in core-shell semiconductor nanocrystals

Transcription

1 Z Axis X Axis Y Axis 8 10 Single and multi-excitons in core-shell semiconductor nanocrystals Prof. Efrat Lifshitz Dept. of Chemistry, Solid State Institute and the Russell Berrie Nanotechnology Institute, Technion, Haifa, Israel

2 Synthesis of PbSe nanocrystals quantum dots TOP:Se + Pb-Ac /Ph-Et/OA/TOP (130 o C) 5nm 400nm Lifshitz et al., Adv. Fn. Mat., 05, J. Phys. Chem. B, 06, Inorg. Chem., 07

3 Nanocyrstals quantum dots (NQDs) active in the near infra-red II-VI:CdTe IV-VI:PbS, PbSe Novel Physics, Applications

4 Photovoltaic cells Gain devices Al Absorption (a.u) Black Body PbSe 8.9nm PbSe 6.3nm PbSe 4.9nm PbSe 2.3nm CdSe 4.2nm Wavelegth(nm) Tessler, Lifshitz et al. Appl. Phys. Lett., 2006 Intensity (counts) 10.0k 8.0k 6.0k 4.0k 2.0k 0.0 Emission intensity (a.u.) Wavelength (nm) B 1260 nm 1460 nm nm nm Pump power (W/cm 2 ) x10 Amplified stimulated emission Spontaneous emission (X10) Lifshitz, Galun et al., Appl. Optics 2007

5 Optical switches Medical platform Pumping PbSe QDs Drug molecules Laser output Q-switch Er:glass Organic molecules M2 M1 Transmission σ abs τ eff γ-fe 2 O 3 NPs Lifshitz, Tannenbaum et al., J.Phys. Chem. C θ x y 0.64 T Intensity (MW/cm 2 ) Lifshitz, Tannenbaum et al, Appl. Optics 2006 N P d Lifshitz, Tannenbaum et al., PRL, submitted

6 SINGLE & MULTIEXCITONS (gain device, solar energy, optical switch, single-photon light source) Generation A single exciton (X) s-shell p-shell E p-shell <E exc <2E g Biexciton (2X) Sequential Filling Triexciton (3X) E exc >2E g Tetraexciton (4X) Impact Ionization [Klimov (04), Nozik&Efros (05), Bawendi, Banin (08)]

7 Auger Relaxation Off On 1. Fluorescence intermittency (Blinking) 2. Intermediate charge species (2X 2X + + e, 2X X + + X) 3. Requires transient measurements

8 Looking for a solution A/C A/B Core-Shell Structures Increase of the effective size Improved surface quality A better match of dielectric const. at the core/shell interface Carriers distribution between the core and the shell Type II Q-Type II Type I alloy Core-shell-shell

9 PbSe PbS PbSe Density of state 5nm PbS PbSe 5.3 nm Density of state PbSe PbS PL, Abs (normalized) PbSe/PbS 4.2/1.1 nm PbSe 4.2 nm PbSe/PbS 3.1/2.2 nm PbSe 3.1 nm Energy (ev) Wise, Lifshitz et al., P RB (07) Lifshitz et al., Adv. Fn. Mat.,( 05), J.Phys. Chem. B (06), Nano Lett.l, submitted (08)

10 αt (1 Ax) T + β 2 1 Eg( T) = Eg(0) + + A2 x 0.95 α core =de g /dt=0.20 (core), 0.42 (core-shell) mev/k Peak PL (ev) PbSe 3.6 nm PbSe 4.1 nm PbSe/PbS 4.1 nm α PbSe-bulk = 0.38 mev/k α PbS-bulk = 0.50 mev/k ===================== Lattice dilation Surface tension Electron-phonon coupling T (K) PbSe PbS E. Lifshitz et al., submitted (08)

11 PbSe in Glassy Solution PL Intensity (*10 4 ) RT 250 K 200 K 150 K 100 K 80 K 60 K 40 K 20 K 6 K 16 mev 12 mev 8 mev Δ B D I PL Energy (ev) I0 ( T) = Δ Eph 1+ aexp( ) + bexp( 1) kt kt B B m Integrated PL (a.u) PbSe 3.6 nm PbSe 4.1 nm PbSe/PbS 4.1 nm /T (1/K)

12 NQD (D total ) E gap QD (ev) PbSe 1.25 E gap (3nm) PbS PbSe PbS PbSe/PbS (4.5nm) x10-9 D h D h PbSe (4.5nm) 0.95 D e di/dv 0.0 P h Sh Band gap energy V [ev] S e P e

13 CdTe/CdSe CdTe E CB 10 nm VB CdTe CdS CdSe CdTe CdSe 5 nm (111) (111) (220) (220) (111) (111) RT ρ e ρ h nm R=1.6nm, Th=0.6nm R=0.6nm, Th=1.6nm Hole Electron Intensity [a.u.] Energy [ev] CdTe/CdSe QE=88% 200ns CdTe QE=81% ~20 ns Radial length [nm]

14 Single dot spectroscopy of Type-I NQDs Room Temp. Ensemble PL [a.u.] Dot II Dot I CdTe Type-I Energy [ev]

15 FIBER-BASED BASED micro-photoluminescence (μ-pl)( setup for single dot spectroscopy at RT and 4.2 K with magnetic field up to 12 T xyz nanopozitioners Attocube, attocfm I

16 Fluorescence Imaging of individual NQDs N x = σ a λ j hc p τ R = Spatial resolution <1 μm Z Axis X Axis Y Axis 8 10 Quartz substrate

17 Typical phenomena in single NQDs: : Spectral Diffusion (shifting) and Fluorescence Intermittency (blinking). Spectral Diffusion Fluorescence Intermittency 300 sec biexciton Auger Process exciton PL Intensity [a.u.] Time (sec) PL [a.u.] 240 sec 180 sec 120 sec 60 sec Energy [ev] Auger recombination process is extremely efficient in NQD cores

18 CdTe/CdSe Intensity [a.u.] ~ 1meV Energy [ev] % 80% 50% Energy [ev] Time [s] Blinking Free

19 10P 0 s-shell p-shell 4X 3X + 6.5P 0 Intensity [a.u.] 3.5P 0 3X 2X 2X X X 0 + 3P 0 2P 0 P Energy [ev] Power density [kw/cm 2 ] X, 4X 2X X, 2X Energy [ev] Intensity (counts/s)

20 B = 3.5 T B = 0 T 2X, J=0 Intensity [a.u.] δ 1 X, J=±1 δ 2 X, J±2 2X X Energy [ev]

21 Summary Applications: Gain devices, optical switches, solar energy and medical platforms, based on single and multiexcitons in NIR active semiconductor NQDs, requiring: Preparation of high quality core-shell NQDs with high QE and chemical robustness, The ground-state exciton of core-shell structures showed a red-shift of the optical transitions, longer lifetime, narrower energy-gap at the STS spectrum, reduced surface strain [see de g /dt], effective increase of the NQD s volume, On top of all, BLINKING FREE process, enabled the detection of MULTIEXCITONS (including s- and p-shell recombination) in the μ-pl of a single core-shell NQD, generated by mild conditions (cw-laser)

22 Acknowledgments Students and Postdocs A. Kiger Dr. M. Brumer R.Ocsovskii Dr. L. Fradkin V. Kloper Dr. D. Cheskis L. Etgar Dr. A. Saschiuk E. Glinkin G. Grinbom M. Saraf M. Muellem G. Maikov Collaborators Dr. J. Konly-Olesiak, Oldenburg, Germany Al. Efros, NRL, A. Nozik (NREL) Prof. R. Tannenbaum and Prof. Y. Assaraf, Technion, Israel Dr. E. Galun and Dr. M. Sirota, ElOp, Israel Funding of this work BSF, ISF, Nofar, Bikura (ISF), Niedersachsen

23 Thank you for your attention!

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25 Optical head Probe pumping and flushing spot Cryostat with the optical head and superconductive magnet Nonmagnetic, floating, optical table Cryostat isolation stage

26 Technion - Projects 1. Semiconductor nanocrystals quantum dots (NQDs), active in the NIR Impact ionization Band-edge absorption Absorbance [a.u] 7.0nm 6.8nm 6.3nm 6.0nm 5.8nm 5.4nm 5.1nm 4.4nm 3.7nm 3.5nm 3.1nm 2.3nm Wavelength [nm] ITO Al Thin film or a blend of NQDs/polymer Efrat Lifshitz (Chemistry) and Nir Tessler (EE)

27 Synthesis of PbSe nanocrystals (NCs): Dots, Rods, Wires & Multipods TOP:Se + Pb-Ac 5nm /Ph-Et/OA/TOP (130 o C) Ethylene-diamine/glycol [templating ligands] ( C); 400nm Lifshitz et al., Adv. Fn. Mat., 05, J. Phys. Chem. B, 06, Inorg. Chem., 07

28 Trapped carrier QE=45% PbSe x S 1-x [ODMR/HR-TEM] PbSe QE=80%

29 &TB 8x8 Inter-valley interaction Efros Al. (2005), Wise (1997), Allen and Deleru, (2004, 2008), Zunger ( 2006)

30 PL Intensity (*10 4 ) PbSe in Glassy Solution Energy (ev) RT 250 K 200 K 150 K 100 K 80 K 60 K 40 K 20 K 6 K Peak PL (ev) PbSe 3.6 nm PbSe 4.3 nm PbSe/PbS 4.7 nm /T (1/K) FWHM (mev) PbSe 3.6 nm PbSe 4.3 nm PbSe/PbS 4.7 nm /T (1/K) E g (T)=E g (T) - α(t 2 /(T+β)) α=de g/ dt= mev/k (0.28 mev/k-bulk) [lattice, strain, el-ph] ELO B Γ ( T) =Γ + σt +Γ ( e 1) inh LO / k T 1

31 nm 10nm 111

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34 Fluorescence imaging of individual fluorescing NQDs 10 9 Spatial Resolution fwhm ~1.3 μm 8 Y (μm) X (μm) Intensity (counts/sec) x - translation (μm) x - translation (μm)

35 Multiexciton: Historical Background Self assembled QDs Colloidal NQDs Shape/ Typical size Pyramid/drop 20 nm*5 nm Spherical/rods 3-4 nm Surrounding Wetting & cladding layers (ε = 8-16) Organic surfactants (ε 2) Generation cw- or pulselaser (E exc <2E g ) Pulse- or q-cw laser (E exc >2E g ) Lifetime nsec psec

36 CdTe/CdSe Time Cd 0 Cd +2 (NPs monomers) 200nm 8nm J. Konly-Olesiak et al. Surface Science (07), Lifshitz et al., J.Phys. Chem C, (07), and J.Phys. Chem. C. (08)

37 How do we create functionalized conjugate structures? PbSe QDs Drug molecules Organic molecules γ-fe 2 O 3 NPs Etgar L.; Lifshitz E.; Tannenbaum R. J. Phys. Chem C, (07)

38 x F fy = 6πηRp ( vpy v fy ) y F 6 πηr v ( v = fx = p px fx 0) d F mx 3μ0N = mp V χ mp mp χ mp + 3 M 2 s R 4 mag ( x + d ) 2 2(( x + d ) + y 2 ) 3 N P F my 3μ0N = mp V χ mp mp χ mp + 3 M 2 s R 4 mag y 2(( x + d ) 2 + y 2 ) 3 F fy [pn] cP 1.73cP 2.13cP 3.71cP 4.13cP 6.35cP F my 0.00 Conjugate structure flow at 0.05ml/hr in 3.71cP fluid viscosity Capacity [ml/hr]