Phase transition in 2D MoS2 is an important issue in transition metal dichalcogenides. The mechanism of the phase transition is investigated extensively, while a comprehensive understanding of the mechanism remains to be elusive. The 2H to 1T phase transition of MoS2 can be induced by intercalation with alkali metals, associated with transformation of its electronic and structural orders. Here, by using in situ transmission electron microscopy, the electrochemical sodiation processes of MoS2 nanosheets are investigated. This paper points out an intermediate phase 2H-Na0.25MoS2 with 2 × √3 superstructure. The phase transition occurs from 2H-Na0.5MoS2 to 1T-Na0.5MoS2, where the ratio of Mo3+ and Mo4+ is 1:1 and intervalence charge transfer (IVCT) between the two ions should not be neglected. Based on this viewpoint it is proposed that the IVCT plays an important role in the formation of the phase transition. The findings provide a new perspective on the mechanism of the phase transition, and help to better understand the phase transition mechanism of other transition metal dichalcogenides.

•Rate mechanism of V2O5/SnO2 nanowire for Li-ion batteries is studied by in-situ TEM.•High cyclability and capacity can be achieved at high rate for V2O5/SnO2 nanowire.•V2O5 coating confined the Sn nanoparticle aggregation during cycling.Correlating composition and structures with battery performance is key aspect of electrode material design and improvement. Here utilizing in situ open cell transmission electron microscopy, we studied the in situ cycling rate performance of vanadium oxide coated tin dioxide nanowire electrode by tuning the lithiation/delithiation current. In situ results show that the good rate performance of such high capacity compositional material lies in the layered vanadium oxide coating strategy. For cycling at high rate, the layered vanadium oxide also serves as fast ions and electrons transportation route while tin nanoparticles aggregate to the surface with sizes controlled by the coating layer, cycle induced volume change is released to the surface and excellent mechanical tolerance of tin nanoparticle and inner nanowire ensure improved cyclability of the electrode.Working mechanism of V2O5 coated SnO2 nanowire electrode for Li-ion batteries with high rate performance is proposed based on operando TEM study to further design high performance compositional materials. Layered V2O5 as shell provides fast Li-ion and electron transport route while well dispersed Sn nanoparticles confined in coating layer that introduced by pre-charge at high rate as core provides high capacity.Download high-res image (206KB)Download full-size image

The dynamic behavior of the interface between few layer graphene (FLG) and tungsten metal tips under Joule heating has been studied by in-situ transmission electron microscopy (TEM) method. High-resolution and real-time observations show the tungsten tip ‘swallow’ carbon atoms of the FLG and ‘spit’ graphite shells at its surface. The tip was carbonized to tungsten carbide (WC, W2C and WCx) after this process. A carbon diffusion mechanism has been proposed based on the diffusion of carbon atoms through the tungsten tip and separation from the surface of the tip. After Joule heating, the initial FLG-metal mechanical contact was transformed to FLG-WCx-W contact, which results in significant improvement on electrical conductivity at the interface.

We investigated the strong coupling between the excitons of ZnO nanowires (NWs) and the localized surface plasmons (LSPs) of individual Ag nanoparticles (NPs) by monochromated electron energy loss spectroscopy (EELS) in an aberration-corrected scanning transmission electron microscopy (STEM) instrument. The EELS results confirmed that the hybridization of the ZnO exciton with the LSPs of the Ag NP created two plexcitons: the lower branch plexcitons (LPs) with a symmetrical dipole distribution and the upper branch plexcitons (UPs) with an antisymmetrical dipole distribution. The spatial maps of the LP and UP excitations reveal the nature of the LSP–exciton interactions. With decreasing size of the Ag NP the peak energies of the LPs and UPs showed a blue-shift and an anticrossing behavior at the ZnO exciton energy was observed. The coupled oscillator model explains the dispersion curve of the plexcitons and a Rabi splitting energy of ∼170 meV was deduced. The high spatial and energy resolution STEM-EELS approach demonstrated in this work is general and can be extended to study the various coupling interactions of a plethora of metal–semiconductor nanocomposite systems.

A lateral piezopotential-gated field-effect transistor is realized by operation of scaning tunneling microscope (STM) tip onto an individual ZnO nanowires inside high-resolution transmission electron microscope (TEM). The electric transport behavior of ZnO nanowires under bending by point contact of STM tip at the cross section of nanowire end shows that, the nanowire conductance decreases up to 2 orders of magnitude when a 2.63% bending strain is applied. Experimental results and their detailed analysis reveal that the regarded change in conductance is not due to Schottky barrier at the contact interface but originates from the carrier depletion caused by the lateral piezoelectric potential within the nanowire. The bending strain-gated transistor is one of the new type piezotronics devices.

Solid electrolyte based-resistive memories have been considered to be a potential candidate for future information technology with applications in non-volatile memory, logic circuits and neuromorphic computing. A conductive filament model has been generally accepted to be the underlying mechanism for the resistive switching. However, the growth dynamics of such conductive filaments is still not fully understood. Here, we explore the controllability of filament growth by correlating observations of the filament growth with the electric field distribution and several other factors. The filament growth behavior has been recorded using in situ transmission electron microscopy. By studying the real-time recorded filament growth behavior and morphologies, we have been able to simulate the electric field distribution in accordance with our observations. Other factors have also been shown to affect the filament growth, such as Joule heating and electrolyte infrastructure. This work provides insight into the controllable growth of conductive filaments and will help guide research into further functionalities of nanoionic resistive memories.

Nanostructured silicon anodes, which possess extremely high energy density and accommodate large strain without pulverization, have been developed rapidly for high-power lithium ion batteries. Here, using in situ transmission electron microscopy, the lithiation behavior of silicon nanowires with diameters smaller than 60 nm was investigated. The study demonstrated a direct dependence of the self-limiting lithiation on the pristine diameter. A “punch-through” lithiation process at the core of nanowires with pristine diameters slightly larger than the self-limiting threshold is suggested to occur with the consequent formation of a stage structure. Our work demonstrates the crucial role of mechanical stress and local defects in determining the migration and geometry of the reaction front at the mesoscopic scale. This intriguing finding holds critical significance for the application of silicon nanostructures in high-power lithium ion batteries.Keywords: in situ transmission electron microscopy; lithium ion battery; reaction front; silicon nanowire; stage structure

Fading mechanism of tin dioxide (SnO2) electrodes in lithium ion batteries has attracted much attentions, which is of great importance for the battery applications. In this paper, electrochemical lithiation-delithiation cycles of individual SnO2 nanowires were conducted in situ in a high-resolution transmission electron microscopy (TEM). Major changes in volume with expansions of 170%∼300% on SnO2 nanowire electrodes were observed during the first lithiation process in electrochemical cycling, including conversion reaction of SnO2 precursor to Li2O matrix and active lithium host Sn, and alloying of Sn with Li to form brittle Li-Sn alloy. SnO2 nanowire electrodes were inclined to suffer from thermal runaway condition in the first two cycles. During cycling, morphology and composition evolution of SnO2 nanowire electrodes were recorded. Cyclic lithiation and delithiation of the electrode demonstrated the phase transition between Li13Sn5 and Sn. Metallic Sn clusters were formed and their sizes enlarged with increasing cycle times. Detrimental aggregation of Sn clusters caused pulverization in SnO2 nanowire electrodes, which broke the conduction and transport path for electrons and lithium ions. The real-time in situ TEM revealed fading mechanism provides important guidelines for the viable design of the SnO2 nanowire electrodes in lithium ion batteries.