Cu2S nanocrystals with disklike morphologies were synthesized by the solventless thermolysis of a copper alkylthiolate molecular precursor. The nanodisks ranged from circular to hexagonal prisms from 3 to 150 nm in diameter and 3 to 12 nm in thickness depending on the growth conditions. High resolution transmission electron microscopy (HRTEM) revealed the high chalcocite (hexagonal) crystal structure oriented with the c-axis ([001] direction) orthogonal to the favored growth direction. This disk morphology is thermodynamically favored as it allows the extension of the higher energy {100} and {110} surfaces with respect to the {001} planes. The hexagonal prism morphology also appears to relate to increased C-S bond cleavage of adsorbed dodecanethiol along the more energetic {100} facets relative to {001} facets. Monodisperse Cu2S nanodisks self-assemble into ribbons of stacked platelets. This solventless approach provides a new technique to synthesize anisotropic metal chalcogenide nanostructures with shapes that depend on both the face-sensitive thermodynamic surface energy and the surface reactivity.

Organic monolayer protected Cu2S nanorods, 4 nm in diameter and 12 nm long, were synthesized using a novel solventless synthetic approach. Thermolytic degradation of a copper thiolate precursor at temperatures ranging from 140 to 200 degrees C produces Cu2S nanorods. Higher temperatures promote isotropic growth of spherical nanocrystals. X-ray diffraction and high-resolution TEM reveal that the nanorods exhibit a hexagonal Cu2S crystal structure, which in the bulk is ferroelectric. The appropriate reaction conditions produce nanorods that are size and shape monodisperse and organize into smectic superlattices. The extent of superlattice ordering and the appearance of extended strands of nanorods provide evidence for strong dipole-dipole coupling between Cu2S nanorods.

Nickel sulfide and copper sulfide nanocrystals were synthesized by adding elemental sulfur to either dichlorobenzene-solvated (copper sulfide) or oleylamine-solvated metal(II) precursors (nickel sulfide) at relatively high temperature to produce the metal sulfide. Nickel sulfide nanocrystals are cubic Ni(3)S(4) (polydymite) with irregular prismatic shapes, forming by a two-step reduction-sulfidation mechanism where Ni(II) reduces to Ni metal before sulfidation to Ni(3)S(4). Despite extensive efforts to optimize the Ni(3)S(4) nanocrystal size and shape distributions, polydisperse nanocrystals are produced. In contrast, copper sulfide nanocrystals can be obtained with narrow size and shape distributions. The copper sulfide stoichiometry depended on the Cu:S mole ratio used in the reaction: Cu:S mole ratios of 1:2 and 2:1 gave CuS (covellite) and Cu(1.8)S (digenite), respectively. CuS nanocrystals formed as hexagonal disks that assemble into stacked ribbons when cast from solution onto a substrate. CuS, Cu(1.8)S, and Ni(3)S(4) differ from the Cu(2)S and NiS nanocrystals obtained by solventless decomposition of metal thiolate single source precursors, in terms of stoichiometry for copper sulfide, and both stoichiometry and morphology for nickel sulfide [Ghezelbash, A.; Sigman, M. B., Jr.; Korgel, B. A. Nano Lett. 2004, 4, 537-542. Sigman, M. B. Ghezelbash, A.; Hanrath, T.; Saunders, A. E.; Lee, F.; Korgel, B. A. J. Am. Chem. Soc. 2003, 125, 16050-16057].

Organic monolayer-coated rhombohedral NiS (millerite) nanorods and triangular nanoprisms were synthesized using a solventless thermolytic decomposition of nickel thiolate precursors in the presence of octanoate. The size and shape distributions are relatively narrow, with nanorod lengths that depend on the growth conditions, ranging from 15 to 50 nm and typically with aspect ratios of approximately 4. For example, a typical procedure yields nanorods 33.9 ± 8.6 nm long and 8.11 ± 1.6 nm wide. The approach also yields triangular nanoprisms under some reaction conditions with nearly a 1:1 ratio of nanorods to nanoprisms. FTIR spectra reveal that octanoate serves as a capping ligand that controls nanorod growth. X-ray diffraction (XRD) shows that the primary reaction byproduct in the synthesis is colloidal Ni3S4 in the form of misshapen needles and particulates. High-resolution transmission electron microscopy (HRTEM) confirm that the well-defined nanorods and triangular nanoprisms are composed solely of rhombohedral NiS (millerite) grown preferentially in the [110] direction.

Crystalline germanium (Ge) nanocrystals were synthesized by arrested precipitation in supercritical hexane and octanol at 400∼550 °C and 20.7 MPa in a continuous flow reactor. Two Ge precursors were explored, diphenylgermane (DPG) and tetraethylgermane (TEG), which undergo thermolysis to crystalline Ge under these conditions. Octanol is added to control particle growth, which appears to serve as a capping ligand that binds to the particle surface through an alkoxide linkage. The nanocrystals were characterized by high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), FTIR spectroscopy, and X-ray diffraction (XRD). The average nanocrystal diameter could be changed over a wide range, from ∼2 nm to ∼70 nm, by varying the reaction temperature and precursor concentration. Relatively size-monodisperse nanocrystals could be produced, with standard deviations about the mean diameter as low as ∼10%. UV−visible absorbance and photoluminescence (PL) spectra of Ge nanocrystals in the 3 to 4 nm diameter size range exhibit optical absorbance and PL spectra blue-shifted by approximately 1.7 eV relative to the band gap of bulk Ge, with quantum yields up to 6.6%.