Local Density of States Modulated by Strain in Marginally Twisted Bilayer Graphene

In marginally twisted bilayer graphene,the Moirépattern consists of the maximized AB(BA)stacking regions,minimized AA stacking regions and triangular networks of domain walls.Here we realize the strain-modulated electronic structures of marginally twisted bilayer graphene by scanning tunneling microscopy/spectroscopy and density functional theory(DFT)calculations.The experimental data show four peaks near the Fermi energy at the AA regions.DFT calculations indicate that the two new peaks closer to the Fermi level may originate from the intrinsic heterostrain and the electric field implemented by back gate is likely to account for the observed shift of the four peaks.Furthermore,the d I/d V map across Moirépatterns with different strain strengths exhibits a distinct appearance of the helical edge states.

Dislocation mechanism of Ni_(47)Co_(53) alloy during rapid solidification

Dislocations and other atomic-level defects play a crucial role in determining the macroscopic properties of crystalline materials,but it is extremely difficult to observe the evolution of dislocations due to the limitations of the most advanced experimental techniques.Therefore,in this work,the rapid solidification processes of Ni_(47)Co_(53) alloy at five cooling rates are studied by molecular dynamics simulation,and the evolutions of their microstructures and dislocations are investigated as well.The results show that face-centered cubic(FCC) structures are formed at the low cooling rate,and the crystalline and amorphous mixture appear at the critical cooling rate,and the amorphous are generated at the high cooling rate.The crystallization temperature and crystallinity decrease with cooling rate increasing.Dislocations are few at the cooling rates of 1×10^(11) K/s,5×10^(12) K/s,and 1×10^(13) K/s,and they are most abundant at the cooling rates of 5×10^(11) K/s and1 × 10^(12) K/s,in which their dislocation line lengths are both almost identical.There appear a large number of dislocation reactions at both cooling rates,in which the interconversion between perfect and partial dislocations is primary.The dislocation reactions are more intense at the cooling rate of 5×10^(11) K/s,and the slip of some dislocations leads to the interconversion between FCC structure and hexagonal close packed(HCP) structure,which causes the twin boundaries(TBs) to disappear.The FCC and HCP are in the same atomic layer,and dislocations are formed at the junction due to the existence of TBs at the cooling rate of 1 ×10^(12) K/s.The present research is important in understanding the dislocation mechanism and its influence on crystal structure at atomic scales.

Effects of buried oxide layer on working speed of SiGe heterojunction photo-transistor

The effects of buried oxide（BOX） layer on the capacitance of SiGe heterojunction photo-transistor（HPT）,including the collector-substrate capacitance,the base-collector capacitance,and the base-emitter capacitance,are studied by using a silicon-on-insulator（SOI） substrate as compared with the devices on native Si substrates.By introducing the BOX layer into Si-based SiGe HPT,the maximum photo-characteristic frequency ft,0 p.of SO1-based SiGe HPT reaches up to 24.51 GHz,which is 1.5 times higher than the value obtained from Si-based SiGe HPT.In addition,the maximum optical cut-off frequency fβ,opt,namely its 3-dB bandwidth,reaches up to 1.13 GHz,improved by 1.18 times.However,with the increase of optical power or collector current,this improvement on the frequency characteristic from BOX layer becomes less dominant as confirmed by reducing the 3-dB bandwidth of SOI-based SiGe HPT which approaches to the 3-dB bandwidth of Si-based SiGe HPT at higher injection conditions.

Monolayer MoS_(2)of high mobility grown on SiO_(2)substrate by two-step chemical vapor deposition

We report a novel two-step ambient pressure chemical vapor deposition(CVD)pathway to grow high-quality Mo S_(2)monolayer on the Si O_(2)substrate with large crystal size up to 110μm.The large specific surface area of the pre-synthesized Mo O_(3)flakes on the mica substrate compared to Mo O_(3)powder could dramatically reduce the consumption of the Mo source.The electronic information inferred from the four-probe scanning tunneling microscope(4P-STM)image explains the threshold voltage variations and the n-type behavior observed in the two-terminal transport measurements.Furthermore,the direct van der Pauw transport also confirms its relatively high carrier mobility.Our study provides a reliable method to synthesize high-quality Mo S_(2)monolayer,which is confirmed by the direct 4P-STM measurement results.Such methodology is a key step toward the large-scale growth of transition metal dichalcogenides(TMDs)on the Si O_(2)substrate and is essential to further development of the TMDs-related integrated devices.