Dependence of the final-state effect on the coupling between a CdSe nanoparticle and its neighbors studied with photoemission spectroscopy

Pin-Jiun Wu, Ku-Ding Tsuei, Meng-Ting Hsieh, Kung-Hwa Wei, and Keng S. Liang

Phys. Rev. B 75, 115402 – Published 5 March 2007

Abstract

A simple model based on an electrostatic calculation of the photoemission effect in the final state is used to characterize the energy shift, relative to bulk material, which is dependent on size in the photoemission spectra of organically passivated CdSe nanoparticles (NPs). For trioctylphosphine oxide/hexadecylamine-passivated NPs, the core-level shifts are well described with a model involving a static effect in the final state. After the NPs are treated with pyridine, the energy shifts are smaller; this decrease is ascribed to a dynamic effect in the final state describing a finite lifetime, on a femtosecond scale, of the photohole residing in the NP. This condition is intimately related to the coupling between the NP and metallic substrate and among NPs themselves. For the valence-band edge, an additional effect of the initial state due to quantum confinement is required to elucidate the energy shift. We find that this initial-state shift is smaller than a prediction according to a semiempirical pseudopotential calculation.

Received 13 September 2006

DOI:https://doi.org/10.1103/PhysRevB.75.115402

Authors & Affiliations

Pin-Jiun Wu^*

Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan and National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan

Ku-Ding Tsuei^†

National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan and Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan

Meng-Ting Hsieh and Kung-Hwa Wei

Department of Material Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan

Keng S. Liang

National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan and Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan

^*Electronic address: pinjiun.ep89g@nctu.edu.tw
^†Author to whom correspondence should be addressed. Electronic address: tsuei@nsrrc.org.tw

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Vol. 75, Iss. 11 — 15 March 2007

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Images

Figure 1
TEM micrographs of CdSe NPs of mean diameter $32 Å$ (a) passivated with TOPO/HDA and (b) treated with pyridine.Reuse & Permissions
Figure 2
Core-level photoemission spectra of bulk CdSe and CdSe NPs of mean diameter $D_{m} 32 Å$ passivated with TOPO/HDA and treated with pyridine. (a) All Cd $3 d_{5 ∕ 2}$ spectra are satisfactorily fitted with a single Voigt component. (b) Se $3 d$ spectra are composed of interior (solid curves) and surface (dotted curves) components except for bulk CdSe.Reuse & Permissions
Figure 3
Photoemission spectra for the valence band of bulk CdSe and CdSe NPs with mean diameter $D_{m} 32 Å$ passivated with TOPO/HDA and treated with pyridine.Reuse & Permissions
Figure 4
Energy shifts of the core levels for CdSe NPs (a) passivated with TOPO/HDA and (b) treated with pyridine vs mean diameter, compared with model calculations. The thick solid curve is the shift calculated with Eq. (5) with the position of the photohole weighted by an exponential function describing attenuation; the dotted curve is overplotted according to a simplified model with an effective photohole position for comparison. The dashed curve in (b) is the calculation based on a model for a dynamic effect in the final state that takes into account the lifetime of the photohole. The thin solid and dotted-dashed curves include the contribution from the nearest-neighboring particles with and without considering the dynamic effect in the final state, respectively.Reuse & Permissions
Figure 5
Schematic diagram of the dynamic effect in the final state illustrates the electrostatic potential-energy curve as the photoelectron travels to infinity at which the zero energy is defined. In the presence of the photohole, the potential energy is the sum of the electron-hole energy $(U_{e − h})$ and the electron-image energy $(U_{e − i m})$ following the solid curve. If the photohole becomes neutralized when the photoelectron travels to $R + v τ$ the potential energy transfers to the dashed curve as only the electron-image energy remains.Reuse & Permissions
Figure 6
Energy shifts of the valence-band edge for CdSe NPs (a) passivated with TOPO/HDA and (b) treated with pyridine vs the mean diameter, compared with model calculations. The dotted curve indicates the calculated final-state shift weighed with a 1S wave function considering the IMFP, and the thick solid curve includes an additional quantum shift [Eq. (7)] reported in Ref. 28. The dashed curve in (b) results from a calculation based on a model for the dynamic effect in the final state with the same parameter used in Fig. 4b. The thin solid and dotted-dashed curves specify the total shift including the contribution from the nearest-neighboring particles with and without the dynamic final-state effect, respectively.Reuse & Permissions

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