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