J . Phys. Chem. 1994,98, 4966-4969
4966
Synthesis and Characterization of InP Quantum Dots Olga 1. MiCiC,' Calvin J. Curtis,' Kim M. Jones, Julian R. Sprague, and Arthur J. Nozik' National Renewable Energy Laboratory, 161 7 Cole Boulevard, Golden, Colorado 80401 -3393 Received: January 27, 1994; In Final Form: February 25, 1994"
Colloidal dispersions of quantum dots (QDs) of crystalline InP were synthesized starting from a chloroindium oxalate complex and P(SiMe3)3. An InP precursor was formed from the reactants at room temperature; heating the precursor in a solution of trioctylphosphine oxide at 270 OC for several days produced InP QDs with a mean diameter of 26.1 f 7.5 A (determined by counting lattice fringes in a TEM micrograph). From the known QD size distribution and its absorption spectrum, we used a theoretical model to calculate the dependence of InP QD bandgap on QD size. From these results we could then calculate the size distribution of any InP colloid from its absorption spectrum. It was found that preparing the InP precursor with an excess of I d + and then heating it in a mixture of trioctylphosphine oxide and trioctylphosphine at 270 OC for 3 days produced a very narrow QD size distribution with a mean diameter of 25 f 1.9 A.
Introduction
Quantum dots (QDs) consisting of colloidal nanocrystalline semiconductor particles that are in the strong quantum confinement regime have aroused considerable interest because of their unique size-dependent, optical, photocatalytic, and nonlinear optical properties.' Most previous experimental studies of colloidalquantum dots have been restricted to group 11-VI, I-VII, IV, and IV-VI and layered chalcogenidesemiconductors;111-V QDs have not been extensively studied because of difficulties with their preparation. Recently,some investigators have reported the synthesis of GaAs," Gap,' and InP*-lo QD particles. However, the InP QDs had generally shown amorphous or partially crystalline structures as well as oxidation effects; annealing at higher temperature improves the crystallization, but this also causes irreversible aggregation and precipitation of the colloidal particles. The resulting precipitate cannot be redissolved to form transparent, colloidal solutions, and the particles also tend to grow sufficiently large to lose the quantum confinement. In this work we report the first synthesis of well-crystallized (zinc blende structure) InP QDs in the strong quantum confinement regime (diameters =25 A). We introduce a theoretical model for the absorption spectrum of a distribution of quantum dots and use it to determine for the first time the dependence of a 111-V QD bandgap on QD size from the measured size distribution of the QDs and the experimental optical absorption data. Thecalculation can be inverted to derive the size distribution of QDs from their absorption spectrum. Wealso report a synthesis for InP QDs that produces a very narrow spread ( E, + 0.075
(2b)
i= 1
where hv is the photon energy and E , is the bandgap (1.35 eV for bulk InP); the form of eq 2 is also valid for QDs where E, > 1.35. The dependence of the QD bandgap on QD size was taken to be of the following f ~ r m : I , - ~ ~ (3) where m*, and m*h are the electron and hole effective masses,
mean diameter,A
standard deviation,A
26.1 26.6 25.2
f7.5 h3.1 f1.9
method ~
~~
TEM
fit to optical spectra fit to optical spectra
r is the particle radius, t is the static dielectric constant, and n and p are exponents of the particle radius (in the effective mass approximation n = 2 and p = llsJ6). The calculation of the expected colloidal QD spectrum was done by first taking the QD size distribution of the colloid, calculating the bandgaps of the particle sizes in the distribution according to eq 3, then applying the appropriate corrections for the dependence of absorption coefficients on wavelength for the QD sizes in the distribution as described in eq 2, then calculating the absorption spectra for the individual QDs in the distribution knowing a and the InP concentration (viz., InP thickness), and finally summing up all the absorption spectra for the particles in the distribution to obtain the final absorption spectrum for the QD colloid. For colloid a, the QD size distribution was determined experimentally, and the experimental absorption spectrum was fitted to our model by adjusting the parameters n and p of eq 3 using nonlinear least-squares procedures. The fit to the experimental spectrum is shown in Figure 4,curve a; the best fit required that n = 1.94 andp = 0.99.A plot of E, vs QD diameter for these n and p values is shown in Figure 5; also shown for comparison are the results expected for the effective mass approximation (EMA), where n = 2 and p = 1 .I6 As already seen for other QD materialsl&17and predicted by theory,'gJ* the EMA approximation greatly overestimates the increase in E, when the QD size is very small (
