Showing papers by "An-Ping Li published in 2016"

Isoelectronic Tungsten Doping in Monolayer MoSe2 for Carrier Type Modulation

Abstract: Carrier-type modulation is demonstrated in 2D transition metal dichalcogenides as n-type monolayer MoSe2 is converted to nondegenerate p-type monolayer Mo1-x Wx Se2 through isoelectronic doping. Although the alloys are mesoscopically uniform, the p-type conduction in monolayer Mo1-x Wx Se2 appears to originate from the upshift of the valenceband maximum toward the Fermi level at highly localized "W-rich" regions in the lattice.

Differentiation of Surface and Bulk Conductivities in Topological Insulators via Four-Probe Spectroscopy.

Abstract: We show a new method to differentiate conductivities from the surface states and the coexisting bulk states in topological insulators using a four-probe transport spectroscopy in a multiprobe scanning tunneling microscopy system. We derive a scaling relation of measured resistance with respect to varying interprobe spacing for two interconnected conduction channels to allow quantitative determination of conductivities from both channels. Using this method, we demonstrate the separation of 2D and 3D conduction in topological insulators by comparing the conductance scaling of Bi2Se3, Bi2Te2Se, and Sb-doped Bi2Se3 against a pure 2D conductance of graphene on SiC substrate. We also quantitatively show the effect of surface doping carriers on the 2D conductance enhancement in topological insulators. The method offers a means to understanding not just the topological insulators but also the 2D to 3D crossover of conductance in other complex systems.

Helical Poly(5-alkyl-2,3-thiophene)s: Controlled Synthesis and Structure Characterization

Abstract: Whereas poly(3-alkyl-2,5-thiophene)s (P3AT), with many potential applications, have been extensively investigated, their ortho-connected isomers, poly(5-alkyl-2,3-thiophene)s (P5AT), have never been reported because of the difficulty in their syntheses. We herein present the first synthesis of regioregular P5AT via controlled Suzuki cross-coupling polymerization with PEPPSI-IPr as catalyst, affording the polymers with tunable molecular weight, narrow polydispersity (PDI), and well-defined functional end groups at the gram scale. The helical geometry of P5AT was studied by a combination of NMR, small-angle X-ray scattering (SAXS), and scanning tunneling microscopy (STM). Particularly, the single polymer chain of poly(5-butyl-2,3-thiophene) (P5BT) on highly oriented pyrolytic graphite (HOPG) substrates with either M or P helical conformation was directly observed by STM. The comparison of UV–vis absorption between poly(5-hexyl-2,3-thiophene) (P5HT) (λ = 345 nm) and poly(3-hexyl-2,5-thiophene) (P3HT) (λ = 45...

High-throughput electrical measurement and microfluidic sorting of semiconductor nanowires

Abstract: Existing nanowire electrical characterization tools not only are expensive and require sophisticated facilities, but are far too slow to enable statistical characterization of highly variable samples. They are also generally not compatible with further sorting and processing of nanowires. Here, we demonstrate a high-throughput, solution-based electro-orientation-spectroscopy (EOS) method, which is capable of automated electrical characterization of individual nanowires by direct optical visualization of their alignment behavior under spatially uniform electric fields of different frequencies. We demonstrate that EOS can quantitatively characterize the electrical conductivities of nanowires over a 6-order-of-magnitude range (10−5 to 10 S m−1, corresponding to typical carrier densities of 1010–1016 cm−3), with different fluids used to suspend the nanowires. By implementing EOS in a simple microfluidic device, continuous electrical characterization is achieved, and the sorting of nanowires is demonstrated as a proof-of-concept. With measurement speeds two orders of magnitude faster than direct-contact methods, the automated EOS instrument enables for the first time the statistical characterization of highly variable 1D nanomaterials.

Interplay between intercalated oxygen superstructures and monolayer h -BN on Cu(100)

Abstract: The confinement effect of intercalated atoms in van der Waals heterostructures can lead to interesting interactions between the confined atoms or molecules and the overlaying two-dimensional (2D) materials. Here we report the formation of ordered Cu(100) $p(2\ifmmode\times\else\texttimes\fi{}2)$ oxygen superstructures by oxygen intercalation under the monolayer hexagonal boron nitride $(h$-BN) on Cu after annealing. By using scanning tunneling microscopy and x-ray photoelectron spectroscopy, we identify the superstructure and reveal its roles in passivating the exposed Cu surfaces, decoupling $h$-BN and Cu, and disintegrating $h$-BN monolayers. The oxygen superstructure appears as a 2D pattern on the exposed Cu surface or quasi-1D stripes of paired oxygen intercalated in the interface of $h$-BN and Cu predominantly oriented along the moir\'e modulations. The oxygen superstructure is shown to etch the overlaying $h$-BN monolayer in a thermal annealing process. After extended annealing, the $h$-BN monolayer disintegrates into nanoislands with zigzag edges. We discuss the implications of these findings on the stability and oxidation resistance of $h$-BN and relate them to challenges in process integration and 2D heterostructures.

Conductivity map from scanning tunneling potentiometry

Abstract: We present a novel method for extracting two-dimensional (2D) conductivity profiles from large electrochemical potential datasets acquired by scanning tunneling potentiometry of a 2D conductor. The method consists of a data preprocessing procedure to reduce/eliminate noise and a numerical conductivity reconstruction. The preprocessing procedure employs an inverse consistent image registration method to align the forward and backward scans of the same line for each image line followed by a total variation (TV) based image restoration method to obtain a (nearly) noise-free potential from the aligned scans. The preprocessed potential is then used for numerical conductivity reconstruction, based on a TV model solved by accelerated alternating direction method of multiplier. The method is demonstrated on a measurement of the grain boundary of a monolayer graphene, yielding a nearly 10:1 ratio for the grain boundary resistivity over bulk resistivity.

Differentiation of Surface and Bulk Conductivities via Four-probe Spectroscopy

Abstract: A large number of materials have localized electronic states present on their surfaces. Understanding and controlling the electronic properties of these states does not only constitute the foundations of surface science but also the modern electronic devices. Being studied with mature surface analytic techniques like STM and ARPES, local electronic properties of surface states can be characterized very well. However, little is known about the electron transport through these 2D surface states which can present completely different characteristics than the transport through the underlying 3D bulk. During the last decade however, the demand for understanding the transport through surface states increased significantly due to constantly decreasing size of microelectronic devices and discovery of new materials like topological insulators (TIs) and other 2D layered materials with surface states hosting new and interesting transport properties.

Electronic Behaviors of Individual Defects and Boundaries in 2D Materials: A Spatially Resolved Study with Multi-Probe Scanning Tunneling Microscopy

Abstract: Electronic properties at the nanoscale are the key to the novel applications of low-dimensional materials in electronic and energy technologies. Due to the restricted dimensionality, one distinctive character of these nano-systems is that the electronic properties are critically dependent on the structural and compositional heterogeneities such as defects and boundaries. Therefore, an important aspect of nanoresearch is to examine the role of defects and boundaries, particularly the correlation between defect structures and electronic properties.