Browsing by Author "Parsi, Amir"

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Open Access
Ion transport induced room-temperature insulator-metal transition in single-crystalline Cu2Se
(Royal Society of Chemistry, 2024-05-09) Suleiman, Abdulsalam Aj; Parsi, Amir; Razeghi, Mohammadali; Başçı, Uğur; Pehlivanoğlu, Doruk; Jeong, Hu Young; Kang, Kibum; Kasırga, Talip Serkan; Oh, Saeyoung
Cu2Se is a superionic conductor above 414 K, with ionic conductivities reaching that of molten salts. The superionic behavior results from hopping Cu ions between different crystallographic sites within the Se scaffold. However, the properties of Cu2Se below 414 K are far less known due to experimental limitations imposed by the bulk or polycrystalline samples that have been available so far. Here, we report the synthesis of ultra-thin, large-area single crystalline Cu2Se samples using a chemical vapor deposition method. The as-synthesized Cu2Se crystals exhibit optically and electrically detectable and controllable robust phases at room temperature and above. We demonstrate that Cu ion vacancies can be manipulated to induce an insulator-metal transition, which exhibits 6 orders of magnitude change in the electrical resistance of two terminal devices, accompanied by an optical change in the phase configuration. Our experiments show that the high mobility of the liquid-like Cu ion vacancies in Cu2Se causes macroscopic ordering in the Cu vacancies. Consequently, phase distribution over the crystals is not dictated by the diffusive motion of the ions but by the local energy minima formed due to the phase transition. As a result, long-range vacancy ordering of the crystal below 414 K becomes optically observable at a micrometer scale. This work demonstrates that Cu2Se could be a prototypical system where long-range ordering properties can be studied via electrical and optical methods.
Open Access
Molten glass-mediated conditional CVD growth of MoS2 monolayers and effect of surface treatment on their optical properties
(Institute of Physics Publishing Ltd., 2024-05-30) Aras, Fikret Gonca; Suleiman, Abdulsalam Aji; Parsi, Amir; Kasırga, Talip Serkan; Yeltik, Aydan
In the rapidly developing field of optoelectronics, the utilization of transition-metal dichalcogenides with adjustable band gaps holds great promise. MoS2, in particular, has garnered considerable attention owing to its versatility. However, a persistent challenge is to establish a simple, reliable and scalable method for large-scale synthesis of continuous monolayer films. In this study, we report the growth of continuous large-area monolayer MoS2 films using a glass-assisted chemical vapor deposition (CVD) process. High-quality monolayer films were achieved by precisely controlling carrier gas flow and sulfur vaporization with a customized CVD system. Additionally, we explored the impact of chemical treatment using lithium bistrifluoromethylsulfonylamine (Li-TFSI) salt on the optical properties of monolayer MoS2 crystals. To investigate the evolution of excitonic characteristics, we conditionally grew monolayer MoS2 flakes by controlling sulfur evaporation. We reported two scenarios on MoS2 films and flakes based on substrate-related strain and defect density. Our findings revealed that high-quality monolayer MoS2 films exhibited lower treatment efficiency due to substrate-induced surface strain. whereas defective monolayer MoS2 flakes demonstrated a higher treatment sensitivity due to the p-doping effect. The Li-TFSI-induced changes in exciton density were elucidated through photoluminescence, Raman, and x-ray photoelectron spectroscopy results. Furthermore, we demonstrated treatment-related healing in flakes under variable laser excitation power. The advancements highlighted in our study carry significant implications for the scalable fabrication of diverse optoelectronic devices, potentially paving the way for widespread real-world applications.
Open Access
Nonlinear optical properties of CVD-synthesized CuS crystals
(AIP Publishing LLC, 2024-12-27) Suleiman, Abdulsalam Aji; Rahighi, Reza; Parsi, Amir; Kasırga, Talip Serkan
Copper sulfide (CuS) is a material of growing interest due to its distinctive electronic, optical, and catalytic properties. In this study, we successfully synthesized ultrathin CuS crystals, with thicknesses as low as 14 nm and lateral dimensions reaching 60 μm, using a single-step chemical vapor deposition process. Detailed structural, compositional, and morphological analyses revealed intrinsic lattice defects, including stacking faults and domain misorientations. These defects disrupt the centrosymmetry of the CuS lattice and are responsible for an unexpected second harmonic generation response, an uncommon behavior in centrosymmetric materials. In addition, we measured the first-order temperature coefficients of Raman shifts, providing insights into the thermal dynamics of the CuS crystal structure. These findings position CuS as a potential material for nonlinear optical applications, while reinforcing its established roles in catalysis and electronics.