Quantitative Characterization of the Anisotropic Thermal Properties of Encapsulated Two-Dimensional MoS2 Nanofilms.

Two-dimensional (2D) semiconductors exhibit unique physical properties at the limit of a few atomic layers that are desirable for optoelectronic, spintronic, and electronic applications. Some of these materials require ambient encapsulation to preserve their properties from environmental degradation. While encapsulating 2D semiconductors is essential to device functionality, they also impact heat management due to the reduced thermal conductivity of the 2D material. There are limited experimental reports on in-plane thermal conductivity measurements in encapsulated 2D semiconductors. These measurements are particularly challenging in ultrathin films with a lower thermal conductivity than graphene since it may be difficult to separate the thermal effects of the sample from the encapsulating layers. To address this challenge, we integrated the frequency domain thermoreflectance (FDTR) and optothermal Raman spectroscopy (OTRS) techniques in the same experimental platform. First, we use the FDTR technique to characterize the cross-plane thermal conductivity and thermal boundary conductance. Next, we measure the in-plane thermal conductivity by model-based analysis of the OTRS measurements, using the cross-plane properties obtained from the FDTR measurements as input parameters. We provide experimental data for the first time on the thickness-dependent in-plane thermal conductivity of ultrathin MoS2 nanofilms encapsulated by alumina (Al2O3) and silica (SiO2) thin films. The measured thermal conductivity increased from 26.0 ± 10.0 W m-1 K-1 for monolayer MoS2 to 39.8 ± 10.8 W m-1 K-1 for the six-layer films. We also show that the thickness-dependent cross-plane thermal boundary conductance of the Al2O3/MoS2/SiO2 interface is limited by the low thermal conductance (18.5 MW m-2 K-1) of the MoS2/SiO2 interface, which has important implications on heat management in SiO2-supported and encased MoS2 devices. The measurement methods can be generalized to other 2D materials to study their anisotropic thermal properties.

Ambient-Stable Two-Dimensional CrI3 via Organic-Inorganic Encapsulation.

Two-dimensional transitional metal halides have recently attracted significant attention due to their thickness-dependent and electrostatically tunable magnetic properties. However, this class of materials is highly reactive chemically, which leads to irreversible degradation and catastrophic dissolution within seconds in ambient conditions, severely limiting subsequent characterization, processing, and applications. Here, we impart long-term ambient stability to the prototypical transition metal halide CrI3 by assembling a noncovalent organic buffer layer, perylenetetracarboxylic dianhydride (PTCDA), which templates subsequent atomic layer deposition (ALD) of alumina. X-ray photoelectron spectroscopy demonstrates the necessity of the noncovalent organic buffer layer since the CrI3 undergoes deleterious surface reactions with the ALD precursors in the absence of PTCDA. This organic-inorganic encapsulation scheme preserves the long-range magnetic ordering in CrI3 down to the monolayer limit as confirmed by magneto-optical Kerr effect measurements. Furthermore, we demonstrate field-effect transistors, photodetectors, and optothermal measurements of CrI3 thermal conductivity in ambient conditions.

Suppressing Ambient Degradation of Exfoliated InSe Nanosheet Devices via Seeded Atomic Layer Deposition Encapsulation.

With exceptional charge carrier mobilities and a direct bandgap at most thicknesses, indium selenide (InSe) is an emerging layered semiconductor that has generated significant interest for electronic and optoelectronic applications. However, exfoliated InSe nanosheets are susceptible to rapid degradation in ambient conditions, thus limiting their technological potential. In addition to morphological changes upon ambient exposure, the mobilities and current modulation on/off ratios of InSe transistors, as well as the responsivities of InSe photodetectors, decrease by over 3 orders of magnitude within 12 h of ambient exposure. In an effort to mitigate these deleterious effects, here we present an encapsulation scheme based on seeded atomic layer deposition that provides pinhole-free growth of alumina without compromising the intrinsic electronic properties of the underlying InSe. In particular, this encapsulation provides reproducible InSe field-effect transistor characteristics and InSe photodetector responsivities in excess of 107 A/W following ambient exposure for time periods on the order of months. Because atomic layer deposition is a highly scalable and manufacturable process, this work will accelerate ongoing efforts to integrate InSe nanosheets into electronic and optoelectronic technologies.

Homocatenation of aluminum: alkane-like structures of Li2Al2 H6 and Li3Al3H8.

Invited for the cover of this issue is Alexander I. Boldyrev and co-workers at Utah State University. The image depicts the magician who symbolizes a chemist synthesizing molecules of "aluminum homocatenated ethane and propane" based on the idea of electronic transmutation. Read the full text of the article at 10.1002/chem.201500298.

Homocatenation of aluminum: alkane-like structures of Li2Al2H6 and Li3Al3H8.

A new class of aluminum homocatenated compounds (LinAlnH2n+2) is proposed based on quantum chemical calculations. In these compounds, Al abstracts an electron from Li, becoming valence isoelectronic with C, Si, and Ge, thus mimicking respective structural features of Group 14 hydrides. Using the Coalescence Kick search program coupled with density functional theory calculations, we investigated the potential energy surfaces of Li2Al2 H6 and Li3Al3H6 . Then single-point-energy coupled-cluster calculations were performed for the lowest energy structures found. Indeed, the global minima established for Li2 Al2 H6 and Li3 Al3 H6 contain the Al2 H6(2-) and Al3H6(3-) kernels, which are isostructural with ethane (C2H6), disilane (Si2H6), digermane (Ge2 H6) and propane (C3H8), trisilane (Si3H8), trigermane (Ge3H8) molecules, respectively. Structural, energetic, and electronic characteristics of the Li2 Al2 H6 and Li3 Al3 H8 compounds are presented and the viability of their synthesis is discussed.

Human epidermal keratinocytes are induced to secrete interleukin-6 and co-stimulate T lymphocyte proliferation by a CD40-dependent mechanism.

Although the expression and function of CD40 on B lymphocytes has been well studied, the significance of CD40 on non-lymphoid cells such as keratinocytes (KC) is not as well characterized. We demonstrate in this report that CD40 is expressed by virtually all human KC, and that it functions as an important signaling molecule. Flow cytometry of undifferentiated and terminally differentiated KC indicated that both cell types expressed CD40, as determined by binding to monoclonal antibodies and a recombinant CD40 ligand fusion protein; interferon-gamma (IFN-gamma) treatment of KC increased CD40 expression. Cultured KC also expressed 1.5-kb CD40 transcripts. Activation of KC cell surface CD40 using the monoclonal antibody G28.5 resulted in the rapid generation of a 50-kDa tyrosine phosphorylated polypeptide, as well as a dose-dependent increase in the secretion of interleukin-6, a cytokine that has been linked to KC proliferation. KC also co-stimulated a significant T lymphocyte proliferative response to the mitogen phytohemagglutinin that was CD40 dependent. These data indicate that KC constitutively express a low level of functional CD40 and regulate their expression in response to IFN-gamma. These data support the concept that KC, via their expression of CD40, have the capacity to amplify inflammation in the skin by interacting with CD40 ligand-bearing T cells.