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Hybrid, Gate-Tunable, van der Waals p–n Heterojunctions from Pentacene and MoS 2
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Kinetically Controlled Epitaxial Growth of Fe 3 GeTe 2 van der Waals Ferromagnetic Films
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Nanoscale Probing of Electrical Memory Effects in van der Waals Layered PdSe 2
Tunable electronic materials that can be switched between different impedance states are fundamental to the hardware elements for neuromorphic computing architectures. This “brain-like” computing paradigm uses highly paralleled and colocated data processing, leading to greatly improved energy efficiency and performance compared to traditional architectures in which data have to be frequently transferred between processor and memory. In this work, we use scanning microwave impedance microscopy for nanoscale electrical and electronic characterization of two-dimensional layered semiconductor PdSe 2 to probe neuromorphic properties. The local resolution of tens of nanometers reveals significant differences in electronic behavior between and within PdSe 2 nanosheets (NSs). In particular, we detected both n-type and p-type behaviors, although previous reports only point to ambipolar n-type dominating characteristics. Nanoscale capacitance–voltage curves and subsequent calculation of characteristic maps revealed a hysteretic behavior originating from the creation and erasure of Se vacancies as well as the switching of defect charge states. In addition, stacks consisting of two NSs show enhanced resistive and capacitive switching, which is attributed to trapped charge carriers at the interfaces between the stacked NSs. Stacking n- and p-type NSs results in a combined behavior that allows one to tune electrical characteristics. In conclusion, as local inhomogeneities of electrical and electronic behavior can have a significant impact on the overall device performance, the demonstrated nanoscale characterization and analysis will be applicable to a wide range of semiconducting materials.
Ethanol Partial Oxidation over VOX/TiO2 Catalysts: The Role of Titania Surface Oxygen on Vanadia Reoxidation in the Mars-van Krevelen Mechanism
The mechanism for the reoxidation step in the Marsvan Krevelen mechanism for ethanol partial oxidation over vanadia anchored on titanium oxide is examined. Kinetic parameters such as ethanol heat of adsorption, the activation energy for the rate-limiting step (alpha-hydrogen abstraction on the adsorbed ethoxide) were obtained while the energetics of the catalyst reoxidation step were explored. A comparison of the parameters obtained from kinetic analysis and the apparent activation energies reported in the literature indicated that a kinetic model that incorporates a catalyst reoxidation step, where molecular oxygen adsorbs into a titania vacancy, accurately predicted the kinetic parameters. In contrast, a model where molecular oxygen directly adsorbs on the reduced vanadia resulted in an underestimation of the ethanol heat of adsorption and activation energy for the a-hydrogen abstraction step. A computational analysis was implemented to elucidate a mechanistic pathway for reduced vanadia that incorporates oxygen adsorption on a titania vacancy. The results indicated that the vanadia reoxidation step involves surface oxygen migration from the titania surface to the reduced vanadia center. The quantification of oxygen uptake by the reduced catalyst validates the premise of this assumption: titania vacancies are created during ethanol partial oxidation and are active sites for oxygen adsorption.
Nanoscale Control of Polar Surface Phases in Layered van der Waals CuInP 2 S 6
Antiferroelectric (AFE) materials, in which alternating dipole moments cancel out to a zero net macroscopic polarization, can be used for high-density energy storage and memory applications. The AFE phase can exist in bulk CuInP 2 Se 6 , CuBiP 2 S 6 , and a few other transition-metal thiophosphates below 200 K. The required low temperature poses challenges for practical applications. In this work, we report the coexistence of ferrielectric (FE) states and a stable surface phase that does not show piezoelectric response (“zero-response phase”) in bulk CuInP 2 S 6 at room temperature. Using piezoresponse force microscopy (PFM) tomographic imaging together with density functional theory, we find that direct and alternating voltages can locally and stably convert FE to zero-response phases and vice versa. While PFM loops show pinched hystereses reminiscent of antiferroelectricity, PFM tomography reveals that the zero-response areas form only on top of the FE phase in which the polarization vector is pointing up. Theoretical calculations suggest that the zero-response phase may correspond to AFE ordering where stacked CuInP 2 S 6 layers have alternating polarization orientations thereby leading to a net-zero polarization. Consistent with experimental findings, theory predicts that the FE polarization pointing down is robust up to the top surface, whereas FE polarization pointing up energetically favors the formation of an AFE surface layer, whose thickness is likely to be sensitive to local strains. AFE order is likely to be more robust against detrimental size effects than polar order, therefore providing additional opportunities to create multifunctional heterostructures with 2D electronic materials.
Advancing Nanoelectronics Applications: Progress in Non-van der Waals 2D Materials
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