Temperature dependent studies on centimeter-scale MoS2and vdW heterostructures.

Transition metal dichalcogenides is an emerging 2D semiconducting material group which has excellent physical properties in the ultimately scaled thickness dimension. Specifically, van der Waals heterostructures hold the great promise in further advancing both the fundamental scientific knowledge and practical technological applications of 2D materials. Although 2D materials have been extensively studied for various sensing applications, temperature sensing still remains relatively unexplored. In this work, we experimentally study the temperature-dependent Raman spectroscopy and electrical conductivity of molybdenum disulfide (MoS2) and its heterostructures with platinum dichalcogenides (PtSe2and PtTe2) to explore their potential to become the next-generation temperature sensor. It is found that the MoS2-PtX2heterostructure shows the great promise as the high-sensitivity temperature sensor.

Understanding the effect of dry etching on nanoscale phase-change memory.

PCM (phase-change memory) is an important class of data storage, operating based on the Joule heating-induced reversible switching of chalcogenide alloys. Nanoscale PCM often requires advanced microfabrication techniques such as dry (plasma) etching, but the possible impacts of process damages or imperfections on the device performance still remain relatively unexplored. This is critical because some chemical etching species are known to cause over-etching with rough edge onto the sidewall of a phase-change material. It is also possible that the phase-change material experiences a composition change due to etching-induced re-deposition of by-products or thermal stress. In this study, a finite-element simulation is performed to understand the effect of dry etching on the RESET characteristics of a nanoscale PCM device to provide a guideline on the PCM manufacturing and cell design.

Molecularly imprinted polymers via reversible addition-fragmentation chain-transfer synthesis in sensing and environmental applications.

Molecularly imprinted polymers (MIP) have shown their potential as artificial and selective receptors for environmental monitoring. These materials can be tailor-made to achieve a specific binding event with a template through a chosen mechanism. They are capable of emulating the recognition capacity of biological receptors with superior stability and versatility of integration in sensing platforms. Commonly, these polymers are produced by traditional free radical bulk polymerization (FRP) which may not be the most suitable for enhancing the intended properties due to the poor imprinting performance. To improve the imprinting technique and the polymer capabilities, controlled/living radical polymerization (CRP) has been used to overcome the main drawbacks of FRP. Combining CRP techniques such as RAFT (reversible addition-fragmentation chain transfer) with MIP has achieved higher selectivity, sensitivity, and sorption capacity of these polymers when implemented as the transductor element in sensors. The present work focuses on RAFT-MIP design and synthesis strategies to enhance the binding affinities and their implementation in environmental contaminant sensing applications.

Proposal for an electrostrictive logic device with the epitaxial oxide heterostructure.

The possible use of electrostrictive materials for information processing devices has been widely discussed because it could allow low-power logic operation by overcoming the fundamental limit of subthreshold swing greater than 60 mV/decade in conventional MOSFETs. However, existing proposals for electrostrictive FET applications typically adopt approaches that are entirely theoretical and simulative, thus lacking practical insights into how an electrostrictive material can be best interfaced with a channel material. Here we propose an electrostrictive FET device, involving the epitaxial oxide heterostructure as an ideal material platform for maximum strain transfer. The ON/OFF switching occurs due to a stress-induced concentration change of oxygen vacancies in the memristive oxide channel layer. Based on finite-element simulations, we show that the application of a minimal gate voltage bias can induce stress in the channel layer as high as 108 N/m2 owing to the epitaxial interface between the electrostrictive and memristive oxide layers. Conductive AFM experiments further support the feasibility of the proposed device by demonstrating the stress-induced conductivity modulation of a perovskite oxide thin film, SrTiO3, that is well known to serve as the substrate for epitaxial growth of other functional oxide layers.

Impact of thermally dead volume on phonon conduction along silicon nanoladders.

Thermal conduction in complex periodic nanostructures remains a key area of open questions and research, and a particularly provocative and challenging detail is the impact of nanoscale material volumes that do not lie along the optimal line of sight for conduction. Here, we experimentally study thermal transport in silicon nanoladders, which feature two orthogonal heat conduction paths: unobstructed line-of-sight channels in the axial direction and interconnecting bridges between them. The nanoladders feature an array of rectangular holes in a 10 µm long straight beam with a 970 nm wide and 75 nm thick cross-section. We vary the pitch of these holes from 200 nm to 1100 nm to modulate the contribution of bridges to the net transport of heat in the axial direction. The effective thermal conductivity, corresponding to reduced heat flux, decreases from ∼45 W m-1 K-1 to ∼31 W m-1 K-1 with decreasing pitch. By solving the Boltzmann transport equation using phonon mean free paths taken from ab initio calculations, we model thermal transport in the nanoladders, and experimental results show excellent agreement with the predictions to within ∼11%. A combination of experiments and calculations shows that with decreasing pitch, thermal transport in nanoladders approaches the counterpart in a straight beam equivalent to the line-of-sight channels, indicating that the bridges constitute a thermally dead volume. This study suggests that ballistic effects are dictated by the line-of-sight channels, providing key insights into thermal conduction in nanostructured metamaterials.

Phonon Conduction in Silicon Nanobeam Labyrinths.

Here we study single-crystalline silicon nanobeams having 470 nm width and 80 nm thickness cross section, where we produce tortuous thermal paths (i.e. labyrinths) by introducing slits to control the impact of the unobstructed "line-of-sight" (LOS) between the heat source and heat sink. The labyrinths range from straight nanobeams with a complete LOS along the entire length to nanobeams in which the LOS ranges from partially to entirely blocked by introducing slits, s = 95, 195, 245, 295 and 395 nm. The measured thermal conductivity of the samples decreases monotonically from ~47 W m-1 K-1 for straight beam to ~31 W m-1 K-1 for slit width of 395 nm. A model prediction through a combination of the Boltzmann transport equation and ab initio calculations shows an excellent agreement with the experimental data to within ~8%. The model prediction for the most tortuous path (s = 395 nm) is reduced by ~14% compared to a straight beam of equivalent cross section. This study suggests that LOS is an important metric for characterizing and interpreting phonon propagation in nanostructures.