Session F71: Defect Engineering and Interfacial Effects in 2D Materials I

Controlled Generation and Understanding of Defects in 2D Materials

The understanding of defects in two-dimensional (2D) materials, such as semiconducting transition metal dichalcogenides (TMDs), is key to exploit their properties for applications in electronics, optoelectronics, and catalysis, among others.

Defects in the form of dopants were introduced in TMD monolayers through the development of a liquid-phase precursor assisted technique. Substitutional dopants in MoS2 and WS2 layers were identified by high-resolution scanning transmission electron microscopy (HR-STEM). This synthesis approach can be tuned to control the doping concentration in the samples up to very high levels (more than 10 at%). The consequent changes in the photoluminescence (PL) emission and Raman spectra show that the proposed synthesis route is effective for engineering functionalities in TMD layers.

Moreover, we induced defects in MoS2 monolayers through controlled gallium ion irradiation, and carried out defect-healing experiments. HR-STEM and Raman spectroscopy characterizations allowed for an estimation of the distance between defects as a function of the longitudinal acoustic (LA) and A’1 modes’ intensities. Temperature dependent Raman spectroscopy studies of MoS2 samples with native defects were also conducted with various excitation energies. The analysis of defect induced Raman modes shows a clear dependence on temperature and excitation energy, as well as a linear trend between the density of defects and the relative intensity of the LA and transversal acoustic (TA) modes with respect to the first-order E' modes. An expression for the quantification of defects in MoS2 was also established at various laser energies across excitonic transitions.

In summary, native and induced defects in TMDs were studied through a combination of electron microscopy, Raman, and PL spectroscopies. These results represent a contribution towards the manipulation, quantification and understanding of defects in TMDs, bringing 2D materials one step closer to applications.   

Long-Range Hydrophobic Attraction Between Graphene and Oil–Water Interfaces

Graphene has become one of the most studied materials over the past two decades due to its outstanding properties. Though it has a wide variety of applications, the mass production of high-quality graphene in a cost-effective way is still a challenge. As a solution for this problem, a simple, inexpensive and scalable method called 'Interface trapping method' was found to produce graphene by exfoliating graphite at the interface of oil and water. To understand this behavior of graphene further, we quantified the interactions between graphene and oil–water interfaces via force spectroscopy using functionalized AFM colloidal probes with graphene at different reduction levels. Our force data showed that graphene is attracted to the oil–water interface with an interaction ranging several 100 nm. The interaction increased with the degree of graphene reduction, which supported our hypothesis that the forces were due to long-range hydrophobic interactions between graphene and the oil–water interface. We were further able to determine the receding and advancing water contact angles on graphene as a function of reduction. This insight on these interactions will enable further development and optimization of the interface trapping method to yield high-quality graphene at industrial scale.   

Tuning electronic band structure of graphene (Gr/SiC) by metal intercalation: Pb and Gd

Controlled metal intercalation under graphene is a very promising way to build a range of hybrid nanostructures with varying novel electronic and topological phases. Despite its importance, information about the growth conditions (temperature, coverage θ), the intercalation mechanism and intercalation location are missing. Pb is especially important for graphene intercalation because of its high spin orbit (SO) coupling and expected 2-d superconductivity. With high resolution surface diffraction, we have studied Pb intercalation on a mixed surface of single layer graphene and buffer layer on SiC. We found that the onset of intercalation is at the relatively low temperature ~200° C. Domain boundaries between these two phases (single layer and buffer layer graphene) serve as the entry portals for Pb atoms, allowing them to intercalant to the most stable intercalation location: underneath the buffer layer. Gd-intercalated bilayer graphene is also studied to identify different intercalated phases, and how the changes in its band structure are correlated to the Gd intercalation location, which is more challenging to determine in experiment due to more possible locations for the intercalation of bilayer graphene. By globally studying the changing morphology as a function of coverage and temperature, we have identified several Gd-intercalated phases. High resolution scanning tunneling spectroscopy (STS) spatial maps of the topography and differential conductance dI/dV are performed to correlate the Gd-intercalated location to the changes in the local band structure as seen in the spectra.   

Two-Dimensional interfaces may not be flat

The interfaces between two-dimensional (2D) materials and 2D-3D materials host rich and diverse phenomena and applications. However, most studies only show a rather local picture of these interfaces. The understanding of the interface interaction including interface flatness on a larger scale remains limited. Here we present the flatness of 2D-2D and 2D-3D interfaces in arrays of field-effect transistors with a total length of over 16 µm. This approach shows a more complete picture of the 2D interfaces from multiple metal contacts and transistor channels, with and without hexagonal boron nitride (hBN) encapsulation. By using cross-sectional transmission electron microscopy (TEM), we observe that transferring hBN onto contacted 2D materials can dramatically alter the interface structure. Intriguingly, the interface between 3D metal and 2D materials is also not flat. We observe highly non-uniform adhesion between evaporated metal on 2D materials. Finally, we correlate the interface characterization with electrical device measurements, highlighting that interface dynamics can profoundly impact device operation in transistors and beyond. Our results reveal the intricacy and complexity of 2D interfaces and challenge the conventional assumption that 2D interfaces are always flat and uniform.   

Disordered Bulk Hydrogenated Graphene Revealed by X-ray and Neutron Scattering

Defects and functionalization can be used to modify graphene’s physical properties such as changing its conductivity or inducing ferromagnetism, which could be of interest for spintronics.  Here we investigate hydrogenated graphene nano-powder that has been produced by Birch reduction of graphite oxide.  One significant challenge in assessing these materials is the ability to determine their atomic structure. By combining x-ray diffraction (XRD), neutron diffraction (ND), and transmission electron microscopy, we show that these materials contain a significant portion of highly disordered carbon. XRD and ND reveal that a small portion of these samples are 2-3 layers thick with a maximum interlayer spacing of 3.85 Å – a significant expansion compared to pristine few-layered graphene, indicating that some H may attach onto the graphene planes. Modeling the coherent diffuse scattering seen in XRD and ND as well as comparing the coherent and incoherent ND, we determine the H/C ratio and we conclude that these materials contain a structure that is quite different from the idealized graphane structure.   

Charging Single Sulfur Vacancies in Monolayer WS2 on Graphite

Point defects in Van der Waals semiconductors can be engineered in many ways beyond their bulk counterparts since screening is reduced when collapsed to the 2D limit. By using scanning tunneling microscopy and spectroscopy (STM/STS), we study the electronic structures of sulfur vacancies in monolayer WS₂ on graphite as a function of their charging state.  For charge-neutral sulfur vacancies, two in-gap states above the Fermi level are observed, consistent with previously reported results.  Moreover, we found a variety of complex defects, including negatively charged point defects which produce pronounced band bending within a length scale of 1-2 nm. Some negatively charged defects create two in-gap states with one above and one below the Fermi level. Some, however, create far more complicated in-gap states. By introducing known chemical species to deterministically dope the sulfur vacancies, we intend to unveil the evolution of the electronic structures associated with sulfur vacancies as a function of their charging state.   

Periodic Strain in Patterned CVD Grown MoS2

Transition metal dichalcogenides (TMD) have emerged as promising materials for novel optical and electronic devices on the nano-scale due to the myriad of unique and tunable properties they display at the single layer. Deformations of TMD materials, such as folds and valleys, result in localized variations in the electronic structure and modification of its optoelectronic properties. We will present results of a process for developing crystalline growth of high-quality MoS₂ with periodic strain resulting from naturally occurring folds. The material is formed from a pre-deposited transition metal pattern which acts as both a seed material and forms as-grown complimentary metal-oxide electrical contacts. We observe lateral material growth on the order of 50 microns with two naturally occurring patterns, one with periodic folds typically at a 2 micron separation occurring through most of the material, the other with folds at a 1 micron separation occurring at the edge of the grown material extending up to 10 microns. Photoluminescence mapping is indicative of MoS₂ and shows increased intensity at separations which correspond to the local periodic growth. Further understanding of this growth process could lead to scalable deterministic induced strain devices.   

Scanning Tunneling Microscopy of Monolayer Hexagonal Boron Nitride Nanoribbons Grown on HOPG

Van der Waals heterostructures involving 2D materials such as graphene (Gr) and hexagonal boron nitride (hBN) exhibit a variety of interesting physics in prototype devices, but require improvements in large-area growth and control over defect densities for wafer-scale implementation. In this work, we use scanning tunneling microscopy (STM) to characterize monolayer ribbons of hBN grown on highly oriented pyrolytic graphite (HOPG) via high-temperature molecular beam epitaxy. STM images confirm that the hBN ribbons grow outward from Gr edges and the morphology depends on the Gr armchair or zig-zag termination. FFT analysis indicates that the hBN and Gr lattices are aligned across the growth interface. We observe large area single-periodicity moire patterns with a spacing consistent with the intrinsic lattice mismatch between rotationally aligned Gr and hBN. Atomic-resolution imaging reveals three distinct periodicities: in addition to the expected hBN honeycomb lattice, there are hexagonal and superstructure periodicities which may reflect variations in local strain and/or the presence of intercalated species. These studies provide atomic-level insights into the growth of single-domain hBN by high-temperature MBE.   

Mechanical exfoliation of single-crystalline spin-crossover molecular crystal

Mechanical exfoliation of two-dimensional van der Waals atomic crystals has led to significant scientific progress and developments in next-generation nanodevice technology. This talk reports the mechanical exfoliation of a novel spin-crossover molecular crystal with strongly anisotropic intermolecular interactions.  Monocrystalline flakes of molecular crystal down to monolayer thickness remain chemically and structurally intact as verified by Raman spectroscopy and atomic force microscopy. Temperature-dependent optical measurements characterize properties of the spin transition as a function of the layer numbers.  Additionally, we build 2D inorganic-organic heterostructures and examine the interlayer coupling. Our results establish a new molecular building block for the library of two-dimensional van der Waals materials.   

Ab initio investigation of the role of the choice transition metal on the structural, electronic, and magnetic properties of transition-metal intercalated 2D materials.

Intercalated transition-metal dichalcogenides (TMDs) host a range of novel magnetic and electronic phenomena including low-power switching, and new potential spin qubits. Manipulation of spins of these intercalating species could open the pathway towards developing new highly tunable magnetic interactions in low-dimensional structures. In this work, we study the effect of varying the intercalant species on the structural, electronic, and magnetic properties of transition-metal intercalated TMDs using first-principles calculations. We systematically study the inclusion of Fe, Ni, and Co intercalants between the TMD layers, in varying concentrations, and at different sites, and track the subsequent exchange interactions in these new magnetic superstructures. Our results indicate that intercalant site selection and concentration, in addition to the atomic species itself play a significant role in determining the ultimate properties of the intercalated TMD, and suggest routes for enabling their tunability. We further discuss the performance of  the exchange-correlation functional (DFT+U and hybrid DFT) on the calculated properties.   

Strain Engineering of Single-layer MoS2 on SiO2 Substrate by Developing a Neural Network Interatomic Potential based on Density Functional Theory.

  Transition metal dichalcogenides (TMDCs) are promising materials for nanoelectronics, energy storage applications, and photonics. However, one of the existing challenges for these materials is the low fracture toughness resulting in rapid crack growth. CVD-grown monolayer MoS₂ (ML-MoS₂) on SiO₂ substrate is susceptible to fracture since there are interatomic forces at the interface, which may not be purely vdW force, leading to crack formation at the edge of MoS₂ triangular crystal layer. Moreover, there are restrictions in molecular dynamics (MD) to study this phenomenon due to the unavailability of a customized interatomic potential (IP) for this interfacial system. In this work, we aim to develop an IP for ML-MoS₂ on SiO₂ substrate using a deep neural network (DNN) based on ab-initio molecular dynamics (AIMD) to study fracture mechanisms. The trained model could recreate interatomic forces and energies of a random frame in the test sets. Furthermore, it could accurately describe the structural properties of the system, such as radial distribution function (RDF) and stress-strain response. Consequently, this model enables MD simulations to study fracture in ML-MoS₂ efficiently with high accuracy.   

h-BN Nanosheets Doped with Transition Metals for Environmental Remediation; A DFT approach and Molecular Docking Analysis

This study compares the catalytic and antimicrobial potential of boron nitride (BN) nanosheets doped with various transition metals-TMs (Co, Cu, Ni, Zr, and Bi). Various characterizations techniques were employed; structural features confirmed via XRD, optical properties that were in deep ultravoilet region accessed through UV-vis, and confirmation of nanosheets were examied through TEM. Evaluation of catalytic activity was accessed to determine the degradation of pollutants. Prepared materials demonstrated significant results that indicate, it can be utilized as efficient nanocatalysts for wastewater management. TMs-doped BN depicted higher bactericidal efficacy against S. aureus compared to E. coli with molecular docking analysis. Density functional theory calculations were also performed to investigate the structural stability and electronic behavior that shows bandgap evolution corroborates well with the experimental trends, exhibiting a diminution of the band gap value with substitutional TM atoms. Moreover, the adsorption energies of NaBH₄ molecule on undoped and TMs doped BN nanosheets are investigated, in which the adsorption energy between the Co-doped BN monolayer and NaBH₄ is greater compared with other doped nanosheets.