AVS 70 Session 2D-ThP: 2D Materials Poster Session

Compositional control of layered materials has the potential for creating new 2D materials with desirable properties. Certain transition metal dichalcogenides (TMDs) may be modified by incorporating excess metals in between the 2D sheets and covalently linking them. Such an incorporation of metals may be achieved in a topotaxy approach. Topotaxy is a solid-state reaction in which the crystal structure of a starting material is modified by incorporation of elements into the existing crystal structure. Here this is explored for NiTe₂ by reacting it with excess Ni. In our study, NiTe₂ thin films were synthesized by molecular beam epitaxy. Subsequently, the composition of these films can be modified by two different approaches; (i) by thermal annealing inducing loss of Te and (ii) by reaction with excess Ni. In both cases the change in the Ni:Te ratio is monitored by XPS and a structural transformation of the film is observed by LEED and STM. With an increase in the Ni-content a (√3 × √3) R30 superstructure is observed. In STM it can be shown that this new superstructure phase forms separate domains from the pure NiTe₂, suggesting that it is compositional line phase in the Ni-Te phase diagram. The superstructure can be explained by Ni insertion in between NiTe₂ layers. From XPS measurements, the Ni-concentration in the intercalation layer is close to 2/3 of the Ni concentration of a NiTe2 layer. The structural models for the intercalation are discussed based on DFT calculations. In STM nanoscale stripe like ripple networks are observed, which may indicate stacking faults in the intercalation layer due to lattice expansion. This study demonstrates a controlled approach to modify the composition of 2D NiTe₂, which may help to control proposed properties of the layered Ni-telluride system, such as superconducting phases [1] or make it a more efficient electrocatalyst [2]. In general, the approach of modifying NiTe₂ by topotaxy may also allow to modify it with other transition metals than Ni and this will be studied in future work.

Reference:

1. Pan, S. et al. On-Site Synthesis and Characterizations of Atomically-Thin Nickel Tellurides with Versatile Stoichiometric Phases through Self-Intercalation. ACS Nano 16, 11444–11454 (2022).
2. Nappini, S. et al. Transition-Metal Dichalcogenide NiTe2: An Ambient-Stable Material for Catalysis and Nanoelectronics. Adv. Funct. Mater. 30, (2020).

Atomic layer etching (ALE) has emerged as a pivotal technique in the precise fabrication of two-dimensional (2D) materials, particularly molybdenum disulfide (MoS₂), which holds promise in the semiconductor industry due to its high mobility in monolayer form. The ability to precisely etch amorphous and crystalline MoS₂ films provides a pathway for controlling thickness, which is critical to achieving desired electrical and optical properties. Previous studies used MoF₆ and H₂O in thermal ALE of MoS₂. Here, we report studies of alternate sources of fluorination and oxygenation and evaluate their impact on thermal ALE of MoS₂. Oxygen sources include water and ozone and fluorine sources include HF/Pyridine and MoF₆. Etch rates, uniformity, and surface chemistry post ALE were characterized using spectroscopic ellipsometry, atomic force microscopy, and X-ray photoelectron spectroscopy. Preliminary results indicate etch per cycle values of 0.5 A/cycle for MoF₆+H₂O at 200 C, 0.32 A/cycle for HF+H₂O at 250 C, and 0.26 A/cycle for HF+O₃ at 250 C. Additionally, ALE processes were combined with ALD to demonstrate thickness control for achieving few-layer MoS2.

The semiconductor industry is increasingly demanding high-performance, highly integrated circuits and the downscaling of conventional cell size has reached its limits. However, using silicon-based technology into downscaled technology has faced diverse challenges in ensuring high cell transistor performance. Therefore, many researchers are exploring the application of next generation semiconductor materials, such as two-dimensional(2D) metal dichalcogenides (TMDs). TMDs have received significant attention as a novel alternative to silicon in the semiconductor industry because of their excellent electrical, mechanical properties.

To achieve the performance of TMD-based devices, it is very important and essential to have good ohmic contact with Schottky barrier (SB) control. The SB is typically controlled by doping such as ion implantation and thermal diffusion. However, these conventional doping methods are impossible to TMDs because they can damage the crystal lattice or cause defects. This is due to the characteristics of TMDs having a thin layer structure.

Several doping schemes have been investigated to reduce contact resistance in TMD-based devices, including edge contact, interlayer depinning, and surface charge transfer doping (SCTD). Among them, SCTD is an effective doping technique for 2D semiconductors due to its non-destructiveness and simplicity.The SCTD mechanism follows an alignment of Fermi level at the semiconductor and dopant material interface. The difference in work function (WF) between semiconductors and metal oxides is the main factor in charge transfer. When the WF of the metal oxide is larger than that of the semiconductor, the electrons in the semiconductor spontaneously move to the metal oxide, the holes accumulate in the semiconductor, and the energy band at the interface bends upward, eventually the semiconductor is doped p-type. In this study, we introduce a method to oxidize WSe₂ to WO_x using ultraviolet (UV) ozone treatment. By this method, the surface of WSe₂ is changed to WO_x, and WOx is a substance having a larger WF than WSe₂, and acts as a dopant material to perform p-type doping of WSe₂. As a result, we confirmed that the device characteristics of 2D WSe₂-based field effect transistor (FET) were improved.

The SB formed at the interface between the 2D semiconductors and the metals obstructs carrier injection and causes contact resistance to increase. Regardless of metals, an unintended high SB exists due to the Fermi level pinning effect. SCTD can selectively dope 2D semiconductors around metal contacts. Contact doping using SCTD is an effective way to lower the SB and improve the contact characteristics of 2D FETs.

Molybdenum disulfide (MoS₂) in its two-dimensional form represents a leading contender for the fabrication of advanced semiconductors. A critical attribute of MoS₂ is its adjustable bandgap, which varies with material thickness. This characteristic is vital for the tailored production of nanoelectronics devices, aligning with industry requirements for precision and flexibility.In this study, we employ ab initio molecular dynamics (AIMD) to investigate the etching mechanisms of Molybdenum disulfide (MoS₂). Our results provide a basic understanding of plasma assisted etching process. By simulating the interaction of plasma with MoS₂ surfaces, we identify the critical factors that can influence the etching rates and patterns. Our calculations indicate that fluorinating the MoS₂ surface before subjecting it to Ar plasma bombardment can enhance etching yield and improve surface roughness. The presence of fluorine (F) at the surface forms a stronger F-S bond compared to the Mo-S bond, facilitating etching of the surface sulfur (S) atoms without significant damage to the underlying layers. This process is crucial for reducing the surface roughness of the unetched layers. Furthermore, the effect of Ar ion energy on etching yield and surface roughness is also addressed.

With the rise of graphene (Gr) since 2004, two-dimensional (2D) have been extensively explored for energy-efficient nanoelectronics due to their novel charge transport properties compared to conventional three-dimensional (3D) bulk materials. However, there are still challenges and issues for practical implementation of 2D materials. Here we take 2D semiconducting MoS2 as an example to review our recent research of energy-efficient nanoelectronics, ranging from materials synthesis to structure engineering and device demonstration. First, by functionalizing the growth substrate, we can achieve on-demand selective-growth of 2D MoS2 using chemical vapor deposition (CVD) and the electron mobility can be up to 20 cm2/Vs at room temperature. At the interface between MoS2 and SiO2 substrates, an interfacial tension can be induced due to a mismatch of thermal expansion coefficients, which creates an anisotropy of in-plane charge transport [1, 2] as well as a self-formed nanoscroll structure [3]. Second, at the interface between MoS2 and metal contact, a monolayer h-BN decoration can enable novel manipulation of charge transport through quantum tunneling, in contrast with conventional thermionic emission [3]. The contact resistance can be suppressed by both localized and generalized doping using transition metals [4]. Third, at the interface between MoS2 and other 2D materials, band-to-band Zener tunneling and cold-source charge injection can be enabled, giving rise to a superior transport factor (<60 meV/decade) in field-effect transistor (FET) configurations. These novel charge transport can be utilized to overcome the fundamental limit of “Boltzmann tyranny”, and realize tunnel FETs and cold-source FETs with sub-60-mV/decade subthreshold swings [5-7] or novel anti-ambipolar FETs [8]. Fourth, at the interface between MoS2 and ferroelectric or ionic dielectrics, excellent electrostatic gating leads to a superior body factor (<1), and also improves the energy efficiency for transistor operation [9].

Reference

1. H. Li and co-workers, under review by Nature Communication.

2. H. Li and co-workers, DRC, Ann Arbor, MI, p. 133, 2019.

3. H. Li and co-workers, Adv. Mater., vol. 32, no. 2002716, 2020.

4. H. Li and co-workers, Nanoscale, vol. 12, pp. 17253, 2020.

5. H. Li and co-workers, IEEE IEDM, virtual, p.251, 2020.

6. H. Li and co-workers, ACS Nano, vol. 15, pp. 5762, 2021.

7. H. Li and co-workers, ACS Nano, vol. 11, pp. 9143, 2017.

8. H. Li and co-workers, JVST B, vol. 41, no. 053202, 2023.

9. H. Li and co-workers, Nano Express, vol. 4, no. 035002, 2023.

Thermal scanning probe lithography (t-SPL), enabled by the NanoFrazor technology, is a nanolithography technique particularly suitable for patterning, contacting, and modifying 2D materials and nanowires [1-6]. t-SPL generates patterns by scanning a heated ultrasharp tip over a sample surface to induce local changes. By using thermal energy as a stimulus, it is possible to perform various modifications to the sample via removal, conversion, or addition of/to the sample surface. Along with an ultrasharp tip, the t-SPL cantilever contains several other important functions such as an integrated thermal height sensor and an integrated heating element both of which are advantageous for generating devices from nanowires and 2D materials.

Nanowires and 2D materials have been the focus of intense academic and industrial research as these materials provide great promise as next generation electronic devices. However, when patterning electrical contacts to nanowires and 2D materials with conventional fabrication techniques (photolithography, electron beam lithography), the fabrication process becomes challenging and time consuming due to overlay requirements. These techniques can also lead to less than desired device performance due to damage from charged particles or ultraviolet irradiation, as well as contamination from residual resist. The issue of time intensive processing comes from the random positioning of nanowires and 2D material flakes on substrates which makes overlay challenging. This point of overlay is addressed with t-SPL by having an integrated thermal height sensor that allows for a non-invasive, in-situ measurement technique to detect buried nanowires or 2D materials prior to patterning. Such capabilities allow for real-time imaging and markerless overlay with high precision.

Within this presentation, the background and workings of t-SPL will be briefly introduced, nanostructuring on nanowires and 2D materials will be discussed along with electrical and optical device performance for nanowire and 2D material-based devices fabricated using t-SPL.

[1] X. Zheng et al., Nat. Commun., 11, 3463 (2020)

[2] X. Liu et al., APL Mater., 9, 011107 (2021)

[3] A. Conde-Rubio et al., ACS Applied Materials & Interfaces, 14, 37 (2022)

[4] S. Ghosh et al., ACS Appl. Mater. Interfaces, 15, 40709-40718 (2023)

[5] H. Ying et al., Device, 1, 100069 (2023)

[6] L. Shani et al., Nanotechnology, 35, 255302 (2024)

MXene grafted with silane combines the distinctive properties of MXene nanosheets with the functional advantages of silane molecules, thereby enhancing versatility and performance across various applications, from materials science to biomedical engineering. The modification of MXene with silane involves chemically bonding silane molecules onto the surface of MXene nanosheets. Despite this, the bonding mechanism between silane and MXene remains incompletely understood. This study introduces an XPS-based methodology to unequivocally determine the silane-MXene coupling. The approach involves comparing XPS results of pristine MXene with silane-modified MXene and MXene modified with silane-free agents like alcohol. The XPS data reveal the formation of Ti-O-Si bonds exclusively following silanization of MXene, with no such bonds observed in alcohol-modified MXene. Therefore, this analysis underscores the capability of XPS in identifying the bonding nature between silane and MXene.

Transition metal dichalcogenides (TMDs) have attracted significant attention in the semiconductor field due to their unique properties. These properties, such as having a direct bandgap in the visible-infrared range, rich valley physical properties, and strong spin-orbit coupling, make them promising for various applications in electronics, optoelectronics, spintronics, and valleytronics. Furthermore, the distinct physical properties of 2D TMD heterostructures further enhance their potential, leading to a growing interest in researching heterostructures composed of different TMD materials.

Chemical vapor deposition (CVD) techniques have proven advantageous in producing TMD materials and their heterostructures with controlled thickness due to their low cost, high yield, and industrial compatibility. However, challenges, such as different growth windows among different TMD materials when growing high-quality large-area TMD heterostructures, need to be addressed.

This work will investigate new strategies for producing large-area TMD heterostructures, such as MoSe2/WS2, using various chemical vapor deposition (CVD) techniques. We will analyze the properties of these materials, including their band gap, optical phonon modes, and excitons, using Raman and photoluminescence spectroscopy.

Two-dimensional (2D) van der Waals materials and their heterostructures have gained significant interest in the past two decades due to their unique characteristics, including high aspect ratio and specific physical, electrical, and mechanical properties. Heterostructures composed of two or more 2D materials have emerged as key components for applications in metaphotonics, energy storage, transistors, photodiodes, memory, and photovoltaics. Understanding the mechanical properties of multilayered 2D materials is essential for practical applications which often involve advanced synthesis methods, developing new transfer techniques and designing custom platforms and architectures for their reliable evaluation and measurements. Techniques such as buckling or bending metrology, nanoindentation, bulge testing, atomic force microscopy (AFM) deflection tests, and tensile testing are necessary to measure these properties. Expanding the mechanical characterization of heterostructures like graphene and boron nitride, especially when capped with a polymeric layer, is vital for practical applications.

This study presents the synthesis, fabrication and mechanical testing of 2D hybrid heterostacks, consisting of few-layer boron nitride and two-three layered graphene heterostructures synthesized via chemical vapor deposition, capped with polymethyl methacrylate layer and suspended across ~200 µm wide trenches using a combined wet-dry transfer method. The mechanical characterization of the heterostacks was performed using two independent approaches: (a) non-local tensile testing (b) local load-displacement testing employing atomic force microscopy probes, complemented by finite element simulations. Both approaches provided new results, which are in good agreement with each other. Our findings offer new insights into a combined load capacity in complex multi-material two-dimensional hybrid systems, and underscore the potential of 2D hybrid heterostructures for advancing micro- and nano-scale device designs, particularly in applications requiring large-area mechanical stability.