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Open Access Publications from the University of California

Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2014)

Recent advances in material science and fabrication techniques enabled development of nanoscale applications and devices with superior performances and high degree of integration. Exotic physics also emerges at nanoscale where confinement of electrons and phonons leads to drastically different behavior from those in the bulk materials. It is therefore rewarding and interesting to investigate and understand material properties at the nanoscale. Optical spectroscopy, one of the most versatile techniques for studying material properties and light-matter interactions, can provide new insights into the nanomaterials. In this thesis, I explore advanced laser spectroscopic techniques to probe a variety of different nanoscale phenomena.

A powerful tool in nanoscience and engineering is scanning tunneling microscopy (STM). Its capability in atomic resolution imaging and spectroscopy unveiled the mystical quantum world of atoms and molecules. However identification of molecular species under investigation is one of the limiting functionalities of the STM. To address this need, we take advantage of the molecular `fingerprints' - vibrational spectroscopy, by combining an infrared light sources with scanning tunneling microscopy. In order to map out sharp molecular resonances, an infrared continuous wave broadly tunable optical parametric oscillator was developed with mode-hop free fine tuning capabilities. We then combine this laser with STM by shooting the beam onto the STM substrate with sub-monolayer diamondoids deposition. Thermal expansion of the substrate is detected by the ultrasensitive tunneling current when infrared frequency is tuned across the molecular vibrational range. Molecular vibrational spectroscopy could be obtained by recording the thermal expansion as a function of the excitation wavelength.

Another interesting field of the nanoscience is carbon nanotube, an ideal model of one dimensional physics and applications. Due to the small light absorption with nanometer size, individual carbon nanotube is not visible under any conventional microscopy and characterization of individual nanotube becomes a focused research interest. Although electron microscopies and optical spectroscopies are developed previously to study carbon nanotubes, none of them permitted versatile imaging and spectroscopy of individual nanotube in a non-invasive, high throughput and ambient way. In this thesis a new polarization-based optical microscopy and spectroscopy is developed with exceedingly better contrast for one dimensional nano-materials and capability of individual carbon nanotube imaging and spectroscopy. This development provides a reliable way to measure the absolute absorption cross-section of individual chirality-defined carbon nanotubes. It also enables fast profiling for growth optimization and in situ characterization for functioning carbon nanotube devices.

Two dimensional systems constitute another important family of nanomaterials, ranging from semi-metal (graphene), semiconductors (transition metal dichalcogenides) to insulators (h-BN). Despite of their scientific significance, they present a complete set of 2D building blocks for two dimensional electronics and optoelectronics. Heterostructures purely made of 2D thin films hold great promises due to functionality, scalability and ultrathin nature. Understanding the properties of the coupled heterolayers will be important and intriguing for these applications. With the advanced ultrafast laser spectroscopy, we study the dynamics of charge transfer process in two dimensional atomically thin semiconductors heterostructures. An extremely efficient charge transfer process is identified in atomically thin MoS2/WS2 system, which is expected to form a type-II heterojunction. Our discovery would greatly facilitate further studies of 2D materials as a photovoltaic device.

    Cover page: Optical Spectroscopy at the Nanoscale
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2015)
    © 2015 Nature Publishing Group Surface plasmons, collective oscillations of conduction electrons, hold great promise for the nanoscale integration of photonics and electronics. However, nanophotonic circuits based on plasmons have been significantly hampered by the difficulty in achieving broadband plasmonic waveguides that simultaneously exhibit strong spatial confinement, a high quality factor and low dispersion. Quantum plasmons, where the quantum mechanical effects of electrons play a dominant role, such as plasmons in very small metal nanoparticles and plasmons affected by tunnelling effects, can lead to novel plasmonic phenomena in nanostructures. Here, we show that a Luttinger liquid of one-dimensional Dirac electrons in carbon nanotubes exhibits quantum plasmons that behave qualitatively differently from classical plasmon excitations. The Luttinger-liquid plasmons propagate at ‘quantized’ velocities that are independent of carrier concentration or excitation wavelength, and simultaneously exhibit extraordinary spatial confinement and high quality factor. Such Luttinger-liquid plasmons could enable novel low-loss plasmonic circuits for the subwavelength manipulation of light.
      Cover page: Observation of a Luttinger-liquid plasmon in metallic single-walled carbon nanotubes
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2014)
      Optical absorption is the most fundamental optical property characterizing light-matter interactions in materials and can be most readily compared with theoretical predictions. However, determination of optical absorption cross-section of individual nanostructures is experimentally challenging due to the small extinction signal using conventional transmission measurements. Recently, dramatic increase of optical contrast from individual carbon nanotubes has been successfully achieved with a polarization-based homodyne microscope, where the scattered light wave from the nanostructure interferes with the optimized reference signal (the reflected/transmitted light). Here we demonstrate high-sensitivity absorption spectroscopy for individual single-walled carbon nanotubes by combining the polarization-based homodyne technique with broadband supercontinuum excitation in transmission configuration. To our knowledge, this is the first time that high-throughput and quantitative determination of nanotube absorption cross-section over broad spectral range at the single-tube level was performed for more than 50 individual chirality-defined single-walled nanotubes. Our data reveal chirality-dependent behaviors of exciton resonances in carbon nanotubes, where the exciton oscillator strength exhibits a universal scaling law with the nanotube diameter and the transition order. The exciton linewidth (characterizing the exciton lifetime) varies strongly in different nanotubes, and on average it increases linearly with the transition energy. In addition, we establish an empirical formula by extrapolating our data to predict the absorption cross-section spectrum for any given nanotube. The quantitative information of absorption cross-section in a broad spectral range and all nanotube species not only provides new insight into the unique photophysics in one-dimensional carbon nanotubes, but also enables absolute determination of optical quantum efficiencies in important photoluminescence and photovoltaic processes.
        Cover page: Systematic determination of absolute absorption cross-section of individual carbon nanotubes
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2014)
        Atomically thin two-dimensional (2D) layered materials, including graphene, boron nitride, and transition metal dichalcogenides (TMDs), can exhibit novel phenomena distinct from their bulk counterparts and hold great promise for novel electronic and optoelectronic applications. Controlled growth of such 2D materials with different thickness, composition, and symmetry are of central importance to realize their potential. In particular, the ability to control the symmetry of TMD layers is highly desirable because breaking the inversion symmetry can lead to intriguing valley physics, nonlinear optical properties, and piezoelectric responses. Here we report the first chemical vapor deposition (CVD) growth of spirals of layered MoS2 with atomically thin helical periodicity, which exhibits a chiral structure and breaks the three-dimensional (3D) inversion symmetry explicitly. The spirals composed of tens of connected MoS2 layers with decreasing areas: each basal plane has a triangular shape and shrinks gradually to the summit when spiraling up. All the layers in the spiral assume an AA lattice stacking, which is in contrast to the centrosymmetric AB stacking in natural MoS2 crystals. We show that the noncentrosymmetric MoS2 spiral leads to a strong bulk second-order optical nonlinearity. In addition, we found that the growth of spirals involves a dislocation mechanism, which can be generally applicable to other 2D TMD materials.
          Cover page: Three-Dimensional Spirals of Atomic Layered MoS2
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