Session S53: Synthesis and Characterizations of Large Scale 2D Materials I

Atomically Thin Wafers and Superlattices

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 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.

In order to build these atomically thin circuits, we developed a series of chemistry-based approaches that are scalable and precise. They include wafer-scale synthesis of three atom thick semiconductors and heterojunctions (Nature, 2015; Science 2018), a wafer-scale patterning method for one-atom-thick lateral heterojunctions (Nature, 2012), and most recently, atomically thin films and devices that are vertically stacked to form more complicated circuitry (Nature, 2017). Once realized, these atomically thin circuits will be foldable and actuatable, which will further increase the device density and functionality.   

Chirality transfer through multistep reaction processes towards the synthesis of enantiopure chiral graphene nanoribbons

Besides its interest for potential optoelectronic devices, molecular chirality is of utmost importance in biology and medicine. Consequently relevant is the selective synthesis of enantiopure molecular compounds, which has been hardly addressed in the growing field of “on-surface synthesis”. In this frame, 2,2′-dibromo-9,9′-bianthracene reactants are known to form chiral graphene nanoribbons (GNR) on coinage metal substrates through a complex multi-step reaction including an initial radical step growth polymerization by Ullmann coupling and following cyclodehydrogenation steps. In this work we show how, starting from enantiopure reactants deposited onto Au(111), their chirality is sequentially transferred to the polymers and finally to the GNRs with an excellent level of selectivity. Unambiguous evidence of this effect is obtained by high-resolution scanning tunneling microscopy images.   

Narrow optical transition linewidth of MoS2 monolayers grown by chemical vapor deposition : impact of dielectric disorder

Chemical vapor deposition (CVD) allows growing transition metal dichalcogenides (TMDs) over large surface areas on inexpensive substrates, but optical transition linewidth of as-grown monolayers (MLs) is usually tens of meV. We correlate the structural quality of CVD grown MoS2 MLs on SiO2/Si wafers studied by high-resolution transmission electron microscopy (HRTEM) with the optical quality revealed in optical emission and absorption from cryogenic to ambient temperatures. With HRTEM we determine a defect concentration of the order of 10¹³/cm² for our samples, comparable to standard exfoliated flakes from bulk crystals. To have access to the intrinsic optical quality of the CVD grown MLs, we remove the MLs from the SiO2 growth substrate and encapsulate them in hBN flakes with low defect density. In encapsluated MLs we show optical transition linewidth of 5 meV at low temperature T=4 K for the free excitons in emission and absorption. This is comparable to the best ML samples obtained by mechanical exfoliation of bulk material. The CVD grown MoS2 ML shows strong photoluminescence that is dominated by neutral excitons and not by defects even at cryogenic temperatures. We study the valley Zeeman effect in magnetic fields up to 9T accessible thanks to the narrow emission linewidth.   

Grain rotation and growth in binary hexagonal two-dimensional materials

Generally polycrystalline structures are formed during large-scale fabrication of 2D materials, with individual crystalline domains separated by grain boundaries. During growth, these grain boundaries move and evolve as neighboring grains interact. We study this dynamic process via Phase Field Crystal (PFC) 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 their implications for grain growth. In particular, due to the continuity of lattice planes 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 motion of the boundary. For binary 2D materials like hBN our PFC study indicates a dynamic property beyond this geometrically-controlled effect, particularly for grain misorientations close to 30 degree which show abnormally fast rotation and strong normal-tangential coupling. This behavior can be associated with the change of angle dependence of grain boundary energy in binary materials with lattice inversion symmetry breaking.   

Van der Waals Epitaxy of the Transition Metal Dichalcogenide WTe2

Thin film transition metal dichalcogenides (TMD) are strong candidates for future technological application, which require controlled means of ultra-high purity synthesis, as provided by molecular beam epitaxy (MBE). So far, MBE-synthesized TMD monolayers have been limited to islands with lateral sizes on the order of tens of nanometers due to the large difference of surface diffusivity of the transition metals and chalcogen species. Here, we adopt a van der Waals epitaxy method [1], based on the metal-organic precursor W(CO)6, with the goal to develop a growth recipe for WTe₂ monolayer films. Employing surface-sensitive measurement techniques, we characterized the as-grown films and observed WTe_x-based, one-dimensional nanostructures at the step edges and two-dimensional islands on the substrate terraces. From these results, we investigate the growth dynamics of WTe₂ few-layer thin films.

[1] S. Tiefenbach et. al. Surf Sci. 318 (1994)   

Synthesis and characterization of large-area single-crystal sheets of borophene on various metal surfaces

Borophene, an atomically thin covalently bonded boron sheet, has attracted great attention as a novel quantum material because of its structural tunability and potential utilization in next-generation electronics. Here, we report the synthesis of borophene on various single-crystal substrates and nanometer-scale thick crystallized metal films on sapphire. With real-time feedback from low-energy electron microscopy and diffraction, we have developed a process of enlarging monocrystal borophene domains, by repeated submersion of boron into substrates at high temperature, resurfacing and re-crystallization at low temperature. We demonstrate the synthesis of borophene as faceted micrometer-size monocrystal islands or as full-monolayer sheets. The process is scalable to wafer size; moreover, metallic buffers could be sacrificed, and sapphire substrate reused. Our work opens the door for new experiments and applications.

R. Wu et al., “Large-area single-crystal sheets of borophene on Cu(111) surfaces”, Nature Nanotechnology 14, 44 (2019).
R. Wu, A. Gozar, I. Bozovic, “Large-area borophene sheets on sacrificial Cu(111) films promoted by recrystallization from subsurface boron”, npj Quantum Materials 4, 40 (2019)   

In situ photo-chemical conversion of layered transition metal ditellurides

WTe₂ and MoTe₂ are good candidates for spintronics, thermoelectric and piezoelectric applications. In general, the 1T`phase of these 2D materials displays metallic behavior, making them interesting candidates for in-plane interconnects when combined with 2D semiconductors. In this study, we synthesize large-area WTe₂ and MoTe₂ films using a simple CVD process. Raman spectroscopy and TEM were used to study the crystal quality as well as phase and chemical composition. Subsequently, the as-grown films were chemically modified using an in situ laser-assisted method recently developed in our group. The entire process takes place within a home-made mini-chamber with an optical quartz window, were the samples are exposed to a laser beam in the presence of a controlled environment rich in a different chalcogen atom (sulfur in this case). For an optimized set of parameters (e.g. exposure time and laser power), the tellurium atoms are replaced by sulfur atoms and the chemical exchange process is monitored in real time via Raman spectroscopy. The site-selective replacement of the chalcogen atoms generate metal-semiconductor heterojunctions. FET-like devices where fabricated to study the electrical response of these junctions.
  

Controllable Growth and Electronic Structures of 2D Transition Metal Dichalcogenides Thin Films

The 2D transition metal dichalcogenides (TMDCs) have attracted extensive interest due to their remarkable properties. Using molecular beam epitaxial (MBE) method, we achieved the controllable growth of atomically thin TMDCs MoSe₂ and WSe₂ films. Combining with the in-situ angle-resolved photoemission spectroscopic (ARPES) measurements, we directly characterized the electronic structures of them and studied the evolution of their electronic structures in 2D limit [1-2]. Moreover, we achieved band structuring engineering in epitaxial TMDCs Mo_xW_1-xSe₂ alloy monolayers with controllable stoichiometric ratio x. We also realized the growth of meta-stable 1T'-WSe₂ monolayer, and studied its thermo-driven structure phase transition to stable 2H-WSe₂ [3]. Our findings not only help understanding of TMDC materials but also enrich the family of epitaxial 2D materials toward a fully MBE grown epitaxial heterostructures for light emission and photon-voltage devices.
References
[1] Yi Zhang et al., Nature Nanotechnology. 9, 111 (2014).
[2] Yi Zhang et al., Nano Letters 16, 2485 (2016).
[3] W. Chen et al., Scientific Reports, 9, 2685 (2019)   

Scaling-up atomically thin coplanar semiconductor-metal circuirty via phase engineered chemical assembly

Phase engineered growth of 2H-MoTe₂ was understood by a solid-to-solid phase transformation mechanism, enabling the controlled growth of chip-size single-crystalline 2H-MoTe₂. Based on that, we report on the large-scale, spatially controlled chemical assembly of the integrated 2H MoTe₂ field-effect transistors with coplanar metallic 1T′ MoTe₂ contacts via phase engineered approaches.   

MOCVD Growth of High Quality Large Area WS2 Films

Tungsten disulphide (WS₂) is of particular interest in optoelectronics because of the band gap transition from indirect to direct as the crystal thickness is reduced from bulk to a single layer [1]. To achieve large area coverage suitable for industrial applications, growth methods such as chemical vapour deposition (CVD) need to the developed. Current powder-based CVD approaches [2] produce crystalline flakes that are useful for fundamental studies but cannot produce continuous wafer scale films [3]. In this work, we produce continuous and uniform thin films of WS₂ via MOCVD techniques at scales up to 200mm wafer sizes.
Our films are deposited at 600C using a tungsten hexachloride precursor gas and a hydrogen sulphide sulphur source. Raman spectroscopy and photoluminescence (PL) measurements demonstrate high quality as-grown WS₂ thin films on sapphire substrates. We further show how a post-growth annealing step in H₂S quenches sulphur vacancy sites in the WS₂ and dramatically increases the PL intensity.
[1] A. Kuc et al., Physical Review B, 83, 245213, 2011
[2] McCreary KM et al., Scientific Reports, 6, 19159, 2016
[3] Kang. K et al., Nature, 520, 656-660, 2015   

Enabling Direct Write Mask Free Fabrication of Low Dimensional Nanoscale Architectures on Different Substrates using Aqueous Inks and CVD synthesis

Low dimensional materials such as nanowires and 2D films, when assembled in vertical or lateral arrangements, often lead to the largely enhanced properties, and new functionalities. While the preparation of layered architectures usually involves multi-step fabrication processes it also relies on mask assisted lithographic techniques. Here we present methodology for controlled selective preparation of 1D and 2D nanostructures of MoS2, WS2 and ZnO in the variety of geometric assemblies by employing parallel direct write patterning (DWP) of aqueous ink precursors on substrates at predefined locations. In a two-step process (1st patterning and 2nd growth) our unconventional fabrication approach enables simple and flexible production of hetero-structures and other architectures based on “mix and match” principle in precisely controlled fashion. Location specific synthesis of materials provides access to as-grown interfaces and rapid testing of materials’ quality, crystallinity and chemical composition which was confirmed by various characterization methods (Raman Spectroscopy, PL, AFM, XRD etc). Acknowledgement. Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Contract No. DE-AC02 06CH11357   

Formation and self-assembly of graphene nanoribbons and nanosheets in metals

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 electrocharging assisted process 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% higher electrical conductivity and enhanced local stiffness, measured by nanoindentation, compared to the pure Al alloys. Molecular dynamic simulations of nanoindentation tests into the nanocarbon metal composite show how the graphene nanoribbons impede dislocation motion and increase hardness. Conductive AFM shows an increase in the local conductivity from these samples compared to the parent aluminum alloy.   

Growth of 10 × 10 cm2 single-crystal hexagonal boron nitride monolayer on the symmetry broken substrate

The ability to grow high-quality large single crystals of essential 2D materials, is at the heart for the industrial applications of 2D devices. Atom-layered hexagonal boron nitride (hBN), with its excellent stability, flat surface and large bandgap, has been reported to be the best 2D insulator. However, the size of single-crystal 2D hBN is still typically less than the wafer scale, mainly due to the extreme difficulties in its single-crystal growth: the three-fold symmetry of hBN lattice leading to antiparallel domains resulting in twin boundaries on most substrates. Here, we report the first epitaxial growth of a 10 × 10 cm² single-crystal hBN monolayer on a low symmetry Cu(110) “vicinal surface” obtained by annealing an industrial Cu foil. Our experimental and theoretical work indicate that epitaxial growth is made by Cu<211> step-zigzag hBN edge coupling that serves to break the equivalence of antiparallel hBN domains, enabling unidirectional domains alignment of > 99%. The findings in this work can significantly boost the applications of 2D devices, and also pave the way for the epitaxial growth of broad non-centrosymmetric 2D materials, such as various transition metal dichalcogenides, into large-sized single crystals.

Reference:
Li Wang et al., Nature, 2019, 570, 9