Measurement of Electronic States of PbS Nanocrystal Quantum Dots Using Scanning Tunneling Spectroscopy: The Role of Parity Selection Rules in Optical Absorption

Bogdan Diaconescu, Lazaro A. Padilha, Prashant Nagpal, Brian S. Swartzentruber, and Victor I. Klimov

Phys. Rev. Lett. 110, 127406 – Published 22 March 2013

Abstract

We study the structure of electronic states in individual PbS nanocrystal quantum dots by scanning tunneling spectroscopy (STS) using one-to-two monolayer nanocrystal films treated with 1, 2-ethanedithiols (EDT). Up to six individual valence and conduction band states are resolved for a range of quantum dot sizes. The measured states’ energies are in good agreement with calculations using the $k \cdot p$ four-band envelope function formalism. A comparison of STS and optical absorption spectra indicates that some of the absorption features can only be explained by asymmetric transitions involving the states of different symmetries (e.g., $S$ and $P$ or $P$ and $D$ ), which points towards the relaxation of the parity selection rules in these nanostructures. STS measurements also reveal a midgap feature, which is likely similar to one observed in previous charge transport studies of EDT-treated quantum dot films.

Received 9 February 2012

DOI:https://doi.org/10.1103/PhysRevLett.110.127406

Authors & Affiliations

Bogdan Diaconescu¹, Lazaro A. Padilha¹, Prashant Nagpal¹, Brian S. Swartzentruber², and Victor I. Klimov^1,*

¹Center for Advanced Solar Photophysics and Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
²Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, USA

^*klimov@lanl.gov

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Vol. 110, Iss. 12 — 22 March 2013

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Images

Figure 1
STM/STS measurements of PbS NCs. (a) An STM constant current image of PbS NCs on the ITO/glass substrate. (b) A transmission electron microscopy image of PbS NCs. (c) Single-dot constant height (inset) $I / V$ (dashed line) and $d I / d V$ (solid line) spectra showing the first three electron and hole states. (d) Variable-gap (inset) $I / V$ (dashed line) and $d I / d V$ (solid line) spectra for the same dot. (e) STS showing charging effects in the case of high tunneling current ( $> 700 pA$ ). STM/STS data were acquired at sample temperature of 100 K.Reuse & Permissions
Figure 2
$d I / d V$ spectra for three different PbS NCs with band gap energies of 0.65 eV (upper trace; apparent STS band gap $Δ V_{NC} = 0.70 eV$ ), 0.69 eV (intermediate trace, $Δ V_{NC} = 0.74 eV$ ), and 0.75 eV (lower trace, $Δ V_{NC} = 0.81 eV$ ). In these spectra, we resolve at least six electron and five hole states. The measured spectra (solid lines) are fit (dashed lines) using a sum of Lorentzians used to identify individual hole and electron states (shown at the bottom by solid red lines). The asymmetric background in the band-gap region can be attributed to the manifold of mid-gap states (MGB) shown by the blue dashed line.Reuse & Permissions
Figure 3
(a) Individual states’ energies derived from the STS data corrected for the contribution from the polarization energy (symbols) as a function of true NC band-gap energy. Lines are theoretical calculations using the $k \cdot p$ four-band envelope function formalism. (b) The comparison of the energies of the first two conduction- and valence-band states plotted as a function of NC band-gap energy. Experimental data are shown by symbols while theoretical calculations by lines.Reuse & Permissions
Figure 4
(a) The first four optical transitions observed in the PL excitation (PLE) spectrum (solid line) and its second derivative (dashed line) of the sample with $E_{g} = 1.09 eV$ . (b–d) The comparison of energies of various symmetric and asymmetric interband transitions derived from the STS measurements (squares and triangles) and optical transitions (circles) from the PLE and single-photon absorption spectra (optical data are from Ref. 10). Based on this comparison, the 2nd peak in the optical spectra (open circles in ‘b’) is best described by the asymmetric STS $1 S_{e} - 1 P_{h}$ and $1 P_{e} - 1 S_{h}$ transitions (open and solid squares in ‘b’, respectively). On the other hand, the 3rd optical transition (open circles in ‘c’) corresponds to the symmetric $1 P_{e} - 1 P_{h}$ STS transition (solid triangles). Finally, the 4th optical transition (open circles in ‘d’) is close to the asymmetric STS $1 P_{e} - 1 D_{h}$ and $1 D_{e} - 1 P_{h}$ STS transitions (open and solid squares in ‘d’, respectively).Reuse & Permissions

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