Session I02: 2D materials

Metal Mediated Exfoliation of Monolayer MoS2

Large area exfoliation of monolayer MoS$_{2}$ and other 2D materials using Au has attracted significant interest in recent years due to its promise in obtaining high quality single crystal in a scalable way. It has been suggested that the success of this method lies on the Au-MoS$_{2}$ binding energy that is stronger than the interlayer VdW interaction of MoS$_{2}$ as well as strain created between the lattice mismatch of Au and MoS$_{2}$. However, other metals (Pd, Pt, Cu, Ni, etc.\textellipsis ) have been predicted to have a stronger binding energy with MoS$_{2\, }$than Au, but a successful exfoliation of MoS$_{2}$ with other metals remains elusive. Raising question whether the metal-MoS$_{2}$ binding energy has any role in the large area exfoliation of monolayer MoS$_{2}$. Here, we present the result of our efforts in exfoliating monolayer MoS$_{2}$ with metals other than gold (such as Pd, Cu and Ni) along with detailed characterizations.   

A molecular magnet vertical tunnel junction.

We have fabricated EGaIn/CoPc/ITO heterojunctions where sublimated CoPc films as thin as 5~nm are sandwiched between transparent bottom-layer ITO and top-layer soft-landing eutectic GaIn (EGaIn) electrodes. Roughness of the CoPc films was determined by AFM to be 1.6~nm, and crystalline ordering of lying-down planar molecules was confirmed by XRD. The current-voltage characteristics of the 5~nm thick sample reveal the onset of a superconducting gap below Tc~at~6~K (the transition temperature of metastable $\beta $-Ga in the EGaIn contact) thereby providing incontrovertible evidence for direct quantum mechanical tunneling processes through the magnetic molecules in our heterojunctions. Simmons' model fits of the conductance data to temperatures as high as 200~K with a weakly temperature-dependent tunnel barrier height near 1.6~eV are consistent with this interpretation. Voltage dependent features in the differential conductance measurements may relate to spin states of single molecules or aggregates of molecules and prove to be important for quantum information device development.   

Large Magnetic Anisotropy in Cobalt Vanadate Thin Films

Cobalt Vanadate is a spinel oxide that displays competition between magnetic frustration, electron itinerancy, and orbital ordering. As a cubic bulk crystal, it is ferrimagnetic below 150 K, with low magnetic anisotropy. However, when CoV2O4 is grown on an [001] SrTiO3 substrate, an orthorhombic structure is formed. Due to this structural distortion, magnetometry and neutron scattering measurements$^{1}$ displays a new 90 K magnetic easy axis change from out-of-plane to in-plane upon cooling, and a K spin-canting transition at 75 K To explore the anisotropy of the thin film, torque magnetometry is performed as a function of temperature and external magnetic field. The resulting torque curves show uniaxial anisotropies in the ferrimagnetic state. Below the 90 K transition, the presence of an extremely hard axis causes the magnetization to lag behind the applied field, creating hysteretic effects around the hard axis. This work demonstrates how strain applied as a thin film can cause dramatic changes to the magnetization of a crystal and encourages further studies behind the role of crystal distortion in magnetic oxides. 1. Thompson, C. J. et al. Phys. Rev. Mater. 2, 104411 (2018).   

Electrical measurement of real-time ion migration dynamics in 1-D perovskite

Organic metal-halide perovskites have shown superior properties amenable to optoelectronic applications. The low-dimensional versions exhibit enhanced chemical stability due to their unique molecular structures. However, the charge transport properties, particularly their electronic stability with respect to ion migration, have yet to be systematically investigated. Here, we report on electrical measurements of real-time ion dynamics in the 1D hybrid metal-halide (R-MBA)PbI$_{3}$. The 4-terminal I-V curves exhibit a number of reproducible features indicative of common ion dynamics, including negative differential resistance, nonlinearity, and hysteresis that depend on the rate and direction of the current sweep. Measurements of the time-dependent voltage at constant current evidence an exponential dynamic of a time constant of \textasciitilde 2 s for the ion migration current. Moreover, all the unusual features in the I-V's can be quantitatively modeled based on this single ion dynamic, which can be enhanced by photo illumination. Our observations are consistent with the photo-activation of mobile ions and field-assisted ion migration. They provide valuable insights into the hysteresis in perovskite solar cells and the general dynamics of ion migration.   

Chirality-induced Spin Selectivity in Molecular Spin Valves: Role of the Nonmagnetic Electrode

Chirality-induced spin selectivity (CISS), an effect in which structural chirality engenders spin polarization in the electrical current from a nonmagnetic metal (NM) electrode, has been observed in a variety of chiral molecules with various experimental probes. However, the microscopic origin and device manifestations of CISS remain controversial. Most theoretical models consider chiral molecules as a spin filter, despite the generally small spin orbit coupling (SOC) in organic molecules. A recent theory posits that chiral molecules act as an orbital polarizer, and the SOC in nonmagnetic electrode converts the orbital polarization to spin polarization. Here, we report a comparison of CISS-induced magnetoconductance (MC) in vertical heterojunctions of (Ga,Mn)As/AHPA-L molecules/NM, between NM of Au and Al. The perpendicularly magnetized (Ga,Mn)As functions as a spin analyzer. The Au junctions show pronounced MC signals, which contain a large nonlinear-response component and a nontrivial-linear response component. In contrast, the MC of Al junctions are significantly diminished. Our observations suggest an important role for SOC in NM electrode in CISS-induced spin valve effect.   

Liquid Phase Exfoliation of SnS Nanosheets in Water

Successful isolation of 2D materials beyond graphene, such as 2D tin monosulfide (SnS), opens a new horizon in material research because unlike graphene SnS is a semiconductor. The cost-effective and large-scale production of SnS nanosheets is a fundamentally important step to realize its applications. The exfoliation of nanosheets in a solution from their bulk counterpart using high-intensity ultrasound waves has proven to be an effective method. Here, we compare the liquid phase exfoliation of SnS nanosheets in isopropyl alcohol and water. We show nanosheets' yield in both solvents significantly increases if the bulk SnS is manually ground prior to the ultrasonication process. The optical measurements show the nanosheets produced are high in quality, whereas the atomic force microscopy measurements suggest SnS is exfoliated into individual layers. Furthermore, it is found that the yield of nanosheets in water is twice as compared to that in isopropyl alcohol. The efficient exfoliation of SnS in an environmentally friendly and low-boiling-point solvent like in water would be advantageous for applications.   

Magnetic-core/Gold-shell Nanoparticles for the Detection of Trace Chemical Contaminants in Food Products

Magnetic-core/gold-shell nanoparticles (MAuNPs) are of interest for enabling portable detection of trace analytes in complex media, such as food. Gold coating provides biocompatibility and facile functionalization, and a magnetic core affords analyte concentration and controlled deposition onto substrates for surface-enhanced Raman spectroscopy. Iron oxide cores were synthesized and coated with gold by reduction of HAuCl$_{4}$ by NH$_{2}$OH. MAuNPs were grafted with polyethylene glycol (PEG) and/or functionalized with 4-mercaptobenzoic acid (4-MBA) and examined using a variety of microscopic, spectroscopic, magnetometric, and scattering techniques. The results showed 4-MBA displaced a significant amount of PEG. PEG displaced less 4-MBA, evidencing denser packing on the MAuNP surface. (4-MBA)-MAuNPs significantly enhanced the Raman signal, thus demonstrating functionality in direct detection of trace analytes. Magnetic deposition rate of MAuNPs and (4-MBA)-MAuNPs was explored. Deposition rate was slowed by 4-MBA. We postulate this originated from NP-substrate interactions. These findings emphasize the importance of ligand choice in reference to the medium, analyte, and substrate, as well as functionalization procedure in the design of similar sensing platforms.