The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Intrinsic ripples in graphene

A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Janus Monolayer Transition Metal Dichalcogenides

Black phosphorus (BP), a burgeoning elemental 2D semiconductor, has aroused increasing scientific and technological interest, especially as a channel material in field-effect transistors (FETs). However, the intrinsic instability of BP causes practical concern and the transistor performance must also be improved. Here, the use of metal-ion modification to enhance both the stability and transistor performance of BP sheets is described. Ag+ spontaneously adsorbed on the BP surface via cation-π interactions passivates the lone-pair electrons of P thereby rendering BP more stable in air. Consequently, the Ag+ -modified BP FET shows greatly enhanced hole mobility from 796 to 1666 cm2 V-1 s-1 and ON/OFF ratio from 5.9 × 104 to 2.6 × 106 . The mechanisms pertaining to the enhanced stability and transistor performance are discussed and the strategy can be extended to other metal ions such as Fe3+ , Mg2+ , and Hg2+ . Such stable and high-performance BP transistors are crucial to electronic and optoelectronic devices. The stability and semiconducting properties of BP sheets can be enhanced tremendously by this novel strategy.

Metal-Ion-Modified Black Phosphorus with Enhanced Stability and Transistor Performance.

Thermoelectric generators have attracted a wide research interest owing to their ability to directly convert heat into electrical power. Moreover, the thermoelectric properties of traditional inorganic and organic materials have been significantly improved over the past few decades. Among these compounds, layered two-dimensional (2D) materials, such as graphene, black phosphorus, transition metal dichalcogenides, IVA–VIA compounds, and MXenes, have generated a large research attention as a group of potentially high-performance thermoelectric materials. Due to their unique electronic, mechanical, thermal, and optoelectronic properties, thermoelectric devices based on such materials can be applied in a variety of applications. Herein, a comprehensive review on the development of 2D materials for thermoelectric applications, as well as theoretical simulations and experimental preparation, is presented. In addition, nanodevice and new applications of 2D thermoelectric materials are also introduced. At last, current challenges are discussed and several prospects in this field are proposed.

/pdf/recent-progress-of-two-dimensional-thermoelectric-materials-3ipszjttc0.pdf

Recent Progress of Two-Dimensional Thermoelectric Materials

Emerging in-plane anisotropic two-dimensional materials

Sugarcane has become an increasingly important first-generation biofuel crop in tropical and subtropical regions. It has a large, complex, polyploid genome that has hindered the progress of genomic research and marker-assisted selection. Genetic mapping and ultimately genome sequence assembly require a large number of DNA markers. Simple sequence repeats (SSRs) are widely used in genetic mapping because of their abundance, high rates of polymorphism, and ease of use. The objectives of this study were to develop SSR markers for construction of a saturated genetic map and to characterize the frequency and distribution of SSRs in a polyploid genome. SSR markers were mined from expressed sequence tag (EST), reduced representation library genomic sequences, and bacterial artificial chromosome (BAC) sequences. A total of 5,675 SSR markers were surveyed in a segregating population. The overall successful amplification and polymorphic rates were 87.9 and 16.4%, respectively. The trinucleotide repeat motifs were most abundant, with tri- and hexanucleotide motifs being the most abundant for the ESTs. BAC and genomic SSRs were mostly AT-rich while the ESTs were relatively GC-rich due to codon bias. These markers were also aligned to the sorghum genome, resulting in 1,203 markers mapped in the sorghum genome. This set of SSRs conserved in sugarcane and sorghum would be the most informative for mapping quantitative trait loci in sugarcane and for comparative genomic analyses. This large collection of SSR markers is a valuable resource for sugarcane genomic research and crop improvement.

Development of microsatellite markers in autopolyploid sugarcane and comparative analysis of conserved microsatellites in sorghum and sugarcane

Accurate measurements of the size-dependent lattice thermal conductivity (κl) of alloy nanostructures are challenging but help to address outstanding questions on the effects of atomic disorder and surface roughness on low-frequency vibrational modes in functional materials. Here, we report sensitive κl measurements of multiple segments of the same individual SiGe nanowires. In contrast to a previous report of ballistic thermal transport over several microns in SiGe nanowires, the obtained κl are nearly independent of the segment length from 2 to 10 μm and the temperature between 150 and 300 K. The results are in agreement with a theoretical calculation based on the virtual crystal approximation of the vibrational modes as phonons with mean free paths suppressed by purely diffuse surface scattering. The findings inform continuing theoretical efforts for understanding the roles of different types of vibrational modes in thermal transport in disordered thermoelectric and electronic materials.

Mean Free Path Suppression of Low-Frequency Phonons in SiGe Nanowires.

https://google.iopscience.iop.org/article/10.1088/1361-6463/abfefa/pdf

Electronic structure of cubic boron arsenide probed by scanning tunneling spectroscopy

The broken reflection symmetry exhibited by the recently reported two-dimensional Janus SMoSe monolayers can lead to unique electronic, thermal, and thermoelectric properties. Thermoelectric measurements of these and other semiconducting materials are often complicated by a loading error associated with a high sample resistance. Here, we report a thermoelectric measurement of a monolayer SMoSe sample with a resistance exceeding 40 MΩ. The loading error associated with the high sample resistance and other contaminations is removed with the use of high-impedance electronics, discrete modulated heating, and Fourier transform analysis of the measured thermoelectric voltage. Under a 20 V gate voltage, the measured Seebeck coefficient of the Janus monolayer changes from −200 µV/K at 350 K to −220 µV/K at 425 K. These results are consistent with an n-type semiconductor behavior of the Janus monolayer sample and demonstrate the effectiveness of this method for thermoelectric measurements of high-resistance semiconductor samples.

Thermoelectric measurements of high-resistance Janus monolayer transition-metal dichalcogenide

Monolithic Graphene–Graphite Integrated Electronics

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