Young In Jhon

Grain boundaries orientation effects on tensile mechanics of polycrystalline graphene

Molecular dynamics simulations were performed to investigate how the orientation of grain boundary (GB) affects the tensile mechanics of polycrystalline graphene, where two opposite GB groups, i.e., armchair (AC) and zigzag (ZZ)-oriented tilted GBs were considered for the anisotropic study. We found very close mechanical similarities between the two groups in misorientation angle effect and critical bond length effect to determine the tensile strength. Mono-atomic carbon chains (MACCs) were commonly generated at tensile failure in both groups, as bridged between fractured sections, yielding the considerably higher population density and achievable length (4.51 nm−2 and 1.47 nm, maximally) compared to pristine graphene. Notably, we found that polycrystalline graphene exhibited distinctly different behaviors in this MACC production depending on GB orientation, being 1.2–3.0 times denser and 1.6–5.0 times longer for ZZ-oriented GBs. Atomic stress analyses indicated that all key reactions emerging before tensile failure would not be affected by the GB orientation of polycrystalline graphene since the reactions only occurred along GBs, explaining why anisotropic mechanical GB response has not been observed so far, in contrast to the MACC dynamics occurring after tensile failure.

Plasma functionalization for cyclic transition between neutral and charged excitons in monolayer MoS2.

Monolayer MoS2 (1L-MoS2) has photoluminescence (PL) properties that can greatly vary via transition between neutral and charged exciton PLs depending on carrier density. Here, for the first time, we present a chemical doping method for reversible transition between neutral and charged excitons of 1L-MoS2 using chlorine-hydrogen-based plasma functionalization. The PL of 1L-MoS2 is drastically increased by p-type chlorine plasma doping in which its intensity is easily tuned by controlling the plasma treatment duration. We find that despite their strong adhesion, a post hydrogen plasma treatment can very effectively dedope chlorine adatoms in a controllable way while maintaining robust structural integrity, which enables well-defined reversible PL control of 1L-MoS2. After exhaustive chlorine dedoping, the hydrogen plasma process induces n-type doping of 1L-MoS2, degrading the PL further, which can also be recovered by subsequent chlorine plasma treatment, extending the range of tunable PL into a bidirectional regime. This cyclically-tunable carrier doping method can be usefully employed in fabricating highly-tunable n- and p-type domains in monolayer transition-metal dichalcogenides suitable for two-dimensional electro-optic modulators, on-chip lasers, and spin- and valley-polarized light-emitting diodes.

Metallic MXene Saturable Absorber for Femtosecond Mode-Locked Lasers

2D transition metal carbides, nitrides, and carbonitides called MXenes have attracted much attention due to their outstanding properties. However, MXenes potential in laser technology is not explored. It is demonstrated here that Ti3 CN, one of MXene compounds, can serve as an excellent mode-locker that can produce femtosecond laser pulses from fiber cavities. Stable laser pulses with a duration as short as 660 fs are readily obtained at a repetition rate of 15.4 MHz and a wavelength of 1557 nm. Density functional theory calculations show that Ti3 CN is metallic, in contrast to other 2D saturable absorber materials reported so far to be operative for mode-locking. 2D structural and electronic characteristics are well conserved in their stacked form, possibly due to the unique interlayer coupling formed by MXene surface termination groups. Noticeably, the calculations suggest a promise of MXenes in broadband saturable absorber applications due to metallic characteristics, which agrees well with the experiments of passively Q-switched lasers using Ti3 CN at wavelengths of 1558 and 1875 nm. This study provides a valuable strategy and intuition for the development of nanomaterial-based saturable absorbers opening new avenues toward advanced photonic devices based on MXenes.

Understanding time-resolved processes in atomic-layer etching of ultra-thin Al2O3 film using BCl3 and Ar neutral beam

We scrutinize time-resolved processes occurring in atomic-layer etching (ALET) of ultra-thin Al2O3 film using BCl3 gas and Ar neutral beam by employing density functional theory calculations and experimental measurements. BCl3 gas is found to be preferentially chemisorbed on Al2O3(100) in trans form with the surface atoms creating O-B and Al-Cl contacts. We disclose that the most likely sequence of etching events involves dominant detachment of Al-associated moieties at early etching stages in good agreement with our concurrent experiments on tracking Al2O3 surface compositional variations during Ar bombardment. In this etching regime, we find that ALET requires half the maximum reaction energy of conventional plasma etching, which greatly increases if the etching sequence changes.

Multi-scale/multi-physical modeling in head/disk interface of magnetic data storage

The model integration of the head-disk interface (HDI) in the hard disk drive system, which includes the hierarchy of highly interactive layers (magnetic layer, carbon overcoat (COC), lubricant, and air bearing system (ABS)), has recently been focused upon to resolve technical barriers and enhance reliability. Heat-assisted magnetic recording especially demands that the model simultaneously incorporates thermal and mechanical phenomena by considering the enormous combinatorial cases of materials and multi-scale/multi-physical phenomena. In this paper, we explore multi-scale/multi-physical simulation methods for HDI, which will holistically integrate magnetic layers, COC, lubricants, and ABS in non-isothermal conditions.

Temperature Profile in the Presence of Hotspots in Heat Assisted Magnetic Recording

Recently, the demands for increasing memory capacities in hard disk drives (HDDs) has resulted in state-of-the-art technologies including heat assisted magnetic recording (HAMR) with significantly higher operating temperatures. HAMR results in swift degradation of current lubricant and carbon overcoat (COC) materials, leading to magnetic media corrosion which is detrimental to HDD operation. In addition, the lack of thorough understanding of the temperature profiles arising from the hotspot and energy management throughout these materials also exacerbates the problem. To address this issue, in this paper we will focus on the COC and investigate the transient heat transfer in various examples of nanoscale thin films when a hot spot is created via lattice Boltzmann method (LBM) since traditional conduction models like Fourier law are not accurate due to dominant sub-continuum effects. LBM originates from the Boltzmann transport equations (BTEs) and is computationally efficient due to easy parallelization with convenient handling of complex geometries. Our results of the heat transfer mechanism and temperature profiles show that Fourier equation under-predicts the peak temperature rise at the center of the hot-spot as the system size approaches the nanoscale domain. Applying LBM to a multilayered system, we observe a temperature slip along the interface of two materials indicated by the broken isothermal contours, as the heat is confined to a single layer. Using LBM, we then explore a novel graphene overcoat which has outstanding thermo-mechanical properties, and thereby extremely compatible in HAMR applications.

An Atomistic Study of Perfluoropolyether Lubricant Thermal Stability in Heat Assisted Magnetic Recording

At the head disk interface (HDI), the stability of the perfluoropolyether (PFPE) lubricant and carbon overcoat (COC) materials must be preserved under HAMR conditions. In this work, we investigate this issue by comparing the effects of transient versus steady heating of Zdol to replicate the precise pulsed heating of the HAMR system. These effects include changes in intermolecular lubricant bonding, molecular decomposition and desorption. In order to accurately account for potential changes in covalent and intermolecular bonds, we utilize the cutting-edge molecular simulation method of ab initio molecular dynamics. To simulate constant heating, a series of constant temperature simulations are performed at temperatures ranging from 300 K-700 K where the temperature is maintained via the Nose Hoover thermostat. For the transient heating simulations, the temperature is ramped over 100 K intervals with initial temperatures ranging from 300 K to 700 K. These heating studies are performed for bulk PFPE systems as well as PFPE-COC configurations to highlight the effect of PFPE-COC adhesion on lubricant thermal stability at the HDI. In the PFPE-COC simulations, we evaluate the amount of desorption versus decomposition as a function of initial temperature. Through our analysis, we are able to reveal the molecular mechanism of PFPE depletion as a function of functional group composition and, thereby, provide design criteria for lubricant molecular architecture in HAMR applications.

Temperature dependent Raman spectroscopic study of mono-, bi-, and tri-layer molybdenum ditelluride

We investigate the thermal properties of mono-, bi- and tri-layer MoTe2 by using temperature-dependent Raman spectroscopy ranging from 90 K to 300 K. The E2g1 and B2g1 modes of MoTe2 blueshift as the temperature decreases. The temperature dependence of the peak positions obtained from mono- to tri-layer MoTe2 is analyzed using the Gruneisen model. The first order temperature coefficients of E2g1 and B2g1 Raman modes of mono- to tri-layer MoTe2 are extracted. This study provides the fundamental information about the thermal properties of MoTe2 layers, which is crucial for developing thermal and electronic applications of MoTe2 based devices.

Perfluoropolyether Lubricant Interactions With Novel Overcoat via Coarse-Grained Molecular Dynamics

In this paper, we investigated physiochemical properties of new lubricant candidates for head-disk interface through various perfluoropolyether lubricant films on diamond, diamond-like carbon, and graphene overcoat surfaces via large scale coarse-grained bead-spring molecular dynamics stemming from the atomistic theory. Lubricant film conformations were characterized by investigating perpendicular component of molecular conformation, which determines the thickness of monolayer lubricant film. The distribution of functional endgroups and the mobility were analyzed via self-diffusion process. Here, we illustrate the effects of endgroup structure and carbon-surface structure on the film conformation and the mobility by expanding the multiscale simulation methodology and select candidates for future HDI design.

Plasma assisted tunable exciton states in monolayer MoS 2

We performed photoluminescence (PL) and Raman spectroscopy for Cl₂ and H₂ plasma modified monolayer MoS₂ (1L-MoS₂) crystals. We demonstrated that PL intensities in 1L-MoS₂ can be tuned by treating Cl₂ and H₂ plasma.