Graduation Year
2022
Document Type
Dissertation
Degree
Ph.D.
Degree Name
Doctor of Philosophy (Ph.D.)
Degree Granting Department
Physics
Major Professor
Dmitri Voronine, Ph.D.
Committee Member
Humberto Rodriguez Gutierrez, Ph.D.
Committee Member
Andreas Muller, Ph.D.
Committee Member
Yusuf Emirov, Ph.D.
Keywords
Quantum plasmonics, Surface-enhanced Raman spectroscopy, Tip-enhanced photoluminescence
Abstract
Chemical vapor deposition (CVD) growth of atomically thin 2D materials, such as transition metal dichalcogenides (TMDs), is a complex process that has not been completely understood. Large-scale growth of monolayer TMDs often leads to polycrystalline films with heterogeneous morphological and optical properties. Previous optical studies of 2D materials by far-field photoluminescence (PL) provided insights into CVD growth mechanisms of simple crystals, which were, however, limited in spatial resolution due to the diffraction limit. Here we performed correlated morphological and tip-enhanced PL (TEPL) imaging of CVD-grown polycrystalline monolayer molybdenum diselenide (MoSe2) flakes with heterogeneous optical response. We observed nanoscale spatial variations of optical band gap due to growth-induced thermal strain, revealing the role of aligned particles (APs) at crystal edges and grain boundaries (GBs) in thermal strain relaxation. TEPL imaging showed the strain-free near-field PL at GBs, revealing the connection between MoSe2 and APs. These results may be used to improve nanoscale bandgap engineering techniques, leading to optimized crystal growth and high-performance optoelectronic nanodevices.
Surface-enhanced Raman scattering (SERS) is based on the ability of a surface substrate to increase Raman signals for sensing and imaging applications. The most widely used Au and Ag SERS substrates are primarily based on the electromagnetic mechanism (EM) with large enhancement factors (EF), which are, however, limited by small gaps due to tunneling. Graphene has been explored as an alternative substrate for graphene-enhanced Raman scattering based on the chemical mechanism (CM) which can be achieved by choosing the right excitation matching the energy of the GO-CNT complex. CM enhancement is the dominant mechanism in non-plasmonic systems, which have advantages over traditional plasmonic substrates. However, the limits of CM EFs in graphene-based substrates have not been well understood, especially as a function of tip-sample distance (TSD). Here we performed tip-enhanced Raman scattering of carbon nanotubes on Au and graphene-oxide (GO) hybrid substrates for different TSDs. We show evidence of quantum plasmonics with GO as a tunneling junction in an Au-GO-Au cavity with a 2 nm gap size. We demonstrate Raman signal enhancement by 4 orders of magnitude beyond the tunneling limit for the resonant GO excitation at small TSD. Tip-induced GO-enhanced Raman scattering (TIGERS) may be used to improve nanoimaging and biosensing on novel hybrid GO/Au substrates.
Finally, we have investigated the complex structure of Mo(1-x)WxS2 alloys with a gradual increase of x from the center to the edge of the monolayer islands. We used nano-optical imaging in the quantum plasmonic regime to investigate the dependence of the fractal dimension of different x components of the Mo(1-x)WxS2 alloys on the excitation energy.
Scholar Commons Citation
Hrim, Hana Nazari, "Nano-optical imaging of 2D materials in the quantum regime" (2022). USF Tampa Graduate Theses and Dissertations.
https://digitalcommons.usf.edu/etd/10300
