ITOH Lab. Hiroaki SAWADA Intraband transitions in semiconductor nanocrystals P. Guyot-Sionnest and M. A. Hines Appl. Phys. Lett. 72, 686 (1998) and P. Guyot-Sionnest et al/ Phys. Rev. B 60, R2181 (1999) (references) ITOH Lab. Hiroaki SAWADA

Abstract Intraband transition of one-electron confined in CdSe quantum dots has been observed by infrared pump-probe spectroscopy. These report ・ The transition energy depends on dot-size. ・ The time profile of transient absorption is influenced by surface modifications of the quantum dots.

Contents Introduction Quantum dot, Quantum-size effect Motivation Experiment (1)Size-dependence of transient absorption (2)Time evolution of transient absorption Summary

Quantum dot A quantum dot is a nanometer-sized semiconductor. It consists of 103~106 atoms. Quantum effects appear due to three dimensionally confined electrons. The energy levels are discrete. bulk DOS E DOS well E Quantum well bulk DOS wire E Quantum wire Quantum dot DOS dot E

Quantum size effect Confined electrons have higher energy than those in bulk crystal, and it depends on dot size. The energy shift is derived by n: principal quantum number h: Planck constant m: effective mass a: dot radius . size energy

Quantum confinement effect Consider the effect on an exciton in a spherical dot. (exciton:an electron-hole pair combined by Coulomb force) Weak confinement Strong confinement aB≪a aB：Bohr radius a:dot radius aB≫a excited state lowest state excited state lowest state lowest state electron excited state 2a hole 2a Motions of electron and hole are confined individually. Center-of-mass motion is confined.

Applications of quantum dot Quantum dots show interesting optical properties and are expected to be used for many optical devices. For example Quantum dot laser Electron-hole pair confinement leads to the efficient recombination. Superior lasing efficiency over existing devices 2. Optical switch The network communication carrier shifts from electric to optical. Large optical nonlinearity of quantum dot realizes optical switch.

Motivation Intraband transition ･･･ electronic transition in conduction band (1S-1P transition etc) Intraband transition energy (1S-1P etc) in quantum dot exists in infrared region, and the energy depends on the dot size. Infrared laser etc. In these reports Authors clarify the details of intraband dynamics of electrons in quantum dots.

Transient infrared absorption To study intraband transition in quantum dots Transient infrared absorption by pump-probe spectroscopy is useful. Pump-probe spectroscopy conduction band excited state By using two beams, we can observe the intraband transition that cannot be observed with the single beam. lowest state pump beam probe beam lowest state valence band

Colloidal CdSe quantum dot CdSe colloids are the best characterized semiconductor quantum dots in the strong confinement regime. colloid method (ref) C. B. Murray et al/ J.Am.chem.Soc. 115,8706(1993) CdSe quantum dot is produced by colloid method. Me2Cd＋TOPSe in TOP TOP:Tri-n-octylphosphine Me:methyl <1sec CdSe-TOPO TOPO 360℃

Experiment Ⅰ Three CdSe samples CdSe The diameters are ・31.5Å ・38Å ・43Å CaF2 window size distributions of samples 31.5Å:7% 38Å:7% 43Å:12% chloroform Optical densities at 532 nm are adjusted　between 0.5 and 1.5 . 1mm

Absorption spectra size energy Absorption spectra of the three colloids in chloroform 545nm 516nm 566nm size energy The dot sizes can be estimated from the peak energy.

Time evolution of transient absorption Experimental setup IR probe beam About 1μm～ VIS pump beam 532nm sample detctor delay Time evolution of transient absorption

Infrared absorption spectra Visible-induced infrared absorption of the colloids. :43Å :38Å :31.5Å The dotted lines are Gaussian fits. exp-(E-E0)2/2δE2 E0:peak energy δE:spectral variance δE Absorption energy increases with decreasing the dot radius. E0

Experimental transition energy Experimental transition energy(solid square) :the 1Se–1Pe transition neglecting the Coulomb interaction. :considering the Coulomb interaction to a 1S hole state. :considering the Coulomb interaction for Se -Pez and Se -Pexy transitions with the hole at the pole. a one-electron transition with the hole being rather localized, possibly as a surface trap The vertical error bars :±δE The horizontal error bars :±r0(δE/2E0)

Time evolution of IR absorption Time evolution of the visible-induced IR absorption. :43Å :38Å :31.5Å All samples exhibit qualitatively similar behavior. Fast(20~60ps) and slow decays are observed.

Experiment Ⅱ New CdSe samples Capping molecules ・TOPO ・thiocresol ・pyridine Sapphire window CdSe nanocrystal 3.5nm and 4.3nm Liquid Optical densities at 532 nm are adjusted　between 1 and 2 . 400μm

IR absorbance change Ⅰ For different surface treatment at 1mJ cm-2 IR absorbance change α as function of delay of the pump visible beam ：thiocresol-capped　in Paeptamethylnonani ：TOPO-capped　in Paeptamethylnonani ：Pyridine-capped　in pyridine solvent at 1mJ cm-2 thiocresol deep S hole traps Fast decay increases. TOPO shallow Se hole traps pyridine charge-separated complex

electron-electron Auger interband relaxation IR absorbance change Ⅱ Excitation density dependence IR absorbance change α for a chloroform　solution of thiocresol-capped CdSe :at 6 mJ cm-2 :at 3 mJ cm-2 :at 1 mJ cm-2 Saturation of the electron density of 1S state fast decay slow decay electron-electron Auger interband relaxation one electron per dot

Summary How transient infrared absorption measurement can be used to study intraband transition in quantum dots is demonstrated. The size dependence of intraband transitions have been measured in strongly confined CdSe quantum dots. The surface modifications of the quantum dots effect on the coupling of the electron with hole.