Desorption Threshold Research Articles

Initial Coupling and Reaction Progression of Directly Deposited Biradical Graphene Nanoribbon Monomers on Iodine-Passivated Versus Pristine Ag(111)

The development of widely applicable methods for the synthesis of C-C-bonded nanostructures on inert and insulating surfaces is a challenging yet rewarding milestone in the field of on-surface synthesis. This would enable studies of nearly unperturbed covalent nanostructures with unique electronic properties as graphene nanoribbons (GNR) and π-conjugated 2D polymers. The prevalent Ullmann-type couplings are almost exclusively carried out on metal surfaces to lower the temperature required for initial dehalogenation well below the desorption threshold. To overcome the necessity for the activation of monomers on the target surface, we employ a recently developed Radical Deposition Source (RaDeS) for the direct deposition of radicals onto inert surfaces for subsequent coupling by addition reactions. The radicals are generated en route by indirect deposition of halogenated precursors through a heated reactive tube, where the dehalogenation reaction proceeds. Here, we use the ditopic 6,11-diiodo-1,2,3,4-tetraphenyltriphenylene (DITTP) precursor that afforded chevron-like GNR on Au(111) via the usual two-staged reaction comprised of monomer-coupling into covalent polymers and subsequent formation of an extended GNR by intramolecular cyclodehydrogenation (CDH). As a model system for inert surfaces, we use Ag(111) passivated with a closed monolayer of chemisorbed iodine that behaves in an inert manner with respect to dehalogenation reactions and facilitates the progressive coupling of radicals into extended covalent structures. We deposit the DITTP-derived biradicals onto both iodine-passivated and pristine Ag(111) surfaces. While on the passivated surface, we directly observe the formation of covalent polymers, on pristine Ag(111) organometallic intermediates emerge instead. This has decisive consequences for the further progression of the reaction: heating the organometallic chain directly on Ag(111) results in complete desorption, whereas the covalent polymer on iodine-passivated Ag(111) can be transformed into the GNR. Yet, the respective CDH proceeds directly on Ag(111) after thermal desorption of the iodine passivation. Accordingly, future work is aimed at the further development of approaches for the complete synthesis of GNR on inert surfaces.

Role of nanoscale surface defects on Sn adsorption and diffusion behavior on oxidized Nb(100)

Nanoscale structural defects such as grain boundaries, atomic dislocations, and surface roughness inhibit the stoichiometrically homogeneous growth of Nb3Sn on Nb. This is a critical technological bottleneck for the implementation of next-generation Nb3Sn superconducting radio frequency cavities, as thin film inhomogeneities are known to degrade superconducting properties that are essential for reaching optimal cavity performance. To determine the influence of structural defects on Nb3Sn film growth, low and moderate surface defect densities were intentionally induced onto a (3 × 1)-O Nb(100) substrate, which serves as a model system to study atomic-scale Sn adsorption and diffusion. Scanning tunneling microscopy shows that, while initial Sn adsorption behavior at room temperature differs between the low and moderate defect density Nb(100) surfaces, the overall diffusion pathways at elevated temperatures are guided by the underlying oxide structure with variations resulting from increased nanoscale surface defects. The (3 × 1)-O Nb(100) surface with a moderate defect density also demonstrates enhanced Sn thermal stability, with the Sn desorption threshold occurring between 850 and 900 °C, approximately 50 °C higher than desorption from both the low defect density and pristine thin oxide surfaces. This suggests that structural surface defects may stabilize adsorbed Sn species on oxidized Nb at the elevated temperatures utilized in Nb3Sn alloy growth procedures. Auger electron spectroscopy shows no significant difference in surface composition following Sn deposition at varying coverages on the pristine and defect-induced (3 × 1)-O Nb(100) surfaces. This indicates that the surface and near-surface composition are not influenced by the presence of nanoscale surface defects despite slight attenuations in Sn diffusion pathways on defected substrates. These results provide the first in situ visualization of Sn adsorption and diffusion behavior on oxidized Nb at the nanoscale, revealing the significance of the underlying Nb oxide surface structure and defect density on Nb3Sn film growth and, ultimately, cavity performance.

On the Control of Plasma Chemistry and Silicon Etching Kinetics in Ternary HBr + Cl2 + O2 Gas System: Effects of HBr/O2 and Cl2/O2 Mixing Ratios

The influences of both HBr/O2 (at constant Cl2 fraction) and Cl2/O2 (at constant HBr fraction) ratios in HBr + Cl 2 + O2 gas mixture on bulk plasma characteristics, active species densities and etching kinetics of silicon were studied. The results indicated that an increase in O2 content in a feed gas at constant Cl2 fraction in a processing gas (1) produces the stronger impact on plasma chemistry by the influence on the kinetics of electron-impact and atom-molecular reaction; and (2) provides the wider adjustments for both halogen atom flux and ion flux with the opposite tendencies with those for variable Cl2/O2 mixing ratio. The experiments demonstrated that the transition toward more oxygenated plasmas in both cases lowers the Si etching rate as well as result is decreasing effective reaction probability and etching yield. These effects may be associated with decreasing amount of adsorption sites for Cl/Br atoms as well as increasing sputtering (ion-stimulated desorption) threshold for reaction products due to the formation of the low-volatile silicon oxy-chlorides and-bromides in heterogeneous SiClx + O/OH and SiBrx + O/OH reactions.

Light-induced atomic desorption of lithium.

We demonstrate loading of a Li magneto-optical trap using light-induced atomic desorption. The magnetooptical trap confines up to approximately 4 × 104 7Li atoms with loading rates up to approximately 4 × 103 atoms per second. We study the Li desorption rate as a function of the desorption wavelength and power. The extracted wavelength threshold for desorption of Li from fused silica is approximately 470 nm. In addition to desorption of lithium, we observe light-induced desorption of background gas molecules. The vacuum pressure increase due to the desorbed background molecules is ≲ 50 % and the vacuum pressure decreases back to its base value with characteristic timescales on the order of seconds when we extinguish the desorption light. By examining both the loading and decay curves of the magneto-optical trap, we are able to disentangle the trap decay rates due to background gases and desorbed lithium. Our results show that light-induced atomic desorption can be a viable Li vapor source for compact devices and sensors.

Ion Desorption from TiO<sub>2</sub>(110) by Low Energy Positron Impact

We have observed positron-stimulated ion desorption from a TiO2(110) surface. H+ and O+ ions were desorbed at incident positron energies above the desorption thresholds for electron impact. However, only O+ ions were detected at energies below those thresholds. These results suggest that by surface ionization positron annihilation as well as by positron impact leads to the O+ ion desorption. By contrast, it is likely that the H+ ions are not desorbed by positron annihilation, but rather by impact ionization.

Salt-induced reentrant stability of polyion-decorated particles with tunable surface charge density

The electrostatic complexation between DOTAP/DOPC unilamellar liposomes and an oppositely charged polyelectrolyte (NaPA) has been investigated in a wide range of the liposome surface charge density. We systematically characterized the reentrant condensation and the charge inversion of polyelectrolyte-decorated liposomes by means of dynamic light scattering and electrophoresis. We explored the stability of this model polyelectrolyte/colloid system at different values of the surface charge of the bare liposomes and by changing two independent control parameters of the suspensions: the polyelectrolyte/colloid charge ratio and the ionic strength of the aqueous suspending medium. The progressive addition of neutral DOPC lipid within the liposome membrane gave rise to an interesting phenomenon which has not been observed previously: the stability diagram of the suspensions showed a novel reentrance due to the crossing of the desorption threshold of the polyelectrolyte. Indeed, at fixed charge density of the bare DOTAP/DOPC liposomes and for a wide range of polyion concentrations, we showed that the simple electrolyte addition first (low salt regime) destabilizes the suspensions because of the enhanced screening of the residual repulsion between the complexes, and then (high salt regime) determines the onset of a new stable phase, originated by the absence of polyelectrolyte adsorption on the particle surfaces. We show that the observed phenomenology can be rationalized within the modified Velegol–Thwar model for heterogeneously charged particles and that the polyelectrolyte desorption fits well the predictions of the adsorption theory of Winkler and Cherstvy [1]. Our findings unambiguously support the picture of the reentrant condensation as driven by the correlated adsorption of the polyelectrolyte chains on the particle surface, providing interesting insights into possible mechanisms for tailoring complex colloids via salt-induced effects.

Role of Low-Energy Electrons (<35 eV) in the Degradation of Fe(CO)5 for Focused Electron Beam Induced Deposition Applications: Study by Electron Stimulated Desorption of Negative and Positive Ions

Fe(CO)5 is one of the most common precursors used in focused electron beam induced deposition (FEBID). Since high-energy electrons interacting with matter produce large amounts of secondary electrons with energies <50 eV, those generated within the substrate in FEBID are expected to play a major role in the dissociation process of the precursor molecules. The aim of this study is to identify the role of the secondary electrons in the deposition process of Fe(CO)5 and the relevant dissociation mechanisms, using an electron stimulated desorption system with 4–33 eV electrons. The desorption of charged fragments from thin films of Fe(CO)5, condensed on Xe or onto a Pt foil, was measured as a function of incident electron energy, incident current, and thickness of both Fe(CO)5 and Xe films. Both dissociative electron attachment (DEA) and dipolar dissociation (DD) are involved in the production of anions, specifically, C– and O–, and Fe(CO)x– (x = 0–4). Cations C+, O+, CO+, and Fe(CO)x+ (x = 0–4) were detected with desorption thresholds in the ∼15–25 eV range. These fragments are produced via direct dissociative processes. This study reveals the detailed dissociation mechanisms of Fe(CO)5 induced by low-energy electron impact by the direct observation of Fe(CO)x fragments under different conditions, thus confirming the frequently proposed mechanisms of deposition of Fe by FEBID.

Positron-annihilation-induced ion desorption fromTiO2(110)

We have investigated the positron-stimulated desorption of ions from a ${\text{TiO}}_{2}(110)$ surface. Desorbed O${}^{+}$ ions were detected in coincidence with the emission of annihilation $\ensuremath{\gamma}$ rays. The energy dependence of the ion yields shows that the O${}^{+}$ ions were detected at energies much lower than the previously reported threshold for electron impact desorption corresponding to the excitation energy of Ti($3p$) core electrons. These results provide evidence that core-hole creation by positron annihilation with electrons in the core levels leads to ion desorption.

Femtosecond laser desorption of ultrathin polymer films from a dielectric surface

By focusing femtosecond pulses on the front and rear surface of a fused silica coverslip, we desorb 8-nm thick polymer films at submicron scale. To determine the role of the substrate in the desorption process, we measure the threshold for nonlinear absorption in fused silica and compare it to the threshold for desorption, taking into account the enhancement of the field at the dielectric-air interface. The results indicate that absorption of energy only occurs in the film. We then measure the beam radius in situ by the knife-edge technique and characterize the desorption by atomic force microscopy. The radius of the laser desorbed area is determined by the desorption threshold intensity and can be a factor of 5 smaller than the beam waist.

Evidence for adsorbate-enhanced field emission from carbon nanotube fibers

We used residual gas analysis (RGA) to identify the species desorbed during field emission (FE) from a carbon nanotube (CNT) fiber. The RGA data show a sharp threshold for H2 desorption at an external field strength that coincides with a breakpoint in the FE data. A comprehensive model for the gradual transition of FE from adsorbate-enhanced CNTs at low bias to FE from CNTs with reduced H2 adsorbate coverage at high bias is developed which accounts for the gradual desorption of the H2 adsorbates, alignment of the CNTs at the fiber tip, and importance of self-heating effects with applied bias.

Dynamics of the sputtering of water from ice films by collisions with energetic xenon atoms

The flow of energy from the impact site of a heavy, translationally energetic xenon atom on an ice surface leads to several non-equilibrium events. The central focus of this paper is on the collision-induced desorption (sputtering) of water molecules into the gas-phase from the ice surface. Sputtering is strongly activated with respect to xenon translational energy, and a threshold for desorption was observed. To best understand these results, we discuss our findings in the context of other sputtering studies of molecular solids. The sputtering yield is quite small; differential measurements of the energy of xenon scattered from ice surfaces show that the ice efficiently accommodates the collisional energy. These results are important as they quantitatively elucidate the dynamics of such sputtering events, with implications for energetic non-equilibrium processes at interfaces.

Effects of Coadsorbed Oxygen on the Infrared Driven Decomposition of N2O on Isolated Rh5+ Clusters

The thermally induced decomposition of nitrous oxide on isolated Rh5+ and Rh5O+ clusters has been investigated using mid-infrared multiple photon dissociation spectroscopy. The presence of a single coadsorbed oxygen atom is observed to have a profound effect on the cluster surface processes that ensue following infrared heating of the cluster. Exciting the infrared active N2O bending transition in Rh5N2O+ results predominantly (≥85%) in molecular desorption of the N2O moiety, while the same excitation in Rh5ON2O+ leads instead to N2O dissociation on the cluster surface producing Rh5O2+ (≥85%). Calculations of the reaction pathway using density functional theory indicate that the change in branching ratio arises from a 0.4 eV greater binding energy of N2O to Rh5O+ compared with Rh5+, taking the desorption threshold above the reaction barrier for the surface reaction channel whose energy is unchanged.

Polarization dependent fragmentation of ions produced by laser desorption from nanopost arrays

Tailored silicon nanopost arrays (NAPA) enable controlled and resonant ion production in laser desorption ionization experiments and have been termed nanophotonic ion sources (Walker et al., J. Phys. Chem. C, 2010, 114, 4835-4840). As the post dimensions are comparable to or smaller than the laser wavelength, near-field effects and localized electromagnetic fields are present in their vicinity. In this contribution, we explore the desorption and ionization mechanism by studying how surface derivatization affects ion yields and fragmentation. We demonstrate that by increasing the laser fluence on derivatized NAPA with less polar surfaces that have decreased interaction energy between the structured silicon substrate and the adsorbate, the spectrum changes from exhibiting primarily molecular ions to showing a growing variety and abundance of fragments. The polarization angle of the laser beam had been shown to dramatically affect the ion yields of adsorbates. For the first time, we report that by rotating the plane of polarization of the desorption laser, the internal energy of the adsorbate can also be modulated resulting in polarization dependent fragmentation. This polarization effect also resulted in selective fragmentation of vitamin B(12). To explore the internal energy of NAPA generated ions, the effect of the post aspect ratios on the laser desorption thresholds and on the internal energy of a preformed ion was studied. Elevated surface temperatures and enhanced near fields in the vicinity of high aspect ratio posts are thought to contribute to desorption and ionization from NAPA. Comparison of the fluence dependence of the internal energies of ions produced from nanoporous silicon and NAPA substrates indicates that surface restructuring or transient melting by the desorption laser is a prerequisite for the former but not for the latter.

Chemistry of fast electrons

A chemicurrent is a flux of fast (kinetic energy approximately > 0.5-1.3 eV) metal electrons caused by moderately exothermic (1-3 eV) chemical reactions over high work function (4-6 eV) metal surfaces. In this report, the relation between chemicurrent and surface chemistry is elucidated with a combination of top-down phenomenology and bottom-up atomic-scale modeling. Examination of catalytic CO oxidation, an example which exhibits a chemicurrent, reveals 3 constituents of this relation: The localization of some conduction electrons to the surface via a reduction reaction, 0.5 O(2) + deltae(-) --> O(delta(-)) (Red); the delocalization of some surface electrons into a conduction band in an oxidation reaction, O(delta(-)) + CO --> CO(2)(delta-) --> CO(2) + deltae(-) (Ox); and relaxation without charge transfer (Rel). Juxtaposition of Red, Ox, and Rel produces a daunting variety of metal electronic excitations, but only those that originate from CO(2) reactive desorption are long-range and fast enough to dominate the chemicurrent. The chemicurrent yield depends on the universality class of the desorption process and the distribution of the desorption thresholds. This analysis implies a power-law relation with exponent 2.66 between the chemicurrent and the heat of adsorption, which is consistent with experimental findings for a range of systems. This picture also applies to other oxidation-reduction reactions over high work function metal surfaces.

Nanostructured porous silicon by laser assisted electrochemical etching

Nanostructured porous silicon (pSi) was fabricated by combining electrochemical etching with 355 nm laser processing. pSi prepared in this way proves to be an excellent substrate for desorption/ionization on silicon (DIOS) mass spectrometry (MS). Surfaces prepared by electrochemical etching and laser irradiation exhibit strong quantum confinement as evidenced by the observation of a red shift in the Si Raman band at ∼520–500 cm −1. The height of the nanostructured columns produced by electrochemical etching and laser processing is on the order of microns compared with tens of nanometers obtained without laser irradiation. The threshold for laser desorption and ionization of 12 mJ/cm 2 using the pSi substrates prepared in this work is lower than that obtained for conventional matrix assisted laser desorption ionization (MALDI)-MS using a standard matrix compound such as α-cyano-4-hydroxycinnamic acid (CHCA; 30 mJ/cm 2). Furthermore, the substrates prepared by etching and laser irradiation appear to resist laser damage better than those prepared by etching alone. These results enhance the capability of pSi for the detection of small molecular weight analytes by DIOS-MS.

Two-color laser desorption of nanostructured MgO thin films

Neutral magnesium atom emission from nanostructured MgO thin films is induced using two-color nanosecond laser excitation. We find that combined vis/UV excitation, for single-color pulse energies below the desorption threshold, induces neutral Mg-atom emission with hyperthermal kinetic energies in the range of 0.1–0.2 eV. The observed metal atom emission is consistent with a mechanism involving rapid electron transfer to three-coordinated Mg surface sites. The two-color Mg-atom signal is significant only for parallel laser polarizations and temporally overlapped laser pulses indicating that intermediate excited states are short-lived compared to the 5 ns laser pulse duration.

Direct Bacterial Profiling by Matrix-Assisted Laser Desorption−Ionization Time-of-Flight Mass Spectrometry for Identification of Pathogenic Neisseria

The present study investigates the suitability of direct bacterial profiling as a tool for the identification and subtyping of pathogenic Neisseria. The genus Neisseria includes two human pathogens, Neisseria meningitidis and Neisseria gonorrhoeae, as well as several nonpathogenic Neisseria species. Here, a matrix-assisted laser desorption/ionization time-of-flight mass spectrometry profiling protocol was optimized using a laboratory strain of E. coli DH5alpha to guarantee high quality and reproducible results. Subsequently, mass spectra for both laboratory and clinical strains of N. gonorrhoeae, N. meningitidis, and several nonpathogenic Neisseria species were collected. Significant interspecies differences but little intraspecies diversity were revealed by means of a visual inspection and bioinformatics examination using the MALDI BioTyper software. Cluster analysis successfully separated mass spectra collected from three groups that corresponded to N. gonorrhoeae, N. meningitidis, and nonpathogenic Neisseria isolates. Requiring only one bacterial colony for testing and using a fast and easy measuring protocol, this approach represents a powerful tool for the rapid identification of pathogenic Neisseria and can be adopted for other microorganisms.

Adsorption and Dissociation of CO on Bare and Ni-Decorated Stepped Rh(553) Surfaces

The adsorption and dissociation of carbon monoxide were studied with plane-wave density functional theory on flat Rh(111), stepped and kinked Rh(553), and Ni-decorated Rh(553) surfaces. The theoretical results were compared to high-resolution X-ray photoelectron spectroscopy (HR-XPS) experiments. The most favorable CO adsorption sites for low coverages were identified by a systematic calculation of the adsorption energies, and their sequence of occupation as a function of CO exposure was determined experimentally in C 1s HR-XPS spectra via their characteristic surface core-level shifts. On the clean, stepped (553) surface, molecular CO is adsorbed more strongly on low-coordinated top sites at the step edge, but on the Ni-decorated surface, the binding is stronger at the terrace sites. The barrier for dissociation with respect to the gas phase is about 1 eV lower on the stepped Rh(553) surface than on the flat Rh(111) surface, implying a substantially higher reaction rate. The presence of kinks at the clean Rh(553) surface does not lead to a significant additional decrease of the dissociation barriers, resulting in dissociation energies just above the desorption threshold for both stepped and kinked surfaces, whereas the barrier can be additionally lowered by about 0.1 eV by decorating the step edges with Ni stripes. Whereas no dissociation of CO was observed by HR-XPS on the clean Rh(553) surface, a minor amount of CO dissociation was found on the Ni-decorated Rh surface, in agreement with the theoretical predictions.

Reversibility, Dopant Desorption, and Tunneling in the Temperature-Dependent Conductivity of Type-Separated, Conductive Carbon Nanotube Networks

We present a comprehensive study of the effects of doping and temperature on the conductivity of single-walled carbon nanotube (SWNT) networks. We investigated nearly type-pure networks as well as networks comprising precisely tuned mixtures of metallic and semiconducting tubes. Networks were studied in their as-produced state and after treatments with nitric acid, thionyl chloride, and hydrazine to explore the effects of both intentional and adventitious doping. For intentionally and adventitiously doped networks, the sheet resistance (R(s)) exhibits an irreversible increase with temperature above approximately 350 K. Dopant desorption is shown to be the main cause of this increase and the observed hysteresis in the temperature-dependent resistivity. Both thermal and chemical dedoping produced networks free of hysteresis. Temperature-programmed desorption data showed that dopants are most strongly bound to the metallic tubes and that networks consisting of metallic tubes exhibit the best thermal stability. At temperatures below the dopant desorption threshold, conductivity in the networks is primarily controlled by thermally assisted tunneling through barriers at the intertube or interbundle junctions.

Reaction threshold and decoherence: current induced desorption of CO on Cu(111) in STM

The transfer of a CO molecule from the Cu(111) surface towards the STM tip, induced by the tunnelling current, occurs only if a threshold voltage of 2.4 eV is exceeded (Bartels L, Meyer G, Rieder K-H, Velic D, Knoesel E, Hotzel A, Wolf M and Ertl G 1998 Phys. Rev. Lett. 80 2004). We present a theory based on the Quantum Nano Dynamics (QND) approach, treating the competition between a coherent and a decoherent mechanism of current induced CO desorption via inelastic tunnelling and vibrational excitation. The keypoint of the theory is to regard the tunnelling process as driving the system in a transient excited state and to include explicitely many-particle relaxation effects in the excited state. Only a decoherent desorption mechanism via a transient negative ion resonance of the sample surface, which is created through electron injection and interacts with the CO molecule, provides an explanation for the existance of the voltage threshold for CO desorption and reproduces the quantum yield and the isotope effect as observed in the STM experiment.