Haibing Peng

Chemical vapor deposition of thin crystals of layered semiconductor SnS2 for fast photodetection application.

Layered two-dimensional (2D) semiconductors, such as MoS(2) and SnS(2), have been receiving intensive attention due to their technological importance for the next-generation electronic/photonic applications. We report a novel approach to the controlled synthesis of thin crystal arrays of SnS(2) at predefined locations on chip by chemical vapor deposition with seed engineering and have demonstrated their application as fast photodetectors with photocurrent response time ∼ 5 μs. This opens a pathway for the large-scale production of layered 2D semiconductor devices, important for applications in integrated nanoelectronic/photonic systems.

High mobility and high on/off ratio field-effect transistors based on chemical vapor deposited single-crystal MoS2 grains

We report the electrical characteristics of field-effect transistors (FETs) with single-crystal molybdenum disulfide (MoS2) channels synthesized by chemical vapor deposition (CVD). For a bilayer MoS2 FET, the field-effect mobility is ∼17 cm2 V−1 s−1 and the on/off current ratio is ∼108, which are much higher than those of FETs based on CVD polycrystalline MoS2 films. By avoiding the detrimental effects of the grain boundaries and the contamination introduced by the transfer process, the quality of the CVD MoS2 atomic layers deposited directly on SiO2 is comparable to or better than the exfoliated MoS2 flakes. The result shows that CVD is a viable method to synthesize high quality MoS2 atomic layers.

Patterned growth of single-walled carbon nanotube arrays from a vapor-deposited Fe catalyst

Single-walled carbon nanotubes have been grown on a variety of substrates by chemical vapor deposition using low-coverage vacuum-deposited iron as a catalyst. Ordered arrays of suspended nanotubes ranging from submicron to several micron lengths have been obtained on Si, SiO 2 , Al 2 O 3 , and Si 3 N 4 substrates that were patterned on hundred nanometer length scales with a focused ion beam machine. Electric fields applied during nanotube growth allow the control of growth direction. Nanotube circuits have been constructed directly on contacting metal electrodes of Pt/Cr patterned with catalysts. Patterning with solid iron catalyst is compatible with modern semiconductor fabrication strategies and may contribute to the integration of nanotubes in complex device architectures.

Field effect transistors with layered two-dimensional SnS2−xSex conduction channels: Effects of selenium substitution

A thorough characterization of field effect transistors with conduction channels made of SnS2−xSex nanocrystals having different selenium content is presented. The main effect of increasing the selenium content is a suppression of the drain-source current modulation by the gate voltage. The temperature dependence of SnS2−xSex conductivity for all compositions is characterized by an activation energy that gradually decreases with x. A simple donor model, with parameters of SnS2 and SnSe2 deduced from density functional theory, suggests that the change in the activation energy is mostly due to enhanced dielectric constants that accompany the band gap reduction in SnS2−xSex.

Room-temperature single charge sensitivity in carbon nanotube field-effect transistors

Electrical current fluctuation studies are reported for coaxial p-type and n-type single-wall carbon nanotube field-effect transistors (FETs). Abrupt discrete switching of the source-drain current is observed at room temperature. The authors attribute these random telegraph signals to charge fluctuating electron traps near the FET conduction channels. Evolution of the current-switching behavior associated with the occupancy of individual electron traps is demonstrated and analyzed statistically. The result strongly indicates room temperature single charge sensitivity in carbon nanotube FETs, which may offer potential applications for single molecule sensors based on suitably prepared FET devices.

Coulomb blockade in suspended Si3N4-coated single-walledcarbon nanotubes

Uniform coaxial coating of suspended single-walled carbon nanotubes with high-quality dielectric silicon nitride has been obtained by low-pressure chemical vapor deposition. A three-terminal device has been demonstrated by coating a suspended metallic nanotube grown directly on contacting metal electrodes with subsequent patterning of a top gate electrode. Large charging energies have been observed in the suspended nanotubes and the conversion factor from gate voltage to the electrostatic potential in the nanotube approaches unity, which can be attributed to the device geometry.

Surface pattern and large low-field magnetoresistance in La0.5Ca0.5MnO3 films

Formation of an ordered surface structure in La0.5Ca0.5MnO3 films due to the mismatch of the thermal expansion coefficient between the film and the substrate has been investigated. The surface pattern consists of grain chains located on regular orthogonal cracks. The cracks serve as weak-link grain boundaries, and unusually enhanced low-field magnetoresistance (−14.4% in 400 Oe at 90 K) has been observed, which may be explained by spin-polarized tunneling across the grain boundaries.

Absence of zero-energy surface bound states in CuxBi2Se3studied via Andreev reflection spectroscopy

CuxBi2Se3 has been proposed as a potential topological superconductor characterized by an odd-parity full bulk superconducting gap and zero-energy surface Andreev bound states (Majorana fermions). A consequence of such Majorana fermions is a peak in the zero-energy density of states which should lead to a persistent zero-bias-conductance-peak (ZBCP) in Andreev reflection (AR) or tunneling experiments. Here we employ a newly developed nanoscale AR spectroscopy method to study normal metal/superconductor (N-S) devices featuring CuxBi2Se3. The results show that a ZBCP can be tuned in or out from CuxBi2Se3 samples depending on the N-S barrier strength. While the appearance of ZBCP may be traced to different origins, its absence under finite barrier strength represents the absence of zero-energy Majorana fermions. The present observations thus call for a reexamination of the nature of the superconducting state in CuxBi2Se3.

AB-Stacked Multilayer Graphene Synthesized via Chemical Vapor Deposition: A Characterization by Hot Carrier Transport

We report the synthesis of AB-stacked multilayer graphene via ambient pressure chemical vapor deposition on Cu foils and demonstrate a method to construct suspended multilayer graphene devices. In four-terminal geometry, such devices were characterized by hot carrier transport at temperatures down to 240 mK and in magnetic fields up to 14 T. The differential conductance (dI/dV) shows a characteristic dip at longitudinal voltage bias V = 0 at low temperatures, indicating the presence of hot electron effect due to a weak electron-phonon coupling. Under magnetic fields, the magnitude of the dI/dV dip diminishes through the enhanced intra-Landau level cyclotron phonon scattering. Our results provide new perspectives in obtaining and understanding AB-stacked multilayer graphene, important for future graphene-based applications.

Hot electron transport in suspended multilayer graphene

We study hot electron transport in short-channel suspended multilayer graphene devices created by a distinct experimental approach. For devices with semi-transparent contact barriers, a dip of differential conductance (dI/dV) has been observed at source drain bias Vd = 0, along with anomalies at higher Vd likely induced by optical phonon scattering. For devices with low contact barriers, only the dI/dV dip at Vd = 0 is observed, and we find a well-fit logarithmic dependence of dI/dV on both the bias Vd and the temperature T. The logarithmic Vd dependence is explained with the hot electron effect and the logarithmic T dependence could be attributed to the weak-localization in two-dimensions.