AVS 71 Session 2D-ThP: 2D Materials Poster Session

InSe, a monochalcogenide two-dimensional (2D) semiconductor¹ with a large room-temperature electron mobility of approximately 10³ cm² V⁻¹ s⁻¹, is a promising material for high-sensitivity Hall sensors², ballistic transistors,³ and non-volatile memory applications.⁴The In-Se system contains many different stable phases⁵ including selenium rich In₂Se₃ and selenium deficient In₄Se₃ phases. The synthesis of InSe is challenging due to the narrow stability range of its stoichiometry on the phase diagram and the need for a surface morphology with large grain lateral growth. Therefore, synthesizing high-quality InSe requires a detailed understanding of how the synthesis parameters affect the structure and stoichiometry of In_xSe_y thin films near and within the narrow range of stability for InSe.

The growth of 2D InSe thin films has been achieved using a novel molecular beam epitaxy (MBE) two-step method involving an indium precursor layer, which effectively suppresses the formation of unwanted phases and allows for high-quality films.⁶ This work builds upon this novel approach by investigating how synthesis parameters, including substrate temperature, precursor flux, and deposition time influence the structural and stoichiometric properties of InSe thin films deposited on sapphire substrates. The chemical bonding, crystalline structure, and morphology of the thin films are characterized by X-ray Photoelectron Spectroscopy, Raman spectroscopy, X-ray diffraction and Atomic Force Microscopy. By understanding how these synthesis parameters impact film quality, the optimal synthesis conditions for InSe thin film deposition can be further refined, enhancing the potential for device applications.

References

1. Camara, M., et al., Physical Review B (2002) 65 (12), 125206
2. Bandurin, D. A., et al., Nature Nanotechnology (2017) 12 (3), 223
3. Jiang, J., et al., Nature (2023) 616 (7957), 470
4. Lu, Y.-Y., et al., ACS Applied Materials & Interfaces (2021) 13 (3), 4618
5. Bergeron, H., et al., Applied Physics Reviews (2020) 7 (4)
6. Hsiao, S.-W., et al., Frontiers in Materials (2022) 9, 871003

The development of high-performance gas sensing devices based on transition metal dichalcogenides (TMDs) has generated considerable research interest in the efficient detection of hazardous gases. The sensing performance of TMDs material-based sensors can be effectively enhanced by increasing the substrate's surface area, i.e., creating the porosity in the substrate to increase the number of sites available for interaction with the target gas molecules. Here, we have grown the porosity on the n-type (100) silicon substrate surface to increase the deposition area of molybdenum disulfide (MoS₂). The DC magnetron sputtering is used to synthesize the MoS₂ on a porous silicon (PSi) substrate for NO₂ gas detection. The fabricated sensor MoS₂@PSi exhibited the maximum response at an operating temperature of 120 ℃ at 100 ppm concentration of NO₂ gas. The sensor is able to detect the significant low gas concentration of 50 ppb NO₂ gas at the operating temperature. Also, the selectivity measurements showed the sensor possessed the best sensitivity for NO₂ among H₂, CO, and NH₃ gases, making it a potential sensor for NO₂ gas sensing.

Understanding how WS₂ interacts electronically with organic molecules is very important for hybrid optoelectronics and energy harvesting applications, where charge separation at the interface governs device function. In this work, we investigate a heterojunction between monolayer WS₂ and 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) to study energy level alignment and interfacial charge transfer.

The combined ultraviolet photoelectron spectroscopy and inverse photoemission spectroscopy (UPS/IPES) measurements show that the pristine monolayer WS₂ has an ionization energy (IE) of 6.00 eV and an electron affinity (EA) of 3.44 eV with a 2.56 eV electronic gap, while films of pristine HAT-CN show an IE of 9.48 eV and EA of 5.66 eV with a 3.82 eV electronic gap. These values suggest a type-II energy level configuration at the interface, providing an energetic driving force for electrons to transfer from WS₂ into HAT-CN.

Raman and Photoluminescence (PL) spectroscopies, and X-ray photoelectron spectroscopy (XPS) were performed on 1L-WS₂ with 0, 3.75, 7.5, and 15nm HAT-CN thicknesses. The Raman signature of WS₂ remains consistent across different thicknesses of HAT-CN, suggesting that the WS₂ lattice remains intact despite HAT-CN presence. XPS confirms HAT-CN adsorption on WS₂. PL spectra reveal a >90% quenching of the intrinsic WS₂ emission at around 633nm after HAT-CN deposition without visible PL peak shift. This significant PL quenching indicates the formation of a charge transfer (CT) state at the interface. The large difference in electronic gaps of 1L-WS₂ and HAT-CN suggests that it is not a Frenkel energy transfer. A device with mechanically transferred 1L-WS₂ and HAT-CN is being constructed and tested for photocurrents. We expect to see an external quantum efficiency (EQE) spectra whose absorption in the energy range below the electronic gap of both 1L-WS₂ and HAT-CN will provide direct evidence for the formation of CT states at the interface.

A systematic study on the evolution of the WS₂ VBM and HAT-CN LUMO positions is performed with combined UPS/IPES by evaporating 0, 0.5, 1, 2, 5, and 10nm HAT-CN on 1L-WS₂ on p-Si. This series of energy alignment studies show that the 1L-WS₂/HAT-CN interface shows a slight relaxation of the CT gap, which aligns well with the loss of electrons from WS₂ into HAT-CN layer.

This work demonstrates how tailoring the energy level alignment in hybrid 2D/organic heterojunctions can enable interfacial charge transfer. Our findings underscore the potential of engineering van der Waals interfaces between TMD monolayers and molecular semiconductors for novel excitonic devices and energy conversion applications.

Transition metal dichalcogenides (TMDs) are a class of layered materials that have garnered significant attention for their unique electronic, optical, and mechanical properties. Their tunable bandgap, high carrier mobility makes them ideal candidates for applications in next-generation electronics, optoelectronics, and energy storage devices. This study, based on ab initio molecular dynamics (AIMD) calculations, suggest that pre-fluorinating the MoS₂ surface before Ar plasma bombardment significantly enhances the etching yield and improves surface smoothness. Additionally, we propose a strategy to remove excess fluorine adsorbed on sulfur using low-energy electrons from the plasma. Our results show that F^- ions migrate much faster than neutral F atoms, facilitating their desorption. We also find that when H atoms are adsorbed on the surface, F^- ions diffuse until they encounter an H adatom, leading to the desorption of stable HF molecules. This approach of utilizing low-energy reactive species from plasmas offers an effective method for surface transport and cleaning of electronegative adsorbates, such as halogens, from the MoS₂ surface.

This research was supported by the Princeton Plasma Physics Laboratory under U.S. Department of Energy Prime Contract No. DE-AC02-09CH11466.

To date, following the discovery and isolation of fullerenes, there have been a large number of both experimental and theoretical studies on their research. A specific group includes the so-called endofullerenes— fullerenes with encapsulated atoms or molecules inside their cavities, such structures are of both theoretical and practical interest. In the current study, the possibility of transition of exohedral fullerene Fe+C₆₀ to endohedral fullerene Fe@C₆₀ through scattering of Fe+C₆₀ on the surface of an iron crystal is explored by methods of molecular dynamics.

In this study, we used the LAMMPS software package in combination with a selected Tersoff potential for modeling purposes [1]. To conduct the model experiment, we created two models: a system consisting of Fe and C₆₀ exofullerenes heated to 900 K, i.e. Fe+C₆₀ model (T = 900K), and an iron crystal heated to 300 K (T = 300K). We then isolated a region on the surface of the iron crystal where exofullerenes were deposited normally at a vertical velocity of 4200 m/s (7.15eV, 400 impact points, process time – 5 ps, timestep – 0.0001 ps)

The primary objective of this study was to isolate those fullerenes where the iron atom was encapsulated within the fullerene structure. Dynamic data was acquired and processed as follows: 12 carbon atoms on the lower hemisphere of each fullerene were selected; Vectors were constructed between the iron atom and each of these selected carbon pairs, and the angles formed by these vectors were measured – a total of 66 angle measurements were taken for each of 400 instances. a 400 × 67 data array was generated, where the potential energy value of the iron atom was recorded in a separate column after 5ps model time.

The output data was analyzed using cluster analysis methods, which allowed the classification of fullerenes into groups based on their characteristic geometries. One of the isolated groups included fullerenes with geometries similar to those with endohedral structures, the bond lengths of these fullerenes were compared to those in the study [2]. Based on the simulation results, it is expected that 21-30% of the exofullerenes will transition to endofullerenes under the given initial conditions.

1. K.O.E. Henriksson et al, Journal of Physics: Condensed Matter, 25(44), 445401
2. Cherepanov et al, Crystallogr. Rep. 65, 404–408 (2020)

Monolayer MoS₂, a 2D material, holds enormous promise for transcending the fundamental limits of silicon-based electronics and continuing the downscaling of transistors and logic circuits for energy-efficient computing. However, major research efforts are needed to overcome many fabrication and integration challenges including wafer-scale growth control, doping, contacts, gate stack, and reliability. In this work, we attempt to address one of the challenges, namely wafer-scale synthesis of MoS2. Atomic Layer Deposition (ALD) is one of the most promising techniques for wafer-scale growth of MoS2 due to its conformal, self-limiting, and low-temperature characteristics. We present here a novel discretized ALD recipe for wafer-scale deposition of uniformly thick MoO3, further, to sulfurized to stoichiometric MoS2. This is an alternative approach for ALD to determine temperature and time based on the Arrhenius equation and first -order reaction kinetics. Wafer-scale uniformity, film morphology, composition and crystallinity were measured using a comprehensive set of characterization techniques including ellipsometry, AFM, XPS, Raman, XRD and Photoluminescence measurements.

Over the past decade, atomically thin 2D van der Waals (vdW) materials have become powerful systems to study light-matter interaction and electronic confinement. In monolayer transition-metal dichalcogenides (TMDCs), optical absorption is dominated by tightly bound excitons due to reduced screening and enhanced Coulomb interactions. I present the optical study of moiré excitons in twisted WSe2 homobilayers, where a small interlayer twist generates a periodic potential landscape that traps excitons at specific locations in real space.

To isolate and study these moiré-localized excitons (MXs), I detail the fabrication process of monolayer WSe2 on SiO2, followed by tear-and-stack assembly to produce a clean interface between the twisted homobilayers. Through power-dependent, polarization-resolved, and magneto-optical spectroscopy, I demonstrate that the narrow, quantum dot-like spectral lines originate from localized MXs. These results reveal how structural symmetry, twist angle, and magnetic fields impact valley physics in moire superlattices. The findings offer new opportunities for creating quantum emitter arrays for integrated quantum photonics.

Despite their potential as of being the excellent candidates for advancing CMOS technology to its physical limits, the presence of an opened energy bandgap in either single- or bilayer- silicene poses a significant challenge, hindering its applications in the main stream semiconductor industry. Previous attempts, including applying external electric field, surface decoration, nanopatterning, and applying uniaxial strain along designate directions, have proven insufficient in meeting the stringent demands of CMOS technology concerning operational reliability, processing environment sensitivity, product yield, and achievable processing standards. Recently a number of research reported that applying of the biaxial in-plane strain leads to energy bandgap opening in AA-stacked bilayer silicene, however the maximum energy bandgap opening is limited to 16 meV.