SYNTHESIS AND SPECTROSCOPIC CHARACTERIZATION OF NANOSTRUCTURED THERMOELECTRIC MATERIALS

Bismuth in the bulk form is a semimetal with a rhombohedral structure. It has a small band overlap between the conduction and valence bands and a highly anisotropic electron effective-mass tensor. Thermoelectric materials, in which one of the three dimensions is in the nanometer regime, exhibit unique quantum confinement properties and have generated much interest in recent years. Theoretical investigations have suggested that nanowires with diameters ≤ 10 nm will possess a figure-of-merit ZT \u3e 2. Prior to this study, it has been shown that Bi nanowires with small enough diameters (~10 nm), prepared via the pulsed laser vaporization method, undergo a transition from a semimetal with a small band overlap to a semiconductor with a small indirect band gap. Infrared absorption and UV-visible measurements were used to confirm this semimetal-to-semiconductor phase transition. In this thesis, we report the synthesis and optical characteristics of a variety of various potential thermoelectric materials including bismuth, nickel sulfide and cadmium sulfide. The infrared absorption in our Bi nanorods is blue-shifted in energy when compared to the corresponding spectra in bulk Bi, and when cooled down to liquid nitrogen temperatures, group theory suggests a strong temperature dependence in the Bi band structure. We also find that the Bi nanorod suspension displays excellent optical limiting properties at both 532 and 1064 nm excitations in the nanosecond laser pulse regime. We have also synthesized nickel sulfide nanoparticles with an average size of 5 nm by a one-step solid phase reaction. The intensity-dependent nonlinear transmission study was carried out using a 7 ns Nd:YAG laser at 532nm using Z-scan, and the nonlinear scattering was found to be the dominant mechanism for the observed response. Importantly, the modified Z-scan method allowed us to measure two competing mechanisms simultaneously - the optical limiting and saturable absorption in surface-modified nickel sulfide nanoparticles suspensions

Laser-assisted synthesis and optical properties of bismuth nanorods

Bismuth in the bulk form is a semimetal with a rhombohedral structure. It has a small band overlap between the conduction and valence bands and a highly anisotropic electron effective-mass tensor. Thermoelectric materials, in which one of the three dimensions is in the nanometer regime, exhibit unique quantum confinement properties and have generated much interest in recent years. Theoretical investigations have suggested that nanowires with diameters \u3c10 nm will possess a figure-of-merit ZT\u3e2. Prior to this research, it has been shown by the Dresselhaus group at MIT that Bi nanowires with small enough diameters (~50 nm), prepared via the template-method, undergo a transition from a semimetal with a small band overlap to a semiconductor with a small indirect band gap. Infrared absorption, temperature-dependent electrical resistance and magneto-resistance measurements were used to confirm this semimetal-to-semiconductor phase transition. In this thesis, we report the synthesis of ~10 nm diameter Bi nanorods using a pulsed laser vaporization method that was previously developed for preparing single-wall carbon nanotubes. The high resolution transmission electron microscopy images of our Bi nanorods show a crystalline Bi core oriented along \u3c012\u3e direction, and coated with a thin amorphous bismuth oxide layer. The infrared absorption and the surface plasmon peaks in our Bi nanorods are blue-shifted in energy when compared to the corresponding spectra in bulk Bi, and relative to those reported by the Dresselhaus group in 45 - 200 nm diameter Bi nanowires

Substrate patterning by electron emission-induced displacement

Disclosed are methods and devices for patterning micro- and/or nano-sized pattern elements on a substrate using field emitted electrons from an element. Disclosed methods and devices can also be utilized to form nano- and micron-sized depressions in a substrate according to a more economical process than as has been utilized in the past. Methods include single-step methods by which structures can be simultaneously created and located at desired locations on a substrate. Methods include the application of a bias voltage between a probe tip and a substrate held at a relatively close gap distance. The applied voltage can promote current flow between the probe and the substrate via field emissions. During a voltage pulse, and within predetermined energy levels and tip-to-surface gap distances, three dimensional formations can be developed on the substrate surface

SYNTHESIS AND SPECTROSCOPIC CHARACTERIZATION OF NANOSTRUCTURED THERMOELECTRIC MATERIALS

Bismuth in the bulk form is a semimetal with a rhombohedral structure. It has a small band overlap between the conduction and valence bands and a highly anisotropic electron effective-mass tensor. Thermoelectric materials, in which one of the three dimensions is in the nanometer regime, exhibit unique quantum confinement properties and have generated much interest in recent years. Theoretical investigations have suggested that nanowires with diameters ≤ 10 nm will possess a figure-of-merit ZT \u3e 2. Prior to this study, it has been shown that Bi nanowires with small enough diameters (~10 nm), prepared via the pulsed laser vaporization method, undergo a transition from a semimetal with a small band overlap to a semiconductor with a small indirect band gap. Infrared absorption and UV-visible measurements were used to confirm this semimetal-to-semiconductor phase transition. In this thesis, we report the synthesis and optical characteristics of a variety of various potential thermoelectric materials including bismuth, nickel sulfide and cadmium sulfide. The infrared absorption in our Bi nanorods is blue-shifted in energy when compared to the corresponding spectra in bulk Bi, and when cooled down to liquid nitrogen temperatures, group theory suggests a strong temperature dependence in the Bi band structure. We also find that the Bi nanorod suspension displays excellent optical limiting properties at both 532 and 1064 nm excitations in the nanosecond laser pulse regime. We have also synthesized nickel sulfide nanoparticles with an average size of 5 nm by a one-step solid phase reaction. The intensity-dependent nonlinear transmission study was carried out using a 7 ns Nd:YAG laser at 532nm using Z-scan, and the nonlinear scattering was found to be the dominant mechanism for the observed response. Importantly, the modified Z-scan method allowed us to measure two competing mechanisms simultaneously - the optical limiting and saturable absorption in surface-modified nickel sulfide nanoparticles suspensions

CLOCK and NPAS2 have overlapping roles in the suprachiasmatic circadian clock

Heterodimers of CLOCK and BMAL1, bHLH-PAS transcription factors, are believed to be the major transcriptional regulators of the circadian clock mechanism in mammals. However, a recent study shows that CLOCK-deficient mice continue to exhibit robust behavioral and molecular rhythms. Here we report that the transcription factor NPAS2 (MOP4) is able to functionally substitute for CLOCK in the master brain clock in mice to regulate circadian rhythmicity

Peripheral circadian oscillators require CLOCK

SummaryIn mammals, the circadian system is hierarchical — a brain pacemaker located within the suprachiasmatic nucleus (SCN) is responsible for regulating locomotor activity rhythms and for synchronizing peripheral oscillators [1,2]. Recent genetic evidence in mice indicates that the bHLH transcription factors CLOCK and NPAS2 have partially redundant functions within the SCN [3,4]. To further examine the roles of CLOCK and NPAS2, we generated CLOCK-deficient (Clock−/−), NPAS2-deficient (Npas2−/−) and double-mutant (Clock−/−;Npas2−/−) mice carrying the mPer2Luciferase reporter gene [5]. We monitored the bioluminescence rhythms of tissue explants in culture and found that while CLOCK or NPAS2 is able to maintain SCN bioluminescence rhythmicity (Supplemental Data) [4], peripheral oscillators are arrhythmic without CLOCK. Thus, there are fundamental differences between the clock machinery of different tissues