Su Ying Quek

Amine−Gold Linked Single-Molecule Circuits: Experiment and Theory

A combination of theory and experiment is used to quantitatively understand the conductance of single-molecule benzenediamine-gold junctions. A newly developed analysis is applied to a measured junction conductance distribution, based on 59 000 individual conductance traces, which has a clear peak at 0.0064 G0 and a width of +/-47%. This analysis establishes that the distribution width originates predominantly from variations in conductance across different junctions rather than variations in conductance during junction elongation. Conductance calculations based on density functional theory (DFT) for 15 distinct junction geometries show a similar spread. We show explicitly that differences in local structure have a limited influence on conductance because the amine-Au bonding motif is well-defined and flexible, explaining the narrow distributions seen in the experiments. The minimal impact of junction structure on conductance permits an unambiguous comparison of calculated and measured conductance values and a direct assessment of the widely used DFT theoretical framework. The average calculated conductance (0.046 G0) is found to be seven times larger than experiment. This discrepancy is explained quantitatively in terms of electron correlation effects to the molecular level alignments in the junction.

Interlayer Breathing and Shear Modes in Few-Trilayer MoS2 and WSe2

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have recently attracted tremendous interest as potential valleytronic and nanoelectronic materials, in addition to being well-known as excellent lubricants in the bulk. The interlayer van der Waals (vdW) coupling and low-frequency phonon modes and how they evolve with the number of layers are important for both the mechanical and the electrical properties of 2D TMDs. Here we uncover the ultralow frequency interlayer breathing and shear modes in few-layer MoS2 and WSe2, prototypical layered TMDs, using both Raman spectroscopy and first principles calculations. Remarkably, the frequencies of these modes can be perfectly described using a simple linear chain model with only nearest-neighbor interactions. We show that the derived in-plane (shear) and out-of-plane (breathing) force constants from experiment remain the same from two-layer 2D crystals to the bulk materials, suggesting that the nanoscale interlayer frictional characteristics of these excellent lubricants should be independent of the number of layers.

Bandgap tunability at single-layer molybdenum disulphide grain boundaries

Two-dimensional transition metal dichalcogenides have emerged as a new class of semiconductor materials with novel electronic and optical properties of interest to future nanoelectronics technology. Single-layer molybdenum disulphide, which represents a prototype two-dimensional transition metal dichalcogenide, has an electronic bandgap that increases with decreasing layer thickness. Using high-resolution scanning tunnelling microscopy and spectroscopy, we measure the apparent quasiparticle energy gap to be 2.40 ± 0.05 eV for single-layer, 2.10 ± 0.05 eV for bilayer and 1.75 ± 0.05 eV for trilayer molybdenum disulphide, which were directly grown on a graphite substrate by chemical vapour deposition method. More interestingly, we report an unexpected bandgap tunability (as large as 0.85 ± 0.05 eV) with distance from the grain boundary in single-layer molybdenum disulphide, which also depends on the grain misorientation angle. This work opens up new possibilities for flexible electronic and optoelectronic devices with tunable bandgaps that utilize both the control of two-dimensional layer thickness and the grain boundary engineering.

Conductance and geometry of pyridine-linked single-molecule junctions

We have measured the conductance and characterized molecule-electrode binding geometries of four pyridine-terminated molecules by elongating and then compressing gold point contacts in a solution of molecules. We have found that all pyridine-terminated molecules exhibit bistable conductance signatures, signifying that the nature of the pyridine-gold bond allows two distinct conductance states that are accessed as the gold-molecule-gold junction is elongated. We have identified the low-conductance state as corresponding to a molecule fully stretched out between the gold electrodes, where the distance between contacts correlates with the length of the molecule; the high-conductance state is due to a molecule bound at an angle. For all molecules, we have found that the distribution of junction elongations in the low-conductance state is the same, while in the high-conductance state, the most likely elongation length increases linearly with molecule length. The results of first-principles conductance calculations for the four molecules in the low-conductance geometry agree well with the experimental results and show that the dominant conducting channel in the conjugated pyridine-linked molecules is through the pi* orbital.

Length Dependence of Conductance in Aromatic Single-Molecule Junctions

Using a scattering-state approach incorporating self-energy corrections to the junction level alignment, the conductance G of oligophenyldiamine-Au junctions is calculated and elucidated. In agreement with experiment, we find G decays exponentially with the number of phenyls with decay constant beta = 1.7. A straightforward, parameter-free self-energy correction, including electronic exchange and correlations beyond density functional theory (DFT), is found to be essential for understanding the measured values of both G and beta. Importantly, our results confirm quantitatively the picture of off-resonant tunneling in these systems and show that exchange and correlation effects absent from standard DFT calculations contribute significantly to beta.

Low resistance metal contacts to MoS2 devices with nickel-etched-graphene electrodes.

We report an approach to achieve low-resistance contacts to MoS2 transistors with the intrinsic performance of the MoS2 channel preserved. Through a dry transfer technique and a metal-catalyzed graphene treatment process, nickel-etched-graphene electrodes were fabricated on MoS2 that yield contact resistance as low as 200 Ω · μm. The substantial contact enhancement (∼ 2 orders of magnitude), as compared to pure nickel electrodes, is attributed to the much smaller work function of nickel-graphene electrodes, together with the fact that presence of zigzag edges in the treated graphene surface enhances tunneling between nickel and graphene. To this end, the successful fabrication of a clean graphene-MoS2 interface and a low resistance nickel-graphene interface is critical for the experimentally measured low contact resistance. The potential of using graphene as an electrode interlayer demonstrated in this work paves the way toward achieving high performance next-generation transistors.

Relating Energy Level Alignment and Amine-Linked Single Molecule Junction Conductance

Using photoemission spectroscopy, we determine the relationship between electronic energy level alignment at a metal-molecule interface and single-molecule junction transport data. We measure the position of the highest occupied molecular orbital (HOMO) relative to the Au metal Fermi level for three 1,4-benzenediamine derivatives on Au(111) and Au(110) with ultraviolet and resonant X-ray photoemission spectroscopy. We compare these results to scanning tunnelling microscope-based break-junction measurements of single molecule conductance and to first-principles calculations. We find that the energy difference between the HOMO and Fermi level for the three molecules adsorbed on Au(111) correlate well with changes in conductance and agree well with quasiparticle energies computed from first-principles calculations incorporating self-energy corrections. On the Au(110) that presents Au atoms with lower-coordination, critical in break-junction conductance measurements, we see that the HOMO level shifts further from the Fermi level. These results provide the first direct comparison of spectroscopic energy level alignment measurements with single molecule junction transport data.

Theoretical study of thermoelectric properties of few-layer MoS2 and WSe2

Molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) are prototypical layered two-dimensional transition metal dichalcogenide materials, with each layer consisting of three atomic planes. We refer to each layer as a trilayer (TL). We study the thermoelectric properties of 1-4TL MoS2 and WSe2 using a ballistic transport approach based on the electronic band structures and phonon dispersions obtained from first-principles calculations. Our results show that the thickness dependence of the thermoelectric properties is different under n-type and p-type doping conditions. Defining ZT1st peak as the first peak in the thermoelectric figure of merit ZT as doping levels increase from zero at 300 K, we found that ZT1st peak decreases as the number of layers increases for MoS2, with the exception of 2TL in n-type doping, which has a slightly higher value than 1TL. However, for WSe2, 2TL has the largest ZT1st peak in both n-type and p-type doping, with a ZT1st peak value larger than 1 for n-type WSe2. At high temperatures (T > 300 K), ZT1st peak dramatically increases when the temperature increases, especially for n-type doping. The ZT1st peak of n-type 1TL-MoS2 and 2TL-WSe2 can reach 1.6 and 2.1, respectively.

Nanoscale Transition Metal Dichalcogenides: Structures, Properties, and Applications

Transition metal dichalcogenides (TMDs), such as MoS2, MoSe2, WS2, and WSe2, are layered materials with strong in-plane ionic-covalent bonds and weak out-of-plane van der Waals interactions, enabling formation of various nanostructures, such as nanotubes, nanoribbons, nanoflakes, and fullerene-like nanoparticles. Various remarkable properties have been found recently in these nanostructures, opening up brand new opportunities for their applications in nanoelectronics, optoelectronics, spintronics and structural materials. In this article, we present recent advances in the study of two-dimensional TMDs and their derivatives with special emphasis on structures, morphologies, properties (electronic, magnetic, thermal, mechanical), and applications (transistors, sensors, catalysts, lubricants, and composite materials). In addition, routes for modifying these properties by chemical doping, defect engineering, strain engineering, and electric fields are discussed. Our intent is to present a state-of-the-art view in this fast evolving field, with a balanced theoretical and experimental perspective.

Large Frequency Change with Thickness in Interlayer Breathing Mode—Significant Interlayer Interactions in Few Layer Black Phosphorus

Bulk black phosphorus (BP) consists of puckered layers of phosphorus atoms. Few-layer BP, obtained from bulk BP by exfoliation, is an emerging candidate as a channel material in post-silicon electronics. A deep understanding of its physical properties and its full range of applications are still being uncovered. In this paper, we present a theoretical and experimental investigation of phonon properties in few-layer BP, focusing on the low-frequency regime corresponding to interlayer vibrational modes. We show that the interlayer breathing mode A(3)g shows a large redshift with increasing thickness; the experimental and theoretical results agree well. This thickness dependence is two times larger than that in the chalcogenide materials, such as few-layer MoS2 and WSe2, because of the significantly larger interlayer force constant and smaller atomic mass in BP. The derived interlayer out-of-plane force constant is about 50% larger than that of graphene and MoS2. We show that this large interlayer force constant arises from the sizable covalent interaction between phosphorus atoms in adjacent layers and that interlayer interactions are not merely of the weak van der Waals type. These significant interlayer interactions are consistent with the known surface reactivity of BP and have been shown to be important for electric-field induced formation of Dirac cones in thin film BP.