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Thermoelectric Effect in Layered Nanostructures
(Izmir Institute of Technology, 2019) Özbal Sargın, Gözde; Senger, Ramazan Tuğrul; Sevinçli, Haldun
In this thesis, ballistic transport and thermoelectric (TE) properties of semiconducting and dynamically stable two-dimensional materials are investigated by combining first-principles calculations with Landauer formalism. Motivated by finding novel promising TE materials, transition metal dichalcogenides (TMDs) and oxides (TMOs) (namely MX2 with M = Cr, Mo, W, Ti, Zr, Hf; X = O, S, Se, Te) are studied systematically in their 2H- and 1T-phases in Chapter 3. Having computed structural, as well as ballistic electronic and phononic transport properties for all structures, we analyze the thermoelectric properties of the semiconducting ones. We report for the first time that, 2H-phases of four of the studied structures have very promising thermoelectric properties, unlike their 1T-phases. Next, ballistic transport and thermoelectric (TE) properties of group IIImonochalcogenides (group III-VI) are presented in a wide range temperature from 100 K to 1000 K. This large family composed of 25 compounds which stands out with their unique electronic band structures. In addition to Mexican hat shaped (quartic energy-momentum relation) valence band character, some of the structures exhibit valley degeneracies which can occur either in their conduction and valence bands. Moreover, TE and transport calculations are performed for BO and BS monolayers which consist of lightest species in group III-monochalcogenides. Surprisingly, BO and BS monolayers exhibit high TE efficiency at low temperatures. Low thermal conductance at low temperatures and stepwise electronic transmission at the valence band edge are the physical mechanisms behind achieving large ZT.
Electronic, Magnetic, and Mechanical Properties of Novel Two Dimensional Monolayer Materials
(Izmir Institute of Technology, 2017) Yağmurcukardeş, Mehmet; Senger, Ramazan Tuğrul; Şahin, Hasan
Layered materials exhibit different properties when they are thinned down to a few monolayers. Following the successful isolation of graphene in 2004, there has been a rapid increase in the number of studies focusing on other novel two dimensional (2D) materials such as hexagonal Boron Nitride (BN), transition metal dichalcogenides (TMDs), post transition metal chalcogenides (PTMCs), and in-plane anisotropic monolayers (Redichalcogenides and blackphosphorus). In addition to their electronic, optical, and magnetic properties, mechanical properties of 2D materials are of fundamental importance. Measurements of elastic constants of 2D materials are still challenging. Therefore, theoretical investigation of the mechanical properties is particularly important. Moreover, investigation of Raman spectra of these materials requires a through understanding of their vibrational properties. In these regards, we investigate the electronic, magnetic, and mechanical properties of some novel monolayer 2D materials (such as, auxetic pentagonal monolayers, flexible monolayers of holey graphene crystals, ultra-flexible monolayers of PTMCs, and in-plane anisotropic monolayers of ReS2 and blackphosphorus) by means of first-principles calculations based on density functional theory (DFT). In addition, tuning electronic properties of a van der Waals heterobilayer structure composed of monolayers of Mg(OH)2 and WS2 upon an external out-of-plane electric field is studied. The effect of biaxial strain on the vibrational properties of novel 2D materials is also studied through their off-resonant Raman activities. Our findings will be useful to clarify several issues related to the experiments of novel 2D materials.

Phd Degree / Doktora

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