Session A71: Growth and Synthesis of 2D Materials and Heterostructures

Templated synthesis of low dimensional films and heterostructures on freestanding graphene membranes

Low dimensional films and layered heterostructures, including topological materials and transition metal dichalcogenides, have incredible potential for a wide array of applications due to the flexibility and tunability of their unique properties.   Key challenges for realizing such potential include understanding how interfacial structure and defects affect desired properties and their incompatibility with existing nanofabrication techniques.  Here we present a “templated synthesis” approach where freestanding monolayer graphene extended over arbitrary surfaces and patterned structures is used as a substrate for the synthesis of high-quality films of topological materials and hybrid interfaces between different materials.  This approach has enabled the synthesis and direct characterization of the atomic and interfacial structure of as-grown low dimensional films and heterostructures using scanning transmission electron microscopy (STEM) for comparison to the electronic structure revealed by scanning tunneling spectroscopy (STS) and angle resolved photoemission spectroscopy (ARPES).  The templated synthesis approach also helps bridge the gap between low dimensional materials and traditional semiconductor nanofabrication techniques, opening a route towards development of device structures with novel functionality.   

Epitaxial Synthesis of Highly Oriented 2D Janus Rashba Semiconductor BiTeCl and BiTeBr

"Janus" 2D crystals Bismuth Tellurohallides (BiTeX, X=Cl, Br, I) are stacked layers of X, Bi and Te planes. The members of this family show giant Rashba spin splitting, which makes them ideal candidates for room temperature spintronics without the need of an external magnetic field¹. Previous experimental studies of BiTeX were performed on the exfoliated samples from bulk BiTeX, which is non-scalable and lacks reproducibility². No thin film growth with controlled thickness and orientation has been reported, which is essential to explore the atomic level phenomena and device integration. Here, the first epitaxial growth of highly crystalline and orientation controlled BiTeCl and BiTeBr is reported which was achieved by a two-step conversion process. The synthesis method includes the physical vapor deposition of Bi₂Te₃ nanosheets on c-cut sapphire and next, conversion of the Bi₂Te₃ sheets to BiTeCl(Br) by exposure to BiCl(Br)₃ vapor. The existence and uniformity of Janus BiTeX was confirmed by Raman mapping of A¹₁ vibrational mode  and energy dispersive X-ray spectroscopy after conversion. Stability of these Janus sheets was also studied by time dependent Raman spectroscopy. Overall, this work offers a new pathway to synthesize few layers of Janus compounds.

¹Kovács-Krausz, Zoltán, et al. Nano letters 20.7(2020):4782-4791

²Jacimovic, J., et al. Scripta Materialia 76(2014):69-72   

Influence of trace oxygen species on chemical vapor deposition (CVD) graphene growth kinetics

Deliberate oxidation of copper substrates prior to CVD graphene growth has proven to significantly alter the nucleation density, grain size, and defect concentration of the resulting film. Here we develop a model to discuss the role of part-per-million oxygen in gas feedstocks on graphene growth kinetics. By eliminating trace oxygen in the gas feedstock and furnace atmosphere, we obtain fast, high-quality growth without the reliance of hydrogen to balance oxygen species. We demonstrate the formation of a pinhole-free, continuous graphene within 1 min due to the lack of oxygen impurities. The O₂/H₂ ratio was explored to understand the balance between grain growth and oxygen-based etching. This study sets the maximum oxygen concentration with respect to hydrogen to facilitate fast, reproducible graphene growth. Our findings highlight the competitive nature of absorbed methane intermediate species as carbon precursors and trace surface oxide as the growth inhibitor and set the limit between impurities limited growth and methane adsorption-dissociation limited growth. We assert that CVD graphene growth in standard ultra-high purity gas feedstock without downstream purification falls within the impurities limited growth regime, thus is held at the mercy of oxidizing etching reactions.   

Determining intrinsic defect densities for high-quality self-flux synthesized transition metal dichalcogenides from first principles and experimental thermodynamics

Two-dimensional transition metal dichalcogenides (TMDs) have gained interest due to their novel optical and electronic phenomena and potential for optoelectronic applications. Chemical vapor deposition (CVD) and chemical vapor transport (CVT) are commonly used to synthesize TMDs due to their fast growths and high yields; however, they produce highly defective TMDs, negatively impacting the novel properties. Alternatively, the self-flux growth method has been shown to yield TMDs with defect densities several orders of magnitude lower than those from CVD and CVT. Yet, it is unknown how close the self-flux TMDs quality is to the intrinsic equilibrium defect density. In this work, we perform density functional theory calculations for intrinsic defects and substitutional oxygen impurities in WSe₂. We use ab initio modeling in conjunction with experimental thermodynamic data to calculate equilibrium formation energies and defect densities as a function of growth temperature. Theoretical defect densities at the self-flux growth conditions were shown to be an order of magnitude less than experimental values. The defect density difference between experiment and theory suggests that equilibrium was not reached, and kinetic or impurity factors alter the self-flux TMDs defect densities.   

Single crystal growth and characterization of RuCl3 intercalated graphite

 

Graphite intercalated material (GIC) refers to inserting atoms, ions or compounds into the crystal lattice of layered graphite. By intercalating other kinds of guest species, new physical behaviors can be tuned. In this process, no chemical bond breaking is involved. Thus, graphite maintains its two-dimensional honeycomb lattice. Recently, a large number of transition-metal halide intercalation compounds have been the subject of 2D magnetism investigations.  For the first time, we have successfully synthesized high stage bulky RuCl₃ intercalated single crystal graphite.  Inspired by photocatalysis reaction, we designed a novel and safer way to introduce a halide atmosphere while eliminating hazardous chlorine exposure in the synthesis process.

XRD and SEM measurements were carried out to confirm the purity of the stage IV and stage II single crystal C-RuCl₃ samples. The number of stages indicate the number of graphite layers between two layers of RuCl_3. These single crystal samples cannot be ground into fine powders for measurements, as they are readily destroyed with the application of pressure. The magnetoresistance was measured for both samples. The higher degree of intercalation in the stage II sample apparently improves the in-plane crystalline quality, evidenced by the more pronounced magnetoresistance quantum oscillations.   

Thin-film transition metal dichalcogenide growth fromheterostructuredbulk metallic patterns

Much research has been dedicated to synthesis of two-dimensional transition metal dichalcogenide (TMD) materials as well as their alloys and heterostructures, particularly by chemical vapor deposition (CVD). We will present initial investigations into potential alloying and heterostructure formation using a CVD method where vertical stacks of bulk transition metal films (Mo and W) are deposited in lithographically defined patterns which act as source material for the CVD reaction. Resultant lateral TMD growth was found to depend on the stacking order of the Mo and W. Interestingly, we have observed that often the predominant TMD formed is based on the first metal deposited, though covered by the second metal. For example, when W is the bottom metal, the lateral growth is predominantly WS₂ in all samples even though the W is ‘capped’ by the Mo. Whereas, when Mo is the bottom metal, the growth is predominantly MoS₂ in half of the samples and WS₂ in the other half, for given growth conditions. For specific growth parameters we observe an enhancement in the WS₂ growth in the presence of the Mo capping. We will present results from characterization using Raman and photoluminescence spectroscopy, SEM imaging, and EDX and discuss possible growth mechanisms.    

Liquid-Solid Assisted synthesis of 2D M2X Materials

Two-dimensional transition metal dichalcogenides (TMDs) MX₂ with M = (W, Mo) and X = (Te, Se, S) are known for their layered structure which yields extraordinary electronic properties. However, other transition metal chalcogenides have been rarely reported. In this work, we have used liquid−solid interface growth which enables us to synthesize layered group 11 chalcogenides with a focus on hexagonal Cu₂X (X=Te, Se, S) crystal system. It is observed that liquid copper plays a significant role in synthesizing these new 2D material systems. Like traditional TMDs, thickness-dependent phonon signatures are observed, and a high-resolution atomic structure reveals a hexagonal layered phase of Cu₂Te that prefers to grow in lattice-matched layers. Theoretical calculations on these structures show thickness-dependent metallic and semiconducting natures, suggesting a promising potential for 2D material electronics. As theoretically predicted ¹ these materials cover a large range of bandgap i.e. 0.2 to 2.8 eV, high carrier mobilities, we believe this work opens a new paradigm in the utility of these materials in a variety of applications.     

Ultra-Confined Lateral Heterostructures in 2D Semiconductors

Spatial confinement drives most quantum effects in semiconductors. In two-dimensional (2D) materials, the gifted confinement along the out-of-plane direction enables a plethora of quantum effects and unprecedented properties. Extra degrees of confinement within the plane of 2D materials requires the development of advanced material synthesis methods combined with state-of-the-art nanofabrication techniques. Towards this goal, the formation of lateral junctions between heterogeneous 2D materials has been intensely pursed. However, the degree of spatial confinements offered by available techniques is far from dimensions at which quantum effects start to emerge. Here, we present a technique that enables the synthesis of lateral heterostructures with dimensions as small as a few tens of nanometers, in isolated or periodic fashions, in predefined locations, and with tunable material compositions. The prospect of the developed method for the realization of quantum devices based on 2D lateral heterostructures will be discussed.   

Growth of two dimensional antiferromagnetic CrCl3 flakes down to monolayer thickness

The van der Waals (vdW) CrCl₃ is a two-dimensional (2D) antiferromagnet. Previous reports have mainly focused on mechanically exfoliated samples, while controlled synthesis of high quality CrCl₃ thin flakes is critical for their application in 2D spintronics. Here, we report the growth of ultrathin CrCl₃ flakes with well-defined shapes down to monolayer thickness (~0.7 nm) via physical vapor transport technique. Confocal Raman measurements show that the CrCl₃ flakes are crystalized in the monoclinic structures at room temperature, consistent with high-resolution transmission electron microscopy results. We carry out atomic force microscopy and electrical characterizations on CrCl₃ flakes with various thicknesses as a function of time. Thick CrCl₃ flakes (>50 nm) show excellent ambient stability for up to 5 months after growth, while ultrathin flakes show sign of degradation in 5 days. We also investigate the electrical properties of graphene/CrCl₃/graphene heterostructures.                

Epitaxial ultraclean, wrinkle-free graphene and in-plane heterostructure growth on Cu(111) surface in an oxygen-free environment

Chemical vapor deposition (CVD)-derived graphene performance has shown to deteriorate with surface wrinkles, folds, and transfer-related contaminations. Towards the stitching-up phase of the graphene growth, the lack of active catalytic copper surface slows down the growth rate and leads to an excess of amorphous carbon formation. With the integration of an oxygen-free growth environment and a Cu(111) growth substrate; flat, clean, and intrinsic defect-free graphene can be reproducibly grown with an enhanced growth rate via low pressure CVD (LPCVD). The resulting sheet of graphene shows an epitaxial relationship with the substrate. Contamination-free graphene surface also enables clean transfer due to the absence of amorphous carbon and structural defects in the graphene sheet. Electrical measurements with h-BN encapsulation demonstrates carrier mobility comparable to exfoliated graphene, with ballistic transport characteristics at cryogenic temperature. In addition, in-plane h-BN-graphene heterostructures can be fabricated for the first time, which is the ideal platform  as a low friction substrate for rotating moiré devices, further enabling possibilities towards 2D circuitry.