Plasma-Enhanced Atomic Layer Deposition of Molybdenum Oxide Thin Films at Low Temperatures for Hydrogen Gas Sensing.

Molybdenum oxide thin films are very appealing for gas sensing applications due to their tunable material characteristics. Particularly, the growing demand for developing hydrogen sensors has triggered the exploration of functional materials such as molybdenum oxides (MoOx). Strategies to enhance the performance of MoOx-based gas sensors include nanostructured growth accompanied by precise control of composition and crystallinity. These features can be delivered by using atomic layer deposition (ALD) processing of thin films, where precursor chemistry plays an important role. Herein, we report a new plasma-enhanced ALD process for molybdenum oxide employing the molybdenum precursor [Mo(NtBu)2(tBu2DAD)] (DAD = diazadienyl) and oxygen plasma. Analysis of the film thickness reveals typical ALD characteristics such as linearity and surface saturation with a growth rate of 0.75 Å/cycle in a broad temperature window between 100 and 240 °C. While the films are amorphous at 100 °C, crystalline ß-MoO3 is obtained at 240 °C. Compositional analysis reveals nearly stoichiometric and pure MoO3 films with oxygen vacancies present at the surface. Subsequently, hydrogen gas sensitivity of the molybdenum oxide thin films is demonstrated in a laboratory-scale chemiresistive hydrogen sensor setup at an operation temperature of 120 °C. Sensitivities of up to 18% are achieved for the film deposited at 240 °C, showing a strong correlation between crystallinity, oxygen vacancies at the surface, and hydrogen gas sensitivity.

CVD Grown Tungsten Oxide for Low Temperature Hydrogen Sensing: Tuning Surface Characteristics via Materials Processing for Sensing Applications.

The intrinsic properties of semiconducting oxides having nanostructured morphology are highly appealing for gas sensing. In this study, the fabrication of nanostructured WO3 thin films with promising surface characteristics for hydrogen (H2 ) gas sensing applications is accomplished. This is enabled by developing a chemical vapor deposition (CVD) process employing a new and volatile tungsten precursor bis(diisopropylamido)-bis(tert-butylimido)-tungsten(VI), [W(Nt Bu)2 (Ni Pr2 )2 ]. The as-grown nanostructured WO3 layers are thoroughly analyzed. Particular attention is paid to stoichiometry, surface characteristics, and morphology, all of which strongly influence the gas-sensing potential of WO3 . Synchrotron-based ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), X-ray photoelectron emission microscopy (XPEEM), low-energy electron microscopy (LEEM) and 4-point van der Pauw (vdP) technique made it possible to analyze the surface chemistry and structural uniformity with a spatially resolved insight into the chemical, electronic and electrical properties. The WO3 layer is employed as a hydrogen (H2 ) sensor within interdigitated mini-mobile sensor architecture capable of working using a standard computer's 5 V 1-wirebus connection. The sensor shows remarkable sensitivity toward H2 . The high, robust, and repeatable sensor response (S) is attributed to the homogenous distribution of the W5+ oxidation state and associated oxygen vacancies, as shown by synchrotron-based UPS, XPS, and XPEEM analysis.

Adsorbed Oxygen Ions and Oxygen Vacancies: Their Concentration and Distribution in Metal Oxide Chemical Sensors and Influencing Role in Sensitivity and Sensing Mechanisms.

Oxidation reactions on semiconducting metal oxide (SMOs) surfaces have been extensively worked on in catalysis, fuel cells, and sensors. SMOs engage powerfully in energy-related applications such as batteries, supercapacitors, solid oxide fuel cells (SOFCs), and sensors. A deep understanding of SMO surface and oxygen interactions and defect engineering has become significant because all of the above-mentioned applications are based on the adsorption/absorption and consumption/transportation of adsorbed (physisorbed-chemisorbed) oxygen. More understanding of adsorbed oxygen and oxygen vacancies (VO•,VO••) is needed, as the former is the vital requirement for sensing chemical reactions, while the latter facilitates the replenishment of adsorbed oxygen ions on the surface. We determined the relation between sensor response (sensitivity) and the amounts of adsorbed oxygen ions (O2(ads)−, O(ads), −O2(ads)2−, O(ads)2−), water/hydroxide groups (H2O/OH−), oxygen vacancies (VO•, VO••), and ordinary lattice oxygen ions (Olattice2−) as a function of temperature. During hydrogen (H2) testing, the different oxidation states (W6+, W5+, and W4+) of WO3 were quantified and correlated with oxygen vacancy formation (VO•, VO••). We used a combined application of XPS, UPS, XPEEM-LEEM, and chemical, electrical, and sensory analysis for H2 sensing. The sensor response was extraordinarily high: 424 against H2 at a temperature of 250 °C was recorded and explained on the basis of defect engineering, including oxygen vacancies and chemisorbed oxygen ions and surface stoichiometry of WO3. We established a correlation between the H2 sensing mechanism of WO3, sensor signal magnitude, the amount of adsorbed oxygen ions, and sensor testing temperature. This paper also provides a review of the detection, quantification, and identification of different adsorbed oxygen species. The different surface and bulk-sensitive characterization techniques relevant to analyzing the SMOs-based sensor are tabulated, providing the sensor designer with the chemical, physical, and electronic information extracted from each technique.

Oxygen Sensing of Pt/PEO-TiO2 in Humid Atmospheres at Moderate Temperatures.

Here, we show that the presence of adsorbed water improves the oxygen-sensing properties of Pt/TiO2 at moderate temperatures. The studied interface is based on porous plasma electrolytic oxidized titanium (PEO-TiO2) covered with platinum clusters. The electrical resistance across Pt/PEO-TiO2 is explained by an electronic depletion layer. Oxygen adsorbates further increase the depletion by inducing extrinsic interface states, which are occupied by TiO2 conduction band electrons. The high oxygen partial pressure in ambient air substantially limits the electron transport across the interface. Our DC measurements at defined levels of humidity at 30 ∘C show that adsorbed water counteracts this shortcoming, allowing oxygen sensing at room conditions. In addition, response and recovery times from temporal oxygen exposure decrease with humidity. We attribute the effects to competing adsorption processes and reactions of water with adsorbed oxygen species and/or lattice oxygen, which involve electron re-injection to the TiO2 conduction band. Elevated temperatures up to 170 ∘C attenuate the effects, presumably due to the lower binding strength to the surface of molecular water compared with oxygen adsorbates.

A new metalorganic chemical vapor deposition process for MoS2 with a 1,4-diazabutadienyl stabilized molybdenum precursor and elemental sulfur.

Molybdenum disulfide (MoS2) is known for its versatile properties and hence is promising for a wide range of applications. The fabrication of high quality MoS2 either as homogeneous films or as two-dimensional layers on large areas is thus the objective of intense research. Since industry requirements on MoS2 thin films can hardly be matched by established exfoliation fabrication methods, there is an enhanced need for developing new chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes where a rational precursor selection is a crucial step. In this study, a new molybdenum precursor, namely 1,4-di-tert-butyl-1,4-diazabutadienyl-bis(tert-butylimido)molybdenum(vi) [Mo(NtBu)2(tBu2DAD)], is identified as a potential candidate. The combination of imido and chelating 1,4-diazadieneyl ligand moieties around the molybdenum metal center results in a monomeric compound possessing adequate thermal characteristics relevant for vapor phase deposition applications. Hexagonal MoS2 layers are fabricated in a metalorganic CVD (MOCVD) process with elemental sulfur as the co-reactant at temperatures between 600 °C and 800 °C. The structure and composition of the films are investigated by X-ray diffraction, high resolution transmission electron microscopy, synchrotron X-ray photoelectron spectroscopy and Raman spectroscopy revealing crystalline and stoichiometric MoS2 films. The new MOCVD process developed for MoS2 is highly promising due to its moderate process conditions, scalability and controlled targeted composition.

From Precursor Chemistry to Gas Sensors: Plasma-Enhanced Atomic Layer Deposition Process Engineering for Zinc Oxide Layers from a Nonpyrophoric Zinc Precursor for Gas Barrier and Sensor Applications.

The identification of bis-3-(N,N-dimethylamino)propyl zinc ([Zn(DMP)2 ], BDMPZ) as a safe and potential alternative to the highly pyrophoric diethyl zinc (DEZ) as atomic layer deposition (ALD) precursor for ZnO thin films is reported. Owing to the intramolecular stabilization, BDMPZ is a thermally stable, volatile, nonpyrophoric solid compound, however, it possesses a high reactivity due to the presence of Zn-C and Zn-N bonds in this complex. Employing this precursor, a new oxygen plasma enhanced (PE)ALD process in the deposition temperature range of 60 and 160 °C is developed. The resulting ZnO thin films are uniform, smooth, stoichiometric, and highly transparent. The deposition on polyethylene terephthalate (PET) at 60 °C results in dense and compact ZnO layers for a thickness as low as 7.5 nm with encouraging oxygen transmission rates (OTR) compared to the bare PET substrates. As a representative application of the ZnO layers, the gas sensing properties are investigated. A high response toward NO2 is observed without cross-sensitivities against NH3 and CO. Thus, the new PEALD process employing BDMPZ has the potential to be a safe substitute to the commonly used DEZ processes.

Spectroscopic Understanding of SnO2 and WO3 Metal Oxide Surfaces with Advanced Synchrotron Based; XPS-UPS and Near Ambient Pressure (NAP) XPS Surface Sensitive Techniques for Gas Sensor Applications under Operational Conditions.

The most promising and utilized chemical sensing materials, WO3 and SnO2 were characterized by means advanced synchrotron based XPS, UPS, NAP-XPS techniques. The complementary electrical resistance and sensor testing experiments were also completed. A comparison and evaluation of some of the prominent and newly employed spectroscopic characterization techniques for chemical sensors were provided. The chemical nature and oxidation state of the WO3 and SnO2 thin films were explored at different depths from imminent surface to a maximum of 1.5 nm depth from the surface with non-destructive depth profiling. The adsorption and amount of chemisorbed oxygen species were precisely analyzed and quantified as a function of temperature between 25-400 °C under realistic operating conditions for chemical sensors employing 1-5 mbar pressures of oxygen (O2) and carbon monoxide (CO). The effect of realistic CO and O2 gas pressures on adsorbed water (H2O), OH- groups and chemisorbed oxygen species ( O 2 ( a d s ) - ,   O ( a d s ) ,   - O 2 ( a d s ) 2 - ) and chemical stability of metal oxide surfaces were evaluated and quantified.

PEALD of HfO2 Thin Films: Precursor Tuning and a New Near-Ambient-Pressure XPS Approach to in Situ Examination of Thin-Film Surfaces Exposed to Reactive Gases.

A bottom-up approach starting with the development of new Hf precursors for plasma-enhanced atomic layer deposition (PEALD) processes for HfO2 followed by in situ thin-film surface characterization of HfO2 upon exposure to reactive gases via near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) is reported. The stability of thin films under simulated operational conditions is assessed, and the successful implementation of HfO2 dielectric layers in metal-insulator-semiconductor (MIS) capacitors is demonstrated. Among the series of newly synthesized mono-guanidinato-tris-dialkyl-amido class of Hf precursors, one of them, namely, [Hf{η2-(iPrN)2CNEtMe}(NEtMe)3], was representatively utilized with oxygen plasma, resulting in a highly promising low-temperature PEALD process at 60 °C. The new precursors were synthesized in the multigram scale and thoroughly characterized by thermogravimetric analyses, revealing high and tunable volatility reflected by appreciable vapor pressures and accompanied by thermal stability. Typical ALD growth characteristics in terms of linearity, saturation, and a broad ALD window with constant growth of 1.06 Å cycle-1 in the temperature range of 60-240 °C render this process very promising for fabricating high-purity smooth HfO2 layers. For the first time, NAP-XPS surface studies on selected HfO2 layers are reported upon exposure to reactive H2, O2, and H2O atmospheres at temperatures of up to 500 °C revealing remarkable stability against degradation. This can be attributed to the absence of surface defects and vacancies. On the basis of these promising results, PEALD-grown HfO2 films were used as dielectric layers in the MIS capacitor device fabrication exhibiting leakage current densities less than 10-7 A cm-2 at 2 MV cm-1 and permittivities of up to 13.9 without postannealing.

Low-Temperature Plasma-Enhanced Atomic Layer Deposition of Tin(IV) Oxide from a Functionalized Alkyl Precursor: Fabrication and Evaluation of SnO2-Based Thin-Film Transistor Devices.

A bottom-up process from precursor development for tin to plasma-enhanced atomic layer deposition (PEALD) for tin(IV) oxide and its successful implementation in a working thin-film transistor device is reported. PEALD of tin(IV) oxide thin films at low temperatures down to 60 °C employing tetrakis-(dimethylamino)propyl tin(IV) [Sn(DMP)4] and oxygen plasma is demonstrated. The liquid precursor has been synthesized and thoroughly characterized with thermogravimetric analyses, revealing sufficient volatility and long-term thermal stability. [Sn(DMP)4] demonstrates typical saturation behavior and constant growth rates of 0.27 or 0.42 Å cycle-1 at 150 and 60 °C, respectively, in PEALD experiments. Within the ALD regime, the films are smooth, uniform, and of high purity. On the basis of these promising features, the PEALD process was optimized wherein a 6 nm thick tin oxide channel material layer deposited at 60 °C was applied in bottom-contact bottom-gate thin-film transistors, showing a remarkable on/off ratio of 107 and field-effect mobility of µFE ≈ 12 cm2 V-1 s-1 for the as-deposited thin films deposited at such low temperatures.

Ultrathin TiO(x) films on Pt(111): a LEED, XPS, and STM investigation.

Ultrathin ordered titanium oxide films on Pt(111) surface are prepared by reactive evaporation of Ti in oxygen. By varying the Ti dose and the annealing conditions (i.e., temperature and oxygen pressure), six different long-range ordered phases are obtained. They are characterized by means of low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). By careful optimization of the preparative parameters, we find conditions where predominantly single phases of TiO(x), revealing distinct LEED pattern and STM images, are produced. XPS binding energy and photoelectron diffraction (XPD) data indicate that all the phases, except one (the stoichiometric rect-TiO2), are one monolayer thick and composed of a Ti-O bilayer with interfacial Ti. Atomically resolved STM images confirm that these TiO(x) phases wet the Pt surface, in contrast to rect-TiO2. This indicates their interface stabilization. At a low Ti dose (0.4 monolayer equivalents, MLE), an incommensurate kagomé-like low-density phase (k-TiO(x) phase) is observed where hexagons are sharing their vertexes. At a higher Ti dose (0.8 MLE), two denser phases are found, both characterized by a zigzag motif (z- and z'-TiO(x) phases), but with distinct rectangular unit cells. Among them, z'-TiO(x), which is obtained by annealing in ultrahigh vacuum (UHV), shows a larger unit cell. When the postannealing of the 0.8 MLE deposit is carried out at high temperatures and high oxygen partial pressures, the incommensurate nonwetting, fully oxidized rect-TiO2 is found The symmetry and lattice dimensions are almost identical with rect-VO2, observed in the system VO(x)/Pd(111). At a higher coverage (1.2 MLE), two commensurate hexagonal phases are formed, namely the w- [(square root(43) x square root(43)) R 7.6 degrees] and w'-TiO(x) phase [(7 x 7) R 21.8 degrees]. They show wagon-wheel-like structures and have slightly different lattice dimensions. Larger Ti deposits produce TiO2 nanoclusters on top of the different monolayer films, as supported both by XPS and STM data. Besides the formation of TiO(x) surfaces phases, wormlike features are found on the bare parts of the substrate by STM. We suggest that these structures, probably multilayer disordered TiO2, represent growth precursors of the ordered phases. Our results on the different nanostructures are compared with literature data on similar systems, e.g., VO(x)/Pd(111), VO(x)/Rh(111), TiO(x)/Pd(111), TiO(x)/Pt(111), and TiO(x)/Ru(0001). Similar and distinct features are observed in the TiO(x)/Pt(111) case, which may be related to the different chemical natures of the overlayer and of the substrate.

Epitaxial TiO2 nanoparticles on Pt(111): a structural study by photoelectron diffraction and scanning tunneling microscopy.

Angle-scanned X-ray photoelectron diffraction (XPD) and scanning tunneling microscopy (STM) are used to characterise the structure of TiO2 nanoparticles grown on a Pt(111) single crystal surface. The nanoparticles grow over a well-ordered oxide interfacial layer that displays a (square root 43 x square root 43) - R7.6 degrees superstructure with a unit cell (18.2 x 18.2 A), as demonstrated by STM and low-energy electron diffraction (LEED). Our XPS Ti 2p core level spectra suggest a significant contribution from reduced titanium ions within the interfacial layer. On the contrary, according to XPS binding energy data, the nanoparticles are mostly composed of Ti(IV) ions. During the initial stage of the growth, the nanoparticles are on the average 2 nm high and about some tens of nm wide, and show a flat on-top surface, while the interparticle region show the structure of the ordered interfacial layer. During later stages of the deposition, the particles become larger and they show a more irregular, globular-like shape as well as coalescence. But, even at this stage of the growth, large interparticle regions are present. Moreover, the nanoparticles produce a distinct XPD pattern which demonstrates that they grow with a preferential azimuthal orientation with respect to the substrate surface. A simple geometrical analysis of the XPD data in terms of forward scattering events suggests that the particles crystallize in the rutile TiO2 structure and expose the (100) surface. This hypothesis is supported by means of multiple scattering simulations of the XPD patterns.