The Effect of Carbon Doping on the Crystal Structure and Electrical Properties of Sb2Te3.

As a new generation of non-volatile memory, phase change random access memory (PCRAM) has the potential to fill the hierarchical gap between DRAM and NAND FLASH in computer storage. Sb2Te3, one of the candidate materials for high-speed PCRAM, has high crystallization speed and poor thermal stability. In this work, we investigated the effect of carbon doping on Sb2Te3. It was found that the FCC phase of C-doped Sb2Te3 appeared at 200 °C and began to transform into the HEX phase at 25 °C, which is different from the previous reports where no FCC phase was observed in C-Sb2Te3. Based on the experimental observation and first-principles density functional theory calculation, it is found that the formation energy of FCC-Sb2Te3 structure decreases gradually with the increase in C doping concentration. Moreover, doped C atoms tend to form C molecular clusters in sp2 hybridization at the grain boundary of Sb2Te3, which is similar to the layered structure of graphite. And after doping C atoms, the thermal stability of Sb2Te3 is improved. We have fabricated the PCRAM device cell array of a C-Sb2Te3 alloy, which has an operating speed of 5 ns, a high thermal stability (10-year data retention temperature 138.1 °C), a low device power consumption (0.57 pJ), a continuously adjustable resistance value, and a very low resistance drift coefficient.

Different photocurrent response of Cs4PbBr6 particles.

Zero-dimensional (0D) all-inorganic cesium lead halide perovskites, particularly Cs4PbBr6, have been attracting wide attention due to their excellent optical properties and stability. The research also focuses on the origin of green emission from Cs4PbBr6, which has a bandgap located in the ultraviolet B (UVB) region. So far, both Cs4PbBr6 without visible emission and with green emission have been successfully prepared; however, the origin of green emission remains controversial. Photocurrent response is one of the effective approaches to explore how the photo-excited carriers influence the photo-physical properties of materials. In our study, Cs4PbBr6 particles without visible emission and with green emission were synthesized and their photocurrent response was investigated. The former showed a positive photocurrent response, while the latter showed a negative photocurrent response. The negative response was believed to be due to a built-in electric field constructed by the charged excitons in green-emissive Cs4PbBr6. From our calculations, numerous vacancies of Br are easier to appear in green-emissive Cs4PbBr6 lattices, which could combine the neutral excitons to form charged excitons. This work may contribute to the explanation of the origin of green emission of Cs4PbBr6 to some extent.

Universal, In Situ Transformation of Bulky Compounds into Nanoscale Catalysts by High-Temperature Pulse.

The synthesis of nanoscale metal compound catalysts has attracted much research attention in the past decade. The challenges of preparation of the metal compound include the complexity of the synthesis process and difficulty of precise control of the reaction conditions. Herein, we report an in situ synthesis of nanoparticles via a high-temperature pulse method where the bulk material acts as the precursor. During the process of rapid heating and cooling, swift melting, anchoring, and recrystallization occur, resulting in the generation of high-purity nanoparticles. In our work, the cobalt boride (Co2B) nanoparticles with a diameter of 10-20 nm uniformly anchored on the reduced graphene oxide (rGO) nanosheets were successfully prepared using the high temperature pulse method. The as-prepared Co2B/rGO composite displayed remarkable electrocatalytic performance for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). We also prepared molybdenum disulfide (MoS2) and cobalt oxide (Co3O4) nanoparticles, thereby demonstrating that the high-temperature pulse is a universal method to synthesize ultrafine metal compound nanoparticles.

Orbital electronic occupation effect on metal-insulator transition in Ti x V1-x O2.

A series of Ti x V1-x O2 (0% ⩽ x ⩽ 4.48%) thin films on c-plane sapphire substrates have been fabricated by co-sputtering oxidation solutions, and the metal-insulator transition temperature (T MIT) of Ti x V1-x O2 films rises monotonically at the rate of 1.64 K/at.% Ti. The x-ray diffraction measurement results show that, after Ti4+ ion doping, the rutile structure expands along the c r axis while shrinking along the a r and b r axis simultaneously. It makes the V-O bond length shorter, which is believed to upshift the π * orbitals. The rising of π * orbitals in Ti-doped VO2 has been illustrated by ultraviolet-infrared spectroscopy and first-principles calculation. With the Ti4+ ion doping concentration increasing, the energy levels of π * orbitals are elevated and the electronic occupation of π * orbitals decreases, which weakens the shielding for the strong electron-electron correlations in the d|| orbital and result in the T MIT rising. The research reveals that the T MIT of VO2 can be effected by the electronic occupancy of π * orbitals in a rutile state, which is helpful for developing VO2-based thermal devices.

Electrochemical Intercalation of Lithium Ions into NbSe2 Nanosheets.

Transition metal dichalcogenides (TMDCs) have been known for decades to have unique properties and recently attracted broad attention for their two-dimensional (2D) characteristics. NbSe2 is a metallic TMDC that has been studied for its charge density wave transition behavior and superconductivity but is still largely unexplored for its potential use in engineered devices with applications in areas such as electronics, optics, and batteries. Thus, we successfully demonstrate and present evidence of lithium intercalation in NbSe2 as a technique capable of modifying the material properties of NbSe2 for further study. We demonstrate successful intercalation of Li ions into NbSe2 and confirm this result through X-ray diffraction, noting a unit cell size increase from 12.57 to 13.57 Å in the c lattice parameter of the NbSe2 after intercalation. We also fabricate planar half-cell electrochemical devices using ultrathin NbSe2 from platelets to observe evidence of Li-ion intercalation through an increase in the optical transmittance of the material in the visible range. Using 550 nm wavelength light, we observed an increase in optical transmittance of 26% during electrochemical intercalation.

The Structural and Electronic Properties of CdS/ZnS Core-Shell Nanowires.

The structural and electronic properties of the CdS/ZnS core-shell nanowires (NWs) oriented along [001] direction have been investigated by means of the first-principles calculation. It is found that CdS core suffers from the compressive strain in the CdS-core/ZnS-shell NWs, and ZnS core is stretched in the ZnS-core/CdS-shell NWs. A thicker ZnS shell can improve the NWs' stability, and a thicker CdS shell would decrease their stability. For both CdS/ZnS core-shell NWs, the band gap decreases linearly with increasing the shell when the core size is fixed. However, when the diameter of NWs is fixed, CdS-core/ZnS-shell NWs with a thicker shell would have larger band gap. The results agree well with that of red-shift or blue-shift of the spectrum in experimental observations. The partial density of states indicates that the contribution to valence band maximum mainly comes from the S-3p state, and the contribution to conduction band minimum mainly comes from Cd-5s state for CdS-core/ZnS-shell NWs. Thus the electrons would be effectively confined in CdS core, and the holes tend to distribute over both the core and shell. It can be deduced that CdS-core/ZnS-shell NWs with a thicker shell may have larger mobility.

Molten Au/Ge alloy migration in Ge nanowires.

Herein, we report time-resolved in situ transmission electron microscopy observation of Au particle melting at a Ge nanowire tip, subsequent forming of Au/Ge alloy liquid, and its migrating within the Ge nanowire. The migration direction and position of the Au/Ge liquid can be controlled by the applied voltage and the migration speed shows a linear deceleration in the nanowire. In a migration model proposed, the relevant dynamic mechanisms (electromigration, thermodiffusion, and viscous force, etc.) are discussed in detail. This work associated with the liquid mass transport in the solid nanowires should provide new insights into the crystal growth, interface engineering, and fabrication of the heterogeneous nanostructure-based devices.

In situ atom scale visualization of domain wall dynamics in VO2 insulator-metal phase transition.

A domain wall, as a device, can bring about a revolution in developing manipulation of semiconductor heterostructures devices at the atom scale. However, it is a challenge for these new devices to control domain wall motion through insulator-metal transition of correlated-electron materials. To fully understand and harness this motion, it requires visualization of domain wall dynamics in real space. Here, domain wall dynamics in VO2 insulator-metal phase transition was observed directly by in situ TEM at atom scale. Experimental results depict atom scale evolution of domain morphologies and domain wall exact positions in (202) and (040) planes referring to rutile structure at 50°C. In addition, microscopic mechanism of domain wall dynamics and accurate lattice basis vector relationship of two domains were investigated with the assistance of X-ray diffraction, ab initio calculations and image simulations. This work offers a route to atom scale tunable heterostructure device application.

Tunable Band Gap and Conductivity Type of ZnSe/Si Core-Shell Nanowire Heterostructures.

The electronic properties of zincblende ZnSe/Si core-shell nanowires (NWs) with a diameter of 1.1-2.8 nm are calculated by means of the first principle calculation. Band gaps of both ZnSe-core/Si-shell and Si-core/ZnSe-shell NWs are much smaller than those of pure ZnSe or Si NWs. Band alignment analysis reveals that the small band gaps of ZnSe/Si core-shell NWs are caused by the interface state. Fixing the ZnSe core size and enlarging the Si shell would turn the NWs from intrinsic to p-type, then to metallic. However, Fixing the Si core and enlarging the ZnSe shell would not change the band gap significantly. The partial charge distribution diagram shows that the conduction band maximum (CBM) is confined in Si, while the valence band maximum (VBM) is mainly distributed around the interface. Our findings also show that the band gap and conductivity type of ZnSe/Si core-shell NWs can be tuned by the concentration and diameter of the core-shell material, respectively.

Melting of metallic electrodes and their flowing through a carbon nanotube channel within a device.

Evidence is presented of a new cause of Joule heating within a simple electronic device involving a multiwalled carbon nanotube (CNT) mounted on two metal electrodes forming an electrical circuit. This time-resolved, high-resolution in situ observation of metal electrode material melting and its flow driven by the thermomigration and electromigration forces through the CNT channel sheds an additional light on the effects affecting the real electrical performance of the CNT-based devices.