Size and shape dependent level structure in CdSe quantum rods

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E17.1.1

Size and shape dependent level structure in CdSe quantum rods Eli Rothenberg1, Taleb Mokari1, Uri Banin1, David Katz2, Tommer Wizansky2, and Oded Millo2 Institute of Chemistry, and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel 2 Racah Institute of Physics and the Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel 1

ABSTRACT Optical spectroscopy and Scanning Tunneling Microscopy are used to study the size and shape dependence of the electronic states in CdSe quantum rods. The quantum rods were grown using colloidal chemistry synthesis methods, with good control over size and size distribution. Samples having average rod dimensions ranging from 10 to 60 nm in length and 3.5 to 7 nm in diameter, with aspect ratios varying between 3 to 12, were investigated. Both optical (at 10 K) and tunneling (at 4.2 K, on single rods) spectra show that the level structure depends primarily on the rod diameter and not on length. With increasing diameter, the band gap and the excited state level spacings shifted to the red. The level structure is assigned using a multi-band effective-mass model, showing relatively good agreement with experiment. We shall also discuss the effect of single electron charging on the tunneling spectra, possibly reflecting the quantum rod level degeneracy. INTRODUCTION Colloidal semiconductor nanocrystals are a class of nanomaterials that manifest the transition from the molecular limit to the bulk solid-state regime [1,2], with significant potential for serving as building blocks of nano-devices in applications ranging from lasers [3,4] and opto-electronic devices [5] to biological fluorescence tagging [6]. Shape control of such colloidally prepared nanostructures has been recently achieved by modifying the synthesis to obtain rod shaped particles - quantum rods (QRs) [7]. QRs exhibit electronic and optical properties different than quantum dots (QDs). For example, unlike the spherical dots, QRs have linearly polarized emission as demonstrated recently by fluorescence measurements on single rods [8], leading also to polarized lasing [4]. Here we combine optical and tunneling spectroscopies and correlate them with a multi-band effective-mass model, to investigate the electronic level structure of CdSe quantum rods, and study its dependence on rod length and diameter. The study provides significant insight on the evolution of the electronic structure from zero dimensional QDs to onedimensional quantum wires. The combination of scanning-tunneling and optical spectroscopies has proven to be a powerful approach to decipher the level structure of spherical nanocrystal QDs [2,9]. While in the optical spectra, allowed valence band (VB) to conduction band (CB) transitions are detected [10,11], in tunneling spectroscopy the CB and VB states can be separately probed yielding complementary information on the level structure [9,12,13,14]. Such data can provide an important benchmark for theoretical

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