The first sixteen members of the {[(VSe 2 ) n ] 1.06 (TaSe 2 ) m } z family of compounds where n and m were varied from 1 to 4 were synthesized by annealing preconfigured reactants. The factor of 1.06 originates from the ab-plane lattice mismatch of VSe 2 and TaSe 2 , reflecting the non-epitaxial structural relationship between the components. Each individual compound was addressed by sequentially depositing n layers of alternating elemental V and Se followed by m layers of alternating elemental Ta and Se in which the thickness of each pair of elemental layers was calibrated to yield a single-crystalline layer of the metal diselenide on annealing at low temperature for a short time. The structural changes during the transformation from initial reactant to crystalline superstructure are followed using X-ray diffraction. The structure of these nanoscale superstructures consist of an integral number of VSe 2 and TaSe 2 layers with minimal cation mixing between the layers. Electrical measurements show that all of the members of this family are metallic.
The development of tools to predict both the properties and structures of unknown materials has created the growing field of crystal engineering-the rational optimization of properties based on structure-function relationships. [1][2][3][4][5] The great potential of crystal engineering has already been realized in ground-breaking studies of semiconductor superlattices, [6] nanostructured cuprate superconductors, [7,8] bismuth-antimony superlattice thermoelectrics, [9] and colossal magnetoresistant cation-ordered manganese oxides.[10] While epitaxialgrowth techniques were used to prepare most of these new materials, this field has been limited by the lack of alternate synthetic approaches that provide similar structural control. In this manuscript we report the preparation of a series of [(VSe 2 ) n ] 1.06 [(TaSe 2 ) n ] superlattices synthesized using a hybrid approach in which a nanostructured reactant prepared using vacuum deposition is subsequently annealed at low temperature. On annealing the initially non-crystalline reactant self-assembles into an ordered crystalline compound with unit-cell control of the desired nanostructure based on the design of the initial reactant. Opportunities enabled by this synthetic approach include the ability to prepare nanostructured materials containing elements with a wide range of vapor pressures and surface mobilities, and the ability to prepare nanostructured materials with large crystallographic misfit between the lattices of the components. The traditional method used in crystal engineering of nanostructured materials is epitaxial growth involving toggling sources to control the nanostructure period.[11] A variety of techniques (evaporation, [12] sputtering, [13] atomic layer deposition, [14] pulsed-laser deposition, [15] metal-organic vapor chemical deposition) [16] have been developed that utilize different sources or deposition techniques for the depositing species.The substrates are heated and the rates are controlled to balance the arrival rate of the depositing species, their evaporation, and their surface mobility to obtain epitaxial layer-bylayer growth. Exquisite control has been achieved in a wide variety of systems resulting in custom compounds with unexpected physical properties. [13,[15][16][17] The opportunities offered by crystal engineering have also encouraged the development of new synthetic approaches to designed solids. Many chemists and materials scientists have focused on self-assembly as a means to prepare ordered nanostructured materials and epitaxial interfaces.[18-22] Self-assembly techniques typically involve the deposition of films from solution, the removal of solvents and ligands, and the annealing of the resulting films to convert the amorphous precursor to the desired crystalline material. In contrast to epitaxialgrowth techniques, self-assembly is potentially scalable to produce materials in bulk and at low cost. The trade-off for these advantages is the loss of exquisite structural control possible with epitaxial-growth techniques.The a...
Tantalum I 5200Synthesis of a Family of {[(VSe2)n]1.06(TaSe2)m}z Compounds. -The first sixteen compounds of the title family in which n and m both vary from 1 to 4 are synthesized by sequentially depositing n layers of alternating elemental V and Se followed by m layers of alternating elemental Ta and Se in an ultrahigh vacuum chamber onto Si wafers followed by annealing at low temperature for a short time. The structural changes during the transformation from initial reactant to crystalline superstructure are monitored by XRD. The structure of these nanoscale superstructures consists of an integral number of VSe2 and TaSe2 layers with minimal cation mixing between the layers. Electrical measurements show that all of the members of this family are metallic. -(NGUYEN, N. T.; HOWE, B.; HASH, J. R.; LIEBRECHT, N.; ZSCHACK, P.; JOHNSON*, D. C.; Chem.
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