Atomic Layer Deposition of Al-Doped MoS2: Synthesizing a p-type 2D Semiconductor with Tunable Carrier Density

Ae3[TO3][SnOQ3] (Ae = Sr, Ba; T = Si, Ge; Q = S, Se) and Ba3[CO3][MQ4] (M = Ge, Sn; Q = S, Se): Design and Syntheses of a Series of Heteroanionic Antiperovskite-Type Oxychalcogenides

The heteroanionic materials (HAMs) have attracted more and more attention because they can better balance the functional properties of materials. However, their rational structural design is still a great challenge. Here, by using the antiperovskite Ba3S[GeS4] as a template and calculating the tolerance factor (t) as a reference, eight heteroanionic oxychalcogenides with balanced properties were finally synthesized by a partially group-substitution method. Among them, Ba3[CO3][MQ4] (M = Ge, Sn; Q = S, Se) are centrosymmetric (CS) crystals and realize optimization of band gaps and birefringence. For Ae3[TO3][SnOQ3] (Ae = Sr, Ba; T = Si, Ge; Q = S, Se), thanks to the novel [TO4SnQ3] polyanionic groups for the regulation to the antiperovskite structures and the contributions to the nonlinear optical (NLO) properties, they achieve the structural transition from CS to noncentrosymmetry and accomplish an excellent balance among the critical performance parameters as the potential candidates for the infrared NLO materials, including phase-matchable behavior, wide band gaps (Eg = 3.26-3.95 eV), high laser damage threshold (LDT = 3.2-4.4 × AgGaS2), suitable birefringence (Δn = 0.065-0.098@2090 nm) and sufficiently strong second-harmonic generation responses (about 0.6-0.9 × AgGaS2). Moreover, benefiting from crystallization in the polar space groups, they exhibit ferroelectricity and piezoelectricity at room temperature. As far as we know, this is the first reported fully inorganic antiperovskite ferroelectric. These demonstrate that our strategy is desirable and can provide some unique insights into the development of HAMs or antiperovskite materials with specific functions or structures.

Two-Dimensional Semiconductors and Transistors for Future Integrated Circuits

Silicon transistors are approaching their physical limit, calling for the emergence of a technological revolution. As the acknowledged ultimate version of transistor channels, 2D semiconductors are of interest for the development of post-Moore electronics due to their useful properties and all-in-one potentials. Here, the promise and current status of 2D semiconductors and transistors are reviewed, from materials and devices to integrated applications. First, we outline the evolution and challenges of silicon-based integrated circuits, followed by a detailed discussion on the properties and preparation strategies of 2D semiconductors and van der Waals heterostructures. Subsequently, the significant progress of 2D transistors, including device optimization, large-scale integration, and unconventional devices, are presented. We also examine 2D semiconductors for advanced heterogeneous and multifunctional integration beyond CMOS. Finally, the key technical challenges and potential strategies for 2D transistors and integrated circuits are also discussed. We envision that the field of 2D semiconductors and transistors could yield substantial progress in the upcoming years and hope this review will trigger the interest of scientists planning their next experiment.

Doped MoS2 Polymorph for an Improved Hydrogen Evolution Reaction

Green hydrogen produced from solar energy could be one of the solutions to the growing energy shortage as non-renewable energy sources are phased out. However, the current catalyst materials used for photocatalytic water splitting (PWS) cannot compete with other renewable technologies when it comes to efficiency and production cost. Transition-metal dichalcogenides, such as molybdenum disulfides (MoS₂), have previously proven to have electronic and optical properties that could tackle these challenges. In this work, optical properties, the d-band center, and Gibbs free energy are calculated for seven MoS₂ polymorphs using first-principles calculations and density functional theory (DFT) to show that they could be suitable as photocatalysts for PWS. Out of the seven, the two polymorphs 3H_a and 2R₁ were shown to have d-band center values closest to the optimal value, while the Gibbs free energy for all seven polymorphs was within 5% of each other. In a previous study, we found that 3H_b had the highest electron mobility among all seven polymorphs and an optimal bandgap for photocatalytic reactions. The 3H_b polymorphs were therefore selected for further study. An in-depth analysis of the enhancement of the electronic properties and the Gibbs free energy through substitutional doping with Al, Co, N, and Ni was carried out. For the very first time, substitutional doping of MoS₂ was attempted. We found that replacing one Mo atom with Al, Co, I, N, and Ni lowered the Gibbs free energy by a factor of 10, which would increase the hydrogen evolution reaction of the catalyst. Our study further shows that 3H_b with one S atom replaced with Al, Co, I, N, or Ni is dynamically and mechanically stable, while for 3H_b, replacing one Mo atom with Al and Ni makes the structure stable. Based on the low Gibbs free energy, stability, and electronic bandgap 3H_b, MoS₂ doped with Al for one Mo atom emerges as a promising candidate for photocatalytic water splitting.

Modulated wafer-scale WS2 films based on atomic-layer-deposition for various device applications

Tungsten disulfide (WS2) is promising for potential applications in transistors and gas sensors due to its high mobility and high adsorption of gas molecules onto edge sites. This work comprehensively studied the deposition temperature, growth mechanism, annealing conditions, and Nb doping of WS2 to prepare high-quality wafer-scale N- and P-type WS2 films by atomic layer deposition (ALD). It shows that the deposition and annealing temperature greatly influence the electronic properties and crystallinity of WS2, and insufficient annealing will seriously reduce the switch ratio and on-state current of the field effect transistors (FETs). Besides, the morphologies and carrier types of WS2 films can be controlled by adjusting the processes of ALD. The obtained WS2 films and the films with vertical structures were used to fabricate FETs and gas sensors, respectively. Among them, the Ion/Ioff ratio of N- and P-type WS2 FETs is 105 and 102, respectively, and the response of N- and P-type gas sensors is 14% and 42% under 50 ppm NH3 at room temperature, respectively. We have successfully demonstrated a controllable ALD process to modify the morphology and doping behavior of WS2 films with various device functionalities based on acquisitive characteristics.

Origin of Decomposition in a Family of Molybdenum Precursor Compounds

The bis(tert-butylimido)-molybdenum(VI) framework has been used successfully in the design of vapor-phase precursors for molybdenum-containing thin films, so understanding its thermal behavior is important for such applications. Here, we report the thermal decomposition mechanism for a series of volatile bis(alkylimido)-dichloromolybdenum(VI) adducts with neutral N,N'-chelating ligands, to probe the stability and decomposition pathways for these molecules. The alkyl groups explored were tert-butyl, tert-pentyl, 1-adamantyl, and a cyclic imido (from 2,5-dimethylhexane-2,5-diamine). We also report the synthesis of the new tert-octyl imido adducts, (^tOctN)₂MoCl₂·L (L = N,N,N',N'-tetramethylethylenediamine or 2,2'-bipyridine), which have been fully characterized by spectroscopic techniques as well as single-crystal X-ray diffraction and thermal analysis. We found that the decomposition of all compounds follows the same general pathway, proceeding first by the dissociation of the chelating ligand to give the coordinatively unsaturated species (RN)₂MoCl₂. Subsequent dimerization results in either an imido bridged adduct, [(RN)Mo(μ-NR)Cl₂]₂, or a chloride bridged adduct, [(RN)₂Mo(μ-Cl)Cl]₂, depending on the size of the R group. The dimeric species then likely undergoes an intramolecular γ-hydrogen transfer to yield a nitrido-amido adduct, (RHN)MoNCl₂, and an alkene. Ultimately, the resulting molybdenum species appears to decompose into free tert-alkylamine and Mo₂N or Mo₂C. The thermolysis reactions have been monitored using ¹H NMR spectroscopy, and the volatile decomposition products were analyzed using gas chromatography-mass spectrometry. A key intermediate has also been detected using electron ionization high-resolution mass spectrometry. Finally, a detailed computational investigation supports the mechanism outlined above and helps explain the relative stabilities of different N,N'-chelated bis(alkylimido)-dichloromolybdenum(VI) adducts.

Plasma-Assisted Nanofabrication: The Potential and Challenges in Atomic Layer Deposition and Etching

The growing need for increasingly miniaturized devices has placed high importance and demands on nanofabrication technologies with high-quality, low temperatures, and low-cost techniques. In the past few years, the development and recent advances in atomic layer deposition (ALD) processes boosted interest in their use in advanced electronic and nano/microelectromechanical systems (NEMS/MEMS) device manufacturing. In this context, non-thermal plasma (NTP) technology has been highlighted because it allowed the ALD technique to expand its process window and the fabrication of several nanomaterials at reduced temperatures, allowing thermosensitive substrates to be covered with good formability and uniformity. In this review article, we comprehensively describe how the NTP changed the ALD universe and expanded it in device fabrication for different applications. We also present an overview of the efforts and developed strategies to gather the NTP and ALD technologies with the consecutive formation of plasma-assisted ALD (PA-ALD) technique, which has been successfully applied in nanofabrication and surface modification. The advantages and limitations currently faced by this technique are presented and discussed. We conclude this review by showing the atomic layer etching (ALE) technique, another development of NTP and ALD junction that has gained more and more attention by allowing significant advancements in plasma-assisted nanofabrication.

Atomic Layer Deposition of Large-Area Polycrystalline Transition Metal Dichalcogenides from 100 °C through Control of Plasma Chemistry

Two-dimensional transition metal dichalcogenides, such as MoS2, are intensely studied for applications in electronics. However, the difficulty of depositing large-area films of sufficient quality under application-relevant conditions remains a major challenge. Herein, we demonstrate deposition of polycrystalline, wafer-scale MoS2, TiS2, and WS2 films of controlled thickness at record-low temperatures down to 100 °C using plasma-enhanced atomic layer deposition. We show that preventing excess sulfur incorporation from H2S-based plasma is the key to deposition of crystalline films, which can be achieved by adding H2 to the plasma feed gas. Film composition, crystallinity, growth, morphology, and electrical properties of MoS x films prepared within a broad range of deposition conditions have been systematically characterized. Film characteristics are correlated with results of field-effect transistors based on MoS2 films deposited at 100 °C. The capability to deposit MoS2 on poly(ethylene terephthalate) substrates showcases the potential of our process for flexible devices. Furthermore, the composition control achieved by tailoring plasma chemistry is relevant for all low-temperature plasma-enhanced deposition processes of metal chalcogenides.

Thermal Stability and Decomposition Pathways in Volatile Molybdenum(VI) Bis-imides

The vapor deposition of many molybdenum-containing films relies on the delivery of volatile compounds with the general bis(tert-butylimido)molybdenum(VI) framework, both in atomic layer deposition and chemical vapor deposition. We have prepared a series of (^tBuN)₂MoCl₂ adducts using neutral N,N'-chelates and investigated their volatility, thermal stability, and decomposition pathways. Volatility has been determined by thermogravimetric analysis, with the 1,4-di-tert-butyl-1,3-diazabutadiene adduct (5) found to be the most volatile (1 Torr of vapor pressure at 135 °C). Thermal stability was measured primarily using differential scanning calorimetry, and the 1,10-phenanthroline adduct (4) was found to be the most stable with an onset of decomposition of 303 °C. We have also investigated molybdenum compounds with other alkyl-substituted imido groups: these compounds all follow a similar decomposition pathway, γ-H activation, with varying reaction barriers. The tert-pentyl, 1-adamantyl, and a cyclic imido (from 2,5-dimethylhexane-2,5-diamine) were systematically studied to probe the kinetics of this pathway. All of these compounds have been fully characterized, including via single-crystal X-ray diffraction, and a total of 19 new structures are reported.

Wafer-Scale Synthesis of WS2 Films with In Situ Controllable p-Type Doping by Atomic Layer Deposition

Wafer-scale synthesis of p-type TMD films is critical for its commercialization in next-generation electro/optoelectronics. In this work, wafer-scale intrinsic n-type WS₂ films and in situ Nb-doped p-type WS₂ films were synthesized through atomic layer deposition (ALD) on 8-inch α-Al₂O₃/Si wafers, 2-inch sapphire, and 1 cm² GaN substrate pieces. The Nb doping concentration was precisely controlled by altering cycle number of Nb precursor and activated by postannealing. WS₂ n-FETs and Nb-doped p-FETs with different Nb concentrations have been fabricated using CMOS-compatible processes. X-ray photoelectron spectroscopy, Raman spectroscopy, and Hall measurements confirmed the effective substitutional doping with Nb. The on/off ratio and electron mobility of WS₂ n-FET are as high as 10⁵ and 6.85 cm² V⁻¹ s⁻¹, respectively. In WS₂ p-FET with 15-cycle Nb doping, the on/off ratio and hole mobility are 10 and 0.016 cm² V⁻¹ s⁻¹, respectively. The p-n structure based on n- and p- type WS₂ films was proved with a 10⁴ rectifying ratio. The realization of controllable in situ Nb-doped WS₂ films paved a way for fabricating wafer-scale complementary WS₂ FETs.

Applications of Plasma-Assisted Systems for Advanced Electrode Material Synthesis and Modification

Research on advanced electrode materials (AEMs) has been explosive for the past decades and constantly promotes the development of batteries, supercapacitors, electrocatalysis, and photovoltaic applications. However, traditional preparation and modification methods can no longer meet the increasing requirements of some AEMs because some of the special reactions are thermodynamically and/or kinetically unfavorable and thus need harsh conditions. Among various recently developed advanced materials synthesis and modification routes, the plasma-assisted (PA) method has received increasing attention because of its unique and different "species reactivity" nature, as well as its wider and adjustable operating conditions. In this Spotlight on Applications, we highlight some recent developments and describe our recent progress by applying PA systems in the synthesis and modification of AEMs, including direct processing, PA deposition, and plasma milling (P-milling). The mechanisms of how plasma works for specific reactions are reviewed and discussed. It is shown that the PA technique has become a powerful and efficient tool in the following areas, including but not limited to materials synthesis, doping, surface modification, and functionalization. Finally, the prospect and challenges are also proposed for AEM preparation and modification using PA systems. This article aims to provide up-to-date information about the progress of PA technology in the fields of chemistry and materials science.