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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 (2023)

Thin films with semiconducting properties can be successfully engineered from chalcogen-basedmaterials, specifically those containing selenium (Se), or/and tellurium (Te). They are vital subjects of study in both their elemental states and various compounds due to their distinct anisotropic properties. These elements play essential roles in compounds like tellurides and selenides, which are crucial in semiconductors, photovoltaic devices, and electronics. However, the quick crystallization tendencies observed in amorphous TexSe1 −x thin films restrict their applicability. It is essential to gain insight into the short- and medium-range structural organization of the amorphous state and comprehend the underlying physics driving film crystallization. Therefore, in the second chapter of the thesis, we provide comprehensive research on the short- to medium-range ordering in amorphous TexSe1 −x thin films through a combination of experimental studies and atomic simulations. This study marked the first instance in which we employed fluctuation electron microscopy (FEM) and Density Functional Theory (DFT) calculations to gain insights into these structural variations across a wide range of compositions, including pure Te. Within the chain network structure, we have identified at least two distinct populations that closely resemble the intrachain distances of Se-Se and Te-Te. In the case of the binary alloy with x greater than 0.61 in TexSe1 −x, we observe an increase in Te-Te-like populations, implying the potential formation of Te fragments. To leverage the anisotropic properties of tellurium, in recent study, the oriented growth of ultra-thin Te layers on a WSe2 substrate is achieved by using a technique called physical vapor deposition (PVD). In this configuration, the atomic chains of Te align with the armchair directions of the substrate, leading to the formation of a moire superlattice. This superlattice consists of micrometer- ´ scale Te flakes positioned on the continuous WSe2 film. Here, we explore the exact orientation relationships and moire lattices by combining electron microscopy with image simulations. We ´ also assess the strain evolution and defects in Te-WSe2 heterostructures with the help of scanning nanodiffraction analysis, shedding light on the complexities of strain transfer and its impact on material properties. In the final chapter of the thesis, we introduce a hybrid approach aimed at addressing challenges in sample preparation for in-situ mechanical testing. It involves depositing samples on flexible, electron-transparent substrates, attaching them to macroscopic copper sheets, and mirroring techniques used in the nanomaterials and thin-film community. With the help of this technique, we were able to study the mechanical responses of sputtered gold (Au) and transition metal dichalcogenides (TMDC), specifically WS2. This method not only streamlines sample preparation but also expands our ability to investigate the mechanical properties of materials at the nanoscale

    Cover page: Multidisciplinary Investigations of Chalcogen-based Semiconductor Materials: Structural Heterogeneity, Strain Analysis, and In-Situ Mechanical Testing
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2022)
    Obsidian is volcanic glass that results from the rapid cooling of silica-rich melt. Nanoscale crystallites precipitate out of the melt prior to solidification and remain embedded in the amorphous matrix. These crystallites provide information on the flow kinetics and composition of the melt. Due to the sparsity and size of nanolites, studies often focus on supramicron crystallites. This research takes advantage of the conchoidal fracture of obsidian by knapping samples with nanometer-thin edges for transmission electron microscopy characterization. Nanolites in the amorphous matrix are studied using energy-dispersive spectroscopy (EDS) and electron diffraction. Certain alkali and alkaline-earth cations exhibit patterns of depletion near Fe-oxide nanolites. EDS is used to identify nanolites and variations in the composition of the matrix. Parallel beam diffraction and radial distribution function analysis of nearest-neighbor distances determine average bond lengths in the matrix near nanolites, showing that nanolites influence the nearby short-range ordering and atomic character of the matrix. Analysis reveals decreased mean nearest-neighbor distances in the matrix adjacent to nanolites compared to the bulk. Our methods exhibit the required sensitivity to detect variations in the composition and structure near nanolites, and our findings indicate that obsidian nanolites contribute to quantifiable localized changes in the amorphous structure.
      Cover page: Chemical and Structural Alterations in the Amorphous Structure of Obsidian due to Nanolites
      • Article
      • Peer Reviewed
      LBL Publications (2023)
      In recent years, there has been an increasing focus on 2D nongraphene materials that range from insulators to semiconductors to metals. As a single-elemental van der Waals semiconductor, tellurium (Te) has captivating anisotropic physical properties. Recent work demonstrated growth of ultrathin Te on WSe2 with the atomic chains of Te aligned with the armchair directions of the substrate using physical vapor deposition (PVD). In this system, a moiré superlattice is formed where micrometer-scale Te flakes sit on top of the continuous WSe2 film. Here, we determined the precise orientation of the Te flakes with respect to the substrate and detailed structure of the resulting moiré lattice by combining electron microscopy with image simulations. We directly visualized the moiré lattice using center of mass-differential phase contrast (CoM-DPC). We also investigated the local strain within the Te/WSe2 layered materials using scanning nanodiffraction techniques. There is a significant tensile strain at the edges of flakes along the direction perpendicular to the Te chain direction, which is an indication of the preferred orientation for the growth of Te on WSe2. In addition, we observed local strain relaxation regions within the Te film, specifically attributed to misfit dislocations, which we characterize as having a screw-like nature. The detailed structural analysis gives insight into the growth mechanisms and strain relaxation in this moiré heterostructure.
        Cover page: Analysis of Strain and Defects in Tellurium-WSe2 Moiré Heterostructures Using Scanning Nanodiffraction.
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2024)
        Black phosphorus (bP) is a promising material for mid-infrared (mid-IR) optoelectronic applications, exhibiting high performance light emission and detection. Alloying bP with arsenic extends its operation toward longer wavelengths from 3.7 μm (bP) to 5 μm (bP3As7), which is of great practical interest. Quantitative optical characterizations are performed to establish black phosphorus-arsenic (bPAs) alloys optoelectronic quality. Anisotropic optical constants (refractive index, extinction coefficient, and absorption coefficient) of bPAs alloys from near-infrared to mid-IR (0.2-0.9 eV) are extracted with reflection measurements, which helps optical device design. Quantitative photoluminescence (PL) of bPAs alloys with different As concentrations are measured from room temperature to 77 K. PL quantum yield measurements reveal a 2 orders of magnitude decrease in radiative efficiency with increasing As concentration. An optical cavity is designed for bP3As7, which allows for up to an order of magnitude enhancement in the quantum yield due to the Purcell effect. Our comprehensive optical characterization provides the foundation for high performance mid-IR optical device design using bPAs alloys.
          Cover page: Quantitative Mid-infrared Photoluminescence Characterization of Black Phosphorus–Arsenic Alloys
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