Philippe K. Chow

Defect-Induced Photoluminescence in Monolayer Semiconducting Transition Metal Dichalcogenides

It is well established that defects strongly influence properties in two-dimensional materials. For graphene, atomic defects activate the Raman-active centrosymmetric A1g ring-breathing mode known as the D-peak. The relative intensity of this D-peak compared to the G-band peak is the most widely accepted measure of the quality of graphene films. However, no such metric exists for monolayer semiconducting transition metal dichalcogenides such as WS2 or MoS2. Here we intentionally create atomic-scale defects in the hexagonal lattice of pristine WS2 and MoS2 monolayers using plasma treatments and study the evolution of their Raman and photoluminescence spectra. High-resolution transmission electron microscopy confirms plasma-induced creation of atomic-scale point defects in the monolayer sheets. We find that while the Raman spectra of semiconducting transition metal dichalcogenides (at 532 nm excitation) are insensitive to defects, their photoluminescence reveals a distinct defect-related spectral feature located ∼0.1 eV below the neutral free A-exciton peak. This peak originates from defect-bound neutral excitons and intensifies as the two-dimensional (2D) sheet is made more defective. This spectral feature is observable in air under ambient conditions (room temperature and atmospheric pressure), which allows for a relatively simple way to determine the defectiveness of 2D semiconducting nanosheets. Controlled defect creation could also enable tailoring of the optical properties of these materials in optoelectronic device applications.

Bonding-induced thermal conductance enhancement at inorganic heterointerfaces using nanomolecular monolayers

Manipulating interfacial thermal transport is important for many technologies including nanoelectronics, solid-state lighting, energy generation and nanocomposites. Here, we demonstrate the use of a strongly bonding organic nanomolecular monolayer (NML) at model metal/dielectric interfaces to obtain up to a fourfold increase in the interfacial thermal conductance, to values as high as 430 MW m(-2) K(-1) in the copper-silica system. We also show that the approach of using an NML can be implemented to tune the interfacial thermal conductance in other materials systems. Molecular dynamics simulations indicate that the remarkable enhancement we observe is due to strong NML-dielectric and NML-metal bonds that facilitate efficient heat transfer through the NML. Our results underscore the importance of interfacial bond strength as a means to describe and control interfacial thermal transport in a variety of materials systems.

Epoxy Nanocomposites with Two-Dimensional Transition Metal Dichalcogenide Additives

Emerging two-dimensional (2D) materials such as transition metal dichalcogenides offer unique and hitherto unavailable opportunities to tailor the mechanical, thermal, electronic, and optical properties of polymer nanocomposites. In this study, we exfoliated bulk molybdenum disulfide (MoS2) into nanoplatelets, which were then dispersed in epoxy polymers at loading fractions of up to 1% by weight. We characterized the tensile and fracture properties of the composite and show that MoS2 nanoplatelets are highly effective at enhancing the mechanical properties of the epoxy at very low nanofiller loading fractions (below 0.2% by weight). Our results show the potential of 2D sheets of transition metal dichalcogenides as reinforcing additives in polymeric composites. Unlike graphene, transition metal dichalcogenides such as MoS2 are high band gap semiconductors and do not impart significant electrical conductivity to the epoxy matrix. For many applications, it is essential to enhance mechanical properties while also maintaining the electrical insulation properties and the high dielectric constant of the polymer material. In such applications, conductive carbon based fillers such as graphene cannot be utilized. This study demonstrates that 2D transition metal dichalcogenide additives offer an elegant solution to such class of problems.

Organic–Inorganic Heterointerfaces for Ultrasensitive Detection of Ultraviolet Light

The performance of graphene field-effect transistors is limited by the drastically reduced carrier mobility of graphene on silicon dioxide (SiO2) substrates. Here we demonstrate an ultrasensitive ultraviolet (UV) phototransistor featuring an organic self-assembled monolayer (SAM) sandwiched between an inorganic ZnO quantum dots decorated graphene channel and a conventional SiO2/Si substrate. Remarkably, the room-temperature mobility of the chemical-vapor-deposition grown graphene channel on the SAM is an order-of-magnitude higher than on SiO2, thereby drastically reducing electron transit-time in the channel. The resulting recirculation of electrons (in the graphene channel) within the lifetime of the photogenerated holes (in the ZnO) increases the photoresponsivity and gain of the transistor to ∼10(8) A/W and ∼3 × 10(9), respectively with a UV to visible rejection ratio of ∼10(3). Our UV photodetector device manufacturing is also compatible with current semiconductor processing, and suitable for large volume production.

A Foldable Lithium–Sulfur Battery

The next generation of deformable and shape-conformable electronics devices will need to be powered by batteries that are not only flexible but also foldable. Here we report a foldable lithium-sulfur (Li-S) rechargeable battery, with the highest areal capacity (∼3 mAh cm(-2)) reported to date among all types of foldable energy-storage devices. The key to this result lies in the use of fully foldable and superelastic carbon nanotube current-collector films and impregnation of the active materials (S and Li) into the current-collectors in a checkerboard pattern, enabling the battery to be folded along two mutually orthogonal directions. The carbon nanotube films also serve as the sulfur entrapment layer in the Li-S battery. The foldable battery showed <12% loss in specific capacity over 100 continuous folding and unfolding cycles. Such shape-conformable Li-S batteries with significantly greater energy density than traditional lithium-ion batteries could power the flexible and foldable devices of the future including laptops, cell phones, tablet computers, surgical tools, and implantable biomedical devices.

Reduced stability of copper interconnects due to wrinkles and steps on hexagonal boron nitride substrates

There is great scientific and technological interest in the use of chemical-vapor-deposition grown hexagonal boron nitride dielectric substrates for microelectronics applications. This interest stems from its superior heat spreading capability compared to silicon dioxide as well as the lack of surface dangling bonds or charge traps in hexagonal boron nitride which results in superior performance for graphene based electronics devices. However, surface heterogeneities, such as wrinkles or steps, are ubiquitous in such devices due to the fabrication and processing of chemical vapor deposition grown hexagonal boron nitride. In this study, we characterize the effect of such surface heterogeneities on the stability of copper interconnects used in microelectronics devices. Based on the theoretical thermo-physical properties of the constituent thin film layers, our simulations predict that copper interconnects deposited on hexagonal boron nitride can withstand ∼1.9 times more power than on a silicon dioxide subs...

Gold-titania interface toughening and thermal conductance enhancement using an organophosphonate nanolayer

We demonstrate that a mercaptan-terminated organophosphonate nanolayer at gold-titania interfaces can give rise to two- to three-fold enhancement in the interfacial fracture toughness and thermal conductance. Electron spectroscopy reveals that interfacial delamination occurs at the metal-molecule interface near the gold-sulfur bonds, consistent with density functional theory calculations of bond energies. Qualitative correlation between interfacial fracture toughness and bond energies suggest that organophosphonate nanolayers are resilient to humidity-induced degradation. These results, and the versatility of organophosphonates as surface functionalization agents for technologically relevant materials, unlock uncharted avenues for molecular engineering of interfaces in materials and devices for a variety of applications.

Tuning of noble metal work function with organophosphonate nanolayers

We demonstrate that weak chemical interactions between untethered moieties in molecular nanolayers on metal surfaces can strongly influence the effective work function Φeff. Electron spectroscopy shows that nanolayers of mercaptan-anchored organophosphonates on Au and Pt decrease Φeff. The measured Φeff shifts correlate with the chemical state of phosphonic acid moieties, and scale with molecular length. These results are contrary to predictions of ab initio calculations of monolayer-capped surfaces, but are consistent with calculations of bilayer-capped surfaces with face-to-face hydrogen-bonded phosphonic acid moieties. Our findings indicate that intra-layer bonding and layering in molecular nanolayers can be key to tailoring heterointerfacial electronic properties for applications.