Electrical And Optical Studies Of Nanoscale Structures

This dissertation focuses on developing new fabrication techniques for fabricating nano-scale devices and studying their emerging properties, especially their coupling to magnetic field and light. We have fabricated ultrasmall ferromagnetic electrodes and studied their magnetoresistance at low temperature. The magnetoresistance fluctuations are enhanced compared to those of the bulk magnetic material, and can be described with a quantum interference theory. A temperature dependence study confirms this interpretation. During magnetoresistance measurement we also observed atomic motion induced two-level conductance fluctuations in ferromgnetic nanoconstrictions. We have developed a self-aligned technique to fabricate gold electrodes with a nanometer scale gap, and used a graphene nanoconstriction as a detector to read out the plasmon resonance of the gold electrodes using a photocurrent measurement. We have fabricated dual-gate graphene devices and used them to perform steady state and time-domain photocurrent studies of graphene PN junctions. The gate dependence study reveals hot carrier transport in the photocurrent response, while the time-resolved photocurrent study provides information about carrier dynamics

Electrical Control of Plasmon Resonance with Graphene

Surface plasmon, with its unique capability to concentrate light into sub-wavelength volume, has enabled great advances in photon science, ranging from nano-antenna and single-molecule Raman scattering to plasmonic waveguide and metamaterials. In many applications it is desirable to control the surface plasmon resonance in situ with electric field. Graphene, with its unique tunable optical properties, provides an ideal material to integrate with nanometallic structures for realizing such control. Here we demonstrate effective modulation of the plasmon resonance in a model system composed of hybrid graphene-gold nanorod structure. Upon electrical gating the strong optical transitions in graphene can be switched on and off, which leads to significant modulation of both the resonance frequency and quality factor of plasmon resonance in gold nanorods. Hybrid graphene-nanometallic structures, as exemplified by this combination of graphene and gold nanorod, provide a general and powerful way for electrical control of plasmon resonances. It holds promise for novel active optical devices and plasmonic circuits at the deep subwavelength scale

Identification of Candidate Genes for the Plateau Adaptation of a Tibetan Amphipod, Gammarus lacustris, Through Integration of Genome and Transcriptome Sequencing

The amphipod Gammarus lacustris has been distributing in the Tibetan region with well-known uplifts of the Tibetan plateau. It is hence considered as a good model for investigating stress adaptations of the plateau. Here, we sequenced the whole-genome and full-length transcriptome of G. lacustris, and compared the transcriptome results with its counterpart Gammarus pisinnus from a nearby plain. Our main goal was to provide a genomic resource for investigation of genetic mechanisms, by which G. lacustris adapted to living on the plateau. The final draft genome assembly of G. lacustris was 5.07 gigabases (Gb), and it contained 443,304 scaffolds (>2 kb) with an N50 of 2,578 bp. A total of 8,858 unigenes were predicted in the full-length transcriptome of G. lacustris, with an average gene length of 1,811 bp. Compared with the G. pisinnus transcriptome, 2,672 differentially expressed genes (DEGs) were up-regulated and 2,881 DEGs were down-regulated in the G. lacustris transcriptome. Along with these critical DEGs, several enriched metabolic pathways, such as oxidative phosphorylation, ribosome, cell energy homeostasis, glycolysis and gluconeogenesis, were predicted to play essential roles in the plateau adaptation. In summary, the present study provides a genomic basis for understanding the plateau adaption of G. lacustris, which lays a fundamental basis for further biological and ecological studies on other resident aquatic species in the Tibetan plateau

A two-step dry process for Cs2SnI6 perovskite thin film

A two-step process for synthesizing stable Cs2SnI6 perovskite thin films is reported in this letter. The two-step process includes the co-evaporation of two precursors SnI2 and CsI onto a glass substrate, followed by a post thermal annealing process in iodine vapor. Using this technique, pure Cs2SnI6 perovskite thin films were successfully synthesized without any wet process. These perovskite thin films are found to be stable under ambient conditions. They also show an electron mobility up to 509 cm2 V−1 s−1, which is higher than the mobilities of films prepared by solution processes reported in the literature

Probing Local Strain at MX₂–Metal Boundaries with Surface Plasmon-Enhanced Raman Scattering

Interactions between metal and atomically thin two-dimensional (2D) materials can exhibit interesting physical behaviors that are of both fundamental interests and technological importance. In addition to forming a metal–semiconductor Schottky junction that is critical for electrical transport, metal deposited on 2D layered materials can also generate a local mechanical strain. We investigate the local strain at the boundaries between metal (Ag, Au) nanoparticles and MX₂ (M = Mo, W; X = S) layers by exploiting the strong local field enhancement at the boundary in surface plasmon-enhanced Raman scattering (SERS). We show that the local mechanical strain splits both the in-plane vibration mode E_2g¹ and the out-of-plane vibration mode A_1g in monolayer MoS₂, and activates the in-plane mode E_1g that is normally forbidden in backscattering Raman process. In comparison, the effects of mechanical strain in thicker MoS₂ layers are significantly weaker. We also observe that photoluminescence from the indirect bandgap transition (when the number of layers is ≥2) is quenched with the metal deposition, while a softened and broadened shoulder peak emerges close to the original direct-bandgap transition because of the mechanical strain. The strain at metal–MX₂ boundaries, which locally modifies the electronic and phonon structures of MX₂, can have important effects on electrical transport through the metal–MX₂ contact

Electrical Control of Optical Plasmon Resonance with Graphene

Surface plasmon has the unique capability to concentrate light into subwavelength volume.− Active plasmon devices using electrostatic gating can enable flexible control of the plasmon excitations, which has been demonstrated recently in terahertz plasmonic structures.− Controlling plasmon resonance at optical frequencies, however, remains a significant challenge because gate-induced free electrons have very weak responses at optical frequencies. Here we achieve efficient control of near-infrared plasmon resonance in a hybrid graphene-gold nanorod system. Exploiting the uniquely strong, and gate-tunable optical transitions, of graphene, we are able to significantly modulate both the resonance frequency and quality factor of gold nanorod plasmon. Our analysis shows that the plasmon–graphene coupling is remarkably strong: even a single electron in graphene at the plasmonic hotspot could have an observable effect on plasmon scattering intensity. Such hybrid graphene–nanometallic structure provides a powerful way for electrical control of plasmon resonances at optical frequencies and could enable novel plasmonic sensing down to single charge transfer events