ALD2023 Session NS-TuP: Nanostructures Synthesis and Fabrication Poster Session

Vapor phase infiltration (VPI) is an emerging method synthesize inorganic materials within polymers using vapor-phase reactants. The incorporation of a secondary molecular species within the polymer via VPI can be used to control the mechanical properties, chemical stability, and thermal resistance of these polymers, making this VPI process applicable to many technologies. For example, VPI has been shown to make commercial membrane filters more stable to organic solvents and high temperatures, potentially enabling their use for emerging separation areas. However, only a handful of VPI process chemistries have been explored. To further expand the properties that can be produced using this technique, more diverse reactants need to be tested. This project focuses on testing organic VPI reactants, such as diethylzinc (DEZ) with ethylene glycol (EG) to synthesize zincones (Zn-organic hybrid material). We expose polyethersulfone (PES) membranes to these reactants, measuring diffusion of reactants and the possible successful reaction of these reactants, along with any changes in the mechanical, thermal, and chemical stability of the resulting hybrid membranes. X-ray photoelectron spectroscopy (XPS) is used to measure any reaction products, while scanning electron microscopy (SEM) along with energy dispersive X-ray (EDX) is used for depth profiling of elemental compositions Ultimately, this project focuses on producing hybrid organic-inorganic membranes with greater stabilities at high temperatures and with various chemicals, in the hope that these membranes could be used to separate materials previously inaccessible to polymer membranes, such as organic solvents.

Chalcogenides, especially transition metal dichalcogenides (TDMCs, but also some other chalcogenides like SnS₂ and Sb₂Se₃) have a layered structure similar to graphene, but instead of being a semimetal they offer a wide variety from semiconducting to conducting materials that are interesting for efficient, fast (and possibly flexible) electronics. Their electronic behavior can be strongly influenced by the thickness of the material (e.g. how many sheets are stacked on top of each other) [1].

With atomic layer deposition (ALD) the layer stacking and individual layer thickness can be precisely controlled in the nm scale and several 2D materials have already been deposited with ALD [2], [3]. ALD is also scalable, unlike other methods for the synthesis of 2D materials e.g. exfoliation.

We utilize ALD to deposit superlattice stacks of 2D materials with “spacing” materials in between to examine their electrical properties. An example is our fabrication of a superlattice consisting of SnS₂ and Sb₂S₃ via ALD but results on other systems including TiS₂, PbS and other sulfides will be presented as well.

References

[1] S. Manzeli, D. Ovchinnikov, D. Pasquier, O. V. Yazyev, and A. Kis, ‘2D transition metal dichalcogenides’, Nat. Rev. Mater., vol. 2, no. 8, Art. no. 8, Jun. 2017, doi: 10.1038/natrevmats.2017.33.

[2] G.-H. Park, K. Nielsch, and A. Thomas, ‘2D Transition Metal Dichalcogenide Thin Films Obtained by Chemical Gas Phase Deposition Techniques’, Adv. Mater. Interfaces, vol. 6, no. 3, p. 1800688, 2019, doi: 10.1002/admi.201800688.

[3]M. Mattinen, M. Leskelä, and M. Ritala, ‘Atomic Layer Deposition of 2D Metal Dichalcogenides for Electronics, Catalysis, Energy Storage, and Beyond’, Adv. Mater. Interfaces, vol. 8, no. 6, p. 2001677, 2021, doi: 10.1002/admi.202001677.

Self-assembled block copolymers (BCPs) are promising for the bottom-up, low-cost lithography of functional nanoarchitectures. Especially, BCP thin films can be directly converted into inorganic replicas by sequential infiltration synthesis (SIS), an organic-inorganic hybridization method derived from atomic layer deposition (ALD), which can selectively infiltrate target inorganic materials into one of the polymer blocks in vapor phase. For the high-fidelity infiltration of target materials, alumina is often first infiltrated (“alumina priming”) to overcome weak binding of the precursors of target inorganic materials with BCP templates. However, the effects of priming alumina–an electrical insulator–on the electrical properties of the final inorganic nanostructures have been rarely studied. In this work, we investigate the effects of alumina priming on the structural and electrical properties of ZnO nanowire fingerprint patterns fabricated by SIS using diethylzinc (DEZ) and water vapor on the lamellar pattern of self-assembled poly(styrene-b-methyl methacrylate) (PS-b-PMMA) BCP thin film as a function of the amount of infiltrated AlOx contents controlled by trimethylaluminum (TMA) exposure time during a single alumina priming cycle. We find that the characteristic dimension, chemical composition, and electrical conductivity of synthesized ZnO nanostructures could be fine-tuned by controlling TMA exposure duration. Specifically, increasing TMA exposure time naturally led to improved ZnO infiltration/structural fidelity and increasing feature dimensions (width and height of nanowires), accompanied by elevating Al contents. Counterintuitively, the electrical resistivity of ZnO nanostructure, extracted via transmission line method (TLM) from the two-terminal current-voltage characteristics, was initially decreasing with increasing TMA exposure time, down to 14.3 kΩ∙cm, with corresponding Al concentration of 5.3 at.%., while a further increased TMA exposure duration beyond rendered the resistivity up to two orders of magnitude higher. The observed enhancement in ZnO electrical conductivity by alumina priming could be understood from the well-known case of Al-doped ZnO (AZO), where optimal Al doping in a similar concentration range as in the current study maximizes the ZnO conductivity. The results show that the alumina priming condition typically used for SIS in the field would generally improve the conductivity of infiltration-synthesized ZnO nanostructures, along with their infiltration and structural fidelities.

TMDs are layered materials that can exhibit semiconducting, metallic and even superconducting behavior. In the bulk formula, the semiconducting phases have an indirect band gap. Recently, these layered systems have attracted a great deal of attention mainly due to their complementary electronic properties when compared to other 2D materials. However, these bulk properties could be significantly modified when the system becomes monolayer; the indirect band gap becomes direct. Such changes in the band structure when reducing the thickness have important implications for the development of novel applications, such as high photoluminescence (PL) quantum yield, excellent flexibility, and thermal stability.

Previous studies have demonstrated direct sulfurization of the metal precursor as an effective route to produce large-area TMDs. In this paper, we have produced WS₂/SiO₂ by depositing WO_x thin films directly onto Si wafer followed by sulfurization to produce WS₂/SiO₂ heterostructures. However, ALD technique is well known for its thickness controllability, reproducibility, wafer-level thickness uniformity and high conformality. Here, we grew WO_x films by ALD method, and the synthesized WS₂ layer retained the inherent benefits of the ALD process. The overall experiments and measurement were carried out on our homemade 6” cluster systems, which include ALD, RTP, and XPS modules. The sample transfer inside were under 5x10^-6 torr to avoid air pollution. WO_x films were deposited on Si wafer at ALD moduleat ~230℃. After that, sulfurization process were progressed at RTP module, which connected the sulfurization equipment. This unit heats TAA powder at ~130℃ and results H₂S gas. Lastly, XPS measurements revealed binding energy shift of W4f_{5/2, 7/2}, indicating mostly WO_x converse to WS₂ during the process.

High-quality vanadium dioxide (VO₂) reveals a phase transition from a dielectric to a metallic state at around 340 K. Despite ongoing discussions regarding the underlying cause of this transition. It has been demonstrated that the complex coupling among lattice, charge, spin, and orbital results in an electronic transition that alters the sample's electrical, thermal, and optical properties at the phase transition. This unique property has led to its utilization exploration in various electronic and optoelectronic applications, including high-temperature thermoelectric materials, thin-film resistors, and optical modulators.

We will present the synthesis and characterization of tailor-made VO₂ thin films (2D), nanotubes (1D), and inverse opals (3D) prepared through a combination of thermal atomic layer deposition (ALD, from TDMAV plus water in a custom-built reactor) and subsequent thermal annealing. Temperature-dependent electrical measurements, Raman spectroscopy, and UV-Vis-NIR characterization comprehensively evaluate the electronic phase transition. First, the report will discuss the impact of various parameters during preparation on the thin film quality and the insulator-to-metal transition (IMT), including substrates, ALD parameters, and annealing conditions. Afterward, the fabrication of VO₂ nanostructured electrical devices will be highlighted, based on the optimized recipe for synthesizing VO₂ thin films with a phase transition temperature of around 335 K, a hysteresis width of approximately 10 K, and a remarkable resistance change of about three orders of magnitude. Specifically, the phase transition in electrically contacted, individual core-shell (Si-VO₂) nanowires is shown. Moreover, we show that the applied voltage can trigger the IMT in such an ALD-based one-dimensional VO₂ device. Finally, we will outline the preparation route for switchable photonic crystals based on inverse VO₂ opals.

Based on the reported results, it can be concluded that ALD of VO₂ holds significant promise for the development of functional, switchable materials in 1D (elongated structures), 2D (thin films), and 3D (inverse opals or bulk-like samples) laying a solid basis for future large-scale applications. Exemplarily, energy-efficient, next-generation nanostructured smart windows are conceivable that can dynamically alter their transmission properties in response to external stimuli, such as temperature, voltage, and current.

Layered two-dimensional molybdenum sulfide (MoS₂) has attracted great interest for a promising candidate material for opto-electronics and photo sensors applications due to its unique characteristics such as tunable bandgap, high electron mobility and high current on/off ratio. Significant efforts have been placed to apply MoS₂ in industrial fields, leading to significant progress in the deposition method of MoS₂. ^[1],[2] However, patterning technology for MoS₂ remains a challenge. In particular, 2D materials like MoS₂ have extremely thin and weak interlayer bonding due to the absence of dangling bonds, making it difficult to apply traditional top-down patterning approach. Therefore, we demonstrated a new area-selective deposition method for MoS₂ using self-assembled monolayer (SAM). To prevent the degradation of SAM, the deposition of MoS₂ was carried out using a pulsed metal-organic chemical vapor deposition (MOCVD) method, which allowed for the synthesis of high-quality MoS₂ at a low temperature. The growth of MoS₂ was effectively prevented by the SAM patterned using photolithography processes. The selectivity for MoS₂ according to the length of the SAM backbone was investigated using X-ray Fluorescence spectroscopy and Raman measurement. Additionally, the influence of the SAM coating process on the crystallinity and impurity concentration of the MoS₂ film was confirmed using X-ray diffraction and X-ray photoelectron spectroscopy. Furthermore, the potential of area-selective deposition of MoS₂ using SAM was demonstrated by fabricating a MoS₂ gas sensor.