Symposium QN02—Defects, Electronic and Magnetic Properties in Advanced 2D Materials Beyond Graphene

In this talk, I will present our work on 2D ferromagnet, antiferromagnet, and multi-phase interconversion. Specifically, it includes the discovery of the intrinsic ferromagnetism in 2D van der Waals materials, the new design of spin field effect transistor based on 2D antiferromagnets, and the interconversion between multiple magnetic phases in 2D magnets. These advances not only deepen the fundamental understanding of the 2D magnets physics, but also suggest possible outlets for the existing challenges in this emerging field such as high-temperature 2D magnets and low-power spintronics.

In this talk, I will discuss the exciting theoretical possibilities for new phases that can emerge and the van der Waals materials within which they may be realized. I will focus on the utility of Raman spectroscopy for characterizing and identifying the novel aspects of the magnetic excitations in these materials.

In this paper, the magnetic and electrocatalytic properties of hydrothermally grown transition metal doped (10% of Co, Ni, Fe, and Mn) 2H-MoS2 nanocrystals (NCs) with a particle size 25–30 nm are reported. The pristine 2H-MoS2 NCs showed a mixture of canted anti-ferromagnetic and ferromagnetic behavior. While Co, Ni, and Fe doped MoS2 NCs revealed room temperature ferromagnetism, Mn doped MoS2 NCs showed room temperature paramagnetism, predominantly. The ground state of all the materials is found to be canted-antiferromagnetic phase. To study electrocatalytic performance for hydrogen evolution reaction, polarization curves were measured for undoped and the doped MoS2 NCs. At the overpotential of η = −300 mV, the current densities, listed from greatest to least, are FeMoS2, CoMoS2, MoS2, NiMoS2, and MnMoS2, and the order of catalytic activity found from Tafel slopes is CoMoS2 > MoS2 > NiMoS2 > FeMoS2 > MnMoS2. The increasing number of catalytically active sites in Co doped MoS2 NCs might be responsible for their superior electrocatalytic activity. The present results show that the magnetic order-disorder behavior and catalytic activity can be modulated by choosing the suitable dopants in NCs of 2D materials.

We have performed an extensive high-throughput screening of known inorganic materials, in order to identify those that could be exfoliated into novel two-dimensional monolayers and multilayers [1]. The screening protocol first identifies bulk materials that appear layered according to a simple and robust chemical definition of bonding, determining then for all of these the binding energies of the respective monolayers, and their electronic state (metallic vs insulating), magnetic configuration (ferro-,ferri- or antiferro-magnetic), and phonon dispersions (to evaluate mechanically stability). Such protocol identifies a portfolio of close to 2,000 inorganic materials that appear either easily or potentially exfoliable, to be investigated further for promising properties. First focus has been on the determination of the effective masses and mobilities (from the full solution of the Boltzmann transport equation) for electronic applications; of topological invariants; of superconductivity and charge-density waves; and of photocatalytic parameters for water splitting. Thanks to the use of the AiiDA (http://aiida.net) materials' informatics platform, all the high-throughput calculations can be performed and streamlined in fully searchable and reproducible ways, they are stored in a database with their full provenance tree of all parent and children calculations, and can be shared with the community at large in the form of raw or curated data via the Materials Cloud (http://www.materialscloud.org) dissemination portal.

[1] Nicolas Mounet, Marco Gibertini, Philippe Schwaller, Davide Campi, Andrius Merkys, Antimo Marrazzo, Thibault Sohier, Ivano Eligio Castelli, Andrea Cepellotti, Giovanni Pizzi and Nicola Marzari, Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds, Nature Nanotechnology 13, 246–252 (2018).

The research on two dimensional (2D) materials keeps attracting much interest as being propelled by compelling features like the emergence of new physical properties in the transition from bulk to the 2D aspect and the promising potential to create a multitude of innovative device structures, with the projected horizon to push beyond the boundaries of current semiconductor device technology. Among the 2D materials, layered transition metal dichalcogenides (TMDs) experience an ever-growing research interest as these could potentially replace traditional semiconductors in next generation nanoelectronic devices. Among this group of TMDs, molybdenite (MoS₂) has surfaced as a particularly interesting TMD.
However, not different from other semiconductors, the presence of structural defects in the 2D crystal lattice considerably affects the performance of TMD-based devices. Hence, defect identification, characterization and quantification, more specifically the evaluation of the impact of processing, is considered indispensable in the progress of device technology research. In this respect, electron spin resonance (ESR) arises as an exclusive technique for atomic assessment and quantification of point defects.
In this regard, we will present on overview of the results obtained from multifrequency ESR study of TMDs, mainly focused on MoS₂ as encountered in various forms according to origin, including p-type geological 2H MoS₂ bulk crystals, synthetic bulk 2H MoS₂ grown by chemical vapour deposition, and large-area few-layer 2H MoS₂ films grown on SiO₂ by Mo sulferization, and metal-organic chemical vapour deposition (MOCVD) grown on c-plane sapphire template wafers after a wet transfer procedure to another substrate.
The results will deal with point defects, either of intinsic or extrinsic origin. With respect to the bulk MoS₂ crystals, a main focus has been on impurities operating as dopants. Accordingly, data will be presented on As and N impurities substituting for S sites, operating as stable, covalently bonded substitutional acceptors. Added to this are the results of one more newly-observed ESR signal, tentatively assigned to Pb impurities substituting for Mo sites, also operating as acceptors.
Besides identification of the impurity-associated defects, largely based on the uniquely revealed hyperfine structure in the ESR spectra, ESR is demonstrated as an exclusive technique for undubious and due impurity-specific inference of the thermal activation energy (level in the bandgap) of dopants, where the As and N acceptors are found to be characterised by an activation energy of E_a = 0.7 ± 0.2 meV and 45 ± 7 meV, respectively.
In a final part, the work will deal with synthetic large-area polycrystalline few-layer 2D MoS₂ films grown on SiO₂ by Mo sulfurization or MOCVD, where the latter films are studied after layer transfer to a target thermal SiO₂/(100)Si substate –this with the intent to probe the impact of the act of layer transfer. The large area synthetic films were found to suffer from substantial amounts of intrinsic defects: In the latter MOCVD grown films, a previously unreported signal has been observed, which based on a comparison with previous ESR data, first principles simulations, and Rutherford backscattering spectrometry data, is assigned to an intrinsic defect, most likely a Mo or MoS₃ vacancy at grain boundaries.
As to the films synthesized by sulfurization of Mo films, the presence of a previously unreported anaisotropic defect of axial symmetry about the c-axis, characterized by g_// = 2.00145 and g_perp = 2.0027, is revealed. Based on the analysis of the ESR sigal features in combination with computational results on defect g factors, the signal has been tentatively assigned to a (di)sulfer antisite defect (S or S₂ substituting for a Mo atom) with energy levels in the MoS₂ bad gap, associated with grain boundaries, the latter thus emerging as intolerably impairing the carrier mobility and layer functionality.

Line defects, such as vacancy agglomerations and grain boundaries, strongly affect the properties of 2D, see Ref. [1] for an overview. The effects can be not only detrimental, but also beneficial in transition metal dichalcogenides (TMDs), so that controlling the density and type of the boundaries in these systems should be important for engineering their properties. In my presentation, I will touch upon the energetics and formation mechanisms of mirror twin boundaries in various TMDs during their growth [2] or under electron irradiation [2-4] and post-synthesis deposition of Mo atoms [5] with the main focus on the theoretical results obtained using DFT calculations in collaboration with several experimental groups. I will also discuss how controllable introduction of mirror twin boundaries and inversion domains can be used to add new functionalities to 2D TMDs.
1. H.-P. Komsa and A. V. Krasheninnikov, Advanced Electronic Materials 3 (2017) 1600468.
2. O. Lehtinen, H.-P. Komsa, A. Pulkin, M.B. Whitwick, M.-W. Chen, T. Lehnert, M. Mohn, O.V. Yazyev, A. Kis, U. Kaiser, and A. V. Krasheninnikov, ACS Nano 9 (2015) 3274.
3. Y.-C. Lin, T. Björkman, H.-P. Komsa, P.-Y. Teng, C.-H. Yeh, F.-S. Huang, K.-H. Lin, J. Jadczak, Y.-S. Huang, P.-W. Chiu, A. V. Krasheninnikov, and K. Suenaga, Nature Communications 6 (2015) 6736.
4. T.Lehnert, M. Ghorbani-Asl, J. Köster, Z. Lee, A.V. Krasheninnikov and U. Kaiser, to be published.
5. P. M. Coelho, H.-P. Komsa, H. C. Diaz, Y. Ma, A. V. Krasheninnikov, and M. Batzill, ACS Nano 12 (2018) 3975.

Two-dimensional (2D) materials especially transition metal dichalcogenides usually have diverse structural and electronic phases, which offer tremendous opportunity to explore and ultimately tailor many optoelectronic properties and quantum phenomena, including charge density waves, magnets, topological insulators, and Weyl semimetals. However, understanding and control the phase transition process in 2D materials is still a great challenge. Our recent work shows that the defects in low symmetry 2D PdSe₂ crystals break the lattice symmetry and induce large structural distortion and ultimately form a new phase, however high symmetry 2D materials such as MoSe₂ still retain their crystalline lattice even under the defect density up to 20%. Various methods triggered the phase transformation in 2D PdSe₂ crystals will be discussed. The defect-mediated transformation process in this material was studied by using in-situ scanning transmission electron microscopy (STEM) combined with theoretical calculation. Through selective phase engineering, single material devices based on few-layer PdSe₂ crystal with the new metallic phase as the contact display the significantly enhanced electrical performance due to the reduced contact resistance and Schottky barrier height. Our results suggest that phase engineering provides a promising way in tuning the properties in 2D materials for applications in electronics, catalysis, or magnet.
Acknowledgment: Synthesis science was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division and characterizations were performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

A persistent challenge in realizing the potential of 2D materials for optoelectronics is the lack of precise control over carrier doping levels and work functions. Here, I focus on results from first-principles calculations, accompanied by complementary experiments, that seek to address these fundamental questions through various interfacial- and defect-engineering strategies. On the one hand, we explore strategies based on noncovalent functionalization of 2D materials with functional polymers that allow for spatially-targeted tuning of electronic properties of the active channel material. On the the other hand, we explore strategies for small-molecule, covalent functionalization of 2D materials that result in unexpected magnetic properties with potential for spintronic applications. A common feature of the approaches proposed here is their amenability to conventional solution-processing and/or lithography that are inherently scalable for technological applications.

A number bi-elemental two-dimensional (2D) nanomaterials (2DNMs) have been experimentally realized in recent years, including stable families of 2DNMs such as transition metal dichalcogenides (TMDs) and M-Xenes. Unlike mono-elemental graphene, MoS₂ (a prototype for TMDs) consists of two atomic layers of close-packed sulfide atoms separated by one close-packed molybdenum atomic layer. TMDs exhibit a number of attractive physical properties, including electronic, optical, and mechanical. However, throughout their lifetime 2DNMs can be exposed to high temperatures and particle irradiation environments, which can dramatically alter the structure-property relationships and degrade their physical properties.

This contribution seeks to understand the physical processes occurring at multiple length scales in MoS₂ and determine the structural stability and property evolution in MoS₂ in conditions far away from equilibrium. Irradiation and high temperatures push materials out of equilibrium, thus producing dynamic systems, which can result in unexpected behaviors. Defect formation and interaction mechanisms in free-standing MoS₂ under irradiation (and various temperatures) were observed in-situ using various experimental techniques, including high resolution transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The phenomena of interest included energy dissipation, defect formation and interaction mechanisms, and the role of defect sinks in the material’s radiation response. The irradiation response of monolayer MoS₂ will be compared to that of a bilayer and a material to discern the differences in their radiation response.

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award # DE-SC0019014.

Van der Waals interactions were believed dominant for interlayer interactions in layered two-dimensional (2D) materials. We recently found wavefunction overlaps do occur at vdW gaps, namely covalent-like quasi-bonding (CLQB)[1-5]. Here, we show that such overlaps strongly affect interlayer magnetism using density functional theory calculations. In particular, the interlayer magnetic couplings (IMCs) of CrS₂ [6] and CrI₃ [7] bilayers were discussed since they are at two extremes for the strength of IMC. The interlayer FM coupling in CrS₂ is very robust and is nearly unable to be tuned under usual external fields, but the intra-layer magnetism does be varied under layer stacking. An opposite case was found in CrI₃ bilayers where the intra-layer FM coupling is, however, very strong but the interlayer magnetism was found governed by a subtle change of interlayer stacking; this shows a decoupled magnetic interaction between intra- and inter-layer directions.

References
[1] J. Qiao et al. Nature Communications 5, 4475 (2014)
[2] Z.-X. Hu et al. Nanoscale 8, 2740 (2016).
[3] Y. Zhao et al. Advanced Materials 28, 2399-2407 (2016).
[4] Y. Zhao et al. Advanced Materials 29, 1604230 (2017).
[5] J. Qiao et al. Science Bulletin 63, 159-168 (2018).
[6] C. Wang, et al. Phys. Rev. B 97, 245409 (2018).
[7] P. Jiang and C. Wang et al. arXiv:1806.09274.

Surface Acoustical Waves (SAWs) and Transition Metal Dichalcogenides (TMDs), separately, are topics of current research due to their present and future use in telecommunications and beyond-CMOS technology. The interaction between a SAW and a 2D electron gas has been previously studied by measuring the absorption of the SAW by GaAs and, more recently, graphene.,,
Here, the interaction between a SAW and a TMD is studied using MoS2 directly grown by chemical vapor deposition on 128°YX-cut LiNbO3.By focusing a 532 nm laser on the sample, the generation of electron-hole pairs is found to enhance the attenuation of the SAW as expected.
We present a hybrid FET-SAW (field effect transistor-surface acoustic wave) device. Our experiments reveal close agreement between the SAW spectroscopy and the FET transport measurements. SAW spectroscopy can be used to spatially resolve variations inside triangular MoS2 islands.
Furthermore, the time dependence of the SAW attenuation with laser exposure is used to distinguish between semiconductor film and the substrate. This technique provides a means of electrically characterizing atomically thin semiconducting film that avoids the limitations of metallic contacts.

1.Weinreich, G., Acoustodynamic effects in semiconductors. Phys. Rev. 104, 321 (1956); http://dx.doi.org/10.1103/PhysRev.104.321
2.Hoskins, M. J.; Morkoç, H.; and Hunsinger, B. J., Charge transport by surface acoustic waves in GaAs. Appl. Phys. Lett. 41, 332 (1982); https://doi.org/10.1063/1.93526
3.
Miseikis, V.; Cunningham, J. E.; Saeed, K.; O’Rorke, R.; and Davies, A. G., Acoustically induced current flow in graphene. Appl. Phys. Lett. 100, 133105 (2012); https://doi.org/10.1063/1.3697403

Robust operation of quantum devices relies on the chemical and structural stability of their parts, including electrical contacts. Given that temperatures and environmental conditions at which quantum devices are synthesized can differ dramatically from their operating conditions, it is imperative to understand how temperature variations, material imperfections, exposure to contaminants, and behavior of metal contacts are inter-related.
Here we focus on metal adhesion to a prototype 2-dimensional metal dichalcogenide WTe₂, which exhibits topological behavior in a monolayer limit. We have developed a reliable procedure to mechanically exfoliate single crystal WTe₂ in ultra-high vacuum in our scanning tunneling microscopy (STM) system. This approach allows for the STM images with atomic resolution to be readily acquired without further treatment. Few defects, attributed to Te vacancies, were observed. Our experiments on Pd deposition on WTe₂surface show that structurally ideal sites and defects, provide stable adsorption sites for small Pd clusters (~2 nm in diameter) even after annealing to 523 K. Tip manipulation experiments demonstrate that Pd particles anchored on defects are more stable than those on non-defective surface sites: we can remove and split Pd nanoparticles above the non-defect sites using STM. The nanoparticles anchored to defects can only be altered but cannot be completely removed demonstrating a strong binding. The atomic-level insight into Pd cluster binding to the ideal and defective sites was obtained using ab initio (density functional theory) simulations. We discuss how the local atomic structure and charge redistribution in the vicinity of the binding site affects the stability and electronic properties of this system.

The physical properties of the vdW heterostructures are significantly affected by their stacking geometries and interlayer coupling (including interlayer charge transfer and energy transfer). In a previous study, we showed that monolayer WS₂ flakes grown onto a graphene substrate exhibit outstanding air stability compared to the flakes grown on a SiO₂/Si substrate.^[1] Using various characterization tools and based on previous reports, we demonstrated that the surface charge transfer significantly affects the prevention of air degradation for the first time. This result suggested that understanding the effect of interlayer coupling in the stacking geometry of vdW heterostructures is highly important for obtaining materials with desirable properties.
Chemical vapor deposition (CVD) has received attention for the scalable synthesis of layered 2D materials and their heterostructures. However, typically synthesized large-scale 2D films by the CVD process are polycrystalline in nature and include a high density of atomic and structural defects, which are associated with a broken lattice symmetry, a modified energy landscape, and quantum confinement. These localized heterogeneities strongly affect the electronic properties of 2D materials. Since dangling bonds at vacancy defects, curved wrinkles, and grain boundaries (GBs) are more reactive and have interaction strengths different from that of a pristine region, defects in the template layer can have a profound effect on the nucleation and growth of heterostructures. However, the effects of atomic and structural defects on the interlayer properties or configuration have not yet been considered; instead, previous theoretical and experimental studies have considered only defect-free templates.
In this study, we demonstrate that the intrinsic properties of subsequent layers of a vdW heterostructure are based on the structural features of the template layer (e.g., the basal plane and GB of graphene), using two types of WS₂ flakes grown directly on graphene defects (D-WS₂) and a pristine basal plane (B-WS₂). Both types of WS₂ flakes have the same crystal structure despite the atomic displacement and lattice distortion beneath the D-WS₂ flakes; however, they strongly influence the interlayer interactions between the stacked layers, thus affecting the layer deformability, thermal stability, and physical and electrical properties. Combined experimental and computational studies showed that the difference in the behavior of D-WS₂ flakes could be attributed to the formation of covalent bonds via W atomic bridges (i.e., formation of larger overlapping hybrid orbitals) at defect sites. Our results reveal the importance of understanding the interlayer coupling in stacking geometries and its correlation effect for designing desirable properties in 2D vdW heterostructures.


[1] S.-Y. Kim, J. Kwak, J. H. Kim, J.-U. Lee, Y. Jo, S. Y. Kim, H. Cheong, Z. Lee, S.-Y. Kwon, 2D Mater. 2017, 4, 011007.

Alloying two-dimensional (2D) semiconductors provides a powerful method to tune their physical properties, especially those relevant to optoelectronic applications. Visualizing the atomic-scale crystal structure of multispecies alloys is crucial in order to correlate their structures-property relationship and engineer their functionality. However, constructing the atomic structure of alloys becomes more difficult as the number of atomic planes and atomic species in an alloy increase. Here, we reveal structural information about the atomic lattice of monolayer and few-layer III-VI alloys using annular dark-field scanning transmission electron microscopy (ADF-STEM) imaging combined with image simulation, second harmonic generation (SHG), and first principles calculations. In addition, we correlate the atomic and electronic structure of the alloys and probe change in their electronic structure as a function of layer number using low-loss electron energy loss spectroscopy (EELS) and density functional theory (DFT) calculations.

To obtain structural and electronic properties of advanced two-dimensional material at the atomic scale is a growing demand in materials science. A new type of transmission electron microscopes operating at electron energies between 80keV and 20keV has been developed. It allows to undercut most of the materials knock-on damage thresholds and enables sub-Angstroem resolution in an 4000x4000 pixels, single-shoot image down to 40keV by correcting not only the geometrical aberrations of the objective lens but also its chromatic aberration [1,2]. During the imaging process, the interaction of the beam electrons with the low-dimensional material can, nevertheless, results in changes of the atomic structure due to ionization and radiolysis [3], and sophisticated sample preparation methods are employed to reduce these effects [4,5]. We briefly outline key instrumental and methodological developments.
To modify properties of two-dimensional materials, we use the electron beam not only for imaging its original structure but also for engineering new properties. Thus we report on the structure as well as on electron-beam-induced transformations from point to extended defects in 2H-MoTe₂ and their changes to the distorted 1T’-MoTe₂ phase. In the case of TaSe₂ we demonstrate the creation of commensurate charge density wave (CDW) in a monolayer 1T-TaSe₂ [6], and show in addition that the CDW’s order varies due to an increase in the S/Se vacancies. For the two-dimensional material MnPS, we show that EELS experiments provide a means to identify single-layer MnPS from bulk structures and give the signature of the low dimensionality in their electronic excitations [7]. Finally we intercalate bilayer graphene by lithium and study the lithiation and delithiation between bilayer graphene, as well as the structure of the new high density crystalline Li- Phase [8].
Most of our studies are accompanied by first principle calculations to understand the structural and electronic properties. [9].

[1] U. Kaiser, J. Biskupek, J.C. Meyer, J. Leschner, L. Lechner, H. Rose, M. S.- Pollach, A.N. Khlobystov, P. Hartel, H. Müller, M. Haider, S. Eyhusen and G. Benner, Ultramicroscopy 111 (2011) 1239.
[2] M. Linck, P. Hartel, S. Uhlemann, F. Kahl, H. Müller, J. Zach, and M Haider, M. Niestadt and M. Bischoff, J. Biskupek, Z. Lee, T. Lehnert, F. Börrnert, H. Rose, and U. Kaiser, PRL 117 (2016) 076101.
[3] R.F. Egerton, Ultramicroscopy, 145 (2014) 85.
[4] G. Algara-Siller, S. Kurasch, O. Lehtinen, U. Kaiser, APL 104 (2014)153115.
[5] G. Algara-Siller, O. Lehtinen, A. Turchanin, U. Kaiser, APL 103 (2013) 20310.
[6] P. Börner, M. K. Kinyanjui, T. Börkmann, T. Lehnert, A. V. Krashenninikov, U. Kaiser, APL, 113 (2018) 173103
[7] M. K. Kinyanjui, J. Koester, F. Boucher, A. Wildes, and U. Kaiser, Phys. Rev. B 98 (2018), 035417
[8] M. Kühne, F. Börrnert, S. Fecher, M. Ghorbani-Asl, J. Biskupek, D. Samuelis, A. V. Krasheninnikov, U. Kaiser, J. H. Smet, Nature accepted
[9] We acknowledge funding from the DFG and the MWK of the Federal state of Baden-Württemberg, Germany, in the frame of the SALVE project (www.salve-project.de.) as well as from the Baden-Württemberg Stiftung GmbH in the frame of the CleanTech program and the European Union in the frame of the Graphene Flagship.

Ferromagnetic ordering in monolayer vanadium dichalcogenides was predicted by DFT, despite the lack of ferromagnetic ordering in its bulk form. Ferromagnetic 2D materials with high Curie temperatures are highly sought to enable spintronic devices based on van der Waals heterostructures. In this talk we present a discussion of VSe₂ and VTe₂ monolayers grown by molecular beam epitaxy and study the difference of their monolayer properties compared to the bulk. In addition, to charge order states and structural variations of the monolayer and the bulk, we critically assess the origin of the measured ferromagnetic properties [1] in (sub)monolayer VSe₂.
[1] M. Bonilla et al. ‘Strong room-temperature ferromagnetism in VSe₂ monolayers on van der Waals substrates’ Nat. Nanotechnol. 13, 289-203 (2018).

Magnetic van der Waals (vdWs) crystals have gained a great deal of attention in the recent past owing to their attractive layer-dependent anisotropic magnetic properties, and for next generation low power spintronic applications. Irradiation with high energetic particles such as protons is an effective route to manipulate the magnetic properties of these materials. In this work, we present the magnetic properties of proton irradiated (10¹⁸/cm²) vdWs crystal, namely, CrSiTe₃ (CST)¹. Pristine CST is a ferromagnetic (Curie temperature, T_C = 32 K) semiconductor (band gap = 0.7 eV). The magnetic measurements were performed both in-plane (IP) and out-of-plane (OOP) geometry as a function of temperature and magnetic field. Most significantly, at 2 K, the IP saturation magnetization increased from 3.85 to 4.77 µ_B upon proton irradiation. Furthermore, proton irradiation causes an increase in the coercive field (H_c) from 465 to 542 Oe and remnant magnetization (M_r) from 0.783 to 1.375 µ_B. In OOP geometry, due to proton irradiation, both H_c (from 138 to 223 Oe) and M_r (0.0483 to 0.247 µ_B) have increased, and noticeable (168 Oe at 2 K) an exchange bias is observed. No significant change in T_C is noted upon proton irradiation. The temperature dependent (10-50 K) X-band (~9.43 GHz) electron paramagnetic resonance (EPR) measurements performed on both pristine and proton irradiated CST crystals show a strong modification in the EPR signal shape from Dysonian to Lorentzian –inferring that the magnetic (super) exchange interactions and electronic behavior are altered. We will present and discuss our comprehensive experimental findings and offer plausible explanation for the observed behavior.

¹A. Milosavljevic et al., Evidence of spin-phonon coupling in CrSiTe₃, PHYSICAL REVIEW B 98, 104306 (2018)

The olivine A2BX4 are potential candidates for thermoelectric and magentic applications. An orthorhombic Pnma symmetry is adopted by A2BX4 where the A site forms a triangle-based low dimensional saw-tooth chain. In the present work, we have studied Mn2SiS4-xSex (0 < x < 4) using magnetization, specific heat, thermal conductivity and density functional theory. In general, the compounds undergo antiferromagnetic transitions and display spin-flop transition upon application of magnetic field. The TN shows a linear decrease from 86 K for Mn2SiS4 towards 66 K for Mn2SiSe4 as x varied; whereas the critical field for spin-flop varied non-linearly. The specific heat of Mn2SiS4-xSex revealed very low magnetic entropy at the TN thereby bringing out the underlying frustrated nature of the Mn2+ ions in the saw-tooth lattice. The density functional theory calculations show an energy gap of 0.5 eV between the valence and conduction bands and accordingly, a semiconducting-like thermal conductivity is revealed by the olivines. Our results are explained based on the emergence of frustration in the triangular units of the saw-tooth Mn lattice.

I will discuss examples of chemically disordered materials recently studied in our laboratory, with a focus on crystallographic and magnetic behaviors in bulk crystals of cleavable, layered halides and chalcogenides. In several cases, both x-ray diffraction and atomic resolution electron microscopy were key in understanding the average structure, local structure, and defect ordering. Much of the work is motivated by efforts to move toward higher magnetic ordering temperatures, and to achieve stronger magnetic anisotropy by incorporating more spin orbit coupling. With these aims in mind, the target materials include metallic systems with strong magnetic interactions and insulating compounds with 4d and 5d transition metals.

The advent of two-dimensional van der Waals crystals creates new possibilities in developing novel spintronic devices. Recent experiments have demonstrated that it is possible to obtain two-dimensional ferromagnetic order in insulating Cr₂Ge₂Te₆ and CrI₃ at low temperatures. Here, we developed a new device fabrication technique, and successfully isolated monolayers from layered metallic magnet Fe₃GeTe₂. We found that the itinerant ferromagnetism persists in Fe₃GeTe₂ down to monolayer. The ferromagnetic transition temperature, T_c, is suppressed in pristine Fe₃GeTe₂ thin flakes. An ionic gate, however, dramatically raises the T_c up to room temperature. The gate-tunable room-temperature ferromagnetism in two-dimensional Fe₃GeTe₂ opens up opportunities for potential voltage-controlled magnetoelectronics.

Layered intrinsically ferromagnetic (FM) semiconductors hold great promise for both fundamental physics and future applications in nanospintronics [1-5]. CrI₃ and Cr₂Ge₂Te₆ have recently attracted wide attention since long-range ferromagnetism persists in atomically thin devices due to large magnetocrystalline anisotropy [5-7]. In this talk I will discuss the nature of the FM transition in bulk crystals based on a detailed study of the critical properties and scaling analysis of magnetic entropy change. The obtained critical exponents suggest three-dimensional (3D) long-range magnetic coupling in CrI₃ and two-dimensional (2D) Ising-like type coupled with a long-range interaction in Cr₂Ge₂Te₆ [8,9]. The rescaled magnetic entropy changes collapse onto a universal curve independent of temperature and field for both materials, confirming a second-order type of the magnetic transition [10,11]. Considering its ferromagnetism can be maintained upon exfoliating bulk crystals down to a single layer, further investigation on the size dependence of critical properties and magnetocaloric effect is of high interest.

Acknowledgements

This work has been supported by the Research supported by the U.S. Department of Energy, Office of Basic Energy Sciences as part of the Computation Material Science Program (Y.L. and C. P.) and by the U.S. DOE under Contract No. DE-SC0012704 (C.P.).

References
1. M. A. McGuire et al., Phys. Rev. Mater. 1, 014001 (2017).
2. M. A. McGuire et al., Chem. Mater. 27, 612 (2015).
3. B. Huang et al., Nature 546, 270 (2017).
4. K. L. Seyler et al., Nat. Phys. 14, 277 (2017).
5. C. Gong et al., Nature 546, 265 (2017).
6. M. W. Lin et al., J. Mater. Chem. C 4, 315 (2016).
7. H. L. Zhuang et al., Phys. Rev. B 92, 035407 (2015).
8. Y. Liu et al., Phys. Rev. B 97, 014420 (2018).
9. Y. Liu et al., Phys. Rev. B 96, 054406 (2017).
10. Y. Liu et al., Phys. Rev. B 97, 174418 (2018).
11. Y. Liu et al., Phys. Rev. Mater. 3, 014001 (2019).

The recent discovery of 2D magnets provides new ways to study 2D magnetism by harnessing the unique features of atomically-thin materials, such as electrical control for magnetoelectronics and van der Waals (vdW) engineering for novel interface phenomena. In this talk, I will describe our recent magneto-optical spectroscopy experiments on vdW magnets. I will discuss the layered antiferromagnetic properties of CrI3 with electrical control, giant tunneling magnetoresistance through spin filtering effect in vdW magnetic tunnel junctions, magnetic dynamics in zigzag antiferromagnets, and progress on exfoliable 2D magnets with stable magnetic order near room temperature.

The representative 2D materials, graphene, h-BN, and MoS2, have interesting mechanical, electrical and optical properties and have exhibited fascinating physical phenomena so far. However, they mostly lack one important physical property in physics, magnetism. The new 2D materials such as CrSiTe3, CrI3, and FePS3, which began to be studied recently, possess ferro- or antiferro-magentic properties even in atomic level thickness and are expected to reveal deep level of physics in 2-dimensional confinement.
In this talk, our recent works on thickness dependence of magnetic properties of a ternary 2D material, Fe3GeTe2, which shows the ferromagnetic behavior, will be presented. Fe3GeTe2 was studied with hall measurement and STM methods. From the hall measurement, it exhibited the anomalous hall effect due to its intrinsic ferromagnetism. Interestingly, the magnetic properties such as coercivity changed significantly with decreasing thickness changing from weak ferromagnet to strong ferromagnet. Also, we found some mixed magnetic behavior of ferromagnetism and anti-ferromagnetism, which seems to be due to partial oxidation. STM study revealed some magnetic domains on the surface. The domain structures barely changed below Curie temperature, but disappeared above the critical temperature, which suggests they are magnetic structures. Since this material is conducting and very thin, and has high Curie temperature of around 220K, it appears to have a high potential for practical applications at micro- and nano-scales.

Several experimental groups have shown that defect structures in 2D WSe₂ result in single photon emission (SPE). However, the origin of SPE is still unknown. We present a first principles study of the nature and optical properties of point defects in 2D WSe₂, together with scanning tunneling microscopy (STM) and scanning transmission electron microscopy images. We predict that O₂ can dissociate easily at Se vacancies, resulting in O-passivated Se vacancies (O_Se) and O interstitials (O_ins), which give STM images in good agreement with experiment. Our GW-Bethe-Salpeter-equation calculations show that O_ins defects give exciton peaks ~50-100 meV below the free exciton peak, in good agreement with the localized excitons observed in independent SPE experiments. No other point defect (O_Se, Se vacancies, W vacancies, and Se_W antisite defects) gives excitons in the same energy range. We conclude that the O_ins defect is a likely source for the SPE previously observed in 2D WSe₂.

Atomically-precise, phosphorus donor devices fabricated in silicon using hydrogen lithography patterning with a scanning tunneling microscope (STM) represents the ultimate limit of fabrication resolution. Devices fabricated with atomic resolution are used as a discovery platform for everything ranging from quantum physics to ultra-efficient tunnel field effect transistors.

Atomically-precise devices are fabricated by hydrogen terminating silicon surfaces and then patterning the hydrogen with the STM tip. By applying a bias to the STM tip hydrogen can selectively be removed patterning the hydrogen resist. After patterning, dopant gas, typically PH3, can be introduced to the chamber to provide dopants to the patterned area. Through an anneal the phosphorus is incorporated into the silicon lattice and finally the atomically thin phosphorus layer is capped with a protective capping layer of silicon.

For many atomically-precise device applications, gating is a critical element of device operation. With hydrogen lithography, in-plane gates made of phosphorus can be added to devices, but it is expected that out-of-plane gates will be more efficient at gating the phosphorus dopant channel. One way to enable out-of-plane gating is to add a surface gate above the atomically precise device. There are several considerations for adding surface gates including the thickness of the silicon cap, choice of gate dielectric, and gate material. In this talk we present we present on fabrication techniques for adding top gates to devices and the resulting low-temperature measurements on tunnel junctions and single electron islands.

This work was supported by the Laboratory Directed Research and Development Program at Sandia National Laboratories, and was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility.

Sandia National Labs is managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a subsidiary of Honeywell International, Inc., for the U.S. Dept. of Energy’s National Nuclear Security Administration under contract DE-NA0003525. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government.

MXenes are a large family of conducting two-dimensional (2D) materials, synthesized by the chemical etching method using acids such as HF or HCl from their parental ternary carbides MAX phases, where M stands for transitional metal elements, A stands for the A group elements, and the X is either C or N. [1-3] MXenes show great promise for the usage of supercapacitors, in part due to pseudocapacitance induced by the reversible surface binding processes. [4-5] However, a comprehensive investigation of the valence and oxidation state changes is still lacking.

In this work, by means of X-ray absorption spectra (XAS) and density functional theory (DFT), we have systematically investigated the valence states of a series of Ti, Mo containing Ti₃AlC₂ (Ti₃C₂T_x), TiMo₂AlC₂ (TiMo₂C₂T_x) as well as Ti₂Mo₂AlC₃ (Ti₂Mo₂C₃T_x), where T stands for terminations, e.g. O, F. Experimentally, the low-dimensional MXenes exhibit greater valence states for metal atoms compared with those in their parent bulk MAX phases. The increasing thickness of the M-X blocks suppresses the valence states of Ti atoms. Our DFT calculations employed special quasi-random structure (SQS) [6] supercells to study intermixing between the surface layer (Mo) and central metal (Ti) layers. We also tested a variety of density functionals to locate the most suitable one to describe the structure and the chemical bonding, including the Perdue-Burke-Ernzerhof (PBE) generalized gradient approximation,[7] Van der Waals corrections, [8] and the Hubbard U correction for the localized d electrons in Ti/Mo.[9] We determine that the intermixing as well as the Van der Waals corrections play significant roles on the structural and electronic properties, which effectively tunes the oxidation state. Including all of these effects leads to the agreement with our experimental observations and suggests that the synthesized MXenes also has significant intermixing. The unveiled tendencies of alloying, varying the thickness, dimensionality and surface terminations give insight to the measured capacitive properties of these materials.

Reference:
[1] M. W. Barsoum, Progress in Solid State Chemistry, 28, 201, (2000).
[2] M. Radovic, and M. W. Barsoum, American Ceramic Society Bulletin, 92, 20, (2013).
[3] M. Nagiub et al., Adv. Mater., 23 (37), 4248-4253, (2012).
[4] M. Hu et. al., ACS Nano, 10, 11344-11350, (2016).
[5] M. R. Lukatskaya, et al., Adv. Energy Mater., 5, 1500589, (2015)
[6] A. Zunger, et al., Phys. Rev. Lett., 65, 353, (1990).
[7] J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett., 77, 3865, (1996).
[8] M. Dion, et al., Phys. Rev. Lett., 92, 246401, (2004).
[9] S. Dudarev, et al., Phys. Rev. B, 57, 1505, (1998).

The demonstrated dependence of the band structure of MoS₂ (and other transition metal dichalcogenides) on layer number and strain – thus enabling tunable electronic and optical properties – has spurred significant interest in a variety of potential applications [1,2]. Practical applications and devices aside, the substantial influence imparted by relatively minor structural perturbations on the band structure provides a large parameter space for fundamental studies of the underlying physics and materials behaviors. For example, the roles of lattice defects and discontinuities (e.g., vacancies, impurities, and grain boundaries) on transport and other basic functionalities have been vigorously explored [3]. These structurally related properties of the materials have stimulated interests in understanding the transient charge-carrier interaction and transport dynamics, which are now extensively studied and better understood. These and other structurally-specific conditions have naturally led to interest in understanding the impacts on energy-carrier (i.e., charge carriers and phonons) dynamics in the time domain, especially as it pertains to emergent macroscopic properties [4]. Accordingly, ultrafast reciprocal-space techniques capable of probing structural responses to femtosecond (fs) photoexcitation have been employed, producing new insights into the inter- and intra-layer dynamics [5]. Relatedly, the development of fs real-space electron imaging in ultrafast electron microscopy (UEM) now provides a means to directly visualize the local, nanoscale dynamics associated with coherent phonon dynamics, providing further routes toward linking the influence of discrete, atomic-scale lattice discontinuities to macroscopic behaviors [6,7].

Here, we describe the direct imaging of the detailed generation and relaxation pathways of highly-coherent acoustic-phonon dynamics resulting from fs photoexcitation of individual multilayer MoS₂ flakes [8]. Using fs electron imaging in UEM, combined with the extreme sensitivity of diffraction-contrast strength to slight local perturbations of the lattice, the complete decay pathways of the coherent modes spanning from the initial fs excitation to tens of microseconds later were resolved. Initiation of coherent modes having tens of GHz frequencies was found to occur preferentially at lattice discontinuities (e.g., crystal step edges and vacuum/crystal interfaces). The initial single wavevectors of these propagating modes quickly decay to incoherent high-frequency but whole-flake motion over the span of a few nanoseconds via scattering of individual coherent phonon wavefronts. In flakes with micrometer lateral dimensions, the initial scattering event leading to phonon interference was directly imaged and isolated to a specific nanoscale region, marking the onset of structural incoherence. This regime of incoherent dynamics continues to evolve from nanoseconds to microseconds as interference effects begin to dominate. This type of motion manifests as complex diffraction-contrast oscillatory patterns in the time domain, rendering Fourier analysis challenging for potentially extracting local mode type. Finally, relaxation of the localized GHz modes to the excitation of low-frequency (MHz) whole-flake oscillations emerges as cantilever-type dynamics in the microsecond UEM image sequence. Altogether, these results produce a comprehensive picture of the effects of local structure and morphology on the excitation and decay of coherent phonon modes in multilayer MoS₂.

[1] K. F. Mak, et al., Phys. Rev. Lett. 105 (2010) 136805.
[2] A. Castellanos-Gomez, et al., Nano Lett. 13 (2013) 5361.
[3] X. L. Zou, et al., Acc. Chem. Res. 48 (2015) 73.
[4] K. A. N. Duerloo, et al., ACS Nano 10 (2016) 289.
[5] E. M. Mannebach, et al., Nano Lett. 15 (2015) 6889.
[6] D. A. Plemmons, et al., Chem. Mater. 27 (2015) 3178.
[7] D. R. Cremons, et al., Nat. Commun. 7 (2016) 11230.
[8] A. J. McKenna, et al., Nano Lett. 17 (2017) 3952.

Symmetry, interaction and topological effects greatly influence the quantum properties of atomically thin two-dimensional (2D) materials. Defects and environmental screening can play equally important roles in determining their behaviors. In this talk I present several interesting phenomena related to point defects and substrate screening on the properties of 2D materials. 1) We study, using ab initio GW-BSE theory, the effect of point-defect chalcogen vacancies on the optical properties of monolayer transition metal dichalcogenides (TMDs). The chalcogen vacancies introduce unoccupied in-gap states and occupied resonant quasiparticle defect states. These defect states give rise to strongly bound defect excitons and hybridize with excitons of the pristine system, reducing the valley-selective circular dichroism. The results suggest a pathway to tune spin-valley polarization and other optical properties through defect engineering. 2) A fascinating property of monolayer semiconductors is that the electronic bandgap ceases to be an intrinsic physical parameter and can be engineered through the surrounding dielectric environment. We have exploited such a dielectric-dependent electronic bandgap and demonstrated the formation of a lateral heterojunction within a homogeneous MoS₂ monolayer through substrate engineering. The capability to tailor the local band structure and electrical transport in 2D semiconductors through dielectric engineering can open exciting new ways to design 2D nanoelectronic and optoelectronic devices. 3) Plasmons in atomically thin 2D materials are distinct from those of the bulk and traditional 2D electron gas systems. We discover that the plasmon dispersion in real quasi-2D metals is qualitatively different, being virtually dispersionless for wavevectors of typical experimental interest. This stems from a lack of continuous translational symmetry, making dispersionless plasmons a universal phenomenon in quasi-2D metals. Moreover, our ab initio calculations show that plasmons on monolayer metallic TMDs are long lived and localizable in wave packets (within ~15 nm) which does not significantly disperse over practical measurement time. Their slow propagation opens the possibilities of tracking plasmon wave packets in real time and of localizing long-lived, boson-like excitations in extended, atomically thin materials.

This work was supported by the U. S. Department of Energy, National Science Foundation, and Office of Naval Research. I would like to acknowledge collaborations with members of the Louie group.

The discovery and subsequent research excitement of new van der Waals magnetic semiconductors such as CrI₃ and CrCl₃, in the post silicon era, offers novel applications in computing and data storage due to the unexpected magnetic properties associated with the reduced dimension and layered structure. Electron spin resonance (ESR) spectroscopy is an ideal local experiment that can pinpoint the atomic origin and underlying mechanisms of magnetic interactions in these materials. This work presents our recent experimental findings on ESR investigations in these compounds as a function of temperature (5-500 K) and microwave frequency (9.43 and 120 GHz). The temperature dependences of the ESR line width and g-value show clear magnetic phase transitions at 61 K and 17 K, in the case of CrI₃ and CrCl₃, respectively. These results are consistent with magnetization and heat capacity data. For CrI₃, at room temperature, the estimated g-value from the single Lorentzian signal is 1.986 and peak-to-peak line width (ΔH_PP) is 336 Gauss (G). In the case of CrCl₃, the estimated g-value is 1.986 and ΔH_PP is 82 G. These are the benchmark signatures of Cr³⁺ (S = 3/2) ions in the octahedral site where all the three spins are in the t2g state, which are responsible for the magnetic behavior of this compound. Interestingly, the ESR signal appearing from CrCl₃ is narrower by a factor of four, which is a direct consequence of distinct magnetic exchange interaction strengths between CrCl₃ and CrI₃ single crystals. This is due to the more covalent nature of the interactions of Cr³⁺ with iodine than with chlorine. This observation not only confirms the presence of Cr³⁺ ion, but also shows the effect of halide ion on the magnetic interaction of these layered compounds. In addition, we will also present light-induced ESR properties of these materials.

There has been an intense research interest in exploring the magnetic properties of van der Waals (vdWs) crystals owing to their interesting layer-dependent anisotropic magnetic properties. These materials are expected to show great potential for next generation low power spintronic applications. Proton irradiation is an effective route to manipulate the magnetic properties of these materials. In this work, we present the magnetic properties of proton irradiated (10¹⁸ cm^-2) Fe_2.7GeTe₂ (FGT) single crystal. FGT is a strongly correlated ferromagnetic metal with a Curie temperature (T_C) of 151 K, which is confirmed through magnetization vs. temperature (M (T)) and heat capacity measurements. The effective moment for pristine FGT was found to be μ_eff = 5.84 μ_B for H//ab and 5.81μ_B for H//c. After proton irradiation, FGT displays the same behavior as the pristine M(T) measurement and exhibits a slight increase in the effective moment to 5.99 μ_B (H//ab) and 5.95 μ_B (H//c). The isothermal magnetization for pristine FGT shows the c-axis as the easy axis for both pristine and irradiated FGT. A slight increase is observed in the magnetization saturation after proton irradiation from M_S = 1.044 μ_B/Fe (H//c) and 0.9535 μ_B/Fe (H//ab) to M_S = 1.0867 μ_B/Fe (H//c) and 0.9796 μ_B/Fe (H//ab). Heat capacity (HC) measurements were also performed on pristine and irradiated samples. Both samples exhibit a clear peak at 151 K, which is consistent with the PM-FM transition. The magnetic heat capacity, which was obtained by subtracting the phonon contribution using the Einstein model, was used to derive the magnetic entropies. The obtained entropies were ~0.153 J mol^-1 K^-1 for pristine and ~0.198 J mol^-1 K^-1 for irradiate samples. Electron Paramagnetic Resonance studies have also been performed on both the crystals to better understand the effects of proton irradiation on an atomic level. We will present and discuss our comprehensive experimental findings.

Reduced graphene oxide (rGO) has potential as a mass-producible and cost-effective substitute for graphene, but displays poor crystal quality due to various types of structural defects. Though many attempts have been reported to improve the quality of rGO, most of them require sophisticated equipment or severe conditions such as toxic chemicals, high temperature, and electron beam or plasma treatment in ultra-high vacuum (UHV) conditions. Here, we report a mild and simple solution-based healing method for obtaining high-quality rGO, with this method involving the use of a simple camera flash light treatment and the nontoxic chemicals L-ascorbic acid and ethanol. Exposing a GO solution containing L-ascorbic acid and ethanol to the flash light irradiation for a short amount of time (<10 min) resulted in the formation of rGO displaying high quality both with respect to its chemical composition and its crystallinity. To elucidate the mechanism of the flash-light-induced healing of rGO, we acquired and analyzed the transient absorption spectrum for the healing reaction by using femtosecond pump-probe spectroscopy; this analysis revealed the photo-catalytic activity of GO by itself to repair chemical and topological defects, and we hence named this process ‘self-catalytic healing’.

2D materials are frequently derived from layered crystalline materials, however bottom-up syntheses present the opportunity to explore amorphous 2D materials with high defect densities. The high concentration of defects along the surface of amorphous 2D materials suggests unique transport properties like faster ion diffusion kinetics than crystalline 2D materials. We report the synthesis of amorphous thin films via Atomic Layer Deposition (ALD) and substrate dissolution. Here, we show 5 nm thick films with lateral sizes up to 1 mm composed of Al₂O₃, HfO₂, ZrO₂ and multilayer metal oxides via this method. The quality of film growth depends on the surface of the NaCl substrate, where extended exposure to ambient environment promotes a transition to continuous growth from island growth, as observed for freshly cleaved crystal faces. Unlike some strong chemical etchants frequently used in nanomaterial transfer, our aqueous dissolution of the NaCl substrate is compatible with many nanomaterials. In addition to large area films, we apply this approach to high surface area NaCl powder substrates to produce a large quantity of metal oxide nanomaterials with inexpensive substrates and gentle aqueous etching. We demonstrate the tunability of the surface chemistry by functionalizing films with organic self-assembling monolayers (SAMs), like alkanoic acids and fluorosilanes. The functionalization of the surface of the metal oxides contributes to the development of a new class of hybrid organic-inorganic amorphous 2D materials. The structure of amorphous 2D materials suggests future applications as membrane materials to with fast diffusion rates and control of ion selectivity.

Due to their profitable band structure features, olivines are potential candidates for thermoelectric and photovoltaic applications. In the present work we study the magnetic properties of sawtooth lattice olivine Mn₂SiS₄ by replacing S with Se progressively. We study the spin flop antiferromagnetic transition in the composition series, Mn₂SiS_4-xSe_x (x = 0 – 4). The magnetic phase transition occurred at 86 K for x = 0 compound and reduced almost linearly in magnitude as Se-doping increased. New magnetic anomalies below 20 K are identified for all these compounds through the present work, which might correspond to spontaneous spin re-orientations. The magnetic phase transitions characterized using specific heat revealed very low magnetic entropy at transition temperature for the x = 0 compound, which suggested that the underlying frustrated nature of the Mn²⁺ ions was in the sawtooth-like triangular arrangement. The spin-flop transition occurred at critical field of 2.7 T for x = 0, and evolved with the composition in a nonlinear fashion. Our studies mark the maximally frustrated compound as the x = 4. Our density functional theory calculations point towards the existence of competing magnetic interactions in the triangular units of Mn as leading to frustration effects.

Using a combination of experiments and density functional theory (DFT) calculations, we probe the magnetic ground state of the low dimensional anisotropic Ising-like system, Co_1-xMg_xTa₂O₆ (x = 0, 0.1, 0.3, 0.5, 0.7, 1). Neutron diffraction data was collected at the high resolution neutron powder diffractometer (PSD) at MURR, Missouri. Co_1-xMg_xTa₂O₆ adopts the tri-rutile space group P42/mnm in the whole composition range and does not undergo structural changes. CoTa₂O₆ displays an antiferromagnetic ordering occurring below 6 K. For the x = 0.1, we discover additional low temperature magnetic transitions. With progressive Mg-doping the magnetic transition disappears and gives rise to short-range correlations with an enhancement in ferromagnetism. Our neutron and specific heat data neatly captures these features of low-dimensional magnetism. We performed DFT calculations with the full-potential local-orbital (FPLO) code using the generalized-gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) scheme. From DFT, the magnetic ground state is found to be ferromagnetic (FM) with the lowest energies among four different magnetic configurations within GGA. However, the small energy difference between the FM and first excited AFM state (~15 meV) is indicative of competing ground state. Here we report results for the FM state where band gap of 0.4 eV was found between the d_xz states in spin-down channel within GGA while that with U = 5 eV for Co, gap rises to ~3.0 eV. Further work is in progress to confirm the antiferromagnetic ground state as found in experiment.

Weak screening of Coulombic interactions lead to strong many-body interactions in monolayer transition metal dichalcogenides (TMDC). This gives rise to the formation of tightly bound excitons, many body particles such as biexcitons, many-body scattering mechanisms such as exciton-exciton annihilation,¹ and screening effects such as bandgap renormalization.² Charged trions, where an exciton couples to a charge carrier, have been observed in the emission spectra of unintentionally doped TMDCs and may form due to defect trapping of charge carriers in those materials.
Recently, it was shown that adsorbed molecules can passivate defects in TMDCs. Specifically, WS₂can be thermally or laser annealed in vacuum to remove atmospheric adsorbates and reveal trion emission;³ excitonic emission is restored under dry air flow. This is likely due specifically to adsorbed oxygen,⁴ which may bind to defect sites in the crystal lattice and neutralize their net charge. It is possible that these defect sites are the same sites responsible for regions of weak emission that have been universally observed in WS₂.⁵ Interestingly, this annealing process can only be done for transferred monolayers indicating that strain in as-grown layers prevents this process.³
Here, we report the ultrafast dynamics of the trions in WS₂. We use the aforementioned annealing process to remove atmospheric adsorbates from defect sites, effectively doping the WS₂, and allowing for excitation of trions. We show that after annealing the trion absorption resonance appears in the ground state absorption spectra in addition to the typical A-exciton peak. We estimate the trion binding energy as 32 meV. In dry air, we see only the A-exciton resonance, which shows that oxygen and not water vapor is responsible for neutralizing defect sites. Ultrafast transient absorption spectroscopy shows that excitons dissociate rapidly into trions. The decay of the photoexcited trions does not appear consistent with many-body Auger mechanisms, but instead appears to be dominated by trapping. We discuss the prospects for using annealing as a methodology for preparing trions states for studies targeting their valley polarization and valley lifetime. We conclude that while this is an effective means of creating trions for fundamental studies that ultimately the trap-limited lifetime may limit the usefulness of such states.

References:
1. Cunningham, et al., J. Phys. Chem. Lett. 75242 (2016)
2. Cunningham, et al., ACS Nano 1112601 (2017)
3. McCreary, et al., Sci. Report. 635154 (2016)
4. Nan, et al., ACS Nano 85738 (2014)
5. Rosenberger, ACS Nano 12, 1793 (2018)

In this present study, we investigate electron wind force driven recrystallization phenomenon in nanocrystalline Molybdenum disulfide (MoS₂) using in-situ Transmission Electron Microscope (TEM). We use a custom made micro electro-mechanical system (MEMS) device to bias the sample inside TEM. To investigate the effect of electron wind force as well as Joule heating on recrystallization phenomenon in MoS₂, we apply DC current at varying current density keeping it below the electro-migration failure limit. For low temperature (400 °C) chemical vapor deposited MoS₂, our electrical annealing scheme resulted in 3x decrease in electrical resistivity. Inside the TEM, we observed changes in selected area aperture diffraction (SAED) pattern at a current density of 1.0×10⁶ A/cm². This is an indication of grain gorwth phenomenon in MoS₂.

To comprehend the grain growth mechanism observed in TEM, we employ classical molecular dynamics (MD) simulation to investigate the underlying mechanism of grain growth using Reactive empirical bond-order (REBO) potential implemented in LAMMPS. In our present study, we build single layer polycrystalline MoS₂ with a grain size of 2.5nm using voronoi-tessellation method. After building the model we checked overlapping of atoms at the grain boundaries. To incorporate both Joule heating and electron wind force effect, we conduct the experiment at different temperatures with an additional imparted electron wind force on each atoms. Grain boundaries of MoS₂ contains different types of ring defects such as 6|8, 5|7, 4|6, 4|8 as well as vacancy. 6|8 ring defects can be generated from 5|7 ring defects by inserting one Molybdenum (Mo) or 1 pair of Sulfur (S) atoms. On the other hand, 4|8 defects can arise from 5|7 defects by deleting one Mo or 1 pair of S atoms. In addition to this 6|8 ring defects can be healed to two perfect 6 rings and one 4 ring defects 4|6|6. In our present simulation, we also notice the similar ring defects recombination phenomena at the GBS of polycrystalline MoS₂. To investigate the defect annihilation at the grain boundaries of nanocrystalline MoS₂, we performed Nudged Elastic Band (NEB) simulation with 192 atoms containing 6|8, 4|8 and vacancy defects. Based on calculation, a defective sample containing these ring defects requires approximately 0.8 eV/atom additional energy to overcome the energy barrier. This additional energy could be provided during the electrical annealing of the sample in terms of Joule heating as well as imparted electron wind force. It is well-known that both electron wind force and Joule heating are concurrent phenomena during electrical annealing. Both Joule heating and electron wind force effect is beneficial for atomic/defect mobility. Due to the disorders and misorientations at the GBs, electron wind force are dominant at these defective sites and contributes pronounced scattering. In addition to the Joule heating, electron wind force could induce high atomic mobility at the GBs and annihilates defects to favor recrystallization. To investigate the effectiveness of annealing on mechanical properties we also conducted tensile testing of pre-annealed and post-annealed sample.

Monolayer transition metal dichalcogenides (TMDCs) are perturbed by the atmosphere; therefore, optoelectronic measurements may vary from day to day and lab to lab. Thus, we quantify changes to the optoelectronic properties of WS₂ monolayers with careful environmental control, namely inert gas, dry air, and humid nitrogen. Our WS₂ monolayers are prepared by chemical vapor deposition, where defects are present. To probe changes to the optoelectronic properties, we employ state-steady photoluminescence, transient absorption, and time-resolved microwave conductivity measurements to monitor the different phenomena resulting from dry air and water vapor. We determine that oxygen and water molecules have disparate impacts on the photoluminescence, exciton dynamics, and photogenerated carriers of monolayer WS₂. Our work disentangles the oxygen and water contributions and how these molecules passivate the WS₂ monolayer surface, which leads to differences in the optoelectronic properties. These effects introduced by water or oxygen molecules can be reversed and the original pristine WS₂ recovered. The conclusion of this work can further develop the interface charge transfer dynamics as well as photocatalysis applications of TMDCs.

Monolayer transition metal dichalcogenides (TMDCs), known as two-dimensional semiconducting materials, exhibit unique features like strong spin-orbit coupling and spin-valley locking. The exotic natures have attracted more and more research interests of new physics in TMDCS such as spintronics and valleytronics. However, a critical problem that limits the application is the relaxation time of spin and valley polarization. In this work, we use time-resolved Kerr rotation spectroscopy to probe the valley dynamics of excitons and free carriers in monolayer tungsten diselenide. Our result shows that the valley relaxation time of free carriers is found around 2 ns at 70 K, about three orders of magnitude longer than the excitons of about 2 ps, and 15 times larger than that of trions (130 ps).

The dynamics of bright excitons in monolayer tungsten diselenides is studied, by pumping at exciton ground state and probing at higher energy, where excitonic excited states can be identified from the pump-probe spectroscopy. Remarkably, the rise time of the differential reflection signal increases significantly with the index of the excited states, which we attribute to the phase-space filling effect that probes the momentum distribution of ground state exciton. Our results reveal fast exciton momentum relaxation as a signature of the strong Coulomb interaction in 2D TMDs.

2D transition metal carbides, carbonitrides, and nitrides, known collectively as MXenes, have been receiving a great deal of attention in several scientific communities due to their outstanding properties such as high electrical conductivity, versatile transition metal chemistry, tuneable surface chemistry, and hydrophilicity. They have shown promise in many applications including electromagnetic interference shielding, energy storage, transparent conductors, catalysis, photothermal therapy, and photonics. To date, about 30 MXenes such as Ti₂C, Ti₃C₂, Ti₃CN, Mo₂C, Mo₂TiC₂, Mo₂Ti₂C₃, V₂C, Nb₂C, and Nb₄C₃ have been experimentally synthesized and a dozens more predicted to be stable with interesting electronic properties. In this talk, an overview of electronic properties of this large 2D materials family is presented and discussed. Transport and magnetoelectronic properties of free-standing MXene films were studied by temperature-dependent resistivity measurements from 2-300 K with and without external magnetic field up to 9 Tesla. Control over MXenes’ electronic properties was achieved by controlling their intercalation and surface termination via high-temperature annealing in inert atmosphere.

MXenes are an emerging class of 2D transition metal carbides, nitrides, and carbonitrides which show promise for applications in energy storage, electromagnetic interference shielding, catalysis, and wireless communication. For these applications, MXenes’ excellent performance is largely due to their high metallic conductivity, and as such there is motivation to better understand and control their electronic properties. In this talk, we will discuss how the MXene surface termination and molecular intercalation can be engineered to maximize conductivity. Specifically, we will describe in situ annealing and electrical biasing measurements of Ti₃C₂, Ti₃CN, and Mo₂TiC₂ performed within the TEM. With annealing, we observe molecular de-intercalation and surface de-functionalization via in situ EELS and ex situ TGA-MS. We show that intercalants strongly influence the inter-flake MXene resistance, and that intercalation can cause macroscopic negative dR/dT for multi-flake MXene samples which display metallic (positive dR/dT) intra-flake conductivity. We also show that high temperature annealing leads to the desorption of surface termination species, and that termination loss increases the MXene conductivity. This work furthers our fundamental understanding of MXene electronic properties and provides clear guidelines for optimizing the MXene conductivity for specific applications.

Surface-enhanced Raman spectroscopy (SERS) has been widely used for the analysis of ultra-low concentration bio and chemical molecules^1-3. Moreover, the SERS signals can be amplified to 10¹²−10¹⁵ times through highly localized surface plasmonic oscillations, enabling the possibility of single molecule detection^1-3. Two major theories such as chemical and electromagnetic enhancement theories are widely accepted in order to understand the enhancement mechanisms although the exact enhancement mechanism is still under debate in the literature. Among these, the electromagnetic enhancement is the first and major contributor for the SERS enhancement, which generally originates from the magnification of the electric field by the excitation of localized surface plasmons of underlying SERS active material^1-3. The second significant chemical enhancement of SERS relies on the charge transfer between chemisorbed species and the metal surface. In order to achieve both the enhancements, we developed a reliable SERS-active substrate by gold nanoparticles (Au NPs) decorated on defect-engineered CVD graphene. It is well known that the local electric field enhancement depends on the particle size, shape and also the gap between the particles in order to get the Raman signal enhancement in SERS. To achieve the Raman signal enhancement and to control the deposition density of the Au NPs, we have used a plasma treatment strategy. Experimentally, it has been observed that Au NPs are stabilized at defect sites^{2, 3}. In appreciation of the above, we have performed a plasma treatment of the monolayer graphene to create the defects. Control of in-plane defects in graphene was achieved by controlling the Ar/H₂ gas flow rates and plasma power, and post-growth functionalization was performed using electrochemical deposition. Quantitative analysis of the defect density and nature of the interaction of Au with graphene was performed using several analytical tools. As compared to the pristine graphene, higher Au NPs density and coverage is observed for plasma modified graphene samples. The intensity ratio of the D to the G band (I_D/I_G) for graphene was 1.30. However, the I_D/I_G ratio in the case of 15-sec sample was 2.31, indicating that the plasma exposure process has altered the structure of graphene with more structural defects. After gold deposition, there is a red shift in G and 2D peaks and enhancement in I_D/I_G ratio which indicates the functionalization of graphene with Au NPs. The morphological features of plasma treated graphene samples were investigated from the STEM. STEM data shows vacancy-type defects and extended line defects, which are formed during the plasma treatment. These substrates show high SERS activity toward the Rhodamine B probe molecules. The observed enhancement in SERS might be due to the structural disorder induced generation of local dipoles and the strong resonant energy transfer between Au NPs and graphene. Moreover, we also observed a strong quench in the background fluorescence signal from the Au and probe molecules due to the graphene. Hence, the defect-mediated efficient gold functionalization and easy charge transfer at the graphene gold interface is expected to be suitable for biosensing and Raman imaging applications.
References
1.Surface Enhanced Raman Spectroscopy, Edited by Sebastian Schlucker, WILEY-VCH Verlag& Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany.
2. Qu Chen, Kuang He, Alex W. Robertson, Angus I. Kirkland, and Jamie H. Warner, Atomic structure and dynamics of epitaxial 2D crystalline gold on graphene at elevated temperatures, ACS Nano,10, 10418-10427 (2016).
3. R.K. Biroju, P.K. Giri, Defect enhanced efficient physical functionalization of graphene with gold nanoparticles probed by resonance Raman spectroscopy, J. Phys. Chem. C, 118 13833-13843 (2014).

Currently, a wide range of potential applications ranging from electronics to catalysis (including modern transistors,¹ photo emitting devices,² solar cells,³ hydrogen storage,⁴ catalysis,^{5, 6} lubricants,⁷ Li-ion batteries,⁸ and supercapacitors⁹) are depended on two-dimensional (2D) materials.^{10, 11} Till date, graphene has been proved best thin layer two-dimensional (2D) material due to its various fascinating properties such as high carrier mobility, good mechanical flexibility, and lower dimension related properties. Recently, graphene is replacing by others 2D materials due to the absence of band gaps and low absorption capacity of light, limits its application in modern electronic devices. Beyond graphene, transition metal dichalcogenides (TMDCs) have been proved their importance in these fields due to tuneable energy band gaps, layered structure, flexible optical, mechanical, and electrical behaviors. Among various TMDCs, WS₂ and MoS₂ behave like a semimetallic graphene and interesting materials to examine their properties.

Hence, we have developed a novel method for the synthesis of surfactant-free hexagonal WS₂ nanosheets using a wet chemical method. Large-scale syntheses (yield = 97 %) were also carried out and obtained good crystalline and pure WS₂ that was confirmed by XRD, FESEM-EDX, and TEM analysis. A temperature and pH-dependent studies were performed to optimize the synthesis process. Prepared highly crystalline hexagonal WS₂ nanosheets were tested for various applications such as a lubricant, as a counter electrode in solar cell and as an anode in batteries. A detailed reaction mechanism was studied using Uv-Visible and FTIR spectroscopy.

References

1. Yue Y, Chen J, Zhang Y, Ding S, Zhao F, Wang Y, Zhang D, Li R, Dong H, Hu W. Two-dimensional high-quality monolayered triangular WS2 flakes for field-effect transistors, ACS applied materials & interfaces 2018.
2. Gutiérrez HR, Perea-López N, Elías AL, Berkdemir A, Wang B, Lv R, López-Urías F, Crespi VH, Terrones H, Terrones M. Extraordinary room-temperature photoluminescence in triangular WS2 monolayers, Nano letters 2012; 13: 3447-3454.
3. Wu M, Wang Y, Lin X, Yu N, Wang L, Wang L, Hagfeldt A, Ma T. Economical and effective sulfide catalysts for dye-sensitized solar cells as counter electrodes, Physical Chemistry Chemical Physics 2011; 13: 19298-19301.
4. Chen J, Kuriyama N, Yuan H, Takeshita HT, Sakai T. Electrochemical hydrogen storage in MoS2 nanotubes, Journal of the American Chemical Society 2001; 123: 11813-11814.
5. Li Y, Wang H, Xie L, Liang Y, Hong G, Dai H. MoS2 nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction, Journal of the American Chemical Society 2011; 133: 7296-7299.
6. Chianelli RR, Siadati MH, De la Rosa MP, Berhault G, Wilcoxon JP, Bearden Jr R, Abrams BL. Catalytic properties of single layers of transition metal sulfide catalytic materials, Catalysis Reviews 2006; 48: 1-41.
7. Rapoport L, Fleischer N, Tenne R. Applications of WS 2 (MoS 2) inorganic nanotubes and fullerene-like nanoparticles for solid lubrication and for structural nanocomposites, Journal of Materials Chemistry 2005; 15: 1782-1788.
8. Whittingham MS. Lithium batteries and cathode materials, Chemical reviews 2004; 104: 4271-4302.
9. Gopalakrishnan K, Sultan S, Govindaraj A, Rao C. Supercapacitors based on composites of PANI with nanosheets of nitrogen-doped RGO, BC1. 5N, MoS2, and WS2, Nano Energy 2015; 12: 52-58.
10. Huang X, Zeng Z, Zhang H. Metal dichalcogenide nanosheets: preparation, properties and applications, Chemical Society Reviews 2013; 42: 1934-1946.
11. Lv R, Robinson JA, Schaak RE, Sun D, Sun Y, Mallouk TE, Terrones M. Transition metal dichalcogenides and beyond: synthesis, properties, and applications of single-and few-layer nanosheets, Accounts of chemical research 2014; 48: 56-64.

I will address recent experimental and theoretical studies of layered 2D materials. The goal is to understand the structural and associated electronic behavior of unconventional crystal phases, stacking sequences, and defect structure, using TEM and STEM as the primary experimental tool. Specific topics include engineering of unusual crystalline phases and stacking sequences via alloying in 2D chalcogenides; aperiodic order in 2D; and realization of on-demand defects, tailored at the single atom limit, in suspended 2D monolayers or low-number multilayers.

Transition metal dichalcogenide (TMD) heterostructures, both lateral and vertical, create atomically thin p-n junctions that have potential application as photodetectors, LED and solar cells.^1,2 The type of interface of TMD heterostructures is critical for all these applications and gives rise to interesting phenomena like ultrafast charge transfer and strong interlayer coupling.^3,4 At present, all these effects are probed by photoluminescence (PL), Raman spectroscopy and pump probe spectroscopy at a micronic resolution. Using electron beam spectroscopy in a scanning transmission electron microscope (STEM) instead of optical light spectroscopy can improve the spatial resolution limit up to 1 nm by which the interesting effects happening at the atomic scale could easily be observed in correlation with structural and morphological information.⁵ In the present work, two electron spectroscopic techniques, Cathodoluminescence (CL) and low loss electron energy loss spectroscopy (EELS) in STEM mode have been used to probe the optical properties such as band gap and exciton states at the interface of laterally confined MoS₂/WS₂ heterostructure. The optical properties were observed in both absorption and emission, allowing the determination of possible Stoke shifts at high spatial resolution.
Keywords: low loss EELS, Cathodoluminescene, STEM, lateral heterostructures
References:
1. Novoselov, K. S., Mishchenko, A., Carvalho, A., Neto, A. H. C. & Road, O. 2D materials and van der Waals heterostructures. Science (80-. ). 353, (2016).
2. Liu, Y. et al. Van der Waals heterostructures and devices. Nat. Rev. Mater. 1, 16042 (2016).
3. Hong, X. et al. Ultrafast charge transfer in atomically thin MoS2/WS2 heterostructures. Nat Nano 9, 682–686 (2014).
4. Fang, H. et al. Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides. Proc. Natl. Acad. Sci. U. S. A. 111, 6198–202 (2014).
5. Kociak, M. & Zagonel, L. F. Cathodoluminescence in scanning transmission electron microscope. Ultramicroscopy 177, 14–19 (2017).

Two dimensional materials have shown great promise in a variety of electrochemical energy conversion and storage applications, and their further developments require an improved understanding of the atomistic mechanisms and effective design principles. I will discuss how the electronic structure of 2D materials controls atoms adsorption, and use this understanding to explain a number of puzzles, including: why structurally similar C forms/defects have distinct binding energies with Li [1], and why graphite has a low Na capacity while a high capacity for other alkaline metals [2]. This understanding also leads us to new catalysts for water splitting [3]. Finally, I will discuss the effects of varying charge and fixed potential in electrocatalysis of 2D materials [4], which are often neglected in modeling but are important for 2D materials due to their peculiar electronic structure, calling for re-evaluation of their electrocatalytic mechanisms by incorporating these effects into simulations.
[1] Y. Liu et al, Phys. Rev. Lett. Phys. Rev. Lett. 2014, 113, 028304
[2] Y. Liu, B. V. Merinov, W. A. Goddard, PNAS 2016, 113, 3735-3739
[3] Y. Liu, J. Wu, K. P. Hackenberg, Nature Energy, 2017, 6, 17127
[4] D. Kim, J. Shi, Y. Liu, 2018, 140, 9127

Electron microscopy is a widespread and often essential tool for structural and chemical analysis at the atomic level. Image resolution is dominated by the energy (or wavelength) of the electron beam and the quality of the lens. Two-dimensional materials are imaged with low beam energies to avoid damaging the samples, limiting spatial resolution to ~1 Å. By combining our new design of electron microscope pixel array detector (EMPAD) which has the dynamic range to record the complete distribution of transmitted electrons at every beam position, and a ptychographic phase retrieval algorithm to process the data, we have been able to increase the spatial resolution well beyond the traditional lens limitations reaching a 0.39 Å resolution for MoS₂, at the same dose and imaging conditions where conventional imaging modes reach only 0.98 Å.
The improved resolution, dose efficiency and robustness to environmental noise enabled by ptychography make it easy to identify defects such as sulfur monovacancies, as well as subtle structural arrangements and tilts on the sulfur sublattice that are undetectable by conventional imaging modes. For twisted bilayers we are able to resolve the shear distortions and interactions between the layers.


[1] Y. Jiang, Z. Chen, Y. Han, P. Deb, H. Gao, S. Xie, P. Purohit, M. W. Tate, J. Park, S. M. Gruner, V. Elser, and D. A. Muller. “Electron Ptychography of 2D Materials to Deep Sub-Ångström Resolution” Nature 559, (2018): 343–349.
[2] Research supported by US National Science Foundation (grants DMR-1539918, DMR-1429155, DMR-1719875) & US Department of Energy Basic Energy Sciences (DE-SC0005827).

We will discuss the diverse basic optics (transmission, absorption, reflection) of numerous 2D materials as it can be derived/predicted from the first principles [1]. Another fundamental property of interest is the thermodynamic shape of 2D-islands, and how to predict/find it in particular challenging cases of the low symmetry crystals [2]. Third, presence of point defects such as vacancies and substitutions and theire complexes (similar to the color centers well studied in 3D) brings about new and often important optical features and functionality [3], where theoretical assessment can guide experimental realization.
[1] S. Gupta, et al. ACS Nano, DOI: 10.1021/acsnano.8b03754 (2018).
[2] L. Wang, et al., to be published.
[3] S. Gupta, J.-H. Yang, and B.I.Y., to be published.

Transition metal dichalcogenide (TMDs) have attracted a significant attention due to their wide range of electronic, photonic, magnetic, and electrochemical properties. TMDs exhibit polymorphism such as trigonal prismatic (1H, 2H) or octahedral (1T) coordination. While the number of layers could tune the band gap energy, the polymorph allows us to discover new intrinsic properties of TMDs. For example, catalytically active basal plane of 1T-molybdenum disulfide (MoS₂) have triggered numerous researches on hydrogen evolution reaction (HER) electrocatalysts. For the polymorphic transition from stable 2H-MoS₂ toward metastable 1T-MoS₂, electron configuration-based strategies including intercalation of alkali metal or ammonium ion and substitutional metal doping were proposed. However, they exhibited the limitations of low process efficiency, low controllability, impurity problem, and lab-scale process. For example, Li⁺ intercalation, the most representative 1T-MoS₂ formation method, requires the soaking process of MoS₂ in n-butyllithium solution which proceeds at inert atmosphere during long time. Therefore, a large-scale and systematic methodology for efficient TMD polymorph control still remains a challenge.
Herein, we develop a new fabrication strategy for controlling MoS₂ polymorph by using gas-solid reaction. For the polymorphic transition from 2H to 1T, the key factor was sulfur vacancy (V_S) generation in MoS₂ nanocrystal. Inspired by classical metallurgy, carbon monoxide (CO)/carbon dioxide (CO₂) mixed gas was applied for S extraction from MoS₂. In particular, processing window for 1T transition was predicted by calculation based on thermodynamics. The polymorph ratio was controlled based on the reaction kinetics according to CO/CO₂ ratio and isothermal time during calcination. The presence of vacancy results in the polymorph transition toward 1T phase as revealed by investigating the Mo-S coordination number and role of V_S with extended X-ray absorption fine structure (EXAFS) and density functional theory (DFT) calculation. The polymorphic transition induced through gas-solid reaction could be applied regardless of form factor and TMD type, even when the MoS₂ layer was incorporated into the carbon matrix. It enabled the fabrication of 1T-MoS₂/carbon nanofibers as HER electrocatalyst and its superior catalytic performance is discussed.

In the recent past, van der Waals crystals have garnered much attention due to their unique and interesting properties, putting them in the spotlight for current and future research. This attention is due to their unique and interesting properties such as cleavability and tunable magnetism. In this study, we explore the magnetic properties of Mn₃Si₂Te₆ (MST), a less-studied analog of Cr₂Si₂Te₆ (CST), after proton irradiation; which has been used as a tool to tune the magnetic properties of various materials. MST is a ferrimagnet below ~78 K, with its moments lying along the ab plane. Here, single crystals were grown using the self-flux method [1]. Crystals were irradiated with a 2 MeV proton beam with a fluency of 10¹⁸ cm^-2. Magnetic measurements, as well as heat capacity (HC) measurements, were performed using a Versalab Vibrating Sample Magnetometer (VSM) with a temperature range of 50 – 350 K and a magnetic field of ±3 T. From M(T) curves, an estimated Curie temperature (T_C) ~74 K is observed, close to reported value of T_C = 74.35 K ±0.05 K [1]. From HC curves, it is apparent that there is a phase transition at ~71.67 K. After proton irradiation, a decrease in magnetization is observed from 14.01 emu/g to 11.78 emu/g in the ab plane. It is believed that proton irradiation may have caused a break in the ordering of magnetic ions, which reduces the exchange interaction in the system, thus causing the decrease in magnetization [2]. The magnetic anisotropy in the H//c direction is also reduced, setting the ab-axis as the easy axis. To corroborate these results, temperature dependent electron spin resonance (ESR) measurements have also been performed before and after proton irradiation. Additional measurements such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy are in progress to gain deeper insights.

There has been tremendous interest in recent years to study two-dimensional (2D) atomic layers which form building blocks of many bulk layered materials. This was started by the spectacular discovery of graphene. This talk will focus on the materials science of the emerging field of 2D atomic layers of various compositions. Several aspects that include synthesis, characterization of defects, doping and manipulation will be explored with the objective of achieving functional structures with atomic layer building blocks. The concept of nanoscale engineering and the goal of creating new artificially stacked van der Waals solids and 3D constructs will be discussed through a number of examples including graphene and other 2D layer compositions. The talk will explore the emerging landscape of 2D materials systems that include hybrid compositions and multi-component 2D alloys. Some of the anticipated applications of these materials will also be discussed.

Magnetic van der Waals materials has been fast emerging as a new exciting field just over the last few years. Among one of the few known examples, TMPS3 with TM=transition metal elements have attracted significant attentions as it can exhibit the three fundamental magnetic model of Ising, XY and Heisenberg Hamiltonian depending on the TM elements. In this talk, I will demonstrate how we can use this unique magnetic property of these materials to learn of the old physics.

[1] Kenneth S. Burch, David Mandrus, and Je-Geun Park, Nature 563, 47 (2018)
[2] So Yeun Kim, et al., Phys. Rev. Lett. 120, 136402 (2018)
[3] Jae-Ung Lee, et al., Nano Lett. 16, 7433–7438 (2016)

We have been using our photoluminescence (PL) based surface analysis technique to investigate the surface of MoS₂ for hydro-desulfurization catalysis. Hydro-desulfurization (HDS) catalysis involves exchange of sulfur between the surface and sulfur containing volatile organic compound. When MoS₂ is at the monolayer limit, it exhibits a direct bandgap which is unique for MoS₂ and other transition metal dichalcogenides (TMDs), therefore has strong photoluminescence. We utilize this special characteristic of MoS₂ and developed a system which uses wide field imaging to drastically shorten the image acquisition time as compared to the traditional point-mapping technique for the same surface analysis. With appropriate bandpass filters centered at around 670nm (the direct band gap energy of MoS)¹, we can study: energy transition induced by thermal expansion ², increasing defect density indicated by dimming of PL, bleaching of PL by hydrogen interaction, and recovery of structure by PL intensity increases. The temperature induced surface defects decrease this PL intensity. It is not a linear decay but rather has some features with frequency differences revealing internal structure. This temperature dependency PL images are presented as a map for the consecutive experiments. We can observe increase in brightness of PL from the surface of MoS₂ film when introducing benzenethiol (containing sulfur) and this brightening is the indication of HDS activities. Comparison between edge and basal plane at different temperature regimes are corelated with HDS actives measured using in-situ mass spectrometry. We have determine the continuous HDS condition with respect to temperature for MoS₂ film which is grown in our newly developed High-Vacuum growth technique³. There is comparison with this homogenous film and in house CVD grown triangle MoS₂ film for correlating PL recovery and HDS reaction. Our unique approach using PL as an indicator for the surface defects, we are able to understand further about the surface actives of this material at different temperature regimes.

Reference
1. Mak, K. F., C. Lee, J. Hone, J. Shan & T. F. Heinz (2010) Atomically Thin MoS₂ : A New Direct-Gap Semiconductor. Physical Review Letters, 105, 136805.
2. Yang, L., X. Cui, J. Zhang, K. Wang, M. Shen, S. Zeng, S. Dayeh, L. Feng & B. Xiang (2014) Lattice strain effects on the optical properties of MoS₂ nanosheets. Scientific Reports, 4.
3. Almeida, K., M. Wurch, A. Geremew, K. Yamaguchi, T. A. Empante, M. D. Valentin, M. Gomez, A. J. Berges, G. Stecklein, S. Rumyantsev, J. Martinez, A. A. Balandin & L. Bartels (2018) High-Vacuum Particulate-Free Deposition of Wafer-Scale Mono-, Bi-, and Trilayer Molybdenum Disulfide with Superior Transport Properties. ACS Applied Materials & Interfaces, 10, 33457-33463.

MXenes, an emerging family of 2D transition metal carbides and nitrides, have shown promise in various applications, such as energy storage, electromagnetic interference (EMI) shielding, conductive thin films, photonics, and photothermal therapy to name a few. Their metallic nature, wide range of optical absorption, and tunable surface chemistry are the key to their success in those applications. The physical properties of MXenes are known to be strongly dependent on their surface terminations. In this study we investigated the electronic properties of Ti₃C₂T_x for different surface terminations, as achieved by different annealing temperatures, with the help of photoelectron spectroscopy, inverse photoelectron spectroscopy and density functional theory calculations. We find that fluorine occupies a single adsorption site, whereas oxygen occupies at least two different adsorption sites. The band structure of Ti₃C₂T_x was measured and found to be largely in agreement with calculated band structures. We further measured the work function of Ti₃C₂T_x as a function of annealing temperature and found that is in the range of 3.9-4.8 eV, depending on its surface composition. Comparing it to detailed DFT calculations shows that it is not simply obtained by averaging the work functions of uniformly terminated Ti₃C₂ surfaces, but that the interplay between the different surface-terminates plays a crucial role.

Transition metal dichalcogenides (TMDs, MoS₂, WSe₂, etc.) possess a range of intriguing optical and electronic properties at the single layer limit including direct bandgap, high exciton binding energies and valley polarization. While significant scientific advances have been achieved using TMD flakes exfoliated from bulk crystals, continued interest remains for the development of wafer-scale processes for TMD films. The synthesis of large area single crystal monolayers requires an epitaxial technique where the substrate provides a template for orientation and lateral growth of TMD domains that merge with minimal grain boundary formation. This is challenging, however, due to rotational symmetry in TMDs which gives rise to twins and anti-phase domains in coalesced films.

Our studies have focused on the epitaxial growth of binary TMD monolayers (MoS₂, WS₂, WSe₂ and MoSe₂) by gas source chemical vapor deposition (CVD) using metal hexacarbonyl and hydride chalcogen precursors. A multi-step precursor modulation method was developed to independently control nucleation density and the lateral growth rate of monolayer domains on the substrate. Using this approach, uniform, coalesced monolayer and few-layer TMD films were obtained on 2” sapphire substrates at growth rates on the order of ~1 monolayer/hour. In-plane X-ray diffraction demonstrates that the films are epitaxially oriented with respect to the sapphire with narrow X-ray full-width-at-half-maximum. Post-growth transmission electron microscopy carried out on monolayers transferred from the sapphire reveals that the films consist of micron-sized single crystal regions formed from merging of clusters of 0^o and 180^o oriented domains which are bounded by anti-phase grain boundaries (APBs). A significant reduction in the density of APBs was observed for WSe₂ grown on hBN flakes transferred onto sapphire. In this case, single atom vacancies in the hBN surface, which act as preferred sites for WSe₂ nucleation, break the surface symmetry leading to a preferred domain orientation. WSe₂ monolayers epitaxially grown on hBN also exhibit increased photoluminescence intensity and improved transport properties (as measured in field-effect devices) compared to comparable layers grown on c-plane sapphire. The results demonstrate hBN as a superior substrate for epitaxial growth of single crystal TMD films with a reduced density of APBs and improved properties and motivate continued efforts in the development of high quality hBN templates for wafer-scale films.

Since the discovery of graphene, a rich family of 2-dimensional (2D) materials spanning transition-metal dichalcogenides (TMDs), metal-organic frameworks (MOFs), oxides, and organics, have broadened the development of nanomaterials and nanotechnology. However, many of the existing 2D materials are naturally occurring van der Waals solids, to achieve stable physical and chemical properties at the nanoscale. To create 2D nanomaterials from a broader range of materials, a comprehensive understanding of the mechanisms behind their syntheses, growth, and stability is crucial. Here we present a novel ionic layer epitaxy (ILE) synthesis of non-layered oxide (ZnO). We demonstrate by using a packed surfactant monolayer at the air-water interface of a ZnO precursor solution, we can guide the 2-dimensional evolution of crystal growth at the interface. The film thickness is controlled by the ionic Stern Layer at the interface, which can be adjusted from one to a few nanometers. We examine the evolution of 2D crystal evolution using x-ray diffraction via synchrotron radiation. Through this method, the size of single-crystalline ZnO can reach more than 50 um. The surfactant caped ZnO nanosheets showed stable p-type conductivity and strong defects-related electronic behaviors. ILE is a promising approach for synthesizing nanometers thick oxides with unique physical and chemical properties at the nanoscale.

Transition metal dichalcogenide (TMD) monolayers, such as WSe₂, WS₂, MoSe₂, and MoS₂, possess distinct physical properties due to the strong coupling between spin and valley degrees of freedom.(1, 2) As monolayer TMDs have a direct bandgap lying in visible range, they have been studied extensively by optical methods.(2, 3) Heterostructures of monolayer TMDs with other functional materials are currently attracting significant attention due to the opportunities to access and utilize their spin-valley degrees of freedom through electrical means.(4) For instance, TMD-ferromagnet heterostructures have been employed recently to study spin current generation in TMDs. (4, 5) The quality of atomically thin TMDs, however, is strongly affected by deposition techniques of metallic layers and have not been fully investigated.(6) In this work, we report the fabrication of Pt/Co multilayer using pulsed laser deposition (PLD) on monolayer WSe₂ grown by metalorganic chemical vapor deposition (MOCVD) on single crystalline (0001)-oriented Al₂O₃ substrates. PLD is a plasma based deposition technique capable of tuning of kinetic and thermodynamic conditions over an expanse range to elucidate and control fundamental structure-property relationships across a wide variety of material classes. (7) Using Raman Spectroscopy, we monitor deposition induced damage on monolayer WSe₂. The pressure of Argon process gas is found to suppress deposition induced defects in WSe_2, which indicates that the primary source of defect generation comes from ion bombardment. Further, we report on magnetometry and spin torque measurements of our WSe₂-ferromagnet heterostructures and demonstrate the generation of spin current from TMD layer. We anticipate that our results will advance the electrical investigation of spin-valley and spin generation phenomena in 2D hybrid heterostructures for spintronics.

1. Z. Y. Zhu, Y. C. Cheng, U. Schwingenschlögl, Giant spin-orbit-induced spin splitting in two-dimensional transition-metal dichalcogenide semiconductors. Physical Review B 84, (2011).
2. H.-L. Liu et al., Optical properties of monolayer transition metal dichalcogenides probed by spectroscopic ellipsometry. Applied Physics Letters 105, (2014).
3. D. Xiao, G. B. Liu, W. Feng, X. Xu, W. Yao, Coupled spin and valley physics in monolayers of MoS₂ and other group-VI dichalcogenides. Phys Rev Lett 108, 196802 (2012).
4. H. Idzuchi, T. Taniguchi, K. Watanabe, P. Kim, Tunneling Devices with Perpendicular Magnetic Anisotropy Electrodes on Atomically Thin van der Waals Heterostructures. ArXiv, (2017).
5. Q. Shao et al., Strong Rashba-Edelstein Effect-Induced Spin-Orbit Torques in Monolayer Transition Metal Dichalcogenide/Ferromagnet Bilayers. Nano Lett 16, 7514-7520 (2016).
6. C. Gong et al., Metal Contacts on Physical Vapor Deposited Monolayer MoS₂. ACS Nano 7, 11350 - 11357 (2013).
7. D. B. Chrisey and G. K. Hubler, Pulsed Laser Deposition of Thin Films (Wiley, New York, 1994).

We performed comprehensive measurements of the structural, magnetic and electronic properties of layered van-der-Waals ferromagnet VI3 down to low temperatures. Despite belonging to a well-studied family of transition metal trihalides such as CrI3, a-RuCl3 and TiCl3, this material has received very little attention. We outline, from high-resolution powder x-ray diffraction measurements, a corrected room-temperature crystal structure to that previously proposed and uncover a structural transition at 79 K, also seen as a peak in the heat capacity. Magnetization measurements confirm VI3 to be a hard ferromagnet (9.1 kOe coercive field at 2 K) with a high degree of magnetic anisotropy, and the pressure dependence of the magnetic properties provide evidence for the two-dimensional nature of the magnetic order. Optical and electrical transport measurements show this material to be an insulator with an optical band gap of 0.67 eV - the previous theoretical predictions of d-band metallicity then lead us to believe VI3 to be a correlated Mott insulator. Our latest band structure calculations support this picture and show good agreement with the experimental data. We suggest VI3 to host great potential in the thriving field of low-dimensional magnetism and functional materials, together with opportunities to study and make use of low-dimensional Mott physics.

Single layer Transition Metal Dichalcogenides (1L-TMDs) are 2D direct bandgap semiconductors that have recently received significant attention for their potential suitability in photonic and optoelectronic applications. Due to the strong quantum confinement, the optical response of 1L-TMDs is strongly renormalized by excitonic effects.
In the electronic bandstructure of single layer MoS₂ we can distinguish A, B and C excitonic peaks at 1.9eV, 2.05eV and 2.87eV, respectively.
The phonon bandstructure of single layer MoS₂ identifies two Raman active optical phonon branches, A₁ (out-of-plane) and E' (in-plane). Raman spectroscopy is extensively used to determine the number of layers in TMDs samples, according to the layer-dependent frequency shift of the characteristic optical phonon frequency. Although vibrational modes of TMDs have been largely investigated by continuous wave Raman scattering, their behaviour in the time domain is almost unexplored. For instance, the generation mechanism of coherent optical phonons is still unclear.
A₁ and E’ modes have characteristic frequencies of 378cm^-1 and 406cm^-1, respectively, and the corresponding coherent oscillation period in time domain is 86 fs for the E' mode and 82 fs for the A₁ mode.
Here we use visible femtosecond pump-probe spectroscopy to detect impulsively excited coherent optical phonons in single layer MoS₂, in order to track the electron-phonon coupling across A, B and C transitions, and to understand what is the fundamental generation mechanism of coherent phonons.
A frequency tunable sub-20fs pump pulse excites electron-hole pairs in the system, while a time-delayed broadband probe pulse, covering A, B and C excitons spectral region (1.8-3.1eV), monitors the transient optical response. We find that optical phonons coherently modulate the transient optical response across all the three excitonic transitions, but predominantly across the C peak. Superimposed on the incoherent electronic relaxation, we can clearly distinguish coherent oscillations, which have a period of 82fs, an amplitude of 2% of the signal peak, and a dephasing time of 2ps. The Fourier transform of this oscillating signal is peaked at the frequency of the A₁ mode (406cm^-1), while no signature of the coupling with E’ mode is detected.
The oscillation amplitude, i.e. electron-phonon coupling strength, which is maximum across the C peak, significantly lowers approaching B and A transitions. These observations are quantitatively supported by ab initio calculations computed in the framework of finite-differences of the dielectric function, also including excitonic effects.
The proposed mechanisms for the generation of coherent optical phonons are Impulsive Stimulated Raman Scattering (ISRS) and Displacive Excitation of Coherent Phonons (DECP). In case of ISRS, femtosecond laser pulses impulsively activate all the Raman modes, if the laser pulse is shorter than the period of the phonon oscillation. Instead in DECP as carriers are photoexcited a sudden change in the energy potential surface acts as a kick for the generation of coherent optical phonons. Different from ISRS, DECP requires a preserved lattice symmetry, therefore only the A₁ mode (out-of-plane) can be excited. This strongly calls for the exclusive coupling with the A₁ mode and the absence of the E' mode, which is exactly what we observe.
The generation mechanism can be also deduced by the trend of the oscillation amplitude with the pump photon energy. As it is tuned from 2eV (A-B peaks) towards 3eV (C peak) the A₁ oscillations amplitude has a dramatic increase, and it lowers again for higher pump photon energies (4.3eV). Ab initio calculations also confirm that this trend tracks the imaginary part of the Raman susceptibility tensor, as required in DECP process.
All these observations reveal that in single layer MoS₂ the fundamental mechanism which is responsible for the generation of coherent optical phonons is the displacive excitation.