Effects of Crystal Shape on the Energy Levels of Zero-Dimensional PbS Quantum Dots

Abstract

Nanometer-size PbS quantum dots have been made by electrodeposition on a Au(111) substrate. The deposited nanocrystals have a flattened cubic shape. We probed the single-electron energy-level spectrum of individual quantum dots by scanning tunneling spectroscopy and found that it deviates strongly from that of spherical PbS quantum dots. The measured energy-level spectrum is successfully explained by considering strong confinement in a flattened cubic box. DOI: 10.1103/PhysRevLett.88.236803 PACS numbers: 73.22. -f, 73.63. -b Insulating nanocrystals with dimensions below 10 nm can show atomiclike discrete energy levels due to quantum confinement [1]. Such nanocrystals are therefore often called "zero-dimensional quantum dots" or "artificial atoms." The energy-level spectrum depends on the chemical identity and crystal structure of the nanocrystals and on their size and shape. Many II-VI and III-V semiconductors have been prepared as insulating nanocrystals by colloidal chemistry. Nucleation and growth in solution leads to (nearly) spherical crystals [2]-unless the growth conditions are carefully manipulated [3]. In contrast, deposition on a surface (molecular beam epitaxy, electrodeposition) typically yields nonspherical structures [4 -6]. The question as to how far the energy level spectrum of very small (zero-dimensional) quantum dots is affected by the shape of the nanocrystals has not been addressed systematically. However, as is clear from recent optical work on insulating quantum rods [3], relatively small deviations from spherical symmetry can have dramatic effects on the optical properties of semiconductor nanocrystals.In this Letter, we report on a study of the energy-level spectrum of electrodeposited PbS "quantum boxes," which show a flattened cubic shape (x:y:z typically between 2:2:1 and 3:3:1, x and y being parallel and z being perpendicular to the substrate surface). Lead chalcogenide nanocrystals could form the ultimate quantum dots due to the low effective mass of both electrons and holes, ensuring strong confinement [7,8]. We probed the energy levels of a number of individual quantum dots, deposited on Au(111), with electron tunneling spectroscopy at 4.2 K. It is found that the quasiparticle gap is significantly larger, and the energylevel separation is significantly smaller than experimentally observed and theoretically predicted for spherical PbS dots [7]. The measured energy-level spectrum of the flattened cubic PbS quantum box is successfully explained by discriminating between the (strong) quantization perpendicular to, and the (weak) quantization parallel to, the gold surface. For the first time, it is experimentally shown that the energy-level spectrum of so-called zero-dimensional quantum dots can be sensitive to deviations from a spherical shape.We deposited PbS nanocrystals from an aqueous solution (1 mM) of Pb͑NO 3 ͒ 2 and Na 2 S 2 O 3 at pH 2.8 [9] on flame-annealed Au(111), graphite (HOPG), and Au (111) covered with a 1,4-dithiane self-assembled monolayer (SAM) ͑Au j SAM͒. The g...

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