Inverse Volcano: A New Molecule-Surface Interaction Phenomenon.

Explosive desorption of guest molecules embedded in amorphous solid water upon its crystallization is known as the "molecular volcano." Here, we describe an abrupt ejection of NH_{3} guest molecules from various molecular host films toward a Ru(0001) substrate upon heating, utilizing both temperature programmed contact potential difference and temperature programmed desorption measurements. NH_{3} molecules abruptly migrate toward the substrate due to either crystallization or desorption of the host molecules, following an "inverse volcano" process considered a highly probable phenomenon for dipolar guest molecules that strongly interact with the substrate.

Chemical Reactivity of Strongly Interacting, Hydrogen-Bond-Forming Molecules Following 193 nm Photon Irradiation: Methanol in Amorphous Solid Water at Low Temperatures.

Mixtures of methanol and amorphous solid water (ASW) ices are observed in the interstellar medium (ISM), where they are subject to irradiation by UV photons and bombardment by charged particles. The charged particles, if at high enough density, induce a local electric field in the ice film that potentially affects the photochemistry of these ices. When CD3OD@ASW ices grown at 38 K on a Ru(0001) substrate are irradiated by 193 nm (6.4 eV) photons, products such as HD, D2, CO, and CO2 are formed in large abundances relative to the initial amount of CD3OD. Other molecules such as D2O, CD4, acetaldehyde, and ethanol and/or dimethyl ether are also observed, but in smaller relative abundances. The reactivity cross sections range from (2.6 ± 0.3) × 10-21 to (3.8 ± 0.3) × 10-25 cm2/photon. The main products are formed through two competing mechanisms: direct photodissociation of methanol and water and dissociative electron attachment (DEA) by photoelectrons ejected from the Ru(0001) substrate. An electric field of 2 × 108 V/m generated within the ASW film during Ne+ ions bombardment is apparently not strong enough to affect the relative abundances (selectivity) of the photochemical products observed in this study.

Distribution of Weakly Interacting Atoms and Molecules in Low-Temperature Amorphous Solid Water.

Understanding the distribution and mixing of atoms and molecules in amorphous solid water (ASW) at low temperatures is relevant to the exploration of the astrochemical environment in the interstellar medium (ISM) that leads to the formation of new complex molecules. In this study, a combination of temperature programmed desorption (ΔP-TPD) experiments and Ne+ ion sputtering is used to determine the extent of mixing and distribution of guest atoms and molecules within thin ASW films deposited at 35 K on a Ru(0001) substrate, prior to sputtering. The mixing of krypton atoms and methyl chloride molecules within thin ASW films is directed by the physical properties of the respective species and the nature of their interaction with the host water molecules. While the Kr-H2O interaction may be described as a weak van der Waals attraction, the CD3Cl-H2O interaction can be characterized as weakly hydrophobic in nature. This leads to differences in the level of homogeneity in mixing and distribution of the guest species in the ASW film. Both krypton atoms and methyl chloride molecules reveal a propensity to migrate toward the ASW-vacuum interface. The krypton atoms migrate through both diffusion and displacement by incoming H2O molecules, while the methyl chloride molecules tend to move toward the vacuum interface primarily via displacement. This behavior results in more homogeneous mixing of Kr in ASW at 35 K compared to the dipole moment containing molecule CD3Cl. As a general outcome of our study, it is observed that mixing in ASW at low temperatures is more homogeneous when the guest atom/molecule is inert and does not possess a constant dipole moment.

Formamidinium Halide Perovskite and Carbon Nitride Thin Films Enhance Photoreactivity under Visible Light Excitation.

Photochemical and photocatalytic activity of adsorbates on surfaces is strongly dependent on the nature of a given substrate and its resonant absorption of the (visible) light excitation. An observation is reported here of the visible light photochemical response of formamidinium lead bromide (FAPbBr3) halide perovskite and carbon nitride (CN) thin-film materials (deposited on a SiO2/Si(100) substrate), both of which are known for their photovoltaic and photocatalytic properties. The goal of this study was to investigate the role of the substrate in the photochemical reactivity of an identical probe molecule, ethyl chloride (EC), when excited by pulsed 532 nm laser under ultrahigh vacuum (UHV) conditions. Postirradiation temperature-programmed desorption (TPD) measurements have indicated that the C-Cl bond dissociates following the visible light excitation to form surface-bound fragments that react upon surface heating to form primarily ethane and butane. Temperature-dependent photoluminescence (PL) spectra of the FAPbBr3 films were recorded and decay lifetimes were measured, revealing a correlation between length of PL decay and the photoreactivity yield. We conclude that the FAPbBr3 material with its absorption spectrum in resonance with visible light excitation (532 nm) and longer PL lifetime leads to three times faster (larger cross-section) photoproduct formation compared with that on the CN substrate. These results contrast the behavior under ambient conditions where the CN materials are photochemically superior due, primarily, to their stability within humid environments.

Same-Energy UV Photons and Low-Energy Electrons Activating Methane and Ammonia Frozen in Amorphous Solid Water.

UV photons and low-energy electrons play an important role in the evolution of various molecules in the interstellar medium (ISM). Here, we examined the product molecule formation as a result of irradiation of 193 nm photons and 6.4 eV electrons (same energy under identical laboratory conditions) on D2O|CH4 + ND3|D2O sandwiched films deposited on Ru(0001) substrate at 25 K in ultrahigh vacuum as a model for processes in the ISM. Temperature-programmed desorption spectra performed following the irradiation revealed the signature of hydrazine and formamide product molecules. These molecules were, however, formed exclusively following the photons' irradiation. These results were compared with the products obtained from a D2O|CH4|D2O sample without ammonia, where deuterated formaldehyde was the dominant product, formed also by photons only. Our results indicate that the photon-induced activation of the cofrozen molecules within D2O occurs via direct (partial) dissociation of the host and embedded molecules, followed by sample annealing. The electron-induced activation occurs through a direct dissociative electron attachment mechanism. The results presented here suggest possible pathways to generate various C-N, C-O, C-C, N-O, and N-H bonds containing molecules in the ISM.

Spontaneous polarization of thick solid ammonia films.

Ammonia molecules have an important role in the radiation-induced chemistry that occurs on grains in the cold interstellar medium and leads to the formation of nitrogen containing molecules. Such grains and surfaces are primarily covered by water ices; however, these conditions allow the growth of solid ammonia films as well. Yet, solid ammonia know-how lags the vast volume of research that has been invested in the case of films of its "sibling" molecule water, which, in the porous amorphous phase, spontaneously form polar films and can cage coadsorbed molecules within their hydrogen-bonded matrix. Here, we report on the effect of growth temperature on the spontaneous polarization of solid ammonia films (leading to internal electric fields of ∼105 V/m) within the range of 30 K-85 K on top of a Ru(0001) substrate under ultra-high vacuum conditions. The effect of growth temperature on the films' depolarization upon annealing was recorded as well. By demonstrating the ability of ammonia to cage coadsorbed molecules, as water does, we show that temperature-programmed contact potential difference measurements performed by a Kelvin probe and especially their temperature derivative can track film reorganization/reconstruction and crystallization at temperatures significantly lower than the film desorption.

The role of thermal history on spontaneous polarization and phase transitions of amorphous solid water films studied by contact potential difference measurements.

Monitoring thermal processes occurring in molecular films on surfaces can provide insights into physical events such as morphology changes and phase transitions. Here, we demonstrate that temperature-programmed contact potential difference (TP-∆CPD) measurements employed by a Kelvin probe under ultrahigh vacuum conditions and their temperature derivative can track films' restructure and crystallization occurring in amorphous solid water (ASW) at temperatures well below the onset of film desorption. The effects of growth temperature and films' thickness on the spontaneous polarization that develops within ASW films grown at 33 K-120 K on top of a Ru(0001) substrate are reported. Electric fields of ∼106 V/m are developed within the ASW films despite low average levels of molecular dipole alignment (<0.01%) normal to the substrate plane. Upon annealing, an irreversible morphology-dependent depolarization has been recorded, indicating that the ASW films keep a "memory" of their thermal history. We demonstrate that TP-∆CPD measurements can track the collapse of the porous structure at temperatures above the growth and the ASW-ice Ic and ASW-ice Ih transitions at 131 K and 157 K, respectively. These observations have interesting implications for physical and chemical processes that take place at the interstellar medium such as planetary formation and photon- and electron-induced synthesis of new molecules.

CsPbBr3 and CH3NH3PbBr3 promote visible-light photo-reactivity.

Inorganic and organic lead halide perovskite materials attract great interest in the scientific community because of their potential for low-cost, high efficiency solar cells. In this report we add a new property of these materials, namely their photochemical activity in the visible light range. Both inorganic (CsPbBr3) and organic (CH3NH3PbBr3-MAPbBr3) perovskite thin films were demonstrated to promote photo-dissociation of adsorbed ethyl chloride (EC), employing 532 nm pulsed laser irradiation under ultra-high vacuum (UHV) conditions. From the post-irradiation temperature programmed desorption (TPD) analysis, the yield of photoproduct formation was found to be up to two orders of magnitude higher than for UV light-excited EC molecules on metallic and oxide surfaces. Photo-reactivity on top of the CsPbBr3 surface is almost an order of magnitude more efficient than on the CH3NH3PbBr3 surface, apparently due to the lower density of defect and surface states. A direct correlation was found between electron-induced luminescence and photoluminescence intensities and the photoreactivity cross-sections. We conclude that both the intense luminescence and the well-known photovoltaic properties associated with these halide perovskite materials are consistent with the efficiency of photo-reactivity in the visible range, reported here for the first time.

Enhanced photochemistry of ethyl chloride on Ag nanoparticles.

Enhanced photodecomposition of ethyl chloride (EC) adsorbed on SiO2/Si (100) supported silver nanoparticles (Ag NPs) under ultrahigh vacuum (UHV) conditions has been studied in order to assess the potential contribution of plasmonic effects. The cross section for photodecomposition of EC and overall photoyield were found to increase with increasing photon energy regardless of the plasmon resonant wavelength and with Ag coverage without any noticeable particle size effect. The influence of EC-Ag NPs separation distance on the rate of EC decomposition was studied in order to examine potential local electric field influence on the photodissociation process. Long (∼5 nm) photoactivity decay distance has been observed which excludes local surface plasmon dominance in the photodecomposition event. These findings suggest that the alignment of excited electron energy and adsorbate affinity levels is central for efficient photochemical reactions, whereas short-range electric field enhancement by plasmon excitation on top and at the immediate vicinity of silver nanoparticles does not have any measurable effect.

Tailor-made oxide architectures attained by molecularly permeable metal-oxide organic hybrid thin films.

Tailor-made metal oxide (MO) thin films with controlled compositions, electronic structures, and architectures are obtained via molecular layer deposition (MLD) and solution treatment. Step-wise formation of permeable hybrid films by MLD followed by chemical modification in solution benefits from the versatility of gas phase reactivity on surfaces while maintaining flexibility which is more common at the liquid phase.

Electron-induced chemistry of methyl chloride caged within amorphous solid water.

The interaction of low energy electrons (1.0-25 eV) with methyl-chloride (CD3Cl) molecules, caged within Amorphous Solid Water (ASW) films, 10-120 monolayer (ML) thick, has been studied on top of a Ru(0001) substrate under Ultra High Vacuum (UHV) conditions. While exposing the ASW film to 3 eV electrons a static electric field up to 8 × 10(8) V∕m is developed inside the ASW film due to the accumulation of trapped electrons that produce a plate capacitor voltage of exactly 3 V. At the same time while the electrons continuously strike the ASW surface, they are transmitted through the ASW film at currents of ca. 3 × 10(-7) A. These electrons transiently attach to the caged CD3Cl molecules leading to C-Cl bond scission via Dissociative Electron Attachment (DEA) process. The electron induced dissociation cross sections and product formation rate constants at 3.0 eV incident electrons at ASW film thicknesses of 10 ML and 40 ML were derived from model simulations supported by Thermal Programmed Desorption (TPD) experimental data. For 3.0 eV electrons the CD3Cl dissociation cross section is 3.5 × 10(-16) cm(2), regardless of ASW film thickness. TPD measurements reveal that the primary product is deuterated methane (D3CH) and the minor one is deuterated ethane (C2D6).

Surface diffusion of gold nanoclusters on Ru(0001): effects of cluster size, surface defects and adsorbed oxygen atoms.

Understanding thermal behavior of metallic clusters on their solid supports is important for avoiding sintering and aggregation of the active supported metallic particles in heterogeneous catalysis. As a model system we have studied the diffusion of gold nano-clusters on modified Ru(0001) single crystal surfaces, employing surface density grating formation via a laser induced ablation technique. Surface modifications included damage induced by varying periods of Ne(+) ion sputtering at a collision energy of 2.8 keV and the effect of pre-adsorbed oxygen on the clean, defect free ruthenium surface. High density of surface damage, obtained at long sputter times, has led to enhanced diffusivity with lower onset temperature for diffusion. It is attributed to reduced cluster-surface commensurability which gives rise to smaller effective activation energy for diffusion. The diffusion of gold nano-clusters, 2 nm in size, was found to be insensitive to the oxygen surface concentration. The adsorbed oxygen acted as an "atomic layer lubricant", reducing friction between the cluster and the underlying surface. This has led to lower diffusivity onset temperatures (150 K) of the nano-clusters, with a stronger effect on smaller clusters.

Reduced oxide sites and surface corrugation affecting the reactivity, thermal stability, and selectivity of supported Au-Pd bimetallic clusters on SiO2/Si(100).

The morphology and surface elemental composition of Au-Pd bimetallic nanoclusters are reported to be sensitive to and affected by reduced silicon defect sites and structural corrugation on SiO2/Si(100), generated by argon ion sputtering under ultrahigh vacuum (UHV) conditions. Metastable structures of the bimetallic clusters, where Au atoms are depleted from the top surface upon annealing, are stabilized by the interaction with the reduced silica sites, as indicated from CO temperature programmed desorption (TPD) titration measurements. Acetylene conversion to ethylene and benzene has been studied as a probe reaction, revealing the modification of selectivity and reactivity enhancement in addition to improved thermal stability on substrates rich in reduced-silica sites. These observations suggest that these unique sites play an important role in anchoring thermodynamically metastable conformations of supported Au-Pd bimetallic catalysts and dictate their high-temperature activity.

Highly efficient photoinduced desorption of N2O and CO from porous silicon.

Photoinduced desorption (PID) of N(2)O and CO from porous silicon (PSi) samples is reported. Both adsorbates exhibit unusually large cross sections for PID at 193 nm, up to 10(-15) cm(2), 2-3 orders of magnitude larger than the literature values for similar processes on flat Si. Under this UV irradiation, N(2)O molecules undergo photodissociation (a competing process leading to surface oxidation) with a cross section that is 2 orders of magnitude smaller than photodesorption. In the case of CO desorption is the exclusive photodepletion mechanism. PID efficiency decreases with increasing CO coverage suggesting PID hindrance by interactions among the desorbing CO molecules leading to re-adsorption at higher coverage. The wavelength and fluence dependence measurements exclude the possibility of laser induced thermal desorption for both adsorbates. The proposed mechanism for this phenomenon is desorption induced by hot electron transfer from the substrate to the adsorbate. Enhanced lifetime of transient negative adsorbate due to stabilization by localized holes on PSi nanotips can explain the observed abnormally large PID efficiency on top of porous silicon.

Reactive-layer-assisted deposition mechanism and characterization of titanium oxide films.

The growth mechanism of TiO(2) films and their morphology are reported using the reactive-layer-assisted deposition (RLAD) method under ultrahigh vacuum conditions. The oxide film formation involves Ti atom deposition on top of amorphous solid water (ASW) condensed on a SiO(2)/Si(100) support at 90 K. Subsequent annealing leads to the desorption of all nonreacted buffer molecules, resulting in the deposition of the titanium oxide film. Employing mass spectrometry and using D(2)O as a buffer, we detected the evolution of deuterium molecules during titanium atom deposition. A solid state sol-gel-like formation mechanism of titanium oxide is proposed on the basis these observations. The morphology of the oxide films is characterized by AFM as a rather uniform amorphous thin film at room temperature. Upon further annealing above 750 K, crystallization of the titanium oxide film has set in, coinciding with a dewetting process of the oxide layer, and information obtained from similar growth procedure on an amorphous carbon-covered TEM grid. It was shown that these films are rather insensitive to the underlying substrate at temperatures below 500 K.

Low energy charged particles interacting with amorphous solid water layers.

The interaction of charged particles with condensed water films has been studied extensively in recent years due to its importance in biological systems, ecology as well as interstellar processes. We have studied low energy electrons (3-25 eV) and positive argon ions (55 eV) charging effects on amorphous solid water (ASW) and ice films, 120-1080 ML thick, deposited on ruthenium single crystal under ultrahigh vacuum conditions. Charging the ASW films by both electrons and positive argon ions has been measured using a Kelvin probe for contact potential difference (CPD) detection and found to obey plate capacitor physics. The incoming electrons kinetic energy has defined the maximum measurable CPD values by retarding further impinging electrons. L-defects (shallow traps) are suggested to be populated by the penetrating electrons and stabilize them. Low energy electron transmission measurements (currents of 0.4-1.5 µA) have shown that the maximal and stable CPD values were obtained only after a relatively slow change has been completed within the ASW structure. Once the film has been stabilized, the spontaneous discharge was measured over a period of several hours at 103 ± 2 K. Finally, UV laser photo-emission study of the charged films has suggested that the negative charges tend to reside primarily at the ASW-vacuum interface, in good agreement with the known behavior of charged water clusters.

Photoinduced desorption of Xe from porous Si following ultraviolet irradiation: evidence for a selective and highly effective optical activity.

Photoinduced desorption (PID) of Xe from porous silicon (PSi) following UV irradiation has been studied. A nonthermal, morphology, and wavelength dependent phenomenon with more than 3 orders of magnitude enhancement of Xe PID within pores over atoms adsorbed on top of flat surfaces has been recorded, displaying extraordinary large cross sections up to σ(Xe/PSi)=2×10(-15) cm(2). A long-lived, photoinduced, charge separated silicon-xenon complex is proposed as the precursor for this remarkable photodesorption process.

Selective ablation of Xe from silicon surfaces: molecular dynamics simulations and experimental laser patterning.

The mechanism of laser-induced removal of Xe overlayers from a Si substrate has been investigated employing MD simulations and evaluated by buffer layer assisted laser patterning experiments. Two distinct regimes of overlayer removal are identified in the simulations of a uniform heating of the Si substrate by a 5 ns laser pulse: The intensive evaporation from the surface of the Xe overlayer and the detachment of the entire Xe overlayer driven by explosive boiling in the vicinity of the hot substrate. Simulations of selective heating of only a fraction of the silicon substrate suggest that the lateral heat transfer and bonding to the unheated, colder regions of the Xe overlayer is very efficient and suppresses the separation of a fraction of the overlayer from the substrate. Interaction with surrounding cold Xe is responsible for significant increase in the substrate temperature required for achieving the spatially selective ablation of the overlayer. The predictions of the MD simulations are found to be in a qualitative agreement with the results of experimental measurements of the threshold laser power required for the removal of Xe overlayers of different thickness and the shapes of metallic stripes generated by buffer-assisted laser patterning.

Structure-reactivity correlations in Pd-Au bimetallic nanoclusters.

The effect of composition and morphology of bimetallic Pd-Au nanoclusters on their chemical reactivity has been studied with acetylene decomposition and conversion to ethylene and benzene as the chemical probe. High resolution transmission electron microscopy (HR-TEM) and CO-Temperature Programmed Desorption (TPD) measurements were employed for structure and chemical composition determination. Pd-Au clusters were prepared in ultrahigh vacuum (UHV) environment on SiO(2)/Si(100) by direct deposition (DD) to form 2D bimetallic nanostructures. Different bimetallic cluster morphology could be obtained by employing the buffer layer assisted growth (BLAG) procedure with amorphous solid water as buffer material. The BLAG bimetallic clusters were found to be more reactive than DD particles toward acetylene hydrogenation to ethylene and trimerization to benzene. The morphology and composition of DD clusters enabled the formation of both tilted (low adsorption energy) and flat laying (high adsorption energy) benzene, while mainly tilted benzene was detected upon adsorption of acetylene on BLAG clusters. Moreover, the reactivity of bimetallic clusters was compared to that of thin Pd film. Strong preference (100:1 ratio) toward acetylene hydrogenation to ethylene over trimerization to benzene has been correlated with the lack of extended Pd(111) facets on the bimetallic clusters that suppress the benzene formation.

Xe interacting with porous silicon.

Thin films of porous silicon (PS), structurally characterized by HR-SEM, were studied using xenon Temperature Programmed Desorption (TPD) as a probe of its inner pores. Geometric hindrance of the depth desorbing population and multiple wall collisions result in a unique double-peak structure of the TPD curve. Surface-diffusion assisted adsorption mechanism into inner pores at 48 K is proposed as the origin of these unique TPD spectra. It is experimentally verified by mild Ne(+) sputtering prior to TPD which preferentially removes Xe population from the top surfaces. A pore-diameter limited desorption kinetic model that takes into account diffusion and pore depth well explains the governing parameters that determine the experimental observations. These results suggest that TPD may be employed as a highly sensitive, non-destructive surface area determination tool.