Kailiang Zhang

Synthesis of Large-Area Highly Crystalline Monolayer Molybdenum Disulfide with Tunable Grain Size in a H2 Atmosphere

Large-area and highly crystalline monolayer molybdenum disulfide (MoS2) with a tunable grain size was synthesized in a H2 atmosphere. The influence of introduced H2 on MoS2 growth and grain size, as well as the corresponding mechanism, was tentatively explored by controlling the H2 flow rate. The as-grown monolayer MoS2 displays excellent uniformity and high crystallinity evidenced by Raman and high-resolution transmission electron microscopy. The Raman results also give an indication that the quality of the monolayer MoS2 synthesized in a H2 atmosphere is comparable to that synthesized by using seed or mechanical exfoliation. In addition, the electronic properties and dielectric inhomogeneity of MoS2 monolayers were also detected in situ via scanning microwave microscopy, with measurements on impedance and differential capacitance (dC/dV). Back-gated field-effect transistors based on highly crystalline monolayer MoS2 shows a field-effect mobility of ∼13.07 cm2 V(-1) s(-1) and an Ion/Ioff ratio of ∼1.1×10(7), indicating that the synthesis of large-area and high-quality monolayer MoS2 with H2 is a viable method for electronic and optoelectronic applications.

Ultra-Low Power Ni/HfO 2 /TiO x /TiN Resistive Random Access Memory With Sub-30-nA Reset Current

In this letter, we report ultra-low power (sub-30-nA reset current, Ireset) Ni/HfO2/TiOx/TiN RRAM devices that were fabricated with the rapid thermal oxidation of evaporated titanium. RRAM devices show forming-free, bipolar resistive switching behavior, low-resistive state (LRS) nonlinearity, good data retention, and stability. The resistive switching mechanism is mainly attributed to Schottky barrier modulation induced by O2- migration at the Ni/HfO2 interface. LRS/high-resistive state current conduction is controlled by Schottky emission/trap-controlled space-charge-limited current. The TiOx film is believed to provide a local high-density current for the device, confirmed by conductive atomic force microscope results.

Crosstalk analysis of carbon nanotube bundle interconnects.

Carbon nanotube (CNT) has been considered as an ideal interconnect material for replacing copper for future nanoscale IC technology due to its outstanding current carrying capability, thermal conductivity, and mechanical robustness. In this paper, crosstalk problems for single-walled carbon nanotube (SWCNT) bundle interconnects are investigated; the interconnect parameters for SWCNT bundle are calculated first, and then the equivalent circuit has been developed to perform the crosstalk analysis. Based on the simulation results using SPICE simulator, the voltage of the crosstalk-induced glitch can be reduced by decreasing the line length, increasing the spacing between adjacent lines, or increasing the diameter of SWCNT.

Scalable Synthesis of Highly Crystalline MoSe2 and Its Ambipolar Behavior

Atomically thin, two-dimensional material molybdenum diselenide (MoSe2) has been shown to exhibit significant potential for diverse applications. The intrinsic band gap of MoSe2 allows it to overcome the shortcomings of the zero-band-gap graphene, while its higher electron mobilities when compared to molybdenum disulfide (MoS2) make it more appropriate for practical devices in electronics and optoelectronics. However, its controlled growth has been an ongoing challenge for investigations and practical applications of the material. Here, we present an atmospheric pressure chemical vapor deposition (CVD) method to achieve highly crystalline, single- and few-layered MoSe2 using a SiO2/Si substrate. Our findings suggested that careful optimization of the flow rate can result in the controlled growth of large-area MoSe2 with desired layer numbers due to the adjustment of gaseous MoSe2 partial pressure and nucleation density. The FETs fabricated on such as-synthesized MoSe2 displayed different transport behaviors depending on the layer numbers, which can be attributed to the formation of Se vacancies generated during low flow rates. Monolayer MoSe2 showed n-type characteristics with an Ion/Ioff ratio of ∼106 and a carrier mobility of ∼19 cm2 V-1 s-1, whereas bilayer MoSe2 showed n-type-dominant ambipolar behavior with an Ion/Ioff ratio of ∼105 and a higher mobility of ∼65 cm2 V-1 s-1 for electrons as well as ∼9 cm2 V-1 s-1 for holes. Our results provide a foundation for property-controlled synthesis of MoSe2 and offer insight on the potential applications of our synthesized MoSe2 in electronics and optoelectronics.

In situ visualization and detection of surface potential variation of mono and multilayer MoS2 under different humidities using Kelvin probe force microscopy

The surface potential (SP) variations in mono and multilayer molybdenum disulfide (MoS2) are visualized in situ and detected using Kelvin probe force microscopy (KPFM) in different humidity conditions for the first time. N-type doping, which originates from the SiO2 substrate, is discovered in the exfoliated MoS2 and is accompanied by a screening length of five layers. The influence of water, which serves as an environmental gating for MoS2, is investigated by controlling the relative humidities (RHs) in the environmental chamber. A monotonic decrease in the SP is observed when the threshold concentration is achieved. This corresponds to the Fermi level variation, which is dominated by different processes. The results also indicate that water adsorption could result in MoS2 p-type doping and provide compensation that partially counteracts the substrate effect. Under this condition, the interlayer screening effect is influenced because of the water dipole-induced electric field. Density functional theory calculations are performed to determine the band structure variations and the interactions between water molecules and between water molecules and the MoS2 surface in mono and trilayer MoS2 under different RHs. The calculations are in excellent agreement with the experimental results. We propose that in situ measurements of the SP using KPFM under different environmental regimes is a noninvasive and effective method to provide real-time visualization and detection of electronic property variations in two-dimensional materials.

Microstructure and Nanometer Scale Piezoelectric Properties of c-BN Thin Films With Cu Buffer Layer by Piezoresponse Force Microscopy

Boron nitride (BN) films were deposited on different metal buffer layer (Cu, Al, Pt) using RF magnetron sputtering. Microstructure and piezoelectric properties of BN films were characterized by FTIR and piezoresponse force microscopy (PFM). After optimizing the deposition condition, the 95% cubic phase volume fraction from FTIR results indicates that BN films with the highest cubic phase is obtained under 120 V bias voltages. Based on the PFM measurements results, the images of the piezoelectric response, the butterfly curve, and the hysteresis loop of a certain grain in the films and polarization image of c-BN/Cu/SiO2/Si structure are confirmed. Compared with Pt and Al, Cu buffer layer is more suitable for depositing c-BN films with better piezoelectric properties by conventional fabrication process.

Electrostatic capacitance extraction for carbon nanotube bundle interconnects

The electrostatic capacitance depending on interconnect geometry is very important for the performance analysis of carbon nanotube (CNT) bundle. In this paper, electrostatic capacitance of each individual CNT in the bundle will be extracted accurately by Ansoft Maxwell. The results show that the ratio of electrostatic capacitance and total capacitance in CNT bundle becomes larger with decreasing the number of CNTs, and the electrical performance is significantly related to the electrostatic capacitance.

Crosstalk analysis of carbon nanotube bundle interconnects

In this paper, the interconnect parameters for a carbon nanotube (CNT) bundle are calculated first, then the equivalent circuit has been developed to perform the crosstalk analysis and in the end the main influencing factors are discussed. On the basis of simulation which is completed by simulation software SPICE, crosstalk voltage can be decreased by increasing spacing between adjacent lines, setting the appropriate position when the length is fixed, decreasing line length and selecting the appropriate frequency.

Effect of AlO x inserting layer on Cu/VO x /TiN RRAM devices performance

In this paper, a low power resistive switching memory (RRAM) device with Cu/VO_x/AlO_x/TiN stack structure is demonstrated. We studied the impact of different thicknesses of AlO_x film on the Cu/VO_x/AlO_x/TiN RRAM device. With the increasing of AlO_x thickness, the resistance of pristine state gets higher. Higher resistance of high resistive states (R_HRS) (up to 10⁹Ω), lower reset current of 40μA and lower operation voltage could be obtained after inserting AlO_x film. Compared with the Cu/VO_x/TiN structure, the Cu/VO_x/AlO_x/TiN stack structure showed better electrical properties. Therefore, this kind of method to insert AlO_x between resistive layer and electrode would have a broader application prospect in the field of RRAM.

Simulation of Hydrogen Diffusion and Adsorption on Single-Walled Carbon Nanotube

With clean fuels increasingly used for transportation due to environmental concerns and limited supply of fossil fuels, hydrogen is attracting more attention as a clean fuel free from carbon dioxide and other greenhouse gas emissions. Analysis of hydrogen diffusion in single-walled carbon nanotube was performed with molecular dynamic simulation. The carbon nanotube is chosen because of a well-known fact that it is an excellent adsorption material with high surface volume ratio. In this paper, diffusivity rate are simulated to study the interaction of molecular and atomic hydrogen with single-walled carbon nanotubes. The adsorption energy and repulsive energy are analyzed to explore the nanotube structure after desorption and the mechanism of desorption. Electric charge density is also studied in order to understand better the process of hydrogen adsorption in CNT. A background of the hydrogen storage problem with carbon nanotubes is provided and the issues to be resolved have been highlighted. Future directions to address these challenges have also been suggested. We make a case that molecular simulation studies can identify the most promising structures and compositions to maximize hydrogen storage.Copyright