Embedding Quantum Dot Monolayers in Al₂O₃ Using Atomic Layer Deposition

Embedding Quantum Dot Monolayers in Al2O3 Using Atomic Layer Depositio

Deposition of MnO Anode and MnO₂ Cathode Thin Films by Plasma Enhanced Atomic Layer Deposition Using the Mn(thd)₃ Precursor

Atomic layer deposition (ALD) of a wide range of Mn oxides (MnO to MnO₂) is demonstrated by combining the Mn­(thd)₃ (tris­(2,2,6,6-tetramethyl-3,5-heptanedionato)­manganese) precursor with different types of plasma activated reactant gases. Typical ALD behavior is found with hydrogen, ammonia, and water plasma, with a fully precursor controlled temperature window (from 140 to 250 °C) and constant growth rate (0.022 ± 0.001 nm/cycle). A purely ligand-exchange chemistry would predict Mn₂O₃ films with the transition metal in the +III state. However, it is found that the nature of the processgas or -plasma, more specific its oxidizing/reducing character, largely determines the oxidation state of the grown films. Our approach provides an effective method for the deposition of MnO₂(+IV), Mn₃O₄(+II/+III), and MnO­(+II) based on the Mn­(thd)₃(+III) precursor. All as-deposited films are found to be smooth (<1.2 nm rms roughness), crystalline and with <6% impurities. The resulting films are tested as lithium-ion battery electrodes, showing the MnO₂ and the MnO films as possible candidate thin-film cathode and anode, respectively

Low Temperature Atomic Layer Deposition of Crystalline In₂O₃ Films

Crystalline In₂O₃ thin films were deposited by atomic layer deposition (ALD) using tris­(2,2,6,6-tetramethyl-3,5-heptanedionato) indium­(III), [In­(TMHD)₃] as an indium source and O₂ plasma. Resulting growth rates were studied as a function of precursor pulse, reactant pulse, deposition temperature, and number of ALD cycles. The film growth rate was found to be 0.14 Å/cycle within the wide ALD temperature window of 100–400 °C. X-ray photoelectron spectroscopic (XPS) and X-ray diffraction (XRD) analysis revealed stoichiometric In₂O₃ thin films with polycrystalline cubic structure, even at 100 °C. All the as-deposited films were smooth, with RMS roughness values between 0.39 to 0.47 nm, as shown by atomic force microscopic (AFM) analysis. The optical properties and electrical resistivities of the films were determined by spectroscopic ellipsometry (SE) and four-point probe measurements. The highly transparent (ca. 94% in the visible region) films had a refractive index of 2.01–2.05 and a resistivity of 2.5–3 mΩ·cm

Role of the Oxidizing Co-Reactant in Pt Growth by Atomic Layer Deposition Using MeCpPtMe₃ and O₂/O₃/O₂‑Plasma

Atomic layer deposition (ALD) of Pt using MeCpPtMe3 and the O2/O3/O2-plasma (O2*) at 300 °C is investigated with in vacuo X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) to gain a better understanding of the Pt growth mechanism. Most notably, the chemical state of the surface Pt atoms and the role of surface O species in Pt growth are revealed. In the MeCpPtMe3/O2 process, the surface Pt atoms remain in a metallic Pt0 state throughout the ALD cycle, and the surface O species generated by the O2 exposure only exist as unstable adatoms, desorbing in vacuum. As for the O3/O2* processes, the surface Pt layer is oxidized to a mixture of Pt0, Pt2+O and Pt4+O2 upon O3/O2* exposure and then fully reduced to Pt0 during the precursor exposure. Surface Pt oxides are stable in a vacuum but can be reduced by hydrocarbon vapors. Quantification analysis shows that the O3/O2* processes have a much higher surface O species content than the O2 process after the coreactant exposure, favoring precursor ligand combustion over dehydrogenation in the next precursor exposure and leading to lower surface C density after the precursor pulse. DFT reveals differences in the combustion mechanism for Me vs Cp species, during the metal precursor and coreactant pulses. Importantly, the differences in the surface O content do not significantly affect the growth per cycle. Moreover, the MeCpPtMe3/O2 process with surface O species and a tailored MeCpPtMe3/O2 process without surface O species, both at 300 °C, yield nearly identical growth rates and as-deposited Pt with the same chemical state. This indicates that surface O species present before the precursor exposure have little impact on the overall Pt growth, in contrast to a previous assumption

Micro-Transfer-Printing of Al₂O₃‑Capped Short-Wave-Infrared PbS Quantum Dot Photoconductors

Quantum dots (QDs) have attracted considerable attention in the development of various optoelectronic applications. The scalable heterogeneous integration of high quality, yet stable QD films is required for low-cost devices based on these materials. Here, we demonstrate the transfer printing of microscale patterns of Al2O3-capped PbS QD films to realize large-scale integrated photodetector arrays with a first excitonic absorption peak at 2.1 μm wavelength. The process provides a facile approach to selectively pick-and-print QD assemblies on device structures with high precision. Transfer-printed photoconductor devices were realized and characterized at low bias voltage and optical power. Under 10 nW surface normal illumination at 2.1 μm wavelength, the responsivity of our devices obtained at 1 V bias reached a maximum value of 25 A/W and 85 A/W for PbS QD films of 88 and 140 nm thick, respectively. Our approach suggests new routes toward scalable and cost-effective integration of multiple high-quality QD stacks on electronic and optoelectronic circuits

ALD-Developed Plasmonic Two-Dimensional Au–WO₃–TiO₂ Heterojunction Architectonics for Design of Photovoltaic Devices

Electrically responsive plasmonic devices, which benefit from the privilege of surface plasmon excited hot carries, have supported fascinating applications in the visible-light-assisted technologies. The properties of plasmonic devices can be tuned by controlling charge transfer. It can be attained by intentional architecturing of the metal–semiconductor (MS) interfaces. In this study, the wafer-scaled fabrication of two-dimensional (2D) TiO₂ semiconductors on the granular Au metal substrate is achieved using the atomic layer deposition (ALD) technique. The ALD-developed 2D MS heterojunctions exhibited substantial enhancement of the photoresponsivity and demonstrated the improvement of response time for 2D Au–TiO₂-based plasmonic devices under visible light illumination. To circumvent the undesired dark current in the plasmonic devices, a 2D WO₃ nanofilm (∼0.7 nm) was employed as the intermediate layer on the MS interface to develop the metal–insulator–semiconductor (MIS) 2D heterostructure. As a result, 13.4% improvement of the external quantum efficiency was obtained for fabricated 2D Au–WO₃–TiO₂ heterojunctions. The impedancometry measurements confirmed the modulation of charge transfer at the 2D MS interface using MIS architectonics. Broadband photoresponsivity from the UV to the visible light region was observed for Au–TiO₂ and Au–WO₃–TiO₂ heterostructures, whereas near-infrared responsivity was not observed. Consequently, considering the versatile nature of the ALD technique, this approach can facilitate the architecturing and design of novel 2D MS and MIS heterojunctions for efficient plasmonic devices

Fe₂O₃–MgAl₂O₄ for CO Production from CO₂: Mössbauer Spectroscopy and in Situ X‑ray Diffraction

Fe2O3/MgFeAlOx materials are promising oxygen storage candidates for chemical looping CO2 conversion. In this work, the cyclic stability of a 50Fe2O3/MgFeAlOx (containing 50 wt % Fe2O3 and 50 wt % MgAl2O4) oxygen storage material is investigated. The evolution of its bulk properties over the course of 1000 H2/CO2 redox cycles has been studied by means of 57Fe Mössbauer spectroscopy and in situ X-ray diffraction. As expected, all iron in the as-prepared oxygen storage material was present as Fe3+, 64% of which in iron-rich phases α-Fe2O3 and α-FeOOH and 36% in the form of a MgFeAlOx spinel. In contrast, after 1000 redox cycles, only 19% of iron was present in an iron-rich spinel such as Fe3O4, γ-Fe2O3, and MgFe2O4. The remaining 81% was present in the form of Mg–Fe–Al–O, including MgxFe1–xO. ILEEMS measurements showed surface enrichment of Fe3+ in 50Fe2O3/MgFeAlOx after 1000 redox cycles, with 36% of all surface Fe present as Fe3+ in iron-rich spinel phases such as γ-Fe2O3 and/or MgFe2O4

Redox Layer Deposition of Thin Films of MnO₂ on Nanostructured Substrates from Aqueous Solutions

In this work, we report a new method for depositing thin films of MnO2 on planar and complex nanostructured surfaces, with high precision and conformality. The method is based on repeating cycles of adsorption of an unsaturated alcohol on a surface, followed by its oxidation with aqueous KMnO4 and formation of thin, solid MnO2. The amount of manganese oxide formed in each cycle is limited by the quantity of the adsorbed alcohol; thus, the growth exhibits the self-limiting characteristics of atomic layer deposition (ALD). Contrary to the typical ALD, however, the new redox layer deposition is performed in air, at room temperature, using common chemicals and simple laboratory glassware, which greatly reduces its cost and complexity. We also demonstrate application of the method for the fabrication of a nanostructured MnO2/Ni electrode, which was not possible with thermal ALD because of the rapid decomposition of the gaseous precursor on the high surface-area substrate. Thanks to its simplicity, the conformal deposition of MnO2 can be easily upscaled and thus exploited for its numerous (electro)­chemical applications