Shaoyun Huang

Covalently bonded single-molecule junctions with stable and reversible photoswitched conductivity

Stable molecular switches Many single-molecule current switches have been reported, but most show poor stability because of weak contacts to metal electrodes. Jia et al. covalently bonded a diarylethene molecule to graphene electrodes and achieved stable photoswitching at room temperature (see the Perspective by Frisbie). The incorporation of short bridging alkyl chains between the molecule and graphene decoupled their pielectron systems and allowed fast conversion of the open and closed ring states. Science, this issue p. 1443; see also p. 1394 Stable molecular conduction junctions were formed by covalently bonding single diarylethenes to graphene electrodes. Through molecular engineering, single diarylethenes were covalently sandwiched between graphene electrodes to form stable molecular conduction junctions. Our experimental and theoretical studies of these junctions consistently show and interpret reversible conductance photoswitching at room temperature and stochastic switching between different conductive states at low temperature at a single-molecule level. We demonstrate a fully reversible, two-mode, single-molecule electrical switch with unprecedented levels of accuracy (on/off ratio of ~100), stability (over a year), and reproducibility (46 devices with more than 100 cycles for photoswitching and ~105 to 106 cycles for stochastic switching).

Electron trapping, storing, and emission in nanocrystalline Si dots by capacitance–voltage and conductance–voltage measurements

Temperature and frequency dependent electrical properties of SiO2/nanocrystalline Si (nc-Si)/SiO2 sandwich structures have been studied. A clear shift of the capacitance–voltage and conductance–voltage characteristics toward positive gate voltage suggests electron trapping in an nc-Si dot. The role of interface states and deep traps in our devices has also been thoroughly examined and shown to be unimportant on the overall device performance. The discharging process is found to be logarithmic with time and weakly temperature dependent. The long memory retention time and the logarithmic time dependence of charge loss in the dots are explained by a buildup of opposing electric field in the tunnel oxide, which hinders the discharge of electrons remaining in the dots.

Patterning two-dimensional chalcogenide crystals of Bi2Se3 and In2Se3 and efficient photodetectors.

Patterning of high-quality two-dimensional chalcogenide crystals with unique planar structures and various fascinating electronic properties offers great potential for batch fabrication and integration of electronic and optoelectronic devices. However, it remains a challenge that requires accurate control of the crystallization, thickness, position, orientation and layout. Here we develop a method that combines microintaglio printing with van der Waals epitaxy to efficiently pattern various single-crystal two-dimensional chalcogenides onto transparent insulating mica substrates. Using this approach, we have patterned large-area arrays of two-dimensional single-crystal Bi2Se3 topological insulator with a record high Hall mobility of ∼1,750 cm2 V−1 s−1 at room temperature. Furthermore, our patterned two-dimensional In2Se3 crystal arrays have been integrated and packaged to flexible photodetectors, yielding an ultrahigh external photoresponsivity of ∼1,650 A W−1 at 633 nm. The facile patterning, integration and packaging of high-quality two-dimensional chalcogenide crystals hold promise for innovations of next-generation photodetector arrays, wearable electronics and integrated optoelectronic circuits.

Quantum confinement energy in nanocrystalline silicon dots from high-frequency conductance measurement

Electron charging and discharging processes in floating gate metal–oxide–semiconductor memory based on nanocrystalline silicon (nc-Si) dots were investigated at room temperature using capacitance–voltage and conductance–voltage (G–V) measurements. From charged nc-Si dots, a sequential electron discharging processes was clearly observed in G–V spectroscopy. The fine structure in the observed conductance peaks has been interpreted in terms of the Coulomb blockade and quantum confinement effects of nc-Si dots, which allowed the electron-addition energy to be estimated at 50 meV. Taking the electron-charging energy between the silicon substrate and the floating dot (30 meV) into account, the quantum confinement energy was found to be as significant as the electron charging energy for nc-Si dots, with ∼8 nm in diameter, embedded in silicon oxide.

Microstructure and origin of dislocation etch pits in GaN epilayers grown by metal organic chemical vapor deposition

Morphology and microstructure of dislocation etch pits in GaN epilayers etched by molten KOH have been investigated by atomic force microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). Three types of etch pits (α, β, and γ) are observed. The α type etch pit shows an inversed trapezoidal shape, the β one has a triangular shape, and the γ type one has a combination of triangular and trapezoidal shapes. TEM observation shows that α, β, and γ types etch pits originate from screw, edge, and mixed-type threading dislocations (TDs), respectively. For the screw-type TD, it is easily etched along the steps that the dislocation terminates, and consequently, a small Ga-polar plane is formed to prevent further vertical etching. For the edge-type TD, it is easily etched along the dislocation line. Since the mixed-type TDs have both screw and edge components, the γ type etch pit has a combination of α and β type shapes. It is also found that the chemical stabilization of Ga-polar surfac...

High electron mobility and quantum oscillations in non-encapsulated ultrathin semiconducting Bi2O2Se

High-mobility semiconducting ultrathin films form the basis of modern electronics, and may lead to the scalable fabrication of highly performing devices. Because the ultrathin limit cannot be reached for traditional semiconductors, identifying new two-dimensional materials with both high carrier mobility and a large electronic bandgap is a pivotal goal of fundamental research. However, air-stable ultrathin semiconducting materials with superior performances remain elusive at present. Here, we report ultrathin films of non-encapsulated layered Bi2O2Se, grown by chemical vapour deposition, which demonstrate excellent air stability and high-mobility semiconducting behaviour. We observe bandgap values of ∼0.8 eV, which are strongly dependent on the film thickness due to quantum-confinement effects. An ultrahigh Hall mobility value of >20,000 cm2 V-1 s-1 is measured in as-grown Bi2O2Se nanoflakes at low temperatures. This value is comparable to what is observed in graphene grown by chemical vapour deposition and at the LaAlO3-SrTiO3 interface, making the detection of Shubnikov-de Haas quantum oscillations possible. Top-gated field-effect transistors based on Bi2O2Se crystals down to the bilayer limit exhibit high Hall mobility values (up to 450 cm2 V-1 s-1), large current on/off ratios (>106) and near-ideal subthreshold swing values (∼65 mV dec-1) at room temperature. Our results make Bi2O2Se a promising candidate for future high-speed and low-power electronic applications.

Optical and electrical properties of Zn1−xCdxO films grown on Si substrates by reactive radio-frequency magnetron sputtering

Zn1−xCdxO films (0⩽x⩽0.179) were grown on Si (001) substrates at 750°C with a radio-frequency reactive magnetron sputtering method. Difference between the photoluminescence (PL) spectra taken at room temperature (RT) and at 12K is reported and is deduced to be the result of PL emission from the ZnCdO phases with wurtzite and zinc blende structures. It is also found that the RT PL intensity is in inverse proportion to the carrier concentration in the films. Cd incorporation results in the transform of conductivity from p type to n type and a decrease of carrier mobility.

Toward long-term retention-time single-electron-memory devices based on nitrided nanocrystalline silicon dots

A memory capacitor with a structure of Si-SiO/sub 2//nc-Si dots/silicon nitride films/SiO/sub 2/ was prepared by means of nc-Si dot deposition followed by N/sub 2/ plasma nitridation processes. The memory device offers dual memory nodes: nc-Si dots and traps in silicon-nitride films. An enlarged memory window in CV characteristics was observed in memory operations, due to the extra traps in silicon-nitrides. The charge-loss rate was found to be much smaller than that of single memory nodes using nc-Si dots only. The provided larger memory window (about twice the width) and longer retention time in the memory operations (three orders of magnitude) are discussed in terms of trap-assisted charging/discharging mechanisms.

Charge storage in nitrided nanocrystalline silicon dots

Nitrided nanocrystalline silicon (nc-Si) dots are proposed to be a candidate of memory nodes for nonvolatile memory device applications to make good use of advantages of both silicon quantum dots and silicon nitride films. The stored charges in the memory nodes are identified not only in nc-Si dots (electron delocalized states) but also in defect states at the nc-Si/silicon-nitride interface (electron localized states) by current-voltage (I-V) spectrum. Temperature dependences of the I-V characteristics demonstrate an evolution of stored charges in the combined system and clarify its storage mechanisms.

Temperature dependence of the specific resistance in Ti/Al/Ni/Au Ohmic contacts on (NH4)2Sx treated n-type GaN

The temperature dependence of the specific contact resistance in annealed Ti/Al/Ni/Au multilayers on (NH4)2Sx treated n-type GaN has been studied in the temperature range from 25 to 600 °C by the transmission line technique. It is found that the specific contact resistivity ρc of the sample treated with (NH4)2Sx solution for 5 min at 90 °C decreases with increasing measuring temperature, while the ρc of the sample treated with (NH4)2Sx solution for 25 min at 90 °C increases with increasing measuring temperature. Excellent agreement with the “5 min treated” sample can be obtained by the field emission model with an average Schottky barrier height (SBH) ϕB=1.05 eV. Meanwhile, a field emission model with a temperature-dependent effective SBH is suggested to be responsible for the “25 min treated” sample in which metal/semiconductor interface potential pinch-off may occur. The high-resolution transmission electron microscope results support the above model.