Public defence in Micro- and nanosciences, M.Sc. MD Gius Uddin

The title of the thesis: Fabrication and characterization of two-dimensional material based devices for photonics and electronics

Thesis defender: MD Gius Uddin
Opponent: Prof. Mircea Guina, Tampere University
Custos: Prof. Ilkka Tittonen, Aalto University School of Electrical Engineering, Department of Electronics and Nanoengineering

The thesis explores the potential of two-dimensional (2D) materials in different practical applications and presents the results divided in three parts. The first part focuses on the miniaturized spectrometers. Unlike conventional tabletop spectrometers, we demonstrate miniaturized (~22×8 μm2) computational spectrometers that rely on the electrically tunable spectral response of 2D materials-based single-junction for spectral reconstruction. We achieve high peak wavelength accuracy (~3 nm) and a broad operation window covering the visible and the near-infrared regions, indicating the great potential of the spectrometers to enable numerous portable applications. 

The second part of this thesis examines different strategies for tuning the optical and electrical properties of 2D materials. We demonstrate that morphological manipulation of 2D indium selenide (InSe) facilitates enhanced light-matter interaction in InSe. Our 2D InSe/1D nanowire heterostructures, exhibit more than 5 times enhanced optical responses compared to that from bare InSe. Moreover, significant optical anisotropy is observed that makes our mixed dimensional heterostructures a good candidate for diverse polarization-dependent optoelectronic applications such as photodetectors. Further, in this thesis, we explore a strain engineering approach to increase the charge carrier mobility of molybdenum ditelluride (MoTe₂) field-effect transistors (FETs). It involves the creation of hole arrays in the substrate, transfer of MoTe2 flakes on the hole arrays, and subsequent deposition of ALD Al2O3 passivation layer on top of the MoTe2 flakes. We achieve ~6 times higher charge carrier mobility in the strained MoTe2 FETs than those MoTe2 FETs without strain. The results offer a bright prospect to realize 2D materials-based high performance devices for future electronics. 

In the final part of this thesis, we experimentally demonstrate a novel concept for the miniaturization of broadband light sources. Coherent broadband light is generated with gallium selenide and niobium oxide diiodide crystals at the deep-subwavelength thickness (<100 nm). The broadband spectrum spans more than an octave (from ~565 to 1906 nm) without the need for dispersion engineering. Compared with conventional methods, our demonstration is ~5 orders of magnitude thinner and requires ~3 orders of magnitude lower excitation power. The results open a new path to create ultra-compact, on-chip broadband light sources.

Key words: 2D materials, Spectrometers, vdW heterostructures, FET, Raman, PL, Nanowire anisotropy, strain engineering, mobility, broadband generation, nonlinear optics

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

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Doctoral theses in the School of Electrical Engineering: https://aaltodoc.aalto.fi/handle/123456789/53