Synthesis Research - Strouse Group Research

A long-standing goal for the preparation of nano-scale materials is the development of a general synthetic methodology that allows low temperature, solution phase growth methods, that yields crystalline materials. We have generalized a strategy that utilizes inorganic clusters as single source precursors for the preparation of II-VI nanocrystals between the sizes of 2-10 nm, including CdSe, ZnSe, CdS, and ZnS. (Figure 1) The use of a cluster based single source precursor allows nanocrystal growth to be initiated without a requisite pyrolytic step for nucleus formation, which is traditionally required for lyothermal growth mechanisms. The elimination of the pyrolytic step allows greater synthetic control, slow thermodynamic growth at lower temperatures, and large scale reactions (> 50 g/L) to be prepared, while maintaining size-dispersity at < 5% and crystallinity. We have continued to explore the properties of nanocrystal growth using inorganic clusters as single source precursors, extending the methodology to Co metal nanoparticles, III-V nanocrystals, and doped nanocrystals.

We have extended our crystal seeding methodology, which employs the introduction of a preformed metal-chalcogenide cluster to a coordinating solvent to CdSe, CdSe, ZnSe, ZnS, and CdSe/ZnS (Core/shell materials). The use of a [M₁₀Se₄SPh₁₆]^4- cluster as the preformed nucleus in a long-chain alkyl amine, hexadecylamine for example, allows growth of crystalline nanocrystals (5% rms batch, <4% rms selectively precipitated) (Figure 2). Owing to the rapid ligand and metal exchange in these materials, nucleation occurs instantly at relatively low temperatures followed by growth of the nanocrystals under thermal control. The reaction mechanism is believed to proceed via a step wise nucleation and growth via addition of a second cluster to an initial nucleus. The size of the nanomaterial can be tuned by solvent temperature in this reaction. It appears this may arise from the fact the surface energy of these structures allow rapid rearrangement with formation of the lowest energy nanocrystal, suggesting controlled thermodynamic growth can be achieved in these materials. Uniquely, the preparative route using an inorganic seeds results in a narrow size distributuion without the observation of small nanocrystals in the reaction mixture. This methodology allows preparation of large scale reactions of organically passivated, spherical nanocrystals (> 50g/L) of CdSe and ZnSe (2-10 nm, <5% rms size selectively precipitated, 5% thermally annealed or 6-12% out of batch). The CdSe can be recapped with an inorganic passivant layer (ZnS) following traditional lyothermal techniques to yield CdSe/ZnS core-shell structures. A major advantage of this new methodology is the attainment of greater synthetic control.

or read the published article:
"Inorganic Clusters as Single Source Precursors for Preparation of CdSe, ZnSe, CdSe/ZnS Nanomaterials." Cumberland, S.L.; Hanif, K.M.; Javier, A.; Khitrov, G.A.; Strouse, G.F.; Woessner, S.M.; Yun, C.S. Chem. Mater., 14, 1576-1584 (2002). [ view article - PDF ]

Doping bulk semiconductors with metal ions, for example Mn(II), Co(II), Eu(II), Cu(II), or V(II), has been demonstrated to significantly tune the observed optical and magnetic properties in the new ternary complex. Interstitial doping has been shown to lead to cooperative effects between the semiconductor host lattice and transition metal guest ions arising from long range exchange interactions. The influence on the physical properties is proportional to the doping level, and material choice. Recent studies on doping nano-scale materials have suggested doping is poorly achieved in materials on the nanoscale due primarily to lattice annealing during lyothermal growth, which tends to lead to surface doping of the materials. Development of new methods for the preparation of quantum dot semiconductor structures doped with transition metal ions could provide a rich opportunity to explore the relationship between structure and transport properties on varying length scales in reduced dimensions.

Systematic core doping of CdSe between 0 and 40% with a selected dopant has been achieved by treatment with appropriately doped precursors. Several manuscripts on this effort will be submitted later this year outlining the results of doping with V, Cu, Co, and Eu dopants (2 of which are currently in preparation). The emphasis of this effort targets the preparation of discrete II-VI nano-crystalline materials doped both interstitially and at the surface with transition metal ions to induce magnetic coupling between quantum dots. Using a novel single source precursor route, our group has prepared a series of transition metal (Co, Cu, V) and lanthanide (Eu) doped CdSe nanoparticles that exhibit a narrow size dispersity (2-8 nm diameter, 5% relative standard deviation of size). The strategy for doping is analogous to the single source precursor method above with the modification of either the addition of an inorganic salt (Eu, V) to the initial reaction mixture, or the addition of an inorganic cluster (Co, Cu). Analysis of the AA, ICP-AES, TEM-EDX and atomic absorption confirm the Cd_1-xM_xSe (M= Cu, Co, Eu, V) materials are doped in the core at levels between x = 0.00 and 0.4.

Corroborating the core-level doping, these materials exhibit a linear shift in the powder X-ray diffraction with doping concentration, in analogy to shifts observed in doped bulk materials. The linear shift in the X-ray parameters is consistent with core-level doping, and provides the first reported evidence of controlled core-doping in II-VI materials. The lattice shift can be understood in terms of a solid-state approximation, in which a similar shift in lattice parameters is observed for statistical doping of the cores of the materials, rather than surface doping (Vegards Law). Furthermore, extrapolation of the M-Se bond distances to a lattice composed of 100% EuSe or CoSe produce a M-Se bond distance consistent with the bonding in pure materials.

Magnetic and optical studies on Co(II) doped CdSe provide further evidence for controlled doping. In the Co(II) doped materials a signature for a Co centered longitudinal optical phonon is evident at ~ 300 cm-1 and scales with Co concentration further corroborating core-level doping in these materials. Recent XPS and EXAFS results suggest the Co may be doped as Co-O, possibly due to core vs surface level doping. Magnetic studies indicate the onset of spin glass behavior below the bulk percolation threshold in these materials, which suggest enhanced quantum confinement may lead to extended magnetic coupling in these materials. Analysis of the magnetic coupling as a function of nanomaterial size and composition is providing unique insight into the structure, and magnetic properties of materials in the quantum confined limit.