Nian-Ke Chen

Novel Zn-doped SnO2 hierarchical architectures: synthesis, characterization, and gas sensing properties

Novel Zn-doped SnO2hierarchical architectures were synthesized by a simple hydrothermal route. The observation of field-emission electron microscopy and transmission electron microscopy showed that Zn-doped SnO2 hierarchical architectures were composed of one-dimensional nanocones. Interestingly, these nanocones were almost parallel to each other and knitted by other parallel nanocones. The morphology of the products could be controlled by varying the concentration of Zn2+. A possible formation mechanism was proposed from the viewpoint of nucleation and the crystal growth habit. Evidences of dopant incorporation were demonstrated in the X-ray diffraction and X-ray photoelectron spectroscopy measurement of Zn-doped SnO2nanocones. The UV-vis absorption spectra of samples exhibited a blue shift with a decrease of the size of nanocones. Moreover, gas sensors based on the hierarchical Zn-doped SnO2nanocones displayed higher response to ethanol compared with the pure urchin-like SnO2 nanostructures. Finally, based on first-principles calculations, the enhancement in sensitivity toward ethanol could be explained by the strong coulomb binding between ZnSn and its neighboring O vacancies.

Crystalline Liquid and Rubber-Like Behavior in Cu Nanowires

Via in situ TEM tensile tests on single crystalline copper nanowires with an advanced tensile device, we report here a crystalline-liquid-rubber-like (CRYS-LIQUE-R) behavior in fracturing crystalline metallic nanowires. A retractable strain of the fractured crystalline Cu nanowires can approach over 35%. This astonishing CRYS-LIQUE-R behavior of the fracturing highly strained single crystalline Cu nanowires originates from an instant release of the stored ultralarge elastic energy in the crystalline nanowires. The release of the ultralarge elastic energy was estimated to generate a huge reverse stress as high as ~10 GPa. The effective diffusion coefficient (D(eff)) increased sharply due to the consequent pressure gradient. In addition, due to the release of ultrahigh elastic energy, the estimated concomitant temperature increase was estimated as high as 0.6 Tm (Tm is the melting point of nanocrystalline Cu) on the fractured tip of the nanowires. These factors greatly enhanced the atomic diffusion process. Molecular dynamic simulations revealed that the very high reverse stress triggered dislocation nucleation and exhaustion.

Amorphous structure and bonding chemistry of aluminium antimonide(AlSb)) alloy for phase-change memory device

With the help of first-principles molecular dynamics calculations, we obtained the atomic picture of amorphous AlSb(a-AlSb) for phase-change memory application. Generally, a-AlSb shows sp3 bonding network, which is the intrinsic characteristic for its good thermal stability. Significant wrong(homogenous) Al-Al bonds can also be observed from the pair correlation function. This hints the amorphous phase may consist of Al cluster and Sb-rich Al-Sb alloy. Recent experiment has observed the Sb-rich region of AlSb alloy can be switched to crystal, on the basis of which, combined with our calculations, we thus propose that on the one hand such a Sb-rich region in a-AlSb can retain the rapid crystallization like pure Sb solid and on the other hand some Al atoms play the important role of stabilizing Sb rich network with sp3 bonding. The present study offers a microscopic view to understand the phase change mechanism of AlSb alloy for information storage device.

Instability Origin and Improvement Scheme of Facial Alq 3 for Blue OLED Application

Degradation phenomenon and poor stability of tris(8-hydroxyquinoline) aluminum(III)(Alq3)-based organic light-emitting diodes(OLEDs) have attracted much attention. In this paper, we discussed the origin of instability of the facial Alq3-based blue luminescent OLEDs with the help of first-principles calculation. The results show that environmental humidity seriously affects the luminescence stability of Alq3-based OLEDs. H2O molecules in environment can be firmly bound to the oxygen atoms of the facial Alq3, which then act as starting points for further degradation of Alq3. Moreover, the interactions between facial Alq3 and different cathode metal layers were investigated to explain the experiment phenomenon. A design guideline for diminishing the strong attraction from oxygen atoms can be proposed to protect Alq3 and improve the stability of materials applied in OLEDs.

Possible n/p-type conductivity of two-dimensional graphene oxide by boron and nitrogen doping: Evaluated via constrained excitation

As the first-principles calculations using the supercell approximation give widely scattered results in a two-dimensional charged system, making the evaluation of defect ionization energy difficult, here an alternative constrained excitation is applied to overcome this problem for defect analysis. As an example in graphene oxide with 50% oxygen coverage (according to the popular epoxy-chain-plus-hydroxyl-chain model), the structures, stabilities, and electronic properties of nitrogen and boron dopants are investigated. Generally, boron prefers to replace carbon in the sp3 region as an acceptor while nitrogen has a tendency to substitute the sp2 carbon close to the boundary between the sp2 region and the sp3 region as a donor. Their ionization energies are 0.24–0.42 eV for boron and 0.32–0.67 eV for nitrogen. However, a special case of nitrogen doped in the boundary-sp3 carbon can change to be an acceptor with the assistance of its neighboring (epoxy) oxygen “Lift-off,” leading to the shallowest ionization...

Slide fastener reduction of graphene-oxide edges by calcium: insight from ab initio molecular dynamics.

The reduction of graphene oxide can be used as a simple way to produce graphene on a large scale. However, the numerous edges produced by the oxidation of graphite seriously degrade the quality of the graphene and its carrier transport property. In this work, the reduction of oxygen-passivated graphene edges and the subsequent linking of separated graphene sheets by calcium are investigated by using first-principles calculations. The calculations show that calcium can effectively remove the oxygen groups from two adjacent edges. The joining point of the edges serves as the starting point of the reduction and facilitates the reaction. Once the oxygen groups are removed, the crack is sutured. If the joining point is lacking, it becomes difficult to zip the separated fragments. A general electron-reduction model and a random atom-reduction model are suggested for these two situations. The present study sheds light on the reduction of graphene-oxide edges by using reactive metals to give large-sized graphene through a simple chemical reaction.

Electrical properties and structural transition of Ge2Sb2Te5 adjusted by rare-earth element Gd for nonvolatile phase-change memory

Phase change memory has been considered as the next generation in non-volatile electronic data storage. The property modulation of such materials by the doping of rare-earth elements has drawn a lot of attention, which motivates us to search for the optimal dopants and reveal the underlying mechanisms. Here, we investigate the role of Gd as a dopant in Ge2Sb2Te5, which exhibits higher crystalline resistance and better thermal stability and antioxidant capacity than the undoped counterpart. Moreover, Gd dopants suppress both the processes of phase transition and grain growth. The crystalline structure remains unchanged with Gd dopants and vacancies are randomly distributed. Furthermore, the bonding mechanism was theoretically investigated. In the amorphous state, Gd atoms modify the local structures around Ge, Sb, and Te atoms. The large coordination number of Gd and the “Gd–Te distorted pentagonal bipyramidal-like” structure can be attributed to the good thermal stability. These microscopic findings figure out some of the key issues about the bonding mechanism, electrical properties, and crystallization behaviors of Gd doped phase change memory materials, which could be useful for storage devices.Phase change memory has been considered as the next generation in non-volatile electronic data storage. The property modulation of such materials by the doping of rare-earth elements has drawn a lot of attention, which motivates us to search for the optimal dopants and reveal the underlying mechanisms. Here, we investigate the role of Gd as a dopant in Ge2Sb2Te5, which exhibits higher crystalline resistance and better thermal stability and antioxidant capacity than the undoped counterpart. Moreover, Gd dopants suppress both the processes of phase transition and grain growth. The crystalline structure remains unchanged with Gd dopants and vacancies are randomly distributed. Furthermore, the bonding mechanism was theoretically investigated. In the amorphous state, Gd atoms modify the local structures around Ge, Sb, and Te atoms. The large coordination number of Gd and the “Gd–Te distorted pentagonal bipyramidal-like” structure can be attributed to the good thermal stability. These microscopic findings figur...

Erratum to “Metal–Insulator Transition of Ge–Sb–Te Superlattice: An Electron Counting Model Study”

Presents corrections to the paper, “Metal–insulator transition ofGe–Sb–Te superlattice: An electron counting model study,” (Chen, N.-K.), IEEE Trans. Nanotechnol., vol. 17, no. 1, pp. 140–146, Jan. 2018.

Exploring long-wave infrared transmitting materials with AxBy form: First-principles gene-like studies.

Long-wave infrared (8–12 μm) transmitting materials play critical roles in space science and electronic science. However, the paradox between their mechanical strength and infrared transmitting performance seriously prohibits their applications in harsh external environment. From the experimental view, searching a good window material compatible with both properties is a vast trail-and-error engineering project, which is not readily achieved efficiently. In this work, we propose a very simple and efficient method to explore potential infrared window materials with suitable mechanical property by first-principles gene-like searching. Two hundred and fifty-three potential materials are evaluated to find their bulk modulus (for mechanical performance) and phonon vibrational frequency (for optical performance). Seven new potential candidates are selected, namely TiSe, TiS, MgS, CdF2, HgF2, CdO, and SrO. Especially, the performances of TiS and CdF2 can be comparable to that of the most popular commercial ZnS at high temperature. Finally, we propose possible ranges of infrared transmission for halogen, chalcogen and nitrogen compounds respectively to guide further exploration. The present strategy to explore IR window materials can significantly speed up the new development progress. The same idea can be used for other material rapid searching towards special functions and applications.

Origin of high data retention for Ge1Cu2Te3 phase-change memory

To meet the high-temperature applications of the phase change memory, Ge1Cu2Te3 (GCT) has been proposed to be a suitable candidate due to its good amorphous stability. Here, we investigate the basic atomic bonding mechanism by first-principles calculations to understand this. The amorphous GCT has significant chemical disorder with large amounts of homopolar bonds. Cage-like clusters (composed of 3-fold rings) are mainly related to Cu atoms. The bonding mechanism in crystalline GCT is proposed and demonstrated by the nonequivalent sp3 hybridization with Te lone-pair electrons. In contrast, amorphous GCT requires Cu d electrons to participate in bonding due to the isolation of Te lone-pair electrons. Thus, the vast difference in atomic and electronic structures between the amorphous and crystalline phase makes the good amorphous stability for high data retention.