In this thesis, nanostructures and nanomaterials ranging from 3D to 0D will be studied compresively, by using optical methods.

Firstly, for 3D and 2D nanomaterials, nanoporous zeolite crystals, such as AFI and AEL are introduced as host materials to accommodate diatomic iodine molecules. Polarized Raman spectroscopy is utilized to identify the two configurations of iodine molecules to stay in the channels of AEL: the lying mode (the bond of the two atoms is parallel to the direction of the channels) and the standing mode (the bond is perpendicular to the direction of the channels). The lying mode and standing mode are switchable and can be well controlled by the amount of water molecules inside the crystal, revealed by both molecule dynamics simulation and experiment observatio...[ Read more ]

In this thesis, nanostructures and nanomaterials ranging from 3D to 0D will be studied compresively, by using optical methods.

Firstly, for 3D and 2D nanomaterials, nanoporous zeolite crystals, such as AFI and AEL are introduced as host materials to accommodate diatomic iodine molecules. Polarized Raman spectroscopy is utilized to identify the two configurations of iodine molecules to stay in the channels of AEL: the lying mode (the bond of the two atoms is parallel to the direction of the channels) and the standing mode (the bond is perpendicular to the direction of the channels). The lying mode and standing mode are switchable and can be well controlled by the amount of water molecules inside the crystal, revealed by both molecule dynamics simulation and experiment observation. With more water molecules inside, iodine molecules choose to stay in the standing mode, while with less water molecules, iodine molecules prefer to lie along the channel. Therefore, the configurations of molecules could be precisely controlled, globally by the surrounding pressure and temperature, and locally by the laser light. Ii is believed that this easy and reversible control of single molecule will be valuable in nanostructured devices, such as molecular sieving or molecular detection. When the iodine molecules inside the crystal can be fixed to certain plane, it could be regarded as the 2D case. Here we take into consideration the standing mode of the systems mentioned above. In AFI, the molecules are able to rotate in the plane parallel to the cross section, since the channel is a ring shape and larger than the dimension of the molecule; while in AEL, with the channel in a elliptical shape, the molecule is fixed in the configuration parallel to the long axis of the elliptical channel. By studying the vibrational behavior of the iodine molecules through Raman spectroscopy, we find that in AEL, where the molecules are more spatially confined, the vibrational behavior is close to a harmonic oscillator, rather than a Morse oscillator as in free space. While in AFI, the molecules behave like in the free space but only with a loosened spring constant. The temperature dependent Raman spectrum validates our observations. To be more specific, the width of Raman degeneration peak of I₂@AEL will not change with temperature since the vibrational energy are equally spaced, whereas the width of I₂@AFI shows a broadening with increasing temperature, as the molecules occupy non-equally spaced vibrational levels following the Boltzmann distribution. Meanwhile, due to the confinement of the nanochannels, vibration to higher level is forbidden if the vibrational amplitude is larger than the dimension of the channels. Thus, by identifying the last observable vibrational mode of the confined molecules, with knowing the exact dimensions of the channels, we can precisely measure the atomic size of iodine molecules, which is far beyond the resolution limit of optical spectroscopy. The size of atom obtained by this method agrees well with that from the literatures. This technique provides a novel and convenient way to determine the size of atoms with high precision. It may suggest a new way of studying the molecular dynamics through optical method.

Secondly, for 1D case, the PL spectrum of ZnO nanowire under uniaxial strain is studied. When a ZnO nanowire is bent, besides the lattice constant induced bandgap change on the tensile and compressive sides, there is a piezoelectric field generated along the cross section. This piezoelectric potential, together with the bandgap changes induced by the deformation, will redistribute the electrons excited by incident photons from valence band to conduction band. As a result, the electrons occupying the states at the tensile side will largely outnumbered the ones at the compressive side. Therefore, the PL spectrum we collected at the whole cross section will manifest a redshift, other than the peak broadening which is caused by the bandgap change. The experimental results confirm our speculation. When we make the nanowire straight again, the redshift disappears. It is believed that this piezoelectric effect is very important to the application of nanowires, and it would benefit the actual design and fabrication for the electronic devices for the next generation.

Lastly, as for the 0D case, the charge transfer mechanism occurring at the interface between graphene and ZnO QDs is investigated. We fabricate a hybrid structure by placing ZnO QDs on top of graphene. With UV light illumination on this device, it will generate electron-hole pairs inside QDs. Before they recombine, the holes will be separated and trapped into the surface states, and discharge the oxygen ions adsorbed on the surface of QDs. The unpaired electrons are then transferred to the graphene layer with a relative long lifetime. After the UV light is switched off, the oxygen molecules will re-adsorb to the QDs surface, capture electrons and recover the graphene’s transport properties. Therefore, this hybrid device shows an ultrasensitive response to on-off of the UV laser, with a photoconductive gain as high as 10⁷, which can be utilized for practical graphene-based UV sensors and detectors with very high responsivity. This gain can be further enhanced by another 2-3 orders by increasing source-drain voltage, shortening the sample’s length, etc.

It is believed that optical spectroscopy provides a convenient, efficient and useful method to study the nanomaterials and nanostructures. It is easy to set up, has no harm or degradation to the sample, and could go beyond the diffraction limit. With appropriate design and creative ideas, optical spectroscopy can be further explored, and will boost the development of nanoscience and technology.