Colloidal semiconductor nanostructures

Abstract

The technology subject of the present application concerns a novel class of fused nanocrystal molecules having unique electronic properties. The application further contemplates methods for their preparation and methods of their use.

Claims

1. A fused nanocrystal molecule comprising two or more semiconductor core/shell structures, the fused nanocrystal molecule comprising two or more core structures and a continuous outermost shell extending the circumference of the two or more core structures, the outermost shell comprising a material identical to a shell material of any of said two or more semiconductor core/shell structures or an alloy material of the shell materials of the two or more semiconductor core/shell structures, wherein each two core/shell structures of the two or more fused semiconductor core/shell structures having a fusion region of a thickness between 0.1 and 5 nm.

2. The fused nanocrystal molecule according to claim 1, consisting of a plurality of semiconductor core/shell structures.

3. The fused nanocrystal molecule according to claim 1, wherein each two fused core/shell structures having a fusion region therebetween, wherein the size, structure, and shape of said fusion region is selected to provide control of at least one electronic property of the fused nanocrystal molecules, the property being carrier wavefunctions, carrier separations, emission characteristics, absorption characteristics or catalytic activity.

4. The fused nanocrystal molecule according to claim 1, wherein the fused nanocrystal molecule comprises a plurality of identical or different core/shell structures.

5. The fused nanocrystal molecule according to claim 1, further comprising one or more quantum dots (QD).

6. The fused nanocrystal molecule according to claim 1, having a fusion region thickness between 0.1 and 0.6 nm.

7. The fused nanocrystal molecule according to claim 1, wherein the core/shell structure is a core/multishell structure.

8. The fused nanocrystal molecule according to claim 1, wherein at least two semiconductor core/shell structures are fused to each other and to at least one other nanocrystal.

9. The fused nanocrystal molecule according to claim 1, wherein each of the core/shell structures having a shape selected from spherical and polygonal structures.

10. The fused nanocrystal molecule according to claim 1, being a semiconductor heterostructure with a type I, reverse type I, quasi-type II or type II band-alignment between a core and a fusion region semiconductor materials or between the two core semiconductor materials.

11. The fused nanocrystal molecule according to claim 1, wherein the semiconductor material of the core and/or shell is selected from elements of Group I-VII, Group II-VI, Group III-V, Group IV-VI, Group III-VI, Group I-VI, Group V-VI, Group II-V, Group I-III-VI.sub.2, Group IV, ternary or quaternary semiconductors and alloys or combinations thereof.

12. The fused nanocrystal molecule according to claim 1, being a molecule selected amongst those listed in the table below: TABLE-US-00002 Material Nano- of the assem- Nano- Nano- Nano- Nano- Fusion bly crystal 1 crystal 2 crystal 3 crystal 4 Region 1 CdSe/CdS CdSe/CdS CdS 2 CdSe/CdS CdSe/CdS/ ZnS-CdS ZnS or CdS- ZnS- CdS 3 CdSe/CdS/ CdSe/CdS/ ZnS or ZnS ZnS CdS- ZnS-CdS 4 CdSe/CdS InP/ZnS ZnS-CdS 5 InP/ZnS InP/ZnS ZnS 6 CdTe/CdS ZnSe/CdS CdS 7 InP/ZnS CdSe/CdS/ ZnS or ZnS ZnS-CdS 8 ZnSe/ZnS ZnSe/ZnS ZnS 9 InAs/CdSe/ InAs/CdSe/ ZnSe or ZnSe ZnSe CdSe- ZnSe- CdSe 10 InAs/InP/ InAs/InP/ ZnS or ZnS ZnS InP-ZnS- InP 11 InAs/CdSe/ InAs/CdSe/ CdS or CdS CdS CdSe- CdS- CdSe 12 CdSe/CdS/ CdSe/CdS/ HgS or HgS HgS CdS- HgS-CdS 13 InAs/GaAs InAs/GaAs GaAs 14 GaP/ZnS ZnO/ZnS ZnS 15 ZnO/ZnS ZnSe/ZnS ZnS 16 ZnSe/ZnS ZnO/ZnS ZnS 17 InGaP/ZnS InGaP/ZnS ZnS 18 InGaP/ZnSe InGaP/ZnSe ZnSe 19 InP/ZnSe InP/ZnSe ZnSe 20 InAs/ZnSe InAs/ZnSe ZnSe 21 InAs/ZnS InAs/ZnS ZnS 22 CdSe/CdS CdSe/CdS CdSe/CdS CdS 23 CdSe/CdS CdSe/CdS CdSe/CdS CdSe/ CdS 24 CdSe/CdS/ CdSe/CdS/ CdSe/CdS/ ZnS or ZnS ZnS ZnS CdS CdS- ZnS-CdS 25 CdSe/CdS/ CdSe/CdS/ CdSe/CdS/ CdSe/ ZnS or ZnS ZnS ZnS CdS/ CdS- ZnS ZnS-CdS 26 CdTe/CdS ZnSe/CdS CdTe/CdS ZnSe/ CdS CdS (barrier for holes) 27 CdTe/CdS CdTe/CdS CdTe/CdS CdTe/ CdS CdS (barrier for holes) 28 ZnSe/CdS ZnSe/CdS ZnSe/CdS ZnSe/ CdS CdS (barrier for holes) 29 CdTe/CdS ZnTe/CdS CdS (barrier for holes only) 30 CdTe/CdS ZnTe/CdS CdTe/CdS ZnTe/ CdS CdS (barrier for holes) 31 ZnSe/CdS ZnSe/CdS CdS (barrier for holes only) 32 ZnSe/CdS ZnSe/CdS ZnSe/CdS ZnSe/ CdS CdS (barrier for holes) 33 ZnTe/CdS ZnTe/CdS ZnTe/CdS ZnTe/ CdS CdS (barrier for holes) 34 ZnSe/ZnTe ZnSe/ZnTe ZnTe (barrier for electrons) 35 ZnSe/ZnTe ZnSe/ZnTe ZnSe/ZnTe ZnSe/ ZnTe ZnTe (barrier for electrons) 36 CdTe/ZnTe CdTe/ZnTe ZnTe (barrier for electrons) 37 CdTe/ZnTe CdTe/ZnTe CdTe/ZnTe CdTe/ CdTe ZnTe (barrier for holes) 38 ZnSe/ZnS ZnSe/ZnS ZnSe/ZnS ZnSe/ ZnS ZnS 39 InP/CdS InP/CdS CdS 40 InP/CdS InP/ZnS CdS-ZnS 41 InP/CdS InP/CdS InP/CdS CdS 42 InP/CdS InP/CdS InP/CdS InP/ CdS CdS 43 PbSe/CdS CdSe/CdS CdS 44 PbSe/CdS PbSe/CdS PbSe/CdS PbSe/ CdS CdS 45 InAs/CdS CdSe/CdS CdS 46 InAs/CdS ZnSe/CdS CdS 47 InAs/CdS InAs/CdS InAs/CdS InAs/ CdS CdS 48 PbSe/CdS InAs/CdS CdS 49 InP/ZnS InP/ZnS InP/ZnS ZnS 50 InP/ZnS InP/ZnS InP/ZnS InP/ ZnS ZnS 51 PbSe/ZnS CdSe/ZnS ZnS 52 PbSe/ZnS PbSe/ZnS PbSe/ZnS PbSe/ ZnS ZnS 53 InAs/ZnS CdSe/ZnS ZnS 54 InAs/ZnS InAs/ZnS InAs/ZnS InAs/ ZnS ZnS 55 PbSe/ZnS InAs/ZnS ZnS 56 ZnO/CdS ZnSe/CdS CdS, Type II 57 ZnO/CdS CdTe/CdS CdS, Type II 58 ZnO/CdS ZnTe/CdS CdS, Type II 59 InP/ZnSe InP/ZnSe InP/ZnSe ZnSe 60 InP/ZnSe InP/ZnSe InP/ZnSe InP/ ZnSe ZnSe 61 ZnSe/ZnS ZnSe/ZnS ZnS 62 ZnSe/ZnS ZnSe/ZnS ZnSe/ZnS ZnSe/ ZnS ZnS 63 ZnSe/ZnS CdSe/ZnS ZnS 64 InAs/CdS InAs/CdS CdS 65 InAs/CdS InAs/CdS InAs/CdS InAs/ CdS CdS 66 InAs/CdTe InAs/CdTe CdS 67 InAs/CdTe InAs/CdTe InAs/CdTe InAs/ CdTe CdTe 68 GaAs/ZnS GaAs/ZnS ZnS 69 GaAs/ZnS GaAs/ZnS GaAs/ZnS GaAs/ ZnS ZnS 70 GaAs/CdS GaAs/CdS CdS 71 GaAs/CdS GaAs/CdS GaAs/CdS GaAs/ CdS CdS 72 GaAs/ZnSe GaAs/ZnSe ZnSe 73 GaAs/ZnSe GaAs/ZnSe GaAs/ZnSe GaAs/ ZnSe ZnSe 74 GaAs/GaP GaAs/GaP GaP 75 GaAs/GaP GaAs/GaP GaAs/GaP GaAs/ GaP GaP 76 CdSe/CdS CdSe CdSe/CdS CdS 77 CdSe/ZnS CdSe CdSe/ZnS ZnS 78 ZnSe/ZnS ZnSe ZnSe/ZnS ZnS 79 InP/ZnSe InP InP/ZnSe ZnSe 80 Au/ZnO ZnS/ZnO ZnO 81 Ag/ZnO Ag/ZnO ZnO 82 Au/ZnS ZnSe/ZnS ZnS 83 Ag/ZnS ZnSe/ZnS ZnS 84 Au/ZnS Au/ZnS ZnS 85 Ag/ZnO Ag/ZnO ZnO 86 Au/ZnS Au/ZnS Au/ZnS ZnS 87 Ag/ZnO Au/ZnO Ag/ZnO Au/ ZnO ZnO 88 Cu/CdS CdSe/CdS CdS 89 Au/ZnS InP/ZnS ZnS 90 Cu.sub.2ZnSnS.sub.4/ Cu.sub.2ZnSnS.sub.4/ ZnS ZnS ZnS 91 ZnS(P- ZnS(N- ZnO doped)/ZnO doped)/ZnO 92 ZnS(P- ZnS(N- ZnSe doped)/ZnSe doped)/ZnSe 93 Pd/Ag.sub.2S Pt/Ag.sub.2S Ag.sub.2S 94 InP(N- InP(P- ZnS doped)/ZnS doped)/ZnS 95 InAs(N- InAs(P- ZnSe. doped)/ZnSe doped)/ZnSe

13. The fused nanocrystal molecule according to claim 11, wherein the material is doped.

14. A fused nanocrystal molecule according to claim 1 exhibiting emission from multicarrier configurations beyond excitonic emission.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIG. 1 schematically depicts various types of coupled dimer structures. (A) Coupled dimer structure with the same size of core. (B) Coupled dimer structure with different sizes of core. (C) Coupled dimer structure of core@shell.sub.1@shell.sub.2 with different sizes of core. (D) Coupled heterodimer structure with the same size of cores of different materials. (E) Coupled heterodimer structure with different sizes of core of different materials.

(3) FIG. 2 is a schematic illustration for an exemplary fabrication processes of coupled CdSe@CdS dimer structure.

(4) FIGS. 3A-3I provide quantum dots characterization. TEM micrographs, absorption and photoluminescence spectra of different sizes of CdSe/CdS core-shell CQDs: (FIGS. 3A-C) 1.9/4.0 nm, (FIGS. 3D-F) 1.4/2.1 nm, and (FIGS. 3G-I) 1.2/2.1 nm.

(5) FIGS. 4A-4L provide a structural characterization of the coupled CQDs. Raw (FIG. 4A) and Fourier filtered (FIG. 4B) HAADF-STEM images of 1.9/4.0 nm CdSe/CdS CQD monomer viewed under [1210] zone axis (ZA). Inset in (FIG. 4A) is a cartoon model built with VESTA software with bounding faces indexed based on the STEM data. Magnified images of edge (shell) (FIG. 4C) and central (core) parts (FIG. 4D) of the CQD shown in (FIG. 4B). Sulfur, selenium, and cadmium atoms are marked in blue, green and purple, respectively. Coherent growth of the shell lattice is identified. (e) and (f) are FFT and atomic structure model of (FIG. 4A), respectively. HAADF-STEM image of CdSe/CdS CQD under ZA [0001] (FIG. 4G) and atomic structure reconstruction imaging calculated for the same orientation (FIG. 4H). (FIG. 4I) High-resolution HAADF-STEM image and atomic structure model (FIG. 4J) of CdSe/CdS CQD viewed under ZA [1210]. The core regions are marked with pink circles in (FIG. 4G) and (FIG. 4I). FFT patterns are inserted in (FIG. 4G), and (FIG. 4I). SAED (FIG. 4K) and XRD pattern acquired at large ensembles of CdSe/CdS CQDs (blue curveexperimental XRD data, red barstheoretical positions for diffraction peaks of hcp) CdS (JCPDF 04-001-6853), black curveintegrated intensity of SAED (FIG. 4K)).

(6) FIGS. 5A-5L provide CdSe/CdS @ SiO.sub.2 characterized with electron microscopy. TEM and SEM images acquired at different magnification of the CdSe/CdS @ SiO.sub.2 NPs produced with different loading ratios of (FIG. 5A-D) 1:2000, (FIG. 4E-H) 1:1000, and (FIG. 4I-L) 1:500. Scale bars are 100 nm.

(7) FIGS. 6A-6L provide TEM and SEM images acquired at different magnification of the SiO.sub.2@CdSe/CdS@SiO.sub.2 NPs produced with different amount of TEOS for masking (FIG. 6A-D) 200, (FIG. 6E-H) 100, and (FIG. 4I-L) 50 L. Scale bars are 100 nm.

(8) FIGS. 7A-7B provide TEM images of the SiO.sub.2@homodimer particles.

(9) FIGS. 8A-8E provide coupled CQDs molecules. (FIG. 8A) Scheme for fabrication of coupled CdSe/CdS CQD molecule. (FIG. 8B) The dimer@SiO.sub.2 CQD structure. The dimer 1.9/4.0 nm CQD molecules (FIG. 8C) before, and (FIG. 8D) after the fusion procedure. (FIG. 8E) The 1.4/2.1 nm fused CdSe/CdS CQD molecules. Schematic structures are illustrated. Scale bars (FIG. 8B-E) are 50 nm and insets show higher magnification images.

(10) FIGS. 9A-9C provide HAADF-STEM (FIG. 9A), line scan (FIG. 9B) and STEM-EDS (FIG. 9C) analysis of the coupled CdSeCdS molecules.

(11) FIGS. 10A-10D provide HRTEM (FIG. 10A), Fast Fourier transform reconstruction imaging (FIG. 10B), electron diffraction (ED) pattern (FIG. 10C), and coupling model (FIG. 10D) of the coupled CdSeCdS molecules with [010] zone axis. While the white arrow was the c-axis for the QDs and insets show zoom-in on regions of the different orientation (Cd atoms are marked with grey and S atoms in dark grey).

(12) FIGS. 11A-11K provide fusion orientation relationships in CQD molecules. Atomic structure model (FIG. 11A, D; cadmium atomsbrown, sulfur atomsblue.), HAADF-STEM images (FIG. 11B, E), and FFT patterns (FIG. 11C, F) of the homo-plane (FIG. 11A-C) and hetero-plane (FIG. 11D-F) attachment of coupled CQD molecules with orientation relationship of attachment on (1010)(1010) and (0002)(1011), respectively. The HAADF-STEM (FIG. 11G, I) and atomic structure model (FIG. 11H, J) of homo-plane attachment on (0002), and (1011) facets, respectively. Dashed orange arrows indicate the CQD fusion/molecular axis in plane of the image normal to projection ZA [1210]. Note that for (1010)(1010) homo-plane attachment, the homonymous (1010) faces of A1 and A2 are parallel (FIG. 11C), while for the hetero-plane attachment the heteronymous faces are parallel (0002)(1010) (FIG. 11C). (FIG. 11K) Distribution of observed homo- and hetero-plane attachment orientations on (1010), (0002), and (1011) faces. Inset shows the CQD model and faces.

(13) FIGS. 12A-12D provide HAADF-STEM images and atomic structure models of the fused 1.9/4.0 nm CdSe/CdS molecules with hetero-plane attachment of (1010)(1011) (FIG. 12A-B), (0002)(1011) (FIG. 12C-D). For the atomic model, the Cadmium atoms are marked in brown and Sulfur atoms in blue.

(14) FIGS. 13A-13D provide characterization of 1.4/2.1 nm CQDs molecule. Fourier filtered HAADF-STEM images of the coupled CdSe/CdS molecules with homo-plane attachment of (0002)(0002) (FIG. 13A) and hetero-plane attachment of (1010)(1011) faces (FIG. 13B). (FIG. 13C) EDS line scan data and (FIG. 13D) STEM-EDS analysis.

(15) FIGS. 14A-14B provide HAADF-STEM image and atomic structure model of the fused 1.4/2.1 nm CdSe/CdS molecules with homo-plane attachment on (1010)(1010) (FIG. 14A), and (1011)(1011) (FIG. 14B). For the atomic model, the Cadmium atoms are marked in purple and Sulfur atoms in light blue.

(16) FIGS. 15A-15B depict HAADF-STEM images and atomic structure model of the fused 1.4/2.1 nm CdSe/CdS molecules with heteronymous-plane attachment on (0002)(1010) (FIG. 15A), (0002)(1011) (FIG. 15B). For the atomic model, the Cadmium atoms are marked in purple and Sulfur atoms in light blue.

(17) FIGS. 16A-16G provide the normalized absorption spectra of monomers (blue), unfused (green), and fused 1.4/2.1 nm CdSe/CdS CQD molecules when normalized with respect to (FIG. 16A) band-edge peak and (FIG. 16B) bulk transitions (300 nm). (FIG. 16C) The potential energy landscape and a cross-section of the calculated first electron wave-function without Coulombic interaction .sub.e (red), with Coulombic interaction .sub.e (green) and of the hole wave-functions .sub.h(blue) of the coupled CQD molecules. (FIG. 16D) Calculated bonding and antibonding 2-dimensional electron wave-functions without (cross-section of the bonding state is the red curve in FIG. 16A), and (FIG. 16E) with Coulombic interaction (cross-section of the bonding state is the green curve in FIG. 16A). (FIG. 16F) Absorption and fluorescence spectra of monomers (blue), unfused (green), and fused 1.4/2.1 nm CdSe/CdS CQD molecules (red). (FIG. 16G) Calculated (red asterisk) and experimental (blue circles) bandgap red shift of monomer-to-respective-homodimer structures for CQD molecules with different core/shell dimensions.

(18) FIGS. 17A-17C provide a representative photoluminescence lifetime decay data at (FIG. 17A) ensemble and (FIG. 17B) single particle level for monomer, unfused dimer and fused dimer composed of 1.4/2.1 nm CQD (FIG. 17C) Histograms summarizing the distribution of the average lifetimes from single particle data for all three types of particles. Further shortening of the lifetime for dimer is observed upon fusion.

(19) FIGS. 18A-18B provides a time-tagged, time-resolved data for single (FIG. 18A) CQD monomer and (FIG. 18B) fused dimer. Shown are, (i) photoluminescence intensity time trace, (ii) second-order photon correlation and (iii) lifetime for the single-particle (the light grey and dark grey lifetime curves were generated from data shaded in the same color in the corresponding time traces).

(20) FIGS. 19A-19C provide a time-tagged time-resolved analysis of the fusion protocol treated monomer CQD (1.4/2.1 nm). (FIG. 19A) A bimodal on-off distribution of the intensity was found (bin-50 ms). The black curve represents the background noise. (FIG. 19B) Fluorescence lifetime of the on-state follows a single exponential decay of 31 ns. (FIG. 19C) Strong photon antibunching with g.sup.2 value of 0.09. All these observations for the fusion protocol treated monomer are highly correlated with the untreated monomer particle as explained in the previous section.

(21) FIG. 20 provides a pump fluence dependent fluorescence lifetime of single 1.4/2.1 nm fused CQD molecule. As the power is stronger the lifetime shortens. The inset shows the possible new multicarrier configurations.

(22) FIGS. 21A-21D provide a TEM imaging of homodimer@SiO.sub.2 (FIG. 21A) and homodimer NCs after etching and release procedure (FIG. 21B). HRTEM and STEM of coupled homodimer QDs after the fusion procedure (FIG. 21C and FIG. 21D, respectively).

(23) FIGS. 22A-22B provide TEM imaging of SiO.sub.2@heterodimers.

(24) FIGS. 23A-23D provide TEM imaging of the heterodimers before (FIGS. 23A, B) and after (FIGS. 23C, D) the fusion procedure with different magnification.

(25) FIGS. 24A-24B provide TEM images of trimer structures.

(26) FIGS. 25A-25D provide TEM image (FIG. 25A), STEM image (FIG. 25B) of the zinc blende (ZB) CdSe/CdS QDs. The TEM of ZB QDs dimer @SiO.sub.2 structure (FIG. 25C). The fused dimer ZB QDs structure (FIG. 25D). Inset shows the HAADF-STEM image of dimer tip attachment QDs structure.

(27) FIGS. 26A-26H provide a HAADF-STEM image (FIG. 26A), Fast Fourier transform reconstruction imaging (FIG. 26B), electron diffraction (ED) pattern (FIG. 26C) on [110] zone axis. HAADF-STEM image of other types of coupled ZB for hetero-orientation attachment CdSeCdS molecules (FIGS. 26D-E). Inset shows the model of the dimer QDs structure. The SAED of the ZB CdSeCdS CQDs (FIG. 26F). HAADF-EDS line scan data (FIG. 26G) and STEM-EDS (FIG. 26H) analysis of the coupled ZB tip attachment CdSe/CdS molecules.

(28) FIGS. 27A-27B provide a scheme for the potential energy landscape of the monomer (left side) and the fused CQDM dimer (right side) along with a cross-section of the highest hole wave-function (blue), the lowest state of the electron without Coulomb interaction (red), with coulomb (dashed red), the first excited state of the electron without Coulomb interaction (green) and with Coulomb (dashed green). (h) Exciton energy level ordering for the monomer CQD (left side) and the dimer CQDM (right side) before (black), and after (magenta) applying the Coulomb interaction. C.sub.m/d refers to the Coulomb energies in the monomer and the dimer respectively, E.sub.f refers to the fusion energy. E refers to the coupling energy, the difference between the symmetric and anti-symmetric electron states in the dimer CQDM.

(29) FIGS. 28A-28C provide the effects of core dimensions, barrier thickness, and band offsets on the coupling of the lowest conduction band state in the CQDM. (FIG. 28A) The wave-functions of the first electronic state (symmetric state) of specific points on the contour graphs 2d-f. core diameter/barrier thickness 2.8 nm/0.9 nm, 2.8 nm/8 nm, 4 nm/5 nm, 5 nm/0.9 nm and 5 nm/8 nm in: (FIG. 28A) 0 eV conduction band offset, (FIG. 28B) 0.1 eV conduction band offset and (FIG. 28C) 0.32 eV conduction band offset.

(30) FIGS. 29A-29C demonstrate the neck effect on the coupling of the lowest electron state in CQDMs. (FIG. 29A) Energy difference between the symmetric and anti-symmetric states as a function of the neck diameter. The blue curve refers to 0.1 eV conduction band offset, and the red curve refers to 0.32 eV conduction offset. Inset: the dimensions considered in the calculation. (FIG. 29B) The wave-functions of the symmetric (bottom) and the anti-symmetric (top) electronic states and the energy difference between them in three points: B1, B2, and B3 which refers to 1.5 nm, 4 nm, and 6.2 nm neck thickness, respectively (0.1 eV band offset). (FIG. 29C) The wave-functions of the symmetric (bottom) and the anti-symmetric (top) electronic states and the energy difference between them in three points C1, C2 and C3 corresponding to 1.5 nm, 4 nm, and 6.2 nm neck thickness, respectively (0.32 eV band offset).

(31) FIGS. 30A-30F depict the excitonic behavior for the CQDM. (FIG. 30A) The overlap integral between the first electron and hole wave-function after applying the Coulomb interaction as a function of the core diameter for the dimer (dashed line) and the monomer (solid line), considering 0.32 eV (red) and 0.1 eV (blue) conduction band offset. (FIG. 30B) The Coulomb energy C.sub.d of the dimer (dashed line) and monomer C.sub.m (solid line), in 0.32 eV (red) and 0.1 eV (blue) conduction band offset as a function of the core diameter. (FIG. 30C) The difference between the monomer and dimer coulomb energy E.sub.c (solid line) and the fusion energy E.sub.f (dashed line) in 0.32 eV (red) and 0.1 eV (blue) conduction band offset as a function of the core diameter. (FIG. 30D) The emission redshift between the monomer and dimer in 0.32 eV (red) and 0.1 eV (blue) conduction band offset as a function of the core diameter. Inset: the dimensions considered in the calculation. (FIGS. 30E-F) A, B, and C show the wave-functions of the first electron and hole states after applying the Coulomb interaction in 1 nm, 1.8 nm and 4.2 nm core diameter respectively, considering 0.1 eV (blue frame) and 0.32 eV (red frame) conduction band offset.

(32) FIGS. 31A-31C depict the absorption spectra of CQD versus CQDMs and comparing theory with experiments (for fused core diameter/shell thickness 2.8/2.1 nm CdSe/CdS. (FIG. 31A) Absorption cross-section as a function of energy for monomer CQD (blue), Non-fused CQD (green) and fused dimer CQDM (red). (FIG. 31B) Monomer absorption cross-section (blue) along with calculated transitions overlap integral (purple) for 0.1 eV band offset. (FIG. 31C) Dimer absorption cross-section (red) along with calculated transitions overlap integral (orange) for 0.1 eV band offset. Electron and hole wave-functions involved in strong transitions are also presented.

(33) FIGS. 32A-32C demonstrate the dual emitting NC heterodimer molecule, made by coupling two CdSe/CdS core/shell NCs with different core sizes, one green emitting and a larger NC with red emission. (FIG. 32A) Excitation above the entire particle gap will lead to dual color emission. (FIG. 32B) Dual-color single photon source. Emission of two color simultaneously is forbidden since the charge carriers will undergo Auger process. (FIG. 32C) Intensity controlled green emission dictated by the intensity of the red excitation. Higher intensity of red excitation will create multi-excitons that will suppress the green emission through Auger process.

(34) FIGS. 33A-33B present QD molecule as an optical switch controlled by electric field. (FIG. 33A) Type I-Type I interface. Under no electric field the exciton can recombine from either of the cores end to emit a photon with wavelength .sub.1 Under electric field the lowest energy photon is an indirect exciton. If the cores are closed different wavelengths will be emitted. If they are far away the emission will be suppressed. (FIG. 33B) Type II-Type II interface. Under no electric field both wavelengths can be emitted, .sub.1 and .sub.2. Under electric field the electron is overlapping with only one of the cores hole, leading to the selection of wavelength by electric field.

(35) FIG. 34 presents QD molecule as a CNOT quantum gate. This structure is using an electronhole pair localized in a QD as elementary excitation representing logical binary: existence (nonexistence) of exciton in the QD corresponds to logical one (zero). The qubit can be manipulated by resonant coherent electromagnetic radiation to pursue quantum manipulation. QD molecule hosting an exciton in each of the dots will manifest different energy levels because of the Coulomb interaction between the excitons. This can be used to facilitate CNOT quantum gate, where 7E pulse with energy matching to the energy of only one exciton in the QD molecule will change the state from 10> to 11> while no operation will be executed if the QD molecule is hosting an exciton in each of the dots.

(36) FIG. 35 provides different types of coupled trimer or other coupled chain nanostructures.

(37) FIG. 36 demonstrates the QD molecule as a building block for superlattices of QDs. These superlattices can be used to create Quantum Cascade Lasers, in which under electric field one electron in the conduction band can undergo intra-band transitions emitting IR photons then tunnel to the next dot and to repeat this process several times. In this way, several IR photons are emitted from one single electron.

DETAILED DESCRIPTION OF EMBODIMENTS

(38) A first example of coupled NC molecules presented herein combines two core/shell nanocrystals, to thereby yield highly controlled coupled systems. The core-shell nanocrystal building blocks can be constructed providing highly diverse geometric patterns. The building blocks are selected from all types of core/shell NCs.

(39) Taking the CdSe@CdS core-shell structure as a non-limiting example, this core/shell system could be synthesized to adopt a spherical-like shape or polygonal shapes such as tetragonal pyramid, hexagonal pyramid, and hexagonal bipyramid. Other core/shell NCs might also have other geometry such as cubes and so on. With these core/shell building blocks, the coupled QDs molecules could be fabricated to yield a rich set of coupled NC molecules with diverse geometries including peanut-like, peasecods-like, snowman-like, calabash-like, matchstick, and additional geometries. In terms of bandgap engineering, in the first instance tuning the size, shape, and composition of the quantum dots (QDs) cores is used to manipulate the wavefunctions and energies of the electron and hole. In addition, the wavefunctions and energies are then further manipulated by the synthesis of shells on top of these cores. Once fusedthese core/shell nanocrystals combine to form the coupled nanocrystal molecules.

(40) There are vast possibilities starting from coupled homodimer structure where two similar core/shell nanocrystals are combined in one nanocrystal molecules. Through control of the shell size, composition and geometry it is then possible to achieve diverse combinations for coupled nanocrystal molecules where the constituent cores are similar. Moreover, using different cores of different sizes or compositions, it is possible to create coupled nanocrystal molecules of heterodimer morphology. It is also possible to combine core/multishell nanocrystals into a coupled nanocrystal molecule to yield rich and further control of the potential landscape between the cores. Additionally, it is possible to combine cores of different materials in the core/shell constituents and thus generate the coupled nanocrystal molecules with different properties.

(41) In essence, we are introducing a whole set of nanocrystal molecules, not limiting only for dimers. It is analogous to going from atoms of the periodic table which is finite, to the infinite possibilities for molecules comprised of these atoms. And our invention offers such potential for the core/shell nanocrystals serving as the building blocks of the coupled nanocrystals molecules.

(42) By way of a non-limiting example, we show one approach we used for the formation of coupled QDs molecules in FIG. 2. This was performed by the following steps:

(43) 1. Fabrication of SiO.sub.2 nanoparticles with a size of 200 nm and coated by (3-mercaptopropyl)trimethoxysilane (MPTMS). This kind of SiO.sub.2 particle presents on the outer surface thiol groups, which are used for the binding of the QDs.

(44) 2. Core/shell NCs binding to the SiO.sub.2 particle surface: mixing a solution with the chosen core/shell NCs, to the SiO.sub.2 nanoparticles allows their binding to the available thiol sites. After this step, the NCs are bound on the surface of the SiO.sub.2 nanoparticles.

(45) 3. (optional step) Secondary thin layer of SiO.sub.2 growth on the SiO.sub.2@QDs for masking: Achieved by a modification of the Stober method, the thin SiO.sub.2 layer is synthesized. In this manner, the NCs are immobilized by this masking SiO.sub.2 layer and cannot rotate or reorient while only a top hemisphere is remaining exposed for further chemical functionalization of the NCs.

(46) 4. Selective surface decoration of the NCs by linker groups: Chemical grafting of a functional structure and group is then applied and it reacts only with the exposed NC hemisphere. For example, a tetrathiol ligand can be added as a linker that also exchanges the NC ligands on the exposed surface (for example oleylamine).

(47) 5. Forming dimer geometry on the silica surface: Addition of a solution of second NCs allows for conjugate formation yielding the controlled formation of a dimer structure by binding to the linkers.

(48) 6. Dimer release: The dimers are released from the silica surface and separated. For example, this can be achieved by selective SiO.sub.2 etching using an HF/NMF solution.

(49) 7. Fusion to form the coupled NC molecule: Dimers are fused. For example by the addition of suitable precursor and heating to form a continuous link between the two shell regions of the pre-made dimers.

(50) Additional optional purification of the dimers versus monomer and higher linked oligomers is possible in between steps 6 and 7, or after fusion step 7. This can be achieved by a multitude of separation methods such as but not limited to size-selective precipitation, density gradient separation, and others. Moreover, other post-coupling processing, such as synthesis of a shell, metal growth and surface engineering are also possible. Additional methods, such as oriented attachment, for the formation of dimers are also possible.

Example 1: Homodimer Formation (with Linkers)

(51) Different sizes of CdSe@CdS NCs as building blocks for the dimer were synthesized by known methods. Taking 1.9/4.0 nm CdSe@CdS NCs for example (FIGS. 3A-C), the CdSe core (3.8 nm) was synthesized at 350 C. by hot-injection method for 70 sec the absorption (580 nm) and photoluminescence (PL) spectra (595 nm) were measured after the purification procedure (FIG. 3). Then the CdSe@CdS NCs (11.5 nm) were fabricated by CdS shell growth achieved by injection of Cd(OA).sub.2 and octanethiol precursor. After the shell growth, the photoluminescence (PL) spectra (637 nm) show a red-shift compared with the cores, which reveal the successful fabrication of quasi-type II QDs. Moreover, the fluorescence quantum yield was greatly enhanced.

(52) Structure analysis for the monomer was shown in high-resolution scanning-transmission electron microscopy (STEM) imaging and high angular annular dark field detector (HAADF) measurement. The core-shell structure was clear depicted by Fast Fourier transform (FFT) reconstruction imaging (FIGS. 4A-B). Furthermore, the lattice planes and structure with different zone axis (ZA) matches well with the structure model. Additionally, the X-ray powder diffraction (XRD) and selected area electron diffraction (SAED) measurement (FIGS. 4K-L) manifests the hexagonal close-packed (hcp) wurtzite structure of CQDs.

(53) The 200 nm diameter SiO.sub.2 nanoparticles (step 1) were prepared as described in the methods section. Then SiO.sub.2@QDs particles were prepared in step 2 by adding the CdSe/CdS QDs to the SiO.sub.2 nanoparticles solution. The resulting particles were characterized by TEM and SEM as shown in FIG. 5. In order to avoid the NCs overlap and aggregation on the SiO.sub.2 surface, the ratio of QDs added to the SiO.sub.2 nanoparticle was controlled. In this specific sample, a 1:500 SiO.sub.2:QD ratio yielded good results of well-separated and clearly resolved QDs as seen in FIG. 5. The SiO.sub.2@QDs nanoparticles solution was cleaned twice from excess of free and weakly bound QDs by centrifugation, discarding the supernatant and re-dispersion in toluene.

(54) After this purification process, the SiO.sub.2@QDs were dispersed in ethanol. A secondary thin SiO.sub.2 masking layer was deposited on the surface of SiO.sub.2@CdSeCdS NCs (step 3) using the Stober method. Briefly, tetraethyl orthosilicate (TEOS), polyvinylpyrrolidone (PVP) and ammonia solution (28%) are added and stirred at room temperature for 10 hours. The secondary masking silica layer has two functions: firstly, the secondary layer can cover the surface thiol group of MPTMS to avoid the adsorption of additional CdSeCdS NCs in the dimerization step 5. This will enhance the efficiency of the dimer structure versus monomers. Secondly, the secondary layer immobilizes the CdSeCdS NCs such that they cannot rotate and their hemisphere that emerges in the solvent can be modified selectively by the chemical grafting of a functional structure and group. In order to control the thickness of the secondary SiO.sub.2 layer, the amount of PVP and TEOS was optimized. As shown in FIG. 6, after the growth of the secondary SiO.sub.2 layer, the surface roughness of the SiO.sub.2@CdSeCdS nanoparticles increases significantly due to this growth, while the emergent QDs can still be discerned.

(55) Next, towards the dimer formation, the chosen linkers are added to be bound to the exposed region of the bound QDs (step 4). In this example, tetrathiol molecule was used as a bi-dentate linker molecule. The thiol binding is strong on the QD surface and can displace the existing surface ligands selectively on the exposed QD hemisphere. In order to enhance the conjugation of the linkers, the surface modification procedure was performed at 60 C. for overnight with Ar flow. Then the excess linker molecules were removed by precipitation and centrifugation of the SiO.sub.2@QDs nanoparticles.

(56) In step 5, the secondary QDs composing the dimers were added. In this example of a homodimer formation, the same type of core/shell QDs were used in this step as those already bound to the SiO.sub.2. The ratio of the added QDs to the original amount used in step 2 was optimized and a ratio of 1:1.5 was used. As shown in FIG. 7, the SiO.sub.2@dimer structure was formed with precise control.

(57) Step 6 is then performed to release and separate the dimers from the SiO.sub.2 spheres. The release of the dimer CdSeCdS NCs was performed by the selective etching process of the SiO.sub.2 using an HF/NMF (10%) etching solution. The freed dimers were separated by centrifugation decanting the supernatant and repeated ethanol/centrifugation cycles for three times. As seen in FIG. 8C, the precisely controlled dimer CdSeCdS NCs were successfully synthesized. Some monomers are identified as well, along with possible aggregates with larger numbers of NCs, and these can be separated optionally after this step or after the fusion in step 7.

(58) Step 7, the fusion of the dimers to form the coupled NC dimer molecule is then performed. In order to get the coupled CdSe/CdS NCs in this example, the fusion procedure was performed while adding Cd(OA).sub.2 and heating to 180 C. for 20 h (FIGS. 8D-E). At this non-trivial important stage, the reaction parameters such as temperature and amount of ligands and precursors play a significant role in the fusion of the CdSeCdS NCs. If the temperature was too high (more than 220 C.), a ripening process of the NCs can be dominant, leading to the collapse of the dimer structures. On the other hand, if the temperature was too low, the fusion rate would be too slow and insignificant. Dimer structure formation is sensitive to both temperature and ligands. In the presence of excess ligands in the solution, the fusion would be inhibited and leads to the decrease of the dimer yield. Thereby, the appropriate tuning of temperature, time and ligands type, and concentration has a significant influence on the formation of the coupled dimer structure. Careful tuning and choice of these reaction parameters is crucial for achieving high dimer yields and lower yields of ripening and collapse while achieving a continuous linking region of the shell materials forming the barrier between the two cores in the fused dimers.

(59) Analysis of TEM and HAADF-STEM images confirms that coupled dimer formation is indeed achieved by fusion of the core/shell QD monomers (FIGS. 9A-B). Closer inspection as shown in FIG. 9, proves clearly that a continuous lattice was formed fusing the two QDs shells together. The high-resolution images (FIG. 9A) show lattice planes that are continuous through the entire structure, indicating coupled dimers. The coupled structure was further proved by the STEM line scan and commensurate energy dispersive spectroscopy (EDS) analysis measurement. As shown in FIG. 9, a continuous distribution of Cd (both in core and shell) and sulfur (only in shell) is identified along the line of the dimer axis. Along the same line, selective regions of the selenium (only in core) are clearly identified signifying the cores locations.

(60) The crystal structure and in particular the interfacial fused region of the coupled nanocrystals was further investigated by HAADF-STEM and Fast Fourier transform (FFT) reconstruction imaging. FIG. 10A shows the as-measured lattice image along with an indication of the identified lattice planes in each of the Monomer QDs A1 and A2. Next, A1 and A2 QDs were marked for selected FFT analysis. The selected FFT data of each region was then filtered in k-space and the filtered image after back-FFT is presented in FIG. 10B. Analysis of the data shows that for QD A1 we clearly identify the (002) and (100) lattice planes with an angle of 90 between them, typical of the hexagonal CdS wurtzite structure observed along the [010] zone axis. For QD A2 the structure also conforms to CdS wurtzite phase with stacking faults. The analysis allows for determining the c-axis direction for each QD as indicated by the white arrows. This is fully consistent with the analysis of the electron diffraction (ED) patterns of A1 and A2 shown in FIG. 10C. Both analyses yield an angle of 60 between the C-axis of each QD in the fused dimer and we term this as the coupling angle.

(61) Additionally, the orientation relationships including homo-plane-attachment and hetero-plane-attachment in the fusion process were observed by HAADF-STEM images. That is, homonymous faces attachment: (1010)(1010), (0002)(0002), and (1011)(1011); heteronymous faces attachment: (1010)(0002), (1011)(0002), and (1010)(1011) (FIGS. 11-12).

(62) Based on this detailed structural data, the statistical analysis was taken for the fused dimer structure. As shown in FIG. 11 K, the (0002) facets, while in minority, is more active during the fusion step, which is a reactive facet manifesting a Cd rich termination with 3 dangling bonds per atom. The fusion relationship for the small CQDs (1.4/2.1), including home-plan attachment and hetero-plan attachment (FIGS. 13-15), is similar to the big CQDs (1.9/4.0) via HAADF-STEM images.

(63) The electronic and wavefunction hybridization for the coupled CQDs molecules was attained by the red-shift both in the absorption and emission spectrum, which was in agreement with the quantum-mechanical calculation as shown in FIG. 16.

(64) Further, the photo-physical properties of the CQD molecules at both ensemble and single-molecule level manifest novel exciton recombination pathways converging to the internal electron rearrangement throughout the artificial CQD molecule. Firstly, we observe the shortening of the lifetime for the fused CQDs molecules as shown in FIG. 17.

(65) The photon statistics of the dimer altered significantly exhibiting fluorescence flickering instead of the ON-OFF blinking feature in the monomers (FIGS. 18-19). A clear off state was not detected for the 1.4/2.1 nm CQD dimers at the experimental resolution. While the monomer CQDs exhibit perfect photon antibunching with a contrast=g.sup.2(0)/g.sub.2() value 0.1 at low excitation power, at a similar condition, the dimers exhibit a much higher value. This is an indication of the formation of enhanced biexciton quantum yield. The deviation of the fluorescence decay profile from single exponential behavior is another important observation in dimer. While the highest intensity trace of the single monomer particles decay in a perfect single exponential manner due to exciton pair recombination, none of the levels in dimer posses single exponential behavior. The formation of the charged trion which is emissive is also possible. These characteristics indicate the introduction of additional recombination pathways and the rich possibilities for multiexciton configurations in the artificial CQD molecule compared with the monomers, also related to coupling within the system.

(66) The absence of a monomer like OFF state in dimer indicates the reduction in the Auger recombination rate and thus an emissive multicarrier can be generated in the homodimers (comprising two identical nanocrystals). Fused homodimers show an enhanced excitation power dependence of the fluorescence lifetime which is possibly the stabilization of emissive biexciton or trions (FIG. 20).

Example 2: Homodimer Formation (Intrinsic Janus Method)

(67) A homodimer structure can also be fabricated with Janus QDs as the building blocks, even without using clear linker molecules. First, QDs with hydrophobic ligands, (combination of oleylamine (OAm) and oleic acid (OA) ligands in this example) were bound to the surface of SiO.sub.2 (by thiol groups in this example) (FIG. 21A). Then, an etching step, with HF/NMF solution, was used to selectively etch the SiO.sub.2 and release Janus QDs, having approximately one hemisphere coated with hydrophobic ligands and the second hemisphere coated with hydrophilic thiolate ligands. Performing the etching in polar solvents, such as the NMF, results in natural homodimer formation due to hydrophobic-hydrophobic interactions while the hydrophilic part of the Janus QDs is facing outwards on both sides of the dimers. FIG. 21B shows the TEM data that manifests the formation of a significant fraction of homodimers (unfused at this step).

(68) Next, this unfused dimer solution has undergone a fusion step performed by adding Cd(OA).sub.2, OAm and heating to 240 C. for 20 hours (FIGS. 21C-D). The HRTEM and HAAD-STEM imaging shows a continuous shell formed after the fusion procedure, which future identified the final coupled homodimer structures.

Example 3: Heterodimer Formation (Different Shell Size)

(69) Heterodimers can be formed in a similar method as presented in example 1, but by the use of primary and secondary quantum dots with different shell dimensions. FIGS. 22A-B present SiO.sub.2@heterodimer of CdSe/CdS Quantum dots, wherein in one QD the shell comprises of 11 layers and in the second QD of 6 layers. FIGS. 23A-B presents the released dimers after the etching of the SiO.sub.2 spheres. This was followed by a fusion process (FIG. 2, steps 6-7 performed similar to example 1 above) providing coupled heterodimers as shown in FIGS. 23C-D.

Example 4: Heterodimer Formation with Different Core Size

(70) Heterodimers with different core sizes can be formed using a similar method as that presented in example 1. For example, coupled heterodimer of CdSe/CdS wherein one CdSe/CdS with a defined core radius are fused with a second QD of the same composition with different core radius.

Example 5: Heterodimer Formation with Different Core/Shell Material

(71) Heterodimers with different core/shell materials can also be formed by a similar procedure to that presented in example 1. These structures can be achieved by introducing a secondary QDs comprise of different core materials than the primary QDs, in step 5 (FIG. 2). As a non-limiting example, InP/CdS can be fused to CdSe/CdS, to form InPCdSe coupled heterodimer with CdS as the barrier and shell.

Example 6: Heterodimer Formation of a Core/Shell QD with QD of the Shell Material

(72) A private case for example 5 is a heterodimer, with one QD having the same composition as the shell of the second. Non limiting examples are coupling CdSe/CdS with CdS NCs or CdSe/ZnS with ZnS NCs.

Example 7: Trimer Formation by Dimer and Monomer Binding

(73) The dimer structures as fabricated in the previous examples can further be utilized as the building blocks for the synthesis of trimer (and in general of more complex) structures. FIG. 24 presents a non-limiting example of trimers produced after the binding and coupling of a quantum dot to a dimer structure. First, a fused dimer structure of CdSeCdSe with CdS barrier and shell were bound to SiO.sub.2 following the above-mentioned method. After that, the secondary QDs of CdSe/CdS were added into the solvent to form the trimer structure and the release and fusion processes were used.

Example 8: Chain Structure Formation

(74) Chain structures of different lengths can be achieved by similar procedures as utilized above. The short-chain structures could be used as the building blocks to grow longer chain structure.

Example 9: Dimer Structure of Core/Multishell Monomer QDs

(75) Dimer structure composed of core/multishell monomer QDs can be achieved by similar procedures as utilized above. A non-limiting example is CdSe/CdS/ZnS QD coupled to secondary CdSe/CdS/ZnS Quantum dots forming CdSe/CdS-CdSe/CdS dimer heterostructure with a ZnS barrier and shell. Another example is CdSe/CdS/ZnS coupled to InP/ZnS forming CdSe/CdSInP dimers with ZnS barrier and shell.

Example 10: Dimer Structure with Outer Shell

(76) Different structures (e.g. dimers, trimers, chains) with multi-shells can be synthesized by the introduction of precursors during or after the fusion step. For example, a ZnS outershell can be grown with Zn(OA).sub.2 and S-ODE precursors on CdSeCdS coupled dimers. In this example, type I band alignment, the outer shell growth is beneficial for passivation of the entire structure to remove possible surface traps.

Example 11: The Controlled Attachment of Dimer Structure Based on Zinc Blende (ZB) Structure

(77) The ZB CdSe@CdS building blocks were generated by previous methods, as shown in TEM and the HAADF-STEM image (FIGS. 25A-B). The formation of the dimers via constrained attachment is then performed. The QDs were first binding to the SiO.sub.2 nanoparticles. Following, the masking for the secondary thin SiO.sub.2 was achieved. Then with the surface modification, the subordinate QDs were added to form the dimer QDs@ SiO.sub.2 nanoparticles. As seen in FIG. 25C, the controlled ZB dimer CdSeCdS were successfully synthesized. Next, the ZB dimer CdSeCdS was released with an etching strategy. The coupled CQDs molecules were formed via fusion step. In our system, the optimized temperature scope for fusion of the ZB CQDs is 180-220 C. Then the fused ZB CdSe@CdS molecules based on the tip-attachment were formed.

(78) The binding relationship and attachment analysis was further investigated by HAADF-STEM and FFT reconstruction imaging. Analysis of the HRTEM for QDs, we clearly identify the (002) and (111) lattice planes, a typical of the tetragonal CdS ZB structure observed along the [110] zone axis which matches well with the Fast Fourier transform (FFT) imaging. Further, the (111) plane of A1 was corresponding with the (002) plane of A2 to form the coupled dimer structure. Additionally, other tip attachment with different orientation was depicted in FIGS. 26D-E.

(79) The coupled structure was further proved by the STEM line scan and EDS analysis measurement. As shown in FIG. 26G, a core-shell dimer molecule based on the ZB CQDs molecules was clearly identified.

Example 12: Quantum Mechanical Calculations of CQD Molecules Energy Levels

(80) Upon fusion, the potential energy landscape is changing from the core/shell type I heterostructure in which a core composed of the smaller bandgap material (CdSe) is embedded inside the shell of the larger bandgap material (CdS), to two closely spaced quantum dots separated by the barrier with height dictated by the band offsets between CdSe and CdS (FIG. 27 A). The resulting wave-functions are those of a symmetric state (red) and anti-symmetric state with a node in the center (green). The hole effective mass is much larger than that of the electron. As a result, the picture for the hole is still that of essentially two separate CQDs which means that in the single exciton regime the hole wave-function is mostly localized inside one of the cores (blue). Upon taking into account the electron-hole Coulomb interaction, the first electron wave-function becomes more localized in the core where the hole resides, while the next electron level will be more localized in the other core (dashed line in FIG. 27A).

(81) A key observation is a redshift of the emission wavelength upon fusion indicative of quantum coupling. The CB 1S, ground level of each CQD is shifted to lower energies due to the presence of the other CQD. However, this shift does not lift the twofold degeneracy of the lowest CB state. This degeneracy is lifted only by the coupling between the two CQDs. The fusion energy, which is the redshift between the ground states before and after fusion, is marked here as E.sub.f. Taking into account the Coulomb energy (magenta levels in FIG. 27B), the monomer ground state energy is red-shifted by C.sub.m that is greater than C.sub.d, the Coulomb redshift of the dimer ground state. The difference between the Coulombic terms E.sub.c defined as: E.sub.c=C.sub.mC.sub.d. The total redshift of the bandgap energy of the dimer with respect to the monomer is then: Red shift=E.sub.fE.sub.c.

(82) We have analyzed the coupling energy dependence on different diameters of CdSe cores and different barrier widths. The barrier width is controlled by overlapping the two outer spheres of the core/shell CQDs. We examined three representative CB offset values of 0.32 eV, 0.1 eV and 0 eV.

(83) As expected, the general trend is that as the core diameter and the barrier width decrease, the coupling energy E increases. In addition, as the band offset decreases the coupling energy increases. However, in small core diameters and small barrier widths, the trend is opposite. As the band offset becomes higher the coupling energy increases.

(84) We next examined how the neck size affects the coupling energy. In order to control the neck we are attaching two CQDs at a center to center distance of 7 nm so their surfaces touch. Then we converted half of the spheres in the side which connects the two CQDs, to half ellipsoid so they will overlap each other. We then merge them and hence the neck size is dictated by the long axis of the ellipsoid.

(85) In order to comprehend the different behavior for 0.1 eV compared to 0.32 eV CB band offsets, the wave-function of the symmetric and anti-symmetric states are presented in FIGS. 29B-C. For 0.1 eV with the largest neck, the wave-functions are delocalized all over the shell. Thus, the reduction of the neck size from 7 nm to 4 nm is significantly reducing the coupling energy. Whereas, for 0.32 eV band offset in all neck sizes the wave-function is concentrated around the cores, so the reduction of the neck from 7 nm to 4 nm is not affecting the coupling energy as dramatically as in the case of 0.1 eV band offset. These calculations demonstrate that the neck has a major effect on the coupling energy and filling the neck by suitable fusion reaction conditions can thus change the emission redshift and additional quantum coupling effects significantly.

(86) A general trend of non-monotonic overlap integral values, holds for all cases. For core sizes up to 2.5 nm the overlap integral decreases with size, while for larger core sizes the overlap integral increases with size. For both band offset values the overlap integral is slightly smaller in the case of the dimers compared to the monomers. In addition, for all core diameters the overlap integral is higher in the case of 0.32 eV band offsets compared to 0.1 eV. The same trend holds also for the 0.32 eV band-offset (FIG. 30F). The main difference is that the overlap integral is larger for all core diameters

(87) Commensurately, the opposite trend holds for the Coulombic interactions. In small core diameters up to 2.5 nm the Coulombic interactions C.sub.m and C.sub.d are increasing with core diameter size. Above 2.5 nm, the Coulombic interactions decrease with core size. For all core sizes the Coulombic interactions are stronger in the case of monomer compared to dimer. As for the different band offsets, except from core sizes below 1.5 nm, C.sub.m and C.sub.d are stronger in the larger band offset of 0.32 eV both for monomers and dimers. Looking on E.sub.f and E.sub.c as a function of the core diameter, one can see that both for 0.32 eV and for 0.1 eV band offsets, E.sub.f is always larger than E.sub.c (FIG. 30C). As a consequence, one should expect a redshift but never blue shift upon fusion (FIG. 30D). In small core diameter, E.sub.f decrease with size because the electron becomes more localized to the core which leads to lesser hybridization. However, beyond 4 nm core diameter, the surfaces of the cores getting closer leading to more coupling and hence to higher fusion energy. These results suggest that one of the signatures of fusion will be the emission redshift.

(88) Another signature for fusion is the absorption cross-section. The absorption cross-section (ACS) of both monomers (core diameter/shell thickness 2.8/2.1 nm) and their corresponding fused and non-fused dimers were extracted from the absorption spectrum and ICP measurements. While the ACS of non-fused dimers (green FIG. 31A) imitating the monomers (blue) one but only doubled, the ACS of the fused dimer is changing significantly. The ACS of fused dimers is losing the distinct features of the monomers and while at energies higher than 2.5 eV the ACS, , is twice the one of the monomer. At the band edge, the a of the dimer is in the height of the monomer but more smeared into lower energies (red in FIG. 31A). Integration on a of the band edge transition gives 80% of the one of the non-fused dimer.

(89) We have calculated the overlap integrals between the electron and hole states. One can see that at higher energies the density of states for the dimer is much higher, and considering the larger density of transitions that are partially allowed for dimers, one can understand the vanishing features in the absorption spectrum (FIGS. 31B-C). At the band edge, the overlap integral of the dimer is slightly lower than the monomer but one should take into account the possibility for the photon to be absorbed in both cores.

(90) Applications:

(91) The unique structure of two fused core-shell nanocrystals opens the way for band structure engineering leading to multiple applications of the coupled nanocrystals structures.

Example 1: Dual Color Emitting Nanocrystals

(92) Heterodimer NC molecules with dual emission: Heterodimers, composed of two types of core/shell NCs with different core sizes (FIG. 32) offer a dual emission system. CdSe/CdS coupled core/shells with cores of different sizes provide an unlimiting example. Since the electron barrier is small, the electron delocalizes. Dual red-green emission related to branching of the relaxation between the hole states in either coupled NC is generated upon excitation above the gap of the smaller dot, which is anti-bunched (termed green excitation) (FIG. 32A). These structures can be used as a dual-color single photon source. Emission of two colors simultaneously is forbidden since the charge carriers will undergo Auger process (FIG. 32B). Upon sufficiently short excitation (termed red excitation), only the larger dot of the dimer is excited and only red emission emerges. Tuning the excitation intensity offers a viable mechanism for switching of emission. Strong red excitation can be used to bring the small NC into the multi-exciton regime. This will attenuate the red emission on account of enhanced Auger relaxation rate. Using green excitation can then provide only green emission. This scheme is applicable for use of these particles in a super-resolution microscopy as a marker for the STED (stimulated emission depletion) technique. The scheme utilizes intense green excitation in bagel shaped mode, and intense to suppress all emission from the outer region. Both green and red emission to be suppressed by the multiexciton occupation (FIG. 32C). And a TM00 spot of red excitation at its center to excite and yield exclusively well-defined super resolved small red emission spot. The obvious virtues of QDs as bio-taggants are well expressed in this case offering highly stable chromophores.

Example 2: Electric Field Controlled Fluorescence

(93) A coupled nanocrystal molecule with dual color emission controlled by electric field is a unique application demonstrated by these particles. Dual-color emitting nanocrystals sensitive to applied electric field can be engineered by carefully choosing the materials of the cores and the shells of the two nanocrystals. As an un-limiting example, FIG. 33A presents a schematic of two fused core-shell nanocrystals with type I band alignment. An unlimiting example is offered by coupling two CdSe/CdS core/shell nanocrystals with suitably tuned core and shell sizes. Under no applied electric field this particle is expected to emit photons at the individual core/shell band-gap energy. However, upon applying electric field which bends the potential energy, the most likely transition is the indirect transition from one of the cores to the other. This transition is red-shifted compared to the transition under no electric field.

(94) FIG. 33B presents a different scenario of a system with two type II interfaces with two different materials in the cores. An un-limiting example is offered by CdTe/CdS coupled to ZnSe/CdS core/shells. Under no electric field two color emission emerges from the two type II transitions between the conduction band of the shell material to the top valence band states of the different cores. Under electric field the bending of the potential selects only one of the type II transitions, the one in which there is better overlap between the electron and the hole.

(95) These structures can serve in multiple applications where electric field controlled emission can be of relevance: First, as an emitter in light emitting diode (LED) devices or liquid crystal display (LCD) screens where an electric field applied by a voltage can tune the color which is emitted from the device. Second, as a multi-color single photon emitter benefitting from both the single photon purity and the multi-color of these nanocrystals, that can be controlled by the electric field. Furthermore, these nanostructures can serve as electric field sensors which change their color when exposed to electric field. These include their integration as bio-labels in Neuroscience for example.

Example 3: Quantum Information and Computation

(96) Another field in which coupled colloidal core-shell semiconductor QDs can play a major role is quantum information processing. Prerequisites for quantum computing are a scalable physical system of qubits, the ability to initialize the state of the qubits, decoherence time much longer than the gate-operation time, a universal set of quantum gates and a specific qubit measurement capability.

Example 3A: Entangled States in Coupled NC Homodimers

(97) In case of CdSe/CdS coupled homodimers with small core of 1.3 nm radius as unlimiting example, and a CdS shell of 1.3 nm thickness, strong hybridization of the coupled dots levels is taking place yielding bonding-antibonding combinations for the lowest lying electron state and similarly for the top hole states (although both barrier height and effective mass for the hole in this system are significantly larger compared to the electron). This offers an entangled levels scheme for a Qubit.

(98) In this scheme, the qubit 10> and 11> represents the location of the charge carrier (electron or hole) either in core 1 or core 2. The coherent evolution of the location of the charge carrier is electric field dependent. When the electric field is turned off, the quantum mechanical tunneling leads to the superposition of two QD states. The quantum gate is built when two different particles, an electron and a hole, are created optically. Under electric field, the particles are localized on opposite dots. After switching off the electric field, the interaction between the two particles should lead to the formation of entangled states. The states can be disentangled at a later time by preventing the tunneling by an electric field.

Example 3B: Coupled NC Heterodimers for Quantum CNOT Gate

(99) In the case of coupled colloidal core-shell semiconductor QDs (CdSe/CdS core/shells as unlimiting example) with different core sizes the existence and nonexistence of electronhole pair localized in each one of the cores serve as a qubit (11.sub.1> represent existence of exciton and 10.sub.1> represent nonexistence of exciton in core 1) as illustrated in FIG. 34. Each of the qubits can be driven by coherent electromagnetic radiation with different wavelengths to address each one of the cores, allowing many operations within the exciton lifetime. In the absence of the radiation, the QD system is in the ground state since the typical energy gap is much larger than the thermal energy. This allows initializing the state of the qubits. The scale-up of the number of quantum bits can be achieved by making a chain of coupled colloidal core-shell semiconductor QDs. The readout of this system is by detecting the spontaneous emission which gives us information on the population of the qubits. The read-out may also be done before the spontaneous decays by probe pulses.

(100) The implementation of any universal quantum computations is known to be decomposed into a series of one-bit rotation gates and two-bit control not (CNOT) gates. The rotation gate essentially results in the qubit population flopping corresponding to the -pulse according to Rabi oscillation induced by external gate radiation. The CNOT gate rotates one qubit (target bit) only when the other bit (control bit) is in the state 10>. QD molecule hosting an exciton in each of the dots will manifest different energy levels because of the Coulomb interaction between the excitons. This can be used to facilitate CNOT quantum gate, where pulse with energy matching to the energy of only one exciton in the QD molecule will change the state from |0> to |1> while no operation will be executed if the QD molecule is hosting an exciton in each of the dots.

Example 4: Beyond DimersColloidal Quantum Cascade Laser Effect in Coupled NC Chain

(101) The extension of the system to more than two coupled colloidal core-shell semiconductor QDs to a chain of QDs separated by a barrier or to any other structure presented in FIGS. 35-36 opens the way to a band structure engineering of a super-lattice. This kind of engineered super-lattice can serve as quantum cascade laser (QCL). In this structure an excited electron under electric field can tunnel from core to core and to decay within the conduction band while emitting an IR photon in a cascade process. The band structure of QCLs should be carefully designed to allow the tunneling between the cores. The colloidal rich chemistry allows the synthesis of different materials and to carefully tune the sizes of the building blocks of the super-lattice.

(102) Photocatalytic Applications

(103) The capacity to couple two or more nanocrystals, can open the path for the production of new heterostructures, with intrinsic charge separation, which will allow better charge transfer to electrodes or for molecular species in solution. These coupled nanocrystals could be used as photocatalysts for wide range of applications in various forms (e.g. dispersed in solution, bound to a substrate or an electrode or embedded in a matrix). Non limiting examples, are the couples being used as photocatalysts for solar to fuel conversion (e.g. hydrogen generation, water splitting, and CO.sub.2 reduction), applications based on reactive species formation (e.g. components in sensors and biochemical kits, agents for phototherapy and antifouling activity, water purification and waste consumption), redox transformation of organic species, and as photoinitiators for adhesives, photocuring of surface, 2D & 3D printing.

(104) Methods:

(105) CdSe core growth: Briefly, 60 mg CdO, 280 mg octadecylphosphonic acid (ODPA) and 3 g trioctylphosphine oxide (TOPO) were added to a 50 mL flask. The mixture was heated to 150 C. and degassed under vacuum for 1 hour. Under argon flow, the reaction mixture was heated to 320 C. to form a colorless clear solution. After adding 1.0 mL trioctylphosphine (TOP) to the solution, the temperature was brought up to 350 C., at which point Se/TOP (60 mg Se in 0.5 mL TOP) solution was swiftly injected into the flask. The reaction was kept for 60 s then finished by removing the heat. The resulting CdSe particles were precipitated by adding acetone and dispersed in 3 mL hexane as a stock solution.

(106) CdSeCdS core-shell NPs synthesis: For the shell growth reaction, a hexane solution containing 200 nmol of CdSe QDs was loaded in a mixture of 1-octadecene (ODE, 6 mL) and oleylamine (OAm, 6 mL). The reaction solution was degassed under vacuum at room temperature for 30 min and 90 C. for 30 min to completely remove the hexane, water, and oxygen inside the reaction solution. After that the reaction solution was heated up to 310 C. under argon flow and magnetic stirring. During the heating, when the temperature reached 240 C., a desired amount of cadmium (II) oleate (Cd-oleate, diluted in 6 mL ODE) and octanethiol (1.2 equivalent amounts refer to Cd-oleate, diluted in 6 mL ODE) began to be injected dropwise into the growth solution at a rate of 3 mL/h using a syringe pump. After finishing precursor infusion, 2 mL oleic acid was quickly injected and the solution was further annealed at 310 C. for 30 min. The resulting CdSe/CdS core/shell QDs were precipitated by adding ethanol, and then redispersed in hexane. The particles were further purified by precipitation-redispersion for two more rounds and finally suspended in 2 ml hexane.

(107) The synthesis of Silica NPs: In briefly, 120 L (3-mercaptopropyl)trimethoxysilane (MPTMS) precursor was mixed with 30 mL ammonia aqueous solution (1%) under strong stirring. After stirring for 1 min, the solution was store for overnight. The SiO.sub.2 NPs were collected by centrifugation and dispersed in ethanol.

(108) The synthesis of SiO.sub.2@CdSeCdS NPs: In brief, SiO.sub.2 NPs (0.0079 nmol) dispersed in 1 mL of hexane and mixed with 0.5 nmol CdSeCdS NPs under vortex for 20 min then 5 mL of ethanol was added in the vails to precipitate and wash for 3 times to remove the unattached NPs. Finally, the SiO.sub.2@CdSeCdS NPs were dispersed in 5 mL of ethanol.

(109) The synthesis of SiO.sub.2@ CdSeCdS@ SiO.sub.2 NPs: The fabrication of the second SiO.sub.2 layer was necessary and decisive for the dimer structure. In brief, the SiO.sub.2@CdSeCdS was dispersed in 5 mL of ethanol. Then 330 L of ammonia solvent (28.5% wt %) was added in the system with stirring for 5 min. Thereafter, 50 L of TEOS was dropwise in the system. After stirring for 10 h, the resulting solvent was centrifuged with 6000 rpm for 5 min and dispersed in 5 mL of THF.

(110) The synthesis of SiO.sub.2@Dimer-CdSeCdS NCs: Before the synthesis of SiO.sub.2@Dimer-CdSeCdS NCs, the tetrathiol linker pentaerythritol-tetrakis (3-mercapto-propionate) (200 L) was used for the ligands exchange progress to remove OAm and OA of the hemisphere and promote the conjunction with the second CdSeCdS NCs. Then 0.6 nmol of CdSeCdS NCs were added in the vials with oil bath at 60 C. for overnight. Finally, the samples were cleaned by centrifugation with 6000 rpm for 5 min and dispersed in vials with 10 mL of THF for storage.

(111) The release of Dimer-CdSeCdS NCs: Briefly, 1 mL of SiO.sub.2@Dimer-CdSeCdS NCs was taken out of the vials and centrifugation with 5000 rpm for 5 min. Then 2 mL of mixed solvent of HF/NMF (10%) was added in the plastic bottle with stirring for 10 h. After the etching procedure, the color of the samples changed to light yellow, which reveals the remove of the SiO.sub.2. Thereafter, the samples were precipitated by centrifugation with 6000 rpm for 10 min and washed for 2 times. Finally, the samples were dispersed in 2 mL of ethanol.

(112) The synthesis of fused Dimer-CdSeCdS NCs: In brief, Dimer-CdSeCdS NCs (in 2 mL of ethanol) mixed with 2 mL of ODE, 50 L of Cd(OA).sub.2 (0.2 M), and 100 L of OAm. The reaction solution was degassed under vacuum at room temperature for 30 min and 90 C. for 30 min. Then, the reaction mixture was heated to 180 C. for 20 h under argon flow. The resulting fused particles were precipitated by adding ethanol and dispersed in 2 mL toluene as a stock solution.

(113) Characterization:

(114) Absorption spectra were measured using a Jasco V-570 UV-Vis-NIR spectrophotometer. Fluorescence spectra and ensemble lifetimes were measured with a fluorescence spectrophotometer (Edinburgh instruments, FL920). Transmission electron microscopy (TEM) was performed using a Tecnai G.sup.2 Spirit Twin T12 microscope (Thermo Fisher Scientific) operated at 120 kV. High-resolution TEM (HRTEM) measurements were done using a Tecnai F20 G.sup.2 microscope (Thermo Fisher Scientific) with an accelerating voltage of 200 kV. High-resolution STEM imaging and elemental mapping was done with Themis Z aberration-corrected STEM (Thermo Fisher Scientific) operated at 300 kV and equipped with HAADF detector for STEM and Super-X EDS detector for high collection efficiency elemental analysis. CQDs atomic structure model were built by the VESTA software. Scanning electron microscopy imaging (SEM) was done with HR SEM Sirion (Thermo Fisher Scientific) operated at 5 kV.