Photoluminescence Spectroscopy on Chemically Synthesized Nanoparticles

Photoluminescence Spectroscopy on Chemically Synthesized Nanoparticles Torben Menke 1 Institut für Angewandte Physik und Zentrum für Mikrostrukturforschung Universität Hamburg Diploma Talk, 18.12.27 1 http://www.entorb.net

Introduction Nanocrystals Nanoneedles Summary Theory Setup Contents Nanocrystals Nanoneedles Images by Weller-Group, HH

Introduction Nanocrystals Nanoneedles Summary Theory Setup Photoluminescence (PL) PL-schema for semiconductors with direct bandgap: CB γ 1 VB e h a) Excitation E E E Egap k k γ2 b) Relaxation c) Recombination k a b c Excitation absorption of photon γ 1 generates e-h-pair Relaxation e and h interact with the lattice until they reach the band extrema Recombination only effective path to the ground state is recombination photon γ 2 is emitted

Introduction Nanocrystals Nanoneedles Summary Theory Setup Confinement nanometer-scaled semiconductors show increased bandgap increasing impact of material borders as size is decreased simple 1D ansatz: -boxpotential n λ 2 = L; k = 2π λ ; E = 2 k 2 2m E n = 2 π 2 2mL 2 n2 E gap,conf. = E gap,bulk + E n bandgap directly tunable via size L

Introduction Nanocrystals Nanoneedles Summary Theory Setup PL of Nanocrystals Image by Weller-Group, HH

Introduction Nanocrystals Nanoneedles Summary Theory Setup Setup a) b) M Laser c) F L S CCD Spectrometer b) Microscope c) Spectrometer Objective Sample G Cryostat CCD

Introduction Nanocrystals Nanoneedles Summary Theory Setup CCD-Detector Spacial mode Spectral mode y (µm) ~ 23 µm y (µm) ~ 23 µm ~ 3 µm x (µm) ~ 25 mev E (mev)

Introduction Nanocrystals Nanoneedles Summary Intro CdSe-Ensemble CdSe-Single NiPt/CdSe Contents Nanocrystals Nanoneedles Images by Weller-Group, HH

Introduction Nanocrystals Nanoneedles Summary Intro CdSe-Ensemble CdSe-Single NiPt/CdSe Structure spherical particle wet chemical synthesis ligands slow down growth process, prevent agglomerate-building and passivate surface core of 2-1 nm often surrounded by one or two shells spin-, dip- or drop-coating onto substrate

Introduction Nanocrystals Nanoneedles Summary Intro CdSe-Ensemble CdSe-Single NiPt/CdSe MBE-Grown Quantum Dots vs. Nanocrystals Quantum Dot Nanocrystal elliptical shape strongest confinement in growth direction embedded growth in other structures (cavities... ) spherical geometry same confinement in all 3 directions Advantages industrial synthesis possible Applications LEDs, photovoltaic cells, biomonitoring, interactions with cavities

Introduction Nanocrystals Nanoneedles Summary Intro CdSe-Ensemble CdSe-Single NiPt/CdSe First Tasks transport experiment to a lab of higher spectral resolution investigate ensemble and single NCs get more information of PL-signal of single NCs verify results Gerwin Chilla found in the another lab

Introduction Nanocrystals Nanoneedles Summary Intro CdSe-Ensemble CdSe-Single NiPt/CdSe Ensemble Measurements on CdSe-NCs 4 3 PL-Intensity (a.u.) 2 gaussian-shape peak energy high above bulk band gap (1.751 ev at 293 K) confinement 1 1.9 2. 2.1 Energy (ev) 2.2 2.3

Introduction Nanocrystals Nanoneedles Summary Intro CdSe-Ensemble CdSe-Single NiPt/CdSe Ensemble Measurements on CdSe-NCs 4 3 PL-Intensity (a.u.) 2 1 1.9 2. 295K 12K 7K 2.1 Energy (ev) 2.2 7K 12K 295K 2.3 gaussian-shape peak energy high above bulk band gap (1.751 ev at 293 K) confinement blueshift increased bandgap decreasing peak width less interactions with phonons 2-fold increased signal less non-radiative recombination channels

Introduction Nanocrystals Nanoneedles Summary Intro CdSe-Ensemble CdSe-Single NiPt/CdSe Resonance Energy Transfer 4 3 PL-Intensity (a.u.) 2 1 295K 12K 7K 7K 12K 295K deviation from gaussian-shape high-band-gap NCs pump low-band-gap NCs Förster Resonance Energy Transfer (FRET) 1.9 2. 2.1 Energy (ev) 2.2 2.3

Introduction Nanocrystals Nanoneedles Summary Intro CdSe-Ensemble CdSe-Single NiPt/CdSe PL of Single CdSe NCs

Introduction Nanocrystals Nanoneedles Summary Intro CdSe-Ensemble CdSe-Single NiPt/CdSe PL of Single CdSe NCs Shifting shift of the PL-energy can be explained by a change of the potential in the surrounding of the NC Stark-effect

Introduction Nanocrystals Nanoneedles Summary Intro CdSe-Ensemble CdSe-Single NiPt/CdSe PL of Single CdSe NCs 2

Introduction Nanocrystals Nanoneedles Summary Intro CdSe-Ensemble CdSe-Single NiPt/CdSe PL of Single CdSe NCs 2 Blinking no PL-signal for some of time (called dark state) can be explained by ionization of the NC Auger recombination

Introduction Nanocrystals Nanoneedles Summary Intro CdSe-Ensemble CdSe-Single NiPt/CdSe Auger Recombination Auger Recombination assumption: dark states caused by charge-separation generation of exciton (e-h-pair) one charge trapped by the surrounding NC is charged photoexcitation of second e-h pair still allowed...... but their recombination energy is more efficiently transfered to the additional charge already present when the trapped charge is release the PL-signal is recovered

Introduction Nanocrystals Nanoneedles Summary Intro CdSe-Ensemble CdSe-Single NiPt/CdSe Single Nanocrystal 12 PL-Intensity (a.u.) 8 1 6.74 mev 11 single spectra summed up aim: find fine-structure of PL-signal signal-to-noise ratio is too low 4 2.125 2.13 2.135 2.14 Energy (ev) 2.145

Introduction Nanocrystals Nanoneedles Summary Intro CdSe-Ensemble CdSe-Single NiPt/CdSe NiPt/CdSe-Nanocrystals Cooperation between Group K and Weller Group: Is it possible to combine magnetic nanocrystals with an optic-active shell? NiPt-NCs show bocked superparamagnetism CdSe-NCs are well known for strong PL-signal magnetic NiPt-core surrounded by CdSe shell

Introduction Nanocrystals Nanoneedles Summary Intro CdSe-Ensemble CdSe-Single NiPt/CdSe 12 Susceptibility (1-6 emu/oe) 4 6 8 1 2 a) T=4.2 zero-field-cooled H=1 Oe 1 2 3 Temperature (K) 4 Magnetization (1-3 emu) -8-4 4 8 b) -4-2 2 4 H-Field (koe) 3K 5K 15 Measurement Gauß-Fit PL-Intensity (a.u.) 5 1 85meV 1.4 1.6 1.8 Energy (ev) 2. 2.2

Introduction Nanocrystals Nanoneedles Summary Intro Ensemble Single Contents Nanocrystals Nanoneedles Images by Weller-Group, HH

Introduction Nanocrystals Nanoneedles Summary Intro Ensemble Single Structure indium-sphere (d 5 nm) indiumphosphide-nanowire (L 4 nm) one-pot synthesis InP: direct bandgap of 1.344 ev at 295 K PL-signal?

Introduction Nanocrystals Nanoneedles Summary Intro Ensemble Single Ensemble-PL PL-Intensity (a.u.) 2 1 no PL-signal at InP-bandgap (1.344 ev) but at higher energies 1.2 1.4 1.6 1.8 Energy (ev)

Introduction Nanocrystals Nanoneedles Summary Intro Ensemble Single Ensemble-PL PL-Intensity (a.u.) 3 2 1 4K 4K 15K 295K no PL-signal at InP-bandgap (1.344 ev) but at higher energies strong increase of PL-intensity at low temp. PL-energy shifts between 4 and 15 K to lower energies (redshift) no shift between 15 and 295 K 15K 295K 1.5 1.6 1.7 Energy (ev) 1.8

Introduction Nanocrystals Nanoneedles Summary Intro Ensemble Single Single Needle in non-spectral Mode spectrometer set to non-spectral mode (direct reflection) filter in front of spectrometer removes laser light laserspot widened to a diameter of 6 µm x and y coordinates of CCD-image correspond to x and y of sample a single needle is moved in y direction

Introduction Nanocrystals Nanoneedles Summary Intro Ensemble Single Single Needle in non-spectral Mode

Introduction Nanocrystals Nanoneedles Summary Intro Ensemble Single Spectral vs. non-spectral Mode

Introduction Nanocrystals Nanoneedles Summary Intro Ensemble Single High Laser Intensity PL-Intensity (a.u.) 15 1 5 a) 1 r (nm) 2 3 non-spectral mode spatial resolution PL from In-head and InP-wire PL-Intensity (a.u.) 6 4 2 b) 1 2 3 r (nm)

Introduction Nanocrystals Nanoneedles Summary Intro Ensemble Single Single Needles: PL and SEM 8 6 4 2 8 6 4 2 Head, PL-Intensity (a.u.) 3 25 2 15 1 5 N1 1.5 1.6 1.7 Energy (ev) 1.8 1 Needle, PL-Intensity (a.u.) intensity from In-head 5-3 higher no correlation between geometry and PL-signal Head, PL-Intensity (a.u.) 3 2 1 N4 In-Head InP-Needle 6 4 2 Needle, PL-Intensity (a.u.) 1.5 1.6 1.7 Energy (ev) 1.8 Head, PL-Intensity (a.u.) 2 4 6 8 1 N8 1 Needle, PL-Intensity (a.u.) 1.5 1.6 1.7 Energy (ev) 1.8

Introduction Nanocrystals Nanoneedles Summary Intro Ensemble Single Single Needles: PL and SEM 8 6 4 2 8 6 4 2 Head, PL-Intensity (a.u.) 3 25 2 15 1 5 N1 1.5 1.6 1.7 Energy (ev) 1.8 1 Needle, PL-Intensity (a.u.) intensity from In-head 5-3 higher no correlation between geometry and PL-signal Head, PL-Intensity (a.u.) 1 2 3 N4 1.5 In-Head InP-Needle 1.6 1.7 1.8 2 4 6 Needle, PL-Intensity (a.u.) Possible Explanations no InP-bulk signal due to trap-states at the surface Head, PL-Intensity (a.u.) 2 4 6 8 1 N8 1.5 Energy (ev) 1.6 1.7 Energy (ev) 1.8 1 Needle, PL-Intensity (a.u.) small InP-nanocrystals surrounding the particle might generate the PL-signal enhancement of NCs PL by the In (metal) spheres

Introduction Nanocrystals Nanoneedles Summary Intro Ensemble Single 15 K vs. 295 K 295K, PL-Intensity (a.u.) 4 8 12 N5 15K 295K 1 2 3 4 5 15K, PL-Intensity (a.u.) 295K, PL-Intensity (a.u.) 1.5 1.6 1.7 1.8 1.5 1.6 1.7 1.8 Energy (ev) Energy (ev) 2 4 6 8 1 N8 15K 295K 295K 15K 4 8 12 15K, PL-Intensity (a.u.) 295K, PL-Intensity (a.u.) 5 1 15 2 25 N9 15K 295K 5 1 15 2 25 15K, PL-Intensity (a.u.) 295K, PL-Intensity (a.u.) 5 1 15 2 5 1 15 1.5 1.6 1.7 1.8 Energy (ev) N1 15K 295K 15K, PL-Intensity (a.u.) 1.5 1.6 1.7 1.8 Energy (ev)

Introduction Nanocrystals Nanoneedles Summary Intro Ensemble Single Polarization PL-Intensity (a.u.) 2 15 1 5 8 1.6 1.7 Energy (ev) 18 45,135 9,18 1.8 45 9 135 18 PL-signal polarized in direction of needle polarization independent of laser s polarization shift the maximum can be found other needles show same behavior PL-Intensity (a.u.) 6 4 2 9 1.6 1.7 Energy (ev) 1.8

Introduction Nanocrystals Nanoneedles Summary Nanoneedles Nanocrystals Summary - Nanoneedles Observations PL-signal 3 mev above InP bulk bandgap strong temperature dependence signal origins mainly from In-spheres no obvious correlation between geometry and PL-energy Possible Explanations no InP-bulk signal due to trap-states at the surface small InP-nanocrystals surrounding the particle might generate the PL-signal enhancement of NCs PL by In spheres

Introduction Nanocrystals Nanoneedles Summary Nanoneedles Nanocrystals Summary - Nanocrystals CdSe-NCs moved experiment to a new lab successfully investigated ensembles and single NCs temperature dependency of ensemble-pl pumping of NCs by others shifting and blinking of single NCs low signal-to-noise ratio

Introduction Nanocrystals Nanoneedles Summary Nanoneedles Nanocrystals Acknowledgment Prof. Dr. Detlef Heitmann Prof. Dr. Horst Weller Dr. Tobias Kipp Gerwin Chilla, Christian Strelow, Hagen Rehberg, Christoph M. Schultz, Tim Köppen, Fabian Wilde, Gernot Stracke Tim Strupeit, JProf. Dr. Christian Klinke, Andreas Kornowski, Jan Niehaus, Klaus Boldt, Andrea Salcher Dr. René Eiselt, Ole Albrecht, Dr. Alexander van Staa We gratefully acknowledge financial support of the Deutsche Forschungsgemeinschaft via the SFB 58 "Quantum Materials" and the Graduiertenkolleg 1286 "Functional Metal-Semiconductor Hybrid Systems".

Introduction Nanocrystals Nanoneedles Summary Nanoneedles Nanocrystals Thank you for your attention