Ultrafast Dynamics of CdTe Nanoparticles in Water Solution

M. Sanz, A. Douhal*

Departamento de Química física, Sección de Químicas, Fac. de Ciencias del Medio Ambiente,

Universidad de Castilla-La Mancha, Avda. Carlos III, S.N. 45071 Toledo (Spain).abderrazzak.douhal@uclm.es

Miguel , M.A. Correa-Duarte, L. M. Liz-MarzánDepartamento de Química Física, Universidade de

Vigo, 36310 Vigo (Spain).

Trans HYZTrans AZO

Cis HYZ

< 30 fs < 1 ps

1 ps (solvent)2 – 15 ps (cavity)

4 ps (solvent)15 – 200 ps (cavity)

S1

So

PNAS, Dec. 2006

Femtochemistry in Nanocavities

Size-dependent luminescence of as-prepared CdTe NCs(concentration of CdTe disperse solution was 0.4 mM, calculated by added amount of NaHTe). The emission color of TGA-stabilized CdTe NCs can be adjusted by their sizeto be cyan, green, yellow, orange, red, and dark red underthe radiation of the UV lamp (left). The colors of the samples were also observed in sunlight (right).

Jia Guo, Wuli Yang, and Changchun Wang*J. Phys. Chem. B, 109 (37), 17467 -17473, 2005.

Living breast cancer cells adherentat the bottom of the flow channelwhich are located (A) 11 mm away, (B) 5 mm away, and (C) just abovethe edge of the permanent magnet. Capsules taken up by the cells are recognizable by their luminescence. All pictures were obtained by overlay of phase contrast andluminescence images; the luminescence of capsules isdetermined by CdTe nanocrystalsand is shown in pseudocolored red.From: W. J. Parak et al., Langmuir, 21 (10), 4262 -4265, 2005.

Under the microscope (5 nm)

CdTe in Water Solution

400 500 600 700 800Wavelength (nm)

0

0.4

0.8

450 500 550 600 650 700

0,0

0,2

0,4

0,6

0,8

1,0

reaction time

(Size dependent on reaction time)CdTe

1,5 h 20 h

Inte

nsity

(u.a

.)

Wavelength (nm)

371 nm

Cd Te 20 horas segunda reciente 23-11-2004 exc=371 nm

630 nm600 nm580 nm540 nm520 nm

0

2000

4000

6000

8000

10000

Cou

nts

0 1 2 3Time (ns)

520 560 600 640 680 720Wavelength (nm)

100 ps200 ps300 ps500 ps1 ns2 ns5 ns10 ns15 ns20 ns

0

10000

20000

30000

40000

Inte

nsity

PICOSECOND OBSERVATION

0

2000

4000

6000

8000

10000

Cou

nts

0 1 2 3Time (ns)

0

2000

4000

6000

8000

10000

Cou

nts

0 1 2 3Time (ns)

630 nm

Size effect on the ps decays of CdTe in Water.

RED: 3 nm, Black: 5 nm and Exit at 371 nm (40 ps).

520 nm

40, 500-700 ps AND 15-25 ns

ESQUEMA DEL SISTEMA DE MEDICIÓN DE DINÁMICAEN FEMTOSEGUNDO, RESOLUCIÓN TEMPORAL, 30 fs

Contador de fotones

Osciloscopio

Láser de FemtosegundoOscilador

~ 60 fs, 82 MHz, 550 mW

Autocorrelador

CN

M

P

DobleMonocromador

SPD

ω0

ω0

ω2

ED

Línea de retraso, 6.5 fs

GTH

ω0,2

E

LLF

CN

E: Espejo; ED: Espejo Dicroico; M: Muestra (Célula rotatoria); P: Polarizador L: Lente; F:Filtro; CN: Cristal No Linear (BBO); GTH: Generador de Tercer Armónico; FD-E: Fotodiodo – Espectrómetro.

Resultado (fs-ps)

Tratamiento

Control Espectral

Control Temporal

0 2000 4000 6000 8000 10000Tiempo (fs)

0

2000

4000

6000

8000

10000

Cue

ntas

266, 400 y 800 nm

Bombeo

1- La INTENSIDAD de la señal es función:

i) del tiempo de RETRASO entre el láser sonda y la emisión de la muestra.

ii) de la INTENSIDAD del LÁSER sonda.

2- La RESOLUCIÓN TEMPORAL depende de los láseres de bombeo y de sonda, de la óptica, PERO NO de la electrónica.

3- La RESOLUCIÓN ESPECTRAL depende del cristal no lineal, del monocromador, y de la óptica.

Retrasotemporal τ

Pulso láser sonda

fs-Excitación Fluorescencia

0 t

0 t

∫∞

∞−−= dttItII laserflsuma )()()( ττ

FluorescenciaCristal No Lineal

Láser sonda

Suma de frecuencias νsum = νláser + νfluo

PRINCIPIO: Óptica No LinealExcitación Láser

INFORMACIÓN INTÍMA SOBRE LA MATERIA

0 2 4 6 8Time (ps)

0

2000

4000

6000

8000

Cou

nts

470 nm

500 nm

540 nm

600 nm

630 nm

580 nm

0 2 4 6 8Time (ps)

0

2000

4000

6000

8000

Cou

nts

460 nm

500 nm

540 nm

580 nm

620 nm

3 nm 5 nm

Femtosecond observation

0

2000

4000

6000

8000

10000C

ount

s

0 4000 8000 12000Time (fs)

NO Fluence excitation effect on CdTe (5 nm in diameter)transients observed upon 60 fs excitation at 371 nm and

gating at 580 nm.

5 mW (Black)45 mW (RED)

0

2000

4000

6000

8000

10000C

ount

s

0 4000 8000 12000Time (fs)

NO solvent-isotope effect on CdTe (5 nm) transients observedupon 60 fs excitation at 371 nm and gating at 630 nm

D2O (Black)

H2O (RED)

0

200

400

600

800

1000

Cou

nts

0 1 2 3Time (ps)

IRF

470 nm

CdTe in water, 5 nm, excit at 371 nm 60 fs.

CdTe 5 nm Exc=368 nm fs

λ (nm) τ1 (fs) % τ2 (ps) % τ3 (ps) % χ2

470 120 100 1.682

500 150 98 1.8 2 2.506

540 325 82 1.4 9 40* 9 1.484

580 150 -47 1.7 20 40* 33 1.805

600 200 -49 1.4 15 40* 36 2.263

630 320 -51 1.2 27 40* 22 2.003

CdTe 3 nm exc=368 nm fs

λ (nm) τ1 (fs) % τ2 (ps) % τ3 (ps) % χ2

470 120 100 2.394

500 280 82 1.8 13 40* 5 1.251

540 200 34 1.5 54 40* 13 1.065

580 200 -47 1.7 37 40* 16 2.376

620 260 -47 1.5 19 40* 34 2.335

520 560 600 640 680 720Wavelength (nm)

100 ps200 ps300 ps500 ps1 ns2 ns5 ns10 ns15 ns20 ns

0

10000

20000

30000

40000

Inte

nsity

- 120 - 350 fs (+ and -)- 1 - 2 ps- 40 ps

0 2 4 6 8Time (ps)

0

2000

4000

6000

8000

Cou

nts

3 nm

5 nm

500 nm

630 nm

Ps- TRES

- 0.5 - 0.7 ns for both sizes

- 14 - 17 ns for 5 nm size- 18 - 25 ns for 3 nm size

Model of ultrafast exciton dynamics in CdTe NPs. Electrons (filled circles) are promoted to the conduction band(CB) by absorption of light of the appropriate energy, leaving a positively charged hole (emptycircles) in the valence band (VB). Twocompeting pathways for emission are available through the band edge anddeep trap routes with short (I) andlong (II) time scales.

I) The initial band edge emission ( BE) results from the immediate recombinationof the electron and hole after excitation. In the deep trap case, an electron froma surface (SS) immediately relaxes to the valence band to radiatively combine with the original photogenerated hole giving rise to the near-IR deep trapemission ( DT).

(II) The long-lived emission at band edge wavelengths results from the excited-stateelectron relaxing to a triplet state (TS) with an energy close to that of the lowestelectronic excited state in the conduction band. Electron migration from the conduction band to the surface hole (or vice-versa) gives rise to the extended deep trap emission.

CONCLUSION

-First observation of femtosecond dynamics of confined CdTein water solution.

-Highly stable under fs-excitation.

-Components of 150-350 fs as decay and rise: connected states by a ultra fast non-radiative channel.

-Increasing the size of the nanoparticle decreases the emission lifetime, most probably to a decrease of the energy gap.- … Might be important for a better design of nanoparticles to nano and biotechnology.

Thanks: MEC for the financial support.