ELECTRO-LUMINESCENT MATERIAL AND ELECTRO-LUMINESCENT DEVICE

Abstract

An electro-luminescent film including a substrate and anisotropic semiconductor nanoparticles distributed on the substrate according to a periodic pattern. The semiconductor nanoparticles have an aspect ratio greater than 1.5, and the repetition unit of the pattern has a smallest dimension of less than 500 micrometer and includes at least one pixel. Also, a process for the manufacture of the electro-luminescent film, and a light emitting device that includes the electro-luminescent film.

Claims

1.-14. (canceled)

15. An electro-luminescent film comprising a substrate and semiconductor nanoparticles distributed on the substrate according to a periodic pattern, wherein semiconductor nanoparticles have an aspect ratio greater than 1.5; wherein the repetition unit of the pattern has a smallest dimension of less than 500 micrometer and comprises at least one pixel.

16. The electro-luminescent film according to claim 15, wherein the pattern is periodic in two dimensions.

17. The electro-luminescent film according to claim 16, wherein the periodic pattern is a rectangular lattice or a square lattice.

18. The electro-luminescent film according to claim 15, wherein semiconductor nanoparticles are inorganic.

19. The electro-luminescent film according to claim 18, wherein semiconductor nanoparticles are semiconductor nanocrystals comprising a material of formula M.sub.xQ.sub.yE.sub.zA.sub.w, wherein: M is selected from the group consisting of Zn, Cd, Hg, Cu, Ag, Au, Ni, Pd, Pt, Co, Fe, Ru, Os, Mn, Tc, Re, Cr, Mo, W, V, Nd, Ta, Ti, Zr, Hf, Be, Mg, Ca, Sr, Ba, Al, Ga, In, Tl, Si, Ge, Sn, Pb, As, Sb, Bi, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Cs; Q is selected from the group consisting of Zn, Cd, Hg, Cu, Ag, Au, Ni, Pd, Pt, Co, Fe, Ru, Os, Mn, Tc, Re, Cr, Mo, W, V, Nd, Ta, Ti, Zr, Hf, Be, Mg, Ca, Sr, Ba, Al, Ga, In, Tl, Si, Ge, Sn, Pb, As, Sb, Bi, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Cs; E is selected from the group consisting of O, S, Se, Te, C, N, P, As, Sb, F, Cl, Br, I; A is selected from the group consisting of O, S, Se, Te, C, N, P, As, Sb, F, Cl, Br, I; and x, y, z and w are independently a rational number from 0 to 5; x, y, z and w are not simultaneously equal to 0; x and y are not simultaneously equal to 0; z and w are not simultaneously equal to 0.

20. The electro-luminescent film according to claim 15, wherein semiconductor nanoparticles have a longest dimension greater than 25 nanometers.

21. The electro-luminescent film according to claim 15, wherein semiconductor nanoparticles have a longest dimension greater than 35 nanometers.

22. The electro-luminescent film according to claim 15, wherein semiconductor nanoparticles are on the substrate with their longest dimension substantially aligned in a predetermined direction.

23. The electro-luminescent film according to claim 15, wherein substrate is selected from a conductive material and a semi-conductive material.

24. The electro-luminescent film according to claim 15, wherein semiconductor nanoparticles on the substrate form layers with a thickness of less than 100 nm.

25. The electro-luminescent film according to claim 15, wherein the repetition unit of the periodic pattern comprises at least two pixels.

26. The electro-luminescent film according to claim 25, wherein semiconductor nanoparticles on the first pixel of the at least two pixels are different from semiconductor nanoparticles on the second pixel of the at least two pixels.

27. A process for the manufacture of an electro-luminescent film comprising a substrate and semiconductor nanoparticles distributed on the substrate according to a periodic pattern, wherein the repetition unit of the pattern has a smallest dimension of less than 500 micrometer and comprises at least one pixel comprising the steps of: i) providing a substrate; ii) creating a surface electric potential on the substrate according to the pattern, so that at least one pixel of the repetition unit is created in the whole pattern; and iii) bringing the substrate in contact with a colloidal dispersion of semiconductor nanoparticles having an aspect ratio greater than 1.5 for a contacting time of less than 15 minutes.

28. The process for the manufacture of an electro-luminescent film according to claim 27, wherein the substrate is an electret substrate and wherein the surface electric potential is written on the electret substrate.

29. The process for the manufacture of an electro-luminescent film according to claim 28, wherein the repetition unit of the pattern comprises at least two pixels and wherein semiconductor nanoparticles on the first pixel of the at least two pixels are different from semiconductor nanoparticles on the second pixel of the at least two pixels; and wherein process further comprises: iv) drying the electret substrate and semiconductor nanoparticles deposited thereon to form an intermediate structure; v) writing a surface electric potential on the intermediate structure according to the pattern, so that the second pixel of the repetition unit is written in the whole pattern; and vi) bringing the electret substrate in contact with a colloidal dispersion of semiconductor nanoparticles having an aspect ratio greater than 1.5 and different from those used in step iii) for a contacting time of less than 15 minutes.

30. The process for the manufacture of an electro-luminescent film according to claim 27, wherein the surface electric potential is induced and maintained on the substrate during contact with colloidal dispersion.

31. The process for the manufacture of an electro-luminescent film according to claim 30, wherein the repetition unit of the pattern comprises at least two pixels and wherein semiconductor nanoparticles on the first pixel of the at least two pixels are different from semiconductor nanoparticles on the second pixel of the at least two pixels; and wherein process further comprises: iv) drying the substrate and semiconductor nanoparticles deposited thereon to form an intermediate structure; v) inducing a surface electric potential on the intermediate structure according to the pattern, so that the second pixel of the repetition unit is induced in the whole pattern; and vi) bringing the electret substrate in contact with a colloidal dispersion of semiconductor nanoparticles having an aspect ratio greater than 1.5 and different from those used in step iii) for a contacting time of less than 15 minutes; while surface electric potential is maintained.

32. A light emitting device comprising an electro-luminescent film comprising a substrate and semiconductor nanoparticles on the substrate according to a periodic pattern, wherein semiconductor nanoparticles have an aspect ratio greater than 1.5; wherein the repetition unit of the pattern has a smallest dimension of less than 500 micrometer and comprises at least one pixel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0167] FIG. 1 illustrates a schematic of an electro-luminescent film (1) comprising a substrate (2). A periodic pattern (here a rectangular lattice) is shown as a network of dotted lines. At each node of the network, a repetition unit (3) of rectangular shape is shown (delimited by a bold dotted line). Smallest size of repetition unit is noted S. In repetition unit are shown three pixels of square section (4a), (4b) and (4c). Semiconductor nanoparticles (not shown) are on the substrate (2), in the volume of each pixel.

[0168] FIG. 2 illustrates an anisotropic nanoparticle, here a nanoplatelet, and defines aspect ratio.

[0169] FIG. 3 shows microscopy images of nanoplatelets used in example 1. Scale bars are 10 nm (3a), 10 nm (3b) and 5 nm (3c).

[0170] FIG. 4 shows emission spectrum (arbitrary unit) of nanoplatelets used in example 1 (emitting in red range: dashed line, green range: dotted line and blue range: solid line) as a function of light wavelength (λ in nanometer).

EXAMPLES

[0171] The present invention is further illustrated by the following examples.

Example 1

[0172] Preparation of a Stamp:

[0173] A photolithographic mask is fabricated on a UV-blue transparent substrate to reproduce a pattern with squared pixels of 5 μm size distributed on a square lattice of period 15 μm. A silicon carrier is covered by a uniform photolithography resin and illuminated by an UV lamp producing a 350 nm light filtered by the lithography mask in order to impress the pattern on the carrier. A proper washing solution for the resin is utilized to develop the polymer and create a tridimensional motif (pixelization).

[0174] A PDMS solution is casted on this tridimensional motif and the silicon carrier, then heated at 150° C. for 24 h to assure the polymerization of the PDMS. The solidified PDMS is thus separated from the silicon carrier. The so patterned PDMS is gold covered by evaporation technique to ensure a conductive pixelated surface. The patterned and conductive PDMS substrate is now called the stamp. It consists of a planar conductive surface on which square pixels of 5 μm size and 20 μm height are distributed on a square lattice. The stamp is a square of size 5 cm.

[0175] Preparation of Substrate:

[0176] A p-doped silicon wafer substrate of 375 μm thickness is used to cast by spin coating a 200 nm thick PMMA solid film by using a solution of 5% in weight of PMMA (Mw: 10.sup.6 g.Math.mol.sup.−1) in toluene.

[0177] Preparation of Nanoparticles Colloidal Dispersions:

[0178] A solution A comprising 10.sup.−8 mole.Math.L.sup.−1 CdSe.sub.0.45S.sub.0.55/CdZnS/ZnS nanoplatelets in cyclohexane is prepared. These nanoplatelets are 25 nm long, 20 nm wide and 9 nm thick (core: 1.2 nm; first shell: 2 nm; second shell: 2 nm) and emit at 630 nm with FWHM of 20 nm.

[0179] A solution B comprising 10.sup.−8 mole.Math.L.sup.−1 CdSe.sub.0.10S.sub.0.90/ZnS/Cd.sub.0.20Zn.sub.0.80S nanoplatelets in cyclohexane is prepared. These nanoplatelets are 25 nm long, 20 nm wide and 8.5 nm thick (core: 1.5 nm; first shell: 1 nm; second shell: 2.5 nm) and emit at 530 nm with FWHM of 30 nm.

[0180] A solution C comprising 10.sup.−8 mole.Math.L.sup.−1 CdS/ZnS nanoplatelets in cyclohexane is prepared. These nanoplatelets are 25 nm long, 20 nm wide and 3 nm thick (core: 0.9 nm; first shell: 1 nm) and emit at 445 nm with FWHM of 20 nm.

[0181] Emission spectra of semiconductor nanoparticles from solution A, B and C are shown in FIG. 4.

[0182] Preparation of Electro-Luminescent Film:

[0183] The substrate is put in contact with the stamp in order to create a capacitive system with the PMMA in the middle (between stamp and p-doped silicon) as dielectric medium. A voltage of 50 V is applied for 1 minute in order to create permanent electrical polarization in the PMMA layer (electret material) only in correspondence with the pixels of the stamp.

[0184] To maintain stable the charges on the electret, humidity level of the environment is kept below 50%.

[0185] Substrate with electrically polarized PMMA layer is dipped in solution A for 10 seconds then rinsed by a clean solvent and dried by a gentle flux of nitrogen.

[0186] Using a microscopic technique of alignment, the stamp is then again placed on the already red pixelated substrate, with pixels of the stamp defining a second pixel on the substrate (different from the red pixel) according to the original periodic patterning chosen. A voltage of 50 V is applied again for 1 minute in order to create permanent electrical polarization in the PMMA layer only in correspondence with the pixels of the stamp, i.e. in correspondence with areas free of nanoparticles.

[0187] Substrate with electrically polarized PMMA layer is dipped in solution B for 10 seconds then rinsed by a clean solvent and dried by a gentle flux of nitrogen.

[0188] Using the same microscopic technique of alignment, the stamp is then again placed on the already red/green pixelated substrate, with pixels of the stamp defining a third pixel on the substrate (different from the red and green pixels) according to the original periodic patterning chosen. A voltage of 50 V is applied again for 1 minute in order to create permanent electrical polarization in the PMMA layer only in correspondence with the pixels of the stamp.

[0189] Substrate with electrically polarized PMMA layer is dipped in solution C for 10 seconds then rinsed by a clean solvent and dried by a gentle flux of nitrogen.

[0190] Electro-Luminescent Film and Device:

[0191] A 25 cm.sup.2 substrate of p-doped silicon coated with a 200 nm PMMA layer with square pixels of 5 μm size and three different types (red, green and blue emitting semiconductor nanoparticles) distributed on a square lattice of period 15 μm is obtained, forming an electro-luminescent film.

[0192] Below the substrate, all necessary other layers and electrical contacts needed for the injection of electric current in each pixel are built by well know techniques in the microelectronic industry of semiconductors, yielding an electro-luminescent device.

Example 2

[0193] Example 1 is reproduced, except that periodic pattern is changed.

[0194] In example 2a, pixels are square with 3 μm size and square lattice has a period of 12 μm.

[0195] In example 2b, four squared pixels of size 5 μm are defined on a square lattice of period 15 μm, with one red pixel, two green pixels and one blue pixel.

Example 3

[0196] Example 1 is reproduced, except that substrate is changed.

[0197] Example 3a: Silicon On Insulator (SOI) having the following structure: Silicon (15 nm)-Insulator (200 nm)-Silicon (200 nm) is used.

[0198] Example 3b: on a glass substrate with TFT matrix are deposited successively the following layers: [0199] 1. a common buried electrode for the periodic array of capacitance in step 3; [0200] 2. a 300 nm silicon oxide insulator; [0201] 3. a periodic array of separately isolated bottom electrode (each configured to make a diode); and [0202] 4. optionally an electron transporting layer for each pixel.

[0203] Example 3c: a LCD glass substrate with TFT matrix are deposited successively the following layers: [0204] 1. a periodic array of bottom electrode; [0205] 2. a ZnO electron transporting layer for each pixel; and [0206] 3. a PMMA layer of 7 nm.

[0207] The same deposition method yields electro-luminescent films which can be implemented as electro-luminescent devices using well know techniques in the microelectronic industry of semiconductors.

Example 4-1

[0208] Example 1 is reproduced, except that semiconductor nanoparticles are changed.

[0209] A solution D comprising 10.sup.−8 mole.Math.L.sup.−1 CdSe.sub.0.45S.sub.0.55/Cd.sub.0.30Zn.sub.0.70S/ZnS nanoplatelets in cyclohexane is prepared. These nanoplatelets are 35 nm long, 25 nm wide and 10.2 nm thick (core: 1.2 nm; first shell: 2.5 nm; second shell: 2 nm) and emit at 630 nm with FWHM of 25 nm.

[0210] After dipping of substrate with electrically polarized PMMA layer in solution D instead of solution A, nanoparticle deposition is observed as for example 1. It is observed that deposition is obtained in shorter exposure time, namely 4 seconds instead of 10 seconds.

Example 4-2

[0211] Example 1 is reproduced, except that semiconductor nanoparticles are changed.

TABLE-US-00001 TABLE I Colloidal dispersions of semiconductor nanoparticles used for deposition on substrate. (MLs refers to the number of monolayers of inorganic material covering the core). Dimensions L W T [NPs] Emission Nanoparticles (NPs) (nm) (nm) (nm) (mol .Math. L.sup.−1) peak FWHM Deposition CORE/SHELL NANOPLATELETS CdS/ZnS 5 MLs 17 17 3.2 5 × 10.sup.−6 465 nm 14 nm observed CdS/ZnSe.sub.0.5S.sub.0.5 5 MLs 15 15 3.2 2 × 10.sup.−6 465 nm 15 nm observed CdS/ZnSe 5 MLs 17 17 3.5 1 × 10.sup.−6 460 nm 15 nm observed CdSe.sub.0.30S.sub.0.70/ZnS 5 MLs 25 20 3.1 0.2 × 10.sup.−6  535 nm 28 nm observed CdSe.sub.0.25S.sub.0.75/Cd.sub.0.05Zn.sub.0.95S 27 22 3.4 2 × 10.sup.−6 550 nm 30 nm observed CdSe.sub.0.20S.sub.0.80/ZnSe 5 MLs 24 18 3.0 2 × 10.sup.−6 540 nm 29 nm observed CdSe.sub.0.20S.sub.0.80/ZnSe.sub.0.50S.sub.0.50 26 20 3.3 0.5 × 10.sup.−6  530 nm 30 nm observed 5 MLs CdSe.sub.0.83S.sub.0.17/Cd.sub.0.50Zn.sub.0.50S 28 18 5 1 × 10.sup.−6 621 nm 29 nm observed 4 MLs CdSe/Cd.sub.0.1Zn.sub.0.9S 4 MLs 16 17 4.9 2 × 10.sup.−6 625 nm 22 nm observed CdSe.sub.0.75S.sub.0.25/Cd.sub.0.50Zn.sub.0.50S 30 20 4.8 4 × 10.sup.−6 645 nm 26 nm observed 4 MLs CdSe/ZnSe.sub.0.50S.sub.0.50 4 MLs 17 17 4 2 × 10.sup.−6 645 nm 28 nm observed CdSe/ZnS 4 MLs 17 17 4 3.5 × 10.sup.−6  617 nm 27 nm observed CORE/SHELL/SHELL NANOPLATELETS ZnSe/ZnSe.sub.0.4S.sub.0.6/ZnS 50 20 3.2 20 × 10.sup.−6  445 nm 15 nm observed CdSe.sub.0.90S.sub.0.10/ZnSe/ZnS 27 19 5 2 × 10.sup.−6 650 nm 28 nm observed 4 MLs CORE/CROWN NANOPLATELETS CdSe/CdS 3 MLs 20 12 0.9 3 × 10.sup.−6 465 nm 10 nm observed CdS/ZnSe 5 MLs 15 15 1.2 2 × 10.sup.−6 468 nm 15 nm observed CdSe/CdS 4 MLs 15 15 1.2 2 × 10.sup.−6 515 nm 10 nm observed CdSe.sub.0.90S.sub.0.10/CdS 5 MLs 27 21 1.5 2.5 × 10.sup.−6  540 nm 14 nm observed CdSe/CdS 5 MLs 26 17 1.5 1 × 10.sup.−6 555 nm 12 nm observed DOT IN PLATE NANOPLATELETS (core: quantum dot, final nanoparticle: nanoplatelet) CdSe/CdS 3 MLs 15 15 0.9 2.3 × 10.sup.−6  462 nm 10 nm observed CdSe.sub.0.50S.sub.0.50/CdS/ZnS 25 25 3.2 2 × 10.sup.−6 540 nm 35 nm observed 4 MLs CORE/CROWN/SHELL NANOPLATELETS CdS/ZnSe/ZnS 5 MLs 17 17 3.5 2 × 10.sup.−6 550 nm 30 nm observed CdSe.sub.0.30S.sub.0.70/CdS/ZnS 27 20 3.4 10 × 10.sup.−6  550 nm 30 nm observed 5 MLs

[0212] After dipping of substrate with electrically polarized PMMA layer in a colloidal dispersion of semiconductor nanoparticles listed in Table I instead of solution A, nanoparticle deposition is observed as for example 1.

Example 5

[0213] Example 1 is reproduced, except that substrate and preparation of electro-luminescent film are changed.

[0214] Substrate is a 50 μm thick square glass slide of size 5 cm. Substrate is held horizontally.

[0215] The stamp is placed below the substrate and in contact with the substrate. A voltage of 50 V is applied in order to induce electrical polarization in the substrate only in correspondence with the pixels of the stamp.

[0216] While voltage is applied, a layer of solution A is poured on the top side of substrate and voltage is maintained for 10 seconds then shut off. Stamp is removed from bottom side of substrate and excess solution A is removed. Substrate is then rinsed by a clean solvent and dried by a gentle flux of nitrogen.

[0217] Using a microscopic technique of alignment, the stamp is then again placed below the already red pixelated substrate, with pixels of the stamp defining a second pixel on the substrate (different from the red pixel) according to the original periodic patterning chosen. A voltage of 50 V is applied in order to induce electrical polarization in correspondence with the pixels of the stamp.

[0218] While voltage is applied, a layer of solution B is poured on the top side of substrate and voltage is maintained for 10 seconds then shut off. Stamp is removed from bottom side of substrate and excess solution B is removed. Substrate is then rinsed by a clean solvent and dried by a gentle flux of nitrogen.

[0219] Using the same microscopic technique of alignment, the stamp is then again placed below the already red/green pixelated substrate, with pixels of the stamp defining a third pixel on the substrate (different from the red and green pixels) according to the original periodic patterning chosen. A voltage of 50 V is applied in order to induce electrical polarization in correspondence with the pixels of the stamp.

[0220] While voltage is applied, a layer of solution C is poured on the top side of substrate and voltage is maintained for 10 seconds then shut off. Stamp is removed from bottom side of substrate and excess solution C is removed. Substrate is then rinsed by a clean solvent and dried by a gentle flux of nitrogen.

Comparative Example C1

[0221] Example 1 is reproduced, except that semiconductor nanoparticles are changed.

[0222] A solution C-A comprising 10.sup.−8 mole.Math.L.sup.−1 CdSe/CdS/ZnS nanoparticles in cyclohexane is prepared. These nanoparticles are spherical (aspect ratio of 1) with a diameter of 6 nm and emit at 620 nm with FWHM of 45 nm.

[0223] A solution C-B comprising 10.sup.−8 mole.Math.L.sup.−1 Cd.sub.0.10Zn.sub.0.90Se.sub.0.10S.sub.0.90/ZnS nanoparticles in cyclohexane is prepared. These nanoparticles are spherical (aspect ratio of 1) with a diameter of 6 nm and emit at 540 nm with FWHM of 37 nm.

[0224] After dipping of substrate with electrically polarized PMMA layer in solution C-A instead of A, no significant nanoparticle deposition is observed: isolated nanoparticles are found on the substrate, but they do not form a layer of nanoparticles. No selective deposition on the pattern occurs.

[0225] After dipping of substrate with electrically polarized PMMA layer in solution C-B instead of B, no significant nanoparticle deposition is observed: isolated nanoparticles are found on the substrate, but they do not form a layer of nanoparticles. No selective deposition on the pattern occurs.

[0226] Nanoparticles of solutions C-A and C-B are too small to form significant deposits on substrate.

[0227] Thus, the deposit with spherical nanoparticles of this size is not conclusive.

[0228] In addition, spherical nanoparticles emitting light in shorter wavelength, typically in blue range, are even smaller in diameter and it was not able to deposit these nanoparticles.

Comparative Example C2

[0229] Example 1 is reproduced, except that semiconductor nanoparticles are changed.

[0230] A solution C-C comprising 10.sup.−8 mole.Math.L.sup.−1 CdSe/CdS/ZnS nanoparticles in cyclohexane is prepared. These nanoparticles are spherical (aspect ratio of 1) with a diameter of 3 nm and emit at 620 nm with FWHM of 45 nm.

[0231] A solution C-D comprising 10.sup.−8 mole.Math.L.sup.−1 Cd.sub.0.10Zn.sub.0.90Se.sub.0.10S.sub.0.90/ZnS nanoparticles in cyclohexane is prepared. These nanoparticles are spherical (aspect ratio of 1) with a diameter of 4 nm and emit at 540 nm with FWHM of 37 nm.

[0232] After dipping of substrate with electrically polarized PMMA layer in solution C-C instead of A, no significant nanoparticle deposition is observed: isolated nanoparticles are found on the substrate, but they do not form a layer of nanoparticles. No selective deposition on the pattern occurs.

[0233] After dipping of substrate with electrically polarized PMMA layer in solution C-D instead of B, no significant nanoparticle deposition is observed: isolated nanoparticles are found on the substrate, but they do not form a layer of nanoparticles. No selective deposition on the pattern occurs.

[0234] Thus, nanoparticles of solutions C-C and C-D do not form significant deposits on substrate because they are too small.

Comparative Example C3

[0235] Example 1 is reproduced, except that semiconductor nanoparticles are changed.

[0236] A solution C-E comprising 10.sup.−8 mole.Math.L.sup.−1 of composite particles comprising CdSe.sub.0.45S.sub.0.55/CdZnS/ZnS nanoplatelets in SiO.sub.2 matrix, in cyclohexane is prepared (nanoplatelets are 25 nm long, 20 nm wide and 9 nm thick). These composite particles are spherical (aspect ratio of 1) with a diameter of 100 nm and emit at 630 nm with FWHM of 20 nm.

[0237] A solution C-F comprising 10.sup.−8 mole.Math.L.sup.−1 of composite particles comprising CdSe.sub.0.10S.sub.0.90/ZnS/Cd.sub.0.20Zn.sub.0.80S nanoplatelets in Al.sub.2O.sub.3 matrix, in cyclohexane is prepared. These composite particles are spherical (aspect ratio of 1) with a diameter of 120 nm and emit at 530 nm with FWHM of 30 nm.

[0238] After dipping of substrate with electrically polarized PMMA layer in solution C-E instead of A, significant nanoparticle deposition is observed.

[0239] After dipping of substrate with electrically polarized PMMA layer in solution C-F instead of B, significant nanoparticle deposition is observed.

[0240] However, the deposition of composite particles of solutions C-E and C-F does not result in an electro-luminescent film because SiO.sub.2 and Al.sub.2O.sub.3 encapsulating the semiconductor nanoplatelets act as insulating, thus no electricity can be transferred directly to the semiconductor nanoplatelets.