(Invited) Photoelectrochemical Properties of All-Inorganic Core/Shell Silicon Quantum Dots

Abstract

Silicon (Si) quantum dots (QDs) are an environmentally friendly nano-functional material potentially suitable for biophotonics applications. Recently, we have developed a new type of all-inorganic Si QDs that can be dispersed in water almost perfectly without organic ligands and exhibit size-dependent photoluminescence (PL) in the near-infrared range in water [1-2]. The unique property of the QDs arises from the unique structure achieved by simultaneous doping of boron (B) and phosphorus (P). The figure shows a typical transmission electron microscope (TEM) image of a B and P codoped Si QD [3]. The lattice fringe corresponds to {111} planes of crystalline Si. On the surface of the crystalline core, an amorphous shell is formed. The shell is composed of Si, B and P, the concentration of B and P being in the range from a few % to 10 at.%. The shell induces negative potential on the surface and prevents agglomeration of Si QDs in polar solvents by the electrostatic repulsion. To the ligand-free surface of codoped Si QDs, molecules in a solution can access easily, and thus a variety of charge-transfer-induced phenomena such as a photocatalytic effect and chemical doping are expected [4-5]. In a codoped Si QD, B and P are also doped in the crystalline core. Formation of donor and acceptor states in the band gap strongly modifies the luminescence property. Combination of the quantum size effect and the doping effect leads to the control of the luminescence energy in the range much wider than that of conventional Si QDs [6, 7]. By comparing the luminescence energy with those obtained by theoretical calculations, we estimate the number of active B and P pairs in a Si QD to be 2-10 depending on the doping concentration and the size. In the presentation, we will discuss the luminescence and photoelectrochemical properties, including photocatalytic hydrogen evolution, of the B and P codoped Si QDs.