Remote Plasma Oxidation and Atomic Layer Etching of MoS₂

Exfoliated molybdenum disulfide (MoS₂) is shown to chemically oxidize in a layered manner upon exposure to a remote O₂ plasma. X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and atomic force microscopy (AFM) are employed to characterize the surface chemistry, structure, and topography of the oxidation process and indicate that the oxidation mainly occurs on the topmost layer without altering the chemical composition of underlying layer. The formation of S–O bonds upon short, remote plasma exposure pins the surface Fermi level to the conduction band edge, while the MoO_<i>x</i> formation at high temperature modulates the Fermi level toward the valence band through band alignment. A uniform coverage of monolayer amorphous MoO₃ is obtained after 5 min or longer remote O₂ plasma exposure at 200 °C, and the MoO₃ can be completely removed by annealing at 500 °C, leaving a clean ordered MoS₂ lattice structure as verified by XPS, LEED, AFM, and scanning tunneling microscopy. This work shows that a remote O₂ plasma can be useful for both surface functionalization and a controlled thinning method for MoS₂ device fabrication processes

Structural Changes in Self-Catalyzed Adsorption of Carbon Monoxide on 1,4-Phenylene Diisocyanide Modified Au(111)

The self-accelerated adsorption of CO on 1,4-phenylene diisocyanide (PDI)-derived oligomers on Au(111) is explored by reflection–absorption infrared spectroscopy and scanning tunneling microscopy. PDI incorporates gold adatoms from the Au(111) surface to form one-dimensional −(Au–PDI)_<i>n</i>– chains that can also connect between gold nanoparticles on mica to form a conductive pathway between them. CO adsorption occurs in two stages; it first adsorbs adjacent to the oligomers that move to optimize CO adsorption. Further CO exposure induces PDI decoordination to form Au–PDI adatom complexes thereby causing the conductivity of a PDI-linked gold nanoparticle array on mica to decrease to act as a chemically drive molecular switch. This simple system enables the adsorption process to be explored in detail. DFT calculations reveal that both the −(Au–PDI)_<i>n</i>– oligomer chain and the Au–PDI adatom complex are stabilized by coadsorbed CO. A kinetic “foot-in-the-door” model is proposed in which fluctuations in PDI coordination allow CO to diffuse into the gap between gold adatoms to prevent the PDI from reattaching, thereby allowing additional CO to adsorb, to provide kinetic model for allosteric CO adsorption on PDI-covered gold

Combustion Synthesis of p‑Type Transparent Conducting CuCrO_2+<i>x</i> and Cu:CrO_<i>x</i> Thin Films at 180 °C

Low-temperature solution processing of p-type transparent conducting oxides (TCOs) will open up new opportunities for applications on flexible substrates that utilize low-cost, large-area manufacturing. Here, we report a facile solution synthesis method that produces two p-type TCO thin films: copper chromium oxide and copper-doped chromium oxide. Using combustion chemistry, both films are solution processed at 180 °C, which is lower than most recent efforts. While adopting the same precursor preparation and annealing temperature, we find that annealing environment (solvent vapor vs open air) dictates the resulting film phase, hence the optoelectronic properties. The effect of annealing environment on the reaction mechanism is discussed. We further characterize the electronic, optical, and transport properties of the two materials, and compare the differences. Their applications in optoelectronic devices are successfully demonstrated in transparent p–n junction diodes and as hole transport layers in organic photovoltaic devices

Defects and Surface Structural Stability of MoTe₂ Under Vacuum Annealing

Understanding the structural stability of transition-metal dichalcogenides is necessary to avoid surface/interface degradation. In this work, the structural stability of 2H-MoTe₂ with thermal treatments up to 500 °C is studied using scanning tunneling microscopy and scanning transmission electron microscopy. On the exfoliated sample surface at room temperature, atomic subsurface donors originating from excess Te atoms are observed and presented as nanometer-sized, electronically-induced protrusions superimposed with the hexagonal lattice structure of MoTe₂. Under a thermal treatment as low as 200 °C, the surface decomposition-induced cluster defects and Te vacancies are readily detected and increase in extent with the increasing temperature. Driven by Te vacancies and thermal energy, intense 60° inversion domain boundaries form resulting in a “wagon wheel” morphology after 400 °C annealing for 15 min. Scanning tunneling spectroscopy identified the electronic states at the domain boundaries and the domain centers. To prevent extensive Te loss at higher temperatures, where Mo₆Te₆ nanowire formation and substantial desorption-induced etching effects will take place simultaneously, surface and edge passivation with a monolayer graphene coverage on MoTe₂ is tested. With this passivation strategy, the structural stability of MoTe₂ is greatly enhanced up to 500 °C without apparent structural defects

Atomic Layer Deposition of a High‑<i>k</i> Dielectric on MoS₂ Using Trimethylaluminum and Ozone

We present an Al₂O₃ dielectric layer on molybdenum disulfide (MoS₂), deposited using atomic layer deposition (ALD) with ozone/trimethylaluminum (TMA) and water/TMA as precursors. The results of atomic force microscopy and low-energy ion scattering spectroscopy show that using TMA and ozone as precursors leads to the formation of uniform Al₂O₃ layers, in contrast to the incomplete coverage we observe when using TMA/H₂O as precursors. Our Raman and X-ray photoelectron spectroscopy measurements indicate minimal variations in the MoS₂ structure after ozone treatment at 200 °C, suggesting its excellent chemical resistance to ozone

Al₂O₃ on Black Phosphorus by Atomic Layer Deposition: An <i>in Situ</i> Interface Study

<i>In situ</i> “half cycle” atomic layer deposition (ALD) of Al₂O₃ was carried out on black phosphorus (“black-P”) surfaces with modified phosphorus oxide concentrations. X-ray photoelectron spectroscopy is employed to investigate the interfacial chemistry and the nucleation of the Al₂O₃ on black-P surfaces. This work suggests that exposing a sample that is initially free of phosphorus oxide to the ALD precursors does not result in detectable oxidation. However, when the phosphorus oxide is formed on the surface prior to deposition, the black-P can react with both the surface adventitious oxygen contamination and the H₂O precursor at a deposition temperature of 200 °C. As a result, the concentration of the phosphorus oxide increases after both annealing and the atomic layer deposition process. The nucleation rate of Al₂O₃ on black-P is correlated with the amount of oxygen on samples prior to the deposition. The growth of Al₂O₃ follows a “substrate inhibited growth” behavior where an incubation period is required. <i>Ex situ</i> atomic force microscopy is also used to investigate the deposited Al₂O₃ morphologies on black-P where the Al₂O₃ tends to form islands on the exfoliated black-P samples. Therefore, surface functionalization may be needed to get a conformal coverage of Al₂O₃ on the phosphorus oxide free samples

Partially Fluorinated Graphene: Structural and Electrical Characterization

Despite the number of existing studies that showcase the promising application of fluorinated graphene in nanoelectronics, the impact of the fluorine bonding nature on the relevant electrical behaviors of graphene devices, especially at low fluorine content, remains to be experimentally explored. Using CF₄ as the fluorinating agent, we studied the gradual structural evolution of chemical vapor deposition graphene fluorinated by CF₄ plasma at a working pressure of 700 mTorr using Raman and X-ray photoelectron spectroscopy (XPS). After 10 s of fluorination, our XPS analysis revealed a co-presence of covalently and ionically bonded fluorine components; the latter has been determined being a dominant contribution to the observation of two Dirac points in the relevant electrical measurement using graphene field effect transistor devices. Additionally, this ionic C–F component (ionic bonding characteristic charge sharing) is found to be present only at low fluorine content; continuous fluorination led to a complete transition to a covalently bonded C–F structure and a dramatic increase of graphene sheet resistance. Owing to the formation of these various C–F bonding components, our temperature-dependent Raman mapping studies show an inhomogeneous defluorination from annealing temperatures starting at ∼150 °C for low fluorine coverage, whereas fully fluorinated graphene is thermally stable up to ∼300 °C

Sub-10 nm Tunable Hybrid Dielectric Engineering on MoS₂ for Two-Dimensional Material-Based Devices

The successful realization of high-performance 2D-materials-based nanoelectronics requires integration of high-quality dielectric films as a gate insulator. In this work, we explore the integration of organic and inorganic hybrid dielectrics on MoS₂ and study the chemical and electrical properties of these hybrid films. Our atomic force microscopy, X-ray photoelectron spectroscopy (XPS), Raman, and photoluminescence results show that, aside from the excellent film uniformity and thickness scalability down to 2.5 nm, the molecular layer deposition of octenyltrichlorosilane (OTS) and Al₂O₃ hybrid films preserves the chemical and structural integrity of the MoS₂ surface. The XPS band alignment analysis and electrical characterization reveal that through the inclusion of an organic layer in the dielectric film, the band gap and dielectric constant can be tuned from ∼7.00 to 6.09 eV and ∼9.0 to 4.5, respectively. Furthermore, the hybrid films show promising dielectric properties, including a high breakdown field of ∼7.8 MV/cm, a low leakage current density of ∼1 × 10^–6 A/cm² at 1 MV/cm, a small hysteresis of ∼50 mV, and a top-gate subthreshold voltage swing of ∼79 mV/dec. Our experimental findings provide a facile way of fabricating scalable hybrid gate dielectrics on transition metal dichalcogenides for 2D-material-based flexible electronics applications

Metal Contacts on Physical Vapor Deposited Monolayer MoS₂

The understanding of the metal and transition metal dichalcogenide (TMD) interface is critical for future electronic device technologies based on this new class of two-dimensional semiconductors. Here, we investigate the initial growth of nanometer-thick Pd, Au, and Ag films on monolayer MoS₂. Distinct growth morphologies are identified by atomic force microscopy: Pd forms a uniform contact, Au clusters into nanostructures, and Ag forms randomly distributed islands on MoS₂. The formation of these different interfaces is elucidated by large-scale spin-polarized density functional theory calculations. Using Raman spectroscopy, we find that the interface homogeneity shows characteristic Raman shifts in E_2g¹ and A_1g modes. Interestingly, we show that insertion of graphene between metal and MoS₂ can effectively decouple MoS₂ from the perturbations imparted by metal contacts (<i>e.g.</i>, strain), while maintaining an effective electronic coupling between metal contact and MoS₂, suggesting that graphene can act as a conductive buffer layer in TMD electronics

Covalent Nitrogen Doping and Compressive Strain in MoS₂ by Remote N₂ Plasma Exposure

Controllable doping of two-dimensional materials is highly desired for ideal device performance in both hetero- and p-n homojunctions. Herein, we propose an effective strategy for doping of MoS₂ with nitrogen through a remote N₂ plasma surface treatment. By monitoring the surface chemistry of MoS₂ upon N₂ plasma exposure using in situ X-ray photoelectron spectroscopy, we identified the presence of covalently bonded nitrogen in MoS₂, where substitution of the chalcogen sulfur by nitrogen is determined as the doping mechanism. Furthermore, the electrical characterization demonstrates that p-type doping of MoS₂ is achieved by nitrogen doping, which is in agreement with theoretical predictions. Notably, we found that the presence of nitrogen can induce compressive strain in the MoS₂ structure, which represents the first evidence of strain induced by substitutional doping in a transition metal dichalcogenide material. Finally, our first principle calculations support the experimental demonstration of such strain, and a correlation between nitrogen doping concentration and compressive strain in MoS₂ is elucidated