Mohammad A. Islam

Binder-Free and Carbon-Free Nanoparticle Batteries: A Method for Nanoparticle Electrodes without Polymeric Binders or Carbon Black

In this work, we have developed a new fabrication method for nanoparticle (NP) assemblies for Li-ion battery electrodes that require no additional support or conductive materials such as polymeric binders or carbon black. By eliminating these additives, we are able to improve the battery capacity/weight ratio. The NP film is formed by using electrophoretic deposition (EPD) of colloidally synthesized, monodisperse cobalt NPs that are transformed through the nanoscale Kirkendall effect into hollow Co(3)O(4). EPD forms a network of NPs that are mechanically very robust and electrically connected, enabling them to act as the Li-ion battery anode. The morphology change through cycles indicates stable 5-10 nm NPs form after the first lithiation remained throughout the cycling process. This NP-film battery made without binders and conductive additives shows high gravimetric (>830 mAh/g) and volumetric capacities (>2100 mAh/cm(3)) even after 50 cycles. Because similar films made from drop-casting do not perform well under equal conditions, EPD is seen as the critical step to create good contacts between the particles and electrodes resulting in this significant improvement in battery electrode assembly. This is a promising system for colloidal nanoparticles and a template for investigating the mechanism of lithiation and delithiation of NPs.

Electrodeposition of patterned CdSe nanocrystal films using thermally charged nanocrystals

A dc electric field is used to attract charged CdSe nanocrystals in hexane to rapidly form very smooth, robust, large-area, several micron-thick films of equal thickness on both electrodes. This deposition on both electrodes implies there are both positively and negatively thermally charged dots, unlike conventional electrophoretic deposition. With patterned electrodes, controllable and locally selective assembly is achieved.

Interdot interactions and band gap changes in CdSe nanocrystal arrays at elevated pressure

Three-dimensional arrays of organically passivated CdSe nanocrystals were investigated under hydrostatic pressure using photoluminescence (PL) and absorption spectroscopies. Interdot separations were varied coarsely by varying the organic ligand on the nanocrystal and finely by applying hydrostatic pressure. The PL and absorption spectra of solutions and arrays of CdSe nanocrystals capped by either tri-n-octylphosphine oxide or tri-n-butylphosphine oxide are the same up to 60 kbar, which suggests that they exhibit no interdot coupling since the interdot separations in the solutions (∼50 nm) are much greater than those in the arrays (≲1 nm). While the variation with pressure is roughly that expected from the increase in band gap energy of bulk CdSe with pressure and the increase in confinement energies of electrons and holes with increased pressure, there is still a significant difference in the energy of the PL peak and the first exciton in absorption (the Stokes shift) for both these solutions and arrays...

Surface chemically-switchable ultraviolet luminescence from interfacial two-dimensional electron gas

We report intense, narrow line-width, surface chemisorption-activated and reversible ultraviolet (UV) photoluminescence from radiative recombination of the two-dimensional electron gas (2DEG) with photoexcited holes at LaAlO3/SrTiO3. The switchable luminescence arises from an electron transfer-driven modification of the electronic structure via H-chemisorption onto the AlO2-terminated surface of LaAlO3, at least 2 nm away from the interface. The control of the onset of emission and its intensity are functionalities that go beyond the luminescence of compound semiconductor quantum wells. Connections between reversible chemisorption, fast electron transfer, and quantum-well luminescence suggest a new model for surface chemically reconfigurable solid-state UV optoelectronics and molecular sensing.

Electrophoretic Deposition of CdSe Nanocrystal Films on Conducting Electrodes

A dc electric field is used to attract thermally charged CdSe nanocrystals in solution to rapidly form large-area, micron-thick films of equal thickness on both electrodes. A pair of Au-on-Si or conducting ITO-on-glass electrodes was submerged in the nanoparticle solution and a dc voltage was applied in a dark room. Uniform, robust, very smooth, and apparently identical films formed on both electrodes. Photoluminescence and absorption of the films showed that they are indeed made of dense arrays of individual nanocrystals. The deposition implies there are both positively and negatively thermally charged dots in solution. These high quality dense arrays of the nanoparticles could be useful in several applications.

THz spectroscopy of nanostructures

Terahertz (THz) time-domain spectroscopy provides a powerful tool to determine the frequency-dependent conductivity of materials using propagating electromagnetic waves. The approach permits probing nanostructures and bulk materials that are difficult to contact. We describe recent measurements on photoexcited semiconductor nanoparticles and insulators.

Probing charge transport by optical-pump/THz-probe spectroscopy

We describe recent results in the application of THz time-domain spectroscopy to examine photo-induced conductivity in insulators. These investigations permit us to determine the fundamental interactions governing the observed charge transport. Applications to non-contact probing of conductivity in nanostructured materials, such as semiconductor nanoparticles, will also be highlighted. Full-text article is not available.