Size-dependent recombination dynamics in ZnO nanowires

J. S. Reparaz1, M. R. Wagner1, A. Hoffmann1, F. Güell2, A. Cornet3, and J. R. Morante2,3

1Institut für Festkörperphysik, Technische Universität Berlin, Germany. 2MIND & M-2E, IN2UB, Departament d’Electrònica, Universitat de Barcelona, Spain.3Institut de la Recerca de l’Energia de Catalunya (IREC), Barcelona, Spain.

Outline

I) Motivation

II) Growth procedure

III) Optical investigation on NWs with different diameters

IV) Single-wire spectroscopy

V) Conclusions

Briefly on some ZnO basic propertiesWurtzite structure 4 at. / cell Band structure direct bandgap

3.3 eV

Growth techniques

rf. magnetronsputtering

Molecular beamepitaxy

Pulsed laser deposition

Chemical vapour deposition

Optical Properties

i) Free excitons (FE)ii) Bound excitons (BE)iii) Donor acceptor pairs (DAP)iv) Two electron satelitesv) Phonon replicas

Γ7

Γ7

Γ7

Γ9

CB

A

B

C

* M. R. Wagner et. al. , PRB 80, 205203 (2009)

*

0200400600800

1000

ZnO nanowires publi-cations

I) Motivation

L. K. Van Vugt. et. Al, Phys. Rev. Lett 97, 147401 (2006)

Single – wire PL spectra

B. Gil and A. V. Kavokin, Appl. Phys. Lett., Vol. 81, 748 (2002)

Theoretically

Size-dependent exciton-polariton

coupling

ΔωLTBulk ≈ 2 to 12 meV

ΔωLTNWs ≈ 60 to 164 meV !!!

• Lowest dimensional system suitable for conductivity measurements

• Non-toxic and highly bio-compatible

• The electronic states in the NWs core are sensitive to the surface states

C. Lao et. al., Nanoletters 7, 1323 (2008)

“The active media is the cavity itself”

Can we learn something on size-dependent polariton fields in the NWs??

II) ZnO NWs growth

Vapour-liquid-solidSEM images – Three samples

HRTEM images

d = 70 nm d = 110 nm d = 170 nm

SiO2/Si substrate

Au deposition

Au drops formation( ≈ 900 ºC)

ZnO atmosphereNWs nucleation

NWs growth

III) Results & Experimental Setup

CCD

MCP

Spectrom.

Beam Splitter

Ti:Sa LBO

63x

Cryostat

Sample

He

Pump

Anti-vibrations

system

70 nmPulsed: 2 ps

355 nm

High spatial resolution

1) 50 x objective 500 nm2) Piezo-XYZ stage 50 nm 3) Horizontally aligned NWs

PiezoXYZ

Photoluminescence spectra

2.7 3.0 3.3

Free exciton

Inte

nsi

ty (arb

. units

)

Energy (eV)

ZnO NWs - 300 K

Room temperature

Low temperatures

- Free exciton

- Free exciton- Bound excitons- Surface excitons- Free to bound- DAP- Two electron satelites

Acceptors

VB

hν

e-

BX

FX

Photoluminescence spectra

DX=3.365 eV observedin all the samples

We use this DX to studythe NWs diameter

Influence on the e.m. filed inside the NWs

J. S. Reparaz, M. Wagner, A. Hofmann, et. al., Appl. Phys. Lett., 96, 053105 (2010)

Time resolved spectra

i) The NWs shape influences the polaritons field

ii) Emission from excitons in different spatial positions in the NWs influence the recombination times.

The different lifetimes probe the influence of the NWs size on the e.m. field

inside the NWs.

The DX are spatially localized states !!!

BULK (λ << d)

NW(d < λlight)

Plane wave E=E0exp(-ikr-wt)

45 60 75 90 105 120 13540

80

120

240

300

360

420

long component

E = 3.357 eV

E = 3.361 eV

Tim

e (ps)

NWs diameter (nm)

E = 3.365 eV

ZnO NWs4 K - Au sputt.

J.S. Reparaz, et. al., Appl. Phys. Lett. 96, 053105

Short: BE

Lifetime vs. NWs diameter

The lifetime of the DX excitons increases approximatelly linearly with NWs diameter. This results from the influence of the NWs size on the e.m. field spatial distribution

J. S. Reparaz, M. Wagner, A. Hofmann, et. al., unpublished (2010)

3.30 3.31 3.32 3.33 3.34 3.35 3.36 3.37 3.38 3.39

Au-colloids

d = 130 nm, L = 1

d = 85 nm, L = 0.8 d = 65 nm, L = 0.8

Inte

nsi

sty

(arb

. units

)

Energy (eV)

ZnO NWs4 K

d = 50 nm, L = 0.7

Au-sputtering

On the precursor influence…

V) Single-wire spectroscopy

380 390 400 410

ZnO single NW300 K

1 (center)

2 (tip)In

tens

ity (ar

b. u

nits

)

Wavelength (nm)

3 (WL)

PL mapscan PL spectra

V) Single-wire spectroscopy

PL mapscan PL spectra

380 390 400 410

Inte

nsity

(ar

b. u

nits

)

Wavelength (nm)

3 nm = 20 meVL = 5

ZnO NWs300 K

Sub-wavelength polariton guiding

V) Single-wire spectroscopy

0 1 2 3

Inte

nsity

(ar

b. u

nits

)

Time (ns)

1) Tcenter = 288 ps

2) Ttip = 211 ps

ZnO single NW300 K= 382 nm

PL mapscan Time resolved spectra

• The lifetime depends on the positionon the NWs.

• We observe the presence of a ZnO WL.

Cavity modesCoupling to theexternal e.m. field

VI) Conclusions

• The DX recombination times have shown to be an useful tool for proving the size influence on the e.m. field inside ZnO NWs.

•The lifetime of the neutral donor bound excitons dependson the NWs size size-dependent polariton field. We find an approximately linear relation for the investigated sizes.

• Single-wire spectroscopy has revealed that the recombination dynamics depend on the position on the NWs, decreasing closer to the tip

• The presence of a ZnO WL was observed by studying single NWs.

Thank you !!

Come downyou messy cat !!!