Session J57: Synthesis of 2D Materials, Superlattices, and Defects

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Manufacturing of paper, which started two thousand years ago, simplified all aspects of information technology: generation, processing, communication, delivery and storage. Similarly powerful changes have been seen in the past century through the development of integrated circuits based on silicon. In this talk, I will first discuss how we can realize these integrated circuits thin and free-standing, just like paper, using two-dimensional materials based on transition metal dichalcogenides and hybrid superlattices. These wafer-scale films and circuits enable novel functionalities previously unavailable; we will discuss two examples including extremely anisotropic thermal conduction for directed heat tranport and delta-waveguides for two dimensional photonics.   

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Two-dimensional (2D) palladium ditelluride (PdTe₂) and platinum ditelluride (PtTe₂) are two Dirac semimetals which demonstrate fascinating quantum properties such as superconductivity, magnetism and topological order, illustrating promising applications in future nanoelectronics and optoelectronics. However, the synthesis of monolayer materials is dramatically hindered by strong interlayer coupling and orbits hybridization. Here, a protocol for the synthesis of monolayer PdTe₂ and PtTe₂ is demonstrated. Epitaxial growth of large-area and high quality monolayers of PdTe₂ and patterned PtTe₂ is achieved by direct tellurization of Pd(111) and Pt(111), respectively. More interestingly, in the case of PtTe₂, different monolayer patterns are obtained by inducing Te vacancies during growth. A well-ordered (2×2) PtTe₂ pattern formed by Te vacancy arrays is successfully grown by controlling the density of defects. The simple and reliable growth procedure of monolayer PdTe₂ and patterned PtTe₂ gives unprecedented opportunities for investigating new quantum phenomena and facilitating practical applications in electronics and catalysts.   

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Bulk Boron Carbide(B₄C) has encouraging properties as a high temperature thermoelectric material, it is extremely hard with a high melting temperature of 2763^oC and a high Seebeck coefficient in the range of 100-300 μV/K. Calculating similar properties for the 2-D form can allow us to establish more varied applications of the material. We start by generating the theoretical structures for non-polar and symmetric 2-D slabs of B4C along multiple cleavage directions and all possible terminations. We then use Van der Waals corrected DFT as implemented in the plane wave code VASP to compute their formation energies and determine the bonding scheme in the slab. Our results show the formation energies for slabs in various directions are similar, indicating that there isn’t a significant preference in the choice for a cleavage direction. Also, three selected planes (001), (012) and (101) have formation energies of 0.056 eV/atom, 0.105 eV/atom and 0.116 eV/atom, respectively, which are all below the threshold energy of 0.2 eV/atom for free-standing single/few-layer nanosheets. We finally compute the thermoelectric coefficients using the open source code BoltzTrap2. Our results indicate that B4C nanosheets are promising thermoelectric materials.   

Live

Black Phosphorene (BlackP) has been widely explored due to its unprecedented properties; however, the method of synthesizing high-quality samples in a large-area is still a daunting challenge. Here we use first-principles approaches to identify a highly desirable semiconducting SnSe substrate which is shown to be an ideal candidate for direct growth of BlackP. Our systematic energetic studies reveal that although both BlackP and its isomer, namely blue phosphorene (BlueP) can be stabilized on SnSe at low temperature, the later monolayer fails to maintain the ordered structure against thermal perturbation due to its weak interaction with the substrate. In contrast, the former retains the structure intact, as confirmed by our molecular dynamics simulations. We further study the atomistic growth mechanisms of the grown structure and find that the mutual interaction between the two phosphorus adatoms is attractive on SnSe(001), leading to the formation of dimers which is essentially favorable to achieve large-area samples. Furthermore, we show that in the ultraviolet region, such system possesses significantly improved optical properties than that of the BlackP, implying that this proposed system has promising potential in industrial applications.   

Live

Composites consisting of carbon nanostructures, such as graphene and carbon nanotubes, and metals are desirable for power transmission lines, interconnects and heat transfer applications due to the combination of excellent charge carrier mobility, thermal conductivity and mechanical strength of the carbon nanostructures and the high density of electrons in the metal. Metal/nanocarbon composites made by chemical vapor deposition, friction stir, ball milling, and plasma spraying have yielded materials with enhanced hardness and tensile strength but their electrical and thermal conductivities usually deteriorate. We use an electrocharge assisted process (EAP) which consists of the application of a high DC current to a mixture of liquid metal and carbon particles to form crystalline graphitic nanoribbons in the liquid metal. The solid composites have shown 5.7% higher electrical conductivity and enhanced local stiffness, measured by nanoindentation, compared to the pure Al alloys. The electrical conductivity shows a linear dependence with graphitic carbon content as well as with total charge infused during the EAP.   

Live

Edge kinetics in two-dimensional structure has been a key to understanding the growth. In this talk, we illustrate intrinsic difficulty of filling a gap of last few atoms on zigzag edges of hexagonal boron nitride, defined as a sealing problem. The physical origin of the problem is due to formation of dimers that largely distorted the edge structure. Specifically, the distortion becomes severe and creates a large growth barrier for the attachment of the last atom and degrades the crystal quality. To solve the sealing problem and reduce the energy barrier, it's possible to passivate the dangling bonds of the edge atoms with H to reduce the edge distortions. This new finding and growth strategy may largely enhance the crystal quality and growth rate.   

Live

Engineering of two-dimensional (2D) materials by modifying edges, creating folds, vacancies or by doping is advantageous to tailor their performance for targeted applications, such as in electronics or catalysis. However, exploring the impact of such defects on the local properties of the layer in which they are introduced is challenging as it requires functional measurements with high lateral resolution and high sensitivity.
Here we show that nanoscale defects can controllably be introduced in the hexagonal boron nitride (h-BN) lattice using the tip of an atomic force microscope (AFM). More specifically, we investigate the dual effect of tip-sample interaction and light on local defects formed in h-BN. We discuss changes in morphology and lattice vibrations observed with different environmental conditions, h-BN thicknesses, light power density and duration of exposure. We show that defects formed under inert environment exhibit changes in morphology, accompanied with the apparition of new IR bands in their spectra signature when exposed to air. This work brings about new means to bridge the gap between conventional characterization of 2D materials and atomistic computational models in view of reaching a deeper understanding of chemisorption and reactions at or near defect sites.   

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Layered 2D materials such as graphene have attracted great interest in thermal management applications. In this work we examined thermal properties of few-layer graphene crystallites arranged vertically with stacking disorder. Thermal conductivity and total interface conductance are obtained using Raman optothermal technique together with finite-element simulation. Multi-layer films consisting of vertically assembled few-layer graphene stacks were prepared using CVD technique and stacking disorder was confirmed using Raman spectroscopy and atomic force microscopy measurements. Thermal conductivity was determined to be in the order of 150 W/mK and it was 70% lower when compared to exfoliated multi-layer graphene that lacked stacking faults. Usually a significant decrease in thermal conductivity in graphitic systems is associated with a decrease in the grain size, whereas the defect density in our samples was very small. We discuss the decrease in thermal conductivity based on increased rate of phonon-phonon scattering due to stacking faults. Furthermore, the negative thermal expansion coefficient was estimated as ~ 3.46 ×10^-6 K^-1 near room temperature, and the phonon modes contributing to the same will be discussed.   

Live

Two-dimensional (2D) materials such as graphene or hexagonal boron nitride (h-BN) form polycrystalline structures during large-scale fabrication, with individual domains separated by grain boundaries which evolve with time. We study this dynamic process via Phase Field Crystal modeling for 2D hexagonal materials. By tracking the motion of circular grains misoriented with respect to the surrounding crystalline matrix we analyze the angle dependence of grain boundary dynamics and its implications for grain growth. Due to the lattice planes' continuity across the boundary, the grain is expected to rotate towards a larger misorientation angle, as governed by the Cahn-Taylor mechanism for the coupled normal-tangential boundary motion. This is found in our simulations of 2D graphene at small enough angles, while at large angles the grain rotation slows down and ceases near 30 degrees. However, this slowing-down and stagnation behavior does not occur for a binary hexagonal material like h-BN, where grains still rotate at high angles. It indicates the important role played by the lattice inversion symmetry breaking in binary materials, causing the change of detailed structures and dynamics of dislocation defects at grain boundaries as compared to single-component materials like graphene.   

Live

The discovery of tunable superconducting and Mott-insulating phases in twisted bilayer graphene (TBG) has galvanized investigations into a number of techniques for dynamically adjusting small twist angles in graphene heterostructures. Here we use low-energy electron microscopy (LEEM) and low-energy electron diffraction (LEED) to characterize the formation of domains of different rotational orders for in-situ grown graphene on various metallic substrates. These results suggest that LEEM could provide a viable method for characterizing small graphene twist angles in inhomogeneous systems.   

Live

A tabletop dry touch-transfer method that employs van der Waals forces was used to transfer multi to few-layer flakes of hexagonal boron nitride (hBN) and graphite onto cleaved, atomically flat GaAs (110) and InSb (110) surfaces. This method offers a cleaner alternative to current polymer-involved transfer methods, which leave polymer traces on the sample surface and require an extensive cleaning process for scanning probe microscopy (SPM) study.
An attractive application of this touch-transfer method is the encapsulation of target molecules between the semiconductor surface and graphene flakes for scanning tunneling microscopy (STM) studies. The conductivity of graphene allows us to probe the electronic states of otherwise challenging molecules and explore their possible applications. We present results of encapsulation of deionized water droplets between GaAs (110) and thinned graphite, characterized by atomic force (AFM), scanning electron (SEM) and optical microscopy techniques. Further studies will utilize nano-squeegeeing techniques using contact-mode AFM to control the placement of trapped water molecules.   

Live

Tungsten disulfide (WS₂) nanosheet is a promising material for potential photocatalysis applications due to its large surface area and novel electronic properties. The economic and large-scale production of WS₂ nanosheets is a fundamentally important step to realize its future applications. The exfoliation of nanosheets in a solution from their layered bulk counterpart using high-intensity ultrasound waves has proven to be a cost-effective method to produce nanosheets with high yield. Here, we show the production of WS₂ nanosheets increases significantly if layered WS₂ bulk powder is ball-milled before the ultrasonication process. We find the ball milling assisted sonication increases nanosheets’ yield by 50% compared to the combined yield if the bulk crystal is solely ball milled or sonicated. The optical absorption studies show the high quality of WS₂ nanosheets with the yield as high as 6.4%. The photodegradation kinetic studies show the decomposition rate of methyl orange with WS₂ nanosheets is at least 1.3 times higher than with WS₂ powder, suggesting nanosheets’ enhanced photocatalytic activity.   

On Demand

Two-dimensional (2D) transition metal dichalcogenides (TMds) shows remarkable physical properties and potential for device applications. Most of the semiconducting TMDs are stable in the 2H phase while MoTe₂ is stable in both 2H (semiconducting) and 1T′ (metallic) phases. It makes MoTe₂ an interesting candidate for in-plane interconnects when combined with 2D semiconductors. Here, we report the synthesize of large-area few-layers MoTe₂ using a simple tellurization CVD process. Subsequently, the as-grown transition metal ditelluride was chemically modified using an in situ laser-assisted method in a reactive gas environment. For an optimized set of parameters, the tellurium atoms are replaced by sulfur atoms. The chemical exchange process is monitored in real time via Raman spectroscopy. The site-selective replacement of the chalcogen atoms generate metal-semiconductor heterojunctions. 2D field effect transistors were fabricated to study the electrical response of these lateral heterojunctions. Additionally, we performed first-principle calculations for different chalcogen compositions in the ternary system MoTe_2(1-x)S_2x and found that the band structure evolves with increasing sulfur composition revealing a systematic transition from metal to semiconductor.