Molecular Desorption Research Articles

A kinetic study of ZnO atomic layer deposition: Effects of surface hydroxyl concentration and steric hindrance

We present a kinetic model for the reaction of diethylzinc (DEZ) adsorption in atomic layer deposition (ALD) of ZnO from DEZ and water. The proposed model has been verified by comparing kinetic experimental data to the prediction of the model in the temperature range of 60–200 °C. In this model, DEZ molecules are molecularly adsorbed on the hydroxyl-terminated surface in the first elementary reaction. Then the molecularly-adsorbed DEZs either desorb from the surface, or undergo an irreversible ligand exchange reaction and form monoethylzinc (MEZ)-terminated surface by liberating ethane molecules. According to the integrated rate law of the model, as the exposure time of DEZ increases, the growth per cycle (gpc) of ALD, i.e., the thickness increment per cycle, rapidly increases and then saturates showing the self-limiting growth behavior. The required DEZ exposure time to reach the saturated gpc value is shortened when the chemical equilibrium between the molecular adsorption and desorption shifts toward the adsorption, as this leads to higher effective rate constant in the overall mechanism of the DEZ adsorption. Although the saturated gpc value is primarily governed by the hydroxyl concentration on the ZnO surface, it is also heavily influenced by the steric hindrance due to the bulkiness of the ethyl ligands. We have determined that the critical temperature at which the steric hindrance disappears is around 150–200 °C, by investigating the variation of the hydroxyl concentration with temperature. At temperatures lower than 150 °C, we have observed that the saturated gpc value is governed by the steric hindrance rather than the hydroxyl concentration. However, the saturated gpc value at 200 °C has been achieved purely by the hydroxyl concentration. In other words, all hydroxyl groups at 200 °C have been consumed and saturated by the MEZ molecules of which the steric hindrance-free concentration has been evaluated to be ∼7.1 /nm2. In addition, the effective activation energy was estimated to be ∼8.50 kJ/mol by using the effective rate constants of all temperatures investigated.

Stimulated CO Dissociation and Surface Graphitization by Microfocused X-ray and Electron Beams

The irradiation with photons or electrons can dramatically influence the chemical stability of a molecule, either free or adsorbed on a surface, inducing its fragmentation or desorption. We revisit here the exostimulated dissociation of CO, a prototypical case, choosing hcp thin cobalt films as model support. Intense, microfocused soft X-rays or electron beams are used to locally stimulate CO dissociation. Fast-XPS gives direct access to the adsorbates’ chemical state and coverage during irradiation, enabling the kinetics of the process to be monitored in real time. The energy-dependent cross sections for photon and electron stimulated molecular dissociation and desorption are estimated for a fixed initial CO coverage of 1/3 ML. In the soft X-ray regime, the desorption channel always prevails over dissociation and is significantly enhanced above the O K edge. The relative dissociation probability increases steadily with increasing photon energy, reaching 30% at 780 eV. Furthermore, we show that low energy electrons in the range 50 to 200 eV dissociate CO more efficiently than X-rays. The prolonged irradiation of the Co surface in CO ambient is found to produce a continuous increase of the carbon coverage, initially promoting the formation of carbides and subsequently accumulating sp2 carbon on the surface. Far from being a detrimental effect, the CO stimulated dissociation can be exploited to lithographically graft carbon-rich microscopic patterns on Co, with resolution well into the nanometer scale. A brief thermal treatment following irradiation results in the formation of a graphitic carbon overlayer, which effectively protects Co from oxidation upon exposure to ambient conditions, preserving its out-of-plane magnetic anisotropy and domain configuration.

Bulk Defect Dependence of Low-Temperature Partial Oxidation of Methanol and High-Temperature Hydrocarbon Formation on Rutile TiO2 (110)

Titanium dioxide (TiO2) is among the most studied model (photo)catalyst materials. The influence of surface point defects, like oxygen vacancies and particularly bulk defects such as Ti3+ interstitials, is usually underestimated or even ignored. We present a systematic study under well-defined UHV conditions illustrating the importance of such defects for the thermal reaction of methanol at the rutile TiO2 (110) single crystal surface by using temperature-programmed reaction spectroscopy (TPRS) and Fourier-transform infrared reflection–absorption spectroscopy (FT-IRRAS). It will be shown that the population of different reaction pathways, namely, the partial oxidation of methanol to formaldehyde and the deoxygenation forming hydrocarbons, especially methane, depends on the bulk defect density and the presence or absence of oxygen adsorbates. While at elevated temperatures molecular desorption is pronounced for the less defective substrates, for higher reduction grades the high-temperature deoxygenation ch...

Guaiacol Adsorption and Decomposition on Platinum

Guaiacol (2-methoxyphenol, C6H4(OH)(OCH3)) adsorption and reactions on a Pt(100) surface were studied with infrared reflection–absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) measurements at different surface coverage values from 100 to 800 K. In addition, density functional theory (DFT) calculations were used to determine geometries, adsorption energies, and vibrational frequencies for adsorption structures. Depending on surface coverage, guaiacol formed one or two physisorbed states. At low coverage, a single state with a desorption peak at 225 K was observed. At high coverage, two physisorbed states were observed with desorption peaks at 195 and 225 K. At temperatures above 225 K, after the desorption of physisorbed layers, a dissociatively adsorbed structure, C6H4O(OCH3) + H, was observed. Recombinative molecular guaiacol desorption was detected at 320 K. The dissociatively adsorbed structure was stable up to 337 K when C–O bonds began to break. Molecularly adsorbed guaiacol...

Molecular dynamics simulations of adsorption and desorption of bone morphogenetic protein-2 on textured hydroxyapatite surfaces

Interactions between bone morphogenetic protein-2 (BMP-2) and biomaterial surfaces are of great significance in the fields of regenerative medicine and bone tissue engineering. In this work, the adsorption and desorption behaviors of BMP-2 on a series of nano-textured hydroxyapatite (HAP) surfaces were systematically investigated by combined molecular dynamic (MD) simulations and steered molecular dynamic (SMD) simulations. The textured HAP surfaces exhibited nanostructured topographies and played a critical role in the mediation of dynamic behaviors of BMP-2. Compared to the HAP-flat model, the HAP-1:1 group (means ridge vs groove = 1:1) showed the excellent ability to capture BMP-2, less conformation change of BMP-2 molecule, and high cysteine-knot stability during the adsorption and desorption processes. These findings suggest that nano-textured HAP surfaces are more capable of loading BMP-2 molecules, and most importantly, they can help maintain a higher biological activity of BMP-2 cargos. In the present study, for the first time, we have deeply clarified the adsorption and desorption dynamics of BMP-2 on various nano-textured HAP surfaces at the atomic level, which can provide significant guidelines for the future design of BMP-2-based tissue engineering implants/scaffolds. Statement of SignificanceBy using combined molecular dynamic (MD) simulations and steered molecular dynamic (SMD) simulations, the adsorption and desorption dynamics of bone morphogenetic protein-2 (BMP-2) dimer on a series of nano-textured hydroxyapatite (HAP) surfaces at the atomic level were presented in details for the first time. We have proved that the HAP-1:1 model (means ridge vs groove = 1:1) possessed excellent ability to capture BMP-2, less conformation change, and high cysteine-knot stability. As a result, the nano-textured topography of HAP-1:1 could maintain a relatively high biological activity of BMP-2 cargos. This work could provide theoretical guidelines for the design of BMP-2-based implants/scaffolds for bone tissue engineering.

The influence on electrical characteristics of amorphous indium tungsten oxide thin film transistors with multi-stacked active layer structure

In order to effectively enhance the carrier mobility and device stability, simultaneously, a multi-stacked active layer of thin film transistor with a novel type of channel material, amorphous indium-tungsten-oxide, is proposed in this work. Atop-cappingoxygen-rich indium-tungsten-zinc-oxide thin film is used for suppressing the influence of the molecular adsorption or desorption from the ambient at the back channel in the gate bias stress test. In addition, a bottom-bufferingoxygen-rich indium-tungsten-zinc-oxide thin film is deposited to avoid the oxygen vacancy generation during the following thermal process. On the other hand, a 1-nm thick WO3 film is inserted between the high-k gate insulator and active layer, which play important roles as interfacial layer for improving the interface quality and keeping the stability of front channel film. Besides, an HfO2 dielectric film is chosen as gate insulator for realizing the low-voltage operation. As a result, the novel tungsten doped channel material with both-sideoxygen-rich active layer exhibits a high ION/IOFF current ratio of ~6 × 107 for low gate leakage current, attributing to the top-cappingoxygen-rich thin film. Then, a high field-effect mobility of ~27.9 cm2/V.s and a low sub-threshold swing of 0.079 V/decade are achieved by the good interface quality. This structure with both-sideoxygen-rich active layer exhibits its potential application for the future high-resolution and large-size display manufacture.

Ultrasensitive and Fully Reversible NO2 Gas Sensing Based on p-Type MoTe2 under Ultraviolet Illumination.

The unique properties of two-dimensional (2D) materials make them promising candidates for chemical and biological sensing applications. However, most 2D material sensors suffer from extremely long recovery time due to the slow molecular desorption at room temperature. Here, we report an ultrasensitive p-type molybdenum ditelluride (MoTe2) gas sensor for NO2 detection with greatly enhanced sensitivity and recovery rate under ultraviolet (UV) illumination. Specifically, the sensitivity of the sensor to NO2 is dramatically enhanced by 1 order of magnitude under 254 nm UV illumination as compared to that in the dark condition, leading to a remarkable low detection limit of 252 ppt. More importantly, the p-type MoTe2 sensor can achieve full recovery after each sensing cycle well within 160 s at room temperature. Finally, the p-type MoTe2 sensor also exhibits excellent sensing performance to NO2 in ambient air and negligible response to H2O, indicating its great potential in practical applications, such as breath analysis and ambient NO2 detection. Such impressive features originate from the activated interface interaction between the gas molecules and p-type MoTe2 surface under UV illumination. This work provides a promising and easily applicable strategy to improve the performance of the gas sensors based on 2D materials.

Theoretical Study of Interaction between Hydrogen and Small Pt–Sn Intermetallic Clusters

Small clusters, which simulate the active sites of Pt–Sn intermetallics exhibiting a high level of activity and selectivity in the deoxygenation reaction of esters without the loss of carbon mass to form C1, C2, and carbon oxides, are constructed and studied with the density functional theory. Molecular adsorption of hydrogen, dissociation of hydrogen molecules at Pt sites, and transition of adsorbed hydrogen atoms from Pt to Sn are considered. The introduction of Sn significantly decreases the affinity of platinum to hydrogen, so that the transition of H atoms to Sn atoms is facilitated with the increase in the amount of Sn. A comparison of the activation energies for such a transition with those of the possible association of hydrogen atoms on tin and the molecular desorption of H2 showed that the hydrogen spillover in the Pt–Sn intermetallics should not lead to a significant accumulation of hydrogen on tin. In other words, in contrast to Pt atoms, Sn atoms probably cannot serve as active sites of hydrogen adsorption in the deoxygenation reaction.

Mechanisms of elementary hydrogen ion-surface interactions during multilayer graphene etching at high surface temperature as a function of flux

In order to optimize the plasma-synthesis and modification process of carbon nanomaterials for applications such as nanoelectronics and energy storage, a deeper understanding of fundamental hydrogen-graphite/graphene interactions is required. Atomistic simulations by Molecular Dynamics have proven to be indispensable to illuminate these phenomena. However, severe time-scale limitations restrict them to very fast processes such as reflection, while slow thermal processes such as surface diffusion and molecular desorption are commonly inaccessible. In this work, we could however reach these thermal processes for the first time at time-scales and surface temperatures (1000 K) similar to high-flux plasma exposure experiments during the simulation of multilayer graphene etching by 5 eV H ions. This was achieved by applying the Collective Variable-Driven Hyperdynamics biasing technique, which extended the inter-impact time over a range of six orders of magnitude, down to a more realistic ion-flux of 1023 m−2s−1. The results show that this not only causes a strong shift from predominant ion-to thermally-induced interactions, but also significantly affects the hydrogen uptake and surface evolution. This study thus elucidates H ion-graphite/graphene interaction mechanisms and stresses the importance of including long time-scales in atomistic simulations at high surface temperatures to understand the dynamics of the ion-surface system.

New insights into mechanisms of material ejection in MALDI mass spectrometry for a wide range of spot sizes

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is widely used for the analysis of large biomolecules in numerous applications. The technique utilizes nanosecond-long laser pulses at various spot sizes to eject and ionize large molecules embedded in a highly absorptive chemical matrix. Despite the methods name, ‘molecular desorption’ from the matrix crystal surface is not the sole mechanism discussed for material ejection in MALDI, but additional ablation of larger clusters has been reported. Here we present results on the influence of laser fluence and spot size on the mechanisms of the initial material ejection in MALDI and subsequent plume development. We used a laser-based postionization (MALDI-2) as well as a complementary photoacoustic method to monitor the material ejection step. The photoacoustic data reveal a quasi-thermal sublimation process up to a transition fluence. Above this threshold fluence additional ablation processes are observed. Complementary investigations on plume dynamics by MALDI-2 showed an ejection of predominantly fast particles for desorption conditions while ablation produces considerably slower ejecta. Additionally the presented results revealed a peculiar influence of the spot size on analyte fragmentation as well as plume development and allows for new insights into the unexplained spot size effect reported for MALDI.

Metal-insulator-metal sensors monitoring charge flow during thermal desorption

Metal-insulator-metal sensors with a new design have been developed which allow us to perform thermal desorption spectroscopy of weakly bound adsorbates. The sensor can be temperature ramped utilising an ITO layer which is electrically isolated from the two metal layers such that a sensitive measurement of the device current is not hampered. An accurate reading of the temperature of the metal surface is derived from the I-V curve of the sensor which is found to be temperature sensitive. Concurrently with the thermal desorption spectrum the device current is recorded allowing us to correlate the latter with distinct desorption processes. For the molecules, H2O, small alcohols, NH3 and HCOOH we obtain thermal desorption spectra. Concurrently with recoding the spectra we detect characteristic device currents. These are found to solely result from interactions between the substrate and the first monolayer of molecules. We suggest that these are due to shifts of the Fermi level of the top metal film in the MIM device induced by the charge transfer processes connected with molecular adsorption and desorption.

Characterization of organic nitrate constituents of secondary organic aerosol (SOA) from nitrate-radical-initiated oxidation of limonene using high-resolution chemical ionization mass spectrometry

Abstract. The gas-phase nitrate radical (NO3⚫) initiated oxidation of limonene can produce organic nitrate species with varying physical properties. Low-volatility products can contribute to secondary organic aerosol (SOA) formation and organic nitrates may serve as a NOx reservoir, which could be especially important in regions with high biogenic emissions. This work presents the measurement results from flow reactor studies on the reaction of NO3⚫ with limonene using a High-Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (HR-ToF-CIMS) combined with a Filter Inlet for Gases and AEROsols (FIGAERO). Major condensed-phase species were compared to those in the Master Chemical Mechanism (MCM) limonene mechanism, and many non-listed species were identified. The volatility properties of the most prevalent organic nitrates in the produced SOA were determined. Analysis of multiple experiments resulted in the identification of several dominant species (including C10H15NO6, C10H17NO6, C8H11NO6, C10H17NO7, and C9H13NO7) that occurred in the SOA under all conditions considered. Additionally, the formation of dimers was consistently observed and these species resided almost completely in the particle phase. The identities of these species are discussed, and formation mechanisms are proposed. Cluster analysis of the desorption temperatures corresponding to the analyzed particle-phase species yielded at least five distinct groupings based on a combination of molecular weight and desorption profile. Overall, the results indicate that the oxidation of limonene by NO3⚫ produces a complex mixture of highly oxygenated monomer and dimer products that contribute to SOA formation.

Single Carbon Nanotubes as Ultrasmall All-Optical Memories

Performance improvements are expected from integration of photonic devices into information processing systems, and in particular, all-optical memories provide a key functionality. Scaling down the size of memory elements is desirable for high-density integration, and the use of nanomaterials would allow for devices that are significantly smaller than the operation wavelengths. Here we report on all-optical memory based on individual carbon nanotubes, where adsorbed molecules give rise to optical bistability. By exciting at the high-energy tail of the excitonic absorption resonance, nanotubes can be switched between the desorbed state and the adsorbed state. We demonstrate reversible and reproducible operation of the nanotube optical memory, and determine the rewriting speed by measuring the molecular adsorption and desorption times. Our results underscore the impact of molecular-scale effects on optical properties of nanomaterials, offering new design strategies for photonic devices that are a few orders of magnitude smaller than the optical diffraction limit.

Ambient-Pressure XPS Studies of Reactions of Alcohols on SrTiO3(100)

Ambient-pressure X-ray photoelectron spectroscopy (APXPS) and temperature-programmed desorption (TPD) have been employed to elucidate the adsorption and reaction of simple alcohols on SrTiO3(100). TPD experiments indicate molecular desorption of alcohols with a small amount of aldehydes below 100 °C, whereas no gas-phase products are observed above this temperature. APXPS spectra at 0.1 Torr show that alcohols adsorb dissociatively onto SrTiO3(100) to form alkoxies. Surface methoxides appear to react with each other to produce acetate as a surface intermediate. These surface species are eventually oxidized to gas-phase CO, CO2, and H2O. Ethoxide is readily oxidized to acetate species that undergo further reactions to form gas-phase products. CO2 is the only C-containing product during ethanol oxidation, whereas methanol can also produce the partial oxidation product, CO. When no O2 is present, alcohol oxidation yields gas-phase CO, CO2, or H2O and creates oxygen vacancies on the surface, resulting in the ...

Analysis of hop acids by thin-layer chromatography and the Molecular Ionization Desorption Analysis Source (MIDAS) for mass spectrometry

A thin-layer chromatography ambient mass spectrometry method was developed for the quantitative analysis of hop acids. Hop acids were extracted from three varieties of hop pellets, Apollo, Cluster and Czech Saaz. Reversed-phase thin-layer chromatography provided the optimum stationary phase for separation and ionization. Using standards, a method was developed to separate α-, iso-α- and β-acids by thin-layer chromatography. Each thin-layer chromatography plate was analyzed directly by the Molecular Ionization Desorption Analysis Source (MIDAS). MIDAS is an atmospheric pressure ambient ionization source for mass spectrometry. Quantitative analysis of the hop extracts was made by successive analysis of known amounts of standards and the determination of a response factor. The expected trend of total α-acid content with Apollo>Cluster>Czech Saaz was observed. Variation in the results was attributed to manual TLC plate spotting and not to the ionization source.

A Theoretical Study of Methanol Oxidation on RuO2(110): Bridging the Pressure Gap

Partial oxidation catalysis is often fraught with selectivity problems, largely because there is a tendency of oxidation products to be more reactive than the starting material. One industrial process that has successfully overcome this problem is partial oxidation of methanol to formaldehyde. This process has become a global success, with an annual production of 30 million tons. Although ruthenium catalysts have not shown activity as high as the current molybdena or silver-based industrial standards, the study of ruthenium systems has the potential to elucidate which catalyst properties facilitate the desired partial oxidation reaction as opposed to deep combustion due to a pressure-dependent selectivity “switch” that has been observed in ruthenium-based catalysts. In this work, we find that we are able to successfully rationalize this “pressure gap” using near-ab initio steady-state microkinetic modeling on RuO2(110). We obtain molecular desorption prefactors from experiment and determine all other ener...

Migration and desorption of hydrogen atom and molecule on/from graphene

Migration and desorption of hydrogen atom and molecule on/from graphene are crucial for understanding mechanism of the hydrogen storage in graphene and graphene synthesis. All possible hydrogen atom migration, hydrogen atom and hydrogen molecule desorption from single layer graphene have been studied systematically by using functional density theory B3LYP/6-31G(d). Our results show that the hydrogen atom desorption is energetically favorable with the lowest desorption barrier of 149.3 kJ mol−1 from stable hydrogen adsorption sites, while the migration is restricted due to high barriers which are greater than 191.0 kJ mol−1. Hydrogen molecular desorption is possible only from edge adsorbed hydrogen atom due to its comparable barrier of 149.9 kJ mol−1. Other hydrogen molecular desorption pathways are less likely due to small population of the reaction species.

Adsorption energy of oxygen molecules on graphene and two-dimensional tungsten disulfide

Adsorption of gas molecules on the surface of atomically layered two-dimensional (2D) materials, including graphene and transition metal dichalcogenides, can significantly affect their electrical and optical properties. Therefore, a microscopic and quantitative understanding of the mechanism and dynamics of molecular adsorption and desorption has to be achieved in order to advance device applications based on these materials. However, recent theoretical calculations have yielded contradictory results, particularly on the magnitude of the adsorption energy. Here, we have experimentally determined the adsorption energy of oxygen molecules on graphene and 2D tungsten disulfide using temperature-programmed terahertz (THz) emission microscopy (TPTEM). The temperature dependence of THz emission from InP surfaces covered with 2D materials reflects the change in oxygen concentration due to thermal desorption, which we used to estimate the adsorption energy of oxygen molecules on graphene (~0.15 eV) and tungsten disulphide (~0.24 eV). Furthermore, we used TPTEM to visualize relative changes in the spatial distribution of oxygen molecules on monolayer graphene during adsorption and desorption. Our results provide much insight into the mechanism of molecular adsorption on the surface of 2D materials, while introducing TPTEM as a novel and powerful tool for molecular surface science.

(Invited) Optical Bistability in Air-Suspended Carbon Nanotubes

Screening is limited in single-walled carbon nanotubes because of their one-dimensional nature, and the strong Coulomb interactions result in optical spectra dominated by excitons with binding energies lager than half an eV. Also because of the limited screening, a charged carrier can be bound to an exciton to form a trion that is stable at room temperature. In particular, air-suspended nanotubes exhibit pronounced effects because of reduced environmental screening [1,2], where adsorption and desorption of molecules on the surface of nanotubes can readily modulate such interactions. Here we show that large screening effects on excitonic energies can cause laser-induced optical bistability [3]. Molecular desorption can occur with sufficient heating by laser, which in turn results in significant blueshift of exciton emission wavelength. When the excitation is detuned to a higher energy shoulder of the E22 absorption resonance, we observe abrupt transition and hysteretic behavior of emission wavelength in the excitation power dependence. We explain the bistability by simultaneous shifts of absorption and emission resonances, and perform time-resolved measurements to obtain relevant time scales. Work supported by JSPS (KAKENHI JP26610080, JP16H05962), The Canon Foundation, and MEXT (Photon Frontier Network Program, Nanotechnology Platform). [1] T. Uda, M. Yoshida, A. Ishii, Y. K. Kato, Nano Lett. 16, 2278 (2016).[2] M. Yoshida, A. Popert, Y. K. Kato, Phys. Rev. B 93, 041402(R) (2016).[3] T. Uda, A. Ishii, Y. K. Kato, in preparation.

Highly sensitive MoTe2 chemical sensor with fast recovery rate through gate biasing

The unique properties of two dimensional (2D) materials make them promising candidates for chemical and biological sensing applications. However, most 2D nanomaterial sensors suffer very long recovery time due to slow molecular desorption at room temperature. Here, we report a highly sensitive molybdenum ditelluride (MoTe2) gas sensor for NO2 and NH3 detection with greatly enhanced recovery rate. The effects of gate bias on sensing performance have been systematically studied. It is found that the recovery kinetics can be effectively adjusted by biasing the sensor to different gate voltages. Under the optimum biasing potential, the MoTe2 sensor can achieve more than 90% recovery after each sensing cycle well within 10 min at room temperature. The results demonstrate the potential of MoTe2 as a promising candidate for high-performance chemical sensors. The idea of exploiting gate bias to adjust molecular desorption kinetics can be readily applied to much wider sensing platforms based on 2D nanomaterials.