Seeds

Current Seeds:

• Magnetic Imaging: In-situ Devices and Novel Imaging Methods
  John Cumings
• Multiscale Modeling Self-Assembled Monolayers and Organic Donor-Acceptor Interfaces
  Daniel Kosov
• Materials Research for Template-Directed Nanostructure Assembly
  Sang Bok Lee & Gary Rubloff

Previous Seeds (On this page):

• Diffusion & Wettability in Porous Nanoparticles
  Douglas English, Sheryl Ehrman, Lyle Isaacs, Michael Zachariah
• Template Synthesis of Nanowire / Nanotube Heterostructures
  Sang Bok Lee
• Spin and spin coherence dynamics of tunable electrochemically synthesized nanostructures
  Min Ouyang

  Diffusion and Wettability in Porous Nanoparticles

Douglas English, Sheryl Ehrman, Lyle Isaacs, Michael Zachariah

Summary of Goals:

Figure 1. Transmission electron microscope image of porous silica nanoparticles, synthesized via an aero-sol-gel process.

There are a large number of possible applications for nanoparticles composed of porous materials with extremely high internal surface areas. Our MRSEC seed efforts will address fundamental questions whose answers are critical in the realization of potential applications. Porous nanoparticles are valuable candidates for molecular transport, release and sequestration important for applications ranging from highly selective sensing and separation processes to drug delivery. In order to realize these applications, a firm knowledge of several key parameters must be established. Most notably, it will be necessary to have a quantitative understanding of the wettability and diffusion rates in nanoparticle interiors as a function of their surface chemistry and morphology, especially pore size. We will focus our efforts on studies of porous silica particles, developed originally by the Zachariah group (see Figure 1), and we will explore the following areas:

Determine wetting behavior of nanoparticle interior pores as a function of pore surface chemistry and pore size. (Ehrman, English, Zachariah)

These efforts will make use of fluorescence-based studies developed in the English group. We will study the filling of nanoscale pores inside of nanoparticles. Materials will be made by a combination aero-sol-gel process, with average diameters on the order of 100 to 200 nm, and average pore diameters ranging from 2 to 20 nm. Experiments will be conducted to test the wettability as a function of surface chemistry ranging from unmodified silica surfaces to derivatized surfaces. Pore sizes will also be varied via control of the concentration of the pore templating material during synthesis. The wettability and rate of wetting will be measured over a range of solvent mixtures to provide the data necessary for understanding how to transport cargo molecules in and out of porous nanoparticles.

Measure diffusion rates as a function of surface chemistry. (English, Isaacs)

The rate of molecular diffusion inside of porous nanoparticles will determine the rate of release or uptake of cargo. We will use conventional uptake/release studies in conjunction with microscopic single-particle studies to evaluate this behavior. Conventional methods evaluate the change in concentration of the cargo molecule in a supernatant while microscopic techniques measure the diffusion of fluorescent cargo molecules inside of the nanoparticles. Combining these techniques will be useful in correlating uptake or release rates with internal diffusion rates.

Control transport rates through nanoparticle properties (Ehrman, English, Isaacs, Zachariah)

Control of loading or release rates will be an overarching theme of this research since it will be a critical aspect of implementing porous nanoparticles in real-life applications. The ability to control the transport rates of specific chemicals may also convey selectivity. The main approach for controlling transport will be the use of surface modifications. Surface modification can be achieved during synthesis or afterwards by silanation techniques. Surfaces will be derivatized using ligands which bind cargo molecules weakly so that diffusion takes place principally along the pore walls.

Develop strategies for differential functionalization of porous nanoparticles (Ehrman, English, Isaacs, Zachariah)

For applications such as separations and drug delivery, it may be possible to take advantage of the material’s pore structure and surface chemistry to modify the interior and exterior surfaces using different moieties to obtain two distinct functionalities within the same material. For this, we will utilize results from the wetting and diffusion studies to design modification processes leading to bifunctional nanomaterials.

  Template Synthesis of Nanowire / Nanotube Heterostructures

Sang Bok Lee

Modulation of composition in nanowires / nanotube heterostructures has received much attention in the material synthesis field due to its importance for wide device functions and various applications such as electronic and photonic devices. For example, studies have shown that well-defined core/shell nanowires heterostructures could overcome the limitation of the injection currents that conventional nanoscale p-n junctions had.

Figure 1. Schematic of template synthesis procedure. (Image by S.B. Lee, UMD)

Nanowires will be fabricated using template synthesis. The template method (Figure 1) is a versatile nanomaterials synthetic strategy that has been developed and perfected over the past 15 years for the synthesis of nanowires and nanotubes with various materials. Nearly any synthetic strategy that is used to make bulk materials can be adapted to the template method to make nanowires and nanotubes. For instance, Lee has used porous polycarbonate filters, prepared via the track-etch method, and porous alumina films, prepared electrochemically from Al film, as the templates. Pore size and pore length can be generally controlled by electrochemistry in the range of 10 – 250 nm and 0.1 – 100 mm, respectively. Due to the cylindrical pore shape in the film, the nanostructures of the produced material are nanowires, nanorods or hollow nanotubes. Especially, electrochemical deposition in these cylindrical pore is an ideal method for fabricating structurally well-defined heterostructure of nanowires.

Our goal is to construct heterogeneous nanowires containing nanoscale metal-semiconductor and semiconductor-semiconductor interfaces in order to tailor the conducting properties of the nanowires. Metal-semiconductor interfaces will be used to create nanoscale depletion regions, and semiconductor-semiconductor interfaces can be used to create built-in p-n junctions, to act as dopant reservoirs for intrinsic semiconductors, and to effectively shorten device lengths. The following three structures will be synthesized by electrochemical deposition.

• Metal-nanoparticle-doped conducting polymer nanowires and nanotubes: Conducting polymer (PEDOT and P3HT) nanowires and nanotubes will be synthesized and then metal nanoparticles will be doped into the nanowires by a reductive electrochemical reaction.
• Radial heterostructure of nanowires and nanotubes: Very interestingly, Lee has developed a new method for electrochemical synthesis of conducting polymer or metal nanotubes and will extend this method to the synthesis of semiconductor and metal oxide nanotubes. By combining the two synthetic methods of conducting polymer and metal nanotubes, various materials (CdSe, Au, TiO2, polymers) will be electrochemically synthesized as core-shell nanowires or nanotube structures.
• Axial heterostructure of nanowires: Axially-segmented nanowires will also be electrochemically synthesized with two or more different materials, such as conducting polymer-metal-conducting polymer, semiconductor (CdSe)-conducting polymer in the composition of nanowires.

Spin and spin coherence dynamics of tunable electrochemically synthesized nanostructures

Min Ouyang

Zero- and one- dimensional nanostructures, such as semiconductor quantum dots and quantum wires, have arisen significant interest in both fundamental science and technological applications. From device point of view, quantum dots and wires represent smallest building blocks and interconnect components for large-scale functional device assembly. From fundamental science point of view, many exotic physics can be expected from these low dimensional systems compared with their higher dimensional counterparts. For example, quantum confinement introduced in these nanostructures make their electron spin states fairly well protected from the dissipate effects of the environment and can thus enhance corresponding phase coherence over significant larger space and time scales. In this MRSEC seed project we propose to study spin and spin coherence dynamics of zero- and one- dimensional semiconductor nanostructures synthesized with tailored optical, electronic and spin properties. We will focus on exploring spin physics that can not be simply derived from previous studies on higher dimensional systems.