Deniz Çakır

As the semiconductor industry faces growing demand for high-performance (HP) nanoscale devices, two-dimensional (2D) materials have emerged as promising candidates to challenge the long-standing dominance of silicon. Among them, 2D GaSe has recently attracted significant interest as a group-III monochalcogenide with a tunable electronic structure and strong mechanical robustness, offering unique opportunities for ultra-scaled device applications. In this work, we perform first-principles density functional theory combined with nonequilibrium Green’s function simulations to systematically investigate the electronic transport properties of GaSe nanodevices with channel lengths of approximately 5 nm across three complementary architectures: p–n junction diodes, p–i–n field-effect transistors (FETs), and double-gate (DG) metal-oxide-semiconductor FETs (MOSFETs). Our results reveal that GaSe p–n junctions exhibit a pronounced backward-diode response, where the reverse current exceeds the forward current due to efficient interband tunneling. Extending the architecture to p–i–n FETs demonstrates that electrostatic gating enables controllable modulation of this tunneling-dominated transport, where moderate gate voltages ( V g ) suppress reverse conduction, whereas stronger gating restores and enhances carrier flow. Furthermore, DG GaSe MOSFETs exhibit competitive performance metrics when benchmarked against the International Technology Roadmap for Semiconductors-2028 (ITRS-2028) HP targets. Specifically, the zigzag n-type configuration achieves an on/off current ratio ( I on / I off ), intrinsic delay time ( τ ), and power-delay product that meet or surpass the projected HP requirements for ultra-scaled logic transistors. Collectively, these findings establish the monolayer 2D GaSe as a versatile semiconductor platform capable of integrating backward-diode functionality, gate-tunable tunneling devices, and HP MOSFET operation within a unified material system. This multifunctional capability highlights the strong technological potential of GaSe for next-generation ultra-scaled nanoelectronic applications.

Vertically aligned tungsten diselenide (WSe₂) nanosheets were synthesized via chemical vapor deposition and subsequently modified through nitrogen plasma treatment to obtain nitrogen-doped WSe₂ (N-WSe₂) with enhanced gas-sensing capabilities. The introduction of nitrogen dopants not only preserves the crystalline integrity of the WSe₂ nanosheets but also significantly improves their electrical conductivity and surface reactivity. Chemiresistive gas sensors based on N-WSe₂ demonstrate markedly enhanced sensitivity and lower detection limits toward both nitrogen dioxide (NO₂) and ammonia (NH₃) compared to their pristine counterparts. First-principles density functional theory calculations reveal that nitrogen doping introduces localized defect states that modulate charge transfer and spin polarization upon gas adsorption, thereby facilitating more efficient carrier modulation. This synergistic combination of structural alignment and electronic tuning offers a versatile strategy for engineering high-performance 2D TMD-based gas sensors for environmental and industrial monitoring applications.

Layered materials, with their distinctive electronic and surface structures, have garnered significant attention in catalysis and energy conversion research. MXenes, a family of two-dimensional transition metal carbides, nitrides, and carbonitrides, exhibit remarkable electrical conductivity and surface chemistry. MXene restacking is commonly observed property during the exfoliation process. Druffel et al. synthesized 3D Y2CF2 MXene through a high-temperature solid-state reaction. We performed density functional theory (DFT) computation to study the structural and electronic property changes when Y2XT2 MXenes formed bilayer and 3D structures. We considered X as C and N, while surface termination T as O, F, and Cl. Except monolayer Y2NO2, the other two Nitride monolayers show metallic character. Our calculations show that Y2NO2 monolayer become a metal when stacking occurs to form bilayers and 3D structures. On the contrary, all the carbide monolayers exhibit non-zero band gaps showing they are semiconductors. Their semiconductor character is preserved even after stacking occurs. Our HSE06 hybrid functional calculations reveal that monolayer Y2CO2, Y2CF2, and Y2CCl2 have band gaps of 2.27, 1.82, and 1.77 eV, respectively. Upon formation of bilayers, the band gaps shift to 2.55, 1.94, and 1.72 eV, while 3D materials exhibit band gaps of 1.98, 2.00, and 1.65 eV, respectively. Moreover, we also studied the electronic properties of bulk MXenes under the external pressure range of 1-10 GPa. Under external pressure, our calculations shows that the band gap change of Y2CO2 and Y2CF2 bulk MXenes is negligible. Conversely, increasing external pressure leads to a reduction in the band gap of bulk Y2CCl2.

Tungsten diselenide (WSe₂) has emerged as a highly promising material for lithium/sodium-ion batteries due to its exceptional electrical and chemical properties and unique sandwich-like two-dimensional structures. In this work, we present a chemical vapor deposition approach for synthesizing densely packed vertically oriented 2H-phase WSe₂ nanosheets uniformly grown on tungsten foil. As an electrode material, the as-synthesized WSe₂ exhibits remarkable electrochemical performance including excellent cycling stability, high-rate capability, and superior electrochemical activity. The WSe₂ nanosheets demonstrate a high reversible specific capacity of 159.3 mAh g^-1 at a current density of 5 A g^-1, maintaining a near-100% Coulombic efficiency over 2400 cycles. The underlying mechanism governing electrochemical performance was further investigated using first-principles density functional theory computations. These findings underscore the potential of WSe₂ nanosheets as high-performance materials for next-generation LIBs.

Ultralow compressible materials, which have high bulk modulus (K), are invaluable in extreme conditions due to their ability to undergo significant compression without structural failure. As large number of borides can be found with high K, this study develop a computational framework to scan the vast chemical space to identify the ultralow compressible borides. Transformer-based networks are helpful to generate new chemical compositions due to their self-attention mechanism, scalability, and ability to capture long-range dependencies. First, we developed a transformer-based network to generate new binary and ternary boride compositions based on the known boride compositions. Next, we trained a hybrid model based on AdaBoost and Gradient Boosting algorithms with mean absolute error (MAE) of 14.1 GPa to scan the high K borides. The CALYPSO code was used to find the possible structures for those materials. After predicting K for broad chemical domain, we found that Re-B and W-B systems are promising ultralow compressible materials. We then performed density functional theory (DFT) calculations to investigate the stability of high K materials. Our computations suggest that Re3B2, Re2B3, W5VB4, and Re5CrB4 materials exhibit K > 300 GPa with negative formation energy and energy-above-hull less than 40 meV. Those materials are mechanically and dynamically stable based on the elastic constant calculations and the phonon dispersion.

We investigated the thermal resistance of SnSe nanoflakes using scanning thermal microscopy (SThM), an extension of atomic force microscopy that allows for nanoscale thermal mapping. The nanoflakes studied ranged in thickness from 5 to 40 nm, enabling us to observe the thermal properties across a wide range of dimensions. Our findings reveal a clear trend of increasing thermal resistance with SnSe nanoflake thickness, suggesting that heat transport in SnSe is governed by diffusive mechanisms that limit the effectiveness of nanostructuring in further reducing thermal conductivity. This investigation showcases the capabilities of SThM for nanoscale thermal analysis, providing critical insights into the intrinsic thermal transport properties of SnSe.

In this study, we employed lateral force microscopy, a specialized technique within atomic force microscopy, to explore the nanoscale frictional properties of 2D SnSe and SnSe$_2$ layers, and we compared these findings with graphene, a commonly used solid lubricant. Our results revealed that SnSe and SnSe2 layers exhibited superior lubricative performance compared to graphene with similar thickness, with SnSe$_2$ demonstrating friction levels up to three times lower. This lower friction observed in SnSe$_2$, as compared to SnSe, was further substantiated by extensive Density Functional Theory (DFT) calculations, confirming the intrinsic material properties contributing to their enhanced lubricative behavior.