NMR study of InP quantum dots: Surface structure and size effects

M. Tomaselli, J. L. Yarger, M. Bruchez, R. H. Havlin, D. deGraw, A. Pines, A. P. Alivisatos
1999 Journal of Chemical Physics  
We report the results of 31 P NMR measurements on trioctylphosphine oxide ͑TOPO͒ passivated InP quantum dots. The spectra show distinct surface-capping sites, implying a manifold of crystalligand bonding configurations. Two In 31 P surface components are resolved and related to different electronic surroundings. With decreasing particle size the In 31 P core resonance reveals an increasing upfield chemical shift related to the overall size dependence of the InP electronic structure. © 1999
more » ... ucture. © 1999 American Institute of Physics. ͓S0021-9606͑99͒70718-X͔ Semiconductor clusters with monodisperse diameters ranging from 10-100 Å manifest quantum dot behavior. 1 The surface composition of these colloidally prepared particles 2 has been shown to be important because of its influence on the discrete electronic structure and quantum confinement 1,3-5 as well as its relation to electronic transport properties, 1,6 structural phase transitions, and thermodynamic stability. 7 In addition, the spectroscopic characterization of the capping molecules can provide valuable information on the morphology and faceting 1 of the nanoparticles. In this letter, we report an initial study on III-V semiconductor InP dots using one-and two-dimensional ͑1D and 2D͒ NMR. We find distinct capping and In 31 P surface sites, implying a variety of ligand-crystal bonding arrangements and structural environments. The chemical shielding of the In 31 P resonance increases with decreasing dot size which can be interpreted as a decrease in the 31 P paramagnetic shift with increasing electronic excitation energy of the quantumconfined nanoparticles. InP samples have been prepared under argon using the dehalosilation reaction of InCl 3 and P͑Si͑CH 3 ͒ 3 ͒ 3 at 540-570 K in TOPO as coordinating solvent. 8 Distinct size distributions of the particles were obtained by size-selective precipitation resulting in InP clusters with average diameters in the range of 20-50 Å and distributions of ϳ20%. 8 For the surface selective experiments, the separation and isolation were carried out in a drybox. The precipitates were dried under vacuum and the resulting powder samples sealed in pyrex tubes ͑0.13 Pa͒. In a second synthesis, the identical precipitation and isolation procedure was carried out in air, yielding TOPO and oxide passivated InP dots. 1,8 X-ray diffraction spectra ͑Siemens D5000, Cu K ␣ radiation͒ and transmission electron microscopy images ͑TEM, TopCon EM002B͒ showed that the InP dots were highly crystalline ͑pure phase, zinc-blende͒, roughly spherical in shape, with indications of faceting. The cluster diameters ͑d͒ were inferred from UV/ vis absorption spectra ͑HP 8452͒ obtained immediately upon separation. 8 Figures 1͑A͒ and 1͑B͒ show typical 1D 31 P (Sϭ1/2) NMR spectra of TOPO-InP (dӍ45 Å) recorded under conditions of magic-angle spinning ͑MAS͒. 9 The experiments were performed at a 31 P Larmor frequency of 0 ϭ75.18 MHz (B 0 ϭ4.36 T) and 0 ϭ161.99 MHz (B 0 ϭ9.39 T) using Chemagnetics CMX spectrometers and 4 mm MAS probe assemblies from the same manufacturer. The MAS frequency ( mas ) was stabilized within 3 Hz for all experiments. The B 1 nutation frequency was matched to 1 ϭ140 kHz on all channels ( 1 H, 31 P, 13 C). Spectrum 1͑A͒ was obtained by a 31 P single-pulse excitation with 1 H decoupling during data acquisition. The inhomogeneously broadened resonance at Ϫ178 ppm ͑relative to 85% H 3 PO 4 ) with a linewidth of ␦ 1/2 ϭ58 ppm ( 1/2 ϭ4400 Hz) is assigned to the In 31 P core ͑interior͒ nuclei: For a spherical InP cluster with a typical diameter of 45 Å, only ϳ20% of all atoms reside at the surface. The lineshape is slightly asymmetric and upfield shifted with respect to bulk In 31 P ͓␦ϭϪ147 ppm, ␦ 1/2 ϭ43 ppm ( 1/2 ϭ3200 Hz)͔, with the main source of broadening being the indirect exchange interaction with the 115 In neighbors (Sϭ9/2), 10 as well as a contribution from the second-order anisotropic ͑pseudo͒dipolar-quadrupolar shift. 11,12 The weak downfield segment of the spectrum ͑ϳ8%͒ in the range Ϫ20Ͻ␦Ͻ80 ppm corresponds to 31 P TOPO resonances at the crystal surface. From the relative TABLE I. 31 P chemical-shift parameters ͑75.18 MHz, 300 K͒; ␦ aniso is the anisotropy and the asymmetry of the chemical-shift anisotropy tensor ͑CSA͒ as defined in Ref. 18.
doi:10.1063/1.478858 fatcat:5a5ohisqhvhyjcpqkwjm577i2u