The syntheses of strongly anisotropic nanocrystals with one dimension much smaller than the two others, such as nanoplatelets, are still greatly underdeveloped. Here, we demonstrate the formation of atomically flat quasi-two-dimensional colloidal CdSe, CdS and CdTe nanoplatelets with well-defined thicknesses ranging from 4 to 11 monolayers. These nanoplatelets have the electronic properties of two-dimensional quantum wells formed by molecular beam epitaxy, and their thickness-dependent absorption and emission spectra are described very well within an eight-band Pidgeon-Brown model. They present an extremely narrow emission spectrum with full-width at half-maximum less than 40 meV at room temperature. The radiative fluorescent lifetime measured in CdSe nanoplatelets decreases with temperature, reaching 1 ns at 6 K, two orders of magnitude less than for spherical CdSe nanoparticles. This makes the nanoplatelets the fastest colloidal fluorescent emitters and strongly suggests that they show a giant oscillator strength transition.

Well documented procedures to grow zero-dimensional systems, 1 dots, and one-dimensional systems, 2 wires and tubes, as colloidal particles in solution have been reported. In contrast, there are no methods of preparation that yield optically active two-dimensional soluble particles. Yet, ultrathin films (quantum wells) of II-VI and III-V semiconductors epitaxially grown on substrates by molecular beam epitaxy for example have proven extremely useful for both fundamental studies and a wealth of applications in optoelectronics. 3 The synthesis of 2D colloidal nanocrystals, nanoplatelets, or nanodisks is limited to a few examples 4 of metal and lanthanide oxide materials as well as CuS and NiS. We extend these studies to show that fluorescent quasi-2D CdSe platelets can be synthesized with different thicknesses quantified by one CdSe monolayer.The nanoplatelet synthesis is based on the solution phase decomposition of cadmium myristate and selenium mesh precursors in the presence of a noncoordinating solvent and an acetate salt. In a typical experiment, 85 mg (0.15mmol) of cadmium myristate and 12 mg (0.15mmol) of Se mesh were mixed in 15 mL of octadecene in a three-neck flask and degassed under vacuum for 10 min. The mixture was then heated at 240°C under argon. When the temperature reached 195°C (the solution is orange), 40 mg (0.15mmol) of cadmium acetate dihydrate were swiftly introduced into the flask. After 10 min at 240°C, the reaction was stopped by removal of the heating mantle. The particles synthesized were isolated by ethanol precipitation and suspended in hexane. Platelets were separated from polyhedral quantum dots by butanol precipitation and resuspended in hexane. TEM observations of the nanoparticles (Figure 1a) demonstrate the formation of CdSe platelets with lateral dimensions from 6 to 40 nm. The platelets have a zinc-blende crystal structure (see Supporting Information) which is consistent with the zinc-blende formation of CdSe polyhedra when the same syntheses are used without acetate salt. 5 The platelet thickness can be measured when they stack on their edge (Figure 1d and Supporting Information) and is found to be 2.2 ( 0.3 nm. Using TEM images, no difference in the platelet thicknesses could be observed. However, as can be seen on Figure 1b, when platelets lay flat on the TEM grid, different gray levels are distinguishable, suggesting the synthesis of platelets of different thicknesses. The platelet formation is induced by the presence of acetate salt in the reaction medium. All the acetate salts we have tested, including Mn(Ac) 2 · 4H 2 O; Zn(Ac) 2 ; Mg(Ac) 2 · 4H 2 O; Co(Ac) 2 · 4H 2 O, and Na(Ac), allow the formation of CdSe platelets, with somewhat different geometries. In all cases, despite the presence of other metallic ions, elementary analysis indicates that, after the washing step, the platelet shaped crystals contained only cadmium and selenium. When the acetate salt is introduced at the beginning of the synthesis, large CdSe quasi 2D films are obtained (Figure 1f). These films' la...

We study the formation of colloidal CdSe nanoplatelets using both tansmission electron microscopy (TEM) and spectroscopic analysis. We show that the platelets form by continuous lateral extension of small (<2 nm) nanocrystal CdSe seeds. The nanoplatelet thickness is fixed by the seed dimension and remains constant during the platelet formation. The nanoplatelet lateral dimensions can be tuned using additional precursor injection. Absorption and fluorescence analysis of the CdSe nanoplatelets as they continuously extend laterally confirms a continuous transition from 3D to 1D confined nanoparticles. The formation of the CdSe platelets is found to be similar for different platelet thicknesses that we control with a precision of one CdSe monolayer.