Surface structure of cadmium selenide nanocrystallites

Abstract

Extended x-ray-absorption fine structure ͑EXAFS͒, Fourier-transform infrared absorption ͑FTIR͒, and elemental analysis were used on a variety of CdSe nanocrystallites ͑NC's͒ to study surface structure. All CdSe NC's were grown by standard inverse micelle techniques. Two sets of NC's samples were made. One set was made so that only the size of the NC's was varied, while the surface treatment was kept the same. The other set was made so that only the surface treatment was varied, while the size distribution was kept the same. For the EXAFS experiments, reference compounds similar in structure were measured. FTIR found surface Cd atoms to be passivated by pyridines and water groups. Fourier-filtered first-shell Cd EXAFS also supports the existence of water groups attached to the surface Cd atoms. The lack of any SeO and Si(CH 3 ) 3 in the FTIR signal indicates that most surface Se atoms have unterminated bonds. Fourier-filtered first-shell Se EXAFS spectra indicate that Se has only Cd as its first-nearest neighbor, and that the coordination number is reduced from the bulk value, suggesting surface Se atoms are unpassivated. Our data support the existence of surface Se lone-pair orbitals that can trap an optically excited hole. ͓S0163-1829͑97͒00908-9͔

Quantitative modeling of the role of surface traps in CdSe/CdS/ZnS nanocrystal photoluminescence decay dynamics

“…These sites are capable of trapping electrons or holes at the surface. Ab initio calculations (27) and positron annihilation spectroscopy on CdSe NCs (24) suggest that Se atoms relax outward irrespective of passivation, indicating, as previously suggested (25,(28)(29)(30), that hole traps constitute the majority surface trap site. The precise number of trap sites around a NC is likely to be both inhomogeneous and sample dependent; although consideration of the number of atoms near the surface in a wurtzite NC puts an approximate upper limit on that number (25).…”

Atomistic theory of coherent spin transfer between molecularly bridged quantum dots

“…According to previous experimental [30,31,32,33,34] and theoretical [14] work, group VI elements (such as S and O) bond preferentially to the Cd atom surface sites on the nanocrystal. Consequently, this calculation is made for each of the sp 3 s * Cd basis orbitals on the 84 surface Cd sites of the 3.4 nm diameter nanocrystal, and for each of the sp 3 s * Cd basis orbitals on the 243 surface Cd sites of the 5.0 nm diameter nanocrystal, over energy intervals of ∆E = 0.01 eV.…”

Analysis and simulation of the structure of nanoparticles that undergo a surface-driven structural transformation

“…A many-shell PDF analysis of nanocrystalline diamond showed no simple relationship between the distance shifts of the first 8 shells (most shells showed contraction, but one showed expansion) [18]. Similarly, Carter et al report a first shell contraction and a second shell expansion from EXAFS analysis of CdSe [11]. In general, small particle size and the presence of bond-bending disorder prevents meaningful structure analysis of second and higher shells in EXAFS data from nanoparticles.…”

“…Studies on tetrahedrally-coordinated semiconductor nanoparticles have found that the first shell bond length may be modified [9,10,11] or unchanged [12,13,14] with respect to the bulk material. Rockenburger et al concluded that the nature of the surface ligand determined the direction of bond length changes [9].…”

“…Extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) spectroscopies offer element-specific sensitivity to coordination, oxidation state and near-neighbour geometry. Many groups have studied the structure and lattice dynamics of nanoparticles with EXAFS [9,10,11,12,13,14]. For example, EXAFS work on carefully characterized nanoparticle systems has revealed structural variations due to synthesis conditions and the nature of the surface ligand.…”