Symposium F.EL03—Emerging Low-Dimensional Chalcogenides for Electronics and Photonics

In this work, we investigate two-dimensional (2D) sheets of germanium sulfide (GeS) and germanium telluride (GeTe) as promising high-capacity and stable materials for energy storage. GeS and GeTe with layered structures are prepared via a simple liquid-phase exfoliation approach. As-synthesized 2D nanosheets can effectively increase the electrolyte-electrode interface area and facilitate metal ion transport. As a result, GeS and GeTe nanosheets deliver a high areal capacity of 1.76 mAh cm^-2 and 1.05 mAh cm^-2, respectively as anodes in lithium-ion batteries (LIBs). Further analysis of GeS and GeTe in sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs) suggest that layered germanium monochalcogenides can be potential anodes in other metal-ion batteries.

Inkjet printing of two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets fabricated by liquid-phase exfoliation (LPE) allows simple, low-cost, and mass-producible photo-electronic devices. Many LPE processes involving toxic and environmentally hazardous solvents; however, dispersants have restricted the extent of applications of 2D-TMD inks. Herein, various 2D-TMD nanosheets, including MoS₂, MoSe₂, WS₂, and WSe₂, in addition to few-layered graphene, are inkjet-printed using a LPE process based on zwitterionic dispersants in water. Zwitterions with cationic and anionic species are water-soluble, while alkyl chain moieties associated with two ionic species adhere universally on the surface of TMD nanosheets, resulting in high throughput liquid exfoliation of the nanosheets. The zwitterion-assisted TMD nanosheets in water are successfully employed as an ink without the need for additives to adjust the viscosity and surface tension of the ink for use in an office inkjet printer; this gives rise to A4 scale, large-area inkjet-printed images on diverse substrates, such as metals, oxides, and polymer substrates patchable onto human skin. The development of mechanically flexible, biocompatible-printed arrays of photodetectors with pixelated MoSe₂ channels and conductive graphene electrodes on a paper exhibits a photocurrent ON/OFF ratio of approximately 10^3.8 and photocurrent switching of 500 ms.

Thermoelectric devices based on Sb₂Te₃ and related alloys are of great interest to academia, industry, and government due to their efficiency in low-temperature applications such as micro-heating and small-scale refrigeration. The search for high performing thermoelectric devices is dominated by manipulating the electronic structure to boost the power factor through an increase in the density of states and/or reduction of the lattice thermal conductivity to maintain a large enough temperature gradient. Superlattice structures are long heralded as potential candidates to increase the thermoelectric power factor as well as significantly reduce the lattice thermal conductivity. In this report, we compare the thermoelectric properties of bilayer p-type Sb₂Te₃ / Bi_0.4Sb_1.6Te₃ film and single layer p-type Bi_0.4Sb_1.6Te₃ film of the same thickness. Preliminary Hall effect measurements showed that the carrier concentration of our bilayer and Bi_0.4Sb_1.6Te₃ samples fall within the desired values common to heavily doped semiconductors. Carrier mobility and electrical conductivity are higher for our bilayer than the single-layer film. We noted a significant increase in the Seebeck coefficient values of our bilayer, 202 µV/K compared to 130 µV/K for our single-layer film. NanoTR thermoreflectance measurements revealed a significant reduction in the overall thermal conductivity of our bilayer system compared to our single-layer film. The low thermal conductivity reduction can be associated with, not only species intermixing -which allows phonon scattering events- interfacial boundary scatterings. When these observations are combined into a device, we would expect to see a great jump in the overall efficiency while maintaining the ease of fabrication allowing for quick scaling of production.

Although the device physics related to the operation of PCM (Phase change Memory) have been widely studied since its discovery in the 1960s, questions relating to the nature of the physical principle responsible for the resistivity drift is still controversial [1, 4].
These problems are deepening at the transition to the nanoscale in the construction of PCM memory [2, 3].
Drift resistivity of the PCM in reset state in terms of subthreshold conductivity is analyzed.
This subthreshold conductivity is analyzed with respect to nanoscaling effects, especially:
the contribution of tunneling and trapping to electrical transport;
contribution of ballistic transport and diffusive transport;
analysis of the scattering in relation to dimension and mean free path in amorphous GST;
limitation of Poole-Frankel approach.

The results are verified on the base of present models of the subthreshold conduction and drift resistivity in reset state of the PCM memory [e.g. 4, 5].

References:

1.M. Le Gallo and A. Sebastian, An overview of phase-change memory device physics, Journal of Physics D: Applied Physics, 53 (2020), Issue 21, 213002
2.M. Le Gallo, Evidence for thermally assisted threshold switching behavior in nanoscale phase change memory cells, J. Appl. Phys. 119, 025704 (2016)
3.Xue-Peng Wang et al., Nanoscale amorphous interfaces in phase-change memory materials: structure, properties and design, J. Phys. D: Appl. Phys. 53 (2020) 114002 (8pp)
4.P. Noé et al., Phase-change materials for non-volatile memory devices: from technological challenges to materials science issues, Semicond. Sci. Technol. 33 (2018) 013002 (32pp)
5.A. Redaelli, Ed., Phase Change Memory Device Physics, Reliability and Applications, 2018, Springer, (Ch. 2, 4)

By virtue of their size, the surface must be one of the dominant aspects of semiconductor nanocrystals. Yet the surface remains the most poorly understood aspect of their basic science. Here, we exploit simple temperature dependence photoluminescence and electron transfer theory to rationalize the nature of surface emission from all nanocrystals. Moreover this work enables prediction of how to rationally generate white light from a single nanocrystal by virtue of mixing emission from the excitonic core and the self trapped excitonic states at the surface.

The paper summarizes our recent TEM (transmission electron microscopy) studies and main former data on novel microcrystals, nanostructures with unexpected, dislocation independent, regular internal bending of the crystal lattice planes [1]. Usually it is revealed in thin films or layers after amorphous – crystalline transitions. Such perfect crystals/grains with regularly bent lattice (built up by simultaneous translation and small regular rotation of the unit cell) demonstrate a new “transrotational” [2] type of solid state order realized in thin films. It is primarily dislocation independent and takes place round an axis (or 2 axes) lying in the film plane of the growing crystal. The maximal values correspond to essential lattice orientation gradients (up to 300 degrees per µm) resulting to several whole rotations for the crystal about 10 µm in length (in film plane).
Thin (10 - 100 nm) crystallized areas with transrotation we study vary from small crystals (0.1 - 100 microns), ribbons, whiskers or spherulites to large-scaled polycrystalline areas with a complex texture. They are grown by heat treatment (or by self-organization upon aging) and local annealing by focused electron/laser beam in amorphous films most easily formed for chalcogenides. The films were prepared primarily by vacuum evaporation mostly with thickness or/and composition gradients across TEM grids (to study these factors directly). We used mainly TEM, primarily bend-contour method [3], in situ studies, and HREM. Correlative TEM-AFM studies were also performed.
The geometry and the magnitude of transrotation can be regulated by film preparation and crystallization conditions, orientation of the crystal nucleus, sublayers, composition and film thickness.
In situ studies in particular include HREM of amorphous-crystalline interface propagation and multiple reversible local transformations "amorphous – transrotational crystalline” inside the fingers in Se-based vacuum deposits. Dynamic changes of TEM diffraction contrast fit the mechanism of transrotation formation based on the surface nucleation that we proposed earlier.
Atomistic mathematical model (based on instrument of conformal transformations) for the atom positions in "transrotational" single crystal and the probable physical reasons are discussed. Transrotational microstructure can be considered as an intermediate one between amorphous and crystalline structures (likewise the structure of liquid crystals, intermediate between crystalline structure and liquid one). We suppose it as one of the reasons for easy phase changes in chalcogen-based films that tend to crystallize in such manner. Transrotational crystals have been eventually recognized/studied in a large variety of thin film systems and well-known chalcogenide compositions [4, 5] used for non-volatile memory and prospective for other kinds of storage and phase-change materials applications (including photonics).
Opposite to other unusual regular nano aggregations of atoms widely recognized by the community in recent 35 years (quasi-crystals, fullerenes and nanotubes and alike nano derivatives as well as graphene-like layers) our less known “transrotational” crystals/structures are less confined in dimensions. Bent atom layers in fine areas can be described as similar to that of hypothetical 2+D halves (180°) of endless (continuous in film plane) multiwall nano- tubes/onions/tori.
The transrotation phenomenon is the basis for novel lattice-rotation crystal nanoengineering of functional, smart thin-film materials including tunable and switchable low-D structures with some features suitable also for strain engineering.
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With the advent of applications for two-dimensional materials in electronics and photonics, a number of theoretical investigations on the optical properties of have emerged. However, these investigations often focus on the linear optical properties with very few focused on the nonlinear optical properties of these two-dimensional materials. For example, measurements on the second harmonic generation (SHG) in monolayer transition metal dichalcogenides (TMDs), primarily MoS₂, WS₂, and WSe₂, have shown promise due to the large second-order susceptibilities. SHG measurements of stacked twisted TMD bilayers, as well as from transition metal monochalcogenide (TMM) monolayers, e.g. GaSe and InSe, have also been reported. Yet, theoretical predictions of these second-order quantities are scarce. Here, we summarize a theoretical analysis of the linear and nonlinear susceptibilities of a series of monolayer TMDs and TMMs in comparison to experiment. Aspects on the importance of including spin-orbit coupling and many-body effects to study excitonic effects in these materials will be discussed in detail. Finally, results for TMD and TMM heterostructures will be described, particularly with respect to the optical response upon changing the relative orientation of the stacked monolayers.

Due to the anisotropic bonding states inherent to transition metal dichalcogenides (TMDs), these materials can withstand as large as 10% elastic strains and display an associated wide range of variable optical and electronic properties [1]. In addition to interests in fundamental behaviors, such properties have driven efforts to develop strain-tunable devices and applications. While spontaneous wrinkle formation in few to monolayer specimens generates local strain variations, controllable and predictable strain-pattern requires rational engineering design and processing principles [2,3]. Nascent approaches have focused on either whole-flake deformation (e.g., indirect handling of elastomer substrates) or single, site-specific mechanical indentation to modulate the global or local strain through repeatable stress application [4,5]. Spatially periodic strain patterning has been introduced to produce predictable, highly-symmetric local structural modulations in a passive, non-continuous manner, e.g., through flake deposition on periodic nanostructures [6,7], or through moiré patterns in twisted layered heterostructures [8,9]. In this way, local strain patterning can be controlled by manipulating nanostructure geometry and patterning periodicity.

Here we report a method for highly-symmetric, spontaneous strain patterning of few-layer MoS₂ using patterned holey substrates based on the principle of tessellation. In this approach, uniformly-spaced substrate holes of the same size, arranged in a hexagonal array throughout the substrate, serve as the tessellation seeds. The resulting tessellation pattern with six-fold symmetry is well-described by centroidal Voronoi tessellation (CVT). In a TEM bright-field image, the tessellation manifests as strongly-scattering bend contours between the holes forming three-fold vertices at the center of sets of three holes. Further, twelve-fold bend contours, indexed to first- and second-order Bragg spots, radiate from the seeds and arise from curvature of the crystal over the hole, as confirmed with correlated bright- and dark-field imaging. Atomistic simulations of a bilayer MoS₂ crystal on a holey substrate show a change in the relative height and display similar bend contour patterns in the hole region [10]. Spatially-averaged strains of 2% to 4% are present, as determined with selected-area diffraction. Further, spatial mapping of relative strain was obtained with nanobeam electron diffraction with a 10-nm beam size. Strains were maximal near the hole centers with a steep decrease toward the substrate, as observed in both experiment and simulation, before increasing again when approaching the hexagonal tessellation patterns over the substrate. In addition to spontaneous formation, tessellated strain-patterning is also influenced by polymer residue from crystal transfer for thicker flakes within the holes. These results suggest a simple, highly-repeatable method based on the principles of CVT for producing highly-symmetric, strain-patterned films of high regular periodicity in TMDs, other layered materials, and generally in ultrathin films and crystals [11].

[1] H.-Y. Chang, et al., ACS Nano 7 (2013), p. 5446.
[2] P. Miró, M. Ghorbani-Asl, and T Heine, Adv. Mater. 25 (2013), p. 5473.
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[8] K. Zhang and E. B. Tadmor, J. Mech. Phys. Solids 112 (2018), p. 225.
[9] H. Yoo, et al., Nature Materlials, 18 (2019), p. 448.
[10] Y. Zhang, et al., submitted (2020).
[11] This work was supported primarily by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013 and partially by the National Science Foundation under Grant No. DMR-1654318.

The emerging characteristics of two categories of nanoscale materials with increasing interests, organic metal chalcogenides (OMC) and two-dimensional (2D) van der Waals layers, can both be elucidated by studying the structure-property-relationship of 2D metal alkanethiolate layers. A detailed study of the crystal structure of this inorganic-organic hybrid system is critical to understand its size and odd/even effect observed in properties such as melting point/enthalpy (NanoDSC), luminescence, and band structures. Crystallography determination with atomic resolution is often challenging for beam-sensitive hybrid materials that do not grow into large scale single crystals (e.g. silver alkanethiolate, AgSCn, in this work). Here we describe a novel methodology combining synchrotron powder X-ray diffraction, nanobeam electron diffraction, and ab initio calculations. Unit cell level information and atomic coordinates of Ag-S segments are well uncovered by diffraction techniques, whereas density functional theory calculations suggest the complete atomic basis including the light/soft organic segments. Crystallography results of AgSCn (n=2-10) show odd/even effect in lamellar thickness consistent with prior experiments. Our preliminary findings of AgSCn structure suggest modifications to the initial simplified model, and consists of three segments: (i) Ag-S backbone: we first report an inorganic backbone deformation modulated by organic chain length, as a result of lattice mismatch between Ag-S framework and the epitaxial hydrocarbon chains that incline towards optimal packing. Such modulation probably leads to various optical luminescence properties inherent in the argentophilic interactions found in the backbone, an emerging topic in gold/silver chemistry. (ii) Interlayer interface: the structural origin of odd/even effect in this layered OMC system lies in the parity-dependent C-H orientations and H-H interactions exhibited among the terminal methyl groups. The resulting denser packing of odd-chain interface explains its higher melting enthalpy. (iii) Alkyl chains: the epitaxial accommodation between the backbone structure (sp³ hybridized sulfur bonding) and the density of alkyl chains triggers the evolution of local chain tilting and twisting towards the optimal packing with longer chain length. This further provides clues that drive the similar variations of chain configurations in other hybrid systems such as self-assembled monolayers.


References:
[1] Ye, Z, de la Rama, L.P., Efremov, M. Y., Schleife, A., and Allen, L.H. J. Phys. Chem. C. 121 (2017): 13916.
[2] Ye, Z., de la Rama, L. P., Efremov, M. Y., Zuo, J.-M., and Allen, L. H. Dalton Trans. 45 (2016): 18954.
[3] de la Rama, L. P., Hu, L., Ye, Z., Efremov, M. Y. and Allen, L. H. J. Am. Chem. Soc. 135 (2013): 14286
[4] Lai, S. L., Guo, J. Y., Petrova, V., Ramanath, G., and Allen, L. H. Phys. Rev. Lett. 77 (1996): 99.
[5] Zuo, J.-M., and Spence, J. CH. Advanced Transmission Electron Microscopy, ISBN 978-1-4939-6605-9. Springer Science+ Business Media New York, (2017).
[6] Kisung, K., Kononov, A, Lee, C. W., Leveillee, J. A., Shapera, E. P., Zhang, X., and Schleife, A. Comput. Mater. Sci. 160 (2019): 207.

Ferromagnetism in two-dimensional (2D) van der Waals materials has attracted increasing attention due to its potential for developing a new generation of spintronic devices. One route that has been explored to induce 2D ferromagnetism is substitutional doping in transition metal dichalcogenides (TMD) with V or with magnetic atoms. In this work, we propose an alternative approach that consists in decorating 2D TMD materials with nanoparticles of manganese oxide. The nanoparticles were deposited on the surface of monolayer MoSe₂ using a chemical vapor deposition technique. Micro-Raman and PL spectroscopy were used to study their optical properties; while the morphology, chemical composition and thickness of the samples were characterized via SEM, STEM and AFM, respectively. Magnetic measurements reveal a weak ferromagnetic behavior in individual MoSe₂ films, probably arising from the presence of selenium vacancies. Similarly, weak ferromagnetism is observed for MnO_x nanoparticles deposited on a SiO₂ substrate. However, when the MnO_x nanoparticles are deposited on 2D MoSe₂, the hybrid MnO_x/MoSe₂ system displays enhanced room-temperature ferromagnetism, suggesting a strong magnetic exchange coupling between the two material components. A detailed analysis on the temperature dependence of saturation magnetization and coercive field indicates stronger magnetic coupling at lower temperatures. Our results provide an alternative route to induce room-temperature ferromagnetism in 2D systems.

Van der Waals materials and heterostructures that manifest strongly bound exciton states at room temperature are of great promise for optoelectronic applications. 2D transitional metal dichalcogenides (TMDCs) of Mo and W are of particular interest since the strong exciton binding manifests in high refractive indices and extinction coefficients near the exciton resonance. Light-exciton interaction has been extensively studied in monolayer direct-band gap TMDC films (Schneider et al. 2018, Nat. Comm.; Brar et al. 2018, Chem. Soc. Rev.). Nevertheless, light-interaction with monolayer films is challenged by the very large disparity between optical wavelength and film thickness; without an external cavity, a TMDC monolayer only achieves 8-10% exciton resonance absorption.

In this work, we demonstrate that nanostructured, multilayer transition metal dichalcogenides by themselves provide an ideal platform for excitation and control of excitonic modes, paving the way to exciton-photonics. We first study reflection spectra of unpatterned micromechanically exfoliated WS₂ flakes on a template-stripped Au substrate. For a WS₂ thickness of 15 nm, we observe an avoided crossing of the A-exciton resonance and an optical cavity mode with a large splitting energy of 170 meV. We then introduced periodic one-dimensional gratings into the WS₂ with e-beam lithography and dry-etching. We measured spectra for grating periods ranging from 300 to 750 nm, WS₂ thicknesses from 5 to 30 nm, and grating widths from 200 to 300 nm. In these heterostructures, we observed hybrid modes in addition to strong dispersion of optical cavity modes and several orders of plasmon polariton modes. For a period of 600 nm, a grating width of 300 nm, and a WS₂ thickness of 10 nm, we observe exciton couplings to optical grating and optical cavity modes with strengths of 116 meV and 89 meV respectively. In addition, we observe complete suppression of the inherently strong exciton absorption resonance as a result of coupled optical modes and their interference. We supplement these experimental results with transfer matrix calculations and finite-difference time-domain simulation. We also model the resonances of this heterostructure with a coupled oscillator model; with this model, we recreate the exciton resonance suppression we observe in experiment, showing that the suppression is indeed due to two polaritonic couplings.

Our results highlight a previously unexplored aspect of TMDC excitonics and optics, opening a new regime in exploring strong light-matter interactions where the TMDC serves as a host for both excitonic and cavity modes (without the need for an external cavity). This work paves the way to a next generation of integrated exciton optoelectronic nano-devices and applications in light generation, computing, and sensing.

References:
Huiqin Zhang, Bhaskar Abhiraman, Qing Zhang, Jinshui Miao, Kiyoung Jo, Stefano Roccasecca, Mark W. Knight, Artur R. Davoyan, Deep Jariwala: “Hybrid Exciton-Plasmon-Polaritons in van der Waals Semiconductor Gratings”, 2019; arXiv:1912.13442. (accepted) Nature Communications

Self-assembled nanocrystal superlattices offer exceptional control over the coupling between nanocrystals, just as solid-state crystals tailor the bonding between atoms. Because nanocrystals are much smaller than the wavelength of light, superlattices that integrate nanocrystals of different properties (e.g. metals, semiconductors, dielectrics) have the potential to generate new light-matter interactions that are not achievable in natural materials [1,2].

In this work [3], we add infrared plasmonic Cu_2-xS nanocrystals to the relatively limited library of materials that have been successfully incorporated into binary superlattices. We create a variety of binary superlattices with small plasmonic (Cu_2-xS) nanocrystals and large excitonic (PbS) nanocrystals, both resonant in the infrared. Then, by controlling the surface chemistry of large Cu_2-xS nanocrystals, we are able to produce structurally analogous superlattices of large Cu_2-xS and small PbS nanocrystals. Transmission electron microscopy (TEM) and grazing-incidence small-angle X-ray scattering (GISAXS) are used to characterize the superlattices. This synthetic achievement is the first step toward understanding how infrared plasmonic and excitonic nanocrystals couple in well-defined lattices that display a variety of symmetries and stoichiometries.

References
[1] F. X. Redl, K.-S. Cho, C. B. Murray, & S. O’Brien, Three-dimensional binary superlattices of magnetic nanocrystals and semiconductor quantum dots. Nature 423, 968-971 (2003).
[2] M. A. Boles, M. Engel, & D. V. Talapin, Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials. Chemical Reviews 116, 11220-11289 (2016).
[3] S. Brittman, N. A. Mahadik, S. B. Qadri, P. Y. Yee, J. G. Tischler, & J. E. Boercker, Binary Superlattices of Infrared Plasmonic and Excitonic Nanocrystals, ACS Applied Materials & Interfaces 12, 24271-24280 (2020).

One of the most distinct features of van der Waals (vdW) heterostructures based on two-dimensional (2D) materials is the ultrafast photo-induced interlayer charge transfer, which is known to occur in sub-picosecond time scales. Such ultrafast dynamics offers promise for the realization of novel hot carrier optoelectronic devices where non-thermalized photocarriers are harnessed to achieve unconventional functions. In this talk, we will discuss a few examples of hot carrier optoelectronic phenomena in heterostructures of 2D transition metal dichalcogenides. First, we will discuss realization of near-infrared (NIR) to visible light upconversion in a metal-insulator-semiconductor (MIS) vdW heterostack tunnel diode [1]. We show that the upconversion is enabled by interlayer transfer of non-thermalized photoexcited holes and its dynamics can be tuned by orders of magnitude by applied electric fields. Second, we will discuss unique photoresponse of these MIS devices arising from hot carriers generated by efficient exciton-exciton annihilation (EEA) in monolayer MoS₂ [2]. We describe how EEA can result in highly energetic electrons and holes due to uneven distribution of annihilation energies following energy- and momentum-conserving scattering.

[1] Linardy et al. “Electro-Optic Upconversion in van der Waals Heterostructures via Nonequilibrium Photocarrier Tunneling” Adv. Mater. In Press (DOI: 10.1002/adma.202001543).
[2] Linardy et al. “Harnessing Exciton–Exciton Annihilation in Two-Dimensional Semiconductors” Nano Lett. 2020, 20, 1647.

Hot carriers have significant impact to energy conversion/storage and photodetection. For instance, a hot carrier solar cell will lead to a ~ 67 % power conversion efficiency (PCE) to overcome the famous Schockley-Queisser (SQ) limit. For purposes of energy conversion and storage, the hot carriers trigger a more energetic and dynamic chemical reaction to split the water molecules and generate hydrogen molecules, leading to an efficient photoelectrochemical (PEC) cell. This is particular important and urgent to 2D transition metal dichalcogenides (TMDCs) family, such as MoS2, WS2, MoSe2 and MoTe2, which have great potential applications in quantum emitters, high performance photo or chemical detector and lithium-ion batteries. However, collecting and utilizing the hot carriers remains one of major challenges in ultrafast phenomena in quantum materials because the hot carriers have a super-short lifetime within couple picoseconds.

In this report, we directly collected hot carrier in monolayer WS2 semiconductor as a photocurrent using ultrafast photocurrent spectroscopy. The hot carriers generation is resulted from the multiple exciton Auger recombination, demonstrated by the ultrafast photocurrent dependence on laser intensity, and temperature.

Second harmonic generation (SHG), as a nonlinear optical effect, has important applications in photonics for frequency conversion and light modulations. Materials with high SHG efficiency and reduced dimensions are favorable for integrated photonics and novel nonlinear optical applications. Here, we fabricate MoS₂ nanoscrolls with different chiralities and study their SHG performances. As a 1D material, MoS₂ nanoscroll shows reduced symmetry and strong chirality dependency in the polarization-resolved SHG characterizations, which is in stark contrast with the 2D MoS₂ layers. This SHG performance can be well explained by the coherent superposition theory of second harmonic field of the nanoscroll walls. MoS₂ nanoscrolls with certain chiralities in our experiment can have SHG intensity up to 98 times stronger than that of monolayer MoS₂. The same chirality-dependent SHG can be expected for nanoscrolls or nanotubes composed of other non-centrosymmetric 2D materials, such as WS₂, WSe₂, and hBN. The characterization and analysis results presented here can also serve as a non-destructive technique to determine the chiralities of these nanoscrolls and nanotubes.

Metal contacts to two-dimensional (2D) semiconductors are critical for high performance and highly scaled devices. Yet, despite several years of effort they remain unoptimized and poorly understood from the perspective of metal semiconductor interface chemistry. Most existing studies rely on cross section electron microscopy images or device transport both of which are indirect ways to measure contact resistance. A key reason for this is because metal-semiconductor contacts are buried interfaces which are difficult to probe directly, at least in the case of bulk semiconductors. However, 2D semiconductors such as transition metal dichalcogenides (TMDCs) of Mo and W offer a unique opportunity to directly probe the metal semiconductor interface from the semiconductor side due to the atomically thin nature of the semiconductor.¹ In this talk, we will present spatially varying electrical and optical characteristics of TMDC-bulk metal buried interface using a simple metal-assisted flipping technique that inverts the interface for direct electronic and optical access of the buried interface.² Using a combination of scanning Kelvin Probe Force Microscopy (KPFM), conductive-AFM and tip enhanced Raman and Photoluminescence Spectroscopy (TERS/TEPL) we concurrently map surface potential, conductivity, charge density dependent-vibration and emission spectra of the interface with ~20 nm spatial resolution. We use three different Au deposition methods (Thermal, e-beam, Sputter) to compare and contrast the quality of different contacts in addition to direct van der Waals stamping. We observe that thermal metal evaporation results in formation of the most uniform, ohmic and lowest resistance contacts (1.49*10⁶ Ω) in comparison to e-beam evaporation (7.20*10⁸ Ω), whereas van der Waals stamping forms Schottky contacts (5.00*10¹¹ Ω). KPFM and TERS data of each buried interfaces reveals distinct change of carrier concentration as a function of evaporation technique.³ We attribute different levels of metal semiconductor intimacy at the interface and different levels of defect creation during metal evaporation to these differences in contact quality. Our result highlights the importance of metal deposition schemes and paves a path forward towards engineering low resistance contacts and also presents a facile method for probing them in atomically thin channel materials.

[1] 2D Mater. 2018, 5, 035003
[2] ACS Appl. Mater. Interfaces, 2019, 11, 38218
[3] Phys. Rev. B, 2012, 85, 161403(R)

Two-dimensional (2D) materials have seen a surge of interest ever since the discovery of graphene, a single 2D layer of carbon atoms (graphene), more than a decade ago. 2D chalcogenides are a fundamental class in the study of low-dimensional science and their synthesis is an important goal of solid state chemistry. The presentation will focus on how novel chalcogenides can form using design concepts such as the “dimensional reduction”. The new materials exhibit diverse centrosymmetric and noncentrosymmetric structures that lead to a variety of attractive optoelectronic properties such as direct bandgaps, high X-ray absorption, neutron detection and strong nonlinear optical responses.

The interaction of macroscopic mechanical object with electron charge and spin plays an important role in today’s information technology and fundamental studies of quantum mechanics. Recently emerged valleytronics encodes information to the electron valley degree-of-freedom and promises exciting applications in computation and communication. Exploring the interplay between the valley physics and macroscopic mechanics will bring new perspectives for valley information processing and exploration of the quantum-classical boundary. Here, for the first time, we realize valley-mechanical interaction in a monolayer MoS₂ resonator and demonstrate direct transduction of the valley information to mechanical states. The valley-mechanical coupling is achieved by exploiting the magnetic moment of valley carriers with a magnetic field gradient. We optically populate the valleys and observe the resulting mechanical actuation using laser interferometry. We are thus able to control the valley–mechanical interaction with the pump laser, the magnetic field gradient and temperature. Our experiment lays the foundation for a new class of valley-controlled mechanical devices. It also facilitates hybridization of valley pseudospin with other quantum information carriers such as two-level qubits and microwave photons.

Transition Metal Dichalcogenides (TMDCs) have been under intensive research due to their unique electronic and optoelectronic properties. Due to their layered structure via weak van der Waals interactions, their bandgap can be modified by the number of layers[1] which directly impacts the range of wavelengths it can intrinsically detect. For the case of MoS₂, monolayer has a direct bandgap of 1.8 eV with a cut-off wavelength of red light (~690 nm) and bulk has an indirect bandgap of 1.2 eV with a cut-off wavelength of ~1 μm. Photodetectors using MoS₂ as a photoactive material have been reported to offer very large responsivities (high light sensitivity) in photodetection applications[2]; however, the larger bandgaps of monolayer and bilayer MoS₂ limits its ability to detect wavelengths deeper in the near-infrared band. One technique to lower the bandgap of these materials is to introduce a very large perpendicular electric field. Unfortunately, conventional dielectric materials like SiO₂ and high-k metal oxides like HfO₂ and Al₂O₃ cannot provide the required high electric fields to enable this effect. However, the use of an electric double layer (EDL) device structure can offer nano-gap capacitances resulting in generating very large electric fields. In this study, we investigated the bandgap tuning of monolayer and bilayer MoS₂ utilizing an electrical double layer device structure. In this structure, under the application of an applied gate bias, Li⁺ ions migrate in the electrolyte layer to form an EDL with MoS₂. As a result, we were able to detect NIR wavelengths and characterize its photoresponse behavior. We further confirmed our results with Density Functional Theory. This work shows the possibility of extending the intrinsic light detection of low dimensional chalcogenide materials for future optoelectronic devices.

[1] H. S. Lee et al., Nano Lett., 12, 3695, 2012.
[2] J.-Y., Wu et al., Adv. Mater., 30, 1705880, 2018.

Nanowire (NW) based photodetectors in the near-infrared wavelength range is of great interest for next-generation single-photon-sources and photodetectors to be used in quantum networks, nanophotonic integrated circuits and solar cell applications. Bandgap engineering in the near-infrared region is facilitated by large bandgap bowing and increased electron confinement upon dilute (<2%) nitrogen incorporation. By leveraging these advantages, this work showcases optimized intrinsic and tellurium (Te)-doped dilute nitride GaAsSbN NW based photodetection. Compensation of point defects in the doped configuration is evidenced by a significant improvement in photoluminescence characteristics, Raman vibrational mode shifts, and spectral shape with improved photodetector device performance relative to intrinsic dilute nitride NWs. Transmission electron microscopy was used further to quantify the quality and composition of NW heterostructures, confirming a planar-defect-free zincblende crystal structure. The first successful demonstration of vertically aligned Te-doped GaAsSbN NW based PD device with a responsivity of ~5 A/W (@860 nm) on single NW configuration and ~3800 A/W (@1100 nm) on an ensemble NW device is reported. Further, a detectivity of 3.2×10¹⁰ Jones in ensemble configuration was confirmed from low-frequency noise characteristics. The responsivity of these NWs is comparable with the best reported values in this wavelength region, though the detectivity needs significant improvement. Encouraging prospects are seen for further bandgap engineering of this material system to develop improved quality and multicolor photodetection spanning the entire near-infrared-region range with potential for further red-shifting photodetection capability to 1.55 µm. A gallium telluride (GaTe) captive source is used as the Te-dopant and molecular beam epitaxy (MBE) is the enabling technology that is used to realize Ga-catalyzed nanowires in axial configuration.

The family of ternary chalcogenide compounds Cu3MX4 (where M is V, Nb, or Ta and X is S, Se, Or Te), also known as sulvanites, recently came to light due to their possible multiple uses in optoelectronic devices. ^1-3 The compounds are isostructural and crystallize in the cubic structure with space group P- 43m . They are characterized by significant values of refractive index, high ionic and electronic conductivity, as well as remarkable photoconductivity and photoluminescence.⁴
Some reports show that Cu₃TaS₄ is transparent in the visible region, having an absorption onset in the range of 2.8-3.0 eV, which could prove useful in light modulators, solar cells, optoelectronic and photocatalytic devices. ^1,3 In general, the sulvanites are p-type semiconductors and show ionic conductivity that is attributed to a small number of interstitial Cu(I) ions. ^5,6
To take advantage of these excellent optoelectronic properties, processability of these materials would be highly desirable. We demonstrated facile solution-phase routes⁷ to synthesize the entire sulfide and selenide suite of compounds, revealing fascinating nanocrystal morphologies, from 2D nanoplates to cubic and pyramidal quantum dots. Photoluminescence measurements reveal strong light emission in both vanadium and niobium compounds. Synthetic and characterization data will be presented in detail, and free-form sensor applications will be discussed.

References
(1) Newhouse, P. F.; Hersh, P. A.; Zakutayev, A.; Richard, A.; Platt, H. A. S.; Keszler, D. A.; Tate, J. Thin film preparation and characterization of wide band gap Cu3TaQ4 (Q = S or Se) p-type semiconductors. Thin Solid Films 2009, 517, 2473-2476.
(2) Tate, J.; Newhouse, P. F.; Kykyneshi, R.; Hersh, P. A.; Kinney, J.; McIntyre, D. H.; Keszler, D. A. Chalcogen-based transparent conductors. Thin Solid Films 2008, 516, 5795-5799.
(3) Lv, X. S.; Deng, Z. H.; Miao, F. X.; Gu, G. X.; Sun, Y. L.; Zhang, Q. L.; Wan, S. M. Fundamental optical and electrical properties of nano-Cu3VS4 thin film. Optical Materials 2012, 34, 1451-1454.
(4) Bougherara, K.; Litimein, F.; Khenata, R.; Ucgun, E.; Ocak, H. Y.; Ugur, S.; Ugur, G.; Reshak, A.; Soyalp, F.; Omran, S. B. Structural, Elastic, Electronic and Optical Properties of Cu3TMSe4 (TM = V, Nb and Ta) Sulvanite Compounds via First-Principles Calculations. Science of Advanced Materials 2013, 5, 97-106.
(5) Arribart, H.; Sapoval, B.; Gorochov, O.; LeNagard, N. Fast ion transport at room temperature in the mixed conductor Cu3VS4. Solid State Communications 1978, 26, 435-439.
(6) Arribart, H.; Sapoval, B. Theory of mixed conduction due to cationic interstitials in the p-type semiconductor Cu3VS4. Electrochimica Acta 1979, 24, 751-754.
(7) Liu, M.; Lai, C.-Y.; Selopal, G. S.; Radu, D. R. Synthesis and optoelectronic properties of Cu3VSe4 nanocrystals. PloS one 2020, 15, e0232184.

There has been a surge in research interests in two-dimensional (2D) materials, and transition metal dichalcogenides (TMDs) in particular, owed to their unique physical, electrical and optical properties. [1,2] Although 2D materials are not at the par with present technologies in terms of device performance, they easily outperform them in terms of functionality in both electronic and optoelectronic applications. TMDs provide a plethora of novel physical properties such as flake-thickness dependent band structure, improved light-matter interaction and easy tuning of transport properties. With the possibility to go below 1-nm channel thickness, mechanically flexible devices and ease of making heterostructures, they have emerged as attractive candidates for next generation (opto)electronic technology.
Tungsten diselenide (WSe₂) has been studied in detail due to its ambipolar transport, large carrier mobility values and good optical response. [3,4] In addition, we can easily dope the TMD channel with electrostatic gating providing us with freedom to easily tune the device performance with applied voltages. [5]
In this abstract, we present a simple yet effective method to leverage the ability to electrostatically tune the device transport to achieve a multi-functional device. The device consists of two set of gates – one to control the WSe₂ channel and two shorted gates under each contact to control the Schottky contact barriers. We are able to achieve not only conventional n-FET and p-FET characteristics, but also ambipolar as well as anti-ambipolar current trends with help of specific gate biasing schemes. Such device can be readily used for modulation of input signals (applied at the two gates) to get phase and frequency coded output current waveforms. Clubbed with an option to illuminate the channel, we can achieve an increased degree of freedom in device operation. We have been able to demonstrate in-phase, out-of-phase and double frequency signals on both ambipolar and anti-ambipolar current trends under dark and illumination condition. These waveforms have been experimentally demonstrated to be useful in two distinct 2-bit symbol encoding schemes. One of these schemes involves transition between ambipolar to anti-ambipolar transport in dark condition using electrostatic gating. This is not possible in conventional ambipolar or anti-ambipolar devices reported on 2D materials due to lack of tunable operation limited by device architecture. The other scheme uses illumination status along with frequency coding to identify frequency shift enabled by light-tunable anti-ambipolar current transport.
We further analysed the output current waveforms to better understand the information coding possibilities. By using a combination of phase, frequency and amplitude coding, we can potentially achieve up to 4-bit coded symbols with appropriate input gate-bias signals and illumination status (on/off). 2-bit and 3-bit transmission schemes are also examined with the same methodology. Phase-amplitude and phase-frequency polar plots are created to observe the constellation chart for any given transmission scheme.
To summarize, a novel device operation is proposed to leverage the tunable operation of 2D TMD channel material enabling both ambipolar and anti-ambipolar transport in the same device. With proper gate biasing schemes and incorporating illumination, we can generate distinct output current waveforms that can be used as modulated signals for multi-bit communication schemes.

References
[1] Y. Cui et al., Adv. Funct. Mater., 1900040 (2019).
[2] M. Long et al., Adv. Funct. Mater., 29, 1803807 (2018).
[3] H. J. Chuang et al., Nano Lett., 14, 3594 (2014).
[4] H. Kim et al., Sci. Adv., 5, 1, eaau4728 (2019).
[5] D. J. Groenendijk et al., Nano Lett., 14, 5846 (2014).

Recent research progress has led to the successful synthesis of numerous low dimensional chalcogenides materials that demonstrate unique electronic and optical properties. In this talk, I will discuss our work in studying the ferroelectric, optical and phonon properties of such materials including CuInP₂S₄ and BaTiS₃. The former demonstrates interesting ferroelectric properties in its layered van der Waals crystal lattice and the latter exhibits quasi-1-dimensional crystal structure giving rise to strong linear dichroism in the infrared and visible wavelength as well as unique phonon properties. I will also discuss the device applications of the respective materials, specifically the application of ferroelectric CuInP₂S₄ in non-volatile memory and infrared detectors based on BaTiS₃. The promising future research directions, challenges and their potential applications in electronics and photonics technologies will also be discussed.

This work is support in part by the Army Research Office (Grant no. W911NF1910111) and the National Science Foundation (Grant no. ECCS-1653870).

Among the layered two dimensional semiconductors, molybdenum disulfide (MoS₂) is considered to be an excellent candidate for applications in optoelectronics and integrated circuits due to the layer-dependent tunable bandgap in the visible region, high ON/OFF current ratio in field-effect transistors (FET) and strong light-matter interaction properties. In this study, using multi-terminal measurements, we report high broadband photocurrent response (R) and external quantum efficiency (EQE) of few-atomic layered MoS₂ phototransistors using multi-terminal measurements, fabricated on a SiO₂ dielectric substrate and encapsulated with a thin transparent polymer film of Cytop. The photocurrent response was measured using a white light source as well as monochromatic light of wavelength λ = 400 nm - 900 nm. We measured responsivity in 2-terminal con- figuration as high as R = 1×10³ A/W under white light illumination with optical power P_opt = 0.02 nW. The R value increased to 3.5×10³ A/W when measured using a 4-terminal configuration. Using monochromatic light on the same device, the measured values of R were 10³ and 6×10³ A/W under illumination of λ = 400 nm when measured in 2- and 4-terminal methods, respectively. The highest EQE values obtained using λ = 400 nm were 10⁵ % and 10⁶ % measured in 2- and 4-terminal configurations, respectively. The wavelength dependent responsivity decreased from 400 nm to the near-IR region at 900 nm. The observed photoresponse, photocurrent-dark current ratio (PDCR), detectivity as a function of applied gate voltage, optical power, contact resistances and wavelength were measured and are discussed in detail.

Active metasurfaces composed of tunable subwavelength structures enable versatile light manipulation on ultrathin devices. Previously, electrically gated indium tin oxide-based metasurfaces have demonstrated dynamic amplitude and phase control of reflected light. However, these devices operate at around 1550nm, which limits their applications to infrared optoelectronics. Two-dimensional transition metal dichalcogenides (2D TMDs) are promising candidates for phase and amplitude modulation of visible light due to their strong excitonic resonances in the visible.
Here, we investigate the gate-tunability of the complex index and the corresponding phase and amplitude modulation of the reflected light from 2D Molybdenum Diselenide (MoSe₂). Our structure uses the large excitonic reflectivity that is dominated by radiative decay at low temperatures. By measuring the gate dependent change in the reflectivity of the MoSe₂, we develop a model for the change in the complex dielectric function. We then enhance the emission of the TMD by integrating the monolayer with a metasurface comprised of subwavelength AlGaAs cylinders that support high quality factor quasi bound states in the continuum (BIC) modes. The subwavelength structure is based on a cylinder that utilizes the destructive interference between optical modes to obtain a significant enhancement in the quality factor of the cavity. The gate tunability of the MoSe₂ monolayer coupled to the high Q resonator allows us to electrically modulate the phase and amplitude of the reflected light. This monolayer TMD-based active metasurface could be a potential platform for ultrathin visible light modulators.

Layered chalcogenides hold promise for manipulating light in confined geometry, such as in photonic integrated circuits, where key material properties include strong and controllable light-matter interaction, and limited optical loss. Layered materials feature strong anisotropy and a panoply of polymorphs that suggest a rich design space for highly-nonperturbative photonic integrated devices based on functional martensitic transformations: phase changes and ferroelastic domain switching. These phenomena are manifest in materials with band gap above the photonics-relevant near-infrared (NIR) spectral band (~ 0.5 – 1 eV), meaning that they can be harnessed in refractive (i.e. non-absorptive) applications.

Transition metal dichalcogenides (TMDs) offer a vast composition space within which to optimize refractive index contrast, optical loss, and switching energy. We are using a strategy of alloy design to optimize new, active materials for optical phase shifters. We will present our theoretical predictions of thermodynamics and optical properties of binary systems of sulfur-based TMDs: ZrS₂-MoS₂, TiS₂-MoS₂, TiS₂-ZrS₂, and MoS₂-CrS₂. We will present our experimental measurements of the NIR optical properties of single crystals. We will then present progress on making large-area thin films of sulfur-based TMDs, including the theoretically-predicted alloys, by a two-step method of metal sulfurization. Finally, we will present the optical properties and phase-change functionality of our designed TMD alloy thin films.

We will also present an alternative paradigm of strong, non-perturbative refractive index modulation: electric field-driven ferroelastic domain switching in low-symmetry layered materials. We predict that in materials such as SnSe, a sufficiently-strong electric field will induce rotation between equivalent crystallographic domains, with strong refractive index contrast for linearly-polarized light. We will present simulation-based design of photonic integrated devices utilizing the trirefringence of materials such as SnSe and black phosphorous, and experimental results demonstrating non-thermal domain switching triggered by fast laser pulses.

Optical properties of atomically thin transition metal dichalcogenides are dominated by exciton resonances exhibiting large oscillator strength and binding energies. In vertically stacked bilayers, a host of new exciton resonances emerge, lending new opportunities to discover new science and enable new devices. In this talk, I will focus on how moire superlattices change exciton resonances and diffusion. We discovered recently that phonon spectra are renormalized in moire superlattices as well. A rich variety of properties can be controlled by tuning the twist angle between the bilayers. I make some comparisons between samples that are grown using chemical vapor deposition and those prepared by mechanical exfoliation and stacking. Our work is supported by DOE under grant DE-SC0019398, NSF DMR-1808042, DMR-1720595, and the Welch Foundation F-1662.

Metalenses are flat surfaces that incorporate subwavelength structures to replace conventional bulky curved lenses. Metasurfaces with the capability of manipulating light-matter interaction at subwavelength are the core components of the metalenses that attract broad research attention. Metalenses are more convenient to be designed with tunable foci than conventional lenses with additional tunability brought by nanoscale features. Nonlinear metalenses are rarely investigated compared to linear counter-parts due to limited efficiency. However, to our interest, nonlinear metalenses provide a future platform for applications in nonlinear information processing, all-optical higher-order Fourier transformations, short laser pulse diagnostics, etc. [1]. Moreover, metalenses conveying information at both linear and nonlinear frequencies could potentially reveal more information of the imaging object. Generating nonlinear signal confocal with linear signal is difficult to realize for conventional high-index material due to material dispersion. Low-loss phase change chalcogenides such as Antimony Sulfide (Sb₂S₃), which introduces substantial change in the refractive index with high-speed thermal-controllable phase changes [2] in addition to intrinsic large third-order nonlinearities, are potential candidates for efficient reconfigurable-foci metalenses that can realize third-harmonic generation (THG) signal co-focusing with the linear signal. We demonstrate a reconfigurable all-dielectric metalens by making use of a unique dispersion relationship between the amorphous state and the crystalline state of Sb₂S₃. The device works at the fundamental wavelength (λ_f) of 950 nm, where the refractive index of amorphous Sb₂S₃(n_a-Sb2S3) at λ_f is close to that of crystalline Sb₂S₃ (n_c-Sb2S3) at THG wavelength (λ_THG) whereas n_c-Sb2S3 at λ_f is close to n_a-Sb2S3 at λ_THG. The principle of phase delay makes THG signal from the metalens of one crystallinity state confocal with the transmitted fundamental signal of the other crystallinity state. The reconfigurable metalens can inspire effective capture of double-frequency imaging of objects.

References
[1] C. Schlickriede, et al. Nano Lett. 20, 4370 (2020).
[2] W. Dong, et al. Adv. Funct. Mater. 29, 1806181 (2019).

The photo-induced conversion of CO₂ to chemical fuels in aqueous solution is limited by the electron transfer efficiency and the mass transfer of the reactants. Herein, Cu was decorated on a Cd_0.5Zn_0.5S twin nanocrystals by photo-deposition, and fixed it on a gas dispenser for photocatalytic CO₂ reduction (CO2RR). The quartz gas dispenser was used as a support for fixing catalysts, which accelerated the diffusion and mass transfer of CO₂ in the electrolyte, thus increasing the energy conversion efficiency of the reaction. Cd_0.5Zn_0.5S provides photovoltage for driving photocatalytic reactions. While Cu provides active sites for the conversion of CO₂. Compared with undecorated Cd_0.5Zn_0.5S, Cu/Cd_0.5Zn_0.5S have a smaller band gap ( ~ -1.98 eV) and a higher selectivity for multi-carbon products (78%). Among the products, the selectivity of ethanol and acetic acid were 45% and 30%, respectively. Moreover, this work provides an efficiently strategy to integrate synthesis and assembly for photocatalytic conversion CO₂.

Investigation of the interplay of metal-organic chemistry will enrich the state-of-the-art of chemical vapor deposition (CVD) technology and open new possibilities for the generation of nanostructured metal sulfide and selenide nanostructures. Therefore, novel single-source precursors exhibiting high volatility and defined thermal decomposition under (plasma-enhanced) CVD conditions through targeted synthesis strategies are reported to elaborate the precursor chemistry – materials synthesis – functional property chain. Synergistic reactivity and sufficient stability through unique ligand design to provide a precise control over compositional purity of metal chalcogenides in CVD deposits without additional chalcogenide sources. CVD-grown ME_x (M = Fe, Co, Ni, Cu, Sn, In; E = S, Se) 2 D nanostructures were tested towards their (electro)catalytic applications, particular in the oxygen evolution reactions (OER). The presented CVD data opens new possibilities in the vapor phase synthesis of chalcogenide nanostructures facilitating the application as mutable earth abundant electro- or photocatalysts in OER and hydrogen evolution reactions (HER).

Hydrogen gas is one of the most important chemical precursors with a wide variety of uses. Currently, over 90% of hydrogen is produced using energy-intensive hydrocarbon reforming. Photocatalytic water splitting introduces an alternative, environmentally friendly pathway for hydrogen production. Water is split into hydrogen and oxygen using energy provided from the absorption of light photons by a photocatalytically active semiconductor. Thermodynamically, splitting of water requires 1.23 eV of energy per molecule of water. However, to overcome the associated overpotential, in semiconductors this energy requirement corresponds to an optimum band gap of 1.8 eV to 2.2 eV. [1] The electronic band structure of a promising material must allow for good mobility of charge carriers as well as align the band edges so that they straddle the redox window of water. [2]

La₅Ti₂AgS₅O₇ (LTA) and La₅Ti₂CuS₅O₇ (LTC) were experimentally shown to exhibit photocatalytic activity for both oxidation and reduction of water. [3,4] The complex crystal structures of systems exhibit one-dimensional chains of perovskite-like Ti-centred octahedra and rock-salt-like metal sulphide tetrahedra. Some experimental studies show the systems’ electronic structure could support high mobility of charge carriers through the structure via these one-dimensional channels. [4]

To better understand the photocatalytic activity, we investigated the electronic structure of bulk LTA and LTC using plane wave density functional theory (DFT) with hybrid functionals as implemented in Vienna ab initio Simulation Package. Low effective charge carrier masses in b-directions were observed with spatial separation of the band edges centred on the Ti octahedra and metal tetrahedra, which confirmed good mobility of charge carriers along the one-dimensional channels with lowered probability of recombination taking place.

All non-polar symmetric surfaces up to a maximum Miller index of 2 were then evaluated to obtain their surface energies and band alignments using generalised gradient approximation (GGA) and hybrid functionals. Wulff constructions based on the GGA surface energies visualised predicted nanoparticle shape, matching the experimental descriptions of crystallites. Combining the Wulff shapes with hybrid DFT band alignment showed the largest surfaces on the sides of columnar shapes had inadequate band edge alignment, so only the ends of crystallites were suitable for photocatalysis. This gave an important insight as photocatalytic activity of LTA and LTC would be greatly improved if the particles length in b-direction decreased in combination with an increase in the area of columnar crystallites’ blunt ends which have more suitable band alignment.

[1] T. Hisatomi and K. Domen, Nat. Catal., 2019, 2, 387–399.
[2] T. Le Bahers, M. Rérat and P. Sautet, J. Phys. Chem. C, 2014, 118, 5997–6008.
[3] V. Meignen, L. Cario, A. Lafond, Y. Moëlo, C. Guillot-Deudon and A. Meerschaut, J. Solid State Chem., 2004, 177, 2810–2817.
[4] T. Suzuki, T. Hisatomi, K. Teramura, Y. Shimodaira, H. Kobayashi and K. Domen, Phys. Chem. Chem. Phys., 2012, 14, 15475.

Chalcogenides have recently attracted attention as alternate materials for photonic applications suffering less from stability and toxicity issues encountered in lead halide perovskites [1]. Also, in the past few years, the computer–aided design of metal chalcogenide semiconductors led to the discovery and synthesis of very promising compounds for numerous application such as light absorbers in the thin–film photovoltaic devices [2–4]. In this presentation, aiming at in–depth exploration for light absorbers and related devices, we discuss the potential of Nb– and Ta–based perovskites (NaMO_(3–x)Q_x; M = Nb, Ta and Q = S, Se) [3], and Bi-based chalcogenides (ABiX_2/3; A = Na, K and X = O, S) [5] for photocatalysis on the grounds of their structural and electronic dimensionality. Our interest focuses on the potential of these materials for hydrogen production from photocatalytic water splitting as well as CO₂ conversion.

To select a photocatalyst, one must look into its photon absorption and charge separation, governed by their electronic properties such as the bandgap, band edge positioning, and the photogenerated charge carrier mobility. Although our density functional theory (DFT) calculation at the Heyd-Scuseria-Ernzerhof functional (HSE06) indicates the materials as good light absorbers, the analysis of effective masses of the charge carriers, along with the electronic dimensionality, imply that several of the considered compounds exhibit a reduced electronic dimensionality despite showing high structural dimensionality.

Analysis of the effective masses of the photogenerated charge carriers along the dominant directions or channels of charge transport show that most of the Bi–chalcogenides, except NaBiS₃ and KBiS₃, could perform well as photocatalysts in terms of charge separation and transport. Even more promising, Nb– and Ta–perovskites have lower effective masses of the photogenerated charge carriers than their Bi–based compounds indicating their superior carrier transport properties. Turning into assessing the electrochemical performance of the Nb–/Ta– and Bi–based chalcogenides for hydrogen evolution reactions and CO₂ conversions, we investigated the band edge alignments with respect to O₂/H₂O, H⁺/H₂, CO₂/CO, CO₂/HCHO, CO₂/CH₃OH, and CO₂/CH₄ redox levels considering aqueous media of different pH (acidic, neutral and alkaline), for photocatalysis reactions. This work provides a comprehensive insight into the Nb–/Ta– and Bi–based compounds on the grounds of relevant properties for hydrogen evolution and CO₂ conversion reactions, calling for a careful analysis of both structural and electronic dimensionality metrics.

References
[1] Buffiere, M., Dhawale, D., El-Mellouhi, F. (2019). Energy Technology 7(11), 1900819.
[2] Park, H., Alharbi, F., Sanvito, S., Tabet, N., El-Mellouhi, F. (2018). The Journal of Physical Chemistry C 122(16), 8814 - 8821.
[3] Park, H., Alharbi, F., Sanvito, S., Tabet, N., El-Mellouhi, F. (2018). ChemPhysChem 19(6), 703 - 714.
[4] El-Mellouhi, F., Tabet, N., Alharbi, F. H., Sanvito, S., Park, H., US10553367B2,
[5] R, S., Akande, A., El-Mellouhi, F., Park, H., Sanvito, S. (2020) Physical Review Materials 4, 075401.

Aknowledgements
AA and SKR thanks Institute of Technology Sligo President's Bursary Award for the funding through the grant PPRES050. FE and HP thank the Qatar National Research Fund (QNRF) through the National Priorities Research Program (NPRP8-090-2-047). SS thanks the Irish Research Council (IRCLA/2019/127) for the financial support. Computational resources have been provided by the research computing group at Texas A&M University at Qatar, Trinity Centre for High Performance Computing (TCHPC) (Project Code: HPC 16 00953), and Irish Centre for High-End Computing (ICHEC) (Project Codes: isphy002c and isphy003c).

A newer generation of solar cells is emerging based on solution-processed materials, primarily PbS colloidal quantum dots (CQD) and lead halide perovskites. However, both contain toxic lead, posing significant safety concerns. Here, we propose to utilize chalcogenide based colloidal quantum dot (CQD) inks to achieve scalable and low-cost solution processing of the state-of-the-art photovoltaic materials. CQDs are used as a means in obtaining bulk direct bandgap semiconductors, which have been demonstrated to be highly efficient light absorbing layers by means of thermally annealing and sintering the QDs

The key challenges in this field revolve around improving CQD packing and further reducing the defect densities. We will demonstrate that inorganic atomic ligands can eliminate the need for organic long chain ligands required for colloidal stability. Furthermore, these inorganic ligands can be used as abundant passivating agents while promoting the sintering of CQDs in the resulting films. We will demonstrate proof-of-principle results with PV devices based on lead-free chalcogenide based CQDs, primarily CuInS2.

Sb₂Se₃ has emerged as a promising absorber material for photovoltaic applications owing to its excellent optoelectronic properties such as suitable bandgap and high absorption coefficient as well as its low-toxic and earth-abundant constituents. Sb₂Se₃ is comprised of quasi-one-dimensional (Q1D) ribbons stacked by weak van der Waals (vdW) interactions without dangling bonds, rendering the Sb₂Se₃ grain boundries (GBs) self-passivated and intrinsically benign. Various growth techniques have been used for the Sb₂Se₃ absorber layer growth, such as close-space sublimation (CSS). However, due to the fast sublimation rate and confined CSS chamber space, the evaporated gas lacks a full and uniform mixture before condensing on the substrates. In the meantime, Se has a much higher saturated vapor pressure than that of Sb₂Se₃, making Se element release first during CSS deposition and dominates the chamber gas at the very initial stage. As a result, the chemical composition (ratio of Se and Sb) of the gas mixture varies with time, causing the deposited films at different growth stages to exhibit different chemical compositions. In this work, the chemical composition (Se and Sb), microstructure, and device performance of Sb₂Se₃ films deposited at different growth stages were systematically investigated. It is demonstrated that the chemical variation dependent growth behavior can be used to control the carrier transport and lead to improved device performance. The work also paves a way to further improve the device performance of Sb₂Se₃ based photovoltaic devices by tuning the scaleable vapor deposition.

Bismuth oxyiodide (BiOI) has gained attention for photovoltaics, photocatalysis and photodetectors owing to its composition of non-toxic elements, tolerance to point defects, and highly-suitable optical properties.^1,2 But like many other bismuth-based compounds, BiOI is a layered material with anisotropic transport properties, making control over the preferred orientation critical for achieving optimal device performance. In this work, we develop new insights into the growth mechanism of BiOI synthesized by chemical vapor deposition (CVD) and show how the preferred orientation can be controlled. By adjusting the precursor and substrate temperatures to tune whether or not we are in a nucleation- or growth-controlled regime, we reproducibly vary the ratio of the (001) and (110) orientations by over two orders of magnitude. As a result, we achieve highly c-axis oriented films, which leads to less shunting than a/b-axis oriented films, resulting in improved open-circuit voltages from a median value of 0.7 V (a/b-axis oriented) to 0.9 V (c-axis oriented) in BiOI solar cells. More broadly, the described mechanisms can be used to control the preferred orientation in other low-dimensional materials, which will be important for achieving improved performance across a wide variety of devices.

[1] Adv. Mater., 2017, 29, 1702176
[2] Adv. Funct. Mater., 2020, 30, 1909983

The field of photovoltaics is undergoing a surge of interest following the recent discovery of the lead hybrid perovskites as a remarkably efficient class of solar absorber. Of these, methylammonium lead iodide (MAPI) has garnered significant attention due to its record breaking efficiencies, however, there are lingering concerns surrounding its long-term stability and toxicity. Compounding these problems is a more fundamental limitation: the hybrid perovskite structure shows a severe lack of compositional tunability. Beyond minor changes to the halide and A site composition it has not been possible to significantly supersede the original MAPI formulation, because the hybrid perovskite structure is not accommodating to significant changes to composition of the A or B sites.[1]
Many of the excellent properties seen in hybrid perovskites are thought to derive from the 6s² electronic configuration of lead, a configuration found in a range of post-transition metal compounds.[2] In this presentation I will outline our computational studies on Sb₂Se₃, a material which has displayed power conversion efficiencies approaching 10% from a limited number of studies,[3] and GeSe, a semiconductor who’s fundamental band gap has been the source of much debate. Our results highlight the complex nature of the defect chemistry of the “simple” binary Sb₂Se₃,[4] explain why as deposited Sb₂Se₃ is n-type,[5] and rationalise the nature of the band gap of GeSe. [6]

[1] W. Travis et al., Chemical Science, 7, 4548 (2016)
[2] A. M. Ganose, C. N. Savory, and D. O. Scanlon, Chemical Communications, 53, 20 (2017)
[3] L. J. Philips et al., IEEE Journal of Photovoltaics, 9, 544 (2019)
[4] C. N. Savory and D. O. Scanlon, Journal of Materials Chemistry A, 7, 10739 (2019)
[5] T. D. C. Hobson et al., Chemistry of Materials, 32, 2621 (2020)
[6] P. A. E. Murgatroyd et al., Chemistry of Materials, 32, 3245 (2020)

Antimony selenide (Sb₂Se₃) has emerged as a strong candidate for practical water splitting due to its suitable bandgap, excellent optoelectronic properties, and resistance to photocorrosion in strongly acidic electrolyte solutions. In this talk, I will discuss the synthesis of Sb₂Se₃ thin films by selenization of antimony metal and the photoelectrochemical properties of these films after coating with a molybdenum sulfide hydrogen evolution catalyst. The performance of these photocathodes was greatly enhanced by a relatively low temperature sulfurization treatment with elemental sulfur, greatly improving the photovoltage and the fill factor. The origin of the improvement upon sulfurization was investigated with time-resolved microwave conductivity (TRMC) measurements and photoluminescence. TRMC of our Sb₂Se₃ reveals high mobilities of the photogenerated charge carriers (>10 cm² V^-1 s^-1) and a low trap density, in contrast to other emerging photoelectrochemical materials such as Fe₂O₃, CuFeO₂, and BiVO₄. We find that the sulfurization treatment of the Sb₂Se₃ thin films yields improved charge carrier lifetimes. Not only is surface recombination curtailed, but deep traps in the bulk are eliminated (most likely Sb-Se antisite defects), which reduces non-radiative recombination of the photogenerated charge carriers. A low temperature sulfurization treatment is thereby identified as a simple method to remove bulk traps in Sb₂Se₃ thin films.

Sb₂Se₃ is becoming a relevant thin film chalcogenide semiconductor with different technological applications such as: superconductivity, electronic component, electrode for sodium-ion batteries, photodetectors and as photovoltaic (PV) absorber. In particular, Sb₂Se₃ have recently experienced incredible progresses as emerging photovoltaic material with a certified record efficiency of 9.2%. This technology has a promising interest due to good optoelectronic properties, earth-abundant and low toxicity constituents. Additionally, the quasi one-dimensional atomic structure provides interesting properties like improved charge carrier transport and minimization of grain boundaries recombination.

Up to date, most of the works in literature have been focused on device structure modifications, synthesis conditions optimization and understanding the fundamental properties of the material itself, to obtain the best self-standing photovoltaic performance. But little attention has been paid on explore new strategies to include extrinsic element that modify the physico-chemical properties of the devices (like doping, recrystallization, passivation, etc.) in order to improve the optoelectronic properties of the devices. In other more mature thin film PV technologies as Cu(In,Ga)Se₂, Cu₂ZnSn(S,Se)₄, the inclusion of extrinsic elements by pre annealing synthesis (PAS) and post deposition treatment (PDT) during the device fabrication have a critical role in the improvement of the optoelectronics properties. These approaches are critical for high efficiency Cu(In,Ga)Se₂ technology, where the inclusion of alkali based treatments (Rb and Na) are critical to achieve record PCE up to 23%, or in the case of the Cu₂ZnSnSe₄ based in sputtered metallic precursor, the introduction of nanometric Ge layer before the synthesis of the absorber and the soft thermal treatment after the synthesis of the complete devices allow to homogenize the material and achieve PCE over 11%, close to record devices.

In this work we propose to explore the effects of the PDT and PAS approaches to include IA and IVA group elements into the Sb₂Se₃ and analyze the effects on its properties. In this case we explored the possibility of alkali inclusion with LiF, NaF and KF, as well as Ge and Sn, using PVD processes. Our synthesis process consists on the reactive annealing under Se atmosphere of antimony layers to obtain a Glass/Mo/Sb₂Se₃/CdS/i-ZnO/ITO system in substrate configuration. In the case of alkali-fluorides we compare post deposition treatments (PDT), consisting in the deposition of alkali fluorides by thermal evaporation on top of the as growth SLG/Mo/Sb₂Se₃ stack and then performing a further soft annealing routine; with pre annealing treatments (PAS) in which the alkali-fluorides are thermally evaporated on top of the Sb layers prior to Sb₂Se₃ synthesis. In the case of Ge and Sn the approach is more flexible and we could explore the PAS and PDT together with the inclusion of GeSe₂ and SnSe during the reactive annealing, in order to obtain more homogeneous incorporation. Additionally, the study is complemented with a wide characterization of the Sb₂Se₃ layers and devices using morphological and physico-chemical characterization (XRF, XRD and Raman spectroscopy), and with a complete analysis of the impact of the doping strategies correlated with the optoelectronic characterization (JV, IQE, CV) of the solar cells.

In the case of the IA inclusion, the optoelectronic characterization shows an improvement of the J_sc for NaF and LiF PDT treatments, and a V_oc increment for NaF PDT treatment, while KF is detrimental for the device performance. The NaF PDT allows slightly increase the power conversion efficiency from the 4.5% for reference devices to 4.9%. In the other hand the case of IVA inclusion, the effect of the SnSe approach do not show impact in the device performance in comparison with a huge degradation observed for the Sn PAS approach.

Metal chalcohalides are candidates for next-generation technologies including energy conversion, information storage, and quantum computing. I will present an exploration of V-VI-VII ternary chalcohalides, where V = Sb, Bi; VI = O, S, Se, Te; and VII = F, Cl, Br, I. These include the photoferroic compound SbSI [1,2] and the solar fuels photoelectrode BiOBr [3,4]. We consider 32 elemental combinations in 4 spacegroups using a combination of density functional theory (structural and thermochemistry) and many-body QSGW theory (relativistic electronic band structures). The materials are found to exhibit a wide range of electronic structures and bandgaps from metallic to over 5 eV. The large spin-orbit coupling in the heavy cations strongly influences the electronic structure. This research is part of a collaboration with the groups of David Scanlon (UCL) and Mark van Schilfgaarde (KCL), and used the UK National Supercomputer ARCHER.

[1] "Ferroelectric materials for solar energy conversion: photoferroics revisited" K. T. Butler et al, Energy Environ. Sci. 8, 838 (2015)

[2] "Quasi-particle electronic band structure and alignment of the V-VI-VII semiconductors SbSI, SbSBr, and SbSeI for solar cells" K. T. Butler et al, Appl. Phys. Lett. 108, 112103 (2016)

[3] "Bismuth oxyhalides: synthesis, structure and photoelectrochemical activity" D. S. Bhachu et al, Chem. Sci. 7, 4832 (2016)

[4] "Interplay of orbital and relativistic effects in bismuth oxyhalides: BiOF, BiOCl, BiOBr, and BiOI" A. M. Ganose et al, Chem. Mater. 28, 7, 1980 (2016)

The diverse properties of transition metal dichalcogenides (TMDCs) make them a materials platform where alloying is an attractive option for tuning properties such as band gaps, charge density waves, superconductivity, and electrochemical activities. Of the many possible combinations of TMDCs, the small subset of Mo- and W-based quasi-binary semiconducting alloys has garnered widespread attention. Outside this limited subset, the synthesizability of alloys remains largely unknown. Furthermore, the usage of multiple principal elements to form so-called high entropy alloys (HEAs) of TMDCs has not been previously explored. Using first-principles calculations, we present a comprehensive analysis of the stability of 25 quasi-binary transition metal- and chalcogen-site TMDC alloys and a select number of 4- and 5-component transition metal-site TMDC HEAs. Critically, we expand knowledge of TMDC alloys beyond group VI transition metals Mo and W to include group V transition metals V, Nb, and Ta, opening the possibility of magnetic, superconducting, and charge density wave phases. We verify our stability predictions through evidence of the successful synthesis of many alloys, including new group VI quasi-binary alloys,¹ and the first synthesized TMDC HEA. First-principle calculations are also used to explain exciting experimental tuning of properties including thermal expansion, catalytic activity, and charge density wave transitions.

Acknowledgements: This work was supported by National Science Foundation (NSF) through grants: DMREF-1729787.

Reference:
1. Z. Hemmat, et al., Quasi-Binary Transition Metal Dichalcogenide Alloys: Thermodynamic Stability Prediction, Scalable Synthesis, and Application. Advanced Materials (2020).

Transition metal dichalcogenides (TMDCs) are of continuing research interest because of their interesting opto-electronical properties as well as catalytic application potential¹. While two-dimensional TMDCs are well investigated one-dimensional micro and nanostructures are sparsely described in literature². When structured in one-dimension these materials offer great potential for catalytic applications like the hydrogen evolution reaction (HER)¹ as they posses the materials intrinsic catalytic property while having additionally a high aspect-ratio. Herein, we report the synthesis of micro and nano-scaled one-dimensional molybdenum disulphide by vapour phase. A proposed mechanism for one dimensional growth is given and discussed.
1) S. R. Kadam, A. N. Enyashin, L. Houben, R. Bar-Ziv, M. Bar-Sadan, J. Mater. Chem. A 2020, 8, 1403–1416.
2) W.-J. Li, E.-W. Shi, J.-M. Ko, Z. Chen, H. Ogino, T. Fukuda, J. Cryst. Growth 2003, 250, 418–422.

Low-dimensional metal chalcogenides with atomic homogeneity are emerging as promising materials due to their unique electronic and optical properties. One of the most interesting behaviors of such ultrasmall inorganic nanocrystals is their ability of self-assembly into higher level structure complexity, which is dictated by the intra/inter molecular interactions through the surfactant ligands. Here, we demonstrate the synthesis and assembly of atomically precise 1.2 nm sulfur-bridged copper (SB-Cu) clusters with tertiary hierarchical complexity, characterized and confirmed with high accuracy through single-crystal X-ray diffraction. The primary structure is clockwise/counterclockwise chiral caps and cores. The caps and cores combine to form full clusters and, due to the cap-core interaction, only two enantiomeric isomers are formed (secondary structure). A tertiary hierarchical architecture is achieved through the self-assembly of alternating enantiomers. More importantly, the hierarchical assembly of SB-Cu clusters is inherently conductive due to the conjugated MBO ligand (MBO = 2-Mercaptobenzoxazole) and the effective intermolecular interactions. The SB-Cu cluster has a characteristic absorption peak at 333 nm and emission at 380 nm, both of which, due to the conductivity of the SB-Cu clusters network, could vary in response to photoirradiation at different wavelengths. These clusters and assemblies make for an interesting nanomaterial for photoconductive devices and optical applications.

PtSe₂ was recently proposed as promising material for low-power, high-performance, and ultra-thin-body electronic application because of its sizeable band gap up to 1.2 eV, high carrier mobility in the order of 1,000 cm²/Vs, stability in air, and possibility of low-temperature fabrication compatible with CMOS back-end-of-line (BEOL) processing. First experimental values of its high carrier mobility of 210 cm²/Vs, that outperforms other 2-dimensional materials such as black phosphorus, MoS₂ and WSe₂, demonstrate the great potential of optimizing the deposition process in order to grow high-quality PtSe₂ material. This work employed molecular beam epitaxy (MBE) to take advantage of its excellent in-situ control and monitoring capabilities to shine light on the kinetic processes involved and to optimize material properties, a strategy that is proven to produce high-quality and large size layers of various transition metal chalcogenides.
In the first step, we show growth of ultrathin Pt layers on c-plane Al₂O₃. We demonstrate how deposition parameters affect the morphology and crystallinity of the Pt films. In-situ reflection high energy electron diffraction (RHEED) and ex-situ atomic force microscopy (AFM) show single crystal film growth with smooth surfaces for growth in the temperature range from 300 to 600 °C, which is confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). We are addressing the epitaxial orientation relationship between Pt and the substrate and determine the growth to proceed in an island growth mode as observed by RHEED, XRD, and TEM.
In the second part we demonstrate the BEOL compatible selenization process done in MBE by exposing optimized Pt layers of 3 nm and 3 monolayer (ML) thickness to Se flux. We discuss the influence of Pt layer configuration and substrate temperature on the selenization process. For 3 nm thick Pt layers, selenization in the optimum temperature range of 200 to 300 °C leads to the formation of epitaxial, smooth, single crystal, PtSe₂ as determined by RHEED, AFM and Raman investigation. We elucidate the epitaxial orientation relationship between PtSe₂ and the underlying Pt as presented in RHEED. We find however from RHEED and Raman, that the thickness of the so formed PtSe₂ is limited to the mono/few-layer regime regardless of the selenization temperature and the Se flux exposure time. We conclude that the reason for the observed self-limited selenization process in the MBE environment is the compared to other physical vapor deposition techniques extremely small Se flux accessible within MBE. With the goal to form a layer of purely PtSe₂ on the Al₂O₃ by the selenization method, we report on the transformation process of 3 ML thick Pt films by Se flux exposure. Using RHEED, AFM and Raman, we still find the transformation rate to be smaller than 100 %, in addition we see that the crystal quality of the PtSe₂ is inferior compared to the PtSe₂ layers formed by selenization of 3 nm thick Pt films. However, we present a post-selenization annealing process in Se atmosphere that enhances the crystallinity of the PtSe₂ significantly as confirmed by Raman investigation.

Chemical vapor deposition (CVD)-grown molybdenum disulfide(MoS₂), one of the most widely used transition metal dichalcogenides (TMDs), have attracted much attention as an emerging semiconductor layer to realize field-effect transistors (FETs) and thin-film optoelectronic devices due to its unique characteristics, such as a high carrier mobility, chemical and thermal stability, absence of dangling bond, and tunable electronic energy band depending on a number of layers.[1] For the last decade, there have been a lot of effort for tuning its intrinsic properties and enhancing device performance via surface charge transfer doping, with exploring its fundamental electrical characteristics.[2,3] The surface charge transfer doping follows the charge transfer mechanism between deposited dopants and host materials which allows to modulate the electrical and optical characteristics of the TMDs by tuning the Fermi level.[4] Especially, the one of the key advantage of surface charge transfer doping is that the electronic structure of the host materials is not changed. This doping strategy is typically conducted via a solution-immersion or vapor-phase doping methods to deposit organic dopants onto the surface of TMDs. However, because the modulation of their electrical characteristics are only determined by immersion time and the dopant concentration, therefore, the effective amount of dopants which critically affects the characteristics of host MoS₂ cannot be precisely controlled.[5]
In this presentation, we will report surface charge transfer doping by using drop-on-demand selective inkjet printing with the use of benzyl viologen (BV) as a surface charge transfer donor. To perform a reliable doping process onto CVD-grown MoS₂ with inkjet printing, we will report how to obtain a stable BV ink droplet considering a specific range of the inverse Ohnesorge number, which is a dominant parameter to determine inkjettability. In addition, systematically analyzed the solvent characteristics depending on the organic dopant candidates will be presented. With these efforts, we can enhance the electrical performance of MoS₂-FETs via surface charge transfer doping using inkjet printing which allows precisely controlled effective doping concentrations.
[1] Z. Yu, Z-Y. Ong, S. Li, J-B. Xu, G. Zhang, Y-W. Zhang, Y. Shi, X. Wang, Adv. Funct. Mater., 27, 1604093 (2017).
[2] X. Zhang, Z. Shao, X. Zhang, Y. He, J. Jie, Adv. Mater., 28, 10409 (2016).
[3] F-Z. Li, L-B. Luo, Q-D. Yang, D. Wu, C. Xie, B. Nie, J-S. Jie, C-Y. Wu, L. Wang, S-H. Yu, Adv. Energy Mater., 3, 579 (2013).
[4] S. Andleeb, AK. Singh, J. Eom, Sci. Technol. Adv. Mater. 16, 035009 (2015).
[5] D. Kiriya, M. Tosun, P. Zhao, JS. Kang, A. Javey, J. Am. Chem. Soc. 136, 7853 (2014).

In recent years tremendous efforts have been devoted to the research on two dimensional materials. Their unique structures and remarkable properties have offered great potential for a wide range of applications in electronics, optoeletronics, valleytronics, catalysis, etc. The synthesis of high quality large area mono- and few-layer 2D materials is highly desirable for their applications. In this talk I will present our recent development on novel 2D material and structures, together with further understanding regarding the chemical vapour deposition (CVD) synthesis process of monolayer MoS2.

As transistor channels scale below 100 nm, the drive current would be limited by the contact resistance. Transition metal dichalcogenides, including MoS2, are promising candidates for ultra-short (< 10 nm) channels as they are immune to mobillity degradation in the < 5nm thickness regime, due to the absence of surface roughnesss scattering. When making metal contacts to MoS2, the metal Fermi level pins slightly below the conduction band edge, leading to n-type Schottky contacts. On the other hand, the problem of n-type contacts has been solved in InAs, a III-V semiconductor, since the metal Fermi level pins above the conduction band edge, leading to ohmic contacts. In this study, we discuss the fabrication of heterostructure contacts to MoS2 by depositing InAs on to monolayer MoS2. The MoS2 has been synthesized at 850 C using the Chemical Vapor Deposition technique and InAs is deposited using the Metalorganic-CVD technique. We fabricate Transmission Line Model (TLM) structures using electron-beam lithography to extract the contact resistance and to study the contacts using temperature-dependent current-voltage (I-V) measurements. We also model the interface using Density Functional Theory (DFT) to gain insight into the physics at the interface.

Reduced dimensionality in known chalcogenides such as the quaternary Cu₂ZnSnSe (CZTSe), enhances the usefulness of these materials, including this known photovoltaic material. Past reports of platinum (Pt)-free counter electrodes for dye-sensitized solar cells (DSSCs) showed that they lacked the high surface area and electrocatalytic activity that could truly drive Pt replacement. CZTSSe, a sulfo-selenide kesterite was synthesized in a cascade reaction path to CZTSe, starting with the binary SnSe2 nanosheets that were sequentially treated with copper and zinc in solution. The templated synthesis enabled retention of the morphology, thus resulting in stand-alone, large area, CZTSSe nanosheets. CZTSSe nanosheets deposited on fluorine-doped tin oxide (FTO) substrates were used as Pt replacement in a dye-sensitized solar cell, yielding a power conversion efficiency of (5.73%), comparable to the performance of a cell using a Pt-based CE (5.78%).[1] An excellent electrocatalytic performance (4.60%)[1] was also recorded for a CZTSSe/carbon paper (CP) utilized as a replacement of Pt/FTO-free CE. Fabrication of CZTSSe nanosheets as well characterization data will be presented.

(1) Mohammadnezhad, M.; Liu, M.; Selopal, G. S.; Navarro-Pardo, F.; Wang, Z. M.; Stansfield, B.; Zhao, H.; Lai, C.-Y.; Radu, D. R.; Rosei, F. Synthesis of highly efficient Cu2ZnSnSxSe4−x (CZTSSe) nanosheet electrocatalyst for dye-sensitized solar cells. Electrochimica Acta 2020, 340, 135954.

Silver phenylselenolate (AgSePh) is an emerging two-dimensional semiconducting member of a hybrid metal-organic chalcogenolate family. In addition to its two-dimensional structure with high exciton binding energy, strong in-plane anisotropy, and a narrow emission spectrum at 467 nm, AgSePh can be synthesized by a scalable vapor-phase method as well as a solution-phase reaction. Here, we show by testing 24 solvents – with different polarities, boiling points and functional groups – that complexation between silver cations and solvent molecules is the key to an increasing size of AgSePh crystals. With the introduction of amine solvents, we are able to increase the size of AgSePh thin films prepared by the vapor-phase tarnish reaction from ~200 nm to >1 µm and that of AgSePh crystals grown by the solution-phase biphasic reaction from ~3 µm to >150 µm. Moreover, the addition of solvent vapors to the vapor-phase tarnish reaction allows the control of crystal orientations between perpendicular and parallel to substrates’ surfaces. The improved syntheses reported in this work will allow easy integrations of AgSePh in both thin-film electronic and nanoelectronic applications and the exploration of strong excitonic anisotropy.

Highly crystalline and conformal growth of metal dichalcogenide based large surface area thin films thermoelectric generator were successfully grown on different substrates using RF sputtering technique. Surface morphology, crystal structure, and electrical properties of both p and n type thin films have been studied thoroughly. X-ray diffraction data revealed that crystallites of MoS₂ and WS₂ films are highly oriented in 00l plane with grain size distribution ranging from 40nm-130nm. Surface roughness study from AFM confirms that it increases with rise in substrate temperature and it plays a big role in electronic and phonon scattering. Interestingly, MoS₂ films also shows low thermal conductivity at room temperature and strongly favors achievement of higher thermoelectric figure of merit value of up to 1.98. Raman spectroscopy data shows two distinct MoS₂ vibrational modes at 380 cm^-1 for E¹_2g and 410 cm^-1 for A_1g. Thermoelectric transport studies further demonstrated that MoS₂ films show p-type thermoelectric characteristics, while WS₂ is an n-type material. We demonstrated high efficient pn-junction thermoelectric generator device for waste heat recovery and cooling applications.

Body-integrated electronic sensing devices is getting great attention in field of human healthcare. Especially, wearable health monitoring sensors allows access to effective and affordable diagnostic tools for sensitive, rapid and accurate detection for biologically relevant molecules such as dopamine (DA) and glucose. In this current work, efforts have been made to develop non-enzymatic direct electrochemical sensors based on transition metal chalcogenides electrocatalyst loaded on flexible polymeric film and graphitic substrate for DA and glucose. Among all the niche applications and properties of transition metal chalcogenides, recently these materials have gained attention due to their favorable electrochemical properties owing to tunable redox behavior of the transition metal center. Binary metal selenides and tellurides grown on different flexible substrates through electrodeposition, hydrothermal synthesis as well as by chemical vapor deposition exhibits exceptional activity for DA detection and electrocatalytic glucose oxidation at low applied potential. These chalcogenides also show enhanced sensing parameters including low limit of detection for glucose concentration and high sensitivity (~19 mA mM^-1 cm^-2 for copper selenide catalyst).The electrochemical studies shows long-term stability as well as sensitivity with a detection region from 5 nm to 2 μM, rapid response time to a set of DA concentrations with excellent reproducibility. Furthermore, for practical application, human blood serum was also tested for accurate detection of DA and glucose level. The results suggested that such nanostructured metal chalcogenides-based sensor may be a potential candidate for electrochemical detection of DA and glucose in practical analysis. Through these studies, we have explored transition metal chalcogenides as electrocatalysts for dopamine sensing and direct glucose oxidation along with an understanding of their structure-property relationship.

2D semiconductor transition metal dichalcogenides (TMDs) have attracted considerable attention due to their layered structure, suitable band gap for visible light absorption, high carrier mobility, electrochemically active unsaturated edges and relatively good stability against photocorrosion [1]. Recently, 2D MoSe₂ has been gaining considerable interest due to its higher electrical conductivity as compared to MoS₂, its wider inter-layer distance (~0.65 nm), narrow bandgap (1.33–1.72 eV), high resistance to photo-corrosion, high surface area layer, electrochemically active unsaturated Se-edges and close to zero Gibbs free energy edges for hydrogen adsorption.These properties are promising for different applications of MoSe₂ including hydrogen evolution [2], photocatalysis [3] and Li-ion batteries [4]. However, their low light absorption efficiency, recombination issues of the photogenerated electron–hole pairs and slow charge transfer of the intrinsic semiconducting 2H-phase are a handicap. An efficient strategy to surpass those intrinsic limitations are hybrid nanostructures using conducting supporting materials. In this regard, anodic TiO₂ nanotubes (TNTs) are excellent photoactive supporting material providing a high surface area, unique directionality for the charge separation, and highly effective charge collection [5]. Accordingly, we present anodic TiO₂ nanotubes homogenously decorated with MoSe₂ nanosheets by atomic layer deposition (ALD). In parallel, we address the current scarcity of convenient ALD Se precursors by the synthesis a set of new selenium precursors - alkysilyl (R₃Si)₂Se and alkytin (R₃Sn)₂Se, and cyclic silylselenides compounds. Those Se precursors were extensicely characterized and their reliability as ALD Se precursors explored [6,7]. Several compounds exhibited promising results to be convenient ALD Se precursor as will be presented in the presentation. The synthesis of the MoSe₂ nanosheets and their composites with TiO₂ NTs, their physical and electrochemical characterization, and encouraging results in electrochemical characterization, hydrogen evolution reaction (HER) and photocatalysis will be presented and discussed.

[1] W. Choi, N. Choudhary, G. H. Han, J. Park, D. Akinwande, Y. H. Lee, Materials Today 20 (2017) 116–130.
[2] S. Mao, Z. Wen, S. Ci, X. Guo, K. Ostrikov, J. Chen, Small 11 (2015) 414–419
[3] Y. Zhao, J. Tu, Y. Sun, X. Hu, J. Ning, W. Wang, F. Wang, Y. Xu, L. He, J. Phys. Chem. C 122 (2018) 26570−26575
[4] Y. Liu, M. Zhu, D. Chen, J. Mater. Chem. A, 3 (2015) 11857-11862
[5] J. M. Macak, H. Tsuchiya, A. Ghicov, K. Yasuda, R, Hahn, S. Bauer, P. Schmuki, Curr. Opin. Solid State Mater. Sci. 11 (2007) 3
[6] , R. Zazpe, J. Charvot, R. Krumpolec, L. Hromadko, D. Pavlinák, F. Dvorak, P. Knotek, J. Michalicka, J. Prikryl, S. Ng, V. Jelínková, F. Bures, J.M. Macak, FlatChem 21 (2020) 100166
[7] J. Charvot, D. Pokorný, R. Zazpe, R. Krumpolec, D. Pavlinák, L. Hromádko, J. Prikryl, J. Rodriguez-Pereira, M. Klikar, V. Jelínková, J. M. Macak, F. Bures, ChemPlusChem 85 (2020) 576-579

MoTe₂ is a transition metal dichalcogenide (TMD), which has been intensively studied in the last few years. It exists in several crystalline phases among which the semimetallic T_d and semiconducting 2H exhibit numerous intriguing physical properties. It is well known however, that MoTe₂ shows significant sensitivity to air, which makes ex-situ experiments, involving sample preparation and processing, very challenging. [1-3] The question of stability is especially important in the case of ultra-thin, single and few-layer samples, which provide access to the most interesting physics.
In this paper we address the problem of low stability of MoTe₂ and propose a very efficient protective capping, that stabilizes transport properties of the layers. A molecular beam epitaxy technique has been used to grow ultra-thin MoTe₂ films in 2H-semiconducting phase on large area GaAs (111)B substrates. A transmission electron microscopy study reveals the high quality of the MoTe₂ films. Directly after the growth, MoTe₂ films have been in-situ capped with a thin (3 nm) layer of Al, which oxidizes after exposure to ambient conditions. This oxide serves as a protective layer to the underlying MoTe₂ and it is thin enough to make good-quality electrical contacts. Wide range temperature resistivity studies showed that charge transport in MoTe₂ is realized by hopping with an anomalous hopping exponent of x ≈ 0.66, reported also previously for ultra-thin, metallic layers. We demonstrate that this approach provides a significant improvement of the MoTe₂ stability. Upon exposure to pure nitrogen and air atmospheres, the resistivity of unprotected sample changes by more than 900%, whereas the protected sample shows electrical stability at the level of 1% over one week and 5% in the timescale of several months. [4]

[1] H. Diaz et al., 2015, 2D Materials 2
[2] M. Yamamoto et al., 2013, J. Phys. Chem., 117
[3] G. Mirabelli et al., 2016, J. Appl. Phys., 2016, 120, 125102
[4] Z. Ogorzalek et al., Nanoscale, 2020, 12, 16535-16542

Since the discovery of buckminsterfullerene in 1985, nanocarbon materials have played a crucial role in modern materials science. Recently, significant efforts have been redirected towards exploring ‘post-nanocarbons’. Over the past decade, 2D layers of transition metal chalcogenides (TMCs) have been widely recognized as ‘beyond graphene’ owing to their versatile chemistry. On the other hand, their 1D counterparts such as nanoribbons, nanotubes, and nanowires could exhibit the unique electrical and optical properties, significantly distinct from 2D TMCs as well as 1D nanocarbons. However, exploring their potential has been hampered by their limited availability. Although these materials have been prepared using chemical and lithographic methods, the reliable production of well-defined 1D TMCs remains a significant challenge.
Here we report atomically precise fabrication of 1D TMCs within carbon/boron-nitride nanotubes (CNTs/BNNTs). Chemical reactions confined inside the NTs promote and stabilize the bottom-up growth of 1D TMCs, allowing their easy handling and characterization. We found that choosing suitable precursors and diameters of the host NTs provides a variety of morphologies including nanoribbons, nanotubes, and nanowires. Atomic-level transmission electron microscopy enabled us to observe dynamic torsions of the nanowires inside CNTs, not seen in the bulk (M. Nagata et al., Nano Lett. 2019; N. Kanda et al., Nanoscale 2020). Furthermore, we have recently investigated electronic structures of 1D TMCs within BNNTs. Our findings suggest that these 1D TMCs could provide new building blocks for future nanoelectronics.

Transition metal chalcogenide (TMC) nanowires have attracted much attention in recent years because of their one-dimensional (1D) metallic properties and electronic device applications [1-5]. To date, many groups have reported the preparation and properties of individual and bundled TMC nanowires including as Mo₆S₆[2], Mo₆Se₆[3], and Mo₆Te₆ [4,5]. Recently, our group has achieved large-area growth of TMC nanowires using chemical vapor deposition (CVD) [6]. Depending on the affinity with substrate surface, this technique also enables network formation of either aligned atomically thin two-dimensional (2D) sheets or random network of three dimensional (3D) bundles composing of individual nanowires. For these samples, it is expected that the morphology such as bundle size will strongly affect the electron transport properties of TMC nanowire networks.
In this study, we have investigated the temperature dependence electron transport of WTe nanowire networks with different bundle sizes. The samples were grown on SiO₂ or Al₂O₃ surface using CVD, and the network formation was confirmed using scanning electron microscope (SEM). At room temperature, the sheet resistances of samples vary from 2 kΩ to 75 MΩ depending on the nanowire density and bundle size. The thick bundle network shows a metallic behavior from room temperature to 20 K, whereas a power-law dependence is observed for the thin bundle network. In the presentation, we will discuss a possible mechanism derived from 1D metallic behavior for the power-law dependence.

[1] L. Venkataraman et al., Phys. Rev. Lett., 83, 5334 (1999).
[2] L. Venkataraman et al., Phys. Rev. Lett., 96, 076601 (2006).
[3] J. Kibsgaard et al., Nano Lett., 8, 11 (2008).
[4] J. Lin et al., Nat. Nanotechnol., 9, 436 (2014).
[5] M. Nagata et al., Nano Lett., 19, 4845–4851 (2019).
[6] H.-E. Lim et al. Submitted.

Two-dimensional (2D) van der Waals (vdW) materials has experienced an explosive growth after graphene, and new familes of 2D systems and block-layered bulk materials have been discovered [1]. The possibility of tuning their electronic properties via structural parameters make the layered vdW materials attractive from both fundamental and device engineering points of view. However, the weakness of vdW interlayer forces lead in general to systems undergoing drastic changes as a function of subtle variation in growth conditions. Finding controllable fabrication processes of such systems has proven challenging [2,3]. Within the scope of exploiting intriguing phenomena related to topological surface states and symmetry-breaking magnetic states, epitaxy of topological insulator materials having atomic layers of Mn has been pursued [4].
Simulations of X-ray scattering and diffraction are well-established procedures for structural analysis at nanometer and subnanometer length scales of layered materials, ranging from amorphous films to crystalline ones such as epitaxial layers on single-crystal substrates. Higher are the ordering in stacking sequences of the atomic layers, the more pronounced are the diffracted intensities at higher angles allowing more refined model structures. X-ray theories are well comfortable at the limiting cases, either amorphous films or perfect periodic layer sequences, that is crystalline films. However, in developing new materials and processing technologies, layered materials with random layer sequences of large d-spacing can often be found. Combined with the very high dynamical range of advanced X-ray sources and instruments, this kind of material represent a challenging in theoretical approach for X-ray diffraction simulation [5].
In this work, we describe how to adapt a general recursive equation for simulating X-ray dynamical diffraction in layered materials to the case of thin films of MnxBi2Te3+x grown by molecular beam epitaxy on BaF2 (111) substrates as a function of MnTe source temperature. The films are staking of n vdW bounded MnBi2Te4 septuple layers (SLs) where the occurance of m Bi2Te3 quintuple layers (QLs) lead to composition x = n/(n+m). Interface quality, random stacking sequences, surface finishing, and evolution of defects during growth are parameters investigated by curve fitting with the recursive equation. It demonstrates the effectiveness of this approach for describing X-ray diffraction in layered systems ranging from random to perfect periodic stacking sequences of atomic layers.

[1] M. M. Otrokov, I. P. Rusinov, M. Blanco-Rey, M. Hoffmann, A. Yu. Vyazovskaya, S. V. Eremeev, A. Ernst, P. M. Echenique, A. Arnau, E. V. Chulkov. Phys. Rev. Lett. 122, 107202 (2019).
[2] S. L. Morelhão, S. W. Kycia, S. Netzke, C. I. Fornari, P. H. O. Rappl, E. Abramof. J. Phys. Chem. C 123, 24818 (2019).
[3] C. I. Fornari, E. Abramof, P. H. O. Rappl, S. W. Kycia, S. L. Morelhão. MRS Advances (2020). doi: 10.1557/adv.2020.202.
[4] J. A. Hagmann, X. Li, S. Chowdhury, S.-N. Dong, S. Rouvimov, S. J. Pookpanratana, K. M. Yu, T. A. Orlova, T. B. Bolin, C. U. Segre, D. G. Seiler, C. A. Richter, X. Liu, M. Dobrowolska, J. K. Furdyna. New J. Phys. 19, 085002 (2017).
[5] S. L. Morelhão, C. I. Fornari, P. H. O. Rappl, E. Abramof. J. Appl. Cryst. 50, 399-410 (2017).

An exciting prospect for 2D materials is their ability to be integrated into heterostructures, where the selection and sequencing of stacked layers facilitate tailored properties. One form of such heterostructures are non-epitaxial chalcogenide superlattices with constituents interleaved at nanometer length scales. These materials can be prepared from amorphous—but intentionally layered—thin films which are gently heated to crystallize into a superlattice that mimicks the layer sequence of the precursor. Such facile synthesis enables superlattices with nearly limitless permutational flexibility, allowing for systematic explorations of how different layer arrangements affect the structure and properties of the composite material.

This talk will primarily focus on materials containing n monolayers of TiSe₂ interleaved with m monolayers of PbSe and/or SnSe in a repeating superlattice. These structures can be described by the chemical formula [(MSe)_1+δ]_m[TiSe₂]_n, where M=Sn or Pb and δ is a compositional misfit parameter. We will discuss several homologous series wherein the relative fractions of constituents are successively modified by changing the integral number of layers (m and n).

Changes to [(MSe)_1+δ]_m[TiSe₂]_n layer sequencing induce changes to the composite materials' semi-metallic transport properties . In both [(PbSe)_1+δ]_m[TiSe₂]_n and [(SnSe)_1+δ]_m[TiSe₂]_n electrons exchanged between the layers are the primary source of mobile carriers while the amount of charge exchange required to equilibrate the chemical potential across the interface is dependent on the thicknesses and identities of layers. Thus, m and n can be used to tune the carrier density with minimal change to carrier mobility since no ionized dopants are introduced into conduction paths. In some cases, the sequencing can also affect the local structure within the layers. For example, in [(PbSe)_1+δ]_m[TiSe₂]_n superlattices both constituents are structurally rigid and insensitive to the thickness of the layers. Conversely, in [(SnSe)_1+δ]_m[TiSe₂]_n the nanolayers of SnSe exhibit a rich polymorphism dependent on thickness, which is attributed to a combination of low polymorph energies and strong interfacial interactions with TiSe₂.

Motivated by successful synthesis of superlattice materials in several selenide systems, we have recently reported on [(SnS)_1+δ]_m[TaS₂]₁ superlattices, marking the first application of this synthesis approach to sulfur-based chemistries. As part of this work, we employ graded precursor films that enable parallel preparation of multiple stacking sequences with similar monolayer control. These results are a promising first step towards similar systematic investigation of structure and properties in another chalcogenide class.

Reducing dimensionality has led to a new realm of materials exhibiting interesting structural, electronic, and catalytic properties. While two-dimensional materials are the focus of many theoretical and experimental research groups, one-dimensional materials are less well studied. Most of the theoretical work on 1D materials is focused on explaining phenomena at the atomic scale rather than performing a high-throughput search of new inorganic nanotube materials.

Here, we investigate by means of density functional theory (DFT) sub-nanometer tubes that exhibit different properties compared to their 2D counterparts while being selective in size and composition. These nanotubes are obtained by rolling up 2D Janus dichalcogenide sheets, where the strain between the two sides of the 2D layer is the driving force for the formation of the nanotube. We will elucidate designing rules to produce nanotubes with controlled dimensions and to guide the search towards nanotubes that can be used in a variety of applications. In detail, we focus on properties like bandgaps, stabilities, and adsorption energies that lead to predictive models assisting in designing new nanotubes before doing DFT calculations. The generalization from existing data helps to accelerate the search for novel materials for applications in batteries, (photo-)catalysis, chemical storage, and nanofluidics.

The Cu₂BaSnS₄ (CBTS) and Cu₂SrSnS₄ (CSTS) compounds have been recently investigated as wide band gap photovoltaic absorbers for tandem solar cells. Unlike their parent compound Cu₂ZnSnS₄, which crystallizes in a fully tetrahedral zincblende-like structure, the large size of Sr and Ba leads to a less-than-3D trigonal structure in CBTS and CSTS. While Cu and Sn remain tetrahedrally coordinated, Ba and Sr are 8-fold coordinated, and their coordinating S atoms form square antiprisms with open spaces between them.

By combining photoluminescence experiments and first-principles defect calculations, we show that the defect properties of CBTS and CSTS are staggeringly different from the defect properties of the much-investigated CZTS compound. Such differences can be explained in terms of their unconventional crystal structure, as well as in terms of cation size mismatch. We demonstrate that the sub-band gap tail states plaguing CZTS photoabsorbers have almost completely disappeared in both CBTS and CSTS. Nevertheless, a high density of (probably interstitial) defects are observed deep into the band gap, due to the open spaces available in the complex crystal structure of CBTS and CSTS.

Based on these observations, we develop a theory to explain the defect properties of a large set of I₂-II-IV-VI₄ chalcogenide semiconductors and predict the most promising chemical spaces to find defect-tolerant compounds. Our analysis includes defects responsible for sub-band gap tail states, as well as defects responsible for deep recombination centers.

Nanoscale specimens of electronic materials as diverse as 2D graphene and MoS₂ and 3D silicon and diamond are known to be deformable to large strains without inelastic relaxation. The introduction of strain within the elastic regime has become a widely employed strategy for providing unique and exciting electronic and photonic properties in both 3D and low-dimensional materials. These discoveries harbinger a new age of deep elastic strain engineering of the band structure and device performance of electronic materials. Many possibilities remain to be investigated as to what graphene can do as the most versatile 2D material or what semiconductor materials such as MoS₂ and hBN can offer after overcoming their present commercial immaturity.

Deep elastic strain engineering explores full six-dimensional (6D) space of admissible nonlinear elastic strain and its effects on physical properties. However, the complexity of controllably engineering materials properties by strains necessitates first-principles computations to first screen for a desirable figure-of-merit and then design an optimal straining pathway. The reduced dimension strain space accessible to low-dimensional materials would thus become easy to navigate if we have the model for the much larger 6D strain space. In our work, to map the 6D strain space fully, we first developed a general method that combines machine learning and ab initio calculations to guide strain engineering whereby properties and performance for 3D and 2D materials could be designed. This method invokes machine learning algorithms and utilizes a limited amount of ab initio data for the training of a surrogate model, predicting electronic bandgap within an accuracy of 8 meV [1]. On top of this, attempts have been made by us in developing a more versatile machine learning framework that adopts convolutional neural networks, solid-state physics informed data representation scheme, and a new active learning algorithm to discover the indirect-to-direct bandgap transition in a material by scanning the entire strain space [2]. Through this framework, we considerably improve the state-of-the-art set by ourselves and achieve enhanced performance in every front, including more accurate bandgap and band structure prediction, band extrema detection, and effective mass calculation. Combining this method with experimentally validated finite-element simulations, we identified the most energy-efficient strain pathways that would reversibly transform an ultrawide-bandgap material, such as hBN or diamond, to a metalized state without phonon stability [3]. Our approach could also be used in finding phase transitions within six-dimensional strain space for different crystallographic orientations of a target material. As an example, we explored the phonon-instability conditions that promote phase transition to graphite in diamond.

The fast and reliable inference of the proposed model opens a path towards analyzing and scrutinizing general band structures (electronic, phonon, magnon, photonic bands) in the vast 6D strain space. Our AI-driven model is even easier to be applied for studying 2D materials such as chalcogenides that are accessible to a lower strain space dimension, auguring unique opportunities for tailoring low-dimensional materials properties via strain engineering for electronic, photonic, quantum and energy applications.

[1] Z. Shi, E. Tsymbalov, M. Dao, S. Suresh, A. Shapeev, J. Li. Proc. Natl. Acad. Sci., (2019) 116, 4117-4122
[2] E. Tsymbalov, Z. Shi, M. Dao, S. Suresh, J. Li, A. Shapeev (2020) submitted
[3] Z. Shi, M. Dao, E. Tsymbalov, A. Shapeev, J. Li, S. Suresh (2020) under review

Complex inorganic materials are usually synthesized by an ‘addition’ approach, whereby simple precursors are reacted to form a target multicomponent phase. However, if the simple precursors are already very stable themselves, then there may be little free energy left to drive a chemical reaction to a higher-component phase, which often results in slow kinetics and long-lived reaction impurities. Here, we demonstrate that metathesis reactions operate in a fundamentally different synthesis paradigm. Instead of a conventional reaction of M + X --> MX, a metathesis reaction is driven by a salt byproduct like NaCl, in a reaction typified by MCl + NaX --> MX + NaCl. Because Na+ and Cl- are separated in the precursor, but are then rejoined in the product, these metathesis reactions are often highly exothermic. The inclusion of additional species like Na+ and Cl- can also relocate the ‘thermodynamic sink’ of the phase diagram to a new composition—enhancing structure-selectivity towards a target multicomponent phase. Here, we demonstrate how metathesis reactions facilitate the synthesis of metastable ternary molybdenum nitrides [1] and the difficult-to-synthesize MgCr2S4 thiospinel chalcogenide for Mg-battery applications [2]. Not only can metathesis reactions facilitate the synthesis of the desired phase, they are also routes to low-dimensional nanoparticles and nano-porous compounds. The additional degrees of freedom afforded by including extra elements into the synthesis reaction opens up a vast and promising design space for creative new metathesis reactions to chalcogenides and pnictides.

[1] J. Odahara et al., "Self-Combustion Synthesis of Novel Metastable Ternary Molybdenum Nitrides." ACS Materials Letters (2019).
[2] A. Miura, et al. "Selective metathesis synthesis of MgCr2S4 by control of thermodynamic driving forces." Materials Horizons 7.5 (2020).

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) have attracted extensive research interests for applications in optoelectronics, spintronics, photovoltaics, and catalysis. Just as in bulk semiconductors, 2D semiconductors contain both intrinsic defects, e.g., vacancies and antisites, and extrinsic defects, e.g., substitutional and interstitial dopants and impurities. Due to the reduced electronic screening in 2D materials, point defects are expected to have an even stronger impact on the electronic properties of these systems compared to in bulk semiconductors. Understanding the effect of defects, dopants, and impurities and how they respond to synthesis and processing conditions is critical to fully realize the potential of 2D TMDCs for applications in electronic and optoelectronic devices.

In this talk, we present first-principles calculations at multiple theory levels to investigate the energetics, electronic structure, and optical response of nitrogen-doped monolayer WS₂. We perform density-functional theory (DFT) calculations with the PBE, SCAN, and HSE functionals to determine the defect formation energies and charge transition levels (CTLs), utilizing the Freysoldt-Neugebauer charge correction [1] to restore the appropriate electrostatic boundary conditions for charged defects in 2D materials. Many-body perturbation theory calculations within the GW approximation using a novel convergence acceleration technique provide accurate quasiparticle levels to correct the DFT band edge positions and CTLs. We show that it is energetically favorable for N atoms to substitute at S sites, forming a paramagnetic defect with a CTL close to the middle of the bandgap. When aligned to a common electrostatic potential reference, the absolute position of the CTL remains almost consistent across all levels of theory, while the positions of the band edges exhibit a strong dependence. Finally, we model the optical response of N-doped WS₂, including excitonic effects, by solving the Bethe-Salpeter equation on top of GW. Based on our results, we suggest possible optical transitions between the defect level and the valence or conduction bands, which may be observable with photoluminescence and other optical spectroscopy techniques. The approach used in this study can be extended to study other defects and dopants in WS₂ and other 2D TMDCs [2].

[1] C. Freysoldt and J. Neugebauer, Phys. Rev. B 97, 205425 (2018)
[2] A. M. Z. Tan, C. Freysoldt, and R. G. Hennig, Phys. Rev. Mater. 4, 064004 (2020)

Thermoelectric figure-of-merit (ZT) is the overall most important physical quantity for a thermoelectric material. It ultimately determines the energy conversion efficiency. However, over the years, higher and higher ZTs were reported without confirmation since it requires zero-resistance contact that is as difficult as, if not more than, the material improvement. Therefore, how to confirm the ZT of any material without making the needed contact is urgently needed. In this report I will present our detailed study on reliable efficiency measurement on materials without making the needed contact.

Following the interest in graphene since its first isolation by mechanical exfoliation in 2004, the broader family of two dimensional (2D) materials has been the subject of extensive attention thanks to their unique properties and atomically thin structure. In particular, transition metal dichalcogenides (TMDC) materials have shown unique optical and electrical properties, such as band structure transitions, semiconducting transport behavior, and strong photoluminescence, which are distinct from those of graphene and other carbon allotropes. TMDC materials are also intriguing for optical, electrical, and thermoelectric applications, especially in few-layer forms. Although electron transport in TMDC materials has been widely studied, there have been only limited experimental data published on thermal transport in WSe₂ which is the crucial parameter in their thermoelectric applications. Therefore, more robust experiments and modeling are needed to explain this trend or to provide more accurate values. Meanwhile, the large disparity in thermal conductivities predicted by theory also motivates further experimental investigation. Here we demonstrate an improved experimental method for measuring thermal conductivities of thin TMDCs. This is used to provide new measurements of the thermal conductivity of single-, bi-, and tri-layer WSe₂ (1L, 2L, 3L WSe₂). The systematic experimental results enable comparisons of experimental results between different layer number, different material conditions.

The optothermal Raman technique has been the most successful method for measurement of thermal conductivity of 2D materials. In this technique, a laser is focused at the center of a flake and used to measure the position of a Raman-active mode. As the laser power is increased, the sample is heated, which enables red-shift Raman mode due to thermal softening. Thermal modeling can then be used to extract the thermal conductivity from the measured shift rate.

The thermal modeling used for determination of thermal conductivity requires additional input of a number of parameters: the rate of mode softening with temperature; optical absorption; and the thermal conductance of the supported area of the flake. However, in previous work, typically only the mode softening rate is directly measured, whereas the values of other parameters are derived from published values or assumed. Here we present measurements of the thermal conductivity of WSe₂ in which all of these parameters are determined experimentally. Molecular Dynamics simulations are performed to confirm the thermal transport properties trend from 1L to 3L, and supports the theoretical model building.

In conclusion, we have used optothermal Raman technique to study thermal transport properties of 1L to 3L WSe₂, an emergent efficient thermoelectric material. This work addresses several important issues in the measurement of thermal conductivity of the 2D materials WSe₂ using Raman spectroscopy. We derive the thermal conductivity values of 1L to 3L WSe₂ and their interfacial thermal conductance with substrate. These results are of significance to discover the thermal transport properties of 2D transition metal dichalcogenides with thermoelectric applications, and thus develop the efficient 2D thermoelectric materials.

One of the most distinct features of van der Waals (vdW) heterostructures based on two-dimensional (2D) materials is the ultrafast photo-induced interlayer charge transfer, which is known to occur in sub-picosecond time scales. Such ultrafast dynamics offers promise for the realization of novel hot carrier optoelectronic devices where non-thermalized photocarriers are harnessed to achieve unconventional functions. In this talk, we will discuss a few examples of hot carrier optoelectronic phenomena in heterostructures of 2D transition metal dichalcogenides. First, we will discuss realization of near-infrared (NIR) to visible light upconversion in a metal-insulator-semiconductor (MIS) vdW heterostack tunnel diode [1]. We show that the upconversion is enabled by interlayer transfer of non-thermalized photoexcited holes and its dynamics can be tuned by orders of magnitude by applied electric fields. Second, we will discuss unique photoresponse of these MIS devices arising from hot carriers generated by efficient exciton-exciton annihilation (EEA) in monolayer MoS₂ [2]. We describe how EEA can result in highly energetic electrons and holes due to uneven distribution of annihilation energies following energy- and momentum-conserving scattering.

[1] Linardy et al. “Electro-Optic Upconversion in van der Waals Heterostructures via Nonequilibrium Photocarrier Tunneling” Adv. Mater. In Press (DOI: 10.1002/adma.202001543).
[2] Linardy et al. “Harnessing Exciton–Exciton Annihilation in Two-Dimensional Semiconductors” Nano Lett. 2020, 20, 1647.

Antimony selenide (Sb₂Se₃) has emerged as a strong candidate for practical water splitting due to its suitable bandgap, excellent optoelectronic properties, and resistance to photocorrosion in strongly acidic electrolyte solutions. In this talk, I will discuss the synthesis of Sb₂Se₃ thin films by selenization of antimony metal and the photoelectrochemical properties of these films after coating with a molybdenum sulfide hydrogen evolution catalyst. The performance of these photocathodes was greatly enhanced by a relatively low temperature sulfurization treatment with elemental sulfur, greatly improving the photovoltage and the fill factor. The origin of the improvement upon sulfurization was investigated with time-resolved microwave conductivity (TRMC) measurements and photoluminescence. TRMC of our Sb₂Se₃ reveals high mobilities of the photogenerated charge carriers (>10 cm² V^-1 s^-1) and a low trap density, in contrast to other emerging photoelectrochemical materials such as Fe₂O₃, CuFeO₂, and BiVO₄. We find that the sulfurization treatment of the Sb₂Se₃ thin films yields improved charge carrier lifetimes. Not only is surface recombination curtailed, but deep traps in the bulk are eliminated (most likely Sb-Se antisite defects), which reduces non-radiative recombination of the photogenerated charge carriers. A low temperature sulfurization treatment is thereby identified as a simple method to remove bulk traps in Sb₂Se₃ thin films.

Two-dimensional (2D) materials have seen a surge of interest ever since the discovery of graphene, a single 2D layer of carbon atoms (graphene), more than a decade ago. 2D chalcogenides are a fundamental class in the study of low-dimensional science and their synthesis is an important goal of solid state chemistry. The presentation will focus on how novel chalcogenides can form using design concepts such as the “dimensional reduction”. The new materials exhibit diverse centrosymmetric and noncentrosymmetric structures that lead to a variety of attractive optoelectronic properties such as direct bandgaps, high X-ray absorption, neutron detection and strong nonlinear optical responses.

Layered chalcogenides hold promise for manipulating light in confined geometry, such as in photonic integrated circuits, where key material properties include strong and controllable light-matter interaction, and limited optical loss. Layered materials feature strong anisotropy and a panoply of polymorphs that suggest a rich design space for highly-nonperturbative photonic integrated devices based on functional martensitic transformations: phase changes and ferroelastic domain switching. These phenomena are manifest in materials with band gap above the photonics-relevant near-infrared (NIR) spectral band (~ 0.5 – 1 eV), meaning that they can be harnessed in refractive (i.e. non-absorptive) applications.

Transition metal dichalcogenides (TMDs) offer a vast composition space within which to optimize refractive index contrast, optical loss, and switching energy. We are using a strategy of alloy design to optimize new, active materials for optical phase shifters. We will present our theoretical predictions of thermodynamics and optical properties of binary systems of sulfur-based TMDs: ZrS₂-MoS₂, TiS₂-MoS₂, TiS₂-ZrS₂, and MoS₂-CrS₂. We will present our experimental measurements of the NIR optical properties of single crystals. We will then present progress on making large-area thin films of sulfur-based TMDs, including the theoretically-predicted alloys, by a two-step method of metal sulfurization. Finally, we will present the optical properties and phase-change functionality of our designed TMD alloy thin films.

We will also present an alternative paradigm of strong, non-perturbative refractive index modulation: electric field-driven ferroelastic domain switching in low-symmetry layered materials. We predict that in materials such as SnSe, a sufficiently-strong electric field will induce rotation between equivalent crystallographic domains, with strong refractive index contrast for linearly-polarized light. We will present simulation-based design of photonic integrated devices utilizing the trirefringence of materials such as SnSe and black phosphorous, and experimental results demonstrating non-thermal domain switching triggered by fast laser pulses.

Optical properties of atomically thin transition metal dichalcogenides are dominated by exciton resonances exhibiting large oscillator strength and binding energies. In vertically stacked bilayers, a host of new exciton resonances emerge, lending new opportunities to discover new science and enable new devices. In this talk, I will focus on how moire superlattices change exciton resonances and diffusion. We discovered recently that phonon spectra are renormalized in moire superlattices as well. A rich variety of properties can be controlled by tuning the twist angle between the bilayers. I make some comparisons between samples that are grown using chemical vapor deposition and those prepared by mechanical exfoliation and stacking. Our work is supported by DOE under grant DE-SC0019398, NSF DMR-1808042, DMR-1720595, and the Welch Foundation F-1662.

Second harmonic generation (SHG), as a nonlinear optical effect, has important applications in photonics for frequency conversion and light modulations. Materials with high SHG efficiency and reduced dimensions are favorable for integrated photonics and novel nonlinear optical applications. Here, we fabricate MoS₂ nanoscrolls with different chiralities and study their SHG performances. As a 1D material, MoS₂ nanoscroll shows reduced symmetry and strong chirality dependency in the polarization-resolved SHG characterizations, which is in stark contrast with the 2D MoS₂ layers. This SHG performance can be well explained by the coherent superposition theory of second harmonic field of the nanoscroll walls. MoS₂ nanoscrolls with certain chiralities in our experiment can have SHG intensity up to 98 times stronger than that of monolayer MoS₂. The same chirality-dependent SHG can be expected for nanoscrolls or nanotubes composed of other non-centrosymmetric 2D materials, such as WS₂, WSe₂, and hBN. The characterization and analysis results presented here can also serve as a non-destructive technique to determine the chiralities of these nanoscrolls and nanotubes.

Metal chalcohalides are candidates for next-generation technologies including energy conversion, information storage, and quantum computing. I will present an exploration of V-VI-VII ternary chalcohalides, where V = Sb, Bi; VI = O, S, Se, Te; and VII = F, Cl, Br, I. These include the photoferroic compound SbSI [1,2] and the solar fuels photoelectrode BiOBr [3,4]. We consider 32 elemental combinations in 4 spacegroups using a combination of density functional theory (structural and thermochemistry) and many-body QSGW theory (relativistic electronic band structures). The materials are found to exhibit a wide range of electronic structures and bandgaps from metallic to over 5 eV. The large spin-orbit coupling in the heavy cations strongly influences the electronic structure. This research is part of a collaboration with the groups of David Scanlon (UCL) and Mark van Schilfgaarde (KCL), and used the UK National Supercomputer ARCHER.

[1] "Ferroelectric materials for solar energy conversion: photoferroics revisited" K. T. Butler et al, Energy Environ. Sci. 8, 838 (2015)

[2] "Quasi-particle electronic band structure and alignment of the V-VI-VII semiconductors SbSI, SbSBr, and SbSeI for solar cells" K. T. Butler et al, Appl. Phys. Lett. 108, 112103 (2016)

[3] "Bismuth oxyhalides: synthesis, structure and photoelectrochemical activity" D. S. Bhachu et al, Chem. Sci. 7, 4832 (2016)

[4] "Interplay of orbital and relativistic effects in bismuth oxyhalides: BiOF, BiOCl, BiOBr, and BiOI" A. M. Ganose et al, Chem. Mater. 28, 7, 1980 (2016)

In recent years tremendous efforts have been devoted to the research on two dimensional materials. Their unique structures and remarkable properties have offered great potential for a wide range of applications in electronics, optoeletronics, valleytronics, catalysis, etc. The synthesis of high quality large area mono- and few-layer 2D materials is highly desirable for their applications. In this talk I will present our recent development on novel 2D material and structures, together with further understanding regarding the chemical vapour deposition (CVD) synthesis process of monolayer MoS2.

An exciting prospect for 2D materials is their ability to be integrated into heterostructures, where the selection and sequencing of stacked layers facilitate tailored properties. One form of such heterostructures are non-epitaxial chalcogenide superlattices with constituents interleaved at nanometer length scales. These materials can be prepared from amorphous—but intentionally layered—thin films which are gently heated to crystallize into a superlattice that mimicks the layer sequence of the precursor. Such facile synthesis enables superlattices with nearly limitless permutational flexibility, allowing for systematic explorations of how different layer arrangements affect the structure and properties of the composite material.

This talk will primarily focus on materials containing n monolayers of TiSe₂ interleaved with m monolayers of PbSe and/or SnSe in a repeating superlattice. These structures can be described by the chemical formula [(MSe)_1+δ]_m[TiSe₂]_n, where M=Sn or Pb and δ is a compositional misfit parameter. We will discuss several homologous series wherein the relative fractions of constituents are successively modified by changing the integral number of layers (m and n).

Changes to [(MSe)_1+δ]_m[TiSe₂]_n layer sequencing induce changes to the composite materials' semi-metallic transport properties . In both [(PbSe)_1+δ]_m[TiSe₂]_n and [(SnSe)_1+δ]_m[TiSe₂]_n electrons exchanged between the layers are the primary source of mobile carriers while the amount of charge exchange required to equilibrate the chemical potential across the interface is dependent on the thicknesses and identities of layers. Thus, m and n can be used to tune the carrier density with minimal change to carrier mobility since no ionized dopants are introduced into conduction paths. In some cases, the sequencing can also affect the local structure within the layers. For example, in [(PbSe)_1+δ]_m[TiSe₂]_n superlattices both constituents are structurally rigid and insensitive to the thickness of the layers. Conversely, in [(SnSe)_1+δ]_m[TiSe₂]_n the nanolayers of SnSe exhibit a rich polymorphism dependent on thickness, which is attributed to a combination of low polymorph energies and strong interfacial interactions with TiSe₂.

Motivated by successful synthesis of superlattice materials in several selenide systems, we have recently reported on [(SnS)_1+δ]_m[TaS₂]₁ superlattices, marking the first application of this synthesis approach to sulfur-based chemistries. As part of this work, we employ graded precursor films that enable parallel preparation of multiple stacking sequences with similar monolayer control. These results are a promising first step towards similar systematic investigation of structure and properties in another chalcogenide class.

Thermoelectric figure-of-merit (ZT) is the overall most important physical quantity for a thermoelectric material. It ultimately determines the energy conversion efficiency. However, over the years, higher and higher ZTs were reported without confirmation since it requires zero-resistance contact that is as difficult as, if not more than, the material improvement. Therefore, how to confirm the ZT of any material without making the needed contact is urgently needed. In this report I will present our detailed study on reliable efficiency measurement on materials without making the needed contact.