1 Mechanism for suppression of free exciton no-phonon emission in ZnO tetrapod nanostructures S. L. Chen 1), S.-K. Lee 1), D. Hongxing 2), Z. Chen 2), W. M. Chen 1), and I. A. Buyanova 1) 1) Department of Physics, Chemistry and Biology, Linköping University, Linköping, Sweden 2) Surface Physics Laboratory, Department of Physics, Fudan University, Shanghai, China

2 Introduction and motivation Experimental ─ Samples ─ Methods Results ─ Temperature dependent PL of tetrapods ensemble and bulk ZnO ─ Spatially resolved CL of individual tetrapods ─ Mechanism for suppression of no-phonon free exciton emission Summary OutlineOutline

3 3 Motivation and Objectives Why ZnO? Attractive fundamental properties: Attractive fundamental properties: Wide and direct band-gap: ~ 3.37 eV Wide and direct band-gap: ~ 3.37 eV Large exciton binding energy: ~60 meV Large exciton binding energy: ~60 meV A large variety of interesting morphologies, e.g. tetrapod nanostructures A large variety of interesting morphologies, e.g. tetrapod nanostructures Promising for various applications: Promising for various applications: Novel nano UV light emitters Novel nano UV light emitters Gas sensors, etc. Gas sensors, etc.  Requires understanding of optical properties

4 ZnO nanostructures: weak no-phonon (NP) free exciton (FX) emission, strong longitudinal optical (LO) phonon-assisted FX transitions ZnO nanostructures: weak no-phonon (NP) free exciton (FX) emission, strong longitudinal optical (LO) phonon-assisted FX transitions ? Enhanced exciton-phonon coupling? Objectives: To clarify this issue by employing PL and cathodoluminescence (CL) spectroscopies Motivation and Objectives

5 Samples and Methods Samples: Tetrapod ZnO nanostructures: Tetrapod ZnO nanostructures: Growth method: thermal evaporation Growth temperature : ~900 ºC Reference, commercially available bulk ZnO Reference, commercially available bulk ZnO Methods: Methods: Scanning electron microsocpy (SEM) Scanning electron microsocpy (SEM) Cathodoluminescence Cathodoluminescence Measurement temperature: 300 K Detection: a grating monochromator with a photomultiplier tube Cw- photoluminescence (PL) Cw- photoluminescence (PL) Excitation: a Verdi/MBD-266 laser system (λ=266 nm) and Ar + laser (λ=351 nm) Measurement temperature: K Detection: a grating monochromator with a charge-coupled device (CCD)

6 PL : bulk & tetrapod ZnO [5K] Bulk ZnO : Neutral donor-bound exciton (BX), eV Bulk ZnO : Neutral donor-bound exciton (BX), eV Tetrapod ZnO : Surface exciton (SX), eV Tetrapod ZnO : Surface exciton (SX), eV  Higher contribution of the SX in the tetrapod structure due to increased surface to volume ratio  Higher contribution of the SX in the tetrapod structure due to increased surface to volume ratio

7 PL : bulk & tetrapod ZnO [Temperature dependence] T↑  FE emission T↑  FE emission Intense FX-nLO transitions Intense FX-nLO transitions  Expected for highly polar ZnO, promoted by a strong Fröhlich interaction No-phonon FX line is more pronounced in bulk ZnO No-phonon FX line is more pronounced in bulk ZnO Origin?  Origin? PL Intensity (a.u)

8 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 distributions of electron and hole charge densities ─ Enhanced in proximity to the surface and, therefore, in nanostructures? PL under different W ex : PL under different W ex : ─ Constant relative intensities of the NP and LO assisted transitions  The surface band bending is NOT the origin of the strong FX-1LO transitions in the tetrapods

9 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  mV a = 5 ke V)  Strong FX line ─ Excitation depth L= 0.2  m (V a = 5 ke V)  Strong FX line ─ Excitation depth L= 2  mV a = 20 keV)  Weak FX line ─ Excitation depth L= 2  m (V a = 20 keV)  Weak FX line  The surface band bending is NOT the origin of the strong FX-1LO transitions in the tetrapods

10 CL : individual tetrapod [RT] SEM: Changes in morphology and shape within the tetrapod SEM: Changes in morphology and shape within the tetrapod ─ multi-faceted shape close to the core ─ round shape in the main part of the leg CL: Weaker NP emission from the core regions CL: Weaker NP emission from the core regions ? Origin ? Strong absorption in tetrapods

11 Ray optics: Ray optics: ─ More probable internal reflections for multi-faceted structures  Suppression of the NP FX line Mechanisms : Re-absorption of the NP FX

12 Refractive index: Refractive index: ─ Strong spectral dependence, ↓ close to the FX resonance  Strong increase of the critical angle in the vicinity of the FX resonance  Strong internal reflections and re-absorption of the NP FX transitions Mechanisms : Re-absorption of the NP FX

13 SummarySummary The NP FX transition is suppressed in ZnO tetrapod structures The suppression is not affected by changes in surface band bending The suppression depends on the structural morphology of the tetrapods and is the most pronounced in the faceted regions The effect is attributed to enhanced re-absorption due to multiple internal reflections, which become especially pronounced in the vicinity of the FX resonance. Appl. Phys. Lett. 96, (2010)