Magnetic Properties of Cd0.5Mn0.5Te Nanoparticles

Jesús GonzálezCENTRO DE ESTUDIOS DE SEMICONDUCTORES

Facultad de Ciencias, Departamento de Física, Universidad de Los Andes – Mérida, VenezuelaCH. Power*, O. Contreras*, E. Calderón*

J.C. Chervin**, E. Snoeck***, J.M. Broto *****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****LNCMP, Université Paul Sabatier, Tolulouse France

Alfa Meeting Highfield Vienne 26th to 30th April 2004

Universidad de Los Andes

Université P. et M. Curie Université Paul Sabatier

PCP Nanosistemas y Nanomedidas (Francia – Venezuela)

WHY?

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

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

Optical Properties

The density of states for an electron in the conduction band of ananocrystal. Here, c(r) denotes the size ependent energy of the conduction band edge.

The band structure for ananocrystal with a crystal size rand 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 indicatedin 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

Structural Phase Transition

Phase diagram of GaAs

Exafs under pressure in GaAs

Transmission electron micrograph (HRTEM) of GaAs

nanocrystal at 17 GPa

Raman scattering in GaAs, laser argon 512.4 nm

Cadmium Selenide CdSe

Lattice Parameters:a = 4.3 Å, c = 7.1 Å, c/a=1.63. Band structure ( eV), and Selection rules:

cv 79 Γ→Γ cv 77 Γ→Γ cv 77 Γ→Γ 1.74 1.77 2.18

Family: II-VI. Structure: Hexagonal wurtzite and Cubic-zinc-blend (ZB) Spatial Group: F-43m Phase transitions:

0.0 - 3.6 GPa. Hexagonal wurtzite.

Bigger than 3.6GPa. NaCl.

Hexagonal wurtzite structure

Hexagonal wurtzite structure (room temperature)

CdSe

Pressure (GPa)0 1 2 3 4 5 6 7 8 9 10

Tem

pera

ture

(K)

300 K

(Pt~3.6 GPa)

HEX Type NaCl

Liquid

PRESSURE (Kbar)

0 5 10 15 20 25 30 35

Ene

rgy

Gap

(eV

)

1.60

1.65

1.70

1.75

1.80

1.85

1.90

Wurtzite phase

NaCl

Hysteresisof thephase

transition

High preassure

phase

Energy Gap Variationunder Pression on CdSe

Single Crystal

PressureThe 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 mm3.

Paris-Edinburgh cell1. Pot2. Piston3. Columns4. System of center5. Plate6. Breech7. Unbleached8. Joint9. Insulation10. Seat11. Anvil12. Samples

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

Electron diffraction pattern of recovered CdSe Quantum dot in the

cubic phase (Z.B)

hkl values and lattice parameter of CdSe nanocrystal in the cubic

phase (Z.B)

CdSe (cubic) F-43m, Lattice parameter: a = 6.077 Å.

(h k l) Interatomic distance. (Å).111 3.51 220 2.15 311 1.83 400 1.52 331 1.39 422 1.24 511 1.17 440 1.07

Nanocrystals of CdSe in thehexagonal phase: 3 nm

Dispersion curve of phonons in CdSe

Raman Spectra of CdSe nanocrystal in the cubic phase (Z.B)

CdSe (7GPa)

Wavelength (cm-1)100 150 200 250 300 350

Inte

nsity

(U.A

.)

0

10

20

30

40

50

60

70

80

90

100

LO

TO

The argon laser line was the 512.4 nm

Confinement Model

( ) ( )( )[ ] ( )

∫Γ+ω−ω

=ωΙ qdq

qCAd 3

20

2

2

2/,

( ) )16/exp( 2222 π−= dqqC

Confinement Model

( ) [ ] ( )4/sin*0 2 qaq ω∆−ω=ω

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, ,Γ0= 6 cm-1, and ∆ω= 12 cm-1

Nanocrystals of CdS in thehexagonal phase: 4 nm

Ramman scattering of CdS 3 nm Nanocristals

CdS 6 GPa LO= 297 cm-1 FWHM= 16 cm-1

2 LO= 598 cm-1 FWHM= 27 cm-1

Phase Diagram of CdTe

0 1 2 3 4 5250

500

750

1000

1250

1500

Liquid

(2.6 ± 0.1 GPa , 735 ± 20 K)

Jayaraman et al. (DT A) I-II (upstroke) I-II (downstroke) II-III (upstroke) II-IIl (downstroke) I-III (upstroke) I-III (downstroke) averages l ines

CdTe II (cinnabar)

CdTe III (NaCl)CdTe I (zincblende)Tem

pera

ture

(K

)

P ressure (GP a)

EXPERIMENTALParis-Edinburgh Large volume cell:

sample

gasket(Cu-Be)

CW toroidalanvils

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 indiameter, with a sample volume of 90 mm3.

HRTEM brith field image shows nearly spherical nanocrystals with diameters ranging from 10 to 30nm. 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.

(h k l) Interatomic distance. (Å).

111 3.75220 2.29311 1.95400 1.62331 1.48422 1.30

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

(h k l) Interatomic distance. (Å).

111 3.75220 2.29311 1.95400 1.62331 1.48422 1.30

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

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

(h k l)(h k l) Interatomic distance. (Å).Interatomic distance. (Å).

111111 3.753.75220220 2.292.29311311 1.951.95400400 1.621.62331331 1.481.48422422 1.301.30

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

nanocrystal

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.

Nanoparticules of Cd0.5Mn0.5Te

20 40 60 80 1003.0

3.5

4.0

4.5

67 °Α 80 °Α

Cd0.5Mn0.5TeField Cooling

χ (1

0-5em

u/g)

Temperatura (K)

Nanoparticules of Cd0.5Mn0.5Te

20 40 60 80 1003.0

3.5

4.0

4.5

67 °Α 80 °Α

Cd0.5Mn0.5TeZero Field Cooling

χ (1

0-5em

u/g)

Temperatura (K)

Phase Diagram

CdTe - MnTe

DTA Measurements

960 1000 1040 1080 1120 1160

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

DTA SIGNALHEATING CYCLECd0.5Mn0.5Te 8nm

DTA

VO

LTAG

E (A

.U)

Temperature (C)

ZB StructureCd

Te

Experimental Details I

High resolution transmission electron microscopy (HRTEM) was realized witha Philips CM30/ST operating at 300 kV.Specimens for electron microscopy were prepared embedding the samplein an epoxy resin and then cut by the ultramicrotomy procedure

Experimental Details II

X- ray diffraction experiments are performed using a Bruker D5005 diffractometer equipped with a graphite monochromator using the Cu Kα line (λ= 1.54059 A0). Silicon powder was used as external standard. The samples were scanned from 2-700 2θ, with scan rate of 0.020/step and 10 s/step. Each reflection was modeled by means of apseudo-Voigt function .

High resolution transmission electron

microscopy (HRTEM) brith field image of Cd0.5Mn0.5Te nanocrystal formed at 6.5

GPa.

The selected area electron diffraction (SAED) pattern of

recovered Cd0.5Mn0.5Te nanocrystal in the cubic

phase (Z.B).

6.5 GPa 6.5 GPa

5 nm

High resolution transmission electron microscopy (HRTEM)brith field image of

Cd0.5Mn0.5Te nanocrystalformed at 8 GPa.

The selected area electron diffraction (SAED) pattern of

recovered Cd0.5Mn0.5Te nanocrystal in the cubic

phase (Z.B).

8 GPa8 GPa

5 nm

XRD pattern of Cd0.5Mn0.5Te Nanocrystals

20 40 60 80 1000.0

0.2

0.4

0.6

0.8

1.0

1.2

111

200

220

311

222

400

331

420

422

333

440 53

1

620

6.5 nm

7.2 nm8.0 nm

Bulk

2*θ

Inte

nsity

(u.a

) ( )θλ

CosD

**9.0

∆Γ=

Standard Scherrer equation

λ is the wavelength∆Γ is the full width at half maximum (FWHM) of the diffraction peaksθ is the Bragg angle

ResultsThese rings correspond respectively to the 111, 220, 311, 400, 331, and 422 lattice planes of the Cd0.5Mn0.5Tenanocrystals in the zinc-blende structure. The values of the lattice parameters and the hkl are given in Table I and are in good accord with previously published values. No other ordered and disordered (amorphous) phase could beidentified. The marked broadening in nanoparticle Cd0.5Mn0.5Te peak is caused by a decrease in domain size upon phase transitions after the pressure cycle.

Volume and lattice parameter pressure dependence

-1 0 1 2 3 4 5 6 7 8 9

6.40

6.41

6.42

6.43

6.44

6.45

6.46

a (°Α

)

Pressure(GPa)

0 2 4 6 8 10

262

264

266

268

270

Cd0.5Mn0.5Te

V (°

A3 )

Pressure(GPa)

Nanocrystal size dependence with pressure

3 4 5 6 7 8 96.5

7.0

7.5

8.0 333 531 MEAN

L (n

m)

Pressure (GPa)

Experimental Details IIIUnpolarized Fourier Transform Raman scattering measurements (FT-Raman) in the near-infrared were performed at room temperature in a 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 and proved to be low enough to avoid spurious effects caused by the laser induced heating of the sample. This was verified by varying the incident power and observing that neither the Stokes to anti-Stokes intensity ratio nor the frequency of theLO mode varied within experimental precision.

Experimental Details IV

Confocal Micro- Raman XY Dilor triple spectrometer, with CCD detector working at liquid nitrogen. We use the 514,5 nm line of an Argon laser at 2mW

Two Mode Like

behaviorCdxMn1-xTe

System

Raman Scattering Cd0.5Mn0.5Te

50 100 150 200 250 3000

2000

4000

6000

8000

10000

12000

14000 *LA (L)

2TA

LO (MnTe-Like)

TO (CdTe-like)

Single Crystal Nanocrystal 6 nm

* Argon Laser Plasma

I R(A

.U)

wave number (cm-1)

Experimental Details VSusceptibility Measurements

Were performed with the MPMS-5 DC SQUID magnetometer(Merida) and the zero-field-cooled (ZFC) and field-cooled (FC) data was obtained in the temperature range from 2 to 300 K. The details of the ZFC–FC procedure was the following: thetemperature of the sample was reduced in zero field (ZFC) down to about 2K; at this temperature a DC field was applied (100 Gauss) and the magnetization was measured with the temperature rising to about 300 K. After that the sample was cooled againkeeping the field constant (FC) and the data was obtained as afunction of decreasing T

On the other hand, data at higher fields, up to 40 T, were obtained using the Laboratory’s pulsed magnetic fields facility in Toulouse.

5 10 15 20 25 30

3.0

3.2

3.4

3.6

3.8

4.0

4.2

4.4

Cd0.5Mn0.5Te

Temperature (K)

χ (1

0-5 e

mu/

g)

χ (bulk ZFC) χ (bulk FC) χ (8.0 nm ZFC) χ (8.0 nm FC) χ (7.3 nm ZFC) χ (7.3 nm FC) χ (6.7 nm ZFC) χ (6.7 nm FC)

Magnetic Phase

transitions

Susceptibility measurements between 2 – 300 K

50 100 150 200 250 300

3.0

3.5

4.0

4.5

Cd0.5Mn0.5Te

Temperature (K)

χ (1

0-5 g

/em

u)

χ (bulk ZFC) χ (bulk FC) χ (8.0 nm ZFC) χ (8.0 nm FC) χ (7.3 nm ZFC) χ (7.3 nm FC) χ (6.7 nm ZFC) χ (6.7 nm FC)

3.6

4.5

5.4

6.3

1/χ

(104 g

/em

u)

1/χ (Bulk ZFC) 1/χ (8.0 nm ZFC) 1/χ (7.3 nm ZFC) 1/χ (6.7 nm ZFC)

From the Curie-Weiss temperature, it is posible to obtain an estimate for the exchange interaction J. For antiferromagnets and ferromagnets,

∑+=

kkk

B

JzKSS

3)1(2θ

where S is the spin of Mn2+ and zk the number of K-th nearest neighbors of given atom. For the case of an fcc or hcp lattice with only nearest-neighbor interaction z1=12 with S=5/2, therefore:

BkxJ 170

=θ

Values for the Exchange interactions between nearest - neighbors

8.911.54314.16.7

6.99.43243.27.2

7.89.80275.18.0

8.19.61282.2Bulk

-J1/KB(K)C(1e-3)-θ (K)D(nm)

High Magnetic Field Measurements

0 10 20 30 400.00

0.05

0.10

0.15

0.20

0.25

T = 2 K

M(B)

M (a

.u.)

B (Tesla)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0.000

0.002

0.004

0.006

0.008

0.7 tesla

M(B)

0

10

20

30

40

dM(B)/dB

dM

(B)/d

B

Nanoparticules of Cd1-XMnXTe

0.0 0.2 0.4 0.6 0.8 1.00

20

40

60

Cd1-xMnxTe

epilayer[3] epilayer[5] epilayer[6] bulk[3] bulk[10] bulk[12] this wordT (K

)

x

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 isostaticallypressed (CIP) compact and the structure is highly disordered and microcrystalline. These nanocrystalsare nearly spherical with diameters ranging from 10 to 30 nm