Nanosheets by Design: The Controllable Synthesis of Group IV-VI Layered Semiconductor Chalcogenide Nanostructures Using Colloidal Chemistry

Open Access
Vaughn, Dimitri Duval
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
December 17, 2013
Committee Members:
  • Raymond Edward Schaak, Dissertation Advisor
  • Raymond Edward Schaak, Committee Chair
  • Thomas E Mallouk, Committee Member
  • John B Asbury, Committee Member
  • Donghai Wang, Committee Member
  • Nanoscience
  • Layered Semiconductors
  • Nanosheets
  • Germanium
  • Colloidal Synthesis
  • Chalcogenide
Nanosheets, a class of nanomaterials with two-dimensional structure and atomic or molecular scale thickness, have attracted a great deal of interest from the scientific community due to excellent physical properties and several promising applications in optoelectronics, energy conversion and storage, and catalysis. While advances in the synthesis of 2D nanostructures using “top-down” chemical and physical strategies such as exfoliation and mechanical cleavage have been achieved, improved synthesis may be realized by applying “bottom-up” colloidal strategies where nanosheets are “built” directly from solution in an atomic layer-by-layer fashion. In this dissertation, I will discuss recent advances in the synthesis of semiconductor nanosheets with controllable lateral dimension, thickness, hierarchical structure, and porosity, specifically focusing on a class of group IV-VI layered semiconductor chalcogenides (GeS, GeSe, SnS, and SnSe) as a model system. Finally, I will highlight my efforts for expanding the synthetic framework mentioned above to access other materials, including the colloidal synthesis of germanium and Ge-based nanostructures. To begin, a solution-based chemical approach for the synthesis of colloidal germanium sulfide (GeS) and germanium selenide (GeSe) nanosheets is described. It is found that the GeS and GeSe nanosheets adopted a uniform hexagonal crystallite morphology with lateral dimensions on the order of several microns and thicknesses that ranged between <10 to 100 nm. The nanosheets were characterized by various analytical techniques including transmission electron microscopy (TEM), selected area electron diffraction (SAED), scanning electron microscopy (SEM), atomic force microscopy (AFM), powder X-ray diffraction (PXRD), diffuse reflectance spectroscopy, and current-voltage (I-V) conductivity measurements. The results of these experiments confirmed the initial hypothesis that the group IV-VI layered semiconductor chalcogenides (LSC) can be made as nanomaterials using colloidal methods and that they indeed form as 2D nanosheets. In a follow-up study it is shown that the chemistry used for producing 2D nanostrucures of GeS and GeSe could be extended to the synthesis of the tin-based analogues including SnSe. The resulting SnSe nanosheets formed as size-uniform square-like crystallites with lateral dimensions on the order of 0.5 microns and thicknesses that could be controlled between 10-40 nm by modifying the starting tin and selenium reagent concentrations. Time-dependent aliquot studies helped to rationalize this observation and a formation pathway for the nanosheets is proposed. The conclusion of this work provided evidence that nanosheets with controlled thickness can be synthesized using “bottom-up” chemical methods and that potential colloidal nanosheet design strategies may be plausible. Utilizing similar chemical methods, this hypothesis was further confirmed in the synthesis of laterally controlled 2D SnS nanostructures. It was demonstrated that nanosheets of SnS with square, disc-like, hexagonal, and rectangular lateral dimensions on the order of 0.5 microns could be prepared by simply modifying the coordinating ligands used in the starting tin precursor. This is significant in that the edge sites of layered materials, in many cases, are the active sites for catalytic reactions and therefore understanding the parameters leading to lateral control in these 2D nanomaterials may afford better catalysts. Further studies using an iodide-based metal tin precursor (SnI2) revealed that the assembly of 2D SnS nanosheets into 3D hierarchical “flower-like” nanostructures could be achieved. These results provided evidence that not only were nanosheets of controlled thickness obtainable, but by the use of bottom-up colloidal strategies, nanosheets with arbitrarily controlled lateral dimension and hierarchal structure were accessible as well. In collaboration with Dr. Ian Sines, a former member of the Schaak lab, the nanosheet design strategy is further validated with the development of an anion exchange pathway that converts SnSe nanosheets to single crystal NaCl-type SnTe nanosheets. The chemical transformation is accomplished by the reaction of the SnSe nanosheet substrates described in the work above with a novel trioctylphosphine-tellerium anion exchange precursor developed by Dr. Sines. The reaction resulted in a SnTe product with preserved 2D nanosheet morphology, however, in the process acquiring a secondary porous structure. The results of this study showed the ability to convert one 2D nanomaterial into another using a colloidal conversion process and that this preservation of 2D morphological structure (SnSe) can occur in a primarily 3D bonded material (SnTe). In addition, it demonstrates a novel pathway for introducing secondary morphology, such as porosity, into colloidally designed 2D nanostructure templates. Next, it is shown that flat 1D nanostructures of GeSe with rectangular cross-section can be synthesized. Maintaining the planar geometry of the underlying 2D crystal structure, the resulting GeSe nanobelts showed an average diameter of approximately 80 nm, a thickness to width ratio of ∼1:2, and lengths that ranged from approximately 1-25 micrometers. Due to the elongated morphology, individual GeSe nanobelts could be easily aligned between gold electrodes for single particle 2- and 4-point electrical measurements. In addition, an indirect band gap of approximately 1.1 eV was determined by diffuse reflectance spectroscopy measurements. Overall, this work improved upon the colloidal design strategy by extendeding capabilities for the modification of the underlying IV-VI LSC 2D crystal structre to give shape-controlled 1D nanomaterials, in addition to the 2D nanostructures presented above. 0D nanocrystals could also be produced in the IV-VI LSC system using a rapid injection protocol. The as-synthesized SnS nanocrystals were both uniform in size and morphology with average diameters of approximately 10 and 12 nm for the SnS spherical polyhedra and nanocubes, respectively. In collaboration with Adam Biachhi, a colleague in the Schaak lab, it was discovered that a modification in crystal structure occurs for tin(II) sulfide when confined to a 0D nanoparticle geometry. For both spherical and cubic SnS nanocrystals, it is found that the typical orthorhombic GeS-type crystal structure adopted by bulk SnS is distorted to give a slightly modified pseudo-tetragonal polymorph for the SnS nanocrystals. Collectively, these results show that the crystallographic driving force for 2D growth in the layered IV-VI materials can be overcome to give spherical particles and that when confined to this 0D non-planar geometry a distortion in crystal structure occurs. Finally, it is demonstrated that the synthetic methods developed for the synthesis of group IV-VI layered semiconductor nanocrystals can be extended toward the synthesis of novel germanium and Ge-based nanomaterials. In this case, elemental germanium nanoparticles were made in a one-pot reaction under milder conditions than those previously used in the literature and the first colloidal synthesis of hexagonal Fe3Ge2 nanocrystals and monoclinic FeGe nanowires is described. These studies represent a rare example of the synthesis of colloidal Ge and Ge-based nanomaterials and suggest that other Ge-based material systems may be accessible using colloidal strategies.