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Nano-optical Spectroscopy of Low Dimensional Semiconductor

  • Author(s): Bao, Wei
  • Advisor(s): Salmeron, Miquel B;
  • Xu, Ting
  • et al.

The necessity to push the spatial resolution of optical microscopy and spectroscopy beyond the diffraction limit has been of high interest for almost three decades starting with the idea of using an aperture smaller than the diffraction limit by Ash and Nicholls (Nature 237, 510 – 512) and first examples on nano spectroscopy by Betzig and Trautman (Science 257, 189-195), who advertised: “two of the most exciting possibilities are localized optical spectroscopy of semiconductors and fluorescence imaging of living cells”. However, albeit its enormous potential for the advancement of nano science to study at the critical length scales physical and chemical properties of nano materials that can be accessed only optically, nano optics has developed only a niche existence. The reasons are many limitations of present nano optics, which advanced specific aspects e.g. high local field intensity via the concept of optical antennae (Science 308, 1607-1609) but with major trade offs such as lack of band width, background of diffraction limited light or intrinsic geometries that enable only the study of e.g. monolayers of molecules squeezed between metal substrate and a metal tip.

Here we present a wildly applicable solution to the nanoscale spectroscopy problem with the concept of a far-field to near field optical transformer that does not require the trade offs made in the past and combines record near field enhancement, enormous bandwidth, background free and complete sample independence to perform nano scale optical spectroscopy. The “campanile” transformer is the missing element that enables to perform the whole bandwidth of optical spectroscopy modalities.

In the first part of this thesis, the finite element method is used compare the properties of this “campanile” structure with conventional aperture and apertureless NSOM tips, as well as state-of-the-art adiabatic-compression-type probes. These benchmarks elucidate a number of advantages of the campanile design, showing that its unique characteristics are crucial for optical techniques such as nano-Raman and nano-IR spectroscopy and nano-photoluminescence studies.

In the second part of the thesis, we have experimentally used the campanile transformer to perform indeed local optical spectroscopy of semiconducting Indium Phosphite nanowires (InP NW),1D semiconductor, taking advantage of enhancement, bandwidth as well as the ability to excite and collect through the campanile, to show the influence of trap states on the local excitation energy and charge recombination rate. InP NWs have fascinating opto-electronic properties (Science 293, 1455-1457) and are expected to be the functional elements of next generation opto-electronic devices. However, many of the observed optical phenomena in nanowire systems are not understood due to the lack of spatial resolution. This work provides the necessary insight to start understanding the optical properties of nanowire and nano crystals systems. We demonstrate how the concept of optical campanile transformers convert bi-directional light with high efficiency between far and near field over a bandwidth spanning the visible to the near IR. Utilizing the campanile to perform hyperspectral nano optical spectroscopy on InP NWs revealed strong heterogeneity of the local photoluminescence, both in local intensity and spectral response, along individual NWs, due to the local influence of trap states.

In the last part of the thesis, we present the first nano-optical investigation of 2D transition metal dichalcogenides (TMDCs). Establishing a breakthrough solution to the “nanospectroscopy imaging” problem for these materials, we cross the boundary from insufficient to sufficient optical spatial resolution, mapping critical optoelectronic properties at their native length scales. In doing so, we uncover new optoelectronic regions and spatially-varying features in CVD-grown MoS2 that were hidden in prior optical studies. We discover an unexpected edge region in synthetic MoS2 (~300 nm wide) that acts as a collection of disordered states effectively localizing carriers and excitons. Moreover, we show that significant nanoscale optoelectronic heterogeneity is present even within more “conventional” regions, and directly visualize the optoelectronic effects of key features such defects and edges – highly-soughtafter information that was unobtainable previously. By revealing key structure-function relationships at the proper length scales, these findings directly impact nearly all anticipated atomically-thin device technologies including novel quantum-optical circuitry, bio sensors and valley-based electronics.

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