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Mechanism for suppression Mechanism for suppression of free exciton no-phonon of free exciton no-phonon emission in ZnO tetrapod emission in ZnO tetrapod

nanostructuresnanostructuresS. L. Chen1), S.-K. Lee1), D. Hongxing2), Z. Chen2), W. M. Chen1), and I. A.

Buyanova1)

1)Department of Physics, Chemistry and Biology, Linköping University, Linköping, Sweden

2) Surface Physics Laboratory, Department of Physics, Fudan University, Shanghai, China

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Introduction and motivationIntroduction and motivation

Experimental Experimental ─ Samples Samples ─ MethodsMethods

ResultsResults─ Temperature dependent PL of tetrapods ensemble Temperature dependent PL of tetrapods ensemble

and bulk ZnO and bulk ZnO ─ Spatially resolved CL of individual tetrapodsSpatially resolved CL of individual tetrapods─ Mechanism for suppression of no-phonon free Mechanism for suppression of no-phonon free

exciton emissionexciton emission

SummarySummary

OutlineOutlineOutlineOutline

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Motivation and ObjectivesWhy ZnO?Why ZnO?

Attractive fundamental Attractive fundamental properties: properties:

Wide and direct band-gap: ~ Wide and direct band-gap: ~ 3.37 eV3.37 eV Large exciton binding energy: Large exciton binding energy: ~60 meV ~60 meV A large variety of interesting A large variety of interesting morphologies, e.g. tetrapod morphologies, e.g. tetrapod nanostructuresnanostructures

Promising for various Promising for various applications:applications:

Novel nano UV light emittersNovel nano UV light emittersGas sensors, etc.Gas sensors, etc.

Requires understanding of optical propertiesRequires understanding of optical properties

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ZnO nanostructures: ZnO nanostructures: weak no-phonon weak no-phonon (NP) free exciton (FX) emission, strong (NP) free exciton (FX) emission, strong longitudinal optical (LO) phonon-assisted longitudinal optical (LO) phonon-assisted FX transitionsFX transitions

? Enhanced exciton-phonon coupling?? Enhanced exciton-phonon coupling?

Objectives:Objectives: To clarify this issue by To clarify this issue by employing PL and cathodoluminescence employing PL and cathodoluminescence (CL) spectroscopies(CL) spectroscopies

Motivation and Objectives

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Samples and MethodsSamples and MethodsSamples and MethodsSamples and Methods

Samples:Samples: Tetrapod ZnO nanostructures:Tetrapod ZnO nanostructures:

Growth method: thermal evaporation Growth method: thermal evaporation Growth temperature : ~900 ºCGrowth temperature : ~900 ºC

Reference, commercially available bulk ZnOReference, commercially available bulk ZnO

Methods:Methods: Scanning electron microsocpy (SEM)Scanning electron microsocpy (SEM) Cathodoluminescence Cathodoluminescence

Measurement temperature: 300 KMeasurement temperature: 300 KDetection: a grating monochromator with a Detection: a grating monochromator with a photomultiplier tubephotomultiplier tube

Cw- photoluminescence (PL)Cw- photoluminescence (PL)Excitation: a Excitation: a Verdi/MBD-266 laser system (λ=266 Verdi/MBD-266 laser system (λ=266 nm) and Arnm) and Ar+ + laser (λ=351 nm)laser (λ=351 nm)Measurement temperature: 5-300 KMeasurement temperature: 5-300 KDetection: a grating monochromator with a Detection: a grating monochromator with a charge-coupled device (CCD) charge-coupled device (CCD)

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PL : bulk & tetrapod ZnO [5K]

Bulk ZnO : Neutral donor-bound exciton (BX), Bulk ZnO : Neutral donor-bound exciton (BX), 3.361 eV 3.361 eV Tetrapod ZnO : Surface exciton (SX), 3.367 eVTetrapod ZnO : Surface exciton (SX), 3.367 eV

Higher contribution of the SX in the tetrapod Higher contribution of the SX in the tetrapod structure due to increased surface to volume structure due to increased surface to volume ratioratio

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PL : bulk & tetrapod ZnO [Temperature dependence]

T↑ T↑ FE emission FE emission

Intense FX-nLO Intense FX-nLO transitionstransitions Expected for highly polar Expected for highly polar ZnO, promoted by a strong ZnO, promoted by a strong Fröhlich interactionFröhlich interaction

No-phonon FX line is No-phonon FX line is more pronounced in bulk more pronounced in bulk ZnOZnO

Origin?Origin?

PL

In

ten

sity

(a.

u)

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Suppression of the NP FE emission : Role of surface band bending

Wavelength (nm)

Photon Energy (eV) Exciton-phonon coupling:Exciton-phonon coupling:─ Depends on spatial Depends on spatial

distributions of electron distributions of electron and hole charge densitiesand hole charge densities

─ Enhanced in proximity to the surface and, therefore, in nanostructures?

PL under different WPL under different Wexex: : ─ Constant Constant relative relative

intensities of the NP and intensities of the NP and LO assisted transitionsLO assisted transitions

The surface band bending The surface band bending is NOT the origin of the is NOT the origin of the strong FX-1LO transitions strong FX-1LO transitions in the tetrapodsin the tetrapods

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Suppression of the NP FE emission : Role of surface band bending

Wavelength (nm)

Photon Energy (eV) Depth profiling using CL:Depth profiling using CL:

─ Excitation depth L= 0.2 Excitation depth L= 0.2 mm (VVaa= 5 keV) = 5 keV) Strong FX line Strong FX line

─ Excitation depth L= 2 Excitation depth L= 2 mm (VVaa= 20 keV) = 20 keV) Weak FX line Weak FX line

The surface band bending is The surface band bending is NOT the origin of the strong NOT the origin of the strong FX-1LO transitions in the FX-1LO transitions in the tetrapodstetrapods

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CL : individual tetrapod [RT]

SEM: Changes in morphology and shape within the SEM: Changes in morphology and shape within the tetrapodtetrapod

─ multi-faceted shape close to the coremulti-faceted shape close to the core─ round shape in the main part of the leground shape in the main part of the leg

CL: Weaker NP emission from the core regionsCL: Weaker NP emission from the core regions? OriginOrigin? Strong absorption in tetrapodsStrong absorption in tetrapods

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Ray optics:Ray optics:─ More probable internal More probable internal

reflections for multi-reflections for multi-faceted structuresfaceted structures

Suppression of the NP Suppression of the NP FX lineFX line

Mechanisms : Re-absorption of the NP FX

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Refractive index: Refractive index: ─ Strong spectral dependence, ↓ close to the FX resonanceStrong spectral dependence, ↓ close to the FX resonance

Strong increase of the critical angle in the vicinity of the Strong increase of the critical angle in the vicinity of the FX resonanceFX resonance

Strong internal reflections and re-absorption of the NP Strong internal reflections and re-absorption of the NP FX transitionsFX transitions

CBAj jTj

TjLjJ

iKn

,,22

,

2,

2,2 )1(

Mechanisms : Re-absorption of the NP FX

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SummarySummarySummarySummary

The NP FX transition is suppressed in ZnO tetrapod The NP FX transition is suppressed in ZnO tetrapod structuresstructures

The suppression is not affected by changes in The suppression is not affected by changes in surface band bendingsurface band bending

The suppression depends on the structural The suppression depends on the structural morphology of the tetrapods and is the most morphology of the tetrapods and is the most pronounced in the faceted regionspronounced in the faceted regions

The effect is attributed to enhanced re-absorption The effect is attributed to enhanced re-absorption due to multiple internal reflections, which become due to multiple internal reflections, which become especially pronounced in the vicinity of the FX especially pronounced in the vicinity of the FX resonance.resonance.

Appl. Phys. Lett. 96, 033108 (2010)Appl. Phys. Lett. 96, 033108 (2010)