Preparation and selective derivatization of azide-terminated self-assembled monolayers on native silicon

This dissertation described progress toward the novel generation of 2 nm surface features in a non-lithographic manner. Our nanopatterning concept arose from a combination of recent scientific and technological advances related to self-assembled monolayers (SAM) on silicon, semiconductor quantum dot (Qdot) photocatalysis, and surface electrophoresis of macromolecules and particles. Briefly, we hypothesized that cadmium selenide (CdSe) Qdots could serve as photocatalytic "pens" to "write" nanopatterns on a reactive, azide-terminated SAM substrate. The CdSe Qdots would be directed about the surface by an electric field of variable strength and direction, while being illuminated with a 10 mW laser that outputted at 407 nm, which would lead to the photocatalytic reduction of the azide to the corresponding amine. The azide groups not in direct contact with the CdSe Qdots would be left untouched. The anticipated end result would be an amine trail with a width similar to the Qdot diameter, 2--3 nm on an azide background. Since it would be difficult for conventional surface analytical tools, such as AFM or STM to detect such small features, 10 nm negatively-charged gold nanoparticles would be used to "develop" the nanopattern in the early stage of the project by selectively binding to the amines, but not to the azides.;The thesis chapters described work toward a demonstration of the nanopatterning concept described above. In Chapter 2, details were given for the synthesis of CdSe quantum dots (Qdots) in water and in organic solvents. When excited by UV-Vis light, Qdots synthesized in organic solvents displayed a strong band gap emission; but fluorescence was quenched when thiol capping ligands were bound to the Qdot surface. To insulate the Qdot core, higher band gap materials were used to synthesize CdSe/ZnSe/ZnS core-shell Qdots, which maintained fluorescence even when capped with thiols in water. Custom-made low band gap dithiocarbamate (DTC) stabilized CdTe Qdots were synthesized for enhancing photovoltaic solar cells. In Chapter 3, the stability of Langmuir films at the air-water interface composed of TOPO-, DTC-, and thiol-capped CdSe Qdots were compared. DTC-capped Qdots appeared to form more stable films than those capped with thiols. In Chapter 4, the preparation of stable azide-terminated SAMs (Az-SAMs) on hydrogen-terminated silicon wafers was described. Az-SAMs were reduced to amine-terminated SAMs with phosphines or with CdSe Qdots after being illuminated with a 407 nm wavelength laser. AuNPs in neutral aqueous solution adsorbed selectively to the surface amines over the azides. In Chapter 5, the electrophoretic mobility of diethylaminoethanethiol (DEAET)-capped CdSe Qdots at pH 7 was nearly size-independent and was valued at approximately 2.0 x 10-4 cm2/V-s, which corresponded to a zeta potential of about 32 mV.