem. Mater. 30, 1799- 1807 (2018).

4. Dong, A. et al. A generalized ligand-exchange strategy enabling sequential surface functionalization of colloidal nanocrystals. J. Am. Chem. Soc. 133, 998-1006 (2011).

5. Song, J. H., Choi, H., Pham, H. T. & Jeong, S. Energy level tuned indium arsenide colloidal quantum dot films for efficient photovoltaic s. Nat. Commun. 9, 4267 (2018).

6. Assaf Aharoni, Taleb Mokari, Inna Popov, and Uri Banin, "Synthesis of InAs/CdSe/ZnSe core/shelll/shell2 structures with bright and stable near-infrared fluorescence", J. Am. Chem. Soc. 128 (1): 257-264 (2006).

GENERAL DESCRIPTION

The inventors of the technology disclosed herein have successfully synthetized uniform luminescent anisotropic wurtzite semiconductor InP nanocrystals and other relevant systems. As disclosed herein, the nanocrystals have been achieved by combining synthetic control, to obtain narrowly distributed copper phosphide NCs, with a post-cation exchange reaction followed by a further reaction with a suitable fluoride agent, such as NOBF4. This unique approach enabled efficient removal of excess copper impurities that remained following the cation exchange, along with achieving suitable stoichiometric In:P ratios. The resultant wurtzite semiconductor nanocrystals, e.g., InP nanocrystals, manifest well resolved and narrow absorption and emission features. Their anisotropic nature was clearly expressed through their polarization characteristics, as studied on ensemble and on a single particle basis, supported by theoretical modelling. This anisotropic character is different from the isotropic cubic InP nanocrystals known in the art.

The uniform optoelectronic grade wurtzite nanostructures of InP of the present invention exhibited highly resolved energy peaks in the absorption spectrum and enhanced photoluminescence (PL) with fluorescence quantum yield reaching as high as 30-40%. Beyond the new type of nanocrystals, the kinetics and mechanism of the process developed by the inventors in achieving the exceptional characteristics was also investigated toward achieving optimized optical properties.

The unique anisotropic characteristics of these InP nanocrystals was manifested in anisotropy measurement by photo -selection. This indeed required high intensity signal (efficient PL and large anisotropy). The measurement of nanocrystals rotation time was made possible by the combination of relatively large anisotropy achieved by the crystal lattice and shape, the achieved intense photoluminescence and the long PL decay lifetime achieved by surface reaction, e.g., employing NOBF4.

Thus, in a first of its aspects, the invention concerns wurtzite anisotropic AX (semiconductor) nanocrystals of optoelectronic grade, wherein A is selected from In and Ga and wherein X is selected from P and As, and alloys thereof. The nanocrystals of the invention may be alternatively described as wurtzite anisotropic AX semiconductor nanocrystals, wherein A is selected from In and Ga and wherein X is selected from P and As, and alloys thereof.

Further provided are wurtzite anisotropic InP nanocrystals.

The invention further concerns a population of optoelectronic-grade wurtzite nanostructures of AX, wherein A is selected from In and Ga and wherein X is selected from P and As, and alloys thereof.

Also provided is an optoelectronic-grade population of anisotropic wurtzite AX nanocrystals, wherein A is selected from In and Ga and wherein X is selected from P and As.

In some embodiments, products of the invention having the formula AX are selected from InX and GaX. In some embodiments, the nanocrystals are of InP or InAs. In some embodiments, the nanocrystals are of GaP or GaAs.

In some embodiments, the product is InP.

In some embodiments, the product is an alloy of AX having the form AX1X2, wherein Xi and X2 are different and each is selected from P and As, and wherein the ratio A to (X1+X2) is between 0.8:1 and 1:0.8, or is substantially stoichiometric (1:1).

In some embodiments, the alloy is of the form InP _n As _m , InAs _n Pm, GaP _n As _m or GaAs _n Pm, wherein each of n and m, independently, is between 0 and 1. In some embodiments, the alloy is InP _n As _m or InAs _n Pm, wherein each of n and m, independently is greater than zero, but smaller than 1 (0<n<l, 0<m<l). In other words, the alloy may be of the form InP _n Asi- _n wherein n is between 0.01 and 0.99.

The invention further provides:

(1) anisotropic InP nanocrystals of optoelectronic grade; or

(2) InP wurtzite anisotropic nanocrystals; or

(3) a population of optoelectronic grade wurtzite nanostructures of InP.

As demonstrated herein, nanocrystals of the invention exhibit one or more of the following characteristics:

1. They are wurtzite in lattice structure, namely having hexagonal lattice symmetry;

2. They are anisotropic in shape;

3. They exhibit an anisotropic (polarized) band-gap emission and absorption;

4. They form narrowly size distributed population of nanocrystals; 5. They are uniform nanocrystals controlled in size in the range 5-15 nm;

6. They have a substantially stoichiometric A:X ratio, wherein A and X are as defined herein, e.g., wherein the ratio A:X is between 0.8: 1.2 or a ratio between 0.8:1 and 1:1.2;

7. They exhibit a narrow band gap emission peak (80-100meV);

8. They exhibit well resolved optical transitions in absorption;

9. They exhibit a photoluminescence quantum yield that is higher than 10%; reaching, in some embodiments, enhancement of 30-40% or higher;

10. They are of an optoelectronic grade; and

11. They may be provided in a variety of shapes including rod-like (quasi ID), plates (quasi 2D), spherical and others.

In some embodiments, the nanocrystals of the invention have a wurtzite lattice and In:P ratio between 0.8 to 1.2.

In some embodiments, the nanocrystals are spherically shaped or non- spherically shaped. In some embodiments, the nanocrystals are oblate in shape. In some embodiments, the nanocrystals are in a shape of platelets. In some embodiments, the nanocrystals are in a shape of rods. In some embodiments, the nanocrystals are amorphous in shape.

In some embodiments, the nanocrystals bear an aspect ratio between 0.6-0.9.

In some embodiments, the nanocrystals are of a thickness between 3-15nm.

In some embodiments, the nanocrystals are substantially monodisperse, having a standard deviation of size of less than 10%.

In some embodiments, the nanocrystals are photoluminescent.

In some embodiments, the nanocrystals have a fluorescence full-width-at-half- maximum of less than 120meV.

In some embodiments, the nanocrystals feature resolution of optical transitions in absorption with well-defined peaks.

The "wurtzite anisotropic AX nanocrystals of the invention or alloys thereof, are semiconductor nanocrystals selected as disclosed herein and exhibit each a hexagonal symmetry. The wurtzite lattice structure is known in the art. The wurtzite lattice structure can be considered formed by penetration of two hep lattices having the same axis (a3-axis), wherein one of the lattices is displaced with respect to the other. In the wurtzite structure, four atoms are present per unit cell. Hence, the average volume per atom in the wurtzite structure is given by (3/8)a2c. The wurtzite structure has uniaxial symmetry. Nanocrystals of the invention are not cubic in lattice structure.

A further discussion of the wurtzite structure may be found in the general state of the art, e.g., DeKisi, E. H. and Elcombe, M. M. (1989). Acta Cryst. C45, 1867-1870.

As used herein, the term “optoelectronic grade refers to the superior quality of nanocrystals of the invention, having high optical quality in terms of resolved absorption transitions and narrow emission, an observation that renders them suitable for use in numerous applications for which highly defined nanocrystals are needed, e.g., for use as free standing emitters in solution or in solid polymers matrices, as single photon emitters for quantum light sources, as devices in photocatalytic applications, in optoelectronic devices such as semiconductor LEDs, PLEDs, OLEDs, laser diodes, conversion layers in display and lighting applications, solar cells/photovoltaic cells, photodetectors, sensors, field effect transistors, or other types of solid-state devices, or as devices for optical conversion from blue to green or red or near infra-red (NIR).

Thus, as nanocrystals of the invention are of optoelectronic grade, and new and superior to others, the nanocrystals may be used, and the invention thus provides use of the nanoparticles as:

(1) free standing emitters when in solution or when embedded in a solid matrix such as a polymer matrix, and/or

(2) single photon emitters, e.g., as quantum light sources, and/or

(3) nanocrystals for photocatalytic applications, and/or

(4) nanocrystals for use or for implementing in optoelectronic devices, e.g., semiconductor LEDs, PLEDs, OLEDs, laser diodes, and others, and/or

(5) nanocrystals for use or for embedding in conversion layers, e.g., for use in display and lighting applications, and/or

(6) nanocrystals for use or for implementing in solar cells or photovoltaic cells, and/or

(7) nanocrystals for use or for implementing in photodetectors, and/or

(8) nanocrystals for use or for implementing in solid-state devices, and/or

(9) nanocrystals for use or for implementing in devices for optical conversion from blue to green or red or to NIR. The invention further provides a device implementing semiconductor nanocrystals of the invention, namely nanoparticles of optoelectronic grade. In some embodiments, the device is an optoelectronic device.

In some embodiments, the optoelectronic device comprises an optical element consisting or implementing a nanocrystal according to the invention.

In some embodiments, the device is an information display device.

Also provided is a process for manufacturing (or synthesizing) wurtzite anisotropic AX semiconductor nanocrystals, as defined herein, the process comprising treating metal phosphide or metal arsenide nanocrystals of a metal cation different from In or Ga under metal cation exchange conditions, to provide the wurtzite anisotropic AX nanocrystals, wherein A is a metal cation being In or Ga and X is an anion being P or As, wherein the wurtzite anisotropic AX nanocrystals are further treated with a fluoride-based agent.

The invention further provides a process for manufacturing (or synthesizing) wurtzite anisotropic AX semiconductor nanocrystals, as defined herein, the process comprising treating metal phosphide or metal arsenide nanocrystals under metal cation exchange conditions to form the wurtzite anisotropic AX nanocrystals and treating the wurtzite anisotropic AX nanocrystals AX nanocrystals with a fluoride -based agent, wherein each of A and X is as defined herein.

In some embodiments, AX is InX. In some embodiments, InX is InP.

In some embodiments, the wurtzite anisotropic AX nanocrystals is InP, InAs, GaP or GaAs.

As disclosed herein, processes of the invention comprise a step of treating metal phosphide or metal arsenide nanocrystals under cation exchange conditions. The “metal phosphide or metal arsenide nanocrysials ' are of a metal that is different from In metal or different from Ga metal or that is different from both metals. The expression “nonindium or non-gallium metal phosphide or arsenide nanocry sials " thus refers to nanocrystals of a metal phosphide or a metal arsenide, respectively, that is/are of a metal cation other than In or Ga. These metals may be copper, silver, zinc, nickel and others.

In some embodiments, a process of the invention comprises:

- obtaining non-indium or non-gallium metal phosphide or arsenide (X=P or As, respectively) nanocrystals; - treating the metal phosphide or metal arsenide nanocrystals under metal cation exchange conditions to form the wurtzite anisotropic AX nanocrystals; and

- treating the AX nanocrystals with a fluoride-based agent , wherein each of A and X is as defined herein.

In some embodiments, the process comprises

- obtaining non-indium or non-gallium metal phosphide or arsenide (X=P or As, respectively) nanocrystals;

- treating the metal phosphide or metal arsenide nanocrystals under metal cation exchange conditions to form AX nanocrystals; and

- treating the AX nanocrystals with a fluoride-based agent to afford the wurtzite anisotropic AX nanocrystals, wherein each of A and X is as defined herein.

In another aspect, there is provided wurtzite anisotropic AX nanocrystals formed according to processes disclosed herein, wherein each of A and X is as defined herein. The nanocrystals may be size varying nanospheres, nanoplates and nanorods or may be amorphous in structure or shape.

As demonstrated herein for products and processes of the invention, and as demonstrated for certain embodiments in Fig. 1, the synthesis of the wurtzite nanocrystals commences with a metal cation exchange of a non-indium metal phosphide nanocrystals. The non-indium metal phosphide, being any metal phosphide that is different from indium phosphide, e.g., copper phosphide, may be formed by any method known in the art, or may be used as commercially available, including the conversion of metal nanocrystals to metal phosphide. The non-indium metal forming the non-indium metal phosphide may be selected amongst various metal cations, e.g., non-trivalent metal cations, including copper, silver, zinc, nickel and others. Thus, in some embodiments, the non-indium or non-gallium metal phosphide material, in a form of nanocrystals, used as disclosed herein may be selected from copper phosphide, silver phosphide, zinc phosphide, nickel phosphide and others.

In some embodiments, a process of the invention may comprise:

- obtaining metal phosphide nanocrystals, wherein the metal phosphide is selected from copper phosphide, silver phosphide, zinc phosphide, and nickel phosphide;

- treating the metal phosphide nanocrystals under metal cation exchange conditions to form InP or GaP nanocrystals; and - treating the InP or GaP nanocrystals with a fluoride-based agent to afford the wurtzite anisotropic InP or GaP nanocrystals as disclosed herein.

In some embodiments, a process of the invention comprises:

- obtaining metal phosphide nanocrystals, wherein the metal phosphide is selected from copper phosphide, silver phosphide, zinc phosphide, and nickel phosphide;

- treating the metal phosphide nanocrystals under metal cation exchange conditions to form InP nanocrystals; and

- treating the InP nanocrystals with a fluoride-based agent to afford the anisotropic wurtzite InP nanocrystals disclosed heren.

In some embodiments, a process of the invention comprises:

- obtaining metal phosphide nanocrystals, wherein the metal phosphide is selected from copper phosphide, silver phosphide, zinc phosphide, and nickel phosphide;

- treating the metal phosphide nanocrystals under metal cation exchange conditions to form GaP nanocrystals; and

- treating the GaP nanocrystals with a fluoride-based agent to afford the wurtzite anisotropic GaP nanocrystals disclosed herein.

In some embodiments, the metal phosphide may be monodisperse.

In some embodiments, the monodisperse metal phosphide is copper phosphide.

In some embodiments, the metal phosphide is copper phosphide (e.g., in the non-deficient form CusP or in the deficient form Cu3- _x P, wherein x<3).

In some embodiments, the process comprises a step of forming the metal phosphide nanocrystals.

In some embodiments, the process comprises a step of forming copper phosphide nanocrystals, e.g., by a modified heat-up procedure utilizing copper acetate and triphenylphosphite as precursors.

In some embodiments, the process comprises obtaining copper phosphide nanocrystals by converting copper acetate into metallic copper and then further converting to copper phosphide in the presence of triphenylphosphite. In some embodiments, the copper phosphide is CusP or Cu3- _x P, wherein x<3. In some embodiments, the C113P nanocrystals used in processes of the invention are in a form of nanocrystals having an average diameter around about 9.4 ± 0.5 nm, e.g., as depicted for an exemplary copper phosphide in Fig. lb. In some embodiments, the CU3P nanocrystals are defined as Cus- _x P, wherein x<3.

In some embodiments, the process comprises treating copper phosphide nanocrystals under metal cation exchange conditions to form the InP or GaP anisotropic wurtzite nanocrystals and treating same with a fluoride-based agent to afford the (optoelectronic grade) InP or GaP nanocrystals.

Irrespective of the type of metal phosphide used, metal cation exchange to replace the metal cation with indium metal cation or a gallium metal cation, depending on the nanocrystals product to be formed, may be achievable by treating the metal phosphide nanocrystals, e.g., CU3P nanocrystals, with an indium cation source, or with a gallium cation source, being an indium salt or complex or a gallium salt or complex, respectively. The indium cation source may be selected amongst indium halide salts, such as indium chlorides, hypochlorites, chlorites, chlorates, perchlorates; indium carbonates; indium carboxylates; indium acetates; indium nitrates; indium cyanates; indium sulfides; indium sulfates; indium phosphates; indium amines; indium phosphines; indium phosphites; indium thiolates; combined cation-anion single source precursors, and others. Similarly, the gallium cation source may be selected amongst gallium halide salts, such as gallium chlorides, hypochlorites, chlorites, chlorates, perchlorates; gallium carbonates; gallium carboxylates; gallium acetates; gallium nitrates; gallium cyanates; gallium sulfides; gallium sulfates; gallium phosphates; gallium amines; gallium phosphines; gallium phosphites; gallium thiolates; combined cation-anion single source precursors, and others.

In some embodiments, the indium cation source or the gallium cation source used for achieving metal exchange to InX or GaX, respectively, as defined, is an indium halide or a gallium halide, such as an indium chloride (e.g., InCi and InCh) or an indium bromide, or a gallium chloride or a gallium bromide.

The cation exchange is achievable under conditions which may be varied and depend on such considerations including the particular indium or gallium source used, the non-indium or non-gallium metal used, the identity of anion X (P or As) and so forth. For example, an exchange to InP may be achievable by adding an indium halide salt, such as InBn, in the presence of at least one coordinating ligand with large volume and low formal charge known as “soft” within the Hard-Soft Acid-Base (HSAB) theory, which is optionally capable of dissolving copper significantly stronger than indium, such a ligand may be triocylpho sphine, in an aprotic solvent, such as octadecene, squalene or hexadecane, under thermal conditions, e.g., at a temperature of between 180 and 220 °C or around ~200°C.

The at least one coordinating ligand, e.g., having a large volume and low formal charge, and which is optionally capable of dissolving copper more significantly than indium, may be selected from such ligands known in the art. Based on the concept of preferred solvation of cooper ions and crystallization of indium ions, the coordinating ligand may be of the L-type. These L-type ligands may include neutral ligands that donate two electrons to the metal. These electrons may be lone pair electrons, pi electrons, or sigma donors. The bonds formed between L-type ligands and the metal are coordinate bonds. Non-limiting examples include C=O, PR3, NH3, amines, H2O, -SH and alkenes.

In some embodiments, the at least one coordinating ligands is selected from alkylphosphines, alkylthiols and alkylamines. In some embodiments, the coordinating ligand is an alkylphosphine such as trioctylphosphine. In some embodiments, the coordinating ligand is a dithiolate or a diamine.

In some embodiments, the process comprises:

- obtaining metal phosphide or arsenide (X=P or As, respectively) nanocrystals, wherein the metal phosphide is selected from copper phosphide, silver phosphide, zinc phosphide, and nickel phosphide;

- treating the metal phosphide or arsenide (X=P or As, respectively) nanocrystals under metal cation exchange conditions in the presence of at least one indium cation source selected from indium halide salts; indium carbonates; indium carboxylates; indium acetates; indium nitrates; indium cyanates; indium sulfides; indium sulfates; indium phosphates; indium amines; indium phosphines; and indium thiolates, or in the presence of at least one gallium cation source selected from gallium halide salts; gallium carbonates; gallium carboxylates; gallium acetates; gallium nitrates; gallium cyanates; gallium sulfides; gallium sulfates; gallium phosphates; gallium amines; gallium phosphines; and gallium thiolates to obtain the wurtzite anisotropic InX or GaX nanocrystals; - treating the wurtzite anisotropic InX or GaX nanocrystals with a fluoride-based agent to afford the InX or GaX nanocrystals of the invention, wherein X is P or As.

In some embodiments, the process comprises treating copper phosphide nanocrystals under metal cation exchange conditions to form the InP nanocrystals, and treating the InP nanocrystals with a fluoride -based agent to afford the anisotropic wurtzite InP nanocrystals, wherein the cation exchange conditions comprise:

(a) treating the copper phosphide nanocrystals with at least one indium cation source selected from indium halide salts; indium carbonates; indium carboxylates; indium acetates; indium nitrates; indium cyanates; indium sulfides; indium sulfates; indium phosphates; indium amines; indium phosphines; and indium thiolates;

(b) presence of at least one coordinating ligand, optionally capable of dissolving copper, such a ligand may be triocylpho sphine;

(c) presence of an aprotic solvent, such as octadecene, squalene or hexadecane.

In some embodiments, the process comprises

- treating copper phosphide nanocrystals with at least one indium cation source selected from indium halide salts; indium carbonates; indium carboxylates; indium acetates; indium nitrates; indium cyanates; indium sulfides; indium sulfates; indium phosphates; indium amines; indium phosphines; and indium thiolates; in the presence of at least one coordinating ligand, in an aprotic solvent, to form InP nanocrystals; and

- treating the InP nanocrystals with a fluoride agent to afford the anisotropic wurtzite InP nanocrystals.

Due to the covalent nature of III-V materials, known to cause a challenging surface chemistry affecting the colloidal stability and compromised low quality optical properties, a divalent cation such as Zn may be added to the cation exchange step in order to improve optical properties and stability of the nanocrystals. Thus, in some embodiments, the cation exchange step may proceed in the presence of divalent cations, such as zinc halides, e.g., Znh, which may be present in a similar concentration to the concentration of the indium cation source, e.g., InBn.

Thus, in some embodiments, the process comprises - treating the metal phosphide or arsenide (X=P or As, respectively) nanocrystals under metal cation exchange conditions in the presence of

(a) at least one indium cation source and at least one divalent cation, wherein the at least one indium cation source is selected from indium halide salts; indium carbonates; indium carboxylates; indium acetates; indium nitrates; indium cyanates; indium sulfides; indium sulfates; indium phosphates; indium amines; indium phosphines; and indium thiolates, to obtain InX nanocrystals; or

(b) at least one gallium cation source and at least one divalent cation, wherein the at least one gallium cation source is selected from gallium halide salts; gallium carbonates; gallium carboxylates; gallium acetates; gallium nitrates; gallium cyanates; gallium sulfides; gallium sulfates; gallium phosphates; gallium amines; gallium phosphines; and gallium thiolates, to obtain GaX nanocrystals, and

- treating the InX or GaX nanocrystals with a fluoride-based agent to afford the anisotropic wurtzite InX or GaX nanocrystals, wherein X is P or As.

In some embodiments, the process comprises treating copper phosphide nanocrystals under metal cation exchange conditions to form the InP or GaP nanocrystals, and treating same with a fluoride -based agent to afford the anisotropic wurtzite InP or GaP nanocrystals, wherein the cation exchange conditions comprise:

(a) treating the copper phosphide nanocrystals with at least one indium cation source selected from indium halide salts; indium carbonates; indium carboxylates; indium acetates; indium nitrates; indium cyanates; indium sulfides; indium sulfates; indium phosphates; indium amines; indium phosphines; and indium thiolates; or at least one gallium cation source selected from gallium halide salts; gallium carbonates; gallium carboxylates; gallium acetates; gallium nitrates; gallium cyanates; gallium sulfides; gallium sulfates; gallium phosphates; gallium amines; gallium phosphines; and gallium thiolates;

(b) presence of at least one divalent cation;

(c) presence of at least one coordinating ligand, optionally capable of dissolving copper, such a ligand may be trioctylphosphine;

(d) presence of an aprotic solvent, such as octadecene, squalene or hexadecane.

In some embodiments, the process comprises - treating copper phosphide nanocrystals with at least one indium cation source and at least one divalent cation, wherein the at least one indium cation source is selected from indium halide salts; indium carbonates; indium carboxylates; indium acetates; indium nitrates; indium cyanates; indium sulfides; indium sulfates; indium phosphates; indium amines; indium phosphines; and indium thiolates; in the presence of at least one coordinating ligand, in an aprotic solvent, to form InP nanocrystals; and

- treating the InP nanocrystals with a fluoride agent to afford the anisotropic wurtzite InP nanocrystals.

In some embodiments, the divalent cation used in processes of the invention may be provided in a form of any salt or complex comprising a divalent cation. The salt or complex of the divalent cation is typically an inorganic salt comprising a divalent metal cation and an inorganic monovalent or divalent anion. The divalent metal cation may be selected from Zn, Cd, Pb, Sn, Mg, Ca, Ba, Co, Mn and others.

In some embodiments, the divalent cation is provided in a form of a salt such as a halide salt or other monovalent anions or organic stabilized (e.g., Zn-oleate, Zn- stearate, Zn-palmitate and others). In some embodiments, the salt is a chloride or a bromide or an iodide salt of Zn, Mg, Ca, Ba, Co, or Mn. In some embodiments, the divalent cation is provided in a form of Znh, ZnCh, or ZnBn.

The process of the invention may comprise a step of allowing the nanocrystals to precipitate, and/or a step of isolating the precipitated nanocrystals. Further, the process may comprise a step of redispersing the nanocrystals in a non-polar solvent which may be the same or different from the aprotic solvent used. Such non-polar solvents may include hexane.

Once the anisotropic InP nanocrystals are obtained after the nanocrystals precipitation, they may be redispersed in a non-polar solvent, such as a non-polar organic solvent, e.g., in hexane. The nanoparticles may then be treated in the presence of at least one fluoride-based agent or a fluoride salt. The fluoride-based agent, which may be a fluoride salt, is selected based on its ability to strip off organic ligands and efficiently etch off, from the surface of the nanocrystals, a layer containing remnants of the non-indium metal, mainly residual copper and some of the zinc and also removing PO _X species from the surface layers.

In some embodiments, the fluoride-based agent is an etchant, optionally in a form of a fluoride salt selected to cause etching off of any of the non-indium or non- gallium metals more efficiently than indium or gallium, respectively, leading to impurity removal and slow etching of InP, thereby causing surface passivation and enhancement of photoluminescence.

In some embodiments, the fluoride -based agent is selected from HF, NOBF4, Et ₃ OBF ₄ , HBF ₄ , NH ₄ BF ₄ and HPF ₆ .

In some embodiments, the fluoride-based agent is NOBF ₄ , optionally provided in a solvent such as dimethyl-formamide.

Figs. 2a-b show ultra-high resolution (UHR)-TEM and Energy Dispersive Spectroscopy (EDS) mapping before and after surface-ligand exchange step, e.g., utilizing a fluoride-based agent such as NOBF ₄ . UHR-TEM images show a decrease in the size of the InP nanocrystals after exposure to NOBF ₄ to a size ranging between 10.6 ± 0.5 to 8.5 ± 0.8 nm. The cation exchanged InP nanocrystals surface contained an amorphous shell of PO _X material with zinc and copper, and their amount is significantly reduced after exposure to NOBF ₄ . Fig. 2c shows the elemental quantification from EDS mapping where the copper amount was decreased significantly from 0.06 to less than 0.01 Cu/In, while the zinc amount was decreased only by half from 0.08 to 0.03 Zn/In. The P:In ratio decreased from nearly 1.5 before exposure to NOBF ₄ to substantially stoichiometric, e.g., 1.1 ratio, after exposure to the fluoride-based agent. The removal of copper and zinc was more pronounced in X-ray Photoelectron Spectroscopy (XPS) compared to EDS, further demonstrating the significant effect on the nanocrystal surface. Further characterization by XPS showed a decrease in a peak at 132eV of P (2p) which could be assigned to remnants of copper phosphide. Taken together, and as depicted in Fig. 2d, it may be concluded that the surface layer was mostly composed of impurities including copper, zinc and PO _X species and its removal led to an absorption spectrum with well resolved transitions.

Remarkably, the absorption of the fluoride -based agent reacted InP nanocrystals, manifested sharp transitions, a clear signature to the optoelectronic grade of the InP nanocrystals nearly stoichiometric In:P ratio, and with minimal impurities.

Moreover, with regards to the photoluminescence, as the reaction time of the InP nanocrystals with NOBF ₄ , was prolonged, the photoluminescence intensity was enhanced, reaching a quantum yield value of 30-40%. This enhancement in photoluminescence was accompanied by a gradual blue shift of the band gap position consistent with surface etching and decreasing size of the nanocrystals. During this process the absorption features were slightly smeared, due to the broadened size distribution. Nonetheless, significant fluorescence quantum yield was attained for these core-only InP nanocrystals, while the absorption features remained resolved.

A narrow emission requires narrow size distribution, but the available copper phosphide in the literature suffers from broad size distribution. In order to achieve uniform samples for resolved absorption and narrow emission, by careful choice of the copper precursors, uniform size was achieved by simple heat-up synthesis. Also, the InP exchanged from CusP are non-emitting and show broad absorption. Even after two times cation exchange and a growth of ZnSe shell the emission intensity is very low and the absorption is without sharp features. The 40% fluorescence quantum yield achieved by the surface reaction here for core only InP is expected to be further enhanced by wide gap semiconductor shell growth, such as ZnSe and ZnS.

The use of NOBF4 according to a process of the invention allows for the selective fast dissolution and removal of copper - which was determined critical for achieving a stoichiometric composition and impurities-free nanocrystals. This led to the optoelectronic grade nanocrystals. Additionally, the slow etching of the InP nanocrystals, removing PO _X and metal oxides, led to an improved surface passivation. These processes were correlated with the appearance of absorption with well-defined features and narrow emission peaks. Indeed, when performing the NOBF4 reaction on InP nanocrystals with cubic (zinblende) lattice structure, prepared by traditional synthesis (by injecting tris(diethylamino)phosphine to a solution containing InCE and ZnL in oleylamine), no improvement in optical properties (absorption peak resolution and emission enhancement) was observed. This non-improvement of optical properties was also demonstrated for InAs.

Thus, the invention generally provides a process for manufacturing optoelectronic wurtzite anisotropic AX semiconductor nanocrystals, the process comprising treating a metal phosphide or metal arsenide nanocrystals of a metal cation different from In or Ga under metal cation exchange conditions to provide the wurtzite anisotropic AX nanocrystals, wherein A is a metal cation being In or Ga and X is an anion being P or As, and reacting the nanocrystals with a fluoride-based agent.

In some configurations of the invention, a process according to the invention comprises treating the metal phosphide or arsenide nanocrystals under metal cation exchange conditions to form the wurtzite anisotropic AX nanocrystals and treating the wurtzite anisotropic AX nanocrystals with a fluoride-based agent.

In some configurations of the invention, the wurtzite anisotropic AX nanocrystals is InP, InAs, GaP or GaAs.

In some configurations of the invention, the wurtzite anisotropic AX nanocrystals are of InP.

In some configurations of the invention, a process according to the invention comprises:

- obtaining the metal phosphide or arsenide nanocrystals;

- treating the metal phosphide or arsenide nanocrystals under metal cation exchange conditions to afford the wurtzite anisotropic AX nanocrystals; and

- treating the wurtzite anisotropic AX nanocrystals with a fluoride-based agent.

In some configurations of the invention, the metal phosphide nanocrystals are of a non-indium metal phosphide.

In some configurations of the invention, a process according to the invention comprises:

- obtaining the non-indium metal phosphide nanocrystals, wherein the nonindium metal phosphide is selected from copper phosphide, silver phosphide, zinc phosphide, and nickel phosphide;

- treating the non-indium metal phosphide nanocrystals under metal cation exchange conditions to form InP nanocrystals; and

- treating the InP nanocrystals with a fluoride-based agent to afford the wurtzite anisotropic InP nanocrystals.

In some configurations of the invention, the metal phosphide or metal arsenide is monodisperse.

In some configurations of the invention, the metal phosphide is copper phosphide.

In some configurations of the invention, a process according to the invention comprises forming the metal phosphide or metal arsenide nanocrystals.

In some configurations of the invention, a process according to the invention comprises forming the copper phosphide.

In some configurations of the invention, a process according to the invention comprises: - obtaining the metal phosphide or arsenide nanocrystals, wherein the metal phosphide is selected from copper phosphide, silver phosphide, zinc phosphide, and nickel phosphide; and wherein the metal arsenide is selected from copper arsenide, silver arsenide, zinc arsenide, and arsenide;

- treating the metal phosphide or arsenide nanocrystals under metal cation exchange conditions in the presence of:

(a) at least one indium cation source selected from indium halide salts; indium carbonates; indium carboxylates; indium acetates; indium nitrates; indium cyanates; indium sulfides; indium sulfates; indium phosphates; indium amines; indium phosphines; and indium thiolates; or

(b) at least one gallium cation source selected from gallium halide salts; gallium carbonates; gallium carboxylates; gallium acetates; gallium nitrates; gallium cyanates; gallium sulfides; gallium sulfates; gallium phosphates; gallium amines; gallium phosphines; and gallium thiolates to obtain the anisotropic wurtzite AX nanocrystals;

- treating the anisotropic wurtzite AX nanocrystals with a fluoride-based agent.

In some configurations of the invention, the metal phosphide is copper phosphide.

In some configurations of the invention, a process according to the invention comprises treating copper phosphide nanocrystals under metal cation exchange conditions to form InP nanocrystals, and treating the InP nanocrystals with a fluoride agent to afford the wurtzite anisotropic InP nanocrystals, wherein the cation exchange conditions comprise:

-treating the copper phosphide nanocrystals with at least one indium cation source selected from indium halide salts; indium carbonates; indium carboxylates; indium acetates; indium nitrates; indium cyanates; indium sulfides; indium sulfates; indium phosphates; indium amines; indium phosphines; and indium thiolates;

-presence of at least one coordinating ligand; and

-presence of an aprotic solvent.

In some configurations of the invention, a process according to the invention comprises

- treating copper phosphide nanocrystals with at least one indium cation source selected from indium halide salts; indium carbonates; indium carboxylates; indium acetates; indium nitrates; indium cyanates; indium sulfides; indium sulfates; indium phosphates; indium amines; indium phosphines; and indium thiolates; in the presence of at least one coordinating ligand, in an aprotic solvent, to form InP nanocrystals; and

- treating the InP nanocrystals with a fluoride -based agent to afford the wurtzite anisotropic InP nanocrystals.

In some configurations of the invention, a process according to the invention comprises

-treating copper phosphide nanocrystals under metal cation exchange conditions to form InP nanocrystals, and treating the InP nanocrystals with a fluoride-based agent to afford the wurtzite anisotropic InP nanocrystals, wherein the cation exchange conditions comprise:

-treating the copper phosphide nanocrystals with at least one indium cation source selected from indium halide salts; indium carbonates; indium carboxylates; indium acetates; indium nitrates; indium cyanates; indium sulfides; indium sulfates; indium phosphates; indium amines; indium phosphines; and indium thiolates;

-presence of at least one divalent cation;

-presence of at least one coordinating ligand, optionally capable of dissolving copper;

-presence of an aprotic solvent.

In some configurations of the invention, the fluoride-based agent is a fluoride salt selected to cause etching of the non-indium metals more efficiently than indium.

In some configurations of the invention, the fluoride-based agent is selected from HF, NOBF ₄ , Et ₃ OBF ₄ , HBF ₄ , NH ₄ BF ₄ and HPF ₆ .

In some configurations of the invention, the fluoride-based agent is NOBF ₄ .

A process for manufacturing wurtzite anisotropic InP nanocrystals, the process comprising:

- treating a metal phosphide different from an indium phosphide under metal cation exchange conditions in the presence of at least one indium cation source selected from indium halide salts; indium carbonates; indium carboxylates; indium acetates; indium nitrates; indium cyanates; indium sulfides; indium sulfates; indium phosphates; indium amines; indium phosphines; and indium thiolates; to obtain InP nanocrystals;

- treating the InP nanocrystals with NOBF ₄ to afford the anisotropic wurtzite InP nanocrystals. In some configurations of the invention, a process according to the invention comprises treating copper phosphide under metal cation exchange conditions in the presence of an indium halide salt to obtain the anisotropic wurtzite InP nanocrystals and treating the InP nanocrystals with NOBF4.

In some configurations of the invention, the cation exchange conditions comprise treating copper phosphide in presence of at least one divalent cation, at least one coordinating ligand, and in an aprotic solvent.

In some configurations of the invention, the cation exchange conditions comprise thermal treatment at a temperature between 180 and 220°C.

A Wurtzite anisotropic nanocrystal of a semiconductor material having structure AX, or an alloy thereof, wherein A is In or Ga and X is P or As.

In some configurations of the invention, a nanocrystal of the invention having a photoluminescence quantum yield higher than 10%.

In some configurations of the invention, a nanocrystal of the invention being of an optoelectronic grade.

A population of wurtzite nanocrystals of semiconductor AX, or an alloy thereof, wherein A is In or Ga and X is P or As.

In some configurations of the invention, a nanocrystal of the invention being InX nanocrystal or an alloy thereof.

In some configurations of the invention, a nanocrystal of the invention being GaX nanocrystal or an alloy thereof.

In some configurations of the invention, a nanocrystal of the invention being AP nanocrystal, or an alloy thereof, wherein A is In or Ga.

In some configurations of the invention, a nanocrystal of the invention being AAs nanocrystals, or an alloy thereof, wherein A is In or Ga.

In some configurations of the invention, a nanocrystal of the invention being InP, InAs, GaP, GaAs or an alloy thereof.

In some configurations of the invention, a nanocrystal of the invention being InP nanocrystal or an alloy thereof.

In some configurations of the invention, a nanocrystal of the invention being an alloy of the form InP _n Asi- _n , wherein n is between 0.01 and 0.9.

A Wurtzite anisotropic InP or InP _n Asi- _n semiconductor nanocrystal, wherein n is between 0.01 and 0.9. In some configurations of the invention, a nanocrystal of the invention having a substantially stoichiometric A:X ratio.

In some configurations of the invention, a nanocrystal of the invention being non-spherical.

In some configurations of the invention, a nanocrystal of the invention being spherical, rod-like or platelet in structure.

In some configurations of the invention, a nanocrystal of the invention being for use:

(1) as a free-standing emitter in solution or embedded in a solid matrix, and/or

(2) as a single photon emitter, and/or

(3) as a nanocrystal for photocatalytic applications, and/or

(4) as a nanocrystal for implementing in optoelectronic devices, and/or

(5) as a nanocrystal for embedding in conversion layers, and/or

(6) as a nanocrystal for implementing in solar cells or photovoltaic cells, and/or

(7) as a nanocrystal for implementing in a photodetector, and/or

(8) as a nanocrystal for implementing in a camera, and/or

(9) as a nanocrystal for implementing in a solid-state device, and/or

(10) as a nanocrystal for implementing in a device for optical conversion.

A device implementing a nanocrystal according to the invention.

In some configurations of the invention, a device of the invention being an optoelectronic device.

In some configurations of the invention, a device of the invention comprising an optic element consisting or implementing the nanocrystal.

In some configurations of the invention, a device of the invention being an information display device.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

Figs, la-f depict CusP NCs conversion to luminescent w-InP. (a) scheme describing the cation exchange of CusP to InP and further reaction by NOBF4. (b) TEM image of C113P NCs (9.4 ± 0.5 nm) (c) TEM image of 6.5 ± 0.6 nm InP after cation exchange and NOBF4 treatment. The scale bars are 50 nm. (d) UHR-TEM images and Fourier Transform on two different NCs oriented with their c-axis perpendicular to each other, showing the lattice and shape anisotropy. The scale bars are 5 nm. (e) absorption before and after cation exchange showing the removal of CU3P plasmon peak and the appearance of the InP band gap. (f) The absorption and emission spectra after reaction with NOBF4.

Figs. 2a-h provide characterization of the cation exchange: (a-c) CU3P, CusP/InP and InP. (d-e) EDS maps of CU3P with copper (green), phosphorous (yellow) and oxygen (purple). Indicating oxidation layer surrounding the NCs before cation exchange, (f-g) Electron diffraction pattern for CU3P and InP, showing their respective lattice periodicities, (h) X-Ray Diffraction data before for CU3P and InP.

Figs. 3a-h demonstrate the effect of reaction with IM NOBF4 on the NCs size: UHR-TEM images w-InP samples, (a) after cation exchange, (b) after short NOBF4 reaction of ~5 minutes, and (c) after reaction of 2 hours, (d-e) The atomic ratios of (d) copper, and zinc of samples a-c by ICP, EDS and XPS. (f-g) The decrease of In-0 XPS signal in O(ls) and In(3d) of InP after cation exchange and after NOBF4 reaction for 0, 2 and 4 hours, (h) The phosphorous signal of copper phosphide, InP after cation exchange and after NOBF4 reaction for 0, 2 and 4 hours.

Figs. 4a-d depict the chemical effects of NOBF4 on w-InP NCs. (a) UHR-TEM and EDS elemental mapping of cation exchanged InP, showing the surface copper, zinc and oxides, (b) UHR-TEM and EDS elemental mapping of NOBF4 reacted InP, showing the decrease in surface copper and zinc, (c) The removal of copper and zinc impurities and the decrease of phosphorous to indium ratio as measured by EDS. (d) A scheme describing the removal of organic ligands and metal oxide by NOBF4 reaction.

Figs. 5a-e demonstrate the NOBF4 effect on optical properties of w-InP NCs. The effect of NOBF4 in time on (a) the absorption spectra, (b) the PL spectra, and (c) on the PL decay fitted to tri-exponential function with 2, 12 and 60ns (the inset shows the decrease in short lifetime coefficient and the increase in long lifetime coefficient), (d) In(3d) XPS signal shift to a higher energy of the peak assigned to In-0 bon indicates an In-F bond formation. O(ls) shows a decrease in the feature assigned to oxidized indium compared to oxidized phosphorous after the addition of NOBF4. F(ls) XPS shows an increase in the signal assigned to In-F beside that of B-F. (e) A scheme describing the slow etching and passivation process by reaction with the fluoride agent.

Figs. 6a-d demonstrate the effect of temperature and molar ratio in NOBF4 reaction, (a) The rate of PL-QY enhancement by NOBF4 treatment in room temperature and various elevated temperature, (b) Arrhenius plot of the initial rate of PL-QY enhancement as a function of inverse reaction temperature, (c) The rate of PL-QY enhancement by NOBF4 reaction in various NOBF4:NC molar ratios (lM=>lmmol NOBF4, the NCs amount - Inmol), (d) initial rate and the maximal PL-QY as a function of molar ratio.

Figs. 7a-d demonstrates the anisotropic properties of w-InP NCs. (a) PLE spectra, measured with excitation and emission in parallel (VV) and perpendicular to each other (VH) and the PLE anisotropy. In the inset, a scheme demonstrates the electronic energy level splitting in the valence band and their transition dipole moment orientation, (b) Time-resolved PL, measured with excitation and emission in parallel (VV) and perpendicular to each other (VH) and the time -resolved anisotropy, (c) Single particle measurement of the emission polarization as a function of polarizer angle of a representative particle, (d) Polarization values for 65 particles.

Figs. 8a-e demonstrate the copper precursor anion effect on copper phosphide NCs size: (a) TEM images of Cu3- _x P NCs synthesized with Cu(acetate)2 (b) Cu(tartrate)2 (c) and Cu(NOs)2 (c). (d) CuCh (e) The number of atoms per NC and the NCs concentration achieved by using Cu(acetate)2, Cu(tartrate)2 and Cu(NOs)2.

Figs. 9a-e demonstrate NCs size increased with precursor concentration. TEM image of Cu3- _x P NCs with different precursor concentration (keeping CuAc2:HDA:TPP = 1:10:10 constant) (a) lOmM. (b) 25mM. (c) 40mM. (d) 66mM. (e) Number of atoms per NC and NCs concentration as a function of initial concentration.

Figs. lOa-b demonstrate NCs size increased by higher ligands to copper ratio. TEM image of Cu3- _x P NCs with varying copper ratio to ligands (a) Cu(acetate) ₂ :HDA:TPP = 1:10:10. (b) Cu(acetate) ₂ :HDA:TPP = 1:30:30. (a) Cu(tartrate) ₂ :HDA:TPP = 1:10:10. (b) Cu(tartrate) ₂ :HDA:TPP = 1:30:30.

Figs, lla-b show the optical spectra of size controlled final w-InP: (a) absorption spectra demonstrating the optical transitions due to the valence band splitting, (b) Emission spectra of same particles. DETAILED DESCRIPTION OF EMBODIMENTS

The manufacturing process of the invention is exemplified in the following discussion relating mainly to the manufacturing of InP. However, the same process may be extended to the manufacturing of InAs as well.

The luminescent w-InP NCs were achieved by three steps as illustrated in Fig. la: (i) synthesis of copper phosphide nanocrystals; (ii) the copper cation exchange by indium; and (iii) the surface ligand exchange and surface traps passivation by NOBF4. Highly monodisperse CU3P NCs were synthesized by a modified heat-up procedure while utilizing copper acetate and triphenylphosphite as precursors. The CU3P nanocrystals diameter was 9.4 ± 0.5 nm as seen in TEM image (Fig. lb). Notably, the size and the size distribution in this case were smaller compared to those obtained by using CuCh.

The CU3P nanocrystals exchange to InP was performed by adding InBr,, triocylpho sphine and octadecene based on a prior method. Particularly, ZnE was added at a similar concentration to the InBn. Beyond improving the colloidal stability of the product, the addition of Zn is known to improve the optical properties of cubic InP in the direct synthesis route of cubic phase InP. The cation exchange was directly accompanied by the disappearance of the CU3P plasmon at ~0.9eV and the appearance of the InP band gap feature at ~1.5eV (Fig. 1c). Direct structural evidence of the exchange process is given by TEM as well as by electron and X-ray diffraction (Fig. 2). Despite the uniform size, the absorption features cannot be distinguished, and no photoluminescence was observed.

In the last stage of the synthesis, the nanocrystals in hexane (~ Inmol in 1ml) where added to NOBF4 molecules in dimethyl-formamide (DMF, lOOmg in 1ml). The DMF and hexane phases were separated, and after several minutes, the nanocrystals were transferred from the hexane phase to the DMF. After the treatment, as will be further detailed below, their size was significantly decreased (TEM image in Fig. Id and Fig. 3). Remarkably, the absorption features of the first three optical transitions were clearly resolved, and a narrow band gap emission peak (80-100meV) was obtained (Fig. le). The photoluminescence quantum yield was enhanced with time up to 30%. To arrest the etching process and for further analysis, the nanocrystals were transferred back to the oleylamine (OAm) in hexane phase. The photoluminescence and absorption spectra were unchanged after the back-transfer, but the photoluminescence was very slowly increased, and after several days in OAm solution up to 39% QY was measured.

The effect of the NOBF4 treatment on the InP nanocrystals was studied next. Fig. 4a-b show Ultra high resolution (UHR)-TEM and Energy Dispersive Spectroscopy (EDS) mapping before and after the NOBF4 treatment. UHR-TEM images show a decrease in the size of the nanocrystals after the exposure to NOBF4 to from 10.6 ± 0.5 to 8.5 ± 0.8 nm. EDS mapping shows a thick layer (~lnm) of oxide before and after the exposure to NOBF4. This layer was formed on the initial CU3P nanocrystals (Fig. 2) and it seems to be persistent in all steps. However, the cation exchanged InP nanocrystals surface contains an amorphous shell of PO _X material with zinc and copper, and their amount is reduced after exposure to NOBF4. Fig. 3c shows the elemental quantification from EDS mapping where the copper amount was decreased significantly from 0.06 to less than 0.01 Cu/In, while the zinc amount was decreased only by half from 0.08 to 0.03 Zn/In. The P:In ratio decreased from nearly 1.5 before the exposure to NOBF4 to nearly stoichiometric, namely to 1.1 after treatment. The removal of copper and zinc is more pronounced in X-ray Photoelectron Spectroscopy (XPS) compared to EDS, further demonstrating that this occurs mainly on the surface (see Fig. 3). Further characterization by XPS showed a decrease in a peak at 132eV of P (2p) which could be assigned to remnants of copper phosphide. Taken together, as shown in Fig. 4d, it may be concluded that the surface layer is mostly composed of impurities including copper, zinc and POx species and their removal directly leads to an absorption spectrum with well resolved transitions. Despite the peel-off, the phosphorous oxidation peak (134eV) relative to the bulk phosphorous remained unchanged (Fig. 4d), suggesting that despite the oxide layer removal, surface phosphorous keeps oxidizing during the etching process performed in ambient conditions.

A further knob for controlling the product outcome in this step of the NOBF4 reaction, is the reaction time. This was followed by measuring the dependence of the absorption and emission at different reaction times (Fig. 5a-b). As described above, immediately after organic ligand removal and transfer from non-polar to polar solvent, the absorption features are well distinguished and a narrow band gap fluorescence signature emerges - this is a strong indication for the optoelectronic grade achieved material. Upon increasing reaction time of the nanocrystals with NOBF4, the photoluminescence intensity was enhanced reaching a quantum yield value of 30% after 7 hours. This enhancement is accompanied by a gradual blue shift of the band gap position consistent with surface etching and decreasing size of the nanocrystals. During this process the absorption features are slowly smeared, due to the broadened size distribution.

Nonetheless, at intermediate times of 3-5 hours, significant fluorescence quantum yield was attained for these core-only InP samples, while the absorption features remained resolved. In this context, it is important to mention that at any desired time it is possible to stop the etching effect by transfer back to hexane with oleylamine ligands. The effect of the NOBF4 reaction time was also followed by time-resolved photoluminescence decay. The photoluminescence enhancement is accompanied by the elongation of the photoluminescence decay (Fig. 5c). Fitting to tri-exponential function, with lifetime components of 2ns, 12ns and 60ns yields an increase of the long lifetime pre-exponential factor (A3) and a decrease of the short one (Al). This is consistent with decreased non-radiative rate competing with the radiative recombination, which is expressed also in the enhanced photoluminescence intensity. HF was reported to have similar effect on the optical properties of cubic phase InP nanocrystals, including the slow enhancement in photoluminescent intensity and elongation of photoluminescence lifetimes.

To examine the NOBF4 reaction mechanism, especially in relation to the emission properties that are governed by the nanocrystal surface due to the presence and positioning of surface traps, XPS analysis was performed at different reaction times (Fig. 5d-e, Fig. 3). The surface P:In ratio, decreases throughout the process, and the nanocrystal surface transforms upon the treatment from phosphorous to indium rich (Fig. 3). The exchange from O to F binding to indium is revealed by the indium (3d) signal, where the In-0 signal at 444.8eV is shifted to 445.4eV, consistent with exchange from In-0 to the more electronegative In-F (Fig. 5d). The indium passivation on the surface is also supported by examining the F(ls) XPS peak (Fig. 5e). Two peaks, at 687eV and 685eV, are observed. The 687eV peak is assigned to remnants of BF4 after the back transfer to OAm ligands. The 685eV peak directly indicates the formation of In-F bonding. Furthermore, the oxygen (Is) peak manifests with time a decrease in the In-0 contribution at 531eV relative to that of P-0 at 532eV (Fig. 3). The XPS results show that the involvement of F- mild etching and passivation is responsible to the PL enhancement and lifetime elongation similar to that reported for HF treatment of cubic phase InP nanocrystals, hence further supporting the F- passivation mechanism. The mechanism here likely involves BFF decomposition to BF3 and F“ analogous to that in Balz-Schieman reaction of aromatic compounds fluorination.

Further enhancement of the NOBF4 reaction process was achieved at higher temperatures and upon increased concentrations as seen from following the photoluminescence intensity with time (Fig. 6). While the optimal quantum yield does not depend on temperature, the enhancement rate strongly depends on it and follows an Arrhenius dependence with an activation energy of ~30 kJ/mol (Fig. 6a-b). This activation barrier can be assigned to the HF formation from BF4 molecules or the surface F“ passivation and etching, as both reactions have shown similar temperature dependence. Changing the molar ratio of NOBF4 to InP shows that it is critical for the final quantum yield that can be achieved. A critical molar ratio around 106 NOBF4 molecules per nanocrystal (or 100 NOBF4/Indium atom) was required (Fig. 6c-d). Slightly below this point the photoluminescence was negligible and slightly above it was saturated. This understanding of the control of this reaction allows to achieve nanocrystals with optimized absorption features and high quantum yield by tuning time, concentration and temperature. The reaction is stopped at the desired time by the phase transfer to hexane with OAm as described above.

By the described procedure, uniform and luminescent wurtzite InP nanocrystals were achieved. The wurtzite crystal structure is expected to be reflected in the electronic energy levels and their polarization. To address this, we used ensemble polarization resolved Photoluminescence Excitation (PLE). This was performed by the excitation photo- selection method, where the exciting light is polarized vertically and the emission is measured in vertical (I_VV) and horizontal (I_VH) polarizations. The PLE in Fig. 7a yields clearly resolved wavelength dependent optical transitions equivalent to those in the absorption spectra in Fig. Id and 3d. I vv, I_VH and the anisotropy R:

R=(I _VV-I_VH)/ (I_VV+2I VH) are plotted in Fig. 7a, showing that peaks/valleys in R are correlated with the three peaks in the PLE and absorption spectra. The three observed peaks are termed A, B and C and are due to spin-orbit and crystal-field splitting.

The measured anisotropy value R depends on both, the emission and absorption anisotropies. The transition from the lowest energy level in the valence band (transition A) to the first excited level in the conduction band is polarized perpendicular to the c- axis. The emission polarization is the same as transition A, so R is maximized for this transition. The next transition (B) is also correlating with a peak in anisotropy, owing to a stronger oscillator strength in the a-b plane, but the smaller anisotropy value indicates higher contribution from polarization parallel to the c-axis. In contrast with the transitions A and B, the peak assigned to transition C correlates with a dip in anisotropy, suggesting that it is more dominantly polarized parallel to the c-axis. These values agree with theoretical and experimental results on w-InP bulk and nanowires grown by physical methods. At the high energy side of the PLE spectrum the anisotropy value is less than 0.01. In this regime the optical transitions are polarized in all directions and expected to be averaged to zero, so this small value can be due to the slightly oblate shape with c < a = b, facilitating photo -selection of the excitation light along the a-b plane by the dielectric effect. The energy splitting are A AB=120meV and A AC=300meV, are much larger compared to the reported bulk splitting value (A AB=30- 50meV and A AC=140-180meV). This is consistent with quantum confinement effect in our system.

The anisotropy value at the band-edge reaches to 0.065, significantly higher compared to similar shapes of wurtzite CdSe NCs. The actual anisotropy value can be even higher. In case where the rotation time (9 rot) is comparable to the average fluorescence lifetime (r ave), the nanocrystals rotation reduces the emission polarization in steady state measurements. To measure this effect, we have performed time-resolved anisotropy decay measurement (Fig. 6b). The excitation wavelength is at 740nm, close to the band gap, and polarized vertically, and the time-resolved photoluminescence is detected at 770nm in vertical (I vv) and horizontal (I VH) polarizations. At short time R=O.O88, higher than that measured in steady state measurement, followed by an exponential decay, R=R_0 e ^A (-t/9_rot), by the depolarization due to the rotation of the NCs. The anisotropy decay time, 9 _ro t=102nm, yields a calculated hydrodynamic nanocrystals diameter of 11.5nm by using the Stokes-Einstein-Debye relation. This agrees with the inorganic measured dimensions by UHR-TEM image (Fig. 4b) considering also the oleylamine ligands length.

The measured anisotropy in the photo -selection method depends on both absorption and emission polarization. To address the polarization of the band-edge emission we have measured single -particle emission polarization of the w-InP nanocrystals embedded in PMMA matrix on a glass coverslip. Upon circularly polarized excitation, the particles emit polarized light, which was separated into two components (vertical and horizontal), and imaged on a fast EMCCD camera. The degree of polarization P is given by:

P=(I _V-I_H)/(I_V+I_H ), as extracted from the images. The DOP (degree of polarization, P) as a function of polarizer angle of a representative particle is shown in Fig. 7c, following a sinusoidal dependence with a maximum and minimum polarization value (P). P values for 65 particles are shown in the histogram (Fig. 7d) giving a mean value of P-0.43. This value is close to the 0.5 value expected for in-plane polarized emission for single w-InP nanocrystals with random orientation on the coverslip. As the band edge transition is polarized perpendicular to the c-axis, this gives a value of P=-0.86 and emission anisotropy of 0.45. The total anisotropy can be calculated from this to be R=0.08, similar to the fitted anisotropy value (R_0) measured by photo-selection.

Size and shape control of wurtzite InP via metallic copper and Cu3-xP

The size and shape control by copper precursor

The wurtzite InP NCs size can be tuned in the first step of copper NCs formation through the use of different copper precursors. Such size control is important for tuning the optical and spectral characteristics of the product InP nanocrystals. For the size control of InP NCs we control the size of Cu3- _x P prepared by use of a heat up synthesis. In this procedure, CuX2 (X = acetate, tartrate, nitrate and chloride) complex is added with hexadecylamine (HD A) and triphenylphosphite (TPP) to octadecene (ODE) as the solvent. Heating up the solution to 280°C under argon results in the formation of uniform Cu3- _x P NCs. Performing the reaction with Cu(acetate)2 led to the formation of uniform Cu° NCs starting at 220°C. These converted into Cu3- _x P at temperature of 260°C from the metallic NCs. TEM image of the resulting Cu3- _x P by using Cu(acetate)2 with a size of 7.4 nm are shown in Fig. 9a.

The chemical identity of the copper precursor anions was found to have an important effect on the determination of the formation mechanism and the final product. Using Cu(tartrate)2 or Cu(nitrate)2 in similar reaction conditions resulted in larger size of 11.3 and 13.6 nm, respectively (Fig. 9b-c). Nanoplates with significantly larger lateral were obtained when using CuCh (Fig. 8b-c). The size is correlated with the reactivity of the precursors, as observed by the temperature at which the NCs formed. The smallest size (Cu(acetate)2) started to nucleate at 220°C and the largest (CuCh) at 270°C. Cu(tartrate)2 and Cu(nitrate)2 in between at 240°C and 260°C.

Size control by the precursor concentration

Further tuning of the NCs size was achieved by changing the concentration of the reaction constituents. Upon lowering the precursors concentration by increasing the volume of ODE solvent, the NCs size was decreased. This effect is demonstrated in Fig. 9a-d by using CuAc2-TPP-HDA in different concentration. In this mode of control, the ratios of the TPP, HDA and CuAc2 is kept constant, only the atoms are farther apart. The NCs volume is increased by more than a factor of three, whereas the change in NCs concentration is less than twice.

Size control by the ligands to copper ratio

A third way to tune the NCs size is by the varying the ratio of TPP and HDA ligand to copper. In Fig. lOa-b the increase in size is observed by decreasing only the amount of Cu(acetate)2 from 90 mg to 30 mg. This results in a higher ligand to copper ratio. Similar results were observed by increasing only TPP or HDA. This effect is also shown on the reaction of Cu(tartrate)2 where using decreasing amounts from 106 mg to 57 mg, resulted in an increase in the NCs diameter from 11.3 to 14.5 nm, respectively.

Optical size effect

Figs. 8-10 show the ability to control the size of copper NCs and thus the size of the parent Cu3- _x P NCs by varying the counter-ion in the copper precursors, the overall reaction concentration and the ratio between the reaction constituents. This directly implies on the available sizes of InP. After the cation exchange process of Cu3- _x P with TOP to form InP NCs we also performed the NOBF4 reaction. Measurements of absorption and photoluminescence of these final products reveals the quantum confinement effect of the InP NCs related directly to control of size and the high quality in terms of surface and lack of remaining copper. This results in hexagonal wurtzite lattice structure and polarized emission as was addressed before in the document (Fig. 7). Using selected samples from Cu(tartrate)2 and Cu(acetate)2 with different concentrations and ratios as described in Figs. 9-10 resulted in tuning the absorption and emission spectra of the final InP in the regime of 700-800 nm as demonstrated in Fig. 11.

Further enhancement of the emission and stability by shell growth

To further enhance the emission and the photochemical stability of the nanocrystals after the reaction with NOBF4 (after back transfer to oleylamine), we used a general procedure of ZnSe/ZnS growth commonly reported to enhance the emission of InP nanocrystals. To this end, the nanocrystals are heated in octadecene as a solvent to 300°C. For shell growth, zinc oleate is slowly injected alternately first with solution of selenium in trioctylphosphine to grow an intermediate buffer shell of ZnSe. After reaching the desired ZnSe shell thickness the ZnS shell at the desired thickness is grown by slow injection alternating between zinc oleate and sulfur in trioctylphosphine. While the ZnSe shell slowly deposited on the nanocrystals the emission is enhanced. By varying the amounts and injection rate of the Zn-oleate and the of trioctylphosphine - Se/S, the shell can be optimized.

Two-dimensional shape anisotropy - InP wutzite nanoplates

As reported above, using copper chloride in the synthesis of the CusP nanocrystals results in stronger shape anisotropy with quasi-two dimensional shape. Similar shapes are achieved by using copper bromide or copper-acetonitryl-PFe as precursors for the CusP nanocrystal synthesis. In this case, the size distribution is typically broader. Various methods are available to obtain narrower size distribution. First, changing the phosphorous precursor from triphenylphosphite to tris(diethylamino)phosphine or trimethylsilyl-phosphine, results in nucleation in lower temperature and smaller nanocrystals with narrower size distribution. Performing the cation exchange to Indium and then performing the NOBF4 surface reaction as results in wurtzite InP nanocrystals with strong two-dimensional anisotropy, not only due to the lattice structure, but also the due to the stronger shape anisotropy.

One-dimensional shape anisotropy - nanorods

Nanorods have one-dimensional shape anisotropy and therefore the emission dipole can have a driving force to be polarized in one axis. Achieving elongated nanorods of InP within the above synthetic scheme, can be achieved by control on either metallic Cu of CusP nanorods. Cu nanorods were achieved through seeded growth starting from gold seeds. First, 9 nm spherical gold nanocrystals were synthesized by injecting 0.25 mmol (22 mg) of TBAB, 1 mL oleylamine and 1 mL toluene into 0.25 mmol (98 mg) of HAuC14-3H2O in 10 mL oleylamine and 10 mL of toluene after purging under argon for 30 min, and kept under stirring for another hour at 15 °C. Following this, the nanocrystals are transformed to Cu nanorods by directional onedimensional attachment by injecting the Au nanocrystals into 0.5 mmol (85 mg) of CuC12-2H2O in oleylamine at 180 °C. The residues of Au in the final nanorods are on the surface. These nanorods are next transformed to CusP by phosphorization via reaction with triphenylphosphite at 260°C as describe above. After cation exchange of the CuaP to InP, the remained metallic impurities on the surface is removed by the abovementioned NOBF4 reaction.