Honglae Sohn

Observation of Negative Charge Trapping and Investigation of Its Physicochemical Origin in Newly Synthesized Poly(tetraphenyl) silole Siloxane Thin Films

A new kind of organic-inorganic hybrid polymer, poly(tetraphenyl)silole siloxane, was invented and synthesized for realization of its unique charge trap properties. The organic portions consisting of (tetraphenyl)silole rings were responsible for negative charge trapping, while the Si-O-Si inorganic linkages provided the intrachain energy barrier for controlling electron transport. The polysilole siloxane dielectric thin films were fabricated by spin-coating and curing of the polymers, followed by characterization with spectroscopic ellipsometry (SE), near edge X-ray absorption fine structure spectroscopy (NEXAFS), and photoemission spectroscopy (PES). The abrupt increase in density and decrease in thickness of the thin film at a curing temperature of 100 °C was attributed to a thermodynamically preferred state in the nanoscopic arrangement of the polymer chains; this was due to cofacial π-π interactions in a skewed manner between peripheral phenyl groups of the (tetraphenyl)silole rings of the adjacent polymer chains. Using the NEXAFS spectrum to assess high electron affinity, the LUMO energy level of the dielectric thin film cured at 150 °C was positioned 1 eV above the Fermi energy level (E(F)). The electron trapping of the dielectric thin films was confirmed from the positive flat band shift (ΔV(FB)) in the capacitance-voltage (C-V) measurements performed within the metal-insulator-semiconductor (MIS) device structure, which strongly verified the polymer design concept. From the simple kinetics model of the electron transport, it was proposed that the flat band shift (ΔV(FB)) or trap density of the negative charges (|ρ|) was logarithmically proportional to the decay constant (β) for the electron-tunneling process. When a phenyl group of a silole ring in a polymer chain was inserted into the two available phenyl groups of another silole ring in another polymer chain, the electron transfer between the groups was enhanced, decreasing the trap density of the negative charges (|ρ|). For the thermodynamically preferred state generating the high refractive index, the distance between the two phenyl groups of the adjacent polymer chains was estimated to be in the range of 0.27-0.36 nm.

Tuning of refractive indices and optical band gaps in oxidized silicon quantum dot solids.

This laboratory has initiated compelling research into silicon quantum dot (Si QD) solids in order to utilize their synergetic benefits with quantum dot solids through fabrication of Si QD thin films. The issues of oxidation concerning the Si QD thin films were confirmed using Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The refractive index value of the Si QD thin film at a 30 degrees C curing temperature was 1.61 and 1.45 at 800 degrees C due to complete oxidation of the Si phases. The optical band gap values of 5.49-5.90 eV corresponded to Si phases with diameters between 0.82 and 0.74 nm, dispersed throughout the oxidized Si QD thin films and modeled by Si molecular clusters of approximately 14 silicon atoms. The photoluminescence (PL) energy (2.64-2.61 eV) in the proposed Si QD thin films likely originated from the Si horizontal lineO bond terminating the Si molecular clusters.

Chemical and Biological Sensors Based on Porous Silicon Nanomaterials

The porous silicon (PSi) material, an electrochemical derivative of silicon, is a natural nano-structured material that can be prepared easily without much sophistication. The history of PSi research is quite remarkable. This material was accidentally discovered by Ulhir at Bell Labs, USA, in 1956[1], followed by Turner[2] during a study on the electropolishing of silicon in hydrofluoric acid solution. After the discovery, only a small amount of interest in the field developed as a result of the poor understanding of the porous structure. In the 1970’s, it was found that by thermal oxidation the porous structure could be easily transformed into silicon dioxide and used as an isolation dielectric material. [3,4] Further advances in electronic isolation technology by Japanese groups in the 1980’s led to the development of full isolated porous oxidized silicon (FIPOS)[5] and the silicon-on-insulator (SOI)[6] process.

Photoluminescence Enhancement of Silole-Capped Silicon Quantum Dots Based on Förster Resonance Energy Transfer.

Photoluminescent porous silicon were prepared by an electrochemical etch of n-type silicon under the illumination with a 300 W tungsten filament bulb for the duration of etch. The red photoluminescence emitting at 650 nm with an excitation wavelength of 450 nm is due to the quantum confinement of silicon quantum dots in porous silicon. HO-terminated red luminescent PS was obtained by an electrochemical treatment of fresh PS with the current of 150 mA for 60 seconds in water and sodium chloride. As-prepared PS was sonicated, fractured, and centrifuged in toluene solution to obtain photoluminescence silicon quantum dots. Dichlorotetraphenylsilole exhibiting an emission band at 520 nm was reacted with HO-terminated silicon quantum dots to give a silole-capped silicon quantum dots. The optical characterization of silole-derivatized silicon quantum dots was investigated by UV-vis and fluorescence spectrometer. The fluorescence emission efficiency of silole-capped silicon quantum dots was increased by about 2.5 times due to F6rster resonance energy transfer from silole moiety to silicon quantum dots.