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    Negative Capacitance Behavior at Low Frequencies of Nitrogen-Doped Polyethylenimine-Functionalized Graphene Quantum Dots-Based Structure
    (American Chemical Society, 2023) Berktaş, Zeynep; Orhan, Elif; Ulusoy, Murat; Yıldız, Mustafa; Altındal, Şemsettin
    Graphene quantum dots (GQDs), zero-dimensional members of the carbon family, have exceptional mechanical, thermal, and electrical properties. Nevertheless, pure GQDs have many challenges in reaching their full potential in electronic applications. Functionalizing or chemical modification of GQDs adjusts the physical and chemical properties, driving GQDs toward high-performance device applications. Nitrogen (N)-doped polyethylenimine (PEI) functionalized GQDs are capturing the interest of researchers specifically for electronic and photovoltaic applications these days. In this context, we present for the first time capacitance/conductance-voltage (C-V and G/ω-V) measurements of the nitrogen-doped PEI-functionalized GQDs-based structure for use in electronic applications in the frequency range from 1 kHz to 2 MHz at 300 K in this study. Capacitance features, the energy density distribution of surface states (Nss), and the relaxation time (τ) of a nitrogen-doped PEI-functionalized GQDs-based structure have been examined by using the admittance/conductance method. Negative capacitance (NC) behavior mostly exhibited by ferroelectric materials has been observed in the GQDs-based structure at low frequencies, and then it starts to disappear. NC is usually attributed to various surface states/interface traps, series resistance (Rs), and minority carrier injection. The NC phenomenon indicates that an increase in voltage gives rise to a decrease in the charge on the electrodes. The control of interfacial charges in such a heterostructure will be critical for NC devices. The results provide a basis for insights into semiconductor device technology.
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    Tunable dielectric characteristics of the nanocomposite diode based on functionalized graphene quantum dots with and without gadolinium
    (Elsevier, 2024) Berktaş, Zeynep; Anter, Aslıhan; Dikicioğlu, Elanur; Ulusoy, Murat; Candan, Can; Yıldız, Mustafa; Di Bartolomeo, Antonio
    In this study, we have separately synthesized and characterized solutions of gadolinium (Gd)-free and Gd-doped polyethyleneimine (PEI)-functionalized graphene quantum dots (GQDs) due to the excellent properties of Gd, a rare earth element, in fluorescence and magnetic resonance imaging (MRI). The dielectric properties of Gd-free/ doped nanocomposite-based diodes have been compared using impedance spectroscopy (IS) in the frequency range from 1 kHz to 500 kHz and voltage range from -3 V to +5 V at 300 K. From our experimental results, the Gd-free diode was found to have a negative dielectric constant (e '). In contrast, the Gd-doped nanocomposite diode exhibited positive e '. The epsilon ' of the Gd-free diode is -80, while the e ' of the Gd-doped diode is 35 at 5 V for 1 kHz. The experimental results showed that the dielectric properties of both structures were strongly dependent on the applied voltage and frequency. The Gd doping in the interface has prevented the domination of loss mechanisms within the structure, eliminated negative dielectric at lower frequencies, and conferred a unique micro-capacitor characteristic to the structure. All these efforts will contribute to the development of functional carbon-based materials and the creation of new electronic devices and tunable dielectric properties.