A detailed study of the adsorption structure of self-assembled monolayers of 4-nitrothiophenol on the Au(111) surface was performed from a theoretical perspective via first-principles density functional theory calculations and experimentally by Raman and vibrational sum frequency spectroscopy (vSFS) with an emphasis on the molecular orientation. Simulations—including an explicit van der Waals (vdW) description—for different adsorbate structures, namely, for (3×3), (2 × 2), and (3 × 3) surface unit cells, reveal a significant tilting of the molecules toward the surface with decreasing coverage from 75° down to 32° tilt angle. vSFS suggests a tilt angle of 50°, which agrees well with the one calculated for a structure with a coverage of 0.25. Furthermore, calculated vibrational eigenvectors and spectra allowed us to identify characteristic in-plane (NO2 scissoring) and out-of-plane (C-H wagging) modes and to predict their strength in the spectrum in dependence of the adsorption geometry. We additionally performed calculations for biphenylthiol and terphenylthiol to assess the impact of multiple aromatic rings and found that vdW interactions are significantly increasing with this number, as evidenced by the absorption energy and the molecule adopting a more upright-standing geometry. © 2021 Author(s).

Surface-sensitive vibrational sum frequency spectroscopy (vSFS) has been utilized to study the adsorption chemistry of small alcohols, namely, methanol, ethanol, 1-propanol, and 2-propanol on TiO2 thin films under near-ambient conditions. The vSF spectra in the C-H region reveal that methanol and ethanol adsorb both molecularly and dissociatively, while 1-propanol and 2-propanol are solely detected in the molecular form. The different adsorption behavior suggests that the extent of dissociation decreases from methanol to propanol. Moreover, polarization analysis of the spectra reveals that the methyl groups are preferentially oriented with their symmetry axis pointing in a direction close to the surface normal for methanol, ethanol, and 1-propanol. However, for 2-propanol, the methyl groups exhibit a larger tilt angle. © 2021 American Chemical Society.

The vibrational dynamics of the various CH stretching modes in a fatty acid Langmuir-Blodgett film was studied using a resonant narrowband infrared (IR) laser pulse for pumping and a broadband femtosecond IR visible pulse pair for detection in a sum frequency spectroscopy setup. The resulting two-dimensional spectra indicate that pumping either the antisymmetric methyl or methylene stretch results in the transfer of energy to the other modes on a time scale faster than 2 ps. This rapid process is followed by energy redistribution to other modes, presumably the bending and internal rotational modes, with a time constant of approximately 85 ps. The formation of gauche defects is not observed within the first 250 ps. The whole spectrum recovers on a time scale of several nanoseconds, indicating dissipation of the excitation energy into the substrate. Copyright © 2019 American Chemical Society.

Surface-sensitive vibrational sum-frequency spectroscopy (vSFS) has been utilized to study the adsorption behavior of methanol on anatase TiO2 thin films under ambient temperature and pressure condition. The vSF spectra in the C-H region establish the presence of both molecular methanol and dissociated methoxy adsorbates. Spectra for the isotopologues of methanol also support the deprotonatation process at the surface. Furthermore, by a systematic study of the vSF spectra of methanol isotopologues in the C-H and C-D stretching regions, we clarify that in the CH spectral region the molecular signal coherently interferes with the near-resonant one from hydrogen bonded OH. The latter is due to adsorption from the water vapor present in the background and its interaction with surface OH. Copyright © 2020 American Chemical Society.

Vibrational sum frequency spectra (vSFS) of 4-nitrothiophenol (4-NTP) have been utilized to study plasmonic effects arising from its interaction with gold nanoparticles (Au NPs). To this end three systems have been studied: 4-NTP adsorbed on deposited Au nanoparticles, a self-assembled monolayer of 4-NTP on flat Au with dispersed Au NPs atop and, as reference, a self-assembled monolayer of 4-NTP. For 4-NTP on 50 and 80 nm Au NPs an enhancement of the SF intensity due to plasmonic effects by less than 1 order of magnitude is inferred in comparison to the monolayer system after taking the particle density into account. The addition of Au NPs to the monolayer system on flat Au also selectively enhances the response to an in-plane field component of the 532 nm upconversion light. © 2019 American Chemical Society.

The photodesorption of CO from Si(1 0 0) has been studied using 355 and 266 nm light obtaining time-of-flight spectra detailing the translation energy released. The spectra clearly exhibit two non-thermal desorption channels, of which one is characterized by an offset energy the size of which is remarkably close to the vibrational quantum of CO. Detailed studies of the spectra as function of coverage and laser fluence used prove that both desorption channels originate from a single adsorbed species. It is suggested that the energy offset is due to vibrational to translational energy transfer along the reaction pathway of the desorbing molecule. © 2018 Elsevier B.V.

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. © 2018 Elsevier B.V.

The structure of calcium arachidate Langmuir-Blodgett monolayers that were prepared at different pH in the subphase, were studied using vibrational sum frequency spectroscopy with a focus on testing their stability against melting upon heating. The pH controls the degree of deprotonation of the arachidic acid molecules. We find that at pH less than 9.5 for which the films are slightly less densely packed, they are also significantly less well ordered. Moreover, these films also develop gauche defects much more rapidly upon heating. Deriving an apparent activation energy for defect formation we find that it is 25% and 60% smaller for pH 7 and 4, respectively, when compared to pH 9.5. If prepared at pH 4, the film looses order already at 350 K. © 2017 Elsevier B.V.

Metal heterostructures have been used in recent years to gain insights into the relevance of energy dissipation into electronic degrees of freedom in surface chemistry. Non-adiabaticity in the surface chemistry results in the creation of electron-hole pairs, the number and energetic distribution of which need to be studied in detail. Several types of devices, such as metal-insulator-metal, metal-semiconductor and metal-semiconductor oxide-semiconductor, have been used. These devices operate by spatially separating the electrons from the holes, as an internal barrier allows only-or at least favours-transport from the top to the back electrode for one kind of carrier. An introduction into the matter, a survey of the literature and a critical discussion of the state of research is attempted. © 2016 The Royal Society of Chemistry.

The energy transferred from a heated platinum surface to an adjacent argon gas of several mbar pressure is studied. The cooling effects during accommodation of the argon atoms when colliding with the surface can be monitored by current changes in a chemoelectronic device, in this case consisting of silicon, silicon-oxide and platinum. A numerical heat flow model and the experimentally observed temperature dependence of the conduction in the device show that the observed signals are due to a cooling of the whole device. Temperature gradients and accompanying thermoelectric effects play only a minor role. © 2014 Published by Elsevier Ltd.

An experimental setup, which combines direct heating and temperature measurement of metal nanofilms allowing temperature programmed desorption experiments is described. This setup enables the simultaneous monitoring of the thermal desorption flux from the surface of chemi-electric devices and detection of chemically induced hot charge carriers under UHV conditions. This method is demonstrated for the case of water desorption from a Pt/SiO2-n-Si metal-oxide-semiconductor nanostructure. © 2014 AIP Publishing LLC.

The conformational order in Ca-arachidate Langmuir-Blodgett films on solid glass supports is investigated by means of vibrational sum-frequency generation spectroscopy (VSFG). The symmetric C-H stretching vibrations of both the terminal methyl and the methylene groups are utilized to monitor the chain conformation at various sample temperatures under ambient conditions. At room temperature the film is well-ordered consisting almost entirely of all-trans configured chains. Between 340 and 430 K we observe a marked increase in gauche-defects before oxidative degeneration starts at sample temperatures above 470 K. The temperature dependence of the data is well represented by apparent enthalpy changes for the formation of gauche-defects, sharply increasing with packing density from 29 to 62 kJ/mol; values, which are an order of magnitude larger than those of the gas phase molecule. These large apparent enthalpies do not prevent the formation of a high degree of conformational disorder at elevated temperatures. © 2014 AIP Publishing LLC.

Vibrational sum frequency generation spectroscopy is used to characterize octadecylsiloxane monolayers on glass substrates at ambient conditions with a focus on thermally induced conformational disorder. Different modes of the C-H stretching vibrations of the terminal methyl groups and the methylene groups are therefore monitored in the frequency range of 2850-3000 cm-1. We observe a progressive increase of conformational disorder of the alkyl chains due to gauche defects over the temperature range from 300 to 510 K. The conformational disorder is reversible over a temperature range from 300 to about 410 K. But after heating to temperatures above 410 K, order is not reestablished on the time scale of the experiment. These results suggest that the assumption of an all-trans configuration of the alkyl chains is an over-simplification which increasingly misrepresents the situation for elevated temperatures which are still well below the one at which decomposition starts. © 2013 AIP Publishing LLC.

The creation of electronic excitations during the reaction of atomic hydrogen on and with coinage and noble metals has been studied using metal-insulator-metal heterostructures. A characteristic current trace is observed when the outer metal surface of the structure is exposed to a 20 s pulse of H atoms. Comparison to the chemical kinetics allows to disentangle the contributions from the different chemical processes to this current. In the case of the coinage metals studied the observation is interpreted to suggest that predominantly electrons excited in connection with the hydrogen recombination reaction are the source of the current. For Pt such phenomenon is not observed. This difference allows insights into the role of the transition state for the non-adiabaticity in this simple chemical reaction.© by Oldenbourg Wissenschaftsverlag, München.

The chemicurrent response of Au-TaOx-Ta metal-insulator-metal junctions exposed to a flux of atomic hydrogen has been studied in detail for device temperatures between 110 and 300 K. Currents of some 100 pA are observed at an H atom flux of 4 × 1014 atoms cm-2 s-1. The steady-state current closely tracks the rate expected for the Langmuir-Hinschelwood recombination reaction as a function of temperature. The current trace reflects the reaction kinetics when the H-atom flux is modulated. The rate constant is directly determined from the individual trace at each temperature. Moreover, it is observed that the Fermi level of the 20 nm thick top metal film shifts with varying H coverage, giving rise to an additional charging/discharging current. For device temperatures below 200 K, subsurface hydrogen becomes significant. © 2013 American Chemical Society.

The possible origins and the magnitude of an electric current arising in nanofilm metal-semiconductor heterostructures when the metal surface is used to catalyze an exothermic chemical reaction are discussed. Two key mechanisms are considered that are responsible for the current generation: electron motion due to a temperature drop across the metal-semiconductor interface (thermionic emission mechanism) and the Seebeck effect in the two layers (thermoelectric mechanism). It is predicted that (i) current up to 10-3 A·cm-2 can arise, (ii) thermoelectric mechanism due to the Seebeck effect in the semiconductor layer plays a dominant role for the current generated under stationary chemical reaction conditions, and (iii) thermoelectric current strongly depends on the temperature. The carrier transport through the metal-semiconductor interface is described by the thermionic emission theory. The obtained results are discussed in view of the experimental data reported earlier for the chemical reaction-induced currents in metal/n-Si structures. © 2013 American Vacuum Society.

Monolayer graphene was prepared on an Ir(111) substrate where it exhibits a 25 × 25 Å2 moiré pattern. Molecular hydrogen was dosed first, allowing it to dissociate on open areas of the Ir substrate. The generated H atoms formed an intercalated reservoir that can bind to the graphene subsequently. Next, atomic hydrogen was dosed, which binds to the graphene sheet and also initiates the transfer of H from the Ir substrate to the graphene sheet. The opposite sides of the sheet can be hydrogenated with isotope selectivity, as a sequence of difference isotopes, H or D, can be chosen at will in the preparation procedure. Sum-frequency generation spectra prove that as consequence of the dosing sequence, C-H bonds are predominantly pointing toward the Ir substrate side when H2 is dosed first and alternatively toward the vacuum side when D2 is dosed first. © 2013 American Chemical Society.

IR-vis sum-frequency generation spectroscopy is used to study the stretching vibration of hydrogen chemically bound to a monolayer graphene sheet prepared on an Ir (1 1 1) substrate. A line characteristic for sp3 bound hydrogen is observed when a mixture of H and D is dosed. The intensity per surface H oscillator is largest when H is strongly diluted in D. This is interpreted to indicate that graphane is formed by concurrent attachment of hydrogen to graphene from the gas phase and from intercalated hydrogen. In this case isotope mixing is a prerequisite for SFG activity. © 2012 Elsevier B.V. All rights reserved.

A direct, noninvasive thermometry method based on the temperature dependence of the in-plane electrical resistance R(T) of 20 nm Pt films on Si-based semiconducting substrates is presented. At the calibration stage, the entire sample is slowly heated or cooled by external means. For moderately doped semiconductors, the R(T) dependence is closely linear at low temperatures (T 300 K), when the nanofilm metal conductance dominates, followed by a maximum and a subsequent decrease attributed to an increasing substrate conductance at higher temperatures. The position of the maximum depends on the Pt-substrate interface conditions, in particular, on the presence of an oxide layer, and may vary from 350 to 500 K. With the R(T) dependence measured in equilibrium one can derive T(R), which provides a highly accurate method for noninvasive measuring and controlling the temperature of the nanofilm with a direct resistive heating technique. Accuracy of the present method for dynamical measurement is shown to be significantly better than that of a standard approach using a Pt resistance temperature detector attached to the nanofilm for heating rates of 1-7 K/s. © 2012 American Vacuum Society.

Associated with chemical reactions at surfaces energy may be dissipated exciting surface electronic degrees of freedom. These excitations are detected using metal-insulator-metal (MIM) heterostructures (Ta-TaOx-Au) and the reactions of H with and on a Au surface are probed. A current corresponding to 510-5 electrons per adsorbing H atom and a marked isotope effect are observed under steady-state conditions. Analysis of the current trace when the H atom flux is intermitted suggests that predominantly the recombination reaction creates electronic excitations. Biasing the front versus the back electrode of the MIM structure provides insights into the spectrum of electronic excitations. The observed spectra differ for the two isotopes H and D and are asymmetric when comparing negative and positive bias voltages. Modeling indicates that the excited electrons and the concurrently created holes differ in their energy distributions. © 2011 American Institute of Physics.

IR-vis sum-frequency generation spectroscopy is used to study the stretching vibration of hydrogen chemically bound to a graphene sheet prepared on an Ir (1 1 1) crystal surface. We observe two distinct resonances at 2563 and 2716 cm-1. Similarly, lines at 1881 and 2027 cm-1 are found after D atom adsorption. The assignment to C-H stretching vibrations is discussed in view of the propensity for dimer formation reported for hydrogen adsorbed on graphene. The lines are assigned to hydrogen bound in either ortho or para configuration to the hexagonal mesh of carbon atoms. © 2011 Elsevier B.V. All rights reserved.

Thin amorphous tantalum films are prepared on Si(111) substrates in a metallic glassy state. The amorphous monoatomic state of the film is characterized by X-ray diffraction studies. The glassy state leads to a negative temperature coefficient of the resistivity (TCR) for low sample temperatures < 200 K which is attributed to incipient localization. Above 200 K a positive TCR is observed as expected for a normal Boltzmann transport regime. Upon heating the Si substrate to 1200 K TaSi2 is formed out of the amorphous tantalum film and the silicon substrate. The TaSi2 layer is crystalline as evident from X-ray diffraction data. Schematic drawing of the evaporation setup on either glass or silicon samples. Scheme of annealing effects. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The optical response of thin-film metal-insulator-metal (MIM) systems of tantalum-tantalum oxide-Au type is studied by recording the macroscopic current across the device resulting from the low-energy electron-hole pairs excited in the metals by red and near-infrared (NIR) light (hν < 2 eV). It is observed that current flows from the top Au to the back Ta electrode, although a larger number of photons is absorbed in the latter. This directional preference is attributed to the built-in electric field across the oxide layer. The yield per photon increases strongly as photon energy becomes comparable to the barrier height. Current exhibits a strong dependence on bias voltages applied across the oxide layer. Photoyields induced by NIR light (hν ∼ 1.5 eV) were found to be comparable to recently observed chemicurrents arising from exposure of a MIM sensor to atomic hydrogen, when compared on a current per photon to current per impinging hydrogen atom basis. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.