Gotas Cuánticas (QD) de Semiconductores II-VI preparadas por ciclos de presión

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1 *C.E.S., Facultad de Ciencias, Departamento de Física, Universidad de Los Andes Mérida, Venezuela **Physique des Millieux Condenses (UA782), Tour 13, E4. Université P. et M. Curie, 4 Place Jussieu 75252, Paris Cedex 05, France ***CEMES, Université Paul Sabatier, Toulouse France Gotas Cuánticas (QD) de Semiconductores II-VI preparadas por ciclos de presión Jesús González CENTRO DE ESTUDIOS DE SEMICONDUCTORES Facultad de Ciencias, Departamento de Física, Universidad de Los Andes Mérida, Venezuela Colaboradores Mérida: CH. Power*, O. Contreras*, E. Calderón* Exterior: J.C. Chervin**, E. Snoeck***

2 Universidad de Los Andes Université P. et M. Curie Université Paul Sabatier PCP Nanosistemas y Nanomedidas (Francia Venezuela)

3 WHY?

5 Density of states in metal (A) and semiconductor (B) nanocrystals. In each case, the density of states is discrete at the band edges. The Fermi level is in the center of a band in a metal, and so kt will exceed the level spacing even at low temperatures and small sizes. In contrast, in semiconductors, the Fermi level lies between two bands, so that the relevant level spacing remains large even at large sizes. The HOMO-LUMO gap increases in semiconductor nanocrystals of smaller size

6 Idealized density of states for one band of a semiconductor structure of 3, 2, 1, and 0 dimensions. In the 3d case the energy levels are continuous, while in the 0d or molecular limit the levels are discrete

7 Optical Properties

8 The density of states for an electron in the conduction band of a nanocrystal. Here, c(r) denotes the size ependent energy of the conduction band edge. The band structure for a nanocrystal with a crystal size r and a size dependent bandgap Eg(r). Again, VB denotes the valence band and CB denotes the conduction band The thick lines indicate the result without any sizevariation. Also indicated in this picture, by the thin lines, is the effect of a low (_2 %) and a high polydispersity (_20 %) of the crystal size on the density of states

12 Structural Phase Transition

13 Phase diagram of GaAs

14 Exafs under pressure in GaAs

15 Transmission electron micrograph (HRTEM) of GaAs nanocrystal at 17 GPa

16 Raman scattering in GaAs, laser argon nm

Cadmium Selenide CdSe Lattice Parameters: a = 4.3 Å, c = 7.1 Å, c/a=1.63. Band structure ( ev), and Selection rules: Γ Γ 9v Γ7c Γ 7v Γ7c 7v Γ7c 1.74 1.77 2.18 Family: II-VI.

17 Cadmium Selenide CdSe Lattice Parameters: a = 4.3 Å, c = 7.1 Å, c/a=1.63. Band structure ( ev), and Selection rules: Γ Γ 9v Γ7c Γ 7v Γ7c 7v Γ7c Family: II-VI. Structure: Hexagonal wurtzite and Cubic-zinc-blend (ZB) Spatial Group: F-43m Phase transitions: GPa. Hexagonal wurtzite. Bigger than 3.6GPa. NaCl. Hexagonal wurtzite structure

18 Hexagonal wurtzite structure (room temperature) Liquid CdSe Temperature (K) HEX Type NaCl 300 K (P t ~3.6 GPa) Pressure (GPa)

19 Energy Gap Variation under Pression on CdSe Single Crystal Wurtzite phase Energy Gap(eV) Hysteresis of the phase transition NaCl High preassure phase PRESSURE (Kbar)

20 Pressure The CdSe samples with the wurtzite structure were compressed using the Paris-Edinburgh cell. This is a low-mass (50 Kg) hydraulic press equipped with opposed toroidal anvils. For pressures up to 12 GPa, the tungsten carbide (WC) anvils have a single toroidal gasket and the sample space is 6 mm in diameter, with a sample volume of 90 mm 3.

21 Paris-Edinburgh cell 1. Pot 2. Piston 3. Columns 4. System of center 5. Plate 6. Breech 7. Unbleached 8. Joint 9. Insulation 10. Seat 11. Anvil 12. Samples

22 Transmission electron micrograph (HRTEM) of CdSe nanocrystal formed at 7 GPa

$Electron diffraction pattern of recovered$

23 Electron diffraction pattern of recovered CdSe Quantum dot in the cubic phase (Z.B)

24 hkl values and lattice parameter of CdSe nanocrystal in the cubic phase (Z.B) CdSe (cubic) F-43m, Lattice parameter: a = Å. (h k l) Interatomic distance. (Å)

25 Nanocrystals of CdSe in the hexagonal phase: 3 nm

26 Dispersion curve of phonons in CdSe

27 Raman Spectra of CdSe nanocrystal in the cubic phase (Z.B) 100 The argon laser line was the nm CdSe (7GPa) Intensity (U.A.) TO LO Wavelength (cm -1 )

28 Modelo de Confinamiento Zonal Ι ( ω, d) = A 2 C( q) 2 [ ω ω( q) ] + ( Γ / 2) 0 2 d 3 q ( ) = exp( q d /16π ) C q

29 Modelo de Confinamiento Zonal ω *sin 2 qa/4 ( ) [ ] ( ) ωq =ω0 ω(0) is the zone center phonon frequency and ω is the difference between the zone-center and the zone - boundary frequencies of the phonon dispersion curve of interest. This confinement model is used for calculating the spectral line shaped of confined-optical phonon LO and diameter d of the QD. The parameters used in the calculation are: ω(0)= 211 cm -1,,Γ 0 = 6 cm -1, and ω= 12 cm -1

30 Nanocrystals of CdS in the hexagonal phase: 4 nm

31 Dispersión Raman de Nanocristales de CdS de 3 nanometros de diámetro CdS 6 GPa LO= 297 cm -1 FWHM= 16 cm -1 2 LO= 598 cm -1 FWHM= 27 cm -1

32 Ancho de línea del fonon LO en CdS en función del tamaño del nanocristal Línea sólida calculado por confinamiento zonal

33 Variación de la frecuencia del fonon LO en nanocristales de CdS

34 Phase Diagram of CdTe 1500 Liquid Temperature (K) CdTe I (zincblende) Jayaraman et al. (DT A) I-II (upstroke) I-II (downstroke) II-III (upstroke) II-IIl (downstroke) I-III (upstroke) I-III (downstroke) averages lines CdTe III (NaCl) 500 (2.6 ± 0.1 GPa, 735 ± 20 K) CdTe II (cinnabar) Pressure (GPa)

$X- ray diffraction: Bruker D5005 diffractometer equipped with a graphite monochromator using the Cu Kα line (λ= 1.54059 A0).$

35 EXPERIMENTAL Paris-Edinburgh Large volume cell: sample gasket (Cu-Be) CW toroidal anvils HRTEM: Philips CM30/ST operating at 300 kv. X- ray diffraction: Bruker D5005 diffractometer equipped with a graphite monochromator using the Cu Kα line (λ= A0). Raman scattering (FT-Raman): RFS 100 Bruker System equipped with Ge cooled detector, in the backscattering geometry. The 1064 nm line of an Nd:YAG laser was used at powers of 10 mw incident on the sample.

$The selected area electron diffraction (SAED) pattern gives the successive interplanar distances which correspond to$

36 HRTEM brith field image shows nearly spherical nanocrystals with diameters ranging from 10 to 30 nm. The selected area electron diffraction (SAED) pattern gives the successive interplanar distances which correspond to the ZB structure

CdTe (cubic) F-43m, Lattice parameter: a = 6.481 Å.

37 CdTe (cubic) F-43m, Lattice parameter: a = Å. (h k l) Interatomic distance. (Å)

39 The LO phonon at 167 cm -1 (169 cm -1 in the bulk) and the TO at 139 cm -1 (the same value in the bulk) are clearly observed. These results are in accordance with the dispersion curves of bulk CdTe: downward curvature of the LO phonon and very small positive dispersion of the TO phonon [4]. bulk

40 nanocrystal

41 The marked broadening in nanoparticle CdTe peak is caused by a decrease in domain size upon phase transitions after the pressure cycle. The standard Scherrer equation yields a particle size of 25 nm.

42 Nanoparticulas de Cd 0.5 Mn 0.5 Te 4.5 Cd 0.5 Mn 0.5 Te Field Cooling χ (10-5 emu/g) Α 80 Α Temperatura (K)

43 Nanoparticulas de Cd 0.5 Mn 0.5 Te 4.5 Cd 0.5 Mn 0.5 Te Zero Field Cooling χ (10-5 emu/g) Α 80 Α Temperatura (K)

44 Nanoparticulas de Cd 1-X Mn X Te T (K) epilayer[3] epilayer[5] epilayer[6] bulk[3] bulk[10] bulk[12] this word 20 Cd 1-x Mn x Te x

45 CONCLUSIONS We have formed II-VI and II-Mn-VI nanocrystals with the zinc-blende structure by the pressure cycled method using the Paris- Edinburgh cell. HRTEM, EDX, X-ray diffraccion and Raman scattering confirm that the microscopic structure of the sample is that of a cold isostatically pressed (CIP) compact and the structure is highly disordered and microcrystalline. These nanocrystals are nearly spherical with diameters ranging from 10 to 30 nm

Crystal Structure. Insulin crystals. quartz. Gallium crystals. Atoms are arranged in a periodic pattern in a crystal.

Crystal Structure. Insulin crystals. quartz. Gallium crystals. Atoms are arranged in a periodic pattern in a crystal. Crystal Structure Atoms are arranged in a periodic pattern in a crystal. The atomic arrangement affects the macroscopic properties of a material. Many important materials (silicon, steel) are crystals