Synthesis of Size-Selected, Surface-Passivated InP Nanocrystals

Abstract

Quantum-confined InP nanocrystals from 20 to 50 Å in diameter have been synthesized via the reaction of InCl3 and P(Si(CH3)3)3 in trioctylphosphine oxide (TOPO) at elevated temperatures. The nanocrystals are highly crystalline, monodisperse, and soluble in various organic solvents. Improved size distributions have been obtained by size-selectively reprecipitating the nanocrystals. The UV/vis absorption spectra of the particles show the characteristic blue shift of the band gap of up to 1 eV due to quantum confinement, a moderately well-resolved first excitonic excited state, and, in some cases, the resolution of a higher excited state. Structurally, the nanocrystals are characterized with powder X-ray diffraction and transmission electron microscopy. Raman spectroscopy reveals TO and LO modes near the characteristic bulk InP positions as well a surface mode resulting from finite size. The Raman line widths, line positions, and relative intensities are all size-dependent . X-ray photoelectron spectroscopy (XPS) shows the nanocrystals have a nearly stoichiometric ratio of indium to phosphorus with TOPO surface coverages ranging from 30% to 100%. We have also used XPS to correlate the oxidation of the nanocrystal surface with photoluminescence intensity. Photoluminescence is observed as both band edge and deep trap emission with both features shifting with nanocrystal size. The luminescence is highly dependent on the surface of the nanocrystal with oxidation being a necessary condition for emission.

Photophysics of size-selected InP nanocrystals: Exciton recombination kinetics

“…13 The general reaction scheme was described in our previous paper, 12 so only a brief description of the synthesis is presented here. All syntheses were carried out using standard glove-box, vacuum line, and other airless techniques.…”

“…In our previous work, we correlated room temperature PL measurements with x-ray photoelectron spectroscopy ͑XPS͒ to demonstrate the effect of oxidation on PL efficiency. 12 We determined that at least partial surface oxidation was necessary for particle luminescence. The particles are fairly stable in air, only exhibiting measurable amounts of oxidation after a day or two of air exposure.…”

“…In our previous work, we correlated particle size ͑as determined by x-ray powder diffraction line broadening and transmission electron microscopy͒ with the lowest energy feature in the room temperature absorption spectrum. 12 This correlation was utilized to extract the sizes of the particles investigated here from their UV/vis spectra.…”

“…11 We have recently reported on the preparation and fundamental physical properties of sizeselected, surface passivated, monodisperse InP nanocrystals. 12 Our synthesis, which is a modification of a scheme reported by the Nozik group, produces nanocrystals that, although not near the state-of-the-art defined for II-VI's, are of suitable quality for investigating InP nanocrystals size-dependent photophysics.…”

Nanostructure Engineering of Size-Quantized Semiconductor Particles for Photoelectrochemical Applications

“…Figure 1 summarizes the size-dependent E g reported for several kinds of semiconductor particles with a spherical shape. [16][17][18][19][20][21][22] In each case, E g was remarkably enlarged with a decrease in particle size due to the quantum size effect when the particle size became smaller than twice the effective exciton Bohr radius of the corresponding bulk semiconductor. Considering the strong size dependence of E g , it is concluded that narrowing the size dispersibility is required for precisely controlling the photoelectrochemical properties of semiconductor nanoparticles as a function of their size.…”