Assembly Research - Strouse Group Research

Our effort on engineering next generation nano-material assemblies forms a major component of our research program bridging traditional inorganic and organic materials. The research program has been successful in demonstrating nano-composite structures via ligand templating. We have demonstrated the formation of a macroscopic, 3-dimensional self-supporting composite that are assembled by reaction of the organic passivant layer forming a composite composed of individual gold and CdSe nanomaterials. The structures can be described as flexible networks producing a non-statistical, glassy composite composed of a regular 6:1 CdSe:Au composition. (figure 3)

The mechanism for the formation of the assembly is pH controlled via acid/base equilibrium due to a proton exchange between the R-NH3⁺ group on 2-aminoethanethiol passivated CdSe and the R-COO- group stabilizing the citrate-passivated Au surface. Analysis of SAED (selected area electron diffraction) patterns in the TEM indicates the free standing composite exhibits a surprising degree of short-range ordering in the material. This suggests these “flexible” composite materials may provide a paradigm for the design of electronic networks based on self-assembling lattices using the competition of VdW, steric, and covalent interactions to drive nano-structural motifs.

Development of nano-scale composites may provide potential new systems for a wide range of material applications. Control over the 3-dimensional assembly of these nanoscale composites can be controlled by thermodynamic equilibrium reactions, in essence a competition between the thermodynamic interaction potentials for steric, van der Waals, and covalent energy terms. The magnitude of the covalent energy term contribution should be manipulated by the choice of the terminal functional group on the passivating layer of one nanomaterial and the energetics of coordination on the surface of a second nanomaterial. By application of acid/base equilibrium control, glassy assemblies composed of a 6:1 ratio of two different nanomaterials can be achieved which exhibit short-range order. Further analysis of this mechanism, by control of chain length, terminal functional group, and nanomaterial may allow tailored composite 3-dimensional structures to be designed. (figure 5)

Nanoparticles of coinage metals can be prepared as near monodisperse materials with surface passivating ligands that act to stabilize the surface of the particle, lower surface energy, and maintain the separation distance between individual particles in a self-assembled structure. Extension of our initial studies into Au-CdSe composites, have led us to investigate the onset of electronic coupling in assemblies of Au nanoparticles passivated by bifunctional ligands, in which the ligand acts as a template to control particle separation. The potential to assemble 3-dimensional architectures of metal nanoparticles opens the possibility to both control and probe the onset of correlated electronic and optical properties arising from the quantum mechanical coupling of individual nanoparticle wavefunctions. The onset of collective electronic behavior in these metal nanoparticle assemblies are modulated by the oscillator strength of the oscillating dipole field for the metal nanoparticle, which is size dependent; and the particle separation, which is a function of both the ligand steric effects and the dispersional forces arising from Van der Waals contributions between particles. The steric repulsion energy (E_steric) between two nanoparticles surface stabilized with alkyl ligands scales directly with the particle diameter and the square of the ligand length. The Van der Waals attractive forces (E_vdW) between particles increase with the square of the ratio of the particle radius and center-to-center separation distances.

We have analyzed the size dependent properties of a series of gold nanoparticles assembled into 3-D aggregate structures covalently linked by hetero-functional ligands. We can systematically modulate the separation distances of Au and the overall coupling of Au nanomaterials by use of a hetero-functional ligand. Steric and Van der Waals effects can be assessed by analysis of TEM spacing and FTIR ligand packing analysis. Variation of the optical properties arising from the onset of quantum mechanical coupling is investigated as a function of the ligand length and the nanoparticle core size. With increasing core size and decreasing ligand lengths, EVdW forces tend to lead to particle coalescence resulting in a loss of the surface plasmon resonance (SPR) of the nanoparticles at 525 nm and the emergence of a collective particle plasmon band around 650 nm. At longer ligand lengths and smaller core sizes E_VdW and E_covalent compete to control assembly formation and smaller shifts in the SPR to lower energy are observed as a function of the interparticle spacing. The observation of collective properties opens the potential to design nano-assemblies that respond to changes in particle spacing for applications in nanoelectronics, memory storage devices, nonlinear optics, sensor science, catalysis, and light energy conversion systems.

Read the published articles:
"Synthesis and Characterization of Metal-Semiconductor Nano-composites." Cumberland, S.L.; Strouse, G.F. Mat. Res. Soc. Symp. Proc., 642, J7.8 (2001). [ view article - PDF ]

"Analysis of the Nature of Oxyanion Adsorption on Gold Nanomaterial Surfaces." Cumberland, S.L.; Strouse, G.F. Langmuir, 18, 269-276 (2002).
[ view article - PDF ]

In a recent study in our lab, we have initiated a study of organic polymers as molecular scaffolding for CdSe nano-composites. These polymer based nanocomposites exhibit efficient energy transfer from the organc oligomers to the nanomaterial. Energy transfer appears to be dominated by a dipole-dipole exchange mechanism in which the size dependent nature of this energy transfer process is clearly evident in the loss of quantum efficiency for transfer above 5 nm. The quenching of the polymer by appended nanoscale CdSe via directed energy transfer proceeds with near unit quantum efficiency in the region where the spectral overlap <J-integral> is favorable. At 5 nm the nature of the directed energy transfer from the polymer to CdSe can be modulated by photolysis.

The observation of a photo-initiated switch in the efficiency of energy transfer coupled to recent crystal structures of the oligomers suggest that structural reorientation of the composite may lead to the opening of transverse optical energy transfer along a pi-pi stacking direction. We are developing a series of oligomerically linked (n = 1, 3, 5) systems based on PAO, phenylethynylene, appended to CdSe. The PAO is linked to the CdSe through a benzylic thiol bond. (figure 7) The size dependent nature of energy transfer allows fine control over energy transport from the surface of the CdSe. (figure 8)